JP5679149B2 - Pickling operation method of stainless steel strip - Google Patents

Description

The present invention relates to a pickling operation method for suppressing the discharge amount of nitrogen oxide (NO _x ) while performing jet pickling when pickling a stainless steel strip with a nitric acid-hydrofluoric acid mixed pickling solution.

  In the manufacturing process of a steel plate product, the steel plate after annealing is usually sent to the next step after being descaled in the pickling step. Pickling at a mass production site is performed by a method of continuously passing a long steel strip while dipping it in a pickling solution in a pickling tank (dip pickling method). In such a continuous pickling line, a plurality of pickling tanks are generally arranged in series, and the steel strip passes through them sequentially.

  Recently, in order to increase the productivity of pickling, the number of cases where “jet pickling” is employed in the dip pickling method has increased. Jet pickling is a pickling method in which a jet of pickling liquid is jetted in a pickling tank, the liquid is stirred, and a steel strip is passed through the flow of pickling liquid by the jet. Patent Document 1 discloses a technique of applying jet pickling with a pickling solution using hydrochloric acid.

  For pickling stainless steel, it is common to use a mixed pickling solution of nitric acid and hydrofluoric acid (hereinafter sometimes simply referred to as “nitric hydrofluoric acid”). Nitrofluoric acid gives beautiful pickling skin to stainless steel in a short time and has a low risk of pitting corrosion, so it is widely applied to various stainless steel types. Even if nitric acid or hydrofluoric acid is used alone, the pickling effect cannot be obtained. Nitric acid is an oxidizing agent and has the property of promoting and maintaining the passivation of stainless steel, but it has no descaling effect by itself. Hydrofluoric acid has the property of eroding not only the scale but also the base metal of stainless steel to roughen the surface, and since it is highly toxic, it is not used alone for pickling. A good descaling effect is exhibited for the first time by the mixture of both.

JP-A-9-268390 JP 2003-13265 A JP 2007-100170 A

In the pickling of stainless steel using nitric hydrofluoric acid, NO _x is generated. The generation mechanism is considered to be mainly based on the following formulas (1) to (3).
4Fe + 10HNO ₃ + 4HF → 4FeF (NO ₃ ) ₂ + 2HNO ₂ + 2H ₂ O (1)
2HNO ₂ = N ₂ O ₃ ↑ + H ₂ O (2)
N ₂ O ₃ = NO + NO ₂ (3)
That is, the pickling reaction of the formula (1) occurs in the nitric hydrofluoric acid pickling solution, and nitric acid (HNO ₃ ) is reduced to nitrous acid (HNO ₂ ). Nitrous acid (2) is further reduced as of dinitrogen trioxide (N ₂ O ₃₎ becomes, NO (nitric oxide) and NO ₂ (nitrogen dioxide) is produced by (3).

NO _x is a causative substance of acid rain and photochemical oxidants. The pickling site stainless steel recovered exhaust gas pickling tank, to discharge after having sufficiently reduced NO _X concentration 100ppm or less by denitrator, concern for the environment have been made. However, since the capacity of the denitration device is limited, careful management is required so that excessive NO _x is not generated in the pickling operation.

One of the factors that the generation amount of the NO _X is increased in pickling stainless steel using nitric-hydrofluoric acid, the liquid temperature rise of the pickling bath can be cited. Since the pickling reaction of the above formula (1) is an exothermic reaction, the liquid temperature easily rises as the reaction proceeds. Liquid temperature is to the (1) pickling reaction accelerator increases, the amount of the NO _X increases. For this reason, the measure which suppresses the excessive raise of liquid temperature by adding industrial water and an acid to a pickling tank is taken. Moreover, since pickling reaction of Formula (1) is promoted also by performing jet pickling, it is difficult to positively employ jet pickling. In particular, when jet pickling is applied to stainless steel grades with a relatively low corrosion resistance level, the pickling reaction is rapidly promoted and NO _X generation becomes intense, so it is easy to improve pickling efficiency by jet pickling. The current situation is not.

