Going for safety: Solutions for the reliable securing of hot coils

The demand for high-strength hot strip grades for use in manifold industrial applications is on the rise. To process these kinds of steel, specific technical requirements in terms of rolling and processing have to be met. This includes safe strip coiling at the hot strip coiler as well as safe coil transport to the downstream stations.

Depending on the yield strength and the strip thickness, a coil has residual stresses that may cause it to spring open. Moreover, an instable coil may roll off the coil transfer car. To make sure the rolled, high-strength coils can safely be handled, SMS group offers custom-tailored technical solutions which are easy to integrate into hot strip mills, including existing and older ones, and which offer maximum safety for both the process and the operators involved.

As a rule, a distinction is to be drawn between two major requirements: on the one hand it is temporary securing of the steel coils during removal from the coiler, and on the other permanent self-securing of the coil due to targeted plastic deformation of the final wraps in the hot strip coiler combined with a significant reduction in the remaining bending moment. The aim must be to safely transport the coil to the strapping machine to further secure the coil.

Before winding, the strip head is mechanically pre-bent to a smaller diameter by a three-roller bending device. In the coiled strip, the bending moment is reduced to make the outer wrap rest against the coil without stress or spring effect. In addition to the mechanical influence on the strip, there may also be a thermal influence.

For this purpose, the coiler is equipped with additional cooling devices that are activated for special material grades. One-sided cooling of the strip generates a temperature stratification. The strip is then coiled with this temperature profile. Once coiling has been completed, temperature equalization takes place. Due to the thermal expansion in the coiler, the strip suffers local deformation. This expansion may counteract the strip spring effect and even completely eliminate it.

Stabilization on the coil car

In order to stabilize coils on the coil stripper car, a hydraulically operated pivoting arm can be installed in the lifting table. This arm is positioned at the entry-end contact area of the coil, thus providing additional support and a secure base for the coil to be taken away. One advantage of this solution is that the coiler design does not require any major adjustments.

Coil clamping from above

With this coil stripper car design, the supporting area is not enlarged. Instead, the coil is clamped at three points around the circumference to prevent the coil from springing open. For that purpose, a hydraulically operated hold-down device is fitted at the operator side of the coil stripper car for clamp-ing the coil from above.

Clamping in the coil eye

The system of clamping in the coil eye is a component of the discharge system for thick strip and high-strength materials. It generally consists of a coil stripper car with pivoting arm, a hold-down device, and an additional holding arm in the coil eye. Depending on the customer’s specification, a movable coil deposit in the area of the coil strapping machine may also be required.

The holding arm gripping the coil’s eye is usually attached to a coil car positioned downstream of the coil stripper car in front of the coiler. The holding arm can be of fixed or movable design to be used in an existing coiler with unfavorable structural conditions. This holding arm in the coil eye ensures that the unstrapped coil is always clamped at a minimum of three points from finish-winding right through to strapping.

Due to the close spacing of the coil rollers (supporting rollers) on the standard coil stripper car, it is necessary for critical coils to implement the above measures either fully or in part.

To avoid the use of this additional equipment, it is also possible to increase the supporting width for the coil leaving the coiler by means of a pallet directly conveying the coil out of the coiler without the need for any intermediate equipment. For new plants and major modification projects in particular, this solution offers distinctly more safety than the conventional stripper car method and can be implemented at moderate investment cost.