JP3046662B2 - Manufacturing method of hot rolled steel sheet with excellent deep drawability - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION The present invention relates to a heat-resistant steel sheet having an excellent Rank-drawing value (hereinafter referred to as an average r-value) of 1.2 or more as an index for evaluating the deep-drawability of a thin steel sheet. The present invention relates to a method for producing a rolled steel sheet.

[0002]

2. Description of the Related Art Generally, the deep drawability of a hot-rolled steel sheet is 1.0 or less in average r value. For this reason, the hot-rolled steel sheet was not used as a material having drawability. However, in recent years,
Various methods for producing hot-rolled steel sheets having a high average r value have been proposed with the aim of reducing the cost of products by saving steps, upgrading the quality of hot-rolled materials, and further improving the material properties after cold rolling. . Of these methods, Japanese Patent Application Laid-Open No. 61-3844 discloses a method of performing lubricating rolling after warm rolling in the γ region and then performing lubricating rolling warmly. Although the average r value is high, lubricating equipment and annealing equipment are required.

[0003]

SUMMARY OF THE INVENTION The present invention is to provide a hot-rolled steel sheet having a deep drawability comparable to that of a cold-rolled steel sheet by making full use of the functions of the current hot-rolling process without having to reinforce or remodel equipment. It is an object of the present invention to provide a method for producing the same.

[0004]

According to the present invention, in order to solve the above-mentioned problems, in terms of% by weight, C ≦ 0.01% and Mn ≦ 0.4.
%, N ≦ 0.01% and the added amount of C and N is Ti, N
b and / or (C / 12 + N
/14)≦1.2(Ti/48+Nb/93)≦0.1
After keeping the slab obtained from the steel having the relationship of not more than the Ar ₃ transformation point at a temperature of 1000 ° C. to 1100 ° C. for 30 minutes or more, the rolling end temperature is 920 ° C. or more and 1000 ° C. 50% total reduction in 920 ° C range
The above-mentioned primary rolling is performed, and then, at 650 ° C. to 830 ° C., the temperature-raising secondary rolling in which each pass rolling reduction is 40% or more is started, and processing is performed.
The temperature is raised from the secondary rolling start temperature by heat generation, 750 ° C
The method for producing a hot-rolled steel sheet having excellent deep drawability, which is characterized by terminating rolling at a temperature lower than the Ar ₃ transformation point and winding and recrystallizing the first means,

[0005] A second means is a method for producing a hot-rolled steel sheet excellent in deep drawability as described in the first means, wherein the final rolling speed of the secondary rolling is 1000 m / min or more,

In terms of% by weight, C ≦ 0.01%, Mn ≦ 0.4
%, N ≦ 0.01% and the added amount of C and N is Ti, N
b and / or (C / 12 + N
/14)≦1.2(Ti/48+Nb/93)≦0.1
After the slab obtained from the steel having the relationship of the above is once brought to a temperature of not more than the Ar ₃ transformation point and then reheated to 1000 to 1100 ° C., the rolling end temperature is 920 ° C. or more, and the total reduction in the range of 1000 ° C. to 920 ° C. rate performs first rolling of 50% or more, followed by the pass rolling reduction at 650 ℃ ~830 ℃ starts heating secondary rolling 40% or more, machining heating by the secondary rolling start temperature
The third method is a method for producing a hot-rolled steel sheet excellent in deep drawability, characterized in that the temperature is raised from 750 ° C. to less than the Ar ₃ transformation point, rolling is completed, and rewinding is performed.

A fourth means is a method for producing a hot-rolled steel sheet excellent in deep drawability according to a third means, wherein the final rolling speed of the secondary rolling is 1000 m / min or more. .

[0008]

The operation of the above means will be described in detail below. The reason why the average r value of the cold-rolled steel sheet is high is that recrystallization of ferrite occurs at the time of annealing after cold rolling, using the strain given by the cold rolling as a driving force. At this time, it is known that the higher the {111} plane strength of the ferrite recrystallization is, the higher the average r value is. It is known that the {111} plane has a high level of no solid solution C or solid solution N. It is said to occur from the grain boundaries of high purity steel. In a normal hot-rolled steel sheet, rolling is completed at or above the Ar ₃ transformation point, so that ferrite is formed by transformation from γ grains. Since it is not recrystallized ferrite, the development of {111} texture is observed. I can't. However, by controlling the steel type, hot rolling temperature, rolling reduction, and the like, the conditions obtained in the cold rolled steel sheet, that is, the ferrite is recrystallized to improve {111}, and the average r
It is possible to increase the value.

