CN100519805C - High-rigidity/high-strength thin steel sheet and manufacturing method therefor - Google Patents
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Abstract
Disclosed is a high-rigidity high-strength thin steel sheet having a tensile strength of not less than 590 MPa and a Young's modulus of not less than 230 GPa at the same time. The high-rigidity high-strength thin steel sheet has a composition consisting of, in mass%, C: 0.02-0.15%, Si: 1.5% or less, Mn: 1.0-3.5%, P: 0.05% or less, S: 0.01% or less, Al: 1.5% or less, N: 0.01% or less, Ti: 0.02-0.50%, and the balance of iron and unavoidable impurities, wherein the C, N, S and Ti contents satisfy the following relations: Ti<*> = Ti - (47.9/14) x N - (47.9/32.1) x S >= 0.01 and 0.01 <= C - (12/47.9) x Ti<*> <= 0.05. The structure of such a steel sheet is mainly composed of a ferrite phase, and contains not less than 1% of a martensite phase in the area ratio.
Description
Technical field
The present invention relates to mainly be applicable to the high-rigidity/high-strength thin steel sheet and the manufacture method thereof of car body.In addition, high-rigidity/high-strength thin steel sheet of the present invention be the thickness susceptibility index of rigidity near 1 column construction element, as the center pillar of automobile, latching system (locker), body side frame, cross member or the like, and it is widely used in rigidity is had the occasion of requirement.
Correlation technique
Owing to, even also implemented exhaust control, therefore reduce body of a motor car weight and be very important mode in automobile industry recently to the showing great attention to of global environment problem.For this reason, can reduce tare by the intensity that improves steel plate effectively to reduce its thickness.
Recently, owing to significantly improving of armor plate strength, thereby to the steel sheet consumption increase of thickness less than 2.0mm.For the intensity that improves steel plate with further minimizing weight, control simultaneously and be absolutely necessary reduce the reduction that causes parts rigidity because of thickness.To produce in tensile strength is not less than the steel plate of 590MPa and thisly especially be not less than in the steel plate of 700MPa in tensile strength owing to steel plate thickness reduces the problem that causes parts rigidity to reduce, this problem is even more serious.
Usually, in order to improve the rigidity of parts, adopting the change component shape or increasing weld quantity or change welding conditions in the spot welding parts all is effective as switching to laser welding or the like.Yet, when these parts are used for automobile, exist the problem that is not easy to change component shape in the finite space in automobile, and the change of welding conditions causes problem of cost increase or the like.
Therefore, do not change the shape or the welding conditions of parts for the rigidity that improves parts, the Young's modulus that improves the parts material therefor is effective.
Usually, the parts rigidity under same parts shape and welding conditions is characterized by the product of the geometrical moment of inertia of the Young's modulus of material and parts.In addition, when the thickness of material was t, geometrical moment of inertia can be expressed as approximately and t
λProportional.Wherein, λ is a thickness susceptibility index and according to the shape value 1～3 of parts.For example, when being a tabular situation as the car panel parts, the value of λ is near 3, yet when for the situation of column such as structure unit, the value of λ is near 1.
When the λ of parts is 3, if reduce 10% the thickness and the rigidity of holding member equivalently, need the Young's modulus of material is improved 37%, yet when the λ of parts be 1, if reduce by 10% thickness, so that the Young's modulus raising 11% of material is just enough.
That is to say that approach at λ under the situation of 1 parts such as cylindrical part, the Young's modulus that improves steel plate self is very effective to the weight that reduces steel plate.Especially, have when steel plate under the situation of high strength and low thickness, the Young's modulus that improves steel plate energetically is the ten minutes needs.
Generally speaking, Young's modulus depends primarily on tissue (texture), and improves towards the closeest direction of atom.Therefore, in comprising with rolling system and heat treated process for making, for the crystal orientation that makes the body centered structure steel develop Young's modulus is favourable 112}＜110〉be effectively, take this to improve with the vertical direction of rolling direction on Young's modulus.
So far, the multiple discussion that improves the steel plate of Young's modulus by the control tissue is arranged.
For example, patent documentation 1 discloses a kind of technology, the steel that obtains in ultra low-carbon steel, adding Nb or Ti wherein, with it at Ar
3～(Ar
3+ 150 ℃) draft is not less than under 85% the condition and carries out hot rolling, to promote from decrystallized austenite to ferritic transformation, thereby the ferritic structure that makes the hot-rolled steel sheet stage for 311}＜011〉and 332}＜113〉crystal orientation, this crystal orientation is an initial orientation, and carry out cold rolling and full annealed and form with 211}＜011〉and be oikocryst to, thereby improve perpendicular to the Young's modulus on the direction of rolling direction.
In addition, patent documentation 2 discloses the manufacture method of the hot-rolled steel sheet that has improved Young's modulus, and in the method, it is in 0.02～0.15% the soft steel that Nb, Mo and B are joined carbon content, and at Ar
3Draft under～950 ℃ be not less than 50% with the development 211}＜011〉crystal orientation.
In addition, patent documentation 3 discloses the method for making hot-rolled steel sheet, wherein Si and Al is joined carbon content and be to be no more than in 0.05% the soft steel to improve Ar
3Transition point, and when hot rolling, make and be lower than Ar
3The draft of transition point is not less than 60%, to improve perpendicular to the Young's modulus on the direction of rolling direction.
Patent documentation 1:JP-A-H05-255804
Patent documentation 2:JP-A-H08-311541
Patent documentation 3:JP-A-H09-53118
Summary of the invention
Technical problem to be solved by this invention
Yet there is following problems in above-mentioned technology.
In patent documentation 1 disclosed technology, thereby adopt carbon content to be no more than the Young's modulus that 0.01% ultra low-carbon steel is controlled tissue raising steel plate, but its tensile strength at most still is low to moderate about 450MPa, therefore uses the problem that this technology exists intensity to have much room for improvement.
In patent documentation 2 disclosed technology, because carbon content is up to 0.02～0.15%, therefore can improve intensity, but because of the target steel plate is a hot-rolled steel sheet, can not adopt the cold processing control tissue, thereby not only existence is difficult to further improve the problem of Young's modulus, and be difficult to stably produce the high tensile steel plate of thickness less than 2.0mm by low temperature finish rolling.
In addition, in patent documentation 3 disclosed technology, owing to, therefore have the significantly problem of deterioration of workability in rolling thickization of crystal grain that make of ferrite area.
