JP5830006B2 - Extruded aluminum alloy with excellent strength - Google Patents

Description

  The present invention relates to a 7000 series aluminum alloy extruded material excellent in strength. The extruded state as used in the present invention refers to the state of the extruded extruded material that has not been subjected to any heat treatment or processing that changes the structure other than cooling from the extrusion temperature after the end of hot extrusion. Say that.

  Conventionally, Zn-Mg- as a material excellent in strength, corrosion resistance, and weight reduction toward structural members in automobiles, bicycles, railway vehicles and other transportation equipment, buildings, structures, and fastening members such as bolts. A 7000 series aluminum alloy extruded material which is a Cu type is used.

  The 7000 series aluminum alloy extruded material has high strength, and various materials having a tensile strength of 700 MPa or more have been proposed in the past as materials that have been subjected to T6 tempering after extrusion.

  For example, in Patent Document 1, a 7000 series aluminum alloy wire rod extruded material having a Zn content exceeding 8% by mass can be easily imparted with a strength exceeding a tensile strength of 720 MPa by aging treatment. However, in this document, since a coarse recrystallized grain layer is inevitably generated, it causes cracks that occur when the extruded material is plastically processed (molded) such as forging or rolling into a large-diameter bolt. For this reason, it is essential to remove the coarse recrystallized grain layer on the surface of the extruded material (surface layer portion) before plastic working.

  In contrast, various attempts to suppress such a recrystallized grain layer have been proposed. For example, in Patent Document 2, the extrusion temperature is set at a relatively low temperature of 480 to 500 ° C., the inside of the extruded 7000 series aluminum alloy is made into a fibrous structure, and the thickness of the recrystallized layer on the surface layer is 10% or less. It is disclosed that the crystal grain size is controlled to 150 μm or less.

JP 2010-236665 A Japanese Patent No. 2908993

  However, even if the extrusion temperature is carried out at a relatively low temperature of 480 to 500 ° C. as in Patent Document 2, the 7000 series aluminum alloy is also recrystallized by extrusion in this temperature range. For this reason, the recrystallization in the surface layer part and the inside of the extruded material is unavoidable, and it is impossible to obtain a strength of 700 MPa or more with good reproducibility.

  The present invention has been made in view of such problems, and provides a 7000 series aluminum alloy extruded material in which recrystallization of the surface layer portion and inside of the extruded material is suppressed and a high strength of 700 MPa or more can be obtained in tensile strength. Objective.

  The summary of the extruded aluminum alloy material excellent in strength of the present invention for achieving the above object is mass%, Zn: 7.0 to 14.0%, Mg: 1.0 to 3.5%, Cu: Zn-Mg-Cu-based aluminum alloy extruded material containing 0.5 to 2.0%, Zr: 0.05 to 0.3%, and the balance consisting of Al and inevitable impurities, and remains extruded As the structure of the cross section parallel to the extrusion direction passing through the axial center portion of the extruded material in the state, the average crystal grain size of the recrystallized grains in the surface layer portion of the extruded material is 100 μm or less, and <001> orientation crystal grains having an average section length of 35 [mu] m or less in the radial direction of crystal grains and an average area ratio of <111> orientation crystal grains in the extrusion direction of 0.5 to 1.0 Of the average area ratio of <111> -oriented grains and <001> It is assumed that >> / <111> is 0.25 or less.

  The present invention, after completion of hot extrusion, except for cooling from the extrusion temperature, no heat treatment or processing to change the structure, the structure state of the extruded material as extruded (extruded), the surface layer portion In addition to suppressing the recrystallization (recrystallization) inside the extruded material, a fine extruded structure (fibrous structure) is obtained. And thereby, the high strength of 700 MPa or more is obtained as the tensile strength after the artificial aging treatment.

  In addition, if the recrystallized grain layer on the surface part of the extruded material (surface part) can be suppressed, there is also an advantage that it can be processed to the member product shape for each application without removing the recrystallized grain layer on the surface part of the extruded material. And corrosion resistance such as intergranular corrosion resistance can be improved.

It is a schematic diagram of the extruded material cross-sectional structure defined in the present invention. It is a unit stereo triangle in the reverse pole figure of the crystal grain orientation of an extruded material.

  Hereinafter, embodiments of the present invention will be specifically described in order for each requirement.

Ingredient composition:
The 7000 series aluminum alloy in the present invention is, as an extruded material, Zn: 7.0 to 14.0% in mass%, Mg: 1 in order to increase the tensile strength after artificial aging treatment to 700 MPa or more. Al-Zn-Mg-Cu composition containing 0.0-3.5%, Cu: 0.5-2.0%, Zr: 0.05-0.3%, the balance being Al and inevitable impurities And Here, the% display of the content of each element means the mass%, including the content of each element described below.

