Effect Of Over-fired Treatment On The Microstructure And Mechanical Properties Of Q345E H-Beams

H-beam is an efficient and economical construction steel developed from I-beam,which can be obtained by hot rolling and welding.Due to its advantages,hot-rolled H-beam has been widely used in various building structures.Although steel is non-flammable,its fire resistance is very poor.One of the biggest problems faced by steel structure buildings is that they are extremely sensitive to high temperature.If they are exposed to fire in service,the mechanical properties of steel will deteriorate,causing structural damage in a short time.There are few studies on the changes of structure and mechanical properties of steel after fire.In this paper,the author simulated the fire environment in the laboratory,by setting different heating temperatures,holding times and cooling modes.The effects of heat treatment conditions on the microstructures were studied by metallographic experiments,SEM and XRD.The effects of different conditions on the mechanical properties were studied by hardness tests and tensile tests at room temperature.Finally,the effects of the changes of the microstructures on the mechanical properties were analyzed.It provides a reliable theoretical basis for the research of fire resistance of steel structure and damage assessment of steel structure after fire.The experimental results show that when the over-fired temperature is lower than the initial austenite transformation temperature,no phase transformation occurs.After cooling,the cementite changes from flake to granular,the hardness and strength of the material decrease,and the plasticity increases slightly.When the temperature of over?fired treatment is in the dual-phase zone,incomplete austenitizing transformation occurs,the banded structure disappears gradually after air-cooling,the plasticity decreases,and the strength and hardness increases.After water-cooling,the mixed structure of martensite and ferrite is obtained,the plasticity decreases sharply,and the strength and hardness increase obviously.Over-fired treatment temperature continues to rise,complete austenitizing transformation occurs,banded structure disappears,grains continue to coarsen,and comprehensive mechanical properties of materials decline.In addition,due to the serious decarburization at the edge of the material,the hardness of the edge is always lower than that of the core.The longer the holding time is,the deeper the decarburization layer on the edge is,and the more obvious the hardness decrease is.In conclusion,the structure,morphology and composition of hot-rolled Q345E H steel will change after different degrees of fire,so the mechanical properties will decrease accordingly Air-cooled steels are prone to instability and collapse,and water-cooled materials are prone to fracture..