Cooling method and apparatus for continuous casting and its mold

Abstract: A cooling mold (2)comprising first and second water cooling jackets (21, 22) which are provided inside the mold (2), a primary cooling water jetting mouth (23) which is located at a distance of 15 to 40 mm from the meniscus of the molten metal (3), and a secondary cooling water jetting mouth (24) which is located at a position with an interval of 20 to 45 mm between a contact point of a primary jet of cooling water and another contact point of a secondary jet of cooling water on an ingot (4). By use of the cooling mold (2) having the primary and secondary cooling water jetting mouths (23, 24) which are respectively set at an angle of 15 to 30 degrees and of 30 to 60 degrees relative to the ingot surface, the primary jet of cooling water from the primary cooling water jetting mouth (23) impinges on the molten metal (3) cooled in the cooling mold (2) at the short distance from the meniscus to establish a transition boiling zone and a film boiling zone, and nextly, the secondary jet of cooling water from the secondary water jetting mouth (24) impinges on initial zones of the transition boiling zone and the film boiling zone to break-out a vapor film generated in the initial zones so as to provoke a nucleate boiling and thereby to produce a firmer solidified shell in the ingot (4), whereby the ingot (4) can be properly and effectively cooled without danger of breakout so that stable high rate casting and high quality ingot can be achieved.

Description: This invention relates to a cooling method and a continuous casting apparatus including an annular cooling mold for continuous casting of ingots from molten aluminum, aluminum alloys, or other metals and more particularly to a method of continuous and direct chill casting and a mold for carrying out the direct chill casting method. In this continuous casting method as shown generally in FIG. 7, a molten metal 13 is injected from a tundish 11 through an orifice plate 15 into a mold 12 which is water-cooled, so that the molten metal is cooled in the mold 12 to cast an ingot 14. The molten metal 13 which is introduced through the orifice plate 15 to the mold 12, is contacted with the wall surface of the mold 12 to form a thin solidified shell and is further cooled and cast with impinging cooling water applied from the mold 12. In the continuous casting, a higher rate of casting is desired to improve the production rate and in order to realize the higher rate of casting, it should be simultaneously required to promote the casting quality such as the surface condition of the ingot by proper cooling. In the high rate casting, when the molten metal is solidified in the cooling mold to form the solid shell, the higher rate of casting requires the greater amount of heat extraction and thereby the larger amount of cooling water. The cooling water is applied from the mold to directly impinge on the high temperature ingot and cool it. However, when the casting rate is increased, since the surface temperature of the ingot becomes higher in a situation of impingement cooling with cooling water, a transition boiling zone and a film boiling zone is produced on the ingot surface and a vapor film which creates an adiabatic phase between the ingot surface and the cooling water is formed thereon. Thus, even if the amount of the cooling water is increased, the cooling water does not effectively function to carry out heat extraction so that the danger of break out increases, and problems such as causing quality defects of the ingot arise. Hence, these problems have been the factors which have considerably reduced the casting stability and the quality stability. In order to solve these problems, cooling methods have been proposed in which directly impinging cooling water is used in two steps as disclosed for example in JP,A Sho 58-212849 (Japanese Patent Laid-Open Application). However, in the two step cooling method using the cooling water as disclosed in the above Japanese Patent publication, since the distance between the first cooling zone and the second cooling zone becomes considerably long, that is half to two times the diameter of the ingot, the surface of the ingot which has been cooled in the first cooling zone is again heated by the time it reaches the second cooling zone due to heat flow from internal region of the ingot. Hence, even when a second cooling is carried out, the transition boiling zone and film boiling phenomena are again produced reducing cooling efficiency. When using hith rate casting, this tendency is more increased which considerably reduces the cooling efficiency. Document US-A-3,713,479 discloses a direct chill casting of metal ingot, wherein an externally solidified ingot having an initially molten core is progressively withdrawn from a shallow, cooled, open-ended mold, the ingot emerging from the mold passes successively through