Inclusion evaluation method and test piece manufacturing method

Abstract

To provide an inclusion evaluation method capable of making a grip part hard to break in a breaking test even when there is almost no change in diameter between the grip part and an effective part.SOLUTION: Provided is an inclusion evaluation method in which hydrogen is charged to a test piece 1 made of a metallic material that gripping portions 3 at both ends and an effective portion 5 provided in an intermediate portion 2 between the gripping portions 3, a destructive test is performed on the hydrogen-charged test piece 1, and the dimensions of a non-metallic inclusion 6 that is the origin of the fracture are measured. The test piece 1 is cut out from a wire and has an outer surface 7 that is formed by an outer surface of the wire, and in the intermediate portion 2, a surface layer 8 is removed from the outer surface 7, and masking 11 is applied to at least the outer surface 7 at the gripping portions 3 to suppress hydrogen penetration, and at least a part of the intermediate portion 2 from which the surface layer 8 has been removed is made into the effective portion 5 without performing the masking 11.SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] The present invention relates to an inclusion evaluation method for evaluating non-metallic inclusions contained in a metallic material and a method for manufacturing a test piece used therein. [Background technology]

[0002] It is known that non-metallic inclusions contained in metallic materials are the starting point for fatigue fracture, and it is therefore important to evaluate non-metallic inclusions.

[0003] A conventional inclusion evaluation method involves performing a tensile test as a destructive test on a test piece made of a metal material into which hydrogen has penetrated, as described in Patent Document 1. In this inclusion evaluation method, nonmetallic inclusions that are the origin of fracture are identified by the tensile test and their dimensions are measured for evaluation.

[0004] In such a conventional inclusion evaluation method, hydrogen penetration makes it easier for fractures to occur originating from non-metallic inclusions during tensile testing, allowing for rapid evaluation of non-metallic inclusions while ensuring stable evaluation.

[0005] In some of these inclusion evaluation methods, as in Patent Document 2, a test piece is cut out from a wire rod, which is a metallic material, and has an outer surface made of the outer surface of the wire rod, thereby obtaining position information from the outer surface of the wire rod of the non-metallic inclusion that is the origin of the fracture.

[0006] However, this test piece had the problem that there was almost no change in diameter between the gripping parts at both ends and the effective part located in the middle between the gripping parts where fracture was expected, making it more likely to fracture at the gripping parts than a typical dumbbell-shaped test piece. [Prior art documents] [Patent documents]

[0007] [Patent Document 1] Patent Publication No. 2009-65789 [Patent Document 2] Patent No. 7408007 Summary of the Invention [Problem to be solved by the invention]

[0008] The problem to be solved is that when there is almost no change in diameter between the gripping portion and the effective portion, breakage is likely to occur at the gripping portion. [Means for solving the problem]

[0009] The present invention provides an inclusion evaluation method in which hydrogen is allowed to penetrate into a test piece made of a metallic material having gripping portions at both ends and an effective portion provided in the middle portion between the gripping portions, a destructive test is performed on the test piece into which hydrogen has penetrated, and the dimensions of the non-metallic inclusion that is the origin of the fracture are measured, wherein the test piece is cut out from a wire made of a metallic material and has an outer surface formed by the outer surface of the wire, the surface layer is removed from the outer surface in at least a part of the middle portion, masking is performed on the outer surface of at least the gripping portion to suppress the penetration of the hydrogen, and the part of the middle portion from which at least the surface layer has been removed is treated as the effective portion without being masked.

[0010] The present invention also provides a method for manufacturing a test piece made of a metallic material, which has gripping portions at both ends and an effective portion provided in the middle between the gripping portions, and which is subjected to a destructive test after hydrogen has penetrated into it, the method comprising: cutting out a wire made of a metallic material and having an outer surface formed by the outer surface of the wire; at least a part of the intermediate portion has a surface layer removed from the outer surface; masking is performed on the outer surface of at least the gripping portion to suppress the penetration of hydrogen; and the part of the intermediate portion from which at least the surface layer has been removed is made the effective portion without being masked. [Effects of the Invention]

[0011] The present invention can make it difficult for breakage to occur at the gripping portion in a breakage test, even when there is almost no change in diameter between the gripping portion and the effective portion. [Brief explanation of the drawings]

