Method for processing a metal workpiece

Abstract

The invention relates to a method for processing a metal workpiece. The method comprises the following known steps: spraying a powder-type auxiliary material (2) onto the workpiece (20) using a spraying device (10), wherein, when exiting the spraying device, the auxiliary material is at least partially melted, before it lands on the surface of the workpiece. According to the method of the invention, in order to be able to easily apply an easily removeable scale-inhibiting coating to the workpiece, the workpiece (20) has a temperature that is lower than the melting point of the powder-type auxiliary material, preferably below 50°C, before it is sprayed with the auxiliary material (2), the powder-type auxiliary material has an oxidation-inhibiting effect, and a homogeneous coating of the auxiliary material is formed on the workpiece by melting the auxiliary material during spraying.

Description

[0001] Method for machining a metallic workpiece

[0002] The invention relates to a method for machining a metallic workpiece.

[0003] In steel production, workpieces such as billets from continuous casting or hollow blocks are often produced in stockpiles for further processing in an extrusion press or radial forge, and only later are they returned to a hot forming process. These workpieces must then be heated to forming temperature in a furnace. This can take several hours, during which a very thick oxide layer often forms on the surface of the workpieces. This oxide layer, also called scale, represents a material loss. Depending on the material, this can amount to 1 to 3 percent of the workpiece's weight.

[0004] In order to prevent the formation of such a thick oxide layer, it is generally known in the prior art to apply, in particular, solid powdered auxiliary materials to the surface of the workpiece. The known auxiliary materials are, for example, in powder form and only develop their desired oxidation-inhibiting effect once they have at least partially melted. Therefore, a known method for applying such powdered auxiliary materials to the surface of a workpiece consists in heating the workpiece, at least on its surface, to a temperature that corresponds to the melting point of the auxiliary material. Melting points of the known auxiliary materials are above 400°C up to 1,000°C. The auxiliary material then becomes liquid and wets the surface. The auxiliary material then forms a protective film on the surface, which protects the surface of the workpiece from oxidation, i.e. the formation of scale.Typical applications for this process include the glazing of billets for extrusion presses, or the deoxidation of hollow blocks for seamless tubes, etc. The necessary heating of the workpiece to the aforementioned high temperatures is costly and time-consuming.

[0005] Another well-known application for applying powdered additives to the surface of a metallic workpiece is thermal spraying, which particularly includes the well-known powder flame spraying process; see Wikipedia. In this process, powdered additives are at least partially melted in a burner flame and then fired at very high speeds onto the surface of the workpiece to be coated. Depending on the additive being applied and the process, the speeds vary from 70 m / s to 800 m / s. Due to this high speed, the particles of the additive deform upon impact with the surface of the workpiece and become embedded in it. This creates a very strong bond between the layer of additive and the surface of the workpiece.The deposition rate and thus the resulting layer thickness of the additive on the workpiece surface are very low, typically only a few microns per spray pass, and the surface must be specially treated beforehand. Flame spraying is a classic process for coating workpieces with metals or oxides.

[0006] However, this known powder flame spraying process is not suitable for applying an oxidation-inhibiting auxiliary material, as provided for in the process according to the invention. This is because, firstly, the present invention does not desire such a strong bond between the layer of auxiliary material and the workpiece. Rather, the layer of auxiliary material applied according to the invention is intended to be merely a "one-time application" that is intended to be partially or completely removed from the surface of the workpiece during a subsequent forming or preparation process. Secondly, the layer thickness achievable with powder flame spraying per spray pass is too small for certain planned subsequent processing steps of the workpiece.

[0007] The invention is based on the object of applying a scale-inhibiting coating to the workpiece in a simple and easily removable manner using a known method for machining a workpiece.

[0008] This object is achieved by the method according to patent claim 1.

[0009] This process is characterized in that the workpiece has a temperature which is lower than the melting point of the powdered auxiliary material - preferably below 50°C - before it is sprayed with the auxiliary material; that the powdered auxiliary material has an oxidation-inhibiting effect; and that a homogeneous coating of the auxiliary material is formed on the workpiece by the melting of the auxiliary material during the spraying of the auxiliary material.

