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Continuous hot-dip galvanizing quality defects and countermeasures

Continuous hot-dip galvanizing quality defects and countermeasures

  • Categories:Industry News
  • Author:
  • Origin:
  • Time of issue:2020-10-13
  • Views:0

(Summary description)Hot-dip galvanized coils have gradually become the mainstream variety in steel coil applications because of their excellent corrosion resistance and good cost performance. At the same time, because of the characteristics of the hot-dip galvanized coil product, it can be used directly in the bare or can be subsequently painted. The complex usage scenarios and the individual requirements of different applications also make users pay more attention to the surface quality of hot-dip galvanized coils.

Understanding the quality defects and countermeasures of hot-dip galvanized coil products will help us better communicate with users and improve trade quality.

一 The zinc layer peels off (the oxide is attached)

Morphology: The galvanized layer is easy to peel off (large or small pieces). The substrate surface under peeling is oxidized, which is black, blue or black and blue. Generally, the plating layer will form pinholes and the surface morphology of the plating layer before peeling occurs. Visual abnormalities (spangles protruding in a three-dimensional shape, the surface is particularly bright... etc.); when the production line stops for a short time due to some reasons, such as poor atmosphere control of the non-oxidizing furnace, peelable oxide scale is formed on the steel surface This part of the zinc pool is blown off by the air knife to form exposed black spots. In addition to the surface of the zinc pool being blown off, some of the scale may also be suspended in the zinc bath, which often takes a long time. Take it all out.

Causes: can be divided into furnace area and non-furnace area factors

furnace area factor:

-Incorrect air/gas combustion ratio and flow control lead to inappropriate CO and O2 content in the furnace atmosphere

- The furnace temperature is too low or not matched with the plate temperature, and the plate temperature is too low

- The dew point is too high, that is, the water content (H2O) is too high

- Sudden change of plate temperature causes excessive changes in furnace temperature, which is related to the thickness, roughness, surface condition of the substrate, etc.

Non-furnace area factors:

- poor cleaning

-Poor water quality in the washing section

Countermeasures:

A) Prevent air leakage in the furnace area

B) Prevent leakage of cooling water systems in the furnace area

C) Carefully and appropriately control non-oxidizing furnace atmosphere CO%: 3-5 %; furnace temperature> 1200 ℃

D) Properly control alkali washing conditions: free alkali, alkali ratio, alkali washing temperature

E) When the water quality is poor, add a bypass soft, pure water manual valve

F) When H2 does not react well with the steel plate, the plate temperature can be increased to promote the reduction rate or slow down to increase the reaction time

二 Zinc layer peeling off (surface carbon)

Form: The coating is easy to peel off, the surface is accompanied by black powder, and the substrate may sometimes be blue. The black powder may be oxide or carbon (C) powder, so you need to do experiments or experience To observe and judge (mostly appear at the head and tail).

Cause:

- Improper control of furnace atmosphere (CO% is too high)

Countermeasures:

A) Properly control furnace atmosphere: CO%: 2 – 3%

三 Excessive zinc dross accumulation on the surface

form: one is block (slag of oxidized peeling flakes); the other is granular (intermetallic compound), which can be divided into two types-top scum and bottom slag. These slags will cause the surface of the steel plate to be rough (distributed on the entire surface of the steel plate or on both sides of the steel plate). In severe cases, it is easy to form warped edges after winding.

Cause:

- The temperature of the zinc pool changes too much, especially when the zinc ingot is immersed too quickly, it will easily cause the surrounding temperature to change

- AL content is too high, if the Al content is too low, it is easy to generate bottom slag, so the Al content should be controlled within 0.18 – 0.22%

-Oxide peeling due to shutdown defects or other factors

- Defects on the surface of the substrate can easily cause dross adhesion

Countermeasures:

A) Try to keep the zinc pool temperature stable. (Because Fe and Al are easy to dissolve in high temperature, but it is easy to precipitate at low temperature and produce zinc dross)

B) When adding large zinc ingots, slowly dip into the zinc bath

C) Do not remove the scum on the surface of the zinc bath too often, just move it to the side and remove it at regular intervals (such as 1 hour)

D) When adding small zinc ingots, don’t put them in one place, but distribute them evenly

E) Try to remove the oxide scale from the zinc bath to prevent sinking to the bottom or suspended in the zinc bath

F) Properly adjust the air knife and its pressure and angle

Continuous hot-dip galvanizing quality defects and countermeasures

(Summary description)Hot-dip galvanized coils have gradually become the mainstream variety in steel coil applications because of their excellent corrosion resistance and good cost performance. At the same time, because of the characteristics of the hot-dip galvanized coil product, it can be used directly in the bare or can be subsequently painted. The complex usage scenarios and the individual requirements of different applications also make users pay more attention to the surface quality of hot-dip galvanized coils.

