KR20010047690A - Method for preventing crack from occuring on hot rolled coil of austenitic stainless steel with high delta-ferrite - Google Patents

Abstract

PURPOSE: A method is provided to prevent surface and edge crack by performing hot rolling process during the heating process and improving resolution of delta ferrite. CONSTITUTION: A method comprises the steps of heating the austenitic stainless steel slab at a temperature of 1150 to 1250 Deg.C for a time period of 0.7 minute x slab thickness to 1.8 minutes x slab thickness, wherein the stainless steel slab contains delta ferrite of 8 or higher and the calculation result of Cr + Mo + 1.5 x Si is 22 or higher from among the steel ingredient; hot rolling the stainless steel slab at a draft percentage of 5 to 50; and heating and hot rolling the stainless steel slab in the above-described process. Thus, hot rolled coil with a superior surface quality having no edge or surface crack is obtained.

Description

Method for preventing crack from occur on hot rolled coil of austenitic stainless steel with high delta-ferrite}

The present invention relates to a method for producing high delta-ferrite-containing austenitic stainless steel, and more particularly, to surface cracking defects and edge cracks of austenitic stainless steel, which contains a large amount of delta-ferrite and is poor in hot workability. It relates to a hot rolling method for preventing.

Most austenitic stainless steels solidified with primary delta-ferrite contain large amounts of delta-ferrite in the slab state after continuous casting. The amount of delta-ferrite is related to the content of alloying elements, and most of the austenitic stainless steels have a high content of alloying elements.

When austenite stainless steel contains an appropriate amount of delta-ferrite in the range of 6.0 to 8.0, recrystallization during hot rolling is promoted to improve hot workability. However, when the amount of delta-ferrite is excessively less than 5, there is little delta-ferrite / austenite interface which acts as a recrystallization nucleation site during hot rolling. In this case, a fine crack-shaped sliver flaw is generated on the surface of the hot rolled coil, but if the flaw is removed by coil grinding, cold rolling is possible. . However, if the excessive amount of delta-ferrite is more than 8, cracks may be generated at the delta-ferrite / austenite interface due to the difference in hot workability between the delta-ferrite and austenite phase, resulting in severe crack surface defects on the edge cracks and surfaces. This causes serious problems in operation and quality.

Conventionally, hot-rolled coils have been manufactured by hot rolling-rolling (crude-thin rolling) -winding in the same state as in the case of a large number of delta-ferrites of 8 or more. In this case, a large amount of surface cracks and edge cracks were generated, resulting in excessive side trimming and coil grinding in order to remove them. In addition, when the edge crack is severe, there was a problem that the edge crack portion hits the side guide during rolling, causing debris to splash into the coil and rolled, which is not removed by the coil grinding.

In order to remedy this problem, a method of decomposing delta-ferrite and rolling after prolonging the heating time by about 50 longer than the normal heating time (heating for about 3 hours in the temperature section of 1210-1300 ° C) was performed. In this case, there was some effect in the 304 series of alloys with relatively small amount of alloys, but steels with high amounts of alloys and high delta-ferrite have insufficient decomposition of delta-ferrite, resulting in cracking defects on the surface of the steel sheet and at the coil edges. There is a problem that an edge crack occurs.

As another method, Japanese Patent Application Laid-open No. Hei 2-54404 discloses that austenitic steel containing 10 or more delta-ferrite in the casting state is processed at a reduction ratio of 5 to 80 in a hot pretreatment prior to rolling, and is subjected to a 1000 to 1300 ° C. After heat treatment for 5 to 30 hours in the temperature range has been proposed a method of heating and hot rolling it normally. Although this method can prevent cracking to some extent, it takes a long time for cracking and the like, which is not suitable for industrial methods.

The present invention aims to produce a high delta-ferrite-containing austenitic stainless steel having an excellent surface quality with no surface cracking defects and edge cracks even with a shorter heating time.

1 is a graph showing the change of the content of delta-ferrite with heating time in austenitic stainless steel

2 is a graph showing an example of a temperature rising curve according to heating time and heating temperature.

