US3260623A - Method of tempering continuously annealed metal sheet - Google Patents

Description

A. J. KLEIN July 12, 1966 METHOD OF TEMPERING CONTINUOUSLY ANNEALED METAL SHEET 2 Sheets-Sheetl l Filed Oct. 4, 1963 INVENTOR ,4 55er Jess/JH /fLf//v BY MJ@ la/- July l2, 1966 A. J. KLEIN 3,260,623

METHOD OF TEMPERING CONTINUOUSLY ANNEALED METAL SHEET United States Patent O 3,260,623 METHQD F TEMPERHNG CONTINUOUSLY ANNEALED METAL SHEET Albert Joseph Klein, Arlington Heights, Ill., assigner to American Can Company, New York, N.Y., a corporation of New Jersey Filed (let. 4, 1963. Ser. No. 314,842 6 Claims. (Cl. 148-12) This is a continuation-in-part of my co-pending application, Serial No. 235,950, tiled November 11, 1962, now abandoned.

This invention relates to the method of manufacturing ferrous metal sheet. More particularly it refers to a method of producing ferrous metal sheet having high stiffness and resistance to bulging.

Prior to this invention, ditlculty has sometimes been encountered with sheet met-al containers that are used in packaging products which are under substantial pressure, such as carbonated beverages and beer. One of the principal difficulties has been .the inability of .the metals from which these containers are made to withst-and the internal pressures produced by the products contained therein, thereby causing the container to bulge and even rupture .the container seams in some cases.

`Since the thickness of the material used in the manufacture of such containers is limited due to economic and other factors, the approach followed in solving this probllern has been to increase the strength of the materials used in the manufacture of the containers without increasing the material thickness. An early attempt at this was made by Goss, U.S. Patent 2,165,368, who utilized certain rolling and heat treating procedures in order to produce a ferrous sheet metal having increased stiffness and workalbility. However, his method left much to be desired, since the strength of the metal sheet produced was considerably lower than .that necessary in the manufacture of metal containers today.

Since the yield point in metals is where metals begin to de-form plastica-Hy, it is necessary to have as high a yield point as possible in order to prevent metal from taking a permanent set when some deformation occurs. By means of Gosss technique, it is possible to produce sheet steel having a yield strength of 55,500 p.s.i., the test being run on strip having a thickness of 0.009 inch. This yield strength is below that in use today, which is 62,000 to 64,000 p.s.i. As vthinner strip is being planned for use in container manufacture, it is apparent that even stronger steel will be required, without substantially decreasing the metals formability.

It has been generally thought that increasing the degree of cold reduction of metal strip will, in turn, proportionally increase the stiffness of the metal. This concept has been found to be not necessarily true. Previous to this invention it has generally been held that the stiffness of sheet metal depends only upon the magnitude of the yield point; or, in other words, the higher the yield point, the stiffer the metal with a corresponding decrease in ductility. `From this it would be reasoned that in order to produce strip having appreciable stiffness, it would be necessary to obtain a material having an extremely high yield strength and therefore an extremely low ductility, a ductility so low as to make the sheet difficult to wor-k.

This invention has found that the biaxial stiffness in continuously annealed, age-hardenable ferrous sheet metal unexpectedly levels olf substantially over an intermediate range of temper rolling, while the yield strength expectedly increases gradually over the same range when the metal is treated according to the teaching of this invention. Thus a metal having a modest yield strength, with good fOrmabiIty, will have a biaxial stiffness substantially equal to a more brittle less formable metal.

It is therefore an object of this invention to provide a method of manufacturing metal sheet having appreciable stiffness, while maintaining its fonmability.

Another object is to provide a method of manufacturing ferrous metal sheet having superior resistance to bulging without materially changing existing sheet metal manufacturing procedures.

Still another object is to provide a method of the character described which can be readily and relatively inexpensively incorporated i-nto present commercial procedures for manufacturing met-al sheet.

A further object is to provide a method of producing thin ferrous metal sheet having superior stiffness without excessively high yield strength.

Another object is to provide a new and improved metallurgical process wherein stiffness properties of ferrous sheet can be improved in a simple and effective manner, without excessive-ly cold working the metal strip.

