JPH0379795A - Method and apparatus for continuously annealing and pickling stainless steel strip - Google Patents

Abstract

PURPOSE:To continuously anneal and pickle a stainless steel strip with superior productivity in small facilities by carrying out annealing by indirect heating in a reducing atmosphere and subsequent pickling by electrolytic pickling with a nitric acid-hydrofluoric acid soln. as an electrolytic soln. CONSTITUTION:A stainless steel strip S is introduced into a vertical annealing furnace filled with a reducing atmosphere, e.g. composed of 3-15% gaseous H2 and the balance gaseous N2. In the furnace, the strip S is indirectly heated with radiant tubes, etc., by passing through a preheating zone 1, a heating zone 2 and a soaking zone 3 and the heated strip S is passed through a cooling zone 4 and cooled to ordinary temp. with a final cooler 5. The strip S is then immersed in an aq. sulfuric acid soln. having 20% concn. at 70 deg.C in a vessel 6 for about 10sec and electrolytic pickling is carried out by passing through a vessel 7 contg. an electrolytic soln. contg. 20% nitric acid and 5-15% hydrofluoric acid. The strip S is further passed through a washing vessel 8 and a drier 9. A stainless steel strip free from oxide scale on the surface is efficiently produced in small facilities.

Description

[Detailed Description of the Invention] <Industrial Application Field> The present invention relates to a continuous lA annealing/pickling method for stainless steel strips and its equipment, and particularly to an advantageous improvement in the continuous annealing/pickling processing ability of stainless steel strips. The aim is to achieve this by effectively shortening the processing line.

<Prior art> Stainless steel T14'4F after cold rolling is generally subjected to continuous annealing and pickling.
Continuous annealing and pickling treatment is carried out by the ing line (APL).

Continuous annealing of stainless steel strips requires extremely high temperatures of 1010 to 1150°C for austenitic steel such as 5US304, so from the viewpoint of productivity and maintainability, the heating means is generally not used for ordinary steel. Instead of the indirect heating type used, a direct flame type is used.

However, in such a direct heating method, an oxide film is formed on the surface of the steel strip, so descaling by pickling is essential after annealing.

The oxide film formed on stainless steel is much denser and stronger than that on Ootsuki steel, and this tendency to densify increases as the 0□ concentration in the furnace approaches zero. In order to facilitate the descaling process in the pickling process, the atmosphere in the furnace was an oxidizing atmosphere with an O1 concentration of about 2 to 3%.

Therefore, a large amount of scale is produced, and this oxidized scale adheres to the hearth roll and further grows, causing so-called pickup flaws in the stainless steel strip.

In order to prevent this pickup flaw, a method is adopted in which the hearth roll is made of asbestos, the number of rolls is minimized, and the stainless steel strip is catenary supported (Japanese Patent Publication No. 52-26723).

In addition, pickling treatment includes descaling and passivation treatment, and conventionally, it has been carried out using a combination of neutral salt electrolysis, 6-acid electrolysis, 61'l acid, nitric-fluoric acid, and the like.

<Sun problem to be solved by the invention> However, the conventional continuous annealing/pickling section (i.e.
In IIF, the processing line is very long, and on the other hand, the problem remains that the production capacity is not very high.

For example, in order to produce a production capacity of 15T/I, approximately 4
This required a 5m long catenary-type direct-fired furnace and an approximately 50m pickling tank, which meant a long facility spanning at least approximately 100m.

In addition, the asbestos roll in the furnace needed to be replaced about once or twice a month in order to avoid the occurrence of pick-up defects.

Furthermore, the pickling section requires not only a long pickling tank but also ancillary equipment such as acid supply and circulation equipment for each pickling tank with a different type of liquid.

In addition, for environmental hygiene reasons, exhaust treatment equipment, waste acid treatment equipment, etc. are also required, making maintenance of the various equipment extremely troublesome.

The present invention advantageously solves the above-mentioned problems, and aims to propose a highly productive continuous annealing and pickling method for stainless steel strips, together with equipment that is compact and easy to maintain.