[0010] For the application of the method according to the invention, the workpiece to be coated advantageously does not need to be heated beforehand; room temperature is typically sufficient. In particular, the workpiece does not need to be heated to the melting point of the auxiliary material to be applied, as is the case with other methods for applying an auxiliary material to a workpiece - especially to a metallic workpiece. This initially saves considerable costs, because heating the workpiece to the melting temperature of the auxiliary material is generally considerably more energy-intensive than the corresponding heating of the auxiliary material. The claimed method can also be used in systems that categorically exclude the use of liquid auxiliary materials, such as the coating of lances in a converter to reduce adhesion.The present invention claims that the sprayed-on auxiliary material has an oxidation-inhibiting effect on the metallic workpiece. This advantageously ensures that, when the metallic workpiece is heated to the forming temperature in a furnace for a subsequent hot forming process, significantly less scale forms than would be the case without the oxidation-inhibiting layer according to the invention on the workpiece. This also means correspondingly less material loss, because the scale that forms on the workpiece during heating ultimately means material loss because it typically has to be removed before further forming. Because only a small amount of scale is formed due to the applied oxidation-inhibiting auxiliary material, the time required to clean the workpiece or to remove any residual scale that does form is also reduced.

[0011] According to one embodiment of the invention, a gas mixture consisting at least of the auxiliary material, ambient air, and a propellant gas is heated at least to the melting temperature of the auxiliary material contained therein, such that the auxiliary material contained therein at least partially melts. The gas mixture can be heated either with the aid of a heat source without flame formation or with a burner with flame formation. With the heat source without flame formation, e.g. in the form of an electrically operated heating coil, the gas mixture flow does not necessarily experience additional translational energy in a conveying direction during heating. This is different, in particular, if the heating is carried out with the aid of the burner with flame formation; in this case, an acceleration of the gas mixture towards the outlet of the injection device can occur due to an explosion of the gas mixture.In this respect, the method chosen for heating the gas mixture to the melting temperature of the auxiliary material also influences its exit velocity from the spray device. In the event of an explosion or flame formation, the exit velocity is significantly higher than without flame formation. The powder is transported to the outlet of the spray device by means of a propellant gas. Therefore, the flow of the propellant gas must be adjusted to the respective amount of auxiliary material. Furthermore, when using a burner, the burner gas flow must be adjusted to the amount of propellant gas to ensure that the gas mixture remains ignitable.

[0012] According to a further embodiment, the spray device is designed in the form of a Venturi nozzle. The use of the Venturi nozzle offers the advantage that the propellant gas only needs to be introduced into the Venturi nozzle at a significantly lower initial volume flow in order to achieve a significantly larger volume flow of the gas mixture at the Venturi nozzle outlet. Due to its design, the Venturi nozzle advantageously functions as a volume flow amplifier.

[0013] According to a further embodiment of the invention, 10 g to 80 g of the at least partially melted auxiliary material can be applied to the surface of the workpiece per second, and / or the liquid or pasty particles of the melted auxiliary material impact the surface of the workpiece at a particle velocity of only 4 m / sec. to 20 m / sec. This required application quantity of the auxiliary material per second onto the surface, or the low particle velocity compared to other application methods for auxiliary materials to metallic workpieces, advantageously increases the application efficiency because, firstly, fewer particles bounce off the surface of the workpiece, and secondly, the particles have more time to heat up in the spray device.In this respect, the required application rate per second and the low particle velocity enable the formation of thicker layers of the auxiliary material on the workpiece than was possible with the prior art. The distance from the spray device outlet to the surface of the workpiece when spraying the auxiliary material is between 15 cm and 60 cm in the process according to the invention. This distance depends on whether the heat source without flame formation or the burner with flame formation is selected for heating the auxiliary material, and in the latter case also on the flame length and the amount of burner gas used.

[0014] Advantageously, the additive applied using the method according to the invention not only has the aforementioned oxidation-inhibiting effect, but also the ability to remove existing oxidation, in particular existing scale, from the surface of the workpiece, and / or a lubricating effect. Depending on which of the aforementioned properties the additive used has, the amount of additive applied to the surface of the workpiece is between 15 g / m 2 and 400g / m 2 .

[0015] In principle, any technical deoxidizer or lubricant containing a proportion of meltable substances can be used as an excipient. The melting temperature typically ranges from 500°C to 1,200°C. Non-meltable components of the excipient are incorporated into the molten particles if necessary. Common base materials in excipients are phosphates, borates, or silicates, which can optionally be enriched with soap and / or graphite, etc.

