Manufacturing method of Al-Mg-Si series aluminum alloy sheet excellent in formability

Abstract

To provide a method for producing an Al-Mg-Si-based aluminum alloy plate excellent in both breaking elongation and work hardening, and excellent in formability.SOLUTION: A method for producing an Al-Mg-Si-based aluminum alloy plate having excellent formability comprises casting an aluminum alloy containing 0.3 mass% or more and 0.45 mass% or less of Mg, 0.6 mass% or more and 1.75 mass% or less of Si, with the balance being Al and unavoidable impurities, and having a ratio of [Si] / [Mg] of more than 2.5, where the Mg content in mass% is [Mg] and the Si content in mass% is [Si], the method also comprises soaking the aluminum alloy, hot rolling it, cold rolling it and solution-treating it, and after the solution treatment, quenching it and cooling it to room temperature, performing a heat treatment in which the temperature is kept in a temperature range of 100°C to 300°C for 5 seconds or more and 300 seconds or less within 1 hour, and then performing a heat treatment in which the temperature is kept in a temperature range of 30°C to 100°C for 5 hours or more and 500 hours or less.SELECTED DRAWING: Figure 1

Description

Technical Field

[0001] The present invention relates to a method for manufacturing a 6000 series aluminum alloy sheet produced by ordinary rolling, and more particularly to a method for manufacturing an Al-Mg-Si series aluminum alloy sheet excellent in formability, having both good elongation at break and work hardenability.

Background Art

[0002] In recent years, due to considerations for the global environment and the like, the social demand for weight reduction of automobile bodies has been increasing. To meet such demands, aluminum alloy materials have been applied to large body panels (outer panels, inner panels) of automobile bodies instead of conventional steel materials such as steel sheets.

[0003] Among the above large body panels, for panels such as outer panels (outer sheets) and inner panels (inner sheets) of panel structures such as hoods, fenders, doors, roofs, and trunk lids, Al-Mg-Si series AA to JIS 6000 series (hereinafter, also simply referred to as 6000 series) aluminum alloy sheets are used as thin and high-strength aluminum alloy sheets.

[0004] This 6000 series (Al-Mg-Si series) aluminum alloy sheet contains Si and Mg as essential components, and particularly, an excess Si type 6000 series aluminum alloy sheet has excellent age hardening ability during artificial aging treatment.

[0005] Since these automotive panel materials are generally subjected to press forming, the applied aluminum alloy sheets are required to have excellent formability. In recent years, with the diversification, sharpness, and complexity of vehicle body designs and character lines, press forming processes have become more complex and processing conditions have become more severe, and it has become necessary to further improve press formability.

[0006] For example, Non-Patent Document 1 describes that in order to improve the press formability of Al-Mg-Si series alloys, it is necessary to improve elongation at break and work hardenability.

[0007] Conventionally, various methods for controlling the Mg-Si-based clusters in 6000 series aluminum alloy sheets as materials for such automotive components have been studied. Specifically, a method has been proposed to achieve both high baking paint curability and high formability due to high elongation at break and low yield strength by controlling the exothermic peak suggesting clusters and strengthening phases.

[0008] For example, in Non-Patent Document 2, in an excess Si type Al-Mg-Si alloy, based on the generation of various precipitation phases such as GP zones (Guinier-Preston zones), strengthening phases, intermediate phases, and equilibrium phases with the increase in temperature over time, it is suggested that alloy microstructure control is possible by controlling the exothermic peak height in differential scanning calorimetry (DSC).

[0009] Further, in Patent Document 1, in a differential scanning thermal analysis curve, there is an endothermic peak with a height A of 3 to 10 μW / mg within a temperature range of 150 to 230°C, and an exothermic peak with a height B of 20 to 50 μW / mg within a temperature range of 230°C or higher and less than 330°C, and an aluminum alloy sheet excellent in formability and baking paint curability is disclosed, characterized in that the ratio B / A of the exothermic peak height B to the endothermic peak height A is more than 3.5 and less than 15.0.

[0010] Furthermore, in Patent Document 2, in a differential scanning thermal analysis curve, within a temperature range of 230 to 330°C, there is only one exothermic peak, or only two exothermic peaks with a temperature difference between the peaks of 50°C or less, and the height of the only one exothermic peak, or the height of the exothermic peak with the larger peak height among the two exothermic peaks is in the range of 20 to 50 μW / mg, and an aluminum alloy sheet is disclosed.

