JP2007268547A - Method for producing aluminum alloy cast plate - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a method for producing an aluminum alloy cast plate where, even in a double roll type continuous casting method for an Al-Mg based aluminum alloy having a wide solid-liquid coexistent temperature region, the defects in the central part of the plate thickness can be suppressed. <P>SOLUTION: Regarding the method for producing an aluminum alloy cast plate, a casting speed is controlled to ≤20 m/min, in double rolls 1, 2 almost simultaneously contacted with molten metal 3, the surface temperature of the roll 2 on one side is controlled to the liquidus temperature of an Al-Mg based aluminum alloy or above and also to the liquidus temperature +30°C or below, on the other hand, the surface temperature of the other roll 1 is controlled to the solidus temperature of the Al-Mg based aluminum alloy or below, and continuous casting is carried out while performing unidirectional solidification of starting the solidification 5 of the molten metal 3 from the surface side of the low temperature side roll 1, thus the internal defects of the cast plate are suppressed. <P>COPYRIGHT: (C)2008,JPO&INPIT

Description

  The present invention provides a method for producing an aluminum alloy cast plate by a twin-roll continuous casting method that can suppress defects at the center of the plate thickness even for an Al-Mg aluminum alloy plate having a wide solid-liquid coexistence temperature range. Is.

  As is well known, various aluminum alloy plates (hereinafter referred to as “Al”) have been conventionally used for transportation equipment such as automobiles, ships, aircraft or vehicles, machines, electrical products, architecture, structures, optical equipment, and components and parts of equipment. Is also widely used depending on the characteristics of each alloy.

  In many cases, these aluminum alloy plates are formed by press molding or the like, and are used as members and parts for the above-described applications. In this respect, from the viewpoint of high formability, among the Al alloys, an Al-Mg Al alloy having an excellent balance between strength and ductility is advantageous.

  For this reason, with regard to Al-Mg-based Al alloy sheets, examination of component systems and optimization of manufacturing conditions have been conventionally performed. As this Al-Mg based Al alloy, for example, JIS A 5052, 5182 and the like are typical alloy component systems. However, even this Al—Mg-based Al alloy is inferior in ductility and inferior in formability compared to cold-rolled steel sheets.

  On the other hand, when the Al-Mg based Al alloy is increased in Mg content by increasing the Mg content, the strength ductility balance is improved. However, it is difficult to industrially manufacture such a high Mg Al—Mg alloy by an ordinary manufacturing method in which an ingot cast by DC casting or the like is subjected to hot rolling after soaking. The reason for this is that Mg is segregated in the ingot during casting, and the normal hot rolling significantly reduces the ductility of the Al—Mg alloy, so that cracking is likely to occur.

  On the other hand, it is also difficult to hot-roll high-Mg Al—Mg alloys at low temperatures while avoiding the above-described temperature range where cracks occur. This is because, in such low-temperature rolling, the deformation resistance of the high-Mg Al—Mg-based alloy material is remarkably increased, and the product size that can be produced is extremely limited by the current rolling mill capability.

  In addition, a method of adding a third element such as Fe or Si has been proposed in order to increase the allowable Mg content of a high Mg Al—Mg alloy. However, when the content of these third elements is increased, a coarse intermetallic compound is easily formed, and the ductility of the aluminum alloy plate is lowered. For this reason, there is a limit to the increase in the Mg content allowable amount, and it was difficult to contain an amount of Mg exceeding 8%.

  For this reason, various proposals have heretofore been made for producing high-Mg Al—Mg-based alloy plates by a continuous casting method such as a twin roll type. In the twin roll type continuous casting method, molten aluminum alloy is poured from a refractory hot water supply nozzle between a pair of rotating water-cooled molds (double rolls) and solidified. This is a method of rapidly cooling to a thin aluminum alloy sheet. As this twin roll type continuous casting method, the Hunter method, the 3C method and the like are known.

  The cooling rate of the twin roll type continuous casting method is 1 to 3 orders of magnitude higher than that of the conventional DC casting method or belt type continuous casting method. For this reason, the obtained aluminum alloy sheet has a very fine structure and is excellent in workability such as press formability. Moreover, the aluminum alloy plate having a relatively thin plate thickness of 1 to 13 mm is obtained by casting. For this reason, steps such as hot rough rolling and hot finish rolling can be omitted as in the case of a conventional DC ingot (thickness 200 to 600 mm). Furthermore, ingot homogenization may be omitted.

Various examples have been proposed in the past in which the structure of the high-Mg Al—Mg alloy plate manufactured using such a twin-roll type continuous casting method is defined in order to improve formability. For example, an aluminum alloy sheet for automobiles with excellent mechanical properties is proposed in which the average size of Al-Mg based intermetallic compounds of Al-Mg based alloy sheets with a high Mg content of 6-10% is 10 μm or less. (See Patent Document 1). In addition, an aluminum alloy sheet for automobile body sheets, in which the number of Al-Mg intermetallic compounds of 10 μm or more is 300 pieces / mm ² or less and the average crystal grain size is 10 to 70 μm has been proposed (Patent Document 2). See).

