WO2022044492A1 - 熱延鋼板 - Google Patents
熱延鋼板 Download PDFInfo
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- WO2022044492A1 WO2022044492A1 PCT/JP2021/022662 JP2021022662W WO2022044492A1 WO 2022044492 A1 WO2022044492 A1 WO 2022044492A1 JP 2021022662 W JP2021022662 W JP 2021022662W WO 2022044492 A1 WO2022044492 A1 WO 2022044492A1
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- 229910000831 Steel Inorganic materials 0.000 title claims abstract description 155
- 239000010959 steel Substances 0.000 title claims abstract description 155
- 229910001566 austenite Inorganic materials 0.000 claims abstract description 50
- 229910000734 martensite Inorganic materials 0.000 claims abstract description 45
- 229910052751 metal Inorganic materials 0.000 claims abstract description 44
- 239000002184 metal Substances 0.000 claims abstract description 44
- 229910000859 α-Fe Inorganic materials 0.000 claims abstract description 36
- 230000000717 retained effect Effects 0.000 claims abstract description 29
- 239000000126 substance Substances 0.000 claims abstract description 19
- 239000000203 mixture Substances 0.000 claims abstract description 15
- 239000013078 crystal Substances 0.000 claims description 32
- 239000002344 surface layer Substances 0.000 claims description 15
- 239000012535 impurity Substances 0.000 claims description 8
- 229910052725 zinc Inorganic materials 0.000 claims description 8
- 229910052721 tungsten Inorganic materials 0.000 claims description 7
- 229910052726 zirconium Inorganic materials 0.000 claims description 6
- 229910052797 bismuth Inorganic materials 0.000 claims description 4
- 238000005098 hot rolling Methods 0.000 description 54
- 238000001816 cooling Methods 0.000 description 42
- 238000010008 shearing Methods 0.000 description 42
- 238000005096 rolling process Methods 0.000 description 39
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- 229910001563 bainite Inorganic materials 0.000 description 12
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- 229910052761 rare earth metal Inorganic materials 0.000 description 10
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- 238000001887 electron backscatter diffraction Methods 0.000 description 7
- 239000011701 zinc Substances 0.000 description 7
- KSOKAHYVTMZFBJ-UHFFFAOYSA-N iron;methane Chemical group C.[Fe].[Fe].[Fe] KSOKAHYVTMZFBJ-UHFFFAOYSA-N 0.000 description 6
- 239000011159 matrix material Substances 0.000 description 6
- 229910052758 niobium Inorganic materials 0.000 description 6
- 238000012545 processing Methods 0.000 description 6
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- 229910052720 vanadium Inorganic materials 0.000 description 6
- 238000006243 chemical reaction Methods 0.000 description 5
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- 238000001878 scanning electron micrograph Methods 0.000 description 5
- 229910052719 titanium Inorganic materials 0.000 description 5
- 238000002441 X-ray diffraction Methods 0.000 description 4
- 229910001567 cementite Inorganic materials 0.000 description 4
- 238000010438 heat treatment Methods 0.000 description 4
- 229910052749 magnesium Inorganic materials 0.000 description 4
- 150000001247 metal acetylides Chemical class 0.000 description 4
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- 230000001376 precipitating effect Effects 0.000 description 4
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- 229910052796 boron Inorganic materials 0.000 description 3
- 229910052791 calcium Inorganic materials 0.000 description 3
- 229910052799 carbon Inorganic materials 0.000 description 3
- 229910052804 chromium Inorganic materials 0.000 description 3
- 229910052802 copper Inorganic materials 0.000 description 3
- 238000000691 measurement method Methods 0.000 description 3
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- 238000005498 polishing Methods 0.000 description 3
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- 238000005728 strengthening Methods 0.000 description 3
- 239000013585 weight reducing agent Substances 0.000 description 3
- CURLTUGMZLYLDI-UHFFFAOYSA-N Carbon dioxide Chemical compound O=C=O CURLTUGMZLYLDI-UHFFFAOYSA-N 0.000 description 2
- HCHKCACWOHOZIP-UHFFFAOYSA-N Zinc Chemical compound [Zn] HCHKCACWOHOZIP-UHFFFAOYSA-N 0.000 description 2
- 238000010521 absorption reaction Methods 0.000 description 2
- 238000009825 accumulation Methods 0.000 description 2
- 229910045601 alloy Inorganic materials 0.000 description 2
- 239000000956 alloy Substances 0.000 description 2
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- 229910052747 lanthanoid Inorganic materials 0.000 description 2
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- 229910052717 sulfur Inorganic materials 0.000 description 2
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- 229910018134 Al-Mg Inorganic materials 0.000 description 1
- 229910021365 Al-Mg-Si alloy Inorganic materials 0.000 description 1
- 229910018467 Al—Mg Inorganic materials 0.000 description 1
- 238000012935 Averaging Methods 0.000 description 1
- UFHFLCQGNIYNRP-UHFFFAOYSA-N Hydrogen Chemical compound [H][H] UFHFLCQGNIYNRP-UHFFFAOYSA-N 0.000 description 1
- 229910001122 Mischmetal Inorganic materials 0.000 description 1
- 229910000990 Ni alloy Inorganic materials 0.000 description 1
- BPQQTUXANYXVAA-UHFFFAOYSA-N Orthosilicate Chemical compound [O-][Si]([O-])([O-])[O-] BPQQTUXANYXVAA-UHFFFAOYSA-N 0.000 description 1
- VYPSYNLAJGMNEJ-UHFFFAOYSA-N Silicium dioxide Chemical compound O=[Si]=O VYPSYNLAJGMNEJ-UHFFFAOYSA-N 0.000 description 1
- 229910001035 Soft ferrite Inorganic materials 0.000 description 1
- UCKMPCXJQFINFW-UHFFFAOYSA-N Sulphide Chemical compound [S-2] UCKMPCXJQFINFW-UHFFFAOYSA-N 0.000 description 1
- 238000007545 Vickers hardness test Methods 0.000 description 1
- 229910007570 Zn-Al Inorganic materials 0.000 description 1
- 239000002253 acid Substances 0.000 description 1
- 230000002411 adverse Effects 0.000 description 1
- 239000012670 alkaline solution Substances 0.000 description 1
- XAGFODPZIPBFFR-UHFFFAOYSA-N aluminium Chemical compound [Al] XAGFODPZIPBFFR-UHFFFAOYSA-N 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
- 229910002092 carbon dioxide Inorganic materials 0.000 description 1
- 239000001569 carbon dioxide Substances 0.000 description 1
- 238000005266 casting Methods 0.000 description 1
- 239000008119 colloidal silica Substances 0.000 description 1
- 230000000052 comparative effect Effects 0.000 description 1
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- 239000000706 filtrate Substances 0.000 description 1
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- 229910052739 hydrogen Inorganic materials 0.000 description 1
- 239000001257 hydrogen Substances 0.000 description 1
- 238000003384 imaging method Methods 0.000 description 1
- 150000002500 ions Chemical class 0.000 description 1
- 239000007788 liquid Substances 0.000 description 1
- 229910052748 manganese Inorganic materials 0.000 description 1
- 230000007246 mechanism Effects 0.000 description 1
- 238000012986 modification Methods 0.000 description 1
- 230000004048 modification Effects 0.000 description 1
- 150000004767 nitrides Chemical class 0.000 description 1
- 229910052757 nitrogen Inorganic materials 0.000 description 1
- 238000010899 nucleation Methods 0.000 description 1
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- 229910052760 oxygen Inorganic materials 0.000 description 1
- 230000000737 periodic effect Effects 0.000 description 1
- 229910052698 phosphorus Inorganic materials 0.000 description 1
- 229910001568 polygonal ferrite Inorganic materials 0.000 description 1
- 230000008569 process Effects 0.000 description 1
- 239000002994 raw material Substances 0.000 description 1
- 238000011160 research Methods 0.000 description 1
- 229910052706 scandium Inorganic materials 0.000 description 1
- 238000004544 sputter deposition Methods 0.000 description 1
- 238000005482 strain hardening Methods 0.000 description 1
- 229910052718 tin Inorganic materials 0.000 description 1
- WFKWXMTUELFFGS-UHFFFAOYSA-N tungsten Chemical group [W] WFKWXMTUELFFGS-UHFFFAOYSA-N 0.000 description 1
- 239000010937 tungsten Substances 0.000 description 1
- 230000000007 visual effect Effects 0.000 description 1
- 229910052727 yttrium Inorganic materials 0.000 description 1
Images
Classifications
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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
- C21D9/00—Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor
- C21D9/46—Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor for sheet metals
-
- C—CHEMISTRY; METALLURGY
- C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
- C22C—ALLOYS
- C22C38/00—Ferrous alloys, e.g. steel alloys
- C22C38/001—Ferrous alloys, e.g. steel alloys containing N
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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
- C22C38/002—Ferrous alloys, e.g. steel alloys containing In, Mg, or other elements not provided for in one single group C22C38/001 - C22C38/60
-
- C—CHEMISTRY; METALLURGY
- C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
- C22C—ALLOYS
- C22C38/00—Ferrous alloys, e.g. steel alloys
- C22C38/005—Ferrous alloys, e.g. steel alloys containing rare earths, i.e. Sc, Y, Lanthanides
-
- C—CHEMISTRY; METALLURGY
- C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
- C22C—ALLOYS
- C22C38/00—Ferrous alloys, e.g. steel alloys
- C22C38/008—Ferrous alloys, e.g. steel alloys containing tin
-
- C—CHEMISTRY; METALLURGY
- C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
- C22C—ALLOYS
- C22C38/00—Ferrous alloys, e.g. steel alloys
- C22C38/02—Ferrous alloys, e.g. steel alloys containing silicon
-
- C—CHEMISTRY; METALLURGY
- C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
- C22C—ALLOYS
- C22C38/00—Ferrous alloys, e.g. steel alloys
- C22C38/04—Ferrous alloys, e.g. steel alloys containing manganese
-
- C—CHEMISTRY; METALLURGY
- C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
- C22C—ALLOYS
- C22C38/00—Ferrous alloys, e.g. steel alloys
- C22C38/06—Ferrous alloys, e.g. steel alloys containing aluminium
-
- 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
- C22C38/08—Ferrous alloys, e.g. steel alloys containing nickel
-
- 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
- C22C38/10—Ferrous alloys, e.g. steel alloys containing cobalt
-
- C—CHEMISTRY; METALLURGY
- C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
- C22C—ALLOYS
- C22C38/00—Ferrous alloys, e.g. steel alloys
- C22C38/12—Ferrous alloys, e.g. steel alloys containing tungsten, tantalum, molybdenum, vanadium, or niobium
-
- C—CHEMISTRY; METALLURGY
- 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
- C22C38/16—Ferrous alloys, e.g. steel alloys containing copper
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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
- C22C38/18—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
- C22C38/44—Ferrous alloys, e.g. steel alloys containing chromium with nickel with molybdenum or tungsten
-
- C—CHEMISTRY; METALLURGY
- C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
- C22C—ALLOYS
- C22C38/00—Ferrous alloys, e.g. steel alloys
- C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
- C22C38/40—Ferrous alloys, e.g. steel alloys containing chromium with nickel
- C22C38/58—Ferrous alloys, e.g. steel alloys containing chromium with nickel with more than 1.5% by weight of 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
- C21D2211/00—Microstructure comprising significant phases
- C21D2211/001—Austenite
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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
- C21D2211/005—Ferrite
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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
- C21D2211/008—Martensite
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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
- C21D8/00—Modifying the physical properties by deformation combined with, or followed by, heat treatment
- C21D8/02—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips
- C21D8/0205—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips of ferrous alloys
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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
- 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
- C21D8/021—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips involving a particular fabrication or treatment of ingot or slab
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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
- 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
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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
- C21D8/00—Modifying the physical properties by deformation combined with, or followed by, heat treatment
- C21D8/02—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips
- C21D8/0247—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips characterised by the heat treatment
- C21D8/0263—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips characterised by the heat treatment following hot rolling
Definitions
- the present invention relates to a hot-rolled steel sheet. Specifically, the present invention relates to a hot-rolled steel sheet that is formed into various shapes by press working or the like and is used, and in particular, a hot-rolled steel sheet having high strength and excellent hole-expanding property and shearing workability.
