JP2001131640A - Method for producing high strength cold rolled steel sheet excellent in deep drawability - Google Patents

Abstract

PROBLEM TO BE SOLVED: To produce a high strength cold rolled steel sheet exhibiting tensile strength equal to or below 440 MPa by which forming is made easy before the forming and having high formability capable of securing the strength of the formed parts after strain aging treatment after the forming by making the tensile strength higher than the strength before the strain aging treatment at least by >=50 MPa. SOLUTION: At the time of subjecting steel in which Ti, Nb and Cu are compositely added, to hot rolling, slab heating temperature and rough rolling finishing temperature are controlled in such a manner that the difference between the A3 transformation point and the starting temperature of austenite- ferrite transformation progressing in the process of the rolling: the Ar3 transformation point, i.e., ΔT = the A3 transformation point-the Ar3 transformation point satisfies ΔT<=100 deg.C, also, rough rolling is executed at the Ar3 transformation point or more, finish rolling starting temperature is controlled to lower than the Ar3 transformation point, and the total draft at least in the temperature region of 500 to 750 deg.C is controlled to >=50%.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for producing a cold-rolled steel sheet having excellent deep drawability used for a steel sheet for automobiles and the like.

[0002] Cold-rolled steel sheets used for automobile panels and the like are required to have excellent deep drawability. In order to improve the deep drawability, as a mechanical property of the steel sheet, a Rankford value (mean r value, hereinafter abbreviated as r value) and ductility (E) are used.
l) is required to be good. On the other hand, in recent years, there has been a rapid increase in the tendency to use higher-strength steel sheets having a tensile strength of 340 to 590 MPa for automobile panels and the like for the purpose of reducing the body weight and improving safety of automobiles. I have. It goes without saying that even such a high-strength steel sheet requires excellent deep drawability during press forming. Therefore, there is a demand for the development of a steel sheet having contradictory characteristics of high strength and excellent deep drawability.

[0003]

2. Description of the Related Art Various means have been proposed for improving the deep drawability of cold rolled steel sheets. For example,
No. 17268, JP-B-44-17269 and JP-B-44-17270 show that the r-value is obtained by repeating cold rolling and annealing twice on a low carbon rimmed steel.
A method for producing cold rolled steel sheets up to 2.18 is disclosed.
However, the cold-rolled steel sheet manufactured by this method cannot always be said to have sufficient deep drawability required today.

On the other hand, JP-A-2-47222 and JP-A-3-150316 use ultra-low carbon steel containing trace amounts of Ti and Nb, and perform rough rolling at 950 ° C. or lower and an Ar ₃ transformation point or higher. the finish rolling lubrication rolling at Ar ₃ transformation point below 600 ° C. or higher, Ar ₃ total rolling reduction of less than transformation point is subjected to a machining rolled 60% or more, a method of obtaining a high r value is disclosed. But,
This method was not a technique for achieving both high strength and high r value. Because in this method, to get high strength
This is because it is necessary to include a large amount of reinforcing components such as Si, Mn, and P, and as a result, there is a problem that an unfavorable texture for deep drawability is formed and the r value decreases.

Further, Japanese Patent Application Laid-Open No. 6-65646 discloses that
Si and M as strengthening components in ultra low carbon steel with Ti and Nb added
Including n, P, Ni, Mo and Cu, finish rolling is started at a temperature not lower than the Ar ₃ transformation point, and 30 ° C. or more at a cooling rate of 20 ° C./s or more without rolling during the rolling. a temperature below Ar ₃ transformation point performs cooling, subsequently Ar ₃ or less transformation point, while subjected to a lubrication rolling at a temperature range of not lower than 500 ° C., Ar
_A method is disclosed in which a rolling process is performed in which the total draft of _three transformation points or less is 50% or more and 95% or less. In this method, since austenite-> ferrite transformation progresses during finish rolling, there is a problem that formation of a sharp {111} texture is easily hindered and product characteristics are not stable.

