Patent Publication Number: US-2016222491-A1

Title: High strength aluminum alloy sheet

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to an Al—Mg—Si alloy sheet. The aluminum alloy sheet referred to in the present invention is a rolled sheet such as a hot-rolled sheet or a cold-rolled sheet, which is an aluminum alloy sheet subjected to tempering such as solid solution treatment and quenching before being subjected to bending fabrication and paint-bake treatment. Hereinafter, aluminum may also be referred to as ALUMI or Al. 
     2. Description of the Related Art 
     Recently, social need for weight reduction of vehicles such as automobiles has increased more and more out of consideration for global environment. To meet such social need, as a material of automobiles, a lighter weight aluminum alloy material having excellent formability and paint-bake hardenability (bake hardenability, hereinafter also referred to as BH property) is used increasingly in place of steel materials such as steel sheets. 
     Aluminum alloy sheets for large panel materials such as outer panels and inner panels of automobiles include, for example, Al—Mg—Si alloy sheets such as AA or JIS 6000-series (also simply referred to hereinafter as 6000-series) are used. The 6000-series aluminum alloy has a composition essentially containing Si and Mg, ensures formability at a low proof stress during formation, improves the proof stress (strength) by heating during artificial aging (hardening) such as paint baking of the panel after formation, and has excellent paint bake-hardenability capable of ensuring necessary strength. 
     For further weight reduction of automobile bodies, extended use of an aluminum alloy materials is desired for automobile structural members such as skeleton materials, for example, frames and pillars or reinforcing materials such as bumper reinforcements and door beams for automobile members except for the panel material. 
     However, further strengthening is required for the automobile structural materials compared with the automobile panels. Accordingly, for applying the 6000-series aluminum alloy sheet used for the automobile panel materials to the skeleton materials or the reinforcing materials needs further strengthening. 
     However, it is not so easy to attain such high strengthening without greatly changing the composition and the manufacturing conditions of existent 6000-series aluminum alloy sheets and without hindering the bendability, etc. 
     It has been variously proposed to control an amount of solute Mg and an amount of solute Si in order to improve the property such as a BH property of the 6000-series aluminum alloy sheet as the panel materials. 
     For example, JP-A 2008-174797 intends to provide an Al—Mg—Si alloy sheet of excellent room temperature stability (suffering from less deterioration of the material by natural aging) as the panel materials. For this purpose, the patent literature proposes an Al—Mg—Si alloy sheet in which an amount of solute Si is 0.55 to 0.80 mass % and an amount of solute Mg is 0.35 to 0.60 mass %, and the amount of solute Si/amount of solute Mg is 1.1 to 2. An example, having a high strength, of the aluminum alloy sheet has a strength of about 210 MPa as 0.2% proof stress after artificial aging of 170° C.×20 minutes after applying 2% strain to the sheet after natural aging for 15 days after the manufacture of the sheet. 
     SUMMARY OF THE INVENTION 
     In the existent control for the amount of solute Mg and the amount of solute Si, for example, in JP-A No. 2008-174797, the strength is insufficient for the application use of the skeleton materials or reinforcing materials since this is originally intended for use as the panel materials. 
     Further, while the skeleton materials or reinforcing materials have no requirement for high press formability as that for the panel material, when the material sheet is fabricated into the skeleton material or the reinforcing material, since the material sheet is mainly subjected to bending fabrication, a bendability of such an extent as not causing cracking by V-bending fabrication is required. 
     The present invention has been accomplished for solving such a subject and intends to provide a high strength 6000-series aluminum alloy sheet that can be manufactured without greatly changing the composition and the manufacturing conditions of the existent 6000-series aluminum alloy sheet as the skeleton material or the reinforcing material, and can also be fabricated into members. 
     According to an aspect of the present invention in order to attain the object, there is provided a high strength Al—Mg—Si alloy sheet comprising, based on mass %, 0.6 to 2.0% of Mg, 0.6 to 2.0% of Si, 1.0% or less of Mn (not including 0%), and 0.5% or less of Fe (not including 0%) respectively with the remainder consisting of Al and inevitable impurities, in which both of the Mg content and the Si content in a solution separated by a residue extraction method with hot phenol are 0.6% or more as an amount of solute Mg and an amount of solute Si of the sheet and, the sum of the amount of solute Mg and the amount of solute Si is 1.4% or more, and the ratio of the amount of solute Si to the amount of solute Mg (solute Si/solute Mg) is 0.8 to 1.2. 
     In the present invention, on the premise of not greatly changing the existent aluminum alloy composition and manufacturing condition, the relation between the amount of solute Mg and the amount of solute Si and the strength of the 6000-series aluminum alloy sheet has been reconsidered. As a result, it has been found that formation of Mg—Si clusters in a good balance between the number of Mg and Si atoms can be promoted by tempering the manufactured sheet as will be described later. The Mg—Si clusters in a good balance between Mg and Si tend to be transformed to Mg—Si precipitates during paint-bake treatment to remarkably contribute to the increase in the strength. As a result, it has been found that the 0.2% proof stress after BH of 185° C.×20 minutes can be increased to 260 MPa or more, preferably 280 MPa or more and, more preferably, 300 MPa or more also after the natural aging without deteriorating the bendability.  
     Accordingly, the aluminum alloy of the present invention is suitable, for example, as skeleton materials or reinforcing materials requiring higher strength than the panel materials of automobiles. 
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
     Preferred embodiments of the present invention are to be described specifically on every constitutions. 
     (Chemical Composition) 
     First, a chemical composition of an Al—Mg—Si based (hereinafter also referred to as 6000-series) aluminum alloy sheet of the present invention is to be described below. In the present invention, skeleton materials or reinforcing materials except for the panel materials are strengthened without deteriorating the bendability and without greatly changing the existent composition and the manufacturing conditions. 
     In order to satisfy such a subject in view of the composition, the 6000-series aluminum alloy sheet has a composition comprising, based on mass %, 0.6 to 2.0% of Mg, 0.6 to 2.0% of Si, 1.0 or less (not including 0%) of Mn and 0.5% or less (not including 0%) of Fe respectively, with the remainder consisting of Al and inevitable impurities. “%” expression for the content of each of the elements means mass %. 
     The range of content and the meaning of each element or the allowable amount thereof in the 6000-series aluminum alloy are to be described. 
     Si: 0.6 to 2.0% 
     Si, together with Mg, is an essential element for obtaining a necessary strength (proof stress) as outer panels of automobiles by forming Mg—Si based precipitates that contribute to the improvement of the strength upon solid solution strengthening and artificial aging such as paint-bake treatment, thereby providing aging hardenability. If the Si content is insufficient, since the amount of solute Si is decreased and the amount of formed Mg—Si based precipitates becomes insufficient before the paint-bake treatment, the BH property is deteriorated remarkably. On the other hand, if the Si content is excessive, coarse constituents and precipitates are formed thereby causing remarkable cracks during hot rolling Accordingly, the Si content is defined within a range from 0.6 to 2.0%. A preferred lower limit of Si is 0.8% and a preferred upper limit thereof is 1.5%. 
     Mg: 0.6 to 2.0% 
