Effect of rare earth La on super-cooled austenite transformation of low carbon copper-containing steel and its mechanism

Abstract

The effect of rare earth element La on the continuous cooling transformation kinetics of undercooled austenite in low carbon copper containing HSLA steel was studied by Formastor-F automatic phase change meter, SEM and TEM. Based on first-principles calculations of Vienna ab-initio simulation package which is under the framework of density functional theory, the effect of La on the transition from fcc-Fe to bcc-Fe was analyzed at atomic scale. The results show that La refined the microstructure, promoted the transformation of granular bainite to lath bainite, and La had a double effect on the phase transformation of low-carbon copper-containing HSLA steel. In the early stage of martensitic transformation, after adding La, the initial martensite transformation temperature Ms decreased from 443 °C to 438 °C, and the CCT curves moved to the lower right, which improved the stability of supercooled austenite. Meanwhile, first-principles calculations results show that the addition of La reduces the driving force of fcc-Fe to bcc-Fe and increases the energy barrier for Fe to undergo martensitic transition through Bain mechanism. The above results show that La delayed the martensitic transition. In the later stage of martensitic transformation, the addition of La decreased the critical transformation speed and improved the hardenability of steel, indicating that La promoted the martensitic transformation.