Inclusions In Molten Steel Research Articles

The Change of Composition of Inclusions in Molten Steel during Ca and Mg Addition

In order to examine the mechanism of the generation of MgO in CaO–MgO–Al2O3 inclusion during Ca addition, the variations of inclusion composition and molten steel composition were measured during and after FeMg and CaSi addition to 2 kg heats of Al killed molten steel at 1873K. From experimental studies, the equilibrium reaction was discussed by the relationship between Ca concentration in molten steel and MgO concentration in inclusion and apparent equilibrium constant of magnesium deoxidation was obtained.The results obtained are summarized as follows;(1) The addition pattern of calcium and magnesium affected composition variation path, although it had no effects on final composition of inclusion.(2) The boundary condition of formation and disappearance of MgO in Ca–Al–O inclusion was given by Ca concentration and Mg concentration in molten steel.(3) The reaction of MgO formation in Ca–Al–O inclusion was expressed as follows.CaO·Al2O3 in inclusion+Mg=MgO·Al2O3 in inclusion+Ca(4) The apparent equilibrium constant log K′Mg was estimated to be −7.2 by thermodynamic calculation.log K′Mg=aMg·aO=−7.2

Mathematical Model for Nucleation, Ostwald Ripening and Growth of Inclusion in Molten Steel

Numerical simulation is a powerful tool to investigate inclusion behavior in the molten steel. Although many mathematical models have been developed to predict inclusion collision-growth behavior in different metallurgical reactors, the inclusion size distribution had to be obtained by experiment or assumption. Thus, a general nucleation-growth model, which involves in chemical reaction, homogeneous nucleation and growth kinetics, is developed to investigate the inclusion nucleation, Ostwald ripening, Brownian collision-growth, Stokes collision-growth and turbulent collision-growth. In order to speed up the calculation, the deoxidation products are divided into two parts. The first part only consists of embryos, and directly numerical simulation is used to solve the differential equations. The second part only consists of inclusion particles, and particle-size-grouping method is introduced to solve the related equations. Numerical results showed that the predicted inclusion size distributions are consistent with previous experimental data. With the increasing diffusion coefficient, the peak-value diameter keeps unchanged and the maximum number density decreases. With the increasing turbulent energy dissipation rate, the peak-value diameter and the maximum number density decrease under the assumption on floating-out of larger inclusions.

Effect of Refining Slag Composition on Inclusions in Molten Steel treated by Barium‐bearing Alloy

AbstractThe effect of the refining slag composition on the inclusions in molten steel was studied in MgO crucible by using MoSi2 furnace. The experimental steel was heavy rail steel and Si‐Ca‐Ba alloy was used as the deoxidizer. The results showed that the total oxygen content of steel, and the total quantities, total areas and average radius of inclusions decreased with the increased basicity of refining slag. The total quantities and total areas of inclusions were the least when Al2O3 content in refining slag was between 13% and 20%, while the proportion of small inclusions increased when Al2O3 content was higher than 20%. The contents of SiO2 and Al2O3 in inclusions were affected directly by the SiO2 and Al2O3 contents in refining slag, and it was very remarkble when the slag composition was CaOAl2O32SiO2. Therefore, it's feasible to control the composition of inclusions by controlling the composition of refining slag.

A New Method of Removing Inclusions in Molten Steel by Injecting Gas from the Shroud

A technology of removing inclusions in molten steel by injecting gas from the shroud has been analyzed and studied by method of mathematical model and water model. Study of position of injecting gas, flow rate of gas and gas nozzle size on the generated bubble size and distribution at the shroud has been investigated; the relationship between blowing parameters and inclusion removal efficiency has been analyzed in order to optimize the blowing parameters. The results show that the stronger turbulent vortex at the enlarged part in the shroud favors fine bubbles generation and uniform distribution; fine bubbles in diameter of 0.3 to 0.5 mm can be generated in the shroud and tundish by controlling flow rate of gas. The superfluous gas makes bubble size largen sharply. The nozzle size has some effect on bubbles size and distribution; the larger nozzle size goes against fine bubbles generation. So the optimum gas injected position is located at the enlarged part in the shroud. The resonable flow rate of gas and nozzle size would greatly improve inclusion removal efficiency.

