US2075385A - Method of heat treating sheets - Google Patents

Description

W. C. WALLIS ET AL METHOD OF HEAT TREATING SHEETS March 30, 1937.

Filed Feb. 1, 1934 2 Sheets-Sheet 1 /////////Vv/// I 22m x- W 522: 1

W4. W M24041; 9

March 30, 1937.

w. c. WALLIS ET AL 2,075,335

METHOD OF HEAT TREATING SHEET-S Filed Feb. 1, 1954 V 2 Sheets-Sheet 2 INYENTORS m-W c, m 4

Patented Mar. 30, 1937 UNITED STATES PATENT OFFICE METHOD OF HEAT TREATING SHEETS ration of Ohio Application February 1, 1934, Serial No. 709,323

6 Claims. (Cl. .14821.5)

. Our invention relates to the manufacture of metal sheets and, in particular, to the-heat treatment of ferrous sheets, although it may be utilized also in the manufacture of nonferrous material.

The method of making sheets previously used includes the rolling of sheet bars to breakdowns, matching or doubling the breakdowns into packs, and further rolling in packform to finished gauge.

Recently, breakdowns made by shearing lengths of hot rolled strip have been substituted for those rolled from bar in pairs. In any case, after the finish rolling, it is necessary according to the present method, to normalize the sheets in order to relieve the distortion of the crystalline structure of the metal introduced by the elongation in rolling. Normalizing is objectionable in that, by the present methods of carrying out the operation, an oxide coating is formed on each sheet which necessitates acid pickling for its removal.

The pickling operation is expensive and, furthermore, is responsible for the formation of blisters, constituting a further objection. After the pickling, the present method of treating sheets includes box annealing. This treatment requires a considerable length of time because the sheets, after being heated, must be permitted to cool gradually. It imposes severe limitations upon the output of a mill and requires the use of a large amount of equipment.

We have invented a method and apparatus for normalizing sheets under conditions such that no oxide is formed on the surfaces thereof. Pickling is therefore unnecessary. The annealing of 5 the sheets, furthermore, is carried out in continuous sequence without loss of heat from the normalizing operation. In the old method above described, the normalized sheets are, of course, cooled before pickling so that they must be re- 40 heated from room temperature for annealing.

In accordance with our invention, we provide a continuous furnace of the mufiie type having a number of zones wherein different temperatures are maintained. The sheets are passed through 45 the zones successively and are thus given the desired heat treatment. The atmosphere within the furnace mufile is maintained so that oxidation is prevented and, furthermore, any oxide present on the sheets may be reduced to the metallic form.

50 A further feature of our invention is the possibility of carbui'izing or de-carburizing the surface of the sheets as desired, by suitably controlling the atmosphere of the furnace. zones.

For a complete understanding of our inven- 55' tion, reference is made to the accompanying drawings illustrating a present preferred embodiment thereof, In the drawings:

Figures 1A and 1B, when placed together along the indicated line of separation, constitute a longitudinal sectional view through the furnace we 5 prefer to employ in practicing our invention;

Figure 2 is a transverse section to enlarged scale taken substantially along the line 11-11 of Figure 1A;

Figure 3 is a partial horizontal sectional view 10 of a slightly modified form of furnace; and

Figure 4 is a sectional view taken substantially along the line IVIV of Figure 3, showing parts in elevation.

Referring now in detail to the drawings, the 15 apparatus we employ comprises a continuous furnace Ill having a plurality of zones ll, l2, l3and I4 therein. A muiiie l5 extends the length of the furnace. A conveyor l6 traverses the muflie for moving material therethrough. The temperature and atmospheric conditions of the different zones are controlled to provide the desired effects upon the metallurgical constitution of the sheets.

The normalizing zone shown at l I may be provided with burners l'l, although the manner of heating the furnace is not material to the invention. A pipe i8 is provided for supplying any desired atmosphere to the interior of the normalizing zone ll.

Adjacent the normalizing zoneis a primary 30 .cooling zone shown at l2 for reducing the temperature of the sheets below the normalizing value. Water-cooling jackets l9 enclose the muffle in the zone l2 and connections 20 provide for the circulation of water therethrough at rates sufficient to obtain any desired cooling of the sheets.

The zone I3 is the annealing zone and its function is to permit the gradual cooling of the material in an atmosphere which will prevent 40 oxidation of the surface thereof. The annealing zone may be heated in any desired manner. A conduit Illa is provided for supplying the desired atmosphere to that portion of the muflie of the furnace within the annealing zone.

The final cooling zone shown at it cools the material to room temperature so that when it is discharged, no oxidation thereof will occur. This zone may be cooled by natural radiation or may be provided with air or water-cooling means (not shown). When discharged from the final cooling zone I4, the sheets may be removed by a conveyor 2| or any other convenient means.

In a modified form of the invention shown in Figures 3 and 4, the annealing zone I3 is only about one-half as long as the annealing zone}! shown in Figures 1A and 1B and the final cooling zone is dispensed with entirely. The sheets, instead of being subjected to a final cooling within he furnace muffle, are discharged and piled on trays within an enclosed chamber 22. As each tray is filled, it is displaced by an empty tray moving into line with the furnace conveyor and as the loaded trays emerge from the chamber 22, they are immediately covered with a box 23. The covered sheets are then permitted to cool slowly out of contact with the atmosphere.

We shall now indicate briefly the methods which we are able to carry out with the apparatus above described.

