Patent Application
20210189524
The present disclosure relates to an aluminum alloy sheet suitable for press forming components, such as components of transport equipment and housings of IT equipment, that require formability, strength, quality of appearance, and the like, and relates to a method of fabricating the aluminum alloy sheet.
In response to the increasing demand for fuel efficiency, more and more aluminum alloys have been recently applied to components of transport equipment in particular. The aluminum alloys are most advantageous in their lightness. Aluminum alloy sheets can substitute for steel sheets, which are widely used as metal materials, to reduce the weights of products. A typical aluminum alloy sheet having a sufficient strength required as a component of transport equipment or the like is inferior in formability to a steel sheet and thus has been required to have higher formability. One of the aluminum alloys that has a relatively good balance between formability and strength is an Al—Fe alloy. Some existing techniques have provided an Al—Fe aluminum alloy sheet having high formability by improving the tensile properties of the sheet in three directions, that is, the 0° direction, 45° direction, and 90° direction relative to the rolling direction (refer to Patent Literature 1). Another existing techniques have achieved high formability by controlling the maximum size and the dispersion density of Al—Fe compound particles (refer to Patent Literature 2).
Unfortunately, the aluminum alloy sheets disclosed in Patent Literatures 1 and 2, which are excellent in formability, do not necessarily have sufficient strengths required as components of transport equipment or the like. Furthermore, typical automobile body panels, which have been increasingly focused as a usage of aluminum alloy sheets, are press-forming and then subject to bake hardening at 170° C. for 20 minutes, which causes softening of the sheets. Unfortunately, existing Al—Fe aluminum alloy sheets are not provided with an appropriate solution against such softening during the bake hardening although such a solution significantly affects the strengths of automobile body panels. In addition, the Al—Fe aluminum alloy sheets are not provided with an appropriate solution in terms of material against streak-like patterns called ridging marks, which readily occur after press forming and impair the appearance.
Patent Literature 1: Japanese Patent No. 3791337
Patent Literature 2: Japanese Patent No. 5276368
An objective of the disclosure, which has been accomplished in view of the above problems, is to provide an aluminum alloy sheet, which has an excellent balance between formability and strength because of the controlled composition and construction of the alloy, and has a good quality of appearance because of reduced ridging marks occurring after press forming, and to provide a method of fabricating the aluminum alloy sheet.
That is, according to a first aspect of the disclosure, an aluminum alloy sheet excellent in formability, strength, and quality of appearance is composed of an aluminum alloy containing: 1.00 to 2.20 mass % of Fe and 0.10 to 1.00 mass % of Mn, with a balance of Al and unavoidable impurities. The sheet is fabricated by rolling. In each of the 0° direction, 45° direction, and 90° direction relative to the rolling direction, the sheet has a total elongation of 34% or more, and has a 0.01% proof stress of 60 MPa or more after application of 2% uniaxial strain and a subsequent heat treatment at 170° C. for 20 minutes. In the following description, the 0.01% proof stress after application of 2% uniaxial strain and a subsequent heat treatment at 170° C. for 20 minutes may be referred to simply as “0.01% proof stress after bake hardening”.
According to a second aspect of the disclosure, the aluminum alloy may further contain one or two elements selected from 0.01 to 0.20 mass % of Cu and 0.005 to 0.10 mass % of Ti in the aluminum alloy sheet according to the first aspect.
According to a third aspect of the disclosure, the aluminum alloy sheet according to the first or second aspect may be applied to an automobile body panel.
The disclosure provides an aluminum alloy sheet suitable for press forming components, such as components of transport equipment (for example, automobile body panels) and housings of IT equipment, that require formability, strength, quality of appearance, and the like, and provides a method of fabricating the aluminum alloy sheet in an industrial scale.
A. Aluminum Alloy Sheet Excellent in Formability, Strength, and Quality of Appearance
An aluminum alloy sheet excellent in formability, strength, and quality of appearance according to the disclosure (hereinafter abbreviated simply as “aluminum alloy sheet according to the disclosure” or “aluminum alloy sheet” in some cases) will now be described in detail.
1. Alloy Composition
The following description will focus on the constituent elements of an aluminum alloy contained in the aluminum alloy sheet according to the disclosure and on the contents of these elements. According to the disclosure, the aluminum alloy contained in the aluminum alloy sheet is an Al—Fe—Mn aluminum alloy composed of Fe and Mn as essential elements so as to achieve sufficient formability, strength, and quality of appearance. This aluminum alloy may contain one or two optional elements selected from Cu and Ti.
