JP2012036430A - Molten salt composition modifying oxidized scale - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a molten salt composition used for removing oxidized scale formed on the surface of a stainless steel plate, wire rod, or the like in a manufacturing process thereof.SOLUTION: This molten salt composition modifying oxidized scale contains 3-20% of NaNO, 0.2-5% of KOH, 5-10% of NaCl, the remainder of NaOH and inevitable impurities. In addition, this molten salt composition modifying oxidized scale contains one or two of 2-10% of NaCOand 0.1-5% of KMnO, the remainder of NaOH and inevitable impurities.

Description

  The present invention relates to a molten salt composition used for removing oxide scale formed on the surface of a manufacturing process such as a stainless steel plate and a wire rod.

  Generally, an oxide scale is generated on the surface of stainless steel in an annealing process after hot rolling or cold rolling. This scale cannot be easily removed by pickling alone. For this reason, a pretreatment method is widely used in which stainless steel on which oxide scale is generated is immersed in a molten salt bath before pickling and is reacted with the salt to be modified to a scale that can be removed by pickling.

  As a pretreatment method for scale modification of stainless steel, a Lusner method is known in which stainless steel with scales is electrolyzed in a neutral salt aqueous solution. In this method, descaling of high alloy type, ferritic stainless steel, etc. is impossible. In addition, this method has a problem that long electrolytic equipment and high current application are required, and equipment costs and electrolysis costs become high.

  On the other hand, as another method for pretreatment of stainless steel, a molten salt method is known. In the pretreatment by this method, a molten salt agent containing an alkali metal hydroxide, an alkali metal nitrate, and further containing an alkali halide, a carbonate, an oxidizing agent, etc. is heated to an operating temperature of 400 to 500 ° C., Form a molten salt. In this method, for example, the stainless steel that has generated scales from the annealing furnace is cooled to about 500 ° C. and immersed in the molten salt. By passing the molten salt through a molten salt, the oxidized scale is modified to a scale that can be easily removed by pickling. In addition, after cooling the stainless steel which came out of the molten salt to a fixed state, the alkaline salt adhering to the surface is removed by washing with water, and then pickling to remove scale.

This molten salt method exhibits an extremely excellent pretreatment effect on any of ferritic stainless steel, austenitic stainless steel, and high alloy (high Cr, Ni-containing) steel, and has a surface gloss after pickling. Uniform and clean stainless steel can be obtained. In addition, the processing equipment is short and simple, and the equipment cost is low. Therefore, it is widely implemented as a method for pre-scaling stainless steel.

However, the conventional melt salt method is based on a high-temperature operation at 450 to 500 ° C, and a patent that examines an appropriate salt composition from the viewpoint of improving reforming properties, suppressing take-out, suppressing sludge formation, etc. is disclosed. Has been. For example, Japanese Examined Patent Publication No. 60-53755 (Patent Document 1) shows that in addition to NaNO ₃ , KOH, and NaOH, an appropriate amount of Na ₂ CO ₃ is added together to reduce the removal of salt. Has been.

In addition, in Japanese Patent No. 4136345 (Patent Document 2), by adding 0.2 to 5% of Na ₂ SO ₄ to a salt containing NaNO ₃ , KOH, NaOH, Na ₂ CO ₃ , and NaCl, A method for suppressing salt removal is disclosed. Further, JP-A-8-291398 (Patent Document 3) also describes KCl, NaCl, Na ₂ SO ₄ , K ₂ SO ₄ , NaNO ₃ , KNO ₃ for the purpose of reducing the amount taken out and descaling the low chromium-containing steel. A composition comprising the remaining NaOH is disclosed as a method for suppressing salt removal.

Japanese Patent No. 3387051 (Patent Document 4) discloses a molten salt composition and a composition comprising molten salt composition Na, K nitrate, Na, K chloride, KOH and the remaining NaOH as a replenishment method thereof, and We have proposed a system that supplies them to the molten salt bath as an aqueous solution.
Japanese Patent Publication No. 60-53755 Japanese Patent No. 4136345 JP-A-8-291398 Japanese Patent No. 3387051

  In recent years, the supply of sodium nitrate has become tapered in the molten salt composition. As a result, price increases have caused supply insecurity. On the other hand, although an alternative oxidizing agent to replace sodium nitrate has been proposed, there is a problem that the price is high. In addition, even if relatively inexpensive nitric acid can be used, additional equipment for supply shown in Patent Document 4 is required.

  As described above, improvement of the molten salt has been studied from the viewpoint of cost reduction, such as suppression of the amount taken out. However, the component composition that essentially affects the modification of the oxide scale, especially the effectiveness of the coexisting molten salt component on the modification, and the quantification thereof, have not been fully studied.

