KR20060135849A - Al-mg alloy sheet with excellent formability at high temperatures and high speeds and method of production of same - Google Patents

Abstract

To provide an aluminum alloy sheet with excellent formability at high temperatures and high speeds with a reduced amount of cavities after forming and a method of production of the same. An aluminum alloy sheet consisting of 2.0-8.0 wt% of Mg, 0.06-0.2 wt% of Si, 0.1-0.5 wt% of Fe, 0. 1-0.5 wt% of Mn, and the balance of Al and unavoidable impurities, wherein a density of inter-metallic compounds having an equivalent circle diameter of 1 to 5 (m is 5000/mm2 or more and an average crystal grain size is 20 (m or less. A method of production of an aluminum alloy sheet comprising the steps of casting an alloy melt having the above described composition by a twin belt casting machine at a cooling rate of 20 to 150°C/sec at the location of 1/4 of the slab thickness during the casting to form a slab having a thickness of 5 to 15 mm, subsequently rewinding up the slab as a coil, cold rolling the slab taken out from the coil at a cold rolling reduction of 70 to 96%, and performing annealing heating the obtained cold rolled sheet at a heating rate of 50°C/sec or more to 420 to 500°C.

Description

Al-Mg ALLOY SHEET WITH EXCELLENT FORMABILITY AT HIGH TEMPERATURES AND HIGH SPEEDS AND METHOD OF PRODUCTION OF SAME}

The present invention relates to an Al-Mg alloy sheet having excellent high temperature formability and high speed formability and a method of manufacturing the same.

Al-Mg alloy is used as a thin plate material or other processing material or molding material for automobile because of its light weight, excellent strength and corrosion resistance. However, since the elongation at room temperature is small, there is a problem in producing an intricate shape by cold working Al-Mg alloy. For this reason, in order to reduce the grain size, an Al-Mg based superplastic alloy has been developed that can suppress recrystallization during hot working and obtain elongation of several hundred percent in the high temperature region of 500 to 550 ° C. have.

Conventional Al-Mg-based superplastic alloys exhibit superplastic properties under a slow molding rate (strain) of 10 ^-4 to 10 ^-3 / sec, which takes a very long time, resulting in high productivity when applied to conventional press molding. It is low and not practical.

Therefore, in the high temperature region to be hot worked, for example, aluminum alloy sheet having sufficient elongation even at high speed of 0.1 / sec strain rate 100 times faster than the existing molding speed and suppressing cavity formation during molding Is being developed.

For example, Japanese Laid-Open Patent Publication No. 10-259441 is an aluminum alloy sheet having excellent superplastic formability during high-speed molding and a reduced amount of cavity after molding, which is in mass%, Mg 3.0 to 8.0%, Cu 0.21 ~ 0.50%, Ti 0.001 ~ 0.1%, Fe 0.06% or less, Si 0.06% or less, and the remainder are Al and impurities, and has proposed an aluminum alloy thin plate, characterized in that the average grain size is 20 to 200㎛. .

However, in the prior art, casting of large slabs by semi-continuous casting, surface scalping, soaking, hot rolling, cold rolling, intermediate to achieve good hot formability and high formability for the final sheet product Many processes, such as annealing, final rolling and final annealing, have to go through, resulting in higher manufacturing costs.

In addition, during large slab casting, for example, Al-Fe-Si intermetallic compounds such as Al ₆ Mn are coarsened to several tens of micrometers or more because the cooling rate is slow, for example, about 1 to 10 ° C / sec. Even after cracking, hot rolling, cold rolling, annealing or the like, coarse intermetallic compounds of 10 µm or more are still present in the final sheet product. The cavities easily appear by peeling at the interface between the intermetallic compound and the matrix during hot forming. As a countermeasure, a method of limiting the content of Fe and Si to 0.1% or less is employed, but there is a problem in that the cost increases in the end because an expensive high purity metal must be used.

It is an object of the present invention to improve the high temperature formability and high formability without using high-purity high-purity metals, which can solve the above problems of the prior art, and to produce aluminum alloy thin plates having reduced cavities after molding and their production. To provide a way.

In order to achieve the above object, according to the present invention, as the aluminum alloy thin plate excellent in high temperature formability and high-speed formability, the amount of the cavity after molding is reduced,

Mg: 2.0-8.0 wt%,

Si: 0.06% to 0.2% by weight,

Fe: 0.1-0.5 wt%,

Mn: 0.1 to 0.5% by weight and the balance include Al and unavoidable impurities, the density of the intermetallic compound having an equivalent circle diameter of 1 to 5㎛ is 5000 / ㎜ or more and the average grain size is 20 An aluminum alloy thin plate is provided, which is characterized in that it is m or less.

