JP6996700B2 - Metal processing method - Google Patents

Description

The present invention relates to a metal processing method.

In recent years, for the purpose of improving fatigue strength, wear resistance, etc. of metal materials, a technique for refining a metal structure using strong strain processing represented by the ECAP method and shot peening has attracted attention (for example, non-patent documents). See 1-3). A metal material having a finely divided metal structure not only has high strength, but also has characteristics different from those of a material having a normal crystal grain size of about several μm in terms of superplastic deformation and corrosion resistance. There is. Further, by miniaturizing the structure, it is possible to improve the strength of the metal material without adding alloying elements, and it is also excellent in recyclability.

Hiroshi Fujiwara, Eiji Oda, Megumi Ameyama, Features of Mechanical Milling Method as Strong Strain Machining Method, Iron and Steel, Vol.94, No.12 (2008), pp.608-615 Minoru Umemoto, Yoshikazu Todaka and Koichi Tsuchiya, Formation of Nanocrystalline Structure in Steels by Air Blast Shot Peening, Materials Transactions, Vol.44, No.7 (2003), pp.1488-1493 Yoshiharu Hotta, Ultra-fine organization of bulk materials and improvement of mechanical properties by ultra-strong processing, Iron and Steel, Vol.94, No.12 (2008), pp.599-607

However, in the processing methods disclosed in these non-patent documents, the processing principle is to introduce a large-scale plastic strain having a true strain of about 4 or more into the material. Therefore, the load on the metal material to be processed is large, and a new processing technology capable of significantly reducing this is required.

The present invention has been made in view of the above circumstances, and provides a metal processing method capable of improving the mechanical properties of a metal material by introducing a fine structure into the metal material while suppressing plastic deformation. With the goal.

The present invention comprises a step of applying a compressive load of less than 200 N to a metal material under the conditions that the load frequency is 50 Hz or more and the scanning speed in one direction is 0.1 mm / sec or more. I will provide a. With such a metal processing method, it is possible to introduce a fine structure into a metal material while suppressing plastic deformation. More specifically, by the same method, the metal structure near the surface of the metal material can be miniaturized.

In the present invention, it is preferable to apply the above-mentioned compressive load at least 1.5 × 10 ⁴ times / mm ² . This makes it possible to further promote the miniaturization of the metal structure.

In the present invention, the scanning speed in the other direction perpendicular to the above one direction may be 0.01 mm / sec or more. This facilitates the introduction of flat crystal grains, which will be described later.

According to the present invention, it is possible to provide a metal processing method capable of improving the mechanical properties of a metal material by introducing a fine structure into the metal material while suppressing plastic deformation.

It is a schematic diagram which shows the Scanning Cyclic Press apparatus. It is a scanning electron microscope (SEM) photograph which shows the cross-sectional structure of the S25C test piece. It is an external view which shows the shape of a test piece. It is an optical image of the appearance of S25C-AH-1. It is a laser microscope image of the center of the reforming part of S25C-AH-1. It is _a graph which shows the arithmetic mean roughness Ra before and after processing of the S25C test piece. It is a graph which shows the decrease amount of the diameter of the parallel part of the S25C test piece. It is a scanning ion microscope (SIM) image of the cross-sectional structure just below the surface layer in the center of the S25C test piece. It is a SIM image of the vertical cross section of the modified part of S25C-AH-1. It is a SIM image of the cross section of the modified part of S25C-AH-1. It is a SIM image of the vertical cross section of the modified part of S25C-AH-2. It is a SIM image of the cross section of the modified part of S25C-AH-2. It is a SIM image of the vertical cross section of the modified part of S25C-AL-1. It is a SIM image of the cross section of the modified part of S25C-AL-1. It is a SIM image of the vertical cross section of the modified part of S25C-NH-1. It is a SIM image of the cross section of the modified part of S25C-NH-1. It is the SN diagram of the S25C-AH-1 and the raw S25C test piece obtained by the fatigue test. FIG. 3 is an SN diagram of AZ31-AL-1 and an unprocessed AZ31 test piece obtained by a fatigue test.

Hereinafter, embodiments of the present invention will be described in detail. The present invention is not limited to the following embodiments.

In the metal processing method of the present embodiment, a step of applying a compressive load of less than 200 N to a metal material under the conditions that the load frequency is 50 Hz or more and the scanning speed in one direction is 0.1 mm / sec or more is added. To prepare. As a result, the metal structure near the surface of the metal material can be miniaturized.

