KR20100073406A - Method for pickling niobium added ferritic stainless steel - Google Patents

Abstract

PURPOSE: A method and an apparatus for continuous casting of unsymmetrical billet are provided to control hydrofluoric acid concentration by controlling the contents of Nb and Cr. CONSTITUTION: A method and an apparatus for continuous casting of unsymmetrical billet comprises: a step of pickling in the neutral salt electrolytic cell in which current more than 0.06A / cm^2 is applied the steel sheet: a step of pickling in a nitric acid electrolytic bath in which current more than 0.06A / cm^3 is applied the steel sheet; and a step of dipping the steel sheet in nitric acid and hydrofluoric acid. The steel sheet is 14Cr system. The content of the niobium in the steel sheet is 0.3% or greater.

Description

Pickling method of niobium-containing ferritic stainless steel {Method for pickling niobium added ferritic stainless steel}

The present invention relates to a pickling method of niobium-containing ferritic stainless steel. More specifically, by adding an electrolytic sulfuric acid pickling process to the hot-rolled or cold-rolled annealing stainless steel sheet and controlling the pickling conditions, it is possible to quickly and uniformly remove the oxidation scale, thereby ensuring excellent surface glossiness. The present invention relates to a pickling method of niobium-containing ferritic stainless steel.

In general, ferritic slabs containing 14 to 20% of chromium are subjected to hot rolling at a high temperature after heating for 2-3 hours in a heating furnace, followed by a pickling process, followed by annealing. After that, the hot rolled steel sheet is finally cold rolled to produce a thin plate, and then subjected to annealing heat treatment. After that, the surface scale is removed again through a pickling process to produce a stainless steel sheet having a smooth surface state.

Conventionally, the pickling process of stainless steel is carried out by neutral salt electrolytic acid washing, nitric acid electrolytic acid washing and mixed acidic deposition of nitric acid and hydrofluoric acid. In addition, a variety of pickling techniques are provided, such as pretreatment in molten salt for pickling the stainless steel sheet, and then passing through nitric acid or a mixed acid bath.

On the other hand, in ferritic stainless steel containing niobium, oxide layers such as Cr, Fe, Si, and Nb are formed on the surface of the stainless steel sheet during the rolling process or annealing heat treatment. The oxide layer may be removed by a pretreatment process or a pickling solution during the pickling process, but it is very difficult to completely remove the Fe and Si oxide layers.

Therefore, there is a problem that the removal of such an oxide layer is not uniform or does not become completely pickled. In this case, the corrosion resistance is remarkably lowered, resulting in a problem that the value of the product is lowered.

The present invention is to improve the above-described problems, the surface gloss recovery rate showing the gloss ratio before heat treatment and the gloss ratio after the pickling treatment when the pickled heat treatment of the annealing ferritic stainless steel sheet after hot rolling or cold rolling is 85 It is an object of the present invention to provide a pickling method of niobium-containing ferritic stainless steel sheet capable of securing excellent surface glossiness of more than%.

In the present invention for achieving the above object, in the electrolytic pickling method of a ferritic stainless steel sheet for removing the scale formed on the surface by annealing heat treatment after hot rolling or cold rolling, the steel sheet is applied with a current of 0.06A / ㎠ or more Pickling in a neutralized salt electrolyzer; Pickling in a sulfuric acid electrolyzer to which a current of 0.06 A / cm 2 or more is applied; Pickling in a nitric acid electrolyzer to which a current of 0.03 A / cm 2 or more is applied; And it relates to a pickling method of niobium-added ferritic stainless steel comprising a step of mixed immersion of nitric acid and hydrofluoric acid.

In addition, in the present invention, the steel sheet is 14Cr-based, when the content of niobium is less than 0.3%, the hydrofluoric acid concentration in the mixed acid immersion step is preferably 10 ~ 15g / l, when the content of niobium is 0.3% or more, hydrofluoric acid concentration Is preferably 5 to 10 g / l.

In addition, in the present invention, the steel sheet is 19Cr-based, when the content of niobium is less than 0.3%, the hydrofluoric acid concentration in the mixed acid immersion step is preferably 15 ~ 20g / l, when the content of niobium is more than 0.3% hydrofluoric acid concentration It is preferable that it is 10-15 g / l.

EMBODIMENT OF THE INVENTION Hereinafter, this invention is demonstrated in detail.

The present inventors have solved the difficulty of removing the Si and Nb oxide layers when pickling niobium-added ferritic stainless steels and researched ways to obtain better surface gloss than conventional pickling processes. As the pickling process including the pickling process is carried out and the conditions are managed specially, it is confirmed through experiments that the excellent surface glossiness can be secured by removing the scale due to Si and Nb oxides. This invention was completed.

