JP4778395B2 - Aluminum alloy sheet with excellent stretch flangeability and high post-baking strength - Google Patents

Description

  The present invention relates to an aluminum alloy sheet excellent in high strength and excellent formability, in particular, stretch flangeability and post-baking strength, and a production method capable of mass-producing this alloy efficiently and inexpensively.

  Since consideration for the global environment is increasingly required in all fields, there is a demand for further weight reduction of automobiles, ships, airplanes, and other various vehicles for the purpose of reducing required fuel consumption. The same applies to various machines, electrical products, buildings, structures, and various other devices. The trend to replace automotive body panels from steel materials to aluminum alloy materials is a remarkable example.

  As a panel material for inner and outer plates used for panel structures such as automobile hoods, fenders, doors, roofs, trunk lids, etc., Al—Mg—Si based alloys (JIS6000 series) are prominent. This alloy containing Mg and Si as essential components has excellent age-hardening properties, and formability can be secured by reducing the yield strength during press molding and bending. Moreover, artificial age hardening by heating during baking after molding has the advantage of improving the yield strength of the material and ensuring the required strength.

  In addition, this alloy material has a lower alloy element content than JIS 5000 series aluminum alloy materials containing a large amount of alloy elements such as Mg. Therefore, the same kind of aluminum alloy material can be recycled economically by recycling the scrap. Convenient for production.

  For the above reasons, JIS 6000 series aluminum alloy materials have already been extensively researched and developed from various viewpoints in terms of both physical properties and manufacturing methods, particularly for automobiles. However, among the press forming and bending processes that are indispensable for this kind of application, JIS 6000 series aluminum alloys are compared with JIS 5000 series in terms of formability, especially stretch flangeability that involves forming a flange by bending when drilling. There is a bad problem.

  Therefore, for example, some of the following patent documents add, as an improvement measure, some other alloy elements in addition to Mg and Si, or in addition to that, control the grain size of the alloy and the dispersion state of the crystallized product. Suggest a way to do it.

  Patent document 1 aims at improving the bending workability and paint bake hardenability of an aluminum alloy sheet for an outer panel of an automobile, and improving the components of JIS6000 series and paying attention to the orientation difference between adjacent crystal grains in the structure. Invention. For this purpose, the present invention incorporates soaking twice.

  Patent Document 2 or 3 is an invention in which homogenization and cooling conditions at the time of manufacture are improved in addition to adjustment of the alloy composition for the purpose of improving formability and paint bake hardenability on the premise of impurities of Fe. .

  Patent Document 4 is also an invention for improving the formability and paint bake hardenability, but also pays attention to the properties of Mg-Si compounds in the alloy and intends to adjust the size and number density. . At the same time, devise manufacturing conditions such as soaking and solution treatment twice.

  Patent Document 5 is an invention that improves the alloy components and properties in consideration of conductivity, and devise manufacturing conditions such as solution treatment for improving the strength.

However, due to these R & D results, aluminum alloy sheets that are highly demanded of stretch flangeability and that do not deteriorate in post-baking strength at the same time as moldability are sufficiently improved to satisfy the user's satisfaction. The situation has not yet reached that it can be supplied at a low price to meet the suitability for mass production.
JP 2003-171726 A JP 2003-105471 A JP 2003-105472 A JP 2002-356730 A Japanese Patent Laying-Open No. 2005-8926 JP 2003-277869 A JP 2003-277870 A

  The main object of the present invention is to provide an aluminum alloy sheet that satisfies both excellent stretch flangeability and high post-baking strength (strength associated with age hardening after baking). For this purpose, an aluminum alloy plate that has been purposely optimized for the crystal structure of the alloy and the properties of the mixed compounds in order to enable it to be supplied at a low cost without using an unnecessarily complicated manufacturing process. The issue is to provide Another object of the present invention is to supply aluminum alloy sheets that are also useful for scrap recyclability and to provide optimum manufacturing techniques for solving these problems.

In order to solve the above-mentioned problems, the present invention makes it possible to provide an aluminum alloy plate that is excellent in stretch flangeability and high post-baking strength by using a structure characterized by the following means as the aluminum alloy plate itself. Is.
(1) Mg: 0.1 to 3.0% by weight (hereinafter simply referred to as%), Si: 0.1 to 2.5%, and Cr: 0.02 to 0.08%, When the measurement area is 0.1 to 0.2 mm ² , the magnification at the time of measurement is × 600, and particles of 0.5 μm or more are measured with EPMA under an acceleration voltage of 20 kV, the number density of Cr-containing compounds is 500. -2000 pieces / mm ² , and the area ratio of the Cr-containing compound ÷ the area ratio of all the compounds is 60 to 98%, the average crystal grain size of the alloy is 50 μm or less, and the maximum particle size of the compound is 20 μm or less. An aluminum alloy sheet characterized by having excellent stretch flangeability with a balance of Al and inevitable impurities and high post-baking strength.
(2) Mn: 0.05% or less The aluminum alloy sheet having excellent stretch flangeability and high post-baking strength as described in 1 above.
(3) The aluminum alloy sheet having excellent stretch flangeability and high post-baking strength according to the above 1 or 2, characterized by containing Fe: 0 to 0.1%.

