Inclusions In Al-killed Steel Research Articles

Evolution and deformability of inclusions in Al-killed steel with rare earth-alkali metals (Ca or Mg) combined treatment

The quality of stainless steel is closely related to the deformability of inclusions, which is significantly affected by their compositions. The present study first investigated the evolution of inclusion compositions in Al-killed steel with rare earth-alkali metals (Ca or Mg) combined treatment through four laboratory-scale experiments. The Ce contents in the final steel are 0.0080 wt%, 0.015 wt%, 0.016 wt% and 0.010 wt%, respectively. The Mg content is 0.0014 wt% in Ce–Mg combined treated steel, and the Ca content is 0.0015 wt% in Ce–Ca combined treated steel. The deformability of inclusions in both Ce2O3–Al2O3–CaO and Ce2O3–Al2O3–MgO systems was subsequently evaluated by calculating their Young's modulus at low temperature. The results show that irregular Al2O3 and MgAl2O4 with poor deformability are modified to CeAlO3 and Ce2O3 by Ce treatment, resulting in the decrease of Young's modulus of inclusions. The deformability of inclusions is further improved due to the transformation from lumped-like CeAlO3 to spherical CaO–Al2O3–Ce2O3 caused by Ca treatment, and some of these inclusions are the ones with low liquidus temperature. Thermodynamic analysis was used to discuss the control condition of the formation and evolution of inclusions. Accordingly, the appropriate addition amounts of Al, Mg, Ce, and Ca are expected to control inclusion compositions and properties, including deformability and liquidus temperature, thereby improving the steel performance.

Calcium Transfer to Oxide Inclusions in Al-Killed Steel Without Calcium Treatment

Alumina inclusions generated from aluminum deoxidation may undergo several transformations depending on steelmaking conditions. Under reducing conditions, in contact with MgO-saturated slag, alumina inclusions are known to transform to spinel. However, there are uncertainties related to CaO pick-up by alumina-based inclusions. This work aims to provide some clarification of CaO pick-up by alumina inclusions through a series of experiments. First, the role of silicon in calcium transfer from slag was examined by conducting two similar experiments: one without any silicon addition and a second with 1 wt pct electronic grade silicon addition. Second, the role of crucibles was tested by conducting experiments in ZrO2 and CaO-3 pct ZrO2 crucibles. It was found that the addition of silicon significantly enhances the rate of CaO pick-up when there was no silica in the slag at the start of reaction. The effect of the crucible on CaO pick-up was found to be weaker than that of the 1 wt pct silicon addition.

Effect of Nozzle Clogging on the Fluid Flow Pattern in a Billet Mold with Particle Image Velocimetry Technology

The flow pattern of molten steel in a mold of continuous casting affects the temperature distribution and initially solidifies the shell thickness homogeneity, inclusion transport, and liquid slag entrapment. Alumina inclusions in Al-killed steel tend to adhere to the inner wall of the nozzle, which changes the nozzle geometry structures and affects the molten steel flow pattern in the mold. In this study, two different clog shapes—the hemispherical clog and tube clog—were identified based on the clogged straight-nozzle dissection results. The hydraulic straight-nozzle casting model with different clog situations was designed to investigate the effect of the clog status on the fluid flow in the mold using particle image velocimetry (PIV) technology. The results show that a tail stream with a length of 80 mm forms near the nozzle wall in the fluid flow direction when the hemispherical clog sticks to the nozzle and the jet stream deflects to the clogged side after the jet stream and tail stream mix. However, when the hemispherical clog sticks to the nozzle outlet, the jet stream deflects to the nonclogged side. The increase in the tube clog thickness causes an increase in the jet stream velocity magnitude near the nozzle outlet.

