CN117940597A - Hot rolled steel sheet - Google Patents
Hot rolled steel sheet Download PDFInfo
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- CN117940597A CN117940597A CN202280060328.5A CN202280060328A CN117940597A CN 117940597 A CN117940597 A CN 117940597A CN 202280060328 A CN202280060328 A CN 202280060328A CN 117940597 A CN117940597 A CN 117940597A
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- 229910000831 Steel Inorganic materials 0.000 title claims abstract description 181
- 239000010959 steel Substances 0.000 title claims abstract description 181
- 229910052751 metal Inorganic materials 0.000 claims abstract description 37
- 239000002184 metal Substances 0.000 claims abstract description 37
- 229910000859 α-Fe Inorganic materials 0.000 claims abstract description 32
- 238000000034 method Methods 0.000 claims abstract description 31
- 229910001562 pearlite Inorganic materials 0.000 claims abstract description 19
- 239000000126 substance Substances 0.000 claims abstract description 19
- 239000000203 mixture Substances 0.000 claims abstract description 15
- 239000011159 matrix material Substances 0.000 claims abstract description 14
- 238000001878 scanning electron micrograph Methods 0.000 claims abstract description 13
- 238000003874 inverse correlation nuclear magnetic resonance spectroscopy Methods 0.000 claims abstract description 7
- 229910001566 austenite Inorganic materials 0.000 claims description 42
- 239000013078 crystal Substances 0.000 claims description 28
- 230000000717 retained effect Effects 0.000 claims description 27
- 239000002344 surface layer Substances 0.000 claims description 18
- 239000012535 impurity Substances 0.000 claims description 9
- 238000001816 cooling Methods 0.000 description 67
- 230000000694 effects Effects 0.000 description 45
- 238000005096 rolling process Methods 0.000 description 43
- 208000010392 Bone Fractures Diseases 0.000 description 40
- 206010017076 Fracture Diseases 0.000 description 40
- 238000005098 hot rolling Methods 0.000 description 39
- 230000009467 reduction Effects 0.000 description 39
- 238000010008 shearing Methods 0.000 description 36
- 238000005452 bending Methods 0.000 description 32
- 229910000734 martensite Inorganic materials 0.000 description 20
- 229910052720 vanadium Inorganic materials 0.000 description 16
- 238000005259 measurement Methods 0.000 description 15
- 238000010583 slow cooling Methods 0.000 description 14
- 230000007423 decrease Effects 0.000 description 12
- 238000004458 analytical method Methods 0.000 description 11
- 230000009471 action Effects 0.000 description 10
- 229910001563 bainite Inorganic materials 0.000 description 10
- 238000005336 cracking Methods 0.000 description 10
- 238000004519 manufacturing process Methods 0.000 description 10
- 229910052761 rare earth metal Inorganic materials 0.000 description 10
- 239000011247 coating layer Substances 0.000 description 8
- 238000007747 plating Methods 0.000 description 8
- 238000005728 strengthening Methods 0.000 description 8
- 238000009864 tensile test Methods 0.000 description 8
- 238000012360 testing method Methods 0.000 description 8
- 230000015572 biosynthetic process Effects 0.000 description 7
- 230000009466 transformation Effects 0.000 description 7
- 229910052721 tungsten Inorganic materials 0.000 description 7
- 238000005520 cutting process Methods 0.000 description 6
- 238000001887 electron backscatter diffraction Methods 0.000 description 6
- 239000010410 layer Substances 0.000 description 6
- 239000000523 sample Substances 0.000 description 6
- 238000005204 segregation Methods 0.000 description 6
- 238000005498 polishing Methods 0.000 description 5
- 238000007670 refining Methods 0.000 description 5
- 239000006104 solid solution Substances 0.000 description 5
- 238000004804 winding Methods 0.000 description 5
- 229910052726 zirconium Inorganic materials 0.000 description 5
- 238000002441 X-ray diffraction Methods 0.000 description 4
- 229910052799 carbon Inorganic materials 0.000 description 4
- 229910001567 cementite Inorganic materials 0.000 description 4
- 238000006243 chemical reaction Methods 0.000 description 4
- 238000009826 distribution Methods 0.000 description 4
- 238000001556 precipitation Methods 0.000 description 4
- 230000008569 process Effects 0.000 description 4
- 239000000047 product Substances 0.000 description 4
- 238000001953 recrystallisation Methods 0.000 description 4
- 229920006395 saturated elastomer Polymers 0.000 description 4
- 230000001133 acceleration Effects 0.000 description 3
- 229910045601 alloy Inorganic materials 0.000 description 3
- 239000000956 alloy Substances 0.000 description 3
- 230000000052 comparative effect Effects 0.000 description 3
- 238000005260 corrosion Methods 0.000 description 3
- 230000007797 corrosion Effects 0.000 description 3
- 238000002354 inductively-coupled plasma atomic emission spectroscopy Methods 0.000 description 3
- KSOKAHYVTMZFBJ-UHFFFAOYSA-N iron;methane Chemical compound C.[Fe].[Fe].[Fe] KSOKAHYVTMZFBJ-UHFFFAOYSA-N 0.000 description 3
- 238000012545 processing Methods 0.000 description 3
- 238000005482 strain hardening Methods 0.000 description 3
- 239000013585 weight reducing agent Substances 0.000 description 3
- OKTJSMMVPCPJKN-UHFFFAOYSA-N Carbon Chemical compound [C] OKTJSMMVPCPJKN-UHFFFAOYSA-N 0.000 description 2
- CURLTUGMZLYLDI-UHFFFAOYSA-N Carbon dioxide Chemical compound O=C=O CURLTUGMZLYLDI-UHFFFAOYSA-N 0.000 description 2
- 238000010521 absorption reaction Methods 0.000 description 2
- 238000009825 accumulation Methods 0.000 description 2
- 238000009749 continuous casting Methods 0.000 description 2
- 229910052802 copper Inorganic materials 0.000 description 2
- 230000007547 defect Effects 0.000 description 2
- 239000012770 industrial material Substances 0.000 description 2
- 239000011261 inert gas Substances 0.000 description 2
- XEEYBQQBJWHFJM-UHFFFAOYSA-N iron Substances [Fe] XEEYBQQBJWHFJM-UHFFFAOYSA-N 0.000 description 2
- 229910052747 lanthanoid Inorganic materials 0.000 description 2
- 150000002602 lanthanoids Chemical class 0.000 description 2
- 239000000463 material Substances 0.000 description 2
- 238000000691 measurement method Methods 0.000 description 2
- 150000002739 metals Chemical class 0.000 description 2
- 238000000386 microscopy Methods 0.000 description 2
- 230000003954 pattern orientation Effects 0.000 description 2
- 230000000737 periodic effect Effects 0.000 description 2
- 239000002244 precipitate Substances 0.000 description 2
- 230000001376 precipitating effect Effects 0.000 description 2
- 230000001737 promoting effect Effects 0.000 description 2
- 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
- 206010003497 Asphyxia Diseases 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
- 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
- UCKMPCXJQFINFW-UHFFFAOYSA-N Sulphide Chemical compound [S-2] UCKMPCXJQFINFW-UHFFFAOYSA-N 0.000 description 1
- 229910007570 Zn-Al Inorganic materials 0.000 description 1
- 229910007567 Zn-Ni Inorganic materials 0.000 description 1
- 229910007614 Zn—Ni Inorganic materials 0.000 description 1
- 239000002253 acid Substances 0.000 description 1
- 230000003044 adaptive effect Effects 0.000 description 1
- 230000002411 adverse Effects 0.000 description 1
- 239000012670 alkaline solution Substances 0.000 description 1
- 238000005269 aluminizing Methods 0.000 description 1
- 238000004364 calculation method Methods 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
- 238000012512 characterization method Methods 0.000 description 1
- 239000011248 coating agent Substances 0.000 description 1
- 238000000576 coating method Methods 0.000 description 1
- 239000010960 cold rolled steel Substances 0.000 description 1
- 239000008119 colloidal silica Substances 0.000 description 1
- 239000000498 cooling water Substances 0.000 description 1
- 230000002950 deficient Effects 0.000 description 1
- 238000002050 diffraction method Methods 0.000 description 1
- 238000009792 diffusion process Methods 0.000 description 1
- 238000003618 dip coating Methods 0.000 description 1
- 239000006185 dispersion Substances 0.000 description 1
- 238000001035 drying Methods 0.000 description 1
- 238000010894 electron beam technology Methods 0.000 description 1
- 230000007613 environmental effect Effects 0.000 description 1
- 238000011156 evaluation Methods 0.000 description 1
- 239000000706 filtrate Substances 0.000 description 1
- 239000000446 fuel Substances 0.000 description 1
- 238000000227 grinding Methods 0.000 description 1
- 238000009499 grossing Methods 0.000 description 1
- 238000010438 heat treatment Methods 0.000 description 1
- 229910052739 hydrogen Inorganic materials 0.000 description 1
- 239000001257 hydrogen Substances 0.000 description 1
- 238000003384 imaging method Methods 0.000 description 1
- 230000001771 impaired effect Effects 0.000 description 1
- 230000006872 improvement Effects 0.000 description 1
- 238000003754 machining Methods 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
- 229910052750 molybdenum Inorganic materials 0.000 description 1
- 229910052758 niobium Inorganic materials 0.000 description 1
- 238000010606 normalization Methods 0.000 description 1
- 230000003287 optical effect Effects 0.000 description 1
- 239000002245 particle Substances 0.000 description 1
- 238000004080 punching Methods 0.000 description 1
- 238000013139 quantization Methods 0.000 description 1
- 239000002994 raw material Substances 0.000 description 1
- 239000011347 resin Substances 0.000 description 1
- 229920005989 resin Polymers 0.000 description 1
- 238000007665 sagging Methods 0.000 description 1
- 229910052706 scandium Inorganic materials 0.000 description 1
- 238000007711 solidification Methods 0.000 description 1
- 230000008023 solidification Effects 0.000 description 1
- 239000000243 solution Substances 0.000 description 1
- 238000005507 spraying Methods 0.000 description 1
- 230000003746 surface roughness Effects 0.000 description 1
- 229910052718 tin Inorganic materials 0.000 description 1
- WFKWXMTUELFFGS-UHFFFAOYSA-N tungsten Chemical compound [W] WFKWXMTUELFFGS-UHFFFAOYSA-N 0.000 description 1
- 239000010937 tungsten Substances 0.000 description 1
- 229910052727 yttrium Inorganic materials 0.000 description 1
- 229910052725 zinc Inorganic materials 0.000 description 1
Classifications
-
- C—CHEMISTRY; METALLURGY
- C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
- C22C—ALLOYS
- C22C38/00—Ferrous alloys, e.g. steel alloys
- C22C38/18—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
- 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
- C21D9/00—Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor
- C21D9/46—Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor for sheet metals
-
- C—CHEMISTRY; METALLURGY
- C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
- C22C—ALLOYS
- C22C38/00—Ferrous alloys, e.g. steel alloys
