JP2003013299A - Descaling method for cold-rolled and annealed austenitic stainless-steel sheet - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a method for efficiently removing scale generating on a cold-rolled austenitic stainless-steel sheet when annealing it, and for imparting a superior grinding property to the steel sheet surface after the removal. SOLUTION: This method is characterized by electrolyzing the cold-rolled and annealed austenitic stainless-steel sheet in an aqueous solution of adjusted pH of 3 or less, which contains sodium sulfate with concentration of 50-300 g/l, and nitrate ion with concentration of 20-250 g/l, to descale the sheet. An immersion in a mixed solution of nitric acid with fluoric acid for descaling may be concomitantly employed after the above electrolysis. An electrolytic descaling in a neutral salt solution also may be concomitantly employed before the electrolysis.

Description

Detailed Description of the Invention

[0001]

TECHNICAL FIELD The present invention relates to a method for removing scale generated during annealing of an austenitic stainless cold rolled steel sheet.

[0002]

2. Description of the Related Art In the process of manufacturing a stainless cold-rolled steel sheet, the step of cold-rolling a hot-rolled stainless steel sheet and the processing strain generated during cold rolling are removed to improve the formability and workability. It consists of an annealing step and a descaling step of removing the oxide scale produced in this annealing step.

In the annealing process, SUS301 steel and
Austenitic stainless steel represented by 04 steel is
It is annealed at a temperature over 900 ° C, but during the annealing, F
An oxide scale mainly composed of e, Cr, etc. is produced. In order to remove this, a descaling step is required after annealing. As a method for descaling this oxide scale, "neutral salt electrolysis treatment" is widely used. In the neutral salt electrolysis treatment, descaling is performed on an annealed stainless steel sheet by alternating electrolysis in which an anodic electrolysis and a cathodic electrolysis are alternately repeated in an aqueous solution of a neutral salt such as sodium sulfate. It is a method of further oxidizing the oxide scale mainly composed of refractory Cr at the time of anodic electrolysis to dissolve it as a soluble ion of Cr ₂ O ₇ ²⁻ .

Another method is "alkaline salt immersion treatment". This is a method in which a mixed salt of sodium hydroxide, sodium nitrate or the like is heated and melted at 450 to 550 ° C., and the annealed stainless steel plate is immersed and descaled. Also in this alkali salt dipping method, the oxide scale is further oxidized to soluble Na ₂ Cr ₂ O ₇ , which is dissolved and removed in the subsequent water washing step.

In either of the above-mentioned neutral salt electrolytic treatment or alkali salt immersion treatment, the above alone is not completely descaled, so that the "nitric hydrofluoric acid immersion treatment" is subsequently carried out. Nitrous-fluoric acid immersion treatment is difficult to remove by alkaline salt immersion treatment or neutral salt electrolytic treatment.
It is performed for the purpose of removing oxide scales other than Cr oxide.

Comparing the alkaline salt immersion treatment and the neutral salt electrolytic treatment, the alkaline salt immersion treatment is superior in the ability to oxidize Cr in the scale, that is, the descaling property, and further, the base material and the oxidized scale are Si concentrated at the interface
It also has excellent ability to remove oxides. However, in the alkali salt dipping method, since the viscosity of the molten salt is high, the sludge accumulated on the scale or the bottom of the tank is caught between the dipping roll and the steel sheet in the salt, and the steel sheet is likely to be flawed. Therefore, the alkaline salt dipping treatment often causes a problem with surface defects as compared with the neutral salt electrolytic treatment.

[0007] In many cases, the quality of the surface of cold-rolled and annealed products is emphasized, so that neutral salt electrolytic treatment is generally used as the descaling method. At this time, since the descaling property is inferior to that of the alkali salt immersion treatment, a plurality of treatment tanks are required, and there is a problem that the equipment becomes large-scale. As described above, the neutral salt electrolytic treatment is inferior in descaling property to the alkaline salt dipping treatment, and thus various proposals have been made.

