JPH05279797A - Cold rolled steel sheet having extremely superior deep drawability and bulging property and its production - Google Patents

Abstract

PURPOSE:To produce a cold rolled steel sheet having superior deep drawability and bulging deformability by preparing a steel which has a specific composition having respectively specified C, S, N, Ti, and Nb contents and where the inplane mean value of Lankford value and the work hardening index in the prescribed tensile strain region are specified, respectively. CONSTITUTION:The cold rolled steel sheet having a composition which consists of, by weight, <=0.0030% C, <=0.05% Si, 0.05-0.50% Mn, <=0.02% P, =0.02% S, 0.03-0.06% Sol. Al, <=0.0040% N, 0.005-0.0015% Nb, 0.04-0.14% Ti, and the balance Fe with inevitable impurities and where (Ti*/-[C]) satisfying equations I,II {where [%C], [%Ti], and [%S] represent C content (%), Ti content (%), and S content (%), respectively} and inequality III {where [%Nb] represents Nb content(%)} is regulated to >=7 is produced. In this cold rolled steel sheet, the inplane mean value (means-r) of Lankford value defined by equation IV {where [r0] means r-value in a steel sheet rolling direction} and the work hardening index evaluated in the region of 10-20% tensile strain are regulated to >=2.8 and >=0.26, respectively.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a cold-rolled steel sheet for super deep drawing which has excellent deep drawing formability and stretch forming property and has improved deep drawing embrittlement resistance, and a method for producing the same.

[0002]

2. Description of the Related Art So-called IF steel (Interstitial), which is made by adding carbon / nitride forming elements such as Nb, Ti, Zr, and B to ultra-low carbon steel.
Free Steel) is attracting attention as a powerful material for producing high deep drawing cold rolled steel sheets (EDDQ) by continuous annealing, which requires deep drawing and non-aging properties, and its importance as the continuous annealing process spreads today. Sex has been recognized.

The IF steel that has been generally used in the past is
Ti-IF steel containing Ti and Nb containing Nb-
IF steel. In particular, Ti is a strong carbon / nitride forming element, and at the same time, it causes S in steel to precipitate and coarsen as sulfides, so Ti-IF steel has extremely excellent deep drawability and ductility in a wide range of components. There are features that can be obtained. However, on the other hand, oxidization is easy and oxide-based surface defects occur during steelmaking, so strict slab care is required. Further, when C in steel is completely fixed as TiC, the grain boundary strength decreases, and problems such as deep drawing embrittlement (secondary work embrittlement phenomenon) occur. It is known that addition of a small amount of boron (B) is effective for this problem, but in that case, if B is added in an amount of 10 ppm or more, the r value is deteriorated (deep drawability is deteriorated). It becomes a problem.

On the other hand, in Nb-IF steel, Ti-I is mainly formed by fixing only C in steel and fixing solid solution C in steel.
Similar to F steel, excellent deep drawability can be obtained, but when Nb is excessively added, the effect of suppressing grain growth due to NbC precipitates becomes remarkable and the material deteriorates. Therefore, there is a problem that the appropriate component range is narrower than that of Ti-IF steel. But T
The surface quality is superior because it does not create oxide-based slab defects compared to i.
It has been clarified that in-plane anisotropy of r value different from that of F steel appears.

As a method for solving the above problems, a technique of adding Nb and Ti in combination (Japanese Patent Publication No. 61-32375) is disclosed from the viewpoint of combining the characteristics of Ti and Nb. The essence of this technology is 0.003-0.025
wt% Nb and 0.010 to 0.037 wt% Ti
Are added in a range satisfying the condition of Nb> 2.23C {(48/14) · (N-0.002)} <Ti <(4C + 3.43N), respectively.
Fusing the differences in texture of Nb and Ti described above,
The contents are to obtain effects such as improving the in-plane anisotropy of the value and reducing the material variation in the coil.

