| Summary: | The expansion curves of the continuous cooling transformation of undercooled austenite of SXQ500/550DZ35 hydropower steel at different heating temperatures and cooling rates were measured by use of a DIL805A dilatometer. Combined with metallography and Vickers hardness measurement, the continuous cooling transformation diagrams (CCT) of the studied steel under two different states were determined. The results show that in the first group of tests, after the hot-rolled specimens were austenitized at 920 °C, when the cooling rate was below 1 °C·s<sup>−1</sup>, the microstructure was composed of ferrite (F), pearlite (P) and bainite (B). With the cooling rates between 1 °C·s<sup>−1</sup> and 5 °C·s<sup>−1</sup>, the microstructure was mainly bainite, and martensite (M) formed as the cooling rate reached 5 °C·s<sup>−1</sup>. When the cooling rate was up to 10 °C·s<sup>−1</sup>, the microstructure was completely martensite and the hardness value increased significantly. In the second group of tests, after the hot-rolled specimens were quenched at 920 °C and then heated at an intercritical temperature of 830 °C, in comparison with the first group of tests, and except for additional undissolved ferrites in each cooling rate range, the other microstructure types were basically the same. Due to the existence of undissolved ferrite, the microstructures of the specimens heated at intercritical temperatures were much finer, and the toughness values at low temperatures were better.
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