Means for Solving the Problem> The first invention provides annealing treatment to a stainless steel strip using continuous annealing equipment and pickling equipment that perform predetermined heat treatment while sequentially conveying steel 41F from a heating zone to a cooling zone. In sequentially performing the pickling treatment, annealing is performed by indirect heating in a reducing atmosphere, and the subsequent pickling is performed by electrolysis using an aqueous nitric acid solution containing at least fluoride, that is, nitric-fluoric acid as the electrolyte. Stainless steel characterized by acid washing! This is a 1F continuous annealing/pickling method.

Further, the second invention is a stainless steel IF equipped with a heating zone, a continuous annealing furnace having a cooling capacity of 4 tF, and pickling equipment.
In the continuous annealing/pickling equipment, each zone of the annealing furnace is mainly composed of a vertical furnace, and the heating zone is configured to perform indirect heating in a reducing atmosphere, and the pickling equipment is configured to provide at least a fluorophore. This continuous annealing/pickling equipment for stainless steel IF is characterized by having an electrolytic pickling tank that uses a nitric acid aqueous solution containing a compound, that is, nitrofluoric acid, as an electrolyte.

<Function> In the present invention, each zone of the annealing furnace is configured mainly as a vertical furnace, so a large number of hearth rolls are arranged above and below the furnace, and the stainless steel strip is wound around these hearth rolls in sequence. However, since the inside of the furnace is maintained in a reducing atmosphere and indirect heating is performed using radiant tubes, there is almost no oxidation scale generated inside the furnace. Even if a roll is used, there is almost no risk of pick-up defects caused by the adhesion and growth of oxide scale on the roll, and the subsequent pickling treatment is only rli'l hydrofluoric acid electrolysis treatment.

<Example> Fig. 1 schematically shows a preferred example of continuous annealing and pickling equipment for stainless steel strip according to the present invention. In the figure, number 1 is the preheating zone, 2 is the heating zone, 3 is the soaking zone, 4 is the cooling zone, 5 is the final cooling device, 6 is the sulfuric acid tank, and 7 is the nitrofluoric acid electrolytic pickling tank. be. In addition, 8 is a cleaning device consisting of a spray, a brush, a scrubber, etc., and 9 is a dryer.

Now, in the above preferred example, each zone of the annealing furnace is configured as a vertical furnace, so a large number of hearth rolls are arranged above and below the furnace, and the stainless steel strip S is attached to these hearth rolls. They are wound one after the other and undergo heat treatment, but the inside of the furnace is maintained in a reducing atmosphere of 11° and Nt, and indirect heating is performed using a radiant tube, so there is no risk of oxidized scale forming inside the furnace. There are almost no
Therefore, even when such a large number of hearth rolls are used, there is almost no risk of occurrence of big amp defects due to the growth of oxide scale on the rolls. Note that as the atmospheric gas, h: 3 to 15%, Nt: balance is appropriate.

In addition, since the furnace is mainly a vertical furnace, the passage length of stainless steel through the furnace can be increased, which far outweighs the negative factor of changing from conventional direct heating to indirect heating. , it is possible not only to significantly improve the sheet passing speed, that is, productivity, but also to significantly shorten the furnace length. Further, there is no need to use expensive rolls such as asbestos rolls as in the past.

In addition, in the preferred example shown in Fig. 1, for the high-temperature part during the heating phase where the above-mentioned problem is particularly likely to occur, it is possible to use a horizontal one-bath furnace and adopt a catenary support type using asbestos hearth rolls as in the past. Of course it's good, but steel 11F
A non-contact catenary squib may be employed using a gas flotation device.

The back-front type is more effective in the present invention.

Next, Figure 2 shows an example of the results of investigating the relationship between the required time and current density when stainless steel 4JF annealed using the annealing equipment shown in Figure 1 is pickled using Gli-hydrofluoric acid electrolysis. Indicated by solid line. 5US430 Teha? il
(Using a nitric-fluoric acid solution with an acid concentration of 20%, a hydrofluoric acid concentration of 5%, and a liquid temperature of 50°C; on the other hand, in 5US304, a boric acid concentration of 20%,
A nitric-fluoric acid solution with a hydrofluoric acid concentration of 15% and a liquid temperature of 60°C was used. In FIG. 2, the area above each [lII line is a good pickling area, while the area below is an insufficient pickling area.

