CN103459638A - High-strength steel sheet with excellent workability and manufacturing process therefor - Google Patents
High-strength steel sheet with excellent workability and manufacturing process therefor Download PDFInfo
- Publication number
- CN103459638A CN103459638A CN2012800158495A CN201280015849A CN103459638A CN 103459638 A CN103459638 A CN 103459638A CN 2012800158495 A CN2012800158495 A CN 2012800158495A CN 201280015849 A CN201280015849 A CN 201280015849A CN 103459638 A CN103459638 A CN 103459638A
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- Prior art keywords
- steel plate
- temperature
- bainite
- high tensile
- tensile steel
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- 229910000831 Steel Inorganic materials 0.000 title claims abstract description 267
- 239000010959 steel Substances 0.000 title claims abstract description 267
- 238000004519 manufacturing process Methods 0.000 title claims abstract description 26
- 229910001563 bainite Inorganic materials 0.000 claims abstract description 304
- 229910052751 metal Inorganic materials 0.000 claims abstract description 83
- 239000002184 metal Substances 0.000 claims abstract description 83
- 229910000734 martensite Inorganic materials 0.000 claims abstract description 75
- 238000000034 method Methods 0.000 claims abstract description 47
- 229910001566 austenite Inorganic materials 0.000 claims abstract description 44
- 229910052799 carbon Inorganic materials 0.000 claims abstract description 35
- 230000000717 retained effect Effects 0.000 claims abstract description 32
- XEEYBQQBJWHFJM-UHFFFAOYSA-N Iron Chemical compound [Fe] XEEYBQQBJWHFJM-UHFFFAOYSA-N 0.000 claims abstract description 24
- 230000005415 magnetization Effects 0.000 claims abstract description 24
- 239000012535 impurity Substances 0.000 claims abstract description 12
- 229910052742 iron Inorganic materials 0.000 claims abstract description 12
- 229910000859 α-Fe Inorganic materials 0.000 claims description 91
- 238000001816 cooling Methods 0.000 claims description 80
- 239000013078 crystal Substances 0.000 claims description 36
- HCHKCACWOHOZIP-UHFFFAOYSA-N Zinc Chemical compound [Zn] HCHKCACWOHOZIP-UHFFFAOYSA-N 0.000 claims description 19
- 229910052725 zinc Inorganic materials 0.000 claims description 19
- 239000011701 zinc Substances 0.000 claims description 19
- 238000007747 plating Methods 0.000 claims description 13
- 229910052761 rare earth metal Inorganic materials 0.000 claims description 13
- 239000011248 coating agent Substances 0.000 claims description 8
- 238000000576 coating method Methods 0.000 claims description 8
- 229910052758 niobium Inorganic materials 0.000 claims description 6
- 230000009772 tissue formation Effects 0.000 claims description 6
- 239000002131 composite material Substances 0.000 abstract description 9
- 229910052748 manganese Inorganic materials 0.000 abstract description 9
- 229910052782 aluminium Inorganic materials 0.000 abstract description 7
- 229910052698 phosphorus Inorganic materials 0.000 abstract description 4
- 229910052717 sulfur Inorganic materials 0.000 abstract description 4
- 229910001568 polygonal ferrite Inorganic materials 0.000 abstract 1
- 238000010438 heat treatment Methods 0.000 description 44
- 238000012360 testing method Methods 0.000 description 33
- 238000002791 soaking Methods 0.000 description 31
- 239000000463 material Substances 0.000 description 30
- 230000000694 effects Effects 0.000 description 27
- 239000012467 final product Substances 0.000 description 27
- 230000009466 transformation Effects 0.000 description 22
- 238000011156 evaluation Methods 0.000 description 20
- OKTJSMMVPCPJKN-UHFFFAOYSA-N Carbon Chemical compound [C] OKTJSMMVPCPJKN-UHFFFAOYSA-N 0.000 description 14
- 239000000203 mixture Substances 0.000 description 13
- 238000005096 rolling process Methods 0.000 description 13
- 238000005097 cold rolling Methods 0.000 description 12
- 238000010791 quenching Methods 0.000 description 12
- 238000005098 hot rolling Methods 0.000 description 10
- 238000012423 maintenance Methods 0.000 description 10
- 230000000171 quenching effect Effects 0.000 description 10
- 150000001875 compounds Chemical class 0.000 description 9
- 230000007797 corrosion Effects 0.000 description 9
- 238000005260 corrosion Methods 0.000 description 9
- 238000001556 precipitation Methods 0.000 description 9
- 238000000137 annealing Methods 0.000 description 8
- 230000008859 change Effects 0.000 description 8
- 239000010960 cold rolled steel Substances 0.000 description 8
- 235000019362 perlite Nutrition 0.000 description 8
- 239000010451 perlite Substances 0.000 description 8
- 230000033228 biological regulation Effects 0.000 description 7
- 229910052804 chromium Inorganic materials 0.000 description 7
- 238000009826 distribution Methods 0.000 description 7
- 229910052750 molybdenum Inorganic materials 0.000 description 7
- 230000008569 process Effects 0.000 description 7
- 238000005516 engineering process Methods 0.000 description 6
- 238000002474 experimental method Methods 0.000 description 6
- 238000005259 measurement Methods 0.000 description 6
- 230000009467 reduction Effects 0.000 description 6
- 239000000126 substance Substances 0.000 description 6
- 238000007669 thermal treatment Methods 0.000 description 6
- 229910000794 TRIP steel Inorganic materials 0.000 description 5
- 238000005452 bending Methods 0.000 description 5
- 229910052802 copper Inorganic materials 0.000 description 5
- HQFCOGRKGVGYBB-UHFFFAOYSA-N ethanol;nitric acid Chemical compound CCO.O[N+]([O-])=O HQFCOGRKGVGYBB-UHFFFAOYSA-N 0.000 description 5
- 229910052759 nickel Inorganic materials 0.000 description 5
- LFQSCWFLJHTTHZ-UHFFFAOYSA-N EtOH Substances CCO LFQSCWFLJHTTHZ-UHFFFAOYSA-N 0.000 description 4
- 229910001335 Galvanized steel Inorganic materials 0.000 description 4
- 238000005275 alloying Methods 0.000 description 4
- 230000002950 deficient Effects 0.000 description 4
- 239000008397 galvanized steel Substances 0.000 description 4
- 230000006698 induction Effects 0.000 description 4
- UUZZMWZGAZGXSF-UHFFFAOYSA-N peroxynitric acid Chemical compound OON(=O)=O UUZZMWZGAZGXSF-UHFFFAOYSA-N 0.000 description 4
- 238000012545 processing Methods 0.000 description 4
- 238000005482 strain hardening Methods 0.000 description 4
- 238000005728 strengthening Methods 0.000 description 4
- 229910052719 titanium Inorganic materials 0.000 description 4
- 229910052720 vanadium Inorganic materials 0.000 description 4
- 230000000007 visual effect Effects 0.000 description 4
- 239000006185 dispersion Substances 0.000 description 3
- 230000007246 mechanism Effects 0.000 description 3
- 238000002156 mixing Methods 0.000 description 3
- 150000004767 nitrides Chemical class 0.000 description 3
- 239000002245 particle Substances 0.000 description 3
- 230000002787 reinforcement Effects 0.000 description 3
- 230000006641 stabilisation Effects 0.000 description 3
- 238000011105 stabilization Methods 0.000 description 3
- 230000009471 action Effects 0.000 description 2
- QVGXLLKOCUKJST-UHFFFAOYSA-N atomic oxygen Chemical compound [O] QVGXLLKOCUKJST-UHFFFAOYSA-N 0.000 description 2
- 230000000254 damaging effect Effects 0.000 description 2
- 238000001125 extrusion Methods 0.000 description 2
- 239000011521 glass Substances 0.000 description 2
- 238000000227 grinding Methods 0.000 description 2
- 230000006872 improvement Effects 0.000 description 2
- 238000002844 melting Methods 0.000 description 2
- 230000008018 melting Effects 0.000 description 2
- 229910052757 nitrogen Inorganic materials 0.000 description 2
- 229910052760 oxygen Inorganic materials 0.000 description 2
- 239000001301 oxygen Substances 0.000 description 2
- 238000005554 pickling Methods 0.000 description 2
- 238000005498 polishing Methods 0.000 description 2
- 238000011160 research Methods 0.000 description 2
- 238000005204 segregation Methods 0.000 description 2
- 239000004575 stone Substances 0.000 description 2
- 229910052684 Cerium Inorganic materials 0.000 description 1
- 208000034189 Sclerosis Diseases 0.000 description 1
- UCKMPCXJQFINFW-UHFFFAOYSA-N Sulphide Chemical compound [S-2] UCKMPCXJQFINFW-UHFFFAOYSA-N 0.000 description 1
- 238000010521 absorption reaction Methods 0.000 description 1
- 229910052787 antimony Inorganic materials 0.000 description 1
- 229910052797 bismuth Inorganic materials 0.000 description 1
- 229910001567 cementite Inorganic materials 0.000 description 1
- 239000003153 chemical reaction reagent Substances 0.000 description 1
- 239000000470 constituent Substances 0.000 description 1
- 238000005520 cutting process Methods 0.000 description 1
- 230000007423 decrease Effects 0.000 description 1
- 230000003247 decreasing effect Effects 0.000 description 1
- 238000013461 design Methods 0.000 description 1
- 230000009977 dual effect Effects 0.000 description 1
- 239000007789 gas Substances 0.000 description 1
- 230000002650 habitual effect Effects 0.000 description 1
- 230000001939 inductive effect Effects 0.000 description 1
- KSOKAHYVTMZFBJ-UHFFFAOYSA-N iron;methane Chemical compound C.[Fe].[Fe].[Fe] KSOKAHYVTMZFBJ-UHFFFAOYSA-N 0.000 description 1
- 229910052746 lanthanum Inorganic materials 0.000 description 1
- 229910052745 lead Inorganic materials 0.000 description 1
- 239000011159 matrix material Substances 0.000 description 1
- 239000002244 precipitate Substances 0.000 description 1
- -1 residual γ Inorganic materials 0.000 description 1
- 229910052706 scandium Inorganic materials 0.000 description 1
- SIXSYDAISGFNSX-UHFFFAOYSA-N scandium atom Chemical compound [Sc] SIXSYDAISGFNSX-UHFFFAOYSA-N 0.000 description 1
- 230000035939 shock Effects 0.000 description 1
- 238000009628 steelmaking Methods 0.000 description 1
- 239000003351 stiffener Substances 0.000 description 1
- 238000005496 tempering Methods 0.000 description 1
- 229910052718 tin Inorganic materials 0.000 description 1
- 230000007704 transition Effects 0.000 description 1
- 238000012795 verification Methods 0.000 description 1
- 229910052727 yttrium Inorganic materials 0.000 description 1
- VWQVUPCCIRVNHF-UHFFFAOYSA-N yttrium atom Chemical compound [Y] VWQVUPCCIRVNHF-UHFFFAOYSA-N 0.000 description 1
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- C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
- C21D8/00—Modifying the physical properties by deformation combined with, or followed by, heat treatment
- C21D8/02—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips
- C21D8/0205—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips of ferrous alloys
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- C—CHEMISTRY; METALLURGY
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- C23C—COATING METALLIC MATERIAL; COATING MATERIAL WITH METALLIC MATERIAL; SURFACE TREATMENT OF METALLIC MATERIAL BY DIFFUSION INTO THE SURFACE, BY CHEMICAL CONVERSION OR SUBSTITUTION; COATING BY VACUUM EVAPORATION, BY SPUTTERING, BY ION IMPLANTATION OR BY CHEMICAL VAPOUR DEPOSITION, IN GENERAL
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- C21D6/004—Heat treatment of ferrous alloys containing Cr and Ni
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- 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
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- C21D8/04—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips to produce plates or strips for deep-drawing
- C21D8/0447—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips to produce plates or strips for deep-drawing characterised by the heat treatment
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- C—CHEMISTRY; METALLURGY
- C21—METALLURGY OF IRON
- C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
- C21D2211/00—Microstructure comprising significant phases
- C21D2211/001—Austenite
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- C—CHEMISTRY; METALLURGY
- C21—METALLURGY OF IRON
- C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
- C21D2211/00—Microstructure comprising significant phases
- C21D2211/002—Bainite
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- C—CHEMISTRY; METALLURGY
- C21—METALLURGY OF IRON
- C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
- C21D2211/00—Microstructure comprising significant phases
- C21D2211/005—Ferrite
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- C—CHEMISTRY; METALLURGY
- C21—METALLURGY OF IRON
- C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
- C21D2211/00—Microstructure comprising significant phases
- C21D2211/008—Martensite
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- Y—GENERAL TAGGING OF NEW TECHNOLOGICAL DEVELOPMENTS; GENERAL TAGGING OF CROSS-SECTIONAL TECHNOLOGIES SPANNING OVER SEVERAL SECTIONS OF THE IPC; TECHNICAL SUBJECTS COVERED BY FORMER USPC CROSS-REFERENCE ART COLLECTIONS [XRACs] AND DIGESTS
- Y10—TECHNICAL SUBJECTS COVERED BY FORMER USPC
- Y10T—TECHNICAL SUBJECTS COVERED BY FORMER US CLASSIFICATION
- Y10T428/00—Stock material or miscellaneous articles
- Y10T428/12—All metal or with adjacent metals
- Y10T428/12493—Composite; i.e., plural, adjacent, spatially distinct metal components [e.g., layers, joint, etc.]
- Y10T428/12771—Transition metal-base component
- Y10T428/12785—Group IIB metal-base component
- Y10T428/12792—Zn-base component
- Y10T428/12799—Next to Fe-base component [e.g., galvanized]
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Abstract
Provided are: a high-strength steel sheet which is improved in both elongation and local formability and thus exhibits excellent workability; and a manufacturing process therefor. The high-strength steel sheet contains C, Si, Mn, Al, P and S with the balance being iron and unavoidable impurities, and has a metal structure which comprises polygonal ferrite, bainite, tempered martensite, and retained austenite and which has the characteristics: (1) the bainite, as the metal structure is observed through a scanning electron microscope, is composed of a composite structure consisting of both a high-temperature -created bainite wherein the average spacing between adjacent retained austenite and/or carbide regions is 1[mu]m or more and a low-temperature-created bainite wherein the average spacing between adjacent retained austenite and/or carbide regions is less than 1[mu]m; and (2) the volume fraction of the retained austenite as determined by a saturation magnetization method is 5% or more relative to the whole metal structure.
Description
Technical field
The present invention relates to high tensile steel plate and the manufacture method thereof of the excellent in workability of tensile strength more than 780MPa or more than 590MPa.
Background technology
At automobile industry, CO
2emission limit set etc. become the task of top priority towards the reply of global environment problem.On the other hand, from this viewpoint of security of guaranteeing the passenger, the collision safety standard of automobile is reinforced, and can fully guarantee that the structure design of the security in space by bus advances.In order to reach these requirements simultaneously, effectively, as the structural member of automobile and use the high tensile steel plate of tensile strength more than 780MPa, and make it thin-walled property more, so that the car body lightweight.But in general, if increase the intensity of steel plate, processibility is deteriorated, and therefore for above-mentioned high tensile steel plate is applied to automobile component, the improvement of processibility just becomes the problem that can't avoid.
As the steel plate of having both strength and processibility, known have a TRIP(Transformation Induced Plasticity: phase change induction plasticity) steel plate.As one of TRIP steel plate, the known parent phase that makes is bainite ferrite, and contain retained austenite (below, be expressed as residual γ.) TBF steel plate (patent documentation 1～4).In the TBF steel plate, can utilize the bainite ferrite of hard to obtain high strength, can utilize the fine residual γ on the border that is present in bainite ferrite to obtain good unit elongation (EL) and stretch flangeability (λ).
As the technology that improves unit elongation and stretch flangeability and improve processibility, known have a patent documentation 5,6.Wherein, in patent documentation 5, realize the high strength of steel plate by applying flexibly martensitic stucture, and generate retained austenite with specified amount, thereby improve processibility.In patent documentation 6, realize the high strength of steel plate by applying flexibly lower bainite tissue and/or martensitic stucture, and generate retained austenite and tempered martensite with specified amount, thereby improve processibility.In these documents, in order to ensure the tensile strength more than 980MPa, polygon ferritic area occupation ratio is suppressed at below 10%.
In order to reach above-mentioned requirements simultaneously, as the structural member of automobile and use tensile strength for the high tensile steel plate more than 590MPa, make it further thin-walled property and make the car body lightweight also effective.But as above-mentioned, in general, if increase the intensity of steel plate, processibility is deteriorated, and therefore for above-mentioned high tensile steel plate is applied to automobile component, the improvement of processibility is the problem that can't avoid.
Steel plate as having both strength and processibility, the known DP(two-phase that has metal structure to be formed by ferrite and martensite: Dual Phase) steel plate, and utilized the TRIP(Transformation Induced Plasticity of the phase change induction plasticity of retained austenite (residual γ): phase change induction plasticity) steel plate.
Wherein, as the TRIP steel plate that possesses intensity and processibility, the known steel plate that patent documentation 7 is arranged for example.Disclose a kind of technology in this document, it is by making the metal structure of steel plate, becomes the complex tissue that is mixed with martensite and residual γ in ferrite, thereby improves intensity and the processibility (particularly unit elongation) of steel plate.
