Cooling method of continuous casting and its mold

Abstract: A cooling method and a cooling mold in a continuous casting wherein even if a continuous casting rate is increased, a proper cooling is carriedout to prevent a danger of a breakout, so as to improve the stability of casting and the high quality of an ingot (4). A cooling mold comprises water cooling jackets (21, 22) which are provided in the inner part of the mold (2), a first cooling water jetting mouth (23) which is arranged at a distance of 20 to 45 mm between a contact position of a primary cooling water and an other contact position of a secondary cooling water on the ingot (2). By use of the cooling mold (2) wherein a primary and a secondary cooling water impinging angles are respectively set at 15 to 30 degrees and at 30 to 60 degrees, the primary cooling water is impinged from the first cooling water jetting mouth (23) to a molten metal (3) which is cooled with the inner surface of the cooling mold (2) to carry-out a primary cooling, and nextly, the secondary cooling water is impinged from the second cooling water jetting mouth (24) to initial zones of a transition boiling zone and a film boiling zone which are produced with the impingement of the primary cooling water, so that a vapor film generated in the transition boiling zoneand the film boiling zone is broken out to provoke a nucleate boiling so as to carry out a direct se condary cooling with the secondary cooling water.

Description: This invention relates to a cooling method and a mold for continuous casting of ingots from molten aluminum, aluminum alloys, or other metals. In this continuous casting method as shown generally in FIG. 4, a molten metal 13 is injected into a mold 12 which is water cooled from a tandish 11 through an orifice of a plate 15, 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 of 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 and it is required to realize the higher rate casting and simultaneously to promote the casting quality due to high cooling. In the high rate casting, in order to form the solified shell in the mold for solidifying the molten metal, it is required to extract a greater amount of heat and thereby to increase the 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 for a given situation of impingement cooling with cooling water, the ingot surface produces a transition boiling zone and a film boiling zone and a vapor film is formed which creates an adiabatic phase between the ingot surface and the cooling water. Thus, even if the amount of the cooling water is increased, the cooling water does not effectively function to cause heat extraction increasing danger of breakout, and generating problems so as to cause quality defects of the ingot. Hence, these problems have been 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 Publication of Unexamined Application). However, in the two step cooling method using cooling water as disdosed 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 when reaching 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 and film boiling phenomena are again produced reducing cooling efficiency. When using high rate casting, this tendency is more increased which considerably reduces the cooling efficiency. Document AT-B-330 387 discloses a cooling method for a continuous casting process comprising a first chill step in which cooling water impinges the ingot at an angle of 5 to 15 degrees and a second chill step in which cooling water impinges the ingot downstream from the impinging location of the cooling water of the first chill step. A small impinging angle is necessary in the first chill step in order to obtain a relatively low cooling intensity in a first cooling zone. It is therefore an object of this invention to provide a method for cooling an ingot in a continuous casting wherein even if the continuous casting rage is increased, a proper cooling may be carried out to prevent a danger of a breakout so as to provide a stable casting and a high quality ingot. This invention concerns a cooling method for a continuous casting process in which an ingot is continuously withdrawn and cast from a cooling mold while cooling a molten metal in said mold, said cooling method comprising a primary chill step of impinging a primary cooling water from the cooling mold to the molten metal which is cooled in the cooling mold, said primary cooling water impinging against the ingot surface at an angle of 15 degrees up to 30 degrees with the exclusion of the 15 degrees value and a secondary chill step of impinging a secondary cooling water at an angle of 30 degrees to 60 degrees to initial zones of a transition boiling zone and a film boiling zone which are generated by the primary cooling water impingement, so that a vapor film generated in said initial zones is broken out to provoke nucleate boiling. When the ingot has a diameter of 15.24 to 22.86 cm (6 to 9 inches), the contact location between the primary impinging cooling water from the mold and the ingot is situated at a distance L1 of 15mm to 40mm from a meniscus, and the distance L2 between the ingot contact location of the primary impinging cooling water from the mold and the other contact location between the secondary impinging cooling water and the ingot in the transition boiling zone and the film boiling zone is preferably 20mm to 45mm. A cooling mold for accomplishing this cooling method comprises water cooling jackets in an inner part thereof, and a primary cooling water jetting mouth and a secondary cooling water jetting mouth which are situated at predetermined distances 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 with exclusion of the 15 degrees value 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. Generally in a casting mold, when a cooling water is impinged 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 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 of a non-solidified part of the ingot in the casting process depends on a highly precise correlation between the cooling water amount, the cooling position and the ingot surface temperature. A shorter length of the non-solidified ingot part prevents most casting cracks and a weaker cooling results in a longer length of the non-solidified ingot part, so that the solid-liquid coexistence phase is extended increasing the danger of casting cracks. This invention intends in view of these phenomena 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 therein 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 by increasing the amount and pressure of the cooling water 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. In a casting of an ingot having a large diameter of 15.24 to 22.86 cm (6 to 9 inches), the contacting location between the primary impinging cooling water and a high temperature ingot is situated at a distance L1 of preferably 15 to 40 nun from a meniscus. When the distance L1 is less than 15 mm, the danger of generating 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 sufficient to generate quality defects. It is also favourable to set a distance L2 of 20 to 45 mm between the contacting location of the primary cooling water with the ingot and the other contacting location of the secondary cooling