JPH0474900A - Method for coating surface of stainless steel with oxide - Google Patents

Abstract

PURPOSE:To form a uniform oxide coating film having adhesion on the surface of stainless steel by successively subjecting the stainless steel having a specified compsn. to cold rolling, annealing in an oxidizing atmosphere and electrolysis in neutral salt. CONSTITUTION:Stainless steel having oxide scale on the surface and contg. >=0.3% Si, >=0.1% Al and >=0.4% each of one or more among Nb, Ti and W is cold rolled and annealed at >=950 deg.C in an oxidizing atmosphere and the oxide scale on the surface of the steel is removed by electrolysis in neutral salt. During the annealing, a coating film of the oxides of elements contained in the steel is formed on the surface of the steel because Si, Al, Nb, Ti and W diffused from the base steel react with oxygen in the atmosphere at the interface between the oxide scale and the base steel. The coating film is thin and has high adhesion to the base steel and uniformity over the entire surface of the steel.

Description

DETAILED DESCRIPTION OF THE INVENTION <Industrial Application Field> The present invention relates to a method for producing an oxide coating layer on the surface of a stainless steel strip or steel plate (hereinafter simply referred to as stainless steel).

<Prior Art> Due to its excellent corrosion resistance and beautiful surface finish, stainless steel is used in a wide range of fields, from large-scale chemical industry plants to household utensils.

Furthermore, as a recent trend, various surface treatments such as coloring, plating, and painting are being applied to enhance the design and functionality of the surface.

However, since stainless steel itself is more expensive and used in smaller amounts than ordinary steel, the fields in which it can be used even after surface treatment are extremely limited.

Furthermore, since stainless steel has a passive film formed on its surface, the surface treatment properties are generally poor and there are few facilities available for use.

<Problems to be Solved by the Invention> Therefore, it is important to establish a method for surface-treated stainless steel that provides high added value and can be processed at low cost.

As a surface treatment method, instead of conventional metal plating or painting, inorganic materials (for example, T i N, S i O
2) coatings have been actively studied in recent years, and various vapor deposition methods are being developed.

Inorganic coatings are seen as promising for improving corrosion resistance and heat resistance, as well as for bonding with inorganic materials, adhesion, and as a base treatment for painting. However, a method for forming a uniform film with good adhesion has not been established.

An object of the present invention is to provide a coating method that inexpensively produces an oxide film that evenly and tightly adheres to the surface of stainless steel in order to improve the functionality of stainless steel through such surface treatment.

<Means for solving the problem> The root cause of the non-uniformity and low adhesion of the coating layer in the conventional technology described above is that the constituent substances of the coating layer are outside the material (that is, they are not contained in the material). The present invention solves this problem by attempting to bond oxides whose physical and chemical properties are significantly different from those of the materials.

That is, according to the present invention, in order to achieve the above object,
It has an oxide scale on the surface, 0.3% or more of Si, 0.
Afl of 1% or more, and Nb of 0.4% or more each,
After cold-rolling stainless steel containing one or more of Ti and W, the surface of the stainless steel is annealed at 950°C or higher in an oxidizing atmosphere, and then subjected to neutral salt electrolysis. Provided is a method for coating a stainless steel surface with an oxide, the method comprising removing oxide scale of the stainless steel and forming a coating layer of the oxide of the contained element on the surface of the stainless steel.

The present invention will be explained in more detail below.

The stainless steel that can be used in the present invention is an austenitic steel type or a ferritic single-phase stainless steel that does not generate an austenite phase even when heated at high temperatures.

Ferritic and martensitic materials, which partially undergo austenite transformation during heating, are not suitable because micro-cracking of oxide scale occurs during cooling due to re-transformation.

Also, during annealing, Si, A, J2. In order to generate enough oxides of Nb, Ti, and W (hereinafter referred to as specific oxides),
In the stainless steel, Si is 0.3% or more, and Aρ is 0.
．． It is necessary to contain at least 1% of Nb, Ti, and W, and at least 0.4% of Nb, Ti, and W.

If the content is less than this, the amount of specific oxide produced will be insufficient and the oxide coating will be incomplete.

Furthermore, if Si, Aj2, Nb, Ti, and W are contained in an amount of 3% or more, the toughness of the material decreases, so it is desirable to limit the content to less than 3%.

The present invention is characterized in that the stainless steel containing one or more of Si, A°C, Nb%Ti, and W is cold-rolled and then annealed in an oxidizing atmosphere, for example, in a combustion gas atmosphere. It is.

The annealing temperature needs to be 950° C. or higher; if it is lower than this, the formation of oxide scale will be reduced and at the same time the specific oxide layer will not be able to be formed sufficiently.

Furthermore, if the annealing temperature exceeds 1250°C, the toughness of the material will decrease, so it is desirable that the annealing temperature be 1250°C or less.

By the annealing, specific oxides can be generated at the interface between the material and the oxide scale, which is mainly composed of oxides of Fe and Cr.

In the present invention, it is essential to remove the oxide scale after annealing by using a neutral salt electrolysis method, and the removal of the oxide scale using sulfuric acid or nitric-fluoric acid is not applicable because the above-mentioned specific oxides are also removed at the same time.

The oxide scale on the surface of stainless steel is composed of oxides of Fe and Cr, and as shown in FIG.
In FIG. 1, it is formed on the outer surface side of Si, Nb).

