FIELD OF THE INVENTION

This invention relates to a process for the manufacture of corrosion resistant metal products and to products produced from the process. The invention has particular but not exclusive application to products comprising a core of corrosion susceptible steel bonded to a cladding comprised of stainless steel, nickel-chrome alloy, nickel-copper or copper-nickel alloy. It is also intended that the invention should cover billets for use in such methods since there may be a market for such billets. .

The susceptibility to corrosion of what are commonly simply called "steels" that are most often used in industry should not require further discussion. Conversely, the corrosion resistant properties of stainless steels and the aforementioned alloys are equally well known. This invention applies, in principle, to any product that is composed of a body of steel that is significantly more susceptible to corrosion than stainless steel or the aforementioned alloys and that is susceptible of having applied to it a cladding of these materials by the techniques described herein. In this

specification, the term "steel" used by itself will refer to such a steel unless it is clear from the context that this is not intended. In particular, it is intended that the term "steel" should cover what are commonly called carbon steels. According to convention, and as used herein, the term "carbon steels" covers various grades of carbon steel, including mild steels, low alloy engineering steels and micro-alloy steels.

The terms "stainless steel", "nickel-chrome alloy" and "nickel-copper alloy" are names that are well known in the metal industry and are generally applied to a range of alloys containing, respectively, significant amounts of chrome, nickel and chrome, and copper and nickel. In nickel copper alloys there is more nickel than copper, in contrast to "copper-nickel alloys" in which the proportions of nickel and copper are reversed. Ranges of alloys under each of the four names appear in lists available from the major producers thereof including Outokumpu, Allegheny Ludlum, Special Metals Corporation (owners of the trade marks Monel for nickel-copper alloys and Inconel for nickel-chrome alloys), Haynes International Inc (owners of the trade mark Hastelloy for nickel-chrome alloys) and Columbia Metals Ltd. Furthermore the alloys in each range are covered by standards issued under the names of the respective alloys and set up by international standards bodies such as ASTM (American Society for Testing Materials) and JSA (Japanese Standards Association) and material classification systems such as UNS (Unified Numbering System). As will become clear, an essential aspect of the invention is the provision of means to avoid oxidation of the named metals in the respective alloys when they are heated in the course of producing ferrous products that are clad with the alloys. As used herein, the three terms are intended to cover such of these alloys in which oxidation of the named metals is avoided or at least reduced in the course of production of such ferrous products according to the techniques of the present invention. For avoidance of doubt, it is intended that the alloys to which this invention applies include, but are not limited to:

Stainless steel: austenitics including ASTM A304 (UNS S30400), ASTM 316 (UNS S31600), ASTM XM-29 (UNS S24000), ASTM XM-28 (UNS S24100);

duplexes including UNS S32101, S32304, S32205, S32760 and 32750.

Nickel-chrome alloys: ASTM B637 (UNS N06002) and ASTM B564 (UNS N10276) Nickel-copper alloys: ASTM B865 (UNS N05500) and ASTM B166 UNS N06600) Copper-nickel alloys: UNS C70600 and UNS C71500

In this specification, the following abbreviations are used in order to avoid excessive repetition:

SS stainless steel

NiCr = nickel-chrome alloys

NiCu = nickel-copper alloys

CuNi = copper-nickel alloys

RT = Starting Rolling Temperature Range

RTa = RT for: austenitic SS NiCr: 1230-1280°C

RTd = RT for: duplex/ferritic SS/NiCu/CuNi: 1100-1200°C

FD = "finely divided" in the sense defined below. BACKGROUND OF THE INVENTION

There are earlier patents that deal with the production of products rolled from a composite billet consisting of a SS cladding on a steel core. Some of these earlier patents are discussed in USP 6706416 to Cacace. In the specification of that patent reference is made to several other patents also in the name of Cacace. These patents and the processes described therein are referred to herein as the "earlier Cacace" patents and processes.

In the earlier Cacace patents, including USP no. 6706416, a billet is prepared by progressively compressing briquettes of carbon steel swarf into a round SS pipe. On perusal of these patents it seems clear that the achievement of a satisfactory metallurgical bond at the interface between the SS cladding and the core of briquetted swarf has been problematical. The root of the problem is the occurrence of oxidation of chrome in the stainless steel at the interface. USP 6706416 discloses a process of dealing with this oxidation. This is possibly the reason that, as far as the applicant is aware, the process is the only one for producing SS clad reinforcing bar that is currently in commercial use.

There are significant disadvantages to the use of a billet that comprises compressed swarf for the core. Modern rolling mills are designed to roll square billets that can be up to 15 m long and are typically 130 mm to 150 mm in cross sectional size. Most modern mills cannot roll round billets. The billets described in the earlier patents are round and are about 2 m long x 100 mm in diameter. Only a limited number of existing rolling mills are able to roll billets of such short length and even fewer that can also roll from a round billet.

The use of small billets is likely to result in the rolling process being inefficient and it is self-evident that costly specialised machinery, some of which is described in USP 5088399 (one of the earlier Cacace patents), is required for preparing even the small billets used in the earlier process. It is not easy to envisage machinery that is capable of producing billets that comprise compressed swarf and have a cross-sectional shape that is not round. Although in principle the size and length of billets that comprise compressed swarf could be increased, and the shape changed, the technical problems involved in achieving suitable machinery for this purpose might well be insuperable.

Furthermore, in a full scale manufacturing operation, it may be difficult to source swarf of a particular grade in a situation in which it is necessary that the end product comply with an international standard or specification.

Apart from the earlier Cacace patents, the applicant is not aware of any prior patent or other prior art in the field of SS clad steel in which the problem of chrome oxidation is mentioned or dealt with. One object of the invention is to provide a billet comprising a solid steel body and a cladding composed of stainless steel, or a nickel-chrome, nickel-copper or copper-nickel alloy in which oxidation which interferes with the bond between the cladding and the steel body in the finished product is reduced, at least to the extent of providing a commercially acceptable finished product.

The applicant has developed a technique for preventing chrome oxidation in billets that are composed of solid mild steel that is clad with SS. This technique is described below and in the specification that accompanies the international patent application filed pursuant to Australian provisional patent application no. 2009905132 and entitled "Corrosion Resistant Metal Products". The present invention is concerned with the production of products from such SS clad billets, including long square billets that can be rolled in existing modern rolling mills.

