JP2006274415A - Aluminum alloy forging for high strength structural member - Google Patents

Aluminum alloy forging for high strength structural member Download PDF

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JP2006274415A
JP2006274415A JP2005098919A JP2005098919A JP2006274415A JP 2006274415 A JP2006274415 A JP 2006274415A JP 2005098919 A JP2005098919 A JP 2005098919A JP 2005098919 A JP2005098919 A JP 2005098919A JP 2006274415 A JP2006274415 A JP 2006274415A
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forging
strength
alloy
flash
amount
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Norifumi Hosoda
Manabu Nakai
学 中井
典史 細田
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Kobe Steel Ltd
株式会社神戸製鋼所
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<P>PROBLEM TO BE SOLVED: To provide an Al-Mg-Si based aluminum alloy forging for a transport airplane structural material which can be produced with high reproducibility, and has high strength, high toughness and high corrosion resistance. <P>SOLUTION: The Al-Mg-Si-Mn based aluminum alloy forging is obtained by subjecting a cast billet having a composition comprising, by mass, 0.6 to 1.0% Mg, 0.8 to 1.5% Si and 0.6 to 1.2% Mn, and in which the content of Cu is regulated to ≤0.15%, and the balance Al with inevitable impurities, and having an electric conductivity of 26 to 32% IACS to hot die forging, and the average area ratio of recrystallized grains in the flash cut face structure in the forging is controlled to ≤20%, so as to improve the corrosion resistance of the forging. <P>COPYRIGHT: (C)2007,JPO&INPIT

Description

  The present invention is a high-strength, high-toughness Al-Mg-Si-based aluminum alloy forging material for high-strength structural members having excellent corrosion resistance such as stress corrosion cracking resistance (hereinafter, aluminum is also simply referred to as Al). It is about.
  As is well known, 6000 series (Al-Mg) in the AA to JIS standards for structural materials and parts of transportation equipment such as vehicles, ships, aircraft, motorcycles and automobiles, especially as suspension parts such as upper arms and lower arms. Al alloy forging such as -Si) is used. The 6000 series Al alloy forging material has high strength, high toughness, and relatively excellent corrosion resistance. The 6000 series Al alloy itself is also excellent in recyclability in that the amount of alloying elements is small and scrap can be reused again as a 6000 series Al alloy melting raw material.
  These 6000 series Al alloy forgings are subjected to homogenization heat treatment of Al alloy castings, followed by hot forging (die forging) such as mechanical forging and hydraulic forging, followed by solution treatment, quenching treatment and artificial age hardening treatment. The so-called tempering treatment is performed. In addition to the cast material, an extruded material obtained by once extruding the cast material may be used as the forged material.
  In recent years, structural materials for these transport aircraft are also required to have higher strength and higher toughness after being made thinner. For this reason, various attempts have been made to improve the microstructure of Al alloy castings and Al alloy forgings. For example, the average particle size of crystal precipitates (crystals and precipitates) of a 6000 series Al alloy cast material is reduced to 8 μm or less, and the dendrite secondary arm interval (DAS) is reduced to 40 μm or less. It has been proposed to increase the strength and toughness of alloy forgings (see Patent Documents 1 and 2).
  In addition, by controlling the average grain size and average interval of crystallized and crystallized precipitates in crystal grains and grain boundaries of 6000 series Al alloy forgings, Al alloy forgings are made stronger and tougher. It is also known. These controls can increase the corrosion resistance against intergranular corrosion and stress corrosion cracking. And in accordance with the control of these crystallized products and crystal precipitates, transition elements having a crystal grain refinement effect such as Mn, Zr, Cr, etc. are added, and the crystal grains are refined or sub-crystallized, Fracture toughness and fatigue characteristics are improved (see Patent Documents 3, 4, 5, and 6).
Furthermore, in order to suppress the generation of coarse crystal grains that have been recrystallized from the processed structure, a transition element having a crystal grain refining effect such as Mn, Zr, Cr, etc. is added, and a relatively high temperature of 450 to 570 ° C. It is also known to perform hot forging at a temperature of (see Patent Documents 7 and 8).
JP 07-145440 A (full text) Japanese Patent Laid-Open No. 06-256880 (full text) JP 2000-144296 A (full text) JP 2001-107168 (full text) JP 2002-294382 A (full text) JP 2004-68076 A (full text) Japanese Patent Laid-Open No. 5-27574 (full text) JP 2002-348630 A (full text)
  However, even if crystal precipitates (crystals and precipitates) and dendrite secondary arm spacing (DAS) are miniaturized at the casting stage, it also has the effect of refining crystal grains such as Mn, Zr, and Cr. Even if a transition element is added, it is difficult to suppress recrystallization with a 6000 series Al alloy forging. When recrystallization occurs, the strength decreases and stress corrosion cracking (SCC) along the grain boundary tends to occur. In particular, recrystallization occurs and coarse crystal grains tend to be generated in the forging and solution treatment processes in the structure of the flash part called a flash part generated during die forging.
  The flash cut surface of the forged material is a cut surface remaining on the product forged material side when the flash portion (burr portion) inevitably generated during hot die forging of the cast billet is cut and removed from the product forged material. As will be described later, the flash part or the flash cut surface structure has a higher processing rate than the processing rate of the other product parts, and processing strain is accumulated. For this reason, even according to the above-described conventional technology, recrystallization and generation of coarse crystal grains may not be prevented in the product portion other than the flash portion of the forged material or the flash cut surface portion.
