Si Bath Research Articles

Effects of Al-Si coating structures on bendability and resistance to hydrogen embrittlement in 1.5-GPa-grade hot-press-forming steel

Hot-press-forming (HPF) steels have attracted great attention as automotive reinforcement parts, but are exposed to the potential risk of hydrogen embrittlement (HE) because H introduced easily during HPF processes is hardly de-trapped through the solidified coating. In particular, H seriously deteriorates bendability, which is one of the main properties to be considered. In this study, the Al-Si coating structures were modified to improve H emission by increasing H diffusivity. The effects of coating structures on bendability and H desorption were investigated by interrupted bending tests, H-permeation tests, and thermal desorption analyses according to elapsed time after H-charging. Immersion in an Al-10%Si bath and the subsequent HPF process (930 °C for 6 min) produced a 33 μm-thick multiple coating structure composed of Fe2Al5, FeAl, and ferrite layers. On the other hand, the reduced Al-Si adhesion amount from the dip bath and the increased time and temperature (950 °C for 30 min) produced a 30 µm-thick body-centered-cubic (BCC)-based coating structure composed of FeAl and ferrite layers. The BCC-based crystal structure, reduced Al content in the FeAl layer, and coarsened ferrite grains effectively enhanced H diffusivity and suppressed H-induced degradation. Moreover, the softened FeAl and thick ferrite layers improved bendability by allowing the large strain accommodation of bending deformation. Thus, this work proposes an optimal Al-Si coating design that enhances both bendability and resistance to H-induced degradation for secure HPF steel applications.

On the distribution of the trace elements V and Cr in an Al–Zn–Si alloy coating on a steel substrate

The Zn–55Al–1.6Si alloy is widely used in hot-dip galvanizing to coat steel and displays a multilayered microstructure (steel substrate/intermetallic compound (IMC) layer/coating overlay) that offers protection from corrosion. Trace level V and Cr are hypothesized to influence the corrosion performance of the coated steel via localized segregation. In this paper, the distribution of trace V and Cr is studied by using synchrotron X-ray fluorescence microscopy (XFM) and scanning transmission electron microscopy (STEM). It is revealed that V and Cr are both primarily segregated into the IMC layer and the top surface layer of the coating and are distributed continuously. The average concentrations for V and Cr are measured by XFM to be 7.7 ppm and 15.2 ppm in the IMC layer, and 4.6 ppm and 19.4 ppm on the top surface, respectively. STEM shows that the V distribution in the IMC layer is localized on the side adjacent to the coating overlayer. It is proposed that the IMC layer forms in two ways. One is the equilibrium phase transformation at 600 °C in the liquid Zn–55Al–1.6Si bath. The other way is the nonequilibrium phase transformation at the IMC/overlayer interface after V and Cr are rejected during solidification of the overlayer.

Inhibitory effect of Ti and La on Fe dissolution in Al–Zn–Si bath by in-situ SXRD, SXRF, TEM and DFT calculation

During the hot-dipping process, there is always a desire to restrain the mutual diffusion between steel and alloy bath for suppressing the formation of bath dross. However, the dissolution of steel sheets is difficult to quantify by the traditional methods. In this work, in-situ X-ray diffraction was used to investigate the solid-liquid reaction between solid Fe and Al–Zn–Si liquid alloy for quantitatively analyzing the consumption of Fe and the formation of Fe–Al compounds. The results indicated that the addition of trace Ti or La could suppress the mutual diffusion. We also observed the microstructures of the Fe–Al intermetallic multilayers and the distribution of Ti or La in the multilayers. A new quaternary phase was identified. The inhibitory effects of Ti and La were also explained by calculating the diffusion barrier energies with density functional theory.

