The MnO–SiO2-type inclusions changed into MnO–Cr2O3 type inclusions in an Fe–Cr alloy deoxidised by Mn and Si with a low Si concentration by heat treatment at 1473 K. At high Si content, the MnO–SiO2-type inclusion remained stable even after heat treatment. The change in the chemical composition of the oxide inclusions by heat treatment depended on the concentrations of Si and Cr in the Fe–Cr alloys. The mechanism of the change in the chemical composition of the oxide inclusions was investigated by using two experimental methods. The solubility of Cr2O3 in MnO–SiO2 was measured from 1473 to 1673 K. The solubility of Cr2O3 decreased with temperature. The other experiment was performed using a diffusion couple method between the Fe–Cr alloy and MnO–SiO2 at 1473 K to investigate a reaction between them. The formation of MnO–Cr2O3 was observed at the interface. It is found that both the decrease in solubility of Cr2O3 and diffusion of Mn, Cr, and Si at the interface between the Fe–Cr alloy and oxide inclusions are important for controlling the change in the chemical composition of the oxide inclusions in the Fe–Cr alloy by heat treatment at 1473 K.
Changing behavior of non-metallic inclusions in composition and size distribution at high temperature was investigated by preparing three kinds of Fe–Al–Ti steels, heating at 1473 K, and observing inclusions by SEM-EDS. In the case of the specimen in which thermodynamically stable inclusion at 1873 K is Al2O3, mostly pure Al2O3 inclusions were observed in an as cast sample, while many dual phase inclusions were observed after heating. Iron content in inclusions increased and many Al–Ti–Fe–O inclusions in narrow composition range were observed. In the case of the specimen in which stable inclusion at 1873 K is Ti3O5, TiOx inclusions were observed in an as cast sample. Inclusion compositions changed to Fe–Ti–O by heating and inclusion size became smaller to 2 to 3 μm. In the case of the specimen which composition is located at the boundary area of Al2O3, Al2TiO5, and Ti3O5 stable regions at 1873 K, various inclusions from pure Al2O3 to TiOx were observed in an as cast sample. By heating, three types of inclusions, namely, Al2O3, Al–Fe–O, and Al–Ti–Fe–O inclusions were observed and average inclusion size decreased. Although the stable phase of oxide equilibrated with Fe–Al–Ti–O system at 1473 K are not clarified, inclusions equilibrated with molten metal are no longer stable in solid state steel and thus inclusion composition and size change by reacting with solid steel.
The most important subject in the steelmaking process is the control of non-metallic inclusions. Non-metallic inclusions with a high melting point do not deform during a hot working process because they are relatively hard. Hence, the inclusion composition should be controlled in order to achieve a low melting point to prevent product defects. Therefore, an MnO–SiO2-based inclusion is considered to be one of the preferred systems. Moreover, the heat treatment of austenitic stainless steel has been reported to influence the composition of MnO–SiO2-type inclusions; these inclusions change into MnO–Cr2O3-type, MnO–Nb2O5-type, and MnO–V2O3-type inclusions. In this study, we investigated the influence of heat treatment on the composition of the inclusions in the martensitic stainless steel. In general, a scanning electron microscope–energy dispersive x-ray spectrometer (SEM–EDS) is used for the quantitative analysis of inclusions; however, SEM–EDS cannot simultaneously analyze a large number of inclusions. Therefore, a new technique using the SEM–EDS along with the image analysis software “Particle Analysis” was used for the chemical analysis and the size measurement of a large number of inclusions (hereafter abbreviated as the PA method). The heat-treatment-induced compositional change of the inclusions in martensitic stainless steel was evaluated by using both the analysis methods.