This oxide scale is easily dissolved during electrolysis, but Si
, Aj2. Since the oxides of Nb, Ti, and W are poorly soluble, when electrolyzed in an electrolytic solution such as a neutral salt that is ineffective in dissolving the stainless steel substrate, only the hardly soluble oxides remain, as shown in Figure 2. As shown, a coating layer is formed on the surface of the material.

This coating layer is formed at the interface between the oxide scale and the material during annealing, and contains the Si and AI that have diffused from the material.
, Nb, Ti, and W elements react with oxygen in the atmosphere, so the resulting coating layer is thin (has high adhesion to the material and is formed very uniformly over the entire surface. , there is no need to install expensive equipment, and existing annealing and
Pickling equipment is available.

In addition, immersion in sulfuric acid, nitric-fluoric acid, or electrolytic treatment in strong acids such as sulfuric acid and nitric acid involves dissolving the stainless steel material, so the hard-to-dissolve specific oxide layer is also removed. The specific oxide coating layer of the present invention cannot be formed using the conventional method of descaling.

In the neutral salt electrolysis of the present invention, Na salts and K salts such as sulfuric acid, nitric acid, and hydrochloric acid can be used alone or in combination as neutral salts, but from the viewpoint of economy and surface finish, sodium sulfate is used. is suitable.

The concentration and temperature of the neutral salt aqueous solution are 100 to 300, respectively.
g/ff, 70 to 90°C is appropriate.

The appropriate current density is 2 to 15 A/dm2 for both the anode reaction current density and the cathode reaction current density.

<Example> The present invention will be specifically explained below based on Examples.

(Example 1) Cold-rolled stainless steel plates (thickness 0.7 mm) having various compositions shown in Table 1 were heated at 850 to 1150°C in an air atmosphere.
After annealing, descaling was performed using the method shown in Table 2.

In addition, for neutral salt electrolysis, sodium sulfate 200g/12
Aqueous solution, 5A/dm" x 10 seconds electrolytic treatment at 80℃, sulfuric acid electrolysis: 5% sulfuric acid, 5A/dm"X at 50℃
5 seconds, nitric acid electrolysis: 10% nitric acid, 5 A/dm at 50°C
” x I did it in 5 seconds.

In addition, nitric-fluoric acid immersion was performed for 40 seconds in a mixed acid consisting of 5% nitric acid and 1% hydrofluoric acid at a bath temperature of 50°C (in the table, A is neutral salt electrolysis, B is sulfuric acid electrolysis, and C is immersion in nitric-fluoric acid). (Nitric acid electrolysis, D indicates nitric-fluoric acid immersion.) For the surfaces subjected to these various electrolytic or immersion treatments, it is judged with an ion microanalyzer whether a specific oxide coating layer has been formed (Table 2, coating layer The effect of the coating layer was determined electrochemically by measuring the pitting corrosion potential.

The results are shown in Table 2.

In Comparative Examples 1 to 4 using steel type S1 in which Si, A°C, Nb, Ti, and W are all outside the range of the present invention, no oxide coating layer was obtained at any annealing temperature, and the pitting potential is also low.

On the other hand, in steel type S2 with a high Si content of 0.52%, when the annealing temperature was 950°C or higher (inventive example 1.2), an oxide coating layer was formed, resulting in a significant increase in pitting potential. .

However, when sulfuric acid electrolysis or nitric acid electrolysis is used as a descaling method (Comparative Example 7.8), an oxide coating layer cannot be obtained.

Further, for steel types S3 to S7 containing one or more of AJ2, Nb%Ti, and W within the scope of the present invention, an oxide coating can be obtained by performing neutral salt electrolysis at an annealing temperature of 1000°C. (Examples 3 to 7 of the present invention)
.

However, similarly, oxide coating cannot be achieved by sulfuric acid electrolysis (Comparative Examples 9 and 10).

Similarly, for steel types S8 and 89, oxide coatings are formed only when neutral salt electrolysis is applied to S9, which has a large number of 81 (invention example 8), but such an effect cannot be obtained with nitric-fluoric acid immersion ( Comparative Example 13).

Table <Effects of the Invention> Since the present invention is configured as explained above, Si, A℃, Nb, which uniformly and closely adhere to the stainless steel surface,
Ti and W oxide coating layers can be produced at low cost.

[Brief explanation of the drawing]

FIG. 1 is a graph showing the surface composition of stainless steel before descaling. FIG. 2 is a graph showing the surface composition of the stainless steel shown in FIG. 1 after neutral salt electrolysis. Note) Pitting potential is a measured value (mV, vs, Ag AgCj2) in a 3.5% NaCρ solution at 30°C.
However, the test piece shall not be polished.

Claims (1)

[Claims] (1) Has oxide scale on the surface and contains 0.3% or more Si
, 0.1% or more Al, and 0.4% or more N each
After cold rolling stainless steel containing one or more of B, Ti, and W, the stainless steel is rolled at 950°C in an oxidizing atmosphere.
By annealing as above and then performing neutral salt electrolysis,
A method for coating a surface of stainless steel with an oxide, the method comprising removing oxide scale on the surface of the stainless steel and forming a coating layer of the oxide of the element contained on the surface of the stainless steel.