two separate cooling zones, the second cooling zone providing a substantially greater intensity of cooling than the first cooling zone. Document DE-A-1,433,021 discloses a method for the continuous casting of a metal ingot comprising impinging a cooling medium onto two separate zones of the ingot. It is therefore an object of this invention to provide a novel cooling method and an apparatus for cooling a molten metal to cast an ingot in a continuous casting wherein even when the continuous casting rate is increased, a proper cooling can be carried out without a danger of breakout so as to provide a stable casting and a high quality ingot. This invention concerns a cooling method for cooling an ingot which is continuously withdrawn and cast from a mold by cooling a molten metal in said mold in a continuous casting process; said method comprising cooling an ingot by impinging the ingot with a primary jet of cooling water from said mold downstream from the meniscus of the molten metal to establish a transition boiling zone and a film boiling zone on the surface of said ingot, said molten metal being cooled in contact with the inner surface of said mold before being cooled by said primary jet of cooling water, and cooling said ingot by impinging the ingot with a secondary jet of cooling water from said mold onto initial zones of the transition boiling zone and the film boiling zone to break-out a vapor film generated in the initial zones to provoke a nucleate boiling, characterized in that it further comprises wiping off said first and second impinging water jets from the surface of the ingot. Thus, there is produced a firmer solidified shell in the ingot without causing casting cracks, whereby the solidifying ingot is properly and effectively cooled to provide stable high rate casting and high quality ingot. Preferably, the impinging angle of the primary cooling water impinging against an ingot surface is 15 to 30 degrees and the impinging angle of the secondary cooling water impinging against the ingot surface is 30 to 60 degrees. When the ingot has a diameter of 15 to 23 cm (6 to 9 inches), the primary impinging cooling water from the mold contacts the ingot at a distance L1 of 15 mm to 40 mm from a meniscus which is a starting point of development of solidifying a shell, and the distance L2 between the contact point of the primary impinging cooling water from the mold and the ingot and the other contact point of the secondary impinging cooling water and the ingot in the transition boiling zone and the film boiling zone is preferably 20 mm to 45 mm. A continuous casting apparatus for accomplishing the above-mentioned cooling method includes an annular cooling cast mold which is disposed to surround an orifice plate secured to an outlet ejecting a molten metal from a tundish, said mold comprising a primary cooling water jetting mouth for establishing a transition boiling zone and a film boiling zone on the surface of an ingot and a secondary cooling water jetting mouth disposed at a predetermined distance in a withdrawing direction of the ingot from said primary cooling water jetting mouth to impinge onto initial zones of said transition boiling zone and said film boiling zone, characterized in that it further comprises: a wiper made of heat- and wear-resistance material arranged in front of the cooling mold to contact with the whole circumferential surface of the ingot which is withdrawn from the tundish, and to wipe off said first and second cooling water jets impinged from the cooling mold to the circumferential surface of the ingot. A third cooling water jetting mouth may be arranged ahead of the wiper. A cooling mold for accomplishing this cooling method comprises first and second cooling jackets inside thereof and a primary cooling water jetting mouth and a secondary cooling water jetting mouth which are disposed at the predetermined distance in the withdrawing direction of an ingot, wherein the primary cooling water jetting mouth is set at an angle of 15 to 30 degrees relative to the ingot surface and the secondary cooling water jetting mouth is set at an angle of 30 to 60 degrees relative to the ingot surface. The primary cooling water jetting mouth has preferably a whole peripheral slit shape and the secondary cooling water jetting mouth has also a grooved or holed shape. This invention will be illustrated in detail with the operation; Generally in a casting mold, when a cooling water impinges directly on a high temperature ingot to cool it, vapor bubbles or vapor films are produced on the high temperature ingot, so that the cooling water coming into contact with the ingot extracts heat from the ingot surface of high temperature. However, even when the cooling water is impinged on a high temperature ingot of about 600°C to promote a forced convection heat transfer, the transition boiling zone and the film boiling zone are produced immediately after the