Analysis Of The Causes Of Surface Blackening In The Stamping Process Of Galvanized Plate

Under the same conditions, the atmospheric corrosion resistance of galvanized sheet is 2~6 times higher than that of galvanized sheet. The coating structure of galvanized sheet is composed of aluminum-rich phase and zinc-rich phase, of which the volume ratio of aluminum-rich phase accounts for about 80%. , the zinc-rich phase accounts for about 20%. This coating structure perfectly combines the isolation protection of aluminum with the durability and the electrochemical protection of zinc, which greatly improves the corrosion resistance of the galvanized sheet. The surface of the galvanized sheet is bright and reflects heat and light. The capacity of the galvanized sheet is twice that of the hot-dip galvanized sheet; the spangle structure has a strong three-dimensional effect, so that the galvanized sheet can be used directly as a bare plate; discoloration. These advantages make galvanized sheet more and more recognized and widely used by domestic end users.

In the past ten years, the domestic aluminized zinc production line has gradually increased, from 2004 to 14.15% in 2015. Although the demand for galvanized sheet continues to increase, there are still problems such as high bath temperature, easy slag formation, and unstable surface quality in the production process, and the introduction of galvanized sheet into China takes time compared to hot-dip galvanizing. Lately, the exchange of its process technology is not sufficient, so the quality of domestic galvanized sheet is generally not high, and the surface blackening phenomenon occurs during the stamping process, which affects the beauty of stamping parts. This paper studies this.

1 existing problems

In 2008, the cold-rolling plant of Anshan Iron and Steel Co., Ltd. underwent self-integrated transformation and became capable of producing galvanized sheets. Through nearly ten years of technical accumulation, the product quality has been continuously improved, and the output has increased from 40,000 t/a to 80,000 t/a. Although the application field is expanding continuously, its application technology research still needs to be improved and accumulated.

The air-conditioning industry is an important potential application area for the future development of Ansteel’s galvanized products. A well-known domestic air-conditioning company and Ansteel have jointly developed a project to replace hot-dip galvanizing with galvanized aluminum. The study found that the surface of Ansteel’s galvanized sheet will appear black during stamping, as shown in Figure 1. Area 1 is the blackened area after relative friction with the mold, and area 2 is the area under pressure from the mold but without relative sliding.

2 Analysis of the causes of blackening

2.1 Macroscopic topography analysis

Wipe the blackened parts with alcohol, and the macroscopic appearance of the blackened samples is shown in Figure 2. With the increase of wiping times, the color of the surface of the sample gradually becomes lighter, indicating that the black substance can be partially wiped off. After repeated wiping, the scratches on the surface of the sample along the moving direction of the mold are clearly visible, and the zinc spangles on the coating are not obvious. During the stamping process, this part is subjected to large force and the mold is seriously polluted. Therefore, it is inferred that the pollutants generated during the stamping process may come from the mold, or may be caused by the anti-fingerprint film or aluminum-zinc powder friction accumulation on the surface of the sample.

2.2 Microscopic topography analysis

Scanning electron microscope has the characteristics of deep projection depth and strong energy spectrum penetration. Using scanning electron microscope, combined with X-ray photoelectron spectrometer (XPS), to measure the composition of the nano-layer range, the blackened parts are divided into blackened parts and unmarked parts. Change part of the two regions, compare the surface state and element valence state of the two regions, analyze the changes of Al and Zn elements, and find out the root cause of stamping black.

2.2.1 SEM and energy spectrum analysis

In order to further analyze the composition of black matter, take the area 1 and area 2 in the stamping part as the analysis object, and divide the area 1 into 1# and 2# according to the stamping process, as shown in Figure 3. Mark 5 positions for parts 1# and 2# to observe the blackened area, and see Figure 4 for the location of the map. In the scanned image in Fig. 4, 5 positions were taken for energy spectrum analysis.

Scanning electron microscope was used to analyze the two areas. After stamping, the aluminum-zinc coating was squeezed and deformed by the die, and the obvious dendrite structure of the coating could not be observed. The first half of stamping was 1#

Part of the surface morphology is metal oxide, and a large number of black strips with a length of about 50 μm are distributed perpendicular to the stamping direction.Part of the surface of 2# in the second half is also gold

It is an oxide, in addition to the black strips, there are also a large number of white strips or granular substances with a length of about 200 μm.

The analysis of areas 1 and 2 by X-ray photoelectron spectrometer increases as the friction increases, the mold and the aluminum-zinc coating are gnawed and scratched, and the surface of the aluminum-zinc coating has plough marks, which are deep and deep.

The surface morphology of area 2 shows that it has a complete fingerprint-resistant film covering the coating surface, that is, the normal fingerprint-resistant aluminum-zinc coating surface composition. The microstructures of region 1 and region 2 are shown in Figure 5.

2.2.2 XPS sample analysis

Further analysis by XPS, taking advantage of its measurement of nanolayer range composition, yields results for region 1 as shown in Figure 6. Among them, it can be seen from Figure 6(a) that the oxygen content distribution in region 1 is significantly increased, and oxygen is mainly combined with aluminum and zinc to form oxides. Figure 6 shows that the surface carbon content of the sample in area 1 is low, indicating that the anti-fingerprint film basically falls off during the stamping process. It can be seen from Figure 6(b) that the binding energy spectrum of aluminum on the surface of the aluminum-zinc layer is the mixed energy spectrum of aluminum oxide and metal aluminum, in which the proportion of aluminum oxide is relatively large, indicating that the surface of the sample is not lubricated by the fingerprint-resistant film. During the stamping process, frictional heat can oxidize aluminum and zinc.

The surface XPS analysis of area 2 shows that the distribution of each component content in the depth direction is shown in Figure 7(a). The surface layer has high C and O content, which is obviously a fingerprint-resistant film component. In addition to C and O, there is Fe and no Zn. , Al element.

However, there is no Fe in the SEM analysis, indicating that the coverage of Fe is very thin, and the scanning electron microscope detection has a certain penetration depth. The full scan spectrum (etching time of 20.0 s) shows that a thin Fe element layer is detected on the surface of the normal fingerprint-resistant film, which indicates that during the stamping process of the die, there is Fe powder contamination on the surface of the die. After etching in the normal area for 870 s ( About 120 ns) and then the surface scan, the Fe element disappears, as shown in Figure 7(c). When the steel plate undergoes plastic flow in the mold, Fe powder is easy to destroy the fingerprint-resistant film on the surface.

3 Conclusion

  • The blackening of galvanized steel sheet stamping is mainly due to the damage and peeling of the fingerprint-resistant film on the surface of the coated steel sheet during the stamping process, and the frictional heat of the die to the steel sheet causes the aluminum-zinc coating to peel off and oxidize with Al and Zn on the surface of the coating to become black.
  • When in contact with the die, there are obvious Fe elements on the surface of the non-stamping area, indicating that the Fe powder contamination on the die surface adheres to the surface of the steel plate during the stamping process, and the Fe contamination on the die surface will play a certain role in destroying the surface of the steel plate in the stamping area.
  • In the stamping process, the galvanized steel sheet has plough marks and scratches along the direction of movement of the die. Under this stamping process condition, the wear resistance of the fingerprint-resistant film should be improved, especially under the condition of frictional heat, the fingerprint-resistant film should be lifted. To lubricate, resist the damaging effect of friction. By adding substances such as polytetrafluoro wax into the anti-fingerprint plating solution, the lubricating performance and wear resistance of the anti-fingerprint film can be effectively improved.

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