[0006]

However, in this way, N
When b and Ti are added in a complementary form, judging from the solubilities of carbon and nitride of each element, assuming that C is distributed in Nb and Ti in an equivalence ratio, it becomes difficult to precipitate either. However, even if it is deposited, it will be finely deposited. Therefore, even if the in-plane anisotropy and material variation in the coil are improved by simply adding Nb and Ti only by the stoichiometric ratio with the C content or N content in the steel, the absolute material In terms of level improvement, it is not completely satisfactory. This is described in Japanese Patent Publication No. 61-323.
It is easy to judge from the balance between the n value and the mean-r value (n value ≈ 0.26 to 0.30, mean-r value ≈ 1.85 to 1.95) in the example disclosed in No. 75. [Mean-r] value-n as is the object of the present invention.
Clearly, it does not achieve value balance.

In recent years, cold-rolled steel sheets used for automobile bodies are used for conventional ultra-deep drawing in response to the complicated shape of vehicle body parts, promotion of integral molding, expansion of applicable parts of galvannealed steel sheet. There is an increasing demand for steel sheets having formability that exceeds that of steel sheets (EDDQ). From this viewpoint, Ti
An example of product development based on the additive IF steel, divided into a deep drawing forming type and a stretch forming type (Shibasaki et al.
"Materials and Process 2 (1989)" p. 1931), but the n value and mean-
The balance of r values is n value = the former (deep drawing forming type).
0.265, mean-r value = 2.50, n value = 0.278, mean-r value = in the latter (protruding molding-oriented type)
It is about 2.15.

The present invention discloses a steel sheet having both deep drawing formability and stretch forming formability from a practical point of view, and a method for producing the same, which is an index for evaluating deep drawing property.
The purpose is to obtain a steel sheet having an r value of 2.8 or more and an n value of 0.26 or more, which is an index for evaluating the overhang property.
Another object of the present invention is to obtain a steel sheet having not only n value and mean-r value but also good deep drawing embrittlement resistance and zinc plating adhesion.

[0009]

Therefore, the present invention has the following configuration. (1) C ≦ 0.0030 wt%, Si ≦ 0.05 wt
%, 0.05 wt% ≦ Mn ≦ 0.50 wt%, P ≦ 0.
02 wt%, S ≦ 0.02 wt%, 0.03 wt% ≦ S
ol. Al ≦ 0.06 wt%, N ≦ 0.0040 wt
%, 0.005 wt% ≤ Nb ≤ 0.015 wt%, 0.
04 wt% ≤ Ti ≤ 0.14 wt% and (Ti * / [C]) ≥ 7 where Ti * / [C] = [wt% Ti *] / 4 [wt
% C] [wt% Ti *] = [wt% Ti]-{(48/14)
-[Wt% N] + (48/32)-[wt% S]} [wt% C]: C content (wt%) [wt% Ti]: Ti content (wt%) [wt% N] : N content (wt%) [wt% S]: S content (wt%) 7 ≦ ([wt% Ti] / [wt% Nb]) ≦ 18 However, [wt% Ti]: Ti content ( wt%) [wt% Nb]: Nb content (wt%) is satisfied, and the steel composition is composed of the balance Fe and unavoidable impurities. r] is 2.8 or more, 10% to 20%
The cold-rolled steel sheet having extremely excellent deep drawing formability and stretch formability, which has a work hardening index n of 0.26 or more evaluated in the tensile strain range. [Mean-r] = ([r _0] +2 [r _45] + [r _90]) / 4 where [r _0]: r value of steel plate rolling direction [r _45]: 45 ° with respect to the steel sheet rolling direction Value in the direction [r ₉₀ ]: r value in the direction of 90 ° with respect to the steel plate rolling direction (2) C ≦ 0.0030 wt%, Si ≦ 0.05 wt
%, 0.05 wt% ≦ Mn ≦ 0.50 wt%, P ≦ 0.
02 wt%, S ≦ 0.02 wt%, 0.03 wt% ≦ S
ol. Al ≦ 0.06 wt%, N ≦ 0.0040 wt
%, 0.005 wt% ≤ Nb ≤ 0.015 wt%, 0.
04 wt% ≤ Ti ≤ 0.14 wt% and (Ti * / [C]) ≥ 7 where Ti * / [C] = [wt% Ti *] / 4 [wt
% C] [wt% Ti *] = [wt% Ti]-{(48/14)
-[Wt% N] + (48/32)-[wt% S]} [wt% C]: C content (wt%) [wt% Ti]: Ti content (wt%) [wt% N] : N content (wt%) [wt% S]: S content (wt%) 7 ≦ ([wt% Ti] / [wt% Nb]) ≦ 18 However, [wt% Ti]: Ti content ( wt%) [wt% Nb]: Steel that satisfies the Nb content (wt%) and has a composition consisting of balance Fe and unavoidable impurities is hot-rolled, cold-rolled and continuously annealed by a conventional method. The in-plane average value [mean-r] of the Rank Ford value defined by the following equation is 2.8 or more,
Work hardening index n evaluated in the tensile strain region of 10% to 20%
Is 0.26 or more, which is a method for producing a cold-rolled steel sheet having extremely excellent deep drawing formability and stretch formability. [Mean-r] = ([r _0] +2 [r _45] + [r _90]) / 4 where [r _0]: r value of steel plate rolling direction [r _45]: 45 ° with respect to the steel sheet rolling direction Value in the direction [r ₉₀ ]: r value in the 90 ° direction with respect to the steel sheet rolling direction (3) In the above (2), the slab heating temperature ≦ 120
After hot rolling at 0 ° C. and hot rolling coiling temperature: 580 to 640 ° C., cold rolling is performed at a rolling ratio: 76 to 84%, and then 800.
A method for producing a cold-rolled steel sheet having extremely excellent deep drawing formability and stretch formability, which is characterized in that continuous annealing is carried out at a temperature of ℃ to 880 ℃.

