Influence of Fused CaZrO3 Addition on Properties of Chrome-free Castables for RH Degassers

This thesis considered specific operating conditions of RH degassers, using corundum and spinel as the main raw material, adding proper micropowder and the efficient compound water reducing agent, discussed the influence of fused CaZrO3 addition on RH degassers used chrome-free castable’s properties, which includes the castable’s strength, permanent line rate, wear resistance and performance of the slag resistance. The results showed: (1) Adding proper fused CaZrO3 can improve the strength of castable and wear resistance;(2) With the increase of the content of CaZrO3, volume density of castable declined slightly, apparent porosity increases, the line rate increased obviously;(3) With the increase of the amount of CaZrO3, slag erosion resistance of castable declined;(4) From the perspective of the comprehensive performance of castable, fused CaZrO3 suitable addition amount of 2 ∼ 4%.


Introduction
RH degassing vacuum cycle method is more popular among steel plants with its advantages as good degassing performance, taking off less heat from molten steel, wide application scope, flexible and reliable operation compared with a variety of other refining outside the furnace method [1].Therefore, RH vacuum refining is widely applied by big steel plants. However, the demanding environment of RH refining forces its lining to use the direct binding or combine magnesite chrome brick, but using magnesite chrome based refractory will produce poisonous hexavalent chromium. Thus, chrome-free refractory materials are widely researched in present domestic universities, enterprises and research institutes. The general research is in corundum-spinel system since this system of materials is of high strength, good abrasion resistance, high temperature volume stability , excellent thermal shock resistance and good resistance to slag erosion, etc [2]..But the class and conditions of iron and steel smelting are complex in RH degassers. Materials of corundum-spinel system is no longer the ideal material facing the great thermal stress change and high basicity (R quartile 2.5) of the molten slag erosion. Therefore, it becomes the focus of attention as how to make corundum-spinel chrome-free material that adapts to RH refining environment, especially with higher resistance to erosion of high basicity slag. The characteristics of fused calcium zirconate are microstructure uniformity, high strength, good resistance to chemical and mechanical attack. Pollution free and environmental friendly, it becomes ideal alternative raw materials to magnesium chromium, magnesium and aluminate spinel [3].This paper based on corundum-spinel system, using fused CaZrO 3 and as the research object, purposed to improve its abrasion resistance, spalling resistance and resistance to high basicity slag erosion, thereby to improve the service life of chrome-free refractory in RH degassers.

Raw materials in experiment
Using densely corundum ( grain size of 5~3 mm ), 3~1 mm, 1~0 mm, ≤0.074 mm fused white corundum, 1~0 mm, 325 mesh fused spinel, CA-80 cement, active alpha Al 2 O 3 micro powder as the main raw materials, added 325BS fused CaZrO 3 , the physical and chemical indexes are shown in table 1.  sintering, a small amount of CaO and Al ℃ 2 O 3 reaction to generate CA6 in matrix, the compactness of material increase, the microstructure was optimized.So when CaZrO 3 addition amount was 4%, the strength of the specimens significantly higher than that of the original, but CaZrO 3 in 4-10% range, along with the increase of the content of CaZrO 3 ,CaO content is increased, so the generated CA6 volume also increased significantly, leading to sample, loose structure, expansion, resulting in significantly reduced intensity [4]. From the perspective of the intensity of the sample, addition amount of 4% of CaZrO 3 achieve more ideal effect.

Influence of CaZrO 3 on castables' permanent linear change
As seen in figure 2, under 110 and 1100 , with the increase of the content of CaZrO ℃ ℃ 3, permanent line rate of castables is not obvious, but the sample under 1500 , with the increase of the content of ℃ CaZrO 3, permanent linear change of castables is obvious increase,the main reason is that under the condition of high temperature, along with the increase of the content of CaZrO 3, CaO content increased, so the generated CA 6 volume also increased significantly, causing sample quantity increase.From the permanent line rate of castables, when the addition amount of 2% of CaZrO 3 , sample volume is more stable.

Figure 2. Effect of CaZrO 3 on permanent linear change of castables 3.3 Influence of CaZrO 3 on castables' wear-resisting
According to the national standard, set the 100×100×30 mm standard sample's test surface vertically to sand blasting tube, blow the wear medium (black silicon carbide) through the gush arenaceous pipe on to sample using compressed air and then measure each sample's wearing volume. Through calculation, the wear-resisting results is shown in figure 3. Permanent linear change/% Figure 3. Influence of CaZrO 3 on castables' wear resistance As seen from the figure 3, for the amount of CaZrO 3 from 2% to 4% range, its wear resistance is better than the same CaZrO 3 (0%);When the content of CaZrO 3 was 4%, the wear resistance of sample is best, its wear resistance increases by about 33.5% than before.Main reason is the system of a small amount of CaO at high temperatures and matrix of Al 2 O 3 reaction CA 6 , its microstructure was optimized, thus improving strength and wear resistance increased. But the content of CaZrO 3 in 6%~10% range, along with the increase of the content of CaZrO 3, CaO content is increased, so the generated CA 6 volume also increased significantly, leading to sample, loose structure, expansion, resulting in wear resistance is decreased obviously.From the point of abrasion resistance index of the sample, CaZrO 3 addition amount of 2%~4%. It can be seen clearly from the figure 4, when adding 2%CaZrO 3 , its best slag resistance, erosion depth of only 1.0 mm, the main reason has two: (1) Small amount of CaO and Al 2 O 3 reaction to generate CA 6 in matrix, the microstructure of the material was optimized, so as to improve the slag resistance of sample;(2) ZrO 2 and CaO itself high temperature resistance, chemical stability, good resistance to alkali slag erosion performance [5].However, can be seen from the figure 4, when the amount of CaZrO 3 from 6% to 10% range, with the improvement of CaZrO 3 content, sample volume expansion, internal structure is loose, porosity, bulk density is reduced, resulting in resistance to slag erosion resistance showed a trend of decrease.But in general, when adding 2%CaZrO 3 , the sample has good resistance to slag erosion resistance.

Conclusions
(1) Adding 2%~4% CaZrO 3 can improve the strength of castables and wear resistance performance; (2) With the increase of the content of CaZrO 3 sample volume density is declined slightly, apparent porosity is increased, the permanent linear rate is increased obviously; (3) With the addition of more CaZrO 3 , slag corrosion resistance of castables is declined; (4) From the perspective of the comprehensive performance of castable, fused CaZrO 3 suitable addition amount is 2 ~ 4%.