Galvannealed Steel Research Articles

A Comparative Study of Zinc Phosphate and Zirconium-Based Conversion Coatings on Galvannealed Steel: Effect on Corrosion Performance of Electrocoated Systems

The efficacy of an electrocoated system that uses a Zr-based conversion coating was assessed to protect galvannealed (GA) steel. The aim was to determine if a Zr-based system has the potential to replace a system with a traditional zinc phosphate (ZnP) conversion coating, where there are concerns for the environmental and economic sustainability. A comparative study between the two conversion coatings, and a control system with no conversion coating, is presented. Data from AC and DC electrochemistry are utilized with surface analysis investigations of pristine specimens, where scanning electron microscopy, x-ray photoelectron spectroscopy, and x-ray diffraction have been used to determine the mechanism through which protection, or coating failure, occurs. Further investigations study surface morphology and stability of specimens during immersion in a simulated electrocoat bath, and their effect on the performance of fully electrocoated systems. The corrosion current density (icorr) for bare, ZnP, and Zr-based conversion coated specimens in chloride-containing solution with no electrocoat were found to be approximately 1.90 µA/cm2, 0.30 µA/cm2, and <0.01 µA/cm2, respectively. Rp values of 0.05 kΩ, 4.2 kΩ, and 20.7 kΩ were recorded for bare, ZnP and Zr-based specimens, respectively. The corrosion current density (icorr) for bare, ZnP, and Zr-based conversion coated specimens under chloride-containing conditions with electrocoat were found to be 3.82 µA/cm2, 0.53 µA/cm2, and 0.06 µA/cm2, respectively. |Z| values at 0.01 Hz (|Z|0.01 Hz) were measured, yielding 45.5 GΩ, 47.4 GΩ, and 31.6 GΩ for bare, ZnP, and Zr-based pretreatments respectively, demonstrating excellent barrier properties after electrocoating. Hydrogen evolution reaction (HER) rate values of 3.52 × 10−2 mol·m−2·s−1, 3.12 × 10−2 mol·m−2·s−1, and 2.94 × 10−2 mol·m−2·s−1 were recorded for the bare, ZnP, and Zr-based systems, respectively. This study produced a ranking order of HER rate. This was bare > ZnP > Zr-based. These results suggest that the Zr-based system is less prone to HER, interfering with the electrocoat deposition process. A performance ranking order for pretreated and electrocoated systems was established, where Zr-based > ZnP > control. The level of thickness and roughness provided by the conversion coating is speculated to be the controlling factor for coating performance, where the ZnP conversion coating was found to provide a thicker layer with greater surface coverage beneficial for a uniform electrocoat film build.

A hybrid coating of organometallic complex with a silane coupling compound to protect galvannealed (GA) steel from white rust without sacrificing weldability

A unique organic-metallic hybrid coating has been formulated to impede white rust formation on galvannealed (GA) steel strips without hindering weldability. With an optimal concentration ratio of methacrylate silane coupling agent to a titanium (IV) lactate metal complex on the surface of zinc were creating of a network-type nanostructure. Controlling the dry film thickness and curing at 120 °C for 60 s with a chemical roll coater produced a thin film coating (<∼2 μm). Spectroscopic instruments like scanning electron microscopy (SEM), Raman spectroscopy, X-ray photoelectron spectroscopy (XPS), dynamic contact resistance measurement, and electrochemical impedance spectroscopy (EIS) were used to investigate the coating's surface microstructure, interfacial bonding, anticorrosion, and electrical conductivity characteristics for weldability. The welding properties were tested using the AWS D8.9 standard, and the material was subjected to a simulated salt spray test under ASTM B117 to evaluate anti-corrosion (white rust formation). It was observed that ‘Silane’ to ‘Titania complex’ at a ratio of 0.02 in the formulation was providing excellent performance in terms of anti-corrosion and weldability. This coating has the potential to replace the traditionally used harmful chromate passivation coating over zinc and zinc alloy surfaces in the entire steel industries.

