US 5,197,533 AGrant
Self-Supporting, Flexible Continuous Casting Starter Bar
Issue Date:1993-03-30
•5 Claims
•8 Drawing Sheets
Abstract
The flexible starter bar has a body which comprises a flexible substantially planar spine on one side, a series of blocks attached to the spine on the other side, and in a top portion thereof adjacent a head for the starter bar, a plurality of block supports disposed between adjacent pairs of blocks adapted to protrude from one block of a pair so as to lie in abutting and supporting relationship with the other block of the pair, thereby increasing the effective length of the operatively outer side of the starter bar body so that the starter bar may assume a curved configuration and be self-supporting. The block supports in the top portion of the starter bar are retractable to shorten the effective length of the starter bar body on the operatively outer side so that the starter bar may resume a straight configuration. The bottom portion of the starter bar is without any block supports and is supported in the curved configuration by a strand guide and support rolls.
Metadata
Inventor
- Gunther Behrends
Application Information
Application Number:US 7958781
Filing Date:1991-11-25
Priority Date:1990-11-23
Art Unit:325
Classifications
IPC:
B22D 1108
Field of Search:
164446;445;426;425
Patent Drawings (8 sheets)
Description
Background of the Invention
This invention relates to a flexible starter bar for use in continuous casting processes The advantages of flexible starter bars over rigid starter bars have been described extensively in the patent literature and generally relate to labour and space-saving improvements in storing the starter bar
Flexible starter bars normally comprise a series of links attached to each other by means which allow the bar to flex through bending rolls which lie in the path of movement of the strand and also through straightening rolls whereby the strand is straightened. Because of strand flexibility, support rolls are also provided to maintain the starter bar in a curved configuration along the casting path. Chain type link structures, in particular, have been used extensively in the construction of flexible starter bars and most improvements to such structures are directed to minimizing and controlling any play between the links so as to prevent jerking strand motions which may result in molten metal breaking out of the mold.
A departure from the chain link structures generally adopted in the industry is to provide a plate type starter bar formed by a relatively thin plate as in U.S. Pat. No. 4,660,616 and U.S. Pat. No. 3,889,740. These structures have not been adopted to any great extent and it is believed that no suitable material has been found which will have the necessary flexibility to withstand repeated flexings through the casting train, the strength to withstand the load of the strand, and the positional stability not to twist as the bar is drawn through the casting train.
To some extent these problems were alleviated in "sandwich" structures comprising top and bottom flat thin steel plates connected to each other by an intervening structure which could assume a variety of shapes as illustrated in a trilogy of patents issued earlier than the aforementioned plate type bars, namely U.S. Pat. Nos. 3,451,466; 3,603,375; and 3,633,653. These patents in turn were alleged to be improvements over flexible starter bars of the type in which a series of links were strung on a flexible tie rod as in U.S. Pat. No. 2,920,359. Other patents of this general class include U.S. Pat. Nos. 3,262,162; 3,351,125 and 3,442,322 in which the tie element is a bar or band. Presumably the improvement in the "sandwich" structures resides in providing a self-supporting bar and a continuous smooth surface which will minimize damage to rolls. There is however an attendant compromise with a reduction in flexibility.
An object of this invention is to provide a starter bar which is self-supporting and flexible.
Summary of the Invention
In accordance with this invention, there is provided a starter bar for closing a mold used in a continuous casting machine and for guiding the leading end of a strand from the mold in a curved path, the starter bar having a head at one end of the bar for attachment to the leading end of the strand, a tail at the other end of the bar for guiding the bar between rollers forming part of the continuous casting machine, and a body disposed between the head and the tail, the body comprising a flexible substantially planar spine on the operatively inner side of the body, the spine extending longitudinally between the head and the tail, and defining an inner radius of curvature for the starter bar lying in said curved path; a series of blocks on the operatively outer side of the body arranged end to end and attached on one side thereof to the operatively outer surface of the spine, the blocks thereby defining an outer radius of curvature for the starter bar; and a plurality of retractable block support means each disposed between adjacent pairs of blocks and adapted to protrude from one block of a pair a predetermined distance x so as to lie in abutting and supporting relationship with the other block of said pair and thereby increase the effective length of the operatively outer side of the body so that the starter bar may assume a curved configuration and be self-supporting for at least a top portion thereof, said block support means being retractable to shorten the effective length of the operatively outer side of the body so that the starter bar may assume a straight configuration.
