JPH09235688A - Method for pickling stainless steel - Google Patents

Abstract

PROBLEM TO BE SOLVED: To reduce acid consumed for the dissolution of scale in a stainless steel peeled by pickling as much as possible, to prevent the deposition of the peeled scale on the bottom part of a pickling tank, to reduce the frequency of periodic cleaning and to increase the pickling efficiency by swiftly separating the scale by relatively simple equipment. SOLUTION: This pickling method is the one in which a stainless steel is immersed in a pickling soln. to remove scale from the steel surface, and, simultaneously with the pickling, scale in a pickling tank is exhausted to the outside of the pickling tank together with the pickling soln., the scale in the discharged pickling soln. is separated from the pickling soln. by magnetic force, and, if required, iron ions in the pickling soln. are removed, and, while the pickling soln. after the treatment is flowed back to the pickling tank, the pickling is executed.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a pickling method of immersing stainless steel in a pickling solution and peeling and removing the scale from the steel surface. More specifically, the present invention relates to a pickling method in parallel with pickling and peeling from the steel surface. The scale in the pickling tank was separated and removed from the pickling solution, and further, iron ions in the pickling solution were removed to prevent aging of the pickling solution and accumulated on the bottom of the pickling tank. The present invention relates to a pickling method for stainless steel, which can significantly reduce the time for which pickling is interrupted due to scale removal work.

[0002]

2. Description of the Related Art In the manufacturing process of stainless steel products such as plates, rods and tubes, alkali molten salt treatment or neutral salt treatment is used as a method for removing scales produced on the steel surface by heat treatment such as hot working and annealing. There are a method of dissolving and removing the scale itself like an electrolytic treatment, and a method of dissolving and peeling the base metal directly below the scale by pickling.

Generally, the former method is often applied when the scale is thin, and is frequently used in, for example, a finish continuous annealing pickling line for stainless steel strips. on the other hand,
The latter method is often used when the scale is relatively thick, such as a hot rolled stainless steel plate, and in this case, the peeled scale accumulates in the pickling solution. Also, a mixed acid of nitric acid and hydrofluoric acid as a pickling solution (hereinafter referred to as nitric hydrofluoric acid)
Continuing the pickling with the solution, the amount of metal ions in the acid solution increases and iron fluoride precipitate occurs.

At the bottom of the pickling tank, the peeled scale, iron fluoride, etc. are accumulated in the form of sludge (called sludge).
However, since it solidifies if left unattended, it is necessary to periodically clean the sludge once a month and discharge the sludge out of the pickling tank. When discharging sludge,
Since the work is performed in the pickling tank, not only the danger of chemical damage due to the acid but also the operation must be stopped, so that the pickling efficiency is reduced.

As a method for continuously removing this sludge, for example, in Japanese Patent Publication No. 10874/1990, the sludge is discharged out of the tank together with the pickling solution by a scraper and an air bubbling device provided in the pickling tank. A facility for separating sludge and pickling solution in a settling tank is also disclosed in JP-A-7-126876.
In the gazette, the pickling solution is jetted from a nozzle provided at the bottom of the pickling tank, the sludge is suspended in the pickling solution, and the sludge is discharged outside the tank together with the pickling solution to precipitate the sludge in the settling tank. A treatment facility for a pickling solution that is removed by the treatment is disclosed. Further, in Japanese Unexamined Patent Publication No. 6-158370, sludge settled and deposited on the bottom of a continuous pickling tank made of stainless steel is sucked together with a pickling solution with a suction nozzle and discharged to the outside of the pickling tank. A method and equipment for separating the washing solution and then refluxing the pickling solution to the pickling tank are disclosed.

As a method for preventing the precipitation of iron fluoride, a method of controlling the dissolved iron ion concentration by a diffusion dialysis device is disclosed in Japanese Patent Publication No. 60-35995. However, although the method described in Japanese Patent Publication No. 60-35995 can suppress fluoride precipitation, the sludge deposited during nitric hydrofluoric acid pickling contains not only iron fluoride precipitate but also a large amount of exfoliation scale. Because
The exfoliation scale is not removed and sludge deposition cannot be completely suppressed.