In addition, disclose a kind of technology in patent documentation 8, it is for the TRIP steel plate, improves the balance (specifically, TS * EL) of intensity (TS) and unit elongation (EL), makes the extrusion molding raising.In this document, in order to improve extrusion molding, metal structure is become and contain ferrite, residual γ, bainite and/or martensitic tissue.And record and narrate in this document, residual γ has the effect of the unit elongation raising that makes steel plate.
As disclosed as above-mentioned patent documentation 7,8, become the tissue that contains residual γ by the metal structure that makes steel plate, on the intensity basis of improving steel plate, can also make the unit elongation characteristic improve.
The look-ahead technique document
Patent documentation
Patent documentation 1: JP 2005-240178 communique
Patent documentation 2: JP 2006-274417 communique
Patent documentation 3: JP 2007-321236 communique
Patent documentation 4: JP 2007-321237 communique
Patent documentation 5: JP 2010-65272 communique
Patent documentation 6: JP 2010-65273 communique
Patent documentation 7: Unexamined Patent 11-279691 communique
Patent documentation 8: JP 2007-126747 communique
Recently for the processibility of steel plate require characteristic strict all the more, such as the steel plate for post and beam etc., require all under strict condition, carrying out stretch forming and drawing and forming than ever.For this reason, for steel plate, just wish especially unit elongation further to be improved among processibility.But generally known, if improve unit elongation, the local deformaton ability of stretch flangeability (λ) and bendability (R) etc. reduces.Therefore in the TRIP steel plate, just require not make intensity and unit elongation deteriorated, and improve the local deformaton ability of stretch flangeability (λ) and bendability (R) etc.But, because residual γ work in-process becomes stone martensite mutually, there is the such problem of local deformaton ability of stretch flangeability and bendability etc. in above-mentioned TRIP steel plate.
Summary of the invention
The present invention is conceived to above-mentioned such situation and forms, its purpose is, providing a kind of is the high tensile steel plate more than 780MPa or more than 590MPa for tensile strength, has improved that unit elongation and local deformability are this two side, high tensile steel plate excellent in workability and manufacture method thereof.
Can solve the what is called high tensile steel plate of the present invention of above-mentioned problem, in quality %, contain C:0.10～0.3%, Si:1.0～3.0%, Mn:1.5～3%, Al:0.005～3%, and meet below P:0.1%, below S:0.05% the steel plate that surplus consists of iron and inevitable impurity.And the metal structure of this high tensile steel plate, contain bainite, polygon ferrite, retained austenite and tempered martensite, there is following some main idea: when (1) observes metal structure with scanning electron microscope, (1a) described bainite, by the retained austenite of adjacency and/or the equispaced of carbide, it is the high-temperature area generation bainite more than 1 μ m, generate the complex tissue formation of bainite lower than the low-temperature region of 1 μ m with the retained austenite of adjacency and/or the equispaced of carbide, the area occupation ratio a that described high-temperature area generates bainite meets 10～80% with respect to metal structure is all, the total area occupation ratio b that described low-temperature region generates bainite and described tempered martensite meets 10～80% with respect to metal structure is all, (1b) described polygon ferritic area occupation ratio c meets 10～50% with respect to metal structure is all, and, (2) take the volume fraction of the described retained austenite that the saturation magnetization method measures with respect to metal structure all as more than 5%.Below, this high tensile steel plate is called to the first high tensile steel plate, it is more than 780MPa that this first high tensile steel plate meets tensile strength.
Above-mentioned the first high tensile steel plate, during with its metal structure of observation by light microscope, in the situation that the compound MA mixed phase of quenched martensite and retained austenite exists, total number with respect to the MA mixed phase, meet the number ratio that the diameter of equivalent circle d observed in cross section surpasses the MA mixed phase of 7 μ m, preferably lower than 15%(, contain 0%).
The average equivalent circular diameter D of described polygon ferrite crystal grain, be preferably 10 μ m following (not containing 0 μ m).
Described the first high tensile steel plate, as other element, also can also contain as inferior:
(a) Cr:1% following (not containing 0%) and/or Mo:1% following (not containing 0%);
(b) below Ti:0.15% (containing 0%), (containing 0%) Nb:0.15% below and (do not contain 0%) V:0.15% below and form group more than one element of selection;
(c) Cu:1% following (not containing 0%) and/or Ni:1% following (not containing 0%);
(d) B:0.005% following (not containing 0%);
(e) (not containing 0%), (not containing 0%) and rare earth element below Mg:0.01% below Ca:0.01%: the element of more than one that select the group that below 0.01%, (not containing 0%) forms.
In the present invention, the surface that is also included within above-mentioned the first high tensile steel plate has the high strength hot dip galvanized steel sheet of molten zinc plating layer, and has the high-strength galvannealed sheet of hot dip alloyed zinc coating on the surface of above-mentioned the first high tensile steel plate.
The first high tensile steel plate of the present invention, can be by comprising the method manufacture of following operation: be heated to { (Ac in order
1point+Ac
3point)/2 }+more than 20 ℃, Ac
3the operation of the temperature province below point+20 ℃; Keep the operation more than 50 seconds in this temperature province; Be cooled to the operation of the temperature T arbitrarily that meets following formula (1) with the average cooling rate more than 2 ℃/secs; Keep the operation of 10～100 seconds in the temperature province that meets following formula (1) (T1 temperature province); Keep the operation more than 200 seconds in the temperature province that meets following formula (2) (T2 temperature province).
400℃≤T1（℃）≤540℃ …（1）
200℃≤T2（℃）＜400℃ …（2）
Can solve the what is called of above-mentioned problem other high tensile steel plate of the present invention, to contain C:0.10～0.3%, Si:1.0～3%, Mn:1.0～2.5%, Al:0.005～3% in quality %, and meet below P:0.1%, below S:0.05% the steel plate that surplus consists of iron and inevitable impurity.And the metal structure of this high tensile steel plate, contain polygon ferrite, bainite, tempered martensite and retained austenite, and there is following some main idea: when (1) observes metal structure with scanning electron microscope, (1a) described polygon ferritic area occupation ratio a is all and surpass 50% with respect to metal structure, (1b) described bainite, generate bainite by the retained austenite of adjacency and/or the high-temperature area of equispaced more than 1 μ m of carbide, generate the complex tissue formation of bainite lower than the low-temperature region of 1 μ m with the retained austenite of adjacency and/or the equispaced of carbide, the area occupation ratio b that described high-temperature area generates bainite meets 5～40% with respect to metal structure is all, the total area occupation ratio c that described low-temperature region generates bainite and described tempered martensite meets 5～40% with respect to metal structure is all, (2) take the volume fraction of the described retained austenite that the saturation magnetization method measures with respect to metal structure all as more than 5%.Below, this high tensile steel plate is called to the second high tensile steel plate, more than this its tensile strength of the second high tensile steel plate meets 590MPa.
Above-mentioned the second high tensile steel plate, during with its metal structure of observation by light microscope, in the situation that the compound MA mixed phase of quenched martensite and retained austenite exists, with respect to total number of MA mixed phase, the number ratio that the satisfied diameter of equivalent circle d observed in cross section surpasses the MA mixed phase of 7 μ m preferably contains 0% lower than 15%().
The average equivalent circular diameter D of described polygon ferrite crystal grain, be preferably 10 μ m following (not containing 0 μ m).
Described the second high tensile steel plate, as other element, also can also contain as inferior:
(a) Cr:1% following (not containing 0%) and/or Mo:1% following (not containing 0%);
(b) below Ti:0.15% (containing 0%), (containing 0%) Nb:0.15% below and (do not contain 0%) V:0.15% below and form group more than one element of selection;
(c) Cu:1% following (not containing 0%) and/or Ni:1% following (not containing 0%);
(d) B:0.005% following (not containing 0%);
(e) (not containing 0%), (not containing 0%) and rare earth element below Mg:0.01% below Ca:0.01%: the element of more than one that select the group that below 0.01%, (not containing 0%) forms.
In the present invention, the surface that is also included within above-mentioned the second high tensile steel plate has the high strength hot dip galvanized steel sheet of molten zinc plating layer, and has the high-strength galvannealed sheet of hot dip alloyed zinc coating on the surface of above-mentioned the second high tensile steel plate.
The second high tensile steel plate of the present invention, can be by comprising the method manufacture of following operation: be heated to Ac in order
1more than point+20 ℃, Ac
3the operation of the temperature province below point+20 ℃; Keep the operation more than 50 seconds in this temperature province; Be cooled to the operation of the temperature T arbitrarily that meets following formula (1) with the average cooling rates of 2～50 ℃/secs; Keep the operation of 10～100 seconds in the temperature province that meets following formula (1); Keep the operation more than 200 seconds in the temperature province that meets following formula (2).
400℃≤T1（℃）≤540℃ …（1）
200℃≤T2（℃）＜400℃ …（2）
Also have, so-called in this manual " and/or ", the meaning is at least to comprise any one party.
According to the present invention, particularly, as bainite, by the two kinds of bainites that form is different that exist that make residual γ and carbide, generate,, the bainite that makes to generate at the high-temperature area more than 400 ℃, below 540 ℃ (below, be expressed as high-temperature area and generate bainite.), and the bainite generated more than 200 ℃, lower than the low-temperature regions of 400 ℃ (below, be expressed as low-temperature region and generate bainite.) this two sides generation, and the polygon ferrite of specified amount is generated, even can realize the high strength territory more than 780MPa, unit elongation and local deformability be good, the first high tensile steel plate excellent in workability also.In addition, according to the present invention, can provide the manufacture method of the first high tensile steel plate that this high strength and good processibility exist side by side.
In addition, according to the present invention, at the area occupation ratio all with respect to metal structure, make polygon ferrite generation to surpass 50% mode after, particularly as bainite, by the two kinds of bainites that form is different that exist that make residual γ and carbide, generate, , make more than 400 ℃, the bainite (high-temperature area generation bainite) that high-temperature area below 540 ℃ generates, with more than 200 ℃, the bainite (low-temperature region generation bainite) generated lower than the low-temperature regions of 400 ℃ this two side generate, even can realize the high strength territory more than 590MPa, unit elongation and local deformability are also good, the second high tensile steel plate of excellent in workability.In addition, according to the present invention, can provide the manufacture method of the second high tensile steel plate that this high strength and good processibility exist side by side.
The accompanying drawing explanation
Fig. 1 means the mode chart of an example of the equispaced of the retained austenite of adjacency and/or carbide.
Fig. 2 is that medelling ground means that high-temperature area generates the figure of the distribution of (low-temperature region generate bainite+tempered martensite) such as bainite and low-temperature region generation bainites.
Fig. 3 means the mode chart of an example of the heating curve in T1 temperature province and T2 temperature province.
Fig. 4 means the graph of the relation of tensile strength (TS) and unit elongation (EL).
Fig. 5 means the graph of the relation of tensile strength (TS) and unit elongation (EL).
Embodiment
At first, for the first high tensile steel plate of the present invention, describe.
The present inventors, the research repeatedly in order to improve processibility, particularly unit elongation that tensile strength is the first high tensile steel plate more than 780MPa and local deformability.It found that,
(1) make the metal structure of steel plate become the mixed structure of containing at bainite, polygon ferrite, residual γ and tempered martensite, particularly as bainite,
If (1a) make between the residual γ of adjacency, between the carbide of adjacency or the carbide of the residual γ of adjacency and adjacency (below, by its unified residual γ etc. that is expressed as.) the equispaced of central position spacing be that the above high-temperature area of 1 μ m generates bainite, and
(1b) two kinds of bainites that the equispaced of the central position spacing of residual γ etc. generates bainite lower than the low-temperature region of 1 μ m generate, and the first high tensile steel plate of unit elongation and the improved excellent in workability of local deformability can be provided,
(2) specifically, above-mentioned high-temperature area generates bainite and contributes to the unit elongation of steel plate to improve, and above-mentioned low-temperature region generates bainite and contributes to the local deformaton ability of steel plate to improve,
(3) in addition, as above-mentioned metal structure, if the polygon ferrite of specified amount is generated, the local deformaton ability that can not make steel plate is deteriorated and can further improve unit elongation,
(4) for the polygon ferrite that makes specified amount generates, by steel plate, at ferrite and austenitic two-phase temperature province, ［ be exactly { (Ac specifically,
1point+Ac
3point)/2 }+more than 20 ℃, Ac
3temperature below point+20 ℃ ］ heating gets final product,
(5) for two kinds of bainites that make specified amount generate, after above-mentioned two-phase temperature province heating, with the average cooling rate more than 2 ℃/secs, be cooled to temperature province more than 400 ℃, below 540 ℃ (below, be called the T1 temperature province.) temperature T arbitrarily, in this T1 temperature province, keep 10～100 seconds and after making high-temperature area generate bainite to generate, more cooling in more than 200 ℃, lower than the temperature provinces of 400 ℃ (below, be called the T2 temperature province.), keep getting final product more than 200 seconds in this T2 temperature province, thereby completed the present invention.
At first, for giving the metal structure of the first high tensile steel plate of the present invention with feature, describe.
" about metal structure "
The metal structure of the first high tensile steel plate of the present invention is the mixed structure consisted of bainite, polygon ferrite, residual γ and tempered martensite.
［ bainite and tempered martensite ］
At first, for the bainite of giving the notable feature of the present invention, describe.Also have, in the present invention, also comprise bainite ferrite in bainite.Bainite is the tissue that Carbide Precipitation is arranged, and bainite ferrite is the tissue that there is no Carbide Precipitation.
The first high tensile steel plate of the present invention is characterized in that bainite generates bainite by high-temperature area, and the strength ratio high-temperature area generates the complex tissue formation of the low-temperature region generation bainite that bainite is high.High-temperature area generates bainite and contributes to the unit elongation of steel plate to improve, and low-temperature region generates bainite and contributes to the local deformaton ability of steel plate to improve.Then by containing this two kinds of bainite structures, on the basis of guaranteeing good local deformaton ability, can improve unit elongation, processibility improves comprehensively.This be considered to due to, owing to making the different bainite structure Composite of strength level produce nonaffine deformation, work hardening capacity rises.
So-called above-mentioned high-temperature area generates bainite, is to be heated to { (Ac
1point+Ac
3point)/2 }+more than 20 ℃, Ac
3in temperature (two-phase temperature province) process of cooling afterwards below point+20 ℃, the bainite structure generated in the T1 temperature province more than 400 ℃, below 540 ℃.High-temperature area generates bainite, is that while with scanning electron microscope (SEM), observing the steel plate cross section of corroding through peroxy-nitric acid ethanol, the equispaced of residual γ etc. reaches the above tissue of 1 μ m.
On the other hand, above-mentioned so-called low-temperature region generates bainite, is in the process of cooling after being heated to above-mentioned two-phase temperature province, at the bainite structure generated more than 200 ℃, lower than the T2 temperature provinces of 400 ℃.Low-temperature region generates bainite, is while being observed for the steel plate cross section through the corrosion of peroxy-nitric acid ethanol with scanning electron microscope (SEM), and the equispaced of residual γ etc. is lower than the tissue of 1 μ m.
In this what is called " equispaceds of residual γ etc. ", when carrying out the SEM observation for the steel plate cross section, central position spacing between the residual γ of measurement adjacency, central position spacing between the carbide of adjacency, or the central position spacing of the carbide of the residual γ of adjacency and adjacency, the value that its result is averaged.Above-mentioned central position spacing, the meaning is while being measured for the residual γ of adjacency the most and/or carbide, tries to achieve the distance between ，Gai central position, central position in each residual γ or each carbide.Above-mentioned central position determines major diameter and minor axis in residual γ or carbide, is the position of long and short diameter intersection.
But, when residual γ or carbide are separated out on the border of lath, many residual γ are connected with carbide, its form becomes needle-like or tabular, therefore the central position spacing needs not to be the distance between residual γ and/or carbide, as shown in Figure 1, if by residual γ and/or carbide the major diameter direction continuously and centered by the interval (lath spacing) of the line formed and line location gap from.
In addition, tempered martensite is to generate with above-mentioned low-temperature region the tissue that bainite has same effect, contributes to the local deformaton ability of steel plate to improve.Also have, above-mentioned low-temperature region generates bainite and tempered martensite, even carry out SEM, observes and can not distinguish, and therefore in the present invention, low-temperature region is generated to bainite and tempered martensite is referred to as " low-temperature region generates bainite etc. ".
In the present invention, by becoming, contain the compound bainite structure that high-temperature area generates bainite and low-temperature region generation bainite etc., can realize the first high tensile steel plate that processibility is improved comprehensively.That is, high-temperature area generates bainite, because compare low-temperature region, generates bainite etc. for soft, so contribute to improve the unit elongation (EL) of steel plate, improves processibility.On the other hand, low-temperature region generates bainite etc., because carbide and residual γ are little, when distortion, stress concentration alleviates, so contribute to improve stretch flangeability (λ) and the bendability (R) of steel plate, improves the local deformaton ability, improves processibility.And in the present invention, because make such high-temperature area generate bainite and low-temperature region generates the mixing such as bainite, so work hardening capacity improves, unit elongation improves and processibility improves.
