Advanced high strength steels (AHSS) are being used in the automotive sector with the aim of supporting the current demands of reducing vehicle weight and structures, but cold forming of such steel grades shows some challenges like springback, high press forces or low stretch-flangeability. To overcome these drawbacks, different ferritic-martensitic dual phase (DP) alloys and martensitic-bainitic complex phase (CP) alloys with retained austenite were designed and novel hot forming routes were applied for processing them. The thermal cycle for the DP alloys included an intercritical reheating whereas in-situ austempering or slow continuous cooling preceded by supercritical reheating was used for the CP alloys. The objective of obtaining similar, or even better, post-formed mechanical properties than the current cold-formable DP1000-Low Yield and CP1000 grades in terms of yield strength (YS), ultimate tensile strength (UTS) and total elongation (TE), combining the new alloys and the proposed hot forming routes was investigated through an intensive testing campaign. First, cold rolled alloys were subjected to hot forming cycles including deformation levels up to 20% in a Gleeble machine and then formability tests at high temperatures (hole tensile tests, omega-shaped parts manufacture) were conducted to compare their performance against the current cold formable alloys. The promising results after hot press forming (YS ≈ 650 MPa; UTS ≈ 1150 MPa; TE ≈ 10% for DP alloys and YS ≈ 850 MPa; UTS ≈ 1250 MPa; TE ≈ 7% for CP alloys) pave the way to promote the use of these new alloys that will allow designing vehicle components with increased geometric complexity, while minimizing the springback effect, reducing the press forces and the material scrap inherent of cold stamping.

Pure martensitic steels have after hot forming limited performance in terms of rest ductility which limits the application in crash relevant parts. New steel grades were designed in the EU project HOTFORM including the corresponding process routes. These steel grades have ferritic-martensitic dual phase (DP) and martensitic-bainitic complex phase (CP) microstructures after hot forming process. The laboratory tests show an improved formability after hot forming. The basic concepts of the new alloys are explained. Furthermore, for validation of upscaling purposes a semi-industrial test is carried out and the results are discussed. The main application is for vehicle safety. This is evaluated by comparing the crash performance of these hot formed grades with cold rolled DP1000 and CP1000 for crash cans in a drop tower test.

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 ).