KAVIČKA František Vysoké učení technické v Brně

Spoluautoři STRÁNSKÝ Karel, DOBROVSKÁ Jana, SEKANINA Bohumil, ŠTĚTINA Josef, MASARIK Miloš

The so called secondary cooling zone, which is divided into thirteen sections is an important part of the CCM. In this zone, specifically in the area of the beginning of slabs straightening a breakout may occur due to an increase of the local and temperature heterogeneity of steel, due to an increase of the stress caused by bending of the slab and by high local concentration of non-metallic slag inclusions. Changes of the chemical composition of the steel during continuous casting are particularly dangerous if the melt of steel with chemical composition A ends and it is immediately followed by the steel B, it may an atypical breakout may take place and thereby automatically stop the CCM. It happened during continuous casting of the slab 250x1530 mm in the area of straightening 20 minutes after flying change of tundish. With use of dimensional analysis altogether 8 criteria of similarity were derived according to the π-theorem. Table of 12 technological, geometrical and thermo-physical dimensional quantities was compiled for these criteria, which characterise both steel grades A and B and process of their continuous casting. Thermo-physical properties of both steels for the table were determined with use of commercial software IDS. Important on temperature field of the slab, such as maximum and minimum length of iso-liquidus and iso-solidus, range so called mushy zone and zone of melt at the place of breakout, temperature of the slab surface, etc. were calculated by the original model. Five of these criteria are at the same time functions of thermo-physical parameters, but also of technological and geometrical parameters. Numerical values of eight criteria were determined for the steels A and B and then standard operations were made with the criteria. This application of the theory of physical similarity clearly proved markedly increased tendency of the steel B to breakouts in comparison with steel A.In order to prevent repetition of this accident of CCM (caster) for another pair of steels cast immediately one after another, it is necessary to assess the derived individual criteria of similarity for both steels and other operations with these criteria. The way to reduce the risk of breakouts may be found mainly in the change of thermo-physical properties of both steels, consisting primarily of reduction of differences of chemical composition of both melts, so that partial shares of similarity criteria were close to the value of one.

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