In order to solve the problems described above, the present inventors have fundamentally reviewed the molten salt component once again because of concerns about the supply of NaNO ₃ , and have intensively studied a salt composition having reforming characteristics higher than those of conventional ones. In other words, quantitatively determine how the limit concentration of coexisting components affecting the oxide scale modification of the molten salt composition is affected, and to what extent the limit concentration can be reduced by the influence of coexisting components. evaluated. As a result, it was possible to find a molten salt composition that has no problem in reforming power.

The gist of the present invention is as follows.
(1) NaNO ₃ : 3 to 20%, KOH: 0.2 to 5%, NaCl: 5 to 10%, the balance NaOH and unavoidable impurities are contained, and the oxide scale modified molten salt composition characterized by the above-mentioned.
(2) In addition to the above (1), Na ₂ CO ₃ : 2 to 10% and KMnO ₄ : 0.1 to 5% of one or two kinds are contained, and the remainder contains NaOH and inevitable impurities The oxide scale modified molten salt composition is characterized in that.

As described above, according to the present invention, based on NaOH, which is the main component of the molten salt composition, coexisting components KOH, NaNO ₃ , NaCl, Na ₂ CO ₃ , and KMnO ₄ are used within a proper addition amount range. Alternatively, by coexisting two or more kinds, an unprecedented efficient and economical molten salt composition can be provided.

The reasons for limiting the scope of the present invention will be described below.
The mechanism to modify the oxide scale by immersing the stainless steel with the oxide scale in the alkali molten salt is generally shown in the patents disclosed so far, but how much each composition contains However, there is no one that stipulates a quantitative concentration to determine the minimum content required.

NaNO ₃ is added to the NaOH molten salt and modifies the scale by its oxidizing action. That is, the oxide scale is 2 (FeO / Cr ₂ O ₃ ) + 7NaNO ₃ + 14NaOH → 2Na ₃ FeO ₃ + 4Na ₂ CrO ₄ + 7H ₂ O + 7NaNO ₂
By this reaction, the trivalent Cr existing in the oxide film is changed to the hexavalent Cr, and the structure is modified from a crystal structure to an amorphous film by X-ray diffraction. As a result, the modified film is easily removed by the pickling treatment after the molten salt immersion treatment. However, it has been said that a low nitrate concentration affects the reforming reaction, but it has been found that there is no problem in the reforming reaction if a certain amount is contained in the molten salt. The lower limit was 3%. The reaction was accelerated if it was present in a large amount, but it was also revealed that it was reoxidized by overreaction, so the upper limit was made 20%. Preferably it is 5 to 15%.

On the other hand, NaNO ₃ is an effective component for promoting descaling in the pickling treatment of the stainless steel strip containing Cr because it increases the reactivity of the alkali molten salt bath. In order to obtain this effect, the content needs to be 3% or more. However, if its content exceeds 20%, the reactivity is too high, the surface scale of the stainless steel strip peels off in the alkali molten salt bath, and the quality defects such as roll indentations increase. The amount was 3 to 20%. Preferably it is 5 to 15%.

  KOH has been actively added so far because it is effective for scale reforming. Since KOH is dissolved in total with NaOH to reduce the melting temperature, it is also added from the viewpoint of reducing the amount of takeout by lowering the viscosity of the molten salt. However, if it is less than 0.2%, the effect cannot be obtained. Moreover, since the effect will be saturated when it exceeds 5%, the range was made 0.2 to 5%.

Further, it has been found that when NaNO ₃ coexists with KOH, the modification of the oxide scale is further promoted. As described above, the KOH coexistence concentration range has a lower limit of 0.2% and an upper limit of 5%. Addition exceeding 5% saturates the reforming performance. Therefore, the range was made 0.2 to 5%. Preferably it is 1 to 4%.

  NaCl is added together. This is added due to the salting-out effect (separation and aggregation) of fine precipitates of sludge (sodium carbonate, etc.) in the molten salt, increasing the conductivity of the molten salt, promoting the flow of electrons (current), and indirectly Involved in the reaction. Although it was considered that the reforming reaction of the oxide scale was not directly involved, it became clear that the reforming was promoted by the simultaneous addition of NaCl. In order to promote the above-mentioned reaction promotion, addition exceeding 5% is necessary. Moreover, since reforming reaction will accelerate | stimulate too much when adding too much, the upper limit was made into less than 10%.

NaNO ₃ is partly produced by reacting with the carbon dioxide gas in the air and NaOH as the main component while using the molten salt, and is dispersed in the molten salt. Na ₂ CO ₃ is not directly involved in oxide scale reforming. However, as shown in the published patents so far, Na ₂ CO ₃ significantly increases the viscosity of the molten salt, increases the basic unit of salt consumption by immersion and removal, and is considered as a factor of cost increase. The main goal is to reduce.

