JPH04304339A - Aluminum alloy sheet for press forming excellent in balance between strength and ductility and baking hardenability and its production - Google Patents

Abstract

PURPOSE:To produce an aluminum alloy sheet excellent in balance between strength and ductility, press formability, and low temp. baking hardenability. CONSTITUTION:The sheet is an aluminum alloy sheet having a composition which consists of, by weight, 1.5-8.0% Mg, 0.25-3.0% Cu, 0.02-0.15% Si, 0.03-0.25% Fe, 0.005-0.15% Ti, 0.0002-0.05% B, 0.10% Zn, and the balance Al with <=0.1% inevitable impurities and where Mg and Cu lie within the range enclosed with lines connecting five points (8.0, 0.25), (4.0, 0.25), (1.5, 0.8), (1.5, 3.0), and (8.0, 1.0) shown in a figure and also having a structure where the average aspect ratio of crystalline grains, L/H, is regulated to <=1.3. After homogenizing treatment is applied to an ingot of aluminum alloy with the above composition at 450-580 deg.C, this ingot is hot-rolled and cold-rolled to the desired sheet thickness. The resulting sheet is heated up to 440-580 deg.C at >=3 deg.C/sec heating rate, held for <=120sec, and cooled down to 100 deg.C at >=2 deg.C/sec cooling rate.

Description

[Detailed description of the invention]

0001

[Field of Industrial Application] This invention relates to an aluminum alloy plate for press forming that has an excellent balance of strength and ductility and bake hardenability, and a method for manufacturing the same, and in particular, an aluminum alloy plate that has an excellent balance of strength and ductility even after being heat treated after rolling. Moreover, the present invention relates to an aluminum alloy plate for press forming suitable for automobile bodies, etc., which has a high hardening ability even in baking coating at a low temperature of about 170°C, and a method for manufacturing the same.

0002

[Prior Art] Surface-treated cold-rolled steel sheets have traditionally been widely used as plate materials for forming automobile body sheets, etc., but in recent years, there has been an increasing demand for weight reduction in automobiles to improve fuel efficiency. In order to meet these requirements, aluminum alloy plates are beginning to be used in automobile body seats and the like.

[0003] As aluminum alloys for automobile body sheets, non-heat-treated Al-M typified by 5182 is used.
g-based alloys, heat-treated Al-Cu-based, Al-Mg-S
It is divided into i-series. As a non-heat treatment type Al-Mg alloy, one has been developed that is intended to be used after heat treatment with the addition of a small amount of Cu or Zn (Japanese Unexamined Patent Publication No. 57-12).
0648, Japanese Unexamined Patent Publication No. 53-103914, etc.).

However, although these have slightly better formability than heat-treated Al alloys, they are inferior to conventional surface-treated cold-rolled steel sheets, and furthermore, no increase in strength can be obtained through the paint baking process. In addition, heat-treated Al-Cu
system 2036, Al-Mg-Si system 6009, 601
0 and 6011 have poor formability, and furthermore, compared to the 200°C baking methods used in Europe and the United States, low-temperature short-time baking methods, which are mainstream in Japan and are held at temperatures below 170°C for less than 30 minutes, have been developed from the perspective of energy conservation. There was also the problem that the strength did not increase with baking, but on the contrary decreased in the 2000 series.

[0005] As described above, at present, conventional aluminum alloys do not fully satisfy the properties required for automobile body sheets, particularly formability and bake hardenability.

[0006]

[Problems to be Solved by the Invention] This invention has been made in view of the above circumstances, and has a good balance of strength and ductility and sufficient press formability for use in automobile bodies, etc.
An object of the present invention is to provide an aluminum alloy plate that has good bake hardenability even at low temperatures and for a short time, and a method for manufacturing the same.

