JPH06184646A - Production of hot rolled steel plate reduced in inplane anisotropy and having high r-value - Google Patents

Abstract

PURPOSE:To stably produce a hot rolled steel plate excellent in workability, reduced in inplane anisotropy, and having high r-value on an industrial scale. CONSTITUTION:A hot steel slab which has a composition containing <=0.0-8% C, <=0.3% Si, 0.01-0.5% Mn, <=0.02% S, 0.01-0.08% solAl, and <=0.01% N or further containing one or more kinds among prescribed amounts of B, Ti, Nb, Zr, V, and Mo in the percentages satisfying an inequality [C equivalent]-[Ti equivalent/4]<=0.0020 is successively treated by means of a process including at least the following treatments: (a) cooling down to a temp. in the region lower than the A3 point; (b) precipitation treatment consisting of holding at a temp. lower than the A3 point for 1-60min; (c) temp. rise up to a temp. ion the region between the A3 point and [A3 point+200 deg.C] to cause inverse transformation from alpha to gamma; (d) cooling from the gamma-phase temp. region; (e) rolling at a temp. between [A3 point-100 deg.C] and 450 deg.C at 70-97% total draft; and (f) recrystallization treatment.

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 hot-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 In recent years, "cold rolled steel sheet" has been used in combination with the demand for cost reduction of steel materials, which has been made in various fields in recent years, and the improvement of hot rolled steel sheet manufacturing technology. Attempts have also been made to apply "hot rolled steel sheets" to other fields. Because, as is well known, cold-rolled steel sheet is manufactured from "hot-rolled sheet obtained by hot rolling" through a process of "pickling → cold rolling → annealing → temper rolling". As is apparent from the above, the disadvantage was that the manufacturing cost was significantly higher than that of the hot-rolled steel sheet.

One of the reasons that the field of application of hot-rolled steel sheets is conventionally limited to "for materials such as cold-rolled steel sheets and welded steel pipes" or "for members that do not require such high workability" Although "the point of inferior workability (especially deep drawability)" is mentioned, as mentioned above, several technologies for improving the workability of hot-rolled steel sheets were proposed instead of cold-rolled steel sheets to hot-rolled steel sheets. It became a great driving force to encourage

As one of the "techniques for improving the workability of hot rolled steel sheet", there is a method disclosed in, for example, Japanese Patent Laid-Open No. 59-153836. This is because aluminum-killed steel containing C, Mn and sol.Al is hot-rolled under specific conditions after precipitation treatment of AlN, then pickled, light-rolled and re-crystallized in order to obtain good deep drawability. It is a technology to obtain hot rolled steel sheet. However, it is pointed out that this method requires a long time of 100 minutes or more for heating during the AlN precipitation treatment, so that the production efficiency is extremely poor.

On the other hand, apart from this, "hot-rolled steel sheet with improved workability" is characterized in that hot rolling under a large pressure in a high temperature range is performed, and then lubrication rolling is performed in a relatively low temperature range. Is also proposed (for example, JP-A-61-384).
No. 4, Japanese Patent Laid-Open No. 61-3845). According to this method, it is reported that a hot-rolled steel sheet having an r value of 1.0 or more and excellent in deep drawability can be obtained. However, in reality, the in-plane anisotropy (0
The difference between the maximum value r _max and the minimum value r _min among the r values r ₀ , r ₄₅ , and r ₉₀ in each direction of °, 45 °, and 90 °) is made small to obtain a uniform high r value. It is difficult to stably impart the workability, and workability comparable to that of cold-rolled steel sheet has not been realized yet.

As another attempt to improve the workability of hot-rolled steel sheet, an extremely low carbon steel slab containing a trace amount of Ti or Nb is roughly rolled in a temperature range of Ar ₃ or more and then 800 ° C. A method of performing finish rolling with a total reduction of 73% in the following ferrite (hereinafter abbreviated as “α”) region has also been reported {Iron and Steel, 74 (1988), pp. 1617-1624}. However, according to the examination results of the present inventors, although this method surely ensures press formability (deep drawability) on a thin steel sheet,
The in-plane anisotropy in the case of thin steel sheets was still as large as 0.5 or more, and the performance equivalent to that of cold-rolled steel sheets could not be expected.

