US 3312576 A
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April 4, 1967 R. R. PALIK 3,312,576
METHOD OF TREATING METAL Filed July 5, 1963 HEATING ZONE 3 HOLDING ZONE COOLING ZONE l9 COIL PEWIND COIL UNWIND COIL UNWIND TOR.
- INVEN ROBERT R. QALIK ATTORNEYS United States Patent ()1 3,312,576 METHOD OF TREATING METAL Robert Richard Palilr, Henrico County, Va., assignor to Reynolds Metals Company, Richmond, Va., :1 corporation of Delaware Filed July 3, 1963, Ser. No. 292,709 6 Claims. (Cl. 148-115) This invention relates to the mechanical and heat treatment of metal, and it particularly concerns a novel method of continuously treating metal in strip form to achieve improved mechanical properties and distinctive metallurgical characteristics. More specifically, the method of this invention comprises controlling the percentage of stretch applied to a metal strip as it is advanced under tension, heated to an elevated temperature to elfect plastic deformation of the metal, and cooled to substantially ambient temperature before the stretching effect is relaxed.
In accordance with the invention, it has been found that various unusual and desirable characteristics can be imparted to metal strip by means of this continuous treatment in which temperature of the metal, the percent stretch applied thereto and the rate of application of stretch are all controlled. Among the desirable characteristics of the treated strip are improved uniformity'of properties, fine grain size in the annealed condition, low caring tendency in subsequent drawing operations and high yield strength in relation to tensile strength.
Strip produced by rolling mills is usually in coil form and in a work-hardened condition which is often unsuitable for subsequent uses which involve forming operations. While coil or batch annealing operations have been employed to alter the properties of such materials, there has long been a need for an effective continuous annealing operation which can be carried out successfully at high speed. The present invention meets this need and provides considerable flexibility in regard to accomplishing other similar operations.
The invention will be described in detail with reference to the accompanying drawings, in which:
FIG. 1 is a schematic view in side elevation of a process line embodying the invention;
FIG. 2 is a schematic of a power drive system for use in the process line shown in FIG. 1;
FIG. 3 shows A a drawn product produced from metal strip treated in accordance with the invention, in comparison with B a similar product made from conventionally prepared metal strip.
The coil stock to be processed is unwound from roll 10, as shown in FIG. 1. The strip is threaded through an S wrap roll bridle 12 which controls the entry and line speed of the strip.' Various idler rolls 13 are provided as shown to set the path of the strip through the annealing furnace 14. At the entry end of the furnace is a heating zone 15 through which the strip passes. The strip next passes through a stabilizing or holding 17, and from there it passes through a cooling zone 19 which cools it to substantially ambient temperature. Upon emerging from the furnace the strip passes through the exit S wrap bridle 16. This bridle is controllably operated at a speed in excess of the entry bridle speed. It has been observed that the percentage speed diiferential between the bridles has a proportional relationship to the stretch which will be imposed upon the strip. After treatmengthe strip is recoiled on a motorized reel 18. Therefore, the actions upon the traveling strip while under the continuous application of stretch are: heating, stabilizing, and cooling.
, The temperatures in the furnace are set according to the alloy being processed and the physical properties desired, and the percent stretch is set according to grain Patented Apr. 4, 1967 size, (flatness, directional and physical properties desired. For a given speed difierential, the strain rate is set by line speed. These major variables are generally interdependent, however, and substantially the same results can be achieved with different combinations.
