US2742691A - Method of making corrosion resistant clad steel - Google Patents

Description

April 24, 1956 H. G. BELITZ ETAL METHOD OF MAKING CORROSION RESISTANT CLAD STEEL Filed April 18. 1950 @QQQQ INVENTORS OLIVER F D S HANS G. B ITZ ATTORNEYS United States Patent METHOD OF MAKING CORROSION RESISTANT CLAD STEEL Hans G. Belitz, Dayton, and Oliver F. Davis, Troy, Ohio, assignors to Commonwealth Engineering Company of Ohio, Dayton, Ohio, a corporation of Ohio Application April 18, 1950, Serial No. 156,696

1 Claim. (Cl. 29480) This invention relates to the deposition of protective coatings on metal bases. More particularly, it relates to the plating of protective coatings by the deposition of metal from readily decomposed volatile metallic compounds. Still more particularly, it relates to the deposition of oxidation resistant coatings on objects which are heated to high temperatures for fonning.

Deposition of thin films of metal, such as nickel and cobalt, on metal bases has been limited to the coating of relatively small objects. The disadvantages of these coatings have been their thinness, lack of adherence and porosity.

Overcoming of these disadvantages now provides an economical method for the elimination of losses of material due to oxidation and scaling of steel billets which has existed in the steel industry for a long time.

When poured from a crucible or like source, the molten steel is cooled sufiiciently to be solid but ductile and then is hammered into billet shape.

Before rolling, the billet is heated in a so-called soaking pit or in a furnace. The cooled strain-free billets are coated with scale which must be removed before rolling to avoid formation of a pitted surface sheet and poor appearance due to the presence of the scale.

The descaling operation removes several per cent of the original metal of the billet from certain types of steel billets which readily oxidize, which loss adds materially to the cost of the metal sheet.

It is an object of the present process to overcome the limitations and disadvantages of the above described processes.

It is a further object of this invention to provide a process which eliminates the formation of anything but grade A sheet material.

It is another object of the present invention to provide a protective coating for billets which insulates them from atmospheric oxidation.

It is another object of this invention to provide a process which eliminates scaling and material loss incident thereto.

It is a still further object of this invention to provide a coating which will be extensible and roll thin with the billet.

It is still another object of the present invention to provide a coating which will be extensible and roll thin with the billet.

It is another object to provide a coating which will adhere under rolling conditions.

It is still another object to provide a process in which a part of the heat in the billet as formed may be utilized to provide relatively thick protective films.

It is a further object to provide a coating which is non-porous and sufficiently thick to be reduced in pro portion to size reduction of the billet and not break exposing base metal.

It is also an object to provide a process wherein a base billet is prepared which yields upon rolling a clad steel having a thin uniform thickness surface layer of nonferrous material.

It is another object to provide clad steel having a coat ing of metals heretofore not available as surface layers because the metals could not be deposited from electrolytic baths.

Other and more specific objects and advantages will be apparent to one skilled in the art as the following description proceeds.

In brief, the process of this invention comprises coating of billets with a relatively thick coating of an oxidation resistant metal by deposition of a metal layer through decomposition of vapors of metal compounds.

The coated billets are then held in the soaking pit or heated in suitable ovens for the necessary strain removing material, following which the billets may be rolled.

The processing of the steel is subject to some variation in sequence of steps while performing the same functional treatment.

Where steel, such as carbon steel (which is readily oxidized ferrous alloy) is being processed on a straight through basis, the poured metal is solidified in a mold.

The solidified metal called ingots or billets may be removed from the molds in an inert atmosphere while still at a temperature up to about 1200 F. and should be held in the inert atmosphere until the temperature is in the neighborhood of 400 F. At this temperature the metal oxidizes relatively slowly.

The hot metal may be subjected to a reducing atmosphere of carbon monoxide or hydrogen before passage into the plating chamber and any oxidized film thereby removed. This is, of course, preferred handling and does not preclude treatment to make the oxidized surface chemically clean.

The billet temperature may be controlled so that upon entering the plating chamber, it has cooled into the effective range for decomposition of vapors and deposition of the metal therefrom.

After plating the coated billet is slowly reheated to a temperature in the range of 1900 to 2500 F. The reheated and equalized billets then are passed to the thickness reducing mills where they pass through a roughing train and a finishing train.

