JPH11172499A - Surface treatment of stainless steel - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a method for surface-treating stainless steel with the smut hardly left on the surface of a pickled member to be treated. SOLUTION: A stainless steel member to be treated with a smut layer consisting essentially of a chromium-contg. carbide formed on its surface is dipped in a processing soln. with the spontaneous potential to the member measured at 20 deg.C using a saturated calomel electrode as the reference controlled to >=-280 mV to remove the smut layer.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

[0001] The present invention relates to a method for surface treating 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 liquid has a great effect on the effect of removing a film such as chromium hydrate, a residue 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 surface treatment method capable of effectively removing a smut layer formed on the surface of a stainless steel workpiece.

[0006]

Means for Solving the Problems and Actions / Effects In order to solve the above-mentioned problems, the surface treatment method of stainless steel according to the present invention is directed to a stainless steel having a smut layer mainly composed of chromium-containing carbide formed on the surface. The smut layer is removed by immersing the member to be processed in a processing liquid having a natural potential of -280 mV or higher with respect to the member measured at 20 ° C. with reference to the saturated calomel electrode. Here, the member to be processed is, for example, a stainless steel wire.

[0007] For example, when processing liquids (for example, pickling liquids) having different components or different compositions are used for the same member to be processed, each processing liquid shows a natural potential peculiar to the member to be processed. Needless to say, even when processing liquids having the same composition are used, the above-mentioned natural potential generally differs depending on the material of the member to be processed or the surface state before the processing. What is important in the present invention is that a processing liquid having a natural potential of -280 mV or more with respect to the processing target member is used regardless of the material of the processing target member and the composition of the processing liquid. That is, the present inventors have conducted intensive studies on desmutting treatment of various stainless steels.
When the natural potential of the processing liquid for the member to be processed is -280 mV or more, it has been found that the smut layer formed on the surface of the member to be processed can be removed very effectively, and the present invention has been completed. It is. It is considered that when the natural potential of the treatment liquid with respect to the member to be treated is -280 mV or more, the adsorption force of the smut is weakened, and the separation and removal of the smut layer are promoted.

The spontaneous potential of the processing solution is determined by a galvanic electromotive force generated between a member to be processed in the processing solution whose temperature has been adjusted to 20 ° C. and a saturated calomel electrode as a reference electrode (however, saturated KCl is used as an electrolyte solution). Can be measured as

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.

Next, as a specific type of the treatment liquid that can be used in the present invention, one that satisfies the above-described condition of the natural potential can be appropriately selected from various pickling solutions such as an aqueous solution of sulfuric acid or an aqueous solution of nitric acid according to the material. . In addition, a solution other than an acid-based aqueous solution such as an aqueous solution of potassium permanganate or an aqueous solution of caustic soda can also be used.

The desmutting action of the processing solution largely depends on the oxidizing power of Fe ³⁺ ions contained in the processing solution, and the natural potential increases as the concentration of the Fe ³⁺ ions increases. ^Tends to increase as the concentration ratio of Fe ³⁺ ions to Fe ²⁺ ions increases. For example, when the Fe ³⁺ ions are reduced to Fe ²⁺ ions and consumed by the progress of the desmutting reaction, the content of the Fe ³⁺ ions decreases, and the natural potential of the treatment liquid, and further, the desmutting ability decreases. It will be.

It is important to increase the content of Fe ³⁺ ions in order to increase the lowered self potential again to fall within the above range. For example, when a pickling solution containing sulfuric acid (hereinafter referred to as a sulfuric acid-based pickling solution) is used as a treatment liquid, an oxidizing agent such as hydrogen peroxide or permanganate (potassium permanganate or sodium permanganate) is used. Is poured into a pickling solution to oxidize Fe ²⁺ ions in the solution into Fe ³⁺ ions, whereby the natural potential can be adjusted. In addition, even when the pickling solution containing no Fe-based ion is used, such as immediately after the bathing, if the natural potential is insufficient, the natural potential can be compensated by adding the oxidizing agent. In this case, the natural potential is increased based on the oxidizing agent component contained in the liquid in a free state.

