Effect of Quenching Temperature on the Microstructure and Mechanical Properties of Ultrahigh Strength Medium Manganese Steel

Author of the article: JIANG Jiale1, LI Yunjie1, LI Xiaolin2, YUAN Guo1

Author's Workplace：1. State Key Laboratory of Rolling and Automation Northeastern University, Shenyang 110819, China; 2. State Key Laboratory of Solidification Processing, Northwestern Polytechnical University, Xi'an 710072, China

Key Words：ultrahigh-strength steel; original austenite; quenching temperature; retained austenite; mechanical properties

Abstract： 2 000 MPa ultrahigh-strength steel is a key structural material for extreme service environments and faces serious problems such as strength-ductility inversion, expensive alloys, and complex production and preparation processes. To this end, a low-cost C-Mn composition system was designed, and the effects of different quenching and tempering temperatures on the microstructure and mechanical properties of the experimental steel were studied. The results show that with increasing quenching and tempering temperatures, the original austenite of the experimental steel changes from banded to equiaxed. When quenched and tempered at 780 ℃, the width and length of the banded original austenite grains are approximately 4 and 14 μm, respectively, and those of the equiaxed original austenite grains are approximately 3 and 5 μm, respectively. The dislocation density of the experimental steel also decreases from 6.87×1015 m-2 to 5.27×1015 m-2 and then to 4.46×1015 m-2 with increasing quenching and tempering temperatures. The volume fraction of retained austenite (RA) of the experimental steel at room temperature decreases from 12.6% to 10.2% and then to 9.2%. The variation in the volume fraction of RA before and after deformation decreases from 7.1% to 6.8% and then increases to 7.1%. Sample Q780 has the best comprehensive mechanical properties, with the highest yield and tensile strengths of 1 665 and 2 107 MPa, respectively, and also a relatively high uniform elongation of 8.9%. The room temperature crack initiation energy (9.8 J) and crack propagation energy (3.9 J) of 5 mm thick Q780 with a V-notch are also relatively high, making its room temperature impact energy as high as 13.7 J. The reasons for its excellent performance are that it has the largest dislocation density (6.87×1015 m-2), the largest RA volume fraction (12.6%) and the smallest average martensite grain size (0.77 μm).