The metal material to which the metal processing method of the present embodiment can be applied is not particularly limited. If an indenter that is harder than the metal to be processed can be prepared as an indenter for applying a compressive load, in principle any metal material can be processed, including carbon steel, non-ferrous metal aluminum alloys, titanium alloys, magnesium alloys, etc. Can be.

In the present embodiment, since it is necessary to refine the metal structure near the surface of the metal material while suppressing the plastic deformation of the metal material, the compressive load is less than 200 N, but preferably less than 150 N, preferably less than 100 N. Is more preferable. The lower limit of the compressive load is not particularly limited, but can be at least about 20 N from the viewpoint of appropriately performing miniaturization. When applying a compressive load to a metal material, for example, an indenter that is harder than the target metal material and has a spherical tip can be used.

The load frequency (vibration frequency of the indenter) when the compressive load is applied is 50 Hz or higher, preferably 100 Hz or higher, and more preferably 200 Hz or higher. When the load frequency is 50 Hz or more, a finer region can be more preferably formed in the metal structure, and the processing time can be shortened. The upper limit of the load frequency is not particularly limited, but may be 50 kHz. An electro-hydraulic servo actuator, an electromagnetic vibration actuator, a piezoelectric actuator, or the like can be used to realize a predetermined load frequency.

The scanning speed in one direction when a compressive load is applied is 0.1 mm / sec or more, preferably 0.2 mm / sec or more, and more preferably 0.4 mm / sec or more. When the scanning speed in one direction is 0.1 mm / sec or more, it is possible to more preferably form a miniaturized region in the metal structure. The upper limit of the scanning speed in one direction is not particularly limited, but if it is 100 mm / sec or less, damage due to friction between the indenter and the metal material can be reduced.

When applying the compressive load, scanning may be performed in the other direction (in-plane perpendicular to one direction) perpendicular to the above one direction. The scanning speed in the other direction can be 0.01 mm / sec or more. When the scanning speed in the other direction is 0.01 mm / sec or more, it becomes easy to introduce the flat crystal grains described later. The upper limit of the scanning speed in the other direction is not particularly limited, but may be 100 mm / sec.

According to the findings of the inventors, the scanning speed while the indenter is in contact with the metal surface (for example, the moving speed of the indenter in the direction perpendicular to the load-bearing direction) has a great influence on the deformation of crystal grains and the like. Therefore, it is extremely important to adjust the load frequency and the scanning speed within the above range when applying the compressive load in order to modify the target structure. When applying the compressive load, the indenter may be moved, the metal material may be moved, or the indenter and the metal material may be relatively moved as long as a desired scanning speed can be obtained.

The shape of the indenter (shape of the tip of the indenter) is not particularly limited, but a spherical surface is preferable from the viewpoint of applying a stable compressive load. Moreover, as such a spherical surface, a spherical surface having a radius of curvature of about 3 mm or more can be mentioned.

In the present embodiment, it is preferable to apply the above compressive load to the metal material at least 1.5 × 10 ⁴ times / mm ² by repeating the above steps. That is, it is preferable that the compression load by the indenter is applied 1.5 × 10 ⁴ times per predetermined unit area (mm ² ) of the metal material under the conditions of the load frequency and the scanning speed. By repeating the scan by the indenter, the desired number of repetitions can be achieved. When the number of repetitions is 1.5 × 10 ⁴ times / mm ² or more, the miniaturization of the metal structure can be further promoted. From this viewpoint, the number of repetitions is preferably 1.5 × 10 ⁵ times / mm ² or more, and more preferably 6.0 × 10 ⁵ times / mm ² or more. On the other hand, the upper limit of the number of repetitions is not particularly limited, but can be set to about 1.0 × ¹⁰⁸ times / mm ² from the viewpoint of saturation of miniaturization and shortening of processing time.

The atmosphere in which the above steps are carried out can be appropriately selected according to the size of the target crystal grains and the element species to be incorporated into the metal structure. The atmosphere is not particularly limited, but the process can be carried out in an atmospheric atmosphere, a nitrogen atmosphere, an argon atmosphere, or the like. By performing processing in an air atmosphere or a nitrogen atmosphere, a unique layer (transition layer) containing a metal atom derived from the metal material to be processed and an oxygen atom or a nitrogen atom is introduced into the surface of the metal material. be able to. More specifically, the layer is introduced on the miniaturized region (region closer to the surface) of the metal material. The thickness of the layer can be adjusted according to the load conditions of the compressive load and the like, but is generally 0.5 to 2 μm. For example, when carbon steel is used as the metal material, the transition layer includes a layer composed of Fe ₂ O ₃ and a layer composed of ζ-Fe ₂ N.

