Zone Analysis of Atypical Breakout during Radial Continuous Casting of a Slab in the Straightening Zone II. Analysis of the Causes of the Breakout by Application of the Theory of Physical Similarity

journal article - other

English

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 resulting from 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. In the event that two melts are cast one right after another, i.e. if the melt of steel with chemical composition A ends and it is immediately followed by the steel B, it may automatically stop the CCM and an atypical breakout may take place. 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 Pi-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 data in temperature field of the slab, such as maximum and minimum length of iso-liquidus and iso-solidus, range of the 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, such as evaluation of partial shares of pairs of the same criteria for both steels. This application of the theory of physical similarity clearly proved markedly increased tendency of the steel B to breakouts in comparison with the 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.

Steel slab; Continuous casting; Chemical and temperature heterogeneity; Breakout; Dimensionless number.

STRÁNSKÝ, K.; KAVIČKA, F.; DOBROVSKÁ, J.; DOBROVSKÁ, V.; SEKANINA, B.; ŠTĚTINA, J.

2013

4. 7. 2013

0018-8069

Hutnické listy

LXVI/2013

3

Czech Republic

17

25

9