The Q345qNH weathering bridge steel was investigated.The phase transition temperature and time of Q345qNH weathering bridge steel containing Nb and V at different cooling rates were measured using Formastor-F Ⅱ phase transforma-tion tester.The metallographic microstructure,hardness and transformation process were analyzed,and the continuous cool-ing transformation(CCT)curve of Q345qNH steel was created.The results showed that the microstructure of tested steel was mainly composed of pearlite and ferrite when the cooling rate was 0.3-2 ℃/s.As the cooling rate was 2-3 ℃/s,the microstructure was composed of uniform pearlite+ferrite+a small amount of granular bainite.When the cooling rate was 3-20 ℃/s,the microstructure of tested steel was mainly composed of uniform ferrite matrix+uniform granular bainite.When the cooling rate was further increased to 20-50 ℃/s,the lath bainite appeared in the microstructure,but the ferrite and granular bainite were dominant.The relationship between hardness and microstructure could be divided into three sta-ges:the stage of ferrite refined crystalline strengthening,the stage of granular bainite incremental strengthening,and the stage of bainite strip morphology strengthening.
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