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Microstructure evolution of 0·2C–5Mn steel during the intercritical annealing at different temperatures was studied. It was found that the microstructure gradually evolved from martensite structure into a structure consisted of austenite and ferrite during the intercritical annealing process. The retained austenite volume fraction reached the maximum value of 36·5 after about 10 min annealing at 680°C. It was also found that carbides precipitated during initial annealing stage and gradually dissolved during following annealing process for all annealing temperatures; and interestingly, the fresh martensite was found, indicating that part of the newly forming austenite was unstable and would transform into martensite when quenching. Based on the microstructural analysis and the calculation by Thermo-Calc software, it was proposed that the different evolution behaviours of the microstructure during annealing were not only controlled by the thermal kinetics of the austenite, but also affected significantly by the thermal stability of the austenite.

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The microstructure and mechanical properties of 30Si2CrNi4MoNb ultrahigh strength steel were investigated after austenitising over a range of temperature between 1133 and 1483 K. The experimental results show that the isotropy of impact toughness and mechanical properties were greatly improved due to the disappearance of undissolved aligned second phase when the austenitising temperature was over 1233 K. When the austenitising temperature was over 1383 K, martensite lath and packet abnormally grew up due to dissolution of spheroidal Nb rich carbonitrides; both the platelet size and morphology of martensite were changed, which has an effect on the mechanical properties of the samples. It was noticed that the finer self-tempered carbides, which strengthened martensitic matrix, appeared after austenitising temperature over 1283 K. The strength profiles show a marked plateau for the samples austenitised from 1283 to 1433 K; however, the strength was deteriorated due to coarsening of these self-tempered carbides at 1483 K. It was confirmed that calcium treatment can help improve the isotropy of mechanical properties by modifying sulphide inclusion morphology.

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