Post-fire mechanical properties of high strength Q690 structural steel

This paper investigates the post-fire mechanical properties of high strength structural steel (HSSS) of Q690, the Chinese Grade of steel with nominal yield strength of 690 MPa. The test specimens were first heated to a temperature level (ranging from 300 °C to 900 °C), and then cooled down to ambient temperature. Two cooling methods were considered: cooling in air and cooling in water. Tensile coupon tests were conducted to determine the yield strength, ultimate strength, elastic modulus, ultimate elongation and stress-strain curves. The study found that high-temperature treatment and cooling method had significant influence on the yield strength, ultimate strength and ultimate elongation of the investigated steel for heating temperature level above 500 °C, while their influence on elastic modulus was negligible. Besides, the post-fire mechanical properties of Q690 steel were compared with S690 steel and Chinese grade steels Q235 and Q460. It was found that difference exists between the post-fire mechanical properties of Q690 steel and S690 steel even though they had the same nominal yield strength, and the effect of fire experience on the mechanical properties were sensitive to the steel grade. Equations were proposed to determine the post-fire material properties of Q690 steel cooled in air and cooled in water, which would provide the fundamentals for appropriate post-fire evaluation of Q690 high strength steel structures.