| Fe-Mn-Al-C TWIP steel is a kind of steels which can be used in automobile industry. The steel meets the requirements for weight saving, fuel efficiency as well as the safety for its high energy absorption capacity, which is continuing to be a major concern on automobile manufacturing. And because it has the advantages of high strength and elongation, well low temperature toughness, non-magnetic and low alloy density etc, these make Fe-Mn-Al-C steel have potential to be applied in high safety train industry. Consequently, Fe-Mn-Al-C steel causes a great interest for applications in automobile and iron and steel industry. Furthermore, Fe-Mn-Al-C steel also can consider to be used in other fields, for example, partly replaces relatively expensive chromium-nickel austenitic stainless steel in the non-corrosive environment, applies to generator rotor with high strength and non-ferromagnetic in order to reduce its weight, and be used for liquid gas storage and transportation in the area of refrigeration technology. The application prospect will be wider with production technology improvement and development. However, the research of Fe-Mn-Al-C TWIP steel has just started, and there are many technical problems to be solved. The microstructure, mechanical property and deformation mechanism of strip cast should be thoroughly investigated.The deformation mechanism in tensile testing nedded to be deeply investigated. And the oxidation behavior of Fe-Mn-Al-C TWIP steel has not studied.In the thesis, deformation behavior and microstructure evaluation in hot compression, microstructure and properties evolution during hot rolling, effect of solution processing on microstructure and properties of cold rolling sheet, and deformation mechanism and influences factors in tensile deformation of Fe-Mn-Al-C steel were investigated by using OM, SEM, EBSD and TEM. Moreover, the oxidation residtance behavior at high temperature of TWIP steel was studied. The main original works of this thesis are presented as follows:(1) Flow stress curves of Fe-Mn-Al-C TWIP steel were obtained by high temperature single-pass compression tests. The TWIP steel deformation behavior and microstructure evolution during high temperature compression process were studied with a constitutive equation for TWIP steel being established. The dynamic recrystallization process of TWIP steel was analyzed through equivalent strain. The results show that, strain rate has an obvious effect on the activation energy for dynamic recrystallization of30Mn20A13steel. The internal deformation of the sample is inhomogeneous during compression tests. The maximum value of equivalent strain distributes in the center, and the middle position of the contact surface with indenter is the minimum. The feasibility of dynamic recrystallization determined by equivalent strain was analyzed.(2) The strain hardening behavior and microstructure evolution of Fe-Mn-Al-C TWIP steel during uniaxial tensile deformation were studied. TWIP steel possesses a multi stage strain hardening behavior during tensile deformation. The process can be divided into three stages. In stage I, planar dislocation structure is the main deformation structure, dislocation gliding is the main deformation mechanism, and strain hardening rate decrease and strain hardening exponent is exhibited. Primary deformation twinning occurred in stage Ⅱ, and the decreased rate of strain hardening rate becomes low and strain hardening exponent increases due to the blockage of dislocations’motion by deformation twin boundaries. But the strain hardening rate of30Mn20A13steel appears the minimum for its yield platform. In stage III, the strain hardening exponent increases to the highest value and suspends. The obstacle effect of twin boundaries to dislocation motion and twin-twin interaction has been observed by TEM, and the interactions between primary and secondary twins were found to cause the additional hardening in addition to the obstacle effect on dislocations’motion. This unique hardening behavior of TWIP steel leads to the TWIP effect in tensile deformation.(3) The mechanical properties of Fe-Mn-Al-C TWIP steel and the microstructure evolution with strain rate and temperature were investigated through tensile tests. Deformation behavior with three stages was observed as the change of strain hardening rate with low strain rate. While at high strain rate, there exist only two stages in the deformation behavior with respect to the strain hardening rate and true strain. Strain hardening exponent of this steel increases with increasing true strain. High density deformation twins can be produced during deformation with different strain rates. The stacking fault energy (SFE) of TWIP steel,（？）, at different temperatures was estimated by thermodynamic equations. The dependence of SFE on deformation mechanism was analyzed. It reveals that when-60℃≤t <100℃,14mJ/m2≤F|≤32mJ/m2, the governing deformation mode is deformation twin;when 100℃<t<300℃,32mJ/m2<F<72mJ/m2, the main deformation mechanism is silp and deformation twin; when t≥300℃, F≥72mJ/m2, the slipping is a predominant deformation mode; and when temperature is higher than600℃, dynamic recrystallizaiton occurs, and much A1N precipitate at austenite grain boundary, and these AlN can generate micro-crack.(4) The oxidation behavior of Fe-Mn-Al-C TWIP steel was studied at high temperature. Thermogravimetric analyses were conducted in isothermal conditions from400to1100℃for5h and microstructural and chemical analyses of the oxide films grown were performed by SEM. The oxide films were also analysed by X-ray diffraction. The results deduced that oxidation resistance property of TWIP steel weaked above800℃. A thick scale is formed on the substrate surface, which, regardless the oxidation temperature, is composed by different oxides arranged in the form of successive layers, so that the surface color changes from blue to khaki, and then to black with increasing temperature. The thickness of oxide film becomes thickening gradually and the upper layer seems tighter as increase the temperature. Regarding kinetics, the mathematical analysis of the collected data shows that the change of the mass of the substrate per unit area vs time is described by a parabolic law above800℃. Hence oxidation is impeded as the exposure time increases. |
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