Design and Processing of Niobium Microalloyed Cost Effective Line Pipe Steel With Enhanced Strength and Fracture Toughness

abstract

The functional role of niobium in the original HTP X-80 design of high niobium (0.1wt%), low interstitial ( C 0.03 to 0.04, N<0.005wt%) cost-effective base chemistry is (i) to use Zener drag from strain induced precipitation of NbC during thermo-mechanical rolling and solute drag from solute niobium to retard static recrystallization, (ii) to impart adequate rolling reduction below temperature of no recrystallization to promote large strain accumulation in pancaked austenite, and (iii) to promote fine ferrite grain size by strain induced phase transformation under accelerated cooling conditions, thereby obtain high strength and fracture toughness at low temperature through grain size effect. Residual niobium in austenite is used to impart additional strength through transformation hardening, dislocation hardening from accelerated cooling and precipitation strengthening of ferrite through accelerated cooling and interrupted cooling at coiling temperature. Recent research has confirmed the importance of control of density and dispersion of crystallographic high angle boundaries which are superimposed on the morphological microstructure in order to prevent the initiation of brittle fracture. Extensive research has been carried out in HTP base chemistry to determine the processing options to control the density and dispersion of high angle boundaries to produce higher grade (>X-80) line pipe steels with enhanced fracture toughness. Whereas the resistance to ductile fracture is measured by Charpy toughness, the resistance to brittle fracture is inferred from ductile to brittle transition temperature and percentage shear in DWTT. The research has underscored the importance of austenite grain refinement in upstream processing of HTP before pancaking in finish rolling to control density and dispersion of high angle boundaries in order to prevent brittle fracture initiation. Experimental results are presented which demonstrate that HTP base chemistry is a cost effective design to produce higher grade line pipe steels, not only to achieve high resistance to ductile and brittle fracture in the base plate, but also in HAZ regions associated with relatively high heat input welding in weld fabrication of pipes from plates, and Girth field welding of pipes involving low heat input multi-pass welding.