Development of X60/X70 Line Pipe Steels through EAF-Thin Slab Casting Technology at Ezz Flat Steel, Ain Sukhna, Egypt

Abstract

API X60/X70 line pipe steels are characterized by their higher strength and excellent toughness properties, which are achieved through grain refinement by addition of micro-alloying elements and controlled rolling. Thin slab casting of Nb micro-alloyed steel is a great challenge to the steel producer because of possible transverse cracking of slabs. The major casting parameters, which affect transverse cracking, were critically examined and modified for production of defect free slabs. The hot strip mill parameters were also designed for controlled rolling of slabs to achieve the desired mechanical properties. Three heats (one X60 and two X70) of each 175 ton liquid steel were made and processed into 6, 8 and 10 mm hot rolled coils. In the heat of X60, Nb and Ti were used as micro-alloying elements where as vanadium was also added in X70 heats to achieve the desired strength. The YS/UTS ratio was designed to be below 0.9 by controlling the amount of Si and Mn. During slab casting, liquid core reduction (LCR) was used to reduce the slab thickness from 90 to 70 mm and to minimize the center line segregation and porosity. The slabs were heated and homogenized in a tunnel furnace at 1150 ±10°C before rolling in the roughing mill. The rolling procedure was designed to achieve the highest possible amount of deformation at the roughing stand for best conditioning of austenite before rolling in the finishing mill. 50% and 35% reductions in slab thickness were given for rolling of 6 and 10 mm coils respectively. The final thickness reduction was then distributed over 5-6 stands of the finishing mill depending on the coil thickness. All parameters were designed to achieve finish rolling temperature 850 ±10°C in the austenite region and very near to Ar3. After rolling, the steel was fast cooled to 570±10°C before coiling. Mechanical properties and microstructure evolutions were extensively investigated. The tensile properties particularly the elongation and YS/UTS ratio achieved were good. The CVN impact energy in transverse direction was found to be very high ranging from 220 to 330 Joules. Furthermore, the impact transition temperature (ITT) was found to be below -70 °C. The results of the drop weight tear test (DWTT) showed fully ductile behavior up to -40°C confirming the impact test results. SEM study was carried out to assess the fracture mechanism of impact-tested specimens at different temperatures.