-
- C—CHEMISTRY; METALLURGY
- C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
- C22C—ALLOYS
- C22C38/00—Ferrous alloys, e.g. steel alloys
- C22C38/001—Ferrous alloys, e.g. steel alloys containing N
-
- C—CHEMISTRY; METALLURGY
- C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
- C22C—ALLOYS
- C22C38/00—Ferrous alloys, e.g. steel alloys
- C22C38/008—Ferrous alloys, e.g. steel alloys containing tin
-
- C—CHEMISTRY; METALLURGY
- C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
- C22C—ALLOYS
- C22C38/00—Ferrous alloys, e.g. steel alloys
- C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
- C22C38/40—Ferrous alloys, e.g. steel alloys containing chromium with nickel
- C22C38/42—Ferrous alloys, e.g. steel alloys containing chromium with nickel with copper
-
- C—CHEMISTRY; METALLURGY
- C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
- C22C—ALLOYS
- C22C38/00—Ferrous alloys, e.g. steel alloys
- C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
- C22C38/40—Ferrous alloys, e.g. steel alloys containing chromium with nickel
- C22C38/44—Ferrous alloys, e.g. steel alloys containing chromium with nickel with molybdenum or tungsten
-
- C—CHEMISTRY; METALLURGY
- C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
- C22C—ALLOYS
- C22C38/00—Ferrous alloys, e.g. steel alloys
- C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
- C22C38/40—Ferrous alloys, e.g. steel alloys containing chromium with nickel
- C22C38/46—Ferrous alloys, e.g. steel alloys containing chromium with nickel with vanadium
-
- C—CHEMISTRY; METALLURGY
- C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
- C22C—ALLOYS
- C22C38/00—Ferrous alloys, e.g. steel alloys
- C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
- C22C38/40—Ferrous alloys, e.g. steel alloys containing chromium with nickel
- C22C38/48—Ferrous alloys, e.g. steel alloys containing chromium with nickel with niobium or tantalum
-
- C—CHEMISTRY; METALLURGY
- C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
- C22C—ALLOYS
- C22C38/00—Ferrous alloys, e.g. steel alloys
- C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
- C22C38/40—Ferrous alloys, e.g. steel alloys containing chromium with nickel
- C22C38/50—Ferrous alloys, e.g. steel alloys containing chromium with nickel with titanium or zirconium
-
- C—CHEMISTRY; METALLURGY
- C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
- C22C—ALLOYS
- C22C38/00—Ferrous alloys, e.g. steel alloys
- C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
- C22C38/40—Ferrous alloys, e.g. steel alloys containing chromium with nickel
- C22C38/52—Ferrous alloys, e.g. steel alloys containing chromium with nickel with 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
- C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
- C22C38/40—Ferrous alloys, e.g. steel alloys containing chromium with nickel
- C22C38/54—Ferrous alloys, e.g. steel alloys containing chromium with nickel with boron
-
- C—CHEMISTRY; METALLURGY
- 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
-
- C—CHEMISTRY; METALLURGY
- C21—METALLURGY OF IRON
- C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
- C21D2211/00—Microstructure comprising significant phases
- C21D2211/005—Ferrite
-
- C—CHEMISTRY; METALLURGY
- C21—METALLURGY OF IRON
- C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
- C21D2211/00—Microstructure comprising significant phases
- C21D2211/009—Pearlite
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- Chemical & Material Sciences (AREA)
- Engineering & Computer Science (AREA)
- Mechanical Engineering (AREA)
- Materials Engineering (AREA)
- Metallurgy (AREA)
- Organic Chemistry (AREA)
- Physics & Mathematics (AREA)
- Thermal Sciences (AREA)
- Crystallography & Structural Chemistry (AREA)
- Heat Treatment Of Sheet Steel (AREA)
Abstract
The hot-rolled steel sheet has a predetermined chemical composition, a metal structure of less than 3.0% by area, ferrite of 15.0% or more and less than 60.0%, pearlite of less than 5.0%, a Entropy (entropy) value of 10.7 or more, a INVERSE DIFFERENCE normalized value of 1.020 or more, a Cluster Shade value of-8.0X10 5～8.0×105, a standard deviation of Mn concentration of 0.60 mass% or less, and a tensile strength of 980MPa or more, which are obtained by analyzing SEM images of the metal structure by a gray scale co-occurrence matrix method.
Description
Technical Field
The present invention relates to a hot rolled steel sheet. More specifically, the present invention relates to a hot-rolled steel sheet which is formed into various shapes by press working or the like and is used, and particularly, to a hot-rolled steel sheet which has high strength and a reduction in ultimate fracture plate thickness, and which has excellent ductility and shearing workability.
The present application claims priority based on 2021, 10 and 11 in japanese patent application No. 2021-166958, the contents of which are incorporated herein by reference.
Background
In recent years, from the viewpoint of global environmental protection, there are many fields in which reduction of carbon dioxide emissions is attempted. Automobile manufacturers are also actively developing a technique for reducing the weight of a vehicle body for the purpose of reducing fuel consumption. However, in order to ensure safety of the occupant, emphasis is placed on improving the collision resistance, and therefore, it is not easy to reduce the weight of the vehicle body.
In order to achieve both the weight reduction of the vehicle body and the collision resistance, the use of high-strength steel sheets to reduce the thickness of the components has been studied. Accordingly, a steel sheet having both high strength and excellent formability has been strongly desired, and several techniques have been proposed so far to cope with these demands. Since automobile parts have various processing patterns, the required formability varies depending on the applied parts, with the ultimate fracture plate thickness reduction rate and ductility being positioned as important indicators of formability. The ultimate breaking sheet thickness reduction rate is a value obtained from the minimum value of the sheet thickness of the tensile test piece before breaking and the sheet thickness of the tensile test piece after breaking. When the limit fracture plate thickness reduction rate is low, early fracture is likely to occur when tensile strain is applied during press forming, which is not preferable.
The automobile parts are formed by press forming, but the press formed blank plate is often manufactured by shearing work with high productivity. In a blank plate manufactured by shearing, it is necessary that the end face precision after shearing is excellent.
For example, if the form of the end face (sheared end face) after shearing is generated as a secondary sheared face of shearing face-fracture face-shearing face, the accuracy of the sheared end face is significantly deteriorated.
For example, patent document 1 discloses a hot-rolled steel sheet which is a raw material of a cold-rolled steel sheet excellent in surface properties after press working in which the Mn segregation degree and the P segregation degree in the center portion of the sheet thickness are controlled.
However, in patent document 1, the limit fracture plate thickness reduction rate and the shearing workability of the hot-rolled steel sheet are not considered.
Prior art literature
Patent literature
Patent document 1: international publication No. 2020/044445
Non-patent literature
Non-patent document 1: J.Webel, J.Gola, D.Britz, F.Mucklich, MATERIALS CHARACTERIZATION, 144 (2018) 584-596
Non-patent document 2: D.L.Naik, H.U.Sajid, R.Kiran, metals 2019,9,546
Non-patent literature 3：K.Zuiderveld,Contrast Limited Adaptive Histogram Equalization,Chapter VIII.5,Graphics Gems IV.P.S.Heckbert(Eds.),Cambridge,MA,Academic Press,1994,pp.474-485
Disclosure of Invention
Problems to be solved by the invention
The present invention has been made in view of the above-described circumstances, and an object thereof is to provide a hot-rolled steel sheet having high strength and a reduction in ultimate fracture plate thickness, and also having excellent ductility and shearing workability.
The gist of the present invention is as follows.
(1) The chemical composition of the hot rolled steel sheet according to one embodiment of the present invention is as follows in mass%:
C：0.050～0.250％、
Si：0.05～3.00％、
Mn：1.00～4.00％、
sol.Al：0.001～2.000％、
P:0.100% or less,
S:0.0300% or less,
N: less than 0.1000 percent,
O:0.0100% or less,
Ti：0～0.500％、
Nb：0～0.500％、
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：0～0.0200％、
As：0～0.100％、
Zr：0～1.00％、
Co：0～1.00％、
Zn：0～1.00％、
W：0～1.00％、
Sn:0 to 0.05%, and
The remainder: fe and the impurities are mixed together,
Satisfies the following formula (A) and (B),
The metal structure is expressed in area percent,
The retained austenite is less than 3.0%,
Ferrite is 15.0% or more and less than 60.0%,
Pearlite less than 5.0%,
A Entropy (entropy) value represented by the following formula (1) obtained by analyzing the SEM image of the metal structure by a gray scale co-occurrence matrix method is 10.7 or more,
A INVERSE DIFFERENCE normalized value represented by the following formula (2) of 1.020 or more,
Cluster Shade represented by the following formula (3) has a value of-8.0X10 5～8.0×105,
The standard deviation of Mn concentration is 0.60 mass% or less,
The tensile strength is 980MPa or more.
0.060％≤Ti+Nb+V≤0.500％(A)
Zr+Co+Zn+W≤1.00％(B)
Wherein each symbol of the elements in the formulas (A) and (B) represents the content of the element in mass%, and 0% is substituted when the element is not contained.
Here, P (i, j) in the following formulas (1) to (5) is a gradation co-occurrence matrix, L in the following formula (2) is the number of gradation levels that can be obtained by the SEM image, i and j in the following formulas (2) and (3) are natural numbers of 1 to L, and μ x and μ y in the following formula (3) are represented by the following formulas (4) and (5), respectively.
[ Mathematics 1]
Entropy＝-∑i∑jP(ij).log(P(i，j))…(1)
[ Math figure 2]
[ Math 3]
[ Mathematics 4]
μx＝∑i∑ji(P(i，j))…(4)
[ Math 5]
μy＝∑i∑jj(P(i，j))…(5)
(2) The hot-rolled steel sheet according to the above (1), wherein the average crystal grain size of the surface layer is less than 3.0. Mu.m.
(3) The hot rolled steel sheet according to the above (1) or (2), wherein the chemical composition comprises, in mass%, a composition selected from the group consisting of
Ti：0.001～0.500％、
Nb：0.001～0.500％、
V：0.001～0.500％、
Cu：0.01～2.00％、
Cr：0.01～2.00％、
Mo：0.01～1.00％、
Ni：0.01～2.00％、
B：0.0001～0.0100％、
Ca：0.0001～0.0200％、
Mg：0.0001～0.0200％、
REM：0.0001～0.1000％、
Bi：0.0001～0.0200％、
As：0.001～0.100％、
Zr：0.01～1.00％、
Co：0.01～1.00％、
Zn：0.01～1.00％、
W:0.01 to 1.00%, and
Sn：0.01～0.05％
One or more of the group consisting of.
