The effect of titanium and nitrogen contents on the microstructure and mechanical properties of plain carbon steels

Abstract

Three series of titanium-containing steels with different nitrogen contents were used to evaluate the effect of a small titanium addition on properties. With the addition of titanium, an obvious ferrite grain refinement was observed in the as-rolled condition. This refinement is attributed to the TiN grain-boundary-pinning effect. The TiN grain-boundary-pinning effect is most effective when the titanium content is around that in the titanium-to-nitrogen stoichiometric ratio, i.e. 3.42. Also the grain refinement effect from TiC might be obtained when the titanium content is higher than that in the titanium-to-nitrogen stoichiometric ratio. In contrast with the observed grain refinement, the yield strength of steels with a titanium content around that in the titanium-to-nitrogen stoichiometric ratio suffered a considerable loss when compared with plain carbon steels. This loss is due to the depletion of free nitrogen for solid solution hardening. However, the toughness was improved through grain refinement and depletion of free nitrogen. When the titanium content was over the stoichiometric ratio, a precipitation-hardening effect resulting from TiC was observed which resulted in the increase in strength and deterioration of toughness.