Galvannealed Research Articles

Improvement in galvanization and galvannealing characteristics of DP 590 steel by prior Cu or Cu–Sn flash coating

Galvanization of a dual phase (DP) steel surface is a challenging subject as occurrence of uncoated spots is a very common attribute in coating. In this study, a novel approach was applied where a thin layer of Cu or Cu–Sn (flash coating) was deposited on the DP steel surface before galvanization to resolve this chronic zinc coatability problem. These thin layers on the steel surface were deposited by displacement processes and both flash coated layers had good surface coverage and excellent adherence to the steel surface. Both flash coated as well as without flash coated steel sheets were annealed and subsequently dipped in molten zinc. It is evident from visual appearance that the galvanized coating obtained on the DP steel surface without any flash coating consists of uncoated spots whereas the galvanized (GI) coatings obtained on the DP steel surfaces with prior flash coatings were compact and free from any defects. Galvannealing operation was done for half the number of the total galvanized samples to obtain galvannealed (GA) coating. SEM-EDS and XRD analysis reveal that galvanized iron (GI) coating for the steel without any flash coating consists of almost pure zinc throughout the depth of the coating. However, GI coatings for the steels with both flash coatings consist of alloy phases up to a certain depth of the coating near steel interface. The GA coatings with both flash coatings mainly consist of the desirable δ phase. Both flash coatings promote controlled Fe–Zn reaction. Tafel, salt spray and open circuit potential measurement tests confirmed that performance of GI coatings against chloride attack was improved for both flash coated steels and GA coating has better resistance capability than GI coating.

抵抗スポット溶接による鋼と Al合金のシールスポット溶接

This research concerns a dissimilar metals joining of steel and aluminum (Al) alloys by means of zinc (Zn) insertion. The authors proposed a joining concept for achieving strong bonded joints between Zn coated steel and Al alloys. A eutectic reaction between Zn in the Zn coating and Al in Al alloy is used to remove the dense, strong oxide film on Al alloy surface at low temperature, thereby resulting in the formation of a thin, uniform Al-Fe intermetallic compound (IMC) layer at the joint interface, leading to a strong bonded joint. The ultimate aim of this research is to apply this joining concept in the resistance spot welding process for manufacturing vehicle bodies.As a practical issue characteristic to dissimilar metals joints, anticorrosion measures against electrochemical corrosion must be undertaken. If there is moisture near a dissimilar metals joint interface, electrochemical corrosion would progress. Therefore, a sealing function that could prevent moisture intrusion is required. By applying above mentioned welding process to a set of metals with thermosetting resin spread in between, we realized seal spot welding which would not only prevent moisture intrusion, but also retain high tensile strength.In this research, first, a cyclic corrosion test was performed on the seal spot welded joint of galvanized (GI) steel and Al alloy, and the anticorrosion measures were found to retain tensile strength and also secure sufficient corrosion resistance. Then galvannealed (GA) steel, a steel grade widely distributed in Japan, and Al alloy was bonded by seal spot welding, and following topics have been discussed. Complete removal of sealant from the joint interface is the key to realize high tensile strength joint, because remaining sealant would lead to reduction of tensile strength. Therefore, heat generation at the interface was monitored by measuring electrical current and potential difference between the two electrodes, and a precise temperature control was performed. Moreover, the bonding process was clarified by stepwise analysis of the joint interface using optical microscopy, and a guideline for producing strong joints was proposed. And finally, a TEM observation also confirmed that the interface structure of seal spot welded joint was the same as joints without the resin; a thin and uniform Al-Fe IMC layer was formed and a strong metallurgical bonding was achieved.