When the heat treatment in an oxidizing atmosphere, the chromium content and annealing temperature of each stainless steel are different so that the composition, thickness and surface color of the oxide scale are different.

When the ferritic stainless steel sheet is heat treated in an oxidizing atmosphere, a Cr-rich scale is mainly formed, and SiO ₂ is formed between the scale interface and the base material. In addition, as the heat treatment temperature and time increases, the scale thickness increases, and the color of the annealed steel sheet becomes blue in the case of ferritic stainless steel.

When the scale is removed from these steel systems, there are several pickling processes. The characteristics of each process are as follows.

First, the ferritic stainless steel cold rolled steel plate is passed through a neutral salt electrolytic cell. When electrolytic pickling is carried out by applying a constant current in an aqueous neutral salt solution containing sodium sulfate, in the case of a ferritic stainless steel plate having a chromium-rich scale, Cr oxide may be dissolved by anodization, but chromium may be dissolved within a short time. It is not possible to remove the oxide completely.

In addition, since the Fe oxide is not dissolved in the neutral saline solution by the electrochemical reaction, it undergoes the nitric acid electrolysis process to remove the Fe oxide in the oxidized scale. In addition, since the Si oxide existing in the scale cannot be removed by neutral salt and nitrate electrolyte, the deposition process is applied in a mixed acid tank composed of nitric acid and hydrofluoric acid.

However, in the case of niobium-added ferritic stainless steel, since the pickling occurs by the Nb oxide layer, the conventional pickling treatment is not sufficient. Therefore, in the present invention, unlike the conventional electrolytic pickling method, the ferritic stainless steel containing Nb is used as the target steel type. It is characterized by the addition of sulfuric acid electrolytic pickling process.

Hereinafter, the pickling conditions of the present invention will be described in detail.

In the present invention, in order to remove the surface scale formed on the surface of the stainless steel sheet by annealing heat treatment after hot rolling or cold rolling, pickling treatment may be performed in the order of neutral salt electrolyte-sulfuric acid electrolyte-nitric acid electrolyte-mixed acidic immersion.

First, the ferritic stainless steel sheet is passed through a neutral salt electrolytic cell. At this time, in a neutral salt electrolyzer to which a current of 0.06 A / cm 2 or more is applied, it may be carried out according to the pickling conditions of a typical temperature and concentration, and is not particularly limited thereto. When the current density of the neutral salt electrolyzer is less than 0.06 A / cm 2, since sufficient pickling is not performed, it is preferable to limit it to 0.06 A / cm 2 or more.

Thereafter, the stainless steel sheet subjected to the neutral salt electrolytic pickling is subjected to a process of pickling in a sulfuric acid electrolytic cell. In the sulfuric acid electrolyzer, a current of 0.06 A / cm 2 or more is applied, and when less than 0.06 A / cm 2, pickling may occur.

In addition, as a subsequent step, the mixture is immersed after undergoing nitric acid electrolytic acid treatment to remove the Fe oxide in the oxide scale. In this case, in a nitric acid electrolyzer to which a current of 0.03 A / cm 2 or more is applied, the nitric acid electrolytic pickling conditions of a normal temperature and concentration may be performed, and the present invention is not particularly limited thereto. When the current density of the nitric acid electrolytic cell is less than 0.03 A / cm 2, sufficient pickling is not performed. Therefore, the nitric acid electrolytic cell is preferably limited to 0.03 A / cm 2 or more.

In addition, the composition of the mixed acid tank during the mixed acid immersion is made of nitric acid and hydrofluoric acid, in the present invention is characterized in that the hydrofluoric acid concentration is controlled according to the content of Cr and Nb.

That is, when the stainless steel sheet of the present invention is 14Cr-based, the niobium content is less than 0.3%, the hydrofluoric acid concentration is preferably 10-15 g / l in the mixed acid immersion step, and when the niobium content is 0.3% or more, the hydrofluoric acid concentration is It is preferable that it is 5-10 g / l. When the hydrofluoric acid concentration is less than the lower limit values, an acid pickling may occur, and when the hydrofluoric acid concentration is higher than a given condition, an overtaxing tax may occur.

In addition, when the stainless steel sheet of the present invention is 19Cr-based, the niobium content is less than 0.3%, the hydrofluoric acid concentration is preferably 15-20 g / l in the mixed acid immersion step, and the hydrofluoric acid concentration when the niobium content is 0.3% or more. It is preferable that it is 10-15 g / l. When the hydrofluoric acid concentration is less than the lower limit values, an acid pickling may occur, and when the hydrofluoric acid concentration is higher than a given condition, an overtaxing tax may occur.