  In the die forging product such as the undercarriage part, a part with a high degree of processing is inevitably generated in the product part, and a flash cut surface to the product part inevitably occurs. In particular, the flash cut surface has a very high processing rate during hot forging. For example, in hot forging that is normally performed a plurality of times without reheating, the processing rate of the flash part tends to be a high processing rate of approximately 90% or more in total for the plurality of forgings. For this reason, on the flash cut surface, recrystallization occurs in the solution treatment step, and coarse crystal grains are likely to be generated.
  On the other hand, when the degree of work in the product is reduced, the crystallized product is less likely to be destroyed, and coarse crystallized product remains in the product, resulting in a decrease in strength and impact value. Further, if the processing rate of the flash cut surface is to be reduced, the degree of processing of the product portion is substantially lowered, and the strength and impact value are reduced. Further, the reduction of the processing degree may cause the cast structure to remain, resulting in a decrease in the strength and toughness of the forged material, and a loss of shape accuracy.
  Moreover, what is important is that when coarse crystal grains are generated in the product portion and the flash cut surface, the stress corrosion cracking resistance is remarkably lowered. This problem is caused when a tensile stress is generated in the environment where the forged material is used, and a crystal grain boundary parallel to the forging flow direction is about to peel off the crystal grain boundary in the 90 ° direction, particularly in a corrosive environment. It can be a serious problem.
  In view of such circumstances, the object of the present invention is to provide an Al-Mg-Si-based aluminum alloy forging material for transport aircraft structural materials that can be manufactured with high reproducibility and has high strength, high toughness, and high corrosion resistance. It is something to be offered.
  In order to achieve this object, the gist of the aluminum alloy forged material for transport aircraft structural materials excellent in stress corrosion cracking resistance of the present invention is an Al-Mg-Si-Mn based aluminum alloy forged material, which is expressed in mass%. Mg: 0.6 to 1.0%, Si: 0.8 to 1.5%, Mn: 0.6 to 1.2%, Cu: 0.15% or less, the balance is Al and inevitable impurities, and the conductivity is 26 to 32% The cast billet, which is IACS, is hot-forged, and the average area ratio of recrystallized grains on the flash cut surface of the forged material is 20% or less.
  In the present invention, at the stage of casting billet, the cooling rate during casting is increased, and Mn is dissolved as much as possible in the cast structure. The above regulation of the conductivity of the cast billet is to reduce the amount of Mn crystallized as an Mn compound from the (added) Mn contained in the cast billet and to increase the solid solution Mn amount or to secure the necessary amount. It is a rule.
  In a normal casting billet for this kind of forging material, if the cooling rate during casting becomes slow due to the composition of the 6000 series Al alloy component and the billet diameter, the Mn contained (added) in the casting billet Of these, the amount of Mn that crystallizes coarsely as an Mn compound tends to increase. For this reason, the amount of solute Mn in the cast billet is inevitably reduced.
  Thus, when the amount of solid solution Mn in the cast billet is reduced, the amount of Mn compound that is newly finely dispersed and precipitated in the structure is reduced in the homogenization heat treatment of the cast billet. On the other hand, the Mn compound crystallized coarsely in the cast billet has a small effect of suppressing recrystallization. As a result, the amount of the Mn compound that suppresses recrystallization is insufficient, and it becomes impossible to suppress recrystallization in the high-workability part of the product part generated during forging and the flash part of the forging. In other words, in the high-workability part of the product part to which processing strain is further applied and the flash cut surface, recrystallization occurs particularly during the solution treatment process, and coarse crystal grains are likely to be generated.
  Conventionally, in the die forgings such as the undercarriage parts, for example, the generation of coarse crystal grains on the flash cut surface was prevented with good reproducibility, and the reason why the subcrystal grains could not be refined with good reproducibility is here. is there.
  In the present invention, Mn is dissolved as much as possible in the cast structure at the stage of casting billet. For this reason, in the homogenization heat treatment, the amount of Mn compound finely dispersed and precipitated in the structure can be increased, and a fine Mn compound sufficient to obtain a crystal grain refining effect can be dispersed and precipitated at high density. As a result, even in a product high processing part, particularly a flash cut surface to which processing strain is further applied, crystal grains can be refined and subcrystal grains can be formed.
  And, in the present invention, in order to further secure the solid solution Mn amount at the stage of the cast billet, the contents of Cu and Mg that have been contained so far in order to ensure the strength are reduced. . Cu and Mg are easily dissolved in the structure of the cast billet, and the amount of Mn solid solution in the cast billet structure is reduced by the amount of Cu and Mg dissolved. As a result, Mn exceeding the solid solution amount crystallizes in the structure as a coarse compound. As a result, even if the Mn content (addition amount) is large, the solid solution Mn amount becomes low. In addition, the necessary amount of the fine Mn compound precipitated during the homogenization heat treatment cannot be secured.
  Furthermore, in the present invention, even if the content of Cu and Mg is reduced, the strength is not so much reduced as compared with the conventional aluminum alloy forging containing a large amount of Cu and Mg, and on the contrary, the toughness is high. Become. This is due to the effect of suppressing the recrystallization and causing the processed structure (sub-grain formation), the solid solution hardening of Mn, and the effect of suppressing the formation of coarse Mn compounds. As a result, compared to conventional aluminum alloy forgings containing a large amount of Cu and Mg, high strength and high toughness can be achieved. In addition, corrosion resistance can be improved by reducing Cu.
  In addition, due to the effect of suppressing the recrystallization of the high-workability part of the product part and the flash cut surface part, tensile stress is generated that tries to peel the grain boundary in the 90 ° direction with respect to the grain boundary parallel to the forging flow direction. Even when used in a corrosive environment under the above condition, it has an effect that the reliability as an aluminum alloy forging material for a high-strength structural member excellent in stress corrosion cracking resistance is excellent.
