JP3925063B2 - Cold-rolled steel sheet excellent in press formability and strain age hardening characteristics and method for producing the same - Google Patents

Description

【００６５】
【表２】

【００６６】
【表３】

【００６７】
本発明例は、いずれも、低い降伏強さＹＳと高い伸びＥｌと、低い降伏比ＹＲを有し、さらに大きな穴拡げ率λを示して、伸びフランジ成形性を含むプレス成形性に優れるとともに、極めて大きなΔＴＳを示し、歪時効硬化特性に優れた鋼板となっている。これに対し、本発明の範囲を外れる比較例では、降伏強さＹＳが高いか、伸びＥｌが低いか、あるいは穴拡げ率λが小さいか、ΔＴＳが小さく、プレス成形性、歪時効硬化特性が低下した鋼板となっている。
【００６８】
（実施例２）
表４に示す組成の溶鋼を転炉で溶製し、連続鋳造法でスラブとした。ついで、これら鋼スラブを1250℃に加熱したのち、仕上圧延終了温度： 900℃、巻取温度： 600℃とする熱間圧延を施す熱延工程により、板厚4.0mm の熱延鋼帯（熱延板）とした。引き続き、これら熱延鋼帯（熱延板）に酸洗、冷間圧延を施す冷延工程により、板厚1.2mm の冷延鋼帯（冷延板）とした。ついで、これら冷延鋼帯（冷延板）に、連続焼鈍ラインで、表５に示す焼鈍温度で再結晶焼鈍を施した。得られた鋼帯（冷延焼鈍板）に、さらに伸び率：0.8 ％の調質圧延を施した。
【００６９】
得られた鋼帯から試験片を採取し、実施例１と同様に、微視組織、引張特性、歪時効硬化特性、穴拡げ性を調査した。なお、プレス成形性は、伸びＥｌ（延性）と降伏強さおよび穴拡げ率から評価した。
これらの結果を表６に示す。
【００７０】
【表４】

