JP2768527B2 - Method for producing thin Cr-Ni stainless steel sheet with excellent workability - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION [Industrial Application Field] The present invention casts a slab having a thickness close to the product thickness by a so-called synchronous continuous casting method in which a mold moves synchronously with the slab, The present invention relates to a method for producing a Cr-Ni stainless steel sheet by cold rolling directly without hot rolling.

[Conventional technology]

Conventionally, to produce a stainless steel sheet by continuous casting, the mold is cast into a slab with a thickness of up to about 100 mm while vibrating in the casting direction, the resulting slab is cleaned, and then heated. After heating to a temperature of 1000 ° C or more in a furnace and performing hot rolling with a hot strip mill consisting of a rough rolling mill and a finishing rolling mill, a hot strip having a thickness of about several mm is formed, and then subjected to cold rolling. Then, necessary annealing, pickling, and temper rolling were performed to obtain a cold-rolled product.

Before cold rolling the hot strip obtained by hot rolling, to ensure the shape (flatness), material (crystal grain size, mechanical properties), and surface quality (prevention of roping) required for the final product In addition, hot plate annealing was performed to soften the hot strip in a state of being hardened by strong hot working, and pickling and grinding were performed to remove oxide scale and the like on the surface.

Such a conventional process requires a large amount of energy for heating and processing the material in a long hot rolling facility, and thus cannot be said to be an excellent manufacturing process from the viewpoint of productivity.

In addition, since the texture developed during hot working remains strongly in the finally obtained thin sheet product, the anisotropy caused by this texture must be taken into account when performing press working etc. by the user. There were many restrictions on the use of the product, such as the need to use it.

Therefore, it does not require long hot rolling equipment and a large amount of energy for hot rolling a slab having a thickness of 100 mm or more to form a hot strip, and it is difficult to use the product due to the texture of hot working. In order to solve the above-mentioned restrictions, a process is being developed in which a continuous casting process is directly connected to a cold rolling process without going through a hot rolling process. That is, it is a process of continuously casting a slab (thin strip) having a thickness equal to or close to that of a hot strip conventionally obtained by hot rolling, and cold rolling the thin slab. Such a process is
For example, it is described in a paper featured in “Iron and Steel”, Vol. 85, A197-A256.As a method of continuous casting, when the thickness of a slab to be obtained is about 1 to 10 mm, a twin is used. A drum type and a twin belt type are considered when the slab thickness is about 20 to 50 mm.

[Problems to be solved by the invention]

However, using any of the above-described methods of continuous casting, a sheet product manufactured by a continuous casting / cold rolling process (hereinafter abbreviated as “strip continuous casting”) without performing hot rolling is a conventional continuous casting. / Hot rolling / Cold rolling process (hereinafter referred to as "conventional method") has a much finer grain structure compared to thin products (grain size GSN 6-8)
Since elongation is low, there is a problem that workability such as press working by a user is reduced. This phenomenon is reported, for example, in a paper featured in `` CAMP ISIJ vo.1, 1988, 1670-1705, and as a countermeasure, δ ferrite remaining in the slab is reduced by annealing the slab. It is described that it disappears.

As a result of a detailed study of the process for manufacturing a Cr-Ni stainless steel sheet by continuous strip casting, the present inventor found that the growth of recrystallized grains during cold rolling and annealing due to the presence of δ ferrite and inclusions remaining in the slab. It was clarified that this was responsible for the fine grain structure of the final product and the resulting decrease in elongation. The residual δ ferrite can be eliminated by heat treatment after casting. However, inclusions cannot be controlled in the steps after casting, and it has been clarified that adjusting the composition of molten steel to be cast is important to reduce the amount of inclusions.