cooling water is contacted with the high temperature ingot, so that they are coated with a vapor film preventing contact between the cooling water and the ingot surface. In order to prevent the vapor film, even if the amount of the cooling water is increased to improve the cooling effects, there is a limit in this improvement of cooling effects, and at the same time, even if the pressure of the cooling water is increased, there is also a limit in the improvement of the cooling efficiency. On one hand, the length and shape of an unsolidified portion of the ingot in the casting process is highly correlated with the cooling water amount, the cooling position and the ingot surface temperature. A hard cooling results in a greater temperature difference between the surface portion and the center portion of the ingot so that the danger of casting cracks increases, and a weaker cooling causes breakout to aggravate the stability of the ingot. In view of these phenomena, this invention intends to produce a firm solidified shell by impinging cooling water in a transition boiling zone and a film boiling zone to break out a continuous vapor film produced thereon using the pressure of the cooling water, and to cool the ingot surface with direct cooling water to generate a nucleate boiling so as to provide an efficient cooling, without compensating for the reduction of the cooling efficiency in the transition boiling zone and the film boiling zone which are produced on the high temperature surface of the ingot by increasing the amount and pressure of the cooling water. In a casting of an ingot having a large diameter of 15 to 23 cm (6 to 9 inches), the contacting point of the primary impinging cooling water and a high temperature ingot is situated at a distance L1 of preferably 15 to 40 mm from a meniscus. When the distance L1 is less than 15 mm, the danger of generating the breakout in the start of the casting and breakout due to slight changes of casting conditions during casting is increased. When the distance L1 exceeds 40 mm, the direct cooling with the cooling water is retarded causing surface defects such as bleeding out and external cracks of the ingot surface. The depth of an inverse segregation layer becomes excessive to generate quality defects. It is also favourable to set a distance L2 of 20 to 45 mm between the contacting point of the primary cooling water with the ingot and the other contacting point of the secondary cooling water with the ingot. When the distance L2 exceeds 45 mm, the cooling is retarded increasing the unsolidified length within the ingot which increases the danger of cast cracks. The cooling water impinging angle relative to the ingot surface is one of the important factors in the efficient casting. It is favourable to set the primary cooling water impinging angle at 15 to 30 degrees and a secondary cooling water impinging angle at 30 to 60 degrees. When the primary cooling water impinging angle is set at less than 15 degrees, the distance from the meniscus which is a starting point of development of solidifying a shell, is increased causing the bleeding out, and when it is set at more than 30 degrees, the cooling water flows inversely at the start of the casting which causes the breakout. It is required to set the secondary cooling water impinging angle at 30 to 60 degrees so as to breakout the vapor film which is generated in the transition boiling zone and the film boiling zone by the primary cooling water. With respect to the shape of a cooling water jetting mouth which is formed in a cooling mold, the whole periphery of the mold is provided with a slit, groove, or hole type opening. The primary cooling water jetting mouth adopts the slit-shaped opening on the whole inner circumferential surface of the mold to cool uniformly the whole outer periphery of the ingot. The secondary cooling water jetting mouth adopts the grooved or holed opening on the whole periphery of the mold to break out the vapor film which is produced in the transition boiling zone and the film boiling zone. Further features and advantages of the invention will be apparent from the detailed description below, together with the accompanying drawings. FIG. 1 is a longitudinal sectional view of the main part which shows a cooling situation of a continuous casting process according to this invention; FIG. 2 is a longitudinal sectional view of the main part which shows a starting situation of the casting process; FIG. 3 is a partial enlarged view of FIG. 1; and FIG. 4 is a longitudinal sectional view of the main part which shows a cooling state of a continuous casting according to a second embodiment of this invention; FIG. 5 is an illustrative view which shows the temperature change of the inner and outer portions of an ingot corresponding to the variation of the distance from the meniscus without a wiper and a third cooling water jetting means ahead of the cooling mold