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A tin mill black plate for canmaking, and method of manufacturing

21 Jul.,2025

A tin mill black plate for canmaking, and method of manufacturing

EPB1 - A tin mill black plate for canmaking, and method of manufacturing - Google Patents

A tin mill black plate for canmaking, and method of manufacturing Download PDF

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Publication number: EPB1
Authority: EP; European Patent Office
Prior art keywords: steel sheet; exceeding; steel; rolling; canmaking
Prior art date: -04-06
Legal status (The legal status is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the status listed.): Revoked

Application number

EPA

Other languages

German (de)

French (fr)

Other versions

EPA1 (en

Inventor

Hideo c/o Chiba Works Kawasaki Kuguminato

Toshikatsu c/o Chiba Works Kawasaki Kato

Chikako c/o Techn. Res. Div. Kawasaki Fujinaga

Kyoko c/o Techn. Res. Div. Kawasaki Hamahara

Current Assignee (The listed assignees may be inaccurate. Google has not performed a legal analysis and makes no representation or warranty as to the accuracy of the list.)

JFE Steel Corp

Original Assignee

Kawasaki Steel Corp

Priority date (The priority date is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the date listed.)

-04-06

Filing date

-04-06

Publication date

-07-02

Family has litigation

First worldwide family litigation filed litigation Critical https://patents.darts-ip.com/?family=&utm_source=google_patent&utm_medium=platform_link&utm_campaign=public_patent_search&patent=EP(B1) "Global patent litigation dataset” by Darts-ip is licensed under a Creative Commons Attribution 4.0 International License.

-04-06 Priority claimed from JPA external-priority patent/JPB2/en

-04-06 Application filed by Kawasaki Steel Corp filed Critical Kawasaki Steel Corp

-10-13 Publication of EPA1 publication Critical patent/EPA1/en

-07-02 Application granted granted Critical

-07-02 Publication of EPB1 publication Critical patent/EPB1/en

-04-06 Anticipated expiration legal-status Critical

Status Revoked legal-status Critical Current

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Classifications

- C—CHEMISTRY; METALLURGY
- C21—METALLURGY OF IRON
- C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
- C21D8/00—Modifying the physical properties by deformation combined with, or followed by, heat treatment
- C21D8/02—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips
- C21D8/04—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips to produce plates or strips for deep-drawing
- C21D8/—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips to produce plates or strips for deep-drawing characterised by the working steps
- C21D8/—Hot rolling
- C—CHEMISTRY; METALLURGY
- C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
- C22C—ALLOYS
- C22C38/00—Ferrous alloys, e.g. steel alloys
- C—CHEMISTRY; METALLURGY
- C21—METALLURGY OF IRON
- C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
- C21D8/00—Modifying the physical properties by deformation combined with, or followed by, heat treatment
- C21D8/02—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips
- C—CHEMISTRY; METALLURGY
- C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
- C22C—ALLOYS
- C22C38/00—Ferrous alloys, e.g. steel alloys
- C22C38/12—Ferrous alloys, e.g. steel alloys containing tungsten, tantalum, molybdenum, vanadium, or niobium
- C—CHEMISTRY; METALLURGY
- C21—METALLURGY OF IRON
- C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
- C21D8/00—Modifying the physical properties by deformation combined with, or followed by, heat treatment
- C21D8/02—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips
- C21D8/04—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips to produce plates or strips for deep-drawing
- C21D8/—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips to produce plates or strips for deep-drawing characterised by the working steps
- C21D8/—Cold rolling
- C—CHEMISTRY; METALLURGY
- C21—METALLURGY OF IRON
- C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
- C21D8/00—Modifying the physical properties by deformation combined with, or followed by, heat treatment
- C21D8/02—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips
- C21D8/04—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips to produce plates or strips for deep-drawing
- C21D8/—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips to produce plates or strips for deep-drawing characterised by the heat treatment
- C21D8/—Final recrystallisation annealing
- C—CHEMISTRY; METALLURGY
- C21—METALLURGY OF IRON
- C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
- C21D8/00—Modifying the physical properties by deformation combined with, or followed by, heat treatment
- C21D8/02—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips
- C21D8/04—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips to produce plates or strips for deep-drawing
- C21D8/—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips to produce plates or strips for deep-drawing involving a particular surface treatment
- Y—GENERAL TAGGING OF NEW TECHNOLOGICAL DEVELOPMENTS; GENERAL TAGGING OF CROSS-SECTIONAL TECHNOLOGIES SPANNING OVER SEVERAL SECTIONS OF THE IPC; TECHNICAL SUBJECTS COVERED BY FORMER USPC CROSS-REFERENCE ART COLLECTIONS [XRACs] AND DIGESTS
- Y10—TECHNICAL SUBJECTS COVERED BY FORMER USPC
- Y10T—TECHNICAL SUBJECTS COVERED BY FORMER US CLASSIFICATION
- Y10T428/00—Stock material or miscellaneous articles
- Y10T428/12—All metal or with adjacent metals
- Y10T428/—Composite; i.e., plural, adjacent, spatially distinct metal components [e.g., layers, joint, etc.]
- Y10T428/—Transition metal-base component
- Y10T428/—Group VIII or IB metal-base component
- Y10T428/—Co- or Ni-base component next to Fe-base component
- Y—GENERAL TAGGING OF NEW TECHNOLOGICAL DEVELOPMENTS; GENERAL TAGGING OF CROSS-SECTIONAL TECHNOLOGIES SPANNING OVER SEVERAL SECTIONS OF THE IPC; TECHNICAL SUBJECTS COVERED BY FORMER USPC CROSS-REFERENCE ART COLLECTIONS [XRACs] AND DIGESTS
- Y10—TECHNICAL SUBJECTS COVERED BY FORMER USPC
- Y10T—TECHNICAL SUBJECTS COVERED BY FORMER US CLASSIFICATION
- Y10T428/00—Stock material or miscellaneous articles
- Y10T428/12—All metal or with adjacent metals
- Y10T428/—Composite; i.e., plural, adjacent, spatially distinct metal components [e.g., layers, joint, etc.]
- Y10T428/—Transition metal-base component
- Y10T428/—Group VIII or IB metal-base component
- Y10T428/—Fe-base component
- Y10T428/—Next to Fe-base component

