Experimental investigation and modelling of the heat transfer during hot stamping

生态因素的日益相关性以及政府对二氧化碳排放的监管，只是过去十年来轻量化设计在汽车行业日益重要的两个因素。尤其是超高强度硼锰钢的热冲压已经发展成为制造与安全相关的车身部件的最先进工艺。热冲压工艺始于半成品零件在合金特定AC3温度以上的完全奥氏体化。在热处理后，热板材被直接转移到压力机，在那里它们随后被成形和模内淬火。只要冷却速度超过某一特定值，奥氏体就会完全转变为马氏体。由于这种相变，热冲压零件表现出至少1500兆帕的极限拉伸强度。工艺链上的温度变化，特别是模内淬火过程中的温度变化，对最终的力学性能有很大的影响。对于精确的数值工艺设计，准确地模拟工件与刀具接触区的热传递是非常重要的。在有限元方法中，相应的换热系数用于建模，可以是整个过程的常量形式，也可以是接触压力的函数。这导致了数值过程建模结果中的重大不确定性。虽然文献中提供了更复杂的模型，但这些模型主要适用于受限的一系列实验。因此，本研究项目的目的是对热冲压过程中的传热机理有一个基本的了解，并为数值工艺设计建立模型。通过一系列有代表性的实验，在系统改变工艺参数的情况下，实现了对热冲压过程的传热分析。在实验过程中，记录了工件和刀具的温度演变，并用于计算随时间变化的换热系数。这些实验数据以及关于表面形貌演变的附加测量结果是开发基于物理的模型的基础，该模型能够确定作为工艺参数的函数的工件与刀具之间接触区的随时间变化的换热系数。由于热传递还取决于不同的接触情况，因此也将对不同的接触条件进行数值模拟。通过将传热学模型和子模型相结合，连续评估工件与刀具的真实接触面积，将提高数值工艺设计的预测质量。

The growing relevance of ecological aspects as well as governmental regulations regarding CO2 emissions are only two factors for the increasing importance of lightweight design in the automotive industry during the last decade. Especially hot stamping of ultra-high-strength boron-manganese steels has developed to a state-of-the-art process for manufacturing safety-relevant car-body parts. The process of hot stamping starts with a full austenitization of the semi-finished parts above the alloy specific AC3 temperature. After the heat treatment, the hot sheets are directly transferred to the press, where they are subsequently formed and in-die quenched. As long as the cooling rate exceeds a specific value, the austenite completely transforms into martensite. Due to this phase transformation, the hot stamped parts exhibit an ultimate tensile strength of at least 1500 MPa. The temperature evolution along the process chain and in particular during in-die quenching has a significant influence on the final mechanical properties. For a precise numerical process design, exact modelling of the heat transfer in the contact area between workpiece and tool is important. In the finite-element method, a corresponding heat transfer coefficient is used for modelling, either in the form of a constant value for the whole process or as a function of the contact pressure. This leads to significant uncertainties in terms of the numerical process-modelling results. Although more complex models are available in the literature, these models are primarily valid for a constrained series of experiments. Therefore, the present research project aims to acquire a fundamental understanding of the heat transfer mechanisms involved in the hot stamping process and to model them for the numerical process design. The analysis of the heat transfer during hot stamping is realized through a representative series of experiments under systematical variation of the process parameters. During the experiments, the temperature evolution of the workpiece and the tool is recorded and used for the calculation of time-dependent heat transfer coefficients. This experimental data as well as additional measurement results on the evolution of the surface topography are the basis for the development of a physically-based model, which has the ability to determine time-dependent heat transfer coefficients in the contact area between workpiece and tool as a function of the process parameters. Since the heat transfer also depends on the respective contact situation, the different conditions of contact will be modelled for the numerical simulation as well. By combining the heat transfer model with a submodel for the continuous evaluation of the true contact area between workpiece and tool, the prediction quality of the numerical process design will be improved.