Influence of physicochemical properties of slags on steel cleanliness during refining and continuous casting processes

Abstract

Non-metallic inclusions, which are potential causes for defects in steel products as well as nozzle clogging during continuous casting, must be removed. Non-metallic inclusions in molten steels can be removed through dissolution into molten slags, and then it is necessary to investigate the physicochemical properties, such as the viscosity and saturation limit for absorbing alumina inclusions, from secondary refining slags.

The viscosity-structure relationship of low-silica calcium aluminate with (or without) CaF2 and iron-containing calcium aluminate melts, which represent typical ladle furnace (LF) and Ruhrstahl-Heraeus degasser (RH) slags, respectively, was investigated using the rotating-cylinder method in conjunction with Raman spectroscopy for analysis of the molecular structure of aluminosilicate melts. The viscosity of low-silica calcium aluminate melts decreases by increasing both CaO/Al2O3 (C/A) and CaO/SiO2 (C/S) ratios. The effect of C/A ratio is larger than that of C/S ratio on the viscosity of low-silica calcium aluminate melts. Because the silicate structural units with various non-bridged oxygen (NBO) are located at the boundary of aluminate networks, it was demonstrated that the increase in C/A ratio effectively modifies the aluminosilicate networks rather than the increase in C/S ratio by employing Raman spectroscopy. Furthermore, it is necessary to understand the effect of CaF2 addition on the viscosity of low-silica calcium aluminate melts, because fluorspar (CaF2 > 98%) is widely used to LF slags. The addition of CaF2 to the low-silica calcium aluminate melts decreases the viscosity of the melts. When the small amount of CaF2 (~5 mass pct) was added to the melts, the CaF2 addition modifies the aluminate networks by the liberation of [SiO4] units from the aluminate networks. However, the addition of CaF2 has less effect on modification of the aluminosilicate networks in the high CaF2 content region (≥ 10 mass pct).

The viscosity of iron-containing calcium aluminate melts decreases by increasing both C/A ratios from 1.0 to 2.0 and total iron content in the melts. In the low iron oxide region, the increase in C/A ratios effectively decreases the viscosity of the iron-containing calcium aluminate melts whereas it rarely decreases the viscosity of the melts with high iron oxide content. The addition of iron oxide to calcium aluminate melts effectively decreases the viscosity of the melts with C/A ratio = 1.0 and 1.3 because it modifies the polymerization of the aluminate networks. On the other hand, the addition of iron oxide to the melts with C/A ratio = 1.6 and 2.0 has less of an effect on the modification of the viscosity of the melts. This is because the polymerization of the aluminate networks is rarely changed in C/A = 1.6 and 2.0 system. Consequently, it was demonstrated that the viscosity of the typical LF an RH slags is strongly dependent on the polymerization of aluminate networks.

Furthermore, the inclusion removal rate of the molten slag was evaluated by considering the interfacial reaction between molten steel and slag layer. The total oxygen content in the steel decreases with reaction time as soon as aluminum is added to the steel melt, which indicates that the alumina-rich inclusions are removed into the molten slag. The apparent rate constant of oxygen is depending on the ratio between concentration difference for absorbing the alumina-rich inclusions (∆C) and the viscosity (η) of the molten slag, i.e., the ∆C/η ratio. However, the apparent rate constant of oxygen is not fully proportional to the ∆C/η ratio, i.e. physicochemical properties, of the slags because the oxygen pick-up is continuously transferred from the iron-containing calcium aluminate slags by the interfacial reaction between molten steel and slag layer when total iron content in the slag is above 10 mass pct at C/A = 2.0. The alumina-rich inclusions in the steel sample were observed using SEM combined with automatic feature analysis (AFA) to characterize the inclusions with reaction time and slag composition. It was revealed that the inclusion removal rate, which is obtained from the decrease in the weight of alumina-rich inclusions, is strongly proportional to the physicochemical properties of the slag.

