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 sour gas resistance was developed. The resulting Fe–C–Mn–Nb slabs were controlled rolled and accelerated cooled. In most of the slabs, mechanical properties equivalent to the above grade were broadly achieved. However, despite achieving excellent mechanical properties, during the test period, several slabs exhibited cracks in the cooling yard, causing their rejection. To elucidate the nature of the crack, several specimens were analysed by employing optical, scanning electron, and transmission electron microscopy. Microstructural characterisation together with microanalysis and thermal analysis carried out on as cast specimens showed the presence of rodlike and/or dendritelike precipitates of the Fe₂ Nb type associated with the cracks. The elimination of the cracks was achieved by increasing the niobium content in the Nb ferroalloy to ensure its dissolution in the liquid bath after its addition during the alloying stage.