JP2669231B2 - 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, in order to obtain a cold rolled steel sheet having low in-plane anisotropy and high r-value and good workability, ferrite (hereinafter abbreviated as “α”) after recrystallization treatment has to be # 11.
It is considered desirable to develop 1｝ texture. Since the {111} texture is generated from the vicinity of the α grain boundary, in order to smoothly develop the {111} texture, the α grain size caused by transformation during hot rolling is reduced and the α grain boundary area is increased. It was necessary to do. That is, the workability of the cold-rolled steel sheet greatly depends on the structure of the hot-rolled steel sheet as the material.

Therefore, as an attempt to produce 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 used in the austenite (hereinafter abbreviated as "γ") region. There has also been reported the results of a test in which quenching was performed after finish rolling to thereby reduce α grains after γ → α transformation. “CAMP-ISIJ” Vol. 3 (1990), No. 785
786}.

[0005] 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 °)
(The difference between the maximum value r _max and the minimum value r _min ) among the values r ₀ , r _{45, and} r ₉₀ ), and a high and uniform r value is stably provided. There wasn't.

This is thought to be because the particle size of α grains is ultimately greatly affected by the size of γ grains before transformation, and recently, before further transformation of α grains, 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) No. 2, pp. 147-64). However, the following problems were pointed out in each of these technologies.

In other words, in the case of the controlled rolling technique, when the γ grains produced by the hot rolling called “controlled rolling” become fine to a certain extent, they cannot practically be further refined anymore. 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 structure refining technique by the large reduction rolling is performed by applying a large reduction with a reduction rate of 30% or more per one pass in the γ non-recrystallization temperature range to obtain the γ grains by "work hardening including a deformation zone in the grains. γ ”and then the γ → α transformation is caused to make the structure finer. In this method, γ before the γ → α transformation
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.

[0010] As described above, in order to develop the {111} texture necessary for improving the deep drawability of a cold-rolled steel sheet, the conventional technology has a limit in reducing the α grain size caused by transformation during hot rolling. Therefore, it is considered that this is a major obstacle in producing a high r-value cold-rolled steel sheet having a small inner surface anisotropy.

[0011] In view of the above, the object of the present invention is to stably produce an "ultra-fine uniform structure" at the hot-rolling stage, which has been difficult to realize by the conventional method. It was to establish means for stably manufacturing a high r-value cold-rolled steel sheet exhibiting excellent workability and having a small in-plane anisotropy on an industrial scale.

[0012]

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 is 0.05% or less (hereinafter,% which represents the component ratio)
Is a percentage by weight) and has a N content of 0.01% or less, or a continuous cast slab or ingot of a low carbon aluminum killed steel to which at least one of Ti, Nb, Zr, and V is added. (Hereinafter collectively referred to as "hot steel slab")
In the γ temperature range, the structure containing α is exposed in advance by rolling it at a predetermined reduction rate in the γ temperature range, and then cooled, and after the structure is rolled at a predetermined reduction rate, the temperature is rapidly raised. Α
When γ is inversely transformed into γ, the γ structure that appears becomes an ultrafine uniform structure that cannot be obtained by conventional rolling.

B) Therefore, if this ultrafine uniform γ structure is cooled as it is, or if it is rolled to such an extent that recrystallization does not occur and then cooled, the α that undergoes transformation takes the ultrafine γ structure as the 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.

C) When the steel sheet having the fine α grain structure is cold-rolled and then recrystallized, the {111} texture is sufficiently developed, and the in-plane anisotropy is small and the cold rolling has 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%
Below, Mn: 0.01 to 0.4% S: 0.02% or less, sol.Al: 0.01 to 0.08%, N: 0.
01% or less, or one or more of Ti, Nb, Zr and V: 0.015 to 0.350% in total, B: 0.0001 to 0.00
One or more of 50% may be [C equivalent]-[Ti equivalent / 4] ≤ 0.0020 A hot slab consisting of Fe and unavoidable impurities is cooled at least after a) rolling at a final pass reduction rate of 30% or more in a temperature range of ₃ or more points of Ar, and b). Rolling with a total reduction of 30% or more in a temperature range below the Ar ₃ point, c) 5 ° C / s from the Ac ₃ point to the [Ac ₃ point + 100 ° C] temperature range
The temperature is raised at the above heating rate to cause a reverse transformation from ferrite to austenite. D) Cooling is performed from the austenite phase temperature range, and cold rolling is performed with a total draft of 50 to 97%. e) By performing recrystallization annealing at a temperature of 550 to 900 ° C. and sequentially performing processing and heat treatment in a process including a process, 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.