In the present invention, by bainite in the above described manner, divide into " high-temperature area generation bainite " and " low-temperature region generates bainite etc. " reason according to the difference of the equispaced of the difference that generates temperature province and residual γ etc., be due in general learned tissue typing, be difficult to distinguish legibly bainite.For example, the bainite of lath-shaped and bainite ferrite, be categorized as upper bainite and lower bainite according to transformation temperature.But as the present invention, make during Si contains the steel grade reached more than 1.0% in a large number, along with the separating out of carbide of bainitic transformation is suppressed, therefore, in SEM observes, also comprising martensitic stucture, it is distinguished and has any problem.Therefore in the present invention, by learned tissue definition, for bainite, do not classified, and according to be as described so, the equispaced of the difference based on generating temperature province and residual γ etc. is distinguished.
The distribution that high-temperature area generates bainite and low-temperature region generation bainite etc. is not particularly limited, can in old γ crystal grain, generate high-temperature area and generate these two sides such as bainite and low-temperature region generation bainite, also can in the γ crystal grain of every Geju City, generate respectively high-temperature area generation bainite and low-temperature region generation bainite etc.
Be presented in Fig. 2 to the distribution medelling of high-temperature area generation bainite and low-temperature region generation bainite etc.In Fig. 2, high-temperature area is generated to the additional oblique line of bainite, low-temperature region is generated to the additional tiny points such as bainite.Fig. 2 (a) is illustrated in mixing in old γ crystal grain and generates the situation that high-temperature area generates these two sides such as bainite and low-temperature region generation bainite, and Fig. 2 (b) means in the γ crystal grain of every Geju City to generate respectively the situation that high-temperature area generates bainite and low-temperature region generation bainite etc.Show shown in Fig. 2 that the black round dot means the MA mixed phase.About MA mixed phase aftermentioned.
In the present invention, if among metal structure is all, the area occupation ratio of shared high-temperature area generation bainite is a, when among metal structure is all, the total area occupation ratio of shared (low-temperature region generate bainite and tempered martensite) such as low-temperature region generation bainites is b, this area occupation ratio a and b all need to meet 10～80%.At this, do not stipulate that low-temperature region generates the area occupation ratio of bainite, but the regulation low-temperature region generates the reason of the total area occupation ratio of bainite and tempered martensite, as aforementioned, be owing in SEM observes, can not distinguishing these tissues.
Above-mentioned area occupation ratio a is 10～80%.If it is very few that high-temperature area generates the growing amount of bainite, the unit elongation of steel plate reduces, and can not improve processibility.Therefore above-mentioned area occupation ratio a is more than 10%, is preferably more than 15%, more preferably more than 20%.But, if generating the growing amount of bainite, high-temperature area becomes superfluous, low-temperature region generates the effect that the Composite of bainite etc. brings and can't be brought into play.Therefore the area occupation ratio a of high-temperature area generation bainite is below 80%, is preferably below 70%, more preferably below 60%, more preferably below 50%.
In addition, above-mentioned total area occupation ratio b is 10～80%.If the growing amount of low-temperature region generation bainite etc. is very few, the local deformaton ability of steel plate reduces, and can not improve processibility.Therefore above-mentioned total area occupation ratio b is more than 10%, is preferably more than 15%, more preferably more than 20%.But, if low-temperature region generate the growing amount of bainite etc. be superfluous, high-temperature area generates the effect that the Composite of bainite brings and can't be brought into play.Therefore to generate the area occupation ratio b of bainite etc. be below 80% to low-temperature region, is preferably below 70%, more preferably below 60%, more preferably below 50%.
The relation of above-mentioned area occupation ratio a and above-mentioned total area occupation ratio b, just be not particularly limited as long as scope separately meets above-mentioned scope, and any one form of a>b, a<b, a=b all comprises.
High-temperature area generates the mixture ratio of bainite and low-temperature region generation bainite etc., according to the desired characteristic of steel plate, determines to get final product.Specifically, among the processibility of steel plate, in order to make local deformaton ability (particularly stretch flangeability (λ)), further improve, reduce as far as possible the ratio that high-temperature area generates bainite, increase as far as possible the ratio that low-temperature region generates bainite etc. and get final product.On the other hand, among the processibility of steel plate, in order to make unit elongation, further improve, increase as far as possible the ratio that high-temperature area generates bainite, reduce as far as possible the ratio that low-temperature region generates bainite etc. and get final product.In addition, in order further to improve the intensity of steel plate, increase as far as possible the ratio that low-temperature region generates bainite etc., reduce as far as possible the ratio that high-temperature area generates bainite and get final product.
［ polygon ferrite ］
Polygon ferrite and bainite are in a ratio of soft, are to improve for the unit elongation that improves steel plate the tissue that processibility plays a role.In order to bring into play such effect, polygon ferritic area occupation ratio is all, more than 10%, to be preferably more than 12% with respect to metal structure, more preferably more than 15%.But, surplus, intensity step-down if polygon ferritic growing amount becomes.Therefore polygon ferritic area occupation ratio is all, below 50%, to be preferably below 45% with respect to metal structure, more preferably below 40%.
Preferably the average equivalent circular diameter D of above-mentioned polygon ferrite crystal grain is 10 μ m following (not containing 0 μ m).By reducing the average equivalent circular diameter D of polygon ferrite crystal grain, make it slightly to disperse, can make the unit elongation of steel plate further improve.Although its detailed mechanism is still not clear clear, but be considered to by making polygon ferrite miniaturization, the polygon ferritic phase dispersion state all for metal structure becomes evenly, therefore is difficult to occur inhomogeneous distortion, and this contributes to the further raising of unit elongation.Think, the metal structure of the first high tensile steel plate of the present invention, because the mixed structure by bainite, polygon ferrite, residual γ and tempered martensite forms, so if the particle diameter of polygon ferrite crystal grain is large, the size of each tissue produces deviation, therefore inhomogeneous distortion occurs, and strain is concentrated locally and is difficult to improve processibility (particularly polygon ferrite generates the unit elongation raising effect brought).Therefore polygon ferritic average equivalent circular diameter D is preferably below 10 μ m, more preferably, below 8 μ m, more preferably, below 5 μ m, is particularly preferably below 3 μ m.
Above-mentioned polygon ferritic area occupation ratio and average diameter of equivalent circle D can observe and be measured by SEM.
［ bainite+tempered martensite+polygon ferrite ］
In the present invention, above-mentioned high-temperature area generates the area occupation ratio a of bainite, the total area occupation ratio b of (low-temperature region generate bainite+tempered martensite) such as above-mentioned low-temperature region generation bainites, with the total (a+b+c) of above-mentioned polygon ferritic area occupation ratio c, preferably all and satisfied more than 70% with respect to metal structure.If add up to area occupation ratio (a+b+c) lower than 70%, unit elongation is deteriorated.Add up to area occupation ratio (a+b+c) more preferably more than 75%, more preferably more than 80%.The upper limit that adds up to area occupation ratio (a+b+c), consider the fill-in ratio of the measured residual γ by the saturation magnetization method and determine, is for example 95%.
［ residual γ ］
Residual γ becomes martensite mutually while deforming by steel plate, being subject to stress, promote the sclerosis of variant part, has the effect of the strain concentrating of preventing, accordingly, homogeneous deformation can improve and give play to good unit elongation.Such effect is commonly referred to as the TRIP effect.
In order to bring into play these effects, the volume fraction of all residual γ with respect to metal structure, while measuring with the saturation magnetization method, need to make it to contain more than 5%.More than residual γ is preferably 8 volume %, more preferably more than 10 volume %.But if the growing amount of residual γ is too much, MA mixed phase described later generates also superfluously, and therefore easy thickization of MA mixed phase make local deformaton ability (stretch flangeability and bendability) reduce.Therefore residual γ on be limited to 30 volume % left and right, be preferably 25 volume %.
Residual γ mainly generates between the lath of metal structure, but also can for example, on the crystal boundary of the aggregate (, lath block and lath bundle etc.) of lath-shaped tissue and old γ, is block as the part of MA mixed phase described later and exists.
［ other ］
The metal structure of the first high tensile steel plate of the present invention, as above-mentioned, contain bainite, polygon ferrite, residual γ and tempered martensite, can only by these, be formed, but in the scope of not damaging effect of the present invention, also can there is the MA mixed phase that (a) quenched martensite and residual γ are compound, and (b) the surplus tissue of perlite etc.
(a) MA mixed phase
The MA mixed phase, generally be known as the compound phase of quenched martensite and residual γ, is the tissue generated in the following way,, to the part of the tissue of the final cooling previous crops austenite existence that be not phase transformation, be phase-changed into martensite final when cooling, all the other are remaining with austenitic state.The MA mixed phase so generated, carbon high density multiviscosisty in the process of thermal treatment (particularly means of isothermal quenching), and also a part becomes martensitic stucture, is therefore stone tissue.The difference of hardness of bainite and MA mixed phase is large for this reason, and stress concentration during distortion easily becomes the starting point that space occurs, if therefore the MA mixed phase generates superfluously, stretch flangeability and bendability reduce, and the local deformaton ability reduces.In addition, if MA mixed phase surplus is generating, intensity has too high tendency.Residual γ amount is more, and Si content is more in addition, and the MA mixed phase more easily generates, but preferred the least possible method of its growing amount.
Above-mentioned MA mixed phase during with the observation by light microscope metal structure, is preferably all below 30 area % with respect to metal structure, more preferably below 25 area %, more preferably below 20 area %.
Above-mentioned MA mixed phase, diameter of equivalent circle d surpasses the number ratio of the MA mixed phase of 7 μ m, preferably with respect to total number of MA mixed phase and lower than 15%(containing 0%).Diameter of equivalent circle d surpasses the thick MA mixed phase of 7 μ m, and local deformability is caused to detrimentally affect.Above-mentioned diameter of equivalent circle d surpasses the number ratio of the MA mixed phase of 7 μ m, more preferably with respect to total number of MA mixed phase and lower than 10%, more preferably lower than 5%.
Above-mentioned diameter of equivalent circle d surpasses the number ratio of the MA mixed phase of 7 μ m, and the cross-sections surfaces parallel with rolling direction with observation by light microscope also calculated and got final product.
Also have, above-mentioned MA mixed phase, its particle diameter is larger, and the more incidental tendency in space, by experimental verification, therefore recommends the MA mixed phase as much as possible little.
(b) perlite
Above-mentioned perlite, carry out SEM while observing for metal structure, preferably with respect to metal structure all below 20 area %.If pearlitic area occupation ratio surpasses 20%, unit elongation is deteriorated, is difficult to improve processibility.Pearlitic area occupation ratio, more preferably with respect to metal structure all below 15%, more preferably, below 10%, be particularly preferably below 5%.
Above-mentioned metal structure can be measured by following step.
High-temperature area generates bainite, low-temperature region generates (low-temperature region generates bainite+tempered martensite), the polygon ferrite and pearlites such as bainite, need only among the cross section parallel in the rolling direction with steel plate, carry out the corrosion of nitric acid ethanol for 1/4 position of thickness of slab, carry out the SEM observation with 3000 times of left and right of multiplying power and just can identify.
High-temperature area generates bainite and low-temperature region generates bainite etc., and that observes is organized as, and what mainly observe is grey, is dispersed with among crystal grain and observes for white or grayish residual γ etc.Therefore observe according to SEM, generate during bainite and low-temperature region generate bainite etc. at high-temperature area, also comprise residual γ and carbide, therefore calculated as the area occupation ratio that also comprises residual γ etc.The crystal grain that polygon ferrite is observed is containing above-mentioned observation, not get up for white or grayish residual γ etc. in the inside of crystal grain.What perlite was observed is organized as, and carbide and ferrite form stratiform.
If carry out the corrosion of nitric acid ethanol for the cross section of steel plate, carbide and residual γ all are observed as white or grayish tissue, distinguish both difficulties.Wherein carbide (for example, cementite), more generate at low-temperature region, more have and compare between lath and to the tendency of separating out in lath, so the interval between carbide can be thought to generate at high-temperature area when wide, when the interval between carbide is narrow, can thinks and generate at low-temperature region.Residual γ generates usually between lath, but the generation temperature of tissue is lower, and the size of lath is less, when the interval between residual γ is wide, can thinks and generate at high-temperature area, when the interval between residual γ is narrow, can thinks and generate at low-temperature region.Therefore in the present invention, be conceived to the SEM observation is carried out in the cross section through the corrosion of nitric acid ethanol, in field of view as white or the light grey residual γ observed etc., while measuring the central position spacing between the residual γ etc. of adjacency, be that the above setup action high-temperature area of 1 μ m generates bainite by its mean value (equispaced), equispaced is generated to bainite etc. lower than the setup action low-temperature region of 1 μ m.
Because residual γ can not observe the evaluation of being organized by SEM, so by saturation magnetization method measurement volumes rate.The value of this volume fraction can directly be read as area occupation ratio.The detailed measurements principle of saturation magnetization method, get final product with reference to " R&D Kobe Steel skill newspaper, Vol.52, No.3,, p.43～46 in 2002 ".
The volume fraction of residual γ like this (area occupation ratio) is measured with the saturation magnetization method, and with respect to this, high-temperature area generates the area occupation ratio of bainite etc. and observes by SEM, comprises residual γ interior and measured, and therefore has it to add up to the situation that surpasses 100%.
The MA mixed phase, need only among the cross section parallel in the rolling direction with steel plate, for 1/4 position of thickness of slab, carries out the corrosion of lepera reagent, with 1000 times of left and right of multiplying power, carries out observation by light microscope, just can be used as white structure observation and arrive.
Then, the chemical composition composition for the first high tensile steel plate of the present invention describes.
" about one-tenth, being grouped into "
The first high tensile steel plate of the present invention, contain C:0.10～0.3%, Si:1.0～3.0%, Mn:1.5～3%, Al:0.005～3%, and meet P:0.1% following (not containing 0%), S:0.05% following (not containing 0%).Stipulate the reasons are as follows of this scope.
C is for improving the intensity of steel plate, and makes residual γ generate needed element.Therefore the C amount is more than 0.10%, is preferably more than 0.13%, more preferably more than 0.15%.But, if contain C superfluously, weldability reduces.Therefore the C amount is below 0.3%, is preferably below 0.25%, more preferably below 0.20%.
Si is as the solution strengthening element, except the high strength that contributes to steel plate, in maintenance under T1 temperature province described later and T2 temperature province, (in means of isothermal quenching) suppresses Carbide Precipitation, effectively making residual γ generate, is very important element.Therefore the Si amount is more than 1.0%, is preferably more than 1.2%, more preferably more than 1.3%.But, if contain Si superfluously, during the heating/soaking in annealing, to the reverse transformation of γ phase, not occurring, polygon ferrite is remaining in a large number, causes undercapacity.In addition, when hot rolling, Si oxide skin occurs and the surface texture of steel plate is worsened in surface of steel plate.Therefore the Si amount is below 3.0%, is preferably below 2.5%, more preferably below 2.0%.
Mn is for obtaining the needed element of bainite and tempered martensite.Mn makes the γ stabilization in addition, for residual γ is generated, is also the element effectively played a role.In order to bring into play such effect, the Mn amount is more than 1.5%, is preferably more than 1.8%, more preferably more than 2.0%.If but contain Mn superfluously, the generation of high-temperature area generation bainite significantly is suppressed.In addition, the surplus of Mn is added, and causes the deteriorated of processibility that the deteriorated and segregation of weldability causes.Therefore the Mn amount is below 3%, is preferably below 2.8%, more preferably below 2.7%.
Al and Si are same, suppress Carbide Precipitation in means of isothermal quenching, are the elements that contributes to residual γ to generate.Al is the element worked as reductor in steel making working procedure in addition.Therefore the Al amount is more than 0.005%, is preferably more than 0.01%, more preferably more than 0.03%.If but contain Al superfluously, the inclusion in steel plate becomes too much and ductility is deteriorated.Therefore the Al amount is below 3%, is preferably below 1.5%, more preferably below 1%, more preferably below 0.5%.
P is the impurity element inevitably contained in steel, superfluous if the P quantitative change obtains, and the weldability of steel plate is deteriorated.Therefore the P amount is below 0.1%, is preferably below 0.08%, more preferably below 0.05%.The P amount is advisable with the least possible aspect, but reach 0%, industrial, has any problem.
S is the impurity element inevitably contained at steel, same with above-mentioned P, is the deteriorated element of weldability that makes steel plate.S forms sulfide-based inclusion in steel plate in addition, if its increase, processibility reduces.Therefore the S amount is below 0.05%, is preferably below 0.01%, more preferably below 0.005%.The S amount is advisable with the least possible aspect, but reach 0%, industrial, has any problem.
The first high tensile steel plate of the present invention, meet mentioned component and form, and the surplus composition is the inevitable impurity beyond iron and above-mentioned P, S.As inevitable impurity, for example, comprise N, O(oxygen), residual element (for example, Pb, Bi, Sb, Sn etc.) etc.Inevitably, among impurity, preferably the N amount is (not containing 0%) below 0.01%, and the O amount is (not containing 0%) below 0.01%.
N separates out nitride in steel plate, is the element that contributes to the strengthening of steel plate, if but contain N, nitride is separated out in a large number, causes the deteriorated of unit elongation, stretch flangeability and bendability. superfluouslyTherefore the N amount is preferably below 0.01%, more preferably below 0.008%, more preferably below 0.005%.
