JP2004232018A - Method for producing complex structure type high tension cold-rolled steel sheet excellent in deep drawability - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a method for producing a complex structure type high tension cold-rolled steel sheet excellent in deep drawability. <P>SOLUTION: A steel slab having the composition by mass% of 0.01-0.05% C, 0.1-1.5% Si, 1.0-3.0% Mn, ≤0.10% P, ≤0.02% S, 0.005-0.1% Al, ≤0.02% N, 0.01-0.2% V and 0.001-0.2% Nb and satisfying the relation of 0.5× C/12 ≤(V/51+Nb/93) ≤2×C/12, is subjected to hot-rolling and cold-rolling, in order. Thereafter, annealing is applied at 650-780°C and again, the cold-rolling is applied and successively, after cooling, this steel sheet is heated at the temperature higher than the temperature obtaining the structure containing ferrite-phase and martensite-phase and higher than recrystallizing temperature and ≤950°C, and the annealing, in which after heating to the above temperature, the cooling is applied at an average cooling speed of at least 5°C/s until it reaches 400°C. <P>COPYRIGHT: (C)2004,JPO&NCIPI

Description

【００７１】
【表２】

【００７２】
表２に示す結果から、本発明例は、いずれも、目標とする、低い降伏比（ＹＲ≦７０％）、高い伸び（Ｅｌ≧２８％）および高いランクフォード値（ｒ値≧１．３）を有し、深絞り成形性に優れた鋼板となっている。特に本発明例では、熱処理条件を規定し、２回冷延２回焼鈍工程を採用することで、１回冷延１回焼鈍工程を採用した場合よりも、ｒ値が飛躍的に上昇し、高強度鋼板ながらｒ値１．３以上を確保できる。これに対し、本発明の範囲を外れる条件で製造した比較例では、ランクフォード値（ｒ値）が低下しているか、あるいは、強度伸びバランスが低下した鋼板となっている。
【００７３】
【発明の効果】
本発明によれば、強度伸びバランスに優れるとともに、深絞り成形性にも優れた冷延鋼板を、工程に対して低負荷で且つ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に適用した場合、プレス成形が容易で、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】ＶとＮｂの含有量とＣの含有量との関係を表す比（Ｖ／５１＋Ｎｂ／９３）／（Ｃ／１２）がランクフォード値（ｒ値）と強度伸びバランス（ＴＳ×Ｅｌ）に及ぼす影響を示した図である。
【図２】一次焼鈍温度とランクフォード（ｒ値）との関係を示した図である。
【図３】本発明の２回冷延−２回焼鈍工程（本発明法）と、従来の１回冷延−１回焼鈍工程（従来法）とで製造した冷延鋼板について、トータル圧下率（％）がｒ値に及ぼす影響を示した図である。[0001]
TECHNICAL FIELD OF THE INVENTION
The present invention relates to a method for producing a composite structure type high-tensile cold-rolled steel sheet excellent in deep drawability and useful in applications such as steel sheets for automobiles and having a tensile strength of 440 MPa or more.
[0002]
[Prior art]
2. Description of the Related Art In recent years, from the viewpoint of preserving the global environment, there has been a demand for improved fuel efficiency of automobiles. In addition, from the viewpoint of protecting occupants in the event of a vehicle collision, it is also required to improve the safety of the vehicle body. Under such circumstances, studies for satisfying both the weight reduction and strengthening of the vehicle body are being actively pursued.
It is said that it is effective to increase the strength of component materials in order to satisfy the weight reduction and strengthening of an automobile body at the same time. Recently, high-tensile steel sheets have been actively used for automobile components.
[0003]
Since many automotive parts made of steel sheets are formed by press working, steel sheets for automobiles are required to have excellent press formability. However, in general, when the strength of a steel sheet is increased, the Rankford value (r value) and the ductility (El) are reduced to deteriorate press formability, and the yield stress is increased to deteriorate the shape freezing property. There is. In particular, the larger the value of the so-called strength-elongation balance, represented by the product TS × El of the tensile strength (TS) and the ductility (El), is more advantageous for press formability. Ductileness has been attempted.
For a steel plate having both high strength and high ductility, a dual-phase steel (having two phases of a ferrite phase and a martensite phase) including a retained austenitic steel (residual γ steel) utilizing a strain-induced plasticity (TRIP) phenomenon. Development research on so-called composite structure steels, such as DP steels, is underway.
[0004]
A typical example of a high-strength steel sheet having good press formability is a composite steel sheet composed of a composite structure of a ferrite phase and a martensite phase. It has low stress (YS) and has both high ductility (El) and excellent bake hardenability. However, although the composite structure steel sheet is generally good in workability, it has a disadvantage that it has a low Rankford value (r value) and is inferior in deep drawability.
[0005]
In recent years, with the increase in strength of automobiles, not only high ductility but also excellent deep drawability, that is, a high r value is required for high-strength steel sheets having a tensile strength of 440 MPa or more, particularly 590 MPa class.
[0006]
Until now, attempts have been made to improve the deep drawability by increasing the Rankford value (r value) of a composite structure steel sheet. For example, in Patent Document 1, after cold rolling, the recrystallization temperature ~ A _c3 A technique is disclosed in which box annealing is performed at the temperature of the transformation point, and thereafter, the steel sheet is heated to 700 to 800 ° C. in order to form a composite structure, followed by continuous annealing accompanied by quenching and tempering. However, in this method, since quenching and tempering are performed during continuous annealing, the yield stress is high, and a low yield ratio cannot be obtained. Here, the yield ratio (YR) is the ratio of the yield stress (YS) to the tensile strength (TS), and YR = YS / TS. The steel sheet having a high yield stress has a disadvantage that the shape freezing property of a pressed part during pressing is poor.
[0007]
[Patent Document 1]
JP-B-55-10650
[0008]
A method for improving the high yield stress is disclosed in Patent Document 2. In this method, box annealing is first performed in order to obtain a high Rankford value (r value). The temperature during box annealing is set to a two-phase region of a ferrite phase (α) -austenite phase (γ), and α Mn is concentrated from the phase to the γ phase. This Mn-enriched phase preferentially becomes a γ phase during continuous annealing, and a mixed structure can be obtained even at a cooling rate of about a gas jet, and the yield stress is low. However, this method requires long-time box annealing at a relatively high temperature in the two-phase region of the α phase and the γ phase for Mn enrichment, so that there is frequent occurrence of adhesion between steel sheets, generation of a temper color, and a furnace body. There are many problems in the manufacturing process such as shortening of the life of the inner cover. Conventionally, it has been difficult to industrially stably produce a high-strength steel sheet having such a high Rankford value (r value) and a low yield stress (YS).
