PRODUCTION PROCESS FOR ALUMINIUM ALLOY ROLLED SHEET

Abstract: A T4 tempered and straightened rolled sheet of Al-Mg series aluminum alloy which contains from 2 to 6% by weight of Mg as the essential alloying element, is provided with an improved formability not degraded by the straightening by, after the T4 treatment and subsequent straightening, subjecting the sheet to heating to a temperature in the range of from 60 to 360°C at a heating rate falling within the hatched region of Figure l attached herewith, then holding within the hatched region of Figure 2 attached herewith, and subsequently, cooling at a cooling rate falling within the hatched region of Figure l.

Description: BACKGROUND OF THE INVENTION l. Field of the Invention The present invention is related to a production method for an Al-Mg series aluminum alloy rolled sheet, which is used for producing formed articles, such as a body sheet, air-cleaner, oil tank, and the like of an automobile, in which a high strength and formability, particularly the elongation, bulging, and bending properties, are required. 2. Description of the Related Arts It is known that the Al-Mg series aluminum alloys exhibit an excellent strength, corrosion resistance, and formability. The representative materials for these series are the 5l82 alloy with O temper or with the addition of Cu or Zn and the alloy described in Japanese Unexamined Patent Publication No. 58-l7l547. Japanese Unexamined Patent Publication No. 53-l039l4 discloses an Al-Mg series aluminum alloy rolled sheet with T4 temper. Japanese Unexamined Patent Publication No. 58-l7l547 proposes to add Cu and Zn, into Al-Mg series aluminum alloy. In addition, these applications propose a rapid cooling to prevent the generation of Lüder's marks and to provide strength and formability. The conventional 5l82 alloy with O temper is usually considered to have a relatively good strength and formability, but likely to form Lüder's marks, which reduce the quality of surface appearance. As described hereinabove, the Al-Mg series aluminum alloy, which is represented by the 5l82 alloy with, occasionally, an additive of Cu, Zn, or the like, and which exhibits an improved balance of strength and formability and does not generate Lüder's marks, can be produced by replacing the conventional batch annealing with a slow heating and slow cooling with the so-called T4 temper with a rapid heating and a rapid cooling. In any production method of rolled sheets of Al-Mg series aluminum alloy with T4 temper, the final heat treatment is carried out while adjusting the heating rate to l°C/sec or more, preferably 5°C/sec or more. In this case, the solid solution treatment and recrystallization takes place simultaneously. The solution treatment in this context is a treatment for dissolving the strengthening elements, such as Mg, Cu, Zn, and the like and is carried out at a temperature which is dependent upon the alloy composition but is usually in the range of from 450 to 600°C, desirably from 450 to 550°C. The heating for the recrystallization and solid solution treatments is usually carried out by loading a rolled sheet in an air-furnace held at a predetermined tempera­ ture, conveying a coil through a furnace while continu­ ously uncoiling same, or loading a rolled sheet into a salt bath. The rapid heating at a heating rate of l°C/sec is realized by any one of these means. On the other hand, the quenching is a treatment succeeding the solid solution treatment and is necessary for obtaining strength. Usually, water-cooling (water quenching), quenching in warm water, and forced air-cooling are carried out. When a rolled sheet in the form of a large-­ sized cut sheet or a coil is subjected to this treatment, for solid solutionizing and quenching, the sheet is deformed due to thermal expansion and shrinkage, thereby generating "warp", "wave", "twist", and the like (these deformations are hereinafter referred to as distortion), and seriously degrading the flatness of a sheet. The sheet to be subjected to forming must have a good flatness. Therefore, a rolled sheet should not be subjected to forming while it is under a distortion, which is generated due to rapid heating and quenching and which will impair the flatness of a sheet. Further­ more, the generation of distortion should be avoided at all costs from the viewpoint of appearance and to prevent flaw generations during forming operations, packaging, and handling. From these viewpoints, it is necessary in the production a rolled sheet of an Al-Mg series aluminum alloy that, subsequent to the recrys­ tallization, solid solutioning and quenching steps, an additional step for straightening the distortion and improving the flatness is