WIDTH REDUCTION OF METAL SLAB

Abstract: A press for reducing the width of a metal slab has a pair of plattens which are arranged to engage opposite edges of the slab. Each platten has a slab edge engaging surface which is configured such that when the slab is pressed there is an increase in slab thickness over a substantial proportion of the width of the slab.

Description: Width Reduction of Metal Slab This invention relates to an apparatus and a method for reducing the width of a hot metal slab. For the manufacture of metal strip, such as steel strip it is becoming increasingly common to continuously cast a work piece in the form of a strand, to cut the strand in to lengths, known as slabs, and to roll each slab into strip. Because strip is required in different widths it is desirable that slabs of different widths are available. To change the width of a slab produced by continuous casting requires that the mould from which the slab is produced be changed and this is inconvenient. It is therefore desirable to be able to cast slabs of a minimal number of widths and, where necessary, to reduce the width of the slab for subsequent rolling into strip. In a known method of reducing the width of a hot metal slab which initially has a width dimension greater than its thickness dimension, the hot metal slab moving in the direction of its length is presented to a pressing machine which uses plattens to apply forces to the opposite edges of the slab to reduce the width dimension of the slab along its length. Normally the slab will be of much greater length than the region to which the forces are applied by the plattens. For this reason, the slab is incremented through the pressing machine so that its width dimension is reduced stepwise along its length by repeated application of forces. Thickening of the slab accompanies each width reduction and the thickening can produce a non-uniform cross section. This non-uniform cross section necessitates a subsequent slab rolling stage to flatten the faces of the width reduced slab. This subsequent rolling stage tends to result in an inefficient and consequently undesirable increase in the width of the slab according to the degree of non-uniformity of the cross section. Furthermore the edges of the slab may not be rolled to the uniform cross section and consequently may require removal before introduction of the slab to the strip mill. The present invention aims to alleviate the aforementioned technical problems. Accordingly there is provided a slab width reduction press comprising two plattens at least one of which is mounted to be displaced relatively towards and away from the other, each platten providing a slab edge engaging surface said engaging surfaces of the plattens being characterised in that they are configured to cause an increase in slab thickness over a substantial proportion of the width of a slab. Preferably each platten is divided in the length direction of the slab into a head part on which the engaging surface is inclined with respect to the length direction and a body part where the engaging surface is parallel to the length direction. The platten may also include a tail part, remote from the head part on which the engaging surface is also inclined with respect to the length direction. Preferably the length of the head part is greater than the maximum incremental length through which the slab is moved after each application of the plattens. The possibility of providing an inclined engaging surface on the head part which is arcuate has been considered, however, because of manufacturing cost it is preferred to provide this as a flat surface. It has been found that the angle of inclination or slope of the inclined engaging surface on the head part is critical to the performance of the press. By analysis of the change in efficiency of width reduction for various platten geometries it has proven possible to determine an optimum range of platten angles. In essence the optimum platten angle will generate a stress field which upon application of both plattens penetrates a substantial proportion of or possibly the full width of the largest size of slab intended for reduction by the press. According to a further aspect of the present invention there is provided a method of reducing the width of a hot metal slab comprising the steps of: (a) feeding the slab in the direction of its length into a pressing machine having two opposing plattens, (b) moving at least one of the plattens towards the other in the direction of the width of the slab to engage and press a part of the length of the edges of the slab to reduce the original slab width (W) by a width reduction (ΔW) , (c) moving at least one of the plattens away from the other platten to disengage the slab edges, (d) advancing the slab by an incremental length (I) to present a new part of the slab for width reduction, (e) repeating the steps (b) to (d) to reduce the width of the whole of the slab, wherein the length (I) is selected to be a maximum in accordance with, the geometry of the plattens, the magnitude of the width reduction and the required uniformity of cross section of the reduced width slab. By controlling the incremental length (I) the deformations generated by sequential applications of the plattens overlap in a manner which results in a reduced width slab of uniform or nearly uniform cross section. Preferably the incremental length (I) is determined in accordance with the formula: Penetration ept (D) = Where alpha is the angle of inclination of the leading engaging surface on the head part of the platten to the length direction. By controlling the incremental length to produce a penetration depth equal to the original width of