Method and apparatus for processing a billet of metal that is circular in cross-section by reducing the cross-section in the end solidification zone

Abstract

A method for processing a billet (17) of metal that is circular in cross-section by reducing the cross-section in the end solidification zone is described. The cross-section in the end solidification zone is reduced by means of at least three deforming tools distributed on the periphery and acting simultaneously on the billet (17). In order to provide favorable processing conditions, it is proposed that the billet (17) be deformed in a longitudinal section, which is at least one quarter of the diameter of the billet before the cross-section reduction, in each deformation stroke by a plurality of forging tools (2, 3) that form the deforming tools, and that the forging tools (2, 3) rotate by an angular pitch around the axis of the billet (17) between the plurality of deformation strokes.

Description

FIELD OF THE INVENTION

[0001] The present invention relates to a method for processing a round cross-section metal billet by reducing the cross-section in the end solidification zone, reducing the cross-section in the end solidification zone by means of at least three deformation tools distributed on the outer periphery and acting on the billet simultaneously, and the present invention also relates to a device for performing this method. BACKGROUND OF THE INVENTION

[0002] In the end solidification zone of a metal billet with a round cross-section, the core temperature drops significantly faster than the surface temperature of the billet, which leads to different thermal contractions and thus to segregation (Seigerung) and core porosity. To counteract the segregation and porosity phenomena, so-called soft reduction is carried out in the end solidification zone to reduce the cross-section. For this purpose, it is known from (DE101 44 234 A1, WO 2018 / 069854 A1) to use at least three deformation tools in the form of rollers distributed on the periphery of the billet and to perform soft reduction on the billet by means of the deformation tools, but only with moderate effects, because the depth of cross-section reduction is limited by using rollers, and a greater cross-section reduction increases the risk of cracking.

[0003] To avoid feeding difficulties during rolling of the billet due to splitting of the billet, (DE 10 2011 012 508A1) proposes to provide a ramp for the billet in the region of the later splitting by means of mutually opposed forging tools, i.e. before the core of the billet has completely solidified. Due to the partially liquid material, the forging force can be kept small. In addition, the cooling rate in the deformation zone can be increased. However, it is not expected to have a significant effect on core porosity.

[0004] To avoid segregation in billets of steel and metal alloys, it is known from (DE 27 33 276 A1) to plastically deform the cast billet during solidification, i.e. by rolling, so as to reduce the cross-sectional area of the billet according to the solidification shrinkage, thereby preventing the melt in the solidified billet from shifting upwards or downwards. However, the core structure remains substantially unaffected by it.

[0005] To improve splicing, it is known from (DE 197 00 486 A1) to subject the preform made from the billet to forging. However, the prerequisite is a solidified preform.

[0006] To ensure favorable extrusion conditions for workpieces with a circular cross-section in a radial press, (EP 0239 875 A2) it is known to support the pressure jaws on axially oppositely inclined wedge surfaces of two axially mutually adjustable adjusting bodies, such that in the opposite pressure loading of the two adjusting bodies, the pressure jaws forming a wedge drive with the adjusting bodies are radially displaced, and a deformation stroke is implemented. When the adjusting bodies are subsequently moved apart from each other, the pressure jaws guided along the wedge surfaces are radially pulled back to the starting position again. Due to the relatively small axial length compared to the extrusion stroke, these radial presses are particularly suitable for extruding hose fittings onto hydraulic pipes. Summary of the Invention

[0007] The technical problem to be solved by the present invention is to design a method for machining a circular billet by light pressing, so that core porosity can be avoided as much as possible without exposing the billet to the risk of cracking.

[0008] Starting from a method of the aforementioned type, the technical problem is solved according to the present invention in that the billet is deformed in a longitudinal section equal to at least one quarter of the diameter of the billet before cross-section reduction by a plurality of forging tools constituting a deformation tool in each deformation stroke, and the forging tools are rotated by an angular pitch or a step angle (Winkelschritt) around the axis of the billet between a plurality of deformation strokes.

