Recent Development of High Strength Steels for Transmission Trunk Lines and Coiled Tubing

Modern high strength steels need accurate control of microstructure in order to meet the steadily increasing requirements on mechanical properties, toughness or corrosion resistance. An appropriate steel making with inclusion control and a sophisticated thermo-mechanical controlled processing are the key factors, but we have to recognize that the process parameters have to be refined continuously to meet the targets. The present paper is going to demonstrate this progress using the examples of two recent development activities at Arcelor. At the same time it is shown that the development is often accompanied by the installation of new testing equipment for a more accurate and target-oriented material characterization. The first example deals with the development of an X70 for a 36” OD spiral welded pipe with 20.6mm wall thickness, a challenge for a hot strip mill when a good toughness is required at this thickness. We will briefly describe the sheet and pipe production and compare the mechanical properties on sheet and on pipe. The toughness performance of the new steel has been characterized in different orientations with respect to the rolling direction. In an attempt to quantify the crack growth propagation during the Batelle test, we equipped the existing testing device with a load cell so that we are now capable to document the energy absorption during crack propagation. The second example concerns the development of a coiled tubing grade 90 with HIC resistance for downhole application in corrosive environment. The challenging combination of high strength level, excellent low cycle fatigue (LCF) performance and corrosion resistance gave rise to the launch of a metallurgical study with the aim to develop a new steel grade. For a more complete material characterization we took advantage of this opportunity to install new CO₂ corrosion test facilities (atmospheric pressure and high pressure). The LCF performance of the steel sheets has been characterized under constant strain amplitude conditions.