Thick aluminum sheet material production method and thick aluminum sheet material

Abstract

Provided is a method for producing a thick aluminum sheet material, the method including: when a longitudinal direction of an aluminum ingot or an aluminum alloy ingot is defined as a Z direction, a first reducing step of reducing the ingot in the Z direction by hot forging of the ingot to reduce a dimension in the Z direction of the ingot and to obtain a hot forged product, and a second reducing step of reducing the hot forged product in the Z direction by hot rolling of the hot forged product to reduce a dimension in the Z direction of the hot forged product and to obtain a thick aluminum sheet material. The present invention can provide a method for producing a thick aluminum sheet material that can produce a thick sheet material with few internal porosities in the production of a thick aluminum or aluminum alloy sheet material with a large sheet thickness.

Description

TECHNICAL FIELD

[0001] The present invention relates to a thick aluminum or aluminum alloy sheet material with few internal porosities.BACKGROUND ART

[0002] Conventionally, thick aluminum or aluminum alloy sheets have been given an important part as processing materials for vacuum chambers, semiconductor manufacturing equipment, and the like. As equipment becomes larger, there is also a demand for extremely thick sheets with a greater sheet thickness. Thick aluminum alloy sheets for this application require a reduction in the number of internal porosities in order to ensure airtightness and reduce outgassing, improve surface treatment uniformity, and improve mechanical strength (especially fatigue strength). In extremely thick sheets in particular, the internal porosities tend to increase in size and number, thus making countermeasures against this problem a technical challenge.

[0003] Reducing the number of internal porosities is also a common issue for materials in other fields requiring high reliability, such as high-speed railroads, ships, and space and aviation.

[0004] Thick aluminum alloy sheets are generally produced by hot rolling with substantially rectangular parallelepipedal DC ingots (slabs) as original materials. At the stage of this slab, they have some internal porosities. Internal porosities tend to be large in size and number especially near a slab thickness center.

[0005] When a slab of a typical thickness of about 500 mm is hot rolled to a sheet thickness of 100 mm or less by normal hot rolling with a relatively high reduction (80% or more), the crimping and elimination of porosities due to material deformation extend to the vicinity of a sheet thickness center to achieve a state of few internal porosities.

[0006] In contrast, at a stage of being a sheet thickness of, for example, 200 to 400 mm by hot rolling, there is a problem in that the size and number of internal porosities are instead larger than those in the original slab. This is thought to be because light to moderate reduction does not cause enough deformation to crimp the existing porosities, but promotes the expansion and consolidation of the porosities instead, and also causes new porosity generation at grain boundaries and at the interface of intermetallic compound particles. In particular, coupled with the fact that the deformation state is different near a sheet thickness center, it is common that the porosities that have existed since the slab state are not eliminated, but increase in their size and number instead.

[0007] Thus, it is well known that the number of internal porosities of a hot rolled sheet can be reduced by increasing the reduction of hot rolling. However, because of industrial limits on the thickness of DC slabs and the maximum material thickness that can enter a hot rolling mill, there is a limit in trying to increase the reduction of hot rolling, especially when producing thick sheets. Therefore, internal porosities tend to remain in extremely thick sheets.

[0008] Given these circumstances, Patent Literature 1 discloses a technology to reduce the number of porosities of thick aluminum sheets by reducing the amount of hydrogen gas by degassing during casting and mainly controlling the reduction of one pass of hot rolling.CITATION LISTPATENT LITERATURE

[0009] Patent Literature 1: Japanese Patent Publication 2009-090372-ASUMMARY OF INVENTIONTechnical Problem

[0010] However, the method of Patent Literature 1 produces an insufficient effect of improving the reduction in the number of porosities in extremely thick sheets.

[0011] Thus, an object of the present invention is to provide a method for producing a thick aluminum sheet material that can produce a thick sheet material with few internal porosities in the production of a thick aluminum or aluminum alloy sheet material with a large sheet thickness.Solution to Problem

[0012] As a result of intensive studies, the inventors of the present invention have found that, in the production of a thick aluminum or aluminum alloy sheet material with a large sheet thickness, the number of internal porosities can be reduced in a thick sheet material with a large sheet thickness by making a thick sheet material by composite hot working that combines hot forging and hot rolling of an ingot to complete the present invention.

[0013] That is, the present invention (1) provides a method for producing a thick aluminum sheet material, the method including: when a longitudinal direction of an aluminum ingot or an aluminum alloy ingot is defined as a Z direction, a first reducing step of reducing the ingot in the Z direction by hot forging of the ingot to reduce a dimension in the Z direction of the ingot and to obtain a hot forged product; and a second reducing step of reducing the hot forged product in the Z direction by hot rolling of the hot forged product to reduce a dimension in the Z direction of the hot forged product and to obtain a thick aluminum sheet material.

[0014] The present invention (2) provides the method for producing a thick aluminum sheet material according to (1), in which a total reduction in the first reducing step and the second reducing step is 75.0% or more.

[0015] The present invention (3) provides the method for producing a thick aluminum sheet material according to (1) or (2), in which a first reduction in the first reducing step is 60.0% or more.

[0016] The present invention (4) provides the method for producing a thick aluminum sheet material according to (1) or (2), in which a second reduction in the second reducing step is 0.5% or more.

[0017] The present invention (5) provides the method for producing a thick aluminum sheet material according to (1) or (2), in which a sheet thickness of the thick aluminum sheet material is 100 mm or more.

[0018] The present invention (6) provides the method for producing a thick aluminum sheet material according to (1) or (2), in which the dimension in the Z direction of the ingot is 800 mm or more.

[0019] The present invention (7) provides the method for producing a thick aluminum sheet material according to (1) or (2), in which, when a short side direction of the ingot is defined as an X direction, a ratio of the dimension in the Z direction of the ingot to a dimension in the X direction of the ingot (the dimension in the Z direction / the dimension in the X direction) is 2.0 or more.

[0020] The present invention (8) provides a thick aluminum sheet material obtained by performing: when a longitudinal direction of an aluminum ingot or an aluminum alloy ingot is defined as a Z direction, a first reducing step of reducing the ingot in the Z direction by hot forging of the ingot to reduce a dimension in the Z direction of the ingot and to obtain a hot forged product; and a second reducing step of reducing the hot forged product in the Z direction by hot rolling of the hot forged product to reduce a dimension in the Z direction of the hot forged product and to obtain a thick aluminum sheet material.