If O(oxygen) contain superfluously, it is the element that causes the reduction of unit elongation, stretch flangeability and bendability.Therefore the O amount is preferably below 0.01%, more preferably below 0.005%, more preferably below 0.003%.
The first high tensile steel plate of the present invention, as other elements, also can also contain as inferior:
(a) Cr:1% following (not containing 0%) and/or Mo:1% following (not containing 0%);
(b) below Ti:0.15% (not containing 0%), the element of more than one that select in the group that Nb:0.15% following (not containing 0%) and V:0.15% following (not containing 0%) form;
(c) Cu:1% following (not containing 0%) and/or Ni:1% following (not containing 0%);
(d) B:0.005% following (not containing 0%);
(e) below Ca:0.01% (not containing 0%), Mg:0.01% following (not containing 0%) and rare earth element: the element of more than one that select in the group that below 0.01%, (not containing 0%) forms.
(a) Cr and Mo, same with above-mentioned Mn, be to be the element effectively played a role for obtaining on bainite and tempered martensite.These elements can be used separately or and use.In order effectively to bring into play such effect, preferably make Cr and Mo contain individually respectively more than 0.1%, more preferably more than 0.2%.But, if the content of Cr and Mo surpasses 1% respectively, the generation of high-temperature area generation bainite significantly is suppressed.In addition, superfluous interpolation causes expensive.Therefore preferably Cr and Mo are respectively below 1%, more preferably below 0.8%, more preferably below 0.5%.And, when Cr and Mo, recommending to make total amount is below 1.5%.
(b) Ti, Nb and V form the precipitate of carbide and nitride etc. in steel plate, make the steel plate strengthening, and are also to have the element that makes the effect that polygon ferrite crystal grain is tiny by the miniaturization of old γ crystal grain.In order effectively to bring into play such effect, Ti, Nb and V preferably make it separately to contain more than 0.01%, more preferably more than 0.02%.If but contain superfluously, at crystal boundary, Carbide Precipitation is arranged, stretch flangeability and the bendability of steel plate are deteriorated.Therefore Ti, Nb and V are preferably separately respectively below 0.15%, more preferably below 0.12%, more preferably below 0.1%.Ti, Nb and V can make it to contain separately, also can contain by optional two or more element.
(c) for making the γ stabilization, to make residual γ generate be the element effectively played a role for Cu and Ni.These elements can be used separately or and use.In order effectively to bring into play such effect, Cu and Ni preferably make it separately respectively to contain more than 0.05%, more preferably more than 0.1%.If but contain Cu and Ni superfluously, hot workability is deteriorated.Therefore Cu and Ni are separately respectively preferably below 1%, more preferably below 0.8%, more preferably below 0.5%.Also have, if Cu is contained, surpass 1%, hot workability is deteriorated, if add Ni, the deteriorated of hot workability is inhibited, and therefore and when Cu and Ni, although cost is high, but can surpass 1% and add Cu.
(d) B and above-mentioned Mn, Cr and Mo are same, for bainite and tempered martensite are generated, are the elements effectively played a role.In order effectively to bring into play such effect, preferably make B contain more than 0.0005%, more preferably more than 0.001%.But, if contain B superfluously, generate boride in steel plate and make ductility deteriorated.If contain B in addition superfluously, same with above-mentioned Cr and Mo, the generation that high-temperature area generates bainite significantly is suppressed.Therefore the B amount is preferably below 0.005%, more preferably below 0.004%, more preferably below 0.003%.
(e) Ca, Mg and rare earth element (REM) are the elements played a role for the fine dispersion of inclusion made in steel plate.In order effectively to bring into play such effect, Ca, Mg and rare earth element preferably contain separately respectively more than 0.0005%, more preferably more than 0.001%.But, if contain superfluously, make castibility and hot workability etc. deteriorated, manufacture and become difficult.In addition, surplus is added to the deteriorated reason of ductility that makes steel plate.Therefore Ca, Mg and rare earth element are separately respectively preferably below 0.01%, more preferably below 0.005%, more preferably below 0.003%.
Above-mentioned so-called rare earth element, to contain lanthanon (15 kinds of elements from La to Lu) and Sc(scandium) and the Y(yttrium) the meaning, among these elements, preferably contain at least one element of selecting from La, Ce and the described group of Y, more preferably contain La and/or Ce.
The first high tensile steel plate of the present invention, tensile strength more than 780MPa, local deformaton ability excellence, and unit elongation is also good, so excellent in workability.This first high tensile steel plate, the former material that is suitable as the structure component of automobile is used.Structure component as automobile, for example, can enumerate before and the shock-resistant absorption part of the reinforcement of the car body constituent part of the reinforcement of the reinforcement of the post class headed by part such as collision such as the front of rear portion curb girder and collision energy absorbing box etc. etc. (such as, central post stiffener), roof rail, curb girder, bottom girder, tread portion etc., safety lever and door anti-collision joist etc., seat part etc.
In addition, above-mentioned the first high tensile steel plate, because the processibility under warm state is good, so also can be suitable for the former material of using for warm working.Also have, so-called temperature processing, the meaning is that the temperature range 50～500 ℃ of left and right is shaped.
Above, for the metal structure of the first high tensile steel plate of the present invention, with becoming to be grouped into, be illustrated.
Secondly, for the method that can manufacture above-mentioned the first high tensile steel plate, describe.Above-mentioned the first high tensile steel plate, can manufacture by the mode that contains in order following operation: the steel plate that will meet the mentioned component composition is heated to { (Ac
1point+Ac
3point)/2 }+more than 20 ℃, Ac
3the operation of the temperature province (two-phase temperature province) below point+20 ℃; Keep the operation more than 50 seconds in this temperature province; Be cooled to the operation of the temperature T arbitrarily that meets following formula (1) with the average cooling rate more than 2 ℃/secs; Keep the operation of 10～100 seconds in the temperature province that meets following formula (1); Keep the operation more than 200 seconds in the temperature province that meets following formula (2).Below, for each operation, describe in order.
400℃≤T1（℃）≤540℃ …（1）
200℃≤T2（℃）＜400℃ …（2）
At first, as being heated to two-phase temperature province ［ { (Ac
1point+Ac
3point)/2 }+more than 20 ℃, Ac
3temperature province below point+20 ℃ ］ high tensile steel plate before, prepare that slab is followed to ordinary method and carry out hot rolling, to resulting hot-rolled steel sheet, carry out cold rolling.It is for example more than 800 ℃ that hot rolling makes finishing temperature, and making coiling temperature is for example to get final product below 700 ℃.In cold rolling, making cold rolling rate is for example that 10～70% scope is rolled and gets final product.
Cold rolling and the cold-rolled steel sheet that obtains, on continuous annealing line, is heated to { (Ac
1point+Ac
3point)/2 }+more than 20 ℃, Ac
3temperature province below point+20 ℃, keep more than 50 seconds and carry out soaking in this temperature province.
Be ferrite and austenitic two-phase temperature province by making Heating temperature, can make the polygon ferrite of specified amount generate.That is, if Heating temperature is too high, become the austenite one phase territory, polygon ferritic generation is suppressed, and therefore can not improve the unit elongation of steel plate, and processibility is deteriorated.Therefore Heating temperature is Ac
3below point+20 ℃, be preferably Ac
3below point+10 ℃, more preferably lower than Ac
3the point.Also have, if be heated to Ac
3more than point, although become the temperature province of austenite one phase, under soaking time degree given to this invention, if Heating temperature is at Ac
3below point+20 ℃, even carry out the soaking maintenance, also there is a small amount of polygon ferrite remaining, therefore, as described later by adjusting the average cooling rate after soaking, can make the polygon ferrite of specified amount generate.If but Heating temperature is lower than { (Ac
1point+Ac
3point)/2 }+20 ℃, polygon ferritic growing amount becomes superfluous, and polygon ferrite generates and surpasses 50 area %, therefore can not guarantee the intensity of wishing.Therefore Heating temperature is { (Ac
1point+Ac
3point)/2 }+more than 20 ℃, be preferably { (Ac
1point+Ac
3point)/2 }+more than 30 ℃, { (Ac more preferably
1point+Ac
3point)/2 }+more than 50 ℃.
If the soaking time under above-mentioned two-phase temperature province, lower than 50 seconds, can not heat steel plate equably, therefore the generation of residual γ is suppressed, and unit elongation and local deformability reduce, and can not improve processibility.Therefore soaking time is more than 50 seconds, is preferably more than 100 seconds.But, if soaking time is long, the austenite crystal diameter becomes large, the consequent is also thickization of polygon ferrite crystal grain, and unit elongation and local deformability have the tendency of variation.Therefore soaking time was preferably below 500 seconds, more preferably below 450 seconds.
Also have, average rate of heating when above-mentioned cold-rolled steel sheet is heated to above-mentioned two-phase temperature province, be for example to get final product more than 1 ℃/sec.
Above-mentioned Ac
1point, Ac
3point, can according to " レ ス リ ー iron steel science " (ball is apt to Co., Ltd., on May 31st, 1985 distribution, P.273) described following formula (a), formula (b) are calculated.In following formula (a), formula (b), ［ ］ means the content (quality %) of each element, and the content of the element that steel plate does not comprise calculates and gets final product as 0 quality %.
Ac
1（℃）＝723－10.7×［Mn］－16.9×［Ni］＋29.1×［Si］＋16.9×［Cr］ …（a）
Ac
3（℃）＝910－203×［C］
1/2＋44.7×［Si］－30×［Mn］－11×［Cr］＋31.5×［Mo］－20×［Cu］－15.2×［Ni］＋400×［Ti］＋104×［V］＋700×［P］＋400×［Al］ …（b）
Be heated to above-mentioned two-phase temperature province, keep more than 50 seconds and, after carrying out evenly heating, be cooled to the temperature T arbitrarily that meets above formula (1) with the average cooling rate more than 2 ℃/secs.By the cooling scope from the two-phase temperature province to the temperature T arbitrarily that meets above formula (1) more than the average cooling rate in regulation, can make the polygon ferrite of specified amount generate, make in addition high-temperature area generate these two sides such as bainite and low-temperature region generation bainite and generate.If the average cooling rate of this temperature province, lower than 2 ℃/secs, pearlitic transformation occurs, perlite generates superfluously, and unit elongation reduces, and processibility is deteriorated.This interval average cooling rate is preferably more than 5 ℃/secs, more preferably more than 10 ℃/secs.The upper limit of the average cooling rate in above-mentioned interval is not particularly limited, if but average cooling rate is excessive, and temperature is controlled difficulty, so the upper limit is for example that 100 ℃/secs of left and right get final product.
After being cooled to the temperature T arbitrarily that meets above formula (1), after the T1 temperature province that meets above formula (1) keeps 10～100 seconds, in the T2 temperature province that meets above formula (2), keep more than 200 seconds.The time kept by suitably being controlled at respectively T1 temperature province and T2 temperature province, can make high-temperature area generate the amount generations in accordance with regulations separately such as bainite and low-temperature region generation bainite.Specifically, by in the T1 temperature province, keeping the specified time, can control the growing amount that high-temperature area generates bainite, by keep the means of isothermal quenching of specified time in the T2 temperature province, can make not transformed austenite become mutually low-temperature region generation bainite, or make it to become mutually martensite, and make carbon to the austenite multiviscosisty, make residual γ generate, thus can make metal structure given to this invention generate.
In addition, the maintenance by being combined in the T1 temperature province and in the maintenance of T2 temperature province, also bringing into play the effect of the generation that can suppress the MA mixed phase.Its mechanism is thought as follows.In general, if add Si and Al, separating out of carbide is suppressed, so has free carbon in steel, in means of isothermal quenching, can confirm, along with bainitic transformation together, carbon is to the phenomenon of transformed austenite multiviscosisty not.To not transformed austenite multiviscosisty, can generate in a large number residual γ by carbon.
At this, for carbon, the phenomenon to not transformed austenite multiviscosisty describes.The multiviscosisty amount of carbon, knownly be limited in the concentration shown in To line that ferrite equates with austenitic free energy, so bainitic transformation also stops.Because temperature is higher, this To line is more in low carbon concentration side, so, if carry out means of isothermal quenching with higher temperature, even extend the treatment time, the place acquired a certain degree at bainitic transformation also can stop.At this moment because the austenitic stability of phase transformation is not low, so thick MA mixed phase generates.
Therefore in the present invention, after above-mentioned T1 temperature province keeps, by keeping in above-mentioned T2 temperature province, can increase to the allowance of the C concentration of transformed austenite not, comparing high-temperature area is a side of low-temperature region, and bainitic transformation carries out, and the MA mixed phase diminishes.In addition, compare situation about keeping in above-mentioned T1 temperature province, when above-mentioned T2 temperature province keeps, because the size decreases of lath-shaped tissue, so even there is the MA mixed phase, the MA mixed phase self also is segmented, and can reduce the MA mixed phase.In addition, because after the T1 temperature province keeps the specified time, just in the T2 temperature province, keep, so start the moment kept in the T2 temperature province, high-temperature area generates bainite and generates.Therefore in the T2 temperature province, high-temperature area generates bainite becomes catalyzer, and the phase transformation that makes low-temperature region generate bainite is promoted, thereby is also bringing into play such effect of time that can shorten means of isothermal quenching.
Also have, even from above-mentioned two-phase temperature province, do not carry out the maintenance in above-mentioned T1 temperature province, and be cooled to the temperature arbitrarily that meets above formula (2), only when the T2 temperature province that meets this formula (2) keeps (, the means of isothermal quenching that simple low temperature keeps), because the size of lath-shaped tissue is little, so also can reduce the MA mixed phase.But in this case, because do not keep in above-mentioned T1 temperature province, so high-temperature area generates bainite, can generate hardly, the dislocation desity of the lath-shaped tissue of matrix becomes large in addition, and unit elongation and local deformability reduce, and processibility is deteriorated.
In the present invention, the T1 temperature province of above formula (1) defined, be exactly more than 400 ℃, below 540 ℃ specifically.By in this temperature province, keeping the specified time, can make high-temperature area generate bainite and generate.That is, if keep in the temperature province that surpasses 540 ℃, the generation of high-temperature area generation bainite is suppressed, and on the other hand, polygon ferrite generates superfluously, and degenerate perlite generates in addition, therefore can not get the characteristic of wishing.Therefore the T1 temperature province on be limited to 540 ℃, be preferably 520 ℃, more preferably 500 ℃.On the other hand, if keep temperature lower than 400 ℃, high-temperature area generation bainite does not generate, so the unit elongation reduction, can not improve processibility.Therefore the T1 temperature province under be limited to 400 ℃, be preferably 420 ℃.
The time kept in above-mentioned T1 temperature province is 10～100 seconds.If the hold-time surpasses 100 seconds, high-temperature area generates bainite and generates superfluously, therefore as described later, even keep the specified time in above-mentioned T2 temperature province, can not guarantee that low-temperature region generates the growing amount of bainite etc.Therefore can not make intensity and processibility exist side by side.In addition, if keep for a long time in the T1 temperature province, carbon excessively multiviscosisty in austenite, even therefore in the T2 temperature province, carry out means of isothermal quenching, still can generate thick MA mixed phase, and processibility is deteriorated.Therefore the hold-time is below 100 seconds, is preferably below 90 seconds, more preferably below 80 seconds.If but too short in the hold-time of T1 temperature province, the growing amount of high-temperature area generation bainite tails off, so the unit elongation reduction, can not improve processibility.Therefore be more than 10 seconds in the hold-time of T1 temperature province, be preferably more than 15 seconds, more preferably more than 20 seconds, more preferably more than 30 seconds.
In the present invention, so-called in the hold-time of T1 temperature province, the meaning is the surface temperature of steel plate, from the moment of the ceiling temperature that arrives the T1 temperature province, to the time of the lower limit temperature that arrives the T1 temperature province.That is, the surface temperature of steel plate, from arriving moment of 540 ℃, to the time that arrives 400 ℃.
For the T1 temperature province meeting above formula (1) keeps, for example, the heating curve shown in employing Fig. 3 (i)～(iii) gets final product.
Fig. 3 (i) is after the two-phase temperature province is chilled to the temperature T arbitrarily that meets above formula (1), the example that carries out specified time constant temperature maintenance with this temperature T, and constant temperature is cooled to the temperature arbitrarily that meets above formula (2) after keeping.In Fig. 3 (i), demonstration be the situation that the constant temperature about having carried out a stage keeps, but be not limited to this, as long as in the scope of T1 temperature province, also can be kept the above constant temperature of two stages that temperature is different to keep.
Fig. 3 is (ii) from the two-phase temperature province is chilled to the temperature T arbitrarily that meets above formula (1), the change speed of cooling, in the scope of T1 temperature province with the specified time carry out cooling after, change speed of cooling once again and be cooled to the example of the temperature arbitrarily that meets above formula (2).Fig. 3 (ii) in, what mean is to carry out cooling situation with the specified time in the scope of T1 temperature province, but the present invention is not limited to this, as long as in the scope of T1 temperature province, also can comprise the operation heated with the specified time, also can suitable cooling and heating repeatedly.In addition, also can as Fig. 3 (ii) as shown in, do not carry out one section cooling, but it is cooling to carry out the multistage more than two sections that speed of cooling is different.In addition, also can carry out one section heating and the multistage heating more than two sections (not shown).