A desulfurization slag is difficult to recycle, since there is a possibility to elute a sulfur containing solution. To promote the recycle of the desulfurization slags, it is important to know the physicochemical properties of the slag. In addition, desulfurization process could be modified, when the sulfur behavior in the slag was understood. In this study, the effect of sulfur and oxygen partial pressure on the crystallization behavior was investigated using Hot Thermocouple Technique. Three different atmospheres were established in the electric furnace through the modification of experimental apparatus. One was an argon without deoxidation, the second was an argon with Ti plate (Ar+Ti) which was located in the electric furnace for deoxidation of argon gas, and the third was an argon with CaS pellet located on the Ti plate (Ar+Ti+CaS) in which the partial pressure of SO2 increased with the decrease of the oxygen partial pressure. The nose position of TTT diagram of CaO–Al2O3 eutectic composition under Ar atmosphere was 7 s at 1100°C, while the nose positions under Ar+Ti and Ar+Ti+CaS were 7 s at 1150°C. TTT diagrams under Ar+Ti and Ar+Ti+CaS, which are lower oxygen potential and higher SO2 potential, showed the earlier crystallization behavior in the higher temperature region than the nose position. From the results of XRD analysis, the sulfur added to the slag existed in the monocalcium aluminate (CaO·Al2O3) primary crystal. When the content of CaS increased, a formation of CaO through oxidation increased and the tricalcium aluminate (3CaO·Al2O3) was formed as a primary crystal, in which the sulfur content was higher than the eutectic structure in the matrix.
It is important to know the behavior of inclusion in a CaO–Al2O3-(SiO2 and/or MgO) system for controlling the properties of steel. The crystallization of inclusion has a large effect on the rolling process. However, there is little study of the effect of sulfur on the crystallization of inclusion. In the previous paper, the authors were investigated the sulfur behavior in the CaO–Al2O3 slag at the eutectic composition (CAEU) under controlled atmosphere. In the present study, the effects of sulfur on the TTT diagrams were measured by the hot thermocouple method under controlled atmosphere. 1 mass%, 5 mass% and 10 mass% CaS were added to the eutectic composition of calcium aluminate (CAEU). The crystal region in TTT diagrams was expanded by the addition of CaS from 1 mass% to 10 mass% under Ar atmosphere. And the start of crystallization became faster with the increase of CaS addition. The crystal phases were monocalcium aluminate (CA) and tricalcium aluminate (C3A) regardless to the CaS content. However, primary crystal was changed from the content of CaS. When 1 mass%CaS was added to CAEU, the primary crystal was CA. On the other hand, the primary crystal was C3A in the cases of 5 mass% and 10 mass%CaS.
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.
In order to clarify both the behavior of non-metallic inclusions during hot deformation and the effects of non-metallic inclusions on the local ductility of steel with aluminum deoxidized and containing lower sulfur content at about 0.002–0.01% were investigated. Both the commercial-quality 440 MPa-class plain carbon steel and super ultra-low carbon steel were studied. To investigate the distribution, morphology, and chemical composition, along with the change in such characteristics, during the hot deformation of non-metallic inclusions, thermo-mechanical treatment with a compression test was carried out. Moreover, the reduction of area with the tensile test species, which refers to the local ductility of steel, was examined, and the effect of the distribution, morphology, and chemical compositions of both the Al2O3 inclusions and the elongated MnS inclusions were studied. Consequently, metal sulfur content of higher than 60 ppm and elongated MnS inclusions of over 10 regarding the aspect ratio were observed. In addition, the elongated MnS inclusions had a stronger influence on local ductility than the smaller Al2O3 inclusions, and drastic effects on the nucleation of voids. Thus, a fracture is most likely to be initiated by void formation at the interface of the elongated MnS inclusions and metal matrix notched by MnS, and thus would experience coalescence in accordance with a brittle fracture in the soft MnS inclusions. The local ductility in steel including elongated MnS inclusions is small because the fracturing and deformation of metal are most likely to be related to the elongated MnS inclusions.
