JP2006037139A - 6000 series aluminum alloy sheet for superplastic forming having excellent paint baking hardenability and its production method - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a 6000 series aluminum alloy sheet for superplastic forming having excellent paint baking hardenability in which coarse abnormal grains are not formed even after superplastic forming, and to provide its production method. <P>SOLUTION: The alloy sheet has a composition containing, by mass, 0.4 to 1.0% Mg, 0.6 to 1.4% Si and >0.4 to 1.0% Mn, and, if required, further comprising prescribed Cr, Ti, B, Fe, Cu and Sn, and the balance Al with inevitable impurities, has a worked structure as cold-rolled, and has an electric conductivity of 40 to 60% IACS. Further, in the method for producing an alloy sheet, a cast ingot having the above components is heated without performing homogenizing annealing, thereafter, hot rolling is started in the temperature range of 450 to 550°C and is finished at ≤350°C, and cold rolling is performed, so as to produce the alloy sheet. <P>COPYRIGHT: (C)2006,JPO&NCIPI

Description

  The present invention relates to a superplastic forming 6000 series aluminum alloy plate suitable for a member that requires difficult forming processing in an automobile body panel or the like and requires high strength after baking coating processing, and a method of manufacturing the same. is there.

  In recent years, application of aluminum alloys to body panels has been progressing as one of means for reducing the weight of automobile bodies. However, in general, aluminum alloys have lower formability than steel plates, and various processing methods have been studied. One of them is a processing method utilizing the superplastic phenomenon.

  The superplastic phenomenon is characterized in that the elongation is so large that it cannot be obtained under normal processing conditions and the deformation stress is small. Therefore, research and development for practical application of superplastic alloys using these characteristics has been actively conducted. In particular, aluminum alloys are characterized by light weight, but are being actively developed due to problems with workability. Among them, 5000 series aluminum alloys (Al-Mg series alloys) have corrosion resistance and appropriate strength. Some alloys have attracted attention and are in practical use because of their excellent surface treatment properties.

Conditions for developing the superplastic phenomenon include (1) an alloy having stable and fine equiaxed crystal grains (-10 μm), (2) heating temperature T is T> 0.5 Tm (absolute temperature of melting point) (3) It is generally said that processing at a low strain rate (10 ⁻⁴ / s˜) is appropriate (see Non-Patent Document 1). Therefore, alloy development so far has been carried out with guidelines such as making crystal grains finer, making a thermally stable structure even when processing at high temperature, suppressing the occurrence of cavities that impair ductility, etc. An alloy of a special component system as disclosed in Patent Document 1 or an alloy that requires special manufacturing conditions as disclosed in Patent Document 2 has been proposed, but is desirable from the viewpoint of manufacturing cost. It was not a thing.

  Further, even under the processing conditions, a low strain rate as described in Non-Patent Document 2 is necessary, so there is a problem with productivity.

  However, the present inventors rarely require a very large elongation such as 500% or 1000% when performing superplastic forming, and if an elongation of about 150 to 200% can be achieved, it is practical. In particular, the invention relates to a 5000 series aluminum alloy sheet that is easy to manufacture without adopting special conditions and that enables superplastic forming with excellent productivity, and an invention relating to a forming method thereof. Proposed (see Patent Document 3). However, since the present invention is based on a non-heat-treatment type 5000 series aluminum alloy plate, it has been difficult to apply it to members that require strength.

  Therefore, the present inventors are able to perform practical superplastic forming with an elongation of 150% or more in order to ensure practical superplastic forming ability and realize further weight reduction of the applied member. Furthermore, an invention relating to a heat treatment type 6000 series aluminum alloy sheet capable of obtaining a high strength of a tensile strength of 300 MPa or more by performing an appropriate heat treatment such as T6 treatment after superplastic forming has been proposed (see Patent Document 4). . Here, T6 process means what was subjected to artificial age hardening after solution treatment. However, in this invention, it is possible to achieve a significant increase in strength by performing a solution treatment after superplastic forming and performing an artificial age hardening treatment, but this is not desirable from the standpoint of manufacturing cost because it increases the number of processes. There wasn't. Therefore, the present inventor can further enhance the strength by utilizing a paint baking process that is usually performed in an automobile manufacturing process or the like without performing a separate solution treatment and artificial aging treatment after superplastic forming. An invention relating to a superplastic forming method of an aluminum alloy high-strength member has been proposed (see Patent Document 5).