  Furthermore, for high-Mg Al-Mg alloys, cast plates with a twin roll peripheral speed and cooling rate higher than specified conditions were further cold-rolled and annealed under specified conditions, and Al-Mg alloys A method for producing an aluminum alloy sheet for automobiles with less intermetallic compounds has also been proposed (see Patent Document 3). In addition, it has been reported that AA6016 aluminum alloy cast plate (1800W x 1 to 2.5mm thickness) was cast in a 6000 series aluminum alloy using a roll casting apparatus called Speed Caster (see Non-Patent Document 1). )

Various proposals have been made to suppress surface defects of the slab in the twin roll continuous casting method. For example, in the twin-roll continuous casting method of aluminum alloy with high cracking sensitivity, the amount of heat removed from one roll is reduced, and the tensile strain due to bending back of the solidified shell at the kiss point (kissing point) of the roll is suppressed, It has also been proposed to prevent surface cracking of the slab (see Patent Document 4). In addition, in horizontal twin-roll continuous casting methods such as alloy incoloy, it is also proposed to preheat the tundish and pouring nozzle after insulating the upper roll side to prevent slab surface defects and plate thickness fluctuations. (See Patent Document 5).
Japanese Patent Laid-Open No. 7-252571 (claims, pages 1 to 2) JP-A-8-165538 (full text) JP 2006-28554 A (full text) JP 7-185748 A (full text) Japanese Patent Laid-Open No. 8-10909 (full text) Continuous Casting, Proceedings of the International Conference on Continuous Casting of Non-Ferrous Metals, DGM2005, p87.

  On the other hand, when producing high-Mg Al-Mg alloy cast plates using the twin-roll continuous casting method, even if the peripheral speed of the twin-roll is increased to improve efficiency and mass production, voids etc. The casting defect is likely to occur. This is because the solidification temperature range of a high Mg Al—Mg alloy is wider than that of an Al—Mg alloy having a low Mg content. For this reason, the gas generated during pouring or during solidification or the gas including the atmosphere is less likely to be released from the inside of the slab, tends to stay in the slab structure, and easily becomes a void.

  In such a high Mg Al-Mg alloy sheet, when the voids in the structure increase in this way, the elongation decreases, and the strength-ductility balance, which is characteristic of the Al-Mg alloy sheet, and the formability based on it are reduced. .

  For this, measures such as increasing the cooling rate in the twin rolls or adding a finer such as Ti are effective. However, these means are also limited in suppressing casting defects such as voids to a range that does not affect molding characteristics such as elongation of the manufactured plate. Moreover, even if the method of the said patent documents 4, 5 etc. which suppress the surface defect of a slab is used, it is still a limit in suppressing internal defects, such as a space | gap, in a high Mg Al-Mg type alloy cast plate. There is.

  Therefore, in the past, when manufacturing high-Mg Al-Mg alloy cast plates using the twin-roll continuous casting method, it has been necessary to tolerate casting defects such as voids to some extent. is there.

  The present invention has been made to solve such a problem, and the purpose of the present invention is to provide a plate thickness even for a twin-roll continuous casting method of an Al-Mg aluminum alloy having a wide solid-liquid coexistence temperature range. An object of the present invention is to provide a method for producing an aluminum alloy cast plate capable of suppressing defects at the center.

  In order to achieve this object, the gist of the method for producing an aluminum alloy cast plate of the present invention in which internal defects are suppressed is as follows. An Al-Mg-based aluminum alloy cast plate containing 5 to 14% by mass of Mg by a twin roll type continuous casting method. In the method of manufacturing, the casting temperature is set to 20 m / min or less, and the surface temperature of one side of the twin rolls that are in contact with the molten metal almost simultaneously is set to (-5.43 × Mg mass% of Al-Mg based aluminum alloy). While the liquidus temperature of the Al-Mg-based aluminum alloy calculated by +663 is set to the liquidus temperature + 30 ° C or lower, the surface temperature of the roll on the other side is (-12.9 × Al-Mg-based (Mg mass% of the aluminum alloy) + 638 calculated to be less than the solidus temperature of the Al-Mg aluminum alloy, and the solidification of the melt starts from the surface side of the roll on one side that is below the solidus temperature. Continuous casting while performing unidirectional solidification It is when.

  In order to achieve the same object, another gist of the method for producing an aluminum alloy cast plate of the present invention in which internal defects are suppressed is the Al-Mg system containing 5 to 14% by mass of Mg by a twin roll type continuous casting method. In the method of producing an aluminum alloy cast plate, the casting speed is set to 20 m / min or less, the timing of contacting the molten metal between the two rolls arranged substantially in the vertical direction is different, and the molten metal is first placed on the lower surface of the roll. After the first contact, the molten metal is not less than the solidus temperature of the Al-Mg aluminum alloy calculated by (-12.9 × Mg mass% of the Al-Mg aluminum alloy) +638. At a temperature of + 10 ° C. or less, unidirectional solidification is performed in which the molten metal starts to solidify from the surface side of the one-side roll disposed below, which is brought into contact with the molten metal at the upper side and previously contacted with the molten metal. Continuous casting while performing It is when.

  As described above, in the production of high-Mg Al-Mg alloy cast plates by the twin-roll continuous casting method, the solidification temperature range of the high-Mg Al-Mg alloy was widened and occurred during pouring and during solidification. A gas such as hydrogen or a gas including an atmosphere is hardly released to the outside from the inside of the slab. For this reason, these gases are likely to stay in the slab structure, and casting defects such as voids are likely to occur.