Brief Description of the Drawings
A preferred embodiment of the invention will now be described with reference to the accompanying drawings in which:
FIG. 1 is a schematic side elevational view of a starter bar made according to the invention and positioned in use between a continuous casting mold at the upstream end and extractor rolls at the downstream end;
FIGS. 2 and 3 (drawn to a larger scale) are detailed views showing the upstream and downstream ends of the starter bar, respectively;
FIG. 4 is an exploded perspective view illustrating the component parts of the starter bar made according to the invention;
FIG. 5 is a partly sectioned side elevational view of the starter bar made according to the invention;
FIG. 6 is a partly sectioned plan view along line 6--6 of FIG. 5;
FIG. 7 is a transverse cross-sectional view through the starter bar taken along line 7--7 of FIG. 6;
FIG. 8 is a detail view drawn to a larger scale of the circled area 8 of FIG. 6 and showing a spacer pin partly sectioned at its inner end where it lies in abutment with a shift pin;
FIG. 9 is a view similar to FIG. 1 showing an alternative embodiment of the invention in which only a top portion of the starter bar is self-supporting;
FIG. 10 is an exploded perspective view showing the components parts of the bottom portion of the starter bar of FIG. 9;
FIG. 11 is a top plan view and partly cut away of the bottom portion of the starter bar of FIG. 9;
FIG. 12 is a cross-sectional view along line 12--12 of FIG. 11; and
FIG. 13 is a schematic side elevation showing storage of the starter bar of FIG. 9.
Detailed Description With Reference to the Drawings
A starter bar generally indicated by numeral 20 is shown in FIG. 1 with its head 22 at the upstream end plugging a chilled mold 24. In use, molten metal 26 held in the mold 24 will freeze to the head 22 and form the leading end of a strand which is pulled through the continuous casting train by the starter bar 20. A tail 21 at the downstream end of the starter bar 20 is gripped by extractor rolls 28 provided in pairs on opposite sides of the starter bar 20 downstream of a guide skid 30 which forms the starter bar into a fixed radius arc.
Opposite pairs of pin shifter rolls 32 and guide rolls 34 of which only one member of each pair are shown in FIG. 1, are positioned adjacent the skid 30 and spaced along the length of the skid to interact with block support means 64 (FIG. 4) whereby the top portion of the bar 20 is made self-supporting. The operation of the pin shifter rolls 32 and the block support means 64 is described in more detail below with reference to FIGS. 4-7.
The starter bar head 22 (FIG. 2) is machined from a block of high carbon steel having a substantially square cross-section and is provided with a bolt 23. The bolt 23 is threaded in one end of the block and anchors the starter bar 20 to the cast strand on solidification of the molten metal 26 about the bolt.
A groove 25 is machined in the other end of the block and is adapted to cooperate with a corresponding tongue 60 formed in blocks 46 which comprise the main body of the starter bar (FIG. 4). Adjacent the groove 25, a depression 27 milled from the operatively upper surface of the block is adapted to receive the leading end of a first segment of a flexible substantially planar spine 40. A through hole 29 is drilled through the upper surface to receive a fastener for securing the spine to the block and a pair of grooves 31 (only one of which is shown in FIG. 2) are formed on opposite sides of the block for location of the pin shifter rolls 32 and guide rolls 34.