[0006]

SUMMARY OF THE INVENTION An object of the present invention is to solve the above problems advantageously and to propose a method for producing a cold-rolled steel sheet having high strength and remarkably excellent deep drawability.
Generally, molding becomes difficult when the strength of the material is high.
A high-strength cold-rolled steel sheet with a high formability that shows a considerably lower tensile strength, and ensures that the strength of the molded part can be secured by increasing the tensile strength after strain aging treatment after molding by at least 50 MPa or more before the strain aging treatment. I will provide a.

[0007]

Means for Solving the Problems The inventors of the present invention have conducted intensive studies to improve the high strength and the deep drawability, and as a result, by specifying the composition of the steel and the manufacturing conditions, it has become possible to obtain an excellent deep drawability. It has been found that a high-strength cold-rolled steel sheet can be obtained.

More specifically, a high-strength and high-strength steel is formed by utilizing a manufacturing process in which a hot-rolled structure is formed by hot rolling in a ferrite region, and {111} texture is developed by recrystallization annealing. In producing a cold-rolled steel sheet having r-value characteristics, C: 0.01 wt% or less, Si: 1.5 wt% or less, Mn: 1.0 wt% or less, Ti: 0.01 to 0.20 wt%, Nb: 0.00
5 to 0.07 wt%, Cu: 0.5 to 1.5 wt%, Al: 0.02 to 0.10 wt%
%, P: 0.05 wt% or less, S: 0.01 wt% or less, and N: 0.01
It contained the following wt%, the balance being substantially when the slab formed in the composition of Fe to hot rolling, the austenite progresses during rolling and A ₃ transformation point - ferrite transformation start temperature difference between the Ar ₃ transformation point: ΔT = A ₃ transformation point−Ar ₃ transformation point is controlled so that ΔT ≦ 100, slab heating temperature and rough rolling end temperature are controlled, rough rolling is performed at Ar ₃ transformation point or higher, and finish rolling starts It has been found that it is important that the temperature be lower than the Ar ₃ transformation point in steels with many strengthening components.

That is, a first aspect of the present invention provides a method for producing a C:
0.01 wt% or less, Si: 1.5 wt% or less, Mn: 1.0 wt% or less,
Ti: 0.01 to 0.20 wt%, Nb: 0.005 to 0.07 wt%, Cu: 0.5
~ 1.5 wt%, Al: 0.02-0.10 wt%, P: 0.05 wt% or less,
When hot-rolling a slab containing S: 0.01 wt% or less and N: 0.01 wt% or less, and the balance is substantially Fe, A ₃
Transformation point and austenite-ferrite transformation start temperature progressing during rolling: difference between Ar ₃ transformation point ΔT = A ₃ transformation point-Ar ₃
The slab heating temperature and the rough rolling end temperature are controlled so that the transformation point satisfies ΔT ≦ 100 ° C., and the rough rolling is performed at the Ar ₃ transformation point or higher, and the finish rolling start temperature is set to less than the Ar ₃ transformation point,
50% total reduction at least in the temperature range of 500 to 750 ° C
After performing the above rolling, after hot rolling, rewinding is performed in the winding or annealing process.
This is a method for producing a high-strength cold-rolled steel sheet excellent in deep drawability, comprising performing recrystallization annealing in a temperature range of 750 ° C. to 900 ° C. after performing the following cold rolling. According to a second aspect of the present invention, in the first aspect, Ni: 0.3 to 1.
Can contain 5 wt%. Further, the third aspect of the present invention.
In the aspect, in the recrystallization treatment after finish rolling in the first or second aspect, the cooling rate between at least 700 to 500 ° C. can be set to 2 ° C./sec or more. further,
In the fourth aspect of the present invention, subsequent to the recrystallization annealing after the cold rolling in the first or second aspect, 250 to 650
490 MPa tensile strength by heating to the temperature range of ℃
A high-strength cold-rolled steel sheet excellent in deep drawability having the above features can be manufactured.

[0010] The cold-rolled steel sheet manufactured according to the present invention has the final characteristics appearing at the shipping stage, and the final characteristics do not appear at the shipping stage. Includes two types that exhibit desired properties. The first, second and third aspects correspond to the case where the final characteristic does not appear in the latter shipping stage, while the fourth aspect corresponds to the case where the final characteristic appears in the former shipping stage.

