| The fire-resistance properties of conventional construction steel is not good enoughto meet the fire-resistance standards, therefore the fire-resistance coating was used toprotect the steel structure, however, the application of fire-resistance coating has lots ofnegative effects, such as environmental pollution, higher construction costs and harm toworkers. The fire-resistance steel is a kind of low-alloy high-strength structural steel,which can resolve the negative effects by fire-resistance coating. The yield strength offire-resistance steel under the temperature of600℃is not lower than two-thirds of theyield strength at room temperature, so it has superior high-temperature mechanicalproperties. Therefore, there are important significances to study the microstructure offire-resistance steel.In this paper, the effects of normalizing temperature and tempering temperature onmicrostructure of two series tested steels are studied by means of OM, SEM, TEM, thefire-resistant mechanism of fire-resistant steel at high temperature is discussed. The resultsshow that: Microstructure of A series (containing Cu, Ni) steels is composed of acicularferrite, blocky ferrite and small amount of martensite and bainite, microstructure of Bseries (containing Mo) steels is composed of ferrite and M-A island. With normalizingtemperature increasing, the acicular ferrite amount of A series steels is decreased, the M-Aisland amount of B series steels first is increased and then decreased. The M-A islandamount reaches the maximum value at normalizing temperature at770℃. The optimalnormalizing temperature of A series steels is770℃, while the best normalizingtemperature of B series steels is740-770℃. Microstructure of A series steels still containsa large number of high-density dislocations of acicular ferrite after tempering, which isgood to keep the steels with high temperature properties. For B series steels aftertempering, part of M-A island does not completely decompose, and a large number of finecarbides precipitate in the grain boundaries and matrix, the stability of M-A island andfine carbides are good for the high temperature properties of B series steels. In thecomparison of two series steels, alloying elements Cu and Ni is beneficial to the formationof acicular ferrite, while Mo elements to promote the formation M-A island. |
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