Field of the Invention

The present invention relates generally to steel hot rolling mills. More specifically the invention relates to coils of hot rolled steel and most specifically to prevention/mitigation of coil collapse of hot rolled steel coils.

Background of the Invention

Hot strip mills are provided with two or sometimes three coilers, each of which in turn coils the hot strip from the mill into a coil and delivers the coil onto a conveyor which carries it away from the mill and allows the hot coil of strip to cool to a temperature at which the coil may be handled without damage. The strip is, of course, coiled on a horizontal mandrel and conventionally the hot coils had been turned 90 degree or "down-ended", as it is called, so as to be conveyed with the hole or the coil eye vertical. This has been done because hot coils set down on a flat surface with the eye horizontal tend to sag and become egg-shaped. The strip when hot does not have enough strength to support itself in this position. Coil sagging occurs during and after hot coiling and is due to a relative displacement of the coil wraps leading to a reduction of the coil inner (coil eye) diameter after the coil is removed from the mandrel. Figure 1 is a schematic depiction of a collapsed coil 1. Specifically, the coil eye 2 can be seen to be oval in shape having an maximum diameter Dmax and a minimum diameter Dmin. The present inventors use the following coil sagging criteria to determine when a coil has sagged beyond acceptable limits: 1 ) Dmin is too small (eg.: For a 760mm I.D. coil, Dmin <710 mm), or; 2) the difference between Dmax and Dmin is too large (for example: >20 - 30 mm).

However, there is a considerable disadvantage in delivering hot coils in the eye vertical position. The successive wraps or turns of hot strip are never coiled with their edges accurately in line and protruding wraps or turns on the lower end of the coil are damaged when the hot coil is down-ended. The coil must, therefore, be trimmed, resulting in a considerable scrap loss.

Recently, to avoid this loss, attention has been directed to means for conveying hot coils with the eye horizontal. Apparatus has been engineered to carry hot coils in V- shaped supporting means. In such apparatus, the coils are supported essentially along two lines of contact, and coil sag is less than it is when the coil is supported only on a bottom line of contact. An optimum angle of the V-shaped conveyor pallets, mathematically determined, has been proposed which minimizes the coil collapse. However, the coil will still sag a certain amount which depends on several factors but mainly tightness and temperature. A tightly wound coil will not sag appreciably if it can be kept tight during transfer. Many coils, however, will loosen up when stripped off the downcoiler mandrel, even when banded as soon as possible. Weight of coil, degree of looseness and temperature will then determine the amount of sagging when the coil rests on its circumferential surface. Continuous rotating or rolling of the coil as it cools can equalize the coil sagging. However, coil rolling is not favored in the industry since it tends to damage the outer turns and also further loosen the coil.

One prior art solution to this problem is disclosed in U.S. Patents 4,271 ,959 and 4,407,407. These patents relate to a walking beam type coil rotating cooling conveyor system. The apparatus works by transferring each coil from station to station along the conveyor in a path normal to the coil axes. Each coil is caused to rotate about its axis through an angle during this transfer, but without rolling, so that the coil is supported on successive regions of its outside wrap as it moves along the conveyor, thus equalizing the tendency to sag and preserving circularity of the coil. The length of the conveyor needed to be long enough to allow the coil to cool to a safe temperature. Unfortunately, this conveyor system is expensive to build and maintain and has not been implemented in steel plants around the world.

There is still needed in the art a simple and inexpensive method/apparatus to avoid or mitigate coil sagging issues related to hot strip mill hot band coils.

Summary of the Invention

The present invention provides for a method for mitigating the effects of coil collapse on hot strip mill coils. The method includes producing a hot rolled steel sheet in a hot strip mill and forming a hot strip coil by coiling the hot rolled steel sheet around a mandrel on a downcoiler. The downcoiler orienting the hot strip coil such that the coil eye axis thereof is in the horizontal direction. The step of producing a hot rolled steel sheet in a hot strip mill may include the steps of providing a steel slab, reheating the steel slab, descaling the steel slab, edging the steel slab, roughing the steel slab, and hot rolling the hot rolled steel strip.

The method for mitigating the effects of coil collapse on hot strip mill coils includes the steps of producing a hot rolled steel sheet in a hot strip mill which is then formed into a hot strip coil by coiling said the hot rolled steel sheet around a mandrel on a downcoiler. Once formed, the hot strip coil is oriented such that the coil eye axis thereof is in the horizontal direction. Then the hot strip coil is removed from the mandrel/downcoiler and placed onto a mandrel of a turnstile. The coil is then allowed to cool sufficiently to reduce the sagging rate of the coil to a negligible level. The coil is then removed from the mandrel on the turnstile.

The turnstile may have a plurality of mandrels each of which may be attached to an arm of the turnstile. Each arm of the turnstile may be attached to a base of the turnstile. Each of the arms of the turnstile may have a first end and a second end. The first end of each of arm includes the mandrel and the second end of each arm may be attached to a central pivot. The central pivot may be attached to the base and the arms may rotate on the base via said pivot in a horizontal direction. The cooled hot strip coil may be placed onto a conveyor and transported to another destination such as a holding yard. The step of removing the hot strip coil from the mandrel/downcoiler may further include banding the hot strip coil with banding strips.

