CN103930585A - 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
CN103930585A
CN103930585A CN201280055523.5A CN201280055523A CN103930585A CN 103930585 A CN103930585 A CN 103930585A CN 201280055523 A CN201280055523 A CN 201280055523A CN 103930585 A CN103930585 A CN 103930585A
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steel
quality
phase
rolling
young
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CN103930585B (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
    • 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
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
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    • 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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    • 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
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    • 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
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    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
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    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/08—Ferrous alloys, e.g. steel alloys containing nickel
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    • 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
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    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/14—Ferrous alloys, e.g. steel alloys containing titanium or zirconium
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    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/16—Ferrous alloys, e.g. steel alloys containing copper
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    • 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
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    • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
    • 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. 0.11<=[%C]-(12/47.9)*[%Ti*]<=0.15 ...(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 that is suitable as car body.The thickness of slab Sensitivity Index that high-strength steel sheet of the present invention is suitable for the rigidity such as center pillar, curb girder, bogie side frame and the crossbeam of automobile approaches 1 column or is similar to structure portion's material of cylindrical cross-section shape, therefore has more than 780MPa tensile strength and ductility also excellent.
Background technology
In recent years, along with the care of global environment problem is grown to even greater heights, also carrying out restriction of exhaust etc. for automobile, therefore the lightness of car body is very important problem.Therefore, reduce thickness of slab by the high strength of steel plate, thereby wish realizes the lightness of car body, recently, significantly, the use that has been 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 that the usage rate of 780MPa level, 980MPa grade steel plate has the trend increasing year by year, and in order to utilize such high strength to realize lightness, also needs to improve the reduction of the parts rigidity being caused by thin-walled property simultaneously.The problem that the parts rigidity being caused by the thin-walled property of steel plate reduces is in steel plate more than 590MPa, to become particularly remarkable in tensile strength.
Conventionally, in order to improve the rigidity of parts, effectively change component shape, or the parts that carried out spot welding are increased to weld, or switch to the change of the welding conditions of laser welding etc.
But when the automotive part, the finite space in automobile changes component shape and is also not easy, in addition, change welding conditions and also have the problem that is 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 for portion's material of parts.
Known Young's modulus is mainly by set tissue left and right, during for the steel of body centered structure, the < 111 > directions of the densest direction that belong to atom are the highest, the contrary little < 100 > direction minimums of atomic density.As everyone knows, be about 210GPa left and right in the Young's modulus of the little general iron of crystal orientation anisotropy, as long as but can make crystal orientation there is anisotropy, improve the atomic density of specific direction, just can improve the Young's modulus of this direction.
In the past, about the Young's modulus of steel plate, improve the Young's modulus of specific direction to gather tissue by control and carried out various research.
For example, in patent documentation 1, disclose following technology, that is, used the steel that has added Nb or Ti in ultra-low carbon steel, in hot-rolled process, made Ar 3～(Ar 3+ 150 DEG C) the draft of temperature province be 85% with on 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 meet at right angles Young's modulus of direction of rolling direction.
In addition, in patent documentation 2, disclose the manufacture method of following hot-rolled steel sheet, that is, by being to add Nb or Mo, B in 0.02～0.15% soft steel in C amount, made Ar 3the draft of the temperature province of～950 DEG C be 50% with on develop 211} < 011 > orientation, thus improve Young's modulus.
And, in patent documentation 3 and 4, following technology is disclosed, , use the steel that has added Nb in soft steel, the C amount that regulation is not fixed as carboritride, and in hot-rolled process, make total reduction below 950 DEG C be 30% with on 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, thereby make that { 112} < 110 > are main orientation, improve and the meet at right angles Young's modulus of direction of rolling direction.
Prior art document
Patent documentation
Patent documentation 1: Japanese kokai publication hei 5-255804 communique
Patent documentation 2: Japanese kokai publication hei 8-311541 communique
Patent documentation 3: TOHKEMY 2006-183131 communique
Patent documentation 4: TOHKEMY 2005-314792 communique
Summary of the invention
But, in above-mentioned prior art, there is problem as described below.
