ALUMINUM ALLOY PLATE HAVING EXCELLENT MOLDABILITY AND BAKE HARDENING PROPERTIES
The purpose of the present invention is to provide an aluminum alloy plate capable of having a 0.2% proof stress during molding of no more than 110 MPa and a 0.2% proof stress after BH of at least 170 MPa. The present invention pertains to an aluminum alloy plate including, in mass %, 0.2%-1.0% Mg and 0.2%-1.0% Si, fulfilling {(Mg content)+(Si content)}≤1.2%, having a 20-50 μW/mg high exothermic peak within a temperature range of 230-330° C. in a differential scanning calorimetry curve, and having both excellent moldability and excellent bake hardening properties.
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The present invention relates to an Al—Mg—Si alloy sheet. The aluminum alloy sheet referred to in the present invention means an aluminum alloy sheet that is a rolled sheet such as a hot rolled sheet or a cold rolled sheet and has been subjected to refining such as a solution heat treatment and a quenching treatment, but is not yet subjected to a press forming and a bake hardening treatment. Further, aluminum is hereinafter also referred to as Al.
BACKGROUND ARTIn recent years, because of environmental awareness and the like, the society's requirement for weight reduction in a vehicle such as an automobile has been steadily increasing. In order to respond to such requirement, as a material for a large body panel structure (an outer panel or an inner panel) of an automobile instead of a steel material such as a steel sheet, application of an aluminum alloy material excellent in formability and bake hardenability and lighter in weight has been increasing.
Among the large body panel structure of an automobile, for an outer panel (outer sheet) such as a hood, a fender, a door, a roof, or a trunk lid, use of an Al—Mg—Si-based AA or JIS 6000-series (hereinafter, also simply referred to as a 6000-series) aluminum alloy sheet, as a thin and high strength aluminum alloy sheet, has been studied.
The 6000-series aluminum alloy sheet contains Si and Mg as essential components. In particular, a 6000-series aluminum alloy with excess Si has a composition in which the Si/Mg mass ratio is 1 or greater, and has excellent age hardenability. Because of this, formability for press forming or bending into the outer panels of automobiles is secured by lowering the proof stress. In addition, it has such bake hardenability (hereinafter referred to also as BH response) that it undergoes age hardening upon heating in an artificial aging (hardening) treatment performed at a relatively low temperature, such as the baking treatment of formed panels, and hence improves in proof stress, thereby ensuring the strength required as a panel.
On the other hand, as is known well, an outer panel of an automobile is manufactured by applying combined formings, such as stretch forming or bending forming in press forming, to an aluminum alloy sheet. For example, in a large outer panel such as a hood or a door, the shape of a formed product is made as an outer panel by press forming such as stretching, and then joining with an inner panel is executed by hem work (hemming) of a flat hem and the like of the outer panel peripheral section to be formed into a panel structural body.
Here, the 6000-series aluminum alloy had an advantage of having excellent BH response, but had a problem of having aging properties at room temperature, that is, of age hardening during retention at room temperature after solution heat treatment and quenching treatment to increase the strength, thereby deteriorating formability into a panel, particularly the bendability. For example, in a case where a 6000-series aluminum alloy sheet is to be used for an automobile panel, it is placed at room temperature (standing at room temperature) for approximately 1 month after the solution heat treatment and the quenching treatment (after manufacturing) at an aluminum manufacturer until forming into a panel at an automobile manufacturer, and comes to be significantly age hardened (room-temperature aged) during that time. Particularly, in the outer panel subjected to severe bending, there was such a case that, although forming was possible without any problem immediately after manufacturing, cracking occurred in hem working after the lapse of 1 month. Therefore, in the 6000-series aluminum alloy sheet for an automobile panel, particularly for an outer panel, it is necessary to suppress room-temperature aging over a comparatively long period of approximately 1 month.
Moreover, in the case where such room-temperature aging is great, there also is a case that the BH response deteriorate and the proof stress is not improved to the strength required as a panel by heating during an artificial aging (hardening) treatment at a comparatively low temperature, such as a bake treatment and the like of the panel after forming described above.
Hereto, in order to cope with such decreases in the formability and BH response of 6000-series aluminum alloy sheets due to room-temperature aging, various proposals have been made on methods for regulating Mg—Si clusters which are formed in the sheets during room-temperature standing after refining (after solution and quenching treatments). Among these proposed methods is a technique in which such Mg—Si clusters are controlled by means of endothermic peaks and exothermic peaks of a differential scanning thermal analysis curve (also called a differential scanning calorimetry curve; hereinafter referred to also as DSC) of the 6000-series aluminum alloy sheet.
For example, Patent Documents 1 and 2 propose that the formation amount of Mg—Si clusters that inhibit room-temperature aging and suppress low-temperature age hardenability, in particular, Si/hole clusters (GPI), is regulated. In these techniques, for regulating the formation amount of GPI, it is regulated that the T4 material (after solution treatment and subsequent natural aging) gives a DSC which has no endothermic peak in the temperature range of 150-250° C., corresponding to the dissolution of GPI. In these techniques, a low-temperature heat treatment of holding at 70-150° C. for about 0.5-50 hours is performed after a solution treatment and quenching to room temperature, in order to inhibit or control the formation of the GPI.
Patent Document 3 proposes a 6000-series aluminum alloy sheet with excess Si which, after a refining treatment including solution and quenching treatments of this aluminum alloy sheet, gives a DSC in which an endothermic peak in the temperature range of 150-250° C. and corresponds to a dissolution of Si/hole clusters (GPI) has a minus height of 1,000 μW or less and an exothermic peak in the temperature range of 250-300° C. and corresponds to a precipitation of Mg/Si clusters (GPII) has a plus height of 2,000 μW or less. This aluminum alloy sheet, after having undergone room-temperature aging for at least 4 months after the refining treatment, has the properties in which a proof stress is in the range of 110-160 MPa, a difference in proof stress with the one just after the refining treatment is 15 MPa or less, an elongation is 28% or greater, and a proof stress, as measured after application of a 2% strain thereto and a subsequent low-temperature aging treatment of 150° C.×20 minutes, is 180 MPa or greater.
Patent Document 4 proposes that a 6000-series aluminum alloy sheet is set to give, after a refining treatment, a DSC in which an exothermic peak in the temperature range of 100-200° C. has a height W1 of 50 μW or larger and a ratio of a height W2 of an exothermic peak in the temperature range of 200 to 300° C. to the exothermic-peak height W1, (W2/W1), is 20.0 or less, in order to obtain BH response in a bake hardening treatment performed at a low temperature for a short period.
The document states that the exothermic peak W1 corresponds to the precipitation of GP zones serving as nucleus formation sites of β″ (Mg2Si phase) in an artificial age hardening treatment, and that the higher the W1 peak height, the more the GP zones serving as nucleus formation sites of β″ in an artificial age hardening treatment have already been formed and secured in the sheet after refining. It states that as a result, the β″ grows rapidly in a bake hardening treatment after forming, thereby attaining an improvement in BH response. It states that the exothermic peak W2, on the other hand, corresponds to a precipitation peak of the β″ itself, and that the height of this exothermic peak W2 is made as small as possible in order to reduce the proof stress of the sheet to be formed to less than 135 MPa and to thereby ensure formability.
