Method for mitigating the effects of coil collapse on hot strip mill coils

Abstract

A method is for mitigating the effects of coil collapse on hot strip mill coils. A hot strip coil is removed from the mandrel/downcoiler and pre-sagged to create an initial sag by allowing gravity cause the coil to sag in a first specific direction for a first period of time. Then, without coil eye support to limit sagging, the direction of the sag caused by the force of gravity is modified to a direction perpendicular to the first specific direction and gravity is allowed to sag the coil for a second period of time. The hot strip coil being cooled enough by the end of the second period of time such that the rate of sagging of the hot strip coil has become negligible.

Claims

1. A method for mitigating the effects of coil collapse on hot strip mill coils without insertion of a mechanical means into the coil eye to limit sagging, the method comprising the steps of: producing a hot rolled steel sheet in a hot strip mill; forming a hot strip coil by coiling the hot rolled steel sheet around a mandrel on a downcoiler; orienting the hot strip coil such that the coil eye axis of the hot strip coil is in a horizontal direction; removing the hot strip coil from the mandrel/downcoiler; pre-sagging the hot strip coil to create an initial sag by allowing gravity to cause the coil to sag in a first specific direction for a first period of time; without inserting a mechanical means into the coil eye to limit sagging, modifying a direction of the sag caused by the force of gravity to a direction perpendicular to the first specific direction and allowing gravity to sag the coil in the modified direction for a second period of time; the hot strip coil being cooled enough by the end of the second period of time such that the rate of sagging of the hot strip coil has become negligible thereby mitigating the initial sag; wherein the first period of time and the second period of time are chosen such that the initial sag created during the first period of time is mitigated by the perpendicular sag during said second period of time; and wherein the pre-sagging step the coil becomes pre-sagged without rotation about said coil eye axis, for said first period of time, and wherein the without inserting step is performed without additional rotation upon said coil eye axis for said second period of time, wherein the step of pre-sagging the hot strip coil includes the steps of: placing the hot strip coil onto a coil car, the coil car having a coil stand to hold and support the coil, the coil resting on a coil bottom; holding the hot strip coil, without rotation about said coil eye axis, for the first period of time, wherein the first period of time is between 2 and 6 minutes, the pre-sagging creating a maximum coil eye diameter (Dmax) dimension in the direction perpendicular to the force of gravity.

2. The method as recited in claim 1 wherein the step of modifying the direction of the sag caused by the force of gravity to a direction perpendicular to the first specific direction includes the steps of: rotating the hot strip coil about the coil eye axis by an angle of Z90 degrees, wherein Z is an odd integer of 1 or greater; thereby rotating the maximum coil axis dimension (D.sub.max) to a direction parallel to the force of gravity; holding the hot strip coil, without additional rotation about the coil eye axis, for the second period of time, the second period of time lasting until the rate of sagging of said hot strip coil is negligible and the initial sag is mitigated.

3. The method as recited in claim 2 further comprising the steps of: placing the hot strip coil onto a conveyor; and transporting the hot strip coil to another destination after the step of rotating the hot strip coil.

4. The method as recited in claim 3 wherein the other destination is a holding yard.

5. The method as recited in claim 1 wherein the step of producing a hot rolled steel sheet in a hot strip mill includes 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 steel slab to form the hot rolled steel strip.

6. The method as recited in claim 1 wherein the step of holding the hot strip coil without rotation about the coil eye axis further includes the step of: banding said hot strip coil with banding strips.

7. The method as recited in claim 1 wherein the coil car includes a set of rollers to implement the step of rotating the hot strip coil about the coil eye axis.

8. The method as recited in claim 7 wherein each roller of the set of rollers includes notches to accommodate the banding strips, thereby preventing the banding strips from being compressed between said hot strip coil and the rollers.

9. The method as recited in claim 1, wherein the first period of time is between 2.5 and 4.5 minutes.

10. The method as recited in claim 1 wherein the step of pre-sagging said hot strip coil includes the steps of: placing the hot strip coil onto a support positioned within the coil eye, the coil hanging by the coil eye from the support; hanging the hot strip coil, without rotation about said coil eye axis, for the first period of time, wherein said first period of time is between 1 and 6 minutes; the pre-sagging creating a maximum coil eye diameter (D.sub.max) dimension in the direction parallel to the force of gravity.

11. The method of claim 10 wherein the step of modifying the direction of the sag caused by the force of gravity to a direction perpendicular to said first specific direction includes the steps of: placing the hot strip coil on a surface, such that a coil bottom of the hot strip coil rests on the surface; and resting the hot strip coil on the coil bottom for said second period of time, the second period of time lasting until the rate of sagging of said hot strip coil is negligible and the initial sag is thereby mitigated.

