Vapor pressure measurement of zn-fe intermetallic compounds

Most galvanized steel sheets are produced by a continuous galvanizing line. Some of the operations include a subsequent heat treatment termed galvannealing to form intermetallic phases on the surface. These galvanizing reactions are essentially regarded as “making intermetallic compounds” in the Fe-Zn-M system. This makes it important to know the thermodynamic properties of the system for any detailed discussion on the formation of these intermetallics; however, experimental difficulties have limited the number of studies. In the present study, two-phase regions of these intermetallics were examined, and the following two-phase mixtures were prepared at 723 K: Fe(α)+Γ, Γ,+Γ₁, Γ₁+δ ₁+ζ. The double cell-type Knudsen mass spectrometer system was developed and employed, in which two sets of Knudsen cells were installed in the same cell holder. The reference material (pure zinc) was placed in one cell and a sample in the other. The cell holder was rotated to the reference and sample positions to measure zinc vapor effused from two cells by a quadrupole mass spectrometer. Material containing the two phases was placed in a Knudsen-type effusion cell and the mass spectrum studied to evaluate the vapor pressure of zinc. The activity of zinc in the intermetallics was determined by comparing the intensity from pure zinc and that from intermetallics. Results for Fe(α)+Γ were 0.46 to 0.48; for Γ+Γ₁, 0.51 to 0.54; for Γ₁+δ ₁, 0.62 to 0.66; and for δ ₁+ζ, 0.80 to 0.83, between the temperatures of 623 and 698 K.     

Metastable Phases of Dross Particles Formed in a Molten Zinc Bath and Prediction of Soluble Aluminum During Galvannealing Processes

The morphology, chemistry, and crystallographic characteristics of metastable dross particles were identified. These particles are formed during the initial stage of precipitation. The particles had aluminum concentrations of 15 to 80 mass pct, with values that decreased gradually as particle size increased. These metastable dross particles were a mixture of the crystalline phase of FeZn₁₀, which is called the “delta phase,” and the high-aluminum amorphous phase, which covered the surface of the crystalline phase. The new “meta Q” concept was proposed to predict the amount of soluble aluminum in the zinc bath by considering nucleation kinetics and particle growth. The results calculated using the “meta Q” concept were compared with the values measured by the aluminum sensor, which were taken during the same period at the commercial galvanizing line. The mean of the absolute values of the differences between the calculated and measured values was 9.7 ppm.     

Influence of Aluminum on the Formation Behavior of Zn-Al-Fe Intermetallic Particles in a Zinc Bath

The shape, size, and composition of dross particles as a function of aluminum content at a fixed temperature were investigated for aluminum added to the premelted Zn-Fe melt simulating the hot-dip galvanizing bath by a sampling methodology. In the early stage, less than 30 minutes after Al addition, local supersaturation and depletion of the aluminum concentration occurred simultaneously in the bath, resulting in the nucleation and growth of both Fe₂Al₅Zn _x and FeZn₁₃. However, the aluminum was homogenized continuously as the reaction proceeded, and fine and stable FeZn₁₀Al _x formed after 30 minutes. An Al-depleted zone (ADZ) mechanism was newly proposed for the “η→η+ζ→δ” phase transformations. The ζ phase bottom dross partly survived for a relatively long period, i.e., 2 hours in this work, whereas the η phase disappeared after 30 minutes. In the early stage of dross formation, both Al-free large particles as well as high-Al tiny particles were formed. The dross particle size decreased slightly with increased reaction time before reaching a plateau. The opposite tendency was observed when the Al content was 0.130 mass pct; with a relatively high Al content, the nucleation of tiny η phase dross was significantly enhanced because of the high degree of supersaturation. This unstable η phase dissolved continuously and underwent simple transformation to the stable δ phase. The relationship between nucleation potential and supersaturation ratio of species is discussed based on the thermodynamics of classical nucleation theory.     

