KR101464845B1 - Steel plate having excellent moldability and shape retention, and method for producing same - Google Patents

Abstract

Provided are a cold-rolled steel sheet having excellent moldability and shape durability and a method for producing the same. 0.001 to 0.0030% of C, 0.05% or less of Si, 0.1 to 0.5% of Mn, 0.05% or less of P, 0.02% or less of S, 0.02 to 0.10% 0.010 to 0.035% Nb, the Al content and the N content satisfying the relation of the following formula (1) and the balance of Fe and inevitable impurities, wherein the cold-rolled steel sheet has an average particle diameter : Has a structure mainly composed of ferrite particles of 8 to 20 mu m, and the area ratio of the ferrite particles within 15 DEG from {211} is 50% or more of the structure thereof.
[% A1] / [% N] ≥ 10 ... (One)
Note that [% M] represents the content of element M (% by mass).

Description

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a cold-rolled steel sheet having excellent moldability and shape-

The present invention relates to a cold-rolled steel sheet which is most suitable as a member of a part having a large flat plate shape in the fields of electric, automobile, building materials and the like, and is excellent in moldability and shape durability, and a manufacturing method thereof.

There is a growing demand to reduce the amount of steel material by reducing the thickness of the steel sheet in order to reduce CO ₂ in the global environment and to reduce the amount of material used for lowering the cost. However, if the steel sheet is made thinner, the component stiffness is lowered, and problems such as warping or breaking occur. In addition, in a steel sheet for merchandise for which a large-size display such as a thin television is progressing, the steel sheet becomes easier to lose, and the problem accompanying the thinning of the steel sheet is increasing.

Further, in order to secure the component rigidity of the steel sheet, embossing or bead processing may be performed to increase the height of the embossing or the bead, or to bend the end of the steel sheet. In this case, New problems such as cracking and deformation will occur. Therefore, there is a growing demand for a steel sheet excellent in moldability and shape durability.

As a steel sheet having excellent shape-formability in the past, for example, Patent Document 1 discloses a technique for suppressing springback of a steel sheet at the time of bending by setting the ratio of {100} plane to {111} plane to 1.0 or more have.

As disclosed in, for example, Patent Document 2, a steel sheet having both formability and shape crystallinity can be used as a steel sheet having an average value of the X-ray random intensity ratios of the {100} <011> to {223} (B) of the X-ray random intensity ratios of the three crystal orientations of {554} <225>, {111} <112> and {111} <110> And at least one of r values in a rolling direction and a direction perpendicular to the rolling direction is 0.7 or less and an average value of r values is 0.8 or more, 1.0? (A) / (B)? 4.0, Discloses a technique for achieving coexistence.

International Publication No. 2000/6791 pamphlet Japanese Laid-Open Patent Publication No. 2004-131754

However, all of the steel sheets described in Patent Documents 1 and 2 have a constant shape freezing property at the time of bending. For example, in the case of machining requiring high ductility such as elongation machining, sufficient shape durability is obtained In the case of embossing or bead processing, in the case of a processing with a still higher extension height, there is a problem that a constricted portion occurs.

It is an object of the present invention to provide a cold-rolled steel sheet having improved formability and shape durability by optimizing the components of the steel sheet and the structure of the steel sheet, and a method for producing the same.

As a result of repeated investigations to solve the above problems, the inventors obtained the following knowledge.

(1) In order to control the ferrite grain size and texture of the ultra-low carbon steel, it is preferable that, in each of the steps after the hot rolling and the cold rolling, the {111} It is necessary to preferentially deposit AlN in the surrounding area.

(2) Further, in the annealing process after the cold rolling, it is necessary to suppress recrystallization of ferrite around {111} by NbC which precipitates AlN as a nucleus at the time of cracking.

(3) It is also effective to control the grain size of ferrite while promoting recrystallization around {211}.

By satisfying the above conditions, it was found that resistance strength (hereinafter referred to as YP), high uniformity softening, and low r-value can be achieved at the same time.

The present invention has been made on the basis of this finding, and its structure is as follows.

1. A ferritic stainless steel comprising, by mass%, 0.0010 to 0.0030% of C, 0.05% or less of Si, 0.1 to 0.5% of Mn, 0.05% or less of P, 0.02% or less of S, 0.02 to 0.10% % And Nb: 0.010 to 0.035%, the Al content and the N content satisfying the relation of the following formula (1) and the balance of Fe and inevitable impurities, wherein the cold- Characterized in that it has a structure mainly composed of ferrite particles having an average particle diameter of 8 to 20 占 퐉 and an area ratio of ferrite particles within 15 占 from {211} is 50% or more of the structure thereof. Cold rolled steel sheet with excellent formation.