The cause is that crystals obtained in a specific direction preferentially exist in the obtained thin steel sheet, and therefore the deformation state changes depending on the relationship between the direction of the crystal and the deformation direction. It is believed that there is. Therefore, a thin steel sheet manufactured by such a method has a "press formability" in terms of the r value {defined as (r ₀ + 2r ₄₅ + r ₉₀ ) / 4} as an average value of r values in each direction. Is said to be “excellent”, but the in-plane anisotropy of the steel sheet (absolute value of “r _max −r _min ”)
Is an inferior value, which does not sufficiently satisfy the performance required as a steel sheet for press working that requires uniform elongation and strength in any direction.

By the way, it has been found from the studies so far that it is desirable to develop a {111} texture in the final α in order to obtain a high r-value steel sheet having a small in-plane anisotropy. Since the {111} texture is generated near the α grain boundary during the recrystallization process, {11}
1) In order to develop a texture, it is necessary to reduce the processed α grain size before the recrystallization treatment and increase the α grain boundary area. Then, in order to realize the reduction of the α particle diameter,
It can be said that it is preferable to refine the γ grain size before transformation from austenite (hereinafter abbreviated as “γ”) to α.

Studies on means for refining γ grains have also been carried out for many years, and as a result, I) controlled rolling, II) high pressure rolling (for example, JP-A-62-253733, JP-A-SHO) 63-145720 gazette), etc. have been created. However, the following problems were pointed out for each of these technologies.

A) In the case of the structure refining technique by controlled rolling, the property that the γ grains produced by hot working called "controlled rolling" cannot actually be further refined when it becomes fine to a certain extent. Therefore, there is a limit to the refinement of γ grains only with controlled rolling.
It was difficult to realize even a uniform microstructure in which the grain size of α transformed from grains is, for example, about 20 μm.

B) In the case of the structure refining technology by large reduction rolling, this is because the reduction ratio per pass is 3 in the non-recrystallization temperature region of γ.
This is a technique for refining the structure by applying a large reduction of 0% or more to make the γ grains “work hardening γ containing a deformation zone in the grains” and then causing the γ → α transformation to refine the structure. "Γ
→ The “γ-grain before α transformation” is simply elongated by large reduction rolling and does not become “fine grain”. Therefore, there is a limit to the refinement of the structure. Therefore, the α-grain size after transformation is 20
It was not possible to realize a uniform fine structure of less than μm.

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

Therefore, the object of the present invention is to stably reveal the "ultrafine uniform structure" in the γ stage, which was difficult to realize by the conventional method, and is excellent on the basis of this. It was to establish means for stably producing a high r-value hot-rolled steel sheet having small in-plane anisotropy exhibiting workability on an industrial scale.

[0014]

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) A low carbon aluminum killed steel having a C content of 0.08% or less (hereinafter,% representing a component ratio is% by weight) and an N content of 0.01% or less, or further Ti, Nb, Zr, V,
Temperature control of continuous cast slabs (usually thick slabs and thin slabs) or ingots (generally referred to as "hot steel slabs") of low carbon aluminum killed steel containing one or more types of Mo (A temperature drop below ₃ points) ) To reveal the α-structure and perform precipitation treatment in this temperature range for rapid precipitation of carbonitrides and sulfides to purify the matrix, and then raise the temperature as it is to convert the α-phase to the γ-phase. Reverse transformation, or by rolling at a predetermined reduction rate and then raising the temperature to reverse transform the α phase to the γ phase, or if the γ grain size of the hot steel billet is 200 μm or more, “α When the "hot steel slab before being exposed" is once rolled at a predetermined reduction rate in the γ temperature range and then subjected to the heat treatment or thermomechanical treatment in the above step to reverse transform α into γ, the γ structure that appears is The ultrafine structure cannot be obtained by controlled rolling.

