JPH02236227A - Production of cr-ni stainless steel sheet excellent in surface quality - Google Patents

Abstract

PURPOSE:To obtain a steel sheet free from surface defects and excellent in quality by continuously casting a Cr-Ni stainless steel into a strip-like slab while specifying cooling velocity at the time of solidification, reheating the above, and then carrying out hot and cold workings. CONSTITUTION:A molten Cr-Ni stainless steel is continuously cast into a strip- like slab of <=10mm thickness while regulating the cooling rate at the time of solidification of >=100 deg.C/sec by using a double roll casting machine of internally water cooled type. The resulting cast slab is heated or cooled without being heated, by which a part of the cast slab is transformed into martensite. Then, reheating is carried out to transform martensite to austenite. Successively, either or both of warm working and cold working are performed. By this method, a stainless steel sheet free from the occurrence of roving and excellent in surface quality can be obtained.

Description

Detailed Description of the Invention [Industrial Field of Application] The present invention involves obtaining a strip-shaped slab with a thickness of 10 mm or less using a continuous casting machine in which the mold wall surface is moved and renewed, and using this as a starting material to cast Cr-Ni & The present invention relates to a method for manufacturing thin stainless steel sheets that can produce products with excellent surface quality.

[Conventional technology] Conventionally, in order to manufacture stainless steel thin plates using the continuous casting method, a mold of 100 mm in thickness was produced while vibrating the mold in the casting direction.
The above slab is cast, the surface of the obtained slab is treated, heated to 1000°C or higher in a heating furnace, and then hot rolled by a hot strip mill consisting of a rough rolling mill and a finishing rolling mill row. It was a hot strip with a thickness of several mm.

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, because the final product is manufactured from a cast slab with a thickness of 100nv+ or more that undergoes a lot of processing, the texture develops, and when the user applies press processing etc. to the product, the anisotropy is There were also many restrictions on use that needed to be taken into account.

Recently, in order to solve the problem that rolling slabs with a thickness of 100 mm or more into hot strip requires a large amount of hot rolling equipment and a large amount of energy and rolling power, a continuous casting process has been recently developed. 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, "Tetsu to Hagane" 85, AI97~
85, A256, a process for obtaining hot strip directly by continuous casting is disclosed. In such a continuous casting process,
The gauge of the slab to be obtained is 1 to 10 m.
When the gauge of the slab is 20 to 50 m+, the twin belt system is being considered.

However, in these continuous casting processes, there are still problems at the casting stage, and the problems regarding the material and surface quality of the product have not yet been resolved.

[Problem to be solved by the invention] In a process that is being developed as a new process and is based on the premise of obtaining a slab (thin strip) with a thickness equal to or close to that of hot strip by continuous casting, , in order to simplify the process from casting to products,
The surface properties of stainless steel products will be sensitively influenced by the properties of the slab. That is, in order to obtain a product with excellent surface properties, it is necessary to obtain an excellent slab.

The present invention has been made with the object of providing a simple manufacturing process capable of obtaining a Cr-Ni stainless steel thin plate free of surface defects called uneven gloss and roping phenomenon that are characteristic of stainless steel thin plate products.

[Means to solve the problem] The gist of the invention is as follows.

For example, Cr-N represented by 18% Cr-84kNi steel
I-series stainless steel is continuously cast into a thin strip slab with a thickness of 10 mm or less using a continuous casting machine in which the mold wall surface is moved and renewed at a cooling rate of 100 ° C / s or more during solidification, and the obtained slab is After heating or cooling without heating to transform a part of the slab into martensite and reheating to reversely transform martensite into austenite, warm-worked and cold-worked IHA or 2 A method for producing a thin Cr-Ni stainless steel sheet with excellent surface quality, characterized in that the product is produced by applying a coating or more.

The details of the present invention will be explained below.

Molten steel whose basic component is SOS 304 steel is cast using an internal water-cooled twin-roll (twin-drum) continuous casting tester to produce 2 to 4 mmJ! ; It was made into a thin ribbon with a thickness of T, cooled, and wound up.

The thus obtained slab (thin ribbon) was descaled, directly cold rolled, finally annealed, and pickled to obtain a 2B product. The surface quality of these products can be obtained by conventional continuous casting of molten steel to obtain a slab with a thickness of 100m+++ or more, which is then reheated, hot rolled in a hot strip mill, and then cold rolled. A detailed comparative study was carried out with the surface properties of other products.

As a result, the molten steel was cast into a thin strip with a thickness of 2 to 4 mm using an internal water-cooled twin-roll (twin-drum) continuous casting machine, which was cooled and wound, which was then descaled and cold-rolled. It has been found that the following surface defects may occur in the final annealed and pickled 2B product.

(1) Roping and orange beer: fine irregularities occur on the surface during cold rolling or product processing.

