微量稀土铈在超洁净IF钢中的作用

 采用化学成分分析、热力学计算、显微观察、硬度测定的方法,研究了稀土铈在氧和硫质量分数均小于0.000 6%的超洁净IF钢中的存在状态,以及铈对钢的Al2O3夹杂物、显微组织和硬度的影响机理。结果表明,当铈质量分数为0.006 1%～0.009 3%时,铈的固溶量为0.004 1%～0.006 7%,约70%的铈以固溶形态存在,其余的铈以夹杂物形态存在,钢中不存在铈铁金属间化合物。铈能够将Al2O3夹杂物转化为尺寸较小的球状CeAlO3夹杂物,具有变质夹杂物的作用。铈具有改善IF钢的显微组织的作用,钢中铈的固溶量越大,晶粒越细。与不含铈的IF钢相比,当钢中铈质量分数为0.006 1%和0.009 3%时,钢的洛氏硬度分别提高了7.6%和12.7%。     

Effect of the conditions of REM microalloying of steel on the corrosion activity of nonmetallic inclusions

Experimental heats of low-alloy steel are performed under various conditions of rare-earth metal microalloying and aluminum and calcium deoxidation. Electron-probe microanalysis of nonmetallic inclusions and a metallographic investigation of a metal are used to show that, when interacting with water, nonmetallic cerium oxide inclusions do not form hydrates and, correspondingly, are not aggressive. When aluminum, calcium, and cerium additions are sequentially introduced into a melt, a continuous cerium oxide shell forms on calcium aluminates, protects corrosive nonmetallic inclusions against interaction with water, and weakens local metal corrosion.     

Role of Mo and W during sensitization of superaustenitic stainless steel—crystallography and composition of precipitates

This study concerns the crystallographic identification and compositions of precipitates formed in superaustenitic stainless steel. Three experimental alloys, all containing 24 wt pct Cr, 22 wt pct Ni, and 0.5 wt pct N but with varying amounts of Mo and W, were investigated after sensitization heat treatment (aging) at 900 °C. The contents of Mo and W in the three alloys were 7 wt pct Mo, (6 wt pct Mo + 2 wt pct W) and (5 wt pct Mo + 5 wt pct W), respectively. While σ and x were the main secondary phases found in the W-free alloy, replacement of Mo by W was found to promote the formation of Laves-related phases with high Mo + W content. The complex crystallographic nature of Laves-related precipitates was exemplified through the formation of intergrowing C14 Laves, μ, and C phases, all with closely related crystal structures. There was no difference in chemical composition between the three phases. Prolonged aging resulted in intragranular precipitation of different intermetallic phases, as well as formation of nitrogen bearing phases, π and Cr₂N, adjacent to previously formed intermetallic precipitates. The content of Mo + W was found to decrease with increasing aging time for all secondary phases.     

Removal of Inclusions from Aluminum Through Filtration

Filtration experiments were carried out using both an AlF₃ slurry-coated and an uncoated Al₂O₃ ceramic foam filter to study the removal of nonmetallic inclusions and impurity elements. The results showed that the 30-ppi ceramic foam filter removed up to 85 pct inclusions from aluminum. Several pictures of two- and three-dimensional morphologies of both nonmetallic and intermetallics inclusions also have been presented. The following contributing mechanisms for the removal of nonmetallic inclusions in the deep-bed filtration mode are proposed: (1) collision with walls and interception effect and (2) the formation of both intermetallic and nonmetallic inclusion bridges during filtration. Fluid dynamics modeling of inclusion attachment to the filter walls showed that most inclusions, especially those with larger sizes, are entrapped at the upper part of the filter, whereas smaller inclusions are dispersed well throughout the filter. The calculated inclusions removal fractions for the 30-ppi filter showed that almost all inclusions >125 μm are removed, and inclusions ~5 μm in size are removed up to 85 pct. The interfacial energy between two collided same-size inclusions was calculated, indicating that a strong clustering of inclusions may result within the filter window. Magnesium impurities were removed up to 86 pct by the AlF₃ slurry-coated filter. The filter acted in active filtration mode in addition to the contribution of the air oxidation of dissolved [Mg], which was calculated to be 13 pct. The total mass transfer coefficient of dissolved [Mg] to the reaction interface was calculated to be 1.15 × 10⁻⁶ m/s.     

