API X 70 Grade HR Coils for ERW Pipes

For supply of API 5L Grade X70 (PSL 2) 1610 mm wide hot rolled (HR) coils of 10.4, 11.2 and 12.8 mm thickness, trial ro lling was undertaken at Bo karo Steel Limited (BSL). Two heats were made and cast into 200 x 1630 mm section slabs with designed alloy chemistry. Rolling parameters of exit roughing (R5) temperature, fin ish rolling temperature (FRT), and coiling temperature (CT) were kept in the range of 1080 - 1100 ° C, 860 - 890° C, and 600 - 630 ° C respectively. Coiling of high strength 10.4, 11.2 and 12.8 mm thick co ils of 1610 mm width at such low temperature was carried out for the first time in SAIL. M icrostructure of hot rolled coils co mpris ed ferrite and pearlite. Average grain size for ferrite varied between 6.3 - 9.2 micron. Microstructure of Centreline segregated zone of cast slab showed presence of primary carb ides of microalloying elements. Optical micrograph clearly shows formation of coarse primary carbides at inter dendritic region. SEM and EDAX examinations confirmed presence of Nb and V in these coarse carbides. Genesis of these coarse primary carbides rich in Nb & V is attributed to segregation and subsequent slow cooling. To ensure comp lete dissolution of coarse primary carbides of Nb and V, a suitable time-temperature combine was adopted during reheating. Elimination of primary carbides in the rolled stock was testified through microstructural evaluation. High yield strength (~ 54 0 MPa) could be achieved accordingly. Excellent YS / UTS rat io ( 100 J even at - 40° C. Lo wering of FRT and CT led to a concurrent increase in strength and toughness. At ambient temperature, 100% shear area was observed after drop weight tear test (DWTT) whereas at 0° C, shear area was above 95%. After pipe forming, tensile propert ies of the parent material and that of fusion -line were found satisfactory. Toughness of parent material were found to be adequate (> 200 J at 0 ° C and > 170 J at - 30 ° C). There had, however, been a few cases of inferior impact resistance of weld -seam. Metallurgical investigation established the genesis of inferior impact toughness of fusion line to be lin ked with incidence of penetrators. Originating crack on the detrimental flow lines propagate easily in presence of non-fusion / penetrators resulting fro m improper selection of welding parameters viz. heat input, pressure and speed of welding etc. During process of ERW pipe making, these inclusions must be ejected. Modified alloy design of API X 70 ensures liquid state of silicates at the end of ERW process which ejects out easily to provide penetrators free seam leading to adequate toughness of the weld seam. W ith modified alloy design, more than 4000 T of API X 70 grade HR coils have been supplied fo r successful production of p ipe, meeting all the metallurgical requirements. The process technology for production of API X 70 grade HR coil has been established. API X 70 is now being regularly made against the commercial o rder at BSL.