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The combined effect of phosphorus grain boundary segregation and hardness on the ductileto-brittle transition was examined for a P-doped 2.25Crl Mo steel by using Auger electron spectroscopy in conjunction with hardness measurements, Charpy impact tests and scanning electron microscopy. With prolonging time at 540 ℃ after water quenching from 980 ℃, the segregation of phosphorus increases and the hardness decreases. The DBTT (FATT) increases with increasing phosphorus segregation and decreases with decreasing hardness. The effect of phosphorus segregation is dominant until 100 h aging and after that the hardness effect becomes dominant. This effect makes the DBTT (FATT) decrease with further prolonging ageing time although the segregation of phosphorus still increases strongly.