US3380277A

US3380277A - Process for gauge control in hot rolled sheet and strip

Info

Publication number: US3380277A
Application number: US520224A
Authority: US
Inventors: Mulflur Walter Harold
Original assignee: Algoma Steel Corp Ltd
Current assignee: Algoma Steel Corp Ltd
Priority date: 1965-10-04
Filing date: 1966-01-12
Publication date: 1968-04-30
Anticipated expiration: 1985-04-30

Description

April 1963 w. H. MULFLUR 3,380,277

PROCESS FOR GAUGE CONTROL IN HOT ROLLED SHEET AND STRIP Filed Jan. 12, 1966 7 Sheeis-Sheet 1 D O O O O O D O O 3 2 LL 3..., K O I) :5

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PROCESS FOR GAUGE CONTROL IN HOT ROLLED SHEET AND STRIP Filed Jan. 12, 1966 7 Sheets-Shet 2 FIG. 2

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April 30, 1968 w. H. MULFLUR PROCESS FOR GAUGE CONTROL IN HOT ROLLED SHEET AND STRIP Filed Jan. 12, 1966 7 Sheets-Sheet 7 /w Mm a, a flan/[71572 5: ME 3% United States Patent 0 ABSTRACT OF THE DISCLOSURE A method for ensuring that hot rolled metal sheet or strip has a uniform thickness after rolling is described. In the method a hot slab is rolled to a wedge shape which is subsequently rolled to sheet or strip, thin end first thereby ensuring a large heat sink in the material rolled last. The material produced by the process is also easier to coil, pickle and otherwise treat as the finishing temperature is uniform throughout.

This invention relates to an improvement in the method for the rolling of hot metal strip and more particularly to a method for obtaining a more uniform mill finishing temperature and a more uniform gauge thickness throughout the entire length of the rolled sheet or strip of metal.

In the rolling of hot metal strip gauge control of the strip passing the finishing stands is of prime importance. The gauge or thickness of the finished strip is regulated by the pressure exerted by each stand of the finishing mill and is related to the temperature of the strip at which the actual rolling operation takes place. As the gauge or thickness of the rolled strip is a function of the mill stands and is set, temperature is the main variable which is capable of control.

From the period that the slab of metal initially leaves the soaking stage, at which point the slab is considered to be at substantially the same temperature throughout the entire area or volume of the slab, heat is radiated from the metal which results in a continual loss or drop in temperature of the metal. After passing the roughing stands and crop shear the temperature loss of the metal is constant throughout the shape. The metal shape so prepared is now ready to enter the finishing stands. On entering the finishing stands a certain time lapse is encountered wherein the first or leading end of the shape enters the first stand until the last or trailing end of the shape enters the stand. As the metal is loosing temperature by heat radiation it is readily seen that this time lapse is sufficient such that the last portion of the shape to enter the finishing stands will be at a lower temperature than that portion of the shape which first entered the stand.

The temperature loss is significant even in the modern high speed or zoom rolling, and effects the rolling operation in two ways. The cooled metal is more difficult to roll which causes a change in gauge thickness. This change is apparent in the finished strip as the thickness gradually increases throughout the length of the strip. Any attempt to control the gauge i.e. by the application of increased pressure during the finishing operation has the effect of altering the physical and metallurgical roperties of the finished strip due to the extra working required.

The temperature loss of the metal shape at the finishing stands is a well known and recognized problem in hot metal rolling operations. Biggert in United States Patent No. 2,095,430 attempted to overcome this problem by maintaining successive portions of the shape at preselected increasing temperatures in order to present each portion 3,380,277 Patented Apr. 30, 1968 of the shape entering the finished stand at substantially the same temperature from the leading to trailing ends. This practice although overcoming to a certain extent the problem would be difficult to incorporate and almost impossible to control precisely due to the speed at which present day rolling operations are carried out.

In normal operations no attempt is made to compensate for the temperature differential between the leading and trailing ends of the slab. The temperature loss or unequal temperature throughout the length of the slab results in unequal reduction of the strip in the finshiing stands which produces a product having a progressively increased gauge from the first portion of the strip to the last'portion of the strip which passes the stands.

It is thus an object of the present invention to provide a method for controlling the temperature of the hot metal material whereby the metal will have the desired substantially constant temperature throughout the finishing rolling.

It is a further object of the present invention to provide a method of temperature control of the hot metal material such that the strip obtained from the finishing stands will have a substantially uniform gauge throughout the length of the strip.

Other objects and advantages of the invention will be readily apparent when the following detailed description is read in conjunction with the accompanying drawings wherein:

FIG. 1 is a schematic block diagram of a hot metal rolling mill.

FIG. 2 is a recorded chart of the gauge deviation with an insert plot of temperature variations of a metal strip passing the finishing stand employing the usual rolling practice.

FIG. 2a is a recorded chart of the gauge deviation with an insert plot of temperature variations of a metal strip passing the finishing stand employing the present invention.

FIGS. 3 and 4 are similar recorded charts as FIG. 2 varying the gauge of the slab.

