WO2016120914A1 - 高強度めっき鋼板およびその製造方法 - Google Patents
高強度めっき鋼板およびその製造方法 Download PDFInfo
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- WO2016120914A1 WO2016120914A1 PCT/JP2015/004173 JP2015004173W WO2016120914A1 WO 2016120914 A1 WO2016120914 A1 WO 2016120914A1 JP 2015004173 W JP2015004173 W JP 2015004173W WO 2016120914 A1 WO2016120914 A1 WO 2016120914A1
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- Prior art keywords
- steel sheet
- less
- cold
- plated steel
- phase
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- 229910000831 Steel Inorganic materials 0.000 title claims abstract description 157
- 239000010959 steel Substances 0.000 title claims abstract description 157
- 238000004519 manufacturing process Methods 0.000 title claims abstract description 18
- 229910000734 martensite Inorganic materials 0.000 claims abstract description 66
- 229910000859 α-Fe Inorganic materials 0.000 claims abstract description 49
- 238000007747 plating Methods 0.000 claims abstract description 43
- 229910001563 bainite Inorganic materials 0.000 claims abstract description 30
- 239000000203 mixture Substances 0.000 claims abstract description 18
- 238000001816 cooling Methods 0.000 claims description 57
- 238000010438 heat treatment Methods 0.000 claims description 45
- 238000000137 annealing Methods 0.000 claims description 31
- 238000005096 rolling process Methods 0.000 claims description 26
- 238000005097 cold rolling Methods 0.000 claims description 15
- 239000000463 material Substances 0.000 claims description 12
- 229910052757 nitrogen Inorganic materials 0.000 claims description 12
- 229910052719 titanium Inorganic materials 0.000 claims description 12
- 238000005098 hot rolling Methods 0.000 claims description 11
- 229910052749 magnesium Inorganic materials 0.000 claims description 10
- 229910052710 silicon Inorganic materials 0.000 claims description 10
- 229910052791 calcium Inorganic materials 0.000 claims description 9
- 229910052748 manganese Inorganic materials 0.000 claims description 9
- 239000012535 impurity Substances 0.000 claims description 8
- 239000002245 particle Substances 0.000 claims description 8
- 238000005275 alloying Methods 0.000 claims description 7
- 229910052759 nickel Inorganic materials 0.000 claims description 7
- 230000000717 retained effect Effects 0.000 claims description 7
- 229910052804 chromium Inorganic materials 0.000 claims description 6
- 229910052735 hafnium Inorganic materials 0.000 claims description 6
- 229910052721 tungsten Inorganic materials 0.000 claims description 6
- 229910052720 vanadium Inorganic materials 0.000 claims description 6
- 238000004804 winding Methods 0.000 claims description 6
- 229910052698 phosphorus Inorganic materials 0.000 claims description 5
- 238000005246 galvanizing Methods 0.000 claims description 4
- 229910052718 tin Inorganic materials 0.000 claims description 4
- 229910052787 antimony Inorganic materials 0.000 claims description 3
- 229910052790 beryllium 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
- 229910052744 lithium Inorganic materials 0.000 claims description 3
- 229910052758 niobium Inorganic materials 0.000 claims description 3
- 238000002203 pretreatment Methods 0.000 claims description 3
- 230000002349 favourable effect Effects 0.000 abstract 1
- 238000000034 method Methods 0.000 description 26
- 230000009466 transformation Effects 0.000 description 24
- 239000010410 layer Substances 0.000 description 23
- 229910001567 cementite Inorganic materials 0.000 description 17
- KSOKAHYVTMZFBJ-UHFFFAOYSA-N iron;methane Chemical compound C.[Fe].[Fe].[Fe] KSOKAHYVTMZFBJ-UHFFFAOYSA-N 0.000 description 17
- 229910001566 austenite Inorganic materials 0.000 description 16
- 230000000694 effects Effects 0.000 description 11
- XEEYBQQBJWHFJM-UHFFFAOYSA-N iron Substances [Fe] XEEYBQQBJWHFJM-UHFFFAOYSA-N 0.000 description 10
- 239000002344 surface layer Substances 0.000 description 10
- 229910052761 rare earth metal Inorganic materials 0.000 description 9
- 238000005482 strain hardening Methods 0.000 description 9
- 238000007598 dipping method Methods 0.000 description 8
- 230000002411 adverse Effects 0.000 description 6
- 230000015572 biosynthetic process Effects 0.000 description 6
- 229910001335 Galvanized steel Inorganic materials 0.000 description 5
- 229910052799 carbon Inorganic materials 0.000 description 5
- 239000008397 galvanized steel Substances 0.000 description 5
- 229910052750 molybdenum Inorganic materials 0.000 description 5
- 150000004767 nitrides Chemical class 0.000 description 5
- 230000006911 nucleation Effects 0.000 description 5
- 238000010899 nucleation Methods 0.000 description 5
- 238000005261 decarburization Methods 0.000 description 4
- 230000007423 decrease Effects 0.000 description 4
- 239000002244 precipitate Substances 0.000 description 4
- 238000009864 tensile test Methods 0.000 description 4
- 238000005452 bending Methods 0.000 description 3
- 229910052742 iron Inorganic materials 0.000 description 3
- 150000001247 metal acetylides Chemical class 0.000 description 3
- 229920006395 saturated elastomer Polymers 0.000 description 3
- UQSXHKLRYXJYBZ-UHFFFAOYSA-N Iron oxide Chemical compound [Fe]=O UQSXHKLRYXJYBZ-UHFFFAOYSA-N 0.000 description 2
- 229910052797 bismuth Inorganic materials 0.000 description 2
- 238000005266 casting Methods 0.000 description 2
- 238000009749 continuous casting Methods 0.000 description 2
- 230000007797 corrosion Effects 0.000 description 2
- 238000005260 corrosion Methods 0.000 description 2
- 230000007547 defect Effects 0.000 description 2
- 238000005516 engineering process Methods 0.000 description 2
- 239000000446 fuel Substances 0.000 description 2
- 238000010191 image analysis Methods 0.000 description 2
- 238000002844 melting Methods 0.000 description 2
- 230000008018 melting Effects 0.000 description 2
- 229910001568 polygonal ferrite Inorganic materials 0.000 description 2
- 238000007670 refining Methods 0.000 description 2
- 239000006104 solid solution Substances 0.000 description 2
- 238000005728 strengthening Methods 0.000 description 2
- OKTJSMMVPCPJKN-UHFFFAOYSA-N Carbon Chemical compound [C] OKTJSMMVPCPJKN-UHFFFAOYSA-N 0.000 description 1
- 229910001035 Soft ferrite Inorganic materials 0.000 description 1
- ATJFFYVFTNAWJD-UHFFFAOYSA-N Tin Chemical compound [Sn] ATJFFYVFTNAWJD-UHFFFAOYSA-N 0.000 description 1
- 238000007545 Vickers hardness test Methods 0.000 description 1
- 229910001297 Zn alloy Inorganic materials 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
- 239000010960 cold rolled steel Substances 0.000 description 1
- 230000000052 comparative effect Effects 0.000 description 1
- 238000005336 cracking Methods 0.000 description 1
- 239000006185 dispersion Substances 0.000 description 1
- 239000012467 final product Substances 0.000 description 1
- 230000001771 impaired effect Effects 0.000 description 1
- 229910052747 lanthanoid Inorganic materials 0.000 description 1
- 150000002602 lanthanoids Chemical class 0.000 description 1
- 238000000465 moulding Methods 0.000 description 1
- 229910001562 pearlite Inorganic materials 0.000 description 1
- 238000005554 pickling Methods 0.000 description 1
- 239000000047 product Substances 0.000 description 1
- 238000009628 steelmaking Methods 0.000 description 1
- 238000009849 vacuum degassing Methods 0.000 description 1
- 239000011800 void material Substances 0.000 description 1
- 239000013585 weight reducing agent Substances 0.000 description 1
Classifications
-
- 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
-
- C—CHEMISTRY; METALLURGY
- C21—METALLURGY OF IRON
- C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
- C21D8/00—Modifying the physical properties by deformation combined with, or followed by, heat treatment
- C21D8/02—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips
-
- C—CHEMISTRY; METALLURGY
- 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
-
- 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
- C22C38/14—Ferrous alloys, e.g. steel alloys containing 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
- C22C38/58—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
- C23C2/0224—Two or more thermal pretreatments
-
- 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
- C23C2/26—After-treatment
- C23C2/28—Thermal after-treatment, e.g. treatment in oil bath
-
- 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
- C23C2/26—After-treatment
- C23C2/28—Thermal after-treatment, e.g. treatment in oil bath
- C23C2/29—Cooling or quenching
-
- 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
- C23C2/34—Hot-dipping or immersion processes for applying the coating material in the molten state without affecting the shape; Apparatus therefor characterised by the shape of the material to be treated
- C23C2/36—Elongated material
- C23C2/40—Plates; Strips
Definitions
- the present invention relates to a high-strength plated steel sheet and a method for producing the same.
