JPH0718016B2 - Method for producing stainless steel with excellent rust resistance - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a method for producing stainless steel having excellent rust resistance, which is characterized by applying a silicon compound in advance to the surface and then applying hydrogen gas or hydrogen gas and nitrogen gas. Heat softening in mixed gas with (annealing)
To form a silica (SiO ₂ or SiO _x ) film on the surface.

The present invention finds that the rust resistance of bright annealed materials of stainless steel, especially chrome-based stainless steel, is the same steel type (component system), but varies widely depending on the material. Contrary to common sense, a new discovery that stainless steel consisting mostly of silicon oxide, especially amorphous silica, exhibits superior rust resistance than stainless steel, which is an oxide containing a large amount of chromium in its surface film. It was made based on.

The current stainless steel manufacturing process is classified according to the final surface finish.After cold rolling, it is annealed in an oxidizing atmosphere by burning light oil, and subsequently the colored oxide film on the surface is removed by nitric acid electrolysis or nitric hydrofluoric acid dipping. Simultaneously with the process of passivating the surface and annealing in a reducing atmosphere containing hydrogen gas to prevent the surface from being oxidized and colored (bright annealing,
Bright annealing (hereinafter abbreviated as BA) is performed and the passivation process is omitted. In the latter process, the surface quality (appearance, rust resistance) of the product is naturally controlled by the final annealing process.

The present invention relates to the stainless steel of the latter process, and even if the stainless steels of the same composition are used, the surface composition of various BA materials is changed to the purpose of clarifying the reason why the rust resistance is greatly different. As a result of detailed investigations by electron spectroscopy and electron diffraction, for example, in the SUS430 system (16 to 19 wt%), the surface composition is mainly composed of Cr and Si oxides, and the surface concentration of Cr (average composition of thickness about 20Å) ) Is 3 to 22 atomic percent, and the surface concentration of Si varies in the range of 14 to 46 atomic percent. Si concentration of the stainless steel (0.3 to
An abnormally high surface Si concentration compared to 0.7 wt% indicates that preferential oxidation of Si occurs in BA. That is, the atmosphere of BA is generally an ammonia decomposition gas (75vol% H ₂ + 25vol
% N ₂ ) is used, but it contains a small amount of water, and its water content is 0.001 vol% to 0.1 vol%, (the dew point is about −60 ° C. to −20 ° C.). Thus, the reducing power of gas is that of hydrogen gas. And the heating temperature. That is, assuming that the metal is M and the oxide is M _x O _y , the redox equilibrium is expressed by the following equation.

Here, ΔG is the Gibbs free energy of the reaction (1). Comparing (1) the easiness of reaction (easiness of oxidation) using ΔG of each of Fe, Cr, and Si, Si is most easily oxidized at any temperature, Cr is next, and Fe is most Hard to be oxidized. That is, if it is 75% H ₂ , Fe is annealed at a dew point of -20 ° C or less (1000 ° C to 100 ° C).
However, Si is in an oxidized state even at dew points of -60 ° C and 1000 ° C, and Cr is oxidized depending on the dew point. When solid solution metal elements in steel become oxides (ions) on the surface,
The solid solution element continuously diffuses from the inside to the surface, and as a result, an oxidized concentrated layer of the component is formed on the surface. The surface oxide of Si is formed in BA by the above principle. Therefore, even with the same steel composition, the surface film composition (the ratio of oxides of Si and oxides of Cr) changes when the annealing atmosphere is different, which directly affects the rust resistance of the product. Therefore, we investigated the relationship between rust resistance and surface composition of various BA materials. The higher the proportion of Si oxide, the better the rust resistance. Furthermore, the Si oxide is amorphous. It was found that the product has good rust resistance.

The formation of Si oxide on the surface can be achieved by controlling the annealing atmosphere components (hydrogen gas concentration and dew point) as described above. However, this method is rigorous for all thermal cycles. Is required, and in continuous annealing, it is difficult to control because there is a rise in the dew point due to the moisture brought in from the coil (steel strip). In addition, there is a problem that the control range of the dew point of the amorphous oxide is narrow. The present inventors have found that applying a Si compound before annealing is effective as a method for easily forming a Si oxide on the surface. That is, after the coil after cold rolling is degreased, it is dissolved in a solution of silica sol in water, and various water-soluble inorganic silicates (sodium orthosilicate and potassium metametasilicate and potassium, or sodium silicofluoride and ammonium silicofluoride). After dipping, apply the amount of SiO ₂ deposited by roll squeezing to a range of preferably 1 to 120 mg / m ² , dry with hot air at 150 ° C, and then anneal under normal heating cycle, for example, 950 ° C for 1 minute. Do. At this time, the dew point does not need to be particularly controlled, but in order to prevent the surface from being colored, it is -20 ° C or lower (75
% H ₂ ) is preferred. Further, the preferable amount of Si deposited is in the above range because rust resistance is not improved as compared with uncoated stainless steel under the same annealing conditions at less than 1 mg / m ² , whereas if it exceeds 120 mg / m ^2. This is because the surface after annealing has poor gloss and the appearance is poor.

As mentioned above, dipping and roll squeezing are the simplest coating methods, but when using a sodium silicate solution, electrolysis is used for the purpose of degreasing (the steel strip can be either positive or negative)
Can also be used together.

Examples of the present invention will be described below.

Example 1 Preliminarily degreased 19Cr stainless steel (Cr19.2%, C0.0005
%, Si0.30%, Mn0.75%, P0.025%, S0.004%, Ni0.30%, Cu
0.4%, N0.009%, Nb0.50%, Fe remaining) cold-rolled sheet was dipped in 3% sodium orthosilicate solution (90 ° C), drained by roll squeezing, dried with hot air, 75% ₂ , Heat treatment was performed at 950 ℃ for 1 minute in an atmosphere with a dew point of -40 ℃. The product taken out into the air after being cooled to 100 ° C. exhibited excellent gloss. Table 1 shows the surface oxide film composition and rust resistance. The surface film was mainly composed of Si oxide and had excellent rust resistance.

Example 2 The same stainless steel (degreasing plate) as in Example 1 was immersed in a silica sol containing 2% solids (normal temperature), pulled up, rolled, and dried. Table 1 shows the surface film and rust resistance of the product annealed in the same manner as in Example 1.

Example 3 A 3% sodium orthosilicate (90 ° C.) was electrolyzed for 2 seconds at 20 A / dm ^{2 using} a stainless steel having the same composition as in Example 1 (with cold rolling oil adhered) as a cathode, followed by washing with water and drying. Was done. The rust resistance of the product annealed in the same manner as in Example 1 was extremely excellent as shown in Table 1.

 ─────────────────────────────────────────────────── ─── Continuation of the front page (72) Takehisa Mizunuma 1618 Ida, Nakahara-ku, Kawasaki-shi, Kanagawa Kanagawa Basic Research Institute (72) Inventor Yuho Omata 1618 Ida, Nakahara-ku, Kawasaki-shi, Kanagawa New Nippon Steel Stock Research Institute

Claims (1)

[Claims] 1. A stainless steel having excellent rust resistance, which is characterized in that a stainless steel or a steel strip is preliminarily treated in an aqueous solution of an inorganic silicate compound or a water solution of silica sol, followed by bright annealing. Manufacturing method.