The details of the present invention will be described below. The present invention is based on Ti-added IF steel, which has a wide allowable range in component design, is stable against manufacturing conditions, and is superior in grain growth compared to Nb-added steel. For the purpose of improving the surface properties, controlling the texture, and improving the resistance to deep drawing embrittlement, the basic feature is to add a small amount of Nb relative to the Ti amount and a limited amount of Nb in relation to the Ti amount. There is.

First, the fundamental difference of the present invention from the above-mentioned prior art is that Ti * / is defined by the following equation.
[C] (atomic weight ratio) is limited to 7 or more. Ti * / [C] = [wt% Ti *] / 4 [wt% C] [wt% Ti *] = [wt% Ti]-{(48/14). [Wt% N] + (48/32 ). [Wt% S]} where [wt% C]: C content (wt%) [wt% Ti]: Ti content (wt%) [wt% N]: N content (wt%) [ wt% S]: S content (wt%)

The present invention aims at the complete fixation of carbon / nitride and the coarsening of their precipitates by adding a sufficient amount of Ti when fixing C in steel. Figure 1 shows T
i: 0.01 to 0.20 wt%, Nb: 0 wt%, and 0.002 to 0.03 wt% of steel, the above-mentioned Ti * / [C] means-r value and Δr value defined below. This is a summary of the effects on the. [Mean-r] = ([r _0] +2 [r _45] + [r _90]) / 4 [Delta] r = ([r _0] + [r _90] -2 [r _45]) / 2 where [r ₀ ]: R value in the rolling direction of the steel plate [r ₄₅ ]: r value in the 45 ° direction with respect to the rolling direction of the steel plate [r ₉₀ ]: r value in the 90 ° direction with respect to the rolling direction of the steel plate When Nb is added, solid solution Nb
It is understood that the level of the mean-r value rises due to the fine graining of the structure of the hot-rolled sheet.

Further, FIG. 2 shows that Ti: 0.01 to 0.20.
wt%, Nb: 0 wt% and 0.002 wt% to 0.
For steel in the range of 03 wt%, the weight% ratio of Ti and Nb ([wt% Ti] / [wt% Nb]) is evaluated in the mean-r value and the work hardening index n (tensile strain range of 10% to 20%). This is a summary of the effect on the n value). According to this, [wt% Ti] / [w
It can be seen that an excellent balance between the mean-r value (2.8 or more) and the n value (0.26) can be obtained only when t% Nb] is set in the range of 7 to 18.