[0021] The present invention (9) provides a thick aluminum sheet material containing aluminum or an aluminum alloy containing aluminum in an amount of 80 mass % or more, the thick aluminum sheet material having a sheet thickness of 100 mm or more, and the thick aluminum sheet material having a number density of internal porosities of 0.15 / mm 2< or less with a Feret's diameter of 50 µm or more in a central part when, in microscopic observation of a cross section parallel to a sheet thickness direction, the cross section is divided into three parts in the sheet thickness direction.

[0022] The present invention (10) provides a thick aluminum sheet material containing an Al-Mg-based alloy containing Mg in an amount of 1.50 mass % or more, the thick aluminum sheet material having a sheet thickness of 100 mm or more, and the thick aluminum sheet material having a number density of internal porosities of 0.15 / mm 2< or less with a Feret's diameter of 50 µm or more in a central part when, in microscopic observation of a cross section parallel to a sheet thickness direction, the cross section is divided into three parts in the sheet thickness direction. Advantageous Effect of Invention

[0023] The present invention can provide a method for producing a thick aluminum sheet material that can produce a thick sheet material with few internal porosities in the production of a thick aluminum or aluminum alloy sheet material with a large sheet thickness.BRIEF DESCRIPTION OF DRAWINGS

[0024] FIG. 1 is a schematic perspective view of a shape example of an ingot to which hot forging is applied according to the present invention. FIG. 2 is a schematic perspective view of a production process for a thick aluminum sheet material of the present invention. FIG. 3 is a schematic cross-sectional view of the thick aluminum sheet material of the present invention when cut parallel to a sheet thickness direction. FIG. 4 is an observation result of a cross section of a thick aluminum sheet material of Example 1 by fluorescence penetrant inspection. FIG. 5 is an observation result of a cross section of a thick aluminum sheet material of Comparative Example 1 by fluorescence penetrant inspection. FIG. 6 is a SEM observation image of a cross section of the thick aluminum sheet material of Example 1. FIG. 7 is a SEM observation image of a cross section of the thick aluminum sheet material of Comparative Example 1. FIG. 8 is an optical microscope observation image of a cross section of the thick aluminum sheet material of Example 1. FIG. 9 is an optical microscope observation image of a cross section of the thick aluminum sheet material of Comparative Example 1. DESCRIPTION OF EMBODIMENTS

[0025] The thick aluminum alloy sheet material of the present invention is a method for producing a thick aluminum sheet material, the method including: when a longitudinal direction of an aluminum ingot or an aluminum alloy ingot is defined as a Z direction, a first reducing step of reducing the ingot in the Z direction by hot forging of the ingot to reduce a dimension in the Z direction of the ingot and to obtain a hot forged product; and a second reducing step of reducing the hot forged product in the Z direction by hot rolling of the hot forged product to reduce a dimension in the Z direction of the hot forged product and to obtain a thick aluminum sheet material. Note that in the present invention, both a thick sheet material made of pure aluminum and a thick sheet material made of an aluminum alloy are collectively referred to as a thick aluminum sheet material.

[0026] The method for producing a thick aluminum sheet material of the present invention includes a composite hot working process that combines a first reducing step of performing forging and the subsequent second reducing step of performing hot rolling.

[0027] The first reducing step is a step of performing hot forging to reduce an aluminum ingot or an aluminum alloy ingot.

[0028] The aluminum ingot is an object to which hot forging is applied in the first reducing step, and is an ingot made of 1000 series pure aluminum. The aluminum alloy ingot is an object to which hot forging is applied in the first reducing step, and is an ingot made of an aluminum alloy. There are no particular restrictions on the kind of the aluminum alloy, and examples thereof include 2000, 3000, 4000, 5000, 6000, 7000, and 8000 series aluminum alloys. In particular, 5000 series and 6000 series aluminum alloys are useful for aluminum alloys for semiconductor manufacturing equipment.

[0029] Examples of the ingot to which hot forging is applied (the ingot before being subjected to hot forging) include an ingot produced by usual semi-continuous casting (DC casting). For semi-continuous casting of aluminum or the aluminum alloy, there is a common method in which molten aluminum or aluminum alloy is fed from the top of a casting mold of a given shape, which is pierced at both ends and placed horizontally, cooled and solidified in the casting mold by, for example, pouring cooling water to the lower part of the casting mold, and discharged downward. In semi-continuous casting, the horizontal direction of the casting mold corresponds to an X direction and a Y direction of the ingot described below, and the casting direction corresponds to a Z direction of the ingot described below.

[0030] In the first reducing step, the shape of the ingot to which hot forging is applied (the ingot before being subjected to hot forging) is a rectangular parallelepiped. The dimensions of the ingot to which hot forging is applied will be described with reference to FIG. 1. FIG. 1 is a schematic perspective view of a shape example of the ingot to which hot forging is applied according to the present invention. In FIG. 1, the shape of this ingot 1 to which hot forging is applied is a rectangular parallelepiped, and the direction in which the longest side out of the three kinds of sides of the ingot 1 extends is a longitudinal direction, and the longitudinal direction is defined as the Z direction. The direction in which the shortest side out of the three kinds of sides of the ingot 1 extends is a short side direction, and the short side direction is defined as the X direction. The direction in which the second longest side out of the three kinds of sides of the ingot 1 extends is defined as the Y direction. Note that the Z direction, the Y direction, and the X direction are orthogonal to each other.

[0031] In the first reducing step, the dimension in the Z direction of the ingot to which hot forging is applied is preferably 800 mm or more, more preferably 1,000 mm or more, and more preferably 1,500 mm or more. In the first reducing step, the dimension in the X direction of the ingot to which hot forging is applied is about 350 to 700 mm.

[0032] In the first reducing step, the ratio of the dimension in the Z direction to the dimension in the X direction of the ingot to which hot forging is applied (the dimension in the Z direction / the dimension in the X direction) is preferably 2.0 or more, more preferably 3.0 or more, and more preferably 3.5 or more. When the ratio of the dimension in the Z direction to the dimension in the X direction of the ingot to which hot forging is applied (the dimension in the Z direction / the dimension in the X direction) is within the above range, the effect of reducing the number of internal porosities increases, and especially the effect of reducing the number of internal porosities near a sheet thickness center increases. The ratio of the dimension in the Z direction to the dimension in the X direction of the ingot to which hot forging is applied (the dimension in the Z direction / the dimension in the X direction) is preferably 6.0 or less in order to ensure safety.