Effect of heat treatment conditions on shape evolution of large-sized elongated MnS inclusions in resulfurized free-cutting rolled steel has been investigated using confocal scanning laser microscope and Si2Mo resistance furnace. The results show that the heating rate, soaking temperature and soaking time impose significant effects upon shape profiles of elongated MnS. The split of slender MnS was oberved in continuous heating with heating rate in the range of 0.5–2 K/s. In addition, split degree of MnS indicated a negative relation with the rise of heating rate. As a result, separation of elongated MnS was not observed at the heating rate of 6 K/s. During soaking experiments, there was no remarkable shape change for MnS at temperature lower than 1073 K. While the elongated MnS splited up and spheroidized obviously at 1473 K. Correspondingly, number density of inclusions increased while mean length reduced as the soaking time increased from 1 h to 4 h at 1473 K. Significant shape change from slender to spindlelike or spherical was identified only when the soaking time exceeds 3 h or 4 h. Based on the Gibbs Thompson relation and the obtained experimental results, mechanism of shape evolution of MnS inclusions was discussed and morphology evolution of MnS was divided into three major steps: (1) first, the shrinkage occurred in the longitudinal direction at the beginning of heating process, (2) expansion and contraction in radial direction followed after the shrinkage which caused the split of slender MnS; (3) eventually, the spherical particles emerged from the split parts.
Equilibrium experiments were carried out with solid Fe and liquid Ag to investigate the interactions between Si and Ti in solid Fe. The thermodynamic properties of Si in γ-Fe and α-Fe phases were studied by determining the distribution equilibrium of Si in the Fe and the Ag phases. Using the activity coefficient and the self-interaction parameter of Si in solid Fe, the interaction parameter of Si and Ti in both the solid Fe phases— γ-Fe and α-Fe—was determined as follows: εSiTi = 460 ± 78 in γ-Fe, εSiTi = 350 ± 7 in α-Fe The analysis results for Si and Ti in the solid Fe phase showed reasonable agreement with previous research in α-Fe region. Furthermore, the influence of the interaction between Si and Ti in the solid Fe phases on the formation of secondary inclusions such as Ti-based inclusions was discussed. It was concluded that, during solidification, the interactions among solutes such as Si and Ti should be properly considered because the formation of not only the secondary inclusions but also ferrites may depend on the thermodynamic properties of the solutes.
Refinement of austenite grains by generating intra granular ferrite (IGF) in austenite grains is one of the most effective methods for improving the toughness of the heat-affected zone (HAZ). It is known that MnS precipitated on fine oxides enhance formation of IGF in austenite grains. Flux composed of CaO–Al2O3 is often used in secondary refining and Ca–Mn–Al complex oxide may form, and it is important to clarify the mechanism of MnS precipitation on oxide inclusions. Therefore, molten CaO–MnO–Al2O3 oxide was equilibrated with (Ca, Mn)S solid solution in Al2O3 crucible in an electric resistance furnace at 1648, 1698 and 1748 K in the present work. Phase relation between CaO·Al2O3 saturated molten CaO–MnO–Al2O3 oxide and (Ca, Mn)S solid solution was clarified.
It is important to determine the equilibrium relationships between the oxide compounds in MgO–Ti2O3–Al2O3 and iron to avoid Al2O3 or MgO·Al2O3 formation and for inclusion control. In this study, the equilibrium relationships between the oxide compounds in MgO–Ti2O3–Al2O3 and molten iron are investigated at 1873 K. The phase-stability diagrams, which show the equilibrium relationships between the oxide compounds in MgO–Ti2O3–Al2O3 and molten iron, are described. It is also important to know the oxides that newly form after solidification. Accordingly, the temperature-dependences of the stable compounds that newly form after solidification were investigated using the thermodynamic calculation software FactSage 6.1. Although the compounds formed after solidification are not necessarily the same as the equilibrium oxide compounds at 1873 K, the correlation between them can be confirmed. In particular, under the unstable condition of MgAl2O4 at 1873 K, it is found that the formation of MgAl2O4 could also be suppressed after solidification.