【００７１】
【表５】

【００７２】
【表６】

【００７３】
本発明例は、いずれも、低い降伏強さＹＳと高い伸びＥｌと、低い降伏比ＹＲを有し、さらに大きな穴拡げ率λを示して、伸びフランジ成形性を含むプレス成形性に優れるとともに、極めて大きなΔＴＳを示し、歪時効硬化特性に優れた鋼板となっている。これに対し、本発明の範囲を外れる比較例では、降伏強さＹＳが高いか、伸びＥｌが低いか、あるいは穴拡げ率λが小さいか、ΔＴＳが小さく、プレス成形性、歪時効硬化特性が低下した鋼板となっている。
【００７４】
【発明の効果】
本発明によれば、優れたプレス成形性を維持しつつ、プレス成形後の熱処理により引張強さが顕著に上昇する冷延鋼板を、安定して製造することが可能となり、産業上格段の効果を奏する。本発明の冷延鋼板を自動車部品用に適用した場合、プレス成形が容易で、かつ完成後の部品特性を安定して高くでき、自動車車体の軽量化に十分に寄与できるという効果もある。
【図面の簡単な説明】
【図１】予変形−熱処理後のΔＴＳと再結晶焼鈍温度の関係におよぼすCu含有量の影響を示すグラフである。
【図２】予変形−熱処理後のΔＴＳと熱処理温度の関係におよぼすCu含有量の影響を示すグラフである。
【図３】λとＹＲとの関係におよぼすCu含有量の影響を示すグラフである。[0001]
BACKGROUND OF THE INVENTION
The present invention mainly relates to cold-rolled steel sheets for automobiles, and in particular, press formability such as bending workability, stretch flange workability and drawability is good, and the tensile strength is remarkably increased by heat treatment after press forming. The present invention relates to a cold-rolled steel sheet having extremely large strain age hardening characteristics and a method for producing the same. The extremely large strain age hardening characteristic referred to in the present invention, that is, “excellent strain age hardening characteristic” means that it has a strain age hardening characteristic of ΔTS: 80 MPa or more. In the present invention, ΔTS is the amount of increase in tensile strength before and after heat treatment when a pre-deformation treatment with a plastic strain amount of 5% or more and a heat treatment with a holding time of 30 seconds or more at a temperature in the range of 150 to 350 ° C. {= (Tensile strength after heat treatment) − (Tensile strength before pre-deformation treatment)}.
[0002]
[Prior art]
In recent years, in connection with exhaust gas regulations due to global environmental conservation issues, the reduction of vehicle body weight has become a very important issue. Recently, in order to reduce the weight of the vehicle body, it has been studied to increase the strength of a steel plate for automobiles and reduce the thickness of the steel plate.
Since many automotive body parts made of steel sheets are formed by press working, the cold-rolled steel sheets used are required to have excellent press formability. In order to obtain a steel sheet having excellent press formability, it is important to secure low yield strength and high ductility. In addition, stretch flange molding is often used, and it is necessary to have a high hole expansion rate. However, generally, when the strength of a steel plate is increased, the yield strength increases and the shape freezing property deteriorates, the ductility decreases, the hole expansion rate decreases, and the press formability tends to decrease.
[0003]
Recently, in order to protect an occupant in the event of a collision, importance is placed on the safety of the automobile body. For this reason, an improvement in impact resistance that is a measure of safety in the event of a collision is required. The higher the strength of the finished vehicle, the more advantageous for improving the impact resistance. Therefore, when forming automotive parts, cold-rolled steel sheets that have low strength, high ductility, excellent press formability, and high strength and excellent impact resistance properties are most strongly desired when finished products are produced. It was.
[0004]
In response to such a demand, a steel sheet having both press formability and high strength has been developed. This steel sheet is a paint bake hardening type steel sheet whose yield stress increases when it is subjected to a paint baking process including holding at a high temperature of usually 100 to 200 ° C. after press working. In this steel sheet, the amount of C remaining in the final solid solution state (solid solution C amount) is controlled within an appropriate range, soft at the time of press forming, ensuring shape freezing and ductility, and baking after press forming. At the time of processing, the remaining solid solution C adheres to the dislocations introduced during press forming, thereby preventing the dislocations from moving and increasing the yield stress. However, in this paint bake hardened automotive steel sheet, although the yield stress can be increased, the tensile strength cannot increase it.
[0005]
Japanese Examined Patent Publication No. 5-24979 discloses a uniform composition containing C: 0.08 to 0.20%, Mn: 1.5 to 3.5% and the balance Fe and unavoidable impurities, and having a uniform structure with a ferrite content of 5% or less. A bake-hardening high-tensile cold-rolled steel sheet made of bainite or bainite partially containing martensite is disclosed. The cold-rolled steel sheet described in Japanese Patent Publication No. 5-24979 is formed by quenching the temperature range of 400 to 200 ° C in the cooling process after continuous annealing and then gradually cooling the structure to make the structure mainly composed of conventional ferrite. From a structure | tissue, it is going to obtain the high bake hardening amount which was not conventionally as a structure | tissue mainly composed of bainite.
[0006]
However, in the steel sheet described in Japanese Patent Publication No. 5-24979, although the yield strength is increased after paint baking and a high bake hardening amount that has not been obtained in the past can be obtained, it still cannot be increased by the tensile strength. There was a problem that improvement in impact resistance characteristics could not be expected.
Several steel sheets that have been subjected to heat treatment after press forming to increase not only the yield stress but also the tensile strength are proposed.
[0007]
For example, Japanese Patent Publication No. 8-23048 discloses steel containing C: 0.02 to 0.13%, Si: 2.0% or less, Mn: 0.6 to 2.5%, sol.Al: 0.10% or less, and N: 0.0080 to 0.0250%. , Re-heated to 1100 ° C or higher, hot rolled to finish finish rolling at 850-950 ° C, then cooled to a temperature of less than 150 ° C at a cooling rate of 15 ° C / s or more, and ferrite and martensite There has been proposed a method for producing a hot-rolled steel sheet having a composite structure mainly composed of. However, in the steel sheet manufactured by the technique described in Japanese Patent Publication No. 8-23048, although tensile strength increases with yield stress due to strain aging hardening, it is wound at an extremely low winding temperature of less than 150 ° C. There was a problem that the fluctuation of mechanical characteristics was large. In addition, there is a large variation in the amount of increase in yield stress after press molding-paint baking, and there is also a problem that the hole expansion rate (λ) is low, stretch flangeability is lowered, and press formability is insufficient.
[0008]
Japanese Patent No. 2805513 proposes a method for producing a hot-dip galvanized steel sheet using a hot-rolled sheet as a plating original sheet. In this method, a steel slab containing C: 0.05% or less, Mn: 0.05 to 0.5%, Al: 0.1% or less, and Cu: 0.8 to 2.0% is hot-rolled at a coiling temperature of 530 ° C. or less. Subsequently, after heating to a temperature of 530 ° C. or lower to reduce the surface of the steel sheet, hot-dip galvanization is performed, whereby significant hardening by heat treatment after forming is obtained. However, in the steel sheet produced by this method, in order to obtain significant hardening by post-forming heat treatment, the heat treatment temperature needs to be 500 ° C. or higher, and the heat treatment temperature is high, leaving a problem in practical use.
[0009]
Japanese Patent Application Laid-Open No. 10-310824 proposes a method for producing an alloyed hot-dip galvanized steel sheet in which a hot rolled plate or a cold rolled plate is used as a plating original plate and an increase in strength can be expected by heat treatment after forming. This method includes C: 0.01 to 0.08%, and Si, Mn, P, S, Al, and N are used in appropriate amounts, and one or more of Cr, W, and Mo are added in a total amount of 0.05 to 3.0. % Hot-rolled steel, or further cold-rolled or temper-rolled and annealed, and then hot-dip galvanized and then heat-alloyed. This steel sheet is said to have an increased tensile strength by heating in a temperature range of 200 to 450 ° C. after forming. However, since the obtained steel sheet has a microstructure of ferrite single phase, ferrite + pearlite, or ferrite + bainite structure, high ductility and low yield strength cannot be obtained, and press formability is deteriorated. was there.
[0010]
[Problems to be solved by the invention]
As described above, the present invention has been made in view of the fact that there has never been a technique for industrially and stably producing a steel sheet that satisfies these characteristics, despite extremely strong demands. Strain aging that advantageously solves the above-mentioned problems, has excellent press formability, suitable as a steel sheet for automobiles, and has extremely high tensile strength due to heat treatment at a relatively low temperature after press forming. An object of the present invention is to propose a high-tensile cold-rolled steel sheet having excellent hardening characteristics and a production method capable of stably producing the high-tensile cold-rolled steel sheet.
[0011]