The starter bar tail 21 (FIG. 3) is similarly to the head machined from a block of high carbon steel having a substantially square cross-section. Chamfered edges 33 are machined from the trailing end of the tail 21 to ensure proper guiding of the starter bar 20 through the extractor rolls 28. At the leading end, a tongue 35 machined in the block is adapted to cooperate with a groove 62 formed in the blocks 46 (FIG. 4). Adjacent the tongue, a depression 37 milled from the operatively upper surface of the block is adapted to receive the trailing end of the last segment of the spine 40 and a through hole 39 is drilled through the upper surface to receive a fastener for securing the spine to the block. A pair of grooves 41 formed on opposite sides of the block are provided for location of the pin shifter rolls 32 and guide rolls 34.
The body of the starter bar, between the head 22 and tail 21, will now be described with reference to FIGS. 4-7. It will be understood that the starter bar 20 is adapted for use with a billet or bloom caster and comprises an elongate body having a generally rectangular cross-section defined by upper and lower parallel surfaces and transverse sides, the accompanying drawings showing the starter bar in this orientation.
The upper surface of the starter bar is defined by a flexible substantially planar spine 40 made from a high strength material with high fatigue resistance. The spine 40 is segmented into a series of plates arranged end to end and having ends cut at a slant (not equal to 90.degree.) so as to provide a smooth transition when the bar travels between the extractor rolls 28 (FIG. 6). Four apertures 42 are provided in each plate, one at each end and two at the centre between the ends. The apertures 42 are countersunk to receive the conical heads of fasteners 44 which secure the spine 40 to a series of underlying blocks 46.
As seen in FIGS. 4 and 6, each segment of the spine 40 is attached to three blocks 46, namely one block at each end and one block at the centre. The blocks 46 are therefore half the length of the spine segments.
The blocks 46 are machined from high carbon steel and are arranged end to end underneath the spine 40, thereby defining a lower surface for the starter bar body. As seen most clearly in FIG. 4, the top surface of the blocks 46 has a pair of oppositely directed shoulders 58 extending along the length of the block and adapted to receive therebetween the spine 40. The thickness of the spine 40 is selected to be greater in height than the depth of the shoulders 58 so that the spine 40 will protrude from the block (FIG. 7). A pair of through holes 48 extending between the top and bottom surfaces of the blocks is machined from the centre of each block 46, the holes 48 having a large diameter at each end to define a reduced diameter portion therebetween.
As indicated above, the fasteners 44 have a conical head at one end which in use locates in the countersunk apertures 42 below the outer surface of the spine 40. The other end of the fasteners 44 have a partially threaded reduced diameter portion. A set of Belville spring washers 50 and a regular washer 52 are secured on the reduced diameter portions of the fasteners by a retaining nut 54 such that the Belville spring washers 50 will bear against a locating shoulder 56 (FIG. 5) defined by the reduced diameter portion machined in the apertures 42.
When the starter bar 20 assumes a curved configuration, the plates comprising the spine 40 will flex and any axial loading applied to the fasteners 44 will operate to compress the Belville washers 50 against the locating shoulder 56. Thus, the fasteners 44 will remain secure in the spine and block assembly even under repeated flexing of the spine 40.
Each block 46 has a tongue 60 at one end and a groove 62 at the other end so as to cooperate with the groove and the tongue, respectively, of adjacent blocks The blocks 46 can thus move independently from each other in conformity with the inclination of the associated portion of the spine 40.
It will be appreciated that the blocks 46 may assume an inclined configuration relative to one another in which the spine 40 is flexed so that the starter bar will assume the curved configuration shown in FIG. 1 but that the arrangement will operate to prevent the spine from flexing in the opposite direction away from the straight configuration of the starter bar.
It will be understood that the effective length of the body of the starter bar 20 is shorter on the inner side defined by the spine than on the outer side defined by the exposed bottom surface of the blocks 46 when it is in a curved configuration. So that the starter bar 20 may be self-supporting in the curved configuration, it is provided with a plurality of retractable block support means generally indicated by numeral 64 in FIGS. 4 and 5, the block support means being adapted to protrude from one block so as to lie in abutting and supporting relationship with a bearing surface on the adjacent block whenever the starter bar is in the curved configuration thereby increasing the effective length of the operatively outer side of the body of the starter bar. Conversely, retraction of the block support means shortens the effective length of the operatively outer side of the body of the starter bar so that it may resume a straight configuration downstream of the extractor rolls 28.
