Analysis of precipitation characteristics of TiC at different quenching and partitioning temperatures and its effect on the mechanical properties

Abstract

Intercritical annealing, quenching and partitioning (I&QP) steel is the most promising one of the third generation advanced high-strength steel. However, iron carbides are inevitable in I&QP steel. They can consume the carbon partitioning from supersaturated martensite to austenite and thus reduce the stability of austenite. Furthermore, the transition iron carbides, i.e., ε and η, are harmful to the toughness and ductility. In this work, the addition of Ti has been proved to effectively suppress the formation of the transition iron carbides. The precipitation characteristics of TiC at different quenching and partitioning temperatures (QT and PT) was investigated mainly by transmission electron microscopy, small angle neutron scattering and three-dimensional atom probe tomography techniques. The results showed that more carbides precipitated at the PT of 450 °C compared with that of 400 °C, and the volume fraction of TiC decreased with the increase of QT at the PT of 450 °C, which are mainly determined by the precipitation kinetics and initial martensite fraction. The nano-sized TiC particles are formed at the dislocations within the martensite and austenite and the interface between the two phases, indicating that the high dislocation density can promote the precipitation of TiC. However, the precipitation of TiC decreases the carbon content of retained austenite and thus affects its stability; moreover, the formation of TiC consumes the carbon atoms pinning the dislocations and thus has an important effect on the yield strength and yield platform. This work is essential for suppressing the formation of transition iron carbides, understanding the precipitation characteristics of TiC and designing the I&QP steel with improved mechanical properties.