1. How does the "reduction" of the cold rolling mill affect the performance of cold-rolled steel?
Reduction (the percentage by which the steel plate thickness is reduced during cold rolling, e.g., rolling from 3mm to 1.5mm is a 50% reduction) is a key parameter:
The greater the reduction, the more severe the work hardening, resulting in higher strength and hardness, but also lower plasticity and stamping properties (prone to cracking), making it suitable for applications requiring high strength (such as structural parts);
The smaller the reduction, the lower the strength and hardness, but better plasticity, but lower production efficiency (requiring multiple rolling passes), making it suitable for subsequent deep drawing (such as automotive panels).
In actual production, adjustments are made based on product requirements. For example, deep-drawing grade DC06 requires a controlled reduction (≤30% per pass) and multiple annealing cycles; standard grade DC01 can use a larger reduction (50% per pass).
2. Why does "sticking" occur during annealing of cold-rolled steel? How can it be prevented? Sticking: After annealing, multiple steel sheets stick together (making them impossible to separate, resulting in scrap). This is caused by a thin oxide film on the steel sheet surface during annealing, which allows metal atoms to diffuse into each other at high temperatures. Preventative measures:
Apply an anti-sticking agent (such as boric acid solution) to the steel sheet surface before annealing;
Control the annealing atmosphere (use nitrogen and hydrogen to prevent air from entering and causing oxidation);
Adjust the annealing temperature and holding time (avoid excessive temperatures or prolonged holding times);
Use a "stack spacer" (place spacers between the steel sheets).
3. Why does "sickling" occur during the unwinding of cold-rolled coils? How can it be corrected? Camber: After the cold-rolled coil is unwound, the edge of the steel plate forms a curved shape (like a sickle). This is caused by uneven reduction on both sides of the steel plate during cold rolling (one side is rolled thinner), resulting in unbalanced stress. Correction methods:
Production side: Adjust the reduction on both sides of the cold rolling mill (using a thickness gauge to monitor in real time to ensure consistent thickness on both sides).
User side: If the camber is minor, it can be corrected using a "straightener" (multi-roll straightening); if it is severe, the edge must be trimmed (losing some material).
4. What do the "yield strength" and "tensile strength" of cold-rolled steel represent? How do they affect their applications?
Both are core indicators for measuring the mechanical properties of cold-rolled steel (unit: MPa):
Yield strength (σs): The minimum stress at which plastic deformation begins (e.g., DC01 yield strength ≥ 140 MPa).
Impact: Low yield strength → Good plasticity and easy stamping (suitable for automotive parts and appliance housings); High yield strength → Strong deformation resistance and suitable for structural parts (such as brackets and frames). Tensile strength (σb): The maximum stress a steel plate can withstand before breaking (e.g., DC01 tensile strength is 270-410 MPa).
Impact: High tensile strength → strong load-bearing capacity, preventing breakage during use (e.g., in mechanical parts); however, excessively high tensile strength can lead to excessive "springback" during forming (e.g., prone to returning to its original shape after bending, requiring process adjustments).
5.
What does "elongation" mean for cold-rolled steel? Why does deep-drawn parts require high elongation? Elongation (δ): After a steel plate breaks, the percentage of elongation at the fracture to its original length (e.g., DC06 elongation ≥41%) is a key indicator of "plasticity." Deep-drawn parts (such as car doors and washing machine drums) require high elongation because the deep-drawing process requires the steel plate to undergo "large-scale, uniform plastic deformation" (e.g., from a flat surface to a curved surface). If the elongation is low (poor plasticity), localized cracking may occur due to excessive deformation. High elongation ensures uniform deformation and avoids cracking. Therefore, the elongation requirement for deep-drawing cold-rolled steel (e.g., DC06 and SPCE) is much higher than that for standard grades (e.g., DC01 elongation ≥28%).
6. What metric is used to measure the "surface roughness" of cold-rolled steel? What applications are different roughness levels suitable for? Commonly used indicators: Ra (arithmetic mean deviation), measured in μm (micrometers). The smaller the value, the smoother the surface. Applications for different roughness levels:
Ra 0.4-0.8μm (smooth surface): Suitable for applications requiring spraying and electroplating (such as home appliance panels and automotive exteriors), with improved coating adhesion and a more refined appearance.
Ra 1.6-3.2μm (medium roughness): Suitable for general structural parts (such as brackets and chassis) that do not require a high-precision appearance and facilitate subsequent soldering (roughness improves solder adhesion).
Ra 6.3-12.5μm (rough surface): Suitable for applications requiring anti-slip properties (such as pedals). However, this roughness is rarely achieved on cold-rolled steel (special flattening rollers are required).
7. How important is the "thickness deviation" of cold-rolled steel? What are the tolerance requirements for different industries? Thickness deviation (the difference between actual and nominal thickness, e.g., nominal 0.5mm, actual 0.49-0.51mm) directly impacts machining accuracy and cost.
Importance: Excessive deviation → uneven force during subsequent stamping (resulting in out-of-tolerance part dimensions) and inconsistent weld penetration during welding (affecting strength); insufficient deviation → increased production costs (requiring a higher-precision rolling mill).
Industry requirements:
Ordinary home appliances (e.g., chassis): Tolerance of ±0.03mm;
Automotive deep-drawn parts (e.g., doors): Tolerance of ±0.01mm (requiring a high-precision cold rolling mill);
Ordinary structural parts: Tolerance of ±0.05mm