How to allocate the extension coefficients of each slot of Single Stand Multi-Groove Reducing Rolling Mill reasonably

        All grooves of a Single Stand Multi-Groove Reducing Rolling Mill share the same roll set, stand and main drive. Multiple tubes are rolled simultaneously. Unlike multi-stand continuous sizing mills, it has no independent stand speed adjustment to compensate flow difference between grooves. Therefore, the core principle of elongation distribution is to keep consistent metal flow rate for every groove. Unbalanced elongation distribution will lead to tube breakage, pile-up, scratching and dimensional out of tolerance.

Basic allocation principle

       Equal volumetric flow rate is the primary rule. At the same roll speed, the product of incoming cross-sectional area and elongation coefficient shall be close for every groove.

       The elongation coefficient of each groove should not differ too much. Limited by stand rigidity, roll elastic deformation and uneven groove wear, the difference between adjacent grooves shall be kept small. Grooves with difficult deformation get lower elongation, while grooves under favorable conditions can take slightly higher elongation.

       Elongation of individual groove cannot exceed the forming limit of the material. For stainless steel and alloy steel with high deformation resistance, single-pass elongation must be reduced. Carbon steel tubes can adopt relatively higher values. The spread of the pass profile should also be considered to avoid fins and folds.

Factors that need to be considered when allocating

       1. Groove position: Grooves at the center and both sides of the stand have different load and cooling conditions. Side grooves are prone to unbalanced load, so their elongation should not be set too high.

       2. Incoming material tolerance: If the incoming tube for one groove has poor wall thickness or ovality, its elongation coefficient should be reduced to avoid buckling and internal folds.

       3. Roll wear tendency: Outer grooves usually wear faster. Allowance shall be reserved in pass design. Roll gap adjustment is preferred for compensation instead of large change of elongation distribution.

       4. Equipment load: Rolling load from all grooves acts on the same stand and spindle. Total torque must be checked besides single groove load to prevent motor overload and stand deflection.

Typical defects caused by unreasonable allocation

       Excessive elongation on one groove: The tube is over-stretched, prone to necking, breakage and thin wall.

       Insufficient elongation on one groove: Slow metal flow causes pile-up, tube scratching, roll marking and oversized outer diameter.

       Large deviation among grooves: Mutual tension interference between tubes, poor dimensional consistency and unstable batch quality.

On site adjustment of ideas

       Elongation is preliminarily allocated during pass design. After trial rolling, outer diameter, wall thickness, surface condition and motor current of each groove are recorded. For grooves with high current and frequent breakage, reduce elongation. For grooves with low current and oversized tube, slightly increase elongation or adjust roll gap.

       Fine tuning of roll gap is preferred. The original elongation distribution scheme of pass design should be kept as far as possible.