The examples described in this specification of products produced according to the methods of the present invention are based on billets that are 14m long and 150 mm x 150 mm in cross sectional size. Clearly, all of these dimensions are by way of example only.

It is difficult to fit a square metal bar that is nominally, say, 14 m long and 150 mm x 150 mm in cross-sectional size, tightly into a preformed square metal tube. In reality, considering the twist and bend tolerances that have to be allowed for, substantial internal gaps exist between the bar and tube. As discussed extensively in the earlier patents, any gaps between the core and the tube are counterproductive to heating and hot rolling into finished products. So, if composite billets are to be useful for rolling into acceptable finished SS clad products, the gaps or clearances must be as small as possible and this requirement increases the difficulty of inserting a bar into a preformed tube. This difficulty increases with increasing length of the bar and tube. It should be clear that many descriptive terms found herein are used in the sense in which they are used the steel industry. Persons skilled in the art will thus be aware that, for example, a metal tube or bar that is described as 'square' will inevitably have corners that are rounded to some extent. For many purposes, metal bars and tubes are purposely formed with round corners and, to be commercially acceptable, many of the characteristics of such products, including the radius applied to the corners of bars and tubes, will be governed by authoritative specifications. To avoid excessive repetition, the following terms have the meanings indicated unless it is clear from the context that this is not intended:

"square bar" and "rectangular bar" include such bars having corners that are intentionally rounded;

"square tube" and "rectangular tube" generally but not essentially refer to tubes of stainless steel and include square tubes and rectangular tubes having corners that are intentionally rounded. Further, although the descriptions are based on the use of a round cornered square bar inserted into a round cornered SS tube, they could equally apply to the use of bars and tubes of any other suitable shapes including both round and out of round shapes, due account being taken of the differences in shape. Most modern bar and rod rolling mills are designed to roll continuous cast square billets of steel in size ranges from 150 mm X 150 mm to 120 mm X 120 mm, with billet lengths as long as possible, typically between 5m and 15m. The examples of billets prepared using the techniques of this invention and described in this specification are therefore based on billets that are 14m long. Clearly, all of these dimensions are by way of example only. STATEMENTS OF INVENTION

In this specification the term "scavenge" implies the removal of gaseous oxygen, as opposed to "reduction" which implies the removal of oxygen from a compound that contains oxygen as one of its components.

According to one aspect of the invention, there is provided a method of producing a billet which can be heated and worked to form a corrosion resistant product, the billet comprising a steel body and a tube of an alloy selected from the group comprising stainless steel, nickel-chrome, nickel-copper and copper-nickel alloys and having a bore of such a size that there is a gap between at least some interfacing parts of the tube and the steel body when the steel body is inserted in the bore, the method including the steps of inserting the steel body in the bore and stretching the tube in such manner that the elastic limit of the stainless steel is exceeded and the size of the gap is reduced.

According to another aspect of the invention there is provided a method of producing a ferrous product from a billet comprising a steel body and a tube of an alloy selected from the group comprising stainless steel, nickel-chrome, nickel-copper and copper- nickel alloys and having a bore of such a size that there is a gap between at least some interfacing parts of the tube and the steel body when the steel body is inserted in the bore, the method including the steps of inserting the steei body in the bore , stretching the tube in such manner that the elastic limit of the stainless steel is exceeded and the size of the gap is reduced, heating the billet and working the heated billet to form a ferrous product in which the interfacing parts are bonded together.

The methods of the invention make use of the fact that stainless steel is capable of a high degree of elongation. A stainless steel tube that is stretched can increase in length by as much as 40% check before it breaks. NiCr alloys containing less than 40% Cr are also capable of about 40% elongation. NiCu and CuNi alloys are capable of 35 - 60% elongation. The cross sectional dimensions of the tube decrease in proportion to the degree of elongation. So that, any clearance or gap that exists between a steel bar and a SS tube in which the bar is inserted, will be reduced when the tube is stretched. Moreover, the reduction will be permanent if the tube is stretched beyond the elastic limit of the stainless steel. The elastic limit is exceeded well before the tube breaks due to excessive elongation. So that, the size of the clearance can be decreased by stretching the tube beyond its elastic limit. In one aspect of the invention, means is provided for preventing oxidation of chrome in the part of the tube located at the interface when the billet is heated.

In one aspect of the invention, at least one end of the tube is closed. In one aspect of the invention, the means for preventing oxidation comprises at least one scavenging metal which is located in the tube adjacent at least one end of the steel body, the method including the steps of heating the billet in such manner that, before the part of the tube at the interface reaches a temperature at which oxides of chrome, nickel or copper can form in the tube at the interface, the scavenging metal is heated to cause oxidising gases at the interface to be scavenged.

In one form of the invention, the scavenging metal is selected from the group comprising aluminium, titanium, magnesium and an alloy of magnesium and aluminium.

In one aspect of the invention, the tube after stretching is substantially longer than the steel body. In one aspect of the invention, the steel body and the tube are of other than round cross sectional shape.

In one aspect of the invention, the steel body and the tube are of rectilinear cross sectional shape. Advantageously, the steel body is square.

In one aspect of the invention, the steel body is hollow.

In one aspect of the invention, the steel body is tubular. Jn one aspect of the invention, the alloy is stainless steel.

The steel body may be a solid bar or hollow bar. Use of a solid bar would typically result in a hot rolled end product such as SS-clad rebar. Use of a hollow bar would typically result in a pipe that is produced by a known technique such as hot piercing and which has an external cladding of stainless steel.

The invention extends to a billet that has been prepared by a method as claimed herein.

DESCRIPTION OF EMBODIMENTS OF THE INVENTION

The invention is further discussed with reference to the accompanying drawings, in which:

Figures 1 to 5 and 10 to 12 show cross sectional views of one end of a billet; Figure 6 is a schematic view of a heating arrangement for the billets;

Figures 7 and 8 are cross sectional view of examples of products that can be produced from the billets;

Figure 9 is a schematic view of an apparatus for evacuating the billets;

Figures 13a and 13b illustrate the typical shape that can be taken up by the components of a round composite billet at the commencement of rolling; Figure 14 similarly illustrates the typical shape that can be taken up by the components of a square composite billet at the commencement of rolling; Figures 15a and 15b are plan views of an apparatus for inserting a bar in a tube, stretching the tube and inserting briquettes into the ends of the tube; Figures 16, 17 and 18 respectively are views in the axial direction of square, round and rectangular billets for the production of products such as rebars, flat bars, angle sections and plates, all externally SS clad;

Figure 19a is a view, also in the axial direction, of a hollow billet for producing an externally clad tubular product;

Figure 19b is a side elevation of the billet shown in Figure 19a; Figure 20 is view in the axial direction of a square billet in which the core body is a square steel tube.

