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 p repared 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 whos e chemistries were determined by microscopic examinations and some thermody namic data were used. The effect of volume fraction of the constituent phas es on the stress-strain relationships of the DR-steels was also examined. A ccording 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 ill. Further, in stage II, the hardening rate is strongly dependent on the volume fraction of the hard er phase. In stage III, the hardening rate for each DP-steel is smaller tha n that in stage II and is related to the difference in tensile strength bet ween the harder and the softer phases. Furthermore the second investigation by means of FEM analysis was carried o ut in order to evaluate the influence of variation of the volume fraction o f the harder phase on the stress-strain behavior of a DR-steel. Tensile tes ts showed that by increasing the amount of the harder phase (bainite) in th e DP-steel, Luders elongation disappears. According to the results obtained by the FEM calculations, the stress-strain behavior is related to the micr ostructure, such as volume fraction and shape of the grains in the DP-steel .