[0033] In the first reducing step, there are no particular restrictions on the method for producing the ingot to which hot forging is applied. For example, normal molten metal treatment can be used. The molten metal treatment results in the amount of hydrogen gas contained in a product of about 0.2 cc / 100 g or less. The resulting ingot can be subjected to homogenization treatment and surface grinding by usual methods. The homogenization treatment temperature can be set as appropriate for each alloy.

[0034] In the first reducing step, the ingot to which hot forging is applied (the ingot before being subjected to hot forging) is reduced by hot forging to reduce the dimension in the Z direction of the ingot. The hot forging performed in the first reducing step is free forging in which the ingot is compressed in the Z direction of the ingot at a temperature that is the recrystallization temperature or more of the aluminum ingot or the aluminum alloy ingot. Free forging more specifically includes upsetting, stretching, and broadening. In hot forging (free forging), constraining force due to friction acts on a surface in contact with a stamping die, and deformation is small; thus, unlike rolling, the strain in a central part is large.

[0035] The temperature of hot forging in the first reducing step has an optimum value for each pure aluminum or aluminum alloy and is not limited to a particular temperature, but the preliminary heating temperature is suitably 300 to 550°C, and the material temperature during hot forging is suitably 300°C to 500°C, for example.

[0036] In the first reducing step, hot forging is performed on an object to be subjected to hot forging until the thickness in the Z direction of a hot forged product obtained by performing hot forging becomes a desired thickness. The number of times of hot forging may be one or two or more. In other words, in the first reducing step, the ingot may be compressed to the desired thickness in a single hot forging operation, or it may be compressed to the desired thickness by performing hot forging operations two or more times on the ingot.

[0037] The first reduction in the first reducing step is preferably 60.0% or more, more preferably 70.0% or more, and more preferably 80.0% or more. The upper limit of the first reduction is not limited here because it is determined by constraints such as equipment, but it is desirably up to about 95%. When the first reduction in the first reducing step is within the above range, the effect of reducing the number of internal porosities increases. Note that in the present invention, the first reduction is a value calculated by the following expression:

[0038] In this way, the first reducing step is performed to obtain a hot forged product. The hot forged product obtained by performing the first step may be subjected to hot rolling in the second reducing step as it is, or the surface of the hot forged product obtained by performing the first step that has been in contact with a stamping die may be surface ground before performing the second reducing step, and the surface ground hot forged product may be subjected to hot rolling in the second reducing step.

[0039] After performing the first reducing step and before performing the second reducing step, the outer shape of the original material for the second reducing step can be adjusted to a rectangular parallelepipedal shape suitable for rolling and adjusted in size by machining, cutting, or side surface grinding as needed. At this time, corners and sides may be rounded or chamfered as needed.

[0040] The second reducing step is a step of performing hot rolling that reduces the hot forged product obtained by performing the first reducing step.

[0041] In the second reducing step, the hot forged product obtained by performing the first reducing step is reduced by hot rolling to reduce the dimension in the Z direction of the hot forged product. The hot rolling performed in the second reducing step is plastic working in which the hot forged product is reduced and extended in the Z direction of a hot rolled product between rotating rolls at a temperature that is the recrystallization temperature of the hot forged product or more. In the second reducing step, the hot forged product obtained by performing the first reducing step is compressed in the Z direction of the hot rolled product to a desired thickness. In hot rolling, shear force is generated by friction between the rolls and the material, and the surface layer part of the material undergoes stronger working than the central part thereof does.

[0042] Note that in the present invention, the Z direction of the hot forged product corresponds to the Z direction of the ingot before being subjected to hot forging in the first reducing step, and is a direction in which the ingot was compressed in the first reducing step. The Z direction of the hot forged product will be described with reference to FIG. 2. FIG. 2 is a schematic perspective view of a production process for the thick aluminum sheet material of the present invention. In FIG. 2, a first reducing step 11 is performed on the ingot 1 to which hot forging is applied to reduce the dimension in the Z direction of the ingot 1. Next, a second reducing step 12 is performed on a hot forged product 2 obtained by performing the first reducing step 11 to reduce the dimension in the Z direction of the hot forged product 2 to obtain a thick aluminum sheet material 3. At this time, the Z direction of the hot forged product 2 is the Z direction of the ingot 1 before being subjected to hot forging, and is a direction in which the ingot was compressed by hot forging. In other words, the Z direction of the hot forged product 2 is the sheet thickness direction of the thick aluminum sheet material 3 obtained by performing the second reducing step 12. In other words, from the perspective of the finally obtained thick aluminum sheet material 3, the thickness direction of the thick aluminum sheet material 3 is the Z direction of the thick aluminum sheet material 3, also the Z direction of the hot forged product 2, and also the Z direction of the ingot 1.

[0043] The temperature of hot rolling in the second reducing step has an optimum value for each pure aluminum or aluminum alloy and is not limited to a particular temperature, but the material temperature during hot rolling is suitably 400°C to 500°C, for example. In hot rolling in the second reducing step, dimensional accuracy and surface smoothness can be managed substantially in the same way as in the hot rolling of normal sheet materials.

[0044] In the second reducing step, hot rolling is performed on the object to be subjected to hot rolling until the thickness in the Z direction of the thick aluminum sheet material obtained by performing hot rolling becomes a desired thickness. The number of times of hot rolling may be one or two or more. In other words, in the second reducing step, the hot forged product may be compressed to the desired thickness in a single hot rolling operation, or it may be compressed to the desired thickness by performing hot rolling operations two or more times on the hot forged product.

