JPS5881979A - Method for descaling cr containing stainless steel - Google Patents

Abstract

PURPOSE:To make the surface of steel beautiful inexpensively without danger, by a method wherein Cr contg. stainless steel is pickled in sulfuric acid under a specific condition and the treated steel is washed in water at a specific temp without drying. CONSTITUTION:Cr contg. stainless steel is pickeled in sulfuric acid with concn. of 50g/l-500g/l at 65 deg.C or more. The treated steel is washed in water at 45 deg.C or more. By this method, descaling can be sufficiently carried out and the surface of the Cr contg. stainless steel is made beautiful.

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a method for descaling CR stainless steel.

Chemical descaling of stainless steel is generally carried out by rough pickling in a strong acid such as sulfuric acid, followed by final pickling in a mixed solution of nitric acid and hydrofluoric acid.

However, the mixed solution of nitric acid and hydrofluoric acid is not only expensive, but also has the problem of generating harmful gases such as N08 and liver gas during processing, which significantly deteriorates the working environment. Its high corrosion resistance and beautiful surface give it great commercial value. Therefore, in the descaling process, we pay close attention not only to residual scale, but also to the residual smut generated by pickling, and strive to obtain a beautiful descaled surface.

One way to compare the beauty of surfaces such as stainless steel is to measure the diffuse reflectance of the two surfaces.

In other words, if the surface is free of dirt and has small irregularities (it looks white to the naked eye)! ! The surface diffuse reflectance shows a large value, but if the surface is contaminated with dirt or minute irregularities become severe (it looks black to the naked eye), the surface diffuse reflectance decreases. Therefore! ! It is evaluated that the larger the value of the surface diffuse reflectance, the more beautiful the surface.

The present inventors have studied in detail the function of each acid in the chemical descaling process of stainless steel, and found that
In the case of ferritic stainless steel and martensitic stainless steel (hereinafter collectively referred to as Cr stainless steel), scale removal must be completed with a pickling process using sulfuric acid, or a pickling process using a mixed solution of nitric acid and hydrofluoric acid. It has been found that the smut that cannot be removed by the sulfuric acid pickling process (it would be more correct to say that it is deposited during the sulfuric acid pickling process) has the function of finishing the surface beautifully. That is, C
In the case of r-series stainless steel, sulfuric acid pickling is sufficient for descaling, and the treatment process with a mixed solution of nitric acid and hydrofluoric acid can be said to be a process for finishing the surface beautifully.

Therefore, the present inventors investigated an inexpensive and less dangerous method of finishing the surface beautifully instead of using a mixed solution of nitric acid and fluoride, which is expensive and has the risk of generating harmful gases. They discovered that by washing with high-temperature water without drying, the surface becomes beautiful without any scale remaining, and the present invention was completed.

The numerical basis of the present invention is shown below.

FIG. 1 is a diagram showing the results of acid pickling of SUB 430 steel hot rolled annealed plates onto which a pickling shot was projected for 1 minute at various concentrations and temperatures, and the presence or absence of scale remaining after washing with water was investigated.

In the figure, the ○ mark indicates completion of descaling, and the x mark indicates scale remaining. The above result is 50? It can be seen that descaling is completed with sulfuric acid at a temperature of /j or higher and 50°C or higher.

Figure 2 shows 80843 with 41 finish after descaling.
FIG. 3 is a diagram showing the relationship between the cleaning water temperature and the surface diffused reflectance when 0 steel was pickled again in a sulfuric acid solution at 80° C.-2001/J and immediately washed with water. It can be seen that when the temperature of the cleaning water is 45° C. or higher, smut is removed, the surface diffuse reflectance increases, and the surface becomes beautiful.

Similarly, Figure 3 shows a surface diffused reflectance of 46 excellent with a Mu1 finish.
It is a figure showing the relationship between acid temperature and surface diffused reflectance when O8430 steel is pickled in 200P/j sulfuric acid aqueous solution at various temperatures and immediately washed with 70°C water. acid temperature is 6
It can be seen that when the temperature is lower than 5° C., the increase in surface diffuse reflectance becomes smaller and the effect becomes smaller.

As mentioned above, with Cr stainless steel, descaling is completed by pickling with high-temperature sulfuric acid and then washing with high-temperature water, and a treatment with high surface diffuse reflectance can be performed. In addition, after descaling! ! ! It was confirmed that sulfuric acid of 101-74 or higher is sufficient if the treatment is to simply finish the surface white.

Next, the reasons for limiting the temperature, concentration, etc. in the present invention will be described.

The sulfuric acid concentration is adjusted according to Figure 1 when descaling is completed.
/! , that's all. In addition, the effect does not change much when the amount exceeds 500 P/J, and there is a risk of intergranular corrosion at higher temperatures and concentrations, so 50 (1/) is not the upper limit.

The temperature of the sulfuric acid was set at the lower limit as shown in FIG. 3, because if it was less than 5°C, smut removal would be incomplete and the value of the surface diffuse reflectance would be small.

The temperature of the washing water is II! As shown in Fig. 2, it is difficult to remove smut and the value of the i-plane diffuse reflectance becomes small at temperatures below 5°C.The lower limit was set at 45°C.

Furthermore, if the material is dried after pickling and before water washing, smut will adhere firmly and become impossible to remove, so it was necessary to avoid drying between pickling and water washing.

Next, further explanation will be given by showing examples and comparative examples.

Example 1 After jetting a hot-rolled and annealed 5US43G steel plate, the steel sheets were subjected to sulfuric acid pickling under various conditions.

The results are shown in Table 1, and it can be seen that in the method of the present invention, descaling is completed without using a mixed solution of nitric acid and hydrofluoric acid, and the surface diffused reflectance is also high. On the other hand, in the comparative method, scale remained and smut remained on the surface, impairing the beauty of the surface, except for further pickling with a mixed solution of nitric acid and hydrofluoric acid.

Annealed BUS 410 steel plates were subjected to sulfuric acid pickling under conditions of pressure of 300 or more.29+ As shown in the table, the method of the present invention removes sludge and has a high reflectance with no smut adhering to the surface. I understand.

SUB 43 with a hot explanatory shot of G tone 1
Diagram showing the results of investigating the scale after pickling and water washing, 8U843020011/l of completed 41 finish in the second round.
Figure 3 shows the cleaning water temperature and surface diffused reflection when pickling in a sulfuric acid solution at a temperature of , is a diagram showing the acid temperature and surface diffused reflection when immediately cleaned at 70°C. 1 Applicant: Nippon Steel Corporation 7th acid secretion E'C] 2nd water/f temperature [°C]

Claims (1)

[Claims] Is Cr stainless steel more than Sol/j soO? A method for descaling Cr-based stainless steel, which comprises pickling in sulfuric acid at a concentration of /7 or less and a temperature of 65°C or higher, followed by washing in water at 45°C or higher without drying.