US2656285A - Production of coated soft iron and steel sheets - Google Patents

Description

Oct 20, 1953 R, s. BURNS ETAT. PRODUCTION oF COATED soET IRON AND STEEL SHEETS Filed June 3, 1948 INVENTUM.

EG. Z.

Patented Oct. 20, 1:953

PRODUCTION OF COATED SOFT IRON AND STEEL SHEETS Robert S. Burns and Robert L. Solter, Middletown, Ohio, assgnors to Armco Steel Corporation, Middletown, Ohio, a corporation of Ohio Application June 3, 1948, Serial No. 30,856

s Claims. l

The principal object of our invention is the provision of a rapid and inexpensive process whereby soft iron and steel sheet stock may be produced from materials which have been cold reduced.

It is an object of the invention to produce from cold reduced iron or mild steel in a rapid and inexpensive manner a sheet stock which hasv a softness the equivalent of soft merchant sheets produced by hot rolling and annealing processes.

It is an object of our invention to provide a process for softening iron or mild steel sheet stock which does not itself involve a rolling, or a critical straining and annealing procedure, and which may be applied to the sheet stock after it has been reduced to gauge and is otherwise finished. l

It is an object of our invention to provide a softening procedure which may be carried on as an incident to other operations, as will hereinafter be set forth.

It is an object of our invention to provide a mode of softening which may be practiced either separately or as a part of a process of hot coating the sheet stock with protective metals.

Novel products are produced by our procedures; and the making of these products constitutes an object of the invention.

These and other objects which will be set forth hereinafter or will be apparent to one skilled in the art upon reading these specifications, we accomplish by those procedures and modes of operation, and in those products of which we will now describe exemplary embodiments. Reference is made to the accompanying drawing wherein: Figure 1 is a diagrammatic representation of an assembly of apparatus with which our process may be carried on, this apparatus including means for coating the treated sheet stock with a molten protective metal.

Figure 2 is a curve showing exemplary temperature conditions obtaining throughout our softening treatment.

By iron or mild steel we mean any of those ferrous materials classified under these terms in the standard literature, and in particular ferrous materials containing less than about 0.25 percent of any one alloying ingredient, except manganese which may be as high as .60%. Such materials, as produced by the best modern methods,

usually contain carbon of the order of .02%- .l% althoughthis may vary considerably depending upon the source of the ferrous material. The presence -of other alloying ingredients is not a limitation on the invention. By sheet stockf we mean a ferrous material of sheet thickness, whether existing in the form of individual sheets, Aor in the form of a continuous strip, regardless Aof width; Our process is applicable to individual A-sheetsibut is most conveniently carried on withv the stock in the f olm .0f a cpntiiluous strip. since Yreduction processes. 165,000 p. s. i. and Rockwell B hardness of 45 rto are those conventionally obtained from cold reduced stocks with any of the usual commercial lafter-treatments, i. e. whether the sheet stocks 'were normalized only, box annealed only, or both :normalized and box annealed. The interruption ,of the cold reduction for an intermediate anneal,

under these circumstances better control is possible. By cold rolled stock We mean a ferrous material which, from an intermediate gauge to which it was hot rolled or otherwise reduced, has been reduced to final sheet gauge by a substantialcold reduction as distinguished from a mere temper rolling. No specific amount of cold reduction performed on the material prior to the application of our processing steps forms a limitation on the invention. Furthermore our procedure is applicable to material which has not been cold reduced at all.

Before the advent of modern cold mill processing, soft steel sheets were made by hot rolling on hand finishing mills, and then box annealing .the product. This gave a very soft sheet, suitable for bending without springback, i. e. it was readily formabie and was known in the trade as a soft merchant sheet. Such sheets were widely used in the galvanized condition. Where an extremely soft condition was not required, hot

rolled sheets in the old art were frequently nor- `malized instead of being box annealed.

Hot rolling followed by box annealing produced sheets having 30,000 to 40,000 p. s. i. bend factor and a Rockwell B" hardness of 30 to 45.

In recent times the old hot mill procedures have largely been supplanted by more economical cold Bend factors of 45,000 to and the use of a final box anneal after the cold reduction will give a greater softness than indi- -cated by the last figures above, but is ecenomi- 40 cally unsound because of the expense involved. Hence there has not hitherto been provided any commercially feasible way of obtaining cold reduced sheets approximating the formability and softness characteristic of the old hot rolled soft merchant sheets at a price comparable to the price of ordinary cold rolled steel.

