US 3247946 A
Description (OCR text may contain errors)
pri 26, 1966 A. J. KLEIN 3,247,946
METHOD OF TREATING METAL Filed Oct. 50, 1962 2 Sheets-Shea?l 1 INVENTOR. Amm? iwf/@Af /af//V BY @M @M /4 f/P/VfJ/S prill 26, w66 A. .1. KLEIN 3,247,94@
METHOD OF TREATING METAL United States Patent() Filed Oct. 30, 1962, Ser. No. 234,069 6 Claims. (Cl. 148--12.3)
This invention relates to a method of treating metal and in particular to a method of mechanically improving the dynamic ductility and reducing the yield strength of a strain-aged metal and subsequently restoring the yield strength by means of hea-t treatment.
In the past fteen years continuously annealed ferrous metal sheet has been increasingly used to replace batch or box annealed steel. The continuously annealed sheet differs from box annealed sheet in that it is stronger, sometimes just as ductile, and often quite brittle. The higher strength characteristics give continuously annealed steel sheet a decided cost advantage over the box annealed material, since a lesser gage can be used for a given application, without sacricing strength.
Unfortunately, the fabrication characteristics of the continuously annealed plate are not consistent and have caused considerable diiculty due to variations in ductility, even within individual lots. Due to frequent brittleness, many items must be fabricated from more expensive box annealed steel sheet. This is especially true in the metal container industry where tin plated steel sheet, such as is used to make tin can ends, must withstand high internal pressures during the heat processing of canned products.
It has been found that continuously annealed steel strip, of the type used in can making, is very susceptible to strain aging thus resulting in the aforementioned brittleness. While this strain aging process causes brittleness, even at room temperature, it is also part of the reason for the increased strength of continuously annealed material.
Aging is a change that occurs in the mechanical properties of certain metal alloys at room or elevated temperatures, after the metal has been rapidly cooled or after cold working. After rapid cooling quench aging is used to denote this change, whereas strain aging is the term used to indicate changes that take place in properties subsequent to cold working. Either of these aging characteristics may result in increased strength and hardness, loss in ductility and impact resistanceand a sharply defined yield point in the stress-strain curve.
In general, aging proceeds slowly at room temperature, with an acceleration in the aging process as the temperature increases. This apparently indicates that aging is a manifestation of a trend toward equilibrium and away from some unstable condition set up by rapid cooling or cold working.
It is thought that strain aging is the reason for the brittleness found in tin plate that has been continuously annealed. It is also part of the reason for the metals strength, because it hardens by precipitation of compounds from solidv solution. Strain aging is considered a time-temperature-mill process dependent mechanism, which begins after temper rolling, and progresses at a rate these factors set for it. Therefore, continuously annealed tin plate is ductile for an unpredictable period of time from the date of production until aging begins to cause embrittlement.
According to some experts, strain aging is a characteristic of cold worked low carbon steels where itis assumed that there are compounds in solid solution which want to precipitate out. During deformation, as in temper rolling, thesecompounds begin to precipitate and continue to do so slowly at room temperature. A rise in temperature will accelerate this aging because it is time-temperature dependent. Nitrides and oxides are assumed to be the aging compounds which presumably harden the metal by precipitation along slip planes. It is claimed that this hardening increases strength noticeably, but lowers ductility a great deal.
Although strain aging is generally thought of as a detriment, this invention proposes to use this property as an asset.
It is therefore an object of this invention to treat strainaged metal to increase its formability.
Another object of this invention is totreat strain-aged, cold rolled, continuously annealed ferrous metal to increase its formability.
Another object is to treat age-hardened ferrous metal sheet to lower its yield strength and increase its ductility, without inducing substantial changes in the metals hardness or ultimate tensile strength.
Another object is to increase the fabricability of agehardened steel sheet Without subjecting the metal to .a heat treatment.
Still another object of this invention is to restore stressrelieved age-hardened lmetal to its age-hardened condition subsequent -to deforming.