The techniques by which the stretch is achieved and controlled can be explained with reference to the block diagram of FIG. 2. A drag brake mechanism on the unwind reel '10 establishes an initial back tension to keep the strip from slipping on the bridle rolls., The entry bridle gear box 22 gears the matched set of rolls together and is driven through a line shaft 24- by a D.C. motor 26 which supplies power for the process and controls line speed. The motor is also coupled to a differential ratio gear box 28 which drives the exit bridle gear box 30, which in turn drives the matched set of rolls in the exit bridle. The D.C. rewind reel motor 32 powers the coiling reel and establishes a front tension in the strip to keep the strip from slipping on the exit bridle rolls. Tachometers 33 and 35 supply voltage to give remote indication of line speed and differential speed of the bridles. Changing the ratio of the variable ratio gear box 28 mechanically fixes the relationship of exit speed to entry speed and therefore controls the percent stretch. Electrically driven bridles, not mechanically geared together, may also be used by voltage speed regulated D.C. motors and appropriate tachometer controls and indicators. There are two advantages in a mechanical arrangement, however, in that once the ratio is me chanically set it cannot vary regardless of variation in line speed, and the total horsepower input to the motor 26 is considerably less than for independent electrical drives which are not mechanically geared together. In either case, mechanical or electrical, a controlled speed ratio of the bridles is necessary to achieve uniform stretching of the metal.
The existing state of the art in commercial continuous annealing lines involves control of tension in the strip by the use of tension controlled bridles. If the yield strength of a metal strip is exceeded, such a tension control does not know when to stop pulling, since it seeks only to re-establish a pre-selected tension. As a result, the strip can be understretched, or overstretched, or even pulled to the point of fracture. Control, in effect, is lost. This loss of predetermined control does not occur in the present stretch anneal process, because the percentage stretch is maintained independently of the magnitude of tension developed in the strip.
An example of accomplishing an improved room tem perature elongation at rupture, without sacrifice to ultimate tensile strength, by use of the stretch anneal process is given below.
Example 1 An aluminum alloy designation R402 cold rolled to the H15 temper condition had an ultimate tensile strength of about 27,000 psi. and an elongation of 3.5 percent. Subjected to the stretch anneal process while running through a heating zone at 550 F., holding zone at 350 F. and an ambient temperature cooling zone, at 200 ft. per minute (f.p.m.), the room temperature elongation was increased significantly, without appreciably affecting the ultimate tensile strength. At a speed differential of 1%, the elongation of this strip was increased to 5%; at 1 /2% speed differential, to 5 /z%; and at 2%, the elongation was improved to 6% The composition of the R402 aluminum alloy was:
Cu Fe Si Mn TiJAl Use of normal batch anneal methods would either reduce the ultimate tensile strength to get the same elongation or, if the ultimate strength was not materially reduced, the improved elongation would not be achieved. Thus parts fabricated with stretch annealed metal will endure more severe forming operations, at high strength levels, than conventionally treated material.
Example 2 Grain size mm. Flash anneal% caring .120 to. .130- 2% stretch anneal-0.68% caring .050 to .060
An illustration of the practical effects of this beneficial result is provided in FIG. 3, which shows ordinary batch annealed metal B drawn into a cuphaving objectionable scalloped edges or ears. The same alloy subjected to a stretch anneal is shown at A with a relatively smooth rim due to its non-caring characteristic.
Another capability of the stretch anneal process is to control uniformity and directionality of properties. Normal rolling processes introduce directionally unequal properties in the strip, with transverse strength normally less than longitudinal strength. By proper application of stretch annealing the transverse properties can be made very close or equal to longitudinal properties. In certain cases, transverse properties can be made to exceed longitudinal properties.
Example 3 An example of uniformity of properties in 5657 aluminum alloy processed at 1.5 stretch, 175 f.p.m. line speed 850 F. in the heating zone, 650 F. in the holding zone, and room temperature in the cooling zone is given below.
Tensile Yield Percent Strength, p.s.i. Strength, p.s.i. Elongation Longitudinal 27, 200 25, 100 8. 5 Transverse 27, 500 25, 300 8. 0
It will be noted that the longitudinal and transverse properties are almost identical. Normal annealing processes would ordinarily show differences in strengths in the order of thousands of pounds per square inch instead of only hundreds.
