Process, Microstructure And Mechanical Property Of High-strength And High-toughness Multiphase Steels And Physical Simulation For Their Mutual Relationship

The relationship between microstructures and mechanical properties of low alloy cold-rolled C-Mn-Al-Si transformation-induced-plasticity (TRIP)-assisted steels was investigated in the present study by utilizing in situ high-energy X-ray diffraction (HEXRD) technique, optical microscopy (OM), scanning electron microscopy (SEM) and transmission electron microscopy (TEM), as well as uniaxial tensile tests. Meanwhile, the micromechanical behavior of investigated steels during deformation was discussed with emphasis, and a constitutive model being able to reveal the stress/flow strain partitioning among constituent phases of multiphase steels was also established and validated. In addition, some helpful efforts have also been devoted, in combination with Thermo-Calc and DICTRA, to the study of relationship between processing parameters and microstructures for the investigated steels.Microstructure and mechanical behavior of the investigated steels with different carbon contents (0.1%and0.2%, mass fraction) were studied after intercritical annealing and subsequent isothermal bainitic transformation at400℃for different times. The steels with high carbon content acquired lower fraction of bainite but larger fraction of martensite, and exhibited higher strengths and larger elongations than those of steels with low carbon content under the condition of the same isothermal transformation time. The excellent ductility of steels with high carbon content was mainly attributed to their strong TRIP effect during deformation, resulted from the larger fraction and higher carbon content of retained austenite in their multiphase microstructures. Yield strengths and elongations were generally raised with isothermal transformation time for both steels, while the opposite case was true for tensile strengths. The value of the product of tensile strength and total elongation, representing the combination of strength and ductility of steels, was found increased linearly with the value of the product of volume fraction and carbon content of retained austenite, which could be approximatively used to characterize the TRIP effect. It was also found that the evolution of transformation rate of retained austenite r during deformation was well corresponding to the variation of incremental strain hardening exponent nincr. with strains for the investigated steels, intuitively indicating the critical influence of TRIP effect on the strain hardening capabilities of TRIP-assisted steels.Stress partitioning among different phases, yield strengths and residual stresses after unloading of constituent phases, as well as the transformation kinetics of retained austenite during deformation, were investigated based on the experiment of in situ HEXRD for the steels. The stress partitioning among phases during elastic deformation was not obvious due to their similar elastic moduli. Significant partitioning was observed once plastic deformation of the steels took place, in such a way:retained austenite undertook the largest load, followed by bainitic ferrite, both of which exhibited as hard phases; while the ferrite matrix phase bore the smallest load, recognized as the soft phase in the multiphase microstructures. Moreover, the transformation kinetics of retained austenite was found to intensively influence this load partitioning and then the final mechanical properties of steels.A constitutive model was established for describing the micromechanical behavior of TRIP-assisted multiphase steels, based on a Gladman-type mixture law (GTML) embedded with the Mecking-Kocks work-hardening formula for each single phase and O-C model for the transformation kinetics of retained austenite. This established model took not only the TRIP effect into account but also the composite effect which is also very important for the mechanical behavior of TRIP-assisted multiphase steels. All the parameters applied in the model had significant physical meanings, particularly for the index n in GTML, an important parameter to be defined to characterize the accommodation of loading stresses among different phases during deformation. Furthermore, the strain partitioning among constituent phases could be approximately acquired after simple calculation based on the results of stress partitioning. Simulations were conducted with the established model to the experimental results of in-situ HEXRD, and well agreements between them were observed. It was also found that the fitting values of index n were severely affected by the transformation kinetics of retained austenite. The quantitative relationship between n and the transformation rate of retained austenite r was also clearly revealed for the investigated steels, which forms the basis for applications of the model in simulations of micromechanical behavior and design of a new type of high-strength and high-plasticity steels.In the present study, the relationship between processing parameters and microstructures for the low alloy cold-rolled C-Mn-Al-Si TRIP-assisted steels was also investigated by simulating the processes of intercritical annealing and subsequent isothermal bainitic transformation. The validity of this predictive method was then evaluated by experiments. As a consequence, this simple and fast method was expected to be helpful in optimizing the processing parameters of low alloy cold-rolled TRIP-assisted steels in industry.