CN108884524B - Aluminum alloy sheet and method for producing aluminum alloy sheet - Google Patents

Aluminum alloy sheet and method for producing aluminum alloy sheet Download PDF

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CN108884524B
CN108884524B CN201780016619.3A CN201780016619A CN108884524B CN 108884524 B CN108884524 B CN 108884524B CN 201780016619 A CN201780016619 A CN 201780016619A CN 108884524 B CN108884524 B CN 108884524B
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temperature
aluminum alloy
height
treatment
alloy sheet
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CN108884524A (en
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宍户久郎
北村智之
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Kobe Steel Ltd
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Kobe Steel Ltd
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Priority to JP2016-067007 priority
Priority to JP2016-213789 priority
Priority to JP2016213789A priority patent/JP6306123B2/en
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Priority to PCT/JP2017/013179 priority patent/WO2017170835A1/en
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    • CCHEMISTRY; METALLURGY
    • C22METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22CALLOYS
    • C22C21/00Alloys based on aluminium
    • C22C21/06Alloys based on aluminium with magnesium as the next major constituent
    • C22C21/08Alloys based on aluminium with magnesium as the next major constituent with silicon
    • CCHEMISTRY; METALLURGY
    • C22METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22CALLOYS
    • C22C21/00Alloys based on aluminium
    • C22C21/02Alloys based on aluminium with silicon as the next major constituent
    • CCHEMISTRY; METALLURGY
    • C22METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22FCHANGING THE PHYSICAL STRUCTURE OF NON-FERROUS METALS AND NON-FERROUS ALLOYS
    • C22F1/00Changing the physical structure of non-ferrous metals or alloys by heat treatment or by hot or cold working
    • C22F1/04Changing the physical structure of non-ferrous metals or alloys by heat treatment or by hot or cold working of aluminium or alloys based thereon
    • C22F1/05Changing the physical structure of non-ferrous metals or alloys by heat treatment or by hot or cold working of aluminium or alloys based thereon of alloys of the Al-Si-Mg type, i.e. containing silicon and magnesium in approximately equal proportions
    • CCHEMISTRY; METALLURGY
    • C22METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22CALLOYS
    • C22C21/00Alloys based on aluminium
    • C22C21/12Alloys based on aluminium with copper as the next major constituent
    • CCHEMISTRY; METALLURGY
    • C22METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22CALLOYS
    • C22C2202/00Physical properties

Abstract

In a differential scanning calorimetry curve of an Al-Mg-Si aluminum alloy sheet having a specific composition in which the total content of Mg and Si is higher than 1.2%, the ratio (B/A) of an endothermic peak having a height A of 3 to 10 [ mu ] W/Mg in a temperature range of 150 to 230 ℃ and an exothermic peak having a height B of 20 to 50 [ mu ] W/Mg in a temperature range of 230 ℃ or higher and lower than 330 ℃ is in a specific range.

Description

Aluminum alloy sheet and method for producing aluminum alloy sheet
Technical Field
The present invention relates to a 6000 series aluminum alloy sheet produced by ordinary rolling, and is a 6000 series aluminum alloy sheet excellent in formability and bake hardenability.
Background
In recent years, in view of global environment and the like, there has been an increasing demand for a lightweight automobile body. In order to meet such a demand, aluminum alloy materials have been applied to large body panels (outer panels and inner panels) of automobile bodies instead of conventional steel materials such as steel sheets.
Among the large body panels, Al — Mg — Si-based AA to JIS 6000-based (hereinafter, all referred to simply as "6000-based") aluminum alloy sheets are used as thin-walled and high-strength aluminum alloy sheets for panels such as outer panels (outer panels) and inner panels (inner panels) of panel structures such as hoods, fenders, doors, roofs, trunk lids, and the like.
The 6000 series (Al-Mg-Si series) aluminum alloy sheet is required to contain Si and Mg, and particularly a high-silicon type 6000 series aluminum alloy, and has excellent age hardening ability in artificial aging treatment. Therefore, in press forming and bending, formability can be ensured by lowering the yield point, and Bake hardenability (hereinafter, Bake Hard property is also referred to as BH property) is provided, that is, even in a relatively low-temperature artificial aging treatment such as a paint baking treatment of a panel after forming, yield strength is improved, and strength required for the panel can be ensured.
On the other hand, as is well known, an outer panel of an automobile or the like is produced by combining aluminum alloy sheets and performing forming such as bulging forming and bending forming in press forming. For example, in a large-sized outer panel such as a hood or a door, the outer panel is formed into a shape of a formed product as an outer panel by press forming such as bulging, and then, the outer panel is joined to an inner panel by hemming (hemming) processing such as flat folding of a peripheral edge portion of the outer panel, thereby forming a panel structure.
In the outer panels and the like of the automobiles and the like, there is a tendency to be thinner for weight reduction, and in addition to the thinner thickness, high strength is required such that the dent resistance is excellent. Therefore, there is a further need for an artificial age hardening ability (bake hardenability), that is, an ability to ensure formability by lowering the yield point of an aluminum alloy sheet during press forming, and an ability to ensure a required strength even after thinning by age hardening by heating at the time of relatively low-temperature artificial aging such as paint baking of a formed panel to improve the yield strength.
Conventionally, various proposals have been made for controlling the bake hardenability of 6000 series aluminum alloy sheets as raw materials for such automobile members, i.e., for controlling Mg — Si series clusters. Recently, techniques have been proposed in which these Mg — Si clusters are measured and controlled based on the endothermic peak and exothermic peak of the differential scanning calorimetry curve (hereinafter also referred to as DSC) of 6000 series aluminum alloy sheets (see patent documents 1 to 5).
For example, in patent document 1, it is specified that in a differential scanning calorimetry curve of a 6000 series aluminum alloy sheet, an exothermic peak height W1 in a temperature range of 100 to 200 ℃ is 50 μ W or more, and a ratio of an exothermic peak height W2 in a temperature range of 200 to 300 ℃ to the exothermic peak height W1, that is, W2/W1, is 20 or less.
In patent document 2, it is specified that, in a differential scanning calorimetry curve of a 6000 series aluminum alloy sheet, when an exothermic peak height in a temperature range of 230 to 270 ℃ in the differential scanning calorimetry curve is a, an exothermic peak height in a temperature range of 280 to 320 ℃ is B, and an exothermic peak height in a temperature range of 330 to 370 ℃ is C, the exothermic peak height B is 20 μ W/mg or more, a/B, which is a ratio of the exothermic peak height A, C to the exothermic peak height B, is 0.45 or less, and C/B is 0.6 or less.
