Galvannealed Steel Research Articles

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]

Degradation mechanism of electrode tip during alternate resistance spot welding of zinc coated and uncoated steel sheets

The operation life and degradation mechanism of electrode during resistance spot alternating current welding of zinc coated galvannealed steel sheets were investigated. The electrode life in alternate resistance spot welding of zinc coated and uncoated steel sheets was extremely short in comparison with welding only on zinc coated steel sheets. During alternate resistance spot welding, the ring shaped nuggets were formed easily. This is attributable to the sticking of the alloy layers formed on the electrode top during the previous welding of zinc coated steel, to the uncoated steel surface at the following welding, so that notable wear took place, and the surface profile of electrode tip became flat. Additionally, an accumulation of highly resistant carbonaceous substance layer at the centre of electrode top caused an annular flow of welding current to hinder the nugget formation.

Mechanical properties of laser-pressure-welded joint between dissimilar galvannealed steel and pure aluminium

Dissimilar metal joints of Zn-coated Galvannealed steel (GA steel) and commercially available pure aluminium (A1050) sheets were produced by changing the laser power and the roller pressure by the laser pressure welding method. By this method, the YAG laser beam was irradiated into a flare groove made by these dissimilar metal sheets. In addition, the laser beam was scanned at various frequencies and patterns through the fθ lens using two-dimensional scanning mirrors. Then the sheets were pressed by the pressure rolls to be joined. The compound layers in the weld interface were observed by an optical microscope and the layer thicknesses were measured. The thicknesses ranged from 7 to 20 μm. The mechanical properties of the welded joints were evaluated by the tensile-shear test and peel test. In the tensile-shear test, the strengths of the joints produced under the most welding conditions were so high that the fracture occurred through the base aluminium sheet. In the peel test of the specimens subjected to a laser beam of 1200–1400 W power under roller pressure of 2.94 kN, the specimen fracture took place in the base aluminium sheet. Even if the compound layer was thick, high joint strength was obtained. On the other hand, the specimen fractured in the weld interface at a laser power of 1500 W. The results of X-ray diffraction on the peel test specimen surface identified that the intermetallic compound on the GA steel side was Fe2Al5Zn0.4. Moreover, the aluminium parts adhering to the GA steel side were confirmed. These results suggest that the fracture in the peel test occurred between the compound layer and A1050 and partly in the base aluminium. A micro-Vickers hardness test was performed to examine the hardness distribution in the compound layer. The hardness values near A1050 and GA steel were about 100 and 470 Hv, respectively, which suggests that the compound layer should not necessarily consist of brittle intermetallic compounds. It is therefore concluded that laser pressure welding could produce high strength joints of GA steel and A1050 dissimilar materials.

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.

合金化溶融亜鉛めっき鋼板の抵抗スポット溶接における電極先端形状の消耗変化

This study investigated the electrode life and the electrode degradation characteristics during resistance spot welding of zinc-coated galvannealed steel sheets. As a result, it was found that the electrode life in alternate resistance spot welding of galvannealed steel sheets and bare steel sheets was extremely short in comparison with welding of only galvannealed steel sheets. During alternate resistance spot welding, the ring-shape nuggets were formed easily. The reasons for this is explained by the sticking of the alloy layers formed on the electrode top during previous welding of the galvannealed steel, to the bare steel sheet surface at the following welding of bare steel, so that notable wear took place and the surface profile of electrode tip became flat. Additionally an accumulation of highly resistant carbonaceous substance layer at the center of the electrode top caused an annular flow of the welding current to hinder the nugget formation.

Influence of the Crack Spacing in the Coating Layer on the Progress of Interfacial Debonding in Galvannealed Steel Pulled in Tension

The coefficient of thermal expansion of the coating layer of the hot-dipped galvannealed steels is higher than that of the substrate steel. Accordingly, the coating layer shows multiple cracking during cooling due to the thermally induced stress. When tensile stress is applied externally on the coated steels, the coating layer exhibits further multiple cracking perpendicular to the tensile direction. As a result, the coating layer is multiply-cracked both in tensile and sample width directions. Under such existent cracks, the coating layer is spalled due to the interfacial debonding induced by the buckling of the coating layer in the sample width direction. In the present work, the influences of the crack spacing in the tensile direction and that in the sample width direction on the spalling process of the coating layer were studied with the finite element stress analysis. The results of analysis showed that (a) the larger the crack spacing of the coating layer in the tensile direction, the more occurs the interfacial debonding and (b) the crack spacing in the tensile direction affects especially on the initial spalling behavior; the larger the spacing, the more the spalling is enhanced. On the other hand, the crack spacing in the sample width direction affects only slightly on the initial spalling behavior, while the larger the spacing, the larger becomes the debonding area in the later stage.

