Decarburization Kinetics during Annealing of a Semi-processed Electrical Steel

Abstract

Semi-processed, non-oriented electrical steels used in motor and transformer cores have their magnetic properties optimized by means of a heat treatment performed under a moistened nitrogen atmosphere containing small amounts of hydrogen. The oxidation of carbon on the steel surface by the oxidizing gases present in this atmosphere decreases the carbon content of the steel. The consequent reduction in the amount of soluble carbon and in the volume fraction of carbides reduces magnetic losses and the susceptibility of the steel to magnetic aging. In this work, a cold-rolled, semi-processed electrical steel was submitted to six thermal treatments covering the most usual conditions employed in industry, combining two types of atmosphere (mixtures of nitrogen with 5 % and 10 % H 2 ) and three dew points (10, 20 and 30°C). The treatments were carried out in a pilot electrical oven especially adapted to simulate the thermal cycle and the moistened atmospheres employed in industry. The results obtained show that, at the beginning of the process, the decarburization rate is controlled by the chemical reaction of carbon on the steel surface. Later, an oxide layer may build up, depending on the humidity level of the atmosphere, reducing the rate of decarburization.KEY WORDS: semi-processed electrical steels; decarburization rate; carbon oxidation; decarburizing atmosphere........ (1) in terms of the initial carbon content C o , the diffusion coefficient of carbon D at the soaking temperature and the strip thickness e. Equation (1) is the solution of the differential equation expressing Fick's second law for diffusion, in this particular case.Regarding the adsorption of water vapor and hydrogen on the steel surface, it is well known that this reaction is very fast 2) and certainly does not control the decarburization process. The third reaction, carbon oxidation at the steel surface, is a thermally activated process obeying Arrhenius law, 2) that is, the reaction rate increases when the temperature and/or concentration of C and H 2 O are increased. Finally, the water vapor reacts also with iron, leading to scale formation, which may isolate the steel from the oxidizing atmosphere and thus inhibit further decarburization. The slowest reaction among those previously mentioned determines the overall decarburization rate.The aim of this work was to identify the phenomenon controlling the kinetics of decarburization in a typical semiprocessed electrical steel, in order to provide relevant information for the users of this type of steel regarding the optimization of magnetic properties during the final annealing cycle applied to the punched lamellae used as core materials. ExperimentalThe semi-processed, cold-rolled steel sheet studied, 0.50 mm in thickness, had the analyzed chemical composition (in mass percent) Fe-0.0052C-0.47Mn-0.73Si-0.015P-0.007S-0.25Al-0.03Sb-0.0035N. Six thermal treatments covering the most usual industrial conditions have been employed, at the soaking temperature of 760°C, combining three dew points (10,...

Prediction of Decarburized Ferrite Depth of Hypoeutectoid Steel with Simultaneous Oxidation

“…The mechanisms by which the decarburization of steels occurs are well understood. 17,18) The phenomenon of decarburization usually takes place at temperatures above about 700°C, with the following reaction. 1,11) ...…”

Effect of Aluminium Addition on the Magnetic Properties of a Semi-Processed Electrical Steel

“…This effect might be linked to the increase in the amount of precipitate (especially AlN) from A1 to A4 steel. Fine precipitates can hinder the kinetic of grain growth during decarburization treatment 3,4 . As hot rolled or annealed condition, the steel samples showed smaller grains without clear influence of aluminum concentration.…”

“…Certainly, this was mainly due to increased electrical resistivity. However, Figure 7 presents the progress of the hysteresis component, and it can be observed that the aluminum content increase caused a discreet reduction on this magnetic loss component, despite the texture randomization and probable increased presence of second phase particles in the metallic matrix 3,9,11 .…”

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