  The aluminum alloy sheet of the present invention is based on a JIS 6000 series aluminum alloy, but additionally contains Cr, and at the same time, the number density of this Cr-containing compound and the area ratio with respect to all compounds are consciously regulated within a specific range. This is a feature. By configuring the properties of the alloy material in this manner, the stretch flangeability of the aluminum alloy plate is improved, and at the same time, a high post-baking strength can be secured at the same time. At the same time, it is possible to carry out manufacturing by recycling scrap materials without difficulty.

  On the other hand, the present invention is characterized by producing an aluminum alloy plate by a method of avoiding so-called roughening in the middle while leaving it to a two-stage homogenization treatment under specific conditions. That is, the aluminum alloy plate of the present invention having the above-described performance is easily and inexpensively industrially produced by this method.

  The aluminum alloy sheet of the present invention is based on a JIS6000-based Al—Mg—Si alloy composition, and Cr is added as another essential element to this, and the existence form of the Cr-containing compound formed in the alloy is changed. The feature is that it was adjusted.

Mg and Si in the alloy form aggregates (clusters) or intermediate phases of the Mg ₂ Si composition called the GP zone and are indispensable elements for improving the strength of the alloy. A quantity is needed. However, if the Mg content exceeds 3.0% and the Si content exceeds 2.5%, coarse crystallized substances increase and become the starting point of fracture during deformation, and the stretch flangeability and bendability of the alloy material are reduced. descend. More preferably, the Mg content is 0.00.4 to 2.5% and the Si content is 0.6 to 2.0%.

  It is a feature of the present invention that Cr is added to the above Al-Mg-Si alloy composition, so that the Cr-containing compound is kept fine throughout the manufacturing process of the alloy, and the strength after baking of the alloy can be improved. It becomes. At the same time, Cr is useful for refining crystal grains and has the effect of improving the stretch flangeability of the alloy material. In order to guarantee these effects, Cr needs a content of 0.02% or more, and if it is less than this, the number of Cr-containing compounds becomes too small, so that there are many compounds not containing Cr, which is coarse. Thus, not only the strength after baking of the alloy plate cannot be expected, but also the stretch flangeability is lowered. Further, even if the Cr content is more than 0.08%, the stretch flangeability is lowered, so 0.02 to 0.10% is more preferable.

  The content of Mn is useful for refining the crystal grains of the alloy as Cr, and improves the workability. However, if it is too large, the amount of α-Al-Fe-Mn-Si generated increases, and the solid solution after solutionization Reduce the amount of Si. As a result, since improvement in the strength after baking of the alloy plate cannot be expected, the Mn content is 0.05% or less, more preferably 0.02% or less.

The present invention has a more important feature of controlling the number density and area ratio of Cr-containing compounds generated by the inclusion of Cr on the premise of the basic alloy composition described above. First, the number of Cr-containing compounds per unit area was measured to be 500 to 2000 / mm ^2.
To be in the range. Research on the correlation between the number of steels and the post-baking strength and stretch flangeability of the alloy material. If the number is less than 500, improvement in post-baking strength and stretch flangeability of the alloy material by adding Cr cannot be expected. Further, it has been clarified that if the number is more than 2000, sufficient stretch flangeability cannot be obtained. It was also found that 600 / mm ² or more and 1800 / mm ² or less are preferable.

As for this correlation, if it is less than 500, there are many compounds that do not contain Cr, and this is coarsened and it is not expected to improve the strength after baking of the alloy plate and the stretch flangeability, and if it is more than 2000, This is due to the fact that the product strength becomes too high, and it is not possible to improve the stretch flangeability.
It is also confirmed that the number ratio of such Cr-containing compounds is simultaneously closely related to the ratio of the area ratio of the Cr-containing compound to the area ratio of all the compounds including other compounds. That is, the ratio expressed by area ratio of Cr-containing compound / area ratio of all compounds is in the range of 60 to 98%, more preferably in the range of 70 to 90%. The improvement of sex can be expected with certainty.