Zn: 7.0 to 14.0%
Zn, which is an essential alloy element, improves the strength together with Mg by forming an aging precipitate, which is an intermetallic compound of Mg and Zn, defined in the present invention, during the artificial aging treatment described later. If the Zn content is less than 7.0%, the strength is insufficient, and if it exceeds 14.0%, ingot cracking is likely to occur during casting and ingot formation becomes difficult. Therefore, the Zn content is in the range of 7.0 to 14.0%, preferably in the range of 8.0 to 12.0%. In addition, when Zn content is high, SCC sensitivity becomes sharp, but in order to suppress it, it is desirable to add Cu or Ag described later.

Mg: 1.0-3.5%
Mg, which is an essential alloying element, together with Zn, forms an aging precipitate that is an intermetallic compound of Mg and Zn as defined in the present invention during the artificial aging treatment described later, thereby improving strength and elongation. If the Mg content is less than 1.0%, the strength is insufficient, and if it exceeds 3.5% by mass, the extrudability at a low temperature below the recrystallization temperature of the cast billet is lowered, and the SCC sensitivity is enhanced. Therefore, the Mg content is in the range of 1.0 to 3.5%, preferably in the range of 1.5 to 3.0%.

Cu: 0.5 to 2.0%
Cu has the effect of improving the SCC resistance of the Al—Zn—Mg alloy. When this is contained, if the Cu content is less than 0.5%, the effect of improving the SCC resistance is small. On the other hand, if the Cu content exceeds 2.0%, cracks during casting lower the extrudability at a low temperature below the recrystallization temperature of the cast billet, and deteriorate the corrosion resistance. Therefore, the Cu content is in the range of 0.5 to 2.0%, preferably in the range of 0.8 to 1.8%.

Zr: 0.05-0.3%
Zr contributes to strength improvement by refining the crystal grains of the ingot and the final product. If the Zr content is less than 0.05%, the effect of miniaturization is small. On the other hand, if the Zr content exceeds 0.3%, a coarse primary crystal compound is formed at the time of casting, and seizure at the time of extrusion and reduction of product elongation are caused. Therefore, the Zr content is in the range of 0.05 to 0.3%, preferably in the range of 0.08 to 0.2%.

Mn: 0.1 to 1.5%, Cr: 0.05 to 0.3%, Sc: 0.05 to 0.3%
Mn, Cr, and Sc contribute to strength improvement by refining the crystal grains of the ingot and the final product. In the case where any one or two or more of these are selectively contained, if the contents of Mn, Cr, and Sc are all less than moderate, the contents are insufficient and the strength is lowered. On the other hand, when the contents of Mn, Cr, and Sc exceed the respective upper limits, the elongation for forming coarse crystals is reduced. Accordingly, Mn: 0.1 to 1.5%, Cr: 0.05 to 0.3%, Sc: 0.05 to 0.3%, preferably Mn: 0.2 to 1.0% , Cr: 0.1 to 0.2%, and Sc: 0.1 to 0.2%.

Ag: 0.01-0.2%
Ag also contributes to strength improvement by refining the crystal grains of the ingot and the final product. When selectively contained, if the Ag content is less than 0.01%, the effect of miniaturization is small. On the other hand, when the Ag content exceeds 0.2%, a coarse primary crystal compound is formed at the time of casting, and seizure at the time of extrusion and reduction of the elongation of the product are brought about. Therefore, the Ag content is in the range of 0.01 to 0.2%.

Ti, B:
Ti and B are impurities in the extruded material, but have the effect of refining the crystal grains of the aluminum alloy ingot, so that each of them is allowed as a 7000 series alloy within the range specified by the JIS standard.

Other elements:
Other elements other than those described above, such as Fe and Si, are unavoidable impurities. As a melting raw material, in addition to pure aluminum ingots, the inclusion of these impurity elements due to the use of aluminum alloy scrap is assumed (allowed), and each content within the range specified by the JIS standard of 7000 series alloys is allowed. . For example, Fe and Si may each be contained within a range of 0.5% by mass or less (including 0%).

Organization:
In this invention, the structure | tissue of the Zn-Mg-Cu type aluminum alloy extrusion material to prescribe | regulate is prescribed | regulated as a structure | tissue in the state as extruded. At the same time, this structure is defined as a structure having a cross section parallel to the extrusion direction passing through the axial center portion of the extruded material. In FIG. 1, the structure of a cross section parallel to the extrusion direction passing through the shaft center portion (axial center) defined in the present invention is shown as a schematic diagram of the cross-sectional structure of a solid round bar-shaped extruded material.