Mathematical Model for Growth and Removal of Inclusions in Molten Steel under Gas‐stirring Conditions

A three‐dimensional mathematical model has been developed to predict growth and removal of inclusions during gas stirring through eccentric tuyeres in a ladle. In the model, the efficiency of inclusion removal is investigated under three different collision mechanisms: Brownian, turbulent and Stokes collision. The Importance of the three approaches of wall adhesion, Stokes flotation and bubble adhesion on inclusion removal is analysed and the efficiency of inclusion removal through three types of tuyeres in central, eccentric and multi‐tuyere form is studied. The results indicate that inclusion growth resulting from turbulent collision is most important and the effect of Stokes collision is remarkable with increased inclusion size, while inclusion growth resulting from Brownian collision is negligible. Removal by Stokes flotation is the main mechanism for large inclusions, while inclusion removal by wall adhesion is negligible. The smaller the bubbles are, the higher the efficiency of inclusion removal is. The type of tuyere arrangement has a great effect on inclusion removal. Inclusion removal in a 135t ladle with one eccentric tuyere is more efficient than in a ladle with central tuyere or multi‐tuyere design.

Mathematical Model for Removal of Inclusion in Molten Steel by Injecting Gas at Ladle Shroud

A three-dimensional mathematical model has been developed to predict steady flow field, pressure distribution inside the shroud and efficiency of inclusion removal in tundish after argon gas injection at the shroud. Effects of the different opening of the slide gate and gas injection on velocity and pressure have been analyzed. The results show that the molten steel velocity gradually decreases and the pressure increases as the opening of the slide plane becomes larger. The velocity increases and the pressure decreases after gas injection. Bubble adhesion plays an important role in inclusion removal. Smaller bubbles and larger inclusions are favorable for inclusions removal. The larger flow rate of gas is, the greater number of bubbles generated is. Thereby the amount of removed inclusion increases. But local boiling will occur in the case of larger rate of gas, so the flow rate of gas should be strictly controlled in practice.

Numerical Modeling of Nucleation and Growth of Inclusions in Molten Steel Based on Mean Processing Parameters

As an important aspect of inclusion engineering, numerical prediction of inclusion behavior in molten steel has attracted much attention in order to control inclusions in steel. Although considerable efforts have been made in the modeling of inclusion growth, it is still necessary to employ an initial particle size distribution (PSD) gained from experiments or assumptions as an input for numerical models. The present work focuses on the construction of a general nucleation-growth model, which combines thermodynamics, classical homogeneous nucleation theories and dynamics of particle collision and coagulation on the basis of mean processing parameters. This can be employed, without any initial PSD of inclusions in advance, to describe the time evolution of PSD in molten steel during inclusion nucleation, Ostwald ripening, and collision growth processes. With regard to collision-coagulation growth mechanisms of inclusions, four approaches, namely Brownian collision, turbulent collision, Stokes collision and gradient collision in laminar shear layers, have been investigated to estimate their role in the evolution of PSD during steel melt deoxidation process. In addition, an approximate numerical technique, termed the ‘DS method’, has been developed as a modification of the so-called (1) discrete-sectional representation and (2) Particle-Size-Grouping method in order for an efficient solution for population balance equations (PBE) at lower cost in CPU time. Finally, as an application of this general model, the evolution of nucleation and growth of alumina inclusions is demonstrated in the Fe-Al-O melt system. The predicted molten steel total oxygen and particle size distribution of alumina inclusions are compared with the experiment data cited from references.

