This paper qualitatively explains the differences in the cathodoluminescence (CL) colors of alumina (Al2O3) inclusions in steels and presents a method of distinguishing MgAl2O4 spinel inclusions from ...other inclusions using CL imaging. It was found that the presence of chromium (Cr) and titanium (Ti) in Al2O3 inclusions increases CL intensity in the red region, while elements that increase oxygen vacancies such as magnesium relatively increase CL intensity in the blue region. We could distinguish MgAl2O4 spinel inclusions, which produce green luminescence, from Al2O3 inclusions by capturing CL images. We were also able to identify MgAl2O4 spinel in an agglomerated inclusion consisting of MgAl2O4 spinel, Al2O3, and MgO from its CL image. Capturing CL images whose wavelength is less than 600nm simplifies the identification of Al2O3 and MgAl2O4 spinel inclusions because all the Al2O3 and MgAl2O4 spinel inclusions produced blue and green luminescence, respectively. The present study suggests that CL image capture is a rapid identification method for inclusions in steels, especially MgAl2O4 spinel and Al2O3 inclusions, because it takes less than 30s to obtain CL images.
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•Differences in the CL colors of alumina inclusions in steels are explained.•Cr and Ti in Al2O3 inclusions increase CL intensity in red region.•Elements increasing oxygen vacancies in Al2O3 increase CL intensity in blue region.•Method to distinguish MgAl2O4 spinel inclusions from other inclusions is presented.
An improvement of automatic ultrasonic testing through a double probe technique along the longitudinal bar axis used in a round bar, with a diameter of several millimeters, is proposed. Non-metallic ...inclusions of several tens of micrometers in the cross-sectional length can be detected using this novel technique, whereas the detectability in a conventional normal beam technique is limited to 100–150 µm. The main advantages of this technique are an increased working sensitivity owing to a decreased surface echo width and the use of a shear wave with a shorter wavelength as compared with a conventional normal beam technique. As another advantage of this technique, malfunctions caused by air bubbles in the coupling medium can be eliminated. Further, the beam paths of the surface echo and the bottom echo are discussed herein using the propagation time difference between both echoes in Appendix.
Determining non-metallic inclusions (NMIs) are essential to engineer ultra-high-strength steel as they play decisive role on performance and critical to probe via conventional techniques. Herein, ...advanced Synchrotron X-ray absorption coupled with photoemission electron microscopy and first-principles calculations are employed to provide the structure, local bonding structure and electronic properties of several NMI model systems and their interaction mechanism within and the steel matrix. B K-, N K-, Ca L2,3- and Ti L2,3-edge spectra show that the additional B prefers to result in h-BN exhibiting strong interaction with Ca2+. Such Ca2+-based phases also stabilize through TiN, revealing the irregular coordination of Ca2+. Observed intriguing no interaction between TiN and BN is further supported with the first-principles calculations, wherein unfavorable combination of TiN and h-BN and stabilization of bigger sized Ca2+-based inclusions have been found. These observations can help to optimize the interaction mechanism among various inclusions as well as steel matrix.
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To effectively remove non-metallic inclusions from the steel during the flowing in a five-strand asymmetric tundish, the novel configuration of the impact pad was developed. For analysis, complex ...numerical modelling in the programme ANSYS Fluent was used. The Lagrangian Discrete Phase Model of inclusion tracking was applied. The distribution of inclusions, with sizes ranging from 2 µm to 100 µm and density from 2500 to 3500 kg·m−3, was considered only through the shroud tube. The residence time distribution (RTD) curves and inclusion removal efficiency were used for evaluation of steady state steel flow character depending on internal configuration of a tundish with an impact pad in two design modifications (Modification 1—M1, Modification 2—M2). The preliminary results showed that in the case of asymmetric geometry plays a role the computational mesh independency. The assembly method with cut cell approach was satisfactory even when the tundish geometry was changed. The RTD curves with an M1 showed a huge dead volume in the tundish. In the case with an M2, the RTD curves are more or less uniform for all casting strands, and the removal of inclusions to slag increased from about 55% up to 70% in comparison with M1.
In this article, the industrial experiments of calcium treatment on the modification of inclusions in NM500 wear-resistant steel were carried out. There are four types of inclusions, which are Type A ...of Al2O3 based inclusions, Type C of inclusions containing CaO without S, Type S of inclusions containing S, and Type M of MnS. Three types of CaO·MgO·Al
2
O
3
inclusions after calcium treatment are embedded MgO·Al
2
O
3
+ CaO·Al
2
O
3
, wrapped MgO·Al
2
O
3
+ CaO·Al
2
O
3
and combined CaO·MgO·Al
2
O
3
+ MgO. There are four types of CaO·MgO·Al2O3·CaS inclusions of multilayer MgO·Al
2
O
3
+ CaO·MgO·Al
2
O
3
+ CaS, embedded MgO·Al
2
O
3
+ CaO·Al
2
O
3
+ CaS, wrapped CaO·MgO·Al
2
O
3
+ CaS and composite MgO·Al
2
O
3
+ CaO·Al
2
O
3
+ CaS·CaO·Al
2
O
3
in the steel after calcium treatment.
