1. A method of controlling the shape of a rolled strip delivered from a rolling mill which contains a work roll and means to vary roll force created by said work roll, comprising: rolling a strip to final gauge while subjecting said strip to from about 0.25 percent to about 6 percent elongation in length while using a work roll size and a roll force within a range that, for said roll size, does not produce significant variation in final strip gage; and varying said roll force within said range during rolling to improve the shape of the strip.
2. A method according to claim 1, wherein: the strip comprises a metal.
3. A method according to claim 2, wherein: the strip comprises a ferrous metal.
4. A method according to claim 1, wherein: said roll force is increased to reduce center fullness of the strip.
5. A method according to claim 1, wherein: said roll force is decreased to reduce edge waves of the strip.
Our invention is generally premised upon the discovery that significant shape control can be effected during the rolling of strip products through the determination and use of a roll size which is compatible with a given desired final gage or thickness. By the expression "compatible with" it is meant that for a given final gage, there is a work roll size or diameter that will permit the use of considerable variations of roll force without the attendant production of significant variations in final gage. Thus, if one is able to significantly vary roll force without a consequent variation in gage, one is able to achieve significant control of the strip shape through the use of controlled variations in roll force.
Prior to further explanation of the invention it is believed that a general discussion of strip shape or flatness would serve to aid in the understanding of certain principles underlying the invention. An extremely important factor in determining the quality of rolled strip is its shape or flatness, i.e., the degree to which buckles or waves exist in the strip when it is permitted to lie free on a flat surface. When strip is rolled, as in a cold rolling operation, it often occurs that certain transverse portions of the strip are reduced and elongated in the longitudinal or rolling direction to a greater extent than other transverse portions. Often, the center of the strip is elongated to a lesser extent than the edges or vice versa. This differential elongation between transverse portions of the strip causes waves or buckles to form in the strip, the extent to which they form being dependent upon the degree to which the various transverse portions of the strip are differentially elongated. Differential elongation of a strip occurs for a variety of reasons. For example, the mechanical alignment and functioning of the rolling mill may be defective, there may be an inbalance of roll forces, or there may be variations in the crown of the rolls.
Because shape control is obviously a vital element of any quality control program, it follows that any control scheme that can effectively minimize the occurrence of poor strip shape is of great interest to the rolling industry. This is especially true in the metals industry where enormous quantities of strip products are produced annually.
Although the prior art, for example, U.S. Pat. Nos. 3,318,124 and 3,404,550 has generally recognized that roll force is a variable which influences strip shape, there does not appear to have been an appreciation of the effect of work roll size and final gage upon effective shape control.
It is thus an object of our invention to provide a method which will effectively control the shape characteristics of rolled strip products.
It is a further objective to provide a method of shape control that is relatively simple to employ from the standpoint that extensive amounts control equipment or systems need not be installed on the rolling mill.
It is yet an additional objective to provide a method of shape control which permits one to take advantage of the discovered relatiohip of work roll size, final strip size, and roll force in an efficient and predictable manner.
It is also an object of our invention to provide a shape control technique which is compatible with existing rolling mill equipment that is adapted for the attainment of relatively small percentages of strip elongation.
FIG. 1 is a graphical depiction of the typical effect of roll force upon final gage for a given work roll diameter.
FIG. 2 is a graphical depiction of the typical effect of roll force upon final gage for a series of work roll diameters.
FIG. 3 illustrates a typical example of improved shape control, as measured by differential elongation, that may be obtained through practice of the invention.
Normally, shape control of rolled strip production is obtained with use of roll bending systems, strip lubricant-coolant spray adjustment, and other techniques. Our method of shape control involves the adjustment of roll force. Typically, roll force adjustment is accomplished through the adjustment of roll screwdown devices. As will be explained further below, the invention has the advantage that relatively simple conventional existing rolling mill equipment can be employed to effect significant shape control when such equipment is operated under the conditions set forth in this application.
As may be seen from the relationship depicted in FIG. 1, as roll force increases, for a given work roll size or diameter, its effect upon final gage becomes less. This effect is especially noticable in the operating region shown in the Figure. Hence, it may be clearly seen that there is a range of work roll force that will permit considerable variations in roll force without producing significant changes in gage. It is by operating in this range where the above relationship is obtained that significant shape control can be effected. Of course, the specific range of roll force and final gage is a function of other variables as well. Such variables would obviously include strip material composition, strip temperature and certain rolling mill characteristics that are a function of a particular type of rolling mill. However, it amounts to a simple matter to empirically determine the particular above discussed parameters that would reproducably define the above discussed relationship for a given material, temperature, and rolling mill.