On the other hand, as a method of suppressing the generation of NO _X, as disclosed in Patent Documents 2 and 3, it is effective to add denitrating agent (e.g. hydrogen peroxide) in the pickling solution of nitric-hydrofluoric acid. However, the addition of a denitration agent increases the pickling cost.

The present invention is produced without the pickling operation stainless steel strip using a nitric-hydrofluoric acid, it ensures stable of the NO _X generation amount, in particular for relatively low grades the corrosion resistance level required for denitration agent addition and by clarifying the simultaneous realization of sexual improvement and NO _X generation suppression.

As a result of detailed investigations, the inventors of the present invention, when performing pickling of a stainless steel strip using nitric hydrofluoric acid, actively perform jet pickling and cool the pickling solution by cooling means using a heat exchanger. As a result, it was found that when the liquid temperature was maintained within a certain range, the productivity was improved by jet pickling and the NO _x generation amount could be suppressed particularly in the steel type having a relatively low corrosion resistance level. Also in austenitic steels and corrosion resistance level is relatively high ferritic steels, that the liquid temperature and the NO _X generation amount becomes more stable was confirmed. The present invention has been completed based on such findings.

That is, in the present invention, in the pickling process in which the stainless steel strip is immersed in the nitric acid-hydrofluoric acid mixed pickling liquid in the pickling tank, the pickling equipment provided with means for cooling the pickling liquid by the heat exchanger is provided. Use
Perform "jet pickling" to expose the surface of the stainless steel strip to the jet of pickling solution discharged from the pickling solution discharge ports provided below the surface of the pickling tank,
By utilizing the heat generated by the reaction heat of the pickling and the heat removal by the heat exchanger, the temperature of the pickling solution during pickling is maintained within a preset temperature range, and by adjusting the jet flow rate, NO. _A method of pickling operation of a stainless steel strip for controlling the amount of _X generated is provided. The said temperature range can be set in the range of 40-70 degreeC, for example.

In the above pickling operation method, using a pickling equipment having an exhaust gas path for discharging exhaust gas from the pickling tanks recovered through the denitrification device into the atmosphere, continuously concentration of NO _X gas flowing through the exhaust gas passage It is possible to adopt a method in which the NO _x concentration on the outlet side of the denitration apparatus is set to 100 ppm or less by measuring automatically or intermittently and controlling the jet flow rate of the pickling solution according to the measured value.

As a stainless steel type that is particularly effective in applying the present invention, a ferritic or martensitic steel type having a Cr content of 10.5 to 20.0% by mass and a Mo content of 0 to 2.5% by mass can be given. . Since such steel types have a relatively low level of corrosion resistance, the effects of improving productivity and suppressing NO _x generation are particularly remarkable. Especially, it is preferable to maintain the temperature of the pickling liquid in the range of 45-60 degreeC in the ferritic or martensitic steel grade with Cr content of 10.5-14.0 mass%. It is not necessary to add hydrogen peroxide, which is a denitration agent, to the pickling solution in order to obtain the above NO _x generation suppression effect.

According to the present invention, the following advantages can be obtained in the pickling operation of a stainless steel strip using nitric hydrofluoric acid.
(A) Since the liquid in the pickling bath by the jet is agitated, the liquid temperature and NO _X generation amount is stabilized, it is possible to stabilize the product quality.
(B) Even a stainless steel type having relatively low corrosion resistance can be applied with jet pickling, which has been difficult in the past, and productivity can be improved. At the same time, a remarkable reduction effect of the NO _x generation amount can be obtained.
(C) In order to maintain the temperature of the pickling solution within a predetermined range by the balance between heat generated by jet pickling and heat removal by the heat exchanger, liquid temperature management mainly including addition of industrial water, acid, steam, etc. It is possible to switch to a method that mainly uses flow rate control and cooling power control of the heat exchanger, and it is possible to reduce the cost of additive substances and increase the efficiency of operation.
Since (d) without addition of denitration agent can reduce the NO _X generation amount, it eliminates the need to inject drugs such as hydrogen peroxide, drug cost can be reduced.