Surface organization:
In the present invention, the average crystal grain size of the recrystallized grains in the surface layer of the extruded material in the extruded state is refined to 100 μm or less. The crystal grain size of the recrystallized grains existing in the surface layer of the extruded material is determined by the extrusion direction and the radial direction of the extruded material in the cross-sectional structure parallel to the extrusion direction passing through the central part (axial center) of the extruded material shown in FIG. Are represented by respective section lengths in the extrusion direction and in the radial direction, each indicated by two arrows orthogonal to each other. In the present invention, the values obtained by summing and averaging the section lengths in the extrusion direction and the radial direction (added and divided by 2) are defined as the average crystal grain size of the recrystallized grains in the surface layer of the extruded material. And this shall be 100 micrometers or less. Incidentally, the surface layer portion of the extruded material means a depth region constituted by recrystallization, and the thickness varies depending on the composition and the production method as in Examples described later.

  By controlling the average crystal grain size of the recrystallized grains in the surface layer part to 100 μm or less, the solution after the solution treatment of the extruded material, or after hot working such as forging the extruded material to the member shape of each application The recrystallized grains in the surface layer portion after the crystallization treatment can be finely controlled. Further, if the recrystallized grain layer on the surface layer of the extruded material can be largely suppressed, there is an advantage that it can be processed to the shape of the member product for each application without removing the recrystallized grain layer on the surface layer of the extruded material. Corrosion resistance such as intergranular corrosion sensitivity can also be improved.

  On the other hand, when the average crystal grain size of the recrystallized grains in the surface layer portion of the extruded material in the extruded state is 100 μm or more, coarse particles are generated in the surface layer portion after the solution treatment of the extruded material, and the strength is increased. Corrosion resistance such as SCC resistance and intergranular corrosion susceptibility deteriorates as well as decreasing. The solution treatment of the extruded material may be after hot working such as forging of the extruded material into the member shape for each application, or after the solution treatment before this hot working. Even in this case, coarse particles are generated in the surface layer portion under the usual solution treatment conditions.

Internal organization:
In the present invention, as a structure of the extruded material as extruded, not only the surface layer portion but also recrystallization inside the extruded material is suppressed to obtain a fine extruded structure (fibrous structure). Thereby, a high strength of 700 MPa or more is obtained as the tensile strength after the artificial aging treatment. Here, the inside of the extruded material is defined as the axial center portion of the extruded material from the viewpoint of the reproducibility of the structure measurement.

  As a reference for such a fine extruded structure (fibrous structure), the average section length in the radial direction of the crystal grains in the axial center portion of the extruded material is set to 35 μm or less. Ensures crystal grain refinement in a direction perpendicular to the direction). The average section length in the radial direction of the crystal grains in the central portion of the extruded material shaft is the central portion of the extruded material shaft in the cross-sectional structure parallel to the extrusion direction passing through the central portion (axial center) of the extruded material shown in FIG. This is expressed by the section length indicated by the arrow in the radial direction of the extruded material. In the present invention, the average intercept length in the radial direction of the crystal grains at the central portion of the extruded material shaft is set to 35 μm or less.

  In this way, by controlling the average intercept length in the radial direction of the crystal grains inside the extruded material (axial center portion) to be equal to or smaller than the above size, the solution is subjected to a solution treatment before hot working of the extruded material, or of the extruded material. The average section length in the radial direction of the crystal grains inside the extruded material after the solution treatment after hot working such as forging into the member shape for each application can also be finely controlled. In the above-mentioned average section length or longer, coarse particles are present in the extruded material after the solution treatment of the extruded material or after the solution treatment after hot working such as forging the extruded material into the member shape for each application. Occurs and the strength after artificial aging treatment decreases.

Crystal grains with <111> orientation in the extrusion direction:
In the present invention, not only this, but also the average area ratio of <111> oriented crystal grains in the extrusion direction is 0.5 or more and 1.0 or less, and the average area ratio of <001> oriented crystal grains is <111> The ratio with the average area ratio of orientational crystal grains, <001> / <111>, is 0.25 or less.

  Indicates that the crystal grains in the extrusion direction are fine and have an extruded structure (fibrous structure). By increasing the <111> -oriented crystal grain component to the above range, the strength in the extrusion direction is increased. Can do. The fine processed structure (fiber structure) of the extruded material has a crystal structure of FCC (face-centered cubic lattice), the <111> face is the closest face, the number of atoms per unit area is the largest, and the face spacing is the same. It will be the widest surface. Incidentally, the <111> direction is the direction indicated by the line passing through (0, 0, 0) and (1, 1, 1) when the xyz axis is defined along the side of the FCC unit cell. The <111> plane is a plane perpendicular to it.

