| Nowadays, lightweight by the application of advanced high strength steels in body in white has been an effective measure to solve the fuel consumption, gas emission and human safety problems. The newly developed austenite TWIP steels has been regarded as the most potential material for automobiles attributed to the combination of high strength, high ductility and high strain hardening property. However, the content of alloyed elements in Fe-Mn-C TWIP steel is extremely higher than the common steels, which leading to the significant importance of the understanding of composition design, solidification characteristic, microstructure and tensile property, and strengthening-toughening mechanism.The reasonability and availability of the sublattice model application in investigating the thermodynamical behavior during γâ†'ε phase transformation was discussed, based on which a new thermodynamic model for calculating stacking fault energy(SFE) of Fe-Mn and Fe-Mn-C alloys was developed. The segregation of interstitial carbon atoms at stacking faults by Suzuki effect was also taken into concideration. The model was verified by the experimental results from previous literatures, and consequently the iso-SFE map with10～30wt%manganese and0～1.2wt%carbon was ploted. The relationship between temperature and grain size on SFE in Fe-Mn-C system was schematically revealed. Seven TWIP steels were designed according to the final prediction for the following investigation.The possibility of single austenite matrix at room temperature was analyzed based on the ternary phase diagram of Fe-Mn-C alloys calculated by ThermoCalc software. The solidus and liquidus of seven TWIP steels was predicted by JmatPro6.0, and then validated by continuous cooling results measured during solidification. The thermal conductivity of TWIP steel was measured by a laser flash system NETZSCH LFA427, which was compared with that of carbon steel Q235, stainless steel304and J4. The grain morphology characteristic and evolution of a TWIP steel of Fe-22Mn-0.7C was observed by macroscopic examination. The common defects including shrinkage, porosity and macrosegregation were investigated. The interaction effect of carbon and manganese microsegregation during ingot solidification was studied according to the concentrations determined by electron probe microanalysis(EPMA), which would be the valuable experiment database for the further microsegregation research on high alloy steels.A cellular automata finite element(CAFE) model to predict solidification microstructure involving heat, flow and solute diffusion with multicomponent, multifield and multilength was built up in the present study. The influence of different physical field on solidification microstructure was demonstrated after the validation by the macroscopic examination results of Fe-22Mn-0.7C TWIP steel involving shrinkage, porosity, macrosegregation and grain morphology. The grain features of columnar to equiaxed transition(CET) was described based on the numerical model. A new mathematical method to quantificationally analyzed CET process and precisely determined the CET position was proposed. The relationship between the solidification microstructure and composition of TWIP steels was illustrated, and the design strategy for high alloy TWIP steel according to solidification quality was finally presented.Three steels in Fe-Mn-C system, with the compostions of Fe-0.7Mn-0.1C, Fe-22Mn-0.1C and Fe-22Mn-0.7C, were produced with a vacuum mid-frequency induction furnace, the hot tensile behavior of which was experimentally studied by Gleeble-1500thermomechanical simulator. The deformation resistances of different steels were compared according to the uniaxial static tensile results. The hot ductility between750and1300℃was described by the reduction of area(R.A). The most important factors influencing the high temperature tensile properties under as cast condition of Fe-Mn-C TWIP steel were revealed based on the analysis of composition, phase structure, SFE and grain-fracture morphology. The characteristics of R.A in the TWIP steels with different carbon content were presented schematically, and the related mechanism was also experimentally investigated. The feasibility of casting Fe-Mn-C TWIP steel by conventional concast equipment was theoretically discussed with the consideration of hot ductility improvement by increasing strain rate and reducing grain size. The probable quality defects of Fe-Mn-C TWIP steel by different casting method were consequently predicted.The stress-strain behavior and work hardening properties of annealing TWIP steels were analyzed by the uniaxial static tensile test, and the twinning and dynamic strain aging induced strengthening-toughening behavior was also studied. The mathematical relationship between Portevin-Le Chatelier band velocity and period as a function of engineering strain was regressed. The grain morphology and phase structure during the preparing and tensile-testing process were investigated by OM, SEM and XRD, based on which the quality control strategy for the optimization of rolling and annealing technology by targeted grain size was proposed. Two excellent TWIP steels, with the tensile strength1030MPa and900MPa, total elongation of60%and90%respectively, were accordingly developed, together with the available processing routes and proper grain size. The strengthening-toughening mechanism of Fe-Mn-C TWIP steel was finally revealed upon the whole research. |
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