Fe:
Fe is an essential element that is dissolved in the alloy or forms Al—Fe compound particles to increase the strengths in the three directions, that is, the 0° direction, 45° direction, and 90° direction relative to the rolling direction. These Al—Fe compound particles serve as nuclei of recrystallized grains and can thus contribute to a reduction in size of recrystallized grains. An Fe content of less than 1.00 mass % (hereinafter abbreviated simply as “%”) leads to an insufficient 0.01% proof stress after bake hardening. As described below, the 0.01% proof stress is regarded as the yield stress of a material according to the disclosure. In contrast, an Fe content exceeding 2.20% causes a reduction in total elongation, resulting in insufficient formability. The Fe content exceeding 2.20% also causes generation of coarse compound particles, which impair castability and material properties. For these reasons, the Fe content is defined within the range of 0.10% to 2.20%. The Fe content should preferably be within the range of 1.20% to 2.00%.
Mn:
Mn increases the strengths in the three directions and contributes to a reduction in size of recrystallized grains, as well as Fe. Mn also reduces ridging marks by suppressing generation of coarse recrystallized grains during hot rolling, which are a cause of ridging marks occurring after press forming. An Mn content of less than 0.10% leads to an insufficient 0.01% proof stress after bake hardening and an insufficient effect of suppressing generation of coarse recrystallized grains. In contrast, an Mn content exceeding 1.00% causes a reduction in total elongation, resulting in insufficient formability. The Mn content exceeding 1.00% also causes generation of coarse compound particles, which impair castability and material properties. For these reasons, the Mn content is defined within the range of 0.10% to 1.00%. The Mn content should preferably be within the range of 0.20% to 0.70%.
Cu:
Cu has an effect of increasing the strengths. This effect cannot be sufficiently achieved at a Cu content of less than 0.01%. In contrast, a Cu content exceeding 0.20% causes a reduction in total elongation, resulting in insufficient formability. For these reasons, the Cu content is defined within the range of 0.01% to 0.20%. The Cu content should preferably be within the range of 0.02% to 0.15%.
Ti:
Ti has an effect of reducing the size of cast structure and thus suppressing casting cracks. This effect cannot be sufficiently achieved at a Ti content of less than 0.005%. In contrast, a Ti content exceeding 0.100% causes a reduction in total elongation, resulting in insufficient formability. For these reasons, the Ti content is defined within the range of 0.005% to 0.100%. The Ti content should preferably be within the range of 0.005% to 0.050%. Some typical alloys also contain B or C as well as Ti. According to the disclosure, the alloy may contain 0.05% or less of B or C as well as Ti.
Other Primary Elements:
In addition to the above-described elements, many typical aluminum alloys contain Si, Mg, Cr, and Zn. These elements mainly increase the strengths, but cause a reduction in total elongation, resulting in insufficient formability. These elements are thus not positively added to the alloy according to the disclosure. The alloy may be contaminated by a small amount of these elements during fabrication, which do not impair the properties of the aluminum alloy sheet according to the disclosure if the content of each of Si, Mg, and Zn is 0.20% or less and if the Cr content is 0.10% or less. The content of each of Si, Mg, and Zn should preferably be 0.10% or less, and the Cr content should preferably be 0.05% or less.
Other Elements as Unavoidable Impurities:
The rest components of the aluminum alloy according to the disclosure are Al and unavoidable impurities. Examples of the unavoidable impurities include Na and Ca. These unavoidable impurities do not impair the properties of the aluminum alloy sheet according to the disclosure if the content of each impurity is less than 0.05% and if the total content is less than 0.15%.
2. Mechanical Characteristics
The following description will focus on the mechanical characteristics of the aluminum alloy sheet according to the disclosure, that is, a total elongation and a 0.01% proof stress after application of 2% uniaxial strain and a subsequent heat treatment at 170° C. for 20 minutes (0.01% proof stress after bake hardening) in each of the 0° direction, 45° direction, and 90° direction relative to the rolling direction.