The addition of rice husk ash (RHA) in tundish flux causes severe reoxidation of molten steel and magnesia refractory corrosion because it generates the self-dissociation of silica by the interfacial reaction between the molten steel and slag layer due to the increase in the silica activity of slag layer. However, the increasing the amount of calcium aluminate-based flux (CA-flux) suppresses the silicon and oxygen pick. The number of inclusions formed by reoxidation in the molten steel was reduced when the ratio of CA-flux to sum of RHA and CA-flux (CA-flux/[RHA + CA-flux] = RCA) is above 0.8. The evolution of inclusions was investigated by comparing experiment results with predictions from a refractory-slag-metal-inclusion (ReSMI) multiphase reaction simulation. Consequently, the formation of alumina inclusions in tundish was investigated using ReSMI simulation by considering the temperature drop of the molten steel in the ladle during a casting procedure. The temperature drop of the steel melts in ladle and the decrease in RCA progress the formation of alumina inclusions in the steel melts at the end of tundish. Therefore, it is necessary to control RCA in tundish and the physicochemical properties of slags in ladle during a casting procedure.

|철강제품의 품질 하락과 연속주조 시 노즐 막힘을 유발하는 비금속 개재물은 반드시 제거되어야 한다. 용강 내 비금속 개재물은 용융 슬래그로 흡수되어 제거되므로, 점도와 알루미나 개재물을 흡수할 수 있는 포화용도 등과 같은 이차정련 슬래그의 물리화학적 특성을 연구하는 것은 매우 중요하다.

래들 정련로 (Ladle Furnace, LF)와 Ruhrstahl-Heraeus degasser (RH) 공정에서 대표적으로 생성되는 CaF2를 함유하는 low-silica calcium aluminate 와 산화철을 함유하는 calcium aluminate 슬래그의 점도와 구조 사이의 상관관계를 각각 점도 측정을 위한 rotating cylinder 방법과 분자 단위의 구조를 분석하기 위한 라만 분광법을 활용하여 연구하였다. CaF2를 함유하는 low-silica calcium aluminate 슬래그 (LF 슬래그) 내 CaO/Al2O3 (C/A) 와 CaO/SiO2 (C/S) 비가 증가할수록 LF 슬래그의 점도는 감소한다. 슬래그 내 C/A 비를 변화하는 경우가 C/S 비를 변화하는 경우보다 슬래그의 점도 변화에 미치는 영향이 더 크게 나타난다. 다양한 개수의 non-bridged oxygen (NBO)와 결합하고 있는 silicate 구조 단위는 aluminate networks 주위에서 aluminate와 결합하고 있기 때문에, C/A 비를 변화는 경우가 C/S 비를 변화는 경우보다 aluminate networks를 더 효율적으로 해리 시킨다는 것을 확인하였다. CaF2를 98% 이상 함유하는 fluorspar는 LF 슬래그에 통상적으로 많이 사용되므로, CaF2 함량변화가 LF 슬래그의 점도거동에 미치는 영향을 이해하는 것은 필요하다. LF 슬래그에 CaF2를 첨가하면 슬래그의 점성은 감소한다. 약 5 mass% 정도의 CaF2를 슬래그에 첨가할 경우, aluminate networks로부터 [SiO4]가 자유롭게 분리되어 aluminate network의 중합도는 크게 감소한다. 하지만, CaF2를 10 mass% 이상 다량으로 첨가할 경우 aluminate 구조 변화에 미치는 영향은 미미하여 슬래그의 점도 변화는 크지 않다.

산화철을 함유하는 calcium aluminate 슬래그 (RH 슬래그) 내 C/A 비를 1.0에서 2.0까지 그리고 철함량을 증가할수록 RH 슬래그의 점도는 감소하다. 슬래그 내 산화철 함량이 낮은 구간에서 슬래그의 C/A를 증가할 경우 RH 슬래그의 점도는 크게 감소하는 반면, 슬래그 내 산화철 함량이 높은 구간에서는 C/A 비 변화가 RH 슬래그의 점도 감소에 미치는 영향은 미미하다. 슬래그 내 C/A 비가 1.0과 1.3인 경우, 슬래그 내 산화철 함량의 증가는 슬래그의 aluminate network를 해리 시키므로, 슬래그의 점도는 급격하게 감소한다. 하지만, 슬래그 내 C/A 비가 1.6과 2.0인 경우, 슬래그의 점도 감소에 미치는 산화철 첨가의 영향은 거의 일정하다. 이는 C/A 비가 1.6과 2.0인 슬래그에 산화철 함량 증가가 슬래그 내 aluminate networks의 중합도를 거의 변화시키지 않았기 때문이다. 결론적으로, 일반적인 LF와 RH 슬래그의 점도는 슬래그 내 aluminate networks의 중합도에 크게 의존한다는 것을 확인하였다.

