New steels for pipelines of strength classes K54–K60 (X70): Production experience at OAO MMK

Abstract

1 Current specifications on pipe for gas and oil pipelines call for high strength, low-temperature strength, and weldability of the metal. In the present work, we consider the production of high-quality sheet (thickness up to 16 mm) of strength classes K54‐K60 (X70), with improved low-temperature strength (even in drop tests) and weldability, on the 2000 mill at OAO Magnitogorskii Metallurgicheskii Kombinat (MMK). This sheet is intended for the manufacture of straight- and spiral-seam pipe without subsequent heat treatment. According to current concepts, satisfactory properties of the metal sheet and pipe are obtained as a result of the formation of fine ferrite—bainite structure [1, 2]. The required structure is ensured by a steel composition with 0.03‐0.07% C and microalloying with Nb, Ti (V), as well as elements (Mo, Cr, Ni, Cu) that increase the stability of austenite in polymorphic transformation during controlled rolling with accelerated cooling. It is necessary here to minimize the content of harmful impurities ( ≤ 0.003‐0.005% S, ≤ 0.010‐0.012% P) and nonmetallic inclusions [1‐3]. It is expedient to produce the sheet for pipe production—straight-seam pipe (diameter 530 mm), twoseam pipe (diameter 1020 mm), and spiral-seam pipe (wall thickness up to 16 mm)—on continuous broadstrip hot-rolling mills. In the manufacture of spiralseam pipe from metal subjected to controlled rolling, there is no need for subsequent heat treatment (quenching + tempering), which is widely used by foreign producers [4]. For the 2000 mill, controlled rolling with accelerated cooling is characterized by a finite number of reductions in successive roughing and finishing cells, limited thickness of the sheet for the finishing group, and small pauses between deformation stages [5]. The sheet with a thickness of 8‐16 mm is cooled more 1 Participants in this research included B. F. Zin’ko, V. V. Savenkov, S. A. Gorshkov, P. A. Stekanov, V. Yu. Salo, and E. A. Kudryakov. slowly than in thick-sheet mills. The most important stage in the operation of the broad-strip mills is winding, which largely determines the final structural type and morphology of the steel, its conditions of dispersional hardening by the deposition of Nb and V carbides, and the cementite distribution.