Fig. 3 is (iii) from the two-phase temperature province is chilled to the temperature T arbitrarily that meets above formula (1), changes speed of cooling, slowly is chilled to the example of the temperature arbitrarily that meets above formula (2) with identical speed of cooling.During Xu Leng like this, the residence time in the T1 temperature province is also to get final product in 10～100 seconds.
The present invention is not defined as Fig. 3's the heating curve intention shown in (i)～(iii), as long as meet important document of the present invention, can adopt the heating curve beyond above-mentioned yet.
In the present invention, by the T2 temperature province of above formula (2) regulation, specifically, be exactly more than 200 ℃, lower than 400 ℃.By in this temperature province, keeping the specified time, can make the not transformed austenite do not undergone phase transition in above-mentioned T1 temperature province, become mutually low-temperature region and generate bainite or martensite.In addition, by guaranteeing the sufficient hold-time, bainitic transformation carries out, and finally generates residual γ, and the MA mixed phase also obtains sectionalization.This martensite at once exists as quenched martensite after phase transformation, but during the T2 temperature province keeps, tempering occurs, residual as tempered martensite.This tempered martensite demonstrates the low-temperature region generated with temperature province martensitic transformation occurs and generates the equal characteristic of bainite.But, if, keeping more than 400 ℃, generate thick MA mixed phase, so unit elongation and the reduction of local deformability, processibility can not be improved.Therefore, the T2 temperature province, lower than 400 ℃, is preferably below 390 ℃, more preferably below 380 ℃.On the other hand, in the temperature lower than 200 ℃, kept, also can not generate low-temperature region and generate bainite, therefore the carbon concentration step-down in γ, can not guarantee residual γ amount, in addition, therefore quenched martensite generates in a large number, thus intensity uprise, unit elongation and local deformability variation.In addition, the carbon concentration step-down in γ, can not guarantee residual γ amount, therefore can not improve unit elongation.Therefore, be limited to 200 ℃ under the T2 temperature province, be preferably 250 ℃, more preferably 280 ℃.
The time kept in the T2 temperature province that meets above formula (2) is more than 200 seconds.If the hold-time is lower than 200 seconds, the growing amount that low-temperature region generates bainite etc. tails off, and the carbon concentration step-down in γ, can not guarantee residual γ amount, in addition because quenched martensite generates in a large number, so intensity uprise, unit elongation and local deformability variation.In addition, because the multiviscosisty of carbon is not promoted, so residual γ quantitative change is few, can not improve unit elongation.In addition, because can not make the MA mixed phase miniaturization generated in above-mentioned T1 temperature province, so can not improve the local deformaton ability.Therefore the hold-time is more than 200 seconds, is preferably more than 250 seconds, more preferably more than 300 seconds.The upper limit of hold-time is not particularly limited, if but keep for a long time,, except productivity reduces, the carbon of multiviscosisty can not make residual γ generate as Carbide Precipitation, cause the reduction of unit elongation, and processibility is deteriorated.Therefore the upper limit of hold-time is for example to get final product in 1800 seconds.
In the present invention, so-called in the hold-time of T2 temperature province, the meaning is the surface temperature of steel plate, from the moment of the ceiling temperature that arrives the T2 temperature province, to the time of the lower limit temperature that arrives the T2 temperature province.That is, from arriving the moment lower than 400 ℃, to the time that arrives 200 ℃.
The method kept in above-mentioned T2 temperature province, just so long as be to be not particularly limited more than 200 seconds in the residence time of T2 temperature province, the heating curve as in above-mentioned T1 temperature province, can keep for constant temperature, also can cooling or heating in the T2 temperature province.In addition, also can at different maintenance temperature, carry out the multistage maintenance.
After above-mentioned T2 temperature province keeps the specified time, by being cooled to room temperature, can manufacture the first high tensile steel plate of the present invention.
On the surface of above-mentioned the first high tensile steel plate, also can form molten zinc plating layer and hot dip alloyed zinc coating.
Condition while forming molten zinc plating layer and hot dip alloyed zinc coating is not particularly limited, and can adopt known condition.
For example preferably make the plating bath temperature be 400～500 ℃ and form the molten zinc plating layer, more preferably 440～470 ℃.The composition of plating bath is not particularly limited, and adopts known molten zinc plating to bathe and gets final product.
For the hot-dip galvanized steel sheet that has formed the molten zinc plating layer, implement the Alloying Treatment of ordinary method, thus can the alloying hot-dip galvanized steel sheet.Alloying Treatment, it carries out for example 450～600 ℃ of left and right (particularly preferably 480～570 ℃ of left and right), keeps (particularly preferably about 10～25 seconds) about 5～30 seconds to get final product.Alloying Treatment, such as using process furnace, straight fire or infrared heating oven etc. to get final product.Heater means also is not particularly limited, such as adopting heated by gas, inductive heater heating habitual means such as (heating of being undertaken by high-frequency induction heating apparatus).
Technology of the present invention, be particularly suitable for the steel sheet of thickness of slab below 3mm and adopt.
Above, for the first high tensile steel plate of the present invention, be illustrated.
Next, for the second high tensile steel plate of the present invention, describe.
The present inventors, the research repeatedly in order to improve processibility, particularly unit elongation that tensile strength is the second high tensile steel plate more than 590MPa and local deformability.It found that,
(1) become polygon ferrite main body (specifically at the metal structure that makes steel plate, with respect to metal structure, all area occupation ratio surpasses 50%) basis on, make it again to become the mixed structure of containing bainite, tempered martensite and residual γ, particularly as bainite
If (1a) make between the residual γ of adjacency, between the carbide of adjacency or the carbide of the residual γ of adjacency and adjacency (below, by its unified residual γ etc. that is expressed as.) the equispaced of central position spacing be that the above high-temperature area of 1 μ m generates bainite, and
(1b) equispaced of the central position spacing of residual γ etc. generates these two kinds of bainites of bainite lower than the low-temperature region of 1 μ m and generates, can provide and can not make unit elongation deteriorated, and improved the second high tensile steel plate of the excellent in workability of local deformaton ability
(2) specifically, above-mentioned high-temperature area generates bainite and contributes to the unit elongation of steel plate to improve, and above-mentioned low-temperature region generates bainite and contributes to the local deformaton ability of steel plate to improve,
(3) for two kinds of bainites are generated with specified amount, after the heating of above-mentioned two-phase temperature province, with the average cooling rate more than 2 ℃/secs be cooled to temperature province more than 400 ℃, below 540 ℃ (below, be called the T1 temperature province.) temperature T arbitrarily, after this T1 temperature province keeps 10～100 seconds and makes high-temperature area generate bainite to generate, be cooled to more than 200 ℃, lower than the temperature provinces of 400 ℃ (below, be called the T2 temperature province.), keep getting final product more than 200 seconds in this T2 temperature province, thereby completed the present invention.
At first, for giving the metal structure of the second high tensile steel plate of the present invention with feature, describe.
" about metal structure "
The metal structure of the second high tensile steel plate of the present invention is the mixed structure consisted of polygon ferrite, bainite, tempered martensite and residual γ.
［ polygon ferrite ］
The metal structure of the second high tensile steel plate of the present invention, take polygon ferrite as main body.So-called main body, the meaning is exactly that the area occupation ratio all with respect to metal structure surpasses 50%.Polygon ferrite and bainite are in a ratio of soft, are to improve for the unit elongation that improves steel plate the tissue that processibility plays a role.In order to bring into play such effect, polygon ferritic area occupation ratio, all surpass 50% with respect to metal structure, is preferably more than 55%, more preferably more than 60%.The upper limit of polygon ferritic area occupation ratio, consider the fill-in ratio of the residual γ that the saturation magnetization method is measured and determine, is for example 85%.
The average equivalent circular diameter D of above-mentioned polygon ferrite crystal grain is preferably 10 μ m following (not containing 0 μ m).By reducing the average equivalent circular diameter D of polygon ferrite crystal grain, make it slightly to disperse, can make the unit elongation of steel plate further improve.Though its detailed mechanism it be unclear that, but think due to by making polygon ferrite miniaturization, with respect to metal structure, all polygon ferritic dispersion staties become evenly, therefore are difficult to occur inhomogeneous distortion, and this contributes to the further raising of unit elongation.Think, because the metal structure of the second high tensile steel plate of the present invention, mixed structure by polygon ferrite, bainite, tempered martensite and residual γ forms, so if the particle diameter of polygon ferrite crystal grain becomes greatly, the size of each tissue produces deviation, therefore inhomogeneous distortion occurs, and strain is concentrated locally and is difficult to improve processibility (particularly by polygon ferrite, generating the unit elongation raising effect brought).Therefore polygon ferritic average equivalent circular diameter D is preferably below 10 μ m, more preferably, below 8 μ m, more preferably, below 5 μ m, is particularly preferably below 4 μ m.
Above-mentioned polygon ferritic area occupation ratio and average diameter of equivalent circle D, can be by being observed to measure with scanning electron microscope (SEM).
［ bainite and tempered martensite ］
The second high tensile steel plate of the present invention is characterized in that bainite generates bainite by high-temperature area, and the strength ratio high-temperature area generates the complex tissue formation of the low-temperature region generation bainite that bainite is high.High-temperature area generates bainite and contributes to the unit elongation of steel plate to improve, and low-temperature region generates bainite and contributes to the local deformaton ability of steel plate to improve.And, by containing this two kinds of bainite structures, can not make the unit elongation of steel plate deteriorated, and can make the local deformaton ability improve, can all improve the processibility of steel plate.This be considered to due to, make the different bainite structure Composite of strength level cause nonaffine deformation to occur, so work hardening capacity rises.
Above-mentioned so-called high-temperature area generates bainite, is to be heated to Ac
1more than point+20 ℃, Ac
3in temperature (two-phase temperature province) process of cooling afterwards below point+20 ℃, the bainite structure generated in the T1 temperature province more than 400 ℃, below 540 ℃.High-temperature area generates bainite, is that while for the steel plate cross section through the corrosion of peroxy-nitric acid ethanol, carrying out the SEM observation, the equispaced of residual γ etc. reaches the above tissue of 1 μ m.
On the other hand, above-mentioned so-called low-temperature region generates bainite, is in the process of cooling after being heated to above-mentioned two-phase temperature province, at the bainite structure generated more than 200 ℃, lower than the T2 temperature provinces of 400 ℃.Low-temperature region generates bainite, is that while for the steel plate cross section through the corrosion of peroxy-nitric acid ethanol, carrying out the SEM observation, the equispaced of residual γ etc. is lower than the tissue of 1 μ m.
The meaning in these " equispaceds of residual γ etc. " is identical with the situation of above-mentioned the first high tensile steel plate.
In addition, tempered martensite, be to have the tissue that generates the same effect of bainite with above-mentioned low-temperature region, contributes to the local deformaton ability of steel plate to improve.Also have, above-mentioned low-temperature region generates bainite and tempered martensite, even carry out SEM, observes and can not distinguish, and therefore in the present invention, low-temperature region is generated to bainite and tempered martensite is referred to as " low-temperature region generates bainite etc. ".
In the present invention, by bainite is become, contain the compound bainite structure that high-temperature area generates bainite and low-temperature region generation bainite etc., can realize having improved the second high tensile steel plate of processibility comprehensively.That is, high-temperature area generates Bainite Phases of Some, and to generate bainite etc. than low-temperature region be soft, so contribute to improve the unit elongation (EL) of steel plate, improves processibility.On the other hand, low-temperature region generates bainite etc., because carbide and residual γ are little, when distortion, stress concentration alleviates, so contribute to improve stretch flangeability (λ) and the bendability (R) of steel plate, improves the local deformaton ability, improves processibility.And in the present invention, because make such high-temperature area generate bainite and low-temperature region generates the mixing such as bainite, so work hardening capacity improves, can not make unit elongation deteriorated and can improve the local deformaton ability.
In the present invention, as described so according to the difference of the equispaced of the difference that generates temperature province and residual γ etc., the reason that Bainite Region is divided into to " high-temperature area generation bainite " and " low-temperature region generates bainite etc. ", be due in the tissue typing of general science, be difficult to distinguish legibly bainite.For example, the bainite of lath-shaped and bainite ferrite, be categorized as upper bainite and lower bainite according to transformation temperature.But a large amount of Si of containing reach in the steel grade more than 1.0% as the present invention, along with bainitic transformation, separating out of carbide is suppressed, and therefore, in SEM observes, also comprises martensitic stucture, and it is distinguished and has any problem.Therefore in the present invention, according to learned tissue, do not define the bainite of classifying, and be based on as described so, the equispaced of the difference based on generating temperature province and residual γ etc. is distinguished.
The distribution that high-temperature area generates bainite and low-temperature region generation bainite etc. is not particularly limited, can generate high-temperature area at old γ intracrystalline and generate these two sides such as bainite and low-temperature region generation bainite, can be also that every Geju City γ crystal grain generates respectively high-temperature area generation bainite and low-temperature region generates bainite etc.
High-temperature area generates the distribution of bainite and low-temperature region generation bainite etc., as above-mentioned Fig. 2 medelling ground shows.
In the present invention, if among metal structure is all, the area occupation ratio of shared high-temperature area generation bainite is b, when among metal structure is all, the total area occupation ratio of shared (low-temperature region generate bainite and tempered martensite) such as low-temperature region generation bainites is c, this area occupation ratio b and c all need to meet 5～40%.At this, do not stipulate that low-temperature region generates the area occupation ratio of bainite, but the regulation low-temperature region generates the reason of the total area occupation ratio of bainite and tempered martensite, as aforementioned, be owing to can not these tissues being distinguished in SEM observes.
Above-mentioned area occupation ratio b is 5～40%.If it is very few that high-temperature area generates the growing amount of bainite, the unit elongation of steel plate reduces, and can not improve processibility.Therefore above-mentioned area occupation ratio b is more than 5%, is preferably more than 8%, more preferably more than 10%.But, if generating the growing amount of bainite, high-temperature area becomes superfluous, with low-temperature region, generate the balance variation of the growing amount of bainite etc., and high-temperature area generates the effect that Composite that bainite and low-temperature region generate bainite etc. brings and can't bring into play.Therefore the area occupation ratio b of high-temperature area generation bainite is below 40%, is preferably below 35%, more preferably below 30%, more preferably below 25%.
In addition, above-mentioned total area occupation ratio c is 5～40%.If the growing amount of low-temperature region generation bainite etc. is very few, the local deformaton ability of steel plate reduces, and can not improve processibility.Therefore above-mentioned total area occupation ratio c is more than 5%, is preferably more than 8%, more preferably more than 10%.But, if the growing amount of low-temperature region generation bainite etc. becomes superfluous, with high-temperature area, generating the balance variation of the growing amount of bainite, the effect that the Composite of low-temperature region generation bainite etc. and high-temperature area generation bainite brings can't be brought into play.Therefore to generate the area occupation ratio c of bainite etc. be below 40% to low-temperature region, is preferably below 35%, more preferably below 30%, more preferably below 25%.
The relation of above-mentioned area occupation ratio b and above-mentioned total area occupation ratio c, be not particularly limited as long as scope separately meets above-mentioned scope, and any one form of b>c, b<c, b=c all comprises.
High-temperature area generates the mixture ratio of bainite and low-temperature region generation bainite etc., according to the desired characteristic of steel plate, determines to get final product.Specifically, among the processibility of steel plate, for local deformaton ability (particularly stretch flangeability (λ)) is further improved, reduce as much as possible the ratio that high-temperature area generates bainite, increase as much as possible the ratio that low-temperature region generates bainite etc. and get final product.On the other hand, among the processibility of steel plate, in order to make unit elongation, further improve, increase as much as possible the ratio that high-temperature area generates bainite, reduce as much as possible the ratio that low-temperature region generates bainite etc. and get final product.In addition, in order further to improve the intensity of steel plate, increase as much as possible the ratio that low-temperature region generates bainite etc., reduce as much as possible the ratio that high-temperature area generates bainite and get final product.
Also have, in the present invention, also comprise bainite ferrite in bainite.Bainite is the tissue that Carbide Precipitation is arranged, and bainite ferrite is the tissue that there is no Carbide Precipitation.
［ polygon ferrite+bainite+tempered martensite ］
In the present invention, preferably the total (a+b+c) of the total area occupation ratio c of (low-temperature region generate bainite+tempered martensite) such as the area occupation ratio b of above-mentioned polygon ferritic area occupation ratio a, above-mentioned high-temperature area generation bainite and above-mentioned low-temperature region generation bainites meets more than 70% with respect to metal structure is all.If add up to area occupation ratio (a+b+c) lower than 70%, unit elongation is deteriorated.Add up to area occupation ratio (a+b+c) more preferably more than 75%, more preferably more than 80%.The upper limit that adds up to area occupation ratio (a+b+c), consider the fill-in ratio of the residual γ that the saturation magnetization method is measured and determine, is for example 95%.
［ residual γ ］
About the specified content of residual γ, since identical with above-mentioned the first high tensile steel plate, so description thereof is omitted.
［ other ］
The metal structure of the second high tensile steel plate of the present invention, as above-mentioned, contain polygon ferrite, bainite, tempered martensite and residual γ, although also can only be formed by these, but in the scope of not damaging effect of the present invention, also can there is the MA mixed phase that (a) quenched martensite and residual γ are compound, and (b) the surplus tissue of perlite etc.
(a) about MA mixed phase and (b) pearlitic specified content, identical with above-mentioned the first high tensile steel plate, therefore description thereof is omitted.
