Patent Publication Number: US-8109320-B2

Title: Method and apparatus for the continuous casting of preliminary steel sections

Description:
This application is a continuation of PCT Application No. PCT/EP06/011972, filed Dec. 13, 2006, which claims the benefit of European Application No. 05028469.4 filed Dec. 24, 2005, the entirety of which are incorporated by reference herein. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The invention relates to continuous casting of preliminary steel sections, such as, for example, preliminary I-sections. 
     2. Description of Related Art 
     Preliminary steel sections represent primary material for producing rolled sectional steel beams of I, H, U and Z cross-sectional shape as well as special sheet pile sections. A method for the continuous casting of preliminary sections of this kind is disclosed, for example, in EP-B-1 419 021. The continuous casting of preliminary sections was introduced on an industrial scale in the seventies and has been increasingly gaining in importance in recent years in consequence of the general trend towards so-called near net shape casting. 
     The preliminary sections are in most cases cast in an I-cross-sectional shape, the molten steel being introduced substantially vertically into a so-called “dog-bone” continuous mold whose mold cavity cross-section is composed of two flange parts and a web part. A preliminary sectional strand with a molten core is fed from the mold to a strand guide with secondary cooling devices. 
     Unlike the continuous casting of conventional long products of a rectangular or round cross section, the continuous casting of preliminary I-sections represents several problems, in particular in the case of preliminary sections with a relatively thin web part, when high strength special steel grades (CaSi or Al-killed and microalloyed steels with V, Nb, inter alia) are cast, or in the case of high-speed casting. For reasons of space, although also governed by economics, the molten steel is only introduced into the mold via one ingate, in most cases asymmetrically at the transition between the web part and one of the flange parts. It is consequently particularly difficult to fill the complicated mold cavity uniformly and without disturbing turbulence and thus create favorable conditions for the initial solidification while preventing near-surface casting defects (gas bubbles, pin holes). It is also difficult to obtain a symmetrical liquid flow inside the strand shell and consequently a symmetrical temperature distribution, which ultimately results in a homogeneous solidification structure. It is equally problematic, where a thin web part is concerned, to prevent arching during solidification and resultant core porosity and/or shrink holes. 
     A continuous mold for the continuous casting of preliminary I-sectional strands is known from JP 08 294746 A. Molten steel is introduced into the two flange parts via 2 submerged nozzles. In order to prevent surface defects on the preliminary sectional strand, it is proposed that a pair of static magnetic poles with S or N poles be disposed outside of the mold cavity both on the two flange outer sides and on both sides of the web part. Through the static magnetic field just below the mouth of the two submerged nozzles, the steel jet emerging from the submerged nozzles is to be slowed down and flow back in a horizontal flow to the mold wall and along this to the liquid surface. The static magnetic fields with N and S poles gives rise to a slowing-down effect of the vertical discharge flow from the submerged nozzles and an uncontrolled deflection from the vertical flow. This prior art does not refer to controlled, reversible traveling fields or flows in the molten crater for creating controlled flow and temperature conditions in the crater of the preliminary sectional strand. 
     SUMMARY OF THE INVENTION 
     The object of the present invention is to provide a method and apparatus by which preliminary steel sections, for example comprising two flange parts and a web part, can be produced with an improved quality, even if the preliminary section comprises a relatively thin web part and/or special steel grades are to be cast. A further aim, depending on the dimensions or the steel quality of the preliminary sectional strand, is to enable a symmetrical or an asymmetrical steel feed with one or with two open or closed ingates into the mold to be selected. 
     According to the invention, electromagnetically induced forces in the region of the flange parts and/or of the web part causes stirring movements in the molten core of the preliminary sectional strand transversely to the strand casting direction. Due to such stirring movements, the molten steel in the crater of the preliminary sectional strand is exchanged between flange parts and the web part. Thus, the flow and temperature conditions in the molten steel crater within the preliminary sectional strand shell are specifically and actively influenced. The invention produces the following beneficial and previously unobtained effects:
         stabilization of the metal surface region by suppressing turbulence, even in the case of varying process parameters, such as, for example, casting speed and metal surface position (for the purpose of preventing non-metallic inclusions as well as gas bubbles in the strand surface);   favorable, controllable flow conditions with a specifiable molten steel exchange between relatively thicker thickened cavity regions through a thinner web part, even in the case of an asymmetrical ingate, and thereby the formation of a uniformly thick strand shell with a favorable solidification structure, while preventing shrink holes and/or core porosity.   prevention of arching during solidification in spite of confined conditions in the web part of the mold cavity cross section.       

