中文摘要：

利用光学显微镜、透射电镜等分别对模拟火灾状态下耐火钢的显微组织、M-A岛的变化进行分析，研究了高温下钢中析出相情况。结果表明，试验耐火钢的组织是由铁素体、少量粒状贝氏体和M-A岛组成的混合组织；在高温状态下，试验耐火钢的强度和显微硬度有所下降，但大部分M-A岛都表现出较好的稳定性；Nb和Ti的碳氮化物在基体或晶界析出所形成的第二相质点，以析出强化的方式来提高钢的高温强度。

英文摘要：

The evolution of microstructure and M-A islands of fire-resistant （ F-R） steels under fire simulation were analyzed by means of OM and TEM.The precipitated phases in F-R steel were studied .The results show that the microstructure of the tested F-R steels is the mixture of ferrite matrix, little granular bainite and M-A islands.At higher temperature, strength and microhardness of F-R steel decrease, most of M-A islands keep stable , Nb and Ti carbonitride form the second phase particles in the matrix or grain boundary , and it improves the strength of the steels at high temperature by precipitation strengthening .