JP3046663B2 - Method for producing hot-rolled steel sheet with excellent deep drawability using thin slab - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

The present invention has a thickness of 20 to 100 mm.
Index for evaluating the deep drawability of thin steel sheets using thin slabs, average Rankford value (hereinafter referred to as average r value)
The present invention relates to a method for producing a hot-rolled steel sheet having excellent deep drawing properties, which is 1.2 or more.

[0002]

2. Description of the Related Art In recent years, a high-speed continuous casting method for thin slabs aiming at cost reduction has been established and put into practical use. However, in this manufacturing method, since the rough rolling step is omitted, the crystal grains are unlikely to become fine grains, and this is a major problem in forming the material properties.

On the other hand, as a new product of a hot-rolled steel sheet, various deep-drawing methods are aimed at reducing the cost of the product by saving steps, upgrading the quality of the hot-rolled material, and further improving the material properties of the cold-rolled material. A method for producing a hot-rolled steel sheet has been proposed. However, these methods do not consider utilizing a thin slab material from which fine grains are difficult to obtain, and in order to create a high average r-value characteristic even when manufacturing from a normal slab instead of a thin slab. Required a variety of capital investments and enhancements. For example, Japanese Unexamined Patent Publication No. Sho 61-3844 discloses a method in which a large rolling reduction is performed in a γ region and then a lubricating rolling is performed in a warm state.
In this method, it is difficult to manufacture a thin slab without rough rolling, and lubrication equipment and annealing equipment are required.

[0004]

SUMMARY OF THE INVENTION The present invention uses a thin slab material without increasing or modifying lubricating equipment and annealing equipment, and has an average r value of 1.2 having a deep drawability comparable to that of a cold rolled steel sheet. An object of the present invention is to provide a method for manufacturing the above hot-rolled steel sheet.

[0005]

According to the present invention, in order to solve the above-mentioned problems, in terms of% by weight, C ≦ 0.01%, Mn ≦ 0.4.
%, N ≦ 0.01% and the amounts of C and N added are Ti, N
b) and the amount of one or both of (b) and (C / 12 +
N / 14) ≦ 1.2 (Ti / 48 + Nb / 93) ≦ 0.
1000 ° C. The cast thin slab of thickness 20~100mm in one relationship between not fall below the temperature Ar ₃ transformation point to 1
After holding at 100 ° C for 30 minutes or more, 1000 ° C to 920
Perform primary rolling with a total draft of 50% or more in the range of ℃,
Immediately after the temperature of the steel sheet falls to 920 ° C. or less, cooling is performed at a cooling rate of 50 ° C./sec or more between the stands.
At the temperature of 830 ° C., the secondary rolling of each pass with a rolling reduction of 40% or more is started, and the temperature is raised from the secondary rolling starting temperature by the heat generated during processing.
Is warm, finished rolling below 750 ° C. to Ar ₃ transformation point,
As a first means, a method for producing a hot-rolled steel sheet having excellent deep drawability using a thin slab characterized by winding and recrystallizing,

A second means is a method for producing a hot-rolled steel sheet excellent in deep drawability using a thin slab according to the first means, wherein the secondary rolling speed is 1000 m / min or more,

In weight%, C ≦ 0.01%, Mn ≦ 0.4
%, N ≦ 0.01% and the amounts of C and N added are Ti, N
b) and the amount of one or both of (b) and (C / 12 +
N / 14) ≦ 1.2 (Ti / 48 + Nb / 93) ≦ 0.
The thin cast slab having a thickness of 20 to 100 mm and having a relationship of 1 is reheated from a temperature lower than the Ar ₃ transformation point to 1000 ° C. to 1100 ° C., and then a total draft of 50% or more in a range of 1000 ° C. to 920 ° C. After primary rolling, the steel sheet temperature was 92
Immediately after the temperature drops to 0 ° C or lower, the cooling rate between the stands is 50 ° C /
Perform the above second cooling starts heating secondary rolling each pass reduction of 40% or more at a temperature of subsequent 650 ℃ ~830 ℃, pressurized
The temperature is raised from the secondary rolling start temperature by the heat generated in the process, and 750
The third means is a method for producing a hot-rolled steel sheet excellent in deep drawability using a thin slab characterized by terminating the rolling at a temperature lower than the transformation point of C to Ar ₃ and winding and recrystallizing the rolled slab.

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

[0009]

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 average r-value increases as the plane orientation of the recrystallization of ferrite increases {111} plane strength. 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, etc., the conditions obtained in the cold rolled steel sheet, that is, by recrystallizing ferrite and improving {111}, it is possible to increase the average r value. become.

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.