     In addition, different traveling field combinations in the flange parts and/or in the web part can be selected in the case of varying steel qualities or different dimensions of the preliminary sectional strand with the same stirrer. It is likewise possible to set traveling fields with completely different direction components in different locations, e.g., the flange parts and/or in the web part if the pouring system is changed, without making any structural changes to the stirrer. 
    
    
     
       DESCRIPTION OF THE DRAWINGS 
       The foregoing and other features of the present invention will be more readily apparent from the following detailed description and drawings of illustrative embodiments of the invention where like reference numbers refer to similar elements throughout and in which: 
         FIG. 1  shows a cross section of a mold in accordance with embodiments of the invention; 
         FIG. 2  shows a cross section of a mold in accordance with additional embodiments of the invention; 
         FIGS. 3-6  shows a cross section of a mold in accordance with further embodiments of the invention with different pole shoe connections; 
         FIGS. 7-8  shows a cross section of a mold in accordance with more embodiments of the invention with different pole shoe connections; 
         FIG. 9  shows a side view of a mold in accordance with yet additional embodiments of the invention; 
         FIG. 10  shows a cross section of a mold in accordance with yet further embodiments of the invention; 
         FIGS. 11-12  shows a cross section of a mold in accordance with yet more embodiments of the invention with different pole shoe connections; 
         FIG. 13  shows a side view of the mold shown in  FIG. 10  in according with embodiments of the invention; 
         FIG. 14  shows a cross section of a mold in accordance with even further embodiments of the invention; and 
         FIG. 15  shows an electrical schematic diagram in accordance with embodiments of the invention containing the mold shown in  FIG. 14 . 
     
    
    
     DESCRIPTION OF EMBODIMENTS OF THE INVENTION 
       FIG. 1  shows in schematic form a mold  1  (in horizontal mold cavity cross section) that is composed of two flange parts  2 ,  3  and a web part  4 . The mold  1  is suitable, as is known in the art, for the continuous casting of preliminary sections, in this example, I-sections. Molten steel is introduced substantially vertically into this continuous mold, in which a strand crust forms and from which a preliminary sectional strand with a molten core is fed to a strand guide with secondary cooling devices. 
     An electromagnetic stirrer  10  uses three-phase current to produce electromagnetically induced forces, preferably in the region of the mold  1  or directly at the exit from the mold  1 , causing stirring movements in the molten core of the preliminary sectional strand generally transversely to the strand casting direction. As a result, molten steel in the crater of the preliminary sectional strand is thereby exchanged between the flange parts  2 ,  3  and the web part  4 . 
     The stirrer  10  which is represented in  FIG. 1  comprises an annular closed yoke  11 , which surrounds the mold  1  at a certain vertical position. Six magnetic poles in the form of pole shoes  12  to  17 , each pole being surrounded by an electromagnetic coil  19 . The pole shoes  12  to  17  are non-uniformly distributed at the circumference of the yoke  11  such that each pole shoe  12 ,  13  is oriented towards the flange parts  2 ,  3  and each two pole shoes  14 ,  15 ;  16 ,  17  are oriented from both sides towards the web part  4 . The stirrer  10 , or in this example the rotating stirrer, works according to the principle of a 6-pole asynchronous motor, in the case of which a traveling field can be generated by means of three-phase current. In this respect, the poles must be correctly interconnected in order to generate a linearly traveling or rotating field or linear or rotating flows. 