Thereby the Young's modulus that improves steel plate by common process is to be object with thick hot-rolled steel sheet or mild steel plate, therefore adopts above-mentioned common process to be difficult to improve the Young's modulus that thickness is no more than the high-strength steel sheet of 2.0mm.
Be increased to the strengthening mechanism that is not less than 590MPa as tensile strength, mainly contain precipitation strength mechanism and change build up mechanism steel plate.
When precipitation strength mechanism is used as strengthening mechanism, can improves intensity and also suppress the reduction of steel plate Young's modulus as much as possible, but but have following difficulty simultaneously.That is: when precipitation strength mechanism is used for separating out the fine of hot-rolled steel sheet, when separating out as the carbonitride of Ti, Nb etc., carry out fine separating out in the coiling process after hot rolling and can improve intensity, but in cold-rolled steel sheet, thickization of precipitate is inevitably and is difficult to improve intensity by precipitation strength in the full annealed step after cold rolling.
When adopt changing build up mechanism as strengthening mechanism, exist since low-temperature transformation mutually as bainite phase, martensite equal in contained strain reduce the problem of steel plate Young's modulus.
Therefore, the objective of the invention is to address the above problem, and provide tensile strength to be not less than 590MPa, preferably to be not less than 700MPa, Young's modulus is not less than 230Gpa, more preferably is not less than the high-rigidity/high-strength thin steel sheet that 240GPa and thickness are no more than 2.0mm, and the favorable method of making the said products.
The means of dealing with problems
In order to achieve the above object, main points of the present invention and being presented below:
(I) high-rigidity/high-strength thin steel sheet, it comprises C:0.02～0.15% in quality %, Si: be no more than 1.5%, Mn:1.0～3.5%, P: be no more than 0.05%, S: be no more than 0.01%, Al: be no more than 1.5%, N: be no more than 0.01%, and Ti:0.02～0.50%, condition is the relation that the content of C, N, S and Ti satisfies following formula (1) and (2):
Ti
*＝Ti-(47.9/14)×N-(47.9/32.1)×S≥0.01％ (1)
0.01％≤C-(12/47.9)×Ti
*≤0.05％ (2)
All the other are essentially iron and unavoidable impurities, and the tissue of this steel plate comprises as the ferritic phase of principal phase and have area than the martensitic phase that is not less than 1%, and have tensile strength that is not less than 590MPa and the Young's modulus that is not less than 230GPa.
(II) according to (I) described high-rigidity/high-strength thin steel sheet, except above-mentioned composition, it further comprises in Nb:0.005～0.04% and V:0.01～0.20% one or both in quality %, and satisfies the relation of above-mentioned formula (1) and following formula (3) replacement formula (2):
0.01％≤C-(12/47.9)×Ti
*-(12/92.9)×Nb-(12/50.9)×V≤0.05％ (3)。
(III) basis (I) or (II) a described high-rigidity/high-strength thin steel sheet, except above-mentioned composition, it further comprises Cr:0.1～1.0% in quality %, Ni:0.1～1.0%, Mo:0.1～1.0%, one or more in Cu:0.1～2.0% and B:0.0005～0.0030%.
(IV) method of manufacturing high-rigidity/high-strength thin steel sheet, it comprises, be not less than at the total draft below 950 ℃ under 30% the condition, the starting material of steel carried out hot-rolled step, and finish finish rolling at 800～900 ℃, batching this hot-rolled steel sheet below 650 ℃, pickling is carried out cold rollingly to be not less than 50% draft, is warming up to 780～900 ℃ by 500 ℃ of temperature rise rates with 1～30 ℃/second and carries out soaking, being cooled to 500 ℃ with the rate of cooling that is not less than 5 ℃/second then anneals
The starting material of described steel comprises C:0.02～0.15% in quality %, Si: be no more than 1.5%, Mn:1.0～3.5%, P: be no more than 0.05%, S: be no more than 0.01%, Al: be no more than 1.5%, N: be no more than 0.01%, and Ti:0.02～0.50%, condition is the relation that the content of C, N, S and Ti satisfies following formula (1) and (2):
Ti
*＝Ti-(47.9/14)×N-(47.9/32.1)×S≥0.01％ (1)
0.01％≤C-(12/47.9)×Ti
*≤0.05％ (2)。
(V) method of basis (IV) described manufacturing high-rigidity/high-strength thin steel sheet, wherein the starting material of steel is except that above-mentioned composition, further comprise in Nb:0.005%～0.04% and V:0.01～0.20% one or both in quality %, and satisfy the relation of above-mentioned formula (1) and following formula (3) replacement formula (2):
0.01％≤C-(12/47.9)×Ti
*-(12/92.9)×Nb-(12/50.9)×V≤0.05％ (3)。
(VI) basis (IV) or (V) method of a described manufacturing high-rigidity/high-strength thin steel sheet, wherein the starting material of steel is except that above-mentioned composition, further comprise Cr:0.1～1.0% in quality %, Ni:0.1～1.0%, Mo:0.1～1.0%, one or more in Cu:0.1～2.0% and B:0.0005～0.0030%.
The invention effect
According to the present invention, can provide tensile strength to be not less than 590MPa, Young's modulus is not less than the high-rigidity/high-strength thin steel sheet of 230GPa.
Promptly, starting material to the soft steel that added Mn and Ti in the hot rolling below 950 ℃ is depressed, to promote becoming ferrite by non-recrystallization austenitic transformation, cold rolling then to develop to improving the favourable crystalline orientation of Young's modulus, produce then and suppressed the low-temperature transformation phase that Young's modulus reduces, thereby and help the ferritic phase that Young's modulus improves in a large number by being controlled at the heating rate in the annealing steps and having kept at cooling stages in the soaking of two-phase region, can make thus and satisfy more high strength and the more steel sheet of high Young's modulus, this produces effective effect in industry.
Further detailed elaboration, in course of hot rolling in the austenite cold zone, starting material to the soft steel that is added with Mn and Ti is depressed, with increase have 112}＜111〉the non-recrystallization austenite structure in crystal orientation, and subsequently cooling stages promoted from 112}＜111〉and non-recrystallization austenite to ferritic transformation so that 113}＜110〉the ferrite orientation is developed.