Patent document 3 states that in a differential scanning calorimetry curve of a 6000 series aluminum alloy sheet, in which the total amount of Mg and Si is 1.2% or less, only 1 exothermic peak exists in a temperature range of 230 to 330 ℃, or only 2 exothermic peaks exist in which the temperature difference between both peaks is 50 ℃ or less, and the height of only 1 exothermic peak, or the height of one of the only 2 exothermic peaks, which has a larger peak height, is in a range of 20 to 50 μ W/Mg.
Patent document 4 states that, in a differential scanning calorimetry analysis curve of a 6000-series aluminum alloy sheet, which is essential for adding Sn, the peak height of an endothermic peak in a temperature range of 150 to 230 ℃ is 8 μ W/Mg or less (including 0 μ W/Mg) as an endothermic peak corresponding to melting of Mg-Si clusters, while the peak height of an exothermic peak in a temperature range of 240 to 255 ℃ is 20 μ W/Mg or more as an exothermic peak corresponding to generation of Mg-Si clusters.
Patent document 5 describes that in a differential scanning calorimetry analysis curve after a thermal refining process including a solution treatment and a quenching treatment of an aluminum alloy material, the height of a negative endothermic peak corresponding to a temperature range of 150 to 250 ℃ at which Si/vacancy clusters (GPIs) melt is 1000 μ W or less, and the height of a positive exothermic peak corresponding to a temperature range of 250 to 300 ℃ at which Mg/Si clusters (GPIIs) precipitate is 2000 μ W or less, and that the aluminum alloy material is excellent in room temperature aging suppression and low temperature age hardening capability.
Documents of the prior art
Patent document
Patent document 1: japanese patent No. 4117243 publication
Patent document 2: japanese laid-open patent publication No. 2013-167004
Patent document 3: japanese laid-open patent publication No. 2015-196852
Patent document 4: japanese laid-open patent publication No. 2015-196853
Patent document 5: japanese patent No. 3819263 publication
However, these conventional techniques for controlling the endothermic peak and exothermic peak of DSC are techniques for obtaining a high BH property by baking finish curing treatment in response to the temperature reduction and shortening of the time of the automobile member, and in the case where the heating temperature is not high, 175 ℃ or lower, 150 ℃ is performed. In other words, BH property in the bake hardening treatment at a high temperature such as 180 ℃ or higher is not intended.
Therefore, in such a bake-coating hardening treatment at a high temperature, there is still room for improvement in order to achieve both good formability and high BH property in the Al — Mg — Si based aluminum alloy sheet aged at room temperature for a long time.
Further, there is still room for improvement in that good formability and high BH properties in the bake hardening treatment at high temperature are both attained, and then high BH properties are obtained also in the bake hardening treatment at low temperature.
That is, there is still room for improvement in the technical problem of having elongation, high-temperature BH properties, and low-temperature BH properties which are considered to be contradictory to each other at the same time.
Disclosure of Invention
In view of the above circumstances, an object of the present invention is to provide a 6000 series aluminum alloy sheet and a method for producing the same, which can achieve both good formability and high BH properties even after long-term aging at room temperature, and which can achieve both high BH properties in the bake hardening treatment at high temperature and also in the bake hardening treatment at low temperature in the related art.
To achieve the above object, an aluminum alloy sheet excellent in formability and bake hardenability according to the present invention contains, in mass%, Mg: 0.3 to 1.5%, Si: 0.6 to 1.5%, and the total content of Mg and Si is higher than 1.2%, and the balance is Al and inevitable impurities, wherein in a differential scanning calorimetry curve of the aluminum alloy sheet, an endothermic peak with a height A of 3 to 10 μ W/Mg exists in a temperature range of 150 to 230 ℃, an exothermic peak with a height B of 20 to 50 μ W/Mg exists in a temperature range of 230 ℃ or higher and lower than 330 ℃, and a ratio B/A of a maximum peak height B among the exothermic peaks to a maximum peak height A among the endothermic peaks is higher than 3.5 and lower than 15.0.
In order to achieve the above object, a gist of a method for producing an aluminum alloy sheet excellent in formability and bake hardenability of the present invention is a method for producing an aluminum alloy sheet containing, in mass%, Mg: 0.3 to 1.5%, Si: 0.6 to 1.5%, and the total of the Mg content and the Si content is higher than 1.2%, and the balance is Al and unavoidable impurities, and within 1 hour after the solution treatment and quenching treatment, the aluminum alloy cold-rolled sheet is subjected to a pre-aging treatment for a long time at a low temperature of 30 to 60 ℃ for 5 to 500 hours, wherein in a differential scanning calorimetry analysis curve before the artificial aging treatment of the sheet, an endothermic peak with a height A of 3 to 10 [ mu ] W/Mg is present in a temperature range of 150 to 230 ℃, an exothermic peak with a height B of 20 to 50 [ mu ] W/Mg is present in a temperature range of 230 to 330 ℃, and a ratio B/A of the exothermic peak height B to the endothermic peak height A is higher than 3.5 and lower than 15.0.
The present inventors have studied clusters that have both good formability and high BH properties even after long-term aging at room temperature, namely, high BH properties in the bake-coating hardening treatment at the high temperature and also in the bake-coating hardening treatment at the conventional low temperature.
As a result, it was found that in order to obtain a high elongation after long-term room temperature aging, it is necessary to set the cluster corresponding to the endothermic peak of the differential thermal curve in the temperature range of 150 to 230 ℃ to a certain value or more.
Further, it was found that, in order to obtain a high BH amount at any baking treatment temperature even when the baking treatment temperature in the baking finish hardening treatment is greatly different from a high temperature to a low temperature, clusters corresponding to an endothermic peak of a differential thermal curve are reduced or clusters corresponding to an exothermic peak in a temperature range of 230 ℃ to 330 ℃ are increased.
That is, it was found that by precisely controlling the endothermic peak of the differential thermal curve in the temperature range of 150 to 230 ℃ and the exothermic peak in the temperature range of 230 to 330 ℃, a high elongation can be obtained even after long-term aging at room temperature, and a high BH property can be obtained at any baking treatment temperature even if the baking treatment temperature greatly varies from high temperature to low temperature.
It has also been found that the differential thermal curve for obtaining a high BH property differs depending on the bake hardening conditions (artificial aging conditions) and particularly the temperature, and that the differential thermal curve needs to be controlled more precisely at a comparatively low bake temperature of 175 ℃ or lower than a comparatively high bake temperature of 180 ℃ or higher.
Based on these findings, the present invention provides a 6000-series aluminum alloy sheet which is a material for automobile members and which has elongation, high-temperature BH properties, and low-temperature BH properties, which are said to be contradictory, by performing texture control for more precisely controlling the differential thermal curve.
Drawings
FIG. 1 is an explanatory view showing a differential scanning calorimetry analysis curve of the aluminum alloy sheet of the present invention.