[0012] [Figure 1] FIG. 1 is a side view schematically showing a test piece used in an inclusion evaluation method according to an embodiment of the present invention. [Figure 2] 2(A) and (B) are cross-sectional views of the test piece of FIG. 1, with FIG. 2(A) showing the effective portion and FIG. 2(B) showing the grip portion. [Figure 3] FIG. 3 is a conceptual diagram showing hydrogen charging to a test piece according to an example. [Figure 4] FIG. 4 is a conceptual diagram showing a tensile test on a test piece according to an example. [Figure 5] FIG. 5 is a conceptual extreme value statistical graph of the dimensions of non-metallic inclusions according to the example. [Figure 6] FIG. 6 is a chart showing the hydrogen concentrations of the test pieces in the examples and comparative examples. [Figure 7] FIG. 7 is a table showing the results of tensile tests carried out on the test pieces of the examples and comparative examples. [Figure 8] FIG. 8(A) is an electron microscope photograph showing a part of the fracture surface of the test piece according to the example, and FIG. 8(B) is an enlarged view of region VIII in FIG. 8(A). DETAILED DESCRIPTION OF THE INVENTION

[0013] Even when there is almost no change in diameter between the gripping portion and the effective portion, the object of making it difficult for fracture to occur at the gripping portion in the fracture test is realized by using a test piece in which at least the outer surface of the gripping portion is masked and the surface layer 8 is removed from the outer surface of the effective portion between the gripping portions.

[0014] That is, in the inclusion evaluation method, hydrogen is allowed to penetrate into a test piece 1 made of a metallic material that has gripping portions 3 at both ends and an effective portion 5 provided in the middle portion 2 between these gripping portions 3, a destructive test is performed on the test piece 1 into which hydrogen has penetrated, and the dimensions of the non-metallic inclusion 6 that is the starting point of the fracture are measured.

[0015] The test piece 1 is cut out from a wire made of a metal material and has an outer surface 7 formed by the outer surface of the wire, and a surface layer 8 is removed from the outer surface 7 in at least a part of the intermediate portion 2, and masking 11 is applied to the outer surface 7 in at least the gripping portion 3 to suppress the penetration of hydrogen, and at least the part of the intermediate portion 2 from which the surface layer 8 has been removed is left unmasked as the effective portion 5.

[0016] The surface layer 8 can be removed by any suitable method, but in one embodiment it can be removed by cutting. It is preferable to polish the cut portion.

[0017] The surface roughness Ra of the outer surface 7 of the effective portion 5 can be set appropriately. <Ra<6.2μmとしてもよい。

[0018] The test piece 1 can have any appropriate cross-sectional shape in a direction intersecting the axial direction, but in one embodiment, the cross-sectional shape may be circular.

[0019] The removal of the surface layer 8 is carried out over the entire intermediate portion 2, and the test piece 1 has a step 9 between the gripping portion 3 and the intermediate portion 2, and the masking 11 may extend from the gripping portions 3 at both ends over the step 9 to part of the intermediate portion 2.

[0020] In one embodiment, the step 9 may be 2.5% or less of the outer diameter of the gripping portion.

[0021] In this case, the step 9 may be 0.3 mm or less.

[0022] The masking 11 may be any suitable material as long as it can prevent hydrogen from penetrating, and in one embodiment, it may be a resin coating. [Example]

[0023] [Inclusion evaluation method and test piece manufacturing method] Fig. 1 is a side view schematically showing a test piece used in an inclusion evaluation method according to an embodiment of the present invention. Fig. 2(A) and (B) are cross-sectional views of the test piece shown in Fig. 1, with Fig. 2(A) showing the effective portion and Fig. 2(B) showing the grip portion.

[0024] In the inclusion evaluation method of this embodiment, hydrogen is made to penetrate into the test piece 1 made of a metallic material shown in Figures 1 and 2, as shown in Figure 3. Hereinafter, the act of making hydrogen penetrate is referred to as "hydrogen charging."

[0025] The test piece 1 was cut out from a wire rod of spring steel, for example, SAE 9254, which is a metal material. Note that the material of the test piece 1 can also be other metal materials.

