For preparing large-scale nano-grained and ultrafine-grained steel sheets by warm rolling and annealing, the effects of deforming temperature on both the flow stress and the microstructure evolution of 09MnNiD steel with lath martensitic microstructure were studied by warm-compression test and transmission electron microscopy （TEM） observation. Thereafter, the steel with the lath martensitic structure was multi-pass warm-rolled and then annealed. TEM results indicate that nano-grained and ultrafine-grained steel sheets are formed by warm rolling at 400℃ and annealing at 400-600℃. In comparison with the as-warm-rolled specimen, the tensile strength at room temperature changes a little when the rolled samples are annealed below 450℃, and the tensile strength is greatly lowered as the annealing temperature increases to above 550℃.
A new plastic deformation process for gray cast iron named cylinder covered compression （CCC） was developed. By CCC process, gray cast iron （GCI） specimens, which are embedded in steel cylinders, were hot compressed up to 80% reduction in height without the cracking problem. It was clearly observed that the uniform distribution of directional graphite flakes appeared after more than 45 % reduction hot compression. The strength, ductility, and microhardness of GCI after 80% reduction deformation were significantly enhanced： the tensile strength varied from 117 MPa to 249 MPa, while the total elongation varied from 0 to 5.2%, and the microhardness varied from 153 HV to 217 HV. It was shown that the tensile fracture surface presented ductility characters after more than 45% reduction hot deformation.
The warm compression tests were performed on the eutectoid steel to investigate the evolution of cementite morphology. Several processing parameters, such as temperature, strain rate and reduction, were changed to analyze the effect of each parameter on spheroidization of cementite. The results showed that the warm compression promoted the fragmentize and the spheroidization of lamellar cementites. When the specimen was compressed with reduction of 50% at 700 ℃ and in the strain rate of 0.01 s-1, the excellent spheroidized cementite was obtained. The mechanism of fragmentation and spheroidization of lamellar cementites during compression was discussed by using transmission electron microscope. The formation of spheroidized cementite was related to the time of compression process. The fragmentize of lamellar cementites was due to the extension of sub-grain boundary in the cementite. The spheroidization of cementite depended on the diffusion of carbon atoms at the tip of bended and breakup cementite.
WU Tao, WANG Ming-zhi, GAO Yu-wei, LI Xiao-pu, ZHAO Yu-cheng, ZOU Qin (State Key Laboratory of Metastable Materials Science and Technology, Yanshan University, Qinhuangdao 066004, Hebei, China)
