KR100514781B1 - Method for pickling martensitic stainless steel wire by pretreating of reduction type - Google Patents

Abstract

The present invention relates to a method of pickling stainless steel in a mixed acid bath after a pretreatment of a pickling method of a reduction method. This method is a pickling method of a martensitic stainless steel wire which is pickled pretreatment and secondary pickling with primary pickling and mixed pickling liquid, wherein the pickling pretreatment is performed in a reducing salt bath.

According to the present invention, the composition of the poorly soluble scale formed in the production process of martensitic stainless steel wire is changed in the pretreatment method of pickling to easily remove the scale without erosion of the surface of the base material during pickling, and has a good surface quality. There is a useful effect that can produce.

Description

METHOD FOR PICKLING MARTENSITIC STAINLESS STEEL WIRE BY PRETREATING OF REDUCTION TYPE}

The present invention relates to a method of pickling martensitic stainless steel wires in a pickling treatment and a mixed acid bath, and more particularly, to a method of pickling stainless steel by a pickling pretreatment of a reduction method of reducing an oxidation scale.

In stainless steel, the scale produced during hot rolling is reoxidized during the annealing heat treatment to form thick and dense material on the surface of the material. Therefore, the scale is removed by pickling. Acids widely used for pickling include sulfuric acid, hydrochloric acid, nitric acid, and hydrofluoric acid, and mixed acids of nitric acid-fluoric acid are also widely used.

The pickling process of stainless steel is largely carried out through pre-pickling, primary pickling and mixed pickling, and the pickling line working process is shown in FIG.

Pickling pretreatment forms fine cracks on the scale to penetrate the acid solution, and shot-blasting or oxidized salt bath treatment methods are known. Shot blasting is a physical impact on the scale of a stainless steel surface causing some descaling and micro cracks on the remaining scale. Oxidation salt treatment is to reoxidize the scale and then quench it with water so that cracks are generated on the scale surface due to thermal shock.

Primary pickling activates the interface between the substrate and the scale and removes some of the scale from the surface. Sulfuric acid pickling is used as the primary pickling.

Mixed pickling completely removes residual scale, and a mixed acid of nitric acid-fluoric acid is widely used. After the mixed pickling process, the passivation process is finally performed to form a passivation film on the surface of the product. The surface is washed with water and dried to complete the pickling process and is sent to a subsequent process such as packaging.

Conventional technologies for pickling treatment of stainless steel are Korean Patent Publication Nos. 1997-0043302 and 2000-0045517. Korean Unexamined Patent Publication No. 1997-0043302 is a technique for pickling alone in a sulfur-nitric acid mixed solution, and Korean Unexamined Patent Publication No. 2000-0045517 is a technique for lowering the concentration of nitric acid in a hydrofluoric acid-nitrate mixing tank. Since these techniques basically use nitric acid in mixed acid baths, environmental problems arise because of the high concentrations of nitrate ions in the waste gases and the emission of toxic fumes of NOx. Therefore, there is a need for a mixed acid free pickling technique. Nitric acid-free acid mixing tank is known as a mixed pickling solution of hydrofluoric acid and sulfuric acid.

However, stainless steel, especially oxidative scales having a chromium content of more than 13% in a nitric acid free acid bath do not descale only by pickling, even if the conditions of the acid solution are changed. As a result, when the pickling bath temperature or concentration is increased or the pickling time is increased to remove the remaining scale, partial pits are generated on the wire rod surface, thereby degrading the quality.

An object of the present invention is to provide a pickling method that is easy to remove an oxide scale when pickling martensitic stainless steel wire with a nitric acid-free pickling solution.

In order to achieve the above object, the present invention, in the pickling method of martensitic stainless steel wire pickling pre-treatment and secondary pickling with the primary pickling and mixed pickling liquid, the pickling pretreatment is configured to include in a reducing salt bath. .

Hereinafter, the present invention will be described in detail.

The present inventors completed the present invention by noting that the scale is easily removed without changing the pickling conditions as a result of converting the pickling pretreatment from the oxidation method to the reduction method in a study for improving the scale removal of the stainless steel.

That is, when the stainless steel is immersed in the reducing salt bath, the oxidation scale is reduced as in Scheme 1, so that it is easily dissolved and peeled off in a later step. A representative example of a reducing salt bath is NaH.

FeO + NaH = Fe + NaOH

Fe ₃ O ₄ + 4NaH = 3Fe + 4NaOH

NiO + NaH = Ni + NaOH

Cr ₂ O ₃ + NaH = 2CrO + NaOH

In this way, the scale whose composition is changed in the pre-pickling process by the reducing salt bath is easily dissolved, peeled off and removed in the post-pickling process. Therefore, it minimizes the occurrence of local pits on the surface during pickling to improve the surface quality.

In the present invention, the pickling pretreatment, the first pickling and the pickling method of the stainless steel pickling second in the mixed pickling tank is characterized in that the pickling pretreatment as a reducing salt bath. Therefore, all steels to which the pickling treatment of the present invention is applied may be applied as long as they are stainless steel. In particular, it is more preferable to apply to martensitic stainless steels where erosion of base metals occurs when severe pickling conditions are applied to remove residual scales.