The effect of iron oxide as an inhibition layer on iron-zinc reactions during Hot-Dip galvanizing

A study was conducted on the effect of a uniform oxide layer on the galvanizing reaction in 0.20 wt pct Al-Zn and pure Zn baths at 450 °C. In the 0.20 wt pct Al-Zn bath, poor wettability of the oxide layer was observed. No significant liquid Zn penetration of the oxide occurred and, therefore, attack of the steel substrate to form localized Fe-Zn growth did not occur. It was found that the iron oxide acted as a physical barrier or inhibition layer in the pure Zn bath, similar to the Fe₂Al₅ inhibition layer that forms at the steel interface in Al-Zn baths. The inhibition effect of the oxide in the pure Zn bath was temporary, since cracks and other macrodefects in the oxide acted as fast diffusion paths for Zn. Localized Fe-Zn growth (outbursts) formed at the steel/coating interface, and the number of outbursts was generally inversely proportional to the oxide layer thickness at constant immersion times. Increased immersion time for a constant oxide layer thickness led to an increase in the number of outbursts. These results simulate the diffusion short circuit mechanisms for Fe₂Al₅ inhibition layer breakdown in Al-containing Zn baths.     

热镀锌钢带沉没辊辊印缺陷成因分析及控制方法

沉没辊辊印缺陷属于镀层缺陷,产生因素很多,结合武钢生产经验,以及对生产中产生缺陷的检测分析,对该缺陷的成因进行探讨,提出控制缺陷的方法,主要包括:控制带钢入锌液板温、优化稳定锌液成分、改进沉没辊沟槽形貌等。     

附加外场力对锌锅表层锌液流动和锌渣分布的影响

实测了热镀锌锌锅表层锌液的流速,并分析了锌锅浮渣的分布特点,提出了一种在锌锅锌液上方主动加载外场力作用以驱动锌液流动进而改变锌渣分布的新方法,称为锌渣流动管理方法(DFMS-Dross Flowing Management System)。通过源力加载的方式修正了流体动量守恒方程,建立了锌液流场数值计算模型,模拟了不同外场力加载布局下锌锅表层锌液的流场。结果表明,附加外场力作用可有效促进锌锅表层锌液的切向流动,最大切向流速可达0.8 m/s;表层锌液的切向流动可拖动锌渣跟随流动,进而改变锌渣分布,促进锌锅排渣,证实了DFMS的有效性。模拟结果还表明,锌锅表层锌液的切向流速受外场力加载时间的影响较小,而随加载外场力的增大而增大,但加载外场力的大小对100~200 mm深锌液的流速影响较小。最后,通过定义锌锅表层锌液的排渣时间因子讨论了不同外场力加载方式的排渣效率。     

Hot-dip Galvanizing of Carbon Steel after Cold Rolling with Oxide Scale and Hydrogen Descaling简

A new processing method for producing hot dip galvanized steel is designed and tested, in which pickling is skipped. Hot-rolled low carbon steel sheets are roiled with oxide scale in an experimental mill at room temperature, prior to annealing under a 20% hydrogen reducing atmosphere and galvanizing on a hot-dip galvanizing simulator. Micro-cracks formed in the oxide scale during cold rolling roughen the steel surface and enlarge the specific surface. Through-thickness cracks provide transport channels for hydrogen, and hence the reduction of oxide scale is en- hanced. When the sheet is dipped in the zinc bath, cracks are submerged by liquated zinc and the defects are not dis- tinct after hot-dip galvanizing. The overlay coating occludes with rough surface of the sheet, whereby a superior coat- ing adherence is realized.     

Kinetics of phase evolution of Zn-Fe intermetallics

The intermetallic phases, Γ (Fe₃3Zn₁₀), Γ₁, (Fe₅Zn₂₁), δ (FeZn₇, and ζ (FeZn₁₃), are mechanically alloyed through ball milling of pure elemental Fe and Zn powders under a controlled atmosphere of argon gas. The state of the as-ball-milled materials was crystalline, except for the Γ phase, which was amorphous. Phase-evolution kinetics through differential scanning calorimeter (DSC) measurements of the as-ball-milled powders show three characteristic transition temperatures for the Γ₁, and ζ phases, two for the Γ phase, and only one for the δ phase. The activation energies for the evolution of the milled powders to their equilibrium crystalline phases are 170 ± 1, 251 ±2, 176± 1, and 737 ±4 kJ/mol for the Γ, Γ₁, δ, and ζ phases, respectively. These values show that the mechanisms for the metastable-to-stable phase transition in these intermetallics are different, whereas diffusion over short distances may be part of the entire process in all cases.     

