Acicular ferrite HSLA steels for line pipe

Summary

An updated state-of-the-art review has been made of the new-generation high-strength low-alloy steels known as the acicular ferrite Mn-Mo-Nb steels. After individual effects of the various alloying elements were reviewed from the results of earlier research and published data, the importance of processing variables in commercial production was stressed. The most important benefits that can be realized pertain to the upgrading of impact toughness properties by the use of: 1) lower slab reheating temperatures, e.g., 1080–1150°C, 2) large total reductions, e.g., 65% below 900°C, and 3) low finish-rolling temperatures, e.g., 740–780°C. The strength properties are relatively insensitive to variations in hot-rolling schedules because there is a good balance of the three main strengthening mechanisms: grain refinement, dislocation substructure, and precipitation strengthening by Nb(C, N).

In the fabrication of line pipe, the acicular ferrite steels exhibit continuous yielding and rapid workhardening, leading to an upgrading of strength in the finished pipe relative to the strength of the starting plate (skelp). Conventional ferrite-pearlite steels in the X60 to X70 grade range do not show this increase because they experience little or no work hardening during pipe fabrication. In fact, a decrease in strength due to the Bauschinger effect is usually encountered when flattened circumferential strip specimens are used for measuring the pipe strength.

As shown in the text, excellent combinations of strength and toughness are available commercially in Aretic-grade X60 to X70 line pipe, using the new low-carbon, weldable, acicular ferrite Mn-Mo-Nb steels.