Effect of high-temperature annealing on elemental distribution in CSP-produced automotive steel billets

The distribution of C, Si, Mn, P and S in Q&P 980 automotive steel billets produced via the Compact Strip Production (CSP) process was investigated using Electron Probe Microanalysis (EPMA). The segregation ratio of each element was calculated. A diffusion kinetics model was established using the finite element method, and the chemical potential equations of different elements were used as the driving conditions for diffusion. Fick's law was applied to numerically calculate the diffusion behaviour and final concentration distribution of C, Si and Mn during the annealing process at 1100 °C. Results indicate that element segregation in the as-cast billet before annealing decreases gradually from the surface to the core. After annealing, the segregation ratio of elements is reduced, leading to a more homogeneous elemental distribution. It was calculated that the diffusion of C at 1100 °C completes in about 10 s, with micrometer-level long-range diffusion occurring in a shorter period. However, the concentration of Mn does not reach equilibrium at the austenite interface even after 20 h of diffusion at 1100 °C.