JPH1150280A - Descaling pickling solution and descaling pickling method for steel - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a descaling pickling method using sulfuric acid or the descaling pickling solution mainly consisting of sulfuric acid for the steel, in particular, the heat-treated steel in which descaling is promoted, erosion of the base metal is suppressed, generation of smut and pitching is prevented, and the smooth and excellent surface can be obtained. SOLUTION: The pickling time is shortened, and generation of smut and pitching is greatly reduced by adding >=1 g/L of thiourea as the descaling promoter to sulfuric acid or the descaling pickling solution mainly consisting of sulfuric acid for steel, and arbitrarily adding the pickling inhibitor less effective in descaling promotion. In order to avoid generation of red iron oxide-like brown smut to be generated in pickling scales on the heat-treated steel, oxy- carboxylic acid, imino-carboxylic acid, or the salt thereof is arbitrarily added as the descaling promotion adjuvant.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a descaling solution and a descaling method for descaling steel, and more particularly, to descaling of heat-treated steel.

[0002]

2. Description of the Related Art In a descaling treatment of a steel material, the steel material is immersed in an aqueous solution of an acid such as hydrochloric acid or sulfuric acid or a mixed acid thereof and pickled (hereinafter referred to as pickling). Dissolution smut remains, and there is a disadvantage of causing pitting corrosion called pitching. This disadvantage is more pronounced with sulfuric acid than with hydrochloric acid. Also, pickling of the annealed scale of the heat-treated steel produces a brownish smut similar to red iron, and a chromium-containing steel (J
In IS G 4105), pitching is apt to occur and black smut often occurs, and these situations cannot be avoided even by pickling with hydrochloric acid.

[0003] Here, "scale" is an oxide formed on the surface when a metal is heat-treated, and the state of the oxide differs between quenching and annealing. In general, in the case of annealing, an annealed scale is generated, and in the case of quenching, a black scale is generated. Here, removing these scales is referred to as "descaling". Rust of general steel is included in the annealed scale, and red rust mainly composed of iron oxide and iron hydroxide is partially mixed. The term "smut" means that when scale, red rust, or steel material is melted by pickling, carbon in the form of fine black powder adhering to the steel surface (the carbon contained in steel turns into scale or red rust). Included). In addition, the term “reddish-like brown smut” refers to a smut in which red rust (iron oxide) is powdered and mixed in addition to carbon. The term “pitting” refers to a pit (pitting corrosion) caused by a needle generated on a steel surface due to material erosion during pickling. The term "base erosion" refers to a state in which a steel base is dissolved and eroded by pickling, and generally involves generation of smut and pitting.

In order to alleviate the drawbacks of descaling pickling, many nitrogen-containing compounds such as amines, sulfur compounds such as thiourea derivatives, surfactants, etc. However, there is a drawback that the rate of descaling is also reduced, and the effect of suppressing the descale is not sufficiently obtained. Therefore, in order to remove smut generated by pickling, the physical force of a high-pressure water shower is used, alkali-permanganate treatment (JP-A-8-41666), or phosphonate treatment (JP-A-7-74). No. 113190), there is a method of immersing it in a smut removing solution, but all of these methods impose a heavy burden on equipment, installation space, wastewater treatment, and the like, and there is no solution for pitching caused by pickling. Not shown.

[0005] Pickling of steel, especially descaling of scales of heat-treated steel, involves a dilemma in which the descaling ability is reduced when an acid pickling inhibitor is added to prevent smut formation and pitting. As a solution to this, the addition of a pickling accelerator has been proposed. For example, addition of a reducing agent (Japanese Patent Application Laid-Open No. 47-34122), addition of a surfactant (Japanese Patent Application Laid-Open No. 57-198273), and addition of an oxidizing agent (Japanese Patent Application Laid-Open No.
No. 3-203780), and addition of sulfinic acid (Japanese Unexamined Patent Publication No.
No. 33171). However, none of these pickling accelerators has a satisfactory descaling acceleration effect on heat-treated steel, especially in sulfuric acid pickling, and conversely erodes the steel material and increases smut formation and pitting. As described above, the treatment is performed by providing a desmutting process after the pickling process or by mechanically providing a heat treatment scale removing device.

