Metallographic study on the structural features of thin slab and rolled products made from it

Abstract

The authors determined the level of zonal and dendritic segregation in slabs poured by thin-slab technology. The calculated variation coefficients of the content of basic and impurity chemical elements over the slab cross-section do not exceed 10 %, zonal segregation is low. The manganese content measured with the area occupied by the dendritic axes and the interstitial spaces showed the level of dendritic segregation. The manganese concentration varies from 0.6 to 1.1 %, respectively. It was established that the use of dynamic soft compression during solidification makes it possible to grind the primary dendritic structure to form additional centers during the phase transformation of δ-ferrite into austenite. Dimensions of the initial austenitic grains formed taking into account the primary dendritic structure are 3 times smaller in a thin slab than in a slab with a thickness of more than 200 mm. Transformations of the dendritic structure during compression show high workability necessary for the formation of uniform austenitic grains in the fullering before finishing rolling. The study has not confirmed the hypothesis that bainite of coarse morphology in the microstructure of hot-rolled products is formed in segregation areas. The hereditary influence of the primary dendritic structure on the structure formation during rolling was revealed. The manganese concentration varies between the bainite and the “neighboring” structure from 0.68 to 1.01 %, similar to the level of the initial dendritic segregation. Difference in the content of chemical elements affects the processes of recrystallization of austenitic grains during high-temperature rough rolling. Bainite was formed within the framework of chemically “depleted” large austenitic grains that are stable during phase transformation.