CN103930585B - Thin steel sheet and process for producing same - Google Patents

Thin steel sheet and process for producing same Download PDF

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Publication number
CN103930585B
CN103930585B CN201280055523.5A CN201280055523A CN103930585B CN 103930585 B CN103930585 B CN 103930585B CN 201280055523 A CN201280055523 A CN 201280055523A CN 103930585 B CN103930585 B CN 103930585B
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steel
phase
rolling
modulus
young
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CN103930585A (en
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河村健二
横田毅
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JFE Engineering Corp
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NKK Corp
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    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • 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
    • 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
    • C21D8/04—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips to produce plates or strips for deep-drawing
    • C21D8/0421—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips to produce plates or strips for deep-drawing characterised by the working steps
    • C21D8/0436—Cold rolling
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    • 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
    • C21D8/04—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips to produce plates or strips for deep-drawing
    • C21D8/0447—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips to produce plates or strips for deep-drawing characterised by the heat treatment
    • C21D8/0463—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips to produce plates or strips for deep-drawing characterised by the heat treatment 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
    • 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
    • C21D8/04—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips to produce plates or strips for deep-drawing
    • C21D8/0447—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips to produce plates or strips for deep-drawing characterised by the heat treatment
    • C21D8/0473—Final recrystallisation annealing
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/001—Ferrous alloys, e.g. steel alloys containing N
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/002—Ferrous alloys, e.g. steel alloys containing In, Mg, or other elements not provided for in one single group C22C38/001 - C22C38/60
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/02—Ferrous alloys, e.g. steel alloys containing silicon
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/04—Ferrous alloys, e.g. steel alloys containing manganese
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/06—Ferrous alloys, e.g. steel alloys containing aluminium
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/08—Ferrous alloys, e.g. steel alloys containing nickel
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/12—Ferrous alloys, e.g. steel alloys containing tungsten, tantalum, molybdenum, vanadium, or niobium
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/14—Ferrous alloys, e.g. steel alloys containing titanium or zirconium
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/16—Ferrous alloys, e.g. steel alloys containing copper
    • 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
    • C22C38/28—Ferrous alloys, e.g. steel alloys containing chromium with titanium or zirconium
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
    • C22C38/38—Ferrous alloys, e.g. steel alloys containing chromium with more than 1.5% by weight of manganese
    • C—CHEMISTRY; METALLURGY
    • C21—METALLURGY OF IRON
    • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
    • C21D2211/00—Microstructure comprising significant phases
    • C21D2211/004—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
    • 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

Abstract

A thin steel sheet that has a composition which contains, in terms of mass%, 0.06-0.12% C, 0.5-1.5% Si, 1.0-3.0% Mn, up to 0.05% P, up to 0.01% S, up to 0.5% Al, up to 0.01% N, and 0.02-0.20% Ti and which satisfies relationships (1) and (2), with the remainder comprising Fe and incidental impurities, and that comprises, in terms of areal proportion, at least 60% ferrite phase and 15-35% martensite phase, the sum of the ferrite phase and the martensite phase being 95% or more, the ferrite and the martensite having average grain diameters of 4.0 [mu]m or smaller and 1.5 [mu]m or smaller, respectively. The steel sheet has a tensile strength and a Young's modulus, as measured in the direction perpendicular to the rolling direction, of 780 MPa or higher and 240 GPa or higher, respectively, and has a strength-elongation balance of 16,500 MPa% or more. 0.05<=[%C]-(12/47.9)*[%Ti*]<=0.10 ...(1), wherein Ti*=[%Ti]-(47.9/14)*[%N]-(47.9/32.1)*[%S] ...(2)

Description

Steel sheet and manufacture method thereof
Technical field
The present invention relates generally to high-strength steel sheet and the manufacture method thereof of the excellent rigidity being suitable as car body.High-strength steel sheet of the present invention be suitable for the center pillar of automobile, curb girder, the rigidity such as side frame and crossbeam thickness of slab Sensitivity Index close to 1 column or be similar to the structure portion material of cylindrical cross-section shape, therefore there is the tensile strength of more than 780MPa and ductility is also excellent.
Background technology
In recent years, grow to even greater heights along with to the care of global environmental problems, for the restriction etc. that automobile is also being exhausted, therefore the lightness of car body is very important problem.Therefore, reduce thickness of slab by the high strength of steel plate, thus for realizing the lightness of car body, recently, significantly, the use being less than the steel plate of 1.6mm to thickness of slab starts to increase the high strength progress of steel plate.Particularly, tensile strength is 780MPa level, the usage rate of 980MPa level steel plate has the trend increased year by year, and in order to utilize such high strength to realize lightness, also needs the reduction simultaneously improving the parts rigidity caused by thin-walled property.The problem that the parts rigidity caused by the thin-walled property of steel plate reduces is become particularly remarkable in the steel plate of more than 590MPa in tensile strength.
Usually, in order to improve the rigidity of parts, effectively change component shape, or weld is increased to the parts carrying out spot welding, or switch to the change of welding conditions of laser welding etc.
But during as automotive part, the finite space in automobile changes component shape and is not easy, in addition, change welding conditions and also there is the problem being attended by cost increase etc.
Therefore, in order to improve the rigidity of parts under the prerequisite not changing component shape, welding conditions, effectively improve the Young's modulus of the portion's material being used for parts.
Known Young's modulus is mainly by set tissue left and right, during steel for body centered structure, the < 111 > direction belonging to the densest direction of atom is the highest, and the < 100 > direction that contrary atomic density is little is minimum.As everyone knows, be about about 210GPa in the Young's modulus of the little general iron of crystal orientation anisotropy, as long as but crystal orientation can be made to have anisotropy, improve the atomic density of specific direction, just can improve the Young's modulus in this direction.
