RU1786129C - Method of decarburizing annealing of cold-rolled low-carbon steel - Google Patents

Abstract

Usage: production of cold rolled low carbon steel sheet for enameling. The inventive steel is heated in a gas atmosphere to annealing temperatures in three stages: first up to 500 ° C at a speed of 15-25 ° C / s, then to a temperature T, determined by the ratio of T (920-3 103% C) ± 20 ° C, where C is the initial carbon content, at a rate of 5-10 ° C / s, further arbitrary. After exposure to the annealing temperature, the steel is cooled. 1 tab.

Description

The invention relates to metallurgy, in particular to the production of cold rolled steel sheets with a particularly low content. burning carbon, intended primarily for enameling without preliminary forming (stamping). Requirements for the enamel strip: no enamel defects (bubbles, shells, pores, fish scales), minimal warping during firing, good adhesion. One of the main factors ensuring the fulfillment of these requirements is a fairly low (0.01%) carbon before enameling. With an increased carbon content in steel, gaseous products are formed at the enamel-metal interface - CH4, H2, CO, Cp2, which cause enamel defects. In addition, carbon lowers the temperature of the y-α transformation, as a result of which austenite appears in the steel structure at an enamel burning temperature above 800 ° C. Since the solubility of hydrogen in austenite is higher than in ferrite, there is a greater risk of fish scales. On the other hand, with subsequent cooling, a transformation occurs, accompanied by volumetric changes causing warpage.

It is possible to get two extremely low carbon in steel, two fundamentally different; by means of decarburization in a liquid state, during smelting / or upon annealing of mountains of a rolled or cold rolled strip. The first way requires special measures to prevent the enrichment of the metal with oxygen, which complicates the process. In addition, the structural and textural characteristics of the sheet are deteriorated due to hot rolling in the single-phase region. The second path is devoid of these drawbacks. The vast majority of related methods include decarburization annealing.

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bosoms in batch furnaces, with slow heating and cooling and. slow shutter speed. As a rule, the temperature of such annealing is 700 ° С, the atmosphere is nitrogen gas with 18-20% Н2 and oxidation potential Рн2О / Рн2 0.2. At a high initial carbon content of 0.12-0.20%, it is recommended that decarburization be carried out in the temperature region between the critical points of AI and Az, so that at least 30% of austenite is present in the structure before decarburization begins to produce sufficiently large grains in the annealed steel.

A disadvantage of the methods listed and similar to them, in addition to low productivity, is the significant heterogeneity of the structure and mechanical properties along the length and width of the strip, as well as the enlargement of the grain. The indicated drawbacks lack decarburization annealing of the strip in a continuous unit, with a short-term (up to 10 min) stay at elevated temperatures. In the patent, a cold-rolled strip for shadow tubes of a picture tube with a thickness of 0.2 mm is recommended to be decarburized at a temperature of 650-850 ° C. However, for thicker steel (0.35-0.50 mm), this mode does not provide deep decarburization (0.01%) in less than 10 minutes. As experiments have shown, the main disadvantage of this method is that it does not regulate the heating rate. : The closest in purpose and its technical implementation to the proposed invention is a method of decarburizing annealing in a continuous furnace, including heating and holding at a temperature between 650 ° C and Az for 20-300 s. For examples illustrating the method, a strip 0.5 mm thick was heated for 60 s at a temperature of 900–920 ° C, i.e. at a sufficiently high speed, although the latter is not regulated in the patent. The method extends to low carbon (0.08% C) steels with the addition of 0.02-0.07% AI. When using it for processing steel without aluminum with 0.04-0.05% C, a coarse-grained uneven structure was obtained (4th ferrite grain number), while in steel with higher carbon (0.06-0.09%) at coarse surface grains did not achieve deep (0.01% C) decarburization. As experiments have shown, the main reason for grain growth in non-aluminum steel is rapid heating. In steel with aluminum, dispersed AIN particles are formed that inhibit

(up to a certain temperature) collective recrystallization processes.

The purpose of the invention is a deep (0.01%) decarburization while maintaining a sufficiently small (5-7th number) homogeneous ferrite grain during short-term (10 min) annealing - is achieved by slowing down to 5-10 ° C / s heating in the flow area recrystallization - from 500 ° C to

temperature T, determined depending on the initial content of C by the equation of T (920-3 10-3% C) ± 20 ° C. Heating to 500 ° C is carried out at a sufficiently high rate of 15-25 ° C / s. The direct decarburization process is carried out at a temperature of 800 ± 20 ° C. From Tn to this temperature, the strip is heated at an arbitrary speed.

