1. What standards are used to classify the substrate materials of galvanized coils?
The substrate materials of galvanized coils are mainly classified according to international or industry standards, the most common being GB/T 2518 (China), ASTM A653 (USA), and EN 10346 (Europe). The core of the classification is based on the substrate's chemical composition, mechanical properties (such as yield strength and tensile strength), and processing applications, dividing them into categories such as general commercial grade, stamping grade, deep drawing grade, and structural grade. Different categories correspond to different controls on carbon, manganese, phosphorus, and sulfur content, as well as subsequent elongation requirements after galvanizing.
2. What are the main differences between Commercial Quality (CQ) and Drawing Quality (DQ) substrate materials?
A: Commercial Quality (CQ) substrates are suitable for simple bending or flat processing. Their carbon content is typically 0.02%~0.10%, yield strength is approximately 140~210 MPa, and elongation is relatively low. Drawing Quality (DQ) substrates, on the other hand, require better ductility. They use aluminum deoxidized killed steel substrates with an even lower carbon content (usually 0.02%~0.06%) and the addition of small amounts of titanium or niobium to refine the grain. Their yield strength is approximately 120~180 MPa, and elongation is 10%~20% higher than CQ. DQ substrates are suitable for manufacturing parts requiring a certain depth of stamping, such as automotive interior panels and air conditioning housings.
3. What are the special material requirements for deep drawing quality (DDQ) and extra-deep drawing quality (EDDQ) substrates?
A: Deep drawing quality requires substrates with extremely high plastic strain ratios (r-value) and work hardening indices (n-value). It typically uses IF steel (interstitial steel) as the substrate, with a carbon content below 0.005%, and adds titanium or niobium to fix residual carbon and nitrogen atoms, resulting in material with no yield elongation and low anisotropy. Extra-deep drawing quality further reduces impurity elements (sulfur < 0.01%, phosphorus < 0.015%) compared to DDQ, and obtains a more uniform grain structure through special hot rolling temperature control, making it suitable for complex drawn parts such as automotive side panels and door panels.
4. How are the substrate materials of structural-grade galvanized coils classified according to strength grade?
A: Structural-grade substrates do not emphasize stamping performance, but are graded according to yield strength or tensile strength. Common grades include S220GD, S250GD, S350GD, and S550GD (according to EN 10346). "S" indicates structural steel, and "GD" indicates hot-dip galvanizing; the numbers represent the minimum yield strength (in MPa). For example, the S350GD substrate has a high carbon and manganese content (manganese can reach over 1.0%), and adds trace amounts of niobium, vanadium, or titanium for microalloying. Through controlled rolling and cooling, fine-grained strengthening is achieved, resulting in a yield strength of not less than 350 MPa and a tensile strength of 420~500 MPa. It can be used for load-bearing components such as building roofs, partition wall framing, and engineering machinery components.
5. Are there differences in the choice of substrate material between different galvanizing processes (hot-dip galvanizing vs. electro-galvanizing)?
A: Yes, there are differences. Hot-dip galvanizing substrates mostly use low-carbon steel or ultra-low-carbon steel (such as IF steel) because the substrate needs to undergo annealing and high-temperature (approximately 460℃) immersion in a zinc bath. This requires the material to have good heat resistance and adhesion to the zinc layer, and it cannot contain elements that easily cause brittleness in the liquid metal. Electro-galvanizing deposits a zinc layer at room temperature, with lower requirements for rheological properties. Therefore, the range of substrate materials is wider, including high-strength steel (such as duplex steel DP, multiphase steel CP) and even some stainless steels. However, the substrate surface must be extremely clean and free of oxide scale; otherwise, the coating uniformity will be poor. Furthermore, electro-galvanizing substrates often use cold-rolled hard-state plates (unannealed) to maintain the original mechanical properties of high-strength steel.