Modelling of strength and continuous cooling transformation behaviour of HSLA-100 plate steel

Continuous cooling transformation (CCT) diagrams were determined for high strength low alloy (HSLA) steel by the dilatometric method as a function of different austenitising temperatures from 900 to 1300°C. The diagrams show significant suppression of polygonal ferrite at higher cooling rates and higher reheat temperatures (1100–1300°C), a prominent transformation region attributed to bainite or acicular ferrite at temperatures intermediate between those of polygonal ferrite and martensite formation. The CCT diagrams for the steel are characterised by the formation of acicular ferrite structure for a wide range of cooling rates. The initial austenite grain size did not affect the transformation temperatures of austenite to acicular ferrite at all cooling rates. The microstructure of steels cooled at very low cooling rates (<0·1 K s−1) exhibited a two-phase structure of acicular ferrite and polygonal ferrite. A model based on the rule of mixtures was used to predict the strengthening components in the steel in the tempered condition. The major contributions to strengthening in the alloy were found to be from solid solution strength, precipitation strength, and ferrite lath strengthening. The contribution to dislocation strengthening in the tempered condition was found to be negligible. The calculated strength is in excellent agreement with the experimental values.