Effect of silicon on the reaction between solid iron and liquid Zn-22.3 wt.% Al bath

Abstract The coating microstructures and thicknesses of the iron panels galvanized in liquid Zn-22.3 wt.% Al baths containing 0.0, 0.7, 1.1, 1.8 and 3.8 wt.% Si for 10 s, 30 s, 60 s, 180 s, 300 s and 600 s have been studied detailedly. With increasing of the Si content in various Zn-Al bath, the phase sequence in the reaction zone of the panel dipped for 600 s was iron substrate/Fe 2 Al 5 /Fe 2 Al 5 + τ 1 /FeAl 3 /FeAl 3 + τ 5C /overlay, iron substrate/Fe 2 Al 5 /Fe 2 Al 5 + τ 1 /FeAl 3 /FeAl 3 + τ 1 /τ 5C + τ 5H /overlay, iron substrate /Fe 2 Al 5 /Fe 2 Al 5 + τ 1 /FeAl 3 /FeAl 3 + τ 1 /τ 5H /overlay, iron substrate/Fe 2 Al 5 /Fe 2 Al 5 + τ 1 /FeAl 3 /FeAl 3 + τ 2 /τ 5H /overlay and iron substrate/Fe 2 Al 5 /Fe 2 Al 5 + τ 1 /FeAl 3 + τ 1 /τ 2 /τ 4 /overlay, respectively. Seven phases were found in these reaction Zones, i.e. Fe 2 Al 5 , FeAl 3 , τ 5C , τ 5H , τ 1 , τ 2 and τ 4 . The growth kinetics of the reaction zone were also studied. The thicknesses of the reaction layer which dipped in 1.8 wt.% Si bath, is the minimum value. The growth of the reaction layer is diffusion controlled for various content Si baths in this study. With the absence of Si in the bath, the liquid phase erodes the iron substrate directly, and the growth of the reaction layer is interface controlled.

Diffusion driven columnar grain growth induced in an Al–Si-coated steel substrate

An alternative method to increase the Al or Si content in steels for electrical applications consists of dipping the steel substrate in a molten Al–Si bath, followed by an annealing treatment to diffuse Al/Si into the substrate. Under specific conditions, a columnar grain structure develops during diffusion annealing. It is demonstrated that a diffusion driven mechanism is responsible for the development of this particular structure. During hot dipping, a 5μm thick layer of the substrate forms a solid solution of Al in Fe, with Al-concentrations up to the solubility limit. The nuclei for the columnar structure are grains of this 5μm thick layer, as this small zone does not transform into austenite during diffusion annealing. The intermetallics formed during hot dipping merely serve as an Al source during diffusion annealing.

Effect of chromium on the formation of intermetallic phases in hot-dipped aluminide Cr–Mo steels

Cr–Mo steels with different chromium contents were coated by hot-dipping into molten baths containing pure aluminum and Al–10wt.% Si for 180s. The effect of chromium content in the steels on the formation of the intermetallic phases in the aluminide coatings was studied. The results show that all the aluminide coatings can be distinguished into an outer pure aluminum or Al–Si topcoat and an inner intermetallic layer. The intermetallic layers, resulting from the steels hot-dipped in pure aluminum, have the same phase constitution, an outer minor FeAl3 and an inner major Fe2Al5. In the aluminide coatings on the steels with 0 and 2.25wt.% chromium after hot-dipping in Al–10wt.% Si, the intermetallic layers were composed of an outer layer of τ5(H)-Al7(Fe,Cr)2Si and an inner one of FeAl3/τ1-(Al,Si)5Fe3/Fe2Al5, while a small amount of polyhedral τ5(H)-Al7(Fe,Cr)2Si and plate-shaped τ6-Al4FeSi were observed in the Al–Si topcoats. In the aluminide coatings on the steels with 5 and 9wt.% chromium after hot-dipping in Al–10wt.% Si, the intermetallic layers were composed of only a τ5(H)-Al7(Fe,Cr)2Si phase. A large amount of scattered granular τ5(C)-Al7(Fe,Cr)2Si and a small amount of plate-shaped τ4-Al3FeSi2 and τ6-Al4FeSi were also found in the Al–Si topcoats. When the chromium content reached 5wt.%, the amount of steel, which dissolved when samples were hot-dipped in Al–10wt.% Si, increased. Also, the rate of dissolving went up as chromium content went up. The increase of dissolution is because the interdiffusion between steels and Al–10wt.% Si bath was enhanced by the formation of scattered granular τ5(C)-Al7(Fe,Cr)2Si, which was stabilized by chromium.

