Patent Publication Number: US-2012043049-A1

Title: Process and apparatus for the continuous casting of a slab

Description:
The invention relates to a method for the continuous casting of a slab, in particular of steel, in which the cast slab is conveyed through a furnace, and in which the slab is subjected to a descaling operation, wherein in at least one section of the furnace the slab or strand surface is subjected to a reduction by maintaining in the section of the furnace an atmosphere which consists of an inert gas and hydrogen or of pure hydrogen. The invention additionally relates to an apparatus for the continuous casting of a slab. 
     A method of the above described type is disclosed in U.S. Pat. No. 5,816,311 A. In the manufacture of a steel strip, the direct rolling from the casting heat is known in the art. In this connection, the method is more interesting the greater the casting speed. The method is known, for example, from EP 0 889 762 B1 and from WO 2006/106376 A1. In this method, initially a slab is manufactured in a continuous casting machine, wherein the slab is discharged from a mold perpendicularly downwardly and is then deflected into the horizontal position. The strip which is still hot is then conducted to a rolling train. A thickness reduction then takes place in each roll stand, until the strip having the desired thickness is produced. The advantages of this method of continuous casting/rolling are a relatively compact construction of the plant and the attendant lower investment costs. Moreover, energy can be saved during the production of the strip. The method makes it possible to produce products which are difficult to roll, for example, very thin strips (thickness of, for example, 0.8 mm), to further process high-strength special materials, and to fabricate strips which are wide and thin in combination. 
     The aforementioned technology is also called CSP-technology. Among these is the manufacture of a steel strip in a thin strip casting/rolling plant which facilitates an efficient production of hot strip if the rigid connection of continuous casting plant, conveying furnace for temperature compensation, and rolling train and its temperature pattern through the total plant can be managed. Accordingly, in this case, the roll stands are arranged directly following the casting machine and the conveying furnace. By using the above mentioned casting/rolling plant, a coupled, fully continuous, direct casting/rolling process is possible. 
     For achieving a sufficient surface quality of the strip to be manufactured, the development of scale is very disadvantageous. The surface scale forming during casting and solidification and during heating and remaining in the slab furnace on the steel strand or on the slab has to be removed prior to hot rolling. 
     A particular disadvantage is the fact that in the aforementioned casting/rolling process, the scale formed during casting and during the solidification of the strand or the slabs cannot be removed. The strand or the slab are cast and heated in an oxidizing atmosphere which causes the surface to form scale. Scale formed on the bottom side of the strand or slab can be pressed in on the support surface in the furnace (rollers, shuffle bars) and may thereby result in a mechanical damage of the surface. The cleaning of the surfaces by means of a scale washer used in the classic manner can take place only outside of the furnace. In addition the removal of scale by means of a scale washer in front of and/or following the furnace leads to a cooling of the strand or slab which must be compensated by a greater introduction of energy into the furnace. 
     DE 199 59 688 A1 discloses a method for heating blanks from metal materials for a rolling or deforming process in which heating is effected by electrical or magnetic fields or by an electric resistance heater, wherein a protective atmosphere is used. In this case, the protective atmosphere is composed especially of carbon dioxide, of argon, of nitrogen, of sulfuric hexafluoride, or of helium. 
     WO 2007/054,237 A1 discloses for a combined casting/rolling process to descale the pre-rolled hot strip immediately prior to the entry into a temperature adjusting device, wherein the strip pretreated in this manner is held in the temperature adjusting device in a protective gas atmosphere. 
     A method for continuously manufacturing strip steel or steel plates is also disclosed in WO 89/11363. A similar method is shown in WO 98/00248. Further similar solutions are shown in WO 02/04145 A2, EP 1 134 296 A2 and CN 101 091 958 A. 
     The present invention is based on the object of reducing the formation of existing scale layers and to prevent the formation and the build up of a scale layer during the annealing treatment. Moreover, it is desirable to avoid or at least reduce surface errors caused by scale. 
     This object is met by the invention with respect to method technology in that in front of and/or behind the at least one section of the furnace with reducing atmosphere in travel direction, a section of the furnace follows in which the slab surface is subjected to an oxidation. This section of the furnace with oxidizing atmosphere may contain oxygen at least partially. 
     The proportion of hydrogen can be between 3% and 100%, preferably between 5% and 50%. The inert gas is preferably nitrogen or argon. 
     Heating of the slab in the furnace can take place by induction. 
     The above described method preferably is the continuous casting in the casting/rolling process. Heating or equalizing annealing of the slab then takes place preferably following the casting process and before the rolling process as seen in travel direction. The slab is conveyed continuously. 
     The apparatus for continuously casting a slab, which includes a furnace through which the slab can be conducted, wherein additionally descaling means are provided by means of which the slab surface can be descaled, is characterized according to the invention in that the furnace has at least two sections, wherein means are provided for maintaining a reducing atmosphere in at least one section of the furnace, and wherein means are provided for maintaining an oxidizing atmosphere in at least another section of the furnace. 
     The means for maintaining a reducing atmosphere preferably includes supply lines for hydrogen and preferably also for an inert gas. The means for maintaining an oxidizing atmosphere preferably includes at least one supply line for oxygen. 
     The furnace may be a muffle furnace. The portions of the furnace can also be formed by steel pipes; this has the advantage that there are no open flames. The furnace may also comprise inductive heating means. 
     With the proposed procedure it is possible to produce a steel strand or a steel slab with improved surface characteristics. This results from a targeted adjustment of the furnace atmosphere during the heating or the equalizing annealing in a stationary furnace or in a roller hearth furnace. 
     In accordance with this proposal, an existing scale layer is reduced and the formation and build up of a scale layer during the annealing treatment is substantially prevented. In addition, smaller surface defects can be removed through a targeted combination of scaling and reducing. 
     As a result, the surface quality of the strand or slab can be improved, which is particularly applicable to directly rolled strip, for example, from the CSP casting-rolling process. This makes it possible to meet high requirements of the surface quality as they exist, for example, with respect to steel plates for outer skin parts in automobile construction. 
     The scale build up on the support surfaces in the furnace (rolling, shuffle bar, etc.) can be avoided or at least reduced. 
     By avoiding scale losses and avoiding reductions in quality, a higher output of the plant can be achieved. 
     There is also a savings of energy due to an improved heat transfer in the furnace. An energy savings and also a savings of water result from the fact that any scale washers that may still be required are operated with reduced water quantity. The resulting lower temperature loss of strand or slab permits the indication of a lower furnace temperature with constant temperature of the slab when entering the first roll stand. 
    
