US3256119A - Method of annealing steel strip - Google Patents

Description

June 14, 1955 J. E. LOGAN E'rAL 3,256,119

METHOD OF ANNEALING STEEL STRIP Filed April 20. 1965 2 Sheets-Sheet 1 Oi O b BIOTO) O O O June 14, 1966 J. E. L'oGAN ETAL 3,256,119

METHOD OF ANNELING STEEL STRIP Filed April 20, 1965 2 Sheets-Sheet 2 ooooo oooV ooo@ ooo oooooooo INVENTORS` BY L7M @any United States Patent O 3,256,119 METHD F ANNEALING STEEL STRIP `lohn E. Logan, Culver City, Calif., and .lohn D. Keller, deceased, late of Pittsburgh, Pa., by Stella P. Keller, administratrix, Pittsburgh, Pa., assignors to George W. Jernstedt, Mount Lebanon, Pa.

Filed Apr. 20, 1965, Ser. No. 453,238 7 Claims. (Cl. 14S-12.9)

This application is a continuation-impart of our application Serial No. 280,607, Iled May 15, 1963, now abandoned.

This invention relates to improved methods of annealing steel strip in short periods of time.

In one conventional method of annealing steel strip in continuous lengths employing equipment which is considered to be high speed, use is made of tower type equipment wherein the steel strip is disposed in loops passing up and down around a plurality of rollers while surrounded with a heated protective lgas atmosphere. In such equipment low carbon steel strip is heated to an annealing temperature of about 1350 F. in a period of time of about twenty-two seconds, it is then soaked or held at this annealing temperature for about Itwenty-nine seconds, then cooled slowly over a period ofI about twentytwo seconds to about 900 F. and iinally cooled at a more rapid rate to a -iinal temperature of about 150 F. in a period of about forty-three seconds. The total time for a steel strip in this equipment is approximately two minutes, land since the strip speed is about 1000 feet per minute, about 2000 feet of the strip are in the furnace at any one time.

Such a conventional high speed annealing equipment comprises .a tower about 60 feet high and about 100 feet lon-g, to which length must be added the loopers at both ends or coiler and uncoiling stands. Commercial equipment of this type has a maximum annealing capacity of some 30 tons of strip per hour. Further, full an- -nealing to a dead soft state is impractical. The steel strip is usually of quarter hard or of slightly softer temper.

An additional problem in annealed sheet steel is that of age hardening. vThus steel which when tested immediately after annealing appears to be fully annealed, exhibits increased hardness with passage of time. This is a highly undesirable characteristic. Steel sheet or strip should not change in hardness due to storage, otherwise subsequent processing will be adversely affected. The problem of age hardening is especially prevalent in rimmed steel strip, particularly if it is subsequently temper rolled. y

Liquid metal 'baths for annealing have been proposed such as in U.S. Patent 2,797,177 issued to one of the coinventors here. The present invention comprises a novel process preferably using the apparatus of Patent 2,797,- 177 or apparatus closely si-milar thereto. However, the process' may .be carried out with .other liquid metal annealing apparatus.

The object of the invention is to provide a method of annealing carbon steel strip to a softness of, for example, 38 to 50 on the Rockwell B scale, in a fraction of the time required in other present day processes, and to secure more uniform and controllable hardness properties inthe annealed steel strip.

Another object of the invention is to subject steel strip to a rapid anneal by applying liquid metal to the surface so as to heat and cool the strip at high rates of thermal change, and to effect at least two cycles of heating to 1300 F. to 1400 F. and down to about 1050 F. to ll50 F., whereby not onlyis the steel annealed to a dead soft condition but has reduced age hardening tendencies.

Patented June 14, 1966 lCeA A further object of the invention is to provide a method of imparting a recrystallizing anneal to a carbon steel strip in a total period of time of from about 5 to 20,

seconds thereby producing a substantially dead soft strip.

A still further object of the invention is to provide a novel annealing process for steel strip comprising heating the steel strip in liquid metal to the desired annealing temperature, cooling the strip at least once to a temperature 200 F. to 400 F. below the annealing temperature, reheating thesteel strip at least once back to the annealing temperature, `and finally cooling the strip rapidly -to about room temperature.

