The estimation of fine inclusion particles is required in order to clarify its effect on the miniaturization of steel grain. In this study, the stability of ZrO2, Ti2O3, TiAl2O5, Ce2O3, and CeS particles during extraction was examined using acid, halogen–methanol, and nonaqueous electrolytes. ZrO2, Ti2O3, and TiAl2O5 particles hardly dissolved in 4%MS (4 v/v% methylsalicylate–1 w/v% tetramethylammonium chloride–methanol) and 10%AA (10 v/v% acetylacetones–1 w/v% tetramethylammonium chloride–methanol) electrolytes, while Ce2O3 and CeS particles did not dissolve in a 2%TEA–Ba (2 v/v% triethanolamine–1 w/v% tetramethylammonium chloride–methanol containing 0.16–0.24 w/v% Ba) electrolyte during potentiostatic extraction. The O content of the extracted ZrO2, Ti2O3, and Ce2O3 inclusion particles agreed approximately with the difference between analyzed total O content and calculated equilibrium O content of the metal, and the S content of the extracted CeS inclusion particles agreed with the analyzed total S content of the metal. The Ce content of the extracted inclusion particles was in agreement with that calculated using the results of two-dimensional measurements.
NMI in electrical steels are known to have a very complex chemical makeup and history of formation, and exert a large influence on the handling of this steel in secondary metallurgy and casting. We have developed routines to analyse the non-metallic inclusion contents of non grain-oriented electrical steels by automated FEG-SEM analysis. We automatically analyze NMI of sizes down to 0.08 μm2 in a steel that has significant amount of alloyed Si (2.3 wt%) by employing a matrix spectrum subtraction routine that leaves only the signal of the NMI itself to be quantified. We describe data reduction procedures for the NMI populations that consist out of duplexing oxides, sulfides, and nitrides. The chemical complexity can be represented and understood in terms of using a multicomponent projection technique, and based on the analysis of the particles it is possible to calculate quantitatively the amount of elements contained in the NMI assemblage of a steel sample. We find that this mass balance gives results in good to excellent agreement with bulk steel analyses for elements that are dominantly inclusion bound, such as Ca, if the complete inclusion size range above 0.08 μm2 is taken into account. Based on these data analysis methods, we compare the NMI development in two heats through the secondary metallurgy process. Although the same steel composition was alloyed, the NMI developed vastly different depending on the details of treatment on the ladle furnace.
A two-dimensionally imaging spectrometer system was employed to obtain an emission image of a test specimen and to estimate the lateral resolution, when it was excited from a glow discharge plasma at a pulsed direct-current voltage. The specimen was a square-shaped copper chip having a dimension of 1.0 × 1.0 mm stuck on a nickel substrate. A Grimm-style glow discharge excitation source, whose hollow anode had an inner diameter of 8 mm, was employed as the excitation source. A conventional discharge condition did not give good lateral resolution being suitable for the actual application, because the emitting zone extended broadly over the size of the copper chip. The gate width of the charge-coupled device (CCD) detector and the on/off-periods of the discharge were investigated to improve the lateral resolution. Degradation of the resolution would be caused by the fact that a sample atom emits at different portions of the plasma body repeatedly, even during a single pulse of the discharge. Therefore, it is suggested that the gate width of the CCD detector should be reduced as small as possible, so that the emission of a sample atom can be detected instantaneously after ejected from the original position in the sample surface.
The amount, chemical composition and distribution of nonmetallic inclusions are important factors when determining steel quality. Therefore, in recent years, a great deal of effort has gone into developing robust detection systems for nonmetallic inclusions. Various methods have been suggested, but most of them require extensive sample preparation. As a result, these methods are only suitable for analyzing micro-inclusions. For estimating the macrocleanliness, these methods are too slow because, due to the rarity of these kinds of inclusions, huge sample volumes are needed. To overcome this problem, we propose a new detection methodology that can detect inclusions and defects, like microcracks and shrinkage holes, at a range of between 5 μm and several thousands of a μm. At the same time, it is fast enough to process samples of very large steel volumes of 300 × 120 × 90 mm³ in size. Similar to the classic metallographic approach using a microscope and polished steel surfaces, the steel surface of a sample is recorded by a moving CCD sensor with a pixel size of 2.75 μm. Inclusions are detected using image processing techniques. Then, the steel surface is milled off, removing a 10 μm chip, and the recording step is repeated. Processing the whole sample in this way allows us to reconstruct the three-dimensional shape of inclusions and other defects and gives us information about the spatial distribution of these inclusions. In this paper, we describe this system in greater detail and describe the initial results of our approach.
