US 3720511 A
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Wfimfih 13, 193 DAVIES, 'ET AL 3,720,511
PRODUCTION OF METAL STRIP FROM POWDEI iED METAL Filed March 17, 1970 OO OOQOOOOOO 3,720,511 Patented Mar. 13, 1973 3,720,511 PRGDUCTION F METAL STRIP FROM POWDERED METAL Idwal Davies and Alan G. Harris, Swansea, Wales, assignor to The British Iron and Steel Research Association, London, England Filed Mar. 17, 1970, Ser. No. 20,184 Claims priority, application Great Britain, Mar. 18, 1969, 14,155/69 Int. Cl. B22f 1/00 US. Cl. 75-214 5 Claims ABSTRACT OF THE DISCLOSURE CROSS-REFERENCES TO RELATED APPLICATIONS remains with the strip during the subsequent processing stages and becomes an integral part of the final strip.
According to a modification of the present invention there is provided a process for the production of strip from powdered metal which comprises depositinga layer of powder metal onto a support surface, rolling the layer to effect compaction of said layer, passing the resultant green strip through an in-line tunnel sintering furnace, coiling the sintered strip before or after a further rolling step and subjecting the coiled strip to a further sintering step in a batch sintering furnace.
Although the modified process described above does not entirely avoid the use of a tunnel furnace, the first sintering step need only be brief, i.e. it need only be sufficient to remove any binder and give the strip enough strength to survive the subsequent mechanical handling before the first batch sintering step is complete.
Both of the processes of the invention described above may, of course, be followed by further batch sintering and rolling steps and normally the resultant strip will be subjected to a final planishing roll. In general, in order to produce steel strip which is fully acceptable for most commercial purposes, it is necessary to subject the strip to at least two compaction and sintering steps.
By the term batch sintering furnace we mean a furnace of the kind normally referred to as batch annealing BACKGROUND OF THE INVENTION strip from powdered metal which comprises depositing a layer of powdered metal onto a support surface, rolling the layer to eflfect compaction and sintering the compacted layer at an appropriate temperature and for a sufficient time to form a coherent metal strip.
In the process described in our above application, the strip is sintered by passage through a tunnel furnace. Use of a furnace of this type involves a number of operating and furnace design problems since in order to achieve a high line speed, the strip must be heated rapidly to a sufficient temperature to sinter the metal particles together as the strip passes through the furnace. Furthermore, a reducing atmosphere must be maintained in the furnace and thus it is necessary to provide seals for the entry and exit of the strip in order to conserve the expensive reducing atmosphere.
SUMMARY OF THE INVENTION It is a primary objective of the present invention to provide a process in which the above-mentioned difficulties can be reduced or avoided.
According to the present invention there is provided a process for the production of metal strip from powdered metal which comprises depositing a layer of powdered metal onto a support surface, rolling the layer to effect compaction of said layer, coiling the resultant green strip and heating the coiled strip to a temperature and for a sufficient period to sinter the metal particles together to form a coherent strip.
Generally speaking where the batch sintering technique is applied to the compacted green strip, the strip should be supported on a substrate, such as a metal foil, which furnaces, although such furnaces have not in the past been used. for sintering powder metal to form a coherent structure. Annealing furnaces are normally filled with a non-oxidising atmosphere and are normally closed to the atmosphere during annealing. Consequently they are more economic than tunnel furnaces in terms of conservation of heat and of the furnace atmosphere. In the practice of the present invention the use of a non-oxidising atmosphere is satisfactory in the batch sintering step, although a reducing atmosphere, e.g. hydrogen or cracked ammonia is preferred.
It is envisaged that the modified process described above will generally be carried out by effecting the batch sintering step after the further rolling step. Thus, in this embodiment of the process, which is preferred for the manufacture of mild steel strip, the layer of powdered metal, which is obtained by deposition on the substrate, is rolled in a first compaction step, sintered in an in-line sintering furnace, rolled again in a second compaction step and then sintered in a batch sintering furnace.
The batch sintering steps can be carried out at normal batch annealing temperatures, e.g. for mild steel strip, 550 C. to 750 C., and at these temperatures sticking together of adjacent laps of the coiled strip does not usually occur. Higher temperatures e.g. up to 950 C. to 1000 C. may be used particularly for alloys such as stainless steel and if lap sticking tends to occur, this problem can be avoided by coating the strip before coiling with a suitable inhibitor such as a metal oxide or salt which is inert at the sintering temperature; e.g. aluminum oxide. Similarly lower temperatures may be used e.g. down to about 500 C. but it must be remembered that the lower the temperature, the longer the time necessary to achieve satisfactory sintering.