Detailed Description
The aluminum alloy sheet (formed material sheet) in the present invention is a rolled sheet such as a hot rolled sheet or a cold rolled sheet, and is a sheet obtained by subjecting the rolled sheet to hardening and tempering (T4) such as solution treatment and quenching treatment, and is a material aluminum alloy sheet before being subjected to artificial aging treatment (artificial aging hardening treatment) such as paint bake hardening treatment before being formed into an automobile member to be used. In the following description, aluminum is also referred to as aluminum or Al.
Hereinafter, the embodiments of the present invention will be described in detail with respect to the respective requirements.
The aluminum alloy comprises the following components:
first, the chemical composition of the aluminum alloy sheet of the present invention will be described below, including the reasons for limiting the respective elements. The content% of each element means all of mass%.
The chemical composition of the aluminum alloy sheet of the present invention is determined so as to satisfy the formability and bake hardenability required as a material for automobile members such as large automobile body panels from the composition of 6000 series aluminum alloy.
From this viewpoint, the chemical composition of the aluminum alloy sheet of the present invention contains, in mass%, Mg: 0.3 to 1.5%, Si: 0.6 to 1.5%, and the total content of Mg and Si is higher than 1.2%, and the balance is Al and unavoidable impurities.
In this composition, Cu: 0.02-0.8%, Fe: 0.05-0.5%, Mn: 0.05 to 0.3%, Zr: 0.04-0.1%, Cr: 0.04-0.3%, V: 0.02-0.1%, Ag: 0.01-0.1%, Zn: 0.01 to 0.3% of one or more kinds.
Si:0.6~1.5%
Si forms age precipitates such as Mg — Si precipitates contributing to strength improvement in the artificial aging treatment such as solid solution strengthening and baking finish treatment together with Mg, and is an essential element for obtaining a desired strength (yield strength) to exert age-hardening ability.
If the Si content is too small, the amount of solid-solution Si before the bake coating treatment (before the artificial aging heat treatment) is reduced, and the amount of Mg — Si precipitates formed is insufficient, so that BH properties are remarkably reduced and the strength is insufficient. On the other hand, if the Si content is too high, coarse crystals and precipitates are formed, and ductility is reduced, which causes cracking during rolling of the raw material sheet. Therefore, the Si content is in the range of 0.6 to 1.5%, preferably 0.7 to 1.5%.
Mg:0.3~1.5%
Mg is an essential element for obtaining a desired strength, which forms age precipitates such as Mg — Si precipitates contributing to strength improvement in the artificial aging heat treatment such as solid solution strengthening and baking and coating treatment together with Si, and exhibits age hardenability. If the Mg content is too small, the amount of solid-dissolved Mg before the baking coating treatment decreases, and the amount of Mg — Si precipitates formed becomes insufficient, so that BH properties remarkably decrease and the strength becomes insufficient. On the other hand, if the Mg content is too large, shear bands are easily formed during cold rolling, which causes cracking during rolling of the raw sheet. Therefore, the content of Mg is in the range of 0.3 to 1.5%, preferably in the range of 0.4 to 0.8%.
In addition, in order to exert excellent artificial age hardening ability in the coating baking treatment after forming into a panel, the total content of Mg and Si is higher than 1.2%. If the total amount is 1.2% or less as in the above-mentioned patent document 3, the endothermic peak and exothermic peak defined in the present invention cannot be generated even if the production conditions of the sheet are within the preferable ranges described later, for example, and the artificial age hardening ability is insufficient, and the required strength cannot be obtained. However, the upper limit of the total of the Mg content and the Si content is determined based on the limit that the sheet can be produced without hot rolling cracks, and is preferably 2.5%.
One or more than two of Cu, Fe, Mn, Zr, Cr, V, Ag and Zn
These elements have an effect of strengthening the sheet in common, and therefore, in the present invention, they are considered to be equivalent elements and selectively contained as necessary, but specific mechanisms thereof have common portions and naturally have different portions.
Cu can improve strength by solid solution strengthening. If the content of Cu is too small, the effect is small, and if too large, the effect is saturated, and the corrosion resistance and the like are deteriorated.
Fe generates a crystal, and serves as a nucleus of recrystallized grains, thereby preventing coarsening of the grains and improving the strength. If the content is too small, the effect is small, and if it is too large, coarse compounds are formed, which become starting points of fracture and deteriorate the strength and formability.
Mn, Zr, Cr, and V contribute to the improvement of strength by refining crystal grains of an ingot and a final plate product. In addition, these elements are present as dispersed particles, contributing to grain refinement and also improving formability. If the content of each is too small, the effect of improving the strength and formability by refining the crystal grains is insufficient. On the other hand, if these elements are too large, coarse compounds are formed, and ductility deteriorates.
Ag is subjected to artificial aging heat treatment after forming into an automobile member, so that aging precipitates contributing to strength improvement are precipitated densely and finely, and an effect of promoting high strength is exhibited. If the content is too small, the strength-improving effect is small, whereas if it is too large, various properties such as rolling property and weldability are rather deteriorated, and the strength-improving effect is saturated, resulting in high cost.
Zn is useful for improving the artificial age hardening ability (BH property), and has an effect of promoting precipitation of compounds such as GP regions of the plate structure into crystal grains by baking coating treatment, thereby increasing the strength.
Therefore, when Cu, Fe, Mn, Zr, Cr, V, Ag, and Zn are contained, as described above, in the case of Cu: 0.02-0.8%, Fe: 0.05-0.5%, Mn: 0.05 to 0.3%, Zr: 0.04-0.1%, Cr: 0.04-0.3%, V: 0.02-0.1%, Ag: 0.01-0.1%, Zn: 0.01 to 0.3% of a solvent, and one or more kinds of the solvents are contained.
Other elements:
other elements such as Ti and B are inevitable impurities other than those described above. Ti forms a coarse compound together with B to deteriorate mechanical characteristics. However, since the small amount of the content also has an effect of refining the crystal grains of the aluminum alloy ingot, the content of each of the aluminum alloy ingots is allowed to fall within the range specified by JIS standards and the like as a 6000-series alloy. As an example of the allowable amount thereof, Ti is 0.1% or less, preferably 0.05% or less. Further, B is 0.03% or less. Incidentally, in the present invention, Sn, which is essential in the above-mentioned patent document 4, is not added. If Sn is added, the Mg-Si clusters are rather reduced under the preferable plate production conditions described later, and the endothermic peak and exothermic peak defined in the present invention cannot be generated, so that the artificial age hardening ability is insufficient and the required strength may not be obtained.
(raw material plate texture)
Assuming the above alloy composition, in the present invention, the structure of the aluminum alloy sheet is defined by DSC (differential scanning calorimetry analysis curve: DSC profile) obtained by differential scanning calorimetry as an index indicating the existence state of artificially aged precipitates in a member using the sheet as a raw material.
