The Influence of Centerline Segregation on the Mechanical Performance and Microstructure of X70 Pipeline Steel

Centerline segregation is an inherent phenomenon during solidification. It is controlled by optimizing the continuous casting process, but is not easy to be avoided. According to the severity of centerline segregation of the slab, it affects the mechanical properties and final microstructure of rolled strip. The objective of this paper is to study the influence of centerline segregation degree on the microstructure and mechanical properties of X70 pipeline steels. In this study, the mechanical properties of strips produced from the slabs are examined and the results show that centerline segregation is some adverse effect on Charpy impact toughness and tensile properties of hot-rolled strips. The strips from slab with higher degree of segregation, exhibits lower Charpy impact toughness, higher tensile strength, higher microhardness. The segregated degree of slab is studied by using the method of Mannesmann, indicating that it could be up to class 3-4. Moreover, the results from Electron Probe Micro Analyzer (EPMA) shows that high level of Mn content is obtained at the centerline of strips processed from the slab with segregation degree of class 3-4. The corresponding microstructure is identified as lath martensite, which should be contributed to the undesirable mechanical properties.