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

• This invention relates to a rolled aluminum alloy strip adapted for mechanical forming and a method for preparing the same. More particularly, it relates to a rolled aluminum alloy strip which can be readily shaped for use in applications where easy forming and high strength are required and paint coatings are applied and baked prior to use, for example, automobile body sheets, various shaped parts and articles.
• Alloys (a) are insufficient in strength, tend to develop a Luders band during shaping, and lose strength during paint baking. Alloys (b) are rather difficult to shape and lose strength during paint baking. Alloys (c) are not satisfactory in forming, especially bending, and lose strength during paint baking. Among alloys (d), 6009 alloy has poor strength and 6010 alloy is insufficient in elongation and bending.
• An object of the present invention is to provide a new and improved rolled aluminum alloy strip which is improved in bake hardening, that is, exhibits high strength after baking of paint coatings, and is easy to mechanically work or form. Another object is to provide a method for preparing such a rolled aluminum alloy strip.
• rolled aluminum alloy strip adapted for forming having improved bake hardening ability and formability.
• the aluminum alloy consists essentially of, in percents by weight, 1.2 to 2.5% of Si, 0.15 to 1.5% of Mg, 0.1 to 1.5% of Cu, less than 0.2% of Fe, less than 0.05% of Mn, less than 0.05% of Cr, less than 0.05% of Zr, less than 0.05% of V, the total amount of Mn, Cr, Zr and V being less than 0.10%, and the balance of aluminum.
• the strip has an electric conductivity of up to 50% IACS and a mean crystal grain size of up to 100 ⁇ m at a surface.
• a method for preparing a rolled aluminum alloy strip adapted for forming having improved bake hardening ability and formability.
• a molten aluminum alloy of the above-defined composition is cast by a semi-continuous casting technique.
• the alloy is heated at a temperature of 480 to 560°C in a heating furnace and then hot rolled into a strip such that the temperature of the alloy being hot rolled drops to 400°C or lower within 30 minutes from the emergence of the alloy from the heating furnace.
• a subsequent solution heat treatment step includes heating the rolled strip at a heating rate of at least 5°C/sec.
• Cold rolling may be carried out between the hot rolling step and the solution heat treatment step.
• a rolled aluminum alloy strip for forming prepared by the inventive method.
• the rolled strip of the present invention is of an aluminum alloy consisting essentially of, in percents by weight, 1.2 to 2.5% of Si, 0.15 to 1.5% of Mg, 0.1 to 1.5% of Cu, less than 0.2% of Fe, less than 0.05% of Mn, less than 0.05% of Cr, less than 0.05% of Zr, less than 0.05% of V, the total amount of Mn, Cr, Zr and V being less than 0.10%, and the balance of aluminum and incidental impurities.
• Si Silicon which forms Mg2Si with magnesium is effective in improving strength through precipitation hardening and at the same time, contributes to an improvement in formability, especially elongation. Less than 1.2% of Si fails to provide a sufficient improvement in strength. Formability improves as the silicon content increases in excess of the stoichiometric ratio of Mg2Si. However, beyond 2.5% of Si, the formability improvement is no longer enhanced and formability, especially bending is rather deteriorated. For this reason, Si is limited to the range of 1.2% to 2.5%. If copper which is effective for strength improvement is not added in excess of 0.3%, Si should preferably be added in excess of 1.5%.
• Mg In the co-presence of silicon, magnesium forms Mg2Si to impart strength as described above. Less than 0.15% of Mg is insufficient to improve strength. In excess of 1.5%, work hardening is enhanced too much and workability, especially elongation is reduced. For this reason, Mg is limited to the range of 0.15% to 1.5%.
• Cu Copper is effective in improving strength and formability, especially elongation. Less than 0.1% of Cu is less effective whereas more than 1.5% of Cu provides extremely high strength at the sacrifice of formability. For this reason, Cu is limited to the range of 0.1% to 1.5%.
• Fe Iron contributes to crystal grain refinement, but lowers formability, especially bending. This tendency becomes outstanding with an iron content of 0.2% or more. Iron should be limited to less than 0.2% for formability.
