Thermodynamics of Oxygen Solutions in Ni–Cr Melts Containing Aluminum

The influence of aluminum on the solubility of oxygen in Ni–Cr melts is considered on the basis of thermodynamic analysis. Very low aluminum concentrations have practically no influence on the oxygen concentration in the melt, which is determined by the chromium content. When the aluminum content exceeds 0.01%, it determines the solubility of oxygen in the melt, in all cases. The minimum oxygen concentration corresponds to about 0.2% Al. With increase in chromium content in the melt, the minimum oxygen concentration increases. It is 2 × 10^–3%, 7 × 10^–3%, and 10^–2% for nickel alloys with 10%, 20%, and 30% Cr, respectively.     

Solubility of oxygen in Fe–Co melts containing titanium

Solutions of oxygen in Fe–Co melts containing titanium are subjected to thermodynamic analysis. The first step is to determine the equilibrium reaction constants of titanium and oxygen, the activity coefficients at infinite dilution, and the interaction parameters in melts of different composition at 1873 K. With increase in cobalt content, the equilibrium reaction constants of titanium and oxygen decline from iron (logK(FeO · TiO₂) =–7.194; logK(TiO₂) =–6.125; logK(Ti₃O₅) =–16.793; logK(Ti₂O₃) =–10.224) to cobalt (logK(CoO · TiO₂) =–8.580; logK(TiO₅) =–7.625; logK(Ti₃O₅) =–20.073; logK(Ti₂O₃) =–12.005). The titanium concentrations at the equilibrium points between the oxide phases (Fe, Co)O · TiO₂, TiO₂, Ti₃O₅, and Ti₂O₃ are determined. The titanium content at the equilibrium point (Fe, Co)O · TiO₂ ? TiO₂ decreases from 1.0 × 10^–4% Ti in iron to 1.9 × 10^–6% Ti in cobalt. The titanium content at the equilibrium point TiO₂?Ti₃O₅ increases from 0.0011% Ti in iron to 0.0095% Ti in cobalt. The titanium content at the equilibrium point Ti₃O₅ ? Ti₂O₃ decreases from 0.181%Ti in iron to 1.570% Ti in cobalt. The solubility of oxygen in the given melts is calculated as a function of the cobalt and titanium content. The deoxidizing ability of titanium decline with increase in Co content to 20% and then rise at higher Co content. In iron and its alloys with 20% and 40% Co, the deoxidizing ability of titanium are practically the same. The solubility curves of oxygen in iron-cobalt melts containing titanium pass through a minimum, whose position shifts to lower Ti content with increase in the Co content. Further addition of titanium increases the oxygen content in the melt. With higher Co content in the melt, the oxygen content in the melt increases more sharply beyond the minimum, as further titanium is added.     

Phase transformations at high pressures and temperatures and the structure of a hypoeutectic 1Ni-99Al alloy

The phase transformations in a hypoeutectic 1Ni-99Al alloy are studied by differential barothermal analysis in the temperature range up to 750°C at a compressed argon pressure up to ～100 MPa. The Al matrix of the initial alloy is found to be saturated by micropores at a concentration of 3.7 × 10¹⁰ cm^?3. After melting and solidification in a compressed argon atmosphere, the micropore concentration increases to 3.2 × 10¹¹ cm^?3. As a result of melting and solidification at a high pressure, the initial fine-grained structure of the alloy with an average grain size of 16 μm transforms into a coarse-grained structure during dendritic solidification. The processing of electron-microscopic images is used to determine the volume content of intermetallic compound Al₃Ni in the Al matrix. The liquidus temperature of the alloy at 100 MPa increases by 10°C, and the solidus temperature is 5°C higher than the eutectic transformation temperature in aluminum-rich Al-Ni alloys. The solid-phase decomposition of the supersaturated solid solution of nickel in aluminum occurs at 630°C. At 100 MPa, the field of solid solutions of nickel in aluminum extends to 1.2 at % Ni as compared to the Al-Ni system at atmospheric pressure. The lattice parameters of Al and Al₃Ni are found to increase in the alloy solidified at 100 MPa. The microhardness of the Al matrix in the alloy is measured after a barothermography cycle. A portion of the Al-Ni phase diagram is proposed for a pressure of 100MPa in the nickel content range 0–4.3 at %.     

