Research On Organization And Performance Of 490MPa Grade Fire-resistant Steel

With the development of the construction industry, Steel structures are used widely in high-rise buildings and large-span buildings. However, fire-resistance performance of steel construction is poor, generally the fire-resistance coating is used of to protect steel, this not only increases construction costs, but also pollutes the environment. The fire-resistant (FR), which is a high strength low alloy steel with desirable fire-resistant property, and compared with ordinary steel, the yield strength at 600℃high temperature of FR steel can be maintained more than 2/3 of the yield strength at room temperature, it can effectively reduce the fire protection coating and have a broad prospect of application. Microstructure and alloy element are the main factor to the material properties, So it has a important significance for enriching fire-resistance mechanism of FR steel to study the relationship between organization and the mechanical properties of FR steel.In this paper, the influence of normalizing temperature and alloying elements on organization and performance were studied by using MTS810 tensile testing machine, OM, SEM, TEM, the fire-resistance mechanism of fire-resistant steel at high temperature was discussed.The results of this study show that:(1) The microstructure character of normalized tested FR steels is the mixture of pro-eutectoid ferrite, granular bainite and granular structure. Granular bainite is comprised of orderly M-A islands distribution in ferrite matrix, and Granular structure is comprised of disordered M-A islands distribution in ferrite matrix. (2) With the increase of normalizing temperature, tensile strength and yield strength of A and B series FR steels decrease, but elongation increases, mechanical properties under normalizing temperature is 800℃are the best. With the increase of cooling rate after rolled, granular bainite and granular structure increase, the grain became smaller, tensile strength and yield strength increase. (3) After tempering at 600℃, part of M-A islands in granular bainite and granular structure decompose, but most M-A islands can remain stable in this condition, the stability of organization is the main reason for FR steel to keep good mechanical properties in high temperature. (4) Mo elements mainly exist as segregation, which increase the stability of the overcooling austenite, promote the formation of bainite in FR steel and improve the stability of M-A islands in high temperature, so fire-resistance performance is guaranteed. (5) B element can be added to replace Mo element in FR steel, it can reduce the content of Mo element in FR steel to make a reasonable controlled rolling and controlled cooling process.