FE-Simulation of the Heat Transfer by Defined Cooling Conditions during the Hot Stamping Process

Abstract

Increasing demands regarding security aspects and light weight construction lead to the application of advanced high strength steels (AHSS) and ultra high strength steels (UHSS) in the automotive sector. Due to high process forces and the reduced formability of these steel grades within cold forming new manufacturing technologies like the hot stamping process are required. Furthermore, crash-performance plays an important role in the automotive industry. Therefore functional optimized components are necessary. Hence, actual research work within the community is focused on manufacturing components with local adjusted mechanical properties. One of the strategies to realize the contradictorily requirements regarding energy absorption and structural integrity is the Tailored tempering process where the cooling rates are adjusted by controlled heating or cooling of different tool zones within the hot stamping process. Thereby knowledge concerning the influence of the different heated tool parts on the heat transfer and the resulting mechanical properties is necessary. Furthermore, the applicability and the accuracy of the calculation approaches used for characteristic values like the heat transfer coefficient in the FE-based simulation have to be analyzed and evaluated. Due to this experiments with a tool which exhibits a heated and a cooled zone were performed according to the Tailored tempering process. During the experiments contact pressures and tool temperatures in the heated tool part were varied and analyzed regarding the influence on the heat transfer. Furthermore, the heat transfer coefficients were calculated and verified by a numerical model built according to the experimental setup and the accuracy of the model was evaluated by the comparison of characteristic values calculated from the experimental and numerical process data.

Analysis of Transition Zone on a Hot‐Stamped Part with Tailored Tool Tempering Approach by Numerical and Physical Simulation

“…Between the blank region in contact with the cold tool and the region in contact with the hot tool, a transition zone is created, due to the thermal conductance of the blank material. [19,20] The width of the transition zone depends on the blank thickness and on the heating or cooling tools layout. [19] The width and the position of the transition zone can influence the crash performance of the component; therefore, an analysis of mechanical properties near the transition zone may be required.…”

Hot Stamping of Two Different High Strength Steel with Tailored Properties

“…As figure 3 shows, when the air gap increases to 8mm, the hardness distribution results are quite different from the existing tailor property test results. For example, the results of T.Sevc [8] et al were that the higher hardness in cooling zone gradually transited to the lower hardness in heating zone. While in the present experiment, when the air gap increased to 8mm, minimum hardness value emerged in the transition zone.…”

“…In the research of George et al [7] , a laboratory-scale hot-formed B-pillar was produced using a segmented die with local heating and cooling zones such that the cooling rate of the blank was controlled locally during the hot forming process. Svec, T et al [8] has conducted the tailoring tempering process with a plate tool comprising a heated and a cooled zone . Merklein, M et al [9] controled the cooling rate of the austenitized blank by local heating of the tool and consequently adjusting microstructure evolution and final mechanical properties.…”