Effects of Initial Microstructures and Annealing Conditions on Mechanical Properties of Cryo-rolled Low Carbon Manganese Steels

The effect of annealing on the microstructural evolution of the matrix in low carbon manganese steels with different initial microstructures, including martensite, bainite and a mixture of ferrite and pearlite, was investigated in conjunction with the charge in mechanical properties. During annealing, steels with a deformed ferrite + pearlite microstructure underwent spheroidization of the lamellar cementite in the pearlite region and recovery or recrystallization in the ferrite region. Martensitic steels and bainitic steels experienced recovery and recrystallization of the matrix and the precipitation of cementite particles during annealing. Although the martensitic steels showed the highest strength up to the annealing temperature of 600 .deg. C, the large driving force from dislocations accumulated during cryogenic deformation, as well as dislocations inherited from the martensitic transformation, accelerated the softening rate during annealing. A small amount of equi-axed grains produced during annealing at 550 .deg. C did not effectively improve ductility. The rapid softening and the improvement in ductility in martensitic steels and bainitic steels annealed at 600 .deg. C could be attributed to the disappearance of elongated substructures, rather than the coarsening of cementite particles during annealing. The bainitic steel showed higher thermal stability and a better combination of strength and ductility than the other microstructures, such as the martensitic microstructure and a mixture of ferrite and pearlite.