Development of Cold-Rolled Dual-Phase Steels with Tensile Strength Above 1000 MPa and Good Bendability

This paper presents the most important results of the study oriented on development of low-silicon (<0.03Si), precipitation-strengthened, fine-grained, cold-rolled dual-phase (DP) steels with tensile strength greater than 1000 MPa, primarily intended for the automotive industry. For this purpose, extensive systematic investigations were conducted with the aim to optimize the composition/processing conditions with regard to the microstructural effects on tensile properties and bendability of DP steels. Within this study, influence of the addition of Mn, Cr, Mo and/or both Mo and Ti on the microstructure and tensile properties of eight steels molded in the form of ~20 kg ingots processed by controlled rolling has been investigated. The effect of simulated coiling temperature on hot-rolled steels followed by cold rolling and intercritical annealing at temperatures 1023 K and 1073 K (750 °C and 800 °C) as well as the interaction between ferrite recrystallization and austenite formation were examined. Investigation of the effect of intercritical annealing on the structure–property relationships was carried out on steels in both the hot-rolled and cold-rolled states. It was found that in spite of strength above 1000 MPa, good bendability (steel strip may have been bent to 180 deg at radius of 0.5 mm) can be achieved in the cold-rolled intercritically annealed steel with nominal composition 0.15-C-1.2Mn-0.02Si-0.2Mo-0.1Ti by more or less homogeneous distribution of fine Ti precipitates (<5 nm) within fine ferrite grains (<2 μm) and about 35 pct martensite volume fraction in the microstructure of DP steel.