Classifications

• • B—PERFORMING OPERATIONS; TRANSPORTING
  • B22—CASTING; POWDER METALLURGY
  • B22D—CASTING OF METALS; CASTING OF OTHER SUBSTANCES BY THE SAME PROCESSES OR DEVICES
  • B22D11/00—Continuous casting of metals, i.e. casting in indefinite lengths
  • B22D11/001—Continuous casting of metals, i.e. casting in indefinite lengths of specific alloys
  • B22D11/003—Aluminium alloys
• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C21/00—Alloys based on aluminium
  • C22C21/06—Alloys based on aluminium with magnesium as the next major constituent
  • C22C21/08—Alloys based on aluminium with magnesium as the next major constituent with silicon
• • B—PERFORMING OPERATIONS; TRANSPORTING
  • B22—CASTING; POWDER METALLURGY
  • B22D—CASTING OF METALS; CASTING OF OTHER SUBSTANCES BY THE SAME PROCESSES OR DEVICES
  • B22D11/00—Continuous casting of metals, i.e. casting in indefinite lengths
  • B22D11/06—Continuous casting of metals, i.e. casting in indefinite lengths into moulds with travelling walls, e.g. with rolls, plates, belts, caterpillars
  • B22D11/0605—Continuous casting of metals, i.e. casting in indefinite lengths into moulds with travelling walls, e.g. with rolls, plates, belts, caterpillars formed by two belts, e.g. Hazelett-process
• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C21/00—Alloys based on aluminium
  • C22C21/06—Alloys based on aluminium with magnesium as the next major constituent
• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22F—CHANGING THE PHYSICAL STRUCTURE OF NON-FERROUS METALS AND NON-FERROUS ALLOYS
  • C22F1/00—Changing the physical structure of non-ferrous metals or alloys by heat treatment or by hot or cold working
  • C22F1/04—Changing the physical structure of non-ferrous metals or alloys by heat treatment or by hot or cold working of aluminium or alloys based thereon
  • C22F1/047—Changing the physical structure of non-ferrous metals or alloys by heat treatment or by hot or cold working of aluminium or alloys based thereon of alloys with magnesium as the next major constituent

Definitions

• the present invention relates to an Al-Mg alloy sheet with excellent formability at high temperatures and high speeds and a method of production of the same.
• An Al-Mg alloy is light and excellent in strength and corrosion resistance, so is being proposed as an automobile sheet material or other worked or formed material.
• its elongation at room temperature is low, therefore th ⁇ re is the problem that an Al-Mg alloy cannot be formed into a complex shape by cold working.
• an Al-Mg-based superplastic alloy suppressing the recrystallization at the time of hot working to reduce the size of the crystal grains and obtaining an elongation of several 100% in a high temperature region of for example 500 to 550°C as been developed and is being used for various applications.
• a conventional Al-Mg-based superplastic alloy manifests its superplasticity at a slow forming speed
• strain rate 10 "4 to 10 ⁇ 3 /sec and requires a long time, therefore is low in productivity when applied to ordinary press forming and is not practical. Therefore, an aluminum alloy sheet able to give a sufficient elongation even with high forming speed of a strain rate of for example 0.1/sec or more in the high temperature region for hot working, that is, 100 times or more than that of the prior art, and able to suppress occurrence of cavities at the time of forming has been developed.
• 10-259441 proposes an aluminum alloy sheet with excellent superplastic formability at high speeds and having a reduced amount of cavities after forming characterized in that it contains 3.0-8.0% (wt%, same below) of Mg, 0.21-0.50% of Cu, and 0.001-0.1% of Ti, contains as impurities Fe to 0.06% or less and Si to
• An object of the present invention is to provide an aluminum alloy sheet solving the above problems of the prior art, not requiring the use of high purity metal accompanied with higher cost, improving the formability at high temperatures and high speeds, and reducing the cavities after forming and a method of production of the same .