Brief Description of the Figures

Figure 1 is a schematic depiction of a collapsed coil;

Figure 2 is a graphical depiction of the normal coil sagging rate the over time;

Figure 3 is a depiction of a turnstile useful in the inventive method; and

Figure 4 is a perspective view of a turnstile useful in the inventive method.

Detailed Description of the Invention

The present invention relates to a method and apparatus for preventing coil sag in hot band coils. Coil sagging occurs during and after hot coiling and is due to relative displacement of the coil wraps leading to a reduction of the coil inner (coil eye) diameter after the coil is removed from the mandrel. Hot band coil collapse/coil sagging results in additional manufacturing cost due to 1 ) the inability to load a collapsed coil onto pickling line mandrel, requiring reprocessing of the coil; and 2) wobbling of coil during subsequent uncoiling, resulting in strip steering issues, lower pickling line productivity and even equipment damage. The issues with coil collapse are increasing with newer high strength steels and will continue to do so.

Sagging rate is defined as the change in the diffidence between maximum and minimum coil diameters over time. Sagging starts at the coiler. The sagging rate is higher in the beginning and could last for 24 hours. Figure 2 is a graphical depiction of the normal coil sagging rate the over time. The graph plots the diameter change rate vs time in hours. In the beginning the coil is hot and the sagging rate is quite high. As time progresses, the sagging rate decreases as the coil cools. Eventually the sagging rate falls to near zero, but by that time the coil has collapsed significantly and is likely to have to be reprocessed to be useful. This reprocessing usually entails recoiling of the band to try to remove the shape eccentricities. As used herein, collapse/collapsing and sag/sagging are used interchangeably for the same concept.

The present invention is a method/apparatus for reducing/preventing coil sag. The process of the present invention relates to the production of coils of hot rolled sheet steel. Thus, the first step is production of hot rolled steel sheet in a hot strip mill. Generally, such a process includes reheating a steel slab, descaling the slab, edging the slab, roughing the slab and then rolling the strip. Then the rolled hot strip is coiled by a downcoiler around a mandrel. The coil has its coil eye axis in the horizontal direction. The coil is then removed from the mandrel/downcoiler by a coil car.

In the inventive method, the coil is placed on a turnstile and allowed to cool until its sagging rate is low enough to allow the coil to stand on its own without coiling. Figure 3 is a depiction of the turnstile 2 used in the method for preventing coil sagging of the present invention. The turnstile 2 include a plurality of arms 3 each of which has a mandrel 4. The turnstile 2 may also preferably have a base 5 upon which the arms 3 thereof may rotate. The method includes taking a coin from the hot strip mill (possibly from a coil car) and placing it onto a mandrel 4 on the turnstile 2. Preferably the mandrel 4 is an expanding mandrel and the expanding mandrel expands to fill the interior of the coil eye, preventing the coil eye from losing its shape. The coil 1 is then left on the mandrel 4 until it has cooled enough that the sagging rate has dropped to a low enough level to ensure that no significant sagging of the coil will occur when the coil is removed from the mandrel. The turnstile has enough arms 3/mandrels 4 to allow each coil 1 to be cooled sufficiently before being removed therefrom and sent to storage, while fresh coil 1 takes its place on the turnstile. That is, the turnstile has enough mandrels to allow the coils to cool and be removed just in time for another coil to be placed onto the now empty mandrel. Thus, the turnstile can contain two or more arms/mandrels as needed to support the production speed of the hot strip mill and cooling rate of the coils 1. The arms 3 of the turnstile may (but do not have to) rotate horizontally on the base 5 and may be connected together by a pivot 6.

As described above, the mandrels 4 are preferably expanding mandrels. This ensures that the coil eye does not sag (lose its shape) as all sagging forces are countered by the outward expansion of the mandrel 4. Expanding mandrels for steel coils are well known in the industry and any suitable mandrel can be used as long and it can hold the coileye in it circular shape without allowing for coil sagging. The expanding mandrel is also preferably actively cooled, such as by water or other coolants. This mandrel cooling allows for quicker cooling of the coils (speeding the phase transformation of the coil and more quickly reducing the sagging rate) and also prevents excessive heat buildup in the mandrels 4.

Once the coil 1 is placed onto a mandrel 4 on the turnstile 3, it will remain on the mandrel until it is cooled sufficiently such that the sagging rate has dropped to a negligible level. Once cooled sufficiently, the coil 1 is removed from the mandrel 4. The turnstile may rotate on its base to bring an empty arm 3/mandrel 4 into position for placement of a freshly coiled hot strip coil thereon. Alternatively, the arms/mandrels of the turnstile may be fixed and a coil car may be used to add and remove coils from the arms/mandrels which are fixed in place.

Figure 4 is a perspective view of a turnstile useful in the inventive method. On the end of each arm 3 there is a mandrel 4 and on each mandrel 4, there is a coil 1 that is cooling. The arms 3 of the turnstile are attached together at a central block pivot 6 which is attached to a base 5. The arms 3 may rotate horizontally around the pivot 6.

The arms/mandrels of the turnstile must be sufficiently sturdy to handle the weight of a steel hotstrip coil thereon. Furthermore, the turnstile must itself be sturdy enough to handle a full load of hotstrip coils thereon.