JP2512650B2 - Method for producing Cr-Ni type stainless steel thin plate excellent in material and surface quality - Google Patents

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to the material and surface of a Cr-Ni-based stainless steel cast into a thin-walled slab having a plate thickness of 6 mm or less, and the thin-walled slab is cold-rolled into a thin plate product. The present invention relates to a method for producing a Cr-Ni-based stainless steel thin plate having excellent quality. In recent years, a technique for directly casting a thin slab having a plate thickness of 10 mm or less from molten steel has been developed, and has already been implemented on an industrial scale. According to this new technology, the hot rolling process of the slab can be omitted, and a great effect can be brought about in terms of energy saving and cost saving. Hereinafter, the process based on the above technology will be referred to as STC process (Strip Casting Process).
Called slab, cast a slab with a plate thickness of 100 mm or more by continuous casting, and hot-roll it into a hot-rolled plate with a plate thickness of about several mm.
A process for producing a cold rolled thin plate product from the hot rolled sheet is referred to as a current hot rolling process.

[0002]

2. Description of the Related Art Conventionally, when a Cr-Ni type stainless steel thin plate product represented by 18% Cr-8% Ni steel is manufactured by the STC process, the material (elongation) of the product is reduced and There was a problem such as rough skin (generally called orange peel or roping).

For example, the Iron and Steel Institute of Japan "Materials and Processes" vol. 1 (1988) P.I. 1694-16
97, P.I. 1698-1701, "Nissin Steelmaking Technique" No. 6
No. 2 1990 P. 62-78, "Materials and Processes" v
ol. 1 (1990) P.I. A paper published in 769 describes a phenomenon in which the material (elongation) of the product becomes low. In these papers, the cause of the deterioration of the material quality (elongation) of the product is that the δ-ferrite remaining in the slab suppresses the growth of recrystallized grains during annealing of the cold-rolled sheet, and the product structure becomes fine-grained. It is written that, or the microsegregation of the slab remains in the product and the elongation decreases due to the non-uniformity of the micro composition.

The following three types of methods are described as measures for improving the growth of products. Hot-roll the slab (eg 5% at 1200 ° C)
Hot rolling). Intermediate annealing is performed and a cold rolling method is performed twice (for example, rolling is performed at a rolling reduction of 60%, annealing is performed at 1050 ° C. for 1 minute, and cold rolling is performed to a total rolling reduction of 80%). Homogenization heat treatment is performed (for example, heat treatment is performed at 1150 ° C. for 3 hours). Further, it is shown that even if Md ₃₀ changes, the thin plate manufactured by the STC process does not affect the elongation.

On the other hand, "Materials and Processes" vol. 1 (1
990) P. A paper published in 770 describes a phenomenon that the surface quality of a product is deteriorated. In this paper, orange peel-like roughening (roping) occurs on the surface of the cold-rolled sheet due to coarse crystal grains before finish cold-rolling. It is described that the grain size is reduced to prevent the surface roughness of the cold-rolled sheet. Hot rolling the slab (for example, 16
% Hot rolling). Intermediate annealing is performed, and a cold rolling method is performed twice (for example, 10% rolling is performed at room temperature, and 11% or more preliminary cold rolling is performed at 500 ° C. by intermediate annealing). Further, in JP-A-02-233529, δ-Fe c
By designing the composition with al (%) of -2 to 10 (%) and quenching the slab at a cooling rate of 100 ° C / sec or more after solidification, the γ grain size of the slab becomes finer and roping is improved. It is shown that

[0006]

[Problem to be Solved by the Invention] In order to produce a thin plate product having good surface quality and workability by the STC process, the above-mentioned workability improving method and surface quality improving method are used to improve productivity and equipment configuration. Various problems occur. For example, in order to perform hot rolling, hot rolling equipment is required,
Since the equipment cost increases, the advantage of the STC process that the equipment cost is lower than the current process cannot be utilized.

