Abstract:
Disclosed is a hot forming tool ( 1 ) which consists of a main tool member ( 2 ) having an at least partial surface coating ( 4 ) and which can be obtained by providing the main tool member with a raised metal relief that is then entirely or partly oxidized and converted into a protective layer.

Description:
FIELD OF THE INVENTION 
       [0001]    The invention is in the field of manufacture of tubular metal workpieces and relates to forming tools, especially piercing plugs, forging mandrels and rolling mandrels with improved stability. 
       STATE OF THE ART 
       [0002]    Seamless steel tubes are generally made in three forming stages on respective rolling mills by hot forming. In a first stage, on a so-called cross-roll piercing mill a solid steel block heated to about 1200° C. is pierced by means of an internal tool, the piercing plug, into the hollow bloom. Here, the block is driven by means of inclined rolls over the piercing plug. In the second forming stage, in a longitudinal rolling process the hollow bloom is reduced over the inner tool, a rolling mandrel, in diameter and wall thickness and stretched in the longitudinal direction. In the third forming stage, the rolling stock is converted to the required dimensions in diameter and wall thickness, where usually no internal tool is used. 
         [0003]    The inner tools in the first two forming stages are exposed in the production to high temperatures and mechanical pressures. In most cases, the inner tools are made of heat resisting steel. In the production, especially with larger rolling times a successive heating of the internal tool is often unavoidable. Due to heating the strength of the tool decreases and the tool can no longer withstand the mechanical stresses. The tool deforms and breaks. 
         [0004]    To achieve a long service life, piercing plugs are provided with natural scale layers. These scale layers inhibit the flow of heat from the workpiece to be shaped into the tool and protect the tool from rapid heating and rapid loss of strength. When forming higher alloyed materials, the scale layer is removed, however, quickly and the thermal protection fails. 
         [0005]    In rolling mandrels scaled tools or tools provided with a chromium layer are used depending on the forming process course. Corresponding piercing plugs are known from German Patent Application DE10 2008 056 988 A1 (SMS MEER). A disadvantage, however, is that the thermal insulation against the heat flow from the workpiece to be shaped into the tool is low. Thus, particularly with internal tools, which are used at reduced speed and length of contact, heating of the inner tool and its failure due to deformation and fracture occur. 
         [0006]    The tool life could be improved if the thickness of the oxide layer could be enlarged. Then, the thermal insulation is better and during abrasive wear the protective layer would remain preserved longer. 
         [0007]    The protective layer is formed naturally from the base material by conversion to iron oxides, but does not have high stability. It is brittle and porous and therefore can be easily destroyed by mechanical and thermal stress. Therefore, these protective layers are limited in thickness. The limit of the layer is about 0.8 mm. The protective effect of such a layer is therefore limited accordingly. Heat penetrates therefore into the main body of the tool and reduces its strength, whereby it then comes to premature failure of the tool. With high-alloyed work-pieces abrasion leads relatively quickly, i.e. by a small length of the rolled material to the removal of the protective layer. 
         [0008]    From the international patent application WO2011 107214 A1 (SMS MEER) piercing plugs or rolling mandrels are known for the production of seamless tubes or forging mandrels for hot forging of tubular workpieces of metal which have a surface profiling, in which the oxide layer is applied. In this way, better adhesion and longer service life to be achieved. 
         [0009]    Similar tools, in which the coating consists of molybdenum, are known from EP0385439A1 (NKK CORP.). 
         [0010]    Subject of the European patent application EP 2404680 A1 (SUMITOMO) is the manufacture of steel tubes according to the Mannesmann process. Claimed is a piercing plug the feature of which is, that it inhibits a channel, through which a lubricant is lead during piercing. According to [0053] the piercing plug can be coated by iron. For this an iron wire is lead to a spraying device where it is molten. The molted iron is then sprayed onto the piercing plug, i.e. a continuous coating is developing. 
         [0011]    In practice, the preparation of such profiled tools, however, proves to be expensive since the profiles have to be individually cut into the piercing plugs, and also lead to material losses. The manufacturing cost of a profile disproportionately increase with the size of the introduced grooves. An economic and feasibility limit is reached at just a few millimeters. Another disadvantage of the profile cut into the base body is the limitation of the material on good oxidizable steels. These have in particular a low chromium content and thus low hardness. 
         [0012]    The object of the present invention has therefore been to provide hot forming tools with improved stability, which are free from the above-stated disadvantages. In particular, these tools should have an oxide layer having a higher strength compared to the prior art, which can be also applied easily and without loss of material. 
     
    
     DESCRIPTION OF THE INVENTION 
       [0013]    A first aspect of the invention relates to a hot forming tool comprising a tool body having at least pro rata surface coating, which can be obtained, by that the basic body is provided with a raised metallic relief, which is subsequently completely or partially oxidized and converted into a protective layer. 
         [0014]    Under ‘raised’ has to be understood, that the relief is raising related to the surface of the tool (‘hill structure’) and thus is contradictory to a profiling where profile grooves are carved into the surface (‘valley structure’). 
