Aluminum alloy rolled plate and its manufacturing method

Abstract

To provide an aluminum alloy rolled sheet having high strength and excellent formability and stress corrosion cracking resistance, and a production method therefor.SOLUTION: The aluminum alloy rolled sheet has a chemical composition that contains 0.25 mass% or more and 0.50 mass% or less of Mn and 2.8 mass% or more and 3.8 mass% or less of Mg, and a balance composed of Al and inevitable impurities. The aluminum alloy rolled sheet has a tensile strength in a rolling direction of 200 MPa or more and 250 MPa or less, a 0.2% proof strength in the rolling direction of 100 MPa or more and 130 MPa or less, and an elongation in the rolling direction of 23% or more. The hardness at a position having a depth of 1 / 2 the thickness of the sheet from the surface of the aluminum alloy rolled plate is 90% or more and 100% or less of the hardness at a position having the depth from the surface of 1 / 4 the thickness. The total orientation density in crystal orientation in which the (001) plane is oriented perpendicular to the surface is 40 or more in a cross section parallel to the rolling direction.SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] The present invention relates to an aluminum alloy rolled sheet and a method for producing the same. [Background technology]

[0002] Aluminum alloy sheets used for automobile bodies and the like are required to have high strength, excellent formability during press working, and excellent stress corrosion cracking resistance. Patent Document 1, for example, describes an invention relating to the production of aluminum automotive structural parts or components from molten aluminum alloys using a continuous casting machine to cast the molten aluminum alloy into slabs. The aluminum automotive structural part of Patent Document 1 essentially has a chemical composition containing 2.7 to 3.6 wt% Mg, 0.1 to 0.4 wt% Mn, 0.02 to 0.2 wt% Si, 0.05 to 0.25 wt% Fe, up to 0.1 wt% Cu, up to 0.25 wt% Cr, up to 0.2 wt% Zn, and up to 0.15 wt% Ti, with the remainder being aluminum, unnecessary elements, and impurities. [Prior art documents] [Patent documents]

[0003] [Patent Document 1] Japanese Patent Application Laid-Open No. 2008-511756 Summary of the Invention [Problem to be solved by the invention]

[0004] In recent years, automobiles have become increasingly multifunctional, and the shapes of their component parts have become more complex. In order to respond to this situation and form aluminum alloy sheets into more complex shapes, there is a demand for aluminum alloy sheets having even higher formability than the aluminum automotive structural parts produced by the technology of Patent Document 1.

[0005] The present invention has been made in view of the above background, and aims to provide an aluminum alloy rolled sheet having high strength and excellent formability and stress corrosion cracking resistance, and a method for manufacturing the same. [Means for solving the problem]

[0006] One aspect of the present invention is an aluminum alloy rolled plate obtained by rolling an aluminum alloy ingot, Mn (manganese): 0.25% by mass or more and 0.50% by mass or less 、 Mg (Magnesium): 2.8% by mass or more and 3.8% by mass or less , Si: 0.30 mass% or less, Fe: 0.40 mass% or less, Ti: 0.10 mass% or less and , as an optional component, C u: 0 mass% or more 0.10% by mass or less, Cr: 0 mass% or more 0.10% by mass or less and Zn: 0 mass% or more 0.10% by mass or more Below The balance is Al (aluminum) and unavoidable impurities, The tensile strength in the rolling direction is 200 MPa or more and 250 MPa or less, The 0.2% proof stress in the rolling direction is 100 MPa or more and 130 MPa or less, The elongation in the rolling direction is 23% or more, the hardness at a position where the depth from the surface of the aluminum alloy rolled plate is 1 / 2 of the thickness is 90% or more and 100% or less of the hardness at a position where the depth from the surface is 1 / 4 of the thickness, The aluminum alloy rolled sheet has a total orientation density of 40 or more of crystal orientations in which the (001) plane in a cross section parallel to the rolling direction is oriented perpendicular to the surface, as calculated by crystal orientation distribution function analysis.

[0007] Another aspect of the present invention is a method for producing an aluminum alloy rolled sheet according to the above aspect, a casting step of producing an ingot having the chemical composition by DC casting; a homogenization treatment step of heating the ingot to a temperature of 450°C or higher and 570°C or lower to subject the ingot to homogenization treatment; a hot rolling step of hot rolling the ingot that has been subjected to the homogenization treatment under conditions such that the temperature of the ingot at the start is 300°C or more and 550°C or less, and the temperature of the aluminum alloy rolled plate at the completion is 150°C or more, to produce an aluminum alloy rolled plate; a cold rolling step of cold rolling the aluminum alloy rolled plate after the hot rolling so that a total rolling reduction ratio is 40% or more and 80% or less; The aluminum alloy rolled sheet after the cold rolling is annealed in a continuous annealing furnace at a temperature T fa and a final annealing step of heating to a temperature of [°C] and performing final annealing, The method for producing an aluminum alloy rolled plate is characterized in that the temperatures of the ingot and the aluminum alloy rolled plate and the thicknesses of the ingot and the aluminum alloy rolled plate after the homogenization treatment step satisfy the relationship of the following formula (3): 400≦T fa ≦550 (1) -1≦ 4[Mn]+[Mg]-0.01T fa ≦1 (2)

[0008]

number

[0009] In the formula (2), [Mn] is the Mn content (unit: mass%) in the aluminum alloy rolled plate, and [Mg] is the Mg content (unit: mass%) in the aluminum alloy rolled plate. In addition, in the formula (3), n is the number of times that the temperature of the ingot and the aluminum alloy rolled plate has reached 405°C or higher after the homogenization treatment step, and τ k0 is the time when the temperatures of the ingot and the aluminum alloy rolled plate exceed 405°C for the kth time, and τ k1 is the time when the temperatures of the ingot and the aluminum alloy rolled plate fall below 405°C for the kth time, T(τ) is the temperature (unit: °C) of the ingot and the aluminum alloy rolled plate at time τ, and t kis the thickness (unit: mm) of the ingot and the aluminum alloy rolled plate at the time when the temperature of the ingot and the aluminum alloy rolled plate exceeds 405°C for the kth time, and the unit of the infinitesimal time dτ is seconds. [Effects of the Invention]

[0010] The aluminum alloy rolled plate (hereinafter referred to as "rolled plate") has the specific chemical composition, thereby reducing stress corrosion cracking susceptibility at room temperature. In addition to having the specific chemical composition, the rolled plate has a crystal orientation in which the (001) plane is oriented perpendicular to the surface, and the total orientation density of the crystal orientation in a cross section parallel to the rolling direction falls within the specific range. This allows the tensile strength, 0.2% proof stress, and elongation in the rolling direction of the rolled plate to fall within the specific ranges, and also allows the ratio of the hardness at a position at a depth of 1 / 2 of the thickness from the surface of the rolled plate to a position at a depth of 1 / 4 of the thickness to fall within the specific range. The aluminum alloy rolled plate having such a distribution of chemical composition, metal structure, mechanical properties, and hardness has an excellent balance between strength and formability, and can improve formability while maintaining high strength.

