汽车超高强钢热成形零件力学性能梯度分布的研究

The strategic cooling approach is proposed for the purpose of obtaining graded distribution of mechanical properties of hot stamping parts of ultra-high strength automotive steels, owing to fulfilling the requirements on matching the mechanical properties with the safety of vehicle body, that is to say, martensite is generated in area higher strength has to be setting up to improve the impact resistance, while pearlite or bainite is generated in area ductility and elongation are required to absorb the impact energy. Laws of the contact heat conduction between the sheet metal and dies are investigated, using thermal contact resistance to represent heat transfer characteristics of the interface, and regularizing functionals for solving the contact thermal conductivity are established. Then, characteristics of microstructure evolution under different temperature changing rate are studied, therefore, the transformation kinetics model of supercooled austenite is established, providing a basis for the choice of cooling path. Material model for ultra-high strength steels hot stamping is set up, taking material properties nonlinearity resulted from stress induced phase transformation, transformation plasticity and latent heat into account. After that, coupled simulation of temperature field, microstructure field, stress and strain field for stamping process is realized, and attention is concentrated on changes of local temperature field and influences on cooling paths. New dies and temperature control system are developed to implement non-uniform cooling of blank. Main factors influenced the graded distribution of mechanical properties are analyzed by experiments. Finally, graded distribution of mechanical properties of parts achieves. New approaches as well as theoretical basis for automobile lightweight and crashworthiness improvement are provided, according to the relationships between the mechanical properties of graded distribution and collision response for typical parts.

为满足汽车超高强钢热成形零件力学性能与车身安全性相匹配的要求，即对强度要求高的区域生成马氏体以提高抗冲击能力，对塑性和延伸率要求高的区域生成珠光体或贝氏体以吸收能量，提出热成形选择性冷却工艺，实现零件力学性能梯度分布。研究成形过程中板料与模具间接触换热规律，采用接触热阻表征接触界面的换热特性，建立求解接触导热系数的正则化泛函；探索不同温变速率下组织演变的规律，建立过冷奥氏体相变动力学模型，为冷却路径的选择提供依据；考虑应力诱导相变、相变塑性、相变潜热问题引起的材料非线性，建立适用于超高强度钢热成形的材料模型，实现成形过程温度场-组织场-应力应变场耦合计算,跟踪局部温度场的变化及其对冷却路径的影响；开发实现零件非均匀冷却的模具和温度控制系统，分析影响零件力学性能梯度分布的主要因素；探究目标零件的力学性能梯度分布与碰撞响应的关系，为汽车轻量化及提高耐撞性提供新方法和理论基础。

随着对汽车轻量化、节能减排和安全性要求的日益提高，高强钢越来越多地被用于车体的重要结构件和安全零部件的制造。热成形技术由于其具有成形性能好、成形载荷小、回弹小和淬火后零件强度高等优势而得以在汽车工程广泛应用，而零部件力学性能与车身安全性能要求相匹配的问题使得高强钢热成形零件TTP（Tailored Tempering Properties）技术应运而生，也即同一零件的不同区域具有不同的力学性能。本项目根据微观组织演变规律，选择高强钢的冷却速率或相变路径，通过不同温度的分块模具、差异化的初始坯料温度和局部回火等方法获得高强钢TTP热成形零件。研究了模具与板料间的接触热阻与压强、间隙和板料表面状况的关系，获得了板料与模具的热交互条件对传热效果的影响。针对热成形过程中板料的热传递状态，考虑相变潜热影响，建立了热成形全过程传热模型，修正Li相变模型，结合数值模拟优化方法准确预测不同冷却路径下的各相含量以及力学性能。利用DIC技术从全应变域角度建立了马氏体、贝氏体和铁素体/珠光体组织的弹塑性本构模型，结合物理实验与数值模拟方法获得单纯试验方法难以确定的断裂模型参数，最终利用线性法则和RVE模型确定混合组织的本构模型和断裂模型。数值模拟分析整车侧面碰撞过程，比较不同强度材料的B柱零件各监测点的侵入量和侵入速度，证明了高强钢材料以及 TTP 工艺对于整车侧面碰撞性能的提升效果。研究了过渡区域位置、软化区材料、过渡区域长度和过渡区域划分数量对于乘客头部和肩部的侵入量及侵入速度大小的影响。基于自主研发的高强钢板高温摩擦磨损试验机，研究热成形过程中坯料与模具之间的摩擦磨损机理，针对TTP工艺改进传统高温摩擦试验机，研究了在不同初始坯料温度、不同模具温度以及不同冷却速率条件下的摩擦磨损行为。本项目的研究成果对于高强钢TTP热成形零件在汽车行业的应用具有重要意义。