Dual phase ferrite-martensitic steel micro-alloyed with V-Nb

For low-carbon V-Nb micro-alloyed steel, dual phase ferrite-martensitic microstructures were prepared with a variable martensite fraction, which ranged from Vm 20 to 88%. By measuring the temperature in the sample centre, the parameters of intercritical annealing and the relationship between Vm and the intercritical annealing temperature 720 deg C < or = t < or = 850 deg C were determined. By analysing the microstructure, the strength and plastic properties of this dual phase steel, the preparation method of dual phase ferrite-martensitic steels was tested. Dual phase ferrite-martensitic microstructures at Vm above 53% formed continuous martensite areas. A change of the microstructure type as compared with the state with Vm < 53% resulted in a change of the increase of R and Rm. With growing Vm the strength properties increased and the plastic properties decreased. Fractures after uniaxial loading had a ductile pit morphology. On extraction carbon replicas, changes of precipitation of fine disperse precipitates were studied on the normalized state and the intercritically annealed state.