KR100395135B1 - Cleaning method of ferritic stainless steel - Google Patents

Abstract

The present invention relates to a clean pickling method of 18Cr stainless steel, the purpose of the conventional pickling method is to use a mixed acid tank (nitric acid + hydrofluoric acid) due to environmental pollution due to NOx generation and metal loss due to excessive base metal erosion and While it was not possible to avoid the cost of reprocessing, in the present invention, by eliminating the mixed acid tank, the ferritic stainless steel which can fundamentally prevent such a problem and reduce the metal loss by reducing the amount of metal loss. To provide a method for pickling rivers.

The present invention for achieving the above object in the method of pickling ferritic stainless steel,

After the steel sheet subjected to the annealing and shot blasting (1) was removed with a grinding brush 18, a portion of the scale was removed, and then sulfuric acid (H ₂ SO ₄ ) was 150-200 g / l, and iron nitrate (Fe ( NO ₃ ) The clean acid pickling method of waste-light stainless steel characterized by pickling with sulfuric acid bath only when the temperature of the sulfuric acid solution composed of ₃ · 9H ₂ O) is set to 90 to 100 ° C. Make a point.

Description

Clean pickling method of ferritic stainless steel

The present invention relates to a pickling method of an 18 Cr ferritic stainless hot rolled steel sheet, and more specifically, to a sulfuric acid bath treatment without omitting a mixed acid bath treatment of a plurality of pickling tanks consisting of a sulfuric acid tank and a mixed acid bath in a conventional method of pickling stainless steel. The present invention relates to a clean pickling method of a waste light stainless steel hot rolled steel sheet which can reduce costs and solve the NOx generation problem which is the most problematic in the conventional mixed acid treatment.

The working process of the continuous annealing pickling line currently being used is briefly described with reference to FIG. 1 as follows.

The dense scale on the surface of heat-treated hot-rolled annealed steel sheet takes a long time to remove when chemical method is used. In addition to removing a portion of the micro-cracks on the residual scale (acid cracks) to allow the acid solution to penetrate well in subsequent processes.

The pickling conditions of the representative ferritic stainless steel SUS430 are shown in Table 1 below, where the role of the sulfuric acid tank (4) not only activates the interface between the base and the scale to facilitate the removal of the scale in the mixed acid tank (6). Remove some.

TABLE 1

In addition, the mixed acid tank 6 is composed of pickling tanks 1 and 2, and the role of the mixed acid tank not only completely removes the residual scale not removed from the sulfuric acid tank, but also forms a protective film on the surface of the stainless steel plate. To give.

After passing through the mixing tank, the steel sheet passes through the brush roll 8 and the washing machine 9, and then completely removes water from the surface of the dryer 10, and evens the shape of the coil in the skin pass roll 11. After winding in 13), it is shipped as a hot rolled coil or sent to a cold rolling mill for cold rolling.

However, in the conventional method as described above, the concentration of HF in the mixed acid bath is more weighted than the sulfuric acid bath, so that the concentration of HF in the mixed acid bath is controlled to 5-10 g / l and the amount of metal ions up to 60 g / l. As the concentration of ions increases rapidly in a short time, the activity of the acid decreases, which not only shortens the acid life, but also sludge (primarily FeF ₃ precipitate) precipitates in the pipe 14 between the acid tank and the heat exchanger, causing a blockage of the acid tank. Interfering with the circulation of acid between the 6 and the heat exchanger 15 reduces the cooling ability of the acid solution and ultimately blocks the piping completely.

In order to solve this problem, the present inventor has proposed a pickling method characterized in controlling the nitric acid and hydrofluoric acid concentration of the grinding brush and the mixed acid tank in the conventional method in the patent application 1997-67750.

However, this method still could not fundamentally solve NOx generation by using a mixed acid bath.

The present invention has been made to solve the problems of the prior art described above, and an object thereof is to provide a clean pickling method for ferritic stainless steel using only sulfuric acid tanks without using a mixed acid tank that causes environmental pollution by NOx generation. have.

1 is a schematic diagram of a pickling line generally used;

2 is a schematic diagram of a pickling line for carrying out the present invention.

<Description of the code | symbol about the principal part of drawing>

1: Shot Blaster 2: Bride Roll 3: Washer 4: Sulfur Tank

5, 9: washing machine 6: mixed acid tank 7: acid spray nozzle 8: brush roll

10: dryer 11: skin pass roll 13: winding machine 14: piping 15: heat exchanger

18: polishing brush roll

In order to achieve the above object, the clean pickling method of the present invention removes a portion of the scale with a grinding brush 18 after annealing and shot blasting, and then sulfuric acid (H ₂ SO ₄ ) is 150 to 200 g / l, Iron nitrate (Fe (NO ₃ ) ₃ .9H ₂ O) for promoting pickling is characterized in that pickling with only a sulfuric acid bath at a temperature of 90 to 100 ° C. for a sulfuric acid solution composed of 15 to 25 g / l.

Hereinafter, the present invention will be described in detail with reference to preferred embodiments.

In the present invention, as shown in Fig. 2, first, a part of the scale is mechanically removed by using a grinding device 18 such as a grinding brush or the like on a steel sheet subjected to annealing and shot blast (Shot Blast) 1.

The grinding brush 18, which is the grinding device, is installed in front of the entrance side of the sulfuric acid tank 4 to remove a portion of the surface scale remaining on the surface of the steel sheet which has passed through the shot blaster (1).

