Cause Analysis and Solutions for Surface Defects of Hot-dip Galvanized Coils

Hot-dip galvanizing is one of the most widely used anti-corrosion technologies, which can greatly improve the atmospheric corrosion resistance of steel materials. Continuous galvanized strip steel is widely used in the fields of automobiles, home appliances and construction due to its high production efficiency, good product quality and large market demand. Compared with continuous electro-galvanized strip, the cost of hot-dip galvanized strip is lower. requirements are particularly stringent.

1 Hot dip galvanizing defect

1.1 Zinc dross defects

Zinc dross is mainly an intermetallic compound formed by iron and zinc or aluminum. The formation principle of zinc slag is: after the strip enters the zinc liquid, the iron atoms on the surface of the strip undergo simultaneous dissolution and metallurgical reactions. When the iron entering the zinc liquid is lower than the saturated solubility of iron in the zinc liquid, the iron mainly exists in the zinc liquid in a dissolved state; when the iron entering the zinc liquid exceeds the saturated solubility of iron in the zinc liquid, the iron will form with zinc or aluminum. Precipitation of intermetallic compounds.

The composition of zinc slag is related to the temperature of molten zinc and its effective aluminum content. At the commonly used temperature of zinc solution, when the effective aluminum content in the zinc solution is lower than 0.135%, the main component of the zinc slag is FeZn7, and the density of the zinc slag is higher than that of the zinc solution, which is called bottom slag;
when the effective aluminum content in the zinc solution is FeZn7 When the content is higher than 0.135%, the main component of zinc slag is Fe2Al5, and the density of this zinc slag is lower than that of zinc liquid, which is called scum.

In the continuous production process, when the effective aluminum content in the zinc liquid changes, the bottom slag can be converted into scum, and the reaction formula is: 2FeZn7+5Al=Fe2Al5+14Zn

The saturated solubility of iron in the zinc solution is proportional to the temperature of the zinc solution and inversely proportional to the effective aluminum content of the zinc solution. When the zinc ingot is added, since the melting of the zinc ingot consumes a lot of heat, the temperature of the local zinc solution will decrease, and the saturated solubility of iron will decrease, so that zinc slag will be precipitated.

When switching from GA to GI, it is necessary to increase the effective aluminum content in the zinc solution. In this process, the saturated solubility of iron in the zinc solution decreases, and a large amount of zinc slag will be precipitated. When the surface of the strip is not thoroughly cleaned before entering the furnace, the iron powder remaining on the surface of the strip will enter the zinc solution with the strip, and these iron powders can quickly form zinc slag after falling off the surface of the strip. When the temperature of the strip into the zinc pot is too high, the dissolution rate and reaction activity of iron atoms on the surface of the strip will increase, which will increase the amount of zinc slag.
Figure 1 shows the dross defects attached to the surface of the strip.

1.2 Air knife streak defect

Because the lip of the air knife has a damaged notch or the air knife gap is blocked, a raised band mark will be generated at the corresponding position of the strip, which is called the air knife streak defect. When the blowing pressure of the air knife is very high and the air knife is very close to the liquid level of the zinc liquid, the splash of the liquid zinc will cause the blockage of the nozzle.

1.3 Leakage plating defects

The main reasons for leakage plating:

(1) Some foreign objects that have not been cleaned are adhered to the original surface of the strip steel, and the distribution of such missing plating defects is irregular in the form of block marks.

(2) When the proportion of atmosphere in the furnace is unreasonable, the hydrogen content is low, and the residual oxygen content is high, some irregularly distributed flake pinhole black dot oxides are attached to the surface of the strip.

(3) There is ash accumulation on the zinc liquid surface inside the furnace nose, or slag inside the furnace nose. When the strip is running, especially when the strip is speeding up, slowing down or passing through a weld, the strip will vibrate here, and zinc dust or slag will adhere to the surface of the strip and cause leakage plating.

1.4 Scratching defects

1.4.1 Scratches before plating

The scratches on the surface of the strip before hot-dip galvanizing are called pre-plating scratches. Such scratches may come from the surface of the raw material, such as scratches on the upper and lower guide plates at the exit of the cold rolling mill;
scratches on the cold rolling or hot rolling rolls Injury; nodules and scratches on the roll surface of the entrance guide table and entrance leveler in front of the zinc pot of the hot-dip galvanizing production line; the strip deviates at the entrance looper and is scratched by the roller;
Scratch; swing the furnace nose to clean up, when the furnace nose is slag, the swing range is too large to scratch the nose of the furnace and the sinking roller scratches and so on. The scratches before plating are all damage to the substrate. After galvanizing, the color of the scratches becomes darker, and it is often more obvious after the sheet is stamped.
1.4.2 Scratches after plating

Such scratches generally only damage the surface of the zinc layer and are not obvious after grinding. For subsequent stamping, the painted surface has little effect. Post-plating scratches may come from the roll surface behind the zinc pot, or foreign objects on the guide table. The scratches are bright in color and can be easily distinguished from scratches before plating, as shown in Figure (b).

1.5 Thermal folds

Thermal buckling is also known as thermal buckling. According to Zhang Qingdong’s research, the main reason for the transverse buckling of the strip is the local buckling caused by the uneven tensile deformation caused by the concentration of tension in the local area of the strip width to the middle. The occurrence of this defect is related to strip thickness, strip width, strip temperature, tension, shape, and roll crown. That is, the thinner the strip, the wider the strip, the higher the temperature of the strip, the greater the tension, the greater the crown, the more serious the buckling of the strip, and the easier it is to produce this defect, as shown in the figure.

