JP2017155251A - Aluminum alloy forging material excellent in strength and ductility and manufacturing method therefor - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a 6000 series aluminum alloy hot forging material having excellent corrosion resistance, high strength and high ductility.SOLUTION: There is provided an aluminum alloy forging material containing, by mass%, each of Si:0.7 to 1.5%, Mg:0.6 to 1.2%, Fe:0.01 to 0.5%, further one or more kind of Mn:0.05 to 1.0%, Cr:0.01 to 0.5% and Zr:0.01 to 0.2% and the balance Al with inevitable impurities and having a structure on an observed surface of a thickness center of the thickest part of the forging material, which has a dislocation density measured by an X ray diffraction in a range of 1.0×10to 5.0×10/mas average, an average percentage of a small tilt angle grain boundary with a tilt angle of 2 to 15° of a crystal grain with an orientation difference within 2° measured by SEM-EBSD method of 50% or more, and an average number density of a precipitate which can be measured by TEM with amplification of 300,000 of 5.0×10/μmor more.SELECTED DRAWING: None

Description

  The present invention relates to an aluminum alloy forged material excellent in strength and ductility and a method for producing the same. Hereinafter, aluminum is also simply referred to as Al.

The forging material referred to in the present invention means an aluminum alloy forging material manufactured (plastic processing) by hot forging.
In the present invention, the term forged material is used as a well-known term that is widely used as a technical term and a patent term for specifying the state of an object, not as a description of a manufacturing method of an object. .
Aluminum alloy materials have different alloy compositions due to differences in plastic working history, such as hot forged materials, extruded materials, rolled materials, etc., but the structure and properties are completely different, unless the plastic working history is specified. The meaning of defining the organization and characteristics disappears.
For this reason, in the present invention, in order to clearly distinguish the target aluminum alloy hot forging material from the other plastic working materials and to clearly specify the state of the object, “hot forging material” or the same meaning is used. The term “forging” is used in the claims and the following description.

In recent years, with respect to global environmental problems caused by exhaust gas and the like, improvement in fuel efficiency has been pursued by reducing the weight of the body of a transport aircraft such as an automobile. For this reason, 6000 series (Al-Mg-Si series) aluminum alloy hots in the AA to JIS standards as structural parts and structural parts of transport equipment such as automobiles, especially automobile underbody parts such as upper arms and lower arms. Forging is used.
As these structural materials and structural parts, 6000 series aluminum alloy hot forged materials have high strength and high toughness, and are relatively excellent in corrosion resistance. Hereinafter, an automobile underbody part will be described as an example of a structural material or a structural part of a transport aircraft.

  In order to further reduce the weight of automobiles, automobile underbody parts are required to have higher strength and higher toughness after being made thinner. In addition, in view of reliability as a safety part, high corrosion resistance against intergranular corrosion and stress corrosion cracking is also required. For this reason, conventionally, various attempts have been made to improve the composition and microstructure of the 6000 series aluminum alloy hot forging as a raw material.

  For example, the crystallization density of a 6000 series aluminum alloy forging material is suppressed to an average area ratio of 1.5% or less, and the grain boundary precipitates observed in the structure of the cross-sectional site including the parting line generated during forging It has been proposed to increase the average interval to 0.7 μm or more (see Patent Document 1).

Moreover, as a 6000 series aluminum alloy forged material obtained by hot forging an aluminum alloy extruded material, a small angle grain boundary with a tilt angle of 2 ° or more and less than 15 ° and a large tilt angle of 15 ° or more in the entire cross section of the forged material. The non-recrystallized region including the tilted grain boundary is provided, and the average grain size of the region surrounded by the boundary having the tilt angle of 2 ° or more in the non-recrystallized region is 10 μm or less and The average area ratio is 75% or more, and the average density of dispersed particles having a maximum length of 10 nm or more and 800 nm or less in this non-recrystallized structure region is 10 particles / μm ³ or more, and the maximum length is 0.5 μm. It has also been proposed to use an aluminum alloy forged material in which the average area ratio of the crystallized material is 2.5% or less (see Patent Document 2).

  On the other hand, although not in the field of hot forging, as a metallurgical technique for increasing the strength of aluminum alloy material, after forging treatment to 6000 series aluminum alloy casting material, warm forging at about 150-250 ° C It is known that the processing is repeated and then artificial aging treatment (artificial age hardening treatment) is performed (see Patent Documents 3 and 4).

Japanese Patent No. 5110938 Japanese Patent No. 5723192 Japanese Patent Laid-Open No. 2014-218685 Japanese Patent No. 5082483

  However, the aluminum alloy forging material which is incompatible with the improvement of the composition and microstructure of the 6000 series aluminum alloy hot forging material, such as Patent Documents 1 and 2, is difficult to combine, and has excellent strength and ductility. There is still room for improvement.

  In addition, a technique for increasing the strength by repeatedly performing a warm forging process on a 6000 series aluminum alloy cast material as in Patent Documents 3 and 4 and then performing an artificial aging treatment is also as high as 500 ° C. If hot forging is performed, the effect of increasing the strength is considered to be small, and it is still unclear whether this technique is effective in improving the mechanical properties of the 6000 series aluminum alloy hot forging.

  The present invention has been made by paying attention to such circumstances, and the object thereof is excellent in strength and ductility (combining high strength and high elongation) on the assumption that it has excellent corrosion resistance. An object of the present invention is to provide a forged aluminum alloy.

In order to achieve this object, the gist of the aluminum alloy forging material excellent in strength and ductility of the present invention is mass%, Si: 0.7 to 1.5%, Mg: 0.6 to 1.2% Fe: 0.01 to 0.5%, respectively, Mn: 0.05 to 1.0%, Cr: 0.01 to 0.5%, Zr: 0.01 to 0.2% %, A balance of Al and unavoidable impurities, and an aluminum alloy forging material, the structure of the thickest part of the forging material at the observation center of the thickness center, X Dislocation density measured by line diffraction is in the range of 1.0 × 10 ^{14 to} 5.0 × 10 ¹⁶ / m ^{2 on} average, and crystal grains having an orientation difference of 2 ° or more measured by SEM-EBSD method The average proportion of the low-angle grain boundaries with an inclination angle of 2 to 15 ° is 50% or more, and the magnification is 300,000 times. The average number density of precipitates that can be measured by TEM is 5.0 × 10 ² pieces / μm ³ or more.