In Japanese Patent Laid-Open No. 247700/1993, the pH of the neutral salt electrolyte is set to 3 or less, and the passivation current density or more is applied to treat the iron, silicon,
A descaling method has been proposed which prevents deterioration of surface gloss due to the remaining oxides of titanium and niobium. However, in the proposed method, when the pH is lowered only by the neutral salt, the skin becomes rough and there is a problem in surface quality.

JP-A-5-287600 discloses that after neutral salt electrolysis, NaOH or KOH and NaNO ₃ or Na ₂ are added.
A method for performing high-speed descaling by electrolyzing with a mixed alkaline aqueous solution of SO ₄ is described. However, in the method described, iron hydroxide is deposited in an alkaline aqueous solution and adheres to the sheet passing roll, which causes indentation flaws, which is a serious problem in surface quality.

Japanese Unexamined Patent Publication (Kokai) No. 11-61500 discloses a method in which neutral salt electrolysis is carried out and then electrolysis is carried out with a sulfuric acid / nitric acid mixed aqueous solution. However, in the disclosed method, the ability to remove Si oxides generated at the interface between the base metal and the oxide scale is poor, and the Si oxides fall off in subsequent temper rolling, which causes indentation flaws. There is.

[0011]

SUMMARY OF THE INVENTION In view of the above problems, an object of the present invention is to remove scales generated during annealing of austenitic stainless cold-rolled steel sheet with high efficiency without using alkali salt dipping treatment. It is intended to provide a descaling method for an austenitic stainless cold-rolled annealed steel sheet which can be done and has good polishing property of the steel sheet surface after removal.

[0012]

[Means for Solving the Problems] As for the descaling method of an austenitic stainless cold-rolled steel sheet after annealing, a descaling method by electrolytic treatment without alkali salt immersion treatment was examined.

First, the structure of oxide scale during annealing of austenitic stainless cold-rolled steel sheet and the findings obtained regarding the descaling behavior in the neutral salt electrolytic treatment step and the nitric-hydrofluoric acid immersion treatment step are described in the following (a). Shown in (e).

(A) The scale produced by annealing has a scale structure in which the outer layer is mainly Fe oxide and the inner layer is mainly Cr oxide, and a scale mainly composed of Si oxide is generated at the interface between the base material and the oxide scale. . In the steel type containing Nb and the like, a scale containing Nb oxide is also formed at this interface.

(B) In the neutral salt electrolytic treatment step, the scale mainly containing Cr oxide is dissolved, but the scale containing oxides of Si and Nb is not dissolved at all.

(C) Oxidation of Si and Nb is not completely removed by immersion pickling in a mixed aqueous solution of nitric-hydrofluoric acid, which follows the neutral salt electrolytic treatment step. In steel types with a high content of, the oxide is thickly formed and is difficult to remove.

In considering the descaling method for austenitic stainless steel sheets in a shorter time without deteriorating the polishing property of the steel sheet surface after descaling,
Various studies were conducted focusing on the neutral salt electrolytic treatment step described in (b). As a result, the following findings (d) and (e) were newly obtained.

(D) In the neutral salt electrolytic treatment step, when nitrate ions are contained in the aqueous solution of sodium sulfate, the scale dissolution rate is increased without lowering the polishing property.

(E) In that case, descaling proceeds with a decrease in pH, and the polishing property increases.

FIG. 1 shows the effect of adding nitrate ions in SUS304 steel.

After annealing the cold rolled material at 1100 ° C., 2
6 in an aqueous solution containing 00 g / l sodium sulfate
Anodic and cathodic electrolysis were alternately repeated at a current density of A / dm ² for 2 seconds each to carry out electrolysis for a total of 40 seconds. Then 2
Electrolysis was performed in an aqueous solution in which 00 g / l of sodium sulfate and nitric acid were mixed, and the thickness of the residual Si oxide in the depth direction was investigated. The conditions at this time were that the current density was 2 A / dm ² , and the electrolysis conditions were anodic electrolysis for 2 seconds and cathodic electrolysis for 2 seconds, which were alternately repeated to perform electrolysis treatment for a total of 80 seconds.

