RU2248412C1 - Method for continuous etching of low-carbon hot-rolled strip steel - Google Patents

Abstract

FIELD: continuous treatment of metal surface, in particular etching of low-carbon steel band in deep bath.

SUBSTANCE: claimed method includes band passing through bath of etching aggregate, divided onto four sections with hydrochloric acid solutions, wherein acid concentration is subsequently increased, and iron concentration is subsequently decreased. Low-carbon steel containing 0.05-0.11 vol.% of carbon is etched at temperature subsequently elevated from first to forth section according to equation: t = 5*(n+10)⁰C, wherein n is a section number.

EFFECT: etching aggregate of improved capability due to increased stimulation etching process; steel without underetching defects.

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Description

The present invention relates to a method for continuous surface treatment, in particular etching of a steel strip of low-carbon steel grades in deep baths, consisting of several compartments located one after another, filled with an etching solution containing hydrochloric acid through which the strip is drawn. The pickling solution is heated through heat exchangers by means of a steam supply. The fresh pickling solution is supplied in the last compartment along the strip, that is, against the strip.

A known method of continuous etching of steel strips in flat baths with turbulent movement of hydrochloric acid. The strip is moved approximately along a horizontal path through the etching bath into which the etching solution is supplied, and the surface to be treated is subjected to the etching solution flow at an increased speed so that a fresh etching solution is fed in the zone of creating an increased flow rate, forming its turbulence (see RF patent No. 2168562, class C 23 G 3/02, publ. 10/20/1997, bull. No. 29).

A known method for the continuous etching of strips of hot rolled tin in flat baths, comprising passing the strip through several adjacent one after another, filled with acid etching solution of bathtubs and supplying a hydrochloric acid solution through nozzles located on the side of the bathtub (see RF patent No. 21115768, class C 23 G, publ. . 20.07.1998, bull. No. 20).

A disadvantage of the known etching methods is a sufficiently high etching temperature for hydrochloric acid (65-85 ° C), which increases the volatility and aggressiveness of hydrochloric acid, which requires the installation of perfect ventilation and etching and washing baths from expensive materials.

The closest analogue to the claimed object is a method of etching rolled metal in deep baths, described in the book of V. D. Gurenko et al. “Etching of strips and sheets in hydrochloric acid”, M., Metallurgizdat, 1971, p. 83.

This method involves the sequential passage of the rolled metal in a deep bath, consisting of four compartments, with a freely hanging loop, and is characterized by a sequential decrease in the temperature of solutions from the first to the fourth compartment of the bath in the range from 65 to 55 ° C (1 compartment - 65 ° C, 2 compartment - 60 ° С, 3 compartment - 55 ° С, 4 compartment - 55 ° С) and the following concentration distribution in the compartments (1 compartment - НСl - 56 g / l, Fe - 110 g / l, 2 compartment - НСl - 102 g / l, Fe - 75 g / l, 3 compartment - HCl - 155 g / l, Fe - 35 g / l, 4 compartment - HCl - 191 g / l, Fe - 7 g / l). The pickling unit works with the processing of solutions in the regeneration unit.

In the known pickling unit, due to the successive decrease in the temperature of the pickling solutions from the first to the fourth compartment of the pickling bath, the pickling time does not reach a minimum.

The basis of the invention includes the task of reducing the time required for the etching process, and, therefore, increasing the productivity of a continuous unit. To this end, maintaining a sufficient acid content and solution temperature in the contact zone between the strip surface and the pickling solution is of particular importance. Moreover, the basis of the invention is the information obtained as a result of many years of operation of such units, according to which the etching process does not occur in the first compartment of the bath, it performs the function of heating the metal. The bulk of the scale is etched in the next three compartments, and the reaction rate between the scale layer and the pickling solution depends on the quality of the material, the thickness and type of the scale, the concentration of acid and iron in the pickling solution and the temperature of the solution. By increasing the temperature of the etching solution from the first to the fourth compartment with a successive increase in the acid concentration and a decrease in the iron concentration, the rate of interaction of the scale with the acid increases, thereby minimizing the etching time and, consequently, increasing the productivity of the continuously etching unit. A constant acid and iron content in the contact zone between the strip surface and the pickling solution is maintained by supplying fresh solution to the fourth compartment of the pickling bath.