Effect of heat input on corrosion behavior of automotive zinc-coated steel joint

Abstract Lately, coated steel sheets have replaced bare steel in the automotive sectors due to their corrosion protection. Among them, zinc-coated high strength interstitial free (HIF) steel is widely used due to its durability, formability, good surface finish, impact resistance, and corrosion resistance. However, joining galvannealed (GA) steel using conventional fusion welding processes is challenging as it results in porosity and potentially harmful fumes. To overcome these challenges, this work uses a pulse MIG (metal inert gas) brazing technique to join the HIF-GA steel sheet with a lap joint configuration utilizing Cu-Si base filler wire (CuSi3Mn1) in an argon atmosphere. The study aims to evaluate the effect of heat input on bead geometry and Zn degradation in MIG brazed joints by adjusting the current from 40 to 60 A and brazing speed from 400 to 500 mm min−1. It is observed that the heat input is inversely proportional to the wetting angle and directly proportional to the bead width, leg length, and cross-sectional area. Heat input influences Zn degradation on both the front (at the toe) and back sides of the lap joint. The corrosion behavior of joints is studied using an electrochemical test (in 3.5% NaCl solution) and a salt spray test (in 5% NaCl solution). Further, the Zn degradation is analyzed with EDS line scanning to understand the corrosion resistance of the MIG brazed joints. A potentiostatic polarization study is also conducted to determine the corrosion rate using a Tafel plot. It is found that the lowest corrosion rate in base metal (BM) increased from 0.88 to 12.26 mm year−1 with the increase in heat input from 61 to 113 J mm−1.

Conflicting behavior between powdering and flaking resistance under skin pass mill process in galvannealed interstitial free steel

&lt;abstract&gt; &lt;p&gt;The failure of galvannealed (GA) coatings during press forming is an important issue for steel companies, because it results in a deteriorated product quality and reduced productivity. Powdering and flaking are thought to be the main failure modes in GA steel. However, these two modes currently lack a clear distinction, despite their different failure types. Therefore, in this study, we demonstrate that the different behaviors of these two failure modes are generated by the skin pass mill (SPM) condition and we discuss the underlying mechanism in detail using microstructural and simulation analyses. With the increase in steel elongation from 0% to 4.0% under milling force from 0 to 6 ton, a high compressive stress is produced up to −380 MPa on the surface of the steel sheet and the interface is correspondingly flattened from 0.96 to 0.53 m in Ra. This flattening weakens the mechanical interlocking effect for adhesive bonding, deteriorating the flaking resistance from 41.1 to 65.2 hat-bead contrast index (hci). In addition, the GA coating layer becomes uniformly densified via the filling of pores under compressive stress in the layer. Furthermore, the ζ phase exhibits significant plastic deformation, leading to a uniform coverage of the coating surface; this helps to suppress crack propagation. Accordingly, the powdering resistance gradually improves from 4.2 to 3.5 mm. Consequently, with the increase in SPM-realized steel sheet elongation, the powdering resistance improves whilst the flaking resistance deteriorates. Significantly for the literature, this implies that the two failure modes occur via different mechanisms and it indicates the possibility of controlling the two coating failure modes via the SPM conditions.&lt;/p&gt; &lt;/abstract&gt;

Effect of Proportions of Γ and Iron–Zinc Solid Solution Phases on the Corrosion Prevention Performance of Mixed Coating Layers on Heated Galvannealed Steel Sheets

