| As the representative of advanced high strength steel, high strength TRIP steels have the unique superiority-transformation induced plasticity. High strength TRIP steels meet these requirements such as the weight saving, consumption and emission reduction, at the same time, the safety of car can be improved, which is a promising material. In recent years, the large scale production of high strength TRIP steels have become a fact with the effective application of microalloying element niobium, vanadium and titanium.The effect of individual elements has its limitations, the interaction of carbon,nitrogen and microalloying elements are often more effective than single action of individual elements, and the application of composite microalloyinghigh strength TRIP steel will be wider.People have researched composite strengthening of niobium, vanadium and titanium, niobium in recent years, and have developed a variety of new composite strengthing steels, but the reports for titanium, vanadium compsite strengthening are rare. In this paper, we added a certain amount of vanadium and titanium on the basis of traditional TRIP steel. The major purpose of this work was to study the effect of hot-rolled technology and heat-treatment technology on the microstructures and mechanical propirties of the experimental steels, and to discuss the effect of microalloying elements and heat-treatment technology on retained austenite content and its mechanical stability. This thesis was supported by National Key Natrual Science Foundation. The contents of the present work include the following main aparts.1. The microstructures of hot-rolled steel are composed of ferrite, pearlite and bainite and ferrite is the matrix. Differtnt cooling methods makes different microstructures. Finer microstructures and smaller pearlite interlamellar spacing were received by water-cooling technology. Big grain size was received by slow-cooing and air-cooling technology. After recrystallization refinement and two phase refinement process, the grains were refined. The effect of heat treatment on grain refinement has its limitations, which makes the experimental steel has good inheritance from the perspective of grain size and tissue types. The mechanical properties of experimental steel are different after heat treatment. Beause water-cooling received finer microstructures, and the distribution of microstructures is relatively uniform, the comprehensive mechanical properties are good after heat treatment.2. The effect of heat treatment process on the microstructures and mechanical properties of the high strength TRIP steel was investigated. The results showed that when the holding time in critical annealing is0.5min, and the isothermal bainitic transformation holding temperature at440℃, the maximum value of tensile strength is925MPa, the total elongation is25.7%, and the strength-ductility reached23773MPa%. Before tensile test,the volume fraction of retained austenite is19.31%, the volume fraction of carbon content of retained austenite is0.8390. After tensile test, the volume fraction of retained austenite is4.76%, the percentage of austenite transformation into martenite is75.35%, which has appropriate mechanical stability. It can continually transform to martensite with high instantaneous n value during the whole deformation process. Above all, when the critical annealing temperature is830℃, holding time is0.5min, isothermal temperature in bainite region is440℃, holding time is3min, the high strength TRIP steel sheet exhibited the best combination.3. The distribution of microalloying elements were analyzed. The results showed that the segregation of Ti, C, N appeared. The Precipitates of Ti(C,N) were mainly distributed on the ferrite matrix. The content of C in matrix on TRIP dual and Q-P-T steel is generally higher than TRIP steel, which is mainly due to the different organization. The higher content of C is retained austenite region, different heat treatment showed different porphology of retained austenite. With the increase of annealing temperature, the Ti-V composite particle size and uneven degree decreases. The distribution of V in different heat treatment has no obvious change, which has good effect of precipitation strengthing.4. TRIP, TRIP dual and Q-P-T processes were studied. The results showed that the matrix of TRIP steel was mainly ferrite; while the TRIP dual steel was mainly bainite and the distribution of microstructures was uniform; the Q-P-T steel was composed with lath martensite and retained austenite. After TRIP dual process, the tensile strength, total elongation and strength-ductility product can reach955MPa,32.3%and30847MPa%. Before tensile test, the volume fraction of retained austenite is20.89%, the volume fraction of carbon content of retained austenite is0.8286. After tensile test, the volume fraction of retained austenite is3.66%, the percentage of austenite transformation into martenite reached82.48%. The retained austenite can continually transform to martensite with high instantaneous n value during the whole deformation process. Thus, TRIP dual steel shows excellent mechanical properties. |
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