(Al alloy forging)
Below, the embodiment of the Al alloy forging material of the present invention will be specifically described. First, the Al alloy forged structural member assumed in the present invention, such as the automobile underbody parts, will be described below.
  The following description will be made using a flash part as a high-working part of the forged material. As shown in the sectional view of the Al alloy forged material 1 after the artificial age hardening treatment in FIG. 1, in the present invention, the product part of the Al alloy forged material means the product part 2 that is the Al alloy forged material body. . The flash cut surface (part) means the part (cut part) of the cut surface 5a between the product part 2 and the flash 3 (in the ST direction perpendicular to the mold parting surface 4).
  As shown in FIG. 2, the Al alloy forging material 1 such as the undercarriage parts is usually a boundary surface (divided into the boundary between the two dies by the upper die 7 and the lower die 8 in the die forging. A split surface 4 (also called a parting line) and a gap 9 between the upper mold 7 and the lower mold 8, and a space for discharging excess Al alloy during forging. Forging is performed with a certain gap 10 referred to as In the forged Al alloy material 1 thus forged, burrs 3 called flash are inevitably generated in the gutta 9. After forging, this flash 3 is separated and cut from the product part 2 in the trim line (flash cutting line) 5, but after cutting, a part of the flash 3 (for example, the root part) is cut so as to remain. Therefore, in the undercarriage part or the like, the product part 2 and the flash cutting surface 5a having a certain length in the direction of the parting surface 4 inevitably exist.
  On the other hand, as shown in FIG. 1, each metal flow (forging line) 6 of the product part 2 of the Al alloy forging material 1 flows into the flash 3 as it is because the interval between the metal flows 6 is narrowed. . However, the processing rate at the time of hot forging differs greatly between the product part 2 and the flash cut surface 5a. In this respect, the processing rate of the flash cut surface 5a (or the old flash part) is usually as high as 90% or more. For this reason, in the hot forging performed a plurality of times, the flash cutting surface 5a having a high processing rate is subjected to processing strain more. Grain tends to occur.
  When crystal grain coarsening occurs in the flash cut surface 5a, in the vicinity thereof, or in the product part, even if the microstructure of the product part is controlled, high strength and high toughness cannot be achieved. In addition, the flash cutting surface 5a and its vicinity become an outer surface against a corrosive environment during use as a structural material, or when tensile stress is applied in the ST direction, or directly joined to an iron-based member. In such a case, there is a high possibility that stress corrosion cracking occurs in this portion due to a synergistic effect with a severe corrosive environment.
(Recrystallization)
Therefore, in the present invention, the average area ratio of recrystallization in the structure of the flash cut surface 5a is set to 20% or less. In this way, by suppressing recrystallization of the flash cut surface 5a structure, it is used as a structural material with tensile stress added, used in a salt water corrosive environment, and used in connection with iron parts. Even in the case of corrosive environment use conditions, reliability as an aluminum alloy forging material for high-strength structural members excellent in stress corrosion cracking resistance can be ensured.
  The recrystallization referred to in the present invention indicates recovery / recrystallization that occurs during the temperature increase rate during the solution treatment. Identification of recrystallized grains (measurement of average grain size) is performed by clarifying each crystal grain in the cross-sectional structure of the product high-processed part and the flash cut surface 5a by an electrolytic etching method, and using a polarizing microscope with a magnification of 25 to 25. Photographed at 100 times, cutting method (counting the number of crystal grains that are completely cut by a line segment of known length, and displaying the average value of the cutting length. Steel specified in JIS G0522 According to the ferrite grain size test method).
  In the present invention, such a high-processed part of the product and recrystallization at the flash cut surface 5a are suppressed by securing the amount of solid solution Mn in the cast billet and reducing (regulating) the contents of Cu and Mg. As long as this product high-working part and recrystallization of the flash cutting surface 5a are controlled, the product part 2 of the Al alloy forging 1 that reduces the processing rate (the part excluding the high-working part in the product) On the other hand, recrystallization and grain coarsening are inevitably suppressed.
(Cast billet)
First, in the present invention, in order to secure the solid solution Mn amount in the cast billet, the solid solution Mn amount is indirectly defined by the conductivity of the cast billet. Within the range of the specific component composition system in the present invention, the conductivity of the cast billet varies greatly depending on whether most of Mn is dissolved or whether most of Mn is crystallized as a compound.
  On the other hand, the amount of solute Mn is difficult to measure directly. Further, the Mn compound having a relatively large particle size that can be observed is measured, and the ratio A (mass%) / B of the element amount A of Mn present as the Mn compound and the total Mn content B in the cast billet Although there is a method of indirectly determining by mass%) or ratio, analysis and measurement are very expensive and complicated.
  Therefore, in the present invention, the solid solution Mn amount in the cast billet is defined by the conductivity that corresponds well with the solid solution Mn amount and is relatively easy to measure.
  In the present invention, this amount of solute Mn is defined as the range of 25-30% IACS in terms of the conductivity of the cast billet. When the conductivity of the cast billet is less than the lower limit of 25% IACS, most of the Mn in the cast billet is crystallized as a compound, and the required amount of solid solution Mn cannot be secured. On the other hand, if the conductivity of the cast billet is 30% IACS or more, the upper limit, the Mn content itself is insufficient, and the necessary solid solution Mn amount cannot be ensured.
  For this reason, when the conductivity of the cast billet is out of the range of 25% to 30% IACS, Mn is finely dispersed and precipitated in the structure during the homogenization heat treatment of the cast billet. The amount of precipitated Mn compound is insufficient to suppress recrystallization of the flash cut surface structure.