[0009] By using a deformation tool in the form of a forging tool, the following prerequisite is first provided, that is, the depth effect of plastic deformation of the billet in the end solidification region is improved, that is, the effect that can be achieved by the axial extension of the forging tool on the corresponding longitudinal section of the billet. If the deformed longitudinal section is selected to be equal to one quarter of the diameter of the billet before cross-section reduction in each deformation stroke, then the influence on the improved quality of the remaining porosity of the billet is already obvious. This influence increases with the axial extension of the tool intervention, so that the longitudinal section of the billet detected by the forging tool in each deformation stroke is preferably in the range between 0.6 times the diameter of the billet and the diameter of the billet.

[0010] However, even when the hollow spaces formed during the solidification of the billet are squeezed together by the deformation stroke of the forging tool during success, the depth effect improved by the axial extension of the forging tool for the plastic deformation of the billet is not sufficient to prevent the core porosity of the casting billet as much as possible, especially. However, by rotating the forging tool around the billet axis between the deformation strokes relative to the casting billet, this is surprisingly successful without having to perform a greater cross-sectional reduction associated with the risk of cracking. The effect associated with the gradual rotation of the forging tool around the billet axis is explained in the following way, that is, the core region with hollow spaces is repeatedly subjected to shear and extrusion stresses in different directions due to the helical billet processing, so that the hollow spaces forming holes can be gradually reduced until they disappear.

[0011] However, the rotation pitch or rotation step amount of the forging tool between the deformation strokes cannot be chosen too small, because otherwise the influence of the forces acting on the volume elements from different directions will be lost. The rotation pitch favorable for the casting billet between the respective deformation strokes can be determined relatively simply by simulation calculation or practical attempts, and can be preset especially according to the casting speed, the number of forging tools, the cross-sectional reduction, the frequency of the deformation strokes, and the material properties.

[0012] If it is assumed that, corresponding to the action length of the forging tool in the longitudinal section of the casting billet, the casting billet is to be processed over its entire circumference, then for the minimum processing in the circumferential region of 360°, a rotation pitch is generated between the respective deformation strokes corresponding to the action length of the forging tool during the billet feed, and the rotation pitch depends on the billet feed amount between the deformation strokes and the action length of the forging tool, because after the casting billet is fed corresponding to the action length of the forging tool, the billet processing must be carried out on the outer peripheral section belonging to each tool. This means that the outer peripheral sections belonging to each forging tool must be divided according to the billet feed amount until the total feed amount is reached and according to the action length of the forging tool in order to determine the angle preset as the lower boundary for the processing of the casting billet on 360°. Therefore, for the minimum angle of the rotation pitch, a pitch of the helical path of the processing is generated by the multiple of the action length of the forging tool corresponding to the number of forging tools. When the billet feed between the deformation strokes is 54 mm and the action length of the forging tool is 350 mm, therefore, when using three forging tools, the minimum rotation pitch angle between two deformation strokes is calculated as 18°, while when using four forging tools it is calculated as 14°.

[0013] Since the deep action of the forging tools is important for suppressing core porosity, it must be noted that the cross-sectional shape of the billet is maintained as much as possible during soft reduction. Therefore, the billet is supported sufficiently in the center by the forging tools distributed on the outer periphery of the billet during the deformation stroke. Advantageously, the billet is deformed by these forging tools in each deformation stroke in a circumferential region of at least 20° related to the average width of the contact surface between the forging tool and the billet, which is subdivided among the individual forging tools, i.e., the forging tool acts on the billet on the outer peripheral section assigned to the forging tool and extending at least 20°.

[0014] For soft reduction, the billet must be processed in the end solidification region, i.e., advantageously on both sides of the bottom tip. The billet is processed in a longitudinal section by the forging tools, which extends from the cross-section of the billet with a solid phase ratio of 80% to the cross-section with a temperature difference of 300 K between the core and the surface. Thus, the preconditions for implementing the soft reduction according to the invention by means of the forging tools are generally well satisfied. The cross-section of the billet should be reduced by at least 8% by the forging tools, so that a significant influence on the core porosity can be exerted.

[0015] To implement a method for processing a billet of metal that is circular in cross-section by reducing the cross-section in the end solidification region, a device is proposed here, which has at least three forging tools that are arranged rotationally symmetrically with respect to the forging axis and are supported in a bracket of a forging press and are connected to a drive for achieving a radial deformation stroke with respect to the forging axis. In such a device, the bracket is rotatably supported about the forging axis in a housing and is connected to a step drive for rotating by one angular pitch each time between a plurality of deformation strokes. Thus, all the preconditions for implementing the method are satisfied. A radial deformation stroke can be implemented by means of the forging tools, and by rotating the bracket accommodating the forging tools about the forging axis between the deformation strokes, a gradual soft reduction of the billet along a spiral can be ensured.