As described above, in the descaling pickling of a heat-treated steel material, there is no means for effectively preventing smut formation and pitting without lowering the pickling rate. Hydrochloric acid having a relatively good degree is frequently used. However, hydrochloric acid not only has insufficient descaling pickling ability but also generates hydrogen chloride gas,
Fatal problems such as severely corroding equipment and facilities and further deteriorating the working environment remain.

[0007]

SUMMARY OF THE INVENTION The present invention does not require a new facility for pickling steel, does not deteriorate the working environment, and can be used not only for black scale but also for a strong annealed scale of heat-treated steel. Pickling containing a descaling accelerator which removes quickly, prevents smut formation and pitting, and makes the steel surface smooth and good surface properties, and optionally a pickling inhibitor and / or a descaling accelerator. A liquid and a descaling method using the same are provided.

[0008]

According to the present invention, in order to solve the above-mentioned problems, as a descaling pickling solution for steel, sulfuric acid or sulfuric acid-based acid which does not generate gas from the pickling solution is used.
As a result of intensive research on descaling accelerators that have the effect of promoting descaling on the heat treatment scale of steel but have little effect on the base erosion of steel materials, the addition of a certain amount of thiourea is effective in promoting descaling. Furthermore, by adding a small amount of pickling inhibitor that does not affect the promotion of descaling, we succeeded in obtaining a smooth and good surface texture with almost no smut generation or pitting, and further assisted in the promotion of descaling. It has been found that the addition of the agent can also prevent the formation of a reddish brown smut during desmutting of the annealed scale.

[0009]

DETAILED DESCRIPTION OF THE INVENTION The present invention will be described in detail. The concentration of sulfuric acid used in the descaling pickling solution of the present invention is 5% by weight to 35% by weight, preferably 10% by weight.
２５25% by weight. The descaling pickling solution containing sulfuric acid having the sulfuric acid concentration range may optionally contain other inorganic acids (such as hydrochloric acid, hydrofluoric acid, nitric acid, and phosphoric acid) and acids such as organic acids (such as carboxylic acid and sulfonic acid). The added sulfuric acid-based pickling liquid has excellent pickling ability. The type of acid to be added is not particularly limited, but the amount of addition is preferably 5% by weight or less in the case of hydrochloric acid or hydrofluoric acid, and 1% by weight or less in the case of nitric acid in order to avoid adverse effects due to corrosion of the steel material. In consideration of the working environment, it is preferable to use an acid which does not generate gas, such as phosphoric acid and sulfonic acid, in combination.

To such a sulfuric acid or desulfurized sulfuric acid-based descaling solution, thiourea is added in an amount of 1 g / L or more, preferably 2 g / L or more.
Addition of L or more has the effect of promoting descaling of the steel material surface, and has a small but effective effect on the erosion of the base material of the steel material, and the effect of suppressing the formation of smut. I found that there was. Conventionally,
Thiourea has long been known as a pickling inhibitor and there are many documents (eg, Metal Surface Technology Handbook, 1976 edition). However, in the case of sulfuric acid pickling in descaling of steel, 0.5 g / It has been found that at L to 1 g / L or more, the behavior of accelerating descaling is changed, and the descaling accelerating effect tends to increase as the amount of addition increases. In addition, there is no adverse effect such as accelerating the dissolution and erosion of the steel material found in the conventional pickling accelerator. Generally, the “pickling accelerator” increases the base erosion of the steel material, generates a lot of smut, and also causes the base to be roughened. On the other hand, the "pickling inhibitor" reduces the base erosion of the steel material and prevents the formation of smut and the roughening of the base, but also has the disadvantage of increasing the descaling time. The sulfuric acid bath of the present invention containing 1 g / L or more of thiourea breaks this general tendency and achieves the effect of reducing the descaling time without causing any base erosion of the steel material. On the other hand, if the concentration of thiourea is excessively increased, for example, if it is increased to 30 g / L, the COD (chemical oxygen demand) of the waste liquid increases, which is not preferable because it has an adverse effect on wastewater treatment. Therefore, the concentration of thiourea is usually used at an upper limit of about 30 g / L.

