JPH06328214A - Method for preventing season cracking of ferritic stainless steel - Google Patents

Abstract

PURPOSE:To provide a producing method, by which season cracking is not generated even if a high purity ferritic stainless steel cast slab cast with a continuous casting machine is cooled to the room temp. CONSTITUTION:In a stainless steel sheet production process of hot-rolling a slab after casting the high purity ferritic stainless steel with the continuous casting machine, the cast slab just after the completion of the solidification in the continuous casting machine is cooled so that the temp. difference between the surface temp. at the center part of the wide surface of the cast slab and the surface temp. at the center part of the narrow surface is controlled within 200 deg.C. Then, at the time of reheating the cast slab cooled to the room temp. once, by controlling the temp. difference between the surface temp. at the center part of the wide surface of the cast slab and the surface temp. at the center part of the narrow surface within 200 deg.C, the season cracking of the high purity ferritic stainless steel is prevented from generating.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a process for producing a ferritic stainless steel sheet having excellent corrosion resistance.

[0002]

2. Description of the Prior Art Ferritic stainless steels having high Cr and reduced C and N have become more important as inexpensive corrosion resistant materials that do not cause chloride stress corrosion cracking peculiar to 18Cr-8Ni steels, and depending on the application. Mo, Ni, Cu, etc. are selectively added, and impurities such as S and O are extremely reduced to improve corrosion resistance, rust resistance, and oxidation resistance, and they are beginning to be used in many applications. On the other hand, one of the drawbacks of the ferritic stainless steels having these excellent characteristics is disclosed in JP-A-54-128464 and JP-A-58-3973.
No. 2, JP-A-60-2628, JP-A-60-
As disclosed in Japanese Patent No. 2622, etc., a slab has a cross section during cooling of a CC slab, during maintenance of the slab surface after cooling, and during transportation or heating of the hot rolling process of the next process. There is a problem that a piercing crack (called a set crack) occurs, and when this set crack occurs, the cast piece cannot be rolled and cannot be manufactured. Especially for high Cr steel, Nb, Mo, A
It is no exaggeration to say that when a large amount of 1 is contained, cracks will always occur.

Conventionally, the above-mentioned measures have been disclosed in the above-mentioned patent documents for the above-mentioned cracks. First, JP-A-54-128464 and JP-A-58-3973.
Japanese Patent Publication No. 2 discloses that the slab is not cooled to 150 ° C. or lower and is not cooled to the transition temperature or lower (300 ° C. in the examples). On the other hand, JP-A-60-2628
In Japanese Patent Laid-Open No. 60-2622, a method of extremely slow cooling when cooling a cast piece, that is, 800 to 130
A method of gradually cooling from 0 ° C to 300 ° C at a cooling rate of 40 ° C / hr or less (800 ° C to 300 ° C for 12.5 hr or more), or a slow cooling rate of 700 ° C to 400 ° C at a cooling rate of 18 ° C / hr or less. (16.7 hours or more), and 200 ° C
It discloses a method of not lowering the value below. As described above, according to the conventional knowledge, two ideas are known, that is, a method of holding the slab at a temperature condition of 150 ° C. or higher or an extremely slow cooling from 700 ° C. to around 400 ° C.

[0004]

As the advantages of ferritic stainless steel become clearer, the production amount of these steel types increases and the steel types expand to materials having even more excellent properties (for example, 25Cr-4Mo type). Is progressing. However, in the process of producing this ferritic stainless steel, the condition that cold pieces (100 ° C. or less at the surface temperature of the slab) cannot be used in order to prevent cracking in place and the gradual cooling conditions in the high temperature range are constraints on production. , Cost, delivery time, and productivity are becoming major obstacles.

For example, in the method of holding the slab at a temperature condition of 150 ° C. or higher, a heat-retaining facility (a furnace having a slab heat-up function equipped with a burner, etc.) for securing the temperature and a heat-retaining carrier are required. Therefore, it is necessary to add these facilities as the production amount of stainless steel increases, and a large amount of capital investment is required. Furthermore, when the steelmaking factory and the hot rolling factory cannot be matched in terms of production schedule, heat retention is required for a long time, which leads to an increase in fuel cost. Further, even in the method of extremely slow cooling from 700 ° C. to around 400 ° C., a slow cooling facility for casting slabs (heat-retaining pit) is required, and since slow cooling takes a long time, productivity is lowered.
As described above, the conventional measures to prevent the laying cracks have problems in terms of capital investment, fuel cost, and productivity associated with the increase in the production amount. There is a strong demand for a possible slab processing method.

