WO2016204043A1

WO2016204043A1 - High strength aluminum alloy hot-forged material

Info

Publication number: WO2016204043A1
Application number: PCT/JP2016/067071
Authority: WO
Inventors: 久郎宍戸; 佳也稲垣; 雅是堀
Original assignee: 株式会社神戸製鋼所
Priority date: 2015-06-16
Filing date: 2016-06-08
Publication date: 2016-12-22
Also published as: US20180148815A1; JP2017002388A; CN107614718A

Abstract

In the texture of a 6000 series aluminum alloy hot-forged material with a specific composition, the present invention refines even small crystal grains with misorientation of at least 2°, and keeps KAM value, which is the mean misorientation of said crystal grains, within a specific range to obtain excellent stress corrosion cracking properties as evaluated by the test method in figure 1 as well as high tensile strength, high yield strength, and high elongation.

Description

High strength aluminum alloy hot forging

The present invention relates to a high-strength aluminum alloy hot forging material. Hereinafter, aluminum is also simply referred to as Al.

In recent years, in response to global environmental problems caused by exhaust gas, etc., improvement in fuel consumption 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 are used as structural materials 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 undercarriage 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, in order to refine crystal grains of a 6000 series aluminum alloy forging, a transition element having a grain refinement effect such as Mn, Zr, or Cr is added, or at a relatively high temperature of about 450 to 570 ° C. It is well known to perform hot forging. In addition, in order to obtain high strength and high toughness, it is also proposed to use an extrudate that has been once hot extruded from the ingot as a material for hot forging, and to refine the unrecrystallized region in the forged structure. (See Patent Document 1).

On the other hand, although it is not in the field of hot forging, as a metallurgical technique for increasing the strength of aluminum alloy materials, after forging 6000 series aluminum alloy casting material, warm forging at about 150-250 ° C It has been proposed to repeat the processing and then perform artificial aging treatment (see Patent Documents 2 and 3).

Also, this is not the field of aluminum alloy, but in the field of rolled sheet of Corson alloy (Cu—Ni—Si based copper alloy), KAM value (KAM value which is the average orientation difference of crystal grains measured by SEM-EBSD method ( Corson alloys have been proposed by controlling the Kernel Average Misorientation value), having low strength anisotropy, particularly high yield strength in the direction perpendicular to the rolling, and excellent balance in bending workability (see Patent Documents 4 and 5). ).

Incidentally, this KAM value is also known as an index for ensuring the balance between strength, elongation and stretch flangeability of a high-strength cold-rolled steel sheet (HITEN) even in the field of steel sheets (see Patent Document 6).

Japanese Unexamined Patent Publication No. 2011-225988 Japanese Unexamined Patent Publication No. 2014-218865 Japanese Patent No. 5082483 Japanese Patent No. 5314663 Japanese Patent No. 5476149 Japanese Patent No. 4977184

When the extruded material is used as the hot forging material as in Patent Document 1, a high proof stress is obtained in a direction parallel to the extrusion direction, but there is a problem that strength anisotropy is high.
In addition, the crystal grain refinement technology in the conventional 6000 series aluminum alloy hot forging described above still has room for improvement in order to achieve higher tensile strength and higher proof stress.

As described in Patent Documents 2 and 3, a method of increasing the strength by repeatedly performing a warm forging process on a 6000 series aluminum alloy cast material and then performing an artificial aging treatment is also a hot process at a high temperature of 500 ° C. It is said that the effect of increasing the strength is small when forging is performed, and it is unclear whether this method is effective in improving the mechanical properties of the hot forging material of 6000 series aluminum alloy.

Even if the KAM value, which is the average orientation difference of the crystal grains, is effective in improving the mechanical properties of a rolled plate such as a copper alloy plate or a steel plate as in Patent Documents 3 to 6, the alloy It is unclear whether it is effective in improving the mechanical properties of 6000 series aluminum alloy hot forgings, which have completely different compositions, properties, and manufacturing methods.

The present invention has been made paying attention to such circumstances, and its purpose is to provide a hot 6000 series aluminum alloy having high tensile strength, high yield strength, and high elongation on the premise of having excellent corrosion resistance. The object is to provide a forging material.

