METHOD FOR MACHINING A METAL CAST STRAND OF ROUND CROSS-SECTION BY REDUCING THE CROSS-SECTION IN THE FINAL SOLIDIFICATION REGION

Abstract

The invention relates to a method for working a metal casting strand (17) that is round in cross-section, by means of a reduction in cross-section in the final solidification region with the aid of at least three forming tools which are distributed around the circumference and act simultaneously on the casting strand (17). In order to provide advantageous working conditions, according to the invention the casting strand (17) is formed by forging tools (2, 3) constituting the forming tools in a longitudinal portion for each forming stroke, which portion corresponds to at least a fourth of the strand diameter before the reduction in cross-section, and the forging tools (2, 3) are rotated by an angle step about the axis of the casting strand (17) between the forming strokes.

Claims

1. Method for machining a metal cast strand of round cross-section, comprising: feeding the metal cast strand through a forging press; and reducing a cross-section in a final solidification region of the metal cast strand using at least three forging tools of the forging press distributed around a circumference of the metal cast strand and acting simultaneously on the metal cast strand wherein, with each forming stroke, the metal cast strand is formed by forging tools in a longitudinal portion that corresponds to at least one quarter of a diameter of the metal cast strand prior to a reduction in cross-section, and wherein, between the forming strokes, the forging tools are rotated through one angle step about the axis of the metal cast strand.

2. Method according to claim 1, wherein, with each forming stroke, the metal cast strand is formed by the forging tools in a circumferential region of at least 20°, divided among the individual forging tools and relating to the mean width of the contact areas between the forging tool and the metal cast strand.

3. Method according to claim 1, wherein the metal cast strand is machined by the forging tools in a longitudinal portion that extends from a cross-section of the metal cast strand with a solid phase content of 80% to a cross-section in which a temperature difference between a core and a surface of the metal cast strand is 300 K.

4. Method according to claim 1, wherein the cross-section of the metal cast strand is reduced by at least 8% by the forging tools.

5. Device for machining a metal cast strand of round cross-section by reducing the cross-section in the final solidification region, comprising: a housing; a frame of a forging press mounted in the housing; at least three forging tools which are arranged in a rotationally symmetrical manner in relation to a forging axis, are mounted in the frame, and are connected to a drive to provide forming strokes radial to the forging axis, wherein the frame is mounted in the housing to be rotatable about the forging axis; and a stepper drive connected to the frame to rotate the frame through in each case one angle step between the forming strokes.

6. Device according to claim 5, wherein the frame has two adjusting discs which are axially displaceable relative to one another and which form wedge surfaces, sloping downwards and outwards in the axial direction, of a wedge gear for the drive.

7. Device according to claim 5, wherein the frame is mounted within the housing in an axially displaceable manner and is connected to an axial actuator.

8. Device according to claim 5, wherein the housing is displaceable along a guide of the metal cast strand.

9. Method according to claim 2, wherein the metal cast strand is machined by the forging tools in a longitudinal portion that extends from a cross-section of the metal cast strand with a solid phase content of 80% to a cross-section in which a temperature difference between a core and a surface of the metal cast strand is 300 K.

10. Method according to claim 2, wherein the cross-section of the metal cast strand is reduced by at least 8% by the forging tools.

11. Method according to claim 3, wherein the cross-section of the metal cast strand is reduced by at least 8% by the forging tools.

12. Method according to claim 9, wherein the cross-section of the metal cast strand is reduced by at least 8% by the forging tools.

13. Device according to claim 6, wherein the housing is displaceable along a guide of the metal cast strand.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0018] The subject matter of the system described herein is shown by way of example in the drawing, in which

[0019] FIG. 1 shows, in a schematic longitudinal section, a forging press according to the system described herein for machining a round, metal cast strand,

[0020] FIG. 2 shows this device in a cross-section along the line II-II of FIG. 1 on a smaller scale, and

[0021] FIG. 3 shows a cast strand with a schematic distribution of the liquid and solid phase over the strand length up to complete solidification as well as in the core and surface region.