     In an embodiment shown in  FIG. 2 , the mold  1  is again surrounded in a certain and preferably adjustable vertical position by an electromagnetic stirrer  20  with an annular, closed yoke  21 , at the circumference of which six pole shoes  22  to  27  are again non-uniformly distributed, with the difference that all six pole shoes  22  to  27  are oriented substantially for linear flows in the web part  4 . 
     According to  FIGS. 3 to 6 , an electromagnetic stirrer  30  is in each case associated with the mold  1 , which stirrer comprises a closed yoke  31  which surrounds the mold  1 , is formed as a rectangular frame, with the longitudinal sides of which three respective pole shoes  34 ,  35 ,  36  and  37 ,  38 ,  39 , distributed over the mold width, are associated, and the narrow sides of which are provided with a respective central pole shoe  32 ,  33  oriented frontally towards the flange parts  2 ,  3 . As is described herein, the stirrer  30  can be operated both as a rotating stirrer and as a linear stirrer, depending on the pole interconnection, i.e., according to which pole shoes are to be energized and with which phase sequence (cf. the phase designation U, V, W; U′, V′, W′). Four different operating possibilities, in which six of the total of eight pole shoes  32  to  39  are in each case organized, are presented on the basis of  FIGS. 3 to 6 . 
     As an example, with respect to  FIG. 3 , pole shoes  32 ,  33  may be disconnected and the pole shoes  34 ,  35 ,  36  on one longitudinal side of the yoke  31  may be phase-shifted with respect to the pole shoes  37 ,  38 ,  39  on the other longitudinal side, resulting in a linear flow in opposite directions in the web part  4  (2×3-pole linear operation, in opposite directions). This pole interconnection is preferable in the case of symmetrically disposed ingates  45 ,  46  in the flange parts  2 ,  3 . 
     Another example, shown with respect to  FIG. 4 , comprises a pole interconnection for a linear operation (central pole shoes  32 ,  33  in the flange region disconnected), with phase sequence U, V, W on both longitudinal sides, resulting in a flow in the same direction in the web part  4  (2×3-pole linear operation, in the same direction). This pole interconnection is preferable in the case of an asymmetrically disposed ingate  47  in the flange part  2  or  3 . 
     In another example, shown in  FIG. 5 , central pole shoes  32 ,  33  in the flange region are energized, but the central of the three pole shoes  34 ,  35 ,  36 ;  37 ,  38 ,  39 , which are associated with the two longitudinal sides, are disconnected (pole shoes  35 ,  38  de-energized). Rotating fields are therefore generated in the flange regions (2×3-pole rotating operation). With phase assignment to the pole shoes  37 ,  32 ,  34  and  36 ,  33 ,  39 , the direction of rotation of the rotating fields in the two flange parts  2 ,  3  is the same, which also results in a flow in the web part  4 , although this is less efficient than in the case of the linear operation according to  FIG. 3 . This pole interconnection is preferable in the case of a symmetrically disposed ingate  48  in the web part  4 . 
     Turning to the example shown in  FIG. 6 , an interconnection of the pole shoes  37 ,  32 ,  34  and  36 ,  33 ,  39  can generate rotating fields with opposite directions of rotation in the flange parts  2 ,  3  by the stirrer  30 . This pole interconnection is preferable in the case of two symmetrically disposed ingates  45 ,  46  in the flange parts  2 ,  3 . 
       FIGS. 7 and 8  show a variant in which two electromagnetic stirrers  40 ,  40 ′ or two yokes  41 ,  41 ′, separated from one another in the width direction of the mold  1 , with three respective pole shoes  42 ,  43 ,  44 ;  42 ′,  43 ′,  44 ′ are associated with the mold  1  at its circumference, each yoke  41 ,  41 ′ being provided with a central pole shoe  42 ,  42 ′ oriented frontally towards the respective pole part  2 ,  3  and two pole shoes  43 ,  44 ;  43 ′,  44 ′ directed towards the flange part  2 ,  3  from both sides. By means of the two stirrers  40 ,  40 ′, a 2×3-pole rotating operation can again be brought about or rotating fields which again have the same direction of rotation ( FIG. 7 ) or opposite directions of rotation ( FIG. 8 ) can be generated in the flange regions  2 ,  3 . Reference  48  indicates a symmetrical ingate, while Reference  49  indicates an asymmetrical ingate. 