; in patent documentation 1 in disclosed technology; by being the ultra-low carbon steel below 0.01% by C amount and controlling set and organize the Young's modulus that improves steel plate; but the tensile strength obtaining is 450MPa left and right at most only; therefore, by applying this technology, to realize further high strength be limited.
In patent documentation 2, there are the following problems for disclosed technology,, owing to being hot-rolled steel sheet as the steel plate of object, control set tissue so cannot utilize cold working, not only be difficult to realize further high Young's modulus, be also difficult to stably manufacture by Low Temperature Finish thickness of slab and be less than the such high tensile steel plate of 2.0mm.
In patent documentation 3, disclosed technology improves tensile strength by increasing alloy addition level and increasing a martensite point rate, but percentage of total elongation reduces, intensity-elongation balance (TS × El) also reduces, so be difficult to improve processibility in realizing high strength.
In addition, in patent documentation 3 and 4 disclosed technology be by hot-rolled process, make total reduction below 950 DEG C be 30% with on improve Young's modulus, but because the rolling load of 950 DEG C of following temperature provinces is high, be difficult to guarantee that total reduction is more than 30% problem so exist.
So, about high Young's modulus, although in existing technology owing to being the hot-rolled steel sheet thicker taking thickness of slab, soft steel plate as object or being high-strength material but lacking ductility or manufacturing has difficulties, so use such prior art, being difficult to make thickness of slab is that the following and TS of 1.6mm is that more than 780MPa high tensile steel plate has high ductibility high Young's modulus simultaneously.
Even if it is below 1.6mm that the object of the invention is to provide a kind of thickness of slab that has solved 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 that meets rolling right angle orientation is the high-strength steel sheet of excellent rigidity more than 240GPa and favourable manufacture method thereof.
The Young's modulus of steel depends primarily on set tissue, during for the ordinary steel of body centered structure, high in the < 111 > directions of the closeest direction as atom, contrary low in the little < 100 > directions of atomic density, if therefore development (112) [1-10] orientation, < 111 > directions are consistent with the rolling right angle orientation of steel plate, so can improve the Young's modulus of this direction.
In addition, the reinforcement of steel has the whole bag of tricks, and for example, the DP steel that the known martensitic phase with hard is strengthened soft ferritic phase has substantially good ductility.But in the ultrahigh-strength steel more than 780MPa, the volume fraction of martensitic phase has the trend totally uprising, therefore not only ductility reduces, and is also difficult to development for effectively (112) [1-10] orientation of Young's modulus that improves rolling right angle orientation.
Therefore, contrivers etc. are in order to address the above problem, be in high-strength steel sheet more than 780MPa at TS, to being studied with the meet at right angles Young's modulus of direction of rolling direction, found that by utilizing solution strengthening, miniaturization strengthening, precipitation strength, even if thereby be under superstrength more than 780MPa at TS, also can suppress martensitic volume fraction compared with lowland, and can improve the set of ferrite to (112) [1-10], when can being high ductibility thus, realizing and have high strength and high rigidization concurrently.
The present invention is based on above-mentioned opinion.
, main composition of the present invention is as described below.
1. a steel sheet, there is following composition: contain below C:0.06～0.12%, Si:0.5～1.5%, Mn:1.0～3.0%, P:0.05%, below S:0.01%, below Al:0.5%, below N:0.01% and Ti:0.02～0.20% in quality %, 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%, and ferritic median size is below 4.0 μ m, and martensitic median size is below 1.5 μ m,
And, the tensile strength (TS) of rolling right angle orientation is more than 780MPa, Young's modulus is more than 240GPa, and it is more than 16500MPa% using tensile strength (TS) and the intensity-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, also contains Nb:0.02～0.10% in quality %, and replaces above-mentioned formula (1) and meet 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, on the basis of above-mentioned composition, also contains and is selected from one kind or two or more in 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 quality %.