The effect of steel chemistry on the formation of Fe-Zn intermetallic compounds of galvanneal-coated steel sheets

The effects of steel chemistry on the formation of Fe-Zn intermetallic compounds in the galvanneal coatings have been investigated by examining the microstructure of galvanneal coat-ings on extra-low-carbon (ELC) steel, interstitial-free (IF) steel, and interstitial-free rephos-phorized (IFP) steel. The layer structure of the coatings was revealed by chemical etching. Phases present in each layer were then identified using electron diffraction in transmission elec-tron microscopy (TEM). A two-layer structure, one consisting of the δ phase with a small fraction of the ζ, phase dispersed on the surface and Γ phases and another consisting of the δ and Γ₁ phases, was observed in the ELC sample and the IFP sample, respectively. A three-layer structure consisting of the δ, Γ₁ + δ, and Γ phases was observed in the IF sample. The presence of C in the steel substrate retarded the alloying between Fe and Zn; while P in the steel favored the formation of the Γ₁, phase over the Γ phase by its surface segregation in the steel substrate. The orientation relationship between coating and substrate was also studied by electron diffraction. Three α-Fe/Γ orientation relationships were frequently observed.     

Studies of the morphology of the Al-Rich interfacial layer formed during the hot dip galvanizing of steel sheet

A small addition of Al in the liquid Zn bath inhibits the Fe/Zn reactions during hot dip galvanizing of steel sheets. Although it is well known that Al and Fe are tied up in an layer located at the interface between Fe and Zn, the effect of the galvanizing parameters on the formation of this Al-rich interfacial layer is not. This study has been carried out to determine the effect of the galvanizing parameters on the formation of this Al-rich layer. Five zinc baths containing 0.10, 0.12, 0.13, 0.16, and 0.18 wt pct Al were used to produce galvanized coatings of commercial ultra-low-carbon (ULC) steel substrate. Full inhibition of the Fe/Zn reactions was achieved in baths with Al content above 0.15 wt pct. The Al-rich layer is isomorphous to Fe₂Al₅ crystals and the Fe/Al ratio is close to Fe₂Al₅ with large amounts of Zn present (22 to 28 wt pct). The morphology of the Al-rich layer is strongly related to the Al content of the bath. Indeed, bath contents above 0.15 wt pct are associated with two sublayers of Fe₂Al₅ crystals, making up colonies of grains of similar crystallographic orientation and directly associated to the crystallographic orientation of the steel grains. In baths with contents below 0.15 wt pct, the Al-rich layer has only one sublayer of crystals and shows colonies of grains with similar orientation. Finally, the Fe₂Al₅ crystals have a strong crystallographic texture. X-ray diffraction (XRD) reveals that the (200) planes are parallel to the surface of the substrate.     

Studies of the morphology of the Al-rich interfacial layer formed during the hot dip galvanizing of steel sheet

A small addition of Al in the liquid Zn bath inhibits the Fe/Zn reactions during hot dip galvanizing of steel sheets. Although it is well known that Al and Fe are tied up in an layer located at the interface between Fe and Zn, the effect of the galvanizing parameters on the formation of this Al-rich interfacial layer is not. This study has been carried out to determine the effect of the galvanizing parameters on the formation of this Al-rich layer. Five zinc baths containing 0.10, 0.12, 0.13, 0.16, and 0.18 wt pct Al were used to produce galvanized coatings of commercial ultra-low-carbon (ULC) steel substrate. Full inhibition of the Fe/Zn reactions was achieved in baths with Al content above 0.15 wt pct. The Al-rich layer is isomorphous to Fe₂Al₅ crystals and the Fe/Al ratio is close to Fe₂Al₅ with large amounts of Zn present (22 to 28 wt pct). The morphology of the Al-rich layer is strongly related to the Al content of the bath. Indeed, bath contents above 0.15 wt pct are associated with two sublayers of Fe₂Al₅ crystals, making up colonies of grains of similar crystallographic orientation and directly associated to the crystallographic orientation of the steel grains. In baths with contents below 0.15 wt pct, the Al-rich layer has only one sublayer of crystals and shows colonies of grains with similar orientation. Finally, the Fe₂Al₅ crystals have a strong crystallographic texture. X-ray diffraction (XRD) reveals that the (200) planes are parallel to the surface of the substrate.     

