PR-256-024 - 2008 Intl. Conf. on New Developments in AHSS

Development of High Manganese TWIP Steel with 980MPa Tensile Strength

S.-K. Kim, G. Kim, K.-G. Chin

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With unique properties such as toughness, non-magnetism, corrosion resistance, damping capacity and shape-memory effect[1,2,3,4,5,6,7], high manganese steels have been studied since the report by Sr. Hadfield[8] in the end of the 19th century. Twinning Induced Plasticity (TWIP) steel is new material for automotive use, which is hardened mainly by twin in addition to dislocation, and its microstructure is high manganese and high carbon austenitic steel with varying aluminum or silicon content, resulted into the extraordinary excellent strength-elongation balance. Recently it has attracted the interests of automotive and steelmaking industries, as the need of lightweighting for better fuel efficiency and high energy absorption of improved crash worthiness increases more and more. In 1994 POSCO has firstly developed an Al-containing high Mn cold rolled steel sheet for automotive parts[9,10]. The steel was trialstamped successfully to a few auto parts including floor front extension, rear package tray, rear window opening inner and cowl inner lower. However, because processing conditions such as continuous casting, hot rolling, strip welding before cold rolling, etc. had not been optimized, commercial production was not attempted at that time. Recently POSCO succeeded in the mill production of TWIP steel of the new alloy system with reduced Mn content. An innovative continuous casting process using molten mold flux was applied to solve the problem of slab surface of high alloy material. It is the objective of this paper to highlight the newly developed high Mn steel for automotive uses. The deformation mechanism will be explained based on the microstructure analysis. The results of formability tests including delayed fracture will also be discussed

Keywords: TWIP, molten mold flux, delayed fracture, SFE, formability