In the past, about the Young's modulus of steel plate, carry out various research to by the charge-coupled Young's modulus improving specific direction of knitting of domination set.
For example, Patent Document 1 discloses following technology, that is, use the steel that with the addition of Nb or Ti in ultra-low carbon steel, in hot-rolled process, make Ar 3~ (Ar 3+ 150 DEG C) the draft of temperature province be more than 85% promote from the austenitic ferrite transformation of not re-crystallization, thus at hot-rolled sheet stage development { 311} < 011 > orientation and the { ferrite in 332} < 113 > orientation, by cold rolling, full annealed thereafter, make that { 211} < 011 > is main orientation, improves and the Young's modulus in rolling direction direction at a right angle.
In addition, Patent Document 2 discloses the manufacture method of following hot-rolled steel sheet, that is, by being add Nb or Mo, B in the soft steel of 0.02 ~ 0.15% in C amount, make Ar 3the draft of the temperature province of ~ 950 DEG C is more than 50% to develop { 211} < 011 > orientation, thus improve Young's modulus.
And, following technology is disclosed in patent documentation 3 and 4, namely, use the steel that with the addition of Nb in soft steel, regulation is not as the C amount that carboritride is fixed, and in hot-rolled process, make the total reduction of less than 950 DEG C be more than 30% promote from the austenitic ferrite transformation of not re-crystallization, thus at the hot-rolled sheet stage development { ferrite in 113} < 110 > orientation, cold rolling by thereafter, full annealed, thus make that { 112} < 110 > is main orientation, improve the Young's modulus with rolling direction direction at a right angle.
Prior art document
Patent documentation
Patent documentation 1: Japanese Unexamined Patent Publication 5-255804 publication
Patent documentation 2: Japanese Unexamined Patent Publication 8-311541 publication
Patent documentation 3: Japanese Unexamined Patent Publication 2006-183131 publication
Patent documentation 4: Japanese Unexamined Patent Publication 2005-314792 publication
Summary of the invention
But, there is problem as described below in above-mentioned prior art.
Namely, in technology disclosed in patent documentation 1, be the ultra-low carbon steel of less than 0.01% by using C amount and domination set is charge-coupled knits the Young's modulus improving steel plate, but the tensile strength obtained is about 450MPa at most only, therefore, by applying this technology, to realize further high strength be limited.
Disclosed in patent documentation 2, there are the following problems for technology, namely, owing to being hot-rolled steel sheet as the steel plate of object, so cold working cannot be utilized, domination set is charge-coupled knits, not only be difficult to realize further high Young's modulus, be also difficult to stably to be manufactured thickness of slab by Low Temperature Finish and be less than the such high tensile steel plate of 2.0mm.
Technology disclosed in patent documentation 3 improves tensile strength by increasing alloy addition level and increasing a martensite point rate, but percentage of total elongation reduces, strength-elongation balance (TS × El) also reduces, so be difficult to improve processibility while realizing high strength.
In addition, technology disclosed in patent documentation 3 and 4 is by making the total reduction of less than 950 DEG C be more than 30% improve Young's modulus in hot-rolled process, but because the rolling load of the temperature provinces of less than 950 DEG C is high, be difficult to guarantee that total reduction is the problem of more than 30% so exist.
So, about high Young's modulus, in existing technology due to be with the thicker hot-rolled steel sheet of thickness of slab, although soft steel plate is for object or be high-strength material but lack ductility or manufacturing has difficulties, so use such prior art, be difficult to make thickness of slab be below 1.6mm and the high tensile steel plate that TS is more than 780MPa has high ductibility simultaneously high Young's modulus.
Even if it is below 1.6mm that the object of the invention is to provide a kind of thickness of slab solving above-mentioned problem, the tensile strength of rolling right angle orientation is also up to more than 780MPa, more preferably up to more than 980MPa, and the Young's modulus meeting rolling right angle orientation is the high-strength steel sheet of the excellent rigidity of more than 240GPa and favourable manufacture method thereof.
The Young's modulus of steel depends primarily on set tissue, during ordinary steel for body centered structure, high on the < 111 > direction in the closeest direction as atom, contrary low on the < 100 > direction that atomic density is little, if therefore develop (112) [1-10] orientation, then < 111 > direction is consistent with the rolling right angle orientation of steel plate, so can improve the Young's modulus in this direction.
In addition, the reinforcement of steel has various method, and such as, the DP steel that the martensitic phase of known hard strengthens soft ferritic phase has substantially good ductility.But, in the ultrahigh-strength steel of more than 780MPa, the volume fraction of martensitic phase has the trend totally uprised, and therefore not only ductility reduces, and is also difficult to develop for effective (112) [1-10] orientation of the Young's modulus improving rolling right angle orientation.
Therefore, contrivers etc. are in order to solve the problem, be in the high-strength steel sheet of more than 780MPa at TS, Young's modulus with rolling direction direction at a right angle is studied, found that by utilizing solution strengthening, miniaturization strengthening, precipitation strength, even if thus be under the superstrength of more than 780MPa at TS, also martensitic volume fraction can be suppressed lower, and the set of ferrite to (112) [1-10] can be improved, high strength and high rigidization can be had concurrently for realizing while high ductibility thus.
The present invention is based on above-mentioned opinion.
That is, main composition of the present invention is as described below.