In the work carried out by the applicant up to the present time in connection with the development of the invention, the billets have been comprised of core bodies of carbon steel and a cladding of A304 SS and UNS S32101 and S32304 duplex stainless steels. The embodiments of the invention described herein are therefore focussed on such billets. However, considering that nickel and copper have a higher "free energy of oxidation" (FEOF) than chrome, the applicant believes that the techniques of this invention can be successfully applied without significant modification to producing products comprising a steel core body that is clad with nickel-chrome, nickel-copper or copper-nickel alloys.

(In the present context, the FEOF provides a measure of whether, at any given temperature, the metal of which an element in the billet is composed will be oxidised in preference to chrome, nickel or copper in the cladding and thus prevent oxidation thereof. A diagrammatic illustration of the FEOF of various metals appears in the Ellingham diagram for the reaction of metals to form oxides. The Ellingham diagram is freely available in the internet and in technical literature).

Referring to the drawings, the common object of the methods used to prepare the billets that are described and illustrated below is to reduce the gaps at the interface between the core bar and the SS jacket. The ultimate purpose of this is to prevent, or at least to reduce as far as possible the possibility of finning of the tube during rolling as aforementioned. Notwithstanding that the gaps have been so reduced, it remains necessary to exclude oxidising gases from the gaps if bonding between the core bar and the SS jacket at the interface is to be satisfactory. Various means for reducing such oxidation will first be described. Apparatus for reducing the size of the gaps is described thereafter.

In general, the drawings are not to scale. In particular and for clarity in the drawings, most of the billets illustrated are not drawn as long as they would in reality be relative to their transverse dimensions. In the discussion that follows, it is convenient to refer first to Figures 13 to 20.

As hereinbefore noted, rolling mills that are currently in use are designed to handle billets of given length and shape, usually square. One object of the methods described and illustrated herein is to prepare composite billets that are of such shape and length. However, the production of composite billets comprising a square core bar inserted in a square tube presents additional problems. The corner radii of continuously cast bars are generally much sharper (smaller) than the corner radii of square tubes. This problem could be dealt with, at least in part, by using for the core a continuously cast bar that is specially rolled to the required shape and size. However, it may not be easy to procure such specially rolled bars. In addition, SS square tubes are more expensive and not as widely available as SS round tubes.

When composite billets comprising a steel core inserted in a SS tube are hot rolled, the core is not initially bonded to the tube and bonding also does not take place during the initial passes through the rolls. In current practice, metallurgical bonding is only achieved by hot rolling through subsequent multiple passes and this is a result of the fact that two different and separate shapes and alloys, which tend to deform and elongate differently, are being rolled together. This difference results in an excess of SS cladding at the periphery, which must be accommodated and distributed as evenly as possible around the bar during subsequent rolling. A core comprised of a solid bar can comprise up to 87-91% of the mass of an equivalent billet of solid steel. This is due to the tolerance gap between the core and tube to allow insertion of the core. Moreover, the size of the gap increases as the billet is heated. The gap is at its greatest at the commencement of rolling due to the initial steep thermal gradient between tube and core and due to the difference in thermal expansion between the steel core and the SS tube.

Rolling will not be successfully completed unless bonding between the core and tube is achieved early in the rolling cycle so that the core and tube start to deform as a single body.

In the case of a round composite billet comprising a steel core inserted in a preformed SS tube, what can happen during initial rolling is schematically illustrated in Figures 13a and 13b. As the billet 47 enters the rolls 45, 46 and is squeezed between them along the Y axis, the billet width increases along the X axis. As illustrated in Figure 13b, the increase in width of the tube 47 is greater than that of the core 50 so the tube tends to separate from the core, substantially increasing the size of the gap 49 therebetween. This causes a number of problems. Once the tube is separated from, and therefore out of contact with, the core, the tube cools down more rapidly and tends to crack during subsequent rolling, even though it is brought back into contact with the hot bar through the next pass. Once separated from the core, the tube can no longer be pushed back evenly in the following pass. This can result in a fold or fin forming in the tube which will either crack or become an unacceptable rolling defect. Furthermore, there is a significantly greater possibility of the occurrence of chrome oxidation in the tube due to the increased volume of the gap 49.

Referring to Figure 14, a similar problem can be expected to occur in the case of a square composite billet comprising a steel core inserted in a preformed SS tube. The profiles of the core and tube after the billet has passed through the first (vertical) roll stand are shown at 55 and 56 respectively. Bonding between the core and tube occurs at the common face 57 which in this case is vertical. The horizontal faces of the bar that were initially flat and in contact with the tube are pushed out along the Y axis as shown at 58. However, the parts of the tube that were initially horizontal are pushed out further so that there is a substantial gap 59 between the inner faces 60 thereof and the faces 58 of the bar. This again creates the possibility of fins or fold forming in the roll passes that follow and increasing the possibility of chrome oxidation in the tube.

For both round and square billets, the problem can be dealt with by the design of special roll passes in the early stands, including the first stand. However, unless the mill is dedicated to the rolling of billets of the shape and size in question, or at least arranged to roll a very considerable tonnage of such billets, this is unlikely to be an economically feasible solution to the problem. It is costly to change rolls in a rolling mill, due to the downtime involved.

In practice, when a core bar is inserted in a tube, there will always be a gap between the bar and tube. It is advantageous that the gap should be as small as possible and, when the core bar and tube are square, the radii of curvature of the corners of the bar should match those of the tube. This may be better understood by reference to the example that follows.

Consider a 14 m length billet comprising a square bar of 90 X 90 mm nominal cross sectional size inserted in a tube of 120 X 120 mm nominal cross-sectional size and 8 mm wall thickness. If the components are produced to typical manufacturing tolerances, there will be a nominal gap equal to a maximum of about 14 mm between the bar and the tube at ambient temperature. Obviously, the force required to insert a solid bar into the tube would increase as the size of the gap between the two decreases. Notwithstanding this, it is thought that the nominal gap could be reduced from 14 mm to between 7 mm and 5 mm, (i.e. 3.5 to 2.5 mm on each side of the bar) by using the apparatus and techniques described and illustrated herein. Problems associated with excess peripheral SS that can occur during rolling would thereby be further reduced.

