WO1992001073A2

WO1992001073A2 - Process for manufacturing metal products using scrap metal

Info

Publication number: WO1992001073A2
Application number: PCT/GB1991/001113
Authority: WO
Inventors: Antonino Giorgio Cacace; Allan Douglas Clark
Original assignee: Camborne Industries Plc
Priority date: 1990-07-12
Filing date: 1991-07-08
Publication date: 1992-01-23
Also published as: WO1992001073A3; AU8201991A; ZA915390B

Abstract

A process for recycling scrap metal swarf is disclosed in which a billet is formed by compacting the swarf into briquettes (86) ina surrounding metal jacket (82). Before being jacketed the swarf is crushed, cleaned and passed through a furnace (34) in which surface oxides are reduced. The swarf is allowed to cool at a controlled rate in non-oxidising conditions so that it is annealed before being briquetted. It may be screened to remove scale and other fines before being briquetted. The swarf may be mild steel or carbon steel and the jacket may be steel or stainless steel. The billets are reheated and rolled or otherwise worked into finished or semifinished products.

Description

TITLE: PROCESS FOR MANUFACTURING METAL PRODUCTS USING SCRAP METAL

BACKGROUND OF THE INVENTION

This invention relates to a process in which, without melting, scrap metal swarf is treated for the removal of oxides in the course of producing useful products therefrom.

The term "swarf" comprehends the off cuts from machining operations in general and is intended to include the off cuts from turning, boring, shaping and milling operations on engineering steels. The fine off cuts from some stamping and punching operations may also be suitable.

A number of proposals have been made to rework swarf into a useful product without remelt. Of these the only process known to the applicant which has been commercially worked is that disclosed in British patent #1313545. In this process steel swarf which has been cleaned and chipped is compacted and enclosed in a steel jacket to form a billet. The billet is heated to about 1200 C prior to being rolled in a substantially conventional rolling mill. In the rolling operation the chips weld together to form a substantially homogeneous mass.

The process is practically successful owing at least partially to the presence of the jacket which prevents atmospheric oxygen from getting to the swarf when it is hot. There have been other prior proposals in which a jacket is not used. In practice it has not been found possible to prevent oxidation of a hot uniacketed billet while it is being worked.

In the commercially worked process referred to above the swarf is introduced into the jacket and compacted into briquettes in the course of forming the billet. The compaction of the swarf is necessary, inter alia, to give the billet inherent strength so that it does not break UD while it is being worked. Up to now no advantage would have been perceived in reducing the oxides prior to insertion in the jacket. It has been found virtually impossible to prevent oxidation of hot uniacketed swarf and the waste of heat involved in allowing the swarf to cool before transferring it to the jacket would have been seen as largely defeating the benefits obtained in recycling the swarf.

SUMMARY OF THE INVENTION

According to the invention there is provided a process for recycling metal swarf including the steps of inserting a quantity of the swarf in a metal jacket to form a billet comprising a core of compacted swarf surrounded by the jacket, the process being characterised in that prior to being inserted in the jacket the swarf is heated and allowed to cool in non-oxidising conditions so that the swarf is annealed and surface oxides initially present on the swarf are at least partially reduced.

In one form of the invention, after the billet is formed it is heated and worked to form a finished or semi-finished product.

In one aspect of the invention the swarf is composed substantially of engineering steel. In another aspect of the invention the jacket is composed of stainless steel.

In an important aspect of the invention the equivalent density of the core is not less than 85% and preferably at least 89%. In this specification the term 'equivalent density' means the ratio (expressed as a percentage) of the densities of two bodies of the same size, the one body being composed of swarf and the other body being composed of solid material of the same type as the swarf.

According to another aspect of the invention discrete quantities of swarf are sequentially inserted in the jacket and compacted into briquettes to form the core.

The applicant has discovered that there are hitherto unforeseen benefits in carrying out the reducing step prior to insertion of swarf into the jacket.

