Application of thermodynamics and kinetics in the designing of new type TRIP steels concerning Al and P effects

Abstract

The influence of C and Al content on phase transformation temperatures is investigated by dilatometric analysis. With the new set of experimental data, an updated thermodynamic description of the Fe-Al-C system is presented, using the thermodynamic data of the limiting binaries and the parameters for the Fe-Al-C ternary system optimized by some of the present authors. A well reproduced vertical section of the Fe-Mn-Si-Al-C system is also presented according to the thermodynamic description of the lower order systems. For the development of P containing transformation induced plasticity steel, the possibility of phosphorous segregating at grain boundary is discussed by thermodynamics as well as kinetics. Lower critical temperature is estimated for the steel to obtain the starting temperature of fast cooling. To understand the minimum rate of fast cooling, pearlite growth kinetics is calculated with self-developed diffusion coefficients of elements at the grain boundary. Over-aging temperature is tentatively determined through the calculation of T₀ temperature by both equilibrium and para-equilibrium assumptions.