Another object of the invention is to provide a process for rapidly annealing carbon steel ystrip in a short period of time by applying high frequency vibrations to the strip while it is being -annealed in liquid metal.

Other objects of the invention will in part be obvious and will in part appear hereinafter.

For a better understanding of the nature and objects l of the invention, reference should be had to the 'following detailed description and drawing, in which:

FIGURE 1 is la vertical cross-section to a liquid metal furnace suitable for protecting the process;

FIG. 2 is a vertical cross-section through a modified liquid metal -furnace suitable for practicing the process; and

iFIGS. 3 and 4 are vertical cross-sections through portions of liquid metal lfurnaces provided with vibratory means for practicing the process.

The present invention is based on the discovery that perature to effect the following:

(a) A continuous length of steel strip is rapidly heated in a few seconds, [for instance from 1 to. 5 seconds, to an annealing temperature of fromabout 13 00" F. to l1400" F. by applying to it the heated liquid metal;

(b) The steel strip `is held at .the annealing temperature for a period of Ifrom 0.5 to 3 seconds;

(c) The steel strip is then reduced in temperature of from about 1050 F. to l150 F. in a period of time of from about l to 5 seconds;

(d) The steel strip is then rapidly reheated in 4a second or two .to the annealing temperature of from 1300 F. to 1400 F.;

(e) `The steel strip is held at the anealing temperature for at least a second; and

(f) The steel strip is rapidly cooled to at least 300 F. Steps (c), (d) and (e) can be repeated one or more times. For certain alloy steels which are especially hard to :anneal to a dead soft annealed state, repetition of the steps (c), (d) and (e) to provide for the steel strip reaching the annealing temperature two or three times will result in exceptionally uniform fullyv annealed, dead soft strip. The triple process will be satisfactory for any strip material.

In one elective method for practicing the process, the continuous length of steel strip is preferably fully immersed in the liquid metal throughout the process. Also for maximum heat eiciency the steel strip should pass in a countercurrent manner after it reaches the zone of highest temperature so that the annealed hot strip gives its heat to the liquid metal and thereby heats the liquid metal which transfers the heat in turn to the entering steel strip. In another method, hot liquid metal is pumped against the surface of the metal strip, and high rates of vthermal change are elected. Further in this latter process the liquid metal does not adhere to the steel surfaceleaving it clear and bright. v

For very thin gauge steel strip, for example 0.002 inch thick sheet or ne wire, the times in steps (b), (c), (d), and (e) may be about 0.5 second, while steps (a) and (f) may be about one second. Accordingly, such thin gauge steel strip may be completely processed in about 5 to 6 seconds. Thicker gauge sheet or strip, for example to mil thick sheet, may require a total annealing time of 13 to 2O seconds, with 25 to 30 seconds for the triple process.

The rate of heating of the steel strip in the liquid metal causes the strip temperature to change at rates of from about 300 F. to 1500 F. per second in the initial heating step (a), and similar rates take place in the cooling step (f), while in the cooling of the strip in step (c) the rate of thermal change in the strip is from 100 F. to 500 F. per second.

Further benefits arel obtained if vibratory energy is applied to the strip during the process. Sonic vibrations increase the rate of heat transfer from the liquid, metal to the steel strip, as well as causing gas lms and surface lms to be rapidly removed. Supersonic vibratory energy, for example from 10,000 to 100,000 cycles per second, exact a signicant effect on the molecules so as to produce a more rapid attainment of a dead soft condition. It appears that the energy of the vibrations further excites the heated molecules of metal so that they more readily undergo the desired conversion from small, elongated and shattered crystal texture, due to rolling or other cold work, to' a large crystal, cubic crystal lattice characteristic of fully annealed steel.

By steel strip we include carbon steel, for example, steel having a carbon content of 0.04% to 0.15%, as well as alloy steels of various compositions. Also strip comprises lengths of sheet of various widths, wire which may be round or of other cross-sectional shape, as well as narrow strip proper. The gauge steel suitable for tin plating is readily annealed to a dead soft state-for example a Rockwell B hardness of from 38 to 42.

