JP2842787B2 - Annealing and descaling of cold rolled stainless steel strip - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for producing a cold-rolled stainless steel strip (including a steel sheet), which enables high-speed and stable descaling and has excellent corrosion resistance after descaling. The present invention relates to a method for annealing and descaling a cold rolled stainless steel strip having excellent characteristics.

[0002]

2. Description of the Related Art Generally, cold-rolled stainless steel is annealed to ensure workability and corrosion resistance. The main methods of annealing stainless steel include a method of treating in a strongly reducing atmosphere and a method of treating in a combustion atmosphere.
The former method is called bright annealing (BA) treatment, in which an oxide film (scale) is extremely thin and a gloss almost as rolled can be obtained. On the other hand, in the latter method, since a scale with a certain thickness or more is generated during annealing, the corrosion resistance,
It has an adverse effect on the die life during molding and processing. For this reason, when annealing is performed in a combustion atmosphere, after the annealing, a pickling treatment for descaling is performed. Further, in recent years, an annealing method in which a steel sheet is passed at a high speed in a reducing atmosphere such as a continuous annealing line (CAL) of ordinary steel is being attempted. In this case, since stainless steel contains Cr that is easily oxidized, even in an atmosphere that is not oxidized by ordinary steel, a temper-colored scale is formed on the surface, and descaling treatment is required after annealing.

[0003] As a descaling treatment performed after annealing, conventionally, after a step such as a salt treatment immersed in a molten alkali salt or an electrolytic treatment in a neutral salt solution, sulfuric acid, nitric acid, nitric hydrofluoric acid or the like is generally used. Or a combination of a step of immersion in an acid solution or a step of electrolytic treatment. The specific method is disclosed in, for example, JP-B-38-12162, JP-A-59-59900, or Stainless Steel Handbook (edited by Masayoshi Hasegawa, Nikkan Kogyo Shimbun, 1973, p. 839). These methods have been used depending on the degree of difficulty of scale removal caused by the type of steel and annealing conditions. However, even if the descaling process of the complicated process as described above is performed, a long-time process is necessary to completely remove the scale, which may hinder the production efficiency of the stainless cold-rolled steel strip. Had become. Therefore, development of a descaling method that can be performed in a short time has been an issue for many years in order to respond to speeding up of a line and improvement of productivity.

[0004] Against this background, many techniques have been proposed to meet the demand for faster descaling. As a promising technique, there is a method in which chloride ions are added to a treatment liquid and a scale erosion function of chloride ions is utilized. For example, in JP-A-53-23245, a method of electrolyzing with an aqueous solution comprising a neutral salt electrolyte aqueous solution- (Nacl or Kcl) is disclosed in JP-B-58-42777 and JP-B-60-543.
No. 91 discloses a method of electrolyzing in a treatment solution consisting of nitric acid-hydrochloric acid-ferric chloride, and Japanese Unexamined Patent Publication No. 1-234600 discloses NaCl.
A method of electrolyzing in an aqueous solution containing as a main component and immersing it in nitric hydrofluoric acid or sulfuric hydrofluoric acid is disclosed. But,
In these methods, the processing time for descaling is limited to about 20 sec, and the only way to increase the line speed is to increase the length of the pickling tank.
High-speed, high-efficiency equipment such as L) could not be put to practical use.
Further, JP-A-1-147100 and JP-A-4-66699 disclose a method of electrolysis in a nitric acid solution. However, in this method, the descaling ability greatly changes depending not only on the dissolved Fe and Cr concentrations in the electrolytic solution but also on the scale state. For this reason, in the actual operation equipment, the scale property generated at the edge of the steel strip is easily affected by the annealing temperature, the atmosphere, etc. There has been a problem that surface roughening due to pickling frequently occurs. In addition, this method has a problem that it is difficult to visually confirm and remove it, and a very small amount of an oxide such as Si remains even after pickling, thereby deteriorating the corrosion resistance of the steel sheet. further,
The limit of the descaling treatment time of this method is at most about 5 sec.

