JPH057100B2 - - Google Patents

Description

[Detailed description of the invention]

(Industrial Application Field) The present invention relates to a continuous casting method for casting ingots or billets for remelting, such as magnesium or magnesium alloys, which are materials for plastic working such as forging, extrusion, and rolling. (Prior art) Ingots and billets for remelting, which are materials for plastic working such as magnesium and magnesium alloys (hereinafter simply referred to as magnesium alloys), are used in the Durville casting method, air-cooled cast iron molds (book molds), continuous casting methods, etc. It is manufactured by The D'Urville casting method is excellent for producing small ingots on a small scale, but when producing large quantities of large ingots, it is more efficient to produce them by the continuous casting method. As shown in Figure 7, in the continuous casting method, molten metal is poured into a water-cooled copper or aluminum alloy mold, and the outer shell of the molten metal is solidified by removing heat from the molten metal within the mold. In this method, a dummy block is pulled down using a hydraulic cylinder or the like, and molten metal is continuously injected and solidified. When the formed ingot reaches a certain length downward, casting is stopped and the ingot is pulled upward. In addition, the solidified ingot is pulled down using pinch rolls, a saw is moved in synchronization with the descent, and the ingot is cut and taken out at certain lengths during the descent, allowing completely continuous pouring and removing the ingot. It can also be manufactured. The supply of molten metal from the sump to the mold is regulated by valve operation. The disadvantages of this casting method are that the molten metal often sweats and enters the outer shell surface during the casting process due to fluctuations or fluctuations in the molten metal level, and cold shut phenomenon is also likely to occur. The problem is that the casting surface deteriorates. Furthermore, since the reverse segregation of alloy components in the surface layer is large, it is necessary to remove the reverse segregation layer by performing many surface scrapings during plastic working. A major advance in continuous casting of non-ferrous metals in recent years is the so-called hot-top casting method, in which a molten metal receiving tank made of an insulating refractory is installed above the mold to hold molten metal under high hydrostatic pressure above the solidified layer of metal. There is a casting method called. In this casting method, the surface of the molten metal exists in the molten metal receiving tank, so unlike traditional casting, there is no need to strictly adjust the height of the molten metal surface in the mold, and labor savings can be expected. . Although it has advantages such as less contamination of oxide film, this method is still far from a perfect technology for casting magnesium alloys whose molten metal tends to be easily oxidized. It is insufficient as a lump, and improvements in improving the casting surface are desired. (Problems to be Solved by the Invention) The existing continuous casting method for manufacturing magnesium alloy ingots has problems in terms of the quality of the ingot, such as poor casting surface and reverse segregation of alloy components in the surface layer. There are many problems such as a large amount of carbon dioxide, and since magnesium is very easily oxidized, many safety considerations are required and the casting operation is complicated. Therefore, the present inventors have developed a continuous casting method for magnesium alloys that is simple in process and provides excellent quality of the cast ingot, and has completed the present invention. Therefore, the present invention provides, from a molten magnesium alloy,
The purpose is to efficiently obtain ingots of excellent quality using a continuous casting method. (Means and effects for solving the problems) The present invention provides a refractory metal molten metal receiving tank provided above a forced cooling mold, and the inner peripheral lower end surface of the molten metal receiving tank is located inside the inner peripheral surface of the mold. A forced cooling mold with a molten metal receiving tank overhanging to form an overhang part is used, the molten metal of magnesium or its alloy to be cast is stored in the molten metal receiving tank, and a lubrication interface is formed on the inner peripheral surface of the mold. A continuous casting method for magnesium or its alloy, which includes the step of holding the molten metal in a columnar or cylindrical shape in close proximity, and cooling the columnar or cylindrical body with a coolant passing through a mold. Introducing a gas containing sulfur hexafluoride directly under the overhang,
The gist of the present invention is a continuous casting method for magnesium or an alloy thereof, characterized in that casting is performed by applying gas pressure to the outer peripheral surface of the columnar or cylindrical molten magnesium or alloy thereof. Since molten magnesium alloy is more easily oxidized than copper, aluminum, etc., it is necessary to cover the surface of the molten metal with a non-oxidizing atmosphere. Conventionally, SO ₂ gas has been used for this purpose, but it has drawbacks such as severe effects on the human body and strong corrosive effects on metal materials of structures such as equipment. The sulfur hexafluoride gas used in the present invention does not have these drawbacks and is effective in preventing oxidation, and is suitable as a gas to be applied to the outer peripheral surface of a molten magnesium alloy metal. The shape of the ingot to which the present invention is applied is mainly as follows:
These are cylindrical ingots that serve as raw materials for extrusion or pultrusion molding, prismatic ingots that serve as raw materials for forming rolled plate materials, thick-walled cylindrical hollow ingots that serve as raw materials for hollow tube extrusion products, or billets for remelting. The present invention relates to an improvement in a continuous casting method called the direct chill method, in which the molten metal is held in a columnar or cylindrical shape in close proximity to a mold. At this time, the columnar or cylindrical body held in the mold comes into contact with the forcedly cooled inner surface of the mold, and the outer peripheral surface of the molten metal is rapidly cooled, forming a thin solidified outer shell, which then becomes thicker. It is believed that the solidified shell is separated from the peripheral surface of the mold, and that solidification starts from a portion near the entrance of the mold. According to the present invention, the gas pressure of a gas containing sulfur hexafluoride is applied to the outer peripheral surface directly below the overhang portion. For example, the gas is introduced parallel to the lower end surface of the magnesium alloy molten metal receiving tank forming an overhang and perpendicular to the axial direction of the casting. That is, the gas is introduced from one or more locations on the interface between the molten metal receiving tank and the mold, is then distributed over all the interfaces, and reaches the outer peripheral surface of the columnar or cylindrical body from all the interfaces. That is, in this case, there is no problem whether the gas branch flow is introduced facing the outer circumferential surface of the magnesium alloy columnar or cylindrical body, or the gas branch flow is introduced obliquely. However, all of the gas may be introduced substantially perpendicular to the outer circumferential surface. The gas is introduced during the entire casting period and around the entire circumference. As long as the introduced gas arrives at the outer peripheral surface of the predetermined area and in a predetermined direction, the flow path of the gas does not matter. However, in practice it is advisable for the gas to pass through the interface. In the present invention, casting is performed by forming a lubricating interface on the inner surface of the mold. As a method for forming a lubricating interface, any of the following known methods may be employed. (1) Liquid lubricating oil is continuously leached from a position below the overhang toward the inner surface of the mold. (2) Apply lubricant to the inner surface of the mold before starting casting. (3) Use a mold made of a material such as graphite, which has a large contact angle with the molten metal and has self-lubricating properties with respect to the solidified metal shell. Formation of the above-mentioned lubricating interface on the inner surface of the mold is one of the essential components for obtaining a smooth and good casting surface, which is a characteristic effect of the present invention. The above (1) and (2) apply to molds made from metals with good thermal conductivity such as aluminum or copper. The casting apparatus used for carrying out the present invention has a refractory metal molten metal receiving tank provided above a forced cooling mold, and the inner peripheral lower end surface of the molten metal receiving tank protrudes inward from the inner peripheral surface of the mold and has an overhang. 1. An apparatus for continuous casting of magnesium alloys, comprising a forced cooling mold with a molten metal receiving tank forming a section, the outer region where the joint between the mold and the molten metal receiving tank are in airtight contact, and a slit between the mold and the molten metal receiving tank. an inner region formed and surrounded by said outer region, said slit being defined therebetween to prevent said molten metal from entering and communicating with a gas source containing sulfur hexafluoride; This is a characteristic feature. Next, the method according to the present invention will be explained based on the drawings. Method of the Invention FIG. 1 is a longitudinal sectional view showing a mold, a molten metal receiving tank, and an ingot for carrying out the method of the invention. Molten magnesium alloy is supplied to the molten metal receiving tank from a molten metal holding furnace (not shown). The surface of the molten metal in the molten metal receiving tank is covered with a gas containing sulfur hexafluoride to prevent oxidation. (Not shown) The mold 1 made of a material such as metal or graphite has a suitable shape defining the outline of the ingot 17, for example, a circular shape in the case of a cylindrical ingot, or a cross section, so that the ingot 17 It surrounds the space that is formed. The mold 1 has a cavity through which a cooling medium such as water 4 flows for forcedly cooling the metal columns 16 and 17. Connected to the mold 1 is a pipe 3 that supplies a cooling medium to the cavity from a cooling medium supply source (not shown). The heat of the molten metal 16 is absorbed from a part of the inner peripheral surface of the mold 1, and the molten metal starts solidifying as shown by diagonal lines. In order to secondarily cool the metal that has been primarily cooled by the mold in this way, the cooling medium is ejected from the ejection port 5 toward the ingot 17 .
