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Abstract As the development of hot-stamping technology, the designing of hot-stamping dies has more and more request. The cooling effect of hot-stamping dies can affect the mechanical properties of hot-stamping parts and the production efficiency of hot-stamping process directly, so it is an important index to evaluate the performance of dies. 31245
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The cooling effect of hot-stamping dies was simulated by CFX software, and the define method of contact thermo resistance was presented in the process of simulation. Besides, the effect of processing parameters, such as pressure-holding time and cooling water velocity, on the cooling effect of hot-stamping dies was investigated and verified by experiments. Comparisons show that numerical simulation results have a good agreement with experimental results, so the FE model is effective in simulation of hot-stamping process. © 2011 Published by Elsevier Ltd. Selection and/or peer-review under responsibility of [name organizer] Keywords: Hot stamping dies;Cooling effect; Numerical simulation; Contact thermal resistance; Processing parameter 1. Introduction Low-carbon transportation is one of important methods to achieve the emissions reduction and develop low-carbon economy. As an important delivery vehicles, the carbon emissions of cars account 5%~8% of the overall carbon emissions. Therefore, the energy reduction of car is very important to develop low-carbon economy. The ultra-high strength steel is more and more used to produce automobile components because of the development of automobile lightweight, so the hot stamping technology gets rapid © 2012 Published by Elsevier B.V. Selection and/or peer-review under responsibility of Garry LeeOpen access under CC BY-NC-ND license. development. The hot stamping technology also called dies quenching technology is put the heated blank into dies to form and quench rapidly. In order to achieve high strength by hot stamping with high strength steels, blanks should be heated above austenitic temperature and then cooled rapidly such that the martensitic transformation will occur. Normally, the tools are heated up to 200°C without active cooling systems in serial production. However, in hot stamping processes, the tool temperature must maintain below 200°C to achieve high strength [Ref.1]. So the cooling effect of hot-stamping dies can affect the mechanical properties of hot-stamping parts and the production efficiency of hot-stamping process directly. Besides, thermal softening effects induced by the dies’ temperature rise gradually and reduce dies’ hardness, eventually causing plastic deformation. The common cooling methods of hot-stamping dies can be pided to cooling by dies themselves and cooling water system, the research of cooling effect on hot-stamping dies is very little until now. In Ref 2, temperature changes of hot-stamping dies were studied for many times cycles by comparing thermal analysis and thermo-mechanical analysis, but the influences of processing parameters on temperature distribution were not studied. In Ref.3, the design method of hot-stamping dies and optimization schemes of cooling system were presented. The heat transfer process was simulated by the method of setting heat-transfer coefficients on the pipe surfaces of cooling system, but the process of turbulent flow in the pipes could not be simulated. In Ref.4 and 5, cooling effects of different cooling circuits in hot-stamping dies were analyzed and the critical flow rate of cooling water was presented, but the contact thermal resistance was not considered in the process of hot forming.
In this paper, BR1500HS steel was selected as AHSS. Regarding hot-stamping dies for square-cup part, influences of processing parameters on effects of dies were investigated by numerical simulation. Because of the complexity of dies, the punch was selected as the study object. The influence of two main processing parameters, namely blank holder force (BHF) and velocity of cooling water, was verified by experiment. So the simulation method could examine the reasonableness of design and well predict the properties of formed parts. 2. FEM model Hot stamping dies of square-cup part concluded four parts: punch, die, holder and depth adjuster. The dies with cooling water system were set tanks to gather pipes together in the entrances and exits, and two pipes were used to connect the tank. According to cooling rates obtained by thermograph cameras, the flow control valves could be adjusted by PLC. As shown in Fig.1, 3D model was simplified, filled with fluid and meshed based on the experiment dies. For the punch, the cooling system was shown in Fig.2 (a). Two feature points were selected as the temperature measuring points, and their positions were shown in Fig.2 (b). Temperatures distributions could be obtained by numerical simulation, and experiment results were recorded by thermograph cameras. The parameters of AHSS were shown in Table 1, and thermo physical parameters and heat capacity of blank were shown in Fig.3. The fitting equations of material thermo physical properties were used for simulation through OriginPro.8.0. 3. Definition of contact The contact thermo resistance was directly affected by BHF,