THE MANUFACTURE OF THIN METAL SLAB

Abstract: In the manufacture of a thin metal slab a workpiece is cast in a continuous casting mould and downstream of the mould, but whilst still connected to the part in within the mould, the workpiece is treated differently across its width to produce slab of a different width from the cast workpiece and of uniform cross section. The treatment may be rolling or forging.

Description: THE MANUFACTURE OF THIN METAL SLAB This invention relates to the manufacture of thin metal slab, particularly steel slab, by a continuous casting route. According to the present invention a method of manufacturing thin metal slab comprises a continuous operation in which a metal workpiece is cast in a continuous casting mould and, downstream of the mould, the workpiece, whilst still connected to the portion thereof in the mould, is dif erentially treated at regions across its width to produce rv.etal slab having a width which is different from the width of the cast workpiece and a thickness which is uniform across its width. The workpiece as cast may be of substantially rectangular cross section and the treatment comprises rolling a longitudinally extending central region of the workpiece to a greater extent than are the longitudinally extending edge regions of one broad face to form a longitudinally extending groove therein and thereafter rolling the edge regions to a greater extent than at the central region to produce slab of uniform thickness. The treatment may be applied to each of the opposite broad faces of the workpiece. Alternatively, the workpiece may be cast with its longitudinally extending edge regions thicker than the longitudinally extending central region and in another embodiment of the invention the workpiece is cast with its longitudinally extending edge regions thinner than the longitudinally extending central region. The slab so produced has a width which may be greater than or less than the width of the workpiece as it leaves the mould. Apparatus for manufacturing thin metal slab comprises a continuous casting mould, guide means positioned below the outlet of the mould passage for guiding a metal workpiece cast in the mould and treatment means for differentially treating the workpiece at regions across its width to produce slab having a width which is different from the width of the cast workpiece and a thickness which is uniform across its width. 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:- Figure 1 is a diagrammatic front elevation, and Figure 2 .is a diagrammatic side elevation of means for widening a thin slab cast in a continuous casting mould. A continuous casting mould 1 is arranged with its mould passage vertical. A refractory feed tube 2 has its lower end projecting into the upper end of the mould passage and a containment zone 3 for guiding the workpiece is arranged below the outlet of tho mould passage. The cross- section of the mould passage is such as to produce a workpiece 4' of generally rectangular cross-section. The workpiece 4' as it leaves the mould has a liquid plus mushy core and the shell thickens to form a solid workpiece 4 at the lower end of the containment zone 3. The thickness of the workpiece 4 is T and its width is . Means for rolling the workpiece to thickness t whilst it is still connected to the portion of the workpiece in the mould is provided downstream of the containment zone. The rolling consists of three pairs of rolls arranged in tandem. The first rolling stage incorporates a pair of rolls 7 each having a working barrel length Bl. This first stage deforms the workpiece differentially by rolling a groove 8 in the central region of each of the opposite faces of the workpiece. The width is increased to l. Means for rolling the workpiece to thickness t whilst it is still connected to the portion of the workpiece in the mould is provided downstream of the containment zone. The rolling consists of three pairs of rolls arranged in tandem. The first rolling stage incorporates a pair of rolls 7 each having a working barrel length Bl. This first stage deforms the workpiece differentially by rolling a groove 8 in the central region of each of the opposite broad faces of the workpiece. The width of the workpiece is increased to Wl. At the second rolling stage a pair of rolls 9 serve to widen the grooves 8 to a width B2 without changing the depth of the grooves and the width of the workpiece is increased to 2. The second rolling stage is optional. The third rolling stage is provided by a pair of cylindrical rolls 10 which roll the longitudinally extending edge regions 11 to a greater extent than the longitudinally extending central region 12 to produce a rectangular thin slab of thickness t and width 3, where W3 is greater than the width of the cast workpiece. The treatment of the workpiece usually commences a safe distance D beyond the final solidification point 6 but the first rolling stage to roll the grooves may commence before complete solidification occurs. The thin slab can be fed to a finishing train of a hot mill and has properties superior to those of a cast only structure. The amount of increase in width of the slab over the cast workpiece can be varied by adjusting the depth of the rolled grooves or by changing the width of the grooves, usually by including additional rolling stages. The pairs of rolls 7, 9, 10 may be fitted to any convenient type of rolling mill and one or more flattening passes may be provided by a planetary rolling mill. As an alternative to the use of rolls, a forging press could be used to apply the treatment to the workpiece. For example a forging press could be used to form the grooves 8, 12 in the slab. The rolls 10 might then be required to flatten the workpiece. Figures 3 and 4 of the accompanying drawings are diagrammatic front and side elevations of a second embodiment of the invention. A continuous casting mould 14 has a mould passage of such a form that a workpiece 15 cast in it has a longitudinally extending central portion 16 with an enlarged end portion 17 at each end of the central portion. The workpiece 15 passes through a guide section 18 constituting a containment zone and at least one pair of rolls 19 is positioned downstream of the containment zone. The rolls 19 roll the workpiece 15 by a greater extent over the end regions 17 as compared with the central region 16 to produce a thin slab 20 of uniform thickness t and width 5 where W5 > W4. Figure 5 is a graph showing the percentage spread achieved in a thin slab of particular dimensions as the width of the groove in the opposite faces is changed. The initial width of the slab as cast is W, the width of the groove is B and the thickness of the slab between the grooves is A. It can be seen from the graphs that where B is 20% of , a spread of about 2% can be achieved for a groove A but a spread of about 6% can be achieved if the depth of the grooves is reduced in order to increase the thickness of the slab between the grooves. Furthermore, the spread of a slab is increased by a factor of about 2 if the width B of the groove is tripled to say 60% of W. Any or all of the rolls used to reduce the thickness of the cast slab may be cooled internally and/or externally. The hardness at elevated temperatures of these rolls may vary across the diameter of the barrel with the surface layer having the greatest hot hardness and good compressive strength, to withstand the thermal stresses, thermal fatigue due to rolling at elevated temperature and lower than normal rolling speeds. Figures 6 and 7 show another casting and rolling technique in which the slab 25 is cast with ends 27 shaped as a rectangle 26 of thickness t^ and a centre section 27 of thickness T where T > t^. The shape of the mould cavity is relatively parallel such that no working of the shell occurs. The containment zone 28 comprises a series .of roll sets 31 each comprising multiple rolls arranged end-to-end and which match the mould exit shape or each support the flat faces of the slab width and incorporating support bearings across the slab width. The final thickness t of the slab is achieved by rolling the solidified centrally thick slab a safe distance D after final solidification point 32 with a width flattening pass with rolls 33 to give a rectangular flat slab of width Wl and thickness t. In Figure 6 the width Wl is shown to be greater than the width of the cast workpiece, but the process can be used to bring about a reduction in the width of the workpiece. Figure 8 is a graph showing the effect of rolling on a thin slab of width W where the end portions are cast with a thickness A and the width of the central region is B. Point J on graph 34 shows that if B is 25% of W a small widening of say 1% of the slab occurs after final rolling but if the thickness A of the cast workpiece is reduced, a reduction of about 5% in the width of the workpiece can be achieved after final rolling. If the width A is reduced and B is extended to about 50%, of W, then a reduction of almost 10% in the width of the slab can be achieved. [See Point H on graph 35]. The dimensions of the mould passage may be changed during the casting operation so as to vary the width of the casting as it leaves the mould. For example, the side walls of the mould which define the mould passage may be movable towards and away from each other in order to adjust the width of the workpiece cast in the mould. This adjustment of the width of the workpiece may simply compensate for the reduction in the width of the workpiece due to shrinkage between the time of casting and the commencement of differential treatment across the width of the workpiece. On the other hand, adjustment of the width of the workpiece cast in the mould may be used in combination with width adjustment downstream of the mould in order to produce thin slabs of uniform width within a considerable range of widthwise dimensions. The feed tube 2 which is employed to supply the molten metal into the mould, two such tubes being shown in Figure 3, may be of the form described and claimed in our co-pending application of even date. The mould 1 shown in Figures 1 and 2 may be of the form described and claimed in our co-pending application of even date.