Graph showing the relationship between the liquid temperature and concentration of generated NO _X in nitric-hydrofluoric acid. The figure which showed typically the installation condition of the heat exchanger in the pickling tank used in the Example. Graph illustrating the liquid temperature and aging of the NO _X generation amount when Tsuban the SUS430. Liquid temperature and NO _X generation amount graph illustrating changes with time of when the Tsuban the SUS304.

[Pickling process]
In the manufacturing process of a stainless steel plate, it is common to perform one or more pickling steps until a steel plate product (hot rolled steel plate or cold rolled steel plate) is obtained using a hot rolled steel strip as a starting material. In the pickling process, nitric hydrofluoric acid is used as the pickling solution as described above. The present invention can be applied to any pickling process performed at any stage as long as it is a pickling process using nitric hydrofluoric acid performed on a stainless steel strip. Because Of these pickling for the purpose of descaling of hot-rolled steel strip is large pickling load, thus the generation amount of the NO _X often, the application of the present invention is particularly effective.

  This pickling process can be carried out in a continuous annealing pickling line. Between annealing and pickling with hydrofluoric acid, damage to scale layers such as scale breakers (apparatus that passes the steel strip while bending it with multiple rolls), shot blasting, hot sulfuric acid immersion, and abrasive brush grinding Inserting the pretreatment step to be applied is effective because the steel substrate under the scale of the stainless steel strip and the acid solution are easily brought into contact with each other during pickling. In particular, in the case of jet pickling, the pickling efficiency is further improved by a synergistic effect with the pretreatment because a fresh acid solution is easily sent to the surface of the steel base. That is, in the present invention, it is more effective to perform a pretreatment that damages the scale layer on the surface of the steel strip by mechanical means or further by immersion in hot sulfuric acid, and then subjected to jet pickling using nitric hydrofluoric acid. .

[Stainless steel grade]
“Stainless steel” is an alloy steel that has improved corrosion resistance with a Cr content of 10.5 mass% or more and a C content of 1.2 mass% or less, as shown in the number 3801 of JIS G0203: 2009. is there. The following composition can be mentioned when the content range of an alloy component is illustrated about each austenitic, ferritic, and martensitic steel types.

Austenitic grades;
In mass%, C: 0.15% or less, Si: 4.0% or less, Mn: 2.5% or less, P: 0.045% or less, S: 0.030% or less, Ni: 6.0-28 0.0%, Cr: 16.0 to 32.0%, Mo: 0 to 7.0%, Cu: 0 to 3.5%, Nb: 0 to 1.0%, Ti: 0 to 1.0% , Al: 0 to 0.1%, N: 0.3% or less, B: 0 to 0.010%, V: 0 to 0.5%, W: 0 to 0.3%, Ca, Mg, Y , REM (rare earth elements) total: 0-0.1%, balance Fe and inevitable impurities.

Ferritic steel grades;
In mass%, C: 0.15% or less, Si: 1.2% or less, Mn: 1.2% or less, P: 0.080% or less, S: 0.030% or less, Ni: 0.6% or less Cr: 10.5 to 32.0%, Mo: 0 to 3.0%, Cu: 0 to 1.0%, Nb: 0 to 1.0%, Ti: 0 to 1.0%, Al: 0 to 0.2%, N: 0.025% or less, B: 0 to 0.010%, V: 0 to 0.5%, W: 0 to 0.3%, Ca, Mg, Y, REM ( Rare earth elements): 0-0.1%, balance Fe and inevitable impurities.

Martensitic steel grades;
C: 0.75% or less, Si: 1.0% or less, Mn: 1.0% or less, P: 0.040% or less, S: 0.030% or less, Cr: 11.5-18 in mass% 0.0%, B: 0 to 0.010%, balance Fe and inevitable impurities.

