Effect of Ti micro-addition on the hot tensile behaviour, microstructure and fractography of low-C high-manganese steels

Abstract

The aim of the work was to determine the effect of Ti micro-addition on the hot tensile behaviour, microstructure and fractography of two low-C high-manganese steels with additions of Si and Al. The hot tensile tests were performed using the Gleeble 3800 thermomechanical simulator. Samples were stretched at a temperature range from 1050 ℃ to 1200 ℃ at a strain rate of 2.5·10 3 s−1. The microstructure of the tested high-manganese steels under conditions of hot deformation was influenced by strain hardening and simultaneous dynamic recrystallization, as well as precipitation processes-depending on the chemical composition of the alloy and plastic deformation parameters. The analysis of the curves registered in the hot tensile tests indicated that a decrease of strain hardening was the result of the dynamic recrystallization. Hot tensile curves of the Ti-micro-alloyed steel were characterized by higher yield stress compared to the Ti-free steel. The Ti micro-addition with a concentration of 0.075 wt.% guaranteeing stable TiN-type nitrides eliminated the possibility of precipitating AlN-type nitrides and complex MnS-AlN type non-metallic inclusions, which are harmful to hot ductility. Fracture modes of the Ti-free steel showed a mixed nature from 1050 ℃ to 1150 °C, i.e. ductile fracture and numerous cavities and voids were identified. As the deformation temperature increased to 1200 °C, the fracture character was brittle with numerous inter-crystalline cracks along austenite grain boundaries. The addition of Ti improved significantly the hot ductility behaviour characterized by higher values of flow stress and reduction in area as well as ductile fracture modes in the entire high deformation temperature range.