Conveniently, the inhibitor is applied to the strip by coating the strip with a solution or dispersion of the material in a suitable liquid medium followed by evaporation of the liquid phase. The solution or dispersion may be applied to a strip by any convenient method, for example roller coating, spraying or by using a rotating absorbent roller impregnated with the solution. The inhibitor may also be applied dry by any conventional method, e.g. sprinkling and/ or electrostatic deposition. The inhibtor is usually removed after the sintering operation.
Batch heating rates are slow and the coiled strips are introduced into the cold furnace and removed after the furnace has heated to the desired maximum temperature and cooled again. 15 hours is a typical period required for the furnace to reach the sintering temperature. The holding time (i.e., the time at which the furnace is maintained at a constant maximum temperature) may vary from zero to several days. The preferred temperature and holding time depend on the particular metal powder being used. In the case of mild steel a batch sintering temperature of 550 C. to 750 C. and a holding time of zero to 48 hours is preferred. For stainless steel it is preferred to batch sinter at a temperature of from 700 to 950 C. for a holding time of from zero to 96 hours.
Because of the nature of the batch sintering as compared with rapid in-line sintering, strip produced in accordance with the present invention tends to have a greater ductility than that produced in accordance with our above-mentioned co-pending application.
It should be noted, however, that where batch sintering is carried out after the compaction step following the first sintering, it may be necessary to carry out this second compaction at a rolling load which results in an extension greater than This may be necessary to avoid critical grain growth (i.e. the excessive grain growth that can occur when annealing metals that have received only a small amount of strain).
The temperature and period of exposure of the green strip in the tunnel furnace must be sufficient to increase the strength of the strip sufficiently to enable it to survive coiling and subsequent handling prior to the second sintering step. This parameter will vary according to the nature of the powdered metal. In the case of iron and steel powders, the tunnel furnace should be at a temperature of 750 to 1400 C. and the time within the hot zone of the furnace should normally be from 5 to 240 seconds. A typical sintering temperature would be 850 C. to 1200 C. and a sintering time of from 8 to 20 seconds. In the case of mild steel strip using a -300 mesh B.S.S. powder we have found that a temperature of 1100 to 1200 C. and a sintering time of 8 to 12 seconds is satisfactory. As described in an above mentioned co-pending patent application, the green strip is passed continuously through the tunnel and the sintering times referred to above are the 7 times which any particular part of the green strip takes to pass from one end of the hot zone to the other.
The strip which emerges from the tunnel furnace will be in a partially sintered condition, i.e. there will be some metal to metal bonds but the strip will be porous and not fully coherent. In the second compaction, the objective is to achieve substantially 100% density and in the batch sintering step to complete the formation of the metal to metal bonds.
The process of the present invention is generally applicable to all of the various embodiments described in our above mentioned cognate application. It is, however, preferred to utilise a binder composition which is a solution or dispersion of a cellulose derivative, such as methyl cellulose, or hydroxypropyl methyl cellulose, in the formation of the green strip. As stated in our co-pending application it is desirable not to remove all the water during the drying step since otherwise the film forming properties of the cellulose derivatives are impaired.
In order to produce ductile and flexible green strip when using methyl cellulose as the binding agent, the amount of water remaining in the strip after drying should be from 7 about 2 to 6%, e.g. 3% by weight of the methyl cellulose.
Methyl cellulose solutions have the property of gelling when heated to temperatures above about 50 C. and this property can be used to advantage in controlling the uniformity of the green strip. Thus, the suspension of powdered metal and methyl cellulose solution may be deposited onto a drum and the wet layer raised quickly above the gel temperature. The gelled layer is then immediately transferred to a continuous band or air bed where it is dried to the residual water content indicated above. The advantages of this procedure are that when depositing onto a drum it is possible to achieve better dimensional accuracy and control of alignment than by depositing directly onto a band. The dimensions of the layer formed on the drum are fixed by the gelling of the methyl cellulose and thus any variation in the gauge of the band has no effect on the dimensions of the gelled layer transferred to the band. Drying the gelled layer on the drum is not generally practicable since in order to achieve the desired line speeds, the diameter of the drum required would be too great.
One embodiment of the process of the present invention will now be described with reference to the accompanying single figure of drawings which is a schematic drawing of one form of apparatus for carrying out the first part of the process.
Referring to the figure, a reservoir 1 contains a supply of a slurry of metal powder and an aqueous binder composition which is transferred by means of the rollers 2 of a roller coater to a drum 3. Drum 3 embodies means for heating its surface above the gelling point of the binder. Disposed downstream of drum 3 to receive the gelled coating therefrom is an endless stainless steel belt whose upper run passes through a drying oven 5 for removing the bulk of the water from the binder composition. Desirably the belt 4 is heated from both above and below in the drying oven 5. A pair of rollers 6 and 7 are arranged to receive the dried metal/binder layer and to apply a compaction of between 2 to 30 tons per inch width of the layer. A tunnel sintering furnace 8 is arranged to receive the green strip. As indicated in the drawing, the furnace has a hot zone A which is heated by a bank of electrical heating elements 9 and a cool zone B. The furnace is filled with a reducing gas such as hydrogen or cracked ammonia and seals may be provided at the entry and exit. In the arrangement shown in the drawing, the reducing gas is burnt off at the entry slot and the strip passes through a roller seal at the exit 11. The strip is conveyed through the furnace on rollers 12. The resultant sintering strip is subjected to a second compaction by passage through a fourhigh rolling mill 13 and coiled at a coiler 14. The resultant partially sintered strip is then subjected to a further sintermg step in a batch sintering furnace.