[0011]

BEST MODE FOR CARRYING OUT THE INVENTION Hereinafter, the present invention will be described more specifically. First, in the present invention, the reason why the component composition of the steel sheet is limited to the above range is as follows. ● C: 0.01 wt% or less C inhibits the development of {111} recrystallization texture, which is closely related to the improvement of the r value, so it is preferable to reduce it as much as possible. it can.

Si: 1.5 wt% or less Si is a useful component that suppresses the decrease in elongation and improves the strength. However, when the Si content exceeds 1.5 wt%, not only does the ductility decrease, The upper limit is set to 1.5 wt% because the surface properties are deteriorated. More preferably, the content is 1.0 wt% or less. Also, when an austenite-forming element is added as a steel strengthening component, Si becomes austenite during hot rolling →
Since it is an effective component for suppressing a decrease in the ferrite transformation start temperature, it is preferable to add 0.2 wt% or more.

Mn: 1.0 wt% or less Mn works effectively as a reinforcing component of steel, but the amount of Mn is 1.0 wt%
%, The ductility is reduced, so the content is limited to 1.0 wt% or less. More preferably, the content is 0.5 wt% or less. Note that Mn
Is an austenite-forming component, and if added in a large amount, lowers the onset temperature of austenite-> ferrite transformation during hot rolling, so is preferably 0.5 wt% or less.

Ti: 0.01 to 0.20 wt% Ti fixes C and N in the steel as precipitates and effectively contributes to the development of {111} recrystallization texture and, consequently, the improvement of the r value. If the content is less than 0.01 wt%, the effect is poor. On the other hand, if it exceeds 0.20 wt%, the ductility is rather reduced.
The content was limited to the range of 0.01 to 0.20 wt%. Note that this
The effect of improving the r value by the addition of Ti can be further enhanced by the combined addition of Cu and Nb, and further by the combination with the low-temperature slab heating.
That is, a preferable hot-rolled work structure is formed at the time of finish rolling of hot rolling due to the addition of Nb and Ti, and Ti does not hinder the progress of austenite → ferrite transformation.
Finish rolling of hot rolling can be easily performed in the ferrite region. Further, in the process before cold rolling and annealing, carbonitrides by Ti and Nb become Cu precipitation sites, and have an effect of promoting the formation of {111} recrystallized texture during recrystallization annealing after cold rolling.

Nb: 0.005 to 0.07 wt% Nb also fixes C and N as precipitates like Ti,
1｝ Contributes effectively to the development of recrystallized texture and, consequently, the improvement of the r-value. The effect of Nb is further enhanced by the addition of Ti and Cu as described above, and contributes to the improvement of the r value.
This effect does not appear when the Nb content is less than 0.005 wt%, while
Addition of 0.07% or more not only causes a decrease in ductility, but also causes austenite → ferrite transformation to progress during finish rolling in order to significantly delay the progress of austenite → ferrite transformation during hot rolling. It is difficult to obtain an r value.

Cu: 0.5 to 1.5 wt% Cu develops a {111} recrystallized texture effective for improving the r-value by complex addition with Ti and Nb and appropriately selecting ΔT described later. Effectively contributes to If the added amount of Cu is less than 0.5 wt%, sufficiently high strength cannot be achieved,
If the content exceeds wt%, a low-temperature transformation phase after hot rolling tends to be formed, and the r value decreases. Therefore, the Cu content is set to 0.5 to 1.5 wt%. In the present invention, the ferrite grain size can be reduced by the combined addition of Cu, Nb and Ti and the low-temperature heating of the slab, which is a problem in steels having a low C content, such as the conventional steel of the present invention. Such secondary working brittleness resistance can also be improved. Therefore, it is not necessary to actively add B, which has been conventionally required to solve this problem, and there is no material deterioration due to the addition of B, so that good workability can be easily secured.

Al: 0.02 to 0.10 wt% Al is added for deoxidization and for improving the yield of carbonitride-forming components. However, if the content is less than 0.02 wt%, there is no effect. On the other hand, if it is more than%, ductility is reduced.

P: 0.05 wt% or less P effectively contributes to the strengthening of steel as a solid solution strengthening component. However, if it is added in excess of 0.05 wt%, FeTiP or FeNbP will be used during heat treatment after hot finish rolling. In order to reduce the sharpness of the {111} texture after cold rolling and recrystallization, the content is preferably 0.05% by weight or less.

