Spray Cooling of Early Extracted Hot Stamped Parts

Hot stamping of automotive parts becomes increasingly important since the hot forming allows manufacturing of parts such as A-pillars with a high specific strength and stiffness. For processing, sheets of suited steels such as the heat treatable steel 22MnB5 are usually firstly austenized, transferred to the forming press, hot stamped and then quenched in the closed die. Cooling rates of at least about 27 K/s are necessary to form the desired martensitic microstructure during quenching. Due to the limited heat transfer to the forming die the cooling below the martensite finish temperature is slow and the process productivity is comparatively low. Regarding the demand for an increased productivity, hot stamped parts were early removed from the die and transferred to an external cooling device where the parts were quenched below the martensite finish temperature by means of a water-air-spray. Various removal temperatures were investigated, process conditions resulting in low distortions identified and mechanical properties characterized.