- This application claims priority based on Japanese Patent Application No. 2020-143745 filed in Japan on August 27, 2020, the contents of which are incorporated herein by reference.
- Patent Document 1 high strength for automobiles having excellent collision resistance and moldability, in which retained austenite having an average crystal grain size of 5 ⁇ m or less is dispersed in ferrite having an average crystal grain size of 10 ⁇ m or less.
- Steel plates are disclosed.
- austenite undergoes martensitic transformation during processing and exhibits large elongation due to transformation-induced plasticity, but the formation of hard martensite impairs the hole-spreading property.
- Patent Document 1 discloses that not only ductility but also hole widening property is improved by miniaturizing ferrite and retained austenite.
- Patent Document 2 discloses a high-strength steel plate having a tensile strength of 980 MPa or more, which has excellent elongation and hole-expanding properties, in which a second phase composed of retained austenite and / or martensite is finely dispersed in crystal grains. There is.
- Patent Documents 3 and 4 disclose a high-strength hot-rolled steel sheet having excellent ductility and hole-expanding property, and a method for manufacturing the same.
- the product is cooled to a temperature range of 720 ° C. or lower within 1 second after the completion of hot rolling, and is allowed to stay in a temperature range of more than 500 ° C. and 720 ° C. or lower for a residence time of 1 to 20 seconds, and then 350 to.
- Patent Document 4 bainite is mainly used, and an appropriate amount of polygonal ferrite and retained austenite are contained, and the average of grains surrounded by grain boundaries having a crystal orientation difference of 15 ° or more in the steel structure excluding retained austenite.
- a high-strength hot-rolled steel sheet having a particle size of 15 ⁇ m or less and having good ductility and stretch flangeability is disclosed.
- Patent Documents 1 to 4 are all techniques for improving strength and press formability at the time of drilling, but there is no mention of a technique for improving shear workability, and parts are press formed. It is presumed that post-treatment will be required at this stage and the manufacturing cost will increase.
- the present invention has been made in view of the above-mentioned problems of the prior art, and an object of the present invention is to provide a hot-rolled steel sheet having high strength and excellent drilling and shearing properties.
- the present inventors have obtained the following findings (a) to (f) as a result of intensive studies on the chemical composition of the hot-rolled steel sheet and the relationship between the metallographic structure and the mechanical properties. Completed the invention.
- having excellent shearing workability means that the straightness of the boundary between the fracture surface and the shearing surface in the end face after shearing is high. This is because if the linearity of the boundary between the fracture surface and the sheared surface on the end face after shearing is high, it can be considered that the end face accuracy after shearing is excellent.
- having excellent strength or high strength means that the tensile strength is 980 MPa or more.
- the matrix structure of the metal structure is hard. That is, it is preferable that the fraction of a soft structure such as ferrite or bainite is as small as possible.
- (D) Specifically, setting the standard deviation of the Mn concentration to a certain value or less, and controlling the periodicity of the metal structure and the uniformity of the metal structure are the fracture surfaces of the end face after shearing. It is effective for improving the linearity of the boundary with the cross section.
- the slab heating step and the subsequent hot rolling step are important. For example, after holding for 900 seconds or more in the temperature range of 700 to 850 ° C., further heating and holding for 6000 seconds or more in the temperature range of 1100 ° C. or higher, and 90% or more in total in the temperature range of 850 ° C. to 1100 ° C. It is effective to perform hot rolling so that the plate thickness is reduced.
- the rolling reduction ratio and rolling temperature of the final stage of hot rolling are controlled within a predetermined range, and the stress applied to the steel sheet after rolling one stage before the final stage of hot rolling and before rolling in the final stage is applied. It is effective to set it to 170 kPa or more and to reduce the stress applied to the steel sheet after rolling in the final stage of hot rolling and until the steel sheet is cooled to 800 ° C. to less than 200 kPa. Under such hot rolling conditions, fine and equiaxed recrystallized austenite grains can be produced, and by combining with the subsequent cooling conditions, the periodicity of the structure morphology can be reduced.
- the gist of the present invention made based on the above findings is as follows.
- (1) The hot-rolled steel sheet according to one aspect of the present invention has a chemical composition of mass%. C: 0.040 to 0.250%, Si: 0.05 to 3.00%, Mn: 1.00 to 4.00%, sol.
- the rest consists of Fe and impurities
- the metallographic structure In area%, Martensite and tempered martensite total more than 92.0% and less than 100.0%, Retained austenite is less than 3.0%, Ferrite is less than 5.0%
- the E value indicating the periodicity of the metal structure is 11.0 or more, and the I value indicating the uniformity of the metal structure is less than 1.020.
- the standard deviation of the Mn concentration is 0.60% by mass or less, It is characterized by having a tensile strength of 980 MPa or more.
- the hot-rolled steel sheet according to (1) above may have an average crystal grain size of less than 3.0 ⁇ m on the surface layer.
- the hot-rolled steel sheet according to (1) or (2) above has a chemical composition of% by mass.
- Ti 0.005 to 0.300%, Nb: 0.005 to 0.100%, V: 0.005 to 0.500%, Cu: 0.01-2.00%, Cr: 0.01-2.00%, Mo: 0.01-1.00%, Ni: 0.02-2.00%, B: 0.0001 to 0.0100%, Ca: 0.0005-0.0200%, Mg: 0.0005-0.0200%, REM: 0.0005 to 0.1000%, and Bi: 0.0005 to 0.020% It may contain one or more selected from the group consisting of.
- the hot-rolled steel sheet according to the above aspect according to the present invention it is possible to obtain a hot-rolled steel sheet having excellent strength, drilling property and shearing workability. Further, according to the above-mentioned preferred embodiment according to the present invention, it is possible to obtain a hot-rolled steel sheet having the above-mentioned various characteristics and further suppressing the occurrence of bending internal cracking, that is, having excellent bending internal cracking resistance. can.
- the hot-rolled steel sheet according to the above aspect of the present invention is suitable as an industrial material used for automobile members, mechanical structural members, and building members.
- the hot-rolled steel sheet according to this embodiment has a mass% of C: 0.040 to 0.250%, Si: 0.05 to 3.00%, Mn: 1.00 to 4.00%, sol. .. Al: 0.001 to 0.500%, P: 0.100% or less, S: 0.0300% or less, N: 0.1000% or less, O: 0.0100% or less, and the balance: Fe and impurities including.
- C 0.040 to 0.250%
- Si 0.05 to 3.00%
- Mn 1.00 to 4.00%
- sol. .. Al 0.001 to 0.500%
- P 0.100% or less
- S 0.0300% or less
- N 0.1000% or less
- O 0.0100% or less
- Fe and impurities including Each element will be described in detail below.
- C 0.040 to 0.250%
- C increases the surface integral of the hard phase.
- C increases the strength of martensite by binding to precipitation-strengthening elements such as Ti, Nb, and V. If the C content is less than 0.040%, it becomes difficult to obtain the desired strength. Further, when the C content is less than 0.040%, the ferrite fraction increases and the I value also increases due to the influence of the flat ferrite structure. Therefore, the C content is 0.040% or more.
- the C content is preferably 0.060% or more, more preferably 0.070% or more.
- the C content exceeds 0.250%, the formation of low-strength pearlite is promoted, and the area fraction of martensite and tempered martensite is lowered, so that the strength of the hot-rolled steel sheet is lowered. Further, when the C content exceeds 0.250%, the flat cementite structure is increased, and the E value is lowered due to the influence of the formation of the carbide region having a small luminance difference. Therefore, the C content is set to 0.250% or less. The C content is preferably 0.150% or less.
- Si 0.05 to 3.00% Si has the effect of delaying the precipitation of cementite.
- the surface integral ratio of martensite and tempered martensite can be increased, and the strength of the steel sheet can be increased by solid solution strengthening.
- Si has an action of deoxidizing the steel to make it sound (suppressing the occurrence of defects such as blow holes in the steel). If the Si content is less than 0.05%, the effect of the above action cannot be obtained. Further, when the Si content is less than 0.05%, the flat cementite structure is increased, and the I value is also increased due to the influence of the formation of the carbide region having a small luminance difference. Therefore, the Si content is set to 0.05% or more.
- the Si content is preferably 0.50% or more and 1.00% or more.
- the Si content exceeds 3.00%, the surface texture and chemical conversion treatment property of the steel sheet, as well as the hole expanding property and weldability are significantly deteriorated , and the A3 transformation point is significantly increased. This makes it difficult to perform hot rolling in a stable manner.
- the Si content exceeds 3.00%, the ferrite fraction increases and the E value decreases due to the influence of the flat ferrite structure. Therefore, the Si content is set to 3.00% or less.
- the Si content is preferably 2.70% or less, more preferably 2.50% or less.
- Mn 1.00 to 4.00% Mn has the effect of suppressing ferrite transformation and increasing the strength of the steel sheet. If the Mn content is less than 1.00%, a tensile strength of 980 MPa or more cannot be obtained. Therefore, the Mn content is set to 1.00% or more.
- the Mn content is preferably 1.50% or more, 2.00% or more, and 2.30% or more.
- the Mn content exceeds 4.00%, the crystal orientation difference of the crystal grains in the hard phase becomes non-uniform due to the segregation of Mn, and the boundary between the fracture surface and the sheared surface at the end face after shearing is performed. The linearity of is reduced. Therefore, the Mn content is set to 4.00% or less.
- the Mn content is preferably 3.70% or less and 3.50% or less.
- Al 0.001 to 0.500%
- Al has the effect of deoxidizing steel to make the steel sheet sound, and by suppressing the precipitation of cementite from austenite, it increases the area fraction of martensite and tempered martensite.
- sol. If the Al content is less than 0.001%, the effect of the above action cannot be obtained. Therefore, sol.
- the Al content is 0.001% or more.
- sol. The Al content is preferably 0.010% or more.
- sol. If the Al content exceeds 0.500%, the above effects are saturated and economically unfavorable.
- the Al content is 0.500% or less. sol.
