Improvement of strength-and-toughness balance of recycled steel through ultrarefinement of grain size.

Abstract

Phosphorus is an element that is highly effective for solid solution strengthening, but its content is kept at minimum in steel making, because an increase in the P content decreases the toughness, especially after quenching and tempering, such as the 50% Charpy absorbed energy transition temperature and the Charpy absorbed energy at low service temperatures. On the other hand, grain refinement improves the toughness of low carbon steels. In this study, an ultrafine grain structure is introduced to low carbon steels to suppress the detrimental effects by the high content of P. The addition of P increases the tensile strength even in an ultra-fine ferrite-cementite microstructure as well as in a ferrite-pearlite microstructure. The 0.1wt. % addition of P decreases the Charpy absorbed energy of the ferrite-pearlite (FLP) and the quench-and-tempered specimens considerably at 233K. On the other hand, the ultra-fine ferrite-cementite specimen with a 0.1wt. % P shows a value that is comparable to that of the F/P specimen without P. The 50% Charpy absorbed energy transition temperature is concurrently improved by the grain refinement.