JP2011058047A - Method for producing aluminum alloy thick plate having excellent strength and ductility - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a method for producing a thick plate of ≥50 mm by hot rolling for an Al-Zn-Mg-Cu-based heat treatment type alloy containing ≥1.0% Cu, wherein, by controlling production conditions, the reduction of coarse intermetallic compounds is achieved, thus the remarkable improvement of its ductility (toughness) is achieved while securing its high strength. <P>SOLUTION: Regarding an Al-Zn-Mg-Cu-based alloy containing 1.0 to 3.0% Cu, in a cooling stage after performing a homogenizing treatment at 450 to 520°C for ≥1 hr to an ingot, the average cooling rate at least to 400°C is regulated to ≥100°C/hr, thereafter, hot rolling is performed to a plate thickness of ≥50 mm at a temperature within the range of 300 to 440°C, and, subsequently, solution treatment, quenching and artificial aging treatment are performed so as to obtain a thick plate in which the total area ratio of intermetallic compounds having an equivalent circle diameter of >5 μm is controlled to ≤2%. Further, electric conductivity of the ingot when measured in a state of being cooled to room temperature after the homogenizing treatment is controlled to be ≤40 IACS%. <P>COPYRIGHT: (C)2011,JPO&INPIT

Description

  The present invention relates to a method for manufacturing a high-strength Al—Zn—Mg—Cu heat-treatable aluminum alloy thick plate having a thickness of 50 mm or more used for aircraft, railway vehicles, automobile parts, etc. by applying hot rolling. More specifically, by optimizing the cooling conditions after homogenization treatment of the ingot, the growth and coarsening of intermetallic compounds after homogenization treatment is suppressed, and it is efficient at low temperature or in a short time. The present invention relates to a method for obtaining a thick aluminum alloy plate having high strength and high ductility (high toughness) as a product plate after solution treatment, solution treatment, quenching and artificial aging treatment.

  As is well known, an Al—Zn—Mg—Cu alloy represented by 7075 alloy and 7050 alloy can obtain extremely high strength by precipitation strengthening as a heat treatment type alloy. However, when used as an aircraft material, for example, there is a problem that the peak toughness due to the T6 refining results in a decrease in fracture toughness and a decrease in stress corrosion cracking resistance (SCC resistance). Therefore, in practice, it is often used as a T7x tempered material in which the strength is lowered by about 10 to 15% from the peak strength of the T6 tempering by performing overaging tempering (T7x) by two-stage aging. .

  For this reason, as an aircraft material, an alloy having higher strength and higher toughness is strongly demanded for further weight reduction. However, in general, there is a negative correlation between strength, toughness, and ductility (elongation), and it is known that toughness and ductility (elongation) decrease when the strength is increased. For this reason, as a method for improving elongation and toughness, a measure for reducing the intermetallic compound that is the starting point of fracture has been conventionally applied. For example, in the 7075 alloy, the toughness has been improved by reducing the contents of Si and Fe, which are impurities in the 7175 alloy or the 7475 alloy, as compared with the prior art. In addition, the contents of Si and Fe as impurities are similarly set low in 7050 alloy, 7055 alloy, 7085 alloy and the like. However, in order to further reduce the content of the impurity elements Si and Fe from the current level, it is necessary to use a high purity Al ingot, so there is a problem that the cost becomes high. It becomes inferior in recyclability, and there are many problems in industrially mass production.

  In addition, as a method of improving fracture toughness, for example, as shown in Patent Document 1, high strength and high toughness can be obtained by maintaining a fiber (fiber) structure by suppressing recrystallization and ensuring a quenching speed. A method of obtaining has been proposed. However, in Patent Document 1, no specific method is described regarding a method for suppressing recrystallization and a method for obtaining a high quenching speed. Further, in Patent Document 1, although the homogenization temperature, the hot rolling start temperature, the solution temperature, and the tempering (aging) temperature / time are defined, there is no specific description thereof. Therefore, it has been difficult to say that it is difficult to reliably obtain an alloy having high strength and high toughness by actually industrially implementing the method proposed in Patent Document 1.