[0007]

[Means and effects for solving the problem] In order to achieve the above object, the inventors of the present application have conducted various studies, and as a result, the present inventors have succeeded in heat treatment by appropriately adjusting the chemical composition and optimizing the manufacturing conditions. The elongation after is 30% or more and 170℃
Even with low-temperature and short-time baking treatment such as 30 minutes, the yield strength after baking is 3 to 5 kgf/mm higher than before baking.
2 or more, and completed the present invention. That is, the present invention aims to improve the balance of strength and ductility, improve formability, and improve dent resistance after paint baking. This improves both characteristics.

In particular, regarding the chemical composition, carbon is added to the Al-Mg alloy from the viewpoint of increasing the strength after baking the paint.
In addition to intentionally adding an appropriate amount of u and controlling the combination of Mg and Cu amounts, in order to improve formability, the crystal grains were made equiaxed and extremely small amounts of Si, Fe, Ti, and B were added. It is something.

That is, the aluminum alloy plate for press forming according to the present invention, which has excellent strength/ductility balance and bake hardenability, contains 1.5 to 8.0% Mg and 0% Cu by weight.
．． 25-3.0, Si 0.02-0.15%, Fe 0.03-0.25%, Ti 0.005-0.15%
, contains B in the range of 0.0002 to 0.05%, Zn in the range of 0.10% or less, and Mg and Cu are at the coordinate point of (Mg%, Cu%) in the Mg-Cu coordinates. When expressed as (8.0, 0.25), (4.0
,0.25), (1.5,0.8), (1.5,3.0
), (8.0, 1.0),
The balance consists of Al and 0.1% or less of unavoidable impurities, and when the axial length of the crystal grain in the rolling direction is L, and the axial length in the plate thickness direction perpendicular to L is H, the average aspect ratio It is characterized in that L/H is 1.3 or less. In addition, with respect to this composition, 0.01 to 0.15% Mn,
One or two of 0.01 to 0.15% Cr, 0.01 to 0.12% Zr, and 0.01 to 0.18% V
It may further contain more than one species. Since these are recrystallization-inhibiting elements, they may be added for the purpose of suppressing abnormal grain growth, but their amounts are limited to the above-mentioned range, which is lower than conventional ones, from the viewpoint of improving formability.

[0010] Furthermore, the method for producing an aluminum alloy plate for press forming with excellent strength/ductility balance and bake hardenability according to the present invention is characterized in that an aluminum alloy ingot having the above composition is heated at a temperature within the range of 450 to 580°C. After performing one-stage or multi-stage homogenization treatment, the ingot is hot-rolled and cold-rolled to a desired thickness, and then 440-58
Heat to a temperature within the range of 0℃ at a heating rate of 3℃/second or more, hold at that temperature for 0 to 120 seconds, then 100℃
It is characterized by cooling at a cooling rate of 2° C./sec or more. As a result, the aluminum alloy plate having crystal grains having an average aspect ratio L/H of 1.3 or less is obtained.

In this case, between hot rolling and cold rolling,
or between cold rolling, or both, 32
Intermediate annealing treatment at a temperature within the range of 0 to 580°C
It is preferable to carry out the process once or twice or more.

The present invention will be explained in detail below. Note that in the following description, % indicates weight %.

First, the reasons for limiting the composition of the aluminum alloy according to the present invention will be explained.

Mg: Mg is an essential basic component in the alloy according to the present invention, and when alloyed in an appropriate amount, it greatly contributes to improving the strength and ductility of the alloy. However, M
If g is less than 1.5%, sufficient strength and bake hardenability cannot be obtained, whereas if it exceeds 8.0%, cracking during hot rolling becomes noticeable and hot workability deteriorates. Therefore, the Mg content is defined in the range of 1.5 to 8.0%.

Cu: Cu is a component that mainly combines with Al--Mg to form Al2CuMg-based precipitates and contributes to hardening by baking. However, if the Cu content is less than 0.25%, the effect will not be sufficiently obtained, and if it exceeds 3.0%, the moldability and corrosion resistance will deteriorate. Therefore, the content of Cu is defined in the range of 0.25 to 3.0%.

