Abstract: Thermal simulation test of European standard R350HT rail steel with different cooling rates was carried out on the Gleeble-3500 thermal simulation testing machine. The microstructure was observed, the hardness was measured, and the continuous cooling transformation (CCT) curves of the tested steel was drawn. The results show that when the cooling rate is between 0.5-2.5 ℃/s, the microstructure consists of pearlite and a small amount of proeutectoid ferrite. When the cooling rate is 3 ℃/s, martensite appears in the structure. Since the pearlite rail steel should not contain martensite, the cooling rate of the tested steel should be less than 3 ℃/s. At the same time, with the increase of cooling rate until 10 ℃/s, the pearlite transformation start temperature decreases, which is because the residence time in the high temperature region is shortened with the increase of the cooling rate, and the pearlite transformation is too late to occur, and the pearlite transformation requires a large degree of undercooling. In addition, with the cooling rate increasing to 20 ℃/s, the microstructure is basically composed of martensite. When the cooling rate is greater than 20 ℃/s, the microstructure only contains martensite. Hence, the critical cooling rate of martensite transformation is 20 ℃/s.

Key words: R350HT rail steel, continuous cooling transformation curves, thermal simulation, microstructure