Experimental Analysis on the Causes of the Breakout of Continuous Cast Steel Slab

The paper deals with investigation into segregation behavior of selected elements in longitudinal cut of continuous steel slab, in the breakout area. The breakout occurred after a flying change of tundish in order to begin casting of another steel grade. Altogether 11 samples were taken from the part of a solidified slab. Concentrations of selected elements (Al, Si, P, S, Cr, Mn, Ni and Mo) were measured in these samples using scanning electron microscope and energy dispersive spectroscopy. Using the original mathematical models the basic micro-segregation characteristics and the parameter of macro-heterogeneity were further determined for each analyzed element. Then a quantitative measurement of inclusions ("micro-purity") in the samples was performed using a metallographic microscope. Then method of differential thermal analysis was used for the measurements of temperatures of phase transformations. The following main results were found: - magnitude of micro-segregation of the analyzed elements in the measured sections of 1000 μm is approximately the same in all the analyzed samples, - chemical macro-heterogeneity is very high across the analyzed slab section, - very uneven mixing of melts of both steels was probably one of the main causes of formation of the breakout.

Micro-Macro Modeling of Continuous Cast Steel to Simulate the Effect of Casting Velocity and Pouring Temperature on Copper Segregation

One of the most serious problems in using recycled scrap for steel production is the occurrence of surface hot shortness during hot deformation due to the presence of copper in scrap. Copper causes surface hot shortness by liquid embrittlement. Therefore, the amount of the liquid copper-enriched phase penetrating into the grain boundaries should be identified in order to keep its effects within acceptable limits. In this regard, understanding the mechanism of segregation during solidification of steel is essential. This paper attempts to demonstrate micro-macro modeling of continuous cast steel to simulate the effect of casting velocity and pouring temperature on copper segregation. First, the temperature profiles at different times were determined using a finite element (macro) model and a segregation (micro) model based on Giovanola-Kurz approach is coupled with the macro model. It is necessary to couple the heat flow calculations from the macro model and the segregation calculations from the micro model because the rate of latent heat liberation is affected by the solute redistribution process, while the solute redistribution process depends on the cooling rates. As the casting velocity and pouring temperature strongly affect the solidification of continuous cast steel, they were chosen as the variables in this study. It is hoped that this study will enable the optimization of these variable to minimize the segregation of copper. This paper also demonstrates that by decreasing casting velocity and pouring temperature of a steel billet during continuous casting, solid-liquid interface moves faster to the center of the billet and there is less chance for diffusion of the residual elements in steel. Therefore, the chance of hot shortness of copper in the steel increases.