Effect of Rare Earth and Cooling Process on Microstructure and Mechanical Properties of an Ultra-Cleaned X80 Pipeline Steel

In order to explore the effect of a small amount of rare earth addition in ultra-cleaned pipeline steel and the influence of the cooling process on the tensile and impact properties, three API X80 pipeline steels were fabricated by varying RE addition and the cooling process at the same time. Three microstructures with different features for a low C high Nb microalloyed high-strength pipeline steel and the corresponding mechanical properties were investigated. The results showed that even in the ultra-cleaned steel with O and S contents less than 10 ppm, the addition of RE would still cause an increase in the volume fraction of inclusions consisting of complicated RE oxysulfide and RE sulfide. More inclusions formed in the 112 ppm RE steel were harmful to the low temperature toughness, while few inclusions formed in the 47 ppm RE steel had almost no influence on the low temperature toughness. The two RE additions had no effect on strength of the steels. As the finishing cooling temperature was increased and the cooling rate was decreased within a certain range, the volume fractions of polygonal ferrite and quasi-polygonal ferrite as well as the number density and size of martensite–austenite islands were increased. Under such combined effect, the strength of the steels had almost no change. As the finishing cooling temperature was increased from 481 to 584 °C and the cooling rate was reduced from 20 to 13 °C/s, for the steel with 112 ppm addition of RE, there was an obvious decrease in the low temperature toughness. The reduced value (about 33 J) of the USE of steel consisted of two parts including the influence (about 18 J) of more inclusions formed due to 112 ppm addition of RE and the effect (about 15 J) of the lower high-angle grain boundaries.