Furthermore, the amount of Mg and Cu is further limited to the range surrounded by the line connecting the coordinate points in FIG. This is because it will be insufficient. That is, Figure 1
At the coordinates of B(4.0, 0.25) and C(1.5
, 0.8), sufficient strength cannot be obtained if the amount of Mg or Cu is smaller than the line connecting them. Also, E(8.0
, 1.0) and D(1.5, 3.0).
When the amount of g or Cu is large, ductility, that is, formability decreases. Furthermore, C(1.5, 0.8) and D(1.5,
If the amount of Mg or Cu is less than the range specified by 3.0), the ductility (namely formability) and strength after baking will not be sufficient.

Si: Si is an element normally contained as an unavoidable impurity, but if added in a small amount, it is an important element that contributes to improving formability. However, if the Si content is less than 0.02%, the effect is not sufficient, and on the contrary, if it exceeds 0.15%, coarse Mg2Si-based precipitates that are precipitated during hot rolling etc. may occur even during solution treatment. does not form a solid solution, reducing moldability. Therefore, the Si content is 0.0
It is defined in the range of 2 to 0.15%. Preferably 0.02
~0.1%.

Fe: Fe is normally contained in aluminum alloys as an unavoidable impurity, and if the content exceeds 0.25%, coarse crystallized substances are likely to be formed due to coexistence with Al, which impairs press formability. Adversely affect. However, addition of a small amount contributes to improving moldability, and if their content is too small, such as less than 0.02%, moldability deteriorates. Therefore, the content of Fe is defined in the range of 0.02 to 0.25%.

Ti, B: Ti and B exist as TiB2, etc., and have the effect of refining the crystal grains of the ingot and improving hot workability, etc., so it is extremely important to add them in combination. It is. However, if these are added in excess, coarse crystallized substances are generated and the formability is deteriorated, so the content of Ti and B should be adjusted to 0.005 to 0.1, respectively.
5%, and within the range of 0.0002 to 0.05%.

Zn: Zn is an element that contributes to improving strength, but if its content exceeds 0.1%, it reduces ductility and hardenability after baking, so the content should be reduced to 0.1%.
% or less.

In the present invention, in addition to the above-mentioned elements, one or more of Mn, Cr, Zr and V may be added in appropriate amounts, if necessary.

Mn, Cr, Zr, V: Since these elements are recrystallization inhibiting elements, they may be added in appropriate amounts for the purpose of inhibiting abnormal grain growth. However, these alloy components have a negative effect on equiaxed recrystallized grains and reduce formability, so their content needs to be defined in a smaller range than in conventional aluminum alloys. Therefore, Mn
, Cr, Zr, and V content from 0.01 to 0.15%, respectively.
, 0.01-0.15%, 0.01-0.12%, 0.
01 to 0.18%.

[0023] In addition to the above elements, unavoidable impurities are contained as in ordinary aluminum alloys, but the amount thereof is allowed as long as the effects of the present invention are not impaired. For example, Be, Na, K, etc. may contain 0.001% or less each and 0.1% or less in total.
There are no problems with the characteristics.

Next, the organization will be explained.

The formability of aluminum alloys largely depends on the shape of the crystal grains. Equiaxed average aspect ratio L/H (
If L: grain axis length in the rolling direction, H: axis length in the plate thickness direction perpendicular to L) exceeds 1.3, a distorted pattern will appear during press forming and formability will decrease, so the equiaxed average aspect The ratio needs to be 1.3 or less.

Next, the manufacturing conditions for the alloy of the present invention will be explained.

[0027] An aluminum alloy whose components and composition are specified in the above range is melted and cast by a conventional method, and the ingot is subjected to one or more stages of homogenization heat treatment at a temperature within the range of 450 to 580°C. . By performing such homogenization treatment, the diffusion solid solution of the eutectic compound crystallized during casting is promoted, and local micro-segregation is reduced. Furthermore, this treatment makes it possible to finely precipitate Mn, Cr, Zr, and V compounds, which play an important role in suppressing abnormal grain growth and achieving uniformity of crystal grains in the final product. However, if the temperature of this treatment is less than 450°C, the above-mentioned effects are insufficient, while if it exceeds 580°C, eutectic melting occurs. Therefore, the temperature of the homogenization treatment was set in the range of 450 to 580°C. Note that if the holding time within this temperature range is less than 1 hour, the above-mentioned effect will not be sufficiently obtained, and if the heating time is longer than 72 hours, the effect will be saturated. is preferably 1 to 72 hours.

