1. What is cold-rolled steel? What is the core difference between it and hot-rolled steel?
Cold-rolled steel refers to steel produced by rolling hot-rolled steel at room temperature (without heating), pickling, cold rolling, annealing, and tempering. Its thickness typically ranges from 0.1 to 3 mm (primarily thin gauges).
2. What do the material codes for cold-rolled steel (such as SPCC and DC01) mean? Different countries/standards have different designations. The most common ones are the Japanese JIS and EU EN standards:
Japanese JIS: For example, SPCC (S - Steel, P - Plate, C - Cold Rolled, C - Common Grade). SPCC stands for "General Cold-Rolled Carbon Steel Plate." There are also SPCD (Stamping Grade, with improved plasticity) and SPCE (Deep Drawing Grade, suitable for complex forming).
EU EN: For example, DC01 (D - Cold Rolled, C - Commercial Grade, 01 - Common Grade), DC03 (Stamping Grade), and DC06 (Deep Drawing Grade), which correspond to ST12, ST14, and ST16 in the Chinese GB standard.
Core logic: The further the letters/numbers after the designation, the better the plasticity and stamping performance (suitable for more complex processing).
3. What is the purpose of the "annealing" process for cold-rolled steel? During the cold rolling process, steel sheets undergo "work hardening" (increased strength, decreased plasticity, and increased brittle cracking). Annealing is a key process for adjusting these properties:
Function 1: Eliminates work hardening, reduces hardness, and restores plasticity (facilitating subsequent forming processes such as stamping and bending);
Function 2: Refines grain size, creates a uniform microstructure, and improves the stability of the steel sheet's performance (avoiding localized cracking during forming);
Function 3: Eliminates internal stress (prevents deformation during storage/processing of the cold-rolled sheet).
Common annealing methods include: hood annealing (batch processing, low cost) and continuous annealing (high efficiency, suitable for high-demand products).
4. What are the types of "coatings" used on cold-rolled sheet? Why is coating necessary? Cold-rolled steel inherently has poor corrosion resistance (prone to rusting). Coatings are applied to improve corrosion resistance and extend service life. Common coating types include:
Galvanized steel (the most popular): such as SGCC (hot-dip galvanized cold-rolled steel) and SECC (electrogalvanized cold-rolled steel). The zinc layer isolates air and moisture and provides sacrificial anode protection (the zinc corrodes first, protecting the steel).
Alu-zinc coating: such as AZ150 (aluminum-zinc alloy coating, 55% aluminum, 43.5% zinc). Its corrosion resistance is 3-5 times that of standard galvanizing (suitable for outdoor applications, such as pre-painted steel sheets).
Tinplate coating (tinplate): such as SPTE offers a bright, non-toxic surface, making it suitable for food packaging (such as cans and beverage cans).
Chrome coating: such as chrome-plated cold-rolled steel offers a high surface hardness and wear resistance, making it suitable for decorative components (such as appliance panels).
5. What is the purpose of the "flattening" process for cold-rolled steel? Leveling is the final process in cold-rolled steel production (after annealing). Its functions include:
Correction: Eliminates the "wavy" shape (ridges on the edges and center of the steel plate) after annealing, ensuring a flat surface (to prevent uneven stress during subsequent processing);
Adjusting surface roughness: Controls the surface gloss of the cold-rolled steel (e.g., matte or semi-matte, to suit different appearance requirements) by adjusting the roughness of the leveling rollers;
Slight strengthening: Slightly increases strength through rolling with a small reduction (1%-3%) (to prevent excessive softening).
6. What is the complete production process for cold-rolled steel? Core Process: Hot-rolled raw material → Pickling → Cold rolling → Annealing → Tempering → Finishing (Shearing/Splitting) → Inspection → Finished Product; Step-by-Step Analysis:
Hot-rolled raw material: 3-6mm thick "hot-rolled coil" (the base material for cold rolling, which requires descaling);
Pickling: Hydrochloric acid/sulfuric acid solution is used to remove the surface oxide scale (Fe₂O₃) on the hot-rolled plate to prevent it from being pressed into the steel surface during cold rolling and affecting quality;
Cold rolling: Continuous rolling (reduction up to 70%) is performed at room temperature using multiple cold rolling mills to reduce the thickness to the target specification (e.g., 0.5mm);
Annealing: The hardened cold-rolled steel plate is placed in an annealing furnace and held at 700-800°C (depending on the material) to eliminate work hardening;
Tempering: The annealed steel plate is rolled using a tempering mill with a small reduction to correct the shape and adjust the roughness;
Finishing: Shearing into cut-to-length steel plates as needed (e.g., 1220×2440mm) or rolled into cold-rolled coils, which are then inspected for surface, dimensions, and performance. Acceptable results are then placed in storage.
7. What are common problems with pickling during cold-rolled sheet production? How can they be avoided? Common Problems and Solutions:
Problem 1: Incomplete pickling (residual oxide scale, black spots on the surface after cold rolling) → Solution: Adjust the acid concentration (18%-22% hydrochloric acid), extend the pickling time, and increase the acid temperature (40-60°C);
Problem 2: Over-pickling (excessive corrosion on the steel plate surface, resulting in pitting) → Solution: Control the pickling time (adjust according to the steel plate thickness) and promptly neutralize the acid (rinse with alkaline solution after pickling);
Problem 3: Acid mist contamination → Solution: Install an acid mist collection device (such as a spray tower) and use an acid mist suppressant.