- the high-strength plated steel sheet of the present invention has a high tensile strength (TS): 780 MPa or more and excellent formability. For this reason, the high-strength plated steel sheet of the present invention is suitable as a material for an automobile skeleton member (structural parts for automotive).
- 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%, or Nb: 0.1% or less, etc., with the balance being a component composition of Fe and impurities, and ferrite in an area ratio of 30 to 95 %,
- the remaining second phase is composed of one or more of martensite, bainite, pearlite, cementite and retained austenite, and the martensite area ratio is 0 to 50 when martensite is contained.
- the steel sheet contains Ti carbonitrides with a grain size of 2 to 30 nm with an average interparticle distance of 30 to 300 nm, and crystallized TiN with a grain size of 3 ⁇ m or more has an average interparticle distance of 50 to 500 ⁇ m. It is said that a high-yield ratio high-strength steel sheet excellent in bending workability and notch fatigue resistance with a tensile strength of 620 MPa or more is obtained.
- 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.
- the low-temperature transformation formation structure contains at least bainite and may further contain martensite.
- the plate surface in a depth of 0.1 mm from the surface of the steel plate is changed in the plate width direction position, and a total of 20 fields of view are observed with a microscope.
- the present invention has been made in view of such circumstances, and an object thereof is to provide a high-strength plated steel sheet having a tensile strength of 780 MPa or more and good workability, and a method for producing the same.
- the present inventors diligently studied the requirements for a steel sheet having a tensile strength of 780 MPa and good workability.
- a high-strength steel sheet we focused on reducing the soft ferrite phase as much as possible and utilizing a low-temperature transformation phase such as a bainite phase or a martensite phase.
- a low-temperature transformation phase such as a bainite phase or a martensite phase.
- the conventional technology if the ferrite phase rich in formability is reduced, good formability cannot be obtained. Therefore, a means for improving the formability of a steel sheet not containing much ferrite phase was studied.
- the fine granular martensite is the martensite phase dispersed in the bainite phase
- the uniform deformation of the bainite phase is promoted, and as a result, the workability is increased and the formability is improved.
- it is effective to finely disperse cementite in the structure before the annealing process and to suppress coarsening of the austenite grain size during annealing. I found out.
- the austenite grain interfacial area which becomes the nucleation site of ferrite transformation, increases with the reduction of the austenite grain size during annealing, the ferrite phase is easily produced.
- the present invention has been completed based on the above findings, and the gist thereof is as follows.
- a high-strength plated steel plate having a steel plate and a plating layer formed on the steel plate, wherein the component composition of the steel plate is% by mass, C: 0.06% to 0.18%, Si : Less than 0.50%, Mn: 1.9% to 3.2%, P: 0.03% or less, S: 0.005% or less, Al: 0.08% or less, N: 0.006% B: 0.0002% or more and 0.0030% or less, Nb: 0.007% or more and 0.030% or less, and Ti containing so as to satisfy the following formula (1), the balance being Fe and inevitable impurities
- the steel structure of the steel sheet has a ferrite phase area ratio of 20% or less (including 0%), a bainite phase area ratio of 35% to 90%, and a martensite phase area ratio of 10% to 65%.
- the component composition further includes, by mass%, Cr: 0.001% to 0.9%, Ni: 0.001% to 0.5%, V: 0.001% to 0.3% % Or less, Mo: 0.001% or more and 0.3% or less, W: 0.001% or more and 0.2% or less, Hf: 0.001% or more and 0.3% or less.
- the component composition further contains, in mass%, one or more of REM, Mg, and Ca in a total of 0.0002% to 0.01% [1] or The high strength plated steel sheet according to [2].
- a steel material having the composition described in any one of [1] to [3] is heated at 1000 ° C. or more and 1200 ° C. or less, and after finish rolling at a finish rolling temperature of 800 ° C. or more, finish rolling temperature To 720 ° C. at an average cooling rate of 10 ° C./s or more and winding at 580 ° C. or more and 720 ° C. or less, and cold rolling to cold-roll the hot-rolled sheet after the hot rolling step After the step and the cold rolling step, the cold-rolled sheet is heated to the Ac3 point, which is the maximum temperature, and the heated cold-rolled sheet is cooled at a cooling rate of 5 ° C / s or higher to 580 ° C.
- the time during which the cold-rolled sheet stays at the Ac3 point or higher is 60 seconds or less
- the dew point at the Ac3 point or higher is ⁇ 45 ° C. or higher and ⁇ 20 ° C. or lower
- the cooling is 440 ° C. or higher and 530 ° C or lower.
- the time that the cold-rolled sheet stays in The average heating rate from the pre-treatment heating step of 0 seconds or more and the cold-rolled sheet after the pre-treatment heating step from 100 ° C. to the highest temperature not less than (Ac3 point ⁇ 10) ° C.
- the cold-rolled sheet heated to the maximum temperature is cooled under the condition that the average cooling rate up to 560 ° C.
