WO2016121388A1 - 高強度冷延鋼板、高強度めっき鋼板及びこれらの製造方法 - Google Patents
高強度冷延鋼板、高強度めっき鋼板及びこれらの製造方法 Download PDFInfo
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- WO2016121388A1 WO2016121388A1 PCT/JP2016/000402 JP2016000402W WO2016121388A1 WO 2016121388 A1 WO2016121388 A1 WO 2016121388A1 JP 2016000402 W JP2016000402 W JP 2016000402W WO 2016121388 A1 WO2016121388 A1 WO 2016121388A1
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- Prior art keywords
- steel sheet
- less
- strength
- rolled steel
- cold
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- 229910000831 Steel Inorganic materials 0.000 title claims abstract description 113
- 239000010959 steel Substances 0.000 title claims abstract description 113
- 239000010960 cold rolled steel Substances 0.000 title claims abstract description 42
- 238000000034 method Methods 0.000 title claims abstract description 35
- 238000004519 manufacturing process Methods 0.000 title claims abstract description 22
- 229910000734 martensite Inorganic materials 0.000 claims abstract description 43
- 229910000859 α-Fe Inorganic materials 0.000 claims abstract description 29
- 229910001563 bainite Inorganic materials 0.000 claims abstract description 21
- 239000000203 mixture Substances 0.000 claims abstract description 18
- 238000009864 tensile test Methods 0.000 claims abstract description 11
- 238000007747 plating Methods 0.000 claims description 33
- 238000001816 cooling Methods 0.000 claims description 28
- 238000005096 rolling process Methods 0.000 claims description 27
- 229910052799 carbon Inorganic materials 0.000 claims description 23
- 238000010438 heat treatment Methods 0.000 claims description 19
- 238000005097 cold rolling Methods 0.000 claims description 17
- 239000000463 material Substances 0.000 claims description 17
- 238000000137 annealing Methods 0.000 claims description 15
- 238000005098 hot rolling Methods 0.000 claims description 13
- 239000012535 impurity Substances 0.000 claims description 10
- 229910052760 oxygen Inorganic materials 0.000 claims description 10
- 229910052804 chromium Inorganic materials 0.000 claims description 9
- 229910052748 manganese Inorganic materials 0.000 claims description 9
- 229910052710 silicon Inorganic materials 0.000 claims description 9
- 229910052782 aluminium Inorganic materials 0.000 claims description 8
- 229910052758 niobium Inorganic materials 0.000 claims description 8
- 229910052719 titanium Inorganic materials 0.000 claims description 8
- 238000004804 winding Methods 0.000 claims description 8
- 238000005275 alloying Methods 0.000 claims description 7
- 229910052759 nickel Inorganic materials 0.000 claims description 7
- 229910052720 vanadium Inorganic materials 0.000 claims description 7
- 229910002091 carbon monoxide Inorganic materials 0.000 claims description 6
- 238000002485 combustion reaction Methods 0.000 claims description 6
- 229910052757 nitrogen Inorganic materials 0.000 claims description 5
- 239000002245 particle Substances 0.000 claims description 5
- 229910052698 phosphorus Inorganic materials 0.000 claims description 4
- 229910052718 tin Inorganic materials 0.000 claims description 4
- 229910052787 antimony Inorganic materials 0.000 claims description 3
- 229910052797 bismuth Inorganic materials 0.000 claims description 3
- 229910052802 copper Inorganic materials 0.000 claims description 3
- 229910052745 lead Inorganic materials 0.000 claims description 3
- 229910052749 magnesium Inorganic materials 0.000 claims description 3
- 230000036961 partial effect Effects 0.000 claims description 3
- 239000002994 raw material Substances 0.000 claims 2
- 239000002344 surface layer Substances 0.000 abstract description 38
- 238000005452 bending Methods 0.000 description 16
- 230000000694 effects Effects 0.000 description 16
- 239000010410 layer Substances 0.000 description 16
- 230000002411 adverse Effects 0.000 description 9
- XEEYBQQBJWHFJM-UHFFFAOYSA-N iron Substances [Fe] XEEYBQQBJWHFJM-UHFFFAOYSA-N 0.000 description 9
- 238000012360 testing method Methods 0.000 description 7
- 230000009466 transformation Effects 0.000 description 7
- 230000015572 biosynthetic process Effects 0.000 description 6
- 238000007598 dipping method Methods 0.000 description 6
- 230000002829 reductive effect Effects 0.000 description 6
- 229910001335 Galvanized steel Inorganic materials 0.000 description 5
- 238000005336 cracking Methods 0.000 description 5
- 230000007423 decrease Effects 0.000 description 5
- 239000008397 galvanized steel Substances 0.000 description 5
- 238000005259 measurement Methods 0.000 description 5
- 239000002184 metal Substances 0.000 description 5
- 229910052751 metal Inorganic materials 0.000 description 5
- 239000002244 precipitate Substances 0.000 description 5
- 238000005728 strengthening Methods 0.000 description 5
- 229910001566 austenite Inorganic materials 0.000 description 4
- 230000003647 oxidation Effects 0.000 description 4
- 238000007254 oxidation reaction Methods 0.000 description 4
- XLYOFNOQVPJJNP-UHFFFAOYSA-N water Chemical compound O XLYOFNOQVPJJNP-UHFFFAOYSA-N 0.000 description 4
- 238000006243 chemical reaction Methods 0.000 description 3
- 230000007797 corrosion Effects 0.000 description 3
- 238000005260 corrosion Methods 0.000 description 3
- 239000000446 fuel Substances 0.000 description 3
- 238000000227 grinding Methods 0.000 description 3
- 238000000465 moulding Methods 0.000 description 3
- 239000006104 solid solution Substances 0.000 description 3
- 229910052717 sulfur Inorganic materials 0.000 description 3
- 239000011800 void material Substances 0.000 description 3
- 238000005266 casting Methods 0.000 description 2
- 229910001567 cementite Inorganic materials 0.000 description 2
- 238000001739 density measurement Methods 0.000 description 2
- 238000011156 evaluation Methods 0.000 description 2
- 239000007789 gas Substances 0.000 description 2
- 238000010191 image analysis Methods 0.000 description 2
- 230000001771 impaired effect Effects 0.000 description 2
- 229910052742 iron Inorganic materials 0.000 description 2
- KSOKAHYVTMZFBJ-UHFFFAOYSA-N iron;methane Chemical compound C.[Fe].[Fe].[Fe] KSOKAHYVTMZFBJ-UHFFFAOYSA-N 0.000 description 2
- 238000002844 melting Methods 0.000 description 2
- 230000008018 melting Effects 0.000 description 2
- 150000001247 metal acetylides Chemical class 0.000 description 2
- 229910052750 molybdenum Inorganic materials 0.000 description 2
- 150000004767 nitrides Chemical class 0.000 description 2
- 229910001568 polygonal ferrite Inorganic materials 0.000 description 2
- 230000001737 promoting effect Effects 0.000 description 2
- 230000005855 radiation Effects 0.000 description 2
- 230000000717 retained effect Effects 0.000 description 2
- 230000000007 visual effect Effects 0.000 description 2
- OKTJSMMVPCPJKN-UHFFFAOYSA-N Carbon Chemical compound [C] OKTJSMMVPCPJKN-UHFFFAOYSA-N 0.000 description 1
- ATJFFYVFTNAWJD-UHFFFAOYSA-N Tin Chemical compound [Sn] ATJFFYVFTNAWJD-UHFFFAOYSA-N 0.000 description 1
- 238000002441 X-ray diffraction Methods 0.000 description 1
- PNEYBMLMFCGWSK-UHFFFAOYSA-N aluminium oxide Inorganic materials [O-2].[O-2].[O-2].[Al+3].[Al+3] PNEYBMLMFCGWSK-UHFFFAOYSA-N 0.000 description 1
- QVGXLLKOCUKJST-UHFFFAOYSA-N atomic oxygen Chemical compound [O] QVGXLLKOCUKJST-UHFFFAOYSA-N 0.000 description 1
- 239000011248 coating agent Substances 0.000 description 1
- 238000000576 coating method Methods 0.000 description 1
- 230000000052 comparative effect Effects 0.000 description 1
- 238000009749 continuous casting Methods 0.000 description 1
- 238000002425 crystallisation Methods 0.000 description 1
- 230000008025 crystallization Effects 0.000 description 1
- 230000003247 decreasing effect Effects 0.000 description 1
- 230000001934 delay Effects 0.000 description 1
- 230000006866 deterioration Effects 0.000 description 1
- 238000009792 diffusion process Methods 0.000 description 1
- 238000009826 distribution Methods 0.000 description 1
- 230000007613 environmental effect Effects 0.000 description 1
- 238000005246 galvanizing Methods 0.000 description 1
- 229910052738 indium Inorganic materials 0.000 description 1
- 238000012423 maintenance Methods 0.000 description 1
- 230000006911 nucleation Effects 0.000 description 1
- 238000010899 nucleation Methods 0.000 description 1
- 230000001590 oxidative effect Effects 0.000 description 1
