WO2023026465A1 - 鋼板およびプレス成形品 - Google Patents
鋼板およびプレス成形品 Download PDFInfo
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- WO2023026465A1 WO2023026465A1 PCT/JP2021/031487 JP2021031487W WO2023026465A1 WO 2023026465 A1 WO2023026465 A1 WO 2023026465A1 JP 2021031487 W JP2021031487 W JP 2021031487W WO 2023026465 A1 WO2023026465 A1 WO 2023026465A1
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Links
- 229910000831 Steel Inorganic materials 0.000 title claims abstract description 170
- 239000010959 steel Substances 0.000 title claims abstract description 170
- 239000000203 mixture Substances 0.000 claims abstract description 15
- 239000000126 substance Substances 0.000 claims abstract description 15
- 239000012535 impurity Substances 0.000 claims abstract description 14
- 238000007747 plating Methods 0.000 claims description 26
- 239000010410 layer Substances 0.000 description 66
- 239000000047 product Substances 0.000 description 28
- 238000005096 rolling process Methods 0.000 description 26
- 230000000694 effects Effects 0.000 description 19
- 238000000034 method Methods 0.000 description 19
- 230000006872 improvement Effects 0.000 description 18
- 238000005097 cold rolling Methods 0.000 description 14
- 238000000137 annealing Methods 0.000 description 13
- 238000004519 manufacturing process Methods 0.000 description 13
- 229910045601 alloy Inorganic materials 0.000 description 12
- 239000000956 alloy Substances 0.000 description 12
- 238000005554 pickling Methods 0.000 description 11
- 229910052761 rare earth metal Inorganic materials 0.000 description 11
- 150000002910 rare earth metals Chemical class 0.000 description 11
- 239000011701 zinc Substances 0.000 description 10
- 229910001297 Zn alloy Inorganic materials 0.000 description 9
- 238000000465 moulding Methods 0.000 description 9
- 238000005204 segregation Methods 0.000 description 9
- 229910052725 zinc Inorganic materials 0.000 description 9
- HCHKCACWOHOZIP-UHFFFAOYSA-N Zinc Chemical compound [Zn] HCHKCACWOHOZIP-UHFFFAOYSA-N 0.000 description 8
- 230000009467 reduction Effects 0.000 description 7
- 238000005275 alloying Methods 0.000 description 6
- 229910052782 aluminium Inorganic materials 0.000 description 6
- 239000011247 coating layer Substances 0.000 description 6
- 229910052749 magnesium Inorganic materials 0.000 description 6
- 239000000463 material Substances 0.000 description 6
- 238000005728 strengthening Methods 0.000 description 6
- 229910052726 zirconium Inorganic materials 0.000 description 6
- 230000015572 biosynthetic process Effects 0.000 description 5
- 230000006866 deterioration Effects 0.000 description 5
- 229910000859 α-Fe Inorganic materials 0.000 description 5
- 229910052791 calcium Inorganic materials 0.000 description 4
- 238000010438 heat treatment Methods 0.000 description 4
- 238000002354 inductively-coupled plasma atomic emission spectroscopy Methods 0.000 description 4
- 229910052710 silicon Inorganic materials 0.000 description 4
- 230000003746 surface roughness Effects 0.000 description 4
- 230000009466 transformation Effects 0.000 description 4
- 229910001335 Galvanized steel Inorganic materials 0.000 description 3
- 229910052804 chromium Inorganic materials 0.000 description 3
- 230000007797 corrosion Effects 0.000 description 3
- 238000005260 corrosion Methods 0.000 description 3
- 239000008397 galvanized steel Substances 0.000 description 3
- 238000005098 hot rolling Methods 0.000 description 3
- 229910052748 manganese Inorganic materials 0.000 description 3
- 229910000734 martensite Inorganic materials 0.000 description 3
- 238000005259 measurement Methods 0.000 description 3
- 229910052757 nitrogen Inorganic materials 0.000 description 3
- 239000002244 precipitate Substances 0.000 description 3
- 238000001953 recrystallisation Methods 0.000 description 3
- 238000007711 solidification Methods 0.000 description 3
- 230000008023 solidification Effects 0.000 description 3
- 239000000758 substrate Substances 0.000 description 3
- 229910052717 sulfur Inorganic materials 0.000 description 3
- 238000012360 testing method Methods 0.000 description 3
- 238000004804 winding Methods 0.000 description 3
- 238000010521 absorption reaction Methods 0.000 description 2
- 238000004458 analytical method Methods 0.000 description 2
- 229910001563 bainite Inorganic materials 0.000 description 2
- 229910052793 cadmium Inorganic materials 0.000 description 2
- 229910052799 carbon Inorganic materials 0.000 description 2
- 239000010960 cold rolled steel Substances 0.000 description 2
- 230000000052 comparative effect Effects 0.000 description 2
- 239000013078 crystal Substances 0.000 description 2
- 230000001186 cumulative effect Effects 0.000 description 2
- 238000009792 diffusion process Methods 0.000 description 2
- 238000011156 evaluation Methods 0.000 description 2
- 229910052732 germanium Inorganic materials 0.000 description 2
- 239000011261 inert gas Substances 0.000 description 2
- 229910052745 lead Inorganic materials 0.000 description 2
- 229910052759 nickel Inorganic materials 0.000 description 2
- 150000004767 nitrides Chemical class 0.000 description 2
- 229910052698 phosphorus Inorganic materials 0.000 description 2
- 238000003825 pressing Methods 0.000 description 2
- 229910052709 silver Inorganic materials 0.000 description 2
- 229910052715 tantalum Inorganic materials 0.000 description 2
- 150000003568 thioethers Chemical class 0.000 description 2
- 238000003466 welding Methods 0.000 description 2
- 229910000838 Al alloy Inorganic materials 0.000 description 1
- 230000001133 acceleration Effects 0.000 description 1
- 229910052787 antimony Inorganic materials 0.000 description 1
- 239000007864 aqueous solution Substances 0.000 description 1
- 229910052785 arsenic Inorganic materials 0.000 description 1
- 229910001566 austenite Inorganic materials 0.000 description 1
- 229910052790 beryllium Inorganic materials 0.000 description 1
- 229910052797 bismuth Inorganic materials 0.000 description 1
- 229910052796 boron Inorganic materials 0.000 description 1
- 229910052792 caesium Inorganic materials 0.000 description 1
- 238000005266 casting Methods 0.000 description 1
- 239000003610 charcoal Substances 0.000 description 1
- 229910052801 chlorine Inorganic materials 0.000 description 1
- 229910052802 copper Inorganic materials 0.000 description 1
- 238000005520 cutting process Methods 0.000 description 1
- 230000006378 damage Effects 0.000 description 1
- 230000007547 defect Effects 0.000 description 1
- 238000006073 displacement reaction Methods 0.000 description 1
- 238000009713 electroplating Methods 0.000 description 1
- 230000007613 environmental effect Effects 0.000 description 1
- 239000010419 fine particle Substances 0.000 description 1
- 229910052733 gallium Inorganic materials 0.000 description 1
- 229910052737 gold Inorganic materials 0.000 description 1
- 238000000227 grinding Methods 0.000 description 1
- 229910052735 hafnium Inorganic materials 0.000 description 1
- 229910052739 hydrogen Inorganic materials 0.000 description 1
- 229910052738 indium Inorganic materials 0.000 description 1
- 239000003112 inhibitor Substances 0.000 description 1
- 229910052740 iodine Inorganic materials 0.000 description 1
- 229910052741 iridium Inorganic materials 0.000 description 1
- 229910052742 iron Inorganic materials 0.000 description 1
- 229910052747 lanthanoid Inorganic materials 0.000 description 1
- 150000002602 lanthanoids Chemical class 0.000 description 1
- 229910052746 lanthanum Inorganic materials 0.000 description 1
- 229910052744 lithium Inorganic materials 0.000 description 1
- 238000004020 luminiscence type Methods 0.000 description 1
- 230000008018 melting Effects 0.000 description 1
- 238000002844 melting Methods 0.000 description 1
- 150000001247 metal acetylides Chemical class 0.000 description 1
- 238000012986 modification Methods 0.000 description 1
- 230000004048 modification Effects 0.000 description 1
- 229910052750 molybdenum Inorganic materials 0.000 description 1
- 229910052758 niobium Inorganic materials 0.000 description 1
- 229910052762 osmium Inorganic materials 0.000 description 1
- 229910052760 oxygen Inorganic materials 0.000 description 1
- 229910052763 palladium Inorganic materials 0.000 description 1
- 229910052697 platinum Inorganic materials 0.000 description 1
- 229910052699 polonium Inorganic materials 0.000 description 1
- 229910052700 potassium Inorganic materials 0.000 description 1
- 238000001556 precipitation Methods 0.000 description 1
- 230000008569 process Effects 0.000 description 1
- 239000002994 raw material Substances 0.000 description 1
- 238000007670 refining Methods 0.000 description 1
- 229910052702 rhenium Inorganic materials 0.000 description 1
- 229910052703 rhodium Inorganic materials 0.000 description 1
- 229910052701 rubidium Inorganic materials 0.000 description 1
- 229910052707 ruthenium Inorganic materials 0.000 description 1
- 239000000523 sample Substances 0.000 description 1
- 229910052706 scandium Inorganic materials 0.000 description 1
- 229910052708 sodium Inorganic materials 0.000 description 1
- 239000000243 solution Substances 0.000 description 1
- 238000009628 steelmaking Methods 0.000 description 1
- 229910052712 strontium Inorganic materials 0.000 description 1
- 238000010301 surface-oxidation reaction Methods 0.000 description 1
- 229910052713 technetium Inorganic materials 0.000 description 1
- 238000009864 tensile test Methods 0.000 description 1
- 229910052718 tin Inorganic materials 0.000 description 1
- 229910052719 titanium Inorganic materials 0.000 description 1
- 229910052721 tungsten Inorganic materials 0.000 description 1
- 229910052720 vanadium Inorganic materials 0.000 description 1
- 238000007740 vapor deposition Methods 0.000 description 1
- 229910052727 yttrium Inorganic materials 0.000 description 1
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
- C21D3/00—Diffusion processes for extraction of non-metals; Furnaces therefor
