硼钢HC1500HS热冲压工艺与热力相变耦合建模仿真

发布时间：2018-09-06 07:09

【摘要】：采用(超)高强度钢板冲压件制造车身零件不仅可以减轻车身重量,降低油耗,而且可以确保、提高车的安全性,是同时实现车体轻量化和提高碰撞安全性的最佳途径。但(超)高强钢板在常温下冲压易开裂、回弹严重。尤其是当强度达到1500MPa时,常规的冷冲压成形工艺几乎无法成形。因此,如何实现高强度钢板的高精度冲压成形就成为一项紧迫需要解决的技术难题。超高强度硼钢板的热冲压新工艺被认为是解决上述难题的有效方法。其工艺原理是：硼钢板加热,使其充分奥氏体化(900～950℃)后,迅速转移到压力机上,利用配有冷却系统的模具成形后,在模具中冷却淬火获得马氏体组织,淬火后钢板的抗拉强度达到1500MPa左右甚至更高。本文主要是围绕超高强硼钢热冲压成形工艺的工程科学问题和技术难题开展研究。通过Gleeble-3500热模拟试验机上进行热膨胀试验。研究了加热速率、加热温度和保温时间对硼钢高温成形性能的影响规律,研究表明：在保证生产节拍的前提下,经过快速加热曲线加热的试件高温成形性能最好；加热温度对硼钢高温下成形的性能有比较明显的影响；保温时间则对硼钢高温下的峰值应力影响显著,对极限应变影响不大。基于奥氏体形核长大理论,考虑加热速率的影响,建立非等温条件下硼钢HC1500HS统一奥氏体相变动力学模型。应用Matlab进化算法工具箱对模型中的材料常数进行求解,获得的统一相变动力学模型能够准确的预测硼钢奥氏体在非等温加热过程中的热膨胀曲线及奥氏体体积百分数。基于响应面法建立了热冲压零件的抗拉强度、屈服强度和延伸率的响应面模型,考虑的工艺参数有加热温度(800-1000℃)、保温时间(60-540 s)、成形温度(560-800℃)以及模具温度(20-220℃)。在获得响应面模型的基础上研究各个工艺参数对热冲压零件机械性能的影响规律,并采用NSGA-Ⅱ多目标进化算法对工艺参数进行了优化,获得了具有良好综合力学性能的热冲压零件。优化结果将为热冲压工艺参数选择提供实验依据和理论指导。设计加工了一套圆台装置用于测定硼钢淬火过程中板料和模具在热冲压保压淬火过程中的温度曲线。计算得到模具与硼钢之间的界面换热系数。并研究了压力和氧化皮厚度对界面换热系数的影响规律。该研究为热冲压冷模具淬火过程中板料与模具温度的计算提供理论依据,为准确计算热冲压过程中的相变提供了数据基础。在Gleeble-3500热模拟实验机上进行了硼钢HC1500HS的热膨胀实验,研究了硼钢HC1500HS连续冷却中冷却速度和变形对过冷奥氏体相变过程的影响,确定了该钢组织转变的临界冷却速度,制定了硼钢HC1500HS的动态奥氏体连续冷却转变曲线(DCCT曲线)。基于合金热力学理论,建立了硼钢HC1500HS的非等温条件下铁素体和贝氏体的相变动力学模型,获得的模型能够很好的预测硼钢在不同冷却速度和变形程度下的转变产物。建立了车门防撞梁热冲压及冷模具淬火过程的有限元模型,进行热冲压过程中零件微观组织的有限元模拟,得到了车门防撞梁热冲压过程中的板料温度、微观组织及维氏硬度的分布特征,并通过实验验证热力相变耦合的有限元模型的数值计算有效性,实现了热冲压零件微观组织演化过程有限元预测,以控制热冲压零件的机械性能。利用有限元模型研究了成形温度,模具温度,保压压力和保压时间对防撞梁热冲压零件的微观组织的影响规律。
[Abstract]:Using (ultra) high strength steel sheet to make body parts can not only reduce body weight and fuel consumption, but also ensure and improve vehicle safety. It is the best way to realize lightweight and improve collision safety at the same time. The conventional cold stamping process is almost impossible to form. Therefore, how to achieve high-precision stamping of high-strength steel sheet becomes an urgent technical problem to be solved. After austenitizing (900-950 C), it is transferred to the press quickly. After forming with a cooling system, the martensite structure is obtained by cooling and quenching in the die. The tensile strength of the quenched steel sheet is about 1500MPa or even higher. This paper mainly focuses on the engineering scientific problems and technical difficulties of hot stamping forming process of ultra-high strength boron steel. The effects of heating rate, heating temperature and holding time on the high temperature formability of boron steel were studied by the thermal expansion test on Gleeble-3500 thermal simulator. Based on the austenite nucleation and growth theory and considering the effect of heating rate, a unified austenite transformation kinetics model of HC1500HS for boron steel under non-isothermal conditions is established. The model is progressed by using MATLAB. The material constants in the model are solved by the algorithm toolbox, and the unified phase transformation kinetic model can accurately predict the thermal expansion curve and austenite volume percentage of boron steel austenite during non-isothermal heating process. The model considers the process parameters such as heating temperature (800-1000 C), holding time (60-540 s), forming temperature (560-800 C) and die temperature (20-220 C). Based on the response surface model, the influence of each process parameter on the mechanical properties of hot stamping parts is studied, and the process parameters are processed by NSGA-II multi-objective evolutionary algorithm. The optimized results will provide experimental basis and theoretical guidance for the selection of hot stamping process parameters. A set of round table device is designed and manufactured to measure the temperature curves of sheet metal and die during hot stamping and pressure-holding quenching of boron steel. The interfacial heat transfer coefficient between boron steel and the influence of pressure and oxide thickness on the interfacial heat transfer coefficient are studied. The study provides a theoretical basis for calculating the temperature of plate and die during quenching of hot stamping cold die, and provides a data basis for accurately calculating the phase transformation in hot stamping process. The thermal expansion test of boron steel HC1500HS was carried out. The effect of cooling rate and deformation on the transformation process of supercooled austenite during continuous cooling of boron steel HC1500HS was studied. The critical cooling rate of microstructure transformation of the steel was determined. The dynamic Austenite Continuous Cooling Transformation Curve (DCCT curve) of boron steel HC1500HS was worked out. The transformation kinetics model of ferrite and bainite of boron steel HC1500HS under non-isothermal condition is established. The obtained model can well predict the transformation products of boron steel at different cooling rates and deformation degrees. Finite element simulation of microstructure is carried out to obtain the distribution characteristics of sheet temperature, microstructure and Vickers hardness in hot stamping process of car door crashproof beam. The numerical calculation validity of coupled thermo-mechanical phase transformation finite element model is verified by experiment, and the finite element prediction of microstructure evolution process of hot stamping parts is realized to control hot stamping parts. The effects of forming temperature, mold temperature, holding pressure and holding time on the microstructure of hot stamping parts of anti-collision beam were studied by finite element model.
【学位授予单位】：北京科技大学
【学位级别】：博士
【学位授予年份】：2016
【分类号】：TG306

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