高强钢热冲压成形性及微观力学性能数值预测

发布时间：2019-05-27 18:27

【摘要】：为了实现汽车轻量化、降低燃油消耗同时保证碰撞安全性能,高强度钢板热冲压技术被广泛应用于制造汽车车身结构零部件。与传统冷冲压成形过程不同,热冲压过程包括板料的加热、成形及淬火流程,是一个温度场、应力场及相变场相互耦合的复杂过程。在该过程中,板料及所使用模具的温度发生变化,板料的成形性会由于冲压工艺参数及模具设计的不合理而发生恶化,板料内部的微观组织也会发生演变。因此,对热冲压过程进行准确建模和数值预测对于指导实际的热冲压工业化生产具有重要的意义。本博士论文针对高强度钢板热冲压过程中的温度场、成形性及微观组织演变进行数值预测和试验验证,主要工作有：(1)基于热冲压过程中温度场、应力场及相变场之间的相互耦合作用,建立了热、力及相变多物理场相耦合的本构方程。通过热力拉伸试验测试了材料在高温奥氏体状态下的流动力学性能,研究了温度及应变率对材料流动行为的影响。结合材料在高温铁素体、珠光体、贝氏体及马氏体状态下的流动力学性能,并考虑奥氏体变形时的等效应力对材料扩散型相变动力学的影响,采用动力显式有限元算法开发了可用于高强度钢板热冲压数值模拟的有限元软件King Mesh Analysis System_Hot Stamping (KMAS_HS)。(2)开发了热冲压三维温度场分析预测模块。概述了热冲压的传热过程,通过考虑板料微观组织相变潜热释放建立了热冲压热传导微分方程。结合热冲压瞬态温度场问题的初始及边界条件,采用Galerkin加权余量法对热冲压三维瞬态温度场微分方程进行求解,导出了热冲压瞬态温度场有限元的一般形式。分别采用温度壳单元及三维四面体单元对热冲压板料和带有冷却系统的模具进行有限元离散。采用该分析模块计算了U型试件热冲压过程中板料、三维实体模具及模具冷却管道的温度分布,并与实际试验进行对比分析,可以看出数值结果与试验结果较一致。(3)开发了热冲压板料成形性分析预测模块。分别将四种非耦合的韧性断裂准则(Oh、Brozzo、Ayada和Rice-Tracey)和基于全耦合连续损伤力学的Lemaitre模型引入到热冲压有限元软件KMAS_HS中,比较了它们对热冲压板料成形性的预测能力。进行了高强度钢板在高温下的成形极限试验,获得了不同温度下的成形极限曲线,通过优化对比采用四种非耦合的韧性断裂准则预测的成形极限曲线与试验所得曲线,确定了四种韧性断裂准则中与温度相关的材料临界常数。对热力拉伸过程进行数值模拟,通过优化方法、数值模拟及试验相结合的方式确定了与温度和应变率相关的材料损伤参数。分别采用四种韧性断裂准则及Lemaitre损伤力学模型预测了一款汽车B柱在热冲压过程中的成形性,并进行相应的试验验证,得出基于连续损伤力学的Lemaitre模型结合有限元的方法可以准确预测热冲压过程中的韧性损伤及断裂。随后基于该损伤力学模型又进一步研究了压边力及摩擦系数对热冲压板料成形性、损伤演化和凸模力的影响。(4)开发了热冲压微观组织演变及成形后零件力学性能分析预测模块。采用Newton-Raphson迭代法对扩散型相变动力学方程进行求解,运用Scheil叠加准则将等温相变动力学模型应用于非等温过程,并考虑奥氏体变形时的等效应力对扩散型相变孕育时间的影响。针对热冲压过程,分别采用Li和Akerstrom扩散型相变动力学模型结合Koistinen-Marburger (K-M)非扩散型相变动力学模型模拟汽车B柱热冲压过程中的微观组织演变,采用Maynier硬度模型预测成形后零件的力学性能,并进行相应的验证试验,结果表明采用Li模型结合K-M模型的数值预测结果与试验结果一致性较好。针对梯度硬度热冲压过程,采用适应能力更强的Lee扩散型相变动力学模型结合Yu非扩散型相变动力学模型预测S型梁梯度硬度热冲压过程。Lee模型中的材料参数基于材料在应力自由状态下的连续冷却转变曲线通过优化的方法确定,此外,还建立和标定了与冷却速率相关的硬度预测模型。并进行相应的验证试验,结果表明采用模具分区加热冷却的方法可实现零件的硬度梯度分布,数值结果与试验结果的一致性证明了所建立的多场耦合本构方程、相变动力学模型及硬度预测模型的正确性。
[Abstract]:In ord to realize that light weight of the automobile, reduce the fuel consumption and ensure the impact safety performance, the high-strength steel plate hot stamping technology is widely applied to the manufacture of automobile body structural parts. In contrast to the traditional cold press forming process, the hot stamping process includes the heating, forming and quenching process of the sheet, which is a complex process of the mutual coupling between the temperature field, the stress field and the phase transition field. In the process, the temperature of the sheet and the used mold is changed, and the formability of the sheet can be deteriorated due to the improper punching process parameters and the die design, and the microstructure in the sheet metal can also change. Therefore, the accurate modeling and numerical prediction of the hot stamping process is of great significance for guiding the practical hot stamping industrial production. This dissertation is based on the mutual coupling between the temperature field, the stress field and the phase change field during the hot stamping process of the high-strength steel plate, and the heat is established. Constitutive equation of the coupling between the force and the phase-change multi-physical field. The effect of temperature and strain rate on the flow behavior of the material was studied by means of thermal tensile test. The dynamic performance of the bonding material in the high temperature ferrite, pearlite, bainite and martensite state is considered, and the effect of the equivalent effect force on the material diffusion type phase change dynamics is taken into consideration. The finite element software King Mesh Analysis System _ Hot Stamps (KMAS _ HS), which can be used for numerical simulation of hot stamping of high-strength steel plates, is developed by the dynamic explicit finite element method. (2) The hot stamping three-dimensional temperature field analysis and prediction module is developed. In this paper, the heat transfer process of hot stamping is introduced, and the heat-stamping heat transfer differential equation is established by considering the latent heat release of the phase change of the micro-structure of the sheet. Combined with the initial and boundary conditions of the transient temperature field of the hot stamping, the differential equation of the three-dimensional