- 239000001301 oxygen Substances 0.000 description 1
- 229910001562 pearlite Inorganic materials 0.000 description 1
- 238000005498 polishing Methods 0.000 description 1
- 238000003825 pressing Methods 0.000 description 1
- 238000004080 punching Methods 0.000 description 1
- 238000007670 refining Methods 0.000 description 1
- 229920006395 saturated elastomer Polymers 0.000 description 1
- 238000010008 shearing Methods 0.000 description 1
- 238000004611 spectroscopical analysis Methods 0.000 description 1
- 238000009628 steelmaking Methods 0.000 description 1
- 239000000126 substance Substances 0.000 description 1
- 238000005496 tempering Methods 0.000 description 1
- 238000009849 vacuum degassing Methods 0.000 description 1
Classifications
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- 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
- 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
- C21D8/0221—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips characterised by the working steps
- C21D8/0226—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
- C21D8/0221—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips characterised by the working steps
- C21D8/0236—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
- C21D8/0247—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips characterised by the heat treatment
-
- 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
- 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
- C21D9/52—Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor for wires; for strips ; for rods of unlimited length
- C21D9/54—Furnaces for treating strips or wire
- C21D9/56—Continuous furnaces for strip or wire
- C21D9/561—Continuous furnaces for strip or wire with a controlled atmosphere or vacuum
-
- C—CHEMISTRY; METALLURGY
- C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
- C22C—ALLOYS
- C22C18/00—Alloys based on zinc
-
- 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
- C22C38/02—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
- C22C38/06—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
- C22C38/38—Ferrous alloys, e.g. steel alloys containing chromium 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
- C22C38/18—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
- 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
- C23C2/02—Pretreatment of the material to be coated, e.g. for coating on selected surface areas
-
- 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
- C23C2/02—Pretreatment of the material to be coated, e.g. for coating on selected surface areas
- C23C2/022—Pretreatment of the material to be coated, e.g. for coating on selected surface areas by heating
-
- 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
- C23C2/02—Pretreatment of the material to be coated, e.g. for coating on selected surface areas
- C23C2/022—Pretreatment of the material to be coated, e.g. for coating on selected surface areas by heating
- C23C2/0222—Pretreatment of the material to be coated, e.g. for coating on selected surface areas by heating in a reactive atmosphere, e.g. oxidising or reducing atmosphere
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- 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
- C23C2/02—Pretreatment of the material to be coated, e.g. for coating on selected surface areas
- C23C2/022—Pretreatment of the material to be coated, e.g. for coating on selected surface areas by heating
- C23C2/0224—Two or more thermal pretreatments
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- 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
- C21D2211/00—Microstructure comprising significant phases
- C21D2211/002—Bainite
-
- 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
- C21D2211/00—Microstructure comprising significant phases
- C21D2211/005—Ferrite
-
- 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
- C21D2211/00—Microstructure comprising significant phases
- C21D2211/008—Martensite
Definitions
- the present invention relates to a high-strength cold-rolled steel sheet having high tensile strength (TS): 780 MPa or more and excellent formability, a high-strength plated steel sheet, and a method for producing the same, which are useful as a material for automobile frame members. .
- TS tensile strength
- Patent Document 1 in a hot-dip galvanized steel sheet provided with a hot-dip galvanized layer on the surface of the steel sheet, by mass%, C: more than 0.02% and 0.20% or less, Si: 0.01 to 2.0 %, Mn: 0.1 to 3.0%, P: 0.003 to 0.10%, S: 0.020% or less, Al: 0.001 to 1.0%, N: 0.0004 to 0 0.15%, Ti: 0.03 to 0.2%, with the balance being a component composition of Fe and inevitable impurities, and containing ferrite in an area ratio of 30 to 95%, with the balance being martensite, It has one or more of bainite, pearlite, cementite and retained austenite and has a steel structure in which the martensite area ratio is 0 to 50% when containing martensite, and the steel plate has a grain size 2-30nm Ti Carbonitrides precipitate comprises a distance 30 ⁇ 300 nm average interparticle and including a particle size 3 ⁇ m or more crystal
- Patent Document 2 by mass%, C: 0.05 to 0.20%, Si: 0.01 to less than 0.6%, Mn: 1.6 to 3.5%, P: 0.05% or less , S: 0.01% or less, sol.
- a steel sheet containing Al: 1.5% or less, N: 0.01% or less, the balance being iron and inevitable impurities, having a polygonal ferrite structure and a low temperature transformation structure, and a low temperature transformation structure Includes at least bainite and may further include martensite.
- Patent Document 2 about the plate
- Patent Document 2 states that a hot-dip galvanized steel sheet having a tensile strength of 780 MPa or more excellent in bending workability and fatigue strength can be obtained.
- the tensile strength of the surface layer portion which is a region from the steel plate surface to 50 ⁇ m in the plate thickness direction, is important.
- An object of the present invention is to provide a high-strength cold-rolled steel sheet, a high-strength plated steel sheet, and a production method thereof having a tensile strength measured using a piece: 780 MPa or more and having good formability.
- the present inventors paid attention to the stress gradient with respect to the plate thickness direction at the time of bending when examining the structural requirements of a steel plate having a tensile strength of the surface layer portion in a desired range and good formability.
- the stress in the vicinity of the central portion in the plate thickness direction is reduced as much as possible, but the stress in the surface layer portion, which is a region of 50 ⁇ m in the plate thickness direction from the steel plate surface, is remarkably increased.
- the stress changes continuously in the thickness direction.
- cracks (cracking) in the bent portion it was found that the void density in the surface layer portion was much higher than the void density at a position deeper than 50 ⁇ m in the plate thickness direction.
- the stress continuously decreases from the steel plate surface to the center of the plate thickness. For this reason, unless the hardness of the surface layer portion, which is a region from the steel plate surface to the thickness direction to 50 ⁇ m in the thickness direction, is continuously changed, a stress concentration portion is generated, which causes cracks. It was.
- Generating the bainite phase, the martensite phase, and the tempered martensite phase in appropriate ranges contributes to the tensile strength of the surface layer portion being in a desired range.
- the hardness of the bainite phase, martensite phase, and tempered martensite phase is determined by the contents of C, Si, Mn, and the like as described later, but the influence of the C amount is the largest.
- Continuously changing the C concentration can be achieved by controlling the furnace atmosphere, dew point and heating temperature in the continuous annealing line or the continuous plating line.
- the present invention has been completed based on the above findings, and the gist thereof is as follows.
- d [% C x ] / dx ([% C x ] ⁇ [% C x ⁇ 10 ⁇ m ]) / 0.01
- [% C x ] represents the C concentration in x
- x represents the distance in the sheet thickness direction from the steel sheet surface, and its value is 50 ⁇ m or less.