1. a steel sheet, there is following composition: in mass % containing C:0.06 ~ 0.12%, Si:0.5 ~ 1.5%, Mn:1.0 ~ 3.0%, below P:0.05%, below S:0.01%, below Al:0.5%, below N:0.01% and Ti:0.02 ~ 0.20%, and meet the relation shown in following formula (1) and formula (2), remainder is made up of Fe and inevitable impurity
Have as undertissue: in area occupation ratio, ferritic phase: more than 60%, martensitic phase: 15 ~ 35%, and the total amount of ferritic phase and martensitic phase is more than 95%, ferritic median size is less than 4.0 μm, and martensitic median size is less than 1.5 μm,
And, the tensile strength (TS) of rolling right angle orientation is more than 780MPa, Young's modulus is more than 240GPa, is more than 16500MPa% by tensile strength (TS) and the strength-elongation balance (TS × El) of the product representation of percentage of total elongation (El);
0.05≤[％C]-(12/47.9)×[％Ti*]≤0.10…(1)
Wherein, Ti*=[%Ti]-(47.9/14) × [%N]-(47.9/32.1) × [%S] ... (2)
[%M] is the content (quality %) of M element
2. the steel sheet as described in above-mentioned 1, wherein, above-mentioned steel plate, on the basis of above-mentioned composition, in mass % also containing Nb:0.02 ~ 0.10%, and replaces above-mentioned formula (1) and meets the relation of following formula (3).
0.05≤[％C]-(12/92.9)×[％Nb]-(12/47.9)×[％Ti*]≤0.10…(3)
3. the steel sheet as described in above-mentioned 1 or 2, wherein, above-mentioned steel plate is on the basis of above-mentioned composition, also one kind or two or more containing what be selected from Cr:0.1 ~ 1.0%, Ni:0.1 ~ 1.0%, Mo:0.1 ~ 1.0%, Cu:0.1 ~ 2.0% and B:0.0005 ~ 0.0030% in mass %.
4. a steel-sheet manufacture method, after following steel is terminated finish rolling in hot-rolled process at 850 ~ 950 DEG C, batches below 650 DEG C, after pickling, the draft with more than 60% carry out cold rolling after, in annealing operation, with (Ac 1-100 DEG C) ~ Ac 1average heating speed: the soaking temperature of heating rate to 780 ~ 880 of 15 DEG C/more than s DEG C, after keeping the time of below 150s, makes the average cooling rate at least to 350 DEG C be that 5 ~ 50 DEG C/s is cooled to less than 350 DEG C in this soaking temperature; Wherein, described steel was made up of following composition, namely, in mass % containing C:0.06 ~ 0.12%, Si:0.5 ~ 1.5%, Mn:1.0 ~ 3.0%, below P:0.05%, below S:0.01%, below Al:0.5%, below N:0.01% and Ti:0.02 ~ 0.20%, and the content of C, N, S and Ti meets the relation shown in following formula (1) and formula (2), remainder is Fe and inevitable impurity;
0.05≤[％C]-(12/47.9)×[％Ti*]≤0.10…(1)
Wherein, Ti*=[%Ti]-(47.9/14) × [%N]-(47.9/32.1) × [%S] ... (2)
[%M] is the content (quality %) of M element.
5. the steel-sheet manufacture method as described in above-mentioned 4, wherein, above-mentioned steel, on the basis of above-mentioned composition, in mass % also containing Nb:0.02 ~ 0.10%, and replaces above-mentioned formula (1) and meets the relation of following formula (3).
0.05≤[％C]-(12/92.9)×[％Nb]-(12/47.9)×[％Ti*]≤0.10…(3)
6. the steel-sheet manufacture method as described in above-mentioned 4 or 5, wherein, above-mentioned steel is on the basis of above-mentioned composition, also one kind or two or more containing what be selected from Cr:0.1 ~ 1.0%, Ni:0.1 ~ 1.0%, Mo:0.1 ~ 1.0%, Cu:0.1 ~ 2.0% and B:0.0005 ~ 0.0030% in mass %.
According to the present invention, can obtain tensile strength is more than 780MPa, is more preferably more than 980MPa, and the Young's modulus of rolling right angle orientation is more than 240GPa, is more preferably more than 245GPa, and TS × El meets the high-strength steel sheet of more than 16500MPa%.
Embodiment
Below, the present invention is illustrated.
First, be described the one-tenth of steel plate being grouped into the reason being limited to above-mentioned scope in the present invention.
Should illustrate, the unit of the content of the element during the one-tenth of steel plate is grouped into is " quality % ", below if not otherwise specified, then only represents with " % ".
C：0.06～0.12％
C is the element making stabilization of austenite, in process of cooling during annealing after cold rolling, improves hardening capacity, significantly promotes the generation that low-temperature phase is covert, thus significantly can contribute to high strength.In order to obtain such effect, need to make C content be more than 0.06%.Be more preferably more than 0.08%.On the other hand, if C content is higher than 0.12%, then point rate that the low-temperature phase of hard is covert becomes large, and steel is extreme high strength not only, and processibility is also deteriorated.In addition, containing the recrystallization that can suppress the orientation being conducive to high Young's modulus in the annealing operation after cold rolling during a large amount of C.Further, containing the deterioration that also can cause weldability during a large amount of C.Therefore, need to make C content be less than 0.12%.
