Investigation of Hardness Change for Spot Welded Tailored Blank in Hot Stamping Using CCT and Deformation-CCT Diagrams
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RODUCTION
HOT stamping is a useful process in the manufacture of high-strength steel parts.[1] In this process, steel sheet is heated in a furnace at a temperature higher than the Ac3 temperature to change its microstructure to austenite. It is then formed into the designated shape and quenched in dies. During the quenching process, the microstructure changes from austenite to martensite. Martensite imparts high strength to the formed parts. The hot stamping process has been applied to parts such as pillars and bumpers, and can contribute to the reduced weight of automobiles. As the applications of hot stamping have expanded, much research has been conducted in areas such as flow stress,[2,3] formability,[4–6] heating method,[7] phase transformation
YASUHIRO YOGO, NOZOMI KURATO, and NORITOSHI IWATA are with the Toyota Central R&D Labs., Inc., 41-1, Yokomichi, Nagakute, Aichi 480-1192, Japan. Contact e-mail: [email protected]
METALLURGICAL AND MATERIALS TRANSACTIONS A
behavior[8–13] impact strength,[14] property tailoring[15–17] and hydrogen-induced delayed fracture.[18,19] When a B-pillar is made with the hot stamping process, an outer panel is first hot stamped. Then, reinforcements are usually spot welded on the inner side of the outer panel. It has been reported that the hardness in the heat-affected zone (HAZ) of the spot welded parts becomes lower than that before welding.[20] During spot welding, the nugget is reheated above the Ac3 temperature. Then, it is quenched, and therefore the microstructure is still martensite after spot welding. However, the HAZ is reheated only between the Ac1 and Ac3 temperatures, and therefore contains both martensite and ferrite. Ferrite in the HAZ lowers the overall hardness. To avoid a hardness reduction after spot welding, a tailored blank approach has been proposed.[20–22] In this process, spot welding is carried out on steel sheets in a lap configuration to form a tailored blank, which is then hot stamped. Because spot welding is already finished before hot stamping, the hot stamped part does not undergo a second reheating. Consequently, a weakened HAZ is not created. If the spot welded tailored blank is hot stamped, the spot welded part is bent, stretched, and drawn. It can be assumed that this part undergoes a larger deformation
than other parts due to the stress concentration. As reported for hot stamped steel[9–11] and other steels,[23–26] pre-strain strongly influences the phase transformation. Pre-strain tends to promote ferrite and pearlite formation. This means that pre-strain suppresses martensite formation, and so a lower hardness area appears around spot welded parts after hot stamping. Simulations based on the kinetics model of Kirkaldy and Venugopalan[27] and the TTT diagram[28] have been conducted for normal hot stamping. Such simulations are useful in designing hot stamping processes to obtain the desired microstructure and hardness. However, they do not incorporate the effects of pre-strain on the phase transformation. Therefore, other techniques are
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