Background of the invention Cooling plates, also called"staves", have been used in blast furnaces for over a hundred years. They are arranged on the inside of the furnace armour and have internal coolant ducts, which are connected to the cooling system of the furnace. Their surface facing the interior of the furnace can be lined with a refractory material.

There are different methods for manufacturing such cooling plates.

According to a first method, a mould for casting a cooling plate body is provided with one or more sand cores for forming the internal coolant ducts.

Liquid cast iron is then poured into the mould. This method has the disadvan- tage that the mould sand is difficult to remove from the cooling ducts and/or that the cooling duct in the cast iron is often not properly formed and that the cooling ducts are often not tight enough.

In order to avoid the above disadvantages it has been suggested to ar- range preformed steel pipes in the mould and to pour the liquid cast iron around the steel pipes. However, these cooling plates have not proved satisfactory.

Indeed, due to carbon diffusion from the cast iron into the steel pipes, the latter become brittle and may crack. Contact between the cooling pipes and the cooling plate body may also be responsible for cracks in the cooling plate body, most probably because of a difference in the coefficient of thermal expansion of both materials.

In order to avoid such cracks in the steel cooling pipes and the cooling

plate body, DE-A-2128827 suggests to provide the preformed steel pipes with a metallic oxide coating so as to prevent carbon diffusion and metallurgical bonding between the cast iron and the steel pipes. In fact, DE-A-2128827 intends to provide an alternative to the prior known cooling plates, in which the steel pipes were coated with graphite or aluminium or plated with copper or tin, since, as mentioned, such layers do not prevent carburising. However, the alternative proposal, which consists in applying a metallic oxide coating to the steel pipe is also unsatisfactory. As a result of casting, the coating has disap- peared and there is a small air gap between the steel pipes and the cooling plate body, whereby pipes and body can expand independently. Unfortunately, these cooling plates have the disadvantage of a bad thermal transmission coefficient, because the small air gap has an insulating effect.

US 4,150, 818 relates to a cooling plate for a metallurgical furnace com- prising a cast cooling plate body wherein the steel cooling pipes are coated with a combination of two layers : a metallic layer in contact with the steel tube and a stable metallic oxide layer thereon. The metallic layer is made from a metal selected from the group consisting of Ni, Co, Mn, and Ag, either individually or including two or more. The metallic layer has a thickness in the range of 40 to 100 jj. m and the metallic oxide layer has a thickness in the range of 30 to 100 pLm, the maximum total thickness of both layers being 200 , m. Although the metallic layer does avoid carburisation of the steel pipe, such cooling staves are however still unsatisfactory i. a. because of the oxide layer, which has a detri- mental effect with regard to heat conductivity.

As an alternative to cast iron cooling plates, copper cooling plates have been developed. So far a number of production methods have been proposed for copper"staves".

Initially an attempt was made to produce copper cooling plates by casting in moulds, the internal coolant ducts being formed by a sand core in the casting mould. However, this method has not proved to be effective in practice, be- cause the cast copper plates often have cavities and porosities, which have an extremely negative effect on the life of the plates, the mould sand is difficult to

remove from the cooling ducts, and/or the cooling duct in the copper is not properly formed.

GB-A-1571789 suggests to replace the sand core by a pre-shaped metal pipe coil made from copper or high-grade steel when casting the cooling plates in moulds. The coil is integrally cast into the cooling plate body in the casting mould and forms a spiral coolant duct. This method has also not proved effective in practice, inter alia because cavities and porosities in the copper cannot be effectively prevented with this method.

A cooling plate made from a forged or rolled copper ingot is known from DE-A-2907511. The coolant ducts are blind holes introduced by mechanical drilling in the rolled copper ingot. With these cooling plates the above- mentioned disadvantages of casting are avoided. In particular, cavities and porosities in the plate are virtually precluded. Unfortunately the production costs of these cooling plates are relatively high, because the drilling of the cooling ducts is complicated, time-consuming and expensive.

WO-98/30345 teaches to cast a preform of the cooling plate with the help of a continuous casting mould, wherein rod-shaped inserts in the casting duct produce ducts running in the continuous casting direction, which form coolant ducts in the finished cooling plate.

While copper cooling plates generally have a far better thermal conductiv- ity than cast iron cooling plates, they however have a far lower wear resistance than the latter. Thus, furnace zones in which the cooling plates are exposed to severe mechanical stresses cannot be equipped with copper cooling plates.

Furthermore, copper cooling plates are more expensive than cast iron cooling plates.

Object of the invention The object of the present invention is to provide a cooling plate that can be . easily manufactured and that nevertheless has a good wear resistance and a low heat transfer resistance. This object is achieved by a cooling plate as claimed in claim 1.

Summary of the invention A cooling plate for a metallurgical furnace in accordance with the present invention comprises a cast cooling plate body made of a ferrous metal and at least one steel cooling pipe cast in the cooling plate body. It shall be appreci- ated that a metallic jacket is provided on the outer surface of the steel cooling pipe in the cooling plate body. The metallic jacket has a thickness in the millimetre range and is made from a metal selected from the group consisting of copper, copper alloys, nickel and nickel alloys.