     Mg, together with Si, is an essential element for obtaining a necessary proof stress as the panels, by forming Mg—Si based precipitates that contribute to the improvement of the strength upon solid solution strengthening and artificial aging such as paint-bake treatment, thereby providing aging hardenability. If the Mg content is insufficient, since the amount of solute Mg is decreased and the amount of formed Mg—Si based precipitates becomes insufficient before the paint-bake treatment, the BH property is deteriorated remarkably. In contrast, if the Mg content is excessive, coarse constituents and precipitates are formed thereby causing remarkable cracks during the hot rolling Accordingly, the Mg content is defined within a range from 0.6 to 2.0%. A preferred lower limit of Mg is 0.8% and a preferred upper limit thereof is 1.5%. 
     Amount of Solute Si and Amount of Solute Mg 
     A typical concept in the prior art, for example, in JP-A 2008-174797 of controlling the amount of solute Mg and the amount of solute Si of the 6000-series aluminum alloy sheet in the use of panel materials for automobiles is that the amount of solute Mg and the amount of solute Si are suppressed to the lowest limit necessary for the BH property in order to suppress formation of Mg—Si, Si—Si and Mg—Mg clusters which cause natural aging of the sheet. 
     On the contrary, in the present invention, it has been found that formation of Mg—Si clusters in a good balance between the number of Mg and Si atoms can be promoted when the amount of solute Mg and the amount of solute Si are increased in a good balance on the premise of not greatly changing the existent compositions and the manufacturing condition of the aluminum alloys. 
     That is, it has been found that when the amount of solute Mg and the amount of solute Si are increased in a good balance, it is possible to suppress the formation of Si enriched Mg—Si clusters (aggregates of Mg atoms and Si atoms) that are formed during tempering of the manufactured sheet (during pre-aging), and ensure the amount of Mg—Si precipitates that are formed during the paint-bake treatment, and contribute to the increase in the strength. 
     The Si enriched Mg—Si clusters described above cause natural aging, also deteriorate the bendability and are less transformed to Mg—Si precipitates during the paint-bake treatment and less contribute to the improvement of the BH property and the increase in the strength. 
     On the contrary, the Mg—Si clusters in a good balance between Mg and Si do not cause natural aging and do not deteriorate the bendability, but tend to be transformed to the Mg—Si precipitates during the paint-bake treatment, thereby remarkably contributing to the increase in the strength. 
     Accordingly, when the Si enriched Mg—Si clusters described above are suppressed and the Mg—Si clusters in a good balance between Mg and Si are increased, the natural aging can be suppressed and, in addition, the BH property can be improved to remarkably increase the strength. That is, even after the natural aging, 0.2% proof stress after BH of 185° C.×20 minutes can be increased to 260 MPa or more, preferably, 280 MPa or more and, more preferably, 300 MPa or more without deteriorating the bendability. 
     Therefore, in the present invention, the amount of solute Mg and the amount of solute Si are increased and the Mg content and the Si content separated in a solution by a residue extraction method with hot phenol are increased as 0.6% or more. That is, both of the amount of solute Mg and the amount of solute Si of the sheet are increased as 0.6% or more thereby ensuring the amount of Mg—Si precipitates that are formed during the paint-bake treatment and contribute to the increase in the strength. In order to increase the effect, both of the amount of solute Mg and the amount of solute Si are defined preferably as 0.7% or more. 
     Naturally, the upper limits for the amount of solute Mg and the amount of solute Si of the sheet are determined based on the Mg content and the Si content of the sheet.  
     Concurrently, the sum of the amount of solute Mg and the amount of solute Si (that is, the sum of the Mg content and the Si content in the solution) is increased as 1.4% or more, preferably, 1.5% or more for ensuring the amount of the Mg—Si precipitates that are formed during the paint-bake treatment and contribute to the increase of the strength. Naturally, also the upper limit for the sum of the amount of solute Mg and the amount of solute Si is determined depending on the Mg content and the Si content of the sheet. 
     Further, for keeping the balance between the amount of solute Mg and the amount of solute Si, the ratio of the amount of solute Si to the amount of solute Mg (solute Si/solute Mg), that is, the ratio of the Si content to the Mg content in the solution) is defined as 0.8 to 1.2. 
     The upper limit 1.2 for the ratio of the amount of solute Si to the amount of solute Mg (solute Si/solute Mg) is defined for suppressing the formation of the Si enriched Mg—Si clusters that tend to be formed when the amount of solute Si is larger to the amount of solute Mg during pre-aging and it is preferably 1.1 or less. 
     In contrast, also in a case where the amount of solute Mg is excessive to the amount of solute Si, formation of the Mg—Si clusters in a good balance between the number of Mg and Si atoms is suppressed. Accordingly, the lower limit of the ratio of the amount of solute Si to the amount of solute Mg is defined as 0.8. 
     Mn: 1.0% or Less (Not Including 0%) 
     Mn improves the strength of the aluminum alloy by the effect of solid solution strengthening and crystal grain refining. However, if Mn is contained excessively by more than 1.0%, the amount of Al-Mn-Fe based intermetallic compounds is increased tending to form fracture origins, and the amount of the Mg—Si based precipitates that contribute to the increase of the strength is decreased. Accordingly, the Mn content is defined as 1.0% or less (not including 0%). 
     Fe: 0.5% or Less (Not Including 0%) 
     Since Fe forms Al—Mn—Fe based intermetallic compounds in the aluminum alloy, as the Fe content increases, the amount of the compounds thereof is increased more tending to form fracture origins and also decrease the amount of the Mg—Si based precipitates that contribute to the increase of the strength. In addition, since Fe is introduced in the aluminum alloys as bare metal impurities and the content is increased as the amount of aluminum alloy scraps increases as the melting material (ratio to the aluminum ingot), the Fe content is preferably as less as possible, and Fe is an element to be restricted. However, since decrease of Fe below the detection limit needs a considerable cost, it is necessary to more or less permit the presence of Fe. Accordingly, the Fe content is defined as 0.5% or less (not including 0%). 
     Other Elements 
     In addition, for increasing the strength of the aluminum alloy sheet in the present invention, one or more of other elements, for example, 1.0% or less (not including 0%) of Cu, 0.3% or less (not including 0%) of Cr, 0.2% or less (not including 0%) of Zr, 0.2% or less (not including 0%) of V, 0.1% or less (not including 0%) of Ti, 0.5% or less (not including 0%) of Zn, 0.2% or less (not including 0%) of Ag, and 0.15% or less (not including 0%) of Sn may be contained. 
     Since the elements described above have an effect, in common, of increasing the strength of the sheet, they can be regarded as elements of providing an equivalent strengthening effect and their specific mechanisms include, naturally, common portions and different portions. 
     Cr, Zr, and V, like Mn, form dispersed particles (dispersion phase) during a homogenizing heat treatment and such dispersed particles have an effect of hindering the movement of grain boundary after recrystallization and serving to refine crystal grains. Further, Ti serves to form constituents as nuclei of recrystallized grains, thereby suppressing crystal grains from coarsening, and refining crystal grains. While Cu improves the strength, since Cu deteriorates the bendability as the strength is increased, the content is defined preferably as 0.7% or less. Zn and Ag are useful for improving the artificial aging hardenability (BH property) and have an effect of promoting precipitation of a compound phase such as a GP zone into the crystal grains of the sheet microstructure under the condition of relatively low temperature and short time in the artificial aging. Sn has an effect of suppressing the diffusion of Mg and Si at a room temperature by capturing atom vacancy, suppressing the increase in the strength at a room temperature (natural aging), and releasing the vacancy captured during the artificial aging and promoting the diffusion of Mg and Si, thereby increasing the BH property. 
     However, if the content of each of the elements is excessive, manufacture of the sheet becomes difficult by formation of coarse compounds, etc. and the strength, the bendability, and the corrosion resistance are also deteriorated. Particularly, Cu remarkably deteriorates the bendability if it is contained excessively. Accordingly, when the elements are contained, their contents are restricted to each of the upper limit values or less as described above. 