On the other hand, the present inventors examined conditions for obtaining a fibrous structure, in particular, ingot homogenization treatment conditions, and as shown in Patent Document 2, fine Al ₃ Zr was precipitated during the ingot homogenization treatment. Has been proposed to improve the fracture toughness by obtaining a fibrous structure even after hot working and solution treatment, but the reduction of other intermetallic compounds that are thought to have a major impact on fracture toughness Has not yet been sufficiently studied, and therefore, it has not been sufficient to reliably obtain an alloy having high strength and high toughness by actually carrying out the method of Patent Document 2 on an industrial scale. .

  Furthermore, in Patent Document 3, it has been proposed to give a hot cycle after hot rolling to a cooling cycle controlled by heating or cooling, but in this Patent Document 3, there is little explanation of the specific action, In addition, the change in the internal structure of the metal has not been sufficiently investigated, and therefore it has been questioned whether a material having high strength and high toughness can be obtained even if the method of Patent Document 3 is actually applied.

On the other hand, although it is completely different from the method of obtaining a thick plate (rolled plate) having a plate thickness of 50 mm or more by applying hot rolling as a main application, which is the object of the present invention, Patent Document 4 and Patent Document 5 describe a method for manufacturing an extruded material that can be used for a vehicle material and a motorcycle material and is relatively thin (less than several millimeters) and capable of dealing with a complicated shape. The conditions, especially the conditions after homogenization of the ingot are specified. And in the method of patent document 4 and patent document 5, while ensuring the outstanding extrudability and ensuring favorable weldability, and also aiming at an improvement in corrosion resistance, the amount of Cu in an alloy shall be 0.4% or less. It is regulated. However, if the amount of Cu is 0.4% or less, it is difficult to sufficiently secure the strength required for the aircraft material used as the main application in the present invention. On the other hand, the thick plate of the aircraft material that is mainly used in the present invention does not require much weldability, and it does not require extrudability because the plate is formed by hot rolling. Therefore, in the thick plate material by hot rolling mainly used for aircraft materials, the Cu amount is restricted to a small amount (0.4% or less) as shown in Patent Document 4 and Patent Document 5 in order to ensure high strength. Therefore, it is considered desirable to increase the amount to about 1.0% or more. However, if the amount of Cu is increased, the precipitation state of the intermetallic compound also becomes significantly different from the case of the methods of Patent Document 4 and Patent Document 5. That is, if the amount of Cu is increased to about 1.0% or more, coarse S phases (AlCu ₂ Mg compounds) and Al ₇ Cu ₂ Fe are likely to be generated as intermetallic compounds, and these coarse intermetallic compounds are It is considered to have a great adverse effect on toughness (ductility). Therefore, when an alloy whose amount of Cu is increased to about 1.0% or more is used, the conditions shown in Patent Document 4 and Patent Document 5 are used in this invention as they are as the production conditions, particularly the homogenization treatment conditions. Even if it is applied to the production of aircraft material having a thickness of 50 mm or more by hot rolling, it cannot be guaranteed that a material having high strength and high toughness as expected can be obtained.

Special Table 2000-504068 JP 2006-257522 A Special table 2007-510061 gazette JP-A-9-310141 JP 2005-307322 A

As already described, heat-treatable Al—Zn—Mg—Cu-based alloys represented by 7075 alloy and 7050 alloy may be used as thick plates having a thickness of 50 mm or more as large structural members such as aircraft members. In many cases, higher strength is required from the viewpoint of weight reduction and fuel efficiency improvement. In such materials, from the viewpoint of damage tolerance design, fracture toughness that shows resistance to crack growth is often emphasized when designing structural members, and improvement in toughness and ductility as well as strength is desired. Yes. However, since there is generally a negative correlation between strength, toughness, and ductility, in order to improve strength, ductility, and toughness at the same time, the reduction of intermetallic compounds by previously suppressing Si and Fe, which are impurity elements, has been achieved. Although measures have been implemented, it has been difficult to sufficiently improve strength, ductility and toughness at the same time. On the other hand, in an Al—Zn—Mg—Cu based alloy in which the Cu content is about 1.0% or more in order to obtain sufficient strength as an aircraft material or the like, a coarse S phase (Al— Cu ₂ Mg compound) and Al ₇ Cu ₂ Fe compound are likely to be generated. Therefore, as a thick plate of such an Al—Zn—Mg—Cu alloy containing about 1.0% or more of Cu, further improvement in ductility and toughness is achieved. In order to achieve this, it is necessary to reduce intermetallic compounds by controlling the manufacturing process conditions. Therefore, in the present invention, as a method for producing a thick plate material by hot rolling of an Al—Zn—Mg—Cu-based (7000) high-strength alloy containing a large amount of Cu, mainly by controlling the production conditions, It is an object to sufficiently reduce the coarse intermetallic compound composed of a compound, thereby ensuring high strength and at the same time greatly improving ductility and toughness.

