US2393363A - Heat treatment of thin metal articles - Google Patents

Description

.L'=1n'. ,22, D GOLD ET AL v HEAT TREATMENT OF THIN METAL ART ICLES Filed March 15, 1943 3 Sheets-Sheet 1 venfo-s af- 1?. dold, I 6. D. 1 01 12758 Jan. 22, 19416;

J. o. sou: ETAL HEAT TRBATI'IENT OF 'THIN METAL ARTICLES 3Sheefs-Sheet 2 Filed marbh 15, 1943 31wcntors v J.' D. sou: & c p. FOUL-KE am eg Jgn. 22, 1946. J. D. GOLD ETAL HEAT TREATMENT OF THIN MET-AL ARTICLES.

Filed March 15, 1943 s Sheets-Sheet s v 0 O O Q Q Q .26 (o 0 v 0 O O 0 2a g zl g 1312 6. 29 t: if

present practice,

however by much stiil'er plate and-bot tle tops, but blackyplate Patented Jan.

UNITED STATE s PATENT foFi-"icE near TREATMENT or rm METAL ARTICLES JohmD. Gold and Clarence D. Foulke, Steubento National Steel Corporation; a corporation of Delaware Application March 15,1943, serial No. 479,246

12 Claims (01. 14s-'10) q ville, Ohio, assignors The present invention relatesto an improved process of producingthin'metal articles having controlled temper and the product thereof.

In the cold working of metals the crystalline structure is modified so that the resulting prodmetal sheet. Although the term net is extremely hard and in the case of sheet metal and wire verystifl. In order to adapt.

7 annealing. Under annealingm'ethods now inuse it is extremely diillcult if not impossible to a rive at an intermediate product with consistently controlled temper. v

As an example of the problem involved and thin sheet steel called black plate for ultimate manufacture into tin plate is steel is hot rolled down to about 16 gauge on a continuous hot stripmill. The hot rolled' strip remove the hot mill scale and through a continuous cold mill where it is re- 4 special heat treatment a worked condition. According to the present inis unrelieved supplies stiffness ing portion modifies the over-all effect'in the now commonly produced as a' product of a continuous cold rolling step. A slab of low carbon" is passed through a'continuous pickling line to is then passed 30 Although sheet described herein, it

duced to black plate gauge, commonly between .025 and .005 ofan inch in thickness. In the usual practice coils of this cold 'reduced'strlp, after the cold mill oil has been removed, are box annealed to dead soft condition; Since. sheet steel in this condition is toosoft for tin plate.

temper must be imparted to it. This isnormally done by passingthe full soft strip through skin pass or temper mills where a very accurately controlledpercentage of reduction gives the desired temper; I

A specific example" of the problem of arriving at the correct temper may be studied relative to tin plate to be used in can bodies. In such prod- ,uct the stiller the tin plate-the lighter the gauge sheet which may be used. .The stiffness is limited the. ability, of the plate to form a side seam. If the plate will not produce a seam which remains flat; it is too stiff. On the other hand, if the plate is too soft, fluting will occur when the cylindrical body is being formed. A isdesired for can ends condition is too still for even these products.

The present invention is a process for produci in its full hard article of determinatelength indeterminate above two terms.

strip metal as developed by cold working, the

term is used herein to designate hardness or stiffnessof sheet or strip metal as arrived atby a of metalinitially in cold vention. afull hard sheet nietal or wire is heat treated under exacting conditions to result directly in a sheet of the desired hardness and stiffnot of controlled temper,

of the metal is relieved for only a portion of the thickness of the sheet or cross section of the wir Thus the portion 01' the thickness in which strain while the remainat least one surface article. Strain is relieved at of the sheet by, recrystallization of the'grain.

structure at andadjacent to that surface. It will be apparent that'in making-abend which puts such surface in the recrystallized and partially recrystallized metal will be beneficial, while the layer of uncrystallized metal will furnish a vent fluting and supply rigidity to the sheet.

will be obvious that metal in strand or wire form will respond as readily to the process. The same applies in this'description and in the appended claims the term .sheetmetal is intended to cover any such and the term metal is intended to cover any such article of length including wire. The term thin metal article as used in this description and in the appended claims is intended to include strip An important object of the present invention is. the provision of. a

heat'treatment of the metal in cold worked contension the greater ductility of.

stillness to preand strip metal are specifically to metal in tubu-' I lar or pipe form. The criterion in any case i the P ocess for producing sheet metal or metal strip of controlled temper by a ing sheet metal or metal strip of controlled temper from the full hard product by eifecting varying degrees of recrystallization of the grain structure through the thickness of the sheet or strip.

