Effect of Tempering Temperature on the Microstructure, Deformation and Fracture Properties of an Ultrahigh Strength Medium‐Mn Steel Processed by Quenching and Tempering

Abstract

Herein, a medium‐Mn steel containing 4 wt% Mn is designed to achieve ultrahigh tensile strength and good ductility by the quenching and tempering process. The effects of tempering temperature on the microstructure, tensile behavior, and fracture properties under different stress states are explored. It is observed that tempering temperature influences the fracture mechanisms of the ultrahigh‐strength steels. Both ductile fracture strain and cleavage fracture strength of the investigated medium‐Mn steel are affected by the tempering temperature. The impacts of tempering temperature on volume fraction, carbon content of retained austenite (RA), and tensile properties are quantitatively analyzed. Detailed microstructure analysis reveals that both volume fraction and carbon content of the RA are increased in the tempered steels, accompanied by improved tensile properties than in quenched conditions. The superior tensile properties are achieved after tempering at 250 °C, with the ultrahigh yield strength of 1590 MPa, ultrahigh tensile strength of 1963 MPa, and total elongation of 12%. The corresponding relationship between microstructure and mechanical properties is investigated, with a focus on the change of residual stress, the occurrence of carbon redistribution/carbide precipitation, the decrease of dislocation density of martensite, and the presence of transformation‐induced plasticity effect during deformation.

A Hot‐Stamped Medium‐Mn Steel with Superior Strength–Ductility–Bendability Combination and Thin Oxidation Layer

“…[10] Besides, some studies confirmed that medium-Mn steels were ideal materials to achieve RA present in the martensite matrix via hot stamping process. [11,12] However, the tensile ductility of most hot-stamped medium-Mn steels without baking or tempering treatment was terrible, [13,14] which may be related to the brittleness of martensite in the hot-stamped medium-Mn steels. In contrast, superior tensile ductility was achieved in warm-stamped medium-Mn steels with the microstructure consisting of RA, ferrite, and martensite, which was processed by low-temperature rolling, heat treatment at intercritical temperature region, and warm stamping (without baking/tempering treatment).…”