B) Therefore, when this ultrafine γ structure is cooled as it is or after further rolling, the α produced by transformation is based on the ultrafine γ structure, so that it is also extremely fine and is conventionally realized. It was extremely difficult to obtain an “isotropic uniform fine α-structure having an α-grain size much smaller than 20 μm”.

C) Then, the intermediate material having a fine α structure in which the precipitate is coarse and the matrix is purified is rolled at a reduction rate of 70 to 97% in the α non-recrystallization temperature range to obtain a desired size. When the steel sheet is used, the {111} texture is sufficiently developed after the recrystallization treatment, and it becomes possible to stably obtain a hot-rolled steel sheet having a small in-plane anisotropy and a high r value.

The present invention has been completed through further studies based on the above findings and the like. "C: 0.08% 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, or further B: 0.0001 to 0.0050%, or in addition to these, Ti, Nb, Zr, V and Mo One or more: 0.015 to 0.350% in total is also the formula [C equivalent]-[Ti equivalent / 4] ≤ 0.0020 A hot steel slab that contains Fe and unavoidable impurities with the balance satisfying the following conditions is cooled to at least a) below the A ₃ point (ie Ae ₃ point), b) above the A ₃ point performing deposition by keeping 1-60 minutes, c) the temperature to a temperature range of a ₃ point - [a _3-point + 200 ° C.] was raised, a ferrite OH - effect reverse transformation to austenite, d)該O -Cooling from the temperature range of the austenite phase, e) Rolling with a total reduction of 70 to 97% in the temperature range of [A ₃ points-100 ° C] to 450 ° C, f) Recrystallization treatment, It is possible to stably manufacture a high r-value hot-rolled steel sheet having a small in-plane anisotropy by sequentially processing and heat treating in a process including ".

FIG. 1 is a schematic view showing an example of the cold-rolled steel sheet manufacturing process according to the present invention. Hereinafter, the reason why the component composition of the material steel and the working / heat treatment conditions are limited as described above in the present invention will be specifically described together with the action and effect.

[0020]

[Action]

<Ingredient composition of raw steel> C C is an element that adversely affects the deep drawability of the steel sheet,
The smaller the content, the better. In particular, when the C content exceeds 0.08%, the deep drawability deteriorates remarkably, so the content was limited to 0.08% or less.

Si Since Si is also an element that adversely affects the deep drawability of the steel sheet, it is preferable that the content of Si is as small as possible. In particular, if the Si content exceeds 0.3%, not only the deep drawability will deteriorate significantly, but also the scale properties will deteriorate and the product quality will be 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 the content thereof is less than 0.01%, the effect due to the above effect is not sufficient and hot brittleness occurs, while 0.4% is added. 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 to improve the yield of deoxidizing element and carbonitride forming element. If the content of sol.Al is less than 0.01%, the above-mentioned action and effect are sufficiently obtained. On the other hand, on the other hand, if the content exceeds 0.08%, the effect will be saturated and it will be uneconomical,
The Al content was defined as 0.01 to 0.08% in terms of sol.Al content.

The lower the N 2 N content is, the smaller the amount of carbonitride or sulfide forming element added is preferable. In particular, when the content exceeds 0.01%, it is unavoidable that the r-value of the steel sheet decreases even if the carbonitride and sulfide forming elements are added, so the N content was set to 0.01% or less. .

[0026] 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%.

Ti, Nb, Zr, V, and Mo. These components reduce the solute C, N, and S by forming carbonitrides and sulfides, and at the same time, the precipitates make the crystal grains moderate. Since it has the effect of making the particles finer, they are added individually or in combination as required. However, if the total content of these is less than 0.015%, the desired effect due to the above action cannot be obtained, while if the total content is more than 0.350%, the strength increases too much and it becomes unsuitable as a steel sheet for working. , Is also disadvantageous economically. Therefore, the total content of these components was set to 0.015 to 0.350%.

