GB2445749A - Dual-phase steel sheet - Google Patents
Dual-phase steel sheet Download PDFInfo
- Publication number
- GB2445749A GB2445749A GB0720437A GB0720437A GB2445749A GB 2445749 A GB2445749 A GB 2445749A GB 0720437 A GB0720437 A GB 0720437A GB 0720437 A GB0720437 A GB 0720437A GB 2445749 A GB2445749 A GB 2445749A
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- GB
- United Kingdom
- Prior art keywords
- steel sheet
- ferrite
- martensite
- high strength
- less
- Prior art date
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- 229910000885 Dual-phase steel Inorganic materials 0.000 title description 9
- 229910000831 Steel Inorganic materials 0.000 claims abstract description 189
- 239000010959 steel Substances 0.000 claims abstract description 189
- 229910000859 α-Fe Inorganic materials 0.000 claims abstract description 166
- 229910000734 martensite Inorganic materials 0.000 claims abstract description 140
- 229910001566 austenite Inorganic materials 0.000 claims abstract description 94
- 230000000717 retained effect Effects 0.000 claims abstract description 73
- 238000001816 cooling Methods 0.000 claims abstract description 66
- 239000011159 matrix material Substances 0.000 claims abstract description 28
- 239000012535 impurity Substances 0.000 claims abstract description 10
- 229910052758 niobium Inorganic materials 0.000 claims abstract description 9
- 229910052719 titanium Inorganic materials 0.000 claims abstract description 9
- 229910052720 vanadium Inorganic materials 0.000 claims abstract description 9
- 229910052748 manganese Inorganic materials 0.000 claims abstract description 7
- 229910052750 molybdenum Inorganic materials 0.000 claims abstract description 4
- 229910052804 chromium Inorganic materials 0.000 claims abstract 3
- 238000010438 heat treatment Methods 0.000 claims description 45
- 239000000463 material Substances 0.000 claims description 38
- 238000012545 processing Methods 0.000 claims description 17
- 238000004519 manufacturing process Methods 0.000 claims description 12
- 230000009467 reduction Effects 0.000 claims description 11
- 238000005096 rolling process Methods 0.000 claims description 8
- 101100366936 Caenorhabditis elegans sto-3 gene Proteins 0.000 claims 1
- XEEYBQQBJWHFJM-UHFFFAOYSA-N Iron Chemical compound [Fe] XEEYBQQBJWHFJM-UHFFFAOYSA-N 0.000 abstract description 9
- 229910052759 nickel Inorganic materials 0.000 abstract description 4
- 229910052802 copper Inorganic materials 0.000 abstract description 3
- 229910052742 iron Inorganic materials 0.000 abstract description 3
- 229910001563 bainite Inorganic materials 0.000 description 40
- 238000000034 method Methods 0.000 description 31
- 230000003247 decreasing effect Effects 0.000 description 28
- 230000036961 partial effect Effects 0.000 description 20
- 230000009466 transformation Effects 0.000 description 19
- 229910001567 cementite Inorganic materials 0.000 description 17
- KSOKAHYVTMZFBJ-UHFFFAOYSA-N iron;methane Chemical compound C.[Fe].[Fe].[Fe] KSOKAHYVTMZFBJ-UHFFFAOYSA-N 0.000 description 17
- 239000000203 mixture Substances 0.000 description 17
- 230000000694 effects Effects 0.000 description 15
- 229910001562 pearlite Inorganic materials 0.000 description 15
- 229910052761 rare earth metal Inorganic materials 0.000 description 12
- 238000012360 testing method Methods 0.000 description 12
- 239000000126 substance Substances 0.000 description 11
- 238000005259 measurement Methods 0.000 description 10
- 230000002950 deficient Effects 0.000 description 9
- 230000006866 deterioration Effects 0.000 description 9
- 230000008569 process Effects 0.000 description 9
- 238000005097 cold rolling Methods 0.000 description 8
- 230000009977 dual effect Effects 0.000 description 8
- 238000005098 hot rolling Methods 0.000 description 8
- 238000001556 precipitation Methods 0.000 description 8
- 230000015572 biosynthetic process Effects 0.000 description 7
- 239000002244 precipitate Substances 0.000 description 7
- 230000003287 optical effect Effects 0.000 description 6
- 238000000879 optical micrograph Methods 0.000 description 6
- 229920006395 saturated elastomer Polymers 0.000 description 5
- 230000000930 thermomechanical effect Effects 0.000 description 5
- 230000007797 corrosion Effects 0.000 description 4
- 238000005260 corrosion Methods 0.000 description 4
- 230000007423 decrease Effects 0.000 description 4
- 230000006872 improvement Effects 0.000 description 4
- 230000007246 mechanism Effects 0.000 description 4
- 239000002245 particle Substances 0.000 description 4
- 238000005498 polishing Methods 0.000 description 4
- 239000000047 product Substances 0.000 description 4
- 229910001035 Soft ferrite Inorganic materials 0.000 description 3
- 229910000794 TRIP steel Inorganic materials 0.000 description 3
- 239000012141 concentrate Substances 0.000 description 3
- 238000010191 image analysis Methods 0.000 description 3
- 238000011835 investigation Methods 0.000 description 3
- 238000005204 segregation Methods 0.000 description 3
- 230000002411 adverse Effects 0.000 description 2
- 150000001875 compounds Chemical class 0.000 description 2
- 230000008034 disappearance Effects 0.000 description 2
- 238000005868 electrolysis reaction Methods 0.000 description 2
- 238000001887 electron backscatter diffraction Methods 0.000 description 2
- 230000001771 impaired effect Effects 0.000 description 2
- 230000005415 magnetization Effects 0.000 description 2
- 239000011347 resin Substances 0.000 description 2
- 229920005989 resin Polymers 0.000 description 2
- 239000006104 solid solution Substances 0.000 description 2
- 230000000087 stabilizing effect Effects 0.000 description 2
- HXKKHQJGJAFBHI-UHFFFAOYSA-N 1-aminopropan-2-ol Chemical compound CC(O)CN HXKKHQJGJAFBHI-UHFFFAOYSA-N 0.000 description 1
- 241000446313 Lamella Species 0.000 description 1
- BPQQTUXANYXVAA-UHFFFAOYSA-N Orthosilicate Chemical compound [O-][Si]([O-])([O-])[O-] BPQQTUXANYXVAA-UHFFFAOYSA-N 0.000 description 1
- UCKMPCXJQFINFW-UHFFFAOYSA-N Sulphide Chemical compound [S-2] UCKMPCXJQFINFW-UHFFFAOYSA-N 0.000 description 1
- 240000008042 Zea mays Species 0.000 description 1
- 235000005824 Zea mays ssp. parviglumis Nutrition 0.000 description 1
- 235000002017 Zea mays subsp mays Nutrition 0.000 description 1
- 239000002253 acid Substances 0.000 description 1
- 235000005822 corn Nutrition 0.000 description 1
- 238000013461 design Methods 0.000 description 1
- 238000011161 development Methods 0.000 description 1
- 238000005516 engineering process Methods 0.000 description 1
- 238000011156 evaluation Methods 0.000 description 1
- 230000001747 exhibiting effect Effects 0.000 description 1
- 229910052737 gold Inorganic materials 0.000 description 1
- 229910052747 lanthanoid Inorganic materials 0.000 description 1
- 150000002602 lanthanoids Chemical class 0.000 description 1
- 230000004807 localization Effects 0.000 description 1
- 238000012986 modification Methods 0.000 description 1
- 230000004048 modification Effects 0.000 description 1
- 238000005554 pickling Methods 0.000 description 1
- 230000002829 reductive effect Effects 0.000 description 1
- 238000003303 reheating Methods 0.000 description 1
- 238000011160 research Methods 0.000 description 1
- 229910052706 scandium Inorganic materials 0.000 description 1
- 229910052709 silver Inorganic materials 0.000 description 1
- 238000002791 soaking Methods 0.000 description 1
- 239000000243 solution Substances 0.000 description 1
- 241000894007 species Species 0.000 description 1
- 238000005728 strengthening Methods 0.000 description 1
- 238000009864 tensile test Methods 0.000 description 1
- 238000012795 verification Methods 0.000 description 1
- 238000004804 winding Methods 0.000 description 1
- 229910052727 yttrium Inorganic materials 0.000 description 1
Classifications
-
- C—CHEMISTRY; METALLURGY
- C21—METALLURGY OF IRON
- C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
- C21D9/00—Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor
- C21D9/46—Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor for sheet metals
-
- C—CHEMISTRY; METALLURGY
- C21—METALLURGY OF IRON
- C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
- C21D8/00—Modifying the physical properties by deformation combined with, or followed by, heat treatment
- C21D8/02—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips
- C21D8/0221—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips characterised by the working steps
- C21D8/0226—Hot rolling
-
- C—CHEMISTRY; METALLURGY
- C21—METALLURGY OF IRON
- C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
- C21D8/00—Modifying the physical properties by deformation combined with, or followed by, heat treatment
- C21D8/02—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips
- C21D8/0247—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips characterised by the heat treatment
- C21D8/0263—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips characterised by the heat treatment following hot rolling
-
- C—CHEMISTRY; METALLURGY
- C21—METALLURGY OF IRON
- C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
- C21D9/00—Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor
- C21D9/46—Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor for sheet metals
- C21D9/48—Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor for sheet metals deep-drawing sheets
-
- C—CHEMISTRY; METALLURGY
- C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
- C22C—ALLOYS
- C22C38/00—Ferrous alloys, e.g. steel alloys
- C22C38/02—Ferrous alloys, e.g. steel alloys containing silicon
-
- C—CHEMISTRY; METALLURGY
- C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
- C22C—ALLOYS
- C22C38/00—Ferrous alloys, e.g. steel alloys
- C22C38/04—Ferrous alloys, e.g. steel alloys containing manganese
-
- C—CHEMISTRY; METALLURGY
- C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
- C22C—ALLOYS
- C22C38/00—Ferrous alloys, e.g. steel alloys
- C22C38/06—Ferrous alloys, e.g. steel alloys containing aluminium
-
- C—CHEMISTRY; METALLURGY
- C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
- C22C—ALLOYS
- C22C38/00—Ferrous alloys, e.g. steel alloys
- C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
- C22C38/32—Ferrous alloys, e.g. steel alloys containing chromium with boron
-
- C—CHEMISTRY; METALLURGY
- C21—METALLURGY OF IRON
- C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
- C21D2211/00—Microstructure comprising significant phases
- C21D2211/001—Austenite
-
- C—CHEMISTRY; METALLURGY
- C21—METALLURGY OF IRON
- C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
- C21D2211/00—Microstructure comprising significant phases
- C21D2211/004—Dispersions; Precipitations
-
- C—CHEMISTRY; METALLURGY
- C21—METALLURGY OF IRON
- C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
- C21D2211/00—Microstructure comprising significant phases
- C21D2211/005—Ferrite
-
- C—CHEMISTRY; METALLURGY
- C21—METALLURGY OF IRON
- C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
- C21D2211/00—Microstructure comprising significant phases
- C21D2211/008—Martensite
Landscapes
- Chemical & Material Sciences (AREA)
- Engineering & Computer Science (AREA)
- Mechanical Engineering (AREA)
- Materials Engineering (AREA)
- Metallurgy (AREA)
- Organic Chemistry (AREA)
- Physics & Mathematics (AREA)
- Thermal Sciences (AREA)
- Crystallography & Structural Chemistry (AREA)
- Heat Treatment Of Sheet Steel (AREA)
Abstract
A micro-duplex steel sheet comprises a ferrite matrix having a secondary phase of martensite or martensite and retained austenite dispersed therein. The above microstructure occupies at least 90% of the steel. The secondary phase has an average grain size of 3 microns or less, occupies 10-60% of the steel and is present in ferrite grains and at grain boundaries with the proportion of the secondary phase present in the ferrite grains is 50% or more. The steel comprises (by weight): 0.02-0.3 % C, 0.01-3 % Si, 0.5-3% Mn, 0.0001-0.005% B, 0.01-1.5 % Al, with the balance being iron and impurities and is processed by cooling from at least the A3 temperature or 600-1000{C to a temperature of Ms or lower at a rate of 0.2-20 {Cs<-1>. 0.03-1 % Mo, 0.01-0.1 % total Nb, Ti and V, 0-0.5 % Ni, 0-0.5 % Cu, 0-1.5 % Cr, 0-0.003 % Ca and/or 0-0.003 % REM may also be present.
Description
ifiGH STRENGTH STEEL SHEET AND
METHOD FOR PRODUCING TRE SAME
TECHNICAL FIELD
The present invention relates to a high strength dual phase steel sheet having excellent elongation and stretch flangeability. In particular, it relates to a high strength dual phase steel sheet having excellent strength-elongation and stretch flangeability in high strength region of 590 MPa or more.
BACKGROUND OF TJi iNVENTION
In industrial fields of automobiles, electrical machinery and appliances, machinery and the like, steel sheets used by press forming are required to combine excellent strength and formability. In recent years, such demand characteristics are increasing more and more. Dual phase steel sheets (DP steel) having dual phase of ferrite and martensite, and TRIP steels comprising a structure containing ferrite, bainite and retained austenite are known as high strength steel sheets having excellent ductility.
However, dual phase steel sheets such as DP steel and TRIP steel have the problem that stretch flangeability is poor.
To improve stretch flangeability in high strength steel sheets, a method of suppressing localization of strain by making the structure a single phase structure to homogenize processability in the structure, and a method of reducing strength difference between a soft phase and a hard phase in a dual phase structure are known. Regarding those two methods, for example, the following technologies are disclosed.
I. Single phase structure (see Patent Document 1) A method of producing a martensite single phase steel sheet having tensile strength of from 880 to 1,170 MPa by bringing more balance to components and heat treatment conditions has been disclosed. For the production of a martensite single
I-
phase structure, soaking temperature necessary for austenitizing is set to 850 C which is generally the industrially attainable temperature condition, and this made it possible to industrially achieve a martensite single phase structure. However, a steel sheet of a martensite single phase structure is that the structure is macroscopically uniform, so that stretch flangeability is excellent, but ductility (elongation) greatly deteriorates (EL<8%).
2. Reduction in strength difference of dual phase structure (see Patent Document 2) A steel sheet having a space factor of a low temperature transformation phase of 90% or more is heated and held in two phase regions of femte and austenite, thereby fine ferrite and austenite succeeding lath of a low temperature transformation phase can be formed. Afler the subsequent cooling, a structure in which ferrite and the low temperature transformation phase are finely dispersed in a lath state is finally formed.
Such a finely dispersed low temperature transformation phase suppresses formation and growth of voids at the time of flange deformation, thereby improving stretch flangeability.
However, in this method, the structure become fine, but martensite which is a hard phase cannot be precipitated in grains. Furthermore, a grain size of martensite is 5.un at the minimum, and the structure cannot sufficiently be homogenized. For this reason, improvement effect of stretch flangeability is not sufficient.
3. Fine retained austenite steel sheet (see Patent Document 3) A steel sheet in which a secondary phase having an average grain size of 500 nm or less is finely dispersed in grains has been disclosed. High elongation and stretch flangeability are achieved by forming a starting point of fracture in the grains.
In this method, very expensive austenite stabilizing element such as Au, Ag or Ni must be added in order to form the secondary phase in the grains. Additionally, a concentrated region of the austenite stabilizing element must be formed in the grains in order to form the secondary phase in the grains, but to achieve this, a solution heat treatment at 1,270 C or higher for 5 hours or more becomes necessary. Consequently, industrial problem in increase of time and cost remains in this method.
Patent Document 1: Japanese Patent No. 3729108 Patent Document 2: JP-A-2005-272954 Patent Document 3: JP-A-2005-179703
SUMMARY OF TIlE INVENTION
Accordingly, the present invention has an object to exhibit both elongation and stretch flangeability particularly in a high strength steel sheet of 590 MPa or more, and further to industrially realize such a high strength steel sheet.
To obtain a high strength steel sheet, it is necessary that a hard phase is a secondary phase, but when the hard phase is present at the grain boundary of a matrix, its interface becomes a starting point of fracture, and local elongation lowers.
