JPH0687054A - Production of stainless steel cast slab - Google Patents

Abstract

PURPOSE:To reduce the surface defect after hot-working by controlling cooling speed of a cast slab, restraining the deposition of carbide and reducing unevenly remained oxide scale while preventing precendent oxidation in the grain boundary in the cast slab, at the time of continuously casting a stainless steel. CONSTITUTION:In the austenitic stainless steel, the continuously cast slab is cooled to the temp. range of >=800 deg.C the surface temp. as the curve 4 in the figure. In the ferritic stainless steel, the continuously cast slab is cooled to the temp. range of >=850 deg.C the surface temp. as the curve 3. These stainless are cooled at >=50 deg.C/min cooling rate from these temp. range.

Description

Detailed Description of the Invention

[0001]

FIELD OF THE INVENTION The present invention relates to a method for manufacturing a stainless steel slab.

[0002]

2. Description of the Related Art With the recent rapid development of continuous casting process, a process for subjecting continuously cast slabs to hot rolling or hot rolling and cold rolling has become common even for stainless steel sheets and the like. Stainless steel sheets are often used for building materials and the like because of their beautiful surfaces, and the required level of surface quality is particularly high. However, a non-uniform oxide film is generated during solution treatment performed for hot rolling, and this oxide film remains on the product, or unevenness occurs during pickling due to this oxide film Had surface defects. Therefore, the plate surface must be ground with a grinder or the like using a hot-rolled plate or a cold-rolled plate, which causes a significant increase in cost.

Therefore, as disclosed in Japanese Patent Application Laid-Open No. 2-228422, a method has been developed in which the primary scale is peeled off and removed by quenching the slab. This method is 1
The slab is water-cooled from a temperature of 00 ° C to 800 ° C, and the difference in expansion coefficient promotes peeling of the scale. However, in such a method, unless cooling in a specific temperature range at a specific cooling rate, carbides precipitate during cooling, resulting in uneven oxidation in the heating furnace in the subsequent step, which promotes product defects. there were.

[0004]

DISCLOSURE OF THE INVENTION The present inventors conducted a detailed investigation on the cause of the formation of a non-uniform oxide film which occurs in a hot rolling furnace. As a result, in the non-uniform oxide film, a (Cr, Fe) -C-based precipitate is generated at the grain boundary when the slab is cooled, and a Cr-deficient layer is formed in the vicinity of the precipitate, thereby lowering the oxidation resistance, It was concluded that grain boundary preferential oxidation is promoted. The present invention proposes a condition for improving the cooling method of a slab and suppressing carbide precipitation in order to prevent uneven oxidation.

[0005]

[Means for Solving the Problems] Precipitation of carbides starts from 800 ° C. in austenitic stainless steel and from 850 ° C. to 900 ° C. in ferritic stainless steel. In the present invention, in order to suppress this phenomenon, the carbide precipitation temperature range is rapidly cooled. That is, as shown in FIG. 1, 400 ° C. to 8 ° C. for austenitic stainless steel.
Since there is a region of carbide precipitation curve 2 at 00 ° C. and a region of carbide precipitation curve 1 in the temperature range of 400 ° C. to 850 ° C. in ferritic stainless steel, this region is rapidly cooled. A higher cooling rate at this time is more advantageous for suppressing the precipitation of carbides, but a sufficient effect can be obtained by cooling the surface temperature of the slab at a rate of 50 ° C./min or more.

[0006]

