JP7318274B2 - Al-Mg-Si-based aluminum alloy cold-rolled sheet and its manufacturing method, and Al-Mg-Si-based aluminum alloy cold-rolled sheet for forming and its manufacturing method - Google Patents

Description

TECHNICAL FIELD The present invention relates to an Al--Mg--Si based aluminum alloy cold-rolled sheet having high strength, high yield strength and excellent impact resistance after solution and aging treatments, and a method for producing the same.

In recent years, climate change due to global warming has become more severe, and there is an urgent need to reduce CO ₂ gas emissions. Therefore, in order to improve the fuel efficiency of automobiles, there is a demand for further weight reduction. For example, 6000 series aluminum alloy sheets are used as various structural members for automobiles. 6000-series aluminum alloy sheets are required to have high strength, excellent formability, and a material capable of absorbing energy at the time of collision. For this reason, aluminum alloy sheets for structural members, which are excellent in balance between strength and crushing properties, have been developed as materials to be applied to various structural members for automobiles.

For example, in Patent Document 1, an aluminum alloy containing Mg: 0.3 to 1.5% and Si: 0.3 to 1.5% by mass, and the balance being Al and unavoidable impurities A plate whose differential scanning calorimetry curve has only one exothermic peak in the temperature range of 200 to 320°C, and the single exothermic peak is in the temperature range of 200 to 270°C and an exothermic peak height of 10 μW/mg or more. According to this, a 6000 series aluminum alloy plate that is manufactured by ordinary rolling and satisfies the balance between strength and crushing properties at the time of an automobile collision without reducing formability into structural members, and aluminum that satisfies the crushing properties It is possible to provide alloy structural members.

Furthermore, in Patent Document 2, the material used for the frame structure of an automobile contains Mg: 0.5 to 1.3% by mass, Si: 0.7 to 1.5% by mass, and Mn: 0.05 to 0.5% by mass. 5% by mass, Zr: 0.04 to 0.20% by mass, and Cr: 0.04 to 0.20% by mass, and the balance is Al and unavoidable impurities. The number density of transition element-based dispersed particles of 0.05 μm or more present in the grain boundary is 0.001/nm or less, and the PFZ width of the grain boundary after artificial aging treatment held at 200 to 250 ° C. for 10 to 30 minutes is An Al--Mg--Si based aluminum alloy plate having a thickness of 60 nm or less has been proposed. According to this, the cold-rolled sheet is subjected to solution treatment, and by strictly controlling the average cooling rate after holding at the solution treatment temperature, the grain boundary of the aluminum alloy plate finally obtained It is said that the number density of dispersed particles of the transition element system existing on the surface can be sufficiently reduced, the PFZ width can be reduced, and the bendability can be improved.

By the way, various structural members for automobiles have generally been manufactured by pressing and forging 6000 series aluminum alloy extruded materials. For example, in Patent Document 3, Si: 0.7 to 1.3% (% by mass, the same applies hereinafter), Mg: 0.55 to 0.95%, Cu: 0.27 to 0.43%, Mn: 0.17 to 0.43%, Cr: 0.07 to 0.23%, and Zr: 0.10 to 0.24%, the balance being Al and unavoidable impurities, and [Si%] × 1.73 - [Mg%] > 0.35%, the central part of the thickness of the cross section has a subgrain structure with an average grain size of 10 μm or less, and the subgrain structure is in the cross section A high-strength and high-toughness aluminum alloy forging material having excellent corrosion resistance is disclosed, which is characterized by having a proportion of 70% or more. According to this, it is possible to provide a high-strength, high-toughness Al--Mg--Si based aluminum alloy forging material which is suitable for use as a vehicle structural member and which is excellent in corrosion resistance.

Furthermore, Patent Document 4 contains Mg: 0.3 to 2.0 mass%, Si: 0.6 to 2.0 mass%, Mn: 0.04% to 0.50 mass% and Cr: 0.04% It is made of an aluminum alloy further containing one or two of ~0.30 mass%, the balance being Al and unavoidable impurities, and after heating to 530 ° C. at a heating rate of 60 ° C./min, adjacent crystal grains It describes an aluminum alloy sheet for hot forming, characterized in that the existence probability of the grain boundary being a small-angle grain boundary of less than 15° is 50% or more. According to this, an aluminum alloy sheet having high high-temperature ductility in a high strain rate region and excellent mass productivity by high-speed forming, and a method for producing the same are provided. In this way, after heating to 530 ° C. at a heating rate of 60 ° C./min, the existence probability that the grain boundaries of adjacent crystal grains are small-angle grain boundaries of less than 15 ° is 50% or more. An aluminum alloy plate is proposed. It is

JP 2017-179469 A JP 2018-100435 A JP 2013-076167 A JP 2016-023354 A Looking Back on Aging Precipitation Studies of Al-Mg-Si Alloys Keikinzoku Vol.63 No.9 (2013), 318-328

Certainly, 6000 series aluminum extrusions or alloy sheets are used as raw materials for press and forging processing, and forged materials are applied to various structural members for automobiles, and have high toughness and excellent impact resistance. . In Patent Document 1, the structure after the artificial aging treatment of the structural member is a structure in which the β″ phase is suppressed and the amount of the β′ phase is increased, and the balance between strength and crushing properties is improved. , defines the exothermic peak value and the number of peaks in the specified temperature range measured by DSC at the stage before the artificial aging treatment of the material plate and structural member.However, the aluminum alloy plate (formed material plate) , Mg, Si, etc., solution treatment and quenching treatment at a holding temperature of 550 ° C. or higher for rolled plates such as hot-rolled plates and cold-rolled plates. Since it is subjected to thermal refining (T4) such as, recrystallized grains may become coarse, depending on the residual plastic strain of the rolled sheet, the alloy composition, and the like.