water with the ingot. When the distance L2 exceeds 45 mm, the cooling is retarded increasing the non-solidified length within the ingot which, in turn increases the danger of cast cracks. The cooling water impinging angle relative to the ingot surface is one of the important factors in an efficient casting. It is favourable to set the primary cooling water impinging angle at 15 to 30 degrees and the 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 is increased causing bleeding out, and when it is set at more than 30 degrees, the cooling water flows inversely at the start of the casting causing breakout. It is required to set the secondary cooling water impinging angle at 30 to 60 degreees so as to break out the vapor film which is generated in the transition boiling zone and the film boiling zone of 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 adapts 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 adapts 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 an important part which shows a cooling state of a continuous casting according to this invention; FIG. 2 is a longitudinal sectional view of an important part which shows a starting state of the casting; FIG. 3 is a partial enlarged view of FIG. 1; and FIG. 4 is a longitudinal sectional view of an important part which shows a cooling state in the conventional continuous casting. A preferred embodiment of this invention will be essentially 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, a 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 water cooling jackets 21, 22 are formed in front and rear positions at a predetermined space 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 mouths 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 location for contacting the cooling water jetted from the jet mouth 23 with the ingot 4. In the ingot diameter of 15.24 to 22.86 cm (6 to 9 inches), the contact location is favourably disposed in the extent L1 of 15 to 40 mm to set the jet mouth at the distance L1 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 location for contacting the primary cooling water with the ingot 4 and the other location for contacting the secondary cooling water with the ingot 4. In the ingot diameter of 15.24 to 22.86 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. In accordance with 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 water and at 30 to 60 degrees in the secondary cooling water. In the continuous casting with the above-mentioned structure, a starting block 7 is inserted into the cooling mold 2 of this invention in the casting start 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 a plate 5. In this state, a molten metal is introduced through orifices 6 of 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 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 is cooled to produce a thin solidified shell. Then, the molten metal is direct-cooled with a primary cooling water which is jetted form the first jet mouth 23 of the mould 2, so as to advance solidification. So, since a transition boiling zone and a film boiling zone are produced on the surface of the ingot 4 with the impingement of the primary cooling water, when a secondary cooling water is impinged from the second jet mouth 24 of the cooling mold 2 toward the vapor film of these zones, the transition boiling zone and the film boiling zone are broken out with the impinging cooling water to provoke a nucleate boiling, so as to produce a firmer solidified shell with the secondary cooling directly 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 location of the primary jet 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 cooling water: 85 1 / min L1 Breakout Bleeding out; Inverse segregation 10 mm exist - 15 mm not existed fine 25 mm not existed fine 35mm not existed fine 40 mm not existed a little 45 mm not existed much (2) The distance L2 between contact locations on the ingot of the first and second impinging cooling water is varied in the following casting conditions to cast the ingot. The results are shown in a Table 2. a. Kinds of alloy : JIS 6063 aluminum alloy b. Diameter of ingot : 7 inches (178 mm) c. Casting rate : 350mm / min d. Casting temperature : 690 ° C e. Amount of primary jet cooling water : 85 1 / min f. Amount of secondary jet cooling water : 45 1/ min g. Distance between meniscus and contact location of primary impinging cooling water : 25 mm L2 Nucleate boiling effects Casting cracks 15 mm small a little 20 mm middle not existed 30 mm large not existed 40 mm large not existed 45 mm large a little 50 mm middle a little As stated hereinabove, in accordance with this invention, advantageous results may be obtained as follows; 1. Since a slight distance from a meniscus produces a firm solidified shell, it is possible to provide a stable high rate casting so as to improve production 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 non-solidified 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 a continuous casting process in which an ingot (4) is continuously withdrawn and cast from a mold (2) while cooling a molten metal (3) in said mold (2), comprising a primary chill step of impinging a primary cooling water from said mold (2) to said molten metal (3) which is cooled in said mold (2), said primary cooling water impinging against ingot surface at an angle of 15 degrees up to 30 degrees with the exclusion of the 15 degrees value and a secondary chill step of impinging a secondary cooling water at an angle of 30 degrees to 60 degrees to initial zones of a transition boiling zone and a film boiling zone which are generated by the primary cooling water impingement, so that a vapor film generated in said initial zones is broken out to provoke a nucleate boiling. A cooling method according to claim 1, wherein the ingot (4) has a diameter of 15.24 to 22.86 cm (6 to 9 inches) and said primary cooling water impinges from said mold (2) at a contact location set at a distance L1 of 15 to 40 mm from a meniscus. A cooling method according to claim 1 or 2, wherein the ingot (4) has a diameter of 15.24 to 22.86 cm (6 to 9 inches) and said secondary cooling water impinges from said mold (2) on said transition boiling zone and said film boiling zone at another ingot contact location set at a distance L2 of 20 to 45 mm from said contact location of the primary cooling water. A continuous casting mold for continuously withdrawing and casting an ingot (4) from said mold (2) while cooling a molten metal (3) in said mold (2) comprising water cooling jackets (21, 22) which are provided in the inner part of said mold (2), and a primary cooling water jetting mouth (23) and a secondary cooling water jetting mouth (24) which are situated at predetermined distances in the withdrawing direction of said ingot (4), said primary cooling water jetting mouth (23) being at an angle ranging from 15 up to 30 degrees relative to an ingot surface with the exclusion of the 15 degrees value and said secondary cooling water jetting mouth (24) being at an angle of 30 to 60 degrees, relative to said ingot surface. A continuous casting mold according to claim 4, wherein the 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: 1992
Main classification: B22D11
IPC codes: B22D11