APPARATUS FOR CONTINUOUSLY CASTING THIN BILLET

Abstract: A reducing belt-type continuous casting apparatus con­ tinuously casts a sheet-bar billet through a casting space formed by the combination of a pair of metal belts which are disposed facing each other and rotatably move such as to form longer sides and a pair of tapered fixed side plates which are disposed between the metal belts and closely contact the same so as to form shorter sides. The sides plates for forming shorter sides are designed in a configuration such that the following conditions are satisfied: d = 5 to 30 mm. D ≧ 60 mm, D/d ≦ 16, ϑ ≦ 30°; ϑ = tan⁻¹(D - d)/H, thereby preventing the production of defects on the side surfaces of the produced billet. Further, as the metal belts are employed one having the yield strength Sy(kg/mm²) and the belt thickness t(mm) which satisfy the following conditions: Sy ≧ 10,500 t/Dr, 0.4 ≦ t ≦ 2.5, where Dr represents the guide roll diameter (mm) thereby preventing failure in the billet quality and also preventing casting accidents due to defor­ mation of the belts.

Description: This invention relates to a belt converging type continuous apparatus according to the first part of claim 1. Background Art In the field of producing steel plates, it has recently been attempted to continuously carry out casting and rolling for the purpose of energy-saving, increase of yield, labor-saving, stock-saving and improvement of quality. In a general method of producing cast sheets using the conventional continuous casting process, cast slabs of about 150-300 mm in thickness are produced from molten steel by means of a continuous casting machine and then subjected to hot-rolling and cold-rolling to produce thin steel sheet of about 0.5-2 mm in thickness. This method is excellent in regard to production yield, labor-saving and energy-saving as compared with the method of obtaining a cast slab from an ingot by blooming. However, when the casting rate is increased to not less than 2.0 m/min in a conventional continuous casting machine, not only does smooth casting become difficult but also there is an increase in surface and inner defects of the cast sheet so that it is very difficult to connect the continuous casting machine to the rolling mills in a continuously operating state. Therefore, even when using the continuous casting process, in order to obtain a thin steel sheet, it is necessary for the slab to be subjected to rough rolling and finish rolling after being reheated at a uniform temperature. If cast sheets of not more than 30 mm in thickness can be directly produced from molten steel by continuous casting, it is possible to omit some procedures from the rough rolling step for obtaining thin steel sheets. Moreover, if thin steel sheets of several mm in thickness can be directly cast from molten steel, the rolling step can considerably be simplified so as to reduce investment cost and processing cost. In view of the above, there have been made various attempts for directly producing cast steel sheets for thin steel plates from molten steel. For instance, there are the techniques described in Japanese Patent laid-open No. 54-61,036 and Japanese Patent laid-open No. 54-139,835 and the like, but they have not yet attained industrial scale. By these techniques, it is particularly difficult to make a broad cast sheet. Fig. 1 of the accompanying drawing is a schematic view illustrating an embodiment of an apparatus in which such attempts have been further improved. This apparatus comprises a casting space for molten metal. The broad sides of this casting space are defined by a pair of endless metal belts 1,1' arranged opposite to each other and supported by guide rolls 2,2',3,3' and 4,4' so as to allow continuous movement of the belts while keeping them spaced by a constant distance. The narrow sides of the casting space are defined by a pair of side plates (not shown) arranged opposite each other and located near both side edges of the metal belts. The apparatus includes metal pads 5,5' arranged behind the opposed portions of the metal belts, and cooling fluid paths (not shown) are provided inside the metal pads for cooling and supporting the molten steel through the metal belts by means of films of cooling fluid flowing between the metal belts 1,1' and the metal pads 5,5', from nozzles for the paths opening at the pad surfaces next to the belts. Molten metal 7 is poured into the casting space defined by the metal belts 1,1' and the side plates from a pouring nozzle 6 and is cooled and solidified along the surfaces of the metal belts and the side plates to obtain a cast sheet 8. During solidification, the metal belts 1,1' support the broad-side surfaces of the sheet and the side plates support the narrow-side surfaces of the sheet. However, in the construction as shown in Fig. 1, it is necessary that the size in the thickness direction of the molten steel flowpath in the pouring nozzle 6 for supplying the molten steel into the casting space is small(for example from several mm to several tens of mm). Also the refractory at the top of the pouring nozzle 6 must be thin. Thus there are fatal drawbacks such as the molten steel becoming solidified in the pouring nozzle 6 and causing clogging and the refractory becoming eroded so that long-term continuous service can not be achieved. As an improved technique for solving the above drawbacks, there have been proposed a combination of casting wheels and belts as disclosed in Japanese Patent laid-open No. 57-32,852 and an apparatus as shown in Fig. 2 of the accompanying drawings. In this latter continuous casting apparatus which is similar to that described in JP-A-58-38641), metal belts 1,1', side plates 9,9' and rolls 10, 10' and 11, 11' are arranged so that as the casting space defined by the pair of opposed metal belts 1, 1' and the pair of opposed side plates 9, 9' advances downward