• an aluminum alloy sheet with excellent formability at high temperatures and high speeds with a reduced amount of cavities after forming characterized in that it consists of: Mg: 2.0-8.0wt%, Si: 0.06-0.2wt%, Fe: 0.1-0.5wt%, Mn: 0.1-0.5wt%, and the balance of Al and unavoidable impurities, wherein a density of an inter-metallic compound having an equivalent circle diameter of 1 to 5 ⁇ m is 5000/mm 2 or more and an average crystal grain size is 20 ⁇ m or less.
• a method of production of an aluminum alloy sheet of the present invention with excellent formability at high temperatures and high speeds with a reduced amount of cavities after forming characterized in that it comprises the steps of: preparing an alloy melt having a composition of the aluminum alloy sheet of the present invention, casting the alloy melt 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 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 with a cold rolling reduction of 70 to 96%, and performing annealing for heating the obtained cold rolled sheet at a rate of temperature rise of 5°C/sec or more to 420 to 500°C.
• the aluminum alloy sheet of the present invention defines ranges of the chemical composition and microstructure and disperses the inter-metallic compounds uniformly and finely so as to improve the formability at high temperatures and high speeds by the increased fineness of the crystal grains without requiring any high purity metal and reduce the cavities after forming. Further, the method of production of the present invention secures a high cooling rate at the time of casting by twin belt casting, restricts the cold rolling reduction, and limits the annealing conditions after the cold rolling so as to realize a uniform fine dispersion of the inter-metallic compounds and increased fineness of the crystal grains. 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 enhanced.
• Mg is an element improving the strength. In order to manifest this effect, it is necessary to set the Mg content to 2.0% or more. However, if the Mg content exceeds 8.0%, the castability of a thin slab is lowered. Accordingly, the Mg content is limited to 2.0 to 8.0%. If stressing the castability, preferably the upper limit of the Mg content is further limited to 6.0% or less.
• Si is precipitated as fine particles of Al-Fe-Si- based, Mg 2 Si, and other inter-metallic compounds at the time of casting and functions as a nucleus generating site of recrystallization at the time of annealing after cold rolling. Accordingly, the larger the number of particles of these inter-metallic compounds, the larger the number of generated recrystallized nucleii and as a result the larger number of fine recrystallized grains formed. Further, the fine particles of the inter-metallic compounds pin the grain boundaries of the generated recrystallized grains and suppress growth due to merging of crystal grains to stably maintain the fine recrystallized grains. In order to manifest these effects, it is necessary to make the Si content 0.06% or more.
• the Si content exceeds 0.2%, the tendency of the precipitated inter-metallic compounds to become coarser becomes stronger, so the formation of cavities is promoted at the time of high temperature deformation. Accordingly, the Si content is limited to 0.06 to 0.2%. The preferred range is 0.07 to 0.15%.
• Si is regarded as an impurity element to be eliminated in the same way as the following Fe, but in the present invention, conversely a suitable amount of Si is made present in order to increase the fineness of the recrystallized grains as described above. Accordingly, high purity metal is not needed and there is no accompanying rise in cost.
• Fe is precipitated as fine grains of Al-Fe-Si-based or other inter-metallic compounds at the time of casting and functions as a nuclei generating site of recrystallization at the time of annealing after cold rolling. Accordingly, the larger the number of particles of these inter-metallic compounds, the larger the number of the generated recrystallized nucleii and as a result the larger the number of fine recrystallized grains formed. Further, the fine particles of the inter-metallic compounds pin the grain boundaries of the generated recrystallized grains and suppress the growth due to merger of crystal grains to stably maintain the fine recrystallized grains. In order to manifest this effect, it is necessary to make the Fe content 0.1% or more.
• the Fe content is limited to 0.1 to 0.5%.
• a preferred range is 0.1 to 0.3%.
• Fe is regarded as an impurity element to be eliminated in the same way as the above Si, but in the present invention, conversely a suitable amount of Fe is made present in order to increase the fineness of the recrystallized grains as described above. Accordingly, high purity metal is not needed and there is no accompanying rise in cost.
• Mn 0.1-0.5%
• Mn is an element increasing the fineness of the recrystallized grains.