[0026] This test piece 1 is in the shape of a round bar according to the shape of the wire, and has gripping portions 3 at both ends in the axial direction and an effective portion 5 provided in the intermediate portion 2 between the gripping portions 3. The gripping portions 3 refer to the end regions other than the intermediate portion 2, and do not refer only to the portion that is actually gripped during destructive testing. The dimensions of the test piece 1 are, for example, an axial length of approximately 150 mm to 250 mm, a diameter of approximately 9 mm to 12.5 mm, a length of the gripping portions 3 of approximately 50 mm to 95 mm, and a gauge length of approximately 50 mm to 150 mm. The size of the test piece 1 is not limited to these.

[0027] The cross-sectional shape of the test piece 1 perpendicular to the axial direction is circular, but may be other shapes. Also, the axis of the test piece 1 is linear, but may be partially or entirely curved.

[0028] The outer surface 7 of test piece 1 is constituted by the outer surface of the wire rod. However, in the intermediate portion 2, the surface layer 8 has been removed. The removal of the surface layer 8 may be performed in a part of the intermediate portion 2, but in this embodiment, it has been removed over the entire intermediate portion 2. A part of the intermediate portion 2 may be at least a portion that becomes the effective portion 5. The surface layer 8 refers to a portion that can promote hydrogen charging to the test piece 1 more than before removal by being removed. The surface layer 8 of this embodiment is an oxide film. The removal of the surface layer 8 is performed by cutting the outer surface 7 in the intermediate portion 2. The outer surface 7 in the intermediate portion 2 of this embodiment is polished after cutting. Thereby, the surface roughness Ra of the outer surface 7 in the intermediate portion 2 is set to be less than 6.2 μm. More preferably, the surface roughness Ra of the outer surface 7 in the intermediate portion 2 is set such that 1 μm < Ra < 6.2 μm.

[0029] The outer diameter of the intermediate portion 2 has decreased due to cutting and polishing. This decrease amount is, for example, 0.3 mm in this embodiment, which is within the tolerance range of a general test piece. Therefore, the test piece 1 has a straight bar shape with almost no change in diameter from the gripping portion 3 to the intermediate portion 2.

[0030] Also, since the decrease amount of the outer diameter of the intermediate portion 2 is within the tolerance range of a general test piece, even if cutting or polishing is performed within that range, the outer surface 7 in the intermediate portion 2 can be regarded as being substantially constituted by the outer surface of the wire rod.

[0031] Moreover, the surface layer 8 of the intermediate portion 2 removed by cutting and polishing becomes a region where breakage due to non-metallic inclusions 6 (see FIG. 4) is unlikely to occur, for example, when the wire rod is shot-peened before being used as a product. Therefore, even if cutting or polishing is performed within the range of such a surface layer 8, the influence on the test piece 1 is low. From this perspective as well, the outer surface 7 in the intermediate portion 2 of this embodiment can be regarded as being substantially constituted by the outer surface of the wire rod.

[0032] On the other hand, in the gripping portion 3, the outer surface 7 is not cut or polished, and the surface layer 8 is not removed. In other words, in the gripping portion 3, the outer surface 7 is formed by the outer surface of the wire itself. Note that the surface layer 8 of the outer surface 7 may also be removed in the gripping portion 3.

[0033] Between the gripping portion 3 and the intermediate portion 2, there is a step 9 corresponding to the reduction in the outer diameter of the intermediate portion 2. The dimension of the step 9 is equal to the amount of reduction in the outer diameter of the intermediate portion 2, and is, for example, 0.3 mm. The dimension of this step 9 is set to 2.5% or less of the outer diameter of the gripping portion 3.

[0034] The test piece 1 has a masking 11 in the gripping portion 3 to suppress the penetration of hydrogen. In this embodiment, the masking 11 is provided from the gripping portions 3 at both ends, over the step 9, and over a portion of the middle portion 2. In this embodiment, the portion of the middle portion 2 is within a range of 5 mm from the step 9. This portion of the middle portion 2 can be set appropriately as long as it does not affect the destructive test. The portion of the middle portion 2 where the masking 11 is not provided is the effective portion 5 where the surface layer has been removed from the outer surface 7 and is expected to be destroyed in the destructive test.

[0035] Since masking 11 is intended to prevent hydrogen from penetrating into gripping portion 3, it is preferable that it be formed including step 9 as described above to cover the entire gripping portion 3. However, masking 11 may also be provided only on gripping portion 3. In this case, the entire intermediate portion 2 becomes effective portion 5.

