JP2607187B2 - Method for producing thin Cr-Ni stainless steel sheet with excellent surface quality and workability - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a so-called synchronous continuous casting method in which there is no relative speed difference between a slab and the inner wall surface of a mold. The present invention relates to a method for manufacturing a steel sheet.

[0002]

2. Description of the Related Art Conventionally, in order to manufacture a stainless steel sheet using a continuous casting method, a mold is cast into a slab having a thickness of 100 mm or less while vibrating a mold in a casting direction, and the surface of the obtained slab is cleaned. After heating to 1000 ° C. or more in a heating furnace, hot rolling was performed by a hot strip mill comprising a row of rough rolling mills and a row of finishing mills to obtain hot strips having a thickness of several mm.

When the hot strip thus obtained is cold-rolled, the hot strip that has been subjected to strong hot working is softened in order to secure the shape (flatness), material and surface properties required for the final product. In addition to performing hot-rolled sheet annealing for removing the surface, scale and the like on the surface were removed by grinding after the pickling step. In this conventional process, a large amount of energy is required for heating and processing the material in a long hot rolling facility, and it is difficult to say that the production process is excellent in terms of productivity. In addition, the final product has a lot of restrictions on its use, for example, the texture has developed and it is necessary for the user to consider the anisotropy when performing press working or the like.

In order to solve the problem that a long hot rolling facility and a large amount of energy and rolling power are required to roll a slab having a thickness of 100 mm or more into a hot strip, a continuous strip has been recently developed. Research on a process for obtaining a slab (thin strip) having a thickness equal to or close to that of a hot strip in the process of casting is ongoing. For example, a paper featured in "Iron and Steel"'85, A197-A256, discloses a process for obtaining hot strip directly by continuous casting. In such a continuous casting process, the gauge of the slab to be obtained is 1 to 10
The twin-drum system is being studied when the gauge is on the order of mm, and the twin-belt system is being studied when the gauge of the slab is on the order of 20 to 50 mm.

[0005]

SUMMARY OF THE INVENTION Twin drum casting,
The SUS304 sheet product manufactured by the cold rolling process has a finer grain structure than the sheet product manufactured by the conventional process,
It is known that elongation is low. For example, "CA
MP ISIJ "vol. 1 1988, 1670-1
The paper featured in 705 also reports that the slab is annealed to eliminate δ ferrite remaining in the slab as a countermeasure. However, reheating and annealing the slab at a high temperature for 1 minute or more is disadvantageous in the process.

[0006] The inventors of the present invention have proposed a method in which
-A detailed study of the Ni-based stainless steel sheet manufacturing process and the elucidation of the factors that suppress the growth of recrystallized grains during cold rolling and annealing revealed that MnS was not sufficiently precipitated in the rapidly cooled slab, and after cold rolling. It was confirmed that MnS was finely precipitated at the time of final annealing, hindered grain growth and lowered elongation.
Therefore, it is necessary to sufficiently precipitate MnS in the slab stage to render it harmless. However, the method of reheating and annealing a slab requires a long-time heat treatment at a high temperature, and a method of efficiently performing the high-temperature long-time heat treatment to facilitate grain growth is desired.

[0007]

Means for Solving the Problems The present inventors have taken the above circumstances into account.
In view of the above , as a result of investigating the heat history of the slab, it has been found that the present invention has a slow cooling region at a temperature range of 1200 to 900 ° C. at a predetermined cooling rate of at least 5 seconds in a cooling process of the slab for at least 5 seconds. We confirmed that we could solve the problem.

That is, the gist of the present invention is as follows. (1) Cr-Ni represented by 18% Cr-8% Ni steel
Cast thin slabs with a plate thickness of 6 mm or less from stainless steel,
In a method of manufacturing a cold-rolled sheet product by omitting hot rolling, in a process of cooling a slab, 1200 to 900 ° C.
When the slab is gradually cooled in the temperature range of 1), the slab temperature at which the slow cooling is started is 1200 to 1050 ° C.
In the case of the above temperature range, a slow cooling range of 20 ° C./sec or less is provided for 5 seconds or more . Slow cooling start temperature T ° C
A method for producing a thin Cr-Ni stainless steel sheet having excellent surface quality and workability, characterized by having a slow cooling region having a cooling rate of CR ° C / sec or less determined by the following formula for 5 seconds or more. T = 57.65 × log (CR) +975