[0045] The second reduction in the second reducing step is preferably 0.5% or more, more preferably 15.0% or more, and more preferably 30.0% or more. The upper limit is not limited to a particular value, but it is desirably up to about 80%. When the second reduction in the second reducing step is within the above range, the effect of reducing the number of internal porosities increases. Note that in the present invention, the second reduction is a value calculated by the following expression:

[0046] The total reduction in the first reducing step and the second reducing step is preferably 75.0% or more, more preferably 85% or more, and more preferably 90% or more. When the total reduction in the first reducing step and the second reducing step is within the above range, the effect of reducing the number of internal porosities increases. Note that in the present invention, the total reduction in the first reducing step and the second reducing step is a value calculated by the following expression: Note that in the case of performing surface grinding (intermediate surface grinding) after hot forging, the above total reduction is calculated as: In other words, it is expressed by the following expression: Total reduction = Z 0 − Z 3 × Z 1 / Z 2 / Z 0 × 100 or Total reduction in first reducing step and second reducing step % = 100 − 100 − RD 1 × 100 − RD 2 / 100 Z0: dimension in Z direction of ingot before being subjected to first hot forging Z1: dimension in Z direction after end of hot forging Z2: dimension in Z direction at start of hot rolling Z3: dimension in Z direction after end of hot rolling RD1: reduction in first reducing step (hot forging) RD2: reduction in second reducing step (hot rolling)

[0047] In the method for producing a thick aluminum sheet material of the present invention, after performing the first reducing step and the second reducing step, cutting to a certain shape, annealing, heat treatment (solution treatment and quenching, aging treatment), or shape correction (stretching, compression, or the like) can be performed as needed.

[0048] Conditions for annealing and heat treatment vary depending on the alloy system, and they can be performed in conformity with standard conditions described, for example, in "Aluminum Handbook, 6th Edition" pp. 9 to 11.

[0049] As to the final temper of the thick sheet material, H112 and O are predominant in non-heat-treated alloys (3000 series, 5000 series, and pure aluminum = 1000 series).

[0050] For heat-treated alloys (2000, 6000, and 7000 series), in addition to H112 and O, precipitation-aged T3, T4, T6, T7 and variations of these (such as T651 and T7) are the final temper of the thick sheet material.

[0051] The temper of each alloy is specified in JIS H0001 and JIS H4000.

[0052] In this way, the method for producing a thick aluminum sheet material of the present invention can produce a thick aluminum sheet material with no internal porosities or a very reduced number of internal porosities, if any.

[0053] In hot forging (free forging), the material near the surface in contact with a stamping die is constrained by friction, and a material flow near the center in the compression direction becomes dominant. In hot rolling, shear force is generated by friction between the rolls and the material, and the surface layer part of the material working than the central part thereof does, and the material flow near the surface becomes dominant. In the method for producing a thick aluminum sheet material of the present invention, hot forging is performed first, followed by hot rolling, thereby combining the above actions and effects of hot forging and hot rolling, and the number of internal porosities can be reduced.

[0054] The thickness of the thick aluminum sheet material obtained by performing the method for producing a thick aluminum sheet material of the present invention is preferably 100 mm or more, and more preferably 150 mm or more. In the conventional method for producing a thick sheet by hot rolling alone, the presence of internal porosities is particularly problematic for a thick sheet material in which the sheet thickness of a thick aluminum sheet to be produced is 100 mm or more. The method for producing a thick aluminum sheet material of the present invention can be used for the production of the thick aluminum sheet with a sheet thickness of less than 100 mm, but the effect of reducing the number of internal porosities by the method for producing a thick aluminum sheet material of the present invention is efficiently shown when a thick sheet material in which the sheet thickness of a thick aluminum sheet material is 100 mm or more is produced. Note that the upper limit of the thickness (the final sheet thickness) of the thick aluminum sheet material depends on the working capability range of a hot rolling mill. For example, if the rolling mill is capable of rolling from 600 mm, it is possible to produce a thick sheet material with a sheet thickness of 570 mm. This makes it possible to produce a thick aluminum sheet material having a larger thickness than the dimension in the X direction (the slab thickness) of the original ingot and to reduce the number of internal porosities. For example, it is possible to produce a thick aluminum sheet material with a reduced number of internal porosities and a sheet thickness of 550 mm using an ingot with a dimension in the X direction of 500 mm.

[0055] The thick aluminum sheet material obtained by performing the method for producing a thick aluminum sheet material of the present invention has very few internal porosities of 50 µm or more, and in particular, the number density of internal porosities of 50 µm or more in the central part of the sheet thickness is very smaller than the number density in the central part of the sheet thickness of the thick sheet material obtained by the conventional method of production. The thick aluminum sheet material obtained by performing the method for producing a thick aluminum sheet material of the present invention has very few internal porosities of 50 µm or more, and thus has higher fatigue strength than that of the thick sheet material obtained by the conventional method of production.

[0056] For example, with an ingot with a dimension in the X direction of 600 mm, by setting the final sheet thickness to about 60 mm or less (a reduction of about 90% or more) by hot rolling that reduces in the X direction alone without performing hot forging, an aluminum sheet with relatively few internal porosities can be obtained. On the other hand, with an ingot with a dimension in the X direction of 600 mm, by setting the final sheet thickness to about 100 mm or more (a reduction of about 83% or less) by hot rolling alone without performing hot forging, a thick aluminum sheet material with few internal porosities cannot be obtained. To obtain an extremely thick sheet with a sheet thickness of 100 mm or more by hot rolling with a reduction of 90%, which is the same as the above 60 mm sheet, a slab with a dimension in the X direction of 1,200 mm or more is required to be reduced by hot rolling in the X direction. Casting and hot rolling of such a slab are difficult to implement within industrial equipment specifications and normal conditions.

[0057] In contrast, the method for producing a thick aluminum sheet material of the present invention differs fundamentally from a normal step in that it reduces in the Z direction an ingot with a dimension in the Z direction of preferably 800 mm or more, more preferably 1,000 mm or more, and more preferably 1,500 mm or more. For example, with a slab having a dimension in the Z direction of 1,200 mm or more, by performing hot forging in the first reducing step, and then performing hot rolling in the second reducing step to reduce it in the Z direction with a total reduction of 90% or more, a thick aluminum sheet material with few internal porosities can be obtained even if the final sheet thickness is 100 mm or more.

[0058] The thick aluminum sheet material of a first aspect of the present invention is a thick aluminum sheet material obtained by performing: when a longitudinal direction of an aluminum ingot or an aluminum alloy ingot is defined as a Z direction, a first reducing step of reducing the ingot in the Z direction by hot forging of the ingot to reduce a dimension in the Z direction of the ingot and to obtain a hot forged product; and a second reducing step of reducing the hot forged product in the Z direction by hot rolling of the hot forged product to reduce a dimension in the Z direction of the hot forged product and to obtain a thick aluminum sheet material.

