Steel cleanness requirements for X65 to X80 electric resistance welded linepipe steels

In Australia, a large proportion of the gas pipelines traverse rural areas of low population density with considerable distances between population centres. As these pipelines transport non-sour gas, there have been considerable economic advantages to be gained from the use of thin walled, small diameter, high strength linepipe grades. The successful development of these X65 to X80 grades has required a balance to be constantly maintained between process parameters, alloy design constraints and quality control measures. This paper considers this mixture of metallurgical and processing challenges in the context of steel cleanness. The steelmaking and casting processes are required to not only produce a low volume fraction of suitably morphology modified inclusions, but also to ensure their dispersion in cast and hot rolled structures. Critical in the achievement of these aims was found to be the order in which calcium treatment and vacuum processing of the liquid steel are carried out. As these steels are complex microalloyed grades, the clean steel concept was extended to include the composition and precipitation of the carbide and nitride species. This broad approach to steel cleanness was achieved while also maintaining a narrow range of casting temperature and chemical composition.