Nb添加極低炭素鋼板における冷延過程でのNbCの析出状態変化

The change in precipitates during cold rolling of Nb-added interstitial-free (IF) steel was studied by an electrochemical analysis and 3D atom probe (3DAP) measurement. The amount of dissolved Nb and C increased with the rolling reduction ratio. In the 3DAP analysis, the size distribution of NbC was changed by cold rolling, as the extremely coarse precipitates and fine precipitates observed in the sheets before cold-rolling decreased markedly. The C/Nb atomic ratio in NbC near the precipitate/matrix interface was also lowered by cold rolling, which corresponded to a change to a high-energy state, as indicated by thermodynamic calculation. A molecular dynamics calculation suggested that the crystal structure of the precipitate surface was randomized by shear deformation and dislocations increased in the matrix steel around the precipitates. This may be due to the interaction between carbon and dislocations, as reported in the dissolution of cementite under heavy deformation. It is inferred that NbC is not completely dissolved in the matrix phase by cold rolling, but rather that the accumulated energy and dislocation propagation due to cold deformation cause structural collapse near the precipitate surface and heavier deformation of finer precipitates, which is evaluated as the resultant solid solution content by electrochemical analysis.