JP2001152286A - Cold rolled steel sheet excellent in composite formability and producing method therefor - Google Patents

Abstract

PROBLEM TO BE SOLVED: To produce a cold rolled steel sheet excellent in breaking resistance in composite forming such as drawing and bulging required for automotive panels such as hoods, doors, fenders and side panels and to provide a method for producing the same. SOLUTION: This cold rolled steel sheet excellent in composite formability has a composition containing, by weight, <=0.0020% C, <=0.05% Si, 0.05 to 0.35% Mn, <=0.025% P, <=0.015% S, 0.01 to 0.06% sol.Alm, <=0.0020% N, 0.010 to 0.040% Nb and 0.003 to 0.035% Ti and also satisfying (12Nb)/(93C)+(12Ti*)/(48C): 1.3 to 5.2 [where Ti*=Ti-(48/14)N-(48/32)S, in the case of Ti*<=0, Ti*=0, and Nb, Ti*, Ti, C, N and S denote weight %] and moreover satisfies the following inequalities (1) and (2): 13.9<=r+50.0(n)...(1), and 2.6<=r+2.0(n)...(2), where r: the average (r) value in the plane, and n: the average tensile strain hardening exponent (n) value in the tensile strain region of 1 to 10%.

Description

DETAILED DESCRIPTION OF THE INVENTION

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a steel sheet used for automobile panels such as hoods, doors, fenders and side panels, and more particularly to a cold-rolled steel sheet having excellent formability in composite forming such as draw forming and stretch forming. And its manufacturing method.

[0001]

2. Description of the Related Art Recently, for steel sheets for automobile bodies,
Extremely high press formability that satisfies both the reduction of the number of parts by the integral molding of parts and the elimination of the pressing step has been required. In particular, when integrally forming a component having a complicated shape such as a front fender or a side panel, not only the draw forming but also the composite formability of the draw formability and the overhang formability is important.

[0002] In order to improve press formability, r
Many techniques for increasing the value and elongation have been proposed. For example, in Japanese Patent Application Laid-Open No. 5-279797, Nb and Ti are added to an ultra-low carbon steel sheet, the average r value is 2.8 or more, and the work hardening index (n value) of 10 to 20% is 0.26 or more. It has been proposed that

[0003] Also, Japanese Patent Publication No. Hei 7-062209 discloses
Is to add Nb and Ti to ultra-low carbon steel sheet,
28.5-31.0kg tensile strength in the direction of 45 °
f / mm ²(279.3-303.8 MPa) and rolling
The technology to make the r value in the direction and 45 ° direction more than 1.90 is proposed.
Is being planned.

[0004]

However, in a composite molded part having a complicated shape such as a front fender or a side panel, the technique for increasing the r value or elongation as described above is not always effective, and frequent press cracking occurs. May be.

[0005] For example, the technique disclosed in the above-mentioned Japanese Patent Application Laid-Open No. Hei 5-279797 is based on deep drawing and a high strain range (1).
(0 to 20%), but is effective for strain forming in a low strain region (10% or less). As a result, excessive distortion occurs in the side wall of the punch, causing cracks.

In the technique disclosed in Japanese Patent Publication No. 7-062209, the target component is a wheel house inner, and when such a component is formed by composite molding, the formability is governed by the draw bead portion. And a deep drawn part. Therefore, it is not always effective for cracks mainly composed of overhang portions. In addition, since the press formability is determined by controlling the flow rate of the steel sheet into the die shoulder R, it is difficult to adjust the wrinkle pressing force, and a formable region may not be obtained. In addition, in such a molding mode, it is possible to improve the moldability and relax the molding conditions by using the strain propagation instead of the inflow, but it is not sufficient.

SUMMARY OF THE INVENTION The present invention has been made in view of the above points, and has been made in view of the above-mentioned requirements for cold forming, which is excellent in rupture resistance in composite molding such as squeezing and overhang required for automobile panels such as hoods, doors, fenders and side panels. An object of the present invention is to provide a rolled steel sheet and a method for manufacturing the same.