  Further, in terms of cost reduction, an invention relating to a superplastic blow molding method using a heat-treatable aluminum-based superplastic alloy that has not been subjected to a solution treatment in plate production has been proposed (see Patent Document 6).

Japanese Patent Laid-Open No. 57-076145 JP 58-081957 A Japanese Patent Application Laid-Open No. 08-199272 Japanese Patent Laid-Open No. 11-131165 JP 2003-301249 A JP 2001-058221 A Osawa, Nishimura: Light Metal, 39-10 (1989), P.C. 765-775 etc. East: Light Metal, 39-11 (1989), P.I. 751-764 The Japan Institute of Light Metals, “Aluminum structure and properties” (1991)

  As a problem when superplastic forming is performed using an aluminum alloy plate having a general crystal grain size, very coarse recrystallized grains are formed after superplastic forming (hereinafter referred to as abnormal grain growth), and forming. There was a case where the appearance of the later product was damaged. In particular, abnormal grain growth tends to occur when superplastic forming is performed at a large strain rate from the viewpoint of productivity. Even in the conventional proposals by the present inventors, this abnormal grain growth sometimes occurs, but it is necessary to further suppress the abnormal grain growth.

  In addition, as in the invention described in Patent Document 6, superplastic forming is often performed by pressurizing at 480 to 530 ° C. for 10 to 20 minutes, and the productivity is lower than that of normal press forming. Further cost reduction has been desired.

  Therefore, the present invention provides a 6000 series aluminum alloy plate for superplastic forming that has excellent paint bake hardenability, which can substantially reduce abnormal grain growth after superplastic forming, and can further reduce material costs, and a method for manufacturing the same. It is intended to do.

The present inventors have made various studies on the abnormal grain growth phenomenon when superplastic forming is performed on a 6000 series aluminum alloy plate. As a result, in the case of superplastic forming performed at a strain rate of 10 ⁻⁴ to 10 / s in a temperature range of 460 to 580 ° C., abnormal grain growth can be almost completely suppressed by adding an appropriate amount of Mn or Cr. I found out. In addition, when superplastic forming is performed under the above conditions, the entire alloy components are adequate even if Mn or Cr is contained in a prescribed amount in addition to the solute atoms of the present aluminum alloy for the purpose of suppressing abnormal grain growth. It was experimentally clarified that an elongation of 150% or more was obtained by prescribing to the above, and that practical superplastic forming ability was sufficiently obtained.

On the other hand, the general solution treatment temperature of the 6000 series aluminum alloy plate is in the range of 480 to 580 ° C., and superplastic forming is also performed in such a temperature range as described above, so the solution treatment is omitted. It is considered possible. However, in order to obtain paint bake hardenability, it is necessary to dissolve solute atoms in a supersaturated state during the cooling process after superplastic forming. At this time, the solute atoms are effectively solidified without cooling at a high cooling rate. It is also important to dissolve, that is, to have good hardenability. In order to obtain a large increase in strength during paint baking, it is generally necessary to increase the amount of Mg ₂ Si or the amount of excess Si, but as disclosed in Non-Patent Document 3, the amount of Mg ₂ Si On the other hand, it is said that the hardenability decreases when the amount of excess Si is increased. Further, Mn and Cr added to suppress abnormal grain growth are said to reduce the strength increase in the coating baking process. Therefore, the present inventor has conducted various studies on the influence of the alloy components on the bake hardenability and hardenability of the paint, and appropriately set the component ranges of the solute atoms Mg and Si and the addition amount of the abnormal grain growth inhibiting element of Mn or Cr It was experimentally confirmed that the paint bake hardenability and the hardenability can be compatible.

