JP2669243B2 - Manufacturing method of high r-value cold rolled steel sheet with small in-plane anisotropy - Google Patents

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 a small in-plane anisotropy and a high rank-order value (r value) and an excellent deep drawability.

[0002]

2. Description of the Related Art Cold rolled steel sheets are generally made of hot rolled steel sheets obtained by hot rolling, and are manufactured by cold rolling and recrystallization annealing. It is widely used in fields where workability is strongly required, such as exterior materials.

By the way, the workability of this cold-rolled steel sheet largely depends on the characteristics of the hot-rolled steel sheet as a raw material.
In recent years, studies on the manufacturing conditions in the hot rolling stage have been actively conducted in order to obtain a cold-rolled steel sheet having excellent deep drawability.

For example, Japanese Unexamined Patent Publication (Kokai) No. 58-133325 discloses that ultra-low carbon steel having a C content of 0.0045% or less (hereinafter, “%” representing a component ratio is referred to as “weight%”) is used as a material, and hot-rolled. A method for producing a cold-rolled steel sheet excellent in deep drawability by controlling the conditions is disclosed.

However, in this proposal, the precipitates and microstructures which influence the workability of the steel sheet have not been examined, and for that reason, the characteristic values of the cold rolled steel sheet obtained according to the method are not intended. Was hard to say. That is, JP-A-5
The above method proposed in Japanese Patent Application Laid-Open No. 8-133325 discloses a method of depositing carbonitrides and sulfides from heating to finish rolling and austenite (hereinafter abbreviated as “γ”).
It cannot be said that it was completed based on an examination focusing on the transformation behavior from ferritic to ferrite (hereinafter abbreviated as “α”) and the change in processing α between finish rolling and winding. It was thought to be the cause that did not lead to satisfactory results.

[0006] On the other hand, in order to obtain a “cold rolled steel sheet with good workability” having a small in-plane anisotropy and a high r-value, {111} texture develops in α after recrystallization treatment. It is desirable to make it. Where the {11
Since the 1｝ texture is generated near the α grain boundary,
In order to smoothly and stably develop the 1 組織 texture, it was necessary to reduce the α grain size caused by transformation during hot rolling and increase the α grain boundary area.

Therefore, as an attempt to manufacture a "hot-rolled steel sheet having a small α grain size" which leads to the realization of a high workability cold-rolled steel sheet, the steel is finish-rolled in the austenite region and then rapidly cooled, whereby γ → The results of the test to refine the α-grains after α-transformation have also been reported {“CAMP-ISI
J "Vol. 3 (1990), pp. 785-786}.

Certainly, according to the above method, a relatively fine α
Although a hot-rolled steel sheet having a grain structure can be obtained, there is a limit to the refinement of α grains, and for example, α grain size is 10 μm.
It was difficult to obtain a uniform structure that was finer than m. Therefore, for cold-rolled steel sheet made of this material, the in-plane anisotropy (r for each direction of 0 °, 45 °, 90 °)
Of the values r ₀ , r _{45, and} r ₉₀ , which is the difference between the maximum value r _max and the minimum value r _min ), until a sufficiently high and uniform r value is stably provided. Was not here.

This is considered to be due to the fact that the particle size of α grains is ultimately greatly affected by the size of γ grains before transformation. It has come to be said that the miniaturization of γ grains is essential.

At present, as "means for refining gamma grains" which have been studied and built for many years, a) controlled rolling, b) large rolling reduction (for example, JP-A-62-253733, JP-A-63-145720) (Refer to Japanese Patent Publication No.) and the like are known. However, the following problems were pointed out in each of these technologies.

That is, in the case of the controlled rolling technique, the γ grains produced by the hot working called "controlled rolling" cannot actually be further refined when it becomes fine to a certain extent. However, it is still difficult to obtain "a uniform microstructure in which the grain size of α transformed from the γ grains is about 10 μm".