The measuring process of above-mentioned metal structure, identical with the step illustrated in above-mentioned the first high tensile steel plate, therefore description thereof is omitted.
Next, the chemical composition composition for the second high tensile steel plate of the present invention describes.
" about one-tenth, being grouped into "
The second high tensile steel plate of the present invention, contain C:0.10～0.3%, Si:1.0～3%, Mn:1.0～2.5%, Al:0.005～3%, and meet P:0.1% following (not containing 0%), S:0.05% following (not containing 0%).Stipulate the reason of such scope, except Si and Mn, all identical with above-mentioned the first high tensile steel plate, therefore description thereof is omitted, below, only for Si and Mn, describe.
Si contributes to the high strength of steel plate as the solution strengthening element, in addition, in maintenance under T1 temperature province described later and T2 temperature province, (in means of isothermal quenching) suppresses Carbide Precipitation, effectively making residual γ generate, is very important element.Therefore the Si amount is more than 1.0%, is preferably more than 1.2%, more preferably more than 1.3%.But, if contain Si superfluously, the reverse transformation to the γ phase during the heating in annealing/soaking does not occur, polygon ferrite is remaining in a large number, causes undercapacity.In addition, when hot rolling, Si oxide skin occurs and the surface texture of steel plate is worsened in surface of steel plate.Therefore the Si amount is below 3%, is preferably below 2.50%, more preferably below 2.0%.
Mn is for obtaining the needed element of bainite and tempered martensite.Mn makes the γ stabilization in addition, is for making residual γ generate the element also effectively played a role.In order to bring into play such effect, the Mn amount is more than 1.0%, is preferably more than 1.5%, more preferably more than 1.8%.If but the superfluous Mn that contains, the generation that high-temperature area generates bainite significantly is suppressed.In addition, the surplus of Mn is added, and causes the deteriorated of processibility that the deteriorated and segregation of weldability causes.Therefore the Mn amount is below 2.5%, is preferably below 2.4%, more preferably below 2.3%.
In the second high tensile steel plate of the present invention, also same with above-mentioned the first high tensile steel plate, as other element, also can also contain as inferior:
(a) Cr:1% following (not containing 0%) and/or Mo:1% following (not containing 0%);
(b) below Ti:0.15% (containing 0%), (containing 0%) Nb:0.15% below and (do not contain 0%) V:0.15% below and form group more than one element of selection;
(c) Cu:1% following (not containing 0%) and/or Ni:1% following (not containing 0%);
(d) B:0.005% following (not containing 0%);
(e) (not containing 0%), (not containing 0%) and rare earth element below Mg:0.01% below Ca:0.01%: the element of more than one that select the group that below 0.01%, (not containing 0%) forms.Stipulate the reason of such scope, since identical with above-mentioned the first high tensile steel plate, so description thereof is omitted.
The second high tensile steel plate of the present invention, because tensile strength is more than 590MPa, the unit elongation excellence, and also the local deformaton ability is also good, so excellent in workability.This second high tensile steel plate, same with above-mentioned the first high tensile steel plate, the former material that is suitable as the structure component of automobile is used.
In addition, above-mentioned the second high tensile steel plate, because the processibility under warm state is good, so also be suitable as the former material use that warm working is used.Also have, so-called temperature processing, the meaning is that the temperature range 50～500 ℃ of left and right forms.
Above, for the metal structure of the second high tensile steel plate of the present invention, with becoming to be grouped into, be illustrated.
Then, for the method that can manufacture above-mentioned the second high tensile steel plate, describe.Above-mentioned the second high tensile steel plate, can be by comprising the method manufacture of following operation: will meet the steel plate that mentioned component forms and be heated to Ac in order
1more than point+20 ℃, Ac
3the operation of the temperature province (two-phase temperature province) below point+20 ℃; Keep the operation more than 50 seconds in this temperature province; Be cooled to the operation of the temperature T arbitrarily that meets following formula (1) with the average cooling rates of 2～50 ℃/secs; Keep the operation of 10～100 seconds in the temperature province that meets following formula (1); Keep the operation more than 200 seconds in the temperature province that meets following formula (2).Below, for each operation, describe in order.
400℃≤T1（℃）≤540℃ …（1）
200℃≤T2（℃）＜400℃ …（2）
At first, as being heated to two-phase temperature province ［ Ac
1more than point+20 ℃, Ac
3temperature province below point+20 ℃ ］ high tensile steel plate before, prepare to follow ordinary method slab is carried out to hot rolling, carry out cold rolling steel plate for resulting hot-rolled steel sheet.Hot rolling, making finishing temperature is for example more than 800 ℃, making coiling temperature is for example to get final product below 700 ℃.In cold rolling, making cold rolling rate is for example that 10～70% scope is rolled and gets final product.
Cold rolling and the cold-rolled steel sheet that obtains, on continuous annealing line, is heated to Ac
1more than point+20 ℃, Ac
3temperature province below point+20 ℃, keep more than 50 seconds and carry out soaking in this temperature province.
By making Heating temperature in ferrite and austenitic two-phase temperature province, can make the polygon ferrite of specified amount generate.That is, if Heating temperature is too high, become the austenite one phase territory, polygon ferritic generation is suppressed, and therefore can not improve the unit elongation of steel plate, and processibility is deteriorated.Therefore Heating temperature is Ac
3below point+20 ℃, be preferably Ac
3below point+10 ℃, more preferably lower than Ac
3the point.Also have, if be heated to Ac
3more than point, although become the temperature province of austenite one phase, under soaking time degree given to this invention, if Heating temperature is Ac
3below point+20 ℃, even carry out soaking, keep still having a small amount of polygon ferrite remaining, therefore, as described later by adjusting the average cooling rate after soaking, can make the polygon ferrite of specified amount generate.If but Heating temperature is lower than Ac
1point+20 ℃, polygon ferritic growing amount becomes superfluous, and the high-temperature area that can not get specified amount generates bainite, low-temperature region generation bainite etc. and residual γ, so processibility is deteriorated.Therefore Heating temperature is Ac
1more than point+20 ℃, be preferably Ac
1more than point+30 ℃, Ac more preferably
1more than point+50 ℃.
If the soaking time of above-mentioned two-phase temperature province, lower than 50 seconds, can not heat steel plate equably, therefore the generation of residual γ is suppressed, and unit elongation and local deformability reduce, and do not improve processibility.Therefore soaking time is more than 50 seconds, is preferably more than 100 seconds.But if soaking time is long, the austenite crystal diameter becomes large, and the consequent is also thickization of polygon ferrite crystal grain, and unit elongation and local deformability have the tendency that becomes evil.Therefore soaking time was preferably below 500 seconds, more preferably below 450 seconds.
Also have, average rate of heating when above-mentioned cold-rolled steel sheet is heated to above-mentioned two-phase temperature province, be for example to get final product more than 1 ℃/sec.
Above-mentioned Ac
1point, Ac
3point is same with above-mentioned the first high tensile steel plate, can according to " レ ス リ ー iron steel science ", (ball be apt to Co., Ltd., on May 31st, 1985 distribution, P.273) described formula (a), formula (b) calculating.
Be heated to above-mentioned two-phase temperature province and keep more than 50 seconds and, after carrying out evenly heating, be cooled to the temperature T arbitrarily that meets above formula (1) with 2～50 ℃/secs of average cooling rates.By cooling two-phase temperature province more than the average cooling rate in regulation to the scope of the temperature T arbitrarily that meets above formula (1), can make the polygon ferrite of specified amount generate, the high temperature area that can make generates these two sides generations such as bainite and low-temperature region generation bainite in addition.If the average cooling rate of this temperature province, lower than 2 ℃/secs, pearlitic transformation occurs, perlite generates superfluously, and unit elongation reduces, and processibility is deteriorated.This interval average cooling rate is preferably more than 5 ℃/secs, more preferably more than 10 ℃/secs.But if the average cooling rate in above-mentioned interval is excessive, can not guarantee the polygon ferrite of specified amount.Therefore average cooling rate is below 50 ℃/secs, is preferably below 40 ℃/secs, more preferably below 30 ℃/secs.
After being cooled to the temperature T arbitrarily that meets above formula (1), after the T1 temperature province that meets above formula (1) keeps 10～100 seconds, then keep more than 200 seconds in the T2 temperature province that meets above formula (2).By suitably controlling respectively the time that remains on T1 temperature province and T2 temperature province, can make high-temperature area generate bainite and low-temperature region generation bainite etc. and generate with specified amount separately.
Concrete condition when remaining on T1 temperature province and T2 temperature province, identical with the condition illustrated in above-mentioned the first high tensile steel plate, therefore description thereof is omitted.
After above-mentioned T2 temperature province keeps the specified time, be cooled to room temperature, can manufacture thus the second high tensile steel plate of the present invention.
On the surface of above-mentioned the first high tensile steel plate, same with above-mentioned the first high tensile steel plate, also can form molten zinc plating layer and hot dip alloyed zinc coating.
Condition while forming molten zinc plating layer and hot dip alloyed zinc coating is not particularly limited, and can adopt known condition.Concrete condition is identical with above-mentioned the first high tensile steel plate, and therefore description thereof is omitted.
Technology of the present invention, can be adapted at adopting on the following steel sheet of thickness of slab 3mm especially.
Above, for this second high tensile steel plate of the present invention, be illustrated.
No. 2011-197670th, the Japanese patent application that the Japanese patent application that the Japanese patent application of the application based on applying on March 31st, 2011 applied on March 31st, No. 2011-080953 1 is applied on September 9th, No. 2011-080954 1 and the Japanese patent application of applying on September 9th, 2011 are advocated preferential interests No. 2011-197671.The full content of the specification sheets that No. 2011-197671st, No. 2011-197670th, the Japanese patent application that the Japanese patent application that the Japanese patent application of applying on March 31st, 2011 is applied on March 31st, No. 2011-080953 1 is applied on September 9th, No. 2011-080954 1 and the Japanese patent application of applying on September 9th, 2011, in application for reference to and quote.
Embodiment
Below, enumerate embodiment and be described more specifically the present invention, but the present invention is not limited by following embodiment, can certainly suitably be changed enforcement in the scope that can meet the forward and backward aim of stating, these all are included in technical scope of the present invention.Also have, following embodiment 1 is the embodiment about above-mentioned the first high tensile steel plate, and following embodiment 2 is the embodiment about above-mentioned the second high tensile steel plate.
［ embodiment 1 ］
The steel that chemical composition shown in the following table 1 of vacuum melting or table 2 forms (surplus is the inevitable impurity beyond iron and P, S, N, O), manufacture the experiment slab.In following table 1 or table 2, it is 50% left and right that the REM use contains La, the mishmetal that Ce is 30% left and right.
Chemical composition based on shown in following table 1 or table 2 and above formula (a) calculate Ac
1point, and calculate Ac based on above formula (b)
3point, its result is presented in following table 3～table 5.
Carry out after hot rolling carrying out again cold rolling for resulting experiment with slab, then carry out continuous annealing and manufacture for the examination material.Concrete condition is as follows.
With 1250 ℃ of heating, keep experiment with slabs after 30 minutes, it is about 90% making draft, makes finishing temperature be 920 ℃ and carries out hot rolling, and the equal speed of cooling from this temperature with 30 ℃/secs is cooled to 500 ℃ of coiling temperatures and is batched.After batching, with this coiling temperature (500 ℃), keep 30 minutes, then stove is chilled to room temperature, manufactures the hot-rolled steel sheet of thickness of slab 2.6mm.
Carry out pickling for resulting hot-rolled steel sheet, after removing surface scale, carry out cold rollingly with cold rolling rate 46%, manufacture the cold-rolled steel sheet of thickness of slab 1.4mm.
By the cold-rolled steel sheet obtained be heated to the temperature shown in following table 3～table 5 (℃), keep the time shown in following table 3～table 5 and after carrying out soaking, any one mode among shown below 4 followed is carried out cooling, continuous annealing and manufacturing for the examination material.
(type of cooling i; (i) of corresponding above-mentioned Fig. 3)
After soaking, with the average cooling rate shown in following table 3～table 5 (℃/sec), be cooled to the beginning temperature T shown in following table 3～table 5 (℃) after, with this start temperature T (℃) keep, then be cooled to beginning temperature in the T2 temperature province shown in following table 3～table 5 (℃), start temperature with this and keep.In following table 3～table 5, the residence time (second) in demonstration T1 temperature province and the residence time (second) in the T2 temperature province.In addition, also show from the T1 temperature province, keeping the complete moment, to the time (second) that arrives the beginning temperature in the T2 temperature province.
(type of cooling ii; Corresponding above-mentioned Fig. 3 (ii))
After soaking, with the average cooling rate shown in following table 3～table 5 (℃/sec) be cooled to the beginning temperature T shown in following table 3～table 5 (℃) afterwards, be cooled to again the end temp shown in following table 3～table 5 (℃), then be cooled to beginning temperature in the T2 temperature province shown in following table 3～table 5 (℃), start temperature with this and keep the time shown in following table 3～table 5 (second).In following table 3～table 5, the residence time (second) in demonstration T1 temperature province and the residence time (second) in the T2 temperature province.In addition, also show from the T1 temperature province and keep the complete moment, to the time (second) that arrives the beginning temperature in the T2 temperature province.
(type of cooling iii; Corresponding above-mentioned Fig. 3 (iii))
After soaking, with the average cooling rate shown in following table 3, table 4 (℃/sec), be cooled to the beginning temperature T shown in following table 3, table 4 (℃) afterwards, be cooled to beginning temperature in the T2 temperature province shown in following table 3, table 4 (℃), start temperature with this and keep.In following table 3, table 4, the residence time (second) in demonstration T1 temperature province and the residence time (second) in the T2 temperature province.
(type of cooling iv)
After soaking, be cooled to beginning temperature in the T1 temperature province shown in following table 3 (℃) or the T2 temperature province in the beginning temperature (℃), start temperature with any one and keep.; the No.8 of following table 3; be not after soaking; keep being retained to again afterwards in 450 seconds room temperature with 420 ℃; but carry out at a dass the example of cooling (average cooling rate is 5 ℃/secs); the residence time in T2 temperature province shown in following table 3, mean by the needed time of T2 temperature province.The No.15 of following table 3, be not after soaking, keep being retained to again afterwards in 450 seconds room temperature with 380 ℃, but carry out at a dass the example of cooling (average cooling rate is 5 ℃/secs), the residence time in T1 temperature province shown in following table 3, mean by the needed time of T1 temperature province.In following table 3, the residence time (second) in demonstration T1 temperature province and the residence time (second) in the T2 temperature province.
Also have, among beginning temperature in beginning temperature in the T1 temperature province shown in table 3～table 5, end temp, T2 temperature province, the value of additional No. ※, it has broken away from T1 temperature province given to this invention or T2 temperature province, but for convenience of explanation, mean heating curve, so record and narrate temperature in each hurdle.
For the examination material, by following step, carry out the observation of metal structure and the evaluation of mechanical characteristics for resulting.
" observation of metal structure "
Among metal structure, (high-temperature area generates bainite, low-temperature region generation bainite etc., low-temperature region generates bainite+tempered martensite) and polygon ferritic area occupation ratio, the result of observing based on scanning electron microscope (SEM) is calculated, and the volume fraction of residual γ is measured by the saturation magnetization method.
［ (1) high-temperature area generates bainite, and low-temperature region generates bainite etc., polygon ferritic area occupation ratio ］
In the parallel cross section of rolling direction with for the examination material, ground for surface, then, after carrying out electrolytic polishing, carried out the corrosion of nitric acid ethanol, use SEM, with 3000 times of 5 visuals field of observations carry out to(for) 1/4 position of thickness of slab of multiplying power.Field of view is the 50 μ m of 50 μ m * approximately approximately.
Secondly, in field of view, as white or the light grey residual γ be observed and the equispaced of carbide, based on aforesaid method, measured.Generate the area occupation ratio of bainite and low-temperature region generation bainite etc. according to the high-temperature area of its equispaced difference, measure by an algorithm.
The area occupation ratio a(% of high-temperature area generation bainite), the total area occupation ratio b(% of low-temperature region generation bainite master tempered martensite), polygon ferritic area occupation ratio c(%) be presented in following table 6～table 8.In addition, the total area occupation ratio (a+b+c) that also shows in the lump above-mentioned area occupation ratio a, total area occupation ratio b and area occupation ratio c.
In addition, measure the diameter of equivalent circle of the polygon ferrite crystal grain confirmed in field of view, try to achieve mean value.Result is presented in following table 6～table 8.In addition, the average equivalent circular diameter D of polygon ferrite crystal grain 10 μ m when following for estimating zero, while surpassing 10 μ m, for estimating △, evaluation result is presented in following table 6～table 8.
［ volume fraction of (2) residual γ ］
Among metal structure, the volume fraction of residual γ is measured with the saturation magnetization method.Specifically, measure the saturation magnetization (I) for the examination material, and, with 400 ℃ of saturation magnetizations (Is) that carried out 15 hours heat treated standard test portions, try to achieve the volume fraction (V γ r) of residual γ according to following formula.The measurement of saturation magnetization, used reason to grind the DC magnetization B-H characteristic self-recording unit " model BHS-40 " of electronics system, makes the maximum additional 5000(Oe of being magnetized to), at room temperature measured.