[0009]
[Patent Document 2]
JP-A-55-100934
[0010]
In addition, in Patent Document 3, a steel having a composition of 0.012 mass% C-0.32 mass% Si-0.53 mass% Mn-0.03 mass% P-0.051 mass% Ti is cold-rolled. Then, after heating to 870 ° C., which is the two-phase temperature of the α-phase and γ-phase, and cooling at an average cooling rate of 100 ° C./s, r value = 1.61, YS = 224 MPa, TS = 482 MPa. There is disclosed a technology that enables production of a composite structure type cold-rolled steel sheet having a very high Rankford value (r value) and a low yield stress. However, since it is difficult to achieve a high cooling rate of 100 ° C./s with a normal continuous annealing line, water quenching equipment is required. Since it becomes obvious, there are problems in terms of manufacturing facilities and materials.
[0011]
[Patent Document 3]
Japanese Patent Publication No. 1-35900
[0012]
In general, it is effective to develop {111} recrystallized texture to increase the r-value. For this purpose, it is necessary to increase the rolling reduction during cold rolling to a certain degree, that is, to introduce strain energy. Become. However, in the case of high-strength steel sheets, when trying to achieve a high cold rolling reduction, the load on the rolls during cold rolling increases, increasing the risk of trouble during cold rolling and reducing productivity. Is concerned. In other words, it is difficult for a high-strength steel sheet to achieve a high cold-rolling reduction rate in one cold rolling step, and therefore, the {111} recrystallized texture hardly develops even if recrystallization annealing is performed continuously. Become.
[0013]
As a method for increasing the r-value of a cold-rolled steel sheet for deep drawing, a so-called two-time cold-rolling-two-time annealing method that combines two times of cold rolling and two times of annealing has been conventionally proposed. For example, in Patent Document 4, Patent Document 5, and Patent Document 6, a cold rolled ferritic single-phase steel having a strength level of TS <440 MPa is obtained by performing twice cold rolling and twice annealing on a very low carbon cold rolled steel sheet. A technique capable of increasing the r-value of a rolled steel sheet to 3.0 or more is disclosed. These techniques aim at accumulation of high strain energy by two cold rollings and accumulation of {111} recrystallization texture formation by two recrystallization annealings.
[0014]
[Patent Document 4]
JP-A-3-97812
[Patent Document 5]
JP-A-3-97813
[Patent Document 6]
JP-A-5-209228
[0015]
However, these techniques are applied to the present invention, a composite structure type high tension cold rolled steel having a basic composition of semi-ultra low carbon steel having a C content of 0.01 to 0.05% by mass and a tensile strength of 440 MPa or more. When applied to a steel sheet, depending on the primary annealing (intermediate annealing), (1) the steel sheet does not show softening due to recovery and recrystallization, and a load on the roll is required to ensure a secondary cold reduction of 40% or more. There are problems such as (2) carbides dissolving during the primary annealing to generate a large amount of solid solution C, and {111} recrystallization texture does not develop during the secondary annealing.
[0016]
Patent Document 7 discloses a composite structure type cold-rolled steel sheet excellent in deep drawability and a method for producing the same. It is disclosed that a property having an r value of 1.9 can be obtained when the tensile strength is 590 MPa or more. However, since the cold rolling reduction at that time is a cold rolling at a high rolling reduction of 70%, there are many restrictions such as rolling due to the narrow width depending on the equipment capacity, and as described above. Problems such as frequent troubles during rolling remain.
[0017]
[Patent Document 7]
JP-A-2002-226941
[0018]
[Problems to be solved by the invention]
The present invention advantageously solves the above problems both in terms of material and production, and regulates the contents of C, V, and Nb as appropriate in the steel composition, and as the production conditions, in particular, the primary annealing temperature. Proposes a technology that can stably produce a composite structure type high-tensile cold-rolled steel sheet having a high Rankford value and excellent in deep drawability by adopting a process of twice cold rolling and two times annealing. The purpose is to:
[0019]
[Means for Solving the Problems]
Means for Solving the Problems In order to achieve the above object, the present inventors have intensively studied the effects of alloying elements, cold rolling conditions and annealing conditions on the microstructure and recrystallization texture of a cold-rolled steel sheet. As a result, by setting the C content to 0.01 to 0.05% by mass and containing V and Nb in appropriate ranges, the amount of solid solution C is reduced as much as possible before the recrystallization annealing to reduce {111} By developing the crystal texture, a high Rankford value (r-value) is obtained, and the temperature is higher than the temperature at which a structure containing a ferrite phase and a martensite phase is obtained after cooling, that is, the two-phase region temperature of the α phase and the γ phase. And then cooled to room temperature at an average cooling rate of at least 5 ° C./s up to at least 400 ° C. to form a martensitic phase as a second phase, which is excellent in ductility despite high strength, Furthermore, it has been found that a composite structure type high-tensile cold-rolled steel sheet having a high Rankford value and excellent deep drawability can be manufactured. In particular, in order to minimize the load on the rolling rolls during cold rolling, increase the cold reduction rate of high-strength steel, and develop {111} recrystallization texture as much as possible, After the cold rolling, the steel is annealed at 650 to 780 ° C., which is a temperature range in which carbide hardly dissolves (that is, the solid solution C is kept low) and the steel sheet recovers, and then the second cold rolling is performed again And then heated to a temperature not lower than a temperature at which a structure containing a ferrite phase and a martensite phase can be obtained after cooling and not lower than a recrystallization temperature and not higher than 950 ° C. Has been found to be effective.
[0020]
Here, the composite structure type cold-rolled steel sheet manufactured by the method of the present invention is a composite structure steel sheet having a ferrite phase as a main phase and a second phase containing a martensite phase having an area ratio of 1% or more. The main ferrite phase is mainly a polygonal ferrite phase, but there is no problem even if a bainitic ferrite phase having a high dislocation density generated by a cooling process from the austenite region is mixed. Further, the second phase may be a martensite phase alone having an area ratio of 1% or more, or a mixture of any of a retained austenite phase, a pearlite phase, and a bainite phase as an auxiliary phase.
[0021]
First, the results of basic experiments performed by the present inventors will be described.
