Cause Analysis And Countermeasures Of Color Difference Defect Of Hot-dip Galvanized Coil

Hot-dip galvanized strip steel is a product with high performance and high added value produced by hot-rolled coil as raw material through cold rolling – annealing – galvanizing. Material saving plays an immeasurable and irreplaceable role, and is currently widely used in the automotive and home appliance industries.

Chromatic aberration defect is a general term for a class of defects on the surface of galvanized strip steel, which directly affects the appearance and coating effect of galvanized strip steel.

From an optical point of view, chromatic aberration defects are caused by abnormal surface roughness at the defect location, resulting in differences in the reflectivity of light in the defect area.
According to the analysis of the current research status at home and abroad, the main factors related to the chromatic aberration defects of the strip are: uneven lateral roughness of the strip surface, foreign matter on the surface of the zinc layer, and uneven grain size of the zinc layer.
edge chromatic aberration

The macroscopic appearance of the edge color difference defect is shown in the figure. Figure (a) The color difference of the edge part of the galvanized sheet is about 5cm at the edge of the strip, and the whole coil exists continuously, which is slightly brighter than the normal plate position. Tested with a hand-held roughness meter, the roughness Ra of the edge chromatic aberration position is 0.56 μm, and the roughness Ra of the normal plate surface position is 0.72 μm.

Figure (b) The chromatic aberration of the rolled edge of the chilled plate is also located at about 5cm on the edge of the strip, and the whole roll exists continuously, slightly whitish compared to the normal plate surface position. Due to the thick iron oxide scale on the surface of the hot-rolled coil, the color difference defect cannot be observed with the naked eye.
Due to the chromatic aberration at the edge, the raw material coil has a strong correspondence with the defect position of the finished coil. It is suspected that the chromatic aberration at the edge of the galvanized sheet is caused by the raw material.

Then, the finished galvanized coils and the same batch of raw material coils with edge color difference were sampled respectively, and the micro-morphology of the surface and cross-section was observed. Figure 2 shows the cross-sectional microscopic morphology of the chromatic aberration position and the normal position of the galvanized curling part under the electron microscope.
It can be seen that in Fig. 2(a) at the edge color difference position, the surface of the galvanized substrate is relatively flat, and the thickness of the zinc layer is about 7-10 μm; in Fig. 2(b) without color difference, the surface of the galvanized substrate has height fluctuations of 3-4 μm, and the zinc layer has a thickness of about 7-10 μm. The layer thickness is about 10-12 μm.

The thickness of the zinc layer at the chromatic aberration position of the rolled edge of this galvanized sheet is thinner and the roughness is lower. This is due to the low roughness of the galvanized raw chilled coil within 5 cm from the edge. During the cooling process of the hot rolling process, the cooling rate of the edge of the strip is faster, resulting in fine grains, higher hardness and strength, and thin and dense iron oxide scale at the edge of the strip.
Hot-rolled coils with such characteristics are easily left with dense iron oxide scale at the edges after pickling. After continuous cold rolling, the roughness of the cold rolled edge is low due to the strengthening effect of edge grain refinement and the lubrication effect of iron oxide scale.

Accordingly, corresponding improvement measures are formulated. In the actual production process of hot-rolled coil, the uniformity control of transverse temperature is mainly by adjusting the water distribution in the upper header to form a convex water flow distribution.
In order to control chromatic aberration defects on the edge of galvanized finished products, the method of edge shielding on the cooling device is adopted, and different shielding strategies are adopted according to different strip widths, thicknesses and steel grades, which improves the horizontal direction during the cooling process of hot rolled coils. Inhomogeneity of temperature, structure, and stress distribution.

Color difference after grinding

Figure (a) shows the macroscopic appearance of chromatic aberration defects on the surface of hot-dip galvanized strip steel after sanding, which is a dark black circle;
When grinding the strip steel at various positions behind the zinc pot, there are chromatic aberration defects after grinding, indicating that the defect occurs at the position of the air knife in the zinc pot.