- 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 of the present invention has high tensile strength (TS): 780 MPa or more and excellent formability. If the high-strength plated steel sheet of the present invention is applied to automobile parts, further weight reduction of the automobile parts can be realized.
- TS tensile strength
- the high-strength plated steel sheet of the present invention includes a steel sheet and a plating layer formed on the steel sheet. It demonstrates in order of a steel plate and a plating layer.
- the component composition of the steel sheet is mass%, C: 0.06% to 0.18%, Si: less than 0.50%, Mn: 1.9% to 3.2%, P: 0.03% S: 0.005% or less, Al: 0.08% or less, N: 0.006% or less, B: 0.0002% or more and 0.0030% or less, Nb: 0.007% or more and 0.030%
- the component composition contains Ti so as to satisfy the following and the above expression (1).
- % representing the content of a component means “mass%”.
- C 0.06% or more and 0.18% or less C has a hardenability which increases the hardness of martensite and suppresses ferrite transformation.
- the C content is set to 0.06% or more and 0.18% or less. Desirably, it is 0.07% or more and 0.18% or less.
- Si Less than 0.50% Si is an element contributing to high strength by solid solution strengthening. On the other hand, since Si raises the transformation point (Ac 3 point) from the ferrite phase to the austenite phase, it makes it difficult to remove the ferrite phase during annealing. Furthermore, since Si reduces the wettability between the plating layer and the steel sheet surface, excessive inclusion of Si causes defects such as non-plating. In this invention, Si content is accept
- Mn not less than 1.9% and not more than 3.2% Mn contributes to high strength by solid solution strengthening, and lowers the Ac 3 transformation point to facilitate removal of the ferrite phase during annealing. It also has the effect of improving the hardenability of the steel sheet.
- the Mn content needs to be 1.9% or more.
- the bainite transformation does not proceed, and as a result, the area ratio of the martensite phase exceeds 65%.
- the upper limit of the Mn content is set to 3.2%.
- a preferable range of the Mn content is 2.0% or more and 3.0% or less.
- P 0.03% or less
- P is an element that has an adverse effect on formability because it segregates at grain boundaries and becomes the starting point of cracking during molding. Therefore, it is preferable to reduce the P content as much as possible.
- the P content is set to 0.03% or less. Preferably it is 0.02% or less. Although it is desirable to reduce it as much as possible, 0.002% may be inevitably mixed in manufacturing.
- S 0.005% or less S exists in the state which became inclusions, such as MnS, in steel. This inclusion becomes wedge-shaped by hot rolling and cold rolling. In such a form, it tends to be a starting point for void generation, and the moldability is also adversely affected. 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. Preferably it is 0.003% or less. It is desirable to reduce the S content as much as possible, but 0.0005% may be inevitably mixed in production.
- Al 0.08% or less
- Al 0.02% or more
- the Al content is 0.08% or less.
- it is 0.07% or less.
- N 0.006% or less
- N combines with Ti and precipitates as coarse Ti-based nitride. Since this coarse Ti-based nitride serves as a nucleation site for ferrite transformation, the N content needs to be reduced as much as possible, and the upper limit is made 0.006%.
- a preferable N content is 0.005% or less. Although it is desirable to reduce the N content as much as possible, 0.0005% may be inevitably mixed in production.
- B 0.0002% or more and 0.0030% or less B has an effect of segregating at the grain boundary of the austenite before transformation to significantly delay the nucleation of the ferrite phase and to suppress the formation of the ferrite phase.
- the B content needs to be 0.0002% or more.
- the B content is set to 0.0002% or more and 0.0030% or less. Desirably, it is 0.0005% or more and 0.0020% or less.
- Nb 0.007% or more and 0.030% or less
- Nb is an important element for suppressing coarsening of austenite grains during annealing.
- Nb content When the Nb content is excessive, coarse carbonitrides containing Nb (generic names for carbides, nitrides, and carbonitrides; hereinafter the same applies to the present invention) are precipitated, and the area ratio of the ferrite phase increases.
- the Nb content needs to be 0.007% or more.
- the Nb content exceeds 0.030%, coarse Nb-based carbonitrides precipitate under the production conditions specified in the present invention. Therefore, the upper limit of Nb content is 0.030%.
- a preferable Nb content is 0.012% or more and 0.027% or less.
- the high-strength plated steel sheet of the present invention is, in mass%, Cr: 0.001% to 0.9%, Ni: 0.001% to 0.5%, V: 0.001% to 0.00%. 1% or more of 3% or less, Mo: 0.001% or more and 0.3% or less, W: 0.001% or more and 0.2% or less, Hf: 0.001% or more and 0.3% or less You may contain.
- Cr, Ni, V, Mo, W and Hf have the effect of delaying the start of ferrite transformation. If there is an effect of these elements in addition to the effect of hardenability by B, it becomes easy to stably obtain a desired steel structure. On the other hand, if the Cr content exceeds 0.9%, the plating property is adversely affected. Further, when Ni is 0.5%, V is 0.3%, Mo is 0.3%, W is 0.2%, and Hf exceeds 0.3%, the effect of hardenability is saturated. From the above, Cr: 0.001% to 0.9%, Ni: 0.001% to 0.5%, V: 0.001% to 0.3%, Mo: 0.001% to 0 .3% or less, W: 0.001% to 0.2%, and Hf: 0.001% to 0.3%.
- the high-strength plated steel sheet of the present invention may further contain 0.0002% or more and 0.01% or less of REM, Mg, or Ca in total by mass%.
- REM lanthanoid element having atomic number 57 to 71
- Mg and Ca spheroidize cementite precipitated in bainite.
- the stress concentration around the cementite is reduced, and the formability is improved.
- the total content of REM, Mg, and Ca exceeds 0.01%, the effect of changing the shape of cementite is saturated and the ductility is adversely affected.
- one or more of REM, Mg, and Ca is 0.0005% or more and 0.005% or less in total.
- Components other than the above components are Fe and inevitable impurities.
- the steel structure of the high-strength plated steel sheet of the present invention has a ferrite phase area ratio of 20% or less (including 0%), a bainite phase area ratio of 35% or more and 90% or less, and a martensite phase area ratio of 10%.
- the number density of inclusions containing 65% or less and having an equivalent circle diameter exceeding 5.0 ⁇ m contains 400 pieces / mm 2 or less.
- the average particle diameter of the granular martensite which comprises the said martensite phase is 4.0 micrometers or less, and the maximum length between martensites is 5.0 micrometers or less.
- the ferrite phase is a soft structure, and when the content of the ferrite phase exceeds 20%, the tensile strength is less than 780 MPa.
- the ferrite phase has a low element solubility, if the ferrite phase content is excessive, the arrangement of cementite finely dispersed in the structure before annealing is changed, and a fine martensite phase cannot be obtained. Therefore, it is desirable to reduce the content of the ferrite phase as much as possible.
- the content of the ferrite phase needs to be suppressed to 20% or less. Desirably, it is 15% or less.
