JPH0617515B2 - Method for producing cold rolled steel sheet or strip of austenitic stainless steel - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION [Industrial field of application] The present invention relates to a cold-rolled austenitic stainless steel or a steel sheet having a plastic anisotropy which is equal to or less than the case where hot-rolled sheet annealing is omitted. It relates to a method of manufacturing a strip.

[Conventional technology]

Conventionally, in the production of cold-rolled steel sheets or strips of austenitic stainless steel (these may be collectively referred to as cold-rolled sheets in this specification), hot-rolled steel sheets obtained by hot rolling or The steel strip (also called hot-rolled sheet) was annealed, pickled, cold-rolled, and finally annealed. In other words, austenitic stainless steel hot-rolled sheet, when subjected to normal hot rolling conditions,
Since it has an unrecrystallized austenite structure and is in a sensitized state in which carbide is precipitated in the austenite grain boundaries and deformation zones, it is annealed at a temperature near 1100 ° C to complete recrystallization and soften and carbide. Has been carried out to improve the corrosion resistance.

This hot rolled sheet annealing is usually performed in a continuous annealing pickling line. Line speed is often controlled by annealing. Since such an annealing treatment requires a large amount of heat energy and has various problems such as troubles caused by the composite line, for example, overheating due to line suspension and generation of overpickling.
In recent years, it has been proposed to omit this hot-rolled sheet annealing to save equipment, energy and labor.

Japanese Patent Application Laid-Open No. 55-70404, which is filed by the same applicant, discloses a technique capable of omitting hot-rolled sheet annealing of this austenitic stainless steel by appropriately controlling hot-rolling conditions.

Japanese Unexamined Patent Publication No. 59-129731 also discloses a technique capable of omitting hot-rolled sheet annealing of austenitic stainless steel by appropriately setting hot-rolling conditions and pickling conditions.

[Problems to be solved by the invention]

When a cold-rolled steel sheet or strip of austenitic stainless steel is manufactured by omitting hot-rolled sheet annealing, the problem of plastic anisotropy is accompanied. That is, there are differences in mechanical properties between the rolling direction, the direction perpendicular to the rolling direction, and the rolling direction and 45 °,
For example, earrings are generated when this thin plate is deep-drawn into a cylinder.

According to the method described in the above-mentioned JP-A-55-70404, the recrystallization of the hot-rolled sheet is completed and the sensitization is suppressed, so annealing of the hot-rolled sheet can be omitted, but plastic deformation of the cold-rolled sheet can be omitted. The issue of orientation is unresolved.

On the other hand, Japanese Unexamined Patent Publication No. 59-129731 discloses a hot rolling condition in which the structure of a hot rolled sheet is an unrecrystallized structure in order to reduce plastic anisotropy (specifically, the hot rolling finishing temperature is lowered to perform hot rolling). However, in this case, the hot-rolled sheet becomes hard and the cold rolling is restricted (for example, the rolling load becomes high and roll defects easily occur, and problems such as coil edge cracking occur. ), And because the sensitization is not completely suppressed, the pickling conditions are limited.

The present invention has been made for the purpose of solving such a problem when the hot-rolled sheet annealing is omitted. More specifically,
Even if the hot-rolled sheet annealing is omitted, the plastic anisotropy of the cold-rolled sheet is improved to the same level as that if it is not omitted or more, and the purpose is to establish the hot-rolling technology for austenitic stainless steel that is soft It is what

[Means for solving problems]

The present invention, when manufacturing a cold-rolled steel sheet or strip of austenitic stainless steel through a hot rolling step, a cold rolling step and a final annealing step, the finish rolling temperature (finish rolling temperature ( T °) and the final pass rolling reduction (R%) within the range surrounded by the straight lines (a) to (e) in Fig. 1, provided that the straight lines (a) to (e) are defined by the following equations. Straight line (a): T = 1000 Straight line (b): T = 1125 Straight line (c): R = 3 Straight line (d): R = 30 Straight line (e): R = 0.4T−390 Cold rolling is performed by omitting the annealing treatment of the hot rolled sheet between the hot rolling step and the cold rolling step, and the cold rolled steel sheet of austenitic stainless steel with small plastic anisotropy is characterized. Alternatively, the present invention provides a method for manufacturing a steel strip.

In the usual method of hot-rolled sheet annealing, the metallographic factor of the hot-rolled sheet as hot-rolled is diluted by the annealing, so the degree of influence of the microstructure on the plastic anisotropy of the cold-rolled sheet is small, When hot-rolled sheet annealing is not performed, the plastic anisotropy must be reduced by appropriately controlling the metallographical factors of the hot-rolled sheet itself. The present inventors have conducted extensive test research to find hot rolling conditions effective for reducing plastic anisotropy for the above-mentioned purpose. As a result, they have found that the above-mentioned objects can be effectively achieved when the finish rolling rate during hot rolling, particularly the final pass reduction rate and the finishing temperature are appropriate.