in the moving direction of the metal belts, the thickness of the resulting cast sheet is reduced from a thickness larger than a given thickness down to the given thickness to thereby define a downwardly tapered molten steel holding portion 12a and a subsequent molten steel solidifying portion 12b having a constant thickness corresponding to the given thickness of the cast sheet. Therefore, according to the continuous casting apparatus shown in Fig. 3 of the accompanying drawings, the molten steel 14 poured into the molten steel holding portion 12a of the casting space through a pouring nozzle 13 forms a solidification shell 15 mainly from its surfaces contacting the metal belts 1, 1', which is led into the molten steel solidifying portion 12b of the casting space while the thickness t is gradually converged during the downward movement and regulated to the desired thickness by the rolls 11, 11'. Then, in this molten steel solidifying portion 12b, as shown in Fig. 4 of the accompanying drawings, the solidification shell 15 grows to complete the solidification at the outlet of the lower end of the solidifying portion which is then drawn out in the form of cast sheet 8. As mentioned above, the continuous casting apparatus as shown in Fig. 2 is constructed so as to gradually reduce the thickness of the poured molten steel in the downwardly tapered or funnel-like molten steel holding portion 12a, so that it is referred to as a belt converging type continuous casting apparatus. In this case, the size in the thickness direction at the upper end of the molten steel holding portion can be made large, so that the problem caused by the use of the thin pouring nozzle 6 as shown in Fig. 1 can be avoided. Also the lower end part of the pouring nozzle 13 can be immersed in the molten steel 14 to pour the molten steel without oxidation. However, as mentioned above, in the belt converging type continuous casting apparatus as shown in Fig. 2, it is necessary to converge the unsolidified cast sheet, formed by enveloping the unsolidified molten steel 14 with the solidification shell 15, in the thickness direction in the molten steel holding portion 12a. For this purpose, the converging rolls 11, 11' are arranged at the transition region between the tapered molten steel holding portion 12a and the molten steel solidifying portion 12b of constant thickness so as to apply a converging force to the unsolidified cast sheet through the metal belts 1, 1'. Accordingly, there are caused not only the problem that the unsolidified cast sheet formed by enveloping the unsolidified molten steel 14 with the solidification shell 15 becomes bulged as a result of the converging force applied by the converging rolls with consequential breaking but also the problem that deep wrinkle-like defects and cracking are produced in the side surface of the resulting cast sheet. As seen from the above apparatus, the guide rolls 2, 2', 3, 3' and 4, 4' for supporting the metal belts 1, 1' conventionally have a diameter of 200-800 mm, while the metal belts 1, 1' are formed of steel materials, of the type used for general structural purposes, having a thickness of 0.4-3.0 mm. However, since the metal belts 1, 1' are used under such very severe conditions that one surface of the metal belt comes into contact with the molten steel while the other surface comes into contact with a film of cooling water flowing from the water cooling pads 5, 5', there are the following various problems: That is, the metal belts 1, 1' are deformed into a wavy form in the widthwise direction. Therefore, the contact between the metal belts 1, 1' and the side plates becomes poor and consequently the molten steel penetrates through the resultant gap to form fins and the surface of the cast sheet is caused to have an uneven wave form. Moreover, the sliding contact portions between the surfaces of the metal belts 1, 1' and the edges of the side plates are easily damaged promoting the aforementioned deformation and fin formation and considerably shortening the lifetime of the belt, which makes the direct connection to rolling equipment more difficult. Furthermore, the endless metal belt is ordinarily joined by butt TIG welding. In this case, however, the thermal deformation of the weld zone is large, so that a poor shape of the belt as mentioned above is caused and also cracking is apt to occur in the weld zone, particularly in the thermally affected zone, occasionally resulting in the breaking of the belt. Additionally, the above metal belt has the following problem. That is, as understood from the above, the side plates 9, 9' for the narrow-sides of the casting space must be so arranged, by heating the side plates during the casting and particularly at the beginning of the casting,that the formation of the solidification shell 15 along the side plates occurs later than the formation of the solidification shell formed along the broad side of the casting space i.e. along the metal belts 1,1'and therefore the rate of growth in the thickness of the solidification shell becomes slower. The reason for this is based on the fact that a considerable converging is required for casting a cast sheet of, for example, about 30 mm in thickness. If the speed of formation of the solidification shell along the side plates 9,9' were equal to or faster than that along the broad-side metal belts 1,1', the cast sheet would suffer a compression at the lower part of the converged casting space and cast wrinkles would be generated thereon. Also, the drawing resistance would become large and, in the extreme case, drawing could not be performed. From the above, the inventors have already proposed apparatuses as disclosed in Japanese Patent laid-open No. 58-32,551 and Japanese Patent laid-open No. 58-32,552 prior to the making of the present invention, wherein it is attempted to slowly form the solidification shell at