High strength-ductility combination by quenching and partitioning of a low carbon microalloyed dual-phase steel

Abstract

This work investigates the quenching and partitioning of hot and cold rolled (HR & CR) low carbon micro-alloyed (Fe-0.05C-1.3Mn-0.4Si-0.6Cr) dual-phase (DP) steel. The quenching (Q) step involved intercritical annealing (IA) at 840°C for 60min and water quenching (200°Cs−1). This is followed by partitioning (P) treatment comprising of below critical annealing at 600°C for 15min and water quenching. The final microstructure after Q and Q&P contains ∼33% martensite within fine ferrite grains with lath for HR and island type martensite morphology for CR sheets. A high strength, ductility and isotropy combination of ∼544±4MPa UTS; 44%(n=0.30) &49% (n=0.32) and Lankford parameter (r‾)=∼1, respectively were obtained for the HR and CR conditions. The strength could be attributed to the equiaxed fine ferrite grain structure (4.3−6.5μm avg. grain size), secondary martensite phase, and dislocation density. The ductility could be a combined effect of the fine ferrite grain size, low dislocation density, geometrically necessary boundaries networking inside the grains, increased co-incidence site lattice boundaries (CSLB), TRIP effect, and texture weakening. ‘P’ treatment successfully stabilized the retained austenite in this lean alloy and induced the TRIP effect. The martensitic transformation during quenching led to ∑3, ∑11, ∑19b, ∑25b, ∑31b and ∑33c CSLB formation (predominantly ∑3), and partitioning treatment could further increase the CSLB density. The presence of CSLB led to the development of weak and partial α-, β-, γ- and θ- fiber texture in the final microstructure. The weak texture led to isotropy (r‾∼1).