SEM (scanning electron microscopy) is a useful technique for the observation of the surface morphology of various materials. Compared to TEM (transmission electron microscope), one of the advantages of SEM is easy sample preparation, although the spatial resolution of SEM is normally less than that of TEM. To improve the spatial resolution of SEM observation, it is well known that a low accelerating voltage SEM is an effective technique. We have proposed the application of glow discharge surface treatment before high-resolution SEM observation. An rf-glow discharge was applied with Ar gas for a steel sample just for 6 sec., leading to surface cleaning, that is, removing the surface oxidation or contamination layer. Besides the surface cleaning, a glow discharge sputtering modified the surface of the steel sample depending on crystal orientation. This surface modification was useful for high-resolution SEM observation. The surface of the steel sample was observed by FE (field emission)-SEM with a low accelerating voltage. A fine structure of grains and inclusions in the sample was clearly observed. The density of the inclusions was roughly determined as being 4 × 104/cm2.
SEM-EDS is a powerful tool for fast, nondestructive x-ray elemental analysis of a localized region. Elemental mapping is also possible by scanning the electron beam. Since the penetration depth of the electron beam is about a few μm, SEM-EDS is useful for determining elemental composition near the surface of the sample. To obtain information regarding the depth of the sample, the surface layer must be removed. For this purpose, we attempted to use glow discharge sputtering. A commercially available rf glow discharge sputtering device (Horiba, Tensec) enables fast sputtering in an area 4 mm in diameter. The combination of SEM-EDS and glow discharge sputtering would be useful for observation of the distribution of inclusions buried in metal samples. Al2O3 particles were mixed with Cu powders, and then pressed into a disk, which was measured. The same area of the sample was analyzed by FE-SEM-EDS and sputtered. The result indicated the possibility of 3D distribution analysis of inclusions in the sample.
The removal of non-metallic inclusions in the metallurgical process greatly affects the properties of the final products. The structure of inclusion clusters plays a key role in inclusion behaviors of their removal process, such as coagulation, flotation and bubble adhesion. However, it is rare to find reports quantitatively investigating the morphology of inclusion clusters in metal system. On the other hand, to quantitatively estimate the inclusion clusters in metal, it is required to distinguish clusters on two-dimensional (2D) cross-sectional images of the as-polished samples. In this study, TiB2 particle clusters were prepared in a mechanically agitated crucible containing molten Al at 1073 K. The samples of Al-TiB2 were measured by X-ray micro-computed tomography (micro-CT) to obtain the three-dimensional (3D) information of TiB2 particles and clusters in solid Al. The images of 3D particle clusters in solid Al were extracted and reconstructed by self-developed programs. A series of parameters were defined to describe the 3D characteristics of clusters and their 2D cross-sections. The effects of agitation time and speed on the cluster structure were investigated. A program was developed to distinguish clusters in 2D cross-sections through the use of the 3D cluster information (DC-2D-3D) obtained from X-ray micro-CT.
A confocal micro-XRF method combined with two individual polycapillary lenses was applied to steel sheets coated with anti-corrosive paint in order to nondestructively observe 3D elemental distribution of paint steels and corroded paint-coated steels. Nondestructive depth analysis and 3D elemental mapping of the painted steel sheets were demonstrated under the confocal XRF configuration. Three different painted steel sheets were prepared by cation electrodeposition coating for automotive onto flat steel sheets modified with a zinc phosphate conversion coating. These painted sheets were then caused to corrode by means of accelerated exposure to a salt bath (5 mass% NaCl) at 55°C for 240 hours. Depth elemental profiles of Ti, Zn, and Fe obtained by confocal micro-XRF measurements were in excellent agreement with that of the prepared sample. Elemental depth profiles and maps of the corroded painted sheets showed some blisters caused by crevice corrosion, which started from the site of a scratch. The distributions of Ti and Fe were approximately homogeneous in both the paint layer and the steel substrate, while the distributions of Zn, Mn, Ca, and Cl were heterogeneous.
