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Dual Phase Steel Characterization for Tube Bending and Hydroforming Applications

The behaviour of dual phase steel tubes at 600, 780, and 980 MPa strength grades undergoing rotary draw bending and pressure sequence hydroforming is examined. Bending was performed using three different bend ratios. Principal strains were measured at several locations on each tube. It was found that the level of strain experienced by the tubes was independent of steel grade. The outcome of the bending process was stochastic in nature; for a given steel grade and bend ratio, some tubes were successfully formed, while others experienced failure. The proportion of failed tubes was found to increase with higher strength steel grades and tighter bend ratios. Metallographic samples from the extrados of the bent tubes revealed that many of the same microstructural features affecting strength and elongation in uniaxial tensile testing, namely martensite banding and non-metallic inclusions, affected dual phase steel tubes undergoing rotary draw bending. Additionally, a nanoindentation technique was investigated to determine the potential for more detailed microstructural characterization.