METHOD AND APPARATUS FOR PRODUCING HOLLOW METAL INGOTS

Abstract: Hollow metal ingots are produced by a hollow metal manufacturing apparatus comprising a mold placed on a stool, and a cylindrical metallic cooling core placed in the mold to form an annular casting space therebetween. During the casting, a cooling fluid is directly blown upon the inner surface of the core while the cylindrical metallic core is allowed to be buckled. The cooling core is constituted by an outermost metallic cylinder to be brought into contact with a molten metal, a cylindrical lattice-fashioned buckling-­ adjusting frame positioned in the metallic cylinder and prov­ ides passages through which the cooling fluid is passed, and a cooling fluid vessel which is positioned inside the buckling-­ adjusting frame and is provided with a number of cooling fluid blowing nozzles.

Description: The present invention relates to a method and an apparatus for producing hollow metal ingots and is concerned with a method for casting hollow metal ingots (hereinafter, typical "steel ingots" will be discussed by way of example), used for the production of cylindrical forged steel articles such as pressure vessel materials, oversized ring materials and the like as well as an apparatus used for performing the above method. Recently, the uses of hollow steel ingots have greatly increased and, with this increase, there has been a demand for hollow steel ingots having a more strict and more diversified shape and quality. For instance, there are demands for producing large size articles exceeding 300 tons and articles having no inverse V-shaped segregation lines in the inner surface thereof. It is now not so difficult to produce such hollow steel ingots themselves. For instance, there are known the following production techniques: (1) By using a metallic cylinder as an outer tube to be brought into contact with the molten steel and by employing a solid core or a core of a hollow metallic cylinder inside the outer tube, hollow steel ingots are produced while a cooling fluid such as air or steam is flowed into the core (see British patent No. 520598). (2) A core consisting of a cylindrical steel pipe and a cylindrical refractory member formed so as to contact the inner wall of the cylindrical steel pipe is placed at the centre of a mold positioned on a stool, and hollow steel ingots are produced by pouring molten steel between the mold and the core (see Japanese patent application laid-open No. 154-117,326). Since the above prior art techniques enable the cores to be formed easily and provide cores having a good cooling performance, it can be said that they are excellent techniques. However, since the demands on the quality of the hollow steel ingots have recently been getting more and more severe and also the size of the steel ingots has become greater, it is the case that such requirements cannot be met by these prior art techniques. More particularly, with an increase in the size of the steel ingots, it has become difficult to produce hollow metallic cylinders which can withstand the static pressure of the molten steel, be appropriately buckled and deformed and, at the same time, still maintain the necessary hollow shape against the subsequent pressure. Further, when the size of the steel ingots becomes larger, cooling of the ingots from the inside of the core becomes insufficient. As a result, inverse V-shaped segregation is liable to appear and hence there occurs a quality problem particularly when the steel ingots are for use in, for example, atomic energy generation plant for which very strict quality is required. For instance, when the thickness of the metallic cylinder itself forming the outer shell of the core is increased to cope with steel ingots of increased size and better quality, the cooling power must be strengthened. On the other hand, fatal cracks can occur in the inner surface of the steel ingots if buckling does not occur. If the thickness of the metallic cylinder of the core is reduced, cracking at the inner surface of the steel ingot can be avoided by allowing an appropriate degree of buckling. However, there is a danger that the core might be crushed because the amount of buckling may be beyond control. If, because of this, buckling of the core is intended to be suppressed midway, it is necessary to install an obstacle between the metallic cylinder and the cooling fluid supply system. Consequently, sufficient cooling cannot be performed. Further, it is known to use water as a cooling fluid. Although in this case the cooling effect is improved, it makes the deformation of the metallic cylinder difficult and there remains the problem of safety. Thus, this technique is not practical. Under the circumstances, it is an object of the present invention to provide an advantageous technique for producing hollow metal ingots of a large size which do not develop cracks in the inner surface thereof and which have excellent internal quality. It is another object of the present invention to provide an advantageous technique for obtaining hollow metal ingots which results in no cracks in the inner surface when oversize hollow metal ingots are produced, which