  Examples of the standard steel types include austenitic, ferritic, and martensitic steel types shown in JIS G4305: 2005, and austenitic and ferritic steel types shown in JIS G4312-11991.

Among the above ferritic or martensitic steel types, the steel type having a Cr content of 10.5 to 20.0% by mass and a Mo content of 0 to 2.5% by mass has a relatively low level of corrosion resistance. When applied to the above, the effects of improving productivity and suppressing NO _x generation can be obtained particularly remarkably.

[Pickling solution]
The present invention is directed to a pickling process using a nitric acid-hydrofluoric acid mixed pickling solution (nitric hydrofluoric acid).
Nitric acid constituting the pickling solution is an effective component for descaling the stainless steel covered with the oxide scale and passivating the steel substrate. If the concentration of nitric acid in the pickling solution is too low, the scale tends to remain, and the passivation is likely to be insufficient, resulting in a decrease in the corrosion resistance of the stainless steel. On the other hand, if the nitric acid concentration is too high, NO _x generation during jet pickling is promoted and the basic unit of acid is increased, resulting in an increase in cost. As a result of various studies, the nitric acid concentration in the pickling solution is preferably adjusted in the range of 20 to 130 g / L, more preferably 25 to 116 g / L.

  The hydrofluoric acid constituting the pickling solution is effective for descaling stainless steel. If the concentration of hydrofluoric acid in the pickling solution is too low, scale tends to remain. On the other hand, if the concentration of hydrofluoric acid is too high, the stainless steel substrate is excessively dissolved, the surface roughness increases, and the quality deteriorates. As a result of various studies, the concentration of hydrofluoric acid in the pickling solution is preferably adjusted in the range of 2 to 20 g / L, and more preferably 3 to 16 g / L.

Fe ³⁺ is present in the pickling solution due to dissolution of the steel substrate, but Fe ³⁺ itself effectively acts as an oxidizing agent. As a result of various studies, when the Fe ³⁺ concentration in the pickling solution is maintained at 10 g / L or more, the action of suppressing the consumption amount of nitric acid appears remarkably, and the NO _x emission amount is further reduced. However, excess Fe ³⁺ causes a complex formation reaction with hydrofluoric acid to form a pickled scale precipitate. In order to prevent the hydrofluoric acid from being consumed by this reaction, it is preferable that the Fe ³⁺ concentration in the pickling solution does not exceed 50 g / L.

(Jet pickling)
Jet pickling is a pickling solution by discharging the pickling solution from a plurality of pickling solution discharge ports provided below the surface of the pickling bath, stirring the pickling solution in the pickling bath, and pickling solution The surface of the steel strip passing through it is exposed to the jet. As the pickling solution discharge port, a nozzle is generally used, but a small hole (injection port) may be provided on the wall surface of the pipe to create a jet. “Exposing the steel strip surface to a jet” means passing the steel strip through the flow of the pickling solution generated by the jet, and does not necessarily apply a strong jet force directly to the steel strip surface. A plurality of pickling liquid discharge ports are provided in the pickling tank so that both the lower surface and the upper surface of the passing steel strip are exposed to the jet. The pickling solution to be discharged is basically collected and circulated by collecting the solution in the pickling tank, but in the circulation path, a part of the liquid collected from the tank is discarded or new liquid is added. May be performed.

  When a stainless steel strip having an oxide scale on the surface is exposed to a jet in the pickling solution, the supply rate of a fresh acid solution to the surface of the steel strip is increased, and the efficiency of the pickling reaction is improved. As described above, if the pretreatment that gives damage to the scale layer on the surface of the steel strip has been performed in advance, the replacement of the acid solution entering the vicinity of the steel substrate by the action of the jet becomes more active, which improves the pickling efficiency. It becomes more prominent. An improvement in pickling efficiency leads to an increase in sheet feeding speed, leading to an improvement in productivity. Further, the pickling liquid in the tank is agitated by the jet and the acid concentration and temperature are made uniform, so that a more stable pickling operation is possible.