In the present invention, the amounts of C and N are regulated.
i and Nb are added by Ti (C, N), Nb (C,
This is for the purpose of precipitating carbonitrides such as N) to provide a substrate free of solid solution C and N. This effect increases as the amounts of Ti and Nb increase. However, if the amount is too large, the cost increases and the workability deteriorates due to the addition of the alloy. The present inventors have proposed (C / 1
2 + N / 14) ≦ 1.2 (Ti / 48 + Nb / 93) is the lower limit of Ti and Nb for eliminating solid solution C and N;
It has been found that 1.2 (Ti / 48 + Nb / 93) ≦ 0.1 is the upper limit of the alloy addition, and 0.4% is the upper limit of the Mn content to maintain the workability.

In order to more effectively precipitate the carbonitride of this material, if the temperature is not lowered below the Ar ₃ transformation point, it is 1000-1000.
Incubate at 1100 ° C for at least 30 minutes or more.
When lowering below the r ₃ transformation point, 1000 to 110
It has been found that reheating to 0 ° C. and applying a strain in the temperature range following the heat retention or the reheating are effective since they induce and promote precipitation.

Next, this material is rolled at a reduction ratio of 50% or more in a temperature range of 920 ° C. to 1000 ° C. in order to reduce ferrite grains before the secondary rolling to reduce γ grains to fine grains. It is to keep it. In order to make crystal grains fine,
It is effective to take the rolling reduction as low as possible and as high as possible. Generally, the transformation point of γ / α of ultra low carbon steel is 9
The rolling reduction was in the range of 920 ° C. to 1000 ° C. in order to ensure recrystallization and fine graining at around 00 ° C. The rolling reduction was set to 50% or more in order to enhance the effect of grain refinement. The higher the rolling reduction, the better. However, as long as the rolling is performed continuously in this temperature range, the degree of grain growth is small. Therefore, a method of securing a rolling reduction of 50% or more by multi-pass rolling is also included in the present invention.

The main points of the secondary rolling are that rolling is all performed in the ferrite region to accumulate strain and facilitate recrystallization.
The purpose is to ensure a temperature at which recrystallization of ferrite occurs after rolling. At this time, the reduction of γ by primary rolling and subsequent cooling contributes to the reduction of the transformed ferrite grains, and it is effective that the reduction of the ferrite easily causes recrystallization.

Therefore, the present invention sets the secondary rolling start temperature between 650 ° C. and 830 ° C. in order to surely accumulate strain in the ferrite region. To less than the Ar ₃ transformation point. As shown in FIG. 1, when rolling is started at 830 ° C. or more, since the recovery of strain is fast, it is difficult to cause a sufficient change in crystal orientation,
The {111} plane is difficult to develop, and a part of it undergoes reverse transformation and may enter the austenite region once.
The 11｝ plane stops developing. As in the present invention, when rolling is started at a low temperature, sufficient strain can be accumulated, and when the temperature rises in the latter half, recrystallization of ferrite is likely to occur, and the strength of the {111} plane is improved. In order to perform such temperature-increasing rolling, it is necessary to perform large rolling with a rolling reduction of 40% or more per pass and use the heat generated during processing. 1
If the rolling reduction per pass is less than 40%, the temperature does not rise, and a high average r value cannot be obtained. 650 ° C
If the secondary rolling start temperature is lower than, the rolling load increases, so the lower limit temperature was set to 650 ° C. This secondary rolling speed is set to 1
When the speed is set to 000 m / min or more, the heat generated during processing is further increased, and a higher average r value can be expected.

In the warm rolling, the {110} plane develops on the surface layer and hinders the improvement of the average r-value. In the invention, the {110} development of the surface layer is small, and there is no need for lubrication rolling. The exact causal relationship is unknown,
It is considered that the strain was greatly accumulated in the surface layer due to the continuous continuation under the large pressure, which caused dynamic recrystallization and became random.

The winding temperature after rolling in the present invention is not particularly limited because the final rolling temperature is high. However, if the end temperature is around 750 ° C., recrystallization may take a long time. Winding at a high temperature using a coiling or proximity coiler is effective in improving the average r value.