[0008]

Means for Solving the Problems The present inventors have studied in detail the formability when forming a steel sheet into a composite material such as a fender or a side panel, and as a result, have found that the composite formability such as drawing and overhanging can be reduced. In addition to improving the r value, it is effective to avoid concentration of strain in a risk of fracture, such as in the vicinity of a punch or die shoulder on the side wall of the formed body during press forming. It has been found that it is effective to optimize the n value of and to promote the strain propagation in the plane strain region of the side wall portion and the plastic flow (strain propagation) in the low strain region at the bottom of the punch.

The present invention has been completed based on the above findings and provides the following (1) to (3). (1) By weight%, C: 0.0020% or less, Si:
0.05% or less, Mn: 0.05 to 0.35%, P:
0.025% or less, S: 0.015% or less, sol. A
l: 0.01 to 0.06%, N: 0.0020% or less,
Nb: 0.010 to 0.040%, Ti: 0.003 to
0.035%, and (12Nb) / (93
C) + (12Ti ^* ) / (48C): 1.3 to 5.2
(However, Ti ^* = Ti− (48/14) N− (48 /
32) S, and in the case of Ti ^* ≦ 0 is set to ^{Ti * = 0, Nb, Ti} *, Ti, C, N, S is the wt%), further, the following equation (1), (2) A cold-rolled steel sheet excellent in composite formability characterized by satisfying the formula. 13.9 ≦ r + 50.0 (n) (1) 2.6 ≦ r + 2.0 (n) (2) where r: average in-plane r value, n: tensile strain range of 10% to 10% Work hardening index n
value

(2) C: 0.0020% or less, Si: 0.05% or less, Mn: 0.05 to 0.35% by weight
%, P: 0.025% or less, S: 0.015% or less, s
ol. Al: 0.01 to 0.06%, N: 0.0020
%, Nb: 0.010 to 0.040%, Ti: 0.
003-0.035%, and C + N: 0.0
030% or less, (12Nb) / (93C) + (12Ti
^* ) / (48C): 2.2 to 4.5 (however, Ti ^* =
Ti- (48/14) N- (48/32) S, T
When i ^* ≦ 0, Ti ^* = 0, and Nb, Ti ^* , T
i, C, N and S are% by weight), and the following (1)
A cold-rolled steel sheet excellent in composite formability, which satisfies the formula (2). 13.9 ≦ r + 50.0 (n) (1) 2.6 ≦ r + 2.0 (n) (2) where r: average in-plane r value, n: tensile strain range of 10% to 10% Work hardening index n
value

(3) In producing the cold-rolled steel sheet of the above (1) or (2), the steel slab is subjected to hot rolling, then cooled to a temperature of 720 ° C. or less by a run-out table cooling, and then 560 650 ° C., and then cold-rolled at a rolling ratio of 70 to 85%,
A method for producing a cold-rolled steel sheet having excellent composite formability, comprising continuously annealing at an annealing temperature of 80 ° C.

[0012]

BEST MODE FOR CARRYING OUT THE INVENTION Hereinafter, the present invention
Specific description will be given separately for the composition, tensile properties, and manufacturing conditions.
1. Ingredient composition The steel sheet in the present invention is, by weight%, C: 0.0020%.
Hereinafter, Si: 0.05% or less, Mn: 0.05 to 0.3
5%, P: 0.025% or less, S: 0.015% or less,
sol. Al: 0.01 to 0.06%, N: 0.002
0% or less, Nb: 0.010 to 0.040%, Ti:
It has a composition of 0.003 to 0.035%, and (12
Nb) / (93C) + (12Ti^*) / (48C):
1.3 to 5.2 (however, Ti^*= Ti- (48/1
4) N- (48/32) S, Ti ^*If ≦ 0
Is Ti^*= 0, Nb, Ti^*, Ti, C, N, S
% By weight). Preferably, the composition has the above composition.
And C + N: 0.0030% or less, (12Nb)
/ (93C) + (12Ti^*) / (48C): 2.2-
4.5 is satisfied. The reasons for the limitation are as follows.
You.

C: not more than 0.0020% C is an element that affects the n value in a low strain region (1 to 10%), and in a solid solution state, interacts with dislocation to reduce the n value. In the state of NbC, the n value is reduced in the case of extremely fine particles. Such a decrease in n value is 0.002
When the content exceeds 0%, the C content is set to 0.0020% or less from the viewpoint of improving the n value.

