Mastering the production of skelp made of naturally alloyed steels to make gas and oil pipelines with a longer service life

Rolled products used to make large-diameter welded pipes must have a combination of high strength, good ductility, good weldability and resistance to cold and corrosion. More than 80% of reported field failures are ascribable to to internal corrosion (especially in Western Siberian pipelines), so any improvement in HSLA corrosion depth will also increase service life of oil and gas pipelines and thus be of great economic benefit. NOSTA has developed a "naturally alloyed" steel by smelting to pig iron an ore rich in Cr and Ni (up to 0.5% each). To evaluate the possible variants, NOSTA conducted statistical analyses the following steels: conventional; "naturally alloyed" (NA) steel (dispersion hardened with niobium); NA steel (dispersion hardened with niobium and vanadium); pipeline steel (dispersion hardened with Nb and V); and pipeline steel (dispersion hardened with Nb and V and naturally alloyed with Cr and Ni); Specifics of steel chemistry adjustments, steel fabrication and testing are described detail in this paper. Changeover to the NA variant for the 17G1S improved the Charpy impact toughness in the 0 to -80 deg C behavior. The NA alloying improved the cryogenic characteristics of the dispersion hardening elements (V, Nb) and also exhibited improved corrosion resistance. Cryogenic and corrosion resistance characteristics are discussed in detail. All testing to date supports the assumption of superior pipeline lifetime with the combined dispersion alloying and natural alloying.