JPS6253569B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a method for producing stainless steel with excellent intergranular corrosion resistance and surface cleanliness.

Austenitic stainless steels are sensitized by slow cooling in the range of 400 to 850°C or by heating to this range. That is, Cr carbides such as CR ₂₃ C ₆ (hereinafter simply referred to as Cr carbides) are precipitated at the grain boundaries, and a Cr-deficient region is generated in the vicinity of the precipitates, resulting in susceptibility to intergranular corrosion. Ferritic stainless steels are similarly sensitized, but the conditions are different: they are more susceptible to intergranular corrosion when cooled from 850°C or higher. As countermeasures against intergranular corrosion, there are a method of reducing the carbon content in steel and a method of fixing carbon as carbides such as Ti and Nb by adding stabilizing elements such as Ti and Nb. However, reducing carbon requires the addition of expensive decarburizing elements during refining, and there is also the problem that the Cr yield decreases due to decarburizing refining. When a stabilizing element is added, in addition to fixing carbon, it combines with N ₂ , O ₂ , and other elements in the molten steel to form nonmetallic inclusions, which precipitate in groups on the surface layer of the slab. The precipitated nonmetallic inclusions are elongated in the processing direction during subsequent processing such as rolling, forging, extrusion, etc., and are exposed on the product surface, resulting in surface flaws and uneven pickling, reducing product yield.

The present invention aims to solve such problems of intergranular corrosion and deterioration of surface quality in stainless steel. The gist is that after heating the surface layer of a stainless steel piece, especially in the semi-finished product stage, to 2000℃ or higher in an inert gas atmosphere using a wide plasma arc, and melting the surface layer to a depth of 1 mm or more and 15 mm or less. The process consists of rapid cooling and solidification.

The stainless steel targeted by the present invention has grain boundaries.
These are steel types where intergranular corrosion is a problem due to the precipitation of Cr carbides, and steel types where non-metallic inclusions are precipitated in the surface layer due to the addition of stabilizing elements. The former includes austenitic stainless steels such as SUS304 that do not contain stabilizing elements or are not low carbonized.
There are ferrite stainless steels such as SUS430.
The latter include Ti-added austenitic stainless steels such as SUS321 and various Ti-added ferritic stainless steels that have not yet been standardized by JIS. The treatment process is suitable for semi-finished products such as continuously cast slabs, blooms, billets, slabs after blooming, blooms, billets, etc. (hereinafter, in the present invention, these are collectively referred to as billets).

The steel slab subjected to the treatment of the present invention may be in a state in which Cr carbide is precipitated at the grain boundaries, or may be in a state in which Cr carbide is not precipitated. In a state where Cr carbide is precipitated at grain boundaries, heating melts the Cr carbide,
Through the subsequent rapid cooling, carbon becomes a solid solution in the steel. Also, during melting, carbon reacts with oxygen in the steel and impurity oxygen in the atmosphere to become CO and decarburize. When Cr carbide is not precipitated at the grain boundaries, the carbon segregated near the grain boundaries is uniformly dissolved and decarburized by melting and rapid cooling. This uniform solid solution of carbon and decarburization prevents Cr from entering the grain boundaries in subsequent steps.
Precipitation of carbides can be prevented. In addition, the steel billet during the treatment of the present invention mainly contains Ti as a stabilizing element.
Nonmetallic inclusions such as nitrides and oxides are precipitated in groups on the surface layer. Such nonmetallic inclusions are finely and uniformly dispersed in the steel by melting and subsequent rapid cooling, and do not deteriorate the surface properties of the product.

The heating temperature of the steel slab needs to be 1800°C or higher to melt the Cr carbide when no stabilizing element is added, and 2000°C or higher when a stabilizing element is added. The melting depth of the steel slab needs to be 1 mm or more and 15 mm or less. In order to improve the intergranular corrosion resistance of a product, it is necessary to remove at least one layer from the product surface.
It is necessary that the grain boundaries of the grains are not sensitized, and considering the amount of scale-off due to subsequent hot working, even if the subsequent reduction rate is low, the fusion depth must be at least 1 mm at the slab stage. is necessary. If it melts beyond 15 mm, the quenching rate becomes too low and the necessary solid solution state cannot be obtained.

A plasma arc is used as a heat source to melt the surface of the steel piece. Plasma arc has a high energy density and can rapidly heat up to a high temperature of over 3000°C, making it possible to melt only the surface layer of a steel billet to the required depth. In order to uniformly melt the surface of the steel billet, the plasma torch must be moved in the width direction of the steel billet, or the torch may be fixed and only the arc can be swung in the width direction of the billet, while the steel billet or plasma torch is moved along the length of the billet. move it in the opposite direction.

(The plasma arc that is moved or vibrated in the width direction of the steel piece in this manner is hereinafter referred to as a wide plasma arc.) The melting depth is adjusted by adjusting the moving speed in the length direction of the steel piece. By continuously melting the surface in this way, the heat of the molten part is transferred to the inside of the steel slab, and it solidifies rapidly. The necessary solid solution state of carbon can be achieved without using any particular cooling means.
A dispersed state of inclusions is obtained.

