Mechanical properties and precipitation behaviors of a low‐Mo Nb‐V‐Ti complex microalloyed fire‐resistant steel with two kinds of microstructure

A new type of low-Mo Nb-V-Ti complex microalloyed fire-resistant steel is designed. Two kinds of controlled rolling and controlled cooling processes are employed to obtain the two steels with predominantly composed of fine-grained proeutectoid ferrite (1#) and coarse-grained full bainite (2#), respectively. The microstructures and precipitates of the experimental steels in room temperature and 600 C-o are characterized by means of the scanning electron microscope (SEM), electron backscattered diffraction (EBSD), transmission electron microscope (TEM), and physicochemical phase analysis, and the strengthening mechanisms at room temperature and elevated temperature are quantitatively calculated and analyzed. The results show that the mechanical properties of the two steels are close to each other at room temperature. However, the fire resistance of bainite steel is much better than that of ferrite steel. Nanosized M(C, N) further precipitates from bainite at elevated temperature, which improves the stability of the bainitic structure and compensates for the loss of yield strength caused by the shear modulus decreased at high temperature. The grain refinement strengthening may not play a role at 600 C-o in proeutectoid ferrite steel.