- C21D3/02—Extraction of non-metals
- C21D3/04—Decarburising
-
- 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
- C21D8/0247—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips characterised by the heat treatment
- C21D8/0257—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips characterised by the heat treatment with diffusion of elements, e.g. decarburising, nitriding
-
- 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
- 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
- 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
- C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
- C22C—ALLOYS
- C22C38/00—Ferrous alloys, e.g. steel alloys
- C22C38/008—Ferrous alloys, e.g. steel alloys containing tin
-
- 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
-
- 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
- C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
- C22C38/40—Ferrous alloys, e.g. steel alloys containing chromium with nickel
- C22C38/42—Ferrous alloys, e.g. steel alloys containing chromium with nickel with copper
-
- 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
- C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
- C22C38/40—Ferrous alloys, e.g. steel alloys containing chromium with nickel
- C22C38/44—Ferrous alloys, e.g. steel alloys containing chromium with nickel with molybdenum or tungsten
-
- 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
- C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
- C22C38/40—Ferrous alloys, e.g. steel alloys containing chromium with nickel
- C22C38/46—Ferrous alloys, e.g. steel alloys containing chromium with nickel with vanadium
-
- 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
- C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
- C22C38/40—Ferrous alloys, e.g. steel alloys containing chromium with nickel
- C22C38/48—Ferrous alloys, e.g. steel alloys containing chromium with nickel with niobium or tantalum
-
- 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
- C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
- C22C38/40—Ferrous alloys, e.g. steel alloys containing chromium with nickel
- C22C38/50—Ferrous alloys, e.g. steel alloys containing chromium with nickel with titanium or zirconium
-
- 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
- C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
- C22C38/40—Ferrous alloys, e.g. steel alloys containing chromium with nickel
- C22C38/54—Ferrous alloys, e.g. steel alloys containing chromium with nickel with boron
-
- 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
- C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
- C22C38/40—Ferrous alloys, e.g. steel alloys containing chromium with nickel
- C22C38/58—Ferrous alloys, e.g. steel alloys containing chromium with nickel with more than 1.5% by weight of 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
- C22C38/60—Ferrous alloys, e.g. steel alloys containing lead, selenium, tellurium, or antimony, or more than 0.04% by weight of sulfur
-
- C—CHEMISTRY; METALLURGY
- C23—COATING METALLIC MATERIAL; COATING MATERIAL WITH METALLIC MATERIAL; CHEMICAL SURFACE TREATMENT; DIFFUSION TREATMENT OF METALLIC MATERIAL; COATING BY VACUUM EVAPORATION, BY SPUTTERING, BY ION IMPLANTATION OR BY CHEMICAL VAPOUR DEPOSITION, IN GENERAL; INHIBITING CORROSION OF METALLIC MATERIAL OR INCRUSTATION IN GENERAL
- C23C—COATING METALLIC MATERIAL; COATING MATERIAL WITH METALLIC MATERIAL; SURFACE TREATMENT OF METALLIC MATERIAL BY DIFFUSION INTO THE SURFACE, BY CHEMICAL CONVERSION OR SUBSTITUTION; COATING BY VACUUM EVAPORATION, BY SPUTTERING, BY ION IMPLANTATION OR BY CHEMICAL VAPOUR DEPOSITION, IN GENERAL
- C23C2/00—Hot-dipping or immersion processes for applying the coating material in the molten state without affecting the shape; Apparatus therefor
- C23C2/04—Hot-dipping or immersion processes for applying the coating material in the molten state without affecting the shape; Apparatus therefor characterised by the coating material
- C23C2/06—Zinc or cadmium or alloys based thereon
Definitions
- the present invention relates to steel sheets and press-formed products.
- panel system parts such as door outers are also being investigated for increased strength and reduced thickness.
- these panel-type parts are required to have a high appearance quality because they are visible to the public. Therefore, even high-strength steel sheets, which have been conventionally applied to frame parts, are required to have excellent appearance quality after molding when applied to panel-type parts.
- a ghost line is a fine unevenness on the order of several millimeters that occurs on the surface due to preferential deformation around the soft phase when a steel sheet having a hard phase and a soft phase is press-formed. Since the unevenness forms a striped pattern on the surface, a press-molded product with ghost lines has poor appearance quality.
- the interface between the hot-dip galvanized layer and the base steel sheet has an average thickness of 0.1 ⁇ m to 2.0 ⁇ m, and the difference between the maximum thickness and the minimum thickness in the width direction of the steel sheet is within 0.5 ⁇ m.
- the difference between the maximum thickness and the minimum thickness of the refined layer in the steel sheet width direction is within 2.0 ⁇ m.
- the Vickers hardness at a depth of 0.05 mm from the front and back surfaces of the steel plate is 100 to 250 Hv, and (the Vickers hardness at a depth of 0.2 mm from the front and back surfaces) ⁇ 0.8 or less, the variation in Vickers hardness in the inner layer portion on the plate thickness center side from the position where the depth from the front and back is 0.2 mm is 100 Hv or less, and the inner layer portion has a total area ratio of bainite and martensite. It discloses a high-strength thin steel sheet containing 80% or more, having a surface roughness Ra of 0.4 to 1.2 ⁇ m, and having a tensile strength of 780 MPa or more.
- Patent document 4 has a chemical composition in which the alloyed hot-dip galvanized layer contains 10 to 15% Fe and 0.20 to 0.45% Al, and the balance is Zn and impurities, It discloses a high-strength galvannealed steel sheet, wherein the interfacial adhesion strength between the steel sheet and the galvannealed layer is 20 MPa or more.
- a high-strength steel sheet with less property deterioration after cutting characterized by:
- An object of the present invention is to provide a press-formed article having high strength and excellent appearance quality, and a steel sheet from which the press-formed article can be produced.
- the gist of the present invention is as follows.
- the steel sheet according to one aspect of the present invention has a chemical composition in mass% of C: 0.040 to 0.100%, Mn: 1.00-2.00%, Si: 0.005 to 1.500%, P: 0.100% or less, S: 0.0200% or less, Al: 0.005 to 0.700%, N: 0.0150% or less, O: 0.0100% or less, Cr: 0 to 0.80%, Mo: 0-0.16%, B: 0 to 0.0100%, Ti: 0 to 0.100%, Nb: 0 to 0.060%, V: 0 to 0.50%, Ni: 0 to 1.00%, Cu: 0 to 1.00%, W: 0 to 1.00%, Sn: 0 to 1.00%, Sb: 0 to 0.200%, Ca: 0 to 0.0100%, Mg: 0-0.0100%, Zr: 0 to 0.0100%, REM: 0-0.0100%, and the balance: Fe and im
- the steel sheet according to (1) above has the chemical composition, in mass %, Cr: 0.01 to 0.80%, Mo: 0.01-0.16%, B: 0.0001 to 0.0100%, Ti: 0.001 to 0.100%, Nb: 0.001 to 0.060%, V: 0.01 to 0.50%, Ni: 0.01 to 1.00%, Cu: 0.01 to 1.00%, W: 0.01 to 1.00%, Sn: 0.01 to 1.00%, Sb: 0.001 to 0.200%, Ca: 0.0001 to 0.0100%, Mg: 0.0001-0.0100%, Zr: 0.0001-0.0100% and REM: 0.0001-0.0100% It may contain one or more selected from the group consisting of.
- the steel sheet according to (1) or (2) above is 3 times the thickness in the thickness direction from the surface of the steel sheet in the thickness direction from a position 1/8 of the thickness in the thickness direction from the surface of the steel sheet.
- the average value of the Mn concentration in the region up to the position separated by /8 is ⁇ and the standard deviation of the Mn concentration is ⁇ , (3 ⁇ / ⁇ ) ⁇ 100 ⁇ 7.0 may be satisfied.
- the steel sheet according to any one of (1) to (3) above may have a decarburized layer having a thickness of 20 ⁇ m or more on the surface of the steel sheet.
- the steel sheet according to any one of (1) to (4) above may have a plating layer on at least one surface of the steel sheet.
- a press-formed product according to another aspect of the present invention is obtained by press-forming the steel sheet according to any one of (1) to (5) above.
- the inventors investigated a method for suppressing the generation of ghost lines after press forming a high-strength steel plate. As a result, the present inventors have found that it is effective to reduce the difference in hardness in steel and control the surface roughness of the steel sheet within a desired range.
- One of the causes of the difference in hardness in steel is band-like Mn segregation that occurs during the solidification process of steel. If Mn segregation occurs in a band shape, the area around the portion with high Mn concentration is likely to transform into austenite during annealing, so hard martensite occurs in a band shape after annealing after cold rolling. As a result, the difference in hardness in the steel increases, and ghost lines are thought to occur during press forming.
- the present inventors have found that in order to suppress the occurrence of ghost lines in press-formed products, it is important to moderately roughen the surface of the steel plate as a material to the extent that the appearance quality does not deteriorate. bottom.
- the steel sheet according to the present embodiment has a chemical composition in mass% of C: 0.040 to 0.100%, Mn: 1.00 to 2.00%, Si: 0.005 to 1.500%, P : 0.100% or less, S: 0.0200% or less, Al: 0.005 to 0.700%, N: 0.0150% or less, O: 0.0100% or less, and the balance: Fe and impurities contains.