(2) In the process of cooling the slab, an additional 9
Cool at 10 ° C / sec or more between 00 ° C and 600 ° C,
2. The method for producing a Cr-Ni stainless steel sheet excellent in surface quality and workability according to the above item 1, wherein the sheet is wound at a temperature of 600 ° C or lower. Hereinafter, the present invention will be described in detail. The reason why the thickness of the cast plate is set to 6 mm or less is that as the thickness of the cast plate increases, the crystal grain size of the slab becomes coarse and the surface quality of the product decreases. However, when thinning the cast thickness becomes bogged sufficient rolling reduction in cold rolling, since it is possible to reduce the surface quality, as the desired range is 3 to 6 mm.
In the present invention, in the process of cooling the slab, 1200
The slab is slowly cooled in a temperature range of up to 900 ° C.
In the case of a temperature range of 050 ° C., a slow cooling range of 20 ° C./sec or less is provided for 5 seconds or more. B) In a case where the slab temperature at which slow cooling is started is a temperature range of 1050 to 900 ° C.,
The slow cooling start temperature T ° C of the slab is determined by the following formula.
That the cooling rate CR ° C. / sec or less for slow cooling zone and to have more than 5 seconds. T = 57.65 × log (CR) +975 In the temperature range from 1200 ° C. to 900 ° C.,
More than one second was secured from 1200 ° C to 900 ° C.
In the temperature range between ° C., the coarsening of MnS tends to proceed, and δ
Ferrite diffusion also proceeds. Therefore, in the present invention, 1
It is necessary to have a slow cooling region defined as above in a temperature region of 200 to 900 ° C.

FIG. 1 is a diagram showing a C steel represented by 18Cr-8Ni steel.
This is a cooling pattern of a cast slab in which r-Ni stainless steel is cast to a thickness of 3 mm by a twin roll method and gentle cooling is started from various temperatures during cooling of the cast slab. a is a cooling pattern of a slab cooled from 1200 ° C. by air, and b and c are cooling patterns of a slab cooled slowly from 1200 ° C. Also, d and e are cooling patterns of the slabs which were air-cooled to 1100 ° C. and 950 ° C., respectively, and then slowly cooled. During the slow cooling pattern of d and e, the cooling pattern of the slab that was air-cooled by changing the time was d
1, d2, d3, d4, and e1.

These slabs were pickled and cold rolled into 0.6 mm product sheets, and the elongation characteristics were examined. The results are shown in Table 1. The slab of the cooling pattern a has insufficient elongation, and the slabs of the cooling patterns b, c, and d to which slow cooling has been applied have improved elongation. In particular, the slab of the cooling pattern d has slow cooling of 1100 ° C. or less for 5 seconds. Excellent elongation was exhibited by the above addition.

In the cooling pattern e slab, 950 ° C. to 1.
Slow cooling at 2 ° C./sec was applied for 40 seconds, but elongation was insufficient. In the first place, an object of the present invention is to achieve an improvement in elongation in a slow cooling zone of 2 minutes or less in the process of cooling a slab. For this purpose, these results show that the temperature during cooling of the slab and the cooling rate at that temperature are important. Thus, the relationship between the slab temperature and the required slow cooling rate was clarified under further conditions, and the result is as shown in FIG.

[0013] Slow cooling start temperature is 1200 to 1050 ° C
In the above temperature range, the elongation is improved by setting the slow cooling range of 20 ° C./sec or less to 5 seconds or more. Start temperature of slow cooling is 1050
In the temperature range of ~ 900 ° C, the slow cooling rate at which the elongation is improved decreases as the slow cooling start temperature decreases. The slab of FIG.
Shows the relationship between the slow cooling start temperature and the slow cooling rate upper limit
In line A, two distinct points on line A, namely point a
(Cooling rate 0.05 ° C / sec for slab temperature 900 ° C
c) and point b (cooling rate 20 for slab temperature 1050 ° C.)
° C / sec) from the same figure and pass through these two points.
Assuming that line A is T = α × log (CR) + β, 2
When simultaneous linear equations are solved by substituting point values, the coefficient α = 5
7.65 and a coefficient β = 975 are obtained. Therefore, in FIG.
The line A is expressed by the following equation when it is mathematically expressed.
If the cooling start temperature is a predetermined T ° C., the temperature is determined by the following equation.
Slow cooling may be performed at a cooling rate of CR ° C./sec or less. T = 57.65 × log (CR) +975 The cooling rate between 1200 and 900 ° C. is preferably as low as possible, but it is economically inefficient to increase the temperature by heating. There is no critical technical basis for limiting the lower limit of the cooling rate. If the slow cooling time is 5 seconds or longer, the longer the slow cooling, the more effective the improvement of elongation is. However, if the slow cooling is performed for a long time, the total length of the equipment becomes longer. Also, 900
Since the temperature range from ℃ to 600 ℃ is a temperature range in which chromium carbide precipitates at the grain boundaries, rapid cooling is desirable. Slow cooling during this period will severely corrode grain boundaries during pickling, deteriorating the surface quality of the product. If the cooling rate is 10 ° C./sec or more, precipitation can be prevented. There is no critical technical basis for limiting the upper limit of the cooling rate, but a range of 10 to 400 ° C./sec is desirable assuming ordinary steam cooling. Similarly, the winding temperature was set to 600 ° C. or lower in order to prevent precipitation of chromium carbide. In addition, there is no critical technical basis to limit the lower limit of the winding temperature, but if it is too low, the deformation resistance of the material increases and a heavy load is imposed on the coiler. Is desirable.