  The present invention makes it clear that it is contained in the salt component in advance and contained in the molten salt to suppress (delay) the production reaction due to carbon dioxide inhalation during use.

FIG. 1 is a graph showing the relationship between the Na ₂ CO ₃ content in the molten alkali salt and the Na ₂ CO ₃ increase rate (final content initial amount / initial amount. That is, CO ₂ gas blowing into the molten alkali salt. It shows the change in alkalinity and Na ₂ CO ₃ according Inclusive, .na ₂ the test conditions, was pure carbon dioxide gas 3kg held in a molten state at each 480 ° C. was continuously fed for 90 minutes 100 ml / min The amount of CO ₃ produced was analyzed by neutralization titration.

As shown in FIG. 1, Na ₂ CO ₃ generated amount of carbon dioxide gas bubbling is when the advance is contained Na ₂ CO _3, suddenly in Na ₂ CO _{3 2%} weight or more, the amount of the molten salt is reduced . Furthermore, it can be seen that as the content increases, the proportion of the production amount gradually decreases, and when it exceeds 10%, it becomes almost constant and the production is remarkably delayed. For this reason, the addition range of Na ₂ CO ₃ was set to the lower limit of 2%. The upper limit concentration that the effect reaches is 10%. Above this, the reaction will be saturated.

It has been found that KMnO ₄ is effective in the oxidation scale reforming reaction, and is further promoted particularly by coexistence with NaNO ₃ . In the range of _{3 to} 20% NaNO ₃ , the effect is exhibited with KMnO ₄ 0.1%. The larger the addition amount, the better. However, it is limited to a range not exceeding 5%. Above this, the cost will increase significantly. Therefore, the range was made 0.1 to 5%. Preferably it is 0.5 to 4%.

  NaOH is the main component of the alkali molten salt. However, with this composition alone, it is difficult to promote the oxidation scale reforming reaction industrially effectively (improvement of productivity). Therefore, the above-mentioned molten salt coexisting component in the NaOH main component is indispensable for promoting the reforming reaction. Therefore, the main component of the present invention is NaOH.

Inevitable components are those in which inorganic substances and molten salts that are inevitably mixed in the process of industrial production such as NaOH, KOH, NaNO ₃ , NaCl, Na ₂ CO ₃ , and KMnO ₄ react with the atmosphere and are inevitably mixed Including. Further, the use temperature of the molten salt of the present invention is preferably 400 ° C. or higher, which is usually used in a molten state, and 450 to 500 ° C. is optimal. Above 530 ° C., sodium nitrate, which is a coexisting salt, is decomposed and the molten salt deteriorates.

  The above-described molten salt is used by melting a required amount of a predetermined composition in a salt bath to be used at a predetermined temperature. Industrially, it is taken along with the processing material for continuous use. Therefore, it is supplied to the molten salt bath as needed. The supply form may be any of powder (flakes and granules), briquette and liquid state (aqueous solution).

Hereinafter, the present invention will be specifically described with reference to examples.
A molten salt having the composition shown in Tables 1 and 2 was prepared, and the scale modification characteristics were evaluated. For the molten salt, a commercially available solid reagent was prepared, mixed in a large amount, put into an iron container having a diameter of 200 mm and a depth of 250 mm, heated to 480 ° C. in an electric furnace, and dissolved by heating. As for the scale reforming characteristics, SUS304 having a width of 50 mm, a length of 150 mm, and a thickness of 1 mm was annealed in an annealing furnace. Annealing was air-cooled at 1100 ° C. for 120 seconds. The annealing plate was dipped in a prepared molten salt. In this test, in order to evaluate the effectiveness of the reforming characteristics of the molten salt components shown in Tables 1 and 2, the immersion time was changed, the surface was observed after taking out and washed with water, and then 10% sulfuric acid solution (30 ° C.). ) For 3 minutes.

According to the method of observing the surface after being immersed in the molten salt and then taken out and washed with water, it can be clearly determined that when the scale is in a modified state, the surface changes from brown after annealing to a bright color mainly composed of yellow. The degree of this modification was evaluated by ○, Δ, and ×.
Furthermore, by immersing in a 10% sulfuric acid solution (30 ° C.) for 3 minutes, surface changes (such as the presence or absence of microscale residue) were observed with a microscope 500 times, and the presence or absence of scale was evaluated. “O” means no residual oxide scale and “x” means residual oxide scale.