The spout ports 5 are holes provided at equal intervals in the lower corner of the mold 1, or are provided in the form of slits around the entire circumference. Although it is not necessary that the medium for primary cooling and secondary cooling be the same, cold water is usually used. Marinite is applied to the upper end surface of mold 1.
A molten metal receiving tank 2 made of a heat-resistant refractory material well known under the trade name of Fiberflax is fixed to the mold 1 with bolts 15. The molten metal receiving tank 2 is arranged coaxially with the mold 1 and has an inner peripheral surface substantially parallel to the inner peripheral surface of the mold 1. As is well known, the molten metal receiving tank 2 stores molten metal and prevents the solidification start surface from changing due to fluctuations in the amount of poured metal. The solidified ingot 17 is continuously drawn downward from the mold 1 by lowering a bottom plate (not shown) at a certain speed (ie, casting speed). Next, the structure of the mold used for introducing gas according to the present invention will be explained based on FIGS. 2 to 4. Three pipes 6, 6', and 6'' branch off from the outer peripheral surface of the mold 1 at an angle of 120° in the diametrical direction, and are communicated with a gas supply source (not shown). 6, 6', and 6'' communicate with an annular channel 7 extending around the entire upper part of the mold 1. The gas is therefore evenly distributed over the entire upper circumference of the mold. It has been confirmed through experiments that the same results can be obtained with one supply pipe as with two or three supply pipes. The outer upper surface 1a of the mold 1 is a flat surface that comes into close contact with the lower surface of the molten metal receiving tank 2, and a groove 12 extending around the entire circumference of the mold is formed in a part of the outer surface 1a. This groove 12 is filled with a packing material such as heat-resistant rubber (not shown) to prevent the gas flowing through the channel 7 from leaking to the outside. The inner upper surface 1b of the mold 1 is slightly lower than the outer surface 1a, and a very small gap 8 is formed between it and the lower surface of the molten metal receiving tank 2. During this time, the hole 8 is in communication with the flow path 7 and is open on the entire inner circumferential surface of the mold. The inner end of the lower surface of the molten metal receiving tank 2 is the mold 1
It extends horizontally so as to cover the inner circumferential surface of the inner circumferential surface, and therefore an overhang portion 9 is formed over the entire inner circumferential surface. Therefore, the gas is introduced directly below the overhang portion from the interval 〓8. A means for supplying liquid lubricating oil between the metal solidified by the primary cooling action of the mold and the inner peripheral surface of the mold is provided inside the mold 1. That is, two supply pipes (not shown) connected to a liquid lubricant supply source (not shown) are fixed to the lubricant inlet 14 . The lubricating oil inlet 14 communicates with a passage 13 extending diametrically inside the mold 1 . Furthermore, this passage 13 communicates with a liquid lubricant supply tubular passage 10 extending circumferentially inside the mold 1 . A large number of liquid lubricant fine supply ports 11 branch from this supply passage 10 and open to the inner peripheral surface of the mold. The supply ports 11 extend radially toward the inside of the mold and are inclined in a direction opposite to the casting direction. The supply opening 11 may extend horizontally or obliquely in the drawing direction, the inclination being defined so that the lubricant flows down from the required height within the mold. Passage 1 in one mold
It is easy to understand that since it is almost impossible to process the passage 10 and the supply port 11, it is better to use two members that have been pre-processed to define the passage 10 and the like, and then fix them together by welding or the like. It will be. According to the structure described above, the liquid lubricating oil supplied from the inlet 14 leaks out from the supply port 11 and is constantly supplied to the inner circumferential surface of the mold from below the overhang portion 9. Furthermore, according to the apparatus shown in FIGS. 1 to 4,
Casting is performed with gas introduced across the outer circumferential surface of the metal column just below the overhang, and with lubricant supplied to the inner surface of the mold. (Example) Experiments were conducted using the method according to the present invention by changing gas introduction conditions, lubricating oil supply conditions, etc. Magnesium alloy AZ31 is shown in Figures 1 to 5.
