JP2007031819A - Method for producing aluminum alloy plate - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a method for producing an aluminum alloy plate having a basic composition of 6000 series and improved BH property, bendability and rough skin prevention property.
As an essential element, Mg: 0.40 to 0.70%, Si: 0.50 to 1.00%, Mn: 0.05 to 0.30%, Fe: 0.10 to 0% by mass% as essential elements. .50%, Ti: 0.005 to 0.10%, B: 0.0005 to 0.01%, and as optional elements, Zr: 0.05% or less, Cr: 0.05% or less An alloy melt containing one or two types and having the balance Al and unavoidable impurities is continuously cast into a thin slab having a thickness of 5 to 15 mm by a twin belt type casting machine, and directly wound on a coil. One cold rolling, intermediate solution forming, second cold rolling, final solution forming, and preliminary aging are sequentially performed, and the second cold rolling is performed at a final cold rolling rate of 15 to 30%. For producing an aluminum alloy plate excellent in BH properties, bendability and rough skin properties
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Description

  The present invention relates to a method for producing an aluminum alloy plate excellent in BH property, bendability and skin roughness. The 6000 series alloy plate according to the present invention can be widely used as an outer plate material or a building material for vehicles such as automobiles and home appliances by taking advantage of its excellent characteristics.

  As a method for producing a 6000 series aluminum alloy plate, a method of performing hot rolling, cold rolling, annealing, etc. in order after subjecting an ingot produced by a semi-continuous casting method or the like to chamfering treatment or homogenizing heat treatment is common. Is.

  However, in recent years, not only has there been a tendency to demand higher strength in order to improve the weight reduction of automobiles, but further improvements have been demanded in baking coating curability, that is, BH properties and bendability, and further productivity improvement. There is also a growing demand for cost reduction.

  In order to meet these requirements, for example, in Patent Document 1, Si 0.4 to 2.5%, Mg 0.1 to 1.2%, Cu 1.5% or less, Zn 2.5% or less, Cr 0.3% or less , Mn 0.6% or less, Zr 0.3% or less of the aluminum alloy containing one or more of the two or more continuously cast into a plate having a plate thickness of 3 to 15 mm, and then subjected to cold rolling, There has been proposed an aluminum alloy plate for forming and a method for producing the same, wherein the maximum size of the intermetallic compound in the matrix is 5 μm or less by solution treatment and quenching.

  Further, in Patent Document 2, in order to obtain an aluminum alloy plate excellent in formability and bake hardenability, for example, by weight, Si: 0.2% or more, Mg: 0.3% or more, and Si + 0.7Mg: It contains Si and Mg in the range of 0.7 to 1.5%, and further Cu: 0.05 to 1%, Zr: 0.01 to 0.15%, Mn: 0.01 to 0.00. 15%, Be: 0.001 to 0.2%, Ti: 0.001 to 0.03%, B: 0.0001 to 0.01%, 1 type or 2 or more types are contained, and the rest Is obtained by homogenizing an Al alloy ingot composed of inevitable impurities with Al and Fe controlled to 0.2% or less, and hot rolling the homogenized Al alloy ingot to obtain a hot-rolled sheet. The cold-rolled sheet is manufactured by cold-rolling the hot-rolled sheet, and the cold-rolled sheet is subjected to a final solution treatment. In the method for producing a metal plate, the cold rolling is performed at least twice, and an intermediate solution treatment and an intermediate aging treatment are performed between the cold rolling and the cold rolling performed at least twice. A method for producing an aluminum alloy plate having excellent formability and bake hardenability has been proposed.

  Similarly, in Patent Document 3, in order to obtain an aluminum alloy thin plate excellent in press formability and bake hardenability, for example, it was obtained by continuously casting a molten aluminum alloy having a composition belonging to Series 6000 defined by JIS. The cast plate material was heated to more than 450 to 570 ° C. with or without cold rolling, held at this temperature for 30 minutes or less, and then subjected to an intermediate solution treatment under the condition of cooling at a cooling rate of 5 to 500 ° C./sec. Thereafter, it is further cold-rolled to produce a cold-rolled sheet. This cold-rolled sheet is heated to 500 to 580 ° C. at a heating rate of 5 to 500 ° C./sec and held at this temperature for 60 seconds or less. Solution treatment under conditions of cooling at ˜500 ° C./sec was performed, then left at room temperature for 135 minutes to 6 days, then heated to 220 to 320 ° C. at 4 to 500 ° C./sec, and this temperature was reduced to 60 seconds or less Cooling after holding Method for producing a molded aluminum alloy plate which comprises treating the restoration has been proposed.

