Abstract:
A method for manufacturing a coiler drum to be used in a coiler furnace and a coiler drum. The method comprises the step of forming a trinickel aluminide containing surface layer onto a coiler drum body.

Description:
FIELD OF THE INVENTION 
       [0001]    The invention relates to a method for manufacturing a coiler drum and to a coiler drum. 
       BACKGROUND OF THE INVENTION 
       [0002]    Reversing hot mills, also called Steckel mills, are used as hot strip mills for hot-rolling slabs of stainless steels, special steel, etc. 
         [0003]      FIG. 1  shows schematically a reversing rolling mill  1  having a relatively high temperature where a cast slab of steel is passed through rolls  2   a  and  2   b  several times. The rolls  2   a  and  2   b  are in nip contact with each other and when the slab travels trough the nip N, the thickness of the slab is reduced to a strip  3  having a thinner thickness. 
         [0004]    On opposite sides of the rolling mill  1  there are arranged two coiler furnaces  4 . When processing the strip with the rolling mill  1 , it is alternately wound up on and unwound from the opposed coiler furnaces  4 , each comprising a rotating, cylindrical coiler drum  5 . The purpose of the coiler furnaces is to maintain the temperature of the strip between passes through the nip N. The coiler furnaces are maintained at a temperature, for example, of about 900 to about 1050° C. 
         [0005]    In operation, the strip  3  passing through the nip N is led to the nearer coiler furnace  4  and wound onto its respective coiler drum  5 . Subsequently, the strip  3  is unwound from the coiler drum  5  as the strip is fed back through the nip N. The process passing the strip through the nip N and winding and unwinding it is repeated until a desired thickness of the strip is reached. 
         [0006]    A coiler drum is a hollow cylinder having an outside diameter of at least about 1000 mm, a length of about 2000 to 5000 mm and a great wall thickness of about 30 to 150 mm. The coiler drum is usually prepared by casting of a heat resistant alloy. The working surface of a coiler drum, that is the surface that becomes next to the strip to be wound, is usually flat or grooved. 
         [0007]    As can be realized from the description above, the coiler drum is working in a very harsh environment. The temperature in the coiler furnace is high. The surface of the coiler drum is also repeatedly subjected to high tightening force when winding the steel strip on the drum. This causes fatigue fractures or other cracks to the surface of the coiler drum. The coiler drums also blister. These influence the durability of the coiler drum. They also leave marks on the steel strip that is being manufactured, thus deteriorating the quality of the product. The surface of the coiler drum is also exposed to an iron oxide film formed on the surface of the steel strip when rolling up the steel strip. In spite of descaling, part of the iron oxide film transfers on and sticks to the coiler drum and subsequently causes defects to the strip. This deteriorates the quality of the product. 
         [0008]    Nowadays the problems caused by the blisters in the coiler drum surface are solved by shutting down the furnace so that the blisters can be hand-ground to reduce the blisters. The frequent shut downs result in high operating costs for maintaining the rolls and reduction of product throughput capability. 
         [0009]    The coiler drums also tend to sag at current operating temperatures, which prevents operating the furnace at even higher temperatures. The sagging causes the drums to become eccentric in their rotation. This causes uneven revolving of the drum, which has an influence on the operation and durability of the coiler furnace and causes defects on the strip. Sagging also causes deformations, i.e. narrowing the slot of the coiler drum. As a consequence, the coiler drum has to be replaced. 
         [0010]    The above mentioned problems have been tried to overcome by using different cast steel compositions when casting the coiler drums. One example is disclosed in U.S. Pat. No. 6,033,626. The composition presented in the patent is only a variation of a standard steel composition. It is soft in high temperatures in which the coiler drums are being used and thus there are problems caused by the deformation of the coiler drum. Also it can not refrain the blistering of the surface of the coiler drum. 
         [0011]    US published patent application 2001/0013383 discloses a trinickel aluminide-based heat resistant alloy for a material for hearth rolls for heating furnaces. The material has a high creep rupture strength in temperature ranges over 1050° C. and excellent weldability. The problem is that trinickel aluminide-based materials are expensive. The material is suitable for casting rolls with a simple structure, i.e. a hollow cylinder, but it can not be used to cast objects with complex geometrical structures, such as a coiler drum. Moreover, when preparing the molten trinickel aluminide alloy, the reaction is strongly exothermical. Thus it is not possible to cast objects that are as big in size as coiler drums are. 
         [0012]    Both of these patent publications disclose casting the entire drum or roll from the same material and as one piece. 
       BRIEF DESCRIPTION OF THE INVENTION 
       [0013]    The purpose of the present invention is thus to provide a method for manufacturing a coiler drum and a coiler drum, which method and coiler drum avoid the above-mentioned problems and by means of which the durability of the coiler drum can be increased remarkably and the quality of the product produced by a rolling mill can be improved. 
