WO2016190409A1

WO2016190409A1 - High-strength aluminum alloy plate

Info

Publication number: WO2016190409A1
Application number: PCT/JP2016/065682
Authority: WO
Inventors: 克史松本; 康博有賀; 久郎宍戸
Original assignee: 株式会社神戸製鋼所
Priority date: 2015-05-28
Filing date: 2016-05-27
Publication date: 2016-12-01
Also published as: CN107532247B; CN107532247A; US20180363113A1; JP6810508B2; JP2016222959A

Abstract

Provided is an aluminum alloy plate of the Al-Mg-Si type with a specific composition containing a transition element. The structure of said aluminum alloy plate is such that: the average crystal grain diameter is reduced in the presence of fine transition element dispersion particles of nanometer order; the bake hardening property (BH property) is improved together with the strength by work hardening when forming a structural element; when used as an automotive structural element, said alloy plate exhibits increased strength after BH treatment without limiting formability, even after room-temperature aging.

Description

High strength aluminum alloy plate

The present invention relates to an Al—Mg—Si aluminum alloy sheet. The aluminum alloy plate referred to in the present invention is used after being subjected to tempering (T4) such as solution treatment and quenching treatment as a rolled plate such as a hot rolled plate or a cold rolled plate. An aluminum alloy plate before being formed into a structural member. In the following description, aluminum is also referred to as aluminum or Al.

In recent years, due to consideration for the global environment, social demands for weight reduction of vehicles such as automobiles are increasing. In order to respond to such demands, the application of lighter aluminum alloy materials with excellent formability and paint bake hardenability (bake hardness, also referred to as BH property) instead of steel materials such as steel plates as automobile materials Is increasing.

Typical aluminum alloy plates for large panel materials such as automobile outer panels and inner panels are Al-Mg-Si AA to JIS 6000 series Ａ (hereinafter also simply referred to as 6000 series) aluminum alloy plates. The This 6000 series aluminum alloy plate has a composition containing Si and Mg as essential components, and has a low yield strength (low strength) during molding and ensures formability, and artificial aging (hardening) such as paint baking treatment of panels after molding. Yield (strength) is improved by heating at the time of processing, and the necessary baking strength can be secured.

In order to further reduce the weight of automobile bodies, aluminum is used not only for the above-mentioned panel materials but also for automobile structural members such as frames, pillars and other skeletal materials, bumper reinforcements, door beams and other reinforcements. It is desirable to expand the application of alloy materials.
These automobile structural members are required to have higher strength than the automobile panels. For this reason, in order to apply the 6000 series aluminum alloy plate currently used for the said automotive panel material to these frame | skeleton materials or a reinforcing material, it is necessary to further strengthen.

However, it is easy to achieve such high strength without significantly changing the composition and manufacturing conditions of the conventional 6000 series aluminum alloy sheet and without disturbing other properties such as formability. is not.

Conventionally, various control of the size and number density of transition element-based dispersed particles has been proposed as a structure control for improving the properties of the 6000 series aluminum alloy plate as the panel material, such as BH properties. .

For example, in Patent Document 1, as the panel material, an average number density of dispersed particles having a size of 0.5 μm or more is set to 3000 to 20000 / mm ² , and a 6000 series aluminum alloy having improved press formability and bending workability. A board has been proposed.
In this example, the aluminum alloy plate was obtained by subjecting a plate after room temperature aging for 3 months (90 days) after the production of the plate (after tempering treatment) to an artificial aging of 170 ° C. × 20 minutes after applying 2% strain. The 0.2% proof stress after the treatment, even a high one is about 205 MPa.

Further, in Patent Document 2, as the panel material, in order to refine the average crystal grain size of recrystallized grains to 45 μm or less, the average diameter of dispersed particles is set to 0.02 to 0.8 μm, and the average number is set. There has been proposed an aluminum alloy plate that is 1 piece / μm ³ or more and excellent in press formability and hem workability. This Patent Document 2 also describes a plate after room temperature aging for 3 months (90 days) after manufacturing the plate (after tempering treatment), after deep drawing, and after artificial aging treatment at 180 ° C. × 20 minutes. It is 2% proof stress, and even high strength is about 205 MPa.

Japanese Unexamined Patent Publication No. 2007-169740 Japanese Patent No. 3802695

In the control of the size and number density of transition element-based dispersed particles in a conventional 6000 series aluminum alloy plate, the panel material may be used, and the strength after artificial aging treatment is 250 MPa or more at 0.2% proof stress. However, the strength is insufficient for the use of the skeleton material or the reinforcing material, which requires high strength.