FIGS. 3a and 4a are similar recorded charts as FIG. 2a varying the gauge of the slab employing the present invention.

Broadly, the present invention provides a method for obtaining a more uniform mill finishing temperature along the entire length of 9. rolled sheet or strip by treating the slab or metal it is desired to roll into the form of a sheet.

or strip on the last pass of the slab or metal through a roughing mill to produce a uniform taper in the slab of a desired slope. The uniform taper so applied may slope from the leading to trailing end of the slab, the trailing end being thicker.

In order that a uniform taper may be imparted to the metal slab it is necessary to raise the setting screws on the rougher mill at a velocity which is dependent upon the speed of the slab through the rolls of the rougher. This may be readily accomplished in the following manner for a mill equipped with screwdown motors supplied from a variable voltage source. If a bar is rolled with a velocity of V feet per seconds and the time for the pass through the rolls is T seconds the length of the slab is V T. In order to impart a taper of h inches per lineal foot wherein the initial is hi inches the final gauge thickness h of the slab will be The distance that the screws must move is then equal to hVT and the speed at which the screws must move becomes hV inches per second. This result may be achieved by setting the desired taper as a programmed reference from a potentiometer or reference bridge. A signal proportioned to the mill speed is obtained from the mill rolls or a rider roll with provision for roll diameter compensation. These two signals can be multiplied in an electronic multiplier and after amplification and scaling in a suitable amplifier the resultant signal is applied as a reference to the screwdown speed or voltage control. The control may be switched in by means of a hot metal detector or load cells when the bar enters the mill and in the case of a reversing rougher on the last pass of the slab through the mill. The above outline is given as an example only of one method of imparting a taper to the metal slab and any other means may be employed to accomplish this same result.

Referring to FIG. 1, which represents a block form diagram of a hot metal rolling mill, the slab 1 is roughed in the roughing mill indicated at 2 in the normal manner down to the last pass through the mill. Before the slab is permitted to enter the rougher for the last pass the screws controlling the setting of the rolls are adjusted to produce a desired thickness of the leading end. As the slab enters the rougher on this last pass the setting screws are started upwardly at a uniform predetermined rate to gradually release the pressure exerted by the rolls to produce a taper of desired slope along the length of the rolled slab.

The slab 1 after the last pass through the rougher mill 2 is in the form, cross-sectionally, of a graduated taper indicated on the drawing in exaggerated form. This tapered form is conveyed by rolls 3 to a crop shear, not shown, and enter the finishing stand indicated at 4.

Quality control of the product issuing from the finishing stands is carefully checked by ways of temperatures which may be taken by use of a recording pyrometer and gauge control of the strip by X-ray or by the use of cobalt radiation detectors employing a recording means. With reference to FIG. 2 a slab which had been roughed to 1.250 inches was introduced into the finishing stands. The slab was reduced in this stand to .100 gauge. The temperature and gauge thickness was checked at the exit end of the finishing stands. It will be noted that a temperature loss of approximately 50 was sustained from the start of the operation to the end and the thickness of the strip increased progressively, as indicated by the graph of thickness deviation, by about 5 thousandths.

In comparison to FIG. 2a employing the taper slab as proposed by the invention, the taper applied to the slab was .800 inch increasing in thickness from the leading end to trailing end to 1.250 inch. This slab was reduced as before in the finishing stands to .100 gauge. The temperature and gauge thickness was checked at the exit end of the finishing stand as previously. It will be noted that the temperature recorded throughout the operation remained substantially constant with only slight variation with no constant drop-off and the thickness deviation of the strip remained substantially constant varying at most by l to 1%. thousands.

Similar results were obtained in rolling to a gauge of .080" as shown in FIGS. 3 and 3a. With reference to FIG. 3 employing a slab roughed to 1.250" a decrease in temperature coupled with a progressive increase in gauge thickness is readily observed. In comparison to FIG. 311 wherein a taper from .800" to 1.250" was imparted to the slab on the last pass through the rougher, the temperature recorded is substantially constant with slight variations both in loss and gain and the thickness deviation of the strip is substantially constant.

Further test work with similar results are shown in FIG. 4 and FIG. 4a again rolling to .080" gauge. The slab, FIG. 4, roughed to 1.250" on passing the finishing stands to form the strip shows the usual pattern of temperature loss acompanied by a progressive increase in thickness during the rolling operation. The slab, FIG. 4a, which had been tapered from .800" to 1.250" from leading to trailing end on the last pass through the rougher shows a substantially constant temperature accompanied lby a relatively low deviation in the thickness of the strip during the rolling operation.

As indicated by the above results the employment of the tapered slab provides a more uniform mill finishing temperature along the entire length of the rolled sheet or strip. This uniform finishing temperature is desirable from a metallurgical standpoint as it will result in more uniform and controlled physical properties as well as in a more uniform and controllable grain size of crystalline structure in the product. By obtaining a uniform finishing temperature, control of the coiling temperature of the strip will be simplified and a more uniform gauge throughout the length of the sheet or strip possible. The use of the invention in producing a uniform finishing tempera ture when employed in conjunction with any mechanical method of gauge control will result in an even better gauge control and such control will be accomplished with less wear on the gauge control equipment. Furthermore, it will be possible to set mechanical gauge control equipment to a narrow gauge range. By the use of the instant invention heavier coils can be rolled with much less variation in finishing and coiling temperatures.