In the preferred embodiment according to the invention, the block support means comprises an axially movable spacer pin 66 disposed to lie parallel to the longitudinal axis of the spine 40 and retained in a corresponding first bore 68 machined in the tongue 60 of each block 46 (FIG. 4, FIG. 8). A longitudinally extending flat 70 is machined in each spacer pin 66 and receives therein a locating pin 72 whereby axial movement of the spacer pin 66 is permitted while preventing rotation thereof.
A second bore 74 is machined in each block 46 transversely of the first bore 68 so that the first bore 68 terminates therein and the second bore 74 terminates in first and second longitudinal grooves 76, 78 formed in respective sides of the block, said grooved sides being transverse to the spine 40. The second bore 74 has an enlarged diameter at one end and is adapted to receive a shift pin 80 having a first large diameter at one end and a second small diameter at the other end, the inner end of the spacer pin 66 being adapted to abut alternately on the large diameter or the small diameter portion of the shift pin 80 in accordance with its axial position in the second bore 74.
Like the spacer pin 66, the shift pin 80 has a flat 82 extending along part of its length and adapted to receive a second locating pin 84, the locating pin 84 being adapted to prevent rotation of the shift pin 80 in the bore 74. The length of the shift pin 80 is selected so that only one end thereof will protrude into one of the grooves 76, 78.
A pair of axially spaced detents 86 is formed in the smaller diameter portion of the shift pin 80 and the detents are adapted to alternately engage a spring biased nib 88 projecting into the path of movement of the shift pin 80. It will be understood that the separation between the detents 86 equals the distance of axial travel of the shift pin 80 and is selected to be approximately equal to the depth of each of the grooves 76, 78.
Corresponding bores 90, 92, 94 are formed in the blocks 46 for receiving the first and second locating pins 72, 84 and a coiled spring fastener 96 associated with the nib 88, respectively.
A sloped channel 98 is machined from the groove 62 at the other end of each block 46 and receives therein a wedged shaped reaction pad 100 made of hardened steel of corresponding slope and retained in position in the channel 98 by a pair of transversely extending dowel pins 102 which traverse the block through respective bores 104 and apertures 106 provided in the pad 100.
In use, the pin shifter rolls 32, on opposite sides of the starter bar are positioned for rolling engagement in the grooves 76, 78, so as to lie in the path of movement of the shift pins 80 and shift the axial position of the shift pins 80 from one groove to another when the starter bar 20 moves along the curved path between the extractor rolls 28 and the mold 24. The upstream pin shifter roll 32A (FIG. 6) nearest the mold 24 and positioned adjacent the groove 78 will cause the shift pins 80 to move axially so that the small diameter end extends into the opposite groove 76 and the large diameter end will be in contact with the spacer pin 66 (FIG. 8) thereby causing the spacer pin 66 to protrude from the block 46 a pre-determined distance x (FIG. 5) into supporting engagement with the reaction pad 100 of the adjacent block. Conversely, the downstream pin shifter roll 32B positioned adjacent the groove 76 will cause the shift pins 80 to move axially so that the large diameter end of the shift pin 80 extends into the groove 78 and thereby allow the spacer pin 66 to be pushed under the load of the upstream end of the starter bar and the strand into contact with the small diameter portion of the shift pin 80.
The effect of the upstream pin shifter roll 32A is thus to extend the effective length of the operatively outer side of the starter bar body and the effect of the downstream pin shifter roll 32B is to allow the effective length of the operatively outer side of the body starter bar to be shortened thereby allowing the starter bar 20 to resume a straight configuration.
Once the head 22 of the starter bar passes through the pin shifter and guide roll pairs (32, 34) during withdrawal of the starter bar, the rolls are moved outwardly away from the casting train.