Simulation of special flow affecting dross formation on steel strip in galvanising bath

The zinc flow in a galvanising bath is numerically simulated for three cases, and the flow field is analysed using three-dimensional views. It is found that the flow near the zinc surface directs from the strip to the ingot side when inductors are equipped and whether ingots are melting or not, and the flow direction is opposite to that near the zinc surface for the case without inductor and ingot, which does not exist during the bath operation process. Whirlpools are found to be formed between the snout and the bath side walls, which will increase the chance of dross particles touching the strip surface. Some effective measures are proposed to stop more dross from adhering to strip. This work will lay foundation for the optimisation of the galvanising process to improve the coating quality of steel strip.     

Change Rules of Γ_2 Particles in Hot-dipped Zn-Ti Coating

Hot-dip Zn-Ti galvanizing can restrain the excess growth of the coating which has better corrosion resistances than hot-dip pure Zn.The change rules ofΓ2intermetallic compound in hot-dip Zn-Ti galvanizing is investigated by immersion time,titanium content and the slag test using scanning electron microscopy(SEM),energy dispersive spectroscopy(EDS)and X-ray diffraction(XRD).And the mechanism of nucleation and growth ofΓ2intermetallic compound is analysed.The results show that,when adding 0.05mass%titanium to the zinc bath,theΓ2particles form in theηlayer,which nucleate by obtaining Ti atoms from theηphase and Fe atoms from the dissolvingζphase layer.Moreover,the more titanium is added into the bath,the more and biggerΓ2 particles appear.TheГ2particles in the coatings grow up noticeably with prolonging the immersion time.     

Effect of ternary additions on the structure and properties of coatings produced by a high aluminum galvanizing bath

A Zn-Al eutectoid galvanizing alloy has been developed to address issues related to the growing environmental concern regarding leaching of zinc into the soil. The basic composition of the bath was selected close to the eutectoid point in the binary Zn-Al system, together with ternary additions in the form of Bi, rare earths (REs) and Si. The effects of these additions on the growth rate, microstructure, and corrosion properties of the coatings were investigated. Both Bi and REs showed a minimal effect on controlling the coating thickness developed with the eutectoid bath composition, which was about an order thicker than the commercially prevalent value of ∼80 μm. Addition of Si to the bath reduced the thickness of the coating to a level of around 30 μm. The morphology of the coatings, investigated in detail using scanning electron microscopy, energy-dispersive spectroscopy, and X-ray diffraction, showed the presence of three layers: interface layer, intermediate layer, and overlay. The dense interfacial layer mainly consisted of the Fe₂Al₅ phase; the intermediate layer contained two phases, an Fe-al phase and a Zn-rich phase; and the overlay had a composition close to that of the liquid bath. The intermediate layer contained a varying amount of porosity in all cases except in the coatings formed by the silicon containing bath. The thin, dense, and smooth coating obtained with the latter bath showed excellent resistance to corrosion, evaluated by field exposure as well as by electrochemical tests. An attempt is made to unravel the role of different elements in the formation of coating microstructure.     