Moreover, in order to achieve the said objective, the summary of the manufacturing method of the aluminum alloy forging material excellent in the intensity | strength and ductility of this invention is the mass%, Si: 0.7-1.5%, Mg: 0.6 -1.2%, Fe: 0.01-0.5%, respectively, Mn: 0.05-1.0%, Cr: 0.01-0.5%, Zr: 0.0. An aluminum alloy ingot containing one or more of 01 to 0.2% and comprising the balance Al and unavoidable impurities is hot-forged after soaking to form a forged material, followed by solution treatment and quenching treatment Dislocation measured by X-ray diffraction as a structure in the observation surface of the thickness center of the thickest part of the forged material after the artificial aging treatment was performed after warming the forged material. range of ^{1.0 × 10 14 ~5.0 × 10 16} / m 2 on average density In addition, the average ratio of crystal grain boundaries with an orientation difference of 2 ° or more and an inclination angle of 2 to 15 ° measured by the SEM-EBSD method is 50% or more, and can be measured by a TEM with a magnification of 300,000 times. The average number density of the precipitates was set to 5.0 × 10 ² pieces / μm ³ or more, respectively.

  In the present invention, for a 6000 series aluminum alloy forged material, a forging material subjected to solution treatment and quenching treatment is given a processing strain due to warm working and then subjected to an artificial aging treatment, and a normal case where no processing strain is imparted It was found that both strength and ductility were improved (higher strength and higher ductility) than those of

  In order to exert or guarantee this effect, in the present invention, as described above, the average dislocation density, the small-gradient grain as the structure in the thickness center part of the thickest part of the forged material after the artificial aging treatment The average ratio of boundaries and the average number density of precipitates are defined respectively.

  According to the present invention, both the strength and ductility of the 6000 series aluminum alloy forged material are improved, so that further weight reduction is possible.

  Hereinafter, embodiments of the present invention will be described in detail.

(Chemical composition)
First, the chemical component composition of the aluminum alloy of the forged material of the present invention and the ingot that is the material of the cast material will be described below.

  The chemical component composition of the 6000 series (Al-Mg-Si series) aluminum alloy in the present invention needs to ensure high strength, high ductility, and high corrosion resistance or durability as the undercarriage forged part. For this reason, the aluminum alloy composition in the present invention in the composition range of 6000 series aluminum is mass%, Si: 0.7 to 1.5%, Mg: 0.6 to 1.2%, Fe: 0.00. In addition to each containing 01-0.5%, Mn: 0.05-1.0%, Cr: 0.01-0.5%, Zr: 0.01-0.2% Or it is set as the aluminum alloy which contains 2 or more types, and consists of remainder Al and an unavoidable impurity.

Further, in order to improve properties such as strength, the aluminum alloy further contains, in mass%, Cu: 0.05 to 1.0%, Ti: 0.01 to 0.1%, Zn: 0.005. You may contain 1 type, or 2 or more types of 0.25%. In addition, all the% display in each element amount means the mass%.
Next, the critical significance and preferable range of the content of each element will be described.

Si: 0.7 to 1.5%,
Si, together with Mg, is an essential element for precipitating in crystal grains mainly as an acicular β ′ phase by artificial aging treatment and imparting high strength to automobile undercarriage parts.
If the Si content is too small, the amount of precipitation during the artificial aging treatment is too small, and high strength cannot be obtained.
On the other hand, if the Si content is too large, coarse single Si particles crystallize and precipitate during casting and during quenching after solution treatment, thereby reducing corrosion resistance and toughness. Moreover, excess Si increases, and high corrosion resistance and high toughness and high fatigue characteristics cannot be obtained. Furthermore, hot forgeability and workability are also hindered, such as elongation becoming low.
Therefore, the Si content is in the range of 0.7 to 1.5%.

Mg: 0.6-1.2%
Mg is an element essential for imparting high strength and high ductility of automobile undercarriage parts by precipitation into crystal grains mainly as acicular β 'phase by artificial aging treatment (aging treatment). is there.
When the content of Mg is too small, the amount of precipitation during the artificial aging treatment is too small to obtain high strength.
On the other hand, when there is too much content of Mg, a coarse Mg containing compound will generate | occur | produce in the grain of a crystal | crystallization, or a grain boundary, and corrosion resistance and toughness will fall. In addition, the strength (proof strength) at high temperatures becomes too high, which hinders hot forgeability and workability.
Therefore, the Mg content is in the range of 0.6 to 1.2%.

Fe: 0.01 to 0.5%
Fe forms an intermetallic compound with Si to produce dispersed particles (dispersed phase), which prevents rebounding and prevents coarsening of crystal grains by preventing grain boundary movement after recrystallization. There is an effect of refining crystal grains.
On the other hand, when the content of Fe is too large, a coarse compound tends to be formed in the crystal grains and in the crystal grain boundaries, and the corrosion resistance and toughness are likely to be lowered. Moreover, since Si is easily contained in the intermetallic compound formed by Fe, the needle-like β ′ phase generated by the artificial aging treatment that requires Si is reduced, and the strength is easily lowered.
Therefore, the Fe content is in the range of 0.01 to 0.5%.

Mn: 0.05 to 1.0%, Cr: 0.01 to 0.5%, Zr: 0.01 to 0.2%, one or more Mn, Cr and Zr are the same as Fe , By generating an intermetallic compound with Si to produce dispersed particles (dispersed phase), preventing grain boundary movement after recrystallization, suppressing recrystallization, and preventing coarsening of crystal grains. There is an effect of miniaturization.
On the other hand, when there is too much content of Mn, Cr, and Zr, it will become easy to form a coarse compound in a crystal grain and a crystal grain boundary, and it will be easy to reduce corrosion resistance and toughness. Moreover, since Si is easily contained in the intermetallic compound formed by these elements, the needle-like β ′ generated by the artificial aging treatment that requires Si is reduced, and the strength is likely to be lowered.
Accordingly, when one or more of these elements are contained, the respective contents are Mn: 0.05 to 1.0%, Cr: 0.01 to 0.5%, Zr: 0.01 It is made into the range of -0.2%.

