JP2016160516A - Aluminum alloy sheet - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a 6000-based aluminum alloy sheet capable of improving impact absorption property during impact of a vehicle without reducing intensity.SOLUTION: Existence of a Sn-based compound which precipitates with necessity in a crystal grain boundary of a 6000-based aluminum alloy sheet having a specific composition and produced by a conventional method, is regulated for preventing breakage of the grain boundary, and a solid solubility of elements including Sn, is secured, thereby, without reducing intensity, impact absorption property during impact of a vehicle which is evaluated in a VDA bending test in Fig. 1 is improved.SELECTED DRAWING: Figure 1

Description

  The present invention relates to a 6000 series aluminum alloy plate produced by ordinary rolling (ordinary method), and relates to a high-strength 6000 series aluminum alloy plate excellent in shock absorption.

  Taking automobiles as an example of structural materials, in recent years, due to consideration for the global environment, social demands for reducing the weight of automobile bodies are increasing. In order to respond to such demands, parts such as panels (outer panels such as hoods, doors and roofs, inner panels), reinforcements such as bumper force (bumper R / F) and door beams, etc. An aluminum alloy material is applied instead of a steel material such as a steel plate.

  However, in order to further reduce the weight of an automobile body, it is necessary to expand the application of aluminum alloy materials to structural members such as frames and pillars that contribute particularly to weight reduction among automobile members. However, these automobile structural members are required to further increase the strength of the material plate and to impart the characteristic of shock absorption = breakdown resistance at the time of collision of the vehicle body as compared with the automobile panel.

  Among high-strength reinforcing materials among automotive structural members such as bumper reinforcing materials and door beams, extruded shapes produced by hot extrusion of JIS or AA7000 series aluminum alloys are already widely used as raw materials. Yes. On the other hand, large structural members such as frames and pillars may be made of a rolled plate manufactured by a conventional method such as hot rolling after soaking or further cold rolling the ingot. preferable. However, the above-mentioned 7000 series aluminum alloy is difficult to make as a rolled plate because of its high alloy, and has not been practically used so far.

  For this reason, as an alloy for a rolled sheet manufactured by normal rolling (ordinary method), it is an Al—Mg—Si aluminum alloy that is easy to make because it is a lower alloy than the 7000 series, JIS to AA6000. Aluminum alloys are attracting attention.

  This 6000 series aluminum alloy plate is already used as a large body panel (outer panel or inner panel such as a hood, fender, door, roof, trunk lid, etc.) of an automobile. For this reason, in order to combine and improve the press formability and BH properties (bake hard properties) required for large body panels of these automobiles, conventionally, metallurgical improvement measures such as component composition and structure, Many proposals have been made.

  As one of them, there is a method in which Sn is positively added to a 6000 series aluminum alloy plate to suppress room temperature aging and improve BH properties. For example, Patent Document 1 proposes a method that combines room temperature aging suppression and BH properties by adding an appropriate amount of Sn and applying preliminary aging after the solution treatment. Patent Document 2 proposes a method for improving formability, baking paintability, and corrosion resistance by adding Sn and Cu for improving formability to a 6000 series aluminum alloy plate.

Further, in Patent Document 3, a 6000 series aluminum alloy plate positively added with these conventional Sns, as a material for an outer panel of an automobile, has good formability after long-time aging, high BH property, and excellent In order to combine and combine various properties such as yarn rust resistance, the structure of the central portion of the cross section perpendicular to the rolling direction of the plate after heat treatment of the 6000 series aluminum alloy plate at 170 ° C. for 20 minutes is obtained. It has been proposed to increase the average number density of precipitates having a size of 2.0 to 20 nm in crystal grains as measured with a transmission electron microscope to 5.0 × 10 ²¹ pieces / μm ³ or more. .

  This precipitate is an intermetallic compound containing Mg and Si that is generated for the first time in the crystal grains of the plate (panel) during the above-mentioned artificial age hardening treatment or the actual baking coating hardening treatment. For this reason, even if it is a high magnification TEM, it cannot observe in the structure | tissue of a raw material board (former structure | tissue) before shaping | molding into a panel, and before the said baking coating hardening process or artificial age hardening treatment.

Therefore, in Patent Document 3, it is determined whether or not the specific structure after the baking coating hardening process or artificial age hardening process is the previous structure even in the state of the plate, not the previous structure of the plate. doing.
And, in this way, the fine structure of 2.0 to 20 nm generated in the crystal grains during the baking coating hardening process is present in the structure of the plate more than a certain number density in the crystal grains. Thus, even after long-term aging at room temperature, heme workability (formability) is ensured with low yield strength during press molding, and high strength can be achieved by high BH properties during the baking coating curing process.
As a result, the yield strength after 100 days of room temperature after the production of the plate is set to 100 MPa or less, and the amount of hardening (BH property) by baking coating can be made 90 MPa or more.

JP 09-249950 A JP-A-10-226894 JP 2014-162962 A

However, these conventional 6000 series aluminum alloy plates with positive addition of Sn are generally used as materials for automobile outer panels, with good formability after long-term aging at room temperature, high BH properties, and excellent yarn rust resistance. The common purpose (problem) is to combine various characteristics such as these.
This is the same for many conventional technologies in which the structure of the area ratio and number density of Mg-Si compounds (precipitates) and clusters is controlled as a 6000 series aluminum alloy plate not containing Sn. It is.

  On the other hand, as described above, the automotive structural members such as frames and pillars used by the present invention do not require press formability and the like, as described above, and further increase the strength. In addition, a characteristic peculiar to this application is required, such as giving a shock absorption property at the time of a vehicle body collision = new pressure resistance.