Figures 15a and 15b illustrate a plant which is capable of inserting a bar 35 into a tube 27, reducing the tolerance gap and pressing into the ends of the tube metal briquettes that act as oxygen scavenging filters as will be described below. The nature of the plant will be well understood by persons skilled in the art and it is not considered necessary to describe it in detail. The plant incorporates two tie rods, four crossheads, four hydraulic cylinders and a chain winch.

At the start of operations, the tube 27 is lowered by a crane onto a roller bed 29. A movable crosshead 30, actuated by two hydraulic cylinders 31, 32 moves forward and, after hydraulically clamping the back end of the tube, pushes it into a fixed crosshead 33 which then clamps the front end of the tube. The hydraulic cylinders 31, 32 are then actuated to pull crosshead 30 and thereby slightly stretch tube 27 so as to remove any twists or bends in the tube but not to exceed the elastic limit thereof. The cross heads can slide on two tie rods 40 and clamps of known type are provided for clamping the crossheads on the tie rods. The bar 35 which, in this example, is 127 mm x 127 mm in cross-sectional size and 15 m long, is lifted by a crane onto a bed of rollers two of which are shown at 34, and which is located to the left of the roller bed 29, as seen in the drawing. A chain link 39 that has already been welded onto the end face of the bar is engaged by a hook mounted on the end of a chain 37. The chain is part of a winch and chain assembly that comprises a winch 38 and is similar to those used in pipe or bar drawing machines. The winch and chain pull the bar through a further crosshead 36, the purpose of which is described below, and then through the crosshead 33 into the tube.

Figure 15a shows the bar inserted about halfway into the tube. The frictional resistance that arises as the bar is inserted in the tube is overcome by winch 38. When insertion is complete, the chain link 39 is cut off and the chain 37 retracted. In this example, it may be noted that after insertion the square bar still protrudes by 500 mm from either end of the tube.

The tube is still clamped in both crossheads 33 and 30. The crosshead 30 is then actuated to move away from crosshead 33 and stretch the tube beyond its elastic limit. Due to the plastic deformation, the length of the tube increases by, for example, 12% from 14m to 15.68m. The tube now overhangs the bar 35 at each end by 340 mm as schematically shown n Figure 15b. In addition to being so stretched, the cross sectional area of the tube is reduced by an equivalent ratio, so that outside dimensions reduce from 150 X 150 mm to approximately 145 X 145 mm and the inside dimensions reduce from 134 X 134 mm to 127 X 127 mm. The wall thickness reduces from 8 to 7.3 mm. The tube is now tightly shrunk onto the bar thereby considerably reducing the tolerance gap.

The method has several additional advantages. The reduced tolerance gap increases the relative density of the billet closer to 100% at ambient temperature which helps to improve subsequent hot rolling characteristics by eliminating excess SS around the periphery of the billet. The possibility of fins being formed in the tube during rolling is significantly reduced. The reduced gap also reduces the amount of oxidising gases that are initially present in the billet and that have to be removed prior to, and during, heating for hot rolling.

The cost of the cladding may be reduced owing to fact that the overhanging ends of the tube may be comprised of carbon steel and not SS as will be described. In addition, referring to Figure 15b, after stretching the tube 27 as just described, the clamps in crossheads 30, 33 need not be released but may act as supporting containers that encase the overhanging ends of the tube while elements such as briquettes, two of which are shown at 41, are being pressed or otherwise inserted in the tube ends. The briquettes may be made from metal granulates in a separate briquetting press and function to reduce or prevent oxidation of the chrome in the SS, as will also be described.

The pressing is carried out by hydraulic cylinders 42, 43 that are slid into position from either the bottom or the top by further hydraulic cylinders (not shown) so as to abut against crossheads 36, 45 respectively. The rams of cylinders 42, 43 are then actuated to press as many briquettes as are required into the end compartments and up against the bar ends. The size of the briquettes is chosen so that they can be pressed into the tube ends and up against, or adjacent to, the ends 46 of the bar 35. Once the briquettes are inserted, crosshead 33 releases the front end of the tube and crosshead 30 pulls the assembled billet comprising the tube, bar and briquettes clear from the crosshead 33. Then crosshead 30 also releases the back end of the tube and moves clear, allowing the assembled billet to be lifted clear with an overhead crane.

Before insertion of the briquettes, the hydraulic cylinders 42, 43 can also be extended up against the bar ends to hold the bar 35 in place while the tube 27 is being stretched.

Figures 16 shows a square billet B 16 comprising a core in the form of a square steel bar 51 that has been inserted in a preformed square SS tube 52. Figure 17 shows a round billet B17 comprising a core in the form of a round steel bar 53 inserted in a round preformed SS tube 54. B16 and B17 are suitable for producing long products such as SS-clad angle bars and other sections, tie rods and ground anchors, round bars and flat bars (including the rebar R and flat bar F shown in Figures 7 and 8.

Figure 18 shows a billet B 18 that is similar in all respects to the billet B 16, except that B18 is rectangular. A billet B18 having suitable dimensions, such as those of a slab, could be used to produce SS clad plates.

Figure 19a shows a round billet B 19 comprising a round hollow steel body or hollow preform 57 inserted in a round preformed SS tube 58. If B19 has suitable dimensions, it is suitable for producing an externally SS-clad seamless steel tube or pipe. Billet B19 is discussed in further detail later with reference to Figure 19b.

Figure 20 shows a billet B20 comprising a steel body 62 that, in this case, is tubular and is bonded to a preformed SS cladding tube 63. B20 could be used to produce an externally SS clad pipe which, if drawn, would have the same appearance as the view shown in Figure 20. So, Figure 20 could also be viewed as showing an externally SS cladded finished pipe, due allowance being made for the differences in dimensions and shape of the components before and after rolling. In the same way, Figures 16 to 19a could be viewed as showing finished products comprising bars with an external SS cladding. In this case, the product shown in Figure 19a would of course be a heavy walled SS clad seamless steel pipe.

The apparatus shown in Figures 15a and 15b can be used to assemble all of the billets shown in Figures 16 to 20. For all of the billets shown, suitable core bars can be produced by conventional techniques including, in the particular case of square bars, rolling or continuous casting procedures.