Japanese Patent Publication No. 2-10874, Japanese Patent Laid-Open No. 7-126
Although the method and equipment described in each of 876 and JP-A-6-158370 are capable of removing sludge from the pickling tank, both adopt the gravity sedimentation method as a method for separating sludge and pickling solution. There is. In this settling method, the sludge settling speed is slow, and powdery scales and fine iron fluoride precipitates are difficult to settle, so they are refluxed to the pickling tank without settling, and it is difficult to completely remove the sludge. is there. Further, as will be described in detail later, since the sludge separation speed is slow, there is also a problem that the dissolution of the exfoliation scale by nitric hydrofluoric acid proceeds and the acid is wasted. JP-A-6-158370
The equipment disclosed in the publication requires a device for reciprocating the suction nozzle along the bottom of the pickling tank to discharge the sludge out of the tank. The amount of suction and discharge is extremely large. Therefore, a large pump is required, and a sludge settling tank, a screw conveyor for transporting two sludges, and the like are also required, resulting in a large-scale facility.

[0008]

SUMMARY OF THE INVENTION According to the present invention, sludge generated by pickling is rapidly separated and removed from the pickling solution and iron ions in the pickling solution are removed with relatively simple equipment. Therefore, it is an object to increase the basic unit of acid, prevent sludge from accumulating at the bottom of the pickling tank, and reduce the frequency of regular cleaning to improve pickling efficiency.

[0009]

Means for Solving the Problems The present inventors have made various studies on the influence of scale in the pickling solution on the aging of the pickling solution and a method for quickly separating sludge from the pickling solution with simple equipment. As a result, the following findings were obtained.

As described above, the current operating method of periodically discharging the peeled scale from the pickling tank at a fairly long interval of one month is that the oxide scale of stainless steel contains Cr, so the pickling solution is used. It is based on the idea that it hardly dissolves. However, from the results of the experiments conducted by the present inventors, it was found that the stainless steel scale, although having a slow dissolution rate, is gradually consumed in nitric hydrofluoric acid and sulfuric acid, and thus the acid is wasted.

FIG. 3 is a graph showing the change over time in the dissolution rate of the scale on the steel surface in the pickling solution. (A) in FIG.
Is the dissolution rate of SUS430 scale of ferritic stainless steel and SUS304 scale of austenitic stainless steel in a 20 wt% sulfuric acid aqueous solution at 80 ° C., and (b) is nitric hydrofluoric acid of 60 ° C. The respective dissolution rates of the scales of the above two steel types in an aqueous solution (10% by weight nitric acid, 2% by weight hydrofluoric acid) are shown. The dissolution rate was obtained from the weight difference between the scales before and after the immersion in the liquid. As is clear from the figure, the dissolution rate of the scale is highest immediately after it is immersed in the pickling solution, so it is desirable to separate and remove the scale from the pickling solution as soon as possible. Also,
The scale solubility (solubility) varies depending on the type of stainless steel, and the scale of ferritic stainless steel is easier to dissolve than austenitic stainless steel, and nitric hydrofluoric acid is more soluble than sulfuric acid. It turned out to be strong. For example, the dissolution rate of SUS430 scale of ferritic stainless steel in nitric hydrofluoric acid aqueous solution is about 1.4 times that of sulfuric acid, and SUS304 scale of austenitic stainless steel is almost soluble in sulfuric acid aqueous solution. do not do. Therefore, in this case, it is not necessary to forcefully and promptly separate and remove. Therefore, as a result of various studies on the method of rapidly separating and removing the scale exfoliated from the stainless steel surface from the pickling solution, the inventors found that the scale of the stainless steel exfoliated by these picklings was ferritic stainless steel. It was found that the scale of austenitic stainless steel, which has no magnetism in the base material, has magnetism, and can be separated and removed from the pickling solution by magnetic force even in the pickling solution.

Further, when nitric hydrofluoric acid is used as the pickling solution, when the iron ions in the pickling solution reach a certain concentration or more, iron fluoride is produced and becomes sludge. Iron fluoride is a non-magnetic substance. Because it exists, it cannot be removed by magnetic force. However, it was confirmed that the sludge deposition in the pickling tank was reduced by removing the iron ions with an anion exchange membrane so that iron fluoride was not generated.

