Post-fire mechanical properties of dual-phase advanced high-strength steel

Abstract

Fire-induced high temperatures significantly affect the microstructure and mechanical properties of advanced high-strength steel (AHSS). This study examines the mechanical properties of cold-rolled dual-phase AHSS after exposure to elevated temperatures, identifies the material's degradation trends, and quantitatively analyzes the relationship between grain size, phase transformation, and yield strength. Tensile coupons were extracted from thin-walled sheets and cold-formed C-sections, heated to 300 °C-1000 °C for soaking times of 15 or 60 min, and naturally cooled to room temperature. Tensile tests were conducted to obtain stress-strain curves as well as key mechanical properties after different fire exposure temperatures. Mechanical property degradation trends of high-strength dual-phase steel differ significantly from those of other high-strength steel. In the 600 °C to 1000 °C range, the strength of dual-phase steel drops from 600 °C to 700 °C, then recovers at higher temperatures. The yield strength of the dual-phase AHSS increases by 48.9 % from its lowest value at 700 °C between 800 °C and 900 °C, while the ultimate strength increases by 45.7 % in the same range. Moreover, longer exposure times led to more pronounced deterioration of mechanical properties. This study proposes a post-fire reduction factor prediction formula and a constitutive model for dual-phase AHSS based on different soaking times. Furthermore, microstructure observations of the dual-phase steel after fire exposure were conducted. A modified Hall-Petch equation, considering grain size distribution and phase transformation after fire exposure, accurately predicts post-fire yield strength, aligning well with experimental results.