Makroskopische Charakterisierung und gefügebasierte Modellierung der Alterungskinetik und der Werkstoffeigenschaften für Dualphasenstähle
Final Report Abstract
The goals of this project was to investigate and simulate the Bake Hardening (BH) process in Dual Phase (DP) Steels with respect to parameters such as temperature, time and prestrain as well as to evaluate the microstructural change and phenomena occurring during this process. An industrially produced DP 500 was used to acquire two dual phase microstructures with 10 % and 20 % martensite by intercritical annealing. In order to investigate the effects of process parameters tensile tests were performed at various aging temperatures (80 °C, 170 °C, 300 °C), times (20 min to 7 days) and prestrains (0 %, 0.5 %, 2 %). Certain conditions were investigated using Scanning Electron Microscopy equipped with EBSD detector for dislocation density measurements and EPMA to investigate the elemental partitioning. Transmission Electron Microscopy (TEM) was used to investigate precipitation in the ferritic phase. The formation of Cottrell atmospheres was simulated utilizing a dislocation- carbon attraction model while the precipitation was modeled using an empirical model. All three mechanisms were combined into Representative Volume Element (RVE) in order to simulate the macroscopic response of the composite microstructure. Strengthening levels of 170- 190 MPa were achieved for both fractions of martensite at 0.5 % prestrain at 170 °C. The effect of prestrain above 0.5 % was negative for Bake Hardening. Three phenomena were recognized to contribute to the increase in strength: Cottrell atmosphere formation (40 MPa), carbide precipitation in ferrite (90-100 MPa) and residual stresses relief (40-50 MPa). It was observed using ECCI and EBSD that the dislocation density rapidly increases with prestrain and create non- uniform dislocation areas in ferrite. Upon further investigation with TEM it was found that carbides precipitate on dislocations are larger than carbides in the bulk which are not effective at pinning dislocation movement hence the decrease in strengthening with prestrain above 0.5 %. EPMA revealed that large cementite particles (0.5 µm), that were inherited by the industrial process, were not dissolved during intercritical annealing and contained a considerable amount of carbon which was depleted form the austenitic islands. This depletion led to lower expansion of martensite and hence lower values of residual stresses to be relieved. All three mechanisms were taken into account during simulation. For Cottrell atmosphere formation a physical model was used that took into account dislocation density, carbon concentration, temperature and time. The results indicated that the amount of carbon in ferrite is not sufficient to occupy all the site of the dislocations produced by prestrain. Precipitation was modeled using an empirical law taking into account time and temperature of the process. The model was able to predict the precipitation behavior in terms of strengthening but not in terms of microstructural evolution. Residual stresses were simulated using a Representative Volume Element (RVE) by considering the expansion of martensite during transformation. During aging a contraction was assigned to the martensitic phase in order to represent the tempering process. The final flow curves extracted from the RVE after aging were in good agreement with experimental curves from Bake Hardening experiments. Overall the three contributing mechanisms (Cottrell effect, precipitation, residual stresses) were analyzed and the difference between DP and traditional ferritic steel was understood and quantified. The effect of process parameters (prestrain, temperature, time) and microstructural parameters (martensite volume fraction, dislocation density) on BH was explained and was connected with fundamental processes on the microscale. BH was successfully simulated utilizing all three contributing mechanisms represented by physical and empirical models.
Publications
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Quantification of bake hardening effect in DP600 and TRIP700 steels, Materials and Design, 2014, 57, pp. 479-486
A. Ramazani, S. Brühl, T. Geber, W. Bleck, U. Prahl
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’The Effect of Bake-Hardening Parameters on the mechanical Properties of Dual-Phase Steels’, Steel Research International, Vol 87 Issue 11, November 2016, Pages 1559-1565
A. Ramazani, S. Brühl, M. Abbasi, U. Prahl, W. Bleck