[0011] Therefore, the aluminum alloy rolled sheet has high strength, excellent formability and stress corrosion cracking resistance.

[0012] In addition, in the method for producing the aluminum alloy rolled plate, the aluminum alloy rolled plate produced through the casting step, the homogenization treatment step, the hot rolling step, and the cold rolling step is annealed at the specific temperature T fa The aluminum alloy is then heated to a temperature of 405°C and subjected to final annealing. Furthermore, in the above-described production method, the production conditions after the homogenization treatment step are set so that the temperatures of the ingot and the aluminum alloy rolled plate, while they are at or above 405°C, and the thickness of the aluminum alloy rolled plate satisfy the relationship of the following formula (3). This makes it possible to easily obtain the aluminum alloy rolled plate.

[0013] As described above, according to the above-described embodiment, it is possible to provide an aluminum alloy rolled sheet having high strength and excellent formability and stress corrosion cracking resistance, and a method for manufacturing the same. [Brief explanation of the drawings]

[0014] [Figure 1] FIG. 1 is a graph showing the relationship between the heating temperature in the final annealing step and the tensile strength and 0.2% proof stress in Example 1. DETAILED DESCRIPTION OF THE INVENTION

[0015] (rolled aluminum alloy plate) The chemical composition, mechanical properties, and crystal orientation distribution of the aluminum alloy rolled sheet will be described below.

[0016] [Chemical composition] ·Mn: 0.25 mass% or more and 0.50 mass% or less The rolled sheet contains 0.25% by mass or more and 0.50% by mass or less of Mn as an essential component. By setting the Mn content within the specific range, the strength and formability of the rolled sheet can be improved in a balanced manner. If the Mn content is less than 0.25% by mass, the strength of the rolled sheet may be reduced. In addition, in this case, the crystal grains of the rolled sheet may be easily coarsened. On the other hand, if the Mn content exceeds 0.50% by mass, the formability of the rolled sheet may be reduced.

[0017] ·Mg: 2.8 mass% or more and 3.8 mass% or less The rolled sheet contains 2.8% by mass or more and 3.8% by mass or less of Mg as an essential component. By setting the Mg content within the specific range, the stress corrosion cracking resistance of the rolled sheet can be improved, and the strength and formability of the rolled sheet can be improved in a balanced manner. If the Mg content is less than 2.8% by mass, the strength of the rolled sheet may be reduced. On the other hand, if the Mg content exceeds 3.8% by mass, the susceptibility to stress corrosion cracking may increase, leading to a deterioration in stress corrosion cracking resistance.

[0018] The aluminum alloy rolled plate contains, in addition to Mn and Mg as essential components, The aluminum alloy plate further contains Si (silicon), Fe (iron), and Ti (titanium). Optional components include Cu (copper) and Cr (chromium) and Zn (zinc )mosquito The metal oxide may contain one or more elements selected from the group consisting of:

[0019] ·Si: 0.30% by mass or less The aluminum alloy rolled plate 、0 Contains 30% or less by mass of Si There are Si has the effect of improving the strength of the rolled sheet. However, if the Si content is excessively high, there is a risk that the formability of the rolled sheet will decrease. By setting the Si content to preferably 0.30 mass% or less, more preferably 0.25 mass% or less, it is possible to further improve the strength of the rolled sheet without impairing the formability. ·Fe: 0.40% by mass or less The aluminum alloy rolled plate 、0 Contains 40% or less Fe by mass There are Fe has the effect of improving the strength of the rolled sheet. However, an excessively high Fe content may lead to a decrease in the formability of the rolled sheet. By setting the Fe content to preferably 0.40 mass% or less, more preferably 0.35 mass% or less, it is possible to further improve the strength of the rolled sheet without impairing the formability.

[0020] ·Cu: 0.10% by mass or less The aluminum alloy rolled sheet may contain Cu in an optional range of 0% by mass to 0.10% by mass. Cu has the effect of improving the strength of the rolled sheet. However, an excessively high Cu content may result in a decrease in the bending workability and corrosion resistance of the rolled sheet. By setting the Cu content to preferably 0.10% by mass or less, it is possible to further improve the strength of the rolled sheet while avoiding a decrease in bending workability and corrosion resistance.

[0021] ·Cr: 0.10% by mass or less The aluminum alloy rolled sheet may contain 0% by mass or more and 0.10% by mass or less of Cr as an optional component. Cr has the effect of refining the crystal grains of the aluminum alloy rolled sheet. However, if the Cr content is excessively high, large crystals containing Cr are likely to be formed during the manufacturing process of the rolled sheet, which may make it difficult to manufacture the rolled sheet. In this case, the crystal grains of the aluminum alloy rolled sheet may be excessively refined, which may make it more likely that a pattern called SS mark (Stretcher-Strain Mark) will occur when the rolled sheet is subjected to forming processing. By setting the Cr content to preferably 0.10% by mass or less, it is possible to avoid the formation of large crystals and appropriately refine the crystal grains of the aluminum alloy rolled sheet, thereby suppressing the occurrence of SS marks.

