Characteristics Of Inclusions In Steel Research Articles

Effect of sulfur content on precipitation behaviour of MnS-containing inclusions in X70 pipeline steel

The presence of oxide-MnS complex inclusions is beneficial for the improvement of hydrogen resistance in pipeline steel. The effect of S and Mg contents on the generation mechanism of MnS, MgAl2O4, and MgO inclusions, and the heterogeneous nucleation principle of MnS on the surfaces of MgAl2O4 and MgO were revealed using laboratory experiments, thermodynamic calculations, and density functional theory (DFT) computations. The inclusion characteristics in steel were evaluated by using a Scanning Electron Microscope in combination with an Energy Dispersive Spectrometer (SEM + EDS). It has been found that as the Mg content increased, MgAl2O4 transformed into MgO, which can be well predicted by thermodynamic calculations. It was found that MnS exhibited the ability to nucleate heterogeneously on the surfaces of MgO and MgAl2O4 inclusions. Furthermore, proper S content (approximately 30 ppm) can facilitate the formation of oxide-MnS complex inclusions, excess S content was not desired. The smaller the oxide inclusion, the easier to form wrapped MnS on the oxides. The nucleation ability of MnS and MgAl2O4 was identified by DFT calculations and found that they followed the order of (111)MnS//(111)MgAl2O4>(100)MnS//(100)MgAl2O4>(110)MnS//(110)MgAl2O4. Furthermore, the heterogeneous nucleation ability of MnS and MgO followed in the order of (111)MnS//(111)MgO > (100)MnS//(111)MgO > (100)MnS//(100)MgO. This study provides theoretical evidence supporting the heterogeneous nucleation ability of MnS on the surfaces of MgO and MgAl2O4. Moreover, it helps bridge the disparity between the calculation of MnS and MgO nucleation using the two-dimensional mismatch degree and the experimental findings.

Inclusion Characteristics in Steel with CeO2 Nanoparticle Addition

The application of Ce oxides in oxide metallurgy has received extensive attention, but until now, the direct adding of CeO2 into molten steel to generate Ce oxides has not occurred. In this paper, a mixture of CeO2 and Si nanoparticles were added into molten steel. The resultant formation of micrometer scale Ce-bearing oxides confirmed its adding validity. This behavior may be interpreted as the reactivity between CeO2 and [Al], and the improved wettability between CeO2 and molten steel with the assistance of Si powder. Thus, when the quantity of CeO2 is kept constant, its added yield should increase when increasing the added quantity of Si. This was verified by the larger percentage of Ce-bearing oxides of the total oxides and the greater average content of Ce in Ce-bearing oxides after normalization. Moreover, compared with the blank sample, statistical results indicated that the oxides in CeO2-modified samples were refined, and their dispersion homogeneity was enhanced. This comparison indicates the effectiveness of the external adding method in oxide metallurgy.

Non-metallic Inclusions in Different Ferroalloys and Their Effect on the Steel Quality: A Review

Ferroalloys have become increasingly important due to their indispensable role in steelmaking. In addition, the demand for improved steel qualities has increased considerably, which in turn highlights the quality of ferroalloys. This is due to the fact that the impurities in ferroalloys directly and significantly influence the quality of steel products. To gain a better understanding of the main trace elements and inclusions in ferroalloys (such as FeSi, FeMn, SiMn, FeTi, FeCr, FeMo, FeNb, FeV, FeB, some complex ferroalloys) and their behaviours in steel melt after the additions of these ferroalloys, information from a large number of previous results on this topic was extensively reviewed in this work. The applications of different ferroalloys and their production trends were discussed. In addition, the effects of some trace element impurities from ferroalloys on the inclusion characteristics in steel were also discussed. The possible harmful inclusions in different ferroalloys were identified. Overall, the results showed that the inclusions present in ferroalloys had the following influence on the final steel cleanliness: (1) MnO, MnS and MnO–SiO2–MnS inclusions from FeMn and SiMn alloys have a temporary influence on the steel quality; (2) the effect of large size SiO2 inclusions (up to 200 μm) in FeSi and FeMo alloys on the steel cleanliness is not fully understood. The effect of Al, Ca contents should be considered before the addition of FeSi alloys. In addition, Al2O3 inclusions and relatively high Al content are commonly found in FeTi, FeNb and FeV alloys due to their production process. This information should be paid more attention to when these ferroalloys are added to steel; (3) except for the existing inclusions in these alloys, the Ti-rich, Nb-rich, V-rich carbides and nitrides, which have important effects on the steel properties also should be studied further; and (4) specific alloys containing REM oxides, Cr–C–N, Cr–Mn–O, Al2O3, Al–Ti–O, TiS and Ti(C, N) have not been studied enough to enable a judgement on their influence on the steel cleanliness. Finally, some suggestions were given for further studies for the development of ferroalloy productions.

Evolution Mechanism of Oxide Inclusions in Titanium-Stabilized AISI 443 Stainless Steel

The evolution mechanism of oxide inclusions in Ti-bearing AISI 443 stainless steel was investigated by industrial experiment and thermodynamic calculation. The chemical compositions of steel and the characteristics of inclusions in steel were analyzed. After the addition of Al, the main type of inclusions in molten steel was irregular MgO·Al2O3 spinel. The MgO·Al2O3 inclusions were modified to be spherical CaO-Al2O3-MgO inclusions after calcium treatment. Thermodynamic calculation results indicated that several ppm Ca could significantly expand the liquid oxide phase field and decrease the stability of spinel. After titanium addition, two types of inclusions were formed: spherical Al2O3-TiOx inclusions and complex CaO-TiOx-Al2O3-MgO inclusions. The compositions of steel after Ti addition were mostly located in Al2O3-TiOx stability phase field. Based on the characteristics of inclusions in steel and thermodynamic calculation, inclusions consisting of liquid and CaTiO3 were formed in molten steel with more than 10 ppm Ca during the Ti addition process. The evolution mechanism of oxide inclusions was discussed with the consideration of the initial calcium content before Ti addition.