Then, the reasons for limiting the most important additional elements in the present invention, Ti and Nb, will be described.
As described above, Ti is a strong carbon / nitride forming element, and is an element that can obtain the above-mentioned merits.
In particular, in order to fix C in steel in equilibrium, Ti * /
[C] ≧ 1 is sufficient, but the size of the precipitates is sufficiently coarsened to provide excellent grain growth and <111> // N
In order to enhance the accumulation of recrystallized grains in the D direction, Ti * /
It is suggested from FIG. 1 that [C] ≧ 7 is preferable. Therefore, in the present invention, Ti * / [C] ≧ 7
Stipulate.

Further, in the present invention, in addition to the above stipulation, Ti
The addition amount is defined as 0.04 wt% ≦ Ti ≦ 0.14 wt%. When Ti is less than 0.04 wt%, C in steel can be fixed, but coarsening of TiC is less likely to occur, and measures such as winding at high temperature during hot rolling are required in the process. On the other hand, not only when the content exceeds 0.14 wt%, a remarkable addition effect is not observed, but also surface defects are revealed and alloy cost is increased.

Nb is an essential addition element in the present invention, but its addition amount is limited to a minute range of 0.005 to 0.015 wt%. In particular, limiting [wt% Ti] / [wt% Nb] to a range of 7 to 18 as shown in FIG. Further, it has been clarified that the addition of a small amount of Nb is effective in improving the deep drawing embrittlement resistance. In order to obtain such effects, the lower limit of Nb addition is 0.005 wt%
Is prescribed. In addition, the upper limit of the addition amount is 0.0 from the viewpoint that the material changes greatly depending on the manufacturing conditions, the material hardens conversely, the alloy cost increases, and the like.
Limited to 15 wt%.

Further, as a secondary effect of the present invention, when a small amount of Nb is added to the steel containing Ti in the range of Ti * / [C] ≧ 7, as shown in FIG. It has been found that the quality is significantly improved. The mechanism by which such an effect is obtained is not necessarily clear, but it is considered that the presence of a small amount of Nb suppresses the oxidation reaction of Ti on the slab surface. From this, it became clear that the technology disclosed in the present invention has utility as a base steel sheet for galvanized steel sheets.

Next, the reasons for limiting other elements will be described. In order to improve the C: n value, it is necessary to limit not only the size of TiC but also the total amount thereof. In the present invention, the upper limit of C is specified to be 0.0030 wt% in order to obtain a high n value. Si: Even at the level of ordinary steel, it does not adversely affect the operation and effect of the present invention, but in order to maintain a high level of ductility, it is set to 0.05 wt% or less. Since Mn: Ti contributes to the fixation of S, Mn may be lower than the level of general steel, but if it is less than 0.05 wt%, the hot metal pretreatment cost increases, so the lower limit is set to 0.
It is specified as 05 wt%. On the other hand, if it exceeds 0.50 wt%, YP increases and the n value decreases due to solid solution strengthening by Mn. Therefore, the upper limit is defined as 0.50 wt%. P: P is a grain boundary embrittlement element, and the upper limit must be strictly controlled particularly in IF steel where the grain boundary is liable to become brittle. Therefore, in the present invention, the upper limit is 0.02 wt%. In particular, in order to make the remarkable improvement effect of the deep drawing embrittlement resistance due to the addition of a trace amount of Nb more stable, P is preferably 0.01 wt% or less. S: S reduces the effective Ti amount (Ti *) by precipitating as TiS. Therefore, in the present invention, the upper limit is specified as 0.02 wt%.

Sol. In the case of Al: Ti-added steel, N is T
Since it is fixed as iN, for the purpose of only fixing N, the addition amount of Al can be reduced within the range where continuous casting is possible. However, in the present invention, Al is added to the same level as ordinary Al-killed steel. This is because, in addition to improving the flowability of the ultra-low carbon steel during casting, deoxidation with Al suppresses the oxidation of Ti and reduces the occurrence of surface defects. From the above viewpoint, Sol. 0.03 as Al
It is specified in the range of wt% to 0.06 wt%. From the viewpoint of the material of the IF steel, N: N is basically preferably as low as possible, and in particular, the mean-r value is improved as the amount of nitrides is reduced. However, since Ti * / [C] is set to a sufficiently high level in the present invention, there is no extreme change in the material due to the fluctuation of the N content at a normal level. Therefore, in the present invention, the upper limit is defined as 0.0040 wt% as the allowable level for the n value and the mean-r value.