  As an example of this, in recent years, due to the level-up (strictening) of crash safety standards for automobiles, in Europe and the like, the automobile structural members such as the frames and pillars are standardized by the German Automobile Manufacturers Association (VDA) “VDA238. -100 Plate bending test for metallic materials (hereinafter referred to as VDA bending test) is required to satisfy the impact absorption (crushing characteristics) at the time of automobile collision.

  With respect to such strict safety standards, the conventional 6000 series aluminum alloy plate for automobile panels described above, even if Sn is added, the shock absorption at the time of a vehicle body collision is enhanced even when the strength is increased. = Insufficient pressure resistance.

  As a means for satisfying the impact absorption (crushing characteristics) at the time of automobile collision without reducing the strength of the 6000 series aluminum alloy sheet produced by ordinary rolling, a 6000 series aluminum alloy to which Sn is added. Not only plates, but effective means are still unknown, and there is still room for clarification.

  In view of such a situation, the object of the present invention is to provide a shock-absorbing property (crushing characteristic) at the time of a car collision without reducing the strength of a Sn-added 6000 series aluminum alloy plate manufactured by ordinary rolling. ).

In order to achieve this object, the gist of the aluminum alloy sheet of the present invention is, in mass%, Mg: 0.2 to 1.5%, Si: 0.4 to 1.5%, Sn: 0.001 to Each contains 0.1%, Cu: 0.02-0.5%, Zr: 0.02-0.15%, Mn: 0.03-0.2%, Cr: 0.02-0 An aluminum alloy plate containing one or more of 15%, the balance being Al and inevitable impurities, the equivalent circle diameter existing at the grain boundary of this plate being 0.2 to 10 μm The average number density of Sn-based compounds in the range of 0.4 / μm ³ or less (including 0 / μm ³ ) is assumed.

  The present invention focuses on the precipitates at the grain boundaries of the 6000 series aluminum alloy plate, and when Sn is added, depending on the manufacturing conditions, the Sn series compounds (precipitates containing Sn) may become grain boundaries in the plate structure. It was found that it is likely to precipitate preferentially.

  When this Sn-based compound is preferentially precipitated at the grain boundaries in the plate structure, Mg-Si-based precipitates may easily precipitate around the Sn-based compound as a nucleus. I found out.

Thus, when Sn-based compounds are preferentially precipitated at the grain boundaries, the impact absorbability (crush characteristics) at the time of automobile collision, which is evaluated by the VDA bending test, is reduced, contributing to solid solution strengthening. The amount of Sn dissolved is also reduced.
Further, when Mg-Si based precipitates are deposited around Sn-based compounds as nuclei, solid solution Mg or solid solution Si that should be generated as a cluster contributing to BH properties during artificial age hardening treatment. The amount is also reduced.
As a result, it is not possible to improve the shock absorption at the time of automobile collision without reducing the strength.

  In contrast, the present invention suppresses precipitation of Sn-based compounds at grain boundaries of Sn-added 6000-based aluminum alloy rolled sheets, and precipitates Mg-Si-based precipitates with Sn-based compounds as nuclei. Also suppress. As a result, the solid solution amount of Sn contributing to solid solution strengthening is ensured, and the amount of solid solution Mg and solid solution Si to be generated during the artificial age hardening treatment as a cluster contributing to BH properties is also ensured. As a result, it is possible to improve the impact absorbability (crush characteristics) at the time of automobile collision without reducing the strength.

  As described above, according to the present invention, a 6000 series aluminum alloy plate suitable for structural members such as automobiles and railway vehicles, which requires high strength and high shock absorption characteristics, can be obtained by a conventional method.

It is a perspective view which shows the aspect of the VDA bending test which evaluates shock absorption.

  The aluminum alloy sheet referred to in the present invention is a cold-rolled sheet that is hot-rolled after the soaking of the ingot and further cold-rolled, and is further subjected to tempering such as solution treatment. It refers to a 6000 series aluminum alloy plate manufactured by the method (ordinary method). In other words, it is manufactured by casting a thin plate by the twin roll method etc. and omitting hot rolling or cold rolling, or by a special manufacturing method that forges the ingot and repeats hot rolling many times. The plates that are made are not included in the scope of the present invention.

  The 6000 series aluminum alloy plate of the present invention manufactured by a conventional method is subjected to molding and processing including stretch flange processing (burring processing, hole expansion processing) and the like, and is a structural member for automobiles, bicycles, railway vehicles, etc. It is said.

  Hereinafter, embodiments of the present invention will be specifically described for each requirement.

Aluminum alloy composition:
First, the chemical component composition of the aluminum alloy sheet of the present invention will be described below, including reasons for limiting each element. In addition,% display of content of each element means the mass% altogether.

The chemical composition of the aluminum alloy plate of the present invention is an Al—Mg—Si based aluminum alloy to guarantee the characteristics required for structural members such as automobiles, which have both strength and shock absorption (crushing characteristics). It is determined. From this point of view, the chemical composition of the aluminum alloy sheet of the present invention is mass%, Mg: 0.2 to 1.5%, Si: 0.4 to 1.5%, Sn: 0.001 to 0.1. %: Cu: 0.02-0.5%, Zr: 0.02-0.15%, Mn: 0.03-0.2%, Cr: 0.02-0.15% 1 type or 2 types or more are contained, and the remainder consists of Al and an unavoidable impurity. This composition may further contain Ag: 0.01 to 0.2% by mass.
In addition,% display of content of each element means the mass% altogether.