Behaviour of Sulphide and Non-alumina-Based Oxide Inclusions in Ca-Treated High-Carbon Steel

Although alumina-based inclusions in Al-killed steel have been widely investigated, few studies focus on non-metallic inclusions in Si-killed steel. Most high-carbon steel, which is expected to have higher potential for the formation of alumina, is deoxidised by silicon to avoid alumina-based inclusions. Many types of high-carbon steel such as free-cutting steel and grinding media for the mining industry include sulphur and calcium; therefore, non-alumina-based oxide and CaS-based inclusions co-exist in molten steel. In this study, the behaviour of sulphide and non-alumina-based oxide inclusions in Ca-treated liquid steel was investigated via transition of the composition and morphology in the liquid state. Using a confocal scanning laser microscope (CSLM), sulphide particles being pulled toward oxide inclusions were directly observed and changes in the composition of inclusions investigated using different analytical techniques. We identified two types of sulphide phase after remelting and cooling, polygonal particles and a ring-shaped layer around the oxide phase. When the sulphide capacity Cs multiplied liquid fraction of oxide had a higher value, the sulphide layer tended to become wider.

Inclusion Characterization in Aluminum-Deoxidized Special Steel with Certain Sulfur Content Under Combined Influences of Slag Refining, Calcium Treatment, and Reoxidation

Inclusions in Al-killed steel with [S] of about 0.0060 to 0.0070 mass pct were characterized and discussed, evaluating the combined effects of basic slag refining and Ca treatment in ladle, together with reoxidation of liquid steel in casting tundish. Inclusions were changed from Al2O3 to MgO-Al2O3 spinel and then to MgO-Al2O3-CaO during basic slag refining. After Ca treatment, many (MgO-Al2O3) + CaS inclusions were formed, featuring the coexistence of MgO-Al2O3 and CaS to form a dual-phased structure. In the following Ar blowing, the number density of (MgO-Al2O3) + CaS inclusions and pure CaS increased obviously, which implied that [Ca] preferentially reacted with [S] rather than [O] in steel. Reoxidation in casting tundish caused the pickup of oxygen in steel, and the rise of total oxygen (T.O) was 0.0002 mass pct; even 55t steel has been poured. As a result, the content of CaO in inclusions increased and MgO-Al2O3-CaO inclusions were formed again. Thermodynamic calculations revealed that the driving force was strong for the formation of CaS-based inclusions. Higher carbon content in steel would help to reduce oxygen content while enhancing the activity of [S] in steel, which further stabilized the existence of CaS-based inclusions. Therefore, inclusions were mostly the solid (MgO-Al2O3) + CaS dual-phase ones, without the formation of liquid calcium aluminates. Contents of CaS and CaO in inclusions were affected by the [mass pct S]/[mass pct O] ratio, which was calculated as about 4.58 K and 5.34 K at 1873 K and 1823 K, respectively. This finding implied that lower oxygen was not favorable to prevent the solid inclusions in the calcium treatment of high carbon special steel.

Determination of Three-Dimensional Morphology and Inner Structure of Second-Phase Inclusions in Metals by Non-Aqueous Solution Electrolytic and Room Temperature Organic Methods

The secondary-phase particles in metals, particularly those composed of non-metallic materials, are often detrimental to the mechanical properties of metals; thus, it is crucial to control inclusion formation and growth. One of the challenges is determining the three-dimensional morphology and inner structures of such inclusions. In this study, a non-aqueous solution electrolytic method and a room-temperature organic technique were developed based on the principle of electrochemistry to determine the three-dimensional morphologies and inner structures of non-metallic inclusions in Al-killed steel, Si-killed steel, and ductile cast iron. The inclusions were first extracted without any damage to the inclusions, and then the collected inclusions were wrapped and cut through Cu ion deposition. The results revealed that the inclusions in Al-killed steel had an irregular morphology, that those in the Si-killed steel were mainly spherical, and that almost all the spheroidal graphite in the ductile cast iron featured a uniform globular morphology. In addition, nucleation was not observed in the inner structures of the inclusions in the Al-killed steel, while some dendritic or rod-like MnS phase precipitates appeared on the silicate inclusion surfaces, and some silicate-rich phases were detected in their inner matrix. For spheroidal graphite, rare-earth oxides (one particle or more) were observed as nuclei in the center of almost every graphite particle. The formation and evolution of inclusions in these types of metals can be better understood by means of the two developed methods.

