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ResumoA condição superficial do aço após o recozimento contínuo é de extrema importância para a qualidade do revestimento galvannealed (GA). A presença de óxidos ou precipitados pode afetar as reações na interface revestimento/ substrato e diminuir a molhabilidade do zinco na superfície, provocando defeitos no produto final. Visando evitar a oxidação do material durante o recozimento, emprega-se uma atmosfera protetora nos fornos. Mas, apesar dessa atmosfera, é possível a oxidação seletiva ou precipitação de partículas de segunda fase, sendo o ponto de orvalho (PO) um dos fatores preponderantes no sentido de atenuar estas ocorrências. Neste estudo, avalia-se o efeito do PO sobre a qualidade de revestimento GA aplicado em aço bake hardenable. Em um Simulador de Processo de Galvanização a Quente realizou-se um recozimento contínuo sob pontos de orvalho de -60°C, -30°C e 0°C, assim como a galvanização a quente. Conclui-se que o PO é determinante para a quantidade, distribuição e tipo de óxidos formados, sendo a condição de -30°C a que proporciona a obtenção de um revestimento com menos falhas e melhor aderência. Palavras chave: Recozimento contínuo; Ponto de orvalho; Aço bake hardenable; Revestimento galvannealed. EFFECT OF DEW POINT OF ATMOSPHERE OF CGL'S ANNEALING FURNACES ON THE QUALITY OF GA COATING ON BAKE HARDENABLE STEEL AbstractThe strip steel surface condition after continuous annealing of strip is extremely important to the quality of galvannealed coating (GA). The presence of oxides or precipitated can affect the reactions at the coating/substrate interface and decrease the zinc wettability on the steel surface, leading to various defects in the final product. To avoid oxidation of the material during continuous annealing, a protective atmosphere is used in the furnaces. Despite this atmosphere, it is possible the selective oxidation or precipitation of second phase particles, being the dew point of atmosphere one of the leading factors to reduce this occurrence. This study aims to evaluate the effect of the dew point on the quality of GA coatings applied to a bake hardenable steel. The continuous annealing under dew points of -60°C, 30°C and 0°C, as well as hot dip galvanizing, were performed in a Hot Dip Process Simulator. It is found that the dew point has a decisive influence on the amount, distribution and type of oxides formed, and the condition of -30°C provided the coating with less failures and better adhesion.

Resumo A umidade nos óleos protetivos temporários e na solução de encruamento, bem como alterações no pH da água desmineralizada, utilizada na lavagem do aço antes do seu oleamento, podem causar manchas na superfície dos aços revestidos com zinco. As condições de transporte e de estocagem nos clientes também podem causar manchas. Essas manchas causam forte impacto visual, podendo gerar dúvidas na aplicação dos aços. Por esse motivo, algumas medidas são utilizadas na Usiminas para evitar manchas, como os controles da umidade no tanque de armazenamento dos óleos, do pH da água desmineralizada e do tempo de estocagem entre o encruamento e o acabamento dos aços. No presente estudo, foram realizadas caracterizações de amostras produzidas e manchadas em laboratório. Foram empregadas técnicas como a microscopia de força atômica, a microscopia eletrônica de varredura com canhão de emissão por efeito de campo e o espectrômetro de emissão ótica por descarga luminescente (GDOES), sendo propostos mecanismos para a formação das manchas que estariam relacionadas com a oxidação superficial do revestimento. Palavras-chave: Aços Galvanizados; Manchas na Camada de Zinco. STUDY OF MECHANISMS AND PREVENTIVE MEASUREMENTS OF STAIN FORMATION IN GALVANIZED STEELS Abstract Moisture in temporary protective oils and hardening solution, as well as uncontrolled in the pH of demineralized water, used in washing the steel before oiling, can cause stains on the surface of zinc coated steels. Transport and storage conditions on customers can also cause stains. These stains have a strong visual impact, its can causes doubts in the application of steels. For this reason, some measures are used in Usiminas to prevent stains, such as moisture controls in the oil storage tank, the pH of the demineralized water and the storage time between the hardening and the finishing of the steels. In the present study, the effect of these parameters on the formation of stains in zinc coated steel was investigated, using samples with simulated stains in the laboratory. Techniques were used to characterize the samples, such as atomic force microscopy, scanning electron microscopy with field emission gun and glow discharge optical emission spectroscopy). In the end, was proposed mechanisms for the formation of stains that would be related to the surface oxidation of the coating.

Purpose -The purpose of this paper is to study the effect of chemical composition on the selective oxidation on the surface of bake hardenable steel, and on the surface of modified bake hardenable steel with titanium addition, annealed at different dew points. Design/methodology/approach -The subject scope of the paper is the selective oxidation. The methodology used is the annealing of steels at different dew points, and the surface characterization by using atomic force microscopy, X-ray photo electronic spectroscopy, and glow discharge optical emission spectroscopy. Findings -The modified bake hardenable steel showed a higher oxidized area than the bake hardenable steel. The optimum condition for annealing of the bake hardenable steel was at 8008C with 2308C dp and 2 608C dp as selective oxidation is less voluminous. Research limitations/implications -One suggestion for future works is the use of transmission electron microscopy for the evaluation of selective oxidation on the steel surfaces. Practical implications -One practical implication is the determination of the optimum condition for annealing of the bake hardenable steel in the steel plant, decreasing the selective oxidation and increasing the quality of galvanized steels. Another implication is the discard by the steel industry of the development of the modified bake hardenable steel. Originality/value -The originality of the present work is to use advanced surface analysis techniques to quantify the selective oxidation on the surface of commercial steels. The selective oxidation on the surface of steels must be minimized in order to increase the wet ability of zinc layer on the steel surface.