However, Patent Document 2 discloses a treatment method in which an intermediate solution treatment is performed in the middle of cold rolling, but it is described that a final cold rolling rate of 50% or more is necessary to stabilize the crystal grain size. ing. Furthermore, the casting method in this case is normal DC casting.
Also, Patent Document 3 discloses a treatment method for performing an intermediate solution treatment in the middle of cold rolling, but the final cold rolling rate is not particularly limited. As an example, (3 mm thickness → 1 mm) Thickness) Only 66.6% can be found. Further, the casting method in this case is twin roll casting.
In addition, 6000 series alloy plates used for automobile body sheets are required to be more easily processed and have high strength characteristics, and it is necessary to further improve BH properties, bendability and skin roughness particularly in continuous cast materials. there were.
JP 62-207851 A JP 07-286251 JP 2003-328095 A

  An object of this invention is to provide the manufacturing method of the aluminum alloy plate which made 6000 type | system | group basic composition and improved BH property, bendability, and rough skin prevention property.

  Therefore, as a result of intensive studies by the present inventors, in the treatment method of performing the intermediate solution treatment during the cold rolling, when the final cold rolling rate in the cold rolling performed after the intermediate solution treatment exceeds 30%, the cold rolling is performed. It has been newly found that the strain accumulation amount due to the increase in the dislocation density increases the grain size of the recrystallized grains to less than 20 μm, which increases the yield strength in the final plate (T4P treated material) and decreases the bendability.

  The present invention has been completed based on the above-mentioned novel findings, and as essential elements, Mg: 0.40-0.70%, Si: 0.50-1.00%, Mn: 0.05- 0.30%, Fe: 0.10 to 0.50%, Ti: 0.005 to 0.10%, B: 0.0005 to 0.01%, and Zr: 0.05 as an optional element % Or less, Cr: One or two of 0.05% or less, and the molten alloy having the balance Al and inevitable impurities is continuously formed into a thin slab having a thickness of 5 to 15 mm by a twin belt type casting machine. After casting and winding directly on the coil, the first cold rolling, intermediate solution forming, second cold rolling, final solution forming, and preliminary aging are sequentially performed. Excellent in BH property, bendability and rough skin, characterized by performing at a ductility of 15-30% It is a manufacturing method of an aluminum alloy plate.

According to the present invention, a molten aluminum alloy having a predetermined composition containing a suitable amount of Mn and the like based on a 6000 series composition is continuously cast into a thin slab having a thickness of 5 to 15 mm by a twin-belt type casting machine. The cooling rate during solidification at the position can be secured at 40 to 150 ° C./s. Thereby, the thin slab which crystallized the fine intermetallic compound can be cast, maintaining the Mg and Si solid solution amount in a matrix high. Furthermore, when this slab is cold-rolled, a transformation from needle-like β-AlFeSi phase to spherical α-Al (Fe · Mn) Si occurs by performing an intermediate solution treatment during the cold rolling. At the same time, Mg ₂ Si diffuses into the matrix and the amount of Mg and Si in the matrix increases. Furthermore, by performing cold rolling at a final cold rolling rate of 15 to 30%, the strain accumulation amount in the final cold rolling process is regulated to an appropriate range, and the dislocation density is controlled, whereby the recrystallized grain size 20 ˜30 μm can be achieved, and the BH property, bendability and rough skin property of the final plate (T4P treated material) can be improved. After the final solution treatment, the transformation from the needle-like β-AlFeSi phase to the spherical α-Al (Fe · Mn) Si phase proceeds, and Si is dissolved and diffused from the β-AlFeSi phase into the matrix. The amount of Si solid solution increases. By subjecting this plate to a pre-aging treatment, clusters that become the core of aging precipitation during baking coating are uniformly and finely dispersed in the matrix, and the plate has excellent BH properties. Furthermore, the spheroidization of the compound and the crystal grain size of an appropriate size are realized to improve the bendability. By these, the 6000 series alloy plate excellent in BH property, bendability, and rough skin property can be manufactured.