[0022] ·Zn: 0.10% by mass or less The aluminum alloy rolled sheet may contain 0% by mass or more and 0.10% by mass or less of Zn as an optional component. Zn has the effect of improving the uniformity of a coating formed by surface treatment, such as chemical conversion treatment or anodizing, when the aluminum alloy rolled sheet is subjected to such surface treatment. However, an excessively high Zn content may result in a decrease in the corrosion resistance of the rolled sheet. By setting the Zn content to preferably 0.10% by mass or less, it is possible to improve the uniformity of a coating formed by surface treatment while ensuring excellent corrosion resistance.

[0023] ·Ti: 0.10% by mass or less The aluminum alloy rolled plate 、0 Contains 10% or less of Ti by mass There are Ti has the effect of refining the ingot structure and improving the productivity of the aluminum alloy rolled sheet. However, an excessively high Ti content may result in a decrease in the corrosion resistance of the rolled sheet. By setting the Ti content to preferably 0.10 mass% or less, it is possible to increase the productivity of the aluminum alloy rolled sheet while ensuring excellent corrosion resistance.

[0024] [Mechanical properties] Tensile strength, 0.2% yield strength and elongation The aluminum alloy rolled plate has a tensile strength in the rolling direction of 200 MPa or more and 250 MPa or less, a 0.2% proof stress of 100 MPa or more and 130 MPa or less, and an elongation of 23% or more. An aluminum alloy rolled plate having a tensile strength, 0.2% proof stress, and elongation each falling within the above-mentioned specific ranges has an excellent balance between strength and formability. From the viewpoint of further improving formability while maintaining high strength, the elongation of the rolled plate in the rolling direction is preferably 25% or more.

[0025] If the tensile strength of the rolled sheet in the rolling direction is less than 200 MPa and the 0.2% proof stress is less than 100 MPa, it may not be suitable for applications requiring high strength, such as structural members. Furthermore, if the tensile strength of the rolled sheet in the rolling direction exceeds 250 MPa and the 0.2% proof stress exceeds 130 MPa, the bending workability of the rolled sheet may deteriorate. If the elongation of the rolled sheet in the rolling direction is less than 23%, the formability of the rolled sheet may be low, making it difficult to form it into complex shapes by press working.

[0026] Hardness The hardness of the aluminum alloy rolled plate at a position half the thickness from the surface is 90% to 100% of the hardness at a position one-quarter the thickness from the surface. When a plate made of a 5000 series alloy is produced by rolling an ingot produced by DC casting under conventional chemical compositions and manufacturing conditions, the hardness near the surface of the plate tends to be higher than that of the central portion in the thickness direction. Furthermore, when the difference in hardness between the surface and the central portion in the thickness direction is different, there is a problem that the formability of the plate, particularly its bending workability, is likely to be deteriorated.

[0027] In contrast, in the case of the rolled plate, as described above, the difference in hardness between the central portion in the thickness direction and the portion shallower from the surface than the central portion is small, so the rolled plate has excellent bending workability and can be easily bent to a high degree.

[0028] [Crystal orientation distribution] In a cross section of the rolled aluminum alloy sheet parallel to the rolling direction (i.e., a cross section parallel to both the rolling direction and the thickness direction), the total orientation density of crystal orientations in which the (001) plane is oriented perpendicular to the surface of the rolled aluminum alloy sheet is 40 or more. In this way, by increasing the orientation density of crystal orientations in which the (001) plane is oriented perpendicular to the surface of the rolled aluminum alloy sheet, the bending workability of the rolled aluminum alloy sheet can be improved. Note that, hereinafter, the crystal orientation in which the (001) plane is oriented perpendicular to the surface will be referred to as the "CubeND orientation."

[0029] If the total orientation density of the CubeND orientation in a cross section parallel to the rolling direction of the aluminum alloy rolled plate is less than 40, the bending workability of the rolled plate may be reduced. From the viewpoint of further improving the bending workability, it is preferable that the total orientation density of the CubeND orientation in a cross section parallel to the rolling direction of the aluminum alloy rolled plate is 44 or more.

[0030] The orientation density of the CubeND orientation described above is a value obtained by crystal orientation distribution function analysis. The specific method for calculating the orientation density of the CubeND orientation is as follows. First, an aluminum alloy rolled sheet is cut along the rolling direction to expose a cross section parallel to the rolling direction. After preprocessing such as polishing, the cross section is observed using a scanning electron microscope (so-called SEM-EBSD) equipped with a crystal orientation measurement device. Next, a measurement region is set from the cross section so that it has a width of 2000 μm in the rolling direction and includes the entire thickness direction of the rolled sheet, and a pole figure of this measurement region is obtained. Note that when obtaining the pole figure, the distance between measurement points (i.e., the step size) may be set appropriately from a range of, for example, 5 μm or less.

[0031] Based on the pole figures obtained in this way, crystal orientation distribution function analysis is performed using the series expansion method with spherical harmonics to calculate the orientation density of the crystal orientations in the cross section. The expansion order in the series expansion method is 16th order, and the half-width is 5°. Of the orientation densities of the various crystal orientations obtained in this way, the total orientation density of the crystal orientations whose Euler angles are (φ1, Φ, φ2) = (5n°, 0°, 0°) (where n is an integer between 0 and 18) is taken as the total orientation density of the CubeND orientations. Crystal orientation distribution function analysis can be performed using analysis software such as "OIM Analysis" by TSL Solutions, Inc.

[0032] [Average grain size] The average crystal grain size in a cross section parallel to the rolling direction of the aluminum alloy rolled sheet is preferably 15 μm or more and 50 μm or less. By making the average crystal grain size in a cross section parallel to the rolling direction of the rolled sheet 15 μm or more, the occurrence of SS marks can be suppressed. Furthermore, by making the average crystal grain size in a cross section parallel to the rolling direction of the rolled sheet 50 μm or less, the surface of the rolled sheet can be made smoother. Therefore, by making the average crystal grain size in a cross section parallel to the rolling direction of the rolled sheet within the above-mentioned specific range, the appearance characteristics of the aluminum alloy rolled sheet can be improved both before and after forming.

[0033] The average crystal grain size of the rolled sheet is a value calculated by the cutting method specified in JIS G0551:2013.