• the Mn content is 0.1% or more.
• the Mn content exceeds 0.5%, a coarse Al- (Fe*Mn) -Si-based inter-metal compound is formed, and the occurrence of cavities is promoted at the time of high temperature deformation. Accordingly, the Mn content is limited to 0.1 to 0.5%. Particularly, when stressing the prevention of occurrence of cavities, preferably the upper limit of the Mn content is further restricted to 0.3%.
• Optional ingredient Cu: 0.1-0.5% In the present invention, Cu can be added within a range of 0.1-0.5% in order to improve the strength of the aluminum alloy sheet.
• the amounts of addition exceed 0.4%, coarse intermetallic compounds are formed at the time of the casting, and the occurrence of cavities is promoted at the time of high temperature deformation.
• the upper limits of the amounts of addition are further restricted to 0.2% or less.
• Ti can be added within a range of 0.001-0.15%. In order to manifest this effect, it is necessary to make the amount of addition of Ti 0.001% or more. However, if the amount of addition of Ti exceeds 0.15%, a coarse compound such as TiAl 3 is generated, the formability at a high temperature is deteriorated, and the occurrence of cavities is promoted.
• a preferred range is 0.006-0.10%.
• the present invention utilizes the fine inter- metallic compound particles as (1) the recrystallized grain nuclei generating sites and (2) means for pinning the grain boundaries of the recrystallized grains and generates finer recrystallized grains by the annealing after the cold rolling.
• the fine grain structure obtained by this gives a high elongation at the time of deformation at high temperatures and high speeds, whereby the formability at high temperatures and high speeds is enhanced.
• the intermetallic compound having the equivalent circle diameter of 1 to 5 ⁇ m must be present in a density of 5000/mm 2 or more.
• inter-metallic compounds such as Al-(Fe-Mn)- Si-based compounds, Mg 2 Si, and Al 6 Mn are precipitated during casting.
• the equivalent circle diameter must be 1 to 5 ⁇ m. If the equivalent circle diameter is less than 1 ⁇ m, the particles are too small to manifest the effects of (1) and (2) described above.
• the inter-metallic compounds having the size within the above described range must be present at a density of 5000/mm 2 or more. If the density is less than 5000/mm 2 , the recrystallized grain diameter at the time of the annealing exceeds 20 ⁇ m, and the elongation at the time of high temperature deformation is lowered. [Average crystal grain diameter of 20 ⁇ m or less] In the alloy sheet of the present invention, the average crystal grain diameter is made 20 ⁇ m or less.
• the twin belt casting method is a continuous casting method injecting a melt into a mould of a pair of water cooled rotating belts facing each other from one end in the vertical direction, solidifying the melt by the cooling from the belt surfaces to form the slab, pulling out the formed slab from the other end of the mould, and taking it up in the form of a coil.
• the thickness of the slab cast by this twin belt casting method is made 5 to 15 mm.
• the thickness is within this range, a high solidification speed can be secured even at the center portion of the sheet thickness, therefore a uniform casting structure can be easily formed.
• the composition of the present invention it is possible to easily suppress the generation of coarse inter-metallic compounds and it becomes easy to control the average grain size of the recrystallized grains in the final sheet product to 20 ⁇ m or less.
• the above described slab thickness range is also suitable from the viewpoint of the twin belt casting. Namely, if the slab thickness is less than 5 mm, the amount of the aluminum alloy melt passing through the casting machine per unit time becomes too small, so the twin belt casting becomes difficult. If the slab thickness exceeds 15 mm, it becomes difficult to rewind it up as a coil.
• the inter-metallic compounds become finer and the compounds less than 1 ⁇ m increase.
• the density of the inter-metallic compounds having the equivalent circle diameter of 1 to 5 ⁇ m becomes less than 5000/mm 2 and the nuclei of the recrystallized grains become fewer at the time of the final annealing (CAL) , so the recrystallized grains become coarse.
