Effect of B Addition on Microstructure and Mechanical Properties of High-Strength 13Mn TRIP Steel with Different Annealing Temperatures

Abstract

The development of advanced high-strength steel has become the research focus of steel in order to meet low emission requirements. Different annealing temperatures from 600 °C to 900 °C were applied to 1200 MPa Grade 13Mn TRIP steels with (30B steel) or without B (0B steel). The effects of B addition on microstructure and mechanical properties with different annealing temperatures were investigated. Except for M2B in 30B steel, both steels annealed at 600 °C or 700 °C contained only austenite. When annealed at 800 °C or 900 °C, ε-martensite and α′-martensite were observed whereas 30B steel had less of them. 30B steel had higher yield strength (YS) and tensile strength (TS) regardless of the annealing temperature. Total elongation (TE) of 30B steel was smaller when annealed at 600 °C or 700 °C but larger at 800 °C or 900 °C. B addition refined austenite grains, and therefore depressed phase transformation to ε-martensite and α′-martensite during annealing. B addition enhanced YS and TS by refining grains, hindering dislocation movement and promoting phase transformation, but changed the fracture mechanism. The best TS × TE (53.62 GPa%) for 30B steel was reached when annealed at 800 °C. The more sufficiently triggered TRIP effect in high-temperature-annealed 30B steel accounts for its scarcely decreased TS and high TE.