Interaction with steel of refractories containing oxygen-free additions

Abstract

Refractories containing in their composition carbon and oxygen-free compounds such as carbides, nitrides, borides, etc. are acquiring increasing practical value [i].Present and future areas of their use include the linings of converters and electric arc furnaces, refractories for ladle gate valves and teeming of steel in continuous billet casting machines, etc.In their interaction with molten metals these refractories exhibit specific features, some of which are discussed in this article.As a first approximation as the result of the high surface activity of oxygen and sulfur in it (their surface activity may exceed the content by tens of times) the surface of molten steel may be represented in the form of the highly electronegative anions 0 =-and S =-in coordination with the cations Fe 2+, Mn =+, etc.In interaction with the oxidizable elements of the refractories the anions form intermediate products all the way to oxides, including gaseous ones.The corrosion of carbon-containing refractories under the action of steel under equilibrium conditions may be represented in three stages [2]:at the steel--refractory contact under the action of surface-active oxygen, oxidation of the carbon occurs and a decarburized zone is formed; the decarburized zone with increased porosity rapidly forms a slag and the products of the interaction have decreased refractoriness, mechanical strength, and thermal coefficient of volumetric expansion differing from the original material; for these reasons the slagged layer of the refractory is rapidly washed away by the steel.The increase in metal resistance of a refractory with the addition of oxygen-free additions, particularly carbon, is the result of the following facts: frequently, the carbon addition is made by impregnation or precipitation from the gaseous or liquid phase into the finished refractory and the carbon partially replaces the porous spaces, mechanically preventing penetration of the molten metal; the products of gasification of the carbon pneumatically prevent penetration of the molten metal into the voids; in the microvolumes carbon deoxidizes the molten metal in contact with it and the highly active oxide forms of iron are converted into less active lower oxides all the way to the metallic state, a confirmation of which is the presence of beads of pure iron in the reaction zones of carbon-containing refractories after their wetting even by slag.To the products of gasification of carbon are added the gases entrapped in the void space and the gaseous products of reduction by carbon of the oxides of the refractory occurring according to reactions of the type:(SiO2, AI2Oa, ZrO~, ZrO=.SiO,, MgO era. )ml+Cso~C0g~+SiOg ~ AIOg~ era.These reactions occur both in the contact zone and in the thickness of the refractory with diffusion of CO and gaseous suboxides into the contact zone. In addition, the decomposition of oxygen-free compounds is possible according to reactions of the form SiaN4--+2N2+3Si.All-Union Refractory Institute.

Use of graphite-containing nozzles for vacuum casting of ingots

“…To the products of gasification of the carbon are added the gases liberated from the pores of the refractory and the gaseous products formed as the result of reactions of solid-phase reduction by carbon of the oxides of the refractory [5]:…”

Horizontal continuous casting machine nozzles

“…ib) or the shell does not separate from the refractory in each step of withdrawal and rupture of it occurs at the junction of the fronts or at the junction of the steps (Fig. Ic).The theory of nonwetting by molten metal of a refractory containing carbon and metal nitrides and carbides was presented in a preceding article [5].During experimental use of refractory nozzles of different compositions it was established that the zone of greatest wear in the absence of flow of metal into the joint of the refractory with the mold is the face portion of the nozzle at a distance of 4-6 mm from the mold (Fig. 2).…”

“…Necessary conditions determining the life of the nozzle in a horizontal continuous casting machine are the absence of a gap at the joint of the refractory nozzle with the mold and the presence in the composition of the refractory material of components not wetted by the molten metal (C, BN, SiaN~, etc.) [i, 4,5].…”

Improving the endurance of graphite-containing submerged nozzles for CCM

“…Other materials (nitrides, carbides) that form a gas layer in the pores during oxidation are known [9]. An especially interesting compound is hexagonal boron nirride, which oxidizes at a substantially slower rate than graphite and whose flakes are much finer than those of graphite (for example, in graphite of grade GT the flakes are 0.7 -1.2 mm long and 0.02-0.04 mm wide, whereas the flakes of hexagonal boron nitride are 0.005 -0.01 mm in size).…”