On the other hand, the microstructure refinement technique by the large rolling reduction is a work hardening method in which a large reduction with a rolling reduction of 30% or more per pass is applied in a γ non-recrystallization temperature range to cause γ grains to “work hardening including a deformation zone in the grains”. γ ”, and then the γ → α transformation is performed to reduce the size of the structure. In this method,“ γ before the γ → α transformation ”is used.
The “grain” is simply elongated by the large rolling reduction and is not an isotropic fine grain, so there is a limit to the refinement of the γ structure as a prerequisite for realizing a fine α structure. It has not been possible to achieve a uniform fine structure having a particle size of, for example, less than 5 μm.

As described above, even if the α grain size generated by the transformation during hot rolling is reduced in order to develop the {111} texture necessary for improving the deep drawability of the cold rolled steel sheet, there is a limit in the prior art. Therefore, it is considered that this is a major obstacle in producing a high r-value cold-rolled steel sheet having a small in-plane anisotropy.

In view of the above, an object of the present invention is to stably produce an “ultrafine uniform structure” at the hot rolling stage, which was difficult to realize by the conventional method. It is an object of the present invention to establish means capable of stably producing a high r-value cold-rolled steel sheet exhibiting excellent workability and having a small in-plane anisotropy on an industrial scale.

[0015]

Means for Solving the Problems The inventors of the present invention have obtained the following findings as a result of earnest studies from various viewpoints in order to achieve the above object.

A) C content of 0.05% or less and N content of 0.0
1% or less low carbon aluminum killed steel, or Ti, N
A continuous cast slab or ingot of low carbon aluminum killed steel to which at least one of b, Zr, and V is added (hereinafter referred to as "hot slab") is subjected to primary rolling including large rolling. After that, when the “precipitation treatment” of maintaining the temperature in a predetermined temperature range is performed, carbonitrides and sulfides precipitate quickly and smoothly due to the precipitation sites introduced by the primary rolling, and a base material suitable for deep drawability is obtained. can get,

B) After the precipitation treatment, the structure is cooled as it is, and a structure containing α is exposed and placed in advance. Subsequently, this structure is rolled at a predetermined reduction ratio, and then the temperature is rapidly raised to set α to γ. When the reverse transformation is performed, the γ structure that appears becomes an ultrafine uniform structure that cannot be obtained by conventional rolling.

C) Therefore, when this ultrafine uniform γ structure is cooled as it is or rolled to such an extent that recrystallization does not occur again, and then cooled, the transformation-generated α becomes the ultrafine γ structure as a starting structure. Since it is extremely fine, it was extremely difficult to realize "α particle diameter 10μ.
An isotropic uniform microstructure well below m can be obtained.

D) Then, when the steel sheet having the fine α-grain structure is cold-rolled and then recrystallized, {111} texture is sufficiently developed, and the cold-rolled steel sheet has a small in-plane anisotropy and a high r value. A steel sheet can be obtained stably.

The present invention has been completed based on the above findings and the like. "C: 0.05% or less, Si: 0.3% or less, Mn: 0.01
~ 0.4%, S: 0.02% or less, sol.Al: 0.01 ~ 0.08%, N: 0.
01% or less, or B: 0.0001 to 0.0050%, or in addition to one or more of Ti, Nb, Zr, and V: 0.015 to 0.350% in total [C equivalent] − [Ti equivalent / 4] ≦ 0.0020 The primary rolling, in which at least a) the final pass rolling is performed at a rolling reduction of 30% or more in a temperature range of 1200 to 900 ° C., with the balance including Fe and unavoidable impurities. B) After the primary rolling, 1 to 1300 in the temperature range of 1200 ° C to 900 ° C.
Holds for 60 minutes to perform precipitation treatment. C) Rolls with a total reduction of 30% or more in a temperature range below Ar ₃ points. D) 5 ° C. to a temperature range from Ac ₃ points to [Ac ₃ points + 100 ° C]. / s
The temperature is raised at the above heating rate to cause a reverse transformation of ferrite to austenite, e) cooling from the austenite phase temperature range and cold rolling with a total reduction of 50 to 97%, f) Perform recrystallization annealing at a temperature of 550 to 900 ° C. By sequentially processing and heat-treating in steps including the following steps, a high r-value cold-rolled steel sheet with small in-plane anisotropy can be stably manufactured. It has a great feature in the point that it was made.