The content of the present invention will be specifically described below with reference to representative examples of the experiments conducted by the present inventors.

FIG. 2 shows an examination of the earring rate when the hot rolling finishing temperature and the final pass reduction rate are changed. The sample steel is C: 0.07
2%, Si: 0.57%, Mn: 1.18%, P: 0.035%, S:
0.009%, Cr: 18.34%, Ni: 8.62%, Cu: 0.10.
%, Mo: 0.08%, N: 0.030% SUS304 steel plate thickness 20 mm
It is a thick steel plate. This was hot rolled under a hot rolling pass schedule as shown in FIG. In FIG. 3, R ₀
Corresponds to rough rolling by a rough rolling mill, and R ₁ and R ₂ represent reductions in the first pass and the second pass in the finish rolling mill. In the test, R ₀ = 50%, R ₁
= 30%, the final pass R ₂ was variously changed and the rolling temperature (finishing temperature) of the final pass was changed. Although the finishing temperature at R ₂ was changed from 925 ° C to 1150 ° C, the rolling temperature at R ₁ was about 25 ° C higher than this, and the cooling time at R ₂ was 4 seconds after rolling. After taking it, it was quenched. This is because when hot-rolled sheet annealing is not performed, the hot-rolled sheet is used so as not to deteriorate the surface properties in the descaling process such as pickling after hot-rolling (to suppress sensitization). This is because it is necessary to suppress the precipitation of grain boundary carbides. Each of the hot-rolled sheets obtained was cut to a thickness of 4.0 mm in order to align the cold-rolling rate, cold-rolled at room temperature to 1.0 mm without annealing the hot-rolled sheet, and then at 1150 ° C for 20 A final annealing for 2 seconds was performed and pickling was performed to obtain a cold rolled sheet. The cold-rolled sheet obtained has a blank diameter of 41 mm and a punch diameter.
A 21 mm conical cup test was used to draw, and the distances of peaks (H _max ) and valleys (H _min ) from the bottom of the cup were measured, and the earring rate was calculated according to the following formula.

FIG. 2 shows a summary of the results of the above test, and the following can be understood from this FIG. First, regarding the finishing temperature, the earring rate drops significantly in the high temperature range above 1000 ° C. In this case, the lower the final pass reduction rate (R ₂ reduction rate), the greater the reduction in the earring rate. More specifically, in order to obtain a level of 7% for the earring rate, at a finishing temperature of 1000 ° C, the final pass rolling reduction is 10% or less, and the finishing temperature is 1050 ° C.
Shows that the final pass rolling reduction must be 40% or less. The level of this earring rate of 7% is the earring rate when the hot rolled sheet of the same steel was annealed according to the conventional method, then cold rolled and finally annealed under the same conditions. Therefore, the results in Fig. 2 show that, if the finishing temperature and the final pass reduction ratio in hot rolling are appropriately set, even if the hot-rolled sheet annealing is omitted, the earring ratio lower than the earring ratio when not omitted is obtained. It shows that you can get. The details of the reason why these results were obtained are not always clear at this stage, but if the finishing temperature is increased and the final pass reduction ratio is decreased, hot-rolled sheet annealing is performed accordingly as the metal structure after hot rolling. It is thought that this is because the crystal grains become larger and soften as much as or more than the case.

As can be seen from the above test results, the plasticity anisotropy decreases as the final pass reduction rate during hot rolling of austenitic stainless steel decreases, but in the actual operation of the present invention. If the final pass reduction rate is less than 3%, it is practically difficult to control the reduction. Therefore, the final pass reduction rate needs to be 3% or more. On the other hand, the final pass reduction has a great influence on the shape control of the hot-rolled sheet, and if the reduction exceeds 30%, the shape deteriorates. Therefore, in order to achieve the object of the present invention in a practically advantageous manner, R ( _min ) =
It is necessary to select the final pass reduction ratio in the range of 3% and R ( _max ) = 30% in relation to the finishing temperature. If the finishing temperature is lower than 1000 ° C, the plastic anisotropy cannot be reduced to the level of 7% earring unless the final pass reduction rate is set too low. Although the plastic anisotropy can be reduced as the finishing temperature becomes higher, the heating temperature before rolling needs to be raised to a high temperature which is disadvantageous in the actual operation. The heating not only causes an increase in the heat intensity but also causes problems such as surface defects due to the oxide scale. Therefore, from the viewpoint of the earring rate, as shown in the results in Fig. 2, the final pass reduction rate at 1000 ° C is 10% or less, and the final pass reduction rate at 1050 ° C is 30% or less. Between ℃ and 1050 ℃, it is necessary to lower the final pass reduction ratio in proportion to the temperature drop. This relationship becomes the relationship shown in FIG. That is, when the horizontal axis represents the finishing degree T and the vertical axis represents the final pass reduction ratio R, hot rolling is performed at the finishing temperature and the final pass reduction ratio within the range surrounded by the straight lines (a) to (e). By doing so, the object of the present invention is effectively achieved. Here, straight lines (a) to (e)
For the above reasons, the straight line (a) has T = 1000, and the straight line (b) has T = 1000.
= 1125, the straight line (c) is R = 3, the straight line (d) is R = 30, and the straight line (e) is R = 0.4T-390.

