Dual-Scale Simulation Of Dynamic Recrystallization Behavior During Hot Compressive Deformation For Bridge Steel

With the comprehensive application of controlled rolling and controlled cooling technology and alloying, various new steels with superior performance get to be developed and applied, in which controlled recrystallization rolling at the austenitic area plays an important role. In order to determine the optimum parameters to achieve reasonable controlled recrystallization rolling, it is necessary to intensively investigate the dynamic recrystallization (DRX) behavior during hot rolling, especially, the research on DRX by computer simulation methods is of theoretical and practical significance.Due to the complex deformation mechanism, a number of influence factors, fast deformation process and the difficulty of establishing microstructure models, the study on simulation of DRX for bridge steel started relatively late, and little attention has been paid on physical or experimental modelling based finite element method (FEM) microstructure simulation in macro scale, and FEM-mesoscopic microstructure modelling combined dual-scale simulation of DRX. Therefore, novel high performance bridge steels, especially advanced HPS485wf steel, were selected as object of this study. Combined with thermo-mechanical simulation experiment, quantitative metallographic analysis, FEM analysis technology, and DRX microstructure simulation technology, choosing MARC software as the FEM modelling support platform, and using the application programs based on phenomenological model or MC model as tools for simulation of DRX microstructure evolutions, physical-FEM modeling-dual scale (macro scale and meso scale) simulation of the DRX behavior during single pass hot compressive deformation for HPS485wf steel was carried out.To start with, systemic isothermal compressive tests of HPS485wf steel and Q420qE steel were conducted. The effects of deformation parameters on mechanical behavior of hot deformation and microstructure evolutions of DRX were investigated. The values of characteristic parameters of DRX were obtained, which could provide experimental foundation not only for establishment and verification of phenomenological model and MC model, but also for macroscopic and mesoscopic microstructure simulation.Secondly, according the true stress-true strain data collected from hot compressive tests, a general constitutive relationship model and a simplified one, which can describe the flow behavior of HPS485wf steel, were established. The critical parameters model of DRX for HPS485wf steel were obtained by using both simplifiedθ—σmodel proposed by Najafizadeh and Jonas and flow stress model proposed by Cingara and McQueen, respectively. These models made it possible to simulate microstructure evolutions by FEM-DRX phenomenological model in macro scale.Thirdly, based on the large commercial finite element software MSC.MARC, necessarily secondary development using the MARC platform, and the thermo-mechanical coupled rigid-plastic FEM-DRX phenomenological model, the evolutions of equivalent effective strain and state of DRX during hot compressive deformation in HPS485wf steel were successfully simulated, which verified the phenomenological model and the critical parameters model. The impacts of deformation conditions on mechanical behavior and microstructure evolutions were analyzed and the macroscopic law of DRX in the steel was totally revealed. These results provide raw data for FEM-mesoscopic microstructure simulation of DRX.In the next place, a mesoscopic DRX MC model, including energy model, nucleation model, R-grain growth model and real time model, was developed. The mesoscopic characteristics of evolutions of microstructure, volume fraction and mean grain size under different deformation conditions for HPS485wf steel were investigated. The phenomenon of coexistence of multi-circles DRX was successfully simulated. Thus, the mesoscopic law of DRX in the steel was displayed. Meanwhile, an effective mesoscopic model was prepared for FEM-mesoscopic simulation for DRX.Finally, mesoscopic DRX behavior of different deformation zones in HPS485wf steel at a certain deformation condition was studied by using the thermal-mechanical coupled FEM—DRX MC model, and the simulation results were compared with those obtained from thermal-mechanical coupled FEM-DRX phenomenological model. The macroscopic and mesoscopic simulation results were well interrelated, indicating the proposed model for macro-meso dual-scale simulation of DRX was reasonable.To conclude, in this study, dual scale simulation of DRX during hot compressive deformation for HPS485wf steel has been realized. The constitutive relationship model, the critical parameter model and the DRX phenomenological model for HPS485wf steel were established. The DRX MC model and corresponding application program was developed, which well simulated the phenomenon of coexistence of multi-circles DRX of this steel. Systemic studies on physical simulation-FEM modeling-mesoscopic DRX microstructure simulation of HPS485wf steel were carried out. The effects of deformation conditions on DRX behavior and involved physical essence were revealed in both macro and meso scale. As a result, it is achievable to simulate and predict DRX behavior and microstructure evolutions of HPS485wf steel.