Nucleation Sites of Cementite in Cold-rolled Steel Sheets during Continuous Annealing

Cold-rolled Al-killed steel sheets with and without boron were laboratory annealed continuously. Nucleation sites of cementite within grains were investigated during aging treatments. The following conclusion was obtained.(1) Single α-MnS and single AlN precipitates were dispersed homogeneously before aging for both kinds of specimens. MnS+AlN complex precipitates were observed only for specimens without boron.(2) Single MnS and MnS+AlN complex precipitates were main nucleation sites of cementite. The nucleation site ratio of both types of precipitates was high and increased as the size of precipitates increased.(3) The distribution density of both types of precipitates over 0.05 μm in size were almost the same as that of cementite within grains. The number of pre-existing precipitates with the high nucleation site ratio was enough for the distribution density of nucleation sites. The nucleation site ratio of AlN was low and did not show any size dependence.(4) The mechanism in which AlN did not reduce the nucleation site ratio of MnS+AlN complex precipitate was considered to be that AlN was a very thin plate and it covered only a small area of MnS/α-Fe interface where nucleation of cementites occurred.(5) One of the mechanisms of preference of MnS as nucleation sites of cementite within grains was considered to be the wettability of nuclei at the MnS surface. According to the classical nucleation theory, the activation energy of heterogeneous nucleation of critical nuclei reduced to less than 2% compared with that in the homogeneous case.