On the other hand, when a pickling solution containing nitric acid (hereinafter, referred to as a nitric acid-based pickling solution) is used as the treatment solution, since the nitric acid itself acts as a strong oxidizing agent for Fe ²⁺ ions, the natural potential Can be adjusted by the content of the nitric acid.

When a sulfuric acid-based pickling solution is used as the pickling solution for desmutting, the sulfuric acid content (concentration) should be 1 to 15%.
It is better to adjust in the range of weight%. If the content of the sulfuric acid component is less than the lower limit of the above range, a sufficient desmutting effect may not be obtained. Further, even if the sulfuric acid component is contained beyond the upper limit of the above-mentioned range, further improvement of the desmutting effect cannot be expected, leading to wasteful cost increase of the pickling solution. In this case, the natural potential of the pickling solution itself does not change much even when the concentration of sulfuric acid is increased, so that in order to obtain a sufficient desmutting effect, the above-mentioned oxidizing agent such as hydrogen peroxide is charged. It is important to adjust the spontaneous potential of the pickling liquid to the member to be processed within the above range.

On the other hand, when a nitric acid-based pickling solution is used, the natural potential of the pickling solution changes depending on the concentration of the nitric acid component.
It is necessary to adjust the concentration of nitric acid so that the natural potential is in the above range. This desirable range of nitric acid concentration is
Depending on the type of steel and its surface condition, for example, in the case of austenitic stainless steel, the nitric acid concentration is 3 to 20.
It is good to set to about weight%. If the nitric acid concentration is less than 3% by weight, the natural potential of the pickling solution may be insufficient, and a sufficient desmutting effect may not be obtained. On the other hand, when the nitric acid concentration is 20% by weight or more, further improvement in the desmutting effect due to the increase in the acid concentration cannot be expected, and only a simple increase in the cost of the pickling solution may be caused. Also, in the case of ferritic stainless steel, the nitric acid concentration is 3 to 20% by weight.
It is good to set to. If the nitric acid concentration is less than 3% by weight, the natural potential of the pickling solution may be insufficient, and a sufficient desmutting effect may not be obtained. On the other hand, if the nitric acid concentration is 20% by weight or more, the surface of the member to be treated may be oxidized and dissolved excessively, causing problems such as rough skin. Further, in the case of martensitic stainless steel, the nitric acid concentration is set to 10 to 10.
It is good to set to 20% by weight. Nitric acid concentration is 10% by weight
If it is less than 30%, the natural potential of the pickling solution may be insufficient, and a sufficient desmutting effect may not be obtained. On the other hand, if the nitric acid concentration is 20% by weight or more, the surface of the member to be treated may be oxidized and dissolved excessively, causing problems such as rough skin.

The sulfuric acid pickling solution or the nitric acid pickling solution can contain an appropriate amount of hydrofluoric acid. Since hydrofluoric acid has a strong dissolving action due to corrosion, it may be possible to enhance the desmutting effect by blending it in the pickling solution or to reduce the content of sulfuric acid or nitric acid while securing the desired desmutting ability. In this case, the optimum blending amount varies depending on the type of steel and the type of the pickling solution, but is appropriately adjusted, for example, in the range of about 1 to 5% by weight so as to produce a certain effect in improving the desmutting ability of the pickling solution. Is done. If too much hydrofluoric acid is added, the spontaneous potential of the pickling solution may be reduced and the desmutting effect may be reduced.
It is preferable to adjust the amount of hydrofluoric acid in such a range that such a problem does not occur.

If the smut layer on the surface of the wire cannot be sufficiently removed by the single-stage desmutting process, the desmutting process can be performed in two or more stages. In this case, the desmutting treatment may be performed using a different pickling solution for each stage. In this case, in any one of the two or more stages of desmutting treatment, the natural potential of the pickling liquid with respect to the material to be treated may be -280 mV or more.