In addition, in Patent Document 2, the characteristic that the PFZ width is sufficiently reduced is that the aluminum alloy plate (that is, T4 material) after solution treatment is subjected to artificial aging treatment, that is, so that it becomes T6 material. By refining, it is said that it can be expressed particularly remarkably. However, in order to obtain a sufficient solid solution amount of each element such as Mg and Si, the aluminum alloy sheet is subjected to solution treatment and quenching treatment at a holding temperature of 550° C. or higher for the cold rolled sheet. Since the steel is tempered (T4), the recrystallized grains may become coarse, depending on the residual plastic strain of the cold-rolled steel sheet, the alloy composition, and the like.

In Patent Document 3, in order to obtain sufficient strength, the amount of Si and the amount of Mg are further controlled to satisfy the relational expression of [Si%]×1.73−[Mg%]>0.35%. It is said that it is necessary. However, in the excessive Si type 6000 series aluminum alloy plate with a high Si/Mg ratio, although the strength is high, depending on the composition and plate making conditions, the grain boundaries are affected by the aging treatment after the solution treatment, depending on the composition and plate making conditions. There is a concern that relatively coarse intermetallic compounds such as β″ and β′ precipitate as intermediate phases in the steel, degrading the impact resistance.

In Cited Document 4, after heating up to 530°C at a heating rate of 60°C/min, an aluminum alloy plate in which the existence probability of the grain boundaries of adjacent crystal grains being small-angle grain boundaries of less than 15° is 50% or more is produced. It is shown. However, nothing is shown about the impact resistance of the molded article.

From the above, as a base sheet for manufacturing various structural members for automobiles, a 6000 series aluminum alloy cold-rolled sheet having high strength, high yield strength, and excellent impact resistance after solution treatment and aging treatment can be manufactured at low cost. There is a need.
The present invention has been devised to solve such problems, and an Al-Mg-Si-based aluminum alloy cold-rolled sheet that has high strength, high yield strength and excellent impact resistance after solution treatment and aging treatment. and a method for producing the same.

The Al--Mg--Si-based aluminum alloy cold-rolled sheet, which has high strength, high yield strength and excellent impact resistance after solution treatment and aging treatment according to the present invention, has a Si content of 0.50 to 0.50 to achieve its purpose. 90% by mass, Fe: less than 0.70% by mass, Cu: 0.10 to 0.90% by mass, Mg: 0.80 to 1.7% by mass, Mn: 0.10 to 1.3% by mass, Cr : 0.20 to 0.90% by mass, Ti: 0.005 to 0.10% by mass, the balance being Al and unavoidable impurities, and the Si/Mg ratio being in the range of 0.4 to 0.9 It is an aluminum alloy cold-rolled sheet having a composition, and the tensile strength of the test piece with the L direction as the longitudinal direction is defined as UTS _L , and the 0.2% proof stress of the test piece with the L direction as the longitudinal direction is _YSL . Defined, the Charpy value of the test piece with the L direction as the longitudinal direction is defined as _SL , and the Charpy value of the test piece with the LT direction as the longitudinal direction is defined as _SLT , solution treatment at 550 ° C. for 5 minutes The metal structure is a recrystallized structure with an average crystal grain size of less than 20 μm, UTS _L is 340 MPa or more, and YS _L is 300 MPa or more. , S _L is 16.0 J/cm ² or more, and S _LT is 13.0 J/cm ² or more.

In addition, the Al-Mg- _Si -based aluminum alloy cold-rolled sheet of the present invention is subjected to solution treatment at 550 ° C. for 5 minutes and further 175 It is preferable that the _ELL measured after the artificial aging treatment of °C x 14 hours is 15.0% or more.

The Al-Mg-Si-based aluminum alloy cold-rolled sheet for forming, which has high strength, high yield strength and excellent impact resistance after solution treatment and aging treatment of the present invention, has Si: 0.50 to 0.90% by mass, Fe: less than 0.70% by mass, Cu: 0.10 to 0.90% by mass, Mg: 0.80 to 1.7% by mass, Mn: 0.10 to 1.3% by mass , Cr: 0.20 to 0.90% by mass, Ti: 0.005 to 0.10% by mass, and the balance consisting of Al and unavoidable impurities, and the Si / Mg ratio is in the range of 0.4 to 0.9 An aluminum alloy cold-rolled sheet having a certain chemical composition, the tensile strength of a test piece with the L direction as the longitudinal direction is defined as UTS _L , and the 0.2% proof stress of the test piece with the L direction as the longitudinal direction is YS Defined as _L , the Charpy value of the test piece with the L direction as the longitudinal direction is defined as _SL , and the Charpy value of the test piece with the LT direction as the longitudinal direction is defined as _SLT . The metal structure is a recrystallized structure with an average crystal grain size of less than 20 μm, UTS _L is 340 MPa or more, and YS _L is 300 MPa. above, S _L is 16.0 J/cm ² or more, and S _LT is 13.0 J/cm ² or more.

In addition, the Al-Mg-Si-based aluminum alloy cold-rolled sheet for forming of the present invention is subjected to solution treatment at 550 ° C. for 5 minutes, when the elongation of the test piece with the L direction as the longitudinal direction is defined as EL _L. Furthermore, it is preferable that the _ELL measured after the artificial aging treatment at 175°C for 14 hours is 15.0% or more.

The method for producing an Al-Mg-Si-based aluminum alloy cold-rolled sheet according to the present invention comprises continuously casting a slab having a thickness of 3 to 15 mm using a thin slab continuous casting machine from a molten aluminum alloy having the above composition, The slab is directly wound on a roll without being homogenized and hot rolled, and then cold rolled at a final cold rolling rate of 50 to 95% without intermediate annealing.
In the method for producing an Al-Mg-Si-based aluminum alloy cold-rolled sheet for forming of the present invention, a molten aluminum alloy having the above composition is continuously cast into slabs having a thickness of 3 to 15 mm using a thin slab continuous casting machine. Then, the slab is wound directly on a roll without being subjected to homogenization treatment and hot rolling, and then subjected to cold rolling at a final cold rolling rate of 50 to 95% without performing intermediate annealing. do.

