EP0829528A2 - Lubricating material for mandrel bars containing no graphite - Google Patents

Abstract

The invention relates to a lubricant for the hot deformation of metals with at least one alkali metal phosphate and, if appropriate, additionally at least one phosphate of a divalent metal, which has improved lubricating properties compared to known high-temperature lubricants and at the same time does not attack the workpiece and the tool or does so less than known lubricants, since it has at least one contains gas-forming additive when heated.

Description

The invention relates to a lubricant for the hot forming of metals with at least one alkali metal phosphate and optionally additionally at least one phosphate of a divalent metal.

When processing metals, such as sheet metal or billets, in rolling or pressing systems, lubricants are required which ensure optimal sliding of the metal between the processing tools at high processing temperatures. Temperatures of 1100 to 1300 ° C can occur in the production of profile sheets or seamless tubes in rolling mills. If hard or difficult-to-deform metals are processed, the processing tools can wear out quickly. Lubricants are intended to reduce the frictional resistance between metal and tool to reduce wear to reduce the tools. The lubricants used must withstand high temperatures. Known lubricants can essentially be divided into graphite-containing and graphite-free lubricants.

For the processing of metals at low temperatures, organic lubricants such as greases, oils or soaps are generally used. In order to improve their properties at high temperatures, it has started to add graphite to these lubricants. Graphite is particularly heat-resistant and, in combination with mineral oils and inorganic salts, has particularly good lubricating properties. A disadvantage of graphite-containing lubricants is that the high carbon content causes carburization of the metal surface of the workpiece. Defective end products with poor further processing or material properties can result. The result is a high workpiece scrap. Another problem that is frequently observed with lubricants containing graphite is that spot welds occur between the tool and the workpiece. The tools in particular are severely attacked and the surface of the workpiece deteriorates.

Other lubricants, which can be both graphite and graphite-free, contain salts or salt mixtures that melt on the hot workpieces and form a lubricating separating layer between the workpiece and the tool. However, only certain salts are suitable for this, and some have such high melting temperatures that the lubricants are only fully operational when the operating temperature is reached. This is particularly disadvantageous, for example, when starting up the processing machines when the tools or workpieces are still cold. In some lubricants, borax is used as a low-melting salt in conjunction with mineral oils. However, it has been shown that with borax-containing lubricants, the tool and workpiece can stick together, so that damage to the tools occurs or the machines come to a standstill. Furthermore, borax-containing lubricants adversely affect the metal surface of the tool or workpiece.

Other known lubricants use raw table salt, which, however, leads to material removal and material application elsewhere on the workpiece and thus leads to scoring. Water-soluble lubricants based on alkali phosphates and alkali borates, which are also used in a mixture with various metal oxides, such as zinc oxide or iron oxide, attack the surface of the metal to be processed.

Another group of high-temperature lubricants contains alkali phosphate glasses or silicate glasses with various additives, such as boron or aluminum. This lubricant have good lubricating properties, but are poorly water-soluble, which makes their removal from the machined workpiece considerably more difficult and requires a high level of technical complexity.

DE 24 30 249 describes a high-temperature lubricant for the hot deformation of metals based on a phosphate borate glass with alkali metal, alkaline earth metal and heavy metal phosphates and borates, lubrication-improving additives such as zinc sulfide, calcium fluoride, sodium fluoride, graphite, sodium chloride and sodium-zinc polyphosphate, a stabilizing agent and a binder. In addition, the lubricant contains an organic blowing agent based on cellulose, starch, synthetic resin or oil, which decomposes at higher temperatures and creates a gas separation layer between the workpiece and the tool in addition to the lubricating separation layer. Compared to the lubricants mentioned above, the blowing agent achieves a significantly improved lubricating effect. The lubricant described in DE 24 30 249, however, has the disadvantages of the above-described graphite-containing and borax-containing lubricants. In addition to the graphite added as a lubricant, the decomposition of the organic blowing agent produces a considerable amount of carbon, which causes the metal surface of the workpiece to carburize. In addition, the proportion of water-insoluble salts and oxides complicates the later removal of the lubricant from the machined workpiece.

DE 14 44 794 describes an inorganic high-temperature lubricant consisting of a mixture of sodium and potassium phosphates, which additionally contains one or more divalent metals, in particular magnesium, zinc or manganese. Since the lubricant from DE 14 44 794 contains no carbon, carburization does not occur on the machined metal surface, but it has poorer lubricating properties than many other known lubricants.