In the present embodiment, the miniaturization of the metal structure is recognized in the vicinity of the surface of the metal material. Here, the vicinity of the surface is assumed to be a region up to a depth of about 50 μm with respect to the surface of the metal material. It is not limited to that.

In the present embodiment, miniaturization means that coarse crystal grains (for example, having a particle size of about 10 to several tens of μm) before applying compressive stress are changed to crystal grains having a fine particle size. The grain size of the finely divided crystal grains is smaller on the surface of the metal material and gradually increases toward the inside of the metal material, so that it cannot be said unconditionally. However, in the case of the metal processing method of the present embodiment, it is approximately 0. A metal structure having fine crystal grains having a size of 5 to 1 μm or less can be formed in the vicinity of the surface of the metal material.

The miniaturized state of the metal structure can be confirmed by observing the cross section of the metal material with, for example, a scanning electron microscope (SEM) or a scanning ion microscope (SIM).

The metal material having a miniaturized structure processed by the processing method of the present embodiment exhibits at least extremely excellent fatigue strength and wear resistance as compared with those before processing. Therefore, it is possible to provide a metal material useful in the fields of aircraft, automobiles and the like.

Hereinafter, the present invention will be specifically described with reference to examples, but the present invention is not limited to the following description.

1. 1. Scanning Cyclic Press Device FIG. 1 is a schematic diagram showing a Scanning Cyclic Press (SCP) device. As shown in FIG. 1, in this apparatus, the columnar test piece 3 provided between the indenter 1 and the reaction force receiver 2 is subjected to rotation and axial feed (Scanning) while indenting. By vibrating 1 perpendicularly to the axial direction, a compression load is repeatedly applied to the surface of the test piece (Cyclic Press). In the SCP method, the microstructure A is introduced into the surface layer of the test piece 3 by repeatedly applying a relatively small compressive load to the metal surface for a long period of time. The shape of the tip of the indenter (contact surface with the test piece) was a spherical surface with a radius of curvature of 7 mm. A cemented carbide harder than the test piece was used for the tip of the indenter. In this example, the SCP method was carried out using an apparatus as shown in FIG. 1, but the embodiment of the metal processing method of the present invention is not limited to those using the apparatus.

2. 2. Test pieces The following two types of test pieces (metal materials) were prepared.
a) Low carbon steel S25C annealed material The chemical composition is C: 0.24, Si: 0.19, Mn: 0.42, P: 0.010, S: 0.019, Cu: 0.01, Ni: 0.02 and Cr: 0.04 (mass%). In addition, annealing treatment (1123K, holding for 3.6 ks and then cooling in a furnace) was performed before processing. The Rockwell hardness (B scale) after the annealing treatment was 62 HRB, and the Micro Vickers hardness (10 g) was 139 HV. FIG. 2 is a scanning electron microscope (SEM) photograph showing the cross-sectional structure of the S25C test piece. The gray structure in the figure is the ferrite phase (α), and the white and gray layered structure is the pearlite phase (P).
b) Magnesium alloy AZ31
The chemical composition is Al: 3.0, Zn: 1.1, Mn: 0.31, Si: 0.007, Fe: 0.002, Cu: 0.001, Ni: 0.001 (mass%). there were. The billet temperature was 673 K, and the product was extruded at a speed of 5 m / min.

FIG. 3 is an external view showing the shape of the test piece. All of the above test pieces have a parallel portion at the center (S25C-AH-1 and AZ31-AL-1 described later are ± 1 mm from the center, and S25C-AH-2, S25C-AL-1 and S25C-NH-1 are from the center. It was an hourglass type with a diameter of ± 3 mm) and a diameter of about 4 mm. S25C was obtained by machining finish by turning. AZ31 was obtained by machining by turning, mirror finishing (polishing with emery paper (# 600 to # 2000), and buffing with alumina abrasive grains (particle size 1 μm)).

