European Oxide Scale Conference 2022

Hydraulic descaling is widely used to remove oxide layer from hot steel. The study of quality of water jets generated by high pressure flat fan nozzles is presented as well as effect of wear during long time usage or under extreme conditions. Optimization of nozzle setup as well as alternative descaling systems are presented and the effect on quality of descaling is shown with focus on maximum impact, overlap, and energy saving. As the water jets hit the hot product, intensive, but in most cases unwanted, cooling occurs. The effect of optimum configuration on heat losses is also presented as well as rapid increase of cooling during low descaling speeds. In often cases the descaled surface is not perfectly clean. The cause of uneven cooling during heat treatment is discussed and it is shown that it can be caused by residual scales.

During high pressure descaling, the local fast very fast cooling creates stresses in the oxide through time-dependent differential contraction. This results in cracks first through the oxide layer, next at the metal scale interface and finally the oxide layer is peeled-off.
In this study we analyze the transformation of the oxide layer by these constraints. For that micro-indentations are carried out, both at room temperature and at the process temperatures. Simulating the experiments by FEM including Cohesive Zone Modelling give insight into crack location and lengths. It also gives fracture stresses criterion which aimed to be used in a future global modeling of hydraulic descaling.
The experimental work consists of oxidizing carbon steel samples and next indenting them out at 25°C, 600, 700 and 800°C with a Berkovich indenter using Alemnis HTM800 inside a SEM.
For all tests done at 25°C, circular and corner cracks are visible in surface. FIB cross-sections show delamination. At 600°C/700°C, only part of the indentations develops cracks, probably depending on local defects. At 800°C, over the brittle to ductile transition, no cracks, whatever the indentation conditions were found. In these tests, the transition temperature depends on strain rate: high speed hardens oxide. This agrees to other mechanical tests on scale found in literature.
As a conclusion to have an effective descaling, the oxide layer should be cooled down below the transition temperature before the underlying metal starts to contract. Fracture stress criteria that were determined here will implemented in future thermo-mechanical FE simulations of descaling.

The hot rolling of some high strength steel grades is a main concern for the hot strip mill to assure a high surface quality.  In the RFCS Infire project new (oxygen depletion, micro dimples) and existing (lubrication, skin cooling) roll bite actuators have been studied on a DP800HpF grade and Strenx700MC grade.  The most promising actuator was oxygen depletion just in front of the first rolling stand.  During these first trials oxygen depletion was obtained on the CRM hot rolling pilot line by installing a tunnel with nitrogen between the reheating furnace and the rolling stand.  Installing a tunnel is however not possible in industry as it would be damaged by ski head ends, cobbles and/or threading refusals.  As already a reduction of oxygen to 10% would have a significant impact a more robust way to apply the nitrogen has been studied during the last years on the continuous hot rolling pilot line.  The principle is to apply nitrogen by a flat jet spray nozzle only directly at the exit of the reheating furnace and by this have a strong influence on the start of tertiary oxidation.  In this presentation the results will be discussed. 

Most of the work done by an expert in oxidation and surface defects is metallographic analysis, in depth process analysis and laboratory furnace experiments. But numerical simulations bring additional understandings of the phenomena occurring on the hot surface. These can be a help finding solutions. 

In this presentation, I illustrate a few practical situations where using computations gave clues about the management of the hot metal surface: scale amount and surface defects during hot rolling.  

High-temperature oxidation occurs at various stages during the steelmaking process. Due to oxidizing in high temperatures and harsh atmospheres, it is practically impossible to observe the compositional changes in the steel surface and the formed oxide scales in-situ during steel production. Therefore, a coupled thermodynamic-kinetic numerical model is developed that predicts the composition profile of the steel alloy's constituents due to external oxidation for linear and parabolic growth. The model is applied to the high-temperature oxidation of Fe-Mn alloys. The kinetic parameters that serve as an input for the model are determined via oxidizing experiments with a Thermogravimetric Analysis (TGA) for different Mn contents (below 10 wt %). The mechanism of oxidation was determined by characterizing the oxidized samples with SEM-EDS and in-situ-XRD. The effect of different parameters such as temperature, oxygen partial pressure, and flow rate on the oxidation rate was studied and shows that the Mn depletion zone is strongly dependent on the oxidation rate. Also, some data analytics approaches for the determination of the kinetic parameters besides experiments are discussed. These serve as the input for the numerical model and high-temperature oxidation of steel during steelmaking could be simulated.

Materials like stainless steel exposed at higher temperatures result in a reduction of corrosion resistance, therefore necessitating protective coatings on them to improve their oxidation resistance at elevated temperatures. 

In this work, the compact and uniform CrSi (with different content of silicon) coatings were deposited on stainless steel alloy samples by a close filed unbalanced magnetron sputtering technique. The coatings on the samples were oxidised in air at 550, 700 °C and 800 °C for varied times. Mass changes after oxidation were measured to evaluate the susceptibility of the coating or scale to spallation. Characterisation of the coating before and after oxidation was evaluated through systematic analyses of surfaces and cross-sections using different techniques, including scanning electron microscopy (SEM), transmission electron microscopy (TEM), energy-dispersive X-ray spectroscopy (EDS) and X-ray diffraction (XRD). The capability of the protective behaviour of the CrSi coatings was verified by the phase constituent change. 

The dense Cr2O3 oxide layer on CrSi coatings can significantly improve the oxidation resistance, and their weight gains are reduced. A small amount of Si in the chromium coating demonstrates a better performance as a large amount of silicon leads to the embrittlement of the coating.  

Infrared video imaging of a moving steel strip following laminar cooling was used to assess and quantify the effectiveness of the strip mill descalers. Anomalous temperature values (i.e., cold spots) were observed on the strip surface. Finite element temperature modeling of the strip determined that the temperature anomalies are not associated with actual thermal variations and are likely due to localized variations in surface emissivity.  Post processing of the infrared images was used to determine the location and area fraction of these temperature anomalies. The transverse spacing of the  cold spots correlated with descaler positioning across the strip. The variation in location and area fraction of the temperature anomalies along the length of an individual strip indicate that the oxides in localized regions are exhibiting enhanced resistance to the descaling operation.