produces ingots having an excellent internal surface quality, and which has high safety during production. EP-A-0174157 describes the production of hollow metal ingots by placing a mold and a core concentrically on a stool to provide an annular casting space therebetween. The core comprises inner and outer tubes. Cooling is achieved by passing an inert gas through the annular gap between the tubes and cooling air is blown towards the inner surface of the inner tube. There is no provision for blowing different cooling fluids towards the inner surface of the inner tube at different stages in the casting process. According to one aspect of the present invention there is provided a process for producing a hollow metal ingot which comprises the steps of (i) placing a cylindrical metallic core in a central portion of a mold so as to form an annular casting space between the mold and the core, (ii) pouring molten metal into the annular casting space, and (iii) solidifying the thus poured molten metal by directly blowing cooling fluid upon an inner surface of the core, characterised in that the core is cooled by using an inert gas as the cooling fluid during the high temperature melt-pouring stage in which the cylindrical metallic core is allowed to buckle and then by using air or water or a mixed mist of water and gas as the cooling fluid during the low temperature solidifying stage. According to another aspect of the present invention there is provided an apparatus for producing a hollow metal ingot, which comprises a mold placed on a stool, a cylindrical metallic core concentrically placed in a central portion of the mold to define an annular casting space therebetween, and a cooling fluid vessel located within the core and having nozzles for directing cooling fluid at the inner surface of the core characterised in that the apparatus includes a switching valve to enable different cooling fluids to be directed at the inner surface of the core at different stages in the casting process. In accordance with a particularly preferred embodiment, the core includes a buckling-adjusting frame which is provided to preliminarily form an appropriate gap to accommodate buckling of the metallic cylinder. In this case, the cooling fluid nozzles are provided opposite openings in the buckling-adjusting frame to appropriately promote the cooling of the metallic cylinder. Since the occurrence of cracks in the steel ingots is diminished and the influence of inverse V-shaped segregation lines is minimized, hollow metal ingots having high quality can be assuredly obtained in accordance with the present invention. For a better understanding of the invention and to show how the same may be carried out, reference will now be made by way of example, to the accompanying drawings, wherein: Fig. 1 is a sectional view of an embodiment of a hollow metal (steel) ingot-producing apparatus according to the present invention; Fig. 2 is a sectional view of another embodiment of a hollow metal (steel) ingot-producing apparatus according to the present invention; Fig. 3 is a perspective view of a buckling-adjusting frame forming part of the hollow metal (steel) ingot-producing apparatus of Figures 1 and 2; and Fig. 4 are schematic views of macrostructures of (b) a hollow metal (steel) ingot obtained according to the present invention and (a) a hollow steel ingot obtained in accordance with the prior art. In accordance with the present invention, hollow steel ingots are obtained principally by concentrically arranging the cylindrical metallic core, which is to be cooled by supplying a cooling fluid thereinto, at the centre portion of a mold, pouring molten steel into the annular casting space formed between the mold and the core, and solidifying the molten steel by cooling it from the inside and the outside thereof. In such a method, as shown in Figs. 1 and 3, core 4 is constituted by a metallic cylinder 6 which is to be in contact with molten steel in the casting space S defined by mold 2 and the metallic cylinder 6. The core includes a cylindrical lattice-like buckling-adjusting frame 7 having openings 7a to serve as cooling fluid passages and a cooling gas vessel 9 having, at its periphery, a number of cooling fluid-blowing nozzles 8 which are opposite the openings 7a. A gap G is provided between the metallic cylinder 6 and the buckling-adjusting frame 7 to allow buckling of the metallic cylinder 6. The mold 2 and the core 4 are placed on a stool 1 having at least one upwardly open sprue 5 leading to the annular casting space S and communicating with a runner 3. The mold includes a heat insulating sleeve 10. In use, the cooling fluid for cooling the metallic cylinder 6 is uniformly blown over the whole inner surface of the metallic cylinder 6 from the fluid blowing nozzles 8 through the openings 7a of the buckling-adjusting frame 7 to uniformly cool the metallic cylinder. Most of the cooling fluid impinges substantially vertically upon the metallic cylinder 6. In this embodiment, an inert gas and air are used as cooling fluids. The inert gas is blown for 5 hours after pouring during which the metallic cylinder 6 is at a temperature of not less