JPH11172475A - Pickling of stainless steel - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a method for pickling stainless steel by which smut is hardly left on the surface of the pickled member. SOLUTION: A stailess steel member to be treated is dipped in a pickling soln. contg. hydrofluoric acid and sulfuric acid in the first pickling stage, and then the member is dipped in a pickling soln. contg. hydrofluoric acid and nitric acid to remove the scales remaining on the surface in the second pickling stage. The first pickling soln. contains 0.5-7 wt.% hydrofluoric acid and 2-20 wt.% sulfuric acid, and the second pickling soln. contains 0.5-3 wt.% hydrofluoric acid and 1-5 wt.% nitric acid.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for pickling stainless steel.

[0002]

2. Description of the Related Art Conventionally, stainless steel wires are often manufactured by hot rolling. However, a scale layer having a considerable thickness is formed on the surface of the wire by hot rolling or subsequent heat treatment. Is widely removed by pickling. Here, stainless steel contains a large amount of chromium in addition to iron, so that in addition to iron oxide scale layers that are relatively easy to dissolve and remove in the scale, dense and strong chromium oxide scales are also formed. .

[0003] Therefore, after the soluble iron oxide-based layer is first removed by pre-pickling using a sulfuric acid-based aqueous solution or the like, then the chromium oxide is dissolved in the soluble chromic acid by a so-called salt treatment by immersion in a molten salt bath. It is converted into a salt or the like, and subsequently dissolved and removed in an acid bath such as hydrochloric acid. However, even in this case, since a considerable amount of a film such as chromium hydrate remains on the surface of the wire, it is necessary to perform a finish pickling treatment in which the film is further immersed in another pickling solution to chemically remove the film. Normal.

[0004]

For example, a pickling solution containing hydrofluoric acid and sulfuric acid containing hydrofluoric acid and sulfuric acid is one of the pickling solutions conventionally used for the above pickling. However, although this pickling solution has a great effect on the effect of removing a film such as chromium hydrate, a residual scale layer called smut may remain on the surface of the treated wire. As a cause of the formation of the smut layer, it is considered that, for example, chromium-based carbide or the like existing in the stainless steel substrate is released by pickling and re-adsorbed on the surface of the substrate. In any case, when the smut layer is formed, the surface of the wire after pickling is blackened and the appearance is impaired. In addition, the wire after processing is drawn to produce a wire processed product such as a spring. In the case of applying a smut layer, friction may be increased by the smut layer, causing problems such as scratches or disconnection, or shortening the life of the wire drawing die.

An object of the present invention is to provide a method for pickling stainless steel in which smut and the like hardly remain on the surface of a member to be processed after pickling.

[0006]

Means for Solving the Problems and Actions / Effects In order to solve the above-mentioned problems, a method for pickling stainless steel according to the present invention comprises: a member to be processed made of stainless steel containing hydrofluoric acid and sulfuric acid; A first pickling step of dipping in a hydrofluoric acid-sulfuric acid-based pickling solution, and a residue layer remaining on the surface of the member to be processed after the first pickling step is completed. Acid
A second pickling step of immersing in a nitric acid-based pickling solution to remove the same. The member to be processed is, for example, a stainless steel wire.

[0007] According to the pickling method of the present invention, in the first pickling step, the hydrofluoric acid-sulfuric acid-based pickling solution containing hydrofluoric acid and sulfuric acid is used to leave the surface of the stainless steel member to be treated. Strong coatings such as chromium hydrate can be easily peeled off. Then, even if a residue layer such as a smut layer is formed on the surface of the member to be processed in the first pickling step, it is immersed in a hydrofluoric acid-nitric acid pickling solution containing hydrofluoric acid and nitric acid. This makes it possible to extremely effectively remove the smut layer and the like, and as a result, obtain a sound and clean treated surface with little smut layer and the like remaining.

Specific examples of the stainless steel to which the present invention can be applied include various stainless steels described in JIS: G4304, for example, SUS201 and SUS20.
2, SUS301, SUS301J, SUS302, S
US302B, SUS304, SUS304L, SUS
304N1, SUS304N2, SUS304LN, S
US305, SUS309S, SUS310S, SUS
316, SUS316L, SUS316N, SUS31
6LN, SUS316J1, SUS316J1L, SU
S317, SUS317L, SUS317J1, SUS
Austenitic stainless steels (stainless steel showing an austenitic structure even at room temperature) such as 321, SUS347, SUSXM15J1, etc., SUS329J1, SU
Austenitic-ferritic stainless steel (stainless steel showing a two-phase structure of austenite and ferrite) such as S329J2L, SUS405, SUS410L, S
US429, SUS430, SUS430LX, SUS
434, SUS436L, SUS444, SUS447
J1, stainless steels such as SUSXM27 (which are not hardened by heat treatment and show a ferrite structure), and precipitation hardening stainless steels such as SUS631 (such as aluminum, copper, etc.). (A stainless steel capable of precipitating and hardening a compound or the like mainly composed of an element).

The concept of “stainless steel” in the present invention includes heat-resistant steel exemplified below.
The present invention can be applied to these steel types. Austenitic heat-resistant steel For example, there are steels whose composition is specified in JIS: G4311 and G4312, and SUS31, SUH35, SUH3
6, SUH37, SUH38, SUH309, SUH3
10, SUH330, SUH660, SUH661, and the like. The heat-resistant austenitic steel is treated as being included in the concept of austenitic stainless steel in the present invention. Ferritic heat-resistant steel For example, there is a steel whose composition is defined in JIS: G4311 and G4312, and SUH446 and the like can be exemplified. The heat-resistant ferritic steel is treated as being included in the concept of ferritic stainless steel in the present invention. Martensitic heat-resistant steel For example, there is a steel whose composition is defined in JIS: G4311 and G4312, and SUS1, SUS3, SUS4, S
US11, SUS600, SUS616 and the like can be exemplified. The heat resistant martensitic steel is treated as included in the concept of martensitic stainless steel in the present invention.