that portion of the molten steel which contacts with the side plate for the narrow-side by heating the side plates 9,9' through heaters embedded therein while making the inner surface of the side plate of refractory, or by radiating the flame of a gas burner at a gap defined between the side plate 9,9' and a partition plate vertically arranged inside the side plate and spaced therefrom prior to the beginning of the casting. However, the above proposals have the drawback that the surfaces of the metal belts 1,1' are oxidized, causing a reduction in their lifetimes, due to the heating of the side plates 9,9'. This tendency is more pronounced when thinner metal belts 1,1' are used because of the cooling effect. Moreover, it has been found that, in the case of the above apparatus, the lubrication between the cast sheet (solidification shell) and the inner surface of the belt is insufficient which causes seizing as compared with ordinary continuous casting in which a mold is lubricated with powder by subjecting it to oscillation. It is, therefore, an object of the present invention to overcome the aforementioned various drawbacks involved in the known belt converging type continuous casting apparatus for the production of cast sheets. According to the present invention there is provided a belt converging type continuous casting apparatus for the production of a cast sheet comprising a casting space defined by a pair of endless metal belts continuously moving around guide rolls and arranged opposite to each other for supporting the broad-side surfaces of the cast sheet and a pair of tapered fixed side plates for supporting the narrow-side surfaces of the cast sheet and each disposed between the metal belts and in intimate contact therewith, characterised in that (i) each side plate has such a shape that the width 2D at the molten metal level, the width 2d at a lower portion corresponding to the thickness of the cast sheet, and the converging angle ϑ satisfy the following requirements: d = 5 - 30 mm D ≧ 60 mm D/d ≦ 16 0 ≦ 30 o [0=tan⁻¹ (D-d)/H (wherein H is the vertical distance from the molten metal level to the upper end of said lower portion and in that (ii) each metal belt has a yield strength Sy satisfying the following requirements: Sy ≧ 10,500 t/Dr 0.4 ≦ t ≦ 2.5 wherein Sy = yield strength (kgf/mm²) Dr = guide roll diameter (mm) t = thickness of the belt (mm) In accordance with the invention, the shape of the side plates is optimizedly designed so as to prevent the occurrence of defects generated on the surfaces of the narrow sides of the cast sheet and make the drawing resistance as small as possible by scarcely producing any narrow-side solidification shell in the molten steel holding portion. Also, the metal belt is designed to have a long-term life without causing cast sheets of reject quality and casting accidents due to deformation of the metal belt. In a particularly preferred embodiment, that surface of the metal belt which contacts the molten metal is covered with a lubricant having an antioxidant function. In this way the belt is designed to be suitable for producing cast sheets having excellent antioxidant properties and lubricating properties and hence an improved surface form. Brief explanation of Drawing For a better understanding of the invention and to show how the same may be carried into effect, reference will now be made, by way of example, to the accompanying drawings, in which: Figs. 1 and 2 are schematical longitudinal section views of the casting portion of two conventional continuous casting apparatus for the production of cast sheets, respectively; Fig. 3 is a transverse section view taken along line III-III of Fig. 2; Fig. 4 is a transverse section view taken along line IV-IV of Fig. 2; Fig. 5 is a schematical longitudinal section view of the molten metal converging portion of a continuous casting apparatus according to the invention; Fig. 6 is a perspective view of the tapered side plate for the narrow-side of the casting space of the apparatus of Fig. 5; Figs. 7 and 8 are views explaining the dimensions of the tapered shape of the molten metal converging portion of the casting space; Figs. 9-11 are perspective views of other embodiments of tapered side plates provided with a refractory lining, respectively; Fig. 12 is a graph showing the relationship between the yield strength of the metal belt and the diameter (Dr) of the guide roll; and Figs. 13(a) and (b) are graphs showing the results of welding tests of the metal belts of apparatus according to the present invention, respectively. Best Mode of Carrying out the Invention Hereinafter, the detailed construction of the apparatus of the invention will be explained with reference to the accompanying drawings. Firstly, the side plates will be examined. In Fig. 5 is diagrammatically shown the molten steel converging portion of the casting space of a continuous casting apparatus for the production of cast sheets according to the invention. As shown in the figure, the inner surface of the side plate 9 defining the narrow-side of molten steel holding portion 12a having a downwardly tapered form is lined with a refractory layer 16 (also see Fig. 6) having a small thermal conductivity so as to prevent substantial growth of the narrow-side solidification shell at this region of the molten steel holding portion 12a, whereby converging rolls (as denoted by reference numerals 11,11' in Fig 2) are omitted. Further, the shape and size of the side plate are properly selected so as to give a predetermined converging action to the region extending from the tapered molten steel holding portion 12a to a molten steel solidifying portion 12b of constant thickness, while the molten steel is supported by the metal belts 1,1' and cooled by films of cooling water jetted under pressure from metal pads 17,17' each arranged behind the metal belt whereby the