One or more of Cu: 0.05 to 1.0%, Ti: 0.01 to 0.1%, Zn 0.005 to 0.25% Cu, Ti, and Zn increase the strength and toughness of the forged material. Since it is a synergistic element to be improved, when these effects are expected, one or more kinds are selectively contained.
Cu contributes to improving the strength and toughness of the forged material by solid solution strengthening, and also has the effect of remarkably accelerating the age hardening of the final product during the aging treatment. When there is too little content of Cu, there will be no these strength improvement effects. On the other hand, if the Cu content is too large, the sensitivity of stress corrosion cracking and intergranular corrosion of the structure of the aluminum alloy forged material is remarkably increased, and the corrosion resistance and durability of the aluminum alloy forged material are lowered. Therefore, if Cu is included, the Cu content is in the range of 0.05 to 1.0%.

  Zn precipitates and forms Zn-Mg precipitates finely and at high density in an artificial aging treatment, thereby improving strength and toughness. Further, the solid solution Zn has the effect of lowering the electric potential in the grains and reducing the corrosion form not from the grain boundaries but as the entire corrosion, resulting in reduction of the intergranular corrosion and stress corrosion cracking. However, when there is too much content of Zn, corrosion resistance will fall remarkably. Therefore, the Zn content in the case of inclusion is set to a range of 0.005 to 0.25%.

  Ti has the effect of refining the crystal grains of the ingot and improving the strength and toughness by using the forged material structure as fine crystal grains. If the Ti content is too small, this effect cannot be exhibited. However, when there is too much content of Ti, a coarse crystallization thing will be formed and the said workability will be reduced. Therefore, when Ti is contained, the content of Ti is set to a range of 0.01 to 0.1%.

  Here, other impurity elements that are likely to be mixed from the melting raw material scrap are inevitable impurities in the remainder of the alloy composition as long as they do not hinder various characteristics of the present invention, and are usually based on the upper limit provisions of JIS standards. It is permissible to include

For example, the impurity elements described below are allowed up to the contents described below. Hydrogen is likely to be mixed as an impurity, especially when the forging material has a low workability, bubbles due to hydrogen will not be crimped by forging and other processes, blisters will be generated, and fracture will occur, leading to toughness and fatigue characteristics Is significantly reduced. In particular, in the undercarriage parts with increased strength, the influence of hydrogen is large. Therefore, the hydrogen concentration per 100 g of Al is preferably 0.25 ml or less, and the content is as low as possible.
Sc, V, and Hf are also likely to be mixed as impurities and hinder the characteristics of the undercarriage parts, so the total of these is made less than 0.3%.
If B is contained in excess of 500 ppm, a coarse crystallized product is formed and the workability is lowered. Therefore, the allowable amount is set to 500 ppm or less.

(Organization)
Based on the above alloy composition, the present invention improves both strength and ductility of structural materials and structural parts of transport vehicles such as automobiles, especially forged materials such as automobile undercarriage forged parts (high strength, high strength). In order to combine ductility), the structure on the observation surface (thickness center portion) of the thickness center of the thickest portion of the forged material is defined.
As this rule, first, the average dislocation density measured by X-ray diffraction is set to a range of 1.0 × 10 ^{14 to} 5.0 × 10 ¹⁶ / m ² .
Further, the average ratio of the low-angle grain boundaries having an inclination angle of 2 to 15 ° measured by the SEM-EBSD method and having an orientation difference of 2 ° or more is set to 50% or more.
Furthermore, the average number density of precipitates that can be measured with a TEM with a magnification of 300,000 times is set to 5.0 × 10 ² pieces / μm ³ or more.

In the 6000 series aluminum alloy forging material having the above alloy composition, when a forging material subjected to solution treatment and quenching treatment is subjected to processing strain due to warm working and then subjected to artificial aging treatment, normal forging material which does not impart the processing strain. Both strength and ductility are improved as compared with the case of.
This is due to dislocations introduced and strengthened by applying a processing strain due to warm working after the uniform fine β ′ phase preliminarily precipitated in the grain of the forging material by heating before warm working described later. It is considered that the β 'phase non-uniform precipitation during the artificial aging treatment is suppressed, and both strength and ductility are improved.
In addition, the uniform fine β ′ phase precipitated in the grains by heating before warm working pinned dislocations introduced by imparting processing strain by subsequent warm working, and dislocation during artificial aging treatment It is assumed that the recovery of the steel is suppressed, the work hardening amount is secured, and the ductility is improved.

In order to exhibit or guarantee these effects, in the present invention, as described above, the average dislocation density, the small thickness of the thickest portion of the forged material after the artificial aging treatment, The average ratio of tilt grain boundaries and the average number density of precipitates are respectively defined.
These organizational requirements will be explained sequentially as follows.

(Dislocation density)
In the present invention, in order to increase the strength and ductility of the forging material, this forging is combined with other structure controls such as the average ratio of the low-angle grain boundaries in the grain boundaries and the average number density of precipitates. The average dislocation density measured by X-ray diffraction on the observation surface at the thickness center of the thickest part of the material is in the range of 1.0 × 10 ^{14 to} 5.0 × 10 ¹⁶ / m ² .
In the present invention, the forged material that has undergone solution treatment and quenching is warm-worked, imparted with processing strain (strain), introduced dislocations again into the forged material, and controlled the dislocation density of the forged material within the specified range. In addition, it suppresses uneven deformation up to a high strain range or up to breakage against external loads when used as automobile undercarriage parts, etc., and has high work hardening characteristics (reduction in yield ratio, elongation Increase). As a result, the 0.2% proof stress is 400 MPa or more and the elongation is 10% or more, together with other structural regulations (requirements) such as the average ratio of the low-angle grain boundaries in the grain boundaries and the average number density of precipitates. High strength and high ductility.

If this dislocation density is too low, less than 1.0 × 10 ¹⁴ / m ² , it becomes the same as a conventional forged material that does not impart distortion due to the warm working, and the work hardening characteristics are lowered. As a result, an early break in a high strain region is caused with respect to an external force load when used as an automobile underbody part or the like.
On the other hand, when this dislocation density exceeds 5.0 × 10 ¹⁶ / m ² and becomes too high, dislocations that can be introduced and accumulated in a high strain range with respect to external force loads when used as automobile underbody parts and the like. As a result, the premature rupture in the high strain region is also caused.