It will be apparent that in using the apparatus and methods of our invention, a portion of the heat supplied to the sheets in the normalizing zone is retained even while the sheets are passing through the primary cooling zone into the annealing zone. In this way, unnecessary cooling and reheating of the material are avoided with a consequent increase in thermal efllciency. In the manufacture of ordinary sheets, they are placed on the conveyor l8 as received after the finish hot rolling, shearing and separation. The sheets are carried into and through the normalizing zone ll of the furnace l0, preferably at a rate of from .ten to forty feet per minute. The

of temperature of the normalizing 'zone is preferably maintained slightly above the upper critical range or, at approximately 1700 F. The furnace heat is adjusted so that the sheets will be subjected to this temperature for a given period of time while passing through the normalizing zone.

Generally it will be preferable to maintain a non-oxidizing or reducing atmosphere in the normalizing zone. This may be done by supplying a suitable gas through the pipe l8.

When the sheets have passed entirely through the normalizing zone ll, they are carried through the cooling zone 12 where a temperature somewhat below the upper critical temperature is maintained. This temperature is approximately 1300 F.

After passing beyond the cooling zone l2, the

sheets are carried through the annealing zone I! wherein the temperature is maintained at approx imately 1150 F. The function of the annealing zone is to retard the rate of cooling and to maintain a non-oxidizing atmosphere about the sheets. The latter may be accomplished by supplying a suitable gas to the zone through the pipe In The function of the final cooling zone, of

course, is to cool the sheets from the annealing temperature gradually to room temperature. For this purpose, as before stated, we may use natural radiation or artificial cooling. the temperature of the sheet when discharged from the final cooling zone is suillciently low to prevent rapid oxidation and should not exceed approximately 600 F. A non-oxidizing atmosphere may be maintained in the zone I by leakage from the zone I! or by a separate supply pipe (not shown).

Substantially the same operations, are carried out in the modified form of the apparatus shown in Figures 3 and 4. Instead of permitting the final cooling to take place in a continuation of the furnace muille, the sheets are stacked within the chamber 22 and, as the stacks are pushed from the chamben they are covered and permitted to cool slowly without contact with the atmosphere. The effect on the sheets is about the Preferably,

same regardless of which of the two forms of apparatus shown is employed.

We have already mentioned the possibility of eifecting carburization or de-carburization of the surfaces of the sheets by using an atmosphere within different portions of the muille which will produce this result. As a specific example, in the manufacture of electrical sheets, it is known that the hysteresis loss depends to a large extent upon the carbon content. Better electrical properties, furthermore, can be obtained by treating a few sheets at a time than by box annealing as heretofore practiced. In making electrical sheets, therefore, our process produces sheets having better electrical properties and much greater uniformity than products of known processes. The sequence of operations we prefer to employ for electrical sheets containing from 4.0 to 4.5% silicon is as follows: After the sheets have been hot rolled to approximately the desired finished gauge, they are cold rolled. The sheets are then passed through the furnace ll wherein they are is in thejmanufacture of stock for vitreous enameling in which it is known that the carbon content should be the lowest possible. For this purpose, we roll sheets from open hearth steel of from .03 to .04% carbon which is reduced by oxidation in both normalizing and annealing zones by using an oxidizing atmosphere instead of a tie-oxidizing atmosphere previously mentioned. This de-carburization gives a much more uniform product than the previous methods and at a considerably reduced cost. We prefer to manufacture enamel stock by normalizing the hot rolled sheets and de-carburizing, as above mentioned, reducing all the surface oxides to the metallic form. The sheets are then cooled in piles in a reducing atmosphere and subsequently cold rolled for the purpose of flattening them.

In making stock for automobile parts or other purposes where good drawing characteristics are desirable, the sheets should have a grain structure of small size. To produce that size. the sheets must have a carbon content which is higher than consistent with the best drawing properties. Our invention makes it possible to specify an analysis for the steel which will give the best drawing properties and, by re-carburizing in the manner aforementioned, the surface requirements of the finished product may be met. A typical treatment of sheets for these purposes would be to normalize the hot rolled sheets in a carburizing atmosphere so that all surface oxides would be reduced. After the primary cooling and annealing, the sheets are piled and cooled in a reducing atmosphere. Subsequently, they may be subjected to final cold rolling and roller leveling.

So-ca1ledblued sheets are produced by methods similar to those known for box annealing, with the added step that steam is introduced when the material is at a temperature. of between surface oxides are all reduced, and discharging them into the air or into a steam chamber at a temperature between 800 and 1100 F., depending upon the color desired. The exposure oi the sheets to air or steam, of course, produces an oxide coat of the desired character.

Although we have illustrated and described only a few or the embodiments of our invention, it will be apparent that .both the apparatus and the methods disclosed herein are susceptible of numerous changes without departing from the spirit of the invention as set forth in the appended claims.

We claim:

5 1. In a method of making sheets, the steps ineluding passing the sheets continuously through a succession of chambers, heating the sheets to about 1700 F. in one chamber, quickly cooling the sheets to about 1300 F. in another chamber, 20 and slowly cooling the sheets to about 1150 F. in another chamber.

2. The method of treating metal sheets including the steps oipassing them through a furnace. muiiie, heating one portion of the muiiie to main- 25 tain a temperature of about 1700 R, cooling an adjacent portion to maintain therein a temperature of about 1300 F., and limiting cooling of a succeeding portion to maintain therein a temperature of about 1150 F.

3. A method of treating sheets including continuously passing them through a succession of zones, heating the sheets to a temperature of about 1700 F. in one zone, cooling the sheets to a temperature of about 1300 F. in another zone, and so limiting cooling of the sheets in another zone as to maintain a temperature of about 1150 F. therein.

4. The method of claim 2 characterized by subjecting the final portion of the muflle to air cooling.

5. The method of claim 2 characterized by subjecting the i'inal portion of the muflie to air cooling, and discharging the sheets therefrom at a temperature of about 600 F.

6. The method of claim 3 characterized by cooling the sheets in a further zone to a temperature of about 600 F.

WILLIAM C. WALLIS. ERNEST B. PLOTT.

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