[% A1] / [% N] ≥ 10 ... (One)

Note that [% M] represents the content of element M (% by mass).

2. The cold-rolled steel sheet as described in 1 above, wherein the cold-rolled steel sheet further contains 0.0003 to 0.0015% of B by mass%.

3. A slab of steel having the composition described in the above 1 or 2 is rolled at a finishing temperature of 870 to 950 캜 at a finishing temperature of 450 to 630 캜 and then subjected to pickling at a rolling reduction of 80% After the cold rolling, annealing is carried out at a temperature of 600 ° C to a temperature of 730 to 850 ° C at a rate of v ₁ satisfying the following formula (2) And then cooled to 600 占 폚 at a speed v of ₂占 폚 / s or higher. The method for producing a cold-rolled steel sheet according to claim 1,

_{v 1 (℃ / s):} ([% Al] / [% N]) / 10 ~ ([% Al] / [% N]) ... (2)

Note that [% M] represents the content of element M (% by mass).

According to the present invention, the aggregate structure and the grain size of the steel sheet are optimized by controlling the heating rate depending on the mass ratio of Al and N in the annealing after the cold rolling, in which Nb or the like is added to the ultra low carbon steel, , The uniform elongation and the r-value can be effectively controlled. As a result, the moldability and shape durability can be effectively improved.

1 is a diagram showing the relationship between the area ratio of the ferrite particles within 15 DEG from {211} and the r value in the rolling direction, the rolling direction at 45 DEG, and the direction perpendicular to the rolling direction.
2 is a graph showing the relationship between the average grain size of ferrite and YP.
Fig. 3 is a graph showing the relationship between the average grain size of the ferrite and the uniform elongation. Fig.
4 is a graph showing the relationship between the value of the heating rate v ₁ / (Al / N) and the area ratio of the ferrite particles within the range of 15 ° from the {211} plane.
5 is a cross-sectional view of a cylindrical extension test press.
6 is a cross-sectional view of the steel sheet after the press.

Hereinafter, the reason why the steel sheet component is limited to the above range in the present invention will be described in detail.

In the following description, the percentages denoting the components in the steel are expressed as mass% unless otherwise specified.

C: 0.0010 to 0.0030%

By presenting C as a solid solution after hot rolling, the introduction of shear deformation into the particles during cold rolling can be promoted, and an increase in r value can be suppressed. Further, at the time of maintaining the crack in the annealing process after the cold rolling, the grain growth and the texture can be optimized by controlling the grain growth of the ferrite by forming Nb and the fine carbide. In order to obtain such an effect, it is necessary to set C to 0.0010% or more. On the other hand, when C is added in an amount exceeding 0.0030%, when C is present as a carbide, the YP increases and the uniform elongation decreases, and when it is present as solid C, an additional increase in YP due to age hardening Therefore, C should be 0.0030% or less. And preferably 0.0020% or less.

Si: not more than 0.05%

When Si is added in a large amount exceeding 0.05%, the steel sheet is hardened to deteriorate the workability, or the plating ability is lowered due to the formation of Si oxide during annealing. Further, when hot rolling is carried out, the temperature at which the structure is transformed from austenite to ferrite rises, making it difficult to finish rolling in the austenite region. Therefore, the Si content should be 0.05% or less.

Mn: 0.1 to 0.5%

Mn deteriorates S in harmful steel as MnS, and therefore, it is necessary to add Mn by 0.1% or more. On the other hand, Mn is required to be not more than 0.5% in view of the deterioration of workability due to hardening of the steel sheet and the recrystallization of ferrite particles at the time of annealing. And preferably 0.3% or less.

P: not more than 0.05%

P is segregated at the grain boundaries of the crystal grains of the steel sheet and is required to be 0.05% or less in that it deteriorates ductility and toughness. And preferably 0.03% or less.

S: not more than 0.02%

S significantly lowers the ductility in hot working, thereby causing hot cracking and significantly deteriorating the surface properties. Further, S not only contributes little to the improvement of the strength of the steel sheet but also deteriorates the ductility by forming coarse MnS as the impurity element. Therefore, it is preferable to reduce the amount of S as much as possible, but it is acceptable if it is 0.02% or less.

