EUROPEAN CONGRESS AND EXHIBITION ON ADVANCED MATERIALS AND PROCESSES - EUROMAT 2021

Nowadays steel research is devoted to improve mechanical properties in order to produce lightweight products, in particular for automotive industry. 3rd generation of advanced high strength steel (AHSS) represent a significant step in the reduction of fuel consumption and the production of light and safe car bodies. In particular, carbide free bainitic TRIP (TRansformation Induced Plasticity) steels represents a serious candidate for the future. In this work a novel composition of a carbide free bainitic steel with high silicon content has been investigated. Specimens have been subjected to different austempering treatment: they were austenitized at 900 °C for 30 minutes and held at different austempering temperatures (AT), in the range (200-370 °C), in order to guarantee bainitic transformation for different dwell times. A carbide free bainite matrix was obtained, with martensite and significative amount of retained austenite. Two morphologies of retained austenite have been found: blocky type and film like austenite, characterized by high carbon content and stability at room temperature. Specimens microstructure have been characterized by optical microscopy, scanning and transmission electron microscopy and by standard mechanical testing (hardness and tensile tests). Retained austenite, martensite and bainitic ferrite volume fraction have been evaluated by X-ray diffraction and performing Rietveld analysis. This investigation was performed before and after tensile tests and after fixed amount of deformation (by interrupted tensile tests) in order to analyze the microstructural evolution, in particular the martensitic transformation related to strain application. Microstructural investigation demonstrated that increasing the holding time at AT there is an increase in retained austenite volume fraction due to completition of bainitic transformation and carbon partitioning process. Moreover, for the same dwell time at different AT different final amount of austenite were found due to different kinetics. Mechanical tests allowed to observe TRIP effect and interesting strength-ductile ratios.