Next, the ingot subjected to such homogenization treatment is subjected to hot rolling and cold rolling according to a conventional method to obtain a predetermined thickness. Further, for strain correction or surface roughness adjustment, stretching of 5% or less, leveling, or skin pass rolling may be carried out both before and after the heat treatment shown below, or either one of them.

After rolling, for such a rolled plate material,
Heating to a temperature within the range of 440 to 580 °C at a heating rate of 3 °C/second or more, and immediately after reaching that temperature, or 12
After holding for a period of 0 seconds or less, heat treatment is performed under conditions of rapid cooling to 100° C. at a cooling rate of 2° C./second or more. This treatment makes the structure uniform, adjusts the average aspect ratio of crystal grains to 1.3 or less, and further eliminates processing strain, resulting in improved press formability. In addition, this heat treatment aims to solutionize the Al-Cu-Mg-based intermetallic compound that greatly contributes to bake hardening.
This achieves improved bake hardenability. In this case,
If the heating temperature is less than 440°C, the above-mentioned effects cannot be sufficiently obtained. Further, if the heating rate is less than 3° C./sec, the heating temperature exceeds 580° C., or the holding time is longer than 120 seconds, some of the crystal grains will undergo abnormal grain growth. Furthermore, if the cooling rate to 100° C. is less than 2° C./sec, the above-mentioned compounds will coarsely precipitate during cooling, which is undesirable in terms of press formability and bake hardenability. Therefore, the manufacturing conditions are defined as described above.

[0030] In addition to such a step, one or more intermediate annealing may be performed between the hot rolling and the cold rolling described above, or between the cold rolling and the cold rolling, or both. It is desirable to apply By performing this intermediate annealing, it is possible to prevent edge cracking during heavy reduction during cold rolling, and Mg compounds that function as recrystallization nuclei are precipitated to homogenize the structure, resulting in improved formability. can be done. However, if the temperature at this time is less than 320°C, the effect is not sufficient, and if it exceeds 580°C, eutectic melting occurs. Therefore, intermediate annealing is performed in the range of 320 to 580°C. Note that this intermediate annealing is not an essential process, and may be omitted from the viewpoint of process saving.

The aluminum alloy plate thus obtained has an excellent balance of strength and ductility, has an elongation at break of 30% or more, and is also excellent in hardenability by low-temperature baking. Therefore, such an aluminum alloy plate is suitable for use in automobile body seats.

[0032]

[Embodiments] Examples of the present invention will be described below. (Example 1) An alloy having the components and compositions shown in Tables 1 and 2 was melted and continuously cast, and the resulting ingot was faced, heated to 530°C for 10 hours, and further heated to 450°C during cooling. A two-stage homogenization process was carried out for 4 hours at ℃, and then the slab was
Heated to 0℃, hot rolled to a plate thickness of 4mm,
Intermediate annealing was performed at ℃ for 1 hour. Thereafter, it was cooled to room temperature and cold-rolled at a rolling rate of 75% to obtain a plate material with a thickness of 1 mm. Note that the finishing temperature of hot rolling was 280°C. Further, intermediate annealing was performed by gradual heating and cooling at a rate of 50° C./hour for both heating and cooling. 530 pieces of this 1mm thick plate material
℃ at a rate of 10℃/sec, and after holding for 10 seconds,
Forced air cooling was performed at a cooling rate of 20°C/sec to 00°C.

