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DESCRIPTION

ALUMINUM ALLOY SHEET EXCELLENT IN HIGH TEMPERATURE HIGH SPEED FORMABILITY AND METHOD OF PRODUCING OF SAME

TECHNICAL FIELD

[0001] The present invention relates to aluminum alloy- sheet excellent in high speed superplastic formability and a method of production of the same.

BACKGROUND ART

[0002] To deal with the increasingly complex formed shapes accompanying the diversification of designs of products formed from aluminum sheet, aluminum sheet excellent in formability has been proposed. With general press forming at ordinary temperature, the elongation of the aluminum sheet is several tens of percent, so sometimes the shape desired by the customer cannot be obtained. As opposed to this, superplastic aluminum sheet enabling giant elongation allows the shapes desired by customers to be easily handled, so is increasingly used. However, conventional Al-Mg-based superplastic alloy exhibits its maximum elongation when the forming speed (strain rate) is a slow 10 ^"4 to 10 ^~3 /sec, so there are the disadvantages that the forming work takes a lot of time and the productivity is therefore inferior. [0003] To solve this problem, Al-Mg-based alloy sheet reducing the number of relatively coarse Al-Fe-Si-based compound particles and defining the range of crystal grain size to enable high speed superplastic deformation is being developed (Japanese Patent Publication (A) No. 09-59736 and Japanese Patent Publication (A) No. 10- 259441). These materials enable the targeted high speed superplastic deformation by strictly restricting the impurities, that is, the amount of Si to 0.06% or less and the amount of Fe to 0.06% or less. However, since the impurities are strictly restricted, there is the problem that the cost of the ground metal becomes higher.

Further, the method of production includes the steps of hot rolling and cold rolling cast slabs after soaking at a high temperature, so there is also the problem that the number of production steps is large.

[0004] To solve this problem, the assignee previously proposed in Japanese Patent Publication (A) No. 2005- 307300 a method of production of Al-Mg-based alloy sheet comprising casting an Al-Mg-based alloy melt by a twin- belt casting machine at a casting cooling rate of 20 to 150°C/sec to obtain a 5 to 15 mm thick slab, then cold rolling this at a cold rolling rate of 70 to 96% and annealing the obtained cold rolled sheet. According to this method of production, even if the alloy contains 0.06% to 0.2% of Si and 0.1% to 0.5% of Fe, it is possible to cause the intermetallic compound to finely disperse in the matrix and possible to obtain Al-Mg-based alloy sheet excellent in high temperature high speed formability and with little cavities after forming work. However, with this material, not only does either of Mn, Cr, or Zr have to be added to make the crystal grains of the annealed material finer, but also further improvement of the high temperature high speed formability has been desired.

DISCLOSURE OF THE INVENTION

[0005] The present invention has as its object the provision of aluminum alloy sheet excellent in high speed superplastic formability produced by a thin sheet continuous casting process without raising the cost of the ground metal, without increasing the number of production steps, and without requiring additive elements for making the crystal grains finer and a method of production of the same.

[0006] To achieve the above object, according to a first aspect of the present invention, there is provided aluminum alloy sheet excellent in high temperature, high speed formability consisting, by mass%, of the following composition:

Mg : 2 . 5 to 5 . 0 % ,

Fe : 0 . 1 to 0 . 3 % ,

Si : 0 . 06 to 0 . 12 % , and a balance of aluminum and unavoidable impurities, in said unavoidable impurities, Mn being restricted to 0.1% or less and Cr to 0.05% or less, and having 20000 particles/mm ² or less of second phase particles with a circle equivalent diameter of 0.2 μm or more and having a fibrous, not yet recrystallized structure.

[0007] Further, there is provided a method of production of aluminum alloy sheet excellent in high temperature high speed formability of the first aspect of the invention comprising, preparing an alloy melt of a composition of the first aspect of the invention, casting said alloy melt by a thin sheet continuous casting machine to a 5 to 15 mm thick slab, coiling up said slab, and cold rolling it, without homogenization, by a cold rolling rate of 70 to 96%.