- C 0.040 to 0.100%
- Mn 1.00 to 2.00%
- Si 0.005 to 1.500%
- P 0.100% or less
- S 0.0200% or less
- Al 0.005 to 0.700%
- N 0.0150% or less
- O 0.0100% or less
- Fe and impurities contains a chemical composition in mass% of C: 0.040 to 0.100%, Mn: 1.00 to 2.00%, Si: 0.005 to 1.500%, P : 0.100% or less, S: 0.0200% or less, Al: 0.005 to 0.700%, N: 0.0
- C 0.040-0.100% C is an element that increases the strength of steel sheets and press-formed products.
- the C content should be 0.040% or more.
- the C content is preferably 0.050% or more, more preferably 0.060% or more, 0.070% or more, or 0.075% or more.
- the C content is made 0.100% or less.
- the C content is preferably 0.095% or less, more preferably 0.090% or less or 0.085% or less.
- the C content is preferably more than 0.075%.
- Mn 1.00-2.00%
- Mn is an element that enhances the hardenability of steel and contributes to the improvement of strength.
- the Mn content should be 1.00% or more.
- the Mn content is preferably 1.05% or more, 1.10% or more, or 1.20% or more, more preferably 1.30% or more, 1.40% or more, or 1.50% or more.
- the Mn content is set to 2.00% or less.
- the Mn content is preferably 1.85% or less, more preferably 1.80% or less, and even more preferably 1.75% or less.
- Si 0.005-1.500%
- Si is an element that improves the strength-formability balance of the steel sheet.
- the Si content should be 0.005% or more.
- Si is 0.010% or more.
- Si is also an element that forms coarse Si oxides that act as starting points for destruction.
- the Si content is set to 1.500% or less.
- the Si content is preferably 1.300% or less, more preferably 1.000% or less.
- P 0.100% or less
- P is an impurity element that embrittles steel.
- the P content is set to 0.100% or less.
- the P content is preferably 0.050% or less, more preferably 0.030% or less or 0.020% or less.
- the lower limit of the P content includes 0%, the production cost can be further reduced by setting the P content to 0.001% or more. Therefore, the P content may be 0.001% or more.
- S 0.0200% or less
- S is an impurity element that forms Mn sulfides and deteriorates the formability of the steel sheet, such as ductility, hole expandability, stretch flangeability and bendability. If the S content is 0.0200% or less, it is possible to suppress a significant deterioration in the formability of the steel sheet. Therefore, the S content should be 0.0200% or less.
- the S content is preferably 0.0100% or less, more preferably 0.0080% or less. Although the lower limit of the S content includes 0%, the production cost can be further reduced by setting the S content to 0.0001% or more. Therefore, the S content may be 0.0001% or more.
- Al 0.005-0.700%
- Al is an element that functions as a deoxidizer. In order to sufficiently obtain the deoxidizing effect of Al, the Al content is made 0.005% or more. The Al content is preferably 0.010% or more or 0.025% or more. Al is also an element that embrittles steel by forming coarse oxides that serve as fracture starting points. By setting the Al content to 0.700% or less, it is possible to suppress the formation of coarse oxides that act as starting points for fracture, and to suppress the slab from becoming easily cracked. Therefore, the Al content is set to 0.700% or less. The upper limit of Al content is 0.600%, 0.400%, 0.200% or . 100% is preferred, and 0.085%, 0.070%, 0.065% or 0.060% is more preferred.
- N is an impurity element that forms nitrides and deteriorates formability such as ductility, hole expandability, stretch flangeability and bendability of the steel sheet.
- N content is made 0.0150% or less.
- N is also an element that causes welding defects during welding and hinders productivity. Therefore, the N content is preferably 0.0120% or less, more preferably 0.0100% or less.
- the lower limit of the N content includes 0%, the production cost can be further reduced by setting the N content to 0.0005% or more. Therefore, the N content may be 0.0005% or more.
- O is an impurity element that forms oxides and impairs the formability of the steel sheet, such as ductility, hole expandability, stretch flangeability and bendability.
- the O content is set to 0.0100% or less. It is preferably 0.0080% or less, more preferably 0.0050% or less.
- the lower limit of the O content includes 0%, the manufacturing cost can be further reduced by making the O content 0.0001% or more. Therefore, the O content may be 0.0001% or more.
- the steel sheet according to the present embodiment may contain the following elements as optional elements instead of part of Fe.
- the content is 0% when the following optional elements are not contained.
- Cr 0-0.80% Cr is an element that increases the hardenability of steel and contributes to the improvement of the strength of the steel sheet. Since Cr does not necessarily have to be contained, the lower limit of the Cr content includes 0%. The Cr content is preferably 0.01% or more, more preferably 0.20% or more, and still more preferably 0.30% or more in order to sufficiently obtain the strength improvement effect of Cr. In addition, when the Cr content is 0.80% or less, it is possible to suppress the formation of coarse Cr carbides that may serve as starting points for fracture. Therefore, the Cr content is set to 0.80% or less. In order to reduce alloy costs, the upper limit of Cr content may be set to 0.60%, 0.40%, 0.20%, 0.10% or 0.05%, as required.
- Mo 0-0.16%
- Mo is an element that suppresses phase transformation at high temperatures and contributes to improvement in strength of the steel sheet. Since Mo does not necessarily have to be contained, the lower limit of the Mo content includes 0%. In order to sufficiently obtain the strength improvement effect of Mo, the Mo content is preferably 0.01% or more, more preferably 0.05% or more, and still more preferably 0.10% or more. In addition, when the Mo content is 0.16% or less, it is possible to suppress a decrease in hot workability and a decrease in productivity. Therefore, Mo content shall be 0.16% or less. In order to reduce alloy costs, the upper limit of Mo content may be set to 0.12%, 0.10%, 0.08% or 0.04% as required. Including both Cr: 0.01 to 0.80% and Mo: 0.01 to 0.16% is preferable because the strength of the steel sheet can be more reliably improved.
- B 0 to 0.0100%
- B is an element that suppresses phase transformation at high temperatures and contributes to improvement in strength of the steel sheet. Since B does not necessarily have to be contained, the lower limit of the B content includes 0%. In order to sufficiently obtain the strength improvement effect of B, the B content is preferably 0.0001% or more, more preferably 0.0005% or more, and even more preferably 0.0010% or more. Further, when the B content is 0.0100% or less, it is possible to suppress the formation of B precipitates and the decrease in the strength of the steel sheet. Therefore, the B content is set to 0.0100% or less. In order to reduce the alloy cost, the upper limit of the B content may be 0.0050%, 0.0030%, 0.0020%, 0.0010% or 0.0005% as required.
- Ti is an element that has the effect of reducing the amounts of S, N, and O that generate coarse inclusions that act as starting points for fracture.
- Ti has the effect of refining the structure and improving the strength-formability balance of the steel sheet.
- the lower limit of the Ti content includes 0%.
- the Ti content is preferably 0.001% or more, more preferably 0.001% or more.
- the Ti content is set to 0.100% or less.
- the Ti content is preferably 0.080% or less, more preferably 0.060% or less.
- the upper limit of the Ti content may be 0.040%, 0.020%, 0.010% or 0.005% as required.
- Nb is an element that contributes to the improvement of the strength of a steel sheet through strengthening by precipitates, grain refinement strengthening by suppressing the growth of ferrite grains, and dislocation strengthening by suppressing recrystallization. Since Nb does not necessarily have to be contained, the lower limit of the Nb content includes 0%. In order to sufficiently obtain the above effects, the Nb content is preferably 0.001% or more, more preferably 0.005% or more, and still more preferably 0.010% or more. Further, when the Nb content is 0.060% or less, it is possible to promote recrystallization and suppress the remaining non-recrystallized ferrite, thereby ensuring the formability of the steel sheet. Therefore, the Nb content is set to 0.060% or less. The Nb content is preferably 0.050% or less, more preferably 0.040% or less. In order to reduce the alloy cost, the upper limit of the Nb content may be 0.030%, 0.020%, 0.010% or 0.005% as required.
- V 0-0.50%
- V is an element that contributes to the improvement of the strength of the steel sheet through strengthening by precipitates, grain refinement strengthening by suppressing the growth of ferrite grains, and dislocation strengthening by suppressing recrystallization. Since V does not necessarily have to be contained, the lower limit of the V content includes 0%. In order to sufficiently obtain the strength improvement effect of V, the V content is preferably 0.01% or more, more preferably 0.03% or more. Further, when the V content is 0.50% or less, it is possible to suppress the deterioration of the formability of the steel sheet due to the precipitation of a large amount of carbonitrides. Therefore, the V content is set to 0.50% or less. In order to reduce the alloy cost, the upper limit of the V content may be 0.30%, 0.20%, 0.10%, 0.05% or 0.02% as required.
- Ni is an element that suppresses phase transformation at high temperatures and contributes to improvement in strength of the steel sheet. Since Ni does not necessarily have to be contained, the lower limit of the Ni content includes 0%. The Ni content is preferably 0.01% or more, more preferably 0.05% or more, and still more preferably 0.20% or more in order to sufficiently obtain the strength improvement effect of Ni. Moreover, it can suppress that the weldability of a steel plate falls as Ni content is 1.00% or less. Therefore, the Ni content is set to 1.00% or less. In order to reduce the alloy cost, the upper limit of the Ni content may be 0.60%, 0.40%, 0.20%, 0.10% or 0.03% as required.
- Cu 0-1.00%
- Cu is an element that exists in steel in the form of fine particles and contributes to the improvement of the strength of the steel sheet. Since Cu does not necessarily have to be contained, the lower limit of the Cu content includes 0%.
- the Cu content is preferably 0.01% or more, more preferably 0.05% or more, and still more preferably 0.15% or more in order to sufficiently obtain the strength improvement effect of Cu. Moreover, it can suppress that the weldability of a steel plate falls that Cu content is 1.00% or less. Therefore, the Cu content is set to 1.00% or less.
