There are many kinds of hot-dip coatings for steel plates and steel strips, and the classification rules in major standards such as the United States, Japan, Europe, and national standards are also similar. We use the European standard EN 10346:2015 as an example to analyze.
Mainstream hot-dip coatings are divided into six categories, namely:
1. Hot-dip pure zinc (Z);
2. Hot-dip galvanized iron alloy (ZF);
3. Hot-dip galvanized aluminum (ZA);
4. Hot-dip aluminum zinc (AZ);
5. Hot-dip aluminum silicon (AS);
6. Hot-dip galvanized magnesium (ZM);
2. Definition and characteristics of various hot-dip coatings
The pretreated steel strip is immersed in the molten pool. Usually different molten metals in the molten pool will obtain different coatings (except for zinc-iron alloy coatings).
1. Hot-dip pure zinc coating (Z)
The zinc content in the molten pool metal liquid should not be less than 99%.
The hot-dip pure zinc process can produce zinc flowers of different sizes. The zinc flowers affect the surface sensory, but do not affect the corrosion resistance.
How is the galvanizing thickness calculated? ., for example, the zinc thickness on one side is 50 μm (microns), that is, the galvanizing weight on one side is 50×7.14=357 g/m^2.
Under normal circumstances, the galvanizing weight per square meter refers to the total weight of double-sided galvanizing.
Excerpt from Article 7.7.2 of GB/T 2518-2008 "Continuous Hot-dip Galvanized Steel Sheets and Steel Strips": The recommended nominal coating weight and corresponding coating code should comply with the provisions of Table 14. After negotiation between the supply and demand parties, the nominal coating weight of equal thickness can also be expressed by the weight of single-sided coating.
For example: hot-dip galvanized coating Z 250 can be expressed as Z 125/125, and hot-dip galvanized iron alloy coating ZF 180 can be expressed as ZF 90/90.
For example, the C-type or Z-type purlins we often talk about are double-sided hot-dip galvanized at 275 g/m^2. The actual single-sided galvanized thickness is 275/2/7.14=19.3 μm (microns), which is 120~200 μm (micron) compared with ordinary coatings. Compared with the paint film thickness of μm, there is a huge difference.
2. Hot-dip galvanized iron alloy coating (ZF)
The main difference from hot-dip pure zinc coating is that an alloying process is subsequently added to diffuse iron in the zinc to form a zinc-iron alloy layer. The iron content in the alloy coating is usually 8~12%.
The welding performance of the zinc-iron alloy coating is better. The color of the coating is matte off-white, which appears dark gray after oiling. The zinc-iron alloy coating has no splatter.
The relationship between the thickness and weight of hot-dip pure zinc and zinc-iron alloy coatings is basically the same: 1μm≈7.1g/m2.
Note 1: Although the requirements for zinc content in the zinc pot are the same for hot-dip pure zinc and zinc-iron alloy coatings, the coatings have different usage characteristics and different corrosion resistance.
3. Hot-dip zinc-aluminum coating (ZA) hot-dip zinc-aluminum coating
The metallic liquid in the molten pool contains about 5% aluminum (Al) and a small amount of rare earth elements, with the remainder being zinc.
The surface of zinc-aluminum coating is brighter than that of pure zinc coating, and its corrosion resistance is stronger than that of pure zinc coating. However, it is rarely used in China and is occasionally mentioned in foreign trade inquiries.
The relationship between coating thickness and weight: 1μm≈6.6g/m2.
4. Hot-dip aluminum-zinc coating (AZ) hot-dip aluminum-zinc coating
The liquid metal in the molten pool contains about 55% aluminum (Al) and about 1.6% silicon, with the remainder being zinc.
Aluminum-zinc coating has excellent corrosion resistance and also has certain high temperature resistance.
The relationship between coating thickness and weight: coating thickness 1μm≈3.8g/m2.
Note: Hot-dip galvanized aluminum (ZA) and hot-dip galvanized aluminum (AZ) are two completely different coatings. Although there are fewer hot-dip galvanized aluminum products in domestic trade, they are still mentioned in foreign trade inquiries. Need to confirm carefully.
5. Hot-dip aluminum-silicon alloy coating (AS) hot-dip aluminum-silicon alloy coating
The metal liquid in the molten pool contains about 8% to 11% silicon (Si), and the remaining component is aluminum.
The aluminum-silicon coating will form an obvious aluminum-iron-silicon alloy layer on the surface of the steel plate. It is characterized by excellent high temperature resistance and can be used in some high-temperature processing and high-temperature working environments.
The relationship between coating thickness and weight: coating thickness 1μm≈3g/m2.
Note 3: Hot-dip aluminum-silicon coated steel plates and steel strips are a type of "aluminized plate" mentioned in trade. In the European standard, pure aluminum plating requires a separate agreement to supply, while in the JIS standard, it is classified according to different applications such as corrosion resistance and high temperature resistance.
6. Hot-dip Zinc-magnesium alloy coating (ZM) hot-dip Zinc-magnesium alloy coating
Hot-dip galvanized magnesium coating is also called zinc aluminum magnesium (ZAM). The composition of the metal liquid in the molten pool is the sum of aluminum and magnesium, usually 1.5~8% (must contain at least 0.2% magnesium), and the remainder is zinc.
The hot-dip galvanized aluminum-magnesium coating is characterized by extremely excellent comprehensive corrosion resistance, especially the protection of the shear fracture of the steel plate.
This kind of coating is quite special. Each manufacturer has its own formula, so there is a large room for adjustment in the content of aluminum and magnesium in the standard. There is no fixed value for the conversion relationship between coating thickness and weight, which is about 1um≈6.2-6.6g/ m2.