Effect of Sulfur in Steel on Transient Evolution of Inclusions During Calcium Treatment

In the current study, the effect of S content in the molten steel on inclusions during calcium treatment was studied using an induction furnace. The calcium in steel decreased from 48 to 2 ppm, and the sulfur in steel changed a little with time. When sulfur content in steel was as low as 25 ppm during calcium treatment, inclusions shifted from CaO-Al₂O₃-CaS to Al₂O₃-CaO with about 35 pct CaO. When the sulfur increased over 90 ppm, more CaS-CaO formed just after the addition of calcium, and then the CaS content decreased from over 45 pct to lower than 15 pct and inclusions were mostly Al₂O₃-CaO-CaS and Al₂O₃-CaO with a high Al₂O₃ content. Thermodynamic calculation predicted the variation of the composition of inclusions, indicating good agreement with the measurement, while a certain deviation existed, especially for heats with 90 and 180 ppm sulfur. A reaction model was proposed for the formation of CaO and CaS, which considered the reaction between calcium vapor bubbles in the zone and the dissolved oxygen and sulfur in the molten steel, as described by a Langmuir-type adsorption isotherm with a reaction occurring on the remaining vacant sites. The variation of transient CaS inclusions was discussed based on the thermodynamic calculation and the morphology evolution of typical inclusions containing CaS.     

The rate of formation of intermetallic layers between aluminum and steel below 660°C

The rate of formation of intermetallic compounds between aluminum and three ferritic steels, one austenitic steel, and Inconel has been determined by an electrolytic method. The steel was held at zero potential with respect to aluminum in a NaCl-AlCl₃ melt, and the current measured. Comparison of measured thicknesses of intermetallic layers with those calculated from the integrated current gives an average deposition efficiency of 95 pct. For the Type 304 austenitic steel thickness (min), andk is given by logk= −6400/T(⁰K) +4.469. The ferritic steels show a linear rate of growth of Al₅Fe₂, with an initial higher rate such that extrapolation of the linear curve back to zero time gives an intercept of 16±7 μm. The rate constants (mm min⁻¹) may be represented by log (rate)=α/T+β, and the values of α and β are respectively −2650 and−0.788 for a plain carbon steel,−6580 and + 3.469 for a 1.3 pct Cr, 0.4 pct Mo steel, and−5950 and +2.466 for a 2.2 pct Cr, 0.9 pct Mo steel. The more highly alloyed steels are thus attacked, more slowly. Results for Inconel could not be fitted to any simple equation. With the ferritic steels growth is by aluminum diffusing inwards; with Inconel it is by nickel diffusing outward.     

Study on the Formation and Characterization of the Intermetallics in Friction Stir Welding of Aluminum Alloy to Coated Steel Sheet Lap Joint

