ULTRAFINED MICROSTRUCTURE OF HYPEREUTECTOID STEEL BY WARM DEFORMATION OF MARTENSITE
马氏体温变形超细化过共析钢

The continuous network of brittle proeutectoid carbide will formed along the grain boundaries when cooled slowly from austenite in hypereutectoid steels. Steels with such high--carbon content have been neglected in industry because of they are inherently brittle. By properly processed, such as hot and warm working (HWW), isothermal warm working (IWW), divorced eutectoid transformation (DET) and divorced eutectoid transformation with associated deformation (DETWAD), the steels will exhibit ultrafine microduplex structure with fine spheroidized cementite (θ) particles dispersed in fine--grained and equiaxed ferrite (α) matrix (grain size is less than 1 μm). This microduplex structure shows superplasticity at elevated temperature and exhibits better mechanical properties at room temperature. However, these processes are relatively complicated and should break the proeutectoid cementiets firstly. By warm deformation of martensite, a simple process and the ultrafined microstructure can be obtained easily. In the present work, the effects of Al on the microstructural ultra--refinement and mechanical properties of hypereutectoid steel during warm deformation of martensite as well as tempering of martensite were investigated by uniaxial hot compression simulation experiment. The results indicate that the warm deformation accelerates the martensite decomposition compared to tempering, leading to the formation of ultrafine (α+θ) microduplex structures. The microstructure evolution of martensite during warm deformation involves the precipitation and coarsen of cementite particles, and the dynamic recovery and dynamic recrystallization of ferrite, while tempering of martensite, the precipitation and coarsen of cementite particles, static recovery and grain growth of ferrite occurred, but no recrystallization of ferrite occurred. With the addition of Al, the decomposition of martensite is impeded during warm deformation and tempering, the microduplex structure is refined, and its strength is improved, while the elongation is not decreased.