JP2002371394A - Method for pickling stainless steel - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a method for effectively removing a scale, without conducting mechanical preliminary descaling such as shot blasting or brush grinding before pickling, when removing an oxidized scale generating in a hot rolling or heat treating process of a stainless steel. SOLUTION: The pickling method for removing the oxidized scale generating on the surface of the stainless steel, comprises sequentially dipping the stainless steel into a hydrochloric acid solution of 100 g/L or higher in the first pickling tank, an acid solution including fluoric acid of 10 g/L or higher in the second pickling tank, a hydrochloric acid solution of 100 g/L or higher in the third pickling tank, and an acid solution including fluoric acid of 10 g/L or higher in the fourth pickling tank. As required, a compound containing a sulfhydryl group of 0.001 mol/L or more, or amine of 0.001 mol/L or more, can be added into the hydrochloric acid bath; concentration of the fluoric acid bath can be 15-100 g /L for fluoric acid and 50-250 g/L for nitric acid; and chloride ions of 0.1 g/L or more with hydrochloric acid and/or thiocyanate ions of 0.17 g/L or more can be added.

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 and descaling stainless steel plates, strips, wires (including those wound in a coil shape), tubes, etc., and more particularly to a hot rolling or heat treatment process. When removing the generated oxide scale,
The present invention relates to a method of removing scale without performing mechanical preliminary descale such as shot blasting or brush grinding prior to pickling.

[0002]

2. Description of the Related Art The oxide scale of stainless steel, unlike ordinary steel, has a high Cr content and is difficult to dissolve in an acid solution when naturally immersed. Therefore, in the case of stainless steel, in order to remove the oxide scale generated in the hot working and heat treatment processes, the base metal is exposed by introducing cracks or partially peeling the scale with shot blasting or a grinding brush. After that, a method of immersing in a mixed aqueous solution of HNO ₃ and HF (nitric hydrofluoric acid) or the like to dissolve the metal base and peel off the scale is implemented.

For example, Japanese Patent Application Laid-Open No. Sho 53-1640 discloses a method of removing annealed scale of stainless steel by immersing the stainless steel in hydrochloric acid after securing a descale rate of 80% or more by blasting. Japanese Patent No. 2354 discloses a method in which cracks are introduced into the scale by bending, and a part of the scale is peeled off, and then immersed in an acid solution to complete the descaling. JP-A-59-
No. 147711 also discloses a technique in which pickling is performed after preliminary mechanical descaling is performed, and finally high-pressure water washing or steam spraying is performed.

[0004] Various studies have also been made on improving the acid solution during pickling. For example, JP-A-5-30218
JP-A-6-65765 discloses a technique of adding Pt, Pd, and Rh ions to a mixed acid of hydrochloric acid and nitric acid, and a technique of adding a precious metal ion to nitric hydrofluoric acid. Organic substances other than noble metals are also being studied, and Japanese Patent Application Laid-Open No. 8-333692 discloses a technique for increasing the efficiency of descaling by adding an organic compound containing a mercapto group to a non-oxidizing acid.

[0005]

In the case of stainless steel having a shape other than a steel plate or a steel strip, such as a coiled wire or a steel pipe, a mechanical force is applied to the entire surface of the product by shot blasting or a grinding brush. Is extremely difficult. For example, in the case of shot blasting, it is difficult to mechanically descale a wire coil in a portion where wires are overlapped, and in a steel pipe, an inner surface of the pipe. For this reason, it is necessary to perform the descaling only by acid immersion, and the acid immersion time becomes extremely long, so that not only the productivity is low, but also the acid solution is deteriorated and the base material is dissolved, so that cost reduction is difficult. In addition, there is a problem that maintenance is complicated because a large number of tanks are used.

[0006] Further, even for a steel sheet or a steel strip which can be easily mechanically descaled such as shot blasting, if mechanical descalement is strengthened, defects such as unevenness and covering of the metal base material increase, and the surface quality of the product deteriorates. The problem exists.

[0007] As described above, a technique for efficiently performing the descale without performing the mechanical preliminary descale has not been developed yet. Incidentally, the mechanical descaling referred to in the present invention refers to a method in which the scale itself is peeled off or cracks are introduced by an external mechanical force, such as blasting or bending. A process such as water washing for washing away the material adhered to the material surface by the force of a water stream shall not belong to mechanical descaling.