[0008] To achieve the object, according to a second aspect of the invention, there is provided aluminum alloy sheet excellent in high temperature high speed formability consisting, by mass%, of the following composition:

Mg: 2.5 to 5.0%,

Fe: 0.1 to 0.3%,

Si: 0.06 to 0.12%, and a balance of aluminum and unavoidable impurities, in said unavoidable impurities, Mn being restricted to 0.1% or less and Cr to 0.05% or less, and having 20000 particles/mm ² or less of second phase particles with a circle equivalent diameter of 0.2 μm or more and having a recrystallized structure with an average crystal grain size of 20 μm or less.

[0009] Further, there is provided a method of production of aluminum alloy sheet excellent in high temperature high speed formability of the second aspect of the invention comprising, preparing an alloy melt of a composition of the first aspect of the invention, casting said alloy melt by a thin sheet continuous casting machine to a 5 to 15 mm thick slab, coiling up said slab, cold rolling it, without homogenization, by a cold rolling rate of 70 to 96%, and final annealing the obtained cold rolled sheet to cause it to recrystallize.

[0010] The aluminum alloy sheet of the present invention is restricted in chemical composition, is strictly restricted in the contents of Mn and Cr, which conventionally had been added for increasing the fineness of the crystal grains of an annealed material, as impurities opposite to the past, utilizes Si and Fe as rather effective essential elements and simultaneously is defined in microstructure as a fibrous not yet recrystallized structure with intermetallic compounds finely and evenly dispersed or a recrystallized structure restricted in grain size so as to form a fine subgrain structure and obtain a high elongation at the time of high temperature, high speed forming work. [0011] Therefore, the aluminum alloy sheet of the present invention does not involve a rise in the cost of the ground metal, does not involve an increase in the number of production steps, and does not require addition of elements for increasing the fineness. [0012] Further, the method of production of the present invention uses a thin sheet continuous casting process to secure a high cooling rate at the time of casting, restricts the cold rolling rate at the time of cold rolling to achieve uniform fine dispersion of the intermetallic compound, and uses cold rolling to obtain a

fibrous structure or uses cold rolling, then final annealing to obtain a fine recrystallized structure and thereby produce aluminum alloy sheet giving a high elongation at the time of high temperature, high speed forming work.

[0013] The method of production of the present invention cold rolls a slab cast by a thin sheet continuous casting machine to the final thickness without homogenization, so is a method of production of aluminum alloy sheet having high speed superplastic characteristics involving few steps and consuming low energy.

[0014] By using the aluminum alloy sheet of the present invention, a high grade formed product is obtained, the forming time is shortened, and the productivity is improved.

BEST MODE FOR WORKING THE INVENTION [0015] The reasons for limiting the chemical composition of the alloy in the present invention will be explained next. In this description, the "%" expressing the chemical composition means "mass%" unless indicated to the contrary. [0016] "Mg: 2.5 to 5.0%"

Mg is an element increasing the strength through solution strengthening. If less than 2.5%, this effect cannot be expressed, a fine subgrain structure cannot be formed at the time of high temperature, high speed deformation, and the elongation is low. If the Mg content is over 5.0%, cold rolling becomes difficult. [0017] "Fe: 0.1 to 0.3%"

Fe precipitates as fine particles of Al-Fe-Si-based compounds or other intermetallic compounds at the time of casting. These function as nucleus forming sites for recrystallization at the time of annealing after cold rolling. Therefore, the greater the number of particles of these intermetallic compounds, the greater the number of recrystallization nuclei formed and as a result the

greater the number of fine recrystallized grains formed. Further, the fine particles of the intermetallic compounds pin the grain boundaries of the recrystallized grains formed to suppress growth due to agglomeration of crystal grains and maintain the fine recrystallized grains stable. To realize this effect, the Fe content must be made 0.1% or more. However, if the Fe content exceeds 0.3%, the precipitated particles of the intermetallic compounds tend strongly to become coarser and, at the time of high temperature forming work, the particles of the intermetallic compounds act as starting points for formation of cavities leading to inferior formability. Therefore, the Fe content is restricted to 0.1 to 0.3%. The preferable range is 0.1 to 0.25%. [0018] "Si: 0.06 to 0.12%"