[Means for solving problems]
  In order to achieve the above-mentioned problems, the present inventors have conducted extensive studies on the influence of alloy elements on strain age hardening characteristics. As a result, the C content is set to a low carbon range, the Cu content within the proper range, or the proper content range.W or evenMo,Cr ofOne or two selected from our houseSeedIn addition, after the steel sheet structure is a composite structure of ferrite and martensite, the pre-deformation amount is set to 5% or more and after heat treatment at a relatively low temperature of 150 ° C. or more and 350 ° C. or less, In addition to increasing the yield stress, it has been found that high strain age hardening can be obtained in which the tensile strength also increases significantly. In addition to such high strain age hardening characteristics, the present inventors have found that the steel sheet has excellent ductility, low yield strength, and high hole expansion ratio, and is excellent in press formability.
[0012]
First, basic experimental results performed by the present inventors will be described.
Containing 0.04%, Si: 0.02%, Mn: 1.7%, P: 0.01%, S: 0.005%, Al: 0.04%, N: 0.002%, Cu 0.3%, 1.3% The sheet bar having the changed composition was heated to 1150 ° C. and soaked, and then subjected to three-pass rolling so that the finish rolling finish temperature was 900 ° C. to obtain a plate thickness of 4.0 mm. In addition, after finishing rolling and coil winding, a heat treatment equivalent to 600 ° C. × 1 h was performed. Subsequently, cold rolling with a rolling reduction of 70% was performed to obtain a cold rolled sheet having a sheet thickness of 1.2 mm. Subsequently, these cold-rolled plates were subjected to recrystallization annealing under various conditions.
[0013]
About the obtained cold-rolled steel plate, the tensile test was implemented and the tensile characteristic was investigated. Furthermore, the strain age hardening characteristics of these cold-rolled steel sheets were investigated.
First, specimens are collected from these cold-rolled steel sheets, pre-deformed with a tensile pre-strain amount of 5%, and then subjected to a heat treatment of 50 to 350 ° C x 20 min. The characteristics were determined. The strain age hardening property was evaluated by the increase in tensile strength ΔTS before and after the heat treatment. ΔTS is the tensile strength TS after heat treatment_HTAnd the difference in tensile strength TS when heat treatment is not performed (= (tensile strength TS after heat treatment_HT)-(Tensile strength TS before pre-deformation treatment)). The tensile test was carried out using JIS No. 5 tensile test pieces.
[0014]
FIG. 1 shows the effect of Cu content on the relationship between ΔTS and the recrystallization annealing temperature. ΔTS is obtained by subjecting the test piece taken from the obtained cold-rolled steel sheet to a pre-deformation treatment with a tensile pre-strain amount of 5%, followed by a heat treatment of 250 ° C. × 20 min, and then carrying out a tensile test. It was.
As shown in FIG. 1, when the Cu content is 1.3% by mass, the recrystallization annealing temperature is set to 700 ° C. or higher, and the steel plate structure is made of a ferrite + martensite composite structure, whereby a high strain age hardening characteristic of ΔTS: 80 MPa or higher is obtained. It turns out that it is obtained. On the other hand, when the Cu content is 0.3% by mass, ΔTS: less than 80 MPa at any recrystallization annealing temperature, and high strain age hardening characteristics cannot be obtained. It can be seen from FIG. 1 that a cold-rolled steel sheet having high strain age hardening characteristics can be produced by optimizing the Cu content and making the structure a ferrite + martensite composite structure.
[0015]
FIG. 2 shows the influence of the Cu content on the relationship between ΔTS and the heat treatment temperature after the pre-deformation treatment. After cold rolling, the steel sheet was annealed at 800 ° C, which is a two-phase region of ferrite (α) + austenite (γ), for a holding time of 40 s, and then at a cooling rate of 30 ° C / s from the holding temperature (800 ° C). What was cooled to room temperature was used. The microstructure of these steel sheets was a composite structure of ferrite and martensite (second phase), and the martensite structure fraction was 8% in terms of area ratio.
[0016]
From FIG. 2, ΔTS increases as the heat treatment temperature increases, but the amount of increase greatly depends on the Cu content. It can be seen that when the Cu content is 1.3% by mass, a high strain age hardening property of ΔTS: 80 MPa or more can be obtained at a heat treatment temperature of 150 ° C. or higher. When the Cu content is 0.3% by mass, ΔTS: less than 80 MPa at any heat treatment temperature, and high strain age hardening characteristics cannot be obtained.
[0017]
Further, for cold-rolled steel sheets with Cu contents of 0.3 mass% and 1.3 mass%, the recrystallization annealing conditions after cold rolling were changed variously, the structure was changed from ferrite + austenite to a ferrite single phase, and the yield ratio YR (= ( A material (steel plate) having a yield strength YS / tensile strength TS) × 100%) of 50 to 90% was produced. About these materials (steel plate), the hole expansion test was implemented and the hole expansion rate ((lambda)) was calculated | required. In the hole expansion test, a punch hole is formed in the specimen by punching with a 10mmφ punch, and then a conical punch with a vertex angle of 60 ° is used, and a crack that penetrates the plate thickness is generated with the beam facing outward. Hole expansion was performed until the hole expansion rate λ was obtained. The hole expansion ratio λ is λ (%) = {(d−d₀) / D₀} × 100. D₀: Initial hole diameter, d: Inner hole diameter when cracking occurs.
[0018]
These results are summarized in the relationship between the hole expansion ratio λ and the yield ratio YR, and the influence of the Cu content on the relationship between the hole expansion ratio λ and the yield ratio YR is shown in FIG.
From FIG. 3, in the steel sheet with Cu: 0.3% by mass, the ferrite + martensite composite structure is formed, and when YR is less than 70%, λ decreases with a decrease in YR. It can be seen that a high λ value is maintained even when the YR becomes low because of a composite structure of ferrite and martensite. On the other hand, in a steel sheet having a Cu content of 0.3% by mass, low YR and high λ cannot be obtained simultaneously.
[0019]
It can be seen from FIG. 3 that a steel sheet satisfying both a low yield ratio and a high hole expansion rate can be produced by setting the Cu content within an appropriate range and forming a composite structure of ferrite and martensite.
In the cold-rolled steel sheet of the present invention, pre-deformation with a strain amount greater than 2%, which is the pre-strain amount when measuring the increase in deformation stress before and after normal heat treatment, and in a relatively low temperature range of 150 ° C. to 350 ° C. By this heat treatment, ultrafine Cu is precipitated in the steel sheet. According to the study by the present inventors, it is considered that high strain age hardening characteristics in which the tensile strength is remarkably increased in addition to the increase in yield stress are obtained by the precipitation of this ultrafine Cu. Such precipitation of ultrafine Cu by heat treatment in a low temperature region was not observed at all in the extremely low carbon steel or low carbon steel reported so far. The reason why ultrafine Cu is precipitated by heat treatment in a low temperature region is not clear until now, but during annealing in the α + γ two-phase region, a large amount of Cu is distributed in the γ phase, It is thought that it was taken over after cooling and Cu was in a supersaturated solid solution in martensite, and was deposited very finely by adding 5% or more of pre-strain and low-temperature heat treatment.
[0020]
  In addition, the detailed mechanism by which the hole expansion rate of steel sheets with a Cu-added microstructure with a composite structure of ferrite and martensite is high has not been clarified so far. It is thought that this is because the difference in hardness between the two is small.
  Based on the above-described novel findings, the present inventors have conducted further intensive studies, and as a result, have found that the phenomenon described above also occurs in a steel sheet not containing Cu. Instead of CuW or evenMo,Cr of1 or 2 of themSeedContaining and making the structure a composite structure of ferrite + martensite, when pre-strain is added and heat treatment is performed at a low temperature, ultrafine carbides are strain-induced precipitated in the martensite and the tensile strength increases. I found. This strain-induced fine precipitation during low-temperature heating isW or evenMo,Cr of1 or 2 of themSeedsIn addition, it has also been found that the addition of one or more of Nb, V, and Ti becomes more prominent.
[0021]
  The present invention has been completed by further study based on the above-described findings, and the gist of the present invention is as follows..
[0022]