Itis within the scope of our invention, however.

knot to carry the softening process so far as to produce the degree of softness of the soft merchant sheet. Our invention provides an inexpensive and rapid way of softening cold rolled materials; and may be practiced to any extent dep sired depending upon the qualities desired in the nal product. It will be understood of course that the cold rolling and finishing procedures current in the art have produced iron and mild steel stocks of excellent drawability, substantial hardness, and a fine grained condition. Hence the utility of our process is primarily realized where a larger grained and softer condition is desired, and where improved bending or forming characteristics are ldrogen bearing atmosphere.

sheet stock is decarburized to the extent of lowering the carbon content to values such as herein'- after set forth, the result ofV a subsequent heat treatment in which the temperature of the material is raised above its A3 point will be the opposite of the result obtained from a normalizing treatment performed on conventional material'. Instead of recrystalizing upon cooling with a fine grain structure and considerable stiffness', the material develops a coarse grain structure and a substantial softness. The decarburizing step, which is a heat treatment, and the step of heating of the material to above itsv As point, and its subsequent cooling, may be practiced very rapidly and with relatively inexpensivel equipment. The heat treatmentsmay be carried on after themanner of strand or continuous anneals, if. e. with individual sheets or a strip of material passing through an elongated furnace in which all surfaces of the material are open to the atmosphere of the furnace. Both these treatments may be carried on, if desired, in a single strand type furnace, preferably one in which different zones may be segregated by baffles or the like. Both these treatments may be practiced upon materials of nal sheet thickness; andv the time durations of the treatments are short. The sheet itself need only be in a reasonably clean condition, i. e. free of scale, though not necessarily free of oxidation, and may be freed of coatings of oil and grease prior to or as an incident of the process.

As a consequence, ordinary cold rolled stock is suitable for use in our procedure usually without other pre-treatment than a simple de-greasing. This may be accomplished in various ways, as by chemical or electrochemical cleaning, but is most cheaply accomplished by passing the stock through a flame or a short furnace in which it is heatedv to around 650 F. to 800 F. in an air atlmosphere in which the oils and greases are burned from the surface of the stock. The decarburizing treatment hereinafter described will, as a matter of fact, remove oils and greases from the surface of the stock; but carbonaceous residues from ready been annealed; but it is also capable of serving as an annealing treatment in itself. Thus our procedure may be practiced upon cold rolled strips or sheets which have neither been box annealed nor normalized, following the final cold rolling.

For the decarburizing step of our process, we prefer to pass thesheet stock through a strand or open annealing furnace containing a wet, hy-

The object of this step is to reduce the carbon content of the sheet, as later Y described without producing excessive oxidation. It is desirable to have hydrogen present primarily to prevent excessive oxidation of iron, and the quantity of hydrogen may vary (and vwill vary withthe source of the atmosphere) roughly between 5% and 100%. Its effect will yvary with variations of temperature and time. Other reducing gases, lsuch as carbon monoxide,

may be used in the place of some' or all of the hydrogen. I n this event some hydrogen'may be Hence we lli LCI

Y 4 produced by reaction between the, carbon monoxide andl theI moisture. Other gases besides water vapor may supply the oxygen in the active form Anecessary to bring about the decarburizing action.

` Carbon dioxide, for example, will produce satisvfactory .decarburizatiom especially when used in the correct ratio with carbon monoxide to prevent excessive oxidation, which can readily be calculatedI fromwell known equilibrium data. The active gases can be diluted with inert gases such as nitrogen without affecting the decarburizing action except. for generally slowing down the rate of the. reaction as the active gases become more dilute.

Our preference for water vapor for the major decarburi'zing mediuml'is based upon its being' in general more rapid' ink its action. Moreover, it is possible to. use combinations of oxidizing gases such as` water vapor and carbon dioxide together with hydrogen and carbon monoxide.

The minimum water vapor content, or minimum dew point in our preferred method is that which will decarburize our sense) in the chosen short 'length of time; the maximum which can be added to a particular atmosphere is that which will decarburize without' producing undesired oxidation. We prefer to decarburize under conditions which are strongly oxidizing to carbon but at the same time reducing to iron at temperatures below the A3 point of the material being treated, and to effect the decarburization in a very brief length of time. Hence our preference for a wet atmosphere containing a gas which will be reducing to iron a't the-*temperatures involved.

By way of example, with dissociated ammonia (about hydrogen, balance nitrogen) we employ a dew point between and 170 F. with a preferred range of to 150 F. The temperatures of the heat treatment will lie between 1300o and 1650 F. with a preferred range of 150 to 1600 F. For other atmospheres containing larger or smaller amountsy of reducing gas, it will be within the skill of the practitioner to vary the water content to avoid undesired oxidation.