Al: 0.02 to 0.10%

Since Al acts as a precipitation site of NbC by forming nitride at the time of elevating the temperature in the annealing process after cold rolling, grain growth of ferrite can be controlled and the grain size and texture of the steel sheet can be optimized. By forming a nitride, it is possible to suppress aging hardening due to solid solution N. In order to obtain these effects, Al must be 0.02% or more.

On the other hand, a large amount of Al exceeding 0.10% promotes precipitation of nitride, and preferential precipitation of ferrite particles around {111} is suppressed. Further, at the time of hot rolling, the temperature at which the steel is transformed from austenite to ferrite rises, making it difficult to finish rolling in the austenite region. Therefore, the Al content needs to be 0.10% or less.

N: 0.0010 to 0.0050%

N can function as an NbC precipitation site by forming Al and nitride at the time of elevating the temperature in the annealing process after cold rolling to control the grain growth of ferrite and optimize grain size and texture. Therefore, N should be 0.0010% or more. On the other hand, if it is added in a large amount exceeding 0.0050%, the slab cracking may occur during hot rolling, and surface scratches may occur. In addition, when it exists as solid solution N after annealing, aging hardening is caused. Therefore, N should be 0.0050% or less.

Nb: 0.010 to 0.035%

Nb can control the grain growth of ferrite by optimizing the grain size and aggregate structure by forming carbide at the time of cracking in the annealing process after cold rolling. Further, in the hot rolling, the ferrite transformation from the non-recrystallized austenite is promoted in the cooling after the finish rolling by being present as the solid solution Nb and suppressing the recrystallization of the austenite, have. In order to obtain such an effect, it is necessary to add 0.010% or more of Nb.

On the other hand, the addition of a large amount of Nb exceeding 0.035% leads to an increase in Nb carbonitride or solid solution Nb, which leads to deterioration of ductility due to hardening of the steel sheet and suppression of recrystallization of ferrite during annealing , The annealing temperature becomes high and the texture can not be controlled. Therefore, Nb should be 0.035% or less. A particularly preferable amount of Nb is in the range of 0.012 to 0.030%.

The basic components of the present invention have been described above. In the present invention, B may be further contained in an amount of 0.0003 to 0.0015% for the purpose of lowering the r value and improving the shape fixability of the steel sheet.

B: 0.0003 to 0.0015%

B is present as solid solution B in hot rolling and suppresses recrystallization of austenite thereby promoting ferrite transformation from non-recrystallized austenite during cooling after finish rolling, . In order to obtain such an effect, B should be added in an amount of 0.0003% or more.

On the other hand, when B is present in a large amount exceeding 0.0015%, it is necessary to raise the annealing temperature in order to suppress the recrystallization of ferrite at the time of annealing after cold rolling, and the aggregate structure of the steel sheet can not be controlled. Therefore, B should be 0.0015% or less.

[% Al] / [% N] ≥ 10

Further, if the content of Al [% Al] is small relative to the content of N [% N], the precipitation of AlN is suppressed at the time of the temperature increase in the annealing process after the cold rolling. Therefore, in the present invention, the Al content [% Al] with respect to the N content [% N] needs to be 10 times or more.

Here, [% M] represents the content (mass%) of the element M, and [% M] represents the content (mass%) of the element M as well.

The remainder of the cold-rolled steel sheet of the present invention other than the above-mentioned components is composed of Fe and inevitable impurities. Here, the inevitable impurities are elements other than the above components, and means trace elements contained so far as they do not impair the action and effect of the present invention.

Next, the structure of the steel sheet according to the present invention will be described.

Average particle size of ferrite: 8 to 20 탆

The steel structure according to the present invention can make low YP and high uniform elongation ratio coexist mainly by using a ferrite phase having an average grain size of 8 占 퐉 or more.

On the other hand, if the grain size of the ferrite exceeds 20 탆, not only the surface shape such as surface roughness becomes visible at the time of press working, but also the control of the texture becomes difficult and the r value becomes high. Therefore, the average particle diameter of the ferrite should be 20 m or less.

In the present invention, the structure other than the ferrite phase is a cementite phase or a bainite phase. However, the term "ferrite phase" as used herein means that the ferrite occupies 90% or more of the area ratio of the steel sheet structure will be. , Preferably 95% or more, and more preferably 100%.

Area ratio of the ferrite particles within 15 DEG from {211} at the plate surface: 50% or more

It is possible to make the r value smaller in the entire rolling direction, such as in the rolling direction and in the direction perpendicular to the rolling direction, by increasing the area ratio of the ferrite particles within 15 DEG from {211} When the area ratio is 50% or more, the r value can be 2.0 or less with respect to the entire surface of the printing plate. Therefore, in the present invention, the area ratio of the ferrite particles within 15 DEG from {211} is set to 50% or more. It is preferably at least 60%.

