超高强度硼钢板热冲压成形数值模拟及试验研究

发布时间：2018-08-05 12:18

【摘要】：超高强钢冲压件在汽车车身上的应用不仅可以实现汽车轻量化,还能够提高汽车抗冲击能力,提高乘车安全性。热冲压成形技术是超高强钢冲压件的专用制造技术,硼钢板是其主要材料之一。该技术首先将硼钢板加热至完全奥氏体化,然后迅速送入带有冷却系统的模具中冲压成形,并在模具内保压淬火,在完成成形的同时实现奥氏体向马氏体的微观组织转变,从而大幅提高成形件的强度及硬度等性能。热冲压成形技术克服了超高强度钢板常温下难以成形的问题,且成形件回弹小、几何精度高。然而,相对传统冲压成形技术,温度的加入使该技术在应用过程中不仅要考虑加热因素,还要考虑冷却和相变等方面的问题。热冲压成形技术更加复杂,需要进一步系统地研究。本文从工艺参数优化、硼钢板不同金相状态下传热性能、热冲压成形模具冷却系统优化设计、热冲压成形数值模拟及试验等方面对热冲压成形技术开展了系统地研究。本文研究有利于促进该技术的工程应用,主要内容及结论如下:(1)热冲压成形过程中淬火阶段工艺参数优化研究通过平板件热冲压成形试验,分析了热冲压成形过程中淬火阶段接触压力和保压时间对成形件微观组织和机械性能的影响;并从生产效率角度考虑,对接触压力和保压时间进行了优化。研究发现:2 mm厚度22Mn B5热冲压成形过程中,当以室温下的水为冷却介质时,若淬火阶段压力小于0.3 MPa,零件淬火后马氏体转变不充分;0.3 MPa压力时,零件淬火后马氏体转变充分,强度及硬度性能良好,但完全马氏体转变所需最短保压时间较长;0.3~1.0 MPa压力时,最短保压时间随压力的增大而显著减少;1.0 MPa以上压力时,最短保压时间随压力的增大变化不显著。因此,当以室温下的水为冷却介质时,2 mm厚度22Mn B5热冲压成形过程淬火阶段最佳的压力选取范围为1.0 MPa以上;在该压力下,钢板以800℃开始冷却时,最短保压时间为8 s。(2)硼钢板不同金相状态下传热性能研究基于牛顿冷却定律微分方程,考虑硼钢板热冲压成形过程中组织转变及相变潜热的释放,构建了板料与模具间热传递系数和板料等效比热容的计算模型。利用该模型,通过平板件热冲压成形试验,揭示了热冲压成形过程中发生完全马氏体转变时硼钢板在不同金相状态下等效比热容和热传递系数的变化规律,以及成形条件对不同金相状态下热传递系数的影响规律。研究发现:淬火过程中,随着板料温度的降低,热传递系数在不同金相状态下变化明显不同,奥氏体状态下的热传递系数变化幅度不大,相变过程中热传递系数急剧增大后缓慢减小,马氏体状态下热传递系数缓慢减小;热传递系数的变化与等效比热容的变化关系密切,马氏体相变过程中大量相变潜热的释放使等效比热容显著变化,从而导致相变过程中热传递系数剧烈变化;成形条件对热传递系数的变化也有一定影响,随着压力的增大和模具型面初始温度的降低,各金相状态下的热传递系数均增大,其中压力的影响更为显著。(3)热冲压成形模具冷却系统设计及优化基于能量守恒定律、湍流理论和形状因子法,以冷却管道直径、管道数量及管道中心与模具型面间距为变量,建立了热冲压成形模具冷却管道参数的选取准则;并基于零件不同位置冷却难易程度,提出了冷却管道结构优化的方法。以汽车车身结构中典型超高强钢热冲压成形件——防撞梁为例,设计了热冲压成形模具冷却管道模型,构建了带有冷却管道的防撞梁热冲压成形二维数值模拟模型,以冷却强度、冷却均匀性及模具强度为衡量标准,利用数值模拟技术分析了所设计冷却系统的冷却效果,进而对冷却管道结构进行了优化。结果表明:基于本文准则所设计的冷却系统,其冷却强度和模具强度均能达到要求;从冷却均匀性角度考虑,利用较小的冷却管道直径所设计的冷却系统冷却效果更好。此外,冷却管道在截面方向上的排布可根据零件相应位置冷却难易程度进行优化。根据冷却管道等间距排布时零件淬火后的温度分布情况,可将零件不同位置的冷却难易程度分为不同等级。在保证模具强度的前提下,对于最难冷却区域,冷却管道可以排布得更密集,且管道中心距模具型面可以更近,管道数量可以更多;对于较难冷却区域,仅取更近的管道中心与模具型面间距即可;对于容易冷却区域,冷却管道排布相对等间距排布时可不改变。模拟结果表明,结构优化后的冷却系统,其冷却均匀性和冷却强度均可得到提高。(4)超高强度硼钢板热冲压成形数值模拟研究以防撞梁为研究对象,构建了三维热冲压成形数值模拟模型,利用数值模拟技术揭示了热冲压成形过程中钢板温度、厚度、微观组织及机械性能的变化规律,分析了冲压阶段和淬火阶段工艺参数对成形件组织及性能等的影响规律,优化了防撞梁热冲压成形工艺参数。研究发现:热成形过程中,板料厚度变化主要发生在快速冲压阶段,其微观组织及性能变化主要发生在淬火阶段,热冲压成形件卸载后的回弹量很小;在500℃以上初始温度下快速冲压时,初始成形温度对板料微观组织和硬度影响不显著,但对板料厚度变化影响显著,较低的初始成形温度易导致板料减薄过大;过低的冲压速度容易导致板料成形过程中热量散失过多,局部发生相变强化,成形性降低,而过高的冲压速度易使板料在高温下塑性变形过大而导致厚度减薄严重;设置压料板对板料热冲压成形过程中温度、微观组织及硬度的影响不显著,但大大降低了板料成形过程中的流动性,从而使板料减薄严重,甚至拉裂;淬火过程中,若淬火压力过小,或保压时间过短,均得不到完全的马氏体组织;对本文所研究的防撞梁热冲压成形而言,最佳的工艺参数为800℃初始成形温度、100mm/s冲压速度、无压料板、5 MPa以上淬火压力和8 s的保压时间。(5)超高强度硼钢板热冲压成形试验研究基于本文热成形模具冷却系统设计及结构优化方法,设计加工了防撞梁热冲压成形模具。利用所加工模具,开展了直接和间接热冲压成形试验,获得了合格的直接和间接热冲压成形件,验证了本文热成形模具冷却管道设计及结构优化方法的合理性。结合防撞梁热成形数值模拟结果,开展了一系列直接热冲压成形试验,通过试验方法研究了工艺参数对防撞梁热冲压成形的影响。试验结果与相应模拟结果吻合,证明了热冲压成形数值模拟的准确性。
[Abstract]:The application of super high strength steel stamping parts on automobile body can not only realize automobile lightweight, but also improve the impact resistance and safety of the car. Hot stamping forming is a special manufacturing technology for super high strength steel stamping parts. Boron steel plate is one of its main materials. The technology first heat the boron steel plate to complete austenitizing. And then quickly sent into the mold with the cooling system, and pressing and quenching in the mould, and realizing the transformation of austenite to martensite microstructure at the same time of forming, thus greatly improving the strength and hardness of the forming parts. The hot stamping technology overcomes the problem that the super high strength steel plate is difficult to form at normal temperature. The forming parts have small springback and high geometric precision. However, relative to the traditional stamping technology, the addition of temperature makes the technology not only consider the heating factors, but also consider the problems of cooling and phase transformation. The technology of hot stamping is more complex and needs further study. Heat transfer performance under metallographic state, optimum design of cooling system for hot stamping die, numerical simulation and test of hot stamping are systematically studied. This paper is helpful to promote the engineering application of this technology. The main contents and conclusions are as follows: (1) the process parameter of