Abstract:
The invention relates to a method for the production of castings made of a copper alloy comprising silicon, nickel, chromium, and zirconium, and also inter-metal primary phases, wherein an ingot is drawn by means of hot forming in only one direction at a ratio of at least 4:1, wherein a casting surface of a casting produced from the drawn ingot, said surface coming into contact with a metal melt, is substantially selected perpendicular to the drawing direction of the ingot. A casting produced in this manner is characterized by high wear resistance and increased service life, particularly when used as a block of a side bank of a double strip casting system.

Description:
BACKGROUND OF THE INVENTION 
       [0001]    1. Field of the Invention 
         [0002]    The present invention relates to a method for producing cast molded parts from a copper alloy containing silicon, nickel, chromium and zirconium as well as intermetallic primary phases. Furthermore, the present invention relates to cast molded parts produced according to this method. 
         [0003]    2. Description of Related Art 
         [0004]    Published European patent document EP 0 346 645 B1 describes the use of a curable copper alloy made up of 1.6 to 2.4% nickel, 0.5 to 0.8% silicon, 0.01 to 0.20% zirconium, the remainder being copper including production-related impurities and the usual processing additives, as material for producing cast molded parts, which are subjected to permanently changing thermal stressing during the casting process, in particular in the form of blocks for side dams of twin-belt casting systems. The capacity of twin-belt casting system depends considerably on the proper functioning of the side dam chain formed by blocks. For example, the blocks must have the highest possible thermal conductivity so that the melting or solidification heat is able to be dissipated as quickly as possible. In order to avoid premature wear of the side edges of the blocks due to mechanical stressing, which leads to the formation of gaps between the blocks and then to the penetration of the molten mass into this gap, the material must exhibit not only high hardness and tensile strength but also a small grain size. 
         [0005]    Finally, optimum fatigue behavior of the material is of the most decisive significance, which will ensure that the thermal stresses arising during cooling of the blocks after they leave the casting line do not lead to cracking of the blocks at the corners of the T groove incorporated for the accommodation of the steel band. If such cracks caused by thermal shock do appear, the respective form block will fall out of the chain after even a short period of time and molten metal is able to run uncontrollably from the casting form cavity and damage parts of the installation. An exchange of the faulty block requires the system to be stopped and the casting operation to be interrupted. 
         [0006]    A testing method in which the blocks are subjected to heat treatment for two hours at 500° C. and are subsequently quenched in water at 20 to 25° C., has proved useful for checking the tendency to crack. Even if this thermal shock test is repeated several times, no cracks must appear in the region of the T groove in the case of a suitable material. 
         [0007]    The zirconium-containing, curable CuNiSiCr alloy described in EP 0 346 645 B1 is extremely suitable for blocks in side dams of twin-belt casting systems. The addition of chromium increases the conductivity of the material. The Fe addition restricts the increase in grain size during the solutionizing treatment without adversely affecting the other properties of the material. 
         [0008]    It is known that intermetallic primary phases occur in the structure of the chromium- and zirconium-containing material, which crystallize in hypoeutectic manner, i.e., with an inhomogeneous distribution, during the solidification of the melt. For method-related reasons, these CrSi-containing and NiZr-containing phases already occur in the cast round ingots that are used as starting material for the production of blocks for the side dams of twin-belt casting systems. In order to adjust a fine-grained structure and to achieve the required hardness and electrical conductivity, the molten material is usually formed while still warm, employing conventional deformation processes such as extrusion, forging or rolling, and subsequently solutionized and cured; in the process, the eutectic, inhomogeneous distribution of the intermetallic primary phases of the casting state are more or less destroyed, and the primary phases are aligned in the form of bands in the main deformation direction. When the blocks are produced in the conventional manner from extruded or hot-rolled rods, then a relatively unevenly distributed primary-phase arrangement in the casting surface of the blocks featuring a distinctly banded orientation is present. During the forging of plates from an unworked cast piece, the net-like distribution of the intermetallic primary phases of the casting state is usually removed only insufficiently since the overall deformation degree is limited, and the plate is formed in approximately the same way in the longitudinal and the transverse direction. 
       SUMMARY OF THE INVENTION 
       [0009]    Using this as the starting point, the present invention is based on the objective of optimizing a method for producing cast molded parts, in particular for producing blocks for side dams of twin-band casting systems, such that the wear of the casting surfaces coming into contact with molten metal sets in later and progresses more slowly, so that a cast metal band featuring a perfect surface quality is able to be produced over a longer period of production using the cast molded parts. Furthermore, a cast molded part having improved properties is to be provided. 
         [0010]    The invention provides a method for producing cast molded parts made from a copper alloy containing at least one alloy element from each of the groups a) and b), group a) including nickel and cobalt, and group b) including chromium, zirconium, beryllium and silicon, as well as intermetallic primary phases, a cast ingot being ironed by hot deformation in only one direction, at a ratio of at least 4:1; an angle of 90°±10° relative to the ironing direction of the cast ingot being selected for a casting surface, which comes into contact with the molten metal, of a cast molded part that is produced from the ironed cast ingot. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0011]      FIG. 1  is a micrograph of a cast round ingot which can be used as a starting material for the production of cast molded parts of side dams of a twin-belt casting system. 
           [0012]      FIG. 2  shows the distribution of the intermetallic primary phases of a cast ingot that already underwent hot deformation, and thus the micrograph in the region of the casting surface of a later cast component. 
           [0013]      FIG. 3  shows a micrograph perpendicular to the casting surface and thus perpendicular to the micrograph of  FIG. 2 . 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0014]    The objective underlying the present invention is achieved in that selective hot deformation is used to orient the intermetallic primary phases included in the copper alloy in such a way that a casting surface, which comes into contact with the molten metal, of a cast molded part that is produced from the ironed cast ingot is selected to be at an angle of 90°±10°, i.e., essentially perpendicular, to the ironing direction of the cast ingot. “Essentially perpendicular” as used in the following text means an angle of 90±10° relative to the ironing direction of the cast ingot. Perpendicular denotes an angle of 90°. 