  In addition, in the present invention, in addition to this, by suppressing the <001> -oriented crystal grains showing coarser recrystallized grains and reducing the <001> / <111> to the above range, <111 > The amount of crystal grain components in the orientation can be ensured, and the strength increase in the extrusion direction can be ensured. Although the crystal grains with the <001> orientation are not all recrystallized grains, the main orientation component of the recrystallized grains is the <001> orientation, and the ratio of the crystal grains with the <001> orientation is a guide for recrystallization. sell. Therefore, in the present invention, <001> / <111> is defined to suppress recrystallization.

  The average area ratio of the <111> -oriented crystal grains in the extrusion direction is less than 0.5, or the ratio of the average area ratio of the <001> -oriented crystal grains to the average area ratio of the <111> -oriented crystal grains <001> When <111> exceeds 0.25 and falls outside the above specified range, it is not a fine extruded structure (fibrous structure) but a recrystallized structure as in the case of conventional extruded materials. It shows that. For this reason, the strength of the extruded material after the artificial aging treatment is lowered.

Tissue measurement:
The above-mentioned structure is defined by the SEM (scanning type), which is generally used for the main structure analysis of the plane orientation of the cross-sectional structure parallel to the extrusion direction passing through the axial center portion of the extruded material in the extruded state. Electron microscope)-Backscattered electron diffraction [ESPSP "Electron Back Scattering Pattern" and EBSD "Electron Back Scattering Diffraction" can be used for measurement and analysis. In these crystal analyses, the Kikuchi line is generated by electron beam backscatter diffraction that occurs on the surface of the material while scanning by irradiating the material with an electron beam in combination with SEM, using a crystal analysis method based on the electron channeling pattern method (ECP method). A diffraction pattern, that is, an EBSD pattern is measured and analyzed. Details of the crystal orientation analysis method in which the EBSP system is mounted on these FESEMs are described in detail in Kobe Steel Engineering Reports / Vol.52 No.2 (Sep.2002) P66-70 and the like.

  Thus, by measuring and analyzing the surface layer portion or axial center portion of the extruded material, the crystal grain size, the slice length of the crystal grains in the radial direction, and the crystal orientation in the minute region of each part of these extruded materials Information is obtained. That is, the average crystal grain size (μm) of the recrystallized grains in the surface layer portion of the extruded material, the average section length of the crystal grains in the radial direction of the axial center portion of the extruded material (μm), and the <111> orientation crystal grains in the extruded direction The average area ratio of <001> / <111> between the average area ratio of <001> oriented crystal grains and the average area ratio of <111> oriented crystal grains can be measured.

  Here, FIG. 2 shows the relationship between the <111> orientation and the <001> orientation defined in the present invention and the crystal orientation of other <101> orientations. Here, FIG. 2 is a unit stereo triangle in the inverse pole figure, and displays the material reference axis direction (extrusion direction) for each crystal grain. All the grains of the aluminum alloy extrudate are present at any position within this unit stereo triangle. The area of the <111> orientation in the extrusion direction is the area of crystal grains present in the region (range) of the orientation within 15 ° from the <111> orientation in the region where the EBSP measurement was performed. The area ratio of the <111> orientation in the extrusion direction is a measurement of the area of crystal grains existing in the region (range) of the orientation within 15 ° from the <111> orientation in the region where the EBSP measurement was performed. This is the percentage of the total area. The same applies to the <001> orientation.

  As a known matter, when displaying the crystal orientation using the stereo projection method as a crystal orientation display method, a method called standard projection is generally used. In the case of a cubic crystal such as an aluminum alloy, the crystal orientation is displayed as a standard projection of the cubic crystal with a projection drawing having a (001) plane at the center of the projection plane. A triangular portion “(001)-(011)-(− 111)” existing in a part of the projected view corresponds to a unit stereo triangle. In addition, many similar triangles including “(001)-(0-11)-(1-11)” and the like exist in this projection view. Are equivalent and all the same. The inverted pole figure of FIG. 2 is displayed using the unit stereo triangle. In other words, the inverse pole figure shows the relative distribution of a sample in a selected direction with respect to the crystal axis, and the projection plane for representing the inverse pole figure uses the standard projection of the crystal, of which the unit It is possible to display everything with only stereo triangles. The present invention is an extruded material, and the reverse pole figure is used to display the relative distribution in the extrusion direction. The above matters are publicly known, and are described in, for example, the column of 2-11 stereo projection of “Karity New Edition X-ray diffraction theory” (translated by Gentaro Matsumura, published by Agne Co., Ltd.). However, in this document, it is called “reverse polar figure”, and the description is described in the column of 9-10 reverse polar figure (pages 290 to 292).