the slab it is possible to produce a reduced width slab of effectively rectangular cross section. Under some circumstances the reduced width slab produced by the present method and apparatus may have a slightly non-uniform cross section. However, the present method may allow the uniformity of the cross section, which is herein called the "slab thickness ratio" to be controlled to produce the maximum efficiency * of width reduction when the slab is subsequently subjected to a flat rolling pass. In order that the invention may be more readily understood, it will now be described, by way of example only, with reference to the accompanying drawings, in which:- Fig. 1 is a side elevation of the plattens of a pressing machine Fig. 2 shows a hot metal slab being squeezed between the two plattens of a pressing machine Fig. 3 is a cross section taken through the slab of Fig. 2 Fig. 4 is a characteristic for slab thickness ratio plotted against platten lead angle, and Fig. 5 is a characteristic for efficiency of width reduction plotted against platten lead angle. Referring to Fig. 1, a pressing machine has a pair of plattens 1, 2 arranged one above the other, conveniently the lower platten 2 is fixed and the upper platten 1 is movable towards and away from platten 2 but, alternatively both plattens may move towards and away from each other. Alternatively the two plattens may be arranged side by side with the movement in the horizontal place. The two plattens have identical geometrical features, namely a head part having a long sloping surface 1A, 2A inclined at a shallow angle "α" to the horizontal plane and a relatively short body part having a horizontal surface IB, 2B. The aforementioned surfaces engage with the edges of the hot metal slab 6 indicated in Fig. 2. This indicates two plattens 4 and 5 performing width reduction on a hot metal slab 6 when the two plattens are part way along the length of the slab. The chain-dotted line 7A indicates the approximate zone of material deformation caused by the application of forces to the slab edges by the two plattens 4 and 5. The broken-line 7B indicates the approximate zone of material deformation cause by the previous application of forces to the slab edges at a position along the slab length equal to the incremental feed length. The incremental feed length 8 is given by the horizontal distance between corresponding points on the chain-dot and broken lines. Selecting a small incremental feed length 8 facilitates an overlapping of mechanical deformation zones thus giving rise to a relatively constant slab cross sectional profile. Fig. 3 indicates a cross section of a hot metal slab 10 after squeezing, utilising a small incremental feed length. The depth of penetration of mechanical work is approximately given by the following formula:- Penetration Depth = ....Equation No. 1 where Δ W = width reduction α = platten slope angle I = incremental feed length From the above formula it can be seen that reducing the incremental feed length I actually reduces the penetration depth of mechanical work, thus it is necessary to reduce the platten slope angle "α" to a shallow angle in order to maintain good material deformation characteristics within the hot metal slab. Referring to Fig. 4, after squeezing the slab thickness dimension will have a tendency to increase, the maximum thickness value will occur somewhere within the body of the slab and is conveniently termed Hmax; shown on the slab cross section after width reduction. Dividing this maximum value Hmax by the minimum slab thickness in this case on the centre-line, conveniently termed He gives a mathematical magnitude termed the "Slab Thickness Ratio". The result of plotting slab thickness ratio against platten lead angle is indicated in Fig. 4 where it can be seen that the slab thickness ratio approaches unity at a very low platten lead angle. The slab thickness ratio defines mathematically the rectangularity of the slab cross section - a value of unity indicates that the penetration of mechanical work is extremely effective. This is the most desirable shape for subsequent flat rolling. Referring to Fig. 5, a slab having initial width Wo undergoes reduction to target width E and then undergoes flat pass horizontal rolling which creates a new width W R owing to the occurrence of lateral spread. The efficiency of width reduction is defined by subtracting the width after flat pass rolling "W R " from the original width "Wo" and dividing by the target width "W E " subtracted from the original width "Wo". The result of this gives a numerical value less than unity, the higher the value, the more efficient is the process. The result of plotting efficiency of width reduction against platten lead angle is indicated in Fig. 5, where it can be seen that the width reduction efficiency diminishes with increasing platten lead angle, i.e., the process is more efficient when the platten lead angle is very shallow. Reducing the platten lead angle to an extremely small magnitude gives excellent slab cross sectional characteristics when incorporated with a very small incremental feed length as defined by Equation No. 1, however, reducing the incremental feed to a very small magnitude gives rise to an excessive cycle time for squeezing a long slab. The parameters must therefore be optimised, which is the recommendation of the present invention, In a practical press for pressing a slab of say 10 metres in length, the minimum incremental feed length is of the order 250-300 mm. For a slab initially 2000 mm wide and to be reduced by 300 mm it can be calculated fron equation 1 that the platten lead angle α is in the range 4-8°. A particular result is α = 5.85 when I = 293 mm.