according to this invention; FIG. 6 is an illustrative view which shows the temperature change of the inner and outer portions of an ingot corresponding to the variation of the distance from the meniscus with the wiper and the third cooling water jetting means ahead of the cooling mold according to this invention; and FIG. 7 is a longitudinal sectional view of the main part which shows a cooling situation in the conventional continuous casting process. An embodiment of this invention will be illustrated with reference to the accompanying drawings. This invention is not only usable in a horizontal casting as illustrated herein, but also may be used in a vertical casting. FIG. 1 is a longitudinal sectional view of a cooling portion in the casting, which is a typical embodiment of this invention. FIG. 2 is a longitudinal sectional view for showing the cooling portion at the start of the casting. And FIG. 3 is a partially enlarged sectional view of the cooling portion. In these drawings, a tundish, a molten metal, an orifice plate, an orifice, a starting block, and a starting pin are respectively indicated by reference numerals 1, 3, 5, 6, 7, and 8. These members have essentially the same structure as the conventional casting members. A cooling mold which is disclosed as the essential part of this invention, is indicated by reference numeral 2. First and second ring shaped water cooling jackets 21, 22 are formed in front and rear positions with a predetermined space inbetween on the same axis of the cooling mold. A part of each water cooling jacket 21, 22 communicates with an external cooling water supply pipe. The first and second water cooling jackets are respectively opened on the inner surface of the cooling mold 2 to form individual jet mouth 23, 24. The jet mouth 23 of the first water cooling jacket 21 which is arranged near the tundish 1, is formed with a slit opening on the whole inner circumferential surface of the mold 2. The jet mouth 24 of the second water cooling jacket 22 which is arranged far from the tundish 1, is formed with a grooved or holed opening on the whole inner circumferential surface of the mold 2. A set position of the jet mouth 23 of the first water cooling jacket 21 is determined by the position in which the cooling water jetted from the jet mouth 23 contacts with the ingot 4. In case of the ingot with the diameter of 15 to 23 cm (6 to 9 inches), the jet mouth should be set at a position such that the contact point is favourably disposed in the extent L1 which is at the distance of 15 to 40 mm from the meniscus. A set position of the mouth 24 of the second water cooling jacket 22 is also determined by the distance L2 between the position where the primary cooling water contacts with the ingot 4 and the other position where the secondary cooling water contacts with the ingot 4. In case of the ingot with the diameter of 15 to 23 cm (6 to 9 inches), the distance L2 is favourable in the extent from 20 to 45 mm. Moreover, commonly in the first and second water cooling jackets 21 and 22, the cooling water impinging angle against the ingot surface exerts a large influence upon the cooling efficiency. According to this invention, the angle formed between the impinging cooling water and the ingot surface is preferably set at 15 to 30 degrees in the primary cooling and at 30 to 60 degrees in the secondary cooling. In the continuous casting with the above-mentioned structure, a starting block 7 is inserted into the cooling mold 2 of this invention at the start of casting as shown in FIG. 2. A starting pin 8 secured to the tip of the starting block 7 is contacted with an end face of an orifice plate 5. In this state, a molten metal is introduced through orifices 6 of the orifice plate 5 into the mold 2, and when the starting block 7 is withdrawn at a predetermined rate from the mold 2, the casting is started. A plurality of orifices 6 are formed in the orifice plate 5. The molten metal 3 in the tundish 1 is introduced through the orifices 6 into the cooling mold 2, and since the molten metal 3 is in contact with the inner surface of the mold 2, the surface of the molten metal 3 is cooled to produce a thin solidified shell. Then, the molten metal 3 is directly cooled with a primary cooling water which is jetted from the primary jet mouth 23 of the mold 2, so as to advance the solidification. So, since a transition boiling zone and a film boiling zone are produced on the surface of the ingot 4 by the impingement of the primary cooling water, when a secondary cooling water impinges from the secondary jet mouth 24 of the cooling mold 2 upon the vapor film of these zones, the transition boiling zone and the film boiling zone are broken out by the impinging cooling water to provoke a nucleate boiling, so as to produce a firmer solidified shell in the secondary direct cooling against the ingot surfaces. This invention is illustrated in the embodied example