On the Microstructure of Plate Steels for API-5L X120 Applications

The very high strength now achievable in low carbon HSLA steel plates is caused by the formation of bainite or martensite during the post-hot rolling cooling in interrupted direct quenching. Modern electron optical examination, especially FEG-SEM, has allowed the microstructural features such as packet, block and lath dimensions and crystallography to be quantitatively determined. Several recent studies have attempted to relate the strength and toughness to these features, with limited success. However, one observation is clear, these microstructural features scale with the prior-austenite grain size and state of recrystallization. The role of microalloying, beyond grain refinement, remains inconclusive. This paper will discuss these microstructures and suggest possible ways of further refining them.