Claims: CLAIMS 1. A method of manufacturing thin metal slab comprising a continuous operation in which a metal workpiece is cast in a continuous casting mould and, downstream of the mould, the workpiece, whilst still connected to the portion thereof in the mould, is differentially treated at regions across its width to produce metal slab having a width which is different from the width of the cast woi piece and a thickness which is uniform across its width. 2. A method as claimed in claim 1, in which the workpiece is of substantially rectangular cross-section and said treatment comprises rolling a longitudinally extending central region of the workpiece to a greater extent than are the longitudinally extending edge regions of one broad face to form a longitudinally extending groove therein and thereafter rolling the edge regions to a greater extent than is the central region to produce slab of uniform thickness. 3. A method as claimed in claim 2, in which the treatment is applied to each of the opposite broad faces of the workpiece. 4. A method as claimed in claim 2 or 3, in which the or each groove is widened by rolling prior to the rolling of the edge regions. 5. A method as claimed in claim 1, in which the workpiece is cast with its longitudinally extending edge regions thicker than the longitudinally extending central region and the treatment comprises rolling the edge regions to a greater extent than is the central region to produce slab of uniform thickness. 6. A method as claimed in claim 1, in which the workpiece is cast with its longitudinally extending edge regions thinner than the longitudinally extending central region and the treatment comprises rolling the central region to a greater extent than are the edge regions to produce slab of uniform thickness. 7. A method as claimed in claim 1, in which the treatment applied to the workpiece comprises forging. ε. A method as claimed in claim 7, in which the forging is followed by rolling. 9. Apparatus for manufacturing thin metal slab comprising a continuous casting mould, guide means positioned below the outlet of the mould passage for guiding a metal workpiece cast in the mould and treatment means for differentially treating the workpiece at regions across its width to produce slab having a width which is different from the width of the cast workpiece and a thickness which is uniform across its width. 10. Apparatus as claimed in claim 9, in which the treatment means comprises one or more sets of rolls. 11. Apparatus as claimed in claim 10, in which the mould passage is such as to produce a workpiece of substantially rectangular cross-section, a first set of rolls serves to form a longitudinally extending groove in the central region of at least one broad face of the workpiece and a subsequent set of rolls serves to roll the longitudinally extending edge regions to a greater extent than the central region to form said slab. 12. Apparatus as claimed in claim 9 or 10, in which the mould passage is such as to produce a workpiece having a non-rectangular cross-section.

Inventors: HOPE THOMAS, LAWSON KENNETH THOMAS, PERRY ROBERT MAIDENS, REYNOLDS TIMOTHY, HOPE, THOMAS, LAWSON, KENNETH, THOMAS, PERRY, ROBERT, MAIDENS, REYNOLDS, TIMOTHY
Applicants: DAVY DISTINGTON LTD, DAVY (DISTINGTON) LIMITED
Application year: 1989
Main classification: B22D11
IPC codes: B21B1, B22D11