JPH03285023A - Production of cr-ni-based stainless steel thin sheet excellent in surface quality and material quality - Google Patents

Abstract

PURPOSE:To produce a Cr-Ni-based stainless steel thin sheet excellent in surface quality and material quality by casting Cr-Ni-based stainless steel having the specified compositional relation into a thin cast piece in N2 and holding it at the specified temp. and thereafter performing cold rolling and final annealing. CONSTITUTION:A thin cast piece having <=6mm sheet thickness is cast from Cr-Ni-based stainless sheet such as 18%Cr-8%Ni. The product of a cold-rolled thin sheet is obtained by omitting hot rolling. In the above-mentioned production, the component delta-Fecal (however delta-Fecal=3(Cr+1.5Si+Mo+Nb+Ti)-2.8(Ni+0.5 Mn+0.5Cu)--84(C+N)-19.8%) of Cr-Ni-based stainless steel is controlled to 0-10%. Molten steel of this component is cast in the atmosphere contg. N2 or He as a main component. Then after the obtained thin cast piece is held at the temp. region of 800-1250 deg.C for <=80min, it is annealed in accordance with necessity and cold-rolled and final annealing is performed. Thereby an austenite-based stainless steel thin sheet excellent in surface quality and material quality is economically obtained in a simple process.

Description

[Detailed Description of the Invention] [Industrial Application Field] The present invention casts a slab of a size close to the product thickness by a so-called synchronous continuous casting method in which there is no relative speed difference between the slab and the inner wall surface of the mold. The present invention relates to a method of manufacturing a Cr-Ni stainless steel thin plate.

[Conventional technology]

Conventionally, in order to manufacture stainless steel thin sheets using the continuous casting method, the mold is vibrated during the casting method to form a sheet with a thickness of 100 mm.
The following slabs are cast, the surface of the resulting slabs is treated, heated to over 1000°C in a heating furnace, and then hot rolled using a hot strip mill consisting of a rough rolling mill and a finishing rolling mill row. It was made into a thick hot strip.

When cold rolling the hot strip obtained in this way, the shape (flatness), material, and
In order to secure the surface quality, hot-rolled sheets were annealed to soften the hot strips that had undergone intense hot working, and scales and the like on the surface were removed by grinding after the pickling process.

This conventional process requires a large amount of energy to heat and process the material in a long hot rolling facility, and cannot be said to be an excellent manufacturing process in terms of productivity. In addition, the final product had a developed texture, and the user had to take into account its anisotropy when applying press processing, etc., and there were many restrictions on use.

Therefore, in order to solve the problem of requiring a long hot rolling equipment and a large amount of energy and rolling power in order to roll slabs with a thickness of 100 ma+ or more into hot strip, we have recently developed a continuous casting process. Research is underway on a process to obtain slabs (thin strips) with a thickness equal to or close to that of hot strip.

For example, a featured article in "Tetsu to Hagane" 85, A197-A256 discloses a process for obtaining hot strip directly by continuous casting.

In such a continuous casting process, the twin drum method is used when the gauge of the slab to be obtained is 1 to 10 mm, and the twin drum method is used when the gauge of the slab to be obtained is 20 to 50 mm. A belt system is being considered.

[Problem to be solved by the invention]

5tl manufactured by twin-drum casting and one-time cold rolling process
It is known that S304FII sheet products have a finer grain structure and lower elongation than thin sheet products manufactured by conventional processes. For example, rcAMPIsIJJνol.

11988, 1670-1705, it is also reported that as a countermeasure to this problem, annealing the slab to eliminate the δ ferrite remaining in the slab is described.

As a result of a detailed study by the present inventors on the Cr-Ni stainless steel-like Tjl'fi manufacturing process by continuous strip casting, we found that there are the following three factors that suppress the growth of recrystallized grains during cold rolling and annealing. has become clear.

(1) δ ferrite remaining in the slab (2) Mn5 finely precipitated during final annealing after cold rolling (3) Minute inclusions δ ferrite in the slab should be removed by reheating and annealing the slab. is possible. However, the δ ferrite remaining in the slab is not sufficiently transformed into the γ phase by short-time annealing as is done in conventional hot-rolled austenitic stainless steel strips, so long-term heat treatment is performed to transform the δ ferrite into γ-phase. It is necessary to transform the γ phase into the γ phase.