  In contrast, in the present invention, as described above, continuous casting is performed while performing unidirectional solidification that starts solidification of the molten metal from the surface side of the roll on one side of the twin roll. However, in order to realize this unidirectional solidification, as a problem specific to the twin roll, either the roll surface temperature of the twin roll or the temperature of the molten metal contacting the roll surface is defined as the liquid phase or solid phase temperature of the molten metal. Therefore, it is necessary to control to a fairly narrow specific range. Outside this specific range, another problem specific to twin rolls arises due to the continuous casting with twin rolls.

  When both twin rolls are in contact with the molten metal almost simultaneously, the surface temperature of the roll on one side is increased to a specific temperature range higher than the liquidus temperature, and the surface side of the roll on the other side where the surface temperature of the other roll is low. To start solidification of the molten metal. In normal twin roll continuous casting, the surfaces of both twin rolls are basically kept at room temperature after cooling.

  Also, in the case of twin rolls arranged in a substantially vertical direction, the timing of contacting the molten metal between the two rolls is different, and one side roll arranged below is first brought into contact with the molten metal and later arranged upward. The molten metal temperature at the time of contacting the roll is raised to a specific temperature range equal to or higher than the solidus temperature, and the solidification of the molten metal is started from the surface side of the one-side roll arranged below.

  Thus, in twin roll continuous casting, unidirectional solidification is realized in which solidification of the molten metal is started from the surface side of the roll on one side. By this unidirectional solidification, gas components such as hydrogen in the molten metal are moved from one side of the cast plate (slab) that is solidifying to the solid phase to one side of the cast plate (slab) that remains in the liquid phase. This facilitates the release of gas components such as hydrogen into the atmosphere from the liquid phase portion that subsequently solidifies into the solid phase. As a result, in normal twin-roll continuous casting, gas components such as hydrogen, which tend to segregate at the center of the cast plate and easily become casting defects such as voids, are reduced, and internal defects are suppressed.

  Therefore, even if the peripheral speed of the twin rolls is increased for efficiency and mass production, and even if it is an Al-Mg aluminum alloy plate with a wide solid-liquid coexistence temperature range, the thickness of the solidified cast plate Defects at the center can be suppressed.

  As a result, even if it is an Al-Mg alloy cast plate with a high Mg content of 5% or more, it is possible to improve the elongation and strength-ductility balance as material properties, such as overhang forming, drawing, bending, drilling, Formability such as hole expansion, punching, or a combination of these forming processes can be improved.

  Hereinafter, the method for producing an Al—Mg-based aluminum alloy cast plate in the present invention will be specifically described for each requirement.

(Double roll type continuous casting method)
1 and 2 schematically show the twin roll type continuous casting method of the present invention. Figure 1 shows a vertical type (vertical type) where both twin rolls placed horizontally contact the molten metal almost simultaneously. FIG. 2 shows a horizontal type (horizontal type) in which the lower roll of the two rolls arranged in the vertical direction comes into contact with the molten metal first.

  In these twin-roll type continuous casting methods, the prerequisite continuous casting process itself is known. That is, in FIGS. 1 and 2, in common, the twin roll type continuous casting is carried out from a hot water supply nozzle made of a refractory (not shown) between the twin rolls 1, 2 or 10, 11 such as a pair of rotating water-cooled molds (arrows). From the direction), molten Al alloy 3 having the above or below described composition is poured. Then, the molten metal is rapidly cooled and solidified between these twin rolls to obtain an Al alloy cast plate 6.

(Double roll casting method)
In the present invention, the twin roll casting method may be vertical (the twin rolls are arranged in a substantially horizontal direction) or horizontal (the twin rolls are arranged in a substantially vertical direction). In the vertical type, the solidification distance can be increased and the contact time becomes longer, so that the casting speed can be increased and the productivity is improved. Therefore, in consideration of these characteristics, the double roll casting of the horizontal type and the vertical type is properly used.

(Casting speed)
For this reason, in the present invention, as a premise, the casting speed or the peripheral speed v of the twin rolls is made slow (small) to 20 m / min or less. When the casting speed or the peripheral speed v of the twin rolls is increased beyond 20 m / min, even if the unidirectional solidification of the present invention is applied, a vortex flow of the molten metal that causes casting defects such as voids is likely to occur. For this reason, in the Al—Mg-based aluminum alloy plate having a wide solid-liquid coexistence temperature region, it becomes impossible to suppress the defects at the center of the thickness of the solidified cast plate. Therefore, the casting speed or the peripheral speed v of the twin rolls is set to 20 m / min or less.

(Unidirectional solidification)
Below, the unidirectional solidification which starts solidification of a molten metal from the surface side of the roll of one side of a twin roll which is the characteristic gist of this invention is demonstrated.

  In the present invention, in the twin roll type continuous casting method, in order to realize unidirectional solidification that can suppress internal defects in the center part of the cast plate, both roll surface temperatures in the twin roll are specified to each other as a problem specific to the twin roll. Control to the range.