Multimaterial fabrication such as joining of steel and aluminum is currently prominent in a variety of industries. Friction stir welding is a novel solid-state welding process that causes good joint strength between steel and aluminum. However, the phenomenon contributing significant strength at the interface is not yet clear. In the present study, the interface of the friction stir lap-welded aluminum and coated steel sheet having joint strength maximum (71.4 pct of steel base metal) and minimum, respectively, under two parameter combinations, i.e., 1000 rpm 50 mm min^?1 and 500 rpm 100 mm min^?1, was exclusively characterized by X-ray diffraction, transmission electron microscopy (TEM), concentration profile, and elemental mapping by electron-probe microanalysis. A TEM-assisted EDS study identifies the morphologies of large size Al₁₃Fe₄ and small size Fe₃Al-type intermetallic compounds at the interface. The diffusion-induced intermetallic growth (thickness) measured from a backscattered image and concentration profile agreed well with the numerically calculated one. The growth of these two phases at 1000 rpm 50 mm min^?1 is attributed to the slower cooling rate (~3.5 K/s) with higher diffusion time (44 seconds) along the interface in comparison to the same for 500 rpm 100 mm min^?1 with faster cooling rate (~10 K/s) and less diffusion time (13.6 seconds). The formation of thermodynamically stable and hard intermetallic phase Al₁₃Fe₄ at 1000 rpm and travel speed 50 mm min^?1 in amounts higher than 500 rpm and a travel speed of 100 mm min^?1 results in better joint strength, i.e., 71.4 pct, of the steel base metal.     

Effects of Cerium Addition on Inclusions and Solidification Structure of a Low-Nickel Si–Mn-Killed Stainless Steel

The effect of cerium on inclusions and solidification structure of a low-nickel Si–Mn-killed stainless steel is studied using laboratory experiments. When the cerium content in steel increased from 0 to 250 ppm, modification sequence of inclusions is Si–Mn(–Al)–O and MnS → Ce–Si–Mn–O–S → Ce(–Si)–O–S → CeS and CeC₂. The number density and area fraction of inclusion first decrease with the increase in the cerium content and then increase due to the formation of CeC₂ inclusions when the cerium content is bigger than 150 ppm, which is precipitated in solid steel during solidification. When the cerium content increases from 0 to 250 ppm, the fraction of equiaxed grain zones of steel ingot first increases and reaches a maximum value when the cerium content is 54 ppm; then the fraction of equiaxed grain zones decreases with the increase of the cerium content. 2D lattice misfit calculations are performed and it is found that there are no heterogeneous nucleation cores in the steel without cerium during solidification. For the steel with cerium, Ce_4.67Si₃O₁₃, Ce₂O₂S, and CeS inclusions act as heterogeneous nucleation cores, increasing the fraction of the equiaxed grain zone. Bigger effective heterogeneous nucleation cores number density leads to a larger fraction of the equiaxed grain zone.     

The effect of mischmetal addition on the structure and mechanical properties of a cast Al-7Si-0.3Mg alloy containing excess iron (up to 0.6 Pct)

The effect of Fe content (0.2 to 0.6 pct) on the microstructure and mechanical properties of a cast Al-7Si-0.3Mg (LM 25/356) alloy has been investigated. Further, 1 pct mischmetal (MM) additions (a mixture of rare-earth (RE) elements) were made to these alloys, and their mechanical properties at room and at elevated temperatures (up to 200 °C) were evaluated. A structure-property correlation on this alloy was attempted using optical microstructure analysis, fractographs, X-ray diffraction, energy-dispersive analysis of X-rays (EDX), and quantitative metallography by image analysis. An increase in Fe content increased the volume percentage of Fe-bearing intermetallic compounds (β and π phases), contributing to the loweryield strength (YS), ultimate tensile strength (UTS), percentage elongation, and higher hardness. An addition of 1 pct MM to the alloys containing 0.2 and 0.6 pct Fe was found to refine the microstructure; modify the eutectic silicon and La, Ce, and Nd present in the MM; form different intermetallic compounds with Al, Si, Fe, and Mg; and improve the mechanical properties of the alloys both at room and elevated temperatures.     

