Predictive Process Optimization for Fracture Ductility in Automotive TRIP Steels

In light of the emerging challenges in the automotive industry of meeting new energy-saving and environment-friendly requirements imposed by both the government and the society, the auto makers have been working relentlessly to reduce the weight of automobiles. While steel makers pushed out a variety of novel Advanced High Strength Steels (AHSS) to serve this market with new needs, TRIP (Transformation Induced Plasticity) steels is one of the most promising materials for auto-body due to its exceptional combination of strength and formability. However, current commercial automotive TRIP steels demonstrate relatively low hole-expansion (HE) capability, which is critical in stretch forming of various auto parts. This shortcoming on ductility has been causing fracture issues in the forming process and limits the wider applications of this steel.;The kinetic theory of martensitic transformations and associated transformation plasticity is applied to the optimization of transformation stability for enhanced mechanical properties in a class of high strength galvannealed TRIP steel. This research leverages newly developed characterization and simulation capabilities, supporting computational design of high-performance steels exploiting optimized transformation plasticity for desired mechanical behaviors, especially for the hole-expansion ductility. The microstructure of the automotive TRIP sheet steels was investigated, using advanced tomographic characterization including nanoscale Local Electrode Atom Probe (LEAP) microanalysis. The microstructural basis of austenite stability, the austenite carbon concentration in particular, was quantified and correlated with measured fracture ductility through transformation plasticity constitutive laws. Plastic flow stability for enhanced local fracture ductility at high strength is sought to maintain high hole-expansion ductility, through quantifying the optimal stability and the heat-treatment process to achieve it.;An additional stabilizing aging treatment was proposed to address the enhancement of hole-expansion ductility in this class of steel, through optimization of retained austenite stability for transformation toughening. Using a commercial galvannealed TRIP steel of 800 MPa strength level as an example, the paraequilibrium model was established with carbon partition stored energy fitted with experimental data and applied in the thermodynamic simulations by Thermo-Calc® and DICTRA®. It is found the specimens aged at 447 °C for 2 minutes demonstrate evident improvement in the fracture ductility compared with original as-received galvannealed TRIP steel. The true fracture strain grows from 0.45 to 0.71, associated with a reduction of Ms σ from 70 °C to 15 °C. Further application of the computational design model on a series of similar TRIP steels confirms the effect of the stability optimization on enhancing the hole-expansion ratio by ∼ 30% on average. The optimum processing parameters along with the new stabilizing aging heat-treatment was predictively designed to successfully improve the material meeting the objectives.