[0006]

Means for Solving the Problems In the process for producing a stainless steel sheet in which a ferritic stainless steel is cast by a continuous casting machine and then rolled by a hot rolling machine,
The slab temperature immediately after the solidification of the slab in the continuous casting machine is controlled such that the temperature deviation between the surface temperature of the wide surface of the slab and the surface temperature of the narrow surface of the narrow surface of the slab is 200 ° C. or less. When the slab is cooled by air cooling or in a heat retaining pit to room temperature and then reheated for hot rolling again, the slab temperature until the temperature of the slab center reaches 200 ° C. The invention has invented a method for preventing misplacement cracking of ferritic stainless steel, which is characterized by controlling the temperature deviation from the surface temperature of the center of the surface to be within 200 ° C. Further, in the present invention, by controlling the total amount of TiS and TiC of the cast slab as shown in the following equation, the effect of the above temperature can be more remarkably exhibited. [Ti]% × [S]% + [Ti]% × [C]% ≦ 0.0
025

The present invention will be described in detail below. Cast slab cracking of ferritic stainless steel is caused by the process of cooling the cast slab cast by a continuous casting machine to room temperature, and by re-heating the slab for hot rolling once by cooling in air or in the heat retaining pit to room temperature. As shown in FIG. 1, a crack that occurs when the slab penetrates in the widthwise direction of the slab and penetrates in the thickness direction at several points in the longitudinal direction. This in-place crack is caused by the embrittlement phenomenon at a low temperature of about 200 ° C. or less, which is a material property of ferritic stainless steel, and conventionally, attention was paid to the improvement of the material property, that is, crystal grains and precipitates. Various studies have been made on the improved means. On the other hand, the inventors of the present invention focused on the form of this placement crack, that is, the phenomenon that cracks penetrating in the thickness direction at the central portion in the width direction occur at several places in the longitudinal direction, and Research on tensile stress,
We have found a manufacturing method that does not cause misplacement cracks.

The concept will be described with reference to the drawings. First, regarding cracks that occur inside the slab in the process of cooling the slab cast by the continuous casting machine to room temperature, as shown in FIG. 2, the slab wide surface central part surface temperature T _C1 and the slab narrow surface central part surface slab situation after cooling to room temperature was found to vary greatly by temperature deviation in temperature _{T C2 (T C1 -T C2)} . That is,
When the temperature deviation in the width direction of the slab is small, the stress remaining inside is small, so many fine cracks exist inside the slab, but these cracks do not propagate to the surface layer of the slab. Here, the narrow surface of the slab refers to two surfaces having a small area out of the four surfaces parallel to the casting direction, and the wide surface refers to two surfaces having a large area out of the four surfaces parallel to the casting direction.

However, when this temperature deviation is large, the tensile stress remaining inside increases, so the cracks generated inside the slab propagate to the surface layer of the slab and cracks that penetrate to the surface of the slab. Form. Once this crack reaches the slab surface, the cracked fracture surface comes into contact with the outside air (air), so a scale layer is formed on the fracture surface by air oxidation, and it becomes impossible to crimp this crack during rolling. Become.

On the other hand, as shown in FIG. 3, with respect to cracks generated inside the slab in the process of reheating and hot rolling the slab cooled to room temperature, the surface temperature T of the wide surface center of the slab during reheating is shown in FIG. _H1 and slab Semamen central surface producing temperature T _H2, the temperature deviation (T _H1
The -T _H2), conditions of the steel strip after hot rolling is found to differ greatly. Reheating and hot rolling were performed using a slab in which cracks did not propagate to the surface when cooled to room temperature, but the temperature deviation in the slab width direction during reheating (T _H1
When -T _H2 ) is small, the thermal stress generated inside the slab during heating is small, so the microscopic cracks that had been generated before heating may develop slightly, but during heating, cracks may develop on the surface of the slab. When the slab is hot-rolled further, the surface of the crack inside the slab is not oxidized, so that the slab does not remain in the steel slab as a defect due to pressure bonding during rolling.

However, when the temperature deviation in the width direction of the slab during reheating is large, the tensile stress generated in the length direction inside the slab increases, so cracks inside the slab are generated in the surface layer of the slab. A crack that propagates and penetrates to the surface of the slab during heating is formed. Once this crack reaches the surface of the slab, the crack fracture surface comes into contact with the outside air (air) as in the case of cooling, so a scale layer is formed on the fracture surface by air oxidation,
It becomes impossible to crimp this crack during rolling.

That is, in order to prevent slab-deposit cracking of high-purity ferritic stainless steel, the fracture surface is not oxidized in the process of hot rolling the fine cracks generated inside the slab,
It becomes possible by pressing the cracks by hot rolling. Regarding the crack generation behavior inside the slab, it can be estimated by the temperature deviation between the surface temperature of the wide surface center and the surface temperature of the narrow surface of the slab. The amount of cooling water is controlled so that the temperature deviation between the surface temperature of the part and the surface temperature of the central part of the narrow surface is within 200 ° C., and the slab, which has been once air-cooled or cooled to room temperature in the heat retention pit, is hot again. The temperature of the slab at the time of reheating for rolling to reach 200 ° C at the center of the slab is controlled so that the temperature difference between the surface temperature at the center of the wide surface and the surface temperature at the center of the narrow surface is within 200 ° C. By doing so, cracking can be avoided.