[0016] The bracket has two axially movable adjusting disks, which form axially outwardly inclined wedge surfaces for a wedge drive of a radial stroke drive. Thus, particularly simple design conditions are created in this case. The forging tools slidably supported on the wedge surfaces with corresponding mating surfaces are thus radially displaced in the opposite adjustment of the adjusting disks.

[0017] Although synchronous movement of the forging tool and the strand is not mandatory during forging due to the typical frequency of the deformation stroke associated with relatively low casting speeds, the forging tool can be moved together with the strand during the deformation stroke in order to return to its initial position during the idle stroke. To this end, the holder that holds the forging tool is axially displaceable in the housing and is connected to an axial adjustment drive. The housing of the forging press itself can be displaced along the strand guide, thereby allowing the forging press to be aligned with the strand's final solidification region. This allows for displacement of the final solidification region due to changing casting parameters to be taken into account. BRIEF DESCRIPTION OF THE DRAWINGS

[0018] The technical solution of the present invention is shown in the accompanying drawings for example. In the accompanying drawings:

[0019] Figure 1 A schematic longitudinal section of a forging press according to the present invention for processing a round, metallic ingot is shown;

[0020] Figure 2 Shows the device according to Figure 1 a cross-sectional view on a smaller scale taken along line II-II of FIG.

[0021] Figure 3 The diagram shows a schematic distribution of the liquid and solid phases over the length of the strand until complete solidification and in the core and surface regions. DETAILED DESCRIPTION

[0022] according to Figure 1 and 2 The forging press has a housing 1 in which a support 4, which accommodates a pair of opposing forging tools 2 and 3, is rotatably mounted about a forging axis 5. The support 4 is formed by two adjusting disks 6, which are mounted axially displaceably in the housing 1 and can be loaded by means of a piston 7 against the force of a return spring 8 supported between the two adjusting disks 6. The piston 7 is embedded in an annular space 9 formed in the housing 1 and can be loaded with a hydraulic medium.

[0023] The two adjusting disks 6 are provided with wedge-shaped surfaces 10 that are inclined in opposite directions to each other. The forging tools 2, 3 are slidably abutted against the wedge-shaped surfaces by means of corresponding mating surfaces, thereby generating a corresponding wedge drive between the adjusting disks 6 and the forging tools. The forging tools 2, 3 are provided with guide bars 11 in the edge region of the wedge-shaped surface 10, and the guide bars are inserted into the guide grooves 12 of the adjusting disks 6. If the adjusting disks 6 are loaded with pressure by the piston 7, then they move relative to each other, which effectively causes the forging tools 2, 3 to slide relative to the wedge-shaped surfaces 10 of the adjusting disks 6, and the forging tools 2, 3 perform a radial deformation stroke. By unloading the piston 7, the adjusting disks 6 move back to the starting position by means of the return spring 8, wherein the forging tools 2, 3 guided along the wedge-shaped surfaces 10 perform an idle stroke.

[0024] The support 4 including the two adjusting disks 6 can be rotated by a pitch between the deformation strokes of the wedge drive by means of a stepping drive 13. According to the illustrated embodiment, the stepping drive 13 includes a gear 15 driven by a stepping motor 14, and the gear meshes with a chain gear 16, and the chain gear is connected to one of the two adjusting disks 6. However, any other rotational stepping drive is also possible. Thus, the billet 17 can be reduced in terms of the cross-section of the billet by the forging tools 2, 3 that are opposed to each other in pairs, wherein between the deformation strokes, the support 4 rotates by a pitch in order to ensure step-by-step machining of the billet 17 around the forging axis 5 along the screw.

[0025] As can be seen from Figure 3 it, the billet 17 is effectively cooled during casting, so that a solid shell 18 is formed around the liquid core 19. After the casting region 20 is a cooling region, in which cooling liquid is sprayed onto the billet 17. Thereby, a gradual solidification of the billet 17 from the outside to the inside occurs, wherein a mixed phase 21 is formed between the liquid core 19 and the solid shell 18, and the mixed phase flows out at the bottom (Sumpf) 22, and the tip 23 of the mixed phase exhibits complete solidification of the billet 17. The solid-phase ratio in the region of the bottom 22 therefore increases from 0% to 100% according to the curve 24. In Figure 3 it, the solid-phase ratios of 0%, 20%, 80% and 100% and the relevant positions of the billet cross-sections Q1, Q2, Q3 and Q4 are shown.