[Liquid temperature management during jet pickling]
The pickling efficiency is improved by jet pickling, but in the pickling of the stainless steel strip using nitric hydrofluoric acid, the progress of the pickling reaction (1) promotes the generation of NO _x. It becomes. To suppress the generation of NO _X, it is effective to lower the temperature of the pickling solution, the in itself known in the prior art. In general, in pickling operations, industrial water and acid are added to renew the pickling solution, and cooling of the pickling solution is usually realized by using such a method. However, in the pickling of stainless steel strip using a nitric-hydrofluoric acid, even when optimizing the liquid temperature in the manner described above, it is not easy to stably suppress the generation amount of NO _X. For these reasons, conventionally, it has been difficult to actively carry out jet pickling in pickling of a stainless steel strip using nitric hydrofluoric acid.

We detailed study results, when combined with the jet pickling, and cooling the pickling solution by the heat exchanger, it is possible to stabilize the liquid temperature variation during pickling operation, NO _X inhibition It has been found that the effect of increases. Even if only the cooling method by adding industrial water or acid is combined with jet pickling, such an effect is not sufficiently exhibited.

  The heat exchanger is a means for transferring the heat of the pickling solution to the refrigerant. The heat exchanger may be installed in the tank or in the pickling liquid circulation path. The pipes that make up the circulation path may be cooled by a heat exchanger from the outside, but a heat exchanger is installed in the pickling tank to remove heat directly from the pickling solution in the tank. Stable cooling can be performed by a simple device configuration. As the heat exchanger installed in the pickling tank, for example, an acid-resistant resin tube or a metal tube whose surface is coated with an acid-resistant resin is wound in a coil shape, and a coolant is allowed to flow through the tube. it can. In addition, you may use the type which raised the heat exchange efficiency more using the fin.

And jet pickling, heat exchanger liquid temperature fluctuations when combined cooling and with is stable, as the reason for occurrence of the NO _X is suppressed, that a large amount of charged (i) industrial water and acid Abrupt fluctuations in acid concentration and liquid temperature due to water can be avoided, (ii) steady heat removal can be achieved by using a heat exchanger, and (iii) the liquid in the tank can be sufficiently stirred by the jet Synergistic effects such as Positional and temporal stabilization of the acid concentration and the liquid temperature leads to stabilization of the NO _X generation amount, and extremely advantageous in preventing emissions regulation value over due to abrupt NO _X generation amount increases Become. Effect stabilization of the acid concentration and the liquid temperature is particularly beneficial in the original corrosion resistance level is relatively low NO _X generation amount is large ferritic and martensitic steels.

If the temperature of the pickling solution is too low, the efficiency of the pickling reaction of the formula (1) is lowered. Whereas that the liquid temperature is too high pickling efficiency is improved, the more the amount of heat generated by the reaction, the amount of the NO _X is increased significantly by the type of steel. In this invention, the liquid temperature at the time of pickling is maintained in an appropriate range by the balance of the calorific value by the reaction and the heat removal amount by the heat exchanger. As a result of various investigations, in order to realize a pickling operation to satisfy simultaneously the reduction of improving the NO _X emissions pickling efficiency as low cost as possible, it is effective to adjust the liquid temperature in the range of 40 to 70 ° C. It is. It is more preferable to adjust in the range of 45 to 65 ° C. In particular, Cr content from 10.5 to 14.0 wt% of the ferritic or martensitic steels corrosion resistance level is relatively low, since the NO _X generation amount due to jet pickling tends to increase, liquid 45 to 60 ° C. It is desirable to manage the temperature.