  The twin belt casting method is a method in which a molten slab is cast by pouring molten metal between rotating belts facing each other up and down and solidifying the molten metal by cooling from the belt surface. In the present invention, a slab having a thickness of 5 to 15 mm is cast by a twin belt casting method. If the slab thickness exceeds 15 mm, it is difficult to wind the thin slab around the coil. If the slab thickness is less than 5 mm, the productivity is lowered and casting of the thin slab becomes difficult.

  By casting a slab having a thickness of 5 to 15 mm by a twin belt casting method, the cooling rate during solidification at a slab thickness of 1/4 can be set to 40 to 150 ° C./s. When the cooling rate is less than 40 ° C./s, the cast structure generated at the time of solidification at the center of the slab becomes rough, leading to a decrease in bendability. When the cooling rate at the time of solidification exceeds 150 ° C./s, the β-AlFeSi phase, The α-Al (Fe · Mn) Si phase becomes finer, the recrystallization nuclei are reduced, the size of the recrystallized grains is coarsened to 30 μm or more, and the bendability and the rough skin property are lowered.

After rolling up the thin slab, cold rolling (first cold rolling) is performed, and intermediate solution treatment is performed in the middle of this cold rolling by a continuous annealing furnace, thereby removing the accumulated strain caused by recrystallization. The strain accumulation amount can be controlled in the subsequent cold rolling step (second cold rolling) with a final cold rolling rate of 15 to 30%, and the grain size of the recrystallized grains is set to 20 to 30 μm. be able to. Further, the intermediate solution treatment promotes the spheroidization of the β-AlFeSi phase and the transformation from the β-AlFeSi phase to the α-Al (Fe · Mn) Si phase, and the relatively fine Mg ₂ Si remaining in the cast structure. Thus, it is possible to obtain a coil in which Mg and Si are dissolved in the matrix, to improve the effect of the final solution treatment after the final cold rolling, and to improve the BH property and the bendability.

  As a solution treatment, it is preferable to heat to 540-580 degreeC with the temperature increase rate of 10 degrees C / s or more with a continuous annealing furnace, and hold | maintain within 30 seconds.

  In the solution treatment, the reason for limiting the temperature increase rate to the solution treatment temperature to 10 ° C./s or more is that when the temperature increase rate is less than 10 ° C./s, the coil feed rate becomes extremely slow, As a result, the processing time becomes long and the cost becomes high. In particular, in the final solution treatment, when the heating rate is less than 10 ° C./s, the crystal grain size exceeds 30 μm, and the rough surface property during molding is reduced.

The reason why the solution treatment temperature is in the range of 540 to 580 ° C. is that when the temperature is less than 540 ° C., the spheroidization of the β-AlFeSi phase is insufficient, and Mg ₂ Si crystallized during casting is dissolved in the matrix. This is because the intermetallic compound existing at the grain boundary melts and causes burning when the temperature exceeds 580 ° C.

The reason for limiting the solution treatment time to 30 seconds or less is that if solution treatment is performed within 30 seconds, Mg ₂ Si crystallized during casting or cooling of the thin slab is sufficiently dissolved in the matrix, so that the solution treatment is 30 This is because Mg and Si in the matrix will be saturated even if it is performed for more than 2 seconds, and the feeding speed of the coil will become too slow, resulting in a longer processing time and higher costs.

  This solution treatment is usually preferably carried out in a continuous annealing furnace (CAL). Continuous annealing furnace (CAL) is equipment for continuously solution treatment of coils, etc., equipped with induction heating device for heat treatment, water tank for water cooling, air nozzle for air cooling, etc. It is characterized by.

Cool rapidly after solution treatment. This rapid cooling is cooling at a cooling rate that does not cause precipitation of age hardening such as Mg ₂ Si during cooling. In particular, the temperature-decreasing curve from the solution temperature is shorter than the precipitation nose of the TTT diagram. Cool to pass. In the case of the 6000 series composition, the tip of the precipitation nose is around 350 ° C. Therefore, it is generally sufficient that the rapid cooling from the solution temperature is carried out to about 250 ° C. at a speed (for example, 50 ° C./s) avoiding the tip of the precipitation nose near 350 ° C.
The specific method of rapid cooling after the solution treatment may be air cooling using an air nozzle, cooling by water mist spray, water quenching through a water tank or hot water quenching. . Moreover, the combination of these cooling methods may be sufficient.

