Galvanized sheet knowledge mini-class
Galvanized steel sheet is a welded steel sheet with a zinc layer formed on its surface through hot-dip galvanizing or electroplating processes. It is divided into electro-galvanized steel sheet (SECC), hot-dip galvanized steel sheet (SGCC), and aluminum-zinc alloy-coated steel sheet (SGLD). This material is mainly used in the fields of construction, home appliances, vehicle and ship manufacturing, and electromechanical equipment. The galvanized layer can improve the corrosion resistance of steel, and its thickness and zinc flake morphology directly affect the performance in use [2]. The product must meet quality inspection standards such as zinc coating weight (measured in grams per square meter), flatness, and camber control.
classification
Galvanized steel sheets are divided into ordinary electrolytic sheets and fingerprint-resistant electrolytic sheets. The fingerprint-resistant sheets are processed with an additional fingerprint-resistant treatment on the basis of ordinary electrolytic sheets, which can resist sweat. They are generally used on parts that do not require any other treatment, and the grade is SECC-N. Ordinary electrolytic sheets are further divided into phosphated sheets and passivated sheets. Phosphating is more commonly used, and the grade is SECC-P, commonly known as P material. Passivated sheets are further divided into oiled and unoiled types.
Example:
Hot-dip galvanized steel sheets (SGCC) have an advantage over electro-galvanized steel sheets (SECC). SECC is prone to rusting when bent or fractured, whereas SGCC exhibits significantly better rust resistance under the same conditions. High-quality computer cases typically use SECC or SGCC galvanized steel sheets. The steel sheets used in such cases are shiny and have a metallic luster. The advantage of this type of steel sheet is its good corrosion resistance.
Electrolytic galvanized steel sheet (SECC): uniform gray color, mainly imported, resistant to fingerprints, possesses excellent corrosion resistance, and maintains the processability of cold-rolled steel sheets. Uses: home appliances, computer case enclosures, as well as some door panels and facades.
Hot-dip galvanized steel sheet (SGCC): immersion plating, bright white, small zinc flakes (hard to see), large zinc flakes can clearly see hexagonal flakes. In China, only two factories, China Steel in Taiwan Province and Shengyu Steel Corporation, can produce it. There is no steel factory in Chinese Mainland that can produce high-quality materials, mainly relying on imports.
Main features:
Corrosion resistance, paintability, formability, and spot weldability.
It has a wide range of applications, primarily used for components of small household appliances that require a good appearance. However, compared to SECC, it is more expensive, so many manufacturers switch to SECC to save costs.
Based on zinc: The size of the zinc flakes and the thickness of the zinc coating can indicate the quality of galvanizing, with smaller flakes and thicker coating indicating better quality. Manufacturers can also add fingerprint resistance treatment. Another way to distinguish is through the coating, such as Z12 indicating a total coating weight of 120g/mm on both sides.
For example:
Galvalume steel sheet (SGLD): It is a multiphase alloy material containing aluminum-rich and zinc-rich phases. Due to the characteristics of aluminum and zinc, it exhibits superior performance compared to hot-dip galvanized steel sheet (SGCC). Main characteristics: corrosion resistance, which is significantly higher than that of SGCC; heat resistance; thermal conductivity and thermal reflectivity; formability; weldability. Uses: Used in places where good reflectivity is required, such as reflective plates inside ovens and reflective plates for electric pots. Hot-dip galvanized steel sheet (SGCC) is generally used, while galvalume steel sheet (SGLD) is used for deep drawing, and SGCE is used for ultra-deep drawing.
Quality inspection standard
The quality requirements for high-grade galvanized sheets encompass several aspects, including dimensions, surface quality, zinc coating weight, chemical composition, sheet shape, mechanical properties, and packaging.
1. Packaging
It is divided into two types of packaging: galvanized sheets cut to a fixed length and rolled galvanized sheets. Generally, they are packed in iron sheets lined with moisture-proof paper, and secured with iron straps to prevent the galvanized sheets inside from rubbing against each other.
2. Specification and dimensions
The product specifications (as mentioned below) list the recommended standard thickness, length, and width of the galvanized sheet, as well as their allowable deviations. Additionally, the width and length of the sheet, as well as the width of the coil, can also be determined according to user requirements.
3. Surface
Overview of surface conditions: Due to different treatment methods in the coating and plating process, galvanized sheets exhibit varying surface conditions, such as ordinary spangle, fine spangle, flat spangle, no spangle, and surfaces treated with phosphating. Galvanized sheets cut to specified lengths and galvanized coils must not have defects that affect their use, but coils are allowed to have minor deformities such as welding areas.
4. Zinc coating weight
Galvanized weight scale value: Measuring galvanized weight is a widely adopted method for determining the thickness of the zinc coating on galvanized steel sheets. There are two types: sheets with equal galvanized weight on both sides (i.e., equal-thickness galvanizing) and sheets with unequal galvanized weight on both sides (i.e., differential-thickness galvanizing). The unit of galvanized weight is g/m2.
5. Machine function
(1) Tensile test: Generally speaking, only galvanized sheets for layout, stretching, and deep stretching have tensile performance requirements.
(2) Bending test: It is an important item for evaluating the process performance of thin plates. However, the requirements for various types of galvanized sheets vary across countries. Generally, it is required that after bending the galvanized sheet through 180 degrees, the outer coating should not have any zinc layer detachment, and the base of the sheet should not have any cracks or fractures.
6. Chemical composition
The requirements for the chemical composition of galvanized steel sheets vary across countries worldwide. For instance, Japan has such requirements, while the United States does not. Generally, product inspection is not conducted.
7. Plate shape
There are two objectives in balancing the black and white of plate shape, namely flatness and camber. The maximum allowable values for flatness and camber of the plate are subject to certain specifications.
Development History
In 1937, the United States established its first continuous hot-dip galvanized strip steel production line, and in 1942, the first continuous electro-galvanized strip steel production line was built in the United States. It has been over 70 years since they entered industrial production.
Hot-dip galvanized products are widely used in construction, home appliances, vehicles, containers manufacturing, electromechanical industry, etc., covering almost all aspects of daily life. In recent years, the global demand for galvanized steel sheets has been continuously increasing, and production has also grown rapidly. In major steel-producing countries such as the United States and Japan, the proportion of hot-dip galvanized steel sheets in steel products has reached as high as 13%-15%.
Recently, the automotive industry has been paying increasing attention to the service life of materials, and the use of painted cold-rolled steel sheets has no longer met the requirements for corrosion resistance. In this context, hot-dip galvanized sheets, which exhibit excellent corrosion resistance and are low in cost, have become popular among automotive manufacturers.
The greatest advantages of coated steel plates lie in their excellent corrosion resistance, paintability, decorativeness, and good formability. In recent years, the usage of coated steel plates in automobiles has been on the rise, making them the mainstay of automotive sheet metal. Countries worldwide are researching ways to expand the varieties and specifications of coated steel plates, and to improve the coating process to enhance coating quality, namely corrosion resistance, resistance to powdering and spalling, paintability, and weldability. The formability of coated steel plates remains a crucial aspect of their application, which primarily depends on the properties of the base plate, the coating process and its performance, and the forming process conditions. Achieving good application results requires the integration of comprehensive advanced technologies.