The chromium-based oxide on the surface of the member to be treated is partially removed in advance by another descaling treatment (prior descaling treatment) prior to the immersion step in a hydrofluoric acid-sulfuric acid pickling solution. Things. For example, when the member to be processed is a stainless steel wire manufactured by hot rolling, the preceding descaling treatment can be as follows. Pre-pickling treatment: This is a pickling treatment for mainly removing a soluble iron oxide-based layer from the scale layer formed on the surface of the stainless steel wire. As the pickling solution, for example, 10-1
A sulfuric acid-based pickling solution containing 5% by weight of sulfuric acid can be used. Salt treatment: When pre-pickling treatment is performed, it is performed thereafter. A molten salt bath mainly containing an alkali metal salt such as a mixed salt bath of sodium hydroxide and sodium nitrate (for example, W1 / W2 is 2 to 5 where W1 is the weight of sodium hydroxide and W2 is the weight of sodium nitrate) is used. Then, the chromium oxide-based scale layer contained in the scale layer is converted into a layer mainly composed of a water-soluble salt component such as sodium dichromate. In addition, the scale layer is cracked by immersion and heating in a molten salt bath of high heat (a temperature of, for example, 400 to 450 ° C.), followed by rapid cooling, thereby causing cracks in the scale layer and promoting penetration of the pickling solution into the scale layer in the subsequent pickling treatment. Effects can also occur. Auxiliary pickling treatment: Dissolves and removes water-soluble salt components generated by the salt treatment. For example, a sulfuric acid-based pickling solution is used.

The first pickling step of the present invention is a finishing step of removing a chromium-based oxide or a chromium-based compound derived from the chromium-based oxide which has not been completely removed in the preceding descaling treatment. An acid washing step can be performed. For example, even when the auxiliary pickling treatment is performed, a considerable amount of a film such as chromium hydrate may remain on the surface of the stainless steel wire rod to be treated. Therefore, such a film can be easily and reliably removed by immersing the wire in a hydrofluoric-sulfuric acid-based pickling solution containing hydrofluoric acid and sulfuric acid.

On the other hand, the preceding descaling treatment may include a step of removing the scale layer on the surface of the member to be treated by mechanical polishing such as shot blasting.

After the first pickling treatment with the above-mentioned hydrofluoric acid-sulfuric acid pickling solution, a chromium-containing carbide (for example, M
_When a smut layer composed of ₂₃ C ₆ and M is a metal element mainly composed of chromium) remains on the surface of the wire, the second pickling step can be a desmutting step of removing the smut layer.

The hydrofluoric-sulfuric acid pickling solution used in the first pickling treatment contains 0.5 to 7% by weight of hydrofluoric acid and 2 to 20% by weight of sulfuric acid. Good to use. When the content of these two components is less than the lower limit of the above range, the effect of removing a film such as chromium hydrate may be insufficient, and the descaling effect may be insufficient. When hydrofluoric acid is contained in an amount exceeding 7% by weight,
Corrosion and dissolution of the member to be processed may be excessive, and the surface state may be worsened. On the other hand, the sulfuric acid content is 20% by weight.
When the viscosity exceeds the above range, the viscosity of the pickling solution may be too high, the mass transfer in the solution may be hindered, the reaction rate may be reduced, and the descaling effect may be rather reduced. The content of hydrofluoric acid in the hydrofluoric acid-sulfuric acid-based pickling solution is more preferably from 1 to 4% by weight, and the content of sulfuric acid is more preferably from 4 to 4% by weight.
The content is preferably 10% by weight.

The weight of Fe ³⁺ ions in the hydrofluoric-sulfuric acid pickling solution is M (III), and the weight of Fe ²⁺ ions is M (I
As (I), M (III) / (M (III) + M (II)) is 0.3 to 1
It is good to adjust so as to be in the range of.

The inventor of the present invention has found that the descaling ability of a hydrofluoric-sulfuric acid-based pickling solution depends on the concentration of Fe-based ions in the pickling solution, particularly Fe ^3+.
They have been found to vary greatly depending on the concentration ratio between the ions and the Fe ²⁺ ions. That is, according to the above-described method, a strong film of chromium hydrate or the like remaining on the surface of the stainless steel member to be treated or a scale layer is formed by a hydrofluoric acid-sulfuric acid-based pickling solution containing hydrofluoric acid and sulfuric acid. A chromium-depleted layer such as a chromium-deficient layer formed on the substrate side can be easily peeled off with the formation. Then, the weight (M (III)) of Fe ³⁺ ions and the weight (M (II)) of Fe ²⁺ ions in the hydrofluoric acid-sulfuric acid pickling solution are calculated as M (III) / (M (III) + M (I
By adjusting I)) to be in the range of 0.3 to 1,
It is possible to stably control the descaling ability of the surface layer of the member to be treated without slowing down the progress of the oxidative dissolution of the member to be treated or, conversely, making it excessive. This allows
It is possible to always stably form an oxidizing and dissolving state with no excess or deficiency on the surface of the member, and thus the scale layer or the dechromized layer is sufficiently removed, and the surface finish can be improved.

When M (III) / (M (III) + M (II)) is less than 0.3, the descaling ability of the pickling solution becomes insufficient.
In some cases, there is a problem that scale remains on the surface of the member to be processed. According to the study of the present inventors, it has been found that the spontaneous potential of the pickling solution increases as M (III) / (M (III) + M (II)) increases. Therefore, M (III) / (M (III)
+ M (II)) is less than 0.3, the descaling effect is insufficient because the spontaneous potential of the pickling solution is too low and the oxidative dissolution current density required to obtain a sufficient descaling state. This is because it is impossible to secure

In the pickling solution, M (III) / (M (III) + M (I
The value of (I)) is adjusted by ^placing an electrolyte (eg, Fe ₂ (SO ₄ ) ₃ ) as an Fe ³⁺ ion source or an electrolyte (eg, FeSO ₄ ) as an Fe ²⁺ ion source in a pickling solution. In order to increase the content of Fe ³⁺ ions in addition to the method of adding them appropriately, an appropriate oxidizing agent (for example, hydrogen peroxide) is added to the pickling solution to reduce the Fe ²⁺ ions in the solution to Fe ²⁺ ions. There is a method of oxidizing to ³⁺ ions. The content of Fe ³⁺ ions and Fe ²⁺ ions in the pickling solution can be identified by a chemical titration method such as a potassium thiocyanate colorimetric titration method.