         [0014]    The invention is based on the idea that the coiler drum is manufactured in such a way that it comprises forming a trinickel aluminide containing surface layer on a coiler drum body. 
         [0015]    The trinickel aluminide alloy can be formed on the coiler drum body by welding or thermal spraying. The surface layer may also be a sleeve of trinickel aluminide alloy that is arranged on the body. The thickness of the surface layer is from about 0,1 mm to about 8 mm, suitably from about 1 mm to about 4 mm. The surface layer defines a working surface that will engage a strip of metal when it is being rolled on the coiler drum. 
         [0016]    The surface layer may be produced to be smooth and even against the steel strip to be wound. The surface layer can be formed on the coiler drum as raised strips of trinickel aluminide alloy extending radially outward from the base surface of the coiler drum body. These raised strips define a working surface that will engage a strip of metal when it is being rolled on the coiler drum. 
         [0017]    The invention has many advantages. Trinickel aluminide alloy has high temperature strength and high temperature corrosion resistance. Consequently, when the surface layer is made from trinickel aluminide alloy, the surface hardness is excellent in high temperatures. When using a conventional H-series austenitic alloy as the surface material, the surface hardness decreases as the temperature increases. It has been noticed that the high surface hardness of trinickel aluminide alloy lasts through the lifetime of a coiler drum in high temperatures. It has also been noticed that the sticking of the iron oxide from the product to the coiler drums is reduced or eliminated, which reduces the pitting marks on the strips. Thus, the quality of the end product is better and the yield from slab to the product is higher. The working life of the coiler drum is also increased because of the high durability in high temperatures. The deformations of the drum are reduced which also increases its lifetime. 
         [0018]    It is also possible to manufacture the coiler drum body from a lower cost material and use the expensive trinickel aluminide material only for forming the surface layer. This reduces the price of the coiler drum. Also by using a material having a high yield strength in the drum body and a trinickel aluminide-based surface layer on it, it is possible to improve the drum&#39;s strength against local stresses and to prevent drum breakages and deformation. 
         [0019]    When using a structure where the surface layer is welded on the coiler drum body, it has been noticed in high temperature FEA analysis that the welded surface structure reduces the temperature gradient in coiler drum wall. This results in lower combined stresses and prevents the drum breakage and deformation. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0020]    In the following, the present invention will be described in more detail with reference to the appended figures, in which 
           [0021]      FIG. 1  shows schematically a conventional reversing rolling mill in a side view, 
           [0022]      FIG. 2  shows a perspective view of a coiler drum, 
           [0023]      FIG. 3  shows the cross section A-A of the coiler drum presented in  FIG. 1 , 
           [0024]      FIG. 4  shows a perspective view of another coiler drum, 
           [0025]      FIG. 5  shows a perspective view of a third coiler drum, and 
           [0026]      FIGS. 6   a  and  6   b  show a cross sectional views of a strip on the coiler drum. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0027]    In this description and claims the term casting means pouring a molten steel alloy to a casting mould where it is solidified as it is cools down. The molten alloy retains the form defined by the casting mould when it cools down. The casting mould can be either stationary or it can rotate in the axial direction during casting. Rotating casting moulds are used to cast axially symmetrical objects, such as cylinders, rolls or pipes. 
         [0028]      FIG. 1  has been explained above, and thus it will not be discussed here any more. 
         [0029]      FIG. 2  shows a coiler drum  5  that has been manufactured according to the method of the invention. The coiler drum comprises a coiler drum body  6  that has a surface layer  7  on it. The coiler drum body  6  can be seen in  FIG. 3 . The coiler drum body is formed by casting. As a cast steel can be used a standard H-series stainless steel, e.g. ASTM A297 grade HK, HT, HP steel or other heat resistant alloy. The body is formed as a circular body with a wall thickness of about 30 mm to about 150 mm. The body is therefore hollow inside. On the body, a slot  12  is provided for grabbing the end of the strip  3  when starting the rolling of the strip ‘on the’ coiler drum  5 . The length of the body can vary and it is determined by the size of the coiler furnace. Trunnions  8  are formed to each end of the coiler drum body  6 . By the trunnions the coiler drum is attached to the coiler furnace. 