As is clear from Figure 2, it takes 2 to 3 seconds when the current density is relatively high, and 10 to 2 seconds even when the current density is low.
Pickling treatment is possible in 0 seconds.

In addition, the - dotted line shows the relationship between current density and pickling time when electrolytic pickling is performed with the above-mentioned nitric-fluoric acid solution after immersion in a sulfuric acid aqueous solution with a concentration of 20% and a temperature of 70°C for 10 seconds. It can be seen that the effect is clearly visible. That is, by performing immersion in sulfuric acid or the like in step 1)1 of electrolytic pickling using a 61'1 hydrofluoric acid solution, the electrolytic pickling time can be shortened, and the electrode length can be shortened.

Next, in Figure 3, lil'l acid concentration is 20%, hydrofluoric acid concentration is 5%,
% nitric hydrofluoric acid, the relationship between current density and liquid temperature that can achieve good pickling is shown below. In addition, the electrolysis time was 2 seconds in all cases. From FIG. 3, it can be seen that when it is desired to lower the current density, increasing the liquid temperature is effective in improving the pickling performance.

Of course, if it is desired to further lower the current density during pickling in nitrofluoric acid electrolytic pickling, it is effective to combine it with another pickling, as shown by the dashed line in FIG.

Note that, as described above, in the present invention, continuous annealing is performed in a reducing atmosphere, so that the subsequent pickling treatment may be performed simply by nitric-fluoric acid electrolytic treatment.

Furthermore, as is clear from the above, the annealing furnace according to the present invention can be used as it is for general cold rolled steel (IP),
Demonstrates great power as a dual-purpose F line.

<Effect of the invention> The effects of this invention can be summarized as follows.

(1) Processing capacity is significantly improved and equipment length can be significantly shortened.

(2) By making the atmosphere inside the annealing furnace a reducing atmosphere and by making the high temperature jtF region in particular during heating g non-contact, it is possible to completely eliminate the occurrence of pick-up flaws.

(3) Since there is no need to use asbestos rolls, there is no need to replace rolls, resulting in cost reduction.

(4) Since almost no oxide scale is present, a decrease in yield due to scale loss can be prevented.

(5) Since fewer pickling tanks are required, chemical management is easy, and additional equipment such as acid supply, waste acid, and circulation equipment can be significantly reduced.

[Brief explanation of drawings]

FIG. 1 is a schematic diagram of continuous annealing and pickling equipment for stainless steel strip according to the present invention, and FIG. 2 is a graph showing the relationship between treatment time and flow density in hydrofluoric acid electrolytic treatment according to the present invention.
FIG. 3 is a graph showing the relationship between current density and liquid temperature in sheath electrolytic pickling. DESCRIPTION OF SYMBOLS 1... Pre-heating zone, 3... Soaking apricot, 5... Layer material cooling device, 7... Nitrofluoric acid electrolytic pickling tank, 9... Drying device, 2... Heating zone, 4... Cooling zone, 6... Sulfuric acid immersion tank, 8... Cleaning device, S... Stainless steel m band. Diagram: Liquid temperature (°C)

Claims (2)

[Claims] (1) When sequentially subjecting a stainless steel strip to annealing treatment and pickling treatment using continuous annealing equipment and pickling equipment that perform predetermined heat treatment while sequentially transporting the steel strip from the heating zone to the cooling zone, Continuous annealing/acidification of a stainless steel strip, characterized in that annealing is performed by indirect heating in a reducing atmosphere, and subsequent pickling is performed by electrolytic pickling using an aqueous nitric acid solution containing at least fluoride as an electrolyte. Washing method. (2) In a continuous annealing/pickling facility for stainless steel strips, which is equipped with a continuous annealing furnace having a heating zone and a cooling zone, and a pickling facility, each zone of the annealing furnace mainly consists of a vertical furnace;
and the heating zone is configured to perform indirect heating in a reducing atmosphere, and the pickling equipment has an electrolytic pickling tank that uses at least a fluoride-added nitric acid aqueous solution as an electrolyte. Continuous annealing and pickling equipment for stainless steel strips.