[0059] The first reducing step and the second reducing step according to the thick aluminum sheet material of the first aspect of the present invention are the same as the first reducing step and the second reducing step according to the method for producing a thick aluminum sheet material of the present invention. The internal porosities are efficiently compressed to disappear by composite hot working by hot forging and rolling.

[0060] The thick aluminum sheet material of a second aspect of the present invention is a thick aluminum sheet material containing aluminum or an aluminum alloy containing aluminum in an amount of 80 mass % or more, the thick aluminum sheet material having a sheet thickness of 100 mm or more, and the thick aluminum sheet material having a number density of internal porosities of 0.15 / mm 2< or less with a Feret's diameter of 50 µm or more in a central part when, in microscopic observation of a cross section parallel to a sheet thickness direction, the cross section is divided into three parts in the sheet thickness direction.

[0061] The thick aluminum sheet material of the second aspect of the present invention is aluminum or an aluminum alloy containing aluminum in an amount of 80 mass % or more, which ensures the ductility of the material, and the internal porosities are efficiently compressed to disappear by composite hot working by hot forging and rolling. The aluminum or the aluminum alloy according to the thick aluminum sheet material of the second aspect of the present invention more desirably contains aluminum in an amount of 90 mass % or more.

[0062] The sheet thickness of the thick aluminum sheet material of the second aspect of the present invention is 100 mm or more, and preferably 150 mm or more.

[0063] In the thick aluminum sheet material of the second aspect of the present invention, the number density of internal porosities is 0.15 / mm 2< or less, and preferably 0.10 / mm 2< or less with a Feret's diameter of 50 µm or more in a central part when, in microscopic observation of a cross section parallel to a sheet thickness direction, the cross section is divided into three parts in the sheet thickness direction. When the number density of internal porosities is within the above range with a Feret's diameter of 50 µm or more in the central part of the cross section parallel to the sheet thickness direction, the fatigue strength increases.

[0064] In the present invention, the number density of internal porosities with a Feret's diameter of 50 µm or more in a central part when the cross section is divided into three parts in the sheet thickness direction is an analysis value in scanning electron microscopy (SEM) of the cross section parallel to the sheet thickness direction. In scanning electron microscopy, first, a cross section parallel to the sheet thickness direction is smoothly polished as appropriate to prepare a cross section to be analyzed, and then the cross section to be analyzed is observed with a scanning electron microscope to obtain a SEM image of the cross section to be analyzed. Next, in the obtained SEM image of the cross section, a central part when divided into three parts in the sheet thickness direction is sectioned, and the number of internal porosities with a Feret's diameter of 50 µm or more present in the section and the area (mm 2< ) of the section are determined to calculate the number density of internal porosities with a Feret's diameter of 50 µm or more. In the present invention, for the analysis of the number density of internal porosities with a Feret's diameter of 50 µm or more in the central part when the cross section is divided into three parts in the sheet thickness direction, optical microscopic observation is used instead of SEM observation to obtain an observation image, and the number density of internal porosities with a Feret's diameter of 50 µm or more can be determined in the obtained optical microscope observation image by the same method as in the above SEM observation.

[0065] The central part and the entire cross-sectional area when the cross section is divided into three parts in the sheet thickness direction will be described with reference to FIG. 3. FIG. 3 is a schematic cross-sectional view of a thick aluminum sheet material 20 when cut parallel to a sheet thickness direction 13. In FIG. 3, when a cross section 11 is divided equally into three parts in the sheet thickness direction 13, the middle part is a central part 14, and the parts on both sides thereof are an outer part 15a and an outer part 15b. The outer part 15a, the central part 14, and the outer part 15b, that is, the area from one surface 12a to another surface 12b of the thick aluminum sheet material 20 is an entire cross-sectional area 16.

[0066] The thick aluminum sheet material of the second aspect of the present invention is suitably produced by performing the method for producing a thick aluminum sheet material of the present invention described above, using an aluminum alloy ingot of aluminum or an aluminum alloy containing aluminum in an amount of 80 mass % or more, and preferably 90 mass % or more as the ingot to be hot forged in the first reducing step.

[0067] The thick aluminum sheet material of a third aspect of the present invention is a thick aluminum sheet material containing an Al-Mg-based alloy containing Mg in an amount of 1.50 mass % or more, the thick aluminum sheet material having a sheet thickness of 100 mm or more, and the thick aluminum sheet material having a number density of internal porosities of 0.15 / mm 2< or less with a Feret's diameter of 50 µm or more in a central part when, in microscopic observation of a cross section parallel to a sheet thickness direction, the cross section is divided into three parts in the sheet thickness direction.

[0068] The thick aluminum sheet material of the third aspect of the present invention contains an Al-Mg-based alloy containing Mg in an amount of 1.50 mass % or more. The Mg content of the Al-Mg-based alloy according to the thick aluminum sheet material of the third aspect of the present invention is 1.50 mass % or more, preferably 2.2 to 6.0 mass %, and more preferably 3.5 to 6.0 mass %. Examples of the Al-Mg-based alloy according to the thick aluminum sheet material of the third aspect of the present invention include 5000 series alloys. The Mg content of the Al-Mg-based alloy being within the above range makes it suitable for a thick sheet used in areas subjected to stress. On the other hand, even if the Mg content of the Al-Mg-based alloy is less than the above range, the effect of reducing the number of porosities can be obtained, but it is not suitable for use as a structural member. Examples of the Al-Mg-based alloy according to the thick aluminum sheet material of the third aspect of the present invention include an Al-Mg-based alloy containing Mg in an amount of 1.50 mass % or more, preferably 2.20 to 6.00 mass %, and more preferably 3.50 to 6.00 mass %, with a balance of Al and unavoidable impurities. Further examples of the Al-Mg-based alloy according to the thick aluminum sheet material of the third aspect of the present invention include an Al-Mg-based alloy containing Mg in an amount of 1.50 mass % or more, preferably 2.20 to 6.00 mass %, and more preferably 3.50 to 6.00 mass %, and further containing one or more selected from the group consisting of 0.05 to 0.35 mass % of Cu, 0.05 to 0.35 mass % of Zn, 0.05 to 0.35 mass % of Cr, 0.05 to 1.0 mass % of Mn, 0.05 to 0.35 mass % of Zr, 0.05 to 0.35 mass % of Sc, 0.05 to 0.35 mass % of V, 0.05 to 0.35 mass % of Ni, 0.005 to 0.20 mass % of Ti, 0.001 to 0.04 mass % of Be, and 0.001 to 0.02 mass % of B, with a balance of Al and unavoidable impurities.