- the Al content is preferably 0.400%% or less and 0.300% or less.
- sol. Al means an acid-soluble Al, and indicates a solid solution Al existing in the steel in a solid solution state.
- P 0.100% or less
- P is an element generally contained as an impurity, but it is also an element having an action of increasing the strength by strengthening the solid solution. Therefore, P may be positively contained, but P is an element that is easily segregated, and when the P content exceeds 0.100%, the decrease in hole widening property due to grain boundary segregation becomes remarkable. .. Therefore, the P content is limited to 0.100% or less.
- the P content is preferably 0.030% or less.
- the lower limit of the P content does not need to be specified, but is preferably 0.001% from the viewpoint of refining cost.
- S 0.0300% or less
- S is an element contained as an impurity and forms sulfide-based inclusions in the steel to reduce the hole expanding property of the hot-rolled steel sheet.
- the S content exceeds 0.0300%, the hole expanding property of the steel sheet is significantly lowered. Therefore, the S content is limited to 0.0300% or less.
- the S content is preferably 0.0050% or less.
- the lower limit of the S content does not need to be specified, but is preferably 0.0001% from the viewpoint of refining cost.
- N 0.1000% or less
- N is an element contained in steel as an impurity and has an effect of reducing the hole expandability of the steel sheet.
- the N content is set to 0.1000% or less.
- the N content is preferably 0.0800% or less, more preferably 0.0700% or less.
- the lower limit of the N content does not need to be specified, but may be 0.0001%.
- the N content is 0.0010% or more in order to promote the precipitation of carbonitride. It is preferably 0.0020% or more, and more preferably 0.0020% or more.
- O 0.0100% or less
- O forms a coarse oxide that becomes a starting point of fracture when it is contained in a large amount in steel, and causes brittle fracture and hydrogen-induced cracking. Therefore, the O content is limited to 0.0100% or less.
- the O content is preferably 0.0080% or less and 0.0050% or less.
- the O content may be 0.0005% or more and 0.0010% or more in order to disperse a large number of fine oxides during deoxidation of the molten steel.
- the balance of the chemical composition of the hot-rolled steel sheet according to the present embodiment may consist of Fe and impurities.
- the impurities mean those mixed from ore as a raw material, scrap, manufacturing environment, etc., and are allowed as long as they do not adversely affect the hot-rolled steel sheet according to the present embodiment. do.
- the hot-rolled steel sheet according to the present embodiment may optionally contain Ti, Nb, V, Cu, Cr, Mo, Ni, B, Ca, Mg, REM, Bi, Zr, Co, Zn, W and Sn. It may be contained as an element. When the above optional element is not contained, the lower limit of the content is 0%.
- the above optional elements will be described in detail.
- Ti 0.005 to 0.300%
- Nb 0.005 to 0.100%
- V 0.005 to 0.500% Since Ti, Nb and V all precipitate as carbides or nitrides in steel and have an action of refining the metal structure by the pinning effect, one or more of these elements are contained. May be good. In order to obtain the effect of the above action more reliably, the Ti content should be 0.005% or more, the Nb content should be 0.005% or more, or the V content should be 0.005% or more. It is preferable to do so. That is, it is preferable that the content of even one of Ti, Nb and V is 0.005% or more.
- the Ti content is 0.300% or less
- the Nb content is 0.100% or less
- the V content is 0.500% or less.
- Cu 0.01 to 2.00%, Cr: 0.01 to 2.00%, Mo: 0.01 to 1.00%, Ni: 0.02 to 2.00% and B : 0.0001-0.0100%
- Cu, Cr, Mo, Ni and B all have the effect of enhancing the hardenability of the hot-rolled steel sheet.
- Cu and Mo have an action of precipitating carbides in steel at a low temperature to increase the strength.
- Ni contains Cu, it has an effect of effectively suppressing the grain boundary cracking of the slab caused by Cu. Therefore, one or more of these elements may be contained.
- the Cu has an action of enhancing the hardenability of the hot-rolled steel sheet and an action of precipitating as carbide in the steel at a low temperature to increase the strength of the steel sheet.
- the Cu content is preferably 0.01% or more, and more preferably 0.05% or more.
- the Cu content is set to 2.00% or less.
- the Cu content is preferably 1.50% or less and 1.00% or less.
- the Cr content is preferably 0.01% or more and 0.05% or more.
- the Cr content is set to 2.00% or less.
- Mo has an action of enhancing the hardenability of the hot-rolled steel sheet and an action of precipitating carbides in the steel to increase the strength.
- the Mo content is preferably 0.01% or more and 0.02% or more.
- the Mo content is set to 1.00% or less.
- the Mo content is preferably 0.50% or less and 0.20% or less.
- Ni has the effect of enhancing the hardenability of hot-rolled steel sheets. Further, when Ni contains Cu, it has an effect of effectively suppressing the grain boundary cracking of the slab caused by Cu. In order to obtain the effect of the above action more reliably, the Ni content is preferably 0.02% or more. Since Ni is an expensive element, it is economically unfavorable to contain it in a large amount. Therefore, the Ni content is set to 2.00% or less.
- B has an effect of enhancing the hardenability of the hot-rolled steel sheet.
- the B content is preferably 0.0001% or more and 0.0002% or more.
- the B content is set to 0.0100% or less.
- the B content is preferably 0.0050% or less.
- Ca 0.0005 to 0.0200%
- Mg 0.0005 to 0.0200%
- REM 0.0005 to 0.1000%
- Bi 0.0005 to 0.020%
- All of Ca, Mg and REM have an effect of enhancing the hole expanding property of the hot-rolled steel sheet by adjusting the shape of the inclusions to a preferable shape.
- Bi has an effect of improving the formability of the hot-rolled steel sheet by miniaturizing the solidified structure. Therefore, one or more of these elements may be contained.
- any one or more of Ca, Mg, REM and Bi is 0.0005% or more.
- the Ca content or Mg content exceeds 0.0200%, or when the REM content exceeds 0.1000%, inclusions are excessively generated in the steel, which in turn reduces the hole expandability of the steel sheet. May be caused. Further, even if the Bi content exceeds 0.020%, the effect of the above action is saturated, which is economically unfavorable. Therefore, the Ca content and Mg content are 0.0200% or less, the REM content is 0.1000% or less, and the Bi content is 0.020% or less. The Bi content is preferably 0.010% or less.
- REM refers to a total of 17 elements composed of Sc, Y and lanthanoids
- the content of REM refers to the total content of these elements. In the case of lanthanoids, they are industrially added in the form of misch metal.
- the chemical composition of the hot-rolled steel sheet described above may be measured by a general analysis method.
- ICP-AES Inductively Coupled Plasma-Atomic Emission Spectrum
- sol. Al may be measured by ICP-AES using a filtrate obtained by heat-decomposing the sample with an acid.
- C and S may be measured by using the combustion-infrared absorption method
- N may be measured by using the inert gas melting-heat conductivity method
- O may be measured by using the inert gas melting-non-dispersive infrared absorption method.
- the metal structure of the hot-rolled steel sheet according to the present embodiment will be described.
- the metal structure is% by area, martensite and tempered martensite are more than 92.0% and 100.0% or less in total, and the retained austenite is less than 3.0%.
- the ferrite is less than 5.0%
- the E value indicating the periodicity of the metal structure is 11.0 or more
- the I value indicating the uniformity of the metal structure is less than 1.020
- Mn The standard deviation of the concentration is 0.60% by mass or less. Therefore, the hot-rolled steel sheet according to the present embodiment can obtain excellent strength, drilling property, and shear workability.
- the metal structure of the cross section parallel to the rolling direction at a depth of 1/4 of the plate thickness from the surface and a center position in the plate width direction is defined. The reason is that the metallographic structure at this position shows the typical metallic structure of the steel sheet.
- Retained austenite is a metal structure that exists as a face-centered cubic lattice even at room temperature. Residual austenite has the effect of enhancing the hole expandability of hot-rolled steel sheets by transformation-induced plasticity (TRIP). On the other hand, retained austenite transforms into high-carbon martensite during shearing, which hinders stable crack generation and reduces the straightness of the fracture surface and the boundary between the sheared surface and the boundary after shearing. It becomes.
- the surface integral of the retained austenite is 3.0% or more, the above effect becomes apparent and the shearing workability of the hot-rolled steel sheet deteriorates (the linearity of the boundary between the fracture surface and the shearing surface on the end face after shearing deteriorates). Not only does it reduce the ability to expand holes. Therefore, the surface integral of retained austenite is less than 3.0%.
- the surface integral of the retained austenite is preferably 1.5% or less, more preferably less than 1.0%. Since the smaller the amount of retained austenite, the more preferable it is, the surface integral of the retained austenite may be 0%.
- Ferrite is generally a soft metal structure. If a predetermined amount or more of ferrite is contained, the desired strength may not be obtained, or the region of the sheared surface on the end face after shearing may increase. Increasing the area of the sheared surface on the end face after shearing is not preferable because the linearity of the boundary between the fracture surface and the sheared surface on the end face decreases.
- the surface integral of ferrite is set to less than 5.0%.
- the surface integral of ferrite is preferably 3.0% or less, more preferably 2.0% or less, and even more preferably less than 1.0%. Since the smaller the amount of ferrite, the more preferable it is, the surface integral of ferrite may be 0%.
- Methods for measuring the area fraction of retained austenite include X-ray diffraction, EBSP (Electron Backscattering Diffraction Pattern) analysis, and magnetic measurement methods, and the measured values may differ depending on the measurement method. ..
- the surface integral of retained austenite is measured by X-ray diffraction.
- the depth position of 1/4 of the plate thickness of the hot-rolled steel plate (1/8 depth from the surface to the plate thickness to 3 from the surface to the plate thickness).
- the surface integral of ferrite is measured by the following method.
- the cross section parallel to the rolling direction is mirror-finished and polished at room temperature with colloidal silica containing no alkaline solution for 8 minutes to remove the strain introduced into the surface layer of the sample.
- Electron backscatter at an arbitrary position in the longitudinal direction of the sample cross section in a region of 50 ⁇ m in length, 1/8 depth from the surface to 3/8 depth from the surface to the plate thickness, at a measurement interval of 0.1 ⁇ m.
- Crystal orientation information is obtained by measuring by diffraction method.
- an EBSD analyzer composed of a thermal field emission scanning electron microscope (JSM-7001F manufactured by JEOL) and an EBSD detector (DVC5 type detector manufactured by TSL) is used.
- the degree of vacuum in the EBSD analyzer is 9.6 ⁇ 10 -5 Pa or less
- the acceleration voltage is 15 kV
- the irradiation current level is 13
- the electron beam irradiation level is 62.
- the area fraction of pearlite can be obtained by identifying the crystal grains in which ferrite and cementite are deposited in layers from the reflected electron image and calculating the area fraction of the crystal grains.
- the obtained crystal orientation information is used for the "Grain Average Misorition" function installed in the software "OIM Analysis (registered trademark)" attached to the EBSD analysis device. Therefore, the region where the Grain Average Measurement value is 1.0 ° or less is determined to be ferrite.
