Zn-coated Steel Sheets Research Articles

Influence of microstructural morphologies on liquid metal embrittlement of Zn-coated dual-phase steels

Dual-phase (DP, ferrite+martensite) steels are crucial for automotive components, predominantly spot-welded using thin Zn-coated cold-rolled sheets. However, the effect of martensite morphologies, namely fine-lamellar (LD) or coarse-globular (GD) shapes, on Zn-assisted liquid metal embrittlement (LME) remains underexplored. This study investigates LME mechanisms during resistance spot welding of Zn-coated DP steel sheets with LD or GD microstructures. Analysis focuses on cracks within the weld shoulder zone, categorized into sub-critical HAZ (SCHAZ), inter-critical HAZ (ICHAZ), and upper-critical HAZ (UCHAZ). The LME susceptibility index, calculated as total crack length divided by total contact length, increased in the order of LD-SCHAZ, LD-ICHAZ, GD-ICHAZ, and GD-SCHAZ. These results indicated that the microstructure of the GD sheet itself was very vulnerable to the cracking because the initial microstructure was almost retained in the SCHAZ. Cracking analysis in the LD-SCHAZ and GD-SCHAZ suggested that martensite grains near ferrite/ferrite boundaries impeded crack growth by deflecting propagation, and that the LD sheet showed higher crack resistance due to unfavorably oriented ferrite/martensite boundaries.

Root cause analysis of indentation mark defect on Zn coated steel sheet in continuous galvanizing line

The coated steel industry is growing as demand of corrosion resistant steels are ever increasing in automotive, construction, appliances, and general engineering segments. The surface requirement of the coated steel is becoming more stringent that demands coated surfaces within specified roughness, waviness, reflectance and free from surface defects. The continuous galvanising line number 2 of Tata Steel Limited, Jamshedpur suffers from a chronic surface defect that appears like a scratch mark in the coated steel. The present study deals with the root cause analysis of the defect focussing on the rolls within annealing furnace in CGL#2, Jamshedpur. The roll profile, roll and strip slippage, roll surface compositions (using profilometer, XRF) are studied in detail. Similarly, the steel strip collected from inside the furnace was also studied for the defect orientation, surface composition (using SEM & EDS), trapped particles etc. The mathematical calculation of slippage along with the characterization evidence confirm the formation of defect through a complex mechanism of particle formation due to non-uniform roll profile, followed by particle coagulation and finally formation of the defect on the strip under tension by those particles. Possible operational strategies to eliminate the defect is also outlined.

Effect of gap clearance on the mechanical properties of weld-brazed lap joints

Gas metal arc brazing (GMAB) is a non-fusion joining process used in the automotive industry for joining thin-gauge Zn-coated steel sheets to other steels or dissimilar metals. Unlike torch brazing, GMAB does not rely on capillary action to form a joint. Consequently, gap clearance has not been considered important for lap joining applications. In this study, the effect of gap clearance on the mechanical properties of arc-brazed lap joints was investigated experimentally and verified using finite element modeling (FEM). The results showed that the gap clearance of lap joints played a critical role in controlling the joint strength.

On the origin of micro-cracking in zinc-coated press hardened steels

Zn-coated press hardened steels (PHS) are in high demand for automotive mass reduction and enhanced passenger safety applications while the Zn coating supplies robust cathodic corrosion protection. However, the mechanism of micro-crack formation during direct hot-press forming (DHPF) has not been adequately described. Thus, the objective of this work was to determine the mechanism for micro-crack formation in Zn-coated DHPF PHS that addressed the relationship between micro-cracking and the coating microstructure created during substrate austenitization. Zn-coated 22MnB5 steel sheets were annealed at 900 °C for annealing times ranging from 30 to 780 s and DHPF at 75 °C s−1 to obtain a fully martensitic substrate microstructure. The inward diffusion between the Zn coating and the substrate during annealing resulted in a dual phase coating microstructure initially comprising Γ-Fe3Zn10 + α-Fe(Zn), transitioning to a single phase α-Fe(Zn) coating after annealing for 240–420 s. Coincident coating α-Fe(Zn) and substrate Zn-enriched austenite (γ-Fe(Zn)) grain boundaries became Zn-enriched, forming a thin layer of α-Fe(Zn) along the γ-Fe(Zn) grain boundaries. It is proposed that coincident coating α-Fe(Zn) and substrate prior austenite grain boundaries (PAGBs) were weakened by this grain boundary α-Fe(Zn) layer. Upon the application of tensile stress, intergranular fracture occurred along the coincident coating α-Fe(Zn) and Zn-enriched PAGBs in the Zn-enriched martensite (M(Zn)) layer. It was further determined that crack propagation ceased and the crack tip was blunted when Zn-enrichment along the PAGBs in the M(Zn) layer was exhausted.