[0069] The sheet thickness of the thick aluminum sheet material of the third aspect of the present invention is 100 mm or more, and preferably 150 mm or more. The internal porosities are efficiently compressed to disappear by composite hot working by hot forging and rolling.

[0070] In the thick aluminum sheet material of the third aspect of the present invention, the number density of internal porosities is 0.15 / mm 2< or less, and preferably 0.10 / mm 2< or less with a Feret's diameter of 50 µm or more in a central part when, in microscopic observation of a cross section parallel to a sheet thickness direction, the cross section is divided into three parts in the sheet thickness direction. When the number density of internal porosities is within the above range with a Feret's diameter of 50 µm or more in the central part of the cross section parallel to the sheet thickness direction, the fatigue strength increases.

[0071] The thick aluminum sheet material of the third aspect of the present invention is suitably produced by performing the method for producing a thick aluminum sheet material of the present invention described above, using an aluminum alloy ingot with an Mg content of 1.50 mass % or more as the ingot to be hot forged in the first reducing step.

[0072] Examples are given below to specifically describe the present invention, but the present invention is not limited to the examples shown below.[Examples](Example 1)

[0073] A DC slab (JIS 5083 alloy, thickness: 600 mm (the dimension in the X direction), width: 1,505 mm (the dimension in the Y direction), length: 2,090 mm (the dimension in the Z direction)) with a chemical composition listed in Table 1 produced by semi-continuous casting was heated to a temperature of 360 to 420°C, which was the recrystallization temperature (350°C) or more, and hot forging in which the DC slab was reduced in the Z direction to be compressed a plurality of times until the dimension in the Z direction became 350 mm was performed with a 15,000 tf hot forging press. The first reduction at this time was 83.2%.

[0074] Next, the material was surface ground until the dimension in the Z direction became 334 mm, reheated to a temperature of 530°C, which was the recrystallization temperature (350°C) or more, reduced in the Z direction in a plurality of passes with a hot rolling mill, and rolled until the dimension in the Z direction became 180 mm to obtain a thick sheet material. The second reduction at this time was 45.6%. The total reduction in the first reducing step and the second reducing step was 90.9%. The total reduction was calculated as follows: (Comparative Example 1)

[0075] A JIS 5083 alloy thick sheet obtained by hot rolling a DC slab with a chemical composition listed in Table 1 in a normal step to be reduced from 600 mm to 180 mm in the dimension in the X direction was used as a comparative material. The reduction of this hot rolling is about 70%.<Evaluation of Internal Porosities by Fluorescence Penetrant Inspection>

[0076] A cross section in the thickness direction of the thick sheet material was observed by fluorescence penetrant inspection. The results are illustrated in FIG. 4 (Example 1) and FIG. 5 (Comparative Example 1).

[0077] The operation and conditions of the fluorescence penetrant inspection are as follows.

[0078] After a cross section in the rolling direction of the thick sheet material was surface ground and cleaned, a fluorescent penetrant (Super Glow OD-2800N, manufactured by Marktec Corporation) was applied to a surface to be observed, and caused to penetrate into porosities, and then the penetrant remaining on the surface was cleaned and removed. This was dried and observed under a black light to visually recognize the part where the fluorescent penetrant was present and seeping out as light spots. The light spots can be regarded as the present positions of porosities.

[0079] In the results of observation, for the same 180 mm thick 5083 alloy thick sheet material, in the hot rolled material with normal hot rolling alone of Comparative Example 1, luminous spots representing internal porosities were present in clusters in the area of about 1 / 3 of the sheet thickness in the central part of the thickness of the cross section. In contrast, in the cross section of the thick sheet material of Example 1, neither significant luminous spots nor clusters of luminous spots were observed. From these facts, it can be seen that with the method of production of the present invention, the internal porosities of the thick sheet material almost disappear, and the problem of internal porosities is almost resolved.<Evaluation of Internal Porosities by Microscopic Observation>

[0080] A cross section parallel to the thickness direction of the thick sheet material was analyzed with a scanning electron microscope (SEM). The results are illustrated in FIG. 6 (Example 1) and FIG. 7 (Comparative Example 1). A cross section in the thickness direction of the thick sheet material was analyzed with an optical microscope. The results are illustrated in FIG. 8 (Example 1) and FIG. 9 (Comparative Example 1). Note that in FIGS. 6 to 9, small black spots with a maximum Feret's diameter (hereinafter simply referred to as the Feret's diameter) of less than 50 µm and mostly 5 to 20 µm are observed, but many intermetallic compound particles of Mg 2 Si, which is composed of lighter elements than Al, appear black in SEM, and it is confirmed by analysis that they are not internal porosities. However, some of the black spots include holes created by the falling of intermetallic compound particles containing Mg 2 Si or transition elements such as Fe during polishing for the preparation of the sample to be observed. Of course, in the comparative material in which the number of internal porosities was not reduced, fine internal porosities also exist and are observed as black spots, and it is not easy to distinguish these from each other. Therefore, as indices for comparing and evaluating the effect of reducing the number of internal porosities in the present invention, the number densities of porosities with a Feret's diameter of 50 µm or more and porosities with a Feret's diameter of 100 µm or more were used.

[0081] Next, the vicinity of the center of the sheet thickness was observed with a SEM, image analysis of the obtained SEM image was performed, and the numbers per unit area (the number densities) of the internal porosities with a Feret's diameter of 50 µm or more and the internal porosities with a Feret's diameter of 100 µm or more in an area with a 1 / 3 thickness of the sheet thickness with the center in the sheet thickness direction as the center (a central part when divided into three parts in the sheet thickness direction) were determined. Table 2 lists the results. [Table 1]Chemical composition (mass %)CuSiFeMnMgZnCrTiBAlExamples 1 and 20.030.140.260.654.340.020.090.020.002BalanceComparative Examples 1 and 20.050.140.240.674.380.020.090.020.002Balance [Table 2] Example 1Comparative Example 1Number density ( / mm 2< )50 µm or more internal porosities00.8100 µm or more internal porosities00.3

[0082] In the results of analysis, the hot rolled material with the normal hot rolling alone of Comparative Example 1 contained internal porosities inside, whereas the internal porosities disappeared in Example 1. From these facts, it can be seen that with the method of production of the present invention, the internal porosities of the thick sheet material almost disappear, and the problem of internal porosities is almost resolved.<Evaluation by Rotary Bending Fatigue Strength>

[0083] The fact that the elimination of porosities by the method of the present invention leads to improvement in mechanical properties will be described through an example of rotary bending fatigue strength.

