Apparatus and method for guiding continuously cast sections

Abstract

 A continuously cast strand guiding apparatus (4) comprises plural sets of walking bars (16, 17) that are paired opposite to each other on both sides of a cast strand, movable along and across the travel line of the cast strand and driven forward by the cast strand intergrally therewith while the cast strand is being gripped thereby, a reciprocating device (41, 42) that causes each set of the paired gripping means to integrally make an approach run from the standby position to the starting point where the cast strand is gripped for guiding and alternately move back and forth along the travel line so that one set moves forward from said starting point to the terminal point while the other set moves backward from the terminal point to the starting point, and a compressing device that applies a gripping force to the walking-bars so that the cast strand is gripped at the starting point and released at the terminal point, each set of walking-bars is independently pressed toward the cast strand at an equal point relative to the travel direction of the cast strand, and each set of walking-­bars grip the cast strand for an overlapped period of time.

Description

Background of the Invention

Field of the Invention

[0001] This invention relates to an apparatus and a method for gripping, carrying forward and guiding such continuously cast sections as slabs, blooms and billets. More particularly, it relates to an apparatus and a method for guiding continuously cast sections that decrease variations in the liquid steel level in the mold when applied on the unsolified portion of the cast strand and substantially eliminate segregation and center porosity when applied on such a portion of the cast strand where solidification is nearly complete.

Description of the Prior Art

[0002] The conventional walking-bar type strand guide, typical examples of which are disclosed in Japanese Patent Publication No. 31321-1969 and Japanese Provisional Patent Publication No. 134055-1981, comprises two sets of walking-bars that alternately hold and guide the cast section. With this type of strand guide, the two sets of walking-bars are actuated to exert their gripping force on different points of the cast section.

[0003] In other words, the two sets of walking-bars apply different compressive gripping forces on equivalent portions of the strand relative thereto. Heretofore, furthermore, the number of walking-bars contained in each set and the area of their contact with the cast strand have not always been equal. Despite this unbalance, conventional walking-bar type strand guides have ordinarily been designed so that substantially equal amount of gripping force is exerted by both sets of walking-bars.

[0004] As a combined result of all this, each set of walking-bars in a conventional strand guide has exerted a different amount of compressive gripping force on the cast strand. If pressure is applied more positively, a resulting compression in the cast strand has varied between one set of bars and the other. When a conventional strand guide is placed immediately under the mold, fluctuation in the liquid metal level in the mold has been caused, resulting in poor surface quality. When it is installed in an area containing the point of complete solidification, creation of segregation and center porosity has been unavoidable because of the dispersion or aggregation of unsolidified steel, in which solute concentration is high, immediately before the completion of solidification.

[0005] At the center of the continuously cast section, there is usually segregation of such elements as carbon, sulphur and phosphorus that have high segregation coefficients. When such a cast section is rolled, the product will have a segregation line along the center of its cross-section. Such products are likely to develop rupture and other defects along the center-line segregation.

[0006] Segregations can be classified into macrosegregation and microsegregation according to their form. Both types of segregation stem from the movement of unsolidified liquid steel resulting from bulging and other irregularities caused between individual rolls by the contraction of the solidifying steel and ferrostatic pressure of the liquid steel. In order to eliminate segregation, therefore, unsolidified steel in the solidifying steel must be kept in a completely non-­mobile state. Simply preventing bulging, however, allows the creation of center porosity as well as segregation, failing to make up for solidification shrinkage.

[0007] The method disclosed in Japanese Patent Publication No. 16551-1984 offers a solution to the above problem, in which the portion of the cast strand near the point of complete solidification is tapered by surrounding surfaces. To be more specific, the continuously cast strand is led to a frame containing a series of spaced guides that is provided near the point of complete solidification where thickness is reduced by 0.5 percent to 2.0 percent per meter.

[0008] The technique according to Japanese Patent Publication No. 16551-1984 avoids the motion of the unsolidified steel in the leading end of the solidifying strand. But the influence of bulging in the area closer to the mold remains uneliminated.