4. a steel-sheet manufacture method finishes following steel after finish rolling in hot-rolled process at 850～950 DEG C, batch below at 650 DEG C, after pickling, with more than 60% draft carry out cold rolling after, in annealing operation, with (Ac 1-100 DEG C)～Ac 1average heating speed: 15 DEG C/speed more than s is heated to the soaking temperature of 780～880 DEG C, keeps after the time below 150s in this soaking temperature, and making at least to the average cooling rate of 350 DEG C is that 5～50 DEG C/s is cooled to below 350 DEG C; Wherein, described steel is by forming following composition,, contain below C:0.06～0.12%, Si:0.5～1.5%, Mn:1.0～3.0%, P:0.05%, below S:0.01%, below Al:0.5%, below N:0.01% and Ti:0.02～0.20% in quality %, 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, also contains Nb:0.02～0.10% in quality %, and replaces above-mentioned formula (1) and meet 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, on the basis of above-mentioned composition, also contains and is selected from one kind or two or more in 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 quality %.
According to the present invention, it is more than 780MPa can obtaining tensile strength, and more preferably more than 980MPa, the Young's modulus of rolling right angle orientation is more than 240GPa, and more preferably more than 245GPa, and TS × El meets high-strength steel sheet more than 16500MPa%.
Embodiment
Below, illustrate the present invention.
First the reason that, is limited to above-mentioned scope to the one-tenth of steel plate being grouped in the present invention describes.
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, only uses " % " to represent.
C：0.06～0.12％
C is the element that makes stabilization of austenite, in process of cooling when annealing after cold rolling, improves hardening capacity, significantly promotes the generation that low-temperature phase is covert, thereby can significantly contribute to high strength.In order to obtain such effect, it is more than 0.06% need to making C content.More preferably more than 0.08%.On the other hand, if C content higher than 0.12%, the covert point rate of the low-temperature phase of hard becomes large, not only high strength extremely of steel, processibility is variation also.In addition, in the annealing operation after cold rolling while containing a large amount of C, can suppress the recrystallization in the orientation that is conducive to high Young's modulus.And, while containing a large amount of C, also can cause the deteriorated of weldability.Therefore, need to make C content is below 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 3when above rolling, can promote to process austenitic recrystallization.Therefore,, while containing a large amount of Si that exceedes 1.5%, cannot obtain the required crystal orientation of high Young's modulusization.In addition, add a large amount of Si and not only make the weldability variation of steel plate, and when heating in hot-rolled process, promote the generation of the fayalite of billet surface, encourage the generation of the so-called surface detail that is called as red oxidization skin.And, while use as cold-rolled steel sheet, make chemical convertibility variation at the Si of Surface Creation oxide compound, in addition, while use as hot-dip galvanized steel sheet, cause not plating part at the Si of Surface Creation oxide compound.Therefore, need to make Si content is below 1.5%.Should illustrate, for needing the steel plate, hot-dip galvanized steel sheet of surface texture, preferably making Si content is below 1.2%.
On the other hand, Si is the element that makes 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, thereby can make stabilization of austenite, promote the covert generation of low-temperature phase.And Si also can improve by solution strengthening the intensity of steel.In order to obtain such effect, it is more than 0.5% need to making Si content.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 covert orientation of the low-temperature phase generating in the process of cooling after soaking, can develop the orientation that is conducive to improve Young's modulus, can suppress the reduction of the Young's modulus of following the covert generation of low-temperature phase.
In addition, in the process of cooling after the equal thermal annealing of Mn in annealing operation, by improving hardening capacity, significantly promote the generation that low-temperature phase is covert, thereby also can significantly contribute to high strength.And, by playing a role as solution strengthening element, thereby also contribute to the high strength of steel.In order to obtain such effect, it is more than 1.0% need to making Mn content.
On the other hand, contain while exceeding cooling that a large amount of Mn of 3.0% can be after annealing and significantly suppress ferritic generation, and also can make the weldability variation of steel plate while containing a large amount of Mn.Therefore, making Mn content is below 3.0%.More preferably below 2.5%.