[0034] (Method of manufacturing aluminum alloy rolled sheet) The aluminum alloy rolled plate is a casting step of producing an ingot having the chemical composition by DC casting; a homogenization treatment step of heating the ingot and subjecting it to a homogenization treatment; a hot rolling step of hot rolling the ingot that has been subjected to the homogenization treatment to produce an aluminum alloy rolled plate; a cold rolling step of cold rolling the aluminum alloy rolled plate after the hot rolling; and a final annealing step in which the cold-rolled aluminum alloy rolled sheet is heated in a continuous annealing furnace and subjected to final annealing. Each step in the manufacturing method will be described in detail below.

[0035] Casting process In the casting step, an ingot of an aluminum alloy having the above chemical composition is produced by DC casting. The casting conditions for DC casting are not particularly limited and may be appropriately selected from known ranges.

[0036] Homogenization process In the manufacturing method, a homogenization treatment step is performed after the casting step is completed. The homogenization treatment step may be performed as a step separate from the hot rolling step, or the heating of the ingot before the hot rolling step may also serve as the homogenization treatment step. That is, in the manufacturing method, the ingot may be cooled after the homogenization treatment step and before the hot rolling step, or the hot rolling step may be started after the homogenization treatment step and before the ingot is cooled.

[0037] The holding temperature in the homogenization treatment step is 450°C or higher and 570°C or lower. By setting the holding temperature in the homogenization treatment step within the above-mentioned specific range, the ingot can be sufficiently homogenized while avoiding melting of the ingot. If the holding temperature is lower than 450°C, the ingot may not be sufficiently homogenized. On the other hand, if the holding temperature is higher than 570°C, the ingot may melt. The holding time in the homogenization treatment step is not particularly limited from the viewpoint of sufficiently homogenizing the ingot, and the homogenization treatment step may be completed when the temperature of the ingot reaches the desired holding temperature, or the desired holding temperature may be held for a desired time. From the viewpoint of avoiding excessive deterioration in the productivity of aluminum alloy rolled sheet, the holding time in the homogenization treatment step is preferably 24 hours or shorter.

[0038] Hot rolling process In the manufacturing method, a hot rolling step is performed after the homogenization step is completed. In the hot rolling step, the ingot is heated as needed and then hot rolled to produce an aluminum alloy rolled plate. The temperature of the ingot at the start of rolling in the hot rolling step is set to 300°C or higher and 550°C or lower. By setting the temperature of the ingot at the start of rolling within the specific range, partial melting of the ingot due to processing heat can be avoided, and the deformation resistance of the ingot can be reduced, thereby improving the productivity of the rolled plate. If the temperature of the ingot at the start of rolling is lower than 300°C, the deformation resistance of the ingot is likely to increase, which may lead to a deterioration in productivity. Furthermore, if the temperature of the ingot at the start of rolling exceeds 550°C, there is a risk of partial melting of the ingot due to processing heat.

[0039] Furthermore, the temperature of the rolled sheet at the completion of the hot rolling step is set to 150°C or higher. By setting the temperature of the rolled sheet at the completion of the hot rolling within the above-mentioned specific range, the productivity of the rolled sheet can be improved. If the temperature of the rolled sheet at the completion of the hot rolling is less than 150°C, there is a risk that the workability during hot rolling will deteriorate.

[0040] The thickness of the aluminum alloy rolled sheet after the hot rolling step may be appropriately set in consideration of the final thickness of the rolled sheet to be obtained and the total rolling reduction ratio in the cold rolling step. The thickness of the rolled sheet after completion of hot rolling is preferably, for example, 10 mm or less at the time of winding into a coil after completion of hot rolling.

[0041] Cold rolling process In the manufacturing method, a cold rolling step is carried out after the hot rolling step is completed. In the cold rolling step, the aluminum alloy rolled sheet after hot rolling is subjected to one or more passes of cold rolling to reduce the thickness of the rolled sheet to a desired thickness. The total rolling reduction ratio in the cold rolling, i.e., the ratio of the amount of reduction in thickness of the rolled sheet in the cold rolling step to the thickness of the rolled sheet before cold rolling, is set to 40% or more and 80% or less. By setting the total rolling reduction ratio in the cold rolling within the specified range, it is possible to impart appropriate processing strain to the aluminum alloy rolled sheet, and to achieve a desired metal structure of the finally obtained rolled sheet.

[0042] Final annealing process In the manufacturing method, a final annealing step is performed after the cold rolling step is completed. In the final annealing step, a continuous annealing furnace is used to anneal the aluminum alloy rolled sheet after cold rolling at a temperature T fa (unit: °C) and perform final annealing. 400≦T fa ≦550 (1) -1≦ 4[Mn]+[Mg]-0.01T fa ≦1 (2)

[0043] In the formula (2), [Mn] is a symbol representing the Mn content (unit: mass%) in the aluminum alloy rolled plate, and [Mg] is a symbol representing the Mg content (unit: mass%) in the aluminum alloy rolled plate.

[0044] In the final annealing process, a continuous annealing furnace is used to heat the rolled aluminum alloy sheet to a temperature of T fa By performing final annealing so that satisfies the above formula (1), the aluminum alloy rolled sheet can be sufficiently recrystallized, and the orientation density of the CubeND orientation in the aluminum alloy rolled sheet after final annealing can be increased, thereby improving the bending workability of the aluminum alloy rolled sheet.

[0045] When the final annealing is performed using a batch furnace in the final annealing process, the orientation density of the CubeND orientation after the final annealing may be low, which may lead to deterioration in the bending workability of the rolled sheet. fa If the temperature T of the rolled sheet in the final annealing step is less than 400°C, the recrystallization of the rolled sheet may be insufficient, which may result in a decrease in the elongation of the rolled sheet. fa If the temperature exceeds 550°C, the rolled sheet may be partially remelted during the final annealing, and the rolled sheet may be separated.

[0046] In addition, in the final annealing process, the temperature T fa However, by performing final annealing so as to satisfy the formula (2) after satisfying the formula (1), the bending workability of the aluminum alloy rolled sheet obtained after the final annealing can be improved and the strength can be increased.

[0047] When final annealing is performed using a continuous annealing furnace, the rolled sheet is transported in the furnace while final annealing is performed. Therefore, the temperature T fa In the final annealing step, the temperature T fa The time during which the formula (1) and the formula (2) are satisfied may be, for example, within one minute.