• Mn, Cr, Zr, V These transition elements are effective in refining recrystallized grains, but adversely affect formability if present in excess. If their content exceeds 0.05% alone or 0.10% in total, formability becomes insufficient. Therefore, the content of the respective elements should be less than 0.05% and the total content of these elements should be less than 0.10%.
• the balance is aluminum. There may be present incidental impurities other than the above-mentioned elements.
• boron should preferably be in the range of 1 to 500 ppm.
• addition of Be in minor amounts is acceptable.
• Beryllium is effective, especially when an alloy containing Mg is melted, for suppressing oxidation of the molten metal and for preventing contaminants such as oxide particles from mixing into the material. Higher Be contents in excess of 100 ppm are economically meaningless since its effect is saturated. Thus the Be content should desirably be limited to 100 ppm or less.
• the rolled aluminum alloy strip of the present invention is defined not only by the above-mentioned alloy composition, but also by an electric conductivity of up to 50% IACS and a mean crystal grain size of up to 100 ⁇ m at a surface.
• the conductivity is related to the quantity of solid solution in the alloy matrix in that the higher the quantity of solid solution, the lower becomes the conductivity. Then conductivity provides a check on the quantity of solid solution.
• Mg, Si and Cu should be present in solid solution form as much as possible because a larger quantity of solid solution of these elements allows the elements to precipitate during baking of paint coatings, contributing to a strength improvement after baking of paint coatings, that is, higher bake hardening ability. If the quantity of Mg, Si and Cu in solid solution form is so small that the conductivity may exceed 50% IACS, then the alloy will increase a little its strength after baking of paint coatings, that is, has poor bake hardening ability. To secure sufficient bake hardening ability, a sufficient quantity of solid solution to provide a conductivity of 50% IACS or lower is necessary.
• the surface crystal grain size is related to skin roughening during forming.
• a mean grain size of up to 100 ⁇ m minimizes skin roughening whereas a mean grain size in excess of 100 ⁇ m leads to skin roughening, detracts from the appearance of a shaped member and in some cases, causes fracture during forming. For this reason, a mean grain size of up to 100 ⁇ m on a surface is necessary.
• a molten alloy of the above-defined composition is prepared in a conventional manner. It is then cast into a slab of rectangular cross section by a semi-continuous casting or direct chill (DC) casting technique.
• the casting rate is not critical although a rate of about 25 to 250 mm/min. is often employed.
• the slab is often subject to soaking prior to hot rolling, desirably by heating it at a temperature of 480 to 560°C for about 1/2 to 48 hours.
• This soaking is effective not only in eliminating any heterogeneity in the slab to improve formability as in the manufacture of conventional aluminum alloys, but also in causing some elements to enter into solid solution to enhance the effect of a subsequent solution heat treatment or even if they precipitate, in rendering the precipitates finer to facilitate a subsequent solution heat treatment.
• the soaking temperature is lower than 480°C or the holding time is less than 1/2 hour, Mg2Si insufficiently enters into solid solution and a hardened phase of Mg2Si or the like becomes coarser during soaking, which phase is difficult to convert into solid solution within a short time by the subsequent solution treatment. This results in poor strength after baking of paint coatings. If the temperature at which the slab is heated exceeds 560°C, eutectic melting occurs. A soaking temperature over 48 hours detracts from economy.
• the slab is subject to preheating again in a heating furnace, immediately followed by hot rolling.
• the preheating immediately before hot rolling requires heating in the temperature range (480 to 560°C) associated with the solution treatment, preferably at relatively higher temperatures within the temperature range, so that the solid solution state of Mg2Si achieved by heating of the slab as mentioned above is maintained as much as possible or even if Mg2Si precipitates, finer precipitates may develop. It is to be noted that this preheating is simply to bring the slab to the above-defined temperature ready for hot rolling to start and does not require to hold the slab for some time at the temperature. If desired, the heat treatment for soaking may be directly followed by preheating for hot rolling without once cooling the slab after the soaking heat treatment.
• the next step is hot rolling.
• the alloy is hot rolled into a strip such that the temperature of the alloy being hot rolled drops from 480°C to 400°C within 30 minutes from the emergence of the alloy from the heating furnace for preheating or combined soaking/preheating.