[0036] The thickness of the masking 11 is, for example, about 1 μm to 10 mm. Any suitable material that is impermeable to hydrogen can be used for the masking 11, for example, a resin coating. The resin coating can be paint, adhesive tape, or the like. In this embodiment, the masking 11 is made of an acrylic coating, a rubber coating, kraft paper tape, or the like.

[0037] Acrylic coatings are primarily made of acrylic resin and are formed, for example, by acrylic spraying. Rubber coatings are made of natural rubber, synthetic natural rubber, butadiene rubber, butadiene styrene rubber, butyl rubber, etc. Kraft paper tapes have a surface laminated with synthetic resin such as polyethylene and an adhesive on the back.

[0038] The test piece 1 may be set to various hardnesses by heat treatment. The heat treatment is performed before removing the surface layer 8 of the intermediate portion 2 and before forming the masking 11. As the heat treatment, tempering, annealing, normalizing, quenching, or the like is appropriately adopted depending on the metal material used for the test piece 1.

[0039] In this example, the heat treatment is, for example, quenching and tempering. By this quenching and tempering, the hardness of the test piece 1 is set to HV 300 to 700. For example, the quenching is performed at about 850 degrees for about 10 minutes, and the tempering is performed at about 700 degrees for about 30 minutes. However, the details of the quenching and tempering, such as the temperature and time, may be appropriately set depending on the size, material, hardness after heat treatment, etc. of the test piece 1.

[0040] In the inclusion evaluation method of this embodiment, hydrogen is charged to the test piece 1 provided with the masking 11 as described above, as shown in FIG.

[0041] Hydrogen charging is performed, for example, by immersing the test piece 1 for a predetermined time in a hydrogen charging solution 4. For example, the test piece 1 is immersed in a 20 mass % ammonium thiocyanate aqueous solution at 50° C. for 48 hours.

[0042] The hydrogen charging method is not limited to this, and includes, for example, a method of exposing the test piece 1 to hydrogen gas, or a method of applying a current while immersing it in an electrolyte such as an aqueous solution of sodium chloride and ammonium thiocyanate or an aqueous solution of sulfuric acid and arsenous acid.

[0043] During this hydrogen charging, the masking 11 prevents hydrogen from penetrating into the gripping portion 3, while promoting hydrogen penetration into the effective portion 5, which is the portion of the intermediate portion 2 from which the oxide film has been removed and which is not covered by the masking 11. As a result, the gripping portion 3 is less susceptible to fracture, while the effective portion 5 is more susceptible to fracture starting from the non-metallic inclusions 6, making the gripping portion 3 even more resistant to fracture.

[0044] In this embodiment, hydrogen is also prevented from penetrating into the step 9 between the gripping portion 3 and the middle portion 2. This more reliably prevents hydrogen from penetrating into the gripping portion 3 and into the boundary between the gripping portion 3 and the middle portion 2, making the gripping portion 3 even more resistant to fracture.

[0045] The hydrogen-charged test specimen 1 is subjected to a tensile test as a destructive test. This tensile test causes fractures in the test specimen 1, originating from the nonmetallic inclusions 6 in the effective portion 5. Note that other destructive tests such as fatigue tests and impact tests may be performed instead of the tensile test. Furthermore, when hydrogen charging is performed on the test specimen 1, the destructive test is preferably performed after hydrogen charging, but may also be performed during hydrogen charging.

[0046] FIG. 4 is a conceptual diagram showing a tensile test on the test piece 1.

[0047] In the tensile test of this example, the gripped portion 3 of the test piece 1 is held by a chuck 13 and pulled at a tensile speed of 20 mm / min, causing fracture to occur between the gauge points of the effective portion 5 of the test piece 1, with the non-metallic inclusion 6 as the initiation point.

[0048] The fracture originating from a non-metallic inclusion 6 refers to a fracture in which the non-metallic inclusion 6, which is the fracture origin, is exposed on the fracture surface 15 of the test piece 1. The gripping position of the chuck 13 can be set arbitrarily. For example, the end of the chuck 13 can be aligned with the step 9 or spaced apart from the step 9.

[0049] After the tensile test, the type of nonmetallic inclusion 6 that caused the fracture is identified. In this example, the nonmetallic inclusion 6 is Al-Ca-Si-Mg-O based. However, the type of nonmetallic inclusion 6 varies depending on the metallic material.

[0050] Here, identification means specifying with a certain degree of certainty the type of nonmetallic inclusion 6. Therefore, in addition to direct identification by detecting the components of the nonmetallic inclusions 6, indirect identification is also possible.