Surface tension of a titanium-containing oxide–fluoride melt calculated by the polymer theory

The surface tension of an aluminum–calcium oxide–fluoride melt is calculated using the polymer theory. It is shown that, at 15 mol % titanium oxide in the melt, aluminum–fluorine–oxygen complexes mainly form. When the titanium oxide content increases further, these complexes disappear and are partly replaced by titanium–oxygen TiO ₄ ^4- and Ti₂O ₇ ^6- anions and titanium–fluorine–oxygen groups.     

Present Technology of Hard and Soft Ferrites

Abstract

In order to improve the mechanical properties of cemented carbides, Ti(C,N)–Ni–Mo alloys, in which carbon in the titanium carbide is replaced by nitrogen, are of potential interest from the viewpoint of grain size control. Since grain size control by nitrogen was also observed in Ti(C,N)–Ni alloys containing no molybdenum, the effect of nitrogen on grain growth of Ti(C,N)–Ni alloys was investigated by comparing TiC–Ni and TiN–Ni alloys. The grain growth rate of Ti(C,N)–Ni alloys showed a minimum value at the carbon content C/C+N = 0·5. From the results of chemical and grain size analyses, it is considered that the growth rate depends on the solubility of Ti(C,N) in the nickel solid solution and the degree of coalescence because the solubility decreases with decreasing carbon content. With nitride, the amount of liquid increases by denitrification of TiN, owing to the formation of the nickel solid solution or TiNi₃ phases, and growth of TiN grains by Ostwald ripening is observed. The apparent activation energy for the growth of TiC and TiN grains is calculated to be ～4·4×10⁵ and 1·9 × 10⁵ J mol^?1 respectively. PM/0201     

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Thermodynamics calculation indicates that the stable inclusion is Al2O3 for Ti-alloyed Al-killed steel. And only when Al content is below 0. 008% does Al2TiO5 emerge. Systematical samples from practical production of Ti-IF steel reveals following discoveries. Unstable Al-Ti composite oxides formed in steel possibly due to the reaction between titanium oxides in ferrotitanium and aluminum in steel. During casting process, the aluminum loss near the melt/slag interface is more sever if the content of reducible oxide such as SiO2 is higher in tundish flux. Therefore Ti enrichment happens in partial areas near the melt and thermodynamic conditions for generating Al2TiO5 or Ti3O5 could be satisfied. In addition, titanium oxides can be easily transported into steel melt by turbulent flow.     

Review of effect of oxygen on room temperature ductility of titanium and titanium alloys

Abstract

Room temperature tensile ductility is an important property of titanium (Ti) and titanium alloys for structural applications. This article reviews the dependency of tensile ductility on oxygen for α-Ti, (α+β)-Ti and β-Ti alloys fabricated via traditional ingot metallurgy (IM), powder metallurgy (PM) and additive manufacturing (AM) or three-dimensional printing methods and recent advances in understanding the effect of oxygen on ductility. Seven mechanisms have been discussed based on case studies of individual titanium materials reported in literature. The dependency of ductility on oxygen is determined by both the composition and microstructure of the titanium alloy. For Ti–6Al–4V (wt-%), as sintered Ti–6Al–4V shows a critical oxygen level of about 0·33 wt-% while additively manufactured Ti–6Al–4V exhibits different critical levels ranging from about 0·22% to well above 0·4% depending on microstructure. Rare earth (RE) elements are effective scavengers of oxygen in titanium materials even just with a small addition (e.g. 0·1 wt-%), irrespective of the manufacturing method (IM, PM and AM). High cycle fatigue experiments revealed no initiation of fatigue cracks from the resulting RE oxide particles over the size range from submicrometres to a few micrometres. A small addition of RE elements offers a practical and affordable approach to mitigating the detrimental effect of oxygen on ductility.     