In order to achieve the above object, according to the present invention, the amount of the cavity after molding according to the present invention is reduced, and as a method for producing an aluminum alloy thin plate excellent in high temperature formability and high speed formability,

Preparing an molten alloy having a composition of an aluminum alloy sheet according to the present invention,

Casting at a cooling rate of 20 to 150 ° C./sec at a quarter point of the slab thickness during casting to produce a 5 to 15 mm thick slab in a twin belt casting apparatus,

Then winding the slab with a coil,

Removing the slab from the coil and cold rolling at a cold rolling reduction rate of 70 to 96%,

There is further provided an aluminum alloy thin plate manufacturing method comprising the step of annealing the cold rolled thin plate to a temperature of 420 to 500 ° C. at a heating rate of 5 ° C./sec.

The aluminum alloy sheet according to the present invention defines the range and microstructure of the chemical composition and finely and uniformly disperses the intermetallic compound to further refine the grains without using high purity metal, thereby improving high temperature formability and high formability and forming Later the cavity is reduced.

In addition, the manufacturing method according to the present invention ensures high cooling rate during twin belt casting, limits cold rolling reduction, and limits annealing conditions after cold rolling to uniformly and finely disperse intermetallic compounds to increase the refinement of grains. Let's do it.

By using the aluminum alloy sheet according to the present invention, a high grade product can be obtained, and the molding time is shortened and the productivity is improved.

BEST MODE FOR CARRYING OUT THE INVENTION

The reason for limitation of the chemical composition of the alloy according to the present invention will be described below. As used herein, "%" refers to "% by weight" unless stated otherwise.

[Mg: 2.0-8.0%]

Mg is an element which improves strength. In order to improve the strength, Mg should be contained 2.0% or more. However, if the Mg content exceeds 8.0%, the castability of the thin slab is lowered. Therefore, the Mg content is limited to 2.0 to 8.0%. If castability should be emphasized, the upper limit of the Mg content is preferably limited to 6.0% or less.

[Si: 0.06-0.2%]

Si precipitates as fine particles of Al-Fe-Si groups, Mg ₂ Si and other intermetallic compounds during casting, and functions as a recrystallization nucleation site during annealing after cold rolling. Therefore, the more particles of such an intermetallic compound, the more recrystallization nuclei are generated, and the number of fine recrystallizations is increased as a result. In addition, the fine particles of the intermetallic compound fix the grain boundaries of the recrystallized grains to suppress growth by fusing the crystal grains so that the fine recrystallized grains are stably maintained.

In order to exhibit this effect, the Si content should be 0.06% or more. However, when the Si content exceeds 0.2%, the intermetallic compound to be precipitated tends to be coarsened, so that the formation of a cavity at high temperature deformation is promoted. The Si content is therefore limited to 0.06 to 0.2%. The preferred range is 0.07 to 0.15%.

In general, Si is considered to be an impurity to be removed in the same manner as Fe described below, but in the present invention, on the contrary, as described above, an appropriate amount of Si must be present to increase the refining of recrystallized grains. Therefore, high purity metals are not needed, and thus a cost increase does not occur.

[Fe: 0.1-0.5%]

Fe precipitates as fine grains of Al-Fe-Si groups or other intermetallic compounds during casting and functions as nucleation sites for recrystallization during annealing after cold rolling. Therefore, the larger the number of such intermetallic particles, the greater the number of recrystallized nuclei to be produced and the more fine recrystallized grains are produced as a result. In addition, the fine particles of the intermetallic compound fix the grain boundaries of the recrystallized grains to suppress growth by fusing the crystal grains so that the fine recrystallized grains are stably maintained. In order to achieve this effect, the Fe content should be 0.1% or more. However, when the Fe content exceeds 0.5%, the precipitated intermetallic compound tends to coarsen, thereby promoting the formation of cavities during high temperature deformation. The Fe content is therefore limited to 0.1 to 0.5%. The preferred range is 0.1 to 0.3%.

In general, Fe is regarded as an impurity to be removed in the same manner as Si above, but in the present invention, on the contrary, as described above, an appropriate amount of Fe must be present to increase the refining of recrystallized grains. Therefore, high purity metals are not needed, and thus a cost increase does not occur.