To solve this problem, in a method involving the passage of strips through the bath of the pickling unit, divided into separate compartments with different temperatures of the hydrochloric acid pickling solution in them, when pickling low-carbon steel with a carbon content in the range from 0.05% to 0.14%, this temperature is sequentially increase from the first to the fourth compartment in the direction of the strip, taking it equal to t = 5 * (n + 10) ° С, where n is the number of the compartment with the following concentration distribution over the compartments (compartment No. 1 - НСl - not less than 50 g / l , Fe - not more than 150 g / l, o tsec No. 2 - НСl - not less than 80 g / l, Fe - not more than 120 g / l, compartment №3 - НСl - not less than 110 g / l, Fe - not more than 100 g / l, compartment №4 - Нсl - not less than 140 g / l, Fe - not more than 60 g / l). The pickling unit works with the processing of solutions in the regeneration unit and the supply of fresh solution to the fourth compartment of the pickling bath.

The essence of the proposed technical solution is to optimize the etching process of strip low-carbon steel containing carbon from 0.05% to 0.14% by changing the temperature of the solution in the bath compartments with increasing it from the first to the fourth compartment, which provides the conditions for the best and fastest descaling by intensification of the process.

The claimed method is implemented as follows. Before etching low-carbon steels of grades 08kp, 08ps, 10ps, 10kp, 08yu, St 2-4, the temperature of the etching solutions in the compartments is set (No. 1 - 55 ± 1 ° С, No. 2 - 60 ± 1 ° С, No. 3 - 65 ± 1 ° C, No. 4 - 70 ± 1 ° C) by appropriate steam heating. Then they start etching with a maximum speed of passage of the strip through the unit, for example, 2.5 m / s.

An experimental verification of the proposed method was carried out in sheet rolling shop No. 8 of OJSC “Magnitogorsk Iron and Steel Works”. For this purpose, when etching hot-rolled strip steel of low-carbon grades with a carbon content in the range of 0.05-0.14%, the temperature of the etching solutions and the etching rate varied from 2.0 m / s to 2.5 m / s, fixing the presence of defects associated with etching (under-etching). The best results (maximum speed in the absence of a defect "undergraduate") were obtained using the proposed method.

The inventive temperature of the etching solutions with a sequential increase from the first to the fourth compartment of the etching bath, taking it equal to t = 5 * (n + 10) ° C, where n is the number of the compartment, is optimal. The speed of etching of the strip metal corresponded to the maximum possible speed of passage of the strip through the unit (2.5 m / s) in the absence of the defect “undergrav”.

A successive decrease in temperature from the first to the fourth compartment of the bathtub according to the technology taken as the closest analogue in the temperature range of the compartments, ° С, No. 1 - 65, No. 2 - 60, No. 3 - 55, No. 4 - 55, leads to undergrazing in the form of dark spots at the front and rear ends of the strips and a decrease in the etching rate by 20-30%.

Thus, the pilot test confirmed the acceptability of the claimed object to solve the problem and its advantages over the known technology.

According to the Central Control Laboratory of OJSC MMK, the use of the technical solution found in the production of strip low-carbon steel will increase the unit's productivity and, accordingly, the profit from sales of products by at least 2%.

An example of a specific implementation.

Pickling of rolled metal of 08ps low-carbon steel with a carbon content of 0.05-0.11% 2.8 mm thick, 475 mm wide is carried out with a sequential increase in temperature from the first to the fourth compartment of the etching bath, ° С, No. 1 - 56, No. 2 - 60, No. 3 64, No. 4 - 70, containing (No. 1 - НСl - 50 g / l, Fe - 120 g / l, No. 2 НСl - 100 g / l, Fe - 80 g / l, No. 3 - Hcl - 150 g / l, Fe - 45 g / l, No. 4 - Hcl - 180 g / l, Fe - 35 g / l). The strip metal etching rate was 2.5 m / s in the absence of a “under-grass” defect.

Claims (1)

A method for continuously etching hot rolled strip low-carbon steel, including passing strips through the bath of the pickling unit, divided into four compartments with hydrochloric acid solutions with a sequential increase in the concentration of acid and a decrease in the concentration of iron in the solution, characterized in that when etching mild steel with a carbon content within 0.05-0.14 vol.% The temperature of the hydrochloric acid solution is successively increased from the first to the fourth compartment along the strip, taking the pace perature equal solution t = 5 · (n + 10) ° C, where n - number of compartments.