That is, in the present invention, in order to achieve both good formability and high BH property after long-term room temperature aging, DSC (differential scanning calorimetry: DSCprofile) is defined by differential scanning calorimetry, wherein the high BH property is a high BH property in a relatively high-temperature bake-coating hardening treatment which is a main object, and the high BH property is naturally also achieved in a relatively low-temperature bake-coating hardening treatment.
Under the relatively low temperature baking coating condition, if the endothermic peak in the temperature range of 150 to 230 ℃ exists, it is difficult to obtain a high BH amount, but under the relatively high temperature baking coating condition which is the main object of the present invention, even if the endothermic peak exists to some extent, a high BH amount can be obtained. It is presumed that, under the conventional baking coating conditions at a relatively low temperature, even if the cluster corresponding to the endothermic peak is melted in the baking coating treatment, the strengthening phase is difficult to regenerate thereafter, and thus a high BH property is not obtained.
On the other hand, it is presumed that under the comparatively high-temperature baking coating conditions which are the main object of the present invention, clusters corresponding to the endothermic peak are melted in a very short time, and then the strengthening compatibility is easily regenerated, and therefore, even if the endothermic peak is present in advance, the high BH property can be obtained. Therefore, under the comparatively high-temperature bake coating hardening conditions targeted by the present invention, the endothermic peak can be increased, and the work hardening can be improved by the presence of the cluster corresponding to the endothermic peak. Therefore, if the heat emission peak in the temperature range of 230 ℃ or higher and lower than 330 ℃ for improving BH property is also present, high work hardenability (moldability) can be achieved in combination with BH property.
The relatively high temperature baking finish hardening treatment conditions are, for example, baking finish hardening treatment under conditions of a heating temperature of 180 to 230 ℃ and a heating holding time of 10 to 30 minutes, and are different from the conventional low temperature and short time baking finish hardening treatment in that the heating temperature is not higher than 175 ℃, particularly in that the heating temperature is not higher.
When such exothermic peak and endothermic peak are caused to coexist, the respective peak heights are important, and the balance of the peak heights with each other is also important. For example, when the ratio of the exothermic peak/endothermic peak is too small, the BH property becomes too low due to the existence of clusters corresponding to the endothermic peak, or the exothermic peak is too low, and the cluster as the reinforcing phase becomes too large, and the elongation is lowered. On the other hand, when the heat emission peak/heat absorption peak is too large, the cluster corresponding to the heat absorption peak is too small, the work hardenability is poor, or the heat emission peak is too high, and the cluster as the reinforcing phase is too small, and the BH property becomes too low.
Based on such recognition, in the present invention, in order to achieve a high elongation in combination with a comparatively high BH property, an endothermic peak having a height A of 3 to 10 μ W/mg exists in a temperature range of 150 to 230 ℃ and an exothermic peak having a height B of 20 to 50 μ W/mg exists in a temperature range of 230 ℃ or higher and lower than 330 ℃ in DSC before the artificial aging treatment of an aluminum alloy sheet, and a ratio B/A of the exothermic peak height B to the endothermic peak height A is higher than 3.5 and lower than 15.0.
In addition, not only in the case of the relatively high temperature but also in the BH property at a relatively low temperature, in order to be compatible with the high elongation, in DSC before artificial aging treatment of an aluminum alloy sheet, an endothermic peak having a height A of preferably 3 to 8 μ W/mg is present in a temperature range of 150 to 230 ℃, and an exothermic peak having a height B of preferably 20 to 40 μ W/mg is present in a temperature range of 230 ℃ or higher and lower than 330 ℃, and a ratio B/A of the exothermic peak height B to the endothermic peak height A is higher than 3.5 and lower than 15.0. Further, it is more preferable that the height A of the endothermic peak is 3 to 7 μ W/mg and the height B of the exothermic peak is 20 to 35 μ W/mg.
The endothermic peak is high on the negative side, and means that the cluster melts in the differential thermal analysis, in other words, that the cluster corresponding to the endothermic peak is large. If the height A of the peak on the negative side is less than 3. mu.W/mg, the work hardening property is lowered and the moldability is lowered. On the other hand, if the height A of the peak on the negative side is too high and exceeds 10. mu.W/mg, the BH property at a relatively high temperature is lowered, and if it is too high and exceeds 7. mu.W/mg, the BH property at a relatively low temperature is lowered.
The exothermic peak is referred to as an exothermic peak height, which means that a large amount of clusters of the reinforcing phase or the nuclei of the reinforcing phase are generated in the differential thermal analysis, in other words, that the number of clusters of the reinforcing phase or the nuclei of the reinforcing phase is small. If the height B of the peak on the positive side is too high and higher than 50 μ W/mg, the reinforcing phase or cluster as a core of the reinforcing phase is too small, and BH properties of bake hardening at a relatively high temperature become low, and if it is too high and higher than 40 μ W/mg, BH properties of bake hardening at a relatively low temperature become low. On the other hand, if the height B of the peak on the forward side is too low and less than 20 μ W/mg, the reinforcing phase or clusters serving as nuclei of the reinforcing phase become too large, and the elongation is lowered. Incidentally, these tendencies are premised on the alloy composition of the sheet satisfying the scope of the present invention.
As described above, the structure defined by DSC at the stage of the raw material sheet is closely related to the behavior of the precipitated phase generated during the artificial aging treatment (BH) of the member such as the automobile panel produced from the raw material sheet. As a result, even if the member is not manufactured, the formability and BH properties of the raw material sheet can be evaluated if the DSC is controlled at the stage of the raw material sheet. In other words, the structure defined by the DSC at the stage of the raw material sheet can be an index of formability and BH properties of a member using the raw material sheet as a forming material.
For example, in the patent document 1, there is no endothermic peak having a height of 3 to 10 μ W/mg in a temperature range of 150 to 230 ℃, and conversely, there is an exothermic peak in a temperature range of 100 to 200 ℃. In the patent document 2, as shown in FIG. 1, there is no endothermic peak having a height of 3 to 10 μ W/mg in a temperature range of 150 to 230 ℃. In patent document 3, the total amount of Mg and Si is 1.2% or less, so that the endothermic peak and exothermic peak defined in the present invention cannot be generated, the artificial age hardening ability is insufficient, and the desired strength cannot be obtained. In the patent document 4, as shown in FIG. 1, there is no endothermic peak having a height of 3 to 10. mu.W/mg in a temperature range of 150 to 230 ℃ and an exothermic peak having a height of more than 20. mu.W/mg in a temperature range of more than 260 ℃ and 330 ℃ or less. In the patent document 5, as shown in fig. 1, there are endothermic peaks with a height of 3 to 10 μ W/mg in a temperature range of 150 to 230 ℃ and exothermic peaks at a position of 260 ℃, but the ratio of exothermic peak/endothermic peak is too small to be less than 3.5 and the ratio of endothermic peak is too high.