[0021]

The reason why the component composition of the raw material steel and the working / heat treatment conditions are limited as described above in the present invention will be specifically explained together with its function and effect.

<Ingredient Composition of Raw Steel> C C is an element that adversely affects the deep drawability of the steel sheet,
It is desirable that the content is small. Especially, when the C content exceeds 0.05%, the deep drawability deteriorates remarkably, so the content is limited to 0.05% or less.

Si Since Si is also an element which adversely affects the deep drawability of the steel sheet, it is preferable that Si is as small as possible. In particular, when the Si content exceeds 0.3%, not only the deep drawability deteriorates remarkably, but also the scale properties deteriorate and the product quality is impaired, so the content is limited to 0.3% or less. did.

Mn Mn has an effect of improving the toughness of the steel sheet, but if its content is less than 0.01%, the effect of the above-mentioned effect is not sufficient and hot brittleness occurs, while 0.4% Since the deep drawability deteriorates remarkably if it is contained in excess, Mn content was set to 0.01 to 0.4%.

The lower S S is, the more the deep drawability of the steel sheet is improved.
Since the adverse effect is not so remarkable when it is reduced to about 02%, the S content is set to 0.02% or less.

Sol.Al Al is added for deoxidation and for improving the yield of carbonitride forming elements. When the content is less than 0.01% by sol.Al, the above-mentioned effects are sufficiently obtained. On the other hand, if the content exceeds 0.08%, the effect is saturated and it becomes uneconomical.
The content was determined to be 0.01 to 0.08% in sol.Al amount.

The lower the N 2 N content is, the smaller the amount of carbonitride forming element added is, which is preferable. In particular, its content is 0.01
%, The addition of carbonitride-forming elements inevitably reduces the r-value of the steel sheet, so the N content is 0.01%.
It is determined as follows.

Each of Ti, Nb, Zr and V has an effect of reducing solid solution C and N by forming carbonitrides and appropriately reducing the size of crystal grains by the precipitates. As necessary, they are added alone or in combination. However, the total content of these
If the content is less than 0.015%, the desired effect cannot be obtained, while if the total content is more than 0.350%, the strength becomes too high to be suitable as a steel sheet for processing, and it is economically disadvantageous. Become. Therefore, the content of these components was determined to be 0.015 to 0.350% in total.

In addition, the formula "[C equivalent]-[Ti equivalent / 4] ≤
"0.0020" indicates the relationship for making the solid solution C and N 0.0020% or less and precipitating the remaining C and N as carbonitrides, and "[C equivalent]-[Ti equivalent / 4]" The value of
If it exceeds 0.0020, solute C and N increase, so {111}
The recrystallization texture does not develop and the deep drawability of the steel sheet deteriorates.

[0030] B B is added as necessary so has the effect of preventing to become "vertical cracks" problem drawing part, the desired effect due to the content of the working is less than 0.0001% On the other hand, if the content exceeds 0.0050%, the effect is saturated and it becomes economically disadvantageous. Therefore, the B content is set to 0.0001 to 0.0050%.

<Working / Heat Treatment Conditions> The raw material billet having the above-described composition used for hot rolling may be one produced by continuous casting, or one produced by decomposition rolling from an ingot. Is also good. In addition, the raw steel slab may be subjected to hot rolling after heating a cold steel slab after continuous casting or slab rolling to a predetermined temperature, or “continuous casting or slab rolling” referred to as “direct feed rolling”. The method in which the steel slab sent from the line at high temperature is used as it is, or is subjected to hot rolling with some auxiliary heating ”may be employed.

