JP2006213942A - Aluminum alloy extruded shape material for rear side rail having excellent corrosion resistance - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a 6000 series aluminum alloy extruded shape material for a rear side rail in a track chassis frame having excellent corrosion resistance even under severe using environment, working environment and stock environment. <P>SOLUTION: The aluminum alloy extruded shape material for a rear side rail in a track chassis frame has a componential composition containing 0.4 to 1.2% Mg and 0.2 to 1.0% Si, and in which each content of Fe, Cu, Mn, Cr, Zn, Zr and Ti is regulated to ≤0.2%, respectively independently in such a manner that the total content thereof is also controlled to ≤1.0%. Further, the average crystal grain size in its recrystallized structure is made the fine one of ≤200 μm, and also, the average aspect ratio of the crystal grains is prescribed to the low one of ≤5. In this way, its corrosion resistance when used as a track chassis frame is improved. <P>COPYRIGHT: (C)2006,JPO&NCIPI

Description

  The present invention relates to an aluminum alloy extruded shape for a rear side rail in a track chassis frame, and more particularly to an aluminum alloy extruded shape for a rear side rail excellent in corrosion resistance such as intergranular corrosion.

  Conventionally, a truck chassis frame of a cab-over vehicle has a rear side rail (rear side frame) on the rear side of the vehicle body (loading side) and a front side rail (front side frame) on the front side of the vehicle body (cabin side of the driver's seat). Is constituted by a single (integral) steel channel member.

  In recent years, with respect to global environmental problems caused by exhaust gas and the like, improvement in fuel efficiency has been pursued by reducing the weight of the body of a transport aircraft such as an automobile. The situation is the same for trucks such as cab-over cars, and the use of lighter aluminum alloy materials such as rolled plates and extruded profiles instead of steel materials that have been used in the past has increased for truck bodies. It's getting on.

However, the application of the aluminum alloy material to the truck body is limited to a truck bed or a truck door frame (see, for example, Patent Documents 1 and 2). In other words, the practical use (application) of the chassis frame 40 and the like to the truck body is delayed.
JP 2004-189125 A (Claims) JP 2004-84235 A (Claims)

  The reason why the practical application (application) of the aluminum alloy material to the truck body such as the chassis frame is delayed is due to the reliability of the aluminum alloy material.

  In the case of a rear side rail in a truck chassis frame, unlike a side frame (side member) of an automobile body housed in a body panel, it is used as a truck body in an exposed state.

  For this reason, the rear side rail in the track chassis frame is required to have high corrosion resistance such as intergranular corrosion resistance. In this respect, among the aluminum alloy extruded profiles, the Al-Mg-Si 6000-series aluminum alloy extruded profiles are more likely to cause intergranular corrosion than the higher-strength 7000-series aluminum alloy extruded profiles. Less is.

  In addition, the 6000 series aluminum alloy extruded profile has higher strength and proof strength than the 5000 series aluminum alloy extruded profile, and can secure the necessary bending strength, rigidity, torsional strength and rigidity for the rear side rail. Is suitable.

  However, when an aluminum alloy extruded shape is drilled or formed by mechanical processing or pressing to be processed into a rear side rail, residual stress is likely to be generated at the processing site. This residual stress is likely to cause corrosion (intergranular corrosion) for a rear side rail used in an exposed state when the truck travels particularly in a corrosive environment such as salt water.

  In addition, the application of aluminum alloy extruded profiles to the rear side rail is a major goal of weight reduction. For this reason, in order to obtain the advantages of weight reduction, the thickness of the aluminum alloy extruded profiles is a maximum of 6 mm or less. It is restricted to thin wall. For this reason, the damage when intergranular corrosion occurs under the use of such a thin wall becomes extremely large as compared with the case of a thicker wall.

  Furthermore, in order to ensure the bending strength / rigidity and torsional strength / rigidity necessary for the rear side rail, the aluminum alloy extruded shape material is required to have a high strength and a high strength of 0.2% proof stress of 200 MPa or more. For this reason, the higher the strength of the extruded aluminum alloy, the greater the sensitivity to intergranular corrosion.

  For this reason, even if it is used as a rear side rail in an exposed state as a usage environment, or even if residual stress is generated in the above-mentioned processed part, the above-mentioned thinning and high-strength can be achieved. There is a need for a 6000 series aluminum alloy extruded profile that has excellent corrosion resistance.

  Therefore, an object of the present invention is to provide a 6000 series aluminum alloy extruded profile for rear side rails that is excellent in intergranular corrosion resistance even under the above-mentioned usage environment, processing environment, and raw material environment.