Patent Document 5 proposes that three exothermic-peak heights (three portions) in a DSC in specific temperature ranges and particularly affect BH response are selected and regulated to enhance the BH response (bake hardenability). The three exothermic peaks are peak A at 230-270° C., peak B at 280-320° C. and peak C at 330-370° C. In the proposed method, the height of the peak B is regulated to 20 μW/mg or larger and the peak ratio (A/B) and the peak ratio (C/B) are regulated to 0.45 or less and 0.6 or less, respectively, thereby attaining an increase in 0.2% proof stress, through an artificial hardening treatment of 170° C.×20 minutes after application of a 2% strain, of 100 MPa or greater.
PRIOR ART DOCUMENTS Patent Documents Patent Document 1: JP-A-10-219382 Patent Document 2: JP-A-2000-273567 Patent Document 3: JP-A-2003-27170 Patent Document 4: JP-A-2005-139537 Patent Document 5: JP-A-2013-167004 SUMMARY OF THE INVENTION Problem that the Invention is to SolveThe various outer panels for automobiles are required to attain strain-free, beautiful curved-surface configurations and character lines, from the standpoint of design. However, since higher-strength aluminum alloy sheet materials are being adopted for the purpose of weight reduction and this results in difficulties in forming, it is becoming difficult year by year to meet such requirements. There is hence a growing desire in recent years for a high-strength aluminum alloy sheet having even better formability. However, with the above-mentioned conventional structure controls with a DSC, it is difficult to meet such requirements.
For example, one cause which renders high-strength aluminum alloy sheets difficult to apply to outer panels is the shapes peculiar to outer panels. Recessed portions having given depths (protrudent portions, embossed portions) for attaching devices or members, such as knob mount bases, lamp mount bases and license (number plate) mount bases, or for drawing wheel arches are partly provided to outer panels.
In the cases when such a recessed portion is press-formed together with consecutive curved surfaces around the recessed portion shape, face strains are prone to occur and it is difficult to attain the strain-free, beautiful curved-surface configuration and character line. Consequently, application of high-strength aluminum alloy sheets to the outer panels has a problem in that it is necessary to obtain a high-strength aluminum alloy sheet which has improved formability and is inhibited from suffering face strains.
The problem concerning such face strains is not for those recessed portions (protrudent portions) but a problem common to automotive panels which partly have a recessed portion (protrudent portion) that may suffer a face strain, such as a saddle-shaped portion of a door outer panel, a vertical wall portion of a front fender, a wind corner portion of a rear fender, a character-line termination portions of a trunk lid or hood outer panel, and a root portion of a rear fender pillar.
From the standpoint of attaining improved formability for inhibiting the occurrence of the face strains to overcome the problem described above, it is desirable that a sheet in press forming, which has undergone room-temperature aging after production, should have a 0.2% proof stress reduced to less than 110 MPa. However, in the cases when the proof stress in forming has been reduced as the above, it is difficult to attain a 0.2% proof stress of 170 MPa or greater after bake hardening (hereinafter also referred to as “after BH”) and to attain an increase in 0.2% proof stress through bake hardening of 70 MPa or greater. As described above, with conventional structure controls with a DSC disclosed in Patent Documents 1 to 5, it is difficult to overcome the problem.
The present invention has been achieved in order to overcome the problem described above. An object thereof is to provide an aluminum alloy sheet which combines formability and bake hardenability, that is, which can have, in automotive-panel forming, a 0.2% proof stress reduced to 110 MPa or less and can have a 0.2% proof stress after BH of 170 MPa or greater.
Means for Solving the ProblemThe present inventors diligently made investigations and, as a result, have discovered that an aluminum alloy sheet which combines formability and bake hardenability can be obtained by adopting a specific composition and specific exothermic peaks in the DSC for an Al—Mg—Si alloy sheet, which contains Mg and Si. The present invention has been thus completed.
The gist of the aluminum alloy sheet of the present invention, which is excellent in terms of formability and bake hardenability, is an Al—Mg—Si alloy sheet containing, in terms of mass %, Mg: 0.2-1.0% and Si: 0.2-1.0% and satisfying {(Mg content)+(Si content)}≤1.2%, with the remainder being Al and unavoidable impurities, in which a differential scanning thermal analysis curve of the aluminum alloy sheet has, in a temperature range of 230-330° C., only one exothermic peak (i) or only two exothermic peaks (ii) having a temperature difference between the peaks of 50° C. or less, and in which the exothermic peak (i) or the peak having a higher peak height of the exothermic peaks (ii) has a height in a range of 20-50 μW/mg.
The differential thermal analysis at each of measurement portions in the sheet is performed under the same conditions including a test apparatus of DSC220G, manufactured by Seiko Instruments Inc., a reference substance of aluminum, a sample container made of aluminum, temperature increase conditions of 15° C./min, an atmosphere of argon (50 mL/min), and a sample weight of 24.5-26.5 mg. The differential thermal analysis profile (μW) obtained is divided by the sample weight and thereby normalized (μW/mg). Thereafter, in the range of 0-100° C. in the differential thermal analysis profile, a region where the differential thermal analysis profile is horizontal is taken as a reference level of 0, and the height of exothermic peak from the reference level is measured.
The aluminum alloy sheet excellent in terms of formability and bake hardenability may further contain one element or two or more elements selected from the group consisting of Fe: more than 0% and 0.5% or less, Mn: more than 0% and 0.3% or less, Cr: more than 0% and 0.3% or less, Zr: more than 0% and 0.1% or less, V: more than 0% and 0.1% or less, Ti: more than 0% and 0.1% or less, Cu: more than 0% and 0.5% or less, Ag: more than 0% and 0.1% or less, and Zn: more than 0% and 0.5% or less.
Effects of the InventionAccording to the present invention, the contents of Mg and Si, which are major elements of an Al—Mg—Si alloy sheet, are regulated to be relatively low, thereby enabling a 0.2% proof stress in forming of the sheet, which has been produced and then subjected to room-temperature aging, to be reduced to 110 MPa or less. Consequently, it can have improved formability when applied to automotive panels or the like, which are particularly problematic in face strains thereof, in automotive panel structures.
In addition, the thermal properties (structure) in the DSC of the aluminum alloy sheet are regulated. As a result, an increased strength which includes a 0.2% proof stress after BH of 170 MPa or greater and an increase in 0.2% proof stress of 70 MPa or greater, which is useful as automotive panels can be ensured. The regulation of thermal properties (structure) in the DSC provides a measure for ensuring the amount of precipitates which precipitate after a bake hardening treatment.
Due to such regulation of composition and structure, an aluminum alloy sheet which combines formability and bake hardenability can be provided merely with a basic composition of Al—Mg—Si alloys, without the need of newly adding any additive element or without the need of giving a large modification to ordinary production processes.
Modes for carrying out the present invention will be specifically explained below with respect to each requirement. In this description, “mass %” has the same meaning as “wt %”.