Batch with pickling after cold-rolled process in, be not less than at draft be rolled under 50% the condition with will 113}＜110〉and crystalline orientation change into to improve Young's modulus favourable 112}＜110〉crystalline orientation, and in the temperature rise period of annealing steps subsequently, with 1～30 ℃/second heating rate from 500 ℃ rise to soaking temperature with promote to have 112}＜110〉the ferritic recrystallization in crystal orientation, and with part keep 112}＜110〉state of not re-crystallization crystal grain provides two phase region, can promote thus 112}＜110〉the not re-crystallization ferritic transformation be austenite.
In addition, in soaking postcooling process, change into the ferritic phase mutually from austenite, have 112}＜110〉thus the ferrite crystal grain in crystal orientation grow up and improved Young's modulus, simultaneously to be not less than 5 ℃/second speed cooling owing to add steel that Mn increases hardenability with formation low-temperature transformation phase, thereby improved the intensity of steel.
In addition, in process of cooling by will by 112}＜110〉austenite that forms of the ferritic transformation in crystal orientation changes mutually once more mutually, therefore thereby produce the low-temperature transformation phase, even in the crystalline orientation of low-temperature transformation phase, also can develop 112}＜110 crystal orientation.
Therefore, by the development ferritic phase 112}＜110〉crystal orientation improved Young's modulus, especially to reduce Young's modulus have the low-temperature transformation of considerable influence go up mutually 112}＜110〉crystal orientation obtained increase, the formation by the low-temperature transformation phase can improve intensity and can suppress to follow the reduction of the Young's modulus that the formation of low-temperature transformation phase produces to a great extent thus.
The accompanying drawing summary
Figure 1 shows that total draft is to the influence of Young's modulus below 950 ℃;
Figure 2 shows that outlet temperature in the hot finishing is to the influence of Young's modulus;
Figure 3 shows that the influence of coiling temperature to Young's modulus;
Figure 4 shows that of the influence of cold rolling middle draft to Young's modulus;
Figure 5 shows that in the annealing process from 500 ℃ of influences that are warming up to the average temperature rise rate of soaking temperature to Young's modulus.
Realize best mode of the present invention
According to high-rigidity/high-strength thin steel sheet of the present invention is that tensile strength is not less than 590MPa, preferably is not less than 700MPa, and Young's modulus is not less than 230GPa, preferably is not less than 240GPa, and thickness is no more than the steel plate of 2.0mm.In addition, target steel plate of the present invention also comprises and carried out the surface-treated steel plate except cold-rolled steel sheet, and described surface treatment is as zinc-plated, electro-galvanizing of containing alloying etc.
The reason that limits the chemical constitution in the steel plate of the present invention will be described below.In addition, be in " quality % " for each constituent content unit of this steel plate chemical constitution, generally it is abbreviated as if no special instructions " % ".
C：0.02～0.15％
C is the element of stable austenite, and in the cooling stages when cold rolling after annealing, it significantly promotes the formation of low-temperature transformation phase by improving hardenability, thereby can greatly help to improve intensity.In addition, cold rolling after, in the temperature rise period of annealing steps, C by promote to have 112}＜110〉and the ferrite crystal grain in crystal orientation from non-recrystallization ferrite to austenitic transformation, thereby help to improve Young's modulus.
In order to obtain above-mentioned effect, need carbon content to be not less than 0.02%, preferably be not less than 0.05%, more preferably be not less than 0.06%.On the other hand, if carbon content surpasses 0.15%, the branch rate of hard low-temperature transformation phase will increase, and make the intensity of steel extremely increase and deteriorated workability.Equally, in the annealing steps after cold rolling, more high-load C will suppress improving the recrystallization in the favourable crystal orientation of Young's modulus.More high-load in addition C will cause the deterioration of weldability.
Therefore, require C content to be no more than 0.15%, preferably be no more than 0.10%.
Si: be no more than 1.5%
In course of hot rolling, Si improves Ar
3Transition point, thereby when stopping when rolling at 800～900 ℃, if contained Si amount surpasses 1.5%, the rolling difficulty and the required crystalline orientation of Young's modulus that can not be improved of becoming of austenitic area then.In addition, more high-load Si makes the weldability deterioration of steel plate, and has promoted to pine in adding of hot-rolled step the formation of the surperficial fayalite of steel billet (slab), thereby has quickened the generation of the surperficial apperance of so-called red fouling.In addition, when the cold-rolled steel sheet, the silicon oxide that the surface produces reduces the processing characteristics of chemical conversion, and with doing steel plate when zinc-plated, the silicon oxide that produces on the surface will bring out and can't electroplate.Therefore, need Si content to be no more than 1.5%.In addition, at the steel plate that requires surface property or to steel plate when zinc-plated, silicone content preferably is no more than 0.5%.
And, Si stablizes ferritic element, in its annealing steps after cold rolling, the cooling stages after the soaking of two phase region has promoted ferritic transformation so that C concentration in the austenite increases, thus can stable austenite to promote the formation of low-temperature transformation phase.Therefore, can improve the intensity of steel if necessary.In order to obtain this effect, require Si content to be not less than 0.2%.
Mn：1.0～3.5％
In the present invention, Mn is one of important element.In course of hot rolling, Mn has the effect that suppresses the processing austenite recrystallization.And, Mn can promote from non-recrystallization austenite to ferritic transformation with development 113}＜110〉crystal orientation and cold rolling and annealing steps subsequently, improve Young's modulus.
In addition, in the temperature rise period of cold rolling after annealing step, Mn has reduced Ac as the austenite stable element
1Thereby transition point promote from non-recrystallization ferrite to austenitic transformation, as for the crystal orientation of the low-temperature transformation phase that produces in the soaking postcooling stage, Mn can develop the orientation that helps the Young's modulus raising is followed the Young's modulus that forms the low-temperature transformation phase and produce with control reduction.
And after soaking in annealing steps and the annealing, thereby Mn can strengthen the formation that the hardenability of cooling stages significantly promotes the low-temperature transformation phase, is a significant benefit to the raising of intensity like this.In addition, Mn is as a kind of solution strengthening element, and it helps the raising of intensity in the steel.In order to obtain above-mentioned effect, require Mn content to be not less than 1.0%, preferably be not less than 1.5%.
On the other hand, when Mn content surpasses 3.5%, Ac in the temperature rise period in the cold rolling after annealing step
3Therefore transition point reduces over-drastic, and the recrystallization at the two-phase region ferritic phase is very difficult, and temperature need be increased to and be higher than Ac
3The austenite one phase district of transition point.Therefore, can not make by process ferritic recrystallization obtain to improve Young's modulus favourable 112}＜110〉the ferrite development in crystal orientation, thereby cause the reduction of Young's modulus.Further, more high-load Mn will worsen the weldability energy of steel plate.In addition, thereby the non-deformability that more high-load Mn can strengthen at the course of hot rolling light plate improves rolling load, and this will cause operational difficulty.Therefore, Mn content is no more than 3.5%.