- the component composition is further in mass%, Mo: 0.01% to 0.5%, Cr: 0.01% to 0.9%, Ni: 0.01% to 0.2%
- Mo 0.01% to 0.5%
- Cr 0.01% to 0.9%
- Ni 0.01% to 0.2%
- the high-strength cold-rolled steel sheet according to [1] containing one or more of the following.
- the component composition is further in mass%, Ti: 0.01% to 0.15%, Nb: 0.01% to 0.1%, V: 0.01% to 0.5%
- the inclusion density with a particle diameter of 0.2 ⁇ m or more in the region from the steel sheet surface to 50 ⁇ m in the sheet thickness direction is 500 / mm 2 or less.
- a high-strength plated steel sheet comprising the high-strength cold-rolled steel sheet according to any one of [1] to [4] and a plating layer formed on the high-strength cold-rolled steel sheet.
- the plating layer contains, by mass%, Fe: 5.0 to 20.0%, Al: 0.001% to 1.0%, and Pb, Sb, Si, Sn, Mg, Contains one or more selected from Mn, Ni, Cr, Co, Ca, Cu, Li, Ti, Be, Bi, and REM in a total of 0 to 3.5%, with the balance being Zn and inevitable impurities
- the high strength plated steel sheet according to [5] or [6].
- the steel material having the component composition according to any one of [1] to [4] is heated to 1150 ° C. or higher, finish-rolled at a finish temperature of rough rolling and finish rolling of 800 ° C. or higher, and 350 ° C.
- the hot rolling step of winding at a winding temperature of 720 ° C. or lower, the cold rolling step of performing cold rolling on the hot-rolled steel sheet after the hot rolling step, and the continuous annealing after the cold rolling step In a line or continuous plating line, using a combustion burner, a temperature range of 580 ° C. to T ° C. with an air ratio of 1.05 to 1.30 (where 580 ⁇ T ⁇ 730), a temperature of 730 ° C.
- the cold-rolled steel sheet is heated to a maximum temperature of 730 ° C. or higher with a dew point of ⁇ 40 to ⁇ 15 ° C., and then the average cooling rate from 700 ° C. to 550 ° C. is ⁇ 10 ° C./s or lower. Cool to a cooling stop temperature of 25-530 ° C, There was optionally heated by, the method of producing a high strength cold rolled steel sheet having a annealing step of holding at 530 temperature range ° C. from 200 ° C..
- a high-strength cold-rolled steel sheet and a high-strength-plated steel sheet having a desired tensile strength of the surface layer portion and good formability can be obtained.
- INDUSTRIAL APPLICABILITY The present invention is suitable for the use of automobile structural members and the like, and its effects are remarkable, such as reducing the weight of automobile parts and improving their reliability.
- the high-strength cold-rolled steel sheet of the present invention is, in mass%, C: 0.06% to 0.20%, Si: 0.01% to 2.0%, Mn: 1.8% to 5.0% %: P: 0.06% or less, S: 0.005% or less, Al: 0.08% or less, N: 0.008% or less.
- component composition of the high-strength cold-rolled steel sheet according to the present invention is, in addition to the above-mentioned components, further by mass, Mo: 0.01% or more and 0.5% or less, Cr: 0.01% or more and 0.9% or less. Ni: One or more of 0.01% or more and 0.2% or less may be contained.
- the component composition of the high-strength cold-rolled steel sheet of the present invention is, in addition to the above-mentioned components, further by mass, Ti: 0.01% or more and 0.15% or less, Nb: 0.01% or more and 0.1% or less. , V: You may contain 1 type or 2 types or more of 0.01% or more and 0.5% or less.
- the high-strength cold-rolled steel sheet of the present invention may further contain B: 0.0002% or more and 0.0030% or less in mass%.
- C 0.06% or more and 0.20% or less
- C has an effect of increasing the hardness of the bainite phase, the martensite phase, and the tempered martensite phase that bear the strength.
- the C content needs to be 0.06% or more.
- C has a hardenability that suppresses the formation of a ferrite phase, and when the C content exceeds 0.20%, the area ratio of the ferrite phase in the region from 1/4 to 3/4 of the plate thickness. Is less than 20%, and ductility and bendability are lost, making practical use difficult. Therefore, the C content is set to 0.06% or more and 0.20% or less. Desirable C content is 0.07% or more and 0.18% or less.
- Si 0.01% or more and 2.0% or less Si is an element contributing to high strength by solid solution strengthening.
- Si delays the diffusion of C and makes it difficult to provide a C concentration gradient. For this reason, there is an optimum value for the Si content. If the Si content exceeds 2.0%, a desired C concentration gradient cannot be obtained in the surface layer portion. Therefore, the upper limit of the Si content is set to 2.0%.
- the lower limit of the Si content is set to 0.01%.
- a preferable Si content is 0.02% or more and 1.6% or less.
- Mn 1.8% or more and 5.0% or less Mn is an element that contributes to increase in strength by solid solution strengthening and suppresses the formation of a ferrite phase.
- the Mn content needs to be 1.8% or more.
- the upper limit of Mn content was 5.0%.
- a preferable range of Mn content is 1.9% to 3.5%.
- P 0.06% or less
- P is an element that segregates at the grain boundary and becomes a starting point of cracking during bending molding, and thus adversely affects moldability. Therefore, it is preferable to reduce the P content as much as possible.
- the P content is set to 0.06% or less.
- a preferable P content is 0.03% or less. Although it is desirable to reduce the P content as much as possible, 0.002% is often inevitably mixed in production.
- S 0.005% or less S is present as an inclusion such as MnS in steel. This inclusion becomes wedge-shaped by hot rolling and cold rolling. In such a form, inclusions tend to be the starting point of void generation, and formability is reduced. Therefore, in the present invention, it is preferable to reduce the S content as much as possible, and set it to 0.005% or less. A preferable S content is 0.003% or less. Although it is desirable to reduce the S content as much as possible, 0.0005% is often inevitably mixed in production.
- Al 0.08% or less
- Al is contained in an amount of 0.02% or more.
- the Al content exceeds 0.08%, the formability deteriorates due to the influence of coarse inclusions such as alumina. Therefore, the Al content is 0.08% or less.
- a preferable Al content is 0.07% or less.
- N 0.008% or less
- N combines with Ti and precipitates as coarse Ti-based nitride. Since this coarse Ti-based nitride has an adverse effect on formability, the N content needs to be reduced as much as possible, and the upper limit is made 0.008%.
- a preferable N content is 0.006% or less. Although it is desirable to reduce the N content as much as possible, 0.0005% is often inevitably mixed in production.
- Mo 0.01% or more and 0.5% or less
- Cr 0.01% or more and 0.9% or less
- Ni 0.01% or more and 0.2% or less
- Mo, Cr and Ni Is an element having an effect of promoting the formation of a bainite phase, a martensite phase, and a tempered martensite phase in addition to solid solution strengthening. Therefore, these elements substantially contribute to high strength. On the other hand, when these elements are contained excessively, moldability deteriorates. From the above, Mo: 0.01% to 0.5%, Cr: 0.01% to 0.9%, Ni: 0.01% to 0.2%.
- Ti 0.01% or more and 0.15% or less
- Nb 0.01% or more and 0.1% or less
- V 0.01% or more and 0.5% or less of Ti
- Nb and V Is an element that combines with carbon to form precipitates.
- This strengthening by precipitates strengthens the tensile strength of the steel sheet over the entire thickness direction.
- softening near the surface layer is more difficult than bainite phase, martensite phase and tempered martensite phase. That is, when the amount of Ti, Nb, and V exceeds the upper limit defined in the present invention, the degree of strengthening by these precipitates becomes excessively large.
- the good bending workability (formability) characteristic of the present invention cannot be obtained. Therefore, Ti: 0.01% to 0.15%, Nb: 0.01% to 0.1%, and V: 0.01% to 0.5%. At this time, the formability may deteriorate due to the influence of the formation of coarse carbonitride containing Ti, Nb and V.
- the inclusion density with a particle diameter of 0.2 ⁇ m or more in the surface layer portion that is a region from the steel sheet surface to the thickness direction of 50 ⁇ m is 500 / mm 2.
