Evaluation of Mechanical Properties and Microstructures of Molybdenum and Niobium Microalloyed Thermomechanically Rolled High-Strength Press Hardening Steel

Abstract

This article studied the effect of molybdenum and niobium on the microstructures and mechanical properties of laboratory control rolled steels based on grade 22MnB5. The constructed continuous cooling transformation diagrams revealed that an addition of Mo significantly increased the hardenability. Especially in the case of austenite compressed below its recrystallization temperature, an Mo addition delayed ferrite and bainite formation, and it increased martensite hardness. Laboratory hot-rolling experiments with a finish rolling temperature of 850°C produced a flattened pancaked austenite. After hot rolling and direct quenching, samples were rapidly reaustenitized at 900°C followed by water quenching to simulate an actual press hardening process. Especially in the case of Nb-Mo steel, a strongly pancaked austenitic structure was achieved after hot rolling, which led to a fine, uniform prior austenite grain structure after re-austenitization and quenching. The Nb-Mo steel had a tensile strength > 1500 MPa and $ 11% total elongation combined with good impact toughness, which can be considered excellent for this type of press hardening steel.

Effect of Dynamic Recrystallization on Microstructural Evolution in B Steels Microalloyed with Nb and/or Mo

“…That means the ferrite-suppressing effect of molybdenum was not measurably influenced by the presence of accumulated strain. This behavior is in good agreement with the results of Hannula et al [7] who investigated the hardenability of very similar DQ steel alloys. Their results indicated the formation very fine-grained equiaxed austenite decorating the boundaries of much larger pancaked austenite grains when finish rolling at 800 • C. Such necklace structures gradually disappeared when molybdenum was added to the CMnB steel in amounts of 0.25 and 0.5 mass%.…”

“…When boron segregation proceeds during austenite conditioning, additionally supported by a strong flux of vacancies towards the austenite boundaries, it is possible that no diffusible boron is left for covering the new austenite grain boundaries generated by dynamic recrystallization. Earlier investigations on the current and other boron-alloyed steels [2,[6][7][8] revealed that the hardenability effect related to boron is weakened when applying substantial strain immediately before quenching. In CCT diagrams reported in the previous works, the ferrite and bainite phase fields are then shifted towards shorter times and higher transformation temperatures under direct quenching conditions.…”

“…austenite grain boundaries generated by dynamic recrystallization. Earlier investigati on the current and other boron-alloyed steels [2,[6][7][8] revealed that the hardenability ef related to boron is weakened when applying substantial strain immediately be quenching. In CCT diagrams reported in the previous works, the ferrite and bainite ph fields are then shifted towards shorter times and higher transformation temperatures der direct quenching conditions.…”

“…This event produces a fraction of very fine equiaxed austenite grains. It has been shown that the application of large deformation strain at low austenite temperature and the presence of dynamically recrystallized austenite compromises the hardenability effect related to boron microalloying [2,[5][6][7][8]. The hardenability related to molybdenum alloying, however, appears to be much more robust under the same processing conditions.…”

Effect of Quenching Strategy and Nb-Mo Additions on Phase Transformations and Quenchability of High-Strength Boron Steels

“…Besides the direct effects of molybdenum, the combination of Nb and Mo leads to pronounced synergetic effects, while Mo significantly contributes to austenite pancaking by solute drag during hot rolling. This effect is even greater in combination with niobium, as the martensitic substructures are formed in the previously pancaked austenite grains, 6 , 19,20 combining drag and straininduced precipitation mechanisms. Molybdenum also delays precipitation of niobium during austenite conditioning, making a higher amount of solute niobium available for precipitation during tempering treatment.…”