[NO _X concentration control in the exhaust gas]
Usually, pickling equipment for pickling stainless steel strips with nitric hydrofluoric acid has an exhaust gas path for collecting exhaust gas from the pickling tank and releasing it into the atmosphere via a denitration device. In the present invention, the amount of NO _x generated is suppressed by optimizing the liquid temperature using the heat exchanger as described above. Depending on the type of steel, pickling conditions (liquid temperature, acid concentration, the jet volume, passing plate speed, etc.) by previously grasping a relationship between the NO _X generation amount, such as NO _X emissions is regulated value Pickling operation becomes possible. However, in order to carry out a smooth continuous pickling operation, it is desirable that the degree of freedom in setting the pickling conditions be as wide as possible. For example, when switching the steel type of a stainless steel strip during continuous pickling, if it becomes possible to temporarily allow the pickling conditions to deviate from the optimum range in terms of NO _x suppression, productivity This is advantageous in preventing the decrease.

To perform such operations flexibility is the concentration of NO _X in the exhaust gas recovered from the pickling tanks continuously or intermittently measured, controls the jet of the pickling solution in accordance with the measured value The method is extremely effective. Although it takes some time to vary the liquid temperature, the adjustment of the jet quantity can be performed quickly, thereby generating amount of the NO _X can also be agile controlled. In some cases, the jet flow rate may be temporarily reduced to zero. The measurement of the NO _x concentration may be performed at any position in the exhaust gas path. The NO _X concentration variation of the gas released into the atmosphere is monitored directly by the denitrator exit side, but may be reflected in the jet amount, in order to cope with rapid change in NO _X generation amount, the NOx removal system It is desirable to measure the NO _x concentration in the gas before entering. In that case, the range of allowable NO _x concentration is determined according to the capability of the denitration apparatus. As an example, by the concentration of NO _X denitrator entry side to adjust the jet amount so as not to exceed the 4000ppm e.g., concentration of NO _X denitration unit exit side is managed to below 100ppm is regulated value, such as Take control.

A pickling experiment using nitric hydrofluoric acid was conducted in a laboratory using hot rolled annealed steel sheets made of various stainless steel types shown below, and the relationship between the liquid temperature and the NO _x generation amount was examined.
[Use steel grade]
-Steel type F1; 17% Cr ferritic (SUS430)
Steel grade M: 12.5% Cr martensite system (SUS410)
Steel type F2: 11% Cr ferrite type Steel type F3: 12% Cr ferrite type (Mn: 0.8%, P: 0.05% contained)
Steel type F4; 22% Cr-1.05% Mo ferritic steel type A; 18% Cr-8Ni austenitic system (SUS304)

These steel plate materials were collected at the stage immediately before entering the nitric hydrofluoric acid pickling tank in the continuous annealing pickling line. After annealing in the line, except for steel type A, “scale breaker → shot blast → heat It has undergone a pretreatment process of “sulfuric acid immersion → abrasive brush grinding”. Steel type A has undergone a pretreatment process of “scale breaker → shot blast”. A test piece of 100 mm × 50 mm was cut out from each steel plate material, and immersed in a nitric acid-hydrofluoric acid mixed pickling solution at 40 ° C., 55 ° C. or 70 ° C. in a thermostatic bath for 5 minutes. At that time, the pickling solution was stirred with a propeller under a certain condition to simulate a flow environment by jet pickling. The liquid temperature was kept constant at each of the above temperatures. The acid concentration of the liquid is shown in Table 1. In either case, the initial concentration of Fe ³⁺ was adjusted to 0 g / L, and it was confirmed that the Fe ³⁺ concentration at the end of immersion did not exceed 50 g / L. The gas generated in the immersion test was introduced to the NO _X meter is harvested by each steel type under the same conditions, to measure the concentration of NO _X in the gas. The results are shown in Table 1 and FIG.