- the upper limit of Cu content may be set to 0.60%, 0.40%, 0.20%, 0.10% or 0.03% as required.
- W 0-1.00%
- W is an element that suppresses phase transformation at high temperatures and contributes to improvement in strength of the steel sheet. Since W does not necessarily have to be contained, the lower limit of the W content includes 0%. In order to sufficiently obtain the strength improvement effect of W, the W content is preferably 0.01% or more, more preferably 0.03% or more, and even more preferably 0.10% or more. In addition, when the W content is 1.00% or less, it is possible to suppress a decrease in hot workability and a decrease in productivity. Therefore, the W content is set to 1.00% or less. In order to reduce the alloy cost, the upper limit of the W content may be 0.50%, 0.20%, 0.10%, 0.05% or 0.02% as required.
- Sn 0-1.00%
- Sn is an element that suppresses the coarsening of crystal grains and contributes to the improvement of the strength of the steel sheet. Since Sn does not necessarily have to be contained, the lower limit of the Sn content includes 0%. In order to sufficiently obtain the effect of Sn, the Sn content is more preferably 0.01% or more. Moreover, when the Sn content is 1.00% or less, it is possible to prevent the steel sheet from embrittlement and breakage during rolling. Therefore, the Sn content is set to 1.00% or less. In order to reduce the alloy cost, the upper limit of the Sn content may be 0.50%, 0.20%, 0.10%, 0.05% or 0.02% as required.
- Sb 0-0.200%
- Sb is an element that suppresses the coarsening of crystal grains and contributes to the improvement of the strength of the steel sheet. Since Sb does not necessarily have to be contained, the lower limit of the Sb content includes 0%. In order to sufficiently obtain the above effects, the Sb content is preferably 0.001% or more, more preferably 0.005% or more. Moreover, when the Sb content is 0.200% or less, it is possible to prevent the steel sheet from embrittlement and breakage during rolling. Therefore, the Sb content is set to 0.200% or less. In order to reduce the alloy cost, the upper limit of the Sb content may be set to 0.100%, 0.070%, 0.040%, 0.010% or 0.005% as required.
- Ca, Mg, Zr, and REM are elements that contribute to improving the formability of steel sheets. Since Ca, Mg, Zr and REM do not necessarily have to be contained, the lower limit of the total content of these elements includes 0%. In order to sufficiently obtain the effect of improving formability, the content of each of these elements is preferably 0.0001% or more, more preferably 0.0010% or more. In order to sufficiently obtain the above effect, it is not necessary to contain all of the above elements, and the content of any one of the above elements should be 0.0001% or more.
- the content of each of Ca, Mg, Zr and REM is 0.0100% or less, it is possible to suppress the decrease in ductility of the steel sheet. Therefore, the content of each of these elements is set to 0.0100% or less. Preferably, it is 0.0050% or less. In order to reduce alloy costs, the upper limits of the contents of Ca, Mg, Zr and REM may be set to 0.0030%, 0.0020%, 0.0010% or 0.0003%, respectively, if necessary.
- REM Rotary Earth Metal
- the REM content refers to the total content of these elements.
- the rest of the chemical composition of the steel sheet according to this embodiment may be Fe and impurities.
- impurities include those that are unavoidably mixed from steel raw materials or scraps and/or during the steelmaking process, or elements that are allowed within a range that does not impair the properties of the steel sheet according to the present embodiment.
- Impurities include H, Na, Cl, Co, Zn, Ga, Ge, As, Se, Y, Tc, Ru, Rh, Pd, Ag, Cd, In, Te, Cs, Ta, Re, Os, Ir, Pt , Au, Pb, Bi, and Po.
- the total amount of impurities may be 0.100% or less.
- the chemical composition of the steel sheet described above may be measured by a general analytical method. For example, it may be measured using ICP-AES (Inductively Coupled Plasma-Atomic Emission Spectrometry).
- C and S can be measured using a combustion-infrared absorption method
- N can be measured using an inert gas fusion-thermal conductivity method
- O can be measured using an inert gas fusion-nondispersive infrared absorption method.
- the coating layer on the surface may be removed by mechanical grinding, and then the chemical composition may be analyzed.
- Arithmetic mean waviness Wa 0.10 to 0.30 ⁇ m
- the present inventors have found that the surface of the steel sheet that is the material should be moderately roughened. By doing so, it was found that the occurrence of ghost lines in the press-molded product can be suppressed. Therefore, the steel sheet according to the present embodiment has an arithmetic mean waviness Wa of 0.10 ⁇ m or more. It is preferably 0.13 ⁇ m or more.
- the arithmetic mean waviness Wa is set to 0.30 ⁇ m or less. It is preferably 0.25 ⁇ m or less.
- the arithmetic mean waviness Wa is the arithmetic mean waviness of the steel sheet when the steel sheet does not have a coating layer, and the arithmetic mean waviness of the coating layer when the steel sheet has a coating layer on its surface.
- the arithmetic mean waviness Wa is obtained by the following method.
- a test piece of 50 mm x 50 mm is cut from a position 10 mm or more away from the end face of the steel plate.
- a laser displacement measuring device Keyence VK-X1000
- three lines of the profile are measured along the direction perpendicular to the rolling direction.
- an undulation curve is obtained by successively applying contour filters with cutoff values ⁇ c and ⁇ f to the cross-sectional curve according to JIS B 0601:2013.
- a wave curve is obtained by removing a component with a wavelength ⁇ c of 0.8 mm or less and a component with a wavelength ⁇ f of 2.5 mm or more from the obtained measurement results.
- the arithmetic mean undulation is calculated according to JIS B 0601:2013, and the average value of a total of 3 lines is calculated.
- the arithmetic mean of the calculated average values of the three lines is taken as the arithmetic mean waviness Wa of the steel plate.
- the surface of the plating layer may be subjected to the line analysis described above.
- the steel plate according to the present embodiment includes a region from a position 1/8 of the plate thickness in the thickness direction from the surface of the steel plate to a position 3/8 of the plate thickness in the thickness direction from the surface (surface of the steel plate).
- the average value of the Mn concentration in the region from 1/8 depth to 3/8 depth from the surface of the steel sheet is ⁇ in unit mass%, and the standard deviation of the Mn concentration is ⁇ in unit mass%, ( 3 ⁇ / ⁇ ) ⁇ 100 ⁇ 7.0.
- (3 ⁇ / ⁇ ) ⁇ 100 is more preferably 6.5 or less.
- the lower limit of (3 ⁇ / ⁇ ) ⁇ 100 is not particularly limited, it may be 0.
- Lowering (3 ⁇ / ⁇ ) ⁇ 100 increases the manufacturing cost, so the lower limit may be 2.0, 4.0 or 5.0.
- the upper limit of (3 ⁇ / ⁇ ) ⁇ 100 may be set to 11.0, 10.0, 9.0 or 8.0 as required.
- the average value ⁇ of the Mn concentration and the standard deviation ⁇ of the Mn concentration are obtained by the following method. After the thickness cross-section of the steel sheet is mirror-polished, the Mn concentration is measured at 600 points at predetermined depth positions in the rolling direction of the steel sheet at intervals of 1 ⁇ m. By calculating the average value of the obtained Mn concentrations, the Mn concentration (% by mass) at a predetermined depth position is obtained. This operation is performed every 1 ⁇ m in the plate thickness direction from a position 1/8 of the plate thickness away from the surface of the steel plate in the plate thickness direction to a position 3/8 of the plate thickness away from the surface in the plate thickness direction.
- the average value ⁇ of the Mn concentration is obtained. Further, the standard deviation ⁇ of the Mn concentration is obtained by calculating the standard deviation from all the obtained Mn concentrations.
- the device used is an electron probe microanalyzer (EPMA), and the measurement conditions are an acceleration voltage of 15 kV.
- the steel sheet according to this embodiment may have a plating layer on at least one surface of the steel sheet.
- the plating layer includes a zinc plating layer, a zinc alloy plating layer, and an alloying zinc plating layer and an alloying zinc alloy plating layer obtained by subjecting these to an alloying treatment.
- the zinc plating layer and the zinc alloy plating layer are formed by a hot dip plating method, an electroplating method, or a vapor deposition plating method.
- the Al content of the galvanized layer is 0.5% by mass or less, the adhesion between the surface of the steel sheet and the galvanized layer can be sufficiently ensured, so the Al content of the galvanized layer is 0.5%. % by mass or less is preferable.
- the galvanized layer is a hot-dip galvanized layer
- the Fe content of the hot-dip galvanized layer is preferably 3.0% by mass or less in order to increase the adhesion between the steel sheet surface and the galvanized layer.
- the galvanized layer is an electrogalvanized layer
- the Fe content of the electrogalvanized layer is preferably 0.5% by mass or less from the viewpoint of improving corrosion resistance.
- the zinc plating layer and the zinc alloy plating layer include Al, Ag, B, Be, Bi, Ca, Cd, Co, Cr, Cs, Cu, Ge, Hf, Zr, I, K, La, Li, Mg, Mn, One or more of Mo, Na, Nb, Ni, Pb, Rb, Sb, Si, Sn, Sr, Ta, Ti, V, W, Zr, and REM, in a range that does not impair the corrosion resistance and formability of the steel sheet and may contain In particular, Ni, Al and Mg are effective in improving the corrosion resistance of steel sheets.
- the zinc plated layer or zinc alloy plated layer may be a zinc alloyed layer or a zinc alloy plated layer that has been alloyed.
- the hot-dip galvanized layer after the alloying treatment is used from the viewpoint of improving the adhesion between the steel sheet surface and the alloyed coating layer.