(Chemical Component Composition)First, the chemical component composition of the Al—Mg—Si (hereinafter referred to also as 6000-series) aluminum alloy sheet (hereinafter also referred to simply as “aluminum alloy sheet”) according to the present invention is explained below.
The 6000-series aluminum alloy sheet targeted by the present invention, as, for example, a sheet for the automotive outer panels, is required to have various properties such as excellent formability, BH response, strength, weldability, and corrosion resistance. Consequently, such requirements are also met by means of the composition. In addition, in the present invention, the contents of Mg and Si, which are major elements, are regulated so as to be relatively low, thereby reducing a 0.2% proof stress in forming of the sheet, which has been produced and then subjected to room-temperature aging, to 110 MPa or less. Thus, the formability into automotive panels or the like, which are particularly problematic in face strains thereof, in automotive panel structures, can be improved. Simultaneously therewith, a 0.2% proof stress after bake hardening of 170 MPa or greater is rendered possible by means of composition.
In order to satisfy such requirements, the aluminum alloy sheet has a composition which contains, in terms of mass %, Mg: 0.2-1.0% and Si: 0.2-1.0% and satisfies {(Mg content)+(Si content)}≤1.2%, with the remainder being Al and unavoidable impurities. In this description, all the content indicated in % of the elements means that in mass %. Furthermore, the “-” in each content means that the content is equal to or more than the lower limit value but is equal to or less than the upper limit value.
In the present invention, elements other than Mg, Si and Al basically are impurities or elements which may be contained. The contents of such other elements are the contents (permissible amounts) on levels in accordance with the AA or JIS standards, etc., or are on levels below such standards. Namely, there are cases, in the present invention also, where not only high-purity Al base metal but also 6000-series alloys containing elements other than Mg and Si as additive elements (alloying elements) in large amounts, other aluminum alloy scrap materials, low-purity Al base metal, and the like are used in large quantities as melted raw materials for the alloy, from the standpoint of resource recycling. In such cases, other elements shown below are inevitably included in substantial amounts. Since refining performed for intentionally diminishing these elements itself leads to an increase in cost, it is necessary to accept an inclusion of some degree of amount. There are content ranges which do not defeat or lessen the object or effects of the present invention, even if included in substantial amounts.
Consequently, in the present invention, examples of the other elements which may be contained in the aluminum alloy include the following elements. The permissible contents thereof are within the ranges of equal to or less than the upper limits according to the AA or JIS standards or the like, and are as shown below.
Specifically, the aluminum alloy sheet may further contain one element or two or more elements selected from the group consisting of Fe: 0.5% or less (exclusive of 0%), Mn: 0.3% or less (exclusive of 0%), Cr: 0.3% or less (exclusive of 0%), Zr: 0.1% or less (exclusive of 0%), V: 0.1% or less (exclusive of 0%), Ti: 0.1% or less (exclusive of 0%), Cu: 0.5% or less (exclusive of 0%), Ag: 0.1% or less (exclusive of 0%), and Zn: 0.5% or less (exclusive of 0%), within those ranges.
In this description, the expression “exclusive of 0%” has the same meaning as that the content is “higher than 0%”.
The content range of each element and the purposes and permissible amount thereof in the 6000-series aluminum alloy sheet according to the present invention are explained below.
Si: 0.2-1.0%Si, together with the Mg, is an essential element for obtaining the strength (proof stress) required as automotive panels because it forms aging precipitates which contribute to an improvement in strength, during an artificial aging treatment such as a baking treatment, and thus exhibits an age hardenability. In the case where the content of Si is too low, the amount of aging precipitates after an artificial aging treatment is too small, resulting in too small an increase in strength after baking. Meanwhile, in the case where the content of Si is too high, not only the strength of the sheet just after production but also the amount of room-temperature aging after the production are increased, resulting in too high a strength before forming. Because of this, the formability into automotive panels or the like, which are particularly problematic in face strains thereof, in automotive panel structures, is reduced. In addition, coarse crystals and precipitates are formed, resulting in a considerable decrease in bendability. A preferred upper limit of the content of Si is 0.8%.
For attaining an excellent age hardenability in a baking treatment performed at a lower temperature for a shorter period after forming into panels, it is preferable to employ a 6000-series aluminum alloy composition in which Si/Mg is 1.0 or larger in terms of mass ratio so that Si has been incorporated further excessively relative to the Mg than in the so-called excess-Si type.
Mg: 0.2-1.0%Mg is also an essential element for obtaining the proof stress required as panels, since it forms, together with the Si, aging precipitates which contribute to an improvement in strength, and thus exhibits an age hardenability. In the case where the content of Mg is too low, the precipitate amount of precipitates after an artificial aging treatment is too small, resulting in too small an increase in strength after baking. Meanwhile, in the case where the content of Mg is too high, not only the strength of the sheet just after production but also the amount of room-temperature aging after the production are increased, resulting in too high a strength before forming. Because of this, the formability into automotive panels or the like, which are particularly problematic in face strains thereof, in automotive panel structures, is reduced. A preferred upper limit of the content of Mg is 0.8%.
{(Mg Content)+(Si Content)}≤1.2%{(Mg content)+(Si content)}, which is the total content of Mg and Si, as the structure of the 6000-series aluminum alloy sheet before forming, considerably affects exothermic peaks present in the temperature range of 230-330° C. in the DSC of this aluminum alloy sheet.
On the assumption that the appropriate production process which will be described later is used, by regulating {(Mg content)+(Si content)} to 1.2% or less, in the case where there exist only two exothermic peaks (ii) in the temperature range of 230-330° C., the difference in temperature between the peaks of the two exothermic peaks (ii) can be 50° C. or less and the one having a higher peak height can have a peak height in the range of 20-50 μW/mg. Meanwhile, in the case where there exists only one exothermic peak (i) in that temperature range, this exothermic peak (i) can have a height in the range of 20-50 μW/mg.
Consequently, it is preferred that {(Mg content)+(Si content)} is as small as possible. However, since there essentially are minimum necessary Mg and Si amounts for exhibiting basic performances as a sheet, a lower limit of {(Mg content)+(Si content)} is determined by the minimum contents of these each. From this standpoint, a lower limit of {(Mg content)+(Si content)} is preferably 0.6% or higher.
Meanwhile, in the case where {(Mg content)+(Si content)} is too high above 1.2%, it is difficult to regulate the DSC exothermic peaks so as to fall within the specified ranges, even if the appropriate production process which will be described later is used. Specifically, in the case where there are two exothermic peaks in the temperature range of 230-330° C., these two exothermic peaks cannot have a temperature difference between the peaks of 50° C. or less. In the case where there is only one exothermic peak in that temperature range, this exothermic peak cannot have a height in the range of 20-50 μW/mg. Because of this, it is difficult to attain both a reduction in strength during forming (before baking) and an enhancement in increase in strength through paint baking. Consequently, an upper limit of {(Mg content)+(Si content)} is 1.2% or less and preferably 1.0% or less.
