The effect of chemical composition on grain structure and texture evolution of hot rough rolled carbon steels

Abstract

Combination of thin slab casting and direct hot rolling to obtain thin steel plate products is an energy efficient process in commercial steel plants. Generally, cracking near the surface is the most serious problem especially during rough rolling of as cast thin slabs. Therefore, it is highly recommended that the microstructure near the surface should be controlled to have enough ductility to withstand the strain. In this study the effect of chemical composition on microstructure evolution and texture has been investigated for the steel plates rough rolled from 70 to 25mm at a commercial plant. In low carbon content of 0.05wt%, the micrograph was seen to comprise mainly ferrites with minor pearlite islands which changed to ferrite–pearlite structure with increasing carbon to 0.17wt%. A detailed investigation was carried out to examine the behavior of the grain structure formed in rough rolled steels, using high resolution SEM microscope fitted with EBSD camera. In all investigated steel plates, a fine grain structure was observed in the plate top surface due to development of rapid static recrystallization after rough rolling which gradually coarsened in mid-section region. Investigation was carried out to understand the effect of alloying elements such as Mn on the texture. Mainly fiber textures of both ND and RD directions were observed after transformation showing inhomogeneity with increasing distance from the plate surface towards the depth. Massive ferrite (bcc) grains transforming from austenite (fcc) has been detected during the rough rolling process. Therefore, tensile test was carried out at 600°C in order to investigate the failure mechanism of ferrite as the work roll chilling has a significant effect on temperature reduction in the plate surface which may lead to lower ductility of massive ferrites and thus formation of crack at the surface during rough rolling process.