[0011] To a carbonitride of this material is more effectively deposited, when not lowered below Ar ₃ transformation point is 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 subjected to primary rolling at 920 ° C. to 1000 ° C. in order to reduce γ grains in order to reduce ferrite grains before secondary rolling. In order to make the 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 the ultra-low carbon steel is near 900 ° C., and the rolling temperature is regulated in the range of 920 ° C. to 1000 ° C. to ensure recrystallization. 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 thickness of the present invention is 20 to 100 mm
When rolling thin slabs, the omission of the conventional rough rolling step is a great merit. The present invention also enjoys this advantage. Therefore, the required material must be obtained only by conventional finish rolling. For this purpose, a means for sufficiently reducing γ crystal grains in place of the rough rolling is required. In the primary rolling described above, the present inventors apply large pressure to a thin slab at a low temperature to reduce the γ crystal grains, thereby reducing the ferrite transformation crystal grains and finally causing the ferrite to recrystallize. It has been found that the above requirements can be easily satisfied.

In order to reduce the grain size of the material to be rolled in the ferrite region as finely as possible, it is necessary to perform inter-stand cooling immediately after the temperature of the rolled material drops to 920 ° C. or less. This is to prevent the crystal grains of γ recrystallized by rolling from growing and coarsening, so that fine ferrite cannot be obtained even when transformed. Further, the cooling is performed at a cooling rate of 50 ° C./sec or more for the same reason.

The main points of the secondary rolling are that all are rolled 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 higher, 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 in the surface layer, which hinders the improvement of the average r value. It is said that lubrication rolling is necessary to alleviate this, but in the present invention, the development of the {110} surface of the surface layer is small and lubrication rolling is not necessary. Although the detailed cause-and-effect 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, and it was considered that dynamic recrystallization was caused to be random.

The winding temperature after rolling in the present invention is not particularly limited since 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.

[0020]

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. Like 1-8,
A high average r value of 1.2 or more is obtained. But E, F
If a material is used, no. As shown in FIGS. 9 and 10, the elongation is reduced and the average r value is reduced due to the remaining solid solution C and N. Also,
No. Although the rolling conditions deviated from 11 to 16, under these conditions, ferrite refinement, 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.

[0021]

According to the present invention, a hot-rolled steel sheet manufactured by using a thin slab can be used as a substitute for a cold-rolled steel sheet. Lubrication equipment and annealing equipment are not required as compared with other methods of manufacturing hot-rolled steel sheets for deep drawing, and the economical advantage 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 starting temperature and an ending temperature of secondary rolling, a range in which strain can be accumulated, and a range of secondary heating and rolling.

──────────────────────────────────────────────────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] C. 0.01%, Mn.ltoreq.0.1% by weight.
4%, N ≦ 0.01% and the added amount of C and N is Ti,
The amount of one or both of Nb and (C / 12
+ N / 14) ≦ 1.2 (Ti / 48 + Nb / 93) ≦
A cast thin slab having a thickness of 20 to 100 mm having a relationship of 0.1 is cooled to 1000 ° C. while the temperature does not reach a temperature below the Ar ₃ transformation point.
After holding at ~ 1100 ° C for 30 minutes or more, 1000 ° C ~ 9
Primary rolling is performed in the range of 20 ° C. with a total draft of 50% or more, and immediately after the steel sheet temperature becomes 920 ° C. or less, cooling is performed at a cooling rate of 50 ° C./sec or more between stands.
Temperature rise of 40% or more at each pass reduction rate at a temperature of ℃ ~ 830 ℃ 2
Start the next rolling, and the temperature of the second rolling starts
A method for producing a hot-rolled steel sheet excellent in deep drawability using a thin slab, wherein rolling is completed at a temperature of 750 ° C. to less than the Ar ₃ transformation point, wound and recrystallized. 2. The method for producing a hot-rolled steel sheet excellent in deep drawability using a thin slab according to claim 1, wherein the secondary rolling speed is 1000 m / min or more. 3. The method according to claim 2, wherein C ≦ 0.01% and Mn ≦ 0.
4%, N ≦ 0.01% and the added amount of C and N is Ti,
The amount of one or both of Nb and (C / 12
+ N / 14) ≦ 1.2 (Ti / 48 + Nb / 93) ≦
1000 ° C. The cast thin slab of thickness 20~100mm in relation 0.1 from Ar ₃ following transformation point temperature to 1100
° C, and then primary rolling is performed at a total draft of 50% or more in the range of 1000 ° C to 920 ° C, and immediately after the steel sheet temperature becomes 920 ° C or less, the cooling rate between the stands is reduced to 50%.
Cooling at a temperature of 650 ° C. to 830 ° C., and starting a secondary rolling at a temperature of 650 ° C. to 830 ° C. with a rolling reduction of 40% or more .
A method for producing a hot-rolled steel sheet having excellent deep drawability using a thin slab, wherein rolling is completed at a temperature of 750 ° C. to less than the Ar ₃ transformation point, wound and recrystallized. 4. The method for producing a hot-rolled steel sheet excellent in deep drawability using a thin slab according to claim 3, wherein the secondary rolling speed is 1000 m / min or more.