In the present invention, the ferrite particles within 15 DEG from {211} means ferrite particles within 15 DEG from {211} obtained by using an EBSD (Electron Backscatter Diffraction) apparatus for a steel sheet surface.

Hereinafter, each manufacturing process in the present invention will be described in detail.

As the solvent method, conventionally known solvent methods such as a conventional converter method and an electric furnace method can be applied. The molten steel, after casting into a slab, is left as it is, cooled or heated, and subjected to hot rolling to finish with a hot plate and then wound. Then, pickling, cold rolling and annealing are carried out.

Finishing temperature during hot rolling: 870 ~ 950 ℃

During the finish rolling during hot rolling, if the steel sheet structure changes from austenite phase to ferrite phase, the rolling load sharply drops and it becomes difficult to control the load of the rolling mill. In this case, there is a risk of breakage of the steel sheet in the passing plate.

In addition, if the finish rolling is carried out from the beginning to the ferrite phase, such a risk can be avoided. However, there is a problem that the rolling temperature is lowered and the structure of the hot plate becomes non-recrystallized ferrite and the load during cold rolling increases Occurs. Therefore, it is important to finish the finish rolling with the austenite phase, and it is necessary to finish at 870 캜 or higher.

On the other hand, when the finish temperature of the finish rolling exceeds 950 DEG C, recrystallization in the austenite region is promoted and the ferrite transformation from the non-recrystallized austenite is suppressed in the cooling after the finish rolling, Rise. Therefore, the finish rolling needs to be finished at 950 캜 or lower. The preferred temperature range is 880 to 920 占 폚. The cooling rate from the hot rolling to the winding of the steel sheet is not particularly limited, but a cooling rate higher than air cooling is preferable. However, if necessary, rapid cooling of 100 ° C / s or more may be performed.

Coiling temperature: 450 ° C to 630 ° C

If the coiling temperature after hot rolling is low, the steel sheet is hardened due to the formation of adherent ferrite, and the load during the subsequent cold rolling becomes high, making it difficult to perform the working. Therefore, the coiling temperature needs to be 450 DEG C or higher.

On the other hand, when the coiling temperature exceeds 630 DEG C, AlN or NbC precipitates at the time of cooling the hot-rolled coil, and the grain size of the ferrite and control of the aggregate structure can not be controlled by the precipitation control of the carbonitride in the annealing process after the cold rolling do. Further, when the precipitation of carbide is promoted in the hot rolling step and the solid solution C disappears, the effect of introducing shear strain into the crystal grain of the steel sheet by solid solution C can not be obtained at the time of cold rolling, and the r value rises. Therefore, the coiling temperature needs to be 630 캜 or less.

Reduction rate: 80%

If the reduction rate at the time of cold rolling is large, the texture of the steel sheet is easy to develop and the r value rises. Therefore, the reduction rate should be 80% or less. On the other hand, although the lower limit is not particularly limited, when the reduction rate is small, it is necessary to bring the plate thickness of the hot plate closer to the predetermined product thickness, so that productivity in hot rolling and pickling is lowered. Therefore, the reduction rate is preferably 50% or more.

[% Al] / [% N]) / 10 to ([% Al] / [% N]) at 600 ° C to the cracking temperature v ₁

When the heating rate v ₁ from 600 ° C to the cracking temperature is low in the temperature raising process after the cold rolling, the precipitation of AlN is promoted and AlN precipitates not only in the vicinity of {111} but also around {211} It is impossible to control ferrite recrystallization. The precipitation of such AlN is, of Al by mass relative to the mass of N ratio, i.e. [% Al] / the larger the value of [% N], since remarkable, v ₁ is [% Al] / [% N ] ([% Al] / [% N]) / 10 ° C / s or higher.

On the other hand, when the heating rate is high, recrystallization at the temperature of the crack progresses without causing precipitation of AlN during the temperature rise, so that the grain size and texture of the ferrite can not be controlled. The precipitate inhibition of such AlN is because by the smaller the value of [% Al] / [% N ] remarkably, v ₁ is the need to below ([% Al] / [% N]) / 10 ℃ / s have.

When the above range is expressed by a value of (v ₁ ) / ([% Al] / [% N]), the range is from 0.1 to 1.0, particularly preferably from (v ₁ ) / N]): 0.2 to 0.8. V1 is the average heating temperature up to the cracking temperature at 600 ° C.