In addition, "[C equivalent]-[Ti equivalent / 4] ≤
The formula "0.0020" represents the relationship for making solid solution C, N, S 0.0020% or less and precipitating the remaining C, N, S as carbonitrides or sulfides. ] −
When the value of [Ti equivalent / 4] exceeds 0.0020, solid solution C, N,
Since the amount of S increases, the {111} recrystallized texture does not develop, and the desired deep drawability cannot be imparted to the steel sheet.

<Processing / Treatment Conditions> The raw material billet having the above-mentioned composition to be subjected to hot rolling may be one produced by continuous casting (usually thick slab or thin slab), and slabbed from an ingot. It may be manufactured by rolling. Further, the raw steel billet may be subjected to hot rolling after heating the cold steel billet after continuous casting or slab rolling to a predetermined temperature,
Even if the "direct rolling" method is used, in which "steel slabs sent from a continuous casting or slab rolling line at a high temperature are used as they are, or with some auxiliary heating, they are subjected to hot rolling" good. Note that FIG. 2 is a schematic view showing another example of the “cold rolled steel sheet manufacturing process according to the present invention” when the reheating rolling or the direct feeding rolling is adopted.

In the present invention, such hot steel pieces are sequentially processed and heat-treated under the above-mentioned conditions, and the processing conditions at this time are specified for the following reasons.

( A ) The temperature of the hot steel piece below A ₃ point (Ae ₃ point)
Reason for cooling to the temperature range The reason why the hot steel billet is once cooled to the temperature range below the A ₃ point is that the method of the present invention has “reverse transformation from α phase to γ phase” as a main requirement. This is because it requires the generation of the α phase. The temperature after cooling at this time is not particularly limited as long as it is a temperature of less than A ₃ point, but in terms of practical operability, a region of a temperature as high as possible near less than A ₃ point { A ₃ points to [A ₃ points-200
C]} is preferable.

However, when the γ grain size of the continuously cast or ingot-cast steel slab is 200 μm or more, the hot steel slab is cooled as it is to a temperature range below the A ₃ point, and α is obtained after rolling. → Even if the γ reverse transformation is caused, the desired uniform ultrafine structure may not be obtained.

However, even in such a case, as shown in FIG. 2, before cooling the hot steel slab as it is or after once inserting it into the heating furnace, rolling in the final pass is performed in the temperature range of A ₃ points or more and finally. By rolling with a pass reduction of 30% or more, the γ grains are recrystallized and refined (γ grain size: 200 μm
Since the processing strain can be introduced into the γ grains, the precipitation sites of the α grains can be increased, and the α grains can be refined in the subsequent cooling process.

Preferably, the rolling reduction in the final pass is 45% or more. If the rolling reduction in the final pass is less than 30%, not only the γ grains will not be recrystallized into fine grains but also the processing strain will be small, so that the α grains will not be fined in the next cooling step. Further, when the temperature of the rolling in the final pass is lower than the A ₃ point, the α phase is mixed and the working strain is concentrated in the soft α phase, and the working strain is not accumulated in the γ phase. The α grains generated by α transformation are not refined. It is preferable that this rolling is carried out for one or more passes, and the final pass is performed under the above conditions.
For the rolling before the final pass, it is not necessary to particularly limit the conditions, and ordinary rolling may be used.

(B) A ₁ to 60 minutes in a temperature range of less than ₃ points
Reason for carrying out the precipitation treatment by holding it The purpose of the precipitation treatment is to precipitate C, N and S in the steel as carbonitrides or sulfides to improve the deep drawability.
For that purpose, the temperature may be maintained in the temperature range of less than A ₃ .
This is because the solubility of the precipitates is generally smaller in the α phase than in the γ phase, so that precipitation will proceed rapidly if cooled and held in the temperature range below the A ₃ point. As a result, the development of the {111} texture after annealing is facilitated and the deep drawability of the final product is improved. The holding time at this time is 1 to 60
It should be minutes. This is because if the holding time is less than 1 minute, the amount of precipitation will be small, and if it is longer than 60 minutes, the precipitation will be saturated, resulting in an increase in manufacturing cost.

