JPS58193319A - Production of hot coil from ferrite-containing austenite stainless steel - Google Patents

Abstract

PURPOSE:To prepare a hot coil free from crack or edge crack in high yield, in preparing the hot coil from the slab of austenite stainless steel, by subjecting said slab prepared by a continuous casting method to low strain processing at a specific temp. before hot rolling. CONSTITUTION:The molten metal of austenite stainless steel containing 20-27% Cr and 10-16% Ni is cast by a continuous casting method to be formed into a slab containing 10-20% ferrite in an as-cast state. Because the solidification of the slab due to the continuous casting method is carried out within a short time of 1-25min, no segregation is generated and the formation of an alpha phase is low. After this continuously cast slab is heated to a temp. equal to or lower than a ferrite forming temp. of 1,220 deg.C, the heated slab is subjected to low strain processing. This processed slab is further subjected to hot rolling at 1,220 deg.C or less to prepare a hot coil reduced in an alpha phase and free from crack or edge crack.

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a method for manufacturing a hot coil of ferrite-containing austenitic stainless steel.

Cr2O~27%, N110~16g! 13US containing + and containing 10-20qb of ferrite in the cast state
High ferrite-containing austenitic steels such as 508.5US309 precipitate α phase in an amount higher than the average composition due to segregation during solidification, resulting in a two-phase structure in steel ingots and slabs, which causes large cracks and cracks during blooming and hot coil rolling. It is extremely difficult to manufacture hot coils due to the occurrence of edge cracks. Even if it were possible to make hot coils, the yield would drop significantly due to eaves removal, full-surface grinding, or truncation in the intermediate process, and furthermore, the work costs and fuel costs would be reduced due to multiple hot rolling operations. The cost has increased, making it virtually impossible to manufacture from a commercial and industrial standpoint.

One of the reasons for this cracking is α at high temperatures.

Since the deformability and deformation resistance of the r-phase are different, it is thought that during hot working, cracks are generated at the grain boundaries of the σ-phase and r-phase, which grow and develop into cracks. Therefore, α has poor hot deformability.
Austenitic stainless steel with many phases 1200~12
Attempts have been made to perform soaking treatment for the purpose of reducing α-phase mineralization by heating at 50°C for a long time (10 to 20 minutes) before hot rolling, but this level of soaking treatment In this case, the effect of reducing the diffusion of α phase is small, and the α phase content which can normally be hot-processed is below 5 cm in the surface layer IQm, so it is not possible to prevent cracking during blooming. Almost all of the steel ingots are cracked. In other words, Figure 1 shows the relationship between the ferrite content (llI) of an austenitic stainless steel containing ferrite and the deformability expressed by the number of twists. If this happens, the number of twists will decrease and hot workability will deteriorate.
If it exceeds 5 inches, it becomes impossible to manufacture hot coils. Furthermore, as is clear from Figure 3, which shows the distribution of ferrite in the thickness direction of the blooming slab, steel ingots with a ferrite content of 5ts or less that can be hot-rolled as shown in Figure 1 above are For materials, the thickness is 10 from the surface layer.
Since it is up to the middle, K is not reached until cracking during blooming is prevented, and cracks occur on the entire surface of most of the lumps.

Therefore, in order to remove cracks caused by blooming and υ cracks, the entire surface is cleaned, but in this case, the part (approximately 10 m) of the surface layer where the α phase has decreased is removed by grinding, and the area below the surface layer (approximately 10 m or less) is removed by polishing. The part, that is, the high ferrite part comes out to the surface, and cracks occur in the next hot working, requiring re-eating.

This repeated cycle of blooming and cracking reduces yield, complicates the process, and increases the number of man-hours. No definitive countermeasures have been taken to deal with these problems, and this has become a major problem in hot working. Ta.