In order to achieve this object, the gist of the present invention is, in mass%, Si: 0.7 to 1.5%, Mg: 0.6 to 1.2%, Fe: 0.01 to 0.5% In addition, one or more of Mn: 0.05 to 0.8%, Cr: 0.01 to 0.5%, and Zr: 0.01 to 0.2% are contained. An aluminum alloy hot forging material composed of the balance Al and unavoidable impurities, and an average grain size of crystal grains having a misorientation of 2 ° or more as a structure in the center portion of the plate thickness measured by the SEM-EBSD method. It is 30 μm or less, and the KAM value, which is the average orientation difference of the crystal grains, is in the range of 0.6 to 2.0 °.

In the present invention, along with the refinement of the crystal grains of the 6000 series aluminum alloy hot forging material, the KAM value obtained by quantifying the average orientation difference of the crystal grains strongly correlates with the tensile strength and yield strength of the forging material. Newly discovered.
This KAM value itself is the amount of average orientation difference of crystal grains measured by the SEM-EBSD method as described in Patent Documents 3 to 6, and as a calculation method of the residual strain amount of crystal grains, It is known in fields other than 6000 series aluminum alloy hot forging.

This KAM value is relatively low in the cold to warm region, with the forged material manufactured by hot forging without changing the already standardized 6000 series aluminum alloy composition of the forged material. It can control suitably by repeatedly forging and artificial aging treatment.

The present invention provides a 6000 series aluminum alloy having high tensile strength, high yield strength, and high elongation without reducing corrosion resistance by refining crystal grains having an orientation difference of 2 ° or more and controlling the KAM value. Hot forging can be provided. For this reason, the reliability of the 6000 series aluminum alloy hot forging as a safety part of the automobile undercarriage part increases.

It is a side view which shows the test piece for stress corrosion cracking resistance evaluation used in the Example. It is a top view which shows the test piece for stress corrosion cracking resistance evaluation used in the Example.

Hereinafter, embodiments of the present invention will be specifically described.

(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 composition of the 6000 series (Al-Mg-Si series) aluminum alloy in the present invention needs to ensure high corrosion resistance and durability such as high strength and stress corrosion cracking resistance as the above-mentioned undercarriage forged parts. is there. 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 to 0.5%, Mn: 0.05 to 0.8%, Cr: 0.01 to 0.5%, Zr: 0.01 to 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 is further, in mass%, Cu: 0.05 to 1.0%, Ti: 0.01 to 0.1%, Zn: 0.005 to You may contain 0.2% of 1 type, or 2 or more types. In addition, all the% display in each element amount means the mass%.

Here, other impurity elements that are inevitably mixed from the melting raw material scrap, etc., are inevitable impurities in the compositional balance, and within the range that does not impair the various characteristics of the present invention, It is permissible to include normal amounts based. Next, the critical significance and preferable range of the content of each element will be described.

Si: 0.7 to 1.5%
Si is an essential element for precipitating mainly as an acicular β ′ phase together with Mg by an artificial aging treatment and imparting high strength and high yield strength when using automobile underbody parts.
When there is too little content of Si, the amount of precipitation at the time of artificial aging treatment will become too small, and high intensity will not 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 also an essential element for precipitating in the crystal grains mainly as an acicular β ′ phase with Si by artificial age hardening (aging treatment) and imparting high strength and high yield strength of automobile undercarriage parts. .
When the content of Mg 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, 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 (yield strength) becomes too high, which hinders hot forgeability and workability. Furthermore, the elongation is also reduced.
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), hinders grain boundary movement after recrystallization, suppresses recrystallization, prevents coarsening of crystal grains, Has the effect of miniaturizing.
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 acicular 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%.

One or more of Mn: 0.05 to 0.8%, Cr: 0.01 to 0.5%, Zr: 0.01 to 0.2% Mn, Cr and Zr are the same as Fe , Si and intermetallic compounds are produced to produce dispersed particles (dispersed phase), which prevents the grain boundary movement after recrystallization, suppresses recrystallization, prevents coarsening of crystal grains, and refines the crystal grains. There is an effect to make it.
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. Further, since Si is easily contained in the intermetallic compound formed by these elements, the acicular β ′ phase generated by the artificial aging treatment that requires Si is reduced, and the strength tends to be lowered.
Accordingly, when one or more of these elements are contained, the respective contents are Mn: 0.05 to 0.8%, Cr: 0.01 to 0.5%, Zr: 0.01 The range is up to 0.2%.