wherein an ingot of an aluminum alloy based on Japanese Industrial Standard 6063 is cast by use of a casting apparatus shown in FIG. 1 in the following casting conditions. ( 1 ) The distance L1 between the meniscus and the contact point of the primary jet of cooling water is varied in the following casting conditions to cast the ingot. The results are shown in Table 1. a. Kinds of alloy : JIS 6063 aluminum alloy b. Diameter of ingot : (7 inches) 178 mm c. Casting rate : 350 mm / min d. Casting temperature : 690 °C e. Amount of primary jet of cooling water : 85 l / min L 1 Breakout Bleeding out ; Segregation 10 mm exist ― 15 mm not exist slightly 25 mm not exist slightly 35 mm not exist slightly 40 mm not exist a little 45 mm not exist much ( 2 ) The distance L2 between contact points of the primary and secondary impinging cooling water on the ingot is varied in the following casting conditions to cast the ingot. The results are shown in Table 2. a. Kind of alloy : JIS 6063 aluminum alloy b. Diameter of ingot : (7 inches) 178 mm c. Casting rate : 350 mm / min d. Casting temperature : 690 °C e. Amount of primary jet of cooling water : 85 l / min f. Amount of secondary jet of cooling water : 45 l / min g. Distance between meniscus of molten metal and contact point of primary impinging cooling water : 25 mm L 2 Nucleate boiling effects Casting cracks 15 mm small a little 20 mm middle not exist 30 mm large not exist 40 mm large not exist 45 mm large a little 50 mm middle a little FIG. 4 shows a second embodiment according to this invention, in which an annular wiper 9 made of felt and non-woven fabric of heat- and wear-resistance fiber material such as aramide fiber, carbon fiber and the like or of leather is secured by a non-illustrated frame in front of the cooling mold 2 with the predetermined space L3. The inner diameter of this annular wiper 9 is set to be slightly smaller than the outer diameter of the ingot 4 which is withdrawn from the tundish 1. The first and second impinging cooling water applied from the cooling mold 2 to the surface of the ingot 4 is intercepted by the wiper 9 which functions to wipe it off the surface of the ingot 4. Moreover, an annular cooling water jetting tube 10 is disposed ahead of the wiper 9 with the predetermined space L4 from the wiper 9 to surround the outer periphery of the ingot 4. The third cooling water is applied from the cooling water jetting tube 10 to the surface of the heat-restored ingot which passed through the wiper. FIG. 5 and FIG. 6 are graphs showing respectively the temperature change of surface and center portions of 7 inches diameter ingot corresponding to the variation of the distance from the meniscus in cases of without or with the wiper 9 and the cooling water jetting tube 10. In these drawings, the dotted line shows the temperature change in the neighbourhood of the ingot surface portion and the solid line shows the temperature change in the neighbourhood of the ingot center portion. Comparison of the both drawings shows that without the wiper 9 and the cooling water jetting tube 10, there is a large temperature difference between the surface portion and the center portion of the ingot 4 for the considerably wide range from the meniscus, and in case of setting the wiper 9 and the cooling water jetting tube 10, the surface portion and the center portion of the ingot 4 are gradually cooled with a smaller temperature difference from the location in which the third cooling water is applied to the ingot, so as to provide a high quality ingot. Futhermore, another wiper like the wiper 9 may be provided ahead of the cooling water jetting tube 10 in the above-mentioned second embodiment. In this case, it is possible to reduce the temperature difference between the surface portion and the center portion of the ingot 4 during cooling. As stated hereinabove, in accordance with this invention, advantageous results may be obtained as follows; 1. Since a firm solidified shell is produced within short distance from the meniscus of the molten metal by proper cooling, it is possible to provide a stable high rate casting so as to improve productivity and yield considerably. 2. Since it is possible to provide effective cooling, the amount of cooling water is considerably reduced allowing miniaturization of the cooling water pumping equipment and energy savings. 3. Since a powerful cooling is carried out at a short distance from the meniscus, it is possible to prevent surface defects such as bleeding out and the like. 4. Since the powerful cooling is carried out in two steps, only a short unsolidified portion is produced in the ingot which prevents internal defects such as casting cracks and the like. 5. Since an internal composition of the ingot becomes fine with the powerful cooling, it is intended to shorten a homogenizing process time, to promote an easy extrusion, and to improve a strength of an extruding material.

