Effects of Lath Boundary Segregation and Reversed Austenite on Toughness of a High-Strength Low-Carbon Steel

Comprehending the mechanisms underlying embrittlement and de-embrittlement during tempering holds paramount importance in the advancement of high-strength and high-toughness steels. In this research, the influence of element segregation at martensite lath boundary and the formation of reversed austenite on toughness after aging were systematically investigated. A high-strength steel has been developed, exhibiting a V-notch impact toughness of 185 J/cm2 at - 80 degrees C, a yield strength of 980 MPa, and a total elongation of 20 pct. The findings indicate that the short aging can result in Mn segregation at the lath boundary, leading to embrittlement of the steel. In contrast, prolonged aging triggers Mo segregation at these boundaries, mitigating the Mn segregation effects. The mechanisms governing post-aging embrittlement and subsequent recovery are chiefly modulated by segregation of Mn and Mo at the martensite lath boundaries. Mn segregation at these boundaries correlates with diminished toughness, while Mo segregation serves to counteract the adverse consequences of Mn segregation, thereby reinstating superior toughness characteristics. Upon extended aging, notable segregation of Mn and Ni occurs at the lath boundaries, leading to a decline in the localized AC1 temperature and facilitating the development of a film-like reversed austenite structure along the martensite lath boundaries. This reversed austenite structure demonstrates remarkable stability, persisting well below - 130 degrees C. Both the segregation of Mo at the lath boundaries and the formation of the film-like reversed austenite significantly contribute to the enhancement of toughness.