Influence of Isothermal Bainite Transformation Time on Microstructure and Mechanical Properties of Hot-Dip Galvanized TRIP Steel

摘要

The influence of isothermal bainitic transformation (IBT) time on microstructure and mechanical properties of hot-dip galvanized TRIP steel with 0.20C-1.50Mn-1.2Al-0.26Si was investigated using optical microscopy, x-ray diffraction (XRD), Transmission Electron Microscope (TEM), dilatometry, and mechanical testing. This steel has high tensile strength of over 780 MPa with elongation more than 22%. The microstructure of the steel mainly consisted of ferrite, bainite, retained austenite, and martensite. The metastable austenite remaining after bainitic transformation will be transformed into martensite at the final cooling stage. The IBT time affects retained austenite content. When the IBT increased from 10 to 60 s, the amount of retained austenite increased from 9.40 to 15.42%, correspondingly. The IBT time also affects the strain hardening behavior. The n value characteristics of samples for IBT time from 10 to 30 s are similar to those of DP steel; however, the n value of specimen with IBT of 60 s shows features typical of TRIP steel.