(Mn compound)
Incidentally, the Mn compound finely precipitated during the homogenization heat treatment referred to in the present invention is an Al- (Fe.Mn.Cr) -Si intermetallic compound. The dispersed particles finely precipitated during the homogenization heat treatment have effects of suppressing recrystallization and hindering grain boundary movement after recrystallization. For this reason, it is possible to prevent recrystallization and coarsening of crystal grains in the product high-processed portion and the flash cut surface 5a.
(Chemical composition)
Next, the chemical composition in the Al alloy forged material or the cast material for forged material of the present invention will be described. The 6000 series Al alloy of the present invention is required to ensure high durability such as high strength, high toughness and stress corrosion cracking resistance for transportation equipment, structural materials or parts such as automobiles and ships.
  Therefore, the chemical component composition of the Al alloy forging of the present invention is, among Al-Mg-Si JIS 6000 series Al alloys, in particular, Mg: 0.6-1.0%, Si: 0.8-1.5%, Mn: 0.6-1.2 %, Cu: 0.15% or less, and the balance is Al and inevitable impurities. In addition,% display in the amount of each element means the mass%.
  Next, the critical significance of the content of each element will be described below.
Mg: 0.6 to 1.0%.
Mg precipitates as β ″ and β ′ phases with Si by artificial aging treatment, and is an essential element for imparting high strength (yield strength) when the final product is used. If the Mg content is less than 0.6%, the age-hardening amount during the artificial aging treatment decreases. On the other hand, if the content exceeds 1.0%, solid solution of Mn in the cast billet is hindered. In addition, the strength (yield strength) becomes too high, which hinders forgeability. Further, a large amount of Mg2 Si is likely to precipitate during the quenching after the solution treatment, and on the contrary, the strength, toughness, corrosion resistance and the like are lowered. Therefore, the Mg content is in the range of 0.6 to 1.0%.
Si: 0.8-1.5%.
Si, together with Mg, is an essential element for precipitating as a β ″ phase and a β ′ phase by artificial aging treatment and imparting high strength (yield strength) when the final product is used. If the Si content is less than 0.8%, sufficient strength cannot be obtained by artificial aging treatment. On the other hand, if the content exceeds 1.5%, coarse single Si particles crystallize and precipitate during casting and during quenching after solution treatment, and as described above, corrosion resistance and toughness are reduced. Moreover, excessive Si increases, and high corrosion resistance, high toughness, and high fatigue characteristics cannot be obtained. Furthermore, workability is also hindered, for example, elongation becomes low. Therefore, the Si content is in the range of 0.8 to 1.5%.
Mn: 0.6-1.2%.
Mn ensures the amount of the solid solution Mn in the cast billet, and thus the amount of Mn compound deposited that finely disperses and precipitates in the homogenized heat treatment of the cast billet, and in particular suppresses the recrystallization of the high-processed part of the product and the flash cut surface. Is essential for. If the Mn content is less than 0.6%, the solid solution Mn amount and thus the Mn compound amount necessary to suppress recrystallization cannot be secured. On the other hand, an excessive content exceeding 1.2% tends to generate coarse intermetallic compounds and crystallized products in the cast billet during melting and casting, which becomes a starting point of fracture and causes toughness and fatigue characteristics to be lowered.
Cr and Zr.
As with Mn, Cr and Zr, which form finely dispersed particles in the homogenized heat treatment of the cast billet, should be Cr: 0.05-0.25% and Zr: 0.05-0.20% as necessary to suppress recrystallization. It may be added. In addition, the Cr compound increases the quenching sensitivity after the solution treatment and tends to cause a decrease in strength. Therefore, when the quenching and cooling rate after solution treatment is slow, the Cr addition amount is desirably reduced. Further, Zr hinders the effect of refining Ti and B and tends to coarsen the crystal grains of the cast billet. Therefore, when the crystal grains of the cast billet impair the product characteristics, it is desirable to reduce the content. In addition, an excessive content of Cr and Zr tends to generate coarse intermetallic compounds and crystallized products during melting and casting, which becomes a starting point of fracture and causes toughness and fatigue characteristics to deteriorate.
Cu: 0.15% or less.
In the present invention, in order to secure the solid solution Mn amount at the stage of the cast billet, the Cu content is reduced to 0.15% or less, contrary to the conventional case. As described above, Cu easily dissolves in the structure of the cast billet. When Cu is contained in an amount exceeding 0.15%, the required amount of Mn cannot be dissolved in the cast billet structure. As a result, Mn crystallizes in the structure as a coarse compound, and even if the Mn content (addition amount) is large in the alloy system, the fine Mn compound precipitated in the homogenization heat treatment is reduced, and the solid solution Mn content is also reduced. Decreases and the strength decreases. Moreover, Cu remarkably increases the sensitivity of stress corrosion cracking and intergranular corrosion of the structure of the Al alloy forging, and lowers the corrosion resistance and durability of the Al alloy forging. Therefore, in the present invention, the Cu content is restricted as much as possible from this viewpoint.
  Furthermore, in the present invention, even if the Cu content is reduced to 0.15% or less, the strength is not significantly reduced as compared with the conventional aluminum alloy forging containing a large amount of Cu, and on the contrary, the toughness is high. Become.
Fe: 0.50% or less.
Fe contained as an impurity in the Al alloy generates a coarse crystallized product which is a problem in the present invention. These crystallized materials deteriorate the fracture toughness and fatigue characteristics as described above. Therefore, the Fe content is preferably regulated to 0.50% or less, more preferably 0.35% or less.
Zn, Ti, B, Be, V, etc.
Zn, Ti, B, Be and the like are elements that are selectively contained depending on the purpose. For example, during the artificial aging, Zn precipitates MgZn 2 finely and at a high density to achieve high strength. However, if the Zn content is less than 0.005%, sufficient strength cannot be obtained by artificial aging. On the other hand, if it exceeds 0.5%, the corrosion resistance is remarkably reduced. Therefore, the Zn content when contained is preferably in the range of 0.005 to 1.0%.