Further, when the cold rolling and the intermediate annealing method are used for manufacturing, the time required for cold rolling and annealing is doubled, and the productivity is remarkably reduced. The homogenizing heat treatment method requires high-temperature heat treatment on the order of several hours, and such long-time heat treatment requires heat treatment of the coil using a batch furnace. Depending on the heat quantity of the furnace and the unit weight of the coil, it takes about 6 hours or more to heat up and heat-treat, and it is very inefficient compared to the fact that the hot-rolled sheet annealing of the current hot-rolled process material is tens of seconds. It's obvious. Further, it is difficult in terms of equipment to rapidly cool the slab immediately after casting. For example, it is possible to perform rapid cooling by performing air-water cooling, water cooling, etc., but steam explosion may occur due to steam generated during cooling, which is extremely dangerous. It is also possible to cool the slab by the contact cooling method, but it is difficult to uniformly cool the entire width of the slab.

The present invention provides a method for efficiently improving the problems of surface quality and workability which are problems in the STC process, without lowering the productivity as compared with the current process and without the need for hot rolling equipment. It was completed for the purpose of providing.

[0009]

The present invention has the following constitution in order to achieve the above object. Its characteristic is 18% C
In a process of casting a Cr-Ni-based stainless steel represented by r-8% Ni steel into a thin slab having a plate thickness of 6 mm or less, and manufacturing a cold-rolled thin plate product from the slab, the above Cr-Ni is used.
Md ₃₀ of system stainless steel component is ₃₀ to 50 ° C., δ-Fe
cal is 0 to 10%, and the cast piece after casting is 1200 to 9
Hold for 5 to 60 seconds in the temperature range of 00 ° C, then 900 to 5
A thin plate product is manufactured by cooling up to 50 ° C. at a cooling rate of 20 ° C./sec or more, and then subjecting the slab to pickling, cold rolling, finish annealing / pickling or bright annealing. However _{Md 30 = 413-462 (C + N} ) -9.2Si-8.1Mn -13.7Cr-18.5Mo-9.1 (Ni + Cu) ( component weight%) δ-Fe cal (% ) = 3 (Cr + 1 .5Si + Mo + 2.5Al) -2.8 (Ni + 0.5Mn + 0.5Cu) -84 (C + N) -19.8 (components are wt%).

[0010]

[Function] The thin-wall continuous cast slab not only has a higher solidification rate than the existing slab continuous cast slab, but also has a higher cooling rate after solidification. In addition, since there is no slab heating step before hot rolling that was performed in the current process, the opportunity to apply heat to the slab is less than in the current process. Therefore, it can be inferred that the state of the δ ferrite phase and the precipitate is significantly different from that of the hot rolled sheet in the current process.

According to the investigation result of the present inventor, the cause of deteriorating the workability (elongation) of the product in the STC process is MnS finely deposited on the cast slab or the cold rolled annealed plate. This fine Mn
Since the grain growth of the product is suppressed by S and a fine grain structure is formed, the workability (elongation) of the product deteriorates. Therefore, it is necessary to coarsely precipitate MnS at the stage of casting.

Reheating the slab once cooled to room temperature,
When coarsening is performed by homogenizing heat treatment, heat treatment at high temperature for a long time is required for coarsening. It is considered that this is because MnS is finely precipitated in the process of cooling the slab to room temperature or in the process of reheating after cooling, and the growth thereof is Ostwald growth.

Therefore, if a method of keeping the slab just after casting at a high temperature (1200-900 ° C.) as in the present invention is adopted, coarse precipitation occurs in a relatively short time because there are few MnS precipitation sites in the room temperature range. MnS can be made harmless.
Therefore, the thin plate product manufactured from the slab obtained by making the MnS harmless by high-temperature winding has good workability (elongation).

FIG. 1 shows an outline of equipment for carrying out the present invention. The slab S cast by the thin wall continuous casting machine (twin drum casting machine) 1 passes through the heat retention furnace (or heating furnace) 2 and is sufficiently Mn in the furnace.
After coarsely depositing S, it is cooled to 500 ° C. or lower in the secondary cooling zone and then wound up. Retaining furnace (or heating furnace)
The configuration of No. 2 may be either a horizontal furnace 2-1 as shown in FIG. 1 (a) or a vertical furnace 2-2 as shown in FIG. 1 (b).