converging action is applied to molten steel in the molten steel holding portion. The converging angle ϑ of the tapered side plate or the reduction rate of the thickness of the molten steel holding portion 12a is required to be not less than 2% of the natural solidification shrinkage of metal per 1 meter of length in the vertical direction. In order to produce cast sheets economically and in large quantities, it is necessary to select the shape and side of the side plate 9 so that, as shown in Figs. 7 and 8, the width 2D at the molten steel level (meniscus) 9a of the side plate 9, the width 2d at the upper end of the portion of the side plate having constant width (corresponding to the desired thickness of the cast sheet), and the converging angle 0 are within the following ranges: d = 5 - 30 mm D ≧ 60 mm D/d ≧ 16 ϑ ≦ 30 o ϑ = tan⁻¹ (D-d)/H wherein H represents the vertical distance from the molten steel level 9a to the upper end 9c of the lower portion 9b with constant width 2d. As to H, a value of not less than 300 mm is usually adopted and a value of not more than 1,000 mm is an upper limit. Fig. 7 shows the case where the broad-sides of the casting space defined by the metal belts 1,1' in the molten steel holding portion 12a are curved along a curve having a constant radius R₁ and Fig. 8 shows the case where the broad-sides of the casting space slope along a straight line from the molten steel level of thickness 2D for a vertical distance H-h and then curve along a curve having a constant radius R₂ for a vertical distance portion h. If the thickness of the cast sheet (2d) is thinner than 10 mm, it is difficult to conduct stable casting, particularly when pouring to produce sheet bar having a broad width. While, if the thickness exceeds 60 mm, it is possible to conduct the casting, but the number of roll stands needed for rolling after the casting becomes large and consequently the advantages obtained by casting are lost. It is difficult to directly supply the cast sheet to a finish mill and also it is impossible to conduct the coiling of the hot cast sheet. If the thickness (2D) at the molten steel level is less than 120 mm, the pouring system suitable for mass production not only gives rise to problems but also the cost of the pouring nozzle becomes high and the thickness of the pouring nozzle cannot sufficiently be maintained, so that the wear rate increases and the serviceable life decreases and consequently the production cost of the cast sheets becomes higher. If D/d exceeds 16 or ϑ exceeds 30 o , the converging resistance increases and it is very difficult to draw the cast sheet. That is, the bending counterforce of the cast sheet increases and pushes the metal belts in the converging portion during drawing, so that it is difficult to form films of flowing water for cooling the metal belt which causes seizing between the belt and the cast sheet. The invention will now be described with respect to a numerical example. A continuous casting apparatus having a molten steel converging portion as shown in Fig. 5 was used for the production of cast sheet for thin steel plate. The molten steel holding portion had a thickness at the molten steel level of 2D=200 mm, a thickness at the molten steel solidifying portion of 2d = 35 mm, a height from the molten steel level of H 500 mm and a width of 1,050 mm. A melt of low-carbon Aℓ killed steel was poured through an immersion nozzle and cast at a casting rate of 15 m/min and the resulting cast sheet was wound into a coil. After the coil had been introduced into a heat holding furnace to uniformize the temperature of the coil, it was immediately rolled to produce a thin steel plate of 0.8 mm in thickness. The quality of the resulting thin steel plate was as good as in the case of rough-rolling and finish-rolling a cast slab produced in a conventional continuous casting apparatus. On the contrary, when the example was repeated using (i) molten steel holding portions having a thickness at the molten steel level of 2D=200 mm, a thickness at the molten steel solidifying portion of 2d=5 mm, a height of H=500 mm and a width of 1,050 mm; (ii) 2D=400 mm, 2d=20 mm, H=500 mm and a width of 1,050 mm; and (iii) 2D=500 mm, 2d=30 mm, H=700 mm and a width of 1,050 mm, molten steel leaked out due to the breaking of the solidification shell and hence continuous casting was difficult. Figs. 9 and 10 show other embodiments of the invention. Fig. 9 shows an embodiment where a refractory layer lining 16 on the inner surface of the metal side plate 9 for the narrow-side of the casting space is composed of an alumina graphite plate 18 and a layer 19 of a refractory consisting mainly of zirconia (ZrO₂) spray-coated on the surface thereof, Fig. 10 shows an embodiment that the refractory layer is composed of only a layer 19 of a refractory consisting mainly of zirconia and directly spray-coated on the surface of the metal side plate 9. When the refractory layer 16 is made by affixing or fitting the refractory plate to the metal side plate 9 as shown in Fig. 9, the refractory plate is required to be effective for preventing erosion by molten steel and slag and to have good bond strength to the metal side plate and spalling resistance. Accordingly, as a refractory plate having such properties, for instance, an alumina graphite containing carbon is preferable. However, since carbon-containing refractory plates of this type generally have a high thermal conductivity, it is necessary that the thickness of the refractory is made as thick as 100-150 mm for preventing the growth of the solidification shell. Refractory plates having such a thickness not only become larger in weight and difficult to attach and detach, but also cannot be subject to partial repair,if cracking or erosion occurs during use, because they are one-piece bodies and consequently it is necessary to replace the refractory plate itself