JPS5983783A - Acid pickling method of hot-rolled cr stainless steel strip - Google Patents

Abstract

PURPOSE:To perform the work of safely acid pickling the titled steel strip without environmental contamination, by descaling said steel strip with an aqueous sulfuric acid solution under a specified condition, and then removing stains with an aqueous nitric acid solution under a different condition while passivating it at the same time. CONSTITUTION:In removing oxide scale from a hot-rolled Cr stainless steel strip with an aqueous sulfuric acid solution, the acid pickling is performed in a manner such that the conditions of C1<=10, C1+0.8T1>=76 and T1>=55 (wherein C1 is the wt% of H2SO4 by 100ml of the aqueous sulfuric acid solution, and T1 is the temp. deg.C of the aqueous sulfuric acid solution) are satisfied. Thereafter, the removal of stains from the surface of the steel strip and passivation are performed in such an aqueous nitric acid solution that satisfies the conditions of -1/6T2+50/3<=C2<=-1/6T2+110/3 and 40<=T2<=80 (wherein C2 is the wt% of HNO3 by 100ml of the aqueous nitric acid solutions, and T2 is the temp. deg.C of the aqueous nitric acid solution). Hereon, the time of dipping said steel strip in each of the aqueous sulfuric and nitric acid solutions is limited within about 120sec.

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a method for pickling stainless steel, and particularly to a novel pickling method that can pickle a Cr-based stainless steel hot-rolled steel strip without using nitric-fluoric acid.

Generally, the method of removing oxidized scale generated on the surface of a hot-rolled Cr-based stainless steel strip that has undergone thermal history such as hot rolling and annealing is to mechanically remove it by shot blasting or the like in a continuous annealing and pickling line (commonly known as an AP line). After preliminary descaling, the material is sequentially immersed in sulfuric acid and nitric-fluoric acid solutions for pickling. In this pickling method, sulfuric acid is used as an auxiliary, and descaling is mainly performed using nitric hydrofluoric acid.

The reason why stainless steel products have a beautiful, shiny surface is largely due to the action of nitric-fluoric acid during the pickling process. However, nitric-fluoric acid generates NOx gas during pickling, which poses a major pollution problem. For this reason, expensive equipment for treating NOx residue and large operating costs are required. Furthermore, since hydrofluoric acid has strong i3i properties, pickling operations using it are always dangerous and must be handled with special care.

Furthermore, when hot-rolled Cr stainless steel strips are pickled with nitric-fluoric acid, the grain boundaries are corroded, so if they are subsequently cold-rolled, surface defects called gold dust (micro-scratches) may occur. Chopsticks also have the disadvantage of being inducing.

For this reason, there has been a desire for a pickling method that is safe and inexpensive and can provide a beautiful, shiny surface without using nitric-fluoric acid.

The object of the present invention is to provide such a pickling method.

The inventors of the present invention reexamined the current pickling method using an actual continuous annealing pickling line, and found that if the nitrofluoric acid immersion step after pickling the Cr-based stainless steel hot rolled steel strip with sulfuric acid is omitted,
It was confirmed that some oxide scale remained and dark smut (dirt) formed and adhered to the surface, causing scratches and uneven gloss during cold rolling, resulting in extremely poor surface quality.

In addition, if the sulfuric acid pickling conditions are selected in particular, oxidized scale can be completely removed after washing, and the resulting copper strip with a completely descaled surface can be treated with a simple aqueous solution containing no hydrofluoric acid. It was confirmed that surface stains could be removed by acid immersion under specific conditions, and no scratches would occur during cold rolling.

The present invention was completed based on the above knowledge, and consists of a first step of immersing a Cr-based stainless steel hot-rolled copper strip in a sulfuric acid aqueous solution under certain conditions to remove the maceration scale, and a second step of immersing a Cr-based stainless steel hot-rolled copper strip in a sulfuric acid aqueous solution under certain conditions to remove the maceration scale. This method is characterized by comprising a second step of immersing it in the liquid to remove dirt on the surface and passivation treatment.

The above certain conditions will be explained below.

Figure 1 shows samples of water in sulfuric acid aqueous solutions at various temperatures and concentrations.
)・Preliminary descaling treatment by blasting 1° 5US4
This shows the descaling status when a 30 hot-rolled rust belt is immersed for 30 seconds to 1 to 20 seconds, with O indicating complete descaling, and oxidized scale or even a small amount of oxidized scale. is represented by an X mark.