[0005]

As described above, all of the above-mentioned known techniques have a problem that the processing time of the descaling step is long (the productivity is low). Further, there is a problem that the descaling ability is unstable, so that descaling failure occurs particularly at the edge portion of the steel sheet, and that the oxides remain on the steel sheet surface to deteriorate the corrosion resistance.

The main object of the present invention is to anneal and descaling a cold rolled stainless steel strip which does not cause the above-mentioned problems of the prior art when annealing and descaling a cold rolled stainless steel strip. It is to provide a method. Another object of the present invention is to provide a method for annealing and descaling a cold rolled stainless steel strip having high productivity and stable descaling ability, and also having excellent corrosion resistance. Still another object of the present invention is to provide a method for annealing and descaling a cold rolled stainless steel strip which can reduce the processing time of the descaling step to 2 sec or less.

[0007]

Means for Solving the Problems In order to realize the above-mentioned object, the present inventors have conducted detailed studies on the state of scale and ground iron after annealing and the descaling behavior in each solution. The instability of descaling ability is affected by the fluctuation of electrolytic current efficiency due to the difference in scale thickness, and the degree of concentration of elements such as iron and scale during annealing, depending on the annealing conditions from the surface layer of the scale. If a certain depth is mechanically ground, stable descaling properties can be obtained, and if treated in a nitric acid solution in the next step, oxides such as Si and Al that adversely affect corrosion resistance do not remain It has been found that a good surface can be obtained in a short time, and the present invention has been completed.

The gist of the present invention is as follows, embodying the above concept. (1) Annealing a cold-rolled stainless steel strip characterized by annealing the cold-rolled stainless steel strip in a reducing atmosphere, then mechanically grinding, and then pickling in a nitric-hydrochloric acid solution.・ Descaling method.

(2) In the above (1), the reducing atmosphere is
H ₂ : 20 vol% or less, the balance being composed of an inert gas,
An annealing and descaling method with a dew point of -10 ° C or less.

(3) An annealing / descaling method according to the above (1) or (2), wherein the mechanical grinding amount is in a range satisfying the following formula in terms of the thickness from the oxide film surface. When A> -40 ° C, 3.0 × 10 ⁻³ × A + 1.3 × 10 ⁻¹ ≦ B ≦
10 A ≦ −40 ° C, 0.01 ≦ B ≦ 10 where A: dew point of annealing atmosphere (° C) B: amount of grinding (μm)

(4) In the above (1) to (3), the solution has a nitric acid concentration of 10 to 300 (g / l) and a hydrochloric acid concentration of 1 to 30 (g / l), a temperature of 35 to 65 ° C., and a current of An annealing and descaling method in which electrolytic treatment is performed at a density of 1 to 30 (A / dm ² ).

[0012]

First, the cold-rolled stainless steel strip in the present invention is:
It refers to a ferritic or austenitic cold rolled stainless steel strip having a Cr content of 5 to 40 wt%, and can be advantageously applied particularly to a cold rolled stainless steel strip having a Cr content of 10 to 30 wt%. Next, in the present invention, the reason why the annealing and descaling conditions of the stainless cold-rolled steel strip are limited as described in the above summary will be described.

[0013] The annealing atmosphere of the present invention must be a reducing atmosphere. The thickness of the scale formed on the surface of the stainless steel strip annealed by the continuous annealing line (CAL),
The degree of concentration of each element at the interface between the base iron and the scale is affected by the composition of the annealing atmosphere and the dew point. In other words, when the dew point of the reducing atmosphere is higher than -10 ° C, the scale formed becomes thicker, and the descaling properties such as Si, Ti, Nb, and Al at the interface between the base iron and the scale and the corrosion resistance after pickling deteriorate. The degree of concentration of the element to be increased increases. Therefore, if the scale becomes too thick or the concentration of these elements in the base iron becomes excessive, it becomes difficult to perform descaling in a short time at high speed even if mechanical grinding is performed in a later step. When the H ₂ concentration in the annealing atmosphere is high, the scale and the thickened layer become thin, but become saturated at around 20 vol%. Incidentally, excessively disadvantageous in terms of concentration of H ₂ is high comes to safety and cost. Therefore, conditions of the annealing atmosphere is preferably H _2: 20 vol% or less, dew point: -10 ℃ or less, more preferably H _2: 2 to 10 vol%, dew point: -25～-
45 ° C.

