Abstract: Based on the analysis of distribution of coarse grain defects, the precipitation of FeTiP in specimen with different degree of coarse grain defects, and the precipitate dissolution temperature of FeTiP in the Ti-IF steel and the grain growth behavior, the causes of coarse grain defects were analyzed, and the method to solve the coarse grain defects was suggested. The results show that annealing at 780 ℃ or slightly higher the temperature becomes the grain coarsening critical temperature, and the dissolution of FeTiP precipitate phase significantly change the distribution state of precipitate phase, which becomes the root cause of abnormal grain growth and coarse grain defects. The local overheating of the outer ring in the steel coil due to the annealing temperature exceeding 780 ℃ causes the abnormal growth of grains in the outer ring of the steel coil and leads to the occurrence of coarse grain defects. The problem of coarse grain defects can be effectively solved by adding a heat insulation platform at a certain temperature during the high temperature rising stage of the batch annealing process.

Key words: Ti-IF steel, coarse grain defect, FeTiP, grain coarsening temperature, batch annealing