The object of the present invention is to provide a high-temperature lubricant which has improved lubrication properties compared to the prior art and at the same time does not attack the workpiece and the tool, or does so less than known lubricants.

This object is achieved by a lubricant with at least one alkali metal phosphate which contains at least one additive which forms a gas when heated.

The gas-forming additive is expediently inorganic and preferably contains at least one inorganic carbonate.

A particular advantage of the high temperature lubricant of the present invention over known graphite containing lubricants is that it contains little or no carbon. At an elevated temperature, the additive contained decomposes spontaneously and, in addition to the phosphate melt, forms a gas separation layer between the workpiece and the tool, which significantly improves the lubricating effect. It is therefore not necessary, as before, to use graphite as an additional lubricant to improve the high-temperature properties of the lubricant. Carburization of the metal surface of the workpiece, as known from graphite-containing lubricants, does not occur with the lubricant of the present invention. If the gas-forming additive has organic anions, such as oxalate, the carbon content of the lubricant during the thermal decomposition of the additive is still so low that carburization does not occur even then. Most of the carbon-containing anions, as well as the preferred inorganic carbonate, but also some organic anions, form unreactive carbon dioxide as a carbon-containing reaction product during thermal decomposition, so that carbon does not form in the free form.

Another advantage of the lubricant of the present invention is the high temperature range in which the lubricant is in the liquid state. It starts to melt at 200 to 250 ° C and is available as a clear melt from 500 ° C. This ensures good lubrication properties even at low temperatures, such as those that occur when the machine tools start up.

A particularly advantageous property of the lubricant of the present invention is its descaling effect. The composition of the lubricant has the property of loosening scale on the metal surface of the workpiece, thereby removing it and thus providing improved end products.

One advantage of the present lubricant, which is particularly useful when machining workpieces that are difficult to access, such as seamless pipes, is the good water solubility of the lubricant. After the workpiece has been machined, the lubricant can be washed off without great technical effort, without any lubricant residues remaining.

In a preferred embodiment of the lubricant of the present invention, the gas-forming additive contains at least one carbonate of at least one divalent metal, preferably an alkaline earth metal. The use of is particularly useful Calcium carbonate as a gas-forming additive.

Calcium carbonate causes the formation of a gas cushion between the tool and the workpiece during the mechanical processing of a workpiece, which greatly reduces the force required for the intended forming. Calcium carbonate is also very soft and has good polishing properties when solid. Undecomposed portions of calcium carbonate thus have additional separating properties, which further improve the separating capacity of the phosphate melt and further reduce the frictional resistance between the workpiece and the tool. The polishing properties of the solid calcium carbonate smooth the surface of the workpiece without sanding it and without creating scratches and scratches. This is particularly advantageous at the start of the machining process of the workpiece, since scale particles are loosened and mechanically removed.

The gas-forming additive is expediently present in the lubricant of the present invention as a powder, preferably with a grain size of 2 to 50 μm, particularly preferably with a grain size of 2 to 30 μm. This special choice of grain size has proven to be very useful since it has a particularly favorable influence on the thermal decomposition properties and the polishing effect of the gas-forming additive.

It is particularly favorable if the gas-forming additive is contained in the lubricant in an amount of 5 to 50% by weight, preferably 20 to 40% by weight, particularly preferably 25 to 30% by weight. The amount of gas-forming additive depends essentially on the processing process, the processing temperature and the material to be processed.

In a further preferred embodiment of the present invention, the lubricant contains a phosphate mixture, corresponding to 55 to 69% by weight of P ₂ O ₅ , 14 to 45% by weight of Na ₂ O, 5 to 27% by weight of K ₂ O.0 up to 10% by weight MO, where M is a divalent metal, preferably zinc, manganese and / or magnesium. It is particularly suitable if the proportion of MO corresponds to 0 to 5% by weight. Such a phosphate mixture has all the desired properties, such as water solubility and a low melting point of 200 to 250 ° C, if the phosphates are used as monophosphates. Up to about 400 ° C, the phosphates in the melt are mainly in the form of diphosphates in addition to mono- and higher-condensed phosphates. At about 500 ° C a clear melt with excellent lubricating properties is formed. For higher temperatures, it has proven expedient to replace small amounts of sodium and / or potassium with equivalent amounts of one or more divalent metals, in particular magnesium, zinc or manganese. The addition of divalent metals increases the adhesion of the lubricant on the metal surfaces to be machined. The divalent metals are also expediently added to the mixture as phosphates. Trivalent metals can advantageously also be present, but because of their poor solubility these should only be present in small amounts, ie below 1%, preferably below 0.2%.

