Microstructure, property and deformation and fracture behavior of 800 MPa complex phase steel with different coiling temperatures

The microstructure characteristics and properties (especially hole expansion property) of 800 MPa hot-rolled complex phase steel with different coiling temperatures were studied. The microstructure consisted of polygonal ferrite and precipitates when the steel was coiled at 550 °C, and when the steel was coiled between 460–520 °C, the microstructure was composed of granular bainite and martensite and austenite (M/A) islands. The morphology of the crack was analyzed by scanning electron microscopy, and the in situ scanning electron microscope tensile test was used to find out the fracture mechanism and deformation behavior of the steel with different coiling temperatures. When the steel was coiled at 550 °C, the cracks initiated at the ferrite grain boundary and propagated through the grains or along the grain boundaries. When the steel was coiled at 520 °C, the cracks first initiated at the junction of ferrite and M/A island and then propagated through the grains. The steel coiled at 520 °C has quite good mechanical properties and relatively high hole expansion ratio.