FIG. 2 shows the condition range of the above secondary rolling.
As is apparent from the figure, in order to obtain a material having an average r value of 1.2 or more, it is necessary to carry out strain accumulation in the recrystallized region of ferrite by heating and rolling.

[0017]

EXAMPLES Table 1 shows the components of the test steels by weight%. Table 2 shows the rolling conditions and the average r values and elongations obtained in the examples and comparative examples.

[Table 1]

[Table 2] Steel grades A to D are components within the scope of the present invention. E is C, Mn
Is deviated at the upper limit, and since F has a small amount of Ti, solute C and N remain. When the materials A to D are rolled under the conditions within the range of the present invention, the rolled material No. of the present invention example. As shown in 1 to 8, a high average r value of 1.2 or more is obtained. Rolled material N
o. Nos. 9 to 16 are comparative examples.
9 and 10 represent the average r due to the decrease in elongation and the residual C and N in the solid solution.
Causes the value to drop. In addition, No. 11 to 16 are primary,
Although the secondary rolling conditions were deviated, under these conditions, grain refinement of ferrite, accumulation of strain, securing of recrystallization temperature, and the like became insufficient, and the average r value was 1.2 in all cases. Did not reach.

[0018]

According to the present invention, a soft hot-rolled steel sheet having an average r value of 1.2 or more can be manufactured and can be used as a substitute for a cold-rolled steel sheet. However, as compared with other methods for producing hot-rolled steel sheets for deep drawing, there is no need for lubrication equipment and annealing equipment, and the economic merit is very large.

[Brief description of the drawings]

FIG. 1 shows an overall view of first and second rolling conditions of the production method of the present invention.

FIG. 2 shows a relationship between a secondary rolling start temperature and an average r value.

FIG. 3 shows a relationship between a starting temperature and an ending temperature of the secondary rolling, a strain accumulable range, and a secondary temperature rising rolling range.

──────────────────────────────────────────────────続 き Continued on front page (58) Field surveyed (Int.Cl. ⁷ , DB name) C21D 9 / 48,8 / 04 C22C 38/00-38/14

Claims (4)

(57) [Claims] 1. C ≦ 0.01% by weight%, Mn ≦ 0.4
%, N ≦ 0.01% and the added amount of C and N is Ti, N
b and / or (C / 12 + N
/14)≦1.2(Ti/48+Nb/93)≦0.1
After keeping the slab obtained from the steel having the relationship of not more than the Ar ₃ transformation point at a temperature of 1000 ° C. to 1100 ° C. for 30 minutes or more, the rolling end temperature is 920 ° C. or more and 1000 ° C. 50% total reduction in 920 ° C range
The above-mentioned primary rolling is performed, and then, at 650 ° C. to 830 ° C., the temperature-raising secondary rolling in which each pass rolling reduction is 40% or more is started, and processing is performed.
The temperature is raised from the secondary rolling start temperature by heat generation, 750 ° C
A method for producing a hot-rolled steel sheet excellent in deep drawability, in which rolling is completed at a temperature lower than the Ar ₃ transformation point, wound and recrystallized. 2. The final rolling speed of the secondary rolling is 1000 m /
The method for producing a hot-rolled steel sheet according to claim 1, wherein the hot-rolled steel sheet has excellent deep drawability. 3. C ≦ 0.01% by weight%, Mn ≦ 0.4
%, N ≦ 0.01% and the added amount of C and N is Ti, N
b and / or (C / 12 + N
/14)≦1.2(Ti/48+Nb/93)≦0.1
After the slab obtained from the steel having the relationship of (1) is once brought to a temperature below the Ar ₃ transformation point and then reheated to 1000 to 1100 ° C., the rolling end temperature is 920 ° C. or more, and the total reduction in the range of 1000 to 920 ° C. rate performs first rolling of 50% or more, followed by the pass rolling reduction at 650 ℃ ~830 ℃ starts heating secondary rolling 40% or more, machining heating by the secondary rolling start temperature
A method for producing a hot-rolled steel sheet excellent in deep drawability, characterized in that the temperature is raised from 750 ° C. to less than the Ar ₃ transformation point, rolling is completed, and the film is rolled up and recrystallized. 4. The final rolling speed of the secondary rolling is 1000 m /
The method for producing a hot-rolled steel sheet according to claim 3, wherein the hot-rolled steel sheet has excellent deep drawability.