(A) Final pass rolling is performed at 1200 to 900 ° C.
Primary in a temperature range of 30% or more
The purpose of the rolled primary rolling, introducing the precipitation sites for depositing the rapidly carbonitride in the following precipitation treatment and sulfides, and for refining the α particle in the course of cooling up to the secondary rolling In addition, the γ grains are refined by recrystallization, and further processing strain is introduced into the γ grains to increase the precipitation sites of α grains. For this reason, it is necessary to perform the primary rolling in the final pass in a temperature range of 1200 to 900 ° C. and a large rolling reduction of 30% or more (preferably, a rolling reduction of 45% or more is good). If not, the above goal cannot be achieved.

That is, if the rolling reduction of the final pass is less than 30%, the processing strain obtained is small and no precipitation sites for precipitates are introduced, and the process of maintaining the temperature in the temperature range of 1200 to 900 ° C. in the next step is performed. Even if it is carried out, it is difficult to form an effective precipitate. In addition, when the rolling reduction of the final pass is small, not only does γ not recrystallize and refine, but also the processing strain is small, so that α grains do not refine in the subsequent cooling process. The final pass temperature is 12
If the temperature is higher than 00 ° C., not only the precipitation site of carbonitride and sulfide is not introduced without accumulating the processing strain, but also the fine grain effect of α by rolling cannot be obtained, while the temperature is lower than 900 ° C. In addition, it becomes difficult to secure the temperature for the precipitation treatment in the next step.

Incidentally, this primary rolling is carried out for one or more passes,
It is advisable to make the final pass of the above conditions. Of course, the rolling before the final pass does not need to be particularly limited, and ordinary rolling may be used.

(B) Precipitation Treatment Performed After Primary Rolling The purpose of the precipitation treatment is to precipitate C, N and S in steel as carbonitrides or sulfides to improve deep drawability.
For this purpose, it is preferable that the rough rolled material after the primary rolling is kept in a temperature range of 1200 to 900 ° C. for 1 to 60 minutes immediately after the rolling without cooling to a room temperature. This is because there are precipitation noises in the γ region in this temperature range. Therefore, if kept at a temperature higher than 1200 ° C, not only the precipitation does not proceed rapidly due to the large solubility, but also the γ grains grow and become coarse, and as a result, the α grains before the next secondary rolling become coarse. Therefore, the deep drawability of the final product is not improved. On the other hand, when the temperature is maintained in the γ range lower than 900 ° C., the precipitation rate is remarkably slow and similarly the precipitation does not proceed rapidly, so that the effect of improving the deep drawability cannot be secured. If the holding time at this time is less than 1 minute, the amount of precipitation will not be sufficient. On the other hand, if the holding time is more than 60 minutes, the precipitation will be saturated and the production cost will increase.

The means for maintaining the steel material after the primary rolling in the above-mentioned temperature range in the rolling line is not particularly limited. For example, it is effective to use a recently developed "coil box". In addition, it does not matter at all that performing the quenching of the steel material after the primary rolling to a predetermined precipitation treatment temperature or to cool the steel material to the starting temperature of the secondary rolling after the precipitation treatment. Rather, the rapid cooling shortens the production time, prevents the α grains from becoming coarse, and improves the deep drawability.

(C) Cooling to a temperature range below the Ar ₃ point
After rolling (secondary rolling) precipitation treatment, the steel material is once cooled to a temperature range lower than the Ar ₃ point and then rolled. This is because the method of the present invention applies the plastic working to the structure containing α and then forms the α phase. The main requirement is to “reversely transform into a γ phase”, and for that purpose, it is necessary to once generate an α phase. Although the cooling temperature at this time is not particularly restricted if falls below _three points Ar, realistic as possible high temperature region near Ar less than ₃ points From workability surface, i.e. "Ar ₃ point - [Ar ₃ point - 100 ° C.] ”. However, α
In reverse transforming the α phase into a γ phase after plastic working on the structure containing, the larger the volume fraction of the α phase at the time of plastic working, the finer the γ grains after reverse transformation, the more the product From the viewpoint of performance, it is desirable that the cooling temperature is set at Ar ₁ point or lower in order to increase the volume ratio of α phase.