[0051] In the case of indirect identification, for example, a fatigue test may be performed in advance on a test piece 1 made from the same type of metallic material without hydrogen charging, the type of nonmetallic inclusion that initiated the fracture may be identified, and the nonmetallic inclusion 6 determined by the inclusion evaluation method of this embodiment may be estimated to be the same type as the nonmetallic inclusion in the fatigue test.

[0052] Furthermore, when applying the inclusion evaluation method to multiple test specimens 1 of the same type, the components of non-metallic inclusions 6 may be detected in some of the test specimens 1, and for the remaining test specimens 1, it may be assumed that the non-metallic inclusions 6 that were the origin of fracture are of the same type as the non-metallic inclusions 6 in some of the test specimens 1 whose components were detected.

[0053] Furthermore, when applying the inclusion evaluation method to multiple test specimens 1 of the same type, the distribution line 17 described below is determined, and the components of non-metallic inclusions 6 are detected in some of the test specimens 1. If the non-metallic inclusions 6 are located within the confidence interval of the distribution line 17, it may be assumed that the non-metallic inclusions in the remaining test specimens 1 are of the same type as the non-metallic inclusions 6 in some of the test specimens 1 whose components have been detected.

[0054] Before or after, or instead of, such identification, the dimensions of the nonmetallic inclusions 6 are measured. In this example, the dimensions of the nonmetallic inclusions 6 are measured by observing the fracture surface using an electron microscope and measuring the major axis, minor axis, and equivalent circle diameter.

[0055] The dimension of the nonmetallic inclusion 6 refers to the equivalent circle diameter of the nonmetallic inclusion 1. The equivalent circle diameter is the diameter of a circle having the same area as the nonmetallic inclusion 1. The dimension of the nonmetallic inclusion 6 may be expressed as the major axis, minor axis, or an average diameter of these, instead of the equivalent circle diameter.

[0056] At this time, the nonmetallic inclusions 6 may be made visible by dry etching or the like. The making visible means removing the material covering the nonmetallic inclusions 6 by utilizing the difference in etching rate (etching speed difference) between the material covering the nonmetallic inclusions 6. This makes the nonmetallic inclusions 6 visible in the test piece 1, thereby reducing variability in the measurement of the dimensions of the nonmetallic inclusions 6 and improving the stability of the evaluation of the nonmetallic inclusions 6.

[0057] In addition, the position of the nonmetallic inclusion 6 in the test specimen 1, which was the fracture initiation point, from the outer surface 7 of the test specimen 1 is measured. This position is obtained as a distance in the radial direction. Here, since the outer surface 7 of the test specimen 1 is the outer surface of the wire, in this example, positional information of the nonmetallic inclusion 6 from the outer surface of the wire can be obtained.

[0058] In the evaluation of this embodiment, a distribution function of the dimensions of the measured non-metallic inclusions 6 is further calculated, and the cleanliness of the metallic material is evaluated using this distribution function. Specifically, a distribution line is calculated as the distribution function using extreme value statistics.

[0059] Note that, when determining the distribution function, destructive testing is performed on a plurality of test specimens 1, and the dimensions of the nonmetallic inclusions 6 that initiated the fracture are measured in advance. Then, as shown in Figure 5, an extreme value statistical graph is generated in which the dimensions of the nonmetallic inclusions 6 that initiated the fracture are plotted, with the vertical axis representing the cumulative probability and the horizontal axis representing the equivalent circle diameter of the largest inclusion. Note that Figure 5 only conceptually illustrates the extreme value statistical graph.

[0060] Based on this extreme value statistical graph, a distribution line 17 can be obtained as a regression line. Using this distribution line 17, the size of the largest non-metallic inclusion 6 in the metallic material can be predicted. In other words, the cleanliness of the metallic material can be evaluated. The cleanliness refers to the degree of non-metallic inclusion 6 contained in the metallic material. In this embodiment, the cleanliness is determined by the size of the largest non-metallic inclusion 6 in the metallic material.

[0061] In this way, the inclusion evaluation method of this embodiment allows accurate prediction of the largest non-metallic inclusion 6, similar to the fatigue test.

[0062] [Experimental Results] FIG. 6 is a chart showing the hydrogen concentrations of the test pieces in the examples and comparative examples.