  In order to achieve this object, the gist of the aluminum alloy extruded profile for the rear side rail in the track chassis frame of the present invention is mass%, Mg: 0.4 to 1.2%, Si: 0.00. Including 2 to 1.0%, Fe, Cu, Mn, Cr, Zn, Zr, and Ti are each independently 0.2% or less, and the total amount of these is also regulated to 1.0% or less, the balance Al and inevitable Made of Al-Mg-Si based aluminum alloy consisting of mechanical impurities, 0.2% proof stress is 200 MPa or more, thickness is 3-6 mm, and the microstructure is recrystallized structure. The crystal grain size is 200 μm or less, and the average of the aspect ratio L / ST, which is the ratio of the length L in the extrusion direction of the crystal grains to the length ST in the thickness direction of the extruded profile, is 5 or less.

  In the present invention, the structure of an extruded shape made of an Al-Mg-Si-based aluminum alloy extends in the extrusion direction of the extruded shape, and as a recrystallized structure made of uniform and fine equiaxed crystals, intergranular corrosion resistance. It shall be excellent.

  Generally, the recrystallized structure is considered to be inferior in corrosion resistance to intergranular corrosion as compared with a fibrous structure having a large aspect ratio L / ST. In this regard, in the present invention, it is found that the intergranular corrosion resistance is improved if the average crystal grain size and the aspect ratio in the recrystallized structure are defined and the recrystallized structure is composed of uniform and fine equiaxed crystals. did.

  As a result, the extruded aluminum alloy profile according to the present invention has a maximum thickness of 6 mm or less even if residual stress occurs, even if the truck travels in a corrosive environment such as salt water, and for weight reduction. Even when the thickness is reduced, the intergranular corrosion resistance can be excellent even if the 0.2% proof stress required for the rear side rail is a high proof stress of 200 MPa or more. As a result, it is possible to reduce the weight by using an aluminum alloy for the rear side rail in the track chassis frame.

(Chemical composition)
First, the Al—Mg—Si aluminum alloy component composition of the extruded profile of the present invention will be described below. The composition of the extruded profile of the present invention has a thickness of 3 to 6 mm, in order to ensure a high yield strength with a 0.2% yield strength of 200 MPa or more, and to ensure intergranular corrosion resistance, in mass%. Mg: 0.4 to 1.2%, Si: 0.2 to 1.0%, and the Al—Mg—Si (6000 series) aluminum alloy composition composed of the balance Al and inevitable impurities.

The preferable content range and significance of each element, or the allowable amount will be described below. Si: 0.2 to 1.0%.
Si forms an age hardening ability by forming a compound phase such as a GP zone together with Mg by solid solution strengthening and artificial aging treatment, and is indispensable for obtaining a required proof stress of 0.2 MPa or 200 MPa or more. It is an element. When Si is less than 0.2%, the compound phase is insufficient even by artificial aging treatment, and the required yield strength cannot be obtained. On the other hand, when Si exceeds 1.0%, workability to the rear side rail and intergranular corrosion resistance are reduced. Accordingly, Si is set in the range of 0.2 to 1.0%.

Mg: 0.4 to 1.2%,
Mg, like Si, exhibits age-hardening ability by forming a compound phase such as a GP zone together with Mg by solid solution strengthening and artificial aging treatment, and a 0.2% proof stress is 200 MPa or more. Is an essential element for. If Mg is less than 0.4%, the compound phase is insufficient even by artificial aging treatment, and the required yield strength cannot be obtained. On the other hand, when Mg exceeds 1.2%, workability to the rear side rail and intergranular corrosion resistance are deteriorated. Therefore, Mg is made 0.4 to 1.2% in range.

(Fe, Cu, Mn, Cr, Zn, Zr, Ti)
When a substantial amount of a transition element such as Fe, Cu, Mn, Cr, Zr, or Ti is included, a fibrous structure having an aspect ratio L / ST of 10 or more is easily obtained during hot extrusion, and the average of the aspect ratio L / ST is 5 The following recrystallized structure cannot be obtained in the extrusion direction of the extruded profile. Furthermore, alloy elements such as Cu and Zn reduce intergranular corrosion resistance. Therefore, it is preferable that these alloy elements are as small as possible.

  However, from the viewpoint of recycling, not only high-purity Al ingots but also 6000 series alloys, other Al alloy scrap materials, low-purity Al ingots, etc. are used as melting raw materials as melting materials. In the case of melting, these other alloy elements are necessarily included. Therefore, in the present invention, Fe, Cu, Mn, Cr, Zn, Zr, and Ti are each independently 0.2% or less (up to 0.2% is allowed), and the total amount of these elements is 1.0. % Or less (content up to 0.1% is allowed).

