2021 Vol. 36, No. 3

Display Method:
Research Progress on Multi-Microalloyed Fire-Resistant Steel
Xin Wang, Zhaodong Li, Ke Zhang, Wentao Wang, Zhongmin Yang, Qilong Yong
2021, 36(3): 1-11. doi: 10.13206/j.gjgS20073101
With the development of social economy, the construction industry has put forward higher requirements for the strength of building materials. The microstructure of construction steel for building has developed from ferrite/pearlite to ferrite/bainite, bainite, multiphase and multiscale metastable (M3). Fire-resistant steel is widely used in high-rise and large-span buildings due to its excellent comprehensive properties and good fire resistant safety. The control of fire resistance properties has also evolved from high-cost high Mo (≥0. 40%) plus single microalloyed mode to economical saving Mo (≤ 0. 30%) plus Nb, V and Ti and other composite multimicroalloyed methods. The development history, product types and application engineering of fire resistance steel at home and abroad, as well as theories and technologies to improve the strength and fire resistance of fire-resistance steel were introduced in detail. The differences in the fire resistance properties of different microstructures and microalloy elements were comparatively studied. The research shows that the room temperature and 600 ℃ properties of the composite multi-microalloyed steel are better than that of the single microalloyed steel, and puts forward a new idea of multi-microalloyed nano-carbide precipitated in fire to enhance the fire resistance properties. For Q345-Q690 fire-resistance steel, the differentiated alloy and microstructure design and its hot rolling/heat treatment technology were formed. The microstructure types of different strength grades of fire-resistance steels were observed by scanning electron microscopy (SEM), and the microstructure and mechanical properties of typical multi-microalloyed fire-resistance steel during reheating-loading process were clarified. The distribution, size and quantity of precipitated phases in hot-rolled state, heat-treated state and 600 ℃ tensile state were observed and counted by transmission electron microscopy (TEM), physical and chemical phase analysis and three-dimensional atom probe (3DAP). The fire resistance mechanism of nano-carbide precipitation strengthening and matrix microstructure stability in high temperature were discussed. The study shows the Q345 grade steel plate after relaxation treatment is proeutectoid ferrite and a small amount of bainite/pearlite microstructure, with high fine grain and precipitation strengthening increments, and the Q345 grade steel plate directly subjected to laminar cooling after rolling is full bainite microstructure with high dislocations and solid solution strengthening increments, its yield strength at 600 ℃ can still reach 327 MPa. The Q460 grade steel plate is bainite microstructure, and the microstructure has good high temperature stability at 600 ℃. With the extension of the holding time at 600 ℃, the nano-precipitated phases that a diameter is less than 10 nm increases significantly. Q690 grade steel plate is martensite, metastable austenite and nano-precipitated phase and low carbon high strength bainite microstructure. It has a yield strength of 690 MPa and good elongation at room temperature. After 600 ℃ tensile test, its yield strength is not lower than 2/3 of the standard yield strength at room temperature. The control idea of multi-microalloyed design and nano-precipitation strengthening in fire can realize the fire resistant property of different grades of Q345-Q690 fire-resistant steel.
Effect of Microalloying Elements on Microstructure and Properties of 690 MPa Fire-Resistance Steel
Jingjing Du, Zhongmin Yang, Xin Wang, Yanguang Cao, Zhaodong Li, Wanhua Yu
2021, 36(3): 12-21. doi: 10.13206/j.gjgS20070804
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.
