Mechanical Properties of Novel Hot-Rolled 9Mn Steel: The Significant Role of Austenite Morphology and Grain Size

A deep cryogenic treatment process before intercritical annealing (IA) to tailor the mechanical stability of the austenite grains was employed in a novel medium-Mn steel. During IA, lath-like and equiaxed morphologies austenite with different grain sizes is formed at the interface of lath-like martensite. Lath-like austenite grains with smaller grain size and adequate strain energy are much more stable than large equiaxed grains, and the difference in stability allows the continuous transformation of austenite into martensite. Austenite grains with different stabilities enable continuous strain hardening, thereby leading to excellent mechanical properties with yield strength of 745 MPa, ultimate tensile strength of 1070 MPa, total elongation of 49.7%, which provides new application prospects to automotive hot-rolled steel sheets with good formability.