Modeling of the Austenitization of Ultra-high Strength Steel with Cellular Automation Method

Abstract

A model for simulating the austenitization of ultra-high strength steel during hot stamping is developed using a cellular automata approach. The microstructure state before quenching can be predicted, including grain size, volume fraction of austenite, and distribution of carbon concentration. In this model, a real initial microstructure is used as an input to simulate austenitization, and the intrinsic chemical difference is utilized to describe the ferrite and pearlite phases. The kinetics of austenitization is simulated by simultaneously considering continuous nucleation, grain growth, and grain coarsening. The UHSS is reduced to a Fe-Mn-C ternary system to calculate the driving force during extent growth in ferrite. The simulation results show that the transformation of ferrite to austenite can be divided into three stages in the condition of a heating rate of 10 K (−263 °C)/s. The transformation rate is determined by two factors, carbon concentration and temperature. The carbon concentration plays a major role at the early stages, as well as the temperature is the main factor at the later stages. The A _c3 calculated is about 1073 K (800 °C) close to the measured value [1067.1 K (794.1 °C)]. Austenite grain coarsening was calculated by a curvature-driven model. The simulated morphology of the microstructure agrees well with the experimental result. Most of the dihedrals of the grain boundaries at the triple junctions are close to 120 deg. Finally, tensile tests were implied, as dwelling time increased from 3 to 10 minutes, the austenite grain size increased from 6.95 to 9.44 μm while the tensile strength decreased from 276.4 to 258.3 MPa.