In the present invention, the means for holding the "steel plate after being cooled to the temperature range of A ₃ point or less" in the above temperature range in the rolling line is not particularly limited, but, for example, a recently developed "coil box" is used. Can be used.
Further, the steel plate may be rapidly cooled in order to attain a predetermined precipitation treatment temperature. By quenching, the manufacturing time can be shortened, and the α grains of the hot-rolled sheet produced by the γ → α transformation can be made finer, and the deep drawability can be improved.

(C) Temperature from A ₃ point to [A ₃ point + 200 ° C.]
Reason for heating up to the temperature range As described above, when the hot steel billet is once cooled to a temperature range below the A ₃ point to generate the α phase and then heated to the A ₃ point or higher, the α → γ reverse transformation causes It is possible to make the γ grains finer. That is, the reason why the temperature is raised to the temperature range of A ₃ point or higher is
“Generates very fine γ grains from α by reverse transformation”
That is to fully exhibit the characteristic actions and effects of the method according to the present invention. In this case, the upper limit of the temperature rise is [A ₃ point + 200 ° C.] because the grain growth of γ occurs when the temperature rises above this temperature and finally the desired uniform ultrafine structure steel sheet is obtained. Therefore, the desired workability and strength cannot be ensured.

If rolling is performed in a temperature range of less than the A ₃ point prior to this temperature rise, strain is accumulated in α and the reverse transformation nuclei to γ are increased by this rolling. It is possible to further reduce the size of the γ grains as compared with the case. Therefore, it may be heated after performing rolling in the temperature range of A less than ₃ points.

From the temperature range below A ₃ point, A ₃ point to
The heating rate when raising the temperature to the temperature range of [A ₃ point + 200 ° C.] is preferably 0.1 ° C./sec or more. As a result, a large number of γ nuclei can be generated from α, the γ grain growth after the reverse transformation can be suppressed, and the processing strain in the temperature range of A ₃ point or lower can be released prior to the α → γ reverse transformation. Therefore, it is possible to realize a desired fine γ grain. As a 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) Reason for cooling from the γ-phase temperature range The steel that is rapidly heated to the γ-phase temperature range to cause the reverse transformation has an isotropic and uniform ultrafine α-structure due to subsequent cooling. 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, the processing strain is accumulated in the γ grains generated by the reverse transformation, and the α-containing structure generated by subsequent cooling is further refined, so that its characteristics are further improved. Is. 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. Then, the steel which has been subjected to the above-described thermomechanical treatment and is made into a plate material is cooled by an arbitrary means, whereby A
Fine α particles having an STM particle size number of 8 or more can be obtained.

The cooling from the γ phase temperature range is from A ₃ point
It is desirable to cool the temperature range of [A ₃ points-100 ° C] at a cooling rate of 5 ° C / s or more. This makes it possible to generate a large number of α nuclei from fine γ and suppress the growth of α grains, so that fine α grains can be obtained. By refining the α grains before rolling in the next step, the area of the α grain boundaries can be increased, and the {111} recrystallized texture that is generated from the α grain boundaries and is favorable for improving the r value is sufficiently developed. Can be made.

(E) From the γ phase temperature range [A ₃ points −100 ° C.]
The reason why the steel is cooled to a temperature range of ~ 450 ° C and rolled as described above, the steel heated to the γ phase temperature range to cause reverse transformation is then [A ₃ point-100 ° C] to a temperature range of 450 ° C. Although it is cooled to, the total rolling reduction in this temperature range: 70 to 97% is performed to obtain the final plate thickness.
The purpose of this rolling is to develop a {111} texture that is favorable for "improving the r value" and "minimizing the in-plane anisotropy of the r value" in the next recrystallization treatment step. To develop.

