KR101676203B1

KR101676203B1 - Heat treatment method of stainless steel slab

Info

Publication number: KR101676203B1
Application number: KR1020150183645A
Authority: KR
Inventors: 이윤석; 이기현
Original assignee: 주식회사 포스코
Priority date: 2015-12-22
Filing date: 2015-12-22
Publication date: 2016-11-15

Abstract

A heat treatment method of a stainless steel slab using a heating furnace including a heating table and a cracking base, characterized in that the MS ratio defined by the following formula (1) is controlled to 1.0 or more in the heating zone and 1.3 or more in the crack zone A heat treatment method for a stainless steel slab is disclosed.
[Formula 1]

Description

HEAT TREATMENT METHOD OF STAINLESS STEEL SLAB [0002]

The present invention relates to a heat treatment method for a stainless steel slab.

Generally, stainless steel is classified according to chemical composition or metal structure. According to the classification of the metal structure, the stainless steel is classified into an austenitic system (300 system), a ferrite system (400 system), a martensitic system and an ideal system.

Such a stainless steel is heated in a slab state through a heating furnace including a heating stand and a crack band, and is produced by hot rolling by a plurality of rolling rolls. A large amount of scratches on the surface of a stainless steel There is a problem.

One of the objects of the present invention is to provide a method for heat treatment of a stainless steel slab in which heat-resistant scale defects are reduced.

According to an aspect of the present invention, there is provided a method for heat treatment of a stainless steel slab using a heating furnace including a heating stand and a crack stand, wherein a MS ratio defined by the following formula 1 is controlled to 1.0 or more in the heating stand, Or more of the temperature of the stainless steel slab.

[Formula 1]

INDUSTRIAL APPLICABILITY According to the present invention, it is possible to effectively prevent the occurrence of scratches in a heating furnace scale.

Figs. 1 (a) and 1 (b) are photographs showing the lower surface of a stainless steel in which a heating furnace scale scratch defect occurred. Fig.
FIG. 2 is a graph showing time-dependent measurement of the temperature and the temperature of the upper portion of the heating furnace in the case where the temperature is equalized by the conventional method.
Fig. 3 shows the result of measuring the deviation from the target temperature in the longitudinal direction of the stainless steel during hot rolling in the case where the temperature is equalized by the conventional method.
Fig. 4 is a graph showing the result of measuring the deviation from the target temperature in the longitudinal direction of the stainless steel during hot rolling in the case where the temperature of the slab is subjected to temperature equalization according to the present invention.
Fig. 5 is a photograph showing the lower surface of stainless steel when temperature uniformity is performed on the slab according to the present invention. Fig.
6 (a) shows operating data obtained by measuring the variation of the actual MS ratio when the temperature of the slab is subjected to the temperature equalization by the conventional method, and FIG. 6 (b) Of the actual MS ratio.

Hereinafter, a heat treatment method of a stainless steel sheet as one aspect of the present invention will be described in detail.

Figs. 1 (a) and 1 (b) are photographs showing the lower surface of a stainless steel in which a heating furnace scale scratch defect occurred. Fig.

Generally, stainless steel is known to have less scale formation in heating furnaces in heating furnaces as compared with ordinary carbon steel because chromium in the steel forms a passive oxide film of Cr ₂ O ₃ to prevent corrosion . However, if the surface temperature of the slabs heated in the furnace is not uniform, non-uniform oxidation may occur in some areas where the surface temperature is low, and the scaling caused by such heterogeneous oxidation may be induced, When the rolling process, the annealing, and the pickling process are performed, the scales caused by the non-uniform oxidation are eliminated, resulting in scratches on the scale of the scale.

Therefore, it is necessary to uniformize the surface temperature of the slab in the heating furnace, particularly the lower surface temperature, in order to prevent the occurrence of such scratches in the heating furnace scale. For this reason, it is difficult to uniformize the surface temperature of the slab.

That is, in the heating furnace, there are a plurality of skid buttons for supporting the slab, and the skid button supports the slab in direct contact with the bottom surface of the slab. By the contact with the skid button, And further, hunting of the flow rate supplied to the slab is caused by the skid button, thereby causing a temperature deviation in the longitudinal direction of the slab.

In order to prevent this, conventionally, a method of controlling the temperature difference between the upper and lower surfaces of the slab to a constant range has been employed.

