JPH04327313A - How to descale stainless steel - Google Patents

Abstract

PURPOSE:To offer the treatment method which can descale stainless steel by a safe work, a simple and compact equipment, and an inexpensive treatment cost. CONSTITUTION:A stock to be treated is, first of all, subjected to electrolytic treatment by using the stock to be treated as the anode in a neutral salt electrolyte whose main component is nitrate, a chrome oxide being a difficult soluble component in scale is eluted into the electrolyte, and also, an iron oxide is eliminated, and moreover, a passive state film is formed. Subsequently, before the electrolyte adhering to the surface of the stock to be treated and iron hydroxide being a precipitate in the electrolyte get dry, the surface of the stock to be treated is washed, while rubbing it with a washing roll, and the adhering electrolyte and the precipitate in the electrolyte are eliminated.

Description

[Detailed description of the invention]

0001

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for removing scale generated on the surface of stainless steel treated in an annealing furnace or the like.

0002

2. Description of the Related Art Generally, when stainless steel is cold-rolled to produce a plate material, softening annealing is performed to soften the work-hardened material due to the cold rolling. This softening annealing is usually achieved by holding the material to be treated at a high temperature of about 800° C. or more and about 1200° C. or less, so that scale is generated on the surface of the material during the treatment. Therefore, it is necessary to remove the generated scale after softening annealing.

Conventionally, methods for removing this scale include:
First, the scale is converted into a lower oxide with good solubility in a molten salt of sodium hydroxide containing sodium nitrate, and then the remaining scale is removed by immersion treatment in a mixed acid of nitric acid and hydrofluoric acid or electrolytic treatment in nitric acid. There is a known method for removing scale and passivating the surface after descaling.

However, in this case, the treatment in the molten sodium hydroxide salt is carried out at a high temperature of about 450° C., so there are problems in that the work is dangerous and heating costs are high. Also, regarding the immersion treatment for descaling, handling of acid is dangerous, and a large number of treatment tanks are required, resulting in a large equipment cost. There are problems such as the need for large-scale environmental measures such as waste acid treatment.

[0005] In addition, as another method,
As disclosed in Japanese Patent No. 162, scale is converted into a highly soluble lower oxide by electrolytic treatment with a neutral salt solution, and then subjected to immersion treatment in a mixed acid of nitric acid and hydrofluoric acid as in the above method, or A method of descaling by electrolytic treatment in nitric acid is known. However, even in this case, there is a problem that the same drawbacks as the above method exist.

On the other hand, as a method that does not use acids, as disclosed in Japanese Patent Publication No. 57-10200, descaling can be carried out at the same time by electrolytic treatment with a neutral salt solution containing nitrate as the main component. A method of passivating the surface after descaling is known. However, in this case, only electrolytic treatment in the neutral salt solution requires a high current density and a long treatment time, so there is a problem that both equipment costs and treatment costs are high.

[0007]

[Problems to be Solved by the Invention] The present invention overcomes the above-mentioned problems of conventional descaling methods and makes it possible to descale stainless steel with safe work, compact equipment, and low processing costs. The purpose is

[0008]

[Means for solving the problems] In the method according to the present invention,
First, the material to be treated is electrolytically treated in a known electrolyte solution containing nitrate as a main component using a DC power source with the material to be treated as an anode. Of course, the electrolysis method at this time may be the indirect electrolysis method that is generally adopted in the case of continuous treatment of strip materials, and the material to be treated may serve as an anode and a cathode alternately within the treatment time, but it must be done only once. must serve as an anode, and it is desirable that the electrolytic treatment be completed with anodic treatment at the end.

[0009] Next, the surface of the material to be treated is coated with cloth, a nylon brush roll, etc. in a dry state in which the electrolyte and the precipitates in the electrolyte that have adhered to the surface of the material are moist. Clean with cleaning water while rubbing with a cleaning roll that does not cause harmful scratches on the surface of the material.

0010

[Function] As an example, the function of the present invention will be explained in the case where an aqueous sodium nitrate solution is used as the electrolyte. First, in the electrolytic treatment of the example, the following reaction proceeds. That is, in the electrolytic solution, sodium nitrate NaNO3 is dissociated into sodium ions Na+ and nitrate ions NO3- according to equation (1) shown later. At the same time, on the surface of the material to be treated, which is the anode of the electrolytic treatment, according to equation (2), chromium oxide Cr2O3, which is a hardly soluble component in the scale, is converted to chromate ions Cr2
It is eluted into the electrolyte as O7 2-.