2-1. Total Elongation in Each of the 0° Direction, 45° Direction, and 90° Direction Relative to the Rolling Direction
In the aluminum alloy sheet according to the disclosure, a total elongation in each of the 0° direction, 45° direction, and 90° direction relative to the rolling direction, which is used as a tensile property, is defined to be 34% or more. In general, a total elongation serves as an index of formability, and a higher elongation indicates superior formability. In an aluminum alloy sheet to be applied to a component, such as an automobile body panel, that requires especially high formability among the components of transport equipment, if a total elongation is less than 34% in at least one of the 0° direction, 45° direction, and 90° direction relative to the rolling direction, the sheet cannot ensure sufficient formability due to the insufficient total elongations. The total elongation should therefore be 34% or more in each of the 0° direction, 45° direction, and 90° direction relative to the rolling direction. In the case where the formability has special importance, the total elongation should preferably be 36% or more.
The upper limit of the total elongations is not particularly defined and consequentially determined depending on the aluminum alloy composition and fabrication method. According to the disclosure, the upper limit is defined to be 50%. The total elongations are measured though a tensile test using JIS 5 tensile test specimens (gauge length: 50 mm) by the matching method in accordance with JIS Z 2241.
2-2. 0.01% Proof Stress After Application of 2% Uniaxial Strain and a Subsequent Heat Treatment at 170° C. for 20 Minutes (0.01% Proof Stress After Bake Hardening) in Each of the 0° Direction, 45° Direction, and 90° Direction Relative to the Rolling Direction
For materials not showing apparent yield phenomena, such as aluminum alloys, a stress that provides a permanent strain after unloading of 0.2% is called a 0.2% proof stress and generally used as a substitute for a yield stress. This 0.2% proof stress is also generally used to predict various characteristics. For example, as a common knowledge about a dent resistance (strength against plastic deformation, such as dents), which is one of the important requirements for an automobile body panel, the critical dent load D is represented by a general formula D=Y×T2, where D (kgf) is a critical dent load, T (mm) is a thickness, and Y (MPa) is a yield stress. In the case of aluminum alloys, a 0.2% proof stress serves as the yield stress Y in general.
The present inventors have carried out studies and found that not the 0.2% proof stress but a 0.01% proof stress is appropriate for the Al—Fe—Mn aluminum alloy according to the disclosure.
It should be noted that automobile body panels are subject to press forming and subsequent bake hardening. That is, the dent resistance is a property necessary for panels after press forming and subsequent bake hardening. According to the disclosure, this series of processes are simulated by application of 2% uniaxial strain and a subsequent heat treatment at 170° C. for 20 minutes.
In the aluminum alloy sheet according to the disclosure to be applied to a component of transport equipment, such as an automobile body panel, the 0.01% proof stress after application of 2% uniaxial strain and a subsequent heat treatment at 170° C. for 20 minutes (simulated press forming and bake hardening) is defined to be 60 MPa or more in each of the 0° direction, 45° direction, and 90° direction relative to the rolling direction. If the 0.01% proof stress after bake hardening is less than 60 MPa in at least one of these directions, the dent resistance should be ensured by, for example, excessively increasing the thickness of the sheet, thereby impairing the advantage of the aluminum alloy in weight reduction. The 0.01% proof stress after bake hardening is therefore required to be 60 MPa or more in each of the directions. In the case where the lightness has special importance, the 0.01% proof stress after bake hardening should preferably be 65 MPa or more in each direction.
The upper limit of the 0.01% proof stress after bake hardening is not particularly defined and consequentially determined depending on the aluminum alloy composition and fabrication method. According to the disclosure, the upper limit is defined to be 85 MPa. The 0.01% proof stresses are measured in accordance with JIS Z 2241 in the same manner as typical measurement of 0.2% proof stresses. As explained above, the press forming and bake hardening are simulated by application of 2% uniaxial strain and a subsequent heat treatment at 170° C. for 20 minutes according to the disclosure.
The above-mentioned dent resistance is one of the important properties required for automobile body panels and the like. A method of evaluating the dent resistance conducted by the present inventors involves: forming a sample panel having a shape illustrated in
3. Thickness of the Aluminum Alloy Sheet According to the Disclosure
The thickness of the aluminum alloy sheet according to the disclosure will now be described. The aluminum alloy sheet according to the disclosure is applied to press forming components, such as components of transport equipment (for example, automobile body panels) and housings of IT equipment, that require formability, strength, quality of appearance, and the like. The thickness necessary for these usages is 0.7 to 3.0 mm in view of the rigidity and the like. The thickness is therefore defined to be within the range of 0.7 to 3.0 mm according to the disclosure. A thickness of less than 0.7 mm leads to an insufficient dent resistance. In contrast, a thickness exceeding 3.0 mm results in no effect of weight reduction.