The steel sheet disclosed in the present invention can obtain the characteristics exceeding the level of the conventional cold-rolled steel sheet even if it is manufactured as a product by a conventional method, but it imparts the best characteristics to the component system defined in the present invention. The manufacturing method for this will be disclosed below. For the component system of the present invention, slab heating temperature ≤1200
C, hot rolling coiling temperature: 580-640C, cold rolling rate:
Most preferably, it is 76 to 84% and the continuous annealing temperature is 800 to 880 ° C.

The most important of these are the hot rolling coiling temperature and the cold rolling rate. In order to obtain a high mean-r value, it is preferable that the carbon / nitride in the hot-rolled sheet becomes coarse and the ferrite grain size be small. For the former, Ti *
By setting / [C] ≧ 7, the winding temperature can be lowered, and this can be achieved. Further, since Nb contributes to grain refinement as solid solution Nb, the latter state can be achieved. As an example showing this effect, the effect of the coiling temperature on the [mean-r] value-n value balance in steel No. 13 (Ti-Nb system) and steel No. 12 (Ti system) in Table 2 in FIG. 4 ( Winding temperature LCT: 620 ° C, winding temperature HC
T: 680 ° C.). As is clear from the figure, Ti-
It can be seen that the mean-r value of the Nb type is higher than the mean-r value of the Ti type, and further that the winding at 620 ° C increases the mean-r value as compared to the winding at 680 ° C. Based on the above results, the upper limit of the winding temperature is preferably 640 ° C. from the viewpoint of the mean-r value. However, when the winding temperature is lower than 580 ° C., TiC is finely precipitated, so that the mean-r value of the product is lowered. Therefore, the lower limit of the winding temperature is 580 ° C.

Next, the cold rolling rate was determined from the viewpoint of the mean-r value and the deep drawing embrittlement resistance. FIG. 5 shows the slab heating temperature H: 1250 ° C., L: 1 for the steel used in FIG.
150 ° C, winding temperature LCT: 620 ° C, HCT: 68
It shows the balance between the deep-drawing embrittlement transition temperature Tth and the mean-r value of the steel sheet produced under the conditions of 0 ° C., cold rolling rate of 75%, 79%, and 82%. As is clear from the figure, the deep drawing embrittlement critical temperature (Tth) of Ti-IF steel is
It is improved by adding a trace amount of Nb. In particular, slab heating temperature:
When manufactured under the conditions of 1150 ° C. and winding temperature: 620 ° C., Tth is improved to about −90 ° C. Also, Tt
Similar to the mean-r value, h has cold pressure dependency. However, while the mean-r value is improved by increasing the cold pressure ratio, Tth is increased on the contrary. This is considered to be a phenomenon related to the change in grain boundary character due to the change in texture. From the viewpoint of the mean-r value, the lower limit of the cold rolling ratio is preferably 76% (desirably 80%), while the upper limit is the deep drawing embrittlement countermeasure and the mean-r value in the rolling 0 ° direction. Considering the decrease of
It is preferably 84%.

Regarding the slab heating temperature and the continuous annealing temperature, the former has an upper limit of 1200 ° C. due to the problem of deep drawing embrittlement shown in FIG. 5, and the latter has a lower limit for achieving sufficient grain growth after recrystallization. Limited to 800 ℃, Ti * / [C]
When the grain growth property is improved by ≧ 7, there is a possibility that abnormal coarsening due to secondary recrystallization may occur when annealing just under the Ac ₃ point, so the upper limit is limited to 880 ° C. The steel sheet of the present invention can also be manufactured by batch annealing, and although the obtained steel sheet material is slightly inferior to the high temperature continuous annealed material, it has excellent characteristics compared to the conventional steel sheet. In particular, when performing batch annealing using Ti-IF steel as a raw material, there is a problem that rough skin occurs due to excessive grain growth, but with the material of the present invention, it is possible to perform batch annealing without causing such a problem. it can.