carried out. The usual methods for straightening the distortion are skin-pass rolling with a light reduction rate, levelling with bending and unbending by passing through the straighten­ ing rolls, if necessary, further together with applying a tension, and stretching to impart a low tensional deformation. SUMMARY OF THE INVENTION As is described above, it is usual in the production process of a rolled sheet of Al-Mg series aluminum alloy with T4 temper to carry out the straightening subsequent to the recrystallization, solid solutioning and quenching steps, to relieve the distortion generated in these steps. A rolled sheet is work hardened in the straightening process, with the result that the excellent formability provided by the recrystallization, solid solution and quenching treatments is reduced. Therefore, a desired formability, particularly the bulging property, may not be exhibited in this case. The present invention was created under the background as described above and aims to provide an Al-Mg series alloy rolled sheet having an improved formability, particularly bulging property, and a process for producing the same. More particularly, it is an object of the present invention to relieve the work hardening induced in the straightening step by a final heat treatment, to restore the formability, particularly the bulging property, reduced by the straightening, to the improved formability attained by the T4 tempering. Basically, the present invention involves a discovery of heat treatment under a particular condition, which may be applied to a rolled sheet of a heat treatable type Al-Mg series aluminum alloy (5000 series), subsequent to the straightening step following the recrystallization, solid solution and quenching treatments. This heat treatment enables the formability to be restored to that attained at the quenched and then T4 tempered state, without the necessity for another straightening, and in addition, the flatness of a rolled sheet is not impaired by the heat treatment. Specifically, the present invention provides a method for producing an Al-Mg series aluminum alloy rolled sheet, wherein a hot rolled sheet or continuous roll-cast sheet of an Al-Mg series aluminum alloy containing from 2 to 6% by weight of Mg as the essential alloying element is cold rolled, subjected to recrys­ tallization, solid solutioning and quenching treatments by rapid heating and rapid cooling, and subsequently to straightening, characterized in that, after the straightening, the rolled sheet is subjected to a heat treatment, wherein heating to a temperature of from 60 to 360°C is carried out at a rate falling within the hatched region in the appended Fig. l, the temperature is held for a time falling within the hatched region in the appended Fig. 2, and then cooling is carried out at a rate falling within the hatched region of Fig. l. BRIEF DESCRIPTION OF THE DRAWINGS Figure l is a graph showing appropriate ranges of heating and cooling rates in connection with the temperature of a final heat treatment. Figure 2 is a graph showing the appropriate range of the holding time and temperature. DESCRIPTION OF THE PREFERRED EMBODIMENTS First, the composition of aluminum alloy which is the subject matter according to the present invention is described. The subject matter of the present invention is an Al-Mg series aluminum alloy, which is known as the 5000 series alloy in practice. In the Al-Mg series aluminum alloy, the solid solution strengthening due to the solute Mg is a fundamental means of strengthening. As an auxiliary strengthening means, a precipitation strengthening by the addition of Cu, Zn, and the like, and grain refining by the addition of Mn, Cr, Zr, V, and the like, may be considered for the Al-Mg series aluminum alloy. Specifically, this alloy contains, as an essential component, Mg in an amount of from 2 to 6%, and further contains, if necessary, one or more of Mn in an amount of from 0.05 to l.0%, Cr in an amount of from 0.03 to 0.3%, Zr in an amount of from 0.03 to 0.3%, and V in an amount of from 0.03 to 0.3%. In addition, one or both of Cu in an amount of from 0.05 to 2.0% and Zn in an amount of from 0.l to 2.0% may be contained, if necessary. The alloying elements are now described in more detail. Mg: Mg not only strengthens but also improves the formability of an aluminum alloy. When the Mg content is less than 2.0%, the strength is not enough that the Al-Mg series aluminum alloy can be used as a body sheet of an automobile. On the other hand, when the Mg content exceeds 6.0%, casting and rolling become difficult. Mn, Cr, Zr, V: These elements refine the recrystallized grains, uniformalize the structure, and enhance the strength. To best attain these