Effects of Reoxidation of Liquid Steel and Slag Composition on the Chemistry Evolution of Inclusions During Electroslag Remelting

Electroslag remelting (ESR) is increasingly used to produce some varieties of special steels and alloys, mainly because of its ability to provide extreme cleanliness and an excellent solidification structure simultaneously. In the present study, the combined effects of varying SiO₂ contents in slag and reoxidation of liquid steel on the chemistry evolution of inclusions and the alloying element content in steel during ESR were investigated. The inclusions in the steel before ESR refining were found to be oxysulfides of patch-type (Ca,Mn)S adhering to a CaO-Al₂O₃-SiO₂-MgO inclusion. The oxide inclusions in both the liquid metal pool and remelted ingots are CaO-Al₂O₃-MgO and MgAl₂O₄ together with CaO-Al₂O₃-SiO₂-MgO inclusions (slightly less than 30 pct of the total inclusions), which were confirmed to originate from the reduction of SiO₂ from the original oxide inclusions by dissolved Al in liquid steel during ESR. CaO-Al₂O₃-MgO and MgAl₂O₄ are newly formed inclusions resulting from the reactions taking place inside liquid steel in the liquid metal pool caused by reoxidation of liquid steel during ESR. Increasing the SiO₂ content in slag not only considerably reduced aluminum pickup in parallel with silicon loss during ESR, but also suppressed the decrease in SiO₂ content in oxide inclusions. (Ca,Mn)S inclusions were fully removed before liquid metal droplets collected in the liquid metal pool.     

Thermodynamics of inclusion formation in Fe-Cr-Ti-N alloys

The thermodynamics of titanium in Fe-Cr alloys and of inclusion formation in Fe-Cr-N-Ti alloys was investigated. A metal-nitride-gas equilibration technique was used to measure the activity of titanium. The equilibrium titanium content of the metal that is in equilibrium with pure solid titanium nitride and nitrogen gas at 1 atm was determined. The activity coefficients of titanium it(f_Ti) relative to 1 wt pct standard state in Fe were calculated for Fe-Cr alloys from the experimental results. The first-order interaction coefficient between titanium and chromium, e _Ti ^Cr , was determined to be 0.024 at 1873 K. The solubility of nitrogen in Fe-Cr alloys was measured and was found to increase with chromium content, which is in agreement with previous work. Thermodynamic calculations were made in order to predict under what conditions titanium nitride will form in 409 stainless steel and was compared with inclusions found in plant samples. The inclusion stability diagrams for 304 stainless steel and Fe-18 pct Cr and Fe-9 pct Cr alloys were computed.     

Effect of sulfide inclusion morphology and pearlite banding on anisotropy of mechanical properties in normalized C-Mn steels

The influence of sulfur content, sulfide shape, and pearlite banding on the anisotropy of mechanical properties was evaluated in a series of 0.2 pct carbon, 1.0 pct manganese steels containing either 0.004 or about 0.013 pct sulfur with and without rare-earth additions. Both globular and stringered sulfide inclusions had a detrimental effect on reduction of area and Charpy shelf energy; this effect was particularly evident in the deterioration of through-thickness properties and was much more severe for stringered inclusions than for globular inclusions. The effect of the sulfide inclusions in the different steels on the through-thickness reduction of area and Charpy shelf energy correlated with differences in their mean free distances or nearest-neighbor distances, both of which depended upon the inclusion characteristics of volume fraction, size, and aspect ratio. The projected length of sulfide inclusions per unit area on a plane perpendicular to the transverse direction seemed to reflect the overall effect of the inclusion characteristics on through-thickness reduction of area and Charpy shelf energy and appeared to be a useful parameter for assessment of steel’s susceptibility to fracture. Pearlite banding had no obvious effect on reduction of area or Charpy shelf energy in any of the steels studied. The improvement observed for the steel with stringered sulfide inclusions as a result of the removal of pearlite banding was a consequence of the inclusions coarsening during the short-time high-temperature treatment used to remove banding.     

