Evolution Of Microstructures And Solute Distributions In Dual-phase Steels: Modeling And Validation

Dual-phase steel is a kind of advanced high strength steels,which consists of a soft ferrite???matrix and dispersed hard second-phase of martensite?and retained austenite???or bainite?.Since the dual-phase steel has an excellent combination of high strength and good ductility,it is widely used in industries.Normally dual-phase steels are produced by intercritical annealing followed by cooling at a fast or moderate cooling rate.In this thesis,the evolution of microstructure and solute distribution during different heat treatments in an Fe-0.323C-1.232Mn-0.849Si?mol.%?dual-phase steel is investigated utilizing combined experiments and simulations.The processing-microstructure-property relationship is intensively studied,which provides guidance for optimizing the processing and improving mechanical properties.The mechanical properties,microstructure and solute distribution of dual-phase steel samples subjected to different heat treatments are studied utilizing tensile tests,scanning electron microscopy?SEM?,transmission electron microscopy?TEM?and three-dimensional atom-probe tomography?3-D APT?.Compared to the as-rolled sample,the sample annealed at 800? and then air cooled?800AC?exhibits a shorter yield platform and a lower yield strength.Martensite containing a twinning substructure and high concentrations of C and Mn is distributed at?/?grain boundaries?GBs?.After tempering the 800AC sample at 400??800AC-400T?,the yield platform becomes longer and the yield strength increases,with martensite distributed at?/?GBs decomposing into the tempered martensite having a lamellar structure.The sample annealed at 760? and then water quenched?760WQ?contains typical lath martensite with a volume fraction of⁰.17,while the sample annealed at 760? and then air cooled?760AC?contains the second-phase with a volume fraction of⁰.09.The second-phase has different substructures and compositional features,including a high density of dislocations?lath martensite?,fine twins?twinned martensite?,carbides with different dimensions?auto-tempered martensite?and a small amount of retained austenite.Compared to the previous cellular automaton models not involving the effect of Mn on the migrating interface,a new CA-SD model coupled with the solute drag effect is developed for the first time.The CA-SD model is then applied to model the isothermal holding at 800?,subsequent air cooling at 6? s^-1 and then tempering at 400? in an Fe-0.323C-1.232Mn-0.849Si?mol.%?dual-phase steel.Simulation results show that with the proceeding ofg??transformation during cooling,the C concentrations in the?-and?-phases increase and the distributions become non-uniform.When cooled to the Ms temperature?¹17??,the remainingg-phase with a volume fraction of⁰.065 transforms to martensite.The simulated microstructures at the end of isothermal holding and cooling,and the C concentrations in martensites agree reasonably well with the SEM micrographs and APT analyses,respectively.After tempering at 400? for 20 min,martensite decomposes and the simulated average C concentration of the?-matrix is higher than that in the air cooled sample.Utilizing the simulation and experimental results,the mechanisms for samples subjected to different heat treatments exhibiting different yield point phenomena are analyzed in detail.As the analytical model involving the effect of substitutional elements for the???transformation has not been addressed so far,a GEB analytical model for the???transformation is developed for the first time based on the Gibbs energy balance approach.The GEB model is then applied to predict the???transformation kinetics during isothermal holding at 760 and 800? in an Fe-0.323C-1.232Mn-0.849Si?mol.%?dual-phase steel.It is found that the Gibbs energy dissipation slowers the transformation rate and reduces the stabilized?-volume fraction.By adjusting the chemical potential difference of Mn between the?-and?-phases,the???transformation kinetics predicted by the GEB model agrees well with the dilatometric results.The?-volume fraction and C concentration at the?/?interface predicted by the GEB model agree well with those simulated by the 1-D CA-SD model.As it is difficult for current numerical models to directly couple with CALPHAD thermodynamic calculations with a high efficiency,a CA-CALPHAD model intergrated with a data management system PanDataNet is developed for the first time.The adoption of PanDataNet shortens the computational time for the required thermodynamic and phase equilibrium data by around two orders of magnitude.The CA-CALPHAD model is applied to simulate the isothermal holding at 760 and 800? and the subsequent cooling at 6? s^-1.The CA-CALPHAD model can not only describe the microstructural evolution and C distributions in the?-and?-phases during different heat treatments,but also predict the partitioning behavior of Mn and Si.The simulated tendencies of the partitioning of Mn and Si agree well with the APT analyses.