Nosov Magnitogorsk State Technical University (Magnitogorsk, Russia)

A. S. Kuznetsova, Cand. Eng., Associate Prof., Dept. of Metal Processing Technology, e-mail: allakuznetsova.mgtu@mail.ru
P. P. Poletskov, Dr. Eng., Prof., Dept. of Metal Processing Technology
N. V. Koptseva, Dr. Eng., Prof., Dept. of Casting Processes and Materials Science
Yu. Yu. Efimova, Cand. Eng., Associate Prof., Dept. of Metal Processing Technology

Demand in Russia for flexible tubing pipes (FTP) increased during last decades by 80 %, and import substitution of foreign technologies in FTP production became a priority task after introduction of sanctions. Coiled rolled sheet of high-strength low-allow steel with increased resistance to atmospheric corrosion is used for FTP manufacture. The operating features in the North regions determine necessity of using for FTP the materials meeting the requirement of ST80 strength grade according to the standard API Specification 5ST: tensile strength ≥ 610 MPa, relative elongation ≥ 20 %, Rockwell hardness ≤ 22 HRC and increased cold resistance down to –60 °С. The paper describes the research of the forming features of structural and phase state of low-alloy steel for FTP with ST80 strength grade, having the following original chemical composition, % (mass.), not exceeding: 0.16C; 0.50Si; 1.00Mn; 0.70Cr; 0.80(Ni+Cu+Mo); 0.050(Nb+V+Ti), depending on thermokinetic transformation conditions. Structural and thermokinetic diagrams were built for the new steel with original chemical composition. To provide the required complex of properties on the base of the selected thermokinetic transformation conditions, the parameters of controlled rolling with accelerated post-deformation cooling were determined. Based on the results of investigations, it was established that achievement of the highest level of viscous-plastic properties (KCV^–60 = 122 J/cm²) at the preset strength level of the steel with ST80 strength grade (σ_u ≥ 610 MPa, HRC ≤ 22) is provided in steel owing to forming of dispersed ferrite-bainite structure. This process occurs during realization of controlled phase transitions: the temperature of the beginning and the end of accelerated cooling 830 °С and 600 °С respectively.

The research was carried out in the Nosov Magnitogorsk State Technical University on the expense of the grant of Russian scientific fund No. 25-29-20085 dated 14.05.2025, as well as on the expense of remedies of the Ministry of Education and Science of the Chelyabinsk region (Agreement No. 30-2025-002767 dated 07.07.2025), https://rscf.ru/project/25-29-20085/.

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