[0084] A rotary bending fatigue test was conducted in conformity with JIS Z2278. A test piece (length: 80 mm, parallel part diameter: 8 mm) was collected from an area with a 1 / 3 thickness of the sheet thickness with the center in the sheet thickness direction as the center (a central part when divided into three parts in the sheet thickness direction) of the thick sheets (5083-H112) after hot rolling with a sheet thickness of 180 mm of Example 1 and Comparative Example 1 such that the rolling direction was the longitudinal direction of the test piece. Evaluation was made in terms of fatigue strength on the basis of 10 to the seventh power times of repeated stress.

[0085] This result showed that the rotary bending fatigue strength was 152 MPa for Example 1 as the present invention example and 69 MPa for Comparative Example 1. In the normally hot rolled material of the comparative example, the internal porosities present near the center of the sheet thickness act in the same way as a notch, reducing the fatigue strength. The example of the present invention, as a result of the elimination of the internal porosities, had significantly higher fatigue strength than that of the comparative example.(Example 2)

[0086] A DC slab (JIS 5083 alloy, thickness: 600 mm (the dimension in the X direction), width: 1,505 mm (the dimension in the Y direction), length: 2,090 mm (the dimension in the Z direction)) with the same chemical composition as that of Example 1 produced by semi-continuous casting was heated to a temperature of 360 to 420°C, which was the recrystallization temperature (350°C) or more, and hot forging in which the DC slab was reduced in the Z direction to be compressed a plurality of times until the dimension in the Z direction became 350 mm was performed with a 15,000 tf hot forging press. The first reduction at this time was 83.2%.

[0087] Next, the material was surface ground until the dimension in the Z direction became 334 mm, reheated to a temperature of 530°C, which was the recrystallization temperature or more, reduced in the Z direction in one pass with a hot rolling mill, and rolled until the dimension in the Z direction became 331 mm to obtain a thick sheet material. The second reduction at this time was 0.9%. The total reduction in the first reducing step and the second reducing step was 83.4%.

[0088] Next, the vicinity of the center of the sheet thickness was observed with a scanning electron microscope (SEM), image analysis of the obtained SEM image was performed, and the numbers per unit area (the number densities) of the internal porosities with a Feret's diameter of 50 µm or more and the internal porosities with a Feret's diameter of 100 µm or more in an area with a 1 / 3 thickness of the sheet thickness with the center in the sheet thickness direction as the center were determined. In the results, the number density of the internal porosities was 0 / mm 2< with a Feret's diameter of 50 µm or more, and the number density of the internal porosities was 0 / mm 2< with a Feret's diameter of 100 µm or more.(Comparative Example 2)

[0089] A JIS 5083 alloy thick sheet obtained by hot rolling a DC slab with the same chemical composition as that of Comparative Example 1 in the normal step to be reduced from 600 mm to 331 mm in the dimension in the X direction was used as a comparative material. The reduction during this hot rolling is about 45%.

[0090] Next, the vicinity of the center of the sheet thickness was observed with a scanning electron microscope (SEM), image analysis of the obtained SEM image was performed, and the numbers per unit area (the number densities) of the internal porosities with a Feret's diameter of 50 µm or more and the internal porosities with a Feret's diameter of 100 µm or more in an area with a 1 / 3 thickness of the sheet thickness with the center in the sheet thickness direction as the center were determined. In the results, the number density of the internal porosities was 0.9 / mm 2< with a Feret's diameter of 50 µm or more, and the number density of the internal porosities was 0.4 / mm 2< with a Feret's diameter of 100 µm or more.

[0091] The rotary bending fatigue strength of the material in an area with a 1 / 3 thickness of the sheet thickness with the center in the sheet thickness direction as the center of the 5083 alloy H112 material was 179 MPa for Example 2 and 76 MPa for Comparative Example 2.

[0092] From these results, it can be seen that with the method of production of the present invention, the internal porosities of the thick sheet material almost disappear, and the problem of internal porosities is almost resolved. In addition, it can be seen that the reduction of the number of internal porosities by the method of production of the present invention also leads to a marked improvement in fatigue strength.(Example 3)

[0093] A DC slab (JIS 5052 alloy, after cutting and surface grinding, thickness: 630 mm (the dimension in the X direction), width: 1,505 mm (the dimension in the Y direction), length: 1,968 mm (the dimension in the Z direction)) with a chemical composition listed in Table 3 produced by semi-continuous casting was heated to a temperature of 360 to 420°C, which was the recrystallization temperature (350°C) or more, and hot forging in which the DC slab was reduced in the Z direction to be compressed a plurality of times until the dimension in the Z direction became 488 mm was performed with a 15,000 tf hot forging press. The first reduction in this case was 75.2%.

[0094] Next, the material was surface ground such that the dimension in the Z direction became 457 mm, reheated to a temperature of 530°C, which was the recrystallization temperature or more, and hot rolled until the dimension in the Z direction became 297 mm in multiple passes with a hot rolling mill. The second reduction at this time was 35.0%, and the total reduction was 83.9%.

[0095] Next, the vicinity of the center of the sheet thickness was observed with an optical microscope, image analysis of the obtained image was performed, and the numbers per unit area (the number densities) of the internal porosities with a Feret's diameter of 50 µm or more and the internal porosities with a Feret's diameter of 100 µm or more in an area with a 1 / 3 thickness of the sheet thickness with the center in the sheet thickness direction as the center were determined. In the results, the number density of the internal porosities was 0 / mm 2< with a Feret's diameter of 50 µm or more, and the number density of the internal porosities was 0 / mm 2< with a Feret's diameter of 100 µm or more.(Comparative Example 3)

[0096] A JIS 5052 alloy thick sheet obtained by hot rolling a DC slab with a chemical composition listed in Table 3 in the normal step to be reduced from 533 mm to 297 mm in the dimension in the X direction was used as a comparative material. The reduction during this hot rolling is about 44%.