S: 0.01 wt% or less When a large amount of S is contained, the amount of inclusions increases and the ductility is reduced. Therefore, it is desirable to reduce the S content as much as possible.
Up to wt% is acceptable.

N: 0.01 wt% or less It is preferable to reduce N as much as possible like C.
% Or less is acceptable.

Ni: 0.3 to 1.5 wt% Ni is a component effective for reducing the surface defects of the steel slab and is also a component effective for improving the strength. Therefore, Ni can be added as necessary. . The effect of Ni appears at an addition of 0.3 wt% or more. On the other hand, when it exceeds 1.5 wt%, the r value is rather lowered, so the Ni content was set to 0.3 to 1.5 wt%.

The component composition range of the present invention has been described above. In the present invention, due to the composite addition of Cu, Ti, and Nb, the austenite-ferrite transformation that proceeds during hot rolling shifts to a lower temperature side, and the two-phase region where austenite + ferrite coexists is expanded. Therefore, just by controlling the finish rolling start temperature below the Ar ₃ transformation point as in the conventional method, the finish rolling start temperature is in the two-phase region of austenite + ferrite. As described above, when the finish rolling start temperature is in the two-phase region where austenite and ferrite coexist, the ferrite grains become extended grains due to the difference in deformation resistance between the austenite phase and the ferrite phase during finish rolling, and the α-fiber structure easily develops. . Therefore, there was a problem that {100} texture is formed by annealing in the next step, and development of {111} texture effective for increasing the r-value is hindered.
The present inventors have found that the difference between the A ₃ transformation point and the austenite → ferrite transformation start temperature Ar ₃ progressing during rolling: ΔT = A ₃ −
A slab heating temperature (SR at which Ar ₃ satisfies ΔT ≦ 100 ° C.
It has been found that by giving T) and the rough rolling end temperature, the austenite crystal grains after rough rolling are refined, and the transformation of austenite to ferrite can be advanced in a short time and at a high temperature before the start of finish rolling.

Next, the manufacturing conditions of the present invention will be described with reference to specific examples. C: 0.002 wt%, Si: 0.7 wt%, Mn: 0.1 wt%, P: 0.
01 wt%, S: 0.01 wt%, Al: 0.049 wt%, Ti: 0.068 wt
%, Nb: 0.015% by weight, Cu: 1.2% by weight, Ni: 0.5% by weight and N: 0.002% by weight, the remainder being a slab having a substantially Fe composition, and a slab heating temperature in a laboratory. ΔT was changed by changing the rough rolling end temperature variously. Under these conditions, the finish rolling start temperature is 840 ° C or 780 ° C, the finish rolling end temperature is 680 ° C, and rolling is performed at a total draft of 60% in the temperature range of 500 ° C to 750 ° C. Then, after cold rolling at a reduction of 80%, recrystallization annealing was performed at a temperature of 880 ° C.

FIG. 1 shows a change in r value after cold rolling and annealing due to ΔT, and FIG. 2 shows a change in ΔT according to the slab heating temperature. From FIG. 2, it can be seen that as the slab heating temperature increases, the austenite → ferrite transformation start temperature during hot rolling decreases, and ΔT increases. As shown in FIG. 1, it can be seen that a high r value is obtained under the condition that ΔT ≦ 100 ° C. and the finish rolling start temperature is less than the Ar ₃ transformation point.

The preferred range of the slab heating temperature satisfying the above conditions is 900 to 1050 ° C. If the temperature is lower than 900 ° C., the ferrite → austenite transformation does not proceed during slab heating, and consequently the ferrite grain size during finish rolling becomes coarse and the r-value decreases. On the other hand, at a slab heating temperature exceeding 1050 ° C., it is difficult to control ΔT to 100 ° C. or less.

