In this work, a medium Mn-Ni steel was treated through Quenching and Partitioning (Q&P) with a partitioning temperature (PT) of 650 °C, which corresponded to the start of the austenite reverse transformation (ART) phenomenon. The influence of the quenching temperature (QT) and partitioning time (Pt) on austenite stabilization and mechanical properties was investigated. A strong influence of the quenching temperature was observed. Results were compared with those obtained after a Q&P treatment with 400 °C partitioning temperature. The Q&P cycle with quenching to room temperature and a high partitioning temperature produced a steel with a high retained austenite (RA) volume fraction and exceptional strength–ductility balance. The analysis of the mechanical stability of the retained austenite revealed a significant stress-induced transformation. Nevertheless, the austenite, which was stable at stresses above the yield stress, provided significant TRIP-assisted ductility. Bending, hole expansion and post-stamping properties were also evaluated for the most promising conditions.

In this work, four low carbon steels with different contents of Mn and Ni were heat treated by quenching and partitioning (Q&P) cycles where high partitioning temperatures, in the range of 550 °C–650 °C, were applied. In order to elucidate the effect of applying these high partitioning temperatures with respect to more common Q&P cycles, the materials were also heat treated considering a partitioning temperature of 400 °C. The microstructure evolution during the Q&P cycles was studied by means of dilatometry tests. The microstructural characterization of the treated materials revealed that austenite retention strongly depended on the alloy content and partitioning conditions. It was shown that the occurrence of austenite reverse transformation (ART) in the partitioning stage in some of the alloys and conditions was a very effective mechanism to increase the austenite content in the final microstructure. However, the enhancement of tensile properties achieved by the application of high partitioning temperature cycles was not significant.

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.