Research and Analysis on Material Properties and Resistance Partial Coefficient of 460FRW Anti-Seismic and Anti-Corrosion Fire-Resistant Steel
Wentao He, Chuhan Liu, Ming Wang, Dongya Nan, Changdong Wu, Peng Liu, Xiaorun Li
2021, 36(3): 22-27. doi: 10.13206/j.gjgS20100210
In order to adapt to the rapid domestic and overseas development of structural steel and to meet the requirements of steel structure on the mechanical properties and special properties of steel plate for construction, Shougang has independently developed 460 MPa antiseismic and anti-corrosion fire-resistant steel(hereafter referred to as 460FRW). In this paper, the 460FRW steel plates of different specifications developed by Shougang are taken as samples for tensile and impact tests. The experimental data are statistically analyzed to research on the yield strength,the partial coefficient of resistance and the design strength value of 460FRW steel, so as to provide the recommended design strength value for the 460FRW anti-seismic and anti-corrosion fire-resistant steel produced by Shougang in engineering design. The 460 FRW steel plate used in the test is provided by Shougang, and the thickness specifications are 10, 14, 20, 28 mm, respectively. According to the strength grade of the steel plate and the section size of the specimen, the yield load of the specimen is between 30% and 60% of the nominal tensile force of the testing machine, which is within the reasonable range of the influence of the flexibility of the testing machine. The testing machine is automatically controlled by computer, which can automatically identify and output the upper and lower yield strength and tensile strength for material uncertainty analysis. The experimental data of yield strength of 460FRW steel are obtained by experiments. Based on the test data, the uncertain statistical parameters of material properties, geometric characteristics and calculation mode of steel plate are further calculated and determined. By synthesizing these three uncertain statistical parameters, the resistance uncertain statistical parameters of 460FRW steel are calculated. Combined with the statistical parameters of load uncertainty, the resistance partial coefficients of 460FRW steel typical members under four load ratios, axial tension, axial compression, eccentric compression (bending moment acting in plane), eccentric compression (bending moment acting out of plane) and steel beam (elastic and plastic failure) are calculated. Through the analysis, when the load ratio ρ = 2. 00, the partial coefficient of elastic failure resistance of steel beam is the highest, which can be used as the statistical analysis value of partial coefficient of resistance of 460FRW steel.
Analysis on the Impact Resistance of Q460FRW for Building Structure at Low Temperature
Zhiming Wang, Shanglin Lyu, Xin Wang, Zhaodong Li, Kun Liu, Longteng Ma
2021, 36(3): 28-33. doi: 10.13206/j.gjgS20061601
In order to verify the design requirements of low temperature impact resistance of Q460FRW steel, its impact toughness at low temperature was tested with SANS pendulum impact testing machine, and the fracture morphology of samples was analyzed. According to GB/T 229—2007 Metallic Materials Charpy Pendulum Impact Test Method, low temperature impact test was carried out on Q460FRW steel specimens of 10 mm and 28 mm at - 20, - 40, - 60 ℃ ambient temperature. The impact energy value AkV was obtained, and the experimental curve of impact strength with temperature was plotted from two series of Q460FRW steel specimens at each low temperature point. The tensile strength, yield strength and elongation of FRW steel at room temperature and low temperature were tested. Scanning electron microscope (SEM, at 1 000 magnification) was used to analyze the fracture morphology of Q460FRW steel specimens in two standard sizes after impact fracture. The results shown that Q460FRW steel can maintain high impact energy and good impact toughness at low temperature. At the same time, the impact toughness increases slightly with the decrease of temperature at -20, - 40, - 60 ℃. The longitudinal impact energy values of the two series Q460FRW were greater than 34 J, according with the requirements of GB/T 19879—2005 Steel Plates for Building Structures, and meeting the design requirements of impact resistance at low temperature. SEM observation and analysis shown that the crack propagates outward from the fiber area in the center of the specimen during impact tension, and a large plastic deformation occurs near the crack area, forming a large shear notch, which indicated that were ductile. There is no significant difference in the size and depth of the fracture dimple from SEM.
Study on the Influence of Microstructure on the Properties of 460 MPa Seismic-Resistant and Fire-Resistant Construction Steel
Jinghua Cong, Xuemin Wang, Jiangwen Li, Ping Du, Fengjuan Wu
2021, 36(3): 34-38. doi: 10.13206/j.gjgS20070801
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.
