Determination of intercritical austenization parameters for development of multiphase steel using dilatometric analysis

ABSTRACT The Multiphase steels are materials with exceptional mechanical properties including high strength, excellent ductility and hardening coefficient and good conformability. These properties are related with the microstructure, which can be a combination of ferrite and martensite (Dual phase steels) or more complex microstructures with ferrite, bainite, martensite and retained austenite as in TRIP steels. One of the most important stages on the fabrication of multiphase steels is the intercritical austenization step which is performed between the A1 and A3 temperatures. This step determines the amount and chemistry (especially carbon) of the high temperature austenite. Aiming to study the intercritical austenization step, steels with variations in carbon (0.2-0.5wt%) and silicon (1.4-2.0wt%) were manufactured and dilatometric studies were conducted. The results allowed to stablish the intercritical austenization temperature (IAT) according with the needed amount of high temperature austenite. Also, the variation in chemistry of the high temperature austenite was studied as a function of the intercritical austenization temperature. (IAT) The results were analyzed according with the possibilities of the step after austenization, i.e., quenching or austempering and its impact on the microstructure and properties of the heat treated alloys.