Below P:0.05%
P, at grain boundary segregation, not only makes ductility and the toughness drop of steel plate, and makes also variation of weldability.In addition, while use as alloyed hot-dip galvanized steel plate, produce the unfavorable condition that alloying speed is postponed.Therefore, making P content is below 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 significantly in the time that S amount exceedes 0.01%, and institute is so that S amount is below 0.01%.Should illustrate, consider from the viewpoint of special raising hole expandability, preferably making S amount is below 0.005%.
Below Al:0.5%
Al is ferrite stabilizer, and the Ac3 point while making to anneal significantly rises, and therefore by suppressing from the ferritic austenite phase transformation of not re-crystallization, thereby when the austenite when cooling generates ferrite, hinders the development in the orientation that is conducive to Young's modulus.Therefore, making Al content is below 0.5%.Be preferably below 0.1%.On the other hand, because Al is useful as the deoxidant element of steel, be more than 0.01% so preferably make Al content.
Below N:0.01%
If contain in a large number N, likely in hot rolling, follow steel billet to break and produce surface imperfection.Therefore, need to make N amount is below 0.01%.
Ti：0.02～0.20％
Ti is most important element in the present invention.; in the heat-processed of Ti in annealing operation, process ferritic recrystallization by inhibition, promote from the ferritic austenite phase transformation of not re-crystallization; about the ferrite generating in the process of cooling after annealing, can develop the orientation that the raising of Young's modulus is had superiority.In addition, the small precipitate of Ti contributes to improve intensity, and also brings into play the effect that is conducive to ferrite and martensitic miniaturization.In order to obtain such effect, it is more than 0.02% need to making Ti content.Be preferably more than 0.04%.
On the other hand, even if add a large amount of Ti, when reheating in common hot-rolled process, also cannot make the whole solid solutions of carboritride, residual thick carboritride, therefore hinders ascending effect in intensity, recrystallization inhibition effect on the contrary.In addition, even after continuous casting without in the temporary transient operation reheating after cooling by steel billet, and in the situation that continuous casting directly starts hot rolling afterwards, the addition of Ti to exceed 0.20% part also little to the help of ascending effect in intensity and recrystallization inhibition effect, and also can cause the increase of cost of alloy.Therefore, need to make Ti content is below 0.20%.
Above, essentially consist explanation of the present invention has been described, but in the present invention, it is inadequate only meeting above-mentioned essentially consist, also needs to make the content of 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 has specified the C amount not being fixed as carbide, if but this C amount exceedes 0.10% and exist in a large number, and not only make martensitic point of rate increase, Young's modulus reduces, and ductility is also reduced.The C not being fixed as the carbide amount that therefore, need to make to be calculated by formula (1) is below 0.10%.Be preferably below 0.09%.On the other hand, if the C not being fixed as carbide amount is less than 0.05%, the C amount in the annealing in 2 phase regions after cold rolling in austenite reduces, and then the martensitic phase of cooling rear generation reduces, and therefore high strength more than 780MPa becomes difficult.Therefore it is more than 0.05%, need to making the C amount being fixed as carbide.Be preferably more than 0.06%.
In addition, in the present invention, can suitably contain the element of following narration.
Nb：0.02～0.10％
Nb is the same with Ti, is the important element in the present invention.In heat-processed in annealing operation after cold rolling, process ferritic recrystallization by inhibition, thereby can promote from the ferritic austenite phase transformation of not re-crystallization, suppress in addition the coarsening of austenite grain, and about the ferrite generating in the process of cooling after annealing soaking, can develop the orientation that is conducive to improve Young's modulus.And the small carboritride of Nb also contributes to the rising of intensity.And also performance is conducive to the effect of ferrite and martensitic miniaturization.In order to have such effect, the content that preferably makes Nb is more than 0.02%.