In mass%, C: 0.02%, Si: 0.5%, Mn: 2.0%, P: 0.05%, S: 0.005%, Al: 0.03%, N: 0. 002% as a basic composition, to which V: 0.01 to 0.1% by mass and Nb: 0.001 to 0.16% by mass to have different V and Nb contents. Various steel materials (sheet bars) were heated to 1250 ° C. and maintained at a constant temperature at this heating temperature, and then subjected to three-pass rolling so that the finish rolling end temperature was 880 ° C., to a sheet thickness of 4.0 mm. After the finish rolling, a heat treatment equivalent to 650 ° C. × 3 h was performed as a coil winding process. Subsequently, the sheet was subjected to cold rolling at a reduction rate of 50% to a sheet thickness of 2.0 mm, subjected to primary annealing at 700 to 850 ° C., and then air-cooled. Further, the sheet is subjected to cold rolling at a rolling reduction of 60% to a sheet thickness of 0.8 mm, and then subjected to secondary continuous annealing (recrystallization annealing) in a temperature range of 700 to 970 ° C. Annealing for cooling to room temperature was performed at a cooling rate of 5 ° C./s or more.
[0022]
The obtained cold-rolled steel sheet was subjected to a tensile test to investigate tensile properties. The tensile test was performed using a JIS No. 5 tensile test piece. The tensile strength TS and the ductility El are values obtained when a tensile test is performed in a direction perpendicular to the rolling direction. The r value is determined in the rolling direction (r _L ), 45 degree direction (r _D ) And the direction perpendicular to the rolling direction (90 degrees) (r _c ) = (R _L + R _c + 2 × r _D ) / 4}.
[0023]
FIG. 1 shows the effect of V on the r-value and strength-elongation balance (TS × El) of the cold-rolled steel sheet obtained in the twice cold-rolling (rolling reduction: 50 to 60%)-twice annealing (700 to 800 ° C.) process. , Nb and C contents, the horizontal axis represents the atomic ratio of the V and Nb contents to the C content ((V / 51 + Nb / 93) / (C / 12)), The axis indicates the r-value and the strength-elongation balance (TS × El) by dividing it into upper and lower parts.
[0024]
From FIG. 1, by limiting the contents of V and Nb in the steel slab to the range of 0.5 to 2.0 in terms of atomic ratio with C, a high r value and a high strength elongation balance can be obtained, It became clear that a composite structure type cold rolled steel sheet having an r value and high ductility El can be manufactured.
[0025]
Next, among the cold-rolled steel sheets used in FIG. 1, a steel material having (V / 51 + Nb / 93) / (C / 12) = 1.2 was subjected to cold rolling, and was subjected to a temperature range of 700 to 850 ° C. The primary annealing is performed, then, the second cold rolling is performed again, and after the heating is further performed at 800 ° C., the secondary cooling is performed at an average cooling rate of at least 5 ° C./s at least up to 400 ° C. FIG. 2 shows the relationship between the primary annealing temperature and Rankford (r value) of the manufactured cold-rolled steel sheet in various cold-rolled steel sheets manufactured by performing crystal annealing.
[0026]
From the results in FIG. 2, by setting the primary continuous annealing temperature to 780 ° C. or lower, a high Rank Ford value is obtained, and a composite structure type cold-rolled steel sheet having excellent deep drawability is obtained by performing twice cold rolling and two times annealing. It can be seen that it is possible to manufacture with.
[0027]
FIG. 3 shows the total rolling reduction of the cold-rolled steel sheet manufactured by the two-time cold rolling and two-time annealing process of the present invention (the method of the present invention) and the conventional one-time cold rolling and one-time annealing process (the conventional method). (%) Shows the effect on the r value.
[0028]
From the results shown in FIG. 3, the r-value increases with the high rolling reduction (this is a general tendency), but even when the cumulative total rolling reduction is the same, the present invention is more effective than the conventional method. It can be seen that the r value is higher in the method.
[0029]
In the cold-rolled steel sheet of the present invention, in the primary annealing process, the annealing process at 650 to 780 ° C recovers the rolled structure (partially recrystallizes), so that the second cold rolling is facilitated, High cold rolling reduction can be achieved in total, and carbide formed in the hot-rolled sheet hardly dissolves, and solid solution C can be kept as small as possible. Therefore, even before the second continuous annealing (recrystallization annealing), The state in which the solid solution C and the solid solution N are small is maintained, and when heated to a temperature higher than the recrystallization temperature during the second continuous annealing, the {111} recrystallization texture is strongly developed and a high r value is obtained.
[0030]
When primary continuous annealing (recrystallization annealing) is performed in a temperature range as disclosed in Patent Documents 4, 5, and 6, a large amount of solute C is present, and this is continuously secondary-rolled-secondarily continuous. It has been found that even after annealing, the {111} recrystallized texture does not further develop, but rather forms a recrystallized texture unfavorable for the r value.
[0031]
That is, the primary annealing temperature should be a temperature at which carbides hardly dissolve, and even if not completely recrystallized at that temperature, after twice cold rolling and twice annealing (recrystallization annealing), It has been found that a strong {111} recrystallized texture develops and the r-value becomes higher than in the single cold rolling and single annealing method. Of course, there is no problem that recrystallization proceeds in a temperature range in which carbides do not dissolve. Further, the primary annealing temperature is at least 650 or higher at which recovery occurs, and the development of a strong {111} recrystallized texture after two-time cold rolling and two-time annealing (recrystallization annealing) and the second cold rolling It has also been found that it is indispensable to reduce the load on the rolling roll at the time.
[0032]
The present invention has been completed by further study based on the above findings, and the gist of the present invention is as follows.
(1) In mass%
C: 0.01 to 0.05%, Si: 0.1 to 1.5%, Mn: 1.0 to 3.0%, P: 0.10% or less, S: 0.02% or less, Al : 0.005 to 0.1%, N: 0.02% or less, V: 0.01 to 0.2% and Nb: 0.001 to 0.2%, and V and Nb and C Content (% by mass)
0.5 × C / 12 ≦ (V / 51 + Nb / 93) ≦ 2 × C / 12
A steel slab having a composition satisfying the following relationship is hot-rolled, then cold-rolled, then annealed by heating to 650 to 780 ° C, cold-rolled again, and after cooling, the ferrite phase After heating to a temperature not lower than the temperature at which the structure containing the martensite phase is obtained and not lower than the recrystallization temperature and not higher than 950 ° C., annealing is performed at an average cooling rate of 5 ° C./s or higher until at least 400 ° C. To produce a composite structure type high-tensile cold-rolled steel sheet having excellent deep drawability.