(Unidirectional solidification of vertical twin rolls)
As shown in FIG. 1, in the case of a vertical type in which both twin rolls 1 and 2 are arranged horizontally and these twin rolls 1 and 2 are in contact with the molten metal 3 almost simultaneously, for example, one roll 2 (see FIG. And the temperature of the surface 2a is made higher than the liquidus temperature of the Al—Mg-based aluminum alloy (molten metal).

  As a result, as shown in FIG. 1, a solidified phase (solid phase) 5 is generated only from the surface 1a side of the other roll 1 whose surface temperature of the roll is low, and the solidification of the molten metal 3 is started and progressed. Directional solidification takes place. At this time, the molten metal 3 still remains in the liquid phase 4 on the surface 2a side of the roll 2 on the high temperature side. The liquid phase 4 also solidifies one after another, and the solid phase (solid phase) 5 develops and becomes thicker toward the slab side (left side in the figure) on the high temperature side roll 2 side, while the thickness of the liquid phase 4 is It becomes thinner gradually.

  Therefore, the molten metal center portion does not remain in an unsolidified state until the end until the kiss point 12 in the twin rolls 1 and 2 as in the normal twin roll method. The liquid phase 4 also solidifies one after another, and a solidified phase (solid phase) 5 develops from the slab side of the low temperature side roll 1 (left side in the figure) to the slab side on the high temperature side roll 2 side (left side in the figure). While the thickness increases, the thickness of the liquid phase 4 decreases gradually.

  Gas components such as hydrogen have low solubility (in the solid phase) in the solid phase and high solubility (in the liquid phase) in the liquid phase. Therefore, the unidirectional solidification prevents the gas component such as hydrogen from remaining in the molten metal central portion. In addition, gas components such as hydrogen in the molten metal flow from the slab side of the low temperature side roll 1 that is solidifying into the solidified phase 5 (left side of the figure) to the slab side of the high temperature side roll 2 that remains in the liquid phase 4. To the right side of the figure.

  The state in which gas components such as hydrogen in the molten metal move from the growing solidified phase 5 side to the liquid phase 4 side that solidifies sequentially continues to the kiss point 12 in the twin rolls 1 and 2, and finally the kiss point. At 12 (including the vicinity), only the solidified phase 5 has grown, and the liquid phase 4 has disappeared. As a result, gas components such as hydrogen are easily released from the liquid phase 4 portion into the atmosphere. For this reason, in normal twin-roll continuous casting, gas components such as hydrogen, which are segregated at the center of the cast plate and easily become casting defects such as voids, are reduced, and internal defects are suppressed.

  In order to exert this effect, the temperature of the surface 2a of the high temperature side roll 2 is set higher than the liquidus temperature of the Al—Mg-based aluminum alloy (molten metal). When the temperature of the surface 2a of the high temperature side roll 2 is lower than the liquidus temperature, the molten metal 3 cannot be held in the liquid phase 4 on the surface side of the high temperature side roll 2. Therefore, it becomes the same as the normal twin roll method, and solidification is started on the surface 2a side of the high temperature side roll 2 as well as the surface 1a side of the low temperature side roll 1, and unidirectional solidification is not performed.

  The upper limit of the temperature 2a of the surface 2a of the roll 2 on the high temperature side is set to the liquidus temperature + 30 ° C. or lower in order to exhibit the unidirectional solidification. If the temperature of the surface 2a of the high temperature side roll 2 exceeds the liquidus temperature + 30 ° C., the temperature of the surface 2a of the high temperature side roll 2 is too high. Therefore, even if the temperature of the surface 1a of the other low temperature side roll 1 is lowered below the solidus temperature, the solidification of the molten metal is not completed up to the kiss point 12 of the twin rolls, and the solidification of the molten metal is completed. It is carried over after the kiss point 12 of the roll. In order to maintain the plate thickness accuracy and the surface smoothness, it is necessary to complete the solidification of the molten metal up to the kiss point 12 of the roll, where there is a restriction of the slab (cast plate) by the twin rolls. Therefore, if the temperature of the surface 2a of the roll 2 on the high temperature side is too high, solidification is completed without being constrained by the twin rolls, and the unevenness of the cast plate becomes severe, and the plate thickness accuracy and surface smoothness are significantly reduced. To do.

  In order to increase the temperature of the surface 2a of the high temperature side roll 2 within the above range, for example, a heater provided outside the roll 2 shown in FIG. It is preferable to heat the surface 2a.

  On the other hand, for the unidirectional solidification of the molten metal from the surface 1a side of the low temperature side roll 1, the temperature of the surface 1a of the low temperature side roll 1 should be as low as possible. In this respect, in the present invention, the upper limit of the temperature of the surface 1a of the low temperature side roll 1 is defined as not more than the solidus temperature of the Al-Mg-based aluminum alloy, but this temperature should be as low as possible including the room temperature region. good. When the temperature of the surface 1a of the low temperature side roll 1 exceeds the solidus temperature of the Al-Mg aluminum alloy, even if the temperature of the surface 2a of the high temperature side roll 2 is appropriate, normal twin roll continuous casting Similarly to the above, the molten metal cannot be unidirectionally solidified from the surface 1a side of the low temperature side roll 1, for example, the molten metal central portion is in an unsolidified state until the end.

  In order to lower the temperature of the surface 1a of the low temperature side roll 1 in this way, a water-cooled mold roll (made of copper or steel) used for a normal twin roll is also used in the present invention, and the low temperature side roll 1 heated by the molten metal is used. The surface 1a is preferably water-cooled from the inside of the roll.