Microwave Heating, Isothermal Sintering, and Mechanical Properties of Powder Metallurgy Titanium and Titanium Alloys

This article presents a detailed assessment of microwave (MW) heating, isothermal sintering, and the resulting tensile properties of commercially pure Ti (CP-Ti), Ti-6Al-4V, and Ti-10V-2Fe-3Al (wt pct), by comparison with those fabricated by conventional vacuum sintering. The potential of MW sintering for titanium fabrication is evaluated accordingly. Pure MW radiation is capable of heating titanium powder to ≥1573 K (1300 °C), but the heating response is erratic and difficult to reproduce. In contrast, the use of SiC MW susceptors ensures rapid, consistent, and controllable MW heating of titanium powder. MW sintering can consolidate CP-Ti and Ti alloys compacted from ?100 mesh hydride-dehydride (HDH) Ti powder to ~95.0 pct theoretical density (TD) at 1573 K (1300 °C), but no accelerated isothermal sintering has been observed over conventional practice. Significant interstitial contamination occurred from the Al₂O₃-SiC insulation–susceptor package, despite the high vacuum used (≤4.0 × 10^?3 Pa). This leads to erratic mechanical properties including poor tensile ductility. The use of Ti sponge as impurity (O, N, C, and Si) absorbers can effectively eliminate this problem and ensure good-to-excellent tensile properties for MW-sintered CP-Ti, Ti-10V-2Fe-3Al, and Ti-6Al-4V. The mechanisms behind various observations are discussed. The prime benefit of MW sintering of Ti powder is rapid heating. MW sintering of Ti powder is suitable for the fabrication of small titanium parts or titanium preforms for subsequent thermomechanical processing.     

Formation of Metallic Phase on Reducing-Gas Injection in Multicomponent Oxide Melt. Part 3. Separation of Ferronickel and Oxide Melt

Using the equations of physicochemical hydrodynamics and experimental results regarding the surface and interphase properties of metallic and oxide melts, the conditions in which metallic phase is formed in the bubbling of carbon monoxide through molten oxidized nickel ore are described. The critical dimensions of the gas bubble (R_b.cr) and the metal droplet (r_d.cr) moving in oxide melt without change in size are determined in the range 1550–1750°C. It is found that R_b.cr increases slightly from 6.35 × 10^–2 m at 1550°C to 6.58 × 10^–2 m at 1750°C. With change in the droplet composition and the temperature, r_d.cr varies from 2.1 × 10^–3 to 2.9 × 10^–3 m. The dimensions of the metal droplet formed at a single bubble during the reduction of nickel and iron from oxide melt are determined. As the content of nickel and iron oxides in the melt decreases with increase in the overall CO consumption, the nickel content in the ferronickel droplets falls from 89 to 18%, while the droplet diameter decreases from 1.4 × 10^–3 to 8.0 × 10^–4 m. The droplet mass falls correspondingly from 9.4 × 10^–5 to 1.6 × 10^–5 kg. The conditions in which the bubble–droplet system rises through the melt are determined. Over the whole range of temperature and Ni content, the bubble–droplet system begins to rise through the oxide melt when r_d/R_b is less than 0.68–0.78. To assess the stability of the bubble–droplet system, with the given bubble and droplet dimensions, the parameters determining their joint motion are calculated. It is found that breakaway of the metal droplet from the bubble is not possible in pyrometallurgical systems. The formation of metal phase as a result of the bubbling of carbon monoxide through the oxide melt is described. In this process, the interaction of the oxide melt with the gas is accompanied by the formation of metal droplets, which become attached to the surface of gas bubbles and move to the surface of the oxide melt. Metal with 80–90% Ni is formed at first. With decrease in the nickel content in the oxide melt, its content in the metal declines to 20%. At the surface of the oxide melt, the metal droplets coalesce. When their diameter is greater than 5 × 10^–3 m, they break away from the surface and fall to the bottom. If the falling drop collides with ascending bubble–droplet systems, they may coalesce with it or flow around it. On coalescence, the small droplets will be assimilated and rise to the surface. The breakaway force of the droplet from the bubble significantly exceeds the gravitational force on the droplet. Therefore, the bubble–droplet system is stable for all the size ratios considered.     