B. Method of Fabricating the Aluminum Alloy Sheet According to the Disclosure
A method of fabricating the aluminum alloy sheet according to the disclosure will now be explained in detail. The aluminum alloy sheet according to the disclosure is fabricated by casting an Al—Fe—Mn aluminum alloy having the above-described composition to produce an ingot, hot rolling the ingot to produce a plate, cold rolling the hot rolled plate, and providing a softening heat treatment to the cold rolled sheet. The ingot after casting may be homogenized. The softening heat treatment may be followed by skin-pass rolling at a draft of 4% to 8% to the rolled sheet. No intermediate annealing is conducted during the period from the hot rolling to the softening heat treatment.
4. Casting
First, an aluminum alloy having the above-described composition is molten by a general method, and then casted by any general casting method, such as a continuous casting method or a semi-continuous casting (DC casting) method, to produce an ingot.
5. Homogenization
The casting may be followed by homogenization. The homogenization in this case is intended to uniformize the added elements, separate Al—Fe compound particles and Al—Fe—Mn compound particles, or adjust the deposited or dissolved state of Fe and Mn. The homogenization causes uniformization of the added elements, separation of Al—Fe—Mn compound particles, and deposition of Fe, thereby increasing the total elongations and thus improving the formability. In the Al—Fe—Mn aluminum alloy, the increase in total elongation due to homogenization is contradictory to an increase in strength. Accordingly, homogenization should preferably be omitted in the case where the strengths have special importance.
The homogenization is performed by heating at a temperature of 380° C. to 620° C. for 1 to 24 hours. A homogenizing temperature exceeding 620° C. leads to a reduction in 0.01% proof stress due to excessive deposition of Fe. From the viewpoint of material properties, the minimum homogenizing temperature is at least a room temperature, so that the homogenization can be omitted. The homogenization, however, does not bring about sufficient effects at a temperature of less than 380° C., resulting in material properties substantially the same as those achieved by the process without homogenization. For these reasons, if homogenization is conducted, the homogenizing temperature is within the range of 380° C. to 620° C. The homogenizing temperature should preferably be within the range of 380° C. to 550° C.
In order to obtain stable effects of homogenization, the temperature must be retained for at least one hour. The upper limit of retention time is not particularly defined but should preferably be 24 hours from the viewpoint of production efficiency and economy. Accordingly, the retention time of homogenization should preferably be within the range of 1 to 24 hours. The retention time should more preferably be within the range of 2 to 10 hours.
6. Hot Rolling
In the hot rolling subsequent to the homogenization (or subsequent to the casting in the process without homogenization), the initial temperature is defined to be within the range of 250° C. to 430° C. and the final temperature is defined to be within the range of 150° C. to 330° C. An objective of this temperature management is to reduce coarse recrystallized grains generated during the hot rolling, which are a cause of streak-like visual deficiencies called ridging marks occurring after press forming. Another objective is to suppress deposition of Fe and Mn during the hot rolling and thus ensure the dissolved state of Fe and Mn effective for increasing the 0.01% proof stresses after bake hardening.
If the initial temperature of the hot rolling is less than 250° C. or if the final temperature is less than 150° C., cracking called edge cracking readily occurs at the widthwise edges of the plate during the hot rolling, resulting in low productivity due to a higher deformation resistance. In contrast, if the initial temperature exceeds 430° C. and the final temperature exceeds 330° C., coarse recrystallized grains are generated during the hot rolling or during the cooling after the hot rolling. These coarse recrystallized grains can cause ridging marks and facilitate deposition of Fe, leading to a reduction in 0.01% proof stress after bake hardening. For these reasons, the initial temperature of the hot rolling is required to be within the range of 250° C. to 430° C., and the final temperature is required to be within the range of 150° C. to 330° C. The initial temperature of the hot rolling should preferably be within the range of 280° C. to 350° C., and the final temperature should preferably be within the range of 170° C. to 300° C.
If homogenization is conducted, the ingot may be temporarily cooled to a room temperature in the step after the homogenization before the hot rolling. Alternatively, the ingot after the homogenization may be cooled to a certain initial temperature of the hot rolling and then be hot rolled.
