The Establishment Of Multi-scale Models And Numerical Simulation During Nb Microalloyed Steel Multi-pass Hot Rolling

As industry develops so rapidly, the integrated service performance such as materialstrength, toughness, compactibility and weldability is eager to be called for. Nbmicroalloyed steel can obtain the performance, so we making use of it all over the world.In contrast to Ti, V, the grain refining effect of Nb is better. By control rolling Nbmicroalloyed steel can obtain the higher strength, toughness and the integrated serviceperformance at a low cost., so it can apply to different kinds of engineering structures suchas bridge, architecture, shipping, car, pressure reservoir, oil production platform, pipelineand so on. At the same time, control rolling technology can dispel the expensive heattreatment procedure, improve the production efficiency, reduce cost and achieve the targetfor energy saving.The product performance of Nb microalloyed steel is determined by themicrostructure morphology, alloying component and rolling process. The metaldeformation, heat transfer, dislocation density evolution and microstructure evolution(including matrix microstructure evolution and second-phase precipitates) of rolled piecehave a complex mutual effect. Deformation process can change the heat quantity by theinternal heat source in rolled piece and state of strain, then the condition of microstructureevolution is changed to effect the heat transfer and microstructure evolution. Heat transfercan change the flow stress and temperature distribution by the constitutive relationship asto effect deformation process and microstructure evolution. Microstructure evolution canchange the flow stress by the constitutive relationship as to effect deformation process andheat transfer. Because of all above, it is necessary to coupling calculate the metaldeformation, heat transfer, dislocation density evolution and microstructure evolution.The valence electron structures of crystal cells in Fe-M-C alloy system (M=Nb, Ti orV) were calculated using the Empirical Electron Theory in Solid and Molecules(EET).Based on the calculational results, the coherent interfacial energy between second phaseand iron matrix in Fe-M-C alloy system (M=Nb, Ti or V) was calculated by intergratingEET into discrete lattice plane, nearest neighbor broken bond(DLP/NNBB) methodthrough covalence bond energy. Considering the effect of microstructure evolution, second phase precipitates andgrain size on dislocation density evolution, the way to calculate the microstructureevolution, second phase precipitates and grain size is proposed during Nb microalloyedsteel hot rolling. The microstructure evolution are integrated with mesoscopic dislocationdensity evolution in order to establish the nonlinear constitutive model based on themicrostructure evolution next chapter and provide the microstructure evolution model formulti-scale finite element numerical simulation during hot rolling.The flow stress curves of Nb microalloyed steel Q345B during hot compressiondeformation were obtained on Gleeble-3500thermo-simulation machine. Based on theexperimental results, the constitutive model of Nb microalloyed steel during hightemperature deformation was established to describe the stress-strain relationship duringdynamic softening (dynamic recovery and dynamic recrystallization) and in the steadyflow state, in which the influences of the thermal deformation parameters (strain rate anddeformation temperature) and dynamic softening mechanism (dynamic recovery anddynamic recrystallization) on the flow stress were considered. Through elasto plasticitymechanics theory, the original work-hardening function is considered as flow stress curvesequivalently. The work-hardening and softening function is propounded. A method wasprovided to solve the work-hardening and softening function and establish the mathematicexpressions of the correlation coefficients. It is shown that the calculated results by themodel are in good agreement with the experimental results, and can represent accuratelythe flow stress during dynamic softening (dynamic recovery and dynamic recrystallization)and in the steady flow state. At last, the elastic-plastic matrix is derived as to calculate theconstitutive relationship during finite element numerical simulation during hot rollingnext.Finally, based on the multi-scale models established above, an approach to realize thecoupling analysis is found. The multi-scale computer modules are integrated on theplatform of MARC2010and FORTRON10.1. The case of Ansteel3450medium plateproduction line is analyzed on the platform. Based on the simulated results, thedistribution of the fields, such as the strain, the strain rate and temperature and the changesof austenite grain size and recrystallization volume fraction in muti-pass hot rolling process is precited and anlyzed. Finally, the calculated flow stress curves and rolling forceby paper's model and others' model are compared with the experimental flow stresscurves and rolling force. it is shown that the calculated results by the paper's model are inbetter agreement with the experimental results than other's model.