[0008]

DISCLOSURE OF THE INVENTION The present invention has been made to solve the above-mentioned problems, and it is intended to remove the oxide scale generated in the hot rolling or heat treatment process of stainless steel by using a shot blast or brush prior to pickling. The present invention relates to a method for efficiently removing scale at low cost without performing a mechanical preliminary descale such as grinding.

The gist of the present invention is as follows. (1) Remove oxide scale generated on the surface of stainless steel
A first pickling tank: 100 g /
Solution with hydrochloric acid concentration of L or more, 2nd pickling tank: 10 g / L or more
Acid solution containing hydrofluoric acid, third pickling tank: salt of 100 g / L or more
Acid concentration solution, fourth pickling tank: containing 10 g / L or more of hydrofluoric acid
A stainless steel characterized by being immersed in an acid bath in the order of
Method for pickling less steel. (2) In one or both of the first and third pickling tanks,
Add a compound containing a rucapto group in an amount of 0.001 mol / L or more
The stainless steel according to the above (1), wherein
How to pickle steel. (3) One or both of the first and third pickling tanks
Characterized by adding at least 0.001 mol / L of min.
Pickling the stainless steel according to the above (1) or (2)
Method. (4) One or both acids of the second pickling tank and the fourth pickling tank
As a liquid, 15 to 100 g / L of hydrofluoric acid and 50 to 2 g of nitric acid
Using a mixed solution of hydrofluoric acid and nitric acid mixed in a range of 50 g / L
In any one of the above (1) to (3),
A method for pickling stainless steel as described. (5) One or both acids of the second pickling tank and the fourth pickling tank
As a liquid, chloride ion: 0.1 g / L or more, thiocyan
Acid ion: 0.17 g / L or more, either one of which
Or an acid solution to which both are added.
The pickling method for stainless steel according to the above (4). (6) It is characterized in that the chloride ion is added with hydrochloric acid.
The pickling method for stainless steel according to the above (5), which is a feature. (7) the stainless steel is a wire rod coil;
Between the pickling tank and the third pickling tank, between the third pickling tank and the fourth pickling tank
Alternatively, after the fourth pickling tank, a pressure of 2.5 kgf / cm^Two
The above is characterized by uniformly spraying water on the steel material
The acid of stainless steel according to any one of (1) to (6)
Washing method. (8) In the water spray, the apparent outer surface of the wire coil
3.0 L / m on the surface ^Two/ S or more flow rate
Pickling of stainless steel according to the above (7), characterized in that:
Method. (9) Between the second pickling tank and the third pickling tank or the second pickling tank
Immersion of molten alkali salt between water spray after washing tank and third pickling tank
Any of (1) to (8) above,
A method for pickling stainless steel as described in Crab.

[0010]

DESCRIPTION OF THE PREFERRED EMBODIMENTS The reasons for limiting the scope of the present invention will be described below. First, the first to fourth chemical treatment
The reasons for limiting the order and configuration of the tanks will be described. The oxide scale of stainless steel has a two-layer structure including an outer layer mainly composed of Fe oxide and an inner layer mainly composed of Cr oxide. At the interface between the inner layer mainly composed of Cr oxide and the metal substrate, there is a Cr-deficient layer in which the amount of Cr is lower than the average concentration of the base material. Hydrochloric acid has the effect of dissolving all of the outer layer and part of the inner layer and creating a path for the acid to reach the metal substrate. Acids containing hydrofluoric acid have the effect of dissolving the Cr-deficient layer and exfoliating the scale. Therefore, in order to perform descale efficiently, it is necessary to use hydrochloric acid as the first tank and an acid containing hydrofluoric acid as the second tank. Most of the scale is peeled off at the end of the treatment in the second tank. However, the scale is thicker in parts where the supply of acid solution is not sufficient because the scale is thick or steel materials overlap with each other due to wire rod coils. Will remain. Therefore, a part of the outer layer and the inner layer mainly composed of Fe is dissolved again to create a path for the acid to reach the metal substrate, and then the Cr-depleted layer is dissolved and the scale is completely peeled off. Need to use an acid containing hydrofluoric acid. In this pickling step, it is desirable to perform water washing between the tanks. This is to prevent the acid from being carried into the next tank with the acid attached to the steel material, so that the different kinds of acids are not excessively mixed.