  The present inventors mainly used an Al—Zn—Mg—Cu heat-treatable alloy thick plate having a thickness of 50 mm or more, particularly an Al—Zn—Mg— containing 1.0% or more of Cu, which is mainly used as an aircraft material. As a manufacturing method by rolling a thick plate of Cu-based alloy, various experiments and examinations were conducted to find a method of manufacturing a thick plate having high ductility (high toughness) and high strength. By appropriately controlling the cooling rate after the heat treatment, and also appropriately controlling the temperature of the hot rolling, the coarsening of the intermetallic compound mainly composed of Cu is suppressed, and the metal during the subsequent solution treatment It became possible to efficiently re-solidify the intermetallic compound, and it was found that a thick plate having high strength and high ductility (high toughness) can be obtained by the subsequent artificial aging treatment, which led to the present invention. .

  Specifically, the manufacturing method of the aluminum alloy thick plate of the invention of claim 1 is Zn 5.0-7.0% (mass%, the same applies hereinafter), Mg 1.0-3.0%, Cu 1.0-3 1.0% and 1 selected from Cr 0.05 to 0.3%, Zr 0.05 to 0.25%, Mn 0.05 to 0.40%, Sc 0.05 to 0.35% Contain two or more seeds in a total amount of 0.05 to 0.5%, further restrict impurities as Si to 0.25% or less and Fe to 0.25% or less, the balance being Al And ingots and other inevitable impurities, the ingot was subjected to homogenization treatment for 1 hour or more at a temperature in the range of 450 to 520 ° C. In the process of cooling the mass, the average cooling rate up to at least 400 ° C. is 1 Restricted to 0 ° C / hr or higher, and then hot-rolled to a thickness of 50 mm or higher at a temperature in the range of 300 to 440 ° C, then subjected to solution treatment, quenching, and artificial aging treatment. It is characterized in that a thick plate having a total area ratio of intermetallic compounds exceeding 5 μm of 2% or less is obtained.

  According to a second aspect of the present invention, in the method for producing an aluminum alloy thick plate according to the first aspect, the ingot conductivity measured in the state of cooling to room temperature after the homogenization treatment is 40 IACS% or less. It is characterized by controlling.

  The invention of claim 3 is the method for producing an aluminum alloy thick plate according to claim 1, wherein the solution treatment is performed at a temperature in the range of 460 to 520 ° C. for 1 to 10 hours. Is.

  According to this invention, as a manufacturing method by hot rolling of an Al—Zn—Mg—Cu heat-treatable aluminum alloy thick plate having a thickness of 50 mm or more and containing 1.0% or more of Cu, it is homogeneous for an ingot. By optimizing the cooling conditions after the heat treatment, and appropriately controlling the temperature of the subsequent hot rolling, the coarsening of the intermetallic compound mainly composed of Cu is suppressed, and the subsequent solution treatment. By re-dissolving the intermetallic compound easily and reliably, and subsequently subjecting to artificial aging treatment, a thick plate having high strength and high ductility (high toughness) can be obtained reliably and easily. In particular, it is optimal as a method for producing thick plate structural materials such as aircraft materials that require high strength and high ductility (high toughness) at the same time.

  The production method of the present invention will be described in detail below.

  First, the reasons for limiting the component composition of the aluminum alloy that is the subject of the manufacturing method of the present invention will be described.

Zn:
Zn is an element that precipitates in the matrix as a η ′ phase (MgZn ₂ ) in a size of 0.01 to 1 μm during the artificial aging treatment and increases the strength by precipitation hardening. If the added amount of Zn is less than 5.0%, sufficient strength cannot be obtained. On the other hand, if it exceeds 7.0%, the effect is saturated and the SCC resistance is lowered. Within the range of 0.0%.

Mg:
Similar to Zn, Mg precipitates as an η ′ phase (MgZn ₂ ) during artificial aging treatment in an alloy of the subject system in the present invention, and exhibits an effect of increasing strength. If the addition amount of Mg is less than 1.0%, a sufficient strength improvement effect cannot be obtained, while if it exceeds 3.0%, the effect is saturated and an intermetallic compound of Mg ₂ Si is easily generated. Since there is a possibility that the toughness (ductility) may be lowered, the Mg amount is set in the range of 1.0 to 3.0%.