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.

When the member to be processed is a stainless steel wire produced by hot rolling, at least one of the following pre-descaling processes, for example, is performed prior to the desmutting process. be able to. 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.

Finishing pickling step: Of the chromium-based oxides or chromium-based compounds derived from the chromium-based oxides, those not completely removed in the auxiliary pickling treatment are removed. 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 processed. Therefore, such a film can be easily and reliably removed by immersing the wire in a hydrofluoric acid-sulfuric acid-based pickling solution containing hydrofluoric acid and sulfuric acid.

Specifically, the following treatment can be suitably adopted as the finish pickling step. That is, it contains hydrofluoric acid and sulfuric acid, and the weight of Fe ³⁺ ions in the liquid is M
(III), the weight of Fe ²⁺ ion is defined as M (II), and M
(III) / (M (III) + M (II)) in a hydrofluoric-sulfuric acid-based pickling solution (hereinafter also simply referred to as pickling solution) adjusted to be in the range of 0.3 to 1. A steel member to be treated (hereinafter, also simply referred to as a member to be treated) is immersed.

The descaling ability of the hydrofluoric-sulfuric acid pickling solution is as follows:
Fe-based ion concentration in pickling solution, especially Fe ³⁺ ion and Fe
^It changes greatly depending on the concentration ratio with ²⁺ ions. By using a hydrofluoric acid-sulfuric acid-based pickling solution containing hydrofluoric acid and sulfuric acid, a strong film such as the above chromium hydrate remaining on the surface of the stainless steel member to be processed or a scale layer is formed. Accordingly, a chromium-depleted layer such as a chromium-deficient layer formed on the substrate side can be easily peeled and removed. Further, the weight (M (III)) of the Fe ³⁺ ions and the weight (M (II)) of the Fe ²⁺ ions in the hydrofluoric-sulfuric acid-based pickling solution are represented by
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 becomes slow or, conversely, excessive. And the descaling ability can be controlled stably. As a result, it is possible to always stably form an oxidizing and dissolving state with no excess or deficiency on the surface of the member, thereby sufficiently removing the aforementioned or dechromized layer and improving the surface finish.

When M (III) / (M (III) + M (II)) is less than 0.3, the descaling ability of the pickling solution becomes insufficient.
There is a problem that scale remains on the surface of the member to be processed. According to the study of the present inventors, the natural potential of the pickling solution is M (III)
It has been found that the higher the value of / (M (III) + M (II)), the higher the value. Therefore, M (III) / (M (III) + M (I
When the value of (I)) is less than 0.3, the descaling effect is insufficient because the spontaneous potential of the pickling solution is too low, and the oxidizing dissolution current density necessary for obtaining a sufficient descaling state is secured. It is thought that it becomes impossible.

The above-mentioned 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.

Next, the hydrofluoric acid-sulfuric acid pickling solution is 0.5 to 7%.
It is preferable to use one containing 2% by weight of hydrofluoric acid and 2 to 20% by weight of sulfuric acid. 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. If hydrofluoric acid is contained in an amount exceeding 7% by weight, corrosion of the member to be treated becomes excessive, and the surface state may worsen.
On the other hand, when the sulfuric acid content exceeds 20% by weight, the viscosity of the pickling solution is too high, the mass transfer in the solution is hindered, the reaction rate is reduced, and the descaling effect is rather reduced. There is. The content of hydrofluoric acid in the hydrofluoric-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 10% by weight. Good.

Next, the total content of Fe ³⁺ ions and Fe ²⁺ ions in the hydrofluoric-sulfuric acid pickling solution is preferably adjusted within the range of 0.05 to 10% by weight. The total content is 0.0
When the content is less than 5% by weight, the value of M (III) / (M (III) + M (II)) greatly changes even if the amount of the iron-based ion in the pickling solution is slightly changed. It becomes difficult to control within the range. On the other hand, if the total content exceeds 10% by weight, the viscosity of the pickling solution will be too high, and the mass transfer in the solution will be hindered, the reaction rate will be reduced, and the descaling effect may be rather reduced.

