Continuous Galvanizing Line Research Articles

Stored Energy Evaluation for High Strength Dual-Phase Steels with Different Pre-annealing Conditions

The purpose of this study was to determine the stored energy of dual-phase (DP) steels after hot rolling, coiling, and cold reduction using electron backscattered diffraction analysis. Three methods for stored energy evaluation were examined and compared: the sub-grain boundary, image quality, and kernel average misorientation methods. The results demonstrated that the sub-grain method, in which stored energy was calculated as a function of sub-grain boundary misorientations and total sub-grain boundary length, can provide more accurate stored energy values, since cold work was responsible for forming numerous dislocation structures such as shear bands, cells, and cell walls. As expected, the steels with the combination of a low coiling temperature of 580 °C and 60 pct cold reduction had the highest stored energy values. This information is important in defining how pre-anneal processing conditions might control the response of DP steels to the various transformations occurring during continuous galvanizing line simulations.

Vortex Dynamics and Fluctuations of Impinging Planar Jet

Smoothness of thin metallic coated strip produced in continuous galvanizing lines is influenced by fluctuations of the impinging wiping pressure. In this paper, vortex dynamics e.g. vortex production frequency and mixing of jet opposing shear layer vortices; and impinging pressure were numerically studied by Large Eddy Simulation (LES). The effects of jet nozzle width, d, and operational parameters (nozzle to strip distance, H, and mean jet velocity, Uo) were investigated. Vortex production rate is almost linearly correlated to Uo and mixing of shear layer vortices occurs when H/d ≥ 6. Dominant frequencies of impinging pressure fluctuation are significantly different between the two possible phenomena of i) Mixing of opposing shear layer vortices prior to jet impingement on the strip, or ii) No mixing of opposing shear layer vortices prior to jet impingement. The impinging pressure of a jet characterised by mixing of vortices is predominantly composed of frequencies lower than 10 kHz with the most significant components at less than 1 kHz. In contrast, for a jet with non-mixing of vortices, the impinging pressure fluctuations are comprised of frequencies greater than 10 kHz and the dominant frequency is approximately one half the vortex production frequency. Utilising existing model results for the coating thickness response to pressure and shear stress fluctuations12) the anticipated degree of coating thickness sensitivity to the mixing and non-mixing impinging jet cases of the present work has been elucidated. It is shown that a mixed vortices jet is most likely to cause surface ripples in the coating.

Mechanical property development of a 0.15C–6Mn–2Al–1Si third-generation advanced high strength steel using continuous galvanizing heat treatments

The effects of starting microstructure and intercritical annealing (IA) temperature on the mechanical properties of a prototype 0.15C–6Mn-1.5Al–1Si (wt%) third-generation medium manganese advanced high strength steel (3G AHSS) were determined. The general 3G AHSS mechanical property targets UTS × TE of 24,000–40,000 MPa% were met after a continuous galvanizing line (CGL)-compatible IA treatments at 710 °C and 740 °C for 120 s from an 80% martensite – 20% ferrite starting microstructure due to the as-annealed microstructure having large volume fractions of stable retained austenite. The deformation-induced retained austenite to martensite transformation – or TRansformation-induced plasticity (TRIP) – kinetics were a strong function of IA temperature and resulting retained austenite stability. In particular, the 665 °C intercritical anneal produced retained austenite which only partially transformed during deformation and the 740 °C intercritical anneal produced retained austenite which exhausted the TRIP effect at significantly lower strains than those observed for the 710 °C treatment. It was also shown that significant deformation-induced mechanical twinning occurred in the retained austenite, aiding in sustaining the work hardening rate. Overall, it was determined that medium-Mn 3G AHSS can be produced using CGL-compatible thermal cycles, which holds promise for cost-effective production of these alloys.

Influence of processing factors on the sheared-edge formability of vanadium bearing dual-phase steels produced using continuous galvanizing line simulations

Dual-phase steels are commercially attractive since they offer good formability at a high strength and reasonable cost. In addition to high strength, these steels also exhibit high “n-values” and high uniform and total tensile elongations. However, one limitation in the application of high-strength dual-phase steels is the fairly limited ductility observed during sheared-edge stretching or punched hole expansion. Therefore, successful high-strength dual-phase steels must have controlled microstructures in order to achieve the desired performance. The failure mechanism observed during stretching of the sheared edge is closely related to the microstructures (i.e., amount, hardness, size and distribution of the different phases and microconstituents), and the cracks were found to initiate predominately at the interphase interfaces separating the ferrite and the martensite. In the present research, the influence of three main variables on the hole expansion ratio (HER) behavior was explored: the metallurgical condition going into the continuous galvanizing line (CGL), different post-anneal CG line thermal processing paths and the influence of vanadium. Depending on conditions, all three variables can influence the sheared-edge ductility as determined in the hole expansion test. Suggestions for improving the sheared-edge ductility while maintaining adequate strength are presented.