Another preferred embodiment of the lubricant of the present invention additionally contains 1 to 10% by weight of solid lubricants, preferably 1 to 5% by weight, based on the weight of the gas-forming additive. Zinc pyrophosphate, iron pyrophosphate and / or boron nitrite are preferably used as solid lubricants.

The lubricant of the present invention is conveniently used by applying the mixture in powder form to the tool or workpiece. In order to achieve a particularly uniform distribution of the lubricant, the powder can also be suspended in water and the surfaces to be lubricated can be coated or sprayed with it. However, it is also possible to heat the workpieces to be machined in a melt pool of the lubricant and then to process them. Other forms of application, such as powder compacts or lubricating pins, are possible and appropriate depending on the application. It is also particularly favorable to immerse the workpiece or the tool in the lubricant mixture when it is hot or to roll it off in order to achieve a coating.

The lubricant of the present invention has proven to be particularly suitable for use in planetary cross-rolling mills or in other longitudinal rolling processes, for example for the production of seamless tubes. In the manufacture of such pipes, for example, a perforated block (slug) is inserted into a planetary cross-rolling mill together with a mandrel bar. As the die is reduced in diameter, it moves axially in the rolling direction. By using the lubricant of the present invention, the force required for the forming of the metallic workpiece was considerably reduced in this process and the subsequent cleaning of the resulting product was made easier since it only had to be rinsed out with water. The lubricant according to the invention was applied before the mandrel rod was threaded in by blowing the powdered lubricant into the perforated hole using a suitable device. Compared to conventional lubricants, a significant acceleration of the rolling process was achieved by using the lubricant according to the invention. Compared to graphite-based or other carbon-containing lubricants, a significant reduction in the carburization of the material was achieved, especially on the inner surfaces of the rolled tubes. This worked out Particularly noticeable when the pipes should be deformed further, because the embrittlement of the material surface caused by the carburization led to cracking and roughening of the material surface with further deformations and made such workpieces unusable.

Further advantages, features and possible uses of the present invention will become clear from the following description of preferred exemplary embodiments.

example 1

• a) 50 Teilen Calciumcarbonat und
  50 Teilen eines Phosphatgemisches,
• b) 25 Teilen Calciumcarbonat und
  75 Teilen eines Phosphatgemisches,
• c) 5 Teilen Calciumcarbonat und
  95 Teilen eines Phosphatgemisches

hergestellt, wobei das Phosphatgemisch

• 60 Gew.-% P₂O₅,
• 20 Gew.-% Na₂O,
• 11 Gew.-% K₂O,
• 3 Gew.-% ZnO
• 3 Gew.-% MnO
• 3 Gew.-% MgO

enthielt.High temperature lubricants were made

• a) 50 parts of calcium carbonate and
  50 parts of a phosphate mixture,
• b) 25 parts of calcium carbonate and
  75 parts of a phosphate mixture,
• c) 5 parts of calcium carbonate and
  95 parts of a phosphate mixture

prepared, the phosphate mixture

• 60% by weight P ₂ O ₅ ,
• 20% by weight Na ₂ O,
• 11% by weight K ₂ O,
• 3% by weight ZnO
• 3% by weight MnO
• 3 wt% MgO

contained.

These mixtures according to the invention were used in the manufacture of seamless tubes in a planetary cross-rolling mill with three conical rolls in a strongly convergent arrangement. Both the mandrel bar used for the production of the tubes and the hot blanks were treated with the lubricants according to the invention before the mandrel bar was threaded into the perforated blanks.

For comparison, the same procedure was carried out with a known graphite-containing high-temperature lubricant.