Vγr＝（1－I/Is）×100
In addition, for the rolling direction with for the examination material, the surface in parallel cross section is ground, and uses opticmicroscope, to observe 1000 times of multiplying powers, in 5 visuals field, is observed, and measures the diameter of equivalent circle d of the compound MA mixed phase of residual γ and quenched martensite.Calculating is with respect to total number of MA mixed phase, and the diameter of equivalent circle d in the observation cross section surpasses the number ratio of the MA mixed phase of 7 μ m.The number ratio is qualified (zero) lower than 15% the time, 15%, is defective (*) when above, and evaluation result is presented in following table 6～table 8.
" evaluation of the characteristic of machinery "
For the characteristic of the machinery that tries material, based on tensile strength (TS), unit elongation (EL), hole expansibility (λ), critical bends radius (R), Erichsen value, estimated.
(1) tensile strength (TS) and unit elongation (EL), carry out tension test and measure based on JIS Z2241.Test film is used, and making with respect to the vertical direction of rolling direction for the examination material is lengthwise direction, so from for the examination material, cutting No. 5 test films from JIS Z2201 defined.Measuring result is presented in following table 6～table 8.
(2) stretch flangeability, estimated by hole expansibility.Hole expansibility (λ), carry out the hole expansibility test and measure based on the specification JFST of iron steel alliance 1001.Measuring result is presented in following table 6～table 8.
(3) critical bends radius (R), carry out the v-shaped bending test and measure based on JIS Z2248.Test film is used, and making with respect to the vertical direction of rolling direction for the examination material is lengthwise direction (crooked crestal line is consistent with rolling direction), so from cut No. 1 test film (thickness of slab: 1.4mm) of JIS Z2204 defined for the examination material.Also have, V-arrangement becomes bent test, and it is in the mode do not occurred be full of cracks, and the end face of the lengthwise direction of test film is implemented to carry out after mechanical grinding again.
The angle of punch die and drift is 90 °, with the 0.5mm of unit, changes the front-end radius of drift and carries out the v-shaped bending test, and the drift front-end radius that trying to achieve can not make to chap occurs and can be crooked is as critical bends radius (R).Measuring result is presented in following table 6～table 8.Also have, use magnifying glass to observe and have or not be full of cracks to occur, judged without hairline crack, to occur as standard.
(4) Erichsen value, carry out drawing test and measure based on JIS Z2247.The test film of 90mm * 90mm that the test film use is cut from confession examination material * thick 1.4mm.Drawing test is used the drift that diameter is 20mm to carry out.Measuring result is presented in following table 6～table 8.Also have, according to drawing test, can estimate the breaking elongation characteristic of steel plate and this two side's of local ductility composite effect.
For the characteristic of the machinery that tries material, the standard of the corresponding unit elongation of tensile strength (TS) (EL), hole expansibility (λ), critical bends radius (R), Erichsen value of following is estimated.That is, because the TS of steel plate is desired EL, λ, R, Erichsen value difference, so, according to the TS rank, follow the characteristic of following standard evaluation machinery.
Based on following judgement criteria, the situation that meets EL, λ, R, Erichsen value all characteristics is qualified (zero), and the situation that any one characteristic does not meet standard value is defective (*), and evaluation result is presented in following table 6～table 8.
(1) situation of 780MPa level
More than TS:780MPa, lower than 980MPa
More than EL:25%
λ: more than 30%
Below R:1.0mm
Erichsen value: more than 10.4mm
(2) situation of 980MPa level
More than TS:980MPa, lower than 1180MPa
More than EL:19%
λ: more than 20%
Below R:3.0mm
Erichsen value: more than 10.0mm
(3) situation of 1180MPa level
More than TS:1180MPa, lower than 1270MPa
More than EL:15%
λ: more than 20%
Below R:4.5mm
Erichsen value: more than 9.6mm
(4) situation of 1270MPa level
More than TS:1270MPa, lower than 1370MPa
More than EL:14%
λ: more than 20%
Below R:5.5mm
Erichsen value: more than 9.4mm
Also have, in the first high tensile steel plate, the TS of take is prerequisite more than 780MPa, lower than 1370MPa, and TS is lower than 780MPa or when 1370MPa is above, even EL, λ, R, Erichsen value are good, also as processing outside object.
According to following table 1～table 8, can investigate as follows.Among No.1 shown in following table 6～table 8～70, No.4,29,31,38,55,65, the 67th, the cooling example of i in the above described manner, No.7,11,14, the 33rd, the cooling example of iii in the above described manner, No.8, the 15th, the cooling example of iv in the above described manner, all the other are cooling examples of ii in the above described manner.
In following table 6～table 8, be attached with zero example in comprehensive evaluation, are all the steel plates that meet the important document of stipulating in the present invention, meet corresponding each TS and the standard value of the characteristic (EL, λ, R, Erichsen value) of definite machinery.Therefore known, high tensile steel plate processibility of the present invention is comprehensively good.
On the other hand, in comprehensive evaluation, be attached with * example (No.8 shown in table 6～table 8,13,15,29,31,34,37,41,46,48,52,60～63), be the steel plate of a certain important document of regulation in discontented unabridged version invention.Details as Follows.
The No.8 of table 6, be that the hold-time under the T1 temperature province is long, and do not keep and carry out cooling example in the T2 temperature province, and the generation of low-temperature region generation bainite etc. is suppressed.In addition, thick MA mixed phase generates in a large number.Therefore λ diminishes, and processibility is deteriorated.The No.13 of table 6, be after the heating of two-phase temperature province keeps, and to the too small example of average cooling rate of the temperature T arbitrarily that meets above formula (1), pearlitic transformation occurs, and can't guarantee residual γ amount, and unit elongation reduces, and processibility is deteriorated.The No.15 of table 6 is after equal thermal treatment, in the T1 temperature province, keeps, and is cooled to the T2 temperature province quickly, and the example kept in this temperature province.Because only keep in the T2 temperature province, so generating bainite, high-temperature area almost do not generate, and unit elongation and local deformability (Erichsen value) reduce, and processibility is deteriorated.The No.29 of table 6 is after equal thermal treatment, in the T1 temperature province, do not keep, but is cooled to the T2 temperature province quickly, and the example kept with two kinds of temperature in this temperature province.Because only keep in the T2 temperature province, so generating bainite, high-temperature area almost do not generate, and unit elongation reduces, and processibility is deteriorated.
The No.31 of table 6 is too short examples of the hold-time under the T1 temperature province, very few because high-temperature area generates the growing amount of bainite, so unit elongation reduces, processibility is deteriorated.The No.34 of table 7, be that the hold-time under the T1 temperature province is long, the example do not kept in the T2 temperature province, and the generation of low-temperature region generation bainite etc. is suppressed.In addition, thick MA mixed phase generates in a large number.Therefore Erichsen value diminishes, and the local deformaton ability reduces, and can not improve processibility.The No.37 of table 7 is because Heating temperature is too high, so do not generate polygon ferrite, unit elongation reduces.Therefore can not improve the processibility of steel plate.The No.41 of table 7 because Heating temperature too low, so polygon ferrite generate superfluously, strength decreased.
The No.46 of table 7 is too short examples of the hold-time in the two-phase temperature province, and the generation of residual γ is suppressed, so unit elongation reduces.In addition, Erichsen value is little, and the local deformaton ability reduces.Therefore can not improve the processibility of steel plate.The No.48 of table 7 is after equal thermal treatment, with the temperature of the temperature that surpasses the T1 temperature province of stipulating in the present invention, is kept, and is kept the example that is cooled to the T2 temperature province and keeps in this temperature province in the T1 temperature province.Because polygon ferrite generates superfluously, and the growing amount of high-temperature area generation bainite is few, so unit elongation reduces, can not improve processibility.The No.52 of table 7 is after the T1 temperature province keeps, be cooled to the temperature lower than the T2 temperature province, the example kept in the T2 temperature province, low-temperature region generates bainite and does not almost generate, observe and can confirm thick MA mixed phase and exist in a large number by SEM, quenched martensite exists in a large number, and it is too high that intensity becomes.
The No.60 of table 8 is that C measures very few example, because the growing amount of residual γ is very few, so unit elongation and Erichsen value diminish, processibility is deteriorated.The No.61 of table 8 is that Si measures too much example, and polygon ferrite generates superfluously, and the generation of high-temperature area generation bainite and low-temperature region generation bainite etc. is suppressed.Therefore can not guarantee the intensity of wishing.The No.62 of table 8 is that Si measures very few example, can not guarantee the growing amount of residual γ.Therefore unit elongation reduces, and processibility is deteriorated.The No.63 of table 8 is that Mn measures very few example, quench and do not carry out fully, so in cooling, polygon ferrite generates superfluously, and on the other hand, the generation of low-temperature region generation bainite etc. is suppressed.Therefore unit elongation and hole expansibility are little, and Erichsen value is also little, and processibility is deteriorated.
As can be known from the above results, according to the present invention, can provide the high tensile steel plate that has improved processibility.
Next, among the steel plate of the 980MPa level shown in above-mentioned table 6, table 7, about the example (No.3～7,9～12,14,16～27,30,32,33,35,36,38～40,42) that meets important document given to this invention, its tensile strength (TS) and the relation of unit elongation (EL) are presented in Fig. 4.In Fig. 4, ● mean the result of average equivalent circular diameter D below 10 μ m of polygon ferrite crystal grain, ■ means that the average equivalent circular diameter D of polygon ferrite crystal grain surpasses the result of 10 μ m.
As shown in Figure 4, test tensile strength (TS) identical, be suppressed at below 10 μ m by the average equivalent circular diameter D by polygon ferrite crystal grain, also can increase unit elongation (EL), can further improve processibility.
[table 1]
[table 2]
[table 3]
[table 4]
[table 5]
[table 6]
[table 7]
[table 8]
［ embodiment 2 ］
The steel that chemical composition shown in the following table 9 of vacuum melting forms (surplus is the inevitable impurity beyond iron and P, S, N, O), manufacture the experiment slab.In following table 9, it is 50% left and right that the REM use contains La, the mishmetal that Ce is 30% left and right.
Chemical composition based on shown in following table 9 and above formula (a) calculate Ac
1point, and calculate Ac based on above formula (b)
3point, result is presented in following table 10, table 11.
The resulting experiment of hot rolling is cold rolling with carrying out after slab again, and then continuous annealing is manufactured for the examination material.Concrete condition is as follows.
After with 1250 ℃, experiment being kept to 30 minutes with heating of plate blank, it is about 90% making draft, makes finishing temperature be 920 ℃ and carries out hot rolling, from this temperature, is cooled to 500 ℃ of coiling temperatures with 30 ℃/secs of average cooling rates and is batched.After batching, with this coiling temperature (500 ℃), keep 30 minutes, then stove is chilled to room temperature, manufactures the hot-rolled steel sheet of thickness of slab 2.6mm.
For resulting hot-rolled steel sheet pickling and after removing surface scale, carry out cold rollingly with cold rolling rate 46%, manufacture the cold-rolled steel sheet of thickness of slab 1.4mm.
By resulting cold-rolled steel sheet be heated to the temperature shown in following table 10, table 11 (℃), keep the time shown in following table 10, table 11 and after carrying out soaking, any one mode among shown below 4 followed is carried out cooling, and continuous annealing is manufactured for the examination material.
(type of cooling i; (i) of corresponding above-mentioned Fig. 3)
After soaking, with the average cooling rate shown in following table 10, table 11 (℃/sec) be cooled to the beginning temperature T shown in following table 10, table 11 (℃) after, with this start temperature T (℃) keep, then be cooled to beginning temperature in the T2 temperature province shown in following table 10, table 11 (℃), start temperature with this and keep.In following table 10, table 11, the residence time (second) in demonstration T1 temperature province and the residence time (second) in the T2 temperature province.In addition, also show from the T1 temperature province, keeping the complete moment, to the time (second) that arrives the beginning temperature in the T2 temperature province.
(type of cooling ii; Corresponding above-mentioned Fig. 3 (ii))
After soaking, with the average cooling rate shown in following table 10, table 11 (℃/sec) be cooled to the beginning temperature T shown in following table 10, table 11 (℃) after, be cooled to the end temp shown in following table 10, table 11 (℃), then be cooled to beginning temperature in the T2 temperature province shown in following table 10, table 11 (℃), start temperature at this and keep keep the time shown in following table 10, table 11 (second).In following table 10, table 11, the residence time (second) in demonstration T1 temperature province and the residence time (second) in the T2 temperature province.In addition, also shown from the T1 temperature province, keeping the complete moment, to the time (second) that arrives the beginning temperature in the T2 temperature province.
(type of cooling iii; Corresponding above-mentioned Fig. 3 (iii))
After soaking, with the average cooling rate shown in following table 10, table 11 (℃/sec) be cooled to the beginning temperature T shown in following table 10, table 11 (℃) after, be cooled to beginning temperature in the T2 temperature province shown in following table 10, table 11 (℃), start temperature with this and keep.In following table 10, table 11, be presented at residence time (second) in the T1 temperature province and the residence time (second) in the T2 temperature province.
(type of cooling iv)
After soaking, be cooled to beginning temperature in the T1 temperature province shown in following table 10 (℃), start temperature with this and keep.; the No.19 of following table 10; be not after soaking; keep being retained to again afterwards in 450 seconds room temperature with 420 ℃; but carry out at a dass the example of cooling (average cooling rate is 5 ℃/secs); the residence time in T2 temperature province shown in following table 10, mean by the needed time of T2 temperature province.In following table 10, the residence time (second) in demonstration T1 temperature province and the residence time (second) in the T2 temperature province.
Also have, among beginning temperature in beginning temperature in T1 temperature province shown in table 10, end temp, T2 temperature province, the value of additional No. ※, it has broken away from T1 temperature province given to this invention or T2 temperature province, but for convenience of explanation, mean heating curve, and record and narrate temperature in each hurdle.
For the examination material, according to following step, carry out the observation of metal structure and the evaluation of mechanical characteristics for resulting.
" observation of metal structure "
Among metal structure, polygon ferrite, high-temperature area generate bainite and low-temperature region and generate bainite etc. (, low-temperature region generation bainite+tempered martensite) area occupation ratio, the result of observing based on scanning electron microscope (SEM) is calculated, and the volume fraction of residual γ is measured with the saturation magnetization method.
［ (1) polygon ferrite, high-temperature area generates bainite, and low-temperature region generates the area occupation ratio of bainite etc. ］
In the parallel cross section of rolling direction with for the examination material, its surface is ground, then, after carrying out electrolytic polishing, made it to carry out the corrosion of nitric acid ethanol, use SEM, with 3000 times of 5 visuals field of observations carry out to(for) 1/4 position of thickness of slab of multiplying power.Field of view is the 50 μ m of 50 μ m * approximately approximately.
Secondly, in field of view, as white or the light grey residual γ be observed and the equispaced of carbide, based on preceding method, measure.Generate the area occupation ratio of bainite and low-temperature region generation bainite etc. according to the high-temperature area of its equispaced difference, measure by an algorithm.
Polygon ferritic area occupation ratio a(%), the area occupation ratio b(% of high-temperature area generation bainite), low-temperature region generates the total area occupation ratio c(% of bainite and tempered martensite) be presented in following table 12, table 13.In addition, the total area occupation ratio (a+b+c) that also shows in the lump above-mentioned area occupation ratio a, area occupation ratio b and total area occupation ratio c.
In addition, measure the diameter of equivalent circle of the polygon ferrite crystal grain confirmed in field of view, try to achieve mean value.Result is presented in following table 12, table 13.In addition, the average equivalent circular diameter D of polygon ferrite crystal grain be 10 μ m when following for estimating zero, while surpassing 10 μ m, for estimating △, evaluation result is presented in following table 12, table 13.
［ volume fraction of (2) residual γ ］
Among metal structure, the volume fraction of residual γ is measured with the saturation magnetization method.Specifically, be the saturation magnetization (I) of measuring for the examination material, and, with 400 ℃ of saturation magnetizations (Is) that carry out 15 hours heat treated standard test portions, try to achieve the volume fraction (V γ r) of residual γ according to following formula.The measurement of saturation magnetization, used reason to grind the DC magnetization B-H characteristic self-recording unit " model BHS-40 " of electronics system, makes the maximum additional 5000(Oe of being magnetized to), at room temperature measure.
Vγr＝（1－I/Is）×100
In addition, for the rolling direction with for the examination material, the surface in parallel cross section is ground, and uses opticmicroscope to be observed for 5 visuals field to observe 1000 times of multiplying powers, measures the diameter of equivalent circle d of the compound MA mixed phase of residual γ and quenched martensite.Calculating is with respect to total number of MA mixed phase, and the diameter of equivalent circle d in the observation cross section surpasses the number ratio of the MA mixed phase of 7 μ m.The number ratio is qualified (zero) lower than 15% the time, 15%, is defective (*) when above, and evaluation result is presented in following table 12, table 13.
" evaluation of the characteristic of machinery "
For the characteristic of the machinery that tries material, based on tensile strength (TS), unit elongation (EL), hole expansibility (λ), critical bends radius (R), Erichsen value evaluation.
(1) tensile strength (TS) and unit elongation (EL), carry out the tension test measurement based on JIS Z2241.Test film is used, and making with respect to the vertical direction of rolling direction for the examination material is lengthwise direction, so from cut No. 5 test films of JIS Z2201 defined for the examination material.Measuring result is presented in following table 12, table 13.
