Continuous Galvanizing Process Research Articles

The effect of Sn microalloying on the selective oxidation of Fe-(6-10 at.%)Mn alloys

To evaluate the suitability of Sn microalloying as strategy to decrease the selective oxidation kinetics of Mn-alloyed advanced steels, the effects of 0.01 and 0.03 at.% Sn micro-additions on oxide morphology and oxidation kinetics for Fe-6Mn and Fe-10Mn (at.%) alloys were determined under annealing conditions characteristic of the continuous galvanizing process. Selective oxidation followed a parabolic rate law, where activation energy analysis revealed that internal oxidation is rate-determined by the grain boundary diffusion of O in Fe. The 0.01 and 0.03 at.% Sn additions significantly reduced the internal oxidation of alloys with Mn contents of 6 and 10 at.%, which was attributed to interfacial Sn segregation reducing the solubility of O in Fe, the occupation of adsorption sites for water vapour molecules, and the reduction of short-circuit diffusion of O along internal interfaces. Sn microalloying decreased the external oxidation of Fe-6Mn-(0.01,0.03)Sn, but did not significantly alter external oxide thickness of the Fe-10Mn-(0.01,0.03)Sn alloys. In this case, Sn reduced the outward diffusion of Mn along grain boundaries, but also affected the balance between the inward O flux and outward Mn flux. Incorporating the effect of Sn occupying adsorption sites into a selective oxidation model revealed that Sn microalloying is predicted to cause a transition from internal to external oxidation for Fe-10Mn. For Fe-6Mn alloys, however, a Sn micro-addition of 0.01 at.% (∼0.022 wt.%) was sufficient to significantly reduce selective oxidation kinetics and produce beneficial alterations to the external oxide morphology in order to ease reactive wetting in the continuous galvanizing process.

Failure behavior of resistance spot welded advanced high strength steel: The role of surface condition and initial microstructure

The continuous galvanizing process (CGP) under high dew point temperature has been recognized as a viable technique for improving the coating quality of advanced high-strength steels (AHSS). However, the effect of this surface modification technique on resistance spot welding (RSW) process is not clear. The aim of this study is to investigate the role of different surface conditions and initial microstructures obtained through CGP on mechanical properties and failure behavior of resistance spot welded AHSS. It has been shown that the formation of internal oxides during the annealing and subsequent coating process reduced the heat input during RSW, resulting in smaller nugget size and consequently, lower tensile-shear peak load values. Finite element modeling (FEM) and experimental results indicated that in the case of the as-received sample, higher fusion zone/base-metal (FZ/BM) hardness ratio leads to a failure location closer to heat-affected zone (HAZ). However, the lower FZ/BM hardness ratio in the case of annealed-bare and annealed-coated specimens leads to failure location more towards FZ. It was shown that nugget size will not control the failure location during lap-shear loading but does affect nugget load-carrying capability. However, failure behavior is controlled by microstructural characteristics of different weld regions.

Optimal Design of Hot-Dip Galvanized DP Steels via Artificial Neural Networks and Multi-Objective Genetic Optimization

This modeling and optimization study applies a non-linear back-propagation artificial neural network, commonly denoted as BPNN, to model the most important mechanical properties such as yield strength (YS), ultimate tensile strength (UTS) and elongation at fracture (EL) during the experimental processing of hot-dip galvanized dual-phase (GDP) steels. Once the non-linear BPNN is properly trained, the most important variables of the continuous galvanizing process, including initial/first cooling rate (CR1), holding time at the galvanizing temperature of 460 °C (tg) and the final/second cooling rate (CR2), are obtained in an optimal way using an evolutionary approach. The experimental development of GDP steels in continuous processing lines with outstanding mechanical properties (550 < YS < 750 MPa, 1100 MPa < UTS and 10% < EL) is possible by using a combined hybrid approach based in BPNN and multi-objective genetic algorithm (GA). The proposed computational method is applied to the specific design of an actual manufacturing process for the first time.

Intelligent design in continuous galvanizing process for advanced ultra-high-strength dual-phase steels using back-propagation artificial neural networks and MOAMP-Squirrels search algorithm

In this research work, the optimization of a back-propagation artificial neural network (BPNN) using a new multi-objective bio-inspired algorithm based on squirrels is proposed in order to optimize the main continuous galvanizing process parameters such as the initial cooling rate (CR1), the isothermal holding time at 460 oC (tg), and the final cooling rate (CR2). The computational approach predicts in a satisfactory way the most important mechanical properties including yield strength (YS), ultimate tensile strength (UTS), and elongation at fracture (EL) of cold rolled low carbon DP steels treated under continuous galvanizing thermal cycle conditions. The experimental production of galvanized ultra-high-strength DP steels from cold rolled low carbon sheets with a minimum UTS of 1100 MPa, YS between 550 and 750 MPa, and a minimum elongation of 10% is possible using the proposed methodology.

The influence of modified annealing during the galvanizing process on the resistance spot welding of the CMn1.8Si advanced high strength steel

Annealing under an atmosphere with controlled oxygen partial pressure is an effective way for improving reactive wetting during the continuous galvanizing process. Hence, understanding the effects of this annealing step on further manufacturing processes such as welding is essential. The present work has shown that the formation of internal oxides during a controlled atmosphere galvanizing process reduced the resistance and heat input resulted in delaying nugget growth in resistance spot welding, compared to the as-received condition. Furthermore, it was shown that by annealing and subsequent zinc coating, the tensile-shear peak load was decreased under the same welding parameters. The nugget formation mechanism of different surface conditions showed a good correlation with dynamic resistance profiles.