  Ti is an element added to refine the ingot crystal grains and improve the workability during forging, as well as the strength and impact value of the product. However, when Ti content is less than 0.001%, this effect cannot be obtained.On the other hand, when it exceeds 0.1%, coarse crystals are formed, workability during forging, product strength, impact Decrease the value. Therefore, when Ti is contained, the content of Ti is preferably in the range of 0.001 to 0.1%.
  B, like Ti, is an element added to refine the ingot crystal grains and improve the workability during forging, as well as the strength and impact value of the product. However, if B content is less than 1 ppm, this effect cannot be obtained.On the other hand, if it exceeds 300 ppm%, coarse crystals are formed, and the workability during forging, product strength, and impact value are reduced. Reduce. Therefore, the content of B 2 when contained is preferably in the range of 1 to 300 ppm.
  Be is an element contained to prevent oxidation of the molten Al in the air. However, when the content is less than 0.1 ppm, this effect cannot be obtained. On the other hand, when the content exceeds 100 ppm, the workability during forging is lowered. Therefore, the content of Be when contained is preferably in the range of 0.1 to 100 ppm.
  V, like Mn, Cr, Zr, etc., forms a compound phase (dispersed particles) mainly during the homogenization heat treatment. These compound phases have the effect of inhibiting recrystallization and hindering grain boundary movement after recrystallization. However, an excessive content tends to generate coarse intermetallic compounds and crystallized substances during melting and casting, which becomes a starting point of fracture and causes a decrease in toughness. Therefore, when V is contained, the content is made 0.15% or less.
(Forging production method)
Next, the preferable manufacturing method of the Al alloy forging material in this invention is demonstrated. In the present invention, the production of the Al alloy forged material itself can be basically produced by a conventional method, which is also an advantage of the present invention. However, there are preferable conditions in which Mn is dissolved as much as possible in the cast structure, or finely dispersed Mn compounds are precipitated by soaking, which will be described below.
  When casting a molten Al alloy melt adjusted within the Al alloy component range, for example, a suitable melt casting method such as a semi-continuous casting method (DC casting method) or a hot top casting method is appropriately selected for casting. To do. However, in the present invention, at the stage of the casting billet, Mn is dissolved as much as possible in the cast structure, so that a small diameter billet (ingot) is used within the range that does not impair the productivity, or the cooling ability is increased, It is preferable to increase the cooling rate as much as possible. When the cooling rate of the cast billet is low, even if the Al alloy component composition is within the range, the required amount of solid solution Mn is insufficient, and a coarse crystallized product may be generated as the Mn compound. In this respect, the cooling rate during casting is preferably 10 ° C./sec or more.
  Next, the homogenization heat treatment temperature of the cast billet is preferably in the temperature range of 500 to 580 ° C. If the homogenization heat treatment temperature exceeds 580 ° C. and is too high, burning or the like occurs, causing forging cracks. In addition, mechanical properties such as toughness and fatigue properties in forged products are reduced. In addition, the Mn compound dispersed particles are coarsened, and the number of dispersed particles themselves exhibiting the effect of suppressing recrystallization is insufficient.
  On the other hand, if the homogenization heat treatment temperature is too low, less than 500 ° C., the homogenization treatment is insufficient and the crystallized product is not sufficiently refined (partially solid solution), making it difficult to increase the toughness of the forged product.
  After this homogenization heat treatment, hot die forging is performed by mechanical forging, hydraulic forging, or the like to form a product-shaped Al alloy forging material. Under the present circumstances, the hot forging start temperature of a product part shall be 550-400 degreeC. If the hot forging start temperature is less than 400 ° C., particularly in the hot forging performed a plurality of times without reheating, the hot forging process itself becomes difficult. In addition, the dislocation density accumulated in the material becomes too high, and recrystallization is likely to occur during the solution treatment, particularly at high product processed parts and flash cut surfaces. On the other hand, when the hot forging start temperature exceeds 550 ° C., local melting is caused by frictional heat, and forging cracks are likely to occur. The final forging end temperature is preferably as high as possible, for example, 350 ° C. or higher in order to reduce the dislocation density accumulated in the material.
  And after forging, T6 (artificial age hardening treatment to obtain maximum strength after solution treatment) to obtain the required strength, toughness and corrosion resistance, T7 (artificial age hardening treatment to obtain maximum strength after solution treatment) Excessive age hardening treatment exceeding the conditions), T8 (artificial age hardening treatment for obtaining maximum strength by performing cold working after solution treatment, and further performing appropriate heat treatment). Moreover, an air furnace, an induction heating furnace, a nitrite furnace, etc. are suitably used for the homogenization heat treatment and the solution treatment. Furthermore, an air furnace, an induction heating furnace, an oil bath, or the like is appropriately used for the artificial age hardening treatment.
The cooling of the quenching treatment after the solution treatment is preferably water cooling. When the cooling rate during the quenching process is lowered, Mg 2 Si, Si, and the like are precipitated on the grain boundaries, and in the product after artificial aging, grain boundary fracture is likely to occur, and the toughness and fatigue characteristics are lowered. Further, during the cooling, stable phases Mg 2 Si and Si are formed in the grains, and the amount of β phase and β phase precipitated during artificial aging is reduced, so that the strength is lowered. On the other hand, when the cooling rate increases, the amount of quenching distortion increases, and after quenching, a new straightening process is required, or the number of steps in the straightening process increases. Therefore, in order to shorten the product manufacturing process and reduce the cost, hot water quenching at 35 to 85 ° C. in which quenching distortion is alleviated is preferable. Here, when the hot water quenching temperature is less than 35 ° C., the quenching strain increases, and when it exceeds 85 ° C., the cooling rate becomes too low, and the toughness, fatigue characteristics, and strength decrease.