Measurement method of dislocation density Although it is widely used to measure the dislocation density with a transmission electron microscope or the like, in the present invention, the dislocation density is measured more easily and reproducibly by X-ray diffraction.
Of the dislocations, regions (cell walls and shear bands) in which linear and streak dislocations are dense are difficult to discriminate with a transmission electron microscope, and can cause measurement errors when determining the dislocation density ρ. On the other hand, in X-ray diffraction, as will be described later, since the dislocation density ρ is calculated from the half-value width of the diffraction peak from each surface in the texture, there is less error even with such a forest dislocation. There are advantages.

  In the structure of a forged material in which dislocations are introduced by applying plastic deformation by forging or warm working, lattice distortion occurs around dislocations. In addition, small tilt grain boundaries, cell structures, and the like develop due to the dislocation arrangement. When such dislocations and the domain structure associated therewith are taken from the X-ray diffraction pattern, a characteristic spread corresponding to the diffraction index and a shape appear in the diffraction peak. By analyzing the shape (line profile) of this diffraction peak (line profile analysis), the dislocation density can be obtained.

Specifically, first, the forged material further subjected to solution treatment and quenching treatment is subjected to artificial aging treatment after imparting strain due to warm working, and any of the thickest parts of the forging material after this artificial aging treatment is given. From the vertical cross section at the position of (2), a measurement sample (three pieces) including the thickness center portion is collected, and the sample is sliced in parallel to the forging material surface and polished so that the thickness center comes out as an observation surface.
That is, the thickness center portion is a plane parallel to the forging material surface at the thickness center (plate thickness center) in the plan view of the forging material, and the forging material surface (for example, a horizontal plane) at the thickness center. ) And a surface extending substantially parallel to the surface.
X-ray diffraction of the structure of the surface of this specimen (surface at the thickness center position), and (111), (200), (220), (311) which are the main orientations in the texture of this surface part , (400), (331), (420), (422) The half width of the diffraction peak from each surface (each azimuth surface) is obtained. The higher the dislocation density ρ, the larger the half width of the diffraction peak on each of these surfaces. In addition, even if the rolled surface used as the measuring object of a X-ray diffraction of a test piece remains in the state of a test piece, the washing | cleaning which does not involve an etching may be given.

Next, after obtaining the lattice strain (crystal strain) ε by the Williamson-Hall method from the half-value width of the diffraction peak of each surface, the dislocation density ρ can be calculated by the following equation. Here, the dislocation density ρ is performed on three samples collected from the central portion of the thickness, and these are averaged to obtain the average of the dislocation density ρ.
ρ = 16.1ε ² / b ²
Here, ρ is the dislocation density, ε is the lattice strain, and b is the size of the Burgers vector.
The size of Burgers vector was 2.8635 × 10 ^-10 m.

  The Williamson-Hall method is a well-known line profile analysis method that is widely used to determine the dislocation density and the crystal grain size from the relationship between the half width of a plurality of diffractions and the diffraction angle. In addition, a series of methods for obtaining the dislocation density by X-ray diffraction is also known, and the series of methods for obtaining the dislocation density by X-ray diffraction are collectively referred to as “dislocation density measured by X-ray diffraction” in the present invention. Dislocation density ".

(Average ratio of low-angle grain boundaries)
In the present invention, in order to increase the strength and ductility of the forging, along with other structure control such as average dislocation density and average number density of precipitates, the center of thickness of the thickest part of the forging The average ratio of small-angle grain boundaries having an inclination angle of 2 to 15 ° of crystal grains having an orientation difference of 2 ° or more, as measured by the SEM-EBSD method, is 50% or more.

In this way, by controlling the ratio of the low-angle grain boundaries to be large as in the specified range, strain is concentrated locally with respect to the load of external force when used as an automobile underbody part or the like. Without causing the tissue to deform uniformly. As a result, local breakage can be prevented, and in combination with other structural rules (requirements) such as average dislocation density and average number density of precipitates, 0.2% proof stress is 400 MPa or more and elongation is 10% or more. High strength and high ductility.
On the other hand, when the average proportion of the low-angle grain boundaries is less than 50%, it becomes the same as a conventional forged material, and the mechanism for achieving the high strength and high elongation is not exhibited, and the elongation is also lowered.

The small tilt grain boundary referred to in the present invention is a grain boundary between crystal grains having a small crystal orientation difference (tilt angle) of 2 to 15 ° among crystal orientations measured by the SEM-EBSD method described later.
In addition, the large tilt grain boundary with respect to this is a grain boundary between crystal grains in which the difference in crystal orientation (tilt angle) exceeds 15 ° and is 90 ° or less.

  As an average ratio of the low-angle grain boundaries, in the present invention, the total length of the grain boundaries of the measured small-angle grain boundaries (the total length of the grain boundaries of all the small-angle grains measured) was also measured. The ratio of the crystal orientation difference with respect to the total length of the crystal grain boundary of 2 to 90 ° (the total length of the measured crystal grain boundaries of all crystal grains) is defined as the ratio of the small tilt grain boundary with the tilt angle of 2 to 15 °. doing. That is, the ratio (%) of the low-angle grain boundary with the specified inclination angle of 2 to 15 ° is [(total length of the crystal grain boundary of 2 to 15 °) / (full length of the crystal grain boundary of 2 to 90 °)] × 100. The average of these values is 50% or more. From the limit of production or hot forging, the upper limit of the ratio of 2-15 ° low-angle grain boundaries is about 90%.

Measurement of average ratio of low-angle grain boundaries by SEM-EBSD method Measurement of average ratio of low-angle grain boundaries with a tilt angle of 2 to 15 degrees of crystal grains having a misorientation of 2 ° or more in the forged structure is performed after artificial aging treatment. The structure of the observation surface (thickness center portion) of the thickness center of the thickest portion of the forged material is measured by SEM-EBSD method.

A specific measurement method is the same as in the sampling of the dislocation density, from a longitudinal section at an arbitrary position of the thickest part of the forged material after the artificial aging treatment, ) And grind so that the observation surface at the center of the wall appears.
And using SEM-EBSD, the length of the longitudinal side of the forging material on the observation surface is 1000 μm × the length of the side in the width direction is 320 μm. The electron beam is irradiated at a pitch of.
Thus, the average ratio of the low-angle grain boundaries per sample is measured, and further averaged with three measured samples.