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.

Effect of Mn and Cr on the selective oxidation, surface segregation and hot-dip Zn coatability

The effect of Mn and Cr on the surface selective oxidation, segregation and hot-dip galvanizing was studied in three Fe-Mn alloys with ∼0.2 wt% Al and two Fe − Mn alloys with ∼0.5 wt% Cr in an HDPS (Hot-dip Process Simulator) following industrial CGL (Continuous Galvanizing Line) conditions. The Mn content was varied from 1.5 to 3.6 wt% in the alloys. The alloys were subjected to the same thermal cycle following continuous annealing (CA) at 800 °C in N2 − 5%H2 gas atmosphere with −40 °C dew point and then hot-dipping in Zn - 0.20 wt% Al bath at 460 °C. The oxidation tendency of Mn, Al and Cr during annealing was calculated using FactSage software. GDOES (Glow Discharge Optical Emission Spectroscopy) measurements showed a higher extent and depth of segregation for Mn with increasing bulk concentration during the CA. It was observed that Cr segregates comparatively less near the annealed surface in the presence of more Mn. XPS (X-ray Photoelectron Spectroscopy) measurements revealed the formation of simple MnO in Fe-Mn-Al alloys and complex MnCr2O4 in Fe-Mn-Cr alloys. Contrary to the thermodynamic calculations, no Al-oxides were detected at the annealed surfaces of Fe-Mn-Al alloys during the XPS study. The hot-dip Zn coatability was determined by measuring the number density of uncoated spots using 3D CLSM (Confocal Laser Scanning Microscopy) and by visual inspection. The Fe-Mn-Cr alloys displayed a better hot-dip Zn-coating compared to Fe-Mn-Al alloys. The conclusions of the present study are also critically discussed considering the literature findings.

Optimization of the Continuous Galvanizing Heat Treatment Process in Ultra-High Strength Dual Phase Steels Using a Multivariate Model

The main process variables to produce galvanized dual phase (DP) steel sheets in continuous galvanizing lines are time and temperature of intercritical austenitizing (tIA and TIA), cooling rate (CR1) after intercritical austenitizing, holding time at the galvanizing temperature (tG) and finally the cooling rate (CR2) to room temperature. In this research work, the effects of CR1, tG and CR2 on the ultimate tensile strength (UTS), yield strength (YS), and elongation (EL) of cold rolled low carbon steel were investigated by applying an experimental central composite design and a multivariate regression model. A multi-objective optimization and the Pareto Front were used for the optimization of the continuous galvanizing heat treatments. Typical thermal cycles applied for the production of continuous galvanized AHSS-DP strips were simulated in a quenching dilatometer using miniature tensile specimens. The experimental results of UTS, YS and EL were used to fit the multivariate regression model for the prediction of these mechanical properties from the processing parameters (CR1, tG and CR2). In general, the results show that the proposed multivariate model correctly predicts the mechanical properties of UTS, YS and %EL for DP steels processed under continuous galvanizing conditions. Furthermore, it is demonstrated that the phase transformations that take place during the optimized tG (galvanizing time) play a dominant role in determining the values of the mechanical properties of the DP steel. The production of hot-dip galvanized DP steels with a minimum tensile strength of 1100 MPa is possible by applying the proposed methodology. The results provide important scientific and technological knowledge about the annealing/galvanizing thermal cycle effects on the microstructure and mechanical properties of DP steels.

Failure analysis of weld joint of high strength quality steel sheets in cold rolling mill

Zinc coating (galvanizing) provides maintenance free, long lasting corrosion protection at a reasonable cost for years. Continuous galvanizing line facilitates the application of such coating onto the steel sheet. For continuous operation, it becomes necessary to join two sheets across their width. Resistance seam welding is one of the widely accepted joining method for cold rolled sheet due to final weld joint quality and efficiency. Quality of the weld is of paramount importance because failure of weld joint can cause breakdown for 1–3 days resulting in huge production loss. One such failure of weld joint of two sheets caused breakdown in CGL line for few hours. Failed samples were investigated to find out the cause of failure. Visual and stereo observations revealed presence of red coloured deposits at one corner of the sheets. Optical microscopy of sheet cross section confirmed the presence of red coloured deposits on the surface. It also revealed presence of scale at weld joint interface and grain coarsening at welding edge of the sheets without any fusion zone. Absence of fusion zone indicates poor welding. Fractography was done by using SEM to identify the mode of failure. Overall analysis suggests improper welding due to presence of scale (insulating material) which caused local temperature to rise and this possibly led to the improper welding and final failure.