  (1) By mass%, C: 0.15% or less, Si: 2.0% or less, Mn: 3.0% or less, P: 0.1% or less, S: 0.02% or less, Al: 0.1% or less, N: 0.02% or less, Cu :1.21~ 3.0%In the press formability characterized in that it has a composition consisting of Fe and inevitable impurities in the balance, and the structure is a composite structure consisting of a ferrite phase and a martensite phase with an area ratio of 2% or more. A cold-rolled steel sheet that is excellent and has excellent strain aging hardening characteristics such as ΔTS: 80 MPa or more.
[0023]
  (2) (1In addition to the above composition, the following groups A to C in mass%:
          Group A: Ni: 2.0% or less
          Group B: One or two of Cr and Mo in total 2.0% or less
          Group C: A total of 0.2% or less of one or more of Nb, Ti and V
A cold-rolled steel sheet having excellent press-formability characterized by containing one group or two or more groups, and having excellent strain aging hardening properties such that ΔTS: 80 MPa or more.
[0024]
  (3) Quality%: C: 0.15% or less, Si: 2.0% or less, Mn: 3.0% or less, P: 0.1% or less, S: 0.02% or less, Al: 0.1% or less, N: 0.02% or less, and, W: 0.05-2.0%Contained, the balance being composed of Fe and inevitable impuritiesHowever, the structure is a composite structure composed of a ferrite phase and a martensite phase with an area ratio of 2% or more.A cold-rolled steel sheet having excellent press formability and excellent strain aging hardening characteristics of ΔTS: 80 MPa or more.
(4) In (3), in addition to the above composition, in mass%, Mo : 0.05 ~ 2.0 %, Cr : 0.05 ~ 2.0 % Of 1 or 2 selected from% 2.0 % Or less, and excellent in press formability and ΔTS: 80MPa Cold rolled steel sheet with excellent strain age hardening characteristics.
[0025]
  (5) In (3) or (4), in addition to the above-mentioned composition, it further comprises, in mass%, one or more of Nb, Ti, and V containing 2.0% or less in total. A cold-rolled steel sheet with excellent press formability and excellent strain age hardening characteristics such as ΔTS: 80 MPa or more.
  (6) By mass%, C: 0.15% or less, Si: 2.0% or less, Mn: 3.0% or less, P: 0.1% or less, S: 0.02% or less, Al: 0.1% or less, N: 0.02% or less, Cu :1.21~ 3.0%
Or further, the following group A to group C
          Group A: Ni: 2.0% or less
          Group B: One or two of Cr and Mo in total 2.0% or less
          Group C: A total of 0.2% or less of one or more of Nb, Ti and V
A steel slab having a composition containing one or more groups of the above, the balance being Fe and inevitable impurities, and hot rolling the material to form a hot-rolled sheet, In the method of manufacturing a cold-rolled steel sheet, a cold-rolling process in which a cold-rolled sheet is subjected to cold rolling and a cold-rolled sheet and a recrystallization annealing process in which the cold-rolled sheet is recrystallized and annealed is sequentially performed. The recrystallization annealing is Ac₁Transformation point ~ Ac_ThreeA method for producing a cold-rolled steel sheet having excellent press-formability and excellent strain age hardening characteristics of ΔTS: 80 MPa or more, characterized in that it is carried out in a two-phase region of ferrite + austenite in the temperature range of the transformation point.
[0026]
  (7) In (6), instead of the steel slab having the above composition, in mass%, C: 0.15% or less, Si: 2.0% or less, Mn: 3.0% or less, P: 0.1% or less, S: 0.02% Below, including Al: 0.1% or less, N: 0.02% or less, and, W: 0.05-2.0%Contains,Or even more Mo , Cr 1 or 2 of them in total with W 2.0 % Or less,Alternatively, it contains one or more of Nb, Ti, and V in total of 2.0% or less.The restA method for producing a cold-rolled steel sheet, which is excellent in press formability and has excellent strain age hardening characteristics such that ΔTS: 80 MPa or more, characterized in that the steel slab has a composition comprising Fe and inevitable impurities.
[0027]
DETAILED DESCRIPTION OF THE INVENTION
The cold-rolled steel sheet of the present invention is a high-tensile cold-rolled steel sheet with a tensile strength of TS: 440 MPa or more, excellent in press formability, and significantly increased in tensile strength by heat treatment at a relatively low temperature after press forming. In addition, the steel sheet is excellent in strain age hardening characteristics such that ΔTS: 80 MPa or more.
[0028]
In the present invention, “excellent in strain age hardening characteristics” means, as described above, after a pre-deformation treatment with a tensile plastic strain amount of 5% or more, at a temperature in the range of 150 to 350 ° C. and a holding time of 30 seconds or more. This means that when heat treatment is performed, the amount of increase in tensile strength ΔTS {= (tensile strength after heat treatment) − (tensile strength before pre-deformation treatment)} before and after this heat treatment is 80 MPa or more. Desirably, ΔTS is 100 MPa or more. It goes without saying that the yield stress is also increased by this heat treatment, and ΔYS {= (yield strength after heat treatment) − (yield strength before pre-deformation treatment)}: 80 MPa or more.
[0029]
The amount of pre-strain (pre-deformation) is an important factor when defining strain age hardening characteristics. The present inventors investigated the influence of the amount of pre-strain on the subsequent strain age hardening characteristics assuming a deformation mode to which the steel sheet for automobiles is applied. As a result, except for extremely deep drawing, it can be organized by the amount of uniaxial equivalent strain (tensile strain), and in actual parts, the amount of uniaxial equivalent strain exceeds about 5%, and the strength of the parts Was found to correspond well with the strength obtained after 5% pre-strain strain aging treatment. Therefore, in the present invention, the pre-strain (deformation) of the heat treatment is set to 5% or more tensile plastic strain.
[0030]
Conventional baking treatment conditions of 170 ° C x 20 min have been adopted as standard. However, when using ultrafine Cu precipitation strengthening as in the present invention, a heat treatment temperature of 150 ° C or higher is required. . On the other hand, when the temperature exceeds 350 ° C., the effect is saturated, and conversely, it tends to soften somewhat. In addition, when heated to a temperature exceeding 350 ° C., the occurrence of thermal strain and temper color becomes remarkable. Therefore, in the present invention, the heat treatment temperature for strain age hardening is set to 150 to 350 ° C. The holding time at the heat treatment temperature is 30 s or longer. With respect to the heat treatment holding time, if it is kept at about 150 to 350 ° C. for about 30 seconds or more, almost sufficient strain age hardening is achieved. In order to obtain a larger and more stable strain age hardening, the holding time is desirably 60 seconds or longer, and more preferably 300 seconds or longer.
[0031]
The heating method in the heat treatment after the pre-deformation is not particularly limited, but, for example, induction heating, non-oxidizing flame, heating by laser, plasma, etc. are all suitable in addition to the atmosphere heating by a furnace as in the ordinary paint baking process. It is. In addition, so-called warm pressing in which the temperature of the steel sheet is increased and pressed is also an extremely effective method in the present invention.
[0032]
  First, the structure of the steel sheet of the present invention will be described.
  The cold-rolled steel sheet of the present inventionTheCelite phase and martensite phase with area ratio of 2% or moreConsisting ofHas a complex tissue.
  In order to obtain a cold-rolled steel sheet having low yield strength YS and high ductility (El) and having excellent press formability, in the present invention, the structure of the steel sheet is composed of a ferrite phase as a main phase and martensite.Composed of phasesIt needs to be a complex organization. The ferrite as the main phase is preferably 50% or more in terms of area ratio. If the ferrite content is less than 50%, it is difficult to ensure high ductility, and press formability deteriorates. Further, when a better ductility is required, the area ratio of the ferrite phase is preferably 80% or more. In order to take advantage of the composite structure, the ferrite phase is preferably 98% or less.
[0033]
  As the second phase, in the present invention, it is necessary to contain martensite in an area ratio of 2% or more. If the martensite is less than 2%, low YS and high El cannot be satisfied simultaneously. The second phase is composed of a martensite phase alone with an area ratio of 2% or more.To do.
[0034]
A cold-rolled steel sheet having the above-described structure is a steel sheet having low yield strength and high ductility, excellent press formability, and excellent strain age hardening characteristics.
Next, the reason for limiting the composition of the cold-rolled steel sheet of the present invention will be described. The mass% is simply written as%.
C: 0.15% or less