On the other hand, even if add a large amount of Nb, when reheating in common hot-rolled process, also cannot make the whole solid solutions of carboritride, residual thick carboritride, therefore cannot obtain the austenitic recrystallization inhibition effect of processing in hot-rolled process and cold rolling after annealing operation in the ferritic recrystallization inhibition effect of processing.In addition, even after continuous casting by steel billet the temporary transient operation reheating after cooling, and in the situation that continuous casting directly starts hot rolling afterwards, the addition of Nb to exceed 0.10% part also little to the help of recrystallization inhibition, and can cause the increase of cost of alloy.Therefore, preferably making Nb content is below 0.10%.More preferably below 0.08%.
In addition, also contain Nb except containing Ti time, 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 passes through to form carbide, thereby the C amount not being fixed as carbide is reduced.Therefore,, for the C amount that makes not to be fixed as carbide is 0.05～0.10%, while adding Nb, the value of make [%C]-(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 that the generation by suppressing cementite improves hardening capacity, has the effect that significantly promotes the generation of martensitic phase in the process of cooling after the soaking in annealing operation.In order to obtain this effect, preferably contain more than 0.1% Cr.On the other hand, even if add in a large number Cr, not only effect reaches capacity, and also can cause the increase of cost of alloy, therefore preferably to add Cr below 1.0%.In addition, while use as hot-dip galvanized steel sheet, because the oxide compound of the Cr of Surface Creation can cause not plating, be below 0.5% so preferably make Cr content.
Ni：0.1～1.0％
Ni is the element that improves 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.And, in the case of having added the steel of Cu, when hot rolling, because the breaking of reduction of following high-temperature ductility causes surface imperfection, but can suppress the generation of surface imperfection by the compound Ni of containing.In order to obtain such effect, it is more than 0.1% preferably making Ni content.On the other hand, a large amount of Ni is added in the process of cooling after soaking and hinders the required ferritic generation of high Young's modulusization, and cost of alloy increases in addition, preferably to contain Ni below 1.0%.
Mo：0.1～1.0％
Mo is the element that improves hardening capacity, in the process of cooling after the soaking in annealing operation, by promoting the generation of martensitic phase, thereby contributes to high strength.In order to obtain this effect, it is more than 0.1% preferably making Mo content.On the other hand, even if add in a large number Mo, not only effect reaches capacity, and cost of alloy increase, and therefore Mo is preferably to contain below 1.0%.More preferably below 0.5%.
Cu：0.1～2.0％
Cu is the element that improves hardening capacity, in the process of cooling after the soaking in annealing operation, by promoting the generation of martensitic phase, thereby contributes to high strength.In order to obtain this effect, it is more than 0.1% preferably making Cu content.On the other hand, because excessive Cu adds, high-temperature ductility being reduced, cause the surface imperfection of breaking while following hot rolling, is below 2.0% so preferably make Cu content.
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, by promoting martensitic generation to contribute to high strength.In order to obtain this effect, it is more than 0.0005% preferably making B content.On the other hand, the ferrite while significantly hindering cooling after soaking due to the excessive interpolation meeting of B generates, and Young's modulus is reduced, so preferably to contain below 0.0030%.
Next, the restriction reason of tissue of the present invention is described.
Steel plate of the present invention is taking ferritic phase as principal phase, has more than 60% ferritic phase in area occupation ratio, the tissue that contains 15～35% martensitic phase.
Because ferritic phase is effective to being conducive to the development of the set tissue that improves Young's modulus, so need to be as more than 60% taking area occupation ratio.In addition, by containing martensitic phase, improve intensity and intensity-elongation balance, therefore need to contain more than 15% martensitic phase in area occupation ratio.On the other hand, if the area occupation ratio of martensitic phase exceedes 35%, cannot guarantee the Young's modulus of rolling right angle orientation, therefore needing to make the area occupation ratio of martensitic phase is below 35%.And in order to improve intensity-elongation balance, the total amount that need to make the area occupation ratio of ferritic phase and the area occupation ratio of martensitic phase is more than 95%.
As the phase beyond ferritic phase and martensitic phase, can enumerate perlite, bainite and cementite, if these are below 5% mutually, though contain also no problem.Preferably below 3%, more preferably below 1%.
