Defects of hot rolled and cold rolled flat steels due to hot dip galvanizing process and their prevention methods

Steel is the most important material and it has a huge usage area in the world in which it has many significant properties such as easy formability, high strength, weldability, paintability, lower cost and so on. One of the most important steel product is flat rolled steels which are divided into two groups: hot rolled steel and cold rolled steel. Steel has a big disadvantage that it corrodes so easily. Corrosion is defined as a degradation of the material properties because of its environmental conditions. As the thermodynamically explanation, the system gets in energy from the environmental to manufacture the steel and the steel tends to give this energy back to the earth which is achieved by the oxidation of iron. Because of that, steel tends to corrode. This is an important issue to use the steel in our lives because the corroded steel has lack of good properties. This situation narrows the usage of the steel in the industry areas including construction, home appliance, automotive sector etc. However, the steel can be protected from the corrosion in which the zinc is an essential element on it. There are many protection methods achieved by zinc element for instance barrier protection method (painting is the most usage one), zinc patina method, cathodic protection and zinc coating. The most effective zinc coating method is the hot dip galvanizing method. Hot dip galvanizing methods differ according to the zinc bath chemical compositions. There are four types of galvanizing which are called as galfan coating (GF); galvalume coating (GL); galvanizing coating (GI) and galvannealed coating. The most important element in the bath is aluminum; for this reason, the coating types are divided by their Al contents. For example, galfan has 5% wt Al; galvalume has 55% Al; galvanizing and galvannealed bath contains Al<1% wt. Apart from the others, galvannealed coating differs from the other types having an extra annealing furnace above the zinc bath. It influenced the microstructure of the coating significantly. Among the galfan, galvalume and galvannealed coating, in this research galvanized coating (GI) is studied which contributes a double corrosion protection by creating a protective film onto the steel surface. The hot dip galvanizing line consists of cleaning section in which the cleanliness of the steel is a quite issue to accomplish a better adhesion; annealing furnace which improves the mechanical properties of the material; galvanizing and cooling section that the steel is immersed into the molten zinc bath (460°C) to obtain zinc coated steel surface and it allows to cool down; skin pass mill which also improves the steel mechanical features and it creates a roughness surface to process the steel such as painting; passivation in which the chromate solution is used to increase the corrosion protection resistance of the material; and the final section is called as inspection area allows to observe the surface defects onto the steel surface. Inspection section is where we can observe the surface defects. According to the defects, some improvements and rearrangements are done in the production line. If it is necessary, the defected parts are cut to make the mechanical tests in the mechanical laboratories to understand and eliminate the problems. Mainly, the surface defects of the flat rolled steel due to hot dip galvanizing process are unappropriate mechanical test result, spangle, dross, edge wave, surface roughness, poor adhesion, coating thickness deviation. In this study, surface defect analysis and their prevention methods are established.