The reason why the total rolling reduction in the rolling process performed in the temperature range lower than the Ar ₃ point is set to 30% or more is that the fine γ grains by the reverse transformation are first obtained when the rolling reduction is 30% or more. This is because stable formation of can be achieved.

That is, when rolling is performed in a temperature range lower than the Ar ₃ point, α is work hardened by this rolling, and the number of reverse transformation nuclei to γ increases. If the number of the reverse transformation nuclei is extremely large, extremely fine γ grains are generated by the subsequent rapid temperature rise to the γ region. However, the number of the inverse transformation nuclei is 3
Only when it becomes 0% or more, a remarkable rapid increase tendency is shown, and the desired ultrafine γ grains can be stably produced.
Although the total reduction ratio in the temperature range below the Ar ₃ point is set to 30% or more, it is preferably 50% or more if possible.

(D) Temperature from Ac ₃ point to [Ac ₃ point + 100 ° C]
The temperature rise to the region Ac is raised to ₃ points or more because the characteristic action and effect of the method according to the present invention that “very fine γ grains are generated by reverse transformation from work-hardened α” is sufficient. It is to show it. In this case, the upper limit of the temperature rise is [Ac ₃ points +10
0 ° C.] is that if the temperature is raised beyond this temperature, γ grains grow and finally a desired uniform ultra-fine structure steel sheet cannot be obtained, and thus desired workability and strength can be secured. Due to disappearance.

If the heating rate at the time of raising the temperature to the temperature range of from Ac ₃ points to [Ac ₃ points + 100 ° C.] is less than 5 ° C./s, distortion due to processing which causes the introduction of reverse transformation nuclei may occur. It is released prior to the α → γ reverse transformation, and the desired fine γ grain structure cannot be realized. For this reason, the above heating rate is set to 5 ° C / s
It was decided above. As the means for raising the temperature, any method such as "utilization of processing heat" or "active heating from the outside (electric heating)" or a combination of both may be adopted.

(E) Cold performed by cooling from the γ phase temperature range
Steel that has undergone reverse transformation by rapidly heating to the rolling γ-phase temperature range is made into an isotropic and uniform ultrafine α-structure by subsequent cooling,
Further, although cold rolling is performed, it is preferable to perform rolling with a total reduction of 50% or less in the γ phase temperature region prior to the cooling. This is because, when rolling is performed in the γ phase temperature range, γ grains generated by the reverse transformation are further refined, and the α-containing structure generated by the subsequent cooling is further refined, so that the characteristics are further improved. In this case, it is desirable that the rolling in the γ phase temperature range is stopped at a total reduction of 50% or less (preferably 30% or less) as described above.
This is because γ is recrystallized and grain-grown when the total rolling reduction exceeds 50%, and α generated by subsequent cooling is not sufficiently miniaturized.

Cooling from the γ-phase temperature range is performed at Ar ₃ points or more.
It is desirable to cool the temperature range of [Ar ₃ points -150 ° C] at a cooling rate of 5 ° C / s or more. As a result, a large number of α nuclei can be generated from γ that has been miniaturized by processing in the γ region and has accumulated processing strain, and fine α grains can be obtained. As described above, the area of the α grain boundary can be increased by refining the α grain before the cold rolling in the next step, and the {111} recrystallization aggregate generated from the α grain boundary is preferable for improving the r value. The tissue can be fully developed. By cooling under the above conditions, fine α grains having an ASTM particle number of 11 or more can be obtained.

By the way, "rolling explained in the item (a)",
Although there is no limitation on where the “treatment and rolling described in the above (b) to (d)” and “the above-mentioned rolling performed in the γ-phase temperature range during reverse transformation” are performed in the hot rolling line,
The rolling described in the section (a) is carried out in a rough rolling step, and the subsequent processing and rolling are preferably performed in a finishing rolling step from the viewpoint of equipment. The winding temperature at this time does not matter.

