The last decade, research on Q&P has been mainly focused on cold rolled steel for the automotive industry for weight savings and increased safety. The limited literature on hot rolled and plate steel however suggested that Q&P may provide attractive combinations of strength and toughness in Si-added plate steel. Within those Q&P-concepts, micro-alloying with Nb can be proposed to get fine prior austenite grain sizes and fine NbC precipitates from a martensitic matrix during tempering, and these precipitates can play an important role in the strengthening of the steel.
In this work, the influence of Nb and combined Nb+Mo additions on hot rolled Q&P steels was investigated. The reference composition was alloyed with 500ppm Nb, and 500ppmNb with 0.15wt.%Mo respectively. Lab hot rolled sheets were re-austenitized and heat treated with variations in the partitioning time and quenching temperature thereby inducing different Nb-precipitation conditions and hence variations in mechanical properties. Besides tensile testing, the bendability was evaluated and correlated with the microstructural evolutions. In-depth characterization of the precipitation state allowed for a more fundamental understanding of the correlation between process conditions, final properties and microstructures.