P: be no more than 0.05%
Because P produces segregation at crystal boundary,, then not only reduce the ductility of steel plate and toughness and make the weldability deterioration if therefore P content surpasses 0.05%.Adopting under the situation of alloy galvanized steel plate, alloying speed is because P and sluggishness.Therefore, require P content to be no more than 0.05%.On the other hand, P can improve intensity and its effectively as the solution strengthening element and has the effect that promotes C enrichment in the austenite as ferritic stabilizer.In the steel that adds Si, P also has the effect that suppresses to produce red fouling.In order to obtain above-mentioned effect, P content preferably is no more than 0.01%.
S: be no more than 0.01%
Thereby S significantly reduces high-temperature ductility and causes thermal crack and make the remarkable deterioration of surface property.In addition, S does not almost act on intensity, and forms thick MnS and reduce ductility and hole diffusion (drill-spreading) performance as impurity element.Become obviously when S content surpasses 0.01% these problems, need to reduce as much as possible S content for this reason.Therefore, S content is no more than 0.01%.From improving the angle of hole diffusion, S content preferably is no more than 0.005%.
Al: be no more than 1.5%
Thereby Al is used for the element that steel-deoxidizing improves steel cleanness.Yet Al is a ferritic stabilizer, and it has significantly improved the Ar of steel
3Change, thus when the finish rolling end temp is 800～900 ℃, if Al content surpasses 1.5%, thus the development that the rolling difficulty that will become in the austenitic area has suppressed to improve the required crystalline orientation of Young's modulus.Therefore, require Al content to be no more than 1.5%.From this viewpoint, Al content is preferably lower, and more preferably is limited in and is no more than 0.1%.On the other hand, Al is as ferrite former, in the annealing steps after cold rolling, in the cooling stages after the two-phase region soaking, thereby Al has promoted ferritic formation to make the C enrichment in the austenite, thus can stable austenite to promote the formation of low-temperature transformation phase.Its result if necessary, can improve the intensity of steel.In order to obtain above-mentioned effect, require Al content to be not less than 0.2%.
N: be no more than 0.01%
N is a harmful element, thereby because it breaks steel billet to cause surface imperfection in course of hot rolling.When N content surpassed 0.01%, steel billet broke more obvious with the generation of surface imperfection.In addition, when adding carbonitride forming element such as Ti, Nb or the like, thereby N at high temperature forms the effect that thick nitride has suppressed to add the carbonitride forming element.Therefore, require N content to be no more than 0.01%.
Ti：0.02～0.50％
In the present invention, Ti is most important element.That is, Ti has controlled the austenitic recrystallization of processing in the finish rolling step in hot rolling, thus promote from non-recrystallization austenite to ferritic transformation and development 113}＜110〉crystal orientation, just can in cold rolling and annealing steps subsequently, improve Young's modulus.And in the temperature rise period of cold rolling after annealing step, suppressed the ferritic recrystallization of processing, thereby promote from non-recrystallization ferrite to austenitic transformation, and the orientation of the low-temperature transformation phase that produces for the soaking postcooling stage, it can develop improving the useful crystal orientation of Young's modulus, the reduction of the Young's modulus that produces to suppress to follow the formation of low-temperature transformation phase.In addition, the fine carbonitride of Ti can help the raising of intensity.In order to obtain these effects, require the content of Ti to be not less than 0.02%, preferably be not less than 0.03%.
On the other hand, when Ti content surpasses 0.50%, then all carbonitrides all can not solid solution and residual thick carbonitride, the therefore effect that can not obtain to suppress to process the effect of austenite recrystallization or suppress to process the ferrite recrystallization in hot-rolled step in cold rolling after annealing step in the reheat of common hot-rolled step.And, even behind continuous casting, just begin to carry out the hot rolling of steel billet, and after the steel billet cooling of continuous casting, do not carry out reheating, when Ti content surpasses 0.50%, suppress the improvement of recrystallization effect still can't discern, and bring the raising of cost of alloy.Therefore, require Ti content to be no more than 0.50%, preferably be no more than 0.20%.
In the present invention, require the content of C, N, S and Ti to satisfy the relation of following formula (1) and (2):
Ti
*＝Ti-(47.9/14)×N-(47.9/32.1)×S≥0.01％ (1)
0.01％≤C-(12/47.9)×Ti
*≤0.05％ (2)
Ti forms thick nitride and sulfide easily in the high-temperature zone.The formation of this type of nitride and sulfide is added the reduction that Ti suppresses the recrystallization effect with causing.Therefore, the Ti amount that requires not to be fixed as nitride and sulfide is Ti
*Amount=Ti-(47.9/14) * N-(47.9/32.1) * S is not less than 0.01%, preferably is not less than 0.02%.
Surpass 0.05% if be fixed as the C amount of carbonitride, the strain that is produced in cold-rolled process will be inhomogeneous and can be suppressed Young's modulus is improved the recrystallization in favourable crystal orientation, therefore requires by (C-(12/47.9) * Ti
*) the C amount that is not fixed as carbide that calculates is no more than 0.05%.On the other hand, be lower than 0.01% when the C that is not fixed as carbide measures, then the C content in the austenite reduces in the two-phase region annealing process of cold rolling back, thereby has suppressed the formation of cooling back martensitic phase, the very difficult thus intensity that improves.Therefore, be not fixed as C amount C-(the 12/47.9) * Ti of carbide
*Be 0.01～0.05%.
In addition, the meaning of term " surplus is essentially iron and unavoidable impurities " is as used herein: the steel that contains other elements that do not damage on a small quantity effect of the present invention and effect within the scope of the present invention.When will further improving intensity, except the chemical constitution of above-mentioned definition, also can add among a kind of in Nb:0.005～0.04% and V:0.01～0.20% or two kinds and Cr, Ni, Mo, Cu and the B one or more.
Nb：0.005～0.04％
Nb is by forming the element that fine carbonitride helps intensity to improve.And Nb is by suppressing the austenitic recrystallization of processing in hot rolled finish rolling step, thereby promotes from non-recrystallization austenite to help to improve the element of Young's modulus thus to ferritic transformation.In order to obtain above-mentioned effect, the content of preferred Nb is not less than 0.005%.On the other hand, when the content of Nb surpasses 0.04%, hot rolling and cold rolling in rolling load significantly improve thereby have difficulties in process of production, the content of therefore preferred Nb is no more than 0.04%, more preferably no more than 0.01%.
