Material Design of Hot-rolled Steel Coils for Heavy Wall X70 Spiral Linepipes

摘要

Higher strength and toughness has been required for linepipe steels using for long-distance pipelines of natural gas transportation. To achieve high strength and drop weight tearing test properties for heavy wall X70 spiral linepipes, the microstructure refinement of bainitic ferrite is essential in hot-rolled steel coils. Niobium addition is very effective to have precipitation strengthening and bainitic ferrite grain refinement through the increase of controlled rolling ratio in austenite non-recrystallization region. Laboratory rolling test results indicated that low-carbon, high-niobium steel had excellent strength-toughness balance by increasing the amount of finer NbC precipitates by 20 nm or less in a fine bainitic ferrite microstructure. From laboratory welding joint test results, low-carbon, high-niobium steel showed good heat-affected zone toughness with a relatively finer prior austenite grain size in spite of a higher martensite-austenite volume fraction.