Radiation losses may lower the temperature of the metal below safe working temperatures. As the thickness is reduced the temperature need not be maintained so high so that reheating seldom need be carried beyond the 1300 to 1600 F. range.

The product issuing from the rolls is a clad steel sheet with a strong, adherent, very thin surface layer of protective metal.

When the steel is not processed on a straight through basis the steel ingots may be cooled to approximately room temperature in the molds. The molded ingots, which are oxidized more or less, are then subjected to chemical treatment as with an alkaline pickling bath to render the surfaces chemically clean.

The clean ingots or billets are then heated to a temperature preferably in the range of 350 to 450 F. and a protective metal coating deposited from metal bearing vapors. The coated ingots may then be stored.

When the ingots are to be rolled they may be raised to a temperature in the range of 1900 to 2500 F. with proper time allowance for equalizing or normalizing as the temperature rises. The hot ingots may then, as before described, be passed through a series of rolls for thickness .reduction.

It at once becomes apparent that such a process eliminates scale breaker and high pressure spray operations where in normal steel processing the surface metal con- -verted to iron oxide is removed.

Elimination of these operations is possible because the protective coating has insulated the billet from the oxidizing atmosphere and is not itself attacked seriously by oxygen of the atmosphere at elevated temperatures.

Steel billets are formed usually by casting and forging by means of rolling or hammering to a slab or billet 3 to 6 inches in thickness and of suitable length and width. At the time of forming. the billet is at a temperature in the range of approximately 1900 to 2500 F., depending upon the composition of the steel.

The formed billet is next subjected to plating by the decomposition of metal bearing vapors. In order to effectively plate from gases, the temperature of the hot billet must be in the range of approximately 350 to 900 F., depending upon the character of the vapors being thermally decomposed.

The cooling may be effected in a reducing or inert atmosphere to eliminate additional surface treatments. Hydrogen, nitrogen, carbon monoxide, carbon dioxide, and the like, are economically feasible materials for use as non-oxidizing atmosphere.

In the plating process, a stream of material is brought into contact with billets having a temperature preferably in the range of 350 to 450 F. The gaseous atmosphere may be formed by mixing an inert gas with the vapors of a volatile metal compound or by atomizing a liquid metal compound into a blast of inert gas or other equivalent method.

Carbon dioxide, helium, nitrogen, hydrogen, the gaseous product of controlled burning of hydrocarbon gasses free of oxygen and the like have been utilized as a carrier medium or inert gas medium.

Metals to be deposited may be introduced as gaseous metal carbonyls or vaporized solutions of certain of the metal carbonyls in readily vaporizable solvents (for example, petroleum ether), also nitroxyl compounds, nitrosyl carbonyls, metal hydrides, metal alkyls, metal halides, and the like.

Illustrative compounds of the carbonyl type are nickel, chromium, molybdenum, tungsten, cobalt, and mixed carbonyls.

Illustrative compounds of other groups are the nitrosyl carbonyls, for example, cobalt nitrosyl carbonyl; hydrides, such as antimony hydride; metal alkyls, such as chromyl chloride; and carbonyls halogens, for example, osmium carbonyl bromide, rhuthenium carbonyl chloride, and the like.

It will at once be recognized that certain of these metals, such as tungsten, cannot be plated from electrolytic plating baths.

Each material from which a metal may be plated has i a temperature at which decomposiiton is complete. However, decomposition may take place slowly at a lower temperature or while the vapors are being raised in temperature through some particular range.

For example, nickel carbonyl completely decomposes at a temperature in the range of 375 F. to 400 F. However, nickel carbonyl starts to decompose slowly at about 175 F. and therefore decomposition continues during the time of heating from 200 F. to 380 F. A large number of the metal carbonyls and hydrides may be effectively and efficiently decomposed at a temperature in the range of 350 F. to 450 F. When working with most metal carbonyls we prefer to operate in a temperature range of 375 F. to 425 F. Certain metals, such as tungsten in form, can be deposited at temperatures up to 875 F.

The plating operation may be divided into a preliminary flash coating with metal and a finish plating operation, in which event an adhesion anneal or heating is interspersed between the two coating operations. However, split plating complicates the coating operation, and it is preferable to insure the chemical cleanness of the metal surface before plating and then effecting adhesion Without controlled heating after the deposit of the protective metal coating.

One of the factors important to successful operation of the preferred form of apparatus hereinafter described in detail is control of gas pressure in the housing.