The present invention was made based on the above findings, and the gist thereof is as follows.

1) A pickling method in which stainless steel is immersed in a pickling solution to remove the scale from the steel surface. In parallel with pickling, the scale in the pickling tank separated from the stainless steel surface is pickled. Discharged together with the washing solution outside the pickling tank, the scale in the pickling solution discharged by magnetic force is separated from the pickling solution, and the separated pickling solution is pickled while being refluxed in the pickling tank. Pickling method for stainless steel.

2) A pickling method in which stainless steel is immersed in a pickling solution to remove the scale from the steel surface, and the scale in the pickling tank peeled from the stainless steel surface is pickled in parallel with the pickling. The scale in the pickling solution discharged by magnetic force is discharged from the pickling solution together with the washing solution, and the pickling solution after separation is refluxed to the pickling tank and the separated pickling solution Part of it is continuously or intermittently introduced into an acid recovery device equipped with an anion exchange membrane to remove metal ions in the pickling solution, and the pickling solution from which the metal ions have been removed is refluxed to the pickling tank. A pickling method for stainless steel, which comprises pickling.

3) In the pickling method of the above 1) or 2), the scale in the pickling tank peeled from the stainless steel surface is adsorbed by an electromagnet and collected near the discharge port of the pickling tank, and the acid is discharged from the discharge port. A method for pickling stainless steel, which comprises discharging the washing liquid and the scale.

[0017]

DESCRIPTION OF THE PREFERRED EMBODIMENTS FIG. 1 is a diagram showing an example of an apparatus for carrying out the pickling method of the present invention in which pickling is carried out while separating and removing a magnetic peeling scale from a pickling solution,
(A) is a plan view, (b) is a side view, and (c) is a sectional view taken along the line AA when the electromagnet 6 ′ is located on the discharge port 5 in the view of (a).

The steel strip 3 is guided by the roll 4 and is pickled.
The scale on the surface of the steel strip is peeled off while continuously passing through the pickling solution inside, and the scale is deposited on the bottom of the pickling tank.

The electromagnet 6'in the pickling tank has a U-shaped cross section as shown in (c), and the surface for adsorbing and supplementing the scale faces the bottom surface of the pickling tank directly above the bottom of the pickling tank. ing. The electromagnet 6'reciprocates in the steel strip passing direction in the pickling tank while adsorbing the scale that has settled on the bottom of the pickling tank or the scale floating in the pickling solution. When the electromagnet comes over the discharge port 5 after several round trips, the current is temporarily stopped to turn off the electric current flowing through the electromagnetic coil to disengage the scale adsorbed on the magnet and drop it to the discharge port. The bottom of the discharge port is inclined, and the scale moves toward the flow path, enters the flow path 9 together with the pickling solution, and is discharged to the magnetic separation tank 8. In the magnetic separation tank, the scale settles at the bottom, is supplemented and fixed by an electromagnet, and is separated from the pickling solution. The pickling solution is returned from the magnetic separation tank to the pickling tank by the pump 10.

Here, it is important that the scale is quickly discharged and removed from the pickling tank in parallel with the pickling. As described above, since the scale is easily dissolved in the pickling solution within a short time after being immersed in the pickling solution, it is necessary to remove it as quickly as possible in order to prevent aging of the pickling solution. Because there is. The pickling solution is not particularly limited since it may be a commonly used 5-30 wt% sulfuric acid aqueous solution or nitric hydrofluoric acid solution (5-20 wt% nitric acid, 0.5-5 wt% hydrofluoric acid).

As described above, when austenitic stainless steel is pickled with a pickling solution of a sulfuric acid aqueous solution, the scale hardly dissolves even if it is immersed in the pickling solution for a long time. The pickling solution does not age even if scale removal is not necessarily performed in parallel with pickling. However, even in this case, it is difficult to remove the thick scale if it is deposited on the bottom of the pickling tank. Therefore, it is desirable to constantly discharge the scale during pickling.

The method of separating the scale from the pickling solution discharged from the pickling tank is not particularly limited, but as shown in FIG. 1, a method of providing an electromagnet in the pickling solution discharge channel to adsorb the scale is known. is there. The equipment using this magnet is simple and it is possible to capture and fix fine powdery scales in the liquid. In the method of simply precipitating the scales in the separation tank to separate them, it takes a long time for the fine scales to settle, which is inefficient, and even if they once settle, they will re-float due to the flow of the pickling solution. .