Coating layers having different ratios of a Γ phase to an Fe–Zn solid solution phase were produced on galvannealed steel sheets by heat treatment. The corrosion prevention characteristics of these layers were then investigated using combined cyclic corrosion test, electrochemical measurements and analyses of corrosion products. A coating layer with a larger proportion of the Γ phase showed better corrosion prevention properties along with a smaller corrosion mass loss. The Γ phase exhibited a sacrificial corrosion protection effect with regard to both the Fe–Zn solid solution and the steel substrate while the Fe–Zn solid solution showed a sacrificial corrosion protection effect on the steel substrate. The corrosion products formed from the Γ phase comprised Zn5(OH)8Cl2·H2O, Zn5(CO3)2(OH)6, ZnO and various amorphous compounds, while the Fe–Zn solid solution generated Zn-containing ferrihydrite and amorphous compounds. The presence of Zn evidently stabilized the ferrihydrite and delayed the formation of crystalline iron oxyhydroxide based on comparisons with corrosion products formed on cold-rolled steel sheets. These effects improved the corrosion prevention performance of the Fe–Zn solid solution. The zinc-based corrosion products formed from the Γ phase provided even better corrosion prevention performance by delaying the formation of crystalline iron oxyhydroxide and were also themselves resistant to reduction.

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. In the present study, high strength interstitial-free steel sheets with a relatively high amount of Mn and P were galvannealed in an industrial continuous galvanizing line and streaky-marks formed on the coating surface were characterized by optical microscopy, scanning electron microscopy, electron probe micro analysis and transmission electron microscopy. Effects of streaky marks on the mechanical properties and paintability of GA coating were also investigated.

Effect of galvannealed coating on corona bond strength of resistance spot welding

The effect of galvannealed (GA) coating on the corona bond strengths of resistance spot welding joints was investigated. A cross tensile test was performed with removal of the nugget, using GA steel sheets and steel sheets whose coating had been removed via pickling. The maximum load with coating was lower than that without coating. When the nugget diameter was small (3√t, t: sheet thickness), GA coating reduced the corona bond strength (MPa). In contrast, when the nugget diameter was large (5√t), GA coating did not change the corona bond strength but reduced the corona bond area.

Plastic deformation of single crystals of the δ1p and δ1k intermetallic compounds in the Fe–Zn system by micropillar compression

The plastic deformation behavior of single crystals of the δ1p and δ1k phase compounds in the Fe–Zn system, which are the major constituent phases in the coating layer of galvannealed (GA) steels, has been investigated by micropillar compression tests at room temperature as a function of crystal orientation and specimen size. (0001)[112‾0] basal slip and (101‾0)[1‾21‾0] prism slip are observed to operate in the δ1p phase compound while (0001)[112‾0] basal slip is observed in the δ1k phase compound. For all cases, a significant strain burst occurs immediately after yielding, forming giant steps on the slip planes along the slip direction and leading in many cases to instantaneous ‘slip plane (shear) failure’. The extent of such strain burst as well as instantaneous ‘slip plane failure’ is reduced by introducing dislocations (pre-straining) prior to micropillar testing, accompanied by the reduction in the stress at which such strain burst occurs as well as that in the extent of strain burst. The critical resolved shear stress (CRSS) for basal and prism slip in the δ1p and δ1k phase compounds all show an inverse power-law scaling against the specimen size with an exponent in the range of 0.02–0.06. The bulk CRSS values are estimated respectively to be approximately 320, 430, and 765 MPa for the basal slip in the δ1p and δ1k compounds and the prism slip in the δ1p compound by taking into account the specimen size effects of CRSS.

Probing the influence of cold wire gas tungsten arc welding current waveforms on the aluminum-steel joining

Abstract Cold wire gas tungsten arc welding (GTAW) process is used to join AA6061-T6 and galvannealed (GA) steel. The welding current distributed in sine, rectangle, square, and triangle AC waveform is used to understand their influence on the arc behavior, temperature distribution, joint profile, and the intermetallic compound (IMC) layer thickness. The instantaneous welding current and voltage waveforms are recorded in synchronization with the high-speed arc images to understand the arc behavior and heat input. The rate of change of instantaneous welding current between the polarities and the arc duration at the peak current is more in square and rectangle waveforms. The peak current in both the positive and negative pulses and the arc heat input of the triangle waveform is the highest among all the waveforms considered in the present work. The wetting length increases along with the reduction in the bead height sequentially with sine, rectangle, square, and triangle waveforms for a constant machine setting welding current. IMC layer thickness is minimum for the sine waveform and maximum for the triangle waveform. Overall, the welding current distributed in sine waveform is found to be suitable among all the waveforms studied in the present work for dissimilar metal joining of aluminum to steel.