The following table sets forth examples of the practice of the present invention in which low carbon-0.04% to 0.15% carbon-rimmed steel strip has been fully annealed, all times being in seconds:

Process A B C D E F G (a) Heat to 1.330-1,350 F 5 3 5 1 2 1 1 (b) Hold at Temperature 2 2 2 1 3 0. 5 0. 5 (e) Cool from 1,330-1,350 F. to 1,100 F 5 3 3 1 3 1 1.0 (d) (e) Rehea't to about 1,330-1,350 F.

and hold time 2 2 2 1 3 1. 5 1. 5 Repeat (c) 0.5 Repeat (d)(e) 1.5 (I) Cool to 300 F--- 2 2 Total Time 19 13 17 7 12 6 8.0

Cycles F and G are applied to the thinnest gauge steels.

Our invention is not limited to any one particular means for heating and/or cooling the strip. Any means which will heat and reheat the strip and will then cool it at a controlled rate may be used; the choice among several means available being governed by the particular requirements of a given installation and by economic feasibility.

For reasons which will appear in the following explanation, we prefer in gene'ral to effect both the rapid heating and the cooling of the strip by bringing it into contact with liquid sodium or other suitable metals, such as lead, bismuth, NaK, lithium and other liquid metals, as disclosed in United States Letters Patent 2,797,173 and 2,- 797,177 granted to J. D. Keller, one of the applicants. Sodium has greater heat transferring capacity than any other known substance. It can therefore produce extremely rapid temperature rise of the strip, or hammerblow heating, while requiring only a small excess of temperature of the sodium above that of the strip. Sodium also protects the surface of the strip, since its avidity for oxygen prevents the formation of any iron oxide on the strip surface. We way, vof course, alternatively use other liquid metals or alloys.

In carrying out our invention by the use of liquid metal or sodium, we preferably use the arrangement shown in the accompanying drawing constituting a part hereof in which like reference characters designate like parts.

As shown in FIGURE l of the drawings, the continuous steel strip 1 passes over guide roll 2 into a container generally designated by the numeral 3 which is divided into compartments or chambers 4, 5, '6 and 7 by partition walls 8, '9 and 10. Container 3 is preferably constructed with a stainless steel liner 11 and an exterior sheet metal housing 12 with suitable thermal insulating material 13 therebetween. The container 3 is lled with a liquid metal such as sodium up -to the level indicated `by the dot and dash lines. A depending partition 14 with short transverse partitions 15, 16, 17 and 18 divide chamber 4 into a plurality of controlled heat zones 19, 20, 21, 22 and 23 in which the temperature of the liquid sodium is regulated by heating elements 24. Strip 1 is rapidly and progressively heated upto 1300 F. to 1400 F. by contact with the liquid sodium.

The strip 1 passes over a series of guide rolls such as lower roll 25 and over rolls 26 and 27, some of which may be driven, into a soaking chamber S which is maintained at the soaking temperature closely similar to that present at the top of the annealing chamber 4, by heating elements 28. A suitable annealing temperature is usually about 1330 F. to 1350 F. for low carbon steel strip and the heating elements 28 in the soaking chamber 5 make up for any loss of heat from the liquid sodium to the walls 8 and 9 of said chamber and the length of the strip travel in the chamber S in relation to the strip speed is such as to allow the strip to be within the chamber for the desired period of time which usually does not exceed several seconds.

From the 'soaking chamber 5 the strip passes into the chamber 6, which is a slow cooling zone where, by means of cooling or heat abstraction elements, such as tubes 29 through which air at a temperature below 1100 F. is passed, the sodium metal is cooled and consequently the strip is cooled from-about l350 F. to about 1050 F to l F. A plurality of guide rolls provide for several loops of strip in chamber 6 and thus the period of time during which the strip is cooled is determined. An insulated cover 30 may be provided over chamber 6- to separate it from adjacent hotter zones. From the chamber `I5 the strip passes into chamber 7 which is a second heating and soaking chamber where, by means of heating elements 31, it is again very rapidly raised to the annealing temperature of between about 1300 F. to 1400 F.

The strip leaving the second soaking chamber`7 is returned by guide rolls 32 and 33 to the top of heating chamber 4 where the temperature of the liquid sodium is about l350 F. The strip passes downwardly around roller 34 thence up and out of the annealing chamber 4 where it is guided around rollers 35 and 36. For liquid sodium metal the strip leaves chamber 4 at a temperature of about 300 F.