On the other hand, even if the artificial aging treatment temperature is increased to, for example, about 200 ° C. to increase the strength, the strength cannot be increased to 250 MPa or more with 0.2% proof stress after the artificial aging treatment.
In addition, the temperature of the artificial aging (curing) treatment is naturally limited and restricted due to processing efficiency and cost, deterioration of the baked paint, strength reduction due to overaging, and there are circumstances where the temperature cannot be increased.

The present invention has been made in order to solve such problems, and for high-strength structural members such as automobiles, without significantly changing the composition and manufacturing conditions of a conventional 6000 series aluminum alloy plate. An object of the present invention is to provide a 6000 series aluminum alloy plate.

In order to achieve this object, the gist of the high-strength aluminum alloy sheet of the present invention includes, in mass%, Mg: 0.3 to 1.5%, Si: 0.3 to 1.5%, As transition elements, Mn: 0.1 to 0.8%, Zr: 0.04 to 0.20%, Cr: 0.04 to 0.20%, Sc: 0.02 to 0.1% An Al—Mg—Si based aluminum alloy plate comprising Al and inevitable impurities, the balance of which is an average crystal grain size of 100 μm or less. In addition, the average equivalent circle diameter of the dispersed particles containing the transition element measured by TEM-EDX at a magnification of 50,000 times is in the range of 50 to 300 nm, and the equivalent circle diameter is in the range of 20 to 400 nm. der average number density of the transition element dispersion particles 5 / [mu] m ³ or more It is assumed that.

In the present invention, on the premise that the conventional aluminum alloy composition and manufacturing conditions are not greatly changed, the influence on the BH property of the transition element-based dispersed particles in the structure of the 6000 series aluminum alloy sheet was reviewed.
As a result, it has been found that when a substantial amount (substantial number) of nanometer-level fine transition element-based dispersed particles are present in the structure of a 6000 series aluminum alloy plate, the BH property is remarkably improved.

This is because the fine transition element-based dispersed particles not only improve the proof strength (strength) by heating during the artificial aging (hardening) treatment such as the paint baking treatment of the structural member formed with the raw material plate, but also in the previous stage This is because work hardening at the time of forming the material plate on the structural member is also improved.
That is, the fine transition element-based dispersed particles have a synergistic effect of improving work hardenability (strength) at the time of forming the raw material plate and improving proof stress (strength) by artificial aging treatment of the molded structural member. There is a unique effect of improving the performance.
Moreover, the fine transition element-based dispersed particles do not hinder the formability of the material plate to the structural member.

For this reason, the present invention is based on the premise that the average crystal grain size at the center of the thickness of the 6000 series aluminum alloy plate is made fine (small), and a substantial amount of the fine transition element-based dispersed particles is present, By defining both the equivalent diameter and the average number density, high strength without hindering moldability for automotive structural members such as skeletal materials and reinforcing materials that require higher strength than conventional panel materials. Can be

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

(Chemical composition)
First, the chemical composition of the Al—Mg—Si (hereinafter also referred to as 6000) aluminum alloy sheet of the present invention will be described below. In the present invention, the strength of the skeleton material or the reinforcing material is increased without greatly changing the conventional composition and manufacturing conditions.

In order to satisfy such a problem from the viewpoint of composition, the composition of the 6000 series aluminum alloy sheet includes Mg: 0.3 to 1.5% and Si: 0.3 to 1.5% in mass%. As transition elements, Mn: 0.1 to 0.8%, Zr: 0.04 to 0.20%, Cr: 0.04 to 0.20%, Sc: 0.02 to 0.1% One or two or more of them are included, and the balance is made of Al and inevitable impurities. In addition,% display of content of each element means the mass% altogether.

The content range and significance of each element in the 6000 series aluminum alloy, or the allowable amount will be described below.

Mg: 0.3-1.5%
Mg, together with Si, forms aging precipitates that contribute to strength improvement during solid solution strengthening and artificial aging treatment such as paint baking treatment, and exhibits age hardening ability, and the required proof strength as a structural member for automobiles, etc. It is an essential element for obtaining.
If the Mg content is less than 0.3%, the strength is insufficient. On the other hand, if the Mg content exceeds 1.5%, a shear band is easily formed during cold rolling, which causes cracks during rolling. Therefore, the Mg content is 0.3% or more, preferably 0.4% or more and 1.5% or less, preferably 1.2% or less.