The degree of oxidation of steel scrap used in the process is very variable. It is dependent, inter alia, on atmospheric conditions and the conditions under which the scrap is stored. It is therefore common to find in the conventional process that some billets comprise lightly oxidised scrap while others comprise more heavily oxidised scrap. Because of this it has been found in practice that there is a degree of inconsistency in reducing the oxides in such billets. The reducing reaction goes to completion in some before it is complete in others. Unless great care is taken to ensure completion of the reaction, metallographic characteristics of products made from these other billets could thus be inferior. In the case of severely oxidised scrap an unoxidised layer on the shavings could prevent proper welding together of the shavings and thus cause internal defects leading to rejection of the entire length of product being rolled.

By carrying out the reduction step prior to jacketing it is much easier to ensure complete reduction of all oxides. In the second place, in the process of the present invention, the shavings can be softened or annealed as the oxides are reduced. This allows the swarf to be more easily subsequently compacted and/or, perhaps more importantly, to be compacted to a greater density. For example, experiments have shown that cold mild steel swarf which has been annealed and reduced is compacted to an equivalent density of 891 under a pressure of about 608 M.p.A. or 40 tons per square inch. This compares to an equivalent density of 75% which is achieved with cold non-annealed mild steel swarf under the same pressure.

One important benefit of preannealing the swarf is that a billet having a core of higher equivalent density can be produced by an existing press; alternatively a smaller press can be used to achieve the same equivalent density which was achieved by the existing press.

An increased density has important advantages. After the billet is formed it is heated prior to being worked. At this stage a secondary reducing reaction takes place which reduces any oxides which form in the core as a result of the presence of oxidising gas between the swarf chips. It is clearly therefore beneficial to reduce the air or other oxidising gas which is available between the chips to a minimum. In the case quoted above, because of the increased density of the compacted swarf, the availability of any oxidising gas is diminished by more than 127%.

toother advantage arising from an increase in the density of the core is an increase in the billet weight. The leads to an increase in the rate of production of billets from a given press. In the example used above, an increase in equivalent density from 75% to 89% has the result that the weight of a billet which is 2 metres long and 10 cm in diameter is increased by about 14%.

The use of annealed swarf also helps to reduce the likelihood of the briquettes becoming separated at the interfaces during the rolling operation. Because of the absence of springback, the chips of swarf at the interfaces deform plastically and weld together much more easily.

Yet another advantage of preannealing and prereducing the swarf lies in the fact that spring steel and medium carbon steel shavings can be used. In the unannealed condition these shavings are hard and springy due to previous cold 'working and consequent strain hardening. They thus do not cold weld together as readily as more ductile steels and they are extremely difficult to compress into a tube or to themselves. If such shavings are preannealed during the reduction of the oxides the cold working and springiness is eliminated enabling them more readily to be compressed and to cold weld together.

The cost savings arising from the abovementioned advantages counterbalance the wasted costs of allowing the swarf to cool after the reduction process.

BRIEF DESCRIPTION OF THE DRAWINGS

Examples of the invention are discussed with reference to the accompanying drawings in which

Figure 1 is a block diagram showing in schematic form successive stages in a process for producing billets from scrap steel swarf;

Figure 2 is a schematic illustration of an installation for reducing and annealing the swarf prior to forming the billets;

Figure 3 is a block diagram similar to Figure 1 showing the stages of a modified process for producing the billets;

Figure 4 is a cross sectional illustration of a billet produced by the process of the invention;

Figure 5 is a cross sectional illustration of part of a flat bar produced by rolling the billet shown in Figure 5;

Figure 6 is a cross sectional illustration of a container for swarf to be processed by the process described with reference to Figure 3.

DETAILED DESCRIPTION OF EMBODIMENTS OF THE INVENTION

In this specification the term "engineering steel" is intended to describe those low alloy steels which are commonly subjected to machining operations including mild steel (a term which itself includes carbon steel), forging steel and axle or shaft steel all of which contain significant amounts of carbon.