    
     
       Embodiments of the invention are illustrated in the drawing. In the drawing: 
         FIG. 1  schematically illustrates a plant for casting/rolling a slab with a casting device, a roller-hearth furnace and a hot rolling train; and 
         FIG. 2  schematically illustrates a plant for the conventional hot rolling process with a casting machine, a slab storage, a heating furnace and a rolling train. 
     
    
    
     In  FIG. 1 , a casting/rolling plant is illustrated, which is known per se. A slab is cast in a casting machine  5 , wherein the slab is discharged from a mold vertically downwardly and is then deflected into the horizontal direction by a plurality of rollers, wherein the cast strand further solidifies. As seen in the conveying direction F, a hot rolling train  6  follows further back, and further in the conveying direction F a cooling group  7  and a reel  8  for the finished strip. 
     In the solution according to  FIG. 1 , a roller-hearth furnace  2  is arranged between the casting machine  5  and the hot rolling train  6 . 
     In the alternative solution according to  FIG. 2 , conventional hot rolling process is used in which the slabs are initially stored behind the casting machine  5  in a slab storage  9 . The slabs are conveyed from there into a furnace  2  which is constructed as a reheating furnace. Subsequently, the additional process described in connection with  FIG. 1  follows, i.e., the rolling, the cooling and the coiling of the strip. 
     The surface scale formed in the slab furnace while the slab is heated and remains the furnace must be removed prior to hot rolling. This takes place classically by means of a scale washer by spraying with water under high pressure (220-400 bar). In contrast to the conventional manufacture by means of strand casting, cooling and subsequent reheating and hot rolling (in accordance with  FIG. 2 ), slabs from the casting/rolling process (see in particular the above mentioned CSP process and  FIG. 1 ) are supplied in a continuous method by means of a roller-hearth furnace directly to the hot strip finishing train. As a consequence, it is no longer possible prior to hot rolling to remove surface defects by flaming or grinding the slabs. 
     With respect to both processes explained above it is intended that a furnace  2  is used in which at least in one portion  3  of the furnace (see  FIG. 1 ), the slab surface is subjected to a reduction by maintaining in the section  3  of the furnace  2  an atmosphere which consists of an inert gas and/or hydrogen (H 2 ). 
     Accordingly, in the section  3  of the furnace  2  mentioned above, a reducing furnace atmosphere is adjusted by means of which a reduction of the surface scale can be effected. The hydrogen (H 2 ) being used may have a content of about 3% to 100%. 
     In the conveying direction F in front of the section  3  with reducing atmosphere is arranged in the embodiment according to  FIG. 1  and  FIG. 2  (however, illustrated only in  FIG. 1 ), a section  4  with oxidizing surface. Accordingly, oxygen (O 2 ) is introduced at this location; it is also possible that the entry of the (oxygen-containing) atmosphere is facilitated at this location. 
     Consequently, smaller surface defects of strand or slab caused during the casting or solidification process can be removed by targeted descaling. By a targeted combination of furnace zones with oxidizing and reducing atmospheres, a clean metallically bright slab surface is achieved following the furnace  2 . 
     A separation of the combustion chamber and the furnace chamber can be achieved by using a muffle furnace or by using spray pipes. Alternatively, it is also possible to carry out heating and equalizing annealing totally or partially by means of inductive heating elements. 
     In this embodiment, two furnace sections or chambers  3 ,  4  are arranged so that—as explained—preferably initially the oxidation of the slab  1  takes place first and the reduction thereof takes place thereafter. It is also possible that more than two sections, especially three sections, are provided in the furnace  2  (not illustrated), in which respectively different atmospheres for the reduction or oxidation are maintained. 
     LIST OF REFERENCE NUMERALS 
     
         
           1 . Slab/strand 
           2 . Furnace 
           3 . Section of furnace 
           4 . Section of furnace 
           5 . Casting machine 
           6 . Hot rolling train 
           7 . Cooling group 
           8 . Reel 
           9 . Slab bearing 
         H 2  Hydrogen 
         N 2  Nitrogen 
         O 2  Oxygen 
         F Conveying direction