It will be appreciated that the length of the flat 70 machined into the spacer pin 66 is at least as great as the axial distance travelled by the spacer pin 66, i.e., the increase in diameter from the small diameter portion to the large diameter portion of the shift pin 80.
In order that the starter bar 20 be self-supporting, the distance x whereby the spacer pin 66 protrudes from the associated block 46 must be equal to the separation between said block and the exposed surface of the reaction pad 100 on the adjacent block. In use, this separation is adjusted to correspond to the length of the spacer pin protruding from a block by positioning the starter bar on a jig which will make it conform to the casting radius and wedging each of the reaction pads 100 along the inclined surface of the channel 98 into contact with the extended spacer pins 66, the bores 104 for receiving the dowel pins 102 only being machined once the vertical displacement of the reaction pads in the channel 98 have been finalized.
The invention thus combines the advantages of flexible starter bars and rigid starter, bars, in that a minimum of space and labour is required to store the starter bar after it has been severed from the continuously cast strand and the starter bar is self-supporting in the curved configuration and therefore does not require support rolls. Because the starter bar does not have any conventional pin connections, problems of wear and link binding are also avoided, less maintenance is required and costs of operation decreased.
It will be understood that several variations may be made to the above described embodiment of the invention without departing from the scope of the appended claims. In particular, the mechanism for controlling the displacement of the spacer pin 66 may vary considerably. It is also within the scope of the patent to provide variations of the block support means whereby the starter bar may straighten progressively from the curved configuration to the straight configuration by passing through several sets of pin shifter rolls and guide skids having varying inclinations.
Finally, in some casting machines, particularly in those having a large radius casting bow requiring a strand guide and support rollers, the starter bar need only be self-supporting at the top for about the first 30.degree. of the casting bow adjacent the mold.
The bottom portion of the starter bar (remaining 60.degree.) may be constructed more simply by omitting spacer and shifter pins. Such an embodiment of the invention is illustrated in FIGS. 9 to 13 of the drawings and is described in more detail below.
In FIG. 9, a starter bar generally indicated by numeral 120, is shown in which the top portion 122 is self-supporting and constructed in accordance with the invention in the manner described with reference to FIGS. 4 to 8. The self-supporting top portion 122 of the starter bar 120 occupies an angle .beta. in the casting bow when positioned to plug the mould 24. (Apparatus common to FIGS. 1 and 9 have been labeled with like reference numerals.) This angle .beta. is selected to leave the area located directly below the mold 24 and in the path of any molten metal break-outs from the mold free. In practice, the angle .beta. will vary in inverse proportion to the radius R of the casting bow and typically will have a magnitude of 25.degree. to 45.degree..
The bottom portion 124 of the starter bar 120 which occupies a complementary angle of (90.degree.-.beta.) is constructed in the manner shown in FIGS. 10-12. It will be seen that the bottom portion 124 is similar to the top portion in that it comprises a flexible substantially planar spine 126 segmented into a series of plates arranged end to end and having apertures 128 countersunk to receive the conical heads of fasteners 130 which secure the spine 126 to a series of underlying blocks 132. The blocks 132 differ from the blocks 46 in that they do not have any spacer pins 66 or shift pins 80 or any of the associated bores 68 and 74. Furthermore, no grooves for receiving the pin shifter rolls 32 and guide rolls 34 are necessary. In all other respects, the blocks 132 and 46 are constructed essentially in the same way. The major features of the blocks 132 are described below.
As seen most clearly in FIG. 10, the top surface of the blocks 132 has a pair of oppositely directed shoulders 134 extending along the length of the block and adapted to receive therebetween the spine 126. The thickness of the spine 126 is selected to be greater in height than the depth of the shoulders 134 so that the spine 126 will protrude from the block. A pair of through holes 136 extending between the top and bottom surfaces of the blocks is machined from the centre of each block 132, the holes 136 having a large diameter at each end to define a reduced diameter portion therebetween.