Cu:
Cu is dissolved in the matrix and exhibits an effect of increasing strength by solid solution hardening. Here, if the amount of Cu is less than 1.0%, the strength is insufficient as a high-strength structural material such as aircraft material, while if the amount of Cu exceeds 3.0%, S phase (AlCu ₂ Mg compound) or Al The volume ratio of ₇ Cu ₂ Fe increases, the fracture toughness value decreases, and Al—Cu—Mg-based precipitates with high corrosiveness increase, which degrades SSC resistance and exfoliation corrosion resistance. Therefore, the amount of Cu is set in the range of 1.0 to 3.0%.

Cr, Zr, Mn, Sc:
Cr, Zr, Mn, and Sc all function as recrystallization suppression and grain growth suppression elements during the hot rolling step or solution treatment. The addition amounts thereof were limited to Cr 0.05 to 0.3%, Zr 0.05 to 0.25%, Mn 0.05 to 0.40%, and Sc 0.05 to 0.35%, respectively. When added, if the amount is less than the lower limit, the above-described effect of addition becomes insufficient. On the other hand, if the amount exceeds the upper limit, the effect is saturated and a coarse intermetallic compound is easily formed. In addition, although sufficient effect is acquired even if these elements are added alone, it is a matter of course that the effect can be obtained even when plural kinds are added simultaneously. However, if the total addition amount of these elements is less than 0.05%, a sufficient addition effect cannot be obtained, while if it exceeds 0.5%, the effect is saturated and a coarse intermetallic compound is easily generated. The total content of these elements was in the range of 0.05 to 0.5%.

Fe:
Fe is an element that is unavoidably contained even in a normal aluminum alloy. In the case of the aluminum alloy thick plate of the present invention, Fe forms an insoluble intermetallic compound as Al ₇ Cu ₂ Fe, and has elongation and toughness. In the present invention, Fe is treated as an impurity and its content is preferably as low as possible in this invention. However, in view of cost, it may be 0.25% or less, preferably 0. .15% or less is good.

Si:
Si is an element that is unavoidably contained in ordinary aluminum alloys as well as Fe, and in the alloys of the present invention, Mg ₂ Si intermetallic compounds are formed to reduce elongation and toughness. However, it is desirable that Si is treated as an impurity, and its content is desirably as low as possible, but industrially, it may be 0.25% or less, preferably 0.15% or less in consideration of cost. .

The balance of the above elements may be basically inevitable impurities other than Al and the above-described Fe and Si. Although not particularly limited, in a normal aluminum alloy, a small amount of a refiner containing TiB ₂ or TiC may be added for the purpose of crystal grain refinement at the time of casting. It is permissible to add about 0.05% of the micronizing agent.

  Further, in the final plate obtained by the method of the present invention, the total area ratio of the intermetallic compound having an equivalent circle diameter exceeding 5 μm is restricted to 2% or less as the distribution density condition of the intermetallic compound. The reason for limiting the intermetallic compound total area ratio condition will be described below.

In the alloy of the system targeted by the present invention, there are intermetallic compounds such as Mg ₂ Si, Al ₇ Cu ₂ Fe, S phase (Al ₂ CuMg), and η ′ phase (MgZn ₂ ) even in the final plate. To do. Of these, a coarse intermetallic compound exceeding 5 μm serves as a void generation starting point or a crack propagation path, thus reducing ductility and toughness. Therefore, in order to improve ductility and toughness, it is effective to reduce the size or number (density) of these intermetallic compounds. Accordingly, as a result of detailed investigations by the inventors on the relationship between the size and total area ratio of the intermetallic compound and ductility (toughness), Mg ₂ Si, Al ₇ Cu ₂ Fe, S phase (Al ₂ CuMg), etc. It was found that excellent ductility could be obtained if the total area ratio of the intermetallic compound having an equivalent circle diameter exceeding 5 μm was 2% or less, and the conditions were defined in the final plate. In the alloy used in the present invention, the η ′ phase is also precipitated as described above. However, the η ′ phase is rarely coarse as an equivalent circle diameter exceeding 5 μm. In any case, regardless of the type of intermetallic compound, it is possible to ensure good ductility and toughness if the coarse intermetallic compound exceeding the equivalent circle diameter of 5 μm is 2% or less in terms of the total area ratio. it can. Here, the total area ratio of the intermetallic compound exceeding 5 μm is a cross section parallel to the plate surface of the thick plate which is the final plate, that is, a cross section of a plane (L-LT plane) composed of the rolling direction and the direction perpendicular to the rolling. The total area ratio of the intermetallic compound observed is

  Further, in the present invention, the thickness of the thick plate to be finally obtained is limited to 50 mm or more, for the following reason.