Therefore, in the conventional technique for controlling the endothermic peak and exothermic peak of DSC, unlike the endothermic peak and exothermic peak of DSC specified in the present invention, under the relatively high temperature bake hardening conditions targeted by the present invention, it is not possible to achieve a combination of high elongation (formability) and bake hardening of 6000 series aluminum alloy sheet after long time room temperature aging.
(method of controlling tissue defined by DSC)
The control of the structure specified by the heat emission peak of the DSC is performed by subjecting an aluminum alloy cold-rolled sheet to a pre-aging treatment for a long time at a temperature of 30 to 60 ℃ for 5 to 500 hours at a low temperature within 1 hour after the solution treatment and quenching treatment, as will be described later.
In order to improve the BH property at a relatively low temperature, as described later, the method is performed by performing a pre-aging treatment in which the solution treatment and the quenching treatment are performed within 1 hour after the solution treatment and the quenching treatment, the pre-aging treatment is performed in a high temperature range of 100 to 300 ℃ for a short time of 5 seconds or more and 300 seconds or less, and then the pre-aging treatment is performed. Therefore, there is an advantage that the composition of 6000 series aluminum alloy which has been standardized as the automobile member of a rolled sheet is not greatly changed, and the rolling process by a conventional method is not greatly changed and can be controlled.
(production method)
The 6000 series aluminum alloy sheet of the present invention is produced by a conventional method in which an ingot is hot-rolled after soaking, and then cold-rolled sheet is further subjected to hardening and tempering such as solution treatment. That is, the hot-rolled aluminum alloy sheet is manufactured through ordinary manufacturing processes of casting, homogenization heat treatment, and hot rolling, and has a thickness of about 2 to 10 mm. Then, the steel sheet is cold-rolled to a cold-rolled sheet having a thickness of 3mm or less.
(melting, casting)
First, in the melting and casting step, an aluminum alloy molten metal prepared to have the above 6000-series composition range is cast and melted by a normal solution casting method such as a continuous casting method or a semi-continuous casting method (DC casting method) as appropriate. In order to control the cluster within the range defined by the present invention, the average cooling rate during casting is preferably as high (fast) as possible, i.e., 30 ℃/min or more from the liquidus temperature to the solidus temperature. If such temperature (cooling rate) control of the high-temperature region at the time of casting is not performed, the cooling rate of the high-temperature region is inevitably slow. In this temperature range of the high temperature region, the amount of coarse crystallized products increases, and variations in the size and amount of crystallized products in the slab width direction and thickness direction of the ingot also increase. As a result, the DSC cannot be controlled within the scope of the present invention, which is highly likely.
Next, the cast aluminum alloy ingot is subjected to a homogenizing heat treatment before hot rolling. In the homogenization heat treatment (soaking treatment), it is important to sufficiently dissolve Si and Mg in a solid solution, in addition to the homogenization of the structure (elimination of segregation in crystal grains in the ingot structure) which is a common purpose.
The homogenization heat treatment temperature is 500 ℃ or higher and 580 ℃ or lower, and the homogenization (holding) time is appropriately selected from the range of 1 hour or longer, so that Si and Mg are sufficiently dissolved in a solid solution. If the homogenization temperature is low, the amount of solid solution of Si and Mg cannot be secured, and the DSC heat emission peak cannot be specified even by the pre-aging treatment (reheating treatment) after the solid solution quenching treatment described later. Further, segregation in the crystal grain cannot be sufficiently eliminated, and this acts as a point of origin of fracture, and therefore formability is lowered.
After the homogenization heat treatment, hot rolling is performed, but it is preferable that the temperature of the ingot is not lowered to 450 ℃ or lower until the rough rolling in the hot state after the homogenization heat treatment is started, in order to secure the solid solution amount of Si and Mg. When the temperature of the ingot is lowered to 450 ℃ or lower by the start of rough rolling, Si and Mg are precipitated, and there is a high possibility that the predetermined amount of solid solution of Si and Mg for achieving DSC is not secured.
(Hot Rolling)
Hot rolling is composed of a rough rolling step and a finish rolling step of an ingot (slab) depending on the thickness of a rolled sheet. In the rough rolling step and the final rolling step, a reversing type or tandem type rolling mill is preferably used.
In the rolling from the start to the end of the hot rough rolling, it is necessary to ensure the solid solution amounts of Si and Mg without lowering the temperature to 400 ℃. In hot rough rolling, if the temperature of the rough rolled sheet is lowered to 400 ℃ or lower, Si and Mg precipitate, and there is a high possibility that the predetermined amount of solid solution of Si and Mg for achieving the DSC described above cannot be secured.
After the hot rough rolling, hot finish rolling is performed at a finish temperature in a range of 250 to 360 ℃. If the soaking temperature and the finishing temperature of the finish rolling are too low, Mg and Si compounds are generated during the soaking and the hot rolling, and the balance of solid solution Mg/Si changes, making it difficult to achieve the DSC specifications.
(annealing of Hot rolled sheet)
The hot-rolled sheet does not require annealing before cold rolling, but can be performed.
(Cold Rolling)
In the cold rolling, the hot rolled sheet is rolled to produce a cold rolled sheet (including a coil) having a desired final thickness. However, the cold rolling reduction is preferably 60% or more in order to further refine the crystal grains, and intermediate annealing may be performed between cold rolling passes for the same purpose as the annealing.
(solution and quenching treatment)
After cold rolling, solution treatment is performed, and subsequently, quenching treatment to room temperature is performed. In the solution quenching treatment, in order to obtain a sufficient amount of solid solution of each element such as Mg and Si, it is preferable to heat the steel to a solution treatment temperature of 500 ℃ or higher and a melting temperature or lower.
From the viewpoint of suppressing the formation of coarse grain boundary compounds that reduce formability, the average cooling rate from the solution temperature to the quenching stop temperature at room temperature is preferably 20 ℃/s or more. When the average cooling rate of the quenching treatment to room temperature after the solution treatment is small, coarse Mg is generated during cooling2Si and Si alone deteriorate the bending workability. Further, the amount of solid solution after solid solution is reduced, and the BH property is reduced. In order to ensure the cooling rate, air cooling, spraying, or spraying with a fan or the like is selected for the quenching treatmentWater, immersion, and the like.