[0026] Due to the external cooling of the billet 17, the surface temperature of the billet 17 is according to Figure 3The curve 25 changes. The core temperature is represented by curve 26. It is confirmed that in the end solidification region, the core temperature 26 drops significantly faster than the surface temperature 25. This temperature gradient can be advantageously used to determine the longitudinally profiled section of the strand 17 that is suitable for soft reduction according to the invention. The depth effect of the deformation stroke of the forging tools 2, 3 furthermore depends, inter alia, on the temperature difference between the surface temperature 25 and the core temperature 26. By limiting this temperature difference 27 to a minimum value that is still effective enough for the depth effect, preferably 300 K, it is thus possible to limit the longitudinally profiled section beyond which soft reduction is no longer meaningful. In Figure 3 the billet cross-section with a temperature difference of 27300 K is denoted by Q5.

[0027] Since in the case of a liquid core, forces acting on the core that influence core porosity cannot be exerted by means of cross-section reduction, soft reduction of the strand 17 can only be carried out with a corresponding solid-phase fraction. In this case, the minimum solid-phase fraction can be determined to be 80%. This means that according to Figure 3 —i.e. between the billet cross-section Q3 with a solid-phase fraction of 80% and the strand cross-section Q5—a favorable longitudinally profiled section 28 for the strand 17 is produced, in which the temperature difference 27 between the surface temperature 25 and the core temperature 26 is 300 K.

Claims

1. A method for processing a billet (17) of metal which is circular in cross-section by reducing the cross-section in the end solidification zone, the cross-section in the end solidification zone being reduced by means of at least three deformation tools distributed on the outer periphery and acting simultaneously on the billet (17), characterized in that The billet (17) is deformed in a longitudinal section (28) by means of a plurality of forging tools (2, 3) forming a deformation tool during each deformation stroke, the longitudinal section (28) being located on both sides of the tip (23) of the bottom (22) of the end solidification zone and being at least one quarter of the billet diameter before the cross-section reduction, and the forging tools (2, 3) being rotated by an angular pitch around the axis of the billet (17) between a plurality of deformation strokes, wherein the forging tools (2, 3) are supported in a bracket (4), the bracket (4) having two axially movable adjusting disks (6), which form axially outwardly inclined wedge surfaces (10) of a wedge drive for a radial stroke drive.

2. The method according to claim 1, wherein By means of these forging tools (2, 3), the billet (17) is deformed during each deformation stroke in a circumferential region of at least 20°, which is subdivided among the individual forging tools (2, 3) and is related to the average width of the contact surface between the forging tool and the billet (17).

3. The method according to claim 1 or 2, characterized in that, The billet (17) is processed in the longitudinal section (28) by means of the forging tools (2, 3), the longitudinal section extending from a cross-section of the billet (17) having a solid phase ratio of 80% to a cross-section where the temperature difference (27) between the core (26) and the surface (25) is 300 K.

4. The method according to claim 1, wherein The cross-section of the billet (17) is reduced by at least 8% by means of the forging tools (2, 3).

5. An apparatus for processing a billet (17) of metal that is circular in cross-section by reducing the cross-section in the end solidification zone, reducing the cross-section in the end solidification zone by means of at least three forging tools (2, 3), which are arranged rotationally symmetrically with respect to a forging axis (5) and supported in a support (4) of a forging press and connected to a drive for achieving a radial deformation stroke with respect to the forging axis (5), characterized in that, The bracket (4) is rotatably supported in the housing (1) around a forging axis (5) and is connected to a step drive (13) for rotating by an angular pitch between a plurality of deformation strokes, wherein the bracket (4) has two axially movable adjusting disks (6), which form axially outwardly inclined wedge surfaces (10) of a wedge drive for a radial stroke drive.

6. The device according to claim 5, characterized in that, The bracket (4) is axially movably supported in the housing (1) and is connected to an axial adjustment drive.

7. The device according to claim 5, characterized in that, The housing (1) of the forging press is displaceable along the guiding portion of the billet (17).

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