Effect of Bainite on Mechanical Properties of Multiphase Ferrite-Bainite-Martensite Steels

摘要

Growing requirements to formability of High Strength Steels in parallel with desirable increased yield strength for resistance to intrusion have brought to agenda so called multi- or complex phase steels with significant portion of bainite. There are numerous publications confirming better hole expansion and bendabilty of similar steels, however, the detailed description how replacement of a part of martensite by bainite changes mechanical properties of initial dual-phase (DP) steel is lacking. The investigation was based on 0.09C-1.8Mn-Si steel that have the various bainite/martensite or bainite morphology by changing isothermal holding time or temperature after annealing at intercritical/supercritical temperatures. As shown the strengthening by bainite appeared to be 30% lower than by martensite. The replacement of martensite by bainite in DP microstructure resulted in an increase in yield strength and a return of yield point elongation. The product of tensile strength and total elongation is improved in multiphase microstructure in spite of lower strength than in DP microstructure. The higher levels of hole expansion in multi-phase steels were confirmed and resistance to crack propagation was measured to explain the phenomena.