Effect of galvannealing temperature on coating microstructure evolution correlated to flaking degradation on galvannealed interstitial-free steel

Galvannealed (GA) steel sheets have been widely used in the automotive industry because of their good weldability, paintability, and corrosion resistance. However, coating failure and exfoliation originating from the brittle and hard FeZn phases during press forming is a serious problem. This study investigates the mechanism of flaking deterioration in GA coatings with high Fe content. For this, specimens with various Fe contents were fabricated with different galvannealing temperatures. As the temperature increased, the η and ζ phases became extinct and both δ and Γ phases grew simultaneously. Accordingly, surface became homogeneously flattened with crater degeneration. Simultaneously, the interface was flattened by lateral Γ phase growth. The flat region where the Γ existed was widened by broadening the Γ phase area, by which the etch-pit shape rough knoll found under craters was gradually eliminated. As a result, the Γ phase growth resulted in interface flattening, which deteriorated the flaking resistance.

Micropillar compression deformation of single crystals of the intermetallic compound Γ-Fe4Zn9

The plastic deformation behavior of single crystals of the Γ (Fe4Zn9) phase compound in the Fe−Zn system, which has been believed to be the most detrimental phase causing brittle failure in the coating layer (such as powdering) on galvannealed (GA) steels, has been investigated by micropillar compression tests at room temperature as a function of crystal orientation and specimen size. All micropillar specimens exhibit rich deformability in compression. Two distinct slip systems, {1¯01}<111> and {1¯01}<010>, are observed to operate. The critical resolved shear stress (CRSS) values for {1¯01}<111> and {1¯01}<010> both obey an inverse power-law scaling against the micropillar size with an exponent of -0.245 and -0.046, respectively. The bulk CRSS values for these two slip systems are estimated respectively to be 238 and 649 MPa by considering the specimen size effects of CRSS. The crystal orientation range for the operation of {1¯01}<010> slip is limited to the vicinity of 〈111〉 because of the considerable difference in the CRSS values for these two slip systems. Dislocations with b (Burgers vector) = 〈010〉 dissociate into two partial dislocations, while those with b = 1/2<111> do not show any dissociation, both of which are consistent with the result of theoretical calculation of the generalized stacking fault energy on the {1¯01} slip plane. Possible reasons for brittle failure related to the Γ phase occurring in the coating layer on GA steels are discussed based on the results obtained.

Corrosion Resistance of Mg-Added Galvannealed Steel Sheets with Nano-Composite Coating

Corrosion Resistance of Mg-Added Galvannealed Steel Sheets with Nano-Composite Coating

Dissimilar laser brazing of aluminum alloy and galvannealed steel and defect control using interlayer

The microstructure and tensile shear strength of the joint by laser brazing between dissimilar metals of aluminum alloy and galvannealed (GA) steel were investigated. A blowhole was formed by zinc vaporization during laser brazing; the tensile shear strength was significantly affected not by intermetallic compounds (IMCs) at the filler metal/GA steel interface but by the blowhole. The vaporization of zinc was suppressed, and the blowhole was controlled by inserting a Ti interlayer. The joint strength was improved through the suppression of blowhole, the thermal deformation of the Ti interlayer, and the increase in brazed filler metal thickness by optimizing the brazing parameter. The hardness test revealed that fracture occurs preferentially in the brazed filler metal; however, as the brazed filler thickness increases, fracture begins to occur near the heat-affected zone of A5052 because the volume of filler metal increases. The maximum strength of the joint was 185 N/mm, which is about 73% of the joint efficiency to 254 N/mm of the A5052 base material, and the base material partly fractured.