[0048] In the production method, in addition to setting the production conditions in each of the casting step, the homogenization treatment step, the hot rolling step, the cold rolling step, and the final annealing step as described above, the temperatures of the ingot and the aluminum alloy rolled sheet and the thicknesses of the ingot and the aluminum alloy rolled sheet after the homogenization treatment step satisfy the relationship of the following formula (3):

[0049]

number

[0050] In the formula (3), n is the number of times that the temperatures of the ingot and the rolled aluminum alloy plate have reached 405°C or higher after the homogenization treatment step, and τ k0 is the time when the temperatures of the ingot and the aluminum alloy rolled plate exceed 405°C for the kth time, and τ k1 is the time when the temperatures of the ingot and the aluminum alloy rolled plate fall below 405°C for the kth time, T(τ) is the temperature (unit: °C) of the ingot and the aluminum alloy rolled plate at time τ, and t k is the thickness (unit: mm) of the ingot and the aluminum alloy rolled plate at the time when the temperature of the ingot and the aluminum alloy rolled plate exceeds 405°C for the kth time, and the unit of the infinitesimal time dτ is seconds.

[0051] As mentioned above, when a sheet made of a 5000-series alloy is produced by rolling an ingot produced by DC casting under conventional chemical compositions and manufacturing conditions, the hardness near the surface of the sheet tends to be higher than that of the central portion in the thickness direction. This variation in hardness in the depth direction is presumably due to the following reason. In ingots obtained by DC casting, the Mg concentration in the interior of the ingot tends to be lower than that near the surface. It is believed that this Mg concentration distribution is not sufficiently resolved after the casting process, and the Mg concentration in the interior of the rolled sheet becomes lower than that near the surface, resulting in a lower hardness in the interior of the rolled sheet than near the surface.

[0052] In contrast, in the formula (3), the value expressed by the following formula (3a) is integrated over time while the temperatures of the ingot and the rolled aluminum alloy plate exceed 405°C in a process subsequent to the casting process. {T(τ)-405} / t k (3a)

[0053] The temperature of 405°C in the above formula (3a) is the temperature at which Mg atoms in the aluminum matrix move by diffusion at a rate of approximately 0.1 μm per second, and it is believed that if the rate of movement of Mg atoms by diffusion is approximately 0.1 μm / s or more, it will be possible to eliminate Mg macrosegregation in a time that is feasible in actual operation. Therefore, the larger the value of the numerator in the above formula (3a), the greater the effect that the diffusion rate of Mg atoms in the aluminum matrix contributes to eliminating Mg macrosegregation.

[0054] Furthermore, in order for Mg atoms present near the surface of an ingot or rolled plate to reach the center in the depth direction, the Mg atoms must move in the depth direction, and the thinner the thickness of the ingot or aluminum alloy rolled plate while the Mg atoms are diffusing, the shorter the distance the Mg atoms must move from near the surface to the interior. Therefore, the smaller the value of the denominator in formula (3a), the shorter the time it takes for Mg atoms in the aluminum matrix to reach the surface of the ingot or rolled plate.

[0055] Based on this concept, in the formula (3), the value obtained by the formula (3a) is integrated over a small time dτ while the temperature of the ingot and the rolled aluminum alloy plate exceeds 405°C. The value obtained by the formula (3) can be used as an index representing the degree of elimination of Mg macrosegregation in the ingot and the rolled aluminum alloy plate. In the production method, by making the temperatures of the ingot and the rolled aluminum alloy plate and the thicknesses of the ingot and the rolled aluminum alloy plate after the homogenization treatment step satisfy the relationship of the formula (3), the macrosegregation of Mg in the finally obtained rolled plate can be sufficiently eliminated. This makes it possible to easily obtain an aluminum alloy rolled plate with excellent bending workability.

[0056] In the manufacturing method, the total rolling reduction R (unit: %) in the cold rolling step and the temperature T of the aluminum alloy rolled sheet in the final annealing step are fa (unit: ° C.) preferably satisfies the relationship of the following formula (4). 0.07T fa-25.7 ( R / 100) 2 >19 ···(4)

[0057] In this case, the average grain size of the aluminum alloy rolled sheet obtained after final annealing can be appropriately increased, which makes it possible to easily obtain an aluminum alloy rolled sheet that is less likely to develop SS marks and has good appearance properties even after forming. [Example]

[0058] Example 1 Examples of the aluminum alloy rolled sheet and its manufacturing method are described below. The aluminum alloy rolled sheet of this example is obtained by rolling an aluminum alloy ingot. The aluminum alloy rolled sheet has a chemical composition containing 0.25% by mass or more and 0.50% by mass or less of Mn and 2.8% by mass or more and 3.8% by mass or less of Mg, with the balance being Al and unavoidable impurities. The tensile strength of the rolled sheet in the rolling direction is 200 MPa or more and 250 MPa or less, the 0.2% proof stress is 100 MPa or more and 130 MPa or less, and the elongation is 23% or more. The hardness at a position where the depth from the surface of the rolled sheet is 1 / 2 of the thickness is 90% or more and 100% or less of the hardness at a position where the depth from the surface is 1 / 4 of the thickness. The sum of the orientation densities of CubeND orientations in a cross section of the rolled sheet parallel to the rolling direction, calculated by crystal orientation distribution function analysis, is 40 or more.

[0059] The aluminum alloy rolled sheet of this example can be produced, for example, by the following method. First, an ingot having the chemical composition shown in Table 1 is produced by DC casting. The thickness of the ingot is not particularly limited, but in this example it is about 500 mm. Note that the symbol "Bal." in Table 1 indicates the remainder.

[0060] Next, the ingot is subjected to a homogenization treatment by maintaining the temperature at 450°C or higher and 570°C or lower (homogenization treatment step). After the homogenization treatment step, hot rolling is started while the ingot temperature is 300°C or higher and 550°C or lower (hot rolling step). After the hot rolling step is completed, the rolled sheet is subjected to one or more passes of cold rolling (cold rolling step).