  However, in particular, Cu and Zn remarkably reduce the intergranular corrosion resistance, so it is preferable to regulate the total content of Cu + Zn to 0.2% by mass or less.

(Micro structure)
In the extruded profile of the present invention, in particular, in order to improve the intergranular corrosion resistance under the use environment as the rear side rail, the microstructure is composed of a recrystallized structure, and the average crystal grain size of the recrystallized structure is It is assumed that the average of the aspect ratio L / ST, which is the ratio of the length L in the extrusion direction of the crystal grains and the length ST in the thickness direction of the extruded shape, is 5 or less.

  When the average crystal grain size of the recrystallized structure exceeds 200 μm and the average aspect ratio of the crystal grains exceeds 5 or less, the grain boundary corrodes preferentially in the use environment as the rear side rail. Prone to corrosion. When this intergranular corrosion occurs, a corrosion form in which crystal grains float and fall off one after another from the surface is propagated and developed along the grain boundary, resulting in an increase in weight loss and loss of the original surface.

  On the other hand, when the average crystal grain size of the recrystallized structure is 200 μm or less and the average aspect ratio of the crystal grains is 5 or less, intergranular corrosion is suppressed under the use environment as the rear side rail, and pitting corrosion and overall corrosion occur. To occur. In the case of these pitting corrosion and overall corrosion, the crystal grains on the surface described above are unlikely to float and fall off, the corrosion weight loss is much less than intergranular corrosion, and the original surface remains.

  In the present invention, uniform and fine equiaxed crystals satisfying the average crystal grain size and the crystal grain aspect ratio in the extrusion direction of the extruded shape, in other words, in the longitudinal direction of the rear side rail. A recrystallized structure consisting of

  There is a recrystallized structure in which the average crystal grain size exceeds 200 μm or the average aspect ratio L / ST of the crystal grains exceeds 5 even partially in the extrusion direction of the extruded shape and the longitudinal direction of the rear side rail. In the use environment, the processing environment, and the raw material environment, there is a high possibility that intergranular corrosion occurs from the portions.

(Production method)
The manufacturing method of the extruded profile of the present invention will be described below. The extruded profile of the present invention can be produced by a conventional extrusion process, but the recrystallized structure after extrusion is not coarsened, and the average of the aspect ratio L / ST of the crystal grains is 5 or less. In particular, conditions such as the temperature until hot extrusion are controlled as follows.

  The homogenization heat treatment (soaking) conditions are a temperature range of 450 to 550 ° C. and a time of less than 4 hours or more than 24 hours. If the soaking temperature is less than 450 ° C., homogenization is sufficient. Also, the soaking temperature need not exceed 550 ° C. However, when the soaking time in this temperature range is 4 to 24 hours, precipitates of transition elements such as Mn and Zr contained as impurities may be generated, and the aspect ratio of the recrystallized structure may increase during extrusion. Is expensive.

  The extrusion temperature is a relatively high temperature exceeding 500 ° C. When the extrusion temperature is less than 500 ° C., the aspect ratio of the crystal grains of the recrystallized structure is likely to increase during the extrusion.

  Under the preferred conditions as described above, an aluminum alloy containing Mg and Si within the above composition range is melted and cast, the ingot is soaked (homogenized), hot-extruded, and press-quenched (extruded) Immediately after quenching, the microstructure of the extruded profile can be made into the recrystallized structure in the extrusion direction and the longitudinal direction of the extruded profile after satisfying the proof stress. .

Examples of the aluminum alloy extruded profile according to the present invention will be described below.
An Al—Mg—Si-based aluminum alloy having chemical components shown in Table 1 is melted by a conventional method, cast into a billet having a diameter of 200 mm by semi-continuous casting, and the billet is subjected to homogenization heat treatment, extrusion start temperature, and extrusion shown in Table 2. Under the speed condition, a rectangular hollow material having a side of 50 mm × 50 mm and a thickness of 4 mm was hot-extruded, subjected to water-cooled press quenching immediately after extrusion, and further subjected to artificial aging treatment after cutting to a product length of 3 m.