[0033] The plate material thus produced was heated at room temperature for 30 minutes.
After standing for several days, it was cut into a predetermined shape and subjected to a tensile test (JIS No. 5, tensile direction: rolling direction) and a conical cup test (JIS No. 5, tensile direction: rolling direction).
SZ2249: test tool type 17) was carried out to measure the average aspect ratio L/H of crystal grains. The conical cup test was conducted as a simulation of press forming, and the combined formability of overhang and deep drawing was evaluated by CCV (mm) (the smaller the CCV, the better the formability). Also,
The crystal grain shape was determined by a cutting method using 50 samples after exposing the microstructure by Ga treatment. Furthermore, in order to simulate paint baking after press forming, the temperature was 170°C.
A heat treatment (corresponding to baking) was performed for 30 minutes, and then a tensile test was performed once again under the same conditions as the test after the heat treatment described above.

The results of these tests are shown in Tables 3 and 4. The surface properties after the conical cup test are also listed. In addition,
Bake hardening was determined by subtracting the yield strength after final heat treatment from the yield strength after simulated baking.

Alloy numbers 1 to 17 in Table 1 are examples within the composition range of the present invention, and alloy numbers 18 to 35 in Table 2
is a comparative example that falls outside the range. In addition, alloy number 3
3 to 35 are alloys conventionally used for body seats, and correspond to 2036, 5182, and 6010, respectively.

As is clear from Table 3, alloy numbers 1 to 17, which are examples, have an average aspect ratio of 1.3 or less, a high elongation at break of 30% or more, a good CCV, and excellent formability. It was done. In addition, the bake hardening has a yield strength of 3 kgf.
YSBH x El (yield strength x
Elongation) and TSBH x El (tensile strength x elongation) were confirmed to be high.

On the other hand, as is clear from Table 4, alloy numbers 18 to 35 of the comparative examples shown in Table 2 have poor formability and bake hardenability even if the average aspect ratio is 1.3 or less. Both or one of them was inferior to the example. For example, alloy number 18, which has a low content of Mg and Cu, which are components that contribute to bake hardening, and alloy number 25, which has a high content of Zn, which reduces bake hardenability, has low bake hardenability and has a 2 kgf/ It was about mm2. Also, Si
, Mn, Cr, Zr, V, Ti-B, and alloy numbers 22, 24, 26, 27, 28, 29, and 31, which have large amounts of Fe, have low elongation and average aspect ratios of 1.3 or more.
It has poor moldability and tends to produce distorted patterns.

Conventional materials with alloy numbers 33, 34, and 35 lacked bake hardenability and had low elongation at break, resulting in poor strength/elongation balance.

As shown by the coordinates in FIG. 1, Alloy Nos. 19 and 20, which contained more Mg and Cu than the Mg-Cu range specified in the present invention, had low elongation. On the other hand, alloy No. 18 with Mg and Cu lower than this range had insufficient strength. That is, it was confirmed that the value of the strength/elongation balance is low when the thickness falls outside the range shown in FIG.

FIG. 2 shows the relationship between Si content and CCV.
As shown in this figure, when the amount of Si is outside the range of the present invention (
It was confirmed that alloy numbers 21 and 22) had low CCV properties and poor formability. (Example 2) Next, an alloy plate material was manufactured under the manufacturing conditions shown in Table 5 using an ingot having the composition of alloy number 4 among the alloys shown in Table 1. Note that for treatments not specifically listed in Table 5, the conditions of Example 1 were adopted (rolling conditions, etc.). Note that symbols A to E in Table 3 are examples within the range of the manufacturing method according to the present invention, and symbols F to M are comparative examples outside of that range.

[0041] Evaluation tests similar to those in Example 1 were conducted on the plate material thus produced. The results are also listed in Table 5.

As is clear from Table 5, it was confirmed that the comparative examples that did not satisfy the conditions of the present invention had insufficient elongation, formability, or bake hardenability.

For example, when the homogenization treatment temperature, intermediate annealing treatment temperature, or solution quenching heating temperature is high, as in Comparative Examples G, I, and L, the formability is poor and distortion patterns occur in G and I. It was confirmed that when the cooling rate of solution hardening was low as in Comparative Example J, or when the homogenization temperature was low as in Comparative Example F, the bake hardenability was poor. Furthermore, in Comparative Example M, in which the heating temperature for solution quenching was low, the average aspect ratio exceeded 1.3 and the elongation was low, resulting in poor formability and distortion patterns. Furthermore, since sufficient bake hardenability was not obtained, the balance between strength and elongation was low. Comparative example K with long heating time for solution quenching
In the case of H, in which the heating rate in solution quenching is low, it was confirmed that abnormal grain growth occurred, the formability was poor, and a distorted pattern was generated.

