Experimental Study on the Behavior of TiN and Ti2O3 Inclusions in Contact with CaO‐Al2O3‐SiO2‐MgO Slags

TiN and Ti2O3 are the predominant inclusion types in Ti-alloyed ferritic chromium stainless steels. In order to ensure the required steel cleanness level, an effective removal of such inclusions in the slag during secondary metallurgy is essential. This inclusion removal predominantly takes place via dissolution of the inclusion in the slag. The dissolution behavior of TiN and Ti2O3 in CaO-SiO2-Al2O3-MgO slags as well as their agglomeration behavior in the liquid steel is investigated using High Temperature Laser Scanning Confocal Microscopy and Tammann Furnace experiments. Thermodynamic calculations are performed using FactSage 7.0. The behavior of TiN is observed to be completely different to that of oxides. Ti2O3 dissolves quickly in slags, and its dissolution behavior is comparable to that of other already well examined oxides. In contrast, TiN shows a very intense gas reaction which is attributed to the release of nitrogen during contact with slag. Slags with higher SiO2 content show a significantly higher ability for the dissolution of TiN as compared to Al2O3-rich slags. The gas reaction is found to also significantly influence the final steel cleanness. Despite the easy absorption of TiN in the slag, the formed nitrogen supports the formation of pinholes in the steel

CHARACTERIZATION OF ACICULAR FERRITE MICROSTRUCTURES USING ETCHING METHODS, OPTICAL MICROSCOPY AND HT-LSCM

ABSTRACT Acicular ferrite is a needle shaped modification of ferrite, which nucleates intergranularly at non-metallic inclusions. Due to the fine grained structure of acicular ferrite, it offers excellent toughness. By increasing the amount of this component within the microstructure, the properties of HSLA steels can be optimized significantly. The formation of acicular ferrite is influenced by four main parameters: Steel composition, cooling rate, austenite grain size and non-metallic inclusions. These parameters are interacting strongly, making a systematic study essential. By using a Laser Scanning Confocal Microscope combined with a High Temperature furnace (HT-LSCM) for the in situ observation of acicular ferrite formation in HSLA steels, fundamental information about the formation mechanism can be gained. Due to the inert furnace atmosphere, the accurate adjustment of austenitizing temperature and the well controllable cooling conditions, the interactions between steel composition, austenite grain size, cooling rate and the acicular ferrite amount can be analyzed in detail. Up to now no automated quantification of the acicular ferrite amount has been described in literature. The present study focuses on the characterization of acicular ferrite microstructures by a combination of metallographical methods. Conventional sample preparation is combined with the in situ observation of the acicular ferrite formation in a HT-LSCM. Special attention is paid to the determination of austenite grain size by optical microscopy and its influence on the acicular ferrite amount. Finally, various etching methods for the illustration of acicular ferrite are presented, focusing on their applicability for an automated quantification of the acicular ferrite amount

Mathematical Modeling of the Early Stage of Clogging of the SEN During Continuous Casting of Ti-ULC Steel

The clogging of the submerged entry nozzle (SEN) during the continuous casting of steel can be divided into two stages: the "early stage," when the initial layer of the clog covers the SEN refractory surface owing to chemical reactions, and the "late stage," when the clog layer continues to grow because of the deposition of non-metallic inclusions (NMIs). In this paper, a mathematical formulation is proposed for the build-up of the initial oxide. The chemical reaction mechanism is based on the work of Lee and Kang (Lee et al. in ISIJ Int 58:1257-1266, 2018): a reaction among SEN refractory constituents produces CO gas, which can re-oxidize the steel melt and consequently form an oxide layer on the SEN surface. The proposed formulation was further incorporated as a sub-model in a transient clogging model, which was previously developed by the current authors to track the late stage of clogging. The thermodynamics and kinetics of CO production, depending on the local pressure and temperature, must be considered for the sub-model of early-stage clogging. Test simulations based on a section of an actual industrial SEN were conducted, and it was verified that the clogging phenomenon is related to the SEN refractory, the chemical reaction with the steel melt, the local temperature and pressure, and the transport of NMIs by the turbulent melt flow in the SEN. The model was qualitatively validated through laboratory experiments. The uncertainty of some parameters that govern the reaction kinetics and permeability of the oxide layer is discussed.11Nsciescopu

Engulfment Behavior of Inclusions in High-Carbon Steel: Theoretical and Experimental Investigation

Previous studies on inclusions behavior at the front of the solidifying steel shell have mainly focused on low-carbon steels. However, with the increasing applications of high-carbon steel in recent years because of its superior properties, it is crucial to understand this behavior in high-carbon steel. Most of the high-carbon steels are deoxidized by silicon, calcium treated, and contain higher sulfur percentage. Also, higher carbon content has a determining influence on the viscosity and surface tension, which will affect the inclusion behavior. In this study, we have investigated the engulfment behaviors of inclusions in front of the solidifying interface in high-carbon steels using concentric solidification method. The critical velocity of the growing shell, at which the particle is engulfed in the solidifying shell, instead of being pushed by this shell, was determined. The inclusion identified in this study is a bi-component form of CaO-SiO2-based oxide and CaS. It was revealed that engulfment behavior is strongly affected by convection of liquid steel that originates from carbon push out in high-carbon steels. This study provides new crucial information to produce high-carbon steel with fewer inclusions, which opens new application pathways for this emerging grade of steel

Dissolution of Sapphire and Alumina-Magnesia Particles in CaO-SiO 2 -Al 2 O 3 Liquid Slags

Understanding the dissolution kinetics of non-metallic inclusions in liquid slag is key in optimization of slag composition for inclusion removal. In this study, the rate of dissolution of high-precision spheres of sapphire and alumina-magnesia particles in CaO-SiO2 -Al2O3 liquid slags was measured in situ using a laser scanning confocal microscope at 1500 °C. It was found that the rate of dissolution of both sapphire and alumina-magnesia particles increased when the slag basicity is increased. A layer was observed around the dissolving sapphire. This layer may be a product layer and/or indicative of a mass transfer rate-controlling system. In the case of alumina-magnesia particle, the kinetics appeared more complex and depended on slag composition. No product layer or mass transfer layer was observed around the particle dissolving in slag with low basicity, whereas for the high basicity slag, a product or stagnant layer was observed, similar to that of the sapphire particle. Assuming a mass transfer-controlled system, measured diffusion coefficients for sapphire particles in slags tested in this study ranged from 10−11 to 10−10 m2 s−1 at 1500 °C