Si: 0.05% or less In the case of a cold-rolled steel sheet, if the Si content exceeds 0.05%, the chemical conversion property deteriorates, and in the case of hot-dip galvanizing, the plating adhesion deteriorates. . Further, in order to maintain high ductility, the content needs to be 0.05% or less. Therefore, S
The i content is set to 0.05% or less.

Mn: 0.05-0.35% Mn is an element that precipitates S in steel as MnS and prevents hot cracking of the slab. In the present invention, Ti
, The Mn content is lower than that of general steel. There is no problem, but if it is less than 0.05%, the cost of hot metal pretreatment increases. On the other hand, if the Mn content exceeds 0.35%, the yield strength increases due to solid solution strengthening, and the n value decreases. Therefore, the Mn content is reduced to 0.05 to 0.35%.
And

P: 0.025% or less P is a grain boundary embrittlement element, and its content should be kept low. However, reducing the P content requires a cost of removing P. On the other hand, P is a solid solution strengthening element like Mn,
If the content exceeds 0.025%, the yield strength increases, and the n value decreases. In consideration of the above points, the P content is set to 0.025% or less.

S: 0.015% or less S exists in steel as an unavoidable impurity. When the S content increases, ductility deteriorates and TiS is formed, so that the effective Ti amount (Ti ^* ) described below decreases. Therefore, the S content is set to 0.015% or less.

Sol. Al: 0.01 to 0.06% sol. Although Al fixes N as AlN, in the present invention, Ti is added to fix N as TiN, so that the amount of Al added can be reduced as compared with a normal case. In the present invention, in order to suppress the oxidation of Ti by deoxidizing Al to secure an effective Ti amount and to suppress the occurrence of surface defects, sol. Al content of 0.01 to
0.06%.

N: 0.0020% or less N is an element that affects the n value in the low strain region (1 to 10%), like C, and in a solid solution state, it interacts with dislocations to reduce the n value. In the case of TiN, the n value is reduced in the case of extremely fine particles. Since such a decrease in the n value occurs when it exceeds 0.0020%, the N content is set to 0.0020% or less from the viewpoint of improving the n value.

Nb: 0.010-0.040% Nb is an important element for fixing solid solution C and improving the n value and the r value. However, when the Nb content is 0.01
If it is less than 0%, C cannot be fixed sufficiently, and a steel sheet having excellent composite formability cannot be obtained. On the other hand, 0.040%
When n exceeds n, the solid solution Nb increases and the n value decreases. Therefore, the Nb content is set to 0.010 to 0.040%.

Ti: 0.003 to 0.035% Ti is an important element for fixing solid solution N and improving the n value and the r value. However, when the Ti content is 0.00
If it is less than 3%, N cannot be fixed sufficiently, and a steel sheet having excellent composite formability cannot be obtained. On the other hand, if it exceeds 0.035%, the precipitation of TiC becomes remarkable and the formation of NbC is suppressed, so that coarse TiC and fine NbC are precipitated, the precipitate size becomes uneven, and a high n value is stably obtained. I can't. Therefore, the Ti content is 0.003 to 0.035%.
And

(12Nb) / (93C) + (12T
i ^* ) / (48C): 1.3 to 5.2 Ti ^* is Ti ^* = Ti− (48/14) N− (48 /
32) S (However, when Ti ^* ≦ 0, Ti ^* = 0, and Nb, Ti ^* , Ti, C, N, and S are% by weight)
Where (12Nb) / (93C) + (1
2Ti ^* ) / (48C) is an important component balance for improving the n value. As described above, Nb fixes C and Ti is an element fixing C and N. However, by adding a sufficient amount of Nb and Ti to C in steel, the carbon / nitride is coarsened. And n value can be stably improved. FIG. 1 shows Nb: 0.005 to 0.05.
%, About the steel containing Ti: 0.04%, (12Nb) / (93C) + (12Ti ^* ) / (48
It is a result of examining the effect of the value of C) on the n value in a low distortion region (1 to 10%). From FIG. 1, (12Nb)
/ (93C) + (12Ti ^* ) / (48C)
It can be seen that a high n value of 0.24 or more is obtained when the value is 3 or more, and an extremely high n value of 0.26 or more is obtained particularly when the value is in the range of 2.2 to 4.5. On the other hand, if it exceeds 5.2, the effect of solid-solution Nb becomes remarkable, and the n value is significantly reduced. Therefore, (12Nb) / (93C) + (12T
i ^* ) / (48C) is in the range of 1.3 to 5.2,
More preferred values are 2.2 to 4.5.

