Investigation On Properties And Microstructure Of Fire-resistant Construction Steel With Low-Mo Containing

Along with the development of construction industry, more and more steel-frame buildings are constructed. But the fire-resistant property of steel-frame buildings is much worse than that of brick-frame buildings and RC-frame buildings. The fire-resistant coating is commonly sprayed to protect the steel, which increases the construction costs, has an injurious effect on the health of people, and causes serious environmental pollution. Therefore, improving the fire-resistant property of steel is the effective means to reduce the fire-resistant coating and enhance the fire-resistant property of the buildings.The fire-resistant of steel is realized by adding a proper amount of alloying elements(Mo,V,Nb,Ti and so on). Mo is one of the most effective elements which are good for the high temperature strength. But with the increase of the Mo content, the construction costs increase greatly. Therefore, designing a fire-resistant steel containing less kinds of alloying elements and less Mo has an important meaning.Based on the research results of Pangang Group and combining the technology of controlled rolling and controlled cooling, this paper designed two kinds of hot-rolled fire-resistant tested steels containing 0.15Mo+B,0.30Mo. The mechanical properties of the tested steels were studied by the CCT curves,tensile test and impact test. Meanwhile, by OM,SEM,TEM , the microstructure character of hot-rolled tested steels, and the effect of alloying elements on the microstructure and properties at elevated temperature were analyzed. And the fire-resistant mechanism was discussed.The important research results have been obtained:①The two series of tested FR steels that were designed have well fire-resistant property,earthquake resistance,weld ability and meet the operating requirements of FR steels.②The microstructure character of hot-rolled tested FR steels is the mixture of pro-eutectoid ferrite, granular structure and granular bainite, without pearlite which is usually considered. The pro-eutectoid ferrite is equiaxial and the content of it of the tested steels containing 0.15Mo+B is more than that of the tested steels containing 0.3Mo. After tempering at 600℃for 30mins, part of M-A islands in granular"structure"/granular bainite decompose, which verifies that the microstructure is not the pearlite. And the substructure of martensite in the M-A islands is twin or dislocation of high density in the lath.③The fire-resistant mechanism of FR steels is that most of M-A islands remain stable at 600℃and there is some precipitation of alloying elements, which restrains the great decrease of the strength at elevated temperature to preserve well elevated temperature properties.④Besides the high-temperature solution strengthening, the more important function of Mo in the FR steels is to increase the stability of the overcooling austenite and promote the achievement of the well fire-resistant property in the FR tested steels with the formation of granular bainite during air cooling after hot rolled.⑤It is feasible to develop a new kind of FR steel, which is"smart FR steel containing V". The"smart FR steel containing V"is the FR steel that a certain amount of V is added into, so there is some precipitation during firing to restrain the great decrease of the strength to guarantee the well high-temperature property.