Materials Science Forum Vols. 654-656

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 high-manganese austenitic steels are an answer for new demands of automotive industry concerning the safety of passengers by the use of materials absorbing high values of energy during collisions. The chemical compositions of two high-manganese austenitic steels containing various Al and Si concentrations were developed. Additionally, the steels were microalloyed by Nb and Ti in order to control the grain growth under hot-working conditions. The influence of hot-working conditions on a recrystallization behaviour was investigated. Flow stresses during the multistage compression test were measured using the Gleeble 3800 thermo-mechanical simulator. To describe the hot-working behaviour, the steel was compressed to the various amount of deformation (4x0.29, 4x0.23 and 4x0.19). The microstructure evolution in successive stages of deformation was determined in metallographic investigations using light microscopy. The flow stresses are much higher in comparison with austenitic Cr-Ni and Cr-Mn steels and slightly higher compared to Fe-(15-25) Mn alloys. Making use of dynamic and metadynamic recrystallization, it is possible to refine the microstructure and to decrease the flow stress during the last deformation at 850°C. Applying the true strains of 0.23 and 0.19 requires the microstructure refinement by static recrystallization. The obtained microstructure – hot-working relationships can be useful in the determination of powerful parameters of hot-rolling and to design a rolling schedule for high-manganese steel sheets with fine-grained austenitic structures.

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: 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: 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: Cold rolled TRIP-600 steels has been developed on a laboratory scale. Composition of the steels is 0.12wt%C-0.30wt%Si-1.6wt%Mn-1.2wt%Al. Effects of Al concentration on phase diagram, TTT curves and T0 line of bainite formation in the steels were calculated and discussed.Cold rolled sheets of 1.7mm thick were annealed between 830 °C and 870 °C followed by austempering at 420 oC. UTS of 660~700MPa, YS 390~420MPa and total elongation 30.5~37.5% is obtained.Volume fraction of the retained austenite is 9~11% and carbon content in the retained austenite is 0.9% ~1.3wt% in the steels. The chemical composition and processing parameters of the experimental steels could be referred for trial-manufacture of TRIP 600 steels.