Experimental and Numerical Study of Coating Thickness Using Multi-slot Air Knives

Gas-jet wiping is a widely employed production technology for controlling the final zinc coating thickness on a moving substrate during continuous hot-dip galvanizing. This paper presents an experimental investigation and numerical analysis of a prototype multi-slot air knife, which offers an increase in wiping efficiency relative to the traditional single-slot jet geometry in the continuous galvanizing process. The applicability of the analytical coating weight model of Elsaadawy et al. (Metall Mater Trans B, 38:413-424, 2007) to predict the final coating weight was determined for the multi-slot geometry, where particular focus was devoted to the effect of geometric parameters. Experimental measurements under a variety of knife geometry and process conditions agreed with the coating weight predictions of the analytical model. It was also shown that the air-knife geometric parameters had a significant effect on the pressure profile and shear stress distribution applied by the air knives to the moving substrate. It was determined that the final coating thickness was significantly affected by the auxiliary jet width, Da, where lighter coating weights at higher strip velocities (up to 5.4 pct at Vs = 1.5 m/s) could be achieved by using the multi-slot air-knives prototype vs. the conventional single-slot configuration.

Numerical analysis of purge jet flow and wiping jet flow in continuous galvanizing process

Numerical analysis of purge jet flow and wiping jet flow in continuous galvanizing process

Characterization of Zn-Mg-Al Based Drosses from the Continuous Galvanizing

The present paper deals with the microstructure characterization of drosses originated during production of a new type of coating called Zinkomag. This coating creates during continuous galvanizing process and it is characterized by addition of 0.8 - 1.0 wt. % Mg and 0.8 - 1.0 wt. % Al in the zinc galvanizing bath. The addition of magnesium in such coatings leads to increasing of hardness, corrosion resistance, and better color adhesion than most common used coatings (GI coatings). In this paper drosses were investigated using AAS, LM, SEM with EDX microanalysis, and XRD.

XPS and EELS characterization of Mn2SiO4, MnSiO3 and MnAl2O4

X-ray Photoelectron Spectroscopy (XPS) and Electron Energy Loss Spectroscopy (EELS) are strong candidate techniques for characterizing steel surfaces and substrate-coating interfaces when investigating the selective oxidation and reactive wetting of advanced high strength steels (AHSS) during the continuous galvanizing process. However, unambiguous identification of ternary oxides such as Mn2SiO4, MnSiO3, and MnAl2O4 by XPS or EELS, which can play a significant role in substrate reactive wetting, is difficult due to the lack of fully characterized standards in the literature. To resolve this issue, samples of Mn2SiO4, MnSiO3 and MnAl2O4 were synthesized and characterized by XPS and EELS. The unique features of the XPS and EELS spectra for the Mn2SiO4, MnSiO3 and MnAl2O4 standards were successfully derived, thereby allowing investigators to fully differentiate and identify these oxides at the surface and subsurface of Mn, Si and Al alloyed AHSS using these techniques.

Microstructure and mechanical properties of a mixed Si–Al TRIP-assisted steel subjected to continuous galvanizing heat treatments

Heat treatments with an isothermal bainitic transformation temperature of 465°C compatible with the continuous galvanizing process were performed on a 0.2C–1.5Mn–1.0Si–0.5Al (wt%) TRIP-assisted steel. The effect of intercritical annealing temperature and isothermal bainitic transformation (IBT) time at 465°C on mechanical properties and microstructure were determined. The best combination of strength and ductility was obtained for the lower intercritical annealing temperature of 750°C for an isothermal bainitic transformation time of 90s at 465°C. However, after intercritical annealing at a higher temperature of 797°C, optimum mechanical properties were obtained for a shorter isothermal bainitic transformation time of 60s at 465°C, likely due to the lower C content of the intercritical austenite bringing about faster transformation kinetics. It was concluded that varying the intercritical temperature and IBT time can play an important role in the development of a suitable process window to obtain the desired mechanical properties in low alloy TRIP-assisted steels given the constraints associated with the continuous galvanizing process.

Coating Weight Model for the Continuous Hot-Dip Galvanizing Process

A coating weight model was developed to describe the pressure and wall shear stress distributions as functions of slot gap (d) and impingement distance (Z), for the air knife wiping of the liquid zinc coatings in continuous hot dip galvanizing at ratios of Z/d ≤ 8. This model was then used in validation studies in order to predict the coating weight as a function of the process parameters. The model was based on improved correlations for pressure and shear stress developed by a combination of experimental and computation techniques, which has resulted in more accurate predictions of coating weight validated using industrial coil average coating weight data, particularly for coating weights of up to 75 g/m2. For this region, the maximum deviation between the predicted and measured coating weights was 8 pct. The coating weight model was further developed by incorporating a lumped heat-transfer analysis to predict the solidification “dry line” of the coating. For a typical continuous galvanizing process, the model predicts an 80 pct coating solid fraction for a coating weight of 130 g/m2 to occur at 15 m from the air knives, which agrees qualitatively with visual observations in continuous galvanizing lines.

A negative effect of the insoluble particles of dross on the quality of the galvanized coatings

In continuous galvanizing process several byproducts are formed. Amongst these byproducts insoluble particles composed by different compounds are grown. The most problematic seems to be a complex oxide of a ZnAl 2− x Fe y O 4 stoichiometry (spinels) with variable lattice parameters, which is formed at the zinc/substrate interface. This oxide accelerates the breakdown of Fe–Al passive layer, resulting in the formation of a series of Fe–Zn intermetallic phases by an outburst growth. The samples used in this study were industrially galvanized wires and byproducts, which were studied by different methods, i.e., Scanning Electron Microscopy (SEM), Transmission Electron Microscopy (TEM) and X-Rays-Diffraction (XRD).