         [0015]    The essential aspect in this procedure is that the hot shaping of the cast ingot not only produces the fine-grained structure recrystallization of the originally coarse-grained casting structure, but also a distinct fiber orientation featuring a reduction in size and an alignment of the intermetallic primary phases in line with these fibers. In this context it is important that if possible, the fiber orientation has fine and evenly distributed primary phases, which in the framework of the present invention is achieved in that the ironing by hot forming takes place in only a single direction, the cast ingot being ironed at a ratio of at least 4:1, preferably more than 7:1. The hot forming may be performed employing methods such as forging or hot rolling. In contrast, a sweeping overall deformation of at least 4:1 or preferably of at least 7:1, in different directions, does not lead to the fiber flow aimed for according to the present invention. 
         [0016]    Another important method feature is that the cast molded parts produced from the ironed cast ingot have a casting surface which comes into contact with the molten metal that is selected essentially perpendicular (=90±10°), preferably precisely perpendicular, to the ironing direction. Only in this case will the wear of the cast surfaces be reduced significantly, thereby making it possible to produce a cast metal band having perfect surface quality over a longer period of production. 
         [0017]    Because of the orientation of the fibers, the intermetallic primary phases in the casting surface essentially manifest themselves only in the form of evenly distributed dots. It is considered useful if the quantitative proportion of the intermetallic primary phases, cut in a micrograph, between the casting surface and the sides of the ironed casting ingot standing perpendicular to the casting surface is set to be greater than 1.5:1. This means that at least 50% more intermetallic primary phases are cut in the casting surface, or in a plane running essentially perpendicular to the ironing direction, than in a side or plane perpendicular to the casting surface. 
         [0018]    The quantitative proportion of the cut intermetallic primary phases adjusted in this manner, in combination with the orientation of the casting surface leads to cast molded parts featuring an optimized application behavior since the introduction of fissures and the spread of fissures in the casting surface is inhibited. This reduces the wear of the cast molded parts during use since the fissure spread proceeds more slowly, which contributes to an increase in service life. The resistance to the formation of fatigue fissures is markedly higher in comparison with cast molded parts in which the intermetallic primary phases are essentially non-aligned. 
         [0019]    The cast molded part produced according to the method of the present invention has a fiber flow that causes the intermetallic primary phases to be arranged in fibers or bands as well. The average length of a primary phase lying in a plane is able to be measured. It is considered advantageous if the ratio between the average length of a band lying in the plane of the casting surface, and the average length of a band that runs essentially perpendicular (=90±)10°, preferably precisely perpendicular, to the casting surface is less than 3:10. In other words, there are bands of intermetallic primary phases in the casting surface whose length corresponds to maximally 30% of the length of a band of an intermetallic primary phase that runs essentially or precisely perpendicular to the casting surface. 
         [0020]    The cast molded part according to the present invention is made of a curable copper alloy, which for this purpose contains alloy components which precipitate as intermetallic phases. The curable copper alloy preferably contains nickel, which may be at least partially replaced by cobalt. In addition, the alloy contains at least one of the following alloy elements: chromium, zirconium, beryllium, silicon. 
         [0021]    The finished cast molded part is characterized by excellent material properties tailored to the specific application case, i.e., especially by a tensile strength of at least 600 MPa at a room temperature of 20° C., and a tensile strength of at least 350 MPa at a temperature of 500° C. 
         [0022]    In the cured state, the copper alloy has an 0.2% yield strength of at least 470 MPA at 20° C., a breaking elongation A 5  of at least 15%, a hardness of at least 190 HV10 and an electric conductivity of at least 40% IACS (IACS=International Annealed Copper Standard, electric conductivity in comparison with copper=100%) at 20° C. The electric conductivity preferably amounts to at least 45%. 
         [0023]    The cured copper alloy is to feature a grain size of maximally 130 μm measured according to ASTM E 112. The U.S. ASTM E 112 standard (American Society for Testing and Materials) is a standard testing method for determining the average grain size. 
         [0024]      FIG. 1  shows a micrograph of a cast round ingot, which is used as starting material for the production of cast molded parts of side dams of a twin-belt casting system. It involves the typical cast structure of a CuNiSiCrZr alloy having CrSi-containing or NiZr-containing intermetallic primary phases in a eutectic arrangement. Subsequently, deformation methods such as extrusion, forging or rolling are used to deform the material in order to adjust a fine-grained structure and to achieve the required hardness and electrical conductivity; then, the material is subjected to a solutionizing treatment and cured, so that a change occurs in the eutectic, inhomogeneous distribution of the intermetallic primary phases. 
         [0025]    If the unworked cast piece shown in  FIG. 1 , which has a net-like distribution of the intermetallic primary phases, is deformed to the same extent both in the longitudinal and the transverse direction, then the phase orientation does not change in the desired manner. 
         [0026]    In contrast,  FIG. 2  shows the distribution of the intermetallic primary phases of a cast ingot that already underwent hot deformation, and thus the micrograph in the region of the casting surface of a later cast component. It can be seen quite clearly that the intermetallic primary phases are very fine and evenly distributed. The fiber orientation, or the orientation of the intermetallic primary phases, runs perpendicular to the casting surface, so that the cut primary phases appear as dots in this figure. 
         [0027]    The number of cut primary phases is approximately 1.7 as high as in  FIG. 3 , which shows a micrograph perpendicular to the casting surface and thus perpendicular to the micrograph of  FIG. 2 . While the phase bands are discernible only in rudimentary form in  FIG. 2  and have a maximum length of approximately 100 μm, a much higher number of primary phase bands can be seen in  FIG. 3 , the phase band lengths ranging from 100 to 400 μm, and partially amounting to more than 400 μm. The following table illustrates the mechanical properties and the fatigue resistance of cast molded parts made from CuNiSiCrZr alloys according to the method of the present invention. 
         [0000]    
       