As indicated above, the fasteners 130 have a conical head at one end which in use locates in the countersunk apertures 128 below the outer surface of the spine 126. The other end of the fasteners 130 have a partially threaded reduced diameter portion. A set of Belville spring washers 138 and a regular washer 140 are secured on the reduced diameter portions of the fasteners by a retaining nut 142 such that the Belville spring washers 138 will bear against a locating shoulder 144 defined by the reduced diameter portion machined in the apertures 136.
Each block 132 has a tongue 146 at one end and a groove 148 at the other end so as to cooperate with the groove and the tongue, respectively, of adjacent blocks.
A sloped channel 150 is machined from the groove 148 and receives therein a wedged shaped reaction pad 152 made of hardened steel of corresponding slope and retained in position in the channel 150 by a pair of transversely extending dowel pins 154 which traverse the block through respective bores 156 and apertures 158 provided in the pad 152.
The bottom portion 124 of the starter bar 120 is assembled so as to assume a straight configuration by suitably positioning the reaction pads 152 in abutting relationship with the tongues 146 of adjacent blocks 132. This is most clearly shown in FIGS. 11 and 12. The bores 156 in the blocks for receiving the dowel pins 154 are machined after the vertical displacement of each of the reaction pads 152 in the respective channels 150 has been finalized.
Such an assembly will operate to allow the bottom portion 124 of the starter bar 120 to assume a curved configuration conforming to the radius R of the casting bow (FIG. 9) while preventing the spine 126 from flexing in the opposite direction away from a flat configuration. The bottom portion 124 is supported in the curved configuration by a strand guide 160 and by support rollers 162 provided in pairs upstream and downstream of the guide.
After withdrawal from the extractor rolls 28, the entire starter bar 120, including the top portion 122 and bottom portion 124, is directed onto a movable storage ramp 164 supported by a scaffold 166 where it is kept flat. The bottom portion 124 is simply restored to its assembled configuration while the withdrawal of the top portion 122 through the pin shifter rolls 32 causes the spacer pins 66 to collapse into the bores 74 as described above with reference to FIG. 7, thereby allowing the top portion to likewise assume a straight configuration. Finally, the starter bar 120 is severed from the leading end of a strand 170 formed during continuous casting from the molten metal 26. A runout table indicated by numeral 168 in FIG. 13 supports the hot strand 170.
It will be appreciated that the embodiment of FIGS. 9 to 13 is a simplified structure which is more economical to manufacture than a starter bar which is made self-supporting in accordance with the invention throughout its length.
Claims
I claim:
1. A starter bar for closing a mold used in a continuous casting machine and for guiding the leading end of a strand from the mold in a curved path, the starter bar having a head at one end of the bar for attachment to the leading end of the strand, a tail at the other end of the bar for guiding the bar between rollers forming part of the continuous casting machine, and a body disposed between the head and the tail comprising a bottom portion adjacent the tail and a self-supporting top portion adjacent the head, the body comprising: a flexible substantially planar spine on an operatively inner side of the body, the spine extending longitudinally between the head and the tail, and defining an inner radius of curvature for the starter bar lying in said curved path; a series of blocks on an operatively outer side of the body arranged end to end and attached on one side thereof to the operatively outer surface of the spine, the blocks thereby defining an outer radius of curvature for the starter bar; and in the self-supporting top portion, a plurality of retractable block support means each disposed between adjacent pairs of blocks and adapted to protrude from one block of a pair a pre-determined distance x so as to lie in abutting and supporting relationship with a bearing surface on the other block of said pair and thereby increase the effective length of the operatively outer side of the body so that the starter bar may assume a curved configuration and be self-supporting, said block support means being retractable to shorten the effective length of the operatively outer side of the body so that the starter bar may assume a straight configuration.