This paper investigates the effect of de-oxidation inclusions on micro-structure in low carbon (0.07 mass%), high Mn (0.9 mass%) steel. De-oxidation tests were carried out by adding either aluminum (0.05 mass%) or titanium (0.05, 0.03 or 0.015 mass%) to an iron melt in a 400 g-scale vacuum furnace. A Confocal Scanning Laser Microscope (CSLM) was used to evaluate the effect of cooling rate by re-melting and quenching during solidification. Fine secondary de-oxidation particles were obtained in the Ti-killed samples, and the particle density increased with increasing oxygen content, and their size decreased with increasing the cooling rate during solidification. The secondary Ti de-oxidation particles were found to have an effect on microstructure evolution, such as solidifying microstructure, austenite grain growth and austenite decomposition. The de-oxidation particles were examined through FE-TEM and were identified to be TiO, MnTiO3 and Mn2TiO4, in low oxygen ([O]=7 ppm) and high oxygen ([O]=56, 81 ppm) Ti-killed steels respectively, which were qualitatively same as those predicted by thermodynamic calculations. Stabilities of TiO, MnTiO3 and Mn2TiO4 are influenced by Mn presence. Composition change and decomposition of oxide were estimated through thermodynamic calculations. The effect of the particles on ferrite formation was evaluated through thermo-mechanical treatments. TiO was the most effective for promoting ferrite formation through heterogeneous nucleation. The particles contributed to ferrite formation in the following order, TiO>TiN>MnS> MnTiO3>Ti2O3. It was found that the secondary Ti de-oxidation particles work are engulfed by the advancing solid phase during solidification based on analysis with PET (Pushing Engulfment Transition) velocity, particle sizes and solidification rates. The particles at dendrite tips and inter-dendritic regions are likely restraining the molten steel flow resulting in a finer solidification microstructure.
The effect of the addition of Ti and Al on the solidification structure of Ni–4%Mo–5%Cu–13%Fe (mass%) alloys was investigated. It was found that the addition of an optimal amount of both Ti and Al affects on the formation of equiaxed fine grains. Oxide films formed on the molten alloy surface (scum) were examined using a transmission electron microscope equipped with energy dispersive spectrometers. The scum was analyzed and found to contain two separated phases of TiO with the monoclinic structure and Al2O3 with the corundum structure. The TiO might be the NaCl structure at the melting point of the matrix phase. According to the lattice disregistry between the fcc Ni and oxides TiO with the NaCl structure has lower disregistry than Al2O3 with the corundum structure. These facts suggest that TiO precipitated with Al2O3 in Ni melt can form heterogeneous nucleation sites for the fcc Ni phase. The distribution conditions of TiO may be different between the Ti-added alloys and those with the addition of both Al and Ti. It is considered that the addition of both Al and Ti causes fine dispersion of TiO and promotes nucleation of the primary fcc Ni phase.
The effect of chemical composition of B1 compounds on the formation of intragranular ferrite during isothermal austenite-to-ferrite transformation was investigated by embedding single crystalline grains of chemically pure B1 compounds in low-alloy steel. Two kinds of B1 compounds having similar misfit against ferrite, TiO and TiN, were adopted in this study. Isothermal transformation experiments were carried out both above and below the bainitic transformation temperature. The orientation relationships between compound and ferrite and between ferrite and prior austenite were characterized by scanning electron microscope equipped with electron backscatter diffraction analyzer. It was found that the Baker-Nutting (B-N) orientation relationship played a dominant role in ferrite formation from B1 compounds at higher transformation temperature, but some low-index orientation relationships other than the Baker-Nutting orientation relationship were also observed as the transformation temperature decreased, which led to an increase in ferrite formation having the Kurdjumov-Sachs (K-S) orientation relationship with prior austenite. Ferrites from TiO had larger misorientation angles from the exact B-N orientation relationship and thus tended to have the K-S orientation relationship with prior austenite at lower transformation temperatures.