A still further object is to at least restore the strength of shaped, stress-relieved, age-hardened metal without further cold working. A
An additional object is to restore the yield strength of stress-relieved shaped tinplate without damaging the tin coating.
Numerous other objects and advantages of the invention will be apparent as it is better understood from the following description, which, taken in connection with the accompanying drawing, discloses a preferred embodiment thereof.
The above objects are accomplished by subjecting continuously annealed, cold rolled, aged metal strip to a mechanical stress-relieving operation either prior to or concurrently with a shaping operation whereby the yield strength of the metal is decreased and the ductility increased. Subsequent to shaping, the yield strength is at least restored to substantially its original value by an artiicial aging process.
Referring to the drawings:
FIG. l shows a diagrammatic View of a method of mechanically stress-relieving metal strip;
FIG. 2 is a graph showing a stress-strain curve for continuously annealed strip which has been flexed after asma;
FIG. 3 is a graph showing stress-strain curve for continuously annealed metal strip which has not been flexed after aging; and
FIG. 4 is a graph showing the effect of aging time and temperature on the yield strength of flexed, continuously annealed metal strip.
As a preferred or exemplary embodiment of the instant invention, FIG. 1 illustrates a method of mechanically relieving the residual stresses in a cold-rolled continuously annealed ferrous metal sheet or strip 1t). Mechanical stress-relieving may be defined as the reduction of residual stresses by mechanical methods at moderate temperatures.
The strip 1t) is fed in the direction of the arrow A between a mill having a plurality of staggered upper rolls l2 and lower rolls 14. These rolls 12 and 14 are verticaly olfset to effect an undulatory path for the strip 1t) to follow, with the axes of the upper rolls 12 lying in substantially the same horizontal plane and the axes of lower rolls, it may lbe understood that the number of rolls used is As the strip 10 advances between the rolls 12 and 14, successive portions of the sheet parallel to the direction of travel A and cold rolling, are ilexed back and forth transversely to the direction of travel A -beginning at one end of the strip 10, and progressing toward the other end of the strip 10. The rolls 12 and 14 have a maximum diameter of approximately 2 inches, with less than 1 inch preferred. Although FIG. l shows only a few offset rolls, it maybe understood that the number of rolls used is dependent upon the degree to which the residual stresses are to be relieved and also upon the hardness and thickness of the metal. Upon leaving the offset rolls 12 and 14, the strip continues in the direction A and passes between two larger rolls and 22 which do not substantially reduce the thickness of the strip 10, but rather tend to atten the strip 10.
There are many other methods for mechanical relieving residual stresses in metals. Among these are direct loading, hydraulic pressure, cyclical loading, peening, pinch or re-temper rolling and even some methods of fabrication. It may be said that such relief may be accomplished by introducing mechanical stresses that cause plastic flow. However, these methods generally introduce some degree of distortion in the shape of the material. Thus the flexing, or roller leveling as it is sometimes called, as hereinbefore described, appears to be the most useful technique,
' from a practical standpoint.
The flexing operation, while increasing the metals static and dynamic ductility, also reduces the yield strength without noticeably reducing hardness nor ultimate tensile strength. Heretofore it has been accepted that roller leveling or flexing after aging further reduces ductility. It is believed that yield strength is intimately related with ductility in the strain aging process. Although yield strength is generally thought of as applying to the entire mass of a metal, often there are areas within the metal that exhibit a considerably lower yield strength than the entire mass. At these areas the problem of localized yielding is found. As a rule, localized yielding occurs at high stress levels where plastic deformation concentrates in preferred sites, instead of uniformly over a given area. This is an undesirable property in sheet metal when it might be stretched or drawn, because premature fractures are easily obtained from it.
In flexing, or other mechanical stress-relieving operations, small residual stresses are induced which mask the yield point, as evidenced in the static stress-strain curve for metals shown in FIG. 2, and in doing so eliminate localized yielding. A sharp yield point, as shown in FIG. 3, is related to brittleness in fabrication of sheet metals, since it promotes yielding in a very local area and, as a consequence, fracturing takes Iplace at that spot before yielding can occur in adjacent areas. The smooth stressstrain curve, such as shown in FIG. 2, exhibited by mechanically stress-relieved sheet is highly desirable, because it represents a material that will yield uniformly before fracture.