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.

[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 an acid pickling method as a surface treatment 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 by being subjected to a heat treatment if necessary after hot rolling or the like, for example, With the traverser 2 moving along the rail 4 shown in FIG. 2, the traverser 2 is sequentially transported to 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. The traverser 2 and the elevating mechanism 5 constitute a member-to-be-processed mechanism and a member-to-be-processed unloading 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 transferred to the finish pickling tank 56, where it is subjected to finish pickling (first pickling of the present invention). In this embodiment, a hydrofluoric-sulfuric acid pickling solution is used in this finishing pickling treatment. The hydrofluoric-sulfuric acid-based pickling solution is specifically 0.5
~ 7% by weight (preferably 1-4% by weight) hydrofluoric acid and
An aqueous pickling solution containing 20% by weight (preferably 4 to 10% by weight) of sulfuric acid; the weight (M (III)) of Fe ³⁺ ions in the hydrofluoric acid-sulfuric acid type pickling solution; The weight (M (II)) of the ²⁺ ions is adjusted to be in the range of 0.3 to 1. 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.

Next, a residue layer called smut may remain on the wire after the finish pickling treatment. The reason why the smut layer is formed is as follows.
As shown in (e), 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 are pickled. It is conceivable that they are released 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 increase, causing problems such as scratches or disconnection, or shortening the life of the wire drawing die.

Therefore, the coil W of the wire is placed in the shower tub 5.
After washing with water in step 7, desmutting treatment is performed in a desmutting tank 58 (FIG. 1). In the desmutting treatment, the coil W is immersed in a pickling liquid having a natural potential of -280 mV or more with respect to the member to be treated measured at 20 ° C. with reference to the saturated calomel electrode. The natural potential of the pickling solution used is-
When the voltage is 280 mV or more, the smut layer formed on the surface of the wire can be removed very effectively.

In order to sufficiently remove and remove the smut layer, it is necessary to dissolve the surface layer of the wire to a necessary and sufficient degree regardless of the material of the wire or the type of the pickling solution. It is considered that the degree of dissolution that occurs is almost uniquely determined by the spontaneous potential of the wire surface. And
As a result of intensive studies, the present inventors have set the level of the self-potential of the solution specifically to -280 mV or more, so that the smut layer is substantially independent of the material of the wire or the type of the pickling solution. They found that they could be removed effectively.

As the pickling solution for desmutting, for example, an aqueous solution of sulfuric acid or an aqueous solution of nitric acid is used. Here, the spontaneous potential of the pickling solution increases as the concentration of Fe ³⁺ ions increases, specifically, as the concentration ratio of Fe ³⁺ ions to Fe ²⁺ ions increases. Then, while repeatedly processing the coil W, the content of Fe ³⁺ ions decreases,
When the natural potential of the processing solution is insufficient, the following is performed. First, when an aqueous sulfuric acid solution is used, an oxidizing agent such as hydrogen peroxide is charged into the pickling solution to reduce Fe ²⁺ ions in the solution to Fe ^3+.
By oxidizing to ions, its natural potential is -280
Adjust so that the value is at least mV. On the other hand, when a nitric acid aqueous solution is used as the treatment liquid, since nitric acid itself acts as a strong oxidizing agent for Fe ²⁺ ions, nitric acid is added to increase the concentration, and the natural potential becomes a value of −280 mV or more. Adjust as follows.