용융 슬래그의 개재물 제거 속도를 용강과 용융 슬래그 사이의 계면 반응을 고려하여 측정하였다. 알루미늄을 용강에 첨가한 이후 용강 내 총 산소함량은 시간에 따라 감소한다. 이는 용강 내 존재하는 알루미나 개재물들이 용융 슬래그로 흡수되어 제거되었다는 것을 의미한다. 용강 내 산소변화의 겉보기 속도상수는 용융 슬래그가 알루미나 개재물을 흡수할 수 있는 농도구배 (∆C)와 용융 슬래그의 점도 (η)의 비, 즉 ∆C/η, 에 의존함을 확인하였다. 그러나, C/A = 2.0에서 철함량이 10 mass pct 이상 슬래그에 첨가되었을 때 용강과 슬래그 사이의 계면반응으로 인해 슬래그로부터 산소가 지속적으로 용강으로 유입되었기 때문에, 산소변화의 겉보기 속도상수는 슬래그의 ∆C/η, 물리화학적 특성, 에 비례하지 않고 오히려 감소하였다. 반응시간과 슬래그의 조성에 따른 용강 내 알루미나 개재물의 변화거동을 자동형상분석 (AFA)기능을 장착한 SEM을 활용하여 관측하였다. AFA 결과를 활용하여 용강 내 알루미나 개재물의 양 변화를 환산하고 개재물의 제거속도를 평가한 결과, 용강 내 개재물의 제거속도는 슬래그의 물리화학적 특성에 비례함을 확인하였다.

왕겨 (RHA)는 우수한 보온성을 가지고 있어 턴디쉬 내 용강의 온도를 유지하기 많은 철강회사에서 사용하고 있지만, 턴디쉬 내 왕겨 함량증가는 용강과 용융 슬래그 계면에서 용융 슬래그 층의 실리카 활동도를 증가시켜 용강의 재산화 뿐만 아니라 magnesia 내화재의 용출을 촉진시킨다. 그러나, calcium aluminate 기반 플럭스 (CA-flux)의 비율 증가는 용강 내 Si과 O 함량증가를 억제함을 확인하였다. CA-flux와 왕겨와 CA-flux 합의 비 (CA-flux/[RHA + CA-flux] = RCA)가 0.8 이상으로 유지되었을 경우 용강 내 재산화로 인해 형성되는 개재물의 량은 급격히 감소함을 확인하였다. 내화재-슬래그-용강-개재물 (ReSMI) 다상반응 시뮬레이션을 활용하여 개재물의 변화거동을 예측한 결과는 실제 실험결과와 매우 유사함을 확인하였다. 마지막으로, 용강을 주조하는 동안 래들 내 용강의 온도하락을 고려한 ReSMI 시뮬레이션을 활용하여 턴디쉬 내 용강에서 개재물 형성거동을 예측하였다. 온도가 감소함에 따라 용강 내 산소의 용해도가 감소하여 주조 시간이 지날수록 개재물의 양은 증가할 뿐만 아니라 턴디쉬 플럭스 내 RCA 비가 감소할수록 용강 내 개재물의 양은 증가하였다.; Non-metallic inclusions, which are potential causes for defects in steel products as well as nozzle clogging during continuous casting, must be removed. Non-metallic inclusions in molten steels can be removed through dissolution into molten slags, and then it is necessary to investigate the physicochemical properties, such as the viscosity and saturation limit for absorbing alumina inclusions, from secondary refining slags.