Analysis of Group-buckling and -debonding Behaviors of Galvannealed Coating Layer on Steel Substrates under Applied Tensile Strain

The hot-dipped galvannealed steels are composed of brittle coating layer with low failure strain and ductile substrate with far higher failure strain. When tensile stress is applied externally on the coated steels, the coating layer exhibits multiple cracking perpendicular to the tensile direction, and then interfacial debonding occurs, following the buckling of the coating layer in the sample width direction. In the buckling behavior, the series of cracked-coating layers arraying along the tensile direction exhibit group-buckling. In the present work, the group-buckling behavior was observed with the scanning electron microscope and analyzed with 3-dimensional finite element models. It was observed that the preceding buckling of the coating enhances the buckling of the neighboring coating. Such a behavior was well accounted for by the present analysis.

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.

Analysis of Buckling and Interfacial Debonding of Galvannealed Coating Layer on Steel Substrates under Applied Tensile Strain

As the hot-dipped galvannealed steels are composed of brittle coating layer with low failure strain and ductile substrate with far higher failure strain. When tensile stress is applied externally on the coated steels, the coating layer exhibits multiple cracking perpendicular to the tensile direction, and then interfacial debonding occurs, following the buckling of the coating layer in the sample width direction. In the present work, the interfacial debonding behavior was observed with the scanning electron microscope and analyzed with 3-dimentional finite element models, to reveal the influence of the crack spacing on the buckling-induced interfacial debonding. In the analysis, three cases with different crack spacing in the coating layer were used. The results of analysis could account well for the experimentally observed buckling-induced spalling process of the coating layer. Furthermore it was revealed that, the shorter the crack spacing of the coating layer in the tensile direction, the less the interfacial debonding takes place.

Measurement of Alloying Degree of Galvannealed Steels by X-Ray Diffraction

Galvannealed steels are produced by reheating of Zinc-coated steels, in which the iron and the zinc are interdiffused to form Zn-Fe intermatallic compound. In recent years, application of galvannealed steel has increased due to its excellent corrosion resistance, good weldability and paintability. Galvannealed production requires strict control of processing conditions to obtain an optimal alloying degree in the coatings. The analysis of the alloying degree is very important since they are closely related to the corrosion and mechanical properties. This study is measurement of alloying degree of the galvannealed steel by using the XRD. Partial least squares (PLS) model is a powerful multivariate tool that has been successfully applied to the quantitative analysis of data in XRD. The alloying degree was determined by using PLS regression to a concentration accuracy of 0.2%. The XRD analysis and proposed PLS model can be successfully used to determine the alloying degree of industrially produced galvannealed steel.

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.

An electrochemical study on deformed galvanneal steel sheets

Corrosion behavior of deformed galvanneal steel sheets has been evaluated as a function of their degree of deformation under four different strain paths namely, biaxial, uniaxial (drawing), plane strain and tensile mode. A correlation between the degree of deformation and the loss in the extent of corrosion protection offered by these coatings in 3.5 wt.% NaCl solution has been established by carrying out electrochemical studies such as potentiodynamic polarization and electrochemical impedance spectroscopy. An almost linear relation has been found to exist between the effective strain applied to the specimen and damage caused to the coating. While uniaxial, plane strain and tensile modes of deformation have been found to affect the coating corrosion resistance almost to the same extent, the biaxial deformation has been found to cause a greater damage to the coating.

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.