However, when a hard phase is not present, high strength cannot be obtained. When a hard phase can finely be present in the grains of a matrix, a starting point of fracture can be minimized, and as a result, local elongation of steel can be improved. It is known that local elongation of steel has a correlation with stretch flangeability thereof, and high stretch flangeability can be obtained by improving local elongation.
Thus, to improve stretch flangeability while maintaining high ductility in a high strength steel sheet of 590 MPa or more, it is important to be a dual phase steel sheet and to uniformly and finely precipitate a hard secondary phase in the grains of a matrix. Furthermore, it is important that the production of steel can industrially be realized.
In view of the above, the present inventors thought that the entire surface of a steel sheet is constituted of bainite in order to provide a steel sheet having improved stretch flangeability while maintaining high ductility even in a high strength region of 590 MPa or more, and thought to control the existence form of cementite in bainite.
The reason that the matrix is bainite is that lath of bainitic ferrite constituting bainite is not grain boundary, and is therefore difficult to become a starting point of fracture. As a result of verification, cementite which is a hard phase could finely be dispersed among laths, but bainite is slightly poor in ductility, and the desired elongation was not obtained.
Subsequently, Si was added to improve elongation. By adding Si, precipitation of cementite is suppressed in the course of cooling, and austenite in which C is concentrated is formed among laths of bainitic ferrite. Furthermore, bainite transformation is completed, and cooling rate was controlled such that lath of bainitic ferrite disappears in the course of the subsequent cooling.
As a result, a structure comprising a ferrite phase and austenite finely dispersed therein was obtained in the cooling process. When austenite reached Ms point or lower, MD structure comprising a ferrite phase and martensite as a hard phase finely dispersed therein was obtained. However, in this method, a ferrite phase in which martensite is not dispersed in the ferrite phase was simultaneously formed. When such ferrite is present in the structure, strength deteriorates, and in addition, stretch flangeability deteriorats due to strength difference at the interface between the ferrite and MD.
In view of the above, the present inventors have further made researches and found that formation of ferrite which does not contain martensite can be suppressed by adding B. As a result, nearly the entire surface can be constituted of MD structure, and the present invention could be completed based on this finding.
When such unique components and heat treatment are employed, martensite having strength difference to a matrix smaller than a hard cementite can be uniformly and finely dispersed in ferrite grains. As a result, it was clarified that a dual phase steel sheet having excellent strength-stretch flangeability while maintaining high ductility can be provided even in a high strength region of 590 MPa or mOre.
Furthermore, DP steel, TRIP steel and the like described above are conventionally developed as the means to combine strength and ductility (elongation).
DP steel comprises hard martensite and soft ferrite. The hard martensite ensures strength, and the soft ferrite ensures ductility (elongation). On the other hand, TRIP steel is that retained austenite generates deformation-induced transformation during deformation, thereby improving ductility (elongation). However, the ductility (elongation) used herein means uniform elongation, and those steel sheets are excellent to combine strength and ductility (uniform elongation), but had the disadvantage that local elongation is poor. It is known that local elongation has a high correlation with stretch flangeability, and improvement in local elongation is indispensable to obtain high stretch flangeability.
Accordingly, as a result of intensive investigations to provide a steel sheet which combines elongation and stretch flangeability even in a high strength region of 590 MPa or more, namely a steel sheet combining uniform elongation and local elongation in addition to strength, the present inventors have succeeded to develop a high strength steel sheet having excellent elongation and stretch flangeability, leading to the present invention.
Development process of the steel of the present invention and mechanism of exhibiting high properties are described below.
To combine strength and ductility, it is advantageous to be a dual phase structure in which a soft phase and a hard phase are combined, and the steel of the present invention has a dual phase structure comprising ferrite as a soft phase and martensite as a hard phase. However, even though a dual phase structure merely comprising soft ferrite and hard martensite is formed as in DP steel as described before, it is effective to combine strength and uniform elongation, but it is not sufficient to ensure excellent stretch flangeability. The reason that DP steel is poor in stretch flangeability includes size and existence position of martensite as a hard phase.
Specifically, martensite in DP steel is generally large as about several ten p.m, and grain boundary between marten site and ferrite is large tilt grain boundary. When deformation is added to a steel sheet, stress concentrates at grain boundary (large tilt 1.0 grain boundary) as compared with the inside of grains. When interface having strength difference is present here, voids are generated in a moment, which is liable to lead to fracture. Such generation of voids and fracture have a correlation with local elongation, and a steel sheet that is liable to cause generation of voids and propagation of cracks is poor in local elongation. Grain boundary between ferrite and martensite in DP steel is just an interface having strength difference, and excellent stretch flangeability cannot be possessed for this reason. Furthermore, although it is considered that this tendency is decreased with decreasing a size of a hard phase, this effect cannot be obtained in a size of martensite in general DP steel.
In view of the above, the present inventors thought that as the means for improving stretch flangeability while maintaining excellent properties of DP steel (combining strength and uniform elongation), a dual phase structure having a soft phase (ferrite) and a hard phase (martensite) is basically formed, but martensite is finely dispersed in ferrite grains (or small tilt grain boundary). They further thought that even in the case of ensuring the same degree of strength, grain size of the individual martensite can be decreased as compared with the general DP steel, and by further making martensite present in grains (or small tilt grain boundary), it becomes possible to suppress generation of voids in the case that deformation has been generated. By those, it is possible to combine elongation and stretch flangeability, and in addition to this, it is expected that ductility (particularly, uniform elongation) is further improved by utilizing TRIP effect as a structure containing retained austenite. The steel of the present invention developed based on the above-described thoughts has a structure comprising ferrite as a matrix, and martensite and retained austenite, finely dispersed mainly in ferrite grains. Due to this structure, the steel combines elongation and stretch flangeability in addition to strength.
BRIEF DESCRIPTION OF THE DRAWING
Fig. I is a schematic view showing thermo-mechanical treatment in the
Examples.
DETAILED DESCRIPTION OF TIlE INVENTION
The present invention relates to the following (1).
(I) A high strength steel sheet having a structure which is mainly composed of MD structure (Micro Duplex structure) comprising a ferrite matrix, and as a secondary phase, martensite or martensite and retained austenite, finely dispersed in said matrix, wherein the proportion that the MD structure occupies in the whole structure is 90% or more, wherein the proportion that the secondary phase present in the whole structure occupies in the whole structure is from 10 to 60%, wherein the secondary phase in the MD structure is present in ferrite grains and at grain boundary, in which the proportion of the secondary phase present in the ferrite grains is 50% or more, and I,-wherein the average grain size of the secondary phase in the whole structure is 3 p.m or less.
In the present invention, the secondary phase is constituted of martensite, or martensite and retained austenite.
The term "proportion" used herein means a proportion of area (area fraction).
Furthermore, the term "whole structure" used herein means the entire of Ml) structure and other structures.
It is desirable that an average grain size of ferrite in the whole structure is 20 p.m or less, and 20 or more secondary phases on the average are present in an observation view of 50 p.m x 50 p.m in the whole structure.
Furthermore, it is desirable in the high strength steel sheet that the proportion that the retained austenite present in the whole structure occupies in the whole structure is 2% or more.
The high strength steel sheet has a composition comprising, in terms of% by mass, C: 0.02 to 0.3%; Si: 0.01 to 3%; Mn: 0.5 to 3%; B: 0.0001 to 0.005%; and Al: 0.01 to 1.5%, with the remainder being Fe and inevitable impurities.
The high strength steel sheet may further contain one or two or more of: (1) Mo: 0.03 to 1%; (2) at least one element selected from the group consisting of Nb, Ti and V in thetotal amount of from 0.01 to 0.1%; (3) at least one of Ni: 0,5% or less (not including 0%) and Cu: 0.5% or less (not including 0%); (4) Cr: 1.5% or less (not including 0%); and (5) at least one of Ca: 0.003% or less (not including 0%) and REM (rare earth element): 0.003% or less (not including 0%). C.
In addition, a method for producing a high strength steel sheet according to the present invention comprises heating a steel sheet material comprising the above-described component compositions, and cooling the steel sheet material from a temperature of A3 point or higher to a temperature of Ms point or lower in a cooling rate of from 0.2 to 20 C/sec.
The steel sheet material is produced by appropriately conducting a hot rolling step and a cold rolling step. Those steps are not particularly limited, and the conditions generally practiced can appropriately be selected and employed. For example, as the hot rolling step, conditions of holding at about 1,200 C for 30 minutes, conducting hot rolling at A3 point or higher, cooling in an average cooling rate of about 30 C/sec, and winding up at about 500 to 600 C can be employed. In the cold rolling step, it is recommended to conduct cold rolling in a cold rolling ratio of about 30 to 70%, but it is not particularly limited to this.
After heating the steel sheet material at a temperature of A3 point or higher and holding the same at that temperature, the steel sheet material is cooled to a temperature of Ms point or lower, generally room temperature. As a result, the structure of the present invention comprising MD structure in major portion can be obtained, and stretch flangeability can be improved while maintaining high ductility, even in a high strength region of 590 MPa or more.
When the steel sheet material is heated to a temperature of A3 point or higher and held at that temperature, the entire surface of the structure converts into austenite.
Then, when the steel sheet material is cooled in a cooling rate of 0.2 to 20 C/sec, a dual phase structure of ferrite-pearlite is generally obtained. However, because Si is added in the present invention, precipitation of cementite is suppressed, and as a result, bainitic ferrite begins to precipitate from the austenite interface. With the growth of lath of bainitic ferrite, austenite decreases its space factor, and austenite is finely dispersed among laths of bainitic ferrite.
The lath of bainitic ferrite is thermally unstable, and therefore disappears in the cooling process after completion of the bainite transformation. As a result, the state that austenite is finely dispersed in the grains of ferrite having bainite block size is obtained, and when the temperature becomes Ms point or lower by further cooling, austenite transforms into martensite. As a result, MD structure comprising ferrite matrix formed by disappearance of the lath of bainitic ferrite, and a secondary phase comprising martensite finely dispersed in the matrix can be obtained. When B is further added, formation of ferrite which does not contain martensite in the grains can be suppressed. Furthermore, when Mo is added, bainite transformation can be completed in a short period of time. Mo is effective to allow the lath of bainitic ferrite to disappear.
Furthermore, the steel sheet having the above-described structure morphology can be obtained by optimizing chemical compositions and production steps (thermo-mechanical treatment conditions) in an industrially possible range. Unclear points still remain regarding appearance mechanism of the above-described structure, and all is not always clarified to the detail. However, at the present, it is considered that the following explanation is possible.
The steel of the present invention can be produced by heating a steel sheet material comprising the above-described component composition controlled in an appropriate range, cooling the material from a temperature of A3 point or higher, subjecting the material to a processing of rolling reduction of 5% or more in a temperature range of 600 to 1,000 C, and then cooling the material to a temperature of Ms point or lower in a cooling rate of 0.2 to 20 C/sec. This production step (thermo-mechanical treatment) may be conducted in a hot rolling step or may be conducted after a hot rolling step and a cold rolling step, generally conducted (in this case, this thermo-mechanical treatment is conducted by reheating).
Accordingly, MD structure comprising ferrite as a matrix, and martensite and retained austenite, which are finely dispersed mainly in ferrites grains is formed.
Thus, the structure of the present invention can be produced by forming an austenite single phase structure at a temperature of A3 point or higher, cooling, adding processing at a predetermined temperature and then cooling in an appropriate range.
As described above, the structure formation mechanism of the steel of the present invention is not clarified to the detail, but one explanation that is considered at the present is described below. Therefore, the following description is not construed to limit the technical scope of the present invention.
One of various structure design guidelines devised to obtain the structure of the present invention is described below.
The present inventors thought to obtain a structure comprising ferrite as a matrix, and mnartensite and retained austenite finely dispersed therein, using bainite transformation generated during the cooling process. For this, bainite is formed during the cooling step after processing, but in the steel of the present invention, a mixed structure of bainitic ferrite and cementite as in general bainite is not formed. Since cementite is harder than martensite, such is harmful to stretch flangeability.
Furthermore, to ensure retained austenite which is essential in the steel of the present invention, it is necessary to concentrate C in austenite, and it is therefore necessary to prevent precipitation of cementite which disturbs to concentrate C. For this reason, in the steel of the present invention, Si and Al are added to suppress precipitation of cementite in the cooling process. Accordingly, austenite in which C is concentrated among laths of bainitic ferrite is formed. This austenite is transformed into martensite in the subsequent cooling step, or remains as retained austenite without transformation.
Lath of bainitic ferrite is small tilt grain boundary, and concentration of strain during deformation as in large tilt grain boundary described before is difficult to be generated. As a result, martensite and retained austemte present among laths of bainitic ferrite are difficult to become a starting point of fracture. The term ferrite grain boundary" used herein means large tilt grain boundary of ferrite, and lath boundary which is difficult to become a starting point of fracture is not grain boundary.
However, it is difficult to observe small tilt grain boundary at the position that martensite and retained austenite are present, in the structure obtained by the present inventors. It suggests that the steel of the present invention has further excellent structure morphology based on the following reasons.
In the steel of the present invention, processing is added at a predetermined temperature region, and as a result, bainite transformation is accelerated and bainite transfonnation is completed in a short period of time. Additionally, since the subsequent cooling rate is set to relatively low condition, it is still present at relatively high temperature for a certain period of time even after bainite transformation. For this reason, it is considered that even lath (small tilt grain boundary) of bainitic ferrite disappears, and as a result, it is considered that a structure (MD structure) comprising ferrite grains and martensite and retained austenite mainly dispersed therein was obtained.
In the above description, one of thoughts considered at present is shown as appearance mechanism of the structure of the present invention, but only the steel comprising the bainitic ferrite as a matrix is not the object of the present invention. In other words, ferrite which is a matrix of MD structure of the present invention is not limited to only ferrite wherein lath of bainitic ferrite becomes unclear, but includes ferrite which is precipitated at relatively high temperature, like pro-eutectoid ferrite.
Thus, the present invention includes all of MD structure steel sheets having structure morphology defined in the present application.
Furthermore, the present inventors have found in the course of the above investigations that when ferrite in which martensite and retained austenite are not present is present, not only strength deteriorates, but stretch flangeability deteriorates by strength difference at the interface between ferrite and MI) structure. Based on this finding, formation of such martensite-free ferrite is suppressed by adding B, thereby nearly the entire surface is constituted of MD structure.
The present invention can provide a dual phase steel sheet having excellent strength-elongation and stretch flangeability while maintaining high ductility even in high strength region of 590 MPa or more by forming a structure comprising martensite, or martensite and retained austenite which have small strength difference to a matrix as compared with hard cementite, uniformly and finely dispersed in ferrite grains.
Furthermore, according to the present invention, such a dual phase steel sheet can be produced by industrially possible means.
The structure of the present invention in the case that the secondary phase is martensite is described below.
Ferrite which is a matrix means ferrite having martensite contained in grains, formed in the cooling process from A3 point, and this ferrite includes pro-eutectoid ferrite and ferrite from which lath of bainitic ferrite has disappeared. On the other hand, this ferrite does not include ferrite which does not contain fine martensite in grains even though it is pro-eutectoid ferrite and ferrite from which lath of bainitic ferrite has disappeared. In the present invention, ferrite containing martensite in grains and ferrite which does not contain fine martensite in grains can be identified by, for example, applying the structure after completion of cooling to repeller corrosion, and showing ferrite in gray and martensite in white by image analysis. The entire surface of ferrite which does not contain fine martensite in grains is gray. On the other hand, the MD structure of the present invention is that fine martensite is contained in a ferrite phase. Therefore, fine white spot can be confirmed in ferrite grains.
The structure of the present invention is mainly composed of MD structure, and the MD structure comprises ferrite as a matrix and martensite as a secondary phase.