(Pre-aging treatment: reheating treatment)
After the solution treatment, the cold rolled sheet is quenched and cooled to room temperature, and then preferably subjected to a pre-aging treatment (reheating treatment) within 1 hour. In order to control the structure specified by the DSC peak, it is preferable to perform the pre-aging treatment at a lower temperature for a longer time than the conventional method, that is, to perform the pre-aging treatment in which the temperature is maintained at a temperature of 30 to 60 ℃ for a long time at a low temperature of 5 hours or more and 500 hours or less. Thus, Mg-Si clusters in which Mg and Si are well balanced can be formed, and the structure specified by the DSC peak can be obtained. Therefore, the high elongation and the BH properties at a relatively high temperature can be achieved by the pre-aging treatment at a low temperature for a long time.
After the quenching treatment to room temperature is completed, if the room temperature holding time until the pre-aging treatment is started (heating is started) is too long, clusters corresponding to the endothermic peak are excessively generated due to room temperature aging, and BH properties are liable to be lowered. Therefore, as the room temperature holding time is shorter, the solution treatment and the quenching treatment and the reheating treatment can be continuously performed with almost no time difference, and the lower limit time is not particularly set.
When the pre-aging temperature is less than 30 ℃ or the holding time is less than 5 hours, clusters corresponding to the endothermic peak are excessively generated, and the BH properties are liable to be lowered, as in the case where the pre-aging treatment is not performed. On the other hand, if the pre-aging condition is higher than 60 ℃ or more than 500 hours, the amount of the reinforcing phase corresponding to the heat emission peak and the cluster which is the nucleus of the reinforcing phase are excessively generated, the strength during press forming before bake coating becomes excessively high, and the formability is easily deteriorated.
Here, in order to further improve BH properties at a relatively low temperature, it is preferable to perform a pre-aging treatment for a short time at a high temperature of 5 seconds to 300 seconds in a temperature range of 100 to 300 ℃ and immediately perform the pre-aging treatment for a long time at a low temperature within 1 hour after the solution and quenching treatment. Thus, the DSC can be controlled reliably by the low-temperature long-time pre-aging treatment, and the endothermic peak height A in the temperature range of 150 to 230 ℃ of the DSC can be controlled to be in the range of preferably 3 to 8. mu.W/mg, more preferably 3 to 7. mu.W/mg. In addition, the height B of the exothermic peak in the temperature range of 230 ℃ or higher and lower than 330 ℃ of the DSC can be controlled to be preferably 20 to 40. mu.W/mg, more preferably 20 to 35. mu.W/mg. If this high-temperature and short-time pre-aging treatment is not performed or if the conditions are not satisfied even if the conditions are performed, the alloy composition and the production process are different, and therefore, the structure cannot be specified by the peak of the DSC, or the BH properties may be lowered at a relatively low temperature.
In this way, the aluminum alloy sheet of the present invention produced as a structure specified by the DSC peak is press-formed as a material into a large body panel of an automobile or the like, and then subjected to a bake-finish hardening treatment (artificial aging treatment) after coating to increase the strength. As described above, the baking finish curing treatment is preferably carried out at a high temperature to achieve the effects of the present invention, and examples of the method include a heating temperature of 180 to 230 ℃ and a heating holding time of 10 to 30 minutes. If the heating temperature is too low or the like, rather than the conditions for the bake-coating hardening treatment, it is necessary to control the structure shown by the differential thermal curve more precisely as described above.
Examples
Next, an embodiment of the present invention is explained. 6000 series aluminum alloy sheets having different structures defined by DSC of the present invention were produced by changing the composition and production conditions. Then, after the plate was kept at room temperature for 100 days after the production, As yield strength (yield strength before bake-finish hardening treatment) and AB yield strength (yield strength after bake-finish hardening treatment), elongation at break, BH properties (bake-finish hardening properties) were measured and evaluated, respectively. The results are shown in tables 1 and 2.
Specifically, the above-described respective manufacturing methods were carried out by variously changing the pre-aging conditions after the solution treatment and the quenching treatment, as shown in table 2, for the 6000-series aluminum alloy sheets having the compositions shown in table 1. Here, in the display of the content of each element in table 1, the numerical value of each element is displayed in blank, indicating that the content thereof is below the detection limit.
(production conditions of aluminum alloy sheet)
The specific production conditions of the aluminum alloy sheet are common (the same) in the following examples except for the above-mentioned pre-aging treatment conditions. Aluminum alloy ingots having respective compositions shown in Table 1 were all melted by a DC casting method. In this case, the average cooling rate during casting was 50 ℃/min from the liquidus temperature to the solidus temperature in the same manner in each example. Then, the ingot is subjected to soaking treatment at 550 ℃ for 10 hours after surface cutting if necessary, hot rough rolling is started at that temperature, and then hot finish rolling is performed at a finishing temperature of 250 to 360 ℃ to obtain a hot-rolled sheet. The hot-rolled sheet was cold-rolled at a reduction ratio of 67% to obtain a cold-rolled sheet having a thickness of 1.0 mm.
The respective cold-rolled sheets were subjected to solution treatment at 550 ℃ for 1 minute in a saltpeter furnace, and then cooled to room temperature by water cooling. Within 1 hour after the cooling, the temperature (. degree.C.) and the holding time (hr) shown in Table 2 were subjected to high-temperature short-time pre-aging using an oil bath and low-temperature long-time pre-aging using an atmospheric furnace, and air-cooled after the pre-aging treatment.
After these thermal refining treatments, the plates were left at room temperature for 100 days, and then test plates (blanks) were cut out from the longitudinal end portions and the width center portion of the product from each of the final product plates to measure and evaluate the DSC and the properties of each of the test plates. The results are shown in Table 2.
(DSC measurement)
The DSC of the structure was measured at 3 points in the center portion of the test plate in the thickness, and the temperature (. degree.C.) and height (. mu.W/mg) of the endothermic peak and the temperature (. degree.C.) and height (. mu.W/mg) of the exothermic peak were measured in the DSC (differential scanning calorimetry) curve) of the plate, respectively, based on the average values at these 3 points. In Table 2 showing the results, for convenience, only the endothermic peak in the temperature range of 150 to 230 ℃ is referred to as "endothermic peak", and the exothermic peak in the temperature range of 230 ℃ or higher and lower than 330 ℃ is referred to as "exothermic peak".
In the differential thermal analysis of the respective measurement positions of these test panels, the test apparatus was operated under the same conditions as follows: セイコ - インスツルメンツ TG/DTA6300, Standard substance: aluminum, sample container: aluminum, temperature rising condition: 10 ℃/min, atmosphere: argon (50ml/min), weight of sample: 39.0 to 41.0mg, dividing the obtained differential thermal analysis curve (. mu.W) by the sample weight (. mu.W/mg), and measuring the endothermic peak height and exothermic peak height from the reference level with the region where the differential thermal analysis curve is horizontal being set as the reference level of 0 in the range of 0 to 100 ℃ of the differential thermal analysis curve.
