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Influence of Annealing Atmosphere on Oxide Removability in a 3.2% Si Non-Grain Oriented Electrical Steel

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Influence of Annealing Atmosphere on Oxide Removability in a 3.2% Si Non-Grain Oriented Electrical Steel ISIJ International, Vol. 57 (2017),ISIJ International,No. 1 Vol. 57 (2017), No. 1, pp. 148–154 Influence of Annealing Atmosphere on Oxide Removability in a 3.2% Si Non-Grain Oriented Electrical Steel Vladimir Vinicio BASABE* Tata Steel, Swinden Technology Center, Rotherham, S60 3AR United Kingdom. (Received on June 17, 2016; accepted on September 16, 2016; J-STAGE Advance published date: November 12, 2016) In this work, the effects of gas composition, elapsed time of reaction and temperature during annealing on scale removability were investigated for a (3.2 wt.% Si) non-grain oriented electrical steel, and the results were discussed from the viewpoint of oxide morphology. The annealing tests were carried out under conditions similar to those of industrial annealing operations. For this purpose, steel samples with their original as-received tertiary scale were annealed over the tem- perature range 900–1 000°C in O2–CO2–H2O–Ar–N2 and N2–H2 gas mixtures and in pure N2. After annealing, oxide/steel samples were cooled in air and some were water quenched to study the effect of thermal shock on oxide scale removability. During the annealing tests, four types of oxide scales were observed: oxide scale without idiomorphic growth (Type I), oxide scale with idiomorphic growth (Type II), neutral scale (Type III) and reduced scale (Type IV). The experiments showed that the annealing atmosphere, annealing time, temperature and cooling media influence the morphology and removability of oxide. In general, the experiments indicated that a reducing atmosphere during annealing and water cooling at the end of annealing are the ideal conditions for oxide removal. KEY WORDS: oxidation-reduction; oxide scale removability; annealing atmosphere; scale morphology; electrical steels. ing in conditions similar to those of industrial processes are 1. Introduction of great interest to better understand scale removability and In hot strip mills, the heating of slabs is necessary to surface quality. soften the steel before rolling. During the reheating of the In the analysis of oxide scale removal, it is necessary slab, a primary scale of around 2–3 mm in thickness is not only to analyse scale removability as a function of sand formed when the slab is reheated to about 1 200–50°C. blasting & pickling processes but also as a process that is After the first descaling, thin secondary and tertiary scales related to the control of the scale during annealing. In the grow fast in the subsequent stages of the hot rolling process. present work, the approach to scale removability is dynamic, In the final stage of coiling, tertiary scales formed during in that several factors and their relationships were analysed hot rolling undergo a cooling process and a phase change as with the purpose of defining better oxide scale removal wüstite is decomposed into magnetite and iron. conditions and heating practices. After hot rolling, hot rolled coils are then directed for further processing. In general, the main steps of the final 2. Experimental production process in non-grain oriented electrical steels are trimming of the strip, first annealing & descaling, cold 2.1. Annealing Test rolling and final annealing & coating. During the first The chemical composition of the investigated steel is annealing operation, the oxide scales formed on the surface given in Table 1. The steel was supplied as hot rolled coil. of hot rolled coils react with the annealing atmosphere. The For the annealing test, the samples were not polished i.e. resultant surface condition after annealing can cause surface the samples were annealed with their original as-received defects. To prevent these defects, the oxide scales formed tertiary scale as it occurs in industrial annealing conditions. during annealing are removed by sand blasting and pickling. The dimensions of the samples were 20 mm wide, 25 mm Although sand blasting and pickling are common and reli- long and 2.3 mm thick. The samples were machined with a able practices, the loss of material due to scale defects still 1 mm hole at the top for wire suspension. occurs in the production of non-grain oriented electrical The oxidation rate increases with gas velocity until a criti- steel. Therefore, studies on scale formation during anneal- cal gas velocity is reached. The critical gas velocity for air1) is 4.2 cm/s, 2.54 cm/s for carbon dioxide and 11.68 cm/s for 2) * Corresponding author: E-mail: [email protected] steam. In this study, a gas velocity of 5 cm/s was used to DOI: http://dx.doi.org/10.2355/isijinternational.ISIJINT-2016-367 