Si: 0.4 to 1.5%
Si, together with Mg, forms Mg-Si-based precipitates that contribute to strength improvement during solid solution strengthening and artificial aging treatment such as baking coating treatment, and exhibits age-hardening ability, strength required as an automotive structural member It is an essential element for obtaining (yield strength). If the Si content is too small, the amount of dissolved Si before baking coating treatment (before artificial aging treatment) decreases and the amount of Mg-Si-based precipitates is insufficient, so that the BH property is significantly lowered. On the other hand, when there is too much Si content, a coarse crystallization thing and a precipitate will be formed and a big board crack will arise during hot rolling. Therefore, the Si content is in the range of 0.4 to 1.5%, preferably in the range of 0.7 to 1.5%.

Mg: 0.2 to 1.5%
Mg, together with Si, forms a Mg-Si-based precipitate that contributes to strength improvement during solid solution strengthening and artificial aging treatment such as baking coating treatment, exhibits age-hardening ability, and necessary proof strength as an automotive structural member It is an essential element for obtaining. If the Mg content is too small, the amount of solid solution Mg before baking coating treatment is reduced, and the amount of Mg-Si based precipitates is insufficient, so that the BH property is remarkably lowered. On the other hand, when there is too much Mg content, a coarse crystallization thing and a precipitate will be formed and a big plate crack will arise during hot rolling. Therefore, the Mg content is in the range of 0.2 to 1.5%, preferably in the range of 0.65 to 1.5%.

Sn: 0.001 to 0.1%
Sn has the effect of suppressing the cluster formation at room temperature and maintaining the excellent formability of the plate after solution treatment and quenching treatment for a long time, and further subjected to artificial aging treatment such as baking coating treatment after that. Increase the strength of the case. For this reason, it is an indispensable element for obtaining the required proof stress and impact absorbability (crush property) as an automobile structural member. If the Sn content is less than 0.001%, the effect is small. If the Sn content exceeds 0.1%, the effect is saturated, and hot brittleness is generated, and hot workability (hot ductility) is remarkably deteriorated. Let Therefore, the Sn content is in the range of 0.001 to 0.1%.

One or more of Cu, Zr, Mn, and Cr These elements can be regarded as synergistic elements because they have the effect of increasing the strength of the plate in common. But there are of course different parts.
Cu improves the strength by solid solution strengthening, and can obtain the necessary proof stress as an automobile structural member. If there is too little content of Cu, the effect will be small, and even if there is too much. The effect is saturated and on the contrary, the corrosion resistance is deteriorated.
Zr, Mn, and Cr improve the strength by refining the crystal grains of the ingot and the plate. If each of these contents is less than the lower limit, the effect is insufficient. On the other hand, when these contents exceed the respective upper limit values, a coarse compound is formed, and the impact absorbability (crush characteristics) is deteriorated.
Therefore, when one or more of these Cu, Zr, Mn, and Cr are contained, Cu: 0.02 to 0.5%, Zr: 0.02 to 0.15%, Mn: 0.00. Each range of 03-0.2%, Cr: 0.02-0.15%, preferably Cu: 0.02-0.3%, Zr: 0.02-0.1%, Mn: 0.0. It is set as each range of 03-0.15% and Cr: 0.02-0.1%.

Ag: 0.01-0.2%
Ag has an effect of closely and finely precipitating aging precipitates that contribute to strength improvement by artificial aging treatment after forming processing into a structural material, and has the effect of promoting high strength. Therefore, Ag is selectively contained as necessary. . If the Ag content is less than 0.01%, the effect of improving the strength is small. On the other hand, if the Ag content is too large, various properties such as rollability and weldability are deteriorated, and the effect of improving the strength is saturated and only expensive. Therefore, the Ag content in the case of inclusion is in the range of 0.01 to 0.2%.

Other elements:
Other elements such as Ti, B, Fe, Zn, and V other than those described above are unavoidable impurities. Ti, together with B, forms a coarse compound and degrades mechanical properties. However, since the inclusion of a small amount also has the effect of refining the crystal grains of the aluminum alloy ingot, each content in the range specified by the JIS standard is allowed as a 6000 series alloy. As an example of this allowable amount, Ti is 0.1% or less, preferably 0.05% or less. Further, B is set to 0.03% or less.

  In addition, other elements such as Fe, Zn, and V are also assumed (allowed) in addition to pure aluminum ingots as raw materials for melting ingots. Each alloy is allowed in the range specified by JIS standard of the base alloy.

Board structure:
As a premise, the 6000 series aluminum alloy plate of the present invention has the same composition and many production steps as the conventional 6000 series aluminum alloy plate and its production method (normal rolling method). For this reason, the fact that a large number of fine nano-sized precipitates exist in the crystal grains as the structure of the plate is the basis for satisfying basic properties such as strength and corrosion resistance. Yes. These fine nano-level precipitates are the intermetallic compounds of Mg and Zn (composition is MgZn _{2 and the} like) that are generated in the crystal grains, and include elements corresponding to the composition. It is a finely dispersed phase.

Grain boundary structure:
On the premise of the composition of the above 6000 series aluminum alloy plate and the structure in the crystal grain, the present invention newly controls the precipitates of the grain boundaries in the plate structure. That is, in order to combine the impact absorption (crushing characteristics) and strength at the time of automobile collision, evaluated by the VDA bending test, a compound (precipitate) at the grain boundary in the crystal grains of this 6000 series aluminum alloy sheet structure. Suppress.

Specifically, the average number density of Sn-based compounds having an equivalent circle diameter in the range of 0.2 to 10 μm existing in the crystal grain boundaries of the aluminum alloy plate having the above composition and grain structure is 0.4. Suppress (control) to the smallest possible number of / μm ³ or less. Here, this average number density includes the case where the Sn-based compound of the above-mentioned size is not detected or does not exist at the crystal grain boundary and is 0 / μm ³ . More preferably, the range is 0.2 pieces / μm ³ or less.