- Bainite phase The bainite phase is higher in hardness than the ferrite phase and is effective for finely forming the martensite phase.
- the content of the bainite phase needs to be 35% or more.
- the maximum length (maximum distance) between the martensites exceeds 5.0 ⁇ m, and good moldability cannot be obtained.
- the preferred bainite phase content is 40% or more and 80% or less in terms of area ratio.
- Martensite phase The content of the martensite phase and the form of the martensite phase have a great influence on the strength and formability.
- the content of the martensite phase is less than 10% in terms of area ratio, the tensile strength is less than 780 MPa.
- the content of the martensite phase exceeds 65% by area ratio, ductility and formability are lost.
- the preferred martensite phase content is 20% or more and 55% or less in terms of area ratio.
- the martensite phase is composed of granular martensite.
- the average particle size of martensite exceeds 4.0 ⁇ m, deformation in the vicinity of coarse martensite is constrained and the steel sheet deforms unevenly during forming. In this case, cracks are likely to occur in the preferentially deformed portion, and good moldability cannot be obtained.
- the average particle size of martensite is preferably 3.0 ⁇ m or less.
- the lower limit of the average particle size of martensite is not particularly limited, but 0.5 ⁇ m or more is preferable from the viewpoint of stably obtaining a martensite fraction of 10% or more.
- the maximum length of the interval between martensites is 5.0 ⁇ m or less. If the maximum length of the interval between martensites is within this range, many bainite phases are in contact with the martensite phase. The bainite phase in contact with the martensite phase is liable to cause dislocations and work hardening. As a result, the work hardening index (work hardening exponent) increases and deforms uniformly, so that good moldability is obtained.
- the maximum distance between martensites (maximum length) is preferably 4.0 ⁇ m or less.
- the lower limit of the maximum distance between martensites is not particularly limited, but when the distance between martensites is too close, dislocations are introduced near the martensite due to transformation strain caused by the occurrence of martensite transformation. By doing so, the occurrence of new dislocations between martensite is hindered and work hardening is difficult, so 1.0 ⁇ m or more is preferable.
- the number density of inclusions having an equivalent circle diameter and a particle size exceeding 5.0 ⁇ m is 400 pieces / mm 2 or less. Inclusions having a particle size exceeding 5.0 ⁇ m are likely to become nucleation sites of the ferrite phase, and the content of the ferrite phase in the area ratio is not in the desired range.
- the inclusions exceeding 5.0 ⁇ m include oxides containing Al or Ti, nitrides containing Ti, and carbonitrides containing Nb.
- the surface hardness of the steel sheet is 95% or less when the hardness at the position of 1/4 t (t is the thickness of the steel sheet) in the thickness direction from the steel sheet surface is 100%. It is. Many cracks at the time of forming occur on the surface of the steel sheet.
- the formability can be improved by improving the ductility of the steel sheet surface layer part by adjusting the coiling temperature and decarburizing the steel sheet surface layer part in the annealing step. Since the hardness and the ductility are in a trade-off relationship, the ductility of the steel sheet surface layer can be estimated by the decrease in the hardness of the steel sheet surface.
- the formability is improved.
- the surface hardness of the steel sheet is 90% or less when the hardness at a position of 1/4 t (t is the thickness of the steel sheet) in the thickness direction from the steel sheet surface is 100%.
- the steel sheet surface layer is in the range from the steel sheet surface to 100 ⁇ m in the thickness direction.
- 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 an alloyed plating layer (a plating layer mainly containing an Fe—Zn alloy formed by diffusing Fe in steel during galvanization by an alloying reaction).
- the manufacturing method of the high strength plated steel sheet of the present invention includes a hot rolling process, a cold rolling process, a pretreatment heating process, an annealing process, and a plating process. Moreover, you may have an alloying process after a plating process as needed.
- the temperature is the surface temperature unless otherwise specified.
- the average heating rate is ((surface temperature after heating ⁇ surface temperature before heating) / heating time), and the average cooling rate is ((surface temperature before cooling ⁇ surface temperature after cooling) / cooling time).
- the hot rolling process means that the steel material having the above composition is heated at 1000 ° C. or more and 1200 ° C. or less, and after finishing rolling at a finish rolling temperature of 800 ° C. or more, the average cooling rate from the finish rolling temperature to 720 ° C. Is a step of cooling at 10 ° C./s or more and winding at 580 ° C. or more and 720 ° C. or less.
- 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. Further, the slab may be formed by a known casting method such as ingot-casting rolling and thin slab continuous casting.
- Heating temperature of steel material 1000 ° C. or more and 1200 ° C. or less
- 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 1200 ° C. the formation of particularly nitrides containing Ti is promoted, and the number density of inclusions exceeding 5.0 ⁇ m increases. Therefore, the heating temperature of the steel material is set to 1000 ° C. or more and 1200 ° C. or less. Desirably, it is 1020 degreeC or more and 1150 degreeC or less. In addition, it does not specifically limit about the rough rolling conditions of the rough rolling after the said heating.
- Finishing rolling temperature 800 ° C. or more
- the finish rolling temperature is 800 ° C. or higher.
- the finish rolling temperature is 820 degreeC or more.
- the surface quality is preferably lowered to 940 ° C. or less because the surface properties are deteriorated due to the biting of the scale.
- the average cooling rate from the finish rolling temperature to 720 ° C. is 10 ° C./s or more.
- the length of the forced cooling zone (runout table) is limited, and when it is cooled at less than 10 ° C./s, the desired winding temperature cannot be achieved. Therefore, an average cooling rate shall be 10 degrees C / s or more.
- the upper limit is not particularly limited, but is substantially 200 ° C./s.
- the average cooling rate from 720 ° C. to the cooling stop temperature may be 10 ° C./s or more and less than 10 ° C./s.
- Winding temperature 580 ° C. or more and 720 ° C. or less
- the coiling temperature is lower than 580 ° C.
- decarburization does not proceed and the hardness of the steel sheet surface layer portion does not decrease.
- the coiling temperature exceeds 720 ° C., an internal oxide layer is formed on the surface layer portion of the steel sheet, which causes cracks during forming.
- the coiling temperature was set to 580 ° C. or higher and 720 ° C. or lower. Preferably they are 600 degreeC or more and 690 degrees C or less.
- the subsequent cold rolling step is a step of cold rolling the hot-rolled sheet after the hot rolling step.
- it is necessary to cold-roll the hot-rolled sheet after the hot rolling process.
- the cold rolling rate is often set to 30% or more and 80% or less due to restrictions on the production line.
- the pretreatment heating step to be performed subsequently refers to heating the cold-rolled plate to the Ac3 point which is the highest temperature after the cold-rolling step, and the cooling rate to 580 ° C is 5 ° C / cooling after the heating. cooling under conditions of s or higher, the heating and cooling allow the cold-rolled plate to stay at the Ac3 point or higher for 60 seconds or less, and the dew point at the Ac3 point or higher to be ⁇ 45 ° C. or higher and ⁇ 20 ° C. or lower.