As described above, in the present invention, Ti * / [C] is aimed at the mean-r value ≥2.8 and the n value ≥0.26.
And [wt% Ti] / [wt% Nb] are limited. By the way, Ti * / [C] is reduced to 7 with low Ti
In order to achieve the above, reduction of C, N, and S is indispensable. However, in this case, the structure of the hot-rolled sheet is likely to be coarsened, and the in-plane anisotropy of the r value after cold rolling and annealing tends to increase. FIG. 6 shows the Ti amount and Ti * /
The results of examining the mean-r value and the Δr value of the material in which the balance of [C] is changed are shown (the diagonal lines in the figure are C: 0.001 wt% and N: 0.001 w).
The value of Ti * / [C] with respect to the Ti amount in the case of t% and S: 0.001 wt% is shown). According to this, Ti * /
Although a mean-r value ≧ 2.8 is obtained in the region of [C] ≧ 7, Δr ≧ 0.5 in the range of 0.04 wt% ≦ Ti <0.06 wt%, whereas Ti ≧ 0. In the range of 06 wt%, Δr <0.5. Therefore, from the viewpoint of today's steelmaking technology level and manufacturing cost, C,
Considering that there is a limit to the reduction of N and S to the limit, it can be said that Ti ≧ 0.06 wt% is more practically advantageous.

[0025]

【Example】

Example 1 Using representative steel types shown in Tables 1 and 2, the pinhole density (evaluation after 2 mm hot scarf) on the surface of the continuous casting slab was examined. The results are shown in Table 3.

Example 2 With respect to the representative steel types shown in Tables 1 and 2, slab heating temperature: 1150 ° C., hot rolling finishing temperature: 900 ° C., winding temperature: 620 ° C., cold pressure ratio: 8
The material of the steel sheet manufactured under the conditions of 2%, annealing temperature: 860 ° C., and pressure regulation rate: 0.5% was examined. The results are shown in Tables 4 and 5.

Example 3 With respect to the steels of the present invention (steel Nos. 8, 11, and 13) shown in Tables 1 and 2, the materials of the steel plates produced under various conditions shown in Tables 6 and 7 were examined. ..
The results are shown in Tables 8 and 9.

Example 4 With respect to the representative steel types shown in Tables 1 and 2, the material which was cold-rolled under the same conditions as in Example 2 was annealed at 850 ° C. by CGL, and then the basis weight was measured. : 55/55 (g / m ² ) galvanized, and then alloyed steel sheets were subjected to 0.5% pressure regulation, and then the amount of stripped plating was evaluated by a draw beat test. The results are shown in Table 10.

[Embodiment 5] Table 11 shows the material properties of the steel sheets subjected to box annealing for the representative steel types shown in Tables 1 and 2. In this example, open coil annealing was performed at 800 ° C. According to this example, it can be seen that the steel sheet of the present invention, which has been subjected to batch annealing, is slightly inferior in material quality to the one continuously annealed at a high temperature, but has superior characteristics to the comparative steel.

[0030]

[Table 1]

[0031]

[Table 2]

[0032]

[Table 3]

[0033]

[Table 4]

[0034]

[Table 5]

[0035]

[Table 6]

[0036]

[Table 7]

[0037]

[Table 8]

[0038]

[Table 9]

[0039]

[Table 10]

[0040]

[Table 11]

[Brief description of drawings]

FIG. 1 is a drawing showing the effect of Ti * / [C] on the mean-r value and Δr value of Ti-added IF steel and Ti-Nb-added IF steel.

FIG. 2 affects the balance between mean-r value and n value [w
It is drawing which shows the influence of t% Ti] / [wt% Nb].

FIG. 3 is a drawing showing the effect of addition of a trace amount of Nb on the number of pinholes (after 2 mm scarf) on the slab surface of Ti-added IF steel.

FIG. 4 is a drawing showing the effect of addition of a trace amount of Nb and hot rolling temperature on the balance between mean-r value and n value.

FIG. 5: mean-r value and deep drawing embrittlement transition temperature (Tt
It is drawing which shows the influence of the slab heating temperature, the coiling temperature, and the cold pressure rate which act on the balance of h).

FIG. 6 Ti amount and Ti that affect mean-r value and Δr value
It is a figure which shows the influence of the balance of * / [C].