- the Fe content of the hot-dip zinc alloy plating layer is 7.0 to 13.0% by mass.
- the Fe content in the plating layer can be obtained by the following method. Only the plated layer is dissolved and removed using a 5% by volume HCl aqueous solution containing an inhibitor. By measuring the Fe content in the obtained solution using ICP-AES (Inductively Coupled Plasma-Atomic Emission Spectrometry), the Fe content (% by mass) in the plating layer is obtained.
- ICP-AES Inductively Coupled Plasma-Atomic Emission Spectrometry
- the steel sheet according to this embodiment may have a decarburized layer with a thickness of 20 ⁇ m or more on the surface of the steel sheet, regardless of the presence or absence of the plating layer.
- the thickness of the decarburized layer is measured by the following method. At three arbitrary points on the steel sheet, the C concentration in the area from the surface of the steel sheet to a position separated by 1/2 of the thickness in the depth direction (thickness direction) is measured every 1 ⁇ m depth. The thickness of the decarburized layer is obtained by regarding the region where the C concentration is 1/2 or less of the C concentration at the position 1/2 of the plate thickness away from the surface as the decarburized layer, and calculating the thickness of the region. For the measurement, a Marcus type high-frequency glow discharge luminescence surface analyzer (GD-Profiler) manufactured by Horiba, Ltd. is used.
- GD-Profiler high-frequency glow discharge luminescence surface analyzer
- the plate thickness of the steel plate according to the present embodiment is not limited to a specific range, but is preferably 0.2 to 2.0 mm in consideration of versatility and manufacturability.
- the plate thickness is preferably 0.2 mm or more. More preferably, it is 0.4 mm or more.
- the plate thickness is preferably 2.0 mm or less. More preferably, it is 1.5 mm or less.
- the steel plate according to this embodiment preferably has a tensile strength of 500 to 750 MPa.
- a tensile strength of 500 to 750 MPa.
- the tensile strength may have a lower limit of 540 MPa, 580 MPa or 600 MPa, and an upper limit of 680 MPa or 660 MPa.
- the tensile strength is evaluated according to JIS Z 2241:2011.
- the test piece shall be JIS Z 2241:2011 No. 5 test piece.
- Tensile test pieces are taken from the 1/4 part from the edge in the width direction, and the direction perpendicular to the rolling direction is taken as the longitudinal direction.
- the press-formed product according to this embodiment which can be manufactured by press-forming the steel plate described above, will be described.
- the press-formed product according to this embodiment has the same chemical composition as the steel plate described above.
- the press-formed product according to the present embodiment may have the above-described plated layer on at least one surface.
- press-formed product according to the present embodiment is obtained by press-forming the above-described steel plate, the occurrence of ghost lines is suppressed and the appearance quality is excellent.
- press-formed products include panel system parts such as door outers for automobile bodies.
- excellent appearance quality means that striped patterns (that is, ghost lines) occurring on the surface at intervals of several millimeters are not observed.
- the maximum length of streak patterns at intervals of several millimeters that can be observed when an arbitrary area of 100 mm ⁇ 100 mm is visually observed is 50 mm or less.
- the maximum length of the streak pattern is preferably 20 mm or less. Moreover, it is more preferable that no streak pattern is observed.
- Wz which is the sum of the maximum peak height Zp and the maximum valley height Zv of the undulation curve
- Wz is 0.60 ⁇ m or less.
- Wz is obtained in accordance with JIS B 0601:2013 by obtaining the undulation curve of the surface of the press-formed product, obtaining the maximum peak height Zp and the maximum valley height Zv, and calculating the sum of these.
- the steel plate according to the present embodiment can obtain the effect as long as it has the above characteristics regardless of the manufacturing method. Moreover, it may be a steel strip instead of a steel plate.
- a steel plate in which the arithmetic mean waviness Wa is preferably controlled can be stably manufactured. can do.
- the condition (VI) in addition to the following conditions (I) to (IV).
- Conditions (V) and (VI) are arbitrary conditions. Each condition will be described below.
- the winding temperature is set to 550° C. or higher.
- the pickling time shall be 50 seconds or longer.
- Arithmetic mean roughness Ra of the roll surface in the final pass of cold rolling is set to 0.2 to 0.7 ⁇ m.
- the reduction ratio of temper rolling is set to 0.3 to 0.7%, and the arithmetic mean roughness Ra of the rolling rolls is set to 1.5 to 3.5 ⁇ m.
- V) The slab is heated to a temperature range of 1200° C. or higher and held in the temperature range for 5 hours or longer.
- Annealing is performed with a dew point (average dew point in the annealing furnace) of ⁇ 20° C. or higher and a residence time of the steel sheet in a temperature range of 700° C. or higher for 50 to 400 seconds.
- the winding temperature is more preferably 600°C or higher, and even more preferably 650°C or higher.
- (II) Pickling Time 50 Seconds or More When the pickling time is set to 50 seconds or more after coiling and before cold rolling, unevenness is likely to occur on the surface of the steel sheet. More preferably, the pickling time is 70 seconds or longer.
- V Slab heating temperature/holding time: 5 hours or longer in a temperature range of 1200°C or higher
- Condition (V) is an arbitrary condition.
- Condition (VI) is an arbitrary condition.
- the cold-rolled steel sheet obtained by the above method may be annealed.
- the dew point during annealing (average dew point in the annealing furnace) is -20 ° C. or higher, and the residence time of the steel plate in the temperature range of 700 ° C. or higher is 50 to 400 seconds, so that the surface of the steel plate is stably removed.
- a decarburized layer having a thickness of 30 ⁇ m or more can be formed on the surface of the steel sheet.
- the upper limit of the dew point need not be set, but may be about 10°C.
- Conditions other than those described above are not particularly limited, but, for example, it is preferable to satisfy the following conditions. After heating the slab to a temperature range of 1100° C. or higher, the billet is hot rolled. After hot rolling, coiling is performed, and then pickling is performed. Cold rolling is performed after pickling. The cumulative rolling reduction in cold rolling is preferably 30 to 90%. Annealing is performed after cold rolling. After that, the plating layer described above is formed as necessary. Moreover, it is preferable to perform temper rolling after that.
- the press molding method is not particularly limited.
- automobile panel parts such as door outers can be formed by pressing a steel plate with a blank holder and a die, applying strain to the steel plate by pressing a punch, and stretching the steel plate.
- Such forming is called draw forming or stretch forming.
- a slab with a thickness of 240 to 300 mm was produced by melting steel having the chemical composition shown in Table 1 and continuously casting it. Using the obtained slabs, cold-rolled steel sheets and plated steel sheets were manufactured under conditions (I) to (V) described later. In Table 2, when the conditions were satisfied, "OK” was written in the condition column, and when the conditions were not satisfied, "NG” was written in the condition column. The thickness of the obtained steel sheet and plated steel sheet was 0.2 to 2.0 mm.
- annealing was performed after cold rolling.
- Manufacturing conditions other than conditions (I) to (VI) were as follows. After heating the slab to a temperature range of 1100° C. or higher, it was hot rolled. After hot rolling, coiling was performed, and then pickling was performed. After pickling, cold rolling was performed with a cumulative rolling reduction of 30 to 90%. Annealing was performed after cold rolling to form an alloyed hot-dip galvanized layer (GA), a hot-dip galvanized layer (GI), and an electroplated layer (EG) as necessary. After that, temper rolling was performed.
- GA alloyed hot-dip galvanized layer
- GI hot-dip galvanized layer
- EG electroplated layer
- Conditions (I) to (VI) in the table are as follows.
- the winding temperature is set to 550° C. or higher.
- the pickling time shall be 50 seconds or more.
- Arithmetic mean roughness Ra of the roll surface in the final pass of cold rolling is set to 0.2 to 0.7 ⁇ m.
- the reduction ratio of temper rolling is set to 0.3 to 0.7%, and the arithmetic mean roughness Ra of the rolling rolls is set to 1.5 to 3.5 ⁇ m.
- the slab is heated to a temperature range of 1200° C. or higher and held in the temperature range for 5 hours or longer.
- Annealing is performed with a dew point (average dew point in the annealing furnace) of ⁇ 20° C. or higher and a residence time of the steel sheet in a temperature range of 700° C. or higher for 50 to 400 seconds.
- a roughly semi-cylindrical simulated part (press-molded product) simulating a door outer was manufactured by press molding.
- the material steel plate or plated steel sheet
- strain in any direction along the surface of the simulated part is applied at any position on the surface of the simulated part.
- the ratio of the strain in the direction perpendicular to the direction (any direction thereof) to the strain was set to about 1. In other words, press molding was performed so that strain anisotropy did not occur at any position on the surface of the simulated part.
- the arithmetic mean waviness Wa, the average value ⁇ and standard deviation ⁇ of the Mn concentration, the tensile strength, and the thickness of the decarburized layer were obtained by the methods described above.
- the obtained tensile strength was 500 MPa or more, it was determined to be high strength and passed. On the other hand, when the obtained tensile strength was less than 500 MPa, it was determined to be unacceptable because the strength was inferior.
- the appearance quality of the simulated parts was evaluated by the following method. Appearance quality was evaluated by the degree of ghost lines generated on the surface of the simulated part after molding.
- the surface after press molding was ground with a grindstone, and striped patterns at intervals of several millimeters on the surface were judged to be ghost lines, and were rated on a scale of 1 to 5 depending on the degree of occurrence of the striped pattern.
- An arbitrary area of 100 mm x 100 mm was visually checked, and the case where no streak pattern was confirmed was rated as "1", and the case where the maximum length of the streak pattern was 20 mm or less was rated as "2", and the maximum length of the streak pattern.
- Wz is the sum of the maximum peak height Zp and the maximum valley height Zv of the undulation curve.