EFFECT OF CALCIUM ON THE QUALITY OF TUBE STEEL

The company Tagmet makes steel in 290-ton open-hearth furnaces by the pig-and-scrap process, tapping the metal into two ladles. The steel is then treated on a two-position “ladle-furnace” unit equipped with a pinch-roller system to introduce cored wire and aluminum wire rod. After the steel has been brought to the desired temperature and chemical composition on the unit and treated with calcium-bearing wire in the final stage, it is bottom-poured into circular tube ingots weighing 1.2‐2.0 tons. In connection with the recent exploitation of gas fields that contain hydrogen sulfide, the pipes used in the equipment for the wells and the gas lines are having to meet higher standards on their content of harmful impurities and nonmetallic inclusions ‐ which affect the physicochemical properties and corrosion resistance of the metal. It is known that micro-additions of different materials to steel can decrease its sulfur content by raising the level of deoxidation of the steel and the refining slag and providing for fuller use of the slag’s sulfide capacity [1]. It has been es tablished that the addition of calcium makes it possible to obtain steel with sulfur content of 0.005% or less [2, 3], and it helps more favorably distribute phosphide eutectics in the steel [4]. The addition of calcium to steel in the needed amount ‐ the amount of calcium being determined by the steel’s content of oxygen, sulfur, and aluminum [3] ‐ helps transform nonmetallic inclusions and convert hard aluminates into globular low-melting calcium aluminates. Proceeding on the basis of the above, Tagmet has developed and introduced a technology for micro-alloying corrosion-resistant tube steels 35G2, 20SA, and other steels used for well casing and drill pipe. The steels are micro-alloyed with a specified amount of calcium-bearing cored wire introduced by a pinch-roller system at a rate of 3‐3.5 m/sec. The wire and the steel are then mixed for 3‐5 min to average out the chemical composition of the latter and ensure that the nonmetallic inclusions rise to the surface of the melt. To study the effect of calcium on the transformation of nonmetallic inclusions in molten steel, we took samples of three heats of steel 20SA having nearly the same chemical composition in terms of their contents of the main elements, aluminum, and harmful impurities (sulfur and phosphorus). Standard procedures were employed on a ladle-furnace unit to obtain the samples with immersion-type samples before and after treatment of the steel (before it was cast). The normal practice was followed in finishing the steel with respect to temperature and chemical composition. Only the steel of two of the heats was treated with the calcium-bearing wire (Table 1). We used the samples to prepare metallographic sections for studying the composition of the nonmetallic inclusions. Thirty fields in all were examined. It was determined that, before the treatment, the nonmetallic inclusions in all three of the heats consisted of accumulations of aluminum spinel and sulfide eutectics present in the form of films along the grain boundaries. After its treatment, the steel of heat No. 1 contained almost no coarse accumulations of oxides (the oxides were probably assimilated by the slag), and the aluminate-type oxides that were precipitated were present in smaller clusters. The sulfides were precipitated in the form of eutectic colonies.

Mathematical Modeling of Iron and Steel Making Processes. Mathematical Modeling of Flow and Inclusion Motion in Vessel with Natural Convection.

In a steel making process, the fine molten steel slab is required to make high quality sheet. From this point of view, it is very important to remove inclusions in molten steel. So a mathematical model, to predict the behavior of various sizes of aluminum inclusions in a molten steel flow, has been developed. In this model, three kinds of collision terms are considered: the first one is due to the flow field acceleration, the second one is due to the gravity acceleration, and the third one is due to the effect of the turbulent eddies. In the process of the collision, total 1 025 particle sizes are taken into consideration. In each particle size, number density should be calculated. But only 13 particles among 1 025 are selected to solve the number density conservation equation. For the rest of the particle sizes, the number density is evaluated by linear interpolation based on selected 13 particles in order to save the computational time. Using this model, the behavior of aluminum inclusions in molten steel, which is dominated by the natural convection, has been simulated. The mechanism of the aluminum collision and coagulation has been studied.