Prior Art Documents

Non-Patent Documents

[0011]

Non-Patent Document 1

Non-Patent Document 2

Patent Document

[0012]

Patent Document 1

Patent Document 2

Summary of the Invention

Problems to be Solved by the Invention

[0013] However, according to the above conventional technology, when Mg is added to enhance the age hardenability for the purpose of achieving both age hardenability and elongation at break, there arises a problem that the elongation at break decreases. Therefore, in order to improve the formability, it is required to improve the elongation at break and the work hardenability.

[0014] The present invention has been made in view of such problems, and an object thereof is to provide a method for manufacturing an Al-Mg-Si-based aluminum alloy sheet excellent in formability, in which both the elongation at break and the work hardenability are good.

Means for Solving the Problems

[0015] The method for manufacturing an Al-Mg-Si-based aluminum alloy sheet excellent in formability according to the present invention comprises the following configuration (1). (1) Mg: 0.3 mass% or more and 0.45 mass% or less, Si: 0.6 mass% or more and 1.75 mass% or less, containing, with the balance being Al and unavoidable impurities, A method for manufacturing an Al-Mg-Si series aluminum alloy sheet, which comprises casting an aluminum alloy in which when the content of Mg is expressed as [Mg] in mass% and the content of Si is expressed as [Si] in mass%, [Si] / [Mg] exceeds 2.5, subjecting the cast alloy to homogenization heat treatment, hot rolling, cold rolling, and solution treatment. After performing quenching treatment after the solution treatment and cooling to room temperature, a heat treatment is performed in which the alloy is held in the temperature range of 100°C to 300°C for 5 seconds or more and 300 seconds or less within 1 hour, and then a heat treatment is performed in which the alloy is held in the temperature range of 30°C to 100°C for 5 hours or more and 500 hours or less, to obtain an Al-Mg-Si series aluminum alloy sheet excellent in formability.

[0016] A preferred embodiment of the method for manufacturing an Al-Mg-Si series aluminum alloy sheet excellent in formability according to the present invention has the configuration of the following (2). (2) The aluminum alloy further contains at least one selected from Cu, Fe, Mn, and Ti in the ranges of Cu: more than 0 mass% and 0.8 mass% or less, Fe: 0.05 mass% or more and 0.5 mass% or less, Mn: 0.05 mass% or more and 0.3 mass% or less, Ti: more than 0 mass% and 0.1 mass% or less, in the method for manufacturing an Al-Mg-Si series aluminum alloy sheet excellent in formability described in the above (1).

Advantages of the Invention

[0017] According to the present invention, it is possible to provide a method for manufacturing an Al-Mg-Si series aluminum alloy sheet excellent in formability, in which both the elongation at break and the work hardening property are good.

Brief Description of the Drawings

[0018]

Figure 1

Modes for Carrying Out the Invention

[0019] Hereinafter, embodiments of the present invention will be described in detail. Note that the present invention is not limited to the embodiments described below, and can be arbitrarily modified and implemented without departing from the gist of the present invention. Also, in this specification, "~" indicating a numerical range is used to mean including the numerical values described before and after it as the lower limit value and the upper limit value.

[0020] As a result of intensive research to solve the above problems, the inventors of the present invention have found that it is effective to increase the Si content and reduce the Mg content compared to conventional aluminum alloy plates, and to appropriately control the ratio of the Si content to the Mg content in the aluminum alloy plate. That is, within the temperature range of 260°C or higher and 370°C or lower in the differential scanning calorimetry curve, an exothermic peak (second exothermic peak) with a peak height of 18 μW / mg or higher can be obtained, thereby improving the elongation at break and work hardenability.

[0021] Also, after solution treatment, quenching treatment is performed and cooled to room temperature, and then heat treatment is performed by holding at a temperature range of 30°C to 100°C for 5 hours or more and 500 hours or less within 1 hour, or after solution treatment, quenching treatment is performed and cooled to room temperature, and then heat treatment is performed by holding at a temperature range of 100°C to 300°C for 5 seconds or more and 300 seconds or less within 1 hour, and then heat treatment is performed by holding at a temperature range of 30°C to 100°C for 5 hours or more and 500 hours or less. As a result, within the temperature range of 210°C or higher and less than 260°C, an exothermic peak (first exothermic peak) with a peak height of 20 μW / mg or higher can be obtained, thereby ensuring a desired elongation at break and improving work hardenability.

