The effect of quenching cooling method on the mechanical properties of 20Cr1Mo1VTiB steel bar after overall heat treatment was studied through microstructure test methods such as mechanical property test， scanning electron microscope （SEM）， transmission electron microscope （TEM）， and electron backscattered diffraction （EBSD） . The results show that when quenching water cooling after austenitizing treatment 1 h at 1 040 °C， good strength and low-temperature impact toughness can be obtained. When quenching water cooling， the strength does not change much， but the low-temperature toughness decreases sharply. When the quenching cooling rate continues to decrease， the low temperature impact toughness performance continues to deteriorate. After austenitizing treatment， the cooling method changes from water cooling→ oil cooling→ air cooling → furnace cooling， the transformation process of the matrix structure is slatted bainite→ granular bainite + slatted bainite→ granular bainite →α ferrite. When tempering at 700°C， the precipitated phase of the slatted bainite is finely dispersed， and M

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C carbides with a certain orientation and continuous distribution are precipitated from the granular bainite M/A island. The slower the quenching cooling rate， the larger the M

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C phase size and the higher the degree of aggregation along the edge of the M/A island. The deterioration of impact toughness performance caused by the decrease of quenching cooling rate from water cooling to oil cooling is mainly caused by three factors， which are the widening of bainite slats， the decrease of the proportion of grain boundaries at large angles， and the continuous distribution of M

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C carbides in the M/A fraction of the granular bainite tissue. The continuous distribution of M

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C carbides providing a low-energy channel for crack growth is the main reason for the rapid decline of low-temperature impact toughness.