  In addition to the above characteristics, the present invention further improves the stretch flangeability of the alloy plate by setting the average crystal grain size of the alloy to 50 μm or less, and preferably has an average crystal grain size of 40 μm or less. Good.

  Further, when the maximum particle size of the contained compound is suppressed to 20 μm or less, this acts as a starting point of fracture, and it is possible to effectively prevent the stretch flangeability of the alloy plate from being deteriorated. More preferably, it is 10 μm or less, and further preferably 8 μm or less.

  The number density, area ratio, and maximum particle size measurement method and display method of the Cr-containing compound described above will be clarified in Examples described later.

  In addition to the above, the present invention can selectively contain some other metal elements in addition to the basic aluminum alloy composition described above.

  First, one or more of Fe: 0 to 0.1%, Zr: 0 to 0.01%, and V: 0 to 0.01% can be contained. These metal elements are effective for refining the crystal grains of the alloy and contribute to the improvement of stretch flangeability, like Mn and Cr described above. However, if it is contained in a large amount exceeding the upper limit of the above numerical value, these synthesize a compound with Si, the amount of Si contained in the compound in the alloy becomes excessive, and the strength after baking cannot be secured. More preferably, Fe: 0.05% or less, Zr: 0.005% or less, and V: 0.005% or less are good.

  Further, one or two of Ti <0.2% and Zn <1.5% can be additionally contained in the above aluminum alloy composition. These elements are also effective for refinement of alloy crystal grains and can further improve the workability of the alloy material, but if they are contained in a large amount exceeding the upper limit of the above numerical value, a coarse compound is formed, which acts as a starting point of fracture. However, workability deteriorates. Preferably, Ti is 0.005 to 0.2% and Zn is 0.1 to 1.5%.

  Furthermore, Cu <1.0% can be contained with respect to the aluminum alloy composition described above, and the strength of the alloy is enhanced by containing 0.1% or more, but if contained in a large amount of 1.0% or more, it is coarse. This compound acts as a starting point of fracture, and stretch flangeability and bendability deteriorate.

  The present invention is a material excellent in stretch flangeability and post-baking strength characterized by the aluminum alloy composition and compound structure as described above, but these alloy plates are effectively produced by the method described below. Is done.

  This normal aluminum alloy sheet is manufactured through the steps of casting → homogeneous heat treatment → hot rolling → cold rolling → final annealing. The method of the present invention is characterized by two-step soaking in which two steps of homogeneous heat treatment before hot rolling are performed under specific conditions.

  The first annealing is performed at a temperature of 500 ° C. or higher and lower than the melting point for 2 hours or more, and the alloy structure is homogenized, that is, the intragranular segregation in the ingot structure is sufficiently eliminated. In fact, when the annealing temperature is lower than 500 ° C., the elimination of segregation within the crystal grains is insufficient, and this becomes the starting point of compound destruction, which deteriorates the stretch flangeability and bendability of the product alloy sheet. The same applies when the heating time is less than 2 hours.

  Cooling subsequent to the first soaking is preferably carried out in the range of 40 to 100 ° C./h in order to suppress the generation of a precipitation phase at the grain boundary, that is, the decrease in stretch flangeability. High-speed cooling exceeding 100 ° C./h is not practical because it requires a forced air cooling device and increases costs.

  Following the first soaking, in the present invention, in preparation for the subsequent hot rolling, the annealing before the hot rolling is performed in a temperature range of 390 to 480 ° C. for the purpose of optimizing the alloy structure. It is the second soaking. If the temperature at this time is 390 ° C. or lower, the formation of a precipitated phase is promoted, and the stretch flangeability and bendability are deteriorated.

  Further, the annealing time in the second soaking is 2 to 15 hours, and if it is shorter than 2 hours, the strength is excessive, and if it is longer than 15 hours, the formation of a precipitated phase at the grain boundary is promoted. Impairs stretch flangeability and bendability.

  Hot rolling is performed under normal heating conditions after soaking twice, but the end temperature is preferably in the range of 170 to 300 ° C. If the temperature is lower than 170 ° C., the crystal anisotropy becomes too large, and if the temperature is higher than 300 ° C., the crystal is partially recrystallized to make the structure non-uniform, and the crystal grains become coarse to deteriorate stretch flangeability.

  In carrying out the final cold rolling in the next step, so-called roughening treatment is not performed in order to avoid an increase in cost.

  Further, the final solution treatment after cold rolling is performed at 500 ° C. or higher, and the solution is sufficiently performed to improve the strength of the product after baking. The holding time during solution treatment is preferably at least 5 seconds so that the compound does not become coarse during cooling, but holding for 120 seconds or more is impractical. In addition, it is good to implement the cooling following this solution solution at 50 degrees C / s or more from the same consideration.