[0061] For example, when producing test material S3 shown in Table 2, the holding temperature during the homogenization treatment is set to 480°C, and the homogenization treatment is terminated when the ingot temperature reaches 480°C. The ingot temperature at the start of hot rolling is set to 460°C, and the ingot temperature at the end of hot rolling is set to 330°C. Then, in the cold rolling process, the thickness of the rolled sheet is reduced from 3 mm to 1 mm. When the thickness of the rolled sheet is reduced from 3 mm to 1 mm in the cold rolling process, the total rolling ratio in the cold rolling process is 71%.

[0062] After the cold rolling process is completed, the rolled sheet is annealed in a continuous annealing furnace at the temperature T shown in Table 2. fa In the final annealing step of this example, the rolled sheet is heated to a temperature T fa The time for which this is maintained should be within 1 minute. The "Value of formula (2')" column in Table 2 shows the value calculated based on the following formula (2'). In formula (2'), [Mn] is the Mn content (unit: mass%) in the rolled plate, and [Mg] is the Mg content (unit: mass%) in the rolled plate. 4[Mn]+[Mg]-0.01T fa (2')

[0063] By the manufacturing method described above, test materials S1 to S3 shown in Table 2 can be obtained. Test materials R1 and R2 shown in Table 2 are test materials for comparison with test materials S1 to S3. The manufacturing method of test materials R1 and R2 is based on the chemical composition and the temperature T fa Except for the differences, the manufacturing method is the same as that of the test materials S1 to S3.

[0064] The tensile strength, 0.2% proof stress, and elongation of the test materials were measured as follows. First, a No. 5 test piece specified in JIS Z2241:2011 was taken from each test material so that the longitudinal direction was parallel to the rolling direction. Using this test piece, a tensile test was performed according to the method specified in JIS Z2241:2011. Then, the tensile strength, 0.2% proof stress, and elongation were calculated based on the load-displacement curve obtained from the tensile test. The tensile strength, 0.2% proof stress, and elongation of each test material are shown in Table 2.

[0065] Figure 1 shows a graph in which the results of the tensile tests for each test material are plotted using the values ​​of formula (2'). The horizontal axis of Figure 1 represents the values ​​of formula (2'), and the vertical axis represents the values ​​of tensile strength and 0.2% proof stress. The dashed line in Figure 1 is a regression line of the data points in the graph, determined by the least squares method.

[0066] The hardness of the test material is measured as follows. First, the test material is cut along the rolling direction to expose a cross section parallel to the rolling direction. In this cross section, the hardness is measured at a position where the depth from the surface of the test material is 1 / 2 of the thickness, and at a position where the depth from the surface is 1 / 4 of the thickness. A micro Vickers hardness tester is used to measure the hardness, and the pressing load of the indenter is 100 gf.

[0067] The "Hardness ratio" column in Table 2 shows the ratio, expressed as a percentage, of the hardness at a position where the depth from the surface is 1 / 2 of the thickness to the hardness at a position where the depth from the surface is 1 / 4 of the thickness of the test material. Note that for test materials for which hardness measurements were not performed, the symbol "-" is entered in the "Hardness ratio" column in Table 2.

[0068] The method for calculating the total orientation density of the CubeND orientations of the test material is as follows. The test material is cut along the rolling direction to expose a cross section parallel to the rolling direction. After preprocessing such as polishing, a scanning electron microscope (SEM-EBSD) equipped with a crystal orientation measurement device is used to obtain a pole figure of the cross section. Based on this pole figure, a crystal orientation distribution function analysis is performed using a series expansion method using spherical harmonics to calculate the orientation density of the crystal orientations in the cross section. Note that the expansion order in the series expansion method is 16, and the half-width is 5°. Of the orientation densities of the various crystal orientations obtained in this way, the total orientation density of the crystal orientations whose Euler angles are (φ1, Φ, φ2) = (5n°, 0°, 0°) (where n is an integer between 0 and 18) is taken as the total orientation density of the CubeND orientations. Crystal orientation distribution function analysis can be performed using analysis software ("OIM Analysis" by TSL Solutions, Inc.). The "Orientation density of CubeND orientation" column in Table 2 shows the total value of the orientation density of CubeND orientation for each test material calculated by crystal orientation distribution function analysis. Note that for test materials for which CubeND orientation measurement was not performed, the symbol "-" is entered in the "Orientation density of CubeND orientation" column in Table 2.

[0069] [Table 1]

[0070] [Table 2]

[0071] As shown in Tables 1 and 2, test material S3 was produced by DC casting an ingot having the specific chemical composition, followed by a homogenization treatment step, a hot rolling step, a cold rolling step, and a final annealing step in this order. In the final annealing step, a continuous annealing furnace was used to heat the cold-rolled aluminum alloy rolled sheet at a temperature T faThe final annealing was performed by heating to 0°C. Therefore, the tensile strength in the rolling direction, 0.2% proof stress, elongation, hardness ratio, and the total orientation density of the CubeND orientation of test material S3 are all within the above-mentioned specific ranges. Test material S3 with these properties has moderate strength and high elongation, making it suitable for applications where it is formed into complex shapes or press-formed under severe processing conditions.

[0072] Furthermore, as shown in Table 2, the tensile strength, 0.2% proof stress, and elongation in the rolling direction of test material S1 and test material S2 are each within the specified ranges. Test material S1 and test material S2 were produced by DC casting an ingot having the specified chemical composition, followed by the same process as test material S3. Therefore, like test material S3, they are presumed to have a hardness ratio and a total orientation density of the CubeND orientation within the specified ranges. Therefore, test material S1 and test material S2 are also suitable for applications requiring forming into complex shapes or press forming under strict processing conditions.

[0073] On the other hand, the Mn content of test material R1 is higher than the specific range, and therefore the strength is higher than the specific range, and therefore test material R1 has inferior bending workability compared to test materials S1 to S3. The Mn content of test material R2 is lower than that within the specific range, and therefore the strength is lower than that within the specific range.