effects, the minimum contents should be 0.05% for Mn, 0.03% for Cr, 0.03% for Zr, and 0.03% for V. The Mn content should not exceed l.0%, or the formability will be reduced. When the content of any one of Cr, Zr, and V exceeds 0.3%, coarse intermetallic compounds are formed. Note, these elements may be added alone or in combination. Cu, Zn: These elements are effective for enhancing the strength by precipitation strengthening and for preventing the generation of Lüder's marks. Therefore these elements can be added to the subject matter alloy according to the present invention, if necessary. The generation of Lüder's marks can be effectively prevented even if the alloys do not contain Cu or Zn, by carrying out a heat treatment after cold-rolling, at a high temperature of 450°C or more. The addition of Cu and/or Zn makes it possible to more easily and stably prevent the generation of Lüder's marks. This effect is not attained when the Cu content is less than 0.05% and the Zn content is less than 0.l%, but on the other hand, when the content of Cu or Zn exceeds 2.0%, the corrosion resistance is impaired. These elements may be added alone or in combination. In addition to the above elements, Fe and Si are contained as the unavoidable impurities in ordinary aluminum alloys. Although neither Fe nor Si are important elements in the present invention, they may be contained in an amount of 0.5% at highest. A content higher than that will increase an amount of crystallites resulting in reducing an improved formability. Further, the above elements Ti or Ti and B may be added to refine the crystal grains of a cast ingot, but preferably, the Ti content should be 0.l5% or less, to prevent the formation of coarse TiAl₃ intermetallics during solidification, and also preferably, the B content should be 0.0l% or less to prevent introducing TiB₂ particles. Next, the steps of the production process according to the present invention are described. The rolling steps prior to the solid solution treatment may be carried out by a conventional process or by any optional process. That is, a cast ingot may be produced by a direct chill casting method, and the cast ingot subjected, if necessary, to homogenizing followed by hot rolling. The hot rolled coil is annealed, if necessary, and is then cold rolled to obtain a rolled sheet having the desired thickness. Obviously, an intermediate annealing is carried out, if necessary, in the middle of the cold rolling. A coiled sheet (continuous cast coil) may be produced directly from the melt by a continuous roll casting. The rolled sheet having a required thickness also can be produced by cold rolling the continuous cast coil. Also, in this case, an annealing may be carried out prior to the cold-rolling or in the middle of cold rolling. The rolled sheet produced as described above is rapidly heated at a heating rate of l°C/sec or more, for the recrystallization and solid solution treatments. The temperature of the recrystallization and solid solution treatments is dependent upon the alloy composition but is usually in the range of from 450 to 600°C, preferably from 450 to 550°C. After the completion of the recrystallization and solid solution treatments, the quenching treatment is then quickly carried out to provide a T4 temper. The cooling rate required for the quenching is dependent upon the alloy composition but should be ordinarily at least l°C/sec or more, preferably at least 5°C/sec or more. The solid solution and quenching treatments may be carried out for a cut sheet or a coil while being continuously unwound. The straightening is carried out as described with reference to the prior art. More specifically, the degree of work hardening during the straightening is dependent upon the degree of distortion after quenching, but as the result, the yield strength is increased by l kgf/mm² or more and the formability is impaired in terms of Erichsen values by 0.2 mm or more. The rolled sheet which has been subjected to flattening (process) and exhibits a reduced formability is subjected to the final heat treatment, in which the sheet is heated to a temperature of from 60 to 360°C, held at this temperature, then cooled, or heated to the above temperature followed by immediate cooling. Referring to Fig. l, the hatched region, i.e., the straight lines or curve connecting the points A, B, C, D, E and F and region surrounded thereby, determines, when the heating temperature is given on an abscissa, that the heating speed on the ordinate is such that the crossing point of the ordinate and abscissa values falls within the hatched region. Referring to Fig. 2, the hatched region, i.e., the straight lines or curve connecting the points a, b, c, d