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

Abstract

The application of direct quenching after hot rolling of plates is being employed in the production of ultra-high-strength hot rolled plates. When heavy gauge plates are produced, the complexity involve in achieving high cooling rates in the plate core is increased and the formation of undesirable soft phases within martensite is common. In the current paper, both direct quenching and conventional quenching (DQ and CQ) processing routes were reproduced by dilatometry tests and continuous cooling transformation (CCT) diagrams were built for four different high-strength boron steels. The results indicate that the addition of Mo and Nb-Mo suppresses the ferritic region and considerably shifts the CCT diagram to lower transformation temperatures. The combination of DQ strategy and the Mo-alloying concept provides the best option to ensure hardenability and the formation of a fully martensitic microstructure, and to avoid the presence of soft phases in the center of thick plates.

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

“…The presence of ferrite colonies (Figure 3) in the former partially recrystallized austenite area indicated an early transformation, probably due to insufficient boron protection. The combination of both effects further extended the ferrite phase field under direct quenching conditions compared to completely recrystallized CMnB steel [6].…”

“…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 Nb and Mo on Austenite Microstructural Evolution During Hot Deformation in Boron High Strength Steels

“…Recently, the effect of combining boron and microalloying elements on phase transformation has been investigated, to select the optimum alloy concept and processing route that ensures the formation of fully martensitic microstructure. [5] Another relevant aspect that has to be considered is the strengthening due to grain size refinement that can be improved reaching a pancaked austenite before phase transformation. It is widely known that the addition of Nb and Mo delays softening kinetics, retards recrystallization of austenite and ensures the accumulation of deformation of the austenite prior to transformation.…”