FIG. 9 schematically shows an anodic polarization curve of a stainless steel workpiece in a hydrofluoric-sulfuric acid-based pickling solution. As mentioned above, the natural potential of the pickling solution is
As M (III) / (M (III) + M (II)) increases, it becomes higher, and the anodic polarization curve shows, from the lower potential side, an active region in which the oxidative dissolution current density relatively increases with an increase in potential, An inactive region in which the current density value is relatively small and the current density change with respect to the potential change becomes slow, and an overpassive region in which the current density value greatly increases again with the potential change appear. In this case, the current density level needs to be set within a range where the descaling effect can be obtained without excess or deficiency (hereinafter referred to as an optimum current density range), and the value of M (III) / (M (III) + M (II)) Is the natural potential of the pickling solution on the anodic polarization curve,
The value is adjusted so as to have a value corresponding to the optimum current density range.

For example, when the optimum current density range exists over the active region and the passive region of the anodic polarization curve, the value of M (III) / (M (III) + M (II)) (ie, pickling) Controlling the spontaneous potential of the solution within the range corresponding to the passive region has a smaller change in the dissolution current density with respect to the potential change of the solution, so that a stable descaling effect is obtained and the surface finish is better. And can be. However, depending on the type of steel, the current density may be slightly excessive even in the passive region.In this case, the current density is appropriately adjusted by adopting a potential region on the negative side of the current density peak point in the active region. Value.

For example, austenitic stainless steel is
It contains a relatively large amount of Ni and has high oxidation resistance.
It is desirable to set the value of (III) / (M (III) + M (II)) to a relatively high value of 0.5 or more in order to obtain a good descaling effect. This is M (III) / (M (III) + M (II))
Is set to 0.5 or more, it is considered that the natural potential of the pickling liquid with respect to the member to be processed is in a passive region. In the case of austenitic stainless steel,
In order to increase the descaling processing speed, it is sufficiently possible to increase the concentrations of hydrofluoric acid and sulfuric acid to near the upper limit values (the former 4% by weight and the latter 15% by weight).

On the other hand, ferritic or martensitic stainless steels do not substantially contain Ni.
Oxidation resistance is slightly lower than that of austenitic stainless steel, and the value of M (III) / (M (III) + M (II)) (ie, the natural potential of the pickling solution) is as high as that of austenitic stainless steel. When set to, the oxidative dissolution rate becomes slightly excessive, which may cause problems such as rough skin. To prevent this, the value of M (III) / (M (III) + M (II)) should be set to 0.3 to 0.5, which is somewhat lower than in the case of austenitic stainless steel. Becomes effective. Also, in order to similarly suppress excessive dissolution of the member to be treated, the content of hydrofluoric acid in the pickling solution is set to 3% by weight or less, and the content of sulfuric acid is set to 7% by weight or less, which is also set slightly lower. Is desirable.

Next, the hydrofluoric acid / nitric acid pickling solution used for the second pickling treatment contains 0.5 to 3% by weight of hydrofluoric acid and 1 to 5% by weight.
It is preferable to use those containing nitric acid. These two
When the content of the component is less than the lower limit of the above-mentioned range, the removing power of the smut layer may be insufficient and the effect of cleaning the surface of the wire may not be sufficiently obtained. Further, when the hydrofluoric acid or nitric acid component is contained in excess of the upper limit of the above range, the improvement of the smut removing effect corresponding to the increase of the acid component can no longer be expected, leading to wasteful cost increase of the pickling solution. . In addition, when the content of these acid components becomes extremely large, the surface condition of the wire may be rather deteriorated due to corrosion by the acid. The content of hydrofluoric acid in the hydrofluoric acid-nitric acid pickling solution is more desirably 1 to 2% by weight. The content of nitric acid is more desirably 1 to 4% by weight.

If the smut layer still remains on the surface of the wire after the second pickling step has been completed, the member to be treated after the second pickling step is reduced to 1 to 4% by weight. Sulfuric acid containing sulfuric acid and 0.1-1% by weight of hydrogen peroxide
A third pickling step of dipping in a hydrogen peroxide pickling solution can be performed. Thereby, the smut can be more reliably removed. The desmutting effect of the third pickling step is particularly remarkable when the member to be treated is a ferrite-based or martensitic-based stainless steel described later.

If the sulfuric acid concentration in the sulfuric acid-hydrogen peroxide pickling solution is less than 1% by weight, the effect of cleaning the surface of the member to be treated cannot be sufficiently achieved. On the other hand, 4% by weight of sulfuric acid
If the amount of sulfuric acid exceeds the above range, the improvement of the surface cleaning effect corresponding to the increase in the concentration of sulfuric acid cannot be expected.

On the other hand, if the concentration of hydrogen peroxide is less than 0.1% by weight, the improvement of the cleaning effect based on the oxidizing power of hydrogen peroxide can hardly be expected. Also,
If the concentration of hydrogen peroxide exceeds 1% by weight, the improvement of the surface cleaning effect corresponding to the increase in the concentration of hydrogen peroxide cannot be expected. As a result, the cost of the pickling solution is unnecessarily increased by the extra hydrogen peroxide. Leads to.

Since the chemical stability of hydrogen peroxide is generally not so high, the above sulfuric acid-hydrogen peroxide-based pickling solution (or the hydrofluoric acid-sulfuric acid-based pickling solution used in the first pickling treatment) is used. ) Is, for example, due to the presence of iron ions in the liquid and other substances that can act as a decomposition catalyst,
Even if the member to be treated is not immersed, self-decomposition occurs to gradually reduce the concentration. Therefore, in order to ensure the hydrogen peroxide concentration in the above-described desirable range immediately before the immersion of the member to be treated, the pickling liquid is added to the pickling liquid in a form to compensate for the decrease due to the preceding treatment of the member to be treated or the decrease due to autolysis. On the other hand, it is desirable to supply fresh hydrogen peroxide continuously or intermittently. It is desirable that the member to be processed is put into the liquid as soon as possible after the supply of hydrogen peroxide.

For example, when a member to be processed such as a wire is divided into predetermined processing units by a sulfuric acid-hydrogen peroxide type pickling solution and batch pickling is performed, each time one unit of processing is completed, In each case, a predetermined amount of hydrogen peroxide can be replenished to the sulfuric acid-hydrogen peroxide-based pickling solution so that a desired hydrogen peroxide concentration is ensured. This makes it possible to reliably adjust the concentration of hydrogen peroxide in the pickling solution to the above-mentioned concentration range when the member to be treated is charged, and to more reliably achieve the effect of surface cleaning, It is sufficient that hydrogen peroxide is replenished during the processing of the next unit by the amount consumed by the above process, so that waste of hydrogen peroxide is suppressed and the process is efficient. Also in this case, after the hydrogen peroxide is supplied, the member to be processed is put into the liquid as soon as possible.
It is desirable to suppress wasteful consumption of hydrogen peroxide based on self-decomposition or the like.

