JPH06306464A - Production of hot rolled austenitic stainless steel plate - Google Patents

Abstract

PURPOSE:To efficiently produce a hot rolled stainless steel plate excellent in surface characteristics from a cast slab of austenitic stainless steel inferior in hot workability. CONSTITUTION:A molten austenitic stainless steel is cast continuously, and the resulting cast slab is heated up to 1200-1280 deg.C and successively subjected to hot roughing at 5-30% draft, by which a breakdown-rolled material is prepared. The surface of this breakdown-rolled material is ground by 0.5-5mm and then reheating is done up to 1200-1280 deg.C. Successively, hot rolling is done down to prescribed plate thickness. By this method, the high-quality hot rolled plate can stably be supplied.

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 austenitic stainless steel sheet having few surface defects.

[0002]

2. Description of the Related Art When austenitic stainless steel is smelted by using a large amount of inexpensive scrap for ferroalloy, impurity elements such as Pb, Zn, Bi, Sn, etc. increase and cracks occur on the surface during hot rolling. Occurs, and surface flaws that are rolled and bite the scale are likely to occur. For this reason, in the hot-rolled sheet having surface defects, it is necessary to remove the surface defects by belt grinding, pickling and lapping, which leads to a significant increase in manufacturing cost.

[0003]

In the conventional method for removing surface flaws by belt grinding of a hot rolled plate or pickling and lapping, the length of the coil is as long as several hundreds of meters, which requires a lot of time and efficiency. Is bad. In addition, in order to improve hot workability and reduce surface defects, scrap materials with limited impurity elements are used to reduce component elements such as Pb and Zn that adversely affect hot workability, and further refining is performed. It is necessary to remove these elements in the process, and there is a problem that the cost becomes high.

The present invention is a method for efficiently producing a hot-rolled steel sheet with few surface defects even when using a slab that is inferior in hot workability, such as a slab using an inexpensive scrap raw material. The purpose is to provide.

[0005]

[Means and Actions for Solving the Problems] In order to solve the above problems, the present inventors have investigated in detail the mechanism of generation of surface defects in hot rolling. As a result, the first of hot rolling
It was found that in the pass or the second pass, surface cracks that are the starting points of surface defects of the hot-rolled sheet were generated at the grain boundaries, and that surface cracks were less likely to occur after the third pass of hot rolling. . Further investigation was conducted on the reason why surface cracking is less likely to occur after the third pass of hot rolling, and the surface layer was recrystallized in the first and second passes of hot rolling to strengthen the grain boundary strength. It turned out to be

From these findings, in order to manufacture a hot-rolled sheet without surface flaws, first, a breakdown corresponding to the first pass and the second pass of hot rolling is applied to prevent surface cracking in hot rolling. Thus, the inventors have found a method in which the surface layer is recrystallized, the surface cracks subsequently generated during breakdown are removed, and grinding is carried out under the condition that a recrystallized region in which the grain boundary strength is strengthened can be secured, followed by hot rolling.

That is, (cracking depth) <(grinding amount) <
The essence of the technical idea of the present invention is to define the breakdown condition and the grinding condition that can satisfy the relationship of (recrystallization depth). That is, the gist of the present invention is that molten steel of austenitic stainless steel is continuously cast to produce a slab, and the slab is produced at 1200 ° C or higher and 1280 ° C.
Heat to the following range, and then the rolling reduction is 5% or more 30
% To produce a breakdown material by rough hot rolling, and subsequently the surface of the breakdown material is 0.5 mm.
The surface is ground within the range of 5 mm or less, and then 120
A method for producing hot-rolled austenitic stainless steel is characterized in that it is reheated to a range of 0 ° C. or higher and 1280 ° C. or lower and then hot-rolled to a predetermined plate thickness.

The reasons for limiting the constituent features of the present invention will be described in detail below. The surface crack depth and recrystallization depth that occur during breakdown are affected by the heating temperature during breakdown and the reduction rate of breakdown. FIG. 1 shows the effects of the heating temperature at breakdown and the rolling reduction at breakdown on the surface crack depth in an austenitic stainless steel in which impurity elements are not particularly reduced. The heating temperature has a great influence on the depth of surface cracks, and the depth of surface cracks rapidly increases as the heating temperature decreases. Particularly, when the heating temperature is lower than 1200 ° C., a severe crack of 5 mm or more occurs.

FIG. 2 shows the influence of the heating temperature at the time of breakdown and the reduction rate of the breakdown on the recrystallization depth in the austenitic stainless steel in which the impurity elements are not particularly reduced. The recrystallization depth is greatly affected by the rolling reduction, and the higher the rolling reduction, the larger the recrystallization depth. In particular, when the rolling reduction is 5% or more, the recrystallization depth increases to 5 mm or more.