     (Manufacturing Method) 
     Then, a method of manufacturing the aluminum alloy sheet of the invention is to be described. The manufacturing process per se of the aluminum alloy sheet of the invention is a customary or known process, in which an aluminum alloy slab having the 6000-series composition is cast and then subjected to a homogenizing heat treatment, hot-rolled and cold-rolled into a sheet of a predetermined thickness and further subjected to tempering such as solid solution treatment and quenching. 
     However, in the manufacturing steps, hot rolling conditions and pre-aging conditions after the solid solution treatment and the quenching are defined within preferred ranges as will be described later in order to obtain the microstructure defined in the present invention (amount of solute Mg and amount of solute Si). 
     (Melting and Casting Cooling Rate) 
     First, in the melting and casting process, a molten aluminum alloy melted and adjusted within the range of the 6000-series composition is cast by properly selecting usual melting and casting process such as a continuous casting, a semi-continuous casting method (DC casting), etc. For controlling the clusters within the range defined in the present invention, an average cooling rate during casting is preferably as high (fast) as possible from the liquidus temperature to the solidus temperature, for example, at 30° C./min or higher.  
     Without such temperature control (cooling rate) in a high temperature region during casting, the cooling rate in the high temperature region is inevitably lowered. If the average cooling rate in the high temperature region is lowered, the amount of constituents formed coarsely in the temperature range in the high temperature region is increased, and the size and the amount of the constituents vary greatly in the direction of the width and in the direction of the thickness of the slab. As a result, there may be a high possibility that clusters cannot be controlled as defined in the range of the present invention. 
     (Homogenizing Heat Treatment) 
     Subsequently, the cast aluminum alloy slab is subjected to a homogenizing heat treatment prior to hot rolling. The homogenizing heat treatment (soaking) is important for sufficiently solid-solutionizing Si and Mg in addition to homogenization of the microstructure (eliminating segregation in the crystal grains in the slab microstructure) as an ordinary purpose. So long as the purpose is attained under the conditions, the conditions are not particularly restricted and the treatment is usually applied for once or in one step. 
     Si and Mg are solid-solutionized sufficiently by properly selecting the homogenizing heat treatment temperature from a range of 500° C. or higher and 560° C. or lower, and the homogenizing (holding) time from a range of one hour or more. If the homogenizing temperature is lower, amounts of solute Si and solute Mg cannot be ensured and the microstructure defined by the present invention (amount of solute Mg and amount of solute Si) cannot be attained even by the pre-aging (reheating) after the solid solution treatment and the quenching to be described later. Further, since the segregation in the crystal grains cannot be eliminated sufficiently and acts as fracture origins, the bendability is deteriorated. 
     After the homogenizing heat treatment, hot rolling is performed in which it is necessary not to lower the temperature of the slab to 500° C. or lower before the start of hot rough rolling after the homogenizing heat treatment thereby ensuring the amounts of solute Si and solute Mg. If the temperature of the slab is lowered to 500° C. or lower before the start of the hot rough rolling, there may be a high possibility that Si and Mg are precipitated failing to ensure the amounts of solute Si and solute Mg for attaining the microstructure defined in the present invention (amount of solute Mg and amount of solute Si). 
     (Hot Rolling) 
     Hot rolling includes a rough rolling step of a slab and a finish rolling step depending on the thickness of the sheet to be rolled. In the rough rolling step or finish rolling step, reversed type, tandem type rolling mill, etc are used properly. 
     During rolling from the start to the end of hot rough rolling, it is necessary not to lower the temperature to 450° C. or lower, thereby ensuring the amounts of solute Si and solute Mg. If the lowest temperature of the roughly rolled sheet between passes is lowered to 450° C. or lower due to increase of the rolling time, etc., Mg—Si based compounds tend to be precipitated to decrease the amount of solute Mg and the amount of solute Si. Accordingly, there may be a high possibility that the amounts of solute Si and solute Mg in order to attain the microstructure defined in the present invention (amount of solute Mg and amount of solute Si) cannot be ensured. 
     After the hot rough rolling described above, a hot finish rolling with an end temperature in a range of 300 to 360° C. is performed. If the end temperature of the hot finish rolling is excessively low, e.g., lower than 300° C., the rolling load is increased to lower the productivity. In contrast, when the end temperature of the hot finish rolling is made higher for obtaining a recrystallized microstructure without leaving much fabrication microstructure, if the temperature exceeds 360° C., Mg—Si based compounds tend to be precipitated to decrease the amount of solute Mg and the amount of solute Si. Accordingly, there may be a high possibility that the amounts of solute Si and solute Mg cannot be ensured for attaining the microstructure defined in the present invention (amount of solute Mg and amount of Si). 
     (Annealing of Hot Rolled Sheet) 
     Annealing before cold rolling of the hot rolled sheet (rough annealing) may be applied although this is not always necessary. 
     (Cold Rolling) 
     In the cold rolling, the hot rolled sheet is rolled to manufacture a cold rolled sheet (also including coil) of a desired sheet thickness. For refining the crystal grains further, the cold rolling compression reduction is preferably 30% or more, and intermediate annealing may be performed between cold rolling passes with the same purpose as that for rough annealing. 
     (Solid Solution Treatment and Quenching) 
     After the cold rolling, solid solution treatment and successive quenching to a room temperature are performed. For the solid solution treatment and the quenching, a usual continuous heat treatment line may be used. However, for obtaining a sufficient amount in solid solution for each of the elements such as Mg and Si, it is preferred to hold the rolled sheet at a solid solution treatment temperature, that is, from 550° C. or higher to a melting temperature or lower for 10 seconds or more and then cool the sheet at an average cooling rate of 20° C./sec or more from the holding temperature to 100° C. If the temperature is lower than 550° C. or the holding time is shorter than 10 seconds, re-solid solution of compounds, for example, Al-Mn-Fe based or Mg—Si based that are formed before the solid solution treatment is insufficient and the amount of solute Mg and the amount of solute Si are decreased. Accordingly, there is a high possibility that the amounts of solute Si or solute Mg for attaining the microstructure defined in the present invention (amount of solute Mg and amount of solute Si) cannot be ensured. 
     If the average cooling rate is less than 20° C./sec, precipitates mainly based on Mg—Si are formed during cooling to decrease the amount of solute Mg and the amount of solute Si and there is also a high possibility that an amount of solute Si and an amount of solute Mg cannot be ensured. In order to ensure the cooling rate, air cooling means such as a blower, water cooling means such as mist, spray or clipping, as well as conditions therefor are selected and used respectively for the quenching. 
     (Pre-Aging: Reheating) 
     After cooling to a room temperature by quenching after the solid solution treatment, the cold rolled sheet is subjected to pre-aging (reheating) within one hour. If the room temperature holding time after the end of the quenching to the room temperature till the start of the pre-aging (start of heating) is excessively long, the Si enriched Mg—Si clusters are formed by the natural aging in which Mg—Si clusters in a good balance between Mg and Si can be less increased. Accordingly, it is preferred that the room temperature holding time is preferably as short as possible, and the solid solution treatment and the quenching may be in continuous with scarce time interval. The lower limit for the time is not particularly defined. 