than 1,000°C. Then inexpensive air is blown during the low temperature solidification stage. For this purpose, according to the present invention, an inert gas pipe line 12 and an air pipe line 13 are connected to a supply system for the cooling gas vessel 9 by way of a switching valve 11. The occurrence of cracks at the inner surface of the steel ingot is avoided by allowing buckling of the metallic cylinder 6 as a consequence of the buckling gap between the metallic cylinder 6 and the buckling-adjusting frame 7. The buckling gap G is preferably from 5 to 40 mm. If it is less then 5 mm, the amount of buckling allowed is small and cracks may occur. If it is more than about 40 mm, the amount of buckling is large and the deformation of the solidified steel may not follow the buckling thereby causing cracks. Further, since the metallic cylinder 6 can be strongly cooled directly through the openings 7a in the buckling-adjusting frame 7, burn-out of the metallic cylinder 6 can not only be prevented but also the internal quality of the steel ingot is enhanced so that the quality of the ingot is improved. The reason why the blowing nozzles 8 are arranged so as to face the openings 7a in the buckling-adjusting frame 7 and hence blow the main stream of the cooling fluid substantially perpendicularly to the metallic cylinder 6 is so that the cooling effect may be further enhanced thereby. In addition, the reason why the buckling-adjusting frame 7 is designed in the manner of a lattice structure is so that the flow of the cooling fluid is not interrupted by the buckling-adjusting frame and so that it can endure the force from the steel ingot after the metallic cylinder 6 has buckled. Next, referring to the use of the cooling fluid, the reason why the inert gas is used at the initial stage and air is used at the latter stage is so as to be able to cope with a large heat capacity when large ingots are being produced. Also, inert gas is used in the initial stage because, when the temperature of the metallic cylinder 6 is 1000°C or more, the metallic cylinder may generate heat through oxidation and cause burn-out if air were to be used. In this respect, when the temperature reaches 1000°C or less, the metallic cylinder 6 does not generate heat through oxidation even when air is blown and air is inexpensive compared to inert gas. Referring now to Fig. 2, parts corresponding to parts of Fig. 1 are denoted by like reference numerals. The core 4 is constituted by metallic cylinder 6 (located at the outermost side) which contacts the molten steel in the casting space S, the cylindrical lattice-like buckling-adjusting frame 7 having openings 7a as cooling fluid passages, and a cooling fluid vessel in the form of a nozzle pipe 39 in which a number of cooling fluid-blowing nozzles 8 are arranged along the longitudinal axis of the pipe and facing the openings 7a. Gap G between the metallic cylinder 6 and the buckling adjusting frame 7 allows buckling of the metallic cylinder 6. In order to cool the metallic cylinder, a cooling fluid comprising an inert gas, water or a mixed mist thereof is uniformly blown over the whole surface of the metallic cylinder 6 from the fluid blowing nozzles 8 through the openings 7a of the lattice-like buckling-adjusting frame 7 to cool the metallic cylinder. The majority of the cooling fluid impinges substantially perpendicularly upon the metallic cylinder 6 to enhance the cooling effect. As the cooling fluid, use may be made of the inert gas, water or a mixed mist thereof depending upon the casting stage. In accordance with the present invention, the inert gas is blown through the nozzles 8 at least during the pouring stage so that the metallic cylinder 6 may be appropriately buckled and thereafter water or the mixed mist is used as the cooling fluid. By so doing, the metallic cylinder 6 is deformed during the pouring or at an early stage after the pouring to prevent cracking of the inner surface of the steel ingot. On the other hand, since water is not used until after the solidified shell has fully formed on the opposite surfaces of the steel ingot, the invention is characterised by being free from the danger of steam explosion. In order to selectively use the cooling fluids according to the present invention depending upon the casting stage, the nozzle pipe 39 is connected to the inert gas pipe line 12 and the water pipe line 33 through a switching valve 11. The mixed mist is obtained by setting the valve 11 so that the pipe 39 is in communication with both line 12 and line 33. As mentioned above, the reason why the inert gas is used at least during the initial stage of the casting process and is then replaced by water is because the metallic cylinder 6 is required to be deformed so as to prevent cracking at the inner surface of the steel ingot. It has been found that cracking occurs at the inner surface of the steel ingot when the solidifying molten steel cannot withstand its tightening action on the core as the solidified shell shrinks during the initial solidifying stage. Therefore, if the stress on the solidified shell is removed, cracking can be prevented. It has been found, from many casting