In the present invention, the heat retention temperature is 1200 to 90.
The reason why the temperature is 0 ° C. will be described. Although MnS precipitates even in the temperature range over 1200 ° C., it enters the stabilization temperature range of δ ferrite, so that the δ ferrite phase increases and the δ ferrite phase remains in the final product, which may cause a problem. Therefore, the upper limit of the holding temperature is 1200 ° C. Further, at a temperature lower than 900 ° C., MnS precipitation requires a long time, so that it is necessary to lengthen the furnace length, which causes a large problem such as an increase in equipment cost. Therefore, the lower limit of the heat retention temperature is set to 900 ° C.

FIG. 2 shows the relationship between the heat retention conditions (temperature, time) and the elongation of the cold rolled annealed thin plate product produced from the slab. 105
It can be seen that the elongation improves at 0 ° C. in about 5 seconds and at 900 ° C. in 60 seconds. Therefore, the heat retention time is set to a range of 5 to 60 seconds.

Since the slab is subjected to the MnS precipitation treatment on-line as described above, the coiling temperature can be lowered to prevent the precipitation of carbides and the slab heat treatment can be omitted. Therefore, it is necessary to completely prevent the precipitation of carbides by cooling the slab after heat retention in the temperature range of 900 to 550 ° C at a cooling rate of 20 ° C / sec or more. This temperature range is 20 ℃ /
When cooling at a cooling rate of sec or less and pickling in a state where carbide is precipitated, microgrooves are generated on the surface of the slab, and the microgrooves remain on the product and deteriorate the surface gloss of the product.

Further, in this process, a slight amount of δ-ferrite remains in the cast piece, so that it is considered that slight segregation remains in the final product. Therefore, it is also effective to subject the slab to a short-time diffusion heat treatment at 1000 to 1200 ° C. for 20 to 180 seconds as described in claim 2.

Occurred on the cold rolled sheet surface when the slab is cold rolled
The surface roughness (roping) is Md ₃₀Ingredients specified in
To Md₃₀= 0 to 10 at δ-Fecal at 30 to 50 ° C
It can be improved by controlling to (%). this
Austenite unstable component system
The amount of processing-induced martensite
By controlling the amount of ellite and refining the γ grain size of the cast slab
If the plastic deformation during cold rolling is made uniform, it will occur during cold rolling.
Rough skin can be prevented. The component (Md₃₀,
5 shows the relationship between δ-Fecal) and roping. Md₃₀,
The roping becomes smaller as δ-Fecal increases.
, Δ-Fecal = 0% or more, Md₃₀= Above 30 ° C
Roping is no longer recognized.

However, if Md ₃₀ is too high, the cold workability of the product sheet deteriorates, and problems such as the appearance of magnetism in the processed product occur, so the temperature is set to 50 ° C. or less. In addition, δ-Feca
If 1 is set to 10% or more, problems such as residual δ-ferrite in the final product may occur, so it was set to 10% or less. Incidentally, in the high Md ₃₀ component system as in the present invention, the problem of age cracking of the product occurs. For example, "iron and steel" vol. 65 (1979) NO. 4, S472 also describes the effect of C and N on age cracking, and it is desirable that the total amount of C + N be 0.09% or less.

[0021]

EXAMPLE Various Cr-Ni type stainless steels having components (Table 1) within the scope of the present invention based on 18% Cr-8% Ni steel shown in Table 1 were melted, and internal water-cooled twin drum casting was used. Machine is used to cast thin slabs with a plate thickness of 2 to 3 mm, 1200 to 900
After holding in the temperature range of ℃ for 5 to 60 seconds, 900 to 550 ℃
Was cooled with water at a cooling rate of 20 ° C./sec or more. The slab was pickled and cold-rolled, and finally annealed, pickled and temper-rolled to obtain a thin plate product. The thin plate products were evaluated for workability (elongation) and surface quality (roping).

Further, as a comparative example, a thin cast piece was cast from a Cr-Ni type stainless steel having a component outside the scope of the present invention by the same method, and the temperature range of 1200 to 900 ° C. was rapidly cooled and held for less than 5 seconds. After the time had passed, the temperature range of 900 to 550 ° C. was cooled at various cooling rates and wound up. The slab was pickled and cold-rolled, and finally annealed, pickled and temper-rolled to obtain a thin plate product. The thin plate products were evaluated for workability (elongation) and surface quality (roping).