The reasons for limiting the concentration in each tank are as follows. First, when the concentration of hydrochloric acid is low, the action of dissolving the outer layer mainly composed of Fe oxide and a part of the inner layer mainly composed of Cr and creating a path for the acid to reach the metal base is weakened,
Efficient descale cannot be performed. Therefore, the first tank and the third tank
The hydrochloric acid concentration in the tank was limited to 100 g / L or more. Similarly,
Acids containing hydrofluoric acid have the effect of dissolving the Cr-deficient layer and exfoliating the scale. However, if the concentration of hydrofluoric acid is low, this effect is weak and efficient descaling cannot be performed. Therefore, the concentration of hydrofluoric acid in the second tank and the fourth tank was specified to be 10 g / L or more.

Next, the reasons for limiting additives to hydrochloric acid are as follows. The addition of mercaptan to hydrochloric acid not only increases the scale dissolution rate in the hydrochloric acid tank, but also shortens the required immersion time in the pickling tank containing hydrofluoric acid following the hydrochloric acid immersion in the pickling process. Hydrofluoric acid is more expensive as an industrial chemical than hydrochloric acid or sulfuric acid, and the immersion time greatly affects the manufacturing cost. In addition, in the descaling method in which several kinds of tanks are sequentially immersed, if any one of the tanks cannot shorten the pickling time, the productivity of the entire process is reduced.

Therefore, it is desirable to add mercaptan to hydrochloric acid in order to achieve high efficiency and low cost of the process of performing descaling without performing mechanical preliminary descaling. At this time, since the effect is weak if the amount of addition is small, the addition concentration is specified to be 0.001 mol / L or more.
Mercaptan is an organic compound having a mercapto group (-SH), and examples thereof include mercaptoacetic acid, mercaptoethanol, mercaptobenzothiazole, mercaptobenzoic acid, mercaptosuccinic acid, and mercaptopropionic acid.

By the way, when mercaptan is added to hydrochloric acid, the solubility of the acid is remarkably increased, so that not only the scale but also the metal matrix may be excessively dissolved in the thin portion of the scale. This not only causes irregularities on the surface of the product, but also leads to acid deterioration due to metal dissolution and a reduction in product yield. To prevent this metal dissolution, it is necessary to add 0.001 mol / L or more of amine to hydrochloric acid. The concentration of amine is 0.001mol / L
If it is less than 1, melting of the metal matrix cannot be prevented efficiently. Here, the amine is a compound in which a hydrogen atom of ammonia NH ₃ is substituted with a hydrocarbon group, and examples thereof include an alkylamine, cyclohexylamine, and aniline.

Next, the composition of the acid containing hydrofluoric acid in the second and fourth tanks and the reasons for limiting the additives will be described. When a mixed solution of hydrofluoric acid and nitric acid is used as the acid containing hydrofluoric acid, the Cr-deficient layer can be more efficiently dissolved than when an acid containing only 10 g / L of hydrofluoric acid is used. On this occasion,
Since hydrofluoric acid is a reducing acid and nitric acid is an oxidizing acid, its concentration greatly affects the descalability. Cr
To dissolve only the Cr-depleted layer in a short time without dissolving the metal matrix other than the depleted layer more than necessary, active dissolution occurs on the low Cr metal surface and the passivated acid concentration on the high Cr surface Is desirable. To this end, 15-100 g / L of hydrofluoric acid is used.
Nitric acid needs to be 50-250 g / L.

In a mixed state with nitric acid, the concentration of hydrofluoric acid is 1
If the amount is less than 5 g / L, the active dissolution of the Cr-deficient layer is insufficient, the descale efficiency is poor, and scale residue may be generated. On the other hand, when hydrofluoric acid is added in excess of 100 g / L, active dissolution occurs actively even when the metal matrix that is not the Cr-deficient layer is exposed, and the yield is reduced due to excessive metal dissolution. Even when the nitric acid concentration is less than 50 g / L, the passivation of the metal matrix is insufficient and excessive metal dissolution occurs. If the nitric acid concentration exceeds 250 g / L, passivation occurs even in a Cr-deficient layer, and efficient descaling cannot be performed.

By the way, chloride ion (Cl-
) And thiocyanate ion (SCN- ion) can increase the active dissolution rate of the Cr-deficient layer. When these ions are added in a small amount, the action is weak and the effect is not exhibited, but the chloride ion is 0.1 g / L,
Thiocyanate ion is effective at 0.17 g / L. By the way, even if thiocyanate ion is excessively added, the degree of inhibiting the passivation of the mother base is weak, but addition of more than 100 g / L of chloride ion inhibits the passivation of the mother ground, resulting in rough skin of the product. The yield decreases due to metal dissolution. For this reason, chloride ion is 0.1 to 100 g / L.
Was limited as an addition range. There is no harm even if chloride ion and thiocyanate ion coexist, and they can be added and used simultaneously. At the time of addition, chloride ions are desirably added with hydrochloric acid. This is because the detailed mechanism of action is unknown, but by using hydrochloric acid, Na
Compared to the case where chloride ion is added with neutral salt such as Cl
This is because the dissolution of the r-deficient layer is performed more efficiently.