examples, that the buckling of the metallic cylinder 6 occurs mainly before the completion of the pouring stage. Consequently, if the stress on the solidified shell is removed when the pouring is completed, no cracks occur in the inner surface of the steel ingot, which has been already solidified, even when strongly cooling the inner side of the steel ingot. However, the growth of the solidified shell is incomplete during the pouring and there is a danger of steel leakage when stress is developed in the core and the solidified shell during strong cooling. Accordingly, in order to remove the stress due to the deformation of the core and ensure safety, it is necessary to cool the molten steel with the inert gas at least during the pouring. Since the heat capacity is large in the case of large size hollow steel ingots, the metallic cylinder 6 may reach temperatures of 1000°C or more. Thus, the reason why the inert gas is used is that if air were to be blown at such temperatures, the metallic cylinder would generate heat through oxidation and burn out. The inert gas and water are used as cooling fluid. Use is preferably made of a construction in which the inert gas and water pipe lines are united together near the mold through a switch valve 11. In another construction, pipe lines for inert gas and water are separately provided. In such a case, the inert gas and the cooling water can be simultaneously introduced, and their flow rates may be independently controlled. This has the merit that the pipe lines can be easily produced. In the embodiment shown in Fig. 2, the cooling inert gas pipe line 12 and the cooling water pipe line 33 are constituted by a concentric double wall pipe. In such a case, when either one of the cooling fluids flows, the pipe line itself is cooled. This has the merit that problems such as abrupt boiling can be avoided when the cooling fluid is changed. The amount of buckling produced in the initial casting stage is controlled by the gap G between the metallic cylinder 6 and the buckling-adjusting frame 7. The gap G is preferably controlled in a range of from about 5 to 50 mm. If it is less then 5 mm, cracks occur due to the limited amount of buckling allowed whereas if it is more than 50 mm the amount of buckling is so large that the solidified shell may crack and there is a danger of steel leakage. As the cooling fluid employed after the completion of the pouring, water is mainly used. A water discharge channel 34 is formed in the central portion of the stool for discharging used water. Thereby, cooling water blown upon the metallic cylinder 6 is rapidly discharged outside the mold. In this way, since there is no need to suck up and remove the used water by means of a pump or the like, safe casting can be performed. Further, if the runner 3 intersects the water discharge channel 34, there is a danger of explosion. Therefore, such must be avoided. For this purpose, the stool 31 is constituted by two plates 31a, 31b with the water discharge channel 34 being formed in the upper plate 31a and the runner 3 being formed in the lower plate 31b as shown in Fig. 2. In this way contact between water and the molten steel can be completely prevented by forming a water discharge outlet in a side of the upper plate and connecting it to a water discharge pipe. By way of example, steel ingots were produced according to the method of the present invention using the steel ingot-producing apparatus. Example 1 A 200 ton hollow steel ingot having an average thickness of 1,150 mm was produced by bottom pouring. The composition of the poured steel was C: 0.17 wt%, Si: 0.23 wt%, Mn: 1.43 wt%. Ni: 0.80 wt%. Cr: 0.14 wt%, Mo: 0.53 wt%, with the balance being Fe and impurities. A chrysanthemum-shaped mold 2 was placed on stool 1 having three up sprues 5. Cylinder 6 formed of mild steel and having an outer diameter of 1,400 mm and an inner diameter of 1,360 mm, buckling-adjusting frame 7 having an outer diameter of 1,320 mm and an inner diameter of 1,020 mm, and cooling gas vessel 9 having an outer diameter of 980 mm and an inner diameter of 964 mm were placed in the centre of the mold in this order from the outside to the inside thereof so that gap G was 20 mm. Starting from the beginning of the pouring, nitrogen gas was passed through nozzles 8 at a flow rate of 100 Nm³/min for 5 hours, and then replaced by air at the same flow rate. The cooling gas was ejected towards the inner surface of the metallic cylinder 6 through the nozzles 8 attached at the side wall of the cooling gas vessel 9 in a direction orthogonal to the inner surface. The side wall of the cooling gas vessel 9 was provided with 350 nozzles having a diameter of 6 mm. Pouring was carried out under the conditions that the melt rising rate was 145 mm/min while the molten steel temperature of 1,598°C was maintained at an overheated degree of 85°C. The metallic cylinder 6 was adhered to the inner surface of the steel ingot, but no burn-out was observed. The maximum deformation was 20 mm. Then the steel ingot was forged and machined, but no cracking occurred in the inner surface of the steel ingot during the forging. No undesirable portion was present in the end