Structure and initial precipitation in a rapidly solidified nickel superalloy

A nickel base superalloy (Nimonic 80A) has been rapidly solidified at cooling rates of between 10⁵ to 10⁶ K.S^-1 by pendant drop melt extraction and by chill block melt spinning in an evacuated chamber backfilled with helium or argon. The internal structure is described in terms of process variables pertaining to rotating chill block quenching techniques. Both transmission electron microscopy and atom-probe field-ion microscopy have been employed to give structural and constitutional data on quenched and aged specimens. The as-quenched structure is homogeneous apart from fluctuations in titanium concentration which upon aging undergoes a spinodal phase decomposition to form disordered Ni₃(Ti,Al,Cr) precipitates in the matrix, which after prolonged aging produces ordered γ (Ni₃(Ti,Al)). inin6 particles form readily on grain boundaries and also appear in conjunction with ordered γ, via a discontinuous reaction, after short aging times.     

超低碳钢中Al?Ti?O夹杂物的形貌演变和生成机理

对超低碳IF钢钛合金化后的非金属夹杂物进行了分析,研究发现钛合金化后的夹杂物主要为Al₂O₃和Al?Ti?O夹杂物,没有发现纯TiO_x夹杂物。钢中生成的Al?Ti?O复合夹杂物从形貌上均可分为七种类型,四种具有Al₂O₃外层,另外三种无Al₂O₃外层。钛合金化后,钢中瞬态生成了大量无Al₂O₃外层的Al?Ti?O夹杂物,随后夹杂物表面生成Al₂O₃外层,导致有Al₂O₃外层的Al?Ti?O夹杂物数量比例逐渐增加至78.0%。热力学计算结果表明,随着钢中钛含量的增加,夹杂物的转变顺序为固态Al₂O₃→液态Al?Ti?O→固态Ti₂O₃。确定了Al?Ti?O夹杂物的生成机理过程分为两步:精炼过程钛合金化后,当钢液局部区域的钛的质量分数高于0.42%时,[Ti]与钢液反应瞬态生成Al₂O₃?TiO_x或TiO_x;随着精炼过程中钛元素的混匀,含TiO_x夹杂物被钢中[Al]还原,Al₂O₃?TiO_x和TiO_x夹杂物逐渐转变,在夹杂物表面生成Al₂O₃。      

Concerning the strength of dynamic strain aging in zirconium

Tensile tests on high purity (6 × 10⁻⁴ oxygen equivalent) and commercial purity (6 × 10⁻³ oxygen equivalent) zirconium were performed between 77 and 1000 K in order to evaluate dynamic strain aging. A comparison with earlier data from two equivalent titanium compositions yielded the following; reducing the interstitial concentration to the zone refined iodide level removes most evidence of strain aging in both zirconium and titanium. At this impurity concentration, zirconium also shows a greatly reduced thermally activated flow stress component. This was not observed in titanium. At the commercial purity level, both metals exhibit strain aging phenomena. These are much weaker, however, in Zr than in Ti. Even at this impurity level Zr does not exhibit a strain aging yield point, the Portevin-Le Chatelier effect or a well defined work hardening rate peak. All of these latter are found in commercial purity Ti. Several other aspects of DSA, while observed in Zr, are less pronounced than in Ti. The principal interstitial impurity in these materials is oxygen. Oxygen in solid solution strongly increases the(c/a) ratio of Ti, but has little effect on this ratio in Zr. This distortion of the Ti hcp lattice may account, in part, for the greater strength of DSA in this metal. Formerly a Post Doctoral Fellow at the University of Florida. Formerly a Post Doctoral Fellow at the University of Florida. Formerly a student at the University of Florida.     