Plating Thickness Evaluation based on Eddy Current Testing for CGL

This paper explains a new technique based on the eddy current testing to evaluate plating thickness in the continuous galvanizing line. Previous techniques cannot apply to the line due to need low and/or fix liftoff. Because test samples are vibrated in the line by gas wipers. The new technique which we propose can evaluate the thickness with high and variable liftoff. Thus, the sensors based on the technique can install to near gas wipers which control the thickness and vibrate the samples in the line. First, the detection principle of the technique and the method of applying are explained on simulations by using the finite element method. Application results on measurement experiments are secondly explained. These results demonstrate the possibility of making thickness measurements using an eddy current with high and variable liftoff.

Effects of pre-annealing conditions on the microstructure and properties of vanadium-bearing dual-phase steels produced using continuous galvanizing line simulations

It is well known that the fracture behavior and weldability of steel often vary inversely with the carbon content. In the case of thin sheet intended for automotive applications, this means spot weldability and tensile ductility. However, the question arises, what is the reasonable maximum strength attainable in a low-carbon dual-phase (DP) steel under the constraints of: (1) with carbon levels at 0.1 wt%, (2) with the product (UTS × % TE) > 18,000 MPa, and (3) with processing on a simulated continuous hot-dipped galvanizing line. This study has focused on experimental compositions intended to meet DP steel grade properties in excess of 780 MPa UTS. This kind of advanced high-strength steel (AHSS) is an integral part of mass reduction programs for the body-in-white for the automotive industry. The family of steels investigated here is based on a low-carbon, aluminum-killed base steel containing Cr and Mo. In this study, the effects of the presence of V were studied. In addition, there were three processing variables investigated in terms of their respective effects on microstructure and mechanical properties: first, hot band coiling temperature; second, % cold reduction; and third, the thermal path through the CGL process. The thermal paths explored included both a standard galvanizing process and a new supercooling process; these were studied using a CGL simulation. This research revealed that all three variables can have a significant effect on the final microstructure and properties.

Thermo-chemical Fluid Flow Simulation in Hot-Dip Galvanizing: The Evaluation of Dross Build-Up Formation

This paper presents a new computational framework to investigate the driving force for the formation of intermetallic dross particles in the zinc bath and dross build-up on the bath hardware. This is a major problem in continuous hot-dip galvanizing lines. The Computational Fluid Dynamics (CFD) model calculates the turbulent thermo-chemical flow conditions within the liquid melt. A detailed modeling of the steel strip–liquid interface enhances this approach, by means of a conjugated heat transfer calculation and a spatially resolved, temperature- and concentration-dependent iron dissolution and aluminum uptake. The heart of the computational framework is a thermodynamic model, which assesses the driving force for the formation or dissolution of dross particles in the bath and dross build-up on stationary and rotating equipment. The simulation results are validated with temperature and species depth profile measurements. The applicability of the CFD model is shown by investigating the locally resolved aluminum uptake and iron dissolution at the steel-strip surface, and the multi-physics conditions in the region near the roll. The novel approach of evaluating the thermodynamic driving force enables the assessment of the formation of dross build-up at the roll surface.

Design and Development of Galvannealed Dual-Phase Steel: Microstructure, Mechanical Properties and Weldability

It was attempted to produce galvannealed dual-phase (DP) steel through a commercial continuous galvanizing line (CGL). The main challenge was to form the DP microstructure even with the slow cooling rate after annealing in the CGL. A two-pronged approach was adopted for this, viz. design of the steel chemistry with calculations and optimization of the CGL parameters with simulation using Gleeble-3800 thermomechanical simulator. The steel thus produced in CGL revealed ferrite and 18 ± 2% martensite microstructure, with tensile strength exceeding 600 MPa and 24% total elongation along with good formability. Weldability study using resistance spot welding indicated the possibility of achieving sound joint with a 4-kA width in weld time–current lobe despite the high carbon equivalence.

Flexible manufacturing technology of controlling exact bake hardening value in continuous galvanizing line

Flexible manufacturing technology for ultra-low-carbon bake hardening (BH) steels with different solute carbon contents in the range of (6–16) × 10−4 wt.% was systematically studied. The effects of soaking temperature, cooling rate and aging time on solute carbon content were explored. The bake hardening value in a continuous galvanizing line with an over-aging section was investigated. Optimal manufacturing parameters were calculated using the design of experiment to obtain the best and most flexible process parameters. Results indicate a relationship among carbon content, soaking temperature and cooling rate. When solute carbon is 9 × 10−6, cooling rate is 50 °C/s, the soaking temperature is 745 °C, and BH value is 45 MPa, the requirements of an automobile factory can be met.