Under conditions such as outlined above, the time required in our process is of the order of one to ten minutes and therefore is well within the commercial possibilities of open annealing 'furnaces and open annealing treatments.

Principal advantages of our process are that in a single continuous furnace or pair of continuous furnaces as herein described, a very rapid treatment may be carried on involving decarburizing at a temperature below the Aa point and a brief recrystallization produced by heating the metal above that point. We have found that by the methods taught herein, within the span usually of two or three minutes, the carbon content of ordinary iron or mild steel may be reduced dependably to .01% or less, and that reductions of carbon content obtained in this way will be effective upon subsequently heating the metal to above its A3 point to produce a very substantial softening of the metal accompanied by a very substantial grain growth.

For the nal heat treatment, in a segregated portion of the same furnace, or in a separate furnace, we heat the sheet stock to a temperature above about 1700 F. Normalizing is defined in Metals'I-Iandbook as a heating to approximately 100c F. above the critical temperature range, followed by a cooling in still air. The

procedure we follow in the second heat treat- 'ment step of our process is not so strictly limited,

it being necessary only to heat the metal to a assenso temperature above its 'A3 point, and hold it there for a short period of time, whereupon it may be cooled atany rate desired. Thisis preferably done in a protective atmosphere which may either be neutral, or reducing,l e. g. adry, hy-

vdrogen-bearing atmosphere, and the cooling is also preferably carried out in such an atmosphere to prevent oxidation as the'temperature falls. The cooling is preferably more rapid than coolingin still air, to save time on a continuous apparatus. Y l

Referring to Figure 1 herein, we have indicated a metal strip I as the material subjected to our process. It is a cold reduced strip and will normally have an oiled surface. While we may clean it in other ways as indicated above, we prefer to pass it iirst through a short oxidizing furnace 2 wherein we heat it and burn the oil from its surfaces. `Open ames may be employed if desired.

'The formation of a light temper-color oxide on the strip is not detrimental, provided it is not permitted to become so heavy that it will not be reduced in the subsequent furnaces. In some cases where the strip is to be given a metallic coating, such thin oxide coating is desirable.

The strip next passes to a furnace indicated generally at 3. This furnace is divided into two parts as by baffles or the like as indicated at 4. The first and longer portion of the furnace is that in which the decarburizing step takes place. This portion of the furnace is indicated at 5, In it will be maintained a (preferably) hydrogen bearing atmosphere which is wet as set forth above. The hydrogen bearing atmosphere may f be derived from any suitable source. Cracked ammonia is a relatively inexpensive source, and one which isl entirely suitable,V providing the amount of undissociated ammonia in the atmosphere is kept low. As indicated above, pure hydrogen may be used, or any atmosphere (e. g. DX gas, a partially burned hydrocarbon gas) vcontaining Vsufficient hydrogen to prevent undesired oxidation with quantities of moisture effective for rapid decarburization. The balance of the atmosphere is preferably inert gas such as nitrogen but may contain minor amounts of .carbonaceous gases provided the combination is not carburizing to iron in accordance with well known equilibrium data.

Hydrogen may, in fact, be omitted altogether, and decarburization secured through the effects of water vapor in an otherwise inert atmosphere, but inasmuch as relatively heavy surface oxidation is likely to occur under these conditions a pickling or cleaning treatment would be added Ato the process, which we prefer to avoid. Nevertheless a sufiicient decarburization at this stage will be productive of the softness after the metal is subsequently heated above its A3 point as taughtherein.

The second portion of the furnace is that indicated at 6, `and is the portion in which the metal is heated above its A3 point. A second or separate furnace may be employed if desired; but there is no advantage in cooling thematerialbetween the furnaces, unless some additional treatment, as a pickling, is to be practiced at this point. It is, of course, possible to carry out our process using entirely separate decarburization and subsequent heating furnaces, the separation being both in space and time. The economical advantages are with the continuous process. If separate furnaces are used in a continuous process, it is preferable to Alocate them closely adjacent to each other so as to minimize cooling and oxidation of the piece between the furnaces.

In Figure 1, we have Ishown the furnaceportion IB as provided with'fa cooling hood 1 communicating with it. This cooling hood may be provided with a closable port 8 permitting the direct removal of the treated strip. It is shown as provided also with a' downwardly extending portion 9 dipping beneath the surface of a bath of molten coating metal Il) in a coating pot Il. The hood and the furnace portion 6 are kept filled with a neutral or reducing atmosphere, as set forth above. 4

A dry, hydrogen-bearing atmosphere may, for example, be introduced intoV the cooling hood portion 9 through a port l2, and caused to pass through the furnace portions in countercurrent to the metal strip. The atmosphere will be dry in the cooling hood and in the furnace portion 6; and its humidity may be raised in the decarburizing portion 5 by ladmission of steam as through a port or ports I3, although other methods of adding moisture may be employed, such as the addition of drops of water which will vaporize at the temperature of the furnace. If the two furnace portions are separate, the atmosphere in the decarburizing portion may be wetted by bubbling it through water, or other- Wise.

