Original Article

Research on the source of internal fault defects in 40Mn2 oil drill pipe

¹ State Key Laboratory of Advanced Metallurgy, University of Science and Technology Beijing, Beijing 10083, China
² Technical Support Center for Prevention and Control of Disastrous Accidents in Metal Smelting, University of Science and Technology Beijing, Beijing 100083, China
³ Xingcheng Special Steel Research Institute Process Research Institute, Xingcheng Special Steel, Jiangyin 214429, China
⁴ Shunde Innovation School, University of Science and Technology Beijing, Shunde 528300, China

^* e-mail: baoyp@ustb.edu.cn
^** e-mail: guchao@ustb.edu.cn

Received: 29 August 2024
Accepted: 9 October 2024

Abstract

As an essential material in various industries, steel’s internal defects can seriously affect its performance and cause huge economic losses. To investigate the source of internal fault defects in oil drill pipes, this study used ultrasonic testing equipment to accurately locate internal fault defects in steel, and obtained the spatial coordinates of the defects as (2.15, 2.29, 0.77). Then, by dissecting layer by layer, the target defect was determined to be a large Al₂O₃ inclusion with a length of 1.609 mm and a width of 0.2 mm. Afterwards, based on the results of automatic scanning of inclusions, this study used extreme value statistical method to estimate the maximum size inclusions that may exist in steel, and found that the target defect size was much larger than the estimated value. Finally, to further determine the source of large inclusions, this study conducted sampling and analysis on immersion nozzles, refining slag, tundish slag, and mold flux. The study found that the matching degree between the composition of large inclusions and the composition of submerged nozzle nodules was the highest, indicating that the fault defect was caused by the peeling of nozzle nodules. This indicates the direction for process optimization.