A Modelling and a New Hybrid MILP/CP Decomposition Method for Parallel Continuous Galvanizing Line Scheduling Problem

A Modelling and a New Hybrid MILP/CP Decomposition Method for Parallel Continuous Galvanizing Line Scheduling Problem

Influence of Steel Strip Temperature in Formation Zinc Dross During Process Production Using Continuous Galvanizing Line (CGL) Machine

To control the occurrence of zinc dross during galvanizing process using production Continuous Galvanizing Line (CGL) machine, it is necessary to control steel strip temperature before entry into zinc bath. The Steel strip temperature controls were variable at 320°C and 400°C and zinc bath temperature was fix at temperature 460°C. At temperature steel strips of 320°C, zinc bath was checked using spectrometry, the results of Fe content 0.1293% and Al 0.0759%, because the solubility of Al is lower than Fe, the dross occurs become bottom dross. If the zinc dross change become bottom dross, the bottom dross could decrease quality of product and life time of zinc pot, because bottom dross settled at the floor. Bottom dross properties very hard and absorb heating, then at that position bottom dross make hot spot in the floor, finally zinc pot leakage. At temperature steel strip 400°C, the results content of Al 0.1667% and Fe 0.1217%, due to high temperature steel strip make solubility of Al become higher than Fe, therefore float at the zinc bath become top dross.

Dissolution Behaviors of Dual Phase Steel during Pickling

Dual phase steels have been widely used in the automotive industry to produce lighter weight vehicles to meet stringent safety and fuel economy regulations. While the phenomena of steel dissolution in the acidic solution have been reported on carbon steel and stainless steel, little is known about the dissolution behavior of dual phase steel, which could compromise the coating adhesion performance in the subsequent electrogalvanzing line (EGL) or continuous galvanizing line (CGL). A fundamental study was undertaken to understand variables that could affect dissolution of dual phase steel in the acidic solution. The results show that the dissolution kinetics is strongly influenced by temperature, acid concentration and the steel microstructure. Different dissolution mechanism is observed in HCl and H2SO4. Dual phase steel displays a non-uniform dissolution behavior of which the ferrite phase dissolves faster than martensite phase. This selective dissolution phenomenon could be attributed to galvanic corrosion between ferrite and adjacent martensite phase and a higher resistance of martensite phase to dissolution than ferrite phase.

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.

Numerical analysis of wiping jet flow in Continuous Galvanizing Line

Numerical analysis of wiping jet flow in Continuous Galvanizing Line

Thermal-Sprayed Coatings on Bushing and Sleeve-Pipe Surfaces in Continuous Galvanizing Sinking Roller Production Line Applications

This paper describes thermal spray techniques for making hard coatings on bushing and sleeve component surfaces. Specifically, plasma-arc welding was used to produce 5-mm thick Co-Cr alloy welding overlays on the bushing, while a high-velocity oxy-fuel spraying technique and laser re-melting technique were used to produce thinner coatings of Co-Cr-Ni+WC of about 1mm thickness on the sleeve-pipe counterparts. The surface-treated components were then submerged in liquid zinc to study the corrosive behaviour of the surface coating and substrate. Both the scanning electron microscope and energy dispersive spectrometer analyses were used to study the microstructure and phase composition of both coatings and substrates prior to and after corrosion experiments. The results show that the microstructure of the bushing consists of γ-cobalt solid solution as well as the eutectic structure of γ-cobalt and carbides, which have good corrosive resistance against molten zinc. Meanwhile, the microstructure of the sleeve pipe consists of a Co-Cr solid solution with various forms of carbides, which displays the combined properties of toughness with good corrosive resistance to molten zinc.

Elasto-plastic bending of steel strip in a hot-dip galvanizing line

A quasi-static model of axially moving steel strips in a continuous hot-dip galvanizing line is presented. The model provides the bending line of the strip and takes into account the history of elasto-plastic deformation. The numerical integration of the material model of elasto-plastic deformation is algorithmically separated from the solution of the boundary value problem of the bending line by pre-computing sets of one-dimensional candidate relations between the strip curvature and the bending moment. Using this model, the influence of different roll positions in the zinc bath on the mean displacement of the strip at the gas wiping dies and the maximum lateral curvature of the strip (crossbow) can be efficiently calculated and analyzed.