so the heat-transfer coefficients between blank and dies were different obviously under different BHFs. The relationship between them was shown in the Eq.1 (Ref.6). Then the relationship between heat-transfer coefficient and pressure could be obtained in Fig.4. In Fig.5, temperatures of point 1 and point 2 were all decreased with the increasing of pressure-holding time. The rate of decreasing temperature was slowed up when the pressure-holding time was more than 8s. In order to obtain a good cooling effect, the pressure-holding time should exceed 8s. So from Fig.5, we could see that from economical angle the optimal time of pressure-holding was 10s. Because of the transient thermal analysis process was adopt in the numerical simulation, the heat conduct process in forming was not considered. So from the experiment result, the temperature of point 2 was higher than point 1. That is because the center of hot blank would bend inwards at the beginning of forming, point 1 could not contact while point 2 contacted closely with the blank. But with the increasing of pressure-holding time, the temperature of point 2 decreased faster than point 1 because the distance between point 2 and cooling water was closer.
4.2. Influence of cooling water velocity on cooling effect of hot-stamping dies The velocity of cooling water of 1.5m/s, 3.5m/s, 5m/s and 10m/s were selected to simulate the quenching process of hot stamping, and temperature distributions of punch were shown in Fig.6. From pictures, the highest temperatures on the punch could be obtained, as shown in Fig.7. From Fig.6, with the increasing of cooling water velocity, the temperature on the fillet of punch decreased from 152.6ć (1.5m/s) to 137.3ć (10m/s). That was because the turbulent flows state of cooling water became more fully in the pipes with the increasing of cooling water velocity. The heat of dies was taken away by the cooling water because the heat conduct was more sufficient between the cooling water and dies. Besides, the decreasing rate of punch temperature would not be obvious by raising current velocity of cooling water when the cooling water velocity exceeded 3.5m/s. And then, under the velocity of cooling water of 3.5m/s and 10m/s, experiments were carried out to verify the simulation results. From Fig.7, the numerical simulation and experiment results agreed well with each other.
5. Conclusion Through carrying out the numerical simulation of quenching process by CFX, the cooling effect of dies was analyzed and influences of two main processing parameters on the temperature of two feature points were obtained. Considering the results achieved by the experimental and numerical simulation methods, it is concluded as follows:
(1) The mathematical simulation model of heat transfer and flow issues was built and the effect of the contact thermal resistance on the quenching process of hot stamping was the primary concern. Besides, the fitting equations of material thermo physical properties used for simulation were fitted according to the values actually measured.
(2) Under the consideration of contact thermal resistance, the temperature distribution of dies after quenching could be obtained by simulating the quenching process of hot stamping. And then the influences of processing parameters on the quenching effect of dies were analyzed, such as press-holding time and cooling water velocity.
(3) Numerical simulation and experiment results agreed well with each other. So the temperature distribution of dies during the quenching process of hot stamping could be predicted effectively, and the evaluation method of design and predicting the service life of hot-stamping dies could be provided. Acknowledgement The authors gratefully thank to M.S. Hongguang WANG of Harbin Institute of Technology for his advices on the hot-stamping experiments of AHSS. 热冲压模具英文文献和翻译(2):http://www.youerw.com/fanyi/lunwen_27333.html