Classifications

• • 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
  • C22C38/001—Ferrous alloys, e.g. steel alloys containing N
• • 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
  • C21D8/0205—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips of ferrous 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
  • 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
  • C21D8/0405—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 of ferrous 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
  • 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
  • C21D8/0421—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
  • C21D8/0426—Hot 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
  • C21D8/0421—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
  • C21D8/0436—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
  • C21D8/0447—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
  • C21D8/0463—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 following hot 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
  • C21D8/0447—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
  • C21D8/0468—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 between cold rolling steps
• • 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
  • C21D9/00—Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor
  • C21D9/46—Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor for sheet metals
• • 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
  • C21D9/00—Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor
  • C21D9/46—Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor for sheet metals
  • C21D9/48—Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor for sheet metals deep-drawing sheets
• • 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
  • C22C38/002—Ferrous alloys, e.g. steel alloys containing In, Mg, or other elements not provided for in one single group C22C38/001 - C22C38/60
• • 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
  • C22C38/004—Very low carbon steels, i.e. having a carbon content of less than 0,01%
• • 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
  • C22C38/02—Ferrous alloys, e.g. steel alloys containing silicon
• • 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
  • C22C38/04—Ferrous alloys, e.g. steel alloys containing manganese
• • 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
  • C22C38/06—Ferrous alloys, e.g. steel alloys containing aluminium

Definitions

• the present invention relates to steel sheets for cans that can be made thinner while maintaining good workability and methods for manufacturing such steel sheets.
• One such approach to reducing can manufacturing costs is to reduce material costs, and the thickness of steel sheets is being reduced for application to three-piece cans, which mainly involve cylinder forming, as well as to two-piece cans, which involve drawing.
• Three-piece cans which are manufactured by forming a cylindrical can body by welding and joining a bottom and a lid to the can body by seaming, are formed of single-reduced (SR) materials, which are manufactured by a single cold rolling process followed by annealing and temper rolling, and steel sheets about 0.175 mm thick are used for beverage cans such as coffee cans.
• SR single-reduced
• DR materials are manufactured by performing cold rolling again after annealing.
• the thickness of DR materials can be more easily reduced than that of SR materials.
• DR materials are mainly used, for example, for drawn cans.
• DR materials are used for three-piece cans
• Cylinder forming is mainly performed by rolling up and electrically welding a rectangular steel sheet. If a DR material is used, the steel sheet may crack near the weld in flange forming.
• welding can bodies along the rolling direction of steel sheets is a prevailing method for manufacturing three-piece beverage cans nowadays. In flange forming, therefore, elongation occurs mainly in a direction perpendicular to the rolling direction of steel sheets; thus, the workability in this direction is important.
• beverage cans such as coffee cans
• This retort sterilization process exposes the cans to the pressure of steam above 100°C, thus requiring can body strength sufficient to withstand the external pressure.
• the can body strength depends on the strength of the steel sheets in the circumferential direction of the can bodies. If the can bodies are welded along the rolling direction of the steel sheets, the strength of the steel sheets in the direction perpendicular to the rolling direction is important.
• PTL 1 discloses a method for improving workability at a weld by adding boron to an ultralow-carbon steel in the amount depending on the carbon content and thickness.
• PTL 2 discloses a method for manufacturing a steel sheet having excellent weldability equivalent to temper grade T3 by appropriately controlling the weight ratio of boron to nitrogen in an ultralow-carbon steel.
• the steel sheet which is disclosed in PTL 2 undergoes two cold rolling reduction steps until a final thickness of about 0.22 mm.
• PTL 3 discloses a method for manufacturing a steel sheet having high workability by controlling the forms, types, and amounts of nitride and sulfide in a boron-containing ultralow-carbon steel to appropriate ranges.
• steel sheets having been subjected to a first cold rolling with a rolling reduction of 86 % and a second cold rolling with a rolling reduction of 10% to 35% such that the final thickness of the sheet is 0.15 mm.
• Further steel sheets are described in JP 10 245655 A , WO 03/031670 A1 , JP 10 237585 A , JP 5 271755 A and JP 2008 163390 A .