  Although there are radiation thermometers for measuring the temperature of these roll surfaces, in order to make them more accurate, they are measured with a contact-type thermometer in contact with the roll surface.

(Liquidus temperature)
The liquidus temperature can also be obtained by actual measurement by melting the alloy. However, the liquidus temperature of the Al-Mg aluminum alloy (molten metal) in the present invention is the coefficient -5.43, and the Mg content of the Al-Mg aluminum alloy Multiply by the amount (% by mass), add the constant 663, and calculate. That is, it is calculated by (−5.43 × Mg mass% of Al—Mg based aluminum alloy) +663.

(Solidus temperature)
In the present invention, the solidus temperature of the Al-Mg-based aluminum alloy (molten metal) is also multiplied by a constant 638 by multiplying the coefficient -12.9 by the Mg content (mass%) of the Al-Mg-based aluminum alloy. And calculated. That is, it is calculated as (-12.9 × Mg mass% of Al—Mg based aluminum alloy) +638.

(Unidirectional solidification of horizontal twin rolls)
When the present invention is applied to a horizontal twin roll, as shown in FIG. 2, it can be realized by changing the timing of contacting the melt between the twin rolls 10 and 11 arranged in a substantially vertical direction (substantially vertical direction). It becomes. That is, first, as shown in FIG. 2, the surface 11a of the roll 11 disposed below is first brought into contact with the molten metal 3. Next, when contacting the surface 10a of the roll 10 disposed above, the temperature of the molten metal (liquid phase) 4 immediately before contacting the surface 10a of the roll 10 as shown by the arrow in FIG. The solidus temperature is 10 ° C. or lower.

  As a result, the solidification is started from the surface 11a side of the lower roll 11 from the lower side of the so-called molten metal 3. At this time, the molten metal 3 still remains in the liquid phase 4 above the molten metal (on the upper roll 10 side), and so-called slab (molten metal) unidirectional solidification proceeds from below to above.

  Then, the temperature of the molten metal 3 when contacting the surface 10a of the upper roll 10 is set to a relatively low temperature not lower than the solidus temperature and not higher than the solidus temperature + 10 ° C. Thereby, the unidirectional solidification from the lower side of the slab (molten metal) to the upper side is completed at the kiss points of both rolls 10 and 11.

  By this unidirectional solidification, gas components such as hydrogen in the molten metal sequentially move from the solidified phase 5 side that grows from below to the upper liquid phase 4 side that solidifies. . This state continues until the kiss point 12 in the twin rolls 1 and 2, and finally, at the kiss point 12 (including the vicinity), only the grown solid phase 5 exists and the liquid phase 4 disappears. As a result, gas components such as hydrogen are easily released from the liquid phase 4 portion into the atmosphere. For this reason, in normal twin-roll continuous casting, gas components such as hydrogen, which are segregated at the center of the cast plate and easily become casting defects such as voids, are reduced, and internal defects are suppressed.

  For this reason, even if the peripheral speed of the twin rolls is increased for efficiency and mass production, and even if it is an Al-Mg aluminum alloy plate with a wide solid-liquid coexistence temperature range, the solidified cast plate Defects at the thickness center can be suppressed. As a result, even if it is an Al-Mg alloy cast plate with a high Mg content of 5% or more, it is possible to improve the elongation and strength-ductility balance as material properties, such as overhang forming, drawing, bending, drilling, Formability such as hole expansion, punching, or a combination of these forming processes can be improved.

  In the present invention, the temperature of the molten metal 4 is the surface temperature of the molten metal (liquid phase) 4 immediately before contacting the surface 10a of the roll 10 in order to measure with a radiation thermometer. That is, the temperature of the molten metal 3 in contact with the surface 10a of the roll 10 is the temperature of the surface of the molten metal (liquid phase) 4 immediately before contacting the surface 10a of the roll 10 as indicated by the black arrow in FIG. Measure.

  The temperature of the molten metal 4 is controlled by controlling the temperature of the original molten metal 3 supplied from the dundish and controlling the temperature of the surface 11a of the lower roll 11 (first roll) that contacts the molten metal 3 and cools the molten metal 3 first. To do. In order to exert the above-described effects, that is, for unidirectional solidification of the molten metal from the surface 11a side of the lower roll 11, the temperature of the surface 11a of the lower roll 11 depends on the temperature of the original molten metal 3. Is preferably as low as possible below the solidus temperature. When this temperature is too high, the temperature of the molten metal 3 when contacting the surface 10a of the roll 10 disposed above is relatively higher than the solidus temperature and lower than the solidus temperature + 10 ° C. It becomes impossible with low temperature.

  In order to lower the temperature of the surface 11a of the lower roll 11 in this way, a water-cooled mold roll (copper, steel) used for a normal twin roll is also used in the present invention, and the surface 11a of the lower roll 11 heated by the molten metal is used. Is preferably water-cooled from the inside of the roll.