[0011] It has been found that a fixed amount of thiourea is added as a descaling accelerator, and that a pickling inhibitor is further added to suppress the base erosion of the steel material and to eliminate the occurrence of smut and pitting. The type and amount of the pickling inhibitor must be carefully selected so as not to hinder the promotion of descaling. In view of this, a study of pickling inhibitors revealed that thiourea derivatives represented by the following formula were particularly effective:

Embedded image R ₁ , R ₂ : alkyl group having 1 to 5 carbon atoms, phenyl group R ₃ : alkylene group having 2 to 5 carbon atoms, phenylene group. By adding this thiourea derivative in an amount of 0.2 to 60 mg / L, preferably 0.5 to 30 mg / L, almost no base erosion of the steel material occurs. A smooth and good surface without smut formation was obtained. The addition amount of these thiourea derivatives must be adjusted, for example, to increase as the concentration of sulfuric acid increases and as the temperature of the processing solution increases. There is an advantage that the control range is wide because the scale speed is not reduced.

As described above, by adding thiourea as a descaling accelerator and a thiourea derivative as a pickling inhibitor to a descaling pickling solution with sulfuric acid, descaling of most steel materials can reduce generation of smut and pitting. Although the surface can have a smooth surface texture without cracks, a reddish brown smut may remain on a part of the annealed scale of the heat-treated steel material. As a countermeasure, in addition to thiourea, a descaling accelerator, auxiliaries for promoting descaling, such as glycolic acid, lactic acid, malic acid, tartaric acid, citric acid, hydroacrylic acid, α-oxybutyric acid, glyceric acid, and tartronic acid By adding at least one of aliphatic oxycarboxylic acids, iminocarboxylic acids such as nitrilotriacetic acid and ethylenediaminetetraacetic acid, and salts of alkali metals such as sodium and potassium, in desmutting of the annealing scale. It has been found that the formation of reddish brown smut can also be prevented. Increasing the concentration of these adjuvants increases their effect,
At the same time, the cost also rises, so that there is usually a certain effect and the cost does not increase extremely.
Preferably, it is used in the range of 1 to 20 g / L.

The treatment method of the present invention can be carried out with existing equipment without requiring any new equipment, and a sufficient effect can be obtained by any of the immersion or spray treatment methods. However, since the descaling pickling liquid of the present invention is a sulfuric acid bath without gas generation, the working environment is not deteriorated even by the spraying treatment, so that the spraying treatment is also effective in physical descaling and desmutting. In particular, the present invention exerts a particularly great effect on shortening the descaling time and improving the finish quality (appearance). The descaling pickling solution of the present invention has an effect of accelerating descaling at a treatment temperature of 30 ° C. or higher, but exhibits a remarkable effect particularly when used at 50 ° C. or higher.

[0014]

Examples 1 to 6 500 cc of a pickling solution having a sulfuric acid concentration of 10% by weight or 20% by weight and containing the thiourea of the present invention, an inhibitor and a descaling accelerator of the composition shown in Table 1 were prepared. A beaker was prepared and heated at 60 ° C., and a heat-treated steel wire with an annealed scale was immersed therein. Was observed. The results are shown in Table 1 together with Comparative Examples. In this test, the effect of the present invention is better as the appearance is better without smut generation or roughening of the ground, and the descaling time is shorter. It can be seen from Comparative Examples 1 and 2 that the appearance was poor only with sulfuric acid. From Comparative Example 3, it can be seen that when only a thiourea derivative is used in combination with sulfuric acid without using thiourea, the descaling time becomes as long as 55 minutes, and a good effect cannot be obtained. In Comparative Example 4, 1
Since thiourea of less than g / L behaves as an inhibitor, the descaling time is prolonged and the appearance is not improved. On the other hand, in Example 1, if thiourea of 1 g / L or more is used, descaling occurs. (The descaling time is 5 times more than that of Comparative Example 1).
Minutes short. Further, it can be seen that the base material erosion of the steel material is suppressed (the amount of smut adhered and the base material roughened compared to Comparative Example 1). Examples 2 to 6 which are within the scope of the present invention
It can be seen that a good appearance and a short descaling time were attained.