Si: 0.3 to 1.5%
Si, together with Mg, forms aging precipitates that contribute to strength improvement during solid solution strengthening and artificial aging treatment such as paint baking treatment, and exhibits age-hardening ability, which is necessary for structural members such as automobiles. It is an essential element for obtaining (yield strength).
If the Si content is less than 0.3%, the strength is insufficient. On the other hand, if the Si content exceeds 1.5%, a coarse compound is formed and ductility is deteriorated. Therefore, the Si content is 0.3% or more, preferably 0.7% or more, and 1.5% or less, preferably 1.4% or less.

Mn, Zr, Cr, Sc
The transition elements of Mn, Zr, Cr, and Sc are main elements constituting the transition element-based dispersed particles defined in the present invention. These form transition element-based dispersed particles in the ingot stage or the final plate to refine the crystal grains and improve the strength. Further, as defined in the present invention, when these transition element-based dispersed particles are refined to the nanometer level, the work hardenability at the time of forming the material plate into the structural member, which is the pre-stage of the artificial aging treatment, is also improved. Together with the improvement of artificial age hardening ability (BH property) of the molded structural member, the strength after BH is remarkably improved.

If the content of Mn, Zr, Cr, and Sc is too small, the number density of the dispersed particles decreases, and the work curability and artificial age hardening ability (BH property) at the time of forming the structural member of the plate decrease. As a result, the increase in yield strength is reduced, the synergistic effect is reduced, and the strength after the artificial aging treatment does not increase to the level required for the structural member.
On the other hand, when there is too much content of Mn, Zr, Cr, and Sc, it becomes easy to form a coarse compound, and it becomes easy to become a starting point of destruction, on the contrary, ductility and intensity | strength are degraded.
Therefore, one of Mn: 0.1 to 0.8%, Zr: 0.04 to 0.20%, Cr: 0.04 to 0.20%, Sc: 0.02 to 0.1% or Including two or more.

Other Elements Others In the present invention, Cu: 0.5% or less (excluding 0%), Ag: 0.01 to 0.2%, Sn: One or more of 0.001 to 0.1% may be included.

By the way, these elements have the effect of increasing the strength of the plate in common, so they can be regarded as high-strength synergistic elements, but the specific mechanism has, of course, common parts and different parts. .
Cu improves the strength by solid solution strengthening or the like. Ag is useful for improving artificial age hardening ability (BH property), and promotes precipitation of compound phases such as GP zones in crystal grains of a plate structure under conditions of artificial aging treatment at a relatively low temperature and short time. effective. Sn captures atomic vacancies, suppresses diffusion of Mg and Si at room temperature, suppresses an increase in strength at room temperature (room temperature aging), releases vacancies captured during artificial aging treatment, It has the effect of promoting the diffusion of Mg and Si and increasing the BH property.

However, if the content of each of these elements is too large, it becomes difficult to produce a plate due to the formation of a coarse compound, and the strength, bending workability, and corrosion resistance also deteriorate. In particular, when Cu content is too large, bending workability is remarkably lowered. Therefore, when these elements are contained, the content is not more than the above upper limit values.

Impurities Other Fe, V, Ti, B, Zn, etc. are unavoidable impurities that are likely to be mixed from scrap as a raw material for melting ingots. The content allowed by standards such as For example, 0.5% or less is preferable for Fe.

(Organization)
In the present invention, the structure of the 6000 series aluminum alloy plate is defined after the chemical component composition of the above 6000 series alloy is used. That is, on the premise that the average crystal grain size at the center of the plate thickness is made fine (small), a substantial amount of the fine transition element-based dispersed particles are present, and the average equivalent circle diameter and the average number density are determined. It prescribes together.

These structure rules are intended to improve the BH property of the 6000 series aluminum alloy plate and increase the strength on the premise that the conventional aluminum alloy composition and manufacturing conditions are not significantly changed or the formability is not deteriorated. Is an important and essential means.

Average grain size:
Making the average crystal grain size finer (smaller) at the center of the plate thickness is a prerequisite for exerting the effect of the fine transition element-based dispersed particles. That is, when the crystal grain structure in the central portion of the plate thickness becomes a fine crystal grain structure having an average crystal grain size of 100 μm or less, the effect of the fine transition element-based dispersed particles is exhibited for the first time. When the crystal grain structure in the central portion of the plate thickness becomes a coarse crystal grain structure with an average crystal grain size exceeding 100 μm, the effect of the fine transition element-based dispersed particles is reduced to half or reduced. In that sense, crystal grain refinement can be said to be a precondition for guaranteeing the effect of fine transition element-based dispersed particles.