[0022] That is, the Al-Mg-Si-based aluminum alloy sheet with excellent formability according to the embodiment of the present invention contains Mg: 0.3% by mass or more and 0.45% by mass or less, and Si: 0.6% by mass or more and 1.75% by mass or less, with the balance being Al and inevitable impurities. When the content of Mg is [Mg] in% by mass and the content of Si is [Si] in% by mass, [Si] / [Mg] is more than 2.5. In the differential scanning calorimetry curve, the height of the first exothermic peak appearing within the temperature range of 210°C or more and less than 260°C is 20 μW / mg or more, and the height of the second exothermic peak appearing within the temperature range of 260°C or more and 370°C or less is 18 μW / mg or more.

[0023] The aluminum alloy sheet (forming material sheet) referred to in the present invention is a rolled sheet such as a hot-rolled sheet or a cold-rolled sheet, which is a sheet subjected to heat treatment such as solution treatment and quenching treatment (T4), before being formed into the automotive members to be used, and before artificial aging treatment (artificial aging hardening treatment) such as paint baking and curing treatment, that is, the raw material aluminum alloy sheet.

[0024] Hereinafter, the embodiments of the present invention will be described more specifically.

[0025] The chemical composition of the Al-Mg-Si-based aluminum alloy sheet with excellent formability according to the present invention is determined to satisfy the required formability and baking paint curability as a raw material for automotive members such as large automotive body panels from the composition of the 6000 series aluminum alloy sheet. From this perspective, the chemical composition of the Al-Mg-Si-based aluminum alloy sheet with excellent formability according to the present invention contains Mg: 0.3% by mass or more and 0.45% by mass or less, and Si: 0.6% by mass or more and 1.75% by mass or less, with the balance being Al and inevitable impurities. When the content of Mg is [Mg] in% by mass and the content of Si is [Si] in% by mass, [Si] / [Mg] is more than 2.5.

[0026] In addition, the Al-Mg-Si-based aluminum alloy sheet excellent in formability according to the present invention may further contain at least one selected from Cu, Fe, Mn, and Ti in the ranges of Cu: more than 0% by mass and 0.8% by mass or less, Fe: 0.05% by mass or more and 0.5% by mass or less, Mn: 0.05% by mass or more and 0.3% by mass or less, and Ti: more than 0% by mass and 0.1% by mass or less.

[0027] Hereinafter, the chemical composition of the Al-Mg-Si-based aluminum alloy sheet excellent in formability according to the present invention will be described in detail, including the reasons for limiting each element.

[0028] (Si: 0.6% by mass or more and 1.75% by mass or less) Si, together with Mg, forms age precipitates such as Mg-Si-based precipitates that contribute to strength improvement during artificial aging treatment such as solution strengthening and baking painting treatment, and exhibits age hardening ability. Further, as the Si addition amount in the alloy increases, the elongation at break and work hardening property increase. Therefore, Si is an essential element for obtaining the required strength (yield strength) and elongation at break and work hardening property. When the Si content in the aluminum alloy sheet is less than 0.6% by mass, the elongation at break decreases, and the amount of Mg-Si-based precipitates formed after artificial aging heat treatment is insufficient, so the BH (Bake Hardening) property is significantly reduced and the strength is insufficient. Therefore, the Si content in the aluminum alloy sheet is 0.6% by mass or more based on the total mass of the aluminum alloy sheet, preferably 1.0% by mass or more, and more preferably 1.2% by mass or more. On the other hand, when the Si content in the aluminum alloy sheet exceeds 1.75% by mass, coarse Si-based precipitates are formed, the ductility decreases, and it causes cracks during the forming of the material plate. Therefore, the Si content in the aluminum alloy sheet is 1.75% by mass or less based on the total mass of the aluminum alloy sheet, preferably 1.6% by mass or less, and more preferably 1.5% by mass or less.