The method of the present invention is intended to prevent the precipitation of carbides by rapidly cooling the slab from the above temperature range. In the usual continuous casting method of stainless steel, the temperature of the slab after cutting the slab with a torch cutter is 600 to 800 ° C on the surface. When air-cooled from this temperature range, carbide precipitation occurs as shown in FIG. 1, preferential oxidation occurs in the Cr-deficient layer, and it becomes a surface defect of the product. Therefore, the precipitation of carbides is prevented by cooling the surface layer of the slab in the continuous casting machine so as to have a temperature pattern as shown in FIG. Curve 3 shows the temperature pattern of the example of ferritic stainless steel, curve 4 shows the surface temperature pattern of the slab of the example of austenitic stainless steel, and curve 5 shows the temperature pattern of the comparative example of ferritic stainless steel. . As shown in FIG. 1, carbide precipitation rate is low in austenitic stainless steel and precipitation start temperature is low at about 800 ° C., but carbide precipitation rate is high and precipitation start temperature is 850 in ferritic stainless steel.
It is as high as ℃. Therefore, in the ferritic stainless steel, as shown by the curve 3 in FIG. 2, it is necessary to gradually cool the secondary cooling in the continuous casting machine to secure the surface temperature of the slab at 850 ° C. or higher. Then, the slab is immersed in the water tank as quickly as possible to quench the carbide precipitation temperature range. This cooling rate is preferably as high as possible from the viewpoint of suppressing the precipitation of carbides, but according to experiments, the surface temperature of the slab is 50 ° C / m.
It was found that a sufficient effect can be obtained by cooling at a cooling rate of in or higher.

[0007]

EXAMPLE In order to confirm the effect of the method of the present invention, an experiment was conducted in which austenitic and ferritic stainless slabs were cast, slabs with different cooling methods were rolled under the same hot rolling conditions, and surface defects were investigated after pickling. went. The experimental conditions are summarized in Table 1. Austenitic stainless steel was evaluated by SUS304, and ferritic stainless steel was evaluated by SUS430. Rolling is performed by heating the slab to 1200 ° C in a heating furnace, soaking for 2 hours, and then 200 mm.
Rolled from thick to 4 mm thick.

FIG. 3 shows the number of surface defects of the sheets after hot rolling and pickling of the cast slabs of Examples and Comparative Examples shown in Table 1 collectively as an index. The experimental condition numbers in FIG. 3 correspond to the numbers in Table 1, respectively. It can be seen that the method of the present invention significantly reduces the number of defects in both austenite and ferrite.

[0009]

[Table 1]

[0010]

According to the method of the present invention, the cooling rate of the slab is controlled to suppress the precipitation of carbides, and the grain boundary preferential oxidation of the slab is prevented. For this reason, it is possible to reduce unevenly remaining oxide scale, reduce surface defects after hot working, improve product quality, and reduce costs by reducing surface maintenance after hot working. It was Furthermore, by increasing the cooling rate of the slab more than usual, the time until the start of hot working can be shortened, and the inventory can be reduced.

[Brief description of drawings]

FIG. 1 is a continuous cooling curve showing the start of precipitation of chromium carbide during isothermal cooling of ferritic and austenitic stainless steels.

FIG. 2 is a schematic diagram showing a transition of a surface temperature of a slab due to a difference in a slab cooling method.

FIG. 3 is a bar graph showing surface defects of the hot rolled sheet during each experiment described in Examples.

[Explanation of symbols]

1 Carbide precipitation curve of ferritic stainless steel 2 Carbide precipitation curve of austenitic stainless steel 3 Cast surface temperature transition curve of ferritic stainless steel (Example) 4 Cast surface temperature transition curve of austenitic stainless steel (Example) 5 Surface temperature curve of slab of ferritic stainless steel (comparative example)

 ─────────────────────────────────────────────────── ─── Continuation of the front page (72) Kenichi Sorimachi, 1 Kawasaki-cho, Chuo-ku, Chiba City, Technical Research Division, Kawasaki Steel Co., Ltd. (72) Haruhiko Ishizuka, 1 Kawasaki-cho, Chuo-ku, Chiba Kawasaki Steel Co., Ltd. Chiba Inside the steel mill

Claims (2)

[Claims] 1. When producing a slab by continuously casting austenitic stainless steel, the slab has a surface temperature of 8
Cool to a temperature range over 00 ° C, and then 50 ° C
A method for producing an austenitic stainless steel slab, characterized by cooling at a cooling rate of not less than / min. 2. When a slab is manufactured by continuously casting ferritic stainless steel, the slab has a surface temperature of 850.
Cooling to a temperature range of ℃ or more, 50 ℃ / mi from the temperature range
A method for producing a ferritic stainless steel slab, which comprises cooling at a cooling rate of n or more.