Papers by Author: Y. Fujimura

Abstract: The work hardening behavior by cold rolling was investigated in ultralow carbon and low carbon martensitic steels containing 12%Cr or 18%Ni, and then the effect of carbon on the work hardening behavior was discussed in terms of the change in dislocation density and the microstructure development during deformation. In the ultralow carbon steel, the hardness is almost constant irrespective of the reduction ratio. On the other hand, the low carbon steel exhibits marked work hardening. The dislocation density of these specimens was confirmed to be never increased by cold rolling. It was also found that cold rolling gives no significant influence on the morphology of martensite packet and block structure. TEM images of the cold-rolled steels revealed that the martensite laths in the ultralow carbon steel are partially vanished, while those in the carbon bearing steel are stably remained. These results indicate that the solute carbon retards the movement of dislocations, which results in the high work hardening rate through the formation of fine dislocation substructure within laths.

Abstract: The behavior of work hardening by cold rolling and tensile deformation was investigated in an ultralow carbon and carbon bearing martensitic steels, and then the effect of carbon on the work hardening behavior was discussed in terms of the change in dislocation density and the microstructure development during deformation. In the ultralow carbon 18%Ni steel (20ppmC), the hardness is almost constant irrespective of the reduction ratio. On the other hand, the carbon bearing 18%Ni steel (890ppmC) exhibits marked work hardening. The dislocation density of these specimens was confirmed to be never increased by cold rolling. It was also found that 10% cold rolling gives no significant influence on the morphology of martensite packet and block structure. TEM images of the 10% cold-rolled steels revealed that the martensite laths in the ultralow carbon steel are partially vanished, while those in the carbon bearing steel are stably remained. These results indicate that the solute carbon retards the movement of dislocations, which results in the high work hardening rate through the formation of fine dislocation substructure within laths.

Abstract: Yield strength of highly dislocated metals is known to be directly proportional to the square root of dislocation density (ρ), so called Bailey-Hirsch relationship. In general, the microstructure of heavily cold worked iron is characterized by cellar tangled dislocations. On the other hand, the dislocation substructure of martensite is characterized by randomly distributed dislocations although it has almost same or higher dislocation density in comparison with heavily cold worked iron. In this paper, yielding behavior of ultra low carbon martensite (Fe-18%Ni alloy) was discussed in connection with microstructural change during cold working. Originally, the elastic proportional limit and 0.2% proof stress is low in as-quenched martensite in spite of its high dislocation density. Small amount of cold rolling results in the decrease of dislocation density from 6.8x1015/m-2 to 3.4x1015/m-2 but both the elastic proportional limit and 0.2% proof stress are markedly increased by contraries. 0.2% proof stress of cold-rolled martensite could be plotted on the extended line of the Bailey-Hirsch equation obtained in cold-rolled iron. It was also confirmed that small amount of cold rolling causes a clear microstructural change from randomly distributed dislocations to cellar tangled dislocations. Martensite contains two types of dislocations; statistically stored dislocation (SS-dislocation) and geometrically necessary dislocation (GN-dislocation). In the early deformation stage, SS-dislocations easily disappear through the dislocation interaction and movement to grain boundaries or surface. This process produces a plastic strain and lowers the elastic proportional limit and 0.2% proof stress in the ultra low carbon martensite.