Effects of the invention
According to the above aspect of the present invention, a hot-rolled steel sheet having high strength and a reduction in ultimate fracture plate thickness, and excellent ductility and shearing workability can be obtained. Further, according to the above preferred embodiment of the present invention, a hot-rolled steel sheet having the above characteristics and further suppressed occurrence of the in-bending cracks, that is, excellent in resistance to the in-bending cracks can be obtained.
The hot-rolled steel sheet according to the above aspect of the present invention is suitable as an industrial material for use in automobile parts, machine structural parts, and building parts.
Drawings
Fig. 1 shows an example of a sheared edge face of a hot rolled steel sheet according to an example of the invention.
Fig. 2 is an example of a sheared edge face of a hot rolled steel sheet according to the comparative example.
Detailed Description
Hereinafter, the chemical composition and the microstructure of the hot-rolled steel sheet according to the present embodiment will be described in more detail. However, the present invention is not limited to the configuration disclosed in the present embodiment, and various modifications can be made without departing from the scope of the present invention.
In the numerical limitation ranges described below with the "to" included therein, the lower limit value and the upper limit value are included in the ranges. In the numerical values expressed as "less than" or "exceeding", the value is not included in the numerical range. In the following description, the% of the chemical composition is mass% unless otherwise specified.
Chemical composition
The chemical composition of the hot-rolled steel sheet according to the present embodiment will be described in detail below.
C：0.050～0.250％
C increases the area ratio of the hard phase and increases the strength of ferrite by bonding with precipitation strengthening elements such as Ti, nb, and V. When the C content is less than 0.050%, the desired strength cannot be obtained. Therefore, the C content is set to 0.050% or more. The C content is preferably 0.060% or more, more preferably 0.070% or more, still more preferably 0.080% or more or 0.090% or more.
On the other hand, when the C content exceeds 0.250%, the area ratio of ferrite decreases, whereby the ductility of the hot-rolled steel sheet decreases. Therefore, the C content is set to 0.250% or less. The C content is preferably 0.200% or less, 0.150% or less, or 0.120% or less.
Si：0.05～3.00％
Si has an effect of promoting the formation of ferrite to improve the ductility of the hot-rolled steel sheet and an effect of solid-solution strengthening ferrite to improve the strength of the hot-rolled steel sheet. Si also has a function of strengthening steel (suppressing defects such as occurrence of voids in steel) by deoxidizing. When the Si content is less than 0.05%, the effect of the above action cannot be obtained. Therefore, the Si content is set to 0.05% or more. The Si content is preferably 0.50% or more, more preferably 0.80% or more, 1.00% or more, 1.20% or more, or 1.40% or more.
However, when the Si content exceeds 3.00%, the surface properties and chemical conversion treatability, and further ductility and weldability of the steel sheet are significantly deteriorated, and the a 3 transformation point is significantly increased. Thus, it is difficult to perform hot rolling stably. In addition, austenite tends to remain after cooling, and the limit fracture plate thickness reduction rate decreases. 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, 2.20% or less, 2.00% or less, or 1.80% or less.
Mn：1.00～4.00％
Mn has an effect of suppressing ferrite transformation and improving the strength of the hot-rolled steel sheet. When the Mn content is less than 1.00%, the desired strength cannot be obtained. Therefore, the Mn content is set to 1.00% or more. The Mn content is preferably 1.30% or more, more preferably 1.50% or more or 1.80% or more.
On the other hand, when the Mn content exceeds 4.00%, the morphology of the hard phase becomes periodic bands due to Mn segregation, and it is difficult to obtain desired shear workability. Therefore, the Mn content is set to 4.00% or less. The Mn content is preferably 3.70% or less or 3.50% or less, more preferably 3.20% or less, 3.00% or less or 2.60% or less.
Ti：0～0.500％，Nb：0～0.500％、V：0～0.500％
0.060％≤Ti+Nb+V≤0.500％(A)
Wherein each symbol of the element in the formula (a) represents the content of the element in mass%, and 0% is substituted when the element is not contained.
Ti, nb, and V are elements that are finely precipitated as carbide and asphyxiation in steel and enhance the strength of the steel by precipitation strengthening. If the total content of Ti, nb and V is less than 0.060, these effects cannot be obtained. Therefore, the total content of Ti, nb, and V is 0.060% or more. That is, the value of the middle side of the formula (A) is set to 0.060% or more. It is not necessary to contain all of Ti, nb and V, and the total content thereof may be 0.060% or more. Therefore, the lower limits of the contents of Ti, nb and V are 0%, respectively. The lower limits of the contents of Ti, nb and V may be set to 0.001%, 0.010%, 0.030% or 0.050%, respectively. The total content of Ti, nb, and V is preferably 0.080% or more, more preferably 0.100% or more.
On the other hand, if the content of any one of Ti, nb, and V exceeds 0.500%, or if the total content of Ti, nb, and V exceeds 0.500%, the workability of the hot rolled steel sheet is deteriorated. Therefore, the content of each of Ti, nb, and V is set to 0.500% or less, and the total content of Ti, nb, and V is set to 0.500% or less. That is, the value of the middle side of the formula (A) is set to 0.500% or less. The content of each of Ti, nb, and V is preferably 0.400% or less or 0.300% or less, more preferably 0.250% or less, and still more preferably 0.200% or less or 0.100% or less. The total content of Ti, nb, and V is preferably 0.300% or less, more preferably 0.250% or less, and further preferably 0.200% or less.
sol.Al：0.001～2.000％
Al has the effect of deoxidizing and strengthening steel, and also has the effect of promoting the formation of ferrite and improving the ductility of a hot-rolled steel sheet, similarly to Si. When the sol.Al content is less than 0.001%, the effect of the above action cannot be obtained. Therefore, the sol.Al content is 0.001% or more. The sol.al content is preferably 0.010% or more, 0.030% or more, or 0.050% or more, more preferably 0.080% or more, 0.100% or more, or 0.150% or more.
On the other hand, when the sol.al content exceeds 2.000%, the above effect is saturated and economically unfavorable, and therefore the sol.al content is 2.000% or less. The sol.al content is preferably 1.700% or less or 1.500% or less, more preferably 1.300% or less, and still more preferably 1.000% or less.
In addition, sol.al refers to acid-soluble Al, and indicates solid-solution Al existing in steel in a solid-solution state.
P: less than 0.100%
P is generally an element contained as an impurity, but is also an element having an effect of improving the strength of a hot-rolled steel sheet by solid solution strengthening. The lower limit of the P content is 0%, but P may be positively contained. However, P is an element that is liable to segregate, and if the P content exceeds 0.100%, the reduction in ductility and ultimate fracture plate thickness reduction rate of the hot-rolled steel sheet due to grain boundary segregation becomes remarkable. Therefore, the P content is 0.100% or less. The P content is preferably 0.050% or less, 0.030% or less, 0.020% or less, or 0.015% or less. The lower limit of the P content is not particularly limited, but the lower limit of the P content is 0%. The lower limit of the P content may be 0.001%, 0.003% or 0.005% 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 steel to reduce the ductility and the ultimate fracture plate thickness reduction rate of the hot-rolled steel sheet. If the S content exceeds 0.0300%, the ductility of the hot-rolled steel sheet and the ultimate fracture plate thickness reduction rate are significantly reduced. Therefore, the S content is 0.0300% or less. The S content is preferably 0.0100% or less, 0.0070% or less, or 0.0050% or less. The lower limit of the S content is 0%, but may be 0.0001%, 0.0005%, 0.0010% or 0.0020% from the viewpoint of refining costs.
N: less than 0.1000%
N is an element contained in steel as an impurity, and has an effect of reducing the ductility and the limit fracture plate thickness reduction rate of the hot-rolled steel sheet. When the N content exceeds 0.1000%, the ductility and the ultimate fracture plate thickness reduction rate of the hot-rolled steel sheet are significantly reduced. Therefore, the N content is 0.1000% or less. The N content is preferably 0.0800% or less, more preferably 0.0700% or less or 0.0300% or less, and still more preferably 0.0150% or less or 0.0100% or less. The lower limit of the N content is 0%, but in the case of further refining the metal structure by containing one or more of Ti, nb, and V, the N content is preferably 0.0010% or more, more preferably 0.0015% or more, or 0.0020% or more, in order to promote precipitation of carbonitrides.
O:0.0100% or less
If O is contained in the steel in a large amount, coarse oxides are formed as starting points of fracture, and brittle fracture or hydrogen induced cracking is caused. Therefore, the O content is 0.0100% or less. The O content is preferably 0.0080% or less, more preferably 0.0050% or less or 0.0030% or less. The lower limit of the O content is 0%, but in order to disperse a large amount of fine oxides during deoxidation of the molten steel, the O content may be 0.0005% or more, or 0.0010% or more.
The hot-rolled steel sheet according to the present embodiment may contain the following elements as optional elements instead of part of Fe. The lower limit of the content in the case where these arbitrary elements are not contained is 0%. Hereinafter, any element will be described in detail.
Cu：0.01～2.00％
Cr：0.01～2.00％
Mo：0.01～1.00％
Ni：0.01～2.00％
B：0.0001～0.0100％
Cu, cr, mo, ni and B each have an effect of improving hardenability of a hot-rolled steel sheet. Cu and Mo precipitate as carbide in steel, and have an effect of improving the strength of the hot-rolled steel sheet. Further, when Cu is contained, ni has an effect of effectively suppressing grain boundary cracking of a slab caused by Cu. Therefore, one or two or more of these elements may be contained.
As described above, cu has an effect of improving hardenability of a hot-rolled steel sheet and an effect of improving strength of the hot-rolled steel sheet by precipitating carbide in the steel at a low temperature. In order to obtain the effect of the above action more reliably, the Cu content is preferably 0.01% or more, more preferably 0.05% or more. However, if the Cu content exceeds 2.00%, grain boundary cracking of the slab may occur. Therefore, the Cu content is 2.00% or less. The Cu content is preferably 1.50% or less, more preferably 1.00% or less, 0.70% or less, or 0.50% or less.
As described above, cr has an effect of improving hardenability of the hot rolled steel sheet. In order to obtain the effect of the above action more reliably, the Cr content is preferably 0.01% or more, more preferably 0.05% or more. However, when the Cr content exceeds 2.00%, the chemical conversion treatability of the hot-rolled steel sheet is significantly lowered. Therefore, the Cr content is 2.00% or less. The Cr content is preferably 1.50% or less, more preferably 1.00% or less, 0.70% or less, or 0.50% or less.
