Experimental investigation and modelling of the heat transfer during hot stamping

在过去的十年中，生态方面的日益相关性以及政府对二氧化碳排放的规定只是汽车行业轻量化设计日益重要的两个因素。特别是超高强度硼锰钢的热冲压工艺已发展成为制造安全相关车身部件的最先进工艺。在合金特定的AC3温度以上，从半成品的完全奥氏体化开始热冲压过程。热处理后，热片直接转移到压力机，在那里它们随后形成和模内淬火。只要冷却速率超过一定值，奥氏体就完全转变为马氏体。由于这种相变，热冲压件表现出至少1500 MPa的极限抗拉强度。沿工艺链的温度变化，特别是模内淬火的温度变化对最终的力学性能有显著影响。为了精确的数值加工设计，工件与刀具接触区域的热传递的精确建模是非常重要的。在有限元法中，使用相应的传热系数进行建模，其形式要么是整个过程的恒定值，要么是接触压力的函数。这就导致了数值过程模拟结果的重大不确定性。虽然文献中有更复杂的模型，但这些模型主要适用于一系列受限的实验。因此，本研究项目旨在对热冲压过程中的传热机制有一个基本的了解，并为数值工艺设计建立模型。通过一系列有代表性的实验，在工艺参数系统变化的条件下，对热冲压过程的传热进行了分析。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传热系数。这些实验数据以及对表面形貌演变的额外测量结果是开发基于物理的模型的基础，该模型能够确定工件和工具之间接触区域中随时间变化的传热系数作为工艺参数的函数。由于传热也取决于各自的接触情况，因此也将对不同的接触条件进行数值模拟。通过将传热模型与连续评估工件与刀具真实接触面积的子模型相结合，提高了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.