Effect of carbon and phosphorus contents on the graphitization of cementite in high carbon sheet steels

In the steel with lower Mn and lower P, carbon occasionaly precipitates as graphite instead of cementite under certain annealing conditions. Then, the elongation became to be independent of amount of C contents because of the decrease in cementite and increase in ferrite grain diameter. In this study, in order to produce graphitized sheet steel with good ductility in short annealing period, the following investigation was carried out. 0.15%Si-0.15%Mn-0.004%S cold rolled sheet steels, with C ranging from 0.05 to 0.65%, with P ranging from 0.001 to 0.05% were provided. These steels were annealed at the temperature ranging from 550 to 700 degrees C for the time ranging from 0.25 to 30h. The microscopic observation was conducted at every annealing condition. Under the condition with higher annealing temperature and longer annealing period, a remarkable stimulation of graphitization was brought in the lower P steel with various C, because of the acceleration of C diffusion. Particularly, in the case of steels with low C, the longer annealing period and the extremely lower P contents is necessary for graphitization. This result was thought to be due to the segregation of P, inhibiting C diffusion, on the graphite nucleation site.