Abstract
ABSTRACT A quenching and two-step bainitic isothermal transformation below Ms (Q-2BT) process was proposed for a medium-carbon steel. The evolution of microstructure and its correlation with mechanical properties were investigated and compared with the samples subjected to a quenching and one-step bainitic isothermal transformation below Ms (Q-1BT) process. Results showed that the Q-2BT sample has a finer martensite/bainite/retained austenite multiphase microstructure than the Q-1BT ones because of the formation of smaller bainite and the decomposition of large untransformed austenite blocks. Meanwhile, an optimal strength (1582 MPa), elongation (20.2%) and toughness (110 J) combination can be obtained after the Q-2BT process. The significantly improved impact toughness can be attributed to refined microstructure, increased filmy retained austenite as well as a high proportion of high-angle boundaries (70.7%). In addition, the Q-2BT sample exhibits a longer hardening plateau and a higher hardening exponent (0.084) because of the larger amount of retained austenite (11.3%) and more filmy austenite with high stability.
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