LAUSR

In this precent study, microstructure evolution of a newly developed 690 MPa composite microalloyed anti-seismic fire-resistant steel plate after quenching at 740 °C , quenching at 740 °C + tempering at 400 °C were studied and the fire resistance mechanism was analyzed. Experimental results show that the microstructure of hot rolled state of the tested steel is lath martensite, acicular ferrite, bainite and M/A island. After quenching at 740 °C, the microstructure of steel was composed of lath martensite, polygonal ferrite and retained austenite. The mixed microstructure of tempered troostite + bainite + a small amount of M/A island was obtained after quenching at 740 °C + tempering at 400 °C , it transforms into tempered sorbite + bainite after holding at 600 °C for 3h. A great number of fine carbides containing Mo, Nb, V and Ti precipitate at 600 °C of the quenched + tempered state of tested steel. The mixed microstructure and fine composite carbides make the quenched +tempered steel have excellent fire resistance.