KR100398398B1 - Method for hot rolling austenitic stainless steel - Google Patents

Abstract

The present invention relates to a hot rolling method of austenitic stainless steel, the purpose of which is to control the heating temperature and the ashing time in the furnace according to the delta ferrite content of the slab to reduce the amount of delta ferrite to prevent breakage or cracks and the surface It is to provide a hot rolling method that can improve the quality.

In order to achieve the above object, the present invention provides a method of heating and hot rolling a 300-series austenitic stainless steel slab, wherein the heating is performed by calculating the content of the delta ferrite of the steel slab by the Krupp formula. Content

(a) For less than 6%, heat 200-270 minutes at 1240-1300 ° C.,

(b) The technical gist of the hot rolling method of austenitic stainless steel comprising heating 250 to 270 minutes at 1240 to 1260 ° C for 6% or more.

Description

Hot rolling method of austenitic stainless steels {Method for hot rolling austenitic stainless steel}

The present invention relates to a hot rolling method of austenitic stainless steel, and more particularly to a hot rolling method that can reduce the amount of delta ferrite by controlling the heating temperature and the ashing time in the furnace according to the delta ferrite content of the slab. will be.

In general, austenitic stainless steel is a representative stainless steel that is widely used in various applications because of excellent corrosion resistance, workability, and weldability. Austenitic stainless steels contain large amounts of delta-ferrite in the slab state after continuous casting. Delta-ferrite contained in steel works effectively for ductility, but when it meets austenite as shown in FIG. (FIG. 2). Therefore, although the element design is designed so that the delta ferrite does not exist much in the slab, it is inevitable to design the element into the steel in which a large amount of delta ferrite is generated depending on the use.

When the delta-ferrite is contained in a large amount, the hot rolling is heated for 300 minutes at a high temperature of about 1270 to 1300 ° C, which is based on the fact that delta-ferrite is reduced when heated at a high temperature for a long time. In addition, the higher the heating temperature is, the higher the hot rolling property is. In Japanese Patent Laid-Open No. 10-17927, delta-ferrite is obtained by performing a cracking treatment for 1 to 2 hours at a heating temperature of 1230 to 1260 ° C, followed by two cracking treatments for 0.5 to 1 hour at 1300 to 1320 ° C. Has been proposed to reduce the However, hot rolling of the 300 series austenitic stainless steel under such conditions may result in inferior surface quality, and occurs intensively when the delta-ferrite content is high.

The present invention is based on the results of metallurgical analysis of the problem of surface quality inferior in 300 austenitic stainless steels, by reducing the content of delta-ferrite by controlling the reheating conditions before hot rolling according to the content of delta-ferrite The purpose is to secure the surface quality stably.

1 is a schematic diagram showing the behavior of delta-ferrite in austenitic stainless steel

2 shows plate breaking and both side crack photos in austenitic stainless steel.

3 is a graph showing the change of Cr and Ni in the delta-ferrite phase according to the heating temperature

4 is a graph showing the change of delta-ferrite in the slab according to the heating temperature

Hot rolling method of the present invention for achieving the above object, the step of calculating the content of the delta ferrite of the 300 series austenitic stainless steel slab by the Krupp formula;

The calculated delta-ferrite content is

(a) For less than 6%, heat 200-270 minutes at 1240-1300 ° C.,

(b) heating 250-270 minutes at 1240-1260 ° C. for at least 6%; And

And hot rolling the heated steel slab.

Hereinafter, the present invention will be described in detail.

In the present invention, among the austenitic stainless steels, 300 austenitic stainless steels called 301, 302, 303, and 304 are used as the target steel grades. Examples of such 300-based austenitic stainless steels include C: 0.08% or less, Cr: 18-20%, Ni: 8.0-15.0%, Si: 1.0% or less, Mn: 2.0% or less, and N: 0.8% or less. In addition, alloy elements such as Nb, Mo, and Ti may be added as necessary.

When the 300-based austenitic stainless steel is heated in a heating furnace and hot rolled, cracks may occur on both sides and the width of the plate because the slab contains a large amount of delta-ferrite. That is, when a large amount of delta-ferrite is contained, the delta-ferrite is not sufficiently separated in the heating furnace, but is connected to each other in a network and coarsened, so that the decomposition rate is decreased. Accordingly, the ductility difference between the austenite and ferrite interfaces is reduced. Therefore, cracks or plate breakage occur. Therefore, it is necessary to lower the content of delta-ferrite in the slab in the furnace as far as possible.

According to the present invention, the decomposition behavior of delta-ferrite in 300-type austenitic stainless steel is directly affected by the solid phase transformation by diffusion of Ni (austenite stabilizing element) and Cr (ferrite stabilizing element) in the delta-ferrite phase. The diffusion of these elements was confirmed to be sensitive to the heating temperature. As shown in Figure 3, when the heating temperature is a low temperature (1230 ~ 1250 ℃) Cr content decreases almost linearly, slightly increases at 1270 ℃ and rapidly increases at 1290 ℃. Ni is reversed.