As described above, mo has an effect of improving hardenability of a hot-rolled steel sheet and an effect of improving strength of the hot-rolled steel sheet by precipitating carbide in the steel. In order to obtain the effect of the above action more reliably, the Mo content is preferably 0.01% or more, more preferably 0.02% or more. However, even if the Mo content exceeds 1.00%, the effect of the above action is saturated, which is not economically preferable. Therefore, the Mo content is 1.00% or less. The Mo content is preferably 0.50% or less, more preferably 0.20% or less or 0.10% or less.
As described above, ni has an effect of improving hardenability of the hot rolled steel sheet. In addition, when Cu is contained, ni has an effect of effectively suppressing grain boundary cracking of a slab caused by Cu. In order to obtain the effect of the above action more reliably, the Ni content is preferably 0.01% or more. However, since Ni is an expensive element, it is economically undesirable to contain Ni in a large amount. Therefore, the Ni content is 2.00% or less. The Ni content is preferably 1.50% or less, more preferably 1.00% or less, 0.70% or less, or 0.50% or less.
As described above, B has an effect of improving hardenability of the hot rolled steel sheet. In order to obtain the effect of the action more reliably, the B content is preferably 0.0001% or more, more preferably 0.0002% or more. However, when the B content exceeds 0.0100%, the formability of the hot-rolled steel sheet is significantly reduced, and therefore the B content is 0.0100% or less. The B content is preferably 0.0050% or less or 0.0025% or less.
Ca：0.0001～0.0200％
Mg：0.0001～0.0200％
REM：0.0001～0.1000％
Bi：0.0001～0.0200％
As：0.001～0.100％
Ca. Mg and REM each have an effect of improving the ductility of the hot-rolled steel sheet by adjusting the shape of inclusions in the steel to a preferable shape. In addition, bi has an effect of improving the ductility of the hot-rolled steel sheet by refining the solidification structure. Therefore, one or two or more of these elements may be contained. In order to obtain the effect of the above action more reliably, the content of any one or more of Ca, mg, REM and Bi is preferably 0.0001% or more. However, if the Ca content or the Mg content exceeds 0.0200% or the REM content exceeds 0.1000%, inclusions are excessively formed in the steel, and the ductility of the hot-rolled steel sheet may be lowered instead. In addition, even if the Bi content exceeds 0.0200%, the effect of the above action is saturated, which is not economically preferable. Therefore, the Ca content and the Mg content are set to 0.0200% or less, the REM content is set to 0.1000% or less, and the Bi content is set to 0.0200% or less. The Ca content, the Mg content and the Bi content are preferably 0.0100% or less, more preferably 0.0070% or less or 0.0040% or less. The REM content is preferably 0.0070% or less or 0.0040% or less. As reduces the austenite single-phase temperature, thereby refining the prior austenite grains and contributing to the improvement of the ductility of the hot-rolled steel sheet. In order to reliably obtain this effect, the As content is preferably 0.001% or more. On the other hand, even if a large amount of As is contained, the effect is saturated, and thus the As content is 0.100% or less.
Here, REM means 17 elements in total composed of Sc, Y and lanthanoid, and the content of REM means the total content of these elements. In the case of lanthanoids, it is industrially added in the form of misch metals.
Zr：0.01～1.00％、Co：0.01～1.00％、Zn：0.01～1.00％、W：0.01～1.00％
Zr+Co+Zn+W≤1.00％(B)
Each symbol of the element in the formula (B) represents the content of the element in mass%, and 0% is substituted when the element is not contained.
Sn：0.01～0.05％
Regarding Zr, co, zn, and W, the inventors of the present invention confirmed that even if these elements are contained in total at most 1.00%, the effects of the hot-rolled steel sheet according to the present embodiment are not impaired. Therefore, one or two or more of Zr, co, zn, and W may be contained in total at most 1.00%. That is, the left side value of the formula (B) may be 1.00% or less, or may be 0.50% or less, 0.10% or less, or 0.05% or less. Zr, co, zn, W and Sn may be 0.50% or less, 0.10% or less, or 0.05% or less, respectively. Since Zr, co, zn, and W may not be contained, the content of each may be 0%. In order to increase the strength by solid solution strengthening of the steel sheet, the contents of Zr, co, zn, and W may be 0.01% or more, respectively.
Further, the present inventors have confirmed that even a small amount of Sn does not impair the effect of the hot-rolled steel sheet according to the present embodiment. However, if a large amount of Sn is contained, defects may occur during hot rolling, and therefore the Sn content is 0.05% or less. Since Sn may not be contained, the Sn content may be 0%. In order to improve the corrosion resistance of the hot-rolled steel sheet, the Sn content may be 0.01% or more.
The remainder of the chemical composition of the hot-rolled steel sheet according to the present embodiment may be Fe and impurities. In the present embodiment, the impurities refer to impurities mixed from raw ores, scraps, manufacturing environments, or the like, and/or impurities allowed within a range that does not adversely affect the hot-rolled steel sheet according to the present embodiment.
The chemical composition of the hot-rolled steel sheet may be measured by a general analytical method. For example, measurement may be performed by ICP-AES (Inductively Coupled Plasma-Atomic Emission Spectrometry ). Alternatively, sol.Al may be measured by ICP-AES using a filtrate obtained by thermally decomposing a sample with an acid. The measurement is performed by the combustion-infrared absorption method, the inert gas dissolution-heat conduction method, or the inert gas dissolution-non-dispersion infrared absorption method.
In the case where the hot-rolled steel sheet has a coating layer on the surface, the coating layer may be removed by mechanical grinding or the like, if necessary, and then the chemical composition may be analyzed.
Metal structure of hot rolled steel sheet
Next, a microstructure of the hot-rolled steel sheet according to the present embodiment will be described.
The hot-rolled steel sheet according to the present embodiment has a microstructure in terms of area% in which retained austenite is less than 3.0%, ferrite is 15.0% or more and less than 60.0%, pearlite is less than 5.0%, entropy (entropy) represented by the following formula (1) is 10.7 or more, INVERSE DIFFERENCE normalized (inverse differential normalization) represented by the following formula (2) is 1.020 or more, cluster shadow represented by the following formula (3) is-8.0x10 5～8.0×105, and standard deviation of Mn concentration is 0.60 mass% or less, which are obtained by analyzing SEM images of the microstructure by a gray scale co-occurrence matrix method.
Therefore, the hot-rolled steel sheet according to the present embodiment has high strength and a limit fracture plate thickness reduction rate, and can obtain excellent ductility and shearing workability. In the present embodiment, in a cross section parallel to the rolling direction, a standard deviation of a metal structure tissue, a Entropy (entropy) value, a INVERSE DIFFERENCE normalized value, a Cluster Shade value, and an Mn concentration at a center position in the plate width direction from a 1/4 depth position (a region from the surface to a 1/8 depth to a 3/8 depth from the surface) of the plate thickness from the surface is defined. The reason for this is that the microstructure at this position represents a representative microstructure of the steel sheet.
In addition, the surface referred to herein means an interface between the coating layer and the steel sheet in the case where the hot-rolled steel sheet has the coating layer.
Area ratio of retained austenite: less than 3.0%
Retained austenite is a metallic structure that exists in the form of a face-centered cubic lattice even at room temperature. The retained austenite has an effect of improving the ductility of the hot rolled steel sheet by transformation induced plasticity (TRIP). On the other hand, since the retained austenite is transformed into high-carbon martensite during the shearing process, stable crack generation is inhibited, and the secondary shearing surface and the reduction in the limit fracture plate thickness are caused. When the area ratio of retained austenite is 3.0% or more, the above-mentioned effect is remarkable, and the shearing workability of the hot-rolled steel sheet is deteriorated. Therefore, the area ratio of the retained austenite is less than 3.0%. The area ratio of the retained austenite is preferably less than 1.5%, more preferably less than 1.0%. Since the smaller the retained austenite is, the more preferable, the area ratio of the retained austenite may be 0%.
Among methods for measuring the area ratio of retained austenite, methods based on X-ray diffraction, EBSP (electron back scattering diffraction image, electron Back Scattering Diffraction Pattern) analysis, magnetic measurement, and the like are known. In the present embodiment, the retained austenite is less susceptible to polishing (if affected by polishing, the retained austenite may be changed to other phases such as martensite, and thus the true area ratio may not be measured), and an accurate measurement result can be obtained relatively easily, and the area ratio of the retained austenite is measured by X-ray diffraction that is less susceptible to polishing.
In the measurement of the retained austenite area ratio by X-ray diffraction in the present embodiment, first, the integrated strength of six peaks in total is obtained by using co—kα line in a plate thickness section parallel to the rolling direction at a 1/4 depth position (a region from 1/8 depth to 3/8 depth from the surface) and a plate width direction center position of the hot-rolled steel plate, and the volume ratio of retained austenite is calculated by using the strength averaging method. The volume fraction of the obtained retained austenite was regarded as the area fraction of the retained austenite.
Area ratio of ferrite: 15.0% or more and less than 60.0%
Ferrite is a structure generated when fcc changes to bcc at a relatively high temperature. Ferrite has a high work hardening rate, and thus has an effect of improving the strength-ductility balance of the hot-rolled steel sheet. In order to obtain the above effect, the area ratio of ferrite is 15.0% or more. Preferably 20.0% or more, more preferably 25.0% or more, and still more preferably 30.0% or more.
On the other hand, ferrite has low strength, and therefore if the area ratio is excessive, the desired strength cannot be obtained. Therefore, the ferrite area ratio is less than 60.0%. Preferably 50.0% or less, more preferably 45.0% or less or 40.0% or less.
Area ratio of pearlite: less than 5.0%
Pearlite is a sheet-like metallic structure in which cementite is layered between ferrite, and is a soft metallic structure when compared with bainite or martensite. If the area ratio of pearlite is 5.0% or more, carbon is consumed by cementite contained in pearlite, and the strength of martensite and bainite, which are the remainder of the structure, is lowered, and a desired strength cannot be obtained. Therefore, the area ratio of pearlite is less than 5.0%. The area ratio of pearlite is preferably 3.0% or less, 2.0% or less, or 1.0% or less.
In order to improve stretch flangeability of the hot-rolled steel sheet, it is preferable to reduce the area ratio of pearlite as much as possible, and it is more preferable that the area ratio of pearlite is 0%.