Particle coagulation plays a key role in steel refining process to remove inclusions. Many research works focus on the behaviors of particle coagulation. To reveal its mechanism water model experiments have been performed by some researchers including the authors’ group. In this paper, experiments of particle coagulation were carried out with molten Al including SiC particles in a mechanically agitated crucible with two baffles. Particle coagulation and formation of clusters were observed on the microscopy images of as-polished samples. Three-dimensional (3D) analysis of the clusters in solidified Al was implemented by X-ray micro CT available at SPring-8. The methods to distinguish clusters on two-dimensional (2D) cross-sectional images were discussed, which were established in the previous works by the present authors’ group. The characteristics of the 3D SiC clusters and their 2D cross-sections were analyzed. The statistical ranges of the parameters for 2D clusters were used as criterions to distinguish the clusters on 2D microscopy images from the as-polished samples. The kinetics of SiC particle coagulation was studied by the measured cluster number density and size using our program to distinguish cluster in 2D cross-sectional images according to 3D information (DC-2D-3D). The calculated and experimental results of the SiC particle coagulation in molten Al agree well with each other.
The probable maximum sizes (PMS) of inclusions in AISI304 stainless steels were predicted by two methods of the statistics of extreme values (SEV) and the particle size distributions (PSD). Firstly, the PMS of inclusions in the molten steel taken from a tundish agreed well with those in the slab sample. The particle size distributions (PSD) of inclusions almost obeyed exponential functions. The results of comparison between the two methods showed that the PMS by the SEV analysis agreed with that by the PSD approximation in the case of the steel with the higher oxygen content. However, in the case of the steel with the lower oxygen content, the PMS by the PSD approximation overestimated the predicted size with the reference to the SEV analysis. In this case, the approximations with elimination of some largest inclusions which were deviated from an exponential distribution were found to be effective to predict the probable largest size in a reference area. This above result suggests that it is necessary to decide if some largest inclusions are employed for adequate predictions. It is considered that necessity of this operation increases with decreasing oxygen content.
Non-metallic inclusions have always been the active subject of steelmaking research to improve the steel cleanliness and to develop the so-called oxide metallurgy technology. Inclusions in molten steel form and grow by the sequence of nucleation, chemical and physical growth and removal. Thus, the size distribution of inclusions evolves continuously with time in molten steel, and significant changes in the steel conditions are reflected in the inclusion size distribution as well as in the inclusion chemistry. This study aims to provide a new approach to interpret the inclusion size distributions. The concept of the Population Density Function (PDF) is introduced to objectively represent a given inclusion size distribution. Several possible applications of PDF analysis are presented to demonstrate the advantages of the utilization of the PDF for understanding the inclusion formation mechanism during the steelmaking process. Several ambitious ideas to utilize the PDF for inclusion size control are also presented.
Modern tundishes plays an important role of refining treatments to improve the quality and purity of casted steel. Purity of steel is defined by the non-metallic inclusions in the steel product, including their size, quantity, distribution, chemical composition and mineralogy. The aim of presented studies was to investigate the number and distribution of non-metallic inclusion in individual billets casted in a six-strand tundish. The industrial measurements, performed during stable production conditions at the continuous steel casting (CSC) plant, were performed for different tundish working space configurations. Analysis of the size and number of non-metallic inclusions has been done on the metallographic samples using light microscope. Experimental studies were supported with numerical simulations using large eddy simulations (LES) method. A modified boundary condition describing inclusion separation at the liquid steel surface was implemented in commercial code AnsysFluent. Experimental results concern size distribution of inclusions in billets for current tundish configuration showed big differences between casted ingots. Numerical results shown the domination in the number of inclusions occurring in the nozzles number 3 and 4 (for basic tundish configuration) and in the nozzles number 2 and 5 (for tundish with turbulence inhibitor). The reason for that is the change in configuration tundish working space, that has an impact on the flow field inside the tundish. Experimental measurements performed for proposed modified tundish configuration (with turbulence inhibitor) shown that those differences are much smaller, which in consequence has an influence in higher quality of continuously casted ingots for individual strand of CSC.