     In the pre-aging, the holding time of the sheet at 60 to 120° C. is defined as 10 hours or more and 40 hours or less. Thus, the Mg—Si clusters in a good balance between Mg and Si defined in the present invention are formed. 
     If the pre-aging temperature is lower than 60° C., or the holding time is less than 10 hours, the result of the treatment is identical with that in a case without the pre-aging, in which the Mg—Si clusters that suppress the Si enriched Mg—Si clusters and in a good balance between Mg and Si are less increased, thereby tending to lower the proof stress after the paint-bake treatment. 
     In contrast, if the temperature is higher than 120° C. or the holding time is longer than 40 hours in the pre-aging conditions, the precipitated nuclei are formed excessively in which the strength during bending fabrication before the paint-bake treatment is excessively high tending to deteriorate the bendability. 
     The present invention is to be described more specifically with reference to examples but it will be apparent that the present invention no way undergoes restriction by the following examples and can be practiced with appropriate modification in a range adaptable to the gists of the invention as described before and to be described later, any of which is included within the technical scope of the present invention. 
     EXAMPLE 
     6000-series aluminum alloy sheets of different microstructures defined by the amount of solute Mg and the amount of solute Si in the present invention were separately fabricated by changing the composition and manufacturing condition, and As proof stress, BH property (paint bake-hardenability), and bendability were measured and evaluated respectively after holding at a room temperature for 100 days after manufacture of the sheets. The results are shown in Tables 1 and 2. 
     Specifically, 6000-series aluminum alloy sheets having compositions as shown in Table 1 were manufactured separately while variously changing the manufacturing conditions, for example, the soaking temperature, the lowest temperature of the roughly rolled sheet between passes of hot rough rolling (described as the lowest temperature in Table 2), the end temperature of the hot finish rolling, the holding temperature and the holding time of solid solution treatment, the average cooling rate, the temperature and the holding time in the pre-aging. In the expression for the content of each of the elements in Table 1, columns for respective elements which are left blank with no numerical values indicate that the contents are below the detection limit 
     Specific manufacturing conditions of the aluminum alloy sheets are as described below. Aluminum alloy slabs of respective compositions shown in Table 1 were melted in common by a DC casting method. In this case, an average cooling rate during casting from a liquidus temperature to a solidus temperature was defined as 50° C./min in common with each of examples. Successively, after subjecting the slabs to soaking for 6 hours in common with each of examples under the temperature conditions shown in Table 2, hot rough rolling was started at that temperature. Table 2 also shows the lowest (pass) temperature in the hot rough rolling. 
     Then, in common with each of the examples, the sheets were hot rolled in the succeeding finish rolling at an end temperature shown in Table 2 to a thickness of 4.0 mm to form hot rolled sheets. After subjecting the aluminum alloy sheets after hot rolling to rough annealing of 500° C.×1 minute in common with each of the examples, the sheets were subjected to cold rolling at a compression reduction of 50% with no intermediate annealing in the course of cold rolling pass to obtain cold rolled sheets of 2.0 mm thickness. 
     Each of the cold rolled sheets was continuously subjected to tempering (T4) while being recoiled and coiled in a continuous heat treatment facility in common with each of the examples. Specifically, solid solution treatment was performed at an average heating rate of 10° C./sec up to 500° C. and holding the sheet for 20 seconds after reaching the aimed temperature of 540° C., and then the sheet was cooled to a room temperature by water cooling at an average cooling rate of 100° C./sec. Just after the cooling, pre-aging was performed at temperature (° C.) and for holding time (hr) shown in Table 2. After the pre-aging, gradual cooling (spontaneous cooling) was performed. 
     Sheet specimens (blanks) were cut out from respective final sheet products which were left for 100 days at a room temperature after the tempering and each of the sheet specimens was measured and evaluated for the microstructure defined by the amount of solute Mg and the amount of solute Si and the properties. The result is shown in Table 2. 
     (Measurement of the Amount of Solute Mg and the Amount of Solute Si) 
     The amount of solute Mg and the amount of solute Si in each of the specimens were measured by dissolving sheet specimens as the object of measurement according to a residue extraction method with hot phenol, separating under filtration the solid solution by using a filter of 0.1 μm mesh, and measuring the contents of Mg and Si separated in the solution as the amount of solute Mg and the amount of solute Si respectively. 
     The residue extraction method with hot phenol was performed specifically as described below. First, after heating phenol contained in a separable flask, each of specimen sheets as the object of measurement was transferred into the separable flask and thermally decomposed. Then, after adding benzyl alcohol, the contents were filtered and separated under suction, and the total content of the separated Mg and Si in the solution was quantitatively analyzed. For the quantitative analysis, atomic absorption spectrometry (AAS), inductively coupled plasma atomic spectrometry (ICP-OES), or the like was used optionally. For the filtration under suction, a membrane filter having 0.1 μm mesh size (size of particle to be captured) and 47 mmφ was used as described above. 
     Measurement and calculation were performed for the three specimens each sampled from the sheet specimen at three points in total, i.e., at one point in a central portion in the direction of the thickness and at two points on both ends in the direction of the width of the sheet specimen, and the amounts of solute Mg and solute Si (mass %) for respective specimens were averaged. Then, the sum of the amount of solute Si and the amount of solute Mg, and the ratio of the amount of solute Si to the amount of solute Mg (amount of solute Si/amount of solute Mg) were calculated based on the amount of solute Si and the amount of solute Mg. 
     (Paint Bake-Harden Ability) 
     As the mechanical properties of the sheet specimen, 0.2% proof stress (proof stress after BH) of the sheet specimen after 2% stretch that simulated the bending fabrication after artificial aging of 185° C.×20 minutes (after BH) was determined by a tensile test in common with each of the sheet specimens. The BH property of the respective sheet specimens was evaluated based on the difference between 0.2% proof stress to each other (increment of the proof stress). 
     In the tensile test, No. 5 test specimen of JIS Z 2201 (25 mm×50 mmGL×sheet thickness) was sampled from each of the sheet specimens and subjected to the tensile test at a room temperature. The tensile direction of the test specimen was in a direction orthogonal to the rolling direction. The tensile speed was set to 5 mm/min up to the 0.2% proof stress and to 20 mm/min after applying the proof stress. The number of times N of the mechanical property measurement was 5 and each of result was calculated as an average value. For the test specimen for measuring the proof stress after the BH. The BH treatment was performed after applying 2% preliminary strain that simulated the press forming of the sheet by a tensile tester. 
     (Bendability) 
     Bendability was measured for each of sheet specimens. In the test, a test specimen of 30 mm width×35 mm length was prepared while taking a major axis in the rolling direction, and 90° V-bending was performed at a bending radius of 2.0 mm while applying a load of 2000 kgf according to JIS Z 2248. 
     The surface state of the V-bent portion, for example, generation of roughening, fine cracks, and large cracks was observed visually and evaluated visually according to the following criteria, and those of 6 or more scores were evaluated as satisfactory (in Table 2, only the acceptance (◯, ×) is described).
     9: no cracks, no roughening,   8: no cracks, slight roughening,   7: no cracks, roughening,   6: slight fine cracks,   5: fine cracks,   4: fine cracks over the entire surface,   3: large cracks,   2: large cracks, immediately to fail,   1: failed.   

