Influence of Nb and V on Microstructure and Mechanical Properties of Hot–Rolled Medium Mn Steels

Abstract

In this paper, the authors investigate effects of microalloying elements, including both V and Nb, on the microstructure and mechanical properties of hot–rolled medium–Mn steels. The additions of both V and V + Nb into 9Mn steel can significantly refine the prior austenite grains, which, however, leads to decreased fractions of austenite retained after hot rolling. During the subsequent intercritical annealing (IA) at 670 °C for 5 h, the blocky austenite grains grow coarser, accompanying with the large increment of RA fractions by 30–50%. The additions of both V and V + Nb result in not only finer blocky austenite grains, but also the extensive precipitation of V‐rich carbide in the annealed microstructures, the latter shall be the main contributor to the significant increases of yield strength in 9Mn–V and 9Mn–V–Nb steels. Almost all of the large blocky austenite grains transform to martensite during deformation, leading to significant work hardening via the TRIP effect. As a result, an excellent combination of mechanical properties, including 950 MPa yield strength, 1230 MPa ultimate tensile strength, and 40% total elongation is achieved in the hot rolled and annealed 9Mn–V steel due to both the largest TRIP effect and extensive precipitation of V–containing carbide.