In the present invention, the temperature of each tank is not particularly specified. This is because sufficient descaling is possible even at a normal temperature of about 25 ° C. However, to increase the descaling efficiency, hydrochloric acid is 50 ° C. or more, preferably 70 ° C. or more, and acid containing hydrofluoric acid is 25 ° C. or more, and if possible, 40 ° C.
It is desirable to make the above.

Next, the reasons for limiting the spraying of water will be described. In the present invention, the acid deficiency layer is dissolved by the pickling of the first tank and the second tank, and the oxide scale is in a state of only adhering to the steel material. If water spraying is performed in this state, the adhered scale is peeled off or washed away, and only the scale remaining during the subsequent pickling can be efficiently descaled. Since the scale will be in the adhered state only after the treatment in the second tank,
The water spray is desirably performed at any one or more places between the second tank and the third tank, between the third tank and the fourth tank, and after the fourth tank. At this time, the scale is not removed if the spray pressure is low. Therefore, the spray pressure was set to 2.5 kg / cm ² or more.

In particular, in the case of spraying water onto the wire coil, the wire is overlapped because the wire is wound in a coil shape. Therefore, the spray pressure does not reach the inside of the coil as it is. Therefore, to effectively perform descale,
It is effective to increase the spray flow rate to carry the momentum into the coil by the flow. At this time, if the flow rate is small, the descaling effect is weak, so the amount of sprayed water needs to be 3.0 L / m ² / s or more on the apparent outer surface of the wire coil.

In the case of the wire coil, water (gas-liquid mixed phase) containing air as air bubbles flows inside the coil with the water spray in the air. The gas-liquid mixed phase can easily pass through the narrow gap between the wires because it also has air bubbles, and has a large flow of water compared to air, so the momentum is carried to the inside of the wire coil through the narrow gap and descaled. Has the characteristic that it can be performed.

By the way, when the heat treatment is performed for a long time, the scale becomes extremely thick, and the treatment time may be extremely long when the acid solution alone or the combination of the acid solution immersion and the water spray is used. In such a case, it is possible to drastically reduce the processing time by immersing in a molten alkali salt bath. This treatment needs to be immersed in hydrochloric acid at least after the outer layer scale has been eliminated, and after the treatment. For this reason, the molten alkali salt treatment needs to be performed between the second tank and the third tank. When water spraying is performed between the second tank and the third tank, it is necessary to perform a molten alkali salt treatment after water spraying. In the present invention, the type and temperature of the molten alkali to be used are not particularly specified, but a small amount (10 to 30%) of alkali or alkaline earth metal hydroxide or oxide mainly containing NaOH heated to about 480 ° C. An alkali molten salt to which an oxidizing agent such as nitrate is added is preferred.

[0023]

DESCRIPTION OF THE PREFERRED EMBODIMENTS The present invention will be described below in detail based on embodiments.

(Embodiment 1) According to Embodiment 1,
Will be described in detail. Using a SUS304 hot-rolled steel sheet (thickness: 3 mm) cut to a width of 50 mm and a length of 50 mm as a test piece, pickling was performed under the conditions shown in Tables 1-1 to 1-2 to evaluate the descaled state. In the descaled state, the surface of the test piece was observed with a loupe of × 30, and it was determined whether the descale was completed or not based on the presence or absence of the residual scale.

Tables 1-1 and 1-2 are obtained by examining the descalability by changing the type and concentration of the acid solution in the first to fourth tanks. In the pickling treatment, the test piece was immersed in a water bath at room temperature between the first to fourth tanks to prevent the acid solution attached to the test piece from being mixed into the next pickling tank. First, the numbers A1 and A
2 is an example of the present invention.
The tank and the fourth tank were pickled with nitric hydrofluoric acid.
If the pickling time is short as in A1, the descaling cannot be completed, but the immersion time in the hydrochloric acid bath is 400 as in A2.
It can be seen that the descaling can be completed in seconds and 200 seconds of immersion in the nitric hydrofluoric acid bath. Hereinafter, the details of the present invention will be described while comparing with A2.

