Effect of Microstructures on the Mechanical Behavior of Fire‐Resistant Dual‐Phase Steels at Elevated Temperature

Abstract

Herein, the start laminar cooling temperature to obtain different volume fractions of ferrite and bainite/martensite is optimized. Lower start laminar cooling temperature yields a higher volume fraction of ferrite and also decreases the transformation temperature of bainite/martensite. The yield strength of the experimental dual‐phase steel at 600 °C increases from ≈484.8 to ≈526.2 MPa by decreasing the start laminar cooling temperature from 650 to 600 °C. To understand the effect of microstructure, the dislocation density, volume fraction of precipitates, and Young's modulus at elevated temperature are statistically analyzed. A machine learning method, known as gradient boosting decision tree, is used to classify the grain boundaries. The statistical results suggest that the steel subject to lower start laminar cooling temperature shows higher Young's modulus and larger volume fraction of precipitates at elevated temperature. Furthermore, at elevated temperature, the boundary is stable and the proportion of Σ3 boundary is greater, when lower start laminar cooling temperature is adopted. Theoretical calculations results suggest that the lower transformation temperature of bainite/martensite improves the fire resistance of dual‐phase steels.