  That is, as already described, when used for large structural members such as aircraft materials, toughness and ductility are required as well as strength. In general, strength and toughness (ductility) are contradictory properties, and as strength increases, toughness (ductility) tends to decrease. When the plate thickness is thin, a sufficiently high cooling rate can be obtained at the time of quenching after the solution treatment, but when the thickness is increased, the cooling rate is lowered, and the characteristics are likely to be lowered. The present invention is a manufacturing method developed for the purpose of obtaining sufficient characteristics even in a thick material of about 2 inches (50.8 mm) or more mainly used for aircraft, and the object of the present invention is 50 mm or more. A thick plate was used.

  Next, manufacturing conditions defined by the method of the present invention will be described.

  In the method of the present invention, basically, as with a general Al—Zn—Mg—Cu alloy (No. 7000 alloy), after the casting, the ingot is homogenized and rapidly cooled. Then, hot rolling is performed to obtain a required plate thickness (50 mm or more), and then an artificial aging treatment is performed after solution treatment and quenching. Accordingly, each of these steps will be described in more detail.

  First, the homogenization treatment is performed mainly for the purpose of reducing component segregation during casting of an Al—Zn—Mn—Cu alloy, solid solution and fragmentation of coarse intermetallic compounds generated during casting, spheroidization, and the like. If the homogenization treatment temperature is less than 450 ° C., the effect of homogenization is insufficient, and coarse intermetallic compounds cannot be divided and spheroidized. If the homogenization temperature exceeds 520 ° C, the matrix may be melted. Further, if the holding time of the homogenization treatment is less than 1 hour, a sufficient homogenizing effect cannot be obtained, so the holding time is set to 1 hour or longer (preferably 4 hours or longer). The upper limit of the homogenization treatment holding time is not particularly limited. However, if the time is too long, energy is wasted and disadvantageous in terms of cost, and therefore it is usually preferably 8 hours or less.

  Next, the limitation of the cooling rate in the cooling process after the homogenization process will be described.

In the so-called heat-treatable aluminum alloy, which is subjected to solution treatment / quenching and artificial aging treatment as in the alloy of the subject system in the present invention, many investigations have already been made regarding the cooling rate during solution treatment / quenching. Has been made. However, in the past, it was thought that the precipitate was sufficiently re-dissolved by the usual solution treatment, so the solid solution / precipitation that occurred during the cooling after the homogenization treatment for the ingot, which was the previous stage The actual situation is that sufficient attention was not paid to the behavior. However, as a result of detailed investigations by the present inventors, an alloy containing 1% or more of Cu, such as an alloy of the subject system in the present invention, particularly in the S phase (Al ₂ There are many coarse intermetallic compounds typified by CuMg), and these coarse intermetallic compounds are difficult to completely re-dissolve even during the solution treatment. As a result, it was found that the ductility and toughness were lowered. As a result of investigating this S phase in more detail, it crystallizes during casting and partly dissolves or spheroidizes due to the homogenization of the ingot, but precipitates and coarsens during the cooling process after the homogenization. There was found. Furthermore, since this precipitation temperature has a peak at about 420 ° C., it has been found that it is important to quickly cool this precipitation temperature region after the homogenization treatment. And based on these findings, as a result of examining the effect of the cooling rate after the homogenization treatment, the temperature range from the homogenization treatment temperature to 400 ° C. is cooled at an average cooling rate of 100 ° C./hr or more, whereby ductility is achieved. It was found that precipitation and coarsening of intermetallic compounds harmful to toughness can be suppressed. Therefore, in the method of the present invention, the cooling after the homogenization treatment is defined as cooling at an average cooling rate of 100 ° C./hr or more up to at least 400 ° C.

