Development of grade API X80 pipeline steel plates produced by controlled rolling

Abstract

The future demand for high-grade pipeline materials (API X80 and higher) calls for the development of steels with microstructures showing acicular phases (acicular ferrite, bainite, and martensite-austenite (M A) constituent). A systematic investigation has heen made (of the mechanical properties and microstructures exhibited hy a wide range of controlled-rolled plates (of final thickness 18 mm) of various chemical compositions, produced on a laboratory scale. Low-carhon (∼ 0·06% C) boron-free and boron-containing steels have been examined, as has a boron steel with a Imrer carbon content (0–02% C). The boron-free steels (carbon equivalent ≤0.46%) always exhihited a polygonal ferrite matrix with a dispersed phase consisting mainly of M A constituent, while the horoncontaining steels (carhon equivalent ≤0.40%) invariahly showed an acicular matrix with a lower volume fraction of finely dispersed M A island. Quantitative relationships are ohtained hetween the volume fractions of the microstructural components and the mechanical properties. These relationships clearly show that the type of microstructure found in the horon steels gives better strength toughness combinations (in terms of the Charpy-V shelf energy). With both the boron-free and the boron-containing steels, the best results have been obtained after adding ∼0·10% Ti (together with ∼0·03% Nb), rather than vanadium, to increase precipitation strengthening. This increase has been confirmed directly for polygonal ferrite hy quantitatively comparing the number and size distrihution of precipitates in a Nb-V and a Nb-Ti steel having otherwise identical compositions.