Claims: Claims 1. A slab width reduction press comprising two plattens at least one of which is mounted ~o be displaced towards and away from the other, each plat-en providing a slab edge engaging surface, said surfaces being arranged to engage the opposing edges of a slab introduced therebetween with its width dimension perpendicular to the direction of motion of the slab whereby the width of the slab can be progressively reduced along its whole lengh by moving the slab incrementally lengthwise subsequent tc each pressing application of the plattens, wherein each engaging surface is configured to ensure that the slab width is reduced with substantially uniform thickening of the slab across its whole width. 2. A press according to claim 1 wherein each platten comprises a head part on which the engaging surface is inclined with respect to the length direction and a body part where the engaging surface is parallel to the longitudinal direction. 3. A press according to claim 2 wherein each platten comprises a tail part on which the engaging surface, remote from the head part, is inclined with respec±. to the length direction. 4. A press according to claim 2 or claim 3 wherein the length of the head part is greater thar. the incremental length through which the slab is moved after each pressing application. 5. A press according to any of claixts 2 to 4 wherein the engaging surface of the head part is flat and inclined at a platten slope angle determined according to the formula:- Penetration depth = i Γ ___J___Λ + I + _____ 1 2 L 2 tanαJ 2 J whereΔW = width reduction α = platten angle of inclination I = incremental feed length W = original slab width 6. A press according to claim 5 wherein the platten angle of inclination is in the range 4 degrees to 8 degrees. 7. A press according to claim 6 wherein the platten angle of inclination is 5.85 degrees. 8. A method of reducing the width of a hot metal slab comprising the steps of: (a) feeding the slab in the direction of its length into a pressing machine having two opposing plattens, (b) moving at least one of the plattens towards the other in the direction of the width to engage and press a part of the length of the edges of the slab to reduce the original slab width (W) by a width reduction (Δ W) , (c) moving at least one of the plattens away from the other platten to disengage the slab edges, (d) advancing the slab an incremental length (I) to present a new part of the slab for width reduction (e) repeating the steps (b) to (d) to reduce the width of the whole of the slab, wherein the length (I) is selected to be a maximum in accordance with, the geometry of the plattens the magnitude of the width reduction and the required uniformity of cross section of the reduced width of the slab * 9. A method according to claim 8 in which the length I is determined in accordance with the formula: Penetration depth D) = i T____J___Λ + I + _____ 1 2 LV.2 tanαj 2 J where α is the angle of inclination of the leading engaging surface of the head part of the platten to the length direction. 10. A method according to claim 9 wherein the length (I) is selected to produce a penetration depth (D) equal to the original width (W) of the slab to produce a width reduced slab of effectively rectangular cross section.

Inventors: BRADSHAW TIMOTHY JOHN, BRADSHAW, TIMOTHY, JOHN
Applicants: DAVY MCKEE SHEFFIELD, DAVY MCKEE (SHEFFIELD) LIMITED
Application year: 1991
Main classification: B21J1
IPC codes: B21B1, B21J1