Crack temperature: 730 ~ 850 ℃

At the cracking temperature after the heating, it is necessary to complete the recrystallization while precipitating NbC, and to control the grain size and aggregate structure of the ferrite. Therefore, the cracking temperature needs to be 730 캜 or higher.

On the other hand, if the crack temperature is higher than 850 DEG C, the amount of Nb or C is increased so that the precipitation of NbC is suppressed and the grain structure of the ferrite is progressed, so that the aggregate structure can not be controlled and C is not precipitated If it is present in a solid state, it causes age hardening. In this respect, the cracking temperature needs to be 850 DEG C or less. Preferably 830 DEG C or less.

Crack time: 30 to 200 s

If the cracking time after the heating is short, since the recrystallization is not completed, the cracking time must be 30 s or more in that the YP of the steel sheet becomes high and the uniform elongation decreases and the workability remarkably deteriorates. On the other hand, if the cracking time exceeds 200 s, the growth of the ferrite particles proceeds and the aggregate structure can not be controlled. Therefore, the cracking time at the time of heating needs to be 200 s or less. Preferably 150 s or less.

Cooling rate from cracking temperature to 600 ° C v ₂ : 3 ° C / s or more

When the steel sheet is cooled, especially when the cooling rate v ₂ to 600 ° C is small, the growth of the ferrite particles is promoted and the aggregate structure can not be controlled. Therefore, the cooling rate v ₂ from the cracking temperature to 600 ° C needs to be 3 ° C / s or more. On the other hand, although the upper limit is not specifically defined, an excessively rapid cooling rate v ₂ is preferably about 30 ° C / s or less in that the cooling is required at a cost, such as requiring a special cooling facility. Also, v ₂ is the average cooling temperature from the cracking temperature to 600 ° C.

Here, the cooling rate in a region lower than 600 占 폚 is not particularly limited. If necessary, plating may be performed by hot-dip galvanizing at about 480 ° C. Further, after plating, the plating may be alloyed by reheating at 500 DEG C or higher, or may be subjected to thermal history such as temperature maintenance during cooling.

If necessary, temper rolling may be performed at a reduction ratio of about 0.5 to 2%. In the case where plating is not carried out during annealing, electro-galvanizing or the like may be performed to improve corrosion resistance. Further, a coating film may be formed on a cold-rolled steel plate or a plated steel plate by chemical conversion treatment or the like.

Example

Hereinafter, an embodiment will be described. After the slab having the chemical composition shown in Table 1 was melted, the slab was heated at 1,200 占 폚 for one hour, and the hot rolling at the finishing temperature (FT) and the coiling temperature (CT) shown in the same table was carried out. After the pickling, cold rolling, heating, cracking and cooling treatment were carried out under the conditions shown in the same table. The plate thickness after cold rolling was 0.6 to 0.8 mm.

Here, the heating rate v ₁ is the average heating rate up to the cracking temperature at 600 ° C, and the cooling rate v ₂ is the average cooling rate from the cracking temperature up to 600 ° C. Further, the temperature was lowered to room temperature at a temperature of 600 ° C or lower and v ₂ . After the annealing, temper rolling with a reduction ratio of 1.0% was performed to examine the structure and mechanical properties. Table 1 summarizes the results of investigating the texture and mechanical properties of the obtained steel sheet.

The samples subjected to the heat treatment were subjected to temper rolling at a reduction ratio of 1% and then JIS No. 5 tensile test specimens were taken from each of the rolling direction (L direction), rolling direction (D direction) and rolling direction (C direction) , Tensile in the L direction, and r values in the L, D, and C directions. In addition, the cross section in the L direction (plate thickness cross section in the rolling direction) was observed with an optical microscope and crystal orientation was measured with EBSD.

(evaluation)

The average particle diameter of the ferrite was determined by the cutting method. That is, the average section length in the rolling direction and the plate thickness direction of each of the test specimens was obtained, and the average section length in the rolling direction was X and the average section length in the plate thickness direction was Y, 2 / (1 / X + 1 / Y) was determined as the average grain size of the ferrite of each specimen.

The area ratio of the ferrite was obtained from the tissue image by image processing.

The texture was measured using EBSD. First, the orientation of the steel in the entire plate thickness direction was measured, and the area ratio of the ferrite particles having {211} within 15 DEG from the steel sheet surface was obtained.