In addition, in the present invention, as shown in Figure 4, while reducing the temperature of the delta-ferrite by the slab depth while varying the crack temperature in the 300 austenitic stainless steel heating furnace, the delta-ferrite rather than 1270 ℃ It was found that the content increased. From these results, it can be seen that the existing fact that the heating temperature is higher than 1270 ℃ is advantageous for the reduction of the delta-ferrite does not fit the 300 series austenitic stainless steel.

In the present invention, the degree of decomposition of the delta-ferrite when heating the 300 series austenitic stainless steel was confirmed to vary depending on the heating conditions according to the content, the classification of the delta-ferrite content is less than 6% or more? It was. That is, according to the present invention, when the content of delta-ferrite is less than 6%, 200-270 minutes is heated at 1240 to 1300 ° C., and at 6% or more, 250 to 270 minutes is heated at 1240 to 1260 ° C. If the delta-ferrite content is reduced to 6% or less in the steel slab, no crack occurs. Even though the delta-ferrite is contained within 6% or less in the steel slab, the delta-ferrite content increases in the slab depending on the heating conditions. do. Therefore, even if the content of the delta-ferrite is less than 6%, it is necessary to strictly control the heating conditions. Here, the heating temperature is 1240 DEG C or higher because the lower the temperature, the lower the deformation of hot deformation in finishing rolling is, the more difficult it is to secure the sheet. The upper limit temperature is set in consideration of the reduction of the delta-ferrite. The heating time of less than 250 minutes is sufficient to lower the delta-ferrite, and productivity is lowered for more than 270 minutes.

As described above, in the present invention, when re-heating and hot rolling a 300-based austenitic stainless steel slab, the content of delta-ferrite in the steel slab is determined, and heating conditions are selected according to the conditions. The content of delta-ferrite in steel slabs can be determined by various methods, but KRUPP's equation is most suitable. As a result of measuring the delta-ferrite content in the thickness direction by cutting the section of the slab produced by continuous casting, it can be seen that it is almost in agreement with the value calculated by the Krupp equation. The Krupp equation is shown in the following relation.

[Relationship 1]

According to the present invention as described above, in the steel slab before hot rolling, the content of delta-ferrite is less than 6% so that cracks or plate breakage do not occur.

Hereinafter, the present invention will be described in more detail with reference to Examples.

EXAMPLE

The changes in the content of delta-ferrite according to heating temperature and heating time were investigated for 300 austenitic stainless steel slabs of Table 1 below.

division Cr Ni Si Mn C N A 19.27 8.53 0.81 1.36 0.041 0.030 B 18.76 8.51 0.41 1.23 0.034 0.030 C 18.38 8.88 0.57 1.23 0.056 0.030 D 18.20 8.86 0.43 0.97 0.030 0.030 The cruising delta ferrite content of A, B, C, D steel grades is A-12%, B-9%, C-6%, D-4%

division Heating temperature (℃) % Delta-ferrite by depth from slab surface Heating time (minutes) 1 mm 2 mm 3 mm 4mm 5 mm 6 mm 7 mm 8 mm 9 mm A (12%) Before heating 8.0 10.1 11.2 9.5 10.02 13.5 12.5 11.8 13.7 200 1230 5.69 7.94 7.24 7.97 9.06 12.3 11.8 8.97 12.3 1250 6.85 7.78 9.27 9.03 8.59 13.7 14.2 12.3 13.3 1270 9.77 9.71 10.5 10.9 12.5 14.4 14.9 14.0 17.2 1290 11.7 11.1 11.9 12.2 15.5 16.1 11.6 15.3 19.6 B (9%) Before heating 4.05 5.25 6.20 7.00 6.39 9.05 7.60 5.25 7.35 1230 0.94 2.30 3.31 5.11 4.89 6.11 7.27 4.54 5.75 1250 1.10 3.00 3.27 5.56 6.38 7.23 7.50 5.53 6.52 1270 2.27 4.94 5.76 6.52 6.36 8.03 8.67 6.02 7.78 1290 5.15 6.99 6.92 8.29 9.41 9.53 11.1 8.88 12.6 C (6%) Before heating 4.02 5.80 5.92 7.00 8.20 9.50 9.98 7.50 7.8 1230 0.40 1.66 3.42 5.73 5.23 6.15 5.20 2.80 5.12 1250 0.53 2.58 3.39 4.66 5.88 6.90 6.25 5.21 8.76 1270 2.75 4.40 4.62 5.91 6.43 7.27 9.68 5.60 8.25 1290 5.79 6.14 6.94 7.47 8.19 9.48 9.05 9.41 8.34 D (4%) Before heating 0.8 1.8 1.4 1.85 2.6 2.7 2.9 2.5 4.5 1230 0.00 0.00 0.00 0.18 0.38 0.71 0.76 0.09 1.22 1250 0.00 0.00 0.00 0.09 0.42 0.55 0.91 1.23 1.38 1270 0.00 0.00 0.00 0.16 0.46 0.59 2.01 0.30 1.15 1290 0.26 0.00 0.00 0.21 0.87 0.88 1.23 1.39 2.86

