This article examines the warming of steel in a tundish during the continuous casting of grades of steel that are used in critical applications. The authors examine how the introduction of an organic addition to the powdered slag-forming mixture affects its thermal conductivity. Quantitative relations are obtained to describe the dependence of the thermal conductivity and bulk density of the mixture on its content of rice husks. A comparative analysis of the warming properties of heat-insulating mixtures used at the leading Russian combines and powdered assimilating mixtures containing rice husks shows that the latter can be used without additional insulation.