In view of the problems that the yield ratio of 690 MPa air-cooled bainite fire-resistant steel is too high to meet the seismic performance, as well as the impact toughness is too low and the yield strength at 600 ℃ is reduced, two kinds of low-carbon bainitic fire-resistant steels, low-V high Nb + Ti and high-v low Nb + Ti, were designed to obtain high strength and toughness fire-resistant steels with room temperature yield strength greater than 690 MPa, yield ratio less than 0. 85, high temperature yield strength at 600 ℃ greater than 2/3 of room temperature yield strength, which is 460 MPa, and low temperature impact toughness at - 0 ℃ greater than 69 J. During the test, the ingot was heated to above 1 200 ℃ for heat preservation and forged at about 900 ℃. After forging, the two-stage controlled rolling process was adopted. In order to improve the mechanical properties and microstructure of the test steel, the final heat treatment method was adopted. The heat treatment process was normalizing air cooling + tempering air cooling. The normalizing temperature was 30-50 ℃ above AC3, and the tempering temperature was in the range of bainite transformation temperature. The microstructure of the hot-rolled and heat-treated specimens was analyzed by metallography and scanning. The mechanical properties of the tested steel were analyzed by normal temperature tensile test, 600 ℃ high temperature tensile test and - 40 ℃ low temperature impact test. Meanwhile, the influence of Nb, V and Ti on CCT curve was studied. The results show that the F + P transition line is obviously shifted to the right and the bainite transition line tends to be flattened by adjusting the Microalloying elements Nb, V and Ti. The transformation structure changes from original granular bainite to a mixed structure of granular bainite and lath bainite. By increasing V and reducing the content of Nb + Ti, the microstructure of hot rolled bainite is compared. It is found that the M-A islands in the obtained granular bainite are finer and more uniform, which is more conducive to improving the plasticity and toughness of the material. By comparing the microstructure and properties of the heat-treated steel, it is found that by increasing V and decreasing Nb + Ti content, the microstructure changes from granular bainite to the mixed structure of granular bainite and lath bainite, and lath bainite is beneficial to the mechanical properties of the steel. The large granular bainite in Y1 test steel is disadvantageous to the low temperature impact toughness of the test steel. The mixed microstructure of fine granular bainite and lath bainite in Y2 test steel can significantly improve the impact toughness of the test steel. Therefore, in the actual production, it is necessary to obtain the refined lath bainite structure as far as possible in order to make the properties of the material better. At the same time, it is found that the tempering temperature has more obvious effect on the yield strength and less effect on the tensile strength of the test steel compared with the test steel after heat treatment of the same composition. By reasonably adjusting the tempering temperature, it is found that Y1 test steel has the best mechanical properties at 350 ℃, but the high temperature yield strength and low temperature impact toughness are not ideal, while Y2 test steel has the best mechanical properties at 400 ℃. In conclusion, after normalizing and tempering at 400 ℃ for 1 h, Y2 test steel has excellent comprehensive properties, that is, tensile strength is 1 009 MPa, yield strength is 855 MPa, yield strength at 600 ℃ is 481 MPa, which meets the requirements of yield strength greater than 2/3 of yield strength at room temperature, and average impact energy at -40 ℃ is 145 J greater than 69 J, which meets the requirements of 690 MPa grade refractory steel.
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