- the time during which the cold-rolled sheet stays at 440 ° C. or higher and 530 ° C. or lower is set to 20 seconds or longer.
- Heating temperature (maximum temperature reached): Ac3 point or higher It is necessary to further advance decarburization in a continuous annealing line to reduce the hardness of the steel sheet surface layer. For this reason, it is necessary to pass through the continuous annealing line before the continuous hot dipping line (the processing performed in the continuous annealing line is pretreatment heating, and the processing performed in the furnace in the continuous hot dipping line is in the present invention. Annealing treatment). Ac3 point is the temperature at which the two-phase region of ferrite and austenite changes to the austenite single-phase region. In the treatment in the continuous annealing line, it is necessary to promote bainite transformation and finely disperse cementite.
- the heating temperature was set to Ac3 point or higher.
- the upper limit of the heating temperature is not particularly limited, but the temperature exceeding 1000 ° C. is preferably 1000 ° C. or less because the load due to the heat of the annealing furnace is large and the equipment life is shortened.
- the cooling rate to 580 ° C. is slow, ferrite transformation starts in the cooling process, and ferrite grain growth proceeds.
- the arrangement of finely dispersed cementite changes, and a fine martensite structure cannot be obtained. For this reason, it is necessary to suppress excessive ferrite grain growth.
- it is 7 degrees C / s or more.
- the cooling stop temperature of this cooling shall be 100 degrees C or less from controlling an average heating rate from 100 degrees C in the annealing process which is the next process.
- Dew point in temperature range of Ac3 point or higher: -45 ° C or higher and -20 ° C or lower Deheating point in the temperature range of Ac3 point or higher in heating and cooling is less than -45 ° C, so decarburization does not proceed. Will not drop.
- the dew point is set to ⁇ 45 ° C. or higher and ⁇ 20 ° C. or lower. Preferably, it is ⁇ 45 ° C. or higher and ⁇ 25 ° C. or lower.
- the dew point in the temperature range below the Ac3 point is not particularly limited and may be determined as appropriate.
- the temperature range in which the bainite transformation proceeds most in the present invention is a temperature range of 440 ° C. or more and 530 ° C. or less. Preferably they are 460 degreeC or more and 520 degrees C or less.
- the lower limit of the residence time was set to 20 seconds. Preferably it is 30 seconds or more.
- the upper limit of residence time is not specifically limited, 900 seconds or less are preferable for the reason of equipment restrictions.
- the steel sheet may be passed twice or more through a continuous annealing line for performing the pretreatment heating step.
- the number of sheet passing be 4 or less.
- the subsequent annealing step is to heat the cold-rolled sheet after the pretreatment heating step under the condition that the average heating rate from 100 ° C. to the highest temperature not less than (Ac3 point ⁇ 10) ° C. is 3.0 ° C./s or more. Then, the cold-rolled sheet heated up to the maximum temperature is cooled under the condition that the average cooling rate up to 560 ° C. is 10 ° C./s or more, and is cold-rolled to (Ac3 point ⁇ 10) ° C. or more in the heating and cooling. In this step, the time for which the plate is retained is 60 seconds or less, and the time for which the cold-rolled plate is retained at 440 ° C. or more and 530 ° C. or less in the cooling is 20 seconds or more and 180 seconds or less.
- Average heating rate from 100 ° C. to the highest temperature: 3.0 ° C./s or more 100 ° C. is a temperature at which C begins to diffuse, and an average heating rate of 100 ° C. or more at which C or Fe diffuses is 3.0 ° C. /
- finely dispersed cementite becomes coarse.
- Cementite becomes a martensite formation site, but fine cementite cannot be obtained when cementite is coarsened.
- austenite is coarsened and a desired average diameter of the martensite phase cannot be obtained.
- the average heating rate from 100 ° C. to the highest temperature reached 3.0 ° C./s or higher.
- a preferable heating rate is 4.0 ° C./s or more.
- the maximum temperature reached is (Ac3 point ⁇ 10) ° C. or higher. Unless heated to at least (Ac3 point ⁇ 10) ° C., the area ratio of the ferrite phase does not become 20% or less.
- a preferable maximum temperature is Ac3 point or higher.
- the average cooling rate up to 560 ° C. needs to be 10 ° C./s or more.
- the cooling stop temperature in this cooling is not particularly limited, but the cooling stop temperature is usually 460 to 540 ° C.
- the cooling rate to the cooling stop temperature after reaching 560 ° C. is not particularly limited, and may be 10 ° C./s or more and less than 10 ° C./s.
- (Ac3 point-10) Time to stay in the temperature range above 60 ° C: 60 seconds or less
- the time of staying in the temperature range of (Ac3 point ⁇ 10) ° C. or higher is 60 seconds or less, and preferably 50 seconds or less.
- Time of residence in a temperature range of 440 ° C. or more and 530 ° C. or less 20 seconds or more and 180 seconds or less.
- cooling is performed at a temperature of 440 ° C. or more and 530 ° C. or less. It is necessary to retain the cold-rolled plate in the region for 20 seconds or more.
- the preferred residence time is 25 seconds or more and 150 seconds or less.
- 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.
- a steel material having a thickness of 250 mm having the composition shown in Table 1 is subjected to a hot rolling process under the hot rolling conditions shown in Table 2 to form a hot rolled sheet, and then subjected to a cold rolling process under the cold rolling conditions shown in Table 2.
- a cold-rolled sheet was prepared, and the pretreatment heat treatment under the conditions shown in Table 2 was applied on a continuous annealing line, and the annealing conditions shown in Table 2 were applied on a continuous hot dipping plating line. Then, the plating process was performed and the alloying process was further performed as needed.
- the temperature of the plating bath immersed in the continuous hot dipping line is 460 ° C.
- the amount of plating 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.
- Ac3 point was measured using a thermal expansion measuring device. The Ac3 point was measured at an average heating rate of 5 ° C./s.
- Specimens were collected from the hot-dip galvanized steel sheet or alloyed hot-dip galvanized steel sheet obtained as described above and evaluated by the following method.
- 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.
- the bainite phase, bainite phase and martensite phase were separated from each other by image analysis, and the area ratio relative to the observation field was obtained.
- the average diameter of the martensite phase the area occupied by each grain of the martensite phase was determined by image analysis, and an equivalent circular diameter equal to the area was determined.
- the martensite connected to the portions is regarded as two, and the respective equivalent circle diameters were determined.
- the maximum distance between the martensites was determined by taking the longest portion in 10 fields as the maximum length.
- interval means the distance of the part where the outer periphery of a martensite and the outer periphery of an adjacent martensite are the nearest.
- the work hardening index is a value determined according to the method defined in JIS Z 2253 (1996), and the true strain range was determined from 0.02 to 0.05. This is because this region is the most sensitive region regarding the crack generation phenomenon due to the effect of work hardening in press working.