The martensite as a secondary phase is formed such that austenite present in the ferrite matrix transforms into martensite at Ms point or lower in the cooling process. When the area fraction of the whole structure is defined as 100%, the proportion of the MD structure is required to be 90% or more. Where the proportion of the MD structure is less than 90%, influence of the remainder increases, uniformity of the structure is impaired, and stretch flangeability deteriorates. The remainder includes retained austenite, pearlite, bainite, and ferrite which is not accompanied with martensite in grains. On the other hand, the proportion that the MD structure occupies in the whole structure is better as large, and the upper limit is not particularly defined. Furthermore, when the area fraction of the whole structure is defined as 100%, the proportion of martensite as a secondary phase present in the whole structure (martensite present in MD structure and other structures) is required to be from 10 to 60%, and the desired strength can be obtained by this proportion. When the proportion is less than 10%, sufficient strength is not obtained, and when it exceeds 60%, elongation deteriorates.
The proportion is preferably in a range of from 20 to 60%, more preferably in a range of from 30 to 55%, and most preferably in a range of from 40 to 50%.
Martensite in the MD structure is nearly uniformly dispersed in ferrite grains and at ferrite grain boundary. When the whole of the martensite is defined as 100%, the proportion of martensite in ferrite grains is required to be 50% or more (namely, the proportion of martensite present at ferrite grain boundary is less than 50%), and the desired stretch flangeability can be obtained by this proportion. When the proportion is less than 50%, martensite present at ferrite grain boundary acts as a starting point of fracture, and stretch flangeability deteriorates. The proportion of martensite in ferrite grains is preferably 60% or more, and more preferably 70% or more.
In the present invention, the term "grain boundary of ferrite matrix" means block boundary of bainitic ferrite formed by bainite transformation during cooling, and, for example, when orientation difference of adjacent ferrites is measured by FE/SEM-EBSP (Electron Back Scatter Diffraction Pattern), large tilt grain boundary in which the orientation difference is 150 or more is defined as block boundary, i.e., grain boundary and a region surrounded by the block boundary is defined as grain.
Next, the structure of the present invention in the case that the secondary phase is mal-tensite and retained austenite is described below.
The structure of the steel of the present invention is mainly composed of a structure (MD structure) comprising ferrite grains as a matrix and contained therein, fine martensite and retained austenite, as a secondaryphase. Ferrite which does not contain a fine secondary phase in grains and MI) structure differ in structure morphology. Furthermore, influence giving to properties differs between those, and it is therefore necessary to distinguish those.
As the identification method, for example, those can be identified by, for example, subjecting a steel sheet containing those structures to repeller corrosion, and showing ferrite in gray and martensite and retained austenite in white by image analysis.
The entire surface of ferrite which does not contain the fine secondary phase in grains is gray. On the other hand, the MD structure is that fine martensite and retained austenite are contained in a ferrite phase. Therefore, fine white spot can be confirmed in ferrite grains.
When the area fraction of the whole structure is defined as 100%, the proportion of the MD structure (ferrite + martensite + retained austenite) is required to be 90% or more. When the proportion of the MD structure is less than 90%, the influence of the remainder increases, uniformity of the structure is impaired and stretch flangeability deteriorates. The remainder includes ferrite which is not accompanied with the secondary phase in grains, pearlite and bainite. On the other hand, the proportion that the MD structure occupies in the whole structure is better as large, and the upper limit thereof is not particularly defined.
When the area fraction of the whole structure is defined as 100%, the proportion of martensite and retained austenite as a secondary phase present in the whole structure (martensite and retained austenite present in MD structure and other structures) is required to be from 10 to 60%, and the desired strength can be obtained by this proportion. When the proportion is less than 10%, sufficient strength is not obtained, and when it exceeds 60%, elongation deteriorates. The proportion is preferably in a range of from 15 to 55%, more preferably in a range of from 20 to 50%, and most preferably in a range of from 20 to 35%.
The proportion of the secondary phase (martensite and retained austenite in MD structure) in ferrite matrix is that when the whole of the secondary phase in MD structure is defined as 100%, the proportion of the secondary phase in ferrite grains is required to be 50% or more (the proportion of martensite and retained austenite on grain boundary is less than 50%), and the desired stretch flangeability can be obtained by this proportion. When the proportion is less than 50%, martensite and retained austenite present on the grain boundary act as a starting point of fracture, and stretch flangeability deteriorates. The proportion is better as large, and is preferably 60% or more, and more preferably 70% or more.
In the present invention, the term "grain boundary" means large tilt grain boundary, and for example, when orientation difference of adjacent grains is measured by FE/SEM-EBSP (Electron Back Scatter Diffraction Pattern), large tilt grain boundary in which the orientation difference is 150 or more is defined as grain boundary.
When the area fraction of the whole structure is defined as 100%, the proportion that retained austenite present in the whole structure (retained austenite present in MD structure and other structures) occupies in the whole structure is desirably 2% or more. When retained austenite is present as the secondary phase, the desired ductility (mainly uniform elongation) can be obtained. When the proportion is less than 2%, the expected ductility improvement effect is diluted. The proportion is preferably 5% or more, and more preferably 8% or more. The proportion of the retained austenite is better as large, but from a practical standpoint, the upper limit is about 20%.
In the structure of the present invention, an average grain size of the secondary phase present in the whole structure (the secondary phase present in MD structure and other structures) is required to be 3 m or less. When the average grain size of the secondary phase is larger than 3 jim, starting points offracture are localized, and as a result, sufficient stretch flangeability is not obtained. The average grain size of the secondary phase is better as small, and is preferably 2.5 jim or less, and more preferably 2 jim or less.
An average grain size of ferrite in the whole structure (ferrite present in MD structure and other structures) is preferably 20 jim or less, and the desired stretch flangeability can be obtained by this grain size. When the grain size exceeds 20 jim, local deformability deteriorates, and stretch flangeability deteriorates, which is not preferable. The average grain size is better as small, and is preferably 15 p.m or less, and more preferably 10 p.m or less.
On the other hand, the secondary phase in the whole structure (martensite, or martensite and retained austenite, present in MD structure and other structures) is required to be finely dispersed, and it is preferred that 20 or more secondary phases on the average are observed in an observation view of 50 p.m x 50 p.m. When the number of the secondary phase is less than 20, the desired strength may not be ensured, or there is the possibility that stretch flangeability deteriorates, which is not preferable. The number of the secondary phase is preferably 30 or more, and more preferably 40 or more. The number is preferable as large, such as 50 or more, 80 or more, and 100 or more.
The MD structure steel sheet having the structure morphology of the present invention has a composition comprising, in terms of% by mass, C: 0.02 to 0.3%, Si: 0.01% to 3%, Mn: 0.5 to 3%, B: 0.0001 to 0.005%, and Al: 0.01 to 1.5%, with the remainder being Fe and inevitable impurities.
The composition may fl.irther contains one or two or more of: (1) Mo: 0.03 to 1%; (2) at least one element selected from the group consisting of Nb, Ti and V in the total amount of from 0.01 to 0.1%; (3) at least one of Ni: 0.5% or less (not including 0%) and Cu: 0.5% or less (not including 0%); (4) Cr: 1.5% or less (not including 0%); and (5) at least one of Ca: 0.003% or less (not including 0%) and REM (rare earth element): 0.003% or less (not including 0%).
The above component composition is described below. In the followings, all the percentages are defined by mass, unless otherwise indicated. Additionally, all the percentages defined by mass are the same as those defined by weight, respectively.
C:0.O2toO.3% C is an effective component to obtain strength of steel. The lower limit 0.02% is the minimum necessary amount to obtain a predetermined space factor of a secondary phase, thereby obtaining the desired strength. The upper limit 0.3% is that where C is added more than this amount, strength of martensite increases, and as a result, stretch flangeability deteriorates. The content of C is in a range of preferably from 0.03 to 0.25%, and more preferably from 0.04 to 0.20%.
Si: 0.01 to 3% Si is an element to decrease the amount of a solid solution C in ferrite, to contribute to improvement of ductility such as elongation, and to suppress precipitation of cementite during bainite transformation. Si is added in an amount of 0.01% or more. When the amount of Si added is less than 0.01%, cementite precipitates during cooling, and martensite is not formed among laths. Additionally, it is difficult to ensure the desired amount of retained austenite. The content of Si is preferably 0.3% or more, and more preferably 0.5% or more. On the other hand, when Si is added in an amount exceeding 3%, cracks may be generated, resulting in deterioration of elongation and stretch flangeability. The content of Si is preferably 2.5% or less, and more preferably 2% or less. a-a..
Mn: 0.5 to 3% Mn enables steel to have high strength by solid-solution hardening, and at the same time, improves hardenability of steel. Mn further has a function to accelerate formation of martensite. Such a function is recognized in the steel having Mn content of 0.5% or more. Mn content is preferably 0.7% or more, and more preferably 1% or more. On the other hand, when Mn is added in an amount exceeding 3%, stretch flangeability deteriorates. Mn content is preferably 2.5% or less, and more preferably 2% or less.
B: 0.0001 to 0.005% B is important for the present invention. When B is added, formation of pro-eutectoid ferrite can be suppressed, thereby preventing precipitation of ferrite which is not accompanied with fine secondary phase in grains, such ferrite adversely affecting stretch flangeability. This effect is exhibited when B is added in an amount of 0.0001% or more. The amount of B added is preferably 0.0002% or more, and more preferably 0.0003% or more. On the other hand, when B is added in an amount exceeding 0.005%, the degree of segregation to grains increases, resulting in deterioration of stretch flangeability. For this reason, 0.005% is defined as the upper limit of B content. The amount of B is preferably 0.004% or less, and more preferably 0.003% or less.
Al: 0.01 to 1.5% Al is used for deoxidation of steel. When the amount of A.l is less than 0.01%, silicate inclusions retain, thereby resulting in deterioration of processability of steel.
Therefore, the amount ofAl added must be 0.0 1% or more. Furthermore, similar to Si, Al has the effect to prevent precipitation of cementite, and to contribute to ensuring retained austenite. The amount of Al added is preferably 003% or more, and more preferably 0.05% or more. However, the amount of Al exceeding 1.5% invites increase of surface mark, and retained austenite and martensite are liable to grow, inviting deterioration of elongation and stretch flangeability. For this reason, the upper limit of Al content is set at 1.5%. The amount of Al is preferably 1% or less, and more preferably 0.5% or less.
The steel sheet material of the present invention may further contain the following components in addition to the above-described essential components.
Mo: 0.03 to 1% Mo is an element effective to accelerate bainite transformation and to accelerate disappearance of lath of bainitic ferrite during heat treatment. Mo further has the effect to suppress ferrite transformation and to improve hardenability. To obtain such effects, Mo is necessary in an amount of 0.03% or more. The amount of Mo is preferably 0.07% or more, and more preferably 0.1% or more. On the other hand, addition of Mo in an amount exceeding 1% results in increase of strength and deterioration of stretch flangeability. Therefore, the upper limit of Mo content is set at 1%. The amount of Mo is preferably 0.8% or less, and more preferably 0.6% or less.
Total of at least one element selected from the group consisting of Nb, Ti and V: 0.01 to 0.1% Nb, Ti and V have the function to form a carbonitride and to increase strength of steel by precipitation strengthening and the function to refine grains, and can be added according to need. Such functions are not exhibited when the total amount of at least one of Nb, Ti and V added is less than 0.0 1%. On the other hand, when at least one of those is added in the total amount exceeding 0.1%, precipitate increases, resulting in remarkable deterioration of stretch flangeability. For this reason, the upper limit is set at 0.1%. The total amount is preferably 0.05% or less, and more preferably 0.03% or less.
At least one of Ni: 0.5% or less (not including 0%) and Cu: 0.5% or less (not including 0%) Ni and Cu are elements effective to realize high strength while maintaining high strength-ductility balance, and are appropriately added. However, even though those are added in excess, the effect is saturated, and additionally, productivity deteriorates, such as generation of cracks during hot rolling. Therefore, the amounts of Ni and Cu added are suppressed to 0.5% or less, respectively. The amounts of those added are preferably 0.3% or less, and more preferably 0.2% or less, respectively. To effectively exhibit the effect, it is recommended to add Ni in an amount of 0.1% or more and/or Cu in an amount of 0.1% or more.
Cr: 1.5% or less (not including 0%) Cr is an element effective to enhance strength of steel by improving hardenability, and is appropriately added. However, even though it added in excess, the effect is saturated, and additionally, ductility deteriorates. Therefore, the amount of Cr added is preferably suppressed to 1.5% or less. The amount of Cr added is preferably 1.0% or less, and more preferably 0.8% or less. To effectively exhibit the effect, it is recommended to add Cr in an amount of 0.1% or more.
At least one of Ca: 0.003% or less (not including 0%) and REM: 0.003% or less (not including 0%) a-Ca and REM (rare earth element) are elements effective to control morphology of a sulfide in steel and to improve stretch flangeability, and are appropriately added.
However, even though those are added in excess, the effect is saturated, and the excessive addition is not economical. Therefore, the amounts of Ca and REM added are 0.003% or less, respectively. To effectively exhibit the effect, it is recommended to add those in an amount of 0.0003% or more, respectively. Examples of REM include Sc, Y and lanthanoid.
Inevitable impurities The composition of the steel sheet according to the present invention is that the remainder other than the above-described components comprises Fe and inevitable impurities. Of the inevitable impurities, P and S are admitted to contain in an amount of P: 0.07% or less (not including 0%) and S: 0.07% or less (including 0%). When processability of a steel sheet is considered, the amount of P and S added is better as small. In particular, where the content of S is high, an inclusion (MnS) increases, which remarkably adversely affects stretch flangeability of a steel sheet. However, where S is added in the above range, the addition does not give influence to properties of a steel sheet.
Next, in the steel sheet according to the present invention, production conditions for obtaining the above-described structure are described below.
The steel sheet of the present invention can be produced by conducting a hot rolling step and a cold rolling step to produce a steel sheet material, and applying a heat treatment step to the steel sheet material. Specific processes of the hot rolling step and the cold rolling step are as described before. However, the production conditions are not limited to those steps, and may appropriately be according to the necessary processes.
The purpose of the heat treatment step is to finely and uniformly disperse martensite in a ferrite phase. When the heating temperature is lower than A3 point, the whole structure does not convert into austenite during heating and holding, and ferrite which is not accompanied with martensite in the state before the heat treatment remains partially. This ferrite has low strength, and forms an interface having large strength difference to MD. As a result, stretch flangeability deteriorates. When the heating temperature is A3 point or higher, such ferrite all disappears. Therefore, the upper limit of the heating temperature is not particularly defined, but from the relationship with practical operating level, it is recommended to appropriately control the heating temperature to a proper value.
To obtain a predetermined structure of the present invention by converting a major part of the structure into MI) structure, in the above-described steel composition, it is necessary to set a cooling rate to a range of from 0.2 to 20 C/sec, and cool to Ms point or lower. In general, it is cooled to room temperature. When the cooling rate is less than 0.2 C/sec, ferrite which is not accompanied with martensite as a secondary phase is formed, and the proportion of ferrite and martensite in the MD structure becomes less than 90%, resulting in deterioration of strength and stretch flangeability.
Furthermore, the proportion of martensite in ferrite grains decreases, and growth of an average grain size of martensite is generated. The cooling rate is preferably 0.5 C/sec or more, and more preferably 1 C/sec or more. On the other hand, when the cooling rate exceeds 20 C/sec, time and temperature sufficient to disappear lath of bainitic ferrite cannot be ensured, and as a result, elongation and stretch flangeability deteriorate. Where the cooling rate further increases, the whole structure is converted into martensite, and as a result, elongation remarkably deteriorates. For this reason, I. the cooling rate is required to be 20 C/sec or less. The cooling rate is preferably 15 C/sec or less, and more preferably 10 C/sec or less.
Next, in the steel sheet according to the present invention, other production conditions for obtaining the structure are described below.
The high strength steel sheet of the present invention, that is, MD steel sheet, can be obtained by heating a steel sheet material satisfying the component system as defined above, cooling the material from a temperature of A3 point or higher (for example, 1,200 C), subjecting the material to a processing of rolling reduction of 5% or more in a temperature range of from 600 to 1,000 C, and then cooling the material to Ms point at a cooLing rate of from 0.2 to 20 C/sec.