  In addition, the electrical conductivity before and after solution treatment of 6000 series aluminum alloy plates manufactured by various manufacturing methods using the phenomenon that the electrical conductivity of the plates decreases as the amount of solute atoms required to ensure the bake hardenability of coating increases. investigated. As a result, if the electrical conductivity before the solution treatment is 40 to 60% IACS, solute atoms equivalent to the solution treatment by solid plastic forming under general conditions are solidified. It was found that the coating bake curability can be obtained. The unit% IACS of conductivity used here is the ratio of substances with the same temperature and volume when the conductivity of standard soft copper (specific resistance 1.7241 μΩ · cm / 20 ° C.) is 100%. The higher the numerical value, the better the conductivity.

  Furthermore, in order to ensure the conductivity of 40 to 60% IACS, by controlling the hot rolling temperature appropriately, in addition to the final solution treatment, it is a process related to the promotion of solid solution of solute atoms. It has been found that the chemical annealing can be omitted, and further, the intermediate annealing that may be performed during the cold rolling in the case of producing a plate by performing the cold rolling after the hot rolling can be omitted.

  That is, the 6000 series aluminum alloy sheet for superplastic forming had a cold-rolled processed structure manufactured by hot rolling under appropriate conditions after melt casting and further cold rolling. A board can be applied.

  Based on these findings, the present inventors do not form coarse abnormal grains even after superplastic forming, and can further reduce the material cost, and can be further reduced in material baking cost. It came to invent the manufacturing method.

  The gist of the present invention is as follows.

  (1) By mass%, Mg: 0.4 to 1.0%, Si: 0.6 to 1.4%, Mn: more than 0.4 to 1.0%, the balance being Al and inevitable impurities A 6000 series aluminum alloy plate for superplastic forming excellent in paint bake hardenability, comprising a cold-rolled processed structure and having an electrical conductivity of 40 to 60% IACS.

  (2) 6000 series aluminum alloy plate for superplastic forming excellent in paint bake hardenability as described in (1) above, further comprising Cr: 0.02 to 0.5% by mass .

  (3) By mass%, further containing one or more of Ti: 0.005-0.15%, B: 0.0001-0.05%, Fe: 0.03-0.4% A 6000 series aluminum alloy plate for superplastic forming excellent in paint bake hardenability as described in (1) or (2) above.

  (4) Excellent in bake hardenability according to any one of the above (1) to (3), characterized by containing Cu: 0.1 to 1.0% by mass%. 6000 series aluminum alloy plate for superplastic forming.

  (5) Excellent in bake hardenability according to any one of (1) to (4) above, characterized by containing Sn: 0.01 to 0.15% by mass%. 6000 series aluminum alloy plate for superplastic forming.

  (6) After heating the ingot having the component according to any one of (1) to (5) above without performing homogenization annealing, hot rolling is started in a temperature range of 450 to 550 ° C, A method for producing a 6000 series aluminum alloy plate for superplastic forming excellent in paint bake hardenability, characterized in that hot rolling is terminated at a temperature of 350 ° C. or less and cold rolling is performed.

  According to the present invention, in particular, a 6000 series aluminum alloy plate excellent in paint bake hardenability, which is suitable as a panel material for superplastic forming of automobiles, does not cause abnormal grain growth even after superplastic forming, and can further reduce material cost, and The manufacturing method can be provided, and the industrial contribution is extremely remarkable.

  Hereinafter, the present invention will be described in detail.

  First, the reasons for limiting the alloy components are shown below.

  Mg and Si are indispensable basic components of the present invention, and are the basic alloy components for securing superplastic formability and increasing the strength when the paint baking process is performed after the forming process, that is, for obtaining the paint bake hardenability. is there. When Mg is less than 0.4% and Si is less than 0.6%, the above-mentioned effects are poor, and when Mg is more than 1.0% and Si is more than 1.4%, superplastic formability, cooling process after processing It is impossible to achieve both hardenability and paint bake hardenability. Therefore, the Mg content is set to 0.4 to 1.0%, and the Si content is set to 0.6 to 1.4%.