FIG. 4 is a diagram showing the outline of another apparatus for separating the scale from the pickling solution by magnetic force. This is a high-gradient magnetic separation device used in wastewater treatment and the like. A fibrous stainless steel matrix 22 is provided in a pressure vessel 21.
Is filled, and the outside of the pressure vessel 21 is the air-core coil 2
It is surrounded by 3 and is further covered with a return frame 24 for preventing the leakage of magnetic lines of force. A direct current is applied to the air-core coil to magnetize the matrix and supplement the scale in the pickling solution. Since a magnetic field with a high gradient can be generated by the air-core coil, it is possible to capture a paramagnetic substance as well as a ferromagnetic substance. The matrix needs to be coated with Teflon so as to withstand nitric hydrofluoric acid and sulfuric acid.

As the means for collecting the scale deposited on the bottom of the pickling tank near the discharge port, it is preferable to use an electromagnet as in the above example. According to this means, not only the scale deposited on the bottom of the pickling tank but also the scale floating in the liquid can be captured. In addition to the magnetic scale collecting means, there is also a means for sliding the scale collecting plate on the bottom of the pickling tank to collect it at the discharge port.

FIG. 5 is a block diagram for explaining another pickling method of the present invention in which the pickling solution and the peeling scale are separated and pickling is performed while removing iron ions in the pickling solution. is there. This method, as compared with the pickling method that only separates the pickling solution and the peeling scale described above, further removes iron ions in the pickling solution, so that the accumulation of sludge in the pickling tank and It is a better method to prevent the aging of pickling.

The pickling tank 1 has a structure as shown in FIG. 1, and the stainless steel strip is pickled by the nitric hydrofluoric acid pickling solution in the tank. The pickling solution in the pickling tank 1 is discharged to the outside of the pickling tank together with the sludge containing the peeling scale, and is supplied to the magnetic separation device 26 through the flow path 25. The magnetic separation device 26 is the magnetic separation tank shown in FIG. 1 or the high gradient magnetic separation device shown in FIG. 4 as long as it has acid resistance capable of withstanding sulfuric acid and nitric hydrofluoric acid.

The peeling scale collected by the magnetic separation device by the magnetic force temporarily suspends the liquid feeding of the flow path 25 and the washing water 36a.
Is supplied to the neutralization tank 34 together with the peeling scale for neutralization. Since this cleaning can be performed in just a few minutes, there is almost no effect of deterioration of the pickling solution due to dissolution of the peeling scale during this period. The pickling solution from which the peeling scale has been removed by the magnetic separation device 26 is normally returned to the pickling tank 1 through the channels 27 and 28.

However, if the iron ion concentration in the pickling solution becomes high, the pickling ability will decrease as it is. Therefore, the iron ion concentration is measured by the analyzer 37, and when the iron ion concentration exceeds a certain level, a part of the pickling solution discharged from the magnetic separator is sent to the acid recovery tank 30 through the flow path 24 to remove the iron ion. Remove. Thus, only when the concentration of iron ions in the pickling solution becomes high, instead of intermittently removing iron ions in the pickling solution from the magnetic separator, the acid from the magnetic separator is removed. A part of the washing solution may be constantly and continuously sent to the acid recovery device 30, and the iron ions in the pickling tank may be constantly controlled to be a fixed amount or less.

The acid recovery device 30 uses a generally known diffusion dialysis method using an anion exchange membrane. When the pickling solution and water are alternately supplied to the chamber separated by the anion exchange membrane, the acid component is recovered by water and passes through the flow passage 31 and the recovered acid tank 32 as recovered acid. It is returned to the pickling tank 1 or temporarily stored in the recovered acid tank. On the other hand, the remaining liquid from which the acid has been recovered passes through the flow path 33 and is processed by the neutralization processing device 6. Also, the acid recovery device 30
Therefore, when the acid is collected or when the other pickling solution is insufficient, a new acid supply device 38 may be used to add a required amount to the pickling tank.

