KR101279246B1 - method of manufacturing austenitic stainless steel with good surface quality - Google Patents

Abstract

The present invention is a continuous process for producing austenitic stainless steel containing 10% to 30% chromium and 5 to 15% nickel by weight in a steelmaking process, a continuous casting process, a hot rolling process and a hot-rolled annealing process. In the casting process, the cut slab is cooled to 600 ° C or less in air, and then deposited in a water cooling tank for 5 minutes to 10 minutes to remove the remaining scale of the slab.The slab is 300 mm on both sides of the slab on a slab using a slab grinding machine. The present invention is directed to a method for producing austenitic stainless steel having excellent surface quality, which is characterized by removing the remaining scale at the center by grinding portions except inside and outside.

Austenitic, Stainless steel, Cooling, Grinding, Rough surface, Wire flaw

Description

Method for manufacturing austenitic stainless steel with good surface quality

The present invention relates to a method for producing austenitic stainless steel having excellent surface quality, and more particularly, cast cooling process and cast casting conditions for cooling the cast from the continuous casting process in order to reduce the line defects and surface roughness defects of hot rolled products The present invention relates to a method for producing austenitic stainless steel having excellent surface quality by appropriately controlling the defects and reducing the surface defects and surface roughness defects of the hot rolled product.

In general, stainless steel hot rolled products produce slabs, which are intermediate materials in steelmaking and performance factories, and reheat slabs in hot rolled mills and roll them to produce black coils. Produces white coil, a stainless steel hot rolled product.

At this time, if the scale of the mold slag component remaining on the surface of the cast slab cannot be removed in the hot rolling process, irregularities are formed on the surface of the hot rolled coil, causing roughness of the surface and cooling the slab to remove the mold slag component. Inadequate cast grinding results in poor surface quality, such as wire defects.

In order to reduce surface roughness defects remaining on the hot rolled coil surface, a method known in the art will be described. First, in order to remove the mold slag layer scale which causes surface roughening, a method of spraying the coolant before cutting the slab or a method of spraying the coolant after the slab cutting is known in the playing process. In addition, a method of immersing the cast in a water-cooled tank for more than 10 minutes immediately after cutting the slab, the method of removing the scale by using a Shot Blaster on the casting cast, or the grinding of the cast cast.

In addition, conventional methods for reducing line defects include changing grinding conditions such as grinding angle, grinding interval, and grinding pressure, controlling slab temperature to 300 degrees or more when charging a hot-rolling furnace, and cooling the cast steel in air. How to do is reported.

However, when these two defects reduce one side, the other side is stretched, so most of the methods specialized on one side are rather raising the other side, and an appropriate method for effectively reducing both defects together is not known.

The present invention has been invented in accordance with the above requirements, and an object of the present invention is to provide a manufacturing condition for reducing surface roughness defects and linear flaw defects in hot rolled products when manufacturing austenitic stainless steels. That is, the present invention is to provide a method for producing austenitic stainless steel having excellent surface quality by reducing the surface roughness and line defects of the hot rolled products by controlling the casting cooling method, cast grinding method of the cast after continuous casting.

The present invention, in order to achieve the above object, by cooling the cast slab completed by a continuous casting process of austenitic stainless steel containing 10% to 30% chromium and 5 to 15% nickel by weight to less than 600 degrees in air This cast is deposited in a water cooling bath for 5 to 10 minutes to provide a method for producing austenitic stainless steel having excellent surface quality to desorb the residual scale of the mold slag layer.

In addition, the present invention provides a method for producing austenitic stainless steel having excellent surface quality to remove the residual scale by grinding only the center portion of the cast steel in the cast steel straightening process except for the side of the upper and lower surfaces of the cast steel. to provide.

In addition, in the present invention, the side surface of the lower face, preferably 300 mm or less.

As described above, according to the present invention, by controlling the cast cooling method and the slab grinding method of the performance casting casts, it is possible to produce the grinding cast of austenitic stainless steel with excellent surface quality with reduced line defects and surface roughness defects. have.

Hereinafter, the present invention will be described in more detail with reference to the drawings and tables.

First, a general stainless steel manufacturing plant and a unit process will be described, and then a process related to the present invention will be described separately.

In the manufacturing process of stainless steel, slab cast steel is produced by correcting the molten steel produced in the steelmaking process through a playing solidification process and a cooling process. Subsequently, black coils are made through charging, reheating, and winding processes after charging and reheating the hot rolling furnace. Since black coil (Black Coil) is subjected to annealing and mechanical / chemical scale removal process in a hot rolled annealing plant to manufacture a white coil (White Coil) of a stainless steel hot rolled coil.

The present invention relates to the manufacturing conditions of the cooling step and the correction step of the cast from the machine. As described in the above stainless steel manufacturing process, if the cast slag from the machine is left with a large amount of mold slag layer scale on the cast before charging, or if the strength and uniformity of descaling cannot be secured during hot rolling, Coarse defects are common, and failure to perform proper slab cooling and correction methods to remove mold slag results in frequent ship defects. 1 and 2 show surface defects related to such roughness defects and linear defects.

In the present invention, in order to reduce the linear scratches and surface roughness defects on the surface of the stainless steel austenitic hot rolled products as shown in Figure 1 and 2, to provide an effective method of detaching the mold slag layer scale remaining in the casting cast. I will propose through the process of cooling and grinding the cutting cast pieces.

In the conventional method, about 800 degrees of the slab cut into the cast slag remaining on the cast slab is deposited in a water cooling tank for 10 minutes or more, and the scale is detached and boiled due to the difference in coefficient of thermal expansion during cooling in the slab surface layer. It was possible to reduce the surface roughing defects by maximizing the scale removal effect.