Pairs of sealing rollers 37 supported in closely fittingA curved bearings 38 are provided to prevent the entrance of atmospheric air and they also prevent the escape of the liquid sodium metal from the container chamber 4. The rollers also squeeze off any sodium lon the strip'. The numeral 40, FIGURE l, designates a liquid metal by-pass having openings 41, 42, 43 and 44 leading to the sodium filled compartments 4, 5, 6 and 7 of the lcontainer to `allow the liquid sodium to reach a nearly equal level in each compartment, but these openings are so small that while they permit equalization of the sodium levels, no appreciable circulation of sodium between the compartments can take place.

Tubes 74 and 74' enable the introduction of an inert gas such as argon above the liquid sodium in the container. v

It shall be noted -that the level of the liquid sodiumis just below the upper rolls 26, 27, 33, 34 and so on. Higher levels of the liquid metal may be provided so as to cover the steel strip during the entire time in the yannealing apparatus.

Because of the proximity of `the strand moving over guide roll 33 downwardly to the guide roll 34 with the strand moving upwardly from guide roll 25 to guide troll 26, there is an elfective transfer of heat from the downwardly passing strand to the cooler upwardly passing strand. This recovery of heat reduces the fuel or energy consumption to only a fraction of what it is in conventional continuous-annealing equipment where but little heat is recovered.

In contrast with the very large tower type annealing apparatus previously described as being 60 feet high and 100 feet in length with several thousand feet of strip being disposed therein, an annealer of equal throughput capacity -using liquid sodium according to our invention is only about 20 feet high times 12 feet long, and only about 150 to 250 feet of strip need be in the annealer container at one time. Furthermore, because of the extraordinarily great -heat transfer characteristics of the sodium, the maximum temperature of the liquid bath ordinarily need not `be higher than about 50 above the desired annealing temperature of the strip, and the movement of the strip can, without harm or detrimental results to the strip, be stopped with the strip in the bath. This permits strip loopers, which are required at both the inlet and the outlet ends, of conventional annealers, to be dispensed with in our invention.

In the modification of the apparatus shown in FIGURE 2 of the drawings, the strip 1 is rapidly heated in an annealing chamber 50 to the desired annealing temperature in liquid sodium, and passes into a first soaking chamber 51 where it is maintained at that temperature for the desired period of time, wherein heating elements 52 make up for any loss of heat from chamber 51. From chamber 51 the strip passes into a cooling chamber 53 provided with cooling -tubes 54. The length and number of passes of the strip in chambers 50 and 53, respectively, is correlated to the speed of movement so as to give the desired time for the strip to remain in each portion.

IFrom chamber 53 the strip re-enters the sodium at the top of chamber 50 for reheating to the annealing temperature of 1300 F. to 1400 F. Chamber 50 thus acts as a second soaking zone. The strip passes over appropriate rollers 5v5, 56 and 57 into a liquid sodium-filled second cooling chamber 58, and after reversing direction over roller 59 at which point its temperature is about 1100 F. and then it proceeds downwardly where it reenters chamber 50 at a point where the temperature of the sodium in 50 is about the same as that of the strip. The strip then passes down around guide rollers 60, 61 and 62 through the lower zones of 50 in close proximity to the upward moving strand of incoming strip passing around roller 63, as shown.

Tubes 64 are provided in chamber 58 through which gases either cooler or hotter than the strip or the sodium may be passed in order to effect quick regulation of the strip temperature when changes of the strip speed or of other factors occur.

lIn order to reduce the energy consumption still further, by recovering the heat given up by the strip in the slow cooling part of the cycle, the slow cooling chamber v 53 may be surrounded by a jacket 65, shown in FIG- URE 2 as a double-walled passage 66, connected to a pump 67 which draws liquid sodium from the cooler parts of the system as at 68 or 69 and causes it to circulate through the jacket in a direction generally counter to the motion of the strip passing through the slow cooling zone 53 as shown by arrows. The hot liquid sodium is then released from the jacket by the pipes shown in dashed lines into the upper portions of chamber 50 and then to inlets 68 and 69. For example, at inlet 68, the

6 sodium is at a temperature of about 700 F., at 69 it is about 300 F. to 500 F. By means of a three-way valve 70, or the equivalent, the relative proportions of 700 F. and 300 F. sodium drawn into the pump may be regulated to give the most suitable temperature of the sodium entering the jacket 65` at 71. The jacket is of course covered externally with thermal insulating material. The circulated sodium in the passage 66, having absorbed the heat given od by the strip in the slow cool- 4 ing zone 53, passes into chamber 50 and is returned to the pump through passages 68 and 69. Chamber 50 is the equivalent of chamber 4 of FIGURE 1.