- a preferable upper limit of inclusion density is 350 pieces / mm 2 or less.
- 50 / mm 2 or more is preferable from the viewpoint of promoting the generation of cracks on the shearing surface during the punching process.
- B 0.0002% or more and 0.0030% or less may be contained.
- B has the effect of segregating at the grain boundaries of austenite before transformation and significantly delaying the nucleation of the ferrite phase and suppressing the formation of the ferrite phase. In order to acquire this effect, it is necessary to contain B 0.0002% or more. On the other hand, if it exceeds 0.0030%, not only the effect of hardenability is saturated, but also an adverse effect on ductility. From the above, the B content is set to 0.0002% or more and 0.0030% or less. A desirable B content is 0.0005% or more and 0.0020% or less.
- Components other than the above are Fe and inevitable impurities.
- these elements shall be included as an unavoidable impurity.
- the steel structure of the high-strength cold-rolled steel sheet of the present invention has a ferrite phase area ratio of 20% or more and 80% or less in the range of 1/4 to 3/4, bainite phase, martensite phase, and tempered martensite.
- the total area ratio of the phases is 20% or more and 80% or less, and the differential amount of the C concentration represented by the above formula (1) in x ⁇ m (where x is 50 ⁇ m or less) from the steel plate surface to the plate thickness direction is 0. 10% by mass / mm.
- the area ratio of the ferrite phase in the region of the thickness 1/4 to 3/4 is 20% or more and 80% or less, and the total area ratio of the bainite phase, martensite phase, and tempered martensite phase is 20%. % To 80%.
- this is the first time that the differential amount of the C concentration represented by the above formula (1) in x ⁇ m (x is 50 ⁇ m or less) in the thickness direction from the steel sheet surface is 0.10% by mass / mm. The bending workability intended by the invention is achieved.
- the tensile strength obtained by the tensile test using JIS No. 5 tensile test piece is the structure in the range from 1/4 to 3/4 with respect to the plate thickness direction. Determined by.
- the ferrite phase is a soft structure, and when the area ratio of the ferrite phase exceeds 80%, the tensile strength is less than 780 MPa. Further, if the area ratio of the ferrite phase exceeds 80% and the tensile strength falls below 780 MPa, the content of the ferrite phase increases in the surface layer portion, so the tensile strength of the surface layer portion is adjusted to a desired range. Not.
- the ferrite phase is a structure that improves the moldability, if the area ratio is less than 20%, the moldability is remarkably lowered and the ductility is also impaired. From this viewpoint, the area ratio of the ferrite phase is set to 20% to 80%. The area ratio of the ferrite phase is preferably 30% or more and 70% or less.
- the metal structure containing these phases is suitable for increasing the strength. In order to bring the tensile strength of the surface layer portion into a desired range, the area ratio of these metal structures needs to be 20% or more in total. On the other hand, these phases are poor in ductility, and inclusion of these phases generally lowers moldability. In the present invention, focusing on the fact that the hardness of these metal structures largely depends on the C content, the present invention is characterized in that formability is improved by continuously decreasing the C concentration of the surface layer portion.
- the total area ratio of the bainite phase, martensite phase and tempered martensite phase exceeds 80%, the desired formability cannot be obtained even if the C concentration in the surface layer is changed.
- a desirable range of the total area ratio of the bainite phase, martensite phase and tempered martensite phase is 30% or more and 70% or less.
- the strength of the high-strength cold-rolled steel sheet according to the present invention largely depends on the C concentration. Therefore, the C concentration, which is the C concentration near the surface, is related to the tensile strength of the surface layer portion.
- the combination of adjusting the C concentration and the tensile strength measured using a JIS No. 5 test piece being 780 MPa or more can indicate that the tensile strength of the surface layer portion is in a desired range. Further, when the C concentration is locally reduced, the workability of that portion is good. Furthermore, since the stress continuously changes in the thickness direction in bending, for example, even if only the hardness of the steel sheet surface is reduced, it is caused by cracks due to insufficient ductility at high hardness or hardness differences.
- the differential amount of the C concentration represented by the following formula (1) in x ⁇ m (where x is 50 ⁇ m or less) from the steel sheet surface to the plate thickness direction is 0.10 mass% / mm or more. do it.
- the differential amount of the C concentration is preferably 6.5% by mass / mm or less.
- the differential amount when x ⁇ 20 ⁇ m is the region where the strain due to bending is the largest, the differential amount is preferably large.
- the region where the strain is the largest when x is 0 ⁇ m or more and less than 20 ⁇ m is large, but the measurement of C concentration in the region of 0 ⁇ m or more and less than 10 ⁇ m has a large error due to the influence of dirt, etc. It is preferable to define the differential amount of the C concentration for the above region.
- the high-strength plated steel sheet of the present invention is composed of the high-strength cold-rolled steel sheet and a plating layer formed thereon.
- the components constituting the plating layer are not particularly limited and may be general components.
- the plating layer contains Fe: 5.0 to 20.0% and Al: 0.001% to 1.0% by mass%, and Pb, Sb, Si, Sn, Mg, Mn, One or two or more selected from Ni, Cr, Co, Ca, Cu, Li, Ti, Be, Bi, and REM are contained in a total of 0 to 3.5%, and the balance is made of Zn and inevitable impurities.
- the plating layer may be a hot-dip plating layer or an alloyed plating layer.
- the manufacturing method of the high-strength cold-rolled steel sheet of the present invention includes a hot rolling process, a cold rolling process, and an annealing process.
- the temperature is the surface temperature unless otherwise specified.
- the average cooling rate is ((surface temperature after cooling ⁇ surface temperature before cooling) / cooling time).
- the hot rolling step refers to heating a steel material having the above composition to 1150 ° C. or higher, performing finish rolling with rough rolling and finish rolling finishing temperature of 800 ° C. or higher, and winding temperature of 350 ° C. or higher and 720 ° C. or lower. It is a process of winding in.
- the melting method for producing the steel material is not particularly limited, and a known melting method such as a converter or an electric furnace can be employed. Further, secondary refining may be performed in a vacuum degassing furnace. Then, it is preferable to use a slab (steel material) by a continuous casting method from the viewpoint of productivity and quality. Also, the slab may be formed by a known casting method such as ingot-bundling rolling or continuous slab casting.
- the steel material obtained as described above is heated under the following conditions.
- Heating temperature of steel material 1150 ° C. or higher
- it is necessary to heat the steel material prior to rough rolling so that the steel structure of the steel material becomes a substantially homogeneous austenite phase.
- the heating temperature exceeds 1400 ° C., scale is generated excessively and the yield decreases, so the heating temperature is preferably 1400 ° C. or less.
- the atmosphere in the heating furnace is (pCO + pCO 2 + pCH 4 ) / (pCO + 2pCO 2 + pH 2 O + 2pO 2 ) ⁇ 1, the temperature is 1150 ° C. or more, and the residence time is 30 minutes or more, the C concentration gradient is stable on the surface layer.
- pCO, pCO 2 , pCH 4 , pH 2 O, and pO 2 mean partial pressures (Pa) of CO, CO 2 , CH 4 , H 2 O, and O 2 , respectively.
- the rough rolling conditions for the rough rolling after the heating are not particularly limited.
- Finishing rolling temperature 800 ° C. or more
- the finish rolling temperature is 800 ° C. or higher.
- a preferable finish rolling temperature is 820 ° C. or higher.
- the finish rolling temperature is preferably 940 ° C. or less because the surface properties are deteriorated due to the bite of the scale.
- the cooling stop temperature in the cooling after the finish rolling may or may not coincide with the coiling temperature. If they do not match, further cooling or heating to the coiling temperature is required.