Understanding the quality defects and countermeasures of hot-dip galvanized coil products will help us better communicate with users and improve trade quality.

一 The zinc layer peels off (the oxide is attached)

Morphology: The galvanized layer is easy to peel off (large or small pieces). The substrate surface under peeling is oxidized, which is black, blue or black and blue. Generally, the plating layer will form pinholes and the surface morphology of the plating layer before peeling occurs. Visual abnormalities (spangles protruding in a three-dimensional shape, the surface is particularly bright... etc.); when the production line stops for a short time due to some reasons, such as poor atmosphere control of the non-oxidizing furnace, peelable oxide scale is formed on the steel surface This part of the zinc pool is blown off by the air knife to form exposed black spots. In addition to the surface of the zinc pool being blown off, some of the scale may also be suspended in the zinc bath, which often takes a long time. Take it all out.

Causes: can be divided into furnace area and non-furnace area factors

furnace area factor:

-Incorrect air/gas combustion ratio and flow control lead to inappropriate CO and O2 content in the furnace atmosphere

- The furnace temperature is too low or not matched with the plate temperature, and the plate temperature is too low

- The dew point is too high, that is, the water content (H2O) is too high

- Sudden change of plate temperature causes excessive changes in furnace temperature, which is related to the thickness, roughness, surface condition of the substrate, etc.

Non-furnace area factors:

- poor cleaning

-Poor water quality in the washing section

Countermeasures:

A) Prevent air leakage in the furnace area

B) Prevent leakage of cooling water systems in the furnace area

C) Carefully and appropriately control non-oxidizing furnace atmosphere CO%: 3-5 %; furnace temperature> 1200 ℃

D) Properly control alkali washing conditions: free alkali, alkali ratio, alkali washing temperature

E) When the water quality is poor, add a bypass soft, pure water manual valve

F) When H2 does not react well with the steel plate, the plate temperature can be increased to promote the reduction rate or slow down to increase the reaction time

二 Zinc layer peeling off (surface carbon)

Form: The coating is easy to peel off, the surface is accompanied by black powder, and the substrate may sometimes be blue. The black powder may be oxide or carbon (C) powder, so you need to do experiments or experience To observe and judge (mostly appear at the head and tail).

Cause:

- Improper control of furnace atmosphere (CO% is too high)

Countermeasures:

A) Properly control furnace atmosphere: CO%: 2 – 3%

三 Excessive zinc dross accumulation on the surface

form: one is block (slag of oxidized peeling flakes); the other is granular (intermetallic compound), which can be divided into two types-top scum and bottom slag. These slags will cause the surface of the steel plate to be rough (distributed on the entire surface of the steel plate or on both sides of the steel plate). In severe cases, it is easy to form warped edges after winding.

Cause:

- The temperature of the zinc pool changes too much, especially when the zinc ingot is immersed too quickly, it will easily cause the surrounding temperature to change

- AL content is too high, if the Al content is too low, it is easy to generate bottom slag, so the Al content should be controlled within 0.18 – 0.22%

-Oxide peeling due to shutdown defects or other factors

- Defects on the surface of the substrate can easily cause dross adhesion

Countermeasures:

A) Try to keep the zinc pool temperature stable. (Because Fe and Al are easy to dissolve in high temperature, but it is easy to precipitate at low temperature and produce zinc dross)

B) When adding large zinc ingots, slowly dip into the zinc bath

C) Do not remove the scum on the surface of the zinc bath too often, just move it to the side and remove it at regular intervals (such as 1 hour)

D) When adding small zinc ingots, don’t put them in one place, but distribute them evenly

E) Try to remove the oxide scale from the zinc bath to prevent sinking to the bottom or suspended in the zinc bath

F) Properly adjust the air knife and its pressure and angle

  • Categories:Industry News
  • Author:
  • Origin:
  • Time of issue:2020-10-13
  • Views:0
Information

Hot-dip galvanized coils have gradually become the mainstream variety in steel coil applications because of their excellent corrosion resistance and good cost performance. At the same time, because of the characteristics of the hot-dip galvanized coil product, it can be used directly in the bare or can be subsequently painted. The complex usage scenarios and the individual requirements of different applications also make users pay more attention to the surface quality of hot-dip galvanized coils.