[0074] Furthermore, as shown in FIG. 1, two regression lines L1 and L2 determined based on the experimental results of test materials S1-S3 and test materials R1-R2 closely approximate the relationship between the tensile strength of each test material and the value calculated based on formula (2'), and the relationship between the 0.2% yield strength and the value calculated based on formula (2'). Of these two regression lines, regression line L1, which approximates the relationship between the tensile strength and the value calculated based on formula (2'), estimates that the tensile strength is approximately 250 MPa when the value of formula (2') is 1.0. Furthermore, regression line L2, which approximates the relationship between the 0.2% yield strength and the value calculated based on formula (2'), estimates that the 0.2% yield strength is approximately 100 MPa when the value of formula (2') is -1.0.

[0075] Therefore, according to FIG. 1, the temperature T of the rolled sheet in the final annealing process fa It can be understood that by satisfying the formula (1) and also the formula (2), the tensile strength and 0.2% proof stress of the aluminum alloy rolled plate can be set within the specific ranges.

[0076] Example 2 In this example, various changes to the manufacturing conditions after the homogenization treatment step will be described. The manufacturing method for the aluminum alloy rolled sheet of this example is the same as the manufacturing method for test materials S1 to S3 in Example 1, except that an ingot having a chemical composition represented by alloy symbol A3 in Table 1 is used, and the manufacturing conditions after the homogenization treatment step are changed as shown in Table 3. This allows the manufacturing of test materials S4 to S10 shown in Tables 3 and 4. Note that test material R3 shown in Tables 3 and 4 is a test material for comparison with test materials S3 to S10. The manufacturing method for test material R3 is the same as the manufacturing method for test materials S3 to S10, except that a batch furnace is used instead of a continuous annealing furnace in the final annealing step, and the holding temperature is set to 330°C.

[0077] The "Value of formula (3')" column in Table 3 shows the value calculated based on the following formula (3'). n in formula (3') is the number of times that the temperature of the ingot and rolled plate has reached 405°C or higher after the homogenization treatment process, and τ k0 is the time when the temperature of the ingot and rolled plate exceeds 405°C for the kth time, and τ k1 is the time when the temperature of the ingot and rolled plate falls below 405°C for the kth time, T(τ) is the temperature of the ingot and rolled plate at time τ (unit: °C), and t k is the thickness (unit: mm) of the ingot and rolled plate when the temperature of the ingot and rolled plate exceeds 405°C for the kth time, and the unit of the minute time dτ is seconds.

[0078]

number

[0079] Table 4 shows the hardness ratio of the test material calculated by the above-mentioned method and the total orientation density of the CubeND orientation.

[0080] The bending workability shown in Table 4 was evaluated as follows. First, strip-shaped test pieces were taken from the test material so that the longitudinal direction was oriented in one of the following directions: parallel to the rolling direction, at an angle of 45° between the longitudinal direction and the rolling direction, or perpendicular to the rolling direction. Hereinafter, a test piece whose longitudinal direction was oriented parallel to the rolling direction will be referred to as a 0° test piece, a test piece whose longitudinal direction was oriented at an angle of 45° with respect to the rolling direction will be referred to as a 45° test piece, and a test piece whose longitudinal direction was oriented perpendicular to the rolling direction will be referred to as a 90° test piece.

[0081] After applying a 12% permanent strain to these test pieces as a pre-strain, a bending jig with a tip curvature radius of 0 mm is pressed against them, and then the bending jig is displaced to bend the strip-shaped test pieces by 90°.

[0082] In the "Bending workability" column of Table 4, if no cracks occurred on the surface of the rectangular test piece after bending, the symbol "A" was entered, and if cracks occurred, the symbol "B" was entered. Note that for test materials for which bending workability was not evaluated, the symbol "-" was entered in the "Bending workability" column of Table 4.

[0083] [Table 3]

[0084] [Table 4]

[0085] As shown in Table 3, test materials S3 to S6 have values ​​represented by formula (3') that are greater than 500, and thus satisfy formula (3). Therefore, in the manufacturing process of these test materials, macrosegregation of Mg atoms can be sufficiently eliminated, and the hardness ratios shown in Table 4 are 90% or more. Furthermore, test materials S7 to S10 also have values ​​represented by formula (3') that are greater than 500, and thus satisfy formula (3), and therefore it is estimated that the hardness ratios shown in Table 4 will be 90% or more.

[0086] For test material S3 and test materials S7 to S10, the annealing furnace used in the final annealing process was a continuous annealing furnace, so the CubeND orientation is likely to form in the test materials after final annealing. Therefore, these test materials have a total orientation density of CubeND orientation of 40 or more, and have excellent bending workability. In addition, test materials S4 to S6 were also subjected to final annealing using a continuous annealing furnace, so it is estimated that the total orientation density of CubeND orientation shown in Table 4 will be 40 or more.

[0087] On the other hand, in test material R3, which was subjected to final annealing using a batch furnace, the CubeND orientation is difficult to form, and the total orientation density of the CubeND orientation is less than 40. The bending workability of test material R3 is inferior to that of test material S10.

[0088] Example 3 In this example, the temperature of the rolled sheet in the final annealing step and the total rolling reduction ratio in the cold rolling step are variously changed. The method for producing an aluminum alloy rolled sheet in this example is the same as the method for producing test materials S1 to S3 in Example 1, except that an ingot having a chemical composition represented by alloy symbol A3 in Table 1 is used and the production conditions after the homogenization treatment step are changed as shown in Table 5. This allows test materials S11 to S20 shown in Tables 5 and 6 to be obtained.

[0089] The "Value of formula (4')" column in Table 5 shows the value calculated based on the following formula (4'). fa is the temperature of the rolled sheet in the final annealing process, and R is the total rolling ratio (unit: %) in the cold rolling process. 0.07Tfa -25.7 ( R / 100) 2 (4')

[0090] The method for calculating the average crystal grain size of the test material and the method for evaluating the appearance characteristics are as follows.

[0091] ·Method for calculating average grain size The test material is cut along the rolling direction to expose a cross section parallel to the rolling direction. After pre-processing such as polishing, a metallurgical microscope is used to obtain a polarizing microscope image of the cross section parallel to the rolling direction. The average crystal grain size in the cross section parallel to the rolling direction of each test material is calculated by applying the cutting method specified in JIS G0551:2013 to this polarizing microscope image. The average crystal grain size in the cross section parallel to the rolling direction of each test material is shown in Table 6.

