Post‐fire Mechanical Behaviors and the Mechanism of their Influence on a 690 MPa Grade High‐Strength Steel for High‐Rise Buildings

Abstract

Herein, the residual mechanical behaviors of a 690 MPa grade high‐strength steel after exposure to fire for an extended duration are investigated, and the factors affecting their variation based on the theories of metallurgy are explored. The corresponding tensile properties and microstructure tests are conducted on this steel soaked at 300–900 °C for 3 h and then cooled in air. The results show that there is negligible influence of temperature on residual mechanical properties at 600 °C or lower. It can be considered that the critical fire temperature is 600 °C, beyond which the strength decreases sharply. This can be attributed to the formation of ferrite and grain coarsening. However, at 900 °C, its residual strength can still maintain approximately half of that at room temperature. It is related to the contribution from fine nanoscaled precipitates of 10–40 nm in size. Compared with steels having the same nominal yield strength, the dependence of residual mechanical properties on elevated temperatures for this steel varies due to the difference in microstructure characteristics. New predicted equations are proposed for this novel structural steel to guide on better prediction of residual mechanical properties in steel structure systems.