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Hot Stamping Die Design for Vehicle Door Beams using Ultra High Strength Steel Chao Jiang Zhongde Shan Bailiang Zhuang Milan Zhang and Ying Xu KEYWORDS Hotforming Ultra highstrengthsteel Mechanical characteristics Automobile lightweight The energy saving and safety is the eternal theme of development of automobile industry Ultra high strength steel automotive body structures of hot stamping have been widely used with these dual advantages that reducing vehicle weight while enhancing safety performance at the same time The forming and quenching integration die was investigated in this paper with a vehicle door beam as an example especially the whole die structure and hot stamping process were optimized through numerical simulation of die strength cooling pipe arrangement and other related factors and beam samples with tensile strength 1550Mpa elongation rate 6 5 and shape accuracy of 0 3mm were produced by this die Moreover the stiffness increased by 2 2 times than the original tube beam and 3 8 times of the intensity which led to full score in C NCAP Crash Test By reducing the thickness cross section and the depth of drawing comparing with the original tubular beam door beam weight could be reducedby9 32 whichhadeffectiveenergy savingand emission reduction effect 1 Introduction Light weight auto body and passive safety of passengers become trend of automotive industry while energy saving and environment protecting deeply wised Application ofultra high strengthsteel with dual advantages of weight light and safety improvement performance grows rapidly also with characteristics of both high strength and high precision has become a industry hotspot On one hand the forming process parameters are the key points of hot stamping technology on the other hand hot stamping die needs to set cooling system to ensure the die function of stamping and quenching which is quite different from the common stamping mold The main parameters including heating temperature holding time forming speed impulse pressure holding time open mold temperature flow velocity and etc should be optimized during hot stamping process primarily for guaranteed high intensity and highprecision of forming parts Taking a Chinese independent brand car door beam as an example ultra high strength steel hot stamping technology and lightweight design were studied in this paper 1 3 2 Development of hot stamping die for ultra high strength steel door beam 2 1 Material optimization of hot stamping die During hot stamping process phase transformation strengthening of parts after forming is completed through the dies so the dies require creation of cooling pipes inside to realize a cooling quenching function 4 5 From the point of view for material properties die material must have high thermal conductivity coefficient in order to achieve rapid and uniform cooling effect better thermal fatigue performance and high heat strength to work under long term alternation of heating and cooling state strong wear resistance to bear thermal friction of high temperature blank and its oxidation skin 6 8 Hot working die steel material of HHD containing high chromium in the composition shown in Table 1 to enhance its corrosion resistance was used Under normal temperature the Table 1 Composition of HHD wt CCrMoNiVWSiMn 0 2 0 3 5 8 0 13 0 1 0 2 00 7 1 30 4 1 00 3 1 00 7 1 30 2 1 0 hardness of HHD was above HRC48 which could keep as HV 498 2 at 600 showing high strength and good thermal stability at high temperature And the material sh ows excellent wear resistance at high temperature which is less than a third of that ofASSAB8407 2 2 Development of ultra high strength steel door beam and stamping die cooling system Tubular beam including seamless tube and seam is a kind of door beam among which the seam welded pipes with a maximum tensile strength of about 400MPa were manufactured by welding steel plates after being bent into pipes and seamless steel pipes manufactured by drawing process had the tensile strength up to 600MPa rare hardened tube got a tensile strength of 1400MPa These door beams had simple structure and low manufacture cost but