Based on these findings, effective and efficient breakdown conditions and grinding conditions were examined. Table 1 summarizes the grounds for limiting the scope of the present invention. The reason for limiting the scope of the present invention will be described with reference to this table. (1) Breakdown Step Heating Temperature The step with the largest load in the present invention is the grinding step, and the smaller the grinding depth, the better. Therefore, considering the time and cost, the breakdown condition is set so that the grinding depth can be 5 mm or less. That is, the surface crack depth rapidly increases when the heating temperature is lowered, and when the heating temperature is lower than 1200 ° C., a large crack having a depth of 5 mm or more is generated. Therefore, the heating temperature of the present invention is set to 1200 ° C. or more.

The heating temperature in hot rolling is usually 1280 ° C.
In the present invention, the upper limit of the heating temperature is set to 1280 ° C. from the practical viewpoint. Reduction ratio If the reduction ratio of breakdown becomes large, the recrystallization depth becomes large, and if the reduction ratio becomes 5% or more, the recrystallization depth becomes 5 m.
m or more. That is, the recrystallization depth becomes deeper than the surface crack depth, and the recrystallized layer can be secured even after the surface cracks are removed by grinding. Therefore, the lower limit of the rolling reduction was set to 5%. Further, when the reduction rate is increased, the surface area of the intermediate slab is increased and the area of the surface grinding that is subsequently performed is increased. Therefore, the reduction rate is limited to 30% or less.

(2) Grinding Depth in Grinding Step In surface grinding, it is essential to secure a recrystallized surface layer while removing surface cracks in the intermediate slab after the breakdown. To meet this condition, 0.5 mm or more 5
Surface grinding of less than mm is required. Since the surface layer of this intermediate slab is recrystallized, the grain boundary strength is high, surface cracking does not occur in the subsequent hot rolling, and a hot rolled sheet having excellent surface properties can be obtained.

(3) Heating temperature in the hot rolling step Even in the hot rolling step, the hot workability is remarkably lowered in the temperature range lower than 1200 ° C. and remarkable surface cracking occurs, so that the heating temperature in the hot rolling is 1200 ° C. That's it. Further, from the practical viewpoint, the temperature is limited to 1280 ° C or lower.

[0014]

EXAMPLES Next, the effects of the present invention will be specifically described with reference to Examples and Comparative Examples. Tables 1 and 2 (continued from Table 1) show the chemical components and production conditions of the examples of the present invention and comparative examples. Each of the test materials was melted in an electric furnace, which is a normal refining process of stainless steel, refined by AOD, and then a cast piece was manufactured by continuous casting. Regarding the presence or absence of surface flaws on the hot-rolled sheet, the number of surface flaws per 1 m ² was visually measured after performing nitric hydrofluoric acid pickling for descaling.

In the example of the present invention, the surface flaw of the hot rolled plate is 0.3.
Month / m ² or less and while the small, and the breakdown process those processing conditions outside the range of the present invention of the comparative example, the one which does not perform the surface grinding process to hot-rolled plates 5 months / m ²
Since the above-mentioned surface flaws have occurred, it can be seen that the examples of the present invention are excellent.

[0016]

[Table 1]

[0017]

[Table 2]

[0018]

As described above, according to the present invention, it is possible to stably supply a high-quality general-purpose austenitic stainless steel material without surface defects, and therefore, the effect of the present invention on industry is Very noticeable.

[Brief description of drawings]

FIG. 1 is a diagram showing influences of heating temperature at breakdown and reduction rate at breakdown on surface crack depth in austenitic stainless steel in which impurity elements are not particularly reduced.

FIG. 2 is a diagram showing the influence of the heating temperature at breakdown and the reduction rate of breakdown on the recrystallization depth in an austenitic stainless steel in which the impurity elements are not particularly reduced.

 ─────────────────────────────────────────────────── ─── Continuation of the front page (72) Inventor Hiroshi Kimoto 1-1, Toibata-cho, Tobata-ku, Kitakyushu-shi, Fukuoka New Nippon Steel Co., Ltd. Yawata Works

Claims (1)

[Claims] 1. A slab is produced by continuously casting molten steel of austenitic stainless steel, and the slab is heated to a range of 1200 ° C. to 1280 ° C., and a rolling reduction of 5% to 30%. Producing a breakdown material by rough hot rolling in the range, subsequently surface grinding the surface of the breakdown material in the range of 0.5 mm or more and 5 mm or less, and subsequently reheating to the range of 1200 ° C. or more and 1280 ° C. or less Then, the method for producing an austenitic stainless steel hot-rolled sheet is characterized by comprising hot rolling to a predetermined sheet thickness.