FIG. 2 is a graph showing time-dependent measurement of the temperature and the temperature of the upper portion of the heating furnace in the case where the temperature is equalized by the conventional method. In FIG. 2, the time during which the slab in the heating furnace is charged is around 1:30 at which the temperature is abruptly dropped, and the time during which the slab is taken out is about 7:30 at which the temperature rises sharply. Referring to FIG. 2, it can be seen that the temperature difference between the upper part and the lower part of the furnace during the charging of the slab is controlled within a certain range.

Fig. 3 shows the result of measuring the deviation from the target temperature in the longitudinal direction of the stainless steel during hot rolling in the case where the temperature is equalized by the conventional method. When the deviation from the target temperature has a negative value, there is a possibility that the heating furnace scale scratch defect will occur. Referring to FIG. 3, there is no problem in the longitudinal direction leading end portion and the leading end portion of the stainless steel, It can be easily predicted that the possibility of occurrence of heat-scale-scale scratches is very high.

Therefore, the inventors of the present invention have conducted intensive studies to find a novel method for preventing the occurrence of scratches on the scale of a heating furnace. As a result, the present inventors have found out the present invention. In the present invention, , It is aimed to achieve uniformity of the bottom surface temperature of the slab by controlling the MS ratio defined by the following formula 1, not by controlling the temperature difference between the upper and lower surfaces of the slab.

[Formula 1]

Fig. 4 is a graph showing the result of measuring the deviation from the target temperature in the longitudinal direction of the stainless steel during hot rolling in the case where the temperature of the slab is subjected to temperature equalization according to the present invention. Referring to FIG. 4, it can be easily predicted that, when the MS ratio is controlled as in the present invention, the occurrence of scratches on the scale of a heating furnace can be effectively prevented.

Fig. 5 is a photograph showing the lower surface of stainless steel when temperature uniformity is performed on the slab according to the present invention. Fig. Referring to FIG. 5, it can be seen that heat-resistant scale defects were not observed on the surface of the stainless steel produced by the present invention.

6 (a) shows operating data obtained by measuring the variation of the actual MS ratio when temperature uniformity is performed on the slab by the conventional method, and FIG. 6 (b) And the variation of the actual MS ratio in the case where it is carried out. For reference, the target MS ratio in Fig. 6 (b) was 1.1. Referring to FIG. 6, it can be seen that when the temperature difference between the top and bottom surfaces of the slab is simply controlled to be within a predetermined range, the variation of the MS ratio is greatly caused. However, It can be seen that the variation of the ratio is managed at a minimal level.

Meanwhile, the inventors of the present invention conducted further studies to derive a condition that can most effectively prevent the occurrence of scratches on the scale of a furnace scale in the heat treatment of a stainless steel using a heating furnace including a heating stand and a crack band. Results The following conditions could be derived.

(1) MS ratio in the heating zone

The MS ratio in the heating zone is preferably controlled to 1.0 or more, more preferably 1.1 or more.

If the MS ratio in the heating zone is less than 1.0, there is a possibility that immature heat and uneven scale due to the temperature deviation in the upper and lower portions of the slab may be generated. On the other hand, as the MS ratio in the heating zone is controlled to be higher, it is advantageous to reduce the occurrence of scratches in the heating furnace scale, so that the upper limit is not particularly limited in the present invention. However, if the MS ratio in the heating zone is excessively high, there is a risk of excessive slab bending due to the heating of the lower slab, and the upper limit can be set to 1.5.

(2) MS ratio in the crack zone

The MS ratio in the crack zone is preferably controlled to 1.3 or more, more preferably 1.35 or more.

If the MS ratio in the crack zone is less than 1.3, there is a fear that an immature heat and a non-uniform scale may be generated due to the upper and lower temperature deviations of the slab. On the other hand, as the MS ratio in the crack zone is controlled to be higher, it is advantageous to reduce the generation of cracks in the scale of the furnace scale, so that the upper limit is not particularly limited in the present invention. However, if the MS ratio in the crack zone is excessively high, there is a risk of excessive slab bending due to the partial heating of the lower portion of the slab, and the upper limit can be set to 1.5 in consideration of this.

Claims

A heat treatment method for a stainless steel slab using a heating furnace including a heating stand and a crack stand,
Wherein the MS ratio defined by the following formula 1 is controlled to be 1.0 or more in the heating zone and 1.3 or more in the case of the crack zone.
[Formula 1]

The method according to claim 1,
Wherein the MS ratio in the heating zone is controlled to be 1.1 or more and 1.5 or less.

The method according to claim 1,
Wherein the MS ratio in the crack zone is controlled to be 1.35 or more and 1.5 or less.