A further important object of the present invention is the provisional. a continuous process for producing a sheet metal or metal strip-of controlled temper from the full hard product by heat treatment.

A further important object of the present invention is the provision of a process for controlling the temper of sheet metal or metal strip by utilizing the skin effect" of high frequency induction he ting.

A furthe important object of the present invention is the provision of an improved product of controlled temper resulting from the present heat treatment of full hard metal.

Other objects andadvantages. of this invention will become apparent from the following detailed description, reference being had to the accompanying drawings which constitute a part of this specification and wherein:

Figure 1 consists of reproductions'enlarged to about 1% of original size of actual photomicrographs at a magnification of 100 of steel treated in accordance with the present invention,

Figure 2 is a schematic showing of one form ing of the surface of a metal article and since the small thickness dimension of the articles involved sometimes necessitates the equivalent of a quenching action to prevent undesirable penetration of the heat into the article, it will be obvious that the invention is limited in such cases to the treatment of those metals which are usually considered nonhardening by quenching methods. special alloy hardening steels cannot be treated by the present invention where a, quenching step is necessary.

In the case of steel, for example, sheets of full hard black plate of .0095" thickness were positioned with both surfaces in contact with heat supplying surfaces at a temperature of 1500 F.

The samples of black plate weremaintained in contact with these heat supplying surfaces for varying intervals of time to control closely the depth of heat penetration. The black plate was water-quenched immediately following contact with the heat supplying surfaces. Photomicrographs of cross sections of 'the treated black plate showing varying degrees of recrystallization through the thickness appear in Figure 1. Tests show the physical properties for the treated specimens as set out in Table I below, the untreated black plate shown at a in Figure 1 being full hard with a Rockwell -T of 81.

Table I Figure 1b 1c 1d Is Time of heat a pllcation. "seconds" 1 2 3 4 Rockwell 30-.II 7 73 6 Can forming N 0 fluting N o fluting No flutlng N o fluting. Side seam Will not seem Will not seam Seams 180 flat'. Se ems 180 flat. 180 flat. 180 flat.

of apparatus for continuously carryingout the process of the present invention, I

Figure 3 is a schematic showing of another form of apparatus for continuously carrying out the process of the present invention,

Figure 4 is a schematic view of apparatus for continuously carrying out a second embodiment of the process of the present invention,

Figure 5 is a schematic showing of apparatus for continuously carrying out the process of the present invention utilizing high frequency induction heating currents Figure 6 is a schematic showing of a cross.

section through a coil adapted to be used in the apparatus of Figure 5, and

Obviously by raising the temperature of the heat I Figures 7 and 8 are views representing photo-s micrographs of across section through articlestreated in accordance with the process of the present invention.

In the-present process an external surface of the cold worked article is subjected to a temperature great enough to raise this surface above a critical value.- when this surface reaches this critical value, recrystallization of the grain structure is initiated. This recrystallization is allowed to proceed until a depth of recrystallized grain structure is arrived at which in the finished article, acting in conjunction with the remaining lunrelieved material, will give the desired over-all temper or physical characteristics. The temperature of that section of the article in which recrystallization is occurring is then reduced to inhibit further recrystallization and further penetration of the heat into the unrelieved section. Since the present invention involves softensupplying surfaces within limits, shorter contact time would be necessary. x

From Table I it will be apparent that the specimen shown at d in Figure 1 would be highly suitable for can bodies.

The remnants of the chilled and free freezing crystals from the ingot may under some conditions influence the results. Such influence is of a favorable nature since the free freezing crystals occur in the intermediate section of the thick ness of the article and therefore retard recrystallization in this section relative to the rate of recrystallization in the sections adjacent the surface where the chilled crystals exist.

In order to treat strip metal continuously in accordance with the present process, apparatus of the type shown in Figure 2 may be utilized.

