Experimental Research on Post-Fire Mechanical Properties of Domestic Q355 Steel After Cooling from High Temperature

As a new type of steel, Q355 has been widely used in the field of construction due to its excellent mechanical properties. However, it has the fatal disadvantage of poor fire resistance, which is dangerous to steel structure buildings. Through the safety appraisal and bearing capacity evaluation of steel structure buildings after fire, the waste caused by demolition and reconstruction can be effectively avoided, and corresponding measures can be taken to repair or replace the components, which can greatly save economic costs. Therefore, it is necessary to study the residual mechanical properties of Q355 steel after fire. In order to simulate the situation of Q355 steel after fire and fire extinguishing, the conditions of 200 to 900℃, natural cooling and water cooling were set to simulate the fire, and the mechanical properties of Q355 steel after high temperature were tested. With the help of high temperature furnace, universal testing machine and electronic extensometer, the stress-strain curve and mechanical properties parameters (yield strength, tensile strength, ratio of yield strength to ultimate tensile strength, elastic modulus and elongation, etc) of Q355 steel after high temperature cooling were obtained. The influence of temperature and cooling method on the stress-strain curve and mechanical parameters was analyzed. The mechanical properties of Q355 steel and Q235, Q460 and Q690 steel after high temperature were compared and analyzed. The mathematical model of mechanical properties of Q355 steel under different cooling methods was fitted by ORIGIN data processing software. It was found that there were significant differences in the different apparent characteristics, failure modes and mechanical properties of Q355 steel for different temperatures and cooling methods. The surface carbonization degree of Q355 steel gradually deepened with the increase of temperature. When the exposure temperature exceeds 600℃, the carbonization phenomenon become more obvious, and even when immersed in water for cooling, the carbonization skin peeled off. When the temperature does not exceed 600℃, the apparent morphology change characteristics were small and the deformation degree of the tensile specimen was light, which was similar to the appearance and deformation of the specimen without high temperature. In addition, 600℃ was also the critical temperature for the change of residual mechanical properties of Q355 steel. When the temperature was lower than 600℃, the effect of temperatures and cooling methods on the mechanical properties of Q355 steel was negligible. However, when the temperature was higher than 600℃, temperatures and cooling methods exerted a distinct influence on the mechanical properties of Q355 steel. After air cooling, the yield strength, tensile strength, and elastic modulus of Q355 steel decreased with the increase in temperature, but the ultimate elongation increased with the increase in temperature. After water cooling, the yield strength and tensile strength of Q355 steel increased with the increase in temperature, while the elastic modulus and ultimate elongation decreased with the increase in temperature. Furthermore, the predictive equations were proposed to determine the post-fire material properties of Q355 steel for cooling in air and cooling in water.