The viscosity-structure relationship of low-silica calcium aluminate with (or without) CaF2 and iron-containing calcium aluminate melts, which represent typical ladle furnace (LF) and Ruhrstahl-Heraeus degasser (RH) slags, respectively, was investigated using the rotating-cylinder method in conjunction with Raman spectroscopy for analysis of the molecular structure of aluminosilicate melts. The viscosity of low-silica calcium aluminate melts decreases by increasing both CaO/Al2O3 (C/A) and CaO/SiO2 (C/S) ratios. The effect of C/A ratio is larger than that of C/S ratio on the viscosity of low-silica calcium aluminate melts. Because the silicate structural units with various non-bridged oxygen (NBO) are located at the boundary of aluminate networks, it was demonstrated that the increase in C/A ratio effectively modifies the aluminosilicate networks rather than the increase in C/S ratio by employing Raman spectroscopy. Furthermore, it is necessary to understand the effect of CaF2 addition on the viscosity of low-silica calcium aluminate melts, because fluorspar (CaF2 > 98%) is widely used to LF slags. The addition of CaF2 to the low-silica calcium aluminate melts decreases the viscosity of the melts. When the small amount of CaF2 (~5 mass pct) was added to the melts, the CaF2 addition modifies the aluminate networks by the liberation of [SiO4] units from the aluminate networks. However, the addition of CaF2 has less effect on modification of the aluminosilicate networks in the high CaF2 content region (≥ 10 mass pct).

The viscosity of iron-containing calcium aluminate melts decreases by increasing both C/A ratios from 1.0 to 2.0 and total iron content in the melts. In the low iron oxide region, the increase in C/A ratios effectively decreases the viscosity of the iron-containing calcium aluminate melts whereas it rarely decreases the viscosity of the melts with high iron oxide content. The addition of iron oxide to calcium aluminate melts effectively decreases the viscosity of the melts with C/A ratio = 1.0 and 1.3 because it modifies the polymerization of the aluminate networks. On the other hand, the addition of iron oxide to the melts with C/A ratio = 1.6 and 2.0 has less of an effect on the modification of the viscosity of the melts. This is because the polymerization of the aluminate networks is rarely changed in C/A = 1.6 and 2.0 system. Consequently, it was demonstrated that the viscosity of the typical LF an RH slags is strongly dependent on the polymerization of aluminate networks.

Furthermore, the inclusion removal rate of the molten slag was evaluated by considering the interfacial reaction between molten steel and slag layer. The total oxygen content in the steel decreases with reaction time as soon as aluminum is added to the steel melt, which indicates that the alumina-rich inclusions are removed into the molten slag. The apparent rate constant of oxygen is depending on the ratio between concentration difference for absorbing the alumina-rich inclusions (∆C) and the viscosity (η) of the molten slag, i.e., the ∆C/η ratio. However, the apparent rate constant of oxygen is not fully proportional to the ∆C/η ratio, i.e. physicochemical properties, of the slags because the oxygen pick-up is continuously transferred from the iron-containing calcium aluminate slags by the interfacial reaction between molten steel and slag layer when total iron content in the slag is above 10 mass pct at C/A = 2.0. The alumina-rich inclusions in the steel sample were observed using SEM combined with automatic feature analysis (AFA) to characterize the inclusions with reaction time and slag composition. It was revealed that the inclusion removal rate, which is obtained from the decrease in the weight of alumina-rich inclusions, is strongly proportional to the physicochemical properties of the slag.

The addition of rice husk ash (RHA) in tundish flux causes severe reoxidation of molten steel and magnesia refractory corrosion because it generates the self-dissociation of silica by the interfacial reaction between the molten steel and slag layer due to the increase in the silica activity of slag layer. However, the increasing the amount of calcium aluminate-based flux (CA-flux) suppresses the silicon and oxygen pick. The number of inclusions formed by reoxidation in the molten steel was reduced when the ratio of CA-flux to sum of RHA and CA-flux (CA-flux/[RHA + CA-flux] = RCA) is above 0.8. The evolution of inclusions was investigated by comparing experiment results with predictions from a refractory-slag-metal-inclusion (ReSMI) multiphase reaction simulation. Consequently, the formation of alumina inclusions in tundish was investigated using ReSMI simulation by considering the temperature drop of the molten steel in the ladle during a casting procedure. The temperature drop of the steel melts in ladle and the decrease in RCA progress the formation of alumina inclusions in the steel melts at the end of tundish. Therefore, it is necessary to control RCA in tundish and the physicochemical properties of slags in ladle during a casting procedure.