[0014] Mechanical grinding is performed subsequent to the annealing.
The mechanical grinding in the present invention means a mechanical grinding process such as a brush roll, abrasive powder, shot blast and the like. As a result of detailed investigation of the steel sheet surface after pickling in the conventional process, the inventors found that oxides such as Si, Ti, Nb, and Al remained, and that these elements were concentrated on the surface layer of the base iron. It was found that these deteriorated descalability and corrosion resistance. Therefore, by performing mechanical grinding, the surface state can be always kept constant irrespective of the annealing conditions, and the stability of the descaling ability can be improved. Furthermore, by this mechanical grinding, it is possible to remove parts that are hardly soluble in the nitric acid hydrochloric acid solution in the subsequent process and degrade the corrosion resistance of the steel sheet, such as Si, Ti, Nb, Al, etc. at the base iron interface concentrated during annealing. . By these actions, descaling can be performed at a higher speed and in a shorter time than before, and the corrosion resistance of the steel sheet after pickling is also improved.

The amount of mechanical grinding in the present invention may be within the range shown by the following equation. When A> -40 ° C, 3.0 × 10 ⁻³ × A + 1.3 × 10 ⁻¹ ≦ B ≦
10 A ≦ −40 ° C, 0.01 ≦ B ≦ 10 where A: dew point of annealing atmosphere (° C) B: grinding amount (μm) The grinding amount is determined depending on the dew point of annealing atmosphere as in the above formula. The reason is as follows. That is, as a result of investigating the effects of annealing conditions on the scale thickness and the enriched layer of each element such as Si, Ti, Nb, Al, etc. of the base iron, the inventors found that it was applied to normal operation.
Is a baked blunt temperature 1050 ° C. or less, the scale thickness and concentrated layer thickness is largely dependent on the dew point of the annealing atmosphere, the thickness from the surface of the concentrated layer is in a reducing atmosphere, dew point -40 ℃ or higher It was found that the thickness was almost proportional to the dew point in the range, and that the thickness became almost constant at -40 ° C or lower irrespective of the dew point. Based on this fact, the lower limit of the above equation was determined. When the grinding amount is less than this range, scale or Si, Ti,
A concentrated layer of Nb, Al, etc. remains, degrading the descaling property, making it impossible to perform stable descaling, and leaving these elements even after pickling, deteriorating the corrosion resistance. On the other hand, if the grinding amount exceeds 10 μm, the surface becomes rough, and problems such as sparks occur during grinding. Therefore, the upper limit of the grinding amount is 10 μm.

After the above mechanical grinding, pickling is performed in a nitric acid / hydrochloric acid solution. The inventors have processed the steel sheet after grinding in various solutions from the viewpoint of the removal of mechanical grinding scratches and the short-time processing of passivation processing to ensure the smoothness of the surface of the processing solution after grinding. It has been found that a method of treating in a nitric-hydrochloric acid solution is suitable. Then, as the treatment in the nitric acid / hydrochloric acid solution, in a mixed solution of nitric acid 10 to 300 (g / l) and hydrochloric acid 1 to 30 (g / l), at a temperature of 35 ° C to 65 ° C and a current density of 1 to 30 ( It is recommended to carry out the electrolytic treatment under the condition of A / dm ² ).