Table 1 and as seen in FIG. 1, the austenitic (steel type A) and corrosion resistance high standard ferritic NO _X generation amount (steel type F4) the liquid temperature rises very little. If other corrosion level of relatively low ferritic or martensitic steels, although less NO _X generation amount either at 40 ° C., NO _X generation amount increases with the liquid temperature increases, the rate of increase The higher the temperature, the larger. As described above, in the jet pickling using nitric hydrofluoric acid, the generation behavior of NO _x accompanying the liquid temperature variation greatly differs depending on the stainless steel type, so that it is understood that the liquid temperature management is important.

  A heat exchanger was installed in the nitric hydrofluoric acid tank of the continuous annealing pickling line, and jet pickling was performed. This continuous annealing pickling line has a pretreatment facility of “scale breaker → shot blasting → hot sulfuric acid immersion → abrasive brush grinding” after the annealing process, and then a plurality of nitrofluoric acid tanks are arranged in series. Has pickling equipment. Among the nitric hydrofluoric acid tanks, a jet pickling apparatus and a heat exchanger were provided in the first tank (the uppermost tank). A total of 64 jet outlets were provided in the pickling tank. The pickling liquid in the tank is collected and pressure is applied by a pump, and it is discharged from the spray nozzle below the liquid level in the tank to create a jet, and the upper and lower surfaces of the steel strip are exposed to the flow of pickling liquid by the jet. It was to so.

  In FIG. 2, the installation condition of the heat exchanger in a pickling tank is shown typically. Fig.2 (a) is a top view inside a pickling tank, (b) is a side view inside a pickling tank. The coil-shaped heat exchanger 3 was placed in the pit 2 provided at the bottom of the pickling tank 1 and installed by a simple method in which it was prevented from moving by the protector brick 4 and the lifting prevention rod 5. The heat exchanger 3 is a tube made of a Teflon (registered trademark) tube, and water cooled by a chiller installed outside the pickling tank 1 is circulated in the tube. Two heat exchangers 1 were arranged in the plate passing direction.

A hot rolled annealed steel strip with oxide scale (so-called black skin) is passed through this pickling tank and pickling is performed. The exhaust gas from the pickling tank is collected and flows through the exhaust gas path until entering the denitration device. the concentration of NO _X gas was continuously measured. Moreover, the liquid temperature of the pickling solution was continuously measured. Here, the measurement result when the hot-rolled steel strip of SUS430 (corresponding to steel type F1 of Example 1) and SUS304 (corresponding to steel type A of Example 1) is passed is illustrated.

Figure 3 illustrates a liquid aging warm and NO _X generation amount when Tsuban the SUS430. Before entering the pickling tank, pretreatment of “scale breaker → shot blast → hot sulfuric acid immersion → abrasive brush grinding” was performed in the line. The pickling solution composition was controlled in the range of nitric acid (HNO ₃ ): 63 to 116 g / L, hydrofluoric acid (HF): 11 to 16 g / L, Fe ³⁺ : 20 to 50 g / L. The transit time was about 35-60 seconds. In the initial stage of feeding, normal immersion pickling without using a jet and heat exchanger was performed. In this case, the liquid temperature rose to about 59 ° C., and the amount of NO _x generated also increased. Therefore, the jet pickling and the cooling by the heat exchanger were started simultaneously from the time shown in FIG. The jet pressure was constant at 0.2 MPa, and the cooling strength of the heat exchanger was automatically controlled so that the liquid temperature was 56 ° C. When starting the cooling by jets pickling and heat exchanger, NO _X generation amount despite the reaction is accelerated by the jet was reduced. This is presumably because the temperature distribution and composition distribution of the pickling solution in the tank were made uniform by stirring by the jet and steady heat removal by the heat exchanger. The liquid temperature in the tank was controlled to the target of 56 ° C., and a stable jet pickling operation with a small amount of NO _x generated could be carried out. Productivity was improved by jet pickling, but the quality of the steel strip surface after pickling was as good as or better than before.