and e and the region surrounded thereby, determines, when the heating temperature is given on an abscissa, that the holding time on the ordinate is such that the crossing point of the ordinate and abscissa values falls within the hatched region. The cooling rate is determined within the hatched region of Fig. l. The points A through F in Fig. l indicate the following temperatures and heating or cooling rates. A: 60°C, 4 × l0³°C/sec B: 60°C, 4 × l0⁻³°C/sec C: 200°C, 4 × l0⁻³°C/sec D: 360°C, 2 × l0⁻¹°C/sec E: 360°C, 3 × l0°C/sec F: 230°C, 4 × l0³°/sec The points ª through e in Fig. 2 indicate the following temperatures and holding times. a: 60°C, l0⁵ sec b: 200°C, 0 sec c: 360°C, 0 sec d: 360°C, 5 × l0² sec e: l60°C, l0⁵ sec In the Al-Mg series aluminum alloy according to the present invention, although the work hardening may be relieved, the precipitation of the β phase (Mg₂Al₃ phase) may occur during the heating, holding, and cooling of the final heat treatment. In addition, if Cu or Zn is contained, the Cu series- or Zn series precipitates may be formed in addition to the β phase. When these precipitates are coarsely precipitated along the grain boundaries, the formability, particularly the bending and elongation properties, is reduced. Accordingly, it is necessary to relieve the work hardening in the final heat treatment while avoiding the problems due to precipitation. It is also necessary to maintain the flatness attained by the straightening. Also, the economics of the process must be considered. The respective ranges are determined in the light of the above points. [Heating Rate] Below the straight line BC, no problems arise as far as the properties of the material are concerned. But, the slow heating at, on or below the line BC necessitates an extremely long time for the temperature elevation, which reduces the productivity. When the heating-temperature and heating rate are in a region below the line CD, precipitation occurs during the temperature elevation and thus the formability is reduced. When the heating temperature is higher than 360°C, i.e., to the right from the line DE, the generation of distortion again occurs during heating or the sheet surface is impaired due to the oxidation of Mg. When the heating temperature and rate are above the line EF, the heating is very rapid in a high temperature region, causing the generation of distortion and thus losing the straightening effect. When the heating rate are above the line AF, i.e., 4 × l0³°C/sec, the heating is more rapid than the heating caused by throwing a sheet into an oil or salt bath having a temperature of from 60 to 230°C. In this case, it is difficult to apply such rapid heating to a rolled sheet in the form of a cut sheet or a coil. When the heating temperature is lower than 60°C, the work hardening due to straightening cannot be relieved. Based on the above reasons, the range of heating-­ temperature and heating rate is in the hatched region surrounded by A, B, C, D, E, and F. The heating means of a cut sheet used for attaining the heating rate in the range A - F are, for example, batch furnace (loading the sheet in a furnace at a required temperature, or heating the sheet together with temperature-elevation of a furnace), continuous furnace, oil bath, salt bath, and metal bath. Such heating means of a coil is a batch furnace or a continuous furnace. Holding Temperature and Time When, upon arrival at the holding temperature of 200 to 360°C, the cooling is immediately initiated with a zero holding time, as illustrated by the line bc, the working strain can be relieved. When the holding temperature is higher than 360°C, the working strain can be relieved but the sheet surface may be deteriorated due to the oxidation of Mg. When the holding temperature and time are above and to the right of the curve de, the work hardening can be relieved but the β phase and coarse precipitates of Cu or Zn series may be formed to impair the formability, particularly the elongation and bending properties. When the holding time is above the straight line ae, the work hardening can be relieved but the holding time exceeds 24 hours, which is economically disadvantageous. When the holding time and temperature are below or to the left of the curve ab, the heat necessary for relieving the work hardening is not imparted and an appreciable recovery of the formability is not attained. Based on the above reasons, the range of holding temperature and time should be in the range surrounded by a, b, c, d, and e. Cooling Rate Below the straight line BC, no problems arise as far as the properties of the material are concerned. But the slow cooling on or below the line BC necessitates an extremely long cooling time, which