Protective Properties of a Sol-Gel Coating on Zinc Coated Steel

Galvanised and galvannealed steels are widely used due to their good corrosion resistance in aqueous solutions. However, when additional protection is required, organic coatings, corrosion inhibitors or conversion coatings are used to improve their corrosion protection. In this work, sol-gel coating was used to improve the corrosion behaviour of these two materials. This paper analyses the final protective properties of a sol-gel coating prepared by basic catalysis and its dependence on the sintering temperature and time of treatment. The influence of the sintering conditions on the galvanised and galvannealed substrates is a decisive factor for the coating quality and for the barrier affect against the aggressive media. While heat treatment time is the controlling factor for the galvannealed steels, the temperature is determining in the case of the galvanised. Corrosion mechanisms for sol-gel galvanised steels did not changed with respect to the uncoated steel. However for galvannealed steel, after coating the mechanism is not purely cathodic.

Evaluation of the corrosion behavior of galvannealed steel in chloride aqueous solution and in tropical marine environment

An investigation was made into the corrosion behavior of commercial galvannealed steel in 10 )2 mol dm )3 NaCl aqueous solutions and in a tropical marine environment, using scanning electron microscopy (SEM), galvanostatic electrochemical stripping (GES), potentiodynamic linear polarization (PLP), and electrochemical impedance spectroscopy (EIS) techniques and open circuit measurements (Eoc). For purposes of comparison, a commercial galvanized steel was also subjected to similar corrosion tests. GES and SEM techniques allowed for the identification of f, d and G intermetallic phases and revealed cracks in the galvannealed steel. The PLP, EIS and Eoc results indicated that the galvannealed coating was more corrosion resistant than galvanized coating in an aqueous medium, but that their corrosion behaviors were similar in the marine environment. The corrosion behavior of the galvannealed steel was affected by the evolution of the cracking process in the Zn–Fe layer due to the dissolution of zinc-rich phases, while the galvanized steel displayed generalized corrosion in the aqueous medium and localized corrosion in the marine environment.

Multiple-cracking phenomenon of the galvannealed coating layer on steels under thermal and tensile stresses

The multiple-cracking phenomenon of the Fe-Zn intermetallic coating layer on the hot-dip galvannealed (GA) steels under thermal and tensile stresses was studied experimentally by tensile tests and analytically by means of the finite-element analysis. The multiple cracking of the coating layer had occurred in the as-supplied samples, and it progressed with increasing applied strain. Based on the calculated dependence of the stress of the coating layer on the crack spacing and applied strain, the multiple cracking in the as-supplied samples was accounted for by the thermally induced residual stress, and the further multiple cracking with increasing applied strain was accounted for by the increased stress of the coating layer. The experimentally observed decrease of the average crack spacing with increasing applied strain was described well, and the tensile strength of the coating layer was estimated to be 260 MPa, by application of the calculated relation between the increased stress of the coating layer and applied strain. The influences of the thickness of the coating layer and the substrate material on the multiple cracking were discussed based the stress analysis. It was revealed that the thinner the coating layer and the higher the flow stress of the substrate, the higher the stress of the coating layer becomes and, therefore, the smaller the crack spacing becomes.

Corrosion behaviour of galvannealed steel sheet

This paper analyzes the influence of galvannealed coating on the corrosion behaviour of press-formed and painted galvannealed steel sheet (GA) against the painted cold rolled and closed annealed (CRCA) sheet evaluated by electro impedance spectroscopy (EIS), salt spray (SST) and combined cyclic corrosion test (CCT). EIS study showed a significant decrease in the polarization resistance from 6.38 to 0.8 Ω/cm2 in case of CRCA painted sheet and from 6.07 to 4.07 Ω/cm2 in case of GA painted material revealing the superior corrosion resistance of galvannealed sheet. This was also confirmed from the scribed samples exposed to salt spray test. The corrosion product was also very low in GA painted compared to CRCA at scribed areas. Studies on under-paint corrosion from a deep scratch or creep-from-scribe in combined cyclic corrosion tests (CCT), simulating drying and wetting conditions in actual service life, have shown drastic reduction in creep-back for GA material. Salt spray test (SST) exposure of 720 h on painted and cross-scribed samples has also demonstrated the ability of GA material to resist corrosion that inevitably occurs on stone chipping on outer surface of auto body panels. Red rust formation started just after two cycles and propagated from crosscut portion with further test cycles. The initiation of red rust across the crosscut in GA material was not visible even after 65 cycles. The corrosion rates of CRCA painted samples after 30 and 45 cycles were 5.17 and 11.56 mpy, respectively, while in the case of GA painted, it was only 0.412 and 0.732 mpy, respectively. In case of CRCA, the corrosion rate increased with the passage of time. After 45 cycles of testing, GA painted sample has shown almost 16 times more corrosion resistance than the painted CRCA material.