In order to insure against leakage of plating gases and decomposition products from the plating chamber and still have openings in the partition walls for passage of the base metal, it is necessary to maintain a metal-vapor free gas atmosphere at a slightly higher gas pressure in the jacket of the housing and in the gas locks preceding and succeeding the plating chamber.

The leakage of inert gas into a plating chamber is limited to small quantities by keeping the pressure differential small. It will be recognized that inert gas leakage into the plating chamber is not a harmful operation because the metal-bearing gases are usually diluted with an inert gaseous medium.

For gas desorption and annealing to insure a uniformly bonded coating, heating at a temperature in the range of 800 to 1200 F. is carried out preferably by causing the metal to warm.

It will be recognized that annealing of the metal deposit occurs in this process when the coated billet is heated with rolling temperatures. Inasmuch, as these tempera tures exceed the above range, an effective desorption, one of the chief requirements, is accomplished.

The invention will be more clearly understood from the following description of one embodiment of the apparatus and its mode of operation.

In the drawings:

Figure 1 is a schematic diagram of the process as carried out on a straight through basis.

Referring to the drawings, the numeral 10 represents a forged steel billet. Billet 10 as illustrated has been hammered to shape at a temperature of about 2000 F., has been cooled by radiation of heat and passage of an oxygen free atmosphere therearound until a temperature of approximately 400 F. has been reached.

Billet 10 is supported by a support track 11 made up of rollers 11a which form a continuous track through the plating and heating equipment.

As illustrated billet 10 is enclosed in a tunnel 12 provided with an inlet door 13. Inert gas is supplied to tunnel 12 through an inlet conduit 14 and removed therefrom through an outlet conduit 15.

From tunnel 12 the billet 10 passes through a gas lock 16 into a housing 17 in which the billet is plated. At the opposite end from the inlet, housing 17 is provided with an outlet lock 18 of a construction similar to gas lock 16.

Gas lock 16 consists of a quadrilateral tube 19 provided with apertured partitions 20 and 21. Roller units 11a journaled in the side walls of the quadrilateral tube 19 are mounted atop the bottom portion of partition walls 20 and 21.

Pivotably mounted at the top of each partition aperture are doors 22 and 23. These doors are adapted to hang vertically and to close the aperture in each partition. Being hinged the doors swing upward and out of the path of entering billets.

Spacing of partitions is preferably such that doors 22 and 23 are not both open at the same time, although this is not a feature essential to the operation of the equipment.

It will be recognized at once that no attempt is made to obtain a gas tight lock. The sealing of plating gas into the inner chamber is obtained by control of gas pressures.

It will further be recognized that the support rollers can be integrated to run continuously and at a predetermined rate to pass billets at a uniform rate of speed through the equipment, or other arrangements can be made such as providing for rotation of groups of rollers as desired in response to manual controls.

Housing 17 consists of an outer wall 24 with inner walls 25 parallel to the vertical front and back walls and wall 26 parallel to the bottom defining a jacketed con struction.

The jacket communicates with the interior of the gas locks 16 and 18. Gas inlet 27 and outlet 2'8 are provided for this jacket. Chamber 29 formed within the housing 17 is provided with a gas inlet 30 and a gas outlet 31. Adjacent the plating housing 17 is positioned a furnace 35. The furnace 35 is adapted for continuous heating. The furnace shell 36 is apertured as at 37 and 38 and provided with doors 39 and 40 pivotably mounted at their top.

From the furnace 35 the track 11 directs the billets 10 to a roughing train 41, for example, a reversing universal mill.

These mills consist of multiple rollers and a number of roller stands, generally four in number, only the contact rollers of which are indicated by the numerals 42, 43 and 44.

In the finishing, billets which have now been reduced to about inch in thickness and are generally designated as strip bars, are passed through the finishing mill, which likewise consist of multiple rollers and a number of roll stands varying from 4 to 8, only the contact rollers of which are indicated by the numerals 45 and 46. The sheet metal product is indicated by the numeral 47.

'In brief, the operation of the apparatus is as follows:

A billet, cooled to a temperature of approximately 400 F., moves through the lock 16 where inert carbon dioxide atmosphere is maintained at a pressure slightly higher than the highest pressure maintained on either side of the lock. This higher pressure insures that gas will flow outward from the lock through unsealed portions of the equipment. The flow of inert gas into the plating chamber, as has heretofore been explained, merely dilutes the gaseous mixture within the plating chamber.

