The Significant Impact on Microstructure-Properties Relationship of 20Mn2Si2Cr Bainitic Steels Produced by Multistep Austempering Process

To elucidate the relationship between different isothermal processes and toughness of 20Mn(2)Si(2)Cr bainite high-strength steel, conventional austempering (AT), multistep austempering (MAT), and inverse multistep austempering (IMAT) heat treatment processes at different temperatures are conducted. The changes of properties with different processes are analyzed according to microstructure characteristics. The results indicate that the fraction of retained austenite is changed slightly for all processes, while the carbon content in retained austenite is from 1.03 wt% (AT-1) to 1.24 wt% (IMAT-2). Toughness is improved significantly by adjusting process parameters, but strength is not greatly affected at the same time. The toughness of AT-1 specimens is only 98 J cm(-2), and it increases to 123 J cm(-2) (AT-2 specimens) by reducing isothermal temperature. Compared with AT-1 specimens, toughness of MAT-2 and IMAT-2 specimens is increased by about approximate to 40% (138 J cm(-2)) and approximate to 65% (162 J cm(-2)), respectively. It is also noteworthy that the tensile strength varies in a small range from 998 to 1080 MPa. The improvement in properties is severely related to modification of microstructure.