Classifications

• • 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
  • C22F1/043—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 silicon as the next major constituent
• • 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
  • C22C21/02—Alloys based on aluminium with silicon as the next major constituent

Definitions

• the present invention relates to a process for manufacturing an aluminum alloy material for forming which has less anisotropy and excellent formability in press working, shape fixability and bake hardenability, and which is especially suitable for the manufacture of transport machinery, such as the body sheet material of automobiles.
• the body sheet materials of automobiles satisfy the requirements for (1) formability, (2) shape fixability (accurate reproduction of the shape of press dies in press working), (3) high strength, (4) dentability, and (5) corrosion resistance, etc.
• 6009, 6111 and 6016 alloys have been developed as the 6000 series AI-Mg-Si alloys having high strength. These alloys acquire high strength by heat treatment at 200 ° C for about 30 minutes in the paint bake step, so that the attainment of marked decrease in thickness, i.e., the attainment of a light-weight structure, is feasible.
• the shape fixability can be improved as the elastic modulus is increased and the yield strength is decreased (see SAE Paper No. 890719). Because the elastic modulus of an aluminum alloy is 7000 kgf/mm 2 which is about one third of 21,000 kgf/mm 2 for steel, it is impossible to obtain a material having the same shape fixability as that of a steel sheet, unless the yield strength of the aluminum alloy sheet in press working is considerably decreased. According to the experiment of the present inventors, it is desired that the yield strength be lower than 14 kgf/mm 2.
• the yield strength of the aluminum alloy sheet manufactured by the conventional method is inevitably increased to about 14 kgf/mm 2 or above in both of the 5000 series alloy and the 6000 series alloy, which is likely to give rise to a poor shape fixability.
• the present invention provides the material especially suitable for press working which has room temperature age hardening properties suppressed so as to improve formability and exhibits improved shape fixability and excellent bake hardenability, as a result of detailed studies of chemical compositions and thermomechanical treatment.
• the present invention provides a process for manufacturing an aluminum alloy for forming with excellent formability, shape fixability and bake hardenability, which comprises the steps of:
• V is further added, as an essential component, in an amount of 0.02% to 0.2% to the above-specified alloy composition with the object of reducing anisotropy.
• Mg It is needed to obtain high strength like Si.
• the amount of Mg is less than 0.3%, no satisfactory strength can be obtained even when heating in paint bake is conducted.
• the amount is 0.5% or above, the yield strength is too high after the solution heat treatment or the completion of final heat treatment, and the formability and the shape fixability are poor.
• V Its addition contributes to a further increase in the strength and an improvement in the isotropy of the mechanical properties. However, when the amount thereof is less than 0.02%, no effect can be exhibited. On the other hand, when the amount of addition of V exceeds 0.2%, the yield strength is too high after the solution heat treatment or the completion of final heat treatment, and the formability and the shape fixability are poor.
• Cu Its addition contributes to a further increase in the strength. However, when the amount of addition exceeds 0.20%, the yield strength is too high after the solution heat treatment or the completion of final heat treatment, and not only the formability and the shape fixability but also the resistance to filiform corrosion are poor.
• Mn Its addition contributes to a further increase in the strength and makes the grains finer so as to improve the formability.
• the amount of addition exceeds 0.20%, the yield strength is too high after the solution heat treatment or the completion of final heat treatment, and not only the formability and the shape fixability are poor but also coarse intermetallic compounds are increased so as to lower the formability.
• Solution heat treatment When the heating rate is below 100 ° C/min, grains become so coarse that the formability is poor. Further, when the heating temperature is below 450 ° C, the dissolution of precipitates is unsatisfactory and no satisfactory strength can be obtained after paint bake. On the other hand, when the temperature is 580 ° C or above, eutectic melting occurs to thereby lower the formability. Still further, when the holding time at 450 ° C or above is less than 10 seconds, the dissolution of precipitates is unsatisfactory and no satisfactory strength can be obtained after paint bake. On the other hand, a holding time of 10 minutes or more does not bring about any further improvement in the performances, so that it is less valuable from the industrial viewpoint.
