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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The aim of this work was to evaluate the adhesiveness of uncoated and zinc-electrogalvanized steel sheets used in the automotive industry. Three types of adhesives, one acrylic and two epoxy resins, were employed to join low carbon cold rolled steels, one uncoated and another electrogalvanized, both previously degreased or chemically pickled. Mechanical strength of the joints was evaluated by the T-peel and tensile strength tests. Steel grade, surface condition and heating below the cure temperatures did not influence the joints' mechanical strength. However, their shear strength decreased drastically as the test temperature increased. The exposure of the joints to an atmosphere with 90% relative humidity at 40 °C caused reduction of their shear strength. Epoxy adhesives showed higher mechanical strength, but exhibited higher degradation by humidity.

Avalia-se a influência da massa de zinco na resistência à corrosão de aços eletrogalvanizados, fosfatizados e pintados. Foram usados dois aços eletrogalvanizados com substratos distintos: um de aço carbono convencional, denominado Usigalve, e outro de aço USI-R-COR-III, com características de resistência à corrosão atmosférica, designado Usigalve/Plus. A resistência à corrosão foi determinada por meio de testes acelerados e não-acelerados de corrosão, utilizando-se câmaras de testes cíclicos e estações de corrosão atmosférica industrial e marinha da Usiminas. Os parâmetros medidos foram o avanço médio e a penetração máxima da corrosão, empregando-se técnicas de análise de imagem. Este estudo mostra que a massa de zinco exerce papel preponderante na resistência à corrosão atmosférica dos aços eletrogalvanizados fosfatizados e pintados. Além disso, verifica-se também que é perfeitamente possível utilizar espessuras de camadas de zinco menores, sem comprometer a resistência à corrosão do material, desde que o substrato metálico apresente propriedades de resistência à corrosão atmosférica, contribuindo assim para melhorar a conformabilidade dos aços revestidos com zinco.

ResumoAs camadas de fosfato do tipo hopeíta, nas quais o zinco é parcialmente substituído por outros metais, como manganês e níquel, são de grande interesse para a indústria automotiva e de eletrodomésticos. Essas indústrias utilizam aços galvanizados, fosfatizados e pintados com eletropintura cataforética. Os fosfatos de zinco modificados com manganês e níquel são isomorfos com a hopeíta, e de difícil identificação usando-se a técnica de difração de raios X. Nesse trabalho, as camadas de conversão de fosfato foram identificadas usando-se a espectroscopia no infravermelho por transformada de Fourier (FTIR). Palavras

PurposeThe purpose of this study was to select an atmospheric corrosion evaluation methodology and to establish a range of relative corrosion penetration and/or progress values, which could be used as reference in the selection of materials for the civil construction industry.Design/methodology/approachSalt spray, field tests, accelerated cyclic tests and accelerated field tests were used to evaluate atmospheric corrosion resistance of civil construction materials. The cyclic accelerated test and the field test with saline solution spray were found to be appropriate for atmospheric corrosion resistance evaluation.FindingsThe corrosion resistance of aluminium killed mild steel, aluminium killed copper added steel, and electrogalvanized steels, all phosphatized and painted, were evaluated by field and accelerated corrosion tests. Of the materials studied, aluminium killed mild steel showed the least resistance to atmospheric corrosion. The use of aluminium killed copper added steel is recommended for material specification in the civil construction industry.Research limitations/implicationsSalt sprays are not adequate to evaluate atmospheric corrosion resistance. There are other cyclic tests that could be tested in future work.Practical implicationsBrazilian technical standards, which specify the metallic materials used in the civil construction industry, will be changed in order to include the construction steel corrosion resistance evaluation methodology, which is proposed in this paper. As a result, the tendency of the construction materials lifetime is set to increase.Originality/valueThis paper contributes to the improvement of the Brazilian Technical Standard by the inclusion of an atmospheric corrosion resistance requirement.