The billet passes into chamber 29 within housing 17 where it contacts decomposable metal bearing gas and emerges through lock 18.

Plating gasses are excluded from the jacket and escape from the plating chamber prevented by passing through the jacket one or more of the above mentioned inert gases at rates within the range of 10 to 20 cubic feet per hour per cubic foot of chamber.

In the plating chamber gas flow of mixed gas containing inert gas, such as carbon dioxide mixed with metal volatile compounds, is in the gas range of 2 to cubic feet per hour per cubic foot of plating chamber.

In the plating of nickel upon a 1020 steel billet or ingot, to a thickness of approximately 0.1 inch, the following conditions may be maintained:

The billet may be washed with alkaline solution to obtain a chemically clean surface. The temperature of the billet may be raised to approximately 410 F. in an electrically heated oven and the billet passed into the plating chamber.

Rate of flow of gas through the plating chamber may be approximately to 30 cubic feet per hour per cubic foot of chamber space, with nickel carbonyl vapors being present when it is desired to deposit nickel plate in the ratio of approximately 10 ounces of carbonyl per cubic foot of carbon dioxide gas passed through the plating chamber.

The temperature of the gas being circulated in the plating chamber may be maintained in the range of 150 F. to 180 F. when not in contact with the hot metal by suitable cooling means, such as finned piping, within which is circulated a suitable refrigerating medium.

The nickel coated billet produced in the plating cham her may then be introduced with or without intermediate cooling into the electric furnace where the temperature of the billet is raised to an equalized temperature of approximately 2200 F.

The hot nickel plated billet is then passed between the rollers of a roughing train.

If the metal cools below 1500 F. it should be reheated to approximately 1650 F. before passage through the finishing train of the rolling mill.

When rollinga billet having a nickel coating, a finished metal sheet may be obtained having a nickel coating approximately .0005 inch thick with good corrosion resistance properties.

It will be understood that this invention is susceptible to modification in order to adopt it to different usages and conditions and, accordingly, it is desired to comprehend such modifications within this invention as may fall within the scope of the appended claim.

We claim:

A method of working and coating cast steel billets prior to rolling and which yields upon rolling a clad steel having a thin uniform thickness surface layer of corrosionresistant metal material, said method comprising the steps of (a) casting a steel billet at a temperature in the range of approximately 1900 to 2500" F (b) lowering the temperature of the cast billet to between about 350-450 F. while retained in a non-oxidizing atmosphere, (c) sub jecting the resultant temperatured billet to gaseous metal plating by contacting the hot billet with a gaseous mixture containing a corrosion-resistant gaseous metal com pound which decomposes depositing corrosion-resistant metal onto the surface of said cast steel billet to provide the same with a relatively thick metal coating of approxi* mately 0.10 inch in thickness, (d) heating the resultant metal plated billet to a temperature between about 1300- 2500 F., and (e) hot working the thus heated billet by passing the same through a roughing and finishing train of rollers to produce a clad steel sheet having a relatively thin protective metal coating which is on the order of 0.0005 inch in thickness.

References Cited in the file of this patent UNITED STATES PATENTS 1,399,044 Bellis Dec. 6, 1921 1,537,839 Lohmann May 12, 1925 1,565,724 Fonda Dec. 15, 1925 1,608,694 Cain Nov. 30, 1926 1,702,387 Kuhn Feb. 19, 1929 1,853,369 Marshall Apr. 12, 1932 1,997,538 Armstrong Apr. 9, 1935 1,998,496 Fiedler Apr. 23, 1935 2,099,874 Trenzen Nov. 23, 1937 2,105,426 McManus Jan. 11, 1938 2,181,093 Ness Nov. 21, 1939 2,183,302 Brauer Dec. 12, 1939 2,225,868 Huston Dec. 24, 1940 2,293,810 Domm Aug. 25, 1942 2,304,182 Lang Dec. 8, 1942 2,337,751 Ingersoll Dec. 28, 1943 2,344,138 Drummond Mar. 14, 1944 2,384,500 Stoll Sept. 11, 1945 2,464,163 Weesner Mar. 8, 1949 2,464,591 Larsen Mar. 15, 1949 2,508,509 Germer May 23, 1950 2,523,461 Young Sept. 26, 1950 2,656,284 Toulmin Oct. 20, 1953

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