Hot-dip Galvanized Steel Sheet Internal Oxidation Coating Quality

The patent applies preliminary action by performing internal oxidation treatment before hot-dip galvanizing. The steel sheet is heated to form Si and Mn oxides in the surface layer (10-100 μm depth) prior to coating, so that these oxides are already present to control the interfacial reaction during galvanizing. This preliminary preparation prevents excessive Fe diffusion and ensures proper coating appearance and adhesion while maintaining the high-strength complex microstructure.

The patent applies local quality by creating a specific oxide-rich surface layer (10-100 μm depth) that is different from the bulk steel composition. The surface layer contains 0.05-0.5 mass% Si and 0.05-0.5 mass% Mn in oxide form, while the bulk maintains the high-strength microstructure. This localized modification at the surface enables good coating quality without compromising the overall mechanical properties.

The patent converts the harmful external oxidation into a beneficial internal oxidation process. Instead of allowing uncontrolled surface oxidation that causes coating defects, the steel sheet is heated in a controlled atmosphere (containing 3-20 mass% H2 and -20°C≤ dew point ≤5°C) to form Si and Mn oxides internally in the surface layer (10-100 μm depth). These internally formed oxides then serve as a protective barrier during hot-dip galvanizing, preventing harmful external oxidation and ensuring good coating quality.

The patent uses an inert atmosphere (hydrogen-containing atmosphere with controlled dew point) during the heating process to control oxidation. The atmosphere contains 3-20 mass% H2 and has a dew point between -20°C and 5°C, which creates a controlled oxidative environment that promotes internal oxidation of Si and Mn without causing excessive external oxidation. This controlled atmosphere ensures the oxides form in the desired location and quantity.