DESCRIPTION OF VARIOUS EMBODIMENTS

[0022] The forging press shown in FIGS. 1 and 2 has a housing 1, in which a frame 4 is mounted in such a way as to be rotatable about a forging axis 5, said frame holding forging tools 2, 3 located opposite one another in pairs. Said frame 4 is formed by two adjusting discs 6, which are mounted in the housing 1 in such a way as to be displaceable in the axial direction and can be acted upon by pistons 7 counter to the force of return springs 8, which are supported between the two adjusting discs 6. The pistons 7 engage in annular chambers 9, which are formed in the housing 1 and to which a hydraulic medium can be supplied.

[0023] The two adjusting discs 6 are provided with wedge surfaces 10 inclined in opposite directions, on which the forging tools 2, 3 bear with corresponding mating surfaces in a sliding manner, so that corresponding wedge gears are formed between the adjusting discs 6 and the forging tools. In the edge region of the wedge surfaces 10, the forging tools 2, 3 are provided with guide strips 11, which engage in guide grooves 12 of the adjusting discs 6. When a compressive force is applied to the adjusting discs 6 by way of the pistons 7, said adjusting discs are moved towards one another, which results in a sliding movement of the forging tools 2, 3 relative to the wedge surfaces 10 of the adjusting discs 6, with the effect that the forging tools 2, 3 execute a radial forming stroke. When the pressure of the pistons 7 is removed, the adjusting discs 6 are moved back to the starting position by the return springs 8, with the forging tools 2, 3 guided along the wedge surfaces 10 executing an idle stroke.

[0024] Between the forming strokes of the wedge gear, the frame 4, which comprises the two adjusting discs 6, can be rotated through one rotation step by means of a stepper drive 13. In the exemplary embodiment shown, the stepper drive 13 comprises a pinion 15, which is driven by a stepper motor 14 and meshes with a ring gear 16 connected to one of the two adjusting discs 6. However, any other rotational stepper drive is also possible. The cast strand 17 can thus be reduced in cross-section by the forging tools 2, 3 located opposite one another in pairs, with the frame 4 being rotated through one rotation step between the forming strokes in order to ensure a stepwise machining of the cast strand 17 along a helical course around the forging axis 5.

[0025] As can be seen from FIG. 3, the cast strand 17 is cooled during the casting, with the effect that a solid shell 18 forms around the liquid core 19. The casting region 20 is followed by a cooling region, in which cooling fluid is sprayed onto the cast strand 17. As a result, a gradual solidification of the cast strand 17 occurs from the outside towards the inside, with a mixed phase 21 being formed between the liquid phase 19 and the solid shell 18, said mixed phase running out into a sump 22, the tip 23 of which indicates the complete solidification of the cast strand 17. The solid phase content in the region of the sump 22 therefore increases from 0 to 100% according to the curve 24. The solid phase contents of 0%, 20%, 80% and 100% and the associated position of the strand cross-sections Q1, Q2, Q3 and Q4 are shown in FIG. 3.

[0026] Due to the external cooling of the cast strand 17, the surface temperature of the cast strand 17 extends along the curve 25 of FIG. 3. The core temperature is illustrated by the curve 26. It has been found that, in the region of the final solidification, the core temperature 26 drops much more quickly than the surface temperature 25. This temperature gradient can advantageously be used to define the longitudinal portion of the cast strand 17 that is advantageous for a soft reduction according to the system described herein. This is because the depth effect of the forming strokes of the forging tools 2, 3 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 that is still sufficiently effective for the depth effect, preferably 300 K, a longitudinal portion can thus be defined, outside of which a soft reduction is no longer useful. The strand cross-section in which the temperature difference 27 is 300 K is denoted Q5 in FIG. 3.

[0027] Since no forces that influence the core porosity can be exerted on the core via a reduction in cross-section in the case of a liquid core, a soft reduction of the cast strand 17 can only be carried out with a suitable solid phase content. In this context, the minimum solid phase content can be set at 80%. This means that, as shown in FIG. 3, a longitudinal portion 28 results for the advantageous soft reduction of the cast strand 17, namely between the strand cross-section Q3 having a solid phase content of 80% and a strand cross-section Q5 in which the temperature difference 27 between the surface temperature 25 and the core temperature 26 is 300 K.