[0029] (Mg: 0.3% by mass or more and 0.45% by mass or less) Mg, together with Si, forms age precipitates such as Mg-Si-based precipitates that contribute to strength improvement during artificial aging heat treatment such as solution strengthening and baking painting treatment, exhibits age hardening ability, and is an essential element for obtaining the required strength. When the Mg content in the aluminum alloy sheet is less than 0.3% by mass, the amount of Mg-Si-based precipitates formed is insufficient, resulting in a significant decrease in BH property and insufficient strength. Therefore, the Mg content in the aluminum alloy sheet should be 0.3% by mass or more based on the total mass of the aluminum alloy sheet. On the other hand, when the Mg content in the aluminum alloy sheet exceeds 0.45% by mass, the material strength during forming increases, and the elongation at break and work hardening property decrease. Therefore, the Mg content in the aluminum alloy sheet should be 0.45% by mass or less based on the total mass of the aluminum alloy sheet.

[0030] ([Si] / [Mg] > 2.5) The inventors have found that the less the added Mg amount is relative to the added Si amount, the more the solid solution Si amount increases. That is, it has been found that the ratio of the Si content to the Mg content can be used to organize the index of the Si solid solution amount, and by appropriately limiting the value of the above ratio, it is possible to obtain a desired elongation at break. When the Mg content in the aluminum alloy sheet is [Mg] in mass% based on the total mass of the aluminum alloy sheet, and the Si content is [Si] in mass% based on the total mass of the aluminum alloy sheet, when [Si] / [Mg] is 2.5 or less, the Si content is less than the Mg content, and the Si solid solution amount decreases, resulting in a decrease in elongation at break. Therefore, [Si] / [Mg] should be more than 2.5, preferably 2.7 or more, and more preferably 3.0 or more.

[0031] The Al-Mg-Si-based aluminum alloy sheet excellent in formability according to the present invention contains 0.6% by mass or more and 1.75% by mass or less of the above Si, 0.3% by mass or more and 0.45% by mass or less of Mg, and the balance is Al and unavoidable impurities. However, in addition to the above Si and Mg, it may contain at least one selected from Cu, Fe, Mn, and Ti. Since these elements all have the effect of strengthening the aluminum alloy plate, they can be regarded as elements having the same effect in the present invention and selectively contained as necessary. Of course, there are both common parts and different parts in their specific mechanisms.

[0032] (Cu: more than 0% by mass and 0.8% by mass or less) Cu is a component that can improve strength by solid solution strengthening. When the Cu content in the aluminum alloy plate exceeds 0% by mass with respect to the total mass of the aluminum alloy plate, the above effect can be obtained. Therefore, when Cu is contained in the aluminum alloy plate, the Cu content is preferably more than 0% by mass with respect to the total mass of the aluminum alloy plate, preferably 0.02% by mass or more, and more preferably 0.1% by mass or more. On the other hand, when the Cu content in the aluminum alloy plate exceeds 0.8% by mass with respect to the total mass of the aluminum alloy plate, not only does the above effect saturate, but the corrosion resistance of the aluminum alloy plate may deteriorate. Therefore, when Cu is contained in the aluminum alloy plate, the Cu content is preferably 0.8% by mass or less with respect to the total mass of the aluminum alloy plate, and preferably 0.6% by mass or less.

[0033] (Fe: 0.05% by mass or more and 0.5% by mass or less) Fe forms compounds to become nuclei of recrystallized grains, refine the crystal grains, and improve the strength. When the Fe content in the aluminum alloy plate is 0.05% by mass or more with respect to the total mass of the aluminum alloy plate, the above effect can be obtained. Therefore, when Fe is contained in the aluminum alloy plate, the Fe content is 0.05% by mass or more with respect to the total mass of the aluminum alloy plate. On the other hand, when the Fe content in the aluminum alloy plate exceeds 0.5% by mass with respect to the total mass of the aluminum alloy plate, coarse compounds are formed, which may become the starting points of fracture and reduce the formability. Therefore, when Fe is contained in the aluminum alloy plate, the Fe content is preferably 0.5% by mass or less with respect to the total mass of the aluminum alloy plate, and preferably 0.3% by mass or less.

[0034] (Mn: 0.05 mass% or more and 0.3 mass% or less) Mn refines the crystal grains of the ingot and the aluminum alloy sheet as the final product, contributing to the improvement of strength. When the Mn content in the aluminum alloy sheet is 0.05 mass% or more based on the total mass of the aluminum alloy sheet, the above effect can be obtained. Therefore, when Mn is contained in the aluminum alloy sheet, the Mn content is set to 0.05 mass% or more based on the total mass of the aluminum alloy sheet. On the other hand, when the Mn content in the aluminum alloy sheet exceeds 0.3 mass% based on the total mass of the aluminum alloy sheet, coarse compounds may be formed, which may deteriorate the ductility. Therefore, when Mn is contained in the aluminum alloy sheet, the Mn content is set to 0.3 mass% or less based on the total mass of the aluminum alloy sheet, and preferably 0.2 mass% or less.