That is, the grinding brush 18 removes a part of the surface scale which remain | survives on the steel plate surface by a pair of brush rolls rotatably installed in the upper and lower parts of the plate | plate steel plate S.

In order to remove the remaining scale in addition to the scale partially removed by the action of the grinding brush 18, in the technique proposed in the prior patent application No. 1997-67750, the sulfuric acid bath conditions the temperature 90 ~ 100 ℃, the concentration 120 ~ It was pickled at 160 g / l, mixed acid bath conditions at a temperature of 40 to 50 ° C, a nitric acid concentration of 100 to 150 g / l, and a hydrofluoric acid concentration of 1 to 5 g / l.

However, in the present invention, in order to fundamentally eliminate the generation of NOx due to the use of the mixed acid tank, without using the mixed acid tank, the combined flow of iron nitrate (Ferric Nitrate) in which the combined water composition can be performed simultaneously from the sulfuric acid tank (4) to the role of the mixed acid tank; 15-25 g / l of Fe (NO ₃ ) ₃ · 9H ₂ O) was added to ensure the existing pickling rate.

Under the above conditions, ferric nitrate (Fe (NO ₃ ) ₃ .9H ₂ O) is added in an amount of not more than 15 g / l but more than 25 g / l, which causes a problem of fine pickling.

In addition, in the present invention, the pickling operation is performed at a sulfuric acid tank temperature of 90 to 100 ° C and a concentration of 150 to 200 g / l. It is a higher numerical range.

When pickling in the above manner, the line speed can be maintained at 30 to 35mpm as in the prior art.

In addition, it is possible to reduce the amount of acid used by reducing the amount of metal dissolved in the solution than the conventional method in which the base metal is excessively eroded by hydrofluoric acid in the mixed acid bath.

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

EXAMPLE

A SUS430 stainless steel hot rolled steel sheet was hot-rolled and annealed by a conventional method, treated with a shot blaster and a grinding brush 18, and then pickled under the conditions shown in Table 2 below. After pickling, pickling and loss of base metal due to pickling were investigated. Pickling evaluation was shown as complete pickling (○), poor whiteness (△), and fine pickling (×).

TABLE 2

As shown in Table 2, the prior art example (1, 2) using a mixed acid tank is secured pickling, it is inevitable that the environmental pollution occurs due to the NOx generation or a reprocessing cost to prevent it.

In addition, the metal loss amount is 5.0-7.5 mg / cm 2 and high, and the reprocessing cost is high in order to recover | recover the metal and acid in a solution.

However, an invention example in which a mixed acid tank to which the present invention is applied is omitted, a portion of the scale is mechanically removed using the grinding brush 18, and the pickling capacity is improved using Fe (NO ₃ ) ₃ .9H ₂ O ( According to 1,2,3), while the pickling property is also secured, the metal loss amount is also low at 2.2 to 2.4 mg / cm 2, which can reduce the cost much more than the conventional pickling method.

On the other hand, in Comparative Example 1 is an example of using the grinding brush 18 without the addition of ferric nitrate (Fe (NO ₃ ) ₃ · 9H ₂ O), the fine pickling occurred, as in Comparative Example 2 Fe (NO ₃ ) When ₃ · 9H ₂ O was added below 10 g / l, the pickling property was secured to some extent, but the whiteness was lower than 70. Also, Comparative Example 3 in which Fe (NO ₃ ) ₃ .9H ₂ O was added 30 g / L or more also showed a whiteness of 70 or less. Here, the mechanism by which Fe (NO3) 3.9H2O promotes pickling property can be described in Equation 1 below.

_{Fe (NO 3) 3 · 9H} 2 O → Fe 3+ + 3NO 3+ 9H 2 O ------- (1)

In the above scheme, Fe ³⁺ ions have the property of promoting pickling as the amount added increases. NO ^3- (nitro group) ion promotes pickling as the concentration increases, but when the concentration is higher than a certain property to interfere with the pickling. This is because the nitro group forms a protective film on the surface of the stainless steel, preventing the acid from reacting with the base metal.

That is, in Comparative Example 2, the amount of ferric nitrate (Fe (NO ₃ ) ₃ .9H ₂ O) was too small to act as an acid pickling accelerator, and in Comparative Example 3, the amount of Fe ³⁺ was too high. The pickling disturbance of the nitro group was greater than the pickling activating capacity, thus preventing complete pickling.

Therefore, the concentration range of ferric nitrate (Fe (NO ₃ ) ₃ .9H ₂ O) in the present invention is limited to 15-25 g / l.

As described above, according to the present invention, in the case of hot-rolled pickling of 18Cr stainless steel, the conventional mixing tank is omitted to prevent environmental pollution due to the generation of NOx and excessive metal loss due to the use of the mixing tank, as well as essentially the use of acid. The cost can be reduced by saving.

Claims (1)

In the method of pickling ferritic stainless steel, After the steel sheet subjected to the annealing and shot blasting (1) was removed with a grinding brush 18, a part of the scale was removed, and then sulfuric acid (H ₂ SO ₄ ) was 150-200 g / l and iron nitrate (Fe (NO) was used to promote pickling. ₃ ) Clean acid pickling method for waste light stainless steel characterized by pickling only with sulfuric acid bath at the temperature of sulfuric acid solution having ₃ · 9H ₂ O) of 15 to 25 g / l at 90 to 100 ° C.