When the average number density of Sn-based compounds having an equivalent circle diameter in the range of 0.2 to 10 μm exceeds 0.4 / μm ³ , both strength and impact absorption at the time of automobile collision are reduced. . As a result, as one standard for structural members, the 0.2% proof stress is 200 MPa or more when the plate is subjected to solution treatment / quenching treatment and artificial aging treatment, and 75 ° or more in the VDA bending test. It becomes impossible to have the crushing characteristic which becomes a bending angle.

  Here, the reason why the equivalent circle diameter of the Sn compound existing in the crystal grain boundary is in the range of 0.2 to 10 μm is that the fine Sn-based compound having an equivalent circle diameter of less than 0.2 μm is one of the grain boundary fractures. It is difficult to cause this, and accurate measurement with a TEM described later is also difficult. In addition, a coarse Sn-based compound having an equivalent circle diameter exceeding 10 μm hardly occurs in the range of the above-described conventional method, and if it occurs, the plate itself is caused by hot rolling cracking or cold rolling cracking. It is because cannot be manufactured.

Paying attention to the compound at the grain boundary of the 6000 series aluminum alloy plate, when Sn is added, depending on the production conditions of the plate, the compound containing Sn, that is, the compound in which Sn is detected in the compound (precipitate) = Sn A system compound (precipitate containing Sn) tends to preferentially precipitate at the grain boundaries in the plate structure.
In other words, in the Sn-added 6000 series aluminum alloy rolled sheet produced by a conventional method, the Sn series compound tends to preferentially precipitate at the crystal grain boundaries.

When this Sn-based compound is preferentially precipitated at the grain boundaries in the plate structure, Mg-Si-based precipitates are likely to be deposited around the Sn-based compound as a nucleus.
As described above, when the Sn-based compound is preferentially precipitated at the crystal grain boundaries, the grain boundary fracture is likely to occur, and the impact absorbability (crush characteristics) at the time of the automobile collision, which is evaluated by the VDA bending test described above, is It decreases, and the solid solution amount of Sn contributing to solid solution strengthening also decreases.

Further, if the Mg-Si-based precipitate is deposited around the Sn-based compound as a nucleus, grain boundary fracture is more likely to occur and should be generated as a cluster that contributes to BH properties during artificial age hardening. The amount of solid solution Mg and solid solution Si is also reduced.
As a result, the strength also decreases, and the shock absorption at the time of a car collision cannot be improved.

Incidentally, in the present invention, only the average number density of Sn-based compounds existing at the grain boundaries is defined, and the average number density of Mg-Si based precipitates present at the grain boundaries is not measured or defined. . This is because the presence state of Mg—Si based precipitates at the crystal grain boundaries can be represented by the average number density of Sn based compounds present at the crystal grain boundaries. That is, when the average number density of the Sn-based compound in the equivalent circle diameter range existing at the grain boundary exceeds 0.4 / μm ³ , the plate manufactured by a conventional method inevitably has Mg This is because the -Si-based precipitate is deposited around the Sn-based compound by an amount that affects the grain boundary fracture.

These are phenomena and problems peculiar to the 6000 series aluminum alloy sheet to which Sn is added.
As long as a normal 6000 series aluminum alloy sheet not containing Sn is produced according to the established production conditions (ordinary method), a specific compound such as Sn is preferentially precipitated at the grain boundaries. In other words, the phenomenon that Mg—Si-based precipitates precipitate around the Sn-based compound as a nucleus does not occur.

These facts are supported by the examples to be described later, but when coarse precipitates (compounds) are precipitated at the grain boundaries, the impact absorbability decreases because of other aluminum alloy plates and extruded materials. It can be understood from known examples of the above and general technical common sense. However, in order to reach such recognition, as a premise, in a Sn-added 6000 series aluminum alloy rolled sheet produced by a conventional method, Sn-based compounds are likely to precipitate preferentially at the grain boundaries. It is indispensable to recognize that Si-based precipitates are likely to precipitate around Sn-based compounds as nuclei.
In addition, the ordinary 6000 series aluminum alloy sheet to which Sn is not added is a common sense that a specific compound will not preferentially precipitate at the grain boundaries as long as it is produced by a conventional method. Unless such a test result (knowledge) is obtained, the recognition that the Sn-based compound preferentially precipitates at the grain boundary cannot be achieved.

As described above, the present invention suppresses the precipitation of the Sn-based compound at the crystal grain boundaries of the Sn-added 6000-based aluminum alloy rolled sheet, thereby using the Sn-based compound of the Mg-Si-based precipitate as a nucleus. Also suppresses precipitation. As a result, the solid solution amount of Sn contributing to solid solution strengthening is ensured, and the amount of solid solution Mg and solid solution Si to be generated during the artificial age hardening treatment as a cluster contributing to BH properties is also ensured.
As a result, it is possible to improve the impact absorbability (crush characteristics) at the time of automobile collision without reducing the strength.

  Specifically, as a material plate subjected to solution / quenching treatment and artificial aging treatment, or as a structural member subjected to artificial aging treatment after forming a solution treatment / quenching treatment plate, 0.2% proof stress of 200 MPa or more is provided. In addition, it may have a crushing characteristic that results in a bending angle of 75 ° or more in the VDA bending test. For this reason, the 6000 series aluminum alloy plate suitable for structural members, such as the said motor vehicle and a railway vehicle, in which such a characteristic is requested | required can be manufactured by a conventional method.