[0014]

EXAMPLE Austenitic stainless steels of various components based on 18% Cr-8% Ni steel shown in Table 2 were melted and the thickness was varied in various atmospheres using an internal water-cooled twin-drum caster. By casting into a slab of 2 to 5.8 mm and passing the slab through a combustion furnace with a changed temperature, 1200 to 900 ° C
Slow cooling was performed at a rate of 20 ° C./sec or less from various temperatures during the heating.

Thereafter, the slab was pickled, cold rolled, bright annealed, and temper rolled to obtain a thin plate product, and the material quality was evaluated.
As a comparative example, a thin plate product was similarly manufactured from a slab having a different cooling rate or time immediately after casting, and the material quality was evaluated. Table 3 shows the results. Workability is determined by elongation in the L direction.
When evaluated, those having an elongation of 48% or more have good elongation (（), 4
Less than 8% was regarded as poor elongation (x).

Slow cooling is started between 1150 and 1000 ° C., and the slow cooling rate is 0.8-20 ° C./sec and 20 ° C./sec.
After setting the cooling time to 5 to 110 seconds, 90
The workability (elongation) of the product of the present invention, which was cooled between 0 to 600 ° C. at 10 ° C./sec or more, was improved. In the comparative method, since slow cooling was not taken, workability was poor.

[0017]

[Table 1]

[0018]

[Table 2]

[0019]

[Table 3]

[0020]

According to the present invention, an austenitic stainless steel sheet having excellent surface properties and workability can be produced by a simple process of continuously casting a strip-shaped slab having a thickness close to the product thickness and directly commercializing the product by cold rolling. Obtainable. Therefore,
The technical effects of the present invention are extremely large in terms of economy and production purpose.

[Brief description of the drawings]

FIG. 1 is a view showing a cooling pattern of a slab after twin roll casting.

FIG. 2 is a diagram showing a relationship between a cooling pattern of a Cr-Ni-based austenitic stainless steel slab and elongation of a product.

 ──────────────────────────────────────────────────の Continued on the front page (72) Inventor Hideki Oka 3434 Shimada, Hikari-shi, Yamaguchi Prefecture Inside Nippon Steel Corporation Hikari Works (72) Inventor Yuji Yoshimura 3434, Shimada, Hikari-shi, Hikari-shi, Yamaguchi New Japan Inside Hikari Steel Works

Claims (2)

(57) [Claims] 1. C represented by 18% Cr-8% Ni steel
In a method of casting a thin slab having a thickness of 6 mm or less from an r-Ni stainless steel and omitting hot rolling to produce a cold-rolled thin sheet product, in a process of cooling the slab, 1200
When the slab is slowly cooled in the temperature range of 900 ° C., a) The slab temperature at which the slow cooling is started is 1200 to 1050 ° C.
In the case of the above temperature range, a slow cooling range of 20 ° C./sec or less is provided for 5 seconds or more . Slow cooling start temperature T ° C
A method for producing a thin Cr-Ni stainless steel sheet having excellent surface quality and workability, characterized by having a slow cooling region having a cooling rate of CR ° C / sec or less determined by the following formula for 5 seconds or more. T = 57.65 × log (CR) +975 2. In the process of cooling the slab, 900
Cool at 10 ° C / sec or more between 600 ° C and 600 ° C.
The method for producing a Cr-Ni stainless steel sheet having excellent surface quality and workability according to claim 1, wherein the sheet is wound at a temperature of not more than ° C.