The effect of C on hot ductility of low alloy steels has been studied in view of surface cracking of continuously cast (CC) slabs. As the ductility was not affected by C content in hot tensile test of the reheated specimens, the well-known C dependency of surface cracking susceptibility in CC slabs can be ascribed to the microstructural change during the solidification process. Austenite grain size of as-cast materials was found to depend largely on C content, i.e., the maximum grain size in 0.10-0.15% C region. This can be explained by the higher austenite formation temperature in these C region. Austenite grain growth rapidly occurred after the complete transformation or solidification into γ phase, as the strong pinning effect of the second phase such as δ-ferrite or liquid phase on γ grain boundary migration was relieved. Carbon dependency of γ grain size became more marked with increasing cooling rate up to that of ordinary continuous casting.
Such coarsening of γ grains enhanced intergranular fracture, resulting in ductility loss inversely proportional to the γ grain size. Uneven surface solidification in the mold due to the peritectic reaction will produce much coarse γ structure because of the local delay of cooling. Surface cracking susceptibility will also be largely accelerated by this mechanism. Carbon range where surface cracking susceptibility was the largest varied with the chemical compositions. This shift can be explained in terms of the effect of alloying elements such as Mn on the peritectic composition.