[0009] The inventors offered a solution to this problem in Japanese Patent Application No. 168534-1984 (Japanese Patent Publication No. 46360-1986). With the proposed technique, the leading end of the continuously cast strand near the point of complete solidification is held and compressed by surrounding surfaces according to the rate of solidification and shrinkage while maintaining the surface temperature of the cast strand between the leading end thereof containing unsolidified steel and a given upstream portion of the same section closer to the mold at 600°C to 900°C. This method makes the solidifying shell rigid enough to withstand the gripping by surfaces, reducing the amount of bulging to 0.05 mm maximum. Accordingly, it prevents the undesirable motion of the liquid core, the solute concentration in a phase where solidified and unsolidified steels coexist, and the occurrence of macro- and microsegregation. Also, it effectively prevents the motion of the unsolidified steel in the solidifying shell and the creation of center porosity due to solidification and shrinking.

[0010] But no apparatus or method of the type that compresses a portion of the cast strand near the point of complete solidification by surrounding surfaces according to the rate of solidification and shrinkage has been put into practical use.

Summary of the Invention

[0011] The object of this invention is to provide an apparatus and a method for guiding the continuously cast strand of the type that offers a solution to the aforementioned problem.

[0012] With a cast section guiding apparatus of this invention, the cast strand is held and carried forward by means of plural sets of gripping means that are adapted to alternately grip the cast strand. Also, provision is made so that the alternately acting sets of gripping means exert an equal amount of compression force on relatively equivalent points of the section being cast. Accordingly, the compression force exerted by each of the plural sets of gripping means, whether it is just for gripping or for more positive compression, is equally distributed. It is also possible to make the product of the total area of contact times the applied pressure equal for all sets of gripping means by making equal the area of contact between the cast section and each set of gripping means or adjusting the exerted compression force according to the difference in the contact area between different sets of gripping means. By so doing, the equal compressive gripping force exerted by the gripping means is uniformly distributed throughout the entirety of the section being cast. This ensures that the cast strand is equally compressed by different sets of gripping means. When the guiding apparatus of this invention is installed immediately under the mold or at a point where the liquid steel in the cast strand still remains unsolidified, the liquid metal level in the mold always remains at rest. When the same apparatus is installed near the point of complete solidification, the unsolidified steel in the solidifying shell, in which solute concentration is high, is kept in a non-mobile state. Consequently, no such defects as segregation and center porosity are produced. The excellent results include an increase in the yield and productivity of continuous casting and a cutdown in production costs.

[0013] With a cast section guiding method of this invention, each set of gripping means applies a required amount of compression force on a relatively equivalent portion of the continuously cast strand being gripped thereby. Furthermore, the surface temperature of the cast strand between the leading end of the portion containing unsolidified steel and a given upstream portion closer to the mold is kept at 600°C to 900°C for a duration of time that ranges from a period in which the steel shell becomes rigid enough to ensure uniform distribution of surface tension (approximately 1 minute) to a period in which the cast strand reaches a point where effective recuperation may no longer be achieved following the completion of solidification in the surrounding gripping surfaces (approximately 7 minutes). These measures increase the rigidity of the solidifying shell gripped by the gripping means and assure uniform distribution of surface tension across the shell. Consequently, uniform distribution of compression force and uniform compression are achieved with greater ease, as a result of which the amount of bulging is reduced to 0.05 mm maximum and the motion of unsolidified steel due to bulging is substantially completely prevented. The gripping means apply uniform compressive force throughout the strand being cast, thereby surely preventing the occurrence of the defects mentioned previously.

[0014] A continuously cast section gripping apparatus according to this invention comprises plural sets of paired gripping means disposed opposite to each other to grip the section being cast, the gripping means being adapted to be moved both along and perpendicular to the travel line of the cast section and driven forward by the cast section integrally therewith while the cast section is in grip, reciprocating drive means that causes each set of the paired gripping means to integrally make an approach run from the standby position to the starting point where the cast strand is gripped for guiding and alternately move back and forth along the travel line so that one set moves forward from said starting point to the terminal point while the other set moves backward from the terminal point to the starting point, and gripping force exerting means that independently pushes each set of gripping means toward the cast strand at the same point relative to the travel direction thereof so that the cast section is gripped at said starting point and released at said terminal point, the gripping force exerting means being adapted to exert the gripping force so that the gripping periods of the plural sets of gripping means overlap one another, and the gripping force being exerted on the cast strand through the plural sets of gripping means at a dynamically equivalent point relative to the cast strand being gripped. In the apparatus just described, provision may be made so that each set of gripping means can be integrally put in and out of the travel line along the plane perpendicular to the direction in which the cast strand is gripped, carried forward and guided. Also, means to detect the displacement of the paired gripping means and a controller to regulate the operation of said gripping force exerting means in accordance with the signals produced by the displacement sensing means may be provided. Furthermore, controlling means that makes equal the gripping force exerted by the plural sets of gripping means on the basis of contact area between each set of gripping means and the cast strand may be provided, as well.