[0016]

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 Si is also an element that adversely affects the deep drawability of the steel sheet, and therefore it is preferable that the content 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 action of improving the toughness of the steel sheet, but if the content thereof is less than 0.01%, the effect due to the above action is not sufficient and hot brittleness occurs, while 0.4% is added. If the content exceeds that, the deep drawability is significantly deteriorated, so the Mn content is 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 the purpose of deoxidizing and improving the yield of carbonitride forming elements, but if the content is less than 0.01% in sol.Al amount, the above-mentioned action and effect are sufficiently obtained. On the other hand, if the content exceeds 0.08%, the effect is saturated and uneconomical.
The content was determined to be 0.01 to 0.08% in sol.Al amount.

NN The lower the N content, the smaller the addition amount of the carbonitride-forming element, 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.

Ti, Nb, Zr and V All of these components have the action of forming carbonitrides to reduce the solid solution C and N, and the precipitates to appropriately refine the crystal grains. 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.

The formula "[C equivalent]-[Ti equivalent / 4] ≤
"0.0020" indicates a relationship for making solid solution C and N 0.0020% or less and precipitating the remaining C and N as carbonitrides. "[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.

[0025] 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 is economically disadvantageous. Therefore, the B content is set to 0.0001 to 0.0050%.

<Working / Heat Treatment Conditions> The raw steel slab having the above-mentioned composition to be subjected to hot rolling may be manufactured by continuous casting, or may be manufactured 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) Ar Final pressure reduction in the temperature range of ₃ points or more
The hot-rolled steel slab that has been subjected to continuous rolling or ingot casting at a rate of 30% or more is cooled to a temperature lower than the Ar ₃ point, so that a desired uniform microstructure can be obtained even if reverse transformation occurs thereafter. There is a fear of not. However, by subjecting the hot steel slab to rolling in which the final pass is rolled in a temperature range of Ar ₃ points or more and a rolling reduction of 30% or more, the γ grains are refined by recrystallization, and at the same time, processing strain is reduced to the γ grains. When introduced, the precipitation sites of α grains increase, and fine α grains can be realized in the next cooling process. The hot slab to be subjected to rolling may be used as it is without continuous cooling or ingot casting without cooling, or may be used after it is once inserted into a heating furnace and reheated.

If the rolling reduction in the final pass in the above rolling is less than 30%, γ will not be recrystallized and the processing strain will be small, so that α particles will not be refined in the next processing step. Therefore,
Although the rolling reduction of the final pass is limited to 30% or more, it is preferably 45% or more if possible. Further, when the rolling in the final pass is lower than the Ar ₃ point, the α phase is mixed, and the work strain is concentrated in the soft α phase and the work strain is not accumulated in the γ phase. Therefore, in the next cooling step. α grains generated by the γ → α transformation are not refined. This primary rolling is preferably performed in one or more passes, of which the final pass is performed under 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) Cooling to a temperature range below the Ar ₃ point
According to the method of the present invention, the rolled hot steel billet is cooled to a temperature range lower than the Ar ₃ point and then rolled. This is because the main requirement is "to", and for that purpose it is necessary to once generate the α phase. The cooling temperature at this time is Ar ₃
Although it is not particularly limited as long as the temperature falls below the point, from the viewpoint of practical operability, the temperature is preferably as high as possible in the vicinity of less than the Ar ₃ point, that is, in the range of “Ar ₃ point to [Ar ₃ point −100 ° C.]”. It can be said that it is preferable to do. However, when the α phase is reversely transformed into the γ phase after the plastic working of the structure containing α, the γ grains after the reverse transformation become finer as the volume ratio of the α phase during the plastic working increases. From the viewpoint of product performance,
In order to increase the volume ratio of the phase, it is desirable that the cooling temperature is set at Ar ₁ point or lower.