It was also revealed for the first time that MnS, which is finely precipitated during the final annealing after cold rolling, strongly suppresses grain growth. Therefore, it is necessary to cause MnS to precipitate sufficiently coarsely at the slab stage to render it harmless. However, the method of reheating and annealing the slab requires heat treatment at a high temperature for a long time, and there is a desire for a method that efficiently performs the heat treatment at a high temperature for a long time and facilitates grain growth.

Further, minute inclusions in the slab also inhibit grain growth in the cold-rolled annealed sheet, but inclusions can be made harmless by controlling components such as deoxidizing elements.

5US manufactured by twin-drum casting and one-time cold rolling process
One problem with thin sheets of 304 thin sheet products is surface problems (roving). Roving is a phenomenon that occurs because the grain size of the slab is coarse, and in order to refine the gamma grains of the slab and suppress roving, it is effective to rapidly cool the slab immediately after casting. However, a large amount of δ ferrite remains in the cooled slab, and the precipitation of MnS into the slab is also suppressed, causing the material to deteriorate as described above.

[Means to solve the problem]

The present inventors have studied the conditions for achieving both the material quality (elongation) and surface quality (roving) of the thin plate, and as a result, it has been found that the γ grains of the slab can be made finer by controlling the casting and solidification atmosphere.
Furthermore, it was revealed that the grain size of the seven slabs could be made finer by adjusting the main components, and that by holding the slabs at high temperatures, it was possible to achieve both the material quality (elongation) and surface quality (roving) of the thin plate. I found it.

That is, the gist of the present invention is to obtain a plate thickness of 6III from Cr-Ni stainless steel represented by 18%Cr-8%Ni steel.
In the process of manufacturing thin plate products by casting a thin slab of 11 or less and directly cold rolling it without hot rolling, the casting and solidifying atmosphere is mainly N2 or He, and δ- Fecalfi(%)=3(Cr+
1.5Si+Mo+Nb+Ti)-2,8(Ni+0.
5Mn+0.5Cu)-84(C+N)-19,8(%
) defined as δ-Fecaj! By setting (%) to 0 to 10%, the primary crystals of solidification are made into δ phase, and the starting temperature of crystallization and precipitation of T phase is lowered, thereby suppressing the growth of γ grains during and after solidification. , then 800℃ or higher 1250℃
The method for manufacturing a Cr--Ni stainless steel thin plate includes holding the sheet at a temperature of .degree.

[For production]

In the process of manufacturing 5US304 thin plate using the continuous strip casting method, one part of the slab is
It was necessary to develop a method for refining the grains and an efficient heat treatment method for reducing the δ ferrite remaining in the slab and for sufficiently coarsely precipitating MnS.

The present inventors have investigated the heat treatment conditions for reducing δ ferrite and coarsely precipitating MnS, and found that by heat-treating the slab immediately after casting in a temperature range of 1250 to 800°C, δ Ferrite disappears and M
It was revealed that nS was coarsely precipitated. In addition, 120
If held at 0-1000°C, then 1000-5
By cooling in a temperature range of 50°C at a cooling rate of 50°C/sec or more, it is possible to prevent precipitation of carbides and omit the heat treatment step of the slab for solid solution of carbides.

In addition, in order to refine the grain size of the slab, the casting and solidification atmosphere must be mainly N2 or He, so that fine chill crystals remain on the surface layer of the slab and Ar is applied throughout the entire thickness of the slab. It has been found that the gamma grain size of the slab becomes finer than that of the slab cast in an atmosphere.

Figure 1 (a) is a microscopic metallographic photograph of a slab cast from molten steel with a composition of δ-Fe can = 3.1% in an N2 atmosphere, and Figure 1 (b) is a photograph of a slab with a composition of δ-Pecaf = 3. ．．
This is a microscopic metallographic photograph of a slab cast from 5% molten steel in an Ar atmosphere. Comparing these two structures, it is clear that the structure shown in Figure (a) is finer.

Furthermore, by controlling the principal component, δ-Pe ca 1 = 3 (
Cr+1.5Si+Mo)-2,8(Ni+0.5Cu
It was revealed that by setting δ-Fecaj2 represented by +0.5Mn)-84(C+N)-19,8 to 0 to 10%, the grain size of the slab 7 becomes finer.