The crystal grain size referred to in the present invention is the crystal grain size in the rolling direction at the center of the plate thickness in the longitudinal section in the rolling direction of the plate (the cross section of the plate cut along the rolling direction). The average crystal grain size is measured by a line intercept method in the rolling direction. That is, a sample at the center of the plate thickness of the longitudinal section was taken from a T4 tempered plate before being formed into a structural member, the sample surface was mechanically polished, and then a solution of tetrafluoroboric acid: water = 5: 400 In this, electrolytic etching is performed at a voltage of 30 V, a solution temperature of 20 to 30 ° C., and a time of 60 to 90 seconds. Then, with a 50 × optical microscope using a polarizing plate, in order to take into account variations in the material of the plate, 10 measurement points at the central portion of the plate thickness (5 lines per field, line length per line is 500 μm) ) By visual observation.

Transition element-based dispersed particles:
Assuming the above average crystal grain size, the average circle equivalent diameter of the transition element-based dispersed particles measured by TEM-EDX at a magnification of 50,000 times as the texture at the center of the plate thickness is 50 to 300 nm. The average number density of the transition element-based dispersed particles in the range and the equivalent circle diameter in the range of 20 to 400 nm is 5 particles / μm ³ or more.
Fine transition elements having an average equivalent circle diameter of transition element-based dispersed particles in the range of 50 to 300 nm and an equivalent circle diameter in the range of 20 to 400 nm in the structure of the 6000 series aluminum alloy plate before artificial aging treatment When the number of system dispersed particles is as many as possible (average number density of 5 particles / μm ³ or more), the BH property is remarkably improved.
Although the mechanism by which these transition element-based dispersed particles improve BH properties is still unclear, improvement in work hardening characteristics when prestraining is applied, and heat treatment equivalent to baking coating treatment of dislocations introduced by prestraining It is presumed that the transition element-based dispersed particles having the above-mentioned size and number density contribute particularly to the suppression of recovery of the above. Moreover, such fine transition element-based dispersed particles also have an excellent effect of not hindering the formability of the material plate to the structural member.

The transition element-based dispersed particles having the average equivalent circle diameter or the transition element-based dispersed particles having the equivalent circle diameter in the above range not only improve the proof strength (strength) due to heating during the artificial aging treatment, but also in the preceding stage. Work hardenability at the time of forming the material plate on the automobile structural member is also improved. Thereby, the fine transition element-based dispersed particles, due to the synergistic effect of improvement of work hardening at the time of molding the raw material plate to the automobile structural member, and improvement of the artificial age hardening ability of the automobile structural member after molding, BH property is remarkably improved.

Even if the average equivalent-circle diameter of the transition element-based dispersed particles is reduced to less than 50 nm, and conversely, it is increased beyond 300 nm, the synergistic effect of improving the strength after BH is not exhibited or reduced.

Even if the average number density of fine transition element-based dispersed particles having an equivalent circle diameter in the range of 20 to 400 nm is less than 5 particles / μm ³ , the number of fine transition element-based dispersed particles is too small, and The effect is not demonstrated. In the present invention, the average number density of the transition element-based dispersed particles that can be measured with the TEM-EDX at the magnification of 50,000 times is not specified regardless of the size (equivalent circle diameter).
From the production limit, the upper limit of the average number density of the fine transition element-based dispersed particles having a circle-equivalent diameter in the range of 20 to 400 nm is about 100 / μm ³ .

The reason why the average number density of such fine transition element-based dispersed particles decreases is that the constituent transition element content is insufficient. However, in addition to this, even if the content of the constituent transition elements is appropriate, the generated transition element-based dispersed particles cannot grow due to the problem of the plate manufacturing method, so that the equivalent circle diameter of 20 nm is not small. There is also a possibility that it is too much (too fine), or conversely, it is too coarse beyond 400 nm.

Measurement of transition element-based dispersed particles:
The average number density (particles / μm ³ ) of fine transition element-based dispersed particles having a circle-equivalent diameter in the range of 20 to 400 nm defined in the present invention is EDX (energy) for identifying (identifying) the dispersed particles. It is measured by a TEM (transmission electron microscope: FE-TEM) with a magnification of 50,000 times having a function of dispersive X-ray spectroscopy.
The plate to be measured is a rolled plate such as a hot-rolled plate or a cold-rolled plate, and after being subjected to tempering such as solution treatment and quenching treatment (T4 material), it is work hardenability. Therefore, the structural member used is made of an aluminum alloy plate before being subjected to forming such as bending.