quenching in the process of hot stamping forming. The influence of contact pressure and pressure time on the microstructure and mechanical properties of the formed parts during hot stamping process was analyzed by hot stamping test of plate parts. The contact pressure and pressure holding time were optimized from the angle of production efficiency. It was found that 2 mm thickness 22Mn B5 hot stamping forming During the process, when the water at room temperature is used as the cooling medium, if the quenching stage pressure is less than 0.3 MPa, the martensite transformation is not sufficient after the quenching of the parts. When 0.3 MPa pressure, the martensite transformation is full and the strength and hardness performance is good after the quenching of the parts, but the shortest holding time required for the complete martensite transformation is longer; the shortest holding time when the 0.3~1.0 MPa pressure is pressed is the shortest holding time. With the pressure increasing, the minimum pressure time of 1 MPa is not significant with the increase of pressure. Therefore, when the water at room temperature is the cooling medium, the optimum pressure range of the quenching stage of the 2 mm thickness 22Mn B5 forming process is 1 MPa, and the steel plate is cooled at 800 C under this pressure. The heat transfer performance of the boron steel plate under the different metallographic state is 8 s. (2), which is based on the differential equation of the Newton's cooling law. The calculation model of the heat transfer coefficient and the equivalent heat capacity between the die and the die is constructed by considering the release of the microstructure transformation and the latent heat of the phase change during the hot stamping process of the boron plate. The change law of the equivalent heat capacity and heat transfer coefficient of boron steel plate under different metallographic state and the influence of forming conditions on the heat transfer coefficient under different metallographic state are revealed in the hot stamping forming test. The thermal transfer coefficient varies obviously in different metallographic states. The change of heat transfer coefficient in austenite is not significant. The heat transfer coefficient decreases rapidly in the process of phase transition, and the heat transfer coefficient decreases slowly in martensitic state; the change of heat transfer coefficient is closely related to the change of equivalent heat capacity, martensitic transformation is closely related. The release of a large amount of latent heat in the process changes the equivalent specific heat capacity, which leads to a sharp change in the heat transfer coefficient in the process of phase transition, and the forming conditions have a certain influence on the change of the heat transfer coefficient. (3) the design and optimization of the cooling system of hot stamping die are based on the law of conservation of energy, the theory of turbulence and the method of shape factor, based on the diameter of the cooling pipe, the number of pipes and the distance between the center of the pipe and the mould surface as variables, the selection criteria for the parameters of the cooling pipe of the hot stamping die are established, and based on the different positions of the parts. The cooling pipe structure optimization method is put forward. The model of hot stamping die cooling pipe is designed by taking the typical super high strength steel hot stamping parts in automobile body structure as an example, and a two-dimensional numerical simulation model with cooling pipe is built for the cooling strength and cooling. The cooling efficiency of the designed cooling system is analyzed by the numerical simulation technology, and the structure of the cooling pipe is optimized. The results show that the cooling strength and die strength of the cooling system based on this criterion can reach the requirements; from the angle of cooling uniformity, the utilization of the cooling system is smaller. The cooling system designed by