表１、表２のＮｏ．１〜４０は本発明例であり、Ｎｏ．４１〜６０は比較例である。

No. in Table 1 and Table 2 1 to 40 are examples of the present invention. Reference numerals 41 to 60 are comparative examples.

Comparative Example No. In Nos. 41 to 42, since the NaNO ₃ concentration was low, the surface was observed after subsequent sulfuric acid pickling even though the immersion time was long. Comparative Example No. In Nos. 43 to 44, since the NaNO ₃ concentration was high, a slight scale residue was observed in the surface observation after subsequent sulfuric acid pickling even though the immersion time was a little longer, 10 seconds. Comparative Example No. In Nos. 45 to 46, since the KOH concentration was low, a minute scale residue was observed in the surface observation after subsequent sulfuric acid pickling even though the immersion time was slightly long.

  Comparative Example No. In No. 47, since the KOH concentration was low, although the immersion time was slightly long as 10 seconds, the surface of the surface after subsequent sulfuric acid pickling was observed to have a minute scale. Comparative Example No. In Nos. 48 to 49, since the KOH concentration was high, even though the immersion time was a little longer, 10 seconds, a minute scale residue was also observed in the surface observation after the subsequent sulfuric acid pickling. Comparative Example No. In Nos. 50 to 51, since the NaCl concentration was low, the surface of the surface after the subsequent sulfuric acid washing was observed to have a minute scale despite the long immersion time.

Comparative Example No. In No. 52, since the NaCl concentration was low, the surface of the surface after subsequent sulfuric acid pickling was observed to have a minute scale despite the slightly longer immersion time. Comparative Example No. In 53 to 55, since the NaCl concentration was high, even though the immersion time was slightly long, microscopic residuals were also observed in the subsequent surface observation after sulfuric acid pickling. Comparative Example No. In No. 56, since the concentrations of NaNO ₃ and NaOH were low, a very small scale residue was observed in the surface observation after subsequent sulfuric acid pickling even though the immersion time was slightly long.

Comparative Example No. In No. 57, since the NaNO ₃ concentration was high and the NaOH concentration was low, a very small scale residue was observed in surface observation after subsequent sulfuric acid pickling even though the immersion time was slightly long. Comparative Example No. In No. 58, since the NaOH concentration was high and the NaCl concentration was low, even though the immersion time was slightly long as 10 seconds, a microscale residue was observed in the subsequent surface observation after the sulfuric acid washing.

Comparative Example No. 59 has a low NaOH concentration, a high NaCl concentration, and a low Na ₂ CO ₃ concentration. Even though the immersion time is a little longer than 10 seconds, the surface observation after subsequent sulfuric acid pickling has a very small scale. Residual was observed. Comparative Example No. No. 60 has a high concentration of NaNO ₃ and a low concentration of KOH, NaCl and KMnO _{4. Even} though the immersion time is a little longer than 10 seconds, the surface observation after subsequent sulfuric acid pickling shows a slight scale residue. It was.

On the other hand, the present invention No. Since all of 1 to 40 satisfy the conditions of the present invention, it can be seen that all the characteristics are excellent. In particular, there is an effect on the oxide scale modification when Na ₂ CO ₃ coexists in NaOH, NaNO ₃ , NaCl, and KOH, and when KMnO ₄ coexists. The coexistence addition of KMnO ₄ further improves the reforming characteristics by adding to NaOH, NaNO ₃ , KOH system. Regardless of whether sodium carbonate is added or not, the effect is exhibited at a content of 0.1% or more. However, the effect is saturated when 5% or more is added.

As described above, according to the molten salt composition of the present invention, NaOH is the main component, and coexisting components NaNO ₃ and KOH are coexistently contained therein, thereby making it possible to improve the oxide scale more efficiently than in the past. High productivity and low cost can be promoted in the stainless steel production process. Moreover, the uneasy supply of sodium nitrate, which is indispensable for promoting the reforming reaction by the molten salt, shakes the foundation of stainless steel production, but no alternative method has been established and is currently alive. Therefore, it is an extremely important technique to establish a composition capable of maintaining the reforming characteristics by reducing the amount of nitric acid added even in a transient state.

Is a diagram showing the relationship between the molten alkali in the salt Na ₂ CO ₃ content and Na ₂ CO ₃ increase (final content initial amount / initial amount.

Claims (2)

An oxide scale modified molten salt composition comprising NaNO ₃ : 3 to 20%, KOH: 0.2 to 5%, NaCl: 5 to 10%, the balance NaOH and unavoidable impurities. In addition to claim 1, it contains one or two of Na ₂ CO ₃ : 2 to 10% and KMnO ₄ : 0.1 to 5%, and contains the remaining NaOH and inevitable impurities Oxide scale modified molten salt composition.

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