V：0.01～0.20％
V is by forming the element that fine carbonitride helps intensity to improve.Because V has this effect, preferred V content is not less than 0.01%.On the other hand, when V content surpassed 0.20%, it was less and can cause the increase of cost of alloy to utilize 0.20% the amount of surpassing to improve the effect of intensity.
Therefore, preferred V content is 0.01～0.20%.
In the present invention, when except Ti, also comprising Nb and/or V, require the content of C, N, S, Ti, Nb and V to satisfy the relation of following formula (3) replacement formula (2):
0.01％≤C-(12/47.9)×Ti
*-(12/92.9)×Nb-(12/50.9)×V≤0.05％ (3)
The C that thereby Nb becomes carbide to reduce is not fixed as carbide with V-arrangement amount.Therefore, be 0.01～0.05% in order to make the C amount that is not fixed as carbide, when adding Nb and/or V, require C-(12/47.9) * Ti
*The value of-(12/92.9) * Nb-(12/50.9) * V is 0.01～0.05%.
Cr：0.1～1.0％
Cr is the element that improves hardenability by the formation that suppresses cementite, thereby and helps the raising of intensity to a great extent by the formation that greatly promotes the low-temperature transformation phase in the cooling stages after the annealing steps soaking.Further, it has suppressed the austenitic recrystallization of processing in hot-rolled step, thus promote from non-recrystallization austenite to ferritic transformation and development 113}＜110〉crystal orientation, and in cold rolling and annealing steps subsequently, can improve Young's modulus.In order to obtain above-mentioned effect, the preferred Cr amount that comprises is not less than 0.1%.On the other hand, when Cr content surpasses 1.0%, the saturated and cost of alloy increase of then above-mentioned effect, the therefore preferred Cr amount that comprises is no more than 1.0%.In addition, when steel sheet of the present invention was used as steel plate galvanized, the Cr oxide compound that produces on the surface can bring out and can not electroplate, and the therefore preferred Cr amount that comprises is no more than 0.5%.
Ni：0.1～1.0％
Thereby Ni is a stable austenite strengthens the element of hardenability, thereby and greatly promotes the formation of low-temperature transformation phase to help the raising of intensity to a great extent in the cooling stages after the annealing steps soaking.In addition, in the temperature rise period of cold rolling after annealing step, Ni has reduced Ac as the austenite stable element
1Transition point, thereby promote from non-recrystallization ferrite to austenitic transformation, and crystal orientation for the low-temperature transformation phase that in the soaking postcooling stage, produces, it has developed for improving the favourable crystal orientation of Young's modulus, can suppress thus to follow the formation of low-temperature transformation phase and the reduction of the Young's modulus that produces.In addition, Ni in course of hot rolling, suppressed processing austenitic recrystallization, thereby promote non-recrystallization austenite to ferritic transformation with the development 113}＜110〉crystal orientation, can in cold rolling and annealing steps subsequently, improve Young's modulus thus.In steel, add under the situation of Cu, follow that the reduction of high-temperature ductility causes surface imperfection by crackle in the hot rolling, but can come the control surface generation of defects by compound interpolation Ni.In order to obtain above-mentioned effect, the preferred Ni amount that comprises is not less than 0.1%.
On the other hand, when Ni content surpasses 1.0%, then at the temperature rise period of cold rolling after annealing step, Ac
3Transition point sharply reduces, and in two-phase region the recrystallization difficulty of ferritic phase, therefore temperature need be increased to Ac
3The austenite one phase district that transition point is above.Consequently, can not develop obtain by processing ferrite recrystallization and to improving the useful ferrite crystal orientation of Young's modulus, thereby cause Young's modulus to reduce.And cost of alloy increases.Therefore, the preferred Ni amount that comprises is no more than 1.0%.
Mo：0.1～1.0％
Mo is the element that reduces to improve hardenability by the movability that makes the interface, and in the cooling stages of cold rolling after annealing step, thereby its formation by very big promotion low-temperature transformation phase helps the raising of intensity to a great extent.Further, it can suppress to process austenitic recrystallization, and promoted from non-recrystallization austenite to ferritic transformation with development 113}＜110〉crystal orientation, and in cold rolling and annealing steps subsequently, can improve Young's modulus.In order to obtain above-mentioned effect, preferred contained Mo amount is not less than 0.1%.On the other hand, when Mo content surpassed 1.0%, above-mentioned effect reached capacity and cost of alloy improves, and therefore, preferred contained Mo amount is no more than 1.0%.
B：0.0005～0.0030％
B suppresses austenite to change ferritic phase mutually into to improve the element of hardenability, and in the cooling stages of cold rolling after annealing step, thereby its formation by very big promotion low-temperature transformation phase helps the raising of intensity to a great extent.Further, it can suppress to process austenitic recrystallization, and promoted from non-recrystallization austenite to ferritic transformation with development 113}＜110〉crystal orientation, and in cold rolling and annealing steps subsequently, can improve Young's modulus.In order to obtain above-mentioned effect, preferred contained B amount is not less than 0.0005%.On the other hand, when the content of B surpassed 0.0030%, then the enhancing of the resistance to deformation in the course of hot rolling caused having improved rolling load and has difficulties in the mill, and therefore, preferred contained B amount is no more than 0.0030%.
Cu：0.1～2.0％
Cu is the element that improves hardenability, and in the cooling stages of cold rolling after annealing step, thereby its formation by very big promotion low-temperature transformation phase helps the raising of intensity to a great extent.For obtaining above-mentioned effect, preferred contained Cu amount is not less than 0.1%.On the other hand, when Cu content surpassed 2.0%, then high-temperature ductility reduced and brings out with the quenching effect of the surface imperfection of crackle and Cu saturatedly in course of hot rolling, therefore preferably contains the Cu amount and is no more than 2.0%.
The reason that limits tissue of the present invention will be described below:
Require to have tissue in steel sheet of the present invention, described tissue comprises as the ferritic phase of principal phase and has area than the martensitic phase that is not less than 1%.
Mean that at this used term " as the ferritic phase of principal phase " the shared area ratio of ferritic phase is not less than 50%.
Because the ferritic phase strain is less, favourable to improving Young's modulus, it is favourable to have good ductility and good workability, therefore requires in the tissue with ferritic phase as principal phase.
