| With the increasing market demand and the increasingly high demand for quality construction, there is an urgent need to develop a low-cost fire-resistant steel with low Mo addition and excellent strength. In order to achieve the purpose of steel structures which low-cost, lightweight, to save raw materials, energy conservation, low-carbon economic. This article is designed with Mo, Mo+Nb, Mo+V and Mo+Nb+V four kinds of ingredients fire-resistant steel. By Nb-V micro-alloyed and controlled rolling and cooling means to reduce the content of Mo. Meanwhile, the four components of steel solution hardening at different temperatures and at different tempering temperatures, to study the precipitation strengthening of the fire-resistant steels can do the greatest contribution. By investigating microstructures and mechanical properties of hot rolled rebars and rebars Quenching-tempering and analyzing the effect of added alloyed elements and rolling process on the phase transformation and mechanical properties of rebars, the research would provides appropriate suggestion for industrial production. The main results are as follows:(1)Typical microstructure of the hot rolled steel consists of equiaxed ferrite+pearlite, in addition, existed certain contents of granular bainite. Mo element of addition increases hardenability, in favor of the formation of bainite.0.3%Mo steel contain a large number of granulation structure,the pearlite content is more in Nb、V microalloyed steels,but contents of granular bainite is less.(2) The hot rolled Mo-Nb-rebar grain refining effect more than Mo-V-rebar, but precipitation hardening ability less Mo-V-based steel. The reason is refining grain capacity of Nb surpassed V,but the behavior of carbide that dispersion precipitation in the matrix will be weaker than V.Nb-V compound microalloyed Mo-Nb-V steel,the combination of grain refinement by Nb and precipitation strengthening by V,its yield strength is signicantly higher than Mo-Nb and Mo-V steel. (3) High temperature tensile experimental results show that will have a lot of Mo, Nb, V carbide precipitation at grain boundary、dislocation and in the ferritic matrix for hot rolled steels keep warm for1h at600℃temperature. Size larger Mo carbide pinning grain boundary effect is stronger at high temperatures, so high temperature yield strength of Mo steel is the largest. Nb, V microalloyed steel, due to Nb and V element for the competition of C atoms, making the Mo carbides precipitate to reduce,result in the high temperature yield strength of the microalloyed steel is low.(4) the test steels tempered at550～650℃after quenching, the hardness and intensity with the increase of tempering temperature showed a trend of first increases and decrease, the best mechanical properties at600℃. Microstructure observation results show that the precipitate size is small and quantity is more at600℃, precipitation comprehensive effect is strongest of Mo、Nb、V carbide. After quenched at950℃and tempered at different temperatures, due to the precipitation strengthening effect of V is better than Nb,the mechanical properties of Mo-V fire-resistant steel is better than that Mo-Nb and Mo-Nb-V steel. Austenitizing at1200℃can ensure Nb carbide completely dissolved, then in the tempered process of Nb precipitation effect is enhanced, therefore, mechanical performance of Mo-Nb steel is better than Mo-V is steel. |
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