The Effect of In Situ Intermetallic Formation on Al-Sn Foaming Behavior

The effect of in situ intermetallic formation on the foaming behavior of Al-3 wt pct Sn alloy has been investigated by introducing five different alloying elements—Co, Mg, Mn, Ni, and Ti. The alloying elements were designed, using thermodynamic calculations, to form various intermetallic phases which are (i) stable until final foaming temperature and (ii) dissolved during the foaming process. Thermal analysis using DSC was carried out to characterize the formation and dissolution of intermetallic phases during the foaming process. The foaming tests of the Al-3 wt pct Sn-X alloy were carried out using a mechanical expandometer and the macrostructure of the foam was scanned with an X-ray tomographer. It is found that the foaming behavior and foam stability of Al-3 wt pct Sn alloy can be actively controlled by the alloying elements.     

Inclusion Characteristics and Acicular Ferrite Nucleation in Ti-Containing Weld Metals of X80 Pipeline Steel

X80 steel weld metals with Ti contents of 0.003 to 0.13 pct were prepared by the single-pass submerged-arc welding process. The effects of Ti content in weld metals on the constituent phases of inclusions and chemical compositions of the constituent phases, as well as the potency of acicular ferrite (AF) nucleation on the inclusions were investigated. Moreover, the crystallographic orientation relationship between the AF and inclusion was examined. The results show that with an increase in Ti content, the primary constituent phases of the inclusions change from the (Mn-Al-Si-O) compound to a mixture of spinel and pseudobrookite solid solutions, and eventually to pseudobrookite. The spinel solid solution is characterized by the MnTi₂O₄ constituent. Compared to pseudobrookite, spinel has a lower Ti concentration, but a significantly higher Mn content. In the case of the presence of a considerable amount of spinel, the Mn element is enriched strongly in the inclusions, resulting in the development of a Mn-depleted zone (MDZ) in the matrix around the inclusions, which enhances the driving force for AF formation. AF shows the Baker–Nutting orientation relationship with MnTi₂O₄. The formation of MDZ and the presence of the Baker–Nutting orientation relationship promote the ability of inclusions to nucleate the intragranular AF.     

Behavior of alumina-magnesia complex inclusions and magnesia inclusions on the surface of molten low-carbon steels

It is well known that alumina inclusions on the surface of molten Al-killed steel quickly attract each other to form clusters. On the other hand, alumina-magnesia complex inclusions on the surface of molten low-carbon steel with a high oxygen content have a much weaker tendency to form clusters. In the present work, the reason for the different behaviors of the two types of inclusions was analyzed in detail. A confocal scanning laser microscope was used to carry out the experiment of in-situ observation of the two types of inclusion on the molten pool. The first type of inclusion was 93 mass pct alumina-7 mass pct magnesian, obtained in a Mg-added Al-killed steel. The second type of inclusion was nearly pure magnesia, obtained in a Mg-killed steel. The attractive force between a pair of inclusions, for both cases, was found to be approximately 1O⁻¹⁷ to 10⁻¹⁶ N and one-tenth of that between a pair of alumina inclusions. The various effects of contact angle, surface tension, and oxygen content of the steel melt on the attractive force are discussed in detail from the viewpoint of the capillary force.     

Microstructure and Thickness of 55 pct Al-Zn-1.6 pct Si-0.2 pct RE Hot-Dip Coatings: Experiment, Thermodynamic, and First-Principles Study

The microstructure and thickness of 55 pct A1-Zn-1.6 pct Si-0.2 pct RE coatings during continuous hot-dip on Q235 steel were investigated in this work. The experimental results revealed that the intermetallic layer was composed of the Fe₂Al₅, FeAl₃, and α-FeAlSi phases. The results of thermodynamic calculations with Pandat software package (CompuTherm, LLC, Madison, WI) indicated that FeAl₃ and α(β)-FeAlSi phase precipitated during the period of temperature cooling, which was consistent with experimental result. Then, the thickness of intermetallic layer was characterized. It was shown that the thickness of intermetallic layer decreased after 0.2 wt pct RE was added. Finally, a first-principles calculation was performed to interpret the effect mechanism of RE on the thickness of intermetallic layer. The results indicated that La substitution in Fe₂Al₅ and FeAl₃ phases could grab electronic charges from Al atoms and weaken the formation of Fe-Al compounds.     

