Study of microstructural evolution and mechanical properties of 1000 MPa low-carbon microalloyed steel prepared by multiple quenching strategies near Ms

In the present study, the microstructural evolution and mechanical properties of low-carbon microalloyed steel prepared by quenching and non-isothermal partitioning (QNP) treatments above and below Ms were investigated. QNP treatments near Ms have been proved to obtain the desirable combination of strength and ductility. Consequently, ultra-high strength bainitic steel was produced at 420 degrees C (slightly above Ms); the yield strength (YS), ultimate tensile strength (UTS), and total elongation (TEL) are 932 +/- 2 MPa, 1042 +/- 2 MPa, and 13.1 +/- 0.2%, respectively. The microstructure contains bainitic ferrite (BF), martensite/austenite (M/A) constituents, and a small amount of retained austenite (RA). Tempered martensite (TM) appears only below Ms and has similar morphology to BF, owing to that they are produced by kinetically continuous transformation and tempered after the prolonged non-isothermal partitioning. As the temperature increases from 360 degrees C to 450 degrees C, the volume fraction of M/A constituents and RA increase. Moreover, higher temperature specimens with lower density dislocations caused by transformation undergo a deeper degree of tempering softening. This contributes to a consistently high strain hardening capacity during uniform deformation and leads to a significant increase in ductility. Fine grain strengthening, nano-scale precipitation strengthening, and high density dislocations allow bainite to achieve ultra-high strength while maintaining its ductility.