US2402013A - Annealing method - Google Patents

Description

gm) WAVE ENERGY.

Patented June 11, 1946 King, Pittsburgh, Pa., assignors to Carnegie- Illinois Steel Corporation, a corporation of New Jersey No Drawing. Application March 31, 1945, Serial No. 586,045

4 Claims.

This invention relates to a method of annealing metal in stacked or coiled condition, and more particularly to a method of annealing sheet or strip steel when stacked or tightly coiled.

The invention has among its objects the provision of an improved method of annealing metal in stacked or tightly coiled condition so that the surface thereof is uniform throughout.

A further object of the invention is a provision of a method of annealing metal, particularlysheet or strip steel, so that the surface thereof is free from deposits or other conditions of non-uniformity which would render nonuniform an electro-deposited metal coating subsequently applied thereto and fused thereon.

In the making of ,Ielectrotinned sheet steel, in which the annealing method of the present invention is particularly advantageous, many steps are necessary in processing the metal from the ingot to the sheet or strip form in which it is coated. Among such steps are the hot rolling of the metal to a thickness of about .08 inch, and then cold rolling the metal, preferably in continuous mills, to approximately final gauge. The cold rolled strip is subsequently cleaned and is then annealed in long lengths in tightly Wrapped coils, usually by being heated to the sub-critical annealing-range for the purpose of soften-.

ing the steel and relieving internal stresses. In order to prevent oxidation of the metal during annealing and to prevent alteration of its carbon content, an atmosphere is employed which is non-oxidizing from fairly low temperatures, for example, 200 R, up to the highest temper ature employed, and which is in at least substantial equilibrium with the metal throughout the annealing range from the standpoint of carburization. Such annealing step is conventionally carried out in a furnace employing an inner cover over the steel, such cover being provided with a sand seal to the furnace base, the atmosphere above described being supplied within the cover by means of a pipe extending through the furnace base.

Prior to the present invention the pressure employed within the inner cover has been one which is sufficient to supply sufficient gas to balance its leakage through the sand seal, and to prevent the ingress of air during the cooling cycle, when the atmosphere within the inner cover tends to contract. Aside from these considerations no particular attention has been paid to the pressure at which the atmosphere was supplied, and the pressure employed within the inner annealing cover has usually been in the neighborhood of one inch of water or above.

Following the annealing operation, the sheet or strip steel is conventionally temper rolled, cleaned, and pickled, after which it is' electrotinned by being run between. tin anodes in a suitable electrolytic bath. After the electrotinning step the coated sheet is Washed and. dried, and is then run through a furnace for the purpose of fusing the tin coating thereon, following which the sheet is quenched in water.

Electrotinned sheet metal produced by such prior method frequently has a surface which is not uniform, such non-uniformity generally taking the form of a dull area on either or both side substantially parallel to the edge of the stripand extending throughout the length of the coil. Such dull area usually extends several inches toward the inside of the strip, and is bordered by a narrow portion on the edge of the strip as Well as by the central portion of the strip, both of which are bright and commercially satisfactory. The appearance characteristics seem to be the only difference between the dull and bright area of the plate, since the weight of the coating and its features as revealed by the microscope in both the dull and the bright areas, both vary well within the range of usual nonuniformity of products of this type. Trade re-. quirements, however, render the elimination of this streakiness imperative, since products made from sheets so streaked have the apparance of being defective, and thus cannot readily be sold. The reason for such streaks or snakes as they are called in the trade, is not altogether known, and themanner with which they could be eliminated was not appreciated before the present invention. It cannot be told whether or not a given strip, at any step prior to the fusion of the electro-deposited coating, will have snakes thereon; that is, there is no detectable difierence in portions of the strip as hot rolled, as cold rolled, annealed, or even electroplated which would lead one to predict accurately exactly when and where such snakes would be formed. It can readily be seen, therefore, that when coated sheet, or strip material having such snakes thereon is rejected, not only is a certain amount of material lost but such rejected strip representsa substantial los in processing time, in view of the many operations to which it has been subjected prior to its quenching following fusion of the coating.

We have found that by holding the, pressure of the annealing atmosphere in contact with the derstood. It is known that no matter how carefully the strip metal is cleaned prior to anneal ing, it cannot in a commercial process be rendered absolutely freefrom water vapor, palm oil which was applied prior to the cold rolling thereof, or from traces'of salts from the cleaning solutions employed prior to annealing. It is thought that such surface contaminants may be Such stock, which is usually below .10% in carbon content, is treated prior to annealing in the same manner as is the sheet or strip in the prior art method outlined above. The tightly coiled strip, after having been rolled in 'a continuous cold mill and cleaned as before, is placed on a furnace base, usually two coils high. The

inner annealing cover is then placed over such coils and the annealing cover proper, or furnace, having heating means, such as radiant tubes thereon, is placed over the inner cover. Before the initiation of the heating step, the air within the inner cover is exhausted by causing the annealing atmosphere, admitted as before, through a pipe'in the base of the furnace within the inner cover, to displace it.

The atmosphere employed is partially burned natural or coke oven gas containing from 5 to 7% carbon dioxide, 8 to 10% carbon monoxide, 10 to 14% hydrogen, less than .13% methane, traces of water Vapor, no free oxygen, the rest being nitrogen. The atmosphere is controlled so that the ratio of CO2 to CO lies between 0.5 and 0.7 and so that the ratio of H to H2 lies between 0.03 and 0.04, which for the hydrogen content given corresponds to a dew point of to F. Such atmosphere is at least slightly reducing at all temperatures from approximately 200 F. to the maximum temperature employed in the cycle, that is 1220 to 1240 F., and is neutral from the carburization standpoint at such maximum temperature used.