(Differential Scanning Thermal Analysis Curve, Differential Scanning Calorimetry Curve, DSC)The composition described above is employed. Furthermore, in the present invention, peaks in the DSC of the aluminum alloy sheet are regulated as a measure for ensuring the amount of precipitates which precipitate after a bake hardening treatment, in order to ensure high strength as automotive panels or the like. Specifically, a structure is configured in which two exothermic peaks, which have conventionally been present in the temperature range of 230-330° C. apart from each other, are present so as to near to each other (with a reduced temperature difference) and to overlap each other. This makes it possible to attain a 0.2% proof stress in forming into automotive panels reduced to 110 MPa or less and to attain a 0.2% proof stress after bake hardening of 170 MPa or greater.
Here, the differential scanning calorimetry curve (DSC) is a heating curve from solid phase, obtained by measuring the thermal changes during melting of aluminum alloy sheet after the refining treatment of the sheet, by differential thermal analysis performed under the following conditions.
Specifically, the differential thermal analysis at each of measurement portions in the aluminum alloy sheet is performed under the same conditions including a test apparatus of DSC220G, manufactured by Seiko Instruments Inc., a reference substance of aluminum, a sample container made of aluminum, temperature increase conditions of 15° C./min, an atmosphere of argon (50 mL/min), and a sample weight of 24.5 to 26.5 mg. The differential thermal analysis profile (μW) obtained is divided by the sample weight and thereby normalized (μW/mg). Thereafter, in the range of 0 to 100° C. in the differential thermal analysis profile, a region where the differential thermal analysis profile is horizontal is taken as a reference level of 0, and the height of exothermic peak from the reference level is measured.
In the DSC, according to conventional techniques, there are two exothermic peaks β″ and β′ in the range of 230-330° C., existing apart from each other so as to have a large temperature difference (distance) between the peaks. In the present invention, the structure of the aluminum alloy sheet has been specified so that the two exothermic peaks are located near to each other (with a reduced temperature difference therebetween) and to overlap each other. Specifically, in a DSC of the aluminum alloy sheet, in the temperature range of 230-330° C., there is only one exothermic peak (i) or there are only two exothermic peaks (ii), having the difference in temperature between the peaks of 50° C. or less. Moreover, the only one exothermic peak (i), or the exothermic peak having a larger (higher) peak height of the only two exothermic peaks (ii) has a height in the range of 20-50 μW/mg.
In 6000-series aluminum alloy sheets, various precipitate phases are yielded, depending on aging temperatures, such as clusters, GP zones, strengthening phase 1 (β″), strengthening phase 2 (β′), and equilibrium phase (Mg2Si). It is presumed that for enhancing the strength after baking (artificial aging treatment), it is effective to yield β″ and β′, among those phases, during the baking. However, the 6000-series aluminum alloy sheet of the present invention, in which the contents of Mg and Si have been regulated so as to be relatively low in order to make the sheet have, in forming after room-temperature aging, a 0.2% proof stress reduced to 110 MPa or less, considerably differs in the appearing behavior (appearing temperature) of the strengthening phase 1 (β″) and strengthening phase 2 (β′) upon BH (artificial aging treatment), from ordinary 6000-series aluminum alloy sheets having relatively high Mg and Si contents.
The changes in the appearing behavior of β″ and β′ upon BH (upon baking treatment) can be simulated with DSC. This is a base of specifying the structure in the present invention by means of DSC.
A simulation with DSC of the appearing behavior of β″ and β′ upon BH shows that in the case of, for example, ordinary 6000-series aluminum alloy sheets having relatively high Mg and Si contents, the exothermic peaks assigned to β″ and β′ are present more widely apart from each other in the range of 230-330° C. More specifically, a conventional exothermic peak assigned to β″ is mostly present around 240-260° C., which is the lower-temperature former half of that temperature range. Meanwhile, a conventional exothermic peak assigned to β′ is present around 310-320° C., which is the higher-temperature latter half of that temperature range, and they have existed in a state that the difference in temperature between the peaks of β″ and β′ has been larger than 50° C.
Such state of conventional exothermic peaks is a representative example, and that appearing behavior of the exothermic peaks varies widely, as a matter of course, depending on the composition of the sheet and production conditions. For example, there are cases where a DSC has three exothermic peaks (three portions) regarding BH response and they are respectively called peak A at 230-270° C., peak B at 280-320° C. and peak C at 330-370° C., as in Patent Document 5.
In contrast, when a simulation with DSC of the appearing behavior of β″ and β′ upon BH is similarly made with respect to the 6000-series aluminum alloy sheet of the present invention, in which the contents of Mg and Si are relatively low, it can be seen that the exothermic peaks assigned to β″ and β′ are characterized in that the positions where the exothermic peaks appear (peak positions) and the distance between the peaks (temperature difference) are nearer to each other (overlapping), as compared with those ordinary 6000-series aluminum alloy sheets. There also is a feature in which this phenomenon occurs as a result of changing the conditions for sheet production, in particular, the conditions for a preliminary aging treatment performed after solution and quenching treatments.
In a 6000-series aluminum alloy sheet of the present invention having relatively low Mg and Si contents, when produced by an ordinary process, exothermic peaks of β″ and β′ exist in the wide temperature range of 230-330° C. as two separate peaks, the distance between whose peaks is 50° C. or larger in terms of temperature difference, like ordinary 6000-series aluminum alloy sheets having relatively high Mg and Si contents. As typical examples thereof, the DSC indicated by the broken line shown in
On the other hand, it has been found that in the cases when a production process is modified to perform the refining after rolling of the sheet so that the conditions for a preliminary aging treatment after solution and quenching treatments are changed, the exothermic peaks of β″ and β′ appear so that the peaks thereof overlap each other (are located near to each other), with the difference in temperature between the peaks being as small as less than 50° C.
According to the finding made by the present inventors, the appearing temperature of the exothermic peak assigned to β″ (also called first or former-half peak) shifts from the position (temperature) around 250-260° C. of low temperature to a position (temperature) around 270-290° C. of high temperature. Meanwhile, the appearing temperature of the exothermic peak assigned to β′ (also called second or latter-half peak) shifts from the position (temperature) around 300-310° C. of high temperature to a position (temperature) around 290-300° C. of low temperature.
It has been found that in the cases when the exothermic peaks assigned to β″ and β′ have appeared so that the peaks are located near to each other or overlap each other, with the temperature difference between the peaks being as small as less than 50° C., then an amount of artificial-aging precipitates which serve to enhance the proof stress after BH can be ensured. Namely, by regulating the exothermic peaks assigned to β″ and β′ so as to be located near to each other or overlap each other, the 0.2% proof stress in panel forming can be reduced to 110 MPa or less and, simultaneously therewith, the 0.2% proof stress of the panel after BH can be increased to 170 MPa or greater. In contrast, in the case where those two exothermic peaks have the difference in temperature between the peaks as large as more than 50° C., those properties cannot be exhibited.
