JPS639023B2 - - Google Patents

Description

[Detailed description of the invention]

The present invention relates to a method for removing pickled smut from 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, mixed solutions of nitric acid and hydrofluoric acid are not only expensive, but also pose problems such as the generation of harmful gases such as _NOx and HF during treatment, which significantly deteriorates the working environment. On the other hand, stainless steel has great commercial value due to its excellent corrosion resistance and beautiful surface. Therefore, in the descaling process, we pay close attention to not only residual scale but also residual smuts caused 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 surface diffuse reflectance. In other words, when there is no dirt attached to the surface and minute irregularities are small (it looks white to the naked eye), the surface diffuse reflectance shows a large value, but if dirt is attached to the surface or minute irregularities are severe, the surface diffuse reflectance will be large. (It looks dark to the naked eye) and the surface diffuse reflectance decreases. Therefore, the larger the value of the surface diffuse reflectance, the more beautiful the surface is evaluated to be. The present inventors investigated 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) , that the removal of scale is completed in the pickling process with sulfuric acid, and that the pickling process with a mixed solution of nitric acid and hydrofluoric acid cannot be removed in the sulfuric acid pickling process (in fact, some people say that scale was attached during the sulfuric acid pickling process) It has been found that it has the function of removing smuts and giving a beautiful surface finish. That is, in the case of Cr 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 inventors of the present invention investigated an inexpensive and less dangerous method of finishing the surface beautifully instead of using a mixed solution of nitric acid and hydrofluoric acid, 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. Figure 1 shows the results of acid-washing SUS430 hot-rolled annealed steel plates with shot shots at various concentrations and temperatures for 1 minute, and examining the presence or absence of scale remaining after washing with water. In the figure, the ○ mark indicates that descaling is completed, and the × mark indicates that scale remains. The above results show that descaling is completed with 50g/or more of sulfuric acid at 50°C or more. Figure 2 shows No. 1 finish after descaling.
It is a figure showing the relationship between the cleaning water temperature and the surface diffused reflectance when SUS430 steel is re-pickled in a sulfuric acid solution of 80° C. and 200 g/ml and immediately washed with water. It can be seen that when the temperature of the cleaning water exceeds 45°C, the smut is removed, the surface diffuse reflectance increases, and the surface becomes beautiful. Similarly, Figure 3 shows the No. 1 finish with a surface diffuse reflectance of 46.
FIG. 3 is a diagram showing the relationship between acid temperature and surface diffuse reflectance when SUS430 steel of 1.5% was pickled in 200g/aqueous sulfuric acid solution at various temperatures and immediately washed with 70°C water. It can be seen that when the acid temperature is lower than 65°C, the increase in surface diffuse reflectance becomes smaller and the effect becomes smaller. As shown above, in Cr stainless steel,
By pickling with high-temperature sulfuric acid and then washing with high-temperature water, descaling can be completed and a treatment with high surface diffuse reflectance can be performed. In addition, it was confirmed that 10 g/or more of sulfuric acid is sufficient if the treatment is simply to whiten the surface after descaling. Next, the reasons for limiting the temperature, concentration, etc. in the present invention will be described. The sulfuric acid concentration is determined according to Figure 1 when descaling is completed.
The amount was 50g/or more. In addition, even if the amount exceeds 500 g/l, the effect does not change much, and at higher temperatures and concentrations, there is a risk of intergranular corrosion, so 500 g/l was set as the upper limit. As shown in FIG. 3, the temperature of the sulfuric acid was set at the lower limit because if it was less than 65°C, the removal of smut would be incomplete and the value of the surface diffuse reflectance would be small. As shown in Figure 2, the lower limit of the cleaning water temperature was set at 45°C, since it would be difficult to remove the smuts and the value of the surface diffuse reflectance would be small if the temperature was less than 45°C. In addition, if the smut is allowed to dry after pickling and before washing with water, the smut will adhere firmly and become impossible to remove, so it was necessary to avoid drying between pickling and washing with water. Next, further explanation will be given by showing examples and comparative examples. Example 1 After projecting a shot onto a normally hot-rolled and annealed SUS430 steel plate, sulfuric acid pickling was performed under various conditions.
The results are shown in Table 1, and in the method of the present invention,
It can be seen that descaling was completed without using a mixed solution of nitric acid and hydrofluoric acid, and the surface diffused reflectance was also high. On the other hand, in the comparative method, scale remains and smuts remain on the surface, impairing the beauty of the surface, unless the material is further pickled with a mixed solution of nitric acid and hydrofluoric acid.

【table】

[Table] Example 2 A normally hot-rolled and annealed SUS410 steel plate was reduced by 3% or more, and then subjected to sulfuric acid pickling under various conditions.
The results are shown in Table 2, and it can be seen that in the method of the present invention, the descaling is completed and no smut is attached to the surface, so the surface diffuse reflectance is also high.

【table】 [Brief explanation of the drawing]

Figure 1 shows the results of acid-washing SUS430 hot-rolled annealed steel plates with various concentrations for 1 minute on which shots were projected, and checking whether there was any remaining scale after washing with water. Figure 2 shows the results of No. Figure 3 shows the relationship between cleaning water temperature and surface diffuse reflectance when No. 1 finish SUS430 steel is pickled in a 200g/sulfuric acid solution at 80℃ and immediately rinsed with water. 200 g of SUS430 steel with a surface diffuse reflectance of 46% at various temperatures
FIG. 3 is a diagram showing the relationship between acid temperature and surface diffused reflectance when pickled in an aqueous sulfuric acid solution and immediately washed with water at 70°C.

Claims (1)

[Claims] 1 Cr stainless steel, 50g/more than 500g/
In the process of pickling in sulfuric acid at a temperature of 65℃ or higher with the following concentration, 45℃ without drying after pickling.
A method for removing pickled smut from Cr-based stainless steel, which comprises washing in water as described above.