     As shown in Tables 1 and 2 respectively, Inventive Examples 1 to 8 are manufactured within the range of the chemical composition and within the range of preferred conditions of the present invention. Accordingly, in each of the inventive examples, as shown in Table 2, both of the amount of solute Mg and the amount of solute Si in the solution separated by the hot phenol residue extraction method are 0.6% or more, the sum of the amount of solute Mg and the amount of solute Si is 1.4% or more, and the ratio of the amount of solute Si to the amount of solute Mg (solute Si/solute Mg) is 0.8 to 1.2. 
     As a result, each of the inventive examples 1 to 8 has high 0.2% proof stress after BH and high strength and is excellent in the bendability even after the natural aging as shown in Table 2. 
     On the contrary, Comparative Examples 9 to 14 in Table 2 use an alloy example 1 identical with that of the inventive example in Table 1. However, in each of the comparative examples, manufacturing conditions such as the soaking temperature, the lowest temperature in the hot rough rolling, the end temperature of the hot finish rolling, the holding temperature and the holding time in the solid solution treatment, the average cooling rate, and the temperature and the holding time in the pre-aging are out of the preferred conditions. As a result, the amount of solute Mg and the amount of solute Si are out of the range defined in the present invention and, the BH property and the bendability after the natural aging are deteriorated when compared with the Inventive Example 1 using the identical alloy composition. 
     Among them, in Comparative Example 9, the lowest temperature in the hot rough rolling and the end temperature of the hot finish rolling are excessively low. Accordingly, the sum of the amount of solute Mg and the amount of solute Si is insufficient being out of the lower limit on one hand, and the ratio of the amount of solute Si and the amount of solute Mg (solute Si/solute Mg) is excessive being out of the upper limit on the other hand, so that the BH property was low and 0.2% proof stress after BH is insufficient. 
     In Comparative Example 10, the lowest temperature in the hot rough rolling is excessively low. Accordingly, the amount of solute Mg and the sum of the amount of solute Mg and the amount of solute Si is insufficient being out of the lower limit on one hand, and the ratio (solute Si/solute Mg) is excessive being out of the upper limit on the other hand, so that the BH property is low and 0.2% proof stress after BH is insufficient. 
     In Comparative Example 11, the holding temperature in the solid solution treatment is excessively low. Accordingly, the amount of solute Mg, the amount of solute Si, and the sum of the amount of solute Mg and the amount of solute Si are insufficient being out of the lower limit, so that the BH property is low and the 0.2% proof stress after BH is insufficient. 
     In Comparative Example 12, the holding time in the solid solution treatment is excessively short. Accordingly, the amount of solute Mg, the amount of solute Si, and the sum of the amount of solute Mg and the amount of solute Si is insufficient being out of the lower limit, so that the BH property is low and the 0.2% proof stress after BH is insufficient. 
     In Comparative Example 13, the average cooling rate after the solid solution treatment is insufficient. Accordingly, the amount of solute Mg and the sum of the amount of solute Mg and the amount of solute Si are insufficient being out of the lower limit on one hand and the ratio (solute Si/solute Mg) is excessive being out of the upper limit on the other hand, so that the BH property is insufficient and the 0.2% proof stress after BH is insufficient. 
     In Comparative Example 14, the soaking temperature and the lowest temperature in the hot rough rolling are excessively low. Accordingly, the amount of solute Mg and the sum of the amount of solute Mg and the amount of solute Si are insufficient on one hand and ratio (solute Si/solute Mg) is excessive being out of the upper limit on the other hand, so that the BH property is low and the 0.2% proof stress after BH is insufficient. The bendability is also poor. 
     While Comparative Examples 15 to 20 in Table 2 are manufactured in a range of preferred conditions but use alloys Nos. 10 to 15 in Table 1 and the contents for Mg, Si, Mn, and Fe as essential elements are out of the range defined in the invention respectively. Accordingly, in such comparative examples, the amount of solute Mg and the amount of solute Si, or relation thereof are out of the ranges defined in the present invention as shown in Table 2, and the strength after BH is deteriorated when compared with that of the inventive examples. 
     Among them, in Comparative Examples 16 and 18, coarse constituents and precipitates are formed, remarkable sheet cracks are formed during the hot rolling, and the sheet per se could not be manufactured, so that the microstructure and the property could not be evaluated. 
     In Comparative Example 15, alloy 10 in Table 1 is used in which Mg is insufficient. 
     In Comparative Example 16, alloy 11 in Table 1 is used in which Mg is excessive. 
     In Comparative Example 17, alloy 12 in Table 1 is used in which Si is insufficient. 
     In Comparative Example 18, alloy 13 in Table 1 is used in which Si is excessive. 
     In Comparative Example 19, alloy 14 in Table 1 is used in which Fe is excessive. 
     In Comparative Example 20, alloy 15 in Table 1 is used in which Mn is excessive. 
     Accordingly, in view of the result of the examples described above, it is supported that all the composition and the microstructure defined in the present invention should be satisfied in order to increase the strength without deteriorating the bendability also after natural aging. 
     