Microwave-induced plasma optical emission spectrometry (MIP-OES) using Okamoto-cavity has a unique feature being suitable for in-situ analysis in steel-making industry. The Okamoto-cavity MIP can be directly loaded with organic solvents including sample solution as well as fine particles of a solid sample in the nitrogen-oxygen mixed gas plasma; furthermore, their emission intensities are much elevated by adding oxygen of up to 10% to the nitrogen matrix gas, thus contributing to better sensitivity in the MIP-OES. The reason for the intensity enhancement was investigated with a spectroscopic method that the spatial distribution of the emission intensity from the plasma was estimated with a two-dimensionally imaging spectrograph. Emission lines of chromium, which was the most important alloying element in steel materials, as well as band heads of nitrogen molecule were observed, indicating that the emission intensity of atomic chromium lines was drastically elevated whereas the intensities of ionic chromium lines and the nitrogen band heads were commonly reduced when oxygen gas was added to the nitrogen plasma. This result implies that the ionization of chromium, which dominantly occurs through collisions with nitrogen excited species, can be suppressed because the nitrogen excited species would be consumed through collisions with oxygen molecules to cause their dissociation. Optimization of the measuring parameters in the Okamoto-cavity MIP-OES was conducted to determine chromium contents with good precision, and finally the analytical performance in the MIP was compared with that in a conventional ICP-OES.
Since selective oxidation of alloying elements on the surface of steel products influences their surface properties, characterization of the surface oxides which can be considered as non-metallic inclusions is of great importance. In this study, X-ray photoelectron spectroscopy (XPS), X-ray diffractometry (XRD) and X-ray absorption spectroscopy (XAS) were used for characterizing the formation process of Mn oxides on the surface of annealed Fe–Mn alloys under a low partial pressure of oxygen. The results obtained by XPS showed that the enrichment and oxidation of Mn occurs on the surface of the Fe–Mn alloys annealed under low oxygen partial pressure, and Mn oxides are formed in the metallic Fe matrix in the surface layers. XAS spectra using grazing-exit X-ray fluorescence measurements showed depth-resolved information on chemical state of Mn. These Mn oxides were identified as MnO (manganosite) by grazing-incident XRD measurements. It was found using in situ XRD measurements at high temperatures that the lattice constant of MnO increased with increasing annealing temperature, which attributed to the non-stoichiometry of MnO. These oxidation characteristics of Mn in the Fe–Mn alloys are discussed on the basis of thermochemical properties of Mn.
The effects of nitrogen content on the formation and changing behaviors of non-metallic inclusions in the four kinds of Fe–Al–Ti–N alloys during heating at 1473 K were studied by observation and analysis of inclusions by FE-SEM and EDS. At the inner part of the specimens, TiN-based inclusions were mainly observed. The size of TiN single phase particles observed in as-cast specimens was much larger in the specimens with larger nitrogen content. The fraction of the number of TiN-based inclusions increased by heating in the specimens with larger nitrogen content, while it decreased in the case of smaller nitrogen content due to the formation of TiS inclusions during heating. The growth of TiN single phase particles in larger nitrogen content specimens was more significant than those in smaller nitrogen content specimens. At the outer part of the specimens, TiN formed after heating on the already existing oxide inclusions. In addition, the size of TiN on the oxide surface formed during quenching also increased by heating. The fraction of the number of oxide inclusions with TiN increased in all specimens by heating. TiN phase on the oxide surface in as-cast specimens was bigger in the case of larger nitrogen contents. TiN growth during heating was also enhanced in larger nitrogen content specimens.