[0033]
(2) In mass%
C: 0.01 to 0.05%, Si: 0.1 to 1.5%, Mn: 1.0 to 3.0%, P: 0.10% or less, S: 0.02% or less, Al : 0.005 to 0.1%, N: 0.02% or less, V: 0.01 to 0.2%, Nb: 0.001 to 0.2%, and Ti: 0.001 to 0.3% And the content (% by mass) of V, Nb and Ti and C is
0.5 × C / 12 ≦ (V / 51 + Nb / 93 + Ti / 48) ≦ 2 × C / 12
A steel slab having a composition satisfying the following relationship is hot-rolled, then cold-rolled, then annealed by heating to 650 to 780 ° C, cold-rolled again, and after cooling, the ferrite phase After heating to a temperature not lower than the temperature at which the structure containing the martensite phase is obtained and not lower than the recrystallization temperature and not higher than 950 ° C., annealing is performed at an average cooling rate of 5 ° C./s or higher until at least 400 ° C. To produce a composite structure type high-tensile cold-rolled steel sheet having excellent deep drawability.
[0034]
(3) The steel slab is excellent in deep drawability as described in (1) or (2) above, further comprising Mo: 0.01 to 0.5% by mass in addition to the above composition. A method for producing a composite structure type high-tensile cold-rolled steel sheet.
[0035]
BEST MODE FOR CARRYING OUT THE INVENTION
The cold-rolled steel sheet manufactured by the method of the present invention is a composite structure type high-tensile cold-rolled steel sheet having a tensile strength TS of 440 MPa or more and excellent in deep drawability.
[0036]
Such a composite structure type high-tensile cold-rolled steel sheet is a composite structure steel sheet having a main phase of a ferrite phase and a second phase containing a martensite phase having an area ratio of 1% or more. The main ferrite phase is mainly a polygonal ferrite phase, but there is no problem even if a bainitic ferrite phase having a high dislocation density generated by a cooling process from the austenite region is mixed. Further, the second phase may be a martensite phase alone having an area ratio of 1% or more, or a mixture of any of a retained austenite phase, a pearlite phase, and a bainite phase as an auxiliary phase. The ferrite phase, which is the main phase, has a {111} texture developed and has a high r value.
[0037]
In order to obtain a cold-rolled steel sheet having a low yield stress (YS) and a high strength-elongation balance (TS × El) and excellent deep drawability, in the present invention, the structure of the cold-rolled steel sheet is changed to ferrite as a main phase. It is necessary to form a composite structure of a phase and a second phase including a martensite phase. It is preferable that the ferrite phase, which is the main phase, has an area ratio of 80% or more. If the area ratio of the ferrite phase is less than 80%, it becomes difficult to secure a high strength-elongation balance, and the press formability tends to decrease. In the case where even better ductility and hole expandability are required, the ratio of the bainitic ferrite phase to the main phase of the polygonal ferrite phase is preferably 5% or more in terms of area ratio. In order to utilize the advantages of the composite structure, the ferrite phase is preferably set to 99% or less.
[0038]
Further, in the present invention, the second phase is a composite structure steel in which a martensitic phase is required to be present and contains 1% or more in terms of an area ratio with respect to the entire structure. If the martensite phase has an area ratio of less than 1%, it is difficult to simultaneously satisfy a low yield ratio and a high strength-elongation balance (TS × El). The second phase may be a martensite phase alone having an area ratio of 1% or more, or a martensite phase having an area ratio of 1% or more and other pearlite, bainite, and residual austenite phases as subphases. It may be mixed with either one.
[0039]
At the time of primary continuous annealing, it is important that carbides are not dissolved as much as possible. Here, the carbides in the steel produced in the method of the present invention are mainly V and Nb-based composite carbides. It is preferable that the amount of solid solution C is as small as possible before the secondary continuous (recrystallization) annealing. That is, the amount of carbide dissolved during the primary continuous annealing is determined, for example, in a hot-rolled hot-rolled steel sheet stage. It is preferable that the content is set to 50% or less of the precipitated carbide.
[0040]
Next, the reason for limiting the composition of the steel slab used in the manufacturing method of the present invention will be described. In addition, mass% is simply described as%.
C: 0.01-0.05%
C is an element that promotes the formation of a composite structure of a ferrite phase and a martensite phase, which is a main phase, and further increases the strength of a steel sheet. In the present invention, it is necessary to contain 0.01% or more from the viewpoint of forming a composite structure. There is. On the other hand, when the content exceeds 0.05%, the development of the {111} recrystallization texture is inhibited, and the deep drawability and ductility are reduced. For this reason, in the present invention, the C content is limited to 0.01 to 0.05%.
[0041]
Si: 0.1 to 1.5%
Si is a useful strengthening element that contributes to improving the ductility at the same time as increasing the strength of the steel sheet, that is, improving the strength-elongation balance. To obtain this effect, the Si content is 0.1%. It is necessary to do above. However, when the Si content exceeds 1.5%, the effect of improving ductility is reduced, and at the same time, the deep drawability is deteriorated, and the surface properties are further deteriorated. For this reason, the Si content was limited to 0.1 to 1.5%.
[0042]
Mn: 1.0-3.0%
Mn has the effect of strengthening the steel, and further lowers the critical cooling rate at which a composite structure of the ferrite phase as the main phase and the martensite phase as the second phase is obtained. That is, it has an effect of promoting the formation of a composite structure of the ferrite phase as the main phase and the martensite phase as the second phase, and is preferably contained according to the cooling rate after annealing. At a slow cooling rate below the critical cooling rate, a martensite phase is not generated, and instead a bainite phase or a pearlite phase is generated. However, when the martensite phase does not exist in the second phase, the strength-elongation balance is reduced. There is a tendency. Therefore, the addition of Mn is effective for facilitating the formation of the martensite phase, that is, for lowering the critical cooling rate. Further, Mn is an effective element for preventing hot cracking due to S, and is preferably contained according to the amount of S contained. Such effects become remarkable when Mn is contained at 1.0% or more. On the other hand, if the Mn content exceeds 3.0%, deep drawability and weldability deteriorate. For this reason, in the present invention, the Mn content is limited to the range of 1.0 to 3.0%.