In the present cooling plate, the steel pipes are protected by a thick metal- lic jacket, which is believed to act as a physical barrier to carbon diffusion. In other words, the thickness of the metallic jacket is such that it is very improb- able that carbon from the liquid ferrous metal, generally cast iron, will reach the steel cooling pipe. This is an important difference with the cooling plates of US 4,150, 818, in which steel pipes are coated by a metallic layer of Ni, Co, Mn or Ag and by a metallic oxide layer having a total thickness of less than 200 Am, these metals being selected for their inability to form metallic carbides, i. e. the metallic layer acts as a chemical barrier to carbon diffusion.

Furthermore, in the present cooling plate, there is no oxide layer to pre- vent welding between the steel pipes and the cast iron, which means that air gaps can be avoided. In fact, the metallic jacket acts as an intermediate layer, which avoids welding between the steel cooling pipe and the cast iron body, and which can absorb strains and stresses. Hence, the metallic jacket is in tight contact with both the steel pipe and the cast iron body. This is extremely advantageous with regard to heat transfer, since copper, nickel and their alloys have a high thermal conductivity. The tight contact between the different materials and the high thermal conductivity of the metallic jacket ensure an intensive heat transfer from the cast iron body to the cooling pipe.

It will be noted that such a cooling plate can easily be manufactured by casting. According to another aspect of the present invention, a method for manufacturing a cooling plate for a metallurgical furnace is proposed. It com- prises the following steps:

- providing at least one steel cooling pipe with a metallic jacket thereon, the metallic jacket having a thickness in the millimeter range and being made from a metal selected from the group consisting of copper, cop- per alloys, nickel and nickel alloys ; - providing a mould for casting a cooling plate body; - arranging in the mould the at least one cooling pipe with the metallic jacket; and - pouring a liquid ferrous metal into the mould around the at least one cooling pipe with the metallic jacket.

Such a method proves relatively simple to implement, since the steel tubes are conventionally cast in the cooling plate body. With regard to handling, the use of a thick metallic jacket is very convenient, as it is more resistant and needs less care than a 100 or 200 Am metallic layer, which can be very easily damaged. Hence, the use of a thick layer ensures a proper coating of the steel pipe and simplifies handling during manufacturing.

The present method thus allows for the manufacturing of cooling plates having an improved thermal conductivity due to the metallic jacket, which ensures an intensive heat transfer from the body to the cooling pipe. Moreover, the obtained cooling plate with a ferrous based plate body has a good wear resistance and thus an increased lifetime, whereby maintenance costs of metallurgical furnaces can be reduced.

It will be noted that copper and copper alloys are particularly preferred for the metallic jacket. Indeed, copper and copper alloys are highly compatible with the surrounding materials, i. e. steel and cast iron, and have a high thermal conductivity. In addition, it will be appreciated that although copper has a lower melting temperature (1 083°C) than the casting temperature of the cast iron (typically between 1 200 and 1 300°C), the use of a thick copper jacket however permits to avoid its dissolution in the liquid cast iron. Indeed, the copper jacket, due to its thick thickness, rapidly absorbs the heat of the liquid cast iron, which then solidifies, without causing the copper to be washed out (i. e. re-melted and

dispersed in the cast iron). The metallic jacket can thus be made from a metal or alloy having a lower melting temperature than the surrounding materials.

Besides, copper and copper alloys have a greater thermal expansion coef- ficient than steel and cast iron, which means that in operation, the copper jacket will really be squeezed between the steel pipe and the cast iron body. There- fore, a very good contact between the different materials in the cooling plate is ensured, which is in favour of good heat transfer.

It will further be appreciated that, by contrast to DE-A-2128827 which mentions that copper plated cooling pipes do not prevent carburisation of the steel pipes, in the present invention the use of a thick copper jacket not only prevents the carburisation of the steel cooling pipe, but also provides an intensive heat transferring layer capable of absorbing mechanical stresses between the steel pipe and the cast iron body. In practice, such a cooling plate has proved to have cooling effect"which is significantly greater (about two to three times) that of a conventional cast iron stave with steel pipes coated with a metallic oxide.

The metallic jacket should preferably have a thickness of at least 2 mm, and of no more than 20 mm. More preferably, the thickness should be in the range of 5 to 10 mm, most preferably about 7 mm. The metallic jacket may advantageously be provided about the steel pipe by casting, since it is eco- nomically the more interesting method for forming such a thick metallic layer.

It is to be noted that the optimal thickness for the metallic jacket depends on the metal or alloy it is made of, and on the casting conditions.

For example, in the case of a cast iron cooling plate with steel tubes sur- rounded by a thick metallic jacket made of copper, the copper jacket has to be sufficiently thick to absorb the heat from the liquid cast iron so as not to be washed out. Besides, if the copper jacket is too thick, there will be a gap between the copper jacket and the cast iron body, due to shrinkage of the copper jacket. In addition, the casting result may vary depending on the casting conditions, such as e. g. temperature, duration and flow of the liquid cast iron.