Study on Formation Mechanism of CaO-SiO<Subscript>2</Subscript>-Based Inclusions in Saw Wire Steel

Attempts were made to elucidate the formation mechanism of CaO-SiO₂-based inclusions in saw wires by both laboratory experiments and industrial trials. The key point was to make clear the origin of CaO in such oxide inclusions. Probable origins of [Ca] in steel were first discussed, which can be taken into steel from the steel-slag reaction or ferrous alloy. As a result, slag-steel chemical reaction equilibrium was carefully evaluated at 1873 K (1600 °C) to classify the changes of dissolved aluminum ([Al]), total magnesium (Mg), and total calcium (Ca) in steel and the caused composition variations of inclusions. With the rise of slag basicity from 0.5 to 1.8, [Al] was remarkably increased from 0.00045 to 0.00139 mass pct, whereas Mg varied in the range of 0.00038 to 0.00048 mass pct. By contrast, Ca was constantly kept below 0.00003 mass pct. Accordingly, Al₂O₃ and MgO in inclusions witnessed obvious rises from 5 to 23 mass pct and from 2 to 8 mass pct, respectively. By contrast, inclusions were free of CaO when slag basicity was below 1.5. With slag basicity further increased to 1.8, CaO witnessed a negligible rise to only 1.0 mass pct on average. This phenomenon agreed well with thermodynamic calculations, which revealed that chemical reaction between steel and CaO in slag (for example, between [Si] and CaO) was weak to hardly supplying sufficient [Ca] to steel to increase CaO in inclusions. Ca contained in ferrous alloys as contaminations was not the cause of CaO-SiO₂-based inclusions, either. The industrial trial results indicated that CaO-SiO₂-based inclusions have been readily produced in short time just after BOF tapping. Also, a percentage of them changed slightly with the proceeding of refining. Based on the good agreement of laboratory, industrial, and thermodynamics calculations results, it can be reasonably concluded that CaO-SiO₂-based inclusions in saw wire were exogenous particles from entrapped/emulsified top slag, but not products of slag-steel-inclusion chemical reactions.     

Physical refining of steel melts by filtration

The removal of nonmetallic inclusions from steel melts prior to casting has significant merit. Laboratory prepared steel melts containing carefully prepared alumina inclusions have been successfully filtered at 1600 °C. Two distinct types of filters were used: (i) tabular alumina packed bed (0.2 to 0.5 cm nominal diameter) and (ii) extruded monolithic alumina (400 cells per square inch). The kinetics of the filtration process have been modeled, and inclusion removal efficiency of up to 96 pct has been achieved in laboratory melts. The results show that inclusion removal efficiency is a strong function of melt velocity in the range of 0.08 to 0.68 cm per second and is weakly dependent on filter length. The type of filter utilized affects inclusion removal efficiency significantly. The inclusion capture kinetics and the filtration characteristics of the filter media tested are discussed. Formerly with Drexel University, Philadelphia, PA.     

Electrode reactions in dc electroslag remelting of steel rod

Abstract

The non-metallic inclusion content increased significantly when a steel rod of Fe-Ni was remelted by dc electroslag remelting. The silicon content increased slightly. The manganese and sulphur contents did not change. The total aluminium content in the ingot was max. 0·7%, while that in the electrode was only 10 ppm. The aluminium cations Al³+ in the slag are reduced to metallic aluminium at the slag/electrode interface, while O² - anions are oxidised to dissolved O in the metal pool. This Al and O subsequently recombine to form alumina inclusions in the metal pool. The inclusion content was dependent on the alumina content in the slag. When a rod of plain carbon steel was remelted, however, the increase in nonmetallic inclusion content was as little as one-tenth of that for the remelted Fe-Ni rod. The non-metallic inclusion content was independent of the polarity of the electrode.