Under the same air knife process parameters, observe the jitter of the strip: when the running speed of the strip is 60m/min, the strip runs smoothly, there is basically no jitter, the surface after grinding is basically the same, and the color difference defect is not obvious; when the strip running speed is 60m/min When it is 100m/min, there is slight shaking of the strip, and the surface color difference defect is obvious after grinding.

This shows that the color difference after grinding has a great correlation with the running speed of the production line and the jitter amplitude after the strip is out of the zinc pot. During the shaking process of the strip, the distance between the upper and lower air knives of the strip will change. Under the same air pressure, the change of the air knife distance will cause the thickness of the zinc layer to change.

The main reasons for the jitter of the strip after the zinc pot are: (1) The installation accuracy of the submerged roller is poor, or the shaft head bushing is seriously worn after the submerged roller is used; (2) The incoming material is bad; (3) The cooling section after plating is fast The spray pressure on the upper and lower surfaces of the cold air box is inconsistent.

The following measures are formulated for the hot-dip galvanizing process to control strip shaking: (1) After the sunken roll is processed and before installation, check its surface roughness and whether there is chamfering in the groove, whether there are burrs and bruises, and whether it will affect the installation accuracy. When installing the sunken roll, check whether the center line of the strip between the sunken roll and the top roll of the cooling tower is consistent;
(2) Adjust the air volume on the upper and lower surfaces of the moving bellows in the cooling section after plating, so that the baffle opening of the upper and lower bellows, The pressure is the same.
Color difference after stamping

After stamping, chromatic aberration defects appear on the stamping parts of hot-dip galvanized steel sheets, and the macroscopic appearance is shown in the figure, showing irregular flakes.
The chromatic aberration defect after stamping is due to the destruction of the zinc layer on the surface of the galvanized steel sheet during the stamping process and the occurrence of cracks, which causes the internal oxidation of the zinc layer, resulting in chromatic aberration defects on the surface of the stamping parts.
There are many factors that affect the surface damage of galvanized steel sheet stamping, such as steel galvanizing process, mold material, lubrication effect, etc.

The surface damage is prevented by measures such as surface strengthening and lubrication of the stamping die. The main measures are: (1) Nitriding the surface of the stamping die; (2) Through the tensile extrusion test, the effect of different lubricants is studied. The research shows that the surface film generated by the lubricant on the surface of the friction pair can reduce the stamping surface damage.
in conclusion

(1) The color difference of the edge of the galvanized sheet is caused by the low roughness of the chilled coil of the galvanized raw material within 5cm from the edge. In the cooling process of the hot rolling process, the edge of the strip has a fast cooling rate, fine grains, high hardness and strength, and thin and dense iron oxide scale.

After pickling, the dense iron oxide scale remains on the edge. In the process of continuous cold rolling, the roughness of the cold rolled edge is low due to the strengthening effect of edge grain refinement and the lubrication effect of iron oxide scale.
In order to control the chromatic aberration defect of the edge of the galvanized finished product, the method of edge shielding on the cooling device is adopted to improve the inhomogeneity of the transverse temperature, structure and stress distribution during the cooling process of the hot-rolled coil. The overall surface quality of the roll is significantly improved.
(2) The chromatic aberration defect after grinding on the surface of the galvanized strip has a great correlation with the running speed of the production line and the jitter amplitude of the strip after the zinc pot.
During the shaking process of the strip, the distance between the upper and lower air knives of the strip will change. Under the same air pressure, the change of the air knife distance will cause the thickness of the zinc layer to change. Therefore, it is necessary to control the shaking of the strip, improve the installation accuracy of the sinking roller, and adjust the air volume on the upper and lower surfaces of the moving bellows in the cooling section after plating, so that the baffle opening and pressure of the upper and lower bellows are consistent.

(3) The chromatic aberration defect appears on the stamping parts of hot-dip galvanized steel sheet after stamping, because the zinc layer on the surface of the galvanized steel sheet is damaged during the stamping process, and cracks appear, which causes internal oxidation of the zinc layer, resulting in chromatic aberration defects on the surface of the stamping parts. . Therefore, it is necessary to carry out nitriding treatment on the surface of the stamping die; add lubricant during the stamping process, and the surface film generated by the lubricant on the surface of the friction pair can reduce the surface damage of the stamping part.

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