A cylindrical ingot with a diameter of 62 mm was cast using the continuous casting apparatus shown in the figure. The temperature of the molten metal in the receiving tank is 680℃, the temperature of the cooling water is 20℃,
The cooling water supply rate is 20/min, and the casting speed is 150/min.
Cast at mm/min. Gas introduction conditions A sulfur hexafluoride cylinder with a pressure of 5 kg/cm ² was used as a gas source, and gas was introduced from supply pipes 6, 6', 6'' via a needle valve and a float type flow meter.Casting period A gas flow adjusted to a specific amount between 0.2 and 4.0/min is applied to the overhang part 9
The test was conducted by introducing it directly under the Castor oil was used as the lubricating oil, and its head pressure was adjusted to +20 mm above the gas pressure value corresponding to each gas flow. When the gas flow rate was low, the surface of the obtained ingot became a baked skin, as shown in the reference photo in Figure 1. The appropriate gas flow rate to obtain a good casting surface is 0.5
~3.0/min, and at flow rates exceeding this upper limit, bubbles were observed blowing out from the molten metal surface of the molten metal receiving tank. In addition, as shown in the reference photo Figure 2,
The optimal flow rate to obtain a very smooth and good casting surface is:
It is in the range of 1.0 to 2.0/min. The appropriate value for the gas pressure of the introduced gas (substantially equal to the gas pressure applied to the outer peripheral surface of the columnar or cylindrical molten metal directly under the overhang) is the value equivalent to the hydrostatic pressure of the molten metal stored in the molten metal receiving tank. The upper limit is determined by the hydrostatic pressure of the molten metal at which gas does not float through the molten metal in the receiving tank, and the lower limit is determined by the contact area between the metal and the inner surface of the mold. is determined by the pressure that can be substantially reduced. in the end,
The introduced gas forms an elastic space with a gas pressure close to the hydrostatic pressure of the molten metal just below the overhang, and the excess gas flows downward from the fine gap at the contact interface between the mold inner surface and the thin solidified shell of the molten metal. do. Conditions for using lubricating oil The minimum head pressure of lubricating oil for continuous lubrication must be higher than the gas pressure applied directly below the overhang if the amount of gas introduced is selected within the appropriate range. However, stable refueling is normally performed at a refueling head pressure 10 to 50 mm above this gas pressure. In this range, a good casting surface can be obtained. If the lubricating oil supply pressure falls below this lower limit, gas enters the oil supply port 11 and pushes back the oil, which prevents continuous oil supply and makes it impossible to obtain a good casting surface. Next, even if the oil head pressure H ₀ increases and the amount of oil used increases, no effect on the casting surface is observed. Therefore, from the viewpoint of reducing the amount of oil used, it is preferable for the oil head pressure to be low unless the oil supply is interrupted. Regarding the type of lubricating oil, the higher the viscosity, the better the results for the casting surface, but if oil spillage is taken into consideration, a viscosity of 5 to 40 poise is industrially appropriate, and castor oil, rapeseed oil, etc. , salad oil, etc. are suitable. Effect of introduced gas In order to investigate how the gas introduced into the overhang is acting, we observed the solidification region of the ingot under (a) gas pressure and (b) no gas pressure. did. During casting, a tracer was introduced into the molten metal in the header, and at the same time, the lower die table of the casting machine was stopped from descending. Thereafter, the solidified ingot was cut along the surface including the shaft and etched with a caustic soda solution. According to this, a solidified shell 50 was observed to grow from just below the header in the case where gas pressure was not effective (Fig. 6a), but a solidified shell 50 was observed in the case where gas pressurization was effective (Fig. 6b). No growth was observed. From this experiment, it is thought that the molten metal solidified