It is well known that subjecting ferrous metal sheets to elevated temperatures in the neighborhood of l000 F. or above will also relieve residual stresses by the annealing mechanism. However, such high temperature treatments cannot be used with tin coated steel or metals that have been coated with materials that will deteriorate at such elevated temperatures. Thus, the aforementioned mechanical stress-relieving processes are applicable.
It must be understood, however, that there are some forming operations that are not particularly severe, wherein the shape of the metal is changed relatively little from a flat sheet to a desired configuration, such as in the stamping of certain can ends. In such situation, forming operations themselves provide sufficient mechanical stress relieving to permit additional forming without cracks. In this case the stress relieving can be accomplished simultaneously with the forming operation.
Once the ductility of the metal has been increased and the yield strength decreased, the material is then more amenable to fabrication. To take the optimum advantage of the restoration of these properties, the desired fabrication should take place as soon as possible, since the aging process will begin again and fabricability will decrease with the passage of time, even at room temperature. In fact, it is desirable that the material be used within a few days of the stress-relieving to optimize uniformity of yield strength and ductility throughout the material. In specific cases, fabrication of the metal may have to be done within 24 hours after mechanical stress relieving.
Among the many manufacturing operations that are most applicable to this process are stamping, drawing, bending, stretching, ironing, and various other methods of sheet metal shaping.
Most of the hereinbefore mentioned sheet metal shaping processes involve a relatively high rate of strain. Generally, the forming speeds will vary between 0.01 to 16 feet per second. If the metal is in the age-hardened condition, it is more than likely that cracks, splits, or tears will occur during forming. This is especially true in the 0.5 to 1.0 foot per second forming rates, which are normal commercial speeds.
When these age-hardened metals are tested at the slow 0.0003 foot-per-second speed of a standard tensile test, evidence of this brittleness will often not be evident. However, Iwhen the brittle aged metal has been stress relieved in a manner as described hereinbefore, little or no rupturing due to such brittleness results upon shaping the metal sheet at commercial forming speeds.
Since the fabricated sheet metal has a yield strength below that which it had in the aged condition, it is desirable that this greater yield strength be restored. It is well known that restoration will progress over an extended period of time at room temperature. However, in order to obtain optimum uniform properties between different batches of fabricated sheet, this invention proposes to artificially age the metal in a relatively short period of time, at a low elevated temperature giving a stress-strain curve as shown in FIG. 3.
It is readily apparent that if the yield strength of the metal can be increased after its fabrication, considerably less material thickness will be necessary for a given strength level. Such a process will thus result in significant cost savings in metal.
It has been found that rapid restoration of the yield strength in deformed low carbon steel, that had been strain aged and subsequently mechanically stress-relieved, can be accomplished at temperatures as low as 200 F. for periods as short as five minutes. However, it is preferred that temperatures between 325 and 450 F. at times from a minimum of 0.4 second to several hours be used for tin coated low carbon steel sheet. Since such low elevated temperatures are utilized in the artificial aging process, no problems in metal oxidation are encountered. FIG. 4 shows the effect of aging time and temperature on the yield strength of freshly flexed low carbon steel strip material. As can be seen there is little or no effect on yield strength at room temperature, whereas the low elevated heat treatment almost immediately increases the yield strength.
Although the yield strength is at least restored to the values found in the age-hardened material, sometimes higher values are obtained. Occasionally the yield strength will even equal the ultimate tensile strength. No increase in hardness nor ultimate tensile strength was noted. This is somewhat surprising since it is quite common for values of these properties to increase when coldrolled low carbon steel is either naturally or artificially aged. No explanation has been found, as yet, to explain this phenomenon, unless the mechanical stress relief has an inhibiting effect on changes in these two mechanical properties.
As an example of this invention, a tin coated strip of low carbon cold-rolled steel, which had been continously annealed, having the following norminal percentage chemical analysis, is selected:
Since the metal is generally found in an age-hardened condition, the properties given for the material may vary -slightly due to difference in the degree of aging between lots.