The tubes can be produced in conventional pipe mills. The drawings show that the corners of both the bars and the tubes are radiused. Before rolling, the core bars and core tubes would not, in reality, fit into the SS tubes as neatly as shown, particularly at the corners. In billets of a size suitable for most existing mills, it is very likely that there will always be a gap of a few millimeters between at least some of the interfacing sides of the core bars (or core tubes) and the SS tubes.

Before any of the billets shown herein are heated preparatory to rolling or otherwise hot working, steps must be taken to prevent oxidation of the chrome in the stainless steel where it interfaces with the steel body. Suitable techniques for this purpose are described below.

Referring to Figures 1-5 and 10-12, each billet B comprises a core bar C of mild steel or any suitable grade of steel that is ordinarily more susceptible to corrosion than stainless steel. The core C can be solid, as shown, or may be hollow. It can be inserted in a tubular SS jacket J using, in the present examples, the apparatus described with reference to Figures 15a and 15b. Except where otherwise stated, the jacket J may optionally comprise a central portion Jl that is composed of stainless steel and an outer portion 12 that is composed of mild steel. The outer portion 12 can be attached to the portion Jl either before or after portion Jl is stretched as previously described. Alternatively, the jacket may be entirely comprised of SS. In each billet there is a zone Z in which there are juxtaposed parts of the core C and the jacket J that become bonded together when the billet is heated and rolled.

Each billet is provided with preventive means for excluding from the zone Z gases that are capable of causing oxidation of chrome in the jacket J. The preventive means includes at least one scavenging metal, usually but not essentially, in the form of an element such as a briquette which is generically referred to as an element E in the examples that follow and which is located in the jacket adjacent at least one end of the core C and is thus displaced from the juxtaposed parts in zone Z.

In relation to the metals that make up the elements discussed herein, the abbreviation 'FD' refers to metals in finely divided form including, as appropriate, turnings, ribbon, powder, wire and so-called wire wool, shot and grit, as well as swarf in the sense in which the latter term is commonly understood by those skilled in the art and as used in the earlier patents.

In this specification, any suggestion that oxidation is 'prevented' or 'reduced' is intended to imply that oxidation is prevented or reduced to the extent that the process results in a product that is industrially acceptable. Persons skilled in the art will recognise that it is probably impossible in practice to expect that oxidation will be prevented or reduced in an absolute sense.

Figure 1 shows one end of a billet Bl in which the ends of the jacket overlie the ends 10 of the core and which comprises at each end an assembly of three elements Es, Ea and Et. A plate 14 is located in the tube 12 against the outer face of the element Et and welded in place to seal the tube. In this example, the opposite end of the billet is similarly arranged so that the jacket J forms a closed metal housing in which the core and the sets of elements at each end are housed and which prevents gases outside the billet from penetrating into the zone Z. These gases include furnace gases and atmospheric gases.

In Figure la, the steel tube 12 and the end plate 14 are substituted by a preformed cap or dome 12a. The cap can be fabricated by deep drawing from plate. The scavenging elements are conveniently compacted or inserted in the cap prior to welding the cap to the end of the centre part Jl of the jacket. Such a cap is less prone to failure during rolling than the welds on the end plate 14. Figure 1 b shows one end of a billet B 1 b that is similar to billet B 1 except that the steel tube 12 is not used. Instead, the entire jacket J is comprised of a SS tube that extends to each end 69 of the billet. The jacket can be closed by welded on plates 14b.

In the present example, the element Et is composed of titanium (Ti) in any suitable FD form; Es comprises FD carbon steel but could alternatively comprise FD titanium; and Ea is composed of FD aluminium (Al) or magnesium (Mg) or an alloy of these. In this assembly, the metal of which Ea composed is thus molten at the rolling temperature of duplex SS ("Rtd") as well as of austenitic SS ("Rta"). Each of the three elements can be formed by compressing the FD metal either directly into the tube 12 or into a briquette before it is pressed into the tube.

Each element acts to prevent oxidation of the chrome in the zone Z as will now be described. Referring to Figure 6, the furnace Fn is provided with induction coils including a first set, indicated schematically at 11 and 12, that in a first stage quickly heat the ends of the billet until the elements E reach a temperature of at least 500°C and preferably 800°C while the rest of the billet, and in particular the part comprising the stainless steel portion Jl, remains below a temperature below which chrome oxides form in the surface of the jacket in the zone Z. Even at the lower temperature, the scavenging metals in elements Ea and Et bond strongly with both nitrogen and oxygen, the principle gases of which air is composed. The elements Ea and Et thus actively scavenge these atmospheric gases from the zone Z to form their equivalent solid oxides and nitrides at each billet end, leaving only inert gases such as argon (Ar). Considering the amount of Ar normally present in the air, a partial vacuum, probably of around 19mm Hg, results at this stage.

Second sets of induction coils 13 in the furnace are then activated together with the coils II and 12 to heat the whole billet to rolling temperature. During this phase, the heating of the carbon steel in the core causes it to decarburise. In the absence of the scavenging elements, the carbon so released would react with any iron oxides on the surface of the core, initially forming C0 ₂ and then, at higher temperatures, CO together with some C. Both C0 ₂ and CO would be oxidising to the chrome in the SS. the scavenging metals however all have a lower "free energy of oxide formation" (hereinafter FEOF) than Cr so are reducing to Cr. They thus combine with any oxygen, including that from the iron oxide, and either prevent oxides of Cr forming or reduce any that have formed. In an alternative arrangement, the scavenging elements can be heated by several high capacity gas- or oil fired burners that are located adjacent the main furnace in which the whole billet is subsequently heated. The main furnace may be an induction furnace as already described or may also be a gas- or oil fired furnace. In the course of tests carried out in connection with the present invention, it has been observed, surprisingly, that the ends of billets prepared as shown in Figure 1 and passed through a particular conventional pusher type furnace have become adequately heated (for the purposes of the invention) before the centre parts without special arrangements being made in the furnace for preheating the ends. The reason for this is not entirely clear but it may be due to any one of several factors or perhaps a combination thereof. In most pusher type furnaces the billets are placed on the furnace floor and eventually exit when they are hottest. The furnace gases can heat the billets only through their top faces and their two end faces since other faces of the billets are not exposed to the furnace gases. The top faces of the billets together however present as a continuous flat mass of steel which acts as a heat sink. The ends therefore heat up more quickly than the central parts of the billets which initially remain relatively cool. In addition, the heat conductivity of both Ti and Al, as well as Mg, is much greater that that of steel or SS. The heated billet Bl is taken to a conventional mill for rolling into a long product such as a rebar shown in cross section at R in Figure 7 or a flat bar F shown in Figure 8. Clearly, products of other suitable shapes and sizes, including plates, could be produced by the processes and from the billets disclosed herein.