One or more of Cu: 0.05 to 1.0%, Ti: 0.01 to 0.1%, Zn: 0.005 to 0.2% Cu, Ti, and Zn are used for the strength of the forging material. Since it is a synergistic element that improves toughness, when one or more of these effects are expected, it is 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 hot forging material is remarkably increased, and the corrosion resistance and durability of the aluminum alloy hot forging material are lowered. Therefore, when Cu is contained, the content of Cu is set in the range of 0.05 to 1.0%.

Zn improves the strength and toughness by depositing and forming Zn-Mg precipitates finely and densely in an artificial aging treatment. 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, if contained, the Zn content is in the range of 0.005 to 0.2%.

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 in the range of 0.01 to 0.1%.

In addition, the elements described below are impurities, and the contents described below are allowed. 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, it is preferable that the hydrogen concentration per 100 g of Al is 0.25 ml or less and the content is as small as possible.

Since Sc, V, and Hf are also likely to be mixed as impurities and impede the characteristics of the undercarriage parts, the total of these should be less than 0.3%. Further, B combines with Ti to enhance the effect of refining Ti ingot crystal grains. However, if the content exceeds 300 ppm, a coarse crystallized product is formed and the workability is lowered. Therefore, B allows up to 300 ppm or less.

(Organization)
On the premise of the above alloy composition, in the present invention, structural materials and structural parts of transport equipment such as automobiles, especially forged materials as automobile undercarriage forged parts, etc. are measured at the center of the plate thickness measured by the SEM-EBSD method. As the structure, the average grain size of crystal grains having an orientation difference of 2 ° or more is set to 30 μm or less, and the KAM value obtained by quantifying the average orientation difference of crystal grains having an orientation difference of 2 ° or more is set to 0.6-2. The range is 0 ° (deg).
The present invention provides a 6000 series aluminum alloy hot forging material having high tensile strength, high yield strength, and high elongation without reducing corrosion resistance by such refinement of crystal grains and control of the KAM value. be able to. If the KAM value is too small, less than 0.6 °, high tensile strength and yield strength cannot be achieved, and if it exceeds 2.0 ° and too large, high tensile strength and yield strength can be achieved. The elongation also decreases.

Here, “a crystal grain having an orientation difference of 2 ° or more” measured by the SEM-EBSD method is “a crystal grain having a grain boundary (boundary) having an orientation difference of 2 ° or more”. Many crystal grains having an orientation difference of 2 ° or more, such as 15 ° and 20 °, are included in the category.
In the present invention, refinement of crystal grains, including those with a relatively small misorientation such as 2 °, greatly affects the improvement of strength (tensile strength and 0.2% proof stress). We know and regulate. That is, by making the average grain size of crystal grains having an orientation difference of 2 ° or more as fine as 30 μm or less, it is possible to increase the strength of the 6000 series aluminum alloy hot forging. The detailed reason is still unclear, but if the grain boundary (boundary) has an orientation difference of 2 ° or more, it has the effect of hindering the movement of dislocations, so the average particle size should be reduced to 30 μm or less. Therefore, it is presumed that the grain boundary that hinders the movement of dislocations is remarkably increased and the forged material is strengthened.

The KAM value (Kernel Averaged Misorientation) measured by the SEM-EBSD method of the present invention is an average orientation difference of the “crystal grains having an orientation difference of 2 ° or more”.
This KAM value itself is known to correlate with the residual strain, for example, “Material” (Journal of the Society of Materials Science, Japan) Vol. 58, No. 7, P568-574, July 2009, etc. .
The KAM value is also known in the above-mentioned patent documents and the like as a value obtained by quantifying a local orientation difference, which is a difference in crystal orientation between adjacent measurement points, as an average orientation difference.
Such a KAM value is defined as (Σy) / n, where n is the number of crystal grains and y is the orientation difference (°) of each measured crystal grain.

However, the KAM value of the present invention uses a large number of crystal grains as the reference for measuring the KAM value, including those having a relatively small misorientation such as an orientation difference of 2 °, as defined above. However, it is different from the conventional one. That is, the KAM value varies greatly depending on the measurement standard or how to define the orientation difference of the target crystal grains.
In the present invention, the KAM value obtained by quantifying the average orientation difference of the “crystal grains having an orientation difference of 2 ° or more” indicates that the tensile strength of the 6000 series aluminum alloy hot forging and the 0 It was found to correlate strongly with 2% yield strength.