2. Starter bar according to claim 1 in which said block support means comprises an axially movable spacer pin disposed to lie parallel to the longitudinal axis of the spine and retained within a respective block at an inner end thereof so that its outer end is exposed, and an axially movable shift pin retained within said block and disposed transversely to the spacer pin in abutting relation with the inner end of the spacer pin, the shift pin having different diameters along its length so that axial movement of the shift pin to change the diameter at the contact area on the shift pin with the spacer pin will bring about a corresponding shift in the axial position of the spacer pin.
3. Starter bar according to claim 2 in which the blocks each have a through hole near one end thereof to receive a shift pin, the hole extending transversely between first and second longitudinal grooves formed in respective sides of the block, said grooved sides being transverse to the spine, and the shift pin being adapted to move axially between the grooves, the grooves being adapted to receive respective pin shifter rolls in rolling engagement therewith as the starter bar moves along said curved path, said pin shifter rolls being longitudinally spaced along said curved path so as to lie in the path of movement of the shift pins and shift the axial position of the shift pins toward opposing guide rolls when the starter bar moves along said curved path, an upstream pin shifter roll nearest the mold causing the spacer pin to protrude from the block and a downstream pin shifter roll remote from the mold allowing the spacer pin to be pushed into the block under the load of the upstream end of the starter bar and the strand.
4. Starter bar according to claim 2 in which said bearing surface against which the block support means abuts in the curved self-supporting configuration of the starter bar is defined by an insert mounted to a ramp surface on the associated block so that the separation between the insert and the adjacent block may be adjusted to equal the distance x.
5. Starter bar assembly comprising a starter bar and associated rolls for use with the starter bar, the starter bar having a head at one end of the bar for attachment to the leading end of a strand, a tail at the other end of the bar for guiding the bar between rollers forming part of the continuous casting machine, and a body disposed between the head and the tail comprising a bottom portion adjacent the tail and a self-supporting top portion adjacent the head, the body comprising: a flexible substantially planar spine on an operatively inner side of the body, the spine extending longitudinally between the head and the tail, and defining an inner radius of curvature for the starter bar lying in said curved path; a series of blocks on an operatively outer side of the body arranged end to end and attached on one side thereof to the operatively outer surface of the spine, the blocks thereby defining an outer radius of curvature for the starter bar; and in the self-supporting top portion, a plurality of retractable block support means each disposed between adjacent pairs of blocks and adapted to protrude from one block of a pair a predetermined distance x so as to lie in abutting and supporting relationship with a bearing surface on the other block of said pair and thereby increase the effective length of the operatively outer side of the body so that the starter bar may assume a curved configuration and be self-supporting, said block support means being retractable to shorten the effective length of the operatively outer side of the body so that the starter bar may assume a straight configuration; said block support means each comprising an axially movable spacer pin disposed to lie parallel to the longitudinal axis of the spine and retained within a respective block at an inner end thereof so that its outer end is exposed, and an axially movable shift pin retained within said block and disposed transversely to the spacer pin in abutting relation with the inner end of the spacer pin, the shift pin having different diameters along its length so that axial movement of the shift pin to change the diameter at the contact area on the shift pin with the spacer pin will bring about a corresponding shift in the axial position of the spacer pin; the blocks each having a through hole near one end thereof to receive a shift pin, the hole extending transversely between first and second longitudinal grooves formed in respective sides of the block, said grooved sides being transverse to the spine, and the shift pin being adapted to move axially between the grooves, the grooves being adapted to receive respective pin shifter rolls in rolling engagement therewith as the starter bar moves along said curved path; and the associated rolls comprising an upstream pair of pin shifter and guide rolls and a downstream pair of pin shifter and guide rolls, the rolls of a pair being disposed on opposite sides of the starter bar in said longitudinal grooves, and said pin shifter rolls being longitudinally spaced along said curved path so as to lie in the path of movement of the shift pins and shift the axial position of the shift pins toward the opposing guide rolls when the starter bar moves along said curved path, the upstream pin shifter roll nearest the mold causing the spacer pin to protrude from the block and the downstream pin shifter roll remote from the mold allowing the spacer pin to be pushed into the block under the load of the upstream end of the starter bar and the strand.