These parameters should thus preferably be taken into account when determin-

ing the optimal thickness for the metallic jacket.

Referring now more specifically to the cooling plate body, it can consist of a variety of ferrous metals. However, the ferrous metal is preferably chosen from the group consisting of cast iron, ductile cast iron, malleable iron and steel.

The protection against carbon diffusion provided by the metallic jacket is particularly important when the cooling plate body is made from cast iron, which has a high carbon content.

Brief description of the drawing The present invention will now be described, by way of example, with ref- erence to the accompanying drawing, in which Fig. 1 : is a sectional view of a preferred embodiment of a cooling plate in accordance with the invention.

Detailed description of a preferred embodiment In Fig. 1 a preferred embodiment of a cooling plate 10 in accordance with the invention is shown in cross-sectional view. The cooling plate 10 comprises a cooling plate body 12 made of a ferrous metal, preferably cast iron. The cooling plate body 12 has the general form of a parallelepiped, whose front side and back side are respectively indicated 14 and 16. The front side 14 of the cooling plate 10 is advantageously provided with a series of regularly spaced parallel ribs 18, so as to increase its heat exchange surface and thus improve the cooling efficiency of the cooling plate 10.

Reference sign 20 indicates a steel cooling pipe cast in the cooling plate body 12. Although not represented, the cooling plate 10 comprises a plurality of such cooling pipes 20. As can be seen, the cooling pipe 20 has a straight portion 22 essentially parallel to the front side 14 of the cooling plate 10. The straight portion 20 terminates at both ends by a bent portion 24 protruding on the rear side 16 of the cooling plate body 12, for connecting the cooling pipe 20 to a cooling circuit of e. g. a blast furnace.

It will be appreciated that the cooling pipe 20 has a metallic jacket 26 sur- rounding its outer surface in the cooling plate body 12. In the present embodi- ment, the metallic jacket 26 is advantageously made of copper or of a copper alloy, and has a thickness in the range of 5 to 10 mm. During casting of the cast iron body, such a thick copper jacket 26 acts as a physical barrier to the diffusion of carbon from the liquid cast iron into the steel cooling pipe 20. The copper jacket 26, which has a high thermal conductibility, is in tight contact with both the steel cooling pipe 20 and the cast iron body 12. The good thermal conductivity between the cast iron body 12 and the steel cooling pipe 20 as well as the intimate contact between the materials allows an intensive heat transfer from the cast iron body 12 to the cooling fluid flowing in the steel cooling pipe 20. Furthermore, the coefficient of thermal expansion of copper and copper alloys is higher than that of steel and cast iron, whereby the good contact between the cooling plate body and the steel pipe is further ensured by the dilatation of the copper jacket when the cooling plate is in operation, i. e. subjected to intense heat. It will be appreciated that although copper has a lower melting temperature (1083°C) than the pouring temperature of cast iron (typically 1200 to 1300°C), the thick copper layer has the ability of absorbing the heat of the liquid cast iron, which then solidifies, without causing the copper to be washed out, i. e. re-melted and dispersed in the cast iron body.

The present cooling plate 10 can be easily manufactured by casting. Ac- cordingly, the manufacture of the cooling plate 10 is preferably carried out as follows. A mould having the dimensions of the cooling plate body 12 is provided and the cooling pipes 20 provided with their metallic jacket 26 are arranged in the mould. Next, molten cast iron is poured into the mould around the cooling pipes and allowed to solidify therein. The obtained cooling plate is then re- moved from the mould.

It will be noted that the metallic jacket is preferably cast about the steel pipe, since casting is economically the more interesting method for forming a thick metallic layer.

According to testing and simulations, a cooling plate 10 in accordance with

the present invention has a proved to have cooling effect"which is signifi- cantly greater (about two to three times) that of a conventional cast iron stave with steel pipes coated with a metallic oxide. This"cooling effect"is determined by measuring the hottest point on the hot side of the cooling plate 10, resp. the conventional stave, when exposed to a same heat source. In particular, where the front side of a conventional stave will be about 600 to 650°C, the front side of a cooling plate 10 in accordance with the invention will be about 200 to 250°C.

Example : A specimen cooling plate was manufactured according to the above de- scribed method. A cooling pipe having an external diameter of 75 mm and a wall thickness of 10 mm was used. The steel cooling pipe was provided with a 7 mm thick copper layer. The steel cooling pipe with its 7 mm thick copper layer was placed in the mould and cast iron was poured therein at a temperature of 1250°C. After solidification, a cooling plate of 200 mm in thickness was ob- tained, which means that the copper jacket was covered by about 55 mm of cast iron.

A transversal cut of the cooling plate was effected to observe the internal structure of the stave. After cutting, a thick and homogeneous copper jacket was observed around the cooling pipe, without any air gap between the cast iron body and the copper jacket. At the pipe/jacket interface, there is always a tight connection due to copper shrinkage. Hence, there was a tight contact at both interfaces of the copper jacket, and the steel cooling pipe could not be moved relatively to the cast iron body.