  Here, the cooling rate in the temperature range lower than 400 ° C. in the cooling process after the homogenization treatment is not particularly limited, but usually it is desirable to cool quickly even in this temperature range, and industrially 20 ° C./hr. It is preferable to cool by the above.

  In addition, there is no particular restriction on the ultimate temperature after cooling. In general, due to the operational status of the hot rolling equipment, cooling to room temperature is often performed after homogenization, but as described above, the temperature range up to 400 ° C. is an average cooling rate of 100 ° C./hr or more. Since the growth of harmful intermetallic compounds is suppressed by cooling at, hot rolling may be started as it is without further cooling. Of course, after cooling to a temperature lower than the hot rolling temperature of 300 to 440 ° C. after the homogenization treatment, reheating for heating the ingot to the hot rolling temperature may be performed. From the viewpoint of energy cost, after the homogenization treatment, it is preferable to cool to a hot rolling temperature range and perform hot rolling as it is.

  In addition, the specific method of cooling the ingot after the homogenization is not particularly limited, but a method of forcibly cooling the fan with cold air or a method of cooling with water using a spray or the like may be applied. .

  Here, after the homogenization treatment and cooling, it is desirable that the conductivity measured at room temperature is 40% or less in terms of IACS%. That is, the electrical conductivity at room temperature after the homogenization treatment / cooling corresponds to the amount of solid solution / precipitation by the homogenization treatment and the subsequent cooling, in other words, the amount of intermetallic compound, and the lower the electrical conductivity, the coarser the intermetallic compound. This means that no crystallization has occurred. If the electrical conductivity after homogenization / cooling is 40 IACS% or less, sufficient elements are maintained in a solid solution state. On the other hand, if the conductivity exceeds 40 IACS%, precipitation and coarsening of intermetallic compounds occur. It has occurred, and it becomes difficult to make the intermetallic compound fine even by the final solution treatment, which causes a decrease in ductility and toughness.

  After the homogenization treatment as described above, the temperature range up to at least 400 ° C. is cooled at an average cooling rate of 100 ° C./hr or more, and is cooled as it is or further to room temperature or a temperature near room temperature. It is reheated to the rolling start temperature and hot rolled to a plate thickness of 50 mm or more. The hot rolling conditions will be described next.

  In the hot rolling process, processing at a high temperature is performed in order to process into a predetermined thickness and shape. However, even if rapid cooling is performed after the homogenization treatment as described above to suppress the coarsening of the intermetallic compound, if the intermetallic compound is maintained for a long time in the precipitation temperature range of the intermetallic compound during hot rolling, the intermetallic compound is again formed. There is a risk of coarsening. Therefore, as a result of a detailed investigation on an appropriate hot rolling temperature, when the hot rolling temperature is less than 300 ° C., the coarsening of the intermetallic compound is small, but the deformation resistance becomes large and industrial production becomes difficult. It has been found that if the hot rolling temperature exceeds 440 ° C., the intermetallic compound is likely to be coarsened, and at the same time, hot workability is deteriorated and hot rolling cracks are likely to occur. Therefore, the hot rolling is performed at a temperature within the range of 300 to 440 ° C.

  In addition, although it does not specifically limit about the total time from the heating for hot rolling to the completion | finish of hot rolling, ie, the time which stays in the temperature within the range of 300-440 degreeC, The coarseness of an intermetallic compound It is desirable to end hot rolling promptly before conversion occurs. That is, from the viewpoint of suppressing the growth and coarsening of intermetallic compounds due to diffusion, the time from reaching the temperature in the range of 300 to 440 ° C. in the heating before hot rolling to the end of hot rolling should be within 8 hours. In view of productivity, it is preferable to end the hot rolling within 4 hours industrially.

  In addition, although it does not specifically limit about the heating rate from room temperature in the heating before hot rolling to hot rolling start temperature, It is preferable to set it as 30 degrees C / hr or more industrially.

  After rolling to a thickness of 50 mm or more by hot rolling as described above, solution treatment is performed.

  This solution treatment is desirably performed in a temperature range of 460 ° C. or more and 520 ° C. or less, like a typical 7000 series alloy specified in, for example, AMS (American Aircraft Material Standard) 2772. If the solution treatment temperature is less than 460 ° C., the effect of solution treatment cannot be sufficiently obtained, while if it exceeds 520 ° C., the matrix may be melted. Further, the solution treatment time (the retention time at the solution treatment temperature) is usually preferably 1 hour or more and 10 hours or less. If the solution treatment time is less than 1 hour, the element is insufficient for solid solution. On the other hand, if the solution treatment time exceeds 10 hours, the effect of solution treatment is saturated and the economic efficiency is only impaired.

