Experimental study to differentiate mechanical behaviours of TMCP and QT high strength steel at elevated temperatures

Abstract

High strength steel (HSS) has gained its popularity in high-rise and large-span structures in which fire is one of the extreme scenarios that needs to be considered in structural design. In view of different types of HSS manufactured from different heat-treatment processes, their mechanical behaviours under fire would be different. However, previous studies and modern design codes show inadequacy of information on their temperature-dependent mechanical properties, especially their mechanical differences due to the different heat-treatment methods. With the research gap being identified, high-temperature coupon tests were conducted on two types of S690 steel respectively manufactured from thermo-mechanically controlled process (TMCP) and quenching and tempering (QT) process. Both steady- and transient-state test methods were adopted. Microstructure changes were discussed and temperature-dependent mechanical properties were compared with each other and with those of mild steel produced through hot-rolling process. The test results revealed that TMCP-S690 steel exhibited similar elastic modulus at elevated temperatures as QT-S690 steel but smaller effective yield strengths when the thermal creep was more significant above 400 °C. Through this study, the heat-treatment dependent thermal elongations, elastic moduli and effective yield strengths at elevated temperatures were recommended for fire resistant design of HSS structural elements. The stress–strain constitutive model was also provided for numerical modelling.