Study on the Influence of Microstructure on the Properties of 460 MPa Seismic-Resistant and Fire-Resistant Construction Steel

We have designed a new kind of Mo-saving low carbon seismic-resistant and fire-resistant constructional steel which consists of bainite and ferrite. The study found that constructional steels with different volume fractions of bainite were obtained by different rolling processes. Due to the deformation of austenite promoted the transformation of ferrite, more volume fraction of ferrite could be obtained by two-stage rolling than by one-stage rolling. After ferrite transformation, the volume fraction of retained austenite decreased and more volume fraction of bainite was obtained. The microstructure, properties at room temperature and properties at elevated temperature were analyzed. They all met the design standard of 460 MPa grade steel. The yield ratio of two experimental steels was lower than 0. 80 and this indicated that they had superior seismic-resistance. It could be found that constructional steels with excellent properties at room temperature could be obtained by either one-stage rolling or two-stage rolling. The yield strength at 600 ℃ of the one-stage rolling experimental steel was about 402. 5 MPa, and that of the two-stage rolling experimental steel was about 294. 1 MPa. The former was ~ 108. 4 MPa higher than the latter. During the tempering process at 600 ℃, a large amount of large-size alloy cementite was formed. Meanwhile, the Young's modulus of the one-stage rolling experimental steel was about 104. 6 GPa, which was significantly higher than 87. 5 GPa of the second-stage rolling experimental steel. By statistically calculating the geometrically necessary dislocation density of the experimental steels after holding at 600 ℃ for 3 h, it could be seen that the experimental steel by one-stage rolling with a higher volume fraction of bainite has a significantly higher dislocation density at 600 ℃ than the experimental steel by two-stage rolling. By calculating the contribution of strengthening, it could be seen that the contribution of dislocation strengthening at 600 ℃ of the one-stage rolling experimental steel was about 141. 7 MPa, while the second-stage rolling experimental steel was only about 91. 7 MPa, which indicated that bainite has better thermal stability. The higher the volume fraction of bainite, the higher the dislocation density and Young's modulus at 600 ℃. The contribution of dislocation strengthening was proved to be the most important reason for the difference of fire-resistance.