FIG. 4 illustrates the change over time of the liquid temperature and NO _x generation amount when SUS304 is passed through. Before entering the pickling tank, pretreatment of “scale breaker → shot blast” was performed in the line. The pickling solution composition was controlled in the range of nitric acid (HNO ₃ ): 48 to 86 g / L, hydrofluoric acid (HF): 5 to 12 g / L, Fe ³⁺ : 20 to 50 g / L. The transit time was about 35-60 seconds. In the initial stage of feeding, normal immersion pickling without using a jet and heat exchanger was performed. Since this steel type generates relatively little heat due to the pickling reaction, the liquid temperature was adjusted by intermittently adding steam to ensure the liquid temperature. After 2 hours, jet pickling was started, and cooling was performed by adding industrial water and acid as usual. In this case, the temperature was not stable. Thereafter, cooling with a heat exchanger was started when 4 hours passed. The cooling strength of the heat exchanger was automatically controlled so that the liquid temperature was 56 ° C. As a result, the liquid temperature and NO _X generation amount was very stable. Productivity was improved by jet pickling, and the quality stability of the steel strip surface after pickling was improved when combined with cooling by a heat exchanger. As seen in this example, even in the corrosion resistance level is relatively high intrinsic NO _X generation amount is small stainless steel species, confirmed that it is possible to more stable jet pickling operation by cooling by jets pickling and heat exchanger It was done.

1 Pickling tank 2 Pit 3 Heat exchanger 4 Protector brick 5 Lifting prevention rod

Claims (7)

In the pickling process of immersing and passing the stainless steel strip in the nitric acid-hydrofluoric acid mixed pickling liquid in the pickling tank, using pickling equipment equipped with a means for cooling the pickling liquid with a heat exchanger,
Perform "jet pickling" to expose the surface of the stainless steel strip to the jet of pickling solution discharged from the pickling solution discharge ports provided below the surface of the pickling tank,
By utilizing the heat generated by the reaction heat of the pickling and the heat removal by the heat exchanger, the temperature of the pickling solution during pickling is maintained within a preset temperature range, and by adjusting the jet flow rate, NO. _A pickling operation method for stainless steel strips to control the amount of _X generated . In a pickling process in which a stainless steel strip is immersed in a nitric acid-hydrofluoric acid mixed pickling solution in a pickling bath, a means for cooling the pickling solution with a heat exchanger and exhaust gas from the pickling bath are collected and a denitration device Using pickling equipment equipped with an exhaust gas path that discharges to the atmosphere via
Perform "jet pickling" to expose the surface of the stainless steel strip to the jet of pickling solution discharged from the pickling solution discharge ports provided below the surface of the pickling tank,
The temperature of the pickling solution during pickling is maintained within a preset temperature range by utilizing the heat generated by the heat of reaction of pickling and heat removal by the heat exchanger, and the NO _{x of the} gas flowing through the exhaust gas path is maintained. A method of pickling operation of a stainless steel strip in which the concentration is measured continuously or intermittently, and the NO _x concentration on the exit side of the denitration device is controlled to 100 ppm or less by controlling the jet flow rate of the pickling solution according to the measured value. The pickling operation method of the stainless steel strip according to claim 1 or 2 , wherein the temperature of the pickling solution during pickling is maintained in a preset temperature range within a range of 40 to 70 ° C. Pickling operation method of a stainless steel strip according to any one of claims 1 to 3, but the heat exchanger is provided in the pickling bath. Stainless steel band, Cr content from 10.5 to 20.0 wt%, and an Mo content of 0 - 2.5 wt% of the ferritic or martensitic steels to any one of claims 1-4 The pickling operation method of the described stainless steel strip. The stainless steel strip is a ferritic or martensitic steel type having a Cr content of 10.5 to 14.0 mass%, and maintains the temperature of the pickling solution during pickling within a range of 45 to 60 ° C. 1 or pickling operation method of a stainless steel strip according to one of 4. Pickling operation method of a stainless steel strip according to any one of claims 1 to 6, without the addition of hydrogen peroxide denitration agent pickling solution.

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