Study of the wettability and the corrosion protection of the hybrid silane (3-aminopropyl) triethoxysilane (APTES) and (3-glycidyloxypropyl) trimethoxysilane (GPTMS) film on galvannealed steel

In this work, the wettability of the galvannealed surface pretreated with the hybrid silane APTES/GPTMS film was measured by sessile drop method. The coatings were applied on the substrate from dip-coating method and dried in an oven. The study followed an experimental factorial design, 3³, with contact angle as response. The corrosion resistance was evaluated by electrochemical techniques and the surface morphology was characterized by scanning electron microscopy. The wettability results showed the best condition for galvannealed steel pretreated with the hybrid silane film for 2:1 APTES:GPTMS ratio, 2% silane concentration and 150 min of hydrolysis time, because the result may be related to a higher rate of hydrolysis of the silanes that is influenced by both the functional group present and by the type and number of hydrolyzed groups, resulting consequently in a higher availability of silanol groups in solution, promoting a greater crosslinking of the film on the metallic substrate. Keywords: hybrid silane, corrosion, galvannealed steel, pretreatment, wettability.

Accurate Online Measurement of Iron Concentration in Galvannealed Coating Layers by Shift of X-ray Diffraction Peak

The authors have developed a new online measurement system for the Fe concentration in galvannealed (GA) coatings based on the proportionality between the peak angle of X-rays diffracted by the δ1 phase and the Fe concentration. The online system was realized by high-speed measurement of the peak angle of diffracted X-rays using a one-dimensional detector in conjunction with correction algorithm for the angular error caused by the displacement of the GA steel strip surface from the measurement position. The performance of the developed online system was demonstrated in a continuous galvanizing line with GA strips of Si-added and Si-free base steels. The developed system showed higher accuracy even with the Si-added base steel, with which enough accuracy could not be obtained with conventional online systems based on the diffraction intensity of the Γ phase. Since the δ1 phase is the major phase of GA coatings, the main advantage of the developed system is that its accuracy is less sensitive to the composition of the base steel.

Prediction of the Weld Qualities Using Surface Appearance Image in Resistance Spot Welding

The quality of the resistance spot weld is predicted qualitatively using information from the weld’s external apparent image. The predicting tool used for weld qualities was a convolution neural network (CNN) algorithm with excellent performance in pattern recognition. A heat trace image of the weld surface was used as information on the external apparent image of welds. The materials used in the experiment were advanced high strength steel (AHSS) with 980 MPa strength, and uncoated cold-rolled (CR) steel sheets and galvannealed (GA) steel sheets were used. The quantitatively predicted weld quality information contained tensile shear strength, nugget diameter, fracture mode of welds, and expulsion occurrence. The predicted performance of the verification step of the model determined through the learning process was as follows; the predicted error rate for tensile shear strength and nugget diameter were 2.2% and 2.6%, respectively. And the predicted accuracy on fracture mode and expulsion occurrence was 100%.

Effects of steel type and sandblasting pretreatment on the solid-liquid compound casting characteristics of zinc-coated steel/aluminum bimetals

Effects of zinc-coated steel type and steel surface sandblasting pretreatment in the solid-liquid compound casting of layered type steel/aluminum bimetals were investigated. The Zn coating behavior and its effects on interfacial microstructure evolution and fracture mechanism were also discussed. The aluminum fluidity in thin plate type flow channels formed by the steel insert and the mold wall primarily depended on the insert surface roughness and secondarily on the wettability. As-galvanized (GI) steel/aluminum bimetal joints showed the bonding strength of 20 MPa and more, while galvannealed (GA) steels showed poor bonding. The interfacial bonding zone consisted of most Al13Fe4, Al8Fe2Si, Al4.5FeSi intermetallic phases, as well as some Si phases. A low temperature and short time of the bonding reaction coupled with a high silicon content of the aluminum alloy suppressed the formation of Al5Fe2 phase. Oxide scales on the GA steel surface prevented the molten Zn coating from mixing with the aluminum melt. The Zn coating of GI steels was rapidly disappeared from the steel surface by the chemical affinity and surface energy-driven fluid flow as well as the diffusion, resulting in the formation of Zn-free intermetallic phases. The Zn coating of GI steels played a role in retarding the onset of bonding reaction. A long time sandblasting caused an excessive growth of intermetallic layers and the formation of Kirkendall voids on the steel side, resulting in the shift of main fracture sites and a slight decrease of the bonding strength.