In addition, if ferritic median size exceedes 4.0 μ m, strength decreased, element is added in point rate, the increase that therefore need to increase martensitic phase, causes the reduction of Young's modulus, the rising of manufacturing cost.Therefore, need to make ferritic median size is below 4.0 μ m.Particularly, in order stably to meet tensile strength more than 780MPa, be preferably below 3.5 μ m.
And, if martensitic median size exceedes 1.5 μ m, while being subject to processing distortion, easily carry out the connection in space, the ductility of result steel plate reduces, and therefore, need to make martensitic particle diameter is below average 1.5 μ m.More preferably below 1.0 μ m.
Should illustrate, the area occupation ratio of ferritic phase and martensitic phase is obtained in the following way,, steel plate cross section is being carried out after the corrosion of nitric acid ethanol, observe with sweep electron microscope (SEM), the photo of taking 3 25 μ m territories, μ m × 30, carries out image processing 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 to average area divided by the number of this phase, using the value of its 1/2 power as median size.
Be grouped into and organize by becoming above one-tenth, can obtain the tensile strength (TS) of rolling right angle orientation for more than 780MPa, Young's modulus is more than 240GPa, and intensity-elongation balance (TS × El) is the high-strength steel sheet of the above such excellent rigidity of 16500MPa%.
Next, the preferable production process of steel plate of the present invention is described.
While manufacturing steel plate of the present invention, first, carry out according to target strength grade the steel of the chemical composition of melting based on above-mentioned composition.Melting method can suitably be applied common converter process, electric furnace process etc.The steel of melting is being cast as after steel billet, directly or temporarily heat after cooling, and at final rolling temperature: implement hot rolling under the condition of 850～950 DEG C.Then, batch below at 650 DEG C, after pickling, carry out cold rolling with more than 60% draft.Thereafter, in annealing operation, at (Ac 1-100 DEG C)～Ac 1temperature province with average heating speed: 15 DEG C/more than s speed heating, keep after the time below 150s the soaking temperatures of 780～880 DEG C, making at least to the average cooling rate of 350 DEG C is that 5～50 DEG C/s is cooled to below 350 DEG C.
Below, the reason that is defined as above-mentioned scope by respectively creating conditions is described.
[final rolling temperature: 850～950 DEG C]
Be below 950 DEG C by making final rolling temperature, thereby carry out, from the austenitic ferrite transformation of not re-crystallization, obtaining small ferritic structure, then by cold rolling and annealing, thereby can improve to the aggregation degree in (112) [1-10] orientation.On the other hand, if final rolling temperature lower than 850 DEG C, Ar 3it is large that the possibility that transformation temperature reduces becomes, and hot rolling microstructure is sneaked in worked structure, and result hinders the set in cold rolled annealed backward (112) [1-10] orientation.In addition, because the increase of resistance to deformation is attended by the difficulty in the manufactures such as rolling load significantly increases.Therefore, need to make final rolling temperature is the scope of 850～950 DEG C.
[coiling temperature: 650 DEG C following]
If the coiling temperature after finish rolling is higher than 650 DEG C, the carboritride coarsening of Ti and Nb, the heating phase in the annealing operation after cold rolling, the effect that suppresses the effect of ferritic recrystallization and the coarsening of inhibition austenite grain diminishes, therefore, making coiling temperature is below 650 DEG C.On the other hand, if coiling temperature lower than 400 DEG C, the low-temperature phase of hard is covert a large amount of generates, thereafter the distortion in cold rolling becomes inhomogeneous, hinder to the set in orientation that is conducive to Young's modulus, its result cannot make the set tissue growth after annealing, improves Young's modulus and becomes difficult.And, because the load in cold rolling after batching increases, be more than 400 DEG C so preferably make coiling temperature.