After cooling from the γ-phase temperature range, cold rolling is performed with a total draft of 50 to 97% to obtain a final sheet thickness. To improve the r-value in the next recrystallization annealing step, and to develop {111} texture which is preferable for minimizing in-plane anisotropy. " When the total rolling reduction in this cold rolling is less than 50% or more than 97%, the {111} texture does not develop even if recrystallization annealing is performed.

(F) Recrystallization annealing Recrystallization annealing is an indispensable step in controlling the texture of α to produce a cold-rolled steel sheet excellent in deep drawability. For that purpose, annealing is performed in the temperature range of 550 to 900 ° C., and α
It is desirable to recrystallize. At a temperature lower than 550 ° C., recrystallization does not sufficiently occur even in batch annealing which is a long-time annealing, and at a temperature higher than 900 ° C., γ-formation remarkably progresses, and it becomes difficult to control a desired α recrystallization texture. There is no particular limitation on the method for carrying out the recrystallization annealing, and there is no problem with using any of a normal continuous annealing process and a batch annealing process. During the skin pass reduction performed after the recrystallization annealing, a reduction of less than 10% may be supplementarily added.

Next, the present invention will be described more specifically by way of examples.

[Examples] Aluminum-killed steel having the chemical composition shown in Table 1 was melted in a 50 kg vacuum melting furnace and then cast into a slab having a thickness of 60 mm. Subsequently, these slabs were hot-rolled under the conditions shown in Table 2, cooled, and wound. Then, the hot-rolled steel sheet after winding is pickled, cold-rolled, and then “800
C. x 2 min continuous annealing (treatment b) "or" 750 C x 5 min.
hr batch annealing (processing b) ".

Test specimens were obtained from the thus obtained cold-rolled steel sheets, and "yield strength", "elongation" and "r value" were examined. Table 3 shows the results.

[0049]

[Table 1]

[0050]

[Table 2]

[0051]

[Table 3]

As is clear from the results shown in Table 3, the hot-rolled steel sheet manufactured according to the conditions specified in the present invention has excellent r-value and elongation, and has extremely small in-plane anisotropy. I understand. Further, it is clear that all the steel sheets according to the present invention have a low yield point and have very excellent workability.

On the other hand, when the manufacturing conditions do not comply with the requirements of the present invention, it can be seen that the α structure is not sufficiently refined, resulting in inferior properties of the obtained steel sheet.

[0054]

[Summary of Effects] As described above, according to the present invention, it is possible to stably manufacture a cold-rolled steel sheet having a small in-plane anisotropy and a high r value. Be done.

[Brief description of the drawings]

FIG. 1 is a schematic view showing a manufacturing process of the present invention.

Claims (4)

