Thickness Section Effect on Ultra-low Temperature Toughness of Extra-thick and Ultra-high Strength Offshore Steel and Its Mechanism Analysis

HU Feng; WANG Tongliang; QU Zhengxiang; QIU Baowen; Serhii Yershov

doi:10.3969/j.issn.1674-6457.2026.09.010

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Journal of Netshape Forming Engineering ›› 2026, Vol. 18 ›› Issue (9) : 90-100. DOI: 10.3969/j.issn.1674-6457.2026.09.010

Iron and Steel Forming

Thickness Section Effect on Ultra-low Temperature Toughness of Extra-thick and Ultra-high Strength Offshore Steel and Its Mechanism Analysis

HU Feng^1,2,3, WANG Tongliang^1,2,4, QU Zhengxiang³, QIU Baowen^1,2,4,*, Serhii Yershov⁵

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Abstract

The work aims to explore the uniformity of toughness of ultra-thick ultra-high strength offshore steel across the entire cross section, especially at -80 ℃ extremely low temperature, and to provide a theoretical basis and technological direction for achieving the coordinated improvement of strength and toughness of key load-bearing components of deep-sea engineering equipment throughout the entire thickness. A 100 mm-thick F690 offshore steel was prepared from a 320 mm thick continuous casting billet through controlled rolling and controlled cooling followed by quenching and tempering. The microstructure evolution patterns at the surface, 1/4 point and the center were studied by OM, SEM and TEM. The correlation mechanism between the microstructure and extremely low-temperature toughness was analyzed based on the fracture mechanics theory. The largest rolling reduction and fastest quenching cooling rate (10 ℃/s) at the surface layer, uniform and fine austenite grains (18.5 μm) were formed, but the excessively high cooling rate led to an increase in the undercooling degree of martensite phase transformation and a decrease in the nucleation rate, resulting in coarse blocky martensite instead. This reduced the crack propagation resistance, and the dislocation walls formed by dislocation pile-up failed to effectively impede the crack, causing the impact toughness at -80 ℃ to drop to 57 J. The slow cooling rate (1-2 ℃/s) and the center segregation of continuous casting at the center, mixed crystal austenite (grain size 26.4 μm) and a large amount of granular bainite (volume fraction 17.8%) were formed. The M/A islands in granular bainite acted as brittle centers, causing stress concentration, and the size effect intensified the cleavage fracture tendency at low temperatures, resulting in an impact energy of only 62 J at -80 ℃. At the 1/4 position, the cooling rate was moderate (5 ℃/s), forming a mixed structure of lath martensite (width 376 nm) and a small amount of granular bainite (volume fraction 9.8%), with the best comprehensive mechanical properties and an impact energy of 176 J at -80 ℃. The non-uniformity of extremely low-temperature toughness of ultra-thick offshore steel is mainly controlled by the microstructure characteristics regulated by the cooling rate across the cross section. By balancing the cooling rate, suppressing the segregation at the center and refining the substructure of martensite at the surface layer, the coordinated improvement of strength and toughness throughout the entire cross-section can be achieved, meeting the requirements of reliable service of materials in extreme environments for deep-sea equipment.

Key words

extra-thick offshore steel / thickness section effect / ultra-low temperature toughness / phase transformation mechanism / crack propagation

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HU Feng, WANG Tongliang, QU Zhengxiang, QIU Baowen, Serhii Yershov. Thickness Section Effect on Ultra-low Temperature Toughness of Extra-thick and Ultra-high Strength Offshore Steel and Its Mechanism Analysis[J]. Journal of Netshape Forming Engineering. 2026, 18(9): 90-100 https://doi.org/10.3969/j.issn.1674-6457.2026.09.010

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