Influence of multilevel martensitic microstructures and carbide on the low-temperature toughness and fracture behavior of MnB steels

This study investigates the microstructural effects on impact toughness of two cost-effective high strength MnB steels: 14MnTiB (MnB) and 14MnMoTiB (MnMoB), particularly at low temperatures. Through a series of quenching and tempering treatments, the mechanical properties were optimized, with MnB steel exhibiting outstanding low-temperature toughness (84.7 J at -40 degrees C with a 5 x 10 x 55 mm sample) and a ductile-to-brittle transition temperature near -110 degrees C. A comprehensive analysis was conducted on the influence of multilevel martensitic structures and carbide distributions on toughness and fracture behavior, employing techniques such as OM, SEM, EBSD, and TEM. Results reveal that Mo addition in MnMoB refines martensitic packets and blocks but results in coarser laths and a higher density of carbides. A comparative analysis of the fracture characteristics of MnB and MnMoB steels at -40 degrees C and -100 degrees C (ductile-to-brittle transition region) reveals that the microstructural differences affect dimple coalescence during ductile fracture and lead to different critical events controlling crack nucleation and propagation at -100 degrees C. This study provides insights into enhancing the lowtemperature toughness of MnB steels, supporting the potential expansion of their applications.