Effect of alloying elements and hot-rolling conditions on microstructure of bainitic-ferrite/martensite dual phase steel with high toughness

This study concerns the contributions of alloying elements and processing parameters to the amount of island martensite in as-hot-rolled dual phase steel. Steels with different Mo, Mn and Ni contents were prepared and hot-rolled. Hot-rolling was finished at the temperature of the austenite region above Ar3 and was followed by water spray cooling. Water spray cooling was stopped within the temperature range of the αB° transformation start and stop temperatures. The result showed that Mo and Mn have a strong impact on the amount of island martensite, but on the other hand, the carbon equivalent (CE=C+Mn/6+(Cr+Mo+V)/5+(Cu+Ni)/15) does not explicitly affect the amount of island martensite. The holding temperature after water spray cooling also has a significant effect. It was possible to obtain the maximum volume fraction of island martensite by setting a coiling temperature at which half of transformation was completed. Furthermore, in this study, the influences of the volume fraction and morphology of island martensite on low-temperature toughness were examined. The results showed that low-temperature toughness below −80 °C was achieved by refining the maximum size of island martensite. In as-hot-rolled bainitic-ferrite/martensite dual phase steel, both a low Y/T ratio and low-temperature toughness were satisfied by controlling Mo and Mn addition, the coiling temperature and controlled rolling.