  The SEM-EBSD (EBSP) method is a general-purpose crystal orientation analysis method in which a scanning electron microscope (SEM) is equipped with an EBSD (Electron Back Scattering (Scattered) Diffraction Pattern) system. It is.

  More specifically, the observation sample of SEM-EBSD is adjusted by mirror-polishing the observation sample (cross-sectional structure) after further mechanical polishing. Then, it is set in a SEM column and irradiated with an electron beam on the mirror-finished surface of the sample to project EBSD (EBSP) on the screen. This is taken with a high-sensitivity camera and captured as an image on a computer. In the computer, the orientation of the crystal is determined by analyzing this image and comparing it with a pattern obtained by simulation using a known crystal system. The calculated crystal orientation is recorded as a three-dimensional Euler angle together with position coordinates (x, y, z) and the like. Since this process is automatically performed for all measurement points, tens of thousands to hundreds of thousands of crystal orientation data in the cross section of the forged material are obtained at the end of the measurement. Based on this crystal orientation data, the crystal grains are discriminated and the difference in crystal grain boundaries is analyzed.

(Precipitate)
In the present invention, in order to increase the strength and ductility of the forged material, along with the control of the average dislocation density and the average ratio of the low-angle grain boundaries, the center of thickness of the thickest portion of the forged material is obtained. The average number density of precipitates that can be measured by a TEM with a magnification of 300,000 times on the observation surface (wall thickness center portion) is set to 5.0 × 10 ² pieces / μm ³ or more.
Accordingly, in combination with other structural provisions (requirements) such as average dislocation density and average proportion of low-angle grain boundaries in crystal grain boundaries, 0.2% proof stress is 400 MPa or more and elongation is 10% or more. Can be highly ductile.

Here, the precipitates that can be measured by a TEM with a magnification of 300,000 times are all precipitates that can be measured (identified) from the shape of the TEM with a magnification of 300,000 times regardless of the composition.
That is, by TEM image analysis, matrix, grain boundaries, or dislocations can be identified (differentiated) from the shape and observed (measured), and can be observed (measured) in various granular or lump-like shapes, rod shapes, needle shapes, etc. This is a general precipitate having a complex shape.
Further, the minimum size of the precipitates that can be measured with a TEM of 300,000 times is out of the range because the average equivalent circle diameter is 5 nm or more, and those smaller than this cannot be measured.

  By the way, although it is the upper limit of the size of precipitates, in automobile underbody parts forgings manufactured by a conventional method, coarse precipitates whose average equivalent circle diameter exceeds 1000 nm cause destruction and are almost present. Do not let (do not exist). For this reason, the substantial upper limit of the size of the precipitates that can be measured by the TEM is 1000 nm.

  Here, the equivalent circle diameter is the diameter (equivalent circle) when the identified precipitate is image-processed, the area of each precipitate within the TEM visual field is calculated, and converted into a circle of the same area. (Diameter) (circle equivalent diameter).

The composition of the precipitates referred to in the present invention is the Mg-Si-based or Al-Mg-Si-Cu-based, Al-Mn-based, Al-Cr-based, Al-Zr-based, produced during the artificial aging treatment in the alloy composition. Or an intermetallic compound mainly containing a composition containing Fe.
By making the average number density of the fine precipitates measurable by these TEM more as 5.0 × 10 ² / μm ³ or more (as much as there are), the strength (BH property) is remarkably increased. improves.
Although the mechanism by which these precipitates improve BH properties is still unclear, improvement in work hardening characteristics at the time of prestraining, and suppression of recovery during artificial aging treatment of dislocations introduced by prestraining Thus, it is presumed that the transition element-based dispersed particles having the size and number density contribute particularly.
Moreover, such fine precipitates have an excellent effect of not reducing the elongation of the forged material.

When the average number density of precipitates that can be measured by a TEM with a magnification of 300,000 times is less than 5.0 × 10 ² pieces / μm ^{3 at the} thickness center of the thickest part of the forging, This is the same as the forging material, and the BH mechanism for achieving the high strength does not appear, and the elongation also decreases.
In addition, from the limit of production or hot forging, the upper limit of the average number density of the precipitates is about 1.0 × 10 ⁵ pieces / μm ³ .

Measurement of the average number density of precipitates The average number density of the precipitates defined in the present invention is 300,000 times the structure of the observation center at the thickness center of the thickest part of the forged material after artificial aging treatment. It measures by TEM (transmission electron microscope: FE-TEM) of magnification.
A specific measurement method is to collect measurement samples (three pieces) including a thickness center portion from a longitudinal section at an arbitrary position of the thickest portion of the forged material after the artificial aging treatment, A thin film sample for TEM is prepared so that the observation surface at.
The thin film sample for TEM was mechanically polished from the measurement sample to 0.05 mm (thickness 0.1 mm) in both thickness directions from the thickness center, and then the thickness center was measured by the twin jet electrolytic polishing method. To a thin film with a thickness of 100 nm.
In addition, a tissue photograph obtained by photographing the thin film (sample) with a TEM at a magnification of 300,000 times can be subjected to image processing, and identification (identification) within the measurement visual field (the total area of the observation visual field is 0.5 μm ² or more) is possible. The number of all precipitates is measured.
And the average number density (pieces / micrometer < ³ >) of the precipitate with respect to a measurement visual field is measured.
Here, the average number density is measured for three samples collected from the central portion of the thickness, and these are averaged to obtain the average number density of the precipitates (pieces / μm ³ ).

  As described above, the structure and characteristics of the forging material defined in the present invention are the structure of the forging material after the artificial aging treatment is applied to the forging material that has been subjected to solution treatment and quenching treatment, and is subjected to artificial aging treatment. It is a characteristic.

(Forging material measurement site)
The measurement site | part of the above structure | tissue and a characteristic was made into the thickness center part of the thickest part of the forged material after artificial aging treatment. If the forging material is a simple shape such as a bar shape, a plate shape, a circular shape or a columnar shape called I-type, the thickness center of the forging material to be measured is the center point of the forging material. It can be specified as a standard.
However, the automobile undercarriage part typically has an overall shape of a substantially triangular shape in plan view, and three ball joints, which are the apex portions of the triangle, are connected to a narrow and thick peripheral edge rib and a wide width. It is composed of a thin central web and has a complicated shape with a cross section connected by a substantially H-shaped or U-shaped arm.
Therefore, the thickness center portion of such an automobile underbody part is the above-described thickness center portion at an arbitrary position of the thick rib as the thickest portion of the forged material.