with a new one. Further, a refractory plate of the aforementioned material has a lifetime of only two heats and the refractory cost increases. Therefore, when the aforementioned refractory plate made of alumina graphite is used to form the refractory layer 16, it is favorable to spray-coat a refractory such as zirconia onto the refractory plate to form a spray coating layer thereon. Thus, if a spray coated zirconia layer with a thickness of 2.5 mm is provided on a refractory plate of alumina graphite of thickness 25 mm, the resultant refracting layer can be used for 6 successive heats. Alternatively, when a spray coated zirconia layer with a thickness of 5 mm is directly applied to the metal side plate 9, it can be used for 4 successive heats. As mentioned above, the use of a spray-coated zirconia layer not only makes the thickness of the refractory layer 16 a reasonable thickness, but also permits the lifetime to be prolonged by partial spray coating repair if a part of the spray coated layer falls off whereby the non-operating time can considerably be shortened compared to that involved in exchanging the metal side plate. Also there is a reduction of the refractory cost. In addition, a CrC or WC series refractory having excellent thermal shock resistance, thermal seizing resistance, molten steel adhesion resistance and high temperature hardness may be spray-coated on to the surface of the side plate. In this case, a composition consisting by weight of CrC₂: 65-90% and NiCr: 35-10% is preferable as the CrC series refractory and a composition consisting by weight of WC: 65-90% and CO: 35-10%, or WC: 65-90% and NiCr: 35-10% is preferably used as the WC series refractory. Fig. 11 shows another embodiment of side plate used in the casting apparatus of the invention, wherein the sliding contact portion at the molten steel level 9a between the tapered side plate 9 and the metal belt is composed of a quenching metal plate. Since the area of the upper portion 9A of the metal plate which contacts the molten steel depends upon the change of the molten steel level during the casting, it is arranged so as to downwardly extend by 100-200 mm, preferably about 150 mm beyond the molten steel level. The illustrated side plate 9 has, for example, such a tapered shape that the width at the upper end 9c is 300 mm, the width at the molten steel level 9a is 200 mm, the width at the lower parallel portion 9b is 30 mm and the total length is 1,050 mm, wherein upper and lower portions 9A, 9B of the side plate facing the molten steel and extending down to 400 mm from the upper end 9c and extending up to 300 mm from the lower end 9b, respectively are composed of the quenching metal plate and the remaining middle area of about 350 mm in length is composed of the refractory layer 16. By using a side plate of the aforementioned construction, a substantially improved effect can be obtained, that is, it is possible to continuously cast a cast sheet of low-carbon Aℓ killed steel sheet having, for example, a width of 850 mm and a thickness of 30 mm at a drawing rate of 7.2 m/min for a long time such as about 2 hours, and accidental leakage of molten steel due to breaking of the solidification shell can substantially be prevented. As shown in the embodiment of Fig. 11, when the sliding contact portion at the molten steel level between the side plate and the metal belt is composed of the quenching metal plate 9A, molten steel is cooled to form a solidification shell by contacting with the quenching metal plate 9A. However, when a cast sheet of several tens of mm in thickness is directly cast, the drawing rate is very high (e.g not less than 5 m/min, and usually 7-30 m/min) as compared with the case of continuously casting thick cast slab at a drawing rate of 1-2 m/min, Thus the thickness of the solidification shell formed by the quenching metal plate 9A near the molten steel level is thin and the temperature thereof is high and consequently this solidification shell can very easily be deformed and hence does not increase the drawing resistance. Particularly, the solidification shell formed on the surface of the quenching metal plate 9A is separated from the quenching metal plate 9A by the solidification shell formed on the surfaces of the metal belts because of the difference in velocity between the rotating metal belts 1, 1' and the fixed side plate 9 and as a result the drawing resistance is scarcely increased. Now, the invention will be considered with respect to the metal belt. According to our investigations, it has been found that the reason why the metal belt used in the conventional apparatus has the aforementioned drawbacks is due to the fact that the belt is not strong enough to be used in conditions where one surface contacts the molten steel and the other surface contacts the cooling water, and due to the fact that the welding technique is not suitable. Considering the fact that the metal belts 1,1' are subjected to bending deformation when they continually move around the guide rolls 2,2',3,3', and 4,4' (the strain due to the bending deformation increases with a decrease in roll diameter and an increase in the thickness of the belt), the strength of the belt must be determined on the basis of the relationship between the diameter D of the guide roll and the thickness t of the belt. Fig. 12 shows the relationship between the yield strength Sy (kgf/mm²) of the belt and the diameter Dr (mm) of the guide roll in the presence or absence of belt deformation. The figures were obtained using belts of 0.4 mm-2.5 mm in thickness under a tension of 3.6 kg/mm². From these figures, the following can be deducted: Sy ≧ 10,500 t/Dr (1) 0.4 ≦ t ≦ 2.5 If t≦0.4, the belt is so thin that on catching foreign matter between the belt and the roll, holes are easily formed and consequently leakage of water and the like tends to occur. Additionally, breaking of