JPH05230681A - Pickling method for ferritic stainless steel hot rolled stock - Google Patents

Abstract

PURPOSE:To efficiently remove surface scales and scale flaws of ferritic stainless steel hot rolled stock. CONSTITUTION:Ferritic stainless steel hot rolled stock is pickled by a soln. of sulfuric acid of 80 to 100 deg.C in which the concn. of sulfuric acid per 1 liter soln. is regulated to 200 to 400g and is thereafter pickled by a soln. of 40 to 80 deg.C whose cocn. is regulated to a one mixed with 20 to 160g nitric acid and 10 to 200g hydrofluoric acid per 1 liter soln. Furthermore, as for stainless steel hot rolled stock in which intergranular erosion is generated by the pickling by nitric acid hydrofluoric acid, its surface layer is dissolved and ground by 20mum by the pickling by sulfuric acid. By the combination of the pickling by sulfuric acid removing scales on the normal part and the pickling by nitric acid/hydrofluoric acid selectively and efficiently removing scale flaws, the productivity of the pickling of ferritic stainless steel hot rolled stock is remarkably improved.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for effectively removing surface oxide scale of hot rolled ferritic stainless steel.

[0002]

2. Description of the Related Art Conventionally, sulfuric acid pickling treatment or nitric hydrofluoric acid pickling treatment has been used for descaling of hot rolled stainless steel. This is a method in which the scale is removed by immersing in an acid solution of sulfuric acid or nitric hydrofluoric acid or by spray pickling to dissolve the metal immediately below.

An acid solution of either sulfuric acid or nitric hydrofluoric acid is often used in consideration of the components of stainless steel and surface properties after pickling. Ferritic stainless steels such as SUS430 have a high dissolution rate in a sulfuric acid solution, and the grain boundary Cr-deficient layer is selectively dissolved in a nitric hydrofluoric acid solution, so that the surface after the nitric hydrofluoric acid pickling treatment becomes rough, It adversely affects the quality such as surface gloss of cold rolled products. For this reason, a sulfuric acid pickling treatment is often applied.

On the other hand, austenitic stainless steel such as SUS304 has a high dissolution rate in a nitric hydrofluoric acid solution, and the grain boundary Cr can be adjusted by optimizing the nitric hydrofluoric acid concentration and temperature.
Since it is possible to prevent selective dissolution of the deficient layer, nitric hydrofluoric acid pickling treatment is often applied. In order to process both austenitic stainless steel and ferritic stainless steel in the same equipment, the sulfuric acid pickling treatment tank and the nitric hydrofluoric acid pickling treatment tank are installed in the annealing pickling treatment equipment for hot rolled stainless steel strip. There are many examples that have been done.

[0005]

By applying the above-mentioned sulfuric acid pickling treatment or nitric hydrofluoric acid pickling treatment, the scale of stainless steel hot rolled material with few surface defects can be removed efficiently. However, in the hot rolling process, due to factors such as rough surface of the roll and adhesion of scale on the surface of the roll, a flaw in the form in which the scale bites into the hot rolled stainless steel material may occur. Hereinafter, this flaw is called a scale flaw. This scale flaw often occurs in ferritic stainless steel due to conditions such as heating temperature during hot rolling. When such scale flaws are present, the scale of the scale flaw cannot be removed by the conventional sulfuric acid pickling treatment and remains. for that reason,
There is a problem that productivity is extremely deteriorated because the acid pickling treatment is performed again after passing through the annealing pickling equipment.

Therefore, there is a strong demand for a pickling method for removing scale flaws in a short time. An object of the present invention is to provide a pickling method for hot-rolled ferritic stainless steel that can meet such demands.

[0007]

The main points of the present invention are as follows. (1) A ferritic stainless steel hot rolled material is subjected to sulfuric acid pickling treatment at a temperature of 80 to 100 ° C. using a sulfuric acid solution having a concentration of 200 to 400 g of sulfuric acid per liter of a solution, and subsequently 20 nitric acid per liter of a mixed solution. ~ 160g and hydrofluoric acid 1
40-80 using the solution of the concentration which mixed 0-200g
A method for pickling a ferritic stainless steel hot rolled material, which comprises performing a nitric hydrofluoric acid pickling treatment at a temperature of ° C.