         
               
               
               
               
               
               
               
               
               
               
             
               
               
               
               
             
               
               
               
               
               
               
               
               
               
               
             
           
               
                   
               
               
                   
                   
                   
                   
                   
                   
                   
                   
                 Response 
                   
               
               
                   
                   
                   
                   
                   
                   
                   
                   
                 Following 
               
               
                   
                   
                   
                   
                   
                 El. 
                   
                 Fatigue % 
                 Thermo- 
               
               
                   
                 R m   
                 R p0.2   
                   
                 Hardness 
                 Cond. 
                 R m   
                 Service 
                 Shock 
                 Grain Size 
               
               
                 Exemplary 
                 MPa 
                 MPa 
                 A s  % 
                 HV10 
                 % IACS 
                 MPa 
                 Life 
                 Testing 
                 ASTM E112 
               
             
          
           
               
                 Embodiment 
                 Testing temp. 20° C. 
                 Testing temp. 500° C. 
                 μm 
               
               
                   
               
             
          
           
               
                 A (R = 5.3:1) 
                   
                   
                   
                   
                   
                   
                   
                   
                   
               
               
                 Fiber 
                 637 
                 514 
                 17 
                 210 
                 51.4 
                 381 
                 117 
                 fissure- 
                 45-65 
               
               
                 perpendicular 
                   
                   
                   
                   
                   
                   
                   
                 free 
               
               
                 to casting 
               
               
                 surface 
               
               
                 (according to 
               
               
                 the invention) 
               
               
                 Fiber parallel 
                 625 
                 502 
                 15.5 
                 210 
                 51.6 
                 371 
                 100 
                 fissure- 
                 45-65 
               
               
                 to casting 
                   
                   
                   
                   
                   
                   
                   
                 free 
               
               
                 surface 
               
               
                 (not standard 
               
               
                 implementation 
               
               
                 according to 
               
               
                 the invention) 
               
               
                 B (R = 7.3:1) 
               