Improvement of High Temperature Oxidation of Low Carbon Steel Exposed to Ethanol Combustion Product at 700 °C By Hot-Dip Aluminizing Coating

Low carbon steel (AISI 1005) was coated by hot-dipping into a molten Al-10% Si bath at 700 °C for 18s. After hotdipping treatment, the coating layers consisted of Al, Si, FeAl3, τ5-Fe2Al8Si, and Fe2Al5. The bare steel and the aluminized steel were isothermally oxidized at 700 °C in ethanol combustion product at atmospheric pressure for 49 h. The aluminized steel shows good performance in high temperature oxidation because the formation of Al2O3 layer on the coating surface. The growth of iron oxide nodules on the surface coating was accelerated by rapid outward diffusion of Fe-ions due to the presence of H2O-vapour generated by ethanol combustion. Thus, the oxidation rate of aluminized steel increased, resulting in a substantial mass-gain as the oxidation time increased. After longer exposure, the τ1(Al,Si)5Fe3 phase was completely transformed to the FeAl in the outer layer. The FeAl formed near the steel substrate was due to Fe-atoms diffusing into the Fe2Al5 layer when the time and temperature increased.

Characterization of Microstructure, Hardness and Oxidation Behavior of Carbon Steels Hot Dipped in Al and Al–1 at% Si Molten Baths

Medium carbon steel was aluminized by hot dipping into molten Al or Al-1 at% Si baths. After hot-dipping in these baths, a thin Al-rich topcoat and a thick alloy layer rich in Al5Fe2 formed on the surface. A small amount of FeAl and Al3Fe was incorporated in the alloy layer. Silicon from the Al-1 at% Si bath was uniformly distributed throughout the entire coating. The hot dipping increased the microhardness of the steel by about 8 times. Heating at 700-1000, however, decreased the microhardness through interdiffusion between the coating and the substrate. The oxidation at 700-1000 in air formed a thin protective a-Al2O3 layer, which provided good oxidation resistance. Silicon was oxidized to amorphous silica, exhibiting a glassy oxide surface.

Microstructure of hot dip coated Fe–Si steels

Hot dipping is a coating technique pre-eminently used in industry to galvanize machine parts or steel sheets for constructional applications. However, other hot dipping applications have been developed in order to have a positive effect on specific material properties. For instance, in Fe–Si electrical steels, a Si/Al rich top layer is applied and followed by diffusion annealing to increase the electrical resistivity of the material and consequently, lower the power losses. Hot dipped aluminised mild steels have been developed with increased corrosion resistance for high temperature applications by the development of a dense Al 2O 3 layer. Regardless of the type of steel coated and the intended application, after the interaction between the molten Al and the solid material, three constituents are formed: Fe 2Al 5, FeAl 3 and an Al-rich alloy. The structural morphology, which can negatively affect the wear resistance and the thermal stability, also appears to be highly dependent on the chemical composition of the base material. To study thermo-mechanical and compositional effects on the coating behavior after hot dipping, cold rolling with different reductions was performed on different Fe–Si materials. It was demonstrated that hardness differences between the layers caused crack formation inside the Fe 2Al 5 layer during subsequent deformation. The present work reports the results obtained on materials that were hot dipped in a hypo-eutectic Al + 1 wt.% Si bath. The bath was used to coat Fe–Si steel substrates with variable silicon content with dipping times ranging from 1 to 20 s. Before dipping, the samples were heated to 700 °C and subsequently immersed in the liquid bath at temperatures of 710 °C, 720 °C and 740 °C. To further evaluate the interactions between Al, Si and Fe, a diffusion annealing treatment at 1000 °C was performed. The main diffusing elements during this treatment are Al and Fe, although small variations in Si content are also observed. At a certain distance from the surface, voids were observed, which most probably can be related to the Kirkendall effect. A characterization of the formed intermetallics was performed by Scanning Electron Microscopy (SEM), Energy Dispersive Spectroscopy (EDS), X-Ray Diffraction (XRD) and Electron Backscatter diffraction (EBSD).