product. Example 2 A 200 ton hollow steel ingot having an average thickness of 1,150 mm was cast by bottom pouring. The composition of the poured steel was C: 0.21 wt%, Si: 0.22 wt%, Mn: 1.49 wt%. Ni: 0.78 wt%. Cr: 0.14 wt%, Mo: 0.54 wt% with the balance being Fe and impurities. A chrysanthemum-shaped mold was placed on a stool having three up sprues, and a mild steel cylinder having an outer diameter of 1,400 mm and an inner diameter of 1,360 mm, a buckling-adjusting frame having an outer diameter of 1,320 mm and an inner diameter of 1,020 mm, and a cooling nozzle pipe were placed in the central portion of the mold in this order from the outside to the inside thereof. During the pouring, nitrogen gas was blown at a flow rate of 40 Nm³/min from the beginning of the pouring. Nitrogen gas was used as cooling fluid for 30 minutes after the completion of the pouring, and was then replaced by water to cool the metallic cylinder by being blown at it in an orthogonal direction thereto. The molten steel (1,597°C) as poured was maintained at an overheating temperature of 89°C, and was poured at a melt rising rate of 150 mm/min. As a result, although the metallic cylinder was adhered to the inner surface of the steel ingot, no burn-out was observed. The maximum deformation was 20 mm. Then, the steel ingot was forged and machined, but no cracks occurred in the inner surface of the steel ingot during the forging and no undesirable portion was present in the end product. A sample was extracted from the product just beneath the feeder head to enable examination of the macrostructure with respect to a sound portion 20, a portion 21 having inverse V-shaped segregation, and a final solidified portion 22. The results shown in Fig. 4 were obtained. As compared with the conventional technique (Fig. 4(a)), the present invention (Fig. 4(b)) is obviously superior in that the inverse V-shaped segregation portion has moved inwardly. As is described in the aforegoing, according to the present invention, the cracking of the steel ingot can be prevented and the influence of inverse V-shaped segregation can be suppressed to a minimum. Therefore, large size hollow steel ingots having high quality can be assuredly obtained. In particular, the effects of the present invention are remarkable with respect to the production of ring-shaped products having a large diameter and ring-shaped products having excellent surface properties can be produced.

Claims: 1. A process for producing a hollow metal ingot which comprises the steps of (i) placing a cylindrical metallic core (4) in a central portion of a mold (2) so as to form an annular casting space between the mold and the core, (ii) pouring molten metal into the annular casting space, and (iii) solidifying the thus poured molten metal by directly blowing cooling fluid upon an inner surface of the core, characterised in that the core is cooled by using an inert gas as the cooling fluid during the high temperature melt-pouring stage in which the cylindrical metallic core is allowed to buckle, and then using air or water or a mixed mist of water and gas as the cooling fluid during the low temperature solidifying stage. 2. A process for producing a hollow metal ingot according to claim 1, wherein the core (4) is constituted by an outermost metallic cylinder (6) and a cylindrical lattice-like buckling-adjusting frame (7) inserted in the outermost metallic cylinder (6) with a gap between the outermost metallic cylinder and the buckling-adjusting frame (7) to allow buckling of the outermost metallic cylinder (6). 3. A process for producing a hollow metal ingot according to claim 2, wherein the gap is set at 5-50 mm. 4. An apparatus for producing a hollow metal ingot, which comprises a mold (2) placed on a stool (1) (31), a cylindrical metallic core (4) concentrically placed in a central portion of the mold (2) to define an annular casting space therebetween, and a cooling fluid vessel (9) (39) located within the core and having nozzles (8) for directing cooling fluid at the inner surface of the core (4) characterised in that the apparatus includes a switching valve (11) to enable different cooling fluids to be directed at the inner surface of the core (4) at different stages in the casting process. 5. An apparatus as claimed in claim 4 wherein the core (4) is constituted by an outermost metallic cylinder (6) for contacting the molten metal and a cylindrical lattice-like buckling-adjusting frame (7) positioned in the metallic cylinder (6) and having openings (7a) for the passage of cooling fluid from the nozzles (8) of the cooling fluid vessel, which is located in the buckling-adjusting frame, to the inner surface of the metallic cylinder. 6. An apparatus as claimed in claim 4 or 5, wherein the cooling fluid vessel is in the form of a nozzle pipe (39).

Inventors: AIZAWA, KANJI MIZUSHIMA WORKS, KATO, TOSHIO@MIZUSHIMA WORKS, OGUCHI, YUKIO C/O TECHNICAL RESEARCH DIVISION, OHZU, HIDESHI MIZUSHIMA WORKS, SAITO, KENJI C/O TECHNICAL RESEARCH DIVISION, YAO, MINORU MIZUSHIMA WORKS, Kato, ToshioµMizushima Works
Applicants: KAWASAKI STEEL CORPORATION
Application year: 1987
Main classification: B22D7
IPC codes: B22D7