[0063] In FIG. 6, Example 1-1 is the test piece 1 of this example, and the masking 11 of the acrylic coating film is formed from the gripping portion 3 through the step 9 to a part of the intermediate portion 2, and the surface roughness of the effective portion 5 is 1 μm. <Ra<6.2μmとしている。

[0064] In Examples 1-2 and 1-3, the material of the masking 11 in Example 1-1 was changed to a rubber coating and a kraft paper tape, respectively. In Example 1-4, the range of the masking 11 in Example 1-1 was changed, and the masking 11 was formed only on the gripping portion 3 so as not to cover the step 9.

[0065] In the comparative example, the masking 11 is omitted from Example 1-1.

[0066] 6, the hydrogen concentration is shown as a result of measurement in the effective portion 5 and the grip portion 3. The hydrogen concentration was analyzed by taking out 10 mm long analysis pieces from the effective portion 5 and the grip portion 3, and heating each analysis piece from room temperature to 300°C in a thermal desorption apparatus (JTF-20A, manufactured by J Science), and integrating the amount of hydrogen released.

[0067] 6, in Examples 1-1 to 1-3, the hydrogen concentration in gripping portion 3 was significantly lower than the hydrogen concentration in effective portion 5, being kept to 0.1 ppm or less. In Example 1-4, the hydrogen concentration in gripping portion 3 was higher than in Examples 1-1 to 1-3, but was kept significantly lower than the hydrogen concentration in effective portion 5. On the other hand, in the comparative example, the hydrogen concentration in effective portion 5 and the hydrogen concentration in gripping portion 3 were equivalent.

[0068] FIG. 7 is a table showing the results of tensile tests carried out on the test pieces of the examples and comparative examples.

[0069] The tensile test was conducted on 15 samples prepared for each of Examples 1-1 to 1-7 and the Comparative Example. Examples 1-1 to 1-4 and the Comparative Example have the same configuration as described above. In all of Examples 1-1 to 1-4 and the Comparative Example, the clamping was performed at a position where the end of the chuck 13 was aligned with the step 9.

[0070] In Example 1-5, the gripping position is changed from Example 1-1, and the end of the chuck 13 is gripped at a position 25 mm away from the step 9. In Examples 1-6 and 1-7, the surface roughness of the effective portion 5 is changed from Example 1-1, with polishing omitted in Example 1-6 and surface roughness Ra<1 μm in Example 1-7. The gripping positions in Examples 1-6 and 1-7 are the same as in Example 1-1.

[0071] In FIG. 7, ◯ indicates fracture at the effective portion 5, Δ indicates fracture at the step 9, and × indicates fracture at the grip portion 3.

[0072] As is clear from FIG. 7, in Examples 1-1 to 1-3 and 1-5 to 1-7, all of the 15 samples broke at the effective portion 5. In Example 1-4, one sample broke at the step 9, and the remaining 14 samples broke at the effective portion 5. On the other hand, in the comparative example, five samples broke at the step 9, and the remaining 10 samples broke at the grip portion 3. In the comparative example, no sample broke at the effective portion 5.

[0073] FIG. 8(A) is an electron microscope photograph showing a part of the fracture surface of the test piece according to the example, and FIG. 8(B) is an enlarged view of region VIII in FIG. 8(A).

[0074] In all of the samples of Examples 1-1 to 1-7 that were fractured at the effective portion 5, nonmetallic inclusions 6 as shown in FIG.

[0075] As described above, in the inclusion evaluation method of this embodiment, hydrogen is charged to a test piece 1 made of a metallic material having gripping portions 3 at both ends and an effective portion 5 provided in the intermediate portion 2 between these gripping portions 3, and a destructive test is performed on the hydrogen-charged test piece 1 to measure the dimensions of the non-metallic inclusion 6 that was the origin of the fracture.

[0076] The test piece 1 to be hydrogen charged has an outer surface 7 cut out from a wire and formed by the outer surface of the wire, and a surface layer 8 is removed from the outer surface 7 in the intermediate portion 2, and masking 11 is applied to the outer surface 7 in the gripping portion 3 to suppress hydrogen penetration, and at least a portion of the intermediate portion 2 from which the surface layer 8 has been removed is left unmasked as the effective portion 5.

