Effect of in situ observation of cooling rates on acicular ferrite nucleation

Abstract

Abstract The effect of cooling rates on acicular ferrite (AF) nucleation was observed in situ by laser scanning confocal microscopy. The precipitation characteristics of TiN–MnS inclusions, the austenite grain size, and the content and morphologies of AF were analyzed and compared under different cooling rates. The results indicated that with the increase of the cooling rate, the size of TiN–MnS inclusions precipitated in the test steel gradually decreased while the inclusion number increased. When the cooling rate increased from 0.2 to 10°C·s−1, the average inclusion size decreased from 2.7 to 1.3 µm. The austenite grain size decreased gradually with the increase of the cooling rate, and the larger the cooling rate was, the more uniform the grain size distribution. The AF proportion increased first and then decreased, it reached the maximum when the cooling rate was 5°C·s−1, which accounted for 82.6%. Meanwhile, the dendrite size decreased gradually, the average length decreased from 30 to 5 µm, and the average width decreased from 2.8 to 0.7 µm when the cooling rate increased from 0.2 to 10°C·s−1.