Characterization Of Microstructures And Mechanical Properties In High Strengh TRIP Steels And Microstructure-based Finite Element Simulation

The reduction in the weight of vehicles has become the driving force for developing high strength steels in order to improve the fuel efficiency and environmental control. The transformation-induced plasticity (TRIP) mechanism has been known as a useful method for developing new grades of steels with optimized high strength and good plasticity. Currently, the development of TRIP steel based on Fe-C-Mn-Si system has been focused on the steels with strength ranging from 600MPa to 800MPa. However, the ultra-high grade of TRIP steels with tensile strength of 1000 MPa combining good plasticity have been developed overseas. In order to catch up with the development of foreign TRIP steel, we must improve our ability to develop new types of TPIP steels.The work of this dissertation was carried out integrating with the project of "Microstructure mechanism in enhancement of plasticity by deformation induced transformation in advanced high strength steel", which is supported by National Natural Science Fund of China. In this study, in order to achieve a strength level of 1000MPa or higher, microalloying concepts have been proposed. Nb, Mo, Ti and V in combination have been used as microalloying additions to refine the microstructures and form carbides or carbonitride precipitates. This study aims at obtain newly TRIP steels with good combination of strength and ductility.The influence of the microstructures, the volume fraction of different phases, the alloying element state, and the stability of retained austenite on the resulting mechanical properties of the experimental steels were investigated. The stable and appropriate proportion of retained austenite will significantly improves the mechanical properties of TRIP steels. In order to study the strength and ductility mechanism of high strength TRIP steels, an actual microstructure of experimental steel is used to build the finite-element model to simulate martensitic phase transition during the deformation. These researches will provide theoretical and experimental basis for factories to develop higher grade TRIP steels. The main work involved as follows:(1) In order to develop ultra-high grade of TRIP steels, the composition of TRIP steels were designed on the core idea of refining austenite grains and precipitating fine carbides inside ferrite. Accordingly, the carbon content of the experimental steels was restricted to 0.2 wt% to ensure good weldability. In comparison to TRIP-type steels used most often, the elaborated steel is characterized by decreasing Si concentration to 0.69 wt%, whose decrement was compensated by the addition of Al. Al addition with the similar amount of Si removal is expected to realize the hot-dip galvanizability.0.5 wt% of Cu in weight was introduced to the alloy system in order to enhance the strength. Furthermore, the additions of Nb, Mo, V and Ti in the experimental steels are expected to achieve further good combination of strength and ductility.The results show that the tensile strength of the microalloyed steel was near to or over 1000MPa, and the elongation was higher than 20%.(2) The effect of intercritical annealing on the microstructures and mechanical properties in high strength TRIP steel was investigated. Direct experimental evidence confirms the partitioning of C to austenite during intercritical annealing is the intrinsic factor that affects the stability of the retained austenite for the high strength TRIP steel. The prolonged intercritical annealing increased the fraction of the austenite phase and improved the C distribution uniformity in austenite that accompanies the growth of the austenite grains, which should lead to a significant decrease in the average C content of the austenite grains. Therefore, the volume fraction of retained austenite and C concentration decreases with increasing intercritical annealing time. When the sample annealed at 830℃ for 1 min and then isothermally treated at 440℃ for 5 min, the investigated low alloy TRIP steel achieved the tensile strength higher than 1000MPa, the total elongation of 22.5%, the combination of tensile strength and total elongation value of 24705MPa%. The microstructure of this sample has two distinct regions:large ferrite grain area and ultrafine grains area. The ultrafine grains area was found to consist of bainite, RA and equiaxed ferrite. C mainly existed among the retained austenite. A few retained austenite grains and a number of M/A islands were in the sample annealed for 30min, and the tensile strength significantly decrease to 869 MPa.(3) The effect of bainitic isothermal temperature on the microstructures and the mechanical properties of the investigated steel were investigated and the mechanical stability of RA was evaluated using Ludwigson and Burger relation. Martensite can be obtained after the isothermal quenching of the steel to the temperature of 320℃. Further increasing the isothermal holding temperature to 480℃, the microstructure of the steel exhibits most bainite and less retained austenite. With the isothermal treatment temperature increasing, the strength initially decreases and then increase while elongation and the combination of strength and elongation raises first and then descends. According the calculated kp values, the RA stability of sample treated at 440℃ is higher than that of other samples. All tensile samples fractured in a ductile manner.(4) Three different heat treatment processes have been proposed as a fundamental method to produce three kinds of TRIP-aided steels with polygonal ferritic matrix (F-matrix), bainitic matrix (B-matrix) and martensitic matrix (M-matrix) in the newly designed low alloy carbon steel. By means of dilatometry study and detailed characterization, the relationships among transformation, microstructure and the resulting mechanical behavior were compared and analyzed for the three cases. The F-matrix sample displays fine microstructure of polygonal ferritic matrix, granular bainite, and a large amount of M/A islands and retained austenite. The B-matrix sample has granular bainite and bainitic ferrite matrix with retained austenite. The M-matrix sample consists of fine martensitic matrix and inter lath second phase consisting of film like retained austenite. The tensile strength of F-matrix sample and B-matrix sample maintained at the level of 1008 MPa and 1045 MPa, respectively. The total elongation of B-matrix sample is 28.9%, and that of F-matrix sample reached 22.4%. The B-matrix sample exhibits superior sustained instantaneous n value at higher strains and the highest combination of tensile strength and total elongation value (30200.5 MPa%) was obtained. During the deformation, the "microtwinning phenomenon" appears in the B-matrix sample. The tensile strength of M-matrix sample is much higher than other samples whereas the total elongation is aboutl6%. The M-matrix sample exhibits the highest instantaneous n value at low strains and subsequent rapid descending at high strains.(5) In this study, a microstructure-based finite element model was developed to capture the complex phase transformation behavior of TRIP steels. The micromechanical finite element model is developed based on the actual microstructure of experimental steel. A user-material subroutine (UMAT) based on the martensite transformation kinetics law considering the stress state effect was developed and implemented into the commercial finite element software (ABAQUS) to simulate the martensite phase transformation during deformation. As the load increases, more area of the austenite phase transforms to the martensite phase. Martensitic phase transformation tends to occur first near the narrow regions located along the loading direction. As the strain continues to increase, the martensitic phase transformation continues to occur along the longitudinal loading direction rather than the transverse direction even under high strain levels, while the transformed martensite grains and bainite continue to carry high stresses. High plastic strain occurs mainly in the ferrite grains. As the load increases, many small localized regions with high plastic strains in the ferrite grains begin to appear in the vertical direction. The final major failure is predicted as the coalescence of the neighboring vertical strain localization zones.