- (Iii) Hardness test The hardness of the steel sheet surface and the steel sheet interior was determined by the Vickers hardness test. The hardness of the steel sheet surface was determined by measuring a total of 20 points with a test load of 0.2 kgf from the steel sheet surface from which the plating layer had been removed by pickling, and the average value was determined. As for the hardness inside the steel plate, a total thickness of 5 points was measured at a test load of 1 kgf at a thickness of 1/4 t in a cross section parallel to the rolling direction, and an average value was obtained. When the average value of the hardness of the steel sheet surface was 95% or less (0.95 or less in the table) of the average value of the hardness inside the steel sheet, the steel sheet characteristics required in the present invention were obtained.
- a steel sheet having a tensile strength TS of 780 MPa or more and a high work hardening index was obtained.
- a comparative example out of the scope of the present invention particularly a steel sheet in which a desired ferrite area ratio has not been obtained, has a low tensile strength.
- the work hardening index was low.
- the hardness of the steel sheet surface was almost the same as that inside the steel sheet.
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Abstract
Description
[%N]-14[%Ti]/48≦0 (1)
(1)式における[%N]はN含有量、[%Ti]はTi含有量を意味する。
本発明の高強度めっき鋼板は、鋼板と、該鋼板上に形成されためっき層とを有する。鋼板、めっき層の順で説明する。
Cはマルテンサイトの硬さを上昇させ、フェライト変態を抑制する焼入性を持つ。引張強さが780MPa以上の鋼板を得るには少なくともC含有量を0.06%以上にすることが必要である。一方、C含有量が0.18%を上回るとマルテンサイト相の面積率が65%を上回り延性および成形性が失われる。そこで、C含有量は0.06%以上0.18%以下とする。望ましくは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はフェライト相からオーステナイト相への変態点(Ac3点)を上昇させるため、焼鈍時でのフェライト相を除去しにくくする。さらにSiはめっき層と鋼板表面との濡れ性を低下させるので、Siの過剰な含有は、不めっき等の欠陥の原因となる。本発明においてSi含有量は0.50%未満の範囲であれば許容される。望ましくは0.30%未満である。下限は特に定めないが、0.01%のSiは不可避的に鋼中に混入する場合がある。
Mnは、固溶強化(solid solution strengthening)により高強度化に寄与するうえ、Ac3変態点を低下させ焼鈍中におけるフェライト相を除去しやすくさせ、また鋼板の焼入性を向上させる効果がある。目的の鋼組織を得るにはMn含有量を1.9%以上とする必要がある。一方、Mn含有量が3.2%を上回るとベイナイト変態が進行せず結果的にマルテンサイト相の面積率が65%を上回る。このため、Mn含有量の上限を3.2%とする。好ましいMn含有量の範囲は2.0%以上3.0%以下である。
Pは、粒界に偏析して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0.03%以下とする。好ましくは0.02%以下である。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場合がある。
Sは、鋼中でMnSなどの介在物となった状態で存在する。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、ボイド生成の起点となりやすく、成形性にも悪影響がある。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。好ましくは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場合がある。
Alを製鋼の段階で脱酸剤として添加する場合、Alを0.02%以上含有することが好ましい。一方で、Al含有量が0.08%を超えるとアルミナなどの介在物の影響でフェライト変態が促進され引張強さが780MPaを下回る。したがって、Al含有量は0.08%以下とする。好ましくは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系窒化物はフェライト変態の核生成サイト(nucleation site)となるため、N含有量は極力低減する必要があり、上限を0.006%とする。好ましいN含有量は0.005%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場合がある。
Bは、変態前のオーステナイトの粒界に偏析しフェライト相の核生成を著しく遅延させる効果がありフェライト相の生成を抑える効果がある。この効果を得るには、B含有量を0.0002%以上にする必要がある。一方、B含有量が0.0030%を上回ると、焼入性の効果が飽和するばかりか、延性に悪影響がでる。以上から、B含有量は0.0002%以上0.0030%以下とする。望ましくは、0.0005%以上0.0020%以下である。
Nbは焼鈍中のオーステナイト粒の粗大化を抑制するため重要な元素である。Nb含有量が過剰になると、Nbを含む粗大な炭窒化物(炭化物、窒化物、炭窒化物の総称。以下この発明に置いて同じ)が析出するためフェライト相の面積率が増大する。オーステナイト粒の粗大化抑制のためには、Nb含有量を0.007%以上にする必要がある。一方、Nb含有量が0.030%を超えると、本発明で規定する製造条件では粗大なNb系炭窒化物が析出する。そこで、Nb含有量の上限を0.030%とする。好ましいNb含有量は、0.012%以上0.027%以下である。
Ti含有量が上記不等式を満たさず[%N]-14[%Ti]/48>0となる場合、NはBと結合するため焼入性が低下し、フェライト相の面積率が20%を上回ることとなる。[%N]-14[%Ti]/48≦0の範囲であれば、NはTiと結合した状態であるため、鋼板の焼入性は失われない。一方、過度にTiを含有させるとCと結合することによって炭化物を形成する。この炭化物は転位上に析出し、転位の運動を著しく阻害するため成形性が低下する要因となる。この観点から、(1)式左辺は-0.010以上であることが好ましい。より好ましくは-0.006以上である。