Heating and holding the steel sheet material at a temperature of A3 point or higher, and then subjecting the material to a processing at rolling reduction of 5% or more in a temperature range of from 600 to 1,000 C: The steel sheet of the present invention has MD structure comprising ferrite as -a matrix and a secondary phase finely dispersed therein, on nearly the entire surface thereof. When the heating temperature is lower than A3 point, the structure during heating does not converts into austenite on the entire surface, and ferrite which is not accompanied with fine secondary phase is formed. The ferrite which is not accompanied with the fine secondary phase in grains contributes to the increase of elongation, but strength difference at the interface to the MID structure increases, and as a result, stretch flangeability. deteriorates. The heating temperature is sufficient to be A3 point or higher, but from the relationship with a practical operating level, it is recommended to appropriately control the heating temperature to a proper value.
The subsequent addition of the processing of rolling reduction of 5% or more in a temperature range of from 600 to 1,000 C is, for example, to accelerate bainite transformation to thereby facilitate obtaining fine MD structure, or to decrease an average grain size of ferrite. When the processing temperature is lower than 600 C, ferrite is precipitated, and when it is higher than 1,000 C, an average grain size of ferrite increases. Furthermore, where processing ratio is lower than 5%, bainite transformation is not accelerated. The upper limit of the processing ratio is practically about 90%, but the processing ratio higher than that has no problem. The processing temperature is preferably in a range of from 620 to 980 C, and more preferably in a range of 650 to 950 C. The rolling reduction is preferably in a range of from 8 to 85%, more preferably from 10 to 80%, and ftirther preferably from 10 to 60%.
Cooling the steel sheet material to room temperature at a cooling rate of from 0.2 to 20 C/sec: To obtain a predetermined structure of the present invention by converting a major part of the structure into MD structure, in the above-described steel composition, it is necessary to set a cooling rate to a range of from 0.2 to 20 C/sec. In general, it is cooled to room temperature. When the cooling rate is less than 0.2 C/sec, ferrite which is not accompanied with a secondary phase in grains is formed, and the proportion of the MD structure becomes less than 90%, resulting in deterioration of strength and stretch flangeability. The cooling rate is preferably 0.5 C/sec or more, and more preferably 1 C/sec or more. On the other hand, where the cooling rate exceeds 20 C/sec, the whole structure converts into martensite, and as a result, elongation remarkably deteriorates. For this reason, the cooling rate is required to be 20 C/sec or less. The cooling rate is preferably 15 C/sec or less, and more preferably 10 C/sec or less. fr
EXAMPLE
Examples in the case that the secondary phase is martensite are described below.
Steels lAto 5F having the component compositions shown in Tables I to 5 were melt to form slabs. The slabs were heated to 1,150 C, hot rolled to a sheet thickness of 3.0 mm at 800 C and then taken up at 550 C. Surface scale was removed by acid pickling, followed by cold rolling to a sheet thickness of 1.2 mm. Each of the steel sheet materials thus obtained was heated and held at each temperature shown in Tables 6 to 9, and then subjected to heat treatment which cools to each stop temperature at a cooling rate shown in Tables 6 to 9.
TABLE 1
_______ Chemical component of specimen Steel Chemical component_(mass%) _______ ______ ________ _______ _______ Ac3 Ms point Remarks code C Si Mn P S Al B Mo Other C) ( C) _____________ 1A 0.008 0. 69 1.89 0.008 0.005 0.23 0.0009 0.87 _______ 950 476 Comparison lB 0.03 1.71 1.22 0.01 0.005 0.17 0.0008 0.56 _______ 969 494 Invention 1C 0.03 1.55 2.35 0.008 0.004 0.03 0.0005 0 _______ 944 448 Invention 1D 0.08 -1.28 -1.31 0.009 0.006 0.13 0.0012 0.06 _______ 912 474 Invention 1E 0.07 0.62 0.67 1 0.005 0.26 0.0034 0 _______ 884 506 Invention iF 0.011 0.09 0.84 OM1 0.005 0.61 0.0011 0.13 _______ 897 531 Invention 1G 0.09 1.07 1.17 0.007 0.004 0.95 0.0019 0 _______ 897 500 Invention iN 0.13 2.43 1.67 0.008 0.005 0.05 0.0016 0.45 _______ 960 437 Invention 11 0.12 1.61 2.27 0.01 0.005 0.08 0.0007 0 _______ 912 421 Invention 1J 018 0.58 1.34 0.008 0.005 0.13 0.0009 0.23 _______ 857 436 Invention 1K 0.19 2.17 0.66 0.01 0.006 0.35 0.0015 0 _______ 919 466 Invention 1M 0.09 0.008 1.78 0.009 0.005 1.09 0.0018 0.56 _______ 867 478 Comparison iN 005 0.02 2.21 0.011 0.005 0.23 0.0023 0.87 _______ 893 448 Invention 0.04 0.03 1.25 0.01 0007 0.81 0.0036 0.13 _______ 875 510 Invention 1P 0.08 0.046 1.81 0.01 0.005 0.33 0.0012 0.08 _______ 857 460 Invention IQ 0.13 0.052 1.19 0.009 0.006 0.76 0.0018 0 _______ 839 479 Invention 1R 0.06.32 1.42 0.011 0.005 0.09 0.0008 0.32 _______ 929 474 Invention 1S 008 1.34 1.46 0.01 0.005 0.12 0.0009 0 _______ 912 468 Invention iT 0.07 1.23 1.51 0.009 0.006 0.22 0.0015 0.28 _______ 920 471 Invention 1U 0.18 1.81 2.34 0.01 0.005 0.07 0.0028 0 _______ 905 396 Invention 1V 0.07 2.35 1.02 0.011 0.005 0.34 0.0019 0.34 _______ 972 493 Invention 1W 0.12 2.33 1.91 0.007 0.004 0.67 0.0007 0 _______ 944 452 Invention 1X 0.04 2.68 1.54 0.01 0.005 0.58 0.0035 0.32 _______ 999 490 Invention 1Y 0.17 2.76 1.83 0.009 0.005 0.32 0.0018 0 _______ 950 427 Invention 1Z 0.08 3.34 0.76 0.011 0.006 0.87 0.0009 0.04 _______ 995 517 Comparison
TABLE 2
________ Chemical component of specimen Steel Chemical component_(mass%) ______ _______ _________ ______ __________ Ac3 Ms point Remarks code C Si Mn P S Al B Mo Other ( C) ( C) _______________ 2A 0.16 1.18 0.38 0.01 0.005 1.07 0.0006 0.23 _________ 889 508 Comparison 2B 0. 14 0.05 0.56 0.008 0.004 0.09 0.0014 0.65 _________ 857 477 Invention 2C 0. 07 0.74 0.52 0.01 0.005 0.32 0.0027 0 _________ 889 514 Invention 2D 0.03 1.42 1.64 0.008 0.005 0.08 0.0011 0.28 _________ 947 476 Invention 2E 0. 06 1.3 0.79 0.009 0.006 0.61 0.0013 0 _________ 918 516 Invention 2F 0.04 208.27 001 0.003 0.88 0.0016 0.18 _________ 968 512 Invention 0.05 0.96 -.29 001 0.004 0.12 0.0035 0 _________ 908 485 Invention 2H 0.04 1.74.85 coi 0.005 0.92 0.0017 0.24 _________ 955 490 Invention 21 0.07 1.42 -.83 0.011 0.005 0.81 0.0014 0 _________ 920 478 Invention 2J 0.13 2.2 2.25 0.009 0.006 0.07 0.0033 0.46 _________ 950 415 Invention 2K 0.18 1.08 2.3 0.01 0.005 0.54 0.0025 0 _________ 872 412 Invention 2L 0.1 0.96 2.81 0.007 0.007 1.23 0.0026 0.34 _________ 899 440 Invention 2M 0.14 0.74 2. 82 0.01 0.006 0.91 0.0044 0 _________ 867 417 Invention 2N 0.08 0.52 3.25 0.011 0.005 0.47 0.0023 0.65 _________ 896 405 Comparison 0.03 1.44 1.36 0.012 0.006 0.006 0.0027 0.56 _________ 957 484 Comparison 2P 0.08 1.37 1.75 0.008 0.005 0.02 0.0015 0.47 _________ 929 451 Invention 2Q 0.12 1. 36 2.41 0.008 0.005 0.02 0.0012 0 _________ 900 413 Invention 2R 0.15 2.33 -.52 0.01 0.005 098 0.0003 0.23 _________ 943 465 Invention 2S 0.04 0.31 -.98 0.01 0.005 0.12 0.002 0 _________ 883 462 Invention 2T 0.05 1.29.1 0.008 0.005 0.81 0.0011 0.78 _________ 947 509 Invention 2U 0.03 1.36 1. 38 0.009 0.005 0.79 0.0015 0 _________ 936 509 Invention 2V 0.08 1.33 1.79 0.01 0.003 1.28 0.0008 0.05 _________ 914 489. Invention 2W 0.06 2.31 0. 78 0.01 -0.004 1.35 0.0004 0 _________ 964 538 Invention 2X 0.08 1.89 1.34 0.01 0.005 1.67 0.0017 0.16 _________ 942 518 Comparison 2Y 0.09 0.86 1. 59 0.011 0.005 0.08 0.0016 0.03 _________ 888 458 Invention 2Z 0.12 1.87 1. 73 0.01 0.005 0.45 0.0024 0.11 _________ 927 452 Invention
TABLE 3
Chemical component of specimen Steel Chemical comtonent mass%) ______ _____ __________ ______ __________ Ac3 Ms point Remarks code C Si Mn P S Al B Mo Other ( C) ( C) _____________ 3A 0.07 2.12 264 0.008 0.007 0.09 0.0014 0.34 __________ 962 423 Invention 3B 0.17 1.19 104 0.008 0. 005 0.73 0.0018 0.81 __________ 905 466 Invention 3C 0.05 1.07 _1_ 0.007 0. 006 0.77 0.0014 1.19 __________ 950 507 Comparison 3D 0.04 0.85 0.66 0.009 0.004 0.33 0 0.3 __________ 917 518 Comparison 3E 0.09 2.52 1.75 0.01 0.005 0.49 0.0001 0.12 _________ 966 463 Invention 3F 0.12 0.12 2.36 0.01 0.004 0.18 0.0001 0 _________ 845 420 Invention 3G 0.15 0:27 1.09 0.01 0.005 0.97 0.0005 0.81 __________ 869 478 Invention 3H 0.11 1.52 2.04 0.008 0.005 0.38 0.0005 0 _________ 911 442 Invention 31 0.03 2.49 0.55 0.012 0.005 1.23 0.0011 0.34 _________ 997 553 Invention 3J 0,08 1.04 1. 67 0.008 0.005 1.03 0.001 0 __________ 899 487 Invention 3K 0,05 1.82 1.78 0.011 0.006 0.49 0.0019 0.23 __________ 953 476 Invention 3L 0.09 *O..... 2.68 0.01 0.004 0.05 0.0022 0 _________ 861 414 Invention 3M 0.11 128 1. 73 0.009 0.005 0.08 0.0047 0.28 _________ 909 444 Invention 3N 0.04 1.82 1. 88 0.01 0.005 0.58 0.0048 0 _________ 951 480 Invention 0.19 1 0.89 0.011 0.005 0.69 0.0077 0.51 _________ 902 465 Comparison 3P 0.05 1.06 1.6 0. 007 0.003 1.10 0.0012 0.77 Ti:0.02 936 499 Invention 3Q 0.12 iY 0.96 0.01 0.005 0.58 0.0009 0.36 Ti:0.02 923 485 Invention 3R 0.08 2.41 1.26 0.01 0.005 0.22 0.0011 0.52 Ti:0.08 -977 476 Invention 3S 0.04 1.09 2.17 0.01 0.004 0.62 0.0004 0 Ti:0.08 918 470 Invention 3T 0.09 0.46 0.97 0.008 0.006 0.08 0.0012 0.89 Ti:0.12 898 478 Comparison 3U 0.12 1.66 1.78 0.009 0.005 0.07 0.0017 0.34 Nb:0.02 925 438 Invention 3V 0.07 1.24 1.83 0.008 0.007 0.32 0.0005 0 Nb:0.02 912 463 Invention 3W 0.17 0.73 2.14 0.011 0.006 1.04 0.0014 0.65 Nb:0.08 879 433 Invention 3X 0.05 0.78 1.48 0.01 0.005 0.62 0.0019 0 Nb:0.08 899 493 Invention 3Y 0.03 1.83 1.54 0.12 0. 005 0.78 0.0021 0.49 Nb:0.12 972 500 Comparison 3Z 0.08 2.22 1.92 0.01 0.004 0.93 0.0016 0.22 V:0.02 961 472 Invention
TABLE 4
______ Chemical component of specimen Steel Chemical corn onent (mass%) _______ ______ ________ ______ __________________ Ac3 Ms point Remarks code C Si Mn P S -Al B Mo Other ( CL ( C) _____________ 4A -0.15 1.69 1.77 0.009 0.005 0.22 0.0034 0 V:0.02 909 433 Invention 4B 0.11 2.3 0.92 0.01 0.003 0.79 0.0008 0.08 -V:0.08 956 495 Invention 4C 0.08 (39 0.59 0.007 0.006 1.17 0.0017 0 V:0.08 878 530 Invention 4D 0.07:71 1.18 0.01 0.005 0.98 0.0014 0.04 V:0.12 946 504 Comparison 4E -0.05 -.96 0.67 001 0.005 0.82 0.0018 0.35 Ti:0.02, Nb:0.02 963 529 Invention 4F 0.09 207 2.45 Q1 0.004 0.05 0.0023 0 Ti:0.02, Nb:0.02 942 423 Invention 4G 0.06 -.37 0.72 0.008 0.007 0.09 0.0031 0.65 Ti:0.03, V:0.03 945 499 Invention 4H 0.04 1.22 0.92 0.008 0.006 0.84 0.0004 0 Ti:0.03, V:0.03 927 524 Invention 41 0.11 1.08 2.15 0.008 0.005 0.48 0.0026 0.31 Nb:0.04, V:0.04 905 438 Invention 4J 0.13 0.11 2.4 0.011 0.005 0.08 0.0011 0 -Nb:0.04, V:0.04 846 410 Invention 4K 0.08 1.47 0.86 0.008 0.005 0.33 0.0013 0.15 Ti:0.02,Nb:0.03 927 495 Invention ______ _____ _____ _____ ______ ______ _____ ________ _____ V: 0.04 _____ __________ 4L 0.05 i.5T 1.39 0.009 0.005 0.38 0.003 1 0 Ti:0.02,Nb:O.03 939 488 Invention ______ _____ __________ ______ ______ _____ _______ _____ V:0.04 _____ _________ 4M -0.03 1.24 1.54 0.0 12 0.005 0.81 0.0014 0.72 Ti:0.05, Nb:0.05 958 498 Comparison ______ _____ _____ -____ ______ ______ _____ ________ _____ V:0.05 _____ __________ 4N 0.05 2.38 1.36 0.011 0.004 1.40 0.0019 0.37 Ni:0.3 966 503 Invention 0.11 1.29 1.08 0.01 0.005 0.38 0.0008 0 Ni:0.3 896 474 Invention 4P oi 0.96 0.78 0.01 0.005 0.09 0.0034 0.79 Ni:0.6 904 472 Comparison 4Q 0.09 2.24 0.94 0.011 0.003 0.03 0.0045 0.16 Cu:0.3 954 478 Invention 4R 0.13 1.54 1.29 0.008 0.007 0.64 0.0029 0.47 Cu:0.3 920 467 Invention 4S -0.18 0.73 2.06 0.007 0.005 0.11 0.0013 0.31 Cu:0.6 866 400 Comparison 4T 0.03 2.08 1.63 0.011 0.005 0.43 0.0006 0.56 Ni:0.2, Cu:0.2 982 484 Invention 4U 0.1 1.39 0.87 0.01 0.005 0.63 0.0003 0 Ni:0.2, Cu:0.2 905 497 Invention
TABLE 5