Yet another object is to provide a method of producing low-carbon steel sheet having strength and fabricability substantially equal .to conventional-ly manufactured continuously annealed sheet, although having a thickness less than conventionally manufactured sheet.

A still further object is to provide low-carbon steel sheet for manufacturing strong metal containers wherein thinner sheet may -be used, thereby reducing the quantity of -meta-l in the container, without substantially sacrificing the required strength and formability. Y

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 drawings, discloses a preferred embodiment thereof.

In accomplishing the above objects, hot-rolled low carbon steel strip is cold rolled until it is reduced in thickness to within 10 to 23% of its final gauge. The strip is then continuously heat treated at a temperature above F., but below the A1 point, thereby producing a spheriodized structure in the steel. Thereafter the strip -is further cold rolled to its iinal thickness. Subsequent to reaching the final gauge the strip preferably is electrotinned and the tin deposit is then flow brightened.

Referring to the drawings:

FIG. l is a schematic view showing the first cold rolling operation followed by cleaning and the spheroidization heat treatment.

IFIG. 2 is a schematic yView showing the cold reduction of the strip to linal gauge.

FIG. 3 is a schematic view showing the coating of the strip.

yFIG. 4 is a graph showing the relationship of stiffness, dynamic ductility, and yield strength to the percent of reduction in the final cold rolling operation.

As a preferred or exemplary embodiment, the instant invention utilizes any -low carbon steel, i.e., that having a carbon content of 0.05 to 0.25%. This steel usually contains other elements such as manganese, 0.40 t0 0.80%, residual elements, and may also contain minor amounts of alloying elements. This steel may be rimmed, semi-killed or fully-killed steel.

A typical steel, that is used in .the manufacture of containers for holding products under substantial pressure, has the following nominal percentage chemical analysis:

`Carbon 008-011 Manganese G50-0.60 Sulphur 0.02-0.03 Phosphorus 0005-0010 Silicon 0.005 max.

Molybdenum 0004-0006 Nickel 0.025-0040 Chromium 0.0l8-0.023

3 Copper 0.009-0.013 Aluminum 0010-0016 Nitrogen 0.008-0.010

Iron Balance The steel, in the form of a strip 10, is cold reduced on a 4 to 6 stand four high tandem rolling mill, generally designated 12, lfrom a hot rolled thickness of approximately 0.125 inch down to about 0.01 inch. In certain cases the cold rolled thickness may be reduced a lesser amount, e.g. to 0.025 inch, depending upon the fin-a1 thickness desired.

The rolling mill is composed of a series of stands 14 having working rolls 16 and back-up rolls 1S. Each lof the stands 14 reduces the thickness of the strip 10 to a thickness less than the preceding stand.

Subsequent to the cold rolling operation the strip 10 is cleaned by .a suitable comercial cleaning method such as illustrated by cleaning tank 20, containing a chemical cleaning composition 22. This removes any accumulation of oil or dirt deposited up-on the strip 10 during cold rollin-g.

Upon leaving the cleaning operation, the strip 10 enters a continuous heat treating furnace 24 where the strip 10 is brought up to a temperatureabove 1000 F., but below the A1 point (1333 F.). The heat-up time is about 20 seconds, followed by -a hold time of about from 10 to 25 and preferably about 15 seconds.

During the hold time of this heat treatment two distinct changes take place in the steel.

Recrystallization of the steel occurs, substantially instantaneously, from a cold worked condition, i.e. elongated -grains, to an annealed condition. With rimmed steel and semi-killed steel the grains in the annealed condition are substantially all equiaxed; whereas with fullykilled steel the grains in the annealed condition are a mixture of equiaxed and elliptical.

Also the shape of the carbides within the steel changes from slightly irregular to spheroidal. This change in carbide shape does not occur instantaneously, but requires a linite time interval, usually la matter of seconds.

It has been found that, for the purposes of the instant invention, the spheroidal shape of the carbides is essential and critical, but the shape of the steel grains is not.

However, it is possible to perform the heat treating step to.obtain a splheroidal structure by heating slightly above the A1 temperature for less than 5 seconds, then cooling slowly below A1, and holding until the desired structure is obtained.