Referring to Figure 1, mild steel shavings are first passed through a crusher 20 where they are crushed into chips of a nominal size of about 7 ram. It is advantageous to reduce the size of the chips to a minimum in order to increase the surface-to-weight ratio thereof so that reduction of surface oxides can take place as rapidly as possible. The crusher is preferably a hammer mill. Crushers of this type are known and do not need to be described here.

After crushing the chips are passed through a heating apparatus 30 where they are heated in a reducing atmosphere to a temperature of between 950 and 1200 Celsius. They are held at this temperature long enough to ensure that all of the surface oxides are reduced. The chips are then allowed to cool to ambient temperature in a cooling apparatus 32 so that the chips are annealed. A non-oxidising atmosphere is maintained in the cooling apparatus 32 so that there is no chance of re-oxidation of the chips occurring.

Referring to Figure 2, the apparatus 30 is preferably a rotary kiln which will be referred to as a reducing kiln 34. One end of the reducing kiln projects into a receiving bin 36. The chips are fed continuously into the bin from a hopper (not shown). From the bin the chips are fed into the open end of the reducing kiln. The reducing kiln is inclined to the horizontal, the bin being located at the high end thereof. The reducing kiln rotates and the chips progress by gravity therethrough.

It is necessary to seal the interior of the reducing kiln from the atmosphere. To this end a suitable seal of known kind is provided between the receiving bin and the outer face 40 of the reducing kiln. Furthermore, an end plate 42 is mounted on the end of the reducing kiln and an open ended tube 44 of small diameter is welded to the end plate 42, coaxial with the reducing kiln. Chips falling into the bin 36 from the hopper form a heap (indicated by the line 46) which ultimately buries the tube 44. The angle at which the top of the heap slopes downwardly into the tube is constant and the chips thus seal the tube and at the same time feed at a constant rate into the tube due to the rotation of the kiln. The size of the tube is selected so that the chips feed into the reducing kiln 34 at the desired rate. The chips in the bin provide an adequate means of sealing the interior of the reducing kiln from the. atmosphere.

The reducing kiln is heated and a reducing atmosphere is maintained therein by burning a suitable fuel such as natural gas. The interior of the reducing kiln is refractory lined. As the chips progress through the reducing kiln surface oxides are reduced. The chips are also softened. The operating parameters including the inclination and the rate of rotation of the kiln are such that the swarf is held at high temperature in the reducing kiln for as long as it takes for surface oxides to be completely reduced. Optimally this is achieved when the swarf reaches the lower or outlet end of the reducing kiln. When the swarf passes out of the reducing kiln 34 it is transferred into a cooling apparatus in the form of a cooling kiln 60 which in the present example is a second rotary kiln. A closed transfer bin 62 is located between the two kilns. The outlet end of the reducing kiln 34 projects through the front wall of the transfer bin. The cooling kiln 60 is inclined downwardly from the transfer bin, the upper or inlet end projecting through the back wall of the transfer bin. Seals of known kind are provided between the respective front and back walls of the transfer bin and the outer faces of the kilns. The interior of the transfer bin is thus sealed from the atmosphere. The chips feeding out of the reducing kiln 34 can gravitate directly to the cooling kiln. It should not normally be necessary to isolate one kiln from the other to minimise the transference of heat therebetween. However if necessary this could be achieved by providing an end plate and tube at the inlet end of the cooling kiln, similar to the end plate 42 and tube 44 fitted to the inlet end of the reducing kiln as described above. The chips can then be arranged to build up in a heap in the transfer chamber in the same way as happens in the receiving bin.

The interior of the cooling kiln is kept non-oxidising, i.e. neutral or slightly reducing. In the present example this is achieved by burning a controlled amo.unt of natural gas in the cooling kiln. At the inlet end the temperature of the cooling kiln is relatively high but at the outlet end 70 it is close to ambient. The outlet end projects into a closed bin 72. The cooling kiln need not be refractory lined and its inner face is provided with longitudinally extending flights which lift and circulate the chips, promoting even cooling. The chips cool to ambient temperature as they progress through the cooling kiln, and are discharged into the bin 72. From there they gravitate through^"a chute 73 provided with double flap valves 74 into a suitable receptacle 75.