[Mn: 0.1-0.5%]

Mn is an element that increases the refining of recrystallized grains. To exhibit this effect, Mn should be added at 0.1% or more. However, when Mn exceeds 0.5% or more, coarse Al- (Fe.Mn) -Si-based intermetallic compounds are formed to promote the formation of voids at high temperature deformation. Therefore, the Mn content is limited to 0.1 to 0.5%. In particular, when it is important to prevent the formation of cavities, it is preferable that the upper limit of the Mn content is further limited to 0.3%.

[Optional Component Cu: 0.1-0.5%]

In the present invention, Cu may be added within the range of 0.1 to 0.5% to improve the strength of the aluminum alloy sheet. In order to obtain sufficient precipitation strengthening effect, the Cu addition amount should be 0.1% or more. However, castability will fall when Cu addition amount exceeds 0.5%. When castability is important, it is preferable that the upper limit of Cu addition amount is further limited to 0.3% or less.

[Optional component Zr and Cr: 0.1 to 0.4%]

In the present invention, at least one of Zr and Cr may be added in the range of 0.1 to 0.4% to contribute to the recrystallization refinement enhancement. Zr and Cr are elements that promote refinement of recrystallized grains. In order to exhibit this effect, the addition amount of both Zr and Cr should be 0.1% or more. However, when the addition amount exceeds 0.4%, coarse intermetallic compounds are formed during casting to promote the formation of voids after high temperature deformation. In particular, when it is important to prevent the expression of the cavity, it is preferable to further limit the upper limit of the amount to 0.2% or less.

[Other elements]

In the present invention, in order to enhance the refinement of the cast structure, Ti may be added within a range of 0.001 to 0.15%. In order to exhibit this effect, the Ti content should be 0.001% or more. However, when the Ti addition amount exceeds 0.15%, coarse compounds such as TiAl ₃ are formed to deteriorate formability at high temperatures and promote the appearance of cavities. Preferable range is 0.006 to 0.10%.

Next, the reason for limiting the microstructure of the alloy thin plate according to the present invention will be described.

[Density of intermetallic compound having circle equivalent diameter of 1 to 5㎛ 5000 / mm2 or more]

The present invention uses the fine intermetallic compound particles as a means for fixing the nucleation site of the recrystallized grains and (2) the grain boundaries of the recrystallized grains, followed by annealing after cold rolling to produce fine recrystallized grains. The fine crystal structure obtained by this method gives high elongation at high temperature and high speed deformation, and improves high temperature formability and high formability.

In order to obtain the above effect, the intermetallic compound having a circle equivalent diameter of 1 to 5 mu m should be present at a density of 5000 / mm 2 or more. As the intermetallic compound, as already mentioned, intermetallic compounds such as Al- (Fe.Mn) -Si group compounds, Mg ₂ Si and Al ₆ Mn are precipitated during casting. In order to exhibit the effects of (1) and (2) by such an intermetallic compound, the equivalent circle diameter should be 1 to 5 mu m. If the circle equivalent diameter is smaller than 1 mu m, the particles are too small to exhibit the effects of (1) and (2) above. On the contrary, when the particle size exceeds 5 mu m, cavities are easily generated at high temperature and high speed processing, and the strength and elongation after the end of processing decrease.

Intermetallic compounds having a size in the above-described range should be present at a density of 5000 / mm 2 or more.

If the density is less than 5000 / mm 2, the diameter of the recrystallized grain at the time of annealing exceeds 20 μm and the elongation at the time of high temperature deformation decreases.

[Average Grain Diameter 20㎛ or Less]

In the alloy sheet according to the present invention, the average grain diameter is 20 mu m or less. If the average grain size exceeds 20 µm, the elongation at the time of high temperature deformation decreases.

The reason for limiting the production method conditions of the present invention will be described below.

[Slab thickness 5-15mm cast by twin belt casting method and wound in coil form]

The twin belt casting method injects molten metal into a mold of a pair of water-cooled belts which rotate to face each other from one end in the vertical direction, solidify the molten metal to form a slab by cooling the surface of the belt, and then fabricate the slab. It is a continuous casting method in which the slab is manufactured by winding the coil at the other end of the mold in the form of a coil.

In the present invention, the thickness of the slab cast by the twin belt casting method is 5 to 15 mm. When the thickness of the slab is in the above range, it is possible to solidify at a high solidification rate even in the center portion in the thickness direction of the thin plate, so that a uniform cast structure can be easily obtained. At the same time, it is possible to easily suppress the formation of coarse intermetallic compounds for the alloy having the component according to the present invention and to easily control the average grain size of the recrystallized grains of the final sheet product to 20 µm or less. It becomes The slab thickness ranges described above are also suitable in terms of twin belt casting methods.

