Effect of isothermal and pre-transformation temperatures on microstructure and properties of ultrafine bainitic steels

Abstract The effects of isothermal and pre-transformation temperatures on the microstructure and mechanical properties of ultrafine bainitic steels are investigated and theoretically analyzed by using optical microscopy, scanning electron microscopy, transmission electron microscopy, and x-ray diffraction analyses in this study. The results show that the QBT (quenching followed by isothermal transformation of bainite) samples exhibit a multiphase microstructure containing lamellar ferrite, lenticular martensite, retained austenite. The isothermal and pre-transformation temperatures have an effect on the morphology, quantity of bainite and the stability of retained austenite. As the pre-transformation temperature is decreased from 245 °C to 210 °C, the content of premartensite increases significantly from 21% to 56%. The premartensite not only refines the bainite structure, but also provides nucleation points for the bainite transformation and accelerates the bainite transformation. Correspondingly, the pre-martensite has a good contribution to the strength of ultrafine bainitic steels, when premartensite transformation temperature is 210 °C, the martensite content is 56%, ultrafine bainitic steels reach the highest tensile strength of 1857 MPa. With the increase of isothermal temperature of bainite, the width of bainite lath increases accordingly, and the retained austenite changes from thin film to blocky, which results in the decrease of strength and improvement of plasticity of ultrafine bainitic steels. However, when the bainitic isothermal temperature is too high, the transformation of bainite gradually stops because of the enrichment of carbon in austenite.