When hydrogen peroxide is mixed in the pickling solution, the pickling treatment using the pickling solution can be efficiently performed by using the following apparatus. In other words, the apparatus is provided with a pickling liquid storage section for storing the pickling liquid, and at least immediately before the immersion of the member to be processed, the processing of the preceding member to be processed so that the expected hydrogen peroxide concentration is secured. A hydrogen peroxide replenishing mechanism for continuously or intermittently replenishing new hydrogen peroxide in a manner to compensate for the accompanying decrease or the decrease due to autolysis.

In the case where the pickling treatment is carried out by the above-mentioned batch treatment, the above-mentioned apparatus can be efficiently carried out if it is constituted as follows. That is, the member to be processed is carried into the pickling liquid storage unit for each predetermined processing unit, and the member to be treated is immersed in the pickling liquid. A processing member unloading mechanism that unloads the unit from the pickling liquid storage unit is provided. The hydrogen peroxide replenishment mechanism ensures the expected hydrogen peroxide concentration every time the processing of one unit of the processing member is completed. In such a case, a predetermined amount of hydrogen peroxide is replenished to the pickling solution each time.

The member to be processed to which the present invention can be applied is not limited to a wire, but may be, for example, a strip steel, a bar steel, a sheet steel, a pipe steel, or the like.

[0032]

Embodiments of the present invention will be described below with reference to the accompanying drawings. FIG. 1 conceptually shows an example of a line for performing the pickling method of the present invention. In this example, the member to be treated is a coil of a stainless steel wire, for example, a coil W of a stainless steel wire having a scale layer formed on the surface, which is manufactured by heat-treating as necessary after hot rolling or the like. The traverser 2 moving along the rail 4 shown in FIG. 2 sequentially transports each process position shown in FIG. In FIG. 2, reference numeral 3 denotes a drive motor for causing the traverser 2 to run on its own, and reference numeral 5 denotes, for example, winding or unwinding a wire (or a chain) 6 for suspending the hanger 7 to raise the coil W. It is an elevating mechanism for elevating between a position and a descending position. In addition,
The traverser 2 and the elevating mechanism 5 constitute a member to be processed loading mechanism and a member to be processed discharging mechanism.

In the line shown in FIG. 1, the pre-pickling tank 50, the shower tank 51, the salt processing tank 52, the cooling tank 53, the auxiliary pickling tank 54, the shower tank 55, the finishing pickling tank 56, the shower tank 57, The smut layer 58, the shower tub 59, and the neutralization tub 60 are arranged in this order, and the wire to be processed is subjected to batch processing in each tub using the coil W as a processing unit. That is, as shown in FIG. 2, the coil W held by the hanger 7 is positioned at the ascending position by the traverser 2 in each tank 8 (50, 52, 54, 5).
6, 58: In the figure, it is moved upward by the reference numeral 8), then lowered to the lowered position by the lifting mechanism 5, and immersed in the liquid 9 in the tank for a predetermined time to perform each processing. When the immersion in the liquid 9 is completed, the coil W is lifted to the ascending position by the elevating mechanism 5, and the next shower bath 51, 55, 57, 59 or the cooling bath 53 is moved by the traverser 2.
To the lowered position again. In each shower tub, a plurality of shower nozzles 10 for washing are arranged in a form surrounding the coil W in the circumferential direction, for example.
This is washed with water by spraying water toward it.

The processing contents in each tank will be described below. FIG. 3A schematically shows an example of a scale formation state on the surface of a stainless steel wire rod before descaling. The stainless steel base contains iron as a main component and contains chromium in a small amount of about 10% by weight and in a large amount of 30% by weight or more. Chromium has a strong affinity for oxygen, and when hot working or heat treatment is performed in an atmosphere containing oxygen, such as air, it is preferentially oxidized in the initial stage of the scale formation process, resulting in a dense and strong dichromium trioxide. A chromium oxide-based scale layer (abbreviated as Cr ₂ O _{3 in the} figure) is formed mainly from the above. Then, the oxidation of the iron component proceeds in accordance therewith, and an iron oxide-based scale layer (abbreviated as Fe-O in the figure) is formed on the chromium oxide-based scale layer. A composite oxide scale layer mainly composed of an iron-chromium composite oxide (F in the figure) is provided between the chromium oxide scale layer and the iron oxide scale layer.
e-Cr-O).

The coil W made of such a wire is first subjected to a pre-pickling treatment in a pre-pickling tank 50 (FIG. 1). As the pickling solution, for example, a sulfuric acid-based pickling solution containing 10 to 15% by weight of sulfuric acid is used. In this processing, FIG.
As shown in (1), by immersing the coil W in the pickling solution, the soluble iron oxide scale layer or the composite oxide scale layer is mainly removed from the scale layer formed on the surface of the wire.

After the coil W is washed with water in the shower tub 51 (FIG. 1), it is subjected to salt treatment in a salt treatment tank 52. In this tank, a molten salt mainly composed of an alkali metal salt, for example, a mixed molten salt bath of sodium hydroxide and sodium nitrate (for example, W1 / W2 is defined as W1 where the weight of sodium hydroxide is W1 and the weight of sodium nitrate is W2) To 5) are formed. The bath temperature is, for example, 400 to 450 ° C. The chromium oxide-based scale layer contained in the scale layer is dense and strong mainly composed of dichromium trioxide as described above, and can hardly be removed by pre-pickling. Therefore, by immersing in the molten salt bath, the chromium oxide scale layer undergoes a chemical reaction with the molten salt as shown in FIG. 3 (c) to remove a water-soluble salt component such as sodium dichromate. Converted to the main layer. Coil W after salt treatment
Is poured into cooling water in a cooling tank 53 (FIG. 1) and rapidly cooled. As a result, a crack is generated in the scale layer on the surface of the wire, and the pickling solution easily penetrates into the scale layer in the subsequent pickling treatment.

Next, the coil W is supplied to the auxiliary pickling tank 54 (FIG. 1).
Is subjected to an auxiliary pickling treatment. As the pickling solution, for example, a sulfuric acid-based pickling solution containing 5 to 10% by weight of sulfuric acid is used.
By immersion in this pickling solution, the water-soluble salt component generated by the salt treatment is dissolved and removed. However, even after the auxiliary pickling treatment, a considerable amount of residual film usually remains on the surface of the stainless steel wire rod as shown in FIG. This residual film is generally said to be a chromium hydrate film having a structure as shown in FIG.