[0035] (Ti: More than 0 mass% and 0.1 mass% or less) Ti is an element that forms coarse compounds and deteriorates the mechanical properties. However, by containing a small amount of Ti in the aluminum alloy sheet, the crystal grains of the aluminum alloy ingot can be refined, and thus the effect of improving the formability can be obtained. Therefore, within the range defined by JIS standards and other standards for 6000 series alloys, Ti may be contained. Since the effect of refining the crystal grains of the aluminum alloy ingot can be obtained by containing a small amount of Ti in the aluminum alloy sheet, when Ti is contained in the aluminum alloy sheet, the Ti content is set to more than 0 mass% based on the total mass of the aluminum alloy sheet. On the other hand, when the Ti content in the aluminum alloy sheet exceeds 0.1 mass% based on the total mass of the aluminum alloy sheet, coarse compounds are formed, deteriorating the mechanical properties. Therefore, when Ti is contained in the aluminum alloy sheet, the Ti content is set to 0.1 mass% or less based on the total mass of the aluminum alloy sheet, and preferably 0.05 mass% or less.

[0036] (Balance: Al and inevitable impurities) The Al-Mg-Si-based aluminum alloy sheet excellent in formability according to the present invention contains the above-mentioned Mg and Si, and preferably at least one selected from Cu, Fe, Mn, and Ti, with the balance being Al and inevitable impurities. Examples of the inevitable impurities include B, Cr, Zn, Zr, Ni, Bi, Sn, etc. Since B is an element that forms coarse compounds and deteriorates mechanical properties, B as an inevitable impurity is regulated to be 0.03 mass% or less. In addition, Cr, Zn, Zr, Ni, Bi, and Sn as inevitable impurities are each regulated to be 0.1 mass% or less.

[0037] (Stock plate structure) Based on the above alloy composition, in the present invention, the structure of the aluminum alloy sheet is defined by the differential scanning thermal analysis curve obtained by differential scanning calorimetry (DSC) as an index indicating in advance the state of artificial aging precipitates in a member made of this sheet. That is, in order to make both the elongation at break and the work hardening property good, the present invention is defined by the differential scanning thermal analysis curve obtained by differential scanning calorimetry.

[0038] Based on such findings, in the present invention, in order to make both the elongation at break and the work hardening property good, in the differential scanning thermal analysis curve, the height of the first exothermic peak appearing within the temperature range of 210°C or more and less than 260°C is 20 μW / mg or more, and the height of the second exothermic peak appearing within the temperature range of 260°C or more and 370°C or less is 18 μW / mg or more.

[0039] (Height of the first exothermic peak: 20 μW / mg or more) The first exothermic peak appearing within the temperature range of 210°C or more and less than 260°C indicates the formation of the strengthening phase (β''). A high height of the first exothermic peak means that a large amount of the strengthening phase is formed during the differential scanning thermal analysis. In other words, it means that the formation of clusters serving as nuclei of the strengthening phase is less during the differential scanning thermal analysis. If the height of the first exothermic peak is less than 20 μW / mg, a reinforcing phase or a cluster serving as the nucleus of the reinforcing phase is formed at the stage before differential scanning calorimetry. As a result, the strength becomes too high, and the elongation at break and work hardening property also decrease. Therefore, the height of the first exothermic peak appearing within the temperature range of 210°C or higher and less than 260°C should be 20 μW / mg or more. On the other hand, although there is no limitation on the upper limit of the height of the first exothermic peak, in terms of being able to control the formation of the reinforcing phase and suppress the strength reduction of the aluminum alloy sheet, the height of the first exothermic peak is preferably 50 μW / mg or less, and more preferably 35 μW / mg or less.

