JP2015010235A - Aluminum alloy material having suppressed stretcher strain mark, and method for producing the same - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide an aluminum alloy material in which SS marks generated upon molding are suppressed without damaging various properties such as moldability and roughening resistance.SOLUTION: Provided is an aluminum alloy material comprising Cu and Mg, in which 0.2% proof stress is controlled below 90 MPa, also, the average area ratio of intermetallic compounds with an equivalent circle diameter of 0.6 to 2.0 μm observed in an alloy structure is controlled below 8.0% to an Al matrix, and further, the crystal grain size of the Al matrix is controlled to less than 70 μm.

Description

  The present invention relates to an aluminum alloy material containing Cu (copper) and Mg (magnesium) and a method for producing the same, and in particular, members / parts of various transport machines such as aircraft fuselage, or building materials and structures. The present invention relates to an aluminum alloy material that is used as a material or the like, has excellent formability, and has excellent resistance to rough skin and SS marks, and a method for producing the same.

  Among aluminum alloys suitable for forming, particularly, 2000 series and 7000 series alloys are known as high-strength aluminum alloys. Of the 2000 series alloys, Al-Cu-Mg series alloys are typical age-hardening alloys, and as represented by the names duralumin and super duralumin, the wrought materials are aircraft and structures that require strength. It is widely used as a high strength aluminum alloy for. In aluminum alloys, T4 treated materials (naturally aged after solution treatment) and T6 treated materials (artificially aged after solution treatment) are high in strength and difficult to form, so O material (soft) A method is known in which molding is performed after treatment, and finally high strength is obtained by preliminary aging or solution treatment. However, if the Al-Cu-Mg alloy is treated with O material and then subjected to forming, a stretcher / strain mark (hereinafter referred to as "SS mark") is generated on the surface of the plate, resulting in poor product appearance. There is a case.

  In general, when the SS mark is subjected to a tensile test, the elongation of the yield point near the yield point on the stress-strain curve, and the serrated or stepped serration on the stress-strain curve at a relatively high strain amount beyond the yield point. It is known that when this occurs, it becomes a surface defect at the time of actual press molding and becomes a serious problem in appearance. The SS mark is generated so as to form a so-called random mark having an irregular belt-like pattern such as a flame generated at a relatively low strain amount portion and about 50 ° with respect to the tensile direction at a relatively high strain amount portion. Divided into parallel bands with parallel strips. It is known that the former random mark is caused by yield point elongation and the latter parallel band is caused by serration on the stress-strain curve.

  In general, the SS mark is more noticeably observed as the crystal grain size becomes finer. Therefore, as one method for eliminating the SS mark, a method of adjusting crystal grains to a certain degree of coarseness has been conventionally known. This method is particularly effective for reducing random marks caused by yield elongation among SS marks, and is also effective to some extent for reducing parallel bands caused by serrations.

  Further, as a conventional method for eliminating the SS mark, there is also known a method in which some processing (pre-processing) such as skin pass processing or leveling processing is given to the O material or the T4 treated material before molding. This method is particularly effective for reducing random marks caused by yield elongation among SS marks. The reason why the generation of random marks can be suppressed by pre-processing is considered as follows.

  That is, in general, in Al—Mg-based alloys, Mg forms a Cottrell atmosphere and fixes dislocations, so that extra stress is required to cause yielding. On the other hand, once yielding is started at a certain location, the deformation propagates avalanche from that location without increasing the stress, and as a result, non-uniform deformation suddenly occurs within the plate. And since the deformation progresses rapidly without increasing the stress in this way, yield elongation appears on the stress-strain curve, and the rapid deformation is non-uniform. Will do. Therefore, if a large number of deformation bands are formed by the pre-processing as described above, these many deformation bands function as the starting point of yielding, so that sudden and non-uniform deformation during yielding does not occur. That is, yield elongation does not occur, and as a result, generation of random marks can be suppressed.