First, the number A3 is an example in which sulfuric acid is used for the first and third tanks in place of the hydrochloric acid of the number A2, but it can be seen that the descaling cannot be completed. Table 1-1
~ In Table 1-2, the underlined part is
It shows that it is out of the scope of the present invention. Similarly, the number A
No. 4 is an example in which only the third tank of the number A2 is replaced with sulfuric acid, and reference numeral A5 is an example in which only the first tank of the number A2 is replaced with sulfuric acid. From the above,
It can be seen that it is necessary to use hydrochloric acid as the acid solution in the first and third tanks.

Next, the number A6 is an example in which the second tank and the fourth tank of the number A2 are changed to nitric acid. It can be seen that the descaling cannot be completed also in this case. Similarly, the number A7 is an example in which only the fourth tank of the number A2 is changed, and the number A8 is an example in which only the second tank of the number A2 is changed to nitric acid. It can be seen that the descale cannot be completed in any case. From the above, it can be seen that it is necessary to use an acid solution containing hydrofluoric acid in the second and fourth tanks.

Next, an example in which the concentration of the acid solution in each tank is examined will be described. Numbers A9 to A12 are obtained by changing the concentrations of hydrochloric acid in the first tank and the third tank of number A2, which are examples of the present invention. FIG. 1 shows the relationship between the hydrochloric acid concentration and the completion / non-completion of the descaling. From this figure, it can be seen that descaling can be completed when the concentration of hydrochloric acid is 100 g / L or more. Table 1
The number A13 of -2 is an example in which only the first tank is less than 100 g / L of the hydrochloric acid in the first tank and the third tank, and the number A14 is an example in which only the third tank is less than 100 g / L. In any case, descale cannot be completed. From the above, it can be seen that it is necessary to use 100 g / L or more of hydrochloric acid in both the first tank and the third tank in order to remove the scale without performing mechanical preliminary descale.

Similarly, the numbers A15 to A21 are for examining the hydrofluoric acid concentration in the second tank and the fourth tank. First, the numbers A15 to A18 are obtained by changing the concentration of hydrofluoric acid in both the second tank and the fourth tank of the number A2 which is an example of the present invention. FIG. 2 shows the relationship between the hydrofluoric acid concentration and the completion / non-completion of the descaling. From this figure, it can be seen that descaling can be completed when the concentration of hydrofluoric acid is 10 g / L or more. Next, in Table 1-2, the number A19 indicates that the hydrofluoric acid concentration was 1 in only the first tank of the second tank and the fourth tank.
Example in which the amount is less than 0 g / L, the number A14 is 10 g only in the fourth tank.
/ L. A21 is an example in which a mixed acid of sulfuric acid and hydrofluoric acid is used in place of nitric acid, but the descaling is completed as in the case of using nitric acid. A22 is an example of phosphoric acid, but also in this case, the descaling is completed. From these facts, in order to remove scale without mechanical preliminary descale, both the second and fourth tanks require 1
It is understood that it is necessary to use an acid containing 0 g / L or more of hydrofluoric acid.

(Embodiment 2) According to Embodiment 2,
Will be described in detail. In the same manner as in Example 1, using a SUS304 hot-rolled steel sheet (thickness: 3 mm) cut into a width of 50 mm and a length of 50 mm, the liquid in the first and third tanks was 150 g / liquid.
L hydrochloric acid, the acid solution in the second and fourth tanks was a mixed solution of 40 g / L hydrofluoric acid and 200 g / L sulfuric acid, and the descaled state when the additive was added to the first and third tanks. evaluated. In the descaled state, as in Example 1, the surface of the test piece was observed with a × 30 magnification, and completion / incompleteness was determined based on the presence or absence of a residual scale.

The results are shown in Tables 2-1 to 2-3. In the pickling treatment, the test piece was immersed in a water bath at room temperature between the first to fourth tanks to prevent the acid solution attached to the test piece from being mixed into the next pickling tank.

Nos. B1 to B5 are compounds obtained by adding a compound containing S to hydrochloric acid in the first tank. The numbers B1 and B2 contain S as sulfate ions and sulfite ions,
The numbers B3 to B5 contain S as a mercaptan (mercapto group). When mercaptans of numbers B3 to B5 were added, A1 and A1 in Table 1 which did not contain these were added.
Descale is completed in a shorter time than that of 2. On the other hand, in the numbers B1 and B2, the scale remains, indicating that the descaleability is low.

