Effect Of Mo And V On The Microstructure And Properties Of Fire-resistant Steel

Steel constructions have such advantages as lightweight, low-cost, quick construction velocity, high industrialization degree, well anti-seismic behavior, recoverability, etc and are widely used in high-rise buildings and large-span buildings. After the"9.11"event, the fire-resistance of the constructional steel is paid more and more attention on by the government and design department. The fire-resistant (FR) steel, which is a high strength low alloy steel with desirable fire-resistant property, has being applied to the steel-structure construction. It is therefore necessary to have an accurate knowledge of FR steel in order to keep up with the increasing demand in the development of steel-structure construction.Two series of tested steels with B and B-free were designed, and then subsequently investigated systematically, considering significant aspects of mechanical properties, microstructure characterization and microstructure evolution after tempering, using tension and impact tests incorporated with various analytical techniques such as optical microscopy (OM), scanning electron microscopy (SEM), transmission electron microscopy (TEM) and energy dispersive x-ray spectroscopy (EDS). It was expected that the effect of the alloy elements, such as Mo and V, on the microstructure and properties of FR steels was analyzed; how to apply the component and technology controlling to improve the properties, especially the fire-resistance of the FR steel was discussed.The paper's main conclusions are as follow:①With well mechanical properties, well fire-resistant property and continuous yielding behavior, the two series of tested FR steels meet the service requirements of FR steels.②Mo and B enhance the stability of the overcooling austenite more effectively than Mo alone, enable the easier formation of bainite in the tested steels after cooling at high rate.③The microstructure of the steels is characterized by a mixture of pro-eutectoid ferrite, granular structure and granular bainite. The substructure of the"island"is the twin martensite and the high-density dislocation in the math martensite.④The well mechanical properties of tested steels at room or high temperature depend on the microstructure strengthen and the stability of it. The M-A island is the essential constitute for the well properties of tested steels.⑤The effect of Mo on the FR steel stems from the effect of it on the microstructure.⑥The influence of V is the precipitate strengthen latter at high temperature.⑦In tested steel B2, the main nucleating mode of V(C,N) in the austenite is nucleating on the dislocation and nucleation rate decaying to zero rapidly. The deformation-stored energy enhances the precipition of V(C,N).⑧After pearlite transformation, the nucleation ratio and precipitate time of V(C,N) in the ferrite are higher or lower than that in the austenite by 2~3 or 1~2 orders of magnitude respectively.