JP2676492B2 - Method for manufacturing low carbon / low nitrogen ferritic stainless steel - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for producing a low carbon / low nitrogen ferritic stainless steel sheet having good surface quality and excellent ridging property from a continuous cast slab.

[0002]

[Prior art and its problems] It is difficult to form a uniform oxide layer on the slab surface because low-carbon / low-nitrogen ferritic stainless steel has a high resistance to oxidation, so surface marks such as oscillation marks and scale marks are difficult to form. Are often observed on the coil surface after hot rolling. Therefore, in order to reduce these defects, it has been proposed to apply an oxidation accelerator and raise the heating temperature. On the other hand, ferritic stainless steels also suffer from streak defects called ridging or roping that occur during rolling or drawing, and these defects tend to worsen at high hot rolling temperatures. Is proposed. However, in these methods, when high-temperature heating is performed to reduce surface defects, the temperature of hot rolling becomes high, resulting in deterioration of ridging property, and in order to avoid this, if cooling time is provided after heating, productivity deteriorates. Not only is this caused, but also the slab temperature becomes uneven, which causes troubles such as misrolling. In addition, excessive high temperature heating may cause local abnormal oxidation, which may lead to deterioration of quality. When an oxidation accelerator is used, there is a problem that the coating method is problematic and troubles such as peeling during heating cannot be avoided.

[0003]

DISCLOSURE OF THE INVENTION The present invention provides a low carbon / low nitrogen ferritic stainless steel sheet having good surface properties without introducing special equipment and without lowering productivity and increasing trouble occurrence rate. It is intended to provide a method of manufacturing from a slab.

[0004]

The gist of the present invention resides in a method for producing a low carbon / low nitrogen ferritic stainless steel comprising the following first to fourth steps. That is,
First step of cleaning the surface of a low carbon / low nitrogen ferritic stainless steel slab with a thickness of Wmm with a # 12-32 grinder, and an atmosphere of Po ₂ = 0.01-0.04 atm and dew point = 40-60 ° C. The second step of heat-treating the slab at a heating temperature (T ° C.) and a heating time (t minutes) satisfying the following equation, and the slab at 1150 ° C. or lower for 20 to 6
A method for producing low carbon / low nitrogen ferritic stainless steel, comprising a third step of soaking for 0 minutes and a fourth step of rolling the slab at a rough rolling start temperature of 1160 ° C. or lower. T = 1150 to 1210 ° C. t ≧ 90 minutes t = {180-3 (T-1150)} × (W / 150)
² to {270-3 (T-1150)} × (W / 150) ²

The method of the present invention obtains a hot-rolled steel sheet having a good surface property by using a known hot-rolling treatment equipment from a slab without introducing special new equipment. Po ₂ used in the second step = 0.01 to 0.04 atm, dew point = 40
The atmosphere of -60 ° C is a typical combustion atmosphere of fuel gas. Under this atmosphere, a slab with a low-carbon, low-nitrogen ferritic stainless steel surface in the as-cast state requires considerable high temperature and long-time heating to obtain a thick and uniform oxide scale layer, but the surface ground with a grinder However, this heating temperature and time can be greatly reduced. This is because the surface area is increased by grinding, the formation of the anti-oxidation layer by the Cr oxide layer does not proceed sufficiently, and as a result, oxidation easily occurs. Therefore, it is better to have finer grinds, but in consideration of the grinding efficiency, the grinder was maintained at # 12 to 32 in the first step. In the second step, excessive oxidation causes local abnormal oxidation, so the upper limit of the heating time was defined with respect to the heating temperature. If the heating temperature exceeds 1210 ° C, it causes abnormal oxidation and it becomes difficult to control the temperature before rough rolling, so the upper limit was made 1210 ° C. The lower limit of 1150 ° C. was set as the lower limit because productivity drops at temperatures below this. The lower limit of the heating time of 90 minutes was set as the lower limit because if the heating time is shorter than this, uniform heating of the slab cannot be performed and problems occur in rolling. Further, in the second step, a slab having a thickness of Wmm is formed by the formula T
= {180-3 (T-1150)} × (W / 150) ²
~ {270-3 (T-1150)} x (W / 150) ²
It is necessary to process the heating time to satisfy the above condition. In the third step, soaking was performed at 1150 ° C. or less for 20 to 60 minutes. The soaking time is limited to 20 to 60 minutes because soaking is insufficient if it is less than 20 minutes and productivity is impaired if it is more than 60 minutes. In the fourth step, the rough rolling start temperature is set to 1160 ° C.
The reason is as follows, because it is necessary to lower the temperature of rough rolling in order to obtain good ridging property, and as a result of research and study, it became clear that there is no problem if the rough rolling start temperature is set to 1160 ° C or lower. is there. Therefore, in the fourth step, rolling is started after lowering the temperature of the soaking furnace and sufficiently lowering the slab temperature.

[0006]

EXAMPLE 18% Cr-0.5Mo-0.3Nb-0.
# 1 of 150mm thick slab with 01C-0.01N composition
Grind with No. 2 grinder, Po ₂ = 0.03at
m, dew point about 55 ℃, heating time 90-300
Min, heating time 1150 to 1210 ° C., then 1
Soaking is performed at 150 ° C for 30 minutes, then the rough rolling start temperature 1
Rolling was performed at 160 ° C or lower. The result is shown in FIG.
In the shaded area in FIG. 1, a coil having good surface quality and excellent ridging property was obtained.

[0007]

[0008]

According to the present invention, a low carbon / low nitrogen ferritic stainless steel sheet having good surface properties can be manufactured at low cost without requiring a special treatment step.

[Brief description of the drawings]

FIG. 1 is a diagram showing a surface property of a steel sheet obtained by performing a test by changing a heating temperature and a heating time in a second step in Examples.

─────────────────────────────────────────────────── ─── Continuation of the front page (51) Int.Cl. ⁶ Identification code Internal reference number FI Technical indication C21D 8/02 9270-4K C21D 8/02 D

Claims (1)

(57) [Claims] 1. A first step of treating the surface of a low-carbon / low-nitrogen ferritic stainless steel slab having a thickness of Wmm with a # 12-32 grinder, Po ₂ = 0.01-0.04 atm, and dew point = 40. ~ 60
Heating temperature (T ° C) that satisfies the following formula in an atmosphere of
A second step of heating the slab for a heating time (t minutes), a third step of soaking the slab at 1150 ° C. or less for 20 to 60 minutes, and a rough rolling start temperature of 1160 ° C. or less. A method for producing low-carbon / low-nitrogen ferritic stainless steel, which comprises a fourth step of rolling. T = 1150 to 1210 ° C. t ≧ 90 minutes t = {180-3 (T-1150)} × (W / 150)
² to {270-3 (T-1150)} × (W / 150) ²