Next, Nos. B6 to B8 indicate the cases where mercaptan was added to the hydrochloric acid in the third tank, but the acidity was higher than in the case where no mercaptan was added (Table 1-1, Nos. A1 and A2) as in the first tank. It can be seen that the descaling can be completed even if the washing time is short. Similarly, when mercaptan was added to the first tank and the third tank of Nos. B9 to B12, no mercaptan was added (Table 1-1, No. A).
1, A2) shows that descaling can be completed even if the pickling time is short. From the above, it can be seen that descaling can be efficiently performed in a short time by adding mercaptan to one or both of the hydrochloric acid in the first tank and the third tank.

Next, Nos. B13 to B17 represent the case where two different compounds were added to the same tank as mercaptan. Also in this case, it can be seen that the descaling can be completed even if the pickling time is shorter than the case where no mercaptan is added (Table 1, Nos. A1 and A2). From the above, it can be seen that if the compounds are classified into the same mercaptan, the descaling can be efficiently performed in a short time even if they are mixed and added.

Nos. B18 to B31 are examples in which the influence of the concentration of the added mercaptan was examined. Mercaptoacetic acid was used as mercaptan. Numbers B18 to B22 are the first tank, numbers B23 to B27 are the third tank, and numbers B28 to B3.
No. 1 is the result at the time of adding to the 1st tank and the 3rd tank. FIG. 3 shows the relationship between the addition concentration of mercaptan and the completion / non-completion of descaling. According to this figure, mercaptan was added to hydrochloric acid in a concentration of 0.1.
It can be seen that descaling can be efficiently performed in a short time by adding 001 mol / L or more.

In particular, Table 1 containing no added mercaptan
In the embodiment of the present invention, in order to complete the descaling, as in the example of A2, the first pickling tank: 400 seconds and the second pickling tank: 200
Second, third pickling tank: 400 seconds, fourth pickling tank: 200 seconds. However, when a predetermined amount of mercaptan is added to the hydrochloric acid tank (Tables 2-1 and B3, B6, and B9), it can be seen that the pickling time in the hydrofluoric acid-containing tank can be shortened.

By the way, hydrochloric acid and hydrofluoric acid have different descaling action mechanisms, and the promotion of descaling in a hydrochloric acid bath is
It does not lead to a reduction in the immersion time of the bath containing hydrofluoric acid. But,
As shown here, mercaptan has the effect of shortening the immersion time of the hydrofluoric acid-containing bath.

(Embodiment 3) A third embodiment will be described using the third embodiment.
Will be described in detail. As a test piece, width 50 mm x length 5
SUS304 hot rolled steel sheet (thickness 3
SUS304 plate (thickness 3 mm)
mm), the solution in the first and third tanks was prepared by adding mercaptoacetic acid to 150 g / L hydrochloric acid to a concentration of 0.01 mol / L, and the acid solution in the second and fourth tanks was used. 40g / L
A mixed solution of hydrofluoric acid and 200 g / L sulfuric acid,
The descaled state and the amount of metal dissolved when the additive was added to the third tank were investigated. The descaled state was determined from the state of the SUS304 hot-rolled steel sheet in the same manner as in Example 1, and the amount of metal cutting (the amount of dissolved metal) was determined from the weight change of the wet-polished SUS304 sheet No. 600.

Tables 3-1 to 3-2 show the completion and non-completion of descaling and the amount of metal cutting (μm) of the metal matrix. The amount of cutting is the first
This is the total amount of fusing between the tank and the fourth tank. In the pickling treatment, the test piece was immersed in a water bath at room temperature between the first to fourth tanks to prevent the acid solution attached to the test piece from being mixed into the next pickling tank. Number C1 is without additive, number C2 is when N is added as ammonium ion, and number C3 is when N is added as nitrate ion. On the contrary,
Numbers C4 to C6 are those to which amines have been added.

In both the case of ammonium sulfate (No. C2) and the case of sodium nitrate (C3), the descaling has been completed, and the amount of ablation is the same as in the case of no addition. It can be seen that descaling can be performed by immersing in the order of the first to fourth tanks using an acid containing hydrochloric acid and hydrofluoric acid specified in the above. On the other hand, the numbers C4 to C4
It can be seen that the amine shown in No. 6 has an action of reducing the amount of ablation while maintaining the descalability. FIG.
It is a summary of the relationship between the amount of amine added and the amount of cutting.
It can be seen that the addition of 0.001 mol / L or more can significantly reduce the amount of cutting (metal loss of the base metal).

