Advances in processing of automotive HSLA steels by application of in-situ materials characterisation and phase field modelling

Advanced High Strength Steels rely upon complex multi-phase microstructures, which result in excellent strength, but can impair stretch-flange formability. An attractive solution to mitigate this is the extension of conventional HSLA grades by addition of micro-alloying elements such as Nb or V combined with Mo, and by lowering C content to prevent the formation of pearlite/cementite. Thermomechanical processing is designed to maximise interphase precipitation of Nb/V containing particles during austenite to ferrite transformation. However salt-bath heat treatment and conventional methods are not able to characterise the austenite to ferrite transformation in-situ and to capture transient events occurring during continuous cooling or isothermal holding. In-situ characterisation, such as High Temperature Confocal Laser Scanning Microscopy and hot-stage Electron Back Scattered Diffraction, allows direct observation of the austenite to ferrite transformation as a function of isothermal holding temperatures and cooling rates. This in turn, enables to enhance understanding of the dynamics of the interphase precipitation process. Complementary to in-situ experimental techniques, the phase-field method is employed to simulate interface precipitation in multicomponent V-Mo steels. This technique generates the Gibbs free energy for all phases from the single function formalism of all three phases in the CALPHAD method without using conventional CALPHAD software. With this approach, the growth of precipitates is simulated in 3D and various anisotropy conditions can be studied. The results reveal that the distribution density of interphase precipitates is maximised by controlling the cooling rate in the temperature region where austenite to ferrite transformation occurs, and by optimising the temperature of the isothermal holding. Phase field modelling shows that the elastic energy contribution to precipitate morphology becomes dominant at later stages of precipitation. Experimental modelling results shows a good correlation between TEM and phase field modelling with regard to shape, size and distribution density associated with interphase precipitation