Conventionally, in the field of iron and steel industry, molten steel reaching to about 1500° C., which has been taken out from a steel making furnace such as a converter, an electric furnace or the like, is serially sent to a casting mold made of copper or a copper alloy having a cooling mechanism on the back face (rear face) via ladle, tundish and tundish nozzle, while reducing the temperature by bringing the molten steel into contact with the mold wall, is discarded in a lower direction while it gradually forms a thick solidification shell and is allowed to make a slab which is serially formed and has a certain size. Furthermore, subsequently, it is sent to the tall end processes such as heating, rolling and the like and then, it is made into a product via cutting off process. Copper or a copper alloy having a high thermal conductivity is usually utilized for such continuous casting mold material for steel, since it has an important function for reducing the temperature from the molten steel.
As described above, although it is needless to say with respect to the contact surface with the molten steel of a mold that the considerations on an effect of reducing the temperature by utilizing copper or a copper alloy having a high thermal conductivity as a material and so on have been done. However, the surface hardening accompanying with the growth of solidification of the molten steel has been progressed, and since the copper member is weared by such surface hardening and the like, a surface cracking which is called as a star crack is occurred by mixing copper into the slab, the prevention of the damage to the slab occurring by utilizing copper as well as the countermeasures of the wear have been conventionally carried out by coating nickel, nickel-cobalt alloy, nickel-iron alloy, nickel-chromium alloy or the like on the contact surface with the molten steel in the mold, using a method of plating, a method of thermal spraying or the like.
With respect to a cooling mechanism installed at the back face of a continuous casting mold, it has a structure in which a back frame (water box) has been attached on the anti-molten steel contact surface of the casting mold as shown in FIG. 7. For example, as shown in FIG. 3, it had a structure in which a hole for a back frame mounting member and a cooling groove for enhancing the cooling efficiency have been also provided on the mold itself. It is natural that the shape and the number of such grooves are accompanied by some changes depending on the conditions specific to the continuous casting machine, however, the thickness of the copper member or copper alloy member used for this casting mold is almost fixed basically in the range from 40 to 50 mm. It should be noted that in a part of the casting field, a continuous casting machine having a so-called electromagnetic stirring function, i.e. the stirring function of the molten steel within the mold by electromagnetic force, has been employed for the purpose of contemplating the improvement of central segregation of the slab and the solidification structure and making the slab with a higher quality, and in recent years, its use has been increased.
The casting mold which is used in a continuous casting machine with an electromagnetic stirring function is often set in a thinner mold than the usual mold, and the thickness of copper member or a copper alloy member is often set in the range from about 25 to about 30 mm for the purpose of enhancing permeability and because the temperature of the casting upper portion is tended to rise by the effect of the molten steel stirring within the mold. Therefore, a certain device is also made to the back frame, however, the thinning of the copper member or the copper alloy member makes it difficult to provide a hole into which the back frame is to be mounted.
Furthermore, in an example of a casting mold which is used in some continuous casting machines with an electromagnetic stirring function, such mold employs a structure in which the back frame is mounted by welding pieces of stud bolts made of stainless steel in a pitch range from 150 to 200 mm by an arc stud welding method as it is in a state of flat surface without providing a cooling groove on the back face from the first. The arc stud welding method is a method wherein a base metal and a portion of stainless steel are melted and welded by generating an arc under the direct current electric source. However, since the object matter (base metal) is copper or an alloy made with copper which has a high thermal conductivity, and since the object matter (base metal) has a heat discharging volume much larger than the stud bolt (made of stainless steel) which is to be welded, it must be said that highly excessive heat input is applied from the viewpoint of the welding conditions. As a result of this, it is difficult to avoid that the material is subjected to the influence of the heat over the wide zone. In addition to this, in this process, it requires a ring made of ceramic in a cylinder shape which is called ferrule and having the all of functions of edge preparation of stud bolt edge, combination use of flux, dispersion prevention of the melted metal, and electric insulation, and thus the welding operation becomes complex and troublesome. Moreover, the casting mold in which the required number of stud bolts have been welded accompanies the variation of 1.6 mm or the like from the viewpoint of a perpendicular degree to the mold copper member, and in addition to the problem of preciseness of mounting stud bolt, it often shows such defects that the heat deformation and strength deterioration of the copper plate, the cracking of the copper member at a higher or lower temperature of the welding portion between the copper plate and stud bolt occur, and the melted slag remains without being completely exhausted from the welded portion between the copper plate and stud bolt. These complex factors require, in some cases, re-welding working, and cause bolt broken incident at the time of back frame assembling, and further, bolt breakage incident by various kinds of action stresses during the mold operation and the like. In FIG. 2(a) and FIG. 2(b), the states of before the arc stud welding is performed and after the arc stud welding has been performed is shown in a summary diagram.
