Q690钢材高温后的力学性能试验研究

通过升温、冷却和拉伸试验，对历经300~900℃高温后的Q690钢材在自然冷却和浸水冷却条件下的力学性能展开试验研究。结果表明：经高温冷却的Q690钢材在不同温度和不同冷却方式下有不同的外观特征；受热温度超过500℃时，高温冷却对Q690钢材的弹性模量影响很小,对其强度和伸长率影响较大；当受热温度不超过700℃时，Q690钢材高温后的强度和伸长率在两种冷却方式下具有基本相同的变化规律；在700~800℃之间，不同冷却方式对Q690钢材高温后强度和伸长率产生影响，且随温度升高差别愈加明显，自然冷却条件下强度降低且伸长率增大，浸水冷却条件下强度增大且伸长率减小。将Q690钢材高温后力学性能与Q235钢材和Q460钢材比较，认为不同强度等级钢材高温后的力学性能差别显著，在自然冷却条件下较高强度钢材（Q690）的强度衰减和延性增长大于较低强度钢材（Q235和Q460）的。根据试验结果，建立了不同冷却条件下的高温后各力学参数与受热温度之间的数学模型，该模型可用于火灾后Q690钢结构的承载能力的评估。

Experimental research on post-fire mechanical properties of Q690 steel

By procedures of heating, cooling and tensile specimen test, an experimental study was carried out to investigate the post-fire （300℃≤T≤900℃） mechanical properties of high strength Q690 steel under two cooling methods, cooling in air and cooling in water. The post-fire appearance characteristics of high strength Q690 steel by different heating temperatures and cooling methods are different. High-temperature treatment and cooling methods have a great influence on the yield strength, ultimate strength and ultimate elongation of Q690 steel, but no obvious influence on elastic modulus, when heated to temperatures higher than 500℃. When the temperature is not higher than 700℃, there is a same trend for the post-fire strength and elongation of Q690 steel under the two cooling methods. The effect of cooling methods on the post-fire mechanical properties of Q690 steel appears between 700℃ to 800℃, and difference becomes bigger with the temperature increasing. When cooled in air, the post-fire strength of Q690 steel decreases with the temperature increasing but the ultimate elongation increases. On the contrary, when cooled in water, the post-fire yield and ultimate strength increase but ultimate elongation decreases with temperature increasing. The mechanical properties of Q690 steel after experiencing elevated temperatures were compared with Q235 and Q460 steel. It is found that steel grades have a significant influence on the post-fire mechanical properties of structural steel. When cooled in air, the higher strength steel (Q690) experiences greater loss of strength but becomes more ductile than the lower strength (Q235 and Q460) steel. Predictive equations were proposed for simple evaluation of post-fire material performance of Q690 steel. The results can be used to evaluate the bearing capacity of Q690 steel structures after fire.