  Mn has the effect of suppressing abnormal grain growth during superplastic forming. The reason why the lower limit of the amount of Mn exceeds 0.4% is that abnormal grain growth may not be sufficiently suppressed when the amount is 0.4% or less. Also, the reason why the upper limit of the amount of Mn is 1.0% or less is that if it exceeds 1.0%, a large number of coarse intermetallic compounds are formed, and the generation of cavities during superplastic forming is remarkably increased. This is because the mechanical properties after molding may be impaired as well as the properties. For the above reasons, the amount of Mn is set in the range of more than 0.4 to 1.0%.

  The present invention may contain the following elements as required.

  Cr, like Mn, is an element having an effect of suppressing abnormal grain growth during superplastic forming. If the Cr content is less than 0.02%, the effect is insufficient. If the Cr content exceeds 0.5%, a coarse intermetallic compound is formed, and many cavities are formed during superplastic forming. There is a risk of impairing the plastic formability and the mechanical properties after molding. Therefore, the Cr content is in the range of 0.02 to 0.5%.

  Moreover, in this invention, you may contain 1 type, or 2 or more types of Ti, B, and Fe as needed. Ti and B have the effect of refining the crystal grain of the ingot by adding a small amount and improving rough skin. When Ti is less than 0.005% and B is less than 0.0001%, the effect of refining the crystal grains of the ingot is slightly insufficient. On the other hand, if Ti exceeds 0.15% and B exceeds 0.05%, a coarse crystallized product is formed, and the superplastic formability may deteriorate. Therefore, the Ti amount is set to 0.005 to 0.15%, and the B amount is set to 0.0001 to 0.05%.

  Fe is an element that contributes to strength improvement and is an element that is effective in suppressing abnormal grain growth during superplastic forming. The effect is insufficient when the Fe content is less than 0.03%. On the other hand, if the amount of Fe exceeds 0.4%, a coarse crystallized product is generated, which may deteriorate superplastic formability and deteriorate mechanical properties after forming due to cavity formation during superplastic forming. is there. Therefore, when Fe is contained, the Fe content is preferably in the range of 0.03 to 0.4%.

  In this invention, you may contain Cu as needed. Cu is an element that contributes to an increase in strength when paint baking is performed after superplastic forming. If Cu is less than 0.1%, the effect of increasing the strength is not sufficiently obtained. If Cu is added in excess of 1.0%, the corrosion resistance is greatly deteriorated. Therefore, the amount of Cu added is in the range of 0.1 to 1.0%.

  Furthermore, in the present invention, Sn may be included as necessary. Sn has the effect of suppressing the increase in strength due to natural aging during standing at room temperature and improving the hardenability. If Sn is less than 0.01%, the effect is small, and if it exceeds 0.15%, the effect of suppressing the increase in strength during natural aging is saturated, and the hardenability is greatly reduced. Therefore, the addition amount of Sn is set to 0.01 to 0.15%. In order to effectively use the effect of Sn, it is important to uniformly dissolve Sn in the Al matrix, and for that purpose, it is desirable to add Sn as an Al—Sn master alloy during melting and casting. . Note that the Sn content in the Al—Sn master alloy does not have to be specified.

  In addition to the above elements, unavoidable impurities are contained, but the amount is within a range that does not impair the effects of the present invention.

  Next, the production method of the present invention will be described in detail.

  As a general manufacturing method of the present 6000 series aluminum alloy sheet, melt casting, homogenization annealing, hot rolling, cold rolling, and if necessary, annealing and solution treatment are performed during the cold rolling. On the other hand, in the present invention, it is possible to omit the solution annealing, homogenization annealing, and intermediate annealing performed in the middle of cold rolling, which are processes related to solid solution of solute atoms. The alloy plate is manufactured by subjecting an ingot melt-cast in accordance with a conventional general method to hot rolling under the conditions defined in the present invention and then cold rolling.

  First, the ingot is heated and hot rolled. In order to set the electric conductivity of the final product plate having the processed structure as cold rolled to 40 to 60% IACS, it is necessary to perform hot rolling with appropriately controlled start and end temperature ranges. The starting temperature in the hot rolling is in the range of 450 to 550 ° C. If the hot rolling start temperature is less than 450 ° C., the solute atoms may not be sufficiently dissolved. If the starting temperature is higher than 550 ° C., heating before hot rolling will be performed at 580 ° C. or higher in consideration of temperature drop due to ingot conveyance from the heating furnace to the hot rolling mill, etc. At temperature, eutectic melting may occur, which is a problem. Therefore, the hot rolling start temperature range was defined as 450 to 550 ° C. On the other hand, the hot rolling end temperature is set to 350 ° C. or lower. This is because although the amount of solid solution is large above 350 ° C., the precipitate becomes relatively large and it is difficult to re-dissolve at the time of superplastic forming.

