Study on Microstructure and Properties of Low-Mo Fire-Resistant Steel

Abstract

The number of steel-frame buildings is increasing as a result of development of social economy. However, the fire-resistant property of steel-frame buildings is much weaker than that of brick-frame buildings and RC-frame buildings. In response to such demand, fire-resistant steel has been developed since the last two decades. Mo is one of the most effective strengthening elements for the high-temperature strength of steels. With the increase of the Mo content in steels, there is a dramatic increase in costs which is unacceptable for the cost-sensitive construction area. Therefore, a fire-resistant steel with a low Mo content is highly desired. Two fire-resistant steels with low Mo content (<0.3%) were designed in this paper. The interdependence of microstructure, properties and temperature was studied and analyzed in hot-rolled testing steels. The results show that the low-Mo fire-resistant steels have improved elevated temperature strength (The yield strength of Nb content steel is 240MPa, which can met the requirements of grade Q345 fire-resistant steel), low yield ratio (<0.6) and good welding performance (Ceq≈0.40). The bainite and fine grains are mainly beneficial to the elevated temperature UTS. Nb is an alloying element effective in increasing elevated temperature strength (especially for YS) as well as Mo.

Effect of Ti Addition on Yield Strength of Low‐Mo Fire‐Resistant Steel at Elevated Temperatures

“…We speculate that the smaller strengths compared to our results were largely due to the mostly ferritic microstructure in those previous studies. In Wan et al's study, [ 8 ] the YS of low‐Mo (below 0.3 wt%) fire‐resistant steel was 252 and 212 MPa at RT and 600 °C, respectively, and increased to 347 and 240 MPa when <0.03 wt% Nb was added. Although the YS of Nb‐added steel was higher by approximately 30 MPa at 600 °C, the YS ratio decreased from 0.84 to 0.69.…”

“…They focused on process parameters such as the cooling rate and finish rolling temperature, and achieved a YS of 521 MPa at RT and 360 MPa at 600 °C in the tested steel. Low-Mo (<0.4 wt%) fire-resistant steels with different combinations of alloying elements such as Nb or V have been designed in other studies, [8,[14][15][16][17][18] and elevated-temperature strength has been successfully achieved via precipitation DOI: 10.1002/srin.202200103 Herein, a series of low-Mo (0.2 wt%) fire-resistant steels with varying amounts of Ti (0.008-0.13 wt%) are investigated to study the effects of Ti on yield strength at elevated temperatures. At room temperature (RT), precipitation strengthening by nanoscale TiC precipitates is found to be the major factor for the enhanced strength.…”

“…Among these elements, Mo is usually considered essential for improving the high-temperature strength not only through solid solution strengthening, but also by promoting the bainite microstructure. [6][7][8][9][10][11] Commercial fire-resistant steels usually include 0.3-0.5 wt% Mo. [5,12] In KS D 3865, it is specified that the Mo content of FR355 steel should be 0.3-0.9 wt%.…”

“…They focused on process parameters such as the cooling rate and finish rolling temperature, and achieved a YS of 521 MPa at RT and 360 MPa at 600 °C in the tested steel. Low‐Mo (<0.4 wt%) fire‐resistant steels with different combinations of alloying elements such as Nb or V have been designed in other studies, [ 8,14–18 ] and elevated‐temperature strength has been successfully achieved via precipitation strengthening. It is well established that Nb and V are effective in increasing the elevated‐temperature strength; however, limited scientific attention has been paid to the precipitation strengthening effect of Ti addition.…”

Research on influence of grain boundaries on the mechanical properties of 460 MPa refractory steel used for high-strength building structures

“…At present, the academic community often improves the refractory properties of steel by adding alloying elements: higher high temperature strength is obtained by solid solution strengthening and precipitation strengthening of alloying elements such as molybdenum, titanium, vanadium, and niobium. Studies have shown that the addition of Mo not only improves the precipitation strengthening and solid solution strengthening of the steel at high temperature, but also promotes the bainite transformation and hinders the dislocation movement [1][2][3][4][5]. However, the addition of alloying elements will increase the cost, so the research and development of Mo-type refractory steel has become a hot spot in the research and development of refractory steel.…”