1.6 Lighting feather marks

Feather marks are formed during the smoothing process and consist of feather-like defects arranged longitudinally in a string. The reason is due to the unstable elongation rolling of the strip in the skin pass gap, and its typical morphology is shown in the figure.

1.7 Imprint

The main reasons for embossing are:

(1) Particles stick to the roller.

(2) Soft deposits stick to the rollers in the annealing furnace due to mechanical damage.

(3) Mosquitoes fly to the surface of the strip steel plate and are rolled in the summer.

(4) The roller surface is damaged.

(5) In the annealing furnace, some impurities are sprayed from the burner to the surface of the strip. There are many reasons for the formation of imprinting defects on the surface of the galvanized sheet, but the imprinting caused by the rollers is periodic. It can be calculated from which roller the imprinting comes from through the cycle, so as to clean or replace the new roller to eliminate the imprinting defects.

2 Defect control measures

2.1 Control measures to reduce zinc slag defects

(1) Control the effective aluminum content in the zinc liquid, so that the zinc slag mainly exists in the form of scum, and at the same time reduce the fluctuation of the effective aluminum content as much as possible, and control the fluctuation of the effective aluminum content below 0.02%.

(2) Determine the reasonable temperature of the strip steel entering the zinc pot, adopt a high-efficiency zinc liquid temperature control model, reduce the temperature fluctuation of the zinc liquid, and control the zinc liquid temperature within the range of (460±3) °C.

(3) Strengthen cleaning to ensure that the iron powder content on the surface of the incoming strip is less than 40mg/m2.

(4) Add ingots evenly to avoid violent fluctuations in effective aluminum content and zinc liquid temperature.

(5) Reduce the number of conversions from GA to GI, or use a double zinc pot to avoid the conversion from GA to GI.

(6)The slag should be picked up in time. Generally, the scum should be removed by the method of picking slag. Be sure to pay attention to the extraction of the scum at the outlet of the strip in the zinc pot, so as to reduce the adhesion of the scum here to the surface of the strip.
(7)
2.2 Air knife streak defect control measures

Once air knife streak defects are found in production, special tools should be used to deal with them in time.

2.3 Control measures for leakage plating defects

(1) Regularly check the cleaning ability of the cleaning section, and do a wiping experiment to check the surface of the strip after cleaning to reduce leakage caused by foreign matter on the original plate surface.

(2) Real-time monitoring of the atmosphere in the furnace and timely adjustment.

(3) Regularly clean and purge the furnace nose.

2.4 Scratch defect control measures

The state of the strip surface can be monitored through the production line inspection table and the online surface inspection system. When continuous scratch defects are found, the first step is to distinguish the type of scratch, whether it is a scratch before plating or a scratch after plating.

If it is the post-plating scratch after the zinc pot, check the running status of each pinch bar, idler and steering roller in time, find the source of the defect and eliminate it. If it is the scratch before plating, also check the running status of each roller before the zinc pot. , and check the raw strip at the entrance of the production line. If the raw material strip is scratched, check the production batch in time, and remove the same batch of raw materials to avoid mass production of defective raw materials.
2.5 Control measures for thermal wrinkle defects

(1) When thermal wrinkle defects occur, the tension of the strip steel in the furnace should be reduced in time, so that the strip steel in the furnace runs with the minimum tension.

(2) Control the shape of the raw material, and those with severe unilateral waves or severe medium waves cannot be used.

(3) To produce materials of different specifications, wide and narrow, thin and thick, should be gradually changed to avoid the direct production of two materials with a large gap between specifications.

2.6 Control measures for finishing feather marks

The feather mark defect of skin pass is produced when the strip steel in the gap of the pass roll is rolled with an unstable elongation, and a feather mark defect occurs. It is necessary to increase the pass strip tension, reduce the rolling force, and increase the pass roll force. When the sheet material is thick, when adjusting the skinning tension and rolling force, there may be serious edge waves. The skinning should be opened in time to avoid belt breakage, and the feather marks can also be eliminated by the tension leveling machine.

2.7 Imprint Defect Control Measures

When the inspector finds imprinting defects, it is necessary to measure the imprinting cycle, and by calculating the roller diameter through the cycle, the position where the imprinting defect occurs can be roughly calculated, thereby eliminating the imprinting. Keeping the production line airtight can reduce dust or mosquitoes attaching to the roller surface and causing imprinting. When it is found that the imprinting cycle does not match the roller diameter of the production line, it is necessary to check the raw materials to avoid mass production.

Imprinting is a common and uncontrollable defect, which cannot be completely eliminated. It needs to be discovered and dealt with in time, so the real-time monitoring of the strip surface is particularly important. The above-mentioned defects are common in daily production. As long as attention is paid to the real-time monitoring of the strip surface, the production personnel can respond quickly and take timely countermeasures, which can greatly reduce degradation and cutting. Simultaneously monitor and inspect raw material quality to reduce unnecessary product defects.
3 Conclusion

The surface quality of hot-dip galvanized strip involves various processes from steelmaking to galvanizing production, and is ultimately reflected in the process of stamping and painting in automobile factories. Therefore, an objective and reasonable evaluation and analysis should be made according to the specific defect form. Identify the causes of various types of surface defects.

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