抄録
The fatigue process preceding the macro crack initiation in cold-rolled low-carbon steel specimens was investigated by the back reflection X-ray microbeam technique. The following results were obtained.
(1) Formation and development of substructure were characteristic of the crystal deformation in fatigue process of cold-rolled specimens, having no relation to reduction percent of cold-rolling, similar to the case of annealed specimens. Therefore, it can be said that fatigue progress correlates with the degree of the formation and development of substructure, independent of initial degree of cold-rolling.
(2) The mechanism of the development or formation of substructure in cold-rolled specimens, however, was different from that in annealed specimens in some respects. In the former case, the tendency that the dislocations introduced by cold-rolling moved to decrease the dislocation density within one subgrain and to make narrow the extended subgrain boundary became more evident as reduction percent increased.
(3) Subgrain size tended to be smaller and total misorientation within one grain increased during the fatigue process, although those changes were not outstanding.
(4) The micro lattice strain range within one grain (strain of 2nd kind), and that within one subgrain (strain of 3rd kind) showed a rapid decrease during the early stage in fatigue process. Their diminution became gradual in the next stage. The trend of the change in the micro lattice strain range corresponds to the change in half-value breadth during the fatigue, similar to the case of annealed specimens.
(5) The dislocation density within one subgrain near the macro crack initiation was small, and equal to 2∼3×108lines/cm2, regardless of their reduction percent. The amount of increased dislocation density at subgrain boundary during the fatigue process of cold-rolled specimens was not widely different from that of annealed specimens.
(6) The well-developed substructure is likely to play an important role in the macro crack initiation of cold-rolled specimens, as in the case of annealed ones.
(7) During the fatigue process of cold-rolled specimens the degree of crystal deformation differred from grain to grain because of the difference in orientation factor, similar to the case of annealed ones.