And, for the tensile strength that makes steel plate is not less than 590MPa, require as the part outside the ferritic phase of principal phase or so-called second mutually in formation as the low-temperature transformation phase of hard phase, thereby obtain compound phase.At this moment, have that to be in low-temperature transformation these characteristics of hard martensitic phase in mutually be useful especially in this tissue, because account for the small portion ferrite mutually and then account for major part, can improve Young's modulus thus and and further improve workability for obtaining second of required tensile strength level.Therefore, require martensitic phase to be not less than whole 1% of the area of organizing.In order to obtain to be not less than the intensity of 700MPa, the area ratio of preferred martensitic phase is not less than 16%.
Steel plate tissue of the present invention is preferably the tissue that comprises ferritic phase and martensitic phase, but has other areas than being no more than 10%, preferably being no more than the also no problem as bainite phase, residual austenite phase, perlite phase, cementite phase or the like of 5% non-ferritic phase and martensitic phase.That is, the area of ferrite and martensitic phase preferably is not less than 90% than sum, more preferably is not less than 95%.
Explained later is for the reason of creating conditions that obtains high-rigidity/high-strength thin steel sheet of the present invention and limit and preferably create conditions.
The composition of the starting material of the used steel of manufacture method of the present invention is identical with the composition of above-mentioned steel plate, therefore at this reason that limits starting material is not described.
Can be by carrying out hot-rolled step, cold rolling step and annealing steps continuously to make steel sheet of the present invention, this hot-rolled step is to carry out hot rolling to obtain hot-rolled sheet with forming identical steel starting material with steel plate, cold rolling step is cold rolling obtaining cold-reduced sheet for the hot-rolled sheet after the pickling is carried out, and annealing steps this cold-reduced sheet of serving as reasons obtains recrystallization and complex tissue.
(hot-rolled step)
Finish rolling: the total draft that is lower than 950 ℃ is not less than 30%, and finishes rolling in the time of 800～900 ℃.
In the finish rolling of hot-rolled step, be rolled at a lower temperature with development have 112}＜111〉the non-recrystallization austenite structure of crystalline orientation, and 112}＜111〉and non-recrystallization austenite can in cooling stages subsequently, change into ferrite with development 113}＜110〉and the ferrite crystal orientation.During tissue in cold rolling and annealing steps subsequently formed, the effect that improves Young's modulus had advantageously been played in this crystal orientation.In order to obtain above-mentioned effect, require total draft of (total draft) below 950 ℃ to be not less than 30%, finishing finish rolling below 900 ℃ in addition.On the other hand, when the outlet temperature of finish rolling is lower than 800 ℃,, there is difficulty this moment in the mill because the increase rolling load of resistance to deformation significantly increases.Therefore, require the outlet temperature of finish rolling to be not less than 800 ℃.
Coiling temperature: be not higher than 650 ℃
When the coiling temperature after the finish rolling surpassed 650 ℃, then the carbonitride of Ti was thick and in the temperature rise period of cold rolling after annealing step the effect that suppresses the ferrite recrystallization is died down, and to be difficult to non-recrystallization ferritic transformation be austenite.Consequently, the crystal orientation of the low-temperature transformation phase that changes in the cooling stages after soaking can not get control, and has greatly reduced Young's modulus mutually by the low-temperature transformation with this type of stress.Therefore, require the coiling temperature after the finish rolling not to be higher than 650 ℃.In addition, when coiling temperature is low excessively, increase and cause existing in the mill difficulty producing a large amount of hard low-temperature transformation load mutually and in subsequently cold rolling, therefore, preferably coiling temperature is not less than 400 ℃.
(cold rolling step)
Being not less than 50% time at draft after pickling carries out cold rolling.
Behind the hot-rolled step, carry out pickling and be formed at fouling on the surface of steel plate with removal.Can adopt common method to carry out pickling.Carry out cold rolling then.By carry out draft be not less than 50% cold rolling, can with develop on the hot-rolled steel sheet 113}＜110〉and the crystal orientation change into can effectively improve Young's modulus 112}＜110〉crystal orientation.Therefore, since by cold rolling the development 112}＜110〉crystal orientation, ferritic in the tissue after subsequent annealing steps 112}＜110〉crystal orientation is enhanced, and further developed in mutually in low-temperature transformation 112}＜110 crystal orientation, can improve Young's modulus thus.In order to obtain above-mentioned effect, require the draft in cold rolling to be not less than 50%.
(annealing steps)
Temperature rise rate from 500 ℃ to soaking temperature is: 1～30 ℃/second, and soaking temperature: 780～900 ℃.
In the present invention, temperature rise rate is an important process condition in annealing steps.In annealing steps, during the soaking temperature of the soaking temperature that is warming up to two-phase region or 780～900 ℃, have 112}＜110 the ferritic recrystallization in crystal orientation obtains promoting, simultaneously a part have 112}＜110〉ferrite crystal grain in crystal orientation arrives two-phase region with non-recrystallization attitude, can promote thus to have 112}＜110〉the ferritic transformation of non-recrystallization in crystal orientation.Therefore, when changing ferrite in the cooling of austenite after soaking, by promote to have 112}＜110〉growth of ferrite crystal grain in crystal orientation can improve Young's modulus.In addition, when when producing low-temperature transformation and improve intensity mutually, by have 112}＜110〉crystal orientation ferritic transformation and the austenite that comes can change in process of cooling once more, therefore the crystalline orientation for the low-temperature transformation phase also can develop 112}＜110〉crystal orientation.By the development ferritic phase 112}＜110〉crystal orientation can improve Young's modulus, simultaneously in the orientation of the low-temperature transformation phase that greatly influence the Young's modulus reduction 112}＜110〉crystal orientation is developed especially, therefore form the low-temperature transformation phase time, following the formation of low-temperature transformation phase and the Young's modulus that causes reduces and can be suppressed.When being austenite by non-recrystallization ferritic transformation, simultaneously when the temperature rise period promotes ferritic recrystallization, requiring from 500 ℃ of average temperature rise rates that are warming up to 780～900 ℃ of soaking temperatures is 1～30 ℃/second, and this average temperature rise rate counterweight crystallization behavior has very big influence.Equally, soaking temperature is that 780～900 ℃ reason is because the following fact, the residual tissue that not re-crystallization is arranged when soaking temperature is lower than 780 ℃, and when soaking temperature is higher than 900 ℃, austenitic generation increases, and be difficult to develop to improve favourable the having of Young's modulus 112}＜110〉ferrite in crystal orientation.