Zn-induced liquid metal embrittlement of galvanized high-Mn steel: Strain-rate dependency

Zn-coated steel sheets suffer from Zn-induced liquid metal embrittlement (LME) during resistance spot welding and hot tensile deformation. By tensile testing at 700 and 800 °C, it was found that LME of an electro-galvanized high-Mn steel sheet was suppressed at low strain rate. Such strain-rate dependency of LME has been observed in various embrittlement metallic couples. Conventional ductile-to-brittle transition theory successfully explains the strain-rate reliance of LME in other embrittlement couples while it is invalid for Fe–Zn system. Owing to the high mutual solubility and several intermetallic compounds in Zn-rich regime of Fe–Zn system, the strain-rate dependency of Zn-induced LME was highly correlated with the existence of liquid Zn, which was verified by analyzing the cross section of the steel sheets.

Verification of Friction Models Implemented in the Simulation Software

Modern approaches in simulations of sheet metal stamping processes are based on definition the friction coefficient as not constant, but the different value in different regions, considering the nature of the actual deformation process in each individual region. In this contribution the regression and analytical models to determine the friction coefficients under blankholder and on the die drawing edge by strip drawing test are presented. These models were verified by experimental strip drawing test under the same contact conditions. Zn coated steel sheets for the automotive industry were used in experiments and simulations – extra deep drawing quality DX54D and high strength steel TRIP. The results indicate, that friction coefficients are not constant and depend on the pressure on the die contact surfaces. Friction coefficients were determined also at the cup test by the regression model of drawing versus blankholding force. Conformity of friction coefficients obtained by the cup and the strip tests was confirmed. Model of friction applied in FEM simulation was verified by cup test and good conformity was found out.

Si effect on Zn-assisted liquid metal embrittlement in Zn-coated TWIP steels: Importance of Fe-Zn alloying reaction

During resistance spot welding of Zn-coated high-strength steel sheets, Zn penetration is caused by Zn-assisted liquid metal embrittlement (Zn LME), and degrades the mechanical properties of the welded parts. When electrogalvanized twinning induced plasticity (TWIP) steels with 0–1.5 wt% Si were welded, Zn LME was exacerbated by Si addition. It was found that Si in the steel delayed the dissolution of Fe into the coating from the steel, which suspended the active formation of α-Fe(Zn). Since α-Fe(Zn) fraction in the coating was insufficient to cover the steel/coating interface in the Si added steel, it could not prevent the steel from contacting the liquid Zn during welding. As a result, liquid Zn generated higher number and deeper Zn penetration into the Si containing steel than into the Si free steel. Overall, it was revealed that Si aggravated Zn LME in TWIP steel by suppressing Fe-Zn alloying reaction, which increased the possibility of steel substrate to contact liquid Zn.

Friction anchor welding between Al alloy and Zn-coated steel using insert steel difference in the mechanism of friction anchor welding between the types of Zn coatings on Zn-coated steel

ABSTRACTThe lap joints of upper Al alloy sheets (1.0-mm-thick A5052) and lower Zn-coated steel sheets (1.2-mm-thick GI steel or GA steel) were welded using insert steel sheets (0.6-mm-thick SPCC) by a spot welding process with a tool having a spherical ceramic tip, i.e. ‘Friction Anchor Welding.’ As a result, straight (not-rugged) steel projections were formed in the Al alloy sheets for both the GI and GA, while steel projections were not formed for the GI, rugged steel projections were formed for the GA without the insert steel sheets. In addition, the tensile shear strength for the GI was greater than that for the GA. In other words, the tensile shear strengths reached about 3.9 kN/point for the GI and about 3.2 kN/point for the GA, which were greater than those of the welds without the insert steel sheets. On the other hand, the cross tensile strengths for the GI and GA were almost the same, which reached about 2.6 kN/point. Additionally, for the GI, the Zn layer on the GI steel sheet melted and was totally removed due to the pressure and heat caused by the rotating tool, which facilitated the welding between the SPCC and GI steel sheets. For the GA, however, the Zn-Fe layer on the GA steel sheet changed to a solid-liquid mixture and was not completely removed, which prevented the welding between the SPCC and GA steel sheets. Therefore, the thickness of the steel-steel welded region (i.e. the SPCC-GI or the SPCC-GA welded region) for the GI was greater than that for the GA. We estimated that the difference in this thickness is significantly related to the fracture mechanism during the tensile shear test and the cross tensile test.

