THERMAL AND THERMO MECHANICAL PROCESSING OF DUAL PHASE STEEL AND ITS CHARACTERIZATION

Abstract

Dual phase steels are being used in automobile industries for last three decades. The mechanical properties of dual phase steels can be altered by varying its martensite volume fraction. However, the benefits obtained in mechanical properties have to be viewed in light of other properties such as corrosion resistance. In this work, dual phase steels have been produced by thermal and thermomechanical processing methods. In thermal processing, dual phase steels with different volume fractions of martensite are obtained using different intercritical soaking times. The mechanical properties of dual phase steels such as Vickers hardness and tensile properties are measured. Corrosion properties are evaluated using potentiodynamic polarization test and immersion test for thermal processing steels. It was observed that the tensile strength and hardness increased and ductility decreased with increase in martensite volume fraction. The corrosion rate for dual phase steels is found to be lower than that for subcritically heat treated ferrite pearlite steel. The higher corrosion resistance of dual phase steels is explained on the basis of microstructural features. In thermomechanical processing, dual phase steels were developed with deformation and without deformation in the intercritical region. The mechanical properties are measured. It was observed that the thermomechanical processing causes the grain refinement and results in improved mechanical properties than normalized steels, and dual phase steels obtained from thermal processing alone. Better mechanical properties were obtained for the steels with deformation performed towards the end of soaking period in the intercritical region.