  The final cold rolling rate in the second cold rolling (final cold rolling) is limited to 15 to 30%. When the final cold rolling rate is less than 15%, the strain accumulation amount during the second cold rolling becomes small, so the dislocation density is low and the nuclei of recrystallized grains are reduced, and the grain size of the recrystallized grains exceeds 30 μm, Rough skin is reduced. When the final cold rolling rate exceeds 30%, the strain accumulation amount in the second cold rolling process increases and the dislocation density increases, so the grain size of the recrystallized grains becomes less than 20 μm, and the final plate (T4P treated material) ) Increases the yield strength and decreases the bendability. A more preferable final cold rolling rate is 15 to 25%. A more preferable final cold rolling rate is 17 to 23%.

In the case where the coil is cooled after the final solution treatment to perform high-temperature winding, the coil-up temperature is set to 70 to 110 ° C. When the coil-up temperature is less than 70 ° C., it is not sufficient to obtain the effect of pre-aging, and when the coil-up temperature exceeds 110 ° C., Mg ₂ Si, which is originally called β ″ that should be deposited during baking coating, This is because the intermediate phase or the strengthening phase equivalent thereto is precipitated, so that the yield strength is increased and the bendability is inferior.

  Specifically, the final solution treatment is performed by a continuous annealing furnace and water-cooled to 250 ° C. or less at a cooling rate of 10 ° C./s or more, and then air to 70 to 110 ° C. at a cooling rate of 1 to 20 ° C./s. It can be cooled, coiled up and cooled to room temperature.

  The coil after high-temperature winding is inserted into a heat insulation box or an annealing furnace, and gradually cooled to room temperature at a cooling rate of 10 ° C./hr or less. This is because if the cooling rate exceeds 10 ° C./hr, it is not sufficient to obtain the effect of preliminary aging.

When the preliminary aging treatment is performed after cooling to room temperature after the final solution treatment, the preliminary aging temperature is set to 60 to 110 ° C. If the holding temperature is less than 60 ° C., it takes a long time to obtain the effect. If the holding temperature exceeds 110 ° C., an intermediate phase of Mg ₂ Si called β ″ that should be deposited during baking coating. This is because a strengthening phase corresponding to that precipitates, resulting in high yield strength and poor bendability.

The holding time in the preliminary aging treatment after the final solution treatment is 3 to 12 hours. If the holding time is less than 3 hours, the effect cannot be obtained. If the holding time exceeds 12 hours, an intermediate phase of Mg ₂ Si called β ″ that should be deposited during baking coating or a strengthening phase equivalent thereto This is because the yield strength increases and the bendability is impaired.

  Next, the significance and reasons for limitation of the alloy components of the present invention will be described. Mg, which is an essential element, is solid-solved in the matrix after the final solution treatment, and precipitates as a strengthening phase together with Si during coating baking heating to improve the strength. The reason why the amount of Mg added is limited to 0.4 to 0.7% is that if the amount is less than 0.4%, the effect is small, and if it exceeds 0.7%, the bendability after solution treatment decreases. A more preferable range of the Mg content is 0.4 to 0.6%.

Si, which is an essential element, precipitates as an intermediate phase of Mg ₂ Si called β ″ together with Mg during coating baking heating or improves the strength. The content of Si is 0.5 to 1.0%. This is because if the amount is less than 0.5%, the effect is small, and if it exceeds 1.0%, the bendability after the final solution treatment is lowered. 5 to 0.9%.

  Fe, which is an essential element, coexists with Si and Mn to produce many acicular β-AlFeSi phases and α-Al (Fe · Mn) Si phases with a size of 5 μm or less at the time of casting. As a result of the increase, the recrystallized grains are made finer and a plate with excellent formability is obtained. When the Fe content is less than 0.1%, the effect is not remarkable. If it exceeds 0.5%, coarse needle-like β-AlFeSi phase and massive α-Al (Fe · Mn) Si phase are produced at the time of casting, which not only lowers the bendability in the final plate, but also a matrix in a thin slab. In order to reduce the amount of solid solution in Si inside, the BH property in a final board falls. Therefore, the preferable range of the Fe content is 0.1 to 0.5%. A more preferable range of the Fe content is 0.1 to 0.3%.

