Effect of helical rolling on the bainitic microstructure and impact toughness of the low-carbon microalloyed steel

• Published in: Materials science & engineering. A, Structural materials : properties, microstructure and processing Vol. 816; p. 141275
• Main Authors: Derevyagina, L.S., Gordienko, A.I., Surikova, N.S., Volochaev, M.N.
• Format: Journal Article
• Language: English
• Published: Lausanne Elsevier B.V 01.06.2021; Elsevier BV
• Subjects: Austenite; Bainite; Energy levels; Ferrite; Fracture toughness; Heat treating; Helical rolling; High strength low alloy steels; Impact strength; Impact toughness; Iron constituents; Low carbon steels; Low temperature; Low-carbon microalloyed steel; Martensite; Martensite–austenite constituent; Microalloying; Microstructure; Helical rolling; Low-carbon microalloyed steel; Martensite–austenite constituent; Bainite; Impact toughness; Microstructure
• ISSN: 0921-5093; 1873-4936
• DOI: 10.1016/j.msea.2021.141275

Ferrite-bainite microstructures and impact toughness of the X65 low-carbon microalloyed steel were investigated after helical rolling at 1000, 920, 850, and 810 °C followed by continuous cooling in air. After helical rolling at 1000 °C, granular bainite with large areas of the massive-shape martensite-austenite constituent (d = 1.5 μm) and a high fraction of twinned martensite (d > 2.0 μm) were observed in the steel. This caused a decrease in impact energy at low test temperatures (for example, 70 J at –70°С). Lowering the helical rolling temperature contributed to a reduction of dimensions of both ferrite-bainite and martensite-austenite constituent areas, as well as the replacement of the latter by a slender type one and an improvement in fracture toughness at the low temperatures. The highest impact energy level (210 J at –70 °C) was achieved after helical rolling at 850 °C due to the formation of a homogeneous microstructure, which included dispersed ferrite grains, granular bainite and small fractions of the slender type martensite-austenite constituent (d = 0.1–0.7 μm). In this case, areas of twinned martensite were absent.