[draft when cold rolling: more than 60%]
After above-mentioned batching, implement to be supplied to after pickling the cold rolling of more than 60% draft.Cold rolling by this, thus make improving favourable (112) [1-10] the orientation set of Young's modulus., by utilizing cold rolling development (112) [1-10] orientation, thereby even also increase the ferrite grain with (112) [1-10] orientation in the tissue after annealing operation thereafter, improve Young's modulus.In order to obtain such effect, it is more than 60% need to making the draft when cold rolling.More preferably more than 65%.On the other hand, large if the draft when cold rolling becomes, rolling load becomes large and manufactures and becomes difficult, therefore, preferably make draft when cold rolling on be limited to 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 in the heat-processed of annealing, suppress to have by the ferritic recrystallization in cold rolling (112) [1-10] orientation developing, become austenite from processing ferritic phase, therefore, need the heat-up rate of 15 DEG C of average out to/more than s.
Wherein, Ac 1the content of C, Si, Mn, Al, Ni, Cr, Cu, Mo, Ti, Nb and B based on representing with quality %, the Ac being 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: 150s are following]
When soaking by annealing operation, make the ferritic phase of q.s become austenite, when cooling, be phase-changed into again ferrite, thereby make to gather tissue growth, improve Young's modulus.In addition, in the situation that soaking temperature is low, rolling structure is residual, and elongation reduces.Therefore it is more than 780 DEG C, need to making soaking temperature.On the other hand, if soaking temperature is too high, austenite grain becomes thick, and after annealing, the ferrite of phase transformation becomes and is difficult to the orientation set at (112) [1-10] when more cooling.Therefore, need to make soaking temperature is below 880 DEG C.
In addition, even in the coarsening that keeps for a long time also can causing austenite grain of this temperature province, therefore needing to make soaking time is below 150s.On the other hand, in order to prevent the residual of rolling structure and to improve elongation, it is more than 15s preferably making soaking time.
[average cooling rate from soaking temperature at least 350 DEG C: 5～50 DEG C/s]
In manufacture method of the present invention, importantly control above-mentioned soaking cooling conditions after treatment.
That is, during by cooling after soaking, make ferrite generate, thereby make improving the favourable set tissue growth of Young's modulus.Therefore, in the time that this is cooling, make more than 60% ferrite generate.For this reason, need to make speed of cooling on be limited to 50 DEG C/s.On the other hand, if cooling excessively slow, do not generate martensite, thus need to make speed of cooling be 5 DEG C/more than s.Be preferably 10 DEG C/more than s.
In addition, the cooling temperature that stops when high, do not generate martensite and generates bainite, perlite, causes the reduction of intensity and the rising of YS/TS ratio.Or, even if generate martensite, also because the tempering in cooling causes martensitic lower hardness, so the help not only intensity being improved diminishes, also can not get good TS-El balance.Therefore, need to carry out cooling at least to 350 DEG C of speed of cooling with regulation.And, in order to make TS-El balance better, preferably carry out cooling with the speed of cooling of regulation till at least 300 DEG C.
Can implement by the processing of overaging band thereafter.In addition, while manufacturing hot-dip galvanized steel sheet, can make steel plate pass through in fused zinc, in addition, when alloying hot-dip galvanized steel sheet, can carry out Alloying Treatment.
Should illustrate, in order to adjust the shape of steel plate, can implement skin-pass, as long as elongation is below 0.8%, just there is not large variation in Young's modulus, tensile properties.Be preferably below 0.6%.
Embodiment
Next, embodiments of the invention are described.Should illustrate, the present invention is not limited only to these embodiment.
Embodiment 1
First, the steel A vacuum melting furnace that the one-tenth shown in table 1 is grouped into carries out melting, after hot rolling, pickling, cold rolling after, implement 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: the thickness of slab after 880 DEG C, hot rolling: 4.4mm, batch condition: with carry out after keeping 1 hour at 600 DEG C stove cold batch suitable processing, 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, cooling till room temperature thereafter: air cooling.This primary condition is shown in to table 2.
And, by (the Ac in the cold rolling draft, the annealing operation that become shown in table 3 in above-mentioned primary condition 1-100 DEG C)～Ac 1heat-up rate, soaking temperature, quenching stop the speed of cooling till temperature and quenching stop temperature.