(Embodiment 4) According to Embodiment 4,
6 will be described in detail. As test pieces, a SUS304 hot-rolled steel plate (thickness: 3 mm) cut to a width of 50 mm x a length of 50 mm and a SUS304 plate (thickness: 3 mm) polished by wet number 600 were used. 150g / L
The hydrochloric acid was added with mercaptoacetic acid at a concentration of 0.01 mol / L, and the descaled state was investigated by changing the acid solution in the second and fourth tanks. The descaled state was evaluated in the same manner as in the examples. In addition, the amount of metal cutting (the amount of metal dissolved) was determined from the weight change of the SUS304 plate polished by wet method No. 600.

Tables 4-1 to 4-4 show that the descaling is completed.
Indicates incomplete and ablation amount. FIG. 5 summarizes the relationship between the descalability and the amount of abrasion with respect to the concentrations of nitric acid and hydrofluoric acid. It can be seen that when the nitric acid concentration is less than 50 g / L, the descaling is not completed, and when the nitric acid concentration exceeds 250 g / L, the descaling is not completed again. It can be seen that the optimum nitric acid concentration is 50 g / L or more and 250 g / L.

Regarding the concentration of hydrofluoric acid, if the concentration is less than 15 g / L, the descaling will not be completed. Therefore, the concentration of 15 g / L or more is preferable. However, if hydrofluoric acid is added in excess of 100 g / L, the amount of cutting will be excessively high and adverse effects will occur due to a reduction in product yield due to metal loss. Therefore, it can be seen that the optimum hydrofluoric acid concentration is 15 g / L or more and 100 g / L or less. In this way, by optimizing the concentration of hydrofluoric acid and nitric acid, and combining it with the addition of mercaptan to the hydrochloric acid tank,
The immersion time of the hydrofluoric acid-containing tank can be reduced. Table 5-
Tables 1 to 5-2 show the effects of adding chloride ions and thiocyanate ions to nitric acid in the second and fourth pickling tanks. FIG. 6 shows the relationship between the chloride ion concentration and the descalability. 0.1 g / L of chloride ion
It can be seen that the above addition improves the descalability. Further, it can be seen that the time of the hydrochloric acid bath can be reduced by adding chloride ions with HCl.

FIG. 7 shows the results obtained by organizing the influence of the thiocyanate ion concentration. From this figure, it is understood that the descaling property is improved by adding thiocyanate ion of 0.1 g / L or more. Furthermore, it can be seen that the combined addition of chloride ion and thiocyanate ion can shorten the immersion time in the hydrochloric acid tank.

(Embodiment 5) Using Embodiment 5, Claim 7
9 will be described in detail. 5.5mm x as a test piece
The hot rolled SUS304 wire cut to a length of 400 mm was used in a 130 mm deep 300 mm wide 450 mm long resin basket. This simulates a part of the coil when the wire after hot rolling is wound into a coil shape, having a gentle curvature associated with winding after rolling,
By adjusting how the wires overlap, the bulk density was adjusted to 25% and the average distance between the wires was adjusted to 5 mm.

Tables 6-1 and 6-2 summarize the results of the descaling test using water spray. The water spray portion, which is a feature of the present embodiment, is underlined. First, F
The position in the process of performing the water spray based on 1 to 6 will be described. F1 is a case where water spraying is not performed. In order to complete the descaling, the first tank and the third tank are placed in the second tank for 400 seconds.
It is necessary to immerse in the tank and the fourth tank for 200 seconds. First, F2
Although water spraying was performed after the treatment in the first tank, it was found that the pickling time could not be reduced. In contrast, F
3 is after the second tank, F4 is between the third tank and the fourth tank, and F5 is the fourth tank.
This is an example in which water spraying is performed after the tank. It can be seen that, unlike F2, the pickling time can be significantly reduced. F6 is an example of spraying at two places, and it can be seen that the effect is further enhanced. FIG. 8 summarizes the situation of shortening the descaling time in Examples F1 to F6.

Next, F7 to F11 are the results of examining the effect of the spray pressure. FIG. 9 is a graph. From this figure, it can be seen that a spray pressure of 2.5 kgf / cm ² or more is required. F12 to F16 are the results of examining the spray flow rate. FIG. 10 shows the results arranged in a graph. From this figure, the effect of increasing the flow rate is extremely large, and 3.0 L
It can be seen that a water amount of / m ² / s or more is required.

Table 7 shows the results of investigating the influence of the alkali molten salt treatment. An underline is added to a portion treated with an alkali molten salt, which is a feature of the present embodiment. G1 is an example in which no alkali treatment is performed. In contrast, G
2 is an example in which the alkali treatment was performed. It can be seen that the pickling time is shorter than that of the alkali molten salt.