  As described above, in the conventional method in which the SS mark is eliminated by adjusting the crystal grains to be coarse, if the crystal grains become too coarse, problems such as surface roughening occur due to press molding, and the molding is performed. It becomes inappropriate as a product. Therefore, in practice, it is difficult to reliably prevent the occurrence of SS marks and to stably avoid the occurrence of rough skin. In addition, this method has a problem that it is not very effective in preventing the occurrence of a parallel band among SS marks.

  In addition, as described above, by giving pre-working, the occurrence of yield elongation is suppressed, and the method for preventing the occurrence of SS marks, particularly random marks, can suppress yield elongation to some extent even in pre-working with a small degree of work. However, it has been difficult to stably and reliably prevent the generation of random marks. That is, in pre-machining with a low degree of machining, slight fluctuations in the thickness of the base plate depending on the location in the board greatly affect the variation in the degree of machining. On the other hand, if the degree of pre-processing is sufficiently large, the yield elongation can be reliably eliminated and the generation of random marks can be stably prevented. However, on the other hand, the yield strength is increased and the ductility and formability of the plate are lowered, which makes it unsuitable for molding applications. For this reason, it is a fact that inevitably increasing the degree of pre-processing.

  Generally, in an Al—Cu—Mg system, when a tensile test is performed immediately after solution treatment and quenching, strong serration occurs on the stress-strain curve. It is known that this phenomenon can occur in the Al-Mg-Si system, Al-Mg-Si-Cu system, and Al-Cu system as well. In all cases, Mg, Cu, and Si, which are the main additive elements, are dissolved in supersaturation in the Al matrix by quenching. As a result, dislocations during deformation are serrated by repeatedly fixing and releasing from the Cottrell atmosphere formed by each element ( B type). It is known that this serration disappears with aging at room temperature in order to reduce the solid solubility in the Al matrix by aging precipitation of each solid solution element. Therefore, in order to avoid the occurrence of serration in the T4 material, it is proposed to leave it at room temperature for an appropriate time after quenching, or to perform a pre-aging treatment before the molding process as in Patent Document 1. ing.

  Here, in Patent Document 1, the SS mark of an Al—Mg—Si based or Al—Cu based alloy is subjected to soaking and hot working, and cold working as necessary, followed by solution treatment and quenching. It has been proposed to perform a heat treatment at 40 to 120 ° C. at an arbitrary timing until the final forming process. This is because each solid solution element is added to G. in order to suppress serration (B type) caused by dislocation fixation and release by the main additive elements such as Cu, Mg, and Si. P. The purpose is to reduce the effect by zoning.

  On the other hand, in the Al—Cu—Mg system, when the tensile test is performed after the O material treatment, serration occurs, which is not as high as that of the T4 material, which may cause a problem as an appearance defect during molding. This serration does not disappear due to room temperature aging. That is, the main additive element G.S. P. Even the method of reducing the solid solubility by zoning could not be suppressed.

JP-A-9-111429

  The present invention has been made in view of the above-mentioned problems, and provides an aluminum alloy material that suppresses SS marks generated during forming without impairing various properties such as formability and rough skin resistance, and a method for producing the same. With the goal.

  As a result of earnestly experimenting and examining to solve the above-mentioned problems, the present inventors have found that serrations generated in the annealed material (O material) have a specific size that precipitates or grows during holding and annealing during annealing. It was found that the distribution of precipitates had an effect. Furthermore, the present inventors have found that the distribution of precipitates of this specific size can be effectively controlled by setting the heating rate during annealing, the heating holding temperature, the heating time, and the cooling rate after heating within a predetermined range, It came to make this invention. The specific solution is as follows.

  The aluminum alloy material that suppresses the SS mark according to the present invention is an aluminum alloy material containing Cu and Mg. The 0.2% proof stress value is less than 90 MPa, and the equivalent circle diameter of 0.6 is observed in the alloy structure. The average area ratio of the intermetallic compound of ˜2.0 μm is less than 8.0% with respect to the Al matrix, and the crystal grain size of the Al matrix is 70 μm or less.