The Al--Mg--Si based aluminum alloy cold-rolled sheet of the present invention exhibits high strength, high proof stress and excellent impact resistance after solution treatment and aging treatment. By limiting the Si/Mg ratio of the Al-Mg-Si-based aluminum alloy to the range of 0.4 to 0.9 to reduce the excess Si amount and excess Mg amount, the width of the PFZ generated during artificial aging treatment is reduced. It is possible to suppress the growth of intermetallic compounds such as β″ and β′ that precipitate as intermediate phases at grain boundaries. can do.

The method for producing an Al--Mg--Si-based aluminum alloy cold-rolled sheet of the present invention comprises an Al--Mg--Si system containing appropriate amounts of Mn and Cr and limiting the Si/Mg ratio to the range of 0.4 to 0.9. When the molten alloy is continuously cast by a thin slab continuous casting machine, by setting the slab cooling rate during casting solidification at 1/4 thickness of the slab to 40 to 200 ° C./sec, Al-Fe (Mn Cr) The size of the -Si crystallized product can be made very fine, and the amount of crystallized product can also be reduced. For this reason, the amount of Si solid solution after solution treatment can be kept high, and it is possible to secure the amount of Si that can be effectively used in the strengthening phase Mg—Si-based compound. The strength and yield strength after aging treatment can be increased.

In addition, the method for producing an Al-Mg-Si-based aluminum alloy cold-rolled sheet of the present invention includes an Al-Mg- A molten Si-based alloy is continuously cast by a thin slab continuous casting machine, directly wound on a roll, and cold-rolled to the final thickness without homogenization or intermediate annealing. Cr solid solution amount can be kept high. For this reason, the metal structure after solution treatment can be made into a recrystallized structure with an average grain size of less than 20 μm, and the existence density of grain boundaries increases, resulting in impact resistance after solution treatment and aging treatment. Excellent for

Therefore, it is possible to provide an Al--Mg--Si based aluminum alloy cold-rolled sheet having high strength, high yield strength and excellent impact resistance after solution treatment and aging treatment.

FIG. 1 shows an instrumented Charpy test piece.

Conventional 6000-series aluminum alloy sheets are subjected to solution treatment, cold forming or hot forming, and then aging treatment, depending on their chemical composition and manufacturing process. When applied to various structural members for use, it may be inferior in impact resistance. Moreover, the impact resistance and the anisotropy of the impact resistance when the 6000 series aluminum alloy plate is subjected to solution treatment and aging treatment have not been sufficiently studied. Therefore, the material to be used is required to have high strength and high yield strength after solution treatment and aging treatment, and to have excellent impact resistance in both the rolling direction and the sheet width direction.

As described above, the excessive Si type 6000 series aluminum alloy plate with a high Si/Mg ratio has high strength, but depending on the chemical composition and plate making conditions, aging treatment after solution treatment Precipitation of relatively coarse intermetallic compounds such as β″ and β′ as intermediate phases at grain boundaries may reduce the impact resistance. Also, it is necessary to investigate the impact resistance after solution treatment and aging treatment.

On the other hand, by controlling the Si / Mg ratio (0.4 to 0.9) and using a 6000 series aluminum alloy plate with a reduced amount of excess Si and an amount of excess Mg, the width of the PFZ generated during artificial aging treatment is reduced. It is possible to suppress the growth of intermetallic compounds such as β″ and β′ that precipitate as intermediate phases at the grain boundaries, and the impact resistance can be excellent. However, the balance type Although the 6000 series aluminum alloy sheet has excellent ductility and impact resistance as various structural members for automobiles, depending on its chemical composition and manufacturing process, there is a risk that the strength after artificial aging treatment will decrease. .

Through investigations of Al-Mg-Si-based aluminum alloy cold-rolled sheets with varying Mg/Si ratios, the present inventors have found that Al-Mg The present invention has been achieved through extensive research to obtain a -Si-based aluminum alloy cold-rolled sheet.
The contents are explained below.

First, the action, appropriate content, etc. of each element contained in the Al--Mg--Si-based aluminum alloy cold-rolled sheet of the present invention will be described.
[Si: 0.50 to 0.90% by mass]
Depending on the cooling rate during ingot casting, Si crystallizes fine intermetallic compounds such as Al--(Fe.Mn)--Si, some of which form a solid solution in the matrix to increase strength. In the artificial aging treatment, the Mg--Si-based compound precipitates uniformly and finely to further increase the strength, so it is an essential element.
If the Si content is less than 0.50% by mass, the precipitation amount of the Mg—Si-based compound is also reduced, so there is a possibility that the desired strength and 0.2% yield strength cannot be obtained. If the Si content exceeds 0.90% by mass, the strength and 0.2% yield strength of the aluminum alloy plate are increased, but the impact resistance may be reduced.
Therefore, the Si content should be in the range of 0.50 to 0.90% by mass. A more preferable Si content is in the range of 0.50 to 0.85% by mass. A more preferable Si content is in the range of 0.50 to 0.80% by mass.

[Fe: less than 0.70% by mass]
Depending on the cooling rate during slab casting, Fe crystallizes fine intermetallic compounds such as Al--Fe(Cr/Mn)--Si and increases the strength of the aluminum alloy plate, so it is an essential element. be.
If the Fe content exceeds 0.70% by mass, intermetallic compounds such as Al--(Fe.Mn)--Si are coarsened, which may make it impossible to obtain the predetermined 0.2% yield strength.
Therefore, the Fe content should be less than 0.70% by mass. A more preferable Fe content is in the range of 0.10 to less than 0.70% by mass. A more preferable Fe content is in the range of 0.15 to less than 0.65% by mass.

[Cu: 0.10 to 0.90% by mass]
Cu content is an element that increases the strength, partly dissolves in the matrix to promote solid solution strengthening, and forms an Al-Mg-Si-Cu-based compound to increase the strength, so it is an essential element. is.
If the Cu content is less than 0.10% by mass, there is a possibility that the desired strength and 0.2% yield strength cannot be obtained. If the Cu content exceeds 0.90% by mass, there is a risk that the impact resistance will decrease.
Therefore, the Cu content should be in the range of 0.10 to 0.90% by mass. A more preferable Cu content is in the range of 0.15 to 0.90% by mass. A more preferable Cu content is in the range of 0.20 to 0.85% by mass.

