High strength, ductility and sheet formability by normalizing and quenching of low carbon microalloyed dual-phase steel

Abstract

This work investigates the possibility of developing high strength, ductility, and normal-anisotropy in cold rolled (CR, ∼84%) 0.05 wt% carbon microalloyed dual-phase (DP) steel sheets by normalizing (N, 920°C, 30 min) followed by intercritical annealing and quenching (IA&Q, 80 0°C, 3 min). CR + N produces ferrite with ∼6.9μm grain size and ∼0.23 bainite fraction with ∼5.3μm packet size. A high engg. (true) strength of ∼605(∼798)MPa; uniform engg. (true) strain eU≈0.32 (εU≈0.28); engg. fracture strain, ef≈0.48; with high normal average anisotropy (r‾) of ∼1.84 and low planar anisotropy, Δr of ∼0.30 could be obtained. Further, IA&Q produced ferrite with ∼6.3μm average grain size, ∼0.25 martensite fraction (Ceq≈0.16) with ∼3.2μm island size. A high engg. (true) strength of 690(864)MPa; engg. (true) uniform strain of ∼0.25 (∼0.22); engg. fracture strain ∼0.39; and high r‾ of ∼1.51, low Δr of ∼0.31, suitable for sheet metal formability, were obtained. The high strength could be attributed to (a) fine ferrite grain size, (b) secondary phase fraction (∼0.23B/∼0.25M), packet size and island morphology, and (c) geometrically necessary boundaries (GNB). High ductility could be due to (a) fine ferrite grains with secondary phase morphology, (b) GNB, coincident site lattice boundaries (CSLB), and random high angle grain boundary (RHAGB) networking. Better anisotropic properties could be associated with strong γ-fiber, and suppression of α-fiber and θ-fiber texture. Loci in the stress and strain space indicate that the hydraulic bulge and plane-strain biaxial strength-ductility improves to 950MPa-0.41 and 1030MPa-0.67 after CR + N, and to 968MPa-0.30 and 1070MPa-0.58 after CR + N + IA&Q.