Definitions

the present invention relates to a tin mill black plate for canmaking, such sheet having temper rolling degrees of T1 - T6 or DR 8 - DR 10. This invention also relates to a method for manufacturing the sheet.
the present invention relates to a plated steel sheet for making a three-piece can, the sheet having small thickness, high strength and excellent welding properties. It further relates to a plated steel sheet for making a two-piece can, the sheet having small thickness and excellent drawability. This invention further relates to a method for manufacturing the sheets.
cans made from steel sheet There are two types of cans made from steel sheet, namely, two-piece cans and three-piece cans.
the former can be further classified as SDC (Shallow-Drawn Cans), DRDC (Drawn & Redrawn Cans), DTRC (Drawn & Thin Redrawn Cans), and DWIC (Drawn & Wall Ironed Cans).
These cans are manufactured by processes such as deep-drawing, ironing, bending, stretching and welding etc. appropriately tin-coated black plate.
the tin mill black plate can be classified, depending on the properties and methods of making the can to be manufactured, into temper degrees of T1 - T6 or DR8 - DR10.
Those black plates having temper degrees of T1 - T3 are called soft-temper tin mill black plates while those of T4 - T6 are called hard-temper tin mill black plates; both types are made by temper rolling a cold rolled steel sheet once.
classes DR8 - DR10 are called DR black plate, manufactured by rolling with a large rolling reduction to the cold rolled steel sheet.
these steel sheets have been manufactured by preparing parent materials having originally different composition, and individually varying the conditions for the hot rolling, the cold rolling, and the annealing etc. for each of them, due to their fundamentally different requirements for strength and processing properties and the like. As a result, the processes have had to be changed each time to meet the requirements for the desired sheet, causing the manufacturing cost to be relatively increased.
Steel sheet for cans must be thin with high strength to reduce cost.
the three-piece can is not an exception, but is further required to have high-speed welding properties. In particular, it must provide a high-quality seam by electric seam welding method at more than 70MPM of welding speed.
the welding current needs to be relatively high to provide sufficient welding strength, thereby causing HAZ cracking.
a coil coating process is carried out on steel sheet. It is desired to apply this coil coating method to steel for high-speed welding, but for this purpose it is necessary to form a non-varnished portion (not a coated portion) in parallel to the rolling direction and to arrange the winding direction of the can body in parallel to the rolling direction.
the steel sheet is generally subjected to tin-plating. Recently the coating weight of tin has been reduced to reduce cost. For example, while the conventional tin coating weight has been 2.8 g/m 2 , in the recent sheet that has sometimes been reduced to less than 1 g/m 2 . In such a case, the corrosion resistance of the steel sheet itself must be improved.
Japanese Patent Publication No. Hei 1- discloses a method for manufacturing steel sheets for T1 - T3 cans by applying continuous annealing and thereafter temper rolling ultra low carbon steel.
this method does not overcome all the aforementioned problems.
a tin mill black plate comprising chemical compositions composed of about C ⁇ 0.004 %, Si ⁇ 0.03 %, Mn: 0.05 - 0.6%, P ⁇ 0.02 %, S ⁇ 0.02 %, N ⁇ 0.01 %, Al: 0. 005 - 0.1 %, Nb: 0.001 - 0.1 %, B: 0. - 0.005 % (all in weight) and incidental impurities, the maximum grain size being less than 30 ⁇ m, and the area ratio of recrystallized grains having a grain size range of 5 - 25 ⁇ m being more than 50 %.
the r value, ⁇ r value and the generation of orange peels are considered to be important factors for the deep-drawability of two-piece cans.
the C content affects the hardness of steel sheet for tinplate, recrystallized grain size and earing.
the influence on hardness is shown in FIG. 1 and that on the earing is shown in FIG. 3. From these data, it is necessary to set the C content to less than about 0.004 % and preferably less than about 0.003 % for obtaining a temper degree of T1 and reducing the generation of earing on continuous annealing.
the generation of earing can be assessed in terms of the following formula: (Hmax - Hmin) / Hmin*100% where Hmax and Hmin stand respectively for the maximum and the minimum height of the can after pressing as shown in FIG. 2.
Si acts to degrade the corrosion resistance of tinplate and further tends to make the steel material extremely hard. It should not be present in an excessive amount. Namely, if the Si content exceeds about 0.03 %, the tinplate tends to become too hard, which makes it impossible to provide the temper degrees of T1 - T3; it should accordingly be less than about 0.03 %.
Mn should be added to prevent the hot rolled coil from cracking at its edge portion. That is, if the Mn content is less than about 0.05 %, the cracking cannot be avoided, while if it exceeds about 0.6 %, the crystal grain size becomes fine and tinplate itself becomes too hard. Therefore, Mn content should be within a range of about 0.05 - 0.06 %.
the Mn amount to be added depends on its relationship to the S content in the steel, as will be mentioned in more detail later.
the element P makes the steel material harder and degrades the corrosion resistance of tinplate and so should be limited to less than about 0.02 % of total content.
the element S may cause cracking of the hot-rolled coil at its edge portion and press defects are caused by sulfide inclusions, and is should be present in an amount less than about 0.02 %. If the ratio Mn/S is less than about 8, the cracking and the press defects would easily arise, so this ratio should exceed about 8.
Al plays a role as a deoxidant in the steel manufacturing process and is added in a proper amount since the cleanliness of the steel would increase proportionally to the increase of the Al content in the steel.
excessive Al would suppress the growth of the recrystallized grain size of the steel at the same time, so it should be less than about 0.10 % in content.
the Al content is less than about 0.005 %, the N content in the steel would increase. Therefore, the Al content should be in the range of about 0.005 - 0.10 %.
N tends to become introduced into the steel during the steelmaking process as a result of mixing of N in the air therewith, but a soft steel sheet cannot be obtained if N is present in the solid-state in the steel. Accordingly the N content should be less than about 0.01 %.
the O content should be less than about 0.01 %.
Nb and B are important elements affecting the recrystallized grain size after annealing. Namely, in an ultra low carbon steel with extremely reduced C content as the steel according to the present invention, the crystal grain size would sometimes become too coarse about 30 ⁇ m, causing orange peel formation as mentioned later. To overcome such a disadvantage and to control the crystal grain size, it is necessary to add both Nb and B together to the steel. Nb is an element necessary to suppress an excessive growth of the crystal particle, and further acts to form carbides or nitrides to reduce the remaining amount of solid-solved C and N, thereby enhancing the processing characteristics of the steel. To obtain these advantages, more than about 0.001 % of Nb should be added.
the Nb content of the steel should be less than about 0.1 %.
B present with Nb contributes to prevent the crystal grains from enlarging too much, and to reduce the secondary work brittleness. Namely, when a carbide forming element is added to an ultra low carbon steel, the strength of the recrystallized grain boundaries would become degraded. Therefore, there is a fear of causing brittle cracking when stored at very low temperature depending on the use of the can and the canning. This can be avoided by adding B to the material. Further, while B forms carbides and nitrides so as to be effective for making the steel softer, it would segregate in the recrystallized grain boundaries during the continuous annealing to retard the recrystallization. Therefore, the B content should be less than about 0.005 %, with the lower limit more than about 0. % which is necessary to manifest the foregoing advantages.
Ti is an element for forming carbide and nitride, and acts to reduce the remaining amount of solid-solved C and solid-solved N for improving the workability of the steel.
the Ti content should be less than about 0.1 % and should be added as required.
Sn, Sb, As and Te are enrichingly concentrated on the steel sheet during the annealing process and can act to prevent C from being enrichingly concentrated, so as to improve the adhesiveness and the corrosion resistance of the tinplate.
Sb and Sn should be added with contents of more than about 0.001 % respectively, while As (more than about 0.001 %) and Te (about 0.%) should be effective when added. Since an excessive addition of these elements would cause a lowering of the press workability, the upper limit of addition for each respective element should be about 0. 01 %.
Ca forms CaO in the molten steel.
Al 2 O 3 which has a very high melting point and hardness, reacts with this CaO, the Al 2 O 3 changes into inclusions having lower melting point and hardness. Therefore, even if Al 2 O 3 remains in the steel sheet by mistake, it would be divided into small pieces in the cold rolling process because of its softness so as not to cause any degradation of the product quality. Accordingly, the Ca content can be more than about 0. %, but with an upper limit of less than about 0.005 % since too much Ca would undesirably increase the non-metallic inclusions.
All of Mo, V, Zr act to increase the recrystallizing temperature during the continuous annealing process. Further, Cr, Cu, Ni, Na, Mg and REM increase the recrystallizing temperature as well as reduce the rolling characteristics of the steel, such that they may make it difficult to anneal the sheet continuously and to cold roll the steel sheet to a very thin gauge. Therefore, it would be preferable to limit the contents of these elements as follows: Mo, V, Zr .... less than about 0.01 %; Cr, Cu, Ni .... less than about 0.1 %; Na, Mg .... less than about 0.001 %; and REM .... less than about 0.005 %.
FIG. 6 shows a relationship between the diameter of maximum crystal grains and HAZ cracking when the winding direction of the can body is in parallel to the rolling direction of the steel sheet, not perpendicular to the rolling direction as in the conventional method.
FIG. 7 shows a relationship between the degree of reduction of thickness of the weld zone and HAZ cracking when the body of the three-piece can is bonded by high-speed welding.
the total thickness of the weld zone is affected by the diameter of the recrystallized grains of the steel sheet. According to experiments carried out by the present inventors, it has been found that if the area ratio of crystal particles of more than 5 ⁇ m exceeds about 50 %, the total thickness of the weld zone would become less than about 1.4 times of the thickness of material steel sheet.
FIG. 4 is a graphic diagram showing a relationship between area ratio of recrystallized particles ranging about 5 - 25 ⁇ m and earing when tin-plated steel sheet of ultra low carbon steel with a C content of less than about 0.004 % is deep-drawn.
the upper limit of the crystal grain size which would generate orange peeling is about 30 ⁇ m, and if the grain size exceeds that value, orange peeling would frequently take place.
the crystal grain size required for the tinplate should be less than about 30 ⁇ m for all the crystal grains, and the area ratio thereof ranging about 5 - 25 ⁇ m should exceed about 50 %.
the crystal grain size can be measured in such a manner that a cross section rolling direction of the tinplate is observed by a microscope, and then the dimensions in the long and short diameter directions are averaged. Further, the area ratio of the recrystallized grains ranging about 5 - 25 ⁇ m refers to the ratio of the recrystallized grains ranging about 5 - 25 ⁇ m, under a microscopic observation, in proportion to the total cross sectional area of the tinplate.
the finishing hot rolling thickness would be so small as about 2 - 3 mm due to the small product thickness.
the rolling time would become long due to its relationship to the capacity of the hot rolling mill, leading to a significant temperature lowering. Therefore, for increase FDT a very high SRT (slab reheating temperature) a problem as will mentioned later would arise and the temperature lowering during the rolling process becomes intense so as to cause dispersion of product quality. Therefore, FDT should be set at about 800 - 900 °C for desirable crystal diameter, product uniformity and less carbide deposition.
CT coiling temperature
CT should be set at less than about 650 °C. Further, since too low CT would cause excessively fine crystal particles, it should be set at more than about 500 °C for lowering the rolling characteristics.
the hot rolled steel strip is pickled, cold rolled, and continuously annealed at about 650 - 800 °C for less than about 60 seconds.
the cold rolling reduction ratio affects the crystal grain size, and if it is too small, the crystal grain size becomes excessively coarse and tends to lower the uniformity of the grain size. Accordingly, the rolling reduction ratio should be more than about 80 %.
annealing temperature makes the product too hard while too high temperature leads to an excessively coarse grain. Accordingly, the continuous annealing is carried out at about 650 °C - 800 °C. For good productivity, annealing time should be less than about 60 seconds.
the steel sheet thus processed is then subjected to temper rolling with a properly selected rolling reduction ratio so as to become a steel sheet for canmaking with a desirable temper degree of T1 - T6 or DR8 - DR 10.
a steel sheet with a temper degree T1 (49 ⁇ 3 in HR30T) can be produced by applying temper rolling to a continuously annealed sheet with several % of rolling reduction ratio.
the rolling reduction ratio may be selected as approximately 10 %.
the rolling reduction ratio can be selected for a desired temper rolling reduction ratio from FIG. 5.
Ni and Fe are completely alloyed to form an Fe-Ni alloy layer having an improved corrosion resistance.
This Fe-Ni alloy layer itself has very excellent corrosion resistance. Further, it has good rust resistance and corrosion resistance because of the potential being closer to Fe than Ni. Therefore, Fe would not easily melt even when any flaw reaching the base steel portion is given.
the weight ratio of Ni/(Fe + Ni) in Fe-Ni alloy layer formed at the surface layer of the steel sheet according to the present invention is less than about 0.01, the corrosion resistance and the rust resistance of Fe-Ni alloy layer itself would be insufficient. If it exceeds about 0.3, when a defect such as a scratch or scrape reaching until the base steel sheet, the base steel sheet would intensely dissolve in solution from the defect portion.
the thickness of the Fe-Ni alloy layer is about 10 - ⁇ , preferably about 200 - ⁇ . If the thickness of the Fe-Ni alloy layer is less than about 10 ⁇ , the rust resistance and the corrosion resistance properties of the steel would be insufficient. Meanwhile, if the thickness exceeds about ⁇ , defects such as come-off or peal off would be easily generated due to the high hardness and brittleness of Fe-Ni alloy when shaping processes such as the neck flange forming process, beat process, deep-drawing process and overhang process are applied to two-piece cans produced from such a steel sheet, thereby reducing the rust resistance and the corrosion resistance of the product.
the Ni diffusion treated steel sheet is manufactured according to the present invention, as firstly providing a cold rolled steel sheet by any known method, next Ni plating of about 0.02 - 0.5 g/m 2 on the surface of the steel sheet obtained by the cold rolling, subsequently forming an Fe-Ni alloy layer having an weight ratio Ni/(Fe + Ni) of about 0.01 - 0.3 and a thickness of about 10 - ⁇ on the steel sheet surface layer by continuously annealing the Ni-plated member in a reducing atmosphere to diffuse Ni into the base steel sheet, temper-rolling the alloy layer-formed steel sheet using a rust-resistant rolling oil; and finally forming a rust-resistant oil film having a dry weight of about 1 - 100 mg/m 2 on the surface of the temper-rolled steel sheet.
the corrosion resistance decreases. Meanwhile if it exceeds about 0.5 g/m 2 , the corrosion resistance cannot be improved any more and a disadvantage in cost would arise.
a steel having a composition shown in Table 1 was melted by a bottom-blowing steel converter of 270 t and was converted into a steel such as that containing 0.03 % C. After decarburizing the steel to not exceed 0.004 % of C by applying an R-H vacuum degassing process, Al and subsequently carbide forming elements, nitride forming elements and elements concentrating on the steel surface were separately added to the steel. These steels were produced by using a continuous casting machine and inclusions were removed after making them float to the top portion of the molten steel so as to provide high cleanliness to the steel.
the steel sheets having been temper-rolled were then subjected to a tin-plating and a reflow treatment (tin-remelting and alloying) successively during a horizontal halogen bath type electrolytic tinning process so as to provide a tinplate having coating weight of 2.8 g/m 2 .
TFS Te Free Steel
TFS was obtained by applying an electrolytic chromium coating process under the following conditions to the temper-rolled steel sheets. Samples were cut off from the thus treated sheets and hardness was measured. The Lankford value, r, was measured by a proper oscillation method. Earing was also measured. In addition, the fruiting resistance was tested by bending the sample.
the distribution of hardness before and after the temper rolling was measured at the widthwise end of the member, the center, and the other widthwise end of the member for estimation of the uniformity of mechanical properties of the steel strip manufactured. This is shown in Table 2. From these results, it is clear that the steel sheet manufactured according to the present invention is superior to the compared reference steel sheet in processing characteristics and uniformity of the material quality.

