Improving flangeability of multiphase steel by increasing microstructural homogeneity

Multiphase microstructure significantly increases the strength, usually at the expense of flangeability because of lacking microstructure homogeneity. To further improve the strength-flangeability of multiphase steel, the microstructural homogeneity was advanced by adjusting the hard martensite/austenite (M/A) islands. The strength-flangeability was measured via uniaxial tensile tests and hole expansion tests. Their microstructures were characterized using a scanning electron microscope equipped with an electron backscatter diffraction detector and a transmission electron microscope. Nanoindentation tests were supplementally used to quantitatively reveal the microstructural homogeneity of the steels. Results show that the adjusted multiphase steel achieves an excellent ultimate tensile strength (similar to 800 MPa) and flangeability (similar to 135% hole expansion ratio). A promising homogeneous multiphase microstructure was obtained by controlling undercooled austenite transformed at about 600 degrees C. This microstructure consists of soft polygonal ferrite, blocky bainitic ferrite, and hard M/A islands. The volume fraction of M/A islands is around 5%, and the average size is less than 1 mu m. Detailed nanoindentation analysis indicated that the participation of M/A islands impressively influenced the microstructural homogeneity. Weakened strain partition and better mechanical compatibility were present in the adjusted multiphase steel since the plasticity initiation started late, which resulted in a positive flangeability. Moreover, avoiding M/A islands distributed in the chain along the rolling direction on the matrix hindered the possibility of voids coalescing into cracks and stabilized the flanging performance.