EBSD study of crystallographic identification of Fe–Al–Si intermetallic phases in Al–Si coating on Cr–Mo steel

5Cr–0.5Mo steel was coated by hot-dipping in a molten Al–10wt.% Si bath at 700°C for 10, 60, 120 or 180s. The identification of the phases in the Fe–Al–Si intermetallic phases formed in the aluminide layers during hot-dipping was carried out by using a combination of scanning electron microscope (SEM) with energy dispersive X-ray spectroscopy (EDS), X-ray diffraction (XRD) and electron backscatter diffraction (EBSD). The EDS results show a τ5(H)-Al7Fe2Si phase, which exhibited 2 distinct morphologies, small particles widely dispersed and a continuous layer. Also revealed by EDS were τ6-Al4FeSi and τ4-Al3FeSi2 phases, which showed plate-shaped morphology, in an Al–Si topcoat. However, the XRD results show the intermetallic phases in the aluminide layer were composed of outer cubic τ5(C)-Al7(Fe,Cr)2Si and inner hexagonal τ5(H)-Al7Fe2Si. EBSPs and mapping functions in EBSD helped to clarify the confused phase identifications yielded by EDS and XRD. In this way, the small intermetallic particles and the continuous intermetallic layer were identified as cubic τ5(C)-Al7(Fe,Cr)2Si and hexagonal τ5(H)-Al7Fe2Si, respectively, and the plate-shaped intermetallic phase was identified as monoclinic τ6-Al4FeSi and tetragonal τ4-Al3FeSi2 with the same metallographic morphology. EBSD proved to be a very effective technique for local phase identification of aluminide layers with complicated multiphase morphologies.

Characterization of the Intermetallic Compounds Formed during Hot Dipping of Electrical Steel in a Hypo-Eutectic Al-Si Bath

In order to improve the magnetic properties of electrical steels, it may be desirable to increase the Si and/or Al content of the steel. A possible and alternative route to realize this is through the application of an Al-Si-rich coating on the steel substrate using a hot dipping process, followed by a diffusion annealing treatment. Previously, a series of compositions were used for dipping, namely: pure Al, Al + 10wt% Si (hypo-eutectic composition) and Al + 25wt% Si (hypereutectic composition). After these dipping experiments, and the subsequent evaluation of the coating and its formed intermetallic phases, the use of a hypo-eutectic Al-Si-bath was recommended for further investigation, because of certain advantages: i.e. hypo-eutectic concentrations allow lower dipping temperatures and reduce the formation of ordered Fe-Si-structures that cause brittleness in the coating and substrate. The present work reports on the results obtained on materials that were hot dipped in a hypo-eutectic Al-Si bath. An Al + 1wt%Si bath was used to coat electrical steel substrates with different silicon contents with dipping times, varying between 0 to 20 seconds, after a preheating of the samples to a temperature of 700°C. A thorough characterization of the formed intermetallics was made by Scanning Electron Microscopy (SEM), Energy Dispersive Spectroscopy (EDS) and X-Ray Diffraction (XRD). Three different compounds were identified as Fe2Al5, FeAl3 and a nearly pure Al phase.

Microstructure and High Temperature Oxidation of the Hot-Dipping Al-Si Coating on Low Carbon Steel in Ethanol, Water Vapor and Air at 700°C

Low carbon steel was coated by hot-dipping into a molten Al-10%Si bath. The high-temperature oxidation was performed at 700oC for 1 h to 49 h in air, air +100% H2O, and air + 30% ethanol under atmospheric pressure. An elemental composition distribution, morphologies of the aluminide layer and the oxide scale were characterized by OM, XRD, and SEM/EDS. After hot-dipping treatment, the coating layers consisted of Al, Si, FeAl3, τ5-Fe2Al8Si, and Fe2Al5. The results of high temperature oxidation tests showed the oxidation rate were parabolic law in three different atmospheres. The polyhedral τ1-(Al,Si)5Fe3 formed at a short time oxidation completely transformed to FeAl2 and FeAl due to the composition gradient and the chemical diffusion. The effect of water vapor on the oxidation resistance of the Al-Si coating may be attributed to increase in Al and Fe ions transport, leading to loss of protective aluminide layer by formation of iron oxide nodules on the coating surface and at interface between aluminide layer and the steel substrate.

