Abstract: The undesirable features in the composition design of traditional transformation induced plasticity （TRIP） steel are described, and composition of the new type TRIP steel is presented based on segregation thermodynamics and kinetics. The lowest cooling rate of steel after intercritical annealing is obtained with diffusion kinetics estimation. Overaging temperature is calculated from both equilibrium and paraequilibrium conditions. Carbon enrichment in Bainite is simulated, and the necessary time obtained based on the Hillert approximation. The calculated parameters are verified by comparison of calculation and experiment results. Conditions for hot dip galvanizing are discussed, and a series of welding tests carried out. The new steel is therefore massproduced in a large steel enterprise. Effects of elements on stacking fault energy (SFE) of austenite in twin induced plasticity (TWIP) steel are calculated. SFE in TWIP steels with different composition is estimated with the thermodynamic parameters proposed by Dumay. To obtain hexagonal close packed (HCP) martensite, composition of TWIP steel is also designed with special care in SFE. Functions of martensite obtained through different treatments on the mechanical properties are discussed, among which the best exhibits very good combination of strength and ductility, i.e., tensile strength over 1 GPa and elongation rate similar to 60%. This well meets the requirement of the 3rdgeneration automobile steel.

Key words: diffusion kinetics, mechanical property, segregation thermodynamics, stacking fault energy (SFE), transformation induced plasticity (TRIP) steel, twin induced plasticity (TWIP) steel