7. Cold Rolling
The hot rolling is followed by cold rolling at a draft of 50% or more without intermediate annealing. The intermediate annealing is not conducted because this process increases the diameters of crystal grains after a softening heat treatment, leading to a reduction in 0.01% proof stress after bake hardening. A draft of less than 50% in the cold rolling also increases the diameters of crystal grains, resulting in a reduction in 0.01% proof stress after bake hardening. The draft in the cold rolling is therefore required to be 50% or more. The draft in the cold rolling should preferably be 75% or more. The upper limit of the draft in the cold rolling is not particularly defined from the viewpoint of material properties. According to the disclosure, the upper limit is defined to be 97% because an excessively large draft increases the number of paths in the cold rolling and thus reduces the productivity.
8. Softening Heat Treatment
The sheet after the cold rolling is then subject to a softening heat treatment.
In the case of a softening heat treatment in a continuous system, the treatment is performed at a temperature of 380° C. to 620° C. for five minutes or less. The range of five minutes or less includes zero minutes, which indicate that heating is terminated immediately after reaching a desired temperature. The softening heat treatment in a continuous system at a temperature of less than 380° C. often causes insufficient recrystallization, thereby reducing the total elongations and thus impairing the formability. This heating temperature also causes insufficient amounts of dissolved Fe and Mn, resulting in a reduction in 0.01% proof stress after bake hardening. In contrast, at a heating temperature exceeding 620° C., the sheet has a reduced high-temperature strength and may be broken in the heating chamber of the continuous annealing furnace. The temperature of the softening heat treatment should preferably be within the range of 500° C. to 620° C. The heating period is defined to be five minutes or less from the viewpoint of productivity, because the heating for a period exceeding five minutes does not further enhance the advantageous effects. The heating period should preferably be within the range of 0 to 0.5 minutes. According to the disclosure, the lower limit of the heating period is defined to be zero minutes (the heating is terminated and followed by cooling immediately after reaching a desired temperature).
In the case of a softening heat treatment in a batch system, the treatment is performed at a temperature of 380° C. to 550° C. for 1 to 24 hours. The softening heat treatment in a batch system at a temperature of less than 380° C. often causes insufficient recrystallization, thereby reducing the total elongations and thus impairing the formability. In contrast, a heating temperature exceeding 550° C. causes an excessive increase in the diameters of crystal grains, leading to a reduction in 0.01% proof stress after bake hardening. The temperature of the softening heat treatment should preferably be within the range of 400° C. to 550° C. A heating period of less than one hour may cause insufficient recrystallization, thereby reducing the total elongations. The upper limit of the heating period is defined to be 24 hours from the viewpoint of productivity (a heating period exceeding 24 hours does not further enhance the advantageous effects) and from the viewpoint of suppressing generation of coarse crystal grains (a heating period exceeding 24 hours causes an excessive increase in the diameters of crystal grains). The heating period should preferably be within the range of one to eight hours.
In the fabrication in an industrial scale, the softening heat treatment in a batch system, in which a sheet is processed in a coiled shape, has a lower rate of temperature increase than that of the softening heat treatment in a continuous system, in which a plate is uncoiled and processed in a plate shape. The softening heat treatment in a batch system therefore more readily causes an increase in the diameters of recrystallized grains, leading to a reduction in 0.01% proof stress after bake hardening and facilitating occurrence of ridging marks. Accordingly, the softening heat treatment in a continuous system is more preferred in the case where the 0.01% proof stresses after bake hardening and the effect of reducing ridging marks have importance.
9. Skin-Pass Rolling
The sheet after the softening heat treatment may be subject to skin-pass rolling at a draft of 4% to 8%. This skin-pass rolling is mainly intended to increase the 0.01% proof stresses after bake hardening. If skin-pass rolling is performed, the rolling at a draft of less than 4% tends to be unstable due to an excessively low load. In contrast, a draft exceeding 8% results in an excessively low total elongation. The skin-pass rolling is therefore defined to be within the range of 4% to 8%.
The skin-pass rolling effectively increases the strengths but significantly reduces the total elongations. In the case where the balance between strength and total elongation has importance, the skin-pass rolling should preferably be not positively performed. Alternatively, any other procedure may be used for applying a slight strain to the entire thickness of the sheet while maintaining the industrial productivity, instead of the skin-pass rolling.
10. Leveling
The softening heat treatment or the skin-pass rolling may be followed by leveling using a device, such as a roller leveler or tension leveler, which process is intended to correct the flatness or the like of the rolled sheet. This leveling applies a negligibly small strain and does not inhibit the effects of the disclosure.