When a sulfuric acid aqueous solution is used for desmutting, the sulfuric acid content is preferably adjusted within the range of 1 to 15% by weight. When an aqueous nitric acid solution is used, and when the wire is austenitic stainless steel, the nitric acid concentration is 3 to 20.
Set to about weight%. In the case of ferritic stainless steel and martensitic stainless steel, the concentration of nitric acid is set to 10 to 20% by weight. In addition, an appropriate amount of hydrofluoric acid can be blended in these pickling solutions. The compounding amount of hydrofluoric acid is appropriately adjusted within a range of, for example, about 1 to 5% by weight so as to produce a certain effect in improving the desmutting ability of the pickling liquid.

Now, the coil W after the desmutting process is completed
1 is carried to the shower tub 59 in FIG. 1, washed with water, and further conveyed to the neutralization tank 60, where the processing liquid remaining after being immersed in a neutralizing liquid consisting of an alkaline aqueous solution such as an aqueous solution of caustic soda is used as the medium. The processing is performed, and the drying is performed by a drying device (not shown).

If the degree of removal of the smut layer by the above desmutting treatment is insufficient, the coil W is washed with water in the shower tub 59 and then carried to the finishing desmutting tank 61 where the finishing desmutting treatment is performed. it can. As shown in FIG. 5, the finish de-smut tank 61 (the pickling liquid storage section) includes
A sulfuric acid-hydrogen peroxide-based pickling solution (hereinafter simply referred to as a pickling solution) 21 composed of an aqueous solution containing 1 to 4% by weight of sulfuric acid and 0.1 to 1% by weight of hydrogen peroxide is contained. ing.
The coil W is then lowered to the lowered position and the sulfuric acid in the tank 61
It is immersed in the hydrogen peroxide-based pickling solution 21 for a predetermined period of time to perform finish desmutting.

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 hydrogen peroxide generally has not so high chemical stability, 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 mechanism 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. The control unit 30 (FIG. 5) opens the electromagnetic valve 26 and sets the electromagnetic valve 2
5 is closed. When the liquid level of the hydrogen peroxide is detected by the liquid level sensor 24, the electromagnetic valve 26 is closed, and the process waits until the pickling of the coil W is completed. FIG. 8 schematically shows a temporal change in the concentration of hydrogen peroxide in the pickling liquid 21 when the loading / pulling of the workpiece (coil W) is repeated. When the member 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 having been subjected to the finish desmutting process is washed with water in the shower tub 62, neutralized in the neutralization bath 60, and then dried by a drying device (not shown) to complete the process. 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.

[0059]

(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: hydrofluoric acid-sulfuric acid aqueous solution (3% by weight of hydrofluoric acid, 7% by weight of sulfuric acid). The proviso Fe ³⁺ ions and Fe ²⁺ ions, Fe ₂
(SO ₄ ) ₃ and FeSO _{4 in} the form of potassium thiocyanate colorimetric titration to determine the content of each ion M (III),
M (II) was identified. Here, M (III) + M (II) = 4% by weight, and M (III) / (M (III) + M (II)) = 0.5. Immersion time: 300 seconds Desmut treatment Pickling solution: sulfuric acid + aqueous solution of hydrogen peroxide; appropriately mixed hydrofluoric acid (compositions adopt various values in Table 1). A 30 cm long measurement electrode piece was sampled from the member to be processed, and this was used as a saturated calomel electrode (Princeton Allied Research, Model No .: K
0077), and immersed in 1000 cc of the pickling solution, and the galvanic electromotive force generated between both electrodes was measured with a potentiometer, and this was used as the measured value of the spontaneous potential of the pickling solution. Immersion time: 60 seconds

After the completion of the pickling treatment, the coil W was neutralized with a prepared aqueous solution, which was then washed and dried to evaluate the state of removal of the smut 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 desmutted region was determined by image processing. If the area ratio was almost 100%, the evaluation was excellent (）), 90% or more. Good (○), 50-90% acceptable (△), 50
% Was judged as unacceptable (x). Table 1 shows the above results.

[0061]

[Table 1]

It can be seen that a good desmutted state was obtained in the case where the treatment was performed using the pickling solution whose natural potential falls within the range of the present invention.