[0016] The effect of the applied oxidation-inhibiting additive on the workpiece is all the better the cleaner the surface of the workpiece is before the additive is applied. It is therefore particularly recommended that any oxidation present on the surface of the workpiece, in particular any existing scale, be removed from there before the additive is applied according to the method according to the invention. This improves the effectiveness of the scaling prevention by the oxidation-inhibiting additive in a reheating furnace. Finally, it should be mentioned that the method according to the invention can also be advantageously used where previously the workpiece had to be heated before an additive could be applied, as is the case, for example, in extrusion, where a billet is coated with a layer of glass.

[0017] Two figures are attached to the description, where

[0018] Figure 1 shows the method according to the invention according to a first variant; and

[0019] Figure 2 illustrates the method according to the invention according to a second variant.

[0020] The invention is described in detail below with reference to the figures mentioned in the form of exemplary embodiments. In both figures, identical technical elements are designated by identical reference numerals.

[0021] Figure 1 shows a spray device 10 in the form of a Venturi nozzle 12 with a heat source 16'. The outlet of the Venturi nozzle 12 and the heat source 16' are connected via a feed line 14. At the heat source 16', a gas mixture is heated without flame formation. The heat source 16' is designed, for example, in the form of an electrically operated heating coil. The heat source 16' forms the outlet of the spray device 10 for the heated gas mixture.

[0022] The method according to the invention is carried out with this spray device 10 as follows: First, the auxiliary material 2 and ambient air 3 are sucked into the spray device 10, where they are mixed with a propellant gas 5, e.g., compressed air, to form a gas mixture 7. Driven by the kinetic energy of the propellant gas, the gas mixture 7 flows as a continuous gas mixture stream in a conveying direction FR to the outlet of the spray device 10, where it flows out.

[0023] When the powdered auxiliary material 2 is sucked into the spray device 10, it typically, but not necessarily, has the temperature of the ambient air 3 sucked in along with it. Only in the spray device, preferably at its outlet, is the gas mixture then heated with the aid of the heat source 16' without flame formation to at least the melting temperature of the auxiliary material 2 contained in the gas mixture, so that this auxiliary material at least partially melts in the gas mixture. After melting, the auxiliary material 2 is contained in the gas mixture in the form of liquid or pasty particles.

[0024] The spraying device 10 sprays the gas mixture 7 emerging at its outlet, containing the molten particles of the auxiliary material 2, onto the surface of the workpiece 20. The previously at least partially molten auxiliary material 2 forms a homogeneous coating on the surface of the workpiece 20, as shown in Figure 1. In the method according to the invention, the workpiece 20 is not preheated to a specific temperature before the application of the auxiliary material, but is then typically at room temperature or ambient temperature, i.e. generally a temperature below 50°C. In particular, the workpiece does not have to be heated to a temperature above the melting point of the applied powdered auxiliary material because the molten auxiliary material has already taken place in the spraying device 10 and not on the workpiece 20.Because the sprayed-on auxiliary material has an oxidation-inhibiting effect in the process according to the invention, the formation of oxidation, in particular a scale layer on the coated surface of the workpiece, is effectively reduced, if not prevented, during subsequent heating of the workpiece.

[0025] When spraying the auxiliary material, the spraying device is typically at a distance of between 15 cm and 60 cm from the surface of the workpiece 20. If a Venturi nozzle 12 is used, the propellant gas is introduced into the Venturi nozzle 12 at an initial volume flow of, for example, 150 l / min. to 300 l / min. The Venturi nozzle amplifies this volume flow so that the gas mixture exits the Venturi nozzle at a volume flow of 800 l / min. to 4,000 l / min. With the gas mixture flow, 10 g to 80 g of the at least partially melted auxiliary material per second are applied to the surface of the workpiece. In other words, the liquid or pasty particles of the melted auxiliary material 2 apply to the surface of the workpiece 20 at a particle speed of 4 m / sec. to 20 m / sec.At this low particle speed compared to other application methods, only a few particles advantageously bounce off the workpiece and therefore a greater layer thickness can be achieved on the workpiece with the method according to the invention.

[0026] In addition to the aforementioned oxidation-inhibiting effect, the auxiliary substance 2 can also be suitable for removing existing oxidation, particularly existing scale, on the surface of the workpiece. Furthermore, it preferably also has a lubricating effect.

