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Reheating of steel slabs for further processing such as hot rolling usually takes place in gas-fired pusher furnaces. Temperatures well above 1000°C, combined with an atmosphere containing H2O, CO2, and O2, lead to substantial oxidation of most steel grades. Newly developed advanced steels often contain significant amounts of Si. This element plays a dominant role in the scaling behaviour near the steel-scale-interface, since fayalite (Fe2SiO4) forms a eutectic with wuestite (Fe1–xO) that melts as low as 1177°C.To better understand the high temperature oxidation behaviour, lab-scale trials were performed with different steel grades containing up to 3 wt.% Si. Possible interactions of Si with other alloying elements present in the samples such as Cr, Mn and Al were also of interest. The atmosphere contained 20% H2O, 7% CO2, and 3% O2, resembling reheating conditions in pusher furnaces, and temperatures ranged from 1100 to 1240°C. For metallographic investigation, the oxidised samples were cold mounted under vacuum using taper section angles. After preparation, the sections were examined through light microscopy, SEM/EDS, XRD, and TEM. The local distribution of the alloying elements could be mapped efficiently, and phase identification was successful in most parts. Under the applied experimental conditions, the elements of interest were present in their oxidic form either as pure or as mixed oxides. Higher Si-contents led to an increased build-up of eutectic melting phase at the steel-scale-interface at temperatures above 1177°C, which in turn further accelerated the oxidation.

We demonstrate the determination of the Poisson’s ratio of steel plates during thermal processing based on contact free laser ultrasound measurements. Our method utilizes resonant elastic waves sustained by the plate, provides high amplitudes, and requires only a moderate detection bandwidth. For the analysis, the thickness of the samples does not need to be known. The trend of the measured Poisson’s ratio reveals a phase transformation in dual-phase steel samples. While previous approaches based on the measurement of the longitudinal sound velocity cannot distinguish between the ferritic and austenitic phase above 770°C, the shown method can. If the thickness of the samples is known, the method also provides both sound velocities of the material. The gained complementary information could be used to analyze phase composition of steel from low temperatures up to its melting point.

Oxidation during steel hot rolling is responsible for various surface defects. Local enrichments of oxygen-affine alloying elements such as silicon or aluminium can cause such defects by complicating oxide scale removal. In this paper, correlations between the temperature profile during oxidation and enrichment formation in electrical steels are investigated. Diffusion and reaction simulations using the numerical Crank-Nicolson-scheme are evaluated by comparing them to laboratory-scale oxidized samples. Furthermore, analytical approximations (spectral methods) are investigated as an alternative approach, focusing on trade-offs between accuracy and calculation times. A link between heating periods during oxidation and Si/Al-rich bands in the scale was established.

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.

There is a growing demand for Advanced High Strength Steels (AHSS) in the automotive industry owing to their high specific strength and good formability. The mechanical properties satisfy the demands for improved passenger safety and decreased vehicle weight due to thinner cross sections. Hot-dip galvanizing is a common procedure to prevent corrosion of steel, galvanized steel forms the basis for further processing like organic coating. Industrially, the steel strip is annealed at 840 °C in 5 % H2 in N2 at a dew point (DP) of -30 °C. These conditions are reducing for Fe, but oxidizing for oxygen-affine alloying elements as Mn, Si and Cr. These ignoble elements form an external, covering oxide layer on the steel surface, which exhibits poor wettability for the Zn(Al, Fe)-bath. The liquid Zn(Al, Fe) has a temperature of 460 °C and contains 0,2 wt% Al to form a Fe2Al5-xZnx-layer to inhibit the growth of Fe-Zn-intermetallics. Along with the increased amount of alloying elements to improve strength and ductility of AHSS the evolving oxide layer after annealing at the steel surface deteriorates hot-dip galvanizing. The question arises what happens to the surface oxides during immersion in the Zn(Fe, Al)-bath. For this purpose annealed as well as annealed and galvanized 0.8Si-AHSS and 1.5Si-AHSS were compared by glow discharge optical emission spectroscopy (GD-OES) depth profiles. Galvanized specimens show fewer oxides at the steel-zinc-interface as annealed specimens. A possible explanation is an aluminothermic reduction of oxides by 0.2 wt% dissolved Al in the Zn(Al, Fe)-bath. Al is thermodynamically more affine to oxygen than Mn and Si and may reduce Mn- and Si-oxides. An alternative theory is the dissolution of Fe in Zn during reactive wetting, as a side effect the oxides are rinsed off too. Additionally, the influence of DP was investigated. According to Wagner’s theory of selective oxidation, a higher amount of oxygen in the annealing atmosphere leads to internal oxidation of the alloying elements. Experiments were carried out with 0.8Si-AHSS and 1.5Si-AHSS by altering the DP during annealing from -30 °C (380 ppm H2O) to 0 °C (6000 ppm H2O). Oxidation mode changed from external (DP -30 °C) to internal oxidation along grain boundaries (DP 0 °C), as predicted by Wagner. These oxide-free surface provides good reactivity to enhance reactive wetting with the Zn(Fe, Al)-bath and form a dense Fe2Al5-xZnx-layer.