• Cooling step The rate of cooling down to 150 ° C or below after the solution heat treatment should be 100 ° C/min or more. When the cooling rate is less than 100 ° C/min, coarse intermetallic compounds are precipitated along the grain boundaries so as to lower the ductility, thus leading to poor formability.
• Holding time at room temperature Preferably, the holding time of the sheet at room temperature should be as short as possible.
• the allowable time is less than 60 minutes.
• the holding time is 60 minutes or longer, GP zones grow and the decomposition of the GP zones take a long time even when baking is conducted at about 170 ° C after the press, which renders hardening difficult.
• Each alloy listed in Table 1 was semicontinuously cast and the surface of the ingot was sculped. Subsequently, the alloy was homogenized at 550 ° C for 24 hours, and the temperature was then allowed to fall to 500 ° C. Hot rolling was started at that temperature, and the alloy was rolled to a thickness of 5 mm. Then, the hot rolled alloy was subjected to intermediate annealing at 350 ° C for 1 hour in a batch furnace and cold-rolled to prepare a sheet having a thickness of 1 mm.
• the sheet was subjected to solution heat treatment under the conditions specified in Table 2 in a continuous annealing line, cooled, allowed to hold still at a predetermined room temperature, subjected to final heat treatment under the conditions specified in Table 2, and leveled.
• the mechanical properties of the obtained materials were evaluated after aging at room temperature for one month subsequent to the final heat treatment.
• the materials of Comparative Examples 1 and 2 exhibited a low yield strength even after heating at 170 ° C for 30 minutes, because the material of Comparative Example 1 had an Si content lower than that defined in the claims and the material of Comparative Example 2 had an Mg content lower than that defined in the claims.
• the Si content and Mg content of the material of Comparative Example 3, the Cu content of the material of Comparative Example 4, the Mn content of the material of Comparative Example 5 and the Mn content and Cr content of the material of Comparative Example 6 were higher than the respective content ranges defined in the claims, and these materials exhibited too high yield strengths after the final heat treatment and yield strengths exceeding 13.5 kgf/mm 2 after the one-month room temperature age hardening, so that the shape fixability and formability were poor.
• each alloy listed in Table 4 was semiconsciously cast and the surface of the ingot was sculped. Subsequently, the alloy was homogenized at 550 ° C for 24 hours, and the temperature was then allowed to fall to 500 ° C. Hot rolling was started at that temperature, and the alloy was rolled to a thickness of 2 mm. Then, the hot rolled alloy was subjected to solution heat treatment under the conditions specified in Table 5 in a continuous annealing line, cooled, allowed to hold still at predetermined room temperature, subjected to final heat treatment under the conditions specified in Table 5, and leveled.
• Comparative Example 17 since the material was held at 20 ° C for a period of as long as 100 minutes between the solution heat treatment and the final heat treatment, a large number of GP zones were formed during the holding, so that no high strength could be obtained even when the material was heated at 170 ° C for 30 minutes.
• the material of Comparative Example 18 since the final heat treatment was so short that the formation of clusters was insufficient and the formability slightly lowered due to the progress of the room temperature age hardening after the heat treatment. Further, no high strength could be obtained even when the material was heated at 170 ° C. In the material of Comparative Example 19, the final heat treatment temperature was so high that the formability lowered due to the hardening of the material.
• each of the alloys listed in Table 7 was semicontinuously cast, and sculping was conducted on the cast surface.
• the Fe and Ti listed in Table 7 are impurities.
• homogenizing treatment was performed at 550 ° C for 24 hours, and the temperature was decreased to 520 ° C, at which hot rolling was started. The rolling was conducted to obtain a thickness of 5 mm, followed by intermediate annealing at 350 ° C for 1 hour in a batch furnace.
• Cold rolling was performed to prepare a sheet having a thickness of 1 mm.
• the sheet was subjected to solution heat treatment in a continuous annealing line under the conditions specified in Table 8, cooled, and allowed to hold still at a predetermined room temperature. Thereafter, the final heat treatment was performed under the conditions specified in Table 8.
• the evaluation of the mechanical properties of the resultant materials were conducted after the one-month room temperature age hardening subsequent to the final heat treatment.
• Materials exhibiting a yield strength increase of 5 kgf/mm 2 or more after heat treatment at 170 ° C for 30 minutes even subsequent to the one-month room temperature age hardening were deemed as having an excellent bake hardenability.