• the residence time in the temperature range of from 480°C to 400°C should be within 30 minutes. This prevents precipitation or coarse growth of Mg2Si during hot rolling immediately after the emergence of the alloy from the heating furnace, thereby facilitating the subsequent solution treatment.
• the basic requirement is a residence time within 30 minutes in the temperature range of from 480°C to 400°C, it is desired to have a residence time as short as possible in order to ensure that precipitation or coarse growth of Mg2Si is inhibited.
• the rolled strip may be directly subject to a solution heat treatment whereupon the strip is available as a product ready for use. Often the hot rolling is followed by cold rolling to a desired strip thickness. If desired, intermediate annealing may be effected between the hot rolling and the cold rolling or midway the cold rolling. After the cold rolling, the rolled strip is subject to a solution treatment.
• the solution heat treatment step includes a series of heating, holding and quenching steps. It is a critical step for imparting bake hardening ability to allow for strength increase after baking of paint coatings and for improving formability through recrystallization.
• the strip should have a mean grain size of up to 100 ⁇ m at the surface, which requires that recrystallization takes place such that recrystallized grains may have a size of up to 100 ⁇ m. Since the transition elements, Mn, Cr, Zr and V themselves adversely affect formability, the content of these elements is limited to less than 0.05% for each element and to less than 0.10% in total as previously described, for the purpose of improving formability.
• a high temperature/long term solution heat treatment is generally desired to form a sufficient solid solution to provide satisfactory bake hardening ability.
• a high temperature/long term solution heat treatment causes recrystallized grains to grow too large to provide a grain size of up to 100 ⁇ m since the contents of Mn, Cr, Zr and V known as crystal grain refining elements or recrystallized grain stabilizing elements are limited to minimal amounts as mentioned above.
• the solution heat treatment is limited to a heating rate of at least 5°C/sec., a heating temperature of 480 to 560°C, and a holding time within 60 seconds in accordance with the invention. Outside these ranges, coarse crystal grains in excess of 100 ⁇ m develop, detracting from formability. It is to be noted that a lower temperature is preferred for the solution heat treatment for obtaining finer crystal grains, but no satisfactory solid solution can form at temperatures below 480°C.
• the present invention employs the heating and hot rolling steps under the above-defined conditions for placing the alloy into conditions ready for the solution heat treatment, more particularly into sufficient conditions to allow a solution heat treatment under the conditions of a higher heating rate, a relatively mild temperature and a shorter holding time to achieve satisfactory solid solution formation.
• the solution heat treatment is carried out by heating the rolled strip at a heating rate of at least 5°C/sec. to a temperature of 480 to 560°C and holding the strip at the temperature for up to 60 seconds, followed by quenching at a cooling rate of at least 5°C/sec.
• This treatment may be accomplished by means of a coil type continuous annealing apparatus such as a gas furnace CAL and electromagnetic induction heating furnace CAL.
• the optional intermediate annealing between hot rolling and cold rolling or midway cold rolling may be either batchwise or continuous, with the continuous annealing being preferred for the subsequent solution heat treatment.
• the alloy is preferably heated to a temperature of 350 to 560°C with or without holding at the temperature within 3 minutes, especially within 60 seconds. If the temperature of the intermediate annealing of the continuous mode exceeds 560°C, coarser crystal grains would develop, detracting from formability. No recrystallization would occur at temperatures below 350°C. A holding time in excess of 60 seconds entails the risk of developing coarser crystal grains if the temperature is above 480°C.
• the cold rolling immediately before the solution heat treatment is carried out to a rolling reduction of at least 30%. If the rolling reduction is below 30%, coarse grains with a size of more than 100 ⁇ m would sometimes result from recrystallization.
• the thus rolled strip may be subject to natural aging in a conventional manner and if desired, leveled for providing a flat surface or removed of strain by skin pass.
• the strain removal if employed, may be followed by a heat treatment as disclosed in Japanese Patent Application Kokai No. 11953/1989, FIGS. 1 and 2, for the purposes of recovering a slight loss of formability due to strain removal and preventing a change of strength with time.
• the rolled aluminum alloy strip according to the present invention is generally used by forming or shaping or forming the strip as by press forming and then applying a paint coating thereto followed by baking.