               
                 Fiber 
                 640 
                 518 
                 16 
                 212 
                 51.4 
                 402 
                 126 
                 fissure- 
                 30-45 
               
               
                 perpendicular 
                   
                   
                   
                   
                   
                   
                   
                 free 
               
               
                 to casting 
               
               
                 surface 
               
               
                 (according to 
               
               
                 the invention) 
               
               
                 Fiber parallel 
                 635 
                 513 
                 15 
                 216 
                 51.2 
                 371 
                 100 
                 fissure- 
                 30-45 
               
               
                 to casting 
                   
                   
                   
                   
                   
                   
                   
                 free 
               
               
                 direction 
               
               
                 (not standard 
               
               
                 implementation 
               
               
                 according to 
               
               
                 the invention) 
               
               
                   
               
             
          
         
       
     
         [0028]    Exemplary embodiment A is based on an alloy having the following composition in weight-%: 
         [0000]    
       
         
               
               
             
           
               
                   
               
             
             
               
                  2.1% 
                 Ni 
               
               
                 0.62% 
                 Si 
               
               
                 0.30% 
                 Cr 
               
               
                 0.15% 
                 Fe 
               
               
                   
               
               
                 remainder = copper, including unavoidable impurities. 
               
             
          
         
       
     
         [0029]    This alloy was smelted in an induction crucible furnace and cast in the form of a round ingot using an extrusion method. The round ingot was preheaded in a forging press within a temperature range between 950° C. and 750° C. and then shaped into a cuboid. The cuboid was subsequently forged into a plate in the longitudinal direction. This blocked plate was then rolled to its final dimensions in a hot rolling mill between 950° C. and 800° C. The overall deformation ratio R in the longitudinal direction, based on the preheaded length and ending with the completely rolled plate length, amounted to 5.3:1. The plate was subsequently solution-annealed and cured. The cooling following the curing was performed in a kiln at a defined cooling rate. Subsequently, the plate was sawed into horizontal strips, and these strips were then used to produce cast molded parts, also referred to as dam blocks, having the dimensions of 70 mm×50 mm×40 mm. 
         [0030]    As an alternative, the cast molded parts having dimensions of 60 mm×50 mm×40 mm or 50 mm×50 mm×40 mm may be obtained in the same manner as well. Preferably, the casting surfaces of the cast molded parts in essence come to lie exactly perpendicular to the longitudinal direction of the plate, and thus preferably in essence also exactly perpendicular to the ironing direction of the deformed cast ingot or the fiber alignment. 
         [0031]    The table reproduces the mechanical/technical properties and also the fatigue resistance of formed molded parts thus produced, in comparison with cast molded parts whose fibers lie parallel to the casting surface and which have not been subjected to a preferred deformation at a ratio of at least 4:1. In laboratory testing, the cast molded parts produced according to the present invention, having an alignment of the intermetallic phases that runs perpendicular to the casting surface, exhibit a fatigue resistance that is 17% higher than that of cast molded parts whose fiber position runs parallel to the casting surface. 
         [0032]    Exemplary embodiment B is based on an alloy having the following composition: 
         [0000]    
       
         
               
               
             
           
               
                   
               
             
             
               
                  2.2% 
                 Ni 
               
               
                 0.60% 
                 Si 
               
               
                 0.33% 
                 Cr 
               
               
                 0.12% 
                 Fe 
               
               
                   
               
               
                 remainder = copper, including unavoidable impurities. 
               
             
          
         
       
     
         [0033]    This alloy, too, was smelted in an induction crucible furnace and cast in the form of a round ingot using an extrusion method. Then, the round ingot was rolled into a plate on a hot rolling mill between 950° C. and 800° C. The overall deformation ratio R in the longitudinal direction relative to the starting length of the cast ingot amounts to 7.4:1, and thus corresponds to the preferred specification according to the present invention of at least 7:1. 
         [0034]    The further treatment of the hot-rolled plate and the removal of the cast molded parts is performed in the manner shown in exemplary embodiment A. 
         [0035]    Table 1 once again reproduces the hardness properties of the cast molded parts having primary phases that run perpendicular to the ironing direction, in comparison with cast molded parts whose intermetallic primary phases run parallel to the casting direction. 
         [0036]    In laboratory testing, the cast molded parts produced according to the present invention and shown in exemplary embodiment B exhibit a fatigue resistance that is even 26% higher in comparison with cast molded parts having a fiber alignment parallel to the casting surface, the mechanical properties being approximately equal. 
         [0037]    The exemplary embodiments illustrate that the cast molded parts produced according to the present invention provide a fatigue behavior of the casting surface that it 17 to 26% better than comparable cast molded parts having a fiber and phase alignment parallel to the casting surface or having no preferred orientation.