Therefore, the main difference

The proposed method from the method of the type is a decarburization annealing with step heating, slowed down in the recrystallization region, from 500 ° C to a temperature T, depending on

carbon source.

Common features of the known and claimed solutions are:.

1) carrying out decarburization annealing in a continuous unit with short-term heating and holding;

2) the temperature of the decarburization annealing in the interval between 650 ° C and the point Az. ..-.-.-..-.

The combination of known and distinctive features of the proposed technical solution provides a positive effect indicated in the purpose of the invention, which allows us to conclude that the claimed solution meets the criteria of the invention for novelty, positive effect and significant differences.

The established mode is based on the following physical premises. With sufficiently fast heating of the steel to the intercritical region, between points Ai and Az (as provided for in the prototype method), decarburization begins simultaneously with recrystallization and, in the absence of AI in steel, is accompanied by the appearance of columnar grains due to the ingrowth of ferrite into austenite. The formation of large surface grains worsens the quality of the enamel, and the presence of a large number of austenitic regions (with an initial carbon of more than 0.05%) inhibits the decarburization process. Slowing down the heating avoids these undesirable effects. Before decarburization, ferrite recrystallization processes undergo and

partial dissolution of cementite in it, as a result of which further heating in the intercritical region produces less austenite and intensifies the decisive removal of carbon diffusion. The upper limit of the recommended range of heating rates above 500 ° C-10 ° C / s is precisely determined by the possibility of a sufficiently complete course of these processes. The existence of a lower limit of 5 ° C / e is associated with the danger of decreasing furnace productivity.

A high heating rate to 500 ° C of 15-25 ° C / s is associated with the need to suppress the return and polygonization processes that inhibit subsequent recrystallization. Since the biphasic region expands and the amount of austenite increases at a given heating temperature with increasing initial carbon content, the latter must be reduced to ensure complete decarburization.

A decrease in the amount of austenite and a partial course of decarburization in the ferrite region also contributes to

the formation of finer grains in decarburized steel - due to a less pronounced process of growing ferrite grains into austenite. In addition, recrystallized ferrite grains formed upon slow heating are less prone to growth.

The choice of the temperature of decarburizing, annealing (holding) - 800 ° С is determined, on the one hand, by the need for a sufficiently high diffusion rate, and, on the other, by preventing grain growth.

The present invention extends to low carbon steels with 0.04-0.10% C, boiling, semi-quiet or deoxidized by aluminum, manufactured by hot and cold rolling in the form of rolls and sheets for enameling or for use in picture tubes.

The proposed method and the prototype method were tested on steel x two industrial swimming trunks of the following chemical composition (%):

Both melts were melted in a converter, rolled on a sling and 2300/1700 mill to a strip 2.5 mm thick at temperatures of the end of rolling 840-880 ° С and winding 650-680 ° С.

After cold rolling to a thickness of 0.5 and 0.35 mm, the steel was decarburized annealed in a horizontal furnace at temperatures in the holding chamber of 800 ° C and in the heating chamber T, which was set depending on the carbon content in the steel according to the invention. The heating rates were regulated (Vi - up to 500 ° С and V2 - from 500 ° С to Тn) by selecting the transportation speed taking into account the thickness of the processed strip. From T to 800 ° C, the strip was heated at an arbitrary rate. The modes deviating from the proposed method and corresponding to the prototype method were also tested. The composition of the atmosphere is 20% At + 80% N2, dew point + 30 ° C.

Annealing conditions and the results are presented in the table.

As can be seen from the presented results, made according to the proposed

In the method, steel differs in both studied thicknesses by a low carbon content and relatively fine grain uniform in cross section, while processing according to

the prototype method and the modes somewhat deviated from the present proposal does not ensure the achievement of the goal.

The proposed method can be implemented in existing units of continuous annealing tower or horizontal type.

Claims (1)

SUMMARY OF THE INVENTION Decarburization Annealing Method low-carbon cold-rolled steel, mainly for enameling, including controlled heating in a gas atmosphere to a predetermined temperature, holding and cooling, characterized in that, in order to intensify decarburization and obtain a fine-grained structure, the heating is carried out in three stages; first up to 500 ° C at a rate of 15-25 ° C / s, then up to the temperature Tn, determined by the ratio of Tn (920-3 103% C) ± 20 ° C, where% C is the initial carbon content, at a rate of 5-10 ° C / s, then arbitrary. 8 numerator - in the surface layer, in the denominator - in the central areas Table continuation