The current study performed thermadynamic calculation, laboratory experiments, and industrial trials for the formation and modification of MgO-Al2O3 spinel inclusions in alloy steels. The stability Mg-Al-O diagram was obtained using the thermodymanic study. The resulting MgO-Al2O3-CaO inclusions from MgO-Al2O3 spinel inclusions after the calcium treatment were spherical, and > 5 μm MgO-Al2O3-CaO inclusions have a two-layer structure: an outside CaO-Al2O3 layer and a MgO-Al2O3 core. The modification of > 5 μm MgO·Al2O3 spinel inclusions by calcium treatment includes two steps: (1) reducing MgO in the inclusion into the dissolved magnesium by the dissolved calcium in the steel and (2) generating a liquid xCaO·yAl2O3 layer at the outside of the spinel inclusion. For <2 μm MgO·Al2O3 spinel inclusions, they can possibly be modified into a xCaO·yAl2O3 inclusion by reducing all MgO component in the spinel inclusions with the added calcium.

Formation and Modification of MgO·Al2O3-Based Inclusions in Alloy Steels

Non-metallic inclusions (NMIs) play a key role in many important properties of steel, influencing both processing and application of steel products. In this work, the current understanding of the origin and classification of NMIs is reviewed, highlighting the dramatic development of the last decades. This includes the discussion of the thermodynamics of inclusion formation and the current knowledge on the effects of melt shop processing variable on NMIs composition, amount and size distribution. The development of inclusion engineering – tailoring the process to obtain the desired NMIs is highlighted and the development in selected areas – tire cord, springs and bearing steels as well as prevention of nozzle clogging – is used to illustrate this development. The promising field of “oxide metallurgy” is also discussed in the context of inclusion engineering. Finally, the difficulties in meaningfully characterizing and quantifying NMIs are briefly commented. In summary it is concluded that inclusion control in steels has evolved significantly in the last decades. This is due to the progress in understanding the interplay between thermodynamics, steel and slag chemical composition as well as melt shop processing. This made possible the tailoring of non-metallic inclusions via processing, to optimize steel properties. Nonetheless, some important problems remain and must still be solved to improve inclusion control and optimization.

Non-metallic inclusions in steels – origin and control

A particle-capture model based on local force balances has been developed, implemented into computational models of turbulent fluid flow and particle transport, and applied to simulate the entrapment of slag inclusions and bubbles during the continuous casting of steel slabs. Turbulent flow of molten steel is computed in the nozzle and mold using transient computational fluid flow models, both with and without the effects of argon gas injection. Next, the transport and capture of many particles are simulated using a Lagrangian approach. Particles touching the dendritic interface may be pushed away, dragged away by the transverse flow, or captured into the solidifying shell according to the results of a local balance of ten different forces. This criterion was validated by reproducing experimental results in two different systems. The implications of this criterion are discussed quantitatively. Finally, the fluid flow/particle transport model results and capture criterion are applied together to predict the entrapment distributions of different sized particles in a typical slab caster. More large particles are safely removed than small ones, but the entrapment rate into the solidifying shell as defects is still very high.

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Transport and Entrapment of Particles in Steel Continuous Casting

In the current study, a three-dimensinal (3D) numerical model is built to investigate the effect of a local-type electromagnetic brake (EMBr) on the fluid flow, heat transfer, and inclusion motion in slab continuous casting strands. The results indicate that the magnetic force affects the jet characteristics, including jet angle, turbulent kinetic energy, and its dissipation rate. To reduce the top surface velocity and stabilize the top surface, the magnetic flux intensity should be larger than a critical value. With a 0.39 T magnetic flux intensity, the top surface velocity and its fluctuation can be well controlled, and less slag is entrained. The motion of argon bubbles is also studied. More bubbles, especially >2.0-mm bubbles, escape from the top surface between the mold submerged entry nozzle (SEN) and \( \frac{1}{4} \) width for the case with a 0.39 T EMBr. This may push the top slag away and create an open “eye” on the top slag. Small bubbles (≤1 mm) tend to escape from one side of wide face no matter with or without EMBr, which is induced by the swirl flow from the SEN outport. EMBr has a little effect on the overall removal fraction of inclusions; however, it affects the local distribution of inclusion in the slab. With EMBr, more inclusions accumulate the region just below the surface, thus a worse subsurface quality, whereas the inner quality of the slab is better than that without EMBr. For heat transfer in the mold, the heat flux on the narrow face and the area of possible break-out zones can be reduced by using EMBr. Prevention of bias flow and/or asymmetrical flow in mold by EMBr is also concluded.

