Low manganese sour service linepipe steel

Linepipe steel technology and metallurgy have evolved over the past four decades since the spectacular failure of a BP line in the Umshaif (Arabian) Gulf. The X65 steel in question was severely controlled rolled and despite having a sulfur content of 0.005 percent, considered very low at that time, featured large quantities of MnS stringer inclusions. Remedial measures involved lowering manganese to below 1.20 percent, development of inclusion shape control technologies and avoidance of low temperature finishing practices. With time, the familiar "BP Solution" (now Solution B in NACE TM0284), was replaced by the more aggressive NACE solution and a new round of steel development occurred. Carbon contents were reduced, sulfur was lowered to below 0.0020 percent, steel cleanliness improved and alloying with chromium and copper became widespread. Additionally, control of pearlite banding and phosphorus segregation was coupled with improved continuous casting practices and the steels became highly resistant to HIC. In a relatively recent development, steels with ultra-low manganese contents (<0.30%) have been developed, which are more forgiving of residual sulfur content and variable casting machine performance and other manufacturing variables. The present paper summarizes the metallurgical basis of the ultra-low manganese concept and presents recent results. (AU) © CBMM