- the surface integral of ferrite is obtained by obtaining the surface integral of the region determined to be ferrite.
- the maximum value of "Grain Average IQ" in the ferrite region is set under the condition that the 5 ° grain boundary is defined as the crystal grain boundary in the remaining region (the region where the Grain Average Misorition value exceeds 1.0 °).
- the region exceeding I ⁇ / 2 is extracted as bainite.
- Total area fraction of martensite and tempered martensite More than 92.0% and 100.0% or less
- the total area fraction of martensite and tempered martensite is 92.0% or less. If there is, the desired strength cannot be obtained. Therefore, the total surface integral of martensite and tempered martensite shall be more than 92.0%. It is preferably 95.0% or more, 97.0% or more, and 99.0% or more. The larger the total surface integral of martensite and tempered martensite, the more preferable, so it may be 100.0%.
- the method for measuring the surface integral of martensite and tempered martensite will be described below.
- a Vickers indentation is imprinted in the vicinity of the observation position.
- the contamination on the surface layer is removed by polishing, leaving the structure of the observation surface, and night-game etching is performed.
- the same visual field as the EBSD observation surface is observed by SEM at a magnification of 3000 times.
- the region having a substructure in the grain and where cementite is precipitated with a plurality of variants is determined to be tempered martensite.
- the region where the brightness is high and the underlying structure is not exposed by etching is judged as "martensite and retained austenite".
- the surface integral of martensite is obtained by subtracting the surface integral of retained austenite obtained by the above-mentioned X-ray diffraction from the surface integral of the obtained "martensite and retained austenite”.
- the sum of the surface integrals of martensite and the surface integrals of tempered martensite the sum of the surface integrals of martensite and tempered martensite is obtained.
- a method such as buffing using alumina particles having a particle diameter of 0.1 ⁇ m or less or Ar ion sputtering may be used.
- the hot-rolled steel sheet according to the present embodiment may contain one or two types of bainite and pearlite having a total area fraction of 0% or more and less than 8.0% as the residual structure.
- E value 11.0 or more
- I value less than 1.020
- E value 11.0 or more
- I value less than 1.020
- the fracture surface and the sheared surface at the end face after shearing are controlled by controlling the E (Entry) value indicating the periodicity of the metal structure and the I (Inverse diffused moment norm) value indicating the uniformity of the metal structure. And enhance the linearity of the boundary.
- the E value represents the periodicity of the metal structure.
- the E value decreases when the luminance is periodically arranged due to the formation of a band-shaped structure or the like, that is, when the periodicity of the metal structure is high.
- the E value is less than 11.0, the linearity of the boundary between the fracture surface and the sheared surface on the end surface after shearing tends to decrease.
- a metal structure having a high periodicity that is, a low E value
- cracks are generated and a fracture surface is formed along a plurality of band-shaped structures existing near the starting point, starting from the periodically arranged structure.
- the E value is set to 11.0 or more. It is preferably 11.1 or more, and more preferably 11.2 or more. The higher the E value, the more preferable, and the upper limit is not particularly specified, but it may be 13.5 or less, 13.0 or less, 12.5 or less, or 12.0 or less.
- the I value represents the uniformity of the metallographic structure, and increases as the area of the region with constant brightness increases.
- a high I value means that the uniformity of the metal structure is high.
- the uniformity of the metal structure is high, that is, when the I value is high, cracks are likely to occur from the tip of the shear tool due to the influence of the precipitate and element concentration difference in the crystal grain and the hardness difference due to the soft ferrite phase. Become. As a result, the linearity of the boundary between the fracture surface and the sheared surface at the end surface after shearing tends to decrease.
- the I value is set to less than 1.020. It is preferably 1.015 or less, and more preferably 1.010 or less.
- the lower limit of the I value is not particularly specified, but may be 0.900 or more, 0.950 or more, or 1.000 or more.
- the imaging region of the SEM image captured for calculating the E value and the I value is a position at a depth of 1/4 of the plate thickness from the steel plate surface (the plate thickness from the surface) in the cross section parallel to the rolling direction. A region from 1/8 depth to 3/8 depth of the plate thickness from the surface) and at the center position in the plate width direction.
- a SU-6600 Shotkey electron gun manufactured by Hitachi High-Technologies Corporation is used to take SEM images, the emitter is tungsten, and the acceleration voltage is 1.5 kV. Under the above settings, an SEM image is output at a magnification of 1000 times and a gray scale of 256 gradations.
- the obtained SEM image was cut out in an area of 880 ⁇ 880 pixels, and the tile grid size was 8 ⁇ 8 smoothing, which was described in Non-Patent Document 3 and the contrast enhancement limit magnification was 2.0. Perform the conversion process. Except for 90 degrees, the SEM image after smoothing is rotated counterclockwise every 1 degree from 0 degrees to 179 degrees, and an image is created every 1 degree to obtain a total of 179 images. .. Next, for each of these 179 images, the frequency values of the luminance between adjacent pixels are collected in the form of a matrix by using the GLCM method described in Non-Patent Document 1.
- the matrix P of is calculated. Further, the E value and the I value are calculated using the following equations (1) and (2) described in Non-Patent Document 2, respectively. In the following equations (1) and (2), the value in the i-th row and j-th column of the matrix P is expressed as Pij .
- Standard deviation of Mn concentration 0.60% by mass or less
- the standard deviation of Mn concentration at the 1/4 depth position of the plate thickness and the center position in the plate width direction from the surface of the hot-rolled steel sheet according to the present embodiment is It is 0.60% by mass or less.
- the standard deviation of the Mn concentration is preferably 0.50% by mass or less, more preferably 0.47% by mass or less.
- the lower limit of the standard deviation of the Mn concentration is preferably smaller as the value is smaller from the viewpoint of suppressing excessive burrs, but the practical lower limit is 0.10% by mass due to the limitation of the manufacturing process.
- the depth position is 1/4 of the plate thickness from the surface of the steel plate (1/8 depth from the surface to the plate thickness to the plate thickness from the surface).
- the standard deviation of the Mn concentration is measured by measuring the center position in the plate width direction with an electron probe microanalyzer (EPMA) (3/8 depth region). The measurement conditions are that the acceleration voltage is 15 kV, the magnification is 5000 times, and the distribution image in the range of 20 ⁇ m in the sample rolling direction and 20 ⁇ m in the sample plate thickness direction is measured. More specifically, the measurement interval is set to 0.1 ⁇ m, and the Mn concentration at 40,000 or more points is measured. Next, the standard deviation of the Mn concentration is obtained by calculating the standard deviation based on the Mn concentration obtained from all the measurement points.
- the bending internal cracking becomes remarkable in the steel sheet having a tensile strength of 980 MPa class or more. Further, the present inventors have found that the finer the crystal grain size of the surface layer of the hot-rolled steel sheet, the more the local strain concentration is suppressed and the less likely it is that internal bending cracks occur.
- the average crystal grain size of the surface layer of the hot-rolled steel sheet is preferably less than 3.0 ⁇ m. More preferably, it is 2.5 ⁇ m or less.
- the surface layer is a region from the surface of the hot-rolled steel sheet to a depth of 50 ⁇ m from the surface.
- the crystal grain size of the surface layer is measured by using the above-mentioned EBSP-OIM method.
- the analysis was performed in a region of 1200 times magnification and 40 ⁇ m ⁇ 30 ⁇ m in at least 5 fields.
- a place where the angle difference between adjacent measurement points is 5 ° or more is defined as a grain boundary, and the crystal grain size of the area average is calculated.
- the obtained area average crystal grain size is taken as the average crystal grain size of the surface layer. Since retained austenite is not a structure generated by a phase transformation at 600 ° C. or lower and has no effect of dislocation accumulation, retained austenite is not included in the analysis in this measurement method. In the EBSP-OIM method, retained austenite having a crystal structure of fcc can be excluded from the analysis target.
- the hot-rolled steel sheet according to this embodiment has a tensile (maximum) strength of 980 MPa or more. If the tensile strength is less than 980 MPa, the applicable parts are limited, and the contribution of weight reduction of the vehicle body is small.
- the upper limit is not particularly limited, but may be 1780 MPa from the viewpoint of suppressing mold wear.
- the tensile strength is measured according to JIS Z 2241: 2011 using the No. 5 test piece of JIS Z 2241: 2011.
- the sampling position of the tensile test piece may be 1/4 of the end portion in the plate width direction, and the direction perpendicular to the rolling direction may be the longitudinal direction.
- the hot-rolled steel sheet according to the present embodiment preferably has a hole expansion ratio ⁇ of 55% or more.
- the hole expansion ratio ⁇ is 55% or more, the applicable parts are not limited, and a hot-rolled steel sheet that greatly contributes to weight reduction of the vehicle body can be obtained.
- the upper limit does not have to be limited.
- the hole expansion ratio ⁇ is measured according to JIS Z 2256: 2010 using a No. 5 test piece of JIS Z 2241: 2011.
- the sampling position of the drilled test piece may be 1/4 of the end portion of the hot-rolled steel plate in the plate width direction.
- the plate thickness of the hot-rolled steel sheet according to the present embodiment is not particularly limited, but may be 0.5 to 8.0 mm.
- the thickness of the hot-rolled steel sheet according to the present embodiment may be 0.5 mm or more. It is preferably 1.2 mm or more and 1.4 mm or more.
- the plate thickness may be 8.0 mm or less. It is preferably 6.0 mm or less.
- the hot-rolled steel sheet according to the present embodiment having the above-mentioned chemical composition and metal structure may be provided with a plating layer on the surface for the purpose of improving corrosion resistance or the like to be a surface-treated steel sheet.
- the plating layer may be an electroplating layer or a hot-dip plating layer. Examples of the electroplating layer include electrogalvanization, electroZn—Ni alloy plating, and the like.
- hot-dip plating layer examples include hot-dip zinc plating, alloyed hot-dip zinc plating, hot-dip aluminum plating, hot-dip Zn-Al alloy plating, hot-dip Zn-Al-Mg alloy plating, and hot-dip Zn-Al-Mg-Si alloy plating.
- the amount of plating adhered is not particularly limited and may be the same as before. Further, it is also possible to further improve the corrosion resistance by subjecting an appropriate chemical conversion treatment (for example, application and drying of a silicate-based chromium-free chemical conversion treatment liquid) after plating.
- an appropriate chemical conversion treatment for example, application and drying of a silicate-based chromium-free chemical conversion treatment liquid
- a suitable method for producing a hot-rolled steel sheet according to the present embodiment having the above-mentioned chemical composition and metal structure is as follows.
- the slab is heated under predetermined conditions and then hot-rolled, accelerated and cooled to a predetermined temperature range, and the cooling history after winding is controlled. This is very important.
- the following steps (1) to (8) are sequentially performed.
- the temperature of the slab and the temperature of the steel plate in this embodiment refer to the surface temperature of the slab and the surface temperature of the steel plate.
- the slab is held in a temperature range of 700 ° C. to 850 ° C. for 900 seconds or longer, then further heated and held in a temperature range of 1100 ° C. or higher for 6000 seconds or longer.
- Hot rolling is performed in a temperature range of 850 to 1100 ° C. so that the total plate thickness is reduced by 90% or more.
