Microstructure and Formability of Aluminum Bearing TRIP-Aided Steels with Annealed Martensite Matrix

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The effects of aluminum content on microstructure, ductility and formability of advanced high strength low alloy TRIP (Transformation-Induced Plasticity)-aided ferrous sheet steels with annealed martensite matrix (or TRIP-aided annealed martensitic steel) were investigated in order to realize hot-dip galvanization. Aluminum addition of 0.5-1.0 mass% (and simultaneous silicon removal of the same amount) to a 0.2C-1.5Si-1.5Mn-0.04Al (mass%) steel refined the matrix structure and retained austenite needles and increased carbon concentration of retained austenite. It also brought on an excellent total elongation, stretch-flangeability and bendability, although the tensile strength decreased. Optimum austempering temperature for the total elongation increased to 450-475°C, due to the increased carbon concentration of retained austenite. On the other hand, optimum austempering temperatures for the stretch-flangeability and bendability were maintained at 350-400°C, mainly due to uniform fine lath matrix and retained austenite needles. If only large total elongation is required for the TRIP-aided steel, it is expected that hot-dip galvanizing immediately after continuous intercritical annealing can be realized.
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