C is an element that increases the strength of the steel sheet and further promotes the formation of a composite structure of ferrite and martensite. In the present invention, C is preferably contained in an amount of 0.01% or more from the viewpoint of forming the composite structure. On the other hand, if the content exceeds 0.15%, the fraction of carbides in the steel increases, and ductility and further press formability decrease. Furthermore, as a more important problem, when the C content exceeds 0.15%, spot weldability, arc weldability, and the like are significantly reduced. For this reason, in the present invention, C is limited to 0.15% or less. From the viewpoint of moldability, the content is preferably 0.10% or less.
[0035]
Si: 2.0% or less
Si is a useful strengthening element that can increase the strength of the steel sheet without significantly reducing the ductility of the steel sheet, but when its content exceeds 2.0%, it causes deterioration of press formability, The surface properties deteriorate. For this reason, Si was limited to 2.0% or less.
[0036]
Mn: 3.0% or less
Mn has the effect of strengthening steel, further lowering the critical cooling rate at which a composite structure of ferrite and martensite can be obtained, and promoting the formation of a composite structure of ferrite and martensite. It is preferably contained depending on the cooling rate after annealing. Moreover, it is an effective element which prevents the hot crack by S, and it is preferable to contain according to the amount of S to contain. Such an effect becomes remarkable when the content is 0.5% or more. On the other hand, if it exceeds 3.0%, press formability and weldability deteriorate. Therefore, in the present invention, Mn is limited to 3.0% or less. More preferably, it is 1.0% or more.
[0037]
P: 0.10% or less
P has an effect of strengthening steel, but can be contained in a necessary amount depending on the desired strength, but if it is contained excessively, press formability deteriorates. For this reason, P was limited to 0.10% or less. When more excellent press formability is required, the content is preferably 0.08% or less.
[0038]
S: 0.02% or less
S is an element present in the steel sheet as an inclusion, and is an element that causes deterioration of the ductility and formability of the steel sheet, particularly stretch flangeability, and is preferably reduced as much as possible. In the present invention, the upper limit of S is 0.02%. When excellent stretch flange formability is required, S is preferably 0.010% or less.
[0039]
Al: 0.10% or less
Al is added as a deoxidizing element for steel and is an element useful for improving the cleanliness of steel. However, even if it exceeds 0.10%, a further deoxidizing effect cannot be obtained. Press formability deteriorates. For this reason, Al was limited to 0.10% or less. In the present invention, it does not exclude a melting method by a deoxidation method other than Al deoxidation. For example, Ti deoxidation or Si deoxidation may be performed. Included in the range. At this time, even if Ca or REM is added to the molten steel, the characteristics of the steel sheet of the present invention are not hindered at all. Of course, steel sheets containing Ca, REM, etc. are also included in the scope of the present invention.
[0040]
N: 0.02% or less
N is an element that increases the strength of the steel sheet by solid solution strengthening and strain age hardening. However, if it exceeds 0.02%, nitride increases in the steel sheet, thereby increasing the ductility of the steel sheet and further press formability. Is significantly deteriorated. For this reason, N was limited to 0.02% or less. In addition, when improvement of press formability is requested | required more, it is suitable to set it as 0.01% or less.
[0041]
  Cu:1.21~ 3.0%
  Cu is an element that remarkably increases the strain age hardening (predeformation—increase in strength after heat treatment) of the steel sheet, and is one of the most important elements in the present invention. If the Cu content is less than 0.5%, even if the predeformation-heat treatment conditions are changed, an increase in tensile strength of ΔTS: 80 MPa or more cannot be obtained. Therefore, in the present invention, Cu is 0.5% or less.upperContaining is required. On the other hand, if the content exceeds 3.0%, the effect is saturated, an effect commensurate with the content cannot be expected, and it is economically disadvantageous, which causes deterioration of press formability and further deteriorates the surface properties of the steel sheet.. YoTo achieve both a large ΔTS and excellent press formabilityIn addition,Cu1.21~3.0%Limited to. Preferably 2.5 % Or less.
[0042]
Moreover, in this invention, in addition to the above-mentioned composition containing Cu, the following A group-C group by mass% further
Group A: Ni: 2.0% or less
Group B: One or two of Cr and Mo in total 2.0% or less
Group C: A total of 0.2% or less of one or more of Nb, Ti and V
It is preferable to contain 1 group or 2 groups or more.
[0043]
Group A: Ni: 2.0% or less
Group A: Ni is an element effective for preventing surface defects generated on the steel sheet surface when Cu is added, and can be contained if necessary. When contained, the content depends on the Cu content, and is preferably about half of the Cu content. Even if the content exceeds 2.0%, the effect is saturated and an effect commensurate with the content cannot be expected, which is economically disadvantageous, and conversely, press formability deteriorates. For these reasons, Ni is preferably limited to 2.0% or less.
[0044]
Group B: One or two of Cr and Mo in total 2.0% or less
Group B: Cr and Mo, like Mn, have the action of lowering the critical cooling rate at which a composite structure of ferrite and martensite can be obtained and promoting the formation of a composite structure of ferrite and martensite. , If necessary. If one or two of Cr and Mo are contained in excess of 2.0% in total, press formability is lowered. For this reason, it is preferable to limit one or two of Group B: Cr and Mo to 2.0% or less in total.
[0045]
Group C: A total of 0.2% or less of one or more of Nb, Ti and V
Group C: Nb, Ti, and V are all carbide-forming elements, and effectively act to increase the strength by fine dispersion of carbides. Therefore, they can be selected and contained as necessary. However, if one or more of Nb, Ti, and V are contained in a total exceeding 0.2%, press formability deteriorates. For this reason, Nb, Ti, and V are preferably limited to 0.2% in total.
[0046]
  Moreover, in the present invention, instead of the above-described Cu content,W: 0.05 ~ 2.0 % Or evenMo: 0.05-2.0%, Cr: 0.05-2.0%ofOne or two selected from our houseSeedYou may contain 2.0% or less in total.
  W: 0.05 ~ 2.0 % Or evenMo: 0.05-2.0%, Cr: 0.05-2.0%ofOne or two selected from our houseSeed2.0% or less in total
  Mo, Cr and W are all elements that significantly increase the strain age hardening of the steel sheet, and are the most important elements in the present invention.W is required or even Mo , Cr TheCan be selected and contained.W or evenMo,Cr of1 or 2 of themSeedFurther, by adding a composite structure of ferrite and martensite, fine carbides are strain-induced finely precipitated during pre-deformation and heat treatment, and an increase in tensile strength of ΔTS: 80 MPa or more can be obtained. If the content of these elements is less than 0.05%, even if the predeformation-heat treatment conditions and the steel sheet structure are changed, an increase in tensile strength of ΔTS: 80 MPa or more cannot be obtained. On the other hand, even if the content of these elements exceeds 2.0%, the effects described above are saturated, an effect commensurate with the content cannot be expected, and it is economically disadvantageous, and the press formability is deteriorated. For this reason,W: 0.05 ~ 2.0 %,Mo: 0.05-2.0%, Cr: 0.05-2.0%ofLimited to range. From the viewpoint of press formability, the total content of Mo, Cr, and W was limited to 2.0% or less.
[0047]
2.0% or less of one or more of Nb, Ti and V in total
Nb, Ti, and V are all carbide-forming elements, and can be selected and contained as needed when one or more of Mo, Cr, and W are contained. By containing one or more of these Nb, Ti, and V, and further forming a composite structure of ferrite and martensite, fine carbides are strain-induced finely precipitated during pre-deformation and heat treatment, and ΔTS: 80 MPa or more An increase in tensile strength is obtained. However, if one or more of Nb, Ti, and V are contained in a total exceeding 2.0%, the press formability deteriorates. For this reason, Nb, Ti, and V are preferably limited to 2.0% or less in total.
[0048]
In addition to the above-mentioned components, although not particularly limited, B: 0.1% or less, Zr: 0.1% or less, Sb: 0.1% or less, Ca: 0.1% or less, REM: 0.1% or less, etc. There is no problem.
The balance other than the above components is Fe and inevitable impurities. As unavoidable impurities, Pb: 0.01% or less, Sn: 0.1% or less, Zn: 0.01% or less, Co: 0.1% or less are acceptable.
[0049]
Below, the manufacturing method of the cold-rolled steel plate of this invention is demonstrated.
The cold-rolled steel sheet of the present invention uses a steel slab having a composition within the above-mentioned range as a raw material, hot-rolls the raw material into a hot-rolled sheet, and cold-rolls the hot-rolled sheet. It is manufactured by sequentially performing a cold rolling process for forming a cold rolled sheet and a recrystallization annealing process for performing a recrystallization annealing on the cold rolled sheet to form a cold rolled annealed sheet.
