Effects of Segregation on the Mechanical Performance of X70 Line Pipe

Elemental segregation during continuous casting of steel is an inherent part of the solidification process. After rolling, the segregation is evident through banding of the microstructure, particularly at the centerline where the enrichment of such elements as carbon, manganese, molybdenum and chromium, may locally increase the hardenability of the steel and result in the formation of harder microstructural features. While operational steps may be taken to minimize segregation during casting, complete elimination of segregation is almost impossible. Various slab rating systems have been defined over the years which are employed as a means to measure slab quality taking into account such factors as internal cracks and segregation. While these slab rating systems were intended to aid mill operators in assessing slab quality, in recent years slab ratings have been prescribed as a means of assessing pipe quality. In this study the properties of pipe produced from a slab with Mannesmann rating of 2 are compared to those of pipe produced from a slab with a rating of 3. The work has been supplemented by microprobe analyses to measure the degree of segregation. Increased levels of Mn and Si were found at the centerline of pipe processed from the Mannesmann 3 slab. In the final pipe, these centerline bands were 10 to 20 μm in thickness and exhibited increased hardness (HV 50g) in the Mannesmann 3 pipe as compared to the Mannesmann 2 pipe. Despite evidence of increased segregation, the mechanical properties (YS, UTS, Charpy, DWTT) of both pipes comfortably met X70 property requirements.