• the paint coating is generally baked at a temperature of about 150 to 250°C.
• the strip can be effectively formed or worked since the mean crystal grain size on the surface is limited to 100 ⁇ m or less and the contents of Mn, Cr, Zr, V and Fe are limited. Since Mg2Si and similar constituents have formed a sufficient solid solution to provide a conductivity of up to 50% IACS, these constituents will precipitate out to increase strength during paint baking, achieving bake hardening.
• a slab of 500 x 1200 x 400 mm was cast by a semi-continuous casting technique.
• the slab was subjected to a soaking heat treatment at 530°C for 10 hours, preheated in a heating furnace at 530°C for 2 hours or at 430°C for 2 hours as shown in Table 2, and then hot rolled into a strip of 3 mm thick.
• the temperature at the end of hot rolling was 280°C when the heating temperature immediately before hot rolling was 530°C and 250°C when the heating temperature immediately before hot rolling was 430°C.
• the time taken from the exit of the slab from the heating furnace to the end of hot rolling was 10 minutes in either case.
• the hot rolled strip was then cold rolled to a thickness of 1 mm and subjected to a solution heat treatment in a continuous annealing furnace.
• the solution heat treatment conditions included a heating/cooling rate of 30°C/sec. and holding at 520°C for 10 seconds or a heating/cooling rate of 30°C/sec. and holding at 550°C for 90 seconds.
• each rolled strip was determined for conductivity and mean crystal grain size on the surface. The results are shown in Table 2 together with main treating conditions.
• the rolled strip was naturally aged for 7 to 10 days before it was measured for mechanical properties, formability and bake hardening ability.
• the results are shown in Table 3.
• the mechanical properties tested included yield strength (YS), tensile strength (TS), and elongation.
• Formability was evaluated in terms of Erichsen value (Er) and 180° minimum bending radius.
• Bake hardening was evaluated by carrying out a 175°C/1 hour heat treatment equivalent to paint baking and then determining the yield strength (YS).
• the rolled strips were naturally aged for 7 to 10 days before they were measured for mechanical properties, formability and bake hardening as in Example 1. The results are shown in Table 6.
• Example 1 Each of alloy Nos. 1 and 5 in Table 1 was cast and soaked as in Example 1. It was preheated in a heating furnace at 530°C for 2 hours or at 430°C for 2 hours as shown in Table 7, and then hot rolled into a strip of 3 mm thick. The temperature at the end of hot rolling was 280°C when the heating temperature immediately before hot rolling was 530°C and 250°C when the heating temperature immediately before hot rolling was 430°C. The time taken from the exit of the slab from the heating furnace to the end of hot rolling was 10 minutes in either case. Without cold rolling, the hot rolled strip was directly subjected to a solution heat treatment using a salt bath. The solution heat treatment conditions included a heating rate of at least 100°C/sec., holding at 520°C for 30 seconds, and a cooling rate of at least 200°C/sec.
• each rolled strip was determined for conductivity and mean grain size on the surface. The results are shown in Table 7 together with main treating conditions.
• the rolled strips were naturally aged for 7 to 10 days before they were measured for mechanical properties, formability and bake hardening as in Example 1. The results are shown in Table 8.
• the final product in this example was obtained by subjecting a strip as hot rolled directly to a solution heat treatment without cold rolling.
• the rolled aluminum alloy strips according to the present invention are improved in formability and bake hardening ability so that they may be readily formed or worked as by press forming without skin roughening.
• the strips increase their strength during baking of paint coatings, eventually offering shaped parts of very high strength having paint coatings baked thereto.
• the strips are thus best suited as automobile body sheets.
• the method of the invention is easy to produce rolled aluminum alloy strips having such improved properties in a commercially acceptable large scale.
• the rolled aluminum alloy strips are suitable not only as automobile body sheets, but also in other applications where the strips are mechanically formed and coated with paint by baking, for example, as automobile parts such as wheels, oil tanks, and air cleaners, various caps, blinds, aluminum cans, household goods, meter covers, and electric equipment chassis.

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• 1992
  • 1992-09-03 EP EP92307980A patent/EP0531118A1/fr not_active Withdrawn

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