(2) Uneven gloss: Uneven gloss occurs due to the sensitization of the material structure, grain boundary oxidation, or coarsening of γ grains during winding of the slab (thin ribbon).

These problems regarding product surface properties are unique to the process, which involves the process of directly obtaining thin slabs (thin strips) by continuous casting, which is not seen in conventional processes.

As a result of a detailed study of the causes of problems related to the surface properties of these products, the inventors found that the γ grain size of the material before cold rolling was large, or that the slab was not sufficiently cooled in the carbide precipitation temperature range. It was found that these surface defects occur significantly in some cases.

In this way, as a countermeasure against roping, the γ grain size of the material before cold rolling is adjusted to the grain size NO. 6 or more, that is, 50 μm or less, and as a countermeasure against uneven gloss, it is necessary to control the cooling of the slab in the high temperature range.When adopting a process that involves obtaining thin slabs directly by continuous casting, revealed what is desirable.

As a means for producing a material for cold rolling with a γ grain size of 504 m or less, there are various ideas as follows. In other words, 1) Reducing the γ grains of the thin slab itself, 2) Hot working the thin slab after casting to make it recrystallized to fine grains, 2 Cold working and annealing the thin slab. , recrystallization and grain refinement, etc.

Although the method (■■) is highly effective, it requires a large number of steps and is not economical. Furthermore, the method (2) is possible by specifying the cooling rate of the slab after casting, and was proposed by the applicant in Japanese Patent Application No. 63-221472.

The inventors of the present invention have found that the above-mentioned problem can be solved even when the apparent grain size is 50 μm or more, especially by reducing the γ grains of the slab itself and making the crystal orientation random. It is based on the findings of

The γ grain size before cold rolling is determined by γ average grain size NO. 6 or more, that is, 50 um or less as an average grain size, roving is less likely to occur, but the inventors believe that although the occurrence of roving is also influenced by the γ grain size itself before cold rolling, We found that the crystal orientation of the γ grains is important.

In other words, if crystals with a specific orientation exist in a group, roping will occur even if the grain size is small, and conversely, even if the grain size is large (50 μm or more), the orientation of the individual crystal grains will change. If they are different from each other, roving will not occur.

The inventors discovered that roving occurs due to the plastic anisotropy of crystals, similar to the mechanism of roving in thin ferritic stainless steel sheets. Therefore, the inventors discovered that in order to make the relationship between the crystal grains of the material as random as possible before cold rolling, δ
If ferrite is present, the slab is heated to form a single γ phase, and if it has a solidified structure of a single γ phase, martensitic transformation occurs in a part of the solidified slab without heating the slab. It has been found that by reheating the cast slab after cooling it to a low temperature range to re-transform this martensite into austenite, the orientation of the transformed austenite changes into about 130 different orientations.

This γ→α→γ transformation theoretically generates 528 types of crystal orientations. Although it was unclear what kind of crystal orientation grains actually occur, the inventors investigated the changes and found that about 130 different crystal orientations occur. FIG. 1 shows an example of a new crystal orientation (150 occurrences) generated from one crystal orientation. When transformed by the method according to the present invention, fine crystals with substantially random crystal orientation are generated in the large-grain γ crystals, and the large γ grains are substantially refined.
The present invention was completed by discovering that roving does not occur when a thin plate is produced using such a slab as a cold-rolled material.

[Example] Austenitic stainless steel was melted and cast into a slab with a thickness of 7.51 from laIII+ using an internal water-cooled twin roll casting machine, and then it was made into a γ single-phase structure either as it was or by reheating. After that, it was immersed in liquid nitrogen.

Thereafter, it was heated to 1000°C, then cooled, and cold rolled. No roping was observed on the surface of the obtained product, and the surface quality was good.

On the other hand, when the material was cold rolled without undergoing such treatment, the γ grain size exceeded 80 μm, roping occurred, and the surface gloss was poor.

The component systems and process conditions in Examples and Comparative Examples of the present invention are shown in Tables 1 and 2.

[Effects of the invention] By using the simple process of the present invention to directly obtain a thin strip with a thickness close to that of the product by continuous casting, it is possible to obtain a Cr-Ni stainless steel thin plate with excellent surface quality and material quality. can.

[Brief explanation of drawings]

FIG. 1 is a diagram showing an example in which one crystal orientation is divided into substantially random crystal orientations due to γ→α′→γ transformation.

Claims (1)

[Claims] 1. The cooling rate of Cr-Ni stainless steel during solidification is reduced to 100% using a continuous casting machine in which the mold wall surface is moved and renewed.
℃/s or more into a thin strip-shaped slab with a thickness of 10 mm or less, the obtained slab is heated or cooled without heating to transform a part of the slab into martensite, and then re-cast. Cr with excellent surface quality, which is produced by heating to reversely transform martensite into austenite, and then subjecting it to one or more of warm working and cold working.
- A method for producing a Ni-based stainless steel thin plate.