Claims: A cooling method for cooling an ingot (4) which is continuously withdrawn and cast from a mold (2) by cooling a molten metal (3) in said mold (2) in a continuous casting process; said method comprising cooling an ingot (4) by impinging the ingot (4) with a primary jet of cooling water from said mold downstream from a meniscus of said molten metal (3) to establish a transition boiling zone and a film boiling zone on the surface of said ingot (4), said molten metal (3) being cooled in contact with the inner surface of said mold (2) before being cooled by said primary jet of cooling water, and cooling said ingot (4) by impinging the ingot (4) with a secondary jet of cooling water from said mold (2) onto initial zones of said transition boiling zone and said film boiling zone so as to break-out a vapor film generated in said initital zones so as to provoke a nucleate boiling, characterized in that it further comprises wiping off said first and second impinging water jets from the surface of said ingot (4). A cooling method according to claim 1, characterized in that said primary jet of cooling water impinges against an ingot surface at an angle of 15 to 30 degrees, and said secondary jet of cooling water impinges against said ingot surface at an angle of 30 to 60 degrees. A cooling method according to claim 1 or 2, characterized in that said ingot (4) has a diameter of 15 to 23cm (6 to 9 inches), and said primary jet of cooling water impinges from said mold (2) onto said ingot (4) at a contact point set at a distance L1 of 15 to 40mm from a meniscus which is a starting point of development of solidifying a shell. A cooling method according to anyone of claims 1 to 3, characterized in that said ingot (4) has a diameter of 15 to 23cm (6 to 9 inches), and said secondary jet of cooling water impinges on said initial zones of said transition boiling zone and said film boiling zone at an other ingot contact point set at a distance L2 of 20 to 45mm from said contact point of the primary jet of cooling water from said mold (2). A cooling method according to anyone of the preceding claims further comprising a third cooling water jet impinging said ingot (4) downstream from said wiping off step. A continuous casting apparatus including an annular cooling cast mold (2) which is situated to surround an orifice plate (5) secured to an outlet ejecting a molten metal (3) from a tundish (1), said mold comprising a primary cooling water jetting mouth (23) for establishing a transition boiling and a film boiling zone on the surface of an ingot (4) and a secondary cooling water jetting mouth (24) is disposed at a predetermined distance in a withdrawing direction of the ingot from said primary cooling watter jetting mouth to impinge onto initial zones of said transition boiling zone and said film boiling zone, characterized in that it further comprises a wiper (9) made of heat- and wear-resistance material arranged in front of said cooling mold (2) to contact with the whole circumferential surface of said ingot (4) withdrawn from said tundish (1) and to wipe off said first and second cooling water jets impinged from said cooling mold (2) to the circumferential surface of said ingot (4). The apparatus of claim 6 further comprising a third cooling water jetting mouth (10) disposed downstream of said wiper (9). A cooling casting apparatus according to claim 6, characterized in that said primary cooling water jetting mouth (23) is at an angle of 15 to 30 degrees relative to an ingot surface and said secondary cooling water jetting mouth (24) is at an angle of 30 to 60 degresse relative to said ingot surface. A cooling casting apparatus according to anyone of claims 6 to 8, characterized in that said primary cooling water jetting mouth (23) provides a slit shape on the whole inner circumferential surface thereof, and said secondary cooling water jetting mouth (24) provides a grooved or holed shape.

Inventors: ARASE MAKOTO, NAGAI YOSHITAKA, OHATAKE NORIO, ARASE, MAKOTO, NAGAI, YOSHITAKA, OHATAKE, NORIO
Applicants: YKK CORP, YKK CORPORATION
Application year: 1993
Main classification: B22D11
IPC codes: B22D11