Growth Kinetics of Al-Si-Fe Intermetallics during Hot Dipping of Steel

Electrical steels are used in flux carrying machines, divided in grain oriented and non oriented electrical steels mainly used in transformers and electrical motors, respectively. Their industrial production is not always easy due to the alloying elements which produce brittle order structures in the steel. Therefore hot dipping was found to be an alternative way of producing electrical steel with a high concentration of Al and/or Si: in a first series of experiments different steel substrates were coated by immersion in an Al + 23 m.-% Si hypereutectic alloy, followed by a high temperature diffusion annealing. The present contribution reports on the growth kinetics of Al-Si-Fe intermetallics formed during the dipping process in a hypoeutectic Al – 5 m.-% Si bath of Fe-substrates with 3 m.-% Si, previously cold rolled to different thickness. This bath composition allows a liquid phase at temperatures lower than the hypereutectic one with 23 m.-% Si and also less amount of eutectic formation. No Na-addition was made to the bath (the occurrence of a needle-like morphology of the Al-Si eutectic was not relevant for these experiments), furthermore this element might lower the magnetic properties of the steel. The preheating of samples and bath temperatures were not varied and set to 670°C. Short dipping times of 1 to 60 sec. were applied. The different layers formed were characterised by Scanning Electron Microscopy (SEM), using the Back Scattered Electron (BSE) detector and Energy Dispersive Spectroscopy (EDS).

Mid-infrared pump–probe spectroscopy of Si–H stretch modes in porous silicon

Using the Dutch free electron laser FELIX, we have investigated vibrational relaxation in free standing porous silicon (p-Si) films. Pump–probe measurements resonant with the SiH, SiH2 and O3SiH stretching modes yield temperature dependent measurements of the decay rates which demonstrate that all the modes decay via at least one internal defect mode with the excess vibrational energy distributed among the Si–Si bath phonons in a fourth order decay process.

Formation of intermetallic phases on 55 wt.%Al–Zn–Si hot dip strip

A study has been conducted to probe the formation of intermetallic phases on steel substrates immersed in 55wt.%Al–Zn–Si hot dip baths as a function of dipping time and bath silicon content. Two bath compositions containing 1.3 and 1.5wt.% Si, respectively, combined with two immersion times of 3 and 9s were studied. It was found that the reaction rate and intermetallic phase formation varied in response to silicon content. Optical microscopy revealed a quantifiable difference in the development of the reaction layer between the two bath compositions. SEM-EDS revealed that the reaction layer that evolved on samples dipped in the 1.5wt.% silicon bath were comprised of two intermetallic species, α-AlFeSi/Fe2Al5, whilst in the 1.3wt.% bath there were three clearly identifiable intermetallic species α-AlFeSi/FeAl3/Fe2Al5. A fourth phase appeared to be present in samples immersed in the 1.3wt.% Si bath that, due to its fine structure, could not be conclusively identified. Experimental results from the literature and from this study have been assessed with reference to the phase stability predicted by MTDATA, a thermodynamic modelling package.

Modeling silicon and aluminum diffusion in electrical steel

High silicon (Si) and aluminum (Al) electrical steel with improved magnetic properties can be produced by hot dipping in a molten Al-25 wt.% Si bath followed by diffusion annealing. The hot dipping deposits a Si-rich layer on top of the substrate, which is subsequently diffused into the bulk at high temperatures. The final properties of the material depend on the resulting concentration profiles, therefore a model capable of simulating the Si and Al diffusion is required. The first steps toward such a model are presented. The lack of knowledge of the ternary interdiffusion coefficients for the Al-Si-Fe system is overcome by reducing the coupled system of diffusion equations to a single partial differential equation (PDE), utilizing the diffusion path. Then by using a Levenberg-Marquardt inverse method, the apparent diffusion along this path can be determined. The direct problem was solved by a second-order space discretization, reducing the diffusion equation to a nonlinear system of ordinary differential equations (ODEs), which consequently can be solved with standard procedures for ODE.

