Papers by Author: Bruno C. De Cooman

Abstract: The temperature dependence of the dynamic Young’s modulus, E, and the damping, Q^-1, of Fe-C-N and Fe-17%Cr-C-N alloys with different C, N contents were studied in the temperature range of 25°C to 600°C by the impulse excitation internal friction technique at 1KHz. Hot rolled samples were cold rolled to a thickness of 1.2mm and recrystallization annealed at 820°C for 30sec then gas jet cooled (-50°C/sec). Samples were subsequently tensile strained 6% and 16%. A Snoek peak was observed at 120°C in the undeformed sample and a Snoek-Koster peak was observed at 400°C after deformation. In the case of the Fe-Cr-C-N alloy, a high damping background due to magneto-mechanical damping was observed in the temperature range of 25°C to 400°C. A broaden Snoek peak in FeCr was observed at 300°C. The internal friction peaks observed in this study showed the strong dependence of the amount of interstitial contents and deformation and were well correlated to previous internal friction studies measured mainly by torsion pendulum and inverted torsion pendulum.

Abstract: The Fe-Mn-N alloys used in the present study showed an unexpected increase of both the yield strength and the ultimate tensile strength after prestraining and strain aging. In order to understand the mechanism of this phenomenon, which is due to solute N, static strain aging tests were carried out in combination with resonant impulse excitation internal friction analysis. It is shown that the interaction between thermal kinks and solute N atoms plays an essential role in the increase of the tensile strength.

Abstract: A new approach to obtain high strength of non-oriented electrical steel by addition of phosphorus is proposed. The method includes B-additions which suppress grain boundary P segregation, strengthen the grain boundary cohesion and enhance the P solid solution hardening. Two 3% Si steels, a B-free 0.1%P steel and a 20 ppm B-added 0.1%P steel were analyzed. The microstructures were studied by EBSD. The B-addition resulted in a pronounced rotated cube component, {100}<011>, after a hot-band annealing treatment. A -fiber texture was developed in the B-free steel. The B-addition caused a retardation of the recrystallization, allowing for the growth of grains with a lower stored energy, such as rotated cube oriented grains. The steels were further cold rolled and recrystallization annealed to observe a similar effect after large cold reductions. The present contribution focuses on the potential of this concept to obtain high strength 3% Si steels with low core losses.

Abstract: Ultra-fine grained TRIP steels (UFG-TRIP) containing 6wt%Mn were produced by intercritical annealing. An ultra-fine grained microstructure with a grain size less than 1μm was obtained. The formation mechanism of the high volume fraction of retained austenite was investigated by dilatometry, XRD and magnetic saturation. The fraction of retained austenite was strongly dependent on the annealing temperature. The tensile properties were also found to be strongly influenced by the annealing temperature with poorer mechanical properties being observed at higher annealing temperatures. It was found that the stabilization of the retained austenite was both a composition and size-effect, made possible by the grain refinement due to the reversely transformed martensite.

Abstract: Edge stretching is an important formability issue when it comes to apply sheet steels to automotive industry. Anisotropy, strain hardening and toughness are closely related to hole expansion properties. In this paper, hole expansion properties of a high Mn fully austenitic Twinning Induced Plasticity (TWIP) steel are compared with three other types of single-phase sheet steels. The effects of r-value, n-value, m-value and post-uniform elongation on the edge stretch-flangeability are discussed. It was found that the post-uniform elongation and the strain rate sensitivity have a pronounced effect on hole expansion properties.

Abstract: A robust TRIP 800MPa tensile strength concept was developed for automotive applications. An optimal TRIP steel composition containing 0.3 mass-% Si and 1.0 mass-% Al was identified. Galvannealing tests revealed that this TRIP steel had an ideal surface structure prior to hot dipping. Galvannealing could be achieved successfully in normal operating conditions. The contribution gives an in depth overview of the development of this new TRIP800 concept, with a special focus on achieving the optimum properties in various CGL line configurations.

Abstract: High Mn steels demonstrate an exceptional combination of high strength and ductility due to their high work hardening rate during deformation. The microstructure evolution and work hardening behavior of Fe18Mn0.6C1.5Al TWIP steel in uni-axial tension were examined. The purpose of this study was to determine the contribution of all the relevant deformation mechanism : slip, twinning and dynamic strain aging. Constitutive modeling was carried out based on the Kubin-Estrin model, in which the densities of mobile and forest dislocations are coupled in order to account for the continuous immobilization of mobile dislocations during straining. These coupled dislocation densities were also used for simulating the contribution of dynamic strain aging on the flow stress. The model was modified to include the effect of twinning.

Abstract: The present study focused on the design and testing of new 3rd generation TWIP steels to find reduced Mn content alternatives to the existing high Mn FeMnC and FeMnAlC alloy systems. In order to investigate the effect of nitrogen addition, 12Mn0.6C-N was examined and 18Mn0.6C-N steel was used as a reference. Effects of nitrogen and manganese on the microstructure and mechanical properties were investigated by X-ray diffraction, optical microscopy, scanning electron microscopy and tensile testing. The deformation microstructures of 12Mn0.6C-N were mixtures of twinned austenite and 5~10% ε-martensite. Only deformation twinning was observed in 18Mn0.6C-N during the deformation due to its higher stacking fault energy. 18Mn0.6C-N steel exhibited higher strength and elongation than 12Mn0.6C-N steel. The effect of the strain rate on the tensile behavior of both materials was also examined. While 18Mn0.6C-N showed clear negative strain rate sensitivity, 12Mn0.6C-N did not show a clear relationship between flow stress and strain rate. The effect of annealing temperature on the tensile behavior and microstructure was also examined.

Abstract: The selective oxidation of Al-free and Al-added Twinning Induced Plasticity (TWIP) steels during full austenitic annealing at 800°C in N2+10%H2 atmosphere at a dew point of -17°C was investigated by means of HR-TEM of FIB cross-sectional samples. For Al-free TWIP steel, a dense MnO layer was formed on the surface. Crystalline c-xMnO.SiO2(x2) particles and amorphous a-xMnO.SiO2(x<0.9) particles were found at the MnO layer/steel matrix interface. In the subsurface, Mn depletion resulted in the transformation of the austenite to the ferrite phase in a narrow zone. For Al-added TWIP steel, a continuous outer MnO layer and a transition layer consisting of amorphous a-xMnO.SiO2(x<0.9) and crystalline c-MnO.Al2O3(0.8

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