An Optimal Punch Design for Multiple Hole Punching Operations and Thickening of Hole Edge in the Hot Stamping Process

• Main Authors: Xiang-Yang Lim, 林翔陽
• Other Authors: 陳復國
• Format: Others
• Language: zh-TW
• Published: 2017
• Online Access: http://ndltd.ncl.edu.tw/handle/32b5cc

碩士 === 國立臺灣大學 === 機械工程學研究所 === 105 === In recent years, with the awareness of environmental protection and energy conservation, the reduction in weight of automotive structures is one of the goal of the automobile industries. At present, the using of high strength steel as the automotive structure in order to meet the strength, safety, and lightweight. However, increasing the strength of steels leads to the defects of low formability and spring back in sheet metal forming, which disadvantageous to the design of the product. While in the hot stamping process the above-mentioned forming defects could be much improved and the tensile strength of the product parts is above 1400MPa. After hot stamping, hot stamped parts generally require a subsequent shearing process such as punching, trimming, and blanking. However, it is not easy to punch hot stamped parts having high strength. The extremely high strength of hot stamped parts cause severe tool wear due to large punching load. Because of high costs for repairing punching tools, laser cutting has been used in almost all automotive industries in spite of the long process time and the high production cost, it is necessary to develop economical shearing methods for use on the hot stamped parts. In this study first focus on the mechanical punching method studies, then the finite element simulations of punching of hot stamped parts are performed to investigate the effects of the process parameters on the sheared surface quality and the maximum punching force. It is found that the punching temperature of 22MnB5 between 550°C and 650°C, the clearance is less than 12.5%, and hot semi punching depth is above 80% are suitable for the process conditions during hot stamping and simultaneous punching in a single press. The parameters from this range are applied to the study of thickening of hole edge in the simulation. The onset and progress of fatigue cracks of punched hot stamped parts are accelerated around holes due to concentration of stress, residual stress and sheared surface quality. The improvement of the quality of the sheared surface and the increasing of thickening of the hole edge improve the fatigue strength of high strength steel in the research. In this study, a punching process including thickening of a hole edge of the hot stamped parts with a punch and die shape to improve fatigue strength of the punched hot stamped parts during the hot stamping process. However, it is not easy to design cooling channels for taking out punching scraps from die, a double taper punch is designed to improve the hot semi punching depth above 90% (thickness of blank), fully punching after quenching. It is shown that the maximum punching force and production time can be reduced effectively. Finally, punches design are investigated by finite element simulation and experiment for the actual process in the future.