However, sometimes, when the water-cooling bath is deposited for 10 minutes or more, boiling forms a steam layer on the lower surface of the cast steel, and bending of the cast steel occurs due to the difference in cooling rate between the lower and lower surfaces. There was this difficult issue. Therefore, the water cooling tank immersion time should be carried out for at least 20 minutes and 30 minutes at the proper time to prevent the cast bending. In addition, cracks developed inside the reheating furnace due to fine cracks and residual stresses in the surface of the cast slab due to thermal shock, and caused linear defects. However, simple cooling water spraying or water cooling deposition of less than 10 minutes did not significantly reduce the surface roughness.

In addition, when the surface was subjected to full grinding in order to remove the scale on which the mold slag adhered after air cooling in air, cracks were generated due to the grinding to cause linear defects. Fig. 3 shows cracks generated by such grinding in preparation for ungrinding.

Hereinafter, embodiments of the present invention will be described in detail.

In the present invention, after the slab is cut by the primary desorption method of the mold slag scale remaining on the cast slab, the slab is cooled to about 600 degrees or less, followed by water cooling for 5 to 10 minutes. As described above, in the case of performing the cooling of the slab, the bending of the slab, which is a problem in the conventional method, does not occur at a low temperature of 600 degrees or less and also in a short time.

However, the scale desorption and boiling due to the difference in thermal expansion coefficient is reduced, so that the overall scale peeling occurs weakly. In particular, the scale of the center of the cast steel is very dense composition because of less contact with air during air cooling. Therefore, in order to eliminate the dense scale of this center part, slab grinding is performed.

However, the grinding of cast steels is to be carried out in consideration of the following line defects.

The linear flaw is a defect that occurs when the crack cracks due to the difference in tensile stress between the center and the side of the hot rolled coil during hot rolling. It is mainly caused within 300 mm of the upper and lower sides of the coil. Therefore, cast grinding should be performed only at the center except 300mm on both sides to eliminate dense scale at the center without making microcracks in 300mm on both sides. Figure 4 shows that only the central portion is ground except for the 300mm on both sides of the cast steel.

As such, the cast is cooled in air and deposited in a water cooling tank at 600 ° C. or less for 5 to 10 minutes to detach the scale in 300 mm on both sides of the non-dense scale, and to reduce the possibility of fine cracks due to thermal shock, thereby reducing the possibility of linear defects. If the cooling is more than 600 degrees, a large amount of fine cracks are generated due to the rapid thermal shock at high temperatures. However, in order to obtain a cooling effect, it is desirable to be maintained at 500 degrees or more. If the temperature falls below 500 degrees, there is a problem in that the scale peeling phenomenon due to the cooling of the water cooling pool becomes weak, which is disadvantageous to surface roughness.

In addition, when the deposition time is 5 minutes or less, there is no cooling effect, so it is difficult to peel the scale, and when it is 10 minutes or more, a bending phenomenon occurs and the possibility of cracking increases.

In addition, the dense scale at the center eliminates the scale by grinding except 300mm on both sides, and eliminates the possibility of linear flaws through fine cracks generated during slab grinding.

Austenitic stainless steel manufacturing method having excellent surface quality according to the present invention as described above is summarized in Table 1 together with the conventional example and the comparative example.

Work process Conventional example Comparative Example 1 Comparative Example 2 Comparative Example 3 Invention Cast cooling method 600-800 degrees
10-20 minutes of deposition 600-800 degrees
20-30 minutes of deposition Air cooling Air cooling More than 500 degrees of air cooling
Less than 600 degrees
5-10 minutes of deposition Cast Grinding Method radish radish radish Front
Grinding Excluding 300mm on Both Sides
And center only
Grinding problem Cast bending phenomenon and
Logistics congestion Line defects caused by thermal shock Surface roughness caused by mold slag layer scale remaining,
Logistics congestion Bottom surface of cast steel
Crack generation
Line defects caused by
Logistics congestion

As described above, the method of the present invention was applied to compare line defects and surface roughness defect rates before and after the present invention. 5 is a graph comparing the present invention and a comparative example with respect to the occurrence of linear defects, and FIG. 6 is a graph comparing the present invention and the comparative example with a surface roughness occurrence rate.

As can be seen in Figures 5 and 6 it can be seen that the linear defects and the rough surface defect rate is significantly reduced compared to the comparative example and the conventional example when applying the present invention.

1 is a photograph and a schematic view of a surface roughness defect in a conventional surface defect.

2 is a photograph and a schematic diagram of a linear flaw defect in a conventional surface defect;

3 is a view showing a cross-sectional view of the cracks generated during grinding of a typical austenitic stainless steel cast pieces compared with the non-grinding.

4 is a schematic view showing a method for grinding a cast steel according to the present invention;

Fig. 5 is a graph showing the effect of reducing line defects according to the present invention.

Figure 6 is a graph showing the surface roughness defect reduction effect according to the present invention.

Claims (6)

After acetone-based stainless steel containing 10% to 30% chromium and 5 to 15% nickel by weight is cooled by air in a continuous casting process, the cast slab is air cooled to 500 ° C to 600 ° C, and the slab is cooled in a cooling bath. The remaining scale of the mold slag layer is desorbed by immersion in water for 10 minutes, A method for producing austenitic stainless steel having excellent surface quality by removing only residual portions by grinding only the center portion of the cast steel in the cast steel straightening process, except for the side surfaces of the upper and lower surfaces of the cast steel. delete The method of claim 1, The cooling tank is a water cooling tank manufacturing method of austenitic stainless steel having excellent surface quality. delete The method of claim 1, wherein the upper and lower sides of the cast steel have a surface quality of 300 mm or less. An austenitic stainless steel produced by any one of claims 1, 3, and 5, and having excellent surface quality with reduced surface roughness and line defects.

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