- a stress of 170 kPa or more is applied to the steel sheet after rolling one step before the final stage of hot rolling and before rolling in the final stage.
- the rolling reduction in the final stage of hot rolling is set to 8% or more, and hot rolling is completed so that the rolling completion temperature Tf is 900 ° C. or higher and less than 960 ° C.
- the stress applied to the steel sheet after the final stage of hot rolling and before the steel sheet is cooled to 800 ° C. is set to less than 200 kPa.
- the average cooling rate is 50 ° C / sec or higher, which is expressed by the following formula ⁇ 1>. Accelerated cooling to a temperature of T1 (° C.).
- T1 (° C.) 770-270 x [C] -90 x [Mn] -37 x [Ni] -70 x [Cr] -83 x [Mo] ... ⁇ 1>
- the [element symbol] in each formula indicates the content (mass%) of each element in the steel. If the element is not contained, 0 is substituted.
- a hot-rolled steel sheet having a metal structure excellent in strength, hole expansion and shear workability can be stably manufactured. That is, by appropriately controlling the slab heating conditions and the hot rolling conditions, the Mn segregation can be reduced and the austenite before transformation can be equiaxed, and in combination with the cooling conditions after hot rolling described later, A hot-rolled steel sheet having a desired metal structure can be stably produced.
- the slab for hot rolling a slab obtained by continuous casting or a slab obtained by casting / slabging may be used. It can be used, and if necessary, hot-rolled or cold-rolled products can be used. It is preferable that the slab to be subjected to hot rolling is held in a temperature range of 700 ° C. to 850 ° C. during heating for 900 seconds or longer, and then further heated and held in a temperature range of 1100 ° C. or higher for 6000 seconds or longer. For holding in the temperature range of 700 ° C. to 850 ° C., the temperature of the steel sheet may be changed in this temperature range or may be constant. Further, in the holding in the temperature range of 1100 ° C. or higher, the temperature of the steel sheet may be changed in the temperature range of 1100 ° C. or higher, or may be constant.
- the holding time in the temperature range of 1100 ° C. or higher is preferably 6000 seconds or longer.
- the temperature held for 6000 seconds or longer is preferably 1100 ° C. or higher.
- Hot rolling reduction rate A total plate thickness reduction of 90% or more in the temperature range of 850 to 1100 ° C.
- the stress applied to the steel sheet is preferably 170 kPa or more. This makes it possible to reduce the number of crystal grains having a crystal orientation of ⁇ 110 ⁇ ⁇ 001> among the recrystallized austenite after rolling one step before the final step. Since ⁇ 110 ⁇ ⁇ 001> has a crystal orientation that is difficult to recrystallize, recrystallization due to reduction in the final stage can be effectively promoted by suppressing the formation of this crystal orientation.
- the band-like structure of the hot-rolled steel sheet is improved, the periodicity of the metal structure is reduced, and the E value is increased.
- the stress applied to the steel sheet is less than 170 kPa, it may not be possible to achieve an E value of 11.0 or more.
- the stress applied to the steel sheet is more preferably 190 kPa or more.
- the stress applied to the steel sheet can be controlled by adjusting the roll rotation speed during tandem rolling.
- the formation of ferrite in the final structure (metal structure of the hot-rolled steel sheet after production) can be suppressed, and a high-strength hot-rolled steel sheet can be obtained.
- Tf to less than 960 ° C.
- coarsening of the austenite particle size can be suppressed, the periodicity of the metal structure can be reduced, and the E value can be set to 11.0 or more.
- the average cooling rate referred to here is the temperature drop width of the steel sheet from the start of accelerated cooling (when the steel sheet is introduced into the cooling equipment) to T1 (° C), and the temperature of the steel sheet is T1 (° C) from the start of accelerated cooling. ) Is divided by the time required to reach.
- Tf-50 ° C or lower After cooling to a temperature range of hot rolling completion temperature Tf-50 ° C or lower, by setting the average cooling rate to T1 (° C) to 50 ° C / sec or higher, ferrite transformation, bainite transformation and / or The pearlite transformation can be suppressed and TS ⁇ 980 MPa can be obtained.
- the average cooling rate of accelerated cooling is preferably 300 ° C./sec or less.
- the cooling after the completion of hot rolling it is more preferable to cool to the temperature range of the hot rolling completion temperature Tf-50 ° C. within 1.0 second after the completion of hot rolling. That is, it is more preferable that the cooling amount for 1 second after the completion of hot rolling is 50 ° C. or higher. This is because the growth of austenite crystal grains that have been refined by hot rolling can be suppressed.
- cooling with a large average cooling rate is performed immediately after the completion of hot rolling, for example, cooling water. May be sprayed onto the surface of the steel sheet.
- the crystal grain size of the surface layer can be miniaturized and the bending internal crack resistance of the hot-rolled steel sheet can be improved.
- the average cooling rate to T1 (° C) or less is 50 ° C / sec. Accelerated cooling may be performed as described above.
- the average cooling rate from T1 (° C.) to the take-up temperature is 50 ° C./sec or more.
- T1 (° C.) It is preferable that the average cooling rate from bainite to the winding temperature is 50 ° C./sec or more.
- the average cooling rate here is a value obtained by dividing the temperature drop width of the steel sheet from T1 (° C.) to the winding temperature by the time required from when the steel sheet temperature reaches T1 (° C.) to winding. It means that.
- the average cooling rate from T1 (° C.) to the winding temperature is set to 50 ° C./sec or more.
- the winding temperature is preferably 350 ° C. or lower.
- the transformation driving force from austenite to bcc can be increased, and the deformation strength of austenite can be increased. Therefore, the hard phase is uniformly distributed during the martensitic transformation from austenite, and the variation can be improved.
- the I value can be reduced, and the linearity of the boundary between the fracture surface and the sheared surface at the end face after shearing can be improved. That is, excellent shear workability can be obtained.
- the winding temperature is preferably 350 ° C. or lower.
- the conditions in the examples are one condition example adopted for confirming the feasibility and effect of the present invention.
- the present invention is not limited to this one-condition example.
- the present invention can adopt various conditions as long as the gist of the present invention is not deviated and the object of the present invention is achieved.
- the area fraction of the metal structure, the E value, the I value, the standard deviation of the Mn concentration, and the average crystal grain size of the surface layer were obtained by the above-mentioned method.
- the obtained measurement results are shown in Tables 5A to 6B.
- the remaining tissue was one or two of bainite and pearlite.
- the tensile strength was evaluated in accordance with JIS Z 2241: 2011.
- the test piece was JIS Z 2241: 2011 No. 5 test piece.
- the sampling position of the tensile test piece was 1/4 from the end in the plate width direction, and the direction perpendicular to the rolling direction was the longitudinal direction.
- the hole expansion ratio was measured according to JIS Z 2256: 2010 using a No. 5 test piece of JIS Z 2241: 2011.
- the sampling position of the drilled test piece was 1/4 from the end of the hot-rolled steel sheet in the plate width direction.
- FIG. 1A is a schematic view of an end surface parallel to the rolling direction of the punched hole
- FIG. 1B is a schematic view of a side surface of the punched hole.
- the sagging is an R-shaped smooth surface
- the shearing surface is a punched end surface separated by shear deformation
- the fracture surface is a punched end surface separated by cracks generated from the vicinity of the cutting edge after the completion of shear deformation.
- a burr is a surface having protrusions protruding from the lower surface of a hot-rolled steel sheet.
- the linearity at the boundary between the fracture surface and the shear was measured by the method described later, and the maximum value of the obtained linearity was calculated.
- the maximum value of the obtained linearity is less than 1.045, it is a hot-rolled steel sheet having high linearity at the boundary between the fracture surface and the sheared surface at the end face after shearing, that is, excellent in shearing workability. As a result, it was judged to be acceptable.
- the maximum value of the obtained linearity is 1.045 or more, the linearity of the boundary between the fracture surface and the sheared surface at the end face after shearing is low, that is, the hot-rolled steel sheet having poor shearing workability. If so, it was judged to be unacceptable.
- the linearity at the boundary between the fracture surface and the shear was obtained by the following method. As shown in FIG. 1 (b), the points of the boundary between the shear plane and the fracture surface (points A and B in FIG. 1 (b)) were determined with respect to the end face. The length of the distance x connecting these points A and B with a straight line was measured. Next, the length y of the curve along the fracture surface-shear plane boundary was measured. The value obtained by dividing the obtained y by x was taken as the linearity at the boundary between the fracture surface and the shear.
- the hot-rolled steel sheet according to the present invention it is possible to provide a hot-rolled steel sheet having excellent strength, drilling property and shearing workability. Further, according to the above-mentioned preferred embodiment according to the present invention, it is possible to obtain a hot-rolled steel sheet having the above-mentioned various characteristics and further suppressing the occurrence of bending internal cracking, that is, having excellent bending internal cracking resistance. can.
- the hot-rolled steel sheet according to the present invention is suitable as an industrial material used for automobile members, mechanical structural members, and building members.
