Influence of Carbon Content on Toughening in Ultrafine Elongated Grain Structure Steels

(0.2-0.6)%C-2%Si-1%Cr-1%Mo steels were quenched and tempered at 773 K and deformed by multipass caliber rolling (i.e. warm tempforming) with a rolling reduction of 78%, in order to obtain ultrafine elongated grain (UFEG) structures. The tensile and Charpy impact properties of the warm tempformed (TF) steels were investigated to determine the influence of the carbon content on toughening in the UFEG structures. The TF samples consisted of UFEG structures with strong <110>//rolling direction (RD) fiber textures. The transverse grain size and aspect ratio in the UFEG structure tended to reduce as the carbon content increased, whilst the carbide particle size became slightly larger. The increase in the carbon content resulted in an increase in the yield strength from 1.68 to 1.95 GPa at room temperature; however, it was accompanied by a loss of tensile ductility. In contrast to quenched and tempered samples exhibiting ductile-to-brittle transitions, the TF samples exhibited inverse temperature dependences of the impact toughness. This was due to delaminations, where cracks were observed to branch in the longitudinal direction (//RD) of the impact test bars. The upper-shelf energy of the TF sample was enhanced as the carbon content decreased, and higher absorbed energy was also achieved as delamination occurred at lower temperatures. The delamination was found to be controlled not only by the transverse grain size, the grain shape, and the <110>1/RD fiber texture but also by carbide particle distribution in the UFEG structure.