Abstract: Effect of rare earth Ce on microstructure and second phase precipitation of BT80s-5Cr oil well pipe steel was studied by adding different contents of rare earth Ce. The prior austenite grain size after quenching was analyzed by using metallographic microscope. The microstructure and second phase after heat treatment were observed and analyzed by using SEM, XRD and TEM. The results show that after water quenching at 900 ℃ for 50 min, when the rare earth Ce content of the tested steel increases from 0 to 0.012%, the average size of the prior austenite grains drops from 14.21 μm to 11.06 μm, and when rare earth Ce content is 0.0025%, the grain distribution is the most uniform, and the proportion of grains within the range of 6-15 μm is 60.26%. The addition of rare earth refines the prior austenite grains and makes the structure more uniform. After tempering at 690 ℃ for 90 min, the microstructure of the quenched steel is a mixed structure of tempered sorbite and tempered troostite. As the content of rare earth Ce increases from 0 to 0.012%, the amount of carbide precipitation decreases, and the addition of rare earth inhibits the precipitation of carbide. The main types of carbides are Cr₂₃C₆ and Cr₇C₃, and the addition of rare earth Ce does not change the type of carbides. When the Ce content is 0.012%, the carbides undergo a transition from continuous distribution to intermittent cluster distribution on grain boundaries.

Key words: rare earth Ce, BT80s-5Cr oil well pipe steel, prior austenite grain, second phase