Abstract: Temperature-expansion curves of the XGTC300L enamelled steel at different cooling rates were obtained by a Gleeble-3500 thermal simulation test machine. The dynamic CCT curve of the steel was drawn by studying the microstructure characteristics and hardness variation of the specimens at different cooling rates. The results show that the matrix of the XGTC300L enamelled steel after hot deformation consists of ferrite when cooled within the range of 0.2-50 ℃/s, and the ferrite grain size decreases with the increase of cooling rates. At the cooling rate of 0.2 ℃/s, the steel exhibits the lowest hardness value, about 96.6 HV0.1. In the range of 0.5-5 ℃/s, the hardness value changes little, about 110.0 HV0.1. In the range of 10-30 ℃/s, the hardness value is about 120.0 HV0.1. At the cooling rate of 50 ℃/s, the steel reaches its highest hardness value, about 137.9 HV0.1. At the fast-cooling rate of 30 ℃/s, boron-containing carbides are found in the ferrite matrix and grain boundary, while only strip cementite precipitates at the grain boundary at the slow cooling rate of 0.2 ℃/s. In the range of 0.2-50 ℃/s, with the increase of cooling rate, the start temperature of transformation is decreased from 850 ℃ to 810 ℃, the end temperature of transformation is decreased from 840 ℃ to 750 ℃, and the transformation time decreases gradually. In the dynamic CCT curve, only the ferrite phase region is found, and the phase region of bainite or martensite is not found. Therefore, the controlled cooling process after rolling of the XGTC300L enamelled steel should adopt a higher cooling rate of more than 30 ℃/s to obtain smaller ferrite grains and a certain number of boron-containing precipitates, so as to improve its comprehensive performance.

Key words: XGTC300L enamelled steel, thermal simulation, dynamic CCT curves, precipitates