Strengthening by this KAM value is achieved by the forging material produced by hot forging without changing the 6000 series aluminum alloy composition already standardized as the automobile undercarriage part of the hot forging material. Further, it can be controlled by repeatedly performing relatively light forging and artificial aging treatment in a cold to warm region.
Therefore, a 6000 series aluminum alloy hot forging material having high tensile strength, high yield strength, and high elongation without lowering corrosion resistance or changing mechanical properties caused by changes in composition or hot forging conditions. Can be manufactured. For this reason, the reliability of 6000 series aluminum alloy hot forgings for safety parts such as automobile undercarriage parts increases.

In addition, since the present invention does not involve changes in hot forging conditions, such a structure and characteristics can be realized even when hot forging is performed at a large processing rate with a minimum thickness reduction rate exceeding 25%. There is.
For example, as for the undercarriage forging part as an application of the present invention, as a general-purpose shape, a substantially triangular overall shape, a substantially Y-shaped arm portion in plan view, and three end portions of this arm portion Each has a complicated shape having ball joint portions (three locations). For this reason, a large processing rate in which the minimum thickness reduction rate exceeds 25% is inevitably required, but this can be realized even when hot forging is performed at such a high processing rate.

(Measurement site by SEM-EBSD method)
Measurement of the average grain size (μm) and KAM value of crystal grains with a misorientation of 2 ° or more is performed at the center of the thickness of the forged material. For example, the plate thickness center portion of the forged material to be measured can be specified based on the center point of the forged material. 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. The cross section is composed of a thin central web and the cross section is connected by an approximately H-shaped or U-shaped arm. In this case, the center portion of the plate thickness is a crystal grain structure at the center portion of the plate thickness at an arbitrary position of the thick rib as a measurement target by the SEM-EBSD method.

(Measuring method)
A specific measurement method is to polish a cross section of measurement samples (three pieces) taken from the central portion of the plate thickness at an arbitrary position of the thick rib. Then, using a SEM-EBSD, an electron beam is irradiated at a pitch of 1.0 μm with respect to a measurement range of 500 μm × 500 μm in a cross section parallel to the compression direction of the forging material of the sample, and the orientation difference is 2 °. The average grain size (μm) of the above crystal grains and the KAM value obtained by quantifying the average orientation difference of the crystal grains are measured, and further averaged with three measured samples.

The SEM-EBSD (EBSP) method uses a field emission scanning electron microscope (Field Emission Scanning Electron Microscope: FESEM) and a backscattered electron diffraction image [EBSD: Electron Back Scattering] Is the law.

More specifically, the observation sample of SEM-EBSD is adjusted by mirror-polishing the observation sample cage (cross-sectional structure) after further mechanical polishing. Then, the EBSP is set on the FESEM column and irradiated with an electron beam onto the mirror-finished surface of the sample to project 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) and the like. Since this process is automatically performed for all measurement points, crystal orientation data of tens of thousands to hundreds of thousands of points in the cross section of the forged material can be obtained at the end of measurement.

The aluminum alloy hot forging material of the present invention having the above alloy composition and structure has a tensile strength of 420 MPa or more, a 0.2% proof stress of 400 MPa or more, and an elongation of 12% or more in consideration of strength and workability. Is preferred.

(Production method)
Next, the manufacturing method of the aluminum alloy hot forging material in the present invention will be described. The manufacturing process itself of the aluminum alloy hot forging material in the present invention is performed by performing a hot forging process after homogenizing heat treatment of the aluminum alloy ingot having the above-described composition. It can be manufactured by a conventional method. That is, it can be manufactured without performing hot extrusion of the ingot, which is an extra step. However, as automobile undercarriage forging parts and the like, there are preferable manufacturing conditions shown below for having the above-described structure and increasing strength, toughness and corrosion resistance.

(casting)
When casting an aluminum alloy melt adjusted to be within the specific aluminum alloy component range, a normal melt casting method such as a continuous casting rolling method, a semi-continuous casting method (DC casting method), or a hot top casting method is appropriately used. Select and cast.

However, when casting a molten aluminum alloy having a specific aluminum alloy component range, an average cooling rate of 100 ° C / 100 ° 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.

(Homogenization heat treatment)
The homogenized heat treatment of the cast ingot is carried out by keeping it in a temperature range of 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 occurs. 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, the minimum thickness reduction rate is 25% or more, and the maximum meat It is preferable to perform hot forging under conditions where the 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, reheating is performed without reheating during forging or as necessary, and rough forging, intermediate forging, finish forging, and hot forging are performed a plurality of times.