(Production method)
Next, the manufacturing method of the aluminum alloy forging material in this invention is described. In the manufacturing process of the aluminum alloy forged material in the present invention, the aluminum alloy ingot having the above composition is subjected to a homogenization heat treatment, followed by a hot forging process, and the forged material is subjected to solution treatment, quenching treatment and artificial aging treatment. It can be produced by a conventional method. That is, it can be manufactured without performing hot extrusion of the ingot.
However, as an automobile undercarriage part, etc., it has the above structure, and in order to improve both strength and ductility (high strength and high ductility) on the premise of high corrosion resistance, solution treatment and quenching treatment There are preferable manufacturing conditions as described below, such as warming in advance after the artificial aging treatment.

Casting When casting a molten aluminum alloy that has been adjusted to be dissolved within the specific aluminum alloy component range, a normal melting casting method such as a continuous casting rolling method, a semi-continuous casting method (DC casting method), or a hot top casting method is used. Select appropriately and cast.
However, when casting an aluminum alloy melt composed of the specific aluminum alloy component range, an average cooling rate of 100 ° C / ° C is used in order to refine the crystallized material and the dendrite secondary arm spacing (DAS). It is preferable to set it as s or more.

Homogenizing heat treatment The cast ingot is homogenized and heat-treated at 450 to 580 ° C for 2 hours or more. If the homogenization heat treatment temperature is less than 450 ° C., the temperature is too low to homogenize the ingot, and if the homogenization heat treatment temperature exceeds 580 ° C., burning of the ingot surface may occur. In addition, after the homogenization heat treatment, an extrusion process prior to hot forging is unnecessary, but may be performed if desired.

Hot forging The ingot after homogenization heat treatment is reheated, the material temperature is in the range of 430 to 550 ° C, the mold temperature is in the range of 100 to 250 ° C, and the minimum thickness reduction rate is 25% or more, It is preferable to perform hot forging under the condition that the maximum thickness reduction rate is 90% or less.
Hot forging is forged into the final product shape (near net shape) of automobile undercarriage parts using mechanical press forging or hydraulic press. In the hot forging, hot forging is performed a plurality of times without reheating during forging or reheating as necessary, crushing, rough forging, finish forging, and so on.

  If the minimum thickness reduction rate is less than 1% as the hot forging processing rate, the above-described complex-shaped automobile undercarriage part may not be forged with high shape accuracy. On the other hand, when the maximum thickness reduction rate exceeds 90%, it is difficult to suppress recrystallization, and the possibility of generating coarse recrystallized grains increases.

  If the forging end temperature after the final forging is less than 300 ° C., it is difficult to suppress recrystallization in the forging and solution treatment process, and the processed structure may be recrystallized to generate coarse crystal grains. . When these coarse crystal grains are generated, even if the structure is controlled, the strength and ductility cannot be increased, and the corrosion resistance also decreases. Moreover, in hot forging at low temperature, it is difficult to refine crystal grains that target the entire region of the cross-section of the forged material. On the other hand, when the material temperature exceeds 550 ° C., burning of the forged material surface occurs and the possibility of generating coarse recrystallized grains increases.

Solution treatment and quenching treatment After this hot forging, solution treatment and quenching treatment are performed. The solution treatment is preferably held in the temperature range of 530 to 570 ° C. for 1 hour or more and 8 hours or less. If the solution treatment temperature is too low, or if the time is too short, the solution treatment is insufficient, the solid solution of the Mg-Si-based compound becomes insufficient, and the precipitation amount of the compound in the subsequent artificial aging treatment is too small. Strength decreases. Although the holding time may be long, the effect is saturated when it exceeds 8 hours.

After this solution treatment, it is preferable to perform a quenching treatment from 500 ° C. to 100 ° C. at an average cooling rate of 25 ° C./s or more. In order to secure this average cooling rate, the cooling during the quenching process is also water cooling, in particular, water cooling (circulating cooling water while bubbling bubbles, It is preferable to carry out by immersion in a water bath. When the cooling rate during the quenching process is lowered, Mg-Si compounds, Si, etc. 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. . Also, during cooling, even in the grains, Mg-Si-based compound is a stable phase, Si is formed, beta phase which precipitates during artificial aging, since the precipitation amount of beta ^'phase is decreased, the strength is lowered.

  However, on the other hand, if the cooling rate is too high (fast), the amount of quenching strain increases, and a new problem arises that a straightening process becomes necessary after quenching, and the number of steps in the straightening process increases. In addition, the residual stress increases, and a new problem arises that the dimensional and shape accuracy of the product is lowered. In this respect, in order to shorten the product manufacturing process and reduce the cost, hot water quenching at 30 to 85 ° C. in which quenching distortion is alleviated is preferable. Here, when the hot water quenching temperature is less than 30 ° C., the quenching strain increases, and when it exceeds 85 ° C., the cooling rate becomes too low, and the toughness, fatigue characteristics, and strength are lowered.

Warm working In the present invention, the hot forged material thus obtained (after solution treatment and quenching treatment) is distorted to form the above-mentioned structure, and to increase the strength and ductility. Warm processing is performed in advance before artificial aging treatment.
As this warm working condition, warm working is performed within 48 hours after solution treatment and quenching treatment. The heating before the warm working is performed in a temperature range of 140 to 220 ° C., a furnace charging (holding) time of 20 minutes to 120 minutes (a temperature increase is performed in a range of 19 minutes to 60 minutes, and a final temperature is 1 minute) It is preferable to perform warm processing without delay (immediately) after that.
Under such heating conditions, a uniform fine β ′ phase is first precipitated in the grains during the heating and holding. Since warm working is performed thereafter, non-uniform precipitation of β ′ phase due to dislocations introduced by warm working is suppressed. In addition, the β ′ phase that has already precipitated pins the dislocation, suppresses the recovery of the dislocation during the artificial aging treatment, and secures the work hardening amount.
On the other hand, if the charging time is short, the β ′ phase is hardly precipitated during the temperature rise and heat retention in the heat treatment before warm working, but the dislocation introduced by the subsequent warm working is artificial. It becomes a precipitation site at the time of aging treatment, and in addition to non-uniform precipitation, dislocation promotes the diffusion of elements, and precipitates are coarsened and sparsely distributed, which may reduce strength.