(2) In the sulfuric acid pickling treatment, the surface layer of the hot rolled material is ablated by 20 μm or more, and the pickling method for hot rolled ferritic stainless steel material according to the above 1 is characterized.

[0009]

The present inventors have obtained the following findings through various experiments. First, claim 1 will be described based on experimental results. Ferritic stainless steel (19Cr-0.3Ni-0.4Cu-0.6Nb- with the chemical composition shown in a of Table 1)
The low C) hot rolled steel strip was subjected to pickling treatment under various conditions using an annealing pickling treatment facility.

First, when the sulfuric acid pickling treatment was carried out, the time until the scale of the normal part was completely removed was investigated. The conditions of the sulfuric acid pickling treatment were changed at a solution temperature of 70 to 100 ° C. using a sulfuric acid solution having a concentration of 100 to 500 g of sulfuric acid per liter of the solution. FIG. 1 shows the influence of the concentration and temperature of the pickling solution on the time (descaling time) until the scale of the normal part (the part without scale defects) is completely removed.

When the concentration of sulfuric acid is 200 g or more per liter of the solution and the solution temperature is 80 ° C. or more, the descaling time is 100 seconds or less, and the descaling can be efficiently performed in a short time. Further, when the concentration exceeds 400 g of sulfuric acid per liter of the solution, even if the concentration is further increased, the descale time is not shortened so much. From this result, it was found that the sulfuric acid pickling treatment conditions for efficiently removing the scale of the normal part in a short time were a concentration of 200 to 400 g of sulfuric acid per liter of the solution and a solution temperature of 80 to 100 ° C. However, only this sulfuric acid pickling treatment left scale flaws.

The properties and composition of the scale flaw remaining after the sulfuric acid pickling treatment were examined in detail using a scanning electron microscope and an X-ray microanalyzer. Figure 2 shows the cross-sectional morphology of the scale flaw. It was found that the scale flaw penetrates into the base metal to a maximum depth of 100 µm, and a Cr-deficient layer with a depth of about 10 µm exists just below the scale flaw. It was

Various experiments have been conducted in order to effectively utilize the existence of the Cr-deficient layer immediately below the scale flaws and to investigate a new pickling treatment method for efficiently removing the scale flaws.
The dissolution rate of ferritic stainless steel in an acid solution largely depends on its Cr content. The smaller the Cr content,
The dissolution rate in the sulfuric acid solution decreases, and conversely, the dissolution rate in the nitric hydrofluoric acid solution increases. Therefore, we considered that nitric-hydrofluoric acid pickling treatment was effective to remove scale flaws efficiently, and repeated experiments.

After the scale of the normal part was completely removed by the sulfuric acid pickling treatment, the nitric hydrofluoric acid pickling treatment was carried out under various conditions in which the solution concentration and temperature of the mixed solution of nitric acid and hydrofluoric acid were changed. As a condition for completely removing the scale of the normal part, 300 g of sulfuric acid per liter of the solution and a solution temperature of 90
A sulfuric acid pickling treatment was carried out at a temperature of 100 ° C. for 100 seconds. As the conditions for the subsequent nitric-hydrofluoric acid pickling treatment, the concentration of the mixed solution was 10-160 g of nitric acid and 5-2 of hydrofluoric acid per liter of the solution.
00g, and the solution temperature was changed in the range of 30 to 80 ° C. Considering the operating efficiency, the treatment time of nitric hydrofluoric acid pickling is 1
It was fixed at 00 seconds. FIG. 3 shows the result of investigation on the removal condition of scale flaws after these pickling treatments. Mixed solution 1
It was found that the scale flaws were completely removed under the pickling conditions of 20 g or more of nitric acid, 10 g or more of hydrofluoric acid, and 40 ° C. or more of temperature per liter.

That is, it has been found that the scale flaws can be efficiently removed by the nitric hydrofluoric acid pickling treatment after the normal part of the scale is removed by the sulfuric acid pickling treatment. The scale removing mechanism in the sulfuric acid pickling treatment and the nitric hydrofluoric acid pickling treatment will be described based on the schematic diagram of the scale removing situation shown in FIG. In the annealed material of ferritic stainless steel, the scale remains on the surface of the steel material in both the normal portion and the scale flaw portion. If sulfuric acid pickling under appropriate conditions is performed, the scale of normal parts will be completely removed. However, the scale flaw remains.