Nature of the inhibition layer in GA baths

The nature of the intermetallic layer which forms on the steel surface during immersion in typical galvanizing baths for galvannealed (GA) sheets production has been investigated on two commercial Titanium-stabilized Interstitial-Free (Ti-IF) steel substrates galvanized in baths with different Al contents. Results from this study show that in both cases the inhibition layer is biphasic and composed of a very thin Al-rich phase layer, identified as Fe2Al5Znx, and a thicker Zn-rich phase layer on top of it, identified as δ. Experimental results also show that the Fe2Al5Znx phase layer becomes discontinuous when decreasing the bath Al content. Discussions about the mechanisms of formation and the final microstructure of this inhibiting layer are also tackled in this paper by means of the Al-Fe-Zn ternary phase diagram at 460 °C and assumptions to justify any deviation from thermodynamic equilibrium are as well proposed.

The Structure and Corrosion Process Characterization of Galvannealed and Galvanized Steel Sheets

The dissolution process of galvannealed (GA) and galvanized (GI) coating was examined by correlating a chemical stripping test in 1MNaCl+0.01MH2SO4solution. The key time of dissolution process of coating was found via measuring the variation of free corrosion potential. Through coating metallographic observations at the key point, the dissolution process of coating can be clear. There are four Fe-Zn intermetallic phases in GA coating, namely gamma-1,gamma,delta and zeta phases, from the substrate. The compressed part of GA and GI coatings was the initial of corrosion process. The net crack will be emerging on the delta phase, which width graduallyincreasing until stripping. Pitting corrosion occurred on the GI coating in the solution containing Cl-ion, the area and depth of pit gradually increasing until stripping. The corrosion potential of Fe2Al5Znxinhibition layer is about-0.8VSCE.

Influence of Gas Atmosphere Dew Point on the Galvannealing of CMnSi TRIP Steel

The Fe-Zn reaction occurring during the galvannealing of a Si-bearing transformation-induced plasticity (TRIP) steel was investigated by field-emission electron probe microanalysis and field-emission transmission electron microscopy. The galvannealing was simulated after hot dipping in a Zn bath containing 0.13 mass pct Al at 733 K (460 °C). The galvannealing temperature was in the range of 813 K to 843 K (540 °C to 570 °C). The kinetics and mechanism of the galvannealing reaction were strongly influenced by the gas atmosphere dew point (DP). After the galvannealing of a panel annealed in a N2+10 pct H2 gas atmosphere with low DPs [213 K and 243 K (−60 °C and −30 °C)], the coating layer consisted of δ (FeZn10) and η (Zn) phase crystals. The Mn-Si compound oxides formed during intercritical annealing were present mostly at the steel/coating interface after the galvannealing. Galvannealing of a panel annealed in higher DP [263 K and 273 K, and 278 K (−10 °C, 0 °C, and +5 °C)] gas atmospheres resulted in a coating layer consisting of δ and Г (Fe3Zn10) phase crystals, and a thin layer of Г 1 (Fe11Zn40) phase crystals at the steel/coating interface. The Mn-Si oxides were distributed homogeneously throughout the galvannealed (GA) coating layer. When the surface oxide layer thickness on panels annealed in a high DP gas atmosphere was reduced, the Fe content at the GA coating surface increased. Annealing in a higher DP gas atmosphere improved the coating quality of the GA panels because a thinner layer of oxides was formed. A high DP atmosphere can therefore significantly contribute to the suppression of Zn-alloy coating defects on CMnSi TRIP steel processed in hot dip galvanizing lines.

A novel iron enrichment isothermal kinetic model proposal for galvanneal coatings

Galvanneal (GA) coatings are extensively used in the automotive industry in car bodies and for exposed parts to enhance corrosion protection. GA coatings are produced in Continuous Galvanizing Lines (CGL) by carrying out a heat treatment immediately after the Zn deposition in the liquid state, where the liquid is constitutionally solidified and transformed into Fe–Zn alloy (composed by its intermetallic compounds (IMCs)). IMCs nucleation and growth kinetics control allows one optimize the coating mechanical performance during steel drawing operations. This work proposes a new kinetic model to describe the GA coating Fe enrichment. Based on the hypothesis that each IMC will be nucleated, Fe enriched, saturated and consumed by an Fe richer IMC, the model was able to describe the Gleeble simulated isothermal Fe enrichment kinetics for unalloyed industrially produced interstitial free (IF) and dual-phase (DP) GA coating steels.