After above-mentioned annealing, from cutting out the test film of 10mm × 50mm with the rectangular direction of rolling direction of steel plate, use the resonant frequency determinator of lateral vibration type, measure Young's modulus (Ec) according to the benchmark (C1259) of American Society to Testing Materials.In addition, from having implemented the cold-rolled steel sheet of skin-pass of elongation 0.5%, along cutting out JIS5 tension test sheet with the rectangular direction of rolling direction, measure tensile properties (tensile strength TS and elongation El).
Should illustrate, the area occupation ratio (α) of ferritic phase, the area occupation ratio (M) of martensitic phase and the average crystallite particle diameter of each phase utilize aforesaid method to obtain.
Acquired results is recorded in to table 2 and table 3 in the lump.
Table 1
C* is the C amount not being fixed as carbide
(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) of making 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 μ m, martensite particle diameter: 0.8 μ m, intensity-elongation balance are good and have a 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), it is more than 780MPa all can obtaining TS, and TS × El is more than 16500MPa% and Ec is the above so excellent characteristic of 240GPa.
Embodiment 2
In addition, the steel B～N of the composition shown in table 4 is carried out to melting with vacuum melting furnace, carry out successively hot rolling, pickling, cold rolling and annealing by the condition shown in table 5.
The cold-rolled steel sheet obtaining is like this carried out to the research identical with embodiment 1.
Acquired results is shown in to table 5 in the lump.
As shown in table 5, it is that 780MPa is above, TS × El is more than 16500MPa% and Ec is the above so excellent characteristic of 240GPa that the steel plate (steel plate: B～G, L～N) obtaining according to the present invention all obtains TS.
On the other hand, become to be grouped in the comparative example (steel plate: H～K) that has departed from zone of reasonableness of the present invention, at least 1 characteristic in tensile strength (TS), intensity-elongation balance (TS × El), Young's modulus (Ec) becomes.
Utilizability in industry
According to the present invention, can provide that to have both tensile strength be that above such high strength, the Young's modulus of 780MPa is the steel sheet of the above so high rigidity of 240GPa.

Claims (6)

1. a steel sheet, there is following composition: contain below C:0.06～0.12%, Si:0.5～1.5%, Mn:1.0～3.0%, P:0.05%, below S:0.01%, below Al:0.5%, below N:0.01% and Ti:0.02～0.20% in quality %, 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%, and ferritic median size is below 4.0 μ m, and martensitic median size is below 1.5 μ m,
And the tensile strength TS of rolling right angle orientation is more than 780MPa, Young's modulus is more than 240GPa, with the intensity-elongation balance TS × El of the product representation of tensile strength TS and percentage of total elongation El be more than 16500MPa%;
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, also contains Nb:0.02～0.10% in quality %, and replaces described formula (1) and meet 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, on the basis of described composition, also contains and is selected from one kind or two or more in 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 quality %.
4. a steel-sheet manufacture method finishes following steel after finish rolling in hot-rolled process at 850～950 DEG C, batch below at 650 DEG C, after pickling, with more than 60% draft carry out cold rolling after, in annealing operation, with (Ac 1-100 DEG C)～Ac 1average heating speed: 15 DEG C/speed more than s is heated to the soaking temperature of 780～880 DEG C, keep after the time below 150s in this soaking temperature, making at least to the average cooling rate of 350 DEG C is that 5～50 DEG C/s is cooled to below 350 DEG C, wherein, described steel is by forming following composition, , with quality, % contains 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%, following and Ti:0.02～0.20% of N:0.01%, 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 %.
5. steel-sheet manufacture method according to claim 4, wherein, described steel, on the basis of described composition, also contains Nb:0.02～0.10% in quality %, and replaces described formula (1) and meet the relation of following formula (3);
0.05≤[％C]-(12/92.9)×[％Nb]-(12/47.9)×[％Ti*]≤0.10…(3)。
6. according to the steel-sheet manufacture method described in claim 4 or 5, wherein, described steel, on the basis of described composition, also contains and is selected from one kind or two or more in 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 quality %.
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