One of the features of the present invention is that the state in which the exothermic peaks assigned to β″ and β′ overlap each other has been specified as above. Specifically, the 6000-series aluminum alloy sheet gives a DSC in which only two (only two in total) exothermic peaks, i.e., a lower-temperature-side exothermic peak assigned to β″ and a higher-temperature-side exothermic peak assigned to β′, that have a difference in temperature between the peaks of 50° C. or less, preferably 30° C. or less, are present in the temperature range of 230-330° C., preferably in the temperature range of 250-320° C., and in which the height of either exothermic peak of these, which has a larger (higher) peak height is in the range of 20-50 μW/mg. In the case where the lower-temperature-side exothermic peak assigned to β″ and the higher-temperature-side exothermic peak assigned to β′ are located nearer to each other to overlap each other so that the difference in temperature between these peaks cannot be recognized (measured), i.e., in the case where it is deemed that there is only one so-called synthesized (superposed) exothermic peak in the temperature range of 230-330° C., then the height of this exothermic peak is in the range of 20-50 μW/mg.
In the present invention, in the case where only two exothermic peaks having a difference in temperature between the peaks of 50° C. or less, preferably 30° C. or less, are present in the temperature range of 230-330° C., preferably in the temperature range of 250-320° C., it is preferable that the exothermic peak assigned to β″ should be present around 270-290° C. as a lower-temperature-side first or former-half peak. It is also preferable that the exothermic peak assigned to β′ should be present around 290-300° C. as a higher-temperature-side second or latter-half peak. Furthermore, the difference in temperature between the peaks of these exothermic peaks is 50° C. or less, and the height of the exothermic peak, which has a higher peak height of these exothermic peaks is in the range of 20-50 μW/mg. Examples thereof are the thick continuous line among the DSCs shown in
Meanwhile, the thin continuous line among the DSCs shown in
Also important for ensuring the BH response is, of course, the height of an exothermic peak which indicates the amount of artificial-aging precipitates in BH. Namely, in the case where there are two exothermic peaks in the temperature range of 230-330° C., the height (μW/mg) of the exothermic peak assigned to β′ (appearing around about 300° C. in Invention Examples in the Examples, which will be described later), which is the exothermic peak having a larger peak height and contributing to BH response, is regulated so as to be in the range of 20-50 μW/mg.
Meanwhile, in the case where there is only one exothermic peak in the temperature range of 230-330° C., that is, in the case where the exothermic peak assigned to β″ (the first or former-half peak, preferably appearing around 270-290° C.) and the exothermic peak assigned to β′ (the second or latter-half peak, preferably appearing around 290-300° C.) overlap each other to form only one synthesized exothermic peak, the height of this exothermic peak is regulated so as to be in the range of 20-50 ρW/mg.
Thus, it is possible to reduce the proof stress in panel forming to 110 MPa or lower and to attain a proof stress after BH of 170 MPa or greater. In other words, aging precipitates of β″ and β′ which are yielded during BH can be ensured in such an amount that a proof stress after BH of 170 MPa or greater is brought about. In the case where the heights of those exothermic peaks are smaller than the lower limit of, or are larger than the upper limit of, the range of 20-50 μW/mg, this means that the amount of the desired aging precipitates of such as β″ and β′, which have influences on BH response through a bake hardening treatment is too small or too large and such precipitates are unable to be yielded in the desired amount. Because of this, it is inevitably impossible to attain both a reduction in proof stress in panel forming to 110 MPa or less and a control of a proof stress after BH to 170 MPa or greater.
(Production Process)Next, a process for producing the aluminum alloy sheet according to the present invention is explained. The aluminum alloy sheet according to the present invention is produced through production steps which themselves are common or known, by subjecting, after casting, an aluminum alloy slab having the 6000-series component composition to a homogenizing heat treatment, hot rolling and cold rolling to obtain a given sheet thickness, followed by a refining treatment such as a solution quenching treatment.
However, for obtaining the structure specified with a DSC according to the present invention, during those production steps, the conditions for a preliminary aging treatment after the solution and quenching treatments are regulated so as to be in a preferred range, as will be described later. With respect to other steps, there are preferred conditions for obtaining the structure specified with a DSC according to the present invention. Unless such preferred conditions are employed, it is difficult to obtain the structure specified with a DSC according to the present invention.
(Melting and Casting Cooling Rate)First, in melting and casting steps, an aluminum alloy molten metal that has been melted and regulated so as to have a component composition within the 6000-series composition range is cast by a suitably selected ordinary melting and casting method, such as a continuous casting method or a semi-continuous casting method (DC casting method). Here, in order to regulate the clusters so as to be in the range specified in the present invention, it is preferable that the average cooling rate, during the casting, from the liquidus temperature to the solidus temperature is as high (quick) as possible at 30° C./min or greater.
In the case where such temperature (cooling rate) control in a high-temperature range during casting is not performed, the cooling rate in this high-temperature range is inevitably low. When an average cooling rate in the high-temperature range is low as the above, the amount of crystals yielded coarsely in the temperature range of this high-temperature range is increased and also unevenness in the size and amount of the crystals along the width direction and thickness direction of the slab is increased. As a result, it is highly probable that the specified clusters cannot be regulated so as to be in the ranges according to the present invention.
(Homogenizing Heat Treatment)Next, the aluminum alloy slab obtained by casting is subjected to a homogenizing heat treatment prior to hot rolling. The purpose of this homogenizing heat treatment (soaking treatment) is to homogenize the structure, that is, to eliminate segregation within the grains in the structure of the slab. The conditions are not particularly limited so long as the purpose is achieved therewith, and the treatment may be an ordinary one conducted once or in one stage.
A homogenizing heat treatment temperature is suitably selected from the range of 500° C. or more and lower than the melting point, and a homogenizing time is suitably selected from the range of 4 hours and longer. In the case where the homogenizing temperature is low, the segregation within grains cannot be sufficiently eliminated, and these act as starting points for fracture, resulting in decreases in stretch flangeability and bendability. When hot rolling is thereafter started immediately or when hot rolling is started after holding and cooling to an appropriate temperature, control within the number density of the clusters specified in the present invention can be achieved.
After the homogenizing heat treatment has been performed, cooling to room temperature may be performed so that the average cooling rate in the range of 300° C. to 500° C. is 20 to 100° C./hour, followed by reheating to 350° C. to 450° C. at an average heating rate of 20 to 100° C./hour to start hot rolling in this temperature range.
In the cases when the average cooling rate after the homogenizing heat treatment and the reheating rate conducted thereafter do not satisfy those conditions, the possibility of forming coarse Mg—Si compounds increases.
(Hot Rolling)The hot rolling is constituted of a slab rough rolling step and a finish rolling step in accordance with the thickness of the plate to be rolled. In these rough rolling step and finish rolling step, rolling mills such as a reverse type and a tandem type are suitably used.
In the cases when the hot-rolling (rough-rolling) start temperature exceeds the solidus temperature, burning occurs and, hence, the hot rolling itself is difficult to carry out. Meanwhile, in the cases when the hot-rolling start temperature is lower than 350° C., the hot-rolling load is too high, rendering the hot rolling itself difficult. Consequently, the hot-rolling start temperature is preferably in the range of 350° C. to the solidus temperature, more preferably in the range of 400° C. to the solidus temperature.