- a waviness curve of the surface of the press-molded product (simulated part) was obtained in accordance with JIS B 0601:2013 by the same method as that used to obtain the arithmetic mean waviness Wa. From this undulation curve, the maximum peak height Zp and the maximum valley height Zv were obtained, and Wz was obtained by calculating the sum of these. When the obtained Wz was 0.40 ⁇ m or less, it was judged that the appearance quality was superior.
- the press-molded product according to the comparative example had inferior strength or deteriorated appearance quality.
- the steel sheets according to the comparative examples have high strength and could not produce press-formed products having excellent appearance quality.
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Abstract
Description
(1)本発明の一態様に係る鋼板は、化学組成が、質量%で、
C :0.040~0.100%、
Mn:1.00~2.00%、
Si:0.005~1.500%、
P :0.100%以下、
S :0.0200%以下、
Al:0.005~0.700%、
N :0.0150%以下、
O :0.0100%以下、
Cr:0~0.80%、
Mo:0~0.16%、
B :0~0.0100%、
Ti:0~0.100%、
Nb:0~0.060%、
V :0~0.50%、
Ni:0~1.00%、
Cu:0~1.00%、
W :0~1.00%、
Sn:0~1.00%、
Sb:0~0.200%、
Ca:0~0.0100%、
Mg:0~0.0100%、
Zr:0~0.0100%、
REM:0~0.0100%、および
残部:Feおよび不純物であり、
算術平均うねりWaが0.10~0.30μmである。
(2)上記(1)に記載の鋼板は、前記化学組成が、質量%で、
Cr:0.01~0.80%、
Mo:0.01~0.16%、
B :0.0001~0.0100%、
Ti:0.001~0.100%、
Nb:0.001~0.060%、
V :0.01~0.50%、
Ni:0.01~1.00%、
Cu:0.01~1.00%、
W :0.01~1.00%、
Sn:0.01~1.00%、
Sb:0.001~0.200%、
Ca:0.0001~0.0100%、
Mg:0.0001~0.0100%、
Zr:0.0001~0.0100%、および
REM:0.0001~0.0100%
からなる群から選択される1種または2種以上を含有してもよい。
(3)上記(1)または(2)に記載の鋼板は、前記鋼板の表面から板厚方向に板厚の1/8離れた位置から、前記表面から前記板厚方向に前記板厚の3/8離れた位置までの領域におけるMn濃度の平均値をμとし、前記Mn濃度の標準偏差をσとしたとき、(3σ/μ)×100≦7.0であってもよい。
(4)上記(1)~(3)のいずれか1項に記載の鋼板は、前記鋼板の表面に厚さが20μm以上の脱炭層を有してもよい。
(5)上記(1)~(4)のいずれか1項に記載の鋼板は、前記鋼板の少なくとも一方の表面にめっき層を有してもよい。
(6)本発明の別の態様に係るプレス成形品は、上記(1)~(5)の何れか1項に記載の鋼板をプレス成形して得られる。
Cは、鋼板およびプレス成形品の強度を高める元素である。所望の強度を得るために、C含有量は0.040%以上とする。強度をより高めるため、C含有量は、好ましくは0.050%以上であり、より好ましくは0.060%以上、0.070%以上又は0.075%以上である。
また、C含有量を0.100%以下とすることで、凝固時のMnの拡散が助長され、これによりバンド状のMn偏析が生じやすくなることを抑制できる。その結果、プレス成形後のゴーストラインの発生を抑制できる。そのため、C含有量は0.100%以下とする。C含有量は、0.095%以下が好ましく、0.090%以下又は0.085%以下がより好ましい。
なお、Mn含有量が1.40%以下である場合は、C含有量は0.075%超であることが好ましい。このように、Mn含有量およびC含有量を厳格に制御することで、高温において鋼中のMn拡散が促進され、Mn偏析を低減することができる。
Mnは、鋼の焼入れ性を高めて、強度の向上に寄与する元素である。所望の強度を得るために、Mn含有量は1.00%以上とする。Mn含有量は、好ましくは1.05%以上、1.10%以上又は1.20%以上、より好ましくは1.30%以上、1.40%以上又は1.50%以上である。
また、Mn含有量が2.00%以下であると、鋼の凝固時にバンド状のMn偏析が生じることを抑制できる。そのため、Mn含有量は2.00%以下とする。Mn含有量は、1.85%以下が好ましく、1.80%以下がより好ましく、1.75%以下がより一層好ましい。
Siは、鋼板の強度-成形性バランスを向上する元素である。この効果を得るために、Si含有量は0.005%以上とする。好ましくは0.010%以上である。
また、Siは、破壊の起点として働く粗大なSi酸化物を形成する元素でもある。Si含有量を1.500%以下とすることで、Si酸化物が形成されることを抑制でき、割れが発生しにくくなる。その結果、鋼の脆化を抑制することができる。そのため、Si含有量は1.500%以下とする。Si含有量は1.300%以下が好ましく、1.000%以下がより好ましい。
Pは、不純物元素であり、鋼を脆化する元素である。P含有量が0.100%以下であると、鋼板が脆化して生産工程において割れ易くなることを抑制できる。そのため、P含有量は0.100%以下とする。生産性の観点から、P含有量は0.050%以下が好ましく、0.030%以下又は0.020%以下がより好ましい。
P含有量の下限は0%を含むが、P含有量を0.001%以上とすることで、製造コストをより低減できる。そのため、P含有量は0.001%以上としてもよい。
Sは、不純物元素であり、Mn硫化物を形成し、鋼板の延性、穴拡げ性、伸びフランジ性および曲げ性などの成形性を劣化させる元素である。S含有量が0.0200%以下であると、鋼板の成形性が著しく低下することを抑制できる。そのため、S含有量は0.0200%以下とする。S含有量は0.0100%以下が好ましく、0.0080%以下がより好ましい。
S含有量の下限は0%を含むが、S含有量を0.0001%以上とすることで、製造コストをより低減できる。そのため、S含有量は0.0001%以上としてもよい。
Alは、脱酸材として機能する元素である。Alによる脱酸効果を十分に得るために、Al含有量は0.005%以上とする。Al含有量は、好ましくは0.010%以上または0.025%以上である。
また、Alは、破壊の起点となる粗大な酸化物を形成し、鋼を脆化する元素でもある。Al含有量を0.700%以下とすることで、破壊の起点として働く粗大な酸化物の生成を抑制でき、鋳片が割れ易くなることを抑制できる。そのため、Al含有量は0.700%以下とする。Al含有量の上限は0.600%、0.400%、0.200%または.100%が好ましく、0.085%、0.070%、0.065%又は0.060%がより好ましい。
Nは、不純物元素であり、窒化物を形成し、鋼板の延性、穴拡げ性、伸びフランジ性および曲げ性などの成形性を劣化させる元素である。N含有量が0.0150%以下であると、鋼板の成形性が低下することを抑制できる。そのため、N含有量は0.0150%以下とする。また、Nは、溶接時に溶接欠陥を発生させて生産性を阻害する元素でもある。そのため、N含有量は、好ましくは0.0120%以下であり、より好ましくは0.0100%以下である。
N含有量の下限は0%を含むが、N含有量を0.0005%以上とすることで、製造コストをより低減できる。そのため、N含有量は0.0005%以上としてもよい。
Oは、不純物元素であり、酸化物を形成し、鋼板の延性、穴拡げ性、伸びフランジ性および曲げ性などの成形性を阻害する元素である。O含有量が0.0100%以下であると、鋼板の成形性が著しく低下することを抑制できる。そのため、O含有量は0.0100%以下とする。好ましくは0.0080%以下、より好ましくは0.0050%以下である。
O含有量の下限は0%を含むが、O含有量を0.0001%以上とすることで、製造コストをより低減できる。そのため、O含有量は0.0001%以上としてもよい。
Crは、鋼の焼入れ性を高め、鋼板の強度の向上に寄与する元素である。Crは必ずしも含有させなくてよいので、Cr含有量の下限は0%を含む。Crによる強度向上効果を十分に得るためには、Cr含有量は、0.01%以上が好ましく、0.20%以上がより好ましく、0.30%以上がより一層好ましい。
また、Cr含有量が0.80%以下であると、破壊の起点となり得る粗大なCr炭化物が形成されることを抑制できる。そのため、Cr含有量は0.80%以下とする。合金コスト低減のため、必要に応じて、Cr含有量の上限を0.60%、0.40%、0.20%、0.10%又は0.05%としてもよい。
Moは、高温での相変態を抑制し、鋼板の強度の向上に寄与する元素である。Moは必ずしも含有させなくてよいので、Mo含有量の下限は0%を含む。Moによる強度向上効果を十分に得るためには、Mo含有量は、0.01%以上が好ましく、0.05%以上がより好ましく、0.10%以上がより一層好ましい。