  Here, the ratio <001> / <111> between the average area ratio of the <001> orientation crystal grains in the extrusion direction and the average area ratio of the <111> orientation crystal grains in the extrusion direction is 15 from the <001> orientation. It is expressed as a ratio of the area ratio of crystal grains existing in a region with an orientation within ± to the area ratio of crystal grains existing in a region with an orientation within 15 ° from the <111> orientation. These are calculated by analysis software attached to the EBSP system. These analyzes are performed at the surface layer portion of the extruded material and the shaft center portion.

In the present invention, basically, those whose orientation deviation (inclination) is ± 15 ° or less from these crystal planes belongs to the same crystal plane (orientation factor). Further, the boundary between crystal grains in which the orientation difference (tilt angle) between adjacent crystal grains is 5 ° or more is defined as a crystal grain boundary. The average crystal grain size is also measured and calculated at a grain boundary having an inclination angle of 5 ° or more. In other words, in the present invention, an orientation shift of ± 5 ° or less is defined as belonging to the same crystal grain, and a boundary between crystal grains having an orientation difference (tilt angle) of 5 ° or more between adjacent crystal grains is defined as a grain boundary. And the average crystal grain size is calculated by the following formula.
Average crystal grain size = (Σx) / n (where n represents the number of crystal grains measured and x represents each crystal grain size)

  A plurality of measured portions of the extruded material are sampled from each part of the surface layer portion or the axial center portion, and the results measured in a cross section parallel to the extrusion direction and parallel to the radial direction are averaged. At this time, a plurality of sample surfaces sampled from each of the above parts are mechanically polished to remove about 0.25 mm from the sample surface by mechanical polishing, and further, a sample whose surface is adjusted by buffing and electrolytic polishing is prepared. .

Production method:
In the present invention, as described above, the structure in the state of the extruded material is not only the surface layer part, but also suppresses recrystallization inside the extruded material to form a fine extruded structure (fibrous structure). Thus, a high strength of 700 MPa or more is obtained as the tensile strength after the artificial aging treatment. The preferable manufacturing method (hot extrusion method) of the extruded material for this will be described below in the order of steps.

(Melting, casting)
First, in the melting and casting process, a molten aluminum alloy melt-adjusted within the above-mentioned 7000 series component composition range is cast by appropriately selecting a normal melting casting method such as a semi-continuous casting method (DC casting method). And

(Homogenization heat treatment)
Prior to hot extrusion, the cast aluminum alloy billet (ingot) is subjected to homogenization heat treatment (soaking) to homogenize the structure (such as eliminating segregation in crystal grains in the ingot structure).

  However, as described above, in order to obtain a fine extruded structure (fibrous structure) by suppressing the recrystallization inside the extruded material as well as the surface layer portion as described above, As the soaking condition, it is preferable to carry out in two or two soaking steps. Two-stage soaking means that after the first (first stage) soaking process, the billet is reheated or cooled to room temperature without cooling to room temperature, and the second (second stage) soaking temperature is reached. Then, the second soaking process is performed, and the extrusion is started at the same temperature or further cooled to the extrusion start temperature. In the second soaking, after the first soaking, the billet is once cooled to room temperature or near room temperature, then reheated to the second soaking temperature, and then the second soaking is performed. Extrusion is started at a temperature of 1 or further cooled to the extrusion start temperature. Incidentally, normal soaking before hot extrusion is performed in one step or once in the soaking step.

  In the two-stage or two-time soaking process, the aim is to refine the crystal grain structure after extrusion / solution treatment by dispersing the transition element compound in the first or first soaking process. In the step or the second soaking step, the solid solution of Zn, Mg, and Cu is promoted, and the strength is improved during the artificial aging treatment.

  By controlling the soaking temperature in the first stage or the first time to 400 to 450 ° C., the Zr compound and the compound composed of Mn, Cr, Sc are finely dispersed, and the crystal grain structure after extrusion or solution treatment To refine. If the temperature is less than 400 ° C., a sufficient fine effect cannot be obtained. If it exceeds 450 ° C., these compounds are coarsened, so the effect of miniaturization is reduced. Preferably, the holding time at 400-440 ° C., first stage or first soaking is about 1-8 hours.

  By controlling the soaking temperature at the second stage or the second time from 450 ° C. to the solidus temperature, the solid solution of Zn, Mg, Cu is promoted and the strength at the time of artificial aging treatment after solution treatment is improved. If it is less than 450 degreeC, the solid solution of these elements cannot fully be obtained and intensity | strength does not increase. Further, when the solidus temperature is exceeded, partial melting occurs and the mechanical properties deteriorate, so the upper limit is made below the solidus temperature. Preferably it is 470 degreeC-solidus temperature. The holding time at the second or second soaking is about 1 to 8 hours.