The warm working rate is preferably 5 to 30%. If it is less than 5%, the amount of strain applied to the forged material due to warm working becomes small, and there are few dislocations introduced into the forged material, and the effect of dislocation strengthening cannot be obtained.
If it exceeds 30%, the accumulated strain increases, so that the driving force for recovery during artificial aging increases, the amount of hardening due to dislocation strengthening is saturated, and the strength improvement effect becomes small.
Furthermore, as the processing amount increases, the grain boundary orientation difference formed by recovery after warm processing or during artificial aging increases, the proportion of low-angle grain boundaries decreases, and the amount of preferential precipitation at the grain boundaries increases. By doing so, the strength decreases.
The mode of warm working is performed according to the shape of the forged material, and rolling or pressing with a roll can be applied as long as it is a simple shape such as a rod, plate, circle or cylinder, and the automobile undercarriage part For complex shapes such as those, warm die forging and free forging are used.
And depending on the processing rate and processing method of the warm processing, when it is desired to increase the processing rate within the above range, the forging material is made into the final product shape by this warm processing (in the above-mentioned hot forging, the near net It is preferable to use a shape).

Artificial aging treatment After the above warm processing, artificial aging treatment (artificial aging hardening treatment) is performed. In order to prevent room temperature aging from proceeding, it is preferable to perform artificial aging treatment immediately after the warm working, for example, within 1 hour as a guideline. In this artificial aging treatment, conditions are preferably selected from a temperature range of 100 ° C. or higher and 250 ° C. or lower and a holding time range of 20 minutes to 8 hours.

However, even within this range of conditions, the optimum conditions should be selected according to the conditions of the previous process such as composition, hot forging, solution quenching, cold or warm processing, and these compositions. If the artificial aging temperature is too low or too high, or the holding time is too short, the desired specified structure, high tensile strength and high yield strength, and high elongation may not be obtained. There is sex.
In addition, an air furnace, an induction heating furnace, a nitrite furnace, etc. are used suitably for the above-mentioned homogenization heat treatment and solution treatment. Furthermore, an air furnace, an induction heating furnace, an oil bath, or the like is appropriately used for the artificial aging treatment.

  The forged material of the present invention may be appropriately subjected to machining or surface treatment before and after the artificial aging treatment for automobile undercarriage parts.

  EXAMPLES Hereinafter, the present invention will be described more specifically with reference to examples. However, the present invention is not limited by the following examples, but may be appropriately modified within a range that can meet the purpose described above and below. It is also possible to implement, and they are all included in the technical scope of the present invention.

  Next, examples of the present invention will be described. Hot forgings with the same production conditions until solution treatment and quenching treatment with each aluminum alloy composition shown in Table 1 are subjected to warm working and artificial aging treatment under different conditions shown in Table 2, Manufactured a forging material to be used as a material for rotating parts. Then, the structure, mechanical properties, and corrosion resistance of this forged material were measured and evaluated as shown in Table 2.

  Specifically, in common with each example, the ingot which consists of a chemical component of the aluminum alloy forging material shown in Table 1 was cast by the semi-continuous casting method which made the average cooling rate 100 degrees C / s or more. In addition, in each example of aluminum alloy shown in Table 1, the hydrogen concentration in 100 g of Al was all 0.10 to 0.15 ml. Here, in the display of the content of each element in Table 1, the display in which the numerical value column for each element is “−” indicates that the content is below the detection limit.

In common with each example, the outer surface of each aluminum alloy ingot is chamfered 3 mm in thickness, cut into a round bar-shaped billet with a length of 120 mm and φ75 mm, and then subjected to a homogenization heat treatment at 520 ° C. for 5 hours. After this homogenization heat treatment, the ingot was forcibly air-cooled using a fan at a cooling rate of 100 ° C./hr or more.
The hot forging of the ingot after the homogenization heat treatment is common to each example, forging three times without reheating to the final wall thickness, the temperature at the start of forging is in the range of 500 to 520 ° C, the mold temperature Was performed by a mechanical press using upper and lower molds under the common conditions of a range of 170 to 200 ° C. and a thickness change rate of the center of the forging material of 75% (over 25%).
In addition, in these hot forgings, in each example, in order to obtain the same final forged material shape in the warm forging described later, each of the near net shape hot forgings corresponding to each processing rate of the warm forging. A material was used.

  These forged materials are commonly used in each example, using an air furnace, and after the solution treatment at 550 ° C. for 5 hours, the water cooling is performed at an average cooling rate of 25 ° C./s or more from 500 ° C. to 100 ° C. (Water bath immersion) was performed.

The hot forged material (after solution treatment and quenching treatment) thus obtained was subjected to warm working and artificial aging treatment under the conditions shown in Table 2 to create the respective structures.
In warm forging, after heating under the pre-warm heating conditions shown in Table 2, it is warm-worked with a mechanical press using upper and lower molds at the pre-warm heating temperature and processing rate in Table 2, Final shape.

  The final shape of the manufactured forged material is common to each example, and is composed of the overall shape of a substantially triangular shape in plan view as described above, and the three ball joints that are the apex portions of this triangle are narrow and thick. The suspension part shape was formed of a peripheral rib having a height of 60 mm and a thin central web having a wide wall thickness (height) of 31 mm and connected by an arm having a substantially H-shaped cross section.

  As described above, the structure, mechanical characteristics, and intergranular stress corrosion cracking resistance of the forged material that made the structure were measured and evaluated by the following methods. These results are shown in Table 2.

(Organization)
Each structure defined in the present invention is obtained by taking a sample from a longitudinal section of an arbitrary thickness center portion of the thickest rib portion in any of the substantially H-shaped arms of the forged material by the measurement method described above. Average dislocation density (/ m ² ), average ratio (%) of grain boundaries with a tilt angle of 2 ° to 15 °, and average number density of precipitates (pieces / μm ³ ). Each was measured in the manner described above.

(Mechanical properties)
A sample is taken from the thickness center portion of an arbitrary part of the thickest rib portion of the forged material, and from this sample, the center of the thickness direction is included in the thickness direction, and the longitudinal direction of the forged material is Three tensile test pieces (L direction) were prepared so that the L direction extends, the outer diameter was 5 mm, the distance between the gauge points was 25 mm. Then, mechanical properties such as 0.2% proof stress (MPa) and elongation (%) of the test piece were measured at room temperature, and average values of these three locations (three test pieces) were obtained. The tensile speed was 5 mm / min up to 0.2% proof stress and 20 mm / min after 0.2% proof stress.
Here, the acceptance criteria for forgings for automobile undercarriage parts were 0.2% proof stress of 400 MPa or more and elongation of 10% or more.

