The effects of Nb and Mo addition on transformation and properties in low carbon bainitic steels

• Whilst the addition of Nb improves the strength of alloy steel through grain refinement, it hinders bainitic transformation.
• The combined effect of Nb and Mo alloying simultaneously promotes grain refinement and bainitic transformation.
• Mo addition is effective to improve the strength of low carbon bainite steel by promoting bainitic transformation.

Four low carbon bainite steels were designed to investigate the effects of Mo and Nb addition on bainitic transformation, microstructures and properties by metallographic method and dilatometry. The results show that single Nb addition retards bainitic transformation in low carbon bainite steels, although it can improve strength by refining microstructures. Moreover, Mo addition is effective to improve the strength of low carbon bainite steel by promoting bainitic transformation and single Mo addition has a better strengthening effect than single Nb addition. Further, in Mo bearing steel, Nb addition refines bainite sheaves, but meanwhile hinders bainitic transformation because of smaller austenite grains. Consequently, the composite strengthening effect of Mo and Nb addition has little improvement compared with individual addition of Mo in low carbon bainite steels.