[0092] ·Evaluation method for appearance characteristics A test piece measuring 200 mm in length and 40 mm in width is taken from each test material, with the longitudinal direction parallel to the rolling direction. The temperature of this test piece is maintained at 40°C, and the initial strain rate is set to 7.5% / sec, giving the test piece a pre-strain of 2% or 5% permanent strain. The surface of the pre-strained test piece is lightly polished with sandpaper or similar, and the appearance of the test piece is then visually inspected to evaluate its appearance characteristics.

[0093] In the "Appearance Characteristics" column of Table 6, the symbol "A" was entered if almost no SS marks appeared, the symbol "B" was entered if the SS marks appeared on only part of the surface of the test material, and the symbol "C" was entered if the SS marks appeared on the entire surface of the test material.

[0094] [Table 5]

[0095] [Table 6]

[0096] As shown in Table 6, the test materials S11 to S16 are subjected to the final annealing process by heating the rolled sheets to a temperature T fa The final annealing was performed by heating to [°C]. Therefore, the average grain size in the cross section parallel to the rolling direction of test materials S11 to S16 is 15 μm or more. Such test materials are less likely to develop SS marks and have excellent appearance properties.

[0097] Furthermore, as shown in Table 6, the average grain size in the cross section parallel to the rolling direction of test material S17 and test material S18 is also 15 μm or more. Therefore, like test materials S11 to S16, test materials S17 and S18 are also less likely to develop SS marks and are presumed to have excellent appearance properties.

[0098] On the other hand, test materials S19 to S20 are subjected to the temperature T fa does not satisfy the formula (4), the average grain size in the cross section parallel to the rolling direction is less than 15 μm. Such test materials are more likely to develop SS marks than test materials S11 to S18.

[0099] As described above, from the results shown in Examples 1 to 3, it can be understood that the aluminum alloy rolled sheet having the specific chemical composition, metal structure, mechanical properties, and hardness distribution has an excellent balance between strength and formability, and can improve formability while ensuring high strength.

[0100] The specific aspects of the aluminum alloy rolled sheet and the manufacturing method thereof according to the present invention are not limited to those shown in Examples 1 to 3, and the configurations can be changed as appropriate within the scope of the present invention.

Claims

1. An aluminum alloy rolled plate obtained by rolling an aluminum alloy ingot, The alloy has a chemical composition comprising Mn: 0.25% by mass or more and 0.50% by mass or less, Mg: 2.8% by mass or more and 3.8% by mass or less, Si: 0.30% by mass or less, Fe: 0.40% by mass or less, and Ti: 0.10% by mass or less, and further comprises, as optional components, Cu: 0% by mass or more and 0.10% by mass or less, Cr: 0% by mass or more and 0.10% by mass or less, and Zn: 0% by mass or more and 0.10% by mass or less, with the balance being Al and unavoidable impurities; The tensile strength in the rolling direction is 200 MPa or more and 250 MPa or less, The 0.2% proof stress in the rolling direction is 100 MPa or more and 130 MPa or less, The elongation in the rolling direction is 23% or more, the hardness at a position where the depth from the surface of the aluminum alloy rolled plate is 1 / 2 of the thickness is 90% or more and 100% or less of the hardness at a position where the depth from the surface is 1 / 4 of the thickness, An aluminum alloy rolled sheet, wherein the sum of orientation densities of crystal orientations in which the (001) plane in a cross section parallel to the rolling direction is oriented perpendicular to the surface, as calculated by crystal orientation distribution function analysis, is 40 or more.

2. 2. The aluminum alloy rolled sheet according to claim 1, wherein the aluminum alloy rolled sheet has an average crystal grain size of 15 μm or more and 50 μm or less in a cross section parallel to the rolling direction.

3. 3. The method for producing an aluminum alloy rolled sheet according to claim 1 or 2, a casting step of producing an ingot having the chemical composition by DC casting; a homogenization treatment step of heating the ingot to a temperature of 450°C or higher and 570°C or lower to subject the ingot to homogenization treatment; a hot rolling step of hot rolling the ingot that has been subjected to the homogenization treatment under conditions in which the temperature of the ingot at the start is 300°C or more and 550°C or less, and the temperature of the aluminum alloy rolled plate at the completion is 150°C or more, to produce an aluminum alloy rolled plate; a cold rolling step of cold rolling the aluminum alloy rolled plate after the hot rolling so that a total rolling reduction ratio is 40% or more and 80% or less; The aluminum alloy rolled sheet after the cold rolling is annealed in a continuous annealing furnace at a temperature T ｆａ and a final annealing step of heating to [°C] and performing final annealing, the temperatures of the ingot and the aluminum alloy rolled plate, and the thicknesses of the ingot and the aluminum alloy rolled plate after the homogenization treatment step satisfy the relationship of the following formula (3): 400≦T ｆａ ≦550 ・・・(1) -1≦4[Mn]+[Mg]-0.01T ｆａ ≦1 ・・・(2) [Equation 1] (wherein, in the formula (2), [Mn] is the Mn content (unit: mass%) in the aluminum alloy rolled plate, and [Mg] is the Mg content (unit: mass%) in the aluminum alloy rolled plate. Also, in the formula (3), n is the number of times that the temperatures of the ingot and the aluminum alloy rolled plate have reached 405°C or higher after the homogenization treatment step, and τ k0 is the time when the temperatures of the ingot and the aluminum alloy rolled plate exceed 405°C for the kth time, and τ k1 is the time when the temperatures of the ingot and the aluminum alloy rolled plate fall below 405°C for the kth time, T(τ) is the temperature (unit: °C) of the ingot and the aluminum alloy rolled plate at time τ, and t ｋ is the thickness (unit: mm) of the ingot and the aluminum alloy rolled plate at the time when the temperatures of the ingot and the aluminum alloy rolled plate exceeded 405°C for the kth time, and the unit of the minute time dτ is seconds.

4. The total rolling ratio R [%] in the cold rolling process and the temperature T of the aluminum alloy rolled sheet in the final annealing process ｆａ The method for producing an aluminum alloy rolled sheet according to claim 3, wherein the temperature [°C] satisfies the relationship of the following formula (4): 0.07T ｆａ -25.7(R / 100) ２ >19 ・・・(4)

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