Improving the stretch flangeability of ultra-high strength TRIP-assisted steels by introducing banded structure

Abstract

A novel ultra-high strength transformation induced plasticity (TRIP)-assisted steel with a better combination of ductility and stretch flangeability was fabricated by introducing a banded structure into the resulting microstructure. The corresponding relationships between microstructures and mechanical properties were studied in detail by the tensile test and the hole expansion test. These tests reveal that some pancake-like ferrite grains obtained in cold rolling are retained during intercritical annealing and finally form a banded structure together with martensite/austenite (M-A) islands along the rolling direction. The steel can be further strengthened in the subsequent punching process, while uniformly-distributed bainite matrix with fine M-A islands dominates the overall microstructure after annealing at higher annealing temperatures. The banded structure of the soft ferrite phase will promote crack propagation along the rolling direction rather than in the thickness direction, thereby favoring improvement of the hole expansion ratio (HER). Instead, the mixed bainite-M-A islands microstructure led to increased micro-voids density at the edge of the shear affected zone (SAZ) after the hole punching process and a great number of cracks rapidly propagated through the thickness direction in the matrix, which is responsible for a decrease in HER. Excellent mechanical properties were obtained in the low-temperature annealed sample with an HER of 29–35%, elongation of 21–26%, tensile strength of 941–1016 MPa, and yield strength of 426–566 MPa.