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Description

The present invention relates to a tin mill black plate for canmaking, such sheet having temper rolling degrees of T1 - T6 or DR 8 - DR 10. This invention also relates to a method for manufacturing the sheet.
More particularly, the present invention relates to a plated steel sheet for making a three-piece can, the sheet having small thickness, high strength and excellent welding properties. It further relates to a plated steel sheet for making a two-piece can, the sheet having small thickness and excellent drawability. This invention further relates to a method for manufacturing the sheets.
There are two types of cans made from steel sheet, namely, two-piece cans and three-piece cans. The former can be further classified as SDC (Shallow-Drawn Cans), DRDC (Drawn & Redrawn Cans), DTRC (Drawn & Thin Redrawn Cans), and DWIC (Drawn & Wall Ironed Cans).
These cans are manufactured by processes such as deep-drawing, ironing, bending, stretching and welding etc. appropriately tin-coated black plate. The tin mill black plate can be classified, depending on the properties and methods of making the can to be manufactured, into temper degrees of T1 - T6 or DR8 - DR10. Those black plates having temper degrees of T1 - T3 are called soft-temper tin mill black plates while those of T4 - T6 are called hard-temper tin mill black plates; both types are made by temper rolling a cold rolled steel sheet once. Meanwhile, classes DR8 - DR10 are called DR black plate, manufactured by rolling with a large rolling reduction to the cold rolled steel sheet.
Conventionally, these steel sheets have been manufactured by preparing parent materials having originally different composition, and individually varying the conditions for the hot rolling, the cold rolling, and the annealing etc. for each of them, due to their fundamentally different requirements for strength and processing properties and the like. As a result, the processes have had to be changed each time to meet the requirements for the desired sheet, causing the manufacturing cost to be relatively increased.
Steel sheet for cans must be thin with high strength to reduce cost. The three-piece can is not an exception, but is further required to have high-speed welding properties. In particular, it must provide a high-quality seam by electric seam welding method at more than 70MPM of welding speed.
However, in the conventional art, reducing the thickness would lead to narrowing of the available welding current range. This is disadvantageous since when a relatively high welding current is supplied, splashing takes place during the welding process to undesirably increase the hardness of the welded portion. As a result, in flange processing step performed after the cylindrical forming, a flange crack tends to occur at a HAZ (Heat Affected Zone) portion in the weld zone.
Nevertheless, the welding current needs to be relatively high to provide sufficient welding strength, thereby causing HAZ cracking.
Further, in recent steel can manufacturing processes, a coil coating process is carried out on steel sheet. It is desired to apply this coil coating method to steel for high-speed welding, but for this purpose it is necessary to form a non-varnished portion (not a coated portion) in parallel to the rolling direction and to arrange the winding direction of the can body in parallel to the rolling direction.
However, if the can body is wound in this direction and a flange forming process is performed thereafter, HAZ cracking is encountered. Accordingly, in the conventional art, the non-varnished portion (not the coated portion) has been arranged perpendicular to the rolling direction. As a result, high-speed welding could not be applied to the coil coated steel strip.
Conventional steel sheets for two-piece cans have been made from soft tempered tin mill black plate having excellent deep drawabilities. Further, since such a steel sheet was generally tin-plated, tin played a role as a lubricant during the process and the r-value was not required to be particularly large.
But in the case of using ultra-thin gauge and high strength steel sheet, since the r-value of the steel sheet is generally small, the drawability of the sheet was not desirable since portions around the bottom of the cup-shaped can cracked during the process.
In addition, the larger △r-value (planer anisotropy of r-value) increases the earing phenomenon during cup processing, requiring the blank diameter to be uneconomically large.
Moreover, due to the lack of rigidity of the very thin steel sheet, creases occur on the can body wall during pressing, and cracking on the shoulder portion of the punch, respectively.
The same problems as in the hard raw sheet aforementioned took place in the DR raw sheet.
The steel sheet is generally subjected to tin-plating. Recently the coating weight of tin has been reduced to reduce cost. For example, while the conventional tin coating weight has been 2.8 g/m2, in the recent sheet that has sometimes been reduced to less than 1 g/m2. In such a case, the corrosion resistance of the steel sheet itself must be improved.
A great deal of effort has been made to cope with the foregoing problems, without success.
For example, Japanese Patent Publication No. Hei 1- discloses a method for manufacturing steel sheets for T1 - T3 cans by applying continuous annealing and thereafter temper rolling ultra low carbon steel. However, this method does not overcome all the aforementioned problems.
Important objects of the present invention are therefore as follows:

(1) To provide an art for manufacturing tin mill black plate having temper degrees of T1 - T6 or DR8 - DR 10 from cold rolled steel sheets manufactured with the same composition and the same rolling conditions, by changing only the temper rolling conditions;
(2) To provide steel sheets for canmaking having high-speed welding characteristics without causing HAZ cracking;
(3) To provide a tin mill black plate which is capable of arranging the winding direction of a can body parallel to the rolling direction of the sheet and of being welded by high-speed welding;
(4) To provide steel sheets for canmaking having excellent deep drawabilities for very small sheet thickness (so-called ultra-thin gauge) and having high strength; and
(5) To provide steel sheets for canmaking having good corrosion resistance even with a small coating weight of tin.