Since the pickling method of the present invention can be applied to any kind of steel as long as it is stainless steel, its components are not limited. Also, the stainless steel to be pickled is
It may be a pipe material, a rod material, a wire material or the like, and a steel strip is particularly suitable.

[0031]

【Example】

(Example 1) An example of a method of pickling while separating the pickling solution and the peeling scale with magnetic force is shown below.

SUS3 of austenitic stainless steel
A hot rolled sheet of No. 04 and SUS430 of ferritic stainless steel was annealed and shot blasted to give cracks to the oxide scale. A large number of test pieces having a width of 100 mm and a length of 150 mm were prepared from this test material.

FIG. 2 is a diagram showing a test apparatus used for carrying out pickling. This equipment is always 100
1 liter of pickling solution is stored, and the pickling solution is made to flow down from the bottom of the pickling tank into the magnetic separation tank 8 through a resin tube, and the scale is captured by the electromagnet 6, Only the pump 10 is pumped up and returned to the pickling layer 8.

Although an electromagnet for capturing the scale in the pickling tank is not shown, an iron core having a diameter of 10 mm and an electromagnet having a diameter of 5 mm are used as the electromagnet for capturing the scale in the magnetic separation tank. And a 1 mm iron plate welded were used. The number of turns of the electromagnetic coil at the end of each iron core is 1
It was set to 00 times and 50 times, and a direct current of 5 amperes was applied to each coil. The portion of the electromagnet immersed in the pickling solution has a thickness of 0.
The one coated with a 2 mm resin film (trade name: Teflon) was used.

The pickling solution is 80 ° C. for SUS430.
20% by weight sulfuric acid aqueous solution was used, and for SUS304, an aqueous solution of 10% by weight nitric acid and 2% by weight hydrofluoric acid at 60 ° C. was used.

With such an apparatus, the test piece was immersed in the pickling layer for 2 minutes, and pickling was continued at 1 hour intervals.

The magnet to which the scale adhered was taken out of the magnetic separation tank at intervals of 10 minutes, electricity was cut off, the scale was rinsed with water, and then immersed again in the magnetic separation tank.

On the other hand, for comparison, the pickling solution at the bottom of the pickling tank is closed, and the scale separated from the test piece is deposited on the bottom of the pickling tank by the conventional method. I went. The state of scale removal on the surface of the test piece after pickling in this manner was visually observed. The results are shown in Table 1. The evaluation was evaluated as ◯ when the scale was completely removed from the test piece, and as x when the scale remained even slightly.

[0039]

[Table 1]

As is clear from Table 1, in the example of the present invention in which the scale was separated and removed by the electromagnet in the middle of the flow path, about 3
Although 0 test pieces could be completely descaled, the test pieces of the comparative example in which the scale was not separated and removed were 24 to 2
Descaling was incomplete after the fifth sheet. After pickling 30 test pieces, the pickling solution was analyzed, and when the scale was separated and removed, the free acid concentration in the solution was higher and the metal ion concentration was higher than when the scale was not peeled off. I found it low. From the above, it can be seen that the acid wasted is reduced and the life of the pickling solution is extended by separating and removing the scale immediately after the pickling.

(Example 2) Next, an example of a pickling method in which the pickling solution and the peeling scale are separated and pickling is performed while removing iron ions in the pickling solution is shown below.

SU which is an austenitic stainless steel
After the hot-rolled steel sheet of S304 was annealed, a shot blast treatment was used to give cracks to the oxide scale, which was used as the test material. From this test material, as in Example 1, a width of 100 m
A large number of test pieces each having a length of m and a length of 150 mm were prepared.

The pickling tank used had an internal volume of 100 liters and was filled with a mixed acid of 10% nitric acid-2% hydrofluoric acid and kept at 60 ° C. As the magnetic separation method for the peeling scale, the high gradient magnetic separation device shown in FIG. 4 was used. As the acid recovery device for removing iron ions in the pickling solution, a diffusion dialyzer TSD-10-66 type manufactured by Tokuyama was used. These devices were arranged as shown in FIG. 5 and pickled. In order to avoid soaking the peeling scale in the acid for a long time, the magnetic separation device was operated for 10 minutes to adsorb and separate the scale, then the supply of the pickling solution was temporarily stopped, the power was turned off, and the peeling scale was removed. The recovered acid was directly returned to the pickling tank without being stored in the recovered acid tank. To control the iron ion concentration, the iron ion concentration in the pickling tank is 20 to 50 g /
A part of the pickling solution after scale separation was continuously supplied to the acid recovery device in the range of 1 liter to remove iron ions.