In addition, soaking time does not have the special restriction that becomes, but preferably is not less than 30 seconds to form austenite, meanwhile, because overlong time will make production efficiency reduce, therefore preferably is no more than 300 seconds.
The rate of cooling of soaking postcooling to 500 ℃ is: be not less than 5 ℃/second
In the cooling stages after the soaking, require formation to comprise martensitic low-temperature transformation to improve intensity.Therefore, require all to be cooled to after the soaking 500 ℃ average rate of cooling and be not less than 5 ℃/second.
In the present invention, at first fusion has steel according to the chemical constitution of target strength level.Melting method can suitably adopt common converter process, eaf process or the like.The fused steel is cast into steel billet, directly carries out then carrying out hot rolling again after hot rolling or cooling and the heating.After the finish rolling, under above-mentioned coiling temperature, batch steel plate and carry out common pickling and cold rolling then under the above-mentioned finish rolling condition in hot rolling.For annealing, it is to heat up under these conditions, and at the soaking postcooling, can improve rate of cooling in the scope that obtains target low transformation phase., this cold-rolled steel sheet is carried out overaging handles thereafter, or under the situation of making steel plate galvanized with it by galvanizing, or further under the situation of alloying steel plate galvanized, further reheat is to being higher than 500 ℃ to carry out Alloying Treatment.
Embodiment
The present invention is provided that following embodiment is described but not as restriction of the present invention.
At first, will have steel A fusion in breadboard vacuum fusion stove of chemical constitution as shown in table 1, make steel ingot (steel raw material) thereby be cooled to room temperature then
Table 1
(notes) Ti
*=Ti-(47.9/14) * N-(47.9/32.1) * S
SC＝C-(12/47.9)×Ti
*
Then, order is carried out hot rolling, pickling, cold rolling and annealing in the laboratory.It creates conditions as follows substantially: 1250 ℃ of down heating after 1 hour, is 40% condition under to carry out hot rolling and finishing temperature be 860 ℃ (corresponding to the outlet temperatures of finish rolling) to obtain hot-rolled sheet of 4.0mm thickness at draft below 950 ℃ with steel ingot.Condition (corresponding to 600 ℃ coiling temperature) is batched in simulation as follows then: after hot-rolled steel sheet reaches 600 ℃, kept 1 hour in 600 ℃ stove, then at this stove internal cooling.To therefrom the hot-rolled steel sheet pickling, under draft is 60% condition, be cold-rolled to the thickness of 1.6mm then.Then, the temperature of cold-rolled steel sheet is risen to 500 ℃ with average 10 ℃/second temperature rise rate, then further with average 5 ℃/second temperature rise rate by 500 ℃ of soaking temperatures that rise to 820 ℃.Then,, be cooled to 500 ℃ with 10 ℃/second of average rate of cooling thereafter, further under 500 ℃ of temperature, kept 80 seconds, then this steel plate is cooled off in air 820 ℃ of following soaking 180 seconds.
In this experiment, creating conditions as under the primary condition, further following condition is changed individually with above-mentioned.Promptly, except the change of following indivedual conditions, all the other all carry out this experiments under primary condition: the total draft below 950 ℃ changes 20～60% into, the final temperature of hot finishing changes 800～920 ℃ into, coiling temperature changes 500～670 ℃ into, cold rolling draft is 40～75%, and is 0.5～35 ℃/second from 500 ℃ of average temperature rise rates that are warming up to soaking temperature (820 ℃) in annealing process.
From annealing after sample with perpendicular to rolling direction as the sample that vertically cuts 10mm * 120mm, by mechanical polishing and chemical rightenning with remove strain with this sample finishing to 0.8mm thickness, adopt transverse vibration type internal friction determinator to measure the resonant frequency of this sample then to calculate Young's modulus.For carrying out the plate of 0.5% sclerosis after cold rolling, then cut the tension test sample of JIS No.5 with direction perpendicular to rolling direction, carry out tension test then.In addition, after peroxy-nitric acid ethanol etching solution (Nital) corrosion, by scanning electronic microscope (SEM) thus observe the type that section structure is judged tissue, and take the photo of 3 30 μ m * 30 μ m area of visual field, measure the area ratio of ferritic phase and martensitic phase then by image processing, thus the mean value of determining each phase as the area of each phase than (branch rate).
The result, in the experiment of method constructed in accordance, the value of the mechanical property under primary condition is, Young's modulus E:242GPa, TS:780MPa, E1:23%, ferrite phase fraction: 67%, the martensite phase fraction: 28%, this shows that clearly this steel sheet has good strength-ductility balanced and high Young's modulus.
In addition, this tissue except ferritic phase and martensitic phase, all the other be bainite phase, residual austenite phase, perlite mutually and cementite any in mutually.
Then, based on above test-results with reference to the accompanying drawings to create conditions and Young's modulus between relation be described.Even under any experiment condition, tensile strength is 730～820MPa, and the branch rate of cable body phase is 55～80%, and the branch rate of martensitic phase is 17～38%, and remaining tissue be bainite phase, residual austenite phase, perlite mutually and cementite any in mutually.
Figure 1 shows that below 950 ℃ that total draft is respectively to the influence of Young's modulus.When total draft be in tolerance interval of the present invention be not less than 30% the time, Young's modulus is then for being not less than the excellent value of 230GPa.
Figure 2 shows that of the influence of the outlet temperature of hot finishing to Young's modulus.When outlet temperature be in tolerance interval of the present invention be not higher than 900 ℃ the time, Young's modulus is then for being not less than the excellent value of 230GPa.
Figure 3 shows that the influence of coiling temperature to Young's modulus.When coiling temperature be in tolerance interval of the present invention be not higher than 650 ℃ the time, Young's modulus is the excellent value that is not less than 230GPa.
Figure 4 shows that of the influence of cold rolling draft to Young's modulus.When draft be in tolerance interval of the present invention be not less than 50% the time, Young's modulus is the excellent value that is not less than 230GPa.
Figure 5 shows that in the annealing process from 500 ℃ of influences that are warming up to the average temperature rise rate of 820 ℃ of soaking temperatures to Young's modulus.When temperature rise rate is that Young's modulus is the excellent value that is not less than 230GPa when being in 1～30 ℃/second of tolerance interval of the present invention.