ensure high oxidising conditions. © 2017 ISIJ 148 ISIJ International, Vol. 57 (2017), No. 1 During annealing operations strip speeds are in the range ing atmosphere. of about 15–30 m/min which in turn results in short anneal- For the annealing test, the steel samples were introduced ing times. Thus, a short standard annealing cycle of 166 s to the furnace that was at the desired soaking temperature. was chosen for the annealing simulation. In addition, an After annealing, the oxide/steel samples were extracted extended annealing cycle of 406 s was selected to observe from the furnace and cooled in air. Some samples were the influence of time, Fig. 1. water quench after annealing to investigate the effect of A vertical tube furnace was used to heat the samples in water-cooling on scale removability. The detailed descrip- the desired atmosphere to either 900, or 950 or 1 000°C. The tion of the annealing conditions and cooling media are selected atmosphere was obtained by controlling the flow given in Tables 2 and 3. These gas mixtures were selected of air, CO2, N2 and Ar gases. The water vapour was added to study annealing in: a) products of combustion (propane), by passing the gas mixtures through a sealed water-heated b) products of combustion and nitrogen, c) nitrogen, and d) container. In the case of a hydrogen-containing atmosphere, nitrogen-hydrogen gas mixture. a premixed N2–3H2 gas mixture was used to create a reduc- 2.2. X-ray Diffraction Analysis Hot rolled coils and annealed samples were analysed by X-ray diffraction (XRD) to reveal the oxide phase compo- sition before and after annealing. The analysis was carried out using a Bruker D8 diffractometer. The X-ray source was a conventional cobalt target X-ray tube set to 30 KV and 40 mA. The diffraction patterns were collected from 20–120° in reflection mode. The qualitative and quantitative analyses were performed by Rietveld analysis and Bruker Topas software package for Rietveld refinement. 2.3. Microscopic Observations Fig. 1. Annealing cycles used to simulate annealing in laboratory conditions. Oxide scales tend to fracture during hot mounting, mask- ing the original oxide morphology. Therefore, for this study, Table 1. Chemical composition of the investigated steel (mass%). the samples were cold mounted and polished with SiC paper to a 1 200 grit surface finish. Diamond paste of 0.25 μm C Si Mn Al PS was used in the final stage. After polishing, the samples 0.004 3.2 0.16 0.1 0.014 0.001 were carbon coated for scanning electron microscopy. In addition, an optical microscope was used to observe oxide/ Table 2. Gas mixtures for annealing test. steel cross-sections. Low magnifications (e.g. X100) and high magnifications (e.g. X8000) were used at ten locations G1 = 0.5O2-11.2CO2-15.6H2O-0.9Ar-N2 in the oxide scales to reveal their morphology in different Oxidising G2 = G1+ 60%N2= 0.2O2-4.5CO2-6.2H2O-0.4Ar-N2 annealing conditions. G3 = G1+ 80%N2= 0.1O2-2.2CO2-3.1H2O-0.2Ar-N2 Neutral G4 = 100N2 3. Results Reducing G5 = N2-3H2 3.1. Industrial Hot Rolled Coil The typical morphology of tertiary oxide scale formed on Table 3. Annealing conditions and cooling media. hot rolled coils is depicted in Fig. 2. The dark areas in the oxide are physical defects such as pores and micro-cracks Standard Extended formed during oxide growth or subsequent cooling. The Temperature Gas (°C) mixture Air Water Air Water grey dotted areas below the oxide scale are internal oxida- cooled cooled cooled cooled tion. 900 G1 * An example of the mole fraction of the oxides formed on 900 G2 * 900 G3 * 950 G1 **** 950 G2 * 950 G3 * 950 G4 ** 950 G5 **** 1 000 G1 * * 1 000 G2 * 1 000 G3 * Fig. 2. Oxide/steel cross-section of hot rolled coil showing inter- Conditions tested are indicated with an asterisk (*) nal oxidation (red oval). 149 © 2017 ISIJ ISIJ International, Vol. 57 (2017), No. 1 hot rolled coil is illustrated in Fig. 3. Notice that the X-ray depending on temperature and atmosphere four types of diffraction analysis corresponds to the entire surface (i.e. oxide scales can be observed during annealing operations. oxide and steel substrate) then the ferrite mole fraction of Type I - This scale corresponds to hot rolled coil annealed the steel substrate is included in the X-ray plot. The X-ray in oxidising atmospheres at 900°C in G1, G2 and G3, at diffraction pattern showed that oxide on hot rolled coil is 950°C in G3 and at 1 000°C in G3, Figs. 4, 5, 6(c), 7(c). In formed mostly of hematite, magnetite, fayalite and a negli- this type, the oxide scale didn’t develop idiomorphic growth gible amount of wüstite. i.e. the oxidation rate was more or less uniform on the initial oxide scale surface. 3.2. Annealing in Laboratory Conditions Type II - It was observed in oxidising atmospheres at the 3.2.1.
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