Microstructure Evolution And Transformation Induced Plasticity Behavior Of Medium Manganese Steel

Transformation induced plasticity (TRIP) steel which has excellent comprehen-sive mechanical properties is one kind of advanced high strength steels developed on the base of transformation induced plasticity effect. The conventional TRIP steel was a multiphase steel which contained ferrite, bainite and amount of retained austenite (5%-15%). As the content of manganese increased, dual-phase microstructure composed of ferrite and retained austenite (20-30%) would be obtained. In order to obtain high strength and high ductility level, not only the proportion of each phase and the amount of retained austenite but also the grain size, morphology and distribution of each phase should be controlled. Grain refinement is a hot issue of microstructure controllment of the transformation induced plasticity steel. The transformation induced plasticity behavior of the steel shows unique characteristics due to increased retained austenite and grain refinement. The ultra-fine-grain medium manganese steel had been prepared by means of the integrated grain refinement technology in this paper. Transformation induced plasticity behavior had been analyzed through quantitative tensile tests. The calculation model of deformation process also had been established to analyze transformation by the finite element simulation software ABAQUS. The main results are as following.The medium manganese steel alloy composition had been employed. The Ac1and AC3temperatures and the intercritical annealing temperature could be reduced when the content of manganese was5%-7%. A pre-quenching treatment before conven-tional annealing process had been adopted, which is beneficial to obtain ultrafine grains, and get more retained austenite. The ferrite and retained austenite grain sizes of10Mn7(0.1C-7Mn-0.04Nb) steel after annealing was found less than1μm and0.5μm respectively. The volume fraction of retained austenitic was up to40.29%. This process with pre-quenching treatment could also improve mechanical properties as well as shorten the optimal annealing time.10Mn7steel achieved optimal mechanical properties after annealing at625℃for4h with tensile strength of1177MPa, total elongation of30.92%and UTS*TE of36.39GPa·%.The microstructure evolution during annealing process had been analyzed. The annealing process with pre-quenching treatment could significantly eliminate the manganese element segregation compared with conventional annealing process and obtain two kinds of retained austenite梐cicular austenite and blocky austenite. The microstructure evolution during annealing was analyzed. Martensitic structure and fine acicular retained austenite could be obtained after the first pre-quenching stage. During re-heating to intercritical annealing, the acicular retained austenite grew on the basis of fine retained austenite that preserved in the pre-quenching stage, while the blocky austenite nucleated and grew at carbon enriched area. Considerable amounts of the two kinds of retained austenite preserved after cooling to room temperature. The acicular austenite that originated from the same prior austenite possessed the same crystal orientation and keep Kurdjumov-Sachs phase boundaries orientation relationship with the a-phase matrix. While no Kurdjumov-Sachs relationships exist between the blocky austenite which nucleated and grew at carbon edriched area and the a-phase matrix. Because the difference of the nucleation and growth mechanism of the two kinds retained austenite, the content of manganese element in the acicular austenite was higher than that in the blocky austenite.The precipitation and distribution of the carbide in the investigated steels after hot rollingduring heating process had been analyzed. It was found that, carbide was finest and most dispersed when insulation at500°C. The dynamic recrystallization softening effect could offset the work hardening effect when deformation at500℃. As a result, the decision of optimal warm rolling at500℃had been carried out. The tensile strength of15Mn7(0.15C-7Mn-0.04Nb) steel up to1135MPa compared with previous1021MPa, total elongation up to35.30%compared with31.16%and UTS*TE up to40.06GPa·%compared with31.81GPa·%was obtained after optimal warm rolling. The proportion of acicular austenite increased and retained austenite volume fraction was up to40.29%compared with previous39.86%.The plastic deformation of medium manganese steel had been analyzed. It was found that, the stress-strain curves of10Mn7steel with different annealing temperatures could be divided into two types. After annealing at lower temperatures of580℃and600℃, the deformation mechanism is mainly Liiders strain due to the weak work hardening behavior caused by fine grain size and little retained austenite. As the annealing temperatures increasing to625℃and650℃, the instantaneous n value was large because of the notable work hardening behavior weak due to the large amount of retained austenite and fresh martensite.The TRIP effect had been studied by means of quantitative tensile tests. It was found that, about20%of retained austenite had transformed into martensite induced by stress in the beginning of deformation. Stress level and the amount of retained austenite remained unchanged during Luders deformation stage when strain was between0.03and0.12. The volume fraction of retained austenite decreased during the work hardening stage when strain was more than0.12. The retained austenite transformed into martensite gradually during plastic deformation and produced TRIP effect even at large strain compared with conventional annealing process. The microstructure of steels with different strain had been observed by TEM and EBSD. It was found that, the retained austenite near small angle grain boundary possessed lower stability than that near large angle grain boundary, and transformed into martensite earlier. Acicular retained austenite had higher stability than blocky retained austenite. The stability of central area in blocky retained austenite increased due to martensitic transformation.The deformation model of medium manganese had been established by the finite element simulation software ABAQUS. It was found that, within the retained austenite, martensitic phase transformation tended to occur fist near the narrow region under uniaxial tension. The stress concentration was discovered around the fresh martensite. Transformation and stress concentration primarily occurred along the tension direction as the load increased. Strain concentration occurred mainly in ferrite grains and kept45°to the tension direction. As the deformation increased, many small localized regions with high strain began to appear in the transverse direction to the tensile loading, where would be the nucleation area of micro crack. The stability reduction of retained austenite would cause the total elongation decreased while the yield strength and tensile strength maintained. Either increased martensite strength or decreased ferrite strength would lead to the decreasing of plasticity obviously. Martensitic phase transformation occurred earlier and faster under equi-biaxial tension. The stress concentration was discovered around the fresh martensite. Strain concentration occurred mainly in the ferrite grains or the phase boundary. The failure mode for equi-biaxial load showed a combination of vertical and horizontal failure bands.