The invention is defined in claims 1 and 5. Preferred embodiments are defined in claims 2-4 and 6-8.
According to the present invention, a tin mill black plate is provided comprising chemical compositions composed of about C < 0.004 %, Si < 0.03 %, Mn: 0.05 - 0.6%, P < 0.02 %, S < 0.02 %, N < 0.01 %, Al: 0. 005 - 0.1 %, Nb: 0.001 - 0.1 %, B: 0. - 0.005 % (all in weight) and incidental impurities, the maximum grain size being less than 30µm, and the area ratio of recrystallized grains having a grain size range of 5 - 25µm being more than 50 %.
Further, according to the present invention, a method is provided for manufacturing tin mill black plate for canmaking with a maximum recrystallized grain size not exceeding 30µm and an area ratio of recrystallized grains having a grain size range of 5 - 25µm being more than 50 %, comprising the steps of:

heating to about 1,000 - 1,200 °C a steel slab containing about C < 0.004 %, Si < 0. 03 %, Mn: 0.05 - 0.6 %, P < 0.02 %, S < 0.02 %, N < 0.01 %, Al: 0.005 - 0.1 %, Nb: 0.001 - 0.1 %, B: 0. - 0. 005 % (all in weight) and incidental impurities;
performing hot rolling of said steel at a finishing temperature of 800 - 900 °C and at a coiling temperature of 500 - 650 °C;
pickling and cold rolling the resulting material; and
performing continuous annealing at 650 - 800 °C for a time not exceeding 60 seconds.

The above and other advantages, features and additional objects of this invention will be manifest to those versed in the art upon making reference to the following detailed description and the accompanying drawings in which embodiments incorporating the principles of this invention are shown by way of illustrative example. And are not intended to define or to limit the scpoe of the invention as defined in the appended claims.

FIG. 1 is a graphic diagram showing a relationship between C content and the hardness of tinplate;
FIG. 2 is a schematic view showing a method for measuring generated earing;
FIG. 3 is a graphic diagram showing a relationship between generated earing and C content;
FIG. 4 is a graphic diagram showing influence of area ratio of recrystallized grain size ranging 5 - 25 µm on the generation of earing;
FIG. 5 is a graphic diagram showing a relationship between a hardness of tinplate and temper rolling reduction;
FIG. 6 is a graphic diagram showing a relationship between a diameter of maximum crystal grain size and HAZ crack generating rate; and
FIG. 7 is a graphic diagram showing a relationship between total sheet thickness at weld zone and HAZ crack generating rate.

Concentrating their energies on researching steel sheets for cans, the present inventors have discovered phenomena (1) - (4), described in detail as follows, which led to discovery of the present invention:
It has been discovered that the C content and the diameter of recrystallized grain size affect the generation of HAZ cracking.

a) Effect of C ... It is typical that the nugget portion becomes harder by subjected to quick heating to near the melting point and quenching on high-speed welding. In the case of very low carbon steel, on the contrary, that nugget portion becomes softer. Therefore, it is possible to reduce the total thickness of the nugget portion during welding process and thus the amount of deformatioin during flange forming process decreases.
b) Effect of crystal grain size ... An optimum value of the crystal grain size exists. When the crystal grain size is too large it suffers from grain boundary cracking due to stress concentration.

The r value, △r value and the generation of orange peels are considered to be important factors for the deep-drawability of two-piece cans.

a) The r value and orange peel ... The r value is enhanced by increase of crystal grain size, but on the other hand orange peels tend to occur. There is discovered to be a certain range for the grain size holding the balance of both. For the adjustment of this range, manufacturing conditions mentioned later play important roles.
b) △r value ... For steel sheet for cans, this △r value in the D direction (direction deflected by 45 degrees from the rolling direction) is degraded due to high cold rolling reduction. But this problem can be overcome by increasing grain size and properly distributing the crystal grains.