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Abstract
Description
本願は、2020年8月27日に、日本に出願された特願2020-143745号に基づき優先権を主張し、その内容をここに援用する。
なお、優れたせん断加工性を有するとは、せん断加工後の端面における破断面とせん断面との境界の直線性が高いことを示す。せん断加工後の端面における破断面とせん断面との境界の直線性が高ければ、せん断加工後の端面精度に優れているとみなすことができるためである。また、優れた強度または高い強度を有するとは、引張強さが980MPa以上であることを示す。
(1)本発明の一態様に係る熱延鋼板は、化学組成が、質量%で、
C:0.040~0.250%、
Si:0.05~3.00%、
Mn:1.00~4.00%、
sol.Al:0.001~0.500%、
P:0.100%以下、
S:0.0300%以下、
N:0.1000%以下、
O:0.0100%以下、
Ti:0~0.300%、
Nb:0~0.300%、
V:0~0.500%、
Cu:0~2.00%、
Cr:0~2.00%、
Mo:0~1.00%、
Ni:0~2.00%、
B:0~0.0100%、
Ca:0~0.0200%、
Mg:0~0.0200%、
REM:0~0.1000%、
Bi:0~0.020%、
Zr、Co、ZnおよびWのうち1種または2種以上:合計で0~1.00%、並びに
Sn:0~0.05%を含有し、
残部がFeおよび不純物からなり、
金属組織が、
面積%で、
マルテンサイトおよび焼き戻しマルテンサイトが合計で92.0%超、100.0%以下であり、
残留オーステナイトが3.0%未満であり、
フェライトが5.0%未満であり、
前記金属組織の周期性を示すE値が11.0以上であり、前記金属組織の均一性を示すI値が1.020未満であり、
Mn濃度の標準偏差が0.60質量%以下であり、
引張強さが980MPa以上であることを特徴とする。
(2)上記(1)に記載の熱延鋼板は、表層の平均結晶粒径が3.0μm未満であってもよい。
(3)上記(1)または(2)に記載の熱延鋼板は、前記化学組成が、質量%で、
Ti:0.005~0.300%、
Nb:0.005~0.100%、
V:0.005~0.500%、
Cu:0.01~2.00%、
Cr:0.01~2.00%、
Mo:0.01~1.00%、
Ni:0.02~2.00%、
B:0.0001~0.0100%、
Ca:0.0005~0.0200%、
Mg:0.0005~0.0200%、
REM:0.0005~0.1000%、および
Bi:0.0005~0.020%
からなる群から選択される1種または2種以上を含有してもよい。
本発明の上記態様に係る熱延鋼板は、自動車部材、機械構造部材さらには建築部材に用いられる工業用素材として好適である。
以下に「~」を挟んで記載する数値限定範囲には、下限値および上限値がその範囲に含まれる。「未満」または「超」と示す数値には、その値が数値範囲に含まれない。以下の説明において、鋼板の化学組成に関する%は特に指定しない限り質量%である。
本実施形態に係る熱延鋼板は、質量%で、C:0.040~0.250%、Si:0.05~3.00%、Mn:1.00~4.00%、sol.Al:0.001~0.500%、P:0.100%以下、S:0.0300%以下、N:0.1000%以下、O:0.0100%以下、並びに、残部:Feおよび不純物を含む。以下に各元素について詳細に説明する。
Cは、硬質相の面積分率を上昇させる。また、Cは、Ti、Nb、V等の析出強化元素と結合することで、マルテンサイトの強度を上昇させる。C含有量が0.040%未満では、所望の強度を得ることが困難となる。また、C含有量が0.040%未満では、フェライト分率が増大し、平坦なフェライト組織の影響によりI値も増大する。したがって、C含有量は0.040%以上とする。C含有量は、好ましくは0.060%以上、より好ましくは0.070%以上である。一方、C含有量が0.250%超では、強度の低いパーライトの生成が促進され、マルテンサイトおよび焼き戻しマルテンサイトの面積分率が低下することで、熱延鋼板の強度が低下する。また、C含有量が0.250%超では、平坦なセメンタイト組織が増大し、輝度差の小さい炭化物の領域が生成する影響によりE値が低下する。したがって、C含有量は0.250%以下とする。C含有量は好ましくは0.150%以下である。
Siは、セメンタイトの析出を遅延させる作用を有する。この作用により、マルテンサイトおよび焼き戻しマルテンサイトの面積分率を高めることができ、また固溶強化により鋼板の強度を高めることができる。また、Siは脱酸により鋼を健全化する(鋼にブローホールなどの欠陥が生じることを抑制する)作用を有する。Si含有量が0.05%未満では、上記作用による効果を得ることができない。また、Si含有量が0.05%未満では、平坦なセメンタイト組織が増大し、輝度差の小さい炭化物の領域が生成する影響でI値も増大する。したがって、Si含有量は0.05%以上とする。Si含有量は、好ましくは0.50%以上、1.00%以上である。しかし、Si含有量が3.00%超では、鋼板の表面性状および化成処理性、さらには穴広げ性および溶接性が著しく劣化するとともに、A3変態点が著しく上昇する。これにより、安定して熱間圧延を行うことが困難になる。また、Si含有量が3.00%超では、フェライト分率が増大し、平坦なフェライト組織の影響でE値が減少する。したがって、Si含有量は3.00%以下とする。Si含有量は、好ましくは2.70%以下、より好ましくは2.50%以下である。
Mnは、フェライト変態を抑制して鋼板を高強度化する作用を有する。Mn含有量が1.00%未満では、980MPa以上の引張強さを得ることができない。したがって、Mn含有量は1.00%以上とする。Mn含有量は、好ましくは1.50%以上、2.00%以上、2.30%以上である。一方、Mn含有量が4.00%超では、Mnの偏析に起因して、硬質相中の結晶粒の結晶方位差が不均一となり、せん断加工後の端面における破断面とせん断面との境界の直線性が低下する。したがって、Mn含有量は4.00%以下とする。Mn含有量は、好ましくは3.70%以下、3.50%以下である。
Alは、Siと同様に、鋼を脱酸して鋼板を健全化する作用を有するとともに、オーステナイトからのセメンタイトの析出を抑制することで、マルテンサイトおよび焼き戻しマルテンサイトの面積分率を増加させる作用を有する。sol.Al含有量が0.001%未満では上記作用による効果を得ることができない。したがって、sol.Al含有量は、0.001%以上とする。sol.Al含有量は、好ましくは0.010%以上である。一方、sol.Al含有量が0.500%超では、上記効果が飽和するとともに経済的に好ましくないため、sol.Al含有量は0.500%以下とする。sol.Al含有量は、好ましくは0.400%%以下、0.300%以下である。
なお、本実施形態においてsol.Alとは、酸可溶性Alを意味し、固溶状態で鋼中に存在する固溶Alのことを示す。
Pは、一般的に不純物として含有される元素であるが、固溶強化により強度を高める作用を有する元素でもある。したがって、Pを積極的に含有させてもよいが、Pは偏析し易い元素であり、P含有量が0.100%を超えると、粒界偏析に起因する穴広げ性の低下が顕著となる。したがって、P含有量は、0.100%以下に制限する。P含有量は、好ましくは0.030%以下である。P含有量の下限は特に規定する必要はないが、精錬コストの観点から、0.001%とすることが好ましい。
Sは、不純物として含有される元素であり、鋼中に硫化物系介在物を形成して熱延鋼板の穴広げ性を低下させる。S含有量が0.0300%を超えると、鋼板の穴広げ性が著しく低下する。したがって、S含有量は0.0300%以下に制限する。S含有量は、好ましくは0.0050%以下である。S含有量の下限は特に規定する必要はないが、精錬コストの観点から、0.0001%とすることが好ましい。
Nは、不純物として鋼中に含有される元素であり、鋼板の穴広げ性を低下させる作用を有する。N含有量が0.1000%超では、鋼板の穴広げ性が著しく低下する。したがって、N含有量は0.1000%以下とする。N含有量は、好ましくは0.0800%以下であり、さらに好ましくは0.0700%以下である。N含有量の下限は特に規定する必要はないが、0.0001%としてもよい。後述するようにTi、NbおよびVの1種または2種以上を含有させて金属組織の微細化を図る場合には、炭窒化物の析出を促進させるためにN含有量は0.0010%以上とすることが好ましく、0.0020%以上とすることがより好ましい。
Oは、鋼中に多く含まれると破壊の起点となる粗大な酸化物を形成し、脆性破壊や水素誘起割れを引き起こす。そのため、O含有量は0.0100%以下に制限する。O含有量は、0.0080%以下、0.0050%以下とすることが好ましい。溶鋼の脱酸時に微細な酸化物を多数分散させるために、O含有量は0.0005%以上、0.0010%以上としてもよい。
Ti、NbおよびVは、いずれも、鋼中に炭化物または窒化物として析出し、ピン止め効果によって金属組織を微細化する作用を有するため、これらの元素の1種または2種以上を含有させてもよい。上記作用による効果をより確実に得るためには、Ti含有量を0.005%以上とするか、Nb含有量を0.005%以上とするか、あるいはV含有量を0.005%以上とすることが好ましい。すなわち、Ti、NbおよびVの1種でもその含有量を0.005%以上とすることが好ましい。しかし、これらの元素を過剰に含有させても、上記作用による効果が飽和して経済的に好ましくない。したがって、Ti含有量は0.300%以下とし、Nb含有量は0.100%以下とし、V含有量は0.500%以下とする。
Cu、Cr、Mo、NiおよびBは、いずれも、熱延鋼板の焼入性を高める作用を有する。また、CuおよびMoは低温で鋼中に炭化物を析出して強度を高める作用を有する。さらに、Niは、Cuを含有させる場合においては、Cuに起因するスラブの粒界割れを効果的に抑制する作用を有する。したがって、これらの元素の1種または2種以上を含有させてもよい。
Ca、MgおよびREMは、いずれも、介在物の形状を好ましい形状に調整することにより、熱延鋼板の穴広げ性を高める作用を有する。また、Biは、凝固組織を微細化することにより、熱延鋼板の成形性を高める作用を有する。したがって、これらの元素の1種または2種以上を含有させてもよい。上記作用による効果をより確実に得るためには、Ca、Mg、REMおよびBiのいずれか1種以上を0.0005%以上とすることが好ましい。しかし、Ca含有量またはMg含有量が0.0200%を超えると、あるいはREM含有量が0.1000%を超えると、鋼中に介在物が過剰に生成され、却って鋼板の穴広げ性を低下させる場合がある。また、Bi含有量を0.020%超としても、上記作用による効果は飽和してしまい、経済的に好ましくない。したがって、Ca含有量およびMg含有量を0.0200%以下、REM含有量を0.1000%以下、並びにBi含有量を0.020%以下とする。Bi含有量は、好ましくは0.010%以下である。
ここで、REMは、Sc、Yおよびランタノイドからなる合計17元素を指し、上記REMの含有量は、これらの元素の合計含有量を指す。ランタノイドの場合、工業的にはミッシュメタルの形で添加される。
Zr、Co、ZnおよびWについて、本発明者らは、これらの元素を合計で1.00%以下含有させても、本実施形態に係る熱延鋼板の効果は損なわれないことを確認している。そのため、Zr、Co、ZnおよびWのうち1種または2種以上を合計で1.00%以下含有させてもよい。