As shown in Table 2, the thin plate products produced by the present invention had excellent workability (elongation) and good surface quality, but the thin plate products produced in Comparative Examples had processability (elongation) or surface quality. Was insufficient.

[0024]

[Table 1]

[0025]

[Table 2]

[0026]

According to the present invention, a Cr-Ni type stainless steel thin plate product having good surface quality and workability can be efficiently produced by the STC process. Therefore, the technical effect is extremely large in terms of economy.

[Brief description of drawings]

FIG. 1 is a schematic diagram of equipment for implementing the present invention.

FIG. 2 is a diagram showing a relationship between heat retention conditions of a slab and elongation of a cold-rolled annealed sheet product manufactured from the slab.

FIG. 3 is a diagram showing the relationship between the components (δ-Fecal, Md ₃₀ ) of the cast slab and the surface roughness after cold rolling.

[Explanation of symbols]

 1 ... Twin-drum casting machine 2 ... Heat-retaining furnace 3 ... Secondary cooling zone 4 ... Winding machine 5 ... Slab

─────────────────────────────────────────────────── ─── Continuation of the front page (51) Int.Cl. ⁶ Identification number Office reference number FI technical display location // C22C 38/00 302 C22C 38/00 302Z 38/44 38/44 (72) Inventor Toshiyuki Suehiro Hikari City, Yamaguchi Prefecture 3434 Shimada, Nippon Steel Co., Ltd., Hikari Steel Works, Inc. (72) Inventor Takashi Arai, Hikari City, Yamaguchi Prefecture, 3434 Shimada, Hikari Steel Works, Nippon Steel Co., Ltd. (72) Inventor, Hideki Oka Hikari City, Yamaguchi Prefecture 3434 Shimada, Shin-Nippon Steel Co., Ltd.

Claims (2)

(57) [Claims] 1. C represented by 18% Cr-8% Ni steel
In a method for producing a cold rolled thin plate product by casting r-Ni stainless steel into a thin cast piece having a plate thickness of 6 mm or less, the Cr-Ni stainless steel component has an Md ₃₀ of ₃₀ to 50.
C, δ-Fecal is set to 0 to 10%, the slab immediately after casting is held in a temperature range of 1200 to 900 ° C for 5 to 60 seconds, and then a temperature range of 900 to 550 ° C is set to 20 ° C.
A method for producing a Cr-Ni-based stainless steel sheet, which comprises cooling at a cooling rate of not less than 1 sec / sec, followed by pickling and cold rolling, and then annealing / pickling or bright annealing to obtain a sheet product. However _{Md 30 (℃) = 413-462 (} C + N) -9.2Si-8.1Mn -13.7C-18.5Mo-9.1 (Ni + Cu) ( component weight%) δ-Fe cal (% ) = 3 (Cr + 1.5Si + Mo + 2.5Al) -2.8 (Ni + 0.5Mn + 0.5Cu) -84 (C + N) -19.8 (Components are wt%) 2. C represented by 18% Cr-8% Ni steel.
In a method for producing a cold rolled thin plate product by casting r-Ni stainless steel into a thin cast piece having a plate thickness of 6 mm or less, the Cr-Ni stainless steel component has an Md ₃₀ of ₃₀ to 50.
C, δ-Fecal is set to 0 to 10%, and a cast piece immediately after casting is held in a temperature range of 1200 to 900 ° C for 5 to 60 seconds, and then a temperature range of 900 to 550 ° C is set to 20 ° C.
1000-1 by cooling the slab at a cooling rate of not less than / sec.
Heat treatment in a temperature range of 200 ° C. for 20 to 180 seconds, and then
A method for producing a Cr-Ni-based stainless steel thin plate, which comprises subjecting to cold rolling after pickling, and then performing final annealing / pickling or bright annealing to obtain a thin plate product. However ^{_{Md 30 (C O) = 413-462}} (C + N) -9.2Si-8.1Mn -13.7Cr-18.5Mo-9.1 (Ni + Cu) ( component weight%) δ-Fecal (%) = 3 (Cr + 1.5Si + Mo + 2.5Al) -2.8 (Ni + 0.5Mn + 0.5Cu) -84 (C + N) -19.8 (Components are wt%)