______ Chemical_component_of specimen ______ ___________ ______________ Steel Chemical_component_(mass%) _______ ______ ________ ______ _____________ Ac3 Ms point Remarks code C Si Mn P S Al B Mo Other ( C) ( C) ______________ 4V 0.08 0.54 1.45 0.01 0.004 1.24 0.0006 0.68 Cr:0.2 898 494 Invention 4W 0.05 1.87 1.9 0.008 0.005 0.45 0.0018 0 Cr:0. 2 948 467 Invention 4X 0.06 0.94 1.44 0.011 0.006 0.98 0.0031 0.22 Cr:0.5 909 490 Invention 4Y 0,09 1.73..51 0.008 0.005 1.19 0.0023 0 Cr:0.5 926 484 Invention 4Z 0.08 1.23 1.63 0.01 0.005 0.97 0.0017 0.75 Cr:0.8 931 467 Invention 5A 0.18 0.57 0.9 0.01 0.005 0.66 0.0022 0 Cr:0.8 849 454 Invention 5B 0.16 1.11 1.27 0.012 0.005 1.29 0.0034 0.36 Cr:1.6 890 448 Comparison 5C 0.07 2.05 2.88 0.01 0.003 0.18 0.002 0.09 Ca:0.002 951 417 Invention SD 0.08 1.06 1.5 0.009 0.005 0.11 0.0005 0 Ca:0.002 900 466 Invention 5E 0.04 1.43 1.27 0.008 0.004 0.30 0.004 0.26 REM:0.002 942 496 Invention SF 0.13 0.92 1.66 0.01 0.006 0.52 0.0013 0.05 REM:0.002 880 454 Invention
TABLE 6
Heat treatment condition __________ Material Steel Heat treatment condition Remarks No. code Heating Cooling Cooling temperature rate stop ( C) ( C/sec) temperature ______ ___ ________ ______ ( C) ________ 1 lÀ 1000 2 10 Comparison 2 lB 1000 2 10 Invention 3 Ic iooo 2 10 Invention 4 ID 800 2 10 Comparison 1D 850 2 10 Comparison 6 ID 900 2 0 Comparison 7 1D 1000 2 0 Invention 8 1D 1100 2 10 Invention 9 1D 1200 2 10 Invention iF 1000 2 10 Invention 11 IF 000 2 10 Invention 12 10 000 2 10 Invention 13 1H 1000 2 10 Invention 14 II 1000 2 10 Invention 1J 1000 2 10 Invention 16 1K 1000 2 10 Invention 18 1M 1000 2 10 Comparison 19 iN 1000 2 10 Invention 10 1000 2 10 Invention 21 IP 1000 2 10 Invention 22 LQ 1000 2 10 Invention 23 IR 1000 0.05 10 Comparison 24 IR 1000 0.1 10 Comparison 1R 1000 0.3 10 Invention 26 IR 1000 0.5 10 Invention 27 IR 1000 1 10 Invention 28 1&_ 1000 5 10 Invention 29 JR 1000 10 10 Invention R 1000 20 10 Invention 31 R 1000 30 10 Comparison 32.R 1000 50 10 Comparison 33 1R 1000 100 10 Comparison 34 IR 1000 500 10 Comparison iS 1000 0.05 10 Comparison
TABLE 7
Heat treatment condition __________ Material Steel Heat treatment condition Remarks No. code Heating Cooling Cooling temperature rate stop ( C) ( Clsec) temperature ______ ____ _______ ______ ( C) ________ 36 is iooo 0.1 10 Comparison 37 j_ 1000 0.3 10 Invention 38 *i_ 1000 05 10 Invention 39 j_ 1000 1 -0 Invention 1_ 1000 5 -0 Invention 41 1_ 1000 10 0 Invention 42 is iooo 20 -0 Invention 43 is -1000 30 -0 Comparison 44 is moo 50 -0 Comparison is iooo 100 -0 Comparison 46 is iooo 500 10 Comparison 47 iT 1000 2 -0 Invention 48 U 1000 2 0 Invention 49 V 1000 2 _0 Invention 1W 1000 2 _1O hjvention_ 51 1X -1000 2 _10 Invention - 52 1Y 1000 2 10 Invention 53 iZ 1000 2 10 Comparison 54 2A 1000 2 10 Comparison 2B_ 1000 2 10 Invention 56 2C -1000 2 10 Invention 57 2D 1000 2 20 Invention 58 2D 1000 2 100 Invention_ 59 2D -1000 2 300 Invention 2D 1000 2 500 Comparison 61 2E 1000 2 10 Invention 62 2F 1000 2 10 Invention 63 2G 1000 2 10 Invention 64 2H 1000 2 10 Invention 21 1000 2 10 Invention 66 2J -000 2 10 Invention 67 2K -000 2 10 Invention 68 2L -000 2 10 Invention 69 2M -000 2 10 Invention 2N -000 2 10 Comparison
TABLE 8
Heat treatment condition __________ Material Steel Heat treatment condition Remarks No. code Heating Cooling Cooling temperature rate stop ( C) ( C/sec) temperature ______ -c) ___ 71 20 1000 2 10 Comparison 72 2P 1000 2 -0 Invention 73 2Q 1000 2 -0 Invention 74 2R 1000 2 -0 Invention 2S 1000 2 ________ Invention 76 2T 1000 2 10 Invention 77 2U 1000 2 10 Invention 78 2V 1000 2 10 Invention 79 2W 1000 2 10 Invention 2X 1000 2 10 Comparison 81 2Y 1000 2 10 Invention 82 2Z 1000 2 10 Invention 83 3A 1000 2 10 Invention 84 3B 1000 2 10 Invention 3C 1000 2 10 Comparison 86 3D 1000 2 10 Comparison 87 3E 1000 2 10 Invention 88 3F 1000 2 10 Invention 89 3G 1000 2 10 Invention 3H_ j000 2 10 Invention 91 31 1000 2 10 Invention 92 3J 1000 2 10 Invention 93 3K 1000 2 10 Invention 94 3L 1000 2 10 Invention 3M 1000 2 10 Invention 96 3N 1000 2 10 Invention 97 30 1000 2 10 Comparison 98 3P 1000 2 10 Invention 99 3Q 1000 2 10 Invention 3R 1000 2 10 Invention 101 -35 000 2 10 Invention 102 3T -000 2 10 Comparison 103 3U -000 2 10 Invention 104 3V 1000 2 10 Invention_ 3W 1000 2 10 Invention
TABLE 9
Heat treatment condition _________ Material Steel Heat treatment condition Remarks No. code Heating Cooling Cooling temperature rate stop ( C) ( C/sec) temperature ______ ____ ________ ______ ( C) ________ 106 3X 1000 2 10 Invention 107 -3Y 1000 2 10 Comparison 108 3Z 1000 2 10 Invention 109 -4A 1000 2 10 Invention 4B 1000 2 10 Invention 111 4C 1000 2 10 Invention 112 4D 1000 2 10 Comparison 113 4E 1000 2 10 Invention 114 4F 1000 2 10 Invention 4G 1000 2 10 Invention 116 4H 1000 2 10 Invention 117 41 1000 2 10 Invention 118 4J 1000 2 10 Invention 119 4K 1000 2 10 Invention - 4L 1000 2 10 Invention 121 4M 1000 2 10 Comparison 122 4N 1000 2 10 Invention 123 40 1000 2 10 Invention - 124 4P 1000 2 10 Comparison 4Q 1000 2 10 Invention 126 4R 1000 2 10 Invention 127 4S 1000 2 10 Comparison 128 4T 1000 2 10 Invention 129 4U 1000 2 10 Invention 4V 1000 2 10 Invention 131 4W 1000 2 10 Invention 132 4X 1000 2 10 Invention 133 4Y 1000 2 -0 Invention 134 4Z j000 2 0 Invention 5A 1000 2 -0 Invention 136 5B 1000 2 0 Comparison 137 SC 1000 2 0 Invention 138 SD 1000 2 0 Invention _139 5E 1000 2 10 Invention SF 1000 2 10 Invention Microstructure and dynamic properties were examined on each of steel sheets obtained above by the following procedures.
Microstructure of each steel was determined by the following method.
Each of steel sheet Nos. 1 to 140 after heat treatment was cut into a test piece for microstructure observation of 10 mm x 10 mm x 1.2 mm. The test piece was embedded with a cold rolled resin such that the observation position is a position of 1/4 of a sheet thickness in a rolled direction. Impression as a landmark was formed with a Vickers tester so as to conduct identification on the structure observation place. The impression was corroded with repeller, and the structure was observed at five positions using an optical microscope in a magnification of 1,000. When the structure photograph after repeller corrosion was image analyzed, ferrite is observed gray, and martensite and retained austenite were observed white. After microstructure observation with an optical microscope, buff polishing and electrolysis polishing were conducted to an extent such that the Vickers impression does not disappear, and* structure observation was conducted on the place at a step distance of 100 nm using FE/SEM-EBSP. A boundary having an orientation difference of 15 or more of grains was considered as grain boundary, and the grain boundary was identified.
Optical microphotograph and FE/SEM-EBSP structure photograph were combined as the standard of Vickers impression, and the structure was evaluation.
Ml) structure: The MD structure contains fine martensite (secondary phase) in ferrite grains as a matrix, as described above. The observation results of optical microscope and FE/SEM-EBSP were combined, a structure in which many martensite are present in the inside of grains surrounded by large tilt grain boundary (i.e., in the grains) was identified as MD structure, and its space factor was obtained.
There is the possibility that pearlite, bainite, retained austenite, ferrite which is not accompanied with martensite in grains, and the like are contained as other structures. Identification method of those structures is as follow.
Retained austenite: Retained austenite has an FCC structure, and therefore can be identified with FE/SEM-EBSP. Area fraction of austenite in the observation view was determined.
Pearlite: Pearlite has a lamella structure of ferrite and cementite, and the cementite can be identified with FE/SEM-EBSP. In the Examples, the area fraction of pearlite in the observation view was determined.
Bainite: Bainite is that lath is present in ferrite grains identified with FE/SEM-EBSP.
Structure in which lath is present in ferrite grains was judged as bainite, and area fraction of bainite in the observation view was determined.
Ferrite which is not accompanied with martensite in grains: Structure in which martensite cannot be confirmed in ferrite grains identified with FE/SEM-EBSP is considered as ferrite which is not accompanied with martensite in grains, and is judged as a structure different from Mi) structure. Area fraction of ferrite which is not accompanied with martensite in grains in the observation view was determined.
Structure in which MD structure is not observed in the observation view is judged that MD is not present. When the whole structure is defined as 100%, in the case that the total of area fractions of other structures (retained austenite, pearlite, bainite, ferrite which is not accompanied with martensite in grains, and the like) exceeds 10%, the defined structure is not fonned. In this case, it was judged that MD is partial.
On the other hand, in the case that the space factor of MD structure occupies 90% or more of the whole structure, the following further detailed investigation was conducted.
Space factor and average grain size of martensite: Space factor of martensite in the whole structure used area fraction of white portion in the whole structure by image analyzing an optical microphotograph. By combining with an image ofFEJSEM-EBSP, martensite present in ferrite grains and martensite present at grain boundary were separated. Retained austenite is also observed as a white portion, but austenite can be distinguished from retained austenite by identifying retained austenite which has an FCC structure, with FE/SEM-EBSP.
Retained austenite was removed by image analysis ofFE/SEM-EBSP, and the space factor of martensite was determined.
On the other hand, an average grai,n size (particle diameter) of martensite was determined by the following method. Five places of the observation view having a size of 20 m x 20 tm were randomly extracted from a SEM observation photograph (magnification 3,000 times), an average grain size (diameter of a corresponding circle) of martensite and retained austenite in the respective observation views was determined, and its average value was used as a grain size of martensite in the whole structure.
Dynamic properties of each steel were determined by the following methods.
Tensile properties: IIS No.5 test piece obtained from a vertical direction of a steel sheet rolled direction was used, and tensile strength (TS) and Elongation (EL) were measured according to MS Z 2241 Test pieces having TS of 590 MPa or more and EL of 10% or more were considered as acceptable products. Elongation from strain corresponding to tensile strength to breaking strain was designated as local elongation.
Stretch flangeability: Hole expanding ratio A was measured as stretch flangeability. The hole expanding ratio A was measured according to 11SF (The Japan Iron and Steel Federation) standard (JFST 1001-1996). Steels having A of 80% or more were considered as acceptable products.
The results obtained are shown in Tables 10 to 15 below.