[0027] In principle, the amount of the auxiliary material applied to the surface of the workpiece 20 is between 20 g / m 2 and 400 g / m 2This specific amount of excipient also represents the layer thickness with which the excipient is applied to the surface of the workpiece. The applied amount of excipient ultimately depends on the effects and properties of excipient 2. Thus, the applied amount of excipient is 15 g / m 2 up to 60 g / m 2 , if the auxiliary substance only has the oxidation-inhibiting effect. If the auxiliary substance 2, in addition to the oxidation-inhibiting effect, also has the property of removing existing oxidation on the workpiece 20, it is preferably used in an amount of 60 g / m 2 up to 120 g / m 2 sprayed onto the workpiece. If the auxiliary material 2, in addition to the oxidation-inhibiting effect and the property of removing existing scale on the workpiece, also has the aforementioned lubricating effect, the method according to the invention recommends applying the auxiliary material in an amount of 120 g / m2 to 400 g / m2 to be applied to the surface of the workpiece.

[0028] In order to apply the stated quantities of the auxiliary material to the surface of the workpiece 20, the above-mentioned gas mixture volume flow of 800 l / min. to 4,000 l / min. is required at the outlet of the Venturi nozzle or the spray device 10.

[0029] The amount of the required initial volume flow of the propellant gas must be adapted to the desired amount of auxiliary material to be applied. The amount or volume flow with which the auxiliary material is introduced into the spray device 10 and the desired resulting layer thickness on the surface of the workpiece depend on the type of auxiliary material and the objective to be achieved by coating the workpiece. The propellant gas 5 is preferably introduced into the spray device in the form of a continuous volume flow in order to achieve a continuous volume flow of the gas mixture at the outlet of the spray device and thus to enable a uniform application of the auxiliary material to the workpiece.

[0030] The at least oxidation-inhibiting excipients used according to the invention are, for example, phosphates, borates, or silicates, or a mixture thereof. Optionally, soap and / or graphite can also be present in the excipient. The melting point of the excipient according to the invention lies in a temperature range between 500°C and 1,200°C. Accordingly, the gas mixture must be heated to a temperature within this temperature range using the heat source in order to at least partially melt the excipient 2.

[0031] The method according to the invention can in principle be carried out on its own, i.e. without any prior machining of the workpiece in a timely manner and without any subsequent machining of the workpiece in a timely manner. However, the method is preferably integrated into an overall machining process for the workpiece, in which prior pre-machining of the workpiece and / or subsequent machining of the workpiece can take place. Prior machining of the coated workpiece can in particular consist in heating the coated workpiece in the reheating furnace to a predetermined forming temperature and subsequently forming it. In particular during the said heating of the workpiece, the applied layer of the auxiliary material then exerts its oxidation-inhibiting effect, i.e. scale formation during heating is reduced.

[0032] Figure 2 shows a second variant of the method according to the invention and of the spray device 10 according to the invention. The spray device according to Figure 2 differs from the first variant shown in Figure 1 in that the gas mixture is heated at the outlet of the spray device 10 with the aid of a burner 16. In order for the burner 16 to function, it requires a fuel, in particular a fuel gas 6. The fuel gas 6 is sucked into the spray device 10 together with the auxiliary material 2 and the ambient air and thus becomes a component of the gas mixture 7 in the spray device. Alternatively or additionally, the fuel gas 6 can also be supplied to the burner directly at its position, i.e. at the outlet of the spray device 10, as shown in Figure 2. In both cases, the fuel gas, which can be propane gas, natural gas or hydrogen, for example, is ignited in the burner 16 to form a flame 8.The auxiliary material 2 contained in the gas mixture 7 is then melted in the flame 8 and sprayed onto the surface of the workpiece 20. Both in the first variant shown in Figure 1 and in the second variant shown in Figure 2 for heating the auxiliary material 2 in the gas mixture, the auxiliary material 2 cools very quickly on the surface of the workpiece 20, so that, at least by the aforementioned application of the auxiliary material alone, there is no significant heating of the workpiece.

[0033] List of reference symbols

[0034] 2 Auxiliary material 3 Ambient air

[0035] 5 Propellant gas, e.g. compressed air

[0036] 6 Fuel gas

[0037] 7 Gas mixture

[0038] 10 Spray device 12 Venturi nozzle

[0039] 14 Conveyor line

[0040] 16 burners as heat source

[0041] 16' heat source without flame formation

[0042] 20 workpiece

[0043] FR conveying direction

Claims

Patent claims:

1. Method for machining a metallic workpiece (20), comprising the following steps, Spraying a powdered auxiliary material (2) onto the workpiece (20) using a spraying device (10), wherein the auxiliary material (2) is at least partially melted upon exiting the spraying device (10) before impacting the surface of the workpiece (20); characterized in that the workpiece (20) has a temperature lower than the melting point of the powdered auxiliary material – preferably below 50°C – before it is sprayed with the auxiliary material (2); that the powdered auxiliary material (2) has an oxidation-inhibiting effect; and that a homogeneous coating of the auxiliary material (2) is formed on the workpiece by the melting of the auxiliary material (2) during the spraying process.

