Study on spalling of hot-dip aluminized layer of cold-rolled steel sheet

Hot-dip aluminizing has good high temperature resistance, oxidation resistance, corrosion resistance and good brightness, and has the advantages of sufficient material sources and concise process. It is a metal surface modification technology with great development potential. In some fields, the service life of aluminized steel can be more than three times that of ordinary carbon steel. Hot dip aluminum plating has been widely used in chemical, metallurgy, petroleum, electric power, aerospace and other fields, and has a very broad prospect. After the cold-rolled steel plate is dipped and plated, the coating will peel off as a whole under the bending of ordinary mechanical external force. The cold-rolled steel plate after hot-dip aluminization was analyzed. The microstructure and morphology of the coating of the sample were observed under an electron scanning electron microscope (SEM). XRD detection was performed on the part of the material, so as to analyze the material composition at the location where the spalling occurred, and study the reason for the spalling. The experimental substrate is 4mm×1.5mm Q195 cold-rolled steel sheet, trace element content (mass fraction, %): 1.5C, 1.0Si, 1.8Mn, 0.9P, 0.9S. The plating solution is pure aluminum (with an aluminum content of 99.999%). The hot-dip plating test adopts the method of pretreatment by reduction method. The specific process flow is as follows: (1) pretreatment, grinding the steel plate with sandpaper to remove the surface rust; (2) using high-frequency induction equipment, in the nitrogen hydrogen (N2: H2 ratio of 3:1) Pre-treatment by reduction method under protective atmosphere; (3) Hot dip plating temperature of 710° C. and dip plating time of 10s. After the immersion plating is completed, some samples are taken to make metallographic samples, and some samples are tested after the coating is peeled off by mechanical external force. The former is used for metallographic analysis, scanning electron microscope observation of morphology and energy spectrum analysis, and the latter is used to analyze the material composition of the exfoliation position by XRD. The hot-dip aluminized coating is divided into two parts: pure aluminum layer and transition layer. The transition layer is in contact with the pure aluminum layer. The part close to the base is Fe2Al5, and the part close to the pure aluminum layer is FeAl3. Its thickness accounts for about 15% of the entire transition layer. Compared with the Fe2Al5 composition area, the structure is relatively fine. Under the action of external mechanical force, due to the great brittleness of FeAl3, fracture occurs, and the ductility of the outer layer of pure Al is very good, so the fracture occurs at the contact position between FeAl3 and pure Al, and the coating is completely peeled off.