Recently, a hot stamping process by shortening the die cooling time has been studied to increase productivity. The phase transformation as martensite is completed in the boron steel below approximately 200 ℃; however, additional cooling time is required to prevent post-deformation due to non-uniform cooling. Therefore, controlling the post deformation with uniform cooling is required to reduce the cooling time rather than merely increasing the cooling performance. In this regard, developing a curved-cooling-channel processed along the die surface as the uniform depth is required rather than the conventional straight-cooling-channel method. Thus, in this study, a cooling performance that differs from the straight-cooling die, the curved-cooling die, was studied. Moreover, the post-deformation of the product using both cooling dies were analyzed through the finite element analysis for a car front pillar. According to the die cooling time, the changes in the mechanical properties of the car front pillar were studied. The change in the mechanical properties was confirmed by heating a 1.6 mm boron steel blank at 930 ℃ for 5 minutes and formed under a load of 500 tons. The die cooling time was changed from 1 second to 13 seconds to produce a prototype. Subsequently, the tensile strength test and microstructure test were fulfilled through specimens obtained from the prototype.
1. Forming, heat transfer, and phase transformation focused on high-temperature forming were examined through hot forming analysis. Moreover, the forming behavior of the product during high-temperature forming could be predicted. During hot stamping, the product had a high cooling process due to the contact with the die; thus, heat transfer by convection and radiation could be neglected. Even if the process is simplified by considering only heat transfer due to contact between the die and the blank, it is possible to confirm the high-temperature formability accurately. However, by merely using the fixed temperature of the die as a boundary condition, the cooling performance according to the shape of the cooling channel of the die and the resulting post-deformation of the product could not be grasped.
2. Through the analysis of the cooling water flow in the curved-cooling-channel die and the heat transfer in the die, it was confirmed that the flow rate of the cooling water directly affects the cooling of the die. In the curved-cooling-channel die, as the distance between the forming surface and the cooling channel is constant and the surface area of the cooling channel is the same, the speed of the cooling water can be an index to grasp the cooling performance. Therefore, the cooling performance can be only inferred by the flow analysis of the cooling water in the actual design stage.
3. The effect of the cooling-channel shape in the hot stamping process on the die cooling was examined through heat transfer in the die through the cooling channel and heat transfer analysis between the die and the product. The average temperatures of the straight-cooling-channel and the curved-cooling-channel dies were found similar; however, the straight-cooling-channel showed a highly non-uniform temperature distribution in the product compared to the curved-cooling-channel. In particular, nonuniform cooling was observed in the longitudinal direction of the channel in the straight-cooling-channel, In addition, even in the curved-cooling-channel die, the analysis confirmed that the temperature was nonuniform in the section in which the block was divided.
4. Through the post-deformation analysis, the effect of the non-uniform temperature distribution of the curved-cooling-channel die on the product deformation during the air cooling process after hot stamping was analyzed. The non-uniform temperature of the die directly affected the cooling of the product, and the temperature change of the product after hot stamping was larger than the temperature difference of the die. After air-cooling was completed, the analysis confirmed that amount of product deformation increased proportionally to the temperature difference of the die. The uncooled section of the die affected the deformation after hot stamping, but the resulting difference in the amount of deformation was less than 0.5 mm.
5. A curved-cooling-channel die was manufactured, and the temperature distribution of the prototype was measured, through a hot stamping experiment. After 4 seconds of cooling the die, the average temperature was measured to be below 200 ℃, and after 5 seconds, the temperature distribution was uniform. The results confirmed that the tensile strength of the blank increased as the die cooling time increased. After 4 seconds of cooling the die, the tensile strength maintained a similar value of 1.4 GPa or more. When the cooling time was short, tempered martensite appeared, and after 5 seconds, complete martensite was confirmed.
In general, in the hot stamping process, the die is developed by performing only hot forming analysis due to time constraints. However, a comprehensive analysis is desirable to predict the actual process accurately. The analysis can consider aspects such as high-temperature forming, the flow of cooling water, heat transfer of die and product, and post-deformation analysis. However, if the distance between the surface area of the cooling channel and the die surface is designed to be constant, it is judged that it will not be unreasonable to develop a hot stamping die using only hot forming analysis.