The hot-rolled steel sheet according to the present embodiment contains, as a residual structure other than retained austenite, ferrite, and pearlite, a hard structure composed of one or two or more of bainite, martensite, and tempered martensite having a total area ratio of more than 32.0% and 85.0% or less. The lower limit of the tissue of the rest part can be 36.0%, 40.0%, 44.0%, 48.0%, 52.0% or 55.0%, and the upper limit can be 82.0%, 78.0%, 74.0%, 70.0% or 66.0%. The remaining structure other than the retained austenite, ferrite, and pearlite may be one or more of bainite, martensite, and tempered martensite.
The area ratio of the metal structure was measured by the following method. The plate thickness cross section parallel to the rolling direction was mirror-finished, and the plate was polished at room temperature for 8 minutes using colloidal silica containing no alkaline solution, to remove strain introduced into the surface layer of the sample. The crystal orientation information was obtained by measuring a region having a length of 50 μm, a depth of 1/4 of the plate thickness from the surface (a region having a depth of 1/8 of the plate thickness from the surface to a depth of 3/8 of the plate thickness from the surface) and a central position in the width direction of the plate at an arbitrary position in the longitudinal direction of the sample cross section by an electron back scattering diffraction method at a measurement interval of 0.1. Mu.m. For the measurement, an EBSD analyzer composed of a thermal field emission type scanning electron microscope (JEOL JSM-7001F) and an EBSD detector (TSL DVC5 type detector) was used. At this time, the vacuum degree in the EBSD analyzer was 9.6X10 -5 Pa or less, the acceleration voltage was 15kV, the irradiation current level was 13, and the irradiation level of the electron beam was 62. The observation area was 40000. Mu.m 2.
Further, a reflected electron image is captured in the same field of view. First, crystal grains in which ferrite and cementite are precipitated in a layered form are determined from a reflected electron image, and the area ratio of the crystal grains is calculated to obtain the area ratio of pearlite. Thereafter, of the crystal grains other than those discriminated as pearlite, those having a body-centered cubic lattice structure were determined, and the obtained crystal orientation information was determined as ferrite using a function of "GRAIN AVERAGE Misorientation (grain average orientation difference)" mounted on the software "OIM Analysis (registered trademark)" attached to the EBSD Analysis apparatus, and a region having a GRAIN AVERAGE Misorientation (grain average orientation difference) value of 1.0 ° or less. At this time, grain Tolerance Angle (grain tolerance angle) was set to 15 °, and the area ratio of the region determined to be ferrite was obtained by obtaining the area ratio of ferrite.
Next, the area ratio of the regions other than the regions identified as pearlite or ferrite was measured as the area ratio of the remaining portion of the structure (i.e., bainite, martensite, and tempered martensite). When the total of the area ratio of bainite and the area ratio of martensite and tempered martensite is to be measured, the area ratio thereof can be measured by the following method. Specifically, when the maximum value of "GRAIN AVERAGE IQ (grain average IQ)" in the ferrite region is defined as iα, the region exceeding iα/2 is extracted (determined) as bainite, and the region equal to or smaller than iα/2 is extracted (determined) as "martensite or tempered martensite". The area ratio of bainite is obtained by calculating the area ratio of the region extracted (determined) as bainite. Further, the total of the area ratios of the martensite and tempered martensite is obtained by calculating the area ratio of the region extracted (determined) as the martensite or tempered martensite.
In this embodiment, since the area ratio of each tissue is measured by X-ray diffraction and EBSD analysis, the total area ratio of each tissue obtained by measurement may not be 100.0%. When the total area ratio of the tissues obtained by the above method is not 100.0%, the area ratio of the tissues is converted so that the total area ratio of the tissues is 100.0%. For example, when the total area ratio of each tissue is 103.0%, the area ratio of each tissue is multiplied by "100.0/103.0" to obtain the area ratio of each tissue.
Entropy (entropy) value: 10.7 or more, INVERSE DIFFERENCE normalized (inverse differential normalized) values: 1.020 or more
In order to suppress the occurrence of the secondary shearing surface, it is important to form the fracture surface after the shearing surface is sufficiently formed, and it is necessary to suppress the occurrence of cracks from the tip of the tool at the time of shearing. For this reason, it is important that the periodicity of the metal structure is low and the uniformity of the metal structure is high. In this embodiment, generation of the secondary shearing surface is suppressed by controlling Entropy (entropy) value (E value) indicating periodicity of the metal structure and INVERSE DIFFERENCE normalized (I value) value indicating uniformity of the metal structure.
The E value represents the periodicity of the metal structure. When the brightness is periodically arranged, that is, the periodicity of the metal structure is high due to the influence of the formation of the band structure or the like, the E value is lowered. In this embodiment, since it is necessary to form a metal structure having a low periodicity, it is necessary to increase the E value. If the E value is less than 10.7, a secondary shearing face is liable to occur. Starting from the periodically arranged tissue, cracks are generated from the cutting edge of the cutting tool at an extremely early stage of the cutting process to form a fracture surface, and thereafter, the fracture surface is formed again. This is because it is estimated that the secondary shearing surface is likely to occur. Therefore, E value is 10.7 or more. Preferably 10.8 or more, more preferably 11.0 or more. The higher the E value, the more preferable, the upper limit is not particularly specified, but may be 13.0 or less, 12.5 or less, or 12.0 or less.
The I value indicates uniformity of the metal structure, and the wider the area of the region having a certain brightness is, the higher the area is. The high I value means that the uniformity of the metal structure is high. In the hot-rolled steel sheet having a metal structure in which the area ratio of ferrite is 15.0% or more and less than 60.0% as the embodiment, it is necessary to form a metal structure having high uniformity. Therefore, in the present embodiment, it is necessary to increase the I value. If the I value is less than 1.020, cracks are generated from the edge of the shearing tool at an extremely early stage of the shearing process due to the influence of the hardness distribution caused by the precipitates in the crystal grains and the element concentration difference, and thereafter, the shearing face is formed again. This is because it is estimated that the secondary shearing surface is likely to occur. Therefore, the I value is 1.020 or more. Preferably 1.025 or more, and more preferably 1.030 or more. The higher the I value, the more preferable, the upper limit is not particularly specified, but may be 1.200 or less, 1.150 or less, or 1.100 or less.
Cluster Shade value: -8.0X10 5～8.0×105
Cluster Shadow (CS) value indicates the bias of the metal structure. The CS value is a positive value if there are many points having a luminance exceeding the average value, and is a negative value if there are many points having a luminance lower than the average value, with respect to the average value of the luminance in the image obtained by photographing the metal tissue.
In a secondary electron image of a scanning electron microscope, the brightness increases at a portion where the surface roughness of an object to be observed is large, and the brightness decreases at a portion where the roughness is small. The irregularities on the surface of the observation target are greatly affected by the particle size and intensity distribution in the metal structure. The CS value in the present embodiment increases when the strength deviation of the metal structure is large or the structure unit is small, and decreases when the strength deviation is small or the structure unit is large.
In the present embodiment, it is important to keep the CS value within a desired range close to 0. If the CS value is less than-8.0X10 5, the reduction rate of the limit fracture plate thickness of the hot rolled steel sheet is lowered. It is assumed that this is because grains having a large grain size exist in the metal structure, and the grains are preferentially broken in the limit deformation. Therefore, CS values are-8.0X10 5 or more. preferably-7.5X10 5 or more, more preferably-7.0X10 5 or more.
On the other hand, if the CS value exceeds 8.0X10 5, the limit fracture plate thickness reduction rate of the hot-rolled steel sheet decreases. It is assumed that this is because the microscopic strength deviation in the metal structure is large, and the strain in the limit deformation is concentrated locally and easily broken. Therefore, CS values are 8.0X10 5 or less. Preferably 7.5X10 5 or less, more preferably 7.0X10 or less 5.
The E value, I value, and CS value can be obtained by the following methods.
In the present embodiment, the scanning area of the SEM image (secondary electron image of scanning electron microscope) that is imaged for calculating the E value, I value, and CS value is a 1/4 depth position (1/8 depth from the surface to 3/8 depth from the surface) of the plate thickness from the surface and the center position in the plate width direction in the plate thickness cross section parallel to the rolling direction. In SEM image photographing, SU-6600 Schottky electron gun manufactured by Hitachi Ltd was used to make the emitter tungsten and the acceleration voltage was 1.5kV. Under the above setting, SEM images were output at a magnification of 1000 times and a gray scale of 256 gray scale.
Next, a smoothing process described in non-patent document 3, in which the limiting magnification of contrast enhancement is set to 2.0 and the tile grid size is 8×8, is performed on an image obtained by cutting the obtained SEM image into a region of 880×880 pixels (the observation region is 160 μm×160 μm in terms of actual size). In addition to 90 degrees, the smoothed SEM images were rotated counterclockwise every 1 degree from 0 degree to 179 degrees, and images were produced every 1 degree, thereby obtaining 179 images in total. Next, for each of these 179 images, the frequency value of the luminance between adjacent pixels is acquired in a matrix form using the GLCM method described in non-patent document 1.
Let k be the rotation angle from the original image, 179 frequencies to be acquired by the above method the matrix of values is expressed as p k (k=0····89; the matrix of values is expressed as p k (k=0····89 ·. For each image, after summing up the generated P k for all k (k=0··89, 91·179), a matrix P of 256×256 normalized so that the sum of the components is 1 is calculated. Further, using the following formulas (1) to (5) described in non-patent document 2, the E value, I value, and CS value are calculated, respectively.
P (i, j) in the following formulas (1) to (5) is a gray-scale co-occurrence matrix, and the value of the ith row and jth column of the matrix P is expressed as P (i, j). Since the calculation is performed using the matrix P of 256×256 as described above, when the point is to be enhanced, the following equations (1) to (5) can be modified to the following equations (1 ') to (5').
Here, L in the following formula (2) is the number of gradation levels (Quantization levels of grayscale) that can be obtained by the SEM image, and in the present embodiment, since the SEM image is output at a gradation level of 256 gradations as described above, L is 256. I and j in the following formulas (2) and (3) are natural numbers of 1 to L, and μ x and μ y in the following formula (3) are represented by the following formulas (4) and (5), respectively.
In the following formulas (1 ') to (5'), the value of the j-th column of the i-th row of the matrix P is expressed as P ij.