また、Mo含有量が0.16%以下であると、熱間加工性が低下して生産性が低下することを抑制できる。そのため、Mo含有量は、0.16%以下とする。合金コスト低減のため、必要に応じて、Mo含有量の上限を0.12%、0.10%、0.08%又は0.04%としてもよい。
なお、Cr:0.01~0.80%およびMo:0.01~0.16%の両方を含むことで、鋼板の強度をより確実に向上することができるため、好ましい。
Bは、高温での相変態を抑制し、鋼板の強度の向上に寄与する元素である。Bは必ずしも含有させなくてよいので、B含有量の下限は0%を含む。Bによる強度向上効果を十分に得るためには、B含有量は、0.0001%以上が好ましく、0.0005%以上がより好ましく、0.0010%以上がより一層好ましい。
また、B含有量が0.0100%以下であると、B析出物が生成して鋼板の強度が低下することを抑制できる。そのため、B含有量は0.0100%以下とする。合金コスト低減のため、必要に応じて、B含有量の上限を0.0050%、0.0030%、0.0020%、0.0010%又は0.0005%としてもよい。
Tiは、破壊の起点として働く粗大な介在物を発生させるS量、N量およびO量を低減する効果を有する元素である。また、Tiは組織を微細化し、鋼板の強度-成形性バランスを高める効果がある。Tiは必ずしも含有させなくてよいので、Ti含有量の下限は0%を含む。上記効果を十分に得るためには、Ti含有量は0.001%以上とすることが好ましく、0.001%以上とすることがより好ましい。
また、Ti含有量が0.100%以下であると、粗大なTi硫化物、Ti窒化物およびTi酸化物の形成を抑制でき、鋼板の成形性を確保することができる。そのため、Ti含有量は0.100%以下とする。Ti含有量は0.080%以下とすることが好ましく、0.060%以下とすることがより好ましい。合金コスト低減のため、必要に応じて、Ti含有量の上限を0.040%、0.020%、0.010%又は0.005%としてもよい。
Nbは、析出物による強化、フェライト結晶粒の成長抑制による細粒化強化および再結晶の抑制による転位強化によって、鋼板の強度の向上に寄与する元素である。Nbは必ずしも含有させなくてよいので、Nb含有量の下限は0%を含む。上記効果を十分に得るためには、Nb含有量は0.001%以上が好ましく、0.005%以上とすることがより好ましく、0.010%以上とすることがより一層好ましい。
また、Nb含有量が0.060%以下であると、再結晶を促進して未再結晶フェライトが残存することを抑制でき、鋼板の成形性を確保することができる。そのため、Nb含有量は0.060%以下とする。Nb含有量は好ましくは0.050%以下であり、より好ましくは0.040%以下である。合金コスト低減のため、必要に応じて、Nb含有量の上限を0.030%、0.020%、0.010%又は0.005%としてもよい。
Vは、析出物による強化、フェライト結晶粒の成長抑制による細粒化強化および再結晶の抑制による転位強化によって、鋼板の強度の向上に寄与する元素である。Vは必ずしも含有させなくてよいので、V含有量の下限は0%を含む。Vによる強度向上効果を十分に得るためには、V含有量は、0.01%以上が好ましく、0.03%以上がより好ましい。
また、V含有量が0.50%以下であると、炭窒化物が多量に析出して鋼板の成形性が低下することを抑制できる。そのため、V含有量は、0.50%以下とする。合金コスト低減のため、必要に応じて、V含有量の上限を0.30%、0.20%、0.10%、0.05%又は0.02%としてもよい。
Niは、高温での相変態を抑制し、鋼板の強度の向上に寄与する元素である。Niは必ずしも含有させなくてよいので、Ni含有量の下限は0%を含む。Niによる強度向上効果を十分に得るためには、Ni含有量は、0.01%以上が好ましく、0.05%以上がより好ましく、0.20%以上がより一層好ましい。
また、Ni含有量が1.00%以下であると、鋼板の溶接性が低下することを抑制できる。そのため、Ni含有量は1.00%以下とする。合金コスト低減のため、必要に応じて、Ni含有量の上限を0.60%、0.40%、0.20%、0.10%又は0.03%としてもよい。
Cuは、微細な粒子の形態で鋼中に存在し、鋼板の強度の向上に寄与する元素である。Cuは必ずしも含有させなくてよいので、Cu含有量の下限は0%を含む。Cuによる強度向上効果を十分に得るためには、Cu含有量は、0.01%以上が好ましく、0.05%以上がより好ましく、0.15%以上がより一層好ましい。
また、Cu含有量が1.00%以下であると、鋼板の溶接性が低下することを抑制できる。そのため、Cu含有量は1.00%以下とする。合金コスト低減のため、必要に応じて、Cu含有量の上限を0.60%、0.40%、0.20%、0.10%又は0.03%としてもよい。
Wは、高温での相変態を抑制し、鋼板の強度の向上に寄与する元素である。Wは必ずしも含有させなくてよいので、W含有量の下限は0%を含む。Wによる強度向上効果を十分に得るためには、W含有量は、0.01%以上が好ましく、0.03%以上がより好ましく、0.10%以上がより一層好ましい。
また、W含有量が1.00%以下であると、熱間加工性が低下して生産性が低下することを抑制できる。そのため、W含有量は1.00%以下とする。合金コスト低減のため、必要に応じて、W含有量の上限を0.50%、0.20%、0.10%、0.05%又は0.02%としてもよい。
Snは、結晶粒の粗大化を抑制し、鋼板の強度の向上に寄与する元素である。Snは必ずしも含有させなくてよいので、Sn含有量の下限は0%を含む。Snによる効果を十分に得るためには、Sn含有量は、0.01%以上がより好ましい。
また、Sn含有量が1.00%以下であると、鋼板が脆化して圧延時に破断することを抑制できる。そのため、Sn含有量は1.00%以下とする。合金コスト低減のため、必要に応じて、Sn含有量の上限を0.50%、0.20%、0.10%、0.05%又は0.02%としてもよい。
Sbは、結晶粒の粗大化を抑制し、鋼板の強度の向上に寄与する元素である。Sbは必ずしも含有させなくてよいので、Sb含有量の下限は0%を含む。上記効果を十分に得るためには、Sb含有量は、0.001%以上が好ましく、0.005%以上がより好ましい。
また、Sb含有量が0.200%以下であると、鋼板が脆化して圧延時に破断することを抑制できる。そのため、Sb含有量は0.200%以下とする。合金コスト低減のため、必要に応じて、Sb含有量の上限を0.100%、0.070%、0.040%、0.010%又は0.005%としてもよい。
Mg:0~0.0100%
Zr:0~0.0100%
REM:0~0.0100%
Ca、Mg、ZrおよびREMは、鋼板の成形性の向上に寄与する元素である。Ca、Mg、ZrおよびREMは必ずしも含有させなくてよいので、これらの元素の含有量の合計の下限は0%を含む。成形性向上効果を十分に得るためには、これらの元素の含有量はそれぞれ、0.0001%以上が好ましく、0.0010%以上がより好ましい。上記効果を十分に得るためには、上記元素の全てを含有する必要はなく、いずれか1種でもその含有量が0.0001%以上であればよい。
また、Ca、Mg、ZrおよびREMのそれぞれの含有量が0.0100%以下であると、鋼板の延性が低下することを抑制できる。そのため、これらの元素の含有量はそれぞれ、0.0100%以下とする。好ましくは0.0050%以下である。合金コスト低減のため、必要に応じて、Ca、Mg、ZrおよびREMそれぞれの含有量の上限を、それぞれ0.0030%、0.0020%、0.0010%又は0.0003%としてもよい。
REM(Rare Earth Metal)は、Sc、Y及びランタノイドからなる合計17元素を指し、REMの含有量とはこれらの元素の合計含有量を指す。
鋼板が表面にめっき層を有する場合は、機械研削により表面のめっき層を除去してから、化学組成の分析を行えばよい。
一般的には、素材となる鋼板の算術平均うねりWaは小さい程、外観品質の観点において好ましいとされる。しかし、本発明者らは、プレス成形品においてゴーストラインの発生を抑制するためには、素材となる鋼板の表面を適度に粗くする、具体的には、算術平均うねりWaを0.10μm以上とすることで、プレス成形品におけるゴーストラインの発生を抑制できることを知見した。そのため、本実施形態に係る鋼板では、算術平均うねりWaを0.10μm以上とする。好ましくは0.13μm以上である。
また、算術平均うねりWaが過度に大きい場合は鋼板自体の外観品質が低下し、プレス成形後においても低い外観品質が維持される。そのため、算術平均うねりWaは0.30μm以下とする。好ましくは0.25μm以下である。
鋼板の端面から10mm以上離れた位置から50mm×50mmの試験片を切り出す。次に、レーザー変位測定装置(キーエンスVK-X1000)を用いて、圧延方向と直角の方向に沿ってプロファイルを3ライン測定する。得られた結果から、JIS B 0601:2013に準拠し、断面曲線にカットオフ値λcおよびλfの輪郭曲線フィルタを順次適用することによってうねり曲線を得る。具体的には、得られた測定結果から、波長λcが0.8mm以下の成分および波長λfが2.5mm以上の成分を除去して、うねり曲線を得る。得られたうねり曲線をもとに、JIS B 0601:2013に準拠し、算術平均うねりを算出し、合計3ラインの平均値を算出する。算出された3ラインの平均値の算術平均を、鋼板の算術平均うねりWaとする。
鋼板が表面にめっき層を有する場合は、めっき層の表面について上述のライン分析を行えばよい。