division Heating temperature (℃) % Delta-ferrite by depth from slab surface Heating time (minutes) 1 mm 2 mm 3 mm 4mm 5 mm 6 mm 7 mm 8 mm 9 mm A (12%) Before heating 8.0 10.1 11.2 9.5 10.02 13.5 12.5 11.8 13.7 230 1230 5.52 7.50 8.66 6.83 8.83 11.5 8.45 9.01 11.3 1250 6.39 7.12 7.70 7.44 8.84 12.6 11.3 10.7 12.7 1270 8.62 8.53 9.04 9.02 9,08 9.41 12.3 14.4 14.1 B (9%) Before heating 4.05 5.25 6.20 7.00 6.39 9.05 7.60 5.25 7.35 1230 0.11 1.96 3.18 3.18 4.21 4.47 4.54 2.50 3.98 1250 0.84 2.23 3.15 5.05 4.40 5.74 6.09 4.44 4.50 1270 1.29 2.34 3.33 5.05 5.32 5.41 7.81 4.18 7.43 C (6%) Before heating 4.02 5.80 5.92 7.00 8.20 9.50 9.98 7.50 7.8 1230 0.12 0.81 1.83 2.68 3.96 5.95 7.15 2.29 3.46 1250 1.48 2.05 2.79 4.46 5.65 5.44 6.73 4.00 8.22 1270 2.16 3.68 3.97 5.13 6.13 6.38 6.07 6.60 7.53 D (43%) Before heating 0.8 1.8 1.4 1.85 2.6 2.7 2.9 2.5 4.5 1230 0.00 0.00 0.00 0.06 0.14 0.15 0.27 0.00 0.15 1250 0.00 0.00 0.00 0.04 0.24 0.39 1.83 0.00 0.24 1270 0.01 0.00 0.00 0.00 0.06 0.03 0.01 0.12 0.01 A (12%) Before heating 8.0 10.1 11.2 9.5 10.02 13.5 12.5 11.8 13.7 250 1230 3.31 3.25 5.66 4.83 5.73 5.61 4.58 5.31 5.63 1250 4.45 5.12 5.68 5.35 5.98 5.97 5.24 5.50 5.98 1270 7.56 7.48 8.24 9.29 8.46 11.38 12.0 11.5 13.5 B (9%) Before heating 4.05 5.25 6.20 7.00 6.39 9.05 7.60 5.25 7.35 1230 2.12 2.41 2.37 3.34 2.71 3.0 2.2 2.02 3.18 1250 2.5 2.45 2.39 4.10 3.8 5.01 4.5 4.3 5.24 1270 2.9 2.97 2.8 5.64 3.5 6.45 5.3 5.2 7.0 C (6%) Before heating 4.02 5.80 5.92 7.00 8.20 9.50 9.98 7.50 7.8 1230 1.25 2.47 2.48 3.6 3.71 4.85 4.88 3.4 3.87 1250 0.14 3.36 3.37 4.5 4.62 5.76 5.77 5.3 5.4 1270 1.13 4.47 4.28 5.45 5.51 6.87 5.92 7.74 7.3 D (4%) Before heating 0.8 1.8 1.4 1.85 2.6 2.7 2.9 2.5 4.5 1230 0.00 0.00 0.00 0.00 0.08 0.05 0.17 0.10 0.21 1250 0.00 0.00 0.00 0.00 0.10 0.09 0.25 0.00 0.35 1270 0.00 0.00 0.00 0.00 0.06 0.01 0.00 0.00 0.20

As shown in Table 2, when the content of the delta-ferrite calculated by the Krupp formula is 6% or more, by heating 250-270 minutes at 1240-1260 ° C., the content of the delta-ferrite may be reduced to 6% or less. When the content of delta-ferrite was less than 6%, it was found that the content of delta-ferrite in the material was not increased by heating 200-270 minutes at 1240-1300 ° C.

In the case of the material heat-treated according to the present invention in Table 2, the content of delta-ferrite was 6% or less, so that not only edge cracks but also surface cracking defects did not occur. On the contrary, in a material (3 mm thick hot rolled sheet) outside the conditions of the present invention, edge cracks were generated at a depth of about 30 mm at both edges of the coil, and surface cracking defects of 50 to 100 mm in length and width were also severely generated on the surface. It was not possible to commercialize.

As described above, according to the present invention, the content of delta-ferrite according to the steel composition is automatically measured by a calculation formula, and the content of delta-ferrite in the material is 6% by applying the heating conditions most suitable for the measured delta-ferrite. It lowers within to prevent the break of the plate during hot rolling, and suppresses the occurrence of cracks on the surface and the side with many deformation, thereby improving the productivity and improving the surface quality.

Claims (1)

In the method of heating and hot rolling a 300-based austenitic stainless steel slab, the heating, the content of the delta-ferrite calculated by calculating the content of the delta ferrite of the steel slab by (a) For less than 6%, heat 200-270 minutes at 1240-1300 ° C., (b) The hot rolling method of austenitic stainless steel which comprises heating 250-270 minutes at 1240-1260 degreeC at 6% or more.