Further, in order to suppress the development of fine cracks generated inside the cast piece, it is effective to improve the material properties, and the inventors of the present invention searched for an improving means focusing on precipitates and the like, It was discovered that the above effect can be more remarkably exerted by controlling the total amount of TiS and TiC precipitated inside by the formula shown below (FIG. 4). [Ti]% × [S]% + [Ti]% × [C]% ≦ 0.0
By adjusting to such a component system and performing the above-mentioned temperature control, it is possible to almost completely prevent the slab from cracking. [T
When i]% × [S]% is higher than 0.0025, TiS
As a result, precipitation occurs at the crystal grain boundaries, and the strength of the grain boundaries is reduced, resulting in cracks. Similarly, when [Ti]% × [C]% is higher than 0.0025, TiC is precipitated at the crystal grain boundary and the strength of the grain boundary is reduced, resulting in cracking. For this reason, [Ti]% ×
By setting [S]% + [Ti]% × [C]% ≦ 0.0025, it is possible to prevent the slab from cracking.

[0014]

[Examples] Ferrite-based stainless steel having the components shown in Table 1 was melted by the usual LD-VOD method. Then, cast a slab with a thickness of 250 mm in a continuous casting machine, change the amount of water for secondary cooling of the continuous casting machine and the cooling water density at the edge of the slab, and change the surface temperature and narrow surface of the wide surface of the slab immediately after solidification is completed. The temperature deviation from the central surface temperature was controlled. Half of the slabs after CC extruding are gradually cooled from about 800 ° C in the heat retention pit for 48 hours, then cooled into pieces, and the remaining slabs are allowed to cool to the atmosphere from about 800 ° C. Slab cooling conditions,
And, the test was carried out by changing the conditions for heating the slab from room temperature as shown in Table 2. As a result, by applying this method, the occurrence rate of misplaced cracks was significantly reduced, and it was possible to almost completely prevent misplaced cracks.

[0015]

[Table 1]

[0016]

[Table 2]

[0017]

EFFECT OF THE INVENTION Ferrite stainless steel of low Cr and high Cr type, which is excellent in corrosion resistance and oxidation resistance, is used in many fields, but it is effective against a crack in production which is a manufacturing defect. It was hoped that some measures would be taken. As a conventional method, a method of gradually cooling gradually during cooling or a method of heat-retaining a slab at 150 ° C. to 350 ° C. or more has been known. On the other hand, the present invention controls the cooling pattern of the continuous casting machine, the heating pattern of the heating furnace, and the precipitate-forming elements (Ti, C, S) to cool the slab to room temperature without causing cracks. Since the process can be performed, the present invention has significant advantages in terms of cost and productivity.

[Brief description of drawings]

FIG. 1 is a diagram showing a situation where a placement crack occurs,

FIG. 2 is a diagram showing the influence of the temperature deviation in the slab width direction immediately after the completion of CC solidification on the crack generation state inside the slab after cooling,

FIG. 3 is a diagram showing the influence of the temperature deviation of the slab width direction on the slab internal crack formation during heating and after rolling, when reheating the slab cooled to room temperature;

FIG. 4 is a diagram showing an influence of a precipitate-forming element (Ti, C, S) on a crack in place.

[Explanation of symbols]

 1 Cast piece 2 Place crack 3 Micro crack

Front page continuation (72) Sumio Suzuki No. 1-1 Tobata-cho, Tobata-ku, Kitakyushu, Fukuoka Prefecture Inside the Yawata Works, Nippon Steel Corporation (72) No. 1 Kazuhisa Tanaka, Tobata-cho, Tobata-ku, Kitakyushu, Fukuoka No. 1 inside Nippon Steel Co., Ltd. Yawata Works

Claims (2)

[Claims] 1. In a stainless steel plate manufacturing process in which ferritic stainless steel is cast by a continuous casting machine and then rolled by a hot rolling mill, the slab temperature immediately after completion of solidification of the slab in the continuous casting machine Cooling is controlled so that the temperature deviation between the surface temperature of the central part and the surface temperature of the narrow part of the slab is 200 ° C or less, and the slab is once air-cooled or cooled to room temperature in the heat retention pit and hot-rolled again. For reheating, the temperature of the slab until the temperature of the slab center reaches 200 ° C is controlled so that the temperature deviation between the surface temperature of the wide center and the surface temperature of the narrow center is within 200 ° C. A preventive method for preventing ferritic stainless steel from placing cracks. 2. In ferritic stainless steel, [T
i]% × [S]% + [Ti]% × [C]% ≦ 0.002
The components were adjusted to 5 so that the temperature of the slab immediately after the solidification of the slab in the continuous casting machine is 200 ° C or less between the surface temperature of the wide surface of the slab and the surface temperature of the narrow surface of the slab. While controlling the cooling, when the slab is once cooled to room temperature in the air-cooled or heat-retaining pit and then reheated for hot rolling, the slab temperature until the temperature of the slab center reaches 200 ° C. A method for preventing disposition cracking of ferritic stainless steel, which comprises controlling the temperature deviation between the surface temperature of the wide surface central part and the surface temperature of the narrow surface central part within 200 ° C.