Si：0.5～1.5％
Si is an important element in the present invention.Because Si makes Ar in hot rolling 3transformation temperature raises, so carry out Ar 3during above rolling, can promote to process austenitic recrystallization.Therefore, during containing a large amount of Si more than 1.5%, the crystal orientation needed for high Young's modulus cannot be obtained.In addition, add a large amount of Si and not only make the weldability of steel plate be deteriorated, and during heating in hot-rolled process, promote the generation of the fayalite of billet surface, encourage the so-called generation being called as the surface detail of red oxidization skin.Further, when using as cold-rolled steel sheet, at the Si oxide compound of Surface Creation, chemical convertibility is deteriorated, in addition, when using as hot-dip galvanized steel sheet, causes not coating section at the Si oxide compound of Surface Creation.Therefore, need to make Si content be less than 1.5%.Should illustrating, for the steel plate of surface texture, hot-dip galvanized steel sheet, preferably making Si content be less than 1.2% for needing.
On the other hand, Si is the element making ferrite stabilization, in the process of cooling after two phase region soaking in the annealing operation after cold rolling, by promoting ferrite transformation, make C multiviscosisty in austenite, thus can stabilization of austenite be made, promote the generation that low-temperature phase is covert.Further, Si also can improve the intensity of steel by solution strengthening.In order to obtain such effect, need to make Si content be more than 0.5%.Be preferably more than 0.7%.
Mn：1.0～3.0％
Mn is also an important element in the present invention.Mn is austenite stabilizer element, in the heat-processed in the annealing operation after cold rolling, makes Ac 1transformation temperature reduces, promote from the ferritic austenitic phase transformation of not re-crystallization, for the orientation that the low-temperature phase generated in the process of cooling after soaking is covert, the orientation being conducive to improving Young's modulus can be developed, the reduction of the Young's modulus with the covert generation of low-temperature phase can be suppressed.
In addition, in the process of cooling after the equal thermal annealing of Mn in annealing operation, by improving hardening capacity, significantly promoting the generation that low-temperature phase is covert, thus also significantly can contribute to high strength.Further, by playing a role as solution strengthening element, thus the high strength of steel is also contributed to.In order to obtain such effect, need to make Mn content be more than 1.0%.
On the other hand, can significantly suppress ferritic generation during cooling after annealing containing a large amount of Mn more than 3.0%, and the weldability of steel plate also can be made to be deteriorated containing during a large amount of Mn.Therefore, Mn content is made to be less than 3.0%.Be more preferably less than 2.5%.
Below P:0.05%
P, at grain boundary segregation, not only makes the ductility of steel plate and toughness reduce, and weldability is also deteriorated.In addition, when using as alloyed hot-dip galvanized steel plate, produce the unfavorable condition making alloying velocity lag.Therefore, P content is made to be less than 0.05%.
Below S:0.01%
S makes the ductility in hot rolling significantly reduce and bring out thermal rupture, makes surface texture significantly deteriorated.In addition, because S forms thick MnS as impurity element, ductility and hole expandability are reduced, so preferably reduce as far as possible.These problems become remarkable when S amount is more than 0.01%, so making S measure is less than 0.01%.Should illustrate, improve hole expandability from the viewpoint of special, preferably making S measure is less than 0.005%.
Below Al:0.5%
Al is ferrite stabilizer, makes Ac3 point during annealing significantly increase, and therefore by suppressing from the ferritic austenite phase transformation of not re-crystallization, thus when generating ferrite from austenite during cooling, hinders the development being conducive to the orientation of Young's modulus.Therefore, Al content is made to be less than 0.5%.Be preferably less than 0.1%.On the other hand, because Al is useful as the deoxidant element of steel, so preferably make Al content be more than 0.01%.
Below N:0.01%
If in a large number containing N, then likely break with steel billet and produce surface imperfection in hot rolling.Therefore, needing N is measured is less than 0.01%.
Ti：0.02～0.20％
Ti is most important element in the present invention.Namely, in the heat-processed of Ti in annealing operation, by suppressing the ferritic recrystallization of processing, promote from the ferritic austenite phase transformation of not re-crystallization, about the ferrite generated in the process of cooling after annealing, the orientation that the raising of Young's modulus is had superiority can be developed.In addition, the small precipitate of Ti contributes to improving intensity, and also plays the effect being conducive to ferrite and martensitic miniaturization.In order to obtain such effect, need to make Ti content be more than 0.02%.Be preferably more than 0.04%.
On the other hand, even if add a large amount of Ti, during reheating in common hot-rolled process, also cannot make the whole solid solution of carboritride, residual thick carboritride, therefore hinder ascending effect, recrystallization inhibition effect in intensity on the contrary.In addition, even if without the operation of carrying out reheating after steel billet is temporarily cooled after continuous casting, and when continuous casting directly starts hot rolling afterwards, the help of part more than 0.20% to ascending effect in intensity and recrystallization inhibition effect of the addition of Ti is also little, and also can cause the increase of cost of alloy.Therefore, need to make Ti content be less than 0.20%.
Above, describe essentially consist of the present invention and illustrate, but in the present invention, it is insufficient for only meeting above-mentioned essentially consist, also needs the content making C, N, S and Ti to meet the relation shown in following formula (1) and formula (2).
0.05≤[％C]-(12/47.9)×[％Ti*]≤0.10…(1)
Wherein, Ti*=[%Ti]-(47.9/14) × [%N]-(47.9/32.1) × [%S] ... (2)
[%M] is the content (quality %) of M element
Above-mentioned relational expression defines the C amount be not fixed as carbide, if but this C amount exist in a large number more than 0.10%, then not only make martensitic point of rate increase, Young's modulus reduces, and ductility is also reduced.Therefore, needing the C be not fixed as carbide calculated by formula (1) is measured is less than 0.10%.Be preferably less than 0.09%.On the other hand, if the C amount be not fixed as carbide is less than 0.05%, then the C amount in the annealing in 2 phase regions after cold rolling in austenite reduces, and then the martensitic phase generated after cooling reduces, and therefore the high strength of more than 780MPa becomes difficult.Therefore, needing the C making not to be fixed as carbide to measure is more than 0.05%.Be preferably more than 0.06%.