Measurement of grain boundary structure:
The average number density (pieces / μm ³ ) of Sn-based compounds having an equivalent circle diameter in the range of 0.2 to 10 μm existing at the grain boundaries defined in the present invention is EDX (identification of compounds containing Sn). It is measured by a TEM (transmission electron microscope) with a magnification of 5000 having an energy dispersive X-ray spectroscopy function.
The average number density of the Sn-based compounds existing at the grain boundaries is almost the same and almost the same in the plate after solution treatment (T4 material) or the plate or structural member after artificial aging treatment (T6 material). is there. Therefore, the measurement target may be either a plate after solution treatment (T4 material), a plate after artificial aging treatment (T6 material), or a structural member.

A specific measurement method is to obtain a thin film sample for TEM by taking a sample at the center of the plate thickness from the T4 material or the T6 material, and filming with a TEM of 5000 times. The structure photograph of the part is subjected to image processing, and the equivalent circle diameter of the Sn-based compound found at the crystal grain boundary within the measurement visual field is measured. Then, the average number density (pieces / μm ³ ) of the Sn-based compound having an equivalent circle diameter in the range of 0.2 to 10 μm is measured.
Measurement of the average number density of the Sn-based compound is performed on 10 samples collected from an arbitrary center portion of the plate thickness, and these are averaged to obtain the average number density of the Sn-based compound (pieces / μm ³ ).

  Here, the equivalent circle diameter is obtained by performing image processing on the Sn-based compound identified (identified) as a compound containing Sn by the DEDX, and calculating the area of the Sn-based compound in the TEM field of view. This is a value (circle equivalent diameter) converted to a diameter (equivalent circle diameter) when converted to a circle of the same area. Therefore, it can be said that the compound containing Sn referred to in the present invention is a compound that can be identified (or can be identified) as a compound containing Sn by the DEDX at the grain boundary.

(Production method)
The 6000 series aluminum alloy sheet of the present invention is a cold-rolled sheet obtained by subjecting an ingot to hot rolling after soaking and then cold rolling, and further subjected to tempering such as solution treatment. Manufactured. That is, an aluminum alloy hot rolled sheet having a thickness of about 2 to 10 mm is manufactured through normal manufacturing processes such as casting, homogenization heat treatment, and hot rolling. Subsequently, it is cold-rolled to obtain a cold-rolled sheet having a thickness of 3 mm or less.

Therefore, the 6000 series aluminum alloy plate of the present invention is not manufactured by a special manufacturing method or rolling method in which cold rolling is performed after continuous thin plate casting such as a twin roll method to omit hot rolling or warm rolling is performed. .
However, as the rapid cooling conditions after heating to obtain the grain boundary structure defined in the present invention, in particular, the end temperature of hot rolling and the rapid cooling (cooling rate) after hot rolling are as usual, as described later. The conditions are different from the process.

(Dissolution, casting cooling rate)
First, in the melting and casting process, an ordinary molten casting method such as a continuous casting method and a semi-continuous casting method (DC casting method) is appropriately selected for the molten aluminum alloy adjusted to be dissolved within the above-mentioned 6000 series component composition range. Cast.

(Homogenization heat treatment)
Next, the cast aluminum alloy ingot is subjected to a homogenization heat treatment prior to hot rolling. The purpose of this homogenization heat treatment (soaking) is to homogenize the structure, that is, eliminate segregation in crystal grains in the ingot structure. The conditions for this soaking process, including the cooling rate, have no effect on the grain boundary structure defined in the present invention, and may be normal one-time soaking, or may be two-time soaking or two-stage soaking. . In one soaking, the hot rolling is started by cooling to the hot rolling start temperature or holding at or near the hot rolling start temperature.

  In the second soaking, after the first soaking, the steel sheet is once cooled to a temperature of 200 ° C. or less including room temperature, reheated, and maintained at that temperature for a certain time, and then hot rolling is started. On the other hand, the two-stage soaking means cooling after the first soaking, but it is not cooled to 200 ° C. or lower, and after stopping the cooling at a higher temperature, the temperature is maintained as it is. The hot rolling is started after reheating to a higher temperature.

  The soaking condition for the first soaking in only one time, or the first soaking in the second soaking, or the soaking condition in the second soaking is in the temperature range of 500 ° C. or higher and lower than the melting point, and the holding time range of 2 hours or more. Is appropriately selected.

  In the second soaking, after the first soaking, the temperature is once cooled to 200 ° C. or less including room temperature for the second soaking. In the two-stage soaking, after the first stage soaking, the temperature is once cooled to a temperature higher than 200 ° C. The soaking condition for the second or second stage is selected from the range of the holding time of 2 hours or more in the temperature range of the hot rolling start temperature or higher and 500 ° C. or lower. It may be heated and cooled to the hot rolling start temperature, or reheated to the hot rolling start temperature and held in the vicinity thereof. Further, the ingot after soaking at the first stage may be cooled to the hot rolling start temperature and held in the vicinity thereof. The soaking temperature at the second or second stage is preferably lower than the soaking temperature at the first or first stage.

(Hot rolling)
In the hot rolling, the hot rolling itself becomes difficult because burning occurs under conditions where the hot rolling start temperature exceeds the solidus temperature. On the other hand, when the hot rolling start temperature is less than 350 ° C., the load during hot rolling becomes too high, and the hot rolling itself becomes difficult. Therefore, the hot rolling start temperature is selected from the range of 350 ° C. to the solidus temperature and hot rolled to obtain a hot rolled plate having a thickness of about 2 to 10 mm. Annealing (roughening) of the hot-rolled sheet before cold rolling is not always necessary, but may be performed.

