The body design with light weight and enhanced safety is a key issue in the car industry. Corresponding to this trend, POSCO is developing various automotive steel products with advanced performance. Conventional advanced high strength steels such as DP and TRIP steels are now expanding their application since the steels exhibit higher strength and ductility than those of conventional solution and precipitation strengthened high strength steels. Efforts have been made to enhance the mechanical performance of these steels such as ductility, hole expansion ratio, deep drawability, etc. Current research is focused on development of extra- and ultra-AHSS. Extra-AHSS are designed to utilize nano-scale retained austenite embedded in fine bainite and martensite. Ultra-AHSS are designed to have austenite as the major phase, and the ductility is enhanced primarily by continuous strain hardening generated during forming. These steels including extra- and ultra-AHSS are believed to be the next generation automotive steels which will replace the existing high strength steels due to their extremely high strength and ductility combinations.

No abstract

A finite element model was developed to simulate the deformation, temperature and phase transformation behavior in high carbon steels. The heat capacity of each phase and the heat evolution due to phase transformation were obtained from the thermodynamic analysis of S45C, 50CrV4 and SK85 steels. Phase transformation kinetics of the steels were derived from continuous cooling experiments. An additivity technique was applied to a modified Johnson-Mehl-Avrami equation to analyze continuous cooling curve. To predict the strain due to TRansformation Induced Plasticity (TRIP), a variant selection model for diffusionless transformation and an accelerative creep model for diffusional transformation were adopted. In order to calculate the deformation behavior, the elastic strain, the volumetric strain due to thermal contraction and phase transformation, the plastic strain and the TRIP strain were taken into account. Using the finite element model developed in this study, the temperature-phase-deformation behavior of the high carbon steels was calculated.

No abstract

The microstructural evolution and the softening behavior of hot rolled and 60% cold rolled 0.85wt% carbon pearlitic steels during spheroidization annealing have been investigated by using the textural and microstructural information contained in the Orientation Imaging Microscopy (OIM) scans. The local boundary energy map, recently suggested by the present authors, is used to monitor the changes of stored plastic strain energy distribution in ferrite during the annealing process, which shows that the spheroidization process of cementite is finished before the completion of recrystallization of the 60% cold-rolled high carbon pearlitic steel.