the belt is apt to occur at the scratched portions. If t≧2.5, as is apparent from Fig. 12, rolls having a very large diameter are required in order to avoid the deformation of the belt which detracts from the merits of the belt type apparatus. The above yield strength required for the metal belt may be achieved by controlling the cooling rate so as to obtain a martensite structure as in a low yield ratio, high strength cold-rolled steel; dual phase steel (CHLY). However, when continuous casting is carried out by using a metal belt of CHLY in a continuous casting apparatus according to the invention, the metal belt is repeatedly subjected to the simultaneous action of heating and cooling during the casting, and consequently the strength of the metal beat considerably lowers. In this regard, as the preferred metal belt according to the invention, use may be made of a high strength steel of a solid-solution strengthening type using P, Si and Mn as a solid-solution strengthening element and having a yield strength of not less than 25 kgf/mm². This material scarcely exhibits a reduction in yield strength even when repeatedly subjected to the simultaneous action of heating and cooling. For instance, cold-rolled steel sheets for automobiles (SPFC40-60), phosphorus-containing high strength cold-rolled steel sheet (CHR40-60), and the like are steels showing substantially no reduction of yield strength and these can very conveniently be used as the metal belt for the continuous casting apparatus according to the invention (see Table 1). Table 1 shows the results obtained when casting Aℓ killed steel by means of the apparatus illustrated in Fig. 2 (roll diameter 400 mm⊘, thickness of cast sheet 95 mm. width of cast sheet 500 mm). In this case, the thickness of the belt was 0.8 mm. From the above, the yield strength of the belt is required to be not less than 20 kgf/mm², preferably not less than 25 kgf/mm² in order to improve the shape of the belt under tension (warp of C surface). Also, clad steel sheet is effective as a material for the metal belt having the yield strength corresponding to the roll diameter and the like as mentioned above. Subsequently, the weld zones formed when the aforementioned materials were welded to form endless metal belts 1, 1' were examined. As a result, it was found that the reverse bend testing described in JIS Z 3126 is very well matched when evaluating the occurrence of cracking from the weld zone and the heat-affected zone during actual operation over the lifetime of the metal belt. The above steel sheets for the metal belts were examined with respect to various welding methods and welding conditions and subjected to the reverse bend testing and as a result, it was found that a method of laser welding was particularly advantageous as shown in Figs. 13a and b. APFC 40 of 0.7 mm in thickness and SPCE of 0.8 mm in thickness were welded by means of butt TIG welding at a welding rate of 30 mm/min and a current of 60-70A and by butt laser welding at a welding rate of 2.5 m/min and a power of 1.1 kw. Specimens were obtained therefrom and subjected to the reverse bend testing while changing the annealing conditions for the removal of strain. The results thus obtained are shown in Figs. 13a and b. In the laser welding, not only did the structure of the weld zone become homogeneous, but also the heat-affected zone could be restricted to a very narrow area as compared with the case of TIG welding, MIG welding, gas welding or the like It was hard to generate cracks resulting from the degradation of these zones and the strength was 2-3 times that of TIG welding as shown in Fig. 13. That is, in the case of the laser welding, a remarkably improved effect has been found. Such an improved effect based on the laser welding was attained not only when using APFC and SPCE, but also when using SPCC-1~4, SUS 304, SUS 430 or clad steel composed of SUS 304 and SS material. The invention will be described with respect to the following Examples. Example 1; Embodiment using high strength steel A metal belt of APFC 60 containing components of C=0.10%, Si=1.10%, Mn=2.00%, P=0.015%, S=0.006%, Aℓ=0.030% and Nb=0.030% and having a yield strength of 50 kgf/mm², a thickness of 1.2 mm and a width of 800 mm was used in the apparatus shown in Fig. 2 (roll diameter 600 mm), whereby a cast sheet having a thickness of 95 mm and a width of 500 mm (low-carbon Aℓ killed steel) was continuously cast at a casting rate of 4.0 m/min. As a result, the deformation of the metal belt was very small as compared with the case of continuous casting using a conventional metal belt of SPCE material (yield strength 16 kgf/mm²), and the shape of the resulting cast sheet and the surface properties of the thin plate after the rolling were good, respectively. Moreover, the lifetime of the metal belt increased by about 1.5 times as high as that of the conventional metal belt. Example 2; Embodiment using high strength steel A metal belt of CHR 40 containing components of C=0.06%, Si=0.01%, Mn=0.50%, P=0.090%, S=0.010% and Aℓ=0.055% and having a yield strength of 26 kgf/mm², a thickness of 0.8 mm and a width of 800 mm was used in the apparatus shown in Fig. 2 (roll diameter 600 mm), whereby a cast sheet having a thickness of 95 mm and a width of 500 mm (low-carbon Aℓ killed steel) was continuously cast at a casting rate of 3.7 mm/min. As a result, the deformation of the metal belt was small as compared with the case of continuous casting using a conventional metal belt of SPCE material (yield strength 16 kgf/mm²), and also the shape of the resulting cast sheet was good. Example 3; Embodiment using clad steel A clad steel composed of 18-8 stainless steel facing molten metal and SS material for general structural purposes facing the cooling water with a cladding ratio of about 1:1 and having a yield strength of 30 kgf/mm², a thickness of 0.8 mm and a width