3 is a graph showing the decomposition amount of delta-ferrite according to the heating temperature

The present invention for achieving the above object, 0.7 minutes × slab thickness of the austenitic stainless steel slab having a delta ferrite content of 8 or more, and the sum of Cr + Mo + 1.5 × Si in the steel component is 22 or more at 1150 to 1250 ° C And heating for a time of ˜1.8 minutes by slab thickness, hot rolling at a reduction ratio of 5 to 50, then heating under the above conditions, and hot rolling in a conventional manner.

Hereinafter, the present invention will be described in detail.

As can be seen in Table 1, the austenitic stainless steel has a higher content of delta-ferrite, which leads to serious problems of edge cracks and surface cracking defects.

Steel grade Calculated amount of delta-ferrite () Chemical composition (weight) Edge crack occurrence degree Surface Crack Defect Cr Ni Si STS309S 5.0 22.2 13.5 0.6 Outbreak Surface 6.5 22.7 14.4 0.6 Outbreak Surface 6.9 22.5 12.7 0.5 Outbreak Surface 7.5 22.6 12.6 0.5 Slight occurrence Minor crack defects occur within 200mm of edge 8.2 22.6 12.7 0.5 Minor Occurrence (Partial 10mm) Crack defect bundle within 300mm 8.9 22.6 12.7 0.5 10mm on both sides Large crack defects 10.5 22.6 13.7 0.5 10mm on both sides Large crack defects

The delta-ferrite content in austenitic stainless steels can be estimated by various formulas. In the present invention, the following KRUPP formula is recommended. The section of the slab produced by continuous casting was cut and the delta-ferrite content was measured in the thickness direction. Table 1 also calculates the delta-ferrite content by the Krupp formula.

[Relationship]

In the present invention, the delta-ferrite content is 8 or more, and in particular, the sum of Cr + Mo + 1.5 x Si is 22 or more steel. Even if the delta-ferrite content is 8 or more, if the sum of Cr + Mo + 1.5 × Si is less than 22, the delta-ferrite is easily decomposed if heated for a long time before hot rolling. A representative example is the STS304 steel grade.

Cr, Mo, and Si are ferrite stabilizing elements. If the content of these components is high, the decomposition of delta-ferrite is insufficient even after heating, and the delta-ferrite content similar to the initial state is maintained, so the sum of Cr + Mo + 1.5 × Si is 22 or more. Steel is treated according to the invention. Figure 1 shows the residual delta-ferrite amount according to the re-heating time when the steel grades shown in Table 2 below at 1250 ℃, showing the effect of the Cr + Mo + 1.5 × Si well.

division Delta-ferrite content measured in slab state () Cr + Mo + 1.5 × Si C Cr Ni Si STS304 0.05 18.3 8.3 0.5 7 STS XM151J1 0.05 19.4 13.3 3.4 6.7 (13.3) 24.6 309S 0.06 22.2 13.5 0.5 9.3 (8.9) 23.3 () Is the content of delta-ferrite calculated by the Krupp formula.

As can be seen in Figure 1, in the case of STS304 steel delta-ferrite is greatly reduced as the length of time is longer, but in the case of STSXM15J1, STS309S steel that the sum of Cr + Mo + 1.5 × Si is 22 or more delta-ferrite It can be seen that the decrease of R is relatively small.

Therefore, in the present invention, the case where the content of delta-ferrite calculated by the Krupp formula is 8 or more and the sum of Cr + Mo + 1.5 × Si is 22 or more is used as the target steel grade of the present invention.

In the present invention, the austenitic stainless steel having the above conditions is heat-treated before hot rolling, and then heat-treated under the same conditions and hot-rolled in a conventional manner. In the casting state, since the gradient of the component concentration is large between the delta-ferrite and austenite phase, the decomposition rate of the delta-ferrite is very fast as shown in FIG. Therefore, in order to accelerate the decomposition of the delta-ferrite, it is very effective to heat treatment immediately after casting, thereby greatly reducing the time for pretreatment.