In the pickling, the test piece was immersed in the pickling tank for 2 minutes and the pickling was continued at 1 hour intervals. The interval of 1 hour was provided because the rate of iron fluoride generation is slow, and the effect of iron fluoride cannot be seen if pickling is continuously performed. As an evaluation method, the number of test pieces that had been completely descaled and the amount of sludge remaining in the pickling tank after the test was evaluated. Table 1 shows the results.

As a comparative example, the case where magnetic separation of the exfoliation scale and the removal of iron ions are not performed and the case where the iron ion concentration in the pickling tank is controlled to 50 g / liter are described together.

[0046]

[Table 2]

As is clear from Table 1, No. 1 in which the scale was separated and recovered from the pickling solution of the present invention and iron ions were controlled.
Nos. 4 and 5 have a large number of test pieces that can be pickled: 45 and 46. That is, the acid basic unit is improved. In addition, the iron ion concentration in the pickling tank was controlled to 20 g / liter. In 5,
There was no accumulation of sludge at the bottom of the pickling tank.

No. of the comparative example. In No. 2, when only the iron ion concentration in the pickling tank was controlled to 50 g / liter, the number of pickled sheets was as large as 30, but the amount of sludge accumulated was large and the pickling solution was aged if pickling was continued. Is promoted,
If the peeling scale is not removed separately, it will not prevent the pickling solution from aging.

[0049]

INDUSTRIAL APPLICABILITY According to the present invention, by removing the exfoliation scale and dissolved metal ions in nitric hydrofluoric acid online, the acid basic unit can be improved and the acid concentration of the waste liquid can be reduced, so that a neutralizing agent can be obtained. And it became possible to reduce neutralization sludge. Furthermore, by removing the exfoliation scale and dissolved metal ions in the pickling solution, it is possible to prevent sludge from accumulating in the pickling tank, reducing or reducing sludge removal work, and shortening the shutdown period due to removal work. It has great industrial value.

[Brief description of drawings]

FIG. 1 is a diagram showing an example of an apparatus for carrying out a pickling method of the present invention.

FIG. 2 is a diagram showing a pickling apparatus used in Examples.

FIG. 3 is a diagram showing a time-dependent change in scale dissolution rate in a pickling solution.

FIG. 4 is a diagram showing a high gradient magnetic separation device.

FIG. 5 is a block diagram for explaining the pickling method of the present invention.

[Explanation of symbols]

 1 Pickling tank 3 Steel strip 6, 6'electromagnet 7 Electromagnetic coil 8 Magnetic separation tank

Claims (3)

[Claims] 1. A pickling method for removing scale from a steel surface by immersing stainless steel in a pickling solution, wherein the scale in a pickling tank separated from the surface of the stainless steel is pickled in parallel with pickling. Discharged together with the washing solution outside the pickling tank, the scale in the pickling solution discharged by magnetic force is separated from the pickling solution, and the separated pickling solution is pickled while being refluxed in the pickling tank. Pickling method for stainless steel. 2. A pickling method of immersing stainless steel in a pickling solution to remove scale from the surface of the steel, wherein the scale in the pickling tank separated from the surface of the stainless steel is pickled in parallel with pickling. Discharged out of the pickling tank with the washing solution, separate the scale in the discharged pickling solution from the pickling solution by magnetic force, and return the separated pickling solution to the pickling tank and remove the separated pickling solution. Part of it is continuously or intermittently introduced into an acid recovery device equipped with an anion exchange membrane to remove metal ions in the pickling solution, and the pickling solution from which the metal ions have been removed is refluxed to the pickling tank. A pickling method for stainless steel, which comprises pickling. 3. The pickling method according to claim 1 or 2, wherein the scale in the pickling tank separated from the stainless steel surface is adsorbed by an electromagnet and collected near the discharge port of the pickling tank, and discharged. A method for pickling stainless steel, characterized in that the pickling solution and scale are discharged from the outlet.