Hardening by baking for coating
As mechanical properties of the test sheet, 0.2% yield strength (As yield strength) and elongation at break (%) were determined by tensile test. The test panels were identical to each other, and the 0.2% proof stress (AB proof stress) of the test panel was determined by a tensile test after 2% elongation of the automobile member by press forming was simulated, and after the artificial age hardening treatment at 185 ℃x20 minutes was performed as the paint bake hardening treatment at high temperature, and after the artificial age hardening treatment at 170 ℃x20 minutes was performed as the paint bake hardening treatment at low temperature (after BH). Then, the 0.2% yield strength after BH property of each test piece was evaluated from the difference between these 0.2% yield strengths (increase in yield strength), and it was judged as acceptable when the paint bake hardening treatment at high temperature (185 ℃ C.. times.20 minutes) was 190MPa or more, and the paint bake hardening treatment at low temperature (170 ℃ C.. times.20 minutes) was at least 160MPa, preferably 180MPa or more, and the elongation at break as an evaluation of press formability was 25% or more. In addition, in the elongation at break as an evaluation of press formability, 24% and 25% differ only slightly by 1%, but it greatly affects whether corners and character lines which sharpen or complicate the shape of, for example, an outer panel of an automobile can be formed in a beautiful and clear curved structure without deformation and wrinkles.
In the tensile test, test pieces No. 13A (20 mm. times.80 mm GL. times.plate thickness) of JIS Z2201 were extracted from the respective test plates, and the tensile test was carried out at room temperature. The tensile direction of the test piece at this time was set to be a direction perpendicular to the rolling direction. The drawing speed was 5 mm/min up to 0.2% yield strength and 20 mm/min after yield strength. The number N of the mechanical property measurements was 5, calculated as each average value. In the test piece for measuring the yield strength after BH, the test piece was prestrained by 2% using a tensile tester, and then BH treatment was performed.
As shown in tables 1 and 2, inventive examples 1 to 8 were produced within the ranges of the composition of the present invention and the preferable conditions, and the pre-aging treatment was performed at a low temperature for a long time in the preferable ranges. Therefore, as shown in Table 2 for each of these inventions, DSC is excellent in moldability and BH property as shown in Table 2 even after long-term aging at room temperature as defined in the present invention.
Specifically, the alloy exhibits a high elongation at break of at least 26%, a high BH property at a high temperature (185 ℃ C.. times.20 minutes) of at least 192MPa, and a low BH property at a low temperature (170 ℃ C.. times.20 minutes) of at least 162 MPa.
In comparison between the invention examples in table 2, invention example 2 in which the low-temperature long-time pre-aging treatment was performed immediately after the high-temperature short-time pre-aging treatment was performed had a BH property higher at a relatively low temperature than that of invention example 1 in which only the low-temperature long-time pre-aging treatment was performed without performing the high-temperature short-time pre-aging treatment. Similarly, in the invention examples 6, 7 and 8 in which the low-temperature long-time pre-aging treatment was performed immediately after the high-temperature short-time pre-aging treatment, the BH properties were higher in average than those at a relatively low temperature, though the alloy composition was different from each other, compared with the invention examples 3, 4 and 5 in which only the low-temperature long-time pre-aging treatment was performed without performing the high-temperature short-time pre-aging treatment.
This is because, in the invention examples 2, 6, 7 and 8, the height a of the endothermic peak in the temperature range of 150 to 230 ℃ in the DSC was more accurately controlled to the preferable range (3 to 8 μ W/mg) or the more preferable range (3 to 7 μ W/mg) by further adding the high-temperature short-time pre-aging treatment, and the height B of the exothermic peak in the temperature range of 230 ℃ or higher and lower than 330 ℃ in the DSC was also more accurately controlled to the preferable range (20 to 40 μ W/mg) and the more preferable range (20 to 35 μ W/mg).
In contrast, comparative examples 1 to 6 in table 2 used alloy example 1 similar to the inventive example. However, in each of these comparisons, as shown in table 2, the production conditions such as the temperature and the holding time of the pre-aging treatment deviate from the preferable conditions. As a result, DSC outside the range defined in the present invention was inferior to invention example 1 having the same alloy composition in either BH property or formability after long-term room temperature aging, and thus the DSC could not be compatible with the invention example 1. Specifically, the above-mentioned pass standard was not satisfied with respect to the BH property at high temperature (185 ℃ C.. times.20 minutes) of less than 190MPa even when the elongation at break was 26% or more, or with respect to the BH property at high temperature (185 ℃ C.. times.20 minutes) of less than 190MPa but the elongation at break was less than 25%.
Of these, comparative example 1 was not subjected to the pre-aging treatment. Therefore, although an endothermic peak exists in a temperature range of 150 to 230 ℃, the height A is excessively high above 10. mu.W/mg, and the exothermic peak height B in a temperature range of 230 ℃ or higher and lower than 330 ℃ is also excessively high above 50. mu.W/mg.
In comparative example 2, the time for the pre-aging treatment on the low temperature side was too short. Therefore, although an endothermic peak exists in a temperature range of 150 to 230 ℃, the height A is excessively high above 10. mu.W/mg, and the exothermic peak height B in a temperature range of 230 ℃ or higher and lower than 330 ℃ is also excessively high above 50. mu.W/mg.
In comparative example 3, the temperature of the pre-aging treatment on the low temperature side was too high. Therefore, although an endothermic peak having a height A of 3 to 10 μ W/mg exists in the temperature range of 150 to 230 ℃, the exothermic peak height B in the temperature range of 230 ℃ or higher and lower than 330 ℃ is too low to be 20 μ W/mg.
In comparative example 4, the time for the pre-aging treatment on the low temperature side was too long. Therefore, although an endothermic peak exists in the temperature range of 150 to 230 ℃, the height A is too low as lower than 3 μ W/mg, and the exothermic peak height B in the temperature range of 230 ℃ or higher and lower than 330 ℃ is also too low as lower than 20 μ W/mg.
In comparative example 5, the time for the pre-aging treatment on the high temperature side was too long. Therefore, although an endothermic peak exists in the temperature range of 150 to 230 ℃, the height A is too low as lower than 3 μ W/mg, and the exothermic peak height B in the temperature range of 230 ℃ or higher and lower than 330 ℃ is also too low as lower than 20 μ W/mg.
In comparative example 6, the temperature of the pre-aging treatment on the low temperature side was too high. Therefore, although an endothermic peak exists in the temperature range of 150 to 230 ℃, the height A is too low below 3 μ W/mg, and the ratio B/A of the exothermic peak height B in the temperature range of 230 ℃ or higher and below 330 ℃ is too high above 15.0.
