Slab cracking after continuous casting of API 5L X-70 grade steel for pipeline sour gas application

An API 5L X-70 grade steel for large diameter pipeline application with sou r gas resistance was developed. The resulting Fe-C-Mn-Nb slabs were control led rolled and accelerated cooled. In most of the slabs, mechanical propert ies equivalent to the above grade were broadly achieved. However, despite a chieving excellent mechanical properties, during the test period, several s labs exhibited cracks in the cooling yard, causing their rejection. To eluc idate the nature of the crack, several specimens were analysed by employing optical, scanning electron, and transmission electron microscopy. Microstr uctural characterisation together with microanalysis and thermal analysis c arried out on as cast specimens showed the presence of rodlike and/or dendr itelike precipitates of the Fe-2 Nb type associated with the cracks. The el imination of the cracks was achieved by increasing the niobium content in t he Nb ferroalloy to ensure its dissolution in the liquid bath after its add ition during the alloying stage.