Si precipitates as fine particles of Al-Fe-Si-based compounds or other intermetallic compounds at the time of casting. These function as nucleus forming sites for recrystallization at the time of annealing after cold rolling. Therefore, the greater the number of particles of these intermetallic compounds, the greater the number of recrystallization nuclei formed and as a result the greater the number of fine recrystallized grains formed. Further, the fine particles of the intermetallic compounds pin the grain boundaries of the recrystallized grains formed to suppress growth due to agglomeration of crystal grains and maintain the fine recrystallized grains stable. To realize this effect, the Si content must be made 0.06% or more. However, if the Si content exceeds 0.12%, the precipitated particles of the intermetallic compounds tend strongly to become coarser and, at the time of high temperature forming work, the particles of the intermetallic compounds act as starting points for formation of cavities leading to inferior formability. Therefore, the Si content is restricted to 0.06 to 0.12%. The preferable range is 0.06 to 0.1%. [0019] "Mn: 0.1% or less"

Mn has conventionally been added to make the recrystallized grains finer and suppress growth of recrystallized grains. As opposed to this, in the present invention, Mn is considered an impurity and restricted in content to 0.1% or less. That is, if the Mn content is over 0.1%, at the time of casting, Al- (Fe -Mn) -Si-based masses of precipitates are formed. Even after the cold rolling step, these are not broken up and remain even after the final annealing. Therefore, at the time of high temperature forming work, the precipitates act as the starting points for formation of cavities leading to inferior formability. In particular, when stressing prevention of formation of cavities, it is preferable to further restrict the upper limit to 0.05% or less. [0020] "Cr: 0.05% or less"

Cr, in the same way as Mn, has conventionally been added to make the recrystallized grains finer and suppress growth of recrystallized grains. As opposed to this, in the present invention, Cr is considered an impurity and restricted in content to 0.05% or less. That is, if the Cr content is over 0.05%, at the time of casting, Al- (Fe ^• Cr) -Si-based masses of precipitates are formed. Even after the cold rolling step, these are not broken up and remain even after the final annealing. Therefore, at the time of high temperature forming work, the precipitates act as the starting points for formation of cavities leading to inferior formability. In particular, when stressing prevention of formation of cavities, it is preferable to further restrict the upper limit to 0.03% or less. [0021] "Ti of any ingredient: 0.001 to 0.1%"

In the present invention, Ti may be added in 0.001 to 0.1% in range to make the crystal grains of the cast slab finer. To realize this effect, the amount of addition of Ti must be made 0.001% or more. However, if the amount of addition of Ti exceeds 0.1%, TiAl ₃ or other coarse intermetallic compounds are formed resulting in

the formation of cavities at the time of high temperature forming work and a drop in the formability. The preferable range is 0.001 to 0.05%. [0022] Next, the reasons for limitation of the microstructure of the alloy sheet in the present invention will be explained.

"20000 particles/mm ² or less of second phase particles with circle equivalent diameter of 0.2 μm or more"

The "second phase particles" in the present invention mean the intermetallic compounds. Specifically, these are the Al-Fe-Si-based, Al- (Fe ^• Mn) -Si-based, Al-

(Fe • Cr) -Si-based, Mg ₂ Si, AlεMn, etc. precipitating at the time of casting.

The present invention restricts the number of second phase particles with a circle equivalent diameter 0.2 μm or more so as to suppress the formation of cavities at the time of high temperature, high speed forming work and realize high ductility at the time of high temperature, high speed deformation, whereby the high temperature high speed formability is improved. To obtain this effect, it is necessary to restrict the density of second phase particles with a circle equivalent diameter of 0.2 μm or more to 20000 particles/mm ² - or less. The number of the second phase particles with a circle equivalent diameter of 0.2 μm or more is preferably reduced as much as possible.

[0023] "Fibrous not yet recrystallized structure" An alloy sheet not subjected to final annealing after cold rolling and given a fibrous, not yet recrystallized structure forms fine recrystallized grains during heating at the time of the high temperature forming work. Further, a fine subgrain structure is formed during the high temperature forming work. Due to this, the elongation is improved.

[0024] "Recrystallized structure with average crystal

grain size of 20 μm or less"

The cold rolled sheet given a recrystallized structure by the final annealing after the cold rolling is restricted to an average crystal grain size of 20 μm or less. If the crystal grain size before the high temperature forming work is fine, surface roughening due to coarse grains will not occur at the time of high temperature deformation and a good appearance will be obtained and, also, the elongation at the time of high temperature deformation will increase. If the average crystal grain size exceeds 20 μm, surface roughening occurs due to the coarse grains and the elongation at the time of high temperature deformation falls. The more preferable average crystal grain size is 13 μm or less. Note that here, "recrystallized structure" includes not only a completely recrystallized structure, but also a partially recrystallized structure (20 μm or smaller recrystallized grains + fibrous not yet recrystallized grains) .

