Causes and characteristics of friction black spots on hot-dip galvanized coils

The friction black spot on the surface of the galvanized coil is a typical product defect named "cause morphology feature. "Friction" refers to the action or process of relative displacement between the internal layers of the steel coil under the pressure of its own gravity or mutual extrusion during the storage and transportation of the galvanized coil. This relative displacement will change the surface morphology and color of the plate. In short, the "black spot" is the visual result of the change of the plate surface caused by the relative displacement between the galvanized coil layer and the layer. Friction black spots are mostly formed in the transportation process, but the low coiling tension in the production process and the loose roll of the outer ring are also one of the reasons for friction black spots.

For the galvanized coil, as shown in the cross-sectional view of FIG. 1, the friction black spots usually occur in the 3 o'clock, 5 o'clock, 7 o'clock and 9 o'clock directions of the steel coil. Among them, 5 points and 7 points are usually the stress parts of the steel coil during storage and transportation, and the steel coil is subjected to the normal force F of the vertical stress parts as shown in the figure. Special attention should be paid to the contact and force between the strap and the bracket. Generally, the friction between the strap and the bracket has the highest probability of black spots. If multiple steel coils are placed side by side during transportation, the front and rear steel coils are squeezed from each other during transportation and are affected by the pressure F of the vertical section. It should be noted that the force F here is generally not in the middle of the steel coils, but mostly in the edge part, because the steel edge part is equipped with a retainer, and most of the edge part is stressed by contact. There are two necessary conditions for the generation of friction black spots, one is the existence of pressure F, and the other is the relative movement between the steel coil layer and the layer. The so-called "black" of the friction black spot is that the surface roughness and morphology of the strip steel have changed due to the formation of different degrees of micro-damage after relative friction. Generally speaking, friction black spots are difficult to completely eliminate, and it is necessary to effectively control the key points that cause friction black spots to occur and minimize the occurrence of friction black spots.

Fig. I Schematic Diagram of Section of Galvanized Coil and Common Position of Friction Black Spot

Four types of typical friction black spots are characterized as follows:

(1) The contact part between the steel coil and the wooden bracket is embossed with friction black spots. As shown in Figure II, the steel coil packaged and transported by wooden bracket is found to have a friction black spot appearance at the corresponding part in contact with the wooden bracket after unwinding, I .e. the 5 o'clock and 7 o'clock directions shown in Figure 1. Such friction black spots are similar to the imprinting of steel coils due to force, but they are not imprints. The friction black spot generated after the contact part of this kind of strip steel and the bracket produces relative sliding is a very typical friction black spot.

Fig. II Friction black spots on the upper and lower surfaces of the strip steel at the contact part between the galvanized coil and the wooden bracket

(2) The steel curling part squeezes each other during transportation to produce friction black spots. As shown in Figure III, the steel coils are placed back and forth in sequence during transportation, and all steel hemming parts are provided with retainer rings. Therefore, when the vehicle shakes back and forth during transportation, the extrusion parts generated are mainly the edges in contact with each other. After unpacking and unpacking, it was found that continuous friction black spots appeared at the part of the steel coil 40cm away from the edge, which corresponded to the contact part of the steel coil.

Fig. III Friction Black Spot on Edge of Galvanized Roll

(3) Friction black spots formed by steel coil banding. As shown in Figure IV, the friction black spot produced by the relative sliding of the contact part between the strip steel and the binding band is completely consistent with the position of the binding band, and the width and morphology of the black spot are also completely consistent with the binding band. The black spot position is the main force position between the steel coil and the strap.

Fig. IV Friction Black Spots Produced by Relative Sliding of Contact Parts of Galvanized Coil and Strap

(4) Other irregular shapes rub black spots. As shown in Figure V, it is a kind of irregular shape friction black spot, which has a large area and a large influence range. The total length of the defects corresponding to this kind of large area friction black spot is also relatively long on the strip. If the plate package material has friction black spots, it is mostly a large area black spot similar to Figure V.

Fig. V Friction Black Spot with Irregular Morphology