TABLE 10
* Measurement results Material Steel Presence Structure parametar ______ ______ Dynamiçproperties _______ ____ Remarks No. code or absence Af of MI) Af of a' Afofct' Grain size TS Total Local of MI) structure in entire in a grain of a' in (MPa) elongation elongation (%) (%) structure of entire entire (%) (%) (%) structure structure ____ _____ _____ ____ (%) (pin) ___ _____ _____ ___ ______ _______ IA _________ 94.5 188 70.9 1.5 532 32.9 25.1 169.2 Comparison 2 lB _________ 95.9 27.2 71.9 1.7 668 27.6 19.8 136.8 Invention 3 IC _________ 95.0 45.5 71.3 1.7 933 18.8 10.2 96.5 Invention 4 1D Partial 28.1 _______ ________ ________ 687 18.3 6.5 39.5 Comparison 1D Partial 50.1 _______ ________ ________ 715 17.5 6.7 45.2 Comparison 6 1D Partial 87.4 ________ ________ ________ 766 16.4 6.3 43.3 Comparison 7 1D _________ 95.4 42.5 77.6 1.6 854 17.2 10.5 105,8 Invention 8 ID _________ 94.1 43.6 77.7 1.6 832 18.4 11.4 108.5 Invention 9 ID _________ 94.1 41.3 78.8 1.7 844 19.4 12.3 110.1 Invention I.E _________ 90.4 31.9 67.8 1.9 729 24.0 18.0 123.4 Invention 11 IF _________ 91.1 29.7 68.3 1.5 696 25.1 18.9 129.3 Invention 12 10 __________ 91.1 39.3 68.3 2.1 849 21.9 14.1 107.3 Invention 13 IH _________ 96.5 58.6 72.4 2.3 1289 14.7 6.2 80.3 Invention 14 II _________ 94.8 57.4 71.1 2.3 1189 14.7 6.3 85.7 Invention 13 _________ 93.6 45.9 70.2 2.6 949 19.6 11.6 95.9 Invention 16 1K _________ 92.3 57.9 69.2 2.7 1119 15.6 9.7 80.4 Invention 18 1M None ________ _______ _______ 843 9.2 4.4 67.4 Comparison 19 IN _________ 94.8 36.7 71.1 1.8 801 21.9 13.5 112. 4 __________ 10 _________ 92.4 23.2 69.3 1.7 597 29.3 22.4 150.7 Invention 21 * LP _________ 93.9 30.1 70.4 2.0 711 26.0 17.2 128.3 Invention 22 j, Q_ _________ 91.2 31.6 68.4 2.3 724 24.2 17.4 124.3 Invention 23 1R None ________ _______ ________ ________ 641 26.2 10.5 41.5 Comparison a: Ferrite, Ct': Maztensite,Af Area fraction, TS: Tensile strength, : Hole expanding ratio
TABLE 11
Measurement results Material Steel Presence Structure parameter ________ ________ Dynamic properties ________ _____ Remarks No. code or absence AfofMl) AIofc' Af of a' Grain size TS Total Local of ML) structure in entire in a grain of a' in (MPa) elongation elongation (%) (%) structure of entire entire (%) (%) (%) structure structure ____ _____ _____ ____ (%) (jmi) ____ _____ _____ ___ ______ 24 IR Partial 72.9 _______ ________ ________ 732 18.1 7.1 52.3 Comparison IR _________ 96.1 38.0 78.4 1.5 867 19.4 14.8 102.5 Invention 26 IR _________ 96.7 39.3 77. 4 1.7 856 20.5 14.9 112.9 Invention 27 IR _________ 96.4 40.9 78.6 1.4 876 20.2 15.9 108.7 Invention 28 LR _________ 96.3 38.5 80.0 1.4 873 19.2 15. 7 112.4 Invention 29 IR _________ 96.0 38.2 78.4 1.5 884 20.9 15.9 115.3 Invention 1R _________ 95.4 38.9 80.3 1.6 871 20.8 16.2 105.8 Invention 31 IR None ________ _______ ________ ________ 1021 9.2 4.7 74.3 Comparison 32 1R None ________ _______ ________ ________ 1101 8.4 4.1 72.1 Comparison 33 IR None ________ _______ ________ ________ 1209 8.1 3.6 79.4 Comparison 34 1R None ________ _______ ________ ________ 1234 7.5 3.8 76.1 Comparison iS None ________ _______ ________ ________ 611 25.9 10.5 38.5 Comparison 36 IS Partial 62.8 _______ ________ ________ 722 17.3 6.9 45.9 Comparison 37 IS _________ 95.1 37.6 776 1.7 856 20.4 15.6 100.5 Invention 38 IS _________ 95.7 38.9 76.7 1.9 844 20.9 14.7 109.2 Invention 39 IS ________ 95.4 40.5 77.8 1.6 860 19.7 16.1 106.5 Invention IS _________ 95.3 38.1 79.2 1.6 861 18.7 15.5 110.9 Invention 41 IS _________ 95.0 37.8 77.6 1.7 863 21.3 15.7 112.3 Invention 42 IS _________ 94.5 38.5 79.5 1.8 859 20.5 16.0 101.6 Invention 43 iS None ________ _______ ________ ________ 989 9. 5 4.5 72.1 Comparison 44 iS None ________ ________ ________ _________ 1088 8.2 3.9 70.9 Comparison IS None ________ _______ ________ ________ 1173 7.9 3.4 76.5 Comparison 46 iS None ________ _______ ________ 1198 7.2 3.5 75.5 Comparison a: Fenite, a': Martensite, Af Area fraction, TS: Tensile strength, X: Hole expanding ratio
TABLE 12
Measurement results Material Steel Presence Structure paramet ______ ______ Dynamic properties ______ ____ Remarks No. code or absence Af of Mi) Afofct' Afofct' Grain size TS Total Local of MD structure in entire in a grain of a' in (MPa) elongation elongation (%) (%) structure of entire entire (%) (%) (%) structure structure _____ _____ ____ (%) (pm) ____ _____ _____ ___ ______ 47 IT _________ 92.1 39.6 69.0 2.0 854 21.7 13.5 106.6 Invention 48 Hi ________ 95.0 64.8 71.2 2.6 1221 14.3 5.8 87.7 Invention 49 IV ________ 93.4 48.4 70.1 1.9 986 18.9 11.4 92.2 Invention 1W ________ 93.6 63.4 70.2 2.3 1201 14.6 6.7 84.9 Invention 51 1X ________ 97.0 62.3 72.7 1.7 1184 14.8 7.5 96.0 Invention 52 IY ________ 93.3 68.1 70.0 2.6 1271 13.8 6.0 90.8 Invention 53 IZ ________ 90.1 66.3 67.5 2.0 1245 9.1 3.0 72.3 Comparison 54 2A Partial 72.5 ________ ________ ________ 661 26.5 20.9 54.7 Comparison 2B ________ 94.5 25.7 70.8 2.4 635 27.6 21. 7 141.8 Invention 56 2C _________ 90.9 24.2 68.2 1.9 623 29.5 22.7 146.7 Invention 57 2D ________ 94.1 45.1 75.6 1.7 865 18.7 13.2 100.4 Invention 58 2D _________ 94.3 44.5 78.9 1.8 878 18.6 13.3 108.1 Invention 59 2D _________ 94.5 48.1 77.5 1.8 855 17.9 12.8 107.3 Invention 2D None ________ ________ ________ ________ 954 9.8 4.8 79.8 Comparison 61 2E _________ 91.2 28.4 68.4 1.9 676 25.9 19.7 133.2 Invention 62 2F _________ 92.9 46.2 69.7 1.7 953 19.6 11.7 95.4 Invention 63 2G _________ 91.5 29.0 68.6 1.8 695 26.6 18.6 131.4 Invention 64 2H _________ 95.4 51.9 71.5 1.7 1038 18.0 9.2 87.5 Invention 21 _________ 93.3 49.9 70.0 1.9 1009 18.5 9. 8 90.1 Invention 66 2J _________ 98.5 53.3 73.9 2.3 1200 14.6 6.2 85.0 Invention 67 2K _________ 94.9 54.0 71.1 2.6 1210 14.5 6.0 84.4 Invention 68 2L _________ 95.4 55.6 71.6 2.1 1100 17.1 6.8 82.6 Invention 69 2M _________ 96.6 58.6 72.4 2.4 1129 15.5 6.3 89.7 Invention a: Ferrite, a': Martensite, Af: Area fraction, TS: Tensile strength, A.: Hole expanding ratio
TABLE 13
Measurement results Material Steel Presence Structure parameter _______ _______ yainic properties ________ ____ Remarks No. code or absence AfofM]) Afofa' Af of a' Grain size TS Total Local of MD structure in entire in a grain of a' in (MPa) elongation elongation (%) (%) structure of entire entire (%) (%) (%) structure structure ____ ___ ___ ___ (pm) __ ____ ____ __ ____ 2N ________ 98.4 55.4 73.8 2.0 1242 15.2 4. 3 73.1 Comparison 71 20 _________ 96.4 35.1 72.3 1.7 787 9.5 3.5 65.8 Comparison 72 2P _________ 96.9 47.5 72.7 2.0 972 19.2 10.6 -93.5 Invention 73 ________ 95.2 567 71.4 2.3 1174 14.9 6.3 86.6 Invention 74 2R ________ 94.2 58.5 70.6 2.4 1128 15.5 8.2 89.8 Invention 25 ________ 93.8 28.8 70.3 1.7 691 26.7 17.7 132.1 Invention 76 2T _________ 97.3 37.6 73. 0 1.8 824 22.5 14.9 110.6 Invention 77 213 ________ 91.8 35.1 68.8 1.7 776 22.5 15.5 116.0 Invention 78 2V _________ 93.6 527 70.2 2.0 1050 17.8 9.1 86.6 Invention 79 2W _________ 92.7 45.6 69.5 1.9 933 18.7 12.6 96.4 Invention 2X _________ 94.8 66.7 71.1 3.8 1100 7.9 4.5 81.8 Comparison 81 2Y ________ 92.7 376 69.6 2.1 824 22.5 14.1 110.6 Invention 82 2Z _________ 93.9 56.1 70.4 2.3 1186 15.9 7.3 86.5 Invention 83 3A _________ 96.8 56.4 72.6 1.9 1106 17.0 7.0 82.2 Invention 84 3B ________ 97.4 55.6 73.0 2.6 1084 16.1 9.6 83.0 Invention 3C Partial 88.7 _______ ________ ________ 803 24.8 18.2 72.1 Comparison 86 3D Partial 41.9 ______ _______ _______ 589 29.7 23.7 52.7 Comparison 87 3E ________ 94.0 57.3 70.5 2.1 1120 16.8 8.1 81.1 Invention 88 3F _________ 95.1 447 71.3 23 920 19.0 10.5 97.8 Invention 89 3G ________ 97.6 41.5 73.2 2.4 873 20.1 13.4 103.1 Invention 3H -94.0 57.8 70.5 2.2 1117 15.7 7.6 80.6 Invention 91 31 _________ 91.9 43.7 68.9 1.7 905 19.3 13.5 99.4 Invention 92 31 _________ 92.8 45.1 69.6 2.0 937 19.9 11.4 97.1 Invention a: Fenite, a': Maxtensite,Af Area fraction, TS: Tensile strength, X: Hole expanding ratio
TABLE 14 -
Measurement results Material Steel Presence Structureparameter ______ ______ Dynamicproperties ______ ____ Remarks No. code or absence AJof MD AfofcL' Af of a' Grain size TS Total Local of MD structire in entire in a grain of a' in (MPa) elongation elongation (%) (%) siructure of entire entire (%) (%) (%) siructure Sthlcture ___ ___ ___ (%) (pm) __ ___ ___ __ ___ 93 3K _________ 95.0 50.8 71.3 1.8 1022 18.3 9.6 89.0 Invention 94 3L _________ 96.1 47.6 72.1 2.1 963 18.2 9.1 93.4 Invention 3M _________ 95.3 -52.9 71.5 2.2 1053 17.8 9.2 86.3 Invention 96 3N _________ 93.5 52.3 70.1 1.7 1044 17.9 9.1 87.0 Invention 97 30 ________ 94.4 62.7 70.8 2.7 1191 14.7 8.4 75.6 Comparison 98 3P _________ 98.9 44. 6 74.2 1.8 929 20.0 11.6 97.9 Invention 99 _________ 93.4 -47.1 70.0 2.3 956 18.3 11.9 94.1 Invention 3R ________ 95.7 54.5 718 2.0 1077 17.4 9.5 -84. 4 Invention 101 3S ________ 94.4 -44.0 70.8 1.7 910 19.2 11.0 98.9 Invention 102 3T _________ 97.8 -27.3 73.4 2.1 -659 26.6 20.1 66.6 Comparison 103 3U ________ 95.9 58.2 72.0 2.3 1132 16.6 7.9 80.2 Invention 104 3V _________ 93.3 43.6 70.0 1.9 914 20.4 11.6 99.5 Invention 3W ________ 96.7 -58.2 72.5 2.6 1273 13.7 5.5 86.7 Invention 106 3X _________ 92.1 -31.7 69.1 1. 8 735 25.1 16.9 124.2 Invention 107 3Y ________ 96.4 47.3 72.3 1.7 960 18.2 10.9 73.8 Comparison 108 3Z _________ 95.4 55.9 71.6 2.0 1248 15.1 6.3 82.7 Invention 109 4A _________ 93.1 56.7 69.8 2.4 1205 14.5 6.9 84.7 Invention 4B _________ 90.9 53.2 68.2 2.2 1049 16.7 10.3 85.8 Invention 111 4C ________ 91.2 27.2 68.4 2.0 658 26.6 20.7 136.9 Invention 112 4D _________ 91.5 44.5 68.6 1.9 928 20.1 12.3 68.1 Comparison 113 4E ________ 92.3 37.4 69.3 1.8 820 22.6 15.6 111.0 Invention 114 4F _________ 95.4 54.6 71.5 2.1 1165 15.0 6.3 83.2 Invention 4G ________ 95.0 30.3 71.2 1.9 -704 24.9 18.8 127.8 Invention 116 4H _________ 90.3 26.8 67.7 1.7 653 26.8 20.4 137.9 Invention a: Femte, a': Martensite, Al': Area fraction, TS: Tensile strength, 1.: Hole expanding ratio
TABLE 15
Measurement results ________________________________ Matenal Steel Presence Structu.re parameter _______ _______ Dynamic properties _______ ____ Remarks No. code or absence AfofMD Alof a' AfofcL' Grain size TS Total Local of MD sucture in entire in a grain of a' in (MPa) elongation elongation (%) (%) stnicture of entire entire (%) (%) (%) sti-uctuie slructiire __ __ __ 1%) (sm) _ __ __ _ __ 117 41 ________ 96.9 58.1 72.7 2.2 1132 16.6 7.4 80.2 Invention 118 4.J _________ 95.2 47.0 71.4 2.3 955 18.3 9.7 94.2 Invention 119 4K _________ 91.3 34.2 68.5 2.0 773 23.9 16.7 118.0 Invention - 4L _________ 91.8 40.0 68.9 1.8 860 21.6 13.5 105.9 Invention 121 4M _________ 98.3 40.6 73.7 1.7 859 20.4 13.1 64.8 Comparison 122 4N _________ 94.8 57.1 71.1 2.0 1207 15.6 7.6 85.2 Invention - 123 40 ________ 90.8 394 68.1 2.2 851 21.8 14.2 107.0 Invention 124 4P ________ 96.4 33.7 72.3 2.1 765 24.2 17.0 59.2 Comparison _________ 91.7 48.0 68.7 2.1 971 18.0 11.6 92.7 Invention 126 4R ________ 95.4 55.8 71.5 2.3 1097 17.1 9.2 82.8 Invention 127 4S ________ 96.6 56.9 72.5 2.6 1164 16.2 7.1 78.0 Comparison 128 4T _________ 97.3 49.0 73.0 1. 7 995 18.8 10.3 91.4 Invention 129 4U ________ 90.1 36.9 67.6 2.1 803 21.8 15.5 112.1 Invention 4V _________ 97.7 40.2 73.3 2.0 863 21.5 -13.3 105.5 Invention 131 4W _________ 93.5 51.6 70.1 1.8 1034 18.1 9.2 87.9 Invention 132 4X _________ 93.8 39.5 70.4 1.9 852 21.8 13.6 106.8 Invention 133 4Y ________ 92.2 55.4 69.2 2.1 091 17.2 8.9 83.3 Invention 134 41 ________ 98.9 51.8 74.1 2.0 1037 18.0 9.6 87.6 Invention 5A ________ 90.2 41.7 67. 6 2.6 876 20.0 13.6 102.7 Invention 136 5B _________ 94.4 58.8 70.8 2.5 1133 7.5 4.8 79.5 Comparison 137 SC _________ 97.5 58.9 73.2 1.9 1133 15.4 6.1 88.4 Invention 138 SD _________ 92.2 36.2 69.1 2.0 803 23.1 14.8 113.5 Invention 139 SE _________ 93.6 36.4 70.2 1.7 805 23.0 15.1 113.2 Invention 5F _________ 93.1 49.0 69.9 2.3 985 17.8 10.3 91.4 Invention a: Ferrite, a': Martensite, Af: Area fraction, TS: Tensile strength, ?.: Hole expanding ratio From the results shown in Tables 10 to 15, it can be considered as follows.
The following alphabets all mean steel codes in Tables 1 to 15, and the following Nos. all mean material Nos. in Tables 6 to 15. Nos. 2 to 3, 7 to 16, 19 to 22, 25 to 30, 37 to 42,47to52, 55to59,61 to69,72to79,81to84,87to96,98to 101, lO3to 106, 108 to 111, ll3to 120, 122to 123, 125 to 126, 128to 135 and l37tol40allareexamples that high strength steel sheets having the structure defined in the present invention were produced by the production method defined in the present invention using steel species satisfying the scope of the present invention (lB to 1K, IN to 1Y, 2B to 2M, 2Pto 2W, 2Yto3B, 3Eto 3N, 3Pto 3S, 3Uto 3X, 3Zto4C,4Eto4L, 4Nto4O,4Qto4R,4Tto 5A and 5C to SF in Tables I to 5). The high strength steel sheets shown by the above Nos. all have excellent tensile strength, elongation and stretch flangeability.
Contrary to this, the following examples that are not satisfied with any one of the requirements specified in the present invention have the following disadvantages.
No. 1 is an example using steel code IA having a small C content. Because the proportion of martensite in the whole structure is lower than the range defined, tensile strength was decreased.
Nos. 4 to 6 each use steel code 1D having a component composition which is satisfied with the provision of the present invention. However, the heating and holding temperature of the heat treatment is not A3 point or higher, and the area fraction of a+a' is less than 90%. Therefore, stretch flangeability was decreased.
No. 18 is an example using steel code 1M having a small Si content. Because it has a bainite structure over the entire surface and MD is not present, elongation is low.
Nos. 23 to 24 each use steel code 1R having a component composition which is satisfied with the provision of the present invention, and Nos. 35 to 36 each use steel code iS having a component composition which is satisfied with the provision of the present invention. However, the cooling rate of the heat treatment is slower than the range defined, and as a result, ferrite which is not accompanied with martensite in grains was formed. No. 23 and No. 35 each have a ferrite-pearlite structure, and MD is not present. No. 24 and No. 36 each are that ferrite which is not accompanied with martensite in grains is formed and the area fraction of a+ct' is less than 90%.
Therefore, stretch flangeability is low.