Here, a method of measuring ΔT will be described.
A here_ThreeThe transformation point is the austenite of the target steel →
Ferrite transformation temperature, measurement of thermal expansion change due to transformation
Or the temperature determined by transformation heat measurement, or the equilibrium state
The temperature obtained from the figure calculation is used. On the other hand,
Austenite to ferrite transformation start temperature: Ar _Three 
The transformation point is a slab of the target steel in the laboratory
It is evaluated from the change in material temperature when rolling. A_ThreeStrange
State and austenite-ferrite transformation progressing during hot rolling
State starting point Ar_Three Is added to increase the strength
Austenite by austenite forming components Cu and Ni
Knight → ferrite transformation temperature caused by low temperature and Nb addition
It is presumed to be caused by a delay in the progress of transformation.

Hot Rolling Conditions In the present invention, it is necessary to control the slab heating temperature and the rough rolling end temperature so that ΔT becomes 100 ° C. or less as described above. Rough rolling needs to be at least the Ar ₃ transformation point in order to reduce the ferrite grain size before finish rolling. vice versa
Below the transformation point of Ar ₃ , the ferrite grains become coarser before finish rolling, and the r value decreases. Further, it is important that the finish rolling start temperature is in the ferrite single phase region by setting the finish rolling start temperature below the Ar ₃ transformation point. When the finish rolling start temperature is in the two-phase region of austenite and ferrite, as described above, the ferrite grains become extended grains due to the difference in deformation resistance between the austenite phase and the ferrite phase, so that the α-fiber structure easily develops, and annealing in the next step is performed. This results in the formation of {100} texture, which hinders the development of {111} texture effective for increasing the r-value. In order to set the finish rolling start temperature in the ferrite single phase region in this way, it is more desirable to set ΔT to
Keep below 70 ° C.

Further, in the present invention, it is essential to perform rolling in a temperature range of at least 500 ° C. to 750 ° C. in a hot rolling step so that the total draft is 50% or more. As described above, in hot rolling, when high-pressure rolling is performed in a low temperature range, strain is accumulated in steel, and the development of {111} texture after recrystallization annealing is promoted. To obtain this effect, the rolling temperature must be 750 ° C. or lower, but if it is lower than 500 ° C., there arises a problem that the rolling load increases. On the other hand, if the total lowering rate in the above temperature range is less than 50%, the strain accumulation effect cannot be obtained. Therefore, in the present invention, at least a temperature range of 750 to 500 ° C. is rolled at a high reduction of 50% or more. When such rolling is performed under lubrication, the accumulation of shear strain applied to the steel sheet surface is suppressed,
As a result, the development of the {111} texture after recrystallization is further promoted, which is desirable. Specifically, the above rolling is performed under oil lubrication.

● Annealing temperature: 750 to 900 ° C. When performing recrystallization by winding, the winding temperature is set to 700 ° C.
The temperature shall be higher than ° C. If the winding temperature is below 700 ° C,
The following recrystallization annealing treatment is performed. In this recrystallization annealing treatment, a recrystallized texture is formed. For this purpose, it is necessary to set the annealing temperature to 750 ° C. or higher. Nevertheless, if the temperature exceeds 900 ° C., the crystal grains become coarse and the surface becomes rough, so the annealing temperature is limited to the range of 750 to 900 ° C.

● Cooling after annealing: at least 700-500 ° C
In the cooling process after forming a recrystallized texture by annealing after hot rolling, if the cooling rate between 700 and 500 ° C is set to 2 ° C / second or more, Is r after cold rolling and annealing.
The value becomes even larger. The finish rolling start temperature shown in FIG.
A hot-rolled sheet having a temperature lower than the Ar ₃ transformation point and satisfying ΔT of 60 ° C. or less is subjected to recrystallization annealing at 880 ° C. for 40 seconds, and then cold-rolled at a rolling reduction of 82%. Recrystallization annealing was performed at a temperature of ° C. Fig. 3 shows the r-value change when the product was obtained by changing the cooling rate between 700 and 500 ° C variously between 0.2 and 10 ° C / s in the cooling process during the recrystallization annealing of the hot rolled sheet.
Shown in FIG. 3 clearly shows that a higher r value is achieved by setting the cooling rate to 2 ° C./s or more. The reason for this is that the nose of Cu precipitation is 600 ° C in the continuous cooling curve diagram.
It is considered that, by cooling this temperature region at a cooling rate of 2 ° C./sec or more, Cu precipitation is suppressed, and {111} texture develops well during cold rolling and recrystallization annealing. Further, when the cooling rate between 700 and 500 ° C. is 2 ° C./sec or more, there is an effect of increasing the amount of increase in tensile strength due to heat treatment after cold rolling and recrystallization annealing or strain aging treatment after forming. The cooling rate in the temperature range exceeding 700 ° C. and the temperature range below 500 ° C. is not particularly limited.