Example 4 An example of achieving high longitudinal yield strength in relation to tensile strength, while retaining usable elongation is given below:
Alloy 5005 Tensile strength -p.s.i. 21,600 Yield strength p.s.i..... 21,200 Elongation percen 3.9
entry speed (about from a small fraction of one percent up to several percent, and the only apparent upper limit in that regard is the physical capacity of the heated metal strip to elongate without actually breaking under the operating conditions involved. In general, however, a relatively slow line or f.p.m.) and higher temperatures (about 1000 F. in the heating zone and 800-900 F. in the holding zone) were found necessary to achieve satisfactory results when treating aluminum strip at speed differentials in the order of 10%.
The present experience comes from use of a furnace capable of heating .019" x 18" aluminum strip to 650 F. metal temperature at 300 f.p.m., with a heating zone about 30 feet in length, a holding zone of 5 feet and a cooling zone of about 20 feet. With this system, a speed differential of one to three percent or a little more is ordinarily sufiicient (in view of the wide range of temperature and speed variations possible) to obtain all of the significant benefits of the invention. The aluminum strip is heated to a maximum temperature (in the heated stretching zone) of at least 350 F. and preferably high enough for recrystallization to take place.
It has been found to be advantageous in most instances to employ different elevated temperatures in the heating and holding zones. It is especially helpful to use a temperature in the heating zone somewhat greater than the actual strip temperature desired, in order to reduce the time required to heat the strip as it advances through the stretching zone and to shorten the length of that zone. A lower temperature in the holding zone, close to the desired maximum temperature of the strip, helps to stabilize the heating effect and assure uniform treatment of the strip.
While certain examples of the practice of this invention have been given with reference to the treatment of aluminum strip, the practice is likewise applicable to other metals which respond in similar fashion to the heating and stretching operations. The term aluminum is used herein in a general sense, furthermore, to specify aluminum and alloys thereof in which aluminum is the principle constituent. Percent stretch is used interchangeably with percent speed differential, since the former is proportional to the latter under specified operating conditions. The speed differential is simply a more convenient process control. The expression stretch annea is used herein to specify an elevated temperature stretching treatment which significantly alters the metal characteristics by causing plastic deformation, whether or not the metal is actually annealed in the ordinary sense.
What is claimed is:
1. A method of treating work-hardened aluminum and aluminum alloy strip to impart improved mechanical properties and metallurgical characteristics thereto comprising continuously advancing the -strip into a heating zone and maintaining the strip under tension throughout the length of said zone by pulling the strip from said zone at a greater speed than the advancing speed; stretching the strip by heating the strip in said zone to a temperature at which the tension causes plastic deformation of the strip; and positively controlling the degree of stretch imparted to the strip by controlling, independently of the magnitude of the tension being applied to the strip, the differential between the rates at which the strip is advanced into and pulled from said zone whereby the strip is uniformly stretched.
2. The method of claim 1 wherein the strip is heated in said zone to a temperature above its minimum recrystallization point, said method further comprising cooling the stretched strip to substantially ambient temperature before releasing the tension on the strip.
3. A method of improving the mechanical properties and metallurgical characteristics of work-hardened aluminum and aluminum alloy strip which comprises continuously advancing the strip into a treatment zone and maintaining the strip under tension throughout the length of said zone by pulling the strip from said zone at a greater speed than the advancing speed; stretching the strip by heating the strip in at least a portion of said zone to a temperature in the range from about 350 F. to about 1000 cause plastic deformation of the strip; and cooling the stretched strip to substantially ambient temperature in said zone before the strip is pulled from said zone.
4. The method of claim 3 in which the entry speed of the strip into said zone is from about 10 to about 350 feet per minute.
5. The method of claim 3 in which the speed differential between the rates at which the strip is advanced into and pulled from said zone is in the order of one to ten percent of the entry speed.
6. The method of claim 3 in which the heat portion of said zone comprises an atmosphere of heated air.
F. and by applying sufiicient tension to 7 References Cited by the Examiner UNITED STATES PATENTS FOREIGN PATENTS Great Britain.
15 DAVID L. RECK, Primary Examiner.
HYLAND BIZCT, Examiner. H. F. SAITO, Assistant Examiner.