Fracture Mechanism And Prediction For Advanced High Strength Steel In Roll Forming Process

Because of the high strength and better energy absorption, advanced high strength steel (AHSS) is widely used in automobile industry to meet the increasing demand of light weight and high stiffness, especially the dual phrase steel. The appearance of advanced high strength steel makes contribution to the light weight and low emission of automobiles.Roll forming is a new method to produce AHSS auto parts, due to its advantage that metal materials can achieve the maximum values of stiffness and strength at lower consumption in this process. So it is suitable to manufacture the long and thin AHSS parts, such as bumpers and B-pillar upper reinforcement. However, because of the lower elongation of AHSS, fracture has become a severe defect in roll forming process for manufacturing AHSS production.In this study, dual phase steel of980MPa is used for investigation. By designing a set of rollers, observing the micro-structure of the fracture surface and establishing the finite element model of roll forming, the fracture mechanism of DP980steel in roll forming process is obtained from micro observation and macro mechanics analysis. Also, the constitutive model and fracture criterion of DP980steel, which is associated with strain ratio, are established to implement the precise simulation of roll forming and predict the appearance of fracture.The uniaxial tensile test, shear test and plane strain tensile test of DP980steel are conducted to attain the mechanics parameter of the material, which is used for creating the static constitutive model of this material. The constitutive model is input into the FE simulation and by contrast of the load-displacement curves both in experiment and simulation, the precision of FE model is verified.According to the number of forming pass and the cross sections in each pass, two sets of rollers which form parts with90°and180°bended corners are designed. Then the two types of roll forming experiments are conducted respectively, and the results are that fracture does not occur on roll formed part with90°bended corner and obvious crack is founded in the bended area of roll formed part with180°bended corner.The Scanning electron microscope is used to observe the micro-structure of the fracture surface on roll formed part, and the result of initial analysis is that the fracture mechanism of roll formed part is complex. The stress states of the bended area dominated by the combination of tensile and shear. Furthermore, the FE model of roll forming process with DP980steel is established, and then, the mechanics data of the simulation result is extracted for macro mechanics analysis, verifying the results obtained from micro-structure observation. Also, bend forming experiment which forms the same bending radius with the roll formed part is conducted, and the fracture mechanism of the DP980bend formed part is confirmed with the same research approach.Series of tensile tests with different strain ratio are conducted to create the strain-ratio coupled KHL constitutive model of DP980steel according to the researched fracture mechanism of the roll formed part. Then the constitutive model is input into ABAQUS, which could ensure the precision of the roll forming simulation. Also, the parameters and fracture critical value of the ductile fracture criterion which associates with DP980steel are calculated. By using these fracture criteria, the fracture mechanism of roll forming and bend forming process can be further analyzed. Associated with KHL constitutive model and Oyane criterion, the fracture prediction of roll forming process can be conducted. The predictions fit well with the experiment results and the validation of this fracture criterion proves suitable for roll forming process.