Mn, which is an essential element, is added as an element that promotes the transformation from the acicular β-AlFeSi phase to the spherical α-Al (Fe · Mn) Si phase and refines the recrystallized grains. In the present invention, the transformation from the acicular β-AlFeSi phase to the spherical α-Al (Fe · Mn) Si phase proceeds by two solution treatments, and Si is dissolved and diffused into the matrix from the β-AlFeSi phase. And the board | substrate excellent in BH property can be obtained because the amount of Si solid solution in a matrix increases.
If the Mn content is less than 0.05%, the effect is not sufficient, the grain size of the recrystallized grains exceeds 30 μm, and the rough skin property decreases. If it exceeds 0.30%, the grain size of the recrystallized grains becomes as fine as less than 20 μm, and the yield strength of the final plate (T4P treated material) becomes high, so that not only the bendability is lowered, but also Al (Fe. Since the crystallization amount of the (Mn) Si phase increases and the solid solution amount of Si in the matrix decreases, the BH property in the final plate decreases. Therefore, the preferable range of the Mn content is 0.05 to 0.30%. A more preferable range of the Mn content is 0.05 to 0.20%.

Even if Ti, which is an essential element, is contained in an amount of 0.10% or less, the effect of the present invention is not hindered, it acts as a crystal grain refining agent for thin slabs, and casting defects such as slab cracks are reliably prevented. can do. When the Ti content is less than 0.005%, the effect is not sufficient, and when the Ti content exceeds 0.10%, a coarse intermetallic compound such as TiAl ₃ is generated during casting, so that the bendability is increased. Is significantly reduced. Therefore, the preferable range of Ti content is 0.005 to 0.10%. A more preferable range of the Ti content is 0.005 to 0.05%.

B, which is an essential element, is inevitably mixed by adding a rod hardener (for example, Al-5% Ti-1% B) as a grain refiner for the ingot. When B is mixed with Ti which is an essential element in the molten metal, the crystal grain refining effect of the ingot is dramatically improved. When the B content is less than 0.0005%, the crystal grain refining effect is not sufficient, and it is difficult to reliably prevent casting defects such as slab cracks. When the B content exceeds 0.01%, not only the crystal grain refining effect of the ingot is saturated, but excess TiB ₂ aggregates act as inclusions in the final plate (T4P treated material). In some cases, the moldability is deteriorated, for example, by generating scratches on the plate surface during molding.

  Cr, which is an optional element, is added as an element that refines the recrystallized grains. For this reason, if it exceeds 0.05%, the grain size of the recrystallized grains becomes as fine as less than 20 μm, and the yield strength in the final plate becomes high. As a result, not only the BH property is lowered, but also Al ( Since the amount of crystallization of the (Fe · Cr) Si phase increases and the amount of Si dissolved decreases, the BH property in the final plate decreases. Therefore, the preferable range of the Cr content is 0.05% or less. A more preferable range of the Cr content is 0.03% or less.

Zr, which is an optional element, is added as an element that refines the recrystallized grains. When the Zr content exceeds 0.05%, a coarse Al ₃ Zr phase is generated during slab casting, and the bendability is lowered. Therefore, the preferable range of the Zr content is 0.05% or less. A more preferable range of the Zr content is 0.03% or less.

  As described above, according to the present invention, it is possible to produce a 6000 series alloy plate for an automobile body sheet that is excellent in BH properties, bendability and skin roughness in a T4P treated material after the final solution treatment at low cost. became. Although an intermediate solution treatment is required, processes such as face milling and hot rolling up to the previous stage are greatly simplified, so that the total manufacturing cost is greatly reduced.

  A synthetic molten metal having the composition shown in Table 1 was melted, degassed, and sedated in a furnace, and then a thin slab having a thickness of 7 mm was cast by a twin belt casting machine. The thin slab was cold-rolled to each plate thickness shown in Table 2 as “plate thickness before intermediate solution”, and then cut into a plate having a predetermined size. However, as shown in Table 2, Sample C is not subjected to intermediate solution.

  Samples A, B, D, E, and F were held in a 560 ° C. × 15 s salt bath (intermediate solution treatment), rapidly quenched with water, and then cold-rolled to a final thickness of 1 mm. . Sample G was kept in an annealer at 350 ° C. for 1 hour and then rapidly quenched with water, and then cold-rolled to a final plate thickness of 1 mm.