From FIG. 1, in the case of the conventional method in which the aqueous solution of sodium sulfate does not contain nitrate ions, Si oxide remains on the surface, but in the case of containing nitrate ions in sodium sulfate, the concentration of nitrate ions increases. Is 20 g / l or more, S
It can be seen that the i oxide is completely removed.

With respect to this mechanism of action, it is presumed that excellent descaling properties can be obtained by the synergistic effect of the following reactions because the following reactions occur at both polarities.

Anodic electrolytic reaction Sodium sulfate contributes to the oxidative dissolution reaction of Cr oxides as described above, and nitrate ion contributes to the oxidative dissolution of Fe oxide and Cr oxide scales.

Cathodic electrolytic reaction Sodium sulfate and nitrate ions contribute to the uniform dissolution of the base material and accelerate the descaling of Si oxide and the like. Nitrate ions also contribute to reducing and dissolving Fe.

However, in the case of steel grades having high Si and Nb concentrations and high annealing temperatures, descaling may be difficult by the electrolytic treatment with sodium sulfate and nitrate ions.
In that case, descaling can be completed by further adding immersion pickling treatment with a nitric-hydrofluoric acid mixed aqueous solution, and even if the production amount of Si oxide and Nb oxide at the interface between the base material and the oxide scale increases. Thus, a steel plate having excellent polishing properties can be obtained.

For steel grades that are more difficult to descale, a normal neutral salt electrolytic treatment process is added before the electrolytic treatment process using sodium sulfate and nitrate ions to enable complete descaling and polishing. A steel sheet having excellent properties can be obtained.

The present invention has been made on the basis of such findings, and the gist thereof is a method for descaling an austenitic stainless cold-rolled annealed steel sheet shown in (1) to (3) below.

(1) Austenitic stainless cold-rolled annealed steel sheet is electrolytically treated in an aqueous solution having a sodium sulfate concentration of 50 to 300 g / l, a nitrate ion concentration of 20 to 250 g / l and a pH of 3 or less. Method for descaling austenitic stainless cold rolled annealed steel sheet.

(2) Following the electrolytic treatment described in (1) above, it is desirable to carry out a dipping treatment with a nitric-hydrofluoric acid mixed aqueous solution.

(3) It is desirable to perform the neutral salt electrolysis treatment before the electrolysis treatment described in (1) above. Here, the "cold rolled annealed steel sheet" refers to a steel sheet that has been annealed after cold rolling.

[0032]

BEST MODE FOR CARRYING OUT THE INVENTION The reason why the scope of the present invention is defined as described above will be described below.

The annealing performed after cold rolling in the manufacturing process of austenitic stainless steel sheet is generally about 9
The temperature range is 00 ° C to 1250 ° C. The details of the annealing conditions are determined by the chemical composition, structure and required formability or workability.

In the descaling step after annealing, electrolysis is performed in a sodium sulfate aqueous solution containing nitrate ions. The sodium sulfate concentration is 50 to 300 g / l, and the nitrate ion concentration is 20 to 250 g / l.

When the sodium sulfate concentration is less than 50 g / l, it takes a long time to complete the descaling. 300
If it exceeds g / l, the effect is saturated and the running cost increases. Therefore, the sodium sulfate concentration is 50-
It was set to 300 g / l. Desirably 80 to 250 g /
It is l.

If the nitrate ion concentration is less than 20 g / l, the descaling effect cannot be obtained. On the other hand, when it exceeds 250 g / l, the polishing property is deteriorated. Therefore, the nitrate ion concentration was set to 20 to 250 g / l. Desirably 50-
It is 150 g / l. As the nitrate ion source, any effect of nitric acid or nitrates such as sodium nitrate and potassium nitrate can be obtained. Even if nitric acid or a plurality of nitrates are mixed and used, good descaling property can be secured if the nitrate ion concentration is within the range of the present invention.

The pH must be kept below 3. If it exceeds 3, descaling does not proceed sufficiently. In order to keep the pH at 3 or less, there is no limitation on the kind of acid such as sulfuric acid, hydrochloric acid, nitric acid, etc. However, when nitric acid is used, the supply of nitrate ions can also be performed, which facilitates liquid management.