The reduction type pickling treatment of the present invention can be applied to a pickling process to which secondary pickling of a mixed acid bath is applied. More preferably, it is applied to the pickling process in which the secondary pickling of the sulfuric acid-fluoric acid mixing tank is applied. The sulfuric acid-hydrofluoric acid mixed acid solution is, for example, sulfuric acid: 60 to 100 g / l, hydrofluoric acid: 10 to 35 g / l, and a concentration of Fe ³⁺ is 5 g / l or more.

The reducing salt bath of the present invention contains 0.1% by weight or more of NaH. If the concentration of NaH is less than 0.1% by weight, the work time required for the pickling treatment is long, so that the removal of the scale is not easy. The reduced salt bath of the present invention changes the composition of the scale when NaH is 0.1% by weight or more, but in consideration of economical efficiency, the concentration of NaH is preferably 0.1 to 0.3% by weight, and the rest is preferably NaOH. Here it is the more preferable that the addition of 23-25% of Na ₂ O to a stabilizing element of NaH. When the Na ₂ O content is more than 23%, the stabilization effect of NaH increases, and when the content exceeds 25%, the effect is saturated.

More preferably, the pickling treatment in the reducing salt bath of the present invention is carried out at a temperature of 350 to 400 ° C. for 10 to 18 minutes. If the temperature of the reducing salt bath is less than 350 ° C., the scale may not be easily removed and residual scale may occur. In addition, when the temperature of the reducing salt bath is higher than 400 ℃ the decomposition rate of NaH is increased to increase the consumption of the drug is not preferable. Even if the treatment time in the reducing salt bath is less than 10 minutes, it is not easy to remove the scale, and the residual scale is likely to occur. The longer the treatment time in the reducing salt bath, the thinner the thickness of the oxidized scale and the better the scale removal during pickling, but the productivity is lowered, it is preferable to be 18 minutes or less in consideration of work efficiency.

According to the present invention, by changing the composition of the poorly soluble scale formed in the production process of stainless steel in the pretreatment of pickling method, it is easy to remove the scale without erosion of the surface of the base material during pickling, and to produce a product having good surface quality. It works.

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

EXAMPLE

The martensitic stainless steel wires of STS410, STS416, and STS420J containing Cr: 13% by weight were heat-treated under the conditions of Table 1, and then pretreated with the conditions of Table 2.

division Heat treatment temperature (℃) Crack time (hr) STS410 810-850 3.2 STS416 810-850 3.2 STS420J 810-850 3.2

division Pickling pretreatment Pretreatment Method Temperature (℃) Minutes Conventional example Coral salt bath 450-490 8-12 Inventive Example Reduction salt bath (NaH: 0.1-0.3%, Na ₂ O: 23-25%, remaining NaOH)) 350-410 10-18

The pickled wire was pickled under the conditions of Table 3.

division Pickling condition Concentration (g / ℓ) Temperature (℃) Pickling time (minutes) Primary pickling (Sulfate pickling tank) Sulfuric acid: 100 ~ 200 40-60 4-10 Rinse - Room temperature 3 Secondary pickling (Fluoride + Sulfuric acid + Hydrogen peroxide) Folic acid: 10-35 Sulfuric acid: 60-100 40-60 4-10 Rinse - Room temperature 3 Passivation tank - Room temperature 3 High pressure water tank - Room temperature 4 Hot water bath - 70 2

Table 4 shows the surface roughness measurement results of the pickled final products as shown in Table 3. In addition, the surface photograph of the final product is shown in FIG.

division STS410 STS416 STS420J Rmax (μm) Rmax (μm) Rmax (μm) Conventional example 16.6 19.4 14.2 Inventive Example 13.2 17.4 13.4

As shown in Table 4, it was confirmed that the surface roughness is improved as a result of applying the powder pretreatment treatment of the reduction salt bath method of the present invention. In addition, in the surface photograph of FIG. 2, the scale is thick in the conventional pickling treatment of the oxidized salt bath, but in the case of the invention example, no scale is present.

As described above, according to the present invention, through the reduction salt bath treatment for reducing the oxidation scale, it is useful to produce a product having a good surface quality by facilitating scale removal without erosion of the surface of the base material during the pickling operation of the mixed acid bath. There is.

1 is a process chart for pickling of martensitic stainless steel wire,

Figure 2 is a photograph of the surface of the final product according to the pickling treatment of the oxidation method (conventional example) and reduction method (invention example).

Claims (3)

delete In the pickling method of martensitic stainless steel wires pickled pretreatment and secondary pickling with primary pickling and mixed pickling solution, the pickling pretreatment is NaH: 0.1 to 0.3% by weight, Na ₂ O: 23 to 25% by weight, the rest A method of pickling martensitic stainless steel wires by a reduction method, which is carried out in a reducing salt bath composed of NaOH. The martensite system according to claim 2, wherein the mixed pickling solution is sulfuric acid: 60-100 g / l, hydrofluoric acid 10-35 g / l, and Fe ³⁺ concentration: 5 g / l or more. Pickling method of stainless steel wire.