  Moreover, the manufacturing method of the aluminum alloy material which suppressed the SS mark which concerns on this invention is the aluminum alloy material which performs a casting process, a homogenization process process, a hot rolling process, a cold rolling process, and an annealing process about the said aluminum alloy material In the manufacturing method, the annealing step is performed at a temperature of 70 ° C./H or higher, heat-treated at 340 to 450 ° C. for 1 minute or more, and further at a cooling rate of 50 to 200 ° C./H to at least 260 ° C. It is the process to cool, It is characterized by the above-mentioned.

  According to the present invention, the distribution of fine precipitates and the crystal grain size are appropriately controlled by optimizing the heating rate, the heating holding temperature, the heating time, and the cooling rate after heating under annealing conditions, and the formability It is possible to suppress the SS mark generated during the molding process without impairing.

  In addition, pre-processing and pre-heating before molding as a countermeasure against the SS mark described above are unnecessary, and a new process is not required. Furthermore, since the SS mark can be suppressed even if the crystal grains are fine, there is an advantage that the problem of rough skin resistance is solved.

  Hereinafter, the aluminum alloy material which suppressed SS mark concerning the present invention, and its manufacturing method are explained in detail.

[Chemical composition of aluminum alloy material]
The alloy material of the present invention may basically be an Al—Cu—Mg alloy, and its specific component composition is not particularly limited, but usually Cu: 3.5% by mass. -5.0%, Mg: 1.0-2.0%, Mn: 0.3% -1.0%, Si: 0.01-0.5%, Fe: 0.01- 0.5%, Zn: 0.01-0.20%, Cr: 0.001-0.10%, Zr: 0.01-0.10%, Ti: 0.01-0.10% It is preferable to use, as a raw material, an alloy aluminum alloy having a component composition containing one or more selected from among them, with the balance being Al and inevitable impurities.

  Next, the reasons for limiting each element will be described.

(Cu)
Cu is an alloy element that is fundamental in the system that is the subject of the present invention, and is an indispensable element for securing strength, and contributes to strength improvement in cooperation with Mg. The amount of Cu is preferably 3.5 to 5.0%. If the amount of Cu is less than 3.5%, sufficient material strength cannot be obtained. On the other hand, if the amount of Cu exceeds 5.0%, it is possible to obtain a very high strength in the previous stage of molding, but the molding processability is reduced, or there is a crack in the casting or hot rolling during the hot rolling. It becomes easy to get up.

(Mg)
Mg, like Cu, is a system alloy that is the subject of the present invention, and is a basic alloy element that contributes to strength improvement. The amount of Mg is preferably 1.0 to 2.0%. If the amount of Mg is less than 1.0%, sufficient material strength may not be obtained. On the other hand, if the amount of Mg exceeds 2.0%, very high strength can be obtained in the stage before molding, but molding processability may be deteriorated.

(Mn)
Mn is an element that contributes to strength improvement in the system of the present invention. The amount of Mn is preferably 0.3 to 1.0%. If the amount of Mn is less than 0.3%, sufficient material strength may not be obtained. If the amount of Mn exceeds 1.0%, in addition to the effect of suppressing recrystallization, an intermetallic compound may be formed and the age hardening behavior may be affected.

(Si, Fe, Zn, Cr, Zr, Ti)
As selective additive elements, Si: 0.01 to 0.5%, Fe: 0.01% to 0.5%, Zn: 0.01 to 0.20%, Cr: 0.001 to 0.10% , Zr: 0.01 to 0.10%, Ti: 0.01 to 0.10%, or one or more selected from among them can be contained. Si acts on the age hardening behavior and is added to improve the alloy strength at room temperature and high temperature. Fe, when added simultaneously with Ni, has the effect of improving heat resistance and alloy strength. Zn is added to improve the alloy strength by solid solution strengthening and precipitation strengthening. Cr and Zr are added for the effect of suppressing recrystallization and the strength of the alloy. Ti is added for grain refinement of the ingot and alloy strength.

(Inevitable impurities)
For example, inevitable impurities such as Ga, V, Ni, etc. within the normally known range contained in the metal and the intermediate alloy do not disturb the effects of the present invention. Inclusion is also allowed.