C + N: 0.0030% or less C and N interact with dislocations in a solid solution state, and n
In order to decrease the value, in the present invention, it is fixed as a precipitate by Nb and Ti, but the amount of the precipitate also affects the n value. FIG. 2 shows C: 0.0030% or less, N:
0.0012% or less, (12Nb) / (93%
C) + (12Ti ^* ) / (48C) is in a range of 2.2 to 4.5, and C + N is in a low strain range (1 to 10).
%) Is the result of investigating the effect on the n value in (%).
FIG. 2 shows that the n value is high in the region where C + N is 0.0040% or less, and that the highest n value is obtained particularly in the region where C + N is 0.0030% or less. Therefore, (12Nb) / (93C) + (12T
When i ^* ) / (48C) is 2.2 to 4.5, C + N
Is preferably 0.0030%.

In the present invention, as a component element other than the above, V and Zr are set to 0.04% in order to fix C.
Hereinafter, in order to fix N, one or more of B may be added in a range of 0.0008% or less.

2. Tensile properties The steel sheet of the present invention has tensile properties satisfying the following equations (1) and (2). 13.9 ≦ r + 50.0 (n) (1) 2.6 ≦ r + 2.0 (n) (2) where r: average in-plane r value, n: tensile strain range of 10% to 10% Is the average work hardening index n value.

Here, the in-plane average r value is calculated as follows. [In-plane average r value] = ([r0] +2 [r45] + [r9
0]) / 4 where [r0]: r value in the steel sheet rolling direction, [r4]
5]: r value in the direction of 45 ° with respect to the rolling direction of the steel sheet, [r9
0]: r value in the direction of 90 ° with respect to the rolling direction of the steel sheet.

The average work hardening index n is calculated as follows. [Average work hardening index n value] = ([n0] +2 [n45]
+ [N90]) / 4 where [n0]: n value in the steel sheet rolling direction, [n4
5]: n value in the direction of 45 ° with respect to the rolling direction of the steel sheet, [n9
0]: n value in the direction of 90 ° with respect to the rolling direction of the steel sheet.

The above equation (1) evaluates the drawability and the equation (2) evaluates the overhang property.
The value and the n value are values obtained by a JIS No. 5 tensile test.
At this time, the r value and the n value are characteristic values of the base material after the plating is stripped in the case of the surface-treated steel sheet.

The distortion range of the n value is not a conventional high distortion region (10 to 20%) but a low distortion region of 1 to 10%. This is based on the results of a detailed survey of actual components such as front fenders and side panels. FIG. 3 shows an example of an equivalent strain distribution in the vicinity of a danger of fracture of the front part fender model molded product of the actual part scale shown in FIG. As shown in FIG. 3, the fracture danger site is the side wall portion, but the distortion generated at the punch bottom contact portion is 0.1%.
0 or less.

FIG. 5 shows the influence of the n value on the horizontal axis and the r value on the vertical axis for the drawability and the overhanging property. From FIG. 5, it can be seen that in the case of the tensile properties satisfying the above equation (1), draw formability (LDR) superior to JSC270F is obtained, and in the case of the tensile properties satisfying the above equation (2), it corresponds to the punch side wall portion. It can be seen that a hat forming height higher than that of JSC270F can be obtained in the plane strain region. The steel of the present invention has tensile properties satisfying the above-mentioned formulas (1) and (2), and is excellent in draw formability and stretch formability. Even if the front fender, the side panel, and the like are integrally formed, press cracking does not occur.

Note that the draw formability was evaluated by the limit draw value (LDR) when forming a cylinder having a diameter of 50 mm, and the stretch formability was obtained by simulating actual panel forming.
Was evaluated by a hat-shaped molding test shown in FIG.

3. Production Conditions In the present invention, in producing the steel sheet, after subjecting the steel slab having the above composition to hot rolling, the steel slab is cooled to an intermediate temperature of 720 ° C. or less by runout table cooling, and then wound at 560 to 660 ° C. And then rolled to 70
Cold rolling is performed at ８５85% and continuous annealing is performed at an annealing temperature of 780 to 880 ° C.