In carrying out the present invention, other hot rolling conditions are not particularly limited as long as the finishing temperature and the final pass rolling reduction ratio satisfy the relationship shown in FIG. For example, when ordinary strip mill rolling is targeted, the heating temperature of the slab may be a generally employed temperature, for example, 1150 to 1260 ° C. The rolling reduction in the preceding pass other than the final stand in finish rolling is appropriately determined depending on the thickness of the slab and the number of rolling mill stands, and may be, for example, the usual 10 to 50%. The control of the winding temperature is not always indispensable for the achievement of the object of the present invention, but in order to avoid the problem of the surface property due to the generation of scale and the problem of the surface scratch at the time of winding, the temperature is, for example, 800 to 400 ° C. It is better to set the range. The hot-rolled coil after winding is descaled prior to cold rolling, which may be mechanical brushing or pickling. When performing descaling by pickling, it is especially desirable to wind in the temperature range of 400 to 600 ° C to prevent sensitization. In the subsequent cold rolling process, the final annealing is performed within the range of the rolling reduction of 40 to 95% to obtain the cold annealed sheet with reduced plastic anisotropy, which is the object of the present invention.

Example Various SUS304 whose chemical composition values (% by weight) are shown in Table 1
A continuous cast slab of equivalent steel was produced by a continuous casting machine. The slab size is 200 mm thick and 1040 mm wide. These 122
After heating at 0 to 1280 ℃, the plate thickness was 25 to 27 mm by rough rolling. Then, using a 6-stand tandem rolling mill, the reduction rate at each stand was reduced to the values shown in F1 to F6 in Table 2,
Hot-rolled at the finishing temperature (rolling temperature at F6) shown in Table 2, air-cooling for 3 to 5 seconds after finishing, water-cooling, and winding at the indicated winding temperature. Was manufactured. All hot-rolled sheets were descaled without annealing and cold-rolled at the indicated cold rolling rate to a sheet thickness of 1 mm.
Final annealing was performed by soaking at 50 ° C for 15 seconds and pickling was performed to obtain cold-rolled sheets. The earring rate of each cold rolled sheet was measured by the test method described in the previous text. The results are shown in Table 2.
Table 2 also shows the presence or absence of recrystallization of the hot-rolled sheet, the hardness of the hot-rolled sheet, and whether or not the relationship shown in FIG.

From the results in Table 2, the cold-rolled sheet obtained by hot rolling under the condition that the relationship of Fig. 1 is established has an earring rate of 7% or less even if the hot-rolled sheet annealing is omitted. It can be seen that the plastic anisotropy is reduced to the same degree as or higher than that in the case of annealing, and the hot-rolled sheet is sufficiently softened.

[Brief description of drawings]

FIG. 1 is a diagram showing the relationship between the finishing temperature and the final pass reduction ratio showing the range of hot rolling conditions for austenitic stainless steel according to the present invention, and FIG. 2 is the hot rolling finishing temperature and the final pass reduction ratio of austenitic stainless steel. Fig. 3 is an experimental result diagram showing the effect of the above on the earring rate of a cold rolled sheet obtained by omitting the hot rolled sheet annealing, and Fig. 3 is a hot rolling pass schedule diagram of the experiment of Fig. 2.

Claims (1)

[Claims] 1. When manufacturing a cold-rolled steel sheet or strip of austenitic stainless steel through a hot rolling step, a cold rolling step and a final annealing step, the finish rolling temperature in the hot rolling step is a finish rolling temperature. (T ° C) and final pass rolling reduction (R%)
Control within the range enclosed by (a) to (e), but with straight lines
(a) to (e) are expressed by the following formulas: straight line (a): T = 1000 straight line (b): T = 1125 straight line (c): R = 3 straight line (d): R = 30 Straight line (e): R = 0.4T-390 And, the cold rolling is performed by omitting the annealing treatment of the hot rolled sheet between the hot rolling step and the cold rolling step. A method for producing a cold-rolled steel sheet or strip of austenitic stainless steel having a low degree of orientation.