Here, in the continuous casting of thin cast slabs and the conventional method of casting thick cast slabs, the casting slabs have significantly different wall thicknesses, so the cooling rate during solidification is significantly different. There is a great difference in the crystallization morphology of inclusions.
That is, in the conventional method, the solidification cooling rate is about 10 ° C./sec, and the inclusions crystallized during solidification are agglomerated, and most of them are floated and separated from the steel and remain in the slab. Most of them are coarse and have a small amount of 5 μm or more. On the other hand, in strip continuous casting, the cooling rate during solidification is extremely fast, about 100 ° C / sec or more,
During solidification, a large amount of fine (1 μm or less) inclusions crystallize, and many of them remain in the steel without agglomeration.

As described above, the fine grain structure and the decrease in elongation in the thin plate product manufactured from the thin cast slab by the strip continuous casting are not present in the conventional method because the inclusion crystallization form peculiar to the strip continuous casting is caused. A completely new solution was needed.

The present invention reduces the amount of inclusions crystallized in a thin cast slab having a thickness of 6 mm or less to reduce the crystal grain size of a thin plate product to GSN.
C controlled to 8 or less to ensure practically sufficient surface quality (roping height 0.2 μm or less) and material (elongation 48% or more)
An object of the present invention is to provide a method for producing an r-Ni stainless steel sheet.

[Means for solving the problem]

According to the present invention, the above object is achieved by a continuous casting method in which a mold moves synchronously with a slab from a Cr-Ni-based stainless steel represented by 18% Cr-8% Ni steel to a thickness of 6 mm or less. In the method of casting a slab of, and performing a direct cold rolling without hot rolling to produce a thin sheet product, Ti in the molten steel before casting,
The components are adjusted so that the total content of Nb, Zr, Al, and REM is 0.05% or less, cast into a ribbon-shaped slab at a solidification cooling rate of 100 ° C / sec or more, and the obtained slab is wound. After taking, pickling and then cold rolling, after final annealing or bright annealing to perform pickling after annealing, rolling reduction 0.5 to 1.5
%, Which is achieved by a method for producing a thin Cr-Ni stainless steel sheet having excellent workability, characterized in that the sheet is subjected to temper rolling of 0.1%.

The present inventors have investigated in detail the relationship between the amount of inclusions in the slab and the deoxidizing conditions and composition of molten steel before casting, and found that Si-Mn
Deoxidation was found to have less inclusions than Al, Ti, and REM deoxidation, and increased inclusions when Zr and Nb were present in the molten steel. Further, as shown in FIG.
By reducing the total content of Ti, Nb, Zr, Al, and REM in molten steel before casting to 0.05% or less, the crystal grain size (GSN) after cold rolling and annealing becomes 8 or less, which is a good material. Was obtained.

On the other hand, if the γ-grain size of the slab increases with the decrease in the amount of inclusions, roping occurs in the sheet product, and it is necessary to suppress the occurrence of roping by temper rolling. Second
As shown in the figure, in order to suppress the roping height to 0.2 μm or less, which is not a problem in practical use, the temper rolling must be performed at a rolling reduction of 0.5 mm.
It is necessary to carry out in more than%. However, in order to secure the elongation of the sheet product to 48% or more which is necessary for practical use, it is necessary to reduce the rolling reduction of the temper rolling to 1.5% or less.

The experiments in FIGS. 1 and 2 were performed as follows.

In order to strictly control the slab components, the raw materials use high-purity alloys and electrolytic metals, and in order to investigate the effects of trace components, Ti, Al, Nb, Zr, REM are added singly or in combination, A SUS304 slab (width 100 mm × sheet thickness 1 mm) containing (Ti + Al + Zr + Nb + REM) = 0 to 0.25% was cast by a laboratory-scale twin-roll continuous casting machine. Cast slab 1200 ° C × 120
After a second HA, descaled. Thereafter, cold rolling (1.0 → 0.3 mm ^t , cold rolling rate 70%) and final annealing (1170
(° C. × 20 seconds), and the result of examining GSN is shown in FIG. In addition, about the cast piece containing (Ti + Al + Zr + Nb + REM) = 0.03%, after final annealing, the tempering rolling (change in temper rolling reduction 0-1.8%) was performed, and the roping and the elongation in the L direction were measured. The results are shown in FIG.

