Microstructure - Properties Relationships in Complex Phase Cold-Rolled High Strength Steels

摘要

It was shown that dilatometry could be done on cold rolled plates to determine the temperatures of phase transformations and the volume fractions of phases formed after continuous annealing simulations. The state of deformation in cold rolled plates was responsible for a decrease in the kinetics of austenite formation during annealing between Ac_1 and Ac_3. Continuous annealing simulations with different cooling patterns were performed on cold rolled plates and the tensile properties were measured. The effects of the different parameters can be summarized as follows: Initial microstructure-A ferrite-pearlite starting microstructure leads to lower tensile strengths and better elongation after annealing, due to the shape of ferrite. However, due to the higher transformation of pearlite to austenite than ferrite to austenite, a banded structure is still present after annealing and may be detrimental for the tensile properties in the transverse direction. A bainite-martensite starting microstructure leads to higher strength levels but poorer elongations. Annealing temperature-Increasing the annealing temperature leads to the formation of more austenite. Hence, according to the cooling pattern, more hard phases bainite and martensite are formed, increasing the strength level. More recrystallisation of the ferrite may be responsible for the excellent elongation. Annealing time-It was shown that the austenite formation was faster at the start of the intercritical annealing. Carbon partitioning took more time. Reducing the annealing time leads to slightly lower fractions of austenite formed. Cooling pattern-The best compromises between high strength and good elongation were obtained for the CP steels with about 12 % ferrite, 25 % bainite and 63 % martensite. It was shown that cycles containing a IBT holding at a temperature at which bainite could be formed were the most efficients.