Various characteristics of these product rectangular hollow materials were evaluated as shown below.
Five test pieces were sampled from the product rectangular hollow shape at intervals of 50 mm along the longitudinal direction, and the microstructure of the thickness center portion was measured. More specifically, the average crystal grain size on the surface of each electrolytically etched specimen was sequentially observed using a 100 × optical microscope, and measured in the direction of the hollow shape L by the line intercept method. And each average crystal grain diameter of five test pieces was further averaged. Note that the length of one measurement line was 0.95 mm and the total measurement line length was 0.95 x 15 mm by observing a total of five fields with three lines per field.

  At the same time, the aspect ratio L / ST, which is the ratio of the length L in the extrusion direction of the extruded shape of the individual crystal grains in the field of view to the length ST in the thickness direction of the extruded shape, is obtained. Divide by number and average.

  A specimen was taken from the product rectangular hollow shape, and 0.2% yield strength was measured by a tensile test. The tensile test was performed according to JIS Z 2201, the shape of the test piece was a JIS No. 5 test piece, the crosshead speed was 5 mm / min, and the test piece was run at a constant speed until the test piece broke. These results are shown in Table 2. The Al alloy numbers in Table 2 correspond to the Al alloy numbers in Table 1.

  The intergranular corrosion test was performed by collecting five C-ring-shaped test pieces at intervals of 50 mm from the product rectangular hollow shape. The test condition was that the C-ring test piece was compliant with the ASTM G47 alternate dipping method. However, the test condition is that the tensile stress is applied to the test piece or the residual stress is applied and used as a rear side rail, and the test condition is 0 in the vertical direction of the C ring of the C ring test piece. .2% proof stress 75% stress was applied. In this state, the immersion of the C-ring test piece in 5% concentration salt water and the pulling were repeated for 60 days, and then the presence or absence of intergranular corrosion of the test piece was confirmed. The confirmation was made by observing the cross-sectional structure of the test piece with a microscope and examining the corrosion state as to whether or not the grain boundary corrosion was developed along the crystal grain boundary from the test piece surface to the inside.

  Evaluation is evaluated as x when the intergranular corrosion has occurred even in one of the five test pieces and the corrosion resistance is not excellent over the longitudinal direction of the rectangular product. did. And, when no intergranular corrosion has occurred in all of the five test pieces, and the entire surface corrosion or pitting corrosion has occurred, the corrosion resistance is excellent over the longitudinal direction of the product rectangular hollow shape material. And judged as “good”. These results are also shown in Table 2.

  As can be seen from Table 2, the alloys A, B, and C in Table 2 satisfying the composition of the present invention were used, and the microstructure consisted of a recrystallized structure. Inventive Examples 1 to 5 having an average aspect ratio L / ST of 5 or less are excellent in intergranular corrosion resistance even under severe use environments. The 0.2% proof stress is 200 MPa or more. This result shows that the aluminum alloy extruded shape for the rear side rail in the track chassis frame is practical.

  On the other hand, using the alloys A and B in Table 2 satisfying the composition of the present invention, even if the microstructure is a recrystallized structure, the average crystal grain size exceeds 200 μm, or the aspect ratio of the crystal grains Comparative Examples 6 to 9 having an L / ST average exceeding 5 are inferior in intergranular corrosion resistance. Further, the 0.2% proof stress is as low as less than 200 MPa.

  In Table 1, Comparative Examples 10 to 12, each using an alloy of D with Zn exceeding 0.2%, E with Cu exceeding 0.2%, and F with Fe exceeding 0.2%, Inferior to intergranular corrosion.

According to the present invention, it is possible to provide a 6000 series aluminum alloy extruded shape for a rear side rail in a track chassis frame having excellent intergranular corrosion resistance even under severe use environment, processing environment and raw material environment. . Therefore, according to the present invention, it is possible to reduce the weight of the truck by expanding the application of the aluminum alloy material to the chassis frame of a truck such as a cab-over vehicle.

Claims (1)

An aluminum alloy extruded shape for a rear side rail in a truck chassis frame, which contains Mg: 0.4-1.2%, Si: 0.2-1.0% by mass, Fe, Cu, Mn, Cr, Zn, Zr, and Ti are each independently 0.2% or less, and the total amount of these is also regulated to 1.0% or less, and Al—Mg—Si based aluminum composed of the balance Al and inevitable impurities It is made of an alloy, 0.2% proof stress is 200 MPa or more, wall thickness is 3 to 6 mm, the microstructure is a recrystallized structure, and the average crystal grain size of this recrystallized structure is 200 μm or less. Aluminum alloy extruded form for rear side rails with excellent corrosion resistance, characterized in that the average of the aspect ratio L / ST, which is the ratio of the length L in the direction and the length ST in the thickness direction of the extruded shape, is 5 or less Wood.