Estudo da influência do chumbo na aparência de revestimentos da linha de zincagem contínua 2 da CSN e otimização na adequação à diretiva Rohs

Abstract

The continuous hot-dip galvanizing is one of the most effective and economical processes employed to protect steel against corrosion. The characteristics of the galvanized product depend mainly on the chemical composition of the zinc and the steel, immersion time, cooling rate and substrate roughness. Solidification of the zinc coating is characterized by the crystallization of a very thin layer of metal. The crystals formed are called a zinc flower, which develop when the molten zinc adheres to the steel plate and is cooled below its melting point. There is another solidification process that leads to the pattern of zinc flowers in the zinc coating, which is dendritic growth. The nature and rate of dendritic growth during the solidification process is affected by the presence of other elements, especially lead. The addition of lead is of great importance in continuous galvanizing and brings many benefits such However, lead is considered to be harmful to man and the environment. The RoHs Directive was created in 2006 mainly because of this toxicity of lead and other elements like hexavalent chromium and mercury. In 2011, the LZC#2/CSN has adapted to the RoHs directive, reducing the lead content in galvanized products, but, because of this decrease, appeared a problem in the appearance of the coating, called "small crystal" or KM. This is a visual decrease of the zinc crystals in the final product. Clearly visible crystals are a requirement of the vast majority of customers and this problem has led to a shift of more than 500 tons of steel from 2011 to 2015, causing operating losses on the LZC#2/CSN. According to this project, it was concluded that the "small crystal" problem arose after the decrease in lead content in the zinc bath to suit the RoHs directive, since the lead content directly influences the size of the zinc flower. It has also been found that this suitability can be optimized since the results indicated a considerable difference (approximately 20%) between the lead content in the zinc bath and the final product and that the roughness of the substrate also significantly influences the quality of the coating.