  The conditions for cooling (quenching) after the solution treatment are not particularly limited, but usually, it is rapidly quenched by water quenching or the like to a temperature of about 50 ° C. or less at a cooling rate of 200 ° C./min or more. good.

  After solution treatment and quenching, if necessary, tension or compression may be performed for the purpose of removing residual stress.

  After the solution treatment and quenching, after performing tension or compression as necessary, an artificial aging treatment is performed to finally obtain the required strength. The treatment temperature and time (aging temperature, time) in this artificial aging treatment are not particularly limited, and may be carried out under the conditions conventionally applied to No. 7000 series alloys. For example, according to AMS 2772 In the case of T6x tempering, the temperature is set to 120 to 135 ° C. × 14 to 48 hr, and in the case of T7x tempering, the first stage aging is performed at 107 to 138 ° C. for about 3 to 24 hr, and the second stage aging is set to 163 to 177. It is desirable to carry out at a temperature of 4 to 30 hours.

  For the alloys A to I in the composition range of the present invention as shown in Table 1, an ingot having a thickness of 400 mm, a width of 1200 mm, and a length of 3500 mm is produced by a DC casting method, and a thickness of 380 mm is obtained by chamfering and cutting. It was cut into 1200 mm and a length of 3000 mm. Subsequently, these ingots were homogenized. As a cooling method from the homogenization temperature to 400 ° C., any one of mist spray cooling using water as a refrigerant, forced fan cooling and air cooling was applied to change the cooling rate. The cooling rate was measured from a cooling curve with a thermocouple embedded in the end of the ingot. For cooling conditions of 400 ° C. or lower, mist spraying, fan cooling, and furnace cooling are performed in combination. 1-No. About 8, it cooled to room temperature. No. For No. 9, after cooling to 390 ° C., reheating for hot rolling was performed. Here, a part of the ingot was sliced and cut, cooled to room temperature as it was, and the conductivity was measured. Table 2 shows the homogenization treatment conditions, the cooling method, the average cooling rate up to 400 ° C. after the homogenization treatment, and the conductivity of the ingot after cooling.

  After performing the homogenization treatment as described above, reheating at a heating rate of 40 ° C./hr for hot rolling, holding at 420 ° C. for 2 hr, starting hot rolling, and rolling with a thickness of 60 mm A board was produced. Thereafter, in accordance with AMS2772, solution treatment at 475 ° C. × 2 hr and water quenching were performed. After quenching, 2% tensile strain was applied by a stretcher for the purpose of removing residual stress, and after natural aging, it was subjected to two-stage artificial aging treatment of 121 ° C x 4hr + 163 ° C x 18hr, and the final product plate of T7451 tempered material It was.

  About the product board (T7451 tempered material), the measurement of the intermetallic compound was performed. For the measurement of this intermetallic compound, a sample was cut out from the central part of the thickness of the product plate, and the surface (hereinafter referred to as L-LT surface) consisting of the rolling direction and the perpendicular direction of rolling was mirror finished by mechanical polishing, and then 20% sulfuric acid was used. Etching was performed, 10 field observations were performed at 100 times with an optical microscope, and the area ratio of intermetallic compounds having an equivalent circle diameter of 5 μm or more was examined by image analysis.

  Further, mechanical properties in the direction perpendicular to the rolling direction (LT direction) were evaluated for evaluation of strength and ductility. That is, a tensile test was performed using a φ6.35 mm round bar test piece defined in ASTM E8, and the tensile strength, proof stress, and elongation were measured. Table 3 shows the results of examining the mechanical properties and the total area ratio of intermetallic compounds exceeding the equivalent circle diameter of 5 μm.

  As shown in Table 3, no. 1-No. In Example 9 of the present invention, by controlling the cooling rate to 400 ° C. after the homogenization treatment to 100 ° C./hr or more, it is confirmed that a sufficiently high solid solubility is maintained even after the homogenization treatment. From the result of. Furthermore, from the results of the tensile test, it was found that both the strength and elongation of the product plate after the T7451 tempering treatment showed high values in the examples of the present invention. It can be understood that such an increase in elongation (ductility) is due to a decrease in the area ratio of the intermetallic compound.