Preferably the binder composition is aqueous methyl cellulose and the powdered metal is an iron or steel powder. In the production of strip of up to about 0.01" final thickness, we prefer to use powder having a particle size of about 300 mesh B.S.S. In the case of strip of greater thickness, a particle size of about mesh B.S.S. is preferred. The preferred procedure for preparing the slurry is as follows: The appropriate quantity of water is added to the powdered metal in a mixing vessel and stirred to wet the powder. The water may include small quantities of a surface-active agent, corrosion inhibitor and a hygroscopic substance, preferably glycerol. This mixture is heated to about 35 C. and the required quantity of methyl cellulose chips are added, stirred and the mixture allowed to cool to below 20 C. to take the methyl cellulose into solution. Mixing is then continued in order to remove any entrained air bubbles.
The relative quantities of metal, methyl cellulose and water depend on the thickness of strip desired. For ultimate strip of thickness in the range of about 0.001" to 0.01, a slurry composition comprising 70% by weight of mild steel powder, and 30% by weight of a solution of 1.5% by weight of methyl cellulose in water is satisfactory. With substantially thicker strip it is desirable to increase the proportion of metal powder at the expense of the water and methyl cellulose.
The invention as described in our co-pending application No. 807,1001 (which is concerned with the incorporation of glycerol in the slurry composition) and as de scribed in our co-pending application 838,325, now Pat. No. 3,658,517 (which is concerned with the application of a release agent such as oleic acid to the belt or drum) may be utilised in the process of the present invention.
Although the batch sintering steps are much slower than the operations of depositing the green strip and subjecting this to compaction and in-line sintering, the differences in processing times can be dealt with by arranging for a plurality of batch' sintering furnaces to be fed from a single in-line strip production plant.
It will be appreciated that by virtue of the present invention, the problems encountered in the use of in-line sintering furnaces are reduced or avoided. The batch furnace can be substantially closed, with minimum flow rates of the reducing atmosphere and lower sintering temperatures may be adopted. These factors simplify the problems of designing the furnace and in avoiding the operating difiiculties of handling green strip at high temperatures and line speeds.
1. A process for the production of metal strip from powdered metal, which comprises the steps of:
(1) depositing on a moving support surface, a coating of a slurry comprising an aqueous suspension of powdered metal in a binder composition containing a gel forming cellulose derivative serving as the binder;
(2) heating the coating on the support surface to form initially, a gelled layer and subsequently a dried coherent self-supporting film;
(3) removing the dried film from the support surface;
(4) rolling the dried film to effect compaction and form a green strip;
(5) passing the green strip through an inline tunnel furnace maintained at a sintering temperature to partially sinter the strips;
(6) rolling the partially sintered strip and then (7) heating the coiled strip at an annealing temperature in a batch annealing furnace.
2. A process according to claim 1 in which the green strip is sintered in the in-line tunnel furnace for a period in the hot zone of from 5 to 250 seconds at a temperature of from 750 C. to 1,400 C. whereby the resultant strip has suflicient strength to withstand coiling and handling.
3. A process according to claim 2 in which the powdered metal is mild steel and wherein the sintering temperature in the tunnel furnace is 850 C. to 1,200 C. and the time in the hot zone is from 8 to 30 seconds.
4. A process according to claim 2 in which the powdered metal is mild steel and in which the coiled strip is heated at an annealing temperature in a batch annealing furnace at a temperature of from 500 C. to 750 C. for a holding time of up to 48 hours.
5. A process according to claim 2 in which the powdered metal is stainless steel and in which the coiled strip is heated at an annealing temperature in a batch annealing furnce at a temperature of from 700 C. to 950 C. for a holding time of up to 96 hours.
References Cited UNITED STATES PATENTS 3,330,654 7/1967 Sweet 7.5-208 CS 3,476,528 11/1969 Buss -208 3,335,002 8/1967 Clark 75221 3,121,631 2/1964 Comstock 75--22l 3,335,000 8/1967 Bliss 75--221 OTHER REFERENCES Jones, W., Fundamental Principles of Powder Metallurgy, Edward Arnold (Publishers) Ltd., 1960, pp. 928-947.
CARL D. QUARFORTH, Primary Examiner B. HUNT, Assistant Examiner U.S. Cl. X.R. 75--211, 221