[0025] The reasons for limiting the conditions in the method of production of the present invention will be explained next.

"Casting by thin sheet continuous casting machine to 5 to 15 mm thick slab"

The thin slab used for the production of the aluminum alloy sheet excellent in high temperature high speed formability of the present invention is cast by a thin sheet continuous casting machine. Thin sheet continuous casting machines include two types: twin-belt and two-roll types.

With a twin-belt type continuous casting machine, a system is employed where the melt is poured between a pair of rotating belts facing each other above and below and cooled by water, the cooling action from the belt surfaces causes the melt to solidify and form a slab, and the slab is continuously pulled out from the opposite

side of the belts where the melt was poured and taken up in a coil.

With a twin-roll type continuous casting machine, a system is employed where the melt is poured between a pair of rotating rolls facing each other above and below and cooled by water, the cooling action from the roll surfaces causes the melt to solidify and form a slab, and the slab is continuously pulled out from the opposite side of the rolls where the melt was poured and taken up in a coil .

[0026] In the present invention, the cast slab is made one with a thickness of 5 to 15 mm. If in this range of thickness, a high solidification rate can be secured even in the middle of the thickness, so a uniform cast structure can be easily formed. Simultaneously, with the composition of the present invention, formation of coarse intermetallic compounds can be easily suppressed, and the density of second phase particles with a circle equivalent diameter of 0.2 μm or more in the final sheet product can be lowered to 20000 particles/mm ² or less. Further, it becomes easy to control the average size of the recrystallized grains after the final annealing to 20 μm or less. This range of slab thickness is also suitable from the practical viewpoint of thin sheet continuous casting. That is, if the slab thickness is less than 5 mm, the amount of aluminum alloy passing through the casting machine per unit time becomes too small and the casting itself becomes difficult. If the slab thickness is over 15 mm, takeup in a coil becomes difficult. [0027] "Cold rolling by a cold rolling rate of 70 to 96%"

Due to the buildup of dislocations around the precipitates caused by the plastic working of the cold rolling, when final annealing is not performed, the final sheet is formed with a fibrous, not yet recrystallized structure, which promotes increased fineness of the crystal grains by heating at the time of high temperature

forming work, or when final annealing is performed, the final sheet is formed with a fine recrystallized structure. If the cold rolling rate is less than 70%, the buildup of dislocations is insufficient and a fine recrystallized structure cannot be obtained during heating at the time of high temperature forming work or after annealing. If the cold rolling rate is over 96%, edge cracks occur during the cold rolling and the cold rolling becomes difficult.

[0028] In the second aspect of the invention of the present application, after cold rolling, the above annealing is performed as the final annealing. This is generally performed in a continuous annealing furnace or batch furnace, but there is no need to limit the invention to this.

"Annealing in continuous annealing furnace at holding temperature of 400 to 500 ⁰ C for holding time of 5 minutes or less"

The annealing temperature of the final annealing by the continuous annealing furnace is made 400 to 500 ⁰ C in range. If the annealing temperature is less than 400 ⁰ C, the recrystallization becomes insufficient and a fine recrystallized structure cannot be obtained. However, if the annealing temperature is over 500 ⁰ C, the recrystallized grain size ends up exceeding 20 μm and a fine recrystallized structure cannot be obtained. [0029] The holding time at the annealing temperature in the continuous annealing furnace should be 5 minutes or less. In the case of a holding time of over 5 minutes, the recrystallized grains end up becoming coarse and a fine recrystallized structure cannot be obtained. [0030] "Annealing in batch furnace at holding temperature of 300 to 400 ⁰ C for holding time of 1 to 8 hours"