An aluminum alloy sheet having excellent stretch flangeability and post-baking strength can be easily manufactured by carrying out a practical combination of the conditions described so far.
(Example)
Table 12 shows the composition of each of the 12 types of aluminum alloys belonging to the regulation range of the present invention as examples, and as a comparative example of a total of 17 types of aluminum alloys whose components were adjusted to deviate reasonably from the range. Shown in

  As shown in Table 2, the ingots of aluminum alloys cast by DC casting or thin plate continuous casting are hot-rolled and finally annealed through two-step soaking that belongs to the regulation range of the present invention. The test material was obtained. In addition, some of the two-stage soaking conditions include comparative examples that reasonably deviate from the regulation scope of the present invention. Tables 2 and 3 show various test results of the specimens having a thickness of 1.0 mm.

First, the number density of the Cr-containing compound was measured for the test material. This measurement is first performed by mechanically polishing the specimen to a depth of 0.25 mm from the rolled surface and scraping the ground surface with an electron beam probe microanalyzer, ie, EPMA (JXA-8000 series manufactured by JEOL Ltd.). It was. The measurement area was about 0.1 to 0.2 mm ² , the magnification at the time of measurement was × 600, and particles of 0.5 μm or more were measured under an acceleration voltage of 20 kV. Note that it is difficult to detect particles of 0.5 μm or less from the resolution of the apparatus.

  Of all the particles thus detected, the Cr-containing compound was extracted as follows. First, constituent elements of Fe, Mn, Mg, Si and Cr contained in individual particles were analyzed by an EPMA apparatus. In this case, since the obtained quantitative value has a problem in analysis accuracy depending on the size and beam diameter of each particle, it was decided to discriminate the Cr-containing compound by the ratio of the main content element as described below. .

  That is, the total amount (T) of Fe + Mn + Mg + Si + Cr is obtained for each at% amount by an EPMA apparatus. Next, about each particle | grain, the content ratio of each element, such as Fe with respect to 5 total amount value (T), was calculated | required by Fe / T, Mn / T, Mg / T, Si / T, and Cr / T. Of these, those having Cr / T of 0.3 or more are determined to be Cr-containing compounds.

  The number density of Cr-containing compounds can be calculated by dividing the number of Cr-containing compounds obtained by the above analysis method by the measurement area. Next, the area of each particle is determined by the pixel value on the image of the Cr-containing compound, that is, the sum of the smallest units of the squares constituting the image of the Cr-containing compound, and the sum of the particles is divided by the measurement area to obtain the Cr-containing compound. The area ratio is obtained.

  Further, the maximum particle size of each compound can be measured by individually determining the maximum shadow length (maximum length when the particles are projected) of each compound for all detected particles.

  Next, the average crystal grain size of the aluminum alloy material is determined by evaluating the texture of the rolled surface. That is, as described above, a sample material having a thickness of 1.0 mm, which is mechanically ground to a depth of 0.25 mm in the direction perpendicular to the rolling direction, is used by buffing and electropolishing the surface. A sample with adjusted was prepared.

  About this sample, SEM (model JEOL JEM 5410) manufactured by JEOL Ltd. was used, and crystal orientation and crystal grain size were measured by EBSP (Electron Back Scattering (Scattered) Pattern). The region was in the range of 150 μm × 1500 μm, and the measurement step interval was 2 μm. The EBSP measurement / analysis system used was EBSP: TSL (OIM).

  In the case where the average crystal grain size is measured as described above, in the examples of the present invention, it is defined that the deviation of the orientation within ± 15 ° belongs to the same crystal grain. Then, after defining the boundary between crystal grains having an orientation difference of 5 ° or more between adjacent crystal grains as a crystal grain boundary, the average crystal grain size was calculated by the following formula 1.

Average crystal grain size = (Σx) / n (1)
n: Number of measured crystal grains
x: Individual crystal grain size Next, the strength after baking of an aluminum alloy specimen is performed using a tensile test piece whose longitudinal direction is 90 ° with respect to the rolling direction. That is, for this test piece, after obtaining a stress-strain curve by JIS No. 5 tensile test, 0.2% proof stress was obtained, and this was performed three times for each test piece, and the average value was calculated and baked. It was set as the post strength.

