Formation Process And Microstructures Characteristics Of A Novel Nano-bainite In A High-carbon Silicon Steel

The lower bainite displays a crucial engineering application value due to its favorable combination of strength and toughness.With the development of society,higher and higher mechanical properties are required for engineering structures,especially high strength and toughness.The research on refining the lower bainite structure,especially the nano-scale lower bainite structure and its forming process has become the focus of the international engineering field.In order to investigate the formation and structural characteristics of nano-bainite,the effect of high-temperature annealing pretreatment above A_ccm on bainitic austempering microstructure evolution in Fe-0.95C-1.37Si-1.04Cr-0.44Mn(wt.%)steel was investigated by X-ray diffraction and high-resolution transmission electron microscope.It was found that a novel nano-bainite can be obtained under conventional bainitic austempering(austenitizing at 860? and austempering at 250? for 20minutes)in the experimental steel after annealing treatment at 950?,which is a temperature above A_ccm.The novel nano-bainite exhibit independent needle-like microscopic morphology in an optical microscope,which is similar to the microstructure of conventional lower bainite,while the fine microstructure of which shows a strip with a width as small as 50 nm and a length as micrometers.The novel nano-bainite with no subunit structure is a single-phase body-centered cubic lattice and a continuous strip,in which the opposite stacking faults is the main crystal defect.In addition,the interface of the novel nano-bainite crystal is composed of both coherent and non-coherent areas,corresponding to concave and convex microscopic morphology,respectively.The test results of mechanical properties show that the micro-hardness of the novel nano-bainite is equivalent to that of the shelf-like nano-bainite,which can contribute more to the strength of the steel than the conventional lower bainite structure,and has superior engineering application value.