If the minimum thickness reduction rate is less than 25% as the hot forging rate, there is a possibility that the above-described complex car undercarriage parts cannot be forged with a good shape system. 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 toughness 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 and quenching treatment)
After this hot forging, solution treatment and quenching 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 the time is too short, the solution treatment is insufficient, the MgSi compound is insufficiently dissolved, the precipitation amount of the compound in the subsequent artificial age hardening treatment is too small, and the strength is low. descend. Although the holding time may be long, the effect is saturated when it exceeds 8 hours.

After the 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, MgSi compounds, Si, and the like are precipitated on the grain boundaries, and in the product after artificial aging, grain boundary fracture is likely to occur, and toughness and fatigue characteristics are lowered. Further, during the cooling, the stable phase MgSi compound and Si are also formed in the grains, and the precipitation amount of the β phase and β ′ phase precipitated during artificial aging is reduced, so that the strength is lowered.

However, on the other hand, if the cooling rate becomes too high (fast), the amount of quenching distortion increases, and a new problem arises that a straightening process becomes necessary after quenching or 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, hot water quenching at 30 to 85 ° C. in which quenching distortion is reduced is preferable in order to shorten the product manufacturing process and reduce the cost. 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.

(Cold or warm processing)
In the present invention, in order to set the hot forged material thus obtained (after solution treatment and quenching treatment) to the prescribed average particle diameter and KAM value, the total sheet thickness reduction rate is further 5%. The cold working or warm working in the temperature range (temperature range) from room temperature to 200 ° C. is repeated at least twice as a combination with the artificial age hardening treatment after these working. preferable.
Even if the combination of these cold work or warm work and each subsequent artificial age hardening treatment is performed only once, or even if cold work or warm work is performed twice, it is artificial after each of these work. When the aging treatment is not performed, there is a possibility that the specified average particle size or KAM value is not achieved.
In other words, when the combination of such cold working or warm working and subsequent artificial age hardening treatment is repeated twice or more, crystal grains whose orientation difference defining the obtained forging material is 2 ° or more. This guarantees the average particle size and KAM value.

If the plate thickness reduction rate per cold working or warm working is reduced to less than 5%, the working effect is lost, and the average grain size of crystal grains having an orientation difference of 2 ° or more exceeds 30 μm and becomes coarse. there's a possibility that. In addition, the KAM value tends to be as small as less than 0.6 °, and the desired high strength may not be obtained.
This is the same even when the working temperature of the warm working exceeds 200 ° C., and the average grain size of crystal grains having a misorientation of 2 ° or more may be coarsened exceeding 30 μm, and the KAM value Also, it tends to be smaller than 0.6 °, and the desired high strength may not be obtained.

On the other hand, the upper limit of the sheet thickness reduction rate per one of these cold working or warm working is preferably 50%, more preferably 40%. If the plate thickness reduction rate is too high and the amount of strain is too large, the KAM value becomes too large, and the elongation may be significantly reduced. In addition, cracks during processing are likely to occur.
In this respect, in Patent Document 2, the equivalent strain is 2 or more and more than 85% in terms of reduction in sheet thickness, and in Patent Document 3, the equivalent strain is less than 2, but in this embodiment, the equivalent strain is 0.8. A very large strain of 55% in terms of thickness reduction rate is applied by warm forging. For this reason, in the 6000 series aluminum alloy hot forging material of the present invention, or even in the 6000 series aluminum alloy cast material of Patent Documents 2 and 3, the strength is increased, but the elongation is significantly reduced.
Therefore, the range of the plate thickness reduction rate per one of these cold working or warm working is preferably 5% or more and 50% or less, more preferably 5% or more and 40% or less.

(Artificial age hardening treatment)
After each cold or warm process, an artificial age hardening process (hereinafter also referred to as an artificial aging process) is repeated at least twice in combination with the process. In order to prevent room temperature aging from proceeding, it is preferable to perform artificial aging treatment immediately after each cold or warm processing, for example, within 1 hour as a guide.
The conditions for the artificial aging treatment are preferably selected from a temperature range of 40 ° 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. For each aluminum alloy composition shown in Table 1, hot forgings that were manufactured under the same manufacturing conditions until solution treatment and quenching were subjected to cold or warm working and artificial aging treatment under different conditions shown in Table 2. The company manufactured hot forgings that are used as materials for automobile undercarriage parts. The structure, mechanical properties, and corrosion resistance of the hot forged material were measured and evaluated as shown in Table 2.