[0015] In alternately gripping and carrying forward the continuously cast strand using plural sets of paired gripping means, a continuously cast section gripping method according to this invention keeps the surface temperature of the cast strand between the point where the leading end of unsolidified steel terminates and a given upstream point closer to the mold at 600°C to 900°C, maintains the above surface temperature while the cast strand is being gripped by each set of gripping means, and exerts the required gripping force on the cast strand through the gripping means at a dynamically equivalent point relative to the cast strand. It is preferable that the continuously cast strand is focibly cooled so that the surface temperature between said two points is kept at approximately 600°C to 900°C for a period of approximately 1 minute to 7 minutes.

Brief Description of the Drawings

[0016] 

Fig. 1 schematically shows the travel of the continuously cast strand from the mold to a walking-bar type gripping apparatus;

Figs. 2 to 4 show a preferred embodiment of the cast section guiding apparatus according to this invention; Fig. 2 is a side elevation; Fig. 3 is a front view; and Fig. 4 is a perspective view;

Fig. 5 is a cross section taken along the lines A to D of Fig. 3 showing the motion of double-eccentric bearings when the outer walking-bar is pressed down for gripping;

Fig. 6 is a system diagram of a control device in the guiding apparatus;

Fig. 7 is a block diagram of the control device;

Fig. 8 is an operational block diagram of the guiding apparatus;

Fig. 9 is a side elevation of another preferred embodiment;

Fig. 10 is a front view of the guiding apparatus shown in Fig. 9;

Fig. 11 is a cross section taken along the line A to D of Fig. 10 showing the motion of double-eccentric bearings and eccentric shafts when the outer walking-bar is pressed down for gripping; and

Figs. 12 and 13 are sketches of standard photographs corresponding to the scores of macrosegregation and V-segregation.

Description of the Preferred Embodiments

[0017] Fig. 1 schematically shows an example of the travel of the continuously cast strand from the mold to a walking-bar type cast-section gripping apparatus.

[0018] After leaving the mold 1, the cast strand S is guided by a series of guide rolls 3 to a walking-bar type guiding apparatus 4. Instantaneous cooling devices 5 disposed along the path of travel spray cooling water on the cast strand S immediately before the strand is gripped by the walking-bar type guiding apparatus 4, thereby keeping the surface temperature of the cast strand S at a temperature between 600°C and 900°C.

[0019] The surface temperature is kept in the range of 600°C to 900°C for a period time between the points at which the cast strand is gripped and released by the walking-bars, ranging from 1 minute to 7 minutes.

[0020] The makeup of the walking-bar type guiding apparatus 4 is as described in the following paragraphs. Figs. 2 to 4 show a preferred embodiment of the guiding apparatus. Fig. 2 is a side elevation, Fig. 3 is a front view and Fig. 4 is a perspective view. The illustrated guiding apparatus is used in an area where the cast strand S is guided horizontally.

