EFEITO DO PONTO DE ORVALHO DA ATMOSFERA DOS FORNOS DE RECOZIMENTO DA CGL NA QUALIDADE DO REVESTIMENTO GA EM AÇO BAKE HARDENABLE
EFFECT OF DEW POINT OF ATMOSPHERE OF CGL’S ANNEALING FURNACES ON THE QUALITY OF GA COATING ON BAKE HARDENABLE STEEL
Guimarães, Juliana Porto; Barbosa, Aldo Henrique de A.; Gonzalez, Berenice Mendonça; Anício, Deyselane de Fátima; Zacarias, José Januário
http://dx.doi.org/10.4322/tmm.2013.038
Tecnol. Metal. Mater. Min., vol.10, n3, p.272-279, 2013
Resumo
A 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
Abstract
The 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.
Keywords
Continuous annealing, Dew point, Bake hardenable steel, Galvannealed coating
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