Effect of Cu Addition on Surface Morphology of Bake Hardenable Galvannealed Steel Sheets

Abstract The effect of chemical composition on the surface characteristics of the hot-dip galvannealed (GA) steel sheets with a carbon content as low as 15 ppm strengthened by Cu and/or Mn sulfides instead of Ti and/or Nb carbides has been studied. The most important goal for developing the new type of steel sheets is focused on the achievement of the extreme surface quality to fulfill the high aesthetical requirements for defect-free automotive outer panels. The shape and distribution of the surface enrichments of Cu, Mn, Al and O on the GA steel sheets were characterized by OM, SEM and TEM. Especially, formation mechanism of the narrow streaky mark with the width of 50–100 µ m occasionally observed on the surface of GA steel sheets will be presented.

ESTUDO DA DISTRIBUIÇÃO DE FASES Fe-Zn EM REVESTIMENTOS GALVANNEALED UTILIZANDO A TÉCNICA GDOES

Galvannealed (GA) coatings are composed of Fe-Zn phases, being its quality greatly affected by the phase’s distribution, making it important to be characterized. Glow Discharge Optical Emission Spectroscopy (GDOES) technique allows a quick and accurate evaluation of the chemical composition profile along the coating thickness. In this study, GA coatings were produced varying the temperature (530°C, 550°C, 580°C) and times (0 s, 5 s, 15 s, 60 s) of galvannealing treatment, in order to change the Fe-Zn phases distribution. The coatings were characterized by SEM and XRD. GDOES analyses were performed to identify the distributions of Fe and Al and correlate them with the η, ζ, δ and Γ phases. It was found that variations in temperature and treatment time influenced the distribution of Fe and Al. For short times, Fe and Al were concentrated near the coating/substrate interface and as the time increase concentrations became more uniform along the coating. The characterization of GA coatings from the Fe and Al distribution were effective for checking the distribution of Zn-Fe phases

Experimental Study on Surface Damage in Forming of Galvanized Steel Sheets

U-channel forming tests were performed to investigate the surface damage behavior of hot-dip galvanized (GI), galvannealed (GA) and electrogalvanized (EG) steels in sheet metal forming (SMF). Experimental results indicate that the surface topographies of galvanized steels are roughening with the number of forming in sheet metal forming (SMF) and the types and mechanisms of surface damages of the three coatings are different. And tool hardness has great effects on surface damages of the GI and GA steels while the influence of tool hardness on surface damage of EG steel is negligible.

Experimental and finite element analysis for fracture of coating layer of galvannealed steel sheet

Mechanical properties of galvannealed (GA) steel sheet used for automotive exposed panel and predicted failure phenomenon of its coating layer were evaluated using finite element method. V-bending test was performed to understand better the fracture of coating layer of GA steel sheet during plastic deformation. Yield strength of the coating layer was calculated by using a relative difference between hardness of coating layer measured from the nano-indentation test and that of substrate. To measure shearing strength at the interface between substrate and coating layer, shearing test with two specimens attached by an adhesive was carried out. Using the mechanical properties measured, a series of finite element analyses coupled with a failure model was performed. Results reveal that the fracture of coating layer occurs in an irregular manner at the region where compressive deformation is dominant. Meanwhile, a series of vertical cracks perpendicular to material surface are observed at the tensile stressed-region. It is found that 0.26-0.28 of local equivalent plastic strain exists at the coating and substrate at the beginning of failure. The fracture of coating layer depends on ductility of the coating layer considerably as well.