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 before molding, and then filming with a TEM at a magnification of 50,000 times. Image of center tissue photograph is processed, and the equivalent circle diameter of the transition element-based dispersed particles that can be identified (identified) within the measurement visual field (total area of observation field of 4 μm ² or more) and the equivalent circle diameter can be measured Measure all.
Then, the average equivalent circle diameter of the transition element-based dispersed particles is measured, and the average number density (number / μm ³ ) of the transition element-based dispersed particles having an equivalent circle diameter in the range of 20 to 400 nm is measured.
Here, the measurement of the average equivalent circle diameter and the average number density is performed on 10 samples collected from an arbitrary central portion of the plate thickness, and these are averaged to obtain a transition element having an equivalent circle diameter in the range of 20 to 400 nm. The average number density of the system dispersed particles (pieces / μm ³ ) is used.

The transition element-based dispersed particles referred to in the present invention are one or more of Mn, Cr, Zr, and Sc by analyzing the visual field at the center of the plate thickness observed by TEM using an X-ray spectrometer (EDX). Are identified as transition element-based dispersed particles (precipitates) containing, and are distinguished from other precipitates (dispersed particles) that do not contain these transition elements. In this identification, the amount containing one or more of Mn, Cr, Zr, Sc may be any amount (a trace amount) that can be detected by the EDX soot, and Mn, Cr, Zr from the precipitate in the field of view. If one or more of Sc can be detected by the EDX, the transition element-based dispersed particles referred to in the present invention are used regardless of the amount.

Here, the equivalent circle diameter is obtained by performing image processing on the dispersed particles identified (identified) as transition element-based dispersed particles containing one or more of Mn, Cr, Zr, and Sc by EDX. The area of each transition element-based dispersed particle in the TEM visual field is calculated and converted into a diameter (equivalent circular diameter) when converted into a circle having the same area (equivalent circular diameter).

(Production method)
Next, a method for producing the aluminum alloy plate of the present invention will be described below. The aluminum alloy sheet of the present invention is a conventional process or a known process, and the aluminum alloy ingot having the above-mentioned 6000 series component composition is subjected to homogenization heat treatment after casting, and then subjected to hot rolling and cold rolling to obtain a predetermined process. And is subjected to a tempering treatment such as a solution hardening treatment.

However, in these production steps, there are preferable production conditions as described later in order to obtain the structure defined by the present invention (average number density and average crystal grain size of transition element-based dispersed particles).

(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. This homogenization heat treatment (uniform heat treatment) is the usual purpose, in addition to the homogenization of the structure (eliminating segregation in the crystal grains in the ingot structure), the transition element-based dispersed particles are refined to the nanometer level. In other words, it is important not to coarsen.
For this reason, the soaking condition may be one soaking, two soaking or two-stage soaking, but it is necessary to control the temperature rise and cooling process.

The second soaking means that after the first soaking, the steel is once cooled to a temperature of 200 ° C. or less including room temperature, reheated, and maintained at that temperature for a certain period of 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 first soaking condition in the first soaking, or the second soaking, or the first soaking condition in the two-stage soaking suppresses the coarsening of the transition element-based dispersed particles in a temperature range of 500 ° C. or higher and lower than the melting point. In order to control the size and the number density to be specified, the size is appropriately selected from a range of holding time of 1 minute or more and 1 hour or less.
The heating rate during the first soaking in the first soaking or the second soaking or the first soaking in the two-stage soaking is rapid heating of 100 ° C./hr or more, and the transition element-based dispersed particles are In order to precipitate as finely as possible to the nanometer level, it is preferable to increase the temperature rising rate. If the rate of temperature rise is as low as less than 100 ° C./hr, the transition element-based dispersed particles become coarse and may not be deposited as finely as possible on the nanometer level.
On the other hand, the cooling rate after the first soaking of the second soaking is 40 ° C./hr or less, preferably 30 ° C./hr or less, the cooling rate after the first soaking of the second step soaking, Contrary to conventional methods, it is preferable to make the cooling rate as small (slow) as possible, such as cooling in the furnace instead of cooling outside.
This promotes precipitation and growth of the nanometer-level fine transition element-based dispersed particles during cooling, and controls the size and number density to be specified.
Further, in the one-time soaking step, it is preferable that the cooling rate after this one-time soaking process is as small as possible (slow) as below 40 ° C./hr.

The soaking conditions for the second or second stage are selected from the range of the holding time of 30 minutes or more in the temperature range of the hot rolling start temperature or higher and 500 ° C. or lower. It is preferable to heat and cool to the hot rolling start temperature, or to reheat to the hot rolling start temperature and hold 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 lower than the soaking temperature at the first or first stage. In addition, even under a single soaking condition, the same effect can be obtained by controlling the time until the hot rolling start temperature after soaking.
In addition, the heating to the soaking temperature in the second or second stage and the cooling rate after soaking are such that the transition element-based dispersed particles of a desired form are formed in the first or first soaking process. Therefore, as long as it satisfies the hot rolling conditions described below, it is not particularly necessary to set the heating rate and cooling rate within the same condition range.