Specifically, in common with each example, an ingot made of a chemical component of a 6000 series aluminum alloy hot forging shown in Table 1 was cast by a semi-continuous casting method with an average cooling rate of 100 ° C./s or more. . In addition, in each aluminum alloy example shown in Table 1, the hydrogen concentration in 100 g of Al was all 0.10 to 0.15 ml.

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, mold temperature Was performed by a mechanical press using upper and lower molds under a common condition of 170 to 200 ° C. and a thickness change rate of the forged material center portion of 75% (over 25%).

The shape of the manufactured hot forged material is common to each example, and is composed of the overall shape of a triangle as described above in plan view. The suspension part shape was formed by a peripheral rib having a thickness (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.

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 forging material thus obtained (after solution treatment and quenching treatment) is subjected to cold working or warm working and artificial aging treatment twice under the conditions shown in Table 2 or 1 The average grain size and KAM value of the crystal grains were made differently by, for example, repeating the test.

As described above, the structure, mechanical properties, and intergranular stress corrosion cracking property of the hot forging material, in which the average grain size and the KAM value of the crystal grains were made, were measured and evaluated by the following methods. These results are shown in Table 2.

(Organization)
The average grain size and KAM value of the crystal grains are determined from the longitudinal cross section of the central part of the plate thickness of the thick rib portion of the substantially H-shaped arm of the forged material by the measurement method described above. A sample was taken, and the average grain size (μm) and KAM value of crystal grains having an orientation difference of 2 ° or more were measured as described above.

(Mechanical properties)
A sample is taken from an arbitrary plate thickness center portion of the thick rib portion of the forged material, and an outer diameter of 5 mm, a distance between gauge points of 25 mm, a tensile test piece (L direction) from any three longitudinal points of the sample. Are measured, and mechanical properties such as tensile strength (MPa), 0.2% proof stress (MPa), and elongation (%) are measured, and the average value of these three points (three test pieces) is calculated. Asked.

(Stress corrosion cracking resistance)
Moreover, the evaluation of the stress corrosion cracking resistance was performed in accordance with the provisions of the alternating immersion method of JIS H8711. FIG. 1A is a side view and FIG. 1B is a plan view showing a test piece for stress corrosion cracking resistance evaluation (C-ring for SCC test) including its dimensions. When the stress corrosion cracking under a load of 300 MPa was less than 30 days, it was evaluated as x, and when it was 30 days to less than 60 days, it was evaluated as ◯.

As is apparent from Tables 1 and 2, each of Invention Examples 1 and 7 to 12 is subjected to cold working or warm working and artificial aging treatment within the component composition range of the present invention and in a preferable condition range. Therefore, as shown in Table 2, each of these invention examples has a structure as defined in the present invention, and the orientation difference is 2 ° or more as the structure at the center of the plate thickness measured by the SEM-EBSD method. The average grain size of the crystal grains is 30 μm or less, and the KAM value of the crystal grains is in the range of 0.6 to 2.0 °.

As a result, each of these inventive examples has excellent stress corrosion cracking resistance, a tensile strength of 417 MPa or more, a 0.2% proof stress of 398 MPa or more, an elongation of 12.6% or more, and a high tensile strength and high. It has proof stress and high elongation, and has various characteristics necessary for undercarriage parts.

On the other hand, as in Comparative Examples 2 to 6 in Table 2, when the alloy composition is within the range, but cold working or warm working and artificial aging treatment are manufactured out of the preferable condition range, It does not meet the structure regulation at the center of the plate thickness measured by SEM-EBSD method. That is, the average grain size of crystal grains having a misorientation of 2 ° or more exceeds 30 μm, the KAM value is too small, less than 0.6 °, or too large, exceeding 2.0 °.
As a result, in Comparative Examples 2 to 6, the tensile strength and the 0.2% proof stress are remarkably inferior to those of the inventive examples, and if the KAM value exceeds 2.0 °, it is stretched. Is significantly inferior to the inventive examples.