[0044]

Effects of the Invention According to the present invention, strength-ductility balance, press formability, and bake hardening ability during low-temperature, short-time paint baking are superior to conventional aluminum alloy plates, and press formability and Provided are an aluminum alloy plate suitable for use in automobile body seats, etc., which requires dent resistance after paint baking, and a method for manufacturing the same.

[0045]

[Table 1]

[0046]

[Table 2]

[0047]

[Table 3]

[0048]

[Table 4]

[0049]

[Table 5]

[Brief explanation of drawings]

FIG. 1 is a diagram showing the range of Mg and Cu contents.

FIG. 2 is a diagram showing the relationship between Si content and CCV characteristics.

Claims (5)

[Claims] Claim 1: 1.5 to 8.0% Mg by weight;
Cu 0.25-3.0%, Si 0.02-0.15%
, Fe 0.03~0.25%, Ti 0.005~0
．． 15%, B 0.0002-0.05%, Zn 0.
Mg and Cu are contained within a range of 10% or less, and Mg and Cu are
-Cu coordinates, set the coordinate point to (Mg%, Cu%
), as shown in Figure 1 (8.0, 0.25),
(4.0, 0.25), (1.5, 0.8), (1.5
, 3.0), (8.0, 1.0), the remainder consists of Al and unavoidable impurities of 0.1% or less, and the axial length of the grain in the rolling direction is Excellent strength/ductility balance and bake hardenability characterized by an average aspect ratio L/H of 1.3 or less, where H is the axial length in the plate thickness direction perpendicular to L and L. Aluminum alloy plate for press forming. 2. 0.01 to 0.15% M by weight %
n, 0.01-0.15% Cr, 0.01-0.12
% of Zr, and 0.01 to 0.18% of V. Aluminum alloy plate for press forming. 3. 1.5 to 8.0% Mg by weight;
Cu 0.25-3.0%, Si 0.02-0.15%
, Fe 0.03~0.25%, Ti 0.005~0
．． 15%, B 0.0002-0.05%, Zn 0.
Mg and Cu are contained within a range of 10% or less, and Mg and Cu are
-Cu coordinates, set the coordinate point to (Mg%, Cu%
), as shown in Figure 1 (8.0, 0.25),
(4.0, 0.25), (1.5, 0.8), (1.5
, 3.0), (8.0, 1.0), and the balance is Al and 0.1% or less of unavoidable impurities for an aluminum alloy ingot. 58
After one-stage or multi-stage homogenization treatment at a temperature within the range of 0°C, the ingot is hot-rolled and cold-rolled to a desired thickness, and then homogenized at a temperature within the range of 440 to 580°C. Heating at a heating rate of 3℃/second or higher until the temperature reaches 0.
A method for producing an aluminum alloy plate for press forming with excellent strength/ductility balance and bake hardenability, which comprises holding the plate for ~120 seconds and then cooling to 100°C at a cooling rate of 2°C/second or more. 4. The aluminum alloy ingot contains 0.01 to 0.15% Mn, 0.01 to 0.1% by weight.
5% Cr, 0.01-0.12% Zr, and 0.0
The aluminum alloy plate for press forming with excellent strength/ductility balance and bake hardenability according to claim 3, further comprising one or more types of V in an amount of 1 to 0.18%. Production method. 5. 320 to 58 between hot rolling and cold rolling, or between cold rolling and cold rolling, or both.
Intermediate annealing treatment at a temperature within the range of 0°C once or twice
The method for producing an aluminum alloy plate for press forming with excellent strength/ductility balance and bake hardenability according to claim 3 or 4, wherein the method is carried out at least once.