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,...

This work refers to the application of a Voltammetric Anodic Dissolution (VAD) Technique in the analysis of coating discontinuities, focusing on pores and cracks that exposed the substrate. An evaluation was made of the influence of several parameters, such as the concentration of the passivation solution and sweep rate (SR), on the substrate passivation process and on the porosity indexes of tricationic phosphate coatings of Fe/Zn/Mn. The phosphatization process used was a commercial tricationic Fe/Zn/Mn phosphate bath applied on a galvanized steel (GS) substrate. Once the best experimental conditions for the use of the VAD technique had been defined, the grain size and layer weight were related to porosity indexes. The porosity was found to show a tendency to decrease with increasing grain size. The VAD technique consists of the anodic polarization of the substrate/coating system and measurement of the charge density involved in the substrate passivation process. A quantitative porosity index was obtained by comparing the passivation charge density of the substrate without coating (standard passivation charge density) and the passivation charge of the coated substrate.

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