Intermediate temperature: 720 ° C. or less The intermediate temperature is an important condition that affects the r value and the n value. In order to obtain a high r-value, it is effective to refine the ferrite grains in the hot-rolled sheet and coarsen the carbon / nitride (precipitate), and to obtain a high n-value, it is important to uniformly coarsen the precipitate. It is. Rapid cooling after finish rolling is necessary for refining ferrite grains, and in the case of adding Nb and Ti in combination, a high r value can be obtained by cooling so that the intermediate temperature is 720 ° C. or lower. On the other hand, (12N
Although b) / (93C) + (12Ti ^* ) / (48C) greatly contributes, when precipitation is not appropriately controlled in precipitation of NbC in a cooling stage until winding after finishing hot rolling, The size distribution of the precipitate becomes uneven, and a high n value cannot be obtained stably. That is, precipitation in the cooling stage before winding is suppressed, and uniform precipitation and coarsening are performed after winding. FIG. 7 shows (12Nb) / (93
C) + (12Ti ^* ) / (48C) = 3.0 It is the result of investigating the influence of the intermediate temperature on the n value in the low strain region (1 to 10%) for the steel of 3.0. According to FIG. 7, the intermediate temperature is 7
It is confirmed that a high n value is stably obtained at 20 ° C. or lower.

Winding temperature: 560 to 660 ° C. In the winding step at the time of hot rolling, precipitates are coarsened and the r value and the n value of the steel sheet are improved. However, if the winding temperature is lower than 560 ° C., the precipitates are not sufficiently coarsened, so that the effect of improving the r value and the n value cannot be obtained. On the other hand, 6
If the temperature exceeds 60 ° C., the crystal grains become coarse, and an excellent r value cannot be obtained. For this reason, the winding temperature is set to 560 to 660 ° C.

Rolling ratio during cold rolling: 70-85% The rolling ratio (cold rolling ratio) during cold rolling affects the r value and the n value, and is excellent when the cold rolling ratio is less than 70%. In the case where the r value is not obtained, while the high cooling pressure ratio exceeds 85%,
The crystal grains become fine and an excellent n value cannot be obtained. For this reason, the cold pressure ratio is set to 70 to 85%.

Annealing temperature: 780-880 ° C. The annealing temperature in continuous annealing affects the r value and the n value. When the annealing temperature is lower than 780 ° C., both r value and n value cannot be obtained sufficiently. On the other hand, when the temperature exceeds 880 ° C., abnormal grain growth may occur to cause deterioration of the material. For this reason, the annealing temperature is in the range of 780 to 880 ° C.

The steel sheet of the present invention is obtained by hot rolling a slab, pickling,
It is manufactured through a series of steps such as cold rolling and annealing, and is subjected to plating if necessary. The hot rolling process may be any of a method of rolling after heating the slab, a method of performing rolling for a short time after continuous casting, and a method of immediately rolling without the heating step after continuous casting. In any of these cases, in order to impart excellent surface properties, it is preferable to sufficiently remove not only the primary scale but also the secondary scale generated during hot rolling. During the hot rolling, heating may be performed by a bar heater. The hot rolling finish temperature is set to Ar
₃ points or more.

The cold-rolled steel sheet of the present invention may be subjected to electroplating or zinc-based plating after annealing to be used as a zinc-based plated steel sheet. Sex can be obtained. Examples of the zinc-based plating include pure zinc plating, alloy plating (zinc plating obtained by subjecting an alloy to heat treatment after zinc plating), and zinc-Ni alloy plating. The same performance can be imparted to a steel sheet subjected to an organic film treatment after plating.

[0039]

[Example 1] The steel sheet No. shown in Table 1 was used. After smelting steel Nos. 1 to 8, it was made into a slab by continuous casting, heated to 1250 ° C, and finished at a temperature of 880 to 910 ° C and an intermediate temperature of 680.
After forming a hot-rolled sheet having a thickness of 3.2 mm at a temperature of 640 ° C. and a winding temperature of 640 ° C., the sheet was cold-rolled to a thickness of 0.80 mm, and then subjected to continuous annealing (annealing temperature: 850 ° C.) and hot-dip galvanizing.
In the continuous annealing / hot-dip galvanizing, hot-dip galvanizing was performed at 460 ° C. after annealing, and immediately, alloying of the plated layer was performed at 500 ° C. in an in-line alloying furnace. After continuous annealing and hot-dip galvanizing, temper rolling at a reduction of 0.7% was performed. Table 2 shows the evaluation results of the mechanical properties and the composite formability of these steel sheets after plating peeling. Table 2 shows that r +
The values of 2.0 (n) and r + 50.0 (n) are also shown. The evaluation of the composite formability was performed by obtaining a limit drawing ratio (LDR) and a hat forming height (H). In Table 1, R
Is (12Nb) / (93C) + (12Ti ^* ) / (4
8C).