is not economical. The cooling means of a cut sheet used for attaining the cooling rate in the range A, B, C, D, E, and F are, for example, furnace cool, cooling in the still air, forced air-cooling, water-cooling, and cooling with must. Such cooling means of a coil is, for example furnace cooling in a batch furnace or cooling outside a batch furnace or forced air cooling in a continuous furnace. When the cooling-temperature and rate are in a region below the line CD, coarse precipitates are formed during the cooling and thus the formability is reduced. Since the heating at a temperature higher than 360°C, i.e., to the right from the line DE, is not carried out according to the present invention, the cooling in this region cannot be carried out. When the cooling temperature and rate are above the line EF, the cooling rate is very rapid in a high temperature region, which causes another distortion due to thermal stress and thus a loss of the effects of straightening prior to the final heat treatment. When the cooling rate are above the line AF, i.e., 4 × l0³°C/sec, the cooling rate is more rapid than water cooling. In this case, it is difficult to apply such a rapid cooling to a rolled sheet in the form of a cut sheet or a coil. Since the heating at a temperature lower than 60°C is not carried out, the cooling in the region to the left of the straight line cannot be carried out. Based on the above reasons, the range of cooling rate and temperature is in the hatched region surrounded by A, B, C, D, E, and F. When the final heat treatment is carried out after the flattening step, the work hardening induced in this step is relieved to restore the formability, particularly the bulging-formability, and an excellent formability, particularly bulging-formability attained in a T4 tempering after the solid solution and quenching treatment, can be fully recovered. In addition, since coarse precipitates are not formed during the heat treatment, the formability is not reduced thereby, and in addition, another distortion is not generated and the improved flatness obtained by the preceding straightening is maintained during the heat treatment. The aluminum alloy rolled sheet is ordinarily subjected to forming, such as press forming, when applied for practical use. Since the aluminum alloy rolled sheet according to the present invention has an improved formability and exhibits no generation of Lüder's marks, there is little possibility of generating defective individuals, and thus the high recovery rate and good productivity are obtained. The present invention is hereinafter described with reference to the Example. Example The Al-Mg series alloy Nos. l through 6 of Table l were melted according to the ordinary method. The ingots, 400 mm × l000 mm × 3000 mm in size, were obtained by DC casting. These ingots were subjected to homogenizing at a temperature of 530°C for l0 hours, followed by hot rolling to a thickness of 4 mm, and further cold rolling to a thickness of l mm. The obtained rolled sheets were subjected to continuous recrystallization, solid solutioning, and quenching treatments in a continuous annealing furnace. In the treatments, the heating rate was 25°C/sec, the heating temperature was 500°C, the holding time was zero second, and the cooling rate was 25°C/sec. After the treatments, the sheets were passed through the tension levelling line to straighten the distortion of the sheets generated by the treatments. In the tension levelling line, the sheets are continuously bent and unbent while passing through rolls, with applying the tension to the sheets. The deformation imparted to the sheets by the tensioning levelling corresponds to a cold-working degree of a few percent cold reduction. The straightened and then cut sheets of l000 mm × 2000 mm in size were subjected to the final heat treatment under the conditions given in Table 2. Table 3 shows the tensile strength σ B , the 0.2% yield strength σ 0. 2 , elongation δ, and the Erichsen value determined at the respective steps after quenching treatments, straightening treatments and final heat treatment. The deformation of the sheets under the final heat treatment-condition was observed with the naked eye and is also shown in Table 3. The appreciable distortion is indicated by an x symbol, and nonappreciable distortion is indicated by an o mark. The symbol L in Tables 2 and 3 indicates the material with an O temper, in which the treatment after the cold rolling is not by rapid heating and rapid cooling but by slow heating and slow cooling. The conditions for the O treatment were a heating rate of 8 × l0⁻³°C/sec, a heating temperature of 350°C, a holding time of 2 hours, and a cooling rate of 8 × l0⁻³°C/sec. As