Nos. 31 to 34 each use steel code 1R having a component composition which is satisfied with the provision of the present invention, and Nos. 43 to 46 each use steel code iS having a component composition which is satisfied with the provision of the present invention. However, the cooling rate of the heat treatment is slower than the range defined. Therefore, Nos. 31 and 32 and Nos. 43 and 44 are that lath of bainite does not completely disappear, thereby a bainitic femte-martensite structure is formed and MD is not present. As a result, elongation and stretch flangeability were decreased. Furthermore, No. 33 and 34 and Nos. 45 and 46 each are that a martensite single phase structure is formed, and elongation and stretch flangeability were decreased. No. 53 is an example using steel code IZ having a large Si content.
Because Si was added in a large amount, elongation and stretch flangeability were decreased.
No. 54 is an example using steel code 2A having a small Mn content.
Because pearlite was formed and a+cx' was less than 90%, stretch flangeability is low.
No. 60 uses steel code 2D which is satisfied with the provision of the present invention. However, because the cooling stop temperature exceeds Ms point, a bainite structure was formed on the entire surface, and elongation and stretch flangeability were decreased.
No. 70 is an example using steel code 2N having a large Mn ôontent, and stretch flangeability was decreased. d.
No. 71 is an example using steel code 20 having a small Al content.
Compounds of Si, Mn and 0 were unavoidably formed, and elongation and stretch flangeability were decreased.
No. 80 is an example using steel code 2X having a large Al content. Course martensite was formed in a large amount, and the area fraction ofct' in the whole structure and an average gain size of a' exceeded the defined ranges. As a result, elongation and stretch flangeability was decreased.
No. 85 is an example using steel code 3C having a large Mo content. Because course retained austenite was formed in a large amount and ct-I-a' was less than 90%, stretch flangeability was decreased.
No. 86 is an example using steel code 3D having a small B content. Because ferrite which is not accompanied with martensite in grains was precipitated and a-I-a' was less than 90%, stretch flangeability was decreased.
No. 97 is an example using steel code 30 having a large B content. Stretch flangeability was decreased due to segregation of B. No. 102 is an example using steel code 3T having a large Ti content. Because precipitate was increased, stretch flangeability was decreased.
No. 107 is an example using steel code 3Y having a large Nb content.
Because precipitate was increased, stretch flangeability was decreased.
No. 112 is an example using steel code 4D having a large V content. Because precipitate was increased, stretch flangeability was decreased.
No. 121 is an example using steel code 4M having a large total addition amount of Ti, Nb and V. Because precipitate was increased, stretch flangeability was decreased.
No. 124 is an example using steel code 4P having a large Ni content. Because Ni was added in a large amount, it is liable to break and stretch flangeability was decreased.
No. 127 is an example using steel code 4S having a large Cu content.
Because Cu was added in a large amount, it is liable to break and stretch flangeability was decreased.
No. 136 is an example using steel code 5B having a large Cr content.
Because Cr was added in a large amount, it is liable to break and stretch flangeability was decreased.
Next, examples in the case that the secondary phase is martensite and retained austenite are described below.
Steels A to W having component compositions shown in Table 16 were melted to form slabs. The slabs were heated to 1,200 C, hot rolled to a sheet thickness of 3.0 mm at 800 C and then taken up at 5 50 C. The thermo-mechanical treatment (heating temperature TI ( C), processing temperature T2 ( C), rolling reduction P (%), cooling rate R ( C/sec)) shown in Fig. I and Table 17 was conducted to cool to room temperature, and thickness reduction processing was conducted so as to be a sheet thickness of 1.2 mm.
TABLE 16
Chemical component of specimen Steel Chemical compent (mass%) _____ _______ ___________________ Ac3 Remarks code C Si Mn P S Al Mo B Other ( C) _________ A 0.05 1.11 0.01 0.005' 0.05 0.3 0.0008 Cr:0.65, Ti:0.015 923 Invention B 0.03 *1_ 14 001 0.005 0.05 03 0.0007 _________________ 927 Invention C 0.08 14 14 1 0.005 0.03 0.3 0.0007 _________________ 902 Invention D 0.25 1.5 0.01 0.005 0.05 0 0.0009 __________________ 858 Invention E 0.35 1.7 2.5 1 0.005 0.05 0.3 0.0005 __________________ 827 Comparison F 0.12 0.008 1.4 0.01 0.005 0.05 0.3 0.0005 __________________ 834 Comparison G 0.10 0.03 [A 0.01 0.005 1.1 03 0.0008 __________________ 1262 Invention H 0.05 028 07 0.01 0.005 0.05 0. 3 0.0003 ________________ 893 Invention I 0.07 3.6 2.5 0.01 0.005 0.05 0.3 0.0008 __________________ 979 Comparison J 0.05 0.4 03 0.01 0.005 0.06 0 0.0005 __________________ 904 Comparison K 0.09 1.5 3.6 0.01 0.005 0.05 0.3 0.0008 _________________ 845 Comparison L 0.05 0 22 0.01 0.005 0.005 03 0.0008 _________________ 818 Comparison M 0.05.5 1.5 0.01 0.005 0.05 0 0.001 __________________ 914 Invention N 0.05.4 1.4 0.01 0.005 0.05 0.7 0.0006 __________________ 934 Invention 0 0.07.2 22 0.01 0.005 0.05 1.3 0.0008 __________________ 912 Comparison P 0.05 2 c9 0.01 0.005 0.05 0.1 0 __________________ 921 Comparison ____ 0.05 5 -.4 0.01 0.005 0.05 0 0. 0035 _________________ 917 Invention R 0.05 5 -3 0.0 0.005 0.05 0.3 0.007 __________________ 929 Comparison S 0.10 1.3 0.01 0.005 0.05 0.3 0.0008 Ti:0.015,Nb:0.012 912 Invention T 0.05 1 -3 OM 0.005 0.05 0.3 0.0008 V:0.0012 926 Invention U 0.05 1.4 -3 0.01 0.005 0.05 0.3 0.0008 Ni:0.1, Cu:0.1 921 Invention V 0.07 1.2 2 0.01 0.005 0.05 03 0.0008 Ca:0.002, REM:0.002 910 Invention W 0.01 0.5 0.01 0.005 0.05 0.3 0.0008 _________________ 913 Comparison X 0.01 0.3 18 0.01 0.005 0.05 0.3 0.0007 __________________ 886 Comparison
TABLE 17
_____Heat treatment condition ________ Material Steel Heat treatment condition ______ _______ Remarks No. code Heating Heating Rolling Cooling temperature temperature reduction temperature TI T2 P R ______ ( C) fC) (%) ( C) _________ 141 A 1200 900 30 10 invention 142 B 1200 900 30 10 Invention 143 C 1250 800 30 10 Invention 144 D 1200 800 20 5 invention E 1200 950 10 18 Comparison 146 F 1200 750 40 7 Comparison 147 G 1300 700 50 10 Invention 14$ H 1250 800 30 10 Invention 149 I 1200 800 30 15 Comparison J 1250 700 40 0.5 Comparison 151 K 1200 900 20 15 Comparison 152 L 1200 900 30 10 Comparison 153 M 1250 800 30 10 Invent- ion 154 N 1200 650 30 0.5 Invention 0 1200 900 10 15 Comparison 156 P 1200 700 30 0.5 Comparison 157 *Q 1200 750 30 0.5 Invention 159 R 1200 900 10 15 Comparison 159 S 1300 700 30 10 Invention T 1300 700 40 10 Invention 161 U 1200 800 30 10 invention 162 V 1200 -700 30 10 Invention 163 W 1200 700 30 0.5 Comparison 164 C 1200 550 30 I Cornjarison C 1300 1050 10 0.5 comparison 166 C 1200 900 2 15 Comparison 167 C 1200 900 10 0.1 Comparison 168 C 1200 900 10 25 Comparison 169 X 1250 900 50 0.5 Comparison Microstructure observation was conducted on each steel sheet obtained above in the following procedures, and dynamic properties were evaluated by a tensile test and a hole expanding test.
Microstructure of each steel sheet was identified by the following method.
Each steel sheet was cut into a test piece for microstructure observation of 10 mm x 10 mm x 1.2 mm. The test piece was embedded in a cold rolled resin, and a position of 1/4 of a sheet thickness in a rolled direction was observed. In such a case, impression as a landmark was formed with a Vickers tester so as to conduct identification on the structure observation place. The impression was corroded with repeller, and the structure was observed at five positions using an optical microscope with a magnification of 1,000. When the structure photograph after repeller corrosion was image analyzed, ferrite was observed gray, and martensite and retained austenite were observed white. After microstructure observation with an optical microscope, buff polishing and electrolysis polishing were conducted to an extent such that the Vickers impression does not disappear, and structure observation was conducted on the place at a step distance of 100 nm using FE/SEM-EBSP. A boundary having an orientation difference of 15 or more of grains was considered as grain boundary, and thus the grain boundary was identified.
Optical microphotograph and FE/SEMEBSP structure photograph were combined as the standard of Vickers impression, and the structure was evaluated.
MD structure: The MD structure contains fine martensite or retained austenite (secondary phase) in ferrite grains as a matrix, as described above. The observation results of optical microscope and FE/SEM-EBSP were combined, a structure in which many martensite or retained austenite are present in the inside of grains surrounded by large tilt grain boundary (i.e., in the grains) was identified as MD structure, and its space factor was obtained.
There is the possibility that pearlite, bainite, ferrite which is not accompanied with martensite or retained austenite in grains, and the like are contained as other structures. Identification method of those structures is as follow.
Pearlite: Pearlite has a lameHa structure of ferrite and cementite, and the cementite can be identified with FE/SEM-EB SP. In the Examples, the space factor of pearlite in the observation view was determined.
Bainite: Bainite is that lath is present in ferrite grains identified with FE/SEM-EBSP.
Structure in which lath is present in ferrite grains was judged as bainite, and space factor of bainite in the observation view was determined.
Ferrite which is not accompanied with martensite or retained austenite in grains: Structure in which martensite or retained austenite cannot be confirmed in ferrite grains identified with FE/SEM-EBSP is considered as ferrite which is not accompanied with martensite or retained austenite in grains, and is judged as a structure different from MD structure. Space factor of ferrite which is not accompanied with martensite or retained austenite in grains in the observation view was determined.
Structure in which MD structure is not observed in the observation view is judged that MD is not present. When the whole structure is defined as 100%, in the case that the total of space factors of other structures (pearlite, bainite, ferrite which is not accompanied with martensite or retained austenite in grains, and the like) exceeds 10%, the defined structure is not formed. In this case, it was judged that MD is partial.
Proportion that secondary phase in the entire phase occupies in the whole structure: Average grain size of ferrite in the entire phase: Proportion of secondary phase present in ferrite grains in Ml) structure: First, the space factor of the secondary phase (martensite and retained austenite) in the whole structure used a space factor of white portion in the whole structure by image analyzing an optical microphotograph. Next, an average grain size (diameter of a corresponding circle) of ferrite was obtained using an image of FE/SEM-EBSP, and by combining with the optical microphotograph, the secondary phase present in the MD structure was separated into the secondary phase present in ferrite grains and the secondary phase present at grain boundary, thereby the space factor of the secondary phase present in grains of the whole secondary phase present in the MD structure was obtained.
Proportion that retained austenite in the whole structure occupies in the whole structure: On the other hand, martensite and retained austenite can be distinguished by identifying retained austenite having an FCC structure with FE/SEM-EBSP. However, retained austenite is an important structure in the present invention, and quantitative measurement of the space factor is indispensable to effectively exhibit the effect of the present invention. For this reason, a method of measuring a space factor of retained austenite by analyzing an image of FE/SEM-EBSP is considered, but in the Examples, the space factor of retained austenite was calculated by a saturated magnetization method which can obtain a measurement value of further high accuracy. It is considered that the value measured by the saturated magnetization method is a volume fraction, but the volume fraction can be regarded as being equivalent with an area fraction.
Average grain size and number of secondary phase in the whole structure: On the other hand, the average grain size (average particle size of secondary phase particles) and the number of the secondary phase (martensite and retained austenite) were obtained by the following methods. Regarding the average grain size, five places of observation view of 20 tm x 20 im are randomly extracted from SEM observation photograph (magnification; 3,000), average grain sizes (diameters of colTesponding circles) of martensite and retained austenite in the respective observation views are obtained, and the average value thereof was used as an average grain size of martensite and retained austenite in the whole structure. Regarding the number, five places of observation view of 50 p. m x 50 p.m are randomly extracted from the above optical microphotograph, and the numbers of the secondary phase particles observed thereon were averaged.
Various dynamic properties of steel were obtained by the following methods.
Tensile properties: Using JIS No. 5 test piece obtained from a vertical direction of a steel sheet rolled direction, tensile strength (TS), uniform elongation (u-EL), local elongation (1-EL) and total elongation (t-EL -u-EL + 1-EL) were measured according to JIS Z 2241.
Test pieces having IS of 590 MIPa or more and t-EL of 10% or more were considered as acceptable products. Strain amount of from the maximum load point to breakage was considered as local elongation.
Stretch flangeability: Hole expanding ratio X was measured as stretch flangeability. The hole expanding ratio was measured according to 11SF (The Japan Iron and Steel Federation) standard (JFST 100 1-1996). Steels having of 80% or more were considered as acceptable products.
The results obtained are shown in Tables 18 and 19 below. *:
TABLE 18
-Measurement results -Test Steel Presence Structure aranieter ______ ______ _______ _______ ______ Dnicpoperties _______ _______ Remarks No. code or Af of Al of Af of Average Average Number Vf of y TS Total Uniform Local absence MD second second gram grain of in (MPa) elongation elongation elongation (%) of Mt) structure structure structure size of size of secondary entire (%) (%) (%) (%) in entire in a in secondary phase structure structure a grain entire phase in entire (%) (%) in structure in entire structure MD (jim) structure structure (jim) 141 A ________ 96.6 28.5 78.4 11.3 0.4 153 0.5 870 25.8 9.2 16.6 101.7 Invention 142 B _______ 95.8 26.1 78.7 11.7 0.6 144 0.3 618 29.5 12.1 17.4 103.4 Invention 143 C ________ 96.9 25.8 74.5 9.6 0.3 168 0.8 784 27.7 13.9 13.8 93.9 Invention 144 D ________ 953 29.4 74.0 9.2 1.4 104 8.8 1205 22.8 9.9 12.9 95.1 Invention E None _______ _______ _______ _______ ________ 1298 4.9 1.8 3. 1 19.3 Comparison 146 F None _______ _______ _______ _______ ________ 992 9.6 7.4 2.2 45.2 Comparison 147 G _______ 97.1 27.3 71.9 6.8 0.7 139 7.3 892 27.4 15.8 11.6 88.6 Invention 148 H _______ 96.5 25.4 74.9 9.5 0.5 161 1.0 811 26.9 12.7 14.2 962 Invention 149 I None _______ _______ _______ _______ ________ 1318 3.1 1.0 2.1 175 Comparison 5 Partial 68.7 17.3 73.4 13.5 0.7 121 1.7 433 30.4 -21.9 8.5 67.5 Comparison 151 K Partial 83.1 31.7 72.5 12.3 3.2 134 0.7 1179 8.4 3.4 5.0 31.3 Comparison 152 L Partial 86.5 22.7 70.2 11.6 0.6 140 0.0 682 9.5 7.7 1.8 327 Comparison 153 M _______ 95.6 24.9 75.5 9.9 0.4 166 0.8 844 26.2 13.2 13.0 942 Invention 154 N ________ 94.2 26.4 72.9 7.8 0.4 159 10 852 24.9 12.8 12.1 93.4 Invention 0 Partial 65.8 62.8 76.2 11.2 4.1 73 0.8 1158 8.5 4.2 4.3 30.6 Comparison a: Ferrite y: Retained austenite, Af: Area fraction, Vf Volume fraction, TS: Tensile strength, X: Hole expanding ratio
TABLE 19
_________ Measurement results Test Steel Presence Structure arameter _______ _______ ________ ________ _______{ Dynamic properties Remarks No. code or Al of Mof Al of Average Average Number Vf of y TS Total Uniform Local absence MD second second grain grain of in (MPa) elongation elongation elongation (%) of MD structure structure structure size of size of secondary entire (%) (%) (%) (%) in entire in in secondary phase structure structure a grain entire phase in entire (%) () in structure in entire structure M.D (urn) structure structure (nm) ___ ___ ___ (%) ___ ____ ____ ___ ___ ____ ____ ____ __ _____ 156 P Partial 63.2 16.4 72.5 13.8 0.7 131 1.7 519 27.7 18.5 9.2 64.7 Comparison 157 _______ 95.5 25.9 76.3 10.1 0.7 149 0.9 993 23.4 12.8 10.6 92.8 Invention 158 R Partial 88.7 24.1 75.2 10.9 0.7 141 0.9 850 18.6 15.0 3.6 36.2 Comparison 159 S _______ 95.6 28.3 70.4 5.4 0.3 172 4.2 987 28.6 18.4 10.2 993 Invention 1.60 T ________ 95.2 27.8 71.0 5.9 0.4 169 4.1 963 28.5 18.3 10.2 100.1 Invention 161 U ________ 93.8 25.9 73.9 9.5 0.5 158 5.8 940 25.4 15.8 9.6 89.9 Invention 1.62 V _______ 96.7 25.7 75.4 8.6 0.6 146 5. 1 699 29.3 18.5.10.8 97.5 Invention 163 W Partial 87.4 22.8 71.6 11.9 0.6 138 1.5 512 27.0 16.7 10.3 89.5 Comparison 164 C Partial 56.7 11.6 72.4 17.5 0.8 99 0.8 719 24.8 15.9 8.9 78.9 Comparison C ________ 94.9 27.5 78.3 22.4 1.1 118 0.8 767 23.9 14.8 9.1 76.3 Comparison 166 C Partial 71. 1 57.3 72.7 15.8 3.6 103 1.1 1087 10.4 8.4 2.0 33.8 Comparison 167 C Partial 60.4 16.5 70.1 17.2 0.7 74 0.9 630 2.6.9 18.8 8.1 77.3 Comparison 168 C None _______ _______ _______ _______ ________ ________ _______ 1302 3.2 0.9 2.3 21.9 Comparison 169 X 92.4 9.2 45.8 8.5 0.6 82 0.0 487 32.2 18.6 13.6 93.2 Comparison a: Ferrite,?: Retained austenite, AL Area fraction, Vf: Volume fraction, TS: Tensile strength, ?.: Hole expanding ratio Test Nos. 141, 142, 143, 144, 147, 148, 153, 154, 157, 159, 160, 161 and 162 each were satisfied with the provision of the present invention in structure morphology, and showed good properties in all of tensile strength TS, total elongation t-EL and hole expanding ratio. Average grain sizes of the secondary phases are all in a range of 3 5.tm or less.