3. 3. Machining with the SCP device Machining with the SCP device was carried out by changing the machining conditions as shown in Table 1. The processing temperature was set to room temperature (20 to 25 ° C.), and the processing region was set to the entire parallel portion of the test piece and a part of the R portion (± 4 mm from the center). The "rotation speed" in the table is the scanning speed in one direction, and 2 rpm in this embodiment is 0.419 mm / sec. In the table, the "feed speed" is the scanning speed in the other direction perpendicular to one direction.

4. Evaluation results 4.1 Observation results of each test piece after processing by the SCP method FIG. 4 is an optical image of the appearance of S25C-AH-1. On the surface of the test piece after processing, a region having a slightly dark luster as shown in the figure was observed. Hereinafter, this region is referred to as a refined part. In order to investigate the surface condition of the modified part in detail, high-magnification images were acquired and the surface roughness was measured before and after processing. A color 3D laser scanning microscope (VK-9700 / 9710 GenerationII, KEYENCE) was used for these measurements. FIG. 5 is a laser microscope image of the center of the reforming portion. From the figure, streaks formed at a pitch of about 300 μm were observed in the modified portion. The pitch of the streaks coincides with the feed amount of 300 μm per rotation of the test piece, which is calculated from the rotation speed and the feed rate in the SCP method.

FIG. 6 is _a graph showing the arithmetic mean roughness Ra (JIS B0601-2001) before and after processing. The measurement region of the arithmetic mean roughness _Ra was set to the range of 820 μm × 60 μm at the center of the parallel portion. Further, FIG. 7 is a graph showing the amount of decrease in the diameter of the parallel portion measured by the laser displacement meter. From these figures, the deformation of the test piece by the SCP method can be regarded as a minute deformation in the category of plastic working. Therefore, in this method of applying a low load repeatedly compression load, large plastic deformation was suppressed.

Similarly, large plastic deformation was suppressed in the other test pieces not shown in the graph.

4.2 Tissue observation Using a focused ion beam processing / observation device, the observation cross section just below the surface layer of the modified part was created and the structure was observed. FIG. 8 is a scanning ion microscope (SIM) image of the cross-sectional structure just below the surface layer in the center of the S25C test piece. The black layer at the top of the figure is a carbon protective film vapor-deposited to protect the surface. From the figure, a ferrite phase and a pearlite phase having a particle size of several tens of μm can be confirmed in the unprocessed structure.

The observation results of each test piece will be described in detail below. As a whole, the crystal grain size in the microstructure could be made smaller, and the microstructure could be formed in a deeper region. Further, in the SCP method, flat crystal grains could be formed in the fine structure. It is presumed that the flat crystal grains can improve the mechanical properties of the metal material from the viewpoint of increasing the fatigue crack growth resistance. The Cyclic Press method referred to here means a processing method in which a compression load is repeatedly applied by an indenter to the surface of a test piece fixed without scanning.

Results of S25C-AH-1 FIG. 9 is a SIM image of a vertical cross section of the modified portion of S25C-AH-1, that is, a cross section parallel to the longitudinal direction of the test piece. As shown in the figure, miniaturization of the tissue was confirmed over the entire observation field of view just below the surface layer. Further, a nanofine layer having nano-order crystal grains was formed to a depth of about 1 μm from the surface. In addition, it was observed that the crystal grain size of the fine structure gradually became coarser from the surface of the test piece toward the inside. Compared with the result by the CP method, the crystal grain size was smaller and the fine structure was formed in a deeper region.

FIG. 10 is a SIM image of a cross section of the modified portion of S25C-AH-1, that is, a cross section perpendicular to the longitudinal direction of the test piece. As shown in the figure, microstructures and nanomicrolayers were observed in the outermost layer in the entire observation field just below the surface layer. On the other hand, a structure having flat crystal grains was formed inside at a depth of about 3 μm from the surface. Since these flat crystal grains are formed along the direction of rotation of the test piece (scanning direction of the indenter), it is presumed that the crystal grains were deformed by the friction between the surface of the test piece and the indenter. Will be done.

Results of S25C-AH-2 FIG. 11 is a SIM image of a vertical cross section of the modified portion of S25C-AH-2. In S25C-AH-1, the crystal grains gradually increase as the depth increases, but in S25C-AH-2, the size of the crystal grains remains as small as the surface layer even in the deep inner region. rice field.

FIG. 12 is a SIM image of a cross section of the modified portion of S25C-AH-2. As shown in the figure, flat crystal grains that seemed to be stretched in the direction of rotation of the test piece were observed. The flat crystal grains are shorter than those of S25C-AH-1. It is presumed that by increasing the number of repetitions, the miniaturization could proceed to a deeper region.