フェライト相は軟質な組織であり、フェライト相の含有量が20%を超えると、引張強さが780MPaを下回る。また、フェライト相は元素の溶解度が小さいため、フェライト相の含有量が過剰になると、焼鈍前組織で微細分散させたセメンタイトの配置を換えてしまい微細なマルテンサイト相も得られなくなる。したがって、フェライト相の含有量は極力低減することが望ましく、本発明においてフェライト相の含有量は20%以下に抑制する必要がある。望ましくは15%以下である。
ベイナイト相はフェライト相よりも硬度が高いうえ、マルテンサイト相を微細に生成させるために有効である。所望の鋼組織を得るために、ベイナイト相の含有量を35%以上とする必要がある。一方、ベイナイト相の含有量が90%を上回るとマルテンサイト間の間隔の最大長さ(最大距離)が5.0μmを上回り、良好な成形性が得られなくなる。好ましいベイナイト相の含有量は面積率で40%以上80%以下である。
マルテンサイト相の含有量およびマルテンサイト相の形態は、強度および成形性に大きな影響を与える。マルテンサイト相の含有量が面積率で10%を下回ると引張強さが780MPaを下回る。一方、マルテンサイト相の含有量が面積率で65%を上回ると延性および成形性が失われる。好ましいマルテンサイト相の含有量は、面積率で20%以上55%以下である。
本発明の高強度めっき鋼板の鋼組織では、円相当径で粒径が5.0μmを超える介在物の個数密度:400個/mm2以下である。粒径が5.0μmを超える介在物はフェライト相の核生成サイトとなりやすく、フェライト相の面積率での含有量が所望の範囲にならなくなる。ここで、5.0μmを超える介在物としては、AlもしくはTiを含む酸化物、Tiを含む窒化物、Nbを含む炭窒化物が挙げられる。
本発明の高強度めっき鋼板では、鋼板の表面硬さが、鋼板表面から厚み方向に1/4t(tは鋼板の厚み)の位置の硬さを100%としたときに、95%以下である。成形時の亀裂の多くは鋼板表面に発生する。本発明では、巻取温度の調整および焼鈍工程での鋼板表層部の脱炭により鋼板表層部の延性を向上させることで、成形性を向上させることができる。硬さと延性とは背反関係にあるため、鋼板表層部の延性は、鋼板表面の硬さの低下によって推測が可能である。鋼板表面の硬さが鋼板内部(鋼板表面から厚み方向に1/4t(tは鋼板の厚み)の位置)の硬さの95%であれば成形性に改善がみられる。好ましくは、鋼板の表面硬さが、鋼板表面から厚み方向に1/4t(tは鋼板の厚み)の位置の硬さを100%としたときに、90%以下である。曲げ加工による成形時の不具合は表面から100μmまでの範囲で亀裂の起点が生じている場合が多い。そのため、鋼板表層とは鋼板表面から厚み方向に100μmまでの範囲とする。
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1000℃を下回ると仕上げ圧延温度が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1200℃を上回ると、特に粗大なTiを含む窒化物の生成が促進され、5.0μmを超える介在物の個数密度が増大する。そのため、鋼素材の加熱温度は1000℃以上1200℃以下とした。望ましくは1020℃以上1150℃以下である。なお、上記加熱後の粗圧延の粗圧延条件については特に限定されない。
仕上げ圧延温度が800℃を下回ると、仕上げ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(duplex grain microstructure)となる。このため、冷間圧延時の板厚精度に悪影響をもたらし、焼鈍前の鋼組織でセメンタイトが微細に分散した形態とならない。したがって、仕上げ圧延温度は800℃以上とする。好ましくは820℃以上である。また、仕上げ圧延温度は過剰に高いとスケールの噛み混みにより表面性状が劣化するという理由で940℃以下が好ましい。
仕上げ圧延後、強制冷却により巻取温度直上まで冷却する。強制冷却帯(ランアウトテーブル)の長さには制約があり、10℃/s未満で冷却した場合は所望の巻取温度にならなくなる。そのため、平均冷却速度は10℃/s以上とする。なお、上限は特に限定されないが実質的に200℃/sである。なお、巻取温度が720℃未満の場合、720℃~冷却停止温度までの平均冷却速度は10℃/s以上でも10℃/s未満でもよい。
巻取で、スケールからの脱炭を促進させ、鋼板表層部のC濃度を低減させる。巻取温度が580℃を下回ると、脱炭が進行せず鋼板表層部の硬さが低下しない。一方、巻取温度が720℃を超えると鋼板表層部に内部酸化層が生成し、成形時に亀裂が発生する原因となる。このため、巻取温度は580℃以上720℃以下とした。好ましくは600℃以上690℃以下である。
連続焼鈍ラインで脱炭をさらに進行させ、鋼板表層部の硬さを低下させる必要がある。このため、連続溶融めっきラインの前に連続焼鈍ラインに通板させる必要がある(連続焼鈍ラインで行われる処理が前処理加熱であり、連続溶融めっきライン内の炉で行われる処理が本発明における焼鈍処理である)。Ac3点はフェライトおよびオーステナイトの二相域からオーステナイト単相域となるときの温度である。連続焼鈍ラインにおける処理では、ベイナイト変態を促進させセメンタイトを微細に分散させる必要がある。しかし、フェライト相の生成を十分に抑えないと、フェライト相は元素(C(炭素))の溶解度が小さいため、フェライト相内部にはセメンタイトは析出せずセメンタイトの分散形態が不均一となり、最終製品でマルテンサイトが微細に分散した形態とならない。そのため、この工程ではフェライト相を十分に除去する必要がある。そこで、加熱温度はAc3点以上とした。加熱温度の上限は特に限定されないが1000℃を超える温度は焼鈍炉の熱による負荷が大きく設備寿命を短くするため、1000℃以下が好ましい。
580℃までの冷却速度が遅い場合、冷却過程でフェライト変態が開始し、フェライト粒成長が進行する。フェライト粒が成長すると、微細分散させたセメンタイトの配置が変わり、微細なマルテンサイト組織が得られなくなる。このため、過度のフェライト粒成長は抑制する必要がある。これには、冷却開始温度(最高到達温度)から580℃までの平均冷却速度は5℃/s以上の強制冷却を施す必要がある。好ましくは7℃/s以上である。なお、この冷却の冷却停止温度は、次工程である焼鈍工程で100℃から平均加熱速度を制御することから、100℃以下とする。
加熱及び冷却においてAc3点以上の温度域での露点が-45℃を下回ると脱炭が進行しないため、鋼板表層部の硬さは低下しない。一方、上記露点が-20℃を上回ると鋼板表層部に内部酸化層や酸化鉄が生成されるため、成形性および表面性状が損なわれる。このため、上記露点は-45℃以上-20℃以下とした。好ましくは、-45℃以上-25℃以下である。なお、Ac3点未満の温度域における露点は特に限定されず適宜決定すればよい。
加熱及び冷却において冷延板がAc3点以上の状態にある時間が長いと、オーステナイト粒が粗大化し、微細にマルテンサイトが分散した鋼組織が得られなくなる。本発明では60秒までは許容できる。
440℃以上530℃以下の温度域でベイナイト変態進行により鋼板中に微細なセメンタイトを形成させることで、連続めっきライン通板時に微細なマルテンサイトを得ることが可能となる。本発明で最もベイナイト変態が進行する温度域は440℃以上530℃以下の温度域である。好ましくは460℃以上520℃以下である。ベイナイト変態を進行させるには、冷却における上記温度域での滞留時間を20秒以上にすることが必要となるため、滞留時間下限を20秒とした。好ましくは30秒以上である。滞留時間の上限は特に限定されないが、設備制約上という理由で900秒以下が好ましい。