The cooling effect of the descaling sprays should be considered. At the same linear speed through the descaling sprays, 'a thicker slab will finish hotter than a thin slab assuming both slabs are at the same entry temperature. However, because the mass flow through the finishing train is constant the linear speed of the tapered bar will be inversely proportional to the increase in thickness. The thicker portion of the tapered slab will therefore pass through the descaler sprays at a slower rate with a consequent greater loss of heat. Some of the heat retention and regeneration as a result of work required for the tapered slab is thus lost to a greater cooling effect of the descaling sprays. If, however, the first descaling in the finishing train is carried out between the first two finishing stands, this rate of heat loss due to descaling will be substantially uniform through the length of the slab and the effect of the taper will be more pronounced. In any event by increasing the amount of taper it is possible to finish the trailing end of the slab at a higher temperature than the leading end of the slab even without the use of zoom or high speed rolling; the use of zoom or high speed rolling, which, overcomes the effect of the greater cooling of the thicker portion of the tapered slab, will magnify the effect of any given amount of taper on finishing the trailing of the slab at a higher temperature than the leading end of the slab.

The optimum slope of the wedge-shaped slab from the roughing mill will vary to a certain extent for each rolling mill. The variations depend in part of the following condition:

(a) Slab size and furnace delivery temperature. (b) Type of roughing mill:

(1) Reversing rougher (2) Continuous rougher (3) Combination reversing-continuous rougher. (c) Average delivery temperature of slab from roughing mill. (d) Drafting capabilities of the finishing train for various finished widths and gauges. (e) Speed range of finishing train. (f) Distance between roughing mill(s) and finishing train. (g) Travel time between roughing mill(s) and finishing train.

For each set of the above conditions which will be repetitive in a given mill a most desirable slope for the wedge shape can be determined. The slope of the wedge itself is a function of the speed of the roughing mill screw on the last pass of the slab through the mill as well as related to the delivery speed of the mill. If it is required or desirous to produce a wedge with a slope of /4" per hundred feet of roughed slab on a roughing mill drawing a delivery rate of 500 ft./rnin., the back-elf speed of the screw will then be set at 1%" per minute. The above is given only as an example and a wedge shape of desired slope may be obtained once the operating characteristics of the roughing mill(s) and finishing trains are known which is readily ascertainable for any milling operation. It should be noted that the slope of the wedge does not necessarily have to be a straight line slope and may vary stepwise with the proviso that the thickness is increased from the leading end to the trailing end of the metal slab. The backoff speed of the screw may be readily computer controlled to obtain the slope characteristics desired.

I claim:

1. A method of obtaining uniform mill finishing temperature and thickness in hot rolled metal strip which comprises rolling a metal slab to a wedge form, and further rolling said wedge form with the thin end of said wedge leading.

2. A method for obtaining more uniform mill finishing thickness in hot rolled metal strip which comprises rolling a metal slab to a wedge form in a roughing mill and introducing said wedge form into a finishing mill in the direction of increasing thickness from the leading end to the trailing end.

3. A method for obtaining more uniform finishing temperatures in hot rolled metal strip which comprises rolling a metal slab to a wedge form in a roughing mill and introducing said wedge form into a finishing mill in the direction of increasing thickness from the leading end to the trailing end.

4. A method for obtaining more uniform metallurgical and physical properties in hot rolled metal sheet and strip material which comprises rolling a metal slab to a wedge form and further rolling said wedge form in the direction of increasing thickness to the trailing end.

5. A method for obtaining more uniform and controlled pickling properties in hot rolled metal sheet and strip material which comprises rolling a metal slab to a wedge form and further rolling said wedge form in the direction of increasing thickness to the trailing end.

6. A method of rolling strip from a hot metal slab comprising passing the slab through at least one roughing pass in a roughing mill, adjusting the pressure exerted by the rougher rolls on the last pass through the rougher mill to impart a wedge shape to said slab and passing said wedge shape through a finishing pass in the direction of increasing thickness of said wedge towards the trailing end.

7. A method of rolling strip from a hot metal slab comprising passing the slab through at least one roughing pass in a roughing mill, releasing the pressure exerted by the rougher rolls on the last pass of the slab through the rougher mill to impart a taper of increasing thickness from the leading end to the trailing end of the slab and passing said tapered slab through a finishing pass from said leading end to said trailing end.

References Cited UNITED STATES PATENTS 2,502,005 3/ 1950 Hansell 72-200 2,710,550 6/ 1955 Sendzimir 72-202 3,199,327 8/1965 Krause 72-215 3,264,856 8/1966 Layard 72202 RICHARD J. HERBST, Primary Examiner.

E. M. COMBS, Assistant Examiner.