Managing Structure and Properties of Pipe Steel Alloyed with Chromium in the Process of Cooling After Thermomechanical Rolling

• Published in: Metallurgist (New York) Vol. 63; no. 7-8; pp. 704 - 716
• Main Authors: Matrosov, M. Yu, Kichkina, A. A., Golovin, S. V., Éfron, L. I., Ringinen, D. A., Shul’ga, E. V., Il’inskii, V. I.
• Format: Journal Article
• Language: English
• Published: New York Springer US 01.11.2019
• Subjects: Characterization and Evaluation of Materials; Chemistry and Materials Science; Materials Science; Metallic Materials; large-diameter pipe steel; chromium alloying; thermomechanical treatment; kinetics of phase transformations; M/A component; cold resistance; mechanical properties
• ISSN: 0026-0894; 1573-8892
• DOI: 10.1007/s11015-019-00880-1

A study was conducted concerning the kinetics of phase transformations and structure of grade K60 (X70) pipe steels alloyed with small amounts of chromium (K60Cr) and molybdenum (K60Mo). It was shown that K60Mo steel differs by a higher structural content of M/A components. The laboratory tests and microstructure studies of the obtained samples by color etching, TEM, and XRD methods have shown that by using the improved cooling conditions after thermomechanical rolling of K60Cr steel, it becomes possible to obtain samples having different matrix microstructure (polygonal, quasi-polygonal, or bainitic ferrite) and various secondary phases (M/A or cementite), which yields different mechanical properties ranging from 467 to 633 MPa (yield point – σ y ); 681 to 804 MPa (ultimate strength – σ t ); 22 to 31.8 % (percent elongation – δ e ); 8.8 to 17.4 % (proportional elongation – δ p ); and 0.68 to 0.88 (σ y /σ t ratio). It was established that the presence of M/A component instead of cementite in the structure contributes to an increase in strength and decrease in σ y /σ t ratio without a considerable decline in cold resistance. At JSC “Vyksa Metallurgical Plant” (VMZ), K60Cr steel, which is more economical compared to K60Mo, and newly developed technology were used to produce commercial batches of rolled products and large diameter pipes, which demonstrated the specified σ y /σ t ratio and higher impact toughness and cold resistance values during a drop weight tear test (DWTT).