Study On Mechanical Properties Of Q345 Steel After Repeated High Temperature

Steel structure is an excellent representative of "green building",high intensity,convenience,green and energy saving have become the mainstream direction of the development of modern building structures.The sudden fire has hindered the promotion and application of steel structure buildings.After the fire,some of the steel structure buildings that have been seriously burned can only be demolished or blasted.For some steel structure buildings,the fire was discovered in time and extinguished in time,or only part of the fire was destroyed by high temperature after a local fire,and the remaining part was not destroyed.If serious damage occurs and its components can continue to meet the service requirements and meet the service strength required by relevant specifications,if the steel structure is resolutely replaced or dismantled as a whole,it will cause huge economic waste,which is not conducive to the development of steel structure and national economic growth.Therefore,it is necessary to appraise and evaluate the steel structure before restoration to determine its loss and remaining bearing capacity and stability.Through the determination of the mechanical properties of steel after high temperature,it is possible to make an accurate judgment on whether the steel structure building after fire is reinforced,and to make a clear distinction for some severely damaged parts that need to be demolished and replaced and urgently needed to be reinforced.The restoration of the function of the steel structure building and the evaluation of reliability and safety provide reasonable test methods and theoretical support.It can also greatly reduce post-disaster losses,which has great social and practical significance.In this paper,considering the wide application of domestic Q345 steel in existing buildings,a series of tests are designed to study the mechanical properties of Q345 steel after repeated high temperatures,mainly including the following work contents:1)Collect statistics on fire data and loss ratios in China and the United States,and introduce the general fire situation in the world today.The focus is on the causes of structural fire hazards and fire fighting methods,as the research background and realistic basis.2)Design three fire extinguishing methods,and innovatively add foam fire extinguisher cooling method.Axial tensile test was carried out on Q345 steel treated by different cooling methods after a single high temperature.Use extensometers,strain collectors and high-precision strain gauges to obtain a series of mechanical performance parameters for comparative analysis,and analyze changes in steel ductility from the macroscopic and microscopic fracture morphologies.The Q345 steel was compared wit h other domestic high-strength steels,and the advantages and disadvantages of the mechanical properties of Q345 steel after fire were summarized.With reference to domestic and foreign literature,the mechanical properties of Q345 steel and American A572 steel with similar strength indexes were compared,and the similarities and differences of the two steel properties were analyzed,and corresponding recommendations were put forward.3)Considering the real fire conditions,design repeated high temperature tests to further explore the changes in the mechanical properties of Q345 steel after complex high temperature fires,compare with single high temperature data,measure the reduction coefficient of mechanical properties,and explore how much repeated high temperatures have on the strength of steel.Secondly weaken the effect,and further study the extent of the effect of fire extinguishing methods on the mechanical properties of steel.4)Based on the test data,use Origin software to sort out the mechanical performance parameter curve,and use the small square method and regression analysis to obtain the theoretical mathematical model of the steel mechanical performance reduction coefficient on the temperature T after repeated high temperature Q345,and compare it with the test data.It is found that the fitting is good,which can provide experimental and theoretical basis for the estimation of the residual strength and residual elongation of Q345 steel after high temperature.