- Winding temperature 350 ° C. or higher and 720 ° C. or lower It is difficult to set the winding temperature to a temperature lower than 350 ° C. due to restrictions on the runout table length. On the other hand, if the coiling temperature is less than 350 ° C., the shape of the plate is deteriorated and cold rolling becomes difficult. On the other hand, if the coiling temperature exceeds 720 ° C., the coiler is damaged due to the high temperature, and the equipment is adversely affected. From the above, the coiling temperature was set to 350 ° C. or more and 720 ° C. or less. Preferably they are 450 degreeC or more and 680 degrees C or less.
- the subsequent cold rolling step is a step of cold rolling the hot-rolled steel sheet after the hot rolling step.
- it is necessary to cold-roll the hot-rolled steel sheet after the hot rolling process.
- the cold rolling rate is preferably 30% or more and 80% or less because of restrictions on the production line.
- the subsequent annealing step is a continuous annealing line or a continuous plating line after the cold rolling step, and the air ratio is 1.05 in a temperature range of 580 to T ° C. (580 ⁇ T ⁇ 730) using a combustion burner.
- the cold rolled steel sheet was heated to a maximum temperature of 730 ° C. or higher under the conditions of ⁇ 1.30 and the dew point in the temperature range of 730 ° C. or higher was ⁇ 40 to ⁇ 15 ° C., and then averaged from 700 ° C. to 550 ° C.
- This is a step of cooling to a cooling stop temperature of 25 to 530 ° C. under a condition where the cooling rate is ⁇ 10 ° C./s or less, then heating as necessary, and maintaining in a temperature range of 200 ° C. to 530 ° C.
- an oxidizing atmosphere is formed, and C on the steel sheet surface reacts with oxygen to cause a C concentration gradient on the steel sheet surface.
- the reaction rate at this time also changes depending on the temperature, and in order to obtain a sufficient reaction rate in the continuous annealing line or the continuous plating line, a temperature range of 580 to T ° C. is used by using a combustion burner with an air ratio of 1.05 or more. It needs to be heated.
- the air ratio of the combustion burner in the above temperature range is set to 1.30 or less.
- a preferable range of the air ratio is 1.07 to 1.26.
- T ° C. was set to 730 ° C. or lower.
- the air ratio when the temperature range of 580 to T ° C. is heated by the combustion burner should be within the above range, and when T is close to 580 ° C. (region where the air ratio is controlled to 1.05 to 1.30) Is also within the scope of the present invention, but T ° C is preferably 600 to 700 ° C in order to obtain a sufficient effect by controlling the air ratio.
- An annealing furnace at this time may be an oxidation furnace in a direct-fired furnace or an oxidation furnace in an oxygen-free furnace, but any heating method using a direct-fired burner can be applied.
- Dew point in the temperature range above 730 ° C: -40 to -15 ° C The reaction of the water vapor in the furnace with C in the steel sheet surface layer also causes a gradient of the C concentration in the steel sheet surface layer.
- the dew point in the temperature range of 730 ° C. or higher during heating is set to ⁇ 40 to ⁇ 15 ° C.
- the dew point in the above temperature range exceeds ⁇ 15 ° C.
- an adverse effect of a decrease in formability due to intergranular cracking becomes obvious due to the effect of intergranular oxidation.
- the dew point is lower than ⁇ 40 ° C.
- the water vapor in the furnace does not react with C in the surface layer portion of the steel sheet, and a desired C concentration gradient cannot be obtained.
- the preferred dew point range is -35 to -20 ° C. Further, in order to prevent water vapor and C in the surface layer portion of the steel sheet from reacting sufficiently, it is necessary to control the dew point in a temperature range of at least 730 ° C. or higher. Further, the dew point is set within the above range up to the maximum temperature.
- Maximum attainment temperature 730 ° C. or higher As described above, when the maximum attainment temperature is less than 730 ° C., a C concentration gradient cannot be formed appropriately, or a desired steel structure may not be obtained.
- the maximum attained temperature is preferably 750 ° C. or higher from the viewpoint of forming a C concentration gradient.
- the upper limit of the maximum attainable temperature is not particularly limited, but if the maximum attainable temperature is excessively high, damage to the annealing furnace equipment due to heat may occur or the fuel consumption rate may be reduced. .
- Average cooling rate from 700 ° C. to 550 ° C . is a temperature range in which ferrite transformation proceeds, and when the average cooling rate in this temperature range exceeds ⁇ 10 ° C./s The area ratio of the ferrite phase exceeds 80%, and the desired steel plate strength cannot be obtained. Therefore, the average cooling rate from 700 ° C. to 550 ° C. is ⁇ 10 ° C./s or less. Desirably, the average cooling rate in the temperature range of 700 ° C. to 550 ° C. is ⁇ 20 ° C./s or less.
- Cooling stop temperature 25-530 ° C
- the cooling method may be any of a gas jet cooling device and water cooling as long as the above average cooling rate condition is satisfied.
- the lower limit of the cooling stop temperature is 25 ° C. corresponding to room temperature.
- Holding temperature 200 ° C to 530 ° C
- a heating method may be IH heating, a gas heating device, or the like. Below 200 ° C, the martensite is not tempered sufficiently and the ductility of the steel sheet is impaired. On the other hand, if the temperature exceeds 530 ° C., the ferrite transformation proceeds and the desired steel plate strength cannot be obtained.
- a preferable holding temperature range is 250 ° C. or more and 520 ° C. or less.
- maintenance should just have steel plate temperature in the said temperature range, and is not limited to constant temperature holding.
- the holding time is not particularly limited, but the holding time is preferably 20 to 1200 seconds from the viewpoint of the above purpose.
- the subsequent plating step is a step of performing plating after the annealing step and forming a plating layer on the annealed plate.
- the above annealing is performed in a continuous hot dipping plating line, followed by cooling after annealing and dipping in a hot dipping bath, and a plating layer on the surface. May be formed.
- the type of plating is preferably galvanizing.
- a steel material having a component composition shown in Table 1 and having a thickness of 250 mm is subjected to a hot rolling process under the conditions shown in Table 2 to obtain a hot rolled steel sheet, and subjected to a cold rolling process under the cold rolling conditions shown in Table 2
- a cold-rolled steel sheet having a thickness of 1.0 mm to 2.0 mm was used.
- the annealing process was given on the conditions shown in Table 2 with the continuous annealing line or the continuous hot dipping line, respectively. Then, the plating process and the alloying process were performed as needed.
- the temperature of the plating bath (plating composition: Zn—0.13 mass% Al) immersed in the continuous hot dipping line is 460 ° C.
- the amount of coating is GI (hot dip plated steel), GA (alloyed)
- Both the hot-dip galvanized steel sheets) were 45 to 65 g / m 2 per side, and the amount of Fe contained in the plating layer was in the range of 6 to 14% by mass.
- Specimens were collected from the cold-rolled steel sheet, hot-dip galvanized steel sheet or alloyed hot-dip galvanized steel sheet obtained as described above and evaluated by the following method.
- (I) Structure observation image The area ratio of each phase was evaluated by the following method.
- the steel sheet was cut out from the steel sheet so that the cross section parallel to the rolling direction becomes the observation surface, the observation surface was corroded with 1% nital, and magnified 2000 times with a scanning electron microscope to obtain a thickness of 1/4 to 3/4. This field was photographed for 10 fields of view.
- the ferrite phase is a structure having a form in which corrosion marks and cementite are not observed in the grains
- the bainite phase is a structure in which corrosion marks and large carbides are recognized in the grains.
- the martensite phase is a structure in which no carbide is observed in the grains and is observed with white contrast.
- Tempered martensite is a structure in which corrosion marks are observed in the grains and fine carbides are observed between the laths.
- the ferrite phase, the bainite phase, the martensite phase, and the tempered martensite phase were separated from each other by image analysis, and the area ratio relative to the observation field was obtained. The results are shown in Table 3.
- Inclusion density measurement was evaluated by the following method. The steel sheet was cut out from the steel plate so that the cross section of the plate thickness parallel to the rolling direction becomes the observation surface, magnified 2000 times with a scanning electron microscope, and 1 mm 2 of the region from the steel plate surface layer to 50 ⁇ m in the plate thickness direction was observed. The number of inclusions of 5 ⁇ m or more was counted and the inclusion density was measured. The results are shown in Table 3.