Research and Development of Anti-Seismic, Corrosion-Resistant and Fire-Resistant Extra Thick Steel Plate for Q460GJEZ35
Jiheng Yuan, Zhongbo Li, Dong Yang, Zhenglei Tang, Yang Yang, Keyi Fu, Yansheng Xue
2021, 36(3): 39-45. doi: 10.13206/j.gjgS20070802
With the construction of more and more super high-rise and large-span building projects, higher requirements are put forward for the high strength, long life and service safety of building structural steel. The development of 460 MPa anti-seismic, corrosion-resistant and fire-resistant composite building steel has broad market application prospects. The steel has the characteristics of high strength and toughness, low yield ratio, lamellar tearing resistance and good weldability, as well as fire resistance and weather resistance. It can reduce the thickness of steel used in high-rise buildings, save the amount of steel, and ensure the long-term bearing capacity of highrise buildings in fire without using fireproof coating. It can be widely used in super high-rise buildings, large factories and other steel structures with high fire resistance and weather resistance. Based on the comparative analysis of the key technologies of 460 MPa anti-seismic, corrosion-resistant and fire-resistant building steel, the process of hot metal KR desulfurization, converter smelting, LF + VD refining, water cooled ingot mold casting of copper plate mould, ingot reheating, rolling mill forming and QLT heat treatment was used for trial production. In the aspect of composition design, the core is the composite microalloying design of fire resistance and corrosion resistance, taking into account its strengthening function, grain refinement function, corrosion resistance function and nano precipitate precipitation behavior. In order to improve its welding performance, the carbon equivalent is controlled within 0. 55, and the composite functions of high strength, toughness, fire resistance and corrosion resistance are realized by reasonable proportion of Cu, Cr, Ni and Mo. At the same time, in order to reduce the cost, the precious alloy element molybdenum is controlled below 0. 30%. In the aspect of molten steel smelting, ultra-low P, ultralow S, ultra-low O and high purity of steel can be achieved through fine material feeding and pure steel smelting technology. The original billet was formed by an improved water-cooled ingot mold with copper plate mold, and the micro defects in the ingot were avoided by directional solidification technology. In the rolling process, the surface hardening is promoted by hard shell rolling, so that the rolling force can penetrate into the center of the steel plate, and the microstructure can be refined uniformly while improving the internal quality. In the heat treatment process, the proper combination of hard and soft phases is achieved by quenching in the two-phase zone, so as to reduce the yield strength ratio, refine the grains and improve the impact toughness. Through the process design of composition design, converter smelting, water cooling mold casting, rolling and off-line heat treatment, 460 MPa anti-seismic and anti-corrosion refractory steel plate was developed. Its yield strength is controlled at 500- 520 MPa, tensile strength is controlled at 630-650 MPa, elongation is 24% -27%, yield ratio is 0. 79-0. 81, average reduction of area in thickness direction is 70%, and its yield strength is 339-367 MPa after holding at 600 ℃ for 3h. The longitudinal impact energy at 40 ℃ is more than 200 J, and the atmospheric corrosion resistance index is more than 7. 5. The maximum thickness of steel plate is 150 mm, the maximum width is 3 800 mm and the maximum length is 120 000 mm.
The Influence of Rolling Temperature on Mechanical Properties of Weathering Resistant Hot-Rolled H-Section Steel
Meng Xia, Hui Chen, Jie Wang, Lin Peng, Junwei He, Jun Xing, Jingcheng Yan
2021, 36(3): 46-51. doi: 10.13206/j.gjgS20031202
According to the deformation characteristics of hot-rolled H-section steel that flange thickness deformation concentrated in the universal rolling stage, the opening rolling temperature of universal stage was set at 800-1 000 ℃, and the other main technological parameters remained unchanged. Through the mechanical property test and microstructure comparison analysis of hot-ralled H-section steel, The result indicated the grain size and shape of ferrite have a significant effect on the mechanical properties of hot-rolled Hsection steel, and the opening rolling temperature of universal stage has a significant effect on the grain size and shape of ferrite. For 1 000-950 ℃ opening rolling temperature, dynamic recrystallization of austenite could be completed, but the initial air cooling after rolling was 900-850 ℃ and recrystallization grain growth was rapid and also easy to grow abnormally. For 1 000 ℃ opening rolling temperature, ferrite grain size was differ and there was an obvious mixed crystal. When the temperature decreased to 950 ℃, mixed crystal situation has improved, but it still cannot eliminate. When the temperature decreased to 900 ℃, austenite dynamic recrystallization could be completed and the