[Mg: 0.80 to 1.7% by mass]
Mg partially dissolves in the matrix and increases strength. In the artificial aging treatment, Mg--Si-based compounds are uniformly and finely precipitated around the fine clusters precipitated in the matrix by natural aging as nuclei, thereby further increasing the strength, so Ni is an essential element.
If the Mg content is less than 0.80% by mass, there is a possibility that the desired strength cannot be obtained. If the Mg content exceeds 1.7% by mass, the specified 0.2% yield strength cannot be obtained, and there is a risk that the impact resistance will decrease. Therefore, the preferred Mg content is in the range of 0.80-1.7% by mass. A more preferable Mg content is in the range of 0.85 to 1.7% by mass. A more preferable Mg content is in the range of 0.85 to 1.6% by mass.

[Mn: 0.10 to 1.3% by mass]
Mn partially dissolves in the matrix to promote solid solution strengthening, and also acts as a grain refiner by delaying recrystallization when solution treatment is performed. Further, within the range of the alloy composition of the present invention, Mn is also an element that constitutes fine intermetallic compounds such as Al--Fe(Mn-Cr)--Si generated during casting and solution treatment, and increases strength. It is an essential element in order to make it high.
If the Mn content is less than 0.10% by mass, the impact resistance may deteriorate. When the Mn content exceeds 1.3% by mass, the amount of intermetallic compounds such as Al—Fe(MnCr)—Si increases, and the precipitation amount of Mg—Si compounds after artificial aging treatment increases. There is a possibility that the predetermined strength and 0.2% proof stress cannot be obtained.
Therefore, the preferred Mn content is in the range of 0.10-1.3% by mass. A more preferable Mn content is in the range of 0.10 to 1.25% by mass. A more preferable Mn content is in the range of 0.15 to 1.2% by mass.

[Cr: 0.20 to 0.90% by mass]
Cr partially dissolves in the matrix to promote solid solution strengthening, and also acts as a grain refining agent by delaying recrystallization during solution treatment. Further, within the range of the alloy composition of the present invention, Cr is also an element that constitutes fine intermetallic compounds such as Al-Fe(MnCr)-Si that are generated during casting and solution treatment. It is an essential element in order to increase the 0.2% yield strength.
If the Cr content is less than 0.20% by mass, the impact resistance may deteriorate. When the Cr content exceeds 0.90% by mass, the amount of intermetallic compounds such as Al-Fe(MnCr)-Si increases, resulting in a decrease in strength and difficulty in producing cold-rolled sheets. There is a possibility that
Therefore, the Cr content is preferably in the range of 0.20-0.90% by mass. A more preferable Cr content is in the range of 0.30 to 0.90% by mass. A more preferable Cr content is in the range of 0.40 to 0.85% by mass.

[Ti: 0.005 to 0.10% by mass]
Ti is an essential element because it acts as a grain refiner during ingot casting and can prevent casting cracks. Of course, Ti may be added alone, but by coexisting with B, a more powerful crystal grain refining effect can be expected, so adding a rod hardener such as Al-5%Ti-1%B There may be.
If the Ti content is less than 0.005% by mass, the effect of refining the steel during ingot casting is insufficient, which may lead to casting cracks, which is not preferable. If the Ti content exceeds 0.10% by mass, coarse intermetallic compounds such as TiAl ₃ may crystallize during ingot casting, which may reduce the press formability and bending workability of the final plate. I don't like it.
Therefore, the Ti content should be in the range of 0.005 to 0.10% by mass. A more preferable Ti content is in the range of 0.005 to 0.07% by mass. A more preferable Ti content is in the range of 0.01 to 0.05% by mass.

[Si/Mg ratio is in the range of 0.4 to 0.9]
By limiting the Si/Mg ratio to the range of 0.4 to 0.9, the width of the PFZ formed during the artificial aging treatment can be reduced, and β″ and β′ precipitated as intermediate phases at the grain boundaries. It is possible to suppress the growth of intermetallic compounds such as, and improve the impact resistance.
If the Si/Mg ratio is less than 0.4, the precipitation amount of the Mg—Si-based compound as a strengthening phase decreases, and the strength decreases, which is not preferable. If the Si/Mg ratio exceeds 0.9, relatively coarse intermetallic compounds such as β″ and β′ are precipitated as intermediate phases at grain boundaries due to aging treatment, which may reduce the impact resistance. Therefore, it is not preferable.
Therefore, the Si/Mg ratio is limited to the range of 0.4-0.9.

[Other unavoidable impurities]
Unavoidable impurities are inevitably mixed from raw material ingots, returned materials, etc., and their allowable contents are, for example, less than 0.20% by mass of Zn, less than 0.10% by mass of Ni, and less than 0.10% by mass of Zr. Less than 0.05% by mass, less than 0.05% by mass of B, less than 0.05% by mass of Ga or V, less than 0.02% by mass each of Pb, Bi, Sn, Na, Ca, and Sr, and others Each content is less than 0.05% by mass, and even if the content of uncontrolled elements is within this range, the effect of the present invention is not impaired.

[The metal structure is a recrystallized structure with an average crystal grain size of less than 20 μm,]
When forming an Al-Mg-Si-based aluminum alloy cold-rolled sheet and applying it to various structural members for automobiles, the metal structure after solution treatment is used as a recrystallized structure, and the crystal grains are made finer. There is a need. If the metal structure is a recrystallized structure and the crystal grains are made finer, the existence density of the grain boundaries increases, and intermetallic compounds such as β″ and β′ precipitate as intermediate phases at the grain boundaries during aging treatment. Growth is suppressed and the impact resistance after aging treatment can be improved.Therefore, in the present invention, solution treatment is performed at 550°C for 5 minutes, and then artificial aging treatment is performed at 175°C for 14 hours. The metal structure measured later was defined as a recrystallized structure with an average grain size of less than 20 μm.