(Annealing of the Hot-Rolled Plate)Annealing (rough annealing) before cold rolling is not always necessary for the hot-rolled plate. However, it may be performed in order to further improve properties such as formability by making the grains smaller and optimizing the texture.
(Cold Rolling)In cold rolling, the hot-rolled sheet is rolled to produce a cold-rolled sheet (including a coil) having a desired final sheet thickness. However, for making the grains even smaller, it is desirable that the cold rolling ratio should be 60% or greater. Intermediate annealing may be performed between cold-rolling passes for the same purpose as in the rough annealing.
(Solution Treatment and Quenching Treatment)After the cold rolling, a solution treatment is performed, followed by a treatment for quenching to room temperature. The solution and quenching treatments may be a heating and a cooling performed on an ordinary continuous heat treatment line, and are not particularly limited. However, from the standpoint of obtaining a sufficient solid-solution amount of each element and because it is desirable that the grains should be finer as stated above, it is desirable that the treatments should be conducted under such conditions of heating at a heating rate of 5° C./sec or greater to a solution treatment temperature which is 520° C. or higher and lower than the melting temperature, and then holding for 0.1-10 seconds.
From the standpoint of suppressing the formation of coarse intergranular compounds that reduce the formability and hem workability, it is desirable that the average cooling rate from the solution treatment temperature to the quenching stop temperature, which is room temperature, should be 3° C./sec or greater. In the case where the average rate of cooling to room temperature after the solution treatment is too low, coarse Mg2Si and elemental Si are yielded during the cooling, resulting in impaired formability. In addition, the solid-solution amount after the solution treatment is reduced, resulting in a decrease in BH response. In order to secure that cooling rate, means such as air cooling with fans or water cooling with mist or spray or by immersion, etc. and conditions therefor are selected and used for the quenching treatment.
(Preliminary Aging Treatment: Reheating Treatment)After having thus undergone the solution treatment and the subsequent quenching treatment to be cooled to room temperature, the cold-rolled sheet is subjected to a preliminary aging treatment (reheating treatment) within 1 hour. In the case where the room-temperature holding period from termination of the treatment for quenching to room temperature to initiation of the preliminary ageing treatment (initiation of heating) is too long, clusters that are prone to dissolve upon room-temperature aging are yielded, making it impossible to form the exothermic peaks, as a prerequisite, specified with a DSC according to the present invention. Consequently, the shorter the room-temperature holding period, the better. The solution and quenching treatments and the reheating treatment may be consecutively performed so that there is substantially no pause therebetween, and a lower limit of the period is not particularly determined.
In this preliminary aging treatment, it is important that periods of holding both in the relatively higher-temperature-side range of 80-120° C. and in the relatively lower-temperature-side range of 60-40° C. should be ensured. Thus, the exothermic peaks specified with a DSC according to the present invention are formed.
Here, the higher-temperature-side range of 80-120° C. and the lower-temperature-side range of 60-40° C. may be divided into stages, e.g., in two stages, in terms of temperature, or may be regulated so that the temperature changes continuously. Furthermore, the temperature holding in the higher-temperature-side range may be a heat treatment in which a constant temperature within that temperature range is maintained or in which the temperature is gradually changed within that temperature range by temperature increase. Meanwhile, the temperature holding in the lower-temperature-side range may be a heat treatment in which a constant temperature within that temperature range is maintained or in which the temperature is gradually changed within that temperature range by temperature decrease. In short, the temperature may be continuously changed by temperature increase, temperature decrease (annealing), etc., so long as the temperature is held in each of the temperature ranges for the necessary holding period. The temperature holding in the higher-temperature-side and in the lower-temperature-side may be a heat treatment of consecutive two stages in which the temperature is divided into stages, or may be heat treatment in which the holding temperature is kept constant within each of the specified temperature ranges or may be a continuous heat treatment in which temperature increase, temperature decrease, natural cooling, etc are suitably combined within each of the specified temperature ranges. The cooling after the preliminary aging treatment may be natural cooling or rapid cooling.
The period of holding in the higher-temperature-side range of 80-120° C. in the former half is preferably regulated to 5-40 hours including the time period during which the sheet is held in the temperature range of 80-120° C. in the temperature increase of the sheet. Meanwhile, the period of holding in the lower-temperature-side range of 60-40° C. in the latter half is preferably regulated to 20-300 hours including the period of temperature decrease from the holding in the higher-temperature-side range or the time period during which the sheet is held in the temperature range of 60-40° C. in the cooling such as natural cooling or rapid cooling.
In the case where those temperatures are too low or those holding periods are too short, similar to in the case where no preliminary aging treatment is performed, the structure according to the present invention specified with a DSC is less apt to be obtained, and no exothermic peak appears in the temperature range of 230-330° C. or, even if two exothermic peaks appear, the temperature difference between the peaks exceeds 50° C. or the specified exothermic peak height exceeds 50 μW/mg.
Conversely, also in the case where those temperatures are too high or those holding periods are too long, the structure according to the present invention specified with a DSC is less apt to be obtained, and no exothermic peak appears in the temperature range of 230-330° C. or the specified exothermic peak height exceeds 50 μW/mg.
ExamplesThe present invention will be explained below in more detail by reference to Examples. However, the present invention should not, of course, be construed as being limited by the following Examples, and can be suitably modified unless the modifications depart from the gist of the present invention described hereinabove and hereinafter. All such modifications are included in the technical range of the present invention.
Examples according to the present invention are explained. 6000-series aluminum alloy sheets were individually produced so as to differ in the structure specified with a DSC in the present invention, by changing the conditions for a preliminary aging treatment performed after solution and quenching treatments. After a holding at room temperature for 30 days after the production of the sheets, BH response (bake hardenability), As proof stress as an index of press formability and hem workability as bendability are examined and evaluated.
For the individual producing, the 6000-series aluminum alloy sheets having the compositions shown in Table 1 was produced by variously changing conditions such as the temperature and holding period in the preliminary aging treatment after the solution and quenching treatments as shown in Table 2. With respect to the indications of the contents of elements within Table 1, a value of the element expressed by a blank indicates that the content is below a detection limit.
Specific conditions for aluminum alloy sheet production were as follows. Slabs of aluminum alloys respectively having the compositions shown in Table 1 were commonly produced through casting by the DC casting method. In this casting, the average rate of cooling from the liquidus temperature to the solidus temperature was set at 50° C./min in common with all the Examples. Subsequently, the slabs were subjected to a soaking treatment of 540° C.×6 hours, followed by initiation of hot rough rolling at that temperature, in common with all the Examples. Thereafter, they were hot-rolled, in the succeeding finish rolling, to a thickness of 3.5 mm to obtain hot-rolled sheets, in common with all the Examples. The hot-rolled aluminum alloy sheets were subjected to rough annealing of 500° C.×1 minute and then to cold rolling at a processing rate of 70% without performing intermediate annealing during the cold-rolling passes, to obtain cold-rolled sheets having a thickness of 1.0 mm, in common with all the Examples.