  On the other hand, when the temperature of the molten metal 3 that is in contact with the surface 10a of the roll 10 disposed above is lower than the solidus temperature, the temperature of the molten metal 3 is too low and unidirectional solidification proceeds, but it is disposed above. Solidification of the slab is completed before the molten metal 3 comes into contact with the roll 10. For this reason, solidification is completed without being constrained by the roll 10 disposed above, and the plate thickness accuracy and surface smoothness of the cast plate are significantly reduced.

  On the other hand, when the temperature of the molten metal 3 in contact with the surface 10a of the roll 10 disposed above exceeds the solidus temperature + 10 ° C., the temperature of the molten metal 3 is too high. For this reason, even if the surface temperatures 10a and 11a of the rolls 10 and 11 are lowered, the solidification of the molten metal is not completed up to the kiss point 12 of the twin rolls, and the completion of the solidification of the molten metal is carried over after the kiss point 12 of the roll. . Therefore, solidification is completed without being constrained by the twin rolls, the unevenness of the cast plate becomes severe, and the plate thickness accuracy and the surface smoothness are remarkably reduced.

(Cast plate thickness)
As described above, in the present invention, the molten metal is completely solidified up to the kiss point of the twin rolls by unidirectional solidification. For this reason, the thickness of the solidified layer at the kiss point is the same as the thickness of the cast plate. In the present invention, the thickness of the cast plate to be cast is freely selected. However, when it is desired to finally obtain a thin plate, if the thickness of the cast plate is too thick, hot rolling or the like is required later, and the advantage of producing the plate by twin roll continuous casting is impaired.

(Other twin roll casting conditions)
Hereinafter, other preferable twin roll casting conditions in the present invention will be described.

(Roll diameter)
Here, for efficiency and mass production, it is preferable to use a large-diameter roll as the twin roll, and the roll diameter D of the twin roll is preferably 100 mm or more. However, the larger the roll diameter D of the twin rolls, the faster the roll peripheral speed v or the casting speed. And when this roll peripheral speed v thru | or casting speed becomes high, it will become easy to generate | occur | produce the eddy current of the molten metal which causes casting defects, such as a space | gap.

(Cooling rate)
Twin roll type continuous casting has an advantage that the cooling rate at the time of casting can be increased as compared with other belt caster type, propel type, block caster type and the like. However, even in casting by twin rolls, the cooling rate is preferably as high as possible at 50 ° C./s or more. If the cooling rate is less than 50 ° C / s, the average grain size of the cast plate exceeds 50 μm and becomes coarse, and the overall intermetallic compounds such as Al-Mg may become coarse or crystallize in large quantities. Get higher. As a result, for this reason, the strength-elongation balance is lowered, and the possibility that the press formability is significantly lowered is increased. In addition, the uniformity of the plate is also reduced.

Since this cooling rate is difficult to measure directly, it is known from the average value of dendrite arm spacing (Dendrite secondary branch interval,: DAS) at multiple points throughout the thickness direction of the cast plate (ingot). (E.g., described in “Method of measuring aluminum dendrite arm spacing and cooling rate” issued by the Japan Institute of Light Metals, issued in 8.20 in 1988). That is, the average distance d between adjacent dendrite secondary arms (secondary branches) in the cast structure of the cast plate was measured using the intersection method (number of fields of view of 3 or more, number of intersections of 10 or more). Using d, the following formula is obtained: d = 62 × C ^−0.337 (where d: dendrite secondary arm interval mm, C: cooling rate ° ^C./s ).

(Roll lubrication)
When a roll lubricant is used, the theoretical or actual cooling rate tends to be substantially less than 50 ° C./s even if the cooling rate is high in theoretical calculation. For this reason, it is desirable to use a roll whose surface is not lubricated as a twin roll. Conventionally, oxide powder (alumina powder, zinc oxide powder, etc.), SiC powder, graphite powder, In general, a lubricant (release agent) such as oil or molten glass is applied to or flowed down on the twin roll surface. However, when these lubricants are used, the cooling rate becomes small and a necessary cooling rate cannot be obtained.

  In addition, when these lubricants are used, cooling unevenness is likely to occur due to the uneven concentration and thickness of the lubricant on the twin roll surface, and the solidification rate tends to be insufficient depending on the part of the plate. For this reason, the higher the Mg content, the larger the macro segregation and micro segregation, and the higher the possibility that it will be difficult to make the balance of strength and ductility of the Al-Mg alloy plate uniform.

(Pouring temperature)
The pouring temperature for pouring the molten Al alloy into the twin rolls is not particularly limited as long as the temperature exceeds the liquidus temperature and is possible in terms of equipment.

(Production method)
The Al-Mg-based Al alloy cast plate of the present invention after twin-roll continuous casting can be used after being molded and processed as it is for the members and parts of each application described above. Moreover, it can be used also as a cast board which gave tempering processes, such as homogenization heat processing and annealing, as needed, and is contained in the scope of the present invention. Alternatively, using the Al-Mg-based Al alloy cast plate of the present invention, it can be further manufactured as a rolled plate by a combination of homogenization heat treatment, cold rolling, annealing, etc. good.

(Chemical composition)
Next, the chemical component composition of the Al—Mg-based Al alloy of the present invention will be described below. The composition of the Al alloy cast plate of the present invention (or the molten metal supplied to the twin rolls) is based on the properties required for the cast plate, such as strength, ductility, and strength-ductility balance. Including the following, the balance is made of Al and inevitable impurities.