[0040] (Height of the second exothermic peak: 18 μW / mg or more) The second exothermic peak appearing within the temperature range of 260°C or higher and 370°C or lower indicates the formation of an intermediate phase (such as β'). In addition, the inventors have clarified that as [Si] / [Mg] increases, the height of the second exothermic peak during differential scanning calorimetry increases. That is, a high height of the second exothermic peak indicates an increase in [Si] / [Mg], and based on this, it is considered that the solid solution amount of Si in the alloy increases, and the elongation at break and work hardening property are improved. If the height of the second exothermic peak is less than 18 μW / mg, it is considered that the solid solution amount of Si in the alloy is small, the elongation at break is likely to be low, and the formability improvement due to the compatibility of the elongation at break and work hardening property cannot be obtained. Therefore, the height of the second exothermic peak appearing within the temperature range of 260°C or higher and 370°C or lower should be 18 μW / mg or more. On the other hand, if the height of the second exothermic peak is too high, precipitates are likely to occur, and the elongation at break and work hardening property decrease. Therefore, although there is no limitation on the upper limit of the second exothermic peak, the height of the second exothermic peak is preferably 50 μW / mg or less.

[0041] Thus, the structure defined by the differential scanning calorimetry curve at the stage of the base material sheet is correlated with the elongation at break and work hardening property of the base material sheet, that is, the formability of members such as automobile panels manufactured from this base material sheet. As a result, at the stage of the base material sheet, if the height of the exothermic peak by the differential scanning calorimetry curve is controlled, the formability of the base material sheet can be evaluated. In other words, the structure defined by the differential scanning calorimetry curve at the stage of the base material sheet can be an index of the formability of members using this base material sheet as a forming material.

[0042] (Method for Controlling the Peak Height of the Differential Scanning Calorimetry Curve) The structure specified by the first exothermic peak of the differential scanning calorimetry curve can be controlled by setting the Mg content in the aluminum alloy sheet to 0.3 mass% or more and 0.45 mass% or less. Further, after solution heat treatment, quenching treatment, and cooling to room temperature, the aluminum alloy cold-rolled sheet with the composition adjusted as described above is subjected to a heat treatment of holding at a temperature range of 30°C to 100°C for 5 hours or more and 500 hours or less within 1 hour. Alternatively, after solution heat treatment, quenching treatment, and cooling to room temperature, a heat treatment of holding at a temperature range of 100°C to 300°C for 5 seconds or more and 300 seconds or less is performed within 1 hour, and then a heat treatment of holding at a temperature range of 30°C to 100°C for 5 hours or more and 500 hours or less is performed to achieve control. Regarding the height of the second exothermic peak of the differential scanning calorimetry curve, it can be controlled by adjusting the Si solid solution amount with the value of [Si] / [Mg] exceeding 2.5.

[0043] (Manufacturing Method) The 6000-series aluminum alloy plate of the present invention is a cold-rolled plate obtained by hot-rolling an ingot after homogenization treatment and then further cold-rolling it, and is manufactured by a conventional method, in which further conditioning such as solution treatment is performed. That is, it is an aluminum alloy hot-rolled plate manufactured through the normal manufacturing processes of casting, homogenization treatment, and hot rolling, with a plate thickness of about 2 to 10 mm. Next, it is cold-rolled to obtain a cold-rolled plate with a plate thickness of 4 mm or less. Also, it may be cooled once after the homogenization treatment. In that case, the cooling rate after the homogenization treatment should be 20°C / hr or more and less than 100°C / hr, and after reheating to a predetermined temperature in the range of 350 to 450°C, hot rolling may be started. During cold rolling, annealing and intermediate annealing may be performed as necessary.

[0044] (Solution treatment and quenching treatment) After cold rolling, solution treatment and subsequent quenching treatment to room temperature are performed. For this solution quenching treatment, in order to obtain a sufficient solid solution amount of each element such as Mg and Si, it is desirable to heat to a solution treatment temperature of 500°C or more and below the melting temperature.

[0045] Also, from the viewpoint of suppressing the formation of coarse grain boundary compounds that reduce formability, it is desirable that the average cooling rate from the solution treatment temperature to the quenching stop temperature at room temperature be 20°C / s or more. If the average cooling rate of the quenching treatment after the solution treatment to room temperature is small, coarse Mg2Si and single-phase Si will be generated during cooling, resulting in deteriorated bending workability. Also, the solid solution amount after solution treatment will decrease, and the BH property will decrease. To ensure this cooling rate, for the quenching treatment, air cooling using a fan, water cooling means and conditions such as mist, spray, and immersion are each selected and used.