Inclusion evolution after calcium addition in Al-killed steel with different sulphur content

ABSTRACTThe inclusion evolutions after calcium treatment in Al-killed steel with different sulphur content were determined by chemical experiments and thermodynamics calculations. The results show that the inclusions are calcium aluminate with a little amount of calcium sulphide in low content sulphur steel after calcium treatment. As the sulphur content in steel increases, the typical inclusions generated in steel are layered or homogeneous irregular oxysulphides. A number of pure sulphides can be observed in higher content sulphur steel, and the morphology of sulphides with different CaS/MnS mass ratio has different shape. Thermodynamics calculations of Al–Ca–O–S–Mn system inclusions in steel at 1873 K and during solidification process were comprehensively conducted, considering all types of inclusions. The experimental results are in good agreement with thermodynamic calculations, which can predict the formation of the inclusions in Al-killed steel with different sulphur content.

Effect of Refractory on Nonmetallic Inclusions in Al-Killed Steel

To understand the effect of refractory on the evolution of inclusions in the secondary steelmaking, steel samples were taken at different stages of the process train. Laboratory experiments were carried out using different refractories (alumina, spinel, and MgO). In the laboratory study, the types of inclusions considered were alumina, spinel, and calcium aluminate. The focus was given to Al-killed steel. The results showed that alumina refractory and spinel refractory had little effect on all the three types of inclusions, while the effect of MgO refractory depended on the activity of dissolved oxygen in liquid steel. With lower oxygen activity, alumina inclusions could transform into spinel inclusions. No evident change could be found for spinel and calcium aluminate inclusions. When the oxygen activity was high enough, spinel inclusions could not be generated from alumina inclusions. The laboratory results helped in understanding the evolution of the inclusions in the industrial process.

Modification of MgO·Al2O3 spinel inclusions in Al-killed steel by Ca-treatment

The modification of MgO·Al2O3 spinel inclusions in Al-killed steel by Ca-treatment has been studied by industrial trials and thermodynamic calculations. In the industrial trials, samples were taken systematically during the refining process in which the molten steel was treated by calcium, and the characters of the inclusions were analyzed using scanning electron microscopy (SEM) and energy dispersive spectra (EDS). The effects of Ca-treatment were evaluated by tracking the compositions of the inclusions. The results show that the modification of MgO·Al2O3 spinel inclusions by Ca-treatment is effective and the transformation sequence of the inclusions during the refining is Al2O3→MgO·Al2O3→liquid complex inclusions. The modification of spinel inclusions by Ca-treatment was calculated by FactSage6.0 utilizing its free-energy minimization routines. The results of thermodynamic calculations indicate that spinel inclusions are easier to be modified than Al2O3 inclusions and the spinel inclusions in 30CrMo steel would transform to liquid complex inclusions when the content of dissolved Ca in the molten steel exceeds 1×10−6. Also, the results show that adding more calcium into the molten steel would lower the contents of Al2O3 and MgO and increase the CaO content of the inclusions, while the change in SiO2 content is little.

Effect of Li2O Addition on Crystallization Behavior of CaO–Al2O3–MgO Based Inclusions

The crystallization behavior of CaO–Al2O3–MgO oxide powder which is one of the major non-metallic inclusions in Al-killed steel was systematically examined in the present study. 33mass%CaO–62mass% Al2O3–5mass%MgO and 30mass%CaO–57mass% Al2O3–5mass%MgO–8mass%Li2O glass powders were heat-treated at 1073–1673 K for 8–512 s under air.CaO–Al2O3–MgO glass was found to be crystallized immediately after the heat treatments. Moreover, it was revealed that the addition of Li2O accelerated the crystallization of the glass powders, and controlled aggregation of crystal particles. Additionally, the indentation hardness of crystallized glasses after 512 s heat-treatments was investigated by using a nano-indenter. The addition of Li2O had the effect of lowering the hardness of samples, which indicates that Li2O would improve the deformation behavior of aluminate based inclusions.