Description

BRIEF DESCRIPTION OF THE FIGURES

(1) FIG. 1 is a schematic depiction of a collapsed coil;

(2) FIG. 2 is a graphical depiction of the normal coil sagging rate the over time;

(3) FIG. 3 is a schematic diagram that shows what happens to a coil which is oval in shape at the beginning of the cooling period with the large axis parallel to the force of gravity;

(4) FIG. 4 is a schematic diagram that depicts the results of inventive process;

(5) FIG. 5 is a simplified depiction of a first embodiment of the method of the present invention; and

(6) FIG. 6 is a simplified depiction of a second embodiment of the method of the present invention.

DETAILED DESCRIPTION OF THE INVENTION

(7) The present invention relates to a method and apparatus for mitigating or eliminating coil sag in hot band coils without inserting a mechanical means into the coil eye to limit sagging. 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.

(8) Sagging rate is defined as the change in the diffidence between maximum and minimum coil diameters over time. Sagging starts at the coiler and sagging rate is higher at the begin and could last for 24 hours. FIG. 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.

(9) In studying the issue of coil collapse, the present inventor has noted that (as in FIG. 2) when the coil is round at the beginning of the coil collapse (hot coil) period, the final coil will be oval shaped with its major axis perpendicular to the force of gravity. It was also noticed (as shown in FIG. 3) that if the coil was oval in shape at the beginning of the cooling period with the large axis parallel to the force of gravity, the coil would collapse to the point that the larger axis shifted to perpendicular to the force of gravity again.

(10) It was then that the inventor discovered that there was a particular range of time in the cooling curve that a coil, which has begun to collapse, can be rotated 90 degrees (or some odd multiple of 90 degrees, such as 270 degrees) and as it continues to cool, the collapse can mitigate or eliminate the effects of coil eye collapse. That is, the coil is initially allowed to collapse, and after a specific period of cooling/collapsing time, the coil is rotated such that the larger axis is shifted from perpendicular to the force of gravity to parallel to the force of gravity. Then, without inserting a mechanical means into the coil eye to limit sagging, the coil is allowed to further cool/collapse, such that the difference between the maximum and minimum coil eye diameters is negligibly small. That is, the difference between the maximum and minimum coil eye diameter is small enough that the coil can be properly used in later processes without the need to be recoiled or scrapped. FIG. 4 depicts a first embodiment of the inventive process, wherein the initially round coil collapses for a first period of time (weave pattern), is rotated and allowed to finish collapsing for a second period of time (dither pattern) and ends up relatively round again. It should be noted that on each of FIGS. 2-4 there is a depiction of coil shape transformation, however, this is just for illustrative purposes only and the position of the shapes along the time line is not indicative of the time of shape transformation.

(11) The present inventor has discovered that the time of initial cooling/collapse (after removal from the coiling mandrel) should be between 1 and 6 minutes. After the initial cooling/collapse time, the coil is rotated such that the diameter dimension of the coil that was perpendicular to the force of gravity is now parallel to the force of gravity. That is, the coil is rotated 90 degrees or some odd multiple of 90 degrees (270, 450, etc. degrees). The equipment needed to perform the rotation is a set of rollers. The rollers may have grooves cut circumferentially therein to allow for placement of the coil banding strips therein. Thus, the capital investment for the equipment is quite small.

(12) The process of the present invention relates to the production of coils of hot rolled sheet. 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 hot rolled steel sheets are conventionally coiled at relatively lower temperatures to allow for the transition from Austenite to Martensite before the coil is removed from the mandrel to prevent excess sagging. In the method of the instant invention, the steel may be coiled at higher temperatures than conventionally used because the transition can be allowed to occur after the coil is removed from the mandrel and any sagging will be compensated for by the present method. The coil has its coil eye axis in the horizontal direction. The coil is then removed from the mandrel/downcoiler by a coil car. The coil is then held without rotation about the coil eye axis for a first hold time of between 1 and 6 minutes. More preferably the first hold time is between 2 to 6 minutes and most preferably between 2.5 to 4.5 minutes. While being held, the coil may optionally be banded and is thereafter placed on a set of rolls. The hold time begins the moment the coil is removed from the mandel. This hold time creates an initial sagging of the coil (a pre-sag) and creates a maximum coil eye diameter (D.sub.max) dimension in the direction perpendicular to the force of gravity. After the hold time, the coil is then rotated upon its coil eye axis via the set of rolls. The coil is rotated Z90 degrees, wherein Z is an odd integer of 1 or greater, thereby rotating said maximum coil axis dimension (D.sub.max) to a direction parallel to the force of gravity. The coil is then held for a second period of time without additional rotation and without any means of mechanical support being inserted into the coileye to prevent the coil from re-sagging past the symmetrically circular state. The second period of time last until the rate of sagging of the coil is negligible, thereby offsetting or substantially mitigating the initial sagging (pre-sag). The initial sag is substantially mitigated when the coil is useable for further processing or sale without having to be recoiled. The rate of sagging is negligible when any further sagging will not affect the coil to the point that the coil would need to be recoiled to be useable. After rotating the coil, it may be placed onto a conveyor and transported to another destination, such as a holding yard.