• the steel sheet disclosed in PTL 1 is unsuitable as a steel sheet for three-piece beverage cans because it has insufficient ductility in the direction perpendicular to the rolling direction due to a high second rolling reduction. Even if the steel sheet is acceptable for welding along the direction perpendicular to the rolling direction, it probably cracks in flange forming after being welded into a can body along the rolling direction.
• the steel sheet manufactured by the method for manufacturing a steel sheet disclosed in PTL 2 has insufficient steel sheet strength for use as a thinner steel sheet for three-piece beverage cans because the hardness thereof is equivalent to temper grade T3.
• the specified rolling reduction in second rolling namely, 3.5% to 6%
• temper rolling equipment which is usually operated with a rolling reduction of 1% to 2%, thus overloading the equipment.
• the rolling reduction is insufficient for second rolling equipment, which uses large amounts of lubricant, thus probably causing a problem with rolling, such as chattering.
• the steel sheet manufactured by the method disclosed in PTL 3 lacks high-temperature ductility because it contains a large amount of sulfur, thus possibly suffering a crack in continuous casting of steel slabs.
• An object of the present invention which has been made in light of the above circumstances, is to provide a high-workability steel sheet for three-piece welded cans that is practical as a steel sheet for three-piece beverage cans with a tensile strength of 400 MPa or more in a direction perpendicular to a rolling direction and excellent flange formability, and a method for manufacturing such a steel sheet.
• the inventors have conducted an intensive study to solve the above problems. As a result, the inventors have obtained the following findings.
• DR materials are harder than SR materials because they are cold-rolled again after annealing. To have good workability, therefore, the steel sheets require sufficient fracture elongation, that is, softness. From this viewpoint, ultralow carbon steels are used in the present invention because carbon steels become softer with decreasing carbon content.
• a DR material which is strained after second cold rolling, is recrystallized by heat given during welding in a region near a weld.
• strain concentrates in the recrystallized region because it is softer than the other region, thus causing a crack.
• the steel sheet needs to be given hardenability.
• An appropriate amount of boron added increases hardenability in welding, thus preventing softening near the weld. If the second rolling reduction is low, however, strain concentrates in the base metal near the weld in flange forming because the hardening effect makes the strength of the weld higher than that of the surrounding base metal, thus causing a crack.
• the rolling reduction in second cold rolling needs to be limited to an appropriate range.
• the present invention provides a high workable steel sheet having the features defined in claim 1. Moreover, a method for manufacturing a high workable steel sheet for three-piece welded cans having the features in claim 2 is provided.
• the percentages showing the steel compositions herein are all percentages by mass.
• a high-workability steel sheet for three-piece welded cans that has a tensile strength of 400 MPa or more in a direction perpendicular to a rolling direction and excellent flange formability.
• a thin steel sheet with excellent workability for three-piece welded cans can be reliably manufactured through second cold rolling by adding boron to an ultralow carbon steel and setting an appropriate second cold rolling reduction.
• the base sheet (steel sheet) with improved workability enables can forming using thin DR materials without causing a crack in flange forming of three-piece cans, thus achieving a significant reduction in the wall thickness of three-piece cans.
• a high-workability steel sheet of the present invention for three-piece welded cans is characterized by a tensile strength of 400 MPa or more in a direction perpendicular to a rolling direction and a fracture elongation of 15% or more in the direction perpendicular to the rolling direction.
• the high-workability steel sheet of the present invention for three-piece welded cans is manufactured as an ultrathin steel sheet through second cold rolling with sufficient flange formability at a weld by adding boron to an ultralow carbon steel to impart hardenability while maintaining its softness and setting an appropriate second cold rolling reduction.
• composition of the high-workability steel sheet of the present invention for three-piece welded cans will now be described.
• the material needs to be a soft steel to ensure sufficient workability after second cold rolling.
• the upper limit of the carbon content is 0.0030%.
• a carbon content exceeding 0.0030% impairs the workability of the steel sheet, thus making it difficult to perform can forming processes such as flange forming.
• the lower limit of the carbon content is more than 0.0015% because reducing the carbon content to 0.0015% or less undesirably increases decarburization cost in a refining process.
• the silicon content is 0.10% or less because a silicon content exceeding 0.10% causes problems such as degraded surface treatment properties and corrosion resistance.