`The use of liquid metal is not essential in chambers 51 and 53 since no heat transfer to the strip is required in 51 and only a low rate of heat transfer is required in 53. Hence chambers 51 and 53, as well as chamber 7, FIGURE 1, may be iilled with a protective gas such, for example, as argon or helium (nitrogen gas can be employed with certain liquid metals to which it is inert), which is admitted through pipe 74. The use of liquid metal in chamber 7, FIGURE 1, though not absolutely essential, is still very desirable, at least in the part where the strip enters this chamber, because here it is desired to raise the strip temperature as rapidly as possible from about 1l00 F. to the annealing and holding temperature.

In FIGURE 3 is shown means for producing vibration in the strip. For supersonic vibration we may preferably provided transducers 75 made for example of ceramic material such as barium titanate and electrically excited by a high frequency source of alternating electric current supplied by connections 76. Such transducers and exciters are regularly available as standard articles of commerce. The vibrations generated by the transducers if mounted outside the sodium container 77, may be transmitted as shown in FIGURE 3, by rods 78 made,

`for example, of titanium metal, to the liquid -sodium 79 in close proximity to the strip.

Another vibration-producing means which we may use is shown in FIGURE 4. The bearing 80 of the shaft of roller 81 over which the strip passes is designed to have -some exibility or resilience, and an electrically operated vibrating device comprising an electrical coil and a magnetostrictivecore of magnetic alloy which vibrates at the frequency of electrical current supplied to the coil, such as is sold under theA trade name Syntron 82, and is a standard article of commerce, vis mounted in connection therewith. The position of mounting may be such as to produce vibration of the strip either in the direction of its length or at right angles to its surface plane. The latter is more effective for increasing the rate of heat transfer, the former for exciting the molecules of the strip metal.

In addition to the examples in the table above, a specific example of the practice of the invention is as follows. Cold rolled steel strip with a carbon content of 0.12% and of a thickness of 9 mils is passed through molten sodium metal which brings it up to the annealing temperature-of 1350 F. in 3.5 seconds, the strip is held at this temperature for 2 seconds, it is then cooled slowly over a period of time of 1.5 seconds to l F., the strip is reheated to 1330 F. in one second, soaked for 2 seconds, then is cooled again over a period of 1.5 seconds to 1100 F., is reheated to 1330 F. in one second, is held at 1330 F. for 2 seconds and then cooled to 300 F. in three seconds. The strip is withdrawn from the molten sodium and nally cooled rapidly to about 100 F. by dipping in cool oil. The resulting annealed steel strip had a Rockwell B hardness of about 40. It resists age hardening and its hardness is constant. The sodium metal results in a bright, clean surface being present on the steel strip. Further, the process is expedited when the steel strip is subjected to vibrations at a frequency of 9600 cycles per second during the preheating step (a).

The high heating rates appear to produce results not obtainable with slower heating rates in conventional processes whereby more complete annealing and a more stable steel strip is obtained.

While we have demonstrated the invention as carried out by the use of liquid metal as the heating and treating medium, it is evident that any means for carrying out the rapid heating to annealing temperature, as for eX- ample an induction coil and blasts of high thermal capacity gases, cooling it and again reheating it, is Within the scope of the present invention. The novel method basically consists of the specied double annealing and cooling steps in the heat treating cycles. The heating and cooling iluids or heat transfer materials may or may not be of the same composition.

Although several embodiments of the invention have been herein illustrated and described, it Wil-l be evident to those skilled in the art that various modifications may be made in the details of construction without departing from the principles herein set forth.