Furthermore, the cold-rolled sheets were each continuously subjected to a refining treatment (T4) with continuous type heat treatment facilities while unwinding and winding each sheet, in common with all the Examples. Specifically, a solution treatment was performed by heating at an average rate of heating to 500° C. of 10° C./sec and holding for 5 seconds after the temperature reached a target temperature of 540° C., followed by cooling to room temperature by performing water cooling at an average cooling rate of 100° C./sec. After this cooling, a preliminary aging treatment was performed in two stages of the higher-temperature-side range and the lower-temperature-side range, using the temperatures (° C.) and holding periods (hr) shown in Table 2. Specifically, this two-stage preliminary aging treatment was performed by holding at the given temperature for the given period by using an oil bath, as the higher-temperature-side range, and thereafter, by holding at the given temperature for the given period by using a thermostatic oven, as the lower-temperature-side range, followed by annealing (natural cooling).
In the preliminary aging treatment, the period of holding in the higher-temperature-side range included the time period during which the sheet was held in the temperature range of 80-120° C. in the temperature increase of the sheet. The period of holding in the lower-temperature-side range included the temperature decrease from the holding in the higher-temperature-side range or the time period during which the sheet was held in the temperature range of 60-40° C. in the cooling by natural cooling.
From the final product sheets which each had been allowed to stand at room temperature for 30 days after the refining treatment, test sheets (blanks) were cut out and the DSC and properties of the test sheets were examined and evaluated. The results thereof are shown in Table 2.
(DSC)The structure in each of ten portions of the central portion in the sheet-thickness direction in each test sheet was examined for the DSC. In the DSC (differential scanning thermal analysis curves) of this sheet, as for the average value for these ten portions, the exothermic peaks present in the temperature range of 230-330° C. were examined. Specifically, in the cases when two exothermic peaks were present, the difference in temperature (° C.) between these exothermic peaks and the peak height (μW/mg) of the exothermic peak having a higher peak height were determined. In the cases when only one exothermic peak was present, the height (μW/mg) of this exothermic peak was determined.
The differential thermal analysis of each of the measurement portions in each test sheet was performed under the same conditions including a test apparatus of DSC220G, manufactured by Seiko Instruments Inc., a reference substance of aluminum, a sample container made of aluminum, temperature increase conditions of 15° C./min, an atmosphere of argon (50 mL/min), and a sample weight of 24.5 to 26.5 mg. The differential thermal analysis profile (μW) obtained was divided by the sample weight and thereby normalized (μW/mg). Thereafter, in the range of 0 to 100° C. in the differential thermal analysis profile, a region where the differential thermal analysis profile was horizontal was taken as a reference level of 0, and the height of exothermic peak from the reference level was measured. The results thereof are shown in Tables 2 and 3.
(Bake Hardenability)The test sheets which had been allowed to stand at room temperature for 30 days after the refining treatment were each examined for 0.2% proof stress (As proof stress) as a mechanical property through a tensile test. Furthermore, these test sheets were aged at room temperature for 30 days, subsequently subjected to an artificial age hardening treatment of 170° C.×20 minutes (after BH), and then examined for 0.2% proof stress (proof stress after BH) through a tensile test, in common with the test sheets. The BH response of each test sheet was evaluated on the basis of the difference between these 0.2% proof stresses (increase in proof stress).
With respect to the tensile test, No. 5 specimens (25 mm×50 mmGL×sheet thickness) according to JIS Z2201 were cut out of each sample sheet to perform the tensile test at room temperature. Here, the tensile direction of each specimen was set so as to be perpendicular to the rolling direction. The tensile rate was set at 5 mm/min until the 0.2% proof stress and at 20 mm/min after the proof stress. The number N of examinations for mechanical property was 5, and an average value therefor was calculated. With respect to the specimens to be examined for the proof stress after BH, a 2% pre-strain as a simulation of sheet press forming was given to the specimens by the tensile tester, followed by performing the BH treatment.
(Hem Workability)Hem workability was evaluated only with respect to the test sheets which had been allowed to stand at room temperature for 30 days after the refining treatment. In the test, strip-shaped specimens having a width of 30 mm were used and subjected to 90° bending at an inward bending radius of 1.0 mm with a down flange. Thereafter, an inner plate having a thickness of 1.0 mm was nipped, and the specimen was subjected, in order, to pre-hem working in which the bent part was further bent inward to approximately 130° and flat-hem working in which the bent part was further bent inward to 180° and the end portion was brought into close contact with the inner plate.
The surface state, such as the occurrence of rough surface, a minute crack or a large crack, of the bent part (edge bent part) of the flat hem was visually examined and visually evaluated on the basis of the following criteria. In the following criteria, ratings of 0 to 2 are on an acceptable level, and ratings of 3 and larger are unacceptable.
0, no crack and no rough surface; 1, slight rough surface; 2, deep rough surface; 3, minute surface crack; 4, linearly continued surface crack.
As shown by alloys Nos. 0 to 9 in Table 1 and Nos. 0, 1, 5, 6, 12, and 15 to 21 in Table 2, the Invention Examples each not only have a component composition within the range according to the present invention and have been produced under conditions within preferred ranges but also have undergone the refining treatment, including the preliminary aging treatment, under conditions within preferred ranges. Because of this, these Invention Examples satisfy the DSC requirements specified in the present invention, as shown in Table 2. That is, these sheets each gave a DSC which had only one or only two exothermic peaks in the temperature range of 230-330° C. and in which when only two exothermic peaks were present, then the difference in temperature between the peaks was 50° C. or less and the exothermic-peak height of one having a higher exothermic-peak height was in the range of 20-50 μW/mg. Furthermore, when only one exothermic peak was present, the height of this exothermic peak was in the range of 20-50 μW/mg.
In Table 2, as for the peak height in the case where only two exothermic peaks were present in the temperature range of 230-330° C., the peak appeared around 300° C. had a larger peak height than the peak appeared on the lower-temperature side, in both Invention Examples and Comparative Examples. Consequently, the peak height (W/mg) of this exothermic peak was determined.
As a result, the Invention Examples each show excellent BH response although the bake hardening is performed after the refining treatment and subsequent room-temperature aging and is a treatment conducted at a low temperature for a short period of time. Furthermore, as shown in Table 2, even after the refining treatment and subsequent room-temperature aging, they each have a relatively low As proof stress and hence show excellent press formability into automotive panels or the like and excellent hem workability. That is, the Invention Examples, even when having undergone an automotive-baking treatment after room-temperature aging, were able to exhibit not only high BH response with a 0.2% proof stress difference of 70 MPa or greater and a 0.2% proof stress after BH of 170 MPa or greater but also press formability with an As 0.2% proof stress of 110 MPa or less and satisfactory bendability.