Morphological Classification of Inclusions in Steel by Image Processing of Micrograph.

The inclusions in stainless steel are of different shapes on a micrograph: circle, triangle, square, bar and so on. A new method to identify the shape of an inclusion was developed with the aid of image processing equipment. Two morphological parameters of y=L2/(4Aπ) and x=πD2/(4A) were selected. On a two dimensional diagram of x and y, the respective area of each shape was determined theoretically and experimentally. The inclusions in specimen taken from molten steel, CC slab and rolled sheet were examined on optical micrographs of 1000 magnification and classified using this diagram. The inclusions in molten steel were more than 96% circular, those in CC slab, about 90% circular and those in rolled sheet, 80% circular. In the last specimen, squares accounted for about 10% and bars, 7%. The inclusions were analyzed by EPMA. The circular inclusions were mostly of oxide, whereas the elongated inclusions were of MnS based complex. Sizes of the circular inclusions were also determined, and the mean diameter in molten steel, CC slab and rolled sheet was 1.4, 2.0 and 1.0 μm, respectively.

Fluid flow modeling in slab tundishes

Nonmetallic inclusions in molten steel can result in poor quality of a continuous casting product. In addition to being used as the reservoir and distributor of molten steel in continuous casting, tundishes are used to remove inclusions in molten steel. The design of a tundish to improve the efficiency of inclusion removal requires a detailed knowledge of the flow behavior of the molten steel in the tundish.

連続鋳造におけるタンディッシュ内溶鋼加熱による非金属介在物の低減

連続鋳造におけるタンディッシュ内溶鋼加熱による非金属介在物の低減

熔鋼中の非金屬介在物に及ぼすクロムの作用に就て(III)

In the foregoing papers it was reported that, with addition of ferro-chromium in the last period of refining, how the non-metallic inclusion in molten steel varied, when chromium steel was melted in a basic arc furnace, basic and acid open-hearth furnaces. (Tetsu-to-Hagane, Vol. 36, 1950, No. 11, p. 15; Vol. 37, 1951, No. 3, p. 19) Further, the effect of chromium contained in the charging materials upon the non-metallic inclusion was investigated.Summarizing the relation between chromium and non-metallic inclusion, the following table was obtained.Namely, the effect of chromium on the non-metallic inclusion in every case had a quite similar tendency at the same furnace, but, comparing the case of basic arc furnace with that of acid open-bearth furnace, an entirely opposite phenomenon was observed. In the case of basic openhearth furnace an intermediate effect of the above two was shown.

熔鋼中の非金屬介在物に及ぼすクロムの作用に就て(II)

The foregoing statement reported that the ferro-chromium addition in the last period of refining at the basic arc furnace melting, caused a conspicuous decrease of non-metallic inclusion in molten steel, and proved the chromium to be one of the strong cleansers.Ever since, the following results have been obtained by further examinations in the same field by means of basic and acid open-hearth meltings.(1) With basic open-hearth melting, the addition caused sometimes the increase and sometimes the decrease of inclusion.(2) With acid open-hearth melting, the inclusion usually increased.(3) It was found that the increase and decrease of inclusion were closely related with the basicity of slags. Namely, with the basic open-hearth melting, in case of the CaO/SiO2 is less than about 2.0-2.2 or the ΣRo/SiO2 is less than about 2.8-3.0, the inclusion increases, and when it is more than the figure the inclusion decreases. This, eventually, induces the fact, that in the acid open-hearth melting, the inclusion duly increases instead of decreasing, and as explained above, it increased without exception.(4) After comparative studies between basic arc furnace melting above described and basic open-hearth melting, the above relation proved to be so relative that, when the slag is basic, and in the range around about 2.0-4.5 of basicity (CaO/SiO2), the addition of ferro-chromium decreases the inclusion, and the maximum decreasing ratio can be reached at the basicity about 2.6-3.0 in both cases.