Influence of boron addition on microstructure and mechanical properties of medium-Mn advanced high-strength steel

This study investigates the effects of 0.12 wt% boron addition on the microstructure and mechanical properties of a Fe-0.08C-4.5Mn-2.5Cr–1Si medium-Mn steel in as-rolled conditions and after the intercritical annealing process. In the as-rolled conditions, boron addition increased the strength and reduced the alloy ductility. In as-hot rolled conditions, an ultimate tensile strength (UTS)/elongation of 1195 MPa/5.5% and 1008 MPa/7% was achieved in the boron-added and boron-free alloys, respectively. However, the effect of boron on the intercritically annealed specimens was diverse. The intercritical annealing had no tangible effect on the mechanical properties of the hot-rolled specimen. However, the intercritical annealing at 700 °C for 600 s led to a UTS/elongation of 1029 MPa/19.5% in the boron-added alloy, the highest UTS/elongation combination among all specimens. The formation of carbo-boride precipitates on lath boundaries of the boron-added alloy possibly locked the boundaries thereby a temper-resistant martensitic structure was attained. The lath structure of the annealed alloy caused an enhanced strength in the boron-added specimen. Nonetheless, in the boron-free alloy, martensite was normally tempered and a considerable reduction in the strength was observed.