Then, the coil W is washed with water in the shower tub 55 (FIG. 1), and then carried to the finishing pickling tank 56, where it is subjected to finishing pickling. In this finish pickling process, the coil W
It is immersed in a hydrofluoric-sulfuric acid pickling solution. The hydrofluoric-sulfuric acid-based pickling solution is specifically 0.5 to 4% by weight (preferably 1 to 4% by weight).
Wt.) Hydrofluoric acid and 2 to 15 wt.% (Preferably 4 to 1 wt.
0% by weight) sulfuric acid.
Weight of Fe ³⁺ ion in hydrofluoric acid-sulfuric acid pickling solution (M (II
I)) and the weight (M (II)) of the Fe ²⁺ ion are 0.3 to 1
It is adjusted to be within the range. Further, the total content of Fe ³⁺ ions and Fe ²⁺ ions is adjusted in the range of 0.05 to 10% by weight.

As a result, the residual film is effectively removed. FIGS. 4 (c) and 4 (d) show the estimated removal mechanism. That is, while the F ^- ions derived from the contained hydrofluoric acid destroy the skeleton of chromium hydrate in the residual film, the sulfuric acid dissolves the iron component of the exposed substrate by its oxidizing power and peels off the residual film. it is conceivable that. Incidentally, sulfuric acid, to oxidize the Fe ²⁺ ions in the pickling solution as Fe ³⁺ ions, the Fe ³⁺ ions of iron component in the green body mechanism entity dissolved out by oxidizing the Fe ²⁺ ions It is presumed that it has become. In some cases, a chromium-deficient layer (dechromized layer) due to chromium diffusion to the scale layer side is formed on the surface layer portion of the base material. In this case, the chromium-depleted layer can be dissolved and removed by contact with the pickling solution.

Further, by adjusting the ratio of M (III) / (M (III) + M (II)) to be in the range of 0.3 to 1, the progress of the oxidative dissolution of the surface layer of the member to be treated may be slowed down. The descaling ability can be stably controlled without becoming excessive or excessive. As a result, it is possible to stably form an oxidizing and dissolving state with no excess or deficiency on the surface of the member, thereby sufficiently removing the scale layer and improving the surface finish.

Here, as the treatment of the wire coil W is repeated, the Fe ³⁺ ions in the hydrofluoric-sulfuric acid-based pickling solution become Fe ²⁺
The amount is gradually reduced by being reduced to ions, and M (III) / (M
(III) + M (II)). In this case, for example, the content of Fe ³⁺ ions and Fe ²⁺ ions in
Periodically identified by the aforementioned chemical titration method, etc.
When M (III) / (M (III) + M (II)) is less than 0.3, an appropriate amount of an oxidizing agent such as hydrogen peroxide is introduced into the pickling solution to convert Fe ²⁺ ions into Fe ²⁺ ions. Oxidized to ³⁺ ions, and the M (III) /
The value of (M (III) + M (II)) is maintained at a value of 0.3 or more.

The temperature of the hydrofluoric acid-sulfuric acid pickling solution may be room temperature (about 20 ° C.). By raising the temperature, the descaling reaction rate is increased to improve the processing efficiency. Can be done. When the temperature of the pickling solution is raised, the solution temperature is set so that the desired descaling reaction rate is obtained according to the material of the wire or the scale formation state of the surface within a range in which steam or the like is not excessively generated. As appropriate (for example, 7
(0 ° C. or less).

The composition of the pickling solution can be appropriately adjusted depending on the material of the wire coil W. For example, when the material of the coil W is austenitic stainless steel, it contains a relatively large amount of Ni, has high oxidation resistance, and has a M (I
It is desirable to set the value of (II) / (M (III) + M (II)) to a relatively high value of 0.5 or more in order to obtain a good descaling effect. On the other hand, in the case of ferritic or martensitic stainless steel, oxidation dissolution slightly proceeds more easily than austenitic stainless steel, so that M (III) /
The value of (M (III) + M (II)) is preferably set to a slightly lower value of 0.3 to 0.5. In order to similarly suppress excessive dissolution, the content of hydrofluoric acid in the pickling solution is preferably 3% by weight or less, and the content of sulfuric acid is also preferably 7% by weight or less, which is set to be slightly lower.

Note that a residue layer called smut may remain on the wire after the finish pickling treatment. The reason for the formation of the smut layer is that particles of metal carbide, for example, chromium-containing carbide (for example, M ₂₃ C ₆ or M ₂ C, M is a metal element containing chromium as a main component) existing in the substrate. May be released by pickling and re-adsorbed on the surface of the substrate. When such a smut layer is formed, the surface of the wire is blackened and the appearance is impaired.In addition, when a wire is subjected to wire drawing after processing in order to manufacture a wire processed product such as a spring, the smut layer is required. As a result, the friction may be increased to cause troubles such as generation of scratches or disconnection, or a problem that the life of the wire drawing die is shortened.

Therefore, the coil W of the wire is placed in the shower tub 5.
After washing with water in step 7, desmutting treatment (second pickling treatment of the present invention) is performed in desmutting tank 58 (FIG. 1). In the desmutting treatment, the coil W is made of a hydrofluoric acid-nitric acid pickling solution, specifically, 0.5 to 3% by weight (preferably 1 to 2% by weight) of hydrofluoric acid and 1 to 5% by weight (preferably 1 to 5% by weight). Is immersed in an aqueous pickling solution containing 1 to 4% by weight of nitric acid. Thereby, the smut layer is effectively removed. It is considered that the reason why the smut layer can be effectively removed by the hydrofluoric-nitric acid-based pickling solution largely depends on the oxidizing power of nitric acid which is more excellent than sulfuric acid.

In this case, desmutting can be performed by using an aqueous solution containing only nitric acid without containing hydrofluoric acid. However, nitric acid contains a nitrogen component, and the pickling waste liquid containing the nitrogen component is discharged. The problem is that the oceans, rivers and lakes become eutrophic by nitrogen. Therefore, in recent years, regulations on the nitrogen content in the waste liquid have been tightened, and accordingly, there has been an increasing movement to reduce the processing liquid containing nitric acid in the wire rod processing line. In the desmutting treatment of the present invention, the amount of nitric acid can be reduced while ensuring sufficient desmutting by adding hydrofluoric acid in the above range to the pickling solution.