[0050]
The steel slab to be used is preferably produced by a continuous casting method in order to prevent macro segregation of components, but may be produced by an ingot casting method or a thin slab continuous casting method. In addition to the conventional method in which a steel slab is manufactured and then cooled to room temperature and then reheated, it is not cooled and inserted into a heating furnace as it is, or rolled immediately after a slight heat retention. Energy saving processes such as direct rolling and direct rolling can be applied without problems.
[0051]
The above-mentioned raw material (steel slab) is heated and subjected to a hot rolling step of hot rolling to obtain a hot rolled sheet. The hot-rolling step is usually a known condition with no particular problem as long as a hot-rolled sheet having a desired thickness can be produced. Preferred hot rolling conditions are as follows.
Slab heating temperature: 900 ℃ or more
In the case of a composition containing Cu, the slab heating temperature is desirably low in order to prevent Cu-induced surface defects. However, if the heating temperature is less than 900 ° C., the rolling load increases and the risk of trouble during hot rolling increases. Note that the slab heating temperature is desirably 1300 ° C. or less because of an increase in scale loss accompanying an increase in oxidized weight.
[0052]
In addition, it goes without saying that using a so-called sheet bar heater for heating the sheet bar from the viewpoint of lowering the slab heating temperature and preventing troubles during hot rolling is an effective method. .
Finishing rolling finish temperature: 700 ℃ or more
By setting the finish rolling finish temperature FDT to 700 ° C. or higher, a uniform hot-rolled base metal structure that provides excellent formability after cold rolling and recrystallization annealing can be obtained. On the other hand, if the finish rolling finish temperature is less than 700 ° C., the hot rolled base metal structure becomes non-uniform, the rolling load during hot rolling increases, and the risk of problems during hot rolling increases. . For these reasons, the FDT in the hot rolling process is preferably 700 ° C. or higher.
[0053]
Winding temperature: 800 ℃ or less
The coiling temperature is preferably 800 ° C. or lower, more preferably 200 ° C. or higher. When the winding temperature exceeds 800 ° C., the scale increases and the yield tends to decrease due to the scale loss. When the coiling temperature is less than 200 ° C., the shape of the steel sheet is significantly disturbed, increasing the risk of causing problems in actual use.
[0054]
Thus, in the hot rolling step of the present invention, after the slab is heated to 900 ° C. or higher, the finish rolling finish temperature is hot rolled to 700 ° C. or higher, and the coiling temperature is 800 ° C. or lower, preferably 200 ° C. or higher. It is preferable to use a rolled hot-rolled sheet.
In the hot rolling process of the present invention, part or all of finish rolling may be lubricated rolling in order to reduce the rolling load during hot rolling. Performing lubrication rolling is also effective from the viewpoint of uniform steel plate shape and uniform material. In addition, it is preferable to make the wear coefficient in the case of lubrication rolling into the range of 0.25-0.10. Moreover, it is preferable to set it as the continuous rolling process which joins the sheet | seat bars which precede and follow, and finish-rolls continuously. The application of the continuous rolling process is also desirable from the viewpoint of the operational stability of hot rolling.
[0055]
Next, a cold rolling process is performed on the hot rolled sheet. In the cold rolling step, the hot rolled sheet is cold rolled to obtain a cold rolled sheet. The cold rolling conditions are not particularly limited as long as the cold rolled sheet having a desired size and shape can be obtained, but the rolling reduction during cold rolling is preferably 40% or more. This is because when the rolling reduction is less than 40%, recrystallization hardly occurs at the time of recrystallization annealing, which is a subsequent process.
[0056]
Next, a recrystallization annealing process is performed on the cold-rolled sheet to form a cold-rolled annealed sheet. The recrystallization annealing is preferably performed in either a continuous annealing line or a continuous hot dip galvanizing line. The annealing temperature for recrystallization annealing is Ac₁Transformation point ~ Ac_ThreeIt is preferable to carry out in the (α + γ) two-phase region in the temperature range of the transformation point. Annealing temperature is Ac₁ Below the transformation point, it becomes a ferrite single phase, whereas Ac_ThreeAt a high temperature exceeding the transformation point, the crystal grains become coarse and become an austenite single phase region, and the press formability deteriorates remarkably. Further, by annealing in the (α + γ) two-phase region, a composite structure of ferrite and martensite is obtained, and a high ΔTS is obtained.
[0057]
In addition, it is preferable that the cooling at the time of recrystallization annealing shall be 1 degree-C / s or more from a viewpoint of martensite formation.
Further, after the recrystallization annealing step, a temper rolling step of 10% or less may be added in order to adjust the shape correction, surface roughness and the like.
The cold-rolled steel sheet of the present invention can be applied not only as a working steel sheet but also as an original sheet of a surface-treated steel sheet for processing. Examples of the surface treatment include galvanization (including alloy system), tin plating, enamel and the like.
[0058]
Further, the cold-rolled steel sheet of the present invention may be subjected to a special treatment for improving chemical conversion treatment properties, weldability, press formability, corrosion resistance, and the like after galvanization.
[0059]
【Example】
Example 1
Molten steel having the composition shown in Table 1 was melted in a converter and made into a slab by a continuous casting method. Next, after these steel slabs were heated to 1150 ° C, a hot rolling process (finishing temperature: 900 ° C, coiling temperature: 600 ° C) was applied to the hot rolling steel strip with a thickness of 4.0 mm (hot Sheet). Subsequently, a cold-rolled steel strip (cold-rolled sheet) having a thickness of 1.2 mm was obtained by a cold-rolling process in which these hot-rolled steel bands (hot-rolled sheets) were pickled and cold-rolled. Subsequently, these cold-rolled steel strips (cold-rolled sheets) were subjected to recrystallization annealing at the annealing temperatures shown in Table 2 using a continuous annealing line. The obtained steel strip (cold-rolled annealed sheet) was further subjected to temper rolling with an elongation of 0.8%.
[0060]
Test specimens were collected from the obtained steel strips and examined for microstructure, tensile properties, strain age hardening properties, and hole expandability. The press formability was evaluated from the elongation El (ductility), the yield strength, and the hole expansion rate.
(1) Microscopic tissue
A specimen is collected from the obtained steel strip, and the cross section (C cross section) orthogonal to the rolling direction is used to image the microstructure using an optical microscope or a scanning electron microscope, and in the main phase using an image analyzer. The structure fraction of a certain ferrite and the type and structure fraction of the second phase were determined.
[0061]
(2) Tensile properties
From the obtained steel strip, a JIS No. 5 tensile test piece was collected and subjected to a tensile test in accordance with the provisions of JIS Z 2241 to determine the yield strength YS, tensile strength TS, elongation El, and yield ratio YR. .
(3) Strain age hardening characteristics
JIS No. 5 specimens were taken from the obtained steel strip (cold-rolled annealed plate) in the rolling direction, subjected to 5% plastic deformation as pre-deformation (tensile pre-strain), and then heat-treated at 250 ° C for 20 min. After that, a tensile test was performed, and the tensile properties after the heat treatment (yield stress YS_HT, Tensile strength TS_HT), ΔYS = YS_HT-YS, ΔTS = TS_HT-TS was calculated. YS_HT, TS_HTIs the yield stress and tensile strength after pre-deformation-heat treatment, and YS and TS are the yield strength and tensile strength of the steel strip (cold-rolled annealed plate).
[0062]
(4) Hole expandability
A test piece taken from the obtained steel strip was punched with a 10mmφ punch to form a punch hole, and then a crack penetrating the plate thickness with a conical punch with an apex angle of 60 ° so that the beam was on the outside Hole expansion was performed until the occurrence of, and the hole expansion rate λ was obtained. The hole expansion ratio λ is λ (%) = {(d−d₀) / D₀} × 100. D₀: Initial hole diameter (punch hole), d: inner hole diameter when cracking occurs.
[0063]
These results are shown in Table 3.
[0064]
[Table 1]