[0046] After such solution treatment and quenching treatment to cool to room temperature, a heat treatment is performed to hold for 5 hours or more and 500 hours or less in a temperature range of 30°C to 100°C within 1 hour. Or, after performing a heat treatment to hold the cold-rolled plate for 5 seconds or more and 300 seconds or less in a temperature range of 100°C to 300°C within 1 hour, a heat treatment is performed to hold for 5 hours or more and 500 hours or less in a temperature range of 30°C to 100°C. Thereby, the height of the peak of the above differential scanning calorimetry curve can be controlled, and the elongation at break and work hardening property can be ensured.

Example

[0047] The present embodiment will be further specifically described with reference to the following examples. However, the present invention is not limited to these examples, and modifications can be made within the scope that conforms to the spirit of the present invention and implemented, and all of them are included in the technical scope of the present invention.

[0048] After manufacturing aluminum alloy plates having various compositions shown in Table 1 below and holding them at room temperature for 7 days, differential scanning calorimetry (DSC) was performed, and the temperature range in which an exothermic peak appeared and the height of the peak were measured. In addition, a tensile test was performed on the obtained aluminum alloy plates to measure the elongation at break and the work hardening index (n-value) as an index of work hardening property. These results are shown in Table 2. In the column of the content of each element in Table 1, the display of "-" indicates that the content was below the detection limit.

[0049] (Manufacturing conditions of aluminum alloy plates) The specific manufacturing conditions of the aluminum alloy plates are shown below. Aluminum alloy ingots of each composition shown in Table 1 were melted together by die casting. Subsequently, after facing the ingot, homogenization treatment at 540 °C for 4 hours was performed, and then hot rolling was performed at that temperature to obtain a hot rolled plate. This hot rolled plate was cold rolled to obtain a cold rolled plate with a thickness of 1.0 mm.

[0050] Furthermore, each of these cold rolled plates was solution-treated at 540 °C for 1 minute, and then water-cooled to room temperature. Within 30 minutes after this cooling, heat treatment at 200 °C or higher for 1 minute or less and heat treatment at 50 °C for 5 hours were performed, and cooling was performed after the heat treatment.

[0051] After these tempering treatments, differential scanning calorimetry was performed on each test plate after leaving it at room temperature for 7 days.

[0052] (Differential scanning calorimetry) Differential scanning calorimetry was performed on the microstructure at the center of the thickness of the test plate, and the temperature (°C) and height (μW / mg) of the exothermic peak of the aluminum alloy test plate were measured.

[0053] The measurement conditions for differential scanning calorimetry at each measurement location of each of these test plates are shown below. Test apparatus: HITACHI DSC7020 Standard substance: Aluminum Sample container: Aluminum Temperature rising condition: 10 °C / min Atmosphere: Argon (60 ml / min) Sample weight: 39.0 - 42.0 mg

[0054] In this example, differential scanning calorimetry was performed under the same above conditions, and after normalizing the obtained heat flow (μW) by dividing it by the weight (mg) of the test plate (μW / mg), within the temperature range of 0 to 100 °C, the region where the differential scanning thermal analysis curve becomes horizontal was set as the reference level of 0, and the height of the exothermic peak from this reference level was measured.

[0055] [Formability] <Elongation at break> As a test for determining the formability of the above test plate, a tensile test was carried out in accordance with JIS Z 2241, and the elongation at break (%) was measured. For the tensile test, No. 13B test pieces (width of parallel part 12.5 mm × gauge length 50 mm × thickness) specified in JIS Z 2241 were taken from each test plate and carried out at room temperature. The tensile direction of the test piece was set to a direction perpendicular to the rolling direction. Also, the tensile speed was 3 mm / min until the strain amount reached 0.5%, and then 20 mm / min. In addition, four test pieces were taken from one aluminum alloy plate, and the average value was calculated.

[0056] The elongation at break was considered qualified if it was 26% or more. Note that the elongation at break, which is an evaluation of press formability, a difference of just 1% between 25% and 26% greatly affects whether, for example, the shape of the outer panel of an automobile at sharpened or complicated corner parts or character lines can be formed into a beautiful and sharp curved surface configuration without strain or wrinkles.