- (Iii) X-ray measurement
- a polished surface obtained by grinding a region having a thickness of 1/4 to 3/4 from the surface of the steel plate in the plate thickness direction and performing chemical polishing of 200 ⁇ m or more.
- the amount of retained austenite was quantified by the X-ray diffraction intensity.
- the incident radiation source was MoK ⁇ radiation, measured from the peaks of (200) ⁇ , (211) ⁇ , (200) ⁇ , (220) ⁇ , and (311) ⁇ to determine the volume fraction.
- the obtained volume ratio is treated as an area ratio.
- the differential amount of the C concentration from the steel sheet surface to a depth of 50 ⁇ m is obtained by differentiating the approximate expression. Also good.
- the tensile strength: 780 MPa or more and the total elongation: 8% or more were set as the steel sheet characteristics required in the present invention.
- the total elongation is set to 8% or more because if it is less than 8%, the formability is poor, and it becomes impossible to use due to problems such as cracking due to insufficient ductility of the steel sheet due to press working or the like.
- the yield strength is preferably 490 MPa or more.
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Abstract
Description
d[%Cx]/dx=([%Cx]-[%Cx-10μm])/0.01 (1)
式(1)における[%Cx]はxにおけるC濃度、xは鋼板表面から板厚方向の距離を示し、その値は50μm以下とする。
(pCO+pCO2+pCH4)/(pCO+2pCO2+pH2O+2pO2)≦1 (2)
式(2)におけるpCO、pCO2、pCH4、pH2O及びpO2はそれぞれ、CO、CO2、CH4、H2O及びO2の分圧(Pa)を意味する。
本発明の高強度冷延鋼板は、質量%で、C:0.06%以上0.20%以下、Si:0.01%以上2.0%以下、Mn:1.8%以上5.0%以下、P:0.06%以下、S:0.005%以下、Al:0.08%以下、N:0.008%以下を含有する。
Cは強度を担うベイナイト相、マルテンサイト相および焼き戻しマルテンサイト相の硬さを上昇させる効果を持つ。鋼板表面から板厚方向に50μmまでの領域である表層部の引張強さが所望の範囲となるようにするためには、C含有量を0.06%以上にする必要がある。一方、Cはフェライト相生成を抑制する焼入性を有しており、C含有量が0.20%を上回ると、板厚1/4から3/4までの領域において、フェライト相の面積率が20%を下回り、延性および曲げ性が失われ実用が困難となる。そこで、C含有量は0.06%以上0.20%以下とする。望ましいC含有量は0.07%以上0.18%以下である。
Siは、固溶強化により高強度化に寄与する元素である。一方で、SiはCの拡散を遅らせてCの濃度勾配を付けにくくする。このためSi含有量には最適値が存在する。Si含有量が2.0%を上回ると、表層部において所望のC濃度勾配が得られなくなる。そこで、Si含有量の上限を2.0%とした。一方、0.01%程度のSiは不可避的に鋼中に混入するため、Si含有量の下限を0.01%とした。好ましいSi含有量は0.02%以上1.6%以下である。
Mnは、固溶強化により高強度化に寄与するうえ、フェライト相の生成を抑える元素である。表層部の引張強さを所望の範囲にするには、Mn含有量を1.8%以上とする必要がある。一方で、Mn含有量が5.0%を上回ると、板厚1/4から3/4までの領域において、フェライト相の面積率が20%を下回り、また、曲げ加工性が劣化する。このため、Mn含有量の上限を5.0%とした。好ましいMn含有量の範囲は1.9%以上3.5%以下である。
Pは、粒界に偏析して曲げ成形時の割れの起点となるため、成形性に悪影響をもたらす元素である。したがって、P含有量は極力低減することが好ましい。本発明では上記問題を回避すべく、P含有量を0.06%以下とする。好ましいP含有量は0.03%以下である。P含有量は極力低減する方が望ましいが、製造上、0.002%は不可避的に混入することが多い。
Sは、鋼中でMnSなどの介在物として存在する。この介在物は、熱間圧延および冷間圧延により楔状の形態となる。このような形態であると、介在物がボイド生成の起点となりやすく、成形性が低下する。したがって、本発明では、S含有量を極力低減することが好ましく、0.005%以下とする。好ましいS含有量は0.003%以下である。S含有量は極力低減する方が望ましいが、製造上、0.0005%は不可避的に混入することが多い。
Alを製鋼の段階で脱酸剤として添加する場合、Alを0.02%以上含有することとなる。一方で、Al含有量が0.08%を超えるとアルミナなどの粗大な介在物の影響で成形性が悪化する。したがって、Al含有量は0.08%以下とする。好ましいAl含有量は0.07%以下である。
本発明においてNは、Tiと結合し粗大なTi系窒化物として析出する。この粗大なTi系窒化物は成形性に悪影響をもたらすため、N含有量は極力低減する必要があり、上限量を0.008%とする。好ましいN含有量は0.006%以下である。N含有量は極力低減する方が望ましいが、製造上、0.0005%は不可避的に混入することが多い。
Mo、CrおよびNiは、固溶強化に加え、ベイナイト相、マルテンサイト相および焼き戻しマルテンサイト相の生成を促進する効果がある元素である。したがって、これらの元素は、実質的に高強度化に寄与する。一方で、これら元素を過度に含有した場合、成形性が悪化する。以上のことから、Mo:0.01%以上0.5%以下、Cr:0.01%以上0.9%以下、Ni:0.01%以上0.2%以下とした。
Ti、NbおよびVは炭素と結合し析出物を生成する元素である。この析出物による強化は、鋼板の引張強さを、板厚方向全体にわたって強化する。一方で、表層付近の軟化はベイナイト相、マルテンサイト相および焼き戻しマルテンサイト相よりも困難である。すなわち、Ti、Nb、V量が本発明で規定する上限を超えると、これらの析出物による強化の度合いが過度に大きくなる。その結果、後述するような、表面から板厚方向に50μmまでのC濃度の微分量が0.10質量%/mm以上とした場合でも、表面から板厚方向に50μmまでの表層部の成形性が劣化し、本発明で特徴とする良好な曲げ加工性(成形性)が得られなくなる。そこで、Ti:0.01%以上0.15%以下、Nb:0.01%以上0.1%以下、V:0.01%以上0.5%以下とした。このとき、Ti、NbおよびVを含む粗大な炭窒化物生成の影響により成形性が悪化することがある。この観点から、これらの元素を上記含有量で含む場合に、鋼板表面から板厚方向に50μmまでの領域である表層部における、粒子径が0.2μm以上の介在物密度が500個/mm2以下とする。なお、好ましい介在物密度上限は350個/mm2以下である。一方、打抜加工時のせん断面における亀裂生成を促進させるという観点からは50個/mm2以上が好ましい。