  In this hot rolling, in order to obtain a grain boundary structure defined in the present invention, it is necessary to control the end temperature of hot rolling and the cooling rate after hot rolling under conditions different from those in the conventional method.

End temperature at hot rolling:
The end temperature at the time of hot rolling is as low as possible below 250 ° C. When the end temperature at the time of hot rolling exceeds 250 ° C., even if the cooling rate from the end temperature of hot rolling to room temperature is fast, for example, the Sn compound is particularly likely to precipitate at the grain boundary. Since it stays in the temperature range, the Sn compound easily precipitates at the grain boundary during cooling, and the Mg—Si based compound also precipitates at the grain boundary starting from the Sn based compound.
Therefore, if the hot rolling end temperature is 250 ° C. or less, the residence time in the temperature range of 200 ° C. to the solidus temperature where the Sn compound is likely to precipitate at the grain boundary is short, and the Sn-based compound is formed during hot rolling. , Suppressing coarsening, reducing the presence of undissolved Sn-based compounds during the subsequent solution treatment, and improving the effect of improving BH properties by Sn. Furthermore, the number density of Sn-based compounds present at grain boundaries and the like can be reduced. However, the end temperature at the time of hot rolling is limited due to the restriction of whether or not it can be hot rolled, and the lower limit of the end temperature at the time of hot rolling is preferably 200 ° C.

Average cooling rate at the end of hot rolling:
The average cooling rate from the end temperature of hot rolling (250 ° C. or lower) to room temperature is 100 ° C./hr or higher. For this reason, it is preferable to quench with a fan or mist immediately after the end of hot rolling and before or after winding on the hot rolling coil. The average cooling rate is preferably as high as possible depending on the alloy composition such as the Sn content, and more preferably 200 ° C./hr or more. More preferably, it is 300 ° C./hr or more.
If the cooling rate is higher than the above, the formation and coarsening of the Sn-based compound during cooling after hot rolling is suppressed, and the number density of the Sn-based compound having the above-mentioned size existing at the crystal grain boundary of the plate is reduced. Can do. As a result, the presence of undissolved Sn-based compound during the subsequent solution treatment of the cold-rolled sheet is reduced, and the Sn-based compound having a circle equivalent diameter in the range of 0.2 to 10 μm that exists at the crystal grain boundary of the sheet. The average number density can be 0.4 pieces / μm ³ or less.

When this cooling rate is slow, even if the end temperature of hot rolling is 250 ° C. or less, the Sn compound stays in the temperature range of 200 to the solidus temperature where it is particularly likely to precipitate at the grain boundary. In this temperature range, the Sn compound is likely to precipitate at the grain boundaries. For this reason, the Mg—Si compound is also precipitated at the grain boundaries starting from the Sn compound.
As a result, even if the solution treatment / quenching process (especially rapid cooling) described later is performed within a preferable range, the undissolved Sn-based compound at the time of the solution treatment cannot be reduced, and exists at the grain boundaries of the plate. The average number density of the Sn-based compounds in the size range cannot be 0.4 pieces / μm ³ or less.

(Cold rolling)
In cold rolling, the hot-rolled sheet is rolled into a cold-rolled sheet (including a coil) having a desired final thickness of about 1 to 5 mm for automobile structural members. The number of cold rolling processes is freely selected according to the relationship between the thickness of the hot rolled sheet and the final thickness of the cold rolled sheet, and the sheet (coil) to the cold rolling mill in this cold rolling process. The number of passes can be freely selected.

  The intermediate annealing in the middle of the roughening or cold rolling may cause the formation of Sn-based compounds and coarsening during cooling after annealing, and the number density of Sn-based compounds existing at the grain boundaries of the plate is reduced. There is a possibility that it cannot be reduced, and is unnecessary. When absolutely necessary, the temperature is in the range of 380 to 500 ° C., and an appropriate time is selected according to the sheet passing conditions in the continuous furnace or batch furnace to be used. Cooling after annealing is performed after the hot rolling. As with rapid cooling, forced cooling with a fan or mist is essential.

(Solution treatment)
After cold rolling, solution treatment is performed as a tempering. The solution treatment is not particularly limited and may be heating and cooling using a normal continuous heat treatment line. However, in order to obtain a sufficient solid solution amount of each element and refinement of crystal grains, solution treatment is performed at a temperature of 500 ° C. or higher and a solidus temperature or lower and a holding time of 1 second to 30 minutes. It is desirable to process.

  The average cooling (temperature decrease) rate after the solution treatment is the same as the above-described hot rolling end temperature and cooling rate, and the range of 200 ° C. to solidus temperature is 40 to 200 ° C./s. That is, in order to suppress grain boundary precipitation of the Sn compound during cooling, the average cooling rate in the temperature range is preferably 40 ° C./s or more. For this reason, the cooling after the solution treatment is performed by selecting and using forced cooling means such as air cooling such as a fan, water cooling means such as mist, spraying, and immersion, or directly in water or hot water at room temperature to 100 ° C. It is preferable to quench.

If this average cooling rate is too slow, the Sn compound tends to precipitate at the grain boundaries, and the plate will stay in the temperature range from 200 ° C. to the solidus temperature for a long time. In addition, Mg-Si compounds are also precipitated at grain boundaries starting from Sn compounds. The upper limit of the average cooling (temperature lowering) rate after the solution treatment is set to 200 ° C./s due to equipment restrictions.
By the way, the solution treatment is basically only once, but when the room temperature age hardening has progressed too much, the solution treatment is performed again under the above-mentioned preferable conditions in order to ensure the formability to automobile members and the like. Then, the room temperature age hardening that has progressed too much may be canceled once.