• Manganese which has the effects of preventing red brittleness due to sulfur during hot rolling and refining crystal grains, is an element necessary to ensure the desired material properties. To achieve these effects, at least 0.20% of manganese needs to be added. On the other hand, the upper limit is 0.80% because an excessive amount of manganese added degrades the corrosion resistance and also degrades flange formability and neck formability by hardening the steel sheet.
• the upper limit of the phosphorus content is 0.020% because phosphorus is a harmful element that degrades the flange formability and neck formability by hardening the steel sheet and also degrades the corrosion resistance.
• reducing the phosphorus content to less than 0.001% involves excessive dephosphorization cost. Accordingly, the lower limit of the phosphorus content is 0.001%.
• Sulfur present as inclusions in the steel, is a harmful element that decreases the ductility and degrades the corrosion resistance.
• an excessive sulfur content results in insufficient high-temperature ductility, thus leading to slab cracking in continuous casting.
• the sulfur content is limited to 0.020% because a sulfur content exceeding 0.020% causes those adverse effects.
• reducing the sulfur content to less than 0.001% involves excessive desulfurization cost, and the steel sheet hardly suffers the above adverse effects at any lower sulfur content. Accordingly, the lower limit of the sulfur content is 0.001%.
• Aluminum is an element necessary as a deoxidizing agent in steelmaking.
• An aluminum content of 0.040% or less results in insufficient deoxidization, thus increasing inclusions and degrading the flange formability.
• an aluminum content exceeding 0.100% results in an increased frequency of occurrence of surface defects due to, for example, alumina clusters. Accordingly, the aluminum content is more than 0.040% to 0.100%.
• Nitrogen 0.030% or less
• the upper limit of the nitrogen content is 0.030%.
• Boron which is an element essential for preventing softening at a weld, fails to provide sufficient performance if the content thereof falls below 0.0002%. Accordingly, the lower limit of the boron content is 0.0002%. On the other hand, a boron content exceeding 0.0050% provides no further improvement in performance and only increases the cost. Accordingly, the upper limit of the boron content is 0.0050%, preferably 0.0011% to 0.0020%.
• the balance is iron and incidental impurities.
• the high-workability steel sheet of the present invention for three-piece welded cans is manufactured using a steel slab manufactured by continuous casting and having the above composition by hot rolling, first cold rolling, annealing, and second cold rolling.
• the steel sheet manufactured by the present invention is assumed to be used as a thinner steel sheet for three-piece beverage cans. Accordingly, the steel sheet is required to have a smaller product thickness than those used conventionally and therefore needs to be rolled to a thickness of about 0.15 mm or less. It is usually difficult to achieve a thickness of 0.15 mm or less by a single cold rolling process. That is, cold rolling to such a small thickness overloads a rolling mill. It is also possible to roll the steel slab to a smaller thickness than usual in hot rolling to reduce the thickness after cold rolling.
• the finish rolling temperature in hot rolling falls below the Ar 3 transformation temperature, the size of recrystallized grains after annealing becomes uneven. If the finish rolling temperature in hot rolling exceeds 960°C, the size of recrystallized grains after annealing becomes larger than necessary. Accordingly, the finish rolling temperature in hot rolling is the Ar 3 transformation temperature to 960°C, more preferably 890°C to 930°C.
• Coiling temperature 560°C to 750°C
• the coiling temperature after hot rolling falls below 560°C, the size of recrystallized grains after annealing becomes extremely small.
• a coiling temperature exceeding 750°C is undesirable because it results in uneven material properties over the entire steel sheet and formation of an excessive amount of scale. Accordingly, the coiling temperature after hot rolling is 560°C to 750°C, more preferably 600°C to 720°C.
• the first cold rolling reduction affects the grain size after annealing; the size of recrystallized grains becomes extremely large if the rolling reduction falls below 89% and becomes extremely small if the rolling reduction exceeds 93%. Accordingly, the first cold rolling reduction is 89% to 93%, more preferably 90% to 92%.
• the annealing temperature affects the recrystallization rate and the grain size. Specifically, if the annealing temperature falls below 600°C, an excessive number of unrecrystallized grains remain, thus impairing the workability. If the annealing temperature exceeds 790°C, the grain size becomes extremely large, thus making it difficult to ensure sufficient strength. Accordingly, the annealing temperature is 600°C to 790°C, more preferably 610°C to 700°C. It should be noted that some unrecrystallized grains may remain after annealing.