of 800 mm was used as a metal belt in the continuous casting apparatus shown in Fig. 2 (roll diameter 400 mm) to continuously cast a common steel (C=0.20%) having a thickness of 130 mm and a width of 500 mm. When using a metal belt of SPCC material (yield strength 18 kgf/mm²) having the same thickness, it was necessary to exchange the belt after casting for about 2 hours due to wavy deformation. On the other hand, when the above clad material according to the invention was used as the metal belt, the continuous casting could be performed for about 10 hours without problems and the surface of the resulting cast sheet was clean. Also, scratches generated on the portion in sliding contact with the fixed side plate were considerably reduced as compared with the conventional metal belt of SPCC material. Example 4; Embodiment using laser welding In the continuous casting apparatus as shown in Fig. 2 (roll diameter: 600 mm) using a pair of metal belts each formed by welding both ends of APFC 40 material having a yield strength of 28 kgf/mm², a thickness of 0.8 mm and a width of 1,350 mm to each other by means of a laser welding machine, low-carbon Aℓ killed steel was continuously cast into a sheet having a thickness of 80 mm and a. width of 1,000 mm. As a result, when using the laser welded metal belt, the lifetime was confirmed to be about 1.5-2 times that of a conventional metal belt formed by TIG welding. Further, the bad shape of the cast sheet, which had frequently been observed when using the TIG welded metal belt, was considerably reduced. Example 5; Embodiment using laser welding In a continuous casting apparatus comprising a rotary caster system (roll diameter 600 mm) using a metal belt formed by welding both ends of SPCC material having a yield strength of 18 kgf/mm², a thickness of 1.6 mm and a width of 300 mm to each other by means of a laser welding machine, and a casting wheel of 3 mm in diameter, a billet (C=0.2%, Mn=0.85%) having a cross section of 130×150 mm² was continuously cast at a drawing rate of 3.5 m/min. In this case, the temperature of the tundish was maintained at 1,535±5°C. When using a TIG welded metal belt made of the same material, cracks were produced from the weld zone or heat-affected zone, so that it was required to exchange the metal belt after the casting of about 3,000 ton. On the contrary, the laser welded metal belt showed no occurrence of cracks even after casting about 6,000 ton and was good. Next, the invention will be described in detail with respect to a means for applying a lubricant in order to prevent the oxidation of the metal belt and increase the lubrication effect to thereby improve the lifetime of the belt and provide a cast sheet having good surface properties. According to this embodiment of the invention, prior to the beginning of the casting or the heating of the side plate, an antioxidant such as an organic resin, BN powder or the like is first applied to the inner surfaces of the metal belts 1, 1' which contact the molten steel. When applying such an antioxidant, the oxidation of the metal belts was scarcely observed and no red-rust occurred. During the casting, and at least prior to contact with the molten steel, a lubricant selected from rapeseed oil, an organic resin, an inorganic antioxidant such as BN or the like or a mixture thereof is applied to the inner surfaces of the metal belts 1, 1' to form a coating thereon, and the casting is continued. In this way, when using a coating of lubricant or antioxidative lubricant, bonding between the cast sheet 8 and the metal belts 1, 1', or seizing is completely prevented by the lubricant Moreover, the method by which the metal belt is coated with the above antioxidant or lubricant is not particularly limited. Thus it may be applied by spraying and the like. The coating material used preferably has both antioxidation and lubrication properties and includes, for example, (1) Teflon, rapeseed oil or heavy oil as an organic material; and (2) BN, zircon powder or zirconia powder as an inorganic material. The coated amount is 50 g/m² ~ 500 g/m². When it is less than 50 g/m², seizing partially occurs, while when it exceeds 500 g/m², solidification of the cast sheet becomes slow because heat conductivity becomes poor. In this connection, an explanation will be given with respect to the following example. Molten steel (C/0.04%, Si/0.2%, Mn/0.3%, P/0.02%, S/0.015%, Aℓ/0.04%) of 5 ton per heat was poured into a belt converging type continuous casting apparatus as shown in Fig. 2 to form a cast sheet having a thickness of 30 mm, a width of 1,000 mm and a length of 23 mm. In this case, each of rapeseed oil, Teflon, BN and BN + rapeseed oil was applied to that surface of the steel belt which contacts the molten steel and then the lifetime of the steel belt and the number of times seizing occurred between the cast sheet and the steel belt were examined. Prior to the casting, a coating material consisting of a mixture of BN and rapeseed oil was applied to the belts contacting the molten steel by means of a brush. The coated amount was 70 g per m² of the belt. Rapeseed oil was sprayed in an amount of 50 g/m² onto the above coated area by means of a spraying system after the beginning of the casting. The lifetime of the belts and the number of seizing times are shown in Table 1 together with the results obtained using different coatings and no coating at all. According to the invention, the lifetime of the belt was prolonged, thermal strain and oxidation were effectively prevented and the number of seizing times was reduced. Industrial Applicability As mentioned above, the belt converging type continuous casting apparatus according to the invention is applicable not only to directly produce thin steel plate such as sheet bar from molten steel, but also to a technique for the continuous casting of aluminum, alloy thereof and the like.