[ Math figure 6]
Entropy＝-∑i∑jP(i，j).log(P(i，j))…(1)
[ Math 7]
[ Math figure 8]
Cluster Shade＝∑i∑j(i+j-μx-μy)3P(i，j)…(3)
[ Math figure 9]
[ Math figure 10]
μy＝∑i∑jj(P(i，j))…(5)
[ Mathematics 11]
[ Math figure 12]
[ Math 13]
[ Math 14]
[ Math 15]
Standard deviation of Mn concentration: 0.60 mass% or less
The hot-rolled steel sheet according to the present embodiment has a standard deviation of Mn concentration of 0.60 mass% or less at a 1/4 depth position from the surface (a region from 1/8 depth to 3/8 depth from the surface) and at a central position in the width direction. This makes it possible to uniformly disperse the hard phase, and to prevent cracking from occurring at the edge of the shearing tool at an extremely early stage of the shearing process. As a result, the occurrence of the secondary shearing surface can be suppressed. The standard deviation of the Mn concentration is preferably 0.55 mass% or less or 0.50 mass% or less, more preferably 0.47 mass% or less or 0.45 mass% or less. The lower limit of the standard deviation of the Mn concentration is preferably smaller from the viewpoint of suppressing excessive burrs, but the substantial lower limit is 0.10 mass% due to the limitation of the manufacturing process. The lower limit may be set to 0.20 mass% or 0.28 mass% as required.
After mirror polishing the plate thickness cross section of the hot-rolled steel plate parallel to the rolling direction, the standard deviation of the Mn concentration was measured at a 1/4 depth position (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 by an Electron Probe Microanalyzer (EPMA). The measurement conditions were set to 15kV for acceleration voltage and 5000 times for magnification, and distribution images were measured in the range of 20 μm in the sample rolling direction and 20 μm in the sample plate thickness direction. More specifically, the Mn concentration was measured at 40000 or more sites with a measurement interval of 0.1. Mu.m. Next, the standard deviation was calculated based on the Mn concentrations obtained from all the measurement points, thereby obtaining the standard deviation of the Mn concentration.
Average crystal grain size of surface layer: less than 3.0 μm
By reducing the crystal grain size of the surface layer, the bending internal cracks of the hot-rolled steel sheet can be suppressed. The higher the strength of the hot-rolled steel sheet, the more likely a crack is generated from the inside of the bend (hereinafter referred to as "in-bend crack") during bending. The mechanism of the crack in the bending is presumed as follows. During bending, compressive stress is generated inside the bend. Initially, the entire inside of the bend is machined while being uniformly deformed, but if the machining amount becomes large, deformation cannot be assumed only by uniform deformation, and strain is concentrated locally, whereby deformation progresses (shear deformation zone is generated). As the shear deformation band further grows, cracks along the shear band are generated from the curved inner side surface and grow. The reason why the in-bending cracks are likely to occur with the increase in strength is that it is difficult to uniformly deform due to the decrease in work hardening energy with the increase in strength, and the deformation deviation is likely to occur, whereby the shear deformation zone is generated at an early stage of processing (or under a slow processing condition).
As a result of studies by the present inventors, it was found that the bending internal cracks become remarkable in steel sheets having a tensile strength of 980MPa or more. Further, the inventors have found that the finer the grain size of the surface layer of the hot-rolled steel sheet, the more localized strain concentration is suppressed, and the more difficult the occurrence of cracks in bending. In order to obtain the above effect, the average crystal grain size of the surface layer of the hot-rolled steel sheet is preferably set to less than 3.0. Mu.m. Therefore, in the present embodiment, the average crystal grain size of the surface layer may be smaller than 3.0 μm. The average crystal grain size of the surface layer is more preferably 2.7 μm or less or 2.5 μm or less. The lower limit of the average crystal grain size in the surface layer region is not particularly limited, and may be 0.5 μm or 1.0 μm.
In the present embodiment, the surface layer refers to a region from the surface to a depth of 50 μm from the surface of the hot-rolled steel sheet. As described above, the surface referred to herein means the interface between the coating layer and the steel sheet in the case where the hot-rolled steel sheet has the coating layer.
The crystal grain size of the surface layer was measured by the EBSP-OIM (Electron Back Scatter Diffraction Pattern-Orientation Image Microscopy, electron back scattering diffraction pattern-orientation imaging microscopy)) method. The EBSP-OIM method is performed using an OIM Analysis (registered trademark) manufactured by ameteek corporation, which is a device combining a scanning electron microscope and an EBSP analyzer. The analyzable region of the EBSP-OIM method is a region that can be observed with SEM. Although also depending on the resolution of the SEM, according to the EBSP-OIM method, analysis can be performed with a resolution of minimum 20 nm.
In a region of a cross section of the hot-rolled steel sheet parallel to the rolling direction, from the surface to a position of 50 μm in depth from the surface and a central position in the sheet width direction, analysis was performed in at least five fields of view in a region of 40 μm×30 μm at a magnification of 1200 times. The area-average crystal grain size was calculated by defining the region where the angle difference between adjacent measurement points was 5 ° or more as the grain boundary. The obtained area-average crystal grain size was used as the average crystal grain size of the surface layer.
In addition, since the retained austenite is not a structure formed by transformation at 600 ℃ or lower, and does not have an effect of dislocation accumulation, the retained austenite is not an object of analysis in the present measurement method (measurement method of the average crystal grain size of the surface layer). In the case where the area ratio of the retained austenite is 0%, the EBSP-OIM method does not need to be performed from the analysis object, except for the case where the measurement of the average crystal grain size of the surface layer may be affected, and the like, the retained austenite having the crystal structure fcc is measured from the analysis object.
Tensile Strength Property
Tensile strength characteristics (tensile strength, total elongation) among mechanical properties of hot rolled steel sheet are based on JIS Z2241: 2011. Test piece was JIS Z2241: 2011 test piece No. 5. The collecting position of the tensile test piece is 1/4 part from the end of the plate width direction, and the direction perpendicular to the rolling direction is taken as the length direction.
The hot-rolled steel sheet according to the present embodiment has a Tensile Strength (TS) of 980MPa or more. Preferably 1000MPa or more. If the tensile strength is less than 980MPa, the application member is limited, and the contribution to the weight reduction of the vehicle body is small. The upper limit is not particularly limited, but may be 1780MPa from the viewpoint of suppressing die wear.
The total elongation of the hot-rolled steel sheet according to the present embodiment is preferably 10.0% or more, and the product (TS×EI) of the tensile strength and the total elongation is preferably 13000 MPa% or more. The total elongation is more preferably 11.0% or more, and still more preferably 13.0% or more. The product of the tensile strength and the total elongation is more preferably 14000MPa·% or more, and still more preferably 15000MPa·% or more. The total elongation is 10.0% or more and the product of the tensile strength and the total elongation is 13000mpa·% or more, whereby the application member is not limited, and the weight reduction of the vehicle body can be greatly facilitated. The upper limit of the product of the tensile strength and the total elongation is not necessarily defined, but may be 22000mpa·% or 18000mpa·%. The upper limit of the total elongation is not necessarily specified, but may be 30.0%, 25.0%, or 22.0%.
Plate thickness
The thickness of the hot-rolled steel sheet according to the present embodiment is not particularly limited, and may be 0.5 to 8.0mm. When the thickness of the hot-rolled steel sheet is less than 0.5mm, it may be difficult to ensure the rolling completion temperature, and the rolling load becomes excessive, and hot rolling becomes difficult. Therefore, the thickness of the hot-rolled steel sheet according to the present embodiment may be 0.5mm or more. Preferably 1.2mm or more, 1.4mm or more, or 1.8mm or more. On the other hand, when the plate thickness exceeds 8.0mm, the metal structure may be difficult to be miniaturized, and the metal structure may be difficult to be obtained. Therefore, the plate thickness may be 8.0mm or less. Preferably 6.0mm or less, 5.0mm or less, or 4.0mm or less.
Coating layer
The hot-rolled steel sheet according to the present embodiment having the above-described chemical composition and metallic structure may be provided with a plating layer on the surface as a surface-treated steel sheet for the purpose of improving corrosion resistance and the like. The plating layer may be a plating layer or a hot dip plating layer. Examples of the plating layer include electrogalvanizing and Zn-Ni alloy plating. Examples of the hot dip coating layer include hot dip galvanization, alloyed hot dip galvanization, hot dip aluminizing, hot dip Zn-Al alloy, hot dip Zn-Al-Mg alloy, and hot dip Zn-Al-Mg-Si alloy. The amount of the plating layer to be deposited is not particularly limited and may be the same as in the prior art. Further, after plating, an appropriate chemical conversion treatment (for example, coating and drying of a silicate-based chromium-free chemical conversion treatment solution) may be performed to further improve corrosion resistance.
Production conditions
A preferred method for producing a hot-rolled steel sheet according to the present embodiment having the above-described chemical composition and metallic structure is as follows.
In a preferred method for producing a hot-rolled steel sheet according to the present embodiment, the following steps (1) to (10) are performed in order. The temperature of the slab and the temperature of the steel sheet in the present embodiment refer to the surface temperature of the slab and the surface temperature of the steel sheet. The stress refers to tension applied in the rolling direction of the steel sheet.
(1) The slab is further heated after being held at a temperature of 700 to 850 ℃ for 900 seconds or more, and is held at a temperature of 1100 ℃ or more for 6000 seconds or more.
(2) Hot rolling is performed in a temperature range of 850 to 1100 ℃ to reduce the total sheet thickness by 90% or more.
(3) After rolling in the preceding stage from the final stage of hot rolling and before rolling in the final stage, a stress of 170kPa or more is applied to the steel sheet.
(4) The hot rolling is completed such that the reduction ratio of the final stage of the hot rolling is 8% or more and the rolling completion temperature Tf is 900 ℃ or more and less than 1010 ℃.
(5) After rolling in the final stage of hot rolling and until the steel sheet is cooled to 800 ℃, the stress applied to the steel sheet is made to be less than 200kPa.
(6) Cooling to a temperature zone below the hot rolling completion temperature Tf-50 ℃ within 1 second after the hot rolling is completed, and then accelerating cooling to a temperature zone of 600-730 ℃ at an average cooling rate of more than 50 ℃/s. Wherein, cooling to a temperature zone of from Tf to 50 ℃ or less, which is the hot rolling completion temperature, is a more preferable cooling condition within 1 second after completion of hot rolling.
(7) In a temperature range of 600-730 ℃, slow cooling is performed with an average cooling rate of less than 5 ℃ per second over 2.0 seconds.
(8) After the slow cooling is completed, the cooling is performed so that the average cooling rate in the temperature range of 450 to 600 ℃ is 30 ℃ per second or more and less than 50 ℃ per second.
(9) The cooling is performed so that the average cooling rate in the temperature range from the winding temperature to 450 ℃ is 50 ℃/s or more.
(10) Coiling is performed in a temperature region below 350 ℃.
By using the above production method, a hot-rolled steel sheet having a metal structure with high strength and a reduction in ultimate fracture plate thickness, excellent ductility, and excellent shearing workability can be stably produced. That is, by properly controlling the slab heating conditions and the hot rolling conditions, mn segregation is reduced and austenite is equiaxed before transformation, and a hot-rolled steel sheet having a desired metal structure can be stably produced by combining the cooling conditions after hot rolling described later.