(Hot rolling)
The hot rolling of the ingot that has been subjected to the homogenization heat treatment includes a rough rolling process of the ingot (slab) and a finish rolling process according to the thickness of the rolled sheet. In these rough rolling process and finish rolling process, a reverse or tandem rolling mill is appropriately used.

The starting temperature of hot rough rolling as the hot rolling start temperature is preferably 350 ° C. or higher and the solidus temperature or lower in the first soaking step, and 350 ° C. or higher and 400 ° C. or lower in the second soaking step. If the starting temperature of hot rough rolling is less than 350 ° C., hot rolling becomes difficult in any soaking process material, and conversely, if it exceeds 400 ° C., transition element-based dispersed particles become coarse in the 2-soaking process material. There is a high possibility that the particles cannot be deposited as finely as possible to the nanometer level. In addition, with regard to the one-time soaking process material, after performing the soaking time in a predetermined time range, immediately performing hot rolling suppresses the coarsening of the transition element-based dispersed particles, and makes a transition in a desired form. Hot rolling can be performed while maintaining the element-based dispersed particles.

After such hot rough rolling, hot finish rolling is preferably performed with an end temperature in the range of 300 to 350 ° C. When the finishing temperature of this hot finish rolling is too low, such as less than 300 ° C., the rolling load becomes high and the productivity is lowered. On the other hand, in order to make a recrystallized structure without leaving a lot of processed structure, when the finish temperature of hot finish rolling is increased, when this temperature exceeds 350 ° C., the transition element-based dispersed particles precipitate coarsely, There is a high possibility that it cannot be deposited as finely as possible on the nanometer level.

(Hot rolled sheet annealing)
Annealing (roughening) of the hot-rolled sheet before cold rolling is not necessary, but may be performed.

(Cold rolling)
In cold rolling, the hot-rolled sheet is rolled to produce a cold-rolled sheet (including a coil) having a desired final thickness. However, in order to further refine the crystal grains, the cold rolling rate is desirably 30% or more, and intermediate annealing may be performed between the cold rolling passes for the same purpose as the above roughening. .

(Solution and quenching)
After the cold rolling, solution treatment and subsequent quenching to room temperature are performed. For this solution hardening treatment, a normal continuous heat treatment line may be used. However, in order to obtain a sufficient solid solution amount of each element such as Mg and Si, after performing solution treatment at a temperature of 550 ° C. or higher and a melting temperature or lower, the average cooling rate to room temperature is 20 ° C./second or higher. It is preferable to do. When the temperature is lower than 550 ° C., the re-solid solution of the compound such as the Mg—Si compound generated before the solution treatment becomes insufficient, and the solid solution Mg amount and the solid solution Si amount decrease.

In addition, when the average cooling rate is less than 20 ° C./second, Mg—Si-based precipitates are mainly generated during cooling, and the solid solution Mg amount and the solid solution Si amount are decreased. The possibility that the amount cannot be secured increases. In order to ensure this cooling rate, the quenching treatment is performed by selecting water cooling means and conditions such as air cooling such as a fan, mist, spray, and immersion, respectively.

(Preliminary aging treatment: reheating treatment)
After such a solution treatment, it is preferable to quench the steel sheet and cool it to room temperature, and then subject the cold-rolled sheet to a pre-aging treatment (reheating treatment) within one hour. If the room temperature holding time from the end of the quenching process to room temperature until the start of the pre-aging treatment (heating start) is too long, Si-rich Mg—Si clusters are generated due to room temperature aging, and the balance between Mg and Si is good It becomes difficult to increase the Mg—Si cluster. Accordingly, the shorter the room temperature holding time is better, the solution treatment and quenching treatment and the reheating treatment may be continued so that there is almost no time difference, and the lower limit time is not particularly set.

In this preliminary aging treatment, the holding time at 60 to 120 ° C. is preferably held for 5 hours or more and 40 hours or less. As a result, Mg—Si clusters having a good balance between Mg and Si are formed.

When the preliminary aging temperature is less than 60 ° C. or the holding time is less than 10 hours, the Si-rich Mg—Si cluster is suppressed and the balance between Mg and Si is the same as in the case where the preliminary aging treatment is not performed. However, it is difficult to increase the Mg-Si cluster, and the proof stress after baking coating tends to be low.