  However, in the present invention, in the above composition, the Al alloy cast plate contains an element (included in the inevitable impurities) that is easily mixed from a melting raw material such as scrap. As these elements, Fe: 1.0% or less, Si: 0.5% or less, Mn: 1.0% or less, Cr: 0.5% or less, Zr: 0.3% or less, V: 0.3% or less, Ti: 0.5% or less B: 0.05% or less, Cu: 0.5% or less, and Zn: 0.5% or less are allowed to be included up to the upper limit value of each of these elements. When these elements exceed the respective upper limit values (allowable amount), the compounds due to these elements become excessive, and the characteristics such as fracture toughness and formability of the Al alloy cast plate are greatly inhibited.

  In the above composition, Mg is an important alloy element that enhances the strength, ductility, and strength-ductility balance of the Al-Mg-based Al alloy cast plate. If the content of Mg is less than 5% by mass, strength and ductility are insufficient. On the other hand, if Mg is contained in excess of 14%, even if the cooling rate during continuous casting is increased, crystal precipitation of the Al—Mg compound increases. As a result, the moldability is also significantly reduced. In addition, the work hardening amount increases and the moldability also decreases. Accordingly, the Mg content is 5 mass% or more and 14 mass% or less, and in order to obtain a high strength ductility balance peculiar to a high Mg Al-Mg-based Al alloy, it preferably exceeds 8%. % Within the following range.

  In addition, this Mg content has a wide solid-liquid coexistence temperature range (solidification temperature range) targeted by the present invention, and the temperature range from the liquidus temperature to the solid phase ratio of 0.8 is 25 ° C. or more. It also has meaning to limit Al-Mg alloy. As described above, the Al—Mg alloy targeted by the present invention tends to cause casting defects such as voids when a large-diameter roll is used or the peripheral speed of a twin roll is increased. On the other hand, in the Al-Mg alloy with Mg less than 5% by mass, the solid-liquid coexistence temperature range is narrow, and the temperature range from the liquidus temperature to the solid phase rate of 0.8 is less than 25 ° C. Is unlikely to occur.

  Examples of the present invention will be described below. Al-Mg-based Al alloy cast plates (Invention Examples A to C, Comparative Examples D and E) having various chemical compositions shown in Table 1 are produced by the twin-roll continuous casting method according to the present invention, and casting such as voids The defect was investigated.

  Regarding the component composition of these Al alloy cast plates, each element of Zr, V, B, and Cu is commonly used as an element other than Ti, Mn, Cr, and Zn, as shown in Table 1 with other total contents. It was below the detection limit.

  As shown in Tables 2 and 3, three types of twin roll continuous casting were performed. That is, type: vertical type, casting type: FIG. 1 is a vertical type as shown in FIG. 1 in which twin rolls are arranged substantially horizontally, and the surface is heated by an external heater 8 as shown in FIG. This is an example using a high temperature roll 2 and a low temperature roll 1 whose surface is cooled by internal cooling water and kept at room temperature. In addition, the type: horizontal type, casting type: FIG. 1 is a horizontal type in which twin rolls are arranged substantially vertically, and a high temperature roll 2 whose surface is heated by an external heater as shown in FIG. This is an example using a low temperature roll 1 whose surface is cooled by an internal cooling water and kept at room temperature.

  Furthermore, the type: horizontal type, casting method: FIG. 2 is a type in which twin rolls are arranged substantially vertically as shown in FIG. 2 and are arranged as a rear roll 10 and a front roll 11 with respect to the molten metal. . In this case, when the surface of the rear roll 10 was heated, the surface was heated with an external heater 8 as shown in FIG. Further, in the example in which the surface temperature was about room temperature to about 50 ° C., the surface was cooled with roll internal cooling water.

  The temperature of the molten metal supplied from the tundish was commonly set to 700 ° C. The size of the cast plate produced by cooling to room temperature after casting is commonly 300mm wide x 5m long. In addition, all examples including the comparative example were continuously cast without lubrication of the twin roll surface (no lubrication) in order to ensure the above-described preferable cooling rate of 50 ° C./s or more.

  And the dendrite spacing of the slab thickness direction immediately after casting discharged | emitted from the roll was observed, and it was confirmed whether the unidirectional solidification had arisen. That is, at the outermost surface portion of each slab on the cooling roll (front roll) side and the high temperature roll (rear roll) side, the cooling rate difference calculated from the dendrite arm spacing, that is, “cooling roll (front roll) side cooling rate / When the “high-temperature roll (rear roll) side cooling rate” was 1.2 or more, it was evaluated that unidirectional solidification occurred. Moreover, the case where it was less than 1.2 evaluated that the unidirectional solidification did not arise. These results are also shown in Tables 2 and 3.

  A test piece was collected from the Al alloy cast plate of each example produced in this manner, and the average area ratio of voids was measured for the plate structure. These results are also shown in Tables 2 and 3.

(Void)
The average area ratio of the voids was evaluated as a pass if 0.5% or less as a range that does not affect the molding characteristics such as elongation of the plate. The average area ratio of the voids is measured by mechanically polishing a sample (test piece) collected from an Al alloy cast plate and observing the cross-sectional structure of the center portion of the plate using a 50 × optical microscope. Then, after image processing in the microscope visual field to identify void defects and normal tissues, the total area of voids that can be identified in the visual field is obtained, and the ratio (%) of the total area of voids in the visual field area is obtained. Calculated as the porosity. Here, the average area ratio of the voids means an average of the area ratios of the voids measured at any 10 locations in the center portion of the plate excluding the front end portion and the rear end portion of the plate.