The strip is sent through a series of offset staggered rolls, having roll diameters of approximately one inch, as hereinbefore described. This process results in an effective mechanical stress relief of the metal.
After flexing, the ultimate tensile strength and the hardness show no discernible change. On the other hand, the yield strength has been reduced to approximately 63,200 p.s.i., the tensile elongation has been increased to 18%, and the impact elongation to 20%.
Once the formability of the tinplate strip is increased, ends for metal cans are stamped from the strip at an impact velocity of approximately 0.5 foot per second.
Since it is desirable to obtain as high a strength in a tin can end as possible in order to withstand the pressures produced during heat processing of canned foods, the formed metal end is then placed in an infra-red oven at 390 F. After 5 seconds at heat, the ductility and yield strength of the metal are restored to at least those values found in the aged metal. Although extremely short periods of time are desirable from a high-speed production standpoint, any time from 0.4 second to several hours may be used without overaging the metal.
Though this invention has been described in terms of ferrous metal sheet, other age hardenable metal alloys of aluminum, copper, magnesium, and nickel are amenable to this process for forming and treating metal.
It is thought that the invention and many of its attendant advantages will be understood from the foregoing description and it will be apparent that various changes may be made in the steps of the method described and their order of accomplishment without departing from the spirit and scope of the invention or sacrificing all of its material advantages, the form hereinbefore described being merely a preferred embodiment thereof.
1. A method of treating and fabricating strain-aged, continuously annealed, cold-rolled age hardenable coated low carbon ferrous metal sheet without substantially decreasing said sheets hardness or ultimate tensile strength, comprising the steps of: mechanically stress-relieving said sheet, whereby the yield strength of said sheet decreases and the ductility of said sheet increases at least 20 percent; shaping portions of said sheet, whereby said portions are strained by said shaping; and thereafter heating said shaped sheet throughout to an elevated temperature between 200 and 450 F. until the yield strength of said sheet is restored to approximately its original value.
2. The method of claim 1 wherein said sheet is 'rnechanically stress-relieved by `flexing successive portions of said sheet back and forth.
3. The method of claim 1 wherein said sheet is mechanically stress-relieved by temper rolling.
4. The method of claim 1 wherein said shaping is effected at a strain rate of 0.3 to 1.5 foot per second.
5. A method of treating strain-aged, age-hardenable, continuously annealed, cold rolled, tin coated, low-carbon steel sheet having a carbon content between 0.05 and 0.12%, a manganese content between 0.25 and 0.60%, sulfur 0.05% maximum, phosphorous 0.02% maximum, silicon 0.05% maximum, copper 0.20% maxi-mum and the balance iron, without substantially changing said sheets ultimate tensile strength or hardness, comprising the steps of: flexing said sheet by passing said sheet through a plurality of staggered, vertically offset rolls, said rolls having a diameter not greater than 2 inches, thereby bending successive portions of said sheet back and forth beginning at one end of said sheet and progressing toward the other end of said sheet, whereby the yield strength of said sheet decreased and the percent impact elongation of said sheet increases at least tenfold; shaping portions of said sheet in a press at an impact velocity of approximately 0.5 foot per second; heating said shaped portions throughout to a temperature above 200 F. but below 450 F. for a period of from 0.4 second to 2 hours until the yield strength and ductility of said sheet portions are restored to at least their original strain-aged values.
6. The method of claim 5 wherein the yield strength of said strain-aged steel sheet is approximately 70,700 p.s.1.
References Cited bythe Examiner UNITED STATES PATENTS 2,165,368 l/l939 Goss 14S-12.3 2,385,627 9/1945 Jones 266-4 2,588,437 3/1952 Ward 148-156 OTHER REFERENCES Metals Handbook (1948 ed.), pub. by the A.S.M., pp. 241-243.
DAVID L. RECK, Primary Examiner. HYLAND BIZOT, Examiner. H. F. sArro, o. MARJAMA, Assistant Examiners.