[0043]
P: 0.10% or less
P has the effect of strengthening steel and can be appropriately contained according to the desired strength. In order to obtain this effect, it is preferable that P be contained at 0.005% or more. However, when the P content exceeds 0.10%, the strength-elongation balance tends to decrease and the deep drawability tends to deteriorate. For this reason, the P content is limited to 0.10% or less. When more excellent press formability is required, the P content is preferably set to 0.08% or less.
[0044]
S: 0.02% or less
S is an element that is present as an inclusion in the steel sheet and causes deterioration of the ductility, formability, and particularly stretch flangeability of the steel sheet. Therefore, it is preferable to reduce S as much as possible. In the present invention, the upper limit of the S content is 0.02%, since it has no significant adverse effect. When more excellent stretch flange formability is required, the S content is preferably 0.01% or less, more preferably 0.005% or less.
[0045]
Al: 0.005 to 0.1%
Al is added as a deoxidizing element of steel and is a useful element for improving the cleanliness of steel. However, if it is less than 0.005%, the effect of addition is ineffective, and if it exceeds 0.1%, it is contained. However, no further deoxidizing effect can be obtained, and on the contrary, deep drawability deteriorates. For this reason, the Al content was limited to 0.005 to 0.1%. Note that, in the present invention, a melting method by a deoxidizing method other than Al deoxidizing is not excluded, and for example, Ti deoxidizing or Si deoxidizing may be performed. Included in the range. At this time, even if Ca, REM, or the like is added to the molten steel, the characteristics of the steel sheet of the present invention are not impaired at all, and a steel sheet containing Ca, REM, or the like is, of course, included in the scope of the present invention.
[0046]
N: 0.02% or less
N is an element that increases the strength of a steel sheet by solid solution strengthening and strain age hardening. However, if it exceeds 0.02%, nitrides increase in the steel sheet, and the deep drawability of the steel sheet is conspicuous. Deteriorates. For this reason, N was limited to 0.02% or less. In the case where further improvement in press formability is required, it is preferable to reduce N, and it is preferable to set N to 0.004% or less.
[0047]
V: 0.01 to 0.2%, Nb: 0.001 to 0.2%, and satisfy the relationship of 0.5 × C / 12 ≦ (V / 51 + Nb / 93) ≦ 2 × C / 12.
V and Nb are the most important elements in the present invention. Before recrystallization, solid solution C is precipitated and fixed as V and Nb-based carbides to develop {111} recrystallized texture to obtain a high Rankford. Value can be obtained. Further, at the time of recrystallization annealing, which is the secondary annealing, V and Nb-based carbides are dissolved, so that a large amount of solute C is concentrated in the austenite phase, and the martensitic transformation is performed in the subsequent cooling process to form the main phase. A composite steel sheet having a ferrite phase and a martensite phase as a second phase is obtained. In order to achieve such effects, the contents of V and Nb are 0.01% or more and 0.001% or more, respectively, and the contents (% by mass) of C, V, and Nb are 0.5 × C / It is necessary to satisfy the relationship of 12 ≦ (V / 51 + Nb / 93). That is, if the V content is less than 0.01% or the Nb content is less than 0.001%, or if 0.5 × C / 12> (V / 51 + Nb / 93), the amount of dissolved C is large. Exists, the {111} recrystallized texture does not develop, and a high r value cannot be obtained. On the other hand, the content of at least one of V and Nb exceeds 0.2%, or the content (% by mass) of C, V, and Nb is (V / 51 + Nb / 93)> 2 × C / 12. In addition, since the dissolution of V and Nb-based carbides hardly occurs during the recrystallization annealing as the secondary annealing, a composite structure of the ferrite phase as the main phase and the martensite phase as the second phase cannot be obtained. Therefore, in the present invention, V: 0.01 to 0.2%, Nb: 0.001 to 0.2%, and 0.5 × C / 12 ≦ (V / 51 + Nb / 93) ≦ 2 × C / 12 Limited to satisfying the relationship.
[0048]
In the present invention, in addition to the above-described composition, it is preferable to contain 0.001 to 0.3% of Ti. In this case, the relationship between the contents (% by mass) of the above C, V, and Nb is preferable. Instead of the expression 0.5 × C / 12 ≦ (V / 51 + Nb / 93) ≦ 2 × C / 12, the above-mentioned relational expression of the contents (% by mass) of C, V, Nb and Ti, that is, 0. It is necessary to satisfy the relational expression of 5 × C / 12 ≦ (V / 51 + Nb / 93 + Ti / 48) ≦ 2 × C / 12.
Ti is a carbide-forming element. Before recrystallization, solid solution C is precipitated and fixed as V, Nb and Ti-based carbides to develop {111} recrystallization texture and obtain a high Rankford value. Further, at the time of recrystallization annealing, which is the secondary annealing, V, Nb and Ti-based carbides are dissolved, so that a large amount of solid solution C is concentrated in the austenite phase, and the main phase is transformed by martensite transformation in the subsequent cooling process. Is obtained, and a composite structure steel sheet of a ferrite phase as a second phase and a martensite phase as a second phase is obtained. To achieve such effects, the V content is 0.01% or more, the Nb content is 0.001% or more, the Ti content is 0.001% or more, and 0.5 × C / 12 ≦ (V / 51 + Nb / 93 + Ti / 48). That is, the V content is less than 0.01%, the Nb content is less than 0.001%, or the Ti content is less than 0.001%, or 0.5 × C / 12> (V / 51 + Nb / In (93 + Ti / 48), a large amount of solute C is present, {111} recrystallization texture is not developed, and a high r value cannot be obtained. On the other hand, the content of at least one of V and Nb exceeds 0.2%, the content of Ti exceeds 0.3%, or (V / 51 + Nb / 93 + Ti / 48)> 2 × C / 12. In some cases, the carbide is less likely to be dissolved in the recrystallization annealing as the secondary annealing, so that a composite structure of the ferrite phase as the main phase and the martensite phase as the second phase cannot be obtained. It forms a solid solution or forms carbon, nitrogen, sulfide, etc., and the ductility of the steel sheet is significantly deteriorated. Therefore, when Ti is contained, Ti is 0.001 to 0.3% and satisfies the relationship of 0.5 × C / 12 ≦ (V / 51 + Nb / 93 + Ti / 48) ≦ 2 × C / 12. Limited to that.
[0049]
In the present invention, it is preferable to further contain Mo: 0.01 to 0.5% in addition to the above composition.