• the second cold rolling reduction is 6.0% or less, the required steel sheet strength cannot be achieved because of insufficient work hardening in second cold rolling. In addition, a hardening effect in welding results in a great difference in strength between the weld, which has increased strength, and the base metal, thus causing a crack near the weld in flange forming.
• the second cold rolling reduction is 10.0% or more, sufficient fracture elongation cannot be achieved because of excessive work hardening in second cold rolling.
• the proportion of crystal grains recrystallized near the weld increases because large strain accumulates in second cold rolling, and the strength near the weld decreases accordingly, thus making the steel sheet prone to cracking in flange forming. Accordingly, the second cold rolling reduction is more than 6.0% to less than 10.0%.
• the subsequent steps are performed in a conventional manner to complete a steel sheet for cans.
• the high-workability steel sheet of the present invention for three-piece welded cans is produced.
• This high-workability steel sheet for three-piece welded cans has a tensile strength of 400 MPa or more in the direction perpendicular to the rolling direction and a fracture elongation of 15% or more in the direction perpendicular to the rolling direction.
• the strength in the direction perpendicular to the rolling direction is important for withstanding external pressure in a retort sterilization process.
• the steel sheet does not dent or buckle when exposed to a retort environment.
• the fracture elongation in the direction perpendicular to the rolling direction is important for preventing a crack in flange forming.
• flange forming can be performed without causing a crack.
• the steel sheets thus manufactured were continuously coated with tin on both sides to form tinplates having 2.8 g/m 2 of tin deposited on each side.
• the resulting coated steel sheets were subjected to heat treatment equivalent to coat baking at 210°C for 20 minutes before a tensile test.
• JIS No. 5 tensile test pieces were used to measure the tensile strength (fracture strength) and the fracture elongation in the direction perpendicular to the rolling direction according to JIS Z2241.
• the steel sheets subjected to the heat treatment equivalent to coat baking were formed into can bodies having an outside diameter of 52.8 mm by seam welding, and the ends thereof were necked in to an outside diameter of 50.4 mm and were flanged to an outside diameter of 55.4 mm before they were evaluated for flange cracks.
• the steel sheets were evaluated as being “poor” if the flanges cracked and as being “good” if the flanges did not crack.
• the can bodies had a size equivalent to that of 190 g beverage cans and were welded along the rolling direction of the steel sheets.
• the necking-in was performed by die necking, and the flanging was performed by spin flanging.
• a paneling test was conducted for evaluation of can body strength.
• Hollow can bodies were formed by the above processes and lid and bottom seaming and were externally pressurized by air pressure in a sealed chamber to measure the pressure at which the can bodies collapsed.
• the steel sheets were evaluated as being “poor” if the collapse pressure fell below 1.7 kg/cm 2 and as being “good” if the collapse pressure was 1.7 kg/cm 2 or more. This criterion was set as the strength with which a steel sheet can withstand the pressure of common retort treatment.
• 15 beads were formed in the centers of the can bodies. The beads had a spacing of 4 mm and a depth of 0.5 mm.
• Nos. 1 to 7 which are examples of the present invention, achieved excellent strength, that is, a tensile strength of 400 MPa or more in the direction perpendicular to the rolling direction, which is required to reduce the wall thickness of three-piece can bodies by several percent.
• the fracture elongation in the direction perpendicular to the rolling direction was 15% or more.
• these steels had excellent workability and did not crack in flanging.
• the can body strength after can forming was also sufficient.
• No. 8 which is a comparative example, lost its ductility after second cold rolling and had poor workability because the carbon content was excessive.
• No. 9 which is a comparative example, became extremely soft in the welding heat-affected zone and cracked in flanging because it contained no boron.
• a steel sheet of the present invention for three-piece welded cans has high workability and excellent flange formability and is suitable for applications such as beverage cans including coffee cans.
• a thin high-workability steel sheet for cans can be provided, thus enabling a significant reduction in the wall thickness of three-piece cans.

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• 2009
  • 2009-08-19 JP JP2009189970A patent/JP5018843B2/ja active Active
• 2010
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