Another method is to heat and cool the metal alternately between te-mperatures that are just above and below the A1 temperature.

Complete recrystallization of the steel occurs almost instantly after the srip 10 reaches 1000 F. resulting in a ne grained material having an ASTM grain size of approximately 10 or greater. In order to minimize surface oxidation of the strip 10 it is generally desired that a non-oxidizing atmosphere, such as nitrogen, be used in the heat treating furnace 24.

To reduce the strip temperature to -a value suitable for subsequent processing after leaving the heat treating furnace 24, the strip 10 enters a cooling chamber 26 Where the strip 10 is cooled for about 15 seconds to a temperature of approximately 900 F. If the A1 temperature is not exceeded during heat treatment to spheroidize the carbides, cooling may be effected more rapidly. Then the strip 10 enters another `chamber 28, where the strip 10 is co-oled as rapidly las possible to a temperature of approximately 250 F. This cooling usually requires 'about 50 seconds. It is desirable that both the cooling chambers 26 and 28 contain non-oxidizing atmosphere to prevent contamination of the metal surface. Once the strip has been cooled to approximately 250 F. it enters a third cooling chamber 30 where the temperature of the strip is brought down to ambient temperature. After heat treatment, the strip 10 is received on a coiler 32.

After the spheroidization heat treatment and coolini the strip 10 is temper rolled to iinal gauge as shown in FIG. 2. A pair of four high tandem rolling mills 34 cold reduce the thickness of the strip 10% to- 23%. It should be understood that the iinal temper rolling of the strip 10 may be done on rolling mills having either more or fewer stands without departing from the scope of this invention. Thus a single pass through a rolling mill giving a reduction of 10 to 23% will have the same desired stiiness and yield strength as will the strip that has been reduced a total of 10 to 23% through a plurality of rolling mills. It is preferred that a reduction of 10 to 18% be used for obtaining the rnost desirable properties, with a reduction from 10 to 14% optimum.

Once the strip 10 has been reduced to nal gauge it may be used without further treatment in the fabrication of parts, or may be coated in order to increase its resistance to corrosion. I-f it is desired that an ordinary tin can be produced, the strip 10 lis then coated by conventional electrotinning according to techniques well known in the art and illustrated in FIG. 3. Subsequent to electroplating, .the tin is flow brightened by means also well known in the art, such as by using resistance or induction heating equipment 40 to produce a bright tinplate, which is commonly .used in the manufacture of metal containers. Thereafter the strip is wound into a coil 42.

The instant invention is designed primarily for inytegration with the commercial production of tinplate; and for this reason it is preferred to temper roll the strip the designated 10 to 23% prior to the tinning operation. However, the desired increase in stilness can also be obtained by temper rolling the strip to its final thickness after it has Ibeen tinned.

Yield strength of sheet steel when subjected to uniaxial tension, such as in a standard tensile test, does increase with the amount of cold working, e.g., thickness reduction by temper rolling, but does not change with the strip thickness, as does biaxial stiffness. Yield strength is shown in the curve in FIG. 4 wherein the yield strength of the low carbon steel strip increases from 55,000 p.s.i. at 3% reduction in thickness by temper rolling to 100,000 p.s.i. after a 50% reduction. Steel strip produced in accordance with the instant invention has an ultimate tensile strength of about from 70,000 to 85,000 p.s.i., a yield strength of about from 60,000 to 80,000 p.s.i. and a dynamic elongation of about 5 .to 15%.

Completely unexpectedly it was found that sheet steel produced in accordance with the instant invention had a higher biaxial stillness, i.e., resistance to bulging or deformation along two of its major axes, for a given thickness and degree of ductility, than steel rolled and heat treated according to conventional practice.

It has generally been accepted that biaxial stiffness is directly proportional to the yield strength of the steel. As can be seen in FIG. 4, the yield strength vs. percent reduction from temper rolling is nearly a linear relationship, resulting in an almost straight-line curve.

