Abstract
Medium Manganese Transformation Induced Plastic (Mn-TRIP) steels are expected to be a new generation of advanced high strength sheet steels due to their excellent balance between material cost and mechanical properties. During the solidification process, AlN precipitates at the grain boundary, which leads to the serious deterioration of hot ductility. However, the precipitation of AlN in Mn-TRIP steel has not been clear. In this study, the chemical compositions, morphology, size distribution, and the precipitation behavior of AlN inclusion in an Fe-0.5Al-2.0Mn alloy were studied under the continuous unidirectional solidification process. The results show that there are two types of nitride inclusions in the Fe-0.5Al-2.0Mn alloy: AlN inclusion and complex inclusion of Al2O3-AlN. The planar sections of most AlN particles are hexagonal. Based on the thermodynamic calculation, it was found that the content of Al has a large effect on the stability of Al2O3 and AlN. When the content of Al increases, the molten iron can be changed from saturated by Al2O3 to saturated by AlN. During the solidification process, the precipitation of Al2O3 inclusions occurred at the beginning of the solidification process. The precipitation of AlN inclusions occurred when the contents of Al and N exceeded the equilibrium value and grew until the end of the solidification. The precipitation conditions of AlN inclusion in the Fe-0.5Al-2.0Mn alloy during the solidification process were discussed. The precipitation and the amount of precipitate of AlN inclusions depend on the initial contents of Al, N, and O. It was found that the precipitation of AlN inclusions can be controlled by reducing the initial content of N to less than 0.0072 mass%.
Highlights
IntroductionThroughout the world there is increasing interest in the development of new Advanced High Strength Steels (AHSS) with enhanced combinations of strength and ductility to provide sheet materials for demanding applications in future vehicles (Matlock et al, 2012)
Nowadays, the global automotive industry has been attempting to address the challenge of weight reduction in automobiles to decrease greenhouse gas emissions and improve fuel efficiency
The sample was longitudinally cut at the middle, on which the morphology and chemical compositions of AlN inclusions were observed and analyzed by a Scanning Electron Microscope with Energy Dispersive Spectroscopy (SEM-EDS) after grinding and polishing
Summary
Throughout the world there is increasing interest in the development of new Advanced High Strength Steels (AHSS) with enhanced combinations of strength and ductility to provide sheet materials for demanding applications in future vehicles (Matlock et al, 2012). The austenitic stainless steels, Twinning Induced Plasticity (TWIP) steels, and lightweight steels are referred to as the second generation of AHSS. The second of AHSS steel exhibits outstanding mechanical properties (UTS × TE ≥ 50 GPa %), but these austenitic grades are highly alloyed resulting in a significant cost increase (Matlock et al, 2012). Medium Mn TRIP steels have been actively investigated and expected to the third generation of AHSS due to their excellent balance between material cost and mechanical properties
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