[0097] Next, the vicinity of the center of the sheet thickness was observed with an optical microscope, image analysis of the obtained image was performed, and the numbers per unit area (the number densities) of the internal porosities with a Feret's diameter of 50 µm or more and the internal porosities with a Feret's diameter of 100 µm or more in an area with a 1 / 3 thickness of the sheet thickness with the center in the sheet thickness direction as the center were determined. In the results, the number density of the internal porosities was 0.23 / mm 2< with a Feret's diameter of 50 µm or more, and the number density of the internal porosities was 0.05 / mm 2< with a Feret's diameter of 100 µm or more. [Table 3]Chemical composition (mass %)CuSiFeMnMgZnCrTiBAlExample 30.020.110.200.022.350.010.200.020.001BalanceComparative Example 30.020.110.220.042.470.010.190.020.000Balance

[0098] The 5052 alloy H112 material of Example 3 clearly has a reduced number of internal porosities compared to that of the normally hot rolled material of Comparative Example 3.

[0099] The rotary bending fatigue strength of the material in an area with a 1 / 3 thickness of the sheet thickness with the center in the sheet thickness direction as the center of these was 115 MPa for Example 3 and 95 MPa for Comparative Example 3, with Example 3 showing a higher value than that of Comparative Example 3.(Example 4)

[0100] A DC slab (JIS 6061 alloy) with a chemical composition in Table 4 was reduced under the same conditions as in Example 3 to obtain a material with a dimension in the Z direction of 297 mm, which was subjected to solution heat treatment that holds at 525°C for 1 h and performs water quenching, and artificial aging treatment that holds at 170°C for 8 h to make a T6 material.

[0101] An image of this 6061 alloy T6 material was analyzed with an optical microscope to determine the numbers per unit area (the number densities) of the internal porosities with a Feret's diameter of 50 µm or more and the internal porosities with a Feret's diameter of 100 µm or more in an area with a 1 / 3 thickness of the sheet thickness with the center in the sheet thickness direction as the center. In the results, the number density of the internal porosities was 0 / mm 2< with a Feret's diameter of 50 µm or more, and the number density of the internal porosities was 0 / mm 2< with a Feret's diameter of 100 µm or more.(Comparative Example 4)

[0102] A 6061 alloy T6 material obtained by hot rolling a DC slab with a chemical composition in Table 4 in the normal step to be reduced from 533 mm to 297 mm in the dimension in the X direction and performing heat treatment in the same manner as in Example 4 was used as a comparative material.

[0103] In the central region of the sheet thickness in this comparative material, the number density of the internal porosities was 0.17 / mm 2< with a Feret's diameter of 50 µm or more, and the number density of the internal porosities was 0.04 / mm 2< with a Feret's diameter of 100 µm or more. [Table 4]Chemical composition (mass %)CuSiFeMnMgZnCrTiBAlExample 40.360.680.320.061.020.040.190.050.002BalanceComparative Example 40.360.690.300.041.000.020.190.050.003Balance

[0104] The 6061 alloy T6 material of Example 4 clearly has a reduced number of internal porosities compared to that of Comparative Example 4 by the normal hot rolling.

[0105] The rotary bending fatigue strength of the material in an area with a 1 / 3 thickness of the sheet thickness with the center in the sheet thickness direction as the center of these was 135 MPa for Example 4 and 95 MPa for Comparative Example 4, with Example 4 showing a higher value than that of Comparative Example 4.

Claims

1. A method for producing a thick aluminum sheet material, the method comprising: when a longitudinal direction of an aluminum ingot or an aluminum alloy ingot is defined as a Z direction, a first reducing step of reducing the ingot in the Z direction by hot forging of the ingot to reduce a dimension in the Z direction of the ingot and to obtain a hot forged product; and a second reducing step of reducing the hot forged product in the Z direction by hot rolling of the hot forged product to reduce a dimension in the Z direction of the hot forged product and to obtain a thick aluminum sheet material.

2. The method for producing a thick aluminum sheet material according to claim 1, wherein a total reduction in the first reducing step and the second reducing step is 75.0% or more.

3. The method for producing a thick aluminum sheet material according to claim 1 or 2, wherein a first reduction in the first reducing step is 60.0% or more.

4. The method for producing a thick aluminum sheet material according to claim 1 or 2, wherein a second reduction in the second reducing step is 0.5% or more.

5. The method for producing a thick aluminum sheet material according to claim 1 or 2, wherein a sheet thickness of the thick aluminum sheet material is 100 mm or more.

6. The method for producing a thick aluminum sheet material according to claim 1 or 2, wherein the dimension in the Z direction of the ingot is 800 mm or more.

7. The method for producing a thick aluminum sheet material according to claim 1 or 2, wherein, when a short side direction of the ingot is defined as an X direction, a ratio of the dimension in the Z direction of the ingot to a dimension in the X direction of the ingot (the dimension in the Z direction / the dimension in the X direction) is 2.0 or more.

8. A thick aluminum sheet material obtained by performing: when a longitudinal direction of an aluminum ingot or an aluminum alloy ingot is defined as a Z direction, a first reducing step of reducing the ingot in the Z direction by hot forging of the ingot to reduce a dimension in the Z direction of the ingot and to obtain a hot forged product; and a second reducing step of reducing the hot forged product in the Z direction by hot rolling of the hot forged product to reduce a dimension in the Z direction of the hot forged product and to obtain a thick aluminum sheet material.

9. A thick aluminum sheet material comprising aluminum or an aluminum alloy containing aluminum in an amount of 80 mass % or more, the thick aluminum sheet material having a sheet thickness of 100 mm or more, and the thick aluminum sheet material having a number density of internal porosities of 0.15 / mm2 or less with a Feret's diameter of 50 µm or more in a central part when, in microscopic observation of a cross section parallel to a sheet thickness direction, the cross section is divided into three parts in the sheet thickness direction.

10. A thick aluminum sheet material comprising an Al-Mg-based alloy containing Mg in an amount of 1.50 mass % or more, the thick aluminum sheet material having a sheet thickness of 100 mm or more, and the thick aluminum sheet material having a number density of internal porosities of 0.15 / mm2 or less with a Feret's diameter of 50 µm or more in a central part when, in microscopic observation of a cross section parallel to a sheet thickness direction, the cross section is divided into three parts in the sheet thickness direction.

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