Tailored tempering without die heating in hot stamping of ultra-high strength steel parts

Abstract Three tailored tempering processes without die heating were developed to manufacture tailored parts having high strength and high ductility zones. In a 1st forming stage, only the high strength zone was formed with the partial dies in order to decrease the cooling rate in the high ductility zone by no contact with the dies, and then both zones were formed with the sufficiently wide dies in a 2nd forming stage. Two cooling stages were introduced to prevent the martensite transformation of the high ductility zone. The 2nd cooling stage was omitted by partial forced cooling in the 1st stage having a higher cooling rate. The 2nd forming stage was further omitted by keeping high temperature in the high strength zone and by slowly cooling the high ductility zone in the 1st cooling stage. A tailored part having hardnesses of about 470 and 230 HV1 in the high strength and high ductility zones, respectively was produced, and the cycle time was 15 s, 4 shots per minute.