       
         
           
               
               
             
               
                 TABLE 1 
               
             
            
               
                   
               
               
                 Alloy 
                 Chemical composition of Al—Mg—Si alloy sheet (mass %, remainder Al) 
               
            
           
           
               
               
               
               
               
               
               
               
               
               
               
               
               
            
               
                 No. 
                 Mg 
                 Si 
                 Mn 
                 Fe 
                 Cu 
                 Cr 
                 Zr 
                 V 
                 Ti 
                 Zn 
                 Ag 
                 Sn 
               
               
                   
               
            
           
           
               
               
               
               
               
               
               
               
               
               
               
               
               
            
               
                 1 
                 0.85 
                 1.03 
                 0.08 
                 0.14 
                   
                   
                   
                   
                   
                   
                   
                   
               
               
                 2 
                 0.86 
                 1.02 
                 0.45 
                 0.16 
                   
                   
                   
                   
                   
                   
                   
                 0.10 
               
               
                 3 
                 0.67 
                 0.80 
                 0.20 
                 0.09 
                   
                 0.05 
                   
                   
                 0.07 
                   
                   
                   
               
               
                 4 
                 0.80 
                 0.66 
                 0.07 
                 0.21 
                 0.70 
                   
                   
                   
                   
                   
                   
                   
               
               
                 5 
                 1.45 
                 1.22 
                 0.34 
                 0.17 
                   
                   
                   
                 0.15 
                   
                   
                 0.10 
                   
               
               
                 6 
                 1.09 
                 1.50 
                 0.08 
                 0.43 
                   
                   
                 0.15 
                   
                   
                 0.35 
                   
                   
               
