| Abstract: | Dual-phase structures were produced in three steels (designated A1, A2 and A3) using two different heat-treatment cycles which incorporate intercritical annealing at a temperature of 750°C. In general, alloy A2 contains the maximum and alloy A1 the minimum volume fraction of martensite, with alloy A3 coming in between. All the cold-rolled alloys show elongated cell-like structures and also some deformation bands. Recrystallisation anneals were carried out at 650°C and 800°C. At the lower recrystallisation temperature of 650°C, the cold-worked ferrite starts to recrystallise, whereas, at the higher temperature of 800°C, re-austenitisation of martensite and recrystallisation of cold-worked ferrite take place simultaneously. During the recrystallisation anneal strain free ferrite grains are found to nucleate at both the deformed ferrite-martensite interfaces as well as inside the deformed ferrite grains. The process of recrystallisation in all three alloys, irrespective of the initial heat-treatment as well as annealing temperature, can be described as an in-situ process. The kinetics of primary recrystallisation of ferrite in the cold-rolled dual-phase steels was analysed from the relevant microhardness data using an Avrami-type relationship. In general, the Avrami plots show straight line segments with two distinct slopes, indicating two different processes during recrystallisation. The activation energies measured from the Arrhenius plots range between 66.88 and 83.6 kJ K mole?1 which are close to the value 84.02 kJ K mole?1 for the diffusion of carbon in α-Fe. |