2. Method according to claim 1, characterized in that at least the auxiliary substance (2) and ambient air (3) are mixed in the spray device (10) with a propellant gas (5), e.g. compressed air, to form a gas mixture (7); and that the gas mixture flows as a continuous gas mixture stream - caused by the propellant gas (5) - in a conveying direction (FR) to the exit of the spray device (10).

3. Method according to claim 2, characterized in that the gas mixture (7) in the spray device (10) is at least applied to the The gas mixture stream is heated to its melting temperature (2) so that the auxiliary substance (2) contained therein melts at least partially; the heating of the gas mixture stream is carried out with a heat source (16') without flame formation, e.g. in the form of an electrically operated heating coil, and / or with a burner (16) with flame formation; and the heat source (16') without flame formation and / or the burner (16) is preferably arranged at the outlet of the spray device (10).

4. Method according to one of claims 2 to 3, characterized in that the spray device (10) has a Venturi nozzle (12); and that the propellant gas (5) is introduced into the Venturi nozzle with an initial volume flow of 150 - 300 ltr / min to generate a gas mixture volume flow of 800 ltr / min to 4000 ltr / min at the outlet of the Venturi nozzle.

5. Method according to claim 3 or 4, characterized in that when using the spray device (10) with the burner (16) at its outlet, a burner gas (6) together with the auxiliary substance (2) is mixed with the propellant gas before the gas mixture stream reaches the burner (16) of the Venturi nozzle (12), and / or that the burner gas (6) is injected directly into the burner (16) at the outlet of the spray device (10), in each case preferably with a burner gas volume flow of 4kg / h to 50kg / h, particularly when using propane or natural gas as burner gas.

6. Method according to one of claims 2 to 5, characterized in that, that 10-80 grams of the at least partially melted auxiliary material (2) are applied to the surface of the workpiece (20) per second by the gas mixture stream; and / or that the liquid or pasty particles of the melted auxiliary material (2) impact the surface of the workpiece (20) at a particle velocity of 4 m / s to 20 m / s. A method according to any of the preceding claims, characterized in that the distance between the outlet of the spray device (10) and the surface of the workpiece (20) during spraying of the auxiliary material (2) is between 15 cm and 60 cm. A method according to any of the preceding claims, characterized in that the auxiliary material (2), in addition to its oxidation-inhibiting effect, also possesses the property of removing existing oxidation, in particular existing scale, from the surface of the workpiece (20), and preferably also possesses a lubricating effect.A method according to one of the preceding claims, characterized in that the amount of the auxiliary material (2) applied to the surface of the workpiece (20) is between 15 g / m² and 400 g / m², preferably between 15–60 g / m² if the auxiliary material (2) only has an oxidation-inhibiting effect, or further preferably between 60 g / m² and 120 g / m² if the auxiliary material (2) has, in addition to the oxidation-inhibiting effect, also the property of removing the existing oxidation on the workpiece (20), or even more preferably between 120 g / m² and 400 g / m² if the auxiliary material (2) is oxidation-inhibiting and removes the existing oxidation. Oxidation on the workpiece (20) and additionally forms with the lubricating effect.

10. Method according to one of the preceding claims, characterized in that the auxiliary material (2) is, for example, phosphates, borates, silicates or a mixture thereof, optionally with the addition of soap and / or graphite.

11. Method according to one of claims 2 to 10, characterized in that the melting temperature of the auxiliary material (2) is in a temperature range between 500°C and 1200°C.

12. Method according to one of the preceding claims, characterized in that any oxidation present on the surface of the workpiece (20), in particular any existing scale, is removed from there before the auxiliary material (2) is applied.

13. Method according to one of the preceding claims, characterized by -Optional: Heating the workpiece (20) homogeneously coated with the auxiliary material to a predetermined forming temperature; and - forming the workpiece coated with the auxiliary material (2).

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