Microstruture Study Of Stretch-bending Failure On Dual Phase Steel Sheet

Advanced high strength dual-phase (DP) steels are widely used instructural beams of automotive because of the good combination of stressand formability, which makes them very important to reduce automotiveweight. However, it is found that different fracture modes appear duringstretch-bending forming in dual-phase steel sheets, for example, shearfracture at a small radius and side-wall necking at a larger radius,furthermore, higher strength leads to shear fracture more easily, andinternal mechanisms of all these phenomena are unclear. Advanced highstrength dual-phase steels are strengthened by martensite phase, which istotally different from those traditional ways. Therefore, the stress andformability of dual phase steels are mainly decided by martensites’volume, shape, distribution, etc. Macro-behavior is the result of themicrostructure, so the study of stretch-bending forming on dual-phasesteels is greatly meaningful to reveal the failure mechanism of shearfracture and predict the failure behavior of dual-phase steels.In this paper，different fracture modes of DP steel sheets underdifferent forming conditions have been studied by using an establishedstretch-bending forming system. Both experimental and computationalapproaches are used to reveal fracture mechanisms of different failuremodes. All the works are hoped to be supports for the further research infailure behavior of advanced high strength steels.The main works are as follows:1、Force situation of the sheets under stretch-bending is analyzed anda stretch-bending ratio is introduced to indicate different stress states.Stretch-bending experiments in HSLA340and DP590are completed forthe further study of their failure patterns.2、Stretch-bending experiments in DP steels having different strengthand thickness are carried out to figure out the impacts of this two factors on stretch-bending behavior. Stretch-bending ratio which determinesthe failure modes of DP steels is discussed. The fracture faces underdifferent conditions are observed by means of metallographic microscopeand scanning electron microscope.3、Mesoscopic modeling of DP steels under stretch-bending isstudied to establish the representative volume element models. And themacroscopic stress states of the fracture position are input as the loadcondition to the RVE models to reveal the micro failure behavior and thefailure mechanism.