また、本発明者らは、Snを少量含有させても本実施形態に係る熱延鋼板の効果は損なわれないことを確認している。しかし、熱間圧延時に疵が発生する場合があるため、Sn含有量は0.05%以下とする。
次に、本実施形態に係る熱延鋼板の金属組織について説明する。
本実施形態に係る熱延鋼板では、金属組織が、面積%で、マルテンサイトおよび焼き戻しマルテンサイトが合計で92.0%超、100.0%以下であり、残留オーステナイトが3.0%未満であり、フェライトが5.0%未満であり、前記金属組織の周期性を示すE値が11.0以上であり、前記金属組織の均一性を示すI値が1.020未満であり、Mn濃度の標準偏差が0.60質量%以下である。そのため、本実施形態に係る熱延鋼板は、優れた強度、穴広げ性およびせん断加工性を得ることができる。なお、本実施形態では、圧延方向に平行な断面の、表面から板厚の1/4深さ位置且つ板幅方向中央位置における金属組織を規定する。その理由は、この位置における金属組織が、鋼板の代表的な金属組織を示すからである。
残留オーステナイトは室温でも面心立方格子として存在する金属組織である。残留オーステナイトは、変態誘起塑性(TRIP)により熱延鋼板の穴広げ性を高める作用を有する。一方、残留オーステナイトは、せん断加工中には高炭素のマルテンサイトに変態するため、安定的なき裂発生を阻害し、せん断加工後の端面における破断面とせん断面と境界の直線性を低下させる原因となる。残留オーステナイトの面積分率が3.0%以上では、上記作用が顕在化し、熱延鋼板のせん断加工性が劣化する(せん断加工後の端面における破断面とせん断面との境界の直線性が低下する)ばかりか、穴広げ性も低下する。したがって、残留オーステナイトの面積分率は3.0%未満とする。残留オーステナイトの面積分率は、好ましくは1.5%以下であり、より好ましくは1.0%未満である。残留オーステナイトは少ない程好ましいため、残留オーステナイトの面積分率は0%であってもよい。
フェライトは一般に軟質な金属組織である。所定量以上のフェライトを含有すると、所望の強度を得られない場合、およびせん断加工後の端面におけるせん断面の領域が増大する場合がある。せん断加工後の端面におけるせん断面の領域が増大すると、端面における破断面とせん断面との境界の直線性が低下するため、好ましくない。フェライトの面積分率が5.0%以上では、上記作用が顕在化し、熱延鋼板のせん断加工性が劣化する。したがって、フェライトの面積分率は5.0%未満とする。フェライトの面積分率は、好ましくは3.0%以下であり、より好ましくは2.0%以下であり、より一層好ましくは1.0%未満である。フェライトは少ない程好ましいため、フェライトの面積分率は0%であってもよい。
本実施形態におけるX線回折による残留オーステナイト面積分率の測定では、まず、熱延鋼板の板厚の1/4深さ位置(表面から板厚の1/8深さ~表面から板厚の3/8深さの領域)、且つ板幅方向中央位置における、圧延方向に平行な断面において、Co-Kα線を用いて、α(110)、α(200)、α(211)、γ(111)、γ(200)、γ(220)の計6ピークの積分強度を求め、強度平均法を用いて算出することで残留オーステナイトの面積分率を得る。
マルテンサイトおよび焼き戻しマルテンサイトの面積分率の合計が92.0%以下であると所望の強度を得ることができない。そのため、マルテンサイトおよび焼き戻しマルテンサイトの面積分率の合計は92.0%超とする。好ましくは95.0%以上、97.0%以上、99.0%以上である。マルテンサイトおよび焼き戻しマルテンサイトの面積分率の合計は多い程好ましいため、100.0%としてもよい。
まず、フェライトの面積分率を測定したEBSD測定領域と同領域をSEMで観察するために、観察位置近傍にビッカース圧痕を打刻する。その後、観察面の組織を残して、表層のコンタミを研磨除去し、ナイタールエッチングする。次に、EBSD観察面と同一視野をSEMにより倍率3000倍で観察する。EBSD測定において、フェライト以外の組織と判別された領域の内、粒内に下部組織を有し、かつ、セメンタイトが複数のバリアントを持って析出している領域を焼き戻しマルテンサイトと判断する。輝度が大きく、かつ下部組織がエッチングにより現出されていない領域を「マルテンサイトおよび残留オーステナイト」と判断する。それぞれの面積分率を算出することで、焼き戻しマルテンサイトの面積分率、並びに「マルテンサイトおよび残留オーステナイト」の面積分率を得る。得られた「マルテンサイトおよび残留オーステナイト」の面積分率から、上述のX線回折により得られた残留オーステナイトの面積分率を差し引くことにより、マルテンサイトの面積分率を得る。マルテンサイトの面積分率および焼き戻しマルテンサイトの面積分率の合計を算出することで、マルテンサイトおよび焼き戻しマルテンサイトの面積分率の合計を得る。
なお、観察面表層のコンタミ除去については、粒子径0.1μm以下のアルミナ粒子を用いたバフ研磨、あるいはArイオンスパッタリング等の手法を用いればよい。
せん断加工後の端面における破断面とせん断面との境界の直線性を高めるには、金属組織の周期性を低減し、且つ金属組織の均一性を低減することが重要である。本実施形態では、金属組織の周期性を示すE(Entropy)値および金属組織の均一性を示すI(Inverce differenced moment norm)値を制御することで、せん断加工後の端面における破断面とせん断面と境界の直線性を高める。
本実施形態において、E値およびI値を算出するために撮影するSEM画像の撮影領域は、圧延方向に平行な断面における、鋼板表面から板厚の1/4深さ位置(表面から板厚の1/8深さ~表面から板厚の3/8深さの領域)、且つ、板幅方向中央位置とする。SEM画像の撮影には、株式会社日立ハイテクノロジーズ製SU-6600ショットキー電子銃を使用し、エミッタをタングステンとし、加速電圧を1.5kVとする。以上の設定のもと、倍率1000倍で、256階調のグレースケールにてSEM画像を出力する。
本実施形態に係る熱延鋼板の表面から板厚の1/4深さ位置且つ板幅方向中央位置におけるMn濃度の標準偏差は0.60質量%以下である。これにより、硬質相を均一に分散させることができ、端面における破断面とせん断面との境界の直線性の低下を防ぐことができる。すなわち、せん断加工性を向上することができる。Mn濃度の標準偏差は、0.50質量%以下が好ましく、0.47質量%以下がより好ましい。Mn濃度の標準偏差の下限は、過大バリの抑制の観点から、その値は小さいほど望ましいが、製造プロセスの制約より、実質的な下限は0.10質量%である。
表層の結晶粒径が細かいと、熱延鋼板の曲げ内割れを抑制することができる。鋼板強度が高くなるほど、曲げ加工時に曲げ内側から亀裂が生じやすくなる(以下、曲げ内割れと呼称する)。曲げ内割れのメカニズムは以下のように推定される。曲げ加工時には曲げ内側に圧縮の応力が生じる。最初は曲げ内側全体が均一に変形しながら加工が進むが、加工量が大きくなると均一な変形のみで変形を担えなくなり、局所にひずみが集中することで変形が進む(せん断変形帯の発生)。このせん断変形帯が更に成長することで曲げ内側表面からせん断帯に沿った亀裂が発生し、成長する。高強度化に伴い曲げ内割れが発生しやすくなる理由は、高強度化に伴う加工硬化能の低下により、均一な変形が進みにくくなり、変形の偏りが生じやすくなることで、加工早期に(または緩い加工条件で)せん断変形帯が生じるためと推定される。
なお、本実施形態において表層とは、熱延鋼板の表面~表面から深さ50μm位置の領域である。
なお、残留オーステナイトは600℃以下の相変態で生成した組織でなく、転位蓄積の効果を有さないので、本測定方法では、残留オーステナイトは解析の対象としない。EBSP-OIM法では、結晶構造がfccである残留オーステナイトを解析対象から除外することができる。
本実施形態に係る熱延鋼板は、引張(最大)強さが980MPa以上である。引張強さが980MPa未満であると、適用部品が限定され、車体軽量化の寄与が小さい。上限は特に限定する必要は無いが、金型摩耗抑制の観点から、1780MPaとしてもよい。
引張強さは、JIS Z 2241:2011の5号試験片を用いて、JIS Z 2241:2011に準拠して測定する。引張試験片の採取位置は、板幅方向の端部から1/4部分とし、圧延方向に直角な方向を長手方向とすればよい。
本実施形態に係る熱延鋼板は、穴広げ率λが55%以上であることが好ましい。穴広げ率λが55%以上であると、適用部品が限定されることなく、車体軽量化の寄与が大きい熱延鋼板を得ることができる。上限は特に限定する必要は無い。
穴広げ率λは、JIS Z 2241:2011の5号試験片を用いて、JIS Z 2256:2010に準拠して測定する。穴広げ試験片の採取位置は、熱延鋼板の板幅方向の端部から1/4部分とすればよい。
本実施形態に係る熱延鋼板の板厚は特に限定されないが、0.5~8.0mmとしてもよい。熱延鋼板の板厚を0.5mm以上とすることで、圧延完了温度の確保が容易になるとともに圧延荷重を低減でき、熱間圧延を容易に行うことができる。したがって、本実施形態に係る熱延鋼板の板厚は0.5mm以上としてもよい。好ましくは1.2mm以上、1.4mm以上である。また、板厚を8.0mm以下とすることで、金属組織の微細化が容易となり、上述した金属組織を容易に確保することができる。したがって、板厚は8.0mm以下としてもよい。好ましくは6.0mm以下である。
(6-1)めっき層
上述した化学組成および金属組織を有する本実施形態に係る熱延鋼板は、表面に耐食性の向上等を目的としてめっき層を備えさせて表面処理鋼板としてもよい。めっき層は電気めっき層であってもよく溶融めっき層であってもよい。電気めっき層としては、電気亜鉛めっき、電気Zn-Ni合金めっき等が例示される。溶融めっき層としては、溶融亜鉛めっき、合金化溶融亜鉛めっき、溶融アルミニウムめっき、溶融Zn-Al合金めっき、溶融Zn-Al-Mg合金めっき、溶融Zn-Al-Mg-Si合金めっき等が例示される。めっき付着量は特に制限されず、従来と同様としてよい。また、めっき後に適当な化成処理(例えば、シリケート系のクロムフリー化成処理液の塗布と乾燥)を施して、耐食性をさらに高めることも可能である。
上述した化学組成および金属組織を有する本実施形態に係る熱延鋼板の好適な製造方法は、以下の通りである。
(1)スラブを700℃~850℃の温度域で900秒以上保持した後、更に加熱し、1100℃以上の温度域で6000秒以上保持する。
(2)850~1100℃の温度域で合計90%以上の板厚減となるような熱間圧延を行う。
(3)熱間圧延の最終段から1段前の圧延後、且つ最終段の圧延前に、170kPa以上の応力を鋼板に負荷する。
(4)熱間圧延の最終段における圧下率を8%以上とし、圧延完了温度Tfが900℃以上、960℃未満となるように熱間圧延を完了する。
(5)熱間圧延の最終段の圧延後、且つ鋼板が800℃まで冷却されるまでに鋼板に負荷する応力を200kPa未満とする。
(6)熱間圧延完了後1秒以内に、熱間圧延完了温度Tf-50℃以下の温度域まで冷却した後、50℃/秒以上の平均冷却速度で、下記式<1>により表される温度T1(℃)まで加速冷却する。ただし、熱間圧延完了後1秒以内に、熱間圧延完了温度Tf-50℃以下の温度域まで冷却することは、より好ましい冷却条件である。
(7)T1(℃)から巻取り温度までを50℃/秒以上の平均冷却速度で冷却する。
(8)巻き取り温度を350℃以下とする。
ただし、各式中の[元素記号]は各元素の鋼中の含有量(質量%)を示す。当該元素を含有しない場合は0を代入する。
