The multiphase steels have complex microstructures containing polygonal ferrite,
martensite, bainite, carbide and a small amount of retained austenite. This microstructure provides
these steels with a high mechanical strength and good ductility. Different thermal cycles were
simulated in the laboratory in order to create the microstructures with improved mechanical
properties. The samples were heated to various annealing temperatures (740, 760 or 780°C), held
for 300 s, and then quickly cooled to 600 or 500°C, where they were soaked for another 300 s and
then submitted to the accelerated cooling process, with the rates in the range of 12-30°C/s. The
microstructure was examined at the end of each processing route. The mechanical behavior
evaluation was made by microhardness testing. The microstructural characterization involved
optical microscopy (OM), X-ray diffraction (XRD), scanning electron microscopy (SEM) with
electron backscattering diffraction (EBSD) and transmission electron microscopy (TEM). The use
of multiple regression analysis allowed the establishment of quantitative relationship between the
microstructural parameters, cooling rates and mechanical properties of the steel.