In the following exemplary evaluations, examples according to the disclosure and comparative examples will be described while being compared with each other. These exemplary evaluations are mere embodiments of the disclosure and should not be construed as limiting the disclosure.
Aluminum alloys having compositions shown in Table 1 were cast by DC casting into ingots. Some of these ingots were homogenized under the conditions shown in Tables 2 and 3. Each of the symbols “−” in Table 1 indicates a value less than the measurable threshold. In Tables 2 and 3, the words “none” in the column of homogenization indicate that no homogenization was conducted. The phrases “to room temperature” in the sub-column of cooling after retention in the column of homogenization indicate that a homogenized ingot was temporarily cooled to a room temperature, reheated to an initial temperature of the hot rolling, and then hot rolled under the conditions shown in Tables 2 and 3. In contrast, the phrases “to hot rolling initial temperature” indicate that a homogenized ingot was cooled from the homogenizing temperature to an initial temperature of the hot rolling, without being temporarily cooled to a room temperature, and then hot rolled under the conditions shown in Tables 2 and 3.
The hot rolled sheets were cold rolled at the draft shown in Tables 2 and 3, without intermediate annealing, subject to a softening heat treatment under the conditions shown in Tables 2 and 3, and then subject to or not subject to skin-pass rolling under the conditions shown in Tables 2 and 3, to yield a final rolled sheet. The thicknesses of the final rolled sheets are also shown in Tables 2 and 3.
The softening heat treatments were conducted in three manners: a treatment using a salt bath furnace for simulating a softening heat treatment in a continuous system; a treatment using an air atmosphere furnace for simulating a softening heat treatment in a batch system; and a treatment using an actual continuous annealing furnace.
Using the rolled sheets fabricated as explained above as samples, total elongations, 0.01% proof stresses after bake hardening, and 0.2% proof stresses after bake hardening (as references) were measured by the above-explained procedures in all of the 0° direction, 45° direction, and 90° direction relative to the rolling direction. The 0.01% proof stress and 0.2% proof stress after bake hardening are respectively represented by “0.01% proof stress after BH” and “0.2% proof stress after BH” in Tables 2 and 3. As explained above, the press forming and bake hardening were simulated by application of 2% uniaxial strain and a subsequent heat treatment at 170° C. for 20 minutes according to the disclosure. It should be noted that total elongations were measured on the final sheets before application of 2% uniaxial strain and a subsequent heat treatment at 170° C. for 20 minutes.
Tables 2 and 3 show the minimum value of the measured values in the three directions for each of the total elongations and the 0.01% proof stresses and 0.2% proof stresses after bake hardening.
As shown in Tables 2 and 3, Examples A1 to A45 according to the disclosure achieved good formability and mechanical characteristics because of the total elongations and the 0.01% proof stresses after bake hardening that fall within the ranges defined in the disclosure.
In contrast, Comparative Examples B1 to B10 and B12 were inferior in at least either of the total elongations and the 0.01% proof stresses after bake hardening that are defined in the disclosure. Furthermore, Comparative Examples B11 and B13 failed to fabricate aluminum alloy sheets.
Specifically, Comparative Example B1 had insufficient 0.01% proof stresses after bake hardening, because of the high Mn content, low Fe content, high Si content, and high initial temperature of the hot rolling that are out of the ranges of composition and fabrication conditions defined in the disclosure.
Each of Comparative Examples B2 to B4 had insufficient 0.01% proof stresses after bake hardening because of the Fe content lower than the range of composition defined in the disclosure.
Each of Comparative Examples B6 and B8 had insufficient 0.01% proof stresses after bake hardening because of the Mn content lower than the range of composition defined in the disclosure.
Comparative Example B13 failed to produce an ingot due to poor flow ability during casting, because of the Fe, Mn, and Ti contents higher than the ranges of composition defined in the disclosure.
Comparative Example B12 had insufficient total elongations because of the Cu content higher than the range of composition defined in the disclosure.
Comparative Example B5 had insufficient total elongations because of the draft in the skin-pass rolling higher than the range of fabrication conditions defined in the disclosure.
Comparative Example B7 failed to conduct stable skin-pass rolling because of the draft in the skin-pass rolling lower than the range of fabrication conditions defined in the disclosure.
Comparative Example B9 had insufficient total elongations because of the temperature of the softening heat treatment lower than the range of fabrication conditions defined in the disclosure.