Study on static and fatigue strength of structural adhesive-bonded joints of steel sheets for automotive application

ABSTRACTThe static and fatigue strength of crush durable structural adhesive-bonded lap joints of steel sheets for automobiles was evaluated by tensile shear tests. The steel sheets used in this study were uncoated and galvannealed (GA) with tensile strength ranging from 270 MPa-grade to 980 MPa-grade and the thickness ranging from 0.7 to 1.8 mm. Also, the effects of the adhesive types were evaluated. The results are as follows: In the static tensile shear tests, when the steel sheets deformed during the tensile test, the tensile shear strength increased with the increase in the base metal properties, such as the yield strength and thickness; however, when the base metal properties were sufficiently high not to undergo plastic deformation, the tensile shear strength exhibited a constant value. On the other hand, the effect of base metal properties on the fatigue joint strength was relatively small. The static joint strength of the GA steel joints was slightly lower than that of the uncoated steel sheets; however, the fatigue strength of the GA steel joints was higher than that of the uncoated steel sheets. The coating failure of the GA was affected by the type of adhesive, base metal properties and type of test. Choosing the proper adhesive can reduce the failure of the GA coating, and the high strength steel showed fewer coating failures than the mild steel.

Evaluation of Weldability for MAG and LASER with Galvannealed Steel.

Lower-arm, one of the components in automotive suspension module, has been fabricated by MAG welding in general which is lap jointed with 2 mm thick galvannealed steel sheet (SGAPH440). This welding process produces some problems such as significantly much spatters and weld defects of porosity in welded joint, which degrades productivity and weldability. Therefore, in order to solve these problems, this study has been tried to apply two types of solid wires with different chemical composition rate of Si and Mn, in MAG welding process. Moreover, the laser welding process has been adopted to fabricate the low-arm and compared with those of MAG welding, in terms of mechanical and metallurgical characteristics. It was observed that in MAG welded joints, much more spatters were occurred in using solid wire with higher Si and Mn contains. The maximum tensile-shear strength in laser welded joints was approximately 16.7 kN, which was almost equal to that of in MAG welded joints. The microstructure showed ferrite and martensite in weld metal in MAG and laser welded joints indicating no significantly grain size change.

Accurate Online Measurement of Iron Concentration in Galvannealed Coating Layers by Shift of X-ray Diffraction Peak

The authors have developed a new online measurement system for the Fe concentration in galvannealed (GA) coating layers based on the proportionality between the X-ray diffraction (XRD) peak angle of the δ1 phase and the Fe concentration. The online system was realized by high speed measurement of the XRD peak angle by a one-dimensional detector and use of a correction algorithm for the angular error caused by the displacement of the GA strip surface from the measurement position. The XRD profile and displacement were simultaneously measured by the one-dimensional detector and a laser displacement meter, respectively. The high accuracy of this new online system was demonstrated in a continuous galvanizing line with GA steel strips of Si-added and Si-free base steels. The developed system showed sufficiently high accuracy even with the Si-added base steel, with which adequate accuracy could not be obtained with conventional online systems, based on the diffracted intensity of the Γ phase, due to the suppression of Γ phase formation. Since the δ1 phase is the major phase of the GA layers, the main advantage of the developed system is that its accuracy is less sensitive to the composition of the base steel. The developed system can be applied to other online measurement applications in the steel industry for various purposes such as measurement of phase transition, orientation, crystallinity, strain and other features.