(1) Slab, slab temperature for hot rolling, and holding time
As the slab to be hot-rolled, a slab obtained by continuous casting, a slab obtained by casting and cogging, or the like can be used. Further, materials to which hot working or cold working is applied can be used as needed.
The slab to be hot-rolled is preferably further heated after being held at a temperature range of 700 to 850 ℃ for 900 seconds or more and then held at a temperature range of 1100 ℃ or more for 6000 seconds or more when the slab is heated. In addition, the temperature of the steel sheet may be varied or kept constant in the temperature range of 700 to 850 ℃. In the holding in the temperature range of 1100 ℃ or higher, the temperature of the steel sheet may be varied or kept constant in the temperature range of 1100 ℃ or higher.
In the austenite transformation in the temperature range of 700 to 850 ℃, mn is distributed between ferrite and austenite, and Mn can be diffused in the ferrite region by extending the transformation time. Thus, mn micro-segregation, which is biased to the slab, can be eliminated, and the standard deviation of Mn concentration can be significantly reduced. Further, by holding the steel sheet at a temperature of 1100 ℃ or higher for 6000 seconds or longer, austenite grains can be made uniform when the steel sheet is heated.
The hot rolling preferably uses a reversing mill or a tandem mill as the multipass rolling. In particular, from the viewpoint of industrial productivity and stress load on a steel sheet being rolled, it is more preferable that at least the final two stages are hot rolling using a tandem mill.
(2) Reduction ratio of hot rolling: the total thickness reduction of 90% or more in a temperature range of 850-1100 DEG C
By hot rolling in a temperature range of 850 to 1100 ℃ in which the total thickness is reduced by 90% or more, the recrystallized austenite grains are mainly miniaturized, and the accumulation of strain energy into the unrecrystallized austenite grains is promoted. Further, it is possible to promote recrystallization of austenite, and promote atomic diffusion of Mn, reducing the standard deviation of Mn concentration. Therefore, it is preferable to perform hot rolling in which the total thickness of the sheet is reduced by 90% or more in a temperature range of 850 to 1100 ℃.
The decrease in the plate thickness in the temperature range of 850 to 1100 ℃ means: when the inlet plate thickness before the initial rolling in the rolling in this temperature range is set to t 0 and the outlet plate thickness after the rolling in the final stage in the rolling in this temperature range is set to t 1, it can be expressed by { (t 0-t1)/t0 } ×100 (%).
(3) Stress after rolling of the preceding stage and before rolling of the final stage from the final stage of hot rolling: 170kPa or more
The stress applied to the steel sheet after rolling in the preceding stage and before rolling in the final stage from the final stage of hot rolling is preferably 170kPa or more. This can reduce the number of crystal grains having {110} < 001 > crystal orientation in the recrystallized austenite after rolling in the preceding stage from the final stage. Since {110} < 001 > is a crystal orientation which is difficult to recrystallize, recrystallization at a pressure based on the final stage can be effectively promoted by suppressing the formation of the crystal orientation. As a result, the strip structure of the hot-rolled steel sheet is improved, the periodicity of the metal structure is reduced, and the E value is increased.
When the stress applied to the steel sheet is less than 170kPa, the E value may not be set to a desired value. The stress applied to the steel sheet is more preferably 190kPa or more.
The stress applied to the steel sheet can be controlled by adjusting the roll rotation speed during tandem rolling, and can be obtained by dividing the load in the rolling direction measured by the rolling stand by the cross-sectional area of the passing sheet.
(4) Reduction ratio of final stage of hot rolling: 8% or more, hot rolling completion temperature Tf:900 ℃ to less than 1010 DEG C
The reduction ratio of the final stage of hot rolling is preferably 8% or more, and the hot rolling completion temperature Tf is 900 ℃ or more. By setting the rolling reduction of the final stage of hot rolling to 8% or more, recrystallization by rolling reduction of the final stage can be promoted. As a result, the strip structure of the hot-rolled steel sheet is improved, the periodicity of the metal structure is reduced, and the E value is increased. By setting the hot rolling completion temperature Tf to 900 ℃ or higher, an excessive increase in the number of ferrite core formation sites in austenite can be suppressed. As a result, the formation of ferrite in the final structure (the structure of the hot-rolled steel sheet after production) can be suppressed, and a high-strength hot-rolled steel sheet can be obtained. Further, by setting the hot rolling completion temperature Tf to less than 1010 ℃, coarsening of the austenite grain diameter can be suppressed, the periodicity of the microstructure can be reduced, and the E value can be set to a desired value.
(5) Stress after rolling in the final stage of hot rolling and until the steel sheet is cooled to 800 ℃): less than 200kPa
The stress on the steel sheet after rolling in the final stage of hot rolling and until the steel sheet is cooled to 800 ℃ is preferably less than 200kPa. By setting the stress (tension) applied to the steel sheet in the rolling direction to be less than 200kPa, recrystallization of austenite proceeds preferentially in the rolling direction, and thus periodic increase of the metal structure can be suppressed. As a result, the E value can be set to a desired value. The stress applied to the steel sheet is more preferably 180MPa or less. The stress applied to the steel sheet in the rolling direction can be controlled by adjusting the rotational speeds of the rolling stand and the coiling device, and can be obtained by dividing the measured load in the rolling direction by the cross-sectional area of the passing sheet.
(6) Cooling to a temperature region below the hot rolling completion temperature Tf-50 ℃ within 1 second after the hot rolling is completed, and then accelerating cooling to a temperature region of 600-730 ℃ at an average cooling rate of more than 50 ℃ per second
In order to suppress growth of austenite grains refined by hot rolling, it is more preferable to cool at 50 ℃ or higher within 1 second after completion of hot rolling. In order to cool the steel sheet to a temperature region of not more than the hot-rolling completion temperature Tf-50 ℃ within 1 second after completion of hot rolling, cooling with a high average cooling rate may be performed immediately after completion of hot rolling, for example, by spraying cooling water onto the surface of the steel sheet. By cooling to a temperature range of Tf-50 ℃ or lower within 1 second after completion of hot rolling, the crystal grain size of the surface layer can be made finer, and the bending crack resistance of the hot-rolled steel sheet can be improved.
Further, after the cooling, the cooling is accelerated at an average cooling rate of 50 ℃/s or more to a temperature range of 730 ℃ or less, whereby the formation of ferrite and pearlite with a small precipitation strengthening amount can be suppressed. This improves the strength of the hot-rolled steel sheet. The average cooling rate referred to herein means: the temperature decrease width of the steel sheet from the start of accelerated cooling (when the steel sheet is introduced into the cooling equipment) to the completion of accelerated cooling (when the steel sheet is introduced from the cooling equipment) is divided by the time required from the start of accelerated cooling to the completion of accelerated cooling.
The upper limit value of the cooling rate is not particularly limited, but if the cooling rate is increased, the cooling equipment becomes large in scale and the equipment cost becomes high. Therefore, in view of equipment cost, it is preferably 300 ℃ per second or less. In order to perform slow cooling described later, the cooling stop temperature of the accelerated cooling may be 600 ℃ or higher.
(7) In a temperature range of 600-730 ℃, slowly cooling at an average cooling rate of less than 5 ℃/s for more than 2.0 seconds
By performing slow cooling at a temperature range of 600 to 730 ℃ for 2.0 seconds or more at an average cooling rate of less than 5 ℃/s, precipitation-strengthened ferrite can be sufficiently precipitated. This makes it possible to achieve both strength and ductility of the hot-rolled steel sheet.
The average cooling rate referred to herein means: the value obtained by dividing the temperature decrease width of the steel sheet from the cooling stop temperature of the accelerated cooling to the stop temperature of the slow cooling by the time required from the stop time of the accelerated cooling to the stop time of the slow cooling.
The time for performing slow cooling is preferably 3.0 seconds or longer. The upper limit of the time for performing slow cooling is determined by the layout of the apparatus, but may be substantially less than 10.0 seconds. The lower limit of the average cooling rate of slow cooling is not particularly set, but the temperature rise may be 0 ℃/s or more because of the large investment in equipment without cooling.
(8) After the slow cooling is completed, the cooling is performed in such a manner that the average cooling rate in the temperature range of 450-600 ℃ is 30 ℃/s or more and less than 50 ℃/s
After the completion of the slow cooling, the cooling is preferably performed so that the average cooling rate in the temperature range of 450 to 600 ℃ is 30 ℃ per second or more and less than 50 ℃ per second. The CS value can be set to a desired value by setting the average cooling rate in the temperature range to 30 ℃/s or more and less than 50 ℃/s. When the average cooling rate exceeds 50 ℃/s, a flat streak-like structure with low brightness is easily produced, and the CS value is less than-8.0X10 5. When the average cooling rate is less than 30 ℃/s, the concentration of carbon in the non-phase-change portion is promoted, the strength of the hard structure increases, and the strength difference from the soft structure increases, so that the CS value exceeds 8.0×10 5.
The average cooling rate referred to herein means: a value obtained by dividing the temperature decrease width of the steel sheet from the cooling stop temperature of slow cooling at an average cooling rate of less than 5 ℃/s to the cooling stop temperature of cooling at an average cooling rate of 30 ℃/s or more and less than 50 ℃/s by the time required from the time when the slow cooling at an average cooling rate of less than 5 ℃/s is stopped to the time when the cooling at an average cooling rate of 30 ℃/s or more and less than 50 ℃/s is stopped.
(9) Average cooling rate in the temperature range of coiling temperature to 450 ℃): 50 ℃/s or more
In order to suppress the area ratio of pearlite and retained austenite and to obtain desired strength and formability, it is preferable that the average cooling rate in the temperature range from the winding temperature to 450 ℃ is 50 ℃/s or more. This makes it possible to harden the parent phase structure.
The average cooling rate referred to herein means: the temperature decrease width of the steel sheet from the cooling stop temperature of the cooling at an average cooling rate of 30 ℃/s or more and less than 50 ℃/s to the coiling temperature is divided by the time required from the time of stopping the cooling at an average cooling rate of 30 ℃/s or more and less than 50 ℃/s to the time required for coiling.
(10) Coiling temperature: 350 ℃ below
The winding temperature is set to be below 350 ℃. By setting the coiling temperature to 350 ℃ or lower, the amount of iron carbide precipitated can be reduced, and the variation in hardness distribution in the hard phase can be reduced. As a result, the I value can be increased, and the occurrence of the secondary shearing surface can be suppressed.