For both two-piece can and three-piece can, it is important to control the size of crystal grains of the steel sheet. In view of this, it is important to add a small amount of Nb and B as chemical components of the steel sheet. It is further important to fix N in the steel by adding a proper amount of Al for work-hardening and strain aging.
Furthermore, it is also important to appropriately establish and control the hot rolling conditions, cold rolling conditions and annealing conditions.
We have revealed preferable range conditions and combination conditions of these conditions.
It has been found that the degrading of the corrosion resistance of lightly tin-coated steel sheet is caused by the precipitation of carbides of crystal grain boundaries on the surface of the steel sheet. For suppressing the precipitation of the carbides, it is preferable to use a composition of Al killed steel containing very low carbon, to perform hot rolling at a temperature lower than the normal temperature, and to apply continuous annealing.
Also, we have found that providing a proper Fe-Ni layer at the surface portion of the plated steel sheet improves corrosion resistance, as well as specific methods for carrying it out.
The C content affects the hardness of steel sheet for tinplate, recrystallized grain size and earing. The influence on hardness is shown in FIG. 1 and that on the earing is shown in FIG. 3. From these data, it is necessary to set the C content to less than about 0.004 % and preferably less than about 0.003 % for obtaining a temper degree of T1 and reducing the generation of earing on continuous annealing.
The generation of earing can be assessed in terms of the following formula: (Hmax - Hmin) / Hmin*100% where Hmax and Hmin stand respectively for the maximum and the minimum height of the can after pressing as shown in FIG. 2.
Si acts to degrade the corrosion resistance of tinplate and further tends to make the steel material extremely hard. It should not be present in an excessive amount. Namely, if the Si content exceeds about 0.03 %, the tinplate tends to become too hard, which makes it impossible to provide the temper degrees of T1 - T3; it should accordingly be less than about 0.03 %.
Mn should be added to prevent the hot rolled coil from cracking at its edge portion. That is, if the Mn content is less than about 0.05 %, the cracking cannot be avoided, while if it exceeds about 0.6 %, the crystal grain size becomes fine and tinplate itself becomes too hard. Therefore, Mn content should be within a range of about 0.05 - 0.06 %. The Mn amount to be added depends on its relationship to the S content in the steel, as will be mentioned in more detail later.
The element P makes the steel material harder and degrades the corrosion resistance of tinplate and so should be limited to less than about 0.02 % of total content.
The element S may cause cracking of the hot-rolled coil at its edge portion and press defects are caused by sulfide inclusions, and is should be present in an amount less than about 0.02 %. If the ratio Mn/S is less than about 8, the cracking and the press defects would easily arise, so this ratio should exceed about 8.
Al plays a role as a deoxidant in the steel manufacturing process and is added in a proper amount since the cleanliness of the steel would increase proportionally to the increase of the Al content in the steel. However, excessive Al would suppress the growth of the recrystallized grain size of the steel at the same time, so it should be less than about 0.10 % in content. On the other hand, if the Al content is less than about 0.005 %, the N content in the steel would increase. Therefore, the Al content should be in the range of about 0.005 - 0.10 %.
N tends to become introduced into the steel during the steelmaking process as a result of mixing of N in the air therewith, but a soft steel sheet cannot be obtained if N is present in the solid-state in the steel. Accordingly the N content should be less than about 0.01 %.
Too much O tends to form oxides with Al and Mn in the steel, with Si in the refractories, and with Ca, Na and F etc. in the flux. Thus, formed oxides tends to cause crack generation during the press working and degradation of the corrosion resistance of the can. Therefore, the O content should be less than about 0.01 %.
Nb and B are important elements affecting the recrystallized grain size after annealing. Namely, in an ultra low carbon steel with extremely reduced C content as the steel according to the present invention, the crystal grain size would sometimes become too coarse about 30 µm, causing orange peel formation as mentioned later. To overcome such a disadvantage and to control the crystal grain size, it is necessary to add both Nb and B together to the steel. Nb is an element necessary to suppress an excessive growth of the crystal particle, and further acts to form carbides or nitrides to reduce the remaining amount of solid-solved C and N, thereby enhancing the processing characteristics of the steel. To obtain these advantages, more than about 0.001 % of Nb should be added.
On the other hand, too much Nb would lead to increased recrystallized temperature due to the pinning effect on crystal grain boundary caused by precipitation of Nb. Therefore, the Nb content of the steel should be less than about 0.1 %.
B present with Nb contributes to prevent the crystal grains from enlarging too much, and to reduce the secondary work brittleness. Namely, when a carbide forming element is added to an ultra low carbon steel, the strength of the recrystallized grain boundaries would become degraded. Therefore, there is a fear of causing brittle cracking when stored at very low temperature depending on the use of the can and the canning. This can be avoided by adding B to the material. Further, while B forms carbides and nitrides so as to be effective for making the steel softer, it would segregate in the recrystallized grain boundaries during the continuous annealing to retard the recrystallization. Therefore, the B content should be less than about 0.005 %, with the lower limit more than about 0. % which is necessary to manifest the foregoing advantages.
For the adjustment of the recrystallized grains, a very important point of the present invention, it is preferable to add simultaneously about 0.003 - 0.02 % of Nb and about 0. - 0.002 % of B.
Ti is an element for forming carbide and nitride, and acts to reduce the remaining amount of solid-solved C and solid-solved N for improving the workability of the steel. On the other hand, when too much Ti is added, microscopic observation of the cross-section of a steel sheet will reveal a pointed and sharp and apparently very hard precipitation. In steel sheet for canmaking, such a precipitation would degrade the corrosion resistance of the steel and become a cause of scratching on press working. Therefore, the Ti content should be less than about 0.1 % and should be added as required.
Sn, Sb, As and Te are enrichingly concentrated on the steel sheet during the annealing process and can act to prevent C from being enrichingly concentrated, so as to improve the adhesiveness and the corrosion resistance of the tinplate.
Sb and Sn should be added with contents of more than about 0.001 % respectively, while As (more than about 0.001 %) and Te (about 0.%) should be effective when added. Since an excessive addition of these elements would cause a lowering of the press workability, the upper limit of addition for each respective element should be about 0. 01 %.
Ca forms CaO in the molten steel. When Al2O3, which has a very high melting point and hardness, reacts with this CaO, the Al2O3 changes into inclusions having lower melting point and hardness. Therefore, even if Al2O3 remains in the steel sheet by mistake, it would be divided into small pieces in the cold rolling process because of its softness so as not to cause any degradation of the product quality. Accordingly, the Ca content can be more than about 0. %, but with an upper limit of less than about 0.005 % since too much Ca would undesirably increase the non-metallic inclusions.
All of Mo, V, Zr act to increase the recrystallizing temperature during the continuous annealing process. Further, Cr, Cu, Ni, Na, Mg and REM increase the recrystallizing temperature as well as reduce the rolling characteristics of the steel, such that they may make it difficult to anneal the sheet continuously and to cold roll the steel sheet to a very thin gauge. Therefore, it would be preferable to limit the contents of these elements as follows: Mo, V, Zr .... less than about 0.01 %; Cr, Cu, Ni .... less than about 0.1 %; Na, Mg .... less than about 0.001 %; and REM .... less than about 0.005 %.
But too large and too small crystal grains frequently cause HAZ cracking.
FIG. 6 shows a relationship between the diameter of maximum crystal grains and HAZ cracking when the winding direction of the can body is in parallel to the rolling direction of the steel sheet, not perpendicular to the rolling direction as in the conventional method.
From FIG. 6 it should be understood that when the can body winding direction is in parallel to the steel sheet rolling direction, HAZ cracking frequently arises unless the diameter of the maximum recrystallized grain is less than about 30 µm, preferably less than about 25 µm.
On the other hand, FIG. 7 shows a relationship between the degree of reduction of thickness of the weld zone and HAZ cracking when the body of the three-piece can is bonded by high-speed welding.
As shown in FIG. 7, a severe stress concentration occurs during the flange forming process when the total thickness of the weld zone exceeds 1.4 times of the thickness of material steel sheet, leading to frequent HAZ cracking.
The total thickness of the weld zone is affected by the diameter of the recrystallized grains of the steel sheet. According to experiments carried out by the present inventors, it has been found that if the area ratio of crystal particles of more than 5µm exceeds about 50 %, the total thickness of the weld zone would become less than about 1.4 times of the thickness of material steel sheet.
FIG. 4 is a graphic diagram showing a relationship between area ratio of recrystallized particles ranging about 5 - 25µm and earing when tin-plated steel sheet of ultra low carbon steel with a C content of less than about 0.004 % is deep-drawn.
As shown in FIG. 4, when the area ratio of the recrystallized grains ranging about 5 - 25µm is less than about 50 %, earing is easily generated and the material is not suitable as a material for two-piece canmaking.
Further, it has been revealed that the upper limit of the crystal grain size which would generate orange peeling is about 30µm, and if the grain size exceeds that value, orange peeling would frequently take place.
In view of foregoing points, the crystal grain size required for the tinplate should be less than about 30µm for all the crystal grains, and the area ratio thereof ranging about 5 - 25µm should exceed about 50 %.
The crystal grain size can be measured in such a manner that a cross section rolling direction of the tinplate is observed by a microscope, and then the dimensions in the long and short diameter directions are averaged. Further, the area ratio of the recrystallized grains ranging about 5 - 25µm refers to the ratio of the recrystallized grains ranging about 5 - 25µm, under a microscopic observation, in proportion to the total cross sectional area of the tinplate.
To obtain crystal grain sizes after the annealing process as mentioned above, it is necessary to appropriately control the hot rolling finishing temperature. Both too high and too low FDT (finishing temperature) would make the recrystallized grain size unnecessarily enlarged.
Also, particularly in steel sheets for canmaking, the finishing hot rolling thickness would be so small as about 2 - 3 mm due to the small product thickness. As a result, the rolling time would become long due to its relationship to the capacity of the hot rolling mill, leading to a significant temperature lowering. Therefore, for increase FDT a very high SRT (slab reheating temperature) a problem as will mentioned later would arise and the temperature lowering during the rolling process becomes intense so as to cause dispersion of product quality. Therefore, FDT should be set at about 800 - 900 °C for desirable crystal diameter, product uniformity and less carbide deposition.
Further, too high SRT would easily cause cracking on the roll surface by thermal shock, which leads to reduced roll service life and more surface defects in the steel strip. Meanwhile, if SRT is less than about °C, it becomes impossible to keep FDT.
If CT (coiling temperature) is increased, the recrystallization and crystal grain growing would be easily generated so as to develop a recrystallized texture that is desirable for improving the deep drawabilities of the steel.
However, when CT is high, the material quality would deteriorate as the temperature increasingly drops at the top and tail ends of the steel strip. Moreover, the pickling properties would be affected due to increased scale developing on the hot rolled steel sheet. Accordingly, CT should be set at less than about 650 °C. Further, since too low CT would cause excessively fine crystal particles, it should be set at more than about 500 °C for lowering the rolling characteristics.
As set forth above, the hot rolled steel strip is pickled, cold rolled, and continuously annealed at about 650 - 800 °C for less than about 60 seconds.
The cold rolling reduction ratio affects the crystal grain size, and if it is too small, the crystal grain size becomes excessively coarse and tends to lower the uniformity of the grain size. Accordingly, the rolling reduction ratio should be more than about 80 %.
Too low continuous annealing temperature makes the product too hard while too high temperature leads to an excessively coarse grain. Accordingly, the continuous annealing is carried out at about 650 °C - 800 °C. For good productivity, annealing time should be less than about 60 seconds.
The steel sheet thus processed is then subjected to temper rolling with a properly selected rolling reduction ratio so as to become a steel sheet for canmaking with a desirable temper degree of T1 - T6 or DR8 - DR 10.
An example of a relationship between the temper degree (HR30T) and the temper rolling reduction ratio is shown in FIG. 5.
As shown in FIG. 5, a steel sheet with a temper degree T1 (49±3 in HR30T) can be produced by applying temper rolling to a continuously annealed sheet with several % of rolling reduction ratio. For that with T2, the rolling reduction ratio may be selected as approximately 10 %. In this manner, the rolling reduction ratio can be selected for a desired temper rolling reduction ratio from FIG. 5. Thus, according to the present invention, steel sheets for canmaking of all temper degrees can be manufactured with the same steel.
By applying Ni plating and annealing for diffusing Ni to the steel sheet, Ni and Fe are completely alloyed to form an Fe-Ni alloy layer having an improved corrosion resistance. This Fe-Ni alloy layer itself has very excellent corrosion resistance. Further, it has good rust resistance and corrosion resistance because of the potential being closer to Fe than Ni. Therefore, Fe would not easily melt even when any flaw reaching the base steel portion is given.
When the weight ratio of Ni/(Fe + Ni) in Fe-Ni alloy layer formed at the surface layer of the steel sheet according to the present invention is less than about 0.01, the corrosion resistance and the rust resistance of Fe-Ni alloy layer itself would be insufficient. If it exceeds about 0.3, when a defect such as a scratch or scrape reaching until the base steel sheet, the base steel sheet would intensely dissolve in solution from the defect portion.
The thickness of the Fe-Ni alloy layer is about 10 - Å, preferably about 200 - Å. If the thickness of the Fe-Ni alloy layer is less than about 10 Å, the rust resistance and the corrosion resistance properties of the steel would be insufficient. Meanwhile, if the thickness exceeds about Å, defects such as come-off or peal off would be easily generated due to the high hardness and brittleness of Fe-Ni alloy when shaping processes such as the neck flange forming process, beat process, deep-drawing process and overhang process are applied to two-piece cans produced from such a steel sheet, thereby reducing the rust resistance and the corrosion resistance of the product.
The Ni diffusion treated steel sheet is manufactured according to the present invention, as firstly providing a cold rolled steel sheet by any known method, next Ni plating of about 0.02 - 0.5 g/m2 on the surface of the steel sheet obtained by the cold rolling, subsequently forming an Fe-Ni alloy layer having an weight ratio Ni/(Fe + Ni) of about 0.01 - 0.3 and a thickness of about 10 - Å on the steel sheet surface layer by continuously annealing the Ni-plated member in a reducing atmosphere to diffuse Ni into the base steel sheet, temper-rolling the alloy layer-formed steel sheet using a rust-resistant rolling oil; and finally forming a rust-resistant oil film having a dry weight of about 1 - 100 mg/m2 on the surface of the temper-rolled steel sheet.
If the Ni-plating amount is less than about 0.02 g/m2, the corrosion resistance decreases. Meanwhile if it exceeds about 0.5 g/m2, the corrosion resistance cannot be improved any more and a disadvantage in cost would arise.
The present invention will now be illustrated specifically on the basis of a selected specific series of embodiments.
A steel having a composition shown in Table 1 was melted by a bottom-blowing steel converter of 270 t and was converted into a steel such as that containing 0.03 % C. After decarburizing the steel to not exceed 0.004 % of C by applying an R-H vacuum degassing process, Al and subsequently carbide forming elements, nitride forming elements and elements concentrating on the steel surface were separately added to the steel. These steels were produced by using a continuous casting machine and inclusions were removed after making them float to the top portion of the molten steel so as to provide high cleanliness to the steel. Thus obtained steel slabs were rolled at the hot-rolling temperature shown in Table 2 to form hot-rolled coils having a thickness of 2.0 mm, and were then pickled and descaled. After cold rolling the hot-rolled coil into a cold rolled strip having a very small sheet thickness of 0.2 mm (rolling reduction ratio 90 %) by a 6 stand tandem cold-rolling mill, the cold rolled strip was continuously annealed in a HNX gas atmosphere (10 % H2 + 90 % N2). The heat cycle was performed at temperatures shown in Table 2 for a level of 60 seconds. Successively, the annealed member was then temper-rolled by a temper-rolling mill with a rolling reduction ratio selected as shown in Table 2 to produce steel sheets of a variety of temper degree.
The steel sheets having been temper-rolled were then subjected to a tin-plating and a reflow treatment (tin-remelting and alloying) successively during a horizontal halogen bath type electrolytic tinning process so as to provide a tinplate having coating weight of 2.8 g/m2. Further, TFS (Tin Free Steel) was obtained by applying an electrolytic chromium coating process under the following conditions to the temper-rolled steel sheets. Samples were cut off from the thus treated sheets and hardness was measured. The Lankford value, r, was measured by a proper oscillation method. Earing was also measured. In addition, the fruiting resistance was tested by bending the sample. The quantification of this fruiting test was made by applying a degree of bending which would correspond to the shape of the can body to the sample, and by judging the generated bend as to whether it was still worthy as an article of commerce (indicated by "○") or not (indicated by "×").
Furthermore, as to the tin mill black plate, the distribution of hardness before and after the temper rolling was measured at the widthwise end of the member, the center, and the other widthwise end of the member for estimation of the uniformity of mechanical properties of the steel strip manufactured. This is shown in Table 2. From these results, it is clear that the steel sheet manufactured according to the present invention is superior to the compared reference steel sheet in processing characteristics and uniformity of the material quality.
The following is provided to indicate the conditions used in Sn plating bathing and in flowing:
The following sets forth the conditions used in chromating process bathing: Composition: CrO3 180g/l H2SO4 0.758g/l Na2SiF6 8g/l Processing conditions: liquid temperature 50 °C current density 80 A/dm2 cathode processing time 1.2 sec

