Development of API X70 Grade Hot Rolled (HR) Coils for Spiral Welded (SW) Pipes

In this paper, efforts made at BSL, SAIL towards developmental of API X70 grade HR coils intended for SW pipes, have been presented. Two trial heats with Nb-Cr-V and Nb-Cr-V-Mo based chemistry aiming (a) ferrite-pearlite and (b) ferrite-pearlite-bainite microstructures respectively were made and cast into 1400 x 210 mm slabs. These trial slabs were processed under suitably designed process variables. Acicular ferrite microstructure engineered through Nb-V-Cr-Mo alloy design has been found more suitable to achieve requisite strength level (in 45° direction) and impact toughness properties consistently in API X70 HR coils. All the properties requirements like, Yield Strength (YS): 515-536 MPa, Ultimate Tensile Strength (UTS): 590-610 MPa, Percentage elongation (%El) in 50mm GL: 25-35%, YS/UTS: 0.86-0.88, Hardness: 184-218 HV 10 and Impact toughness (at -5°C): 305-328 J were successfully achieved in HR coils of Nb-V-Cr-Mo heat. Performance trial for three number of coils of Nb-V-Cr-Mo heat were conducted at customer’s end. The coils were rolled into 28 mm OD x 7.1 mm thick Helical Submerged Arc Welded (HSAW) pipes and forty numbers of pipes were successfully made. Mechanical properties requirements like YS : 511-550 MPa, UTS: 589-646 MPa, %El: 34-38%, YS/UTS: 0.84-0.87), impact toughness in base (280-296 J), weld (184-196 J) and HAZ (184-236 J) and 100% shear area in Drop Weight Tear Test (DWTT) at 0°C were successfully met in all the pipes. Surface of the rolled pipes were found free from any defect. All the pipes passed the hydraulic, ultrasonic and radiography tests.