In other words, if the slab thickness is smaller than 5 mm, the amount of molten aluminum alloy passing through the casting apparatus per unit time becomes too small, making casting by the twin belt casting method difficult. If the slab thickness exceeds 15 mm, it is difficult to wind the slab with a coil.

[20 to 150 ° C / sec cooling rate at the time of casting]

In the production method according to the invention, the slabs of 5 to 15 mm thickness are cast by twin belt casting. When casting, cooling at a quarter point of slab thickness during casting, in order to precipitate an intermetallic compound having a circular equivalent diameter of 1 to 5 탆 to a density of 5000 / mm 2 or more for producing an alloy according to the present invention. The rate should be 20 to 150 ° C./sec. In the aluminum alloy according to the present invention, intermetallic compounds such as Al- (Fe.Mn) -Si group compounds and Mg ₂ Si are precipitated during casting. If the cooling rate is less than 20 ° C./sec, these intermetallic compounds become coarse and the compounds exceed 5 μm. On the contrary, if the cooling rate exceeds 150 ° C./sec, the intermetallic compound becomes fine so that the size of the compound is 1 μm or less. Eventually, in either of the two cases, the density of the intermetallic compound having a circle equivalent diameter of 1 to 5 탆 is smaller than 5000 / mm 2, and the nuclei of the recrystallized grains become smaller and smaller at the time of final annealing (CAL). Becomes coarse.

[Cold rolling reduction rate 70-96% during cold rolling]

Accumulation of dislocations around the intermetallic compound formed by plastic working by cold rolling is indispensable for forming fine recrystallized structures at the time of final annealing. If the cold rolling reduction rate is less than 70%, the accumulation of dislocations becomes insufficient and a fine recrystallized structure cannot be obtained. If the cold rolling reduction rate exceeds 96%, edge cracking occurs during cold rolling, making cold rolling difficult.

[Temperature heating rate 5 ° C./sec or higher for annealing by heating to 420 to 500 ° C.]

In the present invention, the annealing is performed as the final annealing after cold rolling. Although generally carried out in a continuous annealing manner, it is not necessary to specifically limit the annealing to continuous annealing.

The annealing temperature of the final annealing is within the range of 420 to 500 ° C. If the temperature is lower than 420 DEG C, the energy for recrystallization becomes insufficient so that recrystallization becomes insufficient and fine recrystallization structure cannot be obtained. However, if the temperature exceeds 500 ° C., the recrystallized grain diameter exceeds 20 μm, whereby fine recrystallized structures cannot be obtained.

The heating rate up to the annealing temperature should be 5 ° C./sec or higher. If the heating rate is slower than 5 ° C./sec, the recrystallized grains become coarse and fine recrystallized structures cannot be obtained.

Finally, the molding of the aluminum alloy sheet according to the invention is preferably carried out in the 400 to 500 ℃ range. If the molding temperature is lower than 400 ° C., sufficient elongation cannot be obtained. If the molding temperature exceeds 550 ° C., coarsening of grains occurs. In addition, burning occurs in the alloy containing much Mg within the alloy composition range according to the present invention, and the elongation is lowered. It is preferable that the strain rate at the time of shaping | molding is 0.1 / sec or more. If the strain is less than 0.1 / sec, coarsening of the crystal grains occurs at the time of molding, resulting in a lowering of the elongation.

The aluminum alloy molten metal of the component shown in Table 1 was cast by the twin belt casting system, and the slab of thickness 7-9 mm was formed. Each slab was cold rolled to a thickness of 1 mm, annealed at 450 ° C., cut into specimens described in JIS H7501, and subjected to a tensile test, followed by elongation. In addition, the cross section of the broken sample was polished to measure the cross-sectional ratio of the cavity (cavity ratio) using an image analyzer. The production process and the characteristics of the specimens are shown in Table 2.

As evident from the alloying components of Table 1, the thin plates obtained by cold rolling the thin slab cast by twin belt casting apparatus, despite the Fe content of all samples being 0.1% or more and the Si content of 0.06% or more (Product of this invention, the sample numbers 1-7) have the density of the intermetallic compound whose round equivalent diameter is 1-5 micrometers, and are 5000 / mm <2> or more, and a grain size is 20 micrometers or less. For this reason, a good elongation at a tensile temperature of 500 ° C. was 200% or more, and a good cavity ratio after a high temperature tensile test was less than 0.15 to 0.27% or 1%.