[0021] A housing 7 which supports walking-members comprises vertical frames 8 erected on both sides of the travel line of the cast strand and a horizontal frame 9 that connects the two vertical frames 8 at the top thereof. A hydraulic cylinder 91 for taking in and out traversing wheels 93 is provided in the lower part of each vertical frame 8, the traversing wheels 93 being connected to the cylinder rod thereof. Rails 97 are laid on a floor 95 in such a manner as to cross the cast-strand travel line, with the housing 8 mounted thereon through the traversing wheels 93. Each rail 97 has a rack-like teeth cut thereon, and a car (not shown) equipped with a worm gear engaging with the teeth and a motor to drive the worm gear travels thereover to put the housing 7 in and out of the cast-strand travel line. The position of the housing 7 is set by means of a stopper 98 fastened at the far end of the rail 97 and a vertically retractable stopper 98 provided at a midway point of the rail 97. The direction of the in-and-out motion of the guiding apparatus is not necessarily limited to horizontal. The guiding apparatus may also be moved up and down, like an elevator, along veticaly disposed rails by means of a hydraulic unit. The movable guiding apparatus is easy to inspect and maintain. Also, provision may be made so that the housing 7 is moved by means of a nut attached to the vertical frame 8 and a threaded shaft fastened on the floor.

[0022] Paired walking-members, one lying on top of the other, are disposed above and below the continuously cast strand S. There are two sets of such paired walking-members, one set consisting of outer walking-­members 11 and the other of inner walking members 12, which are supported by the housing 7 as described below. The outer and inner walking-members 11 and 12 respectively have outer walking-bars 16 and inner walking-bars 17, which are the lower and upper portions of a body 14 extending toward the cast-strand travel line, adapted to press the top and bottom surfaces of the cast strand S. While the outer walking-bars 16 are four in number, the inner walking-bars 17 are three. But the area of contact between each set of walking-bars and the cast strand is designed to be equal. The outer walking-bars 16 and inner walking-bars 17 are arranged to cover the entire width of the cast strand S in an alternately offsetting pattern.

[0023] The outer and inner top walking-bars 11 and 12 are designed to fall under their own weight. To permit their release from the cast strand S, therefore, a walking-member lifting device 21 is provided to the horizontal frame 9 of the housing 7. The walking-member lifting device 21 has an arm 22 whose one end id pivoted to a base 24 and the other end to a spring- or hydraulically loaded supporting device 25. The supporting device 25 upholds the other end of the arm 22 by means of a disk spring or the like (not shown). The upper end of a downward-extending connecting rod 27 is pivoted to a midway point of the arm 22. The lower end of the connecting rod 27 is pivoted to the top surface of the top outer and inner walking-members 11 and 12. Accordingly, the outer and inner walking-members 11 and 12 can be swung back and forth (to the right and left in Fig. 2). The top and bottom outer and inner walking-­members 11 and 12 are connected by means of reciprocating link mechanisms 31 and 32 to be described below. Therefore, the outer and inner walking-members 11 and 12 at the top and bottom are integrally supported by the housing 7.

[0024] Each of the outer and inner walking-member reciprocating link mechanisms 31 and 32 that cause the outer and inner walking-members 11 and 12 to make an approach run and a return trip has a rotary shaft 34 supported by bearings 33 fastened to the vertical frames 8 of the housing 7. Four sets of the bearings 33 and rotary shafts 34 are provided to support the two each pair of the outer and inner walking-members 11 and 12 at the top and bottom. To each rotary shaft 34 are fastened arms 35, with one end of a rod 36 being pivoted to the tip of each arm 35. The other end of the rod 36 is pivoted to the end surface of the body of the outer and inner walking-members 11 and 12. Arms 37 and 38 are fastened to one end of the top and bottom rotary shafts 34. The top and bottom arms 37 and 38 are connected to each other by means of a rod 39 pivoted thereto. The housing 7 carries the tubes 43 of reciprocating hydraulic cylinders 41 and 42. The rods 44 of the reciprocating hydraulic cylinders 41 and 42 are pivoted to the other end of the arms 38. When the reciprocating hydraulic cylinders 41 and 42 are driven, therefore, the outer and inner walking-members 11 and 12 are interlockingly moved back and forth along the travel line of the cast strand.

[0025] Four horizontally and vertically spaced chucking shafts 46 for the paired outer and inner walking-members 11 and 12 at the top and bottom are fastened to the vertical frames 8 of the housing 7. To each chucking shaft 46 is rotatably and concentrically attached a wheel 49 having a bearing-supported eccentric sleeve 51 for the outer walking-member and a wheel 50 having a bearing-supported eccentric sleeve 51 for the inner walking-member.