  As shown in Tables 2 and 3, Invention Examples 1 to 8 and 9 to 16 using the composition alloys within the scope of the present invention of A to C in Table 1 are the surface of the twin roll in the present invention regardless of the casting type. The temperature condition or the molten metal temperature condition is satisfied. For this reason, unidirectional solidification occurs, the average area ratio of voids is small, and internal defects are suppressed.

  On the other hand, Comparative Examples 19 to 21 and 24 to 26 use an alloy having a composition within the range of the present invention of B in Table 1, but the casting speed is too high or the surface temperature of the high temperature side roll 2 Is out of the scope of the present invention, or the molten metal temperature in contact with the rear roll 10 is out of the scope of the present invention. As a result, unidirectional solidification does not occur, the average area ratio of the voids is large, and internal defects are not suppressed.

  Among these, Comparative Examples 20 and 25 use an alloy having a composition within the range of the present invention of B in Table 1, but the surface temperature of the high-temperature side roll 2 is too high or the molten metal temperature in contact with the rear roll 10 Is too expensive. As a result, the unevenness of the slab was severe, there was no flatness, and it was not a cast plate, and the casting was stopped midway.

  Further, Comparative Examples 17 and 22 use an alloy having a composition outside the scope of the present invention of D in Table 1 whose Mg is less than 5% by mass. Therefore, although the casting speed or the peripheral speed v of the twin roll is increased to 30 m / min, exceeding the upper limit of the present invention of 20 m / min, the surface temperature condition of the twin roll of the present invention or the molten metal temperature condition Is applied, unidirectional solidification occurs, and defects in the central portion of the solidified cast plate can be suppressed.

  The results of Comparative Examples 17 and 22 show that, as in Comparative Examples 21 and 26, in the Al-Mg-based aluminum alloy within the scope of the present invention, if the casting speed is too high, the surface temperature condition of the twin rolls of the present invention, or Contrast with the result that the unidirectional solidification does not occur even when the molten metal temperature condition is applied, and the internal defects are not suppressed. From these points, the special property of twin-roll continuous casting due to the wide solid-liquid coexistence temperature range of the high Mg content Al-Mg aluminum alloy with Mg of 5% by mass or more can be seen.

  Therefore, the above examples support the critical significance of the requirements or preferred conditions of the present invention for suppressing porosity.

  As described above, according to the present invention, even in a twin-roll continuous casting method of an Al-Mg-based aluminum alloy having a wide solid-liquid coexistence temperature range, an aluminum alloy casting capable of suppressing defects at the center of the plate thickness. A method for manufacturing a plate can be provided. As a result, the application can be expanded to applications that require formability, such as transportation equipment such as automobiles, ships, airplanes, and vehicles, machines, electrical products, architecture, structures, optical equipment, and members and parts of equipment.

It is explanatory drawing which shows one embodiment of a vertical twin roll type continuous casting method. It is explanatory drawing which shows one embodiment of a horizontal type twin roll type continuous casting method.

Explanation of symbols

1, 2, 10, 11: twin roll, 3 molten metal, 4: liquid phase, 5: solid phase, 6: cast plate,
7, 9: Partition, 8: Heater, 12: Kiss point,

Claims (2)

  In a method for producing an Al-Mg-based aluminum alloy cast plate containing 5 to 14% by mass of Mg by a twin-roll type continuous casting method, the casting speed is set to 20 m / min or less, and the number of twin rolls in contact with the molten metal is almost the same. The surface temperature of the roll on one side is equal to or higher than the liquidus temperature of the Al-Mg aluminum alloy calculated by (−5.43 × Mg mass% of the Al—Mg aluminum alloy) +663. The surface temperature of the roll on one side on the other side is set to (−12.9 × Mg mass% of Al—Mg aluminum alloy) +638 or less than the solidus temperature of the Al—Mg aluminum alloy calculated as 638 An aluminum alloy cast plate with reduced internal defects, characterized in that continuous casting is performed while unidirectional solidification is started from the surface side of the roll on one side having a surface temperature equal to or lower than the solidus temperature. Manufacturing method.   In a method for producing an Al-Mg aluminum alloy cast plate containing 5 to 14% by mass of Mg by a twin roll type continuous casting method, the casting speed is set to 20 m / min or less, and the twin rolls arranged substantially vertically The timing of contact with the molten metal is different, the molten metal is first contacted with the roll surface on one side disposed below, and then the molten metal is calculated as (-12.9 × Mg mass% of Al-Mg based aluminum alloy) +638 At the temperature above the solidus temperature of the Al-Mg-based aluminum alloy and at a temperature of the solidus temperature + 10 ° C or below, the upper surface is brought into contact with the surface of the roll on the one side, and the lower side in contact with the molten metal previously A method for producing an aluminum alloy cast plate with reduced internal defects, characterized in that continuous casting is performed while performing unidirectional solidification that initiates solidification of the molten metal from the surface side of the arranged roll on one side.

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