Properties and application of TRIP‐steel in sheet metal forming

A further development of dual‐phase‐steels are represented by TRIP (transformation induced plasticity) ‐steels. TRIP‐steels contain austenite, which is metastable at room temperature. It transforms to martensite during straining (TRIP effect). This process improves the strength‐ductility balance of these steels. Two types of TRIP‐steels, low alloyed (L‐TRIP) and high alloyed (H‐TRIP), can be applied in sheet forming processes and exhibit different forming characteristics. Basing on results of uniaxial tensile tests and the evaluation of Young's modulus the forming limits in deep drawing processes and the component properties of deep drawn parts are discussed. The Young's modulus decreases significantly with increasing pre‐strain, especially demonstrated for the L‐TRIP material TRIP700. Forming limit curves determined at different forming temperatures indicate its influence on the forming limits. Martensite transformation is suppressed at a temperature of approximately T = 200 °C and therefore the major strain ϕ1 decreases significantly. For the investigated stainless steel AISI304 (H‐TRIP) different lubricant types in comparison to chlorinated paraffins have been tested. Lubricants consisting of sulphur additives led to good forming conditions in forming processes, even better than lubricants based on chlorinated paraffins. The evaluation of component properties, compared between L‐TRIP and H‐TRIP, was done based on the analysis of springback and dent resistance. The L‐TRIP material TRIP700 shows higher springback angles than AISI304 resulting from higher yield strength and decreased Young's modulus, resulting from the forming process. The dent resistance of TRIP‐steel was exemplarily demonstrated for AISI304. Uniaxial pre‐strained sheet specimen were analysed to show the dent resistance depending on dent depth. During elastic denting pre‐strain has no influence on dent resistance. Further increasing dent depth lead to increased dent forces for pre‐strained specimens.