Behavior of carbon and nitrogen during the continuous casting of IF-type steels with the use of different slag-forming compositions

In attempting to restrain the increase in the carbon content of IF steels during their casting, two of the key factors to be considered are lowering the carbon content of the charge materials and selecting the correct processes for forming the slag in the tundish and the mold. Optimizing slag formation in turn better protects the steel from secondary oxidation and results in a smaller increase in the steel’s nitrogen content. Optimization of the use of slag-forming mixtures has allowed steelmakers to reduce the increase in the carbon and nitrogen contents of the steel by 20%. The results of the study reported here have made it possible to improve the IF steel production method used in the continuous-casting section of the oxygen converter shop at the Severstal’ Metallurgical Combine.     

Isotherm velocity in the axial zone of a continuously cast steel ingot

The behavior of the isotherm velocity in a continuously cast solidifying steel ingot is analyzed. An expression is obtained to describe the accelerated motion of isotherms near the thermal center of a cooled body (the center of symmetry of the body) without a phase transition. In the axial zone of the solidifying ingot, where the thermal overheating is insignificant, the isotherm velocity is caused by the action of two opposing factors: (i) the accelerated motion of isotherms that is characteristic of bodies cooled without a phase transition when the thermal center is approached and (ii) the release of the heat of phase transition. As a result, in the axial zone of an ingot made of a low-carbon steel (the initial carbon concentration C ₀ ≤ 0.2%), the liquidus isotherm velocity is almost constant, whereas the isotherm motion at the end of solidification is sharply accelerated, as in the case of purely thermal cooling. For high-carbon steels (C ₀ ≥ 1.0% C), the liquidus isotherm velocity increases, and the velocity of the isotherm at the end of solidification is constant (the effect of the eutectic transformation manifests itself). As a consequence, in low-carbon steels, the pool calculated from the liquidus and pouring-boundary isotherms has the shape of an acute-angled wedge, and the pool calculated from the isotherm of the end of solidification has a rounded shape. In contrast, in high-carbon steels, the pool shape calculated from the liquidus and pouring-boundary isotherms is rounded, and the pool shape calculated from the isotherm of the end of solidification is wedgelike. As a result of the analysis, a mathematical procedure is proposed for the calculation of the isotherm velocity in the two-phase zone and the shape and position of the pool bottom (from the corresponding isotherms) in a continuously cast solidifying steel ingot.