In Comparative Example 2, warm processing and artificial aging treatment are performed only once. For this reason, the average grain size of crystal grains having an orientation difference of 2 ° or more exceeds 30 μm, and the KAM value is too small at less than 0.6 °.
In Comparative Example 3, warm processing and artificial aging treatment are repeated twice in this order, but the plate thickness reduction rate (processing rate) of warm processing is too small. For this reason, the average grain size of crystal grains having an orientation difference of 2 ° or more exceeds 30 μm, and the KAM value is too small at less than 0.6 °.
In Comparative Example 4, warm processing and artificial aging treatment are repeated twice in this order, but the temperature of warm processing is too high in both cases. For this reason, the average grain size of crystal grains having an orientation difference of 2 ° or more exceeds 30 μm, and the KAM value is too small at less than 0.6 °.
In Comparative Example 5, although the warm working is repeated twice, the artificial aging treatment is not performed after the second warm working. For this reason, the average grain size of crystal grains having an orientation difference of 2 ° or more is 30 μm or less, but the KAM value exceeds 2.0 and is too large.
In Comparative Example 6, warm processing and artificial aging treatment are repeated twice in this order, but the temperature of the artificial aging treatment is too high in both cases. For this reason, the average grain size of crystal grains having an orientation difference of 2 ° or more exceeds 30 μm, and the KAM value is too small at less than 0.6 °.

Further, Comparative Examples 13 to 24 in Table 2 using alloys whose compositions deviate as shown in Alloy Nos. 8 to 18 in Table 1 are manufactured in a condition range in which cold working or warm working and artificial aging treatment are preferable. Even if it has been met, either the tensile strength, the 0.2% proof stress, the elongation, or the stress corrosion cracking resistance is met, regardless of whether or not it satisfies the structure rule at the center of the plate thickness measured by the SEM-EBSD method. Low compared to the inventive examples.
That is, the examples of high tensile strength and high yield strength as in the invention examples are low in elongation or stress corrosion cracking resistance is significantly inferior to the invention examples. In addition, the examples where the elongation is as high as the invention examples or the stress corrosion cracking resistance is excellent are significantly inferior to the invention examples in tensile strength and 0.2% proof stress.

In Comparative Example 13, Mg is insufficient (alloy number 8 in Table 1).
In Comparative Example 14, Mg is excessive (Alloy No. 9 in Table 1).
In Comparative Examples 15 and 16, Si is insufficient (alloy number 10 in Table 1). In Comparative Example 16, warm working and artificial aging treatment are performed only once.
In Comparative Example 17, Si is excessive (Alloy No. 11 in Table 1).
Comparative Examples 18 and 19 contain neither Mn, Cr nor Zr, or contain too little (Alloy Nos. 12 and 13 in Table 1).
Comparative Example 20 has too much Mn (Alloy No. 14 in Table 1).
Comparative Examples 21, 22, 23, and 24 have too much Cr, Zr, Cu, and Zn (alloy numbers 15, 16, 17, and 18 in Table 1).

From the above results, on the premise of having excellent corrosion resistance, it is possible to realize a 6000 series aluminum alloy hot forging material having high tensile strength, high yield strength, and high elongation. I understand the significance.

Although the present invention has been described in detail and with reference to specific embodiments, it will be apparent to those skilled in the art that various changes and modifications can be made without departing from the spirit and scope of the invention.
This application is based on a Japanese patent application filed on June 16, 2015 (Japanese Patent Application No. 2015-121043), the contents of which are incorporated herein by reference.

According to the present invention, it is possible to obtain a 6000 series aluminum alloy hot forged material having high tensile strength, high yield strength, and high elongation 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

In mass%, Si: 0.7-1.5%, Mg: 0.6-1.2%, Fe: 0.01-0.5%, respectively, and Mn: 0.05- Hot aluminum alloy containing one or more of 0.8%, Cr: 0.01 to 0.5%, Zr: 0.01 to 0.2%, the balance being Al and inevitable impurities As a structure in the center of the plate thickness measured by the SEM-EBSD method, the forged material has an average grain size of crystal grains having an orientation difference of 2 ° or more of 30 μm or less, and an average orientation difference of the crystal grains. A high-strength aluminum alloy hot forging material having a KAM value in the range of 0.6 to 2.0 °.
The aluminum alloy hot forging material may further comprise, in mass%, Cu: 0.05 to 1.0%, Ti: 0.01 to 0.1%, Zn: 0.005 to 0.2%, or The high-strength aluminum alloy hot forging material according to claim 1 containing two or more kinds.
The high-strength aluminum alloy hot forging material according to claim 1 or 2, wherein the aluminum alloy hot 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 12% or more.