MECHANICAL PROPERTIES AND DEFORMATION BEHAVIOR OF MARTENSITE-FERRITE DUAL-PHASE STEELS

The martensite-ferrite dual-phase (DP) steels were produced using a rapid resistance heating method. Starting with a commercial cold drawn 1018 pearlitic structure, DP steels with a variety of properties were obtained depending upon the dislocation structure of the ferrite matrix in combination with the carbon content and volume fraction of the martensite. A strength maximum accompanied by a ductility trough for these DP steels was observed at 34 percent martensite by volume. The structure-property relationships of the DP steels were studied in terms of the effects of the amount of martensite and its carbon content as well as the changes in ferrite properties. The strength maximum was reconciled in terms of the combined effect of increasing amounts of martensite with accompanying lowering of the strength (carbon content) of the martensite as a function of increasing intercritical heat treatment temperature for this single grade of steel. A model was proposed to explain the variation of strength of DP steels as a function of the amount and carbon content of martensite and the strength of ferrite. The annealing of the cold-worked dislocation cell structures of the ferrite regions was found to result in a high initial strain hardening rate with concomitant restricted ductility.