Our preferred mode of operation is one wherein the two furnace .portions are connected, where the decarburization is so accomplished as to leave the sheet surfaces very lightly oxidized if at all, and with the production at most of only a light temper color on the iron or steel, and where the atmosphere in the higher temperature portion not only protects the sheets from oxidation but insures the reduction of any oxide of iron on their surfaces.

If separate furnaces are employed for the portions 5 and 6, and some oxidation and drop in temperature occurs between them, the length of the furnace portion 6 may be increased in order to provide a sufficient length of time for reheating and reduction. Between such separate furnaces it is sometimes desirable to introduce a strip cleaning means, such as, for example, an oxidizing furnace like furnace 2. However where the furnace portions 5 and 6 are part of the same apparatus and are connected as shown, the strip will ordinarily be received in a clean and oxide-free condition from the furnace portion 5, and the furnace portion 6 may be relatively short.

The total overall length of the furnace portions will be chosen in relation to the speed of travel of the materials therethrough, in order to obtain the required time durations for the heat treatments. From the figures hereinafter given, it will be seen that a ratio of lengths of the decariburizing furnace portion to the higher temperature furnace portion of 31/2 to 1 is a satisfactory operating length ratio'V for materials having an initial carbon content of around 0.20%.

In order to make an exemplary showing herein, we give complete data for a commercial ingot iron of the following analysis:

f Percent onlyr necessary tohold the strip at the required temperature for a length of time at least as long as indicated above, the speed of travel of the strip in any given apparatus may usually be regulated for the decarburizing treatment.

Our process is useful for the general softening of cold rolled iron and mild steel stock of sheet gauge, as has been indicated. It is also useful for a number of special purposes. For example, reference is made to the Sendzimir Patents No. 2,136,957 and No. 2,197,622 having to do with the hot coating of metal strip or articles. 1n Figure 1 of this application we `have shown and described a furnace mechanism terminating in a cooling hood with, a bell leading to a point beneath a bath of molten coating'metal. In the Sendziinir patents, a process of hot coating is described, resulting in exceptional adherence of the coating. In the use of the mechanism illustrated in our Figure l, the same effect is secured and by the same metallurgical phenomena, although other phenomena are present resulting in an unusual softness of the base material, as has been described.

Prior practice of the Sendzimir process has heretofore resulted in a relatively stiff sheet, be-

cause of the grain refining action of the nal heat treatment in the reducing furnace and cooling zone, although the adherence of the coating,

especially when of Zinc to which a minor quantity of aluminum is added, has been excellent.

By the practice of our process, we can make the base metal, albeit the base metal is a cold rolled product, as soft and easily formable as a soft merchant sheet made by hot rolling and i annealing; and in the practice of our invention we have produced for the first time such an iron or mild steel sheet stock which is very soft and easily formable with low springback in combination with a galvanized coating which is strongly adherent and of suicient ductility to take without peeling or fiaking all of the bends which the base metal is capable of withstanding.

An excellent product may be made by treating iron or mild steel stock in accordance with the teachings above and coating it with aluminum in accordance with the Sendzimir method.

Modications may be made in our invention without departing from the spirit of it. Having thus described our invention in an exemplary embodiment, what we claim as new and desire to secure by Letters Patent is:

1. A process of softening cold reduced iron or mild steel sheet stock which comprises subjecting said stock to a strand anneal in an elongated furnace with two successive portions separated by baffles, introducing into said furnace in countercurrent to the stock passing therethrough a dry, hydrogen-bearing atmosphere, which lls the final portion of said furnace beyond said baffles, introducing moisture into the initial portion of said furnace ahead of said baffles whereby to bring the hydrogen-bearing atmosphere therein to a dew point of substantially 110 to 170 F., maintaining the said forward portion of said furnace at a temperature between substantially 1300 to 1650 F., maintaining the final portion of said furnace at a temperature above substantially 1700* F., cooling said stock from the temperature of the final portion of said furnace in a hood connected therewith and supplied with said dry hydrogen-bearing atmosphere, and, without reexposing said stock to the external atmosphere, passing it beneath the surface of a molten coating metal.