100℃はCが拡散し始める温度であり、CもしくはFeが拡散する100℃以上の平均加熱速度が3.0℃/sを下回る加熱条件では、微細分散したセメンタイトが粗大化する。セメンタイトはマルテンサイト生成サイトとなるがセメンタイトが粗大化した状態では、微細なマルテンサイトを得ることができなくなる。さらに微細なマルテンサイトを得るには焼鈍中のオーステナイトの粗大化も抑制する必要がある。平均加熱速度が3.0℃/sを下回ると、オーステナイトが粗大化し所望のマルテンサイト相の平均径が得られなくなる。以上の通り、100℃から最高到達温度までの平均加熱速度が3.0℃/s以上とした。好ましい加熱速度は4.0℃/s以上である。ここで、最高到達温度は(Ac3点-10)℃以上である。少なくとも(Ac3点-10)℃まで加熱しなければ、フェライト相の面積率が20%以下とならない。好ましい最高到達温度はAc3点以上である。
上記加熱後の冷却において、560℃までの冷却速度が遅い場合、冷却過程でフェライト変態が開始し、過度にフェライト相が生成される。これを回避するには、560℃までの平均冷却速度を10℃/s以上にする必要がある。また、この冷却での冷却停止温度は特に限定されないが、通常、冷却停止温度は460~540℃になる。また、560℃になった以降の冷却停止温度までの冷却速度は特に限定されず、10℃/s以上でも10℃/s未満でもよい。
加熱及び冷却において(Ac3点-10)℃以上の温度域に冷延板が滞留される時間を60秒超えとすると、焼鈍中のオーステナイトが粗大化し微細なマルテンサイトが得られなくなる。以上の観点から、(Ac3点-10)℃以上の温度域に滞留される時間は60秒以下とし、50秒以下とすることが好ましい。
ベイナイト変態を促進させ、微細なマルテンサイトを含むベイナイト組織を得るには、冷却において440℃以上530℃以下の温度域に冷延板を20秒以上滞留させる必要がある。一方で、滞留時間が180秒を超えると、過度にベイナイト相が生成され、マルテンサイト相に接しないベイナイト相が多くなる。好ましい滞留時間は25秒以上150秒以下である。
各相の面積率は以下の手法により評価した。鋼板から、圧延方向に平行な断面(鋼板を置いた場合の鉛直かつ圧延方向に対し平行となる断面)が観察面となるよう切り出し、板厚中心部を1%ナイタール(nital)で腐食現出し、走査型電子顕微鏡で2000倍に拡大して板厚1/4t部を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。これらを画像解析によりベイナイト相、ベイナイト相および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。マルテンサイト相の平均径も画像解析によってマルテンサイト相の各粒が占める面積を求め、その面積と等しい相当円直径を求めた。マルテンサイト相が長さ0.5μm以下で連結した部分に対しては、その部分に連結するマルテンサイトをふたつと見なして、それぞれの相当円直径を求めた。マルテンサイト間の最大間隔長さは10視野で最も長い部分を最大長さとして求めた。上記間隔は、マルテンサイトの外周と、隣のマルテンサイトの外周とが最も近い部分の距離を意味する。
得られた鋼板から圧延方向に対して垂直方向(直角方向)にJIS5号引張試験片を作製し、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。表3において、引張強さ:780MPa以上、加工硬化指数(n値):0.16以上を本発明鋼で求める鋼板の機械的性質とした。ここで、加工硬化指数はJIS Z 2253(1996)で定める方法に従って求められる値であり、真ひずみ域が0.02から0.05から求めた。この領域はプレス加工において加工硬化の影響による亀裂発生現象に関して最も感受性が高い領域であるためである。
鋼板表面と鋼板内部の硬さはビッカース硬さ試験によって求めた。鋼板表面の硬さは、めっき層を酸洗により除去した鋼板表面から試験荷重0.2kgfで計20点測定し、平均値を求めた。鋼板内部の硬さは圧延方向に平行な断面の板厚1/4t部を試験荷重1kgfで計5点測定し、平均値を求めた。鋼板表面の硬さの平均値が鋼板内部の硬さの平均値の95%以下(表中の0.95以下)であれば、本発明で求める鋼板特性とした。
Claims (7)
- 鋼板と該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であって、
前記鋼板の成分組成は、質量%で、C:0.06%以上0.18%以下、Si:0.50%未満、Mn:1.9%以上3.2%以下、P:0.03%以下、S:0.005%以下、Al:0.08%以下、N:0.006%以下、B:0.0002%以上0.0030%以下、Nb:0.007%以上0.030%以下、および下記(1)式を満たすようにTiを含有し、残部がFeおよび不可避的不純物からなり、
前記鋼板の鋼組織は、フェライト相を面積率で20%以下(0%を含む)、ベイナイト相を面積率で35%以上90%以下、マルテンサイト相を面積率で10%以上65%以下含有し、かつ円相当径が5.0μmを超える介在物を個数密度で400個/mm2以下含有し、
前記マルテンサイト相を構成する粒状のマルテンサイトの平均粒径が4.0μm以下、マルテンサイト間の最大長さが5.0μm以下であり、
前記鋼板の表面硬さが、鋼板表面から厚み方向に1/4t(tは鋼板の厚み)の位置の硬さを100%としたときに、95%以下であることを特徴とする高強度めっき鋼板。
[%N]-14[%Ti]/48≦0 (1)
(1)式における[%N]はN含有量、[%Ti]はTi含有量を意味する。 - 前記成分組成は、さらに、質量%で、Cr:0.001%以上0.9%以下、Ni:0.001%以上0.5%以下、V:0.001%以上0.3%以下、Mo:0.001%以上0.3%以下、W:0.001%以上0.2%以下、Hf:0.001%以上0.3%以下の1種または2種以上を含有する成分組成であることを特徴とする請求項1に記載の高強度めっき鋼板。
- 前記成分組成は、さらに、質量%で、REM、Mg、Caの1種または2種以上を合計で0.0002%以上0.01%以下含有することを特徴とする請求項1または2に記載の高強度めっき鋼板。
- 前記めっき層が溶融めっき層、合金化溶融めっき層のいずれかであることを特徴とする請求項1~3のいずれかに記載の高強度めっき鋼板。
- 請求項1から3のいずれかに記載の成分組成を有する鋼素材を、1000℃以上1200℃以下で加熱し、800℃以上の仕上げ圧延温度で仕上げ圧延終了後、仕上げ圧延温度から720℃までの平均冷却速度が10℃/s以上で冷却し、580℃以上720℃以下で巻き取る熱間圧延工程と、
前記熱間圧延工程後に熱延板を冷間圧延する冷間圧延工程と、
前記冷間圧延工程後に、冷延板を最高到達温度であるAc3点以上まで加熱し、該加熱後の冷延板を580℃までの冷却速度が5℃/s以上の条件で冷却し、該加熱及び該冷却においてAc3点以上に冷延板が滞留される時間を60秒以下、Ac3点以上における露点を-45℃以上-20℃以下とし、該冷却において440℃以上530℃以下に冷延板が滞留される時間を20秒以上とする前処理加熱工程と、
前記前処理加熱工程後の冷延板を100℃から(Ac3点-10)℃以上の最高到達温度までの平均加熱速度が3.0℃/s以上の条件で加熱し、最高到達温度まで加熱された冷延板を560℃までの平均冷却速度が10℃/s以上の条件で冷却し、該加熱及び該冷却において(Ac3点-10)℃以上に冷延板が滞留される時間を60秒以下とし、該冷却において440℃以上530℃以下に冷延板が滞留される時間を20秒以上180秒以下とする焼鈍工程と、
前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めっき工程とを有することを特徴とする高強度めっき鋼板の製造方法。 - 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ことを特徴とする請求項5に記載の高強度めっき鋼板の製造方法。
- 前記めっき処理が、溶融亜鉛めっき処理、合金化溶融亜鉛めっき処理のいずれかであることを特徴とする請求項5又は6に記載の高強度めっき鋼板の製造方法。
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WO2023013372A1 (ja) * | 2021-08-02 | 2023-02-09 | 日本製鉄株式会社 | 高強度鋼板 |
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