, The rate of flow of the gaseous atmosphere into the inner cover is adjusted to balance the leakage through the sand seal between the inner cover and the base, and the pressure of such atmosphere in excess of atmospheric pressure is maintained within the range of .20 to .30 inch of water. A pressure of .25 inch of water in excess of atmospheric pressure is preferred. "A typical annealing cycle for strip of this type involves a slow rise in temperature for about twenty-four hours from room temperature to '1230 F., a holding of the charge for eight hours at such temperature, and a slow cooling to 200 F., during which time the gaseous atmosphere is continually introduced. When the charge is cooled to 200 F., the heating cover is removed, and the charge is allowed to cool in' air for forty-eight hours after which the inner cover is removed. The total heating cycle consumes four days.

Steel strips or sheets so annealed, which have been treated in the same manner as'has previously been done, prior to electrotinning, and which are coated and quenched as in the prior art, are substantially free from surface defects, such as streaks or snakes. It has been further gasified upon the heating of the metal during the annealing operation, and that such gasified contaminants or the products thereof may seek to escape from the tightly coiled metal. The I presence of a gaseous annealing atmosphere at the pressures employed in the prior art on the outside of such coils is thought may retard the escape of such gaseous contaminants or products,

' and to result in their deposition upon the strip found that, when so annealed, the strip metal may be run through the electrotinning line at substantially greater speeds than formerly without the development of an unduly high percentage of rejections due to such surface defects. This, of course, means that sheet metal so annealed may be electrocoated with a smaller thickness of coating without the development of such surface defects. 7 s

The reason for such results are not fully unmetal in a location spaced slightly from the outer edges thereof. There appears to be some evidence that the production of the streaks or snakes is due to the presence of 'an extremely thin layer of organic material, probably monomolecular in thickness, at the portions where the snakes form. The presence of such layer'cf organic material or the manner in which it is deposited, if it does exist, however, have not as yet been fully substantiated, and it is to be-understood that we are not to be bound by such theory.

The annealing method of the present invention has been found to work equally well with stacks of flat sheet or strip steel products, subsequently to be electrotinned and such coating fused thereon, as with such material in tight coil form. It is obvious that stacks of flat sheets or strips in which such products are usuallyannealed are of great weight and that in most of the stack, except perhaps the very top thereof, the pressures between adjoining sheets or strips are high; the conditions existing between the tightly wrapped layers of a coil during annealing are therefore substantially duplicated in such the present invention whether the sheet or strip stacks of fiat products.

Having thus fully disclosed the preferred em-. bodiment of the method of our invention,we desire to claim as new the following:

1. The method of annealing coldrolled low carbon steel for making tin plate comprising dis posing such metal in an annea'lingfurnace so that'successive layers of metal are positioned in intimate contact with each other, providing an atmosphere about the metal in the furnace which is reducing in the high temperature portion of the annealing cycle and which is substantially in equilibrium with the metal at the maximum temperature of the annealing cycle as regards carburization, maintaining such atmosphere about the metal during the annealing cycle at a pressure of from 0.20 to 0.30 inch of water in excess of atmospheric pressure, heating the metal up to the maximum temperature, holding it at such temperature for an appreciable length of time, and cooling the thus heated metal in such gaseous atmosphere to prevent oxidation thereof. 2. The method. of annealing cold rolled low carbon steel for making tin plate comprising disposing such metal in an annealing furnace .so that successive layers of metal are positioned in tight and intimate contact with each other, pro viding an atmosphere about the metal inthe furnace which is reducing in the high tempera ture portion of the annealing cycle and which is metal is annealed in tightly wrapped coils or in substantially in equilibrium with the metal at the maximum temperature of the annealing cycle as regards carburization, maintaining such atmosphere about the metal during the annealing cycle at a pressure of from 0.20 to 0.30 inch of water in excess of atmospheric pressure, heating the metal up to the maximum temperature, holding it at such temperature for an appreciable length of time, and cooling the thus heated metal in such gaseous atmosphere to prevent oxidation thereof.

3. The method of annealing cold rolled low carbon steel for making tin plate comprising disposing such metal in fiat form in an annealing furnace so that successive layers of the metal are positioned tightly and in intimate contact with each other, providing an atmosphere about the metal in the furnace which is reducing in the high temperature portion of the annealing cycle and which is substantially in equilibrium with the metal at the maximum temperature of the annealing cycle as regards carburization, maintaining such atmosphere about the metal during the annealing cycle at a pressure of from 0.20 to 0.30 inch of Water in excess of atmospheric pressure, heating the metal up to the maximum temperature, holding it at such temperature for an appreciable length of time, and cooling the thus heated metal in such gaseous atmosphere to prevent oxidation thereof.

4. The method of annealing cold rolled low carbon steel for making tin plate comprising placing tightly coiled cold rolled steel in an annealing furnace so that successive layers of metal lie in intimate contact with each other, providing an atmosphere about the metal in the furnace which is reducing in the high temperature portion of the annealing cycle and which is substantially in equilibrium with the metal at the maximum temperature of the annealing cycle as regards carburization, maintaining such atmosphere about the metal during the annealing cycle at a pressure of from 0.20 to 0.30 inch of water in excess of atmospheric pressure, heating the metal up to the maximum tempenature, holding it at such temperature for an appreciable length of time, and coolin the thus heated metal in such gaseous atmosphere to prevent oxidation thereof.

ARNO L. BILLETER. MICHAEL C. KING.