Claims: A belt converging type continuous casting apparatus for the production of a cast sheet comprising a casting space defined by a pair of endless metal belts (1,1') continuously moving around guide rolls (2,2',3,3',4,4') (10,10',11,11') and arranged opposite to each other for supporting the broad-side surfaces of the cast sheet (8) and a pair of tapered fixed side plates (9,9') for supporting the narrow-side surfaces of the cast sheet (8) and each disposed between the metal belts and in intimate contact therewith, characterised in that (i) each side plate (9,9') has such a shape that the width 2D at the molten metal level (9a), the width 2d at a lower portion (9b) corresponding to the thickness of the cast sheet, and the converging angle ϑ satisfy the following requirements: d = 5 - 30 mm D ≧ 60 mm D/d ≦ 16 ϑ ≦ 30 o [ϑ=tan⁻¹ (D-d)/H (wherein H is the vertical distance from the molten metal level (9a) to the upper end of said lower portion (9b) and in that (ii) each metal belt (1,1') has a yield strength Sy satisfying the following requirements: Sy ≧ 10,500 t/Dr 0.4 ≦ t ≦ 2.5 wherein Sy = yield strength (kgf/mm²) Dr = guide roll diameter (mm) t = thickness of the belt (mm) A continuous casting apparatus according to claim 1, wherein that portion of each side plate which contacts molten metal is composed of a refractory (18) or a metal plate (9), and the surface of the refractory or metal plate is provided with a spray coated layer (19) of a refractory having erosion resistance and low heat conductivity such as zirconia. A continuous casting apparatus according to claim 1, wherein that portion of each side plate which contacts molten metal is composed of a metal plate (9), and the surface of the metal plate is spray-coated with a CrC series or WC series refractory (19) having excellent thermal shock resistance, molten steel adhesion resistance and high-temperature hardness. A continuous casting apparatus according to any one of claims 1, 2 and 3, wherein each side plate includes a quenching metal plate (9A) at the level (9a) of the molten metal and a refractory layer (16) below the level of the molten metal. A continuous casting apparatus according to any one of claims 1 to 4, wherein each metal belt (1,1') is formed of a high strength cold-rolled steel of a solid-solution strengthening type using P, Si and Mn as a solid-solution strengthening element. A continuous casting apparatus according to any one of claims 1 to 4, wherein each metal belt (1,1') is formed of a clad steel composed of a stainless steel at the side in contact with the molten metal and a steel for general structural purposes at the other side. A continuous casting apparatus according to any one of claims 1 to 6, wherein each metal belt is obtained by endlessly joining a metal sheet using laser welding. A continuous casting apparatus according to any preceding claim, wherein that surface of each metal belt which contacts molten metal is coated with a lubricant having an antioxidation function.

Inventors: HABU, YASUHIRO KAWASAKI STEEL CORPORATION, KIMURA, TOMOAKI HITACHI LTD, HITACHI WORKS, KINOSHITA, KATSUO KAWASAKI STEEL CORPORATION, NAKATO, HAKARU KAWASAKI STEEL CORPORATION, NOZAKI, TSUTOMU KAWASAKI STEEL CORPORATION, OHMIYA, SHIGERU KAWASAKI STEEL CORPORATION, OHNUMA, HIROAKI KAWASAKI STEEL CORPORATION
Applicants: HITACHI, LTD., KAWASAKI STEEL CORPORATION
Application year: 1984
Main classification: B22D11
IPC codes: B22D11