  By the above process, as one standard for structural members, the 0.2% proof stress when the plate is artificially aged is set to 200 MPa or more, and the crushing becomes a bending angle of 75 ° or more in the VDA bending test. Can have properties.

  The aluminum alloy plate of the present invention is used as a structural member for automobiles, bicycles, railway vehicles and the like after being subjected to press molding and processing including burring processing, hole expansion processing, and the like as materials. Further, from the viewpoint of securing moldability, after being molded or processed into these structural members, an artificial age hardening treatment is separately performed to increase the strength as necessary.

(Artificial age hardening treatment)
This artificial age hardening treatment may be performed at the stage of the material plate or after the material plate is formed into a structural member. The treatment conditions may be general artificial aging conditions (T6, T7), and the temperature and time conditions are freely determined from the desired strength, the strength of the 6000 series aluminum alloy plate of the material, or the progress of aging at room temperature. The For example, in the case of one-stage aging treatment, aging treatment at 100 to 150 ° C. is performed for 12 to 36 hours (including an overaging region). In the two-stage process, the first-stage heat treatment temperature is in the range of 70 to 100 ° C. for 2 hours or longer, and the second-stage heat treatment temperature is in the range of 100 to 170 ° C. for five hours or longer (over-aged region). Select from).

  Mechanical properties such as strength and VDA bending test of the grain boundary structure of the cold-rolled sheet of 6000 series aluminum alloy having each component composition shown in Table 1 with various production conditions as shown in Table 2 The impact absorbability (crushing property) evaluated in (1) was evaluated. These results are also shown in Table 2.

  As shown in Table 2, the grain boundary structure of the cold-rolled sheet was controlled by changing the soaking condition and the solution treatment condition. Specifically, in common with each example, a 6000 series aluminum alloy molten metal having each component composition shown in Table 1 below is DC-cast, and the resulting ingot is subjected to soaking conditions and hot rolling conditions shown in Table 2. Then, hot rolling was performed to produce a hot rolled sheet having a thickness of 5 mm. In particular, the hot rolling conditions controlled the hot rolling end temperature and the cooling rate after the hot rolling, and in the case of rapid cooling, each example was rapidly cooled to room temperature at that cooling rate.

  These hot-rolled sheets were cold-rolled without any roughening (annealing) and intermediate annealing in common with each example, and a cold-rolled sheet having a thickness of 2 mm was obtained in common. And these cold-rolled plates are shown in Table 2 with the solution treatment temperature (attainment temperature: ° C.), holding time (s), and average cooling rate (200 ° C. to solidus temperature range: ° C./s). Solution treatment was performed under each condition to obtain a T4 material.

  After aging these T4 materials at room temperature for 1 week, sample materials were collected, textures and fine precipitates (reference) were investigated, and mechanical properties were examined by a tensile test described later. . These results are shown in Table 2, respectively.

(Grain boundary structure)
As the grain boundary structure of the T4 material, the average number density (pieces / μm ³ ) of Sn compounds having an equivalent circle diameter in the range of 0.2 to 10 μm was measured by the method described above.

(Fine precipitates in crystal grains)
At this time, in each case, as a reference, when the grain boundary structure of the T4 material was measured with a transmission electron microscope with a magnification of 5000 times, the equivalent circle diameter in the range of 2.0 to 20 nm in the crystal grains of the same sample was used. The average number density (pieces / μm ³ ) of 10 precipitates was measured. As a result, in each of the inventive examples and the comparative examples, the average number density of the precipitates having a size of 2.0 to 20 nm was in the range of 2 to 9 × 10 ⁴ pieces / μm ^{3 on} average. Here, the size of the precipitate was measured in terms of the diameter of a circle having an equivalent area.

  Further, after aging the T4 material at room temperature for 1 week, as a T6 treatment (BH treatment) simulating an artificial age hardening treatment after forming the structural member, the T4 material is subjected to 185 ° C. after 2% stretching. An artificial aging treatment was performed for 20 minutes.

(Mechanical properties)
In each example, the plate test piece of the T6 material or the T4 material was processed into a JIS No. 5 test piece, and a room temperature tensile test was performed so that the tensile direction was parallel to the rolling direction, and the tensile strength (MPa). 0.2% proof stress (MPa) was measured. The room temperature tensile test was performed at a room temperature of 20 ° C. based on JIS2241 (1980), and was performed at a constant speed until the test piece broke at a distance of 50 mm between the ratings and a tensile speed of 5 mm / min.

(Shock absorption)
The bending test for evaluating shock absorption was performed as a VDA bending test in accordance with “VDA238-100 Plate Bending Test for Metallic Materials” in the standards of the German Automobile Manufacturers Association (VDA). This test method is shown in perspective view in FIG.
First, the plate-like test piece of the T6 material is placed on two rolls arranged parallel to each other with a roll gap, as shown by dotted lines in FIG. .
Specifically, the T6 material plate-shaped test piece is rolled so that the rolling direction and the extending direction of the plate-shaped pushing and bending jig arranged vertically upright are perpendicular to each other. It is placed on two rolls horizontally and equally in the left and right so that the central part is located in the center of the gap.
Then, the pressing and bending jig is pressed against the center of the plate-shaped test piece from above to apply a load, and the plate-shaped test piece is bent toward the narrow roll gap (bending) to bend and deform. The center part of the plate-shaped test piece is pushed into the narrow roll gap.

  At this time, the angle of the bending outside of the center part of the plate-like test piece when the load F from the pushing bending jig from the maximum is measured as a bending angle (°), and the magnitude of the bending angle. Evaluate the shock absorption. The larger the bending angle, the more the plate-like test piece is not crushed in the middle, the bending deformation is continued, and the shock absorption (crushing property) is higher.

