Investigation of thermal spring back of a hot formed 22MnB5 A-pillar with tailored properties
Sammanfattning: In a world where fuel economy and crash safety is becoming an important factor in the automotive industry, the need for materials with very high strength-to-weight ratio is growing rapidly. One of the materials used for this purpose is the boron steel 22MnB5. Since the material has very high mechanical yield limit it is hard to produce parts using cold forming, which is the standard procedure for sheet metal forming. Therefore, the use of hot stamping is increasing. Hot stamping enables manufacturing of boron steel parts with good dimensional accuracy and low spring back. However, some amount of spring back is still present during the process. In this thesis, spring back of a hot formed 22MnB5 A-pillar is investigated using computer simulations in the software LS-DYNA. The main focus was to develop a process for simulating spring back in hot stamping. The work started with simulations of the forming and quenching stages of the hot stamping process, both on a full size and on a smaller section of the blank. Simultaneously as the simulations, a literature study was also conducted. The literature study was aimed at finding hints and information on how to build the simulations. Furthermore, interviews were made with experts on both LS-DYNA and hot stamping. A process for spring back evaluation was then created and written as an LS-DYNA keywordfile. In the developed spring back simulation, the part is taken out of the press right after the quenching is finished and placed in a space where it can cool and move freely. The simulation is conducted until the part reaches room temperature. After the quenching is done, data containing temperatures, stresses and strains of the part is exported. This data is then included in the spring back simulation where the part is cooled to room temperature. During the cooling, the stresses, strains and temperatures are equalized until the spring back reaches a steady state. The results indicate that the developed method for spring back evaluation can be used to foresee shape deviations for the intended part and process.
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