the diameter of the cooling pipe has a better cooling effect. In addition, the arrangement of the cooling pipe in the direction of the section can be optimized according to the difficulty of cooling the parts according to the corresponding position. The cooling difficulty of the parts can be divided into different degrees in different positions according to the temperature distribution of the parts quenched at the same distance between the cooling pipes. Grade. On the premise of guaranteeing the strength of the die, the cooling pipe can be arranged more densely for the most difficult cooling area, and the pipe center can be closer to the mold surface and the number of pipes can be more; for the difficult cooling area, only the closer distance between the pipe center and the mold surface can be taken; for the easy cooling area, the cooling pipe arrangement phase The simulation results show that the cooling uniformity and the cooling strength of the cooling system after the optimized structure can be improved. (4) the numerical simulation of the hot stamping forming of the ultra high strength boron steel plate is studied with the collision beam as the research object, and the numerical simulation model of the 3D hot stamping is constructed and the numerical simulation technology is used to uncover the numerical simulation technology. The change law of the temperature, thickness, microstructure and mechanical properties of the steel sheet during hot stamping was shown. The influence rules of the process parameters on the microstructure and properties of the forming parts were analyzed, and the parameters of the hot stamping process were optimized. The study found that the thickness of the sheet was mainly changed during the process of hot forming. In the rapid stamping stage, the change of microstructure and properties mainly occurs in the quenching stage, and the rebound of the hot stamping parts is very small after unloading. The initial forming temperature has no significant influence on the microstructure and hardness of the sheet material at the initial temperature above 500 degrees C, but it has a significant influence on the sheet thickness and the lower initial forming temperature. It is easy to reduce the thinning of the sheet material easily; the low stamping speed can easily lead to excessive heat loss during the sheet forming process, the local phase transformation and the formability decrease, and the high stamping speed causes the thickness of the sheet to be too large at high temperature, which causes the thickness to be thinned seriously, and the temperature of the sheet material during the hot stamping process is set up. The effect of microstructure and hardness is not significant, but it greatly reduces the fluidity in the process of sheet metal forming, which makes the sheet thinning serious and even crack. In the process of quenching, if the quenching pressure is too small or the pressure time is too short, the complete martensitic structure can not be obtained. For the initial forming temperature of 800 C, 100mm/s stamping speed, no press plate, 5 MPa quenching pressure and 8 s holding time. (5) experimental research on hot stamping forming of ultra high strength boron steel plate, based on the design of cooling system and structure optimization method of the hot forming die, the hot stamping die of anti collision beam was designed and processed. The direct and indirect hot stamping tests have been made. The qualified direct and indirect hot stamping parts are obtained. The rationality of the design and structure optimization of the cooling pipe of the hot forming die is verified. A series of direct hot stamping tests are carried out in combination with the results of the numerical simulation of the thermal forming of the bump beam. The process parameters are studied by the test method. The experimental results agree well with the corresponding simulation results, which proves the accuracy of the numerical simulation of hot stamping.
【学位授予单位】：吉林大学
【学位级别】：博士
【学位授予年份】：2016
【分类号】：TG306

【相似文献】