SU1723156A1 - Process for producing aluminum-deoxidized cold-rolled sheet steel - Google Patents

Abstract

This invention relates to the field of metallurgy, to methods of annealing cold-rolled strip of low carbon steel. The purpose of the invention is to improve stampable ™ by increasing the proportion of favorable texture orientations. The invention includes the following operations: hot slab rolling, pickling, cold rolling and annealing in a bell furnace. Moreover, the heating rate under annealing varies depending on the carbon content in the steel. When the carbon content is not more than 0.02%, it is set to 60-80 ° C / h. and with an increase in carbon content by every 0.02%, the heating rate decreases by 20-25 ° C / h. The invention allows to increase the coefficient of normal plastic anisotropy to 1.78-1.95. 2 tabl .. cl C

Description

The invention relates to ferrous metallurgy, in particular to the production of sheet metal, and improves the process of heat treatment of cold-rolled aluminum type 08U steel.

The purpose of the invention is to improve stamping capacity by increasing the proportion of favorable texture orientations.

The proposed method for the production of aluminum-cold-rolled steel sheets is carried out as follows.

In a hot rolling mill strip, slabs of 08U steel with a rolling end temperature of 860-880 ° C are rolled. Hot rolled strips are wound into rolls at 560-620 ° C, cooled to 40-60 ° C and etched to remove scale. Etched rolls are rolled on

cold rolling mill with a total degree of reduction and transferred to the thermal compartment of the bell-type furnaces for annealing. After the formation of the coil foot t on the bench, based on the carbon content in the blast furnace steel, the rate of heating of the metal to the annealing temperature is determined. In determining the heating rate, the initial minimum carbon content of 0.02% is subtracted from the percentage of carbon in this melt and the difference is attributed to the initial minimum carbon content. The difference between the maximum heating rate (75-80 ° C / h) and the product relative to the carbon content by the amount of decrease in the heating rate (20-25 C / h) is the necessary heating rate to the annealing temperature for the coils of this melting. Original

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The minimum carbon content of 0.02% was chosen based on the current level of development of steelmaking production. Steel with a lower carbon content is almost impossible to obtain without special types of thermochemical processing.

An example of a specific determination of the heating rate. The carbon content in steel is 0.06%. Relative content

0.06 - 0.02 p. Carbon equal to GG) 2. Speed

heating is (75-80) - 2 (20-25) 25-40 ° C / h.

The method involves heating to the annealing temperature both with intermediate exposure at 550 ° C and without it.

After annealing at 650–700 ° C, the coils are cooled and subjected to further technological operations: training and cutting.

The improvement of stamped ™ finished sheets using the proposed method is achieved by changing the heating rate of the metal to the annealing temperature depending on the carbon content in the steel. In steel type 08U, in order to obtain elongated grains with predominant orientation, which determine high forging properties, in the annealing process, it is necessary that aluminum nitrides precipitate before the recrystallization begins. In this case, the completeness and rate of release of nitrides (AIM) are significantly affected by the total amount of carbon, the shape and dispersion of carbide inclusions. The more dispersible the carbide inclusions isolated from the solid ferrite solution, the more complete the process of separating aluminum nitrides. In the case of carbide inclusions enlarged or joined into chains, the number of nitrides is slowed down, the number of ferrite grains with orientation {III} decreases. The processes of nitride isolation, spheroidization and coagulation of carbides occur simultaneously in the annealing process. Moreover, the process of precipitation of nitrides can occur in the process of cooling hot rolled strips simultaneously with the pressure of carbides. If the cooling rate of the bands is low, then the AIN ferrite stable in the ferrite is released, which further leads to the formation of grains with an unfavorable orientation of {110} and {100} grains during annealing. Therefore, the 08U steel production technology provides for the accelerated cooling of hot rolled strips, preventing the precipitation of aluminum nitrides and ensuring the formation of

fine carbides. However, it is not possible to completely prevent the release of carbides. It has been established that if a significant part of the carbides precipitated during the slow cooling of hot rolled strips, during the annealing process their rate of release and coagulation slow down.