Further, in a nitric hydrofluoric acid solution, a stainless steel having a smaller amount of Cr is characterized by a higher dissolution rate.
Therefore, when the scale of the normal part is removed by sulfuric acid pickling and then nitric hydrofluoric acid pickling is performed, the Cr-deficient layer immediately below the scale flaw is selectively dissolved, and as a result, the scale flaw is peeled off from the base metal. be able to. Next, claim 2 will be described.

When the hot rolled steel strip of ferritic stainless steel is subjected to nitric hydrofluoric acid pickling treatment, grain boundary erosion may occur. This is because the grain boundary Cr-deficient layer existing in the surface layer of the steel strip is selectively dissolved by nitric hydrofluoric acid. Grain boundary Cr
Whether or not a depletion layer is generated is mainly determined by the composition of stainless steel. The mechanism for depositing the chromium carbide is as follows.

When soaking a stainless steel slab, a decarburized layer is formed on the surface by the reaction between the solid solution C in the surface layer and oxygen gas. Since the decarburized layer has a low C which is an austenite forming element, martensite is less likely to be formed during hot rolling as compared with the inside. In addition, since the ferrite structure has a smaller solid solution limit amount of C than the martensite structure, chromium carbide easily precipitates at grain boundaries in the decarburized layer on the surface, and Cr carbide precipitates to form a Cr-deficient layer. To be done.

In order to suppress the formation of the grain boundary Cr-deficient layer, it is effective to reduce the C concentration and add an element such as Nb or Ti that is more likely to combine with C than Cr. However,
From the viewpoint of steel composition, it is inevitable that a grain boundary Cr deficient layer is formed.
Experiments were also conducted to apply the pickling treatment of the present invention also to (for which an element that easily bonds with is not added).

The hot-rolled steel strip of 19Cr stainless steel having the chemical composition shown in Table 1b was subjected to annealing, sulfuric acid pickling and nitric acid / hydrofluoric acid pickling treatment using annealing pickling equipment, and then grain boundaries. The erosion situation was observed with a scanning electron microscope. Table 2 shows the pickling conditions. After performing the sulfuric acid pickling treatment in which the amount of scraping was changed by changing the time, the nitric hydrofluoric acid pickling treatment was performed under certain conditions.

FIG. 5 shows the relationship between the amount of ablation by the sulfuric acid pickling treatment and the grain boundary erosion depth after the nitric hydrofluoric acid pickling treatment. When the amount of ablation by the sulfuric acid pickling treatment was 15 μm or less, severe intergranular erosion was observed by the subsequent nitric hydrofluoric acid pickling, but when the amount of ablation by the sulfuric acid pickling treatment was 20 μm or more, the grain boundary erosion was I can't see it at all. The reason why grain boundary erosion occurs in the subsequent nitric hydrofluoric acid pickling treatment only when the amount of smelting by the sulfuric acid pickling treatment is small can be explained as follows.

The grain boundary Cr-deficient layer exists at most about 20 μm from the surface. If the grain boundary Cr-deficient layer is dissolved and removed by the sulfuric acid pickling treatment, the grain boundary Cr-deficient layer is also removed by the subsequent nitric-hydrofluoric acid pickling treatment. It is considered that no field erosion occurs. That is, even with respect to ferritic stainless steel in which generation of a grain boundary Cr-deficient layer is unavoidable in terms of steel composition, C
It has been found that if the surface layer having the r-deficient layer is ablated to 20 μm, a smooth surface can be obtained even after pickling, and the pickling treatment of the present invention can be applied.

[0023]

EXAMPLES 19Cr-0.3Ni-0.4Cu-0.6
Nb-Low C (a in Table 1), and 19Cr (b in Table 1)
The two types of hot rolled steel strips of ferritic stainless steel in Table 3
-The pickling treatment shown in Table 6 was performed. This stainless steel composition is the same as shown in Table 1. There are two types of steel history, hot rolled material and hot rolled / annealed material. The pickling method is
There are two types: immersion and spray.