Microstructures of galvannealing coats plated on an iron substrate deformed by indentation

Two different galvannealed (GA) steels with different formability performances were deformed at room temperature with a micro-Vickers indenter. Cross-sectional specimens for transmission electron microscopy were prepared from the indented areas using a focused ion beam technique. Direct evidence of plastic deformation of the GA coats was obtained. Cracks were observed to nucleate at triple points of two neighbouring Gamma and substrate Fe. The GA coat with better formability has finer grains than the GA coat, with worse formability.

Forming characteristics and mechanical parameter sensitivity study on pre-phosphated electro-galvanized sheet steel

With the development of modern automotive manufacturing technology, and the increased demands of automotive use, coated sheet steel has been applied in the manufacturing of large-scaled auto-panels more widely. It presents many challenges in characterizing, and understanding the behavior of coated products in terms of forming. In recent years, the increasing application of galvanized steels has highlighted the urgent need for work in this area in relation to both formability, and stamping technology. The forming characteristics of pre-phosphated electro-galvanized (EG) steel are analyzed in this paper, including the effect of a zinc coating on formability, zinc damage characteristics, and relationship between the coefficient of friction, and the viscosity of lubricants. In addition, the sensitivity of the material mechanical parameters of sheet steel to the stamping process of an auto-panel is discussed. It is shown that compared with other zinc-coated steel the application of pre-phosphated film to electro-galvanized steel prior to its being subjected to stamping operations will increase the formability, and reduce the coefficient of surface friction. With the increase of the viscosity of lubricants, the coefficient of friction of both pre-phosphated EG, and EG sheet steel decrease. Electro-zinc coatings have little influence on the mechanical properties, and the forming limit of sheet steel, but they will affect the strain distribution in the stamping process. Because of their relatively low hardness, the coatings of electro-galvanized, and pre-phosphated electro-galvanized steel are much easier to deform with the substrate than galvannealed steel, so that cracks occur later, and are to a lesser extent. It can be seen that the mechanical parameters, σs/σb, N value, R value, and δ, are relatively sensitive to the forming process under the bulging-deep drawing deformation of pre-phosphated electro-galvanized sheet steel.

Forming characteristics and mechanical parameter sensitivity study on pre-phosphated electro-galvanized sheet steel

With the development of modern automotive manufacturing technology, and the increased demands of automotive use, coated sheet steel has been applied in the manufacturing of large-scaled auto-panels more widely. It presents many challenges in characterizing, and understanding the behavior of coated products in terms of forming. In recent years, the increasing application of galvanized steels has highlighted the urgent need for work in this area in relation to both formability, and stamping technology. The forming characteristics of pre-phosphated electro-galvanized (EG) steel are analyzed in this paper, including the effect of a zinc coating on formability, zinc damage characteristics, and relationship between the coefficient of friction, and the viscosity of lubricants. In addition, the sensitivity of the material mechanical parameters of sheet steel to the stamping process of an auto-panel is discussed. It is shown that compared with other zinc-coated steel the application of pre-phosphated film to electro-galvanized steel prior to its being subjected to stamping operations will increase the formability, and reduce the coefficient of surface friction. With the increase of the viscosity of lubricants, the coefficient of friction of both pre-phosphated EG, and EG sheet steel decrease. Electro-zinc coatings have little influence on the mechanical properties, and the forming limit of sheet steel, but they will affect the strain distribution in the stamping process. Because of their relatively low hardness, the coatings of electro-galvanized, and pre-phosphated electro-galvanized steel are much easier to deform with the substrate than galvannealed steel, so that cracks occur later, and are to a lesser extent. It can be seen that the mechanical parameters, σs/σb, N value, R value, and δ, are relatively sensitive to the forming process under the bulging-deep drawing deformation of pre-phosphated electro-galvanized sheet steel.