Effect of Volume Fraction of Constituent Phases on the Stress-Strain Relationship of Dual Phase Steels.

Ferrite-martensite and ferrite-bainite dual phase steels (DP-steels) were prepared by applying accelerated cooling (AcC) process on a linepipe steel. Their stress-strain relationships were predicted by micromechanics. In the predictions, the stress-strain relationships of the constituent phases whose chemistries were determined by microscopic examinations and some thermodynamic data were used. The effect of volume fraction of the constituent phases on the stress-strain relationships of the DP-steels was also examined. According to the applied model, a simple stress-strain curve can be divided into three stages. As a result of this investigation, work hardening takes place in stage II and at the beginning of stage III. Further, in stage II, the hardening rate is strongly dependent on the volume fraction of the harder phase. In stage III, the hardening rate for each DP-steel is smaller than that in stage II and is related to the difference in tensile strength between the harder and the softer phases. Furthermore the second investigation by means of FEM analysis was carried out in order to evaluate the influence of variation of the volume fraction of the harder phase on the stress-strain behavior of a DP-steel. Tensile tests showed that by increasing the amount of the harder phase (bainite) in the DP-steel, Lüders elongation disappears. According to the results obtained by the FEM calculations, the stress-strain behavior is related to the microstructure, such as volume fraction and shape of the grains in the DP-steel.