• Materials exhibiting a yield strength increase of 13.5 kgf/mm 2 or more after heat treatment at 170 ° C for 30 minutes even subsequent to the one-month room temperature age hardening were deemed as having excellent dentability.
• Materials satisfying all of the above criteria were deemed as acceptable. With respect to the grain diameter, the sheet surface was observed, and materials having a grain diameter of 100 /1.m or less were deemed as acceptable.
• the material of Comparative Example 20 had poor formability, because the heating rate to the solution heat treatment was lower than that defined in the claims so that grains become coarse.
• the material of Comparative Example 21 was poor in both of formability and bake hardenability, because the cooling rate in the solution heat treatment was lower than that defined in the claims so that solute atoms were precipitated on the grain boundaries to thereby diminish the hardening effect.
• the materials of Comparative Examples 22 and 23 had poor bake hardenability, because the former had a holding temperature in the solution heat treatment of below that defined in the claims for patent and the latter had a holding time in solution heat treatment of shorter than that defined in the claims for patent, resulting in an insufficient solution heat treatment.
• the evaluation of the material of Comparative Example 24 was halted because the temperature in the solution heat treatment was so high that partial eutectic melting occurred.
• the materials of Comparative Examples 25 and 26 had poor bake hardenability, because the former had a room temperature holding time after the solution heat treatment of longer than that defined in the claims for patent and the latter had a final heat treatment time of shorter than the lower limit of that defined in the claims for patent.
• the materials of Comparative Examples 27 and 28 were poor in both of bake hardenability and formability, because the former had a final heat treatment time of longer than the upper limit of that defined in the claims for patent and the latter had a final heat treatment temperature of higher than the upper limit of that defined in the claims for patent.
• the material of Comparative Example 29 had poor bake hardenability, because the final heat treatment temperature is lower than the lower limit of that defined in the claims for patent.
• Comparative Examples 30 and 31 were poor in both of bake hardenability and dentability, because the former contained Si in an amount smaller than the lower limit of that defined in the claims for patent and the latter contained Mg in an amount smaller than the lower limit of that defined in the claims for patent.
• an anisotropy was observed in the mechanical properties because the content of V was smaller than the lower limit of that defined in the claims for patent.
• each of the materials of Comparative Examples 32 to 35 exhibited a too high yield strength after the one-month room temperature age hardening and were poor in both of shape fixability and formability because the materials of Comparative Example 32 contained Si in an amount larger than the upper limit of that defined in the claims for patent, the material of Comparative Example 33 contained Cu in an amount larger than the upper limit of that defined in the claims for patent, the material of Comparative Example 34 contained Mn and Cr in amounts larger than the upper limits of those defined in the claims for patent, and the material of Comparative Example 35 contained V in an amount larger than the upper limit of that defined in the claims for patent.
• the present invention can provide a material for press which has improved formability by virtue of suppression of room temperature age hardening, improved shape fixability and excellent bake hardenability. Further, it becomes possible to manufacture various forming materials each having a smaller thickness than that of the conventional forming material, so that the reduction in the weight of the formed article can be further promoted.
• the present invention has been described mainly on examples of the sheet material, it can be applied also to other manufacturing processes, such as a process for manufacturing an extruded material, because the alloy used can be manufactured on the same principle.
• the paint bake temperature may be lowered to 150 ° C or below in the near future, the effects attained by heating at 170 ° C cannot be expected at that temperature.
• the performance of the product is clearly superior to that of the prior art.

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• Chemical & Material Sciences (AREA)
• Engineering & Computer Science (AREA)
• Materials Engineering (AREA)
• Mechanical Engineering (AREA)
• Metallurgy (AREA)
• Organic Chemistry (AREA)
• Physics & Mathematics (AREA)
• Thermal Sciences (AREA)
• Crystallography & Structural Chemistry (AREA)
• Shaping Metal By Deep-Drawing, Or The Like (AREA)

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Cited By (20)

Citations (4)

• 1991
  • 1991-10-09 EP EP19910117216 patent/EP0480402B1/de not_active Revoked
  • 1991-10-09 DE DE1991607392 patent/DE69107392T2/de not_active Revoked

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