Further, in the T7 tempered material, the proportion of β phase precipitated on the grain boundary becomes high. This β ' phase is difficult to elute in a corrosive environment, lowers intergranular corrosion sensitivity, and increases stress corrosion cracking resistance. On the other hand, the β phase that precipitates a lot in the T6 material is easily eluted in a corrosive environment, increases the intergranular corrosion sensitivity, and decreases the stress corrosion cracking resistance. Therefore, when the Al alloy forged material is the T7 material, the proof stress is slightly lowered, but the corrosion resistance is higher than that of other tempering treatments.
  In order to eliminate the cast structure remaining in the Al alloy forging, destroy and refine the crystallized material, and improve the strength, toughness and fatigue properties, the Al alloy ingot is subjected to extrusion and rolling after homogenization heat treatment. After that, the forging may be performed.
  Next, examples of the present invention will be described. Each Al alloy cast billet (Al alloy ingot) whose chemical composition is shown in Table 1 was cast by a semi-continuous casting method. At this time, as shown in Table 2, each billet was produced by changing the ingot size from 70 mm to 400 mm in diameter. As a result, the cooling rate during casting (the center of the ingot) was varied from 1 to 25 ℃ / sec. 1 to 3 shown in Table 1 are alloy compositions within the scope of the present invention, and 4 to 8 are alloy compositions outside the scope of the present invention.
  Table 2 shows the measurement results of the conductivity (% IACS) of each cast billet. The conductivity was measured by cutting each cast billet, polishing the cut surface with emery paper # 1000, and measuring the center of the cut surface (the center of the billet) with an eddy current conductivity measuring device. The number of measurement samples (number n) was 10, and the conductivity was obtained as an average value of these.
  Next, the outer surface of each cast billet was chamfered to a thickness of 3 mm, cut to a length of 500 mm, and then subjected to a homogenization heat treatment at 520 ° C. for 4 hours in common. The heating time to the homogenization heat treatment temperature is 6 hours. Thereafter, hot forging was performed to produce a forged material having a substantially undercarriage member shape as shown in FIGS.
  Forging conditions were commonly set to a forging start temperature of 500 ° C. and a forging end temperature of 450 to 500 ° C. In each example, hot forging was performed by mechanical forging using the upper and lower molds shown in Fig. 2 above, with a flashland gap of 1.5 to 3 mm, and the processing rate of product part 2 was the sum of the three forgings. Forging was performed three times without reheating so that the processing rate of the flash cut surface 5a was constant at 80 to 85% and 90 to 95% in total of the three forgings. The processing rate C is calculated by using C = [(BA) / B] × 100, using forging line spacing (interval between crystallized columns) A and ingot average cell layer size B. Calculated by% formula.
  This forged material was commonly subjected to a solution treatment of 560 ° C. × 3 hours and an artificial age hardening treatment of 190 × 5 hours to obtain a tempered T6 material. The solution treatment was performed using an air furnace with a heating time of 1 to 2 hours. After solution treatment, the solution was quenched in warm water at 60 ° C., and the artificial age hardening treatment was performed within 15 minutes. .
  Table 2 shows the evaluation results of the characteristics of these forgings. The Al alloy numbers in Table 2 correspond to the Al alloy numbers in Table 1. At this time, the average crystal grain size (μm) of the product part 2 and the sub-crystal grain area ratio (%) of the flash cut surface 5a were measured as described above. In addition, from each forged product part 2, five tensile test pieces A (L direction) and five Charpy test pieces B (LT direction) were sampled, and tensile strength (MPa), 0.2% proof stress (MPa), elongation ( %), Charpy impact value, etc. were measured, and each average value was determined.
  In addition, the stress corrosion cracking test was performed by collecting a C-ring test piece including the flash cut surface 5a from each forged material. The stress corrosion cracking test conditions were determined in accordance with the ASTM G47 alternate dipping method for the C-ring test piece. However, the test condition is that the forging material is used by applying tensile stress in the ST direction to the cut surface 5a portion, and testing the mechanical properties in the ST direction of the C-ring test piece. The test piece was loaded with 75% of the proof stress in the LT direction. In this state, the C-ring test piece was repeatedly immersed and pulled up in salt water for 90 days, simulating use in a salt water corrosive environment, and the presence or absence of stress corrosion cracking in the test piece was confirmed.
  As a result, the case where stress corrosion cracking has occurred in the C-ring test piece is ×, and the case where intergranular corrosion which is not stress corrosion cracking but is likely to lead to stress corrosion cracking has occurred △, Table 2 shows the cases where no stress corrosion cracking or intergranular corrosion has occurred (including the case where superficial overall corrosion has occurred).
    As is clear from Table 2, Invention Examples 1, 2, 4, 5, 7, and 8 have chemical composition compositions within the scope of the present invention (Al alloys 1 to 3 in Table 1), and the conductivity of the cast billet is 26. It is in the range of ~ 32% IACS.
  Further, in each of the above-mentioned invention examples, coarse Mn crystallized material was not observed in the visual field observation at any 10 positions by SEM (scanning electron microscope) 200 to 500 times as large as each cast billet structure, and Mn was large. Was estimated to be dissolved in the cast billet structure.