• CAL final annealing
• the above annealing is conducted as the final annealing after the cold rolling. This is generally conducted by the continuous annealing, but it is not particularly necessary to limit the annealing to this.
• the annealing temperature of the final annealing is made a range of 420 to 500°C.
• the temperature is less than 420°C, the energy required for recrystallization is insufficient, therefore the recrystallization becomes insufficient and a fine recrystallization structure cannot be obtained.
• the recrystallized grain diameter exceeds 20 ⁇ m, and the fine recrystallization structure cannot be obtained.
• the heating rate to the annealing temperature is made 5°C/sec or more. If the temperature is slowly elevated by a rate less than 5°C/sec, the recrystallized grains become coarse, so the fine recrystallization structure cannot be obtained.
• the forming of the aluminum alloy sheet of the present invention is preferably conducted at a temperature of 400-500°C.
• the forming temperature is less than 400°C, a sufficient elongation cannot be obtained. If the forming temperature exceeds 550°C, the coarsening of the crystal grains occurs. Further, burning occurs in an alloy having a high Mg content within the range of the present invention, and the elongation is lowered.
• the strain rate at the time of the shaping is preferably 0.1/sec or more. If the strain rate is less than 0.1/sec, the coarsening of the crystal grains occurs during the forming, so a drop of the elongation is induced. Examples Aluminum alloy melts having the compositions shown in Table 1 were cast by the twin belt casting method to form slabs having thicknesses of 7 to 9 mm.
• the recrystallized grains were measured by the cross-cut method.
• the cooling rate was calculated from DAS measurement results at 1/4 thickness of cast slab.
• Sheets obtained by cold rolling thin slabs cast by a twin belt casting machine (products of the present invention, Sample Nos. 1 to 7), as apparent also from the alloy compositions of Table 1, irrespective of the fact that the Fe contents were 0.1% or more and the Si contents were 0.06% or more in all samples, had densities of the inter-metallic compounds having equivalent circle diameters of 1 to 5 ⁇ m of 5000/mm 2 or more and crystal grain sizes of 20 ⁇ m or less. For this reason, the elongations at the tensile temperature of 500°C were good ones of 200% or more and also the cavity ratios after the high temperature tension were good ones of the range of 0.15-0.27% or less than 1%.
• a sheet obtained by cold rolling a thin slab cast by a twin roll casting machine (comparative example, Sample No. 8) had a large number of very fine intermetallic compounds having equivalent circle diameters less than 1 ⁇ m since the cooling rate at the time of the casting was a relatively high 300°C/sec, therefore the density of ' the inter-metallic compounds having an equivalent circle diameter of 1 to 5 ⁇ m in the final sheet became less than 5000/mm 2 or coarse exceeding the crystal grain size of 20 ⁇ m or more. For this reason, the cavity ratio after the high temperature tension was a relatively low good one of 0.12%, but the elongation at the tensile temperature of 500°C was a poor 80%.
• a sheet obtained by cold rolling a thin slab cast by a twin belt casting machine down to the sheet thickness of 2 mm, then intermediate annealing the slab at 350°C, then cold rolling down to 1 mm had a density of inter-metallic compounds of an equivalent circle diameter of 1 to 5 ⁇ m in the final sheet of 5000/mm 2 or more, but the cold rolling reduction before the final annealing was a low one of less than 70%, therefore the crystal grains became slightly coarse exceeding the crystal grain size of 20 ⁇ m.
• the elongation at the tensile temperature of 500°C was a poor one of less than 200%.
• a sheet obtained by cold rolling a thin slab cast by a twin belt casting machine comparativelongation at the tensile temperature of 500°C was a poor one of less than 200%.
• the tensile speed in the tensile test was a relatively slow 0.01/sec, therefore the cavity ratio after the high temperature tension was also a poor 1.8% and the elongation at the tensile temperature of 500°C was a poor one of less than 200%.
• INDUSTRIAL APPLICABILITY According to the present invention, aluminum alloy sheet with excellent formability at high temperatures and high speeds with a reduced amount of cavities after the forming and the method of production of the same are provided.

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