relatively poor protection performance Another kind of anticollision door beam known as hat shape beam was mainly divided into single cap shape U type and double hat shape W type whose tensile strength could reach above 1500MPa and get higher safety performance through hot stamping process more widely used in European andAmerican cars Adoor beam was optimized from seamless pipe to two hat shape thickness of 2mm length of 1071mm and width of 99 9mm with the following shape shown in Fig 1 Cooling pipes were arranged in even distribution to maintain good cooling efficiency as shown in Fig 2 bolt seal method Was used on the end of die and O shape sealing rings were used at the bottom to prevent leakage of high speed cooling water circulation The cooling rate was guaranteed by water velocity regulated according to production cycle and the appropriate temperature of cooling water was selected 9 10 2 3 Design of cooling parameters Cooling system of hot stamping die influences not only the completion of forming and quenching but also final properties of the parts The parameters include such three aspects as depth from die surface to cooling pipe spacing between pipes pipe center distance and the diameter of cooling pipe that is location arrangement and pipe shape Pre condition for a numerical simulation as initial die temperature of 20 initial blank temperature of 890 cooling water flow rate of 1m s and other cooling parameters are given in Table 2 The simulation results were shown in Fig 3 As shown in Fig 3 with increasing of depth from die surface to cooling pipe and space between pipes the average cooling rate decreases with increasing of pipe diameter the average cooling rate linearly increases And the greatest influence factor on cooling effectiveness is depth of pipe from the surface followed by pipe spacing and finally pipe diameter that is calculative determination of depth from die surface to cooling pipe should be considered first during design of die cooling system and it is also the basis of the reasonable design of pipe spacing and pipe diameter The depth from die surface to cooling pipe of 10mm pipe spacing of 15mm and pipe diameter of 10mm was the optimized result of the Simulation The design of cooling pipe should make sure that the die could keep ensuring sufficient strength during hot stamping process so the overall strength intensity of the die needs to be checked firstly The next numerical simulation boundary conditions were as friction coefficient of 0 03 forming speed of 50mm s the stress field and force were shown in Fig 4 The results showed that there was no damage on die because the maximum deformation was only 0 027mm which was in the elastic deformation range The stress simulation results showed that the stress is far less than that of the blank mechanical strength cracking phenomena would not happen The corresponding door beam die entities was shown in Fig 5 超高强度钢在车辆门梁热冲压模具设计中的应用 关键字 热成型 超高强度钢 机械特性 汽车轻量化 纵观汽车的发展 节能和安全可靠性是它永恒的主题 由于超 高强度钢既可以降低车身的重量并且大大的提高安全可靠性这双重 优点 它已经被广泛的应用在热冲压车身结构中 在这些研究中 以 汽车门梁的热冲压模具作为一个例子 特别是通过数值模拟模具的强 度来对整个模具结构和热冲压工艺进行优化 而冷却管和其他相关的 因素 以及梁的平均抗拉强度为 1550MP 伸长率 6 5 和 0 3mm 的 形状精度 以此来确定该如何生产该模具 除此之外 该零件的刚度 比原来材料的管梁强度提高了 2 2 倍和 3 8 倍 这是在 C NCPA 碰撞 试验中得来的 通过减少横截面的厚度和深度的比较 试验样本比原 来的管状梁的重量可以减少 9 32 其中节能减排的效果显著 1 导论 汽车车身轻量化 更加注重乘客的被动安全性已经成为了汽车行 业发展的趋势 而与此同时节约能源和环保的意识也在被社会所关 注 而超强度钢的重量轻与很好的安全性能这双重优点已使得它的需 求量在迅速增长 而其兼具的高强度和高精确度的特色 也成为业界 关注的热点 一方面 热冲压技术的成型过程参数是关键点 另一方 面 热冲压模具需要设置冷却系统 从而保证冲压模具的各种功能和 淬火的实现 这与其他的普通冲压模具的表现并不相同 而其中的主 要参数包括加热温度 保温时间 成型速度 冲击压力 保压时间 开模温度 流速等等 在热温度方面的优化 主要是为了保证冲压工 艺成型件的高强度 高精度 以中国的一个自主品牌汽车作为例子 门梁的超高强度钢板的热冲压技术和轻量化设计进行了研究 2 1 超高强度钢门梁的热冲压模具的发展 在对热冲压模具的材料在冲压过程中的工艺优化 变相通过模具 加强了零件的形成 所以需要在模具内部设计冷却水管 从而实现了 冷却淬火 从材料的属性角度来看 模具材料必须具有很高的热传导 系数 以实现快速的并且均匀的冷却效果 在长期交替加热和冷却的 状态下 这需要更好的耐热疲劳性和较高的高温强度 高强度的耐磨 损性 承受热摩擦的高温和表面氧化 高温工作下的模具钢材料含有较最高的HHD 在合金材料中铬 示 于表 1 用来提高其耐腐蚀性 在正常温度下 HHD 的组成部分如表 1 重量 CCrMoNiVWSiMn 0 2 0 35 8 0 1 3 0 1 0 2 0 0 7 1 3 0 4 1 0 0 3 1 0 0 7 1 3 0 2 1 0 表 1 硬度高于 HRC48 的 HHD 可以保持 HV498 2600 在高温下依旧 拥有很高的强度和良好的热稳定性 和一般的材料相比 在同样温度 下 耐磨性要优于三分之一 2 2 超高强度钢门梁和冲压模具冷却系统的的开发 门梁包括无缝管和有缝管两种 其中有缝管最多的制作方式是通 过焊接制造 用抗拉强度约 400MPa 焊管弯曲而成 无缝管是在拉丝 工艺生产之后 钢板的拉伸强度高达 600MPa 极少的硬质管可以得 到抗拉强度为 1