In addition, can suitably containing the following element described in the present invention.
Nb：0.02～0.10％
Nb and Ti is the same, is the important element in the present invention.In heat-processed in annealing operation after cold rolling, by suppressing the ferritic recrystallization of processing, thus can promote from the ferritic austenite phase transformation of not re-crystallization, suppress the coarsening of austenite grain in addition, and about the ferrite generated in the process of cooling after annealing soak, the orientation being conducive to improving Young's modulus can be developed.Further, the small carboritride of Nb also contributes to the rising of intensity.Further, the effect being conducive to ferrite and martensitic miniaturization is also played.In order to have such effect, the content of Nb is preferably made to be more than 0.02%.
On the other hand, even if add a large amount of Nb, also carboritride whole solid solution cannot be made during reheating in common hot-rolled process, residual thick carboritride, therefore cannot obtain processing austenitic recrystallization inhibition effect in hot-rolled process and cold rolling after annealing operation in the ferritic recrystallization inhibition effect of processing.In addition, even if carry out the operation reheated after temporarily being cooled by steel billet after continuous casting, and when continuous casting directly starts hot rolling afterwards, the part more than 0.10% of the addition of Nb is also little to the help of recrystallization inhibition, and the increase of cost of alloy can be caused.Therefore, Nb content is preferably made to be less than 0.10%.Be more preferably less than 0.08%.
In addition, except containing when also containing Nb except Ti, replace above-mentioned formula (1) and meet the relation shown in following formula (3).
0.05≤[％C]-(12/92.9)×[％Nb]-(12/47.9)×[％Ti*]≤0.10…(3)
Nb by forming carbide, thus makes the C amount be not fixed as carbide reduce.Therefore, in order to make the C amount be not fixed as carbide be 0.05 ~ 0.10%, when adding Nb, make [%C]-value of (12/92.9) × [%Nb]-(12/47.9) × [%Ti*] is 0.05 ~ 0.10%.Be preferably 0.06 ~ 0.09%.
Cr：0.1～1.0％
Cr is the element by suppressing the generation of cementite to improve hardening capacity, has the effect of the generation significantly promoting martensitic phase in the process of cooling after the soaking in annealing operation.In order to obtain this effect, preferably contain the Cr of more than 0.1%.On the other hand, even if add Cr in a large number, not only effect reaches capacity, and also can cause the increase of cost of alloy, therefore preferably adds Cr with less than 1.0%.In addition, when using as hot-dip galvanized steel sheet, because the oxide compound of the Cr of Surface Creation can cause not plating, so preferably make Cr content be less than 0.5%.
Ni：0.1～1.0％
Ni is the element improving hardening capacity, in the process of cooling after the soaking in annealing operation, can promote the generation of martensitic phase.In addition, Ni also contributes to the high strength of steel effectively as solution strengthening element.Further, when with the addition of the steel of Cu, during hot rolling, causing surface imperfection due to the breaking of reduction with high-temperature ductility, but contain Ni by compound and the generation of surface imperfection can be suppressed.In order to obtain such effect, Ni content is preferably made to be more than 0.1%.On the other hand, hinder the ferritic generation needed for high Young's modulus in the process of cooling after a large amount of Ni is added on soaking, cost of alloy increases in addition, preferably contains Ni with less than 1.0%.
Mo：0.1～1.0％
Mo is the element improving hardening capacity, in the process of cooling after the soaking in annealing operation, by promoting the generation of martensitic phase, thus contributes to high strength.In order to obtain this effect, Mo content is preferably made to be more than 0.1%.On the other hand, even if add Mo in a large number, not only effect reaches capacity, and cost of alloy increases, and therefore Mo preferably contains with less than 1.0%.Be more preferably less than 0.5%.
Cu：0.1～2.0％
Cu is the element improving hardening capacity, in the process of cooling after the soaking in annealing operation, by promoting the generation of martensitic phase, thus contributes to high strength.In order to obtain this effect, Cu content is preferably made to be more than 0.1%.On the other hand, because excessive Cu adds, high-temperature ductility is reduced, cause with the surface imperfection of breaking during hot rolling, so preferably make Cu content be less than 2.0%.
B：0.0005～0.0030％
B is the element by suppressing to improve from austenite to ferritic phase transformation hardening capacity, in the process of cooling after the soaking in annealing operation, contributes to high strength by promoting martensitic generation.In order to obtain this effect, B content is preferably made to be more than 0.0005%.On the other hand, because excessive interpolation of B can significantly hinder the ferrite during cooling after soaking to generate, Young's modulus is reduced, so preferably contain with less than 0.0030%.
Next, the restriction reason of tissue of the present invention is described.
Steel plate of the present invention take ferritic phase as principal phase, has the ferritic phase of more than 60%, the tissue containing the martensitic phase of 15 ~ 35% in area occupation ratio.
Because ferritic phase is effective to the development being conducive to the set tissue improving Young's modulus, so need for more than 60% in area occupation ratio.In addition, by containing martensitic phase, improve intensity and strength-elongation balance, therefore need the martensitic phase containing more than 15% in area occupation ratio.On the other hand, if the area occupation ratio of martensitic phase is more than 35%, then cannot guarantee the Young's modulus of rolling right angle orientation, therefore need the area occupation ratio making martensitic phase to be less than 35%.Further, in order to improve strength-elongation balance, the total amount making the area occupation ratio of ferritic phase and the area occupation ratio of martensitic phase is needed to be more than 95%.