Referring again to Figure 1, as long as the jacket remains completely intact, there is no possibility that atmospheric gases can enter the billet Bl through its ends as a result of the cooling that occurs when the billet is removed from the furnace. After the billet has passed through as many roll stands as are needed to ensure that the jacket is bonded to the core, the ends of the now more elongated billet incorporating the parts that house the remains of the scavenging elements are cropped off.

The function of Es when composed of carbon steel is discussed later. It is convenient first to discuss the properties of Ti, Al and Mg together.

One reason that Ti is selected for Et or as an alternative to the carbon steel in Es in this example is because it has a melting point that is higher than either Rtd or Rta. There is therefore no need to make any provision to keep Et separate from the core as is the case with Al and Mg and some of the other metals that could be used, as discussed below and both Es and Et can act as barriers for containing molten Mg orAl as will be discussed. Notwithstanding the high melting point of Ti, the oxides that it forms in the billet are absorbed into the Ti metal so that the formation of further oxides is not inhibited. Unlike the case when Al and Mg are in the solid phase, Ti is thus able to react continuously with any oxygen that is formed in the billet while it is being heated. Ti therefore does not need to melt in order to function as an efficient oxygen scavenger. Furthermore, Ti is reactive even at low temperatures. As is the case with Al and Mg, dried and cleaned titanium turnings (suitable for briquetting) are readily available due to their high intrinsic value. This avoids the need for a scrap- processing plant to clean and dry steel swarf such as is required in the processes described in the earlier Cacace patents.

Of all of the metals named herein as being suitable for use in connection with the present invention, aluminium is the most widely available and the least expensive. It is perceived as being safe to handle. As noted in USP 6706416, it is an aggressive oxygen scavenger but, in the context of the present invention, its usefulness in this regard may be limited by the fact that its oxide, AI2O3, once formed, remains in the solid state on the surface of the Al metal and forms a barrier to scavenging. This barrier disappears when the metal melts at about 660°C. This temperature is easily achieved by induction pre-heating the end of the billet. This is one advantage of using Al. The boiling point (hereinafter "BP") of aluminium is well above Rtd and Rta and is thus too high to make aluminium in the gaseous state useful as an oxygen scavenger. On the other hand, the melting point ("MP") of Mg is about 650°C and its BP is about 1100°C. In addition, it is a more aggressive oxygen scavenger than Al. Mg is however commonly perceived as being unsafe to handle. This view is expressed in USP 6706416. Contrary to this view however, information that has been provided by industrial suppliers of Mg suggests that, provided simple, easily achievable, safety steps are taken, the use of Mg for Ea, in the working conditions in which the present invention is put into practice, is unlikely to prove so hazardous as to render the use of Mg unacceptable. It appears that this will certainly be the case when the Mg is in the form of turnings or ribbon and is likely to be the case even when the Mg is in powder form.

Both aluminium and magnesium form stable oxides, nitrides, hydrides and carbides and, as noted, are active scavengers of atmospheric and other gases. They also have the advantage of low cost. They are most reactive on melting, at which point the surface oxide layers cease to inhibit their scavenging action. The FEOF of each is lower than that of titanium and of course much lower than that of Cr.

For a billet such as B 1, there are some disadvantages to the use of an element Ea comprising Al or any of the other metals named herein, including Ti, that do not boil below rolling temperatures. In this case, the gas pressure inside the billet at the commencement of rolling will be lower than atmospheric so that air would enter the billet if an end of the tube 12 was to fail before the jacket is bonded to the core during rolling or through pinhole leaks in the welding of plate 14. The modification to the billet, described below with reference to Figure 2, addresses this problem. Conversely, a significant advantage of the use of Mg for Ea is that, when Mg is raised above its boiling point, a positive gas pressure is created inside the billet, replacing the partial vacuum that it creates in the billet as a result of forming solid oxides. Mg vaporises at 1100°C at atmospheric pressure but at a lower temperature under the partial vacuum. At RTd the pressure of the vapourised Mg in the billet is close to atmospheric. At RTa the pressure of the vapourised Mg in the billet is above atmospheric. The possibility of entry of air during rolling if the jacket fails is thereby much diminished. The vaporised Mg acts as a strong reducing gas for any CO and C(½ that might occur in the billet. CO starts to form from about 780°C and reduces Cr only at above 1225°C.

The element Ea may also comprise an alloy of aluminium and magnesium. As is known, the BP of such an alloy can be controlled by adjusting the proportions of the constituent metals. Thus the BP of the alloy can be made' higher or lower than rolling temperatures, as desired. The ratio of Al to Mg can be chosen to cause the alloy to vaporise anywhere between 850°C and 1260°C. In essence, this process relies on the Mg vapour, rather than CO, to reduce Cr oxides.

It may prove unacceptable in practice to use elements composed of a metal such as magnesium or an alloy thereof that vaporises below RT of the billet concerned, because the vapour that penetrates into the zone Z may leave unacceptable inclusions at the interface in the finished product. On the other hand, the same elements may be acceptable for use in billets whose RT is below the temperature at which the elements vaporise. Experience will determine the circumstances in which such elements can be used.

Because Mg and Al melt at temperatures lower than rolling temperatures, it is considered necessary to prevent molten Mg and/or Al, when used for Ea in billet Bl, from reaching the interface of the core and the SS jacket. This is achieved by the presence of Es which, whether it is comprised of FD steel or Ti, does not melt below rolling temperatures and acts as a barrier to the molten metal. This is one function of Es. If FD steel is used for Es, it is preferably of medium- to high- carbon grade, which typically contains 0.4%-l of carbon. Graphite could be added to the FD steel to increase the carbon content if necessary. At elevated temperatures, CO will be evolving from the FD steel and any graphite present. At RTa, CO is reducing to any oxides in the chrome. Even at RTd, CO may be reducing to Cr in the presence of Al or Ti.