However, when the biaxial stiifness is plotted against the percent reduction from temper rolling, a slight peak is encountered at 18% reducti-on, with a leveling oil, or plateau in the curve between 18 and 23% reduction and a sharp rise at about 25%. This was found for both strip Irolled to ll mils thickness and 7 mils thickness. It is apparent, though, that the biaxial stiffness will vary for a given rolling reduction for steel strip of different thicknesses.

Anoth'er factor that enters into the consideration of the` degree of temper rolling to be done to produce the optimum strength with good formability in the strip is the dynamic ductility, as measured by percent elongation in a standard tensile test, except that the strain rate was 0.5 foot per second rather than the standard 0.0003 foot per second. The dynamic ductility thus gives a close estimate of the forming properties 'of the metal during such operations as drawing, stamping, and punching.

It is evident in FIG. 4 that the dynamic `ductility decreases sharply between and 18%, while the yield strength increases gradually and biaxial stiffness increases gradually and tends to level off near 18% As indicated previously, the ductility ofthe sheet steel generally decreases as the yield strength increases. Contrary to the apparent teachings of the prior art, as was hereinbefore discussed, FIG. 4 shows that biaxial stiltness does not continue to increase with yield strength. Therefore, by means of the instant invention, Ifor any given sheet thickness, high biaxial stittn'ess is obtained without greatly increasing the yield strength of the sheet and thereby substantially lowering its ductility; or in other words ductility is maintained at an operable level while obtaining high biaxial stiffness.

It is thought that the invention and many of its attendant advantages will be understood from the foregoing description and it will be apparent from the foregoing description that 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.

I claim:

1. In a method of producing low carbon ferrous sheet metal strip having high stiffness and yield strength with satisfactory ormability, whereby said strip is hot rolled to an intermediate thickness, then cold reduced at least 80%, the steps comprising: continuously heat treating said cold reduced ferrous strip at a temperature above 1000 F. and below 1400" F. for a time of about from 10 to 25 seconds, whereby the carbides in said ferrous strip assume a spheroidal structure; cooling said heated strip to substantially ambient temperature; and temper rolling the cooled strip to a thickness whereby a reduction of from 10 to 23% is made in the thickness of the strip, thus producing a strip having increased stillness and adequate formability.

2. The method of :claim 1 wherein said cold rolled strip is heated between 1000 F. and the A1 temperature.

3. The method of claim 1 wherein said cold rolled strip is heated above the A1 temperature for less than 5 seconds and then -cooled slowly to a temperature slightly below the A1 temperature and held at said temperature below the A1 temperature until a spheroidized structure is obtained in the ferrous metal.

4. The method of claim 1 wherein said cold rolled strip is alternately heated and cooled between temperatures `slightly above and slightly below the A1 temperature until a spheroidzed structure is obtained in the ferrous metal.

5. The method of claim 1 wherein the heat treated strip is temper rolled -to a thickness reduction of from 10 to 18%.

6. The method of claim 1 wherein said temper rolled strip is electrolytically :coated with tin.

References Cited by the Examiner UNITED STATES PATENTS 2,381,435 8/1945 Burns 148-12 2,497,164 2/1950 George et al. 148-12 2,606,848 8/1952 Farling et al. 148-12 2,832,711 4/1958 Krahe et al. 148-156 3,058,856 10/1962 Miller 148-16 3,139,359 6/1964 Morgan 14S-12.4

DAVID L. RECK,P1-imary Examiner.

H. F. SAITO, Assistant Examiner.

Claims (1)

1. IN A METHOD OF PRODUCING LOW CARBON FERROUS SHEET METAL STRIP HAVING HIGH STIFFNESS AND YIELD STRENGTH WITH SATISFACTORY FORMABILITY, WHEREBY SAID STRIP IS HOT ROLLED TO AN INTERMEDIATE THICKNESS, THEN COLD REDUCED AT LEAST 80%, THE STEPS COMPRISING: CONTINUOUSLY HEAT TREATING SAID COLD REDUCED FERROUS STRIP AT A TEMPERATURE ABOVE 1000*F. AND BELOW 1400*F. FOR A TIME OF ABOUT FROM 10 TO 25 SECONDS, WHEREBY THE CARBIDES IN SAID FERROUS

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