Thermal characterization by dilatometry tests of a ferritic-bainitic (FB) steel for automotive application

The main objective of this article is to deep the knowledge of the thermal behavior of ferrite-bainite (FB) steel in correlation with the structural aspects that justify the mechanical properties. Samples of a DP-590 steel were tested by dilatometry considering a heating rate of 0.17 °C/s, up to a temperature of 1150 °C and different cooling rates between 0.03 °C/s to 100 °C/s, in air atmosphere. The results of the thermal behaviour were correlated with a microstructural study using optical microscopy in order to establish the final proportion of ferrite and bainite that is achieved at each cooling rate considered. As the cooling rate increases, the diffusion processes are limited so that hard second phases are obtained. In this steel, the presence of bainite was corroborated. The mechanical strength evolution associated with the cooling rate was determined through microhardness measurements. On this base, the steel's continuous CCT curves was built.