No. 1-No. 25 is an example of the present invention.
As a comparative example, no. 26-No. No. 33 is a sulfuric acid pickling treatment alone or a nitric hydrofluoric acid pickling treatment alone. 34
~ No. In No. 43, the concentration and temperature conditions of the pickling solution were out of the limited range. Tables 7 and 8 show the scale removal status of the normal part, the scale removal status of the scale flaw part, and the intergranular corrosion status after pickling. In the example of the present invention, the scale can be completely removed from both the normal portion and the scale flaw portion, and no intergranular corrosion has occurred, so that the effect of the descaling of the present invention is remarkable.

On the other hand, in the case of the sulfuric acid pickling treatment alone, the nitric acid / hydrofluoric acid pickling treatment alone, and the treatments in which the pickling conditions are out of the proper condition range of the present invention, which are shown in the comparative examples, the scale of the normal part or the scale flaw part Remaining was seen. Further, when the amount of fusing of the steel material b in the sulfuric acid pickling treatment was smaller than 20 μm, intergranular corrosion was observed.

[0026]

[Table 1]

[0027]

[Table 2]

[0028]

[Table 3]

[0029]

[Table 4]

[0030]

[Table 5]

[0031]

[Table 6]

[0032]

[Table 7]

[0033]

[Table 8]

[0034]

EFFECT OF THE INVENTION Ferrite is produced by the pickling method of the present invention, which is a combination of a nitric hydrofluoric acid pickling treatment capable of selectively and efficiently removing scales on a scale flaw portion and a sulfuric acid pickling treatment for removing scales on a normal portion. The productivity of pickling treatment, which completely removes the scale flaws of hot-rolled stainless steel, is greatly improved.

[Brief description of drawings]

FIG. 1 19Cr-0.3Ni-0.4Cu-0.6N
b-In the sulfuric acid pickling treatment of the low C stainless steel hot rolled sheet,
It is a figure showing the influence of the concentration and temperature of the sulfuric acid pickling treatment solution on the time required until the scale of the normal part is completely removed.

FIG. 2 is a view showing a form of a scale flaw cross section generated in a ferritic stainless steel hot rolled sheet.

FIG. 3 19Cr-0.3Ni-0.4Cu-0.6N
FIG. 6 is a diagram showing the influence of nitric acid concentration, hydrofluoric acid concentration, and solution temperature conditions on the removal of scale flaws after the scale of the normal part of a b-low C stainless steel hot-rolled sheet was completely removed by sulfuric acid pickling treatment. ..

FIG. 4 is a diagram schematically showing a mechanism for removing scale flaws by the sulfuric acid pickling treatment and the nitric hydrofluoric acid pickling treatment of the present invention.

FIG. 5 shows the effect of the amount of ablation in the sulfuric acid pickling treatment on the depth of intergranular erosion that occurs when the sulfuric acid pickling treatment and the nitric acid hydrofluoric acid pickling treatment of the present invention are applied to a 19Cr stainless steel hot rolled sheet. It is the figure shown.

 ─────────────────────────────────────────────────── ─── Continuation of front page (72) Inventor Masafumi Takai 3434 Shimada, Hitsu-shi, Yamaguchi Shinko Nippon Steel Corporation (72) Inventor Shingo Furukawa 3434 Shimada, Hikari-shi, Yamaguchi Prefecture New Japan (72) Inventor Yohei Fujii, Yamaguchi Prefecture, Hikari City 3434 Shimada, Nippon Steel Co., Ltd. Hikari Steel Co., Ltd.

Claims (2)

[Claims] 1. A hot rolled ferritic stainless steel is subjected to sulfuric acid pickling treatment at a temperature of 80 to 100 ° C. using a sulfuric acid solution having a concentration of 200 to 400 g of sulfuric acid per liter of solution,
Then, using a solution having a concentration of 20 to 160 g of nitric acid and 10 to 200 g of hydrofluoric acid per liter of the mixed solution, 4
A pickling method for hot-rolled ferritic stainless steel, which comprises performing a nitric hydrofluoric acid pickling treatment at a temperature of 0 to 80 ° C. 2. The pickling method for a hot rolled ferritic stainless steel material according to claim 1, wherein the surface layer of the hot rolled material is ablated by 20 μm or more in the sulfuric acid pickling treatment.