[0017] nitric acid concentration is less than 10 g / l, able to handle short time passivation difficult, whereas 300 g / l occurs an adverse effect that the NO _X generation amount increases exceeds. If the hydrochloric acid concentration is less than 1 g / l, it is difficult to descalate at a high speed, and if it exceeds 30 g / l, the skin becomes rough. For the above reasons, the treatment solution of nitric acid and hydrochloric acid is nitric acid 10-300 (g / l) and hydrochloric acid 1-
30 (g / l), preferably 50-200 (g / l) nitric acid and 3-20 (g / l) hydrochloric acid.

The temperature of the nitric acid solution is
If the temperature is lower than 35 ° C., it is difficult to perform the treatment in a short time. If the temperature is higher than 65 ° C., adverse effects such as an increase in the amount of generated NO _X and rough skin are caused. For the above reasons, the temperature of the nitric acid solution is
35 to 65 ° C, preferably 40 to 60 ° C.

Further, if the electrolytic current density in the nitric acid solution is less than 1 A / dm ² , the treatment cannot be performed in a short time.
If it exceeds A / dm ² , adverse effects such as an increase in the amount of generated NO _X and rough skin will occur. For the above reasons, the current density is 1 to 30 (A / dm ² )
, Preferably 5 to 25 A / dm ² . In the present invention, the rolling reduction during hot rolling, annealing conditions, and the rolling reduction during cold rolling do not need to be particularly limited, and may be appropriately determined according to the sheet thickness and the like.

[0020]

EXAMPLES Example 1 SUH409 stainless steel (Cr: 11 wt%, Ti: 0.5 wt%)
The hot-rolled sheet was cold-rolled into a cold-rolled steel strip having a thickness of 1.0 mm as a test material. This test material was annealed under the conditions shown in Table 1,
Grinding and pickling (nitric acid / hydrochloric acid) were performed, and the state of descaling and corrosion resistance were examined. Here, the annealing pattern is
Temperature rises to 900 ° C in about 200 sec seconds → Hold at 900 ° C x 60 sec →
Air cooling was performed, and grinding was performed by a method using a nylon brush. For comparison, a conventional method using neutral salt electrolysis without grinding (condition: Na ₂ SO ₄ 200g /
l, temperature 80 ° C, current density 10A / dm ² , electrolysis time 10sec → nitric acid 100g / l, hydrofluoric acid 10g / l, temperature 50 ° C, immersion 30sec → nitric acid 10
0 g / l, temperature 60 ° C., a current density of 20A / dm ^2, the electrolysis time 10 sec)
But I tried descaling. The descalability was determined by comparing with a completely descaled representative sample, and by visual judgment, ○, Δ, × (○: good descaling, Δ: fine scale remaining, ×: scale remaining) . In addition, the determination of corrosion resistance was evaluated by measuring the pitting corrosion potential (JIS G 0577). Table 1 shows the results.

[0021]

[Table 1]

From Table 1, it can be seen that according to the method of the present invention, stable descaling can be performed in a very short time and the corrosion resistance after pickling is good. In contrast,
In the comparative examples outside the scope of the present invention, it is difficult to process at a high speed and in a short time compared to the present invention, and at least one of poor descaling failure, particularly scale residue at an edge portion, poor corrosion resistance after pickling. It can be seen that the following disadvantage occurs.
In addition, it was shown that the conventional example using neutral salt electrolysis was inferior to the invention method, particularly in terms of corrosion resistance, despite the long processing time.

