Abstract:

When the pierced billets were cooled by two different rates before reheating, the E470 steel pipes manufactured by online normalization process were found to have performance anomaly. The analysis results show that, when the pierced billet cooling rate exceeds the critical cooling rate of bainite during 719-600 ℃, the austenite does not transform into ferrite and pearlite completely during the cooling process but partial undercooled austenite can be found. When the undercooled austenite is reheated, it continues to grow into coarse grains and transforms into bainite during the consequential cooling process, the final microstructure is composed of coarse bainite, ferrite, and pearlite, so that the impact toughness of the material reduces greatly. When the cooling rate of pierced billet does not exceed the critical cooling rate of bainite during 719-600 ℃, the final microstructure is composed of refined ferrite and pearlite, so the good match of strength and toughness can be achieved. In order to ensure the online normalization effect, it is proposed that the cooling rate of the pierced billet after piercing rolling shall be controlled as one important process parameter.

Key words: E470 steel pipes, online normalization, undercooled austenite, critical cooling rate, bainite