When Es is formed from Ti, Es not only acts as a scavenger to oxygen that is initially present, or that evolves, inside the zone Z, but also helps to scavenge atmospheric oxygen before it gets into the zone Z through cracks or pinholes in the welding or jacket failure as already noted.

One advantage of the present process is that the steel body that comprises the core can be round, square, rectangular or of any other suitable shape and the billets can have can have various cross sectional sizes and lengths. In particular, the billet size can be chosen to suit an existing rolling mill.

The steel body that comprises the core can also be hollow. It may be tubular or comprise a hollow block of steel so that the billet can be used to produce, for example, a seamless steel pipe having an internal external SS cladding. The ability to make rectangular billets enables them to be used to roll SS clad plates as well as long products.

The bar that is to be used for the core may first be mechanically ground. This would also have the result of descaling the bar. All bars that are commercially produced for the present purpose will need to be descaled, a process normally carried out by shot blasting. Such shot blasting would be unnecessary if the bar is ground.

In order to assist the removal of atmospheric oxygen from any of the billets described herein, it may be advantageous to evacuate the billet by connecting one or both ends of the billet to a vacuum pump P prior to any heating. This is shown schematically in Figure 9. Before the billet is transferred to the furnace, the pump is disconnected from the billet, and the apertures in the billet by which the pump is connected are closed. The means of evacuating the billet in this way are well known and need not be described in detail. Instead of evacuating the billet, or in addition thereto, the pump P could be of a type arranged to pump an inert gas such as AT into the billet to displace the residual air. In Bl , the elements Ea, Et and, at temperatures close to RT, Es each serve as means to scavenge oxygen, particularly from atmospheric air that may get into the billet in any of the ways previously described. The potential for oxidation of the Cr to occur as a result of such failure is exacerbated if the temperature of the interior of the billet and the incoming air is lower than 1225°C. The modification to the billet, shown in Figure 2, addresses this problem.

Figure 2 shows one end of a billet B2 that is provided at each end with three elements Es, Ea and Et that, subject to what is said below about Ea, are comprised of the same metals, and serve the same functions as, the identically named elements in Bl. The ends of B2 are initially sealed by plates 15 but each plate is provided with a temperature-dependent plug 16 that melts and allows the billet to be vented inside the furnace at a temperature which can be preselected but is in any case not less than 1225°C. A suitable material for such a plug is 30% copper-nickel which fully melts at 1237°C. When the plug melts, vacuum conditions in the billet cause hot oxidising furnace gases, which are normally at temperatures of around 1300°C and in any event well above 1225°C, to be rapidly sucked into the billet. These furnace gases would pass through Et, Ea and Es and thus through three layers of reducing and scavenging metals. First through the outer elements Et and Ea which are aggressive oxygen scavengers. Any remaining oxygen or C0 ₂ when passing through the final element Es would be converted into CO, with an increase in pressure due to the formation of two CO molecules for every molecule of CO ₂ or 0 _2. The CO entering the zone Z at temperatures well above 1225°C would have a reducing effect on any Cr oxide traces still present in zone Z. The elements pressed into each end of billet B2 also provide additional protection as a precaution against oxidation occurring in the zone Z in the event of failure of the jacket ends during rolling. The elements therefore serve as CO converters not only when the plug melts but also if the jacket should fail during rolling. In what follows, it is not considered necessary to repeat in every instance the description of the elements in some arrangements thereof and such elements may be identified by the simple letter E. Notwithstanding that a billet contains elements comprising the metals, particularly aluminium and titanium, that have so far been suggested, it is possible that, after the ends are preheated, conditions in the interior of the billet may still allow some oxidation of the Cr, despite the fact that the atmospheric air has been scavenged or evacuated from the billet prior to heating.

Figure 3 shows the end of a billet B3 that addresses this issue. B3 comprises an assembly of four elements Eu, Es, Ea and Et. The latter three can be identical to those already described and serve the same respective functions. The plate 14 can be omitted or, alternatively, a plate 17 with a vent hole 18 may be provided to help hold the elements in place during rolling. Eu is sandwiched between Es and the end 10 of the core and is a briquette comprising NtLjClor urea. The usefulness of this assembly is that the NH4CI or urea dissociates at a low temperature, as described in the earlier patents, and forms large volumes of gas that are able to escape from the billet through vent hole 18, since Es, Ea and Et can be made sufficiently porous to allow this to happen. These gases displace residual air in zone Z of the billet. The dissociation of NH4CI or urea commences at a temperature below 200°C and continues until the temperature reaches somewhere below 600°C at which point the NH4CI or urea are spent and the flow of gases out of the ends of the billet ceases. The billet B3 does not therefore need to be evacuated or purged to remove the atmospheric gases inside the billet. Although the porosity of Es, Ea and Et also allows atmospheric air to be drawn into the billet when the ends are being heated, Es, Et and the molten constituents of Ea scavenge any oxygen that may remain, or evolve, in the billet and also scavenge oxygen and other gases in the air before they can penetrate into the interior of the billet.

In the billets Bl, Bla, Bib, the jacket J that houses the core body and is closed to the atmosphere provides means for preventing oxidising gases from outside the billet penetrating the zone Z until the interfacing parts of the core and SS jacket become bonded together. In a billet such as B3, this means is effectively provided by the element Eu in combination with an array of scavenging elements such as Es, Ea and Et. Eu is active in the lower temperature ranges to scour oxidising gases from the zone Z and the scavenging elements not only allow these gases to escape but also provide a sufficient sealing action at the lower temperatures to stop atmospheric or furnace gases from penetrating to the zone Z. As the temperature rises, the scavenging elements become more active and, although atmospheric and furnace gases may be able to penetrate to the zone Z, any oxygen in these gases is scavenged by Es, Ea and Et before they do so. The elements also act to scavenge oxidising gases that evolve in the zone Z until the interfacing parts become bonded together.

It may be found unnecessary to provide as many as three scavenging elements in a billet such as B 1 or even B3. For example, the element Et in B 1 may be active enough to allow the element Ea to be omitted as shown in the billet B4 in Figure 4. Since Et does not melt, the barrier element Es may also not then be needed as shown in the billet B5 in Figure 5.

The elements might typically be 10 -150mm thick. This is however by way of example and they could be of any suitable thickness. It will probably always be necessary to prevent the raw scavenging metals from the elements E being present in the zone Z before the billet is heated. The residue of any significant quantity of these metals is likely to be deleterious to bonding between the faces of the core and jacket and the parts of the billet that contain such residue after rolling are in any case discarded. It is therefore thought that the scavenging elements E should initially be located in a position that is separate from the faces of the core and jacket.

