Study of TRIP-Aided Bainitic Ferritic Steels Produced by Hot Press Forming

A study is reported to produce high strength ductile steels by controlled cooling following hot press forming, instead of quenching, as is practiced in the traditional press hardened steels. Heat treatments of several specially designed low carbon steels were carried out by interrupting the fast cooling from the austenization temperature at temperatures between T (0) and Ms and then cooling in controlled rates to room temperature. The effect of the interrupt temperature and the cooling rate afterward on the microstructures and tensile properties was studied. The microstructures were characterized using dilatometry, optical microscopy, X-ray diffraction, and TEM. A multi-phase microstructure including bainite, martensite, and retained austenite was obtained in the simulated hot press forming process. Volume fraction bainite was found to increase with an increase in interrupt temperature and a decrease in cooling rate. Structure-property correlations of the studied steels heat treated at different conditions were developed. Improved tensile properties were obtained by controlling the interrupt temperature and cooling rate which produced an optimum bainite content of 60 to 75 pct and retained austenite. Unfortunately, the bainite in the simulated samples was not completely carbide free even though the steels contained about 1.6 wt pct of Si. (C) The Minerals, Metals & Materials Society and ASM International 2013