Claims (8)

A steel sheet for canmaking consisting of: C not exceeding 0.004 %; Si not exceeding 0.03 %; Mn 0.05 - 0.6 %; P not exceeding 0.02 %; S not exceeding 0.02 %; N less than 0.01 %; Al 0.005 - 0.1 %; Nb 0.001 - 0.1 %; B 0. - 0.005 %; optionally Ti ≤ 0.1 % Sn 0.001-0.01% Sb 0.001-0.01% As 0.001-0.01% Te 0.-0.01% and the remainder Fe except incidental impurities; wherein the recrystallized grain size of said steel sheet does not exceed 30µm; and
the area ratio of recrystallized grains which ranges from 5 - 25µm is equal to or more than 50 %.
A steel sheet for canmaking according to claim 1, wherein said steel sheet comprises Ti in an amount less than 0.1 %.
A steel sheet according to either of claims 1 or 2, wherein said steel sheet comprises any of the following elements in the specified amounts: Sn equal to or more than 0.001 %; Sb equal to or more than 0.001 %; As equal to or more than 0.001 %; or Te equal to or more than 0. %.
A steel sheet for canmaking according to any of claims 1, 2 or 3, wherein said impurities are composed of the following elements in the specified amounts: Cr not exceeding 0.1 %; Cu not exceeding 0.1 %; Ni not exceeding 0.1 %; Mo not exceeding 0.01 %; O not exceeding 0.01 %; V not exceeding 0.01 %; Zr not exceeding 0.01 %; Ca not exceeding 0.005 %; Rem not exceeding 0.005 %; Mg not exceeding 0.001 %, and Na not exceeding 0.001%.
A method for manufacturing a steel sheet for canmaking comprising the steps of: heating at a temperature of 1,000 - 1,200°C a continuously molded slab having a composition as defined in any of the preceding claims, hot-rolling the heat slab with a finishing temperature of 800 - 900 °C and a coiling temperature of 500 - 650°C for providing a hot-rolled steel strip; pickling and cold rolling the hot-rolled steel sheet to provide a cold rolled steel strip; continuously annealing the cold rolled steel sheet at 650 - 800 °C for not exceeding 60 seconds; and thereafter temper-rolling the continuously annealed steel sheet, which obtains a recrystallized structure as defined in claim 1.
A method according to claim 5, wherein the finished steel sheet has a temper degree ranging from T1 - T6 or DR8 - DR10.
A steel sheet according to any of claims 1, 2 or 3, wherein said steel sheet includes an Fe-Ni alloy layer having a weight ratio Ni/(Fe+Ni) of 0.01 - 0.3 and a thickness of 10 - Å at the surface part.
A method for manufacturing the steel sheet defined in claim 7 according to the method of claim 5, wherein said method further comprises the step of plating Ni in an amount of 0.02 - 0.5g/m2 on the cold rolled steel strip.

EPA -04-06 -04-06 A tin mill black plate for canmaking, and method of manufacturing Revoked EPB1 (en)

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Tinplate and Process of Tinning - IspatGuru

Tinplate and Process of Tinning

Tinplate and Process of Tinning

Tinning or tinplating is the process of thinly coating sheet or strip of steel with tin (Sn), and the resulting product is known as tinplate. Tinplate is light gauge, cold-reduced low-carbon steel sheet or strip, coated on both faces with commercially pure tin. It combines the strength and formability of steel and the corrosion resistance, solderability and good appearance of tin. Within this broad description, there exists today an extremely wide range of tinplate products, tailor-made to meet particular end-use requirements.

Tinplates are widely used for making various types of cans by soldering or welding. They are characterized by the attractive metallic luster. Tinplates with various kinds of surface roughness are produced by selecting the surface finish of the substrate steel sheet. They have excellent paintability and printability. Printing is beautifully finished using various lacquers and inks. Appropriate formability is obtained for various applications as well as the required strength after forming by selecting a proper temper grade. Also, appropriate corrosion resistance is obtained against container contents by selecting a proper coating weight.

Tinplate is used for making all types of containers such as food cans, beverage cans, and artistic cans. Its applications are not limited to containers. Tinplate has also been used for making electrical machinery parts and many other products.

Production of the steel base and its subsequent coating with tin are independent of each other, so that any set of properties in the steel, can in theory be combined with any tin coating. The composition of the steel used for tinplate is closely controlled and according to the grade chosen and its manner of processing, various types with different formabilities (also known as tempers) can be produced. Tinplate is sold in a range of steel thicknesses, generally ranging from around 0.15 mm to 0.6 mm.

The steel sheets can be coated with different thicknesses of tin. Even different thicknesses on the two faces (differential coatings) can also be produced to cater for varying conditions at the internal and external surfaces of a container. Several surface finishes are also produced for diverse applications. Tinplate has a special passivation treatment to stabilize the surface and improve adhesion of lacquers. It also carries a very thin film of an oil to improve its handling and fabrication properties. This oil is, of course, compatible with food products. The resulting wide variety of materials gives the user a great flexibility in choice and the ability to select precisely the right material for a given end use.

Tinplate and packaging of food material

Tin is present in the diet only in small quantities of complex bound Sn (+2) ions. It occurs in most of the food materials. Tin levels are to be as low as practicable because of the possibility of the gastric irritation. Levels are usually less than 1 mg/kg (milligrams per kilogram) in unprocessed food materials. Higher concentrations are found in canned food materials because of the dissolution of the tinplate to form inorganic tin compounds or complexes. Generally a maximum limit of 250 mg/kg for tin in solid foods in cans and a maximum level of 200 mg/kg for liquid foods in cans are specified. Stannous chloride is authorized as a food additive for canned food products upto 25 mg/kg (as tin).

The present major source of tin in the diet is food contact materials, especially the release from the tin cans to acidic food materials. Tin cans are actually steel cans with a thin coating of metallic tin (tinplate). There is often an internal resin-based coating on the tinplate. Tinplate is mainly used in cans, can ends, and closures mainly for glass bottles and jars. However, the use of tin cans is decreasing. Tin is also used to coat kitchen utensils.

Tin is amphoteric, reacting with both strong acids and bases, but is relatively unreactive to nearly neutral solutions. The presence of oxygen greatly accelerates reaction in solution. Tinplate used in food containers is only slowly oxidized. The tin content in food materials depends on (i) whether the tin cans are lacquered, (ii) the presence of any oxidizing agents or corrosion accelerators, (iii) the acidity of the food product in the tin can, (iv) how long, and at what temperature, the tin cans are stored before being opened, and (v) the length of time the product is kept in the tin can after it has been opened.

The oxidation of the tinplate followed by unavoidable migration of the tin ions formed into the food material is the physiochemical mechanism, known as the sacrificial anode effect, which protects the underlying steel from being corroded by the food material. The dissolution of the tin protects the can from possible perforation, and protects the contents from degradation (changes in colour and flavour) during heat sterilization and storage, which is having a typical shelf life of 2 years.

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Tin concentrations in food materials in unlacquered cans can exceed 100 mg/kg while food materials stored in lacquered cans have tin levels generally below 25 mg/kg. However, storing food materials in opened unlacquered cans result in substantial increases in the tin concentration in the food materials. Canned vegetables and fruits in unlacquered cans make up only a small percentage by weight of total food intake, while they may contribute 85 % of the total intake of tin. The lacquer coating thickness greatly affects the performance of the lacquered food can.