               
                 7 
                 0.85 
                 1.25 
                 0.90 
                 0.19 
                 0.15 
                   
                   
                   
                 0.02 
                   
                   
                   
               
               
                 9 
                 0.84 
                 1.03 
                 0.10 
                 0.13 
                   
                 0.25 
                 0.03 
                   
                   
                   
                   
                 0.02 
               
               
                 10 
                 0.52 
                 0.89 
                 0.08 
                 0.15 
                   
                   
                   
                   
                   
                   
                   
                   
               
               
                 11 
                 2.25 
                 1.22 
                 0.20 
                 0.25 
                   
                   
                   
                   
                   
                   
                   
                   
               
               
                 12 
                 1.20 
                 0.54 
                 0.10 
                 0.14 
                   
                   
                   
                   
                   
                   
                   
                   
               
               
                 13 
                 0.70 
                 2.25 
                 0.06 
                 0.23 
                   
                   
                   
                   
                   
                   
                 0.10 
                   
               
               
                 14 
                 0.85 
                 1.00 
                 0.15 
                 0.72 
                   
                   
                   
                   
                   
                   
                   
                   
               
               
                 15 
                 0.85 
                 1.03 
                 1.15 
                 0.15 
               
               
                   
               
            
           
         
       
     
     
       
         
           
               
               
               
               
               
               
               
               
               
               
               
               
             
               
                 TABLE 2 
               
             
            
               
                   
               
               
                   
                   
                   
                   
                   
                   
                   
                   
                   
                   
                   
                 Aluminum alloy sheet after holding 
               
            
           
           
               
               
               
               
               
            
               
                   
                   
                   
                 Manufacturing conditions of aluminum alloy sheet 
                 at room temperature for 100 days 
               
            
           
           
               
               
               
               
               
               
               
               
               
               
               
               
               
            
               
                   
                   
                   
                   
                 Hot 
                 Hot 
                   
                   
                   
                   
                   
                   
                 Property 
               
            
           
           
               
               
               
               
               
               
               
               
               
               
               
               
            
               
                   
                   
                   
                   
                 rough 
                 finish 
                 Solid solution treatment 
                   
                   
                   
                 0.2% 
                   
               
            
           
           
               
               
               
               
               
               
               
               
               
               
               
               
               
               
            
               
                   
                   
                 Alloy  
                 Soaking 
                 rolling 
                 rolling 
                   
                   
                 Average 
                   
                   
                   
                 proof 
                 Bend- 
               
            
           
           
               
               
               
               
               
               
               
               
               
               
               
               
               
            
               
                   
                   
                 No. 
                 Soaking 
                 Lowest 
                 End 
                 Holding 
                 Hold- 
                 cooling 
                 Pre-aging 
                 Solid solution amount 
                 stress 
                 ability 
               
            
           
           
               
               
               
               
               
               
               
               
               
               
               
               
               
               
            
               
                   
                   
                 in  
                 temper- 
                 temper- 
                 temper- 
                 temper- 
                 ing 
                 rate 
                 Temper- 
                   
                 (mass %) 
                 after  
                 90° 
               
            
           
           
               
               
               
               
               
               
               
               
               
               
               
               
               
               
               
               
               
            
               
                   
                   
                 Table  
                 ature 
                 ature 
                 ature 
                 ature 
                 time 
                 (° C./ 
                 ature 
                 Time 
                   
                   
                 Mg + 
                 Si/ 
                 BH 
                 V- 
               
               
                 Section 
                 No. 
                 1 
                 (° C.) 
                 (° C.) 
                 (° C.) 
                 (° C.) 
                 (sec) 
                 sec) 
                 ° C. 
                 hr 
                 Mg 
                 Si 
                 Si 
                 Mg 
                 MPa 
                 bending 
               
               
                   
               
            
           
           
               
               
               
               
               
               
               
               
               
               
               
               
               
               
               
               
               
            
               
                 Inventive  
                 1 
                 1 
                 540 
                 470 
                 330 
                 570 
                 15 
                 25 
                 100 
                 20 
                 0.83 
                 0.94 
                 1.77 
                 1.13 
                 285 
                 ∘ 
               
               
                 Example 
                   
                   
                   
                   
                   
                   
                   
                   
                   
                   
                   
                   
                   
                   
                   
                   
               
               
                 Comparative  
                 9 
                 1 
                 530 
                 410 
                 280 
                 570 
                 15 
                 25 
                 100 
                 20 
                 0.61 
                 0.75 
                 1.36 
                 1.23 
                 252 
                 ∘ 
               
               
                 Example 
                   
                   
                   
                   
                   
                   
                   
                   
                   
                   
                   
                   
                   
                   
                   
                   
               
               
                 Comparative  
                 10 
                 1 
                 540 
                 420 
                 300 
                 570 
                 15 
                 25 
                 100 
                 20 
                 0.59 
                 0.73 
                 1.32 
                 1.24 
                 244 
                 ∘ 
               
               
                 Example 
                   
                   
                   
                   
                   
                   
                   
                   
                   
                   
                   
                   
                   
                   
                   
                   
               
               
                 Comparative  
                 11 
                 1 
                 540 
                 490 
                 330 
                 530 
                 15 
                 25 
                 100 
                 20 
                 0.49 
                 0.52 
                 1.01 
                 1.06 
                 220 
                 ∘ 
               
               
                 Example 
                   
                   
                   
                   
                   
                   
                   
                   
                   
                   
                   
                   
                   
                   
                   
                   
               
               
                 Comparative  
                 12 
                 1 
                 540 
                 490 
                 330 
                 570 
                 3 
                 25 
                 100 
                 20 
                 0.50 
                 0.54 
                 1.04 
                 1.08 
                 223 
                 ∘ 
               
               
                 Example 
                   
                   
                   
                   
                   
                   
                   
                   
                   
                   
                   
                   
                   
                   
                   
                   
               
               
                 Comparative  
                 13 
                 1 
                 540 
                 490 
                 330 
                 570 
                 15 
                 10 
                 100 
                 20 
                 0.54 
                 0.68 
                 1.22 
                 1.26 
                 239 
                 ∘ 
               