initial air cooling temperature decreased to 800 ℃. The growth of recrystallized grains was inhibited, and the small and uniform initial austenite structure was formed. At this time, the ferrite grains were 10-30 μm at equiaxial shape. When the temperature further reduced to 850-800 ℃, the thermal activation energy required to promote dynamic recrystallization of austenite cannot be met, so the deformation only appeared in the unrecrystallized zone, and flat ferrite grain was formed with the ratio of the long axis to the short axis being nearly 2 ∶1. The size of the long axis was not significantly reduced, but the size of the short axis was further reduced. Because of this, as the open rolling temperature decreased from 1 000 ℃ to 900 ℃, the ferrite grain boundary area was increased with the decrease of grain size, reduced the stress concentration degree, and increased the ability of distribution with the instantaneous deformation. The product yield strength increased from 369 MPa to 415 MPa. The tensile strength increased from 508 MPa to 546 MPa, the percentage elongation after fracture increased from 30. 0% to 31. 5%, the impact energy at low temperature increased from 36 J to 99 J. When the temperature decreased to 850-800 ℃, flat ferrite grain further increased the grain boundary area, made the product yield strength and tensile strength further to 468 MPa and 567 MPa respectively. When the distortion increased due to coordinate rotation between different size grains in plastic deformation, the percentage elongation after fracture decreased to 27. 0%, the impact energy at low temperature increased to 109 J, and the yield to tensile ratio reached 0. 83. In view of the fact that the lower rolling temperature affects the production output, and considering the universal mill load, energy consumption and economic factors, 900-850 ℃ is an ideal open rolling temperature range. At this time, not only the strength and plasticity indicators of the product are maintained at a relatively high level, but also the toughness index has been greatly improved. The comprehensive mechanical properties of weathering resistant hot-rolled H-section steel have been significantly improved.
Atmospheric Corrosion of Weathering Steels for Assembly Steel Structure
Jianwen Fan, Guoqiang Sun, Yonggang Gao, Ying Chen, Qicheng Wang, Han Dong
2021, 36(3): 52-57. doi: 10.13206/j.gjgS20021001
Assembly steel structure buildings with characterization of energy saving, environmental protection and source recycling, are in line with the future development direction of Chinese buildings. But, steel corrosion protection have to be concerned in the steel structure design since ordinary steels are susceptible to corrosion. The high cost of rust cleaning and air and soil pollution caused by powders during rust cleaning, organic chemicals or deposited metals of corrosion protection coatings, restrict the development of steel structure buildings. Because of cost saving and reduction of environmental pollution, the research and development of new self corrosion protection steels without additional coatings is of great significance to the development of steel structure buildings. Therefore, Q390 weathering and anti-seismic hot-rolled H-section steel named Q390NHD, was developed with the addition of Ni, Cr and Cu alloy elements to meet the requirements of atmospheric corrosion resistance, and with vanadium micro-alloying, V (C, N) precipitation strengthening during cooling process after hot rolling, combined with the characteristics of section steel hot rolling process. The corrosion resistance was studied by periodic immersed comparison test, and corrosion problem during long term atmospheric exposure was analyzed, and the feasibility of the use of Q390NHD steel without coatings in the assembly steel structure buildings was demonstrated. The chemical contents of Q390NHD section steel are mainly composed of m(C) ≤ 0. 12,m(Si) ≤ 0. 50,m(Mn) ≤ 1. 50,0. 20≤ m(Ni) ≤0. 65,0. 30≤m(Cr) ≤1. 25,0. 20≤m(Cu) ≤0. 55,m(V) ≤0. 12 and so on. The mechanical properties of Q390NHD steel are as follow, lower yield strength ReL is above 400 MPa, tensile strength Rm was botween 560-600 MPa, yield ratio was not more than 0. 80, elongation with short proportional specimens not less than 25%, and V-notch impact test energy at - 20 ℃ was not less than 34 J/cm2, which meet the requirements of anti-seismic design of steel structure parts assigned in Chinese standard GB 50017—2017 Standard Design of Steel Structure. The corrosion rate of Q390NHD steel for the 72h period immersed test was 1. 665g/(m2·h), 47% of ordinary steel's, which meant the former corrosion resistance was more than 2 times of the latter, equivalent to that of 09CuPCrNiA steel. According to ISO 11844-1 ∶2006 standard, the 70 year total corrosion thickness of Q390NHD steel is not more than 0. 031 mm at inner corrosivity class IC3, less than 0. 3 mm at the more severe class IC4, and below 0. 7 mm at the most severe class IC5. So, the new developed steel, Q390NHD, can be used without coating in steel structure buildings for common offices and residences in most part of China.