Activities in liquid Fe−Al−O and Fe−Ti−O alloys

The solubility and activity of oxygen in Fe?Al and Fe?Ti melts at 1600°C were measured. The activity was measured electrochemically using the following galvanic cells: Cr-Cr₂O₃(s) ? ThO₂(Y₂O₃) ? Fe-Al-O(l), Al₂O₃(s) Cr-Cr₂O₃(s) ? ThO₂(Y₂O₃) ? Fe-Ti-O(l, saturated with oxide) Cr-Cr₂O₃(s) ? ZrO₂(CaO) ? Fe-Ti-O(l, saturated with oxide) Aluminum and titanium decrease the solubility of oxygen in liquid iron to a minimum of 6 ppm at 0.09 wt pct Al and 40 ppm at 0.9 wt pct Ti, respectively. The value of the interaction coefficients ε ₀ ^(Al) and ε ₀ ^(Ti) are ?433 and ?222, respectively. the activity coefficient of aluminum at infinite dilution in liquid iron is 0.021, while that of titanium is 0.038. The value of the aluminum equilibrium constant, the solubility product at infinite dilution, is 5.6×10^?14 at 1600°C. The ThO₂(Y₂O₃) electrolyte exhibited insignificant electronic conductivity at 1600°C down to oxygen partial pressures of 10^?15 atm, which corresponds to about 0.3 ppm O in unalloyed iron.     

Thermodynamics and kinetics analyses of ZrB2 formation in molten aluminium alloys

Smelter grade aluminium can be used as a source for electrical conductor grade aluminium after the transition metal impurities such as zirconium (Zr), vanadium (V), titanium (Ti) and chromium (Cr) have been removed. Zirconium (Zr), in particular, has a significant effect on the electrical conductivity of aluminium. In practice, the transition metal impurities are removed by adding boron-containing substances into the melt in the casthouse. This step is called boron treatment. The work presented in this paper, which focuses on the thermodynamics and kinetics of Zr removal from molten Al–1?wt-%Zr–0.23?wt-%B alloy, is part of a broader systematic study on the removal of V, Ti, Cr and Zr from Al melt through boron treatment carried out by the authors. The thermodynamic analyses of Zr removal through the formation of ZrB₂ were carried out in the temperature range of 675–900°C using the thermochemical package FactSage. It was predicted that ZrB₂ is stable compared to Al–borides (AlB₁₂, AlB₂) hence would form during boron treatment of molten Al–Zr–B alloys. Al–Zr–B alloys were reacted at 750?±?10°C for 60 minutes, and the change in the chemistry and microstructure were tracked and analysed at particular reaction times. The results showed that the reaction between Zr and AlB₁₂/B was fast as revealed by the formation of boride ring at the early minutes of reaction. The presence of black phase (AlB₁₂), i.e. the original source of B, after holding the melt for 60 minutes advocated that the reaction between Zr and AlB₁₂/B was incomplete, hence still not reached the equilibrium state. The kinetics data suggested a higher reaction rate at the early minutes (2 minutes) of reaction compared to at a later stage (2–60 minutes). Nevertheless, a simple single-stage liquid mass transfer controlled kinetic model can be used to describe the overall process kinetic. The analysis of integrated rate law versus reaction time revealed that the mass transfer coefficient (k_m) of Zr in molten alloy is 9.5?×?10^?4?m?s^?1, which is within a typical range (10^?3 to 10^?4?m?s^?1) observed in other metallurgical solid–liquid reactions. This study suggests that the overall kinetics of reaction was predominantly controlled by the mass transfer of Zr through the liquid aluminium phase.     