(57) [Claims] 1. A weight ratio of C: 0.05% or less, Si: 0.3% or less, Mn: 0.01 to
0.4%, S: 0.02% or less, sol.Al: 0.01 to 0.08%, N: 0.
A hot steel slab containing at most 01% and the balance consisting of Fe and unavoidable impurities is sequentially worked and heat-treated at least in a process including the following processes a) to f). Manufacturing method of small high r value cold rolled steel sheet. a) Perform primary rolling in which rolling in the final pass is performed in a temperature range of 1200 to 900 ° C at a rolling reduction of 30% or more. b) After primary rolling, a primary rolling is performed in a temperature range of 1200 ° C to 900 ° C.
Holds for 60 minutes to perform precipitation treatment. C) Rolls with a total reduction of 30% or more in a temperature range below Ar ₃ points. D) 5 ° C. to a temperature range from Ac ₃ points to [Ac ₃ points + 100 ° C]. / s
The temperature is raised at the above heating rate to cause a reverse transformation of ferrite to austenite, e) cooling from the austenite phase temperature range and cold rolling with a total reduction of 50 to 97%, f) Perform recrystallization annealing at a temperature of 550 to 900 ° C. 2. A weight ratio of C: 0.05% or less, Si: 0.3% or less, Mn: 0.
01 to 0.4%, S: 0.02% or less, sol.Al: 0.01 to 0.08%, N: 0.
01% or less, B: 0.0001 to 0.0050%, the balance being characterized in that a hot steel slab consisting of Fe and unavoidable impurities is processed and heat-treated sequentially in a process including at least the following processes a) to f). A method for producing a high r-value cold-rolled steel sheet having a small in-plane anisotropy. a) Perform primary rolling in which rolling in the final pass is performed in a temperature range of 1200 to 900 ° C at a rolling reduction of 30% or more. b) After primary rolling, a primary rolling is performed in a temperature range of 1200 ° C to 900 ° C.
Holds for 60 minutes to perform precipitation treatment. C) Rolls with a total reduction of 30% or more in a temperature range below Ar ₃ points. D) 5 ° C. to a temperature range from Ac ₃ points to [Ac ₃ points + 100 ° C]. / s
The temperature is raised at the above heating rate to cause a reverse transformation of ferrite to austenite, e) cooling from the austenite phase temperature range and cold rolling with a total reduction of 50 to 97%, f) Perform recrystallization annealing at a temperature of 550 to 900 ° C. 3. By weight ratio, C: 0.05% or less, Si: 0.3% or less, Mn: 0.01-
0.4%, S: 0.02% or less, sol.Al: 0.01 to 0.08%, N: 0.
Not more than 01%, and at least one of Ti, Nb, Zr and V: 0.015 to 0.350% in total, also in the formula [C equivalent]-[Ti equivalent / 4] ≦ 0.0020 In the in-plane anisotropy, characterized by sequentially processing and heat-treating a hot steel slab containing Fe and unavoidable impurities at least in the steps including the following processes a) to f). Manufacturing method of small high r value cold rolled steel sheet. a) Perform primary rolling in which the rolling of the final pass is performed at a rolling reduction of 30% or more in a temperature range of 1200 to 900 ° C. b) After the primary rolling, the primary rolling is performed in a temperature range of 1200 to 900 ° C.
Holds for 60 minutes to perform precipitation treatment. C) Rolls with a total reduction of 30% or more in a temperature range below Ar ₃ points. D) 5 ° C. to a temperature range from Ac ₃ points to [Ac ₃ points + 100 ° C]. / s
The temperature is raised at the above heating rate to cause a reverse transformation of ferrite to austenite, e) cooling from the austenite phase temperature range and cold rolling with a total reduction of 50 to 97%, f) Perform recrystallization annealing at a temperature of 550 to 900 ° C. 4. A weight ratio of C: 0.05% or less, Si: 0.3% or less, Mn: 0.
01 to 0.4%, S: 0.02% or less, sol.Al: 0.01 to 0.08%, N: 0.
01% or less, B: 0.0001 to 0.0050%, and one or more of Ti, Nb, Zr and V: 0.015 to 0.350% in total is also represented by the formula [C equivalent]-[Ti equivalent / 4] ≤0.0020. In the in-plane anisotropy, characterized by sequentially processing and heat-treating a hot steel slab containing Fe and unavoidable impurities at least in the steps including the following processes a) to f). Manufacturing method of small high r value cold rolled steel sheet. a) Perform primary rolling in which the rolling of the final pass is performed at a rolling reduction of 30% or more in a temperature range of 1200 to 900 ° C. b) After the primary rolling, the primary rolling is performed in a temperature range of 1200 to 900 ° C.
Holds for 60 minutes to perform precipitation treatment. C) Rolls with a total reduction of 30% or more in a temperature range below Ar ₃ points. D) 5 ° C. to a temperature range from Ac ₃ points to [Ac ₃ points + 100 ° C]. / s
The temperature is raised at the above heating rate to cause reverse transformation from ferrite to austenite. E) Cooling is performed from the austenite phase temperature range, and cold rolling is performed at a total draft of 50 to 97%. f) Perform recrystallization annealing at a temperature of 550 to 900 ° C.