Mo: 0.01 to 0.5%
Mo, like Mn, lowers the critical cooling rate at which a composite structure of the ferrite phase as the main phase and the martensite phase as the second phase is obtained. That is, it has an effect of promoting the formation of a composite structure of a ferrite phase and a martensite phase, and can be contained as necessary. The effect is exhibited by containing Mo of 0.01% or more. However, when the Mo content exceeds 0.5%, the deep drawability decreases, so the Mo content was limited to 0.01 to 0.5%.
[0050]
In the present invention, there is no particular limitation on the components other than the above-mentioned components, but there is no problem even if B, Ca, REM, and the like are contained within the range of a normal steel composition.
[0051]
B is an element having an effect of improving the hardenability of steel, and can be contained as necessary. However, if the B content exceeds 0.003%, the effect is saturated, so that B is preferably 0.003% or less. Note that a more desirable range is 0.0001 to 0.002%.
Ca and REM have the effect of controlling the form of the sulfide-based inclusions, and thereby have the effect of improving the stretch flangeability of the steel sheet. Such an effect is saturated when the content of one or two selected from Ca and REM exceeds 0.01% in total. Therefore, the content of one or two of Ca and REM is preferably set to 0.01% or less in total. Note that a more preferable range is 0.001 to 0.005%.
[0052]
The balance other than the above components is substantially Fe and inevitable impurities. The inevitable impurities include, for example, Sb, Sn, Zn, and Co. The allowable ranges of these contents are as follows: Sb: 0.01% or less, Sn: 0.1% or less, Zn: 0.01%. Hereinafter, Co: 0.1% or less.
[0053]
Next, the reason for limiting the manufacturing conditions in the present invention will be described.
The present invention uses a steel slab having a composition within the above-described range as a raw material, a hot-rolling step of performing hot rolling on the raw material to form a hot-rolled sheet, and performing a cold rolling on the hot-rolled sheet to obtain a cold-rolled sheet. A first cold rolling step, a first annealing step of heating the cold rolled sheet to a temperature range of 650 to 780 ° C., a second cold rolling step of cold rolling the cold rolled sheet again, and a recrystallization annealing And a secondary annealing (recrystallization annealing) step of sequentially producing a cold-rolled steel sheet.
[0054]
The steel slab to be used is preferably manufactured by a continuous casting method in order to prevent macro segregation of components, but may be manufactured by an ingot casting method or a thin slab casting method. In addition, after the steel slab is manufactured, it is cooled to room temperature, and then heated again.In addition to the conventional method, it is not cooled, but inserted into the heating furnace as a hot piece, or after performing a slight heat retention. Energy saving processes such as a direct rolling method and a direct rolling method in which rolling is performed immediately can be applied without any problem.
[0055]
The above-mentioned material (steel slab) is heated, subjected to hot rolling, and subjected to a hot rolling process to form a hot rolled sheet. The hot-rolling step may be performed under conditions that can produce a hot-rolled sheet having a desired thickness, and there is no particular problem even if ordinary rolling conditions are used. In addition, suitable hot rolling conditions are shown below for reference.
[0056]
Slab heating temperature: 900 ° C or more
The slab heating temperature is desirably lower because the precipitate is coarsened to develop {111} recrystallized texture and improve deep drawability. However, when the heating temperature is lower than 900 ° C., the rolling load increases, and the risk of occurrence of trouble during hot rolling increases. Therefore, the slab heating temperature is preferably set to 900 ° C. or higher. In addition, the upper limit of the slab heating temperature is more preferably set to 1300 ° C. because of an increase in scale loss due to an increase in oxidation weight. From the viewpoint of reducing the slab heating temperature and preventing problems during hot rolling, it is needless to say that utilizing a so-called sheet bar heater for heating the sheet bar is an effective method.
[0057]
Finish rolling end temperature: 700 ° C or more
The finish rolling finish temperature (FT) is preferably set to 700 ° C. or higher in order to obtain a uniform hot-rolled base plate structure that can obtain excellent deep drawability after cold rolling and recrystallization annealing. That is, when the finish rolling end temperature is less than 700 ° C., the hot-rolled base plate structure becomes non-uniform, the rolling load during hot rolling increases, and the risk of trouble occurring during hot rolling increases. is there.
[0058]
Winding temperature: 800 ° C or less
The winding temperature is preferably set to 800 ° C. or lower. That is, if the winding temperature exceeds 800 ° C., the scale tends to increase and the yield tends to decrease due to scale loss. If the winding temperature is lower than 200 ° C., the shape of the steel sheet is remarkably disturbed, and the risk of causing troubles in actual use increases. Therefore, it is more preferable to set the lower limit of the winding temperature to 200 ° C.
[0059]
As described above, in the hot rolling step of the present invention, after the steel slab is heated to 900 ° C. or higher, hot rolling is performed at a finish rolling end temperature of 700 ° C. or higher, and winding at 800 ° C. or lower, preferably 200 ° C. or higher. It is preferable to wind at a temperature to form a hot-rolled sheet.
In the hot rolling step of the present invention, lubricating rolling may be performed between some or all of the finishing passes in order to reduce the rolling load during hot rolling. In addition, performing lubricating rolling is effective from the viewpoint of uniformizing the shape of the steel sheet and uniforming the material. In addition, it is preferable that the friction coefficient at the time of lubricating rolling be in the range of 0.10 to 0.25.
[0060]
Further, it is preferable to adopt a continuous rolling process in which successive sheet bars are joined and finish rolling is continuously performed. Applying the continuous rolling process is also desirable from the viewpoint of operational stability of hot rolling.
[0061]
Next, primary cold rolling is performed on the hot-rolled sheet to obtain a cold-rolled sheet. The hot-rolled sheet is preferably cold-rolled after pickling as usual, and the pickling may be performed under normal conditions. The conditions of the cold rolling are not particularly limited as long as a cold rolled sheet having a desired size and shape can be obtained, and the rolling reduction during the cold rolling is preferably 30% or more. If the rolling reduction is less than 30%, recovery or recrystallization hardly occurs in the subsequent primary annealing step, and ultimately excellent deep drawability cannot be obtained.
For example, in the case of the semi-ultra low carbon steel as described above, the present invention has a relatively low primary rolling ratio of less than 70% without applying a primary cold rolling reduction of 70% or more, which makes it difficult to roll a wide material. Even at the cold rolling reduction, sufficient deep drawability can be obtained.
