钢结构以其结构自重轻、材质塑性和韧性好、结构抗震性能卓越、构件加工适合标准化生产、以及施工方便且周期短等优势在结构工程领域占据着越来越重要的地位。但是钢结构也具有一定的缺点，钢材虽为非燃烧材料，但耐火性较差。随着钢结构的广泛使用，越来越多的钢结构火灾案例出现，其火灾后性能鉴定也对相关规范以及研究提出更高要求。目前国内外相关研究及规范都较为匮乏，火灾后钢结构性能需要更充分的研究。本文为配合规程编制的需要，采用试验研究、文献调研、拟合分析的方法对我国Q235和Q345结构钢材母材及其焊缝的高温过火后不同冷却方式下的力学性能进行研究，并探究在火灾现场使用里氏硬度进行结构钢材力学性能无损判断的方法，同时结合试验结果以及文献数据，提出了适用于我国结构钢材高温过火后力学性能折减公式。本文主要完成了以下工作：（1）对Q235及Q345结构钢材母材高温过火后包括应力－应变曲线、弹性模量、屈服强度、极限强度、断后伸长率、断面收缩率、外观特征、里氏硬度的力学性能及其折减系数等进行试验研究和拟合分析，探究最高过火温度及冷却方式对结构钢材母材高温过火后力学性能变化情况的影响，并对比文献进行验证。（2）以试验研究的方法对Q235及Q345钢材的焊缝高温过火后包括应力－应变曲线、弹性模量、极限强度、断面收缩率的力学性能及其折减系数等进行研究和拟合分析，探究最高过火温度及冷却方式对结构钢材焊缝高温过火后力学性能变化情况的影响，并对比文献验证。（3）基于里氏硬度试验结果，结合现有里氏硬度－极限强度转换方法，探究高温过火后结构钢材母材里氏硬度－屈服强度、里氏硬度－极限强度、里氏硬度－断后伸长率转换关系，并依据试验数据进行拟合分析。（4）结合试验结果及文献数据，参考中国规范公式形式进行拟合分析，得出以钢材牌号与最高过火温度为自变量的多项式形式的结构钢材母材及焊缝高温过火后不同冷却条件下力学性能折减系数公式 。本论文获得国家自然科学基金项目（51478244）和国家自然科学基金优秀青年基金项目（51522806）资助。
Steel structure occupies an increasingly important position in the field of structural engineering because of its light weight, good plasticity and toughness, convenience for standardized production, convenient construction process, and short construction time. While, except the above advantages, steel structure also has some weeknesses. The fire hazard of steel structure is one of them. Although steel is non-combustion material, its fire resistance is unsatisfactory. With the widespread use and promotion of steel structure, more and more cases of steel structure fires have emerged. The post-fire performance evaluation of steel structure gives out higher requirements for existing researches and standards. While the researches and standards on the post-fire performance of steel structure are far from enough in China and abroad, the post-fire performance of steel structure need more research. The post-fire mechanical properties of Chinese structural steel Q235 and Q345 steel materials and welds under different cooling conditions were investigated by experimental research, literature investigation, and fitting analysis in this thesis to meet the need of standards compilation. And the possibility of using Leeb hardness as an index to evaluate the post-fire mechanical properties of steel structure without distrcution. In the mean time, formulas for reduction factors of post-fire mechanical properties of Chinese structural steels are given out based on the data from experiments and literatures. The research works in this thesis are as follows:(1) Mechanical properties and its reduction factors of Q235 and Q345 materials including the stress-strain curve, elastic modulus, yield strength, ultimate strength, elongation after fracture, section reduction, appearance characteristics, Leeb hardness are studied based on experimental research and fitting analysis. The effects of the elevated temperature and cooling methods on the post-fire mechanical properties of materials were also investigated. The fitting formulas are verified with the data collected from literatures.(2) Mechanical properties and its reduction factors of Q235 and Q345 wields including the stress-strain curve, elastic modulus, ultimate strength, section reduction are studied based on experimental research and fitting analysis. The effects of the highest heating temperature and cooling methods on the post-fire mechanical properties of wields were also investigated. The fitting formulas are verified with the data collected from literatures.(3) The relationships between Leeb hardness and yield strength, Leeb hardness and ultimate strength, Leeb hardness and elongation after fracture were studied based on the experiments, existing researches and fitting analysis. New fitting formulas are given out.(4) Based on the fitting analysis on data from experiments and literatures, fitting formulas of the reduction factors of post-fire mechanical properties in polynomial form were given out considering the effect of the highest heating temperature and the standard value of the yield strength of the steel.This work was supported by the National Natural Science Foundation of China (No. 51478244), and the Excellent Young Scientist Programme of the National Natural Science Foundation of China (No. 51522806).