     Practically the same effect can be achieved with the two stirrers  40 ,  40 ′ or yokes  41 ,  41 ′, separated from one another in the width direction of the mold  1 , as with the stirrer  30  provided with the closed yoke  31  and connected, for example, according to  FIG. 5  or  6 . However the former solution affords additional advantages. Electromagnetic stirrers can be constructed with two independent stirrers or half-stirrers that can be brought up to/mounted on the mold  1  relatively easily from outside. Scope for the designer is acquired through the free sector. Not least, this solution also allows the two stirrers  40 ,  40 ′ to be disposed in a vertically staggered manner, as shown, for example, in  FIG. 9 , in which case the vertical position of the stirrers  40 ,  40 ′ with respect to one another and/or related to the mold height can preferably be adjusted according to requirements. 
     Similar characteristics are provided by embodiments shown in  FIGS. 10 to 12 , in which two electromagnetic stirrers  50 ,  50 ′ ( FIGS. 10 and 13 ) or  60 ,  60 ′ ( FIGS. 11 and 12 ) are again associated with the mold  1  at its circumference, although these stirrers comprise yokes  51 ,  51 ′ separated from one another in the thick direction of the mold  1  rather than in the width direction thereof in other embodiments such as shown in  FIGS. 7 and 8 . Each yoke is in each case provided with three pole shoes  52 ,  53 ,  54 ;  52 ′,  53 ′,  54 ′ or  62 ,  63 ,  64 ;  62 ′,  63 ′,  64 ′. 
     In the embodiment according to  FIG. 10  the three pole shoes  52 ,  53 ,  54 ;  52 ′,  53 ′,  54 ′ are in each case distributed over the entire width of the preliminary section and two of them (pole shoes  52 ,  54 ;  52 ′,  54 ′) are directed at the sides towards the flange parts  2 ,  3 , and the central pole shoe  53 ,  53 ′ projects up to the web part  4 . 
     In the embodiment according to  FIGS. 11 and 12  all three pole shoes  62 ,  63 ,  64 ;  62 ′,  63 ′,  64 ′ of the respective stirrer  60 ,  60 ′ are only distributed over the web and project towards the web part  4 . Two symmetrical ingates are represented by  45 ,  46 . 
     The stirrers  50 ,  50 ′ and  60 ,  60 ′, respectively, are operated as linear stirrers, in the same manner as described above, in which case flows in opposite directions ( FIGS. 10 and 11 ) or a flow in the same direction ( FIG. 12 ) can be produced in the web part  4 . The setting takes place in accordance with the casting and/or product parameters. 
     Finally,  FIG. 14  shows an electromagnetic stirrer  70  with an 8-pole structure, composed in a similar way to the stirrer  30  according to  FIGS. 3 to 6  (with a yoke  71  which is formed as a rectangular frame, with the longitudinal sides of which three respective pole shoes  74 ,  75 ,  76 ;  77 ,  78 ,  79 , distributed over the mold width, are associated, and the narrow sides of which are provided with a respective central pole shoe  72 ,  73  oriented frontally towards the flange parts  2 ,  3 ). However in these embodiments, rather than either a rotating or linear operation being created by disconnecting two of the eight poles, linear fields are generated in the web part  4  using a 1×6-pole linear stirrer (pole shoes  74 ,  75 ,  76 ;  77 ,  78 ,  79 ) and rotating fields in the flange parts  2 ,  3  using 2×3-pole rotating stirrers (pole shoes  74 ,  72 ,  77  and  76 ,  73 ,  79 ) at the same time. 
       FIG. 15  shows an electrical schematic diagram of the stirrer  70  with this 8-pole structure or this 8-pole system, in which the linear fields are generated by means of a 1×6-pole linear stirrer and the rotating fields using these 2×3-pole rotating stirrers at the same time. This electromagnetic stirrer  70  is fed from the network, for example with three-phase current 50 Hz, by means of lines  81 ,  82 , these lines  81 ,  82  in each case leading to a frequency converter  83 ,  84 . These frequency converters  83 ,  84  are connected to a converter control  85 , and the individual phases are set by this to a desired predetermined frequency. 
