Mechanism and Control of Sulfide Inclusion Accumulation in CET Zone of 37Mn5 Round Billet
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OIL casing, as a part of well bore configuration, is used for side wall protection and stratum sealing. The requirements of strength, composition homogeneity, corrosion stability, and wear resistance for oil casing steel are relatively high due to the severe service conditions and high load. Therefore, the composition and inclusion distribution of the billet should be controlled in a reasonable range to avoid defects formed during rolling and tempering. The distribution of sulfide inclusions on the cross section of the billet is bound to the segregation of sulfur, which is caused by the uneven distribution of solute concentration. The precipitation of sulfide inclusions, as well as the select crystallization which is the primary cause of element segregation,[1] takes place during solidification.[2] Therefore, locations of the element segregation and sulfide precipitation are related to solidification structures of the billet. For the relationship between inclusions and segregation, the existing studies predict the segregation of solutes and composition change of nonmetallic inclusions during solidification by microsegregation models and thermodynamic calculations. But these predictions are for micro-regions in the size of micrometers to YI WANG, LIFENG ZHANG, HAIJIE ZHANG, XING ZHAO, SHENGQIAN WANG, and WEN YANG are with the School of Metallurgical and Ecological Engineering, University of Science and Technology Beijing (USTB), Beijing 100083, China. Contact e-mail: [email protected] Manuscript submitted August 9, 2016. METALLURGICAL AND MATERIALS TRANSACTIONS B
millimeters.[3,4] The macroscopic transport of solute and nonmetallic inclusions has not been considered yet. For the connection between macrostructure and macrosegregation in the billet, the studies mainly focus on the mechanism of center segregation.[5–8] The element segregation in positions besides the center has got attention recently. Several attempts have been taken out to reveal the formation mechanism of segregation in a quarter position of the billet by experiments and numerical simulations. The 1/4 segregation has been related to the interlaced dendrites in these studies.[9,10] For studies on sulfide inclusions and the solidification structure, steady-state unidirectional solidification models have been proposed to describe the relationship between solidification variables and the morphology of MnS inclusions. But it is still in the size of dendrite structures instead of the macrostructure of billet.[2,11,12] Although it is almost intuitive to notice the connection among sulfide inclusion distribution, macrosegregation of sulfur, and macrostructure of billet, these characteristics have not been clearly related and thoroughly studied, especially for the whole transverse section of the billet. In the current study of the round billet of 37Mn5 oil casing steel, after the detection of sulfide inclusions, sulfur content, and macrostructure of the billet, it was found that sulfide inclusions gathered near R/2 of the billet, where the columnar-to-equiaxed trans
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