• 85 Teile Graphit
• 10 Teile Vinylacetat-maleinsäurebutylester (Mowilith-Typ, Hoechst)
• 5 Teile Hydroxyethylzellulose (Tylose, Hoechst)
  Der Vergleichsschmierstoff wurde als wäßrige Suspension der vorgenannten Zusammensetzung in einer Konzentration von 25 bis 30 Gew.-% angewendet.

Composition of the comparative lubricant:

• 85 parts of graphite
• 10 parts vinyl acetate-maleic acid butyl ester (Mowilith type, Hoechst)
• 5 parts of hydroxyethyl cellulose (Tylose, Hoechst)
  The comparison lubricant was used as an aqueous suspension of the aforementioned composition in a concentration of 25 to 30% by weight.

Subsequently, the rolled tubes were further shaped into elbows and the mechanical properties of the processed materials were compared. When the graphite-containing lubricant was used, carburization caused embrittlement of the material surface due to the high graphite content, which led to cracking and roughening of the material during the deformation and mechanical aftertreatment, such as pulling and cold pilgrimage. The workpiece had to be discarded.

No carburization occurred when the lubricants according to the invention were used, and no cracks, embrittlement or roughening were evident.

In addition, the lubricants according to the invention were distinguished in that less force was required by the tool when the perforated blanks were formed into seamless tubes than with the graphite-containing lubricant.

Example 2

• 10 Teilen Calciumcarbonat,
• 85 Teilen eines Phosphatgemisches, entsprechend dem aus Beispiel 1, und
• 5 Teilen Hydroxyethylzellulose hergestellt

A high temperature lubricant was made

• 10 parts calcium carbonate,
• 85 parts of a phosphate mixture, corresponding to that from Example 1, and
• 5 parts of hydroxyethyl cellulose

The hydroxyethyl cellulose serves as an organic-based thickener. It advantageously delays that sedimentation of calcium carbonate in the aqueous suspension. Instead of hydroxy cellulose, other alkyl celluloses, alginates, polysaccharides or mixtures thereof are also suitable.

When compared to the lubricants from Example 1, the mixture described was distinguished by a higher viscosity and thus lower flowability. This resulted in a particularly good adhesion to the surfaces to be machined and a lower outflow of molten lubricant from the spaces between the workpiece and the tool.

Claims (13)

Lubricant for the hot forming of metals with at least one alkali metal phosphate and optionally additionally at least one phosphate of a divalent metal, characterized in that it contains at least one additive which forms a gas when heated. Lubricant according to claim 1, characterized in that the gas-forming additive is inorganic. Lubricant according to one of claims 1 or 2, characterized in that it contains at least one inorganic carbonate as gas-forming additive. Lubricant according to one of claims 1 to 3, characterized in that it contains at least one carbonate of at least one divalent metal, preferably an alkaline earth metal, as the gas-forming additive. Lubricant according to one of claims 1 to 4, characterized in that it contains calcium carbonate as the gas-forming additive. Lubricant according to one of claims 1 to 5, characterized in that the gas-forming additive is contained as a powder, preferably with a grain size of 2 to 50 µm, particularly preferably with a grain size of 2 to 30 µm. Lubricant according to one of claims 1 to 6, characterized in that it contains the gas-forming additive in an amount of 5 to 50% by weight, preferably 20 to 40% by weight, particularly preferably 25 to 30% by weight. Lubricant according to one of claims 1 to 7, characterized in that it corresponds to a phosphate mixture 55 to 69% by weight of P ₂ O ₅ , 14 to 35% by weight Na ₂ O, 5 to 27% by weight of K ₂ O, 0 to 10 wt% MO where M is a divalent metal, preferably zinc, manganese and / or magnesium. Lubricant according to claim 8, characterized in that the proportion of MO corresponds to 0 to 5% by weight. Lubricant according to one of claims 8 or 9, characterized in that the phosphate mixture is contained in an amount of 50 to 95% by weight, preferably 60 to 80% by weight, particularly preferably 70 to 75% by weight. Lubricant according to one of claims 1 to 10, characterized in that it additionally contains 1 to 10% by weight of solid lubricants, preferably 1 to 5% by weight, based on the weight of the gas-forming additive, preferably zinc pyrophosphate, iron pyrophosphate and / or boron nitrite . Use of a lubricant according to one of claims 1 to 11 for the hot forming of metals in a rolling mill. Use of a lubricant according to claim 12, for lubricating mandrel bars and / or slugs.