Example 2 A hot-rolled sheet of SUS430 stainless steel (Cr: 18 wt%) was cold-rolled to form a 1.0 mm-thick cold-rolled steel strip, which was used as a test material.
The test material was annealed, ground, and pickled (nitric acid / hydrochloric acid) under the conditions shown in Table 2, and examined for descaling and corrosion resistance. Here, the annealing pattern was heated to 850 ° C. in about 250 sec seconds → maintained at 850 ° C. × 60 sec → air cooling, and the grinding was performed by a method using a nylon brush. For comparison, a conventional method using neutral salt electrolysis without grinding (conditions: Na ₂ SO ₄ 200 g / l, temperature 80
° C, current density 10A / dm ² , electrolysis time 10sec → nitric acid 100g / l,
Descaling was attempted even at a temperature of 60 ° C., a current density of 20 A / dm ² , and an electrolysis time of 10 sec). Judgment of descalability is compared with a representative sample that has been completely descaled, and by visual judgment,
△, △, × (○: good descaling, Δ: minute scale remaining, ×: scale remaining) Also,
The corrosion resistance was evaluated by measuring the pitting potential (JIS G 0577). Table 2 shows the results.

[0024]

[Table 2]

From Table 2, it can be seen that according to the method of the present invention, stable descaling can be performed in a very short time and the corrosion resistance after pickling is good. In contrast,
In the comparative examples outside the scope of the present invention, it is difficult to process at a high speed and in a short time compared to the present invention, and at least one of poor descaling failure, particularly scale residue at an edge portion, poor corrosion resistance after pickling. It can be seen that the following disadvantage occurs.
In addition, it was shown that the conventional example using neutral salt electrolysis was inferior to the invention method, particularly in terms of corrosion resistance, despite the long processing time.

[0026]

As described above, according to the method of the present invention,
The problem of annealing and descaling of a cold rolled stainless steel strip which the conventional method has is solved. In addition, according to the method of the present invention, the stainless steel cold-rolled steel strip having high productivity and stable descaling ability, and also having excellent corrosion resistance is obtained.
Descaling becomes possible. According to the method of the present invention,
The processing time of the descaling step can be reduced to 3 sec or less. For this reason, a stainless steel strip of good quality can be manufactured at low cost, the conventional long pickling equipment can be greatly reduced, and equipment management can be greatly facilitated.

──────────────────────────────────────────────────の Continued on the front page (51) Int.Cl. ⁶ Identification symbol FI C23G 1/08 C23G 1/08 C25F 1/06 C25F 1/06 B (72) Inventor Junsuke Takasaki Kawasaki-cho, Chuo-ku, Chiba-shi, Chiba No. 1 Kawasaki Steel Corporation Chiba Works (72) Inventor Takashi Shiokawa 1 Kawasaki-cho Chuo-ku Chiba City Chiba Prefecture Kawasaki Steel Corporation Chiba Works (56) References JP-A-6-15308 (JP, A) JP-A-62-136528 (JP, A) JP-A-3-56657 (JP, A) JP-B-6-7951 (JP, B2) (58) Int.Cl. ⁶ , DB name) B21B 45/06 C21D 1 / 26,9 / 46 C23G 1/08 C25F 1/06

Claims (4)

(57) [Claims] 1. A cold-rolled stainless steel strip, comprising: annealing a cold-rolled stainless steel strip in a reducing atmosphere, mechanically grinding the strip, and then pickling in a nitric-hydrochloric acid solution. Annealing and descaling method. 2. The annealing / descaling method according to claim 1, wherein the reducing atmosphere is H ₂ : 20 vol% or less, the balance is composed of an inert gas, and the dew point is -10 ° C. or less. 3. The mechanical grinding amount is in a range that satisfies the following relationship in terms of the thickness from the oxide film surface.
Or the annealing / descaling method according to 2. When A> -40 ° C, 3.0 × 10 ⁻³ × A + 1.3 × 10 ⁻¹ ≦ B ≦
10 A ≦ −40 ° C, 0.01 ≦ B ≦ 10 where A: dew point of annealing atmosphere (° C) B: amount of grinding (μm) 4. A solution composition comprising a nitric acid concentration of 10 to 300 (g / l) and a hydrochloric acid concentration of 1 to 30 (g / l), a temperature of 35 to 65 ° C., and a current density of 1 to 30 (A / dm ² ). The annealing / descaling method according to any one of claims 1 to 3, wherein the electrolytic treatment is performed under the following conditions.