Further, G3 and G4 are examples in which water spraying was performed in addition to the alkali molten salt treatment. G3 is an example in which water spraying was performed before the alkali molten salt treatment, and G4 was an example in which water spraying was also performed after the fourth tank. G5 is the case where mercaptan was added to hydrochloric acid, and it can be seen that the pickling time can be further reduced.

[0050]

[Table 1]

[Table 2]

[0051]

[Table 3]

[Table 4]

[Table 5]

[0052]

[Table 6]

[Table 7]

[0053]

[Table 8]

[Table 9]

[Table 10]

[Table 11]

[0054]

[Table 12]

[Table 13]

[0055]

[Table 14]

[Table 15]

[0056]

[Table 16]

[0057]

According to the present invention, a stainless steel plate,
Prior to pickling, mechanical pre-descaling such as shot blasting and brush grinding should be performed before removing strips, wires (including those wound in coils), pipes, etc., especially oxide scales generated in the hot rolling or heat treatment process. Without doing so, scale can be efficiently removed at low cost.

[Brief description of the drawings]

Fig. 1 Effect of hydrochloric acid concentration on descaleability

FIG. 2 Effect of hydrofluoric acid concentration on descaling properties

FIG. 3 Effect of mercaptan concentration on descalability

Fig. 4 Effect of amine concentration on descaleability and amount of dissolved metal matrix

FIG. 5: Effect of nitric acid and hydrofluoric acid concentrations on descaleability and amount of cutting

FIG. 6: Effect of chloride ion on descalability

FIG. 7: Effect of thiocyanate ion on descalability

FIG. 8: Relationship between water spray location (example number) and descaleability in the pickling process

FIG. 9: Effect of water spray pressure on descaleability

FIG. 10: Effect of water spray flow rate on descaleability

Continuation of the front page (72) Inventor Akira Teramae 3434 Shimada, Omitsu-shi F-term in the Nippon Steel Corporation Hikari Works 4K053 PA03 PA12 PA13 PA14 PA18 QA01 RA06 RA07 RA16 RA17 RA19 RA40 RA46 RA47 RA52 RA54 RA57 RA59 SA04 SA06 TA02 TA04 TA14 TA16 TA18 YA03 YA04

Claims (9)

[Claims] 1. A pickling method for removing oxide scale generated on the surface of stainless steel, comprising: a first pickling tank: a solution having a hydrochloric acid concentration of 100 g / L or more; a second pickling tank: 10 g / L An acid solution containing L or more hydrofluoric acid, a third pickling tank: a solution having a hydrochloric acid concentration of 100 g / L or more, and a fourth pickling tank: an acid solution containing 10 g / L or more hydrofluoric acid. Pickling a stainless steel. 2. A compound containing a mercapto group in one or both of the first and third pickling tanks at a concentration of 0.001 mol / mol.
The method for pickling stainless steel according to claim 1, wherein L or more is added. 3. The pickling of stainless steel according to claim 1, wherein an amine is added in an amount of 0.001 mol / L or more to one or both of the first pickling tank and the third pickling tank. Method. 4. An acid solution of one or both of the second pickling tank and the fourth pickling tank, wherein hydrofluoric acid mixed with 15 to 100 g / L of hydrofluoric acid and 50 to 250 g / L of nitric acid are mixed. 4. The method for pickling stainless steel according to claim 1, wherein a mixed solution of nitric acid is used. 5. An acid solution for one or both of the second pickling tank and the fourth pickling tank, wherein chloride ion: 0.1 g / L or more, and thiocyanate ion: 0.17 g / L or more. The pickling method for stainless steel according to claim 4, wherein an acid solution to which one or both of them is added is used. 6. The method for pickling stainless steel according to claim 5, wherein said chloride ion is added by hydrochloric acid. 7. The stainless steel is a wire rod coil,
A pressure of 2.5 kgf is applied between the second pickling tank and the third pickling tank, between the third pickling tank and the fourth pickling tank or after the fourth pickling tank.
The method for pickling stainless steel according to any one of claims 1 to 6, wherein water is uniformly sprayed on the steel material at a pressure of / cm ² or more. 8. The method for pickling stainless steel according to claim 7, wherein in the water spray, a flow rate of 3.0 L / m ² / s or more is applied to an apparent outer surface of the wire rod coil. 9. A method of immersing molten alkali salt between the second pickling tank and the third pickling tank or between the water spray after the second pickling tank and the third pickling tank. Item 10. The method for pickling stainless steel according to any one of Items 1 to 8.