JIS5号引張試験片を用いた引張試験で得られる引張強さは板厚方向に対して1/4から3/4までの領域の組織によって決定される。フェライト相は軟質な組織であり、上記フェライト相の面積率が80%を上回ると、引張強さが780MPaを下回る。また、上記フェライト相の面積率が80%を上回ることで上記引張強さが780MPaを下回ると、表層部でもフェライト相の含有量が多くなるため、表層部の引張強さが所望の範囲に調整されない。一方で、フェライト相は成形性を向上させる組織であるため、上記面積率が20%を下回ると成形性が著しく低下し、延性も損なわれる。この観点からフェライト相の面積率は20%以上80%以下とした。好ましいフェライト相の面積率は、30%以上70%以下である。
ベイナイト相、マルテンサイト相および焼き戻しマルテンサイト相は、フェライト相よりも硬いため、これらの相を含有する金属組織は、高強度化に適する。表層部の引張強さを所望の範囲にするには、これら金属組織の面積率を合計で20%以上とする必要がある。一方で、これらの相は延性に乏しく、これらの相の含有は一般的には成形性を低下させる。本発明では、これら金属組織の硬度がC含有量によるところが大きいことに着目し、表層部のC濃度を連続的に低下させることで成形性を改善したことに特徴を有する。しかしながら、ベイナイト相、マルテンサイト相および焼き戻しマルテンサイト相の面積率合計が80%を上回ると表層部のC濃度を変化させても所望の成形性が得られないため、これら組織の面積率合計は80%以下とした。ベイナイト相、マルテンサイト相および焼き戻しマルテンサイト相の面積率の合計の望ましい範囲は30%以上70%以下である。
前述の通り、本発明の高強度冷延鋼板の強度はC濃度によるところが大きい。したがって、表面近傍のC濃度である上記C濃度は表層部の引張強さと関連性を有する。上記C濃度を調整することとJIS5号試験片を用いて測定した引張強さが780MPa以上であることの組み合わせによって、表層部の引張強さが所望の範囲にあることを表すことができる。また、C濃度が局所的に低下した場合は、その部分の加工性は良好なものとなる。さらに、曲げ加工では板厚方向に対して連続的に応力が変化するため、例えば鋼板表面の硬さのみを低下させても、硬度が高いところで延性不足に起因した亀裂や、硬度差に起因した亀裂が発生する。これらの悪影響を回避するには、鋼板表面から板厚方向にxμm(ただし、xは50μm以下)における下記式(1)で表されるC濃度の微分量を0.10質量%/mm以上とすればよい。一方、上記微分量が6.5質量%/mmを上回ると板厚方向に対する硬度差が大きくなり、亀裂の原因となる可能性がある。そこで、C濃度の微分量は6.5質量%/mm以下とすることが望ましい。
本発明の高強度めっき鋼板は、上記高強度冷延鋼板と、その上に形成されためっき層とから構成される。
次に、本発明の高強度冷延鋼板の製造方法について説明する。本発明の高強度冷延鋼板の製造方法は、熱間圧延工程と、冷間圧延工程と、焼鈍工程を含む。以下、各工程について説明する。なお、以下の説明において、温度は特に断らない限り表面温度とする。また、平均冷却速度は((冷却後の表面温度-冷却前の表面温度)/冷却時間)とする。
本発明においては、粗圧延に先立ち鋼素材を加熱して、鋼素材の鋼組織を実質的に均質なオーステナイト相とする必要がある。また、粗大な介在物の生成を抑制するためには加熱温度の制御が重要となる。加熱温度が1150℃を下回ると仕上圧延温度が800℃以上で熱間圧延を完了させることができない。一方、加熱温度が1400℃を上回ると、過度にスケールが生成され歩留まりが低下するため、加熱温度は、好ましくは1400℃以下である。
仕上圧延温度が800℃を下回ると、仕上圧延中にフェライト変態が開始してフェライト粒が伸展された組織となるうえ、部分的にフェライト粒が成長した混粒組織となる。このため、800℃未満の仕上圧延温度は、冷間圧延時の板厚精度に悪影響をもたらす。したがって、仕上圧延温度は800℃以上とする。好ましい仕上圧延温度は820℃以上である。また、仕上圧延温度はスケールの噛み混みによる表面性状が劣化するという理由で仕上圧延温度は940℃以下が好ましい。仕上圧延後は、仕上圧延温度から560℃までの平均冷却速度が-30℃/s以下の条件で冷却することが望ましい。平均冷却速度が-30℃/sを上回る場合、ランアウトテーブル長さの制約上、低温で巻き取ることが困難となる。この仕上圧延後の冷却における冷却停止温度は巻取温度と一致してもよいし、一致しなくてもよい。一致しない場合には、さらに、巻取温度まで冷却又は加熱が必要になる。
巻取温度が350℃を下回る温度に設定するのは、ランアウトテーブル長さの制約上、困難である。また、巻取温度が350℃未満であると、板の形状が悪化し冷間圧延が困難となる。一方、巻取温度が720℃を上回るとコイラーが高温による影響で損傷し、設備に対して悪影響をもたらす。以上から、巻取温度は350℃以上720℃以下とした。好ましくは450℃以上680℃以下である。
空気比が1.05以上では酸化雰囲気となり、鋼板表面のCと酸素とが反応することで鋼板表面のC濃度の勾配が生じる。このときの反応速度は温度によっても変化し、連続焼鈍ラインもしくは連続めっきラインで十分な反応速度を得るには、空気比が1.05以上で燃焼バーナーを用いて580~T℃の温度域を加熱する必要がある。一方、空気比が1.30を上回ると粒界酸化による粒界割れによる成形性低下の悪影響が顕在化するため、上記温度域での燃焼バーナーの空気比は1.30以下とした。好ましい上記空気比の範囲は1.07~1.26である。
炉内の水蒸気と鋼板表層部のCとが反応することによっても、鋼板表層部のC濃度の勾配が生じる。この効果を発現させるには、加熱の際の730℃以上の温度域の露点が-40~-15℃になるようにする。上記温度域での露点が-15℃を上回ると、粒界酸化の影響により粒界割れによる成形性低下の悪影響が顕在化する。一方、上記温度域で露点-40℃を下回った場合、炉内の水蒸気と鋼板表層部のCとが反応せず、所望のC濃度勾配が得られない。好ましい露点の範囲は-35~-20℃である。また、水蒸気と鋼板表層部のCとが十分に反応しないことを防ぐため、少なくとも730℃以上の温度域は露点を制御する必要がある。また、最高到達温度まで露点を上記範囲にする。
最高到達温度が730℃未満であると上記の通り、適切にC濃度勾配を形成することができない場合や、所望の鋼組織を得られない場合がある。最高到達温度はC濃度勾配形成の観点からは750℃以上が好ましい。なお、最高到達温度の上限は特に限定されないが、最高到達温度を過度に高温にすると、焼鈍炉の設備に対する熱による損傷が生じたり、燃料原単位の低下につながったりするため880℃以下が好ましい。
700℃から550℃まではフェライト変態が進行する温度域であり、この温度域の平均冷却速度が-10℃/sを上回るとフェライト相の面積率が80%を上回り、所望の鋼板強度が得られない。したがって、700℃から550℃までの平均冷却速度:-10℃/s以下とする。望ましくは700℃から550℃の温度域の平均冷却速度が-20℃/s以下である。
ベイナイト相、マルテンサイト相を十分に得るには、530℃以下に冷却する必要がある。冷却する方法は、上記平均冷却速度の条件を満たせば、ガスジェット冷却装置や水冷等のいずれでもよい。安定的に、ベイナイト相、マルテンサイト相および焼き戻しマルテンサイト相の面積率の合計を20%以上にするには、冷却停止温度を520℃以下にすることが望ましい。本発明では、あえて室温を下回る温度まで冷却する必要が無いことから冷却停止温度の下限は室温に相当する25℃とする。
前工程で得られたマルテンサイト相を焼き戻す、もしくはベイナイト変態を促進させる目的で200℃から530℃で保持する。保持温度よりも上記冷却停止温度の方が低い場合は加熱する必要があり、加熱する方法はIH加熱、ガス加熱装置などを用いればよい。200℃以下では十分にマルテンサイトが焼き戻されず、鋼板の延性が損なわれる。一方で、530℃を上回る場合にはフェライト変態が進行し、所望の鋼板強度が得られなくなる。好ましい保持温度の範囲は250℃以上520℃以下である。また、上記保持は、鋼板温度が上記温度域にあればよく、定温保持に限定されない。また、保持時間は特に限定されないが、上記目的の観点から保持時間は20~1200秒であることが好ましい。