[0026] Two each compressing link mechanisms 55 and 56, each having a rotary shaft 57, for the outer and inner walking-members 11 and 12 at the top and bottom are attached to the housing 7. The rotary shafts 57 for the outer walking-members are supported by bearings 58 fastened to the vertical frames 8. Arms 60 are fastened to each rotary shaft 57. While one end if a rod 62 is pivoted to the tip of the arm 60, the other end of the rod 62 is pivoted to the ear 52 of said eccentric sleeve 51.

[0027] While the tubes 67 of compressing hydraulic cylinders 65 and 66 are connected to the housing 7 by pins P, the rods 68 thereof are pivoted to the free end of arms 61 fastened to said rotary shafts 57. When the compressing hydraulic cylinders 65 and 66 are driven, therefore, the eccentric sleeves 51 concentrically rotate around the chucking shafts 46 to push the outer and inner walking-members 11 and 12 toward the cast strand S through the wheels 49 and 50.

[0028] Fig. 5 shows the wheel 49 having the bearing-­ supported eccentric sleeve 51 in operation when the outer walking-bar 16 is compressing the cast strand S at A and D, and the wheel 50 out of operation when the inner walking-bar 17 is not compressing the cast strand S.

[0029] When the wheel 50 having the bearing-supported eccentric sleeve 51 for the inner walking-bar is rotated by the inner walking-bar compressing hydraulic cylinder 66 through the inner walking-bar compressing link mechanism 56, as in exerting force through the outer walking-bar 16, the inner walking-bars 17 at the top and bottom respectively mode down and up to grip the cast strand S. Then, the wheel 50 having the bearing-­supported eccentric sleeve 51 comes in contact with the inner walking-bar 17, thus falling into the condition shown at A and D. On the other hand, the condition at A and D is reversed to the one at B and C.

[0030] Because of the four chucking shafts 46, two each at the top and bottom, attached to the vertical frames, no gripping force is exerted on the cast strand S. Accordingly, the gripping force becomes an internal force working between the two shafts 46 at the top and bottom. Essentially, therefore, no more load than the self weight of the apparatus works on the foundation.

[0031] The compressing hydraulic cylinders 65 and 66 are controlled by a controller. Figs. 6 and 7 are a control system diagram and a block diagram. Hydraulic fluid is supplied from a hydraulic source 73 to the compressing hydraulic cylinders 65 and 66 through a servo valve 74 that is controlled by a PI-actuated pneumatic controller 71. To the controller 71 are inputted signals from a pressure gauge 76 that senses the pressure in the compressing hydraulic cylinders 65 and 66 and a walking-­bar displacement sensor 77. The walking-bar displacement sensor 77 detects the displacement of the top and bottom walking-bars 16 and 17 as shown in Fig. 3 and 4. A loadcell 78 to determine the compressing load is interposed between the housing 7 and the chucking shaft 46 as shown in Fig. 3.

[0032] Next, the operation of the above-described guiding apparatus will be described. Essentially, the outer and inner walking-bars 16 and 17 of this preferred embodiment perform compression in an overlapped pattern as shown in Fig. 8.

[0033] To be concrete, the inner walking-bars 17 actuate the inner walking-bar compressing hydraulic cylinder 66 for chucking while the outer walking-bars are compressing the cast strand S, thereby lowering the inner walking-bars 17 through the inner walking-bar compressing link mechanism 56 as described previously. At the same time, the inner walking-bar reciprocating hydraulic cylinder 42 is actuated to move the inner walking-bars 17 at substantially the same speed as the casting speed so that no excessive force is exerted on the cast strand S in chucking. By the action of the inner walking-bar reciprocating hydraulic cylinder 42, the inner walking-bars 17 at the top and bottom are simultaneously accelerated through the inner walking-bar reciprocating link mechanism 32. The inner walking-bars 17 are accelerated to a given speed by the time when chucking is effected. The acceleration is completed when chucking is performed. On completion of chucking, the inner walking-bars 17 move forward while gripping the cast strand S to the point of releasing, keeping pace with the travel speed of the strand.

[0034] the outer walking-bars 16 release the cast strand S after it has been chucked by the inner walking-bars 17. The release of the cast strand S is effected through the outer walking-bar compressing link mechanism 55 by extracting the hydraulic fluid from the outer walking-­bar compressing hydraulic cylinder 65.