Examples
Next, the effects of one embodiment of the present invention will be described more specifically by way of examples, but the conditions in the examples are one condition example employed for confirming the possibility of implementation and effects of the present invention, and the present invention is not limited to this one condition example. The present invention can adopt various conditions as long as the object of the present invention is achieved without departing from the gist of the present invention.
Steels having chemical compositions shown in Table 1 and Table 2 were melted, and slabs having a thickness of 240 to 300mm were produced by continuous casting. Using the obtained slab, hot-rolled steel sheets shown in table 5 and table 6 were obtained under the manufacturing conditions shown in table 3 and table 4.
In addition, the average cooling rate of slow cooling is less than 5 ℃/s. In addition, since the winding temperature shown in table 4 is set to 50 ℃ as the measurement lower limit, the actual winding temperature shown as an example of 50 ℃ is 50 ℃ or lower.
The area ratio of the metal structure, the E value, the I value, the CS value, the standard deviation of Mn concentration, the average crystal grain size of the surface layer, the tensile strength TS, and the total elongation EI were obtained for the obtained hot-rolled steel sheet by the above-described method. The measurement results obtained are shown in tables 5 and 6.
The remainder of the structure is one or more of bainite, martensite, and tempered martensite.
Method for evaluating characteristics of hot-rolled steel sheet
Tensile characteristics
When the Tensile Strength (TS) was 980MPa or more, the total Elongation (EI) was 10.0% or more, and the Tensile Strength (TS). Times.the total Elongation (EI) was 13000 MPa.about.% or more, the steel sheet was judged to be a hot-rolled steel sheet having high strength and excellent ductility. If either of these is not satisfied, the steel sheet is determined to be a hot-rolled steel sheet that does not have high strength and has excellent ductility.
Ultimate fracture plate thickness reduction rate
The ultimate fracture plate thickness reduction rate of the hot-rolled steel sheet was evaluated by a tensile test.
The tensile test was performed by the same method as in the case of evaluating the tensile characteristics. When the plate thickness before the tensile test is t1 and the minimum value of the plate thickness at the widthwise central portion of the tensile test piece after the fracture is t2, the limiting fracture plate thickness reduction rate is obtained by calculating the value of (t 1-t 2) ×100/t 1. The tensile test was performed five times, and the average value of three times excluding the maximum value and the minimum value of the limiting fracture plate thickness reduction rate was calculated to obtain the limiting fracture plate thickness reduction rate.
When the limit fracture plate thickness reduction rate is 60.0% or more, the steel sheet is judged to be acceptable as a hot-rolled steel sheet having a high limit fracture plate thickness reduction rate. On the other hand, when the limit fracture plate thickness reduction rate is less than 60.0%, the hot-rolled steel sheet is determined to be unacceptable as a hot-rolled steel sheet not having a high limit fracture plate thickness reduction rate.
Shear workability (secondary shear surface evaluation)
The shear workability of the hot-rolled steel sheet was evaluated by a blanking test.
Three punched holes were produced for each example at a hole diameter of 10mm, a clearance of 10% and a punching speed of 3 m/s. Next, the cross section of the punched hole perpendicular to the rolling direction and the cross section parallel to the rolling direction were each buried in the resin, and the cross section shape was photographed by a scanning electron microscope. In the obtained observation photograph, the sheared edge face shown in fig. 1 or 2 can be observed. Fig. 1 is an example of a sheared edge face of a hot-rolled steel sheet according to an example of the present invention, and fig. 2 is an example of a sheared edge face of a hot-rolled steel sheet according to a comparative example. In fig. 1, the shear end face is the collapse-shear face-fracture face-burr. On the other hand, in fig. 2, the shear end face is a collapse-shear face-fracture face-burr. Here, the sagging means an R-shaped smooth surface region, the shearing surface means a punched end surface region separated by shearing deformation, the breaking surface means a punched end surface region separated by a crack generated from the vicinity of the cutting edge, and the burr means a surface having a protrusion protruding from the lower surface of the hot-rolled steel sheet.
Of the obtained sheared edge surfaces, two surfaces perpendicular to the rolling direction and two surfaces parallel to the rolling direction are determined to form secondary sheared surfaces when the sheared surface-fracture surface-sheared surface shown in fig. 2 is found, for example. Four sides were observed for each punched hole, and if none of the sides where the secondary shearing surface appears was found, the steel sheet was judged to be acceptable as a hot-rolled steel sheet having excellent shearing workability, and the steel sheet was "none" in the table. On the other hand, even when one secondary shearing surface is formed, the steel sheet is judged as being defective as a hot-rolled steel sheet which is not excellent in shearing workability, and is described as "present" in the table.
Resistance to internal cracking during bending
The bending resistance to internal cracking was evaluated by the following bending test.
A rectangular test piece of 100mm X30 mm was cut from a position 1/2 in the width direction of a hot-rolled steel sheet to obtain a bending test piece. Both a bending (L-axis bending) in which the bending ridge line is parallel to the rolling direction (L-direction) and a bending (C-axis bending) in which the bending ridge line is parallel to the direction (C-direction) perpendicular to the rolling direction were performed in accordance with JIS Z2248: 2006V block method (bending angle θ of 90 °). Thus, the minimum bending radius at which no crack is generated was obtained. The resistance to internal cracking by bending was examined. The value obtained by dividing the average value of the minimum bending radii of the L-axis and the C-axis by the plate thickness is used as the limit bending R/t, and is used as the index value of the bending crack resistance. When R/t is 2.5 or less, it is determined that the hot-rolled steel sheet is excellent in resistance to bending internal cracking.
However, regarding the presence or absence of cracks, after mirror polishing a cross section obtained by cutting a test piece in a plane parallel to the bending direction and perpendicular to the plate surface, the cracks were observed with an optical microscope, and when the crack length observed inside the bending of the test piece exceeded 30 μm, the presence of cracks was determined.
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TABLE 3 Table 3
The underline indicates that the manufacturing conditions are not preferable.
TABLE 4 Table 4
The underline indicates that the manufacturing conditions are not preferable.
TABLE 5
Underlined indicates characteristics outside the scope of the present invention or which are not preferred.
TABLE 6
Underlined indicates characteristics outside the scope of the present invention or which are not preferred.
As is clear from examination of table 5 and table 6, the hot rolled steel sheet according to the example of the present invention has high strength and a reduction in the ultimate fracture plate thickness, and also has excellent ductility and shear workability. Further, it was found that the hot-rolled steel sheet having the surface layer with an average crystal grain size of less than 3.0 μm in the examples of the present invention has excellent resistance to internal cracking during bending in addition to the above-mentioned properties.
On the other hand, it was found that the hot-rolled steel sheet according to the comparative example was deteriorated in any one or more of strength, ductility, ultimate fracture plate thickness reduction ratio, and shear workability.
Industrial applicability
According to the above aspect of the present invention, a hot-rolled steel sheet having high strength and a reduction in ultimate fracture plate thickness, and excellent ductility and shearing workability can be provided. Further, according to the above preferred embodiment of the present invention, a hot-rolled steel sheet having the above characteristics, in which occurrence of cracks in bending is suppressed, that is, excellent in resistance to cracks in bending can be obtained.
The hot-rolled steel sheet according to the present invention is suitable as an industrial material for automobile parts, machine structural parts, and building parts.
Claims (3)
1. A hot rolled steel sheet is characterized by comprising the following chemical components in mass percent:
C：0.050～0.250％、
Si：0.05～3.00％、
Mn：1.00～4.00％、
sol.Al：0.001～2.000％、
P:0.100% or less,
S:0.0300% or less,
N: less than 0.1000 percent,
O:0.0100% or less,
Ti：0～0.500％、
Nb：0～0.500％、
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：0～0.0200％、
As：0～0.100％、
Zr：0～1.00％、
Co：0～1.00％、
Zn：0～1.00％、
W：0～1.00％、
Sn:0 to 0.05%, and
The remainder: fe and the impurities are mixed together,
Satisfies the following formula (A) and (B),
The metal structure is expressed in area percent,
The retained austenite is less than 3.0%,
Ferrite is 15.0% or more and less than 60.0%,
Pearlite less than 5.0%,
A Entropy (entropy) value represented by the following formula (1) obtained by analyzing the SEM image of the metal structure by a gray scale co-occurrence matrix method is 10.7 or more,
A INVERSE DIFFERENCE normalized value represented by the following formula (2) of 1.020 or more,
Cluster Shade represented by the following formula (3) has a value of-8.0X10 5～8.0×105,
The standard deviation of Mn concentration is 0.60 mass% or less,
The tensile strength is over 980MPa,
0.060％≤Ti+Nb+V≤0.500％ (A)
Zr+Co+Zn+W≤1.00％ (B)
Wherein each symbol of the elements in the formulas (A) and (B) represents the content of the element in mass%, and the symbol is substituted with 0% when the element is not contained,
Here, P (i, j) in the following formulas (1) to (5) is a gradation co-occurrence matrix, L in the following formula (2) is the number of gradation levels that can be obtained by the SEM image, i and j in the following formulas (2) and (3) are natural numbers of 1 to L, μ x and μ y in the following formula (3) are represented by the following formulas (4) and (5), respectively,
[ Mathematics 1]
Entropy＝-∑i∑jP(i，j).log(P(i，j)…(1)
[ Math figure 2]
[ Math 3]
Cluster Shade＝∑i∑j(i+j-μx-μy)3P(i，j)
[ Mathematics 4]
μx＝∑i∑j i(P(i，j))…(4)
[ Math 5]
μy＝∑i∑jj(P(i，j))…(5)。
2. The hot rolled steel sheet according to claim 1, wherein the surface layer has an average crystal grain size of less than 3.0 μm.
3. A hot rolled steel sheet according to claim 1 or 2, characterized in that,
The chemical composition contains one or more selected from the group consisting of the following elements in mass%:
Ti：0.001～0.500％、
Nb：0.001～0.500％、
V：0.001～0.500％、
Cu：0.01～2.00％、
Cr：0.01～2.00％、
Mo：0.01～1.00％、
Ni：0.01～2.00％、
B：0.0001～0.0100％、
Ca：0.0001～0.0200％、
Mg：0.0001～0.0200％、
REM：0.0001～0.1000％、
Bi：0.0001～0.0200％、
As：0.001～0.100％、
Zr：0.01～1.00％、
Co：0.01～1.00％、
Zn：0.01～1.00％、
W:0.01 to 1.00%, and
Sn：0.01～0.05％。
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| JP2021166958 | 2021-10-11 | ||
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| PCT/JP2022/034395 WO2023063010A1 (en) | 2021-10-11 | 2022-09-14 | Hot-rolled steel plate |
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| JP6152782B2 (en) * | 2013-11-19 | 2017-06-28 | 新日鐵住金株式会社 | Hot rolled steel sheet |
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