| Generally, ultra-high strength martensitic steel exhibited a poor toughness. Grain refinement is the only way which can be used to improve both strength and toughness. Many works about the manufacturing method of ultra-fine grained martensitic steels have been done by materials scientists in an attempt to further improve the performance of the ultra-high strength martensitic steel. And a lot of grain refinement methods have been developed, such as cyclic heat treatment, rapid heating method, deformation heat treatment and so on. However, these methods have many shortcomings, such as complicated process, high cost and so on, and thus are difficult to be applied in the current industrial process. Therefore, it is necessary to develop a novel, simple and available method for grain refinement of ultra-high strength martensitic steel.In the present paper, the complex microalloying design of Nb-Ti and V-Ti were employed. A new process for producing ultra-fine grained martensitic steels with excellent mechanical properties was proposed as follow:obtaining ultra-fine grained austenite by thermomechanical control process, reheated-quenching process and controlling the precipitation behavior of the second phases in austenite and ferrite. Based on the solubility products of secondary phases, an analytical model for describing the precipitation thermodynamic of quadruple-element precipitation was built, which can provide a theoretical basis for the determination of microalloying design and soaking process. The microstructure of direct-quenched steels was characterized by means of scanning electron microscopy (SEM), transmission electron microscopy (TEM) and electron backscatter diffraction (EBSD) and so on. The microstructure and the strengthening and toughening mechanism of direct-quenched steel were analyzed. The reheating process was carried out in an attempt to further refine the austenite grain size. The effects of controlled rolling parameters, matrix structure, reheating temperature and holding time on the austenite grain refinement were investigated. The mechanism of austenite grain refinement during reheating process in the direct-quenched steel was discussed. The microstructure and mechanical properties of ultra-fine grained martensitic steel was characterized.The thermodynamic and kinetic results of complex carbonitride precipitation in asutenite indicated that, compared to the (V, Ti)(C, N) precipitation, the (Nb, Ti)(C, N) precipitation was easier to precipitate during the soaking process and rolling process. For the Nb-Ti complex microalloyed design, combining the thermomechanical control process and direct-quenching technology, through controlling the precipitation behavior of (Nb, Ti)(C, N) in austenite, ultra-fine grained direct-quenched steel with the prior austenite grain thickness smaller than5μm was obtained. The soaking process carried out at1180℃for1h and the rough and finish rolling process were carried out at1020℃and780℃, respectively. After the two-stage rolling, the steel was directly quenched by water. The micro structure of the direct-quenched Nb-Ti microalloyed steels was composed of lath martensite, equiaxial ferrite, deformed ferrite and a very small amount of retained austenite. Compared to the direct-quenched V-Ti microalloyed steel, the amount of ferrite was smaller and the density of boundary was higher in the direct-quenched Nb-Ti microalloyed steel, in which the better balance between strength and toughness was obtained. The transverse tensile strength and yield strength were1750MPa and1300MPa, respectively and the longitudinal Charpy impact energy at-40℃was37J. Moreover, the effects of tempering temperature on the microstructure and mechanical properties of direct-quenched steel were also investigated. The results indicated that the mechanical properties of direct-quenched steels could be further improved by combining the direct quenching technology with low temperature tempering process. After tempering at200℃for1h, the direct quenched Nb-Ti microalloyed steel obtained the best mechanical properties. The transverse tensile strength and yield strength were1730MPa and1400MPa, respectively and the longitudinal Charpy impact energy at-40℃was increased to43J, which exceeded the level of toughness in the conventional martensitic steels with the same strength grade.According to the results of physical and chemical phase analysis for the direct-quenched steel, the reheating process was carried out in the V-Ti microalloyed steel and ultra-fine grained austenite was obtained. The ultra-fine austenite grains with the size of2μm was obtained in the V-Ti microalloyed steel as reheated at880 ℃for1s. The experimental results indicated that increasing flattening degree of austenite, using martensite as matrix structure, decreasing reheating temperature and holding time could refine austenite grain to some extent. The globular austenite nuclei intensely formed at the high angle boundaries in V-Ti microalloyed steel during the reheating process, such as prior austenite grain boundary, ferrite grain boundary, ferrite/martensite boundary and so on. Compared to the conventional martensitic steel, the amount of austenite nucleation site was more in the direct quenched V-Ti microalloyed steel during the reheating process. Moreover, the (V, Ti)C precipitates would spontaneous nucleate in the matrix during the reheating process. And the precipitation behavior of (V, Ti)C was mainly controlled by atomic diffusion. With the reheated temperature increased, the PTT curve of (V, Ti)C precipitates in ferrite monotonically trends to the short time direction. However, when the reheating temperature was above AC3, the (V, Ti)C particles will be redissolved and coarsened. However, the undissolved particles in the matrix can pin the austenite grains and inhibit the austenite grain growth effectively.Martensite transformation from austenite with different grain sizes obeyed K-S orientation relationship. The martensite transformed from ultra-fine austenite had a multi-scale structure consistent with conventional martensite. However, with the decreasing of austenite grain size, the numbers of martensite variant in a prior austenite grain decreases and the density of high angle boundary, especially the density of prior austenite grain boundary also significantly increases. Moreover, the packet size and block width were decreased with the austenite grain size. Compared to the conventional martensitic steel and the direct quenched steel, the best balance between strengthen and toughness was achieved in the ultra-fine grained martensite V-Ti microalloyed steel due to the grain refinement. The transverse tensile strength and yield strength were1670MPa and1460MPa, respectively and the longitudinal Charpy impact energy at-40℃was57J. |
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