Christine Dessain

Development of Estimation Procedure of Contact Heat Transfer Coefficient at the Part–Tool Interface in Hot Stamping Process

Energy efficiency, a high level of passenger safety, and weight reduction of the vehicles still constitute the main functional elements for the design of modern car body structures. Toward this objective, the usage of advanced sheet-metal forming technologies like hot stamping of quenchable boron manganese steel is developing nowadays. Hot stamping is a combination of hot forming and simultaneously quenching and hardening of the blank. In this article, an experimental procedure developed to estimate the thermal conductance at the part–tool interface during a hot stamping procedure is presented. The tools set (punch and die) has been designed to form omega-shaped samples. This work represents the first stage of the procedure development where the standard experiment is a simple compressive load of sample in the bottom of the omega-shape die, considering the real hot-stamping conditions. Tests are carried out under different contact pressure values for two different blank materials (Usibor 1500P and a material B). The results show that the thermal conductivity increase contributes to the reduction of the thermal contact resistance and therefore to a more rapid cooling of the part.

Thermal Study of Hot Stamping with Heated and Cooled Tooling to Obtain Tailored Properties

To produce parts with tailored properties, i.e. parts with high strength in some areas and high ductility on some other areas, one of the most popular method, called the tailored tempering process, is to heat up locally the tools. In the hot areas, the blank follows a different thermal path leading to different microstructure evolutions and thus different final mechanical properties. In this paper, a tool is designed to have a side heated up to 500°C and a water cooled side. The hot side is heated up thanks to heated cartridges. A PID regulation is used to control the temperature of the hot side (from 200°C to 500°C) while the cold side is maintained at a low temperature using a thermostated water circulation. A uniform temperature on the working surface is successfully reached on both sides. Instrumentation by thermocouples is designed to be able to fully characterize the heat transfer: solving 2D heat conduction problems, the temperature fields in the tools and the thermal contact resistances at the blank/tool interfaces are estimated. Hardness measurements are also performed on the blank: the possibility to confer a distribution of mechanical properties is highlighted.

Estimation of the Heat Transfer Conditions in a die Radius during Hot Stamping

Established process in the automotive industry, the hot stamping process consists in heating a blank until complete austenitization in a furnace before transferring it to a press where it is formed at high temperature before being quenched by contact with the cold tools. During the forming step the hot blank slides on the die radius. Locally, the contact pressure can reach very high values. Due to this contact, heat transfer between the hot blank and the die can be significant. Using an omega die instrumented with eight thermocouples localized in the die radius, a 2D inverse method is used to estimate the heat flux that crosses the Blank/Die interface and the temperature field in the die radius and on the die surface. Four thermocouples are located in the blank thickness and a FE analysis is performed to estimate their positions as function of the time. The temperature in the thickness of the blank is considered as uniform according to Biot number value. This assumption is checked afterward. Thus, it is possible to estimate the sliding thermal contact resistance between the blank and the die as a function of time in front of each thermocouple of the blank. The estimation of the temperature field in the die can be useful for investigating the fatigue that occurs in the die. On the other hand, the knowledge of the interface condition in the die radius can present a high interest for improving the numerical simulations of this process.

Latent Heat Estimation of the Martensite Transformation Through Inverse Methods During the Hot Stamping Process

In the hot stamping process, the temperature evolution drives the metallurgy, as well as the metallurgical transformation influences the temperature evolution through the exothermic nature of the austenite to martensite transformation. This heat release has already been highlighted by previous experimental work. This heat release leads to a source term in the heat equation in the blank. This source term must be quantified in order to accurately predict blank temperature evolution. Moreover, the end of the heat release corresponds to the end of the metallurgical transformation. It allows the determination of the minimum quenching time, relevant information for industry to minimize the process time.This paper presents a method to quantify the heat released by the metallurgical transformation during the hot stamping of Usibor 1500P® ArcelorMittal steel, solving inverse conduction problems. It allows the determination of this heat release as a function of temperature or time. Then, integrating it, the latent heat of the transformation can be estimated. This can be done for different contact pressures between tool and blank. Finally, it can be linked to the martensite proportion to estimate it as a function of time or temperature and determine the Koistinen-Marburger model parameters. These results should improve the accuracy of numerical simulations of the hot stamping process.Copyright

Metallurgical Model Fitted to Experimental Data Using a Genetic Algorithm

The hot stamping process is an established process in the automotive industry to satisfy challenges concerning security aspects and lightweight construction. Now, innovative processes have arisen which consist in heating locally the tools and thus adjust local final mechanical properties of the parts. To simulate accurately this so called tailored tempering process, a coupling between thermal and metallurgical phenomena must be considered as the metallurgical transformations lead to a source term in the heat equation and the thermal evolution drives the transformation. To improve the model, a genetic algorithm optimizes the metallurgical model parameters to fit both the CCT and TTT diagrams, taking in account the cooling rate dependence. This method for creating a metallurgical data file, that is directly usable by the industrial software and that fits the TTT diagram and the final constituent proportions of the different constituents, is presented. This method, tested on hypothetical experimental data, is then validated and results are presented. Moreover, the principle of this work can be adapted to various softwares that industries use.