(2) stretch flangeability is by the hole expansibility evaluation.Hole expansibility (λ), carry out the hole expansibility test and measure based on the specification JFST of iron steel alliance 1001.Measuring result is presented in following table 12, table 13.
(3) critical bends radius (R), carry out the v-shaped bending test and measure based on JIS Z2248.Test film is used, and making with respect to the vertical direction of rolling direction for the examination material is lengthwise direction (crooked crestal line is consistent with rolling direction), so from cut No. 1 test film (thickness of slab: 1.4mm) of JIS Z2204 defined for the examination material.Also have, the v-shaped bending test, implement to carry out after mechanical grinding to the end face of the lengthwise direction of test film in the mode that does not make be full of cracks occur.
The angle of punch die and drift is 90 °, with the 0.5mm of unit, changes the front-end radius of drift and carries out the v-shaped bending test, tries to achieve be full of cracks and does not occur and can carry out crooked drift front-end radius as critical bends radius (R).Measuring result is presented in following table 12, table 13.Also have, use magnifying glass to observe and having or not be full of cracks to occur, take and hairline crack does not occur judged as standard.
(4) Erichsen value, carry out drawing test and measure based on JIS Z2247.The test film of 90mm * 90mm that the test film use is cut from confession examination material * thick 1.4mm.Drawing test is used the drift that diameter is 20mm to carry out.Measuring result is presented in following table 12, table 13.Also have, according to drawing test, can estimate the breaking elongation characteristic of steel plate and this two side's of local ductility composite effect.
For the characteristic of the machinery that tries material, follow the standard evaluation of the corresponding unit elongation of tensile strength (TS) (EL), hole expansibility (λ), critical bends radius (R), Erichsen value.That is, because the TS of steel plate is desired EL, λ, R, Erichsen value difference, so corresponding TS rank is followed the characteristic of following standard evaluation machinery.
Based on following judgement criteria, the situation that meets EL, λ, R, Erichsen value all characteristics is qualified (zero), and the situation that any one characteristic does not meet standard value is defective (*), and evaluation result is presented in following table 12, table 13.
(1) situation of 590MPa level
More than TS:590MPa, lower than 780MPa
More than EL:34%
λ: more than 30%
Below R:0.5mm
Erichsen value: more than 10.8mm
(2) situation of 780MPa level
More than TS:780MPa, lower than 980MPa
More than EL:25%
λ: more than 30%
Below R:1.0mm
Erichsen value: more than 10.4mm
(3) situation of 980MPa level
More than TS:980MPa, lower than 1180MPa
More than EL:19%
λ: more than 20%
Below R:3.0mm
Erichsen value: more than 10.0mm
(4) situation of 1180MPa level
More than TS:1180MPa, lower than 1270MPa
More than EL:15%
λ: more than 20%
Below R:4.5mm
Erichsen value: more than 9.6mm
Also having, in the second high tensile steel plate, more than 590MPa, is prerequisite lower than 1270MPa with TS, and TS is lower than 590MPa or the situation more than 1270MPa, even EL, λ, R, Erichsen value are good, also as processing outside object.
By following table 9～table 13, can be investigated as follows.Among No.1 shown in following table 12, table 13～43, No.1,3,4,11,14,15,20, the 28th, the cooling example by aforesaid way i, No.2, the 6th, the cooling example by aforesaid way iii, No.19 is the example cooling by aforesaid way iv, and all the other are examples cooling by aforesaid way ii.
In following table 12, table 13, be attached with zero example in comprehensive evaluation, are all the steel plates that meet important document given to this invention, meet according to each TS and the standard value of the characteristic (EL, λ, R, Erichsen value) of definite machinery.So known high tensile steel plate of the present invention, unit elongation and local deformability excellence, processibility is comprehensively good.
On the other hand, in comprehensive evaluation, be attached with * example (No.4 shown in table 12, table 13,8,9,12,15,18～20,31,34～36), be the steel plate of discontented foot a certain important document given to this invention.Details as Follows.
The No.4 of table 12, be after the heating of two-phase temperature province keeps, and the too small example of average cooling rate while being cooled to the temperature T arbitrarily that meets above formula (1), pearlitic transformation occurs, and can not get the residual γ amount of wishing.So undercapacity.The No.8 of table 12 is too short examples of the hold-time in the two-phase temperature province, can not guarantee the growing amount of residual γ, so undercapacity.The No.9 of table 12, be after equal thermal treatment, and the temperature maintenance with the temperature over T1 temperature province given to this invention, do not keep in the T1 temperature province, but the example that is cooled to the T2 temperature province and keeps in this temperature province.Polygon ferrite generates in a large number, thereby causes the growing amount of high-temperature area generation bainite few, and the growing amount of residual γ is also few, so unit elongation and Erichsen value reduction, can not improve processibility.
The No.12 of table 12, after the T1 temperature province keeps, be cooled to the temperature lower than the T2 temperature province, therefore the example do not kept in the T2 temperature province, low-temperature region generates bainite and does not almost generate, observe and confirm thick MA mixed phase and exist in a large number by SEM, quenched martensite exists in a large number.Therefore unit elongation, hole expansibility, critical bends radius, Erichsen value all are discontented with foot criterion of acceptability given to this invention, can not improve processibility.The No.15 of table 12, be that the hold-time under the T1 temperature province is long, there is no the example kept in the T2 temperature province, and the generation of low-temperature region generation bainite etc. is suppressed.In addition, thick MA mixed phase generates in a large number.Therefore hole expansibility, critical bends radius are little, and Erichsen value is also little, and the local deformaton ability reduces, and can not improve the processibility of steel plate.
The No.18 of table 12 is because Heating temperature is too high, so polygon ferrite does not almost generate, the growing amount of high-temperature area generation bainite and low-temperature region generation bainite etc. becomes superfluous in addition.Therefore unit elongation reduces, and can not improve the processibility of steel plate.The No.19 of table 12 is that the hold-time of T1 temperature province is long, and does not keep just carrying out cooling example in the T2 temperature province, and the generation of low-temperature region generation bainite etc. is suppressed.In addition, thick MA mixed phase generates in a large number.Therefore hole expansibility is little, and Erichsen value diminishes in addition, and the local deformaton ability reduces, and processibility is deteriorated.The No.20 of table 12, after being equal thermal treatment, do not keep in the T1 temperature province, is cooled to the T2 temperature quickly, the example kept with two kinds of temperature in this temperature province.Because only keep in the T2 temperature province, so generating bainite, high-temperature area almost do not generate, residual γ does not almost generate yet in addition.Therefore unit elongation and Erichsen value reduce, and processibility is deteriorated.The No.31 of table 13 is the too low examples of Heating temperature, and polygon ferritic generation quantitative change is many, and high-temperature area generates bainite, low-temperature region generation bainite etc. and residual γ does not all generate.Therefore unit elongation reduces, and can not improve processibility.
The No.34 of table 13 is that C measures very few example, because the growing amount of residual γ is very few, so unit elongation and Erichsen value diminish, processibility is deteriorated.The No.35 of table 13 is that Si measures very few example, because the growing amount of residual γ is very few, so unit elongation reduces, processibility is deteriorated.The No.36 of table 13 is that Mn measures very few example, quench and fully do not carry out, so in cooling, polygon ferritic generation is promoted, and on the other hand, the generation of low-temperature region generation bainite etc. is suppressed.Therefore unit elongation, hole expansibility, critical bends radius diminish, and processibility is deteriorated.
As can be known from the above results, according to the present invention, can provide the high tensile steel plate that has improved processibility.
Then, among the steel plate of the 780MPa level shown in above-mentioned table 12, table 13, about the example (No.3,5～7,11,14,16,17,23～26,30,32,37～43) that meets important document given to this invention, its tensile strength (TS) and the relation of unit elongation (EL) are presented in Fig. 5.In Fig. 5, ● mean the result of average equivalent circular diameter D below 10 μ m of polygon ferrite crystal grain, ■ means that the average equivalent circular diameter D of polygon ferrite crystal grain surpasses the result of 10 μ m.
As shown in Figure 5, even tensile strength (TS) is identical, by the average equivalent circular diameter D by polygon ferrite crystal grain, be suppressed at below 10 μ m, also can strengthen unit elongation (EL), further improve processibility.
[table 9]
[table 10]
[table 11]
[table 12]
[table 13]
Claims (20)
1. the high tensile steel plate of an excellent in workability, is characterized in that, in quality, % contains:
C：0.10～0.3%、
Si：1.0～3.0%、
Mn：1.5～3%、
Al：0.005～3%，
And, meet:
Below P:0.1%,
Below S:0.05%,
Surplus consists of iron and inevitable impurity,
The metal structure of this steel plate comprises bainite, polygon ferrite, retained austenite and tempered martensite,
(1) while observing metal structure with scanning electron microscope,
(1a) described bainite generates bainite by the retained austenite of adjacency and/or the high-temperature area of equispaced more than 1 μ m of carbide, generate the complex tissue formation of bainite lower than the low-temperature region of 1 μ m with the retained austenite of adjacency and/or the equispaced of carbide
The area occupation ratio a that described high-temperature area generates bainite meets 10～80% with respect to metal structure is all,
The total area occupation ratio b that described low-temperature region generates bainite and described tempered martensite meets 10～80% with respect to metal structure is all,
(1b) described polygon ferritic area occupation ratio c meets 10～50% with respect to metal structure is all, and,
(2) take the volume fraction of the described retained austenite that the saturation magnetization method measures with respect to metal structure all as more than 5%.
2. high tensile steel plate according to claim 1, wherein, during with the described metal structure of observation by light microscope, when the MA mixed phase be composited at quenched martensite and retained austenite exists, with respect to total number of MA mixed phase, the number ratio that the satisfied diameter of equivalent circle d observed in cross section surpasses the MA mixed phase of 7 μ m contains 0% lower than 15%().
3. high tensile steel plate according to claim 1 and 2, wherein, the average equivalent circular diameter D of described polygon ferrite crystal grain is (containing 0 μ m) below 10 μ m.
4. high tensile steel plate according to claim 1, wherein, described steel plate also contains Cr:1% following (containing 0%) and/or Mo:1% following (containing the 0%) element as other.
5. high tensile steel plate according to claim 1, wherein, described steel plate also contain from by (containing 0%) below Ti:0.15%, Nb:0.15% below (containing 0%) and (do not contain 0%) V:0.15% below and form group in more than one element of selection as other element.
6. high tensile steel plate according to claim 1, wherein, described steel plate also contains Cu:1% following (containing 0%) and/or Ni:1% following (containing the 0%) element as other.
7. high tensile steel plate according to claim 1, wherein, described steel plate also contains the element of B:0.005% following (containing 0%) as other.
8. high tensile steel plate according to claim 1, wherein, described steel plate also contains from (containing 0%) and rare earth element by (containing 0%) below Ca:0.01%, Mg:0.01% below: during 0.01% below, (containing 0%) forms group, more than one element of selection is as other element.
9. high tensile steel plate according to claim 1, wherein, have molten zinc plating layer or hot dip alloyed zinc coating on the surface of described steel plate.
10. the manufacture method of the high tensile steel plate of an excellent in workability, is characterized in that, is the method for manufacturing high tensile steel plate claimed in claim 1, wherein, comprises in order following operation:
Be heated to { (Ac
1point+Ac
3point)/2 }+more than 20 ℃, Ac
3the operation of the temperature province below point+20 ℃;
Keep the operation more than 50 seconds in this temperature province;
Be cooled to the operation of temperature T arbitrarily that meets following formula (1) with the average cooling rate more than 2 ℃/secs;
Keep the operation of 10～100 seconds in the temperature province that meets following formula (1);
Keep the operation more than 200 seconds in the temperature province that meets following formula (2);
400℃≤T1（℃）≤540℃…（1）
200℃≤T2（℃）＜400℃…（2）。
11. the high tensile steel plate of an excellent in workability, is characterized in that, in quality, % contains:
C：0.10～0.3%、
Si：1.0～3%、
Mn：1.0～2.5%、
Al：0.005～3%，
And, meet:
Below P:0.1%,
Below S:0.05%,
Surplus consists of iron and inevitable impurity,
The metal structure of this steel plate comprises polygon ferrite, bainite, tempered martensite and retained austenite,
(1) while observing metal structure with scanning electron microscope,
(1a) described polygon ferritic area occupation ratio a surpasses 50% with respect to metal structure is all,
(1b) described bainite generates bainite by the retained austenite of adjacency and/or the high-temperature area of equispaced more than 1 μ m of carbide, generate the complex tissue formation of bainite lower than the low-temperature region of 1 μ m with the retained austenite of adjacency and/or the equispaced of carbide
The area occupation ratio b that described high-temperature area generates bainite meets 5～40% with respect to metal structure is all,
The total area occupation ratio c that described low-temperature region generates bainite and described tempered martensite meets 5～40% with respect to metal structure is all,
(2) volume fraction of the described retained austenite of measuring with the saturation magnetization method with respect to metal structure all more than 5%.
12. high tensile steel plate according to claim 11, wherein, during with the described metal structure of observation by light microscope, when the MA mixed phase that quenched martensite and retained austenite are composited exists, with respect to total number of MA mixed phase, the number ratio that the satisfied diameter of equivalent circle d observed in cross section surpasses the MA mixed phase of 7 μ m contains 0% lower than 15%().
13., according to the described high tensile steel plate of claim 11 or 12, wherein, the average equivalent circular diameter D of described polygon ferrite crystal grain is (containing 0 μ m) below 10 μ m.
14. high tensile steel plate according to claim 11, wherein, described steel plate also contains Cr:1% following (not containing 0%) and/or (does not contain 0%) below Mo:1% the element as other.
15. high tensile steel plate according to claim 11, wherein, described steel plate also contain from by (containing 0%) below Ti:0.15%, Nb:0.15% below (containing 0%) and (do not contain 0%) V:0.15% below and form group in more than one element of selection as other element.
16. high tensile steel plate according to claim 11, wherein, described steel plate also contains Cu:1% following (not containing 0%) and/or (does not contain 0%) below Ni:1% the element as other.
17. high tensile steel plate according to claim 11, wherein, described steel plate also contains the element of B:0.005% following (not containing 0%) as other.
18. high tensile steel plate according to claim 11, wherein, described steel plate also contains from (containing 0%) and rare earth element by (containing 0%) below Ca:0.01%, Mg:0.01% below: during 0.01% below, (containing 0%) forms group, more than one element of selection is as other element.
19. high tensile steel plate according to claim 11, wherein, have molten zinc plating layer or hot dip alloyed zinc coating on the surface of described steel plate.
20. the manufacture method of the high tensile steel plate of an excellent in workability, is characterized in that, is the method for manufacturing the described high tensile steel plate of claim 11, wherein, comprises in order following operation:
Be heated to Ac
1more than point+20 ℃, Ac
3the operation of the temperature province below point+20 ℃;
Keep the operation more than 50 seconds in this temperature province;
Be cooled to the operation of the temperature T arbitrarily that meets following formula (1) with the average cooling rates of 2～50 ℃/secs;
Keep the operation of 10～100 seconds in the temperature province that meets following formula (1);
Keep the operation more than 200 seconds in the temperature province that meets following formula (2);
400℃≤T1（℃）≤540℃…（1）
200℃≤T2（℃）＜400℃…（2）。
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| JP2011-080954 | 2011-03-31 | ||
| JP2011080953 | 2011-03-31 | ||
| JP2011-080953 | 2011-03-31 | ||
| JP2011080954 | 2011-03-31 | ||
| JP2011197670A JP5685166B2 (en) | 2011-03-31 | 2011-09-09 | High-strength steel sheet with excellent workability and method for producing the same |
| JP2011-197671 | 2011-09-09 | ||
| JP2011197671A JP5685167B2 (en) | 2011-03-31 | 2011-09-09 | High-strength steel sheet with excellent workability and method for producing the same |
| JP2011-197670 | 2011-09-09 | ||
| PCT/JP2012/057210 WO2012133057A1 (en) | 2011-03-31 | 2012-03-21 | High-strength steel sheet with excellent workability and manufacturing process therefor |
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| EP (2) | EP2695961B1 (en) |
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| CN107109557A (en) * | 2014-11-21 | 2017-08-29 | 株式会社神户制钢所 | High-strength high-tractility steel plate |
| CN107923007A (en) * | 2015-08-21 | 2018-04-17 | 新日铁住金株式会社 | Steel plate |
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Also Published As
| Publication number | Publication date |
|---|---|
| EP2942416A1 (en) | 2015-11-11 |
| EP2695961A4 (en) | 2014-12-17 |
| KR101574400B1 (en) | 2015-12-03 |
| CN104762565A (en) | 2015-07-08 |
| EP2942416B1 (en) | 2017-06-07 |
| EP2695961A1 (en) | 2014-02-12 |
| KR101604963B1 (en) | 2016-03-18 |
| CN103459638B (en) | 2015-07-15 |
| US20140044988A1 (en) | 2014-02-13 |
| EP2695961B1 (en) | 2019-06-19 |
| KR20130125829A (en) | 2013-11-19 |
| CN104762565B (en) | 2017-04-12 |
| US20160355920A1 (en) | 2016-12-08 |
| KR20150050592A (en) | 2015-05-08 |
| WO2012133057A1 (en) | 2012-10-04 |
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