The higher the temperature of the electrolytically treated solution, the greater the descaling effect, but the generation of NO _X and the evaporation of the acid solution pose a problem. Therefore, it is desirable to set the temperature to 40 to 90 ° C.

The higher the electrolytic current density is, the higher the dissolution rate and the descaling effect are. However, if the electrolytic current density is too high, the base material may be dissolved to cause rough skin and the polishing property may be deteriorated.
In addition, melting of the electrode material may occur, so 0.5
It is desirable to set it to 10 A / dm ² .

Materials that are difficult to descale only by electrolytic treatment in the above-mentioned sodium sulfate aqueous solution containing nitrate ions,
For example, it is desirable that a steel type steel sheet containing one or more kinds of Si, Nb, Mo, etc. be subsequently subjected to a nitric hydrofluoric acid immersion treatment. In this case, the nitric acid concentration is 30 to 20.
It is desirable that the concentration is 0 g / l and the hydrofluoric acid concentration is 5 to 30 g / l. The liquid temperature at this time is preferably 30 to 60 ° C.

In the case of a steel type having a high Cr content, the scale layer mainly composed of Cr oxide is thickly formed in the scale inner layer. In this case, before the electrolytic treatment in the sodium sulfate aqueous solution containing nitrate ions, the electrolytic treatment is carried out in a neutral salt aqueous solution such as sodium sulfate to further improve the descaling property. The conditions for the neutral salt electrolysis treatment at this time may be those that are usually carried out, and the concentration is 50 to 3 in the case of sodium nitrate.
00g / l, liquid temperature 40-90 ° C, current density 0.5-
10 A / dm ² is desirable.

[0042]

EXAMPLES SUS304 having the chemical composition shown in Table 1
Steel was used as the test material.

[0043]

[Table 1] This test material is 0.8 mm in thickness as cold-rolled. 100 × 150m from this test material
A test piece of m was cut out and annealed. An electric furnace was used for annealing, and the annealing atmosphere was a hydrocarbon combustion gas atmosphere of an oxidizing atmosphere. The annealing temperature was 1100 ° C., and the soaking time was 30 seconds in each case. After annealing, descaling was performed in an electrolytic solution in which the concentrations of sodium sulfate and nitrate ions were changed. The current densities were both ± 2 A / dm ² , and the electrolysis conditions were alternating anodic electrolysis for 2 seconds and cathodic electrolysis for 1 second, and the total electrolysis time was 36 seconds.

For some of the steel sheets, a neutral salt electrolytic treatment and a nitric hydrofluoric acid dipping treatment were added before and after the electrolytic treatment with an aqueous solution of sodium nitrate mixed with nitrate ions.

In the neutral salt electrolysis treatment, sodium sulfate having a concentration of 200 g / l was used, the current density was 6 A / dm ² , and the electrolysis conditions were anodic electrolysis for 2 seconds and cathodic electrolysis for 2 seconds, which were alternately repeated. The time was 80 seconds. The nitric-hydrofluoric acid immersion treatment was performed at the following concentrations A or B. In each case, the liquid temperature was 40 ° C. and the immersion time was 40 seconds.

A: nitric acid concentration = 80 g / l, hydrofluoric acid concentration = 30 g / l B: nitric acid concentration = 80 g / l, hydrofluoric acid concentration = 15 g / l For both steel types, the evaluation after descaling is as follows. Evaluation was made by the methods shown, and in all cases, “⊚” and “∘” were evaluated as good (pass).

First, the "descaling property" means observing with a 100 times optical microscope, and ◎ when completely descaled.
○, 90% if the area rate is 97% or more
The case of descrusting is represented by Δ, and the case of less than 90% descaling is represented by x.

Next, the "presence or absence of residual Si oxide" is observed by a scanning electron microscope (SEM). When these oxides are completely removed, ⊚, and very slight residual is recognized. The case where it was judged that there was no problem was represented by ◯, the case where some of the particles remained near the grain boundaries was represented by Δ, and the case where some of them remained on the entire surface was represented by x.