Fluid Flow-Related Transport Phenomena in Steel Slab Continuous Casting Strands under Electromagnetic Brake

The nucleation, growth, transport, and entrapment of nonmetallic inclusions during the steel casting process are briefly reviewed in this article. The current main research accomplishments as well as future topics that should be focused on in this field are summarized.

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Nucleation, Growth, Transport, and Entrapment of Inclusions During Steel Casting

Clogging of the submerged entry nozzle (SEN) is a serious problem during the continuous casting of steel, due to its influence on the casting operations and product quality. Fluid-flow-related phenomena in the continuous casting mold region with the SEN clogging are investigated in the current article, including the quantitative evaluation of inclusion removal, slag entrainment, heat transfer, and the prediction of breakouts. The calculations indicate that, in order to accurately simulate the fluid flow in the mold region, the SEN should be connected with the mold region and the two should be calculated together. In addition, the whole mold region has to be calculated. Clogging at the SEN at one side induces asymmetrical jets from the two outports; thus, the fluid flow in the mold is asymmetrical. In addition, more inclusions are carried by the flow to the top surface of the nonclogged side, and the slab at the nonclogged side has a lower quality. With SEN one-sided clogging, inclusions travel a much larger distance, on average, before they escape from the top or move to the bottom. The overall inclusion entrainment fraction from the entire top surface for inclusions of any size is less than 10 pct. A higher turbulence energy and a larger surface velocity induce more inclusion entrainment from the top surface. Smaller inclusions are more easily entrained into the steel than are larger ones. More >200-μm inclusions can be entrained into the molten steel from the top slag with SEN clogging than without clogging. The SEN one-sided clogging generates an asymmetrical temperature distribution in the mold; it also generates temperatures higher than the liquidus temperature at some locations of the solidified shell, which increases the risk of breakouts. The SEN clogging should be minimized in order to achieve a uniform steel cleanliness, a cleaner steel, and a safe continuous casting operation.

Flow Transport and Inclusion Motion in Steel Continuous-Casting Mold under Submerged Entry Nozzle Clogging Condition

This work applied automated particle analysis to study non-metallic inclusions in steel. Compared with traditional methods, the approach has the advantage of capturing the morphology, measuring the size, recording the original positions, and identifying the composition of inclusions on a selected area in a short time. The morphology and composition of typical inclusions were analyzed using partial acid extraction and discussed through thermodynamic calculation. Steel samples were collected from the entire cross section of billets cast during times of steady state and ladle change. The spatial distribution of inclusions agreed well with the measurement of the total oxygen. The spatial distribution of inclusions was plotted to represent the entrapment positions of inclusions on the casting strand and their concentration on the cross section of the billet. Also, regarding the different size and type of inclusions, the spatial distribution of classified inclusions was explored such as the distribution of sulfide, oxide, and high sodium and potassium content inclusions. The sufficient information could be used to identify the source of inclusions and guide the steel refining process.

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Detection of Non-metallic Inclusions in Steel Continuous Casting Billets

A full-scale water model and mathematical simulation were used to study the fluid flow-related phenomena in a water model of an aluminum electrolysis cell. The time-dependent, multiphase fluid flow model was developed to represent the complex transient flow in the electrolysis bath. The accuracy of the mathematical model was validated by the ink dispersion and laser doppler velocimetry measurements in the water model. The shape, motion, release frequency of large-size bubbles, the fluid flow pattern, and the electrolyte–metal interface were predicted by the mathematical simulation. The design and operation of the anode were discussed, including the effect of the anode edge corner shape, the presence of a tilted bottom angle, and the magnitude of applied current density. The results indicated that coupling using a curved corner, with slot and with tilted angle at the anode, is effective for the release of bubbles and for the stability of the electrolyte–metal interface.

Simulation of the Fluid Flow-Related Phenomena in the Electrolyte of an Aluminum Electrolysis Cell

The clogging of the Submerged Entry Nozzle (SEN) during
 billet continuous casting of mid-carbon steel is studied.
 Clogging materials and inclusions in steel samples taken at
 ladles, tundish and billets are investigated. The total oxygen on
 the whole section of the billet is measured. Steel cleanliness at
 unsteady casting states, including cast start, ladle change, SEN
 change, cast end, and the special unsteady pouring period
 induced by SEN clogging, are studied. Fluid flow and inclusion
 motion and entrapment to SEN surface are also simulated.

Xiangjun Zuo

Papers

Flow Transport and Inclusion Motion in Steel Continuous-Casting Mold under Submerged Entry Nozzle Clogging Condition

Detection of Non-metallic Inclusions in Steel Continuous Casting Billets

Simulation of the Fluid Flow-Related Phenomena in the Electrolyte of an Aluminum Electrolysis Cell

Inclusions, lining materials and nozzle clogging during middle carbon steel billet continuous casting process