Furthermore, on the surface of the material to be treated which is the anode,
According to the equation (3), nitric acid HNO3 is generated from the dissociated nitrate ion NO3 -, and the nitric acid HNO3 is converted into (4)
According to the formula, iron oxide FeO remaining on the surface of the material to be treated is removed. On the other hand, at the cathode of electrolytic treatment, sodium hydroxide NaOH is generated from the dissociated sodium ions Na+ according to equation (5), but the sodium hydroxide NaOH is eluted from the surface of the treated material (ferrous nitrate Fe) (
NO3)2 reacts with ferrous hydroxide Fe(OH)2
and regenerates sodium nitrate, NaNO3.

Further, a part of sodium nitrate, NaNO3, becomes sodium peroxonitrate, NaNO4, through the oxidation reaction represented by equation (7). Since this sodium peroxonitrate NaNO4 has strong oxidizing ability, it forms a healthy passive film on the surface of the treated material.

On the other hand, however, this sodium peroxonitrate, NaNO4, oxidizes ferrous nitrate Fe(NO3)2 to form ferric nitrate Fe(NO3)3. Therefore, the iron hydroxide generated is Fe(OH)2 instead of Fe(OH)2.
(OH)3.

Since this ferric hydroxide Fe(HO)3 has poor solubility in the electrolyte, it becomes solid and precipitates.
A part of it adheres to the surface of the treated material. The ferric hydroxide Fe(OH)3 adhering to the surface of the treated material is gradually removed by the gas generated by continuing electrolysis, but when only electrolytic treatment is used, the ferric hydroxide is removed. Removal requires high current densities and long processing times.

As a result, if descaling is attempted only by electrolytic treatment, the current density will be high and the treatment time will be long. In addition, ferric hydroxide Fe(O
Once H)3 dries, it becomes Fe2O3 and remains strongly on the surface of the treated material.

Therefore, in the present invention, the electrolytic treatment in an aqueous sodium nitrate solution is completed when scale elution from the surface of the material to be treated and formation of a passive film on the surface of the material to be treated are completed. And ferric hydroxide F attached to the surface of the treated material
In the dry state where e(OH)3 still contains water,
The surface of the material to be treated is washed with washing water while being rubbed with a cleaning roll that does not cause harmful scratches on the surface of the material to be treated, such as a cloth or nylon brush roll. Ferric hydroxide Fe(OH) attached to the surface
Remove 3. With the above-described treatment, descaling of the material to be treated can be performed only by electrolytic treatment at low current density and in a short time.

[0017] NaNO3 →Na+ +NO3 −

(1) Cr2 O3 +4H2 O→Cr
2 O7 2-+8H+ +6e-
(2) 4NO3 − +2H2 O→4HNO
3 +O2 ↑+4e- (3)
FeO+2HNO3 →Fe(NO3)2
+H2O (4)
2Na+ +2H2 O+2e- →2NaOH+H
2 ↑ (5) Fe(
NO3 )2 +2NaOH→Fe(OH)2 +2N
aNO3 (6) 2NaNO3 +O2 →2
NaNO4
(7)

[0018]

[Example] 8 cold-rolled SUS430 plates with a thickness of 1.0 mm
Table 1 shows the results of annealing at 50° C. and then electrolytic treatment and washing with water using a brush roll under various conditions. As the electrolytic solution, a sodium nitrate NaNO3 aqueous solution with a liquid temperature of 80° C. and a concentration of 200 g/l was used.

[0019]

[Table 1]

As can be seen from Table 1, if water washing using a brush roll is not performed, in order to obtain a surface with good descaling properties, the treatment time must be at least 40 seconds at a current density of 0.6 A/cm2 or the current density must be Processing time of 60 seconds or more is required at 0.2 A/cm2. On the other hand, when the electrolytic solution adhering to the surface of the treated material and the precipitates contained therein are undried and washed with water using a brush roll after electrolytic treatment, the current density is 0. ．．
Descaling can be achieved with a treatment time of 10 seconds or more at 2A/cm2 or a treatment time of 10 seconds or more at a current density of 0.1A/cm2, and a good surface with no deposits can be obtained, greatly reducing the power required for treatment. At the same time, we were able to significantly shorten the processing time.

[0021]

[Effects of the Invention] The present invention provides safe work that does not require dangerous acids such as nitric acid or hydrofluoric acid, compact equipment,
Stainless steel can be descaled at low cost and in a short time.

Claims (1)

[Claims] Claim 1: A material to be treated is electrolytically treated in a neutral salt electrolytic solution containing nitrate as a main component, using the material as an anode, and then the electrolytic solution adhering to the surface of the material to be treated and the precipitate in the electrolytic solution are removed. A method for descaling stainless steel, characterized in that the surface of the material to be treated in a wet and undried state is washed with washing water while being rubbed with a washing roll.