2. A process of softening cold rolled iron or mild steel sheet stock which'comprisessubjecting said stock to a strand anneal in an elongated furnace with two successive portions separated by baffles, introducing into said furnace in counter-current to the stock passing therethrough a dry, hydrogen-bearing atmosphere, which fills the final portions of said furnace beyond said baffles, introducing moisture into the initial portion of said furnace ahead of said baiiles whereby to bring the hydrogen-bearing atmosphere therein to a dew point of substantially to 170 F., maintaining the said forward portion of said furnace at a temperature between substantially 1300 to 1650 F., maintaining the final portion of said furnace at a temperature above substantially i700 F., cooling said stock from the temperature of the final portion of said furnace in a hood connected therewith and supplied with said dry hydrogen-bearing atmosphere, and without re-exposing said stock to the external atmosphere, passing it beneath the surface of a molten coating metal, which molten coating metal is zinc containing a small quantity of aluminum.

3. The process of claim 2 including the initial step of subjecting the stock to heat under oxidizing conditions to burn grease and oil from its surfaces and form thereon a thin, uniform oxide coating.

4. The process of claim 2 in which the stock, before entering said furnace is cleaned, and its surfaces are provided with a thin, reducible oxide layer.

5. A process of treating cold reduced iron or mild steel sheet stock which comprises forming upon the surfaces thereof a thin coating or oxide of the ordinary thickness of a temper color, thereafter heat treating the stock in a strand annealing treatment in a wet decarburizing atmosphere at a temperature below its A3 point to reduce the carbon therein to a value not greater than substantially .01 per cent, subjecting the stock to a strand annealing treatment in a dry reducing atmosphere whereby to reduce any oxide on its surfaces at a temperature above the A3 point whereby to cause a substantial grain growth and substantial softening, cooling said stock, and finally passing the stock beneath the surface of a molten coating metal without re-exposing it to the external atmosphere.

6. A process of softening and coating cold reduced iron or mild steel sheet stock which comprises subjecting said stock to a strand anneal in a wet hydrogen-bearing atmosphere having a dew point of substantially to 150 F. at a temperature of substantially 1500 to 1600 F. and thereafter raising the temperature of said stock to above 1700 F. in said strand annealing treatment, in a dry, hydrogen-bearing atmosphere, cooling said stock, and thereafter leading said stock beneath the surface of a bath of molten coating metal without re-exposing it to air.

7. A process of softening and coating cold reduced iron or mild steel sheet stock which comprises subjecting said stock to a strand anneal in a furnace at a temperature between substantially 1300 and 1650 F. in a reducing decarburizing atmosphere having a dew point of substantially 110 to F., and thereafter raising the temperature of said stock to above 1700 F. in a strand annealing treatment in a dry hydrogen-bearing atmosphere, cooling said stock, and thereafter leading said stock beneath the surface of a bath of molten coating metal while still surrounded by a non-oxidizing atmosphere.

Claims (1)

1. A PROCESS OF SOFTENING COLD REDUCED IRON OR MILD STEEL SHEET STOCK WHICH COMPRISES SUBJECTING SAID STOCK TO A STRAND ANNEARL IN AN ELONGATED FURNACE WITH TWO SUCCESSIVE PORTIONS SEPARATED BY BAFFLES INTRODUCING INTO SAID FURNACE IN COUNTERCURRENT TO THE STOCK PASSING THERETHROUGH A DRY, HYDROGEN-BEARING ATMOSPHERE, WHICH FILLS THE FINAL PORTION OF SAID FURNACE BEYOND SAID BAFFLES, INTRODUCING MOISTURE INTO THE INITIAL PORTION OF SAID FURNACE AHEAD OF SAID BAFFLES WHEREBY TO BRING THE HYDROGEN-BEARING ATMOSPHERE THEREIN TO A DEW POINT OF SUBSTANTIALLY 110* TO 170* F., MAINTAINING THE SAID FORWARD PORTION OF SAID FURNACE AT A TEMPERATURE BETWEEN SUBSTANTIALLY 1300 TO 1650* F., MAINTAINING THE FINAL PORTION OF SAID FURNACE AT A TEMPERATURE ABOVE SUBSTANTIALLY 1700* F., COOLING SAID STOCK FROM THE TEMPERATURE OF THE FINAL PORTION OF SAID FURNACE IN A HOOD CONNECTED THEREWITH AND SUPPLIED WITH SAID DRY HYDROGEN-BEARING ATMOSPHERE, AND, WITHOUT REEXPOSING SAID STOCK TO THE EXTERNAL ATMOSPHERE, PASSING IT BENEATH THE SURFACE OF A MOLTEN COATING METAL.

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