Both kilns operate in counter current mode. Air for combustion is introduced by a fan into the cool (outlet) end 70 of the cooling kiln. The interior of the cooling kiln (as also the reducing kiln) is thus under positive pressure. Waste gases from the cooling kiln pass into the reducing kiln. Waste gases from the reducing kiln are extracted through an outlet manifold 76 located adjacent the inlet end of the reducing kiln.

After being discharged from the cooling kiln the chips are compacted in a briquetting press 80 to form a jacketed billet. However, before this step they are passed through a screening apparatus where fine particles (i.e. minus 2 mm), principally comprising mill scale, are screened out. Scale comprises a high percentage of non-reducable oxides. A suitable briquetting press is described in international application #PCT/GB90/01438. Details of suitable billets appear in international applications #PCT/GB90/01437 and PCT/GB90/01440. The billet is rolled or otherwise hot worked in substantially the same manner as described in British patent #1313545. The process of heating to remove surface oxides followed by slow cooling as described above will have the result that the chips have been annealed and are soft and malleable for the briquetting process.

It is desirable that the briquetting and subsequent operations be carried out as soon as possible after the chips are removed from the cooling furnace so that reoxidation thereof is minimised. Such reoxidation as does take place will be removed when the jacketed billets are heated prior to rolling.

Typically, mild steel shavings are held at 1000 C for thirty minutes in the reducing kiln. They are held, also for thirty minutes, in the cooling kiln from which they emerge at ambient temperature. A typical analysis shows 0.15% carbon; 0.05% sulphur; 0.03% phosphorous; 0.05% manganese; and 0.1% silicon. This demonstrates that little carburisation takes place. The shavings are bright, indicating substantially complete removal of oxides, and soft. The benefits of annealing have already been discussed.

In another example of the invention spring steel and medium carbon steel shavings treated in the same way as described with reference to Figures 1 and 2 show similar results except that the carbon content of the chips after reduction is typically 0.40% and the silicon content is higher. Because they are annealed in the process of reducing the surface oxides the hardness and springiness which such shavings normally exhibit is much reduced and they can be compressed into briquettes which are comparable in density to briquettes of mild steel.

A typical billet produced by the apparatus described in the abovementioned application #PCT/GB90/01438 is shown in Figure 4. The billet is 2 metres long and has a jacket comprised of a welded ASTM A304L grade stainless steel pipe 82 of 89 mm bore and 6 mm wall thickness. The jacket has mild steel end pieces 84 about 75 mm long welded to each end. The billet has a core 86 made up of briquettes comprised of compacted mild steel swarf. The swarf comprises chips of 7 mm average size which have been cleaned, de-oxidised and annealed as described herein. A space of about 3 cm is left between each end of the jacket and the end face of the core. Typically about 86 kg swarf are loaded into the press to make up the core. By calculation a solid bar of mild steel of the same size as the core would weigh 95.5 kg indicating that the core has an equivalent density of about 90%.

A similar billet made up of carbon steel swarf has a core with an equivalent density of about 89%.

Billets of this kind are suitable for rolling in a conventional rolling mill into products such as, purely by way of example, 50 mm x 6 mm flat bars 88 (see Figure 5) and 40 mm x 40 mm x 5 mm angle bars. These products have a cladding of stainless steel of substantial thickness and are useful for use in environments which are corrosive to mild steel.

The billets can also be forged into useful products.

The process of the invention may also be used to produce billets comprising a core of engineering steel in a copper jacket. Such billets are described in greater detail in the abovementioned international patent applications.

It may be of advantage for some applications to compact the chips into unjacketed briquettes or indeed into a larger unjacketed mass comprising several briquettes pressed together prior to reduction or alternatively after reduction but prior to jaccketing. In this case the briquettes or the larger mass are of such a size as to enable them to be pressed into the a jacket. In either case the chips in the briquettes or the larger mass will have been annealed during the reduction and cooling stages and they can be further compressed when they are inserted in the jacket.