The coil W having been subjected to the above processing is carried to the shower tub 59 shown in FIG.
Then, the remaining processing liquid or the like immersed in a neutralizing liquid composed of an alkaline aqueous solution such as an aqueous caustic soda solution is neutralized, dried by a drying device (not shown), and the processing is completed.

If the desmutting effect of the hydrofluoric-nitric acid pickling solution is insufficient, the coil W is placed in the shower tub 5.
After washing with water in step 9, it is transported to the finish de-smutting tank 61, where the finish de-smutting treatment (third pickling treatment of the present invention) can be performed. As shown in FIG.
(Pickling liquid storage section) contains 1 to 4% by weight of sulfuric acid,
A sulfuric acid-hydrogen peroxide-based pickling liquid 21 composed of an aqueous solution containing 〜1% by weight of hydrogen peroxide is contained. The coil W is then lowered to the lowering position, immersed in the sulfuric acid-hydrogen peroxide-based pickling solution 21 in the tank 61 for a predetermined time, and the finish desmutting process is performed.

In the pickling solution 21, the iron ions present in the solution are substantially all Fe ³⁺ ions due to the blending of hydrogen peroxide. It is contained in. Thus, the smut layer can be reliably and effectively removed from the wire of the coil W by the strong oxidizing action of hydrogen peroxide. In addition, Fe ³⁺ ions in the liquid can also contribute to cleaning of the surface of the wire due to its oxidizing action.

Since the chemical stability of hydrogen peroxide is generally not so high, the hydrogen peroxide contained in the above-mentioned sulfuric acid-hydrogen peroxide-based pickling solution contains, for example, iron ions and other decomposed substances in the solution. Due to the presence of a substance that can serve as a catalyst, self-decomposition occurs without immersion of the coil W to gradually reduce the concentration. Therefore, as shown in FIG. 5, a new amount is compensated for by a decrease due to the preceding treatment of the coil W or a decrease due to self-decomposition so that the above-mentioned hydrogen peroxide concentration is secured immediately before the immersion of the coil W. A hydrogen peroxide supply mechanism 15 for supplying hydrogen peroxide is provided.

The hydrogen peroxide supply mechanism 15 includes a coil W
Each time one batch is processed, a predetermined amount of hydrogen peroxide is replenished to the sulfuric acid-hydrogen peroxide pickling solution each time. That is, the hydrogen peroxide supply mechanism 15
A main storage unit 27 for storing hydrogen peroxide (H ₂ O ₂ );
And a temporary storage unit 23 for temporarily storing hydrogen peroxide from the main storage unit 27 for one charge. Hydrogen peroxide from the main storage unit 27 is supplied to the temporary storage unit 23 via a pipe 28 having an electromagnetic valve 26. Further, the hydrogen peroxide in the temporary storage section 23 is charged into the tank 61 by opening the electromagnetic valve 25. Further, a liquid level sensor 24 is arranged in the temporary storage section 23 at a height position corresponding to the above-mentioned one-time injection amount of hydrogen peroxide. Reference numeral 22
Is a coil detection sensor for detecting whether or not the coil W exists in the tank 61. Then, the coil detection sensor 22, the liquid level sensor 24, the valve 25, the valve 26, the timer 39, and the like are transmitted to the control unit 30 (configured by a microprocessor or a hardware circuit) that controls the operation sequence of the hydrogen peroxide supply mechanism 15. It is connected.

The operation of the hydrogen peroxide supply 15 will be described below. FIG. 6A shows a state where the coil W is pickled in the pickling liquid 21. The temporary storage unit 23 is filled with hydrogen peroxide using the immersion time. Control unit 30
(FIG. 5) opens the electromagnetic valve 26 and closes the electromagnetic valve 25. Then, the liquid surface of the hydrogen peroxide is
Is detected, the electromagnetic valve 26 is closed, and the process waits until the pickling of the coil W is completed. Fig. 8 shows the material to be treated (coil W)
4 schematically shows a temporal change in the concentration of hydrogen peroxide in the pickling liquid 21 when charging / pulling is repeated. When the material to be treated is charged, hydrogen peroxide in the liquid is consumed due to the progress of pickling, and the concentration decreases.

The coil W measured by the timer 39 (FIG. 5)
When the immersion time is increased, the coil W is pulled up as shown in FIG. However, the hydrogen peroxide in the pickling liquid 21 still undergoes the above-mentioned self-decomposition even after being pulled up. Therefore, if left as it is, the concentration will further decrease as shown by the broken line in FIG. Therefore, FIG.
As shown in (c), when the coil W is pulled up and the coil detection sensor 22 (FIG. 5) enters the non-detection state, the control unit 3
At 0 (FIG. 5), the electromagnetic valve 25 is opened, and all the hydrogen peroxide in the temporary storage section 23 is charged into the tank 61. This allows
As shown in FIG. 8, the concentration of hydrogen peroxide in the pickling liquid 21 increases again. When the supply of hydrogen peroxide is completed, the electromagnetic valve 25 is closed, and the next coil W is immersed in the pickling liquid 21, as shown in FIG. It is.

This ensures that the concentration of hydrogen peroxide in the sulfuric acid-hydrogen peroxide-based pickling solution 21 when the coil W is charged is adjusted to the above-mentioned concentration range (the upper limit is CU and the lower limit is CL). This makes it possible to reliably clean the surface of the wire, that is, to remove the smut layer and the like. Further, hydrogen peroxide may be replenished during the processing of the next coil W by the amount consumed by the processing of one batch (unit) of the coil W, so that waste of hydrogen peroxide is suppressed and the efficiency is improved. It is desirable that the immersion time t of the coil in the tank 61 be set so that the hydrogen peroxide concentration is maintained at the above CU to CL during the immersion period. in this case,
If the absolute amount of the pickling liquid 21 in the tank 61 is small, the immersion time t
Becomes short, and conversely, if it is large, t becomes long. Therefore, it can be said that it is desirable to adjust the amount of the pickling liquid 21 in the tank 61 so that the immersion time t necessary and sufficient to clean the wire surface can be secured.