  In addition, although the rolling reduction rate for each pass is not particularly defined, it may be set so that the hot rolling end temperature is in the above range.

  The present invention alloy sheet manufactured by performing hot rolling controlled to satisfy this condition and then performing cold rolling has a conductivity of 40 to 60% IACS, and is used as a superplastic forming material. It has sufficient moldability and exhibits good paint bake hardenability.

  In the following, suitable conditions from superplastic forming to paint baking treatment for sufficiently exerting the performance of the aluminum alloy plate of the present invention will be described.

As the superplastic forming conditions, the aluminum alloy sheet according to the present invention has abnormal grain growth even in a wide range of conditions such as a strain rate dε / ds of 10 ⁻⁴ to 10 / s in a temperature range of 480 to 580 ° C. In addition, it exhibits a total elongation of about 150% or more and has a practically sufficient superplastic forming ability. Moreover, sufficient solution treatment is performed.

Although the aluminum alloy plate according to the present invention has excellent hardenability, it is preferably cooled at 2 ° C./s or higher up to 200 ° C. or lower after superplastic forming. If it is less than 2 ° C./s, the Mg ₂ Si phase, the Si phase, etc. may precipitate during the cooling process, the amount of supersaturated solid solution will be insufficient, and paint bake hardenability may be difficult to obtain.

  Further, in order to greatly increase the strength in the paint baking process, it is preferable that the room temperature standing time from the superplastic forming to the paint baking process is as short as possible. If the standing time at room temperature becomes longer, heat treatment at a higher temperature for a longer time is required to obtain a large paint bake hardenability. Preferably, the coating baking process is preferably performed within 12 hours after superplastic forming, and the baking conditions are preferably 180 ° C. or higher and 30 minutes or longer.

  A 6000 series aluminum alloy having the component composition shown in Table 1 was melted and cast by a DC casting method. The obtained ingot was hot-rolled at 510 ° C, and the hot-rolling was finished at 280 ° C with a plate thickness of 5 mm. Thereafter, it was cold-rolled to 1 mm to obtain a test material.

These aluminum alloy rolled sheets were subjected to a high-temperature tensile test at a temperature corresponding to superplastic forming conditions of 500 ° C. and a strain rate of 10 ⁻² / s. 150% or more was regarded as good superplastic elongation.

These aluminum alloy plates were subjected to superplastic forming using a small high temperature blow molding tester. The mold is a square tube mold with a side of 250 mm, the plate is preheated to 500 ° C. for 1 minute, and then pressed to form an average strain rate of about 10 ⁻² / s to form a height of 60 mm. It was. Moreover, it cooled at the speed | rate of 3 degrees C / s after shaping | molding. Moreover, after cooling, it was left at room temperature for 8 hours, and then a heat treatment equivalent to coating baking at 180 ° C. for 60 minutes was performed. In order to investigate the strength after superplastic forming, a JIS No. 5 tensile test piece was sampled in the rolling direction from the center of the upper surface of the square tube molded product and subjected to a tensile test. Since the upper limit of the yield strength when a 5000 series aluminum alloy superplastic material is used is about 140 MPa, the yield strength of 150 MPa or more was determined to be good paint bake hardenability.

  Further, in order to investigate the presence or absence of abnormal grain growth, a test piece was collected from the vicinity of the center of the upper surface of the rectangular tube molded product, and the cross section of the plate was etched to observe the crystal grain structure over the entire plate thickness. Table 2 shows the evaluation results. Equiaxial recrystallized grains of 200 μm or less are considered good, and are shown in Table 2 as ◯. Further, in the case where coarse crystal grains exceeding 200 μm were partially observed, abnormal grain growth occurred, and it was indicated as “x” in Table 2.