[Method for producing aluminum alloy material]
Hereinafter, the manufacturing method of the aluminum alloy material of the present invention will be described separately for each main item.

(Manufacture of aluminum alloy rolled sheets)
In the present invention, an Al—Cu—Mg-based aluminum alloy rolled sheet is preferably used. This aluminum alloy rolled sheet can be produced by a method generally employed in the aluminum alloy manufacturing industry.

  That is, an aluminum alloy melt adjusted to a predetermined component is cast by appropriately selecting a normal melting casting method. Here, as a normal melt casting method, for example, a semi-continuous casting method (DC casting method), a thin plate continuous casting method (roll cast method or the like), or the like can be used.

  Next, the aluminum alloy ingot is subjected to homogenization at a temperature of 470 ° C. or higher. Homogenization treatment mitigates microsegregation of alloying elements during solidification of molten metal, and when various transition elements including Mn / Cr are included, dispersion of intermetallic compounds mainly composed of these elements This is a process necessary to deposit particles uniformly and densely in the matrix. The heating time for the homogenization treatment is usually 1 hour or more, and is usually terminated within 48 hours for economic reasons. Before or after this homogenization treatment process, after appropriate chamfering, hot rolling is started in a temperature range of 430 to 480 ° C., and then cold rolling is performed at a reduction ratio of 30 to 60%. Then, an aluminum alloy rolled plate having a predetermined plate thickness is manufactured.

(Annealing treatment)
After the cold rolling, as the annealing treatment, heat treatment is performed at a temperature of 70 ° C./H (hours) or more, more preferably 100 ° C./H or more and holding at 340 to 450 ° C. for 1 minute or more. The heating and holding time may be 1 minute or longer, but is preferably less than 1 hour in consideration of productivity. Further, after the heating and holding, cooling at 50 to 200 ° C./H is performed. Here, the cooling rate indicates the cooling rate from the holding temperature to 260 ° C. or less.

  The reason why the rate of temperature rise is defined as 70 ° C./H or more is that if it is less than 70 ° C./H, it becomes a coarse recrystallized structure exceeding 70 μm, and rough skin becomes a problem after molding. The upper limit of the rate of temperature rise is not particularly specified, and there is no problem even with rapid heating as in a continuous annealing furnace, but the upper limit on equipment is 50 ° C./second (180000 ° C./H). In addition, the holding temperature was set to 340 to 450 ° C. If it is less than 340 ° C., it may be less than the recrystallization temperature, and even if the rate of temperature rise is within the above range, recrystallization may not occur and various product characteristics may not be obtained. This is because when the temperature exceeds 450 ° C., not only a large amount of intermetallic compounds involved in the SS mark are generated, but also solid solution is promoted and age hardening is increased.

  The target intermetallic compound that causes the generation of the SS mark is an Al-Cu-based or Al-Cu-Mg-based precipitate from component analysis. The intermetallic compound precipitates and grows during cooling in the annealing, and becomes a particle size significant for serration generation. In order to suppress the SS mark so that there is no problem in the appearance of the product, it is effective to suppress the formation of the intermetallic compound. Here, particles of less than 0.6 μm are produced in large quantities during furnace cooling regardless of the production conditions, but do not have an advantageous size for interacting with dislocations during deformation. The impact is very small. On the other hand, the larger the particle size, the stronger the influence on the generation of serrations. However, in the case of particles exceeding 2.0 μm, the interaction with the dislocations during deformation becomes small and does not affect the generation of serrations. Therefore, in the present invention, an intermetallic compound having a circle-equivalent diameter of 0.6 μm or more and 2.0 μm or less is targeted, and the range in which the occurrence of SS marks can be suppressed by the average area ratio of the intermetallic compound with respect to the Al matrix. For the intermetallic compound having an equivalent circle diameter of 0.6 μm or more and 2.0 μm or less, the average area ratio with respect to the Al matrix is less than 8.0%, more preferably 6.0% or less, most preferably from the results of Examples described later. Is 5.0% or less.