On the other hand, if the pre-aging condition exceeds 120 ° C or exceeds 40 hours, the amount of precipitation nuclei is too much, the strength during bending before baking coating becomes too high, and the bending workability deteriorates. It's easy to do.

EXAMPLES Hereinafter, the present invention will be described more specifically with reference to examples. However, the present invention is not limited by the following examples, but may be appropriately modified within a range that can meet the purpose described above and below. It is also possible to implement, and they are all included in the technical scope of the present invention.

As an example of the present invention, 6000 series aluminum alloy plates having different average number density and average crystal grain size of the fine transition element based dispersed particles at the nanometer level were produced by changing the composition and production conditions. And BH property (paint bake hardenability) after board manufacture was measured and evaluated. These results are shown in Tables 1, 2, and 3.

The specific method of making the 6000 series aluminum alloy sheet having the composition shown in Table 1 is performed by changing production conditions such as soaking conditions and hot rough rolling start temperature as shown in Tables 2 and 3. It was.
Here, in the display of the content of each element in Table 1, the display in which the numerical value of each element is blank indicates that the content is below the detection limit (0%).

The specific production conditions for the aluminum alloy plate were as follows. Aluminum alloy ingots having respective compositions shown in Table 1 were commonly melted by DC casting. Subsequently, the ingot was subjected to soaking treatment under different conditions shown in Tables 2 and 3 in each example, and then hot rough rolling was started at different temperatures shown in Tables 2 and 3, respectively.
In each example, hot finish rolling was performed with an end temperature in the range of 300 to 350 ° C. to obtain a hot rolled sheet having a thickness of 4.0 mm. In common with each example, this hot-rolled sheet is cold-rolled at a processing rate of 50% without rough annealing after hot-rolling or intermediate annealing in the middle of the cold-rolling pass. A board was used.

Furthermore, each cold-rolled sheet was tempered (T4) with heat treatment equipment under the same conditions as in each example. Specifically, the solution treatment is performed at 550 ° C. × 30 minutes. At this time, the average heating rate up to the solution treatment temperature is 10 ° C./second, and after the solution treatment, the average cooling rate is 100 ° C./second. It was cooled to room temperature by performing water cooling. Immediately after this cooling, the preliminary aging treatment was immediately carried out at 100 ° C. for 8 hours, and after the preliminary aging treatment, it was gradually cooled (cooled).

A test plate (blank) is cut out from each final product plate after being left at room temperature for 2 weeks after the tempering treatment, and the average equivalent circle diameter (nm) of the transition element-based dispersed particles of each test plate is 20 to 400 nm. Structures such as the average number density (particles / μm ³ ) and the average crystal grain size (μm) of the transition element-based dispersed particles having a circle-equivalent diameter in the range of were measured by the measurement method described above.
Moreover, the BH property of each test plate was measured and evaluated. These results are shown in Tables 2 and 3.

(BH property)
0.2% proof stress (As proof strength: T4 material = proof strength of the plate before forming and BH), 0.2% proof stress after stretching 2%, artificial age hardening treatment after 2% stretching Then, 0.2% yield strength after BH was obtained by a tensile test.
The 2% stretch simulates bending as forming of a material plate into a structural member, and the artificial age hardening (BH) was performed at 185 ° C. for 20 minutes.
Tables 2 and 3 show the As 0.2% yield strength, the 0.2% yield increase after the 2% stretch, and the 0.2% yield strength after the BH in this order.

In the tensile test, JIS Z 2201 No. 5 test piece (25 mm × 50 mmGL × plate thickness) was sampled from each test plate, and a tensile test was performed at room temperature. The tensile direction of the test piece at this time was made parallel to the rolling direction. Further, the distance between the scores was 50 mm, the tensile speed was 5 mm / min, and the test was performed at a constant speed until the test piece broke. The N number for the measurement of mechanical properties was 5, and each was calculated as an average value.

As shown in Tables 1 and 2, Invention Examples 1 to 13 are produced within the component composition range of the present invention and in a preferable condition range. For this reason, as shown in Table 2, each of these invention examples has a structure as defined in the present invention (transition element-based dispersed particles having an average equivalent circle diameter of the transition element-based dispersed particles and a circle equivalent diameter in the range of 20 to 400 nm. Average number density, average crystal grain size).

As a result, as shown in Table 2, each of these inventive examples has high 0.2% proof stress after BH and high strength even after aging at room temperature. That is, it can be seen that the amount of increase in 0.2% proof stress after the 2% stretch is as high as 33 MPa or more, and the work hardenability at the time of forming the material plate into the automobile structural member is improved.
And it turns out that BH property has improved remarkably with 262 MPa or more by the synergistic effect with the improvement of the yield strength (strength) at the time of the artificial aging treatment of the automotive structural member after shaping | molding.