In addition, steel B-Z and AA-AI fusion in the vacuum fusion stove of laboratory that will have chemical constitution shown in the table 2 are cooled to room temperature then, are used to make steel ingot (steel raw material).Under the conditions shown in Table 3 respectively in proper order carry out hot rolling, pickling, cold rolling and annealing thereafter.Heating is after 1 hour down at 1250 ℃ with steel ingot, and hot rolling is to obtain the hot-rolled sheet of 4.0mm thickness under different rolling temperatures.Then, in being in the smelting furnace of coiling temperature, kept 1 hour, subsequently at the stove internal cooling, thus the condition of batching behind the simulated target coiling temperature.With different drafts with the hot-rolled steel sheet pickling, be cold-rolled to the thickness of 0.8～1.6mm, be warming up to 500 ℃ and further be warming up to the target soaking temperature with 10 ℃/second average temperature rise rate then with the average temperature rise rate shown in the table 3.Soaking was cooled off with average rate of cooling different shown in the table 3 after 180 seconds under soaking temperature, and steel plate keeps being cooled to then in 80 seconds room temperature down at 500 ℃ in air.
Table 4 shows the characteristic that obtains by above-mentioned test.At this moment, this tissue except ferritic phase and martensitic phase, remaining be bainite phase, residual austenite phase, perlite mutually and cementite any in mutually.
Table 3
Table 4
In steel D, C content is low to moderate 0.01%, and martensitic minute rate be 0%, and TS is littler than the acceptable scope of the present invention.In steel E, the C content that is not fixed as carbide (SC) up to 0.08% and the branch rate of ferritic phase be low to moderate 30%, and Young's modulus is littler than the acceptable scope of the present invention.In steel F, SC is up to 0.06%, and Young's modulus is littler than the acceptable scope of the present invention.In steel K, Mn content is up to 3.6%, and Young's modulus is littler than the acceptable scope of the present invention.In steel AD, C content up to 0.16% and SC up to 0.14%, and the branch rate of ferritic phase is low to moderate 25%, and Young's modulus is littler than the acceptable scope of the present invention.In steel AF kind, Mn content is low to moderate 0.9%, and TS and Young's modulus are littler than the acceptable scope of the present invention.In steel AI, the content of Ti is low to moderate 0.01% and Ti
*Content be low to moderate 0.00%, and Young's modulus is littler than the acceptable scope of the present invention.
As for other steel grades, all clauses are all in the acceptable scope of the present invention, and TS and Young's modulus also satisfy the acceptable scope of the present invention.
Industrial applicability
Can provide tensile strength to be not less than 590MPa according to the present invention, and Young's modulus is not less than The high-rigidity/high-strength thin steel sheet of 230GPa.
Claims (6)
1. high-rigidity/high-strength thin steel sheet, it comprises C:0.02～0.15% in quality %, Si: be no more than 1.5%, Mn:1.0～3.5%, P: be no more than 0.05%, S: be no more than 0.01%, Al: be no more than 1.5%, N: be no more than 0.01%, and Ti:0.02～0.50%, condition is the relation that the content of C, N, S and Ti satisfies following formula (1) and (2):
Ti
*＝Ti-(47.9/14)×N-(47.9/32.1)×S≥0.01％ (1)
0.01％≤C-(12/47.9)×Ti
*≤0.05％ (2)
All the other are essentially iron and unavoidable impurities, and the tissue of this steel plate comprises as the ferritic phase of principal phase and have area than the martensitic phase that is not less than 1%, and have tensile strength that is not less than 590MPa and the Young's modulus that is not less than 230GPa.
2. high-rigidity/high-strength thin steel sheet as claimed in claim 1, except above-mentioned composition, it further comprises in Nb:0.005～0.04% and V:0.01～0.20% one or both in quality %, and satisfies the relation of above-mentioned formula (1) and following formula (3) replacement formula (2):
0.01％≤C-(12/47.9)×Ti
*-(12/92.9)×Nb-(12/50.9)×V≤0.05％ (3)。
3. high-rigidity/high-strength thin steel sheet as claimed in claim 1 or 2, except above-mentioned composition, it further comprises Cr:0.1～1.0% in quality %, Ni:0.1～1.0%, Mo:0.1～1.0%, one or more in Cu:0.1～2.0% and B:0.0005～0.0030%.
4. make the method for high-rigidity/high-strength thin steel sheet, it comprises, be not less than at the total draft below 950 ℃ under 30% the condition, the starting material of steel carried out hot-rolled step, and finish finish rolling at 800～900 ℃, batching this hot-rolled steel sheet below 650 ℃, pickling is carried out cold rollingly to be not less than 50% draft, is warming up to 780～900 ℃ by 500 ℃ of temperature rise rates with 1～30 ℃/second and carries out soaking, being cooled to 500 ℃ with the rate of cooling that is not less than 5 ℃/second then anneals
The starting material of described steel comprises C:0.02～0.15% in quality %, Si: be no more than 1.5%, Mn:1.0～3.5%, P: be no more than 0.05%, S: be no more than 0.01%, Al: be no more than 1.5%, N: be no more than 0.01%, and Ti:0.02～0.50%, condition is the relation that the content of C, N, S and Ti satisfies following formula (1) and (2):
Ti
*＝Ti-(47.9/14)×N-(47.9/32.1)×S≥0.01％ (1)
0.01％≤C-(12/47.9)×Ti
*≤0.05％ (2)。
5. the method for manufacturing high-rigidity/high-strength thin steel sheet as claimed in claim 4, wherein the starting material of steel is except that above-mentioned composition, further comprise in Nb:0.005%～0.04% and V:0.01～0.20% one or both in quality %, and satisfy the relation of the following formula (3) of above-mentioned formula (1) and replacement formula (2):
0.01％≤C-(12/47.9)×Ti
*-(12/92.9)×Nb-(12/50.9)×V≤0.05％ (3)。
6. as the method for claim 4 or 5 described manufacturing high-rigidity/high-strength thin steel sheets, wherein the starting material of steel is except that above-mentioned composition, further comprise Cr:0.1～1.0% in quality %, Ni:0.1～1.0%, Mo:0.1～1.0%, one or more in Cu:0.1～2.0% and B:0.0005～0.0030%.
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| KR20140048348A (en) * | 2011-11-15 | 2014-04-23 | 제이에프이 스틸 가부시키가이샤 | Thin steel sheet and process for producing same |
| KR101726130B1 (en) * | 2016-03-08 | 2017-04-27 | 주식회사 포스코 | Composition structure steel sheet having excellent formability and method for manufacturing the same |
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