  The intermetallic compound having a particle size is likely to generate SS marks because its number density increases as the holding temperature is higher and the cooling time is longer, thereby causing serration. Accordingly, the holding temperature is 450 ° C. or lower, more preferably 410 ° C. or lower. The cooling rate is important not only for determining the particle size of the intermetallic compound but also for determining the strength characteristics as the O material. That is, when the cooling rate is high, solid solution of Cu or Mg as the main additive element into the Al matrix is promoted, and a solid solution state close to that of the T4 material is obtained. As a result, room temperature age hardening after cooling is promoted, so that the strength as the O material after annealing cannot be maintained. In general, the elongation of the material is important for the stretchability and bendability in drawing, and a soft material (O material) is required. Therefore, when the strength is increased, the moldability may be deteriorated. Therefore, the strength as the O material is 0.2% proof stress, preferably 90 MPa or less, more preferably 80 MPa or less. Therefore, the cooling rate is set to 200 ° C./H or less from the viewpoint of strength characteristics as the O material. On the other hand, if the cooling rate is too slow, it takes a long time for cooling and there is a problem in productivity, and the production of the intermetallic compound having the particle diameter is promoted. For this reason, a cooling rate is 50-200 degreeC / H, More preferably, it is 100-200 degreeC / H, Most preferably, it is 120-200 degreeC / H.

  Further, when the final plate is molded, rough surface may occur on the surface of the plate, which may cause defects in the appearance of the product. Rough skin is related to the recrystallized structure of the surface layer of the plate, and the rougher the recrystallized structure, the more the rough skin becomes. Therefore, from the viewpoint of preventing rough skin, the crystal grain size in the surface layer portion of the plate is preferably 70 μm or less, more preferably 50 μm or less.

  Examples of the present invention will be described below together with comparative examples. The following examples are for explaining the effects of the present invention, and the processes, conditions and performance values described in the examples do not limit the technical scope of the present invention.

As shown in Table 1, the alloy symbols A1 to A5 within the component composition range of the present invention were melted in accordance with conventional methods, and each slab obtained by casting was made to have a plate thickness of 400 mm by face milling. 480 ° C. × 5 hours homogenization treatment, hot rolling from hot starting temperature 470 ° C. to plate thickness 6 mm, then cold rolling to plate thickness 2.5 mm, under annealing conditions shown in Table 2 Finally, it was heat-treated and cooled to 260 ° C. or less to obtain a product plate. The product plate obtained as described above was evaluated as follows.

(Intermetallic compound distribution)
In order to measure the average area ratio of the intermetallic compound having an equivalent circle diameter of 0.6 μm or more and 2.0 μm or less defined in the present invention in the Al matrix, first, from the rolling surface of each test material to the plate pressure direction (1 / 4) It cut out to t part and the surface was mirror-polished. Thereafter, two fields of view were photographed in an area of 20 μm × 25 μm by observation of a 5000 times reflected electron image with a scanning electron microscope, and image processing was appropriately performed using commercially available image analysis software, and the distribution of intermetallic compounds was measured. The average area ratio of the intermetallic compound having a particle diameter in the above range was an average value from two visual fields.

(Evaluation of surface properties and strength characteristics)
Tensile test pieces were taken from the product plate under each condition, and the length of the test piece was taken in parallel with the rolling direction, and 0.2% proof stress value was measured as tensile properties. Furthermore, the surface of the test piece after the tensile fracture was visually determined. Also, the cross section in the rolling direction of the plate was mechanically polished, etched with Keller solution, then observed with an optical microscope, and taken in two fields of view at a magnification of 200 μm and 600 μm × 800 μm at a 1/4 t portion including the surface layer. did. The crystal grain size was measured from the photographed image using a cutting method defined by JIS H 0501. The crystal grain size used for the evaluation was an average value from two fields of view.

