| Mg O-C refractories have been widely used as the lining of basic oxygen furnaces, electric arc furnaces, refining furnaces and the slag line of steel ladles because of their outstanding thermal shock resistance and slag resistance. So, their development plays a decisive role on the technical progress of ironmaking and steelmaking industry. With the progress of ironmaking and steelmaking technology with high efficiency and low cost and requirements of energy saving and resource consumption reducing, carbon content in Mg OC refractories must be controlled effectively and such refractories have the excellent comprehensive performances. Firstly, as for Mg O-C refractories containing more than 12 wt% carbon content used in the severe locations, their properties need to be further improved to achieve their high efficience and long sevice life. Secondly, the investigations on low carbon Mg O-C refractories(lower than 8 wt% carbon content) and ultra-low carbon Mg OC refractories(lower than 3 wt% carbon content) have important practical significance. Generally, the thermal shock resistance of Mg O-C refractories is improved by increasing the content of flaky graphite, but their oxidation resistance is deteriorated; meanwhile, the anisotropy of such refractories becomes more remarkable due to the orientation of flaky graphite in the manufacturing process. Also, the mechanical strengths of Mg O-C refractories are often enhanced via the incorporation of the additives, but their toughness and thermal shock resistance decreased. Furthermore, the thermal shock resistance of Mg OC refractories is evaluated only by measuring the residual strength and calculating the residual strength ratio after thermal shocks.Based on the problems above, in this thesis, the relationship between the mechanical properties of Mg O-C refractories and flaky graphite content was firstly investigated, and effect of flaky graphite on thermal shock resistance of such refractories was also presented to provide the reference for researching and developing Mg O-C refractories with various carbon content. Then, in order to study the relationship between the particle shapes of carbon source and mechanical properties and thermal shock resistance of Mg O-C refractories, the particle-like graphite was incorporated into Mg O-C refractories containing 10 wt% graphite to partially and fully replace flaky graphite due to the fact that its addition can dramatically decrease the orientation of flaky graphite in such refractories, which can provide theoretical guidance for optimizing high carbon Mg O-C refractories containing more than 12 wt% carbon content. Furthermore, low carbon Mg O-C refractories containing less than 8 wt% carbon content were prepared with the addition of nanocarbon sources(such as graphene oxide nanosheets, etc), and their effect on the microstructure, mechanical properties and thermal shock resistance of such refractories was inveatigated to reveal the synergetic strengthening and toughening mechanism of nanocarbon and in-situ formed ceramic phases. Based on the work above, effect of metallic Al and Si powder on the microstructure and mechanical properties of ultra-low carbon Mg O-C refractories containing less than 3 wt% carbon content was studied with the incorporation of Ni containing catalyst loaded magnesia powder, and the catalytical formation of nanocarbon structure and ceramic phases in the refractories as well as their formation mechanisms were analyzed. Finally, the fracture behavior and crack propagation of Mg O-C refractories were studied with the aid of wedge splitting test, fractal theory and fractographic analysis to evaluate quantitatively their thermal shock resistance; the correlations between the microstructure and thermal shock resistance were further revealed. On the basis of the work above, the main conclusions can be made as follows:1. As for Mg O-C refractories containing different flaky graphite content, the increase of flaky graphite content enhanced carbon network of Mg O-C refractories, and thus increased their maximal fracture displacement; Mg O-C refractories containing 18 wt% flaky graphite had the best thermal shock resistance. Also, Al4C3, Al N, Mg Al2O4 and Si C formed in all the Mg O-C refractories after treating at elevated temperatures; much more insitu formed ceramic phases were observed for the refractories after firing at 1400 ℃ compared to those after firing at 1000℃, which led to the improvement of their mechanical strength and the decrease of their maximal fracture displacement.2. In case of Mg O-C refractories containing particle-like graphite(10 wt% graphite), the addition of particle-like graphite decreased considerably the anisotropy of their coefficient of thermal expansion and thermal conductivity, but increased their fracture displacement, which led to the fact that their thermal shock resistance was close to or better than that of Mg O-C refractories containing 14 wt% flaky graphite.3. Concerning Mg O-C refractories containing nanocarbon sources(suah as carbon black(CB), carbon nanotubes(CNTs), graphene oxide nanosheets and expanded graphite), the incorporation of different nanocarbons promoted the in-situ formation of much more Al4C3 and Al N in the refrectories, and thus led to the improvement of their mechanical properties. Compared with Mg O-C refractories containing 5 wt% flaky graphite, such refractories containing different nanocarbons had better thermal shock resistance. Also, the addition of either CNTs or CB contributed to a higher level of residual flexural strength and residual strength ratio even though it had only an amount of 5 wt% carbon, which was close to thermal shock resistance of Mg O-C refractories with 10 wt% flaky graphite.4. With respect to Mg O-C refractories with Ni containing catalyst, the addition of such catalyst can be favorable for the growth of Mg O and Mg Al2O4 whiskers as well as CNTs and carbon onions, and improved considerably their mechanical properties and thermal shock resistance. In comparison with Mg O-C refractories with metallic Al or Si powder as the additives, much more whisker-shape ceramic phases(such as Si C whiskers and hollow spinel whiskers) formed in Mg O-C refractories containing metallic Al and Si powder, which led to their better mechanical strength and thermal shock resistance.5. The results from wedge splitting test, fractal theory and fractographic analysis demonstrated that the incorporation of nanocarbon sources led to a more tortuous crack path during wedge splitting test of Mg O-C refractories, and increased their specific fracture energy, characteristic length and thermal shock resistance parameter, which improved their thermal shock resistance. In all the investigated compositions, the best thermal shock resistance was for Mg O-C refractories containing CNTs. The correlation analysis showed that the thermal shock resistance of Mg O-C refractories was positive correlative with their specific fracture energy and the interface crack propagation, while was negative correlative with the modulus of elasticity, thermal expansion coefficient and the transgranular crack propagation. |
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