As the phase beyond ferritic phase and martensitic phase, perlite, bainite and cementite can be enumerated, if these are less than 5% mutually, even if then containing also no problem.Preferably less than 3%, more preferably less than 1%.
In addition, if ferritic median size is more than 4.0 μm, then intensity reduces, and therefore needs point rate, the increase Addition ofelements that increase martensitic phase, causes the reduction of Young's modulus, the rising of manufacturing cost.Therefore, need to make ferritic median size be less than 4.0 μm.Particularly in order to stably meet the tensile strength of more than 780MPa, be preferably less than 3.5 μm.
Further, if martensitic median size is more than 1.5 μm, then easily carry out the connection in space when being subject to processing distortion, the ductility of result steel plate reduces, and therefore, needs to make martensitic particle diameter be average less than 1.5 μm.Be more preferably less than 1.0 μm.
Should illustrate, the area occupation ratio of ferritic phase and martensitic phase is obtained in the following way, namely, after the corrosion of nitric acid ethanol is carried out to steel plate cross section, observe with sweep electron microscope (SEM), the photo in shooting 3 25 μm × 30 μm of territories, carries out image procossing to these photos, measures the area of ferritic phase and martensitic phase.In addition, median size is that the summation of the ferritic phase of visual field in SEM photo and each area of martensitic phase is obtained average area divided by the number of this phase, using the value of its 1/2 power as median size.
Be grouped into by becoming above one-tenth and organize, the tensile strength (TS) that can obtain rolling right angle orientation is more than 780MPa, Young's modulus is more than 240GPa, and strength-elongation balance (TS × El) is the high-strength steel sheet of the such excellent rigidity of more than 16500MPa%.
Next, the preferable production process of steel plate of the present invention is described.
When manufacturing steel plate of the present invention, first, the steel of melting based on the chemical composition of above-mentioned composition to be carried out according to target strength grade.Melting method can suitably apply common converter process, electric furnace process etc.The steel of melting, after being cast as steel billet, directly or after temporarily cooling heats, at final rolling temperature: implement hot rolling under the condition of 850 ~ 950 DEG C.Then, batch below 650 DEG C, after pickling, the draft with more than 60% carries out cold rolling.Thereafter, in annealing operation, at (Ac 1-100 DEG C) ~ Ac 1temperature province with average heating speed: the heating rate of 15 DEG C/more than s, 780 ~ 880 DEG C soaking temperatures keep below 150s time after, make the average cooling rate at least to 350 DEG C be that 5 ~ 50 DEG C/s is cooled to less than 350 DEG C.
Below, the reason each manufacturing condition being defined as above-mentioned scope is described.
[final rolling temperature: 850 ~ 950 DEG C]
By making final rolling temperature be less than 950 DEG C, thus carry out, from the austenitic ferrite transformation of not re-crystallization, obtaining small ferritic structure, then by cold rolling and annealing, thus the aggregation degree to (112) [1-10] orientation can be improved.On the other hand, if final rolling temperature is lower than 850 DEG C, then Ar 3the possibility that transformation temperature reduces becomes large, and worked structure is mixed into hot rolling microstructure, result, hinders the set in cold rolled annealed backward (112) [1-10] orientation.In addition, the difficulty in the manufactures such as rolling load significantly increases is attended by due to the increase of resistance to deformation.Therefore, final rolling temperature is needed to make to be the scope of 850 ~ 950 DEG C.
[coiling temperature: less than 650 DEG C]
If the coiling temperature after finish rolling is higher than 650 DEG C, the then carboritride coarsening of Ti and Nb, the heating phase in the annealing operation after cold rolling, suppresses the effect of ferritic recrystallization and suppresses the effect of the coarsening of austenite grain to diminish, therefore, coiling temperature is made to be less than 650 DEG C.On the other hand, if coiling temperature is lower than 400 DEG C, then the low-temperature phase of hard in a disguised form generates in a large number, thereafter the distortion in cold rolling becomes uneven, hinder the set to the orientation being conducive to Young's modulus, its result cannot make the set tissue growth after annealing, improves Young's modulus and becomes difficult.Further, because the load in cold rolling after batching increases, so preferably make coiling temperature be more than 400 DEG C.
[draft time cold rolling: more than 60%]
Above-mentioned batch after, implement to be supplied to the cold rolling of the draft of more than 60% after pickling.Cold rolling by this, thus make improving favourable (112) [1-10] the orientation set of Young's modulus.That is, cold rollingly develop (112) [1-10] orientation by utilizing, even thus also increase the ferrite grain with (112) [1-10] orientation in tissue after annealing operation thereafter, improve Young's modulus.In order to obtain such effect, draft when making cold rolling is needed to be more than 60%.Be more preferably more than 65%.On the other hand, if cold rolling time draft become large, then rolling load becomes large and manufactures and become difficult, and therefore, the upper limit of draft when preferably making cold rolling is 85%.
[(Ac 1-100 DEG C) ~ Ac 1average heating speed: 15 DEG C/more than s]
In order to improve the Young's modulus of the steel plate after annealing, need to suppress in the heat-processed of annealing to have by the cold rolling and ferritic recrystallization in (112) [1-10] orientation that is development, austenite is become from processing ferritic phase, therefore, the heat-up rate of average out to 15 DEG C/more than s is needed.
Wherein, Ac 1the content based on C, Si, Mn, Al, Ni, Cr, Cu, Mo, Ti, Nb and the B represented with quality %, the Ac obtained by following formula (4) 1transformation temperature.
