Investigation On The Friction Behavior In The Hot Stamping Of Ultra High Strength Steel

Due to the demand for improvement in safety and the weight reduction of vehicles,the need to manufacture automobile structural components from ultra high strength steelsis apparent. However, the formability and shape controllability of high strength steelsdecrease with increasing strength. In order to decrease the springback, hot stamping (or ahot press forming) process, which results in better formability and lower springback atelevated temperature is becoming increasingly used in the automotive industry.Compared with conventional cold sheet metal forming, high temperature and high surfacehardness means high wear rates. Therefore, it is in desperate need of the research on thefriction and wear behavior between the mould material and high strength steel at elevatetemperature at its most fundamental.To investigate the friction and wear behavior of high strength steel in hot stampingprocess, a hot strip drawing tribo-simulator is developed and the friction coefficient,which is an important parameter in the finite element modeling, is measured. The resultshave shown that the friction coefficient remains almost unchanged until temperaturereaches500oC. It then increases sharply as temperature is increased from500oC to600oC. It has also been shown that the friction coefficient decreases as the drawing speedincreases. The change in the dominate wear mechanism as the temperature and thedrawing speed increases has been identified from SEM analyses of the worn surface. Thedominate wear mechanism is the groove cutting at temperatures between roomtemperature and500oC, which changes to the adhesive wear at temperatures above500oC. The main wear mechanism is the adhesive wear at25mm/s, which changes to theslight groove cutting at75mm/s.By the means of the finite element simulation of the hot stamping of U shaped part,the simulation result was obtained using nonlinear finite element method andmulti-physics coupling method. The value of friction coefficient obtained from theexperiment was used in the simulation. The value of the forming force was influenced bythe value of friction coefficient greatly. By revising the friction boundary condition, the forming force obtained from the finite element simulation is in close proximity to theexperimental result. The error is only5.6%. It can be seen that the accuracy of the finiteelement was enhanced to a great extent.