  In contrast, No. of the comparative example. In No. 10, since the temperature during the homogenization treatment was low, it was found that the intermetallic compound was not sufficiently divided and re-dissolved, and therefore both strength and elongation were reduced. The comparative example No. 11-No. In No. 14, the cooling rate to 400 ° C. after the homogenization treatment is slow, and it can be seen from the conductivity that precipitation and coarsening of intermetallic compounds occur during cooling. For this reason, even when solution treatment / quenching and aging treatment were performed, the area ratio of the intermetallic compound was high, and in the mechanical properties of the product plate, a decrease in strength and elongation was observed.

  Next, the influence of the temperature at the time of hot rolling was confirmed using the alloy A within the component composition range of the present invention. The ingot was made to have a thickness of 400 mm, a width of 1200 mm, and a length of 3500 mm in the same manner as in Example 1, and was cut into a thickness of 380 mm, a width of 1200 mm, and a length of 3000 mm by chamfering and cutting. This was subjected to a homogenization treatment at 475 ° C. × 12 hr. After the homogenization treatment, it was rapidly cooled by cooling with a fan and cooled to room temperature. The average cooling rate to 400 ° C. after the homogenization treatment was 152 ° C./hr, and the electrical conductivity at room temperature was 32.9 IACS%. Thereafter, reheating for hot rolling was performed. The heating rate during heating was fixed at 40 ° C./hr, and hot rolling was performed under the conditions shown in Table 4. 21-No. 28 rolled plates having a thickness of 60 mm were produced. Furthermore, water quenching was performed following the solution treatment at 475 ° C. × 2 hr. After quenching, 2% tensile strain was applied by a stretcher for the purpose of removing residual stress, and after natural aging, it was subjected to two-stage artificial aging treatment of 121 ° C x 4hr + 163 ° C x 18hr. It was.

  The product plate was measured for intermetallic compounds in the same manner as in Example 1, and the total area ratio of intermetallic compounds having an equivalent circle diameter of 5 μm or more was investigated.

  Further, regarding the evaluation of mechanical properties, the mechanical properties in the LT direction were evaluated using a round bar test piece having a diameter of 6.35 mm in the same manner as in Example 1. These results are shown in Table 5.

  No. 26-No. Each of the 28 comparative examples is an example in which the hot rolling temperature condition is out of the range of the present invention, but in these cases, as shown in Table 5, the area ratio of the intermetallic compound is high as the characteristics of the final plate. It was found that strength, proof stress and elongation were reduced. On the other hand, No. of the present invention example. 21-No. 25, the area ratio of the intermetallic compound was 2% or less, indicating that both strength and elongation were good.

Claims (3)

  Zn 5.0 to 7.0% (mass%, the same applies hereinafter), Mg 1.0 to 3.0%, Cu 1.0 to 3.0%, Cr 0.05 to 0.3%, Zr0.05 -0.25%, Mn 0.05-0.40%, Sc 0.05-0.35% selected from one or more kinds within the range of total amount 0.05-0.5% In addition, using an Al—Zn—Mg—Cu-based aluminum alloy, in which Si is controlled to 0.25% or less and Fe is 0.25% or less as impurities, and the balance is Al and other inevitable impurities, The ingot is subjected to homogenization treatment for 1 hour or more at a temperature in the range of 450 to 520 ° C., and then the average cooling rate up to at least 400 ° C. is 100 ° C./hr or more in the process of cooling the ingot. At a temperature in the range of 300-440 ° C. After hot rolling to a plate thickness of 0 mm or more, solution treatment / quenching and artificial aging treatment are performed to obtain a plate with a total area ratio of intermetallic compounds exceeding 5 μm in equivalent circle diameter of 2% or less. A method for producing an aluminum alloy thick plate having excellent strength and ductility. In the manufacturing method of the aluminum alloy thick plate of Claim 1,
A method for producing an aluminum alloy thick plate excellent in strength and ductility, wherein the ingot conductivity measured in the state of cooling to room temperature after the homogenization treatment is controlled to be 40 IACS% or less. In the manufacturing method of the aluminum alloy thick plate of Claim 1,
The method for producing an aluminum alloy thick plate excellent in strength and ductility, wherein the solution treatment is performed at a temperature in the range of 460 to 520 ° C. for 1 to 10 hours.