The strip metal I being fedfrom a coil of cold reduced metal is passed into an insulatedi'urnace structure ll, wherein it passes between two heated rolls l2. The strip passes from between the rolls out of furnace l I and into immediate contact with two cooling folls II. The temperature of heated rolls I2 is such that the heat supplied to the surface of the strip is sufficient to initiate recrystallization and carry on the same through the thickness of the strip by heat penetration. Cooling rolls I: are positioned relative to heating rolls l2 so that with the strip moving at the desired speed, heat is abstracted from the surfaces of the strip to prevent further recrystallization and penetration of the heat. A controlled depth of recrystallized grain structure is thus achieved, leaving an intermediate layer of unre- Thus the high carbon steels and strip is next contacted with hot roll again the temperature of. the heating rolls and the speed of the strip are.

ing roll 23. 'An idler hot rolls by an internal flame,

I includes an insulated furnace ing therein a duction coil 26. A strip 21 passes into furnace 2,5

'. plied to coil 26.

. designed that sage of the strip 'With'the apparatus shown in Figure 3 a higher strip speed can be attained, the strip being mainwith theheating and apparatus the strip l0 tained in contact longer cooling rolls. With this enters an insulated furnace l4 and makes pro-- longed contact with heating roll 15. The strip is then immediately contacted in the ner'with cooling roll it. The other same manside of the i1 followed by cold roll I8. Here so related-that the desired degree of recr'ystalliza tion part way through the strip is accomplished to be more prevalent before the cooling rolls abstract 'sufllcient heat to stop recrystallization.

In Figure 4 an apparatus is shown for carrying out an embodiment of the present invention wherein only one surface of a strip is treated. A strip it enters an'insulated furnace structure and is guided by idler roller 2! onto the surface of aheating roll 22. The strip leaves the heating roll and passes into contact with a cool roller 24 guides the treated strip to a recoiling (notshown). Strip produced in the;apparatus of Figure 4 will have the metal at one surface in strained and unrelieved condition while at the other surface the metal will be in relieved condition with the grain structure completely to partially recrystallized.

The hot rolls are formed of heat resistant material having a smooth surface to avoid marrin the strip. The cooling rolls are formed with copper surfaces or some such heat conductivit Heat may be supplied to the heating elements or in any desired manner.

hot fluid, electrical or further treating station material having good apparatus of Figure 5.

The apparatus schematically shown in Figure structure 25 havhigh frequency electromagnetic in- The disposition'of the induced currents in this to supply heat only at manner can thus be used the points where recrystalliaztion of ture is desired. is most favorably treated in this manner.

grain struc- Prompt abstraction of the heat following pas- 1 through the coil inhibits further 7 but this may not be necessary is not excessive. Although a recrystallization, if the heat supplied liquid bath 29 is shown, anydesirable form of be 'used when this step is desired. The desired frequency must quenching or cooling means may be arrivedat by experimentation for'eachshape of article being treated. 'A frequency in the neighborhood of one megacycle will produce sumcient skin effect to successfully treat 98# black plate in accordance with the present invention.

Where flat strip material is being treated an induction coil surrounding the strip will not give the proper distribution of heat itis prop Strip metal of magnetic nature in Figure 1b to Figure 'shown for introducing inert gas enclosingthe cooling rolls. Sealing means 34 at the strip exit point force the gas to circulate into the recrystallization erly shaped. The edges of the strip will receive energy from both the sides and the ends. of the coil. In Figure 6,- strip such that'some sections of the'sheet-receive more energy than the others. Since segregation tends in one section of a strip such' as the center, giving a greater annealing efiect at such a section may result ina product having more width. p

Obviously, where wire is being treated, a cylin drical coil will be effective.

In orderto prevent oxidation of the material aninert atmosphere may be desired. In the ap paratus of Figures 2 to 4 inclusive, means 32 is into a casing 33 the furnace and out at the strip entrance point. In the case of the apparatus of Figure. 5, the gas is introduced by means .35 directly into-the furnace.

Where desired or necessary suiiicient'tension is maintained on the strip to buckling of the hot'strip.

Figure 7 shows a cross section of a product produced by the process carried out 'in the apparatus of Figure 4. In this case it is apparent that is initiated from only one side of the articl the material at the other surface being in unrelieved sult in a sheet having better bend in one direction than the other due to ductility.

in accordance with the present invention in the The article having been cold drawn has a grain orientation in the direction of the length. A center of the wire of D will have the same appearance as the showing 1! depending onthe degree of treatment desired.