         [0015]    Another aspect of the invention includes a method for producing a hot forming tool comprising a tool body having at least pro rata surface coating, in which
   (a) the body is provided with a raised metallic relief, and   (b) successively the metallic relief is completely or partially oxidized and converted into a protective layer.   
 
         [0018]    The application of raised reliefs is the reverse case to a profiling of the tool. For the purposes of the invention, therefore, material is added and not removed. Surprisingly, it was found that the relief formation is in contrast to the profiling not only much easier to realize, but by complete or partial conversion of the relief material, even a considerably harder and thus more stable oxide film is obtained, which leads to a significant improvement in tool life. The invention also provides the possibility of selecting the relief material to vary the quality of surface protection and adjust the process conditions. 
         [0019]    The economic benefit of the invention is obvious and is in particular in the reduction of tool costs during the production of steel products, as well as the extension of the rolling time, which is usually associated with larger lengths of the rolling stock and reduced material waste. 
         [0020]    Tools 
         [0021]    Hot forming tools of the present invention are preferably a piercing plug or a forging mandrel, which are typically made of steel. The invention includes under this preamble, however, in principle, any other metallic workpiece, in which the body is to be protected against heat influx. The term metal is not limited to iron and steel, but also includes other metallic materials including metal-composite materials that are to be supplied to a hot forming. 
         [0022]    But not only in piercing plugs, the inner tools in piercing by cross rolling, with the other inner tools that are used in the production of seamless steel tubes, the surface coating of the invention may be advantageously employed. In the rolling mandrels, the inner tools in the rolling mills with several successively arranged roll stands in the second forming stage is particularly important to ensure that the friction between tool and rolling stock is low. Therefore, the surface layer of the invention has to be grinded and polished for this application. Also, an additional layer can be applied, made of chrome on the protective layer according to the invention. 
         [0023]    The raised relief, which is applied to the base body can be pronounced quite differently, the alternative embodiments are all suitable in principle to fulfill the task in full. 
         [0024]    In a first embodiment, it may be simply a wrapping of the body with a wire, preferably a steel wire at the raised relief. 
         [0025]    In a second embodiment, the raised relief can represent a metal fabric or metal mesh, which is applied to the base body. 
         [0026]    The metallic bodies applied to the surface of the tool are preferably made from a steel mesh, for example with a steel wire thickness of about 1 to about 5 mm and preferably about 1.5 mm and a mesh spacing of about 1 to 5 mm and in particular about 2.5 mm. Under the mesh spacing the distance of the center lines of two adjacent fabric elements is to be understood. 
         [0027]    In a third embodiment the raised relief may be an irregular coating, as is achieved by chemical or physical deposition of metal from the vapor phase. 
         [0028]    Relief Formation 
         [0029]    The application of the raised reliefs can by very different—simple and complex—procedures, which yet solve all the object of the invention to the full extent. 
         [0030]    In a first embodiment, the base body is simply wrapped with a wire, preferably a metal wire. 
         [0031]    In a second alternative embodiment, a metal fabric or a metal mesh is used instead of the wire. This may be preformed, for example by forming the shape of the tool and then mounted on the base body. In order to increase the strength, it is advisable to weld the wire coil or the metal fabric to the base body. 
         [0032]    In a third alternative embodiment, it is possible to produce the relief on the surface of the base body by chemical or physical vapor deposition (Chemical/Physical Vapor Phase Deposition, CVD/PVD). 
         [0033]    The term chemical vapor deposition is a group of coating methods which are used inter alia in the manufacture of microelectronic components and optical waveguides. At the heated surface of a substrate, a solid component is separated due to a chemical reaction from the gas phase. A prerequisite is that volatile compounds of the component layers exist, the entrained solid layer at a given reaction temperature. The method of the chemical vapor deposition is characterized by at least one reaction on the surface of the workpiece to be coated. This reaction must be at least a gaseous starting compound (starting material) and at least two reaction products—to be involved—of which at least one in the solid phase. To over competing vapor phase reactions to promote those reactions at the surface and thus to avoid the formation of solid particles, the process is preferably carried out at reduced pressure. 
         [0034]    Unlike the CVD, the starting material is converted into the gas phase using the preferred PVD. The gaseous material is then led to the substrate to be coated, where it condenses and forms the target layer. Examples are classical evaporation processes, such as thermal evaporation, electron beam (electron beam evaporation) or laser beam evaporation (pulsed laser deposition). For the purposes of the present invention, preferred is sputtering in which the starting material is sputtered by ion bombardment and transferred into the gas phase from which it can then be deposited on the basic body. All these processes have in common that the material to be deposited is in solid form in the mostly evacuated coating chamber. By bombardment with laser beams, magnetically deflected ions or electrons as well as by arc discharge, the target is evaporated. The proportion of atoms, ions or larger clusters in the vapor varies from procedure to procedure. The vaporized material moves either ballistically or performed by electric fields through the chamber and impinges on the parts to be coated, where it comes to the layer formation. 
