Effect On Microstructure And Mechanical Properties Of Q345 H-Beam After Fire

Hot-rolled H-beam is widely used in the construction field due to its comprehensive mechanical properties,excellent cross-section design,etc.The building,especially the high-rise building steel structure will suffer the damage of variable(changeful)degrees when it is exposed to fire.At present,the research on the fire resistance of high-rise building steel structure is mainly focused on the change of mechanical properties under high-temperature.There are few studies which explore the microstructure and mechanical properties of steel after building suffered a fire.However,it's very important to evaluate the stability and risk of the steel structure in the case that the building have to be used after the fire.In this study,we investigate the microstructure and mechanical properties of Q345B,refined Q345B and Q345E,which are commonly used in the building,under different conditions.The results will provide theoretical support for the risk assessment of the undamaged steel structure after the fire and the development of the new refractory steel.In order to observe the changes of materials microstructure and mechanical property before and after the fire,we simulate the fire scene by a laboratory method.The fire temperature is set at 650?,700?,750?,800?;850?,900? and 950?,and the holding time is set as 1 h and 2h.The changes of material microstructure is observed by metallographic experiments while the changes of mechanical property is observed using hardness test and tensile test.The analysis of relationship between the mechanical property changes and microstructure changes of the material shows that the performance of mechanical property after fire is changed by many factors such as the microstructure and composition of the steel structure,fire contact area on steel structure,over-temperature and over-fire and so on.The three kinds of steels used in our experiments have occur obvious decarburization and sudden changes of microstructure and mechanical properties when the over-temperature reaches 750? or above.The mechanical properties of the core part of the experimental materials are better than external part.The mechanical properties of refined Q345B decrease most,followed by Q345B and Q345E reduce the least after the fire.In addition,the fire resistance of the three materials are not only related to its fire resistance structure design,but also the original composition and the way of Thermo Mechanical Control Process(TMCP)of raw materials.Q345E has better fire resistance since it contains Nb,Ti,V,B and other micro-alloying elements.while refined Q345B,which produces some Widmannstatten structure during hot rolling process,has the worst fire resistance.