     The function of the control  85  is to tune the frequencies of the two converters to one another to synchronize the stirring movements which are produced in the web and in the transition region to the two flange parts. The control is also to prevent the occurrence of beat phenomena when the two stirrers are at slightly different frequencies. A beat would cause the one and the other pole to be under full load simultaneously in the course of time, which would result in a highly non-uniform network load. 
     The individual phases U, V, Woof the one converter  84  and the phases U 1 , V 1 , W 1  of the other converter  83  are routed from these frequency converters  83 ,  84  to the coils that are wound around the pole shoes  74 ,  75 ,  76 ;  77 ,  78 ,  79 . The phases U, V, W lead to the coils  77 ′,  78 ′,  79 ′ at the pole shoes  77 ,  78 ,  79  in the web part and further to the coils  76 ′,  75 ′,  74 ′, disposed symmetrically with respect to the latter, of the pole shoes  76 ,  75 ,  74 , the connecting lines being routed from the coils  77 ′,  79 ′ crosswise to the coils  76 ′,  74 ′ (connected in series). The lines are routed from these coils to the star point  87 . The same applies to the phases U 1 , V 1 , W 1 , although this is not illustrated in detail. In the case of the linear operation the phase W 1  is routed to the coil  72 ′ and further to the opposite coil  73 ′ and further to a star connection. 
     As already mentioned, it is therefore possible, by means of the electromagnetic stirrers  10 ;  20 ;  30 ;  40 ,  40 ′;  50 ,  50 ′;  60 ,  60 ′;  70  and using electromagnetically induced forces, in the region of the flange parts and/or of the web part to generate stirring movements in the molten core of the preliminary sectional strand transversely to the strand casting direction, and thereby exchange of the molten steel in the crater of the preliminary sectional strand between flange parts and the web part. It is as a result possible to specifically and actively influence the flow and temperature conditions in the molten steel crater within the preliminary sectional strand shell as desired and therefore produce the following effects:
         stabilization of the metal surface region by suppressing turbulence, even in the case of varying process parameters, such as, for example, casting speed and metal surface position (for the purpose of preventing non-metallic inclusions as well as gas bubbles in the strand surface);   favorable, controllable flow conditions with a specifiable molten steel exchange between relatively thicker thickened cavity regions through a thinner web part, even in the case of an asymmetrical ingate, and thereby the formation of a uniformly thick strand shell with a favorable solidification structure, while preventing shrink holes and/or core porosity.   prevention of arching during solidification in spite of confined conditions in the web part of the mold cavity cross section.       

     As a result of the choice of interconnection of the poles with the individual phases of the 3-phase current, it is possible, without making any structural changes to the stirrer, to produce different direction components and thereby different flows in the molten crater of the preliminary sectional strand in accordance with the casting parameters, such as the ingate system with regard to the number of ingates, open or closed pouring, casting speed, casting temperature, steel composition, etc. However it is also possible to use the same stirring device for molds with different product parameters, such as preliminary section dimensions, etc. and at the same time vary the pole interconnection such that rotating traveling fields can be generated in the flange part and/or linear traveling fields generated in the web part in accordance with the product parameters in order to specifically obtain flows in the molten crater. 
     It is noted that tubular molds are represented schematically in the figures. However, instead of tubular molds, it is also possible to operate all mold constructions which are suitable for preliminary sections, such as ingot molds or plate molds, etc., as known in the art, with the method according to the invention or to use these with the device according to the invention. 
     Those skilled in the art will recognize that the materials and methods of the present invention will have various other uses in addition to the above described embodiments. They will appreciate that the foregoing specification and accompanying drawings are set forth by way of illustration and not limitation of the invention. It will further be appreciated that various modifications and changes may be made therein without departing from the spirit and scope of the present invention, which is to be limited solely by the scope of the appended claims.