Correlation Between Microstructure and Yield Strength in Low-Carbon High-Strength Microalloyed Steels

Microalloyed pipeline and structural steels are currently graded according to their yield strength. In this work, different microstructural factors that affect the yield strength of the steels are assessed and their contributions to the strength are estimated for several low-carbon microalloyed steels, used in pipeline or structural applications. Emphasis is placed on the relative contribution of grain/sub-grain size, precipitate distribution and dislocation density. Accurate grain/sub-grain size measurements were only possible through electron microscopy imaging. It was found that the increased strength is mainly due to the formation of bainitic structures with fine grain/sub-grain sizes. The contribution from other strengthening sources such as precipitates, dislocations and atoms in solid solution is limited and does not vary much among the several grades examined here. The variation in hardness among the fine-grained heat-affected zone samples (heat input range 0.5–2.5 kJ/mm) of one of the steels was also explained based on the microstructural changes.