[0062]
Next, a primary annealing step is performed on the cold-rolled steel sheet. The primary annealing may be box annealing, but is preferably performed on a continuous annealing line from the viewpoint of manufacturing cost. It is necessary to perform the primary annealing at 650 to 780 ° C., which is a temperature range in which carbides of the hot-rolled sheet hardly dissolve. In the high-temperature annealing in which the primary annealing temperature exceeds 780 ° C., a large amount of carbide is dissolved, so that a large amount of solute C is present. In the subsequent secondary cold rolling and secondary annealing (recrystallization annealing) steps, {111} is used. Recrystallized texture is not sufficiently developed, and a high r value cannot be obtained. If the primary annealing temperature is lower than 650 ° C., the structure after cold rolling is not recovered, so that it has the same strength as the cold-rolled material after primary annealing, At the time of rolling, a predetermined cold rolling reduction cannot be ensured, and further, during the secondary annealing (recrystallization annealing), the {111} recrystallization texture does not sufficiently develop. Therefore, the primary annealing temperature was limited to 650-780 ° C.
[0063]
Further, the cold-rolled annealed steel sheet is subjected to secondary cold rolling. The cold rolling conditions are not particularly limited as long as a cold rolled sheet having a desired size and shape can be obtained, but the rolling reduction during cold rolling is preferably 40% or more. A high cold rolling reduction is generally effective for a high r value, and the next step corresponds to a recrystallization annealing step. Therefore, a higher rolling reduction is preferable. If the rolling reduction is less than 40%, the {111} recrystallized texture does not sufficiently develop in the recrystallization annealing step, and ultimately excellent deep drawability cannot be obtained.
In the present invention, sufficient deep drawability can be obtained even when the secondary cold rolling reduction is relatively low, for example, a low reduction of less than 70%.
[0064]
Subsequently, the cold-rolled steel sheet is subjected to a recrystallization annealing to perform a secondary annealing (recrystallization annealing) step to form a cold-rolled annealed sheet. The secondary annealing (recrystallization annealing) is preferably performed in a continuous annealing line to secure the cold rolling speed required in the present invention. The annealing temperature of the recrystallization annealing must be not lower than the temperature at which a structure containing a ferrite phase and a martensite phase is obtained after cooling, and not lower than the recrystallization temperature and not higher than 950 ° C. If the annealing temperature is low and the annealing temperature is lower than the temperature at which a structure containing a ferrite phase and a martensite phase is obtained after cooling, a desired structure cannot be obtained, and a low yield stress and a high strength-elongation balance are simultaneously satisfied. It becomes difficult to use a steel plate. If the temperature is lower than the recrystallization temperature, an unrecrystallized structure remains, sufficient elongation cannot be obtained, a {111} recrystallized texture does not develop, and a high r value cannot be obtained. On the other hand, at a high temperature exceeding 950 ° C., the recrystallized grains are significantly coarsened, and the characteristics tend to be significantly deteriorated.
Here, the temperature at which a structure containing a ferrite phase and a martensite phase is obtained after cooling is defined as annealing after simulating annealing and heating to various temperatures and then cooling to 400 ° C at an average cooling rate of 5 ° C / s. The heating temperature at which the microstructure of the applied steel sheet is observed and a structure including a ferrite phase and a martensite phase is confirmed. Further, the recrystallization temperature is a temperature at which no unrecrystallized structure is observed in the microstructure observation and the recrystallization rate becomes 100%.
In order to obtain a temperature at which a structure containing a ferrite phase and a martensite phase is obtained after such cooling, the temperature in the two-phase region of a ferrite phase (α phase) and an austenite phase (γ phase) or more during annealing, that is, Ac _l What is necessary is just to be above the transformation point. In the present invention, by annealing at a temperature not lower than the two-phase region temperature of the α phase and the γ phase, V and Nb-based carbides are dissolved, and a structure containing a ferrite phase and a martensite phase can be formed after cooling. The annealing temperature is lower than the two-phase region temperature of α phase and γ phase, that is, Ac _l If the temperature is lower than the transformation point, a ferrite single phase structure is obtained after cooling, and a structure containing a ferrite phase and a martensite phase cannot be obtained.
In the present invention, by setting the annealing temperature to approximately 750 ° C. or higher, the temperature can be set to a temperature at which a structure containing a ferrite phase and a martensite phase is obtained after cooling, and to a recrystallization temperature or higher. That is, in the steel of the present invention, when the temperature is approximately 750 ° C. or higher, the V and Nb-based carbides are dissolved, and the transformation from the α phase to the γ phase is easily promoted. At the same time, recrystallization proceeds and the recrystallization temperature can be increased.
That is, if the temperature is lower than 750 ° C., the dissolution of V and Nb-based carbides is insufficient, and it is difficult to raise the temperature to the two-phase region temperature of the α phase and the γ phase. It becomes difficult to obtain, and an unrecrystallized structure tends to remain.
[0065]
The cooling during the secondary annealing (recrystallization annealing) needs to be cooled to room temperature at an average cooling rate of 5 ° C./s or more at least up to 400 ° C. from the viewpoint of martensite formation. If the average cooling rate is less than 5 ° C./s, a martensitic phase is hardly formed, and a ferrite single phase structure is formed, and the strength-elongation balance is lowered. Therefore, in the present invention, since the presence of the second phase including the martensite phase is indispensable, the average cooling rate up to 400 ° C. must be 5 ° C./s or more, which is higher than the critical cooling rate. is necessary. The cooling at 400 ° C. or lower does not need to be particularly limited, and may be continued cooling or air cooling.
[0066]
Further, the cold-rolled annealed steel sheet after the second continuous annealing may be subjected to temper rolling for shape correction, adjustment of surface roughness and the like. Further, there is no inconvenience even if a treatment such as resin or oil coating, various kinds of painting or electroplating is performed.
[0067]
The above description is only an example of the embodiment of the present invention, and various changes can be made within the scope of the claims.