Some aspects for the application of Laser Ablation-Inductively Coupled Plasma Mass Spectrometry (LA-ICP-MS) are considered and discussed from the viewpoint of the rapid analysis of oxide inclusions in metal samples. The inclusion characteristics in Fe–10% Ni alloy samples such as number, size and particle size distribution obtained by LA-ICP-MS method are compared with those from three-dimensional observation of particles on a film filter after electrolytic extraction. Though some limits had to be considered regarding to the content of soluble elements, the number and dispersion of analyzed inclusions in metal matrix, it was found that the LA-ICP-MS technique can be successfully applied for the rapid analysis of oxide inclusions containing Al2O3, MgO and others.
Extraction of inclusion particles from a metal matrix allows for accurate three-dimensional estimation of their morphology, size, and composition. In this study, the stability of MgO and MgAl2O4 particles was examined using acid, halogen-methanol, and nonaqueous electrolytes. These particles hardly dissolved in a 2% TEA-Ba (2 v/v% triethanolamine-1 w/v% tetramethylammoniumchloride-methanol containing 0.05–0.20 w/v% Ba) electrolyte. The potentiostatic extraction method using nonaqueous electrolytes with various Ba and H2O contents was examined for the extraction of MgO inclusion particles from metals. The O content of the extracted MgO and MgAl2O4 inclusion particles agreed approximately with the analyzed total O content of the metal. The Mg contents of the extracted inclusion particles were in agreement with those calculated using the results of two-dimensional measurements. Finally, it was concluded that 2% TEA-Ba was the most suitable for the electrolytic extraction of MgO and MgAl2O4 inclusion particles from steel.
A statistics of extreme values was applied to determine the largest inclusion sizes in the Type 304 stainless steel. The samples taken from a tundish, slab and hot rolled steel in one heat were examined by using a two dimensional observation of inclusions on a metal cross section. It was found that the molten steel sample contained two different types of inclusions, which were deoxidation products (SiO2–CaO–MgO–Al2O3) and reoxidation products (SiO2–MnO–Cr2O3). As a result, the extreme value distribution (EVD) for different types of inclusions in the melt has two different slopes. Meanwhile, the inclusions in the slab sample provided a good linearity in one EVD. Moreover, the correlation coefficients of the regression lines for both the slab and rolled steel samples increased significantly with an increased number of measurements from 40 to 80 unit areas. It was found also that the EVD data for fractured inclusions on a parallel cross section of rolled steel agreed satisfactorily well with that for the initial spherical inclusions in the slab sample. Based on the geometrical considerations of inclusion deformation and fracture during hot-rolling, the maximum length of fractured inclusions in rolled steel can be estimated reasonably well from the EVD for initial undeformed inclusions in the slab sample.
The effects of observed number of inclusions on the diameter distribution at two-dimensional inspection were evaluated in this work. Microscopic observations were reproduced by numerical calculations. Numerically calculated diameters of circles at cross-sectional planes were aggregated as size and density histograms and those were compared with theoretically calculated histograms. Deviations between the calculated and true density in each class were evaluated quantitatively with the error evaluation index. The reliability of two-dimensional particle size and density histogram was affected by the observed number of particles rather than the observed number of fields and independent of the input three-dimensional particle distribution. Misunderstandings in reading the histograms could be prevented by setting small number of classes when the observed number of particles is small. It is better to decide the goal of reliability level before inspection to optimize the observation time and cost.
During routine metallographic investigations of some fully processed, non-oriented, electrical steel sheets, the typical MnS inclusions were not observed in the microstructures. In the MS-type sulphides, the manganese was substituted by magnesium. A systematic ex-situ characterisation of the non-metallic, magnesium-containing inclusions was carried out and the origin of the inclusions was proposed. The inclusions' chemistry and morphology were investigated by light microscopy and field-emission scanning electron microscopy. The non-metallic, magnesium-containing inclusions were classified as complex sulphides, oxides and spinels. Magnesium was also detected as being co-precipitated with other non-metallic inclusions, like nitrides. The form of the co-precipitated magnesium inclusions was predetermined by the shape of the thermodynamically most stable inclusions.