In contrast, Comparative Examples 2 to 4, 7 to 11, 13, and 14 in Table 2, which employed alloy example 1, 2 or 3 in Table 1 like Invention Examples, each have the preliminary aging treatment conditions outside the preferred ranges, as shown in Table 2. As a result, they each gave a DSC which was outside the range specified in the present invention, and show enhanced room-temperature aging and, in particular, a relatively high As proof stress after 30-day room-temperature holding, as compared with the Invention Examples having the same alloy composition. Because of this, they are poor in press formability into automotive panels or the like and in hem workability and are poor also in BH response.
In Comparative Examples 2 and 9, among these, the period from the solution treatment and the quenching treatment to room temperature to the preliminary aging treatment (initiation of heating) is 120 minutes, which is too long. Because of this, Mg—Si clusters that do not contribute to strength have been yielded in a large amount. Although the two exothermic peaks present in the temperature range of 230-330° C. have a difference in temperature between the peaks of 50° C. or less, the exothermic-peak height exceeds 50 μW/mg.
In Comparative Example 3, the period of holding in the higher-temperature-side range in the preliminary aging treatment is 48 hours, which is too long. Because of this, the one exothermic peak present in the temperature range of 230-330° C. has too small a height less than 20 μW/mg.
In Comparative Examples 4, 11 and 14, the period of holding in the lower-temperature-side range in the preliminary aging treatment is 2 hours, which is too short. Because of this, although the two exothermic peaks present in the temperature range of 230-330° C. have a difference in temperature between the peaks of 50° C. or less, the exothermic-peak height exceeds 50 μW/mg, or in the case where one exothermic peak is present in the temperature range of 230−330° C., this exothermic peak has a height exceeding 50 μW/mg.
In Comparative Examples 10 and 13, the period of holding in the higher-temperature-side range in the preliminary aging treatment is 2 hours, which is too short. Because of this, in the case where one exothermic peak is present in the temperature range of 230-330° C., this exothermic peak has a height exceeding 50 μW/mg.
In Comparative Example 7, the temperature in the higher-temperature-side range in the preliminary aging treatment is 70° C., which is too low. Because of this, although the two exothermic peaks present in the temperature range of 230-330° C. have a difference in temperature between the peaks of 50° C. or less, the higher exothermic peak has a height exceeding 50 μW/mg.
In Comparative Example 8, the temperature in the higher-temperature-side range in the preliminary aging treatment is 130° C., which is too high. Because of this, in the case where one exothermic peak is present in the temperature range of 230-330° C., this exothermic peak has a height less than 20 μW/mg.
Comparative Examples 22 to 30 in Table 2 have been produced under preferred conditions, including the conditions for the preliminary aging treatment. However, since they employed alloys Nos. 10 to 18 shown in Table 1, the contents of Mg and Si, which are essential elements, therein are outside the ranges according to the present invention or the content of impurity elements therein is too high. Because of this, these Comparative Examples 22 to 30 each show, in particular, a relatively too high As proof stress after 30-day room-temperature holding as compared with the Invention Examples, as shown in Table 2. They hence are poor in press formability into automotive panels or the like and in hem workability or are poor in BH response. The compositions of Comparative Examples 22 to 30 are described in detail below.
Comparative Example 22 is alloy 10 shown in Table 1, in which the Si content is too low.
Comparative Example 23 is alloy 12 shown in Table 1, in which the Mg+Si content is too high.
Comparative Example 24 is alloy 11 shown in Table 1, in which the Si content is too high and the Mg+Si content is too high.
Comparative Example 25 is alloy 13 shown in Table 1, in which the Fe content is too high.
Comparative Example 26 is alloy 14 shown in Table 1, in which the Mn content is too high.
Comparative Example 27 is alloy 15 shown in Table 1, in which the Cr and Ti contents are too high.
Comparative Example 28 is alloy 16 shown in Table 1, in which the Cu content is too high.
Comparative Example 29 is alloy 17 shown in Table 1, in which the Zn content is too high.
Comparative Example 30 is alloy 18 shown in Table 1, in which the Zr and V contents are too high.
DSCs selected from those of the Invention Examples and Comparative Examples are shown in
In the DSC of Invention Example 1, a first exothermic peak of β″ appears around 270° C. and a second exothermic peak of β′ appears around 300° C. near the first peak, and the difference in temperature between these peaks is 27° C. as shown in Table 2, which is 50° C. or less as specified.
In the DSC of Invention Example 12, a first exothermic peak of β″ and a second exothermic peak of β′ overlap each other to form one synthesized peak. This synthesized peak appears around 290° C. and, as shown in Table 2, has a peak height of 35.9 μW/mg, which is in the range of 20-50 μW/mg.
In contrast, in the DSC of Comparative Example 23, a first exothermic peak of β″ appears around 260° C. and a second exothermic peak of β′ appears around 310° C., and the difference in temperature between these peaks is 53° C. as shown in Table 2, which exceeds the specified temperature of 50° C.
Those results of the Examples support that, for improving formability and BH response after room-temperature aging, it is necessary that all the requirements concerning composition and DSC specified in the present invention should be satisfied.
While the present invention has been described in detail and with reference to specific embodiments thereof, 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 of the present invention. This application is based on a Japanese patent application filed on Mar. 31, 2014 (Application No. 2014-074044), the contents thereof being incorporated herein by reference.
INDUSTRIAL APPLICABILITYAccording to the present invention, it is possible to provide 6000-series aluminum alloy sheets which combine BH response and formability after room-temperature aging. As a result, the 6000-series aluminum alloy sheets are usable in applications extended to automotive panels, in particular, outer panels in which problems may arise concerning the design of beautiful curved-surface configurations, character lines, etc.
Claims
1. An aluminum alloy sheet excellent in terms of formability and bake hardenability, which is an Al—Mg—Si alloy sheet comprising, in terms of mass %, Mg: 0.2-1.0% and Si: 0.2-1.0% and satisfying {(Mg content)+(Si content)}≤1.2%, with the remainder being Al and unavoidable impurities,
- wherein a differential scanning thermal analysis curve of the aluminum alloy sheet has, in a temperature range of 230-330° C., only one exothermic peak (i) or only two exothermic peaks (ii) having a temperature difference between the peaks of 50° C. or less, and
- wherein the exothermic peak (i) or the peak having a higher peak height of the exothermic peaks (ii) has a height in a range of 20-50 μW/mg.
2. The aluminum alloy sheet excellent in terms of formability and bake hardenability according to claim 1, further comprising one element or two or more elements selected from the group consisting of Fe: more than 0% and 0.5% or less, Mn: more than 0% and 0.3% or less, Cr: more than 0% and 0.3% or less, Zr: more than 0% and 0.1% or less, V: more than 0% and 0.1% or less, Ti: more than 0% and 0.1% or less, Cu: more than 0% and 0.5% or less, Ag: more than 0% and 0.1% or less, and Zn: more than 0% and 0.5% or less.
Type: Application
Filed: Mar 23, 2015
Publication Date: Jul 5, 2018
Applicant: Kabushiki Kaisha Kobe Seiko Sho (Kobe Steel, Ltd.) (Kobe-shi)
Inventors: Hisao SHISHIDO (Hyogo), Katsushi MATSUMOTO (Hyogo), Yasuhiro ARUGA (Hyogo)
Application Number: 15/128,281