[0068]
【Example】
Molten steel having the composition shown in Table 1 was smelted in a converter and made into a slab by a continuous casting method. Then, after heating these steel slabs to 1250 ° C., a hot rolling step of performing hot rolling at a finish rolling end temperature of 880 ° C. and a winding temperature of 650 ° C. is performed to obtain a hot-rolled steel strip having a thickness of 4.0 mm. Hot-rolled sheet). Subsequently, after pickling these hot-rolled steel strips (hot-rolled sheets), a cold-rolled steel strip (cold-rolled sheets) was obtained by a cold-rolling step of performing primary cold rolling at a rolling reduction shown in Table 2. Next, these cold-rolled steel strips (cold-rolled sheets) were subjected to primary continuous annealing at the temperatures shown in Table 2 in a continuous annealing line. Subsequently, secondary cold rolling was performed at a rolling reduction shown in Table 2. Here, the steel sheet after the secondary cold rolling is simulated in the laboratory by secondary continuous annealing, and after heating to the annealing temperature at a heating rate of 5 ° C./s, the cooling rate to 400 ° C. is immediately increased to 5 ° C./s. After cooling, the structure was observed, and after cooling, a structure containing a ferrite phase and a martensite phase was obtained, and the lower limit temperature (T) of the temperature at which the recrystallization ratio was 100% and no unrecrystallized structure was observed was determined. The above investigation was conducted by changing the annealing temperature from 700 ° C. at a pitch of 10 ° C. Table 2 shows the determined lower limit temperature T.
Next, secondary continuous annealing was performed in a continuous annealing line. Further, for some steel strips (steel No. 4 in Table 2), the first cold rolling and the first annealing step were omitted, and the first cold rolling and the first annealing step were performed. Further, each of the obtained steel strips (cold rolled annealed steel sheets) was subjected to temper rolling at an elongation of 0.5%.
[0069]
Further, a test piece is collected from a steel strip obtained by the above method, and a microstructure is imaged using a light microscope or a scanning electron microscope for a cross section (L cross section) parallel to the rolling direction, The area ratio of the ferrite phase, which is the main phase, and the type and area ratio of the second phase were determined using an image analyzer. Further, a JIS No. 5 tensile test piece was sampled from the obtained steel strip and subjected to a tensile test in accordance with the provisions of JIS Z 2241, yield stress (YS), tensile strength (TS), elongation (El) , Yield ratio (YR). In addition, YS, TS, El, and YR are values when a tensile test was performed in a direction perpendicular to the rolling direction. The r value was determined by using a JIS No. 5 tensile test specimen collected from the obtained steel strip, obtaining an average r value (average plastic strain ratio) in accordance with the provisions of JIS Z 2254, and defining this as the r value. . Table 2 shows the results.
[0070]
[Table 1]

[0071]
[Table 2]

[0072]
From the results shown in Table 2, all of the examples of the present invention are aimed at low yield ratio (YR ≦ 70%), high elongation (El ≧ 28%) and high Rankford value (r value ≧ 1.3). And has excellent deep drawability. In particular, in the example of the present invention, by defining the heat treatment conditions and adopting the twice cold-rolling twice annealing step, the r value is dramatically increased as compared with the case where the single cold rolling once annealing step is adopted, An r value of 1.3 or more can be ensured in spite of a high-strength steel sheet. On the other hand, in the comparative example manufactured under the condition out of the range of the present invention, the steel sheet has a reduced Rankford value (r value) or a reduced strength-elongation balance.
[0073]
【The invention's effect】
ADVANTAGE OF THE INVENTION According to this invention, while being excellent in intensity | strength elongation balance, it becomes possible to manufacture the cold rolled steel sheet which was also excellent in the deep drawing formability with low load and a stable process, and the industrially remarkable effect is obtained. Play. When the cold-rolled steel sheet of the present invention is applied to an automobile part, there is also an effect that press forming is easy and it is possible to sufficiently contribute to reducing the weight of an automobile body.
[Brief description of the drawings]
FIG. 1 shows the ratio (V / 51 + Nb / 93) / (C / 12) representing the relationship between the contents of V and Nb and the content of C. The ratio between the Rankford value (r value) and the strength-elongation balance (TS × El) FIG.
FIG. 2 is a diagram showing a relationship between primary annealing temperature and Rankford (r value).
FIG. 3 shows the total rolling reduction of the cold-rolled steel sheet manufactured by the two-time cold rolling and two-time annealing process of the present invention (the method of the present invention) and the conventional one-time cold rolling and one-time annealing process (the conventional method). FIG. 9 is a diagram showing the effect of (%) on the r value.

Claims (3)

C: 0.01-0.05%, Si: 0.1-1.5%, Mn: 1.0-3.0%, P: 0.10% or less, S: 0.02% by mass% Hereafter, it contains Al: 0.005 to 0.1%, N: 0.02% or less, V: 0.01 to 0.2%, and Nb: 0.001 to 0.2%. The content (% by mass) of Nb and C is
0.5 × C / 12 ≦ (V / 51 + Nb / 93) ≦ 2 × C / 12
A steel slab having a composition satisfying the following relationship is hot-rolled, then cold-rolled, then annealed by heating to 650 to 780 ° C, cold-rolled again, and after cooling, the ferrite phase After heating to a temperature not lower than the temperature at which the structure containing the martensite phase is obtained and not lower than the recrystallization temperature and not higher than 950 ° C., annealing is performed at an average cooling rate of 5 ° C./s or higher until at least 400 ° C. To produce a composite structure type high-tensile cold-rolled steel sheet having excellent deep drawability. C: 0.01-0.05%, Si: 0.1-1.5%, Mn: 1.0-3.0%, P: 0.10% or less, S: 0.02% by mass% Hereinafter, Al: 0.005 to 0.1%, N: 0.02% or less, V: 0.01 to 0.2%, Nb: 0.001 to 0.2%, and Ti: 0.001 to 0 0.3%, and the contents (% by mass) of V, Nb and Ti and C are:
0.5 × C / 12 ≦ (V / 51 + Nb / 93 + Ti / 48) ≦ 2 × C / 12
A steel slab having a composition satisfying the following relationship is hot-rolled, then cold-rolled, then annealed by heating to 650 to 780 ° C, cold-rolled again, and after cooling, the ferrite phase After heating to a temperature not lower than the temperature at which the structure containing the martensite phase is obtained and not lower than the recrystallization temperature and not higher than 950 ° C., annealing is performed at an average cooling rate of 5 ° C./s or higher until at least 400 ° C. To produce a composite structure type high-tensile cold-rolled steel sheet having excellent deep drawability. The composite structure type high tensile cold steel having excellent deep drawability according to claim 1 or 2, wherein the steel slab further contains Mo: 0.01 to 0.5% by mass in addition to the above composition. Manufacturing method of rolled steel sheet.

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