         [0035]    For achieving that the vapor particles reach the components, and are not lost by scattering at the gas particles, the work must be done in vacuum. Typical operating pressures are in the range of  10   −4  Pa to 10 Pa. Since the vapor particles propagate straight, areas that are not visible seen from the steam source, are coated with a lower deposition rate. In order to produce a relief and no homogeneous coating a rotation of the substrate will be omitted different from the usual procedure. 
         [0036]    A fourth alternative embodiment of the relief formation comprises the so-called thermal spraying. Here additional materials, the so-called spray additives are melted off, at or on inside or outside a spray burner, accelerated in a gas stream in the form of spray particles and thrown on the surface of the component to be coated. The component surface in this case (in contrast to the cladding) is not melted and thermally loaded only slightly. A layer formation takes place, as the spray particles are flattening more or less depending on process and material when impinging on the component surface, stick primarily by mechanical bonding and layer by layer to build the spray layer. Quality characteristics of spray coatings are low porosity, easy bonding to the component, avoidance of cracks and homogeneous microstructure. The layer properties obtained are significantly influenced by the temperature and the speed of the spray particles at the time of incidence to the surface to be coated. The surface state (purity, activation temperature) also exerts a significant influence on quality characteristics such as adhesion. 
         [0037]    As energy carrier for the melting of the spray additive material are used electric arc (arc spraying), plasma jet (plasma spraying), fuel-oxygen flame or fuel-oxygen high-speed flame (conventional and high-velocity flame spraying), fast, preheated gas (cold gas spraying) and laser beam (laser beam spraying). According to EN 657 DIN standard spray methods are classified according to these criteria. 
         [0038]    Using this method, the base body may be coated not only with metals but also oxide-ceramic materials and carbide materials (or in general composites). Preferably in this embodiment the coating takes place with a steel/ceramic mixture. 
         [0039]    While the base body is preferably made of steel, it is valid for the material forming the raised relief, the requirement that it is at least capable rata for forming an oxide layer. Preferably, this is iron or steel so that a layer of iron oxide, preferably scale is generated. It can be used as said, a mixture of iron/steel and ceramics for example in a weight ratio of about 20:80 to about 80:20. 
         [0040]    It is understood that the relief may have different forms, ranging from regular (round, square, etc.) to any freeform. It can also be used composite materials, i.e. for example, a molybdenum fabric that is applied to the steel body. The fabric element can also consist of a composite of hard chrome steel (inside) and well oxidizable steel (outside). As the spacer, also combustible materials may be employed. It is also possible, for better heat insulation to embed ceramic. 
         [0041]    Oxidation 
         [0042]    The complete or partial conversion of the metallic reliefs in an oxidic protective layer may be produced by known methods of the prior art, for example by flame spraying, plasma spraying, or is carried out by a thermochemical process. 
         [0043]    In the oxidation of the tool with the metallic body applied on its surface, for example a steel fabric, a part of the surface of the tool base body and a part of the relief deposited on the surface is converted into oxide. At the same time, an additional oxide layer is formed on all surfaces, typically to about 3000 microns, and especially about 1.500 to about 2,500 microns. Thus, also oxide is formed in the spaces between the bodies, for example, between the tool body and the applied steel fabric and within the meshes of the steel fabric. The result is a particularly thick protection layer which is reinforced by means of an internal body. In particular, the layer thickness is different than in the production of grooves not limited to a few millimeters. Layer thicknesses of 10 mm and more can be produced without difficulty and at low cost. 
       INDUSTRIAL APPLICABILITY 
       [0044]    Another object of the invention relates to the use of the new tool described in detail above, especially as a piercing plug, forging mandrel or rolling mandrel for the production of seamless tubes or hot-forging tubular workpieces of metal. 
       EXAMPLES 
     Example 1 
       [0045]    On the surface of a piercing plug a mesh preformed by forming the shape of the base steel was laid on with a steel wire thickness of 1.5 mm and a mesh width of 2.5 mm and welded. Successively, the composite has been exposed to a thermo-chemical oxidation. A coherent continuous oxide layer of 2500 microns thickness has been obtained. 
         [0046]      FIG. 1  shows a hot forming tool in the form of a piercing plug in a side view. The tool  1  has a tool body  2  having a work area  3 , which extends in the direction of an axis A over a certain length. In the work area  3 , the tool is provided with a coating  4  which protects the tool  1  against thermal and mechanical stress. 
         [0047]      FIGS. 2 and 3  show the detail “Z” in the horizontal section through the tool according to  FIG. 1  once for the material body with raised relief before and after the production of the oxide protective layer (“scaling”). 
         [0048]    In  FIG. 2 a    one recognizes the saw-shaped relief which is formed according to Example 1 by applying a wire mesh. In this case, the base body is characterized by the reference numeral  6 , the mesh by the numeral  7 . In  FIG. 2 b    it is seen that a part of the surface of the relief has been converted to oxide, but also between the loops of the mesh, the surface of the base body has been oxidized (hatching with reference numeral  8 ). 
         [0049]    The  FIGS. 3 a  and 3 b    are analogue, however, the relief here has no square, but a round cross section. Again, one can see that the oxide layer (hatching) is developing in equal proportions above and beneath the original surface of the ferrous body.