Transient Behavior of Inclusion Chemistry,Shape, and Structure in Fe-Al-Ti-O Melts: Effect of Titanium Source and Laboratory Deoxidation Simulation

During ladle processing of interstitial-free (IF) steel melts, it is possible for transient titanium-containing oxides to be formed if the local titanium/aluminum (Ti/Al) ratio is locally and temporarily increased after aluminum killing. The phase stability diagrams suggest that if the Ti/Al ratio is increased, then Al₂TiO₅ and/or a liquid Al-Ti-O region can become stable, and eventually at even higher Ti/Al ratios, Ti₃O₅ becomes stable. In this study, the Ti/Al ratio was successively altered to investigate (1) how the inclusions evolved after titanium addition to aluminum-killed iron melts and (2) whether the inclusions present after sufficient time were those predicted by thermodynamics. When the Ti/Al ratio was maintained at 1/4, such that Al₂O₃ is the only thermodynamically stable oxide, the results show that transient titanium-containing oxides exist temporarily after titanium addition, but with time, the predominant inclusion was Al₂O₃, which would generate little shape change and produce transient stage inclusions with less titanium contents. When the Ti/Al ratio was increased to 1/1 (Al₂O₃ still being the only thermodynamically stable oxide), the results show a more distinct increase in the titanium content of the transient inclusions. The transient reaction was, in this case, accompanied by an irreversible shape change from spherical to irregular inclusions. When the Ti/Al ratio in the melt was increased to 15/1 within the Al₂TiO₅ stable phase region, the inclusion population evolved from spherical-dominant ones to irregular ones. It was found that the final inclusion chemistry has more titanium but less aluminum content compared with the expected from the Al₂TiO₅ chemistry. Besides, the transmission electron microscopy (TEM) results showed the existence of Ti₂O. When the Ti/Al ratio in the melt was increased such that Ti₃O₅ is the thermodynamically stable inclusion (Ti/Al ratio of 75/1 or ∞), the inclusions evolved after titanium addition toward TiO_x inclusions, which is accompanied by a shape change from spherical to irregular. The TEM results revealed and confirmed the existence of metastable Ti₂O besides the thermodynamically stable Ti₃O₅, and it was consistent with the results based on oxidation studies of thin layers of titanium with Al₂O₃ substrate. It was discovered that Ti₂O has the tendency of transforming into the thermodynamically stable phase Ti₃O₅ under certain conditions.     

十六烷基三甲基溴化铵-硫氰酸钾-水体系浮选分离钛

研究了十六烷基三甲基溴化铵（CTMAB）-硫氰酸钾-水体系浮选分离钛的行为。结果表明, 在水溶液中, Ti与CTMAB和硫氰酸钾形成不溶于水的三元缔合物［Ti(SCN)₆^2- ］［CTMAB⁺］₂,此三元缔合物沉淀浮于水相上层形成界面清晰的液-固两相。当溶液中CTMAB和硫氰酸钾的浓度分别为2.0×10^-3mol/L和7.0×10^-2mol/L时, Ti可与CdⅡ、Cr、MnⅡ、V、Al、NiⅡ、FeⅡ、MgⅡ和Ga离子定量分离。该方法简便迅速,不污染环境,在微量钛的分离和富集分析中有很好的实用价值。     

钛与钛合金的低成本清洁制备新技术

提出了多级深度热还原的理念,发明了镁热/铝热自蔓延-深度多级还原制备钛与钛合金的关键技术,成功制备出纯度为99.69%高纯还原钛粉,Ti6Al4V合金粉：Ti:89.5-90.2%、Al:5.85-6.57%、V:3.90-4.17%,氧含量<0.15%;制备出20kg级规模的TiAl合金铸锭,Ti/Al原子比为1:1,氧含量为0.09%。成果已在山东傅山集团成功转化应用,500t/a还原钛粉工业示范线于2018年12月08日试车成功,可使金属钛与钛合金的生产成本降低30%以上,项目的推广应用为钛材低成本清洁利用奠定工业化基础。     

Fabrication of aluminum–ceramic skeleton composites based on the Ti<Subscript>2</Subscript>AlC MAX phase by SHS compaction