That is, FIG. 1(C) shows the component δ-Fe car! =
- This is a microscopic metallographic photograph of a slab cast in a N atmosphere with 2.1% molten steel.
It can be seen that the grain size of slab 7 is larger than that of slab a).

Figures 2 and 3 show N! using a twin-drum continuous casting machine. 1200-8 immediately after casting for JIS304 stainless steel slab (thickness 2++m+) cast in atmosphere
FIG. 3 is a diagram showing the relationship between holding conditions at 00° C., elongation of the final product, and roving. By holding the slab at high temperature for a long time, δ ferrite decreases and MnS precipitates, so the grains during cold rolling annealing grow and exhibit good elongation. but,
If the temperature exceeds 1250°C, one grain of the slab will grow even for a short time, resulting in roving during cold rolling. Therefore, in order to manufacture thin plate products with excellent surface quality and material quality, it is necessary to hold the slab in a temperature range of 1250 to 800°C for 80 minutes or less.

〔Example〕

Austenitic stainless steels of various compositions based on 18% Cr-8% N1JLIIL shown in Table 1 are melted and cast into slabs with a thickness of 2 m in various atmospheres using an internal water-cooled twin drum casting machine. Cast at 800-1250°C
It was maintained at a temperature of The slab is annealed, pickled, cold rolled,
After annealing, it was temper rolled to produce a thin plate product, and the surface quality and material quality were evaluated.

In addition, as a comparative example, heat treatment conditions immediately after casting, δFecaI
! , Thin plate products were similarly manufactured from slabs cast under casting atmosphere conditions outside the range of the present invention, and the surface quality and material quality were evaluated.

These evaluations are shown in Table 2. According to this table, the thin plates manufactured by the method of the present invention (Nos. 1 to 9) had excellent material quality and good surface quality, but the thin plates manufactured by the comparative method (Nα10 to 12) had poor material quality (elongation). Or the surface quality (roving) was poor.

The following margin [Effects of the invention] According to the present invention, an austenitic stainless steel thin plate with excellent surface properties is obtained by a simple process of commercializing a thin strip slab with a thickness close to the product thickness by continuous casting and direct cold rolling. be able to. Therefore, its technical effects are extremely large in terms of economy and manufacturing purpose.

[Brief explanation of drawings]

Figure 1 is a microscopic metallographic photograph of a thin slab obtained by the continuous casting method, in which (a) is a photograph of the metallographic structure of a thin slab obtained by the method of the present invention, and (b) and (C) are photographs of the comparative method. Fig. 2 is a photograph of the metallographic structure of a thin slab cast by the method of the present invention.
FIG. 3 is a diagram showing the state of elongation in the L direction when held for 80 minutes, and FIG. 3 is a diagram showing the state of roving when a thin slab cast by the method of the present invention is held under the same conditions as in FIG. 2. It is. 139- Elongation in L direction ○ Good/° Bad 102030405060708° Holding time (minutes) Surface quality (roving) Procedural amendment (voluntary) Good 1990 Date

Claims (1)

[Claims] 1. Thin slabs with a thickness of 6 mm or less are cast from Cr-Ni stainless steel such as 18%Cr-8%Ni steel, and hot rolling is omitted to produce cold-rolled thin plates. In the method of manufacturing a product, δ-Fec of the Cr-Ni stainless steel component
Molten steel with al (%) controlled to 0 to 10% is heated to N_2 or H
Casting in an atmosphere containing e as the main component, then holding the thin slab obtained by the casting in a temperature range of 800°C or more and 1250°C or less for 80 minutes or less, cold rolling, and final annealing. Cr-Ni with excellent surface quality and material characteristics
A method for manufacturing thin stainless steel sheets. However, δ-Fecal (%) = 3 (Cr+1.5Si+
Mo+Nb+Ti)-2.8(Ni+0.5Mn+0.
5Cu)-84(C+N)-19.8(%)2, the thin slab is held in a temperature range of 800°C or higher and 1250°C or lower for 80 minutes or less, then annealed, cold rolled, and final annealed. The method according to claim 1.