[Cold Rolling] If the rolling reduction of the cold rolling is less than 50%, a {111} texture does not develop, so that a sufficient r value cannot be obtained. On the other hand, if it exceeds 95%, the r-value will rather decrease. Desirably, the lowering rate is 80% or more.

● Cold-rolled sheet recrystallization annealing The cold-rolled texture having {111} texture is 700 to
By performing recrystallization annealing at 950 ° C., {111} texture is developed and a high r value is obtained. When the annealing temperature is lower than 700 ° C., the recrystallized structure is not developed, and when the temperature exceeds 950 ° C., the crystal orientation is randomized due to austenite → ferrite transformation in the cooling process, and both have low r values.

In this cold-rolled and recrystallized annealed sheet, since no precipitation hardening occurs, the tensile strength is low and it can be said that it is advantageous from the viewpoint of formability. For example, on the customer side,
By subjecting a 10% molded article to heating at 250 ° C. for 20 minutes, strain aging is effected, and the tensile strength can be increased by 50 MPa or more than that before processing. The recrystallization annealing after the cold rolling can be used also in an annealing process such as hot-dip galvanizing, which is a heat treatment at a high temperature for a short time.

According to the present invention, it is of course possible to increase the strength at the time of shipment to the consumer. In this case, the following heat treatment is performed. ● Heating temperature: 250 to 650 ° C By this heat treatment, fine Cu is precipitated and the necessary strength for molded parts is secured more than enough. For this purpose, it is necessary to heat to 250 ° C or higher.
° C range. A high strength is achieved by forming the cold-rolled and recrystallized annealed sheet and the cold-rolled and recrystallized annealed hot-dip galvanized steel sheet on the customer side and performing this heat treatment after the forming. it can.

Thus, by satisfying the component composition and the production conditions according to the present invention, a hot-rolled steel sheet excellent in deep drawability having a tensile strength of 490 MPa or more and an r-value of 2.5 or more can be obtained.

[0036]

EXAMPLE 1 A continuous cast slab having the composition shown in Table 1 was heated to 975 ° C., and rough rolling was completed so that ΔT = A ₃ transformation point−Ar ₃ transformation point was less than 100 ° C. temperature was adjusted to a finish rolling start temperature is less than Ar _3, 500 rolling reduction at to 750 ° C.: subjected to 60% of the lubricating rolled to a hot rolled steel plate having a plate thickness of 4mm under the conditions of the coiling temperature 550 ° C.. Next, the soaking condition
Cooling rate between 700-500 ° C after soaking at 880 ° C for 40 seconds, 5
After performing recrystallization annealing at ℃ / sec, cold rolling was performed at a rolling reduction of 82.5% to obtain a cold-rolled steel sheet having a final thickness of 0.7 mm. Next, recrystallization annealing was performed by heating at 860 ° C. for 40 seconds. Table 2 shows the r value and the tensile strength (TS) at this time. After applying a pre-strain of 10% to these cold-rolled steel sheets,
Value after aging treatment for 20 minutes at 500 ° C
Table 2 also shows the TS value after the heat treatment for 60 minutes. Table 2
As can be seen, in all cases satisfying the requirements of the present invention, before processing (after cold rolling recrystallization annealing), the tensile strength is 440 MPa or less, and the tensile strength after aging treatment after imparting prestrain is 440 MPa. MPa or more, the tensile strength after strain aging treatment after processing at least 50 MPa or more increases before strain aging treatment,
In addition, excellent mechanical properties such that the tensile strength after the heat treatment was 490 MPa or more and the r value after cold rolling and annealing was 3.0 or more were obtained.

