Designing environment‐friendly chromium‐free Spinel‐Periclase‐Zirconia refractories for Ruhrstahl Heraeus degasser

Abstract

The Ruhrstahl Heraeus (RH) degasser (Figure 1) is the globally preferred equipment to manufacture high quality vacuum treated steel purified to ppm level of C, H, and N. 1-3 Examples of such steel are ultra-low carbon steel (<30 ppm C) for lightweight automobile applications, high carbon manganese steel (<1.5 ppm H 2 , <40 ppm N 2) for crack resistant railway tracks and electrical steel for transformer cores. Faster operation and the ability to perform gas removal, decarburization, desulphurization, and alloy addition with precise composition control makes RH degassers superior to other metallurgical equipment like the vacuum arc degasser. This paper explores refractories for use in a RH degasser which is used to manufacture steel consumed primarily by the automotive industry, one of the world's largest industries in terms of revenue. The automotive steel market is worth US$

Use of dilatometer to screen refractory raw materials

“…However, Figure 2 shows when rutile is used (light red curve), firing expansion is 4.4%, but when anatase is used in Figure 6 (green curve), the expansion reduces to 3.3%. In a refractory brick, such small differences in expansion can push the coarse (1–3) mm particles apart exacerbating the pore formation to even above 28% 5 . Figure 6 also shows, though the final expansion of MgAl 2 O 4 is double that of Mg 2 TiO 4 , when their raw materials are mixed together, the firing expansion is much lower than both which could be due to the heterovalent substitution 2Al 3+ → Mg 2+ + Ti 4+ forming complex qandilite‐spinel solid solution, which affected the overall Mg 2 TiO 4 ‐phase formation mechanism and the associated expansion 10 .…”

“…For example, choosing between two clays with different free quartz and gibbsite content, or two bauxites, one diaspore‐rich, the other gibbsite‐rich 4 . It might also be choosing between the TiO 2 polymorphs rutile and anatase to form Mg 2 TiO 4 , which is an emerging spinel‐like refractory, 5 or it could be the selection between kyanite and sillimanite for a particular mullite‐rich brick. The options in each case would have different amount of expansion, different temperatures at which they expand, and different sintering temperature, each of which can create warpage during firing or produce high porosity. Finally, higher cost of excellent refractory raw materials like spinel and mullite might not be justified for a low‐cost product, and hence in‐house, cost‐effective reactive spinel and mullite could be developed.…”

Alumina‐spinel castable for steel ladles: An overview

“…Magnesia chromecontaining refractories were initially used in the degassers, but due to the health hazards of chrome-containing refractories, the magnesium aluminate-spinel-based refractories have replaced them. 111 It has been observed that silica-free alumina-spinel castables with high thermal shock and corrosion resistance are highly successful in RH degasser applications. 19…”

“…Vacuum degassing techniques are part of the secondary refining of steel and becoming very common for steelmaking processes nowadays to meet the demands for higher grades and clean steel. Magnesia chrome‐containing refractories were initially used in the degassers, but due to the health hazards of chrome‐containing refractories, the magnesium aluminate–spinel‐based refractories have replaced them 111 . It has been observed that silica‐free alumina‐spinel castables with high thermal shock and corrosion resistance are highly successful in RH degasser applications 19 …”

Impact on aggregate/matrix bonding when a refractory contains zinc aluminate instead of spinel and magnesia‐chrome

“…The aggregate/matrix bonding interface is considered the weakest link in a castable microstructure and greatly influences its mechanical performance 3 . However, a good aggregate/matrix bonding increases the high‐temperature three‐point bending strength also called the hot modulus of rupture (HMOR) and is directly correlated with erosion resistance in service 4 . A well‐engineered matrix also resists slag corrosion and increases the overall life of a refractory.…”