[0065]
[Table 2]

[0066]
[Table 3]

[0067]
Each of the examples of the present invention has a low yield strength YS, a high elongation El, a low yield ratio YR, shows a larger hole expansion ratio λ, and is excellent in press formability including stretch flangeability, The steel sheet exhibits an extremely large ΔTS and has excellent strain age hardening characteristics. On the other hand, in the comparative example outside the scope of the present invention, the yield strength YS is high, the elongation El is low, the hole expansion ratio λ is small, ΔTS is small, press formability, and strain age hardening characteristics are obtained. It is a lowered steel plate.
[0068]
(Example 2)
Molten steel having the composition shown in Table 4 was melted in a converter and made into a slab by a continuous casting method. Next, after heating these steel slabs to 1250 ° C, a hot rolling process with hot rolling at a finish rolling finish temperature of 900 ° C and a coiling temperature of 600 ° C is used to produce a hot rolled steel strip with a thickness of 4.0 mm (hot Sheet). Subsequently, a cold-rolled steel strip (cold-rolled sheet) having a thickness of 1.2 mm was obtained by a cold-rolling process in which these hot-rolled steel bands (hot-rolled sheets) were pickled and cold-rolled. Subsequently, these cold-rolled steel strips (cold-rolled sheets) were subjected to recrystallization annealing at the annealing temperatures shown in Table 5 using a continuous annealing line. The obtained steel strip (cold-rolled annealed sheet) was further subjected to temper rolling with an elongation of 0.8%.
[0069]
Test specimens were collected from the obtained steel strips, and the microstructure, tensile properties, strain age hardening properties, and hole expansibility were investigated in the same manner as in Example 1. The press formability was evaluated from the elongation El (ductility), the yield strength, and the hole expansion rate.
These results are shown in Table 6.
[0070]
[Table 4]

