Effect of finishing rolling temperature on fire resistance and dynamic strain aging behavior of a structural steel

The influence of finishing rolling temperature (FRT) on dynamic strain aging (DSA) behavior and high-temperature resistance of a fire resistant steel microalloyed with Mo and Nb was investigated by means of tensile tests performed at temperatures ranging from 25 to 600 °C and strain rates of 10⁻⁴ to 10⁻¹ s⁻¹. In these steels, DSA manifestations are less intense than those observed for carbon steels and they take place at higher temperatures. The precipitation behavior of the steels was also considered. Hardness of samples heat treated at 100–600 °C displayed a maximum at 400 °C. Samples treated at this temperature and tensile tested at 600 °C did not show a higher yield stress than the untreated specimens. Results obtained indicated that DSA in the fire resistant steel might have a contribution for its fire resistance. The empirical activation energies related to the appearance of serrations on the stress–strain curves and to the maxima on the variation of tensile strength with temperature suggested that the high-temperature strengthening associated with DSA in this steel is the dynamic interaction of interstitial-substitutional solute dipoles and dislocations. The steel with lower FRT is more susceptible to DSA because of its higher amount of carbon in solid solution and showed better results in terms of high-temperature resistance.