Development of 980MPa Grade Galvannealed Advance High Strength Steel Sheets for Automobile

Main issues in the automotive industry are the reduction of vehicle body weight for energy savings and improvement of crashworthiness for passenger safety. In order to address both these issues, there has recently been increasing application of galvannealed advance high strength steel (GA AHSS) sheets for automobiles. However, GA AHSS sheets have some surface defects such as coating bare spots due to the addition of solid-solution strengthening elements, which result in the deterioration of the galvannealing reaction. In this study, the effects of galvannealed manufacturing conditions on surface and mechanical properties, resistance spot weldability on a laboratory scale, and GA 980 MPa steel sheets produced by commercial continuous galvannealing line (CGL) were investigated.

The Characteristics of Cr-Free Coating Hot Dip Galvanized Sheet Steel

The greatest purpose of chromate treatment is to improve anti-corrosion by stabilizing a metal surface. Because metal surface forms a compound by absorbing oxygen or water in the air by being generally unstable, it is necessary to improve anti-corrosion of the metal by forming the metal surface with a stable film. When considering the economical efficiency and requirements together because the film of the metal surface treated with chromate has good anti-corrosion and the stability also in the air by being compact and strong, Chromate treatment has been used most up to the electronics industry from the auto industry. However, these days, because hexavalent chromium is both a toxic agent to be able to cause cancers and deadly poisonous environmental pollutant, the strong legal controls on its use is being imposed all over the world. Because of this reason, a new anti-corrosion method is being required. Also, by users' various demands, the passivations that have recently been developed require various characteristics such as conductivity, chemical resistance, alkali cleaning resistance as well as anti-corrosion. We could confirm the results such as excellent anti-corrosion compared to chromate, conductivity, chemical resistance and detergent resistance as the result of analysis of various characteristics of the galvannealed sheet steels coated with Cr-Free solution developed in this research.

Effects of Pre-Electrodeposition with Fe and Cu Before Annealing in Reducing Atmosphere on the Corrosion of Hot-Dip Galvanized and Galvannealed Steels

not Available.

Effects of Steel Coatings on Electrode Life in Resistance Spot Welding of Galvannealed Steel Sheets

The effects of different galvannealed (GA) coatings, containing Fe varying from 7.0 to 11.4 mass%, on steel sheets on the electrode life in resistance spot welding (RSW) have been investigated with metallurgical analysis of the coating microstructures and properties, and the surfaces and cross-sections of failed electrodes. The results showed that the electrode life in RSW of GA steel with 11.4 mass% Fe in coating was 110% higher than that with coatings containing 7.0 or 9.6 mass% Fe. The improvement was believed to be caused by the build-up of a Fe-rich alloy layer on the electrode surface, which could serve as a barrier to prevent copper loss from the electrode surfaces to the steel sheets, thus reducing the growth rate of the electrode tip face diameters. In addition, higher Fe content in the coating resulted in increased contact resistance and hence a lower welding current needed in RSW. [doi:10.2320/matertrans.M2010239]

Surface topography evolvement of galvanized steels in sheet metal forming

U-channel forming tests were performed to investigate the surface topography evolvement of hot-dip galvanized(GI) and galvannealed(GA) steels and the effects of die hardness on sheet metal forming(SMF). Experimental results indicate that the surface roughness values of the two galvanized steels increase with the number of forming, i.e., the surface topographies of galvanized steels are roughened in SMF. Moreover, GI steel has a better ability of damage-resistance than GA steel. The mechanisms of topography evolvement are different in the forming of GI and GA steels. Scratch is the main form of surface damage in the forming of GI steels. The severity of scratch can be decreased by increasing die hardness. GA steel results in exfoliating of the coating firstly and then severe scratching. The surface topography of galvannealed steels can be improved by increasing die hardness. However, the hardness should not be too high.