The temperature of the heat treatment should be determined taking into account the decomposition of delta-ferrite in the slab, segregation, aging and final rolling temperature during slab heating. The temperature to be heated may be different depending on the equipment characteristics of each hot rolling, but heating to 1150 to 1250 ° C. is appropriate for decomposition of delta-ferrite by heating. If the heating temperature is higher than 1250 ℃, the amount of delta-ferrite is increased because of the delta-ferrite stable zone in the state diagram. If the heating temperature is lower than 1150 ℃, the decomposition rate of delta-ferrite is low due to the austenite stable zone or temperature in the state diagram. It is not suitable late. Figure 2 shows the amount of delta-ferrite in the steel when heated STS304 steel 210 minutes by heating temperature. As shown in Figure 2, if the heating temperature is excessively low or less than 1150 ℃ excessively high or more than 1300 ℃ insufficient decomposition, in particular it can be seen that the increase rather than 1300 ℃.

The longer the heating time is, the more favorable for the decomposition of delta-ferrite, but the longer the heating time, the more severe grain boundary oxidation of the slab surface is likely to cause oxidative defects. As it gets longer, the decomposition rate becomes slower. Considering this, the upper limit of the heating time is slab thickness x 1.8. In addition, heating time should be heated more than slab thickness × 0.7 minutes to ensure minimum cracking temperature for hot rolling. 2 shows a heating curve according to heating time and heating temperature for reference.

After heating as above, hot rolling at a reduction ratio of 5 to 50 and heating under the same conditions again, are as follows.

The slab is charged into a furnace to decompose the delta-ferrite in the slab upon heating. Delta-ferrite has a higher decomposition temperature (approximately 1250 ° C) in the austenite stable zone in the state diagram, and the longer the heating time, the better the decomposition of delta-ferrite. However, the longer heating time does not mean that delta-ferrite continues to decompose indefinitely. In the initial stage of heating, delta-ferrite is rapidly decomposed, but as time passes, the decomposition rate is slowed down, so that little decomposition occurs later (FIG. 1). As shown in FIG. 1, when the rework time exceeds about 200 minutes, the decomposition of the delta-ferrite hardly occurs even if the rework time is longer regardless of the steel type. In this state, even if the slab is cooled and then heated again, the decomposition rate hardly increases. Therefore, the driving force for delta-ferrite decomposition is required for the further delta-ferrite decomposition, in which rolling is most effective. Rolling the slab stretches the tissue and changes the delta-ferrite and austenite interface, which in turn promotes the decomposition of delta-ferrite again upon reheating.

Table 3 below shows the decomposition rate of delta-ferrite according to the change of the intermediate rolling reduction rate when heating-rolling-reheating STS309S steel containing delta-ferrite 8.7.

Steel grade The amount of delta-ferrite after casting Slab thickness (mm) Primary heat treatment condition Rolling rate of intermediate rolling Secondary heat treatment condition Cooling condition Amount of delta ferrite after rolling () 309S 8.7 200 1250 ℃ × 10 minutes 4 (192 mm) 1255 ℃ × 191 minutes Water cooling 4.1 8 (184 mm) 3.5 14 (172 mm) 2.3 () Is the slab thickness after intermediate rolling

As can be seen in Table 3, it can be seen that the decomposition rate of delta-ferrite increases when reheating after heating-rolling.

This heating, hot rolling and then reheating are effective for the decomposition of the delta-ferrite, wherein the hot rolling is carried out at a reduction ratio of 5 to 50. As the reduction ratio increases, the decomposition rate of delta-ferrite increases, so the reduction ratio should be at least 5, but if the reduction ratio is higher than 50, cracks start at the interface of delta-ferrite and austenite, and thus the reduction ratio cannot be increased indefinitely. .

In this invention, it consists of the process of hot-rolling by a heating-hot rolling-heating-normal method, These processes can be performed as follows in a normal hot rolling facility. The slab is heated in accordance with the present invention and hot rolled at a rolling reduction ratio of 5 to 50 in the rough rolling mill of the hot rolling mill, and then cooled and heated again to hot roll the slab to a predetermined thickness in the rough rolling mill and the finishing mill of the hot rolling mill. will be.