as shown in Figure 5. That is, the molten metal is removed from the area directly below the overhang portion 9 by the applied gas pressure. The molten metal starts contacting the inner peripheral surface of the mold at a position considerably lower than the top end of the mold 1, immediately forms a thin solidified shell, and then separates from there. The length of the molten magnesium alloy in the casting direction in contact with the inner peripheral surface of the mold is quite short, and therefore the effect of primary cooling is reduced. The realization of the solidification process as shown in FIG. 5 is considered to be one of the reasons for the effects of the present invention. Another reason is that the effects of fluctuations in the molten metal level in the receiving tank and disturbances in the flow path of the inflow into the receiving tank are alleviated by the gas that exists directly under the overhang, and It does not reach the molten metal that solidifies directly, and therefore solidification progresses under certain conditions. Such a process is considered to be an important cause of the effects of the present invention. Reverse segregation of alloy components in an ingot The quality of an ingot formed by continuous casting is evaluated not only by the cast surface but also by the magnitude of the reverse segregation of alloy components in the ingot. Magnesium alloy AZ31 by the method of the present invention,
The ingots were cast by the conventional continuous casting method shown in FIG. 7 to obtain cylindrical ingots each having a diameter of 62 mm.
The results shown in Table 1 were obtained by comparing the segregation layer just below the surface of the ingot and the reverse segregation layer inside the ingot.

[Table] (Effects of the invention) An excellent ingot with a smooth casting surface and few reverse segregation layers was obtained by continuous casting of a magnesium alloy using a hot-top casting method using pressurized gas containing sulfur hexafluoride. Therefore, plastic processing such as extrusion and rolling can be performed by omitting skin shaving or by only a very small amount of skin shaving, and it is also possible to use the material directly as a forging material after cutting without going through the extrusion process. Ta.

[Brief explanation of drawings]

Fig. 1 is a vertical sectional view showing a specific example of a casting device according to the present invention, Fig. 2 is a plan view of a mold of the device in Fig. 1, Fig. 3 is a sectional view AB of Fig. 2, and Fig. 4 is the second
Figure 5 is a partial enlarged view of Figure 1 to explain the cooling mechanism, Figure 6 is (a) under gas pressure.
(b) A sketch of a cross section of an ingot showing the solidification region of the ingot without gas pressurization. FIG. 7 is an explanatory diagram showing a conventional continuous casting method. 1...Mold, 2...Refractory molten metal receiving tank, 3...Cooling medium supply hole, 4...Cooling medium, 6...Sulfur hexafluoride gas supply port, 8...Between, 9...Overhang part, 1
0...Lubricating oil supply tubular port, 11...Lubricating oil outflow port, 1
6... Molten magnesium alloy metal, 17... Magnesium alloy ingot, 20... Valve, 21... Magnesium alloy molten metal transfer pipe, 22... Nozzle, 24... Distribution cup.

Claims (1)

[Claims] 1. A molten metal receiving tank made of a refractory material is provided above the forced cooling mold, and the lower end surface of the inner periphery of the molten metal receiving tank extends inward from the inner peripheral surface of the mold to form an overhang part. The molten metal of magnesium or its alloy to be cast is stored in the molten metal receiving tank using a forced cooling mold, and the molten metal is held in a columnar or cylindrical shape close to the mold with a lubricated interface formed on the inner peripheral surface. A continuous casting method for magnesium or its alloy including the step of cooling the columnar body or cylindrical body by a coolant passing through the mold, and in which sulfur hexafluoride is placed directly under the overhang part. 1. A continuous casting method for magnesium or its alloy, characterized in that casting is carried out by introducing a gas containing gas and applying gas pressure to the outer circumferential surface of the columnar or cylindrical molten magnesium or its alloy.