| With the continuous development of modern aviation manufacturing technology, more and more overall die forgings with large size and complex shape are applied for load-bearing structures of aircraft to meeting the extreme mechanical properties. For this goal, high-alloy ultra-high strength steel, which has strength of1930Mpa and grain size of22.5μm (level8), has been developed to manufacture aircraft landing gear. However, the high requirements of grain size and its homogeneity bring big challenge to the overall die forging process, especially for large-sized landing gear. Because, material goes through heating, deformation and heat treatment during the whole forming process, and its microstructure evolution mechanism is very complicated. Meanwhile, China is lack of basic research and technical reserve in the field of overall die forging. Now, grain dissatisfaction and instability of deformation have become the main problem for obtaining high quality aircraft forgings. In this paper, a series of research work were done on newly developed ultra-high strength steel23Co13Ni11Cr3Mo for solving the problem listed above. The evolution of microstructure and mechanical properties during the whole hot forming process, including heating, deformation, cooling and heat treatment, were studied. And, the corresponding models were developed. Then, microstructure evolution of landing gear during the overall die forging process was investigated. Based on these research works, a new forging process on800MN hydraulic press with high stability and efficiency was proposed, and deformation parameters for high quality forgings were acquired. The main conclusions are listed as following:(1) Isothermal compression tests of23Co13Ni11Cr3Mo steel were carried out. Deformation behaviors during high temperature and microstructure evolution rules were studied. Dynamic recrystallization models, including recrystallized fraction and grain size, were developed. A new approach for establishing the model of flow stress was developed, wihch is successive approximation method. Consequently, the deformation active energy and critical strain conditions of dynamic recystallization were obtained. Then, hot processing map for 23Co13Ni11Cr3Mo steel was developed on the basis of experimental flow stress and deformation instability rules. Hot deformation zone with high stability and efficiency was recognized through processing map. The results show that in this zone the degree of dynamic recrystallization is increased by increasing deformation degree, and small even grain is achieved.(2) The overall die forging process of large-size products was characterized for complex microstructure evolution mechanism and varied physical fields. For this reason, cellular automaton(CA) model based on dislocation density was developed to describe grain evolution. During this process, the identification method on the basis of optimization theory was proposed for acquiring parameters of microstructure evolution. The relationships between macroscopic deformation parameters and hardening exponent, two-phase particle pinning effect were developed. Through these measures, the microstructure evolution through macroscopic and mesoscopic was realized. According to the actual forging processing characteristics, a series of CA improving algorithm were developed. Moreover, a software for visualized microstructure simulation was developed coupled with FEM platform, such as DEFORM and QFORM. The experimental results show the proposed CA model and developed software can predict grain shape, distribution and its evolution rule during multiple dynamic recrystallization very well.(3) For large-sized forgings, its grain is easily to becoming coarse during static process of transferring, waiting, cooling and heat treatment. Therefore, microstructure evolution of ultra-high strength steel with high temperature during static processes was studied by experimental methods. Models of static behaviors, such as grain growth, meta-dymamic and static recrystallization were established, by which microstructure evolution during the whole deformation process can be expressed quantitatively. The results show that grain refinement is determined by the final forging process and its following cooling process. Grain size of static recrystallization is larger than that of meta-dynamic and dynamic recrystallization, which is the main reason for grain dissatisfaction of landing gear forging.(4) Experimental forging of ultra-high strength steel was carried out on the industrial conditions. Influences of deformation parameters on grain evolution and mechanical properties were investigated. Consequently, deformation conditions which could fulfill the demands of grain and mechanical properties were reached, and grain evolution during the whole forming process could be controlled. The results show that a proportional relationship exists between forging grain size and heat treatment grain size, forging grain size should reach level8for the final grain size of level8. Deformation paramaters has little effects on mechanical properties. Under a deformation temperature of980-1140℃, strain of12-30%, the variation of strength is3.5%, and the variation of KIC is8.7%.(5) Landing gear under the existing overall die forging process was dissectted and analyzed. The results of microstructure and mechnical properties show KIC can be improved about20.5%by applying overall die forging process. However, grain size can only reach level5-6.5. Microstructure simulation and experimental research during the whole hot forming process of landing gear show that the static recrystallization and grain growth caused by insufficient and discontinous deformation are the main reason for coarse grain and instable deforamtion. Lower temperature, higher strain rate and larger defromation degree lead to smaller grain size. So, a new forging process on800MN hydraulic press with high stability and efficiency was proposed. With the deformation degree of the final forging process increasing to33.7%, static recrystallization can be restrained, and grain evolution mechanism changes to meta-dynamic recrystallization, grain is refined greatly. |
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