Effect of Porosity on the Fatigue Behavior of Gas Metal Arc Welding Lap Fillet Joint in GA 590 MPa Steel Sheets

Weld defects such as blowholes and surface pores occur due to zinc vaporization during the gas metal arc welding of lap fillet joints of Zn-coated steel. In this study, the effect of porosity on fatigue behavior was investigated. A Zn-coated steel sheet with the strength of 590 MPa and a thickness of 2.3 mm was used as the base material. Three kinds of specimens with weld pore defects, such as blowholes and surface pores, were prepared and the tensile shear strength, hardness and fatigue behavior were investigated. The pore defects in the welds reduced the tensile shear strength. In the fatigue test, at higher load stresses between 122 and 366 MPa, pore defects reduced the fatigue life of the weld. However, the pore defects in the welds did not significantly affect the fatigue life of the welds at stresses below 92 MPa.

On the Corrosion Performance of Module-Mounting Assemblies for Ground-Mounted Photovoltaic Power Station

The present paper reports the corrosion performance results of construction materials employed for the manufacture of mounting assemblies for ground-mounted photovoltaic (PV) power stations. For this purpose, the corrosion behavior of industrial hot-dip Zn-coated steel sheets was compared with that of Magnelis® type steel coating. Experiments involved samples in continuous exposure in 3 wt% NaCl solution along with regular assessment of their corrosion parameters by means of major (impedance spectroscopy, linear and Tafel polarization plots) electrochemical techniques. Analyses of surface/cross-sectional morphology and elemental compositions of metal-coated samples were carried out by means of SEM and EDX spectroscopy techniques.Graphical ᅟ

Continuous nanoscale Al2Fe transition layer strengthened magnesium-steel spot joints

Cold metal transfer welding was employed to produce Mg to steel spot joints. A continuous and compact plate-like intermetallic Al2Fe layer, with a thickness of 100–300nm, was formed at the interface between solidified AZ61 Mg filler alloy and Zn-coated DP600 steel sheet, resulting in a very high tensile-shear strength of up to 180MPa. On the contrary, when 100nm thick, island-type Al2Fe was observed at the interface between Mg and uncoated DP600 steel sheet, a much lower joint strength was observed regardless of energy input.

Study of martensitic-ferritic dual phase steels produced by hot stamping

The effects of heat treatment and initial microstructure on tensile properties of 22MnB5 and 30MnB5 high-strength hot stamping steels with martensite-ferrite matrix were investigated. Hot stamping steels possessed limited elongations of about 5% in a tensile strength ranging from 1300 to 1500 MPa when quenched at temperatures above A3 temperatures. The total elongations were tried to improve by partial austenization between Ac1 and Ac3 temperature and quenching. Ac1 and Ac3 temperatures were calculated via ThermoCalc. Microstructural characterization was made by using Light Microscope and Scanning Electron Microscope. Microstructure is composed of ferrite+martensite. It was seen that annealing temperature affects the volume fraction of phases. It was concluded that initial microstructure is an important parameter for the final microstructure. This method can be used for automobile parts which require higher TE with sufficient yield and tensile strength. Also this process may be a way of using Zn coated steel sheets in hot stamping process.

Liquid Zn assisted embrittlement of advanced high strength steels with different microstructures

In the present study, liquid metal embrittlement (LME) phenomenon during high temperature deformation was investigated for 3 grades of Zn-coated high strength automotive steel sheets consisting of different phases. Hot tensile tests were conducted for each alloy to compare their LME sensitivities at temperature ranges between 600 and 900 °C with different strain rates. The results suggest that Zn embrittles all the Fe-alloy system regardless of constituent phases of the steel. As hot tensile temperature and strain rate increase, LME sensitivity increases in every alloy. Furthermore, it is observed that the critical strain, which is experimentally thought to be 0.4% of strain at temperatures over 700 °C, is needed for LME to occur. It is observed via TEM work that Zn diffuses along grain boundaries of the substrate alloy when the specimen is strained at high temperatures. When the specimen is exposed to the strain more than 0.4% at over 700 °C, the segregation level of Zn at grain boundaries seems to become critical, leading to occurrence of LME cracks.

Formability and Weldability of Zn Coated Steel Sheet for Hot-stamping

Formability and Weldability of Zn Coated Steel Sheet for Hot-stamping

Study on monitoring of plasma emission signal in lap welding of Zn coated steel sheet using CO2 laser

In this study, induced plasma that is created during the CO2 laser lap welding of 6-mm-thick Zn coated steel for shipbuilding were measured through photo-diode and microphone. Then, we compared and analyzed the RMS value of plasma emission signal and spatter generation amount as a function of lap clearances with Zn coated thickness. The results indicated that as RMS values increased depending on the amount of Zn vaporization, and RMS also changed abruptly when defects occurred depending on lap joint conditions. Also, as a result of obtaining FFT value of raw signal by welding conditions, the obtained frequency signal was about 30 Hz at coating thickness 15 μm and about 50 Hz at coating thickness 30 μm in light signal, and in acoustic signal, peak value appeared at about 1 kHz. In addition, it was found that when the frequency value was set as the bandwidth of bandpass-filter, and the RMS values which were obtained after filtering raw signals were matched to weld bead. Thus we have considered that the signals and defects were very closely related.