As test conditions for this VDA bending test, the symbols shown in FIG. 1 are used to indicate that the plate-shaped test piece has a square shape of width b: 60 mm × length l: 60 mm, and each of the two roll diameters D is 30 mm. The roll gap L was 4 mm, which is 2.0 times the plate thickness of the plate-shaped test piece. s is the depth of intrusion into the roll gap at the center of the plate-like test piece when the load F is maximum.
In addition, as shown in FIG. 1, the plate-like pushing / bending jig presses against the center portion of the plate-like test piece so that the lower end side has a radius of 0.2 mmφ at the tip (lower end). It has a sharp tapered shape.
In each example, the above bending test was carried out by three plate-like test pieces (three times), and the average value of the bending angles (°) was adopted.

As is apparent from Tables 1 and 2, each of the inventive examples is within the composition range of the aluminum alloy of the present invention, and is manufactured within the range of the preferred hot rolling conditions and solution treatment conditions described above. As a result, the average number density of Sn-based compounds having an equivalent circle diameter in the range of 0.2 to 10 μm existing at the crystal grain boundaries of this plate is 0.4 / μm ³ or less.

  As a result, it is excellent in impact absorbability (crush property) evaluated in the VDA bending test. Further, the 0.2% proof stress of the T6 material is 220 MPa or higher, and a high strength is 240 MPa or higher.

  On the other hand, in each comparative example, the alloy composition deviates from the scope of the present invention as shown in Table 1, or the alloy composition is within the scope of the present invention as shown in Table 1, but from the range of the preferred hot rolling conditions described above. Each is manufactured. As a result, even if a desired grain boundary structure cannot be obtained or has been obtained, the strength and impact absorbability (crush characteristics) evaluated by the VDA bending test are inferior.

In Comparative Examples 9 to 16, the same alloy examples 1 and 2 as the inventive examples in Table 1 are used. However, in these comparative examples, as shown in Table 2, the hot rolling end temperature is too high (Comparative Examples 9, 10, 13, 14), and the average cooling rate after hot rolling is slow (Comparative Examples 9, 11, 13). 15), and the average cooling rate after the solution treatment is low (Comparative Examples 12 and 16).
Incidentally, Comparative Examples 9 and 13 are models of manufacturing by a conventional method in which a plate containing Sn has a high hot rolling end temperature and a low average cooling rate after hot rolling.
For this reason, in these comparative examples, the average number density of Sn-based compounds having an equivalent circle diameter in the range of 0.2 to 10 μm existing at the crystal grain boundaries of the plate exceeds 0.4 / μm ^3. The strength is lower than the example, and the shock absorption (crushing property) is inferior.

  Although Comparative Examples 17-22 are manufactured in the preferable condition range except Comparative Examples 17 and 19, Alloy Nos. 9 to 14 in Table 1 are used. These alloys have low Sn (Alloy Nos. 9 and 10), too much Sn (Alloy No. 11), low Mg (Alloy No. 12), low Si (Alloy No. 13), Cu, Zr, Mn, Each of them does not contain Cr or Sc (alloy number 14), etc., and is out of the composition range of the present invention.

Among them, Comparative Example 17 is a model for manufacturing a 6000 series plate by a conventional method, which does not contain Sn, has a high hot rolling end temperature, and has a low average cooling rate after hot rolling. For this reason, in Comparative Example 17, as in Comparative Example 18 in which the Sn content is too low, naturally, there is little or no Sn-based compound (average number density) present at the crystal grain boundaries of the plate. Therefore, these comparative examples 17 and 18 are low in strength and do not have both strength and impact absorbability, as can be seen from comparison with invention examples 1 and 5 having the same impact absorbability (crush characteristics).
Although the comparative example 19 had too much Sn content and produced the hot rolling crack, it was not able to manufacture.
In Comparative Examples 20 to 22, the required amount of each element is too small (Alloy Nos. 12, 13, and 14), and the strength is remarkably low as compared with the inventive examples for the impact absorption (crushing characteristics).

  From the above results, the critical significance of each requirement of the present invention is confirmed in order for the aluminum alloy plate of the present invention to have both the impact absorbability (crushing property) evaluated by the VDA bending test and the high strength.

  As described above, the present invention can provide a 6000 series aluminum alloy plate that is manufactured by conventional rolling and has improved impact absorption (crushing characteristics) at the time of automobile collision without reducing strength. . Therefore, the present invention is suitable for structural members such as automobiles, bicycles, and railway vehicles that contribute to weight reduction.

Claims (3)

By mass%, Mg: 0.2-1.5%, Si: 0.4-1.5%, Sn: 0.001-0.1%, respectively, and Cu: 0.02-0. 5%, Zr: 0.02 to 0.15%, Mn: 0.03 to 0.2%, Cr: 0.02 to 0.15%, one or more of them are contained, the balance being Al And an average number density of Sn-based compounds having an equivalent circle diameter in the range of 0.2 to 10 μm, which is present in the crystal grain boundary of the plate, is 0.4 / μm ^3. An aluminum alloy plate characterized by the following (including 0 / μm ³ ).   The aluminum alloy plate according to claim 1, wherein the aluminum alloy plate further contains Ag: 0.01 to 0.2% by mass%. The aluminum alloy plate is for a structural member. The aluminum alloy plate has a 0.2% proof stress of 200 MPa or more as a characteristic after solution treatment, quenching treatment and artificial aging treatment, and 75 in a VDA bending test. The aluminum alloy sheet according to any one of claims 1 and 2, wherein the aluminum alloy sheet has a crushing characteristic that results in a bending angle of not less than °.