Influence of Two-Step Bainite Transformation on Toughness in Medium-Carbon Micro/Nano-Structured Steel

Micro/nano-structured bainitic steel provides a unique combination of ultra-high strength and high ductility due to their structure consisting of micro/nano-scale bainitic-ferrite and retained austenite, but the toughness is a little bit low. The retained austenite plays a leading role for the toughness, and it can significantly increase the toughness of micro/nano-structured bainitic steel by refining the size of blocky retained austenite and improving the content of film retained austenite. Simultaneously, the structure of retained austenite affects the stability of retained austenite, and even can change the micro-deformation and determine the toughness. This work has been refined retained austenite of medium-carbon bainitic steel by using two-step bainitic transformation to study phase transformation of retained austenite through heat treatment. The effect of retained austenite on the impact toughness in medium-carbon micro/nano-structured steels was analyzed by one (300 degrees C for 6 h) and two-step (300 degrees C for 2 h, then 250 degrees C for 24 h) bainitic transformation processes. The microstructure, phase fraction, misorientation, crystallographic grain size and impact energy of different heat treatment steels were observed, detected and analyzed. The results showed that the impact property of two-step bainitic transformation was significantly higher than that of one-step bainitic transformation in medium-carbon steel, which the impact energy in -40 degrees C increased from 31 J to 42 J. The main reason is the new bainitic ferrite was formed in two-step bainitic transformation, the untransformed retained austenite was divided and refined by new bainitic-ferrite, reducing the formation of massive martensite during water quenching after isothermal bainite process. It significantly improve the toughness of the steel because the fracture energy was increased, owing to making crack bifurcation and even preventing the propagation of cracks in the impact process. Through the above-mentioned studies, this research not only precisely refines the retained austenite structure, reveals the effect of retained austenite stability on deformation mechanism and resolves toughness mechanism, but also provides the theoretical guidance for the production of micro/nano-structured bainitic steels in combination with good toughness.