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

AbstractThe 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 c...