  Table 2 shows the results of the average area ratio, crystal grain size, and 0.2% proof stress value of the intermetallic compound of 0.6 μm or more and 2.0 μm or less defined in the present invention together with the manufacturing conditions in the Al matrix. . In Table 2, the mark “◯” in rough skin and SS mark evaluation indicates a rough skin or a weak SS mark, and the cross indicates a rough skin or a strong SS mark. In the table, the ◯ marks in the strength characteristics indicate that the yield strength is less than 90 MPa, and the X marks indicate 90 MPa or more. If either the rough skin or the SS mark is strong, there will be a problem as the appearance of the product after molding. On the other hand, even if the skin roughness and the SS mark are both minor, if the strength of the O material is deviated and the strength becomes very high, molding becomes difficult.

  Production numbers 1 to 5 are all within the range defined by the present invention for the component composition of the alloy, and the production process is also within the range defined by the present invention. The cooling rate also satisfies the conditions specified in the invention. In these cases, the surface roughness was excellent with almost no SS mark being observed regardless of the alloy components, and the strength and productivity as an O material were also good.

  On the other hand, in production numbers 6 to 10, the alloy component composition is within the range specified in the present invention, but any of the final annealing conditions is out of the range of the present invention. Among these, production number 6 was only the rate of temperature rise outside the lower limit of the range of the present invention, and since the rate of temperature rise was low, a coarse recrystallized structure was formed, and even if other characteristics were satisfied, rough skin occurred. In production numbers 7 and 8, only the holding temperature is outside the upper limit or the lower limit of the range of the present invention. If the upper limit of the holding temperature is exceeded, not only a large amount of intermetallic compounds are involved in the SS mark, but also solid solution is promoted and age hardening is increased even within the range of the cooling rate defined in the present invention. As a result, the strength also increased. Further, if the lower limit of the holding temperature was exceeded, complete recrystallization was not possible and various product characteristics could not be obtained. In production numbers 9 and 10, only the cooling rate is outside the upper limit or the lower limit of the range of the present invention. If the upper limit of the cooling rate is exceeded, solid solution is promoted and age hardening is increased, so that the strength as the O material cannot be maintained. When the lower limit of the cooling rate was exceeded, there was no problem with the rough skin and the strength of the O material. However, since a long time was required for cooling, the intermetallic compound involved in the SS mark grew relatively coarsely during cooling, and SS It became a factor of the mark generation. Therefore, if any one of the annealing conditions is out of the scope of the present invention, good surface properties and strength characteristics cannot be obtained.

  As for cooling, if it was cooled to at least 260 ° C., no difference was observed in the characteristics even if it was cooled to a temperature lower than that.

  The present invention has been described in detail based on the above specific examples. However, the present invention is not limited to the above specific examples, and various modifications and changes can be made without departing from the scope of the present invention.

Claims (3)

  In an aluminum alloy material containing Cu and Mg, the average area ratio of an intermetallic compound having a 0.2% proof stress of less than 90 MPa and an equivalent circle diameter of 0.6 to 2.0 μm observed in the alloy structure is Al. An aluminum alloy material that suppresses stretcher strain marks, characterized by being less than 8.0% of the matrix and having an Al matrix crystal grain size of 70 μm or less.   As a composition of the said alloy material, Cu: 3.5-5.0%, Mg: 1.0-2.0%, Mn: 0.3-1.0% are contained by the mass%, Furthermore, Si: 0.01-0.5%, Fe: 0.01-0.5%, Zn: 0.01-0.20%, Cr: 0.001-0.10%, Zr: 0.01-0. It contains 1 type or 2 or more types chosen from 10%, Ti: 0.01-0.10%, The remainder consists of Al and an unavoidable impurity, The characterized by the above-mentioned. Aluminum alloy material with reduced stretcher and strain marks.   About the aluminum alloy material of Claim 1 or 2, In the manufacturing method of the aluminum alloy material which performs a casting process, a homogenization process process, a hot rolling process, a cold rolling process, and an annealing process, the said annealing process is 70 degreeC. Stretcher characterized in that it is a step of heating to at least 260 ° C. at a cooling rate of 50 to 200 ° C./H after heating at 340 to 450 ° C. and holding for 1 minute or more. A method for manufacturing an aluminum alloy material with suppressed strain marks.