[0035] When the outer walking-bars 16 are away from the cast strand S by a given distance, the outer walking-bar reciprocating hydraulic cylinder 41 is actuated to return the outer walking-bars 16 to a predetermined position through the outer walking-bar reciprocating link mechanism 31. Then, the chucking process of the outer walking-bars begins. This process is performed in the same manner as the chucking by the inner walking-­bars. Namely, the outer walking-bar compressing hydraulic cylinder 65 is actuated to respectively move down and up the outer walking-bars 16 at the top and bottom through the outer walking-bar compressing link mechanism 55. At the same time, the outer walking-bar reciprocating hydraulic cylinder 41 is actuated to accelerate the outer walking-bars 16 to a given speed through the outer walking-bar reciprocating link mechanism 31.

[0036] The release and return of the inner walking-bars 17 are also performed in the same manner as those of the outer walking-bars 16. Namely, the hydraulic fluid is extracted from the inner walking-bar compressing hydraulic cylinder 66 to cause the inner walking-bars 17 to release the cast strand S through the inner walking-­bar compressing link mechanism 56. When the inner walking-bars 17 are away from the cast strand S by a given distance, the inner walking-bar reciprocating hydraulic cylinder 42 is actuated to return the inner walking-bars 17 to a predetermined position, where they stand ready for the next operation, through the inner walking-bar reciprocating link mechanism 32.

[0037] Control of the strand gripping or compressing action will be described in the following (refer to Figs. 6 and 7).

[0038] After the cast strand S has been chucked by the outer walking-bars 16 or inner walking-bars 17, a given amount of compression force is applied thereon at a given rate depending on the rate of solidification and shrinkage. A host computer (TOSHIBA TOSBAC 70G) 72 determines the desired compression force and rate on the basis of casting conditions, with the obtained results set in a controller 71. The pressure corresponding to the bulging force, which is to be exerted by the outer and inner walking-bar compressing hydraulic cylinders 65 and 66, is calculated beforehand.

[0039] For gripping the cast strand S, hydraulic fluid is fed from a fluid source 73 to the outer and inner walking-bar compressing hydraulic cylinders 65 and 66. The point at which a pressure gauge 76 senses a pressure corresponding to said bulging force is established as the zero point. On recognizing the zero point based on the signals sent from the pressure gauge 71 and an outer and inner walking-bar displacement sensor 77, the controller 71 instructs a compression rate in accordance with the elapsed time from the zero point. The controller 71 translates the compression rate into the lever ratio of the outer and inner walking-bar compressing link mechanisms 55 and 56 and the amount of eccentricity of the outer and inner wheels 49 and 50 and outputs the result to a hydraulic valve 74 as a control singal of the PI operation. While the cast strand S is being guided under compression by the outer and inner walking-bars 16 and 17, the outer and inner walking-bar displacement sensor 77 determines the displacement or compression rate of the outer and inner walking-bars 16 and 17, feeding back the results to the controller 71. The controller 71 adjusts the amount and feed rate of the hydraulic fluid that is supplied to the outer and inner walking-bar compressing hydraulic cylinders 65 and 66 through the hydraulic valve 74. Consequently, the amount of eccentricity of the outer and inner wheels 49 and 50 vary with the rod stroke of the outer and inner walking-bar compressing hydraulic cylinders 65 and 66 to attain the desired amount and rate of compression. Also, provision may be made so that the controller 71 recognizes the zero point on the basis of the signal from the loadcell 78.

[0040] Now another preferred embodiment of the guiding apparatus will be described by reference to Figs. 9 to 11. Since this embodiment is analogous to the previously described one in fundamental structure and operation, similar parts are designated by similar reference characters, without detailed description.

