20~08~

S P E C I ~ I C A T I ~ N

TITLE OF THE lN V~;N l lON
COOLING MEl~OD AN~ APPARATUS FOR CONTINUOUS CASTING
AND ITS MOLD

B~CKGROU~D OF THE IN~-ENTION
Field of the Invention;
This invention relates to a cooling method and a cooling mold for continu~us casting of ingots from molten alumin~, 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.
Description of the Prior Art;
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 ~hich is water-cooled, so that the molten metal is cooled in -the mold 12 to cast an ingot 14. The molten metal 13 ~hich is introduced through the orifice plate 15 to the mold 12, is contacted with the ~all surface of the mold 12 to form a thin solidified shell and is further cooled and cast with imping-ing cooling ~ater 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 reali~e 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 '2~ 08'~

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 ~one is produced on the ingot surface and a vapor film which creates an adiabatic phase between the ingot surface and the cooling ~ater 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 209S0~

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.

S~MMARY OF THE lN~NllON
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 a continuous casting process in which an ingot is continuously withdrawn and cast from a cooling mold ~hile cooling a molten metal in said mold.
The cooling method of this invention comprises a primary direct chill step in which primary cooling water from the cooling mold impinges on the molten metal cooled in contact with the cooling mold at a ~short distance from the meniscus of the molten metal to establish a transition boiling zone and a film boiling zone, and a secondary direct chill step in which a secondary cooling water impinges on the 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 and thereby to produce 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 ~he primary cooling water 2~9~8~

;mpinging agairlst an ingot surface is 15 to 30 degrees and the impin&ing angle of the secondary cooling water impinging against the ingot: surface -is 30 to 60 degrees. Whcn the ingot has a dia-meter of 6 to 9 inches, the primary impinging cooling water from the mold contac~s the ingot at a distance Ll 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 cooling apparatus for accomplishing the above-mentioned cooling method is disposed to surround an orifice plate which is secured to an outlet ejecting a molten metal from a tundish. The conti-nuous casting apparatus includes an annular cooling mold having cooling water jetting mouths in an inner face thereof. The cooling mold comprises water cooling jackets in an inner portion thereof, and primary and secondary cooling water jetting mouths which are disposed at the predetermined distance in the withdrawing direction of the ingot. A wiper made of heat- and wear-resistance material is arranged in front of the cooling mold and is contacted with the ~hole circumferential surface of the ingot which is withdrawn from the tundish. This ~iper serves to wipe off cooling water which is applied from the cooling mold to the ingot surface.
~ third cooling water jetting mouth is arranged ahead of the wiper.
A cooling mold for accomplishing this cooling method comprises first and second water cooling jackets inside thereof, and a pri-20gS085 mary cvoling water jetting mouth and a secondary cooling waterjetting mou-th which are disposed a-~ the predetermined distance in the w1thdrawing 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 peri-pheral 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, ~here is a limit in this improvement of cooling effects, and at the same time, even if -the pressure of the cooling ~ater is increased, there is also a limit in the improvement of the cooling efficlency.
On one hand, the length and shape of an unsolidified portion 20~0~

of the ingot in the casting process is highly correlated with the cooling water amount, the cooling position and the ingot surface temperatllre. 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 ~one and a film boiling ~one 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 ~one 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 6 to 9 inches, the contacting point of the primary impinging cooling water and a high temperature ingot is situated at a distance Ll of preferably 15 to 40 mm from a meniscus. When the distance Ll is less than 15 mm, the danger of generating the breakGut in the start of the casting and breakout due to slight changes of casting conditions during casting is increased. When the distance Ll 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 qualily defects. It is also favourable to 2~9~0~

set a distaTlce L2 of ~0 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 ~msolidified ]ength 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 1~ 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 ~one 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. ~he 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 secolldary 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 ~one and the film boiling ~one.

2~9~S

Fur~her features and advantages of the invention will be apparent from the detailed description below, together with the accompanying drawings.

BRIEF DESCRIPTION OF THE 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. l; and FIG. 4 is a longitudinal sectional Yiew 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.

209508~

DETAII.ED DESCRlr~ION
A preferred embodiment of this invention will be illustrated with refercnce 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 t~pical 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 ~iew 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.
h 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 ~ith a predetermined space inbetween on the same axis of the cooling mold. A part of each water cooling jacket 21, 22 communicates with an e~ternal 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 209508~i (~)en;ng on ~he ~ ole im]er 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 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 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 se-t at 15 to 30 degrees in the primary cooling and at 30 to 60 degrees in the secondary coo]ing.
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 t;p of the starting block 7 is contacted with an end face of an orifice plate 5. In this s~ate, 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 ra~:e rrom ~:he 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 wit~ 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 wi-th 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 ~one and a film boiling zone are produced on the surface of the ingot ~ 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 ~one and the film boiling ~one 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 ~herein 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 20~S085 d. Casting temperature : 690 ~C
e. Amount of primary jet of cooling water : 85 l / min [ Table 1 ]

L 1BreakoutBleeding out ; Segregation 10 mmexist 15 mmnot exist slightly 25 mmnot exist slightly 35 mmnot exist slightly 40 mmnot exist a little 45 mmnot 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. ~iameter of ingot : 7 inches 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 ~ater : 45 l / min g. Distance between meniscus of molten metal and contact point of primary impinging cooling water : 25 mm 2~3~085 [ Table 2 ]

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 alamide fiber, carbon fiber and the like or of lea-ther is secured by a non-illustrated frame in front of the cooling mold 2 ~ith ~he 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 L.4 from the ~iper 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~

2~9S08~

FIG. ~ and ~IG. 6 are graphs showing respective]y the temperature change of surface and cen-ter portions of 7 inches diameter ingot corresporlding to ~he variation of the distance from the meniscus in cases of withollt or wi-th 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.

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~ . 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.
~ . 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 ~n extruding material.
It is to be understood that the invention is not limited to the features and an embodiment hereinabove specifically set forth but may be carried out in other ways without departure from its spirit.