(Hot extrusion)
In order to suppress not only the surface layer portion but also the recrystallization inside the extruded material, the structure in the state of the extruded material remains as low as 300 to 400 ° C. below the recrystallization temperature (billet extrusion). Extrusion is preferably carried out at the starting temperature and the temperature during extrusion. By setting the extrusion temperature in the range of 300 to 400 ° C., not only the surface layer portion of the extruded material of the 7000 series aluminum alloy whose Zn content exceeds 7 mass%, but also suppressing internal recrystallization. A fine extruded structure (fibrous structure) can be obtained.

  When this extrusion temperature exceeds 400 ° C., the temperature at the time of extrusion rises, and in a 7000 series aluminum alloy whose Zn content exceeds 7% by mass, recrystallization at a high temperature occurs immediately after extrusion, and the surface layer of the extruded material A coarse recrystallized structure is formed in the part and inside, resulting in a deterioration in corrosion resistance and a decrease in strength. On the other hand, the lower the extrusion temperature, the better. However, since the deformation resistance increases on the low temperature side and extrusion becomes difficult, the lower limit is about 300 ° C. Preferably it is set as 320-380 degreeC.

  Further, the extrusion speed is preferably 5 m / min or less in order to suppress processing heat generation during extrusion and suppress the recrystallization during extrusion. More preferably, it is 1 m / min or less.

  As for the cooling after the hot extrusion, any means and cooling rate including cooling can be used, but it is preferable to forcibly cool by air or water for the efficiency of the extrusion process.

  As the extrusion method, direct extrusion or indirect extrusion may be used. However, when a lot of seizure occurs under the above-described extrusion conditions, it is preferable to carry out by hydrostatic extrusion.

  Direct extrusion and indirect extrusion are more efficient than hydrostatic extrusion, but the recrystallized grain layer on the surface of the extruded material (surface) is relatively fine inside the extruded material. Compared to the crystal grain (extrusion process) structure, there is a problem that the grains tend to be coarse grains. In addition, when extruding a 7000 series aluminum alloy having a Zn content exceeding 7% by mass as in the present invention, in the case of direct extrusion or indirect extrusion, extrusion processing below the recrystallization temperature range is considerably performed. There are difficulties.

  This is because even if the billet, which is an extrusion material, has a low heating temperature lower than the recrystallization temperature range, in direct extrusion or indirect extrusion, the billet is extruded in contact with the container wall or die due to the structure of the extruder. As a result, frictional heat is generated, and as a result, the temperature during extrusion inevitably falls within the recrystallization temperature range. For this reason, it is because the coarse recrystallized (grain) layer which inevitably causes a problem in Patent Document 1 itself is easily formed on the surface layer portion of the extruded material.

  On the other hand, in hot isostatic pressing, a lubricant is put between the container and the billet, and a state in which the billet for extrusion floats in this lubricant is pushed out by a stem (with a dummy block). . For this reason, the billet does not come into direct contact with the container or the die, unlike direct extrusion or indirect extrusion, due to the action of the lubricant. That is, the billet is in direct contact only while it passes through about 5 mm of the die thickness. As a result, friction and frictional heat are reduced, and the metal flow becomes nearly uniform. As a result, even a billet of a 7000 series aluminum alloy like the present invention having a high Zn content can be extruded even at a low temperature lower than the recrystallization temperature, and the recrystallized grains in the surface layer part and inside of the extruded material The layer can be suppressed (miniaturized).

  For this reason, the extruded material by hot isostatic pressing includes the recrystallized grain layer, or even if there is a recrystallized grain layer, the structure from the surface layer to the inside can be made uniform. As a result, the drawability, drawability, workability, and formability of the wire rod or wire rod product are significantly improved. In addition, if the recrystallized grain layer is suppressed as in the present invention, the basic characteristics such as sag resistance required for wire rod products such as aluminum alloy bolts can be ensured by the fine extruded structure. .

Product processing and heat treatment of extruded material The extruded material after hot extrusion described above is further processed into the product shape of the member for each application. For example, in order to further process the extruded material cold, an annealing treatment may be performed before this processing. Specifically, in order to reduce the diameter of the extruded material, when drawing is performed by drawing, an annealing treatment may be performed on the extruded material before drawing or during drawing.

  Then, before completion of such product processing or after completion of product processing, solution treatment and quenching treatment are performed, and further, artificial aging treatment is performed to improve the strength.