As described above, according to the present invention, the pickling method of the niobium-added ferritic stainless steel sheet of the present invention can be pickled in a short time and at the same time secured excellent surface gloss can minimize product defects of the stainless steel sheet and improve productivity Provide effect.

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

EXAMPLE

In this example, 14,19Cr, 1% Mo, and 0.5% Cu grades were carried out. Table 1 shows the glossiness recovery rate, which is the result of pickling according to the hydrofluoric acid concentration. Here, the glossiness recovery rate (%) represents the ratio between the glossiness before heat treatment and the glossiness after pickling treatment, and the glossiness measurement was measured by irradiating light at 60 degrees in the rolling direction.

Si Cr
Nb Neutral Salt Electrolysis
(A / ㎠) Sulfuric acid
(A / ㎠) Nitric acid
(A / ㎠) Foshan concentration
(g / L) Gloss recovery
(%)
Inventive Example 1 0.525 14.06 0.53 0.06 0.06 0.03 10 90 Comparative Example 1 0.795 13.97
0.46 0.06 0.06 0.03 15 60
Comparative Example 2 0.796 14.00
0.45 0.047 0.047 0.016 15 65
Comparative Example 3 0.795 19.42
0.45 0.06 0.06 0.03 20 70
Inventive Example 2 0.276 19.30
0.45 0.06 0.06 0.03 10 95 Comparative Example 4 0.768 19.37
0.44 0.06 0.06 0.03 20 65
Comparative Example 5 0.940 14.02
0.35 0.047 0.047 0.016 15 60
Inventive Example 3 0.521 19.31
0.34 0.06 0.06 0.03 15 92
Inventive Example 4 0.092 19.42 0.34 0.06 0.06 0.03 10 90 Inventive Example 5 0.532 14.03
0.33 0.06 0.06 0.03 10 90
Inventive Example 6 0.509
14.10
0.33 0.06 0.06 0.03 10 95 Comparative Example 6
0.940
19.30
0.32 0.06 0.06 0.03 20 68 Inventive Example 7 0.278 14.10
0.22 0.06 0.06 0.03 10 92
Comparative Example 7 0.801 14.15
0.21 0.047 0.047 0.016 20 60
Comparative Example 8 0.792 14.00 0.21 0.047 0.047 0.016 20 65 Inventive Example 8 0.278 19.30
0.21 0.06 0.06 0.03 15 90
Comparative Example 9 0.539 14.00
0.15 0.047 0.047 0.016 20 75
Inventive Example 9 0.555 19.28
0.126 0.06 0.06 0.03 20 95

As shown in Table 1, in the invention examples (1-9) satisfying the scope of the present invention, the glossiness recovery rate was 90 to 95%, showing excellent surface glossiness.

On the other hand, Comparative Examples (1 to 9) that do not satisfy the mixed acid pickling conditions of the present invention did not secure more than 85% of the glossiness recovery target of the present invention.

Claims (5)

An electrolytic pickling method of a ferritic stainless steel sheet for removing a scale formed on a surface by annealing heat treatment after hot rolling or cold rolling, comprising: pickling the steel sheet in a neutral salt electrolyzer to which a current of 0.06 A / cm 2 or more is applied; Pickling in a sulfuric acid electrolyzer to which a current of 0.06 A / cm 2 or more is applied; Pickling in a nitric acid electrolyzer to which a current of 0.03 A / cm 2 or more is applied; And a step of mixed immersion of nitric acid and hydrofluoric acid. The method of claim 1, wherein the steel sheet is 14Cr-based, the niobium content is less than 0.3%, and the hydrofluoric acid concentration in the mixed acid immersion step is 10 ~ 15g / l, the pickling method of niobium-added ferritic stainless steel. The method of claim 1, wherein the steel sheet is 14Cr-based, niobium content is 0.3% or more, and the hydrofluoric acid concentration in the mixed acid immersion step is 5 ~ 10g / l, the pickling method of niobium-added ferritic stainless steel. The method of claim 1, wherein the steel sheet is 19Cr-based, the niobium content is less than 0.3%, the hydrofluoric acid concentration in the mixed acid immersion step is a niobium-added ferritic stainless steel pickling method, characterized in that. The method of claim 1, wherein the steel sheet is 19Cr-based, the niobium content is 0.3% or more, the hydrofluoric acid concentration in the mixed acid immersion step is 10 ~ 15g / l, the method of pickling niobium-added ferritic stainless steel.