On the other hand, Comparative Examples 14 to 21 in Table 3 use the same alloy examples 1 and 4 as the invention examples in Table 1. However, in each of these comparative examples, as shown in Table 3, the production conditions such as the soaking condition and the hot rough rolling start temperature are not preferable.
In Comparative Example 14, the soaking temperature is less than 500 ° C. and too low.
In Comparative Examples 15, 18, and 21, the heating rate during soaking is less than 100 ° C./hr, which is too slow.
In Comparative Example 19, the cooling rate after soaking exceeds 40 ° C./hr and is too fast.
Comparative Examples 15, 16, and 18 are two soaking steps, but the start temperature of hot rough rolling exceeds 400 ° C. and is too high.
Although the comparative examples 17 and 20 are 2 times soaking processes, the starting temperature of hot rough rolling is less than 350 degreeC, and is too low.
For this reason, in these comparative examples, the average number density and average crystal grain size of the transition element-based dispersed particles having an equivalent circle diameter in the range of 20 to 400 nm deviate from the structure defined in the present invention.
As a result, compared with the invention example having the same alloy composition, even if the 0.2% yield strength increase after the 2% stretch is high, it is as low as about 30 MPa, and a synergistic effect with the improvement in the yield strength at BH is exhibited. Furthermore, even if the BH property after aging at room temperature is high, it is inferior to about 248 MPa.

Although Comparative Examples 22 to 27 in Table 3 are manufactured in a preferable range of conditions, Alloy Nos. 9 to 14 whose alloy compositions in Table 1 are out of the scope of the present invention are used.
In Comparative Examples 22 and 23, the contents of Mn, Zr, Cr, and Sc of alloy numbers 9 and 10 in Table 1 are too small.
In Comparative Example 24, the Mg content of Alloy No. 11 in Table 1 is too small.
In Comparative Example 25, the Mg content of Alloy No. 12 in Table 1 is too large.
In Comparative Example 26, the Si content of Alloy No. 13 in Table 1 is too small.
In Comparative Example 27, the Si content of Alloy No. 14 in Table 1 is too large.
For this reason, as shown in Table 3, these comparative examples show the average number density and average crystal grain size of the transition element-based dispersed particles having an equivalent circle diameter in the range of 20 to 400 nm from the structure defined in the present invention. It is off.
As a result, even if the amount of increase in 0.2% yield strength after 2% stretch is high, it is as low as 30 MPa, a synergistic effect with improvement in yield strength during BH is not exhibited, and even if the BH property after aging at room temperature is high. It is inferior to about 244 MPa.

Therefore, the results of the above examples confirm that it is necessary to satisfy all the compositions and structures defined in the present invention in order to increase the strength even after aging at room temperature.

Although the present invention has been described in detail and with reference to specific embodiments, it will be apparent to those skilled in the art that various changes and modifications can be made without departing from the spirit and scope of the invention.
This application is based on a Japanese patent application filed on May 28, 2015 (Japanese Patent Application No. 2015-108596), the contents of which are incorporated herein by reference.

According to the present invention, a high-strength 6000 series aluminum alloy plate can be provided. As a result, the application of the 6000 series aluminum alloy plate can be expanded as an automobile structural member such as a frame material such as a frame or a pillar excluding the panel material, or a reinforcing material such as a bumper reinforcing material or a door beam.

Claims

In mass%, Mg: 0.3-1.5% and Si: 0.3-1.5% are contained, respectively, and as transition elements, Mn: 0.1-0.8%, Zr: 0.04 Al—Mg containing one or more of Cr: 0.04 to 0.20%, Sc: 0.02 to 0.1%, the balance being Al and inevitable impurities A transition element measured by TEM-EDX having an average crystal grain size of 100 μm or less and having a mean grain size of 100 μm or less as a structure at the center of the thickness of the Si-based aluminum alloy plate The average equivalent circle diameter of the dispersed particles containing is in the range of 50 to 300 nm, and the average number density of the transition element-based dispersed particles in the range of the equivalent circle diameter in the range of 20 to 400 nm is 5 particles / μm 3 or more. High strength aluminum alloy sheet.
The aluminum alloy plate further includes one of Cu: 0.5% or less (excluding 0%), Ag: 0.01 to 0.2%, Sn: 0.001 to 0.1%, or The high-strength aluminum alloy sheet according to claim 1, comprising two or more kinds.