The total rolling reduction of the rolling process carried out in the temperature range below the Ar ₃ point is set to 30% or more because the fine γ grains due to reverse transformation are not produced until the rolling reduction becomes 30% or more. This is because a stable formation of the compound can be achieved.

That is, when rolling is performed in a temperature range below 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.

(C) 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 for raising the temperature from the Ac ₃ point to the [Ac ₃ point + 100 ° C.] is less than 5 ° C./s, strain due to working that causes the introduction of reverse transformation nuclei will 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.

(D) Cold work 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.

It should be noted that cooling from the γ-phase temperature range is performed from Ar ₃ points to
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)",
There is no restriction on where in the hot rolling line the "rolling / treatment described in the above (b) and (c)" and "the above rolling performed in the γ phase temperature range at the time of reverse transformation" are carried out.
It is advantageous in terms of equipment that the "rolling described in section (a)" is a rough rolling step and the subsequent rolling and treatment is a finish rolling step.

After cooling from the γ phase temperature range, cold rolling with a total reduction ratio of 50 to 97% is performed to obtain the final plate thickness. The purpose of this cold rolling is to "roll rolling texture. To improve the r value in the subsequent recrystallization annealing step, and to develop a {111} texture that is preferable for minimizing the 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.

(E) Recrystallization Annealing Recrystallization annealing is an essential step for controlling the texture of α and producing 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 pieces were taken from the cold-rolled steel sheet thus obtained, and "yield strength", "elongation" and "r value" were investigated. Table 3 shows the results.

[0041]

[Table 1]

[0042]

[Table 2]

[0043]

[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, a sufficient fine α structure is not developed, resulting in inferior properties of the obtained steel sheet.

[0046]

[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 (2)

(57) [Claims] (1) C: 0.05% or less by weight, 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 01% or less and the balance consisting of Fe and unavoidable impurities is sequentially processed and heat-treated in a step including at least the following treatments a) to e). Manufacturing method of small high r value cold rolled steel sheet. a) Rolling with a final pass draft of 30% or more in the temperature range of Ar ₃ points or more, and then cooling. b) Rolling with a total draft of 30% or more in a temperature range below the Ar ₃ points, c). 5 ℃ / s up to the temperature range from Ac ₃ points to [Ac ₃ points + 100 ° C]
The temperature is raised at the above heating rate to cause a reverse transformation from ferrite to austenite. D) Cool from the austenite phase temperature range and perform cold rolling with a total draft of 50 to 97%. e) Recrystallization annealing is performed at a temperature of 550 to 900 ° C. 2. By weight ratio, 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.
Not more than 01%, and at least one of Ti, Nb, Zr and V: 0.015 to 0.350 in total
%, B: 0.0001 to 0.0050% of the formula [C equivalent]-[Ti equivalent / 4] ≤ 0.0020 In the in-plane anisotropy, the hot steel slab containing Fe and unavoidable impurities is sequentially processed and heat-treated at least in steps including the following a) to e). Manufacturing method of small high r value cold rolled steel sheet. a) Rolling with a final pass draft of 30% or more in the temperature range of Ar ₃ points or more, and then cooling. b) Rolling with a total draft of 30% or more in a temperature range below the Ar ₃ points, c). 5 ° C / s up to the temperature range of Ar ₃ points to [Ar ₃ points + 100 ° C]
The temperature is raised at the above heating rate to cause a reverse transformation of ferrite to austenite, d) cooling from the austenite phase temperature range and cold rolling with a total reduction of 50 to 97%, e) Recrystallization annealing is performed at a temperature of 550 to 900 ° C.