熱間圧延に供するスラブは、連続鋳造により得られたスラブや鋳造・分塊により得られたスラブなどを用いることができ、必要によってはそれらに熱間加工または冷間加工を加えたものを用いることができる。熱間圧延に供するスラブは、加熱時の700℃~850℃の温度域で900秒以上保持し、その後更に加熱し、1100℃以上の温度域で6000秒以上保持することが好ましい。なお、700℃~850℃の温度域での保持では、鋼板温度をこの温度域で変動させてもよく、一定としてもよい。また、1100℃以上の温度域での保持では、鋼板温度を1100℃以上の温度域で変動させてもよく、一定としてもよい。
また、Mn濃度の標準偏差を低減するためには、1100℃以上の温度域での保持時間は6000秒以上とすることが好ましい。所望量のマルテンサイトおよび焼き戻しマルテンサイトを得るためには、6000秒以上保持する温度は1100℃以上とすることが好ましい。
850~1100℃の温度域で合計90%以上の板厚減となるような熱間圧延を行うことにより、主に再結晶オーステナイト粒の微細化が図られるとともに、未再結晶オーステナイト粒内へのひずみエネルギーの蓄積が促進され、オーステナイトの再結晶が促進されるとともにMnの原子拡散が促進され、Mn濃度の標準偏差を小さくすることができる。したがって、850~1100℃の温度域で合計90%以上の板厚減となるような熱間圧延を行う。
なお、850~1100℃の温度域の板厚減とは、この温度域の圧延における最初のパス前の入口板厚t0とし、この温度域の圧延における最終パス後の出口板厚をt1としたとき、(t0-t1)/t0×100(%)で表すことができる。
熱間圧延の最終段から1段前の圧延後、且つ最終段の圧延前の鋼板に負荷する応力を170kPa以上とすることが好ましい。これにより、最終段から1段前の圧延後の再結晶オーステナイトのうち、{110}<001>の結晶方位を有する結晶粒の数を低減することができる。{110}<001>は再結晶し難い結晶方位であるため、この結晶方位の形成を抑制することで最終段の圧下による再結晶を効果的に促進することができる。結果として、熱延鋼板のバンド状組織が改善され、金属組織の周期性が低減し、E値が上昇する。鋼板に負荷する応力が170kPa未満の場合、11.0以上のE値を達成することができない場合がある。鋼板に負荷する応力は、より好ましくは190kPa以上である。鋼板に負荷する応力は、タンデム圧延中のロール回転速度の調整により制御可能である。
熱間圧延の最終段における圧下率は8%以上とし、熱間圧延完了温度Tfは900℃以上とすることが好ましい。熱間圧延の最終段における圧下率を8%以上とすることで、最終段の圧下による再結晶を促進することができる。結果として熱延鋼板のバンド状組織が改善され、金属組織の周期性が低減し、E値が上昇する。熱間圧延完了温度Tfを900℃以上とすることで、オーステナイト中のフェライト核生成サイト数の過剰な増大を抑制することができる。その結果、最終組織(製造後の熱延鋼板の金属組織)におけるフェライトの生成を抑えられ、高強度の熱延鋼板を得ることができる。また、Tfを960℃未満とすることで、オーステナイト粒径の粗大化を抑制でき、金属組織の周期性を低減してE値を11.0以上とすることができる。
熱間圧延の最終段の圧延後、且つ鋼板が800℃に冷却されるまでの鋼板に負荷する応力は200kPa未満とすることが好ましい。鋼板に負荷する応力を200kPa未満とすることで、オーステナイトの再結晶が圧延方向に優先的に進み、金属組織の周期性の増大を抑制できる。その結果、E値を11.0以上とすることができる。鋼板に負荷する応力は、より好ましくは180MPa以下である。
熱間圧延完了後、且つ熱間圧延完了温度Tf-50℃以下の温度域まで冷却した後、50℃/秒以上の平均冷却速度でT1(℃)以下まで加速冷却を行うことで、フェライト、ベイナイトおよびパーライトの生成を抑制できる。これにより、熱延鋼板の強度が向上する。なお、ここでいう平均冷却速度とは、加速冷却開始時(冷却設備への鋼板の導入時)からT1(℃)までの鋼板の温度降下幅を、加速冷却開始時から鋼板温度がT1(℃)に達する時までの所要時間で除した値のことをいう。熱間圧延完了温度Tf-50℃以下の温度域まで冷却した後、T1(℃)までの平均冷却速度を50℃/秒以上とすることで、鋼板内部でのフェライト変態、ベイナイト変態および/またはパーライト変態を抑制でき、TS≧980MPaを得ることができる。熱間圧延完了後、T1(℃)まで加速冷却する間に空冷等を行うと、50℃/秒以上の平均冷却速度を実現することができなくなり、上記効果が得られない。
平均冷却速度の上限値は特に規定しないが、冷却速度を速くすると冷却設備が大掛かりとなり、設備コストが高くなる。このため、設備コストを考えると、加速冷却の平均冷却速度は300℃/秒以下が好ましい。
熱間圧延完了後1.0秒以内に、熱間圧延完了温度Tf-50℃の温度域まで冷却した後は、上述のように、T1(℃)以下までの平均冷却速度を50℃/秒以上とするように加速冷却を行えばよい。
フェライト、ベイナイトおよびパーライトの面積分率を抑え、TS≧980MPaの強度を得るために、T1(℃)から巻取り温度までの平均冷却速度を50℃/秒以上とすることが好ましい。これにより、母相組織を硬質にすることができる。なお、ここでいう平均冷却速度とは、T1(℃)から巻取り温度までの鋼板の温度降下幅を、鋼板温度がT1(℃)に達した時から巻取りまでの所要時間で除した値のことをいう。上記平均冷却速度を50℃/秒以上とすることで、フェライト、ベイナイトおよびパーライトの面積分率を抑制し、強度およびせん断加工性を確保することができる。したがって、T1(℃)から巻取り温度までの平均冷却速度は50℃/秒以上とする。
巻取り温度は350℃以下とすることが好ましい。巻取り温度を350℃以下とすることで、オーステナイトからbccへの変態駆動力を大きくすることができ、また、オーステナイトの変形強度を大きくすることができる。そのため、オーステナイトからマルテンサイト変態する際に硬質相が均一に分布し、ばらつきを向上できる。その結果、I値を低減することができ、せん断加工後の端面における破断面とせん断面との境界の直線性を高めることができる。すなわち、優れたせん断加工性を得ることができる。また、残留オーステナイトの影響により穴広げ性が低下することも抑制できる。したがって、巻取り温度は350℃以下とすることが好ましい。
なお、残部組織はベイナイトおよびパーライトの1種または2種であった。
(1)引張強さおよび穴広げ率
引張強さは、JIS Z 2241:2011に準拠して評価した。試験片はJIS Z 2241:2011の5号試験片とした。引張試験片の採取位置は、板幅方向の端部から1/4部分とし、圧延方向に直角な方向を長手方向とした。
穴広げ率は、JIS Z 2241:2011の5号試験片を用いて、JIS Z 2256:2010に準拠して測定した。穴広げ試験片の採取位置は、熱延鋼板の板幅方向の端部から1/4部分とした。
穴広げ率≧55%であった場合、穴広げ性に優れるとして合格と判定した。一方、穴広げ率<55%であった場合、穴広げ性に劣るとして不合格と判定した。
熱延鋼板のせん断加工性は、打ち抜き試験により、破断面とせん断との境界における直線度を求めることで評価した。板幅中央位置に、穴直径10mm、クリアランス15%、打ち抜き速度3m/sで5個の打ち抜き穴を作製した。次に、5個の打ち抜き穴について、10箇所の圧延方向に平行な端面(1個の打ち抜き穴につき2箇所の端面)の様子を光学顕微鏡観で撮影した。得られた観察写真では、図1(a)に示すような端面を観察することができる。図1(a)および(b)に示すように、打ち抜き後の端面では、ダレ、せん断面、破断面およびバリが観察される。なお、図1(a)は打ち抜き穴の圧延方向に平行な端面の概略図であり、図1(b)は、打ち抜き穴の側面の概略図である。ダレとはR状の滑らかな面であり、せん断面とはせん断変形により分離した打ち抜き端面であり、破断面とはせん断変形終了後、刃先近傍から発生したき裂によって分離した打ち抜き端面であり、バリとは熱延鋼板の下面からはみ出した突起を有する面である。
図1(b)に示すように、せん断面と、破断面との境界の点(図1(b)の点Aおよび点B)を、端面に対して決定した。これらの点Aおよび点Bを直線で結んだ距離xの長さを測定した。次に、破断面―せん断面境界に沿った曲線の長さyを測定した。得られたyをxで除することによって得られた値を、破断面とせん断との境界における直線度とした。
曲げ試験片は、熱延鋼板の幅方向1/2位置から、100mm×30mmの短冊形状の試験片を切り出し、以下の曲げ試験により耐曲げ内割れ性を評価した。
曲げ稜線が圧延方向(L方向)に平行である曲げ(L軸曲げ)と、曲げ稜線が圧延方向に垂直な方向(C方向)に平行である曲げ(C軸曲げ)の両者について、JIS Z 2248:2014(Vブロック90°曲げ試験)に準拠して耐曲げ内割れ性を調査し、亀裂の発生しない最小曲げ半径を求め、L軸およびC軸の最小曲げ半径の平均値を板厚で除した値を限界曲げR/tとして曲げ性の指標値とした。R/t≦3.0であった場合、耐曲げ内割れ性に優れた熱延鋼板であると判断した。
ただし、亀裂の有無は、Vブロック90°曲げ試験後の試験片を曲げ方向と平行でかつ板面に垂直な面で切断した断面を鏡面研磨後、光学顕微鏡で亀裂を観察し、試験片の曲げ内側に観察される亀裂長さが30μmを超える場合に亀裂有と判断した。
得られた測定結果を表6Aおよび表6Bに示す。
一方、化学組成、金属組織が本発明で規定する範囲内でない比較例は、特性(引張強さTS、穴広げ性、せん断加工性)のうちいずれか一つ以上が劣った。
本発明に係る熱延鋼板は、自動車部材、機械構造部材さらには建築部材に用いられる工業用素材として好適である。
Claims (3)
- 化学組成が、質量%で、
C:0.040~0.250%、
Si:0.05~3.00%、
Mn:1.00~4.00%、
sol.Al:0.001~0.500%、
P:0.100%以下、
S:0.0300%以下、
N:0.1000%以下、
O:0.0100%以下、
Ti:0~0.300%、
Nb:0~0.300%、
V:0~0.500%、
Cu:0~2.00%、
Cr:0~2.00%、
Mo:0~1.00%、
Ni:0~2.00%、
B:0~0.0100%、
Ca:0~0.0200%、
Mg:0~0.0200%、
REM:0~0.1000%、
Bi:0~0.020%、
Zr、Co、ZnおよびWのうち1種または2種以上:合計で0~1.00%、並びに
Sn:0~0.05%を含有し、
残部がFeおよび不純物からなり、
金属組織が、
面積%で、
マルテンサイトおよび焼き戻しマルテンサイトが合計で92.0%超、100.0%以下であり、
残留オーステナイトが3.0%未満であり、
フェライトが5.0%未満であり、
前記金属組織の周期性を示すE値が11.0以上であり、前記金属組織の均一性を示すI値が1.020未満であり、
Mn濃度の標準偏差が0.60質量%以下であり、
引張強さが980MPa以上であることを特徴とする熱延鋼板。 - 表層の平均結晶粒径が3.0μm未満であることを特徴とする請求項1に記載の熱延鋼板。
- 前記化学組成が、質量%で、
Ti:0.005~0.300%、
Nb:0.005~0.100%、
V:0.005~0.500%、
Cu:0.01~2.00%、
Cr:0.01~2.00%、
Mo:0.01~1.00%、
Ni:0.02~2.00%、
B:0.0001~0.0100%、
Ca:0.0005~0.0200%、
Mg:0.0005~0.0200%、
REM:0.0005~0.1000%、および
Bi:0.0005~0.020%
からなる群から選択される1種または2種以上を含有することを特徴とする請求項1または2に記載の熱延鋼板。
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