  The solution treatment is preferably performed at 450 ° C. or higher and a solidus temperature or lower, more preferably 480 ° C. or higher and a solidus temperature or lower, and the holding time is in the range of 1 to 4 hours. Immediately after the solution treatment, rapid cooling (quenching) is performed at a cooling rate of 10 ° C./second or more. Cooling after solution treatment is performed by mist cooling or water cooling, or by quenching in warm water from room temperature to 100 ° C. Exceeding these conditions may reduce the mechanical properties of the product.

  The artificial age hardening treatment may be performed under the general artificial aging conditions (T6, T7) of this kind 7000 series aluminum alloy extruded material. In other words, it is an advantage of the present invention that a high strength with a tensile strength of 700 MPa or more is obtained under such general artificial aging conditions. As a standard, artificial aging treatment at 100 to 200 ° C. is performed for 12 to 36 hours (hr).

  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. After casting a 7000 series aluminum alloy that is a Zn-Mg-Cu system having the composition shown in Table 1, hot isostatic pressing is performed according to the soaking conditions, extrusion start temperature and extrusion speed shown in Table 2, in common. A wire rod material having a circular cross section of 15 mmφ was produced. Here, in all the inventive examples, soaking was performed by the above-described two-stage soaking or two-time soaking, and hot isostatic pressing was performed in an unrecrystallized region. On the other hand, the comparative example was made by changing various conditions such as composition, soaking, and hot isostatic pressing.

  As the structure of the extruded material in the extruded state, the average crystal grain size (μm) of the recrystallized grains in the surface layer portion, the average section length (μm) in the radial direction of the crystal grains in the axial center portion of the extruded material The average area ratio of <111> oriented crystal grains in the extrusion direction, and the ratio <001> / <111> between the average area ratio of <001> oriented crystal grains and the average area ratio of <111> oriented crystal grains It was measured. These results are shown in Table 3.

  This tissue was measured by the measurement method described above. Specifically, a JEM SEM (JEOL JSM 6500F) equipped with the TSL EBSP measurement / analysis system (OIM) was used. In each example, five test pieces taken from arbitrary positions in the longitudinal direction of the extruded material were used in the structure having a cross section parallel to the extrusion direction passing through the central portion of the extruded material shaft shown in FIG. The structures of the surface layer part and the inner shaft center part were measured, and these measured values were averaged. The measurement area of each test piece was 400 μm in the direction of the extrusion axis and 800 μm in the radial direction, and the measurement step interval was also set to 2.0 μm.

  The extruded wire rod material is simulated at 480 ° C. for 3 hours by tempering the product after being processed into the member for each application without removing the recrystallized grain layer of the surface layer portion (surface portion). Immediately after the completion of the solution treatment and the solution treatment, quenching was performed by quenching in water at 40 ° C., holding for 10 minutes, and cooling to room temperature. Thereafter, artificial aging treatment was further performed at 120 ° C. for 24 hours.

  And various characteristics, such as a mechanical characteristic and corrosion resistance, of the extrusion material after this artificial aging treatment were evaluated. These results are also shown in Table 3.

mechanical nature:
As for the mechanical properties of the tempered extruded material, test pieces were collected and subjected to a tensile test using a tensile tester at a crosshead speed of 5 mm / min at room temperature until breakage. From the stress-strain rate, tensile strength (MPa) and 0.2% yield strength (MPa) were measured. Elongation (%) was calculated from the spacing of the marking lines before and after the tensile test during the tensile test (interval of 10 mm before the tensile test). In addition, these measured values were made into the average value of each measured value of three test pieces in each example.

Corrosion resistance:
In order to evaluate the corrosion resistance, the tempered extruded material was subjected to a corrosion test in accordance with the method described in Section 4.4.3 of JIS-W1103 method. That is, the corrosion test conditions were as follows. First, after immersing in an etching solution at 93 ° C. (composition of 70% concentrated nitric acid 50 ml, 48%, hydrofluoric acid 5 ml, distilled water 945 ml), washed with distilled water and dried. It was. Thereafter, it was immersed in a corrosion accelerating solution at 30 ° C. (57 g of NaCl, 10 ml of 30% hydrogen peroxide solution diluted to 1 liter with distilled water) for 6 hours. Then, the test specimen parallel section was immersed in an etching solution (composition of 70% concentrated nitric acid 2.5 ml, concentrated hydrochloric acid 1.5 ml, 48% hydrofluoric acid 1.0 ml, distilled water 95.0 ml) for 10 seconds, followed by distillation. Washed with water and dried. The corrosion state of this test piece parallel cross section was observed with a 200-fold metal microscope. In the observation of the corrosion, the corrosion resistance was judged by comparing with the comparative example (circle) in Table 3 in which the corrosion resistance was not good after distinguishing it from other pitting corrosion and general corrosion in the microscope field of view. That is, compared with this comparative example (circle), the case where the degree of corrosion was large evaluated as x, and the case where it was small was evaluated as (circle).