Solid-State Reaction Between Fe-Al-Ca Alloy and Al2O3-CaO-FeO Oxide During Heat Treatment at 1473 K (1200 °C)

The aim of this study was to control the physicochemical characteristics of inclusions in steel through appropriate heat treatment. Using a confocal scanning laser microscope (CSLM) and pipe furnace, the solid-state reactions between Fe-Al-Ca alloy and Al2O3-CaO-FeO oxide during heat treatment at 1473 K (1200 °C) and the influence of these reactions on the compositions of and phases in the alloy and oxide were investigated by the diffusion couple method. Suitable pretreatment of the oxide using a CSLM and production of the diffusion couple of Fe-Al-Ca alloy and Al2O3-CaO-FeO oxide gave good contact between the alloy and oxide. The diffusion couple was then sealed in a quartz tube with a piece of Ti foil to lower oxygen partial pressure and a block of Fe-Al-Ca alloy was introduced to conduct heat treatment experiments. Solid-state reactions between the alloy and oxide during heat treatment at 1473 K (1200 °C) were analyzed and discussed. A dynamic model to calculate the width of the particle precipitation zone based on the Wagner model of internal oxidation of metal was proposed. This model was helpful to understand the solid-state reaction mechanism between Fe-Al-Ca alloy and Al2O3-CaO-FeO oxide.

Effects of deoxidizing degree on the pitting corrosion behavior of carbon and manganese steels

Nine steels with different deoxidizing degrees and two comparative steels were selected. Their pitting initiation susceptibility was compared by means of potentiodynamic polarization tests in 3wt% NaCl solution. The pit propagation rate was evaluated in artificial sea water and 3wt% sea salt solution by simulating occluded corrosion cell (SOCC) test and hanging plate test, respectively. The composition of inclusions and corrosive feature were studied by scanning electron microscopy (SEM), electron probe micro-analysis (EPMA), and optical microscopy (OM). The results indicate that sulfide inclusions in steel are the sites for pit nucleation. The sulphide inclusions vary in shape from short spindle-like to long strip-like with increasing deoxidizing degree. Under the same conditions, the lower the deoxidizing degree gets, the lower the pitting initiation susceptibility becomes, and the stronger the resistance to pit propagation exhibits. For steels with different deoxidizing degrees, their pitting initiation susceptibility is mainly influenced by thermodynamic stability, while the pit propagation rate is primarily subject to the characteristics of inclusions in steel.

Morphology and growth of alumina inclusions in Fe–Al alloys at low oxygen partial pressure

The degree of supersaturation is a factor that influences the Al2O3 inclusion characteristics in steel. The influence of the addition of a large amount of Al in the molten steel on the formation, growth and morphology of Al2O3 inclusions was investigated by laboratory scale experiments. Consecutive steel samples were taken during the deoxidation process and subjected to chemical analysis (ICP-AES), automated image analysis (AIA) and scanning electron microscopy (SEM) assessment with respect to the extracted inclusions. The characterisation and quantification of Al2O3 particles show different growth processes, leading to variations in particle size distribution as well as in the morphology.

On the Role of Non-metallic Inclusions in the Nucleation of Acicular Ferrite in Steels

The effects of non-metallic inclusions in nucleating acicular ferrite in steels during cooling from a weld or cooling from an austenitic temperature are reviewed. The influence of the acicular ferrite (AF) structure on mechanical properties of steels such as strength and toughness is briefly mentioned. The different factors affecting the formation of acicular ferrite, such as the soluble content of alloying elements in steel, cooling rate from austenitizing temperature, austenite grain size and inclusion characteristics in steel, are discussed. The mechanisms of acicular ferrite formation on non-metallic inclusions, such as reduction of interfacial energy, mismatch strain between the inclusion and ferrite/austenite, thermal strains at the inclusions and changes in matrix composition near the inclusions are also discussed. Finally, the effects of inclusion characteristics, such as size, number and composition are described and their effectiveness in nucleating acicular ferrite is discussed.

The Concept of the Liquid Sampling and Hot Rolling Method for Determination of Macro Inclusion Characteristics in Steel

A novel method for determination of semi-macro and macro size distributions of inclusions in steel from samples in ladle, tundish and mould has been developed. The samples are worked and surface treated before inspection. Mainly immersed ultrasonic scanning is performed to determine the semi-macro and macro inclusion content. The LSHR method has been tested in several different steel plants and for different steel grades. It is concluded that new information regarding large inclusions during steelmaking can be obtained using the method.

Influence of calcium carbide slag additions on inclusion characteristics in steel

An investigation into the cleanness of steel manufactured by a major Scandinavian producer of steel for railway wheels has been carried out. Samples taken before and after secondary refining in the ladle and after degassing were compared with samples taken similarly from a heat including the addition of a calcium carbide (CaC2 ) slag modifier. The amount, size distribution, and composition of non-metallic inclusions, as well as the slag compositions of both heats, were determined. With the addition of CaC2 , the total number of inclusions in the sample taken before casting was found to decrease, and the size distribution revealed the presence of mostly small inclusions (between 2·8 and 5·6 μm). The assessment of railway wheel specimens revealed that the inclusion content in CaC2 treated steel was lower than that for standard steel.