Influence of technological heating used in the manufacture of parts on the initial structure of 10G2FBYu steel after controlled rolling

Abstract

Modern pipe steels for the main pipelines belong are of the high-strength class of X70 and higher. The steels must simultaneously still have sufficient viscosity at low operating temperatures, hardenability and good weldability. This set of properties is achieved by combining the optimal material composition and the method of manufacturing a sheet billet using controlled rolling. Subsequent operations for the manufacture of pipes, connecting parts of oil and gas pipelines (welding, heat treatment, etc.), accompanied by inevitable high-temperature heating, can significantly affect the initial structure of the sheet and the level of its properties. In this case, the possible growth of austenite and ferrite grains, as well as the loss of hot work hardening, should negative effect the mechanical properties of pipe steel. However, taking into account the peculiarities of the chemical composition and structure of sheet pipe steels subjected to controlled rolling, containing hardly soluble fine particles of carbides and carbonitrides based on vanadium, niobium, titanium, which inhibit recrystallization, one can expect the preservation of a fine-grained structure and a part of hot work hardening with a rational scheme for manufacturing parts. The article presents data on the study of the structure and hardness of low-carbon low-alloy steel 10G2FBYu of strength class K60 after repeated normalization from temperatures of 850, 880, 910 and 940°C. Structural studies performed using metallographic and electron microscopes Carl Zeiss Axio Observer and JEOL JSM-7001F showed that after additional heating at the indicated temperatures, the grain sizes increase, but not significantly. The hardness of steel is reduced by 20% or less, depending on the normalization mode