Grey-Taguchi analysis and experimental assessment of 1 GPa HSLA steel treated by quenching and tempering

The effect of quenching and tempering (QT) process on the mechanical properties of the experimental high-strength low-alloy (HSLA) steel was analyzed by Grey-Taguchi method, thus achieving the optimum combination of parameters based on the appropriate nine sets of experiments on an orthogonal array. The grey relational analysis (GRA) reveals that the quenching temperature T-1 has the greatest effect on each response variable, followed by the tempering temperature T-2, while the quenching time t(1) and tempering time t(2) have similar and the least effect. Experimental assessment of microstructure evolution was performed by multi-scale characterizations and in-situ investigation combined with modeling, focusing on martensite transfomation kinetics that controlled by the quenching process. Results indicate that the prior austenite grain (PAG) size and the substructure of martensite are significantly refined, as the quenching temperature decreased from 950 to 850 degrees C. The PAG refinement leads to an increase in driving force for martensite transformation initiation, thus shifting the martensite transformation temperature to lower levels for a higher degree of undercooling, consistent with the experimental and modeling results. The refined microstructure obtained at low quenching temperature contributes to strength improvement, and the various carbides precipitated during tempering process offset the tempering softening of the steels. In general, the value increase of process parameters (T-1, T-2, t(1) and t(2)) leads to a decrease of strength property, but an increase of ductility and toughness. Based on the theoretical and experimental basis, a 1 GPa grade HSLA steel with improved comprehensive mechanical properties can be produced.