Comparative Example B10 had insufficient 0.01% proof stresses after bake hardening because of the initial and final temperatures of the hot rolling higher than the ranges of fabrication conditions defined in the disclosure.
Comparative Example B11 failed to proceed to the step subsequent to the hot rolling due to the occurrence of significant edge cracking during the hot rolling, because of the initial and final temperatures of the hot rolling lower than the ranges of fabrication conditions defined in the disclosure.
In Examples A3, A4, A7, A10 to A12, A17 to A20, A21 to A25, A31 to A37, A38 to A41, A43, and A44 among the examples according to the disclosure, the amounts of added Fe and Mn (primary elements) and the conditions of hot rolling, cold rolling, softening heat treatment, and skin-pass rolling (the skin-pass rolling should preferably be omitted if the balance between total elongation and strength has importance, as explained above) fall within the more preferable ranges. These examples tend to have achieved a large sum of the total elongations and the 0.01% proof stresses after bake hardening, that is, achieved a good balance between total elongation and strength.
Some of the final rolled sheets shown in Tables 2 and 3 were evaluated in terms of dent resistance. The evaluation was conducted by the above-explained procedure and provided the results shown in Table 4. Table 4 includes an item of critical dent load in addition to the items of Tables 2 and 3.
Some of the final rolled sheets shown in Tables 2 and 3 were evaluated in terms of ridging resistance (scarceness of occurrence of ridging marks). Specifically, a JIS 5 test specimen in the 90° direction relative to the rolling direction was provided with uniaxial strains of 2% to 10% (2% intervals), was manually polished in the rolling direction and the 90° direction, and then visually observed on the surface using the Polinet A-800 manufactured by KOYO-SHA Co., Ltd., to determine the occurrence of ridging marks. The results of evaluation are shown in Table 5. Table 5 includes an item of ridging resistance in addition to the items of Tables 2 and 3. It should be noted that the mechanical characteristics in the table indicate the results in the three directions relative to the rolling direction, as described above.
Comparative Example B8, which is a conventional Al—Fe aluminum alloy having an insufficient ridging resistance, was used as a reference sample for evaluation of ridging marks. A sample having an effect of increasing the minimum strain that provides visible ridging marks by 2% or more and less than 4% compared to the reference sample was evaluated as “B”, a sample having the effect by 4% or more was evaluated as “A”, and a sample having the effect by less than 2% was evaluated as “C”.
Table 5 shows that a reduction of ridging marks depends on hot rolling conditions and addition of Mn.
Examples A5, A15, A19, A25, A32 to A35, A40 to A42, and A45 achieved a good ridging resistance because of the Mn content within the range defined in the disclosure and the hot rolling conditions within the preferable ranges defined in the disclosure.
Example A30 had a lower effect of improving the ridging resistance than that provided by a softening heat treatment in a continuous system because the softening heat treatment in Example A30 was conducted using an air atmosphere furnace for simulating a softening heat treatment in a batch system, regardless of the Mn content within the range defined in the disclosure and the hot rolling conditions within the ranges defined in the disclosure.
Example A37 achieved an improved ridging resistance because of the Mn content within the range defined in the disclosure and the hot rolling conditions within the ranges defined in the disclosure. The effect of improving the ridging resistance, however, is lower than that in the case of a preferable initial temperature of the hot rolling.
Comparative Example B1, which uses a typical 3003 alloy, had no effect of improving the ridging resistance because of the initial temperature of the hot rolling higher than the range defined in the disclosure.
Each of Comparative Examples B2 to B4 achieved a good ridging resistance because of the Mn content within the range defined in the disclosure and the hot rolling conditions within the preferable ranges defined in the disclosure, regardless of the low Fe content and the low 0.01% proof stresses after bake hardening.
Comparative Example B10 had no effect of improving the ridging resistance because of the initial and final temperatures of the hot rolling higher than the ranges defined in the disclosure, regardless of the Mn content within the range defined in the disclosure.
Provided are an aluminum alloy sheet, which has an excellent balance between formability and strength because of the controlled composition and construction of the alloy, and has a good quality of appearance because of reduced ridging marks occurring after press forming, and a method of fabricating the aluminum alloy sheet.
| Number | Date | Country | Kind |
|---|---|---|---|
| 2018-102809 | May 2018 | JP | national |
| Filing Document | Filing Date | Country | Kind |
|---|---|---|---|
| PCT/JP2019/021092 | 5/28/2019 | WO | 00 |
