Research On Numerical Simulation Of Hot Stamping Process Of Quenchable Ultra High Strength Steel

Hot stamping of ultra high strength steel is a new sheet metal forming method for modern automotive manufacture to adapt to energy crises, environmental deterioration and increasing safety requirements. It is an advanced technology that can make the strength of parts rise sharply by changing the physical chemical properties of material. Its principle is to heat the manganese boron steel plate which has strong hardenability to a temperature above the recrystallization and holding for minutes to make it austenized, then transfer it to the tools quickly for stamping, after holding for some time the microstructure of workpiece transforms to uniform martensite completely. The ultimate tensile strength of the hot stamping part can increase up to 1500MPa from about 600MPa before, and the part will get excellent wear resistance and weldability. Hot stamping process is becoming a trend of auto body safety parts forming for its apparent advantages.With the rapid development of numerical methods and computer hardware, numerical simulation is increasingly applied to engineering practice including stamping process, and it becomes a powerful tool to engineers for product development and process design. However, it is difficult for the numerical simulation of hot stamping process because of not only the inclusion of nonlinear, contacts and large deformation, but also the interaction of displacement, temperature and phase transformation.The hot stamping process of ultra high strength steel is simulated based on dynamitic explicit thermo-mechanically coupled finite element method using ABAQUS software. According to the product of actual hot stamping for auto body is usually U-shaped part, the numerical simulation of hot stamping of U-shaped part with 22MnB5 is researched on after finite element modeling for the "forming-quenching-springback" process. In this paper, there are main issues discussed that the effect of blank holder force and initial temperature of blank on forming and springback, and the effect of time and temperature of cooling water on quenching process.The dynamic explicit thermo-mechanically coupled finite element model of hot stamping process is built, and in which the effect of solid phase transformation on stress-strain state and temperature of workpiece is simplified as the volume expansion and the internal heat source of the workpiece during transformation. The cooling effect of cooling system on tools is controlled by the film condition of convection heat transfer between piper wall and cooling water. After forming and quenching simulation, the springback of the part is simulated through the explicit-implicit method. Then the whole hot stamping process of ultra high strength steel simulation is completed.For researching on the process under different BHF conditions, the U-shaped part finite element model with blank holder is built. Form the result, it could be found that the increasing BHF will conduce to the increasing plastic strain of workpiece, and even induce thinning phenomenon of the blank. The vertical edge of the U-shaped part is the area most likely thinned, and the local necking will arise at junction of vertical edge and bilge rounded corner when severe thinning happens. Through the researching, the local thinning and the temperature distribution effecting on each other is found. Usually, the area with has the highest temperature must be the thinnest. With the increasing BHF, the heat resistance increases on the horizontal wing edge while decreases in the local thinning area, that will exacerbates the nonuniformity of the part temperature. In addition, through the simulation researches on hot stamping in condition of different initial blank temperature, it is severely that the formability is effected on by the initial blank temperature. The blank with the high initial temperature has good formability and will generate less residual stress during forming. When the temperature of blank decreases, the material will become "hard" and the plastic deformation becomes difficult during forming, that induces the forming force is larger than which at high temperature. Furthermore, high temperature workpiece has larger contact area with tools, that it is benefit for the quenching later.The quenching phase of hot stamping is simulated in this paper. In hot stamping process, holding tools on closed comes to two fruits. The one makes the material of the part transform to martensite completely, the other one makes the internal stress of the part uniform through improving the distribution of stress-strain state. During quenching phase, the cooling rate of the part falls. To make the part transform to martensite completely, it must be sure that the cooling rate is above critical cooling rate of martensite transformation before the maximum temperature of the part falls below finish temperature of martensite transformation.The springback of the U-shaped hot stamping part is simulated and analysed through importing simulation result of forming and quenching. Although the springback of hot stamped U-shaped part is little, it is effected on by BHF. In one hand larger BHF makes less springback, and solid phase transformation is another factor inducing springback of hot stamped U-shaped part in the other hand. But the transformation is not a major force to springback, so it takes a low place. With the increasing BHF, the effect of phase transformation on springback decreases.Through the simulation of hot stamping of U-shaped part, the law of the part deformation and temperature distribution during Forming and quenching process has been got, and some process parameters effecting on the process was researched. The simulation could provide a theoretical guidance in industrial production.