DOI: 10.1002/adem.201000065 Texture Dependent Mechanical Anisotropy of X80 Pipeline Steel By N. Sanchez Mourin ˜o * , R. Petrov, J.-ho Bae, K. Kim and Leo A. I. Kestens High strength and toughness are the most common requirements for pipeline steels. High strength of the steel contributes to cost reductions of the fuel transportation by decreasing construction, material, and compression costs which increases transportation efficiency. Improvements in strength are generally achieved, though, at the expense of reducing toughness and ductility; but toughness is an imperative property to ensure the structural integrity of the pipeline over a long period of time. [1–4] Optimized micro- structure and texture are necessary to further improve the strength and toughness of pipeline steels. An appropriate design of the chemical composition together with a thermo- mechanically controlled process (TMCP) contribute to the achievement of effective microstructures and textures, ensu- ing improved mechanical properties. [6,10] It has been largely reported in the literature that mechanical properties of pipeline steels like yield strength and toughness are strongly affected by thermo-mechanical controlled processing. The properties are mainly dependent on the rolling temperatures, the finish cooling temperature and the cooling rate on the runout table after hot rolling. [11] However, these mechanical properties often display high anisotropy and it is not well understood to which micro- structural or crystallographic elements this can be attributed. Mechanical anisotropy has been extensively studied by analyzing the effect of microstructural parameters [4–7] and by analyzing the effect of texture. [8–10] Nevertheless, the effect of texture and microstructure on the anisotropy in the ductile- to-brittle transition region (DBTR) is not yet well known. Toughness is most commonly measured by the amount of energy absorbed by a Charpy V-notch specimen during impact testing. Low energy levels are associated with brittle behavior at low temperatures due to cleavage-type fracture. At higher temperatures, high-energy ductile fracture occurs by microvoids coalescence. [12] In numerous studies the low temperature toughness has been linked to microstructural parameters, like the effective grain size represented by the effective free path length available to a moving dislocation. This implies that various types of obstacles such as grain or phase boundaries may act as critical sites of stress concentra- tion and hence can be activated as crack nucleation sites. It is already known that the effective grain size in ferritic steels corresponds to the ferrite grain size [13–15] and larger ferrite grains contribute to an increase in the transition temperature and reduction of the impact toughness. [16] However, the effective grain size for acicular ferritic, bainitic, martensitic, or multiphase steels as API steel grades is not well COMMUNICATION [*] N. Sanchez Mourin ˜o, Dr. R. Petrov, Dr. L. A. I. Kestens Department of Materials Science and Engineering, Ghent University Technologiepark Zwijnaarde 903, B-9052 Ghent, Belgium E-mail: nuria.sanchezmourino@ugent.be Dr. J.-ho Bae, Dr. K. Kim Sheet Products & Process Research Group, POSCO Jeonnam, 545-090, South Korea Dr. L. A. I. Kestens Department of Materials Science and Engineering, Delft University of Technology Mekelweg 2, 2628 CD, Delft, The Netherlands Pipeline steel grades API X80 are used for oil and gas transport. The most important mechanical properties of these steel grades are strength and fracture toughness. A remarkable directional anisotropy of toughness and strength were often observed in the plates. The effect of the thermo- mechanical processing parameters on the mechanical properties and texture of several commercial pipeline API X80 plates were investigated with a particular emphasis on the effect of the slab reheating temperature and the finish cooling temperature on the directional anisotropy. The crystallographic texture gave rise to a specific anisotropy profile in yield strength and in Charpy fracture energy in the ductile–brittle transition temperature domain. The observed anisotropy pattern could be understood on the basis of crystal plasticity and crystal fracture principles. ADVANCED ENGINEERING MATERIALS 2010, 12, No. 10 ß 2010 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim wileyonlinelibrary.com 973