  Alloy No. In Nos. 1 to 12, the alloy components and production conditions were within the scope of the present invention, had superplastic elongation of 150% or more, excellent paint bake hardenability, and no abnormal grain growth was observed.

  On the other hand, Alloy No. In No. 13, the amount of Mg and Si was less than the range of the present invention, so the bake hardenability was low, and Mn was less than the specified amount, and therefore abnormal grain growth occurred. Alloy No. No. 14 had a low amount of Mg, and the amounts of Si and Cr were larger than the range of the present invention, so the paint bake hardenability was low. Alloy No. In No. 15, the amount of Mg and Si was outside the range of the present invention, so the paint bake hardenability was low, and abnormal grain growth occurred because Mn was less than the specified amount. Alloy No. In No. 16, the amount of Mg and Si was within the range of the present invention. Alloy No. In No. 17, since the Sn amount was larger than the range of the present invention, the hardenability was lowered and the paint bake curability was insufficient.

  The ingots of the alloys 1 and 11 of the present invention shown in Table 1 were subjected to hot rolling and cold rolling under the conditions shown in Table 3 to obtain test materials. Table 3 shows the hot rolling start temperature and end temperature, and the hot rolling end plate thickness. The hot-rolled sheet was cold-rolled to obtain a test material having a thickness of 1 mm.

  The specimens thus produced were cooled using the same high-temperature blow test machine as in Example 1 under the conditions shown in Table 4 and superplastic forming and square tube molding. At this time, the same mold as in Example 1 was used, and the molding height was also set to 60 mm.

  The superplastic elongation of the test material was obtained by performing a high-temperature tensile test at the superplastic forming temperature at a strain rate equivalent to that of superplastic forming. Moreover, evaluation of the presence of paint baking curability and abnormal grain growth was performed in the same manner as in Example 1. Table 5 shows the evaluation results.

  Alloy No. When superplastic forming is performed on the alloy plate of the present invention produced by the methods A to D under the condition A, the ingot having the component 1 has superplastic elongation of 150% or more, and paint bake hardenability. And no abnormal grain growth was observed. On the other hand, when manufactured by the methods E and F outside the present invention, good paint bake hardenability was not obtained even under the same superplastic forming conditions.

  In addition, Alloy No. Even when the ingot having 11 components is superplastically formed on the alloy plate produced by the manufacturing methods A to D of the present invention under the condition (a), it has a superplastic elongation of 150% or more, and is baked by coating. Excellent curability and no abnormal grain growth was observed. However, when manufactured by the methods E and F outside the present invention, the paint bake hardenability was lowered even under the same superplastic forming conditions.

Claims (6)

% By mass
Mg: 0.4 to 1.0%,
Si: 0.6 to 1.4%,
Mn: more than 0.4 to 1.0%
The balance is made of Al and inevitable impurities, has a cold-rolled processed structure, and has an electrical conductivity of 40 to 60% IACS, superplastic forming with excellent paint bake hardenability 6000 series aluminum alloy plate. In mass%,
Cr: 0.02-0.5%
The 6000 series aluminum alloy plate for superplastic forming with excellent paint bake hardenability according to claim 1, characterized in that In mass%,
Ti: 0.005 to 0.15%,
B: 0.0001 to 0.05%,
Fe: 0.03-0.4%
The 6000 series aluminum alloy plate for superplastic forming excellent in paint bake hardenability according to claim 1 or 2, characterized by containing one or more of the following. In mass%,
Cu: 0.1 to 1.0%
The 6000 series aluminum alloy plate for superplastic forming excellent in paint bake hardenability according to any one of claims 1 to 3, characterized by comprising: In mass%,
Sn: 0.01 to 0.15%
The 6000 series aluminum alloy plate for superplastic forming excellent in paint bake hardenability according to any one of claims 1 to 4, characterized by comprising: After heating the ingot having the component according to any one of claims 1 to 5 without performing homogenization annealing, hot rolling is started in a temperature range of 450 to 550 ° C, and at a temperature of 350 ° C or less. A method for producing a 6000 series aluminum alloy plate for superplastic forming excellent in paint bake hardenability, characterized in that hot rolling is finished and cold rolling is carried out.