[0040]

[Table 1]

[0041]

[Table 2]

As shown in these tables, it has the component composition of the present invention and (12Nb) / (93C) + (12T
i ^* ) / (48C), r + 50.0 (n) and r +
By setting the value of 2.0 (n) within the scope of the present invention,
It was confirmed that a cold-rolled steel sheet having excellent composite formability could be obtained. It was also confirmed that when C + N was 0.0030% or less (Steel No. 4), the composite moldability was particularly excellent.

On the other hand, the comparative steel has a composite formability that is equal to or lower than that of the conventional material.
(12Nb) / (93C) + (12Ti ^* ) / (48
When the value (R value) of C) is out of the range of the present invention, the n value becomes low and an excellent hat forming height (H) cannot be obtained. Further, when the values of r + 50.0 (n) and r + 2.0 (n) are out of the range of the present invention, the hat forming height (H) and the limit drawing ratio (LDR) are lowered, respectively. Composite moldability cannot be obtained.

Example 2 After smelting steels Nos. 1 and 4 shown in Table 1, forming a slab by continuous casting, heating to 1250 ° C., finishing temperature 880-910 ° C., intermediate temperature 660-76.
0 ° C, coiling temperature 500-700 ° C, thickness 2.4-6.0
After a hot-rolled sheet having a thickness of 0.8 mm, the sheet was cold-rolled at a rolling reduction of 67 to 87% to a sheet thickness of 0.80 mm. Then, annealing temperature 750-
Continuous annealing or continuous annealing / galvanizing was performed at 900 ° C. In the continuous annealing / hot-dip galvanizing, hot-dip galvanizing was performed at 460 ° C. after annealing, and immediately, alloying of the plated layer was performed at 500 ° C. in an in-line alloying furnace. After continuous annealing and hot-dip galvanizing, the rolling reduction is 0.
A temper rolling of 7% was performed. Table 3 shows the evaluation results of the mechanical properties and the composite formability of these steel sheets after plating peeling.
The evaluation of the composite formability was performed by obtaining a limit drawing ratio (LDR) and a hat forming height (H).

[0045]

[Table 3]

As shown in Table 3, the production conditions of the present invention are satisfied and (12Nb) / (93C) + (12T
i ^* ) / (48C), r + 50.0 (n) and r +
By setting the value of 2.0 (n) within the scope of the present invention,
It was confirmed that a cold-rolled steel sheet having excellent composite formability could be obtained. In addition, it was confirmed that Steel No. 4 having C + N of 0.0030% or less was particularly excellent in composite moldability.

On the other hand, the comparative steel which does not satisfy the production conditions of the present invention has a composite formability that is equal to or lower than that of the conventional material, and particularly has a high intermediate temperature, a low winding temperature, and a low cold rolling ratio. If the annealing temperature is high and the annealing temperature is low, the n value becomes low, and a steel sheet having an excellent hat forming height cannot be obtained. Also, r + 50.0 (n) and r + 2.0 (n)
Is out of the range of the present invention, the hat forming height (H) and the limit drawing ratio (LDR) become low, and excellent composite moldability cannot be obtained.

[0048]

As described above, according to the present invention,
C, N, Nb, which affect the propagation of strain in the low strain region
By appropriately controlling Ti and r values, n values, hot rolling conditions, cold rolling rates, and annealing conditions, the composite formability consisting of draw formability and stretch formability is excellent, and front fenders and sides can be formed without causing press cracking. It is possible to obtain a cold-rolled steel sheet that can be composite-formed into a complicated shape such as a panel, which is industrially extremely significant.

[Brief description of the drawings]

FIG. 1: (12Nb) / (93C) + (12Ti ^* ) /
The graph which shows the relationship between (48C) and n value.