Advances in the production of high-silicon electrical steel by thermomechanical processing and by immersion and diffusion annealing

Steel sheet with 6.5% Si is an excellent soft magnetic material, but its industrial production in low thickness still remains problematic or expensive. Using an adequate thermomechanical routing makes it possible to obtain steel with silicon up to 6.5% Si and thickness below 0.5 mm with excellent magnetic properties. Also a second production process is based on an immersion in a hypereutectic Al–Si bath at the adequate temperature in order to deposit a Si-rich layer on a steel substrate, followed by a proper diffusion annealing. The process is leading to homogenous concentrations of, e.g. 6.5% Si over the thickness being the power losses comparable with those industrially produced by chemical vapor deposition (CVD) method.

Effect of Mg and Si on the Microstructure and Corrosion Behavior of Zn-Al Hot Dip Coatings on Low Carbon Steel.

Coatings formed on a low carbon steel by a two step hot dipping, primarily in a Zn bath and secondarily in a Zn-6Al bath with or without 0.5 mass% Mg and 0.1 mass% Si addition. The effects of the Mg and Si addition on the surface morphologies, microstructures, and corrosion characteristics of the coatings were investigated using a scanning electron microscope, a laser scanning microscope, and an electron probe microprobe analyzer. The coating formed in the Zn-6Al bath showed flaws as pores and cracks in the outer adhered layer, while there were few flaws in the Zn-6Al-0.5Mg-0.1Si bath coating. The Zn-6Al-0.5Mg-0.1Si coating has a lamellae structure developed well in the outer adhered layer and the initially formed α-Al was finer than that of the Zn-6Al coating. Corrosion in a 5 % NaCI solution commenced from the α-Al phase at a very early stage and the Zn phase corroded preferentially. The Zn-6Al-0.5Mg-0.1Si coating corroded slowly and relatively homogeneously, while the Zn-6Al coating degraded locally due to a preferential corrosion along flaws. The Zn-6Al-0.5Mg-0.1Si coating incorporates Mg and Si in the outer adhered layer and Si in the inner alloy layer, making the Zn-6Al-0.5Mg-0.1 Si coating more anti-corrosive.

The effects of silicon on the reaction between solid iron and liquid 55 wt pct Al−Zn baths

The effect of various silicon levels on the reaction between iron panels and Al-Zn-Si liquid baths during hot dipping at 610°C was studied. Five different baths were used: 55Al−0.7Si−Zn, 55Al−1.7Si−Zn, 55Al−3.0Si−Zn, 55Al−5.0Si−Zn, and 55Al−6.88Si−Zn (in wt pct). The phases which formed as a result of this reaction were identified as Fe2Al5 and FeAl3 (binary Fe−Al phases with less than 2 wt pct Si and Zn in solution),T1, T2, T4, T8, andT 5H (ternary Fe−Al−Si phases), andT 5C (a quaternary Fe−Al−Si−Zn phase). Compositional variations through the reaction zone were determined. The phase sequence in the reaction zone of the panel dipped for 3600 seconds in the 1.7 wt pct Si bath was iron panel/(Fe2Al5+T 1)/FeAl3/(T 5H+T 5C)/overlay. In the panel dipped for 1800 seconds in the 3.0 wt pct Si bath the reaction zone consisted of iron panel/Fe2Al5/(Fe2Al5+T 1)/T 1/FeAl3/(FeAl3+T 2)/T 5H/overlay. In the panel dipped for 3600 seconds in the 6.88 wt pct Si bath the phase sequence was iron panel/Fe2Al5/(Fe2Al5+T1)/(T1+FeAl3)/(T1+T2)/T2/T8/T4/overlay. The growth kinetics of the reaction zone were also studied. A minimum growth rate for the reaction zone which formed from a reaction between the iron panel and molten Al−Zn−Si bath was found in the 3.0 wt pct Si bath. The growth kinetics of the reaction layers were found to be diffusion controlled in the 0.7, 1.7, and 6.88 wt pct Si baths, and interface controlled in the 3.0 and 5.0 wt pct Si baths. The presence of the interface between theT2/T5H, Fe2Al5/T 1, orT 1/FeAl3 phases is believed responsible for the interface controlled growth kinetics exhibited in the 3.0 and 5.0 wt pct Si baths.