[0037]

[Table 1]

[0038]

[Table 2]

(Example 2) Using the A and B steels shown in Table 1, hot rolled steel sheets (sheet thickness 4 mm) were manufactured by lubricating rolling.
At this time, various conditions at the time of hot rolling, the annealing temperature and cooling rate at the time of annealing of the hot-rolled sheet, the reduction ratio of the subsequent cold rolling, and the recrystallization annealing temperature of the cold-rolled sheet were changed as shown in Table 3. . Some of these cold-rolled steel sheets were subjected to a strain aging treatment at each temperature after a pre-strain of 12% was applied. In addition, all of the remaining cold-rolled steel sheets were subjected to a heat treatment at each temperature for 60 minutes. Table 4 shows the results of examining the mechanical properties of the obtained steel sheet. As is evident from Table 4, high r values were obtained in all cases when manufactured under the proper conditions according to the present invention. A cold-rolled steel sheet having high strength and excellent deep drawability that satisfies the desired properties could be produced.

[0040]

[Table 3]

[0041]

[Table 4]

[0042]

Thus, according to the present invention, before forming, it exhibits a tensile strength of 440 MPa or less, which is easy to form. It is possible to provide a high-strength cold-rolled steel sheet having high formability capable of securing the strength of a formed part by increasing the tensile strength after the strain aging treatment after forming by at least 50 MPa or more than the strength before the strain aging treatment.

[Brief description of the drawings]

FIG. 1 is a graph showing the effect of ΔT = A ₃ transformation point−Ar ₃ transformation point and the finish rolling start temperature on r value after cold rolling and annealing.

FIG. 2 shows a slab heating temperature and a rough rolling end temperature of ΔT.
₃ is a graph showing the effect on the Ar and transformation point.

FIG. 3 is a graph showing the effect of the cooling rate during hot-rolled sheet-annealing on the r value after cold-rolling-annealing.

 ────────────────────────────────────────────────── ─── Continuing from the front page (72) Inventor Osamu Furukun 1 Kawasaki-cho, Chuo-ku, Chiba-shi, Chiba F-term in the Technical Research Institute, Kawasaki Steel Co., Ltd. 4K037 EA01 EA04 EA13 EA15 EA18 EA19 EA20 EA23 EA25 EA27 EA28 EA31 FB03 FB05 FE01 FF03 FJ05 FJ06 FK02 FK03 FL01 FL05 FM01 FM04 JA06

Claims (4)

[Claims] C: 0.01 wt% or less, Si: 1.5 wt% or less, Mn: 1.0 wt% or less, Ti: 0.01 to 0.20 wt%, Nb: 0.005 to 0.07 wt%, Cu: 0.5 to 1.5 wt%, When hot rolling a slab containing Al: 0.02 to 0.10 wt%, P: 0.05 wt% or less, S: 0.01 wt% or less, and N: 0.01 wt% or less, with the balance being substantially Fe composition, austenite progresses during rolling and a ₃ transformation point - ferrite transformation start temperature difference between the Ar ₃ transformation point △ T =
A ₃ transformation point-Ar ₃ transformation point should satisfy ΔT ≦ 100 ° C.
Controls slab heating temperature and rough rolling end temperature, and the rough rolling performed by Ar ₃ transformation point or higher, the finish rolling start temperature and Ar less than ₃ transformation point, the total reduction ratio in the temperature range of at least 500 to 750 ° C. is Rolling to 50% or more, hot rolling, rewinding in the winding or annealing process, then rolling reduction
After cold rolling of 50% or more and 95% or less, 750 ° C or more
A method for producing a high-strength cold-rolled steel sheet having excellent deep drawability, wherein recrystallization annealing is performed in a temperature range of 900 ° C or lower. 2. The method for producing a high-strength cold-rolled steel sheet having excellent deep drawability according to claim 1, wherein Ni is contained in an amount of 0.3 to 1.5 wt%. 3. The high drawability according to claim 1, wherein a cooling rate between 700 ° C. and 500 ° C. is set to 2 ° C./sec or more in the recrystallization treatment after finish rolling. Manufacturing method of high strength cold rolled steel sheet. 4. Following recrystallization annealing after cold rolling, 25
The method for producing a high-strength cold-rolled steel sheet excellent in deep drawability according to claim 1, wherein the steel sheet is heated to a temperature range of 0 to 650 ° C. and has a tensile strength of 490 MPa or more.