Ac 1＝750.8-26.6[％C]+17.6[％Si]-11.6[％Mn]-169.4[％Al]-23.0[％Ni]+24.1[％Cr]-22.9[％Cu]+22.5[％Mo]-5.7[％Ti]+232.6[％Nb]-894.7[％B]…(4)
Wherein, [%M] is the content (quality %) of M element
[soaking temperature: 780 ~ 880 DEG C, soaking time: below 150s]
Make the ferritic phase of q.s become austenite during soaking by annealing operation, be phase-changed into ferrite again during cooling, thus make set tissue growth, improve Young's modulus.In addition, when soaking temperature is low, rolling structure remains, and elongation reduces.Therefore, need to make soaking temperature be more than 780 DEG C.On the other hand, if soaking temperature is too high, then austenite grain becomes thick, and after annealing, during cooling, the ferrite of phase transformation becomes and is difficult in (112) [1-10] orientation set again.Therefore, need to make soaking temperature be less than 880 DEG C.
In addition, even keep the coarsening that also can cause austenite grain for a long time in this temperature province, therefore need to make soaking time be below 150s.On the other hand, in order to prevent remaining and improving elongation of rolling structure, soaking time is preferably made to be more than 15s.
[from soaking temperature to the average cooling rate of at least 350 DEG C: 5 ~ 50 DEG C/s]
In manufacture method of the present invention, importantly control the cooling conditions after above-mentioned equal thermal treatment.
That is, by during cooling after soaking, ferrite is generated, thus make improving the favourable set tissue growth of Young's modulus.Therefore, the ferrite of more than 60% is made to generate when this cooling.For this reason, the upper limit making speed of cooling is needed to be 50 DEG C/s.On the other hand, if slowly cooled, then do not generate martensite, so need to make speed of cooling be 5 DEG C/more than s.Be preferably 10 DEG C/more than s.
In addition, when cooling stops temperature height, do not generate martensite and bainite, perlite, cause the reduction of intensity and the rising of YS/TS ratio.Or, even if generate martensite, also cause martensitic hardness to reduce because of the tempering in cooling, so not only diminish to the help that intensity improves, also can not get good TS-El balance.Therefore, need to cool with the speed of cooling of regulation at least to 350 DEG C.Further, better in order to make TS-El balance, preferably cool with the speed of cooling of regulation till at least 300 DEG C.
Thereafter the process by overaging band can be implemented.In addition, when manufacturing hot-dip galvanized steel sheet, steel plate can be made to pass through in fused zinc, in addition, during alloying heat-transmission dip galvanizing steel sheet, can Alloying Treatment be carried out.
Should illustrating, in order to adjust the shape of steel plate, can implement skin-pass, as long as elongation is less than 0.8%, just there is not large change in Young's modulus, tensile properties.Be preferably less than 0.6%.
Embodiment
Next, embodiments of the invention are described.Should illustrate, the present invention is not limited only to these embodiments.
Embodiment 1
First, the steel A vacuum melting furnace be grouped into by the one-tenth shown in table 1 carries out melting, and after hot rolling, pickling, after cold rolling, implements annealing, make cold-rolled steel sheet.Now, using following condition as primary condition: the heating condition before rolling: 1250 DEG C of heating 1 hour, the final rolling temperature of hot rolling: 880 DEG C, thickness of slab after hot rolling: 4.4mm, coiling condition: with keep 1 hour at 600 DEG C after carry out stove cold batch suitable process, cold rolling draft: 68%, the thickness of slab after cold rolling: 1.4mm, (Ac 1-100 DEG C) ~ Ac 1average heating speed: 20 DEG C/s, soaking temperature: the hold-time at 830 DEG C: 60s, the average cooling rate till 300 DEG C: 15 DEG C/s, the cooling till room temperature thereafter: air cooling.This primary condition is shown in table 2.
Further, by (the Ac in the cold rolling draft become table 3 Suo Shi in above-mentioned primary condition, annealing operation 1-100 DEG C) ~ Ac 1heat-up rate, soaking temperature, the quenching speed of cooling that stops temperature and quenching to stop till temperature.
After above-mentioned annealing, the test film of 10mm × 50mm is cut out from the direction at a right angle with the rolling direction of steel plate, use the resonant frequency determinator of lateral vibration type, the benchmark (C1259) according to American Society toTesting Materials measures Young's modulus (Ec).In addition, from the cold-rolled steel sheet of skin-pass implementing elongation 0.5%, cut out JIS5 tension test sheet along the direction at a right angle with rolling direction, measure tensile properties (tensile strength TS and elongation El).
Should illustrate, the average crystallite particle diameter of the area occupation ratio (α) of ferritic phase, the area occupation ratio (M) of martensitic phase and each phase utilizes aforesaid method to obtain.
Acquired results is recorded in table 2 and table 3 in the lump.
Table 1
C* is not as the C amount that carbide is fixed
(C*＝[％C]-(12/92.9)×[％Nb]-(12/47.9)×[％Ti*])
Wherein, Ti*=[%Ti]-(47.9/14) × [%N]-(47.9/32.1) × [%S]
As shown in table 2, the cold-rolled steel sheet (steel plate: A1) made according to primary condition is TS:1064MPa, El:16.3%, TS × El:17343MPa%, Ec:252GPa, ferritic area occupation ratio: 67%, martensitic area occupation ratio: 33%, ferrite particle diameter: 2.9 μm, martensite particle diameter: 0.8 μm, strength-elongation balance is good and have the cold-rolled steel sheet of high Young's modulus.