               
                 Example 
                   
                   
                   
                   
                   
                   
                   
                   
                   
                   
                   
                   
                   
                   
                   
                   
               
               
                 Comparative  
                 14 
                 1 
                 480 
                 430 
                 300 
                 570 
                 15 
                 25 
                 100 
                 20 
                 0.53 
                 0.66 
                 1.19 
                 1.25 
                 234 
                 x 
               
               
                 Example 
                   
                   
                   
                   
                   
                   
                   
                   
                   
                   
                   
                   
                   
                   
                   
                   
               
               
                 Inventive  
                 2 
                 2 
                 540 
                 470 
                 330 
                 560 
                 20 
                 25 
                 100 
                 20 
                 0.84 
                 0.84 
                 1.68 
                 1.00 
                 304 
                 ∘ 
               
               
                 Example 
                   
                   
                   
                   
                   
                   
                   
                   
                   
                   
                   
                   
                   
                   
                   
                   
               
               
                 Inventive  
                 3 
                 3 
                 550 
                 470 
                 330 
                 570 
                 15 
                 25 
                 120 
                 10 
                 0.65 
                 0.76 
                 1.41 
                 1.17 
                 263 
                 ∘ 
               
               
                 Example 
                   
                   
                   
                   
                   
                   
                   
                   
                   
                   
                   
                   
                   
                   
                   
                   
               
               
                 Inventive  
                 4 
                 4 
                 540 
                 460 
                 320 
                 560 
                 20 
                 25 
                 60 
                 40 
                 0.77 
                 0.64 
                 1.41 
                 0.83 
                 275 
                 ∘ 
               
               
                 Example 
                   
                   
                   
                   
                   
                   
                   
                   
                   
                   
                   
                   
                   
                   
                   
                   
               
               
                 Inventive  
                 5 
                 5 
                 520 
                 450 
                 310 
                 550 
                 15 
                 20 
                 80 
                 30 
                 1.10 
                 0.98 
                 2.08 
                 0.89 
                 298 
                 ∘ 
               
               
                 Example 
                   
                   
                   
                   
                   
                   
                   
                   
                   
                   
                   
                   
                   
                   
                   
                   
               
               
                 Inventive  
                 6 
                 6 
                 540 
                 460 
                 320 
                 560 
                 20 
                 25 
                 100 
                 20 
                 1.02 
                 1.15 
                 2.17 
                 1.13 
                 312 
                 ∘ 
               
               
                 Example 
                   
                   
                   
                   
                   
                   
                   
                   
                   
                   
                   
                   
                   
                   
                   
                   
               
               
                 Inventive  
                 7 
                 7 
                 540 
                 470 
                 330 
                 560 
                 15 
                 20 
                 70 
                 30 
                 0.79 
                 0.88 
                 1.57 
                 1.11 
                 308 
                 ∘ 
               
               
                 Example 
                   
                   
                   
                   
                   
                   
                   
                   
                   
                   
                   
                   
                   
                   
                   
                   
               
               
                 Inventive  
                 8 
                 9 
                 540 
                 480 
                 350 
                 570 
                 10 
                 25 
                 100 
                 20 
                 0.78 
                 0.90 
                 1.68 
                 1.15 
                 305 
                 ∘ 
               
               
                 Example 
                   
                   
                   
                   
                   
                   
                   
                   
                   
                   
                   
                   
                   
                   
                   
                   
               
               
                 Comparative  
                 15 
                 10 
                 550 
                 470 
                 330 
                 570 
                 15 
                 25 
                 100 
                 20 
                 0.48 
                 0.84 
                 1.32 
                 1.75 
                 237 
                 ∘ 
               
               
                 Example 
                   
                   
                   
                   
                   
                   
                   
                   
                   
                   
                   
                   
                   
                   
                   
                   
               
            
           
           
               
               
               
               
               
               
            
               
                 Comparative  
                 16 
                 11 
                 510 
                 Crack in hot oiling 
                 — 
               
               
                 Example 
                   
                   
                   
                   
                   
               
            
           
           
               
               
               
               
               
               
               
               
               
               
               
               
               
               
               
               
               
            
               
                 Comparative  
                 17 
                 12 
                 530 
                 470 
                 330 
                 570 
                 15 
                 25 
                 100 
                 20 
                 1.08 
                 0.48 
                 1.56 
                 0.44 
                 209 
                 ∘ 
               
               
                 Example 
                   
                   
                   
                   
                   
                   
                   
                   
                   
                   
                   
                   
                   
                   
                   
                   
               
            
           
           
               
               
               
               
               
               
            
               
                 Comparative  
                 18 
                 13 
                 520 
                 Crack in hot oiling 
                 — 
               
               
                 Example 
                   
                   
                   
                   
                   
               
            
           
           
               
               
               
               
               
               
               
               
               
               
               
               
               
               
               
               
               
            
               
                 Comparative  
                 19 
                 14 
                 540 
                 470 
                 330 
                 560 
                 15 
                 25 
                 100 
                 20 
                 0.84 
                 0.75 
                 1.59 
                 0.89 
                 253 
                 x 
               
               
                 Example 
                   
                   
                   
                   
                   
                   
                   
                   
                   
                   
                   
                   
                   
                   
                   
                   
               
               
                 Comparative  
                 20 
                 15 
                 520 
                 460 
                 320 
                 560 
                 15 
                 20 
                 100 
                 20 
                 0.75 
                 0.57 
                 1.32 
                 0.76 
                 246 
                 x 
               
               
                 Example 
                   
                   
                   
                   
                   
                   
                   
                   
                   
                   
                   
                   
                   
                   
                   
                   
               
               
                   
               
            
           
         
       
     
     According to the present invention, a 6000-series aluminum alloy sheet increased in the strength without deteriorating bendability can be provided. As a result, application use of the 6000-series aluminum alloy sheet can be extended as automobile structural materials except for panel materials, for example, skeleton materials such as frames and pillars or reinforcing materials such as bumper reinforcements and door beams.