In the above-described example, hydrogen peroxide is intermittently supplied to the pickling liquid 21 in the tank 61 every time one batch of pickling is completed in the finishing desmutting process. Alternatively, while monitoring the concentration of hydrogen peroxide (or information reflecting the concentration: for example, the spontaneous potential of the pickling solution), the hydrogen peroxide may be continuously supplied so as to maintain the concentration within a predetermined range. Good. FIG. 7 shows a configuration example of the hydrogen peroxide supply mechanism 15 in that case. The pickling liquid 21 in the tank 61 is circulated by a circulation pipe 31 communicating with the tank 61 and a pump 32 provided therein. Further, hydrogen peroxide is supplied from the storage section 27 into the tank 61 via a variable flow rate valve 125 such as a proportional control valve. on the other hand,
The spontaneous potential of the pickling solution 21 in the circulation line 31 is caused by the iron-based electrode immersed in the pickling solution (however, stainless steel or the like is used to prevent corrosion etc.) and the standard electrode And a potential measurement unit 33 including a potentiometer or the like (both not shown) for measuring the galvanic electromotive force between the electrodes. The control unit 30 receives the measurement result of the natural potential and controls the opening amount of the variable flow rate valve 125 so that the natural potential of the pickling liquid 21 is within a predetermined range, and adjusts the supply amount of hydrogen peroxide. I do.

Returning to FIG. 1, the coil W after the finish de-smut processing is washed with water in the shower tank 62 and neutralized in the neutralization tank 60, and then dried by a drying device (not shown) to complete the processing. . The hydrogen peroxide supply mechanism 1
5 can be similarly provided in the finishing pickling tank 56 (FIG. 1) when hydrogen peroxide is contained in the hydrofluoric acid-sulfuric acid pickling solution used for the finishing pickling treatment.

[0057]

(Example 1) An austenitic stainless steel wire rod (SUS304, wire diameter: 5.000) which was hot-rolled by a predetermined rolling mill and then heat-treated at a temperature of 1000 ° C. in air.
5 mm) was wound around a coil of a predetermined size. The coil W was pickled in the line of FIG. The conditions for each process are as follows. Pre-pickling treatment Pickling solution: 10% by weight sulfuric acid aqueous solution Immersion time: 300 seconds Salt treatment Molten salt: 60% by weight of sodium hydroxide, balance sodium nitrate Bath temperature: 450 ° C. Immersion time: 350 seconds Auxiliary pickling treatment Pickling solution : 10% by weight sulfuric acid aqueous solution Immersion time: 180 seconds Finish pickling treatment Pickling liquid: aqueous solution of hydrofluoric acid and sulfuric acid: Various compositions shown in Table 1 were adopted. However, Fe ³⁺ ion and Fe ²⁺
The ions were blended in the form of Fe ₂ (SO ₄ ) ₃ and FeSO ₄ , and the contents M (III) and M (II) were identified by potassium thiocyanate colorimetric titration, and M (III) / (M (III)
+ M (II)). Immersion time: 300 seconds

After the completion of the pickling treatment, the wire W was neutralized with an aqueous solution of caustic soda, which was further washed and dried to evaluate the state of removal of the scale layer on the surface. In the evaluation, a magnified photograph (magnification: 10 times) of the surface of the wire was taken, and the area ratio of the descaled area was determined by image processing. If the area ratio was almost 100%, it was excellent (優), 90% or more. Good (○), 50-90% acceptable (△), 50%
Less than that was evaluated as unacceptable (x). On the other hand, for each sample, the maximum height Rmax of the surface roughness was measured by the method described in Japanese Industrial Standard B0601, and those having a value of Rmax less than 15 μm were excellent (優) and those having a value of 15 to 30 μm were excellent. Good (O), acceptable for 30-40 μm (△), 40 μ
Those exceeding m were judged as unacceptable (x). Table 1 shows the above results.

[0059]

[Table 1]

That is, it can be seen that a good descaling state was obtained by performing the finish pickling treatment with an aqueous solution of hydrofluoric acid and sulfuric acid.

Next, those having good results in the overall evaluation of descaling were desmutted (second pickling treatment) under the following conditions. Desmut treatment Pickling solution: hydrofluoric acid-nitric acid aqueous solution (hydrofluoric acid concentration 1.5% by weight, nitric acid concentration 3% by weight). As a result, good desmutting results were obtained in each case.

Further, the finish pickling treatment with an aqueous solution of hydrofluoric acid and sulfuric acid is performed at a hydrofluoric acid concentration of 3% by weight, a nitric acid concentration of 7% by weight and M (II
Table 3 shows the results of the same experiment when the desmutting treatment was performed with various concentrations of hydrofluoric acid-nitric acid aqueous solution under the fixed conditions of (I) / (M (III) + M (II)) = 0.7. It is shown in FIG. However, the evaluation of the surface roughness after the treatment is the same as described above. The desmutted state was evaluated by taking an enlarged photograph (magnification: 10 times) of the surface of the wire and determining the area ratio of the desmutted region by image processing. , 90% or more is good (○), 50 ~
90% is acceptable (可) and less than 50% is unacceptable (x).

[0063]

[Table 2]

That is, the concentration of hydrofluoric acid is 0.5 to 3% by weight,
When the desmutting treatment was performed using a hydrofluoric acid-nitric acid aqueous solution whose nitric acid concentration was adjusted to 1 to 5% by weight, a good desmutting state was obtained, and no abnormality in surface roughness due to rough skin or the like occurred. .

(Example 2) A ferritic stainless steel wire (SUS430, wire diameter 5.5 mm), which was hot-rolled by a predetermined rolling device and then heat-treated at 850 ° C in a nitrogen atmosphere, was heated to a predetermined size. And wound it up.
The coil W was pickled in the line of FIG. The conditions for each process are as follows. Pre-pickling treatment Pickling solution: 10% by weight sulfuric acid aqueous solution Immersion time: 300 seconds Salt treatment Molten salt: 60% by weight of sodium hydroxide, balance sodium nitrate Bath temperature: 450 ° C. Immersion time: 350 seconds Auxiliary pickling treatment Pickling solution : 10% by weight sulfuric acid aqueous solution Immersion time: 180 seconds Finish pickling treatment Pickling liquid: aqueous solution of hydrofluoric acid and sulfuric acid: Various compositions shown in Table 3 were adopted. However, Fe ³⁺ ion and Fe ²⁺
The ions are mixed in the form of Fe ₂ (SO ₄ ) ₃ and FeSO ₄ , and each content M is determined by potassium thiocyanate colorimetric titration.
(III) and M (II) were identified, and M (III) / (M (III) + M (I
I)) is calculated. Immersion time: 250 seconds