FIG. 2 illustrates another important facet of the invention. This is that there is a different curve defining the relationship (same as that depicted in FIG. 1) between roll force and final gage for different workroll diameters. As may be seen, work roll size or diameter has an influence upon the specific parameters that define the region or range in which the beneficial results of the invention are manifested. It should also be noted that the curves contained in FIG. 2 shift from right to left upon the use of decreasing work roll sizes or diameters. This means, generally speaking, that smaller work roll sizes should be utilized with the production of lower final gages in order to achieve significant shape control according to the invention. Thus, it may be seen that work roll size or diameter is an important factor to consider when one wishes to obtain a particular final strip gage and yet be able to operate in a range of work roll force values which permit considerable variation in roll force with little consequent variation in gage; a situation in which roll force can primarily be beneficially directed to the aspect of shape control.
From the above discussion of FIGS. 1 and 2, it may be seen that once one has determined the final gage or thickness at which shape control is to be effected, it is but a relatively simple matter to select or determine a particular work roll size that is compatible with the desired final gage and effective shape control. Once operating in the desired shape control work roll force operating range, it is then possible to vary roll force within said range in order to effect the particular type of shape control that is required to improve or correct an existing poor strip shape, i.e., to increase or decrease work roll force. Roll force variations within the desired range produce extremely small gage reductions which are necessary to effect significant shape control due to consequent changes in differential elongation on the order of several one-hundredths of a percent.
Variations in work roll force are suitably accomplished with the use of appropriate roll screwdown mechanisms that are conventional for strip rolling mills. Center fullness is reduced through downward movement of the screwdown mechanism which in turn increases roll force. On the other hand, roll force may be decreased by upward movement of the screwdown mechanism in order to reduce edge waves on the strip.
The invention is applicable to the production of any material which may be rolled into a strip shape by a rolling mill. Obviously included in such class of materials are metals and alloys, both of the ferrous and nonferrous type. An especially suitable metal is low carbon steel.
The invention may be practiced on a wide variety of rolling mill equipment. For example, rolling mills which effect a relatively light degree of strip reduction, on the order of about 0.25 to 6 percent elongation in length, are especially suitable for conducting the inventive method. Temper mills are representative of the class of rolling mills in which light reductions are obtained. Of course, the invention can also be effectively practiced on multiple-stand tandem mills if desired. In the use of tandem mills it is often convenient to utilize the invention during rolling on the final roll stand where relatively small reductions are attained when contrasted to the overall amount of strip reduction effected by the entire roll train. The invention is equally applicable to rolling mills whether or not back-up rolls are utilized in combination with work rolls.
In accordance with the process of the invention, low carbon steel strip thirty-five inches in width, was cold rolled on a three-stand double reduction mill to a final gage of 0.0061 inch. The conditions during rolling on the third or last stand were adjusted in order control strip shape. During rolling at the last stand a percentage of reduction of 4.7 percent was effected at a roll speed of 440 feet per minute. Backup rolls of 54 inches in diameter were employed in combination with two work rolls, each having a diameter of 21.5 inches. An initial work roll force of 309 tons was increased to a final work roll force of 343 tons in order to achieve a reduction in center fullness of the strip. Work roll size and roll force operating values were selected so as to be compatible with final gage in accordance with the previously enumerated practice of the invention.
As may be observed from FIG. 3, a substantial improvement or control of strip shape resulted from the above procedure. The initial shape of the low carbon steel strip is depicted by the solid line contained in the graph. It should be noted that the percentage of differential elongation is plotted for various portions of the strip cross-section to depict shape characteristics. The graphical profile is representative of a typical initial strip shape which can be characterized as having a medium center and flat edges. The final strip shape is represented by the dashed lines on the Figure. The relative improvement in strip shape due to the net increase of approximately 34 tons of work roll force can be readily seen. The final shape can be characterized as having a light center and full edges. Hence, this example demonstrates the improvement in strip shape that can be attained through the practice of this invention by the expedient of selecting or determining the proper combination of work roll size, final gage, and variation in work roll force. Of course, it should be understood that the above presented values are merely representative of operational parameters for a specific type of material, rolling temperature, and rolling mill and that other operational parameters could be required in the event that one were producing a different product or utilizing different rolling mill apparatus. one processing different material or utilizing different equipment would merely be required to routinely develop a different set of operating parameters through use of the teachings contained in this specification in order to control the shape characteristics of such other product.