Agglomeration of Non-metallic Inclusions at Steel/Ar Interface in Ti-added Ultra Low C Steel

Abstract

제강공정에서 극저탄소강 생산 시 개재물(Al2O3)에 의한 연주 침지 노즐 막힘 및 개재물성 품질 결함 문제가 지속 발생하고 있는 가운데 특히, Ti첨가강이 Ti미첨가강 대비 개재물에 의한 품질 결함 문제가 더 심각한 상황이며 이를 해결하기 위한 노력이 지속되고 있음. 본 연구에서는 Ti첨가강 생산시 개재물 요인에 의한 품질 결함 발생 매커니즘을 규명하고 원인 파악을 위해 Ti첨가에 따른 개재물의 합체 거동을 확인하였음. 실험은 Ti첨가 극저탄소강 [%Ti] 0.0735 샘플과 Ti미첨가 극저탄 [%Ti] 0.0018샘플간 개재물 합체 거동을 비교 분석하는 방법으로 진행하였고, 실험 장치는 공초점 현미경(Confocal)과 SEM을 이용함. 실험의 주요한 특징으로는 시편 주변에 Ti foil를 설치하여 산소 분압을 극력으로 낮게 제어함으로써 시편 용융 시 알루미늄과 산소가 반응하여 개재물이 순식간에 다량 발생하는 현상을 방지하였음. 실험 결과 개재물의 크기가 클수록 개재물간 군집력이 상승함을 확인하였고, 군집가속도는 개재물의 크기와 상관성을 보이지 않았으며 Ti첨가 유무에 따른 군집가속도는 차이를 보이지 않았고 동등 수준이었음. 이는, 일반적으로 Ti함량이 증가할수록 표면 장력은 감소하지만 산소 분압이 극력으로 낮게 제어된 조건에서는 Ti 0.0735% 정도의 소량의 Ti 첨가 시 표면장력 감소는 거의 발생하지 않기 때문에 군집 가속도 변화에 영향 없는 것으로 판단됨. 향후에는 재산화성 분위기 조건 및 합금철 투입 등 다양한 조건에서 Ti첨가 유무가 개재물 합체 거동에 미치는 영향을 연구해볼 필요가 있음.

As competition among steel companies continues to intensify due to oversupply of global steel products, the demand of quality requirements for steel materials by automobile manufacturers becomes substantial. In particular, with the increasing variety of automotive designs, the processing volume of steel materials is increasing and the level of surface quality requirements continues to rise. In order to satisfy the increasing level of customers' requirements, the technology of manufacturing clean steel, with minimum level of inclusions is very important. The material of Ti-added ultra low carbon steel, which is mainly used as automotive outer panel, suffers clogging of the submerged entry nozzles and surface quality issues caused by the inclusions. To resolve these issues, it is necessary to understand removal mechanism of non-metallic inclusions produced in the steel. Removal of the inclusions is done by various factors. Physically, inclusions can be removed only when they are raised from steel liquid to slag by buoyancy. In order to realize this, the agglomeration of inclusions can be considered a very important factor. In this study, the characteristics of agglomeration behavior of inclusions in steel/Ar interface of molten steel in Ti-added steel were studied. The experiments were designed in order to observe the agglomeration behavior of inclusions in ultra low C steel employing a CSLM (Confocal Scanning Laser Microscope). The attraction force among inclusions was confirmed with the same tendency as reported previously. The larger the size of inclusions, the greater attraction force between the inclusions. The acceleration among the inclusions was then used to compare the agglomeration character of the inclusions in Ti-added and in Ti free steel. The results showed that the acceleration among the inclusions virtually irrespective to the addition of Ti under conditions employed in the present study where oxygen partial pressure was significantly low. This indicated that in the case of Ti addition ([%Ti] = 0.0735), the reduction in surface tension of the steel was almost negligible[9]. Therefore its impact on the attractive acceleration was not significant. In the near future, it is necessary to study whether the addition of Ti affects the agglomeration behavior of inclusions under conditions where re-oxidation occurs during the production process of ultra low carbon steel in the steel-making process.