Effect of Nb and Mo on Austenite Microstructural Evolution During Hot Deformation in Boron High Strength Steels

Abstract

This work has focused on the study of hot working behavior of boron high strength steels microalloyed with different combinations of Nb and/or Mo. The role of Nb and Mo during the hot deformation of low carbon steels is well known: both mainly retard austenite recrystallization, leading to pancaked austenite microstructures before phase transformation and to refined room temperature microstructures. However, the design of rolling schedules resulting in properly conditioned microstructures, requires microstructural evolution models that take into account the effect of the different alloying elements. In this specific case, the effect that high levels of molybdenum (0.5 pct) have in the recrystallization delay was evaluated. In that respect, hot torsion tests were performed in this work to investigate the microstructural evolution during hot deformation of four boron steels, with different Nb (0.025 pct) and Mo (0.5 pct) combinations. The retardation in recrystallization kinetics was modeled in all cases and measured kinetics agree with those predicted by equations previously developed for Nb–Mo microalloyed steels with lower Mo concentrations (< 0.3 pct). The strain-induced precipitation in the Nb and Nb–Mo bearing steels was also characterized. Finally, the fractional softening evolution during multipass rolling simulations was compared with MicroSim® model predictions, showing a good agreement with experimental results.

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

“…The T nr temperatures for the current steels were previously determined as 955, 980, 1010, and 1024 • C for the CMnB, CMnNbB, CMnMoB, and CMnNbMoB steels, respectively [4]. Accordingly, all finishing temperatures shown in the current experiments were below T nr .…”

“…Microstructural heterogeneity in austenite can be generated at different stages during the hot working Materials 2022, 15, 1424 2 of 16 process. Alloy additions of boron, niobium, and molybdenum induce strong solute drag on austenite boundaries, thus delaying austenite recrystallization at temperatures between 1000 and 1100 • C [4]. If the austenite temperature during the last passes of recrystallizing rolling (roughing) drops into that range, the recrystallization of austenite, especially in the plate center, may not completely occur and individual non-recrystallized grains may not be as refined as the recrystallized ones.…”

Understanding the static recrystallization of Ni-Mo alloyed steels for rolling optimization in high strength thick plates

“…The results suggest that reasonable prediction could be achieved in most of the cases taking into account this approach. In a recent work, and considering this approach, good correlation is also obtained with high Mo and boron microalloyed steels [5]. However, for the LowMoNi steel, the predictions underestimate the experimental measurements to a higher degree.…”

“…However, for very thick plates, to increase hardenability and ensure good toughness properties, Mo addition is not sufficient and additional alloying elements such as Ni are required [4]. It is widely known that the addition of Mo retards static recrystallization due to solute drag effect [5]. When Ni is alloyed to austenitic steel it increases the stacking fault energy (SFE) of the iron matrix [6].…”

Hot Deformation Behavior and Microstructure Evolution of 2Cr12Ni4Mo3VNbN Steel Used for Last‐Stage Large Blades of Steam Turbine

“…When the Z value reaches a certain level, DRV becomes difficult and the material is completely in the work-hardening stage. [26,27]…”