As a result, from Table 2, in each of the above invention examples, the average area ratio of recrystallized grains in the structure of the forged material flash cut surface 5a is 20% or less. This is because the solid Mn in the cast billet structure was precipitated during the homogenization heat treatment, and the effect of refining crystal grains of fine Mn precipitates was exhibited. Therefore, the proof stress (σ 0.2 ) is 350 MPa or more and the Charpy impact value is 20 J / cm 2 or more, and the strength and toughness are high. In the stress corrosion cracking test that simulates a severe use environment and a corrosive environment, the forged material flash cut surface 5a is also excellent in stress corrosion cracking resistance.
On the other hand, Comparative Examples 3, 6, and 9 have chemical composition compositions within the scope of the present invention (Al alloys of 1 to 3 in Table 1), but have a slow cooling rate during casting, and the conductivity of the cast billet is the upper limit. It exceeds 32% IACS. For this reason, the average area ratio of recrystallized grains in the structure of the forged material flash cut surface 5a is only high with an upper limit of 20% or more. As a result, in each comparison of Invention Examples 1, 2, 4, 5, and 7, 8 of Al alloys having the same composition, particularly in the stress corrosion cracking test, the stress resistance of the forged flash flash surface 5a Corrosion cracking property is remarkably inferior to that of the inventive examples. Also, the proof stress (σ 0.2 ) and Charpy impact value are significantly inferior to those of the inventive examples.
  In the comparative examples 3, 6 and 9, in the SEM observation of each cast billet structure similar to the inventive example, many coarse Mn crystallized products are observed, and the Mn dissolved in the cast billet structure is small. It was speculated. Therefore, in Comparative Examples 3, 6, and 9, it is surmised that the solid solution Mn in the cast billet structure was precipitated during the homogenization heat treatment, and the number or amount of fine Mn precipitates was small, and the recrystallization suppression and prevention effects were not exhibited. Is done.
Further, in Comparative Example 10 using the Al alloy 4 with excessive Cu, the conductivity of the cast billet is less than the upper limit of 26% IACS even though the cooling rate at the time of casting is as fast as the invention example. This is because, as described above, in the Al alloy 4 in which a substantial amount of Cu is dissolved, the amount of Mn that can be dissolved in the cast billet structure is reduced, and the contained Mn is crystallized in the structure as a coarse compound. Means. In fact, in SEM observation of the cast billet structure similar to that of the inventive example in Comparative Example 10, coarse Mn crystallized material was observed, and it was estimated that Mn dissolved in the cast billet structure was small. Therefore, in Comparative Example 10, it is presumed that the solid solution Mn in the cast billet structure is small in the number or amount of fine Mn precipitates precipitated during the homogenization heat treatment, and the effect of suppressing or preventing recrystallization is not exhibited.
For this reason, in Comparative Example 10, the average area ratio of recrystallized grains in the forged material flash cut surface 5a is only as high as 30%. As a result, in particular, in the stress corrosion cracking test, the stress corrosion cracking resistance of the forged material flash cut surface 5a is remarkably inferior to that of the inventive example.
On the other hand, in Comparative Example 11 using Al alloy 5 with a low Mn, the conductivity of the cast billet is lower than the upper limit of 32% IACS even though the cooling rate at the time of casting is as fast as the invention example. This indicates that, as described above, the amount of Mn itself is insufficient, and the amount of solute Mn in the cast billet structure is reduced. In fact, in SEM observation of the cast billet structure in Comparative Example 11 similar to that of the inventive example, there were also few coarse Mn crystallized products. Therefore, in Comparative Example 11, it is presumed that the solid solution Mn in the cast billet structure is small in the number or amount of fine Mn precipitates precipitated during the homogenization heat treatment, and the effect of suppressing or preventing recrystallization is not exhibited.
Therefore, in Comparative Example 11, the average area ratio of recrystallization in the texture of the forged material flash cut surface 5a is only as high as 80%. As a result, in particular, in the stress corrosion cracking test, the stress corrosion cracking resistance of the forged material flash cut surface 5a is remarkably inferior to that of the inventive example. Moreover, the proof stress and the Charpy impact value are remarkably inferior to those of the inventive examples.
On the contrary, in Comparative Example 12 using Al alloy 6 with excessive Mn, the casting billet conductivity is less than the upper limit of 26% IACS even though the cooling rate at the time of casting is as fast as the invention example. This indicates that the amount of Mn is too large and the amount of solid solution Mn in the cast billet structure is large, but there are also many coarse Mn crystals. In fact, in SEM observation of the cast billet structure in Comparative Example 11 similar to the invention example, there were many coarse Mn crystals.
For this reason, in Comparative Example 12, the effect of refining the crystal grains of fine Mn precipitates precipitated during the homogenization heat treatment is hindered by coarse Mn crystallized substances. Coarse Mn crystallized products are likely to become nuclei for recrystallization. For this reason, the average area ratio of recrystallization in the texture of the forged material flash cut surface 5a is only as high as 60%. As a result, in particular, in the stress corrosion cracking test, the stress corrosion cracking resistance of the forged material flash cut surface 5a is remarkably inferior to that of the inventive example. Also, the proof stress and Charpy impact value are significantly inferior to those of the inventive examples.
In Comparative Example 13 using Al alloy 7 with a small amount of Mn and a large amount of Cr, the conductivity of the cast billet is less than the upper limit of 26% IACS even though the cooling rate at the time of casting is as fast as the invention example. . This indicates that there are many crystallized substances containing coarse Cr. In fact, in SEM observation of the cast billet structure in Comparative Example 11 similar to the inventive example, there were many coarse crystals containing Cr.
Therefore, in Comparative Example 13, since the solid solution Mn in the cast billet structure is precipitated during the homogenization heat treatment, the number or amount of fine Mn precipitates is small, and a coarse crystallized product containing Cr tends to become a nucleus of recrystallization. It is presumed that the effect of suppressing and preventing recrystallization is not exhibited. For this reason, the average area ratio of sub-crystal grains in the forged material flash cut surface 5a structure is only as high as 55%.
