Third generation of AHSS with increased fraction of retained austenite for the automotive industry

Third generation of advanced high-strength steels for the automotive industry contains a high volume fraction of fine-grained ferrite, carbide-free bainite, martensite and retained austenite. The level of strength and ductility is highly dependent on the fraction and mechanical stability of austenitic phase. One of the methods to obtain an increased fraction of γ phase is trough its chemical stabilization by Mn. Two 0.17C–3Mn–1.5Al–0.2Si–0.2Mo steels with and without Nb microaddition were developed in the present study. The steels were subjected to the thermomechanical processing designed on the basis of the DCCT diagram (deformation – continuous cooling transformation). The paper presents the results of the multi-stage compression tests and multiphase microstructures obtained as a result of the controlled multi-stage cooling. It was found that the hot workability of a new generation of AHSS is very challenging due to high values of flow stresses required. However, the thermomechanical processing enables to obtain very fine-grained bainite-based microstructures with a fraction of retained austenite up to 20%. The highest fraction of fine grains and interlath austenite was obtained for the temperature range between 400 and 450 °C. The effect of Nb results in higher flow stresses and better distribution of all the microstructural constituents.