transient temperature field of the hot stamping is solved by the Galerkin weighted margin method, and the general form of the finite element of the thermal stamping transient temperature field is derived. Respectively adopting a temperature shell unit and a three-dimensional tetrahedral unit to carry out finite element dispersion on the hot stamping plate and the mould with the cooling system respectively. The temperature distribution of the plate, the three-dimensional solid mold and the mold cooling pipeline during the hot stamping process of the U-type test piece is calculated by the analysis module, and compared with the actual test, it can be seen that the numerical results are consistent with the test results. (3) The hot stamping sheet forming analysis and prediction module was developed. Four non-coupled ductile fracture criteria (Oh, Brozzo, Ayada, and Rice-Tracey) and the Lemaitre model based on the full-coupled continuous damage mechanics were introduced into the hot-stamping finite element software KMAS _ HS, and their prediction of the formability of the hot stamping sheet was compared. the forming limit test of the high-strength steel plate at high temperature is carried out, the forming limit curve at different temperatures is obtained, The critical constants of the material related to the temperature in the four ductile fracture criteria are determined. The thermal stretching process is numerically simulated, and the material damage parameters related to the temperature and strain rate are determined by means of the optimization method, the numerical simulation and the test. The formability of a B-column in a hot stamping process is predicted by four ductile fracture criteria and the Lembaitre damage mechanics model, and the corresponding test verification is carried out. It is concluded that the Lemeritre model based on the continuum damage mechanics can accurately predict the toughness damage and fracture in the hot stamping process. Then, based on the model of the damage mechanics, the effects of the blank holder force and the friction coefficient on the formability, damage evolution and the convex die force of the hot stamping sheet are further studied. (4) The microstructure evolution of the hot stamping and the prediction of the mechanical properties of the parts after forming are developed. By using the Newton-Raphson iteration method to solve the diffusion-type phase-change dynamic equation, the isothermal phase-change dynamic model is applied to the non-isothermal process by using the Scheil superpose criterion, and the effect of the isodynamic force on the time of the diffusion-type phase change inoculation is taken into consideration. In the process of hot stamping, the microstructure evolution of the B-pillar hot stamping process was simulated by using the Li and Akerstrom diffusion-type phase-change kinetics model in combination with the Kostinian-Marburger (K-M) non-diffusion type phase change dynamics model, and the mechanical properties of the molded parts were predicted by the Maynard hardness model. The results show that the result of the combination of the Li model with the K-M model is better than that of the test results. The thermal stamping process of the S-beam gradient was predicted with a stronger adaptive Lee diffusion-type phase-change dynamic model and a Yu non-diffusion-type phase-change dynamic model for the gradient-hardness thermal stamping process. The material parameters in the Lee model are determined based on the continuous cooling transition curve of the material in the stress free state, and the hardness prediction model associated with the cooling rate is also established and calibrated. The results show that the hardness gradient distribution of the part can be realized by the method of heating and cooling of the die zone, and the consistency of the numerical results with the test results shows that the established multi-field coupling constitutive equation, the phase change dynamics model and the hardness prediction model are correct.
【学位授予单位】：大连理工大学
【学位级别】：博士
【学位授予年份】：2015
【分类号】：TG306

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