[UTS _L is 340 MPa or more, YS _L is 300 MPa or more, S _L is 16.0 J/cm ² or more, and S _LT is 13.0 J/cm ² or more]
When forming an Al-Mg-Si based aluminum alloy cold-rolled sheet and applying it to various structural members for automobiles, it is necessary to have high strength, high proof stress and excellent impact resistance after solution treatment and aging treatment. High strength, high yield strength, and excellent impact resistance after solution treatment and aging treatment increase the degree of freedom in designing various structural members for automobiles.

The strength of the material is the tensile strength when the tensile test is performed, the yield strength is the 0.2% yield strength when the tensile test is performed, and the impact resistance is the Charpy impact value when the Charpy impact test is performed. can be known by
Therefore, in the present invention, the tensile strength of the test piece with the L direction (rolling direction) as the longitudinal direction is defined as UTS _L , and the 0.2% proof stress of the test piece with the L direction (rolling direction) as the longitudinal direction is YS Defined as _L , the Charpy value of the test piece with the L direction (rolling direction) as the longitudinal direction is defined as _SL , and the Charpy value of the test piece with the LT direction (plate width direction) as the longitudinal direction is defined as _SLT . Then, the UTS _L is 340 MPa or more, the YS _L is 300 MPa or more, and the _SL is measured after performing solution treatment at 550 ° C. for 5 minutes and further artificial aging treatment at 175 ° C. for 14 hours. It was defined as being 16.0 J/cm ² or more and _SLT being 13.0 J/cm ² or more.
The heat treatment conditions applied before measuring the properties of UTS _L , YS _L , S _L , S _LT , etc. are as follows. It may be aged and then subjected to artificial aging treatment at 175 ° C. x 14 hours, or after solution treatment at 550 ° C. x 5 minutes, for example, without natural aging and further at 175 ° C. x 14 hours. Artificial aging treatment may be applied.

Although the details will be described in the examples below, the Al-Mg-Si-based aluminum alloy cold-rolled sheet of the present invention applied to various structural members for automobiles is subjected to solution treatment at 550 ° C. for 5 minutes, Furthermore, the metal structure measured after artificial aging treatment at 175 ° C. for 14 hours is a recrystallized structure with an average crystal grain size of less than 20 μm, UTS _L is 340 MPa or more, YS _L is 300 MPa or more, and S It is preferable that _L is 16.0 J/cm ² or more and _SLT is 13.0 J/cm ² or more.

In addition, although the details will be given to the description of the examples below, in any case, it has the above-mentioned specific component composition, is subjected to solution treatment at 550 ° C. for 5 minutes, and further subjected to artificial aging treatment at 175 ° C. x 14 hours. The metal structure measured after the application is a recrystallized structure with an average grain size of less than 20 μm, UTS _L is 340 MPa or more, YS _L is 300 MPa or more, and _SL is 16.0 J/cm ² or more. , S _LT of 13.0 J/cm ² or more is an Al-Mg-Si-based aluminum alloy cold-rolled sheet that exhibits high strength, high yield strength, and excellent impact resistance after the solution treatment and aging treatment of the present invention. becomes.

Next, an example of a method for producing an Al--Mg--Si-based aluminum cold-rolled sheet having high strength, high yield strength and excellent impact resistance after solution treatment and aging treatment as described above will be briefly introduced.
[Melting/smelting]
After the raw material is put into the melting furnace and reaches a predetermined melting temperature, flux is appropriately added and stirred, and if necessary, the inside of the furnace is degassed using a lance or the like. Sediment is separated from the surface of the melt.
In this melting and smelting, it is important to re-insert the raw material such as the master alloy in order to obtain the predetermined alloy composition, but the above-mentioned flux and slag should be left in the molten aluminum alloy until it floats and separates on the surface of the molten aluminum alloy. It is extremely important to take It is desirable that the sedation time is usually 30 minutes or more.

The aluminum alloy melt produced in the melting furnace may be cast after being temporarily transferred to a holding furnace, but may also be tapped directly from the melting furnace and cast. A more desirable sedation time is 45 minutes or longer.
If necessary, in-line degassing and filtering may be performed.
In-line degassing is mainly of a type in which an inert gas or the like is blown into molten aluminum from a rotating rotor to diffuse and remove hydrogen gas in the inert gas bubbles. When nitrogen gas is used as the inert gas, it is important to control the dew point to, for example, −60° C. or lower. The amount of hydrogen gas in the ingot is preferably reduced to 0.20 cc/100 g or less.

If the amount of hydrogen gas in the ingot is large, porosity may occur in the final solidified portion of the ingot. It is preferable to crush it. In addition, hydrogen gas dissolved in a supersaturated solid solution in the ingot, depending on the heat treatment conditions of the cold-rolled coil, is precipitated during, for example, spot welding even after press forming of the final plate, and many blowholes may occur. Therefore, the hydrogen gas amount in the ingot is more preferably 0.15 cc/100 g or less.

[Thin slab continuous casting machine]
Thin slab continuous casters include both twin belt casters and twin roll casters.
The twin-belt casting machine includes a pair of rotating belt units that are provided with endless belts and face each other vertically, a cavity formed between the pair of rotating belt units, and cooling means provided inside the rotating belt units. A molten metal is supplied into the cavity through a refractory nozzle to continuously cast a thin slab.
The twin roll casting machine includes a pair of rotating rolls provided with endless rolls and facing each other vertically, a cavity formed between the pair of rotating rolls, and cooling means provided inside the rotating rolls. A molten metal is supplied into the cavity through a refractory nozzle to continuously cast a thin slab.

[Thickness of slab 3 to 15 mm]
The thin slab continuous casting machine can continuously cast thin slabs with a thickness of 3 to 15 mm. If the slab thickness is less than 3 mm, even if casting is possible, it is difficult to achieve a final rolling reduction of 50 to 95%, which will be described later, depending on the thickness of the final plate. If the slab thickness exceeds 15 mm, it becomes difficult to wind the slab directly onto a roll. In this range of slab thickness, the cooling rate of the slab is about 40 to 1000° C./sec at around 1/4 of the slab thickness, and intermetallic compounds such as Al—Fe(Cr/Mn)—Si are formed. Crystallize finely.