A one-stage manufacturing technology of aluminum–ceramic skeleton composites by combining the processes of self-propagating high-temperature synthesis (SHS) of a porous skeleton formed by the MAX phase of the Ti₂AlC composition and its impregnation by the aluminum melt under pressure (SHS compaction) is considered. A composition of the exothermic charge 2Ti + C + 22.5 wt % Al + 10 wt % TiH₂, which provides the formation of a porous skeleton of the Ti₂AlC phase without impurity phases by the SHS technology, is selected. It is shown that, when impregnating the hot SHS skeleton with aluminum, new phases are formed such as the MAX phase (Ti₃AlC₂), titanium carbide (TiC), and titanium aluminide (Al₃Ti). However, the content of the basic MAX phase remains high, and the ceramic component of the material consists of Ti₂AlC by 76%. When analyzing the microstructure, it is revealed that the composite has certain residual porosity after impregnation and cooling. The influence of the impregnation pressure (q = 22, 28, and 35 MPa) on the distribution of the aluminum content over the height and radius of the diametral sample section is investigated experimentally. It is shown that the nonuniform Al distribution over the sample bulk is caused by the nonuniform pressure and temperature fields, as well as the different compactibility of hot inner and colder outer sample parts. The degree of compaction of characteristic zones is leveled as the impregnation pressure increases, and the composition inhomogeneity over the sample bulk decreases. The difference in aluminum concentration over the sample bulk at q = 35 MPa does not exceed 5%. The SHS-compacted aluminum–ceramic skeleton composite based on the Ti₂AlC MAX phase corresponds to high-strength Al-Zn–Mg–Cu aluminum alloys by the hardness level (HB ≈ 150 kg/mm²).     

Cleanliness of Alloying Structural Steel

 Alloying structural steel used for mechanical structures has a high requirement for cleanliness because its failures are greatly affected by non-metallic inclusions and total oxygen content in steel. It has been reported by some steelmaking plants to have some problems in controlling total oxygen content and inclusions during alloying structural steel production. For this purpose, cleanliness control in 02C-03Si-06Mn-1Cr-02Mo steel was investigated. Firstly, low melting temperature zone (≤1873 K) of CaO-Al2O3-MgO system and formation condition of low melting temperature inclusions were investigated through thermodynamic equilibrium calculation. On this basis, industrial tests were carried out. Through sampling at different stages, transformation of oxide inclusions and change of total oxygen content in steel were studied. The results show that: in order to form CaO-Al2O3-MgO system inclusions with low melting temperature, mass percent of Al2O3, MgO and CaO in inclusions should be controlled from 376% to 708%, 0 to 174% and 255% to 606%; For the condition of 1873 K and 005% (mass percent) dissolved aluminum in steel, the activities of dissolved oxygen, magnesium and calcium should be controlled as 0298×10-4-2×10-4, 01×10-5-40×10-5 and 08×10-8-180×10-8 respectively. With secondary refining proceeding, average total oxygen content and inclusion amount decrease, the type of most inclusions changes from Al2O3 after tapping to Al2O3-MgO after top slag is formed during ladle furnace refining and finally to CaO-Al2O3-MgO after RH treatment. In the final products, average total oxygen content was 127×10-6 and most inclusions were in spherical shape with size less than 5 μm.     

Oxygen activities in Cr-containing Fe and Ni-based melts

Plug-type, ZrO₂-based oxygen sensors have been used for long-term measurements of oxygen activity in Fe–O–Cr and Ni–O–Cr melts. In these melts, equilibrated with chromium oxide, oxygen activities a_O were determined as a function of Cr content. From the experimental results, data were derived for activity coefficients f_O and of 1st and 2nd order interaction parameters e_O^Cr and r_O^Cr. Cr₂O₃ has been identified as the oxide phase in equilibrium with the metal melt at ≥ 5 wt.% Cr in the case of iron and at ≥ 0.2 wt.% Cr in the case of nickel. Oxygen activities and oxygen contents in Cr-containing iron melts are lowered with increasing additions of nickel. Further investigations were directed to a_O determination in Fe–O–Cr–C and Fe–O–Cr–Al melts.