本実施形態に係る鋼板は、鋼板の表面から板厚方向に板厚の1/8離れた位置から、前記表面から板厚方向に板厚の3/8離れた位置までの領域(鋼板の表面から1/8深さ~鋼板の表面から3/8深さの領域)におけるMn濃度の平均値を単位質量%でμとし、前記Mn濃度の標準偏差を単位質量%でσとしたとき、(3σ/μ)×100≦7.0であることが好ましい。(3σ/μ)×100を7.0以下とすることで、鋼板中のMn偏析の発生をより低減でき、ゴーストラインの発生をより抑制することができ、より外観品質に優れるプレス成形品を得ることができる。(3σ/μ)×100は6.5以下とすることがより好ましい。(3σ/μ)×100の下限は特に限定しないが、0としてもよい。(3σ/μ)×100を低くするためには製造コストが高くなるため、その下限を2.0、4.0または5.0としてもよい。必要に応じて、(3σ/μ)×100の上限を11.0、10.0、9.0又は8.0としてもよい。
鋼板の板厚断面を鏡面研磨した後に、所定の深さ位置において、鋼板の圧延方向に、測定間隔1μmで600点におけるMn濃度を測定する。得られたMn濃度の平均値を算出することで、所定の深さ位置におけるMn濃度(質量%)を得る。この操作を、板厚方向に1μm毎に、鋼板の表面から板厚方向に板厚の1/8離れた位置から、前記表面から板厚方向に板厚の3/8離れた位置まで行う。得られたすべてのMn濃度の平均値(算術平均)を算出することで、Mn濃度の平均値μを得る。また、得られたすべてのMn濃度から標準偏差を算出することで、Mn濃度の標準偏差σを得る。
使用する装置は電子プローブマイクロアナライザ(EPMA)とし、測定条件は加速電圧を15kVとする。
亜鉛めっき層が溶融亜鉛めっき層の場合、鋼板表面と亜鉛めっき層との密着性を高めるため、溶融亜鉛めっき層のFe含有量は3.0質量%以下が好ましい。
亜鉛めっき層が電気亜鉛めっき層の場合、電気亜鉛めっき層のFe含有量は、耐食性の向上の点で、0.5質量%以下が好ましい。
鋼板の任意の3か所について、鋼板の表面から深さ方向(板厚方向)に板厚の1/2離れた位置までの領域におけるC濃度を1μm深さ毎に測定する。表面から板厚の1/2離れた位置におけるC濃度の1/2以下のC濃度である領域を脱炭層とみなし、その厚さを求めることで、脱炭層の厚さを得る。
測定には(株)堀場製作所製のマーカス型高周波グロー放電発光表面分析装置(GD-Profiler)を用いる。
一方、板厚が2.0mm以下であると、製造過程で、適正なひずみ付与および温度制御を行うことが容易になり、均質な組織を得ることができる。そのため、板厚は2.0mm以下が好ましい。より好ましくは1.5mm以下である。
また、3σ/μが好ましく制御された鋼板を用いてプレス成形品を製造することで、外観品質により優れたプレス成形品を得ることができる。つまり、うねり曲線の最大山高さZpと最大谷高さZvとの和であるWzが0.40μm以下であるプレス成形品を得ることができる。
Wzは、JIS B 0601:2013に準拠して、プレス成形品の表面のうねり曲線を得て、最大山高さZpと最大谷高さZvと求め、これらの和を算出することで得る。
本実施形態に係る鋼板は、製造方法に依らず、上記の特徴を有していればその効果が得られる。また、鋼板ではなく、鋼帯であってもよい。しかし、上述した化学組成を有する鋼を用いて、例えば、下記条件(I)~(IV)を複合的且つ不可分に制御することで、算術平均うねりWaが好ましく制御された鋼板を安定して製造することができる。また、3σ/μを好ましく制御するためには、下記条件(I)~(IV)に加えて更に、条件(V)を制御することが好ましい。また、脱炭層の厚さを好ましく制御するためには、下記条件(I)~(IV)に加えて更に、条件(VI)を制御することが好ましい。なお、条件(V)および(VI)は任意条件である。
以下、各条件について説明する。
(II)酸洗時間を50秒以上とする。
(III)冷間圧延の最終パスの圧延ロール表面の算術平均粗さRaを0.2~0.7μmとする。
(IV)調質圧延の圧下率を0.3~0.7%とし、圧延ロールの算術平均粗さRaを1.5~3.5μmとする。
(V)スラブを1200℃以上の温度域に加熱し、当該温度域で5時間以上保持する。
(VI)露点(焼鈍炉内の平均的な露点)を-20℃以上とし、700℃以上の温度域における鋼板の滞在時間を50~400秒とする焼鈍を行う。
熱間圧延後の巻取り温度を550℃以上の高温域とすることで、鋼板の表面にスケールが生じやすくなる。その結果、酸洗後の鋼板の表面に凹凸が生じやすくなる。巻取り温度は、より好ましくは600℃以上であり、より一層好ましくは650℃以上である。
巻取り後、且つ冷間圧延前の酸洗において、酸洗時間を50秒以上とすることで、鋼板の表面に凹凸が生じやすくなる。酸洗時間は70秒以上とすることがより好ましい。
酸洗後、冷間圧延における最終パスの圧延ロール表面の算術平均粗さRaを0.2~0.7μmとすることで、冷間圧延時に鋼板の表面に適度な凹凸を形成することができる。圧延ロールの算術平均粗さRaは0.3μm以上とすることがより好ましい。
焼鈍後(めっき材であれば、めっき後)の調質圧延において、圧下率を0.3~0.7%とし、圧延ロール表面の算術平均粗さRaを1.5~3.5μmとすることで、鋼板の表面に凹凸を形成することができる。調質圧延時の圧下率は0.5%以上とすることがより好ましく、圧延ロール表面の算術平均粗さRaは2.3μm以上とすることがより好ましい。
条件(V)は任意条件である。スラブを1200℃以上の温度域で5時間以上加熱することによって、鋼板の表面から板厚方向に板厚の1/8離れた位置から、前記表面から板厚方向に板厚の3/8離れた位置までの領域(鋼板の表面から1/8深さ~鋼板の表面から3/8深さの領域)における3σ/μを好ましく制御することができる。その結果、鋼板中のMn偏析の発生をより低減でき、より外観品質に優れるプレス成形品を得ることができる。
条件(VI)は任意条件である。本実施形態では、上述の方法により得た冷間圧延後の鋼板に対し、焼鈍を施してもよい。焼鈍時の露点(焼鈍炉内の平均的な露点)は-20℃以上とし、700℃以上の温度域における鋼板の滞在時間を50~400秒とすることで、安定して鋼板の表面を脱炭することができる。これにより、鋼板の表面に厚さが30μm以上である脱炭層を形成することができる。なお、露点の上限は特に定める必要はないが、10℃程度としてもよい。
鋼片を1100℃以上の温度域にスラブを加熱した後、熱間圧延する。熱間圧延後は巻取りを行い、次いで酸洗を行う。酸洗後は冷間圧延を行う。冷間圧延における累積圧下率は30~90%とすることが好ましい。冷間圧延後は焼鈍を行う。その後、必要に応じて、上述しためっき層を形成する。また、その後に調質圧延を施すことが好ましい。
(I)巻取り温度を550℃以上とする。
(II)酸洗時間を50秒以上とする。
(III)冷間圧延の最終パスの圧延ロール表面の算術平均粗さRaを0.2~0.7μmとする。
(IV)調質圧延の圧下率を0.3~0.7%とし、圧延ロールの算術平均粗さRaを1.5~3.5μmとする。
(V)スラブを1200℃以上の温度域に加熱し、当該温度域で5時間以上保持する。
(VI)露点(焼鈍炉内の平均的な露点)を-20℃以上とし、700℃以上の温度域における鋼板の滞在時間を50~400秒とする焼鈍を行う。
外観品質は、成形後の模擬部品の表面に発生するゴーストラインの程度により評価した。プレス成形後の表面を砥石掛けし、表面に生じた数mmオーダー間隔の縞模様を、ゴーストラインと判断し、筋模様の発生程度によって、1~5で評点付けした。100mm×100mmの任意の領域を目視で確認し、筋模様が全く確認されなかった場合を「1」とし、筋模様の最大長さが20mm以下の場合を「2」とし、筋模様の最大長さが20mm超、50mm以下の場合を「3」とし、筋模様の最大長さが50mm超、70mm以下の場合を「4」とし、筋模様の最大長さが70mmを超える場合を「5」とした。評価が「3」以下であった場合、外観品質に優れるとして合格と判定した。一方、評価が「4」以上であった場合、外観品質に劣るとして不合格と判定した。
Claims (6)
- 化学組成が、質量%で、
C :0.040~0.100%、
Mn:1.00~2.00%、
Si:0.005~1.500%、
P :0.100%以下、
S :0.0200%以下、
Al:0.005~0.700%、
N :0.0150%以下、
O :0.0100%以下、
Cr:0~0.80%、
Mo:0~0.16%、
B :0~0.0100%、
Ti:0~0.100%、
Nb:0~0.060%、
V :0~0.50%、
Ni:0~1.00%、
Cu:0~1.00%、
W :0~1.00%、
Sn:0~1.00%、
Sb:0~0.200%、
Ca:0~0.0100%、
Mg:0~0.0100%、
Zr:0~0.0100%、
REM:0~0.0100%、および
残部:Feおよび不純物であり、
算術平均うねりWaが0.10~0.30μmである、ことを特徴とする鋼板。 - 前記化学組成が、質量%で、
Cr:0.01~0.80%、
Mo:0.01~0.16%、
B :0.0001~0.0100%、
Ti:0.001~0.100%、
Nb:0.001~0.060%、
V :0.01~0.50%、
Ni:0.01~1.00%、
Cu:0.01~1.00%、
W :0.01~1.00%、
Sn:0.01~1.00%、
Sb:0.001~0.200%、
Ca:0.0001~0.0100%、
Mg:0.0001~0.0100%、
Zr:0.0001~0.0100%、および
REM:0.0001~0.0100%
からなる群から選択される1種または2種以上を含有する、ことを特徴とする請求項1に記載の鋼板。 - 前記鋼板の表面から板厚方向に板厚の1/8離れた位置から、前記表面から前記板厚方向に前記板厚の3/8離れた位置までの領域におけるMn濃度の平均値をμとし、前記Mn濃度の標準偏差をσとしたとき、(3σ/μ)×100≦7.0である、ことを特徴とする請求項1または2に記載の鋼板。
- 前記鋼板の表面に厚さが20μm以上の脱炭層を有する、ことを特徴とする請求項1~3の何れか一項に記載の鋼板。
- 前記鋼板の少なくとも一方の表面にめっき層を有する、ことを特徴とする請求項1~4の何れか一項に記載の鋼板。
- 請求項1~5の何れか一項に記載の鋼板をプレス成形して得られる、ことを特徴とするプレス成形品。
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