Example 2 An austenitic stainless steel wire coil W similar to that of Example 1 was pickled and evaluated in the same manner as in Example 1 except for desmutting.
The conditions of the desmutting treatment are as follows. Desmut treatment Pickling solution: nitric acid aqueous solution (compositions adopt various values in Table 2). The measurement of the spontaneous potential of the acid solution was performed in the same manner as in Example 1. Immersion time: 60 seconds Table 2 shows the results.

[0064]

[Table 2]

It can be seen that a good desmutting state was obtained in the case where the treatment was performed using the pickling solution whose natural potential falls within the range of the present invention.

(Example 3) 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. 1 and evaluated in the same manner as in Example 1. The conditions for each process are as follows. Pre-pickling treatment Pickling liquid: 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 liquid : 10% by weight sulfuric acid aqueous solution Immersion time: 180 seconds Finish pickling treatment Pickling liquid: hydrofluoric acid-sulfuric acid aqueous solution (hydrofluoric acid 2.5% by weight, sulfuric acid 5
weight%). However, Fe ³⁺ ion and Fe ²⁺ ion
e ₂ (SO ₄ ) ₃ and FeSO ₄ , and the content of each ion M (II
I) and M (II) were identified. Here, M (III) + M (II) = 4
% By weight, M (III) / (M (III) + M (II)) = 0.4. Immersion time: 250 seconds De-smut treatment Pickling solution: nitric acid aqueous solution (compositions adopt various values in Table 3). The measurement of the spontaneous potential of the acid solution was performed in the same manner as in Example 1. Immersion time: 60 seconds Table 3 shows the results.

[0067]

[Table 3]

It can be seen that a good desmutted state was obtained in the case where the treatment was performed using the pickling solution whose natural potential falls within the range of the present invention.

Example 4 An austenitic stainless steel wire (SUS304, wire diameter 5.5 mm), which was hot-rolled by a predetermined rolling device and then heat-treated at a temperature of 1000 ° C. in the atmosphere, was heated to a predetermined size. Wound on a coil.
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 then 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 4 shows the above results.

[0071]

[Table 4]

That is, M (III) / (M (III) + M (II))
It can be seen that a sample that has been subjected to finish pickling using a pickling solution having a value of 0.3 or more has a good descaling state.

Next, those which gave good results in the overall evaluation of descaling were desmutted under the following conditions. Desmut treatment Pickling solution: nitric acid aqueous solution (concentration 20% by weight, natural potential 550)
mV). The measurement of the spontaneous potential of the pickling solution was performed in the same manner as in Example 1. Immersion time: 60 seconds As a result, good desmutting results were obtained in each case.

(Example 5) 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 2 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 then 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 5 shows the above results.

[0076]

[Table 5]

That is, M (III) / (M (III) + M (II))
It can be seen that a sample that has been subjected to finish pickling using a pickling solution having a value of 0.3 or more has a good descaling state.

Next, those which gave good results in the comprehensive evaluation of descaling were desmutted under the following conditions. Desmut treatment Pickling solution: nitric acid aqueous solution (concentration 20% by weight, natural potential 295)
mV). The measurement of the spontaneous potential of the pickling solution was performed in the same manner as in Example 1. Immersion time: 60 seconds As a result, good desmutting results were obtained in each case.

[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 (3)

[Claims] 1. A stainless steel member having a smut layer mainly composed of chromium-containing carbide formed on its surface has a natural potential of -280 mV or more when measured at 20 ° C. with reference to a saturated calomel electrode. A surface treatment method for stainless steel, wherein the smut layer is removed by immersing the smut layer in a treatment liquid. 2. The method according to claim 1, wherein the treatment liquid is a pickling liquid containing sulfuric acid, and the natural potential is adjusted by adding hydrogen peroxide. 3. The surface treatment method for stainless steel according to claim 1, wherein the treatment solution is a pickling solution containing nitric acid, and the natural potential is adjusted by the content of the nitric acid.