There are other metals that have a lower FEOF than Cr and that therefore might be used instead of Al, Mg or Ti. Although it appears at present that these other metals are less likely to be used, this is not discounted. These other metals include zirconium lithium, calcium, silicon, vanadium, manganese and uranium.

Yet another possibility is illustrated in Figure 10. The billet B10 contains one or more elements in substantially the same arrangements as any heretofore described. However, the elements are not placed directly in the jacket ends but are prepacked instead in a cartridge 20 which can be of mild steel. In this example, three such elements Es, Ea, Et are illustrated which are identical to those previously described. The cartridge is a close fit in the tube 12 and comprises a longitudinally extending, tubular outer body 21 with end plates 22, 23 at its inner and outer ends. The end plates are welded to, or integral with, the body 21 so that the joints between the plates and body 21 are sealed. The end plate 22 is located against the end 25 of the core C and is provided with a central aperture 24. After the cartridge is inserted in the billet end, it is fixed in place by a plate 26 welded to the tube 12. The function of the plate 26 is similar to that of the plate 14 so that, as necessary and depending on the nature of the element or elements E inserted in the cartridge, the plate 26 may have an aperture or may be provided with a plug that melts at a predetermined temperature or

alternatively (as shown) may have no aperture, all as previously described. In the first two of these cases, the end plate 23a will be provided with an aperture 27 (as shown in Figure 11) that is aligned with the aperture 28 in the plate 29 and is similar to the aperture 24a in the end plate 22a.

The inner end plate 22a serves, in the first place to hold the element or elements in place in the cartridge. It is one aspect of the invention that the elements E, in any of the arrangements described herein, can be packed into cartridges and transported separately from the billets. This could have the result that simpler machinery might be required for assembling the billets. Where one of the elements E that is inserted in the cartridge is composed of a scavenging metal that melts below rolling temperature as previously described, each end plate in the cartridge can also act as a barrier for holding the molten metal. The quantity of metal could be chosen so that, when molten, its upper surface lies below the apertures 24, 24a, 27, 28. This would help prevent molten Al or other metal from spilling out of the cartridge and finding its way into the gap between the core and the jacket when the hot billet is being handled. In Figure 12, the cartridge 30 is of larger cross sectional size than the jacket J and has a skirt 31 that fits over the end of the billet B 12.

Figure 19b is a side elevation of billet Bl 9 already mentioned with reference to Figure 19a. Bl 9 comprises a core in the form of a tube or hollow body of steel 57 housed in a SS tube 58. The ends 70 of the tube project clear of the core which is provided with a cylindrical passage or bore 71. An array of annular elements E are mounted in an annular casing at each end of the billet. The casing comprises a cylindrical skirt 73 that is welded to the end face of the block around the periphery of the passage 71. After insertion of the elements E, the casing is closed to outside gases by an annular plate 74 welded to the skirt 73 and the tube end 70. The elements E are similar to any that have been heretofore described and prevent oxidation of the SS in zone Z at the interface between the tube 58 and the core 57, As noted, the billet B 19 is suitable for producing an externally SS clad, seamless steel pipe by known conventional techniques.

As exemplified in Figure lb, in any of the foregoing examples, it may be preferable to omit the use of carbon steel pipe ends 12 welded to the SS jacket. Instead, the elements E are inserted in the ends of the SS jacket, which is made longer for the purpose.

It is possible that, in some cases, only one end of the jacket is closed. The other end could, for example, be provided with an array of elements including Eu and arranged as described with reference to Figure 3.

In all cases the cartridge can be formed of carbon steel which is less prone to cracking than SS if the cartridge cools excessively during rolling.

The cores and jackets of the billets heretofore described and shown in the drawings are typically, but not essentially, of square cross sectional shape. This is because it is thought that it will be most practical to form a square shaped core with the requisite degree of longitudinal straightness and uniformity of cross sectional dimensions. Clearly, however, billets of other cross sectional shapes (including round and rectangular shapes) may be used.

In a trial, two commercially produced carbon steel core bars 84 mm x 84 mm in size and 2 m long were descaled. The bars were inserted into square tubes, also commercially produced, of ASTM A 304 grade SS 100 mm x 100 mm in outside size of 6 mm wall thickness. Initially, there was thus a nominal clearance gap of 4 mm between the core bar and the tube. After insertion of the bars, the tubes were stretched beyond the elastic limit of the SS to result in a 12% elongation of the tube. In this procedure, the tube was shrunk tightly over the core bar to the point that the rounded corners of the tube distorted to adapt to the different radii of curvature of the core bar. The tube became longer than the core bar and shrank to a size of 91 mm x 91 mm at its projecting ends where they were not restrained by the core bar.

After the stretching procedure, tubular carbon steel end pieces 70 mm long were welded to the ends of the SS casing using the same Inertfil 309 (TM) welding wire. A single element 35 mm long and composed of a compacted mass of Ti turnings was pressed into each end piece before a closing plate was inserted in the end piece and welded thereto as exemplified in billet Bl.

The ends of each billet were preheated to around 800 °C leaving the central part of the billet at ambient temperature. After this the entire billets were heated in a rolling mill furnace to 1200 °C.

The billets were then rolled through the first six roughing passes of a conventional rolling mill in a conventional diamond-square roll pass configuration. In this procedure, the billets were reduced in size to 70mm X 70mm and the partially rolled product was sectioned and examined. There was no sign of significant oxidation in the SS casing at the interface with the core bar at a distance of more than 50 mm from the billet ends. Furthermore, there appeared to be complete bonding between the core bar and the casing at the interface. No finning was observed which would have resulted from de-bonding of the SS casing from the core bar into the roll gaps. In commercial production, the ends of the billets containing the remnants of the end pieces would be cropped off as soon as bonding is known by experience to be complete. In the present case, it was therefore concluded that, in practice, the ends could be safely cropped off after the sixth pass.

It should be clear that the techniques described and illustrated in the drawings can be used without significant modification to produce steel products that are clad with the alternative alloys suggested above instead of SS. Consequently, the descriptions of the examples shown in the drawings may be read as if applying to the alternative alloys instead of SS, due allowance being made for differences in the rolling temperatures of billets clad with the alternative alloys.