Further, the "polishing property" means that the descaled steel sheet is temper-rolled at about 1%, and then the # 1200 abrasive grains are used to make a one pass pass using a rotary polishing machine. Buffing was performed. The polished surface was judged by making a comprehensive visual judgment on the uniformity of polishing, the glossiness, and the like. Especially good was marked with ⊚, good was marked with ◯, somewhat poor was marked with Δ, and bad was marked with x.

The obtained results are summarized in Tables 2 to 4.

[0051]

[Table 2]

[Table 3]

[Table 4] Reference numerals 1, 2, 8 and 9 are examples of the conventional method of neutral salt electrolysis with sodium sulfate, but the descaling property is inferior even if the pH is lowered or the electrolysis time is taken longer, and Si oxide is also present. It remains and the polishing property is inferior.

On the other hand, reference numerals 4, 5, 6, 11, 12 and 13 indicate the cases where nitric acid ions are contained as nitric acid or sodium nitrate in the range of 20 to 250 g / l specified in the present invention in sodium sulfate. Yes, descaling was completed only by electrolytic treatment. Since the Si oxide was also almost removed, it was found that the nitric-hydrofluoric acid dipping treatment, which was carried out subsequently, was unnecessary. As a result, with this steel type, the time required for descaling can be greatly shortened. But,
When the nitrate ion concentration is lower than the specified amount of the present invention as shown by reference numerals 3 and 10 or when the nitrate ion concentration is high as shown by reference numerals 7 and 14, the polishing property is poor.

Further, as indicated by reference numerals 16, 17 and 18, when the neutral salt electrolysis is carried out before the electrolysis with an aqueous solution of sodium sulfate mixed with nitrate ions within the scope of the present invention, the descaling property is The polishing property is further improved.

However, when the nitrate ion concentration is lower than the specified amount of the present invention as indicated by reference numeral 15 or when it is high as indicated by reference numeral 19, the polishing property is poor. Reference numerals 21, 24, and 27 represent the cases in which, within the scope of the present invention, electrolytic treatment was performed with an aqueous solution in which sodium sulfate was mixed with nitrate ions, and then dipping treatment was performed with nitric hydrofluoric acid. It has been removed. In particular, before and after electrolysis with an aqueous solution of sodium sulfate mixed with nitrate ions, when the neutral salt electrolysis treatment and the dipping treatment with nitric hydrofluoric acid were performed, the descaling was completed completely and the Si oxide was also completely removed. It is removed and the polishing property becomes extremely good.

However, when the nitrate ion concentration is lower than the specified amount of the present invention as in the reference numerals 20, 23 and 26, or when it is high as in the reference numerals 22, 24 and 28, the polishing property is inferior.

From the above results, when the electrolytic treatment with the aqueous solution of the present invention in which sodium sulfate and nitrate ions are mixed, the descaling time is greatly shortened and the ability to remove Si oxide at the interface between the base material and the scale is improved. It has been found that a steel plate having excellent polishing properties is obtained.

[0057]

According to the descaling method of the present invention, the descaling rate of an austenitic stainless cold-rolled steel sheet after annealing can be significantly improved, and a surface excellent in polishability can be obtained. It can be expected to prevent indentation flaws by omitting the alkali salt dipping process and to improve productivity by shortening the neutral salt electrolytic descaling process time.

Further, it is also excellent in descaling efficiency of Si oxide and the like produced at the interface between the base material and the oxide scale.

[Brief description of drawings]

FIG. 1 is a graph showing the relationship between the thickness of residual Si oxide on the surface of SUS304 steel after electrolysis and the content of nitrate ions.

Claims (3)

[Claims] 1. An austenitic stainless cold-rolled annealed steel sheet is electrolyzed in an aqueous solution having a sodium sulfate concentration of 50 to 300 g / l, a nitrate ion concentration of 20 to 250 g / l and a pH of 3 or less. Descaling method for cold-rolled austenitic stainless steel sheet. 2. A method for descaling an austenitic stainless cold-rolled annealed steel sheet, which is characterized by immersing in a mixed aqueous solution of nitric-fluoric acid after the electrolytic treatment. 3. The method of descaling an austenitic stainless cold-rolled annealed steel sheet according to claim 1, wherein a neutral salt electrolytic treatment is performed before the electrolytic treatment.