If oxygen is present in the atmosphere during melting of the surface of the steel billet, the melted portion will be oxidized, resulting in quality deterioration such as the formation of new oxide inclusions. Therefore, it is necessary to provide an inert atmosphere in the melting zone and to keep the air concentration to 1% or less.

Hereinafter, specific examples of the present invention will be explained based on the drawings.
FIG. 1 shows an example of a preferred apparatus for carrying out the present invention, in which plasma torches 1 are arranged in a row in the width direction of a steel piece 4, and the area around the molten part on the surface of the steel piece is controlled to be an inert atmosphere. plasma torch 1
is surrounded by a seal case 3 to prevent oxidation.
An electromagnetic coil 2 is placed between the plasma torch 1 and the steel piece 4.
The arc width 6 is expanded by applying an alternating magnetic field perpendicular to the plasma arc current to cause the plasma arc 5 to reciprocate.
When the surface layer of a steel slab is heated and melted with such a plasma arc, a band-shaped molten zone is formed in the width direction of the steel slab. By moving this steel piece in the longitudinal direction of the steel piece, the molten zone continuously moves upstream in the direction of movement of the steel piece, and the downstream side is continuously rapidly solidified, reducing the surface layer of the steel piece to its length. The melting and solidifying process can be performed continuously along the direction.

Next, examples of the present invention will be shown.

Example 1 SUS304 continuous casting bloom 210mm x 210mm x 5000
Approximately 4 mm of the surface layer was melted using a plasma arc heat source and subjected to rapid solidification treatment. FIG. 2 shows the carbon distribution in the surface layer region of the steel slab after the treatment. It can be seen that the surface layer of the steel slab became a low-carbon steel due to the decarburization reaction caused by high-temperature melting.

A seamless steel pipe was manufactured by hot rolling the bloom, followed by hot extrusion and cold drawing.

FIG. 3A is a photograph of the structure of a vertical section of the seamless steel pipe subjected to an etching test using the JIS method.

FIG. 3B is an enlarged photograph of the structure of part a in FIG. 3A, which corresponds to the region to which the present invention is applied, and shows a stepped structure that is not susceptible to intergranular corrosion.
FIG. 3C is an enlarged photograph of the structure of section b in FIG. 3A, which corresponds to the area to which the present invention was not applied, and the structure shows a groove-like structure susceptible to intergranular corrosion. As described above, by remelting and quenching, the precipitated layer is dissolved in the steel and a low carbon steel is produced, which has an excellent effect on intergranular corrosion.

Example 2 SUS321 continuous casting slab 130mm x 1000mm x 5000mm
The entire surface layer of 4 mm was melted.

FIG. 4 shows the cleanliness distribution in the surface layer region of the steel slab in the treatment system.

It can be seen that non-metallic inclusions of TiN, TiC, and TiO ₂ are dissolved in the steel up to a depth of 4 mm from the surface of the steel slab subjected to this treatment, and are not precipitated as non-metallic inclusions.

The slab was hot rolled and then cold rolled to produce a thin plate with a thickness of 1 mm. Conventionally, streak-like defects and pickling defects occur in thin steel sheets, and the average yield is reduced.
However, by implementing the method of the present invention, there were no surface defects caused by non-metallic inclusions, and the average yield was 96%.
It was 100%.

[Brief explanation of the drawing]

FIG. 1 is an explanatory diagram showing an example of an apparatus implementing the present invention, and FIG. 2 is a diagram showing the carbon concentration distribution in the surface layer region of a steel slab in which the present invention is applied to SUS304 stainless steel.
Figure 3A is a micrograph (x100) showing the results of a JIS etch test on a longitudinal section of a seamless steel pipe. FIG. 3B is a micrograph showing the tissue in the area where the present invention was applied in Bloom [an enlarged view of section a in FIG. 4A (×500)]. FIG. 3C is a microscopic photograph showing the structure of the area in Bloom where the present invention was not applied [an enlarged photograph (×500) of part b in FIG. 4A]. Figure 4 is
FIG. 2 is a diagram showing the cleanliness distribution in the surface layer region of a steel billet obtained by applying the present invention to SUS321 stainless steel. In the figure, 1 is the plasma torch, 2 is the electromagnetic coil, 3 is the seal case, 4 is the steel piece, 5 is the plasma arc, 6 is the arc width, 7 is the surface after melting the steel piece surface layer, 8 is the steel piece surface layer This is the surface before dissolution.

Claims (1)

[Claims] 1 The surface layer of a stainless steel piece is heated to over 2000℃ using a wide plasma arc in an inert atmosphere,
A method for producing stainless steel with excellent intergranular corrosion resistance and surface cleanliness, characterized by melting the surface layer to a depth of 1 mm or more and 15 mm or less, followed by rapid solidification.