Tinplate-its corrosion and uses

For hot dipped and electroplated tin, an oxide film forms on the tin in air. The film is fairly stable and provides a barrier to further oxidation. At pH values between 3 and 10 and in the absence of complexing agents, the oxide barrier protects the metal from food. Outside this pH range, however, corrosion of the tin occurs.

Some corrosion can be expected from tin or tin coatings exposed outdoors. In normal indoor exposure, tin is protective on iron, steel, and their alloys. Corrosion can be expected at discontinuities in the coating (such as pores) due to galvanic couples formed between the tin and the underlying steel through the discontinuities, especially in humid atmospheres.

Tinning is an extremely cost-effective process, since tin is readily available and it is much less expensive. It also offers excellent solderability, as well as superior protection against corrosion.

Tinplating can produce a whitish-gray colour which is preferable when a dull or matte appearance is desired. It can also produce a shiny, metallic look when a bit more luster is preferred. Tin offers a decent level of conductivity, making tinning useful in the manufacturing of various electronic components. Tin is also used for food packaging. Because of several advantages, tin is the metal of choice for plating applications in a wide range of industries such as (i) aerospace, (ii) food packaging, (iii) electronics, (iv) telecommunications, and (v) jewelry manufacturing.

Formation of tin whiskers can occur during the tinning process and can negatively impact the final outcome. Tin has the strong tendency to form whiskers. Tin whiskers are small, sharp protrusions which can form on the surface of the pure tinplated sheets long after the conclusion of the plating process. Whiskers have a diameter of 1 mm to 2 mm and can reach a length of about 3 mm. Whiskers can cause significant damage to the finished tinplates. Since the whiskers are electrically conductive, they can cause short circuits in electronic components. Although the exact mechanism of whisker growth is not yet understood in detail, tin whiskers can occur only in electroplated pure tin coatings. As a preventive measure, lead is required to be added to the tin by at least 2 %, or the pure tin plating is to be heated above the melting temperature of tin.

Tinning process

Tinplate is basically a steel product, since it is essentially light gauge steel strip coated with tin on both surfaces. Hence, the production of tinplate falls conveniently into two main stages namely (i) the production of thin low carbon steel strip or sheet having the required dimensions and mechanical properties, and (ii) the tin coating process. Here only the tin coating process is described. The thin low carbon steel strip or sheet on which the tin coating is applied is called ‘black plate’.

Large quantities of relatively strong tinplate are now manufactured by the technique of double reduction. Thinner yet stronger tinplate can be produced by double reduction method, which allows for more efficient material utilization in can making. After an initial cold rolling and annealing, instead of temper rolling, the steel is given a second cold reduction with lubrication, of around 10 % to 50 %. The work hardening effect gives the steel additional strength, whilst the strip retains sufficient ductility for it to be formed into can ends and bodies. Final thickness can be as low as 0.12 mm, the typical range being 0.14 mm to 0.24 mm. A two-stand or three-stand rolling mill can be used for double reduction. In some plants, a dual purpose mill is used which can produce double-reduced material and operate as a conventional temper (skin pass) mill. Double-reduced steel shows very marked directional properties and the grain direction is always to be indicated and taken into account during forming operations with the final tinplate.

Before entering the tinning line the strip is normally edge trimmed and inspected on a coil preparation line. A strip thickness gauge can also be installed so that off-gauge or sub-standard black plate can be cut out. Coils of optimum weight are produced by welding strip lengths together.

There are two processes for the tinning of the black plates namely (i) hot dip tinning process and (ii) electroplating process.

Hot dip tinning process

Hot dip tinning process is the process of immersing the steel black plate into a bath of pure molten tin at a temperature greater than 232 deg C. The coating produced consists of a very thin intermetallic layer which first forms at the interface of the base material and the tin (e.g. when dipping the black plate, an iron/tin alloy is formed) followed by a layer of pure tin.

The steel strip to be tin-coated is first uncoiled and then subjected to a thorough cleaning and, optionally, pickling cycle. Thereafter, its entire surface is wetted with a fluxing agent suitable for the application, usually a standard commercial product. This flux or ‘soldering fluid’ activates the strip surface in preparation of the tinning process. The so-called fluxing bath is followed by the heated tin bath. Typically this is a resistance heated pot, but for high outputs the use of an induction-heated pot can also be considered. Here the molten tin is held at the specified temperature, and the amount of energy removed by the coated strip is substituted. Gas heating system can also be used but it tends to be disadvantageous due to the installation complexity.

Strip speeds reach upto 200 metres per minute (m/min). The tin bath has a temperature of around 250 deg C to 290 deg C (the melting temperature of tin is around 230 deg C). Given the relatively low heat conductivity of tin, bath temperature management needs to be carefully addressed. Downstream of the tin bath, which is to be adequately sized, the core of the system is the design and process integration of the wiping and blow-off unit since it is decisive for the coating thickness and uniformity over the width and length of the strip. Optionally, the air wiper can be coupled with a non-destructive inline coating gauge. This forms a closed control loop ensuring uniform product quality. From the air wiper the newly coated strip enters a non-contacting high-convection cooling zone and then passes through the coating gauge before it is wound up again on the recoiler. The special operating regime of tinning line in stop-and-go-mode provides a dramatic reduction in tin-coated reject material.

The advantages of hot dip tinning process are (i) no waste from production process, (ii) no hazardous substance (such as cyanogen, lead, etc.) is used at all in production process, (iii) plating speed is very high (several times higher than electrolytic plating, (iv) both thick coating and thin coating can be produced at around same speed, (v) thickness of tin layer is set by computer-controlled air knives system, a contact free process which ensures particularly high surface qualities, (vi) tin coating and base metal is strongly bonded as inter-metallic layer is formed during the hot dip process, (vii) risk of whisker growth is very small since hot dip process makes crystal structure of tin uniform and minimizes its inner stress which minimizes risk of whisker growth. The advantages of hot dip tinning when compared to electroplated tin coating include (i) less porous than electroplating, (ii) more ductile than electroplating, (iii) virtually stress-free, (iv) more economical than electroplating, and (v) better corrosion resistance than electroplating. The disadvantages of hot dip tinning is that the thickness of the coating provided by hot dip tinning is not as well controlled when compared to that provided by electroplating methods. Hot dip tinning is not to be used when tight tolerances are needed.

Tinning by electroplating

In electroplating, the item to be coated is placed into a vessel containing a solution of one or more tin salts. The item is connected to an electrical circuit, forming the cathode (negative) of the circuit while an electrode typically of the same metal to be plated forms the anode (positive). When an electric current is passed through the circuit, metal ions in the solution are attracted to the item. For producing a smooth, shiny surface, the electroplated sheet is then briefly heated above the melting point of tin.

Presently, tinplate is virtually produced only by the electroplating of tin on to the steel base by a continuous process (Fig 1). The major reason for electro-tinning of steel strip replacing hot dip tinning process is because it gives a very high degree of thickness control, including differential thicknesses of coating on the two sides of the steel sheet. The electro-tinning process also gives higher outputs of tinplate with superior quality and at lower production cost. Further, with the improvements in the plating technology and steel base chemistry, the thicknesses of steel base and tin coating have been gradually significantly reduced. These days a typical coating thickness is in the range 0.1 to 1.5 microns depending on the end use.

Fig 1 Schematic process flow diagram of a continuous electro-tinning line

There are four basic choices of electrolytic plating processes which can be used to deposit tin. These are (i) alkaline stannate, (ii) acid sulphate, (iii) acid fluoborate, and (iv) acid sulphonate. The stannate process is based on either sodium or potassium stannate. For high-speed plating applications, the potassium stannate is used since it has very high solubility as compared to the sodium salt. For achieving current densities upto amperes per square metre (A/sqm), a formulation containing 210 grams per litres (g/L) of potassium stannate and 22 g/L of potassium hydroxide is used. The potassium stannate concentration can be doubled in order to reach current density of A/sqm. Anode efficiencies in the range of 75 % to 95 % and cathode efficiencies in the range of 80 % to 90 % are typical for the alkaline process.

Of all the tin plating processes, the alkaline process has superior throwing power. The process does not require the use of organic addition agents but is to operate at elevated temperatures (70 deg C to 90 deg C). The most important aspect of alkaline tin plating is the critical need for proper control of the anode. If the tin anodes are not properly controlled during the plating process, rough porous deposits result. A yellow-green film is to be present on the anode during the plating operation in order to ensure excellent plating.

Plating solutions based on stannous sulphate (7 g/L to 50 g/L) and sulphuric acid (50 g/L to 150 g/L) can deposit either a bright decorative deposit or a matte finish depending on the type of grain refiner / brightening system used. A semi-bright matte tin finish can be obtained using gelatin and an organic compound, beta-naphthol. A large variety of organic brighteners are commercially available to produce bright, decorative adherent deposits from the stannous sulphate electrolyte. These additives are generally based on aliphatic aldehydes and an aromatic amine. Improved versions of the above consist of wetting agents such as water-soluble polyethylene glycol and a water soluble derivative of ethylene as a primary brightening agent. The bright bath has several advantages over the matte process which include improved corrosion resistance, reduced porosity, resistance to fingerprints, improved solderability as well as its cosmetic appearance.