We claim as our invention:

1. In a continuous method of annealing steel strip by passing the strip through hot liquid metal, the steps comprising:

(a) passing the strip into the liquid metal to rapidly heat it to an annealing temperature of from 1300 F. to 1400 F. in a period of time of from about 1 to 5 sceonds,

(b) maintaining the strip at the annealing temperature `for a period of from about 0.5 to 3 seconds,

(c) cooling the strip ,to a temperature of from 1050 F.

to 1l50 F. in a period of from about 1 to 5 seconds,

(d) reheating the strip to the annealing temperature of from about 1300 F. to 1400 F. in a period of from about 1 to 3 seconds,

(e) maintaining the strip at vthe annealing temperature for a brief period, and

(f) cooling the strip 4in the liquid metal to a temperature of about 300 F. and lower in a period of from 1 to 5 seconds.

2. The process of claim 1, wherein the cooling (c), reheating (d) and maintaining (e) steps are repeated at least once. i

3. The process of claim 1, wherein the steel strip is subjected to vibrations during at least the heating step (a) whereby heat is imparted more rapidly, surface hns are removed from the surface of the strip, and improved annealing are secured.

8 4. The process of claim 1 wherein the liquid metal is sodium.

S. The process `for producing substantially fully annealed carbon steel strip which comprises:

(a) rapidly heating the steel strip to an annealing temperature of from about 1300 F. to 1400 F. in a period of from about 1 to 5 seconds,

(b) maintaining the strip at the annealing temperature `for a period of from about 0.5 to 3 seconds,

(c) cooling the strip to a temperature of from 1050 F. to 1l50 F. in a period of from about 1 to 5 seconds,

(d) reheating the strip to the annealing temperature of from about 1300 F. to 1400" F. in a period of from about 1 to 3 sceonds,

(e) maintaining the strip at the annealing temperature for a brief period, and

(f) cooling the strip to a temperature of about room temperature in a period of about l to 5 seconds.

6. The process of claim 5, wherein steps (c), (d) and (e) are repeated at least once.

7. In the process of producing a non-aging, fully annealed steel strip wherein annealing is effected at temperatures of from about 1300 F. to 1400 F., the improvement comprising cooling the steel strip several times from the annealing temperature to about 1050 F. to 1150 F. and reheating it back to the annealing temperature of from about 1300o F. tol400 F., each cooling and reheating step being eifected in a short period of time of the order of 1 to 3 seconds.

References Cited by the Examiner UNITED STATES PATENTS 1,792,573 2/1931 Cox et al 14S- 144 `2,125,128 7/1938 rRobinson 1 148-134 :2,798,832 7/1957 Harvey 14S-12.9 2,848,775 8/ 1958 Ettenreich 14S-12.9 2,920,988 1/1960 Bulat 14S-12.9

OTHER REFERENCES Metals Handbook (1948 Ed), published by the A.S.M., pp. 617 and i744 relied upon.

DAVID L. RECK, Primary Examiner.

Claims (1)

1. IN A CONTINUOUS METHOD OF ANNELAING STEEL STRIP BY PASSING THE STRIP THROUGH HOT LIQUID METAL, THE STEPS COMPRISING: (A) PASSING THE STRIP INTO THE LIQUID METAL TO RAPIDLY HEAT IT TO AN ANNELAING TEMPERATURE OF FROM 1300*F. TO 1400*F. IN A PERID OF TIME OF FROM ABOUT 1 TO 5 SECONDS, (B) MAINTAINING THE STRIP AT THE ANNEALING TEMPERATURE FOR A PERIOD OF FROM ABOUT 0.5 TO 3 SECONDS, (C) COOLING THE STRIP TO A TEMPERATURE OF FROM 1050*F. TO 1150*F. IN A PERIOD OF FROM ABOUT 1 TO 5 SECONDS, (D) REBEATING THE STRIP TO THE ANNEALING TEMPERATURE OF FROM ABOUT 13000*F. TO 1400F. IN A PERIOD OF FROM ABOUT 1 TO 3 SECONDS, (E) MAINTAINING THE STRIP AT THE ANNEALING TEMPERATURE FOR A BRIEF PERIOD, AND (F) COOLING THE STRIP IN THE LIQUID METAL TO A TEMPERATURE OF ABOUT 300*F. AND LOWER IN A PERIOD OF FROM 1 TO 5 SECONDS.

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