[0071]
[Table 5]

[0072]
[Table 6]

[0073]
Each of the examples of the present invention has a low yield strength YS, a high elongation El, a low yield ratio YR, shows a larger hole expansion ratio λ, and is excellent in press formability including stretch flangeability, The steel sheet exhibits an extremely large ΔTS and has excellent strain age hardening characteristics. On the other hand, in the comparative example outside the scope of the present invention, the yield strength YS is high, the elongation El is low, the hole expansion ratio λ is small, ΔTS is small, press formability, and strain age hardening characteristics are obtained. It is a lowered steel plate.
[0074]
【The invention's effect】
According to the present invention, while maintaining excellent press formability, it becomes possible to stably manufacture a cold-rolled steel sheet whose tensile strength is remarkably increased by heat treatment after press forming. Play. When the cold-rolled steel sheet of the present invention is applied to automobile parts, there is an effect that press forming is easy and the characteristics of the parts after completion can be stably increased, and it can contribute sufficiently to the weight reduction of the automobile body.
[Brief description of the drawings]
FIG. 1 is a graph showing the effect of Cu content on the relationship between ΔTS after pre-deformation and heat treatment and the recrystallization annealing temperature.
FIG. 2 is a graph showing the effect of Cu content on the relationship between ΔTS after pre-deformation and heat treatment and the heat treatment temperature.
FIG. 3 is a graph showing the influence of Cu content on the relationship between λ and YR.

Claims (10)

% By mass
C: 0.15% or less, Si: 2.0% or less,
Mn: 3.0% or less, P: 0.1% or less,
S: 0.02% or less, Al: 0.1% or less,
N: 0.02% or less, Cu: 1.21 to 3.0 %
And the balance is Fe and inevitable impurities, and the structure is a composite structure comprising a ferrite phase and a martensite phase with an area ratio of 2% or more. A cold-rolled steel sheet that is excellent and has excellent strain aging hardening characteristics such as ΔTS: 80 MPa or more. The cold-rolled steel sheet according to claim 1, further comprising one group or two or more groups of the following groups A to C in mass% in addition to the composition.
Record
Group A: Ni: 2.0% or less
Group B: One or two of Cr and Mo in total 2.0% or less
Group C: A total of 0.2% or less of one or more of Nb, Ti and V % By mass
C: 0.15% or less, Si: 2.0% or less,
Mn: 3.0% or less, P: 0.1% or less,
S: 0.02% or less, Al: 0.1% or less,
N: 0.02% or less, W: 0.05-2.0%, the balance is composed of Fe and inevitable impurities, the structure is a ferrite phase and martensite with an area ratio of 2% or more A cold-rolled steel sheet having excellent press formability, characterized by being a composite structure composed of phases, and having excellent strain age hardening characteristics such as ΔTS: 80 MPa or more.   In addition to the above composition, the composition further comprises one or two selected from Mo: 0.05 to 2.0% and Cr: 0.05 to 2.0% in terms of mass% in total of 2.0% or less with W. The cold-rolled steel sheet according to claim 3.   The cold-rolled steel sheet according to claim 3 or 4, further comprising 2.0% or less in total of one or more of Nb, Ti, and V in addition to the composition. . % By mass
C: 0.15% or less, Si: 2.0% or less,
Mn: 3.0% or less, P: 0.1% or less,
S: 0.02% or less, Al: 0.1% or less,
N: 0.02% or less, Cu: 1.21 to 3.0 %
A steel slab having a composition composed of Fe and inevitable impurities as a raw material, and hot rolling the hot rolled sheet by subjecting the raw material to hot rolling, and cold rolling the cold rolled sheet In the method of manufacturing a cold-rolled steel sheet, in which a cold-rolling step for forming a sheet and a recrystallization annealing step for performing a recrystallization annealing on the cold-rolled sheet to sequentially perform recrystallization annealing, the recrystallization annealing is performed at Ac ₁ transformation point to A method for producing a cold-rolled steel sheet excellent in press formability, which is performed in a two-phase region of ferrite + austenite in the temperature range of the Ac ₃ transformation point, and strain age-hardening characteristics of ΔTS: 80 MPa or more. In addition to the said composition, it is set as the steel slab of the composition containing 1 group or 2 groups or more of the following A group-C group by the mass%, The cold-rolled steel plate of Claim 6 characterized by the above-mentioned. Production method.
Record
Group A: Ni: 2.0% or less
Group B: One or two of Cr and Mo in total 2.0% or less
Group C: A total of 0.2% or less of one or more of Nb, Ti and V Instead of the steel slab having the above composition, in mass%,
C: 0.15% or less, Si: 2.0% or less,
Mn: 3.0% or less, P: 0.1% or less,
S: 0.02% or less, Al: 0.1% or less,
The cold-rolled steel sheet according to claim 6, wherein the steel slab contains N: 0.02% or less, further contains W: 0.05-2.0%, and the balance is composed of Fe and inevitable impurities. Production method.   In addition to the above composition, a steel slab having a composition containing, by mass%, one or two selected from Mo: 0.05 to 2.0% and Cr: 0.05 to 2.0% in total with W in an amount of 2.0% or less. The method for producing a cold-rolled steel sheet according to claim 8.   10. A steel slab having a composition containing not less than 2.0% in total of one or more of Nb, Ti, and V in addition to the above composition. A method for producing a cold-rolled steel sheet according to 1.

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