[Cold rolling]
A thin slab continuous casting machine is used to continuously cast slabs, and the slabs are directly wound on rolls without being hot-rolled, and then cold-rolled. Therefore, it is possible to omit the facing process, homogenization process, and hot rolling process that are required for cast slabs manufactured by conventional semi-continuous casting. The rolls from which the thin slabs are directly wound are passed through a cold rolling mill and are usually subjected to several passes of cold rolling.

[Final cold rolling rate 50 to 95%]
After performing cold rolling with a final cold rolling rate of 50 to 95%, solution treatment is performed. If the final cold rolling rate is within this range, the metal structure after solution treatment will be a recrystallized structure with an average crystal grain size of less than 20 μm, and after solution treatment and aging treatment, high strength, high yield strength and excellent impact resistance will be obtained. It can be an Al--Mg--Si based aluminum alloy cold-rolled sheet. Therefore, dislocations are accumulated by applying cold working while securing the solid solution amount of the transition metal element while keeping the processing cost low, and recrystallization adjusted to an average crystal grain size of less than 20 μm by the solution treatment process. organization can be obtained.
If the final cold rolling reduction is less than 50%, the amount of working strain accumulated during cold rolling is too small to obtain a recrystallized structure with an average grain size of less than 20 μm by solution treatment. When the final cold-rolling ratio exceeds 95%, the amount of work strain accumulated during cold rolling is too large, resulting in severe work hardening and edge cracking, making rolling difficult. Therefore, the preferred final cold rolling rate is in the range of 50-95%. A more preferable final cold rolling rate is in the range of 60 to 95%. A more preferred final cold rolling rate is in the range of 70 to 90%.

By going through the continuous casting process and the cold rolling process as described above, it is possible to obtain an Al-Mg-Si-based aluminum alloy cold-rolled sheet that has high strength, high yield strength, and excellent impact resistance after solution treatment and aging treatment. can.

Next, an example of application of the Al--Mg--Si based aluminum alloy cold-rolled sheet of the present invention to molded articles will be described below.
[Solution treatment]
The solution treatment is preferably, for example, a continuous annealing furnace at a temperature of 540° C. to 570° C. for 10 to 300 seconds, followed by rapid cooling. Air cooling by air injection or water cooling by mist injection is desirable as means for rapid cooling. Since the solution treatment is performed for a relatively short period of time, the Mn and Cr dissolved in the matrix act as a grain refiner by delaying recrystallization without causing the intermetallic compound to grow significantly. The average grain size of the crystals can be less than 20 μm.
When the holding temperature is lower than 540°C, it becomes difficult to obtain a recrystallized structure, although it depends on the alloy composition. If the holding temperature exceeds 570°C, burning may occur, depending on the alloy composition. If the holding time is less than 10 seconds, the actual temperature of the plate may not reach the predetermined temperature, resulting in insufficient solution treatment. If the holding time exceeds 300 seconds, the processing will take too long and the productivity will decrease.

[Natural aging]
When natural aging is performed after the solution treatment, it may be left at room temperature for several hours to 6 months, but the storage temperature of the coil should be appropriately controlled, for example, the holding time should be 16 to 72 hours. From the viewpoint of quality control, it is desirable to regulate

〔Press molding〕
In the press molding, after the obtained coil is split into strips, it is further cut into plates of a predetermined size with a shear, and formed into a predetermined shape by, for example, cold press molding. After the solution treatment, it is preferable to perform natural aging, cold forming or hot forming, and further artificial aging treatment. Of course, after the solution treatment, cold forming or hot forming may be performed without performing natural aging, and then artificial aging may be performed.

[Artificial aging treatment]
The artificial aging treatment is performed by inserting the molded article into a heating furnace, and is preferably held at a holding temperature of 150 to 200° C. for 1 to 72 hours. If the holding temperature is less than 150°C, the strength and 0.2% yield strength of the molded product will not be sufficiently increased, making it difficult to obtain the desired mechanical properties, which is not preferred.
If the holding temperature exceeds 200° C., the strength of the molded product is lowered, making it difficult to obtain the desired mechanical properties, which is not preferred. If the holding time is less than 1 hour, there is a possibility that the treatment will end while the substance temperature of the molded product is uneven, which is not preferable. If the holding time exceeds 72 hours, productivity will decrease, which is not preferable.

First, in order to investigate the effect of the content of each essential element on various properties of the test material, the following test material was produced.
[Preparation of thin slab continuous casting simulation material]
5 kg each of various ingots mixed with 20 levels of compositions (alloy Nos. 1 to 20) shown in Table 1 were inserted into a #20 crucible, and the crucible was heated in a small electric furnace to melt the ingots. Next, a lance was inserted into the molten metal, and N ₂ gas was blown for 5 minutes at a flow rate of 1.0 L/min for degassing. After that, the mixture was allowed to settle for 30 minutes, and the slag floating on the surface of the molten metal was removed with a stirring rod. Next, the crucible was taken out from the small electric furnace, and the molten metal was poured into a water-cooled mold with inner dimensions of 200×200×16 mm to prepare a thin slab. Disc samples of each test material (alloy Nos. 1 to 20) collected from the molten metal in the crucible were subjected to composition analysis by emission spectroscopic analysis. Table 1 shows the results. Both sides of this thin slab were chamfered by 3 mm to a thickness of 10 mm, and then cold-rolled without homogenization and hot rolling to obtain a cold-rolled material with a thickness of 2.5 mm. . No intermediate annealing treatment was performed during the cold rolling process. The final cold rolling reduction in this case was 75%.

Next, after cutting these cold-rolled materials (alloy Nos. 1 to 20) into predetermined sizes, the cold-rolled materials are inserted into a salt bath, heated and held at 550 ° C. for 5 minutes, and salt bath It was quickly removed from the mold, cooled with water, and subjected to solution heat treatment. After that, natural aging was performed at room temperature for 24 to 72 hours to obtain a T4 material. These T4 materials were inserted into an annealer and subjected to artificial aging treatment at 175° C. for 14 hours to obtain test materials (Examples 1 to 10, Comparative Examples 1 to 10).

