Microstructure and Mechanical Properties of an Austempered Nanostructured Bainitic Steel
Based on Thermo-Calc calculation,a nanostructured bainite steel,which is suitable for making steel wire,with composition of Fe-0.8C-2Mn-1.5Si-1.5Cr-0.25Mo-0.25Ni-1Al-0.25Co-0.1V was de-signed.The influence of austempering treatment on microstructure and mechanical properties of the steel was investigated by means of scanning electron microscope(SEM),X-ray diffractometer(XRD),transmis-sion electron microscope(TEM),thermal dilatometer,and tensile tester in terms of microstructures,ther-mal and mechanical properties etc.Results show that after austempering at lower temperature,the micro-structure of the nanostructured bainitic steel is composed of nanostructured bainite ferrite lath,retained austenite and a little proportion of martensite.With the increase of austempering temperature,the transfor-mation rate and the volume fraction of bainite ferrite increase.As the austempering time increases,the vol-ume fraction of bainite ferrite increases,while the amount of undercooled austenite decreases,and the size and volume fraction of the massive M/A islands obtained at room temperature decrease.Due to the carbon partitioning sufficiently between the bainite ferrite and austenite,the stability of undercooled austen-ite is improved,and the proportion of brittle martensite in M/A islands is reduced significantly.This induces the transition of tensile fracture from mixed fracture to quasi-cleavage fracture.After being heated at 230℃ for 48 hours,the tensile strength and yield strength of the experimental steel were 1625 and 1505 MPa,respectively,with an elongation of 34.5％,indicating the best match for strength and toughness.
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