Design and properties of 1000 mpa strength level hot-formed steels possessing dual-phase and complex-phase microstructures

In cold forming for automotive lightweight design, advanced high strength steels (AHSS) lead to limited formability, high springback and press forces, low stretch flangeability, multiple operations for complex geometries and large scrap rates. Two sets of AHSS, namely ferritic-martensitic dual-phase (DP) steel and martensitic-bainitic complex-phase (CP) steel with some amounts of retained austenite (RA), were designed for the hot-forming route, which eliminates the above drawbacks and guarantees higher performance in the body-in-white (BIW). Design of four DP and four CP alloys was accomplished using JMatPro6.0 thermodynamic software and available literature. The alloys were manufactured in the laboratory in cold-rolled gauge of ~1.5 mm and subjected to hot-forming cycles including hot deformation (up to 20% strain), using a dilatometer and a Gleeble 3800 machine. The thermal cycles of the DP alloys included an intercritical reheating whereas in-situ austempering or slow continuous cooling followed by supercritical reheating was used for the CP alloys. The results showed that yield strength (YS) of 605MPa & 695MPa, ultimate tensile strength (UTS) of 1097MPa & 1242MPa with a total elongation (TE) of 12.6% & 14.1% can be achieved in the best performing DP alloys with a martensite content of 65% & 60 vol.%. The best CP alloys with austempering achieved YS of 673MPa & 699MPa, UTS of 983MPa & 1026MPa and TE of 9.2% & 13.6% with RA of 4%-12 vol.%. The continuously-cooled alloys achieved even better properties. Higher bendability at 1.0 mm gauge in the critical direction was achieved in the CP alloys (90^o &107^o ) than in the DP alloys (73^o &76^o ).