갈바어닐링온도변화가 합금화용융아연코팅의 합금상과 마찰특성에 미치는 영향

This paper is aimed to understand the effect of different galvannealing temperatures on the frictional properties and Fe-Zn intermetallic phases of the galvannealed (GA) coatings on steel sheets. Their galvannealing treatments were conducted at 465, 505, 515 and 540℃ for about 10s in the additional heating furnace of an industrial continuous hot-dip galvanizing line. The mechanical and the frictional properties of the coatings were estimated using nanoindentation, nanoscratch, micro vickers hardness tests and flat friction tests, which were performed at contact pressures of 4, 20 and 80㎫. Also, the correlation between the microstructure and the frictional properties of the GA coatings were investigated by SEM observation for the cross-section of the GA coating after and before flat friction tests. The results showed that the mechanical and the frictional properties of the coatings are strongly dependent on their phase distributions and microstructure. Especially, in low contact pressure of 4㎫ the frictional properties of the coatings were dependent on the surface phases and morphology, while in high contact pressure of 80㎫ it was influenced by their mechanical properties based on the dominant phase distributions.

Powdering in Coating Layer of Galvannealed Nb-Ti Free Extra Low Carbon Steel Sheet

We have studied the fracture behavior of coating layer when low and high alloying galvannealed (GA) steels are subject to forming process. To understand better powdering features in the coating layer of the steel sheets, we carried out V-bending test and a series of finite element analysis which simulates damage characteristics in the coating layer. Results showed that the powdering behavior in the coating is significantly affected by the soundness and volume fraction of phases in the coating layer. The hardness variation of coating layers attributed to different phases leads to different deformation behavior of the coating layer itself.

In-situ observation in a scanning electron microscope on the exfoliation behavior of galvannealed Zn–Fe coating layers

Exfoliation behavior of galvannealed (GA) coating layers was continuously observed in-situ under 3-point bending load in a scanning electron microscope (SEM). Two kinds of substrate, Al-killed steel and Ti-stabilized interstitial free (IF) steel, were used. For GA coating layers formed on Al-killed steel, powdering was dominant at the region under compressive stress during the bending. Exfoliation occurred at the Γ/ δ 1 interface, and the amount of exfoliation decreased with an increase in Fe content in the coating layer. Meanwhile, for GA layers coated on IF steel substrate, flaking was dominant. One of the perpendicular cracks that were pre-induced during galvannealing grew and propagated up to the Γ/Fe matrix interface with increasing strain. On the other hand, the crack initiated at the Γ/Fe matrix interface grew and propagated along the interface. Then, this crack combined with a nearby perpendicular crack, resulted in flaking.

Enhanced forming properties of galvannealed steel sheet by polymanganese phosphate coating

Polymanganese phosphate coating is obtained on galvannealed (GA) steel surface by dipping process. Coating weight of 1.1 g/m2 is found to give dense with uniform crystallites of size 1–2 μm. Energy Dispersive X-ray Spectroscopy and X-ray Diffraction results show that the coating composes of manganese phosphate hydrate and rockbridgeite. The coefficient of the friction of polymanganese coated galvannealed steel sheet is reduced significantly from 0.22 to 0.11 on the die. Manganese phosphate in the coating behaves as an anti-sticking base, thereby the contact zone is found to be smooth. Further, the treated sheet removes only 5.9 mg of Fe powder in the form of flakes as compared to the untreated sheet producing 12 mg Fe powder from its surface.

Investigation of Streaky Marks Formed on the Coating Surface of Hot-Dip Galvannealed Steel Sheets

Abstract Interstitial-free High strength (IF-HS) steels, the tensile strength ranging from 340MPa to 440MPa, are widely used in automobile industry. The major strengthening mechanism employed to increase the tensile strength of ultra-low carbon steels is solution hardening by the additional elements of P, Mn, Si, Cr and Cu. It has been well known that the surface condition of steel sheet substrates before dipping into a molten zinc pot significantly affects the final products quality of galvannealed (GA) steel sheets, i. e., the occurrence of the visual defects so-called “streaky mark” and others in GA process are strongly related to the surface enrichment of oxides in terms of the chemical composition of the steel sheet substrate.