is apparent from Table 3, in all of Conditions A through K, the elongation δ and the Erichsen value after levelling are lower than those before levelling, and therefore, the formability is reduced. In the case of Conditions A through F, in which the final heat treatment was carried out within the range of the present invention, the elongation and the Erichsen value under the finally heat treated condition are virtually the same as those under the T4 tempering before the levelling. This indicates that the formability was satisfactorily restored by the final heat treatment. A deformation such that it would impair the flatness of the sheets was not generated under the inventive conditions A through F. The condition G is an example of a too slow heating; the condition H is an example of a too short holding time at the holding temperature; the condition I is an example of a too long holding time; and the condition J is as example of a too slow cooling at the final heat treatment. In these cases, the formability was not restored to that before levelling, or the formability was reduced to some extent. In addition, Condition K is an example of a too fast cooling in the final heat treatment. In this case, the formability was restored but the rolled sheet was deformed, degrading the flatness. Accordingly, it is apparent that, to restore the formability to that attained by the T4 tempering and to maintain the flatness attained by the levelling, the conditions of the final heat treatment must fall within the range of present invention. Since the quenching (rapid cooling) is not carried out under Condition L of Table 3, the distortion of a sheet does not occur and the levelling is therefore unnecessary. But the formability is inferior to that of the material having T4 temper with rapid heating and cooling. The aluminum alloy rolled sheet according to the present invention is most appropriate for application for the automobile body, and can also exhibit excellent characteristics when used for automobile parts, such as an air cleaner and the like, and for various instruments for home use.

Claims: 1. A production process of an aluminum alloy rolled sheet of an Al-Mg series aluminum alloy which contains from 2 to 6% by weight of Mg as the essential alloying element, wherein a hot-rolled sheet or a continuous roll cast sheet of said alloy is subjected to cold-rolling and T4 treatment, and then to a straightening, characterized in that, after the straightening, the heating to a temperature in the range of from 60 to 360°C is carried out at a heating rate falling within the hatched region of Figure l attached herewith, then holding is carried out within the hatched region of Figure 2 attached herewith, and subsequently, cooling is carried out at a cooling rate falling within the hatched region of Figure l. 2. A production method according to claim l, wherein said Al-Mg series aluminum alloy further contains one or more of Mn in an amount of from 0.05 to l.0%, Cr in an amount of from 0.03 to 0.3%, Zr in an amount of from 0.03 to 0.3%, V in an amount of from 0.03 to 0.3%. 3. A production method according to claim l or 2, wherein said Al-Mg series aluminum alloy further contains one or both of Cu in an amount of from 0.05 to 2.0% and Zn in an amount of from 0.l to 2.0%. 4. A T4 tempered and straightened rolled sheet of Al-Mg series aluminum alloy which contains from 2 to 6% by weight of Mg as the essential alloying element, wherein said rolled sheet is produced by a process wherein a hot rolled sheet or a continuous roll cast sheet of said alloy is subjected to cold rolling and T4 treatment, and then to straightening, characterized in that, after the straightening, the heating to a temperature in the range of from 60 to 360°C is carried out at a heating rate falling within the hatched region of Figure l attached herewith, then holding is carried out within the hatched region of Figure 2 attached herewith, and subsequently, cooling is carried out at a cooling speed falling within the hatched region of Figure l. 5. A T4 tempered and straightened rolled sheet according to claim 4, wherein said aluminum alloy further contains one or more of Mn in an amount of from 0.05 to l.0%, Cr in an amount of from 0.03 to 0.03%, Zr in an amount of from 0.03 to 0.3%, V in an amount of from 0.03 to 0.3%. 6. A T4 tempered and straightened rolled sheet according to claim 4 or 5, wherein said aluminum alloy further contains, one or both of Cu in an amount of from 0.05 to 2.0% and Zn in an amount of from 0.l to 2.0%.

Inventors: KOMATSUBARA, TOSHIO, MATSUO, MAMORU, MURAMATSU, TOSHIKI
Applicants: SKY ALUMINIUM CO., LTD.
Application year: 1987
Main classification: C22F1
IPC codes: C22F1, B21B3, C22C21