In Tables 1 to 3, the invention numbers 1 to 12 having the same serial numbers are as shown in Table 1, and the aluminum alloy is in the composition range of the present invention (however, alloy number 11 in Table 1 is a missing number) . Moreover, as shown in Table 2, it is produced under preferable production conditions in which soaking is performed by two-stage soaking or twice soaking in a specific temperature range, and hot isostatic pressing is performed in an unrecrystallized region. (However, number 11 in Table 2 is a missing number) . As a result, as shown in Table 3, recrystallization is suppressed including the surface layer portion and the inside, and a fine extruded structure (fibrous structure) is formed. As a result, the tensile strength is 700 MPa or more and the elongation is 7% or more. Moreover, it is excellent also in corrosion resistance (however, the number 11 in Table 3 is a missing number) .

On the other hand, in each comparative example of 13 to 24, which is the same serial number from Table 1 to Table 3, as shown in Table 1, the aluminum alloy is out of the composition range of the present invention.
In Comparative Example 13, Zn deviates from the lower limit.
In Comparative Example 14, Zn deviates from the upper limit.
In Comparative Example 15, Mg deviates from the lower limit.
In Comparative Example 16, Mg deviates from the upper limit.
In Comparative Example 17, Cu deviates from the lower limit.
In Comparative Example 18, Cu deviates from the upper limit.
In Comparative Example 19, Zr deviates from the lower limit.
In Comparative Example 20, Zr deviates from the upper limit.
In Comparative Example 21, Mn deviates from the upper limit.
In Comparative Example 22, Cr deviates from the upper limit.
In Comparative Example 23, Sc deviates from the upper limit.
In Comparative Example 24, Ag deviates from the upper limit.

  For this reason, although these comparative examples are manufactured by a preferable manufacturing method, as shown in Table 3, recrystallization is not suppressed including those that cannot be manufactured due to occurrence of casting cracks, and the surface layer portion and the inside. Although the tensile strength is less than 700 MPa or the tensile strength is increased, the overall evaluation is inferior because the elongation and the corrosion resistance are deteriorated.

Moreover, each comparative example of 25-29 which is the same serial number from Table 1 to Table 3 is manufactured out of the preferable manufacturing conditions as shown in Table 2, although the aluminum alloy is within the composition range of the present invention as shown in Table 1. Has been. As a result, as shown in Table 3, recrystallization including the surface layer portion and the inside is not suppressed, and the tensile strength is less than 700 MPa.
In Comparative Example 25, the heat treatment was performed only once.
Comparative Examples 26 and 27 are two-stage soaking or two-time soaking, but one of the soaking temperatures deviates from the preferred condition and is too high or too low.
In Comparative Examples 25 to 29, the extrusion start temperature is too high for the composition, and hot isostatic pressing is performed in the recrystallization region.

  ADVANTAGE OF THE INVENTION According to this invention, the surface layer part and internal recrystallization of an extrusion material are suppressed, and the high strength 7000 series aluminum alloy extrusion material with a tensile strength of 700 Mpa or more can be provided. For this reason, this invention can be used suitably as a member of each said use reduced in weight.

Claims (3)

  In mass%, Zn: 7.0-14.0%, Mg: 1.0-3.5%, Cu: 0.5-2.0%, Zr: 0.05-0.3% As a structure of a cross section parallel to the extrusion direction passing through the axial center portion of the extruded material in the state of being extruded, the balance being a Zn-Mg-Cu-based aluminum alloy extruded material consisting of Al and inevitable impurities The average crystal grain size of the recrystallized grains in the surface layer portion of the extruded material is 100 μm or less, the average section length in the radial direction of the crystal grains in the central portion of the extruded material shaft is 35 μm or less, and < The ratio of the average area ratio of the <001> -oriented crystal grains to the average area ratio of the <111> -oriented crystal grains when the average area ratio of the 111> -oriented crystal grains is 0.5 to 1.0, <001 >> / <111> is 0.25 or less Gold extrusion material.   The aluminum alloy extruded material is further in mass%, Mn: 0.1 to 1.5%, Cr: 0.05 to 0.3%, Sc: 0.05 to 0.3%, The aluminum alloy extruded material according to claim 1, containing two or more kinds. The aluminum alloy extruded material according to claim 1 or 2, wherein the aluminum alloy extruded material further contains Ag: 0.01 to 0.2% by mass%.

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