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.

Near‐Atomic Scale Characterization of Mn‐Gradient in Reverted Austenite in Hot‐Rolled Medium‐Mn Steels

“…The (V,Nb)C carbide at α'/γ interface reveals a radius size of 4 nm, while those carbides inside the austenite grain are much smaller (1-2 nm). The growth of austenite phase is accompanied by the dissolution of metastable carbides [24,25] and precipitation of face-centered cubic structured (V,Nb)C carbides. In the investigated microalloyed MMnS, the austenite grew and the α'/γ interface continuously encountered (V,Nb)C carbides.…”

Mechanism and Application of Reverse Austenitic Transformation in Medium Mn Steels: A Systematic Review

“…In some cases, microalloying elements, such as Nb and V, have been added to MMSs to further improve strength via precipitation hardening. [41,80] These atoms may precipitate as nonpartitioned carbides (e.g., NbC and VC) before or during the IA process, which usually has a higher dissolution temperature than cementite. On the one hand, their extensive precipitation consumes a certain amount of C atoms, reducing the chemical driving force for phase transformation.…”

Investigation on Yield Point Elongation and Yield Strength of Fe–9Mn–1.5Si–1Al–0.05C Water‐Quenched and Air‐Quenched Steel Prior to Intercritical Annealing

“…One of the features on medium Mn steels is martensitic transformation from austenite during plastic deformation, which commonly provides a localized work-hardening effect and ultimately improves tensile strength and total elongation. [4][5][6][7][8][9] The majority of studies are focused on the effects of alloying elements and annealing parameters on microstructure evolution and mechanical properties. [1,3,9] Medium Mn steels with globular-shaped α grains due to recrystallization and γ grains transformed from deformed martensite (α') generally reveal discontinuous yielding and large yield point elongation (YPE).…”

“…[4][5][6][7][8][9] The majority of studies are focused on the effects of alloying elements and annealing parameters on microstructure evolution and mechanical properties. [1,3,9] Medium Mn steels with globular-shaped α grains due to recrystallization and γ grains transformed from deformed martensite (α') generally reveal discontinuous yielding and large yield point elongation (YPE). [6,8,10,11] It is reported that Lüders bands on the surface of products propagate within the gauge portion during YPE, indicating heterogeneous deformation.…”