Result of S25C-AL-1 FIG. 13 is a SIM image of a vertical cross section of a modified portion of S25C-AL-1. As shown in the figure, microstructures and nanomicrolayers were observed in the outermost layer in the entire observation field just below the surface layer. However, the particle size, especially in the microstructure, was larger than that of S25C-AH-1. Moreover, the region to be miniaturized was shallower than that of S25C-AH-1.

FIG. 14 is a SIM image of a cross section of the modified portion of S25C-AL-1. Flat crystal grains were observed, but their thickness was larger than that of S25C-AH-1. It is presumed that the degree of miniaturization could be reduced by reducing the load.

Result of S25C-NH-1 FIG. 15 is a SIM image of a vertical cross section of a modified portion of S25C-NH-1. Even in nitrogen, a nano-miniaturized layer just below the surface layer was observed, and it was observed that the particle size tended to gradually increase as it became deeper.

FIG. 16 is a SIM image of a cross section of the modified portion of S25C-NH-1. Flat crystal grains were formed as in S25C-AH-1. Miniaturization progressed even in nitrogen, and the degree of miniaturization was similar to that under the same load conditions in the atmosphere.

4.3 Surface hardness The hardness of the modified part of S25C-AH-2 was measured by a Micro Vickers hardness tester (measured load: 10 g, measured at 11 points). The average hardness was 361 HV. Since the hardness of the S25C test piece before processing was 139 HV, the hardness of the test piece (modified portion) was increased about 2.5 times by the SCP method.

4.4 Corrosion resistance S25C-AH-2 and an unprocessed S25C test piece were prepared and covered with a modified silicone-based filler except for the range of 6 mm (± 3 mm from the center) in the center of the parallel portion of the test piece. These were immersed in a 3.5% NaCl aqueous solution for 4 days. Then, the mass change before and after immersion was measured using an electronic balance. As a result, the mass loss of S25C-AH-2 was 0.0120 g, while the mass loss of the raw S25C test piece was 0.1257 g. Surface modification by the SCP method greatly improved the corrosion resistance of the test piece.

4.5 Fatigue characteristics Results of S25C-AH-1 Fatigue tests were performed on S25C-AH-1 and raw S25C test pieces using an axial load hydraulic servo fatigue tester (load: stress ratio R = -1, Repeat frequency: 30Hz sine wave). In order to suppress the heat generation of the test piece, a fatigue test was performed while the test piece was forcibly air-cooled using compressed air from a compressor. The surface temperature of the test piece during the fatigue test was maintained at room temperature.

FIG. 17 is an SN diagram of the S25C-AH-1 and the raw S25C test piece obtained by the fatigue test. The arrow pointing to the right in the figure indicates that the test piece did not break even when the number of load repetitions exceeded ¹⁰⁷ times. From the figure, the life of S25C-AH-1 (marked with ●) was improved to 2 to 50 times as much as that of the unprocessed S25C test piece (marked with ○). It is presumed that the nano-fine layer formed on the outermost layer of the test piece suppressed the occurrence of cracks.

Results of AZ31-AL-1 Fatigue tests were performed on AZ31-AL-1 and raw AZ31 test pieces in the same manner as above (load: stress ratio R = -1, repetition frequency: 120 Hz sine wave. ). FIG. 18 is an SN diagram of the AZ31-AL-1 and the raw AZ31 test piece obtained by the fatigue test. From the figure, the life of AZ31-AL-1 (marked with ●) is improved to about 16 to 100 times as compared with the unprocessed AZ31 test piece (marked with ○).

1 ... indenter, 2 ... reaction force receiving, 3 ... test piece, A ... microstructure.

Claims (4)

For metal materials, using an indenter with a spherical tip, a compressive load of less than 200 N, under conditions where the load frequency is 50 Hz or higher and the scanning speed in one direction is 0.1 mm / sec or higher. , At least 1.5 × 10 ⁴ times / mm ² A metal processing method comprising a step of adding. The metal processing method according to claim 1, wherein the scanning speed in the other direction perpendicular to the one direction is 0.01 mm / sec or more. The metal processing method according to claim 1 or 2, wherein the compression load is 20 N or more. The metal processing method according to any one of claims 1 to 3, wherein the step is carried out in a nitrogen atmosphere.

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