As is clear from Figure 1, the effective sulfuric acid pickling conditions are as follows (a) and (b) by connecting the critical points where the effect was seen in the figure.
, I'11 revealed that it is effective for the percentage that satisfies formula (c).

C1≧10 ・・・・・・・・・(a)
C+ +0.8T1 ≧ 76 ・・・・・・・・・
...(b) T1 ≧ 55 ...
...(C) Here.

C1・Wart 1. The weight percentage (%) of H2so4 per 100 m of water-resistant solution T1 is the temperature of the sulfuric acid aqueous solution (°C).

In addition, it was confirmed that the above results can be similarly applied to the case of Cr-based stainless steel hot-rolled steel strips other than 5US430.

Next, the steel strip was completely descaled by sulfuric acid pickling by varying the concentration and temperature of the nitric acid aqueous solution.

In the figure, ○ marks indicate cases in which immersion is effective, and × marks indicate cases in which immersion is not effective. Soaking? j'j Effective means that there is no generation of NOx residue, etc.
This indicates that smut from the sulfuric acid pickling on the surface is removed by immersion for 2 seconds or more, resulting in a beautiful finish. This is a case where the immersion time exceeds 120 seconds.

In other words, the critical points A and B where the effect is seen under nitric acid immersion conditions
, D, and E to satisfy the following formulas (d) and (e).

40≦T2≦86 -------・(e:
+Here, the weight percentage (%) of C2 nitric acid aqueous solution OHNO per unit. T2: Temperature of nitric acid aqueous solution (°C).

The reason why the immersion time in the sulfuric acid and nitric acid aqueous solutions is limited to 120 seconds at the maximum as mentioned above is to take into account the production speed determined by the length of the washing tank of the current continuous annealing and washing line. It is.

As described in 1-1 below, according to the present invention, toxic substances such as hydrofluoric acid, which are considered essential for improving the surface finish in the pickling process of Cr stainless steel, are not used at all, so the washing process is safe and easy. There is no need to worry about environmental pollution caused by NOx residue,
It is extremely advantageous for exhaust gas treatment and also has the advantage of low manufacturing costs. Furthermore, the product obtained according to the present invention has excellent surface properties without jiE or uneven gloss.

Each hot-rolled steel strip of 4 mn) thickness 5 US 430.410.420J2 (7) hot-rolled and annealed under the same conditions as in the examples was preliminarily descaled by shot blasting, and was first descaled at a temperature of I&80. 45 seconds immersion in 20% H2SO4 at a temperature of 60 °C, then 45 seconds in 10% HNO3 at a temperature of 60 °C.
Pickling was carried out under the pickling conditions of second immersion.

Next, among these, 5US430.410 are all 0.
After cold-rolling to a thickness of 7 mm, 5US420J2 is subjected to No.2B finish specified in JIS G4305.5US420J2 is hot-rolled steel strip with No.2B finish specified in JIS G4305.
o, when I turned to finish 1, both 1 go were JI
A copper strip was obtained that passed the No. 2B or No. 1 finish specified by S.

In addition, No.
However, it was confirmed that the occurrence of gold tast was significantly reduced when compared to the conventional NitrofluoroM milk washing.

[Brief explanation of drawings]

Figure 1 is a graph showing the relationship between sulfuric acid concentration and temperature during sulfuric acid pickling and descaling, and Figure 2 is a graph showing the relationship between nitric acid concentration and temperature during nitric acid pickling after sulfuric acid pickling and wash resistance effect. . Special mention 1 Agent Masu Physician Ii Kosugi 4o male Figure 1 40 50 60 70 80 90 100 liquid
On Wangji C)

Claims (1)

[Claims] I Cr stainless steel hot rolled steel strip is described below (a). (b), the first step of removing oxide scale by deep immersion in a sulfuric acid aqueous solution that satisfies the conditions of formulas (c), and the following (d), (
e) A method for pickling a hot-rolled Cr-based stainless steel strip, which comprises the second step of removing dirt on the surface and passivating it by immersing it in an aqueous nitric acid solution that satisfies the conditions of formula. CI≧10 ・・・・・・(a)C,
+0,8Tl ≧ 76 ......(b) Tl
≧ 55 ・・・・・・(c)・・・
...(d) 40≦T2≦go -・----Ce) Here, C1: Weight percentage (%) of O H2SO4 per 100 m of sulfuric acid aqueous solution T1: Temperature of sulfuric acid aqueous solution (°C) C2: Nitric acid aqueous solution Weight percent of HNO per 100 m liters (%) T2: Temperature of nitric acid aqueous solution (°C)