With respect to the tensile properties, the tensile test specimen of JIS No. 5 was cut out from the rolling direction and a tensile test (according to JIS Z 2241) was carried out at a tensile rate of 10 mm / min to measure the values of YP and uniform elongation.

The r value was measured at 15% of the JIS No. 5 tensile test specimen cut out from each direction in the rolling direction (L direction), the rolling direction (D direction) and the rolling direction (C direction).

1 shows the relationship between the area ratio of the ferrite particles on the surface of the ferrite particles within 15 DEG from {211} and the ratio of the r value in the rolling direction, the rolling direction of 45 DEG and the direction perpendicular to the rolling direction 2 and 3 show the relation between the average grain size of ferrite and the YP and the degree of uniform elongation for the steel sheets No. 1 to 3, 5, 6, 8, and 10 having the area ratio of 50% 4 shows the relationship between the area ratio on the surface of ferrite particles within 15 DEG from {211} and the ratio of v ₁ / ([% Al] / [% N ]) &Quot;, respectively.

It can be seen from Fig. 1 that, when the area ratio of the ferrite particles within 15 DEG from {211} to the steel of the specified steel No. 1 to 11 is 50% or more in the rolling direction, in the rolling direction, in the rolling direction, , The r value is 2.0 or less.

2 and 3, it can be seen that by making the average grain size of the ferrite 8 mu m or more, the uniformity of the YP is 230 MPa or less and the uniform drawing degree is 22% or more, have.

4, when the value of "v ₁ / ([% Al] / [% N])" is in the range of 0.1 to 1.0, the area ratio of the ferrite particles within 15 ° from {211} % Or more.

The shape freezing property at the time of elongation processing was evaluated by a cylinder extension test. 5 shows a cross section of the press.

Punch diameter: 30 mm, punch shoulder radius: 5 mm, die diameter: 45 mm, and die shoulder radius: 1 mm. A sample machined to 100 mmφ was used and an extension of 8 mm height was carried out with a wrinkle restraining force of 200 kN. Fig. 6 shows a section after pressing.

The evaluation of the shape freezing property was made by visually observing the twist after elongation, showing that there was no twist, &amp; cir &amp;, and slightly twisted &amp; cir &amp;

The results are shown in Table 1. In the steel of the present invention, it can be seen that a press is formed without defects in shape.

Industrial availability

INDUSTRIAL APPLICABILITY According to the present invention, it is possible to provide a cold-rolled steel sheet having excellent formability and shape crystallinity and a manufacturing method thereof, as compared with a conventional cold-rolled steel sheet.

Claims (4)

0.001 to 0.0030% of C, 0.05% or less of Si, 0.1 to 0.5% of Mn, 0.05% or less of P, 0.02% or less of S, 0.02 to 0.10% 0.010 to 0.035% of Nb, and the Al content and the N content satisfy the relation of the following expression (1) and the balance of Fe and inevitable impurities, wherein the cold-rolled steel sheet has an average grain size : A structure having a main body of ferrite particles of 8 to 20 占 퐉 and an area ratio of ferrite particles within 15 占 from {211} is 50% or more of the structure thereof; Excellent cold-rolled steel sheet.
[% A1] / [% N] ≥ 10 ... (One)
[% M] represents the content of element M (% by mass). The method according to claim 1,
Wherein the cold-rolled steel sheet further contains 0.0003 to 0.0015% of B by mass%. 0.001 to 0.0030% of C, 0.05% or less of Si, 0.1 to 0.5% of Mn, 0.05% or less of P, 0.02% or less of S, 0.02 to 0.10% A steel slab containing 0.010 to 0.035% of Nb and having an Al content and an N content satisfying the relation of the following formula (1) and the balance being Fe and inevitable impurities, at a finishing temperature of 870 to 950 And then cold rolled at a reduction ratio of 80% or lower after the pickling, and then subjected to annealing at 600 캜 for cracks at 730 to 850 캜 Temperature is maintained at a speed v ₁ satisfying the relation of the following formula (2), maintained for 30 to 200 s in the crack temperature range, and then cooled to 600 ° C at a rate v ₂ of 3 ° C / s or higher By mass or more, and is superior in shape and dimensional stability.
[% A1] / [% N] ≥ 10 ... (One)
_{v 1 (℃ / s):} ([% Al] / [% N]) / 10 ~ ([% Al] / [% N]) ... (2)
[% M] represents the content of element M (% by mass). The method of claim 3,
Characterized in that the steel slab further contains, by mass%, B: 0.0003 to 0.0015%. &Lt; / RTI &gt;

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