[0041] This embodiment differs from the previous one in the following points. The cast strand S is gripped by the inner walking-bars 17 that are actuated by means of inner walking-bar compressing wheels 83 that are moved up and down by the eccentric chucking shafts 47 supported by bearings 82 that are driven by an electric motor (not shown) through spindles 81. Fig. 11 shows, like Fig. 5, the operating conditions of the wheels 83 and 49 of the second preferred embodiment in a state in which the inner walking-bars 17 are not compressed. This embodiment automatically performs fine adjustment of compression rate using a hydraulic cylinder 85 for an automatic gauge controller of the known type and allowing for the elongation of the vertical frames 8, deflection of the bars, and other variables. The hydraulic cylinder 85 is controlled based on the signals supplied from the loadcell 78. The outer walking-bars 16 grip the cast strand S when the outer walking-bar compressing hydraulic cylinders 65 exert force through the outer walking-bar compressing link mechanisms 55 on the wheels 49 equipped with the bearing-supported eccentric sleeves 51 for the outer walking-bars. Fine adjustment of the outer walking-bars 16 is also performed by the hydraulic cylinder 85, as with the inner walking-bars 17.

[0042] Using the guiding apparatus of this invention described above, cast strands were gripped and carried forward while keeping the surface temperature of the strands between the leading end of the portion containing unsolidified steel and a given upstream point closer to the mold at 600°C to 900°C and compressing the individual sets of walking-bars in accordance with the rate of solidification and shrinkage. The results are shown in Table 1, along with the results of comparative examples obtained by use of a conventional apparatus.

[0043] As is obvious from the table, the examples according to this invention (Nos. 1 to 11) produced no macro-segregation, V-segregation and center porosity. The scores of semimacro-segregation were expressed by the ratios of the phosphorus concentration in the segregated phase, which is determined by a computer-­aided micro analyzer (CMA) of a known type, to the phosphorus concentration determined by ladle analysis. With Nos. 5 and 8, the score was 1, meaning that no segregation has occurred. Even with Nos. 1 to 4, 6, 7 and 9, the score was as good as 2, although their phosphorus concentrations determined by ladle analysis were high. These figures attest the remarkable improving effect of our invention.

[0044] In examples Nos. 12 and 13, on the other hand, the surface temperature between said two points was not kept at 600°C to 900°C for a period of 1 minute to 7 minutes. With these examples, the incidence of macro-, V- and semimacro-segregations and center porosity was higher than in the examples obtained with the apparatus of this invention. Still worse results were obtained in examples Nos. 14 to 20.

[0045] This invention should not be limited to the particular embodiments described hereabove. For example, the guiding apparatus may be installed immediately under the mold. In a curved-type continuous caster, the walking-bars are curved along the profile of the cast strand. The reciprocating and compressing hydraulic cylinders may be replaced with pneumatic cylinders of electric motors. Also, the pneumatic controller may be replaced with an electronic controller.

Claims

1. A gripping apparatus for gripping and guiding a continuously cast strand over a given distance along the line of travel thereof comprises:
plural sets of paired gripping means disposed opposite to each other to grip the strand being cast, the gripping means being adapted to be moved both along and perpendicular to the travel line of the cast strand and driven forward by the cast strand integrally therewith while the cast strand is in grip;
reciprocating drive means that causes each set of the paired gripping means to integrally make an approach run from the standby position to the starting point where the cast strand is gripped for guiding and alternately move back and forth along the travel line so that one set moves forward from said starting point to the terminal point while the other set moves backward from the terminal point to the starting point; and
gripping force exerting means that independently pushes each set of gripping means toward the cast strand so that the gripping means grips the cast strand at an equivalent point relative to the travel direction thereof with a substantially equal amount of force and releases the cast strand at said terminal point, the gripping force exerting means being adapted to exert the gripping force in such a manner that the gripping periods of the plural sets of gripping means overlap one another.

2. A gripping apparatus according to claim 1, in which said gripping means comprises plural walking-bars extending along the travel line of the cast strand and compressing the surface thereof, the walking-bars of adjoining sets of gripping means being arranged in an alternately offsetting pattern.

3. A gripping apparatus according to claim 2, in which each set of gripping means has an equal number of walking-bars.

4. A gripping apparatus according to claim 2, in which each set of gripping means has an equal area of contact between the walking-bars and the cast strand.

5. A gripping apparatus according to any one of claims 1 to 4, in which said reciprocating drive means comprises a reciprocating link mechanism that connects a pair of oppositely disposed gripping means and a hydraulic cylinder to drive the reciprocating link mechanism to reciprocate the gripping means.