Furthermore, as an embodiment of mold which is utilized in a continuous casting machine accompanied with another electromagnetic stirring function as shown in FIG. 4, there is also exemplified such an example having a structure wherein only the copper member of the back frame attachment portion is thicker, a back frame attachment seat is provided by performing the screw hole processing to the structure which has been made thicker and further, a cooling groove is provided by performing the cutting processing. In such an example, there is no heat input accompanied with an arc stud welding, and there is also no fear about the raw material deformation and strength deterioration since the back frame mounting seat in a cylindrical shape is provided on the rear surface of the copper member, and cooling grooves at other sites except for the rear surface are processed. On the other hand, it cannot help becoming a complex processing of the processing of mounting seat, the processing of back frame mounting hole (screw hole processing and tapp for HELI-COIL insert (hereinafter, referred to as helisert) made of stainless steel mounting) and the like. Further, 40 to 50% by weight of the original copper or copper alloy member is actually cut and removed by this serial mechanical processing. The cut pieces will be utilized in any way as refreshable scraps, however, it accompanies the occurrence of a large amount of industrial waste.
As described above, in the case of a conventional casting mold, for the purpose of setting the back frame mounting hole and imparting the function of efficient cooling, the processing such as the processing of mounting hole, the fitting of a helisert made of stainless steel for the purpose of enhancing and protecting a screw thread, and further the processing of a cooling groove which is to be a pathway of cooling water and the like is performed on the back face of the thick copper member. On the other hand, in a casting mold in the casting machine having the conventional electromagnetic stirring function, there is used an arc stud welding method in which there are many problems in view of heat deformation of the material, strength deterioration, welding defects, welding position preciseness and the welding strength, or there is performed the processing of back frame mounting seat, the attachment of tapp for HELI-COIL insert (hereinafter, referred to as helisert) made of stainless steel, the processing groove or the like which is accompanied with a long time processing and the material loss from the thick copper member. In this way, it is understood that the technical problems common to the conventional anti-molten steel contact surface (rear surface) of steel continuous casting mold are largely summarized to two problems of efficient cooling and the mounting of back frame. More concretely, solution of the various problems such as improvements in cutting processing loss of copper member or a copper alloy member, long time of processing, precision of welding position with respect to copper member or a copper alloy member, reliability of welding strength, and heat deformation and heat deterioration of copper member and the like, contributes to the saving of the resources, the reduction of industrial waste, the shortening of the processing time, and the enhancement of the reliability in the welding between the copper alloy member and stud bolt, therefore, which contribution is enormous to the industries.
Moreover, a large problem of the casting mold in which the conventional general cooling groove is provided by performing the cutting processing is in that from the necessity of providing a mounting hole for the purpose of mounting the back frame on the side of the copper plate, the copper plate is confined to a thicker one and the use of the thicker copper plate causes the processing of a cooling groove. If a novel method of mounting a back frame which solves such a problem is found, the thinning of the copper plate and the processing of a cooling groove themselves can be made null, the problem of the copper plate used for the electromagnetic stirring of a method in which the thicker copper plate is still used for the purpose of providing a back frame mounting seat is also solved.
Moreover, in the case of the mold copper plate for electromagnetic stirring which utilizes an arc stud welding method of stud bolt, if a stud bolt welding method which can improve the welding position precision and the welding strength as well as the heat deformation and heat distortion of the copper plate can be suppressed is found, the position leveling of bolt, stud bolt breakage incident and the like can be avoided.
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