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

EXAMPLE

The surface quality state according to the conventional hot rolling method and the hot rolling method of the present invention is investigated for slabs having a thickness of 8.9 mm and a thickness of 200 mm of STS309S steel whose Cr + Mo + 1.5 × Si is 23.3. saw.

Conventional Method

The slab was heated to a hot rolled sheet having a thickness of 4 mm after heating for 229 minutes at a cracking temperature of 1250 ° C.

(Method of the present invention)

The slab was first heated at a cracking temperature of 1255 ° C. for 209 minutes, then rolled to 14 (slab thickness 200 → 172) in two steps and air cooled, then reloaded and re- charged at 1259 ° C. for 204 minutes. After heating, it was rolled into a 4 mm hot rolled sheet.

division Delta Ferrite Amount Pretreatment Condition Before Rolling Furnace Condition (Cracking Temperature × Ashing Time) Hot rolling result Product Availability Conventional example 8.9 Not carried 1250 ° C × 229 minutes 10mm edge crack occurs at the coil side and 50-100mm crack defects occur at the coil surface. Impossible Inventive Example Primary crack: 1255 ° C × 209 minutes Middle rolling ratio: 14 (200 → 172mm) 1259 ° C × 204 minutes No edge cracks or surface crack defects possible

As can be seen from Table 4, when the conventional example is rolled with a hot-rolled sheet, edge cracks are generated at about 10 mm deep at both corners of the coil, and surface cracking defects of 50 to 100 mm in length and width are also severely generated on the surface. It was not possible to commercialize. By the way, in the case of the present invention it was possible to obtain a hot rolled coil of good quality without the occurrence of surface cracking defects.

Example 2

The conventional hot rolling method and the present invention of a slab having a thickness of 200 mm of STSXM15J1 steel grade (19.4Cr-13.3Ni-3.4Si) having a delta ferrite amount of 13.3 and Cr + Mo + 1.5 × Si is 24.6 The surface quality condition of hot rolling was investigated.

Conventional Method

The slabs were rolled into a hot rolled sheet having a thickness of 4 mm after 197 minutes of heating at a crack temperature of 1249 ° C.

(Method of the present invention)

The slab was first heated at a cracking temperature of 1251 ° C. for 188 minutes, then rolled to 15 (slab thickness 200 → 170 mm) in two steps, air-cooled, and then re-loaded into the heating furnace for 219 minutes at 1250 ° C. After the secondary heating for 4 mm hot rolled plate was rolled.

division Delta Ferrite Amount Pretreatment Condition Before Rolling Furnace Condition (Cracking Temperature × Ashing Time) Hot rolling result Product Availability Conventional example 13.3 Not carried 1249 ℃ × 197 minutes Severe crack defect occurs within 200mm of coil side (no edge crack) Impossible Inventive Example 1st crack: 1251 ℃ × 188 minutes Intermediate rolling rolling rate: 15 (200 → 170mm) 1250 ° C × 219 minutes No edge cracks or surface crack defects possible

As described above, the present invention provides a hot rolled coil having a good surface quality with little cracking and edge crack and surface cracking defects by almost no operation load by rolling after delta ferrite decomposition by primary heating-medium rolling-secondary heating-rolling. You can get it. Therefore, through the present invention, the delta-ferrite loading is high while increasing the real error rate of the steel species having a high content of ferrite stabilizing elements, and has a useful effect of improving the coil grinding treatment error rate from 100 to 5 or less.

Claims (1)

An austenitic stainless steel slab having a delta ferrite content of 8 or more and a sum of Cr + Mo + 1.5 x Si of 22 or more is heated at 1150 to 1250 ° C for 0.7 minutes x slab thickness to 1.8 minutes x slab thickness. And hot rolling at a reduction ratio of 5 to 50, followed by heating under the above conditions and hot rolling in a conventional manner. 2. A method of preventing crack defects in a high delta-ferrite-containing austenitic stainless steel hot rolled coil.