6. A gripping apparatus according to any one of claims 1 to 5, in which a common shaft extending perpendicularly to the plural sets of gripping means (in the direction of the strand width) is provided so that cast strand gripping force is applied to the gripping means through rotary cams mounted on the common shaft.

7. A gripping apparatus according to any one of claims 1 to 6, in which said gripping force exerting means comprises a cam held in contact with the gripping means, a gripping force exerting link mechanism connected to said cam and a gripping force exerting hydraulic cylinder that actuates the gripping force exerting link mechanism to press the gripping means against the surface of the cast strand.

8. A gripping apparatus according to any one of claims 1 to 6, in which said gripping force exerting means comprises cams respectively held in contact with two sets of gripping means, a gripping force exerting link mechanism and a gripping force exerting hydraulic cylinder actuating the gripping force exerting link mechanism to press the gripping means against the surface of the cast strand that are connected to the cam held in contact with one set of gripping means, and a gripping force exerting drive means that rotates the cam held in contact with the other set of gripping means through a common shaft shared by said first cam to press the gripping means against the surface of the cast strand.

9. A gripping apparatus according to any one of claims 1 to 8 that is integrally movable in the direction that is perpendicular in a horizontal plane to the direction in which the cast strand is gripped, carried forward and guided.

10. A gripping apparatus according to any one of claims 1 to 9 that is integrally movable in the direction that is perpendicular in a vertical plane to the direction in which the cast strand is gripped, carried forward and guided.

11. A gripping apparatus according to any one of claims 7 to 10 which comprises means to detect the displacement of said paired gripping means and a controller that regulates the operation of said gripping force exerting hydraulic cylinder in accordance with signals supplied from said displacement detecting means.

12. A gripping apparatus according to any one of claims 1 to 11, in which said gripping means comprises means to adjust the gripping force in accordance with the area of contact with the cast strand.

13. A gripping apparatus for gripping and guiding a continuously cast strand over a given distance along the line of travel thereof comprises:
a housing;
plural sets of paired walking-members disposed opposite to each other on both sides of the cast strand;
means supporting the walking-members on said housing in such a manner that the walking-members can move along and across the travel line of the cast strand and that the walking-members are driven forward by the cast strand intergrally therewith while the cast strand is being gripped thereby:
a reciprocating drive link mechanism that connects the oppositely paired walking-members;
a hydraulic cylinder that drives the reciprocating drive link mechanism to cause each set of the paired gripping means to integrally make an approach run from the standby position to the starting point where the cast strand is gripped for guiding and alternately move back and forth along the travel line so that one set moves forward from said starting point to the terminal point while the other set moves backward from the terminal point to the starting point;
a shaft attached to said housing in such a manner as to cross the travel line of the cast strand;
cams attached to the shaft in such a manner as to come in contact with the walking-members in the same position relative to the travel direction of the cast strand, thereby ensuring that the gripping force is applied to the walking-members at a dynamically equivalent point relative to the cast strand;
a gripping force exerting link connected to said cam; and
a gripping force exerting hydraulic cylinder that drives said gripping force exerting link in such a manner that the cast strand is gripped at said starting point and released at said terminal point, the gripping force exerting hydraulic cylinder applies the gripping force to the walking-members so that the plural sets of walking-members grip the cast strand for an overlapped period of time.

14. A method of guiding a continuously cast strand using plural sets of vertically paired gripping means that alternately grip and carry forward the cast strand which comprises the steps of keeping the surface temperature of the cast strand at 600°C to 900°C between the leading end of the portion containing unsolidified steel and a given upstream point closer to the mold while the cast strand is being gripped by each set of gripping means and imparting a substantially equal amount of force to each set of the gripping means at an equivalent point relative to the direction of the length of the cast strand.

15. A method of guiding a continuously cast strand according to claim 14, which comprises the steps of forcibly cooling the cast strand to attain the surface temperature of 600°C to 900°C between the leading end of the portion containing unsolidified steel and a given upstream point closer to the mold and maintaining the above surface temperature for a period of 1 minute to 7 minutes.

Drawing