(13) As used herein, collapse/collapsing and sag/sagging are used interchangeably for the same concept.

Example

(14) Examples of coils/coiling both with and without the first embodiment of the method of the present invention are shown in Table 1. The steel alloy of the coils has the following nominal composition in wt. %: C: 0.07-0.25, Mn: 1.5-2.5, Cr: 0-0.3, S: 0.-0.3, Mo: 0-0.3, Nb: 0-0.03, B: 0-0.0030, Ti: 0.-0.05 and the remainder Fe and inevitable impurities.

(15) TABLE-US-00001 TABLE 1 1 Gauge range of coils, mm 2.0-2.5 2.6-3.0 3.1-3.5 3.6-4.0 4.1-4.5 4.6-5.0 total 2 Number of coils produced without 2 58 341 96 69 73 639 inventive method 3 Number of sagging coils without 2 50 243 59 27 20 401 inventive method 4 sagging rate in % without 100.0% 86.2% 71.3% 61.5% 39.1% 27.4% 62.8% inventive method 5 Average Sagging of coils without 30.0 37.1 37.0 25.2 29.8 21.3 33.2 inventive method, mm 6 Number of coils produced with 3 9 224 185 65 24 510 inventive method 7 Number of sagging coils with 0 1 31 5 0 0 37 inventive method 8 sagging rate in % with 0.0% 11.1% 13.8% 2.7% 0.0% 0.0% 7.3% inventive method 9 Average Sagging of coils with 15.0 16.3 22.5 18.5 17.3 13.3 19.8 inventive method, mm

(16) The first row of Table 1 indicates the gauge range of the steel strip comprising the coils. The second row is the number of coils in a particular gauge range produced without the method of the present invention. The third row is the number of coils from row two that sagged without inventive method. The fourth row is the percentages of coils from row two that sagged (i.e. the number of coils in row 3 divided by the number of coils in row 2 times 100). The fifth row shows the average sagging (in mm) of coils without inventive method.

(17) The sixth row of Table 1 indicates the number of coils in a particular gauge range produced with the method of the present invention. The seventh row is the number of coils from row six that sagged with inventive method. The eighth row is the percentages of coils from row six that sagged (i.e. the number of coils in row 7 divided by the number of coils in row 6 times 100). The ninth row shows the average sagging (in mm) of coils with inventive method.

(18) As can be seen, the method of the present invention reduces the number/percentage of coils that have deleterious sagging throughout all gauge ranges investigated. Averaging all of the coils tested, the coil collapse sagging rate in percent was reduced from 62.8% down to 7.3%. Thus increasing yield and saving money by reducing scrap/downgraded coils.

(19) Generically, the invention can be described as allowing a hot coil to pre-sag for a period of time under the influence of gravity and then modifying the direction of the force of gravity on the coil to allow the coil to further sag in such a manner as to offset or mitigate the pre-sag, returning the coil to a relatively circular shape.

(20) FIG. 5 is a simplified depiction of a first embodiment of the present invention. The steel coil is circular when removed from the coiling mandrel, it is allowed to rest on its coil bottom and pre-sag for a period of time designated Time 1. After Time 1, the coil is rotated Z90 degrees, wherein Z is an odd integer of 1 or greater and the coil is again allowed to rest on its coil bottom to sag for a second period of time, designated Time 2. After this second period of time, the coil has sagged enough to counteract the pre-sag and is once again in a relatively circular shape.

(21) FIG. 6 is a simplified depiction of the second embodiment of the present invention. The steel coil is circular when removed from the mandrel, it is allowed to hang from a support in the coil eye and pre-sag for a period of time designated Time 1. After Time 1, the coil is allowed to rest on its coil bottom and continue sagging for a second period of time, designated Time 2. After this second period of time, the coil has sagged enough to counteract the pre-sag and is once again in a relatively circular shape.