Properties of dual phase steels - Influence of morphology of martensite on tensile and strain hardening properties of dual phase steels

Two different heat treatment (intercriticaly and intermediate heat treatments) have been applied to steels containing 0.16 % C in order to obtain dual-phase steels (DPS) with different microstructures. One set of intermediate quenched samples were tempered. The specimens that have different microstructures were subjected to tensile testing and compared according to their heat treatment conditions. The strain-hardening exponent and strength coefficient of DPSs were determined by using Ludwick equation. It was observed that volume fraction of martensite and grain size increased as the annealing temperature and time increased in the intercriticaly annealed steels. The tensile strength and strength coefficient of DPSs decreased as the annealing temperatures and time increased but the strain and strain hardening coefficient of DPSs increased. In the case of intermediate quenching, the volume fraction of martensite increased with the increase in annealing temperature. Tempered DPS exhibited a lower strength than samples obtained with intermediate quenching. Consequently, it was determined that the tensile strength and the strength coefficient of DPS increased. However the total strain and strain hardening exponent decreased with the increment of the annealing temperature in the case of intermediate quenching. Deformation hardening in these steels has occurred in three different specific areas.