Evaluation of Friction Coefficient of Stamping

This In order to define the friction coefficient the simulator of strip drawn test between flat dies and cup test were applied. To evaluate the coefficient between flat dies the modified Coulumb law was used and for defining the friction at drawing edge of die the modified Ouehler formula was applied. Due to these both evaluation processes it was enabled to point out the difference of friction coefficient values in various contact areas of rolling product and the tool. In his paper there are presented the measurement and evaluation processes of friction coefficient by applying the cup test. For experimental research the following types of materials are suitable: DC 05- extra deep drawable steel sheet, DX 54D - extra deep drawable Zn coated steel sheet, austenitic steel sheet DIN 1.4301.

The effects of coating parameters on the quality of TiB2–TiC composite phase coating on the surface of Cu–Cr–Zr alloy electrode

To improve the lifetime of electrodes used for the resistance spot welding of Zn-coated steel sheets, a TiB2–TiC composite coating was fused onto the surface of spot welding electrodes using an electro-spark deposition (ESD) process. Different processing parameters were studied in order to improve the quality of the electrode coating. The microstructure, elemental composition, phase structure, and mechanical properties of the coatings were characterized by scanning electron microscopy (SEM), energy dispersive X-ray analysis (EDX), X-ray diffraction (XRD) and microhardness testing. When TiB2–TiC is deposited onto the electrodes in air, extensive coating defects occur within the bulk TiB2–TiC coating and at the interface with the electrode; however, coating defects are significantly reduced when the TiB2–TiC is deposited in argon. Delamination and oxidation of the TiB2–TiC coating were prevented and the number of cracks diminished. When TiB2–TiC was deposited in air on an electrode pre-coated with Ni, almost all coating defects were eliminated. The TiB2–TiC coated directly onto electrodes in air was found to have the lowest average microhardness of HV50 650, and the average hardness values obtained in argon and with Ni-coated electrodes were about HV50 1050 and HV50 1080, respectively. A heat-affected zone (HAZ) of about 20μm in depth within the substrate was observed near the interface between the coating and the substrate for all coating conditions.

Numerical Modeling of Electrode Degradation During Resistance Spot Welding Using CuCrZr Electrodes

Resistance spot welding is a technique widely used by the automotive industry to assemble thin steel sheets. The cyclic thermo-mechanical loading associated with the accumulation of weld spots progressively deteriorates the electrodes. This study addresses the development of a comprehensive multi-physical model that describes the sequential deterioration. Welding tests achieved on uncoated and Zn-coated steel sheets are analyzed. Finite element analysis is performed using an electrical-thermal-metallurgical model. A numerical experimental design is carried out to highlight the main process parameters and boundary conditions which affect electrode degradation.

Two-beam Laser Brazing of Thin Sheet Steel for Automotive Industry Using Cu-base Filler Material

This work shows the potential of two-beam laser brazing for joining both Zn-coated steel and 22MnB5. Brazing of Zn-coated steel sheets using Cu-Si filler wire is already state of the art in car manufacturing. New press-hardened steels like 22MnB5 are more and more used in automotive industry, offering high potential to save costs and improve structural properties (reduced weight / higher stiffness). However, for joining of these ultra-high strength steels investigations are mandatory. In this paper, a novel approach using a two-beam laser brazing process and Cu-base filler material is presented. The use of Cu-base filler material leads to a reduced heat input, compared to currently applied welding processes, which may result in benefits concerning distortion, post processing and tensile strength of the joint. Reliable processing at desired high speeds is attained by means of laser-preheating. High feed rates prevent significant diffusion of copper into the base material.

Coaxial monitoring of the fibre laser lap welding of Zn-coated steel sheets using an auxiliary illuminant

In the laser welding process, weld defects are often caused by many factors, such as variations in the laser power, welding speed and gaps between two workpieces. In an auto-welding system, the on-line monitoring of the welding quality is very important in avoiding weld defects. In this paper, an on-line coaxial monitoring system with an auxiliary illuminant was built for the fibre laser welding of galvanised steel; images of the weld pool were taken during the welding process. Profiles of the weld pool and the keyhole were obtained by processing the images using the region-growing algorithm and the Canny algorithm. In this research, we used the on-line monitored weld pool width to monitor the weld surface width. The weld penetration status was divided into the three categories of incompletely penetrated, moderately penetrated and over-penetrated using the value of d (diameter at the bottom of the keyhole)/D (diameter at the top of the keyhole). Thus, the weld width and weld penetration status of fibre laser welding can be monitored on-line.