FIG. 2 C: 0.0030% or less, N: 0.0030%
Containing the following, (12Nb) / (93C) + (12Ti
^The graph which shows the result which investigated the influence which C + N gives to n value in a low distortion area | region (1-10%) about the steel which ^* ) / (48C) was in the range of 2.2-4.5.

FIG. 3 is a graph showing an example of an equivalent strain distribution in the vicinity of a fracture danger site of the front fender model molded product of the actual part scale shown in FIG. 4;

FIG. 4 is a schematic diagram showing a front fender model molded product and its fracture danger site.

FIG. 5 is a graph showing a relationship between an n value and an r value, draw formability and stretch formability.

FIG. 6 is a schematic diagram of a hat molding test.

FIG. 7: (12Nb) / (93C) + (12Ti ^* ) /
The graph which shows the result of having investigated the influence which the intermediate temperature gives to the n value of a low distortion area | region (1-10%) about the steel of (48C) = 3.0.

 ──────────────────────────────────────────────────続 き Continuing on the front page (72) Inventor Manabu Tano 1-2-1, Marunouchi, Chiyoda-ku, Tokyo Nihon Kokan Co., Ltd. (72) Koji Matsubayashi 1-2-1, Marunouchi, Chiyoda-ku, Tokyo Sun (72) Inventor Kenichi Mitsuka 1-1-2 Marunouchi, Chiyoda-ku, Tokyo, Japan In-house Steel Pipe Co., Ltd. (72) Katsumi Nakajima 1-2-1, Marunouchi, Chiyoda-ku, Tokyo, Japan Co., Ltd. (72) Inventor Kohei Hasegawa 1-2-1, Marunouchi, Chiyoda-ku, Tokyo Nihon Kokan Co., Ltd. F-term (reference) 4K037 EA01 EA04 EA15 EA18 EA19 EA23 EA25 EA27 EA31 EB01 FE02 FE03 FG03 FH01 FJ05 FJ06

Claims (3)

[Claims] C .: 0.0020% or less by weight, C:
i: 0.05% or less, Mn: 0.05 to 0.35%,
P: 0.025% or less, S: 0.015% or less, so
l. Al: 0.01 to 0.06%, N: 0.0020%
Hereinafter, Nb: 0.010 to 0.040%, Ti: 0.0
03-0.035%, and (12Nb) /
(93C) + (12Ti ^* ) / (48C): 1.3 to
5.2 (However, Ti ^* = Ti- (48/14) N-
(48/32) S, and when Ti ^* ≦ 0, Ti ^*
= 0, Nb, Ti ^* , Ti, C, N, and S are% by weight.)
And a cold-rolled steel sheet excellent in composite formability, characterized by satisfying the following expressions (1) and (2). 13.9 ≦ r + 50.0 (n) (1) 2.6 ≦ r + 2.0 (n) (2) where r: average in-plane r value, n: tensile strain range of 10% to 10% Work hardening index n
value 2. In% by weight, C: 0.0020% or less, S
i: 0.05% or less, Mn: 0.05 to 0.35%,
P: 0.025% or less, S: 0.015% or less, so
l. Al: 0.01 to 0.06%, N: 0.0020%
Hereinafter, Nb: 0.010 to 0.040%, Ti: 0.0
03-0.035%, and C + N: 0.00
30% or less, (12Nb) / (93C) + (12T
i ^* ) / (48C): 2.2 to 4.5 (however, Ti ^*
= Ti- (48/14) N- (48/32) S,
When Ti ^* ≦ 0, it is assumed that Ti ^* = 0, and Nb, Ti ^* ,
A cold-rolled steel sheet having excellent composite formability, characterized by satisfying the following formulas (1) and (2). 13.9 ≦ r + 50.0 (n) (1) 2.6 ≦ r + 2.0 (n) (2) where r: average in-plane r value, n: tensile strain range of 10% to 10% Work hardening index n
value 3. In producing the cold-rolled steel sheet according to claim 1 or 2, after subjecting the steel slab to hot rolling, the steel slab is cooled to a temperature of 720 ° C. or lower by a run-out table cooling, and then 560 to 660. A method for producing a cold-rolled steel sheet excellent in composite formability, comprising: winding at a rolling rate of 70 to 85%, followed by cold rolling at a rolling rate of 70 to 85%, and continuous annealing at an annealing temperature of 780 to 880 ° C. [0001]