(Operation)

The present invention reduces the amount of fine oxides, nitrides, and other inclusions that crystallize during solidification, promotes crystal growth during cold rolling and annealing, and appropriately reduces the occurrence of roping due to this growth. By controlling the rate by temper rolling, a Cr-Ni stainless steel sheet having both practically sufficient surface quality and material (elongation) can be manufactured.

According to the method of the present invention, it is not necessary to perform annealing for eliminating residual ferrite in the slab or cold rolling at a high temperature for a long time, which is extremely advantageous in terms of productivity and production cost.

Hereinafter, the present invention will be described in more detail with reference to examples.

〔Example〕

Various austenitic stainless steels having the basic composition of 18% Cr-8% Ni stainless steel shown in Table 1 were melted,
The components were adjusted such that the total amount of Ti, Nb, Zr, Al, and REM in the molten steel was 0.05% or less. This molten steel is 3 mm thick and 1 width wide by a twin-drum continuous casting machine with internal water cooling.
Cast into a slab of 00mm, cool the slab to a secondary temperature of 400 ° C
And wound up as a coil. After annealing and pickling this coil and cold rolling to 0.6 mm, annealing and pickling,
Temper rolling was performed at a rolling reduction of 1.0% to obtain a thin plate product.

(Comparative example)

For comparison, a thin product was manufactured from molten steel having a total content of Ti, Nb, Zr, Al, and REM exceeding 0.05% in the same procedure as in the example.

Table 2 summarizes the results of grain size measurement, tensile test, and roping height measurement performed on the thin sheet product samples of the examples and comparative examples.

Samples A to C manufactured according to the present invention had a grain size (GSN) of 8 or less, and had good elongation and good surface quality (roping height).

[Effects of the Invention] As described above, according to the present invention, in a production method by continuous casting in which a cold rolling process is directly performed without going through hot rolling from a casting process, the amount of inclusions crystallized in a slab By reducing temper rolling and optimizing the temper rolling, it is possible to produce a Cr-Ni stainless steel sheet having a surface quality and material sufficient for practical use, equivalent to the conventional method of hot rolling.
As a result, compared to the conventional method in which a thick slab up to about 100 mm is hot-rolled, a production method of a Cr-Ni stainless steel sheet with extremely high productivity which has significantly reduced the production cost is realized. You.

[Brief description of the drawings]

FIG. 1 is a graph showing the relationship between the total content of Ti, Al, Nb, Zr, and REM in a slab and the crystal grain size of a sheet product, and FIG. 2 is a reduction ratio of temper rolling and roping of a sheet product. It is a graph which shows the relationship with height and elongation.

──────────────────────────────────────────────────続 き Continued on front page (58) Field surveyed (Int.Cl. ⁶ , DB name) C21D 9/46 C21D 8/02

Claims (1)

(57) [Claims] 1. A slab having a thickness of 6 mm or less is cast from a Cr-Ni stainless steel represented by 18% Cr-8% Ni steel by a continuous casting method in which a mold moves in synchronization with the slab. In a method of manufacturing a thin sheet product by directly performing cold rolling without hot rolling, the content of Ti, Nb, Zr, Al, and REM in molten steel before casting is adjusted to be 0.05% or less in total. Adjust the ingredients to
Cast into ribbon-shaped slab at a solidification cooling rate of 100 ° C / sec or more,
The obtained slab is taken up, pickled and then cold-rolled, and then subjected to final annealing by performing pickling after annealing or final annealing by bright annealing, and then temper rolling at a rolling reduction of 0.5 to 1.5%. Cr- with excellent workability, characterized in that
Manufacturing method of Ni-based stainless steel sheet.