Alloy Design And Study On Microstructure And Properties Of Fire-resistant Reinforced Bar For Building

In this day, the frequent occurrence of earthquake and fire disaster has arisen people’s focus on the seismic and fire resistance of rebar for Building. In order to meet the engineering application,400MPa-grade fire-resistant rebar with seismic resistance was developed by adopting niobium or vanadium microalloying method and the thermo-mechanical control process on the basis of adding lower content of molybdenum in the research. Four species of fire-resistant rebars as single-Mo rebar, Mo+Nb-rebar, Mo+V-rebar, Mo+Nb+V-rebar were introduced. By investigating microstructures and mechanical properties of hot rolled rebars and rebars after normalizing 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) The microstructure of both the hot rolled rebars and rebars after normalizing consists of ferrite, pearlite and bainite. Because of the increase of hardenability by adding the molybdenum, it is easier to obtain granular bainite in steel after transformation. For the reason that the rebars were deformed in the y-transition two-phase zone, the grain sizes of four rebars lie between2-5μm.The normalizing process doesn’t alter compositions but inscrease grain sizes of rebars.(2) The yield-tensile ratios of Mo+Nb-rebar, Mo+V-rebar and Mo+Nb+V-rebar are above0.8, which can not meet the norm of rebars. The research adopted normalizing at820℃for Mo+Nb-rebar and Mo+Nb+V-rebar, and these heat treatments can decrease the yield-tensile ratio to below0.8as well as promote plastic property on the basis of achieving400MPa in yield strength.(3) Ratios between yield strength at room temperature and yield strength at600℃of hot rolled rebars are below0.67in the research. The decrease of yield strength of hot rolled rebars is due to the growth of grain size, the reduction of dislocation and the growing of cellular structure. The effect of Mo+Nb alloying is better than that of Mo+V alloying in order to promote the fire resistance of rebars(4) Ratios between yield strength at room temperature and yield strength at600℃of rebars after normalizing are below0.67because part of the precipitates dissolve and grow when reheated in normalizing process, and the number is close to that of hot rolled rebars without heat treatment. It is concluded that grain refinement strengthening and dislocation strengthening can contribute to the high temperature yield strength, but it can not rely on these two strengthening methods to strengthen the rebars extensively.