         [0030]    In one embodiment of the invention, a surface layer  7  of trinickel aluminide (Ni 3 Al) alloy is provided on the coiler drum body  6 . The surface layer covers the coiler drum substantially completely, extending, in the axial direction of the coiler drum, from one end of the coiler drum to the other end and defining a working surface  9  that will engage the strip of metal when it is being processed in a reversing rolling mill. It also covers the coiler drum body around its entire circumference. In  FIG. 3  can be seen the cross section A-A of the coiler drum presented in  FIG. 1 . The surface layer may be formed on the coiler drum body by overlay welding. In other words, the surface layer is formed by superimposing several layers of trinickel aluminide alloy on the coiler drum body. The welding may be done by plasma, laser, MIG/MAG or TIG welding. In welding, a filler metal comprising trinickel aluminide is used. The number of overlays is determined by the desired thickness of the surface layer. The surface layer may also be formed on the coiler drum body by thermal spraying. Thermal spraying is conducted by melting the trinickel aluminide alloy into droplets and impinging these droplets on the coiler drum. Different welding techniques and thermal spraying are well known to the skilled person in the art and thus they will not be described here in more detail. The surface layer is finished to have a smooth working surface by heat treating and machining and/or grinding the drum after the surface layer is formed on the drum body. 
         [0031]    The surface layer can also be a sleeve of trinickel aluminide alloy that is arranged on the body. The sleeve may be prepared from a sheet of trinickel aluminide alloy having a suitable thickness. The sheet is processed to a sleeve of appropriate size and it is fitted on the drum body. The sleeve is attached to the coiler drum body by welding. 
         [0032]      FIGS. 2 and 3  show a coiler drum comprising a surface layer of trinickel aluminide alloy defining a smooth working surface  9  that will engage the strip of metal when it is being processed in a reversing rolling mill. 
         [0033]    In another embodiment of the invention, the coiler drum  5  comprises a plurality of raised strips of trinickel aluminide alloy extending radially outward from the base surface of the coiler drum body. 
         [0034]    In  FIG. 4  is presented a coiler drum having raised strips  10 . The raised strips  10  are formed to the outer surface i.e. the base surface  11  of the coiler drum body. When winding the metal strip  4  on to the coiler drum  5 , the base surface  11  will not be in contact with the metal strip  4 , but it is engaged by the raised surfaces of the raised strips  10 . These raised surfaces form the working surface  9  of the strips that will engage the strip of metal. 
         [0035]    The orientation of the raised strips  10  on the base surface  11  relative to the coiler drum may vary. In the embodiment shown in  FIG. 4 , the raised strips  10  are substantially parallel to the longitudinal axis of the coiler drum  5 . The length of the raised strips  10  extends to the entire length of the base surface  11 . The raised strips  10  are arranged around the circumference of the base surface  11  at regular intervals from each other. The raised strips  10  may also be arranged to have variable spacing on the base surface  11 . Also the length and frequency of the raised strips  10  may vary. They may extend only to a portion of the axial length of the base surface. The raised strips  10  of shorter length can also be laterally offset or staggered across the base surface  11 . Also, certain cross sections of the base surface  11  may not include any raised strips  10 . In the embodiment shown in  FIG. 5  the raised strips  10  are circumferentially oriented relative to the longitudinal axis of the coiler drum  5 . In this embodiment, the raised strips  10  are formed as circumferentially continuous separate rings around the coiler drum  5 . The circumferential raised strips  10  may also consist of separate staggered segments of a ring. There may also be only one raised strip  10  that is arranged as a helix around the base surface  11 . The raised strips  10  are produced on the base surface by overlay welding. The strip is formed by overlaying several weld beads  10   a - 10   n  on top of another. In  FIGS. 6   a  and  6   b  this is presented in more detail. The strips can be finished by machining and/or grinding to have a suitable height H and shape. In the finishing, it is possible only to treat the weld beads of the strips  10 . This is shown in  FIG. 6   a.  The machining and/or grinding has been done in such a way that it only affects the weld beads  10   a - 10   n  to form a desired shape and height strip  10 . The base surface  11  retains as it was before finishing. It is also possible to extend the machining to the base surface  11 . When doing this, both the base surface and upper surfaces of the strips are treated. This is shown in  FIG. 6   b.  The machining and/or grinding has been done in such a way that also material from the base surface has been removed. The base surface  11  around the finished strip  10  is below the original base surface  11   a,  because the base surface of the coiler drum has also been machined. 
         [0036]    The height H of the raised strips is about 2 to about 10 mm, suitably from about 4 to about 8 mm. The raised strips  10  have a flat upper surface. When winding the metal strip  4  on the coiler drum  5 , the upper surfaces of the raised strips  10  form the working surface  9  that will engage the strip of metal when it is being processed in a reversing rolling mill. 
         [0037]    The trinickel aluminide alloy consists of, in % by weight,
       4.0 to 12.0% Al   0 to 16.0% Cr   0 to 1.5% Mo   0 to 1.5% Zr   0 to 0.003% B
 
the balance being Ni and inevitable impurities. The alloy has a metal structure comprising Ni 3 Al as the main phase thereof.
       
 
         [0043]    The coiler drums that are manufactured according to the invention are especially suitable for use in Steckel Mill coiler furnaces. 
         [0044]    The intention is not to restrict the invention to the embodiments described by way of example, but it is intended that the invention can be interpreted widely within the scope of protection defined by the claims presented hereinbelow.