Abstract

Refractory products such as ladle shrouds, monoblock stoppers and submerged entry nozzles find their application in the continuous casting of steel. They are responsible for the steel flow control and its protection against oxidation. During casting, refractories are subjected to severe thermal, mechanical and chemical conditions which may have a deleterious impact on their lifetime. Thus, seeing a need for properties improvement of these materials, the main goal of this work was to investigate the evolution of carbon-bonded refractories during firing and thereby understand their mechanical properties. The particular emphasis was put on the influence of additives, the so-called antioxidants.

Model materials having simplified composition compared to the real industrial ones were investigated in order to facilitate the comprehension of interactions between different constituents (alumina, graphite, binder, additives). The influence of four different antioxidants such as boron carbide, a metallic additive and two low melting point compounds were studied.

The evolution of Young’s modulus during firing was followed thanks to an ultrasonic pulse echography device. Damage occurrence within the material was registered by the acoustic emission technique. X-ray diffraction measurements with Rietveld refinement enabled identification and quantification of phases after firing. Stress-strain curves were obtained by tensile testing at room temperature.

Obtained results show that the carbonaceous binder undergoes transformation into pyrolytic carbon and that micro-cracks in material’s matrix are created during firing. It results in a non-linear stress-strain behavior. What is more, boron carbide transforms partially into boron oxide and carbon which does not have important influence on the mechanical properties. The metallic additive reacts with surrounding carbon to form carbides which rigidify the refractory. One of the low melting point compounds forms a liquid phase at high temperature which causes micro-cracks healing thereby leading to a stiffening of the material. The second one undergoes irreversible softening and only slightly modifies the mechanical properties.

Antioxidants are added to carbon-bonded refractories so as to protect all forms of carbon from oxidation. Nevertheless, this study showed that even a small quantity of certain additives might significantly modify their mechanical properties and thus have an influence of their reliability and performance.