After the completion of the pickling treatment, the wire W was neutralized with an aqueous solution of caustic soda, which was further washed and dried to evaluate the state of removal of the scale layer on the surface. In the evaluation, a magnified photograph (magnification: 10 times) of the surface of the wire was taken, and the area ratio of the descaled area was determined by image processing. If the area ratio was almost 100%, it was excellent (優), 90% or more. Good (○), 50-90% acceptable (△), 50%
Less than that was evaluated as unacceptable (x). On the other hand, for each sample, the maximum height Rmax of the surface roughness was measured by the method described in Japanese Industrial Standard B0601, and those having a value of Rmax less than 15 μm were excellent (優) and those having a value of 15 to 30 μm were excellent. Good (O), acceptable for 30-40 μm (△), 40 μ
Those exceeding m were judged as unacceptable (x). Table 3 shows the above results.

[0067]

[Table 3]

That is, it can be seen that a good descaling state was obtained by performing the finish pickling treatment with an aqueous solution of hydrofluoric acid and sulfuric acid.

Next, those having good results in the overall evaluation of descaling were desmutted (second pickling treatment) under the following conditions. Desmut treatment Pickling solution: hydrofluoric acid-nitric acid aqueous solution (hydrofluoric acid concentration 1.5% by weight, nitric acid concentration 3% by weight). As a result, good desmutting results were obtained in each case.

Further, the finish pickling treatment with an aqueous solution of hydrofluoric acid and sulfuric acid is performed at a hydrofluoric acid concentration of 3% by weight, a nitric acid concentration of 7% by weight and M (II
Table 3 shows the results of the same experiment when the desmutting treatment was performed with various concentrations of hydrofluoric acid-nitric acid aqueous solution under the fixed conditions of (I) / (M (III) + M (II)) = 0.7. 4 (number 1
-10). However, the evaluation of the surface roughness after the treatment is the same as described above. The desmutted state was evaluated by taking an enlarged photograph (magnification: 10 times) of the surface of the wire and determining the area ratio of the desmutted region by image processing. , 90% or more is evaluated as good (○), 50 to 90% is evaluated as acceptable (△), and less than 50% is evaluated as unacceptable (x).

[0071]

[Table 4]

That is, the concentration of hydrofluoric acid is 0.5 to 3% by weight,
When the desmutting treatment was performed with a hydrofluoric acid-nitric acid pickling solution adjusted to a nitric acid concentration of 1 to 5% by weight, a good desmutting state was obtained, and the surface roughness was abnormal due to rough skin. Has not occurred.

Further, desmutting treatment (second pickling treatment) was performed with a hydrofluoric acid-nitric acid aqueous solution having a hydrofluoric acid concentration of 1.5% by weight and a nitric acid concentration of 3% by weight, and then sulfuric acid of various concentrations.
The results of the finish de-smut treatment (third pickling treatment) with an aqueous hydrogen peroxide solution are also shown in Table 4 (numbers 11 to 17). According to this, a finish desmutting treatment performed with a sulfuric acid-hydrogen peroxide aqueous solution containing 1 to 4% by weight of sulfuric acid and 0.1 to 1% by weight of hydrogen peroxide has a better desmutting state. It can be seen that is obtained.

[Brief description of the drawings]

FIG. 1 is a conceptual diagram showing an example of a line for performing the pickling method of the present invention.

FIG. 2 is a schematic diagram showing an example of a transport mechanism of a coil of the wire.

FIG. 3 is an estimated schematic view showing a desmutting process of a stainless steel wire rod.

FIG. 4 is a schematic view following FIG. 3;

FIG. 5 is a conceptual diagram showing an example of a hydrogen peroxide supply mechanism used in a finishing desmutting step.

FIG. 6 is an operation explanatory view of FIG. 5;

FIG. 7 is a conceptual diagram showing a modification of the hydrogen peroxide supply mechanism.

FIG. 8 is an explanatory diagram showing how the concentration of hydrogen peroxide in a sulfuric acid-hydrogen peroxide-based pickling liquid changes when a member to be processed is subjected to batch pickling.

FIG. 9 is a schematic diagram of an anodic polarization curve of a stainless steel material in a hydrofluoric-sulfuric acid-based pickling solution.

Claims (6)

[Claims] 1. A first pickling step in which a member to be processed made of stainless steel is immersed in a hydrofluoric acid-sulfuric acid-based pickling solution containing hydrofluoric acid and sulfuric acid, and the first pickling step is completed. A second pickling step of immersing the residue layer remaining on the surface of the member to be treated in a hydrofluoric acid-nitric acid-based pickling solution containing hydrofluoric acid and nitric acid to remove the same; Pickling treatment method for stainless steel. 2. The chromium-based oxide on the surface of the member to be treated has been partially removed in advance by another descaling treatment (hereinafter, referred to as a prior descaling treatment) prior to the first pickling step. Wherein the first pickling step is a finish pickling for removing the chromium-based oxide or a chromium-based compound derived from the chromium-based oxide that cannot be completely removed in the preceding descaling treatment. The second pickling step is a desmutting step of removing a smut layer mainly composed of chromium-containing carbide remaining on the surface of the member to be processed after the finish pickling step.
A method for pickling stainless steel according to the above. 3. The hydrofluoric acid-sulfuric acid pickling solution has a hydrofluoric acid content of 0.5 to 7% by weight and a sulfuric acid content of 2 to 20% by weight.
3. The method for pickling stainless steel according to claim 1, wherein the amount is adjusted in a range of weight%. 4. The pickling solution of a hydrofluoric acid-nitric acid system according to claim 1, wherein
4. The method for pickling stainless steel according to claim 1, wherein a material containing hydrofluoric acid of 1% by weight and nitric acid of 1 to 5% by weight is used. 5. A sulfuric acid-hydrogen peroxide-based acid containing 1 to 4% by weight of sulfuric acid and 0.1 to 1% by weight of hydrogen peroxide, wherein the member to be treated after the second pickling step is completed. 5. The method for pickling stainless steel according to claim 1, further comprising a third pickling step of dipping in a washing liquid. 6. The method for pickling stainless steel according to claim 5, wherein the member to be processed is ferritic or martensitic stainless steel.