Research Progress on Multi-Microalloyed Fire-Resistant Steel

With the development of social economy, the construction industry has put forward higher requirements for the strength of building materials. The microstructure of construction steel for building has developed from ferrite/pearlite to ferrite/bainite, bainite, multiphase and multiscale metastable (M³). Fire-resistant steel is widely used in high-rise and large-span buildings due to its excellent comprehensive properties and good fire resistant safety. The control of fire resistance properties has also evolved from high-cost high Mo (≥0. 40%) plus single microalloyed mode to economical saving Mo (≤ 0. 30%) plus Nb, V and Ti and other composite multimicroalloyed methods. The development history, product types and application engineering of fire resistance steel at home and abroad, as well as theories and technologies to improve the strength and fire resistance of fire-resistance steel were introduced in detail. The differences in the fire resistance properties of different microstructures and microalloy elements were comparatively studied. The research shows that the room temperature and 600 ℃ properties of the composite multi-microalloyed steel are better than that of the single microalloyed steel, and puts forward a new idea of multi-microalloyed nano-carbide precipitated in fire to enhance the fire resistance properties. For Q345-Q690 fire-resistance steel, the differentiated alloy and microstructure design and its hot rolling/heat treatment technology were formed. The microstructure types of different strength grades of fire-resistance steels were observed by scanning electron microscopy (SEM), and the microstructure and mechanical properties of typical multi-microalloyed fire-resistance steel during reheating-loading process were clarified. The distribution, size and quantity of precipitated phases in hot-rolled state, heat-treated state and 600 ℃ tensile state were observed and counted by transmission electron microscopy (TEM), physical and chemical phase analysis and three-dimensional atom probe (3DAP). The fire resistance mechanism of nano-carbide precipitation strengthening and matrix microstructure stability in high temperature were discussed. The study shows the Q345 grade steel plate after relaxation treatment is proeutectoid ferrite and a small amount of bainite/pearlite microstructure, with high fine grain and precipitation strengthening increments, and the Q345 grade steel plate directly subjected to laminar cooling after rolling is full bainite microstructure with high dislocations and solid solution strengthening increments, its yield strength at 600 ℃ can still reach 327 MPa. The Q460 grade steel plate is bainite microstructure, and the microstructure has good high temperature stability at 600 ℃. With the extension of the holding time at 600 ℃, the nano-precipitated phases that a diameter is less than 10 nm increases significantly. Q690 grade steel plate is martensite, metastable austenite and nano-precipitated phase and low carbon high strength bainite microstructure. It has a yield strength of 690 MPa and good elongation at room temperature. After 600 ℃ tensile test, its yield strength is not lower than 2/3 of the standard yield strength at room temperature. The control idea of multi-microalloyed design and nano-precipitation strengthening in fire can realize the fire resistant property of different grades of Q345-Q690 fire-resistant steel.