In addition, even if change cold rolling draft, annealing conditions, if these conditions meet scope of the present invention (steel plate: A3, A4, A7), then the characteristic of the excellence that all can to obtain TS be more than 780MPa, TS × El to be more than 16500MPa% and Ec be more than 240GPa is such.
Embodiment 2
In addition, steel B ~ N vacuum melting furnace of the composition shown in table 4 is carried out melting, carry out hot rolling, pickling, cold rolling and annealing successively by the condition shown in table 5.
The research identical with embodiment 1 is carried out to the cold-rolled steel sheet obtained like this.
Acquired results is shown in table 5 in the lump.
As shown in table 5, the characteristic of the excellence that the steel plate (steel plate: B ~ G, L ~ N) obtained according to the present invention all obtains TS be more than 780MPa, TS × El to be more than 16500MPa% and Ec be more than 240GPa is such.
On the other hand, become to be grouped in the comparative example (steel plate: H ~ K) having departed from zone of reasonableness of the present invention, at least 1 characteristic in tensile strength (TS), strength-elongation balance (TS × El), Young's modulus (Ec) becomes.
Utilizability in industry
According to the present invention, can provide have both that tensile strength is the such high strength of more than 780MPa, steel sheet that Young's modulus is the such high rigidity of more than 240GPa.

Claims (6)

1. a steel sheet, there is following composition: in mass % containing C:0.06 ~ 0.12%, Si:0.5 ~ 1.5%, Mn:1.0 ~ 3.0%, below P:0.05%, below S:0.01%, below Al:0.5%, below N:0.01% and Ti:0.02 ~ 0.20%, and meet the relation shown in following formula (1) and formula (2), remainder is made up of Fe and inevitable impurity
Have as undertissue: in area occupation ratio, ferritic phase: more than 60%, martensitic phase: 15 ~ 35%, and the total amount of ferritic phase and martensitic phase is more than 95%, ferritic median size is less than 4.0 μm, and martensitic median size is less than 1.5 μm,
Further, the tensile strength TS of rolling right angle orientation is more than 780MPa, and Young's modulus is more than 240GPa, is more than 16500MPa% with the strength-elongation balance TS × El of the product representation of tensile strength TS and percentage of total elongation El;
0.05≤[％C]-(12/47.9)×[％Ti*]≤0.10…(1)
Wherein, Ti*=[%Ti]-(47.9/14) × [%N]-(47.9/32.1) × [%S] ... (2)
[%M] is the content of M element, and unit is quality %.
2. steel sheet according to claim 1, wherein, described steel plate, on the basis of described composition, in mass % also containing Nb:0.02 ~ 0.10%, and replaces described formula (1) and meets the relation of following formula (3);
0.05≤[％C]-(12/92.9)×[％Nb]-(12/47.9)×[％Ti*]≤0.10…(3)。
3. steel sheet according to claim 1 and 2, wherein, described steel plate is on the basis of described composition, also one kind or two or more containing what be selected from Cr:0.1 ~ 1.0%, Ni:0.1 ~ 1.0%, Mo:0.1 ~ 1.0%, Cu:0.1 ~ 2.0% and B:0.0005 ~ 0.0030% in mass %.
4. a steel-sheet manufacture method, after following steel is terminated finish rolling in hot-rolled process at 850 ~ 950 DEG C, batches below 650 DEG C, after pickling, the draft with more than 60% carry out cold rolling after, in annealing operation, with (Ac 1-100 DEG C) to Ac 1average heating speed be the mode of the speed of 15 DEG C/more than s, be heated to the soaking temperature of 780 ~ 880 DEG C, keep the time of below 150s in this soaking temperature after, the average cooling rate at least to 350 DEG C is made to be that 5 ~ 50 DEG C/s is cooled to less than 350 DEG C, wherein, described steel is formed by forming as follows, namely, C:0.06 ~ 0.12% is contained with quality %, Si:0.5 ~ 1.5%, Mn:1.0 ~ 3.0%, below P:0.05%, below S:0.01%, below Al:0.5%, below N:0.01% and Ti:0.02 ~ 0.20%, and C, N, the content of S and Ti meets the relation shown in following formula (1) and formula (2), remainder is Fe and inevitable impurity,
0.05≤[％C]-(12/47.9)×[％Ti*]≤0.10…(1)
Wherein, Ti*=[%Ti]-(47.9/14) × [%N]-(47.9/32.1) × [%S] ... (2)
[%M] is the content of M element, and unit is quality %,
Further, Ac 1the content based on C, Si, Mn, Al, Ni, Cr, Cu, Mo, Ti, Nb and the B represented with quality %, the Ac obtained by following formula (4) 1transformation temperature,
Ac 1＝750.8-26.6[％C]+17.6[％Si]-11.6[％Mn]-169.4[％Al]-23.0[％Ni]+24.1[％Cr]-22.9[％Cu]+22.5[％Mo]-5.7[％Ti]+232.6[％Nb]-894.7[％B]…(4)
[%M] is the content (quality %) of M element.
5. steel-sheet manufacture method according to claim 4, wherein, described steel, on the basis of described composition, in mass % also containing Nb:0.02 ~ 0.10%, and replaces described formula (1) and meets the relation of following formula (3);
0.05≤[％C]-(12/92.9)×[％Nb]-(12/47.9)×[％Ti*]≤0.10…(3)。
6. the steel-sheet manufacture method according to claim 4 or 5, wherein, described steel is on the basis of described composition, also one kind or two or more containing what be selected from Cr:0.1 ~ 1.0%, Ni:0.1 ~ 1.0%, Mo:0.1 ~ 1.0%, Cu:0.1 ~ 2.0% and B:0.0005 ~ 0.0030% in mass %.
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