The acid sulphate process operates between 20 deg C to 30 deg C at essentially 100 % anode and cathode efficiencies. The acid bath does not need the careful anode monitoring of the alkaline stannate bath but does need organic addition agents. However, the throwing power of the acid bath is normally less when compared to the alkaline stannate process.

Another acidic plating process based on tin fluoborate (75 g/L to 115 g/L) and fluoboric acid (50 g/L to 150 g/L) is designed to plate pure matte tin deposits. A major advantage of this process over the tin sulphate is that it can be operated at much higher cathode current densities, up to 10,000 A/sqm (in agitated plating solutions). Gelatin and beta-naphthol are typically used as grain refiners in this process, which is operated in the temperature range of 20 deg C to 30 deg C. Anode and cathode efficiencies are around 100 %.

Recently tin plating formulations based on methane-sulphonic acid (15 % to 25 % by volume) are gaining acceptance because the solutions require simple waste treatment, contain no fluorides or boron, and are less corrosive than the electrolytes based on fluoboric acid. The methane-sulphonic electrolytes, similar to the fluoborate baths, can hold high concentrations of metal in solution (upto 100 g/L tin) permitting plating at high speeds. A major drawback of the methane-sulphonic acid process is its high chemical make-up cost.

All of the acidic tin plating electrolytes mentioned above deposit tin from the divalent state (+2) as compared to the +4 state for the alkaline stannate solutions. The acidic processes thus deposit tin twice as fast as the stannate process and operate at essentially 100 % cathode efficiency. The acid tin processes are easier to control and maintain than the stannate solution. They have the additional advantage of operating at ambient temperatures.

While considering the process flow in the continuous electro-tinning line (Fig 1), black plate coils are fed onto the tinning line, being loaded onto the uncoiler. Two uncoilers are needed for continuous operation. The tail end of the coil being processed is welded to the head end of the next coil to be processed, which necessitates the two coils being stationary during welding. To avoid shut down during welding, lines are fitted with looping towers or accumulators which can hold varying quantities of uncoiled black plate (often upto 600 metres). Modern electro-tinning lines incorporate side trimmers after the accumulator to cut the strip to the correct width. Also, many lines now incorporate tension or stretch levellers, which apply controlled tension across the strip to remove distortions.

In the continuous electro-tinning lines, the cleaning time is very short (around 1 second to 2 seconds). Hence, there is need of effective cleaning of the black plate strip. This need is met with the use of electrolysis to aid chemical dissolution of rolling oil residues and other organic contaminants. Heavy current which is passed during the electrolysis produces gases at the strip surface. This results into lifting of the dirt and residue from the strip. The cleaning agent is generally a 1 % to 5 % solution in water of a mixture of phosphates, wetting agents and emulsifiers in a sodium hydroxide / carbonate base. Temperature is generally in the range of 80 deg C to 90 deg C with current density of A/sqm is normally adequate.

After cleaning, the strip is thoroughly washed, ideally in hot water (70 deg C) using high-pressure sprays. Pickling removes oxide and rust layers and leaves the surface etched for better deposition of tin. During the process the strip is usually made anodic then cathodic with current densities ranging between 500 A/sqm and A/sqm being employed.

Different types of electrolytes can be used in the tinplating section. The plating cells consist of a series of vertical tanks through which the strip passes in serpentine fashion. The number of plating tank passes in use, the anode length, and the width of the strip determine the effective plating area. This, together with the available plating current, decides the maximum line speed for a particular coating weight. Present day tinning lines achieve speeds of 600 m/min or more with typical strip widths between mm and mm. The steel strip is guided through the tanks by sink rolls located at the bottom of the tanks and conductor rollers with rubber covered hold-down rollers at the top. These collect electrolyte from the strip and return it to the plating cell. The conductor rolls need to have good electrical conductivity and low contact resistance between the roll and the wet strip. These rolls are generally made from steel coated with copper and then chromium.

Each plating tank has four anode bus bars and four banks of anodes, one for each face of the down and up passes of the strip. Traditionally anodes are made of 99.9 % pure tin and are 76 mm wide, 50 mm thick and around 1.8 m long. The anode is consumed in the process and is replaced when it is reduced in thickness by around 70 %. A worn anode is removed from one end of the bank and a new one inserted at the other, the others being moved across to make room. In recent years, inert anodes made from titanium coated with platinum or iridium oxide have become more popular. Nippon Steel was the first to use inert anodes in electro-tinning line. In this case stannous ions are produced off line in a generation plant in which high pressure oxygen is bubbled through the electrolyte solution containing pure tin beads, dissolving the tin and making fresh electrolyte.

Inert anodes are positioned parallel to the steel strip in a fixed position. There is no necessity for frequent renewal of these anodes. This results into minimal variations in the tin coating thickness across the strip width. Adjustable edge masks ensure correct anode width to avoid tin build-up on the edges of the strip. Since there is no need to cast and replace tin anodes, use of inert anodes also reduces requirement of manpower.

An alternative system of parallel tin anodes has also been used. In this system the anode bridges are aligned parallel to the strip and are loaded with conventional tin anodes. The anode bank is placed close to the strip reducing the initial voltages required. As the anodes slowly dissolve the voltage is increased to maintain a given current. When the anodes have been reduced to a specified thickness the whole bank is replaced. This system is claimed to give similar control over tin thickness as with inert anodes.

At the end of the plating section there is a drag-out control section which essentially removes residual electrolyte from the strip for subsequent recovery. Tin is deposited as a whitish coating having a slight metallic lustre. Where needed this is flow melted by induction or resistance heating (or a combination) to produce a bright mirror-like finish. In resistance heating, a high alternating current is passed through the strip via conductor rolls. With induction heating the strip passes through a series of internally cooled copper coils through which a high frequency current is passed. The induced eddy current and hysteresis losses heat up the strip and melt the tin coating. This flow melting process enhances the corrosion resistance of the product by formation of an inert tin-iron alloy layer.

Prior to flow melting, the plate is fluxed by treating with dilute electrolyte or proprietary chemicals to prevent surface defects appearing on the plate. Flow melted tin plate has a thin tin oxide film on the surface, which if untreated can grow during storage. In order to improve the tarnish resistance and laquerability a chemical or electrochemical passivation is applied to the strip. The most common form of passivation involves cathodic treatment at temperatures between 50 deg C and 85 deg C in dichromate or chromic acid solution containing 20 g/L dichromate (other treatments which are now seldom used are the use of phosphates or carbonates). This treatment deposits a complex layer of chromium and its hydrated oxides, which inhibits the growth of tin oxides, prevents yellowing, improves paint adhesion and minimizes staining by sulphur compounds. Prior to oiling the tinplate is to be thoroughly dried. Oiling with dioctyl sebacate or acetyl tributyl citrate is carried out in an electrostatic spray process.

Quality inspection is by in-line inspection prior to recoiling and involves checking of the strip thickness, detection of pinholes and tin thickness.

There is another electro-tinning process which has horizontal rather than vertical plating tanks. This configuration together with the high current densities used ( A/sqm), enables lines to be run fast, with above 600 m/min speeds being common. The plating tanks are on two decks with each level containing upto 18 plating tanks (1.8 m long by 300 mm deep) with banks of small anodes supported on conducting carbon rests, over which the strip passes. The anodes extend around 130 mm beyond the strip edge and the supports are inclined at an angle across the tank width which ensures constant spacing between strip and anode surfaces for anodes of progressively diminishing thickness. At the entry and exit of each plating level and between adjacent individual plating cells the strip passes between a pair of rolls, the upper conducting roll being termed the cathode roll. Tin is plated on the underside in the first deck. The steel is then turned through 180 deg and enters the second deck where the other side is plated.

The pH of this system (around 3) is high for an acid system, but no free acid is added to the bath. The bath contains tin chloride (around 35 g/L as Sn 2+), sodium and potassium fluorides, sodium chloride and potassium hydrogen fluoride together with organic additives such as poly-alkylene oxides or naphthalene sulphonic acid. The electrolyte continually circulates in the system, overflows the ends of the tanks and is recirculated. In the lower deck the electrolyte is sprayed onto the top of the strip to wet it. After plating the strip passes through rinsing tanks, wringer rolls and a hot air dryer all located in a top third deck. In this process, flow melting is usually by induction heating. The electrolyte contains tin fluoro-borate (30 g/L as Sn 2+), fluoro-boric acid and boric acid to prevent hydrolysis of the fluoro-borate ions. Also, proprietary additives are used. It is claimed that these lines can operate over a wider current density range allowing greater line flexibility. Although the first lines to be built were horizontal, later lines are vertical, containing up to 16 plating tanks and running at line speeds of 640 m/min or higher.

In the production of tinplate, the manufacture of the steel base and the application of the tin coating are independent of each other so that theoretically any tin coating, or combination of coatings, can be applied to any steel base. Thus the range of materials classified as tinplate can run into many thousands, indeed tinplate is available in more qualities than virtually any other light gauge sheet metal product. In practice the range of steel base thickness is from 0.13 mm to 0.60 mm and the tin coating from 0.5 g/sqm to 15.2 g/sqm tin per surface. There are international and national standards which specify the ranges and tolerances for the various characteristics, and methods of verifying them.

For more information, please visit Tin Mill Steel.

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