热 镀 锌 液 中 锌 渣 的 电 磁 分 离

 锌液中的锌渣是影响热镀锌钢板表面质量的重要因素。从电磁净化理论分析和电磁分离实验研究两个角度出发,对解决热镀锌液中锌渣的去除进行了探讨。结果表明,在磁场频率175 kHz、磁感应强度均方根值005 T时,电磁作用10 s,可有效去除粒度大于10 μm的锌渣;通过对锌液的连续净化处理,粒度小于15 μm的锌渣去除效率可达到825%。     

Kinetics and phase transformation evaluation of Fe-Zn-Al mechanically alloyed phases

Fixed composition ratios of Fe and Zn corresponding to γ-(Fe₃Zn₁₁₀), Γ₁-(Fe₅Zn₂₁), δ-(FeZn₇), and ζ-(FeZn₁₃) with the addition of 5 pct Al (wt) were ball milled in an argon gas atmosphere to form homogenous alloys. Nonisothermal kinetic analyses of the mechanically alloyed materials, based on differential scanning calorimetry (DSC) measurements, revealed two diffusion-controlled processes during the evolution of the δ+5 pct Al and ζ+5 pct Al compositions with activation energies of 227±2 and 159±1 kJ/mole, respectively. Other endothermic and exothermic reactions detected for these compositions are consistent with the Fe-Zn-Al equilibrium phase systems with respect to the formation of the Fe₃Al, Fe₂Al₅, and δ-FeZn₇ phases Based on the evidence of FeAl₂ formation at 440 °C for the ζ+5 pct Al composition from X-ray diffraction (XRD) and DSC measurements, the revision/re-evaluation of the Fe-Zn-Al equilibrium phase diagrams is proposed. The Γ+5 pct Al and Γ₁+5 pct Al compositions evolved similarly through the same fields, except at 400 °C, where the former consisted of α-Fe + Γ + δ, while the later was without the Γ phase.     

Recylcing of the Dross Formed in Hot Galvanizing

A comparison is made of the characteristics of dross formed on lines that make rolled products with anti-corrosion coatings. Two methods of recycling dross to rid it of harmful impurities are examined. In distillation sublimation, an increase in temperature to 1280°C helps maximize the amount of zinc that is transferred from the dross to the storage container. In this case, 70–85% of the zinc is recovered from the dross. The resulting melt contains 99.5% zinc, 0.03% iron, and 0.04% aluminum. Up to 70% of the metallic zinc in dross is recovered when the dross is dissolved in an aluminum melt containing 10–25% aluminum. The use of ammonium chloride increases the amount of iron removed from the secondary zinc and shortens the recycling operation. Yu. A. Kabanov, V. V. Pol'shikov, V. D. Petrov, and R. A. Mukaev also participated in this study. __________ Translated from Metallurg, No. 7, pp. 60–61, July, 2005.     

On the wear of a cobalt-based superalloy in zinc baths

Journal bearings made of a cobalt-based superalloy, trade marked STELLITE 6, were evaluated in zinc baths with and without the presence of aluminum and iron. The sleeve and the bushing wore evenly when tested in a pure zinc bath. The surfaces were generally smooth and covered by some rather fine grooves after the test. The wear of the bearings was much more severe when tested in aluminum-containing zinc baths. The worn surfaces of the bearings were fully covered by wide and deep grooves. The coefficient of friction of the superalloy generally increased with increases in the aluminum content of the molten zinc. The iron addition to the molten zinc appeared to affect the friction and wear characteristics of the superalloy. Detailed metallographic and microchemical analyses were performed to elucidate the wear mechanisms. The superalloy was found to react readily with the molten zinc alloys to form intermetallic compounds. In a pure zinc bath, zinc-based intermetallic compounds formed on the bearing surfaces; in baths containing aluminum, a compact intermetallic layer, based on the cobalt-aluminide phase, was detected on the bearing surfaces. Evidence collected in this study indicated that wear debris reacted with aluminum in the molten zinc and transformed itself into hard and abrasive cobalt-aluminide particles. During the test, these particles reattached to the bearing surfaces and built up. Acting as blunt microcutters, these particles plowed the bearing surfaces and created deep grooves. The dominant wear mechanism in the aluminum-containing zinc baths was identified as abrasion; fatigue and corrosive wear, however, served as precursors of the dominant wear mechanism.