Eliminating Mo-Segregation Bands and Mechanical Property Improvement in High-Grade Oil Casing Steel via a Combination of Hot Rolling and Reheating Diffusion Process
摘要
The banded defect in steel pipes caused by Mo segregation during continuous casting results in extremely unstable mechanical performance. A novel combination strategy of rolling–reheating diffusion–rolling (3R) process has been developed to eliminate the Mo-segregation bands in high-grade oil casing steel pipes. It was found that the increased specific surface area of alloying segregation after the first hot rolling process notably mitigated the Mo solute drag effect and facilitated grain boundary migration during the reheating diffusion period. The average austenite grain size in the rolling segregation bands substantially increased from 12 to 41 μm, significantly faster than the expansion observed in the segregation rings of the round bloom, which grew from 9 to 25 μm upon isothermal holding at 1200 °C for 300 seconds. Then, the Mo element diffused more thoroughly, with its concentration decreasing from 3.65 wt pct in the initial round bloom to 1.43 wt pct after reheating diffusion process, ultimately leading to the disappearance of segregation bands. Moreover, the purpose of the hot rolling in the third step of the 3R process is designed to refine the coarse grains formed during the reheating diffusion process. Following the quenching and tempering (Q&T) process, the sizes of the four carbide phases MC, M2C, M7C3 and M23C6 along with their aggregation clusters decreased and became uniformly dispersed. Finally, a uniform tempered sorbite microstructure steel was obtained by 3R and subsequent Q&T treatment, which led to a significant improvement in mechanical properties with an ultimate tensile strength of 1240 MPa and a ductility of 20.7 pct.