Next, in order to investigate the effects of natural aging on various properties of test materials, the following test materials were produced.
After cutting the cold-rolled material of alloy No. 3 into a predetermined size, the cold-rolled material was inserted into a salt bath, heated and held at 550°C for 5 minutes, quickly removed from the salt bath, and cooled with water to melt. It has undergone a quenching treatment. After that, some of them were inserted into an annealer without being naturally aged and subjected to artificial aging treatment at 175° C. for 14 hours to obtain a test material (Example 11). In addition, some of them were naturally aged at room temperature for 24, 48, and 72 hours to make T4 materials, and these T4 materials were inserted into an annealer and artificially aged at 175°C for 14 hours to obtain test materials. (Examples 12, 13, 14).

Next, in order to investigate the effects of press forming (plastic deformation) after natural aging on various properties of the test materials, the following test materials were produced.
After cutting the cold-rolled material of alloy No. 3 into a predetermined size, the cold-rolled material was inserted into a salt bath, heated and held at 550°C for 5 minutes, quickly removed from the salt bath, and cooled with water to melt. It has undergone a quenching treatment. After that, natural aging was performed at room temperature for 48 hours to obtain a T4 material. For this T4 material, 2% prestrain was introduced at room temperature using a tensile tester to simulate cold press forming, and then it was inserted into an annealer and subjected to artificial aging treatment at 175 ° C. for 14 hours. It was used as a test material (Example 15).

Next, various characteristics were measured and evaluated for each test material thus obtained.
[Measurement of properties by tensile test]
The mechanical properties of each test material obtained were evaluated based on the values of tensile strength, 0.2% yield strength, and elongation (elongation at break) in a tensile test.
Specifically, a JIS No. 5 test piece is taken from the obtained test material so that the tensile direction is parallel to the rolling direction (L direction), and a tensile test is performed according to JISZ2241. Strength (MPa), 0.2% yield strength (MPa), and elongation (%) were determined. These tensile tests were performed three times (n=3) in the L direction of each test material, and the average value was calculated.
A test piece with a tensile strength UTS _L of 340 MPa or more with the L direction as the longitudinal direction was evaluated as good strength (O), and a test material with less than 340 MPa was evaluated as poor strength (x). The test piece whose 0.2% yield strength YS _L of the test piece with the L direction as the longitudinal direction was 300 MPa or more was evaluated as good yield strength (○), and the test material with less than 300 MPa yield strength evaluation was poor (x). and Table 2 shows the evaluation results.

[Evaluation of impact resistance by instrumented Charpy impact test]
The evaluation of the impact resistance properties of each of the obtained test materials was performed based on the absorbed energy, Charpy impact value, and downward slope value in an instrumented Charpy impact test.
Specifically, from the obtained test material, as a test piece for an instrumented Charpy impact test, a sample with the 0 ° direction (L direction) / 90 ° direction (LT direction) with respect to the rolling direction as the longitudinal direction was collected and processed into test pieces (n=3) of dimensions 55 mm×10 mm×2.5 mm (45° V notch, depth 2 mm) as shown in FIG. From the results of the instrumented Charpy impact test (load-displacement curve), the absorbed energy (J), Charpy value (J/cm ² ), and downward slope value (N/mm/mm ² ) of each test material were calculated. .
The Charpy value S _L of the test piece with the L direction as the longitudinal direction is 16.0 J/cm ² or more, and the Charpy value S _LT of the test piece with the LT direction as the longitudinal direction is 13.0 J/cm ² or more. The test material was evaluated as having good impact resistance (○). The Charpy value S _L of the test piece with the L direction as the longitudinal direction was less than 16.0 J/cm ² or / and the Charpy value S _LT of the test piece with the LT direction as the longitudinal direction was less than 13.0 J/cm ² The test material that was , was regarded as poor impact resistance evaluation (x). Table 2 shows the evaluation results.

[Measurement of average grain size]
The RD-ST surface of each test material obtained was polished, placed in a scanning electron microscope manufactured by JEOL (JSM-6490A), and a range of 500 μm×500 μm was measured using a crystal orientation analyzer. The obtained analysis results were analyzed by OIM ANALYSIS manufactured by TSL, and the area average of the crystal grain size was defined as the average crystal grain size. Table 2 shows the evaluation results.

[Evaluation of characteristics of each test material]
Examples 1 to 10 and 11 to 15 in Table 2 showing the characteristics evaluation results of the test material are within the composition range of the present invention, and the tensile strength UTS _L of the test piece with the L direction of the test material as the longitudinal direction , 0.2% proof stress YS _L of the test piece with the L direction as the longitudinal direction, Charpy value S _L of the test piece with the L direction as the longitudinal direction, and Charpy value S _LT of the test piece with the LT direction as the longitudinal direction. , met the criteria.
Specifically, the tensile strength UTS _L of the test piece whose longitudinal direction is the L direction of the test material: 340 MPa or more, and the 0.2% yield strength YS _L of the test piece whose longitudinal direction is the L direction: 300 MPa or more, L direction Charpy value S _L of the test piece with the longitudinal direction: 16.0 J/cm ² or more, Charpy value S _LT of the test piece with the LT direction as the longitudinal direction: 13.0 J/cm ² or more. . That is, in Examples 1 to 10 and 11 to 15, the strength evaluation was good (◯), the yield strength evaluation was good (◯), and the impact resistance evaluation was good (◯).

