1.What is the definition of flatness and what are the common types of problems?
Edge wave: A wavy pattern at the edge of the strip.
Center wave: A wavy pattern in the center of the strip.
Bow: A small, bulging-like warp that occurs in the center or edge of the strip.
Camber: A bend in the length of the strip that curves to one side.
2.How is waviness/straightness measured?
Definition: The ratio of the vertical distance between the highest and lowest points of a wave (wave height) within any given measurement range (e.g., 2000mm or 1000mm) to the measured length (wave distance), usually expressed as a percentage or millimeters per meter.
Measurement Method:
Professional Instrument: Continuous measurement using an online laser straightness meter or flatness gauge.
Simple On-site Measurement: Lay the coil sample flat on a platform. Use a long ruler or taut string against the surface. Use a feeler gauge to measure the gap between the highest point of the wave and the ruler (wave height). The difference is then divided by the measured length (wave distance) for calculation.
3.How to detect sickle curve?
Definition: The maximum deviation between the side of a steel strip and a straight line after it has been stretched flat under tension.
Measurement Method: Align a long rope or ruler with a straight line on one side of the strip and measure the maximum distance between the strip edge and the line.
4.What are the factors that affect flatness?
Cold-rolled substrate shape: This is the most fundamental factor. If the substrate shape after cold rolling is poor, it will be difficult to correct after galvanizing.
Galvanizing Line Process:
Annealing Process: The annealing temperature and uniformity affect the internal stress of the material.
Air Knife Control: Controls the uniformity of the zinc layer thickness.
5.What are the flatness selection and recommendations for galvanized coils?
When placing an order, be sure to clearly specify the flatness requirements in the technical agreement, for example, "Waviness no greater than 0.8% (over a 2000mm measuring length)." This serves as the basis for subsequent quality inspections and dispute resolution.
For high-end applications (such as appliance exterior panels and automotive exterior panels), select stamping-grade or higher materials, and specify strict flatness specifications.
A simple method for determining quality: Upon receipt, cut a sample and lay it flat on a flat surface. Observe the waviness at the edges and center to assess the quality.
Tension Control: Maintains the stability and uniformity of tension on the production line.
Leveler: This is a key piece of equipment for improving plate shape, eliminating internal stress and improving flatness through stretching and bending.
Cooling Uniformity: Uneven cooling after galvanizing can lead to uneven shrinkage in different parts of the material, resulting in warping.