The effect of the dew point (and therefore oxygen partial pressure) on the selective oxidation of Advanced High Strength Steels was investigated. Steels with different Si contents, 0.2 wt% Si, 0.8 wt% Si and 1.5 wt% Si were used. The steel samples were annealed at 840 °C for 60 s and various gas atmospheres prior to hot-dip galvanized at 460 °C. The dew point of the 5 % H2-N2 annealing atmosphere was lowered from-30 °C (equivalent to 380 ppm H2O) to-58 °C (equivalent to 14 ppm H2O) in order to investigate surface segregation of alloying elements Si, Mn and Cr. These conditions are reducing for Fe, but oxidizing the oxygen-affine elements. Oxide morphology changed from a complete covering surface at high dew point to separated oxide spots at grain boundaries at low dew point. At the low dew point Cr was not oxidized. Oxides with a low Mn/Si-ratio seems to be amorphous. The Si-oxides are especially located at grain boundaries, Mn-oxides tend to cover the surface. Oxides covering the steel surfaces are detrimental for subsequent procedures as hot dip galvanizing, painting and welding.

Gas-Jet wiping is a widely applied technology in continuous galvanizing mills, which enables the adjustment of the specific coating mass by an impinging turbulent gas-jet. The unsteady impingement conditions, however, are reported to cause surface non-uniformities such as waves. In this study, proper orthogonal decomposition (POD) is used to analyze an industrial gas-jet wiping process. POD allows to objectively extract the most dominant flow structures (modes) and their dynamics from the impinging jet. The acquisition of the necessary temporally and spatially highly resolved flow data is done by a LES-VOF simulation model. The POD analysis shows that jet flapping and axial fluctuations of the jet core are the most dominant spatial modes in the studied case. The frequency range of the modes as well as the frequency range of the height fluctuations of the impinged film are compared in a spectral analysis. Additionally, it is shown that slight changes, for example, the absence of the thin liquid film on the impinged surface, alters the frequency range of the dominant POD modes, whereas the modes themselves remain mostly unchanged.

In addition to corrosion resistance and processing properties, high coating uniformity is a key quality criterion for galvanized steel sheets. Hydrodynamic gas jet wiping has proved to be an efficient method to control the coating thickness. However, the occurrence of non-uniformities is attributed to the unsteadiness of the impinging jet. For the first time, vertical surface non-uniformities resulting from the interaction of the impinging jet with the liquid coating are numerically predicted under industrial boundary conditions using the ANSYS Fluent®. The turbulent flow field of the compressible wiping gas is accomplished by the LES (Large-Eddy-Simulation) turbulence model, whereas the interphase between the wiping gas and the liquid coating is modeled by the VOF (Volume-of-Fluid) method. It is found that the liquid coating reacts relatively slowly to the high frequent flapping gas jet. Only, when the jet is deflected for a comparatively long period, significant waves are able to develop. The waviness predicted by the simulation model is in good agreement with experimental results. Thus, the model enables a careful study of process settings on the vertical coating uniformity characteristics. For the studied case, an increase of nozzle inclination is found to enhance the performance in terms of coating uniformity significantly.