Referring now to Figure 3, the apparatus 30 may alternatively be a conventional hearth-type heating furnace 90 in which a reducing atmosphere is maintained. In this case the chips will have first been cleaned and degreased to remove oil, water and other impurities in a cleaning apparatus 92. Apparatus suitable for this purpose is known. The chips must also be crushed in a crusher such as a hammer mill 92 to an average size of 7 mm average size. The cleaned and crushed chips are placed in reusable containers of heat resistant stainless steel or other suitable material. One such container 50 is shown in Figure 6. It has a lid 52. After being filled with chips, the containers are closed and sealed from the atmosphere apart from having small vents 54 in the lids to enable gas. evolved therein to escape. The cooling apparatus 32 may also be a conventional hearth-type furnace 96. In this case the chips are left in their containers 50 while being moved to the cooling furnace and while being allowed to cool in the cooling furnace. It may be advantageous or even necessary to ensure that the vent 54 in a container is sealed when the container is removed from the reheating furnace to prevent ingress of atmospheric oxygen.

Instead of sealing the vent it may be preferable, immediately after the container is removed from the heating furnace, to mount over the vent a cap or nozzle 56 which provides a supply of gas such as nitrogen or hydrogen (or a mixture thereof) and thus keeps the interior of the container under a neutral or reducing atmosphere until the chips are cool.

After cooling, the chips are removed from the containers, briquetted and formed into billets and hot worked in the same manner and using the same apparatus as described with reference to Figures 1 and 2.

It is believed that the reduction of oxides in the swarf by this latter method is due largely to decarburisation of the steel. Because of this it was initially thought that carbon or other reducing agent would have to be added to the swarf to ensure the occurrence of the secondary reducing reaction referred to above which takes place when the swarf is jacketed and reheated prior to rolling. Surprisingly, this has been found unnecessary, even where the swarf comprises low carbon steel.

It is not intended that the scope of a patent granted in pursuance of the application of which this specification forms a part should exclude modifications and/or improvements to the embodiments described and/or illustrated which are within the scope of the invention as defined in the claims or be limited by details of such embodiments further than is necessary to distinguish the invention from the prior art.

Claims

1. A process for recycling metal swarf including the steps of inserting a quantity of the swarf in a metal jacket (82) to form a billet comprising a core of compacted swarf (86) surrounded by the jacket, CHARACTERISED IN THAT prior to being inserted in the jacket the swarf is heated and allowed to cool in non-oxidising conditions so that the swarf is annealed and surface oxides initially present on the swarf are at least partially reduced.

2. A process according to claim 1, CHARACTERISED IN THAT the swarf is composed substantially of engineering steel.

3. A process according to claim 1 or claim 2, CHARACTERISED IN THAT the jacket (82) is composed of stainless steel.

4. A process according to any one of claims 1 to 3, CHARACTERISED IN THAT the equivalent density of the core is at least 85%.

5. A process according to claim 4, CHARACTERISED IN THAT the equivalent density of the core is at least 89%.

6. A process according to any one of claims 1 to 5, CHARACTERISED IN THAT discrete quantities of swarf are sequentially inserted in the jacket and compacted into briquettes to form the core.

7. A process according to any one of claims 1 to 6, CHARACTERISED IN THAT there are residual impurities such as oil and moisture in the swarf which can be driven off by heating, and the swarf is heated for reducing the surface oxides in a heating apparatus in which the residual impurities are also driven off.

8. A process according to any one of claims 1 to 7, CHARACTERISED IN THAT after the billet is formed it is heated and worked to form a finished or semi-finished product (88).

9. A billet comprising a core of compacted swarf in a metal jacket, CHARACTERISED IN THAT the billet is produced by a process according to any one of claims 1 to 7.

10. A finished or semi-fmished metal product CHARACTERISED IN THAT it is produced by a process according to claim 8.