 ─────────────────────────────────────────────────── ─── Continuation of the front page (72) Inventor Shuji Kanto 1-2-1, Marunouchi, Chiyoda-ku, Tokyo Nihon Kokan Co., Ltd. (72) Hiroshi Wakasa 1-2-1 Marunouchi, Chiyoda-ku, Tokyo Date Inside the steel pipe company

Claims (3)

[Claims] 1. C ≦ 0.0030 wt%, Si ≦ 0.0
5 wt%, 0.05 wt% ≤ Mn ≤ 0.50 wt%, P
≦ 0.02 wt%, S ≦ 0.02 wt%, 0.03 wt
% ≦ Sol. Al ≦ 0.06 wt%, N ≦ 0.0040
wt%, 0.005 wt% ≤ Nb ≤ 0.015 wt%,
0.04 wt% ≦ Ti ≦ 0.14 wt% and (Ti * / [C]) ≧ 7, where Ti * / [C] = [wt% Ti *] / 4 [wt
% C] [wt% Ti *] = [wt% Ti]-{(48/14)
-[Wt% N] + (48/32)-[wt% S]} [wt% C]: C content (wt%) [wt% Ti]: Ti content (wt%) [wt% N] : N content (wt%) [wt% S]: S content (wt%) 7 ≦ ([wt% Ti] / [wt% Nb]) ≦ 18 However, [wt% Ti]: Ti content ( wt%) [wt% Nb]: Nb content (wt%) is satisfied, and the steel composition is composed of the balance Fe and unavoidable impurities. r] is 2.8 or more, 10% to 20%
The cold-rolled steel sheet having extremely excellent deep drawing formability and stretch formability, which has a work hardening index n of 0.26 or more evaluated in the tensile strain range. [Mean-r] = ([r _0] +2 [r _45] + [r _90]) / 4 where [r _0]: r value of steel plate rolling direction [r _45]: 45 ° with respect to the steel sheet rolling direction Value in the direction [r ₉₀ ]: r value in the 90 ° direction with respect to the rolling direction of the steel sheet 2. C ≦ 0.0030 wt%, Si ≦ 0.0
5 wt%, 0.05 wt% ≤ Mn ≤ 0.50 wt%, P
≦ 0.02 wt%, S ≦ 0.02 wt%, 0.03 wt
% ≦ Sol. Al ≦ 0.06 wt%, N ≦ 0.0040
wt%, 0.005 wt% ≤ Nb ≤ 0.015 wt%,
0.04 wt% ≦ Ti ≦ 0.14 wt% and (Ti * / [C]) ≧ 7, where Ti * / [C] = [wt% Ti *] / 4 [wt
% C] [wt% Ti *] = [wt% Ti]-{(48/14)
-[Wt% N] + (48/32)-[wt% S]} [wt% C]: C content (wt%) [wt% Ti]: Ti content (wt%) [wt% N] : N content (wt%) [wt% S]: S content (wt%) 7 ≦ ([wt% Ti] / [wt% Nb]) ≦ 18 However, [wt% Ti]: Ti content ( wt%) [wt% Nb]: Steel that satisfies the Nb content (wt%) and has a composition consisting of balance Fe and unavoidable impurities is hot-rolled, cold-rolled and continuously annealed by a conventional method. The in-plane average value [mean-r] of the Rank Ford value defined by the following equation is 2.8 or more,
Work hardening index n evaluated in the tensile strain region of 10% to 20%
Is 0.26 or more, which is a method for producing a cold-rolled steel sheet having extremely excellent deep drawing formability and stretch formability. [Mean-r] = ([r _0] +2 [r _45] + [r _90]) / 4 where [r _0]: r value of steel plate rolling direction [r _45]: 45 ° with respect to the steel sheet rolling direction Value in the direction [r ₉₀ ]: r value in the 90 ° direction with respect to the rolling direction of the steel sheet 3. A slab heating temperature ≦ 1200 ° C., a hot rolling coiling temperature: 580 to 640 ° C., and a rolling ratio of 7 after hot rolling.
The method for producing a cold-rolled steel sheet having extremely excellent deep drawing formability and stretch formability according to claim 2, wherein cold rolling is performed at 6 to 84%, and then continuous annealing is performed at 800 ° C to 880 ° C. ..