Therefore, it is desirable that this rolling is carried out in the temperature range of α unrecrystallized temperature [A ₃ point −100 ° C.] or lower.
However, in the temperature range below 450 ° C., the energy required for rolling increases due to the increase in deformation resistance, and the economic merit decreases. The rolling reduction in the above rolling is 70%.
If it is less than 100% or exceeds 97%, the {111} texture is not sufficiently developed even if the recrystallization treatment is performed.

That is, [A ₃ points-100 ° C] to 450 ° C
By performing recrystallization treatment after rolling with a rolling reduction of 70 to 97% in the temperature range of 1, the {111} texture is sufficiently developed, and the r value is high and the in-plane anisotropy is small. That is, the rolled steel sheet can be stably obtained in the actual operation.

In order to improve the r value, the development of {110} texture generated during rolling is suppressed and the strain is evenly distributed in the thickness direction so that the texture at the center of the thickness and the surface portion It is preferable to reduce the difference between. For this purpose, it is preferable to carry out rolling while lubricating so that the friction coefficient (μ) between the steel sheet and the rolling roll is 0.20 or less, preferably 0.15 or less.

By the way, the r-value of the steel sheet recrystallized after the rolling, in other words, the formation of texture has a great influence on the strain rate during the rolling. That is, from the experimental results on hot rolling, the strain rate in the final pass was 100 s ^−1.
In the above cases, it was also found that the {111} texture can be developed only by securing a reduction rate of 30% or more.

The rolling pass is performed once or more, and the number of passes may be determined in consideration of the plate thickness of the raw material and the product plate thickness. In this rolling, it is optional to add not only the above-mentioned lubricated large reduction rolling but also ordinary non-lubricated rolling.

By the way, "rolling explained in the item (a)"
Also, there is no restriction on where in the hot rolling line the "rolling heat treatment described in (c) to (e)" is performed, but the former is performed in the rough rolling process and the latter is performed in the finish rolling process. Is advantageous in terms of equipment. At that time, the hot rolling winding temperature does not matter.

(F) Recrystallization Treatment Recrystallization treatment is an essential step for giving excellent workability to the steel sheet after rolling. The recrystallization method is not particularly limited, and may be recrystallized by self-annealing during cooling after completion of hot rolling or in a state of being wound on a coil, or by heating after winding. It may be recrystallized.

The hot-rolled steel sheet manufactured according to the above-mentioned conditions has extremely excellent workability comparable to that of the "conventional cold-rolled steel sheet" despite being a hot-rolled steel sheet.

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 slabs of 20 mm thickness and 100 mm thickness. Subsequently, these slabs were hot-rolled under the conditions shown in Table 2, cooled and wound up, and then
The recrystallization treatment was performed under any of the conditions shown in a) to d).

[Recrystallization Treatment Conditions] a) After being wound on a coil, recrystallized by its own heat during slow cooling [treatment a], b) After winding, once cooled to room temperature and then at a temperature of 800 ° C. Apply a heat history equivalent to continuous annealing held for 2 minutes to recrystallize [treatment b], c) After winding, cool to room temperature once, and then hold for 10 seconds at a temperature of 850 ° C for hot dip galvanizing line Re-crystallize by applying a heat history equivalent to continuous annealing at [process c], d) After winding, cool to room temperature once, and then hold at 700 ° C for 5 hours To recrystallize [treatment d].

Test pieces were taken from the hot-rolled steel sheet thus obtained, and the "yield strength", "elongation" and "r value" were investigated. The results are shown in Table 3.

[0054]

[Table 1]

[0055]

[Table 2]

[0056]

[Table 3]

As is clear from the results shown in Table 3, the hot-rolled steel sheet produced according to the conditions specified in the present invention exhibits 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 satisfy the specified conditions of the present invention, the α structure must be sufficiently refined as pointed out in the hot-rolled steel sheets obtained in Test Nos. 16-22. In other words, it can be seen that the obtained steel sheet has poor properties. Further, even if the α structure is fine, if the reduction ratio and the rolling end temperature of the α region rolling are inappropriate, the steel sheet properties may be inferior as seen in the hot rolled steel sheets obtained in Test Nos. 23 to 25. I understand.