A thin plate produced by cold rolling a thin slab cast by a twin roll casting device (Comparative Example, Sample No. 8) is a very fine metal having a circle equivalent diameter of less than 1 μm because the cooling rate is relatively high at 300 ° C./sec during casting. Contains large amounts of liver compounds. Therefore, the density of the intermetallic compound having a circle equivalent diameter of 1 to 5 탆 in the final thin plate is less than 5000 / mm 2 and the grain size is 20 탆 or more. For this reason, the void ratio after high temperature tension is relatively low so that the elongation in the tensile test at 500 ° C. is not as good as 80%.

The hot rolled sheet (Comparative Example, Sample No. 9) after hot-rolling a slab with a crack of about 7 mm or less in a cracked sheet of a normal slab cast by a DC casting apparatus has a relatively low cooling rate at the time of casting. The low 5 ° C./sec results in intermetallic compounds with circle equivalent diameters exceeding 5 μm. Therefore, the density of the intermetallic compound having a circle equivalent diameter of 1 to 5 µm in the final thin plate becomes less than 5000 / mm 2 and the grains slightly coarse to exceed 20 µm. For this reason, the void ratio after the high temperature tensile test is high as 15.% and the elongation in the tensile test at 500 ° C. is not as good as 160%.

The thin slab cast by twin belt casting apparatus is cold rolled, the sheet is cold rolled to a thickness of 1 mm after the intermediate annealing of the slab at 350 ℃ (Comparative Example, Sample No. 10) has a circle equivalent diameter in the final thin plate 1 to The density of the intermetallic compound having a thickness of 5 µm is 5000 / mm 2 or more. However, since the cold reduction rate before final annealing is low, less than 70%, the grains slightly coarse beyond 20 µm. Elongation in the tensile test at 500 ° C. is not good, less than 200%.

A thin plate produced by cold rolling a thin slab cast by a twin belt casting device (Comparative Example, Sample No. 11) has a density of 5000 / mm 2 or more in an intermetallic compound having a circle equivalent diameter of 1 to 5 μm in the final thin plate. The crystal grain is 20 mu m or less. However, the elongation is less than 200% because the tensile temperature in the tensile test is relatively low 350 ℃.

A thin sheet produced by cold rolling a thin slab cast by a twin belt casting device (Comparative Example, Sample No. 12) has a density of 5000 / mm 2 or more between the intermetallic compounds having a circle equivalent diameter of 1 to 5 µm in the final sheet. The crystal grain is 20 mu m or less. However, the void ratio after high temperature tension is not good as 1.8% and the elongation of tensile test at 500 ° C. is not good as less than 200% because the tensile speed at the tensile test is relatively low 0.01 / sec.

The present invention provides an aluminum alloy sheet and a method for producing the same, in which the amount of the cavity is reduced after molding and excellent in high temperature formability and high speed formability.

Claims (6)

An aluminum alloy sheet that reduces the amount of cavities after molding and is excellent in high temperature formability and high speed formability. Mg: 2.0 to 8.0% by weight, Si: 0.06 ~ 0.2% by weight, Fe: 0.1 to 0.5% by weight, Mn: 0.1-0.5 wt%, and Remainder: Contains Al and inevitable impurities, An aluminum alloy thin plate, wherein the intermetallic compound having a circle equivalent diameter of 1 to 5 µm has a density of 5000 / mm 2 or more and an average grain size of 20 µm or less. The method of claim 1, Cu: aluminum alloy sheet, characterized in that it further comprises 0.1 to 0.5% by weight. The method according to claim 1 or 2, Aluminum alloy sheet, characterized in that it further comprises at least one of Zr: 0.1 to 0.4% by weight and Cr: 0.1 to 0.4% by weight. The method according to any one of claims 1 to 3, An aluminum alloy thin plate, characterized in that the elongation is at least 200% in the case of tensile strain at a strain rate of 0.1 to 1.0 / sec in the temperature range of 400 to 550 ℃. The method of claim 4, wherein An aluminum alloy thin plate, wherein the void ratio in the cross section after breakage due to tensile deformation is 1% or less. A method for producing an aluminum alloy sheet, wherein the amount of cavities after molding according to any one of claims 1 to 5 is reduced and is excellent in high temperature formability and high speed formability. Preparing the molten alloy of the composition according to any one of claims 1 to 3, Casting the molten alloy with a twin belt casting apparatus within a range of 20 to 150 ° C./sec cooling rate at a quarter point of the slab thickness during casting to produce a slab with a thickness of 5 to 15 mm, Winding the slab in the form of a coil, Cold rolling the coil to a cold reduction ratio of 70 to 96%, and And performing an annealing of the cold rolled thin plate at a heating rate of 420 to 500 ° C. at a heating rate of 5 ° C./sec or higher.