  Finally, the stretch flangeability of the aluminum alloy specimen is evaluated by a hole expansion test. First, a 10 mm diameter hole is punched into a 70 × 70 mm test plate, and then a hole is expanded using a conical punch having a diameter of 33 mm. . That is, the test plate was placed with the burrs on the upper surface side (dies side), a wrinkle holding force of 3 tons, and a punch speed of 10 mm / min. Then, the punch is stopped when the edge of the punched hole is broken.

  Then, the hole expansion rate (λ) is calculated by the following formula 2.

λ = (d _s −d ₀ ) / d ₀ × 100 (2)
d _s : bore diameter after fracture
d ₀ : Initial hole diameter before the test Note that the hole inner diameter ( _ds ) after the fracture was measured in the rolling direction and in the direction perpendicular thereto, respectively, and the hole expansion ratio was obtained, and the average value was used to expand the hole of each test plate. Rate. Further, this operation was performed three times for each test plate, and the average value was finally used as the hole expansion rate (λ) of the test plate.

  Examples and comparative examples manufactured and tested according to the above procedure are shown in three tables.

  No. Nos. 1 to 12 are aluminum alloys that selectively satisfy the alloy composition characterized by the present invention from the upper and lower limits to the intermediate range (Table 1). The processing conditions for these 12 alloys were also selected within the range characterized by the present invention (Table 2). These test results are shown in Table 3.

  No. No. 1 is a product manufactured by processing a basic aluminum alloy of the present invention containing only Mg, Si, Mn and Cr according to the basic conditions of the present invention. Therefore, this alloy sheet clearly shows the characteristics that the average crystal grain size, the number density and area ratio of the Cr-containing compound are within the appropriate range, and the hole expansion ratio indicating stretch flangeability and the strength after baking are sufficiently good. .

  No. No. 2 is an alloy of the present invention that does not actively contain Mn. It retains good characteristics comparable to that of one alloy.

  No. 3, 4 and 5 are the alloys of the present invention additionally containing Fe, Ti or Cu, respectively, and all have good performance.

  No. Nos. 6 and 7 are obtained by increasing the contents of Mg and Si to near the upper limit regulated by the present invention, and an alloy having a performance that may be such an amount of Mg and Si is obtained.

  No. No. 8 is an element in which the optional element Fe is increased to near the upper limit, and the performance is satisfactory.

  No. No. 9 has Mn near the lower limit and Cr near the upper limit. No. 10 has Mn near the upper limit and Cr near the lower limit, both of which satisfy the performance.

  No. 11 and 12 have both Mn and Cr near the lower limit and near the upper limit, but satisfactory results are obtained.

  Next, No. 1 in Table 1 was obtained. The alloy materials 13 to 21 are comparative examples in which the contents of Cr, Mn, Si or Cu, Mg, Fe, Zr, Ti, etc. are reasonably deviated from the regulation range of the present invention. Is the same as in the examples within the scope of the method of the present invention. These alloy materials are uniformly inferior in stretch flangeability, while the post-baking strength varies, and none of these performances is satisfied.

  Finally, no. Nos. 22 to 29 are comparative examples in which an alloy containing an essential element within the regulation range of the present invention was processed under conditions that deviated from the regulation range of the method of the present invention. Although the strength after baking is slightly lower overall, it is observed that the stretch flangeability is uniformly inferior. The cause is that the maximum particle size of the compound in the alloy is generally large, and exceeds the upper limit of 20 μm of the present invention, and the area ratio of the Cr-containing compound is uniformly less than the upper limit of 60% of the present invention. by.

  Although the required levels of the stretch flangeability λ value and the post-baking strength value required for the aluminum alloy sheet are not always determined, A numerical range of 1 to 12 is practically satisfactory.

Claims (3)

Mg: 0.1 to 3.0% by weight (hereinafter simply referred to as%), Si: 0.1 to 2.5%, and Cr: 0.02 to 0.08%, and measuring area 0.1-0.2 mm ² , magnification at the time of measurement is x600, and the number density of Cr-containing compounds is 500-2000 when particles of 0.5 μm or more are measured with EPMA under an acceleration voltage of 20 kV. / Mm ² , and the area ratio of the Cr-containing compound / the area ratio of all the compounds is 60 to 98%, the average crystal grain size of the alloy is 50 μm or less, and the maximum particle size of the compound is 20 μm or less. An aluminum alloy sheet characterized by excellent stretch flangeability with a balance of Al and inevitable impurities and excellent post-baking strength. Mn: aluminum alloy plate having excellent excellent stretch flangeability and high bake strength after according to claim 1, characterized in that it contains 0.05% or less. The aluminum alloy plate having excellent stretch flangeability and high post-baking strength according to claim 1 or 2 , characterized by containing Fe: 0 to 0.1 % .