When the article being treated is wire it will be apparent that 4 a product. of high tensile strength and great flexibility will result. The core of the wire being in full hard unrelieved condition will have a high tensile strength while the outer annealed portions will form a jacket of soft ductile material.

' for the present process by The heat can be imparted to the strip or 'wire v any of'the well known heating baths such as molten salt baths.

We claim:

1. A process for softening a thin, elongated"- .ferrous metal-article 'w ich is in cold. work hardened condition comprising raising the temcrystallization adjacent the surface and control-- ling the temper of penetratlonof the'h'eat through the thickness of j V 'perature of an elongated surface of initiate recrystallization adjacent the surface and controlling the temper oi the sumciently to product by controlling the heat penetration into the article to prevent complete recrystallization throughout the thickness of the article.-

2. A process for softening a thin, elongated is in cold work ferrous metal article which hardened condition comprising supplying heat at an elongated surface of the article to effect rethe product by controlling uniform characteristics across its prevent objectionable conditon. This will recharacteristics the -mate--' rial adjacent one surfa'ce beingof much greatersection taken through the Figure 8 along the length the article the article to effect a decreasing degree of crystallization from the surfaceinwardly.

3. A process for treating a thin, elongated ferrous metal article vwhich is in cold rolled condition, the article being formed of a ferrous metal substantially nonhardening by quenching methods, the steps comprising passing an elongated surface of the article into contact with a heat supplying medium to initiate recrystallization adjacent the surface and passing the article into contact witha cooling medium to control the heat penetration into the article to prevent complete recrystallization throughout the thickness of the article to thereby control the temper of the product.

4. A process of softening cold drawn, work hardened, elongated wire of ferrous metal comprising supplying heat at an elongated surface of the wire to effect recrystallization adjacent thesurface and controlling the temper of the product by controlling the heat penetration into the wire to give a decreasing degree of recrystallization toward the center of the wire.

5. A process for treating a 'thin, elongated ferrous metal article which is in cold worked condition, the article being formed of a ferrous metal substantially nonhardening by quenching methods, the steps comprising setting up a high frequency electromagnetic field in the article to induce heating currents therein, the frequency of the field being suchvthat skin effect confines the major portion of the currents adjacent the elongated surface of the article, supplying suminduced at the elongated surface of the article and withdrawing the article from the field when recrystallization has been effected adjacent the elongated surface and before heat penetration can effect recrystallization throughout the thickness of the article to thereby control the temper of the product. a

7. A process for treating a thin, elongated ferrous metal'strip which is in cold worked condition, the strip being formed of a ferrous metal substantially nonhardening by quenching methods, the steps comprising passing the strip through an alternating electromagnetic field having a frequency such that heating currents are induced at the elongated surface of the strip,

withdrawing the strip'from the field when recrystallization has been effected adjacent the elongated surface and before heat penetration can effect recrystallization throughout the thickness of the sheet, and passing th strip through a quenching zone to abstract heat from the surface to thereby arrive at a controlled temper in the product.

8. A process for treating an elongated ferrous metal sheet or strip which is in cold worked condition, the sheet or strip being formed of a ferrous metal substantially nonhardening by quenching methods, the steps comprising supplying heat at one of the elongated surfaces of the sheet or strip to effect recrystallization adjacent that surface and controlling the temper of the product by. controlling penetration of the heat through the thickness of the sheet or strip to prevent recrystallization adjacent the other surface of the sheet or strip.

9. A thin ferrous-metal article of controlled I temper produced by the process of claim 1.

10. Athin'ferrous'metal article of controlled temper produced by the process of claim 2.

11. A thin ferrous metal article of controlled temper produced by the process of claim 8.

12. A thin, elongated ferrous metal article of controlled temper formed from a metal substantially nonhardenin'g by quenching methods,

the article being characterized by having a portion of its cross-section'in strained condition as a .result of a cold working operation and a portion of its cross-section adjacent an elongated surface in relieved condition as a. result of a heat treatment, the extent of the portion in relieved condition determining the temper of the article.

JOHN D. GOLD. CLARENCE D. FOULKE.

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