続いて行うめっき工程とは、上記焼鈍工程後にめっきを施し、焼鈍板上にめっき層を形成する工程である。例えば、めっき処理として、自動車用鋼板に多用される溶融めっきを行う場合には、上記焼鈍を連続溶融めっきラインで行い、焼鈍後の冷却に引き続いて溶融めっき浴に浸漬して、表面にめっき層を形成すればよい。また、上記めっき工程後に、必要に応じて、合金化処理を行う合金化工程を設けてもよい。なお、めっきの種類は亜鉛めっきが好ましい。
各相の面積率は以下の手法により評価した。鋼板から、圧延方向に平行な板厚断面が観察面となるよう切り出し、観察面を1%ナイタールで腐食現出し、走査型電子顕微鏡で2000倍に拡大して板厚1/4~3/4の領域を10視野分撮影した。フェライト相は粒内に腐食痕やセメンタイトが観察されない形態を有する組織であり、ベイナイト相は粒内に腐食痕や大きな炭化物が認められる組織である。マルテンサイト相は粒内に炭化物が認められず、白いコントラストで観察される組織である。焼き戻しマルテンサイトは粒内に腐食痕が認められ、ラス間に微細な炭化物が認められる組織である。これらを画像解析によりフェライト相、ベイナイト相、マルテンサイト相および焼き戻しマルテンサイト相を分離し、観察視野に対する面積率を求めた。結果を表3に示した。
介在物密度測定は以下の手法により評価した。鋼板から、圧延方向に平行な板厚断面が観察面となるよう切り出し、走査型電子顕微鏡で2000倍に拡大して鋼板表層から板厚さ方向に50μmまでの領域を1mm2観察し、0.5μm以上の介在物の数をカウントし、介在物密度を測定した。結果を表3に示した。
圧延方向に平行な板厚断面において、鋼板表面から板厚方向に板厚1/4~3/4の領域を研削加工し、200μm以上の化学研磨を施した、研磨面のX線回折強度により残留オーステナイト量を定量した。入射線源はMoKα線を用い、(200)α、(211)α、(200)γ、(220)γ、(311)γのピークから測定し、体積率を求めた。なお、本発明においては、得られた体積率は面積率として扱う。
鋼板表面を逐次板厚方向に研削し、表れた表面に対してスパーク放電発行分光分析を行うことによりその表面でのC濃度を求めた。これにより研削量に応じて複数回測定することで、板厚方向のC濃度分布が測定できる。
得られた鋼板から圧延方向に対して垂直方向にJIS5号引張試験片を作製し(もとの厚さのままであり、試験片は表層を含む)、JIS Z 2241(2011)の規定に準拠した引張試験を5回行い、平均の降伏強度(YS)、引張強さ(TS)、全伸び(El)を求めた。引張試験のクロスヘッドスピードは10mm/minとした。降伏強さは連続降伏の場合は0.2%耐力の値を読みとり、不連続降伏の場合は下降伏点を読みとった。表3において、引張強さ:780MPa以上、かつ全伸び:8%以上を本発明で求める鋼板の特性とした。ここで、全伸び:8%以上としたのは、8%を下回る場合、成形性が悪く、プレス加工などで鋼板の延性不足によって割れなどの問題で実用不可となるためである。なお、降伏強さは490MPa以上であることが好ましい。
鋼板の板厚方向に対して垂直方向が試験片の長手方向となるようにJIS Z2248に記載の3号試験片を採取し、Vブロック法で曲げ試験を行った。曲げ稜線に割れが認められたときの押しつけ金具先端の半径を板厚で割ることにより限界曲げ半径(R/t)を求めた。実際は強度によって求められる曲げ性が異なる。そのため、(R/t)/TSが2.0×10-3以下であれば非常に良好な結果として“○”、2.0×10-3超2.5×10-3以下は良好な結果として“△”の評価とし本発明で求める範囲とした。(R/t)/TSが2.5×10-3を上回る場合には、“×”として評価し本発明で求める範囲外とした。
Claims (11)
- 質量%で、C:0.06%以上0.20%以下、Si:0.01%以上2.0%以下、Mn:1.8%以上5.0%以下、P:0.06%以下、S:0.005%以下、Al:0.08%以下、N:0.008%以下を含有し、残部がFeおよび不可避的不純物からなる成分組成と、
板厚1/4から3/4までの領域におけるフェライト相の面積率が20%以上80%以下、ベイナイト相、マルテンサイト相および焼き戻しマルテンサイト相の面積率の合計が20%以上80%以下であり、
鋼板表面から板厚方向にxμmにおける下記式(1)で表されるC濃度の微分量が0.10質量%/mm以上である鋼組織と、を有し、
JIS5号引張試験片を用いて引張試験を行ったときの引張強さが780MPa以上である高強度冷延鋼板。
d[%Cx]/dx=([%Cx]-[%Cx-10μm])/0.01 (1)
式(1)における[%Cx]はxにおけるC濃度、xは鋼板表面から板厚方向の距離を示し、その値は50μm以下とする。 - 前記成分組成は、さらに質量%で、Mo:0.01%以上0.5%以下、Cr:0.01%以上0.9%以下、Ni:0.01%以上0.2%以下の1種または2種以上を含有する請求項1に記載の高強度冷延鋼板。
- 前記成分組成は、さらに質量%で、Ti:0.01%以上0.15%以下、Nb:0.01%以上0.1%以下、V:0.01%以上0.5%以下の1種または2種以上を含有し、
前記鋼組織において、鋼板表面から板厚方向に50μmまでの領域における、粒子径が0.2μm以上の介在物密度が500個/mm2以下である請求項1または請求項2に記載の高強度冷延鋼板。 - 前記成分組成は、さらに質量%で、B:0.0002%以上0.0030%以下を含有する請求項1~3のいずれかに記載の高強度冷延鋼板。
- 請求項1~4のいずれかに記載の高強度冷延鋼板と、
該高強度冷延鋼板上に形成されためっき層と、を有する高強度めっき鋼板。 - 前記めっき層は、溶融めっき層又は合金化溶融めっき層である請求項5に記載の高強度めっき鋼板。
- 前記めっき層は、質量%で、Fe:5.0~20.0%、Al:0.001%~1.0%を含有し、さらに、Pb、Sb、Si、Sn、Mg、Mn、Ni、Cr、Co、Ca、Cu、Li、Ti、Be、Bi、REMから選択する1種または2種以上を合計で0~3.5%含有し、残部がZn及び不可避的不純物からなる請求項5又は6に記載の高強度めっき鋼板。
- 請求項1から4のいずれかに記載の成分組成を有する鋼素材を、1150℃以上に加熱し、粗圧延、仕上圧延終了温度が800℃以上の仕上圧延を施し、350℃以上720℃以下の巻取温度で巻取る熱間圧延工程と、
前記熱間圧延工程後に、熱延鋼板に対して冷間圧延を施す冷間圧延工程と、
前記冷間圧延工程後に、連続焼鈍ラインもしくは連続めっきラインで、燃焼バーナーを用いて、580℃以上T℃以下の温度域を空気比が1.05~1.30の条件(ただし、580<T≦730)、730℃以上の温度域の露点が-40~-15℃の条件で、冷延鋼板を730℃以上の最高到達温度まで加熱し、次いで700℃から550℃までの平均冷却速度が-10℃/s以下の条件で25~530℃の冷却停止温度まで冷却し、次いで必要に応じて加熱して、200℃から530℃の温度域で保持する焼鈍工程と、を有する高強度冷延鋼板の製造方法。 - 前記熱間圧延工程において、前記鋼素材を加熱する際の、雰囲気が式(2)を満たし、前記鋼素材が1150℃以上の温度域に滞留する時間が30分以上である請求項8に記載の高強度冷延鋼板の製造方法。
(pCO+pCO2+pCH4)/(pCO+2pCO2+pH2O+2pO2)≦1 (2)
式(2)におけるpCO、pCO2、pCH4、pH2O及びpO2はそれぞれ、CO、CO2、CH4、H2O及びO2の分圧(Pa)を意味する。 - 請求項8又は9に記載の製造方法で製造された高強度冷延鋼板にめっきを施すめっき工程を有する高強度めっき鋼板の製造方法。
- 前記めっき工程後に、合金化処理を行う合金化工程を有する請求項10に記載の高強度めっき鋼板の製造方法。
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