Although comparative examples 7 and 8 in Table 2 were produced in the preferred ranges including the above-mentioned pre-aging treatment conditions, alloy Nos. 7 and 8 in Table 1 were used, and the alloy compositions were out of the range of the present invention. Therefore, as a result of these comparisons, as shown in table 2, DSC and the like do not deviate from the ranges specified in the present invention, and the BH properties and moldability after long-term room temperature aging do not have both of the variances. Specifically, even when the elongation at break is 25% or more, the BH property at a high temperature (185 ℃ C.. times.20 minutes) is only about 138 to 146MPa, and the BH property at a low temperature (170 ℃ C.. times.20 minutes) is only about 133 to 139 MPa.
Comparative example 7 is alloy 7 of table 1, and Mg is too small, and the total content of Mg and Si is too small. Therefore, although an endothermic peak exists in the temperature range of 150 to 230 ℃, the height A is too low as lower than 3 μ W/mg, and the exothermic peak height B in the temperature range of 230 ℃ or higher and lower than 330 ℃ is also too low as lower than 20 μ W/mg.
Comparative example 8 is alloy 8 of table 1, and the total content of Mg and Si is too small. Therefore, although an endothermic peak exists in the temperature range of 150 to 230 ℃, the height A is too low as lower than 3 μ W/mg, and the exothermic peak height B in the temperature range of 230 ℃ or higher and lower than 330 ℃ is also too low as lower than 20 μ W/mg.
The DSC selected from the inventive examples and comparative examples is shown in fig. 1. In fig. 1, the unit of the vertical axis indicated as "Heat Flow" is μ W/m, the thick solid line indicates invention example 1 in table 2, the thick dotted line (broken line) indicates invention example 2, and the thin dotted line indicates comparative example 3. As shown in FIG. 1, in the present invention examples, an endothermic peak having a height A of 3 to 10. mu.W/mg exists in a temperature range of 150 to 230 ℃, an exothermic peak having a height B of 20 to 50. mu.W/mg exists in a temperature range of 230 ℃ or more and less than 260 ℃, and an exothermic peak having a height of 20. mu.W/mg or more does not exist in a temperature range of 260 ℃ or more and less than 330 ℃.
From the results of the above examples, it is possible to demonstrate the significance of the composition and the criticality of each condition of DSC defined in the present invention, which is used for the bake-finish hardening treatment at high temperature and after long-term aging at room temperature, and which makes the material have both good formability and high BH properties.
[ Table 1]
The numerical value of each element is in the blank column meaning below the detection limit
The present invention has been described in detail and with reference to specific embodiments thereof, but it will be apparent to one skilled in the art that various changes and modifications can be made therein without departing from the spirit and scope thereof.
The present application is based on Japanese patent application No. 2016-.
Industrial applicability
According to the present invention, it is possible to provide a 6000 series aluminum alloy sheet which can achieve both good formability and high BH properties even after long-term aging at room temperature and bake hardening treatment at high temperature. That is, the excellent formability and the high BH property can be achieved at the same time even after the long-term room temperature aging, and the high BH property can be achieved at the same time in the bake-coating hardening treatment at high temperature, not to mention the bake-coating hardening treatment at low temperature in the conventional bake-coating hardening treatment. As a result, the 6000 series aluminum alloy sheet can be widely used as an automobile member including a panel material.

Claims (6)

1. An aluminum alloy sheet having excellent formability and bake hardenability, characterized by containing, in mass%, Mg: 0.3 to 1.5%, Si: 0.6 to 1.5%, and the total of the Mg content and the Si content is higher than 1.2%, and the balance is Al and unavoidable impurities, wherein in a differential scanning calorimetry analysis curve of the aluminum alloy sheet, an endothermic peak with a height A of 3 to 8 [ mu ] W/Mg exists in a temperature range of 150 to 230 ℃, an exothermic peak with a height B of 20 to 40 [ mu ] W/Mg exists in a temperature range of 230 ℃ or higher and lower than 330 ℃, and a ratio B/A of a maximum peak height B among the exothermic peaks to a maximum peak height A among the endothermic peaks is higher than 3.5 and lower than 15.0.
2. The aluminum alloy sheet excellent in formability and bake hardenability according to claim 1, further comprising, in mass%, Cu: 0.02-0.8%, Fe: 0.05-0.5%, Mn: 0.05 to 0.3%, Zr: 0.04-0.1%, Cr: 0.04-0.3%, V: 0.02-0.1%, Ag: 0.01-0.1%, Zn: 0.01 to 0.3% of one or more kinds.
3. A method for producing an aluminum alloy sheet having excellent formability and bake hardenability, characterized by comprising, in terms of mass%, Mg: 0.3 to 1.5%, Si: 0.6 to 1.5%, wherein the total of the Mg content and the Si content is higher than 1.2%, the balance is Al and unavoidable impurities, the temperature is not reduced to 400 ℃ or lower in the rolling from the start to the end of the hot rough rolling, and after the hot rough rolling, the hot finish rolling is performed at an end temperature in the range of 250 to 360 ℃ within 1 hour after the solution and quenching treatment, first, the pre-aging treatment is performed for a short time at a high temperature of 5 seconds to 300 seconds in a temperature range of 100 to 300 ℃, then, the pre-aging treatment is immediately performed for a long time at a low temperature of 5 hours to 500 hours in a temperature range of 30 to 60 ℃, whereby in a differential scanning calorimetry analysis curve of the sheet before the artificial aging treatment, an endothermic peak having a height A of 3 to 10 μ W/Mg is present in the temperature range of 150 to 230 ℃, and an exothermic peak having a height B of 20 to 50 μ W/Mg is present in the temperature range of 230 ℃ to 330 ℃ A heat peak, and a ratio B/A of the exothermic peak height B to the endothermic peak height A is higher than 3.5 and lower than 15.0.
4. The method for producing an aluminum alloy sheet excellent in formability and bake hardenability according to claim 3, wherein the aluminum alloy sheet further contains, in mass%, Cu: 0.02-0.8%, Fe: 0.05-0.5%, Mn: 0.05 to 0.3%, Zr: 0.04-0.1%, Cr: 0.04-0.3%, V: 0.02-0.1%, Ag: 0.01-0.1%, Zn: 0.01 to 0.3% of one or more kinds.
5. The method for manufacturing an aluminum alloy sheet excellent in formability and bake hardenability according to claim 3, wherein the aluminum alloy sheet is coated after forming, and is subjected to bake hardening treatment under conditions of a heating temperature of 180 to 230 ℃ and a heating holding time of 10 to 30 minutes.
6. The method for manufacturing an aluminum alloy sheet excellent in formability and bake hardenability according to claim 4, wherein the aluminum alloy sheet is coated after forming, and is subjected to bake hardening treatment under conditions of a heating temperature of 180 to 230 ℃ and a heating holding time of 10 to 30 minutes.
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