The present inventors have found that the above-mentioned α phase formation is due to segregation during solidification, and that the longer the aging time, the more α phase is generated.
We focused on manufacturing hot coils from continuously cast slabs, which can easily achieve this reverse phenomenon. In other words, in the case of continuous casting, the solidification time for a regular large steel ingot is about 4 hours, whereas the solidification time for a continuously cast slab is only 15 to 25 minutes, so segregation is less likely to occur, and therefore less alpha phase is formed, resulting in a finer surface. Furthermore, from Figure 82, which shows the relationship between hot working speed and deformability, that is, the influence of ferrite amount and strain rate on hot workability, when the ferrite amount is small (5 (Below) It is known that there is no cracking even when processed with Fi, and that even if the amount of ferrite is large, there is no cracking if processed with low strain.However, it is known that the disappearance of the α phase requires long heating at around 1200℃. As shown in Figure 4, the amount of 7-elite decreases if heated for a long time, but if processing distortion occurs due to rolling down, the amount of α
Based on the above-mentioned findings, the present invention has discovered that a phenomenon of phase diffusion and disappearance can be obtained.The present invention is based on the above-mentioned findings.The present invention is based on the above-mentioned findings.The present invention is based on the above-mentioned findings. By post-low strain processing, the slab is given processing strain, but it is made into a bloomed slab without cracks, and by further heat rolling, the α phase is reduced, and we have succeeded in producing hot fill. After heating the continuous casting slab 1.
The product is subjected to low strain processing at a strain rate of 0.05 ec-" or less (see Figure 2), followed by 10 to 40 slow reductions, followed by 2 to 4 soaking and hot rolling at 1220°C or less to form a hot coil. .

In this way, in the present invention, since the amount of ferrite generated is small and a thin continuously cast slab is adopted, not only can the ferrite be easily eliminated during the subsequent heat treatment, but also the ferrite can be processed to have low distortion and cracks can be avoided. The ferrite-reduced layer can be preserved because the slab does not have any blooming, and the yield can be greatly improved by reducing surface grinding. Then, the subsequent heat treatment promotes a reduction in the amount of ferrite more than long-time soaking treatment.
Since it can be heated in the same time as ordinary austenitic stainless steel, it saves energy, lowers costs, and simplifies the process. It is also rolled out before hot rolling, so it can be rolled from intermediate thickness to hot rolled. Therefore, it has many industrial effects, such as being able to prevent edge cracking due to a drop in temperature at the edge.

Next, examples of the present invention will be shown.

Test component CSi Mn P S Cr N
1 (1) Q, 020 0.42 2.0 0.0
18 o, oos 21.5 11.3 (2)
0.018 0.41[750,0200,00321
,811,5(3) 0.021 0.46 2.0
7 0.018 0.005 21.2 11.1 Continuously cast slab (approximately 8tOn/
After heating at 1200°C for 2 to 4 hours, the strain rate was o-7.
1 sec' (roll rotation speed: 20 rpm) to obtain 13o(t) x 1030 (・n) I was able to get a hot coil.

On the other hand, when using the steel ingot method, peeling is done 5 to 4 times.
In addition, it takes 2 to 3 times of heating to obtain a blooming piece, a long heating time of 10 to 20 hours, and 4 to 6 times of full-surface grinding. In some cases, the hot workability can be significantly improved.

[Brief explanation of drawings]

In the attached drawings, the flX1 diagram shows the relationship between deformability and ferrite; Figure 2 shows the relationship between the amount of ferrite and strain rate on hot workability; Figure 3 shows the ferrite distribution in the thickness direction of the blooming slab; The figure is a chart showing the effect of rolling on the diffusion and disappearance of ferrite. Ya l Figure 212 3 Figure 4 pseudo, 9! Nomenme t-self 2 good, big 4 to, -yu,
4 Shibu and Pφ<Jj ri, E Pei one enemy - x side IIL O word i1 + lL gooto + 4 1:u addition,! Jun Keno () 4r) Continued from page 1 0 Author: Tatsuo Chinju Shikisha Wakayama Steel Works Applicant: Sumitomo Metal Industries, Ltd. 5-15 Kitahama, Higashi-ku, Osaka

Claims (1)

[Claims] In manufacturing a hot coil from a slab of austenitic stainless steel containing Cr2O~2796, N110~16H and 10~20H of ferrite in the cast state, the slab is manufactured by continuous casting, and this slab is 1
A method for producing a hot coil of ferrite-containing austenitic stainless steel, which comprises heating the heated slab to a temperature of 220° C. or lower, subjecting the heated slab to low strain processing, and subsequently heating it to a temperature of 1220° C. or lower and hot rolling as before.