The amount of carbides released during the cooling process depends on the carbon content of the steel. With a content in the steel of 0.02%, the volume of selected carbides is insignificant, they are finely dispersed, and the process of separating aluminum nitrides is practically not difficult and is possible at any heating rate during the annealing process. With increasing carbon content in steel, the number and size of carbide inclusions increase, carbon diffusion makes nitride formation difficult and, accordingly, the release rate of aluminum nitrides decreases. Therefore, in the process of coil annealing, slowing down the heating rate is necessary to ensure priority and possibly more complete release of aluminum nitrides on the crystallization annealing stage.

The relationship between the heating rate, the carbon content in steel, and the stampability criteria (ov, 5 / i, and R) are given in Table 1.

From the data presented in Table 1. it follows that to improve the stampability of the finished cold-rolled sheets, i.e. to obtain, 6 and (possibly by reducing the heating rate with an increase in the carbon content in the steel. At the same time, for every 0.02% increase in carbon, the heating rate should be reduced by 20-25 ° C / h. The decrease in the heating rate by less than 20 ° C / h leads to a decrease in the normal plastic anisotropy coefficient, 6 Reducing the heating rate by more than 25 ° C / h is not advisable, since this reduces the performance of bell-type furnaces, and the stampability criteria do not significantly increase.

The proposed method of producing aluminum-deoxidized cold-rolled steel sheet. Tested under industrial conditions. For this purpose, we took two smelting slabs of Yu8Yu steel with the initial content of carbon in each Ci 0.04%; C2 0.06%, Could be heated in a furnace up to 1260 and rolled in a hot rolling mill to a cavity size of 3.2 1340 mm. The end temperature of the rolling was 880 ° C, the coiling temperature 620 ° C. Horus churned rolls cooled to

60 ° C, pickled and sent to the cold rolling mill, where they were rolled with a total reduction ratio of 71%, i.e. Got cold-rolled strip, 1.0 mm thick. After rolling, the rolls were transferred to the thermal compartment of the bell-type furnaces for annealing. Moreover, 1/2 of each heat was annealed using a known method, and the second part of each heat was annealed by the proposed method.

The mode of annealing of the metal by known annealing for both floats included heating at a rate of 75 ° C / h to 550 ° C. holding at this temperature is 18 h, heating at a rate of 70 ° C / h to the annealing temperature and holding for 20 h.

During annealing of cold-rolled steel with a carbon content of 0.04%, the heating rate to 550 ° C was 60 ° C / h, holding at 550 ° C for 16 h. To 690 ° C, the heating rate was equal to 55 ° C / h, holding at this temperature 19 hours

When annealing steel with a carbon content of 0.06%, the heating rate to 550 ° C was set to 35 ° C / h, to 690 ° C – 40 ° C / h. Accordingly, exposures at these temperatures were 9 and 18 h.

The annealed rolls were cooled, pressed, cut, and an experienced stamping of parts of a particularly complex stretch was made.

Table 2 shows the test data of cold-rolled sheet, processed by the known and proposed methods.

Data analysis of Table 2 shows that the use of the proposed method allows to significantly improve the stamping properties of the finished sheets. The plastic anisotropy coefficient is consistently above 1.6. The yield for stamping products is 96.5-97.4%, i.e. higher than when using the known method on average by 4-6%.

Thus, the use of the proposed method for the production of cold-rolled aluminum sheet deoxidized by aluminum in comparison with the prototype makes it possible, by ensuring sufficient dispersion of carbide inclusions and increasing the proportion of favorable texture orientations, to improve the stampability of steel. As a result, the yield for stamping products is increased by 4-6%.

Claims (1)

Claims The method for producing aluminum-deoxidized cold-rolled sheet steel, including hot slab rolling, pickling, cold rolling and annealing in a bell furnace, is also different in that with the aim of improving the stampability by increasing the proportion of favorable texture orientations} warming. Annealing with carbon content in steel no more than 0.02% is conducted at a rate of 60-80 ° C / h, and with an increase in carbon content for every 0.02%, the heating rate is reduced by 20-25 ° C / h. Table 1  Numbers, yield strength, denominator - coefficient. M Relative lengthening, t. Table 2