On the other hand, No. 145 used steel E having C content larger than the range defined in the present application. As a result, nearly the entire surface was converted into martensite structure, and total elongation and X were deficient.
No. 146 used steel F having Si content smaller than the range defined in the present application. As a result, the structure of bainite + martensite was formed, and total elongation and X were deficient.
No. 149 used steel I having Si content larger than the range defined in the present application. As a result, nearly the entire surface was converted into martensite structure, and total elongation and ? were deficient.
No. 150 used steel J having Mn content smaller than the range defined in the present application. As a result, hardenability was poor, and ferrite was precipitated.
As a result, TS and were deficient.
No. 151 used steel K having Mn content larger than the range defined in the present application. As a result, the structure of MD + martensite was formed, and total elongation and X were deficient.
No. 152 used steel L having Al content smaller than the range defined in the present application. As a result, compounds of Si, Mn and 0 were unavoidably formed, and total elongation and were deficient.
No. 155 used steel 0 having Mo content larger than the range defined in the present application. As a result, the structure of MD + martensite was formed, and total elongation and were deficient. r,.
No. 156 used steel P having B content smaller than the range defined in the present application. As a result, ferrite was precipitated, and TS and A. were deficient.
No. 158 used steel R having B content larger than the range defined in the present application. As a result, deterioration of A. which was considered due to grain segregation was generated.
No. 163 used steel W having C content smaller than the c range defined in the present application. As a result, ferrite was precipitated, and TS was deficient.
No. 164 was that the processing temperature T2 is lower than the range defined in the present application. As a result, ferrite was precipitated, and A. was decreased.
No. 165 was that the processing temperature T2 is higher than the range defined in the present application. As a result, the average grain size of the matrix ferrite is larger than the prescribed range, and A. was decreased.
No. 166 was that rolling reduction is lower than the range defined in the present application. As a result, bainite transformation is not accelerated, thereby forming the structure of MD + martensite. As a result, A. was decreased.
No. 167 was that the cooling rate was lower than the range defined in the present application. As a result, ferrite was precipitated, and A. was decreased.
No. 168 was that the cooling rate is higher than the range defined in the present application. As a result, martensite structure was formed, and total elongation and A. were decreased.
No. 169 used steel X having C content smaller than the range defined in the present application. As a result, the proportion of the secondary phase in the whole structure was reduced, and TS was decreased.
While the present invention has been described in detail and with reference to specific embodiments thereof, it will be apparent to one skilled in the art that various changes and modifications can be made therein without departing from the scope thereof This application is based on Japanese Patent Application No. 2006-283517 filed October 18, 2006 and Japanese Patent Application No. 2006-283579 filed October 18, 2006, the entire contents thereof being hereby incorporated by reference.
Further, all references cited herein are incorporated in their entireties.
Claims (21)
- WHAT IS CLAIMED IS: 1. A high strength steel sheet having a structurewhich is mainly composed of MD structure (Micro Duplex structure) comprising a ferrite matrix, and as a secondaiy phase, martensite or martensite and retained austenite, finely dispersed in said matrix, wherein the proportion that the MD structure occupies in the whole structure is 90% or more, wherein the proportion that the secondary phase present in the whole structure occupies in the whole structure is from 10 to 60%, wherein the secondary phase in the MD structure is present in ferrite grains and at grain boundary, in which the proportion of the secondary phase present in the ferrite grains is 50% or more, and wherein the average grain size of the secondary phase in the whole structure is 3.tm or less.
- 2. The high strength steel sheet as claimed in claim 1, wherein the secondary
- 3. The high strength steel sheet as claimed in claim 1, wherein the secondary phase is constituted of martens ite and retained austenite.
- 4. The high strength steel sheet as claimed in claim I or 2, wherein an average grain size of ferrite in the whole structure is 20 J. tm or less, and wherein 20 or more secondary phases on the average are present in an observation view of 50 m x 50 p.m in the whole structure.
- 5. The high strength steel sheet as claimed in claim I or 3, wherein an average grain size of ferrite in the whole structure is 20 im or less, and wherein 20 or more secondary phases on the average are present in an observation view of 50 p.m x 50 p.m in the whole structure.
- 6. The high strength steel sheet as claimed in claim 1, 3, or 5, wherein the proportion that the retained austenite present in the whole structure occupies in the whole structure is 2% or more.
- 7. The high strength steel sheet as claimed in claim 1, 2, or 4, comprising, in terms of % by mass, C: 0.02 to 0.3%; Si: 0.01 to 3%; Mn: 0.5 to 3%; B: 0.0001 to 0.005%; and Al: 0.01 to 1.5%, with the remainder being Fe and inevitable impurities.
- 8. The high strength steel sheet as claimed in claim 1, 3, 5 or 6 comprising, in terms of % by mass, C: 0.02 to 0.3%; Si: 0.01 to 3%; Mn: 0.Sto3%; B: 0.0001 to 0.005%; and Al: 0.01 to 1.5%, with the remainder being Fe and inevitable impurities.
- 9. The high strength steel sheet as claimed in claim 7, further comprising Mo: 0.03 to 1%.
- 10. The high strength steel sheet as claimed in claim 8, further comprising Mo: 0.03 to 1%.
- 11. The high strength steel sheet as claimed in claim 7 or 9, further comprising at least one element selected from the group consisting of Nb, Ti and V in the total amount of from 0.01 to 0.1%.
- 12. The high strength steel sheet as claimed in claim 8 or 10, further comprising at least one element selected from the group consisting of Nb, Ti and V in the total amount of from 0.01 to 0.1%.
- 13. The high strength steel sheet as claimed in claim 7, 9 or 11, further comprising at least one of Ni: 0.5% or less, and Cu: 0.5% or less.
- 14. The high strength steel sheet as claimed in claim 8, 10 or 12, further comprising at least one of Ni: 0.5% or less, and Cu: 0.5% or less.
- 15. The high strength steel sheet as claimed in claim 7, 9, 11 or 13, further comprising Cr: 1.5% or less.
- 16. The high strength steel sheet as claimed in claim 8, 10, 12 or 14, further.comprising Cr: 1.5% or less.
- 17. The high strength steel sheet as claimed in claim 7, 9, 11, 13 or 15, fuither comprising at least one of Ca: 0.003% or less, and REM: 0.003% or less.
- 18. The high strength steel sheet as claimed in claim 8, 10, 12, 14 or 16, further comprising at least one of Ca: 0.003% or less, and REM: 0.003% or less.
- 19. A method for producing a high strength steel sheet, which comprises: heating a steel sheet material comprising, in terms of % by mass, C: 0.02 to 0.3%, p. Si: 0.01 to 3%, Mn: 0.5 to 3%, B: 0.0001 to 0.005%, and Al: 0.01 to 1.5%, with the remainder being Fe and inevitable impurities; and cooling the steel sheet material from a temperature of A3 point or higher to a temperature of Ms point or lower in a cooling rate of from 0.2 to 20 C/sec.
- 20. A method for producing a high strength steel sheet, which comprises: cooling, from a temperature of A3 point or higher, a steel sheet material comprising, in terms of% by mass, C: 0.02 to 0.3%, Si: 0. 01 to 3%, Mn: 0.5 to 3%, B: 0.0001 to 0.005%, and Al: 0.01 to 1.5%, with the remainder being Fe and inevitable impurities; subjecting said material to a processing of rolling reduction of 5% or more in a temperature range of from 600 to 1,000 C; and then cooling said material to a temperature of Ms point or lower in a cooling rate of from 0.2 to 20 C/sec.
- 21. A steel sheet substantially as described hereinbefore.
Applications Claiming Priority (2)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| JP2006283517A JP4725973B2 (en) | 2006-10-18 | 2006-10-18 | High strength steel plate with excellent stretch flangeability and method for producing the same |
| JP2006283579A JP4743076B2 (en) | 2006-10-18 | 2006-10-18 | High strength steel plate with excellent elongation and stretch flangeability |
Publications (3)
| Publication Number | Publication Date |
|---|---|
| GB0720437D0 GB0720437D0 (en) | 2007-11-28 |
| GB2445749A true GB2445749A (en) | 2008-07-23 |
| GB2445749B GB2445749B (en) | 2009-06-03 |
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| Application Number | Title | Priority Date | Filing Date |
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| GB0720437A Expired - Fee Related GB2445749B (en) | 2006-10-18 | 2007-10-18 | High strength steel sheet and method for producing the same |
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| Country | Link |
|---|---|
| US (1) | US20080178972A1 (en) |
| KR (1) | KR100960775B1 (en) |
| GB (1) | GB2445749B (en) |
Cited By (2)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| WO2010015251A2 (en) * | 2008-08-08 | 2010-02-11 | Sms Siemag Aktiengesellschaft | Method for producing a semi-finished product, especially steel strip, having a dual-phase microstructure |
| EP1633894B1 (en) | 2003-06-18 | 2017-04-26 | SMS group GmbH | Method and installation for the production of hot-rolled strip having a dual-phase structure |
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| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| JP4998756B2 (en) | 2009-02-25 | 2012-08-15 | Ｊｆｅスチール株式会社 | High-strength hot-dip galvanized steel sheet excellent in workability and manufacturing method thereof |
| EP2374910A1 (en) * | 2010-04-01 | 2011-10-12 | ThyssenKrupp Steel Europe AG | Steel, flat, steel product, steel component and method for producing a steel component |
| US9745639B2 (en) | 2011-06-13 | 2017-08-29 | Kobe Steel, Ltd. | High-strength steel sheet excellent in workability and cold brittleness resistance, and manufacturing method thereof |
| KR101439628B1 (en) * | 2012-10-15 | 2014-09-11 | 주식회사 포스코 | Wear resistant steel and method for manufacturing the same |
| KR101439629B1 (en) * | 2012-10-15 | 2014-09-11 | 주식회사 포스코 | Wear resistant steel having excellent wear-resistance and method for manufacturing the same |
| JP5632947B2 (en) | 2012-12-12 | 2014-11-26 | 株式会社神戸製鋼所 | High-strength steel sheet excellent in workability and low-temperature toughness and method for producing the same |
| KR101505292B1 (en) * | 2013-05-31 | 2015-03-31 | 현대제철 주식회사 | High strength steel and manufacturing method of the same |
| KR101797401B1 (en) * | 2016-12-07 | 2017-11-13 | 주식회사 포스코 | Hot dip zinc-based plated steel sheet having excellent bake hardenability and strain aging resistance at room temperature and method for manufacturing same |
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| EP0969112A1 (en) * | 1997-03-17 | 2000-01-05 | Nippon Steel Corporation | Dual-phase high-strength steel sheet having excellent dynamic deformation properties and process for preparing the same |
| JP2001220641A (en) * | 2000-02-02 | 2001-08-14 | Kawasaki Steel Corp | High strength thin steel sheet and high strength gavlanized thin steel sheet excellent in ductility and low in yield ratio and producing method therefor |
| EP1382703A2 (en) * | 2002-07-10 | 2004-01-21 | Nippon Steel Corporation | Steel pipe having low yield ratio |
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| JPS595650B2 (en) | 1980-02-06 | 1984-02-06 | 川崎製鉄株式会社 | Method for manufacturing high-tensile steel sheet with good ductility |
| EP1096029B1 (en) * | 1999-04-21 | 2006-01-25 | JFE Steel Corporation | High tensile hot-dip zinc-coated steel plate excellent in ductility and method for production thereof |
| WO2001081640A1 (en) * | 2000-04-21 | 2001-11-01 | Nippon Steel Corporation | Steel plate having excellent burring workability together with high fatigue strength, and method for producing the same |
-
2007
- 2007-10-16 US US11/873,048 patent/US20080178972A1/en not_active Abandoned
- 2007-10-17 KR KR1020070104346A patent/KR100960775B1/en active IP Right Grant
- 2007-10-18 GB GB0720437A patent/GB2445749B/en not_active Expired - Fee Related
Patent Citations (3)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| EP0969112A1 (en) * | 1997-03-17 | 2000-01-05 | Nippon Steel Corporation | Dual-phase high-strength steel sheet having excellent dynamic deformation properties and process for preparing the same |
| JP2001220641A (en) * | 2000-02-02 | 2001-08-14 | Kawasaki Steel Corp | High strength thin steel sheet and high strength gavlanized thin steel sheet excellent in ductility and low in yield ratio and producing method therefor |
| EP1382703A2 (en) * | 2002-07-10 | 2004-01-21 | Nippon Steel Corporation | Steel pipe having low yield ratio |
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| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| EP1633894B1 (en) | 2003-06-18 | 2017-04-26 | SMS group GmbH | Method and installation for the production of hot-rolled strip having a dual-phase structure |
| WO2010015251A2 (en) * | 2008-08-08 | 2010-02-11 | Sms Siemag Aktiengesellschaft | Method for producing a semi-finished product, especially steel strip, having a dual-phase microstructure |
| WO2010015251A3 (en) * | 2008-08-08 | 2010-07-01 | Sms Siemag Aktiengesellschaft | Method for producing a steel strip having a dual-phase microstructure |
Also Published As
| Publication number | Publication date |
|---|---|
| KR20080035471A (en) | 2008-04-23 |
| US20080178972A1 (en) | 2008-07-31 |
| GB2445749B (en) | 2009-06-03 |
| KR100960775B1 (en) | 2010-06-01 |
| GB0720437D0 (en) | 2007-11-28 |
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