The annealing temperature of the final annealing by the batch furnace is made 300 to 400 ⁰ C in range. If the

annealing temperature is less than 300°C, the recrystallization becomes insufficient and a fine recrystallized structure cannot be obtained. However, if the annealing temperature is over 400°C, the recrystallized grain size ends up exceeding 20 μm and a fine recrystallized structure cannot be obtained. [0031] The holding time at the annealing temperature in the batch furnace should be 1 to 10 hours in range. If the holding time is less than 1 hour, while depending on the rate of temperature rise, the coil as a whole is not uniformly heated, so a uniform, fine recrystallized structure cannot be obtained. If the holding time is over 10 hours, the production cost becomes too great, so this is not preferable. [0032] The aluminum alloy sheet of the present invention is preferably formed at 400 to 550°C in temperature. If the temperature of the forming work is less than 400 ⁰ C, sufficient elongation cannot be obtained. If the temperature of the forming work is over 550°C, the crystal grains become enlarged. Further, with a high Mg composition alloy in the range of the present invention, local melting (burning) occurs and the elongation drops. The strain rate at the time of forming work is preferably 2xlO ^~2 /sec to 8XlO ^-1 ZSeC in range. If the strain rate is less than 2xlO ^~2 /sec, during the forming work, a fine subgrain structure cannot be formed and the elongation drops. If the strain rate exceeds 8XlO ^-1 ZSeC, the crystal grains become coarser and the elongation becomes lower. A more preferable range of strain rate is 5xlO ^~2 Zsec to 5xlO ^" Vsec.

EXAMPLES

[0033] An aluminum alloy melt of each of the chemical compositions shown in Table 1 was cast by a twin-belt type continuous casting machine to a 10 mm thick thin slab. The cast slab was then cold rolled to the final sheet thickness of 1 mm without homogenization. Part of

the obtained sheets was final annealed at 450 ⁰ C for 15 seconds and designated as tempering O. After this, samples for examination of the microstructure were taken from all of the sheet materials, polished at their cross- sectional surfaces, then counted for the number of second phase particles with a circle equivalent diameter of 0.2 μm or more present in a predetermined area by an image analyzer (LUZEX) . Further, test pieces prescribed in JIS H7501 were prepared, were heated to 440°C, were subjected to a high temperature tensile test at a strain rate of IxIO ^-1 ZSeC, and were measured for elongation (%). The results are summarized in Table 2.

Table 1

(mass% )

(Note) Balance of Al and unavoidable impurities (other than Mn and Cr) . Double underlined values indicate outside range of present invention.

Table 2

[0034] Sample No. 1 is a sample within the scope of composition of the present invention. In its metal structure, there were 20000 particles/mm ² or less of second phase particles with a circle equivalent diameter of 0.2 μm or more and the average crystal grain size was

20 μm or less, so the elongation was 250% or more. [0035] Sample No. 2 is a sample within the scope of composition of the present invention. In its metal structure, there were 20000 particles/mm ² or less of second phase particles with a circle equivalent diameter of 0.2 μm or more and the structure was a fibrous not yet recrystallized structure, so a fine recrystallized grain structure was obtained during heating at the time of a high temperature tensile test and the elongation was 250% or more.

[0036] Sample No. 3 has an Mn content of over 0.1% and therefore is a sample outside the scope of composition of the present invention. In its metal structure, there were 20000 particles/mm ² or more of second phase particles with a circle equivalent diameter of 0.2 μm or more, so a large number of cavities formed during the high temperature tensile test and the elongation was less than 250%.

[0037] Sample No. 4 has a Cr content of over 0.05% and therefore is a sample outside the scope of composition of the present invention. In its metal structure, there were 20000 particles/mm ² or more of second phase particles with a circle equivalent diameter of 0..2 μm or more, so a large number of cavities formed during the high temperature tensile test and the elongation was less than 250%.

[0038] Sample No. 5 has an Mg content of less than 2.5% and therefore is a sample outside the scope of composition of the present invention. In its metal structure, there were 20000 particles/mm ² or less of second phase particles with a circle equivalent diameter of 0.2 μm or more and the average crystal grain size was

20 μm or less, but a fine subgrain structure could not be obtained at the time of high temperature deformation and the elongation was less than 250%. [0039] Sample No. 6 has an Mg content of over 5.0% and

therefore is a sample outside the scope of composition of the present invention. While a thin slab could be cast, during the subsequent cold rolling, edge cracks occurred, so a 1 mm thick sheet material could not be obtained.

INDUSTRIAL APPLICABILITY

[0040] According to the present invention, there are provided aluminum alloy sheet excellent in high speed superplastic formability produced by a thin sheet continuous casting process without raising the cost of the ground metal, without increasing the number of production steps, and without requiring additive elements for making the crystal grains finer and a method of production of the same.