(Corrosion resistance)
Corrosion resistance was evaluated according to intergranular corrosion properties in accordance with the provisions of the alternating immersion method of JIS H8711. That is, stress of 300 MPa was applied to a test piece for stress corrosion cracking resistance evaluation (C-ring for SCC test), and the time (number of days) until grain boundary corrosion cracking occurred regardless of the size of the crack was measured for 30 days. In the case of less than x, it was evaluated as x, and the case of 30 days or more to less than 60 days was evaluated as ◯.

As is apparent from Tables 1 and 2, each of the inventive examples is subjected to warm working and artificial aging treatment within the component composition range of the present invention and within a preferable condition range. Therefore, as shown in Table 2, each of the inventive examples has a structure as defined in the present invention, and the average dislocation density measured by X-ray diffraction is 1.0 × 10 ^{14 to} 5.0 ×. 10 ¹⁶ / m ² , measured by the SEM-EBSD method, the average proportion of the low-angle grain boundaries with an inclination of 2 ° to 15 ° of crystal grains having an orientation difference of 2 ° or more is 50% or more, and the magnification The average number density of precipitates that can be measured with a TEM of 300,000 times is 5.0 × 10 ² pieces / μm ³ or more.

  As a result, each of the inventive examples has excellent corrosion resistance, 0.2% proof stress of 400 MPa or more, elongation of 10% or more, high strength and high ductility, and is necessary as an undercarriage part. Various characteristics are combined.

On the other hand, as in Comparative Examples 13 to 19 in Table 2, the alloy composition is within the range of Alloy No. 1 in Table 1, but when the warm working is manufactured out of the preferable condition range, Does not meet the organizational regulations of the present invention. As a result, in these comparative examples, 0.2% proof stress and elongation are significantly inferior to those of the inventive examples.
In Comparative Example 13, warm processing was not performed before artificial aging treatment.
In Comparative Example 14, the heating temperature during warm working is too low.
In Comparative Example 15, the heating temperature during warm working is too high.
In Comparative Example 16, the heating and holding time during the warm working is too short.
In Comparative Example 17, the heating and holding time during the warm working is too long.
In Comparative Example 18, the processing rate of warm processing is too low.
In Comparative Example 19, the processing rate of warm processing is too high.

Further, in Comparative Examples 20 to 23 in Table 2, warm working is within a preferable condition range, but the alloy composition is out of the range and does not satisfy the structure rule of the present invention at the thickness center portion. As a result, in these comparative examples, 0.2% proof stress and elongation are significantly inferior to those of the inventive examples.
In Comparative Example 20, as shown in Alloy No. 11 in Table 1, Mg deviates from the lower limit.
In Comparative Example 21, as shown in Alloy No. 12 in Table 1, Si deviates from the lower limit.
Comparative Example 22 does not contain Fe as shown by Alloy No. 13 in Table 1.
Comparative Example 23 does not contain any of Mn, Cr, and Zr as shown in Alloy No. 14 in Table 1.

  From the above results, on the premise of having excellent corrosion resistance, the critical significance of the composition of the present invention and the structure definition that can obtain a 6000 series aluminum alloy forged material having high strength and high ductility can be understood.

  According to the present invention, a 6000 series aluminum alloy forged material having high strength and high ductility can be obtained on the premise of having excellent corrosion resistance. Therefore, the 6000 series aluminum alloy hot forging material has a great industrial value in that it can be used for a transportation machine such as an automobile undercarriage part.

Claims (4)

In mass%, Si: 0.7 to 1.5%, Mg: 0.6 to 1.2%, Fe: 0.01 to 0.5%, respectively, and Mn: 0.05 to Aluminum alloy forging containing 1.0%, Cr: 0.01 to 0.5%, Zr: 0.01 to 0.2%, or two or more of the balance, Al and inevitable impurities The average dislocation density measured by X-ray diffraction is 1.0 × 10 ^{14 to} 5.0 × 10 ¹⁶ / as the structure in the observation surface at the thickness center of the thickest part of the forged material. in the range of m ^2, and measured by SEM-EBSD method, the average proportion of the low-angle grain boundaries of the orientation differences 2 ° or more crystal grains in the inclination angle 2 to 15 ° is 50% or more, magnification 300,000 × The average number density of precipitates that can be measured by TEM is 5.0 × 10 ² / μm ³ or more. This is a forged aluminum alloy with excellent strength and ductility.   The aluminum alloy forging material is further one or two kinds of Cu: 0.05 to 1.0%, Ti: 0.01 to 0.1%, Zn: 0.005 to 0.25% by mass%. The aluminum alloy forging material excellent in strength and ductility according to claim 1 containing the above.   The aluminum alloy forging material according to claim 1 or 2, wherein the aluminum alloy forging material has a tensile strength of 420 MPa or more, a 0.2% proof stress of 400 MPa or more, and an elongation of 10% or more. In mass%, Si: 0.7 to 1.5%, Mg: 0.6 to 1.2%, Fe: 0.01 to 0.5%, respectively, and Mn: 0.05 to Aluminum alloy ingot containing 1.0%, Cr: 0.01-0.5%, Zr: 0.01-0.2%, or two or more, and the balance Al and inevitable impurities After forging, the forged material is hot-forged into a forged material, and further subjected to artificial aging treatment after warming the forged material that has undergone solution treatment and quenching treatment. The thickest forged material after this artificial aging treatment Measured by the SEM-EBSD method with the average dislocation density measured by X-ray diffraction in the range of 1.0 × 10 ^{14 to} 5.0 × 10 ¹⁶ / m ² as the structure on the observation surface at the thickness center of the part. Average fraction of low-angle grain boundaries with an inclination angle of 2 to 15 ° of crystal grains having an orientation difference of 2 ° or more The total number density of precipitates that can be measured by a TEM with a magnification of 300,000 times is 5.0 × 10 ² pieces / μm ³ or more, and has excellent strength and ductility. A method for producing aluminum alloy forgings.