1.What are the basic principles of correcting the corrugated shape of cold-rolled coils? What are the core strategies?
The basic principle of strip straightening is to lengthen the "short" portions of the strip width or shorten the "long" portions, ultimately making the lengths of all parts more uniform and eliminating internal stress. Based on this principle, there are three main strategies:
Strategy 1: Roll Gap Control (Correction During Rolling): During the rolling process, the roll gap shape is changed by adjusting the degree of bending (bent rolls), the position of cross-sectional movement (CVC), or the inclination of the rolls. This allows the roll gap shape to precisely match the profile of the incoming material, thereby controlling the uniformity of the stretching from the source.
Strategy 2: Subsequent Leveling/Rework (Small Deformation Straightening): For rolled steel coils, a leveling mill is used to apply small deformation rolling. By adjusting the rolling force and bending roll force, minor shape defects are improved.
Strategy 3: Stretch bending straightening (straightening with large deformation): For steel coils with severe shape defects, a special stretch bending straightening machine (stretch straightening machine) is used. Under the combined action of tension and repeated bending, the entire length of the strip steel is plastically extended, forcibly stretching the "short" part, thereby obtaining extremely high straightness.
2.How to correct different types of waves online during the cold rolling process?
For single-sided edge waves (linear wavy patterns): Correction is primarily achieved by tilting the rolls. This involves slightly increasing the pressure at one end of the roll and decreasing it at the other to compensate for the difference in elongation on both sides of the strip.
For double-sided waves or center waves (secondary wavy patterns): Correction is primarily achieved through bending rolls and CVC (continuously variable crown) roll movement. Double-sided waves indicate excessive elongation in the middle of the strip, requiring a reduction in the positive bending roll force or an increase in the negative bending roll force; center waves, conversely, require an increase in the positive bending roll force. CVC roll movement continuously alters the roll crown to accommodate different rolling requirements.
For quarter-waves or composite waves (higher-order wavy patterns): These patterns are more complex and difficult to completely eliminate using conventional methods, necessitating the use of segmented roll cooling. By locally spraying coolant onto the rolls, the thermal expansion of that area is altered, thereby fine-tuning the roll gap shape and achieving precise control over the local strip shape.
3.Can a steel coil with existing wavy defects be repaired using a leveling machine? What would be the effect?
Yes, but it's mainly suitable for slight, uniform waviness. Handan Iron and Steel Group (Handan Steel) has had successful rework experience: they used a cold rolling leveling mill to rework pickled corrugated plates.
Key control parameters: The core of leveling rework is controlling elongation and rolling force. Using a constant rolling force mode combined with wet leveling can effectively improve the plate shape. Handan Steel's experience shows that by optimizing the process, the first-grade yield of reworked pickled corrugated coils gradually increased from 20% to 50%, significantly reducing downgraded products caused by waviness.
Limitations: The reduction in leveling is very small (usually <3%), mainly improving mechanical properties and minor plate shape defects. For severe, complex waviness (such as warping or oblique waviness), leveling is difficult to completely eliminate.
4.What is the most effective correction method for severe or complex wave defects?
The most effective method is stretch bending straightening (stretch straightening). It can eliminate not only common edge and center waviness, but also three-dimensional shape defects such as flaring and sickle bends.
Working Principle: The stretch straightening machine consists of multiple sets of bending and straightening rollers, applying a large tension (approximately 1/3 to 1/2 of the material's yield strength) to the strip. Under this tension, the strip repeatedly bends, redistributing its internal stress and stretching the previously insufficient length to match the longer portion, thus achieving extremely high straightness.
Stunning Results: After installing and using the stretch straightening machine, Taigang's plate straightness improved from 15 I-units to 4 I-units, significantly improving plate shape quality.
Special Technology: For thick plates or specific parts where high tension cannot be used, there are more precise straightening machines. For example, the straightening machine from the German company KELER has a straightening roller bending function, allowing the straightening rollers to bulge in the middle or on both sides, specifically stretching specific areas of the material to precisely eliminate edge or center waviness.
5.As a downstream user, what remedial measures can be taken if the cold-rolled coils you purchased are wavy?
Slight Wavy Patterns – Adjust Stamping Process: Slight wavy patterns can sometimes be "flattened" during the forming process by adding draw beads and adjusting blank holder force.
Moderate Wavy Patterns – Use a Precision Leveling Machine: Add a precision leveling (flattening) process before uncoiling or cutting. This is the most direct and effective method. By adjusting the insertion and reduction of the leveling machine, most wavy deformation can be effectively eliminated, providing a flat sheet for subsequent stamping.
Severe Wavy Patterns – Return or Claim: If the wavy pattern is very severe (such as producing difficult-to-eliminate "dead creases" or severe warping), ordinary leveling cannot solve the problem, meaning the material is scrapped. In this case, a quality objection should be raised with the supplier for a return or claim.