Comparative Examples 1 to 10 in Table 2 showing the property evaluation results of the test materials are outside the composition range of the present invention, and the tensile strength UTS _L of the test piece with the L direction of the test material as the longitudinal direction. At least one of the 0.2% yield strength YS _L of the test piece in the longitudinal direction, the Charpy value S _L of the test piece with the L direction as the longitudinal direction, and the Charpy value S _LT of the test piece with the LT direction as the longitudinal direction did not meet the standard values.
In Comparative Example 1, since the Si content was too low, the strength evaluation was poor (x) and the yield strength evaluation was poor (x). In Comparative Example 2, the Si content was too high, so the impact resistance evaluation was poor (x).
In Comparative Example 3, the Fe content was too high, so the yield strength evaluation was poor (x).
In Comparative Example 4, since the Cu content was too low, the strength evaluation was poor (x) and the yield strength evaluation was poor (x). In Comparative Example 5, the impact resistance evaluation was poor (x) because the Cu content was too high.
Comparative Example 6 had a poor strength evaluation (x) because the Mg content was too low. In Comparative Example 7, since the Mg content was too high, the yield strength evaluation was poor (x) and the impact resistance evaluation was poor (x).
In Comparative Example 8, the Mn content was too low, so the impact resistance evaluation was poor (x). In Comparative Example 9, since the Mn content was too high, the strength evaluation was poor (x) and the yield strength evaluation was poor (x).
In Comparative Example 10, the Cr content was too low, so the impact resistance evaluation was poor (x).

From the above, the tensile strength UTS _L of the test piece having the specific composition and having the L direction as the longitudinal direction: 340 MPa or more, and the 0.2% yield strength YS _L of the test piece having the L direction as the longitudinal direction : 300 MPa or more, Charpy value S _L of the test piece with the L direction as the longitudinal direction: 16.0 J/cm ² or more, Charpy value S _LT of the test piece with the LT direction as the longitudinal direction: 13.0 J/cm ² or more It can be seen that the Al--Mg--Si based aluminum alloy cold-rolled sheet, which has high strength, high proof stress and excellent impact resistance after solution treatment and aging treatment, exhibits the value.

Claims (6)

Si: 0.50 to 0.90 mass%, Fe: 0.10 mass% to less than 0.70 mass%, Cu: 0.10 to 0.90 mass%, Mg: 0.80 to 1.7 mass% , Mn: 0.10 to 1.3% by mass, Cr: 0.20 to 0.90% by mass, Ti: 0.005 to 0.10% by mass, and the balance consists of Al and unavoidable impurities, Si / Mg An aluminum alloy cold-rolled sheet having a composition in which the ratio is in the range of 0.4 to 0.9,
Define the tensile strength of the test piece with the L direction as the longitudinal direction as UTS _L , define the 0.2% proof stress of the test piece with the L direction as the longitudinal direction as _YSL , and the test piece with the L direction as the longitudinal direction The Charpy value of is defined as _SL , and the Charpy value of the test piece with the LT direction as the longitudinal direction is defined as _SLT . Measured after natural aging for 24, 48, or 72 hours and further artificial aging treatment at 175 ° C. for 14 hours, the metal structure is a recrystallized structure with an average grain size of less than 20 μm, and UTS _L is 340 MPa. An Al—Mg—Si-based aluminum alloy characterized in that it has a YS _L of 300 MPa or more, an S _L of 16.0 J/cm ² or more, and an S _LT of 13.0 J/cm ² or more. Cold-rolled sheet. When the elongation of the test piece with the L direction as the longitudinal direction is defined as _ELL , the aluminum alloy cold-rolled sheet is subjected to solution treatment at 550 ° C. for 5 minutes and then natural aging for 24, 48, or 72 hours. The Al-Mg-Si-based aluminum alloy cold-rolled according to claim 1, characterized in that the EL _L measured after being subjected to artificial aging treatment at 175 ° C. for 14 hours is 15.0% or more . board. Si: 0.50 to 0.90 mass%, Fe: 0.10 mass% to less than 0.70 mass%, Cu: 0.10 to 0.90 mass%, Mg: 0.80 to 1.7 mass% , Mn: 0.10 to 1.3% by mass, Cr: 0.20 to 0.90% by mass, Ti: 0.005 to 0.10% by mass, and the balance consists of Al and unavoidable impurities, Si / Mg An aluminum alloy cold-rolled sheet having a composition in which the ratio is in the range of 0.4 to 0.9,
Define the tensile strength of the test piece with the L direction as the longitudinal direction as UTS _L , define the 0.2% proof stress of the test piece with the L direction as the longitudinal direction as _YSL , and the test piece with the L direction as the longitudinal direction The Charpy value of is defined as _SL , and the Charpy value of the test piece with the LT direction as the longitudinal direction is defined as _SLT . Measured after natural aging for 24, 48, or 72 hours and further artificial aging treatment at 175 ° C. for 14 hours, the metal structure is a recrystallized structure with an average grain size of less than 20 μm, and UTS _L is 340 MPa. or more, YS _L is 300 MPa or more, S _L is 16.0 J/cm ² or more, and S _LT is 13.0 J/cm ² or more. Aluminum alloy cold-rolled sheet. When the elongation of the test piece with the L direction as the longitudinal direction is defined as _ELL , the aluminum alloy cold-rolled sheet is subjected to solution treatment at 550 ° C. for 5 minutes and then natural aging for 24, 48, or 72 hours. The Al-Mg-Si-based aluminum alloy for molding according to claim 3, characterized in that the EL _L measured after being subjected to artificial aging treatment at 175 ° C. for 14 hours is 15.0% or more. Cold-rolled sheet. The method for producing the Al-Mg-Si-based aluminum alloy cold-rolled sheet according to claim 1 or 2, wherein the molten aluminum alloy having the chemical composition according to claim 1 is cast using a thin slab continuous casting machine. Cast continuously into 3-15mm slabs,
The slab is wound directly into a coil without being subjected to homogenization treatment and hot rolling, and then cold rolled at a final cold rolling rate of 50 to 95% without performing intermediate annealing. - A method for producing a Mg-Si-based aluminum alloy cold-rolled sheet. The method for producing the Al-Mg-Si-based aluminum alloy cold-rolled sheet according to claim 3 or 4, wherein the molten aluminum alloy having the chemical composition according to claim 3 is cast using a thin slab continuous casting machine. Cast continuously into 3-15mm slabs,
Forming characterized in that the slab is directly wound into a coil without being subjected to homogenization treatment and hot rolling, and then subjected to cold rolling at a final cold rolling rate of 50 to 95% without performing intermediate annealing. A method for producing an Al-Mg-Si-based aluminum alloy cold-rolled sheet for

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