[0059]

[Summary of Effects] As described above, according to the present invention, it is possible to stably produce a hot-rolled steel sheet having a small in-plane anisotropy and a high r value, and it is possible to obtain an extremely useful effect in industry. Be done.

[Brief description of drawings]

FIG. 1 is a schematic view showing an example of a hot-rolled steel sheet manufacturing process according to the present invention.

FIG. 2 is a schematic view showing another example of the hot-rolled steel sheet manufacturing process according to the present invention.

Claims (4)

[Claims] 1. A weight ratio of C: 0.08% 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: In-plane anisotropic, characterized by sequentially processing and heat treating a hot steel slab containing 0.01% or less and the balance Fe and unavoidable impurities in a step including at least the treatments a) to f) below. A method for producing a high r-value hot-rolled steel sheet having low properties. cooled to a temperature range below a) A _3-point, b) performs the A precipitation treatment to hold 1-60 minutes at a temperature range of less than ₃ points, c) the temperature of the A ₃ point - [A ₃ point + 200 ° C.] Up to the temperature range to cause the reverse transformation of ferrite to austenite, d) cooling from the temperature range of the austenite phase, e) in the temperature range of [A ₃ points-100 ° C] to 450 ° C. Rolling with a total reduction of 70 to 97%, f) Recrystallization treatment is performed. 2. A weight ratio of C: 0.08% 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 Fe and inevitable impurities, hot steel slabs are sequentially processed and heat treated in a process including at least the following a) to f). A method for producing a high r-value hot-rolled steel sheet having small in-plane anisotropy. cooled to a temperature range below a) A _3-point, b) performs the A precipitation treatment to hold 1-60 minutes at a temperature range of less than ₃ points, c) the temperature of the A ₃ point - [A ₃ point + 200 ° C.] Up to the temperature range to cause the reverse transformation of ferrite to austenite, d) cooling from the temperature range of the austenite phase, e) in the temperature range of [A ₃ points-100 ° C] to 450 ° C. Rolling with a total reduction of 70 to 97%, f) Recrystallization treatment is performed. 3. By weight ratio, C: 0.08% 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 is contained and Ti, Nb, Zr,
One or more types of V and Mo: 0.015 to 0.350% in total is also represented by the formula [C equivalent]-[Ti equivalent / 4] ≤ 0.0020. A hot steel slab that contains Fe and unavoidable impurities with the balance satisfying the following conditions, characterized by sequentially processing and heat treating in a step including at least the following treatments a) to f). A method for manufacturing a small high-r hot-rolled steel sheet. cooled to a temperature range below a) A _3-point, b) performs the A precipitation treatment to hold 1-60 minutes at a temperature range of less than ₃ points, c) the temperature of the A ₃ point - [A ₃ point + 200 ° C.] Up to the temperature range to cause the reverse transformation of ferrite to austenite, d) cooling from the temperature range of the austenite phase, e) in the temperature range of [A ₃ points-100 ° C] to 450 ° C. Rolling with a total reduction of 70 to 97%, f) Recrystallization treatment is performed. 4. By weight ratio, C: 0.08% 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%, Ti, Nb,
One or more types of Zr, V and Mo: 0.015 to 0.350% in total is also represented by the formula [C equivalent]-[Ti equivalent / 4] ≤0.0020. A hot steel slab that contains Fe and unavoidable impurities with the balance satisfying the following conditions, characterized by sequentially processing and heat treating in a step including at least the following treatments a) to f). A method for manufacturing a small high-r hot-rolled steel sheet. cooled to a temperature range below a) A _3-point, b) performs the A precipitation treatment to hold 1-60 minutes at a temperature range of less than ₃ points, c) the temperature of the A ₃ point - [A ₃ point + 200 ° C.] Up to the temperature range to cause the reverse transformation of ferrite to austenite, d) cooling from the temperature range of the austenite phase, e) in the temperature range of [A ₃ points-100 ° C] to 450 ° C. Rolling with a total reduction of 70 to 97%, f) Recrystallization treatment is performed.