The effect of the tundish flux on the evolution of non-metallic inclusions in Si-killed 304 (18%Cr-8%Ni) stainless steel has been investigated at 1773 K. The interfacial reaction between molten steel ...and the CaO–Al2O3–MgO flux causes the aluminum pick-up from the liquid slag into the steel melt, resulting in a decrease in the oxygen content in the steel. The aluminum originating from the slag modifies the pre-existing Mn-silicate inclusions into alumina-rich inclusions in the steel. Because the oxygen content in the steel decreases as it reacts with the CaO–Al2O3–MgO flux, the degree of supersaturation for alumina formation is too low to precipitate new-born alumina particles in the steel. By analyzing the population density function (PDF) results for inclusions, it can be observed that the growth of spinel-type inclusions occurs by the diffusion of aluminum and magnesium in the steel. On the other hand, the composition of the steel, as well as the evolution of inclusions, is negligibly changed when the CaO–SiO2–MgO flux is added to the molten steel. Furthermore, the computational simulation for predicting the evolution of inclusions in molten steel during a continuous casting tundish process was carried out based on a refractory-slag-metal-inclusion (ReSMI) multiphase reaction model.
A study was carried out to observe inclusions during the secondary refining process of case-hardening steel to understand the factors to determine inclusion compositions. During the LF refining, ...inclusions changed from the primary deoxidation product of Al2O3 to MgO·Al2O3 and the CaO–Al2O3 system. At the end of LF refining, the compositions were placed on the tie-line connecting the areas of MgO·Al2O3 and CaO–Al2O3–MgOliq in the CaO–Al2O3–MgO diagram. This change took place by the composition evolution targeting the thermodynamic equilibrium states. After the RH treatment, the inclusion compositions changed mainly to Al2O3 and the CaO–Al2O3 system because MgO·Al2O3 inclusions were removed, while the CaO–Al2O3 system inclusions, the most stable oxide, were remained. This behavior could be described by the interfacial properties between the oxides and the molten steel. Al2O3 inclusions were considered to be newly generated during the RH treatment. It was confirmed that three factors of (1) equilibrium state (2) removal and (3) generation of inclusions dominated to determine the inclusion compositions.
The slag-metal reactions between Si-killed 316L stainless steel and the fluxes rice hush ash (RHA), RHA + CaO–SiO2 (flux A), and RHA + CaO–Al2O3 (flux B) were conducted in an induction furnace at ...1773 K to investigate the effects of the various fluxes on the reoxidation behavior of the liquid steel. The total oxygen (T.O) content in the liquid steel increased when the steel reacted with RHA and RHA + A fluxes, whereas T.O decreased when the liquid steel reacted with RHA + B flux. The number density of non-metallic inclusion (NMI) in the steel reacted with RHA, RHA + A, and RHA + B fluxes increases, stays constant, and decreases, respectively. The increase in the number density of NMI is due to the reoxidation of the molten steel caused by the dissociation reaction of SiO2. However, the number density of NMI in the steel decreased since B flux has a higher value of physicochemical factor, i.e., log(ΔC/η), where η is slag viscosity and ΔC represents concentration difference between initial content and saturation limit of target oxide in slag. The reoxidation reaction caused by the presence of SiO2 in the fluxes and inclusion absorption (dissolution) reactions are competitive or coupled each other. Higher SiO2 activity in the fluxes (such as RHA) resulted in easier formation of SiO2-rich inclusions. However, the SiO2 activity in Flux A and Flux B is lower than that in RHA, and the MnO–SiO2 and SiO2 inclusions in the Si-killed steel were reduced by Al to form MnO–SiO2–Al2O3(-Cr2O3) system inclusions in the steels reacted with Flux A and B, respectively.
•VHCF crack initiation is caused by FGA formation around inclusions.•The VHCF behavior is highly dependent on the types of inclusions in the material.•Different inclusion types have specific ...threshold values for VHCF crack initiation.•An absolute threshold value for VHCF crack initiation was found.
This study aims to clarify the influence of different inclusion types on the crack initiation in the very high cycle fatigue regime. For this purpose ultrasonic tension–compression fatigue tests (R=−1) with the bainitic high-strength steel 100Cr6 (SAE 52100) were carried out until an ultimate number of cycles of 109. Additionally, runout specimens were tested repeatedly with higher stress amplitudes until failure occurred. By this method threshold values for the stress intensity factors in the very high cycle fatigue regime were found to be dependent on the inclusion type. So beside the threshold value for crack propagation of long cracks inside a fish-eye, Kth, another threshold value can be found for the formation of the so called fine granular area which is characteristic for very high cycle fatigue failure. This threshold value, below which no initiation in form of a fine granular area can be observed, is further dependent on the chemical composition of the crack initiating inclusion. Thus, a true fatigue limit in the very high cycle fatigue regime depends on the inclusion size and inclusion type. Further the inclusion type with the lowest threshold value indicates an absolute threshold value for very high cycle fatigue crack initiation due to the formation of the fine granular area.
To determine the effect of the CaO/Al
2
O
3
ratio on desulphurization and inclusion evolution in low-density steel, a series of laboratory-scale experiments are performed. The results show that ...violent reaction occurs at the interface between refining slag and molten steel. As the slag-steel reaction progresses, (CaO) and (MgO) are reduced to Ca and Mg by dissolved Al. The deoxidation product Al
2
O
3
dissolves into the slag and accumulates at the slag-steel interface. The evolution process of the sulphide inclusions is CaS → MgS → MnS. The sulphide outer layer wraps AlN and spinel inclusions, floats up and is absorbed by the top slag. During refining process, the accumulated Al
2
O
3
at the slag-steel interface easily forms MgO·Al
2
O
3
. Finally, a dense spinel layer can be formed at the slag-steel interface, which inhibits the desulphurization reaction. Reducing the C/A ratio can reduce the formation of MgO·Al
2
O
3
and has a positive effect on the desulphurization in low-density steel. The influence of the C/A ratio on desulphurization kinetics should be considered in the design of slag composition.
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