As a result, in Comparative Example 13, particularly in the stress corrosion cracking test, the stress corrosion cracking resistance of the forged material flash cut surface 5a is remarkably inferior to that of the inventive example. Also, the proof stress and Charpy impact value are significantly inferior to those of the inventive examples.
  Comparative Example 14 using an Al alloy 8 with a low Si content and Comparative Example 15 using an Al alloy 8 with a low Mg content are excellent in resistance to stress corrosion cracking. However, the yield strength (strength) is low because the amount of precipitates such as β ′ phase contributing to the strength is low, and it is not suitable as a structural member such as an automobile undercarriage part.
  Therefore, from these results, the critical significance of each requirement in the present invention, such as the Al alloy component composition, the conductivity of the cast billet, and the average area ratio of the recrystallized grains in the forged material flash cut surface structure, can be understood.
  ADVANTAGE OF THE INVENTION According to this invention, the Al-Mg-Si-Mn type aluminum alloy forging material for structural members which can be manufactured with sufficient reproducibility, is high strength, high toughness, and has high corrosion resistance can be provided. Therefore, the use of Al-Mg-Si-Mn-based aluminum alloy forgings for transportation equipment can be expanded.
It is sectional drawing which shows the macro structure of the ST direction cross section of the structure | tissue part prescribed | regulated with this invention Al alloy forging material. It is sectional drawing which shows the metal mold | die for Al alloy forging of this invention.
Explanation of symbols
1: Al alloy forging, 2: Product part, 3: Flash, 4: Split surface,
5: Flash cutting line, 6: Metal flow, 7: Upper mold, 8: Lower mold,
9: Gutta, 10: Flash stand,

Claims (4)

  1.   Al-Mg-Si-Mn-based aluminum alloy forging, including mass: 0.6 to 1.0%, Mg: 0.8 to 1.5%, Mn: 0.6 to 1.2%, Cu: 0.15% or less The cast billet consisting of the balance Al and inevitable impurities and having an electrical conductivity of 26 to 32% IACS is hot-die forged, and the average area ratio of recrystallized grains in the flash cut surface structure of the forging is An aluminum alloy forging material for high-strength structural members excellent in stress corrosion cracking resistance, characterized by being 20% or less.
  2.   Furthermore, the aluminum alloy for high-strength structural members excellent in stress corrosion cracking resistance according to claim 1, further comprising one or two of Cr: 0.05 to 0.25% and Zr: 0.05 to 0.20% by mass%. Forging material.
  3. The high-strength structure excellent in stress corrosion cracking resistance according to claim 1 or 2, wherein the 0.2% proof stress is 350 MPa or more on average and the Charpy impact value is 20 J / cm 2 or more on average after T6 treatment of the forged material. Aluminum alloy forgings for parts.
  4. The aluminum alloy forging material for a high-strength structural member excellent in stress corrosion cracking resistance according to any one of claims 1 to 3, wherein the forging material is an automobile underbody part.
JP2005098919A 2005-03-30 2005-03-30 Aluminum alloy forging for high strength structural member Pending JP2006274415A (en)

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WO2013114928A1 (en) * 2012-02-02 2013-08-08 株式会社神戸製鋼所 Forged aluminum alloy material and method for producing same
WO2014170946A1 (en) * 2013-04-15 2014-10-23 日本軽金属株式会社 PRODUCTION METHOD FOR Al-Mg-Si-BASED ALUMINIUM ALLOY MEMBER FOR RESIN BONDING, AND Al-Mg-Si-BASED ALUMINIUM ALLOY MEMBER FOR RESIN BONDING OBTAINED USING SAID METHOD
CN104805340A (en) * 2015-05-21 2015-07-29 广西友合铝材有限公司 Rare earth aluminium-magnesium-silicon alloy material and preparation method thereof
EP3464659B1 (en) 2016-06-01 2020-03-11 Aleris Aluminum Duffel BVBA 6xxx-series aluminium alloy forging stock material and method of manufacting thereof
CN110952049A (en) * 2019-09-23 2020-04-03 山东南山铝业股份有限公司 Thermomechanical treatment method for toughening high-performance wrought rare earth aluminum alloy

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JP2004084058A (en) * 2002-06-27 2004-03-18 Kobe Steel Ltd Method for producing aluminum alloy forging for transport structural material and aluminum alloy forging
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JP2003277868A (en) * 2002-03-19 2003-10-02 Kobe Steel Ltd Aluminum alloy forging having excellent stress corrosion cracking resistance and stock for the forging
JP2004084058A (en) * 2002-06-27 2004-03-18 Kobe Steel Ltd Method for producing aluminum alloy forging for transport structural material and aluminum alloy forging
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JP2008196009A (en) * 2007-02-13 2008-08-28 Toyota Motor Corp Method for manufacturing aluminum alloy material, and heat treatment type aluminum alloy material
WO2013114928A1 (en) * 2012-02-02 2013-08-08 株式会社神戸製鋼所 Forged aluminum alloy material and method for producing same
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CN104805340A (en) * 2015-05-21 2015-07-29 广西友合铝材有限公司 Rare earth aluminium-magnesium-silicon alloy material and preparation method thereof
EP3464659B1 (en) 2016-06-01 2020-03-11 Aleris Aluminum Duffel BVBA 6xxx-series aluminium alloy forging stock material and method of manufacting thereof
CN110952049A (en) * 2019-09-23 2020-04-03 山东南山铝业股份有限公司 Thermomechanical treatment method for toughening high-performance wrought rare earth aluminum alloy

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