[0015]

[Table 1]

Next, there will be described examples in which good results were obtained for the annealed material and the black scale material, for which good results could not be obtained by the conventional pickling with sulfuric acid. Example 7 After annealing a wire coil having a weight of about 2 tons, 7 g / L of thiourea and 5 mg / dibutylthiourea were added to a sulfuric acid solution having a concentration of 15% by weight.
L, 3 g / L of sodium nitrilotriacetate were added, and the resultant was immersed in a descaling pickling solution of the present invention heated to 72 ° C. to remove the annealed scale. As a comparative example, at the same sulfuric acid concentration and processing solution temperature as in the example, 0.5 g / L of a pickling inhibitor Aid 48 (manufactured by Nippon Surface Chemical Co., Ltd.) using a conventional thiourea and a nonionic surfactant was used. Table 2 also shows the results of descaling of the same material with the added pickling solution.

[0017]

[Table 2]

EXAMPLE 8 A hot-rolled wire rod having a weight of about 2 tons and a density of 15
7 g / L of thiourea, 5 mg / L of dibutylthiourea, and 3 g / L of sodium nitrilotriacetate were added to a sulfuric acid solution of 5% by weight, immersed in a descaling pickling solution of the present invention heated to 72 ° C. Table 3 shows the results of the removal. Also,
As a comparative example, at the same sulfuric acid concentration and processing solution temperature as in the example, the descaling of the same material was carried out with a pickling solution containing 0.5 g / L of a conventional pickling inhibitor Aid 48 (manufactured by Nippon Surface Chemical Co., Ltd.). The results are shown in Table 3.

[0019]

[Table 3]

[0020]

According to the present invention, the pickling bath mainly composed of sulfuric acid or sulfuric acid having a good working environment because of no generation of gas is improved, and not only the scale of black scale but also firmly fixed without greatly changing the equipment. Significantly shortens the descaling time of the annealing scale and suppresses the base erosion of the steel material, preventing smut formation and preventing pitting, making it white and smooth which could not be obtained until now. We succeeded in obtaining excellent surface properties.

Continuation of the front page (72) Inventor Takashi Sugimoto 1-1, Marumemachi, Nakagawa-ku, Nagoya-shi, Aichi Prefecture Miyazaki Seiko Co., Ltd.

Claims (5)

[Claims] 1. 1 g of thiourea as a descaling accelerator
A descaling pickling solution for a steel material comprising sulfuric acid or an acid mainly composed of sulfuric acid, characterized by containing at least / L. 2. 1 g of thiourea as a descaling accelerator
/ L or more, and a descaling pickling solution for a steel material comprising sulfuric acid or an acid mainly composed of sulfuric acid, further comprising a pickling inhibitor. 3. The descaling pickling solution for steel according to claim 2, wherein the pickling inhibitor is at least one of the following thiourea derivatives: R ₁ , R ₂ : alkyl group having 1 to 5 carbon atoms or phenyl group R ₃ : alkylene group having 2 to 5 carbon atoms or phenylene group. 4. The steel material according to claim 2, wherein the pickling liquid contains at least one of oxycarboxylic acid, iminocarboxylic acid and a salt thereof as an auxiliary agent for promoting descaling. Descaled pickling solution. 5. A method for descaling a steel material using the descaling solution according to any one of claims 1 to 4.