Understanding the quality defects and countermeasures of hot-dip galvanized coil products will help us better communicate with users and improve trade quality.

一 The zinc layer peels off (the oxide is attached)

Morphology: The galvanized layer is easy to peel off (large or small pieces). The substrate surface under peeling is oxidized, which is black, blue or black and blue. Generally, the plating layer will form pinholes and the surface morphology of the plating layer before peeling occurs. Visual abnormalities (spangles protruding in a three-dimensional shape, the surface is particularly bright... etc.); when the production line stops for a short time due to some reasons, such as poor atmosphere control of the non-oxidizing furnace, peelable oxide scale is formed on the steel surface This part of the zinc pool is blown off by the air knife to form exposed black spots. In addition to the surface of the zinc pool being blown off, some of the scale may also be suspended in the zinc bath, which often takes a long time. Take it all out.

Causes: can be divided into furnace area and non-furnace area factors

furnace area factor:

-Incorrect air/gas combustion ratio and flow control lead to inappropriate CO and O2 content in the furnace atmosphere

- The furnace temperature is too low or not matched with the plate temperature, and the plate temperature is too low

- The dew point is too high, that is, the water content (H2O) is too high

- Sudden change of plate temperature causes excessive changes in furnace temperature, which is related to the thickness, roughness, surface condition of the substrate, etc.

Non-furnace area factors:

- poor cleaning

-Poor water quality in the washing section

Countermeasures:

A) Prevent air leakage in the furnace area

B) Prevent leakage of cooling water systems in the furnace area

C) Carefully and appropriately control non-oxidizing furnace atmosphere CO%: 3-5 %; furnace temperature> 1200 ℃

D) Properly control alkali washing conditions: free alkali, alkali ratio, alkali washing temperature

E) When the water quality is poor, add a bypass soft, pure water manual valve

F) When H2 does not react well with the steel plate, the plate temperature can be increased to promote the reduction rate or slow down to increase the reaction time

二 Zinc layer peeling off (surface carbon)

Form: The coating is easy to peel off, the surface is accompanied by black powder, and the substrate may sometimes be blue. The black powder may be oxide or carbon (C) powder, so you need to do experiments or experience To observe and judge (mostly appear at the head and tail).

Cause:

- Improper control of furnace atmosphere (CO% is too high)

Countermeasures:

A) Properly control furnace atmosphere: CO%: 2 – 3%

三 Excessive zinc dross accumulation on the surface

form: one is block (slag of oxidized peeling flakes); the other is granular (intermetallic compound), which can be divided into two types-top scum and bottom slag. These slags will cause the surface of the steel plate to be rough (distributed on the entire surface of the steel plate or on both sides of the steel plate). In severe cases, it is easy to form warped edges after winding.

Cause:

- The temperature of the zinc pool changes too much, especially when the zinc ingot is immersed too quickly, it will easily cause the surrounding temperature to change

- AL content is too high, if the Al content is too low, it is easy to generate bottom slag, so the Al content should be controlled within 0.18 – 0.22%

-Oxide peeling due to shutdown defects or other factors

- Defects on the surface of the substrate can easily cause dross adhesion

Countermeasures:

A) Try to keep the zinc pool temperature stable. (Because Fe and Al are easy to dissolve in high temperature, but it is easy to precipitate at low temperature and produce zinc dross)

B) When adding large zinc ingots, slowly dip into the zinc bath

C) Do not remove the scum on the surface of the zinc bath too often, just move it to the side and remove it at regular intervals (such as 1 hour)

D) When adding small zinc ingots, don’t put them in one place, but distribute them evenly

E) Try to remove the oxide scale from the zinc bath to prevent sinking to the bottom or suspended in the zinc bath

F) Properly adjust the air knife and its pressure and angle

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