[0057] <Work hardening index (n value)> As another test for determining the formability of the test plate, a tensile test was carried out in accordance with JIS Z 2253, and the work hardening index (n value) was measured. The work hardening index (n value) was calculated from the true strain and true stress, plotted on a logarithmic scale with the strain on the horizontal axis and the stress on the vertical axis, and the slope of the straight line represented by the measurement points was calculated by the least squares method for the logarithm of the true stress and true strain in the plastic strain range of the nominal strain of 4 to 6%, and taken as the n value (4 - 6%). Note that the n value was considered acceptable if it was 0.29 or more.

[0058]

Table 1

[0059]

Table 2

[0060] As shown in Table 1 and Table 2, in Invention Examples No. 1 to No. 8, since the chemical composition of the aluminum alloy plate is within the range defined by the present invention, the temperature and peak height of the first exothermic peak and the temperature and peak height of the second exothermic peak in the differential scanning calorimetry curve are within the range defined by the present invention, and both the elongation at break and the n value are good values. Specifically, the elongation at break is a high value of 26% or more, and the n value is a high value of 0.29 or more, resulting in excellent formability.

[0061] In Comparative Examples No. 1 and No. 5, since the Mg content of the aluminum alloy plate exceeds the upper limit of the present invention range and [Si] / [Mg] is 2.5 or less, both the height of the first exothermic peak and the height of the second exothermic peak are less than the lower limit of the present invention range, and as a result, the n value is low.

[0062] In Comparative Examples No. 2 and No. 4, since the Mg content of the aluminum alloy sheet exceeds the upper limit of the present invention's range, the height of the first exothermic peak is less than the lower limit of the present invention's range. As a result, the n value became low. In Comparative Example No. 3, since [Si] / [Mg] is 2.5 or less, the height of the second exothermic peak is less than the lower limit of the present invention's range. As a result, the elongation at break decreased. In Comparative Example No. 6, since the Si content of the aluminum alloy sheet is less than the lower limit of the present invention's range and [Si] / [Mg] is 2.5 or less, the first peak did not appear, and the height of the second exothermic peak was also less than the lower limit of the present invention's range. As a result, the elongation at break decreased. In Comparative Example No. 6, since the first peak did not appear, the "first exothermic peak temperature" and "first exothermic peak height" in Comparative Example No. 6 of Table 2 are indicated by "-".

[0063] The differential scanning calorimetry curves of Invention Example No. 1, Invention Example No. 2, and Comparative Example No. 1 are shown in FIG. 1. In FIG. 1, the thick solid line represents Invention Example No. 1, the thick dotted line (broken line) represents Invention Example No. 2, and the thin dotted line represents Comparative Example No. 1. As shown in FIG. 1, in Invention Examples No. 1 and No. 2, the first exothermic peak appears within the temperature range of 210°C or more and less than 260°C, and its height is 20 μW / mg or more. Also, the second exothermic peak appears within the temperature range of 260°C or more and 370°C or less, and its height is 18 μW / mg or more.

[0064] On the other hand, in Comparative Example No. 1, although the first exothermic peak and the second exothermic peak appeared within a predetermined temperature range, their heights were low, and good formability could not be obtained.

Claims

1. Mg: 0.3% by mass or more and 0.45% by mass or less, Si: 0.6% by mass or more and 1.75% by mass or less, with the balance being Al and unavoidable impurities; A method for producing an Al-Mg-Si-based aluminum alloy sheet, comprising the steps of casting an aluminum alloy having a ratio of [Si] / [Mg] of more than 2.5, where [Mg] is the content of Mg in mass%, and [Si] is the content of Si in mass%, and casting the aluminum alloy soaked in heat treatment, hot rolling, cold rolling, and solution treatment, The method for producing an Al-Mg-Si-based aluminum alloy sheet having excellent formability comprises quenching treatment after the solution treatment, cooling to room temperature, and then performing a heat treatment in which the sheet is maintained at a temperature range of 100°C to 300°C for 5 seconds or more and 300 seconds or less within 1 hour, and then performing a heat treatment in which the sheet is maintained at a temperature range of 30°C to 100°C for 5 hours or more and 500 hours or less.

2. The aluminum alloy further contains at least one selected from Cu, Fe, Mn and Ti in the ranges of Cu: more than 0 mass% and 0.8 mass% or less, Fe: 0.05 mass% or more and 0.5 mass% or less, Mn: 0.05 mass% or more and 0.3 mass% or less, and Ti: more than 0 mass% and 0.1 mass% or less. The method for producing an Al-Mg-Si-based aluminum alloy plate having excellent formability according to claim 1.

Images