Research On Hot Deformation Behaviors And Phase Transformation Kinetics Of Nb-Microalloyed Ship Plate Steels

In this thesis, the hot deformation behaviors and phase transformation kinetics of two ship plate steels with different compositions were studied. The purpose was to explore the feasibility for acceding to high content Nb in ship plate micro-alloying process.Solid solution process and austenite grain growth of high-Nb ship plate steels were discussed by reheat treatment process and chemical analysis technology, confirmed the temperature of anstenization. Dynamic recrystallization, static recrystallization behavior and strain-induced precipitation were studied in Gleeble-3500 thermo mechanical simulator, thus, the influences of acceding to high content Nb on the hot deformation behaviors were analyzed, furthermore, in the processes of continuous cooling transformation of high-Nb ship plate steels, the phase transformation kinetics and the microstructure were researched, the influences of deformation processes and high content Nb on the phase transformation were analyzed.The results showed that the temperature at 1100℃was grain coarsening temperature in the process of austenite grain growth, under this temperature, the growth of austenite grain size was in a low speed, the size of two steels austenite grain had small discrepancy, while austenite grain growth of two steels increased with a high speed over coarsening temperature.In hot deformation processes, it was found that steel B have more higher activation energy than steel A by calculating. It was found that steel B have a more longer gestation period than that in steel A by analyzing statically soft fraction, so the difficulty level of the recrystallization in steel B was greater than that in steel A. Precipitation-start time of steel B was shortened while precipitation temperature was lower than those of steel A by analyzing the stain induced carbide precipitation curves (PTT curves). Under-cooled austenite continuous cooling phase transition kinetics and the microstructure of steels were analyzed, the results showed that grain size reduced and the beginning point of phase transformation for acicular ferrite raised notablely. The influence of high content Nb addition on the phase transformation mainly manifested as, austenite had good thermal stability and the ferrite phase transformation point decreased by the function of the solid solution drag force, while grain size reduced observably because strain-induced precipitation of Nb increased nucleation rate.