  These cold-rolled plates (samples A to G) were held in a 560 ° C. × 15 s salt bath (final solution treatment), then immediately quenched with 85 ° C. hot water, and directly subjected to a heat treatment of 85 ° C. × 8 hr (preliminary). Aging treatment) was performed, and then the mixture was allowed to cool to room temperature and then allowed to stand at room temperature for 1 week. This material was used as the final plate (T4P treatment material) before baking coating. Further, this T4P-treated material was subjected to an aging treatment at 180 ° C. for 30 minutes in an annealer, imitating the thermal history during paint baking. This material was used as a T6P treatment material.

  The room temperature tensile test was performed on both the T4P treated material and the T6P treated material. The results are shown in Table 2. The difference in 0.2% proof stress between the T4P treated material and the T6P treated material was evaluated as BH property. If this value is 100 MPa or more, it is evaluated that the BH property is excellent. Table 2 also shows the evaluation results of the bendability, crystal grain size, and skin roughness of the T4P material.

  Regarding the bendability evaluation, 5% pre-strain was applied to the T4P treated material, and then 180 ° bending was performed at r / t = 0.5 (bending radius r, plate thickness t), and the surface of the bending portion was visually observed. The degree of cracking was evaluated. The evaluation score is 7 levels of 1, 1.5, 2, 2.5, 3, 4, and 5. If the score is 2.5 or less, it is evaluated that the bendability is excellent.

  The crystal grain size was measured by a cross-cut method at a quarter thickness of the cross section in the rolling parallel direction.

  About rough skin property, three-stage evaluation of (circle) (good), (triggering), and x (defect) was performed by visual observation of the surface near the test piece fracture part after the tensile test of the T4P treated material.

  For sample A and sample B which are examples (product of the present invention), an intermediate solution treatment is performed during cold rolling, and the final cold rolling rates are 23% and 17%, respectively. It has excellent properties and has a crystal grain size of 20 to 30 μm and excellent skin roughness.

  In contrast, Sample D and Sample E, which are comparative examples, are subjected to an intermediate solution treatment in the middle of cold rolling, but the final cold rolling rate is 50% and 33%, so the bendability is inferior. . About F sample which is a comparative example, since a final cold rolling rate is 9%, a crystal grain will coarsen and it is inferior to rough skin property. About the sample C which is a comparative example, since the intermediate solution treatment is not performed, both BH property and bendability are inferior. About the sample G which is a comparative example, since it replaced with the intermediate solution treatment and performed the intermediate annealing process, although it is excellent in bendability, it is inferior to BH property and rough skin property.

  ADVANTAGE OF THE INVENTION According to this invention, the manufacturing method of the aluminum alloy plate which made 6000 type | system | group basic composition and improved BH property, bendability, and rough skin prevention property is provided.

Claims (4)

  As essential elements, Mg: 0.40 to 0.70%, Si: 0.50 to 1.00%, Mn: 0.05 to 0.30%, Fe: 0.10 to 0.50%, Ti: 0.005 to 0.10%, B: 0.0005 to 0.01%, Zr: 0.05% or less and Cr: 0.05% or less as optional elements A molten alloy containing a seed and containing the balance Al and inevitable impurities is continuously cast into a thin slab having a thickness of 5 to 15 mm by a twin-belt casting machine, wound directly on a coil, and then first cold rolled. , Intermediate solution, second cold rolling, final solution, and preliminary aging in order, BH property characterized by performing the second cold rolling at a final cold rolling rate of 15-30%, A method for producing an aluminum alloy sheet excellent in bendability and rough skin.   2. The method according to claim 1, wherein the intermediate solution treatment is carried out in a continuous annealing furnace in a temperature range of 540 to 580 [deg.] C. within 30 seconds. In claim 1 or 2, preliminary aging is defined by the following (1) (2):
(1) Treatment of gradually cooling to room temperature at a cooling rate of 10 ° C./hr or less from the temperature range of 70 to 110 ° C. in the cooling process after final solution, or (2) After cooling once after final solution, A method characterized in that it is carried out by any of the processes of reheating to 110 ° C. and holding for 3 to 12 hours.   The method according to any one of claims 1 to 3, wherein heating is performed at a heating rate of 10 ° C / second or more to a temperature of intermediate solution and final solution.