[0077] Therefore, in this embodiment, the masking 11 prevents hydrogen from penetrating into the gripping portion 3, while promoting hydrogen penetration into the effective portion 5 from which the oxide film of the surface layer 8 has been removed. In other words, the gripping portion 3 becomes less susceptible to fracture, while the effective portion 5 becomes more susceptible to fracture starting from the non-metallic inclusions 6, making the gripping portion 3 even more resistant to fracture.

[0078] Therefore, even when there is almost no change in diameter between the gripped portion 3 and the effective portion 5, fracture is unlikely to occur at the gripped portion 3. As a result, fracture can be reliably caused at the effective portion 5, allowing for reliable evaluation of the nonmetallic inclusions 6.

[0079] Furthermore, in this embodiment, since the oxide film is removed by cutting and the cut portion is polished, the outer surface 7 after cutting can be made flat, thereby suppressing fracture from this portion during the tensile test. Therefore, in this embodiment, the non-metallic inclusion 6 can be surely made the starting point of fracture, and the evaluation of the non-metallic inclusion 6 can be performed more surely.

[0080] Since the surface roughness of the effective portion 5 is 1 < Ra < 6.2 μm, it is easy for hydrogen to penetrate into the effective portion 5, and fracture from the cut outer surface 7 can be suppressed.

[0081] The removal of the surface layer 8 is performed over the entire intermediate portion 2. The test piece 1 has a step 9 between the gripping portion 3 and the intermediate portion 2, and the masking 11 extends from the gripping portion 3 beyond the step 9 over a part of the intermediate portion 2.

[0082] Therefore, in this embodiment, the penetration of hydrogen into the gripping portion 3 and the step 9 is surely suppressed, making the gripping portion 3 more difficult to fracture and also suppressing fracture between the gripping portion 3 and the intermediate portion 2. As a result, fracture can be surely caused in the effective portion 5 more surely, and the evaluation of the non-metallic inclusion 6 can be performed.

[0083] Note that if the region of the intermediate portion 2 adjacent to the effective portion 5 in the axial direction has the surface layer 8 removed and is masked 11 and has no step with respect to the effective portion 5, the same effect can be obtained. In this case, the step 9 may be formed between the said region of the intermediate portion 2 and another region, and in addition, may be formed between the said other region of the intermediate portion 2 and the gripping portion 3. When the said other region of the intermediate portion 2 has the same diameter as the gripping portion 3, it is also possible to consider it as the gripping portion 3.

Explanation of Reference Numerals

[0084] 1 Test piece 2 Intermediate portion 3 Gripping portion 5 Effective portion 6 Non-metallic inclusion 7 Outer surface 9 Step 11 Masking

Claims

1. Hydrogen is allowed to penetrate into a test piece made of a metal material having gripping portions at both ends and an effective portion provided in the middle between the gripping portions, A destructive test is performed on the test piece into which hydrogen has penetrated, An inclusion evaluation method for measuring the size of the non-metallic inclusion that is the origin of the fracture, comprising: The test piece is cut out from a wire made of a metal material and has an outer surface formed by the outer surface of the wire, a surface layer is removed from the outer surface in at least a part of the middle portion, masking is performed on the outer surface of at least the gripping portion to suppress the penetration of hydrogen, and the part of the middle portion from which at least the surface layer has been removed is left unmasked and is used as the effective portion. Inclusion evaluation method.

2. The inclusion evaluation method of claim 1, The removal of the surface layer is carried out by cutting. Inclusion evaluation method.

3. The inclusion evaluation method according to claim 2, grinding the cut portion; Inclusion evaluation method.

4. The inclusion evaluation method according to any one of claims 1 to 3, The surface roughness Ra of the outer surface of the effective portion is 1 μm<Ra<6.2 μm. Inclusion evaluation method.

5. The inclusion evaluation method according to any one of claims 1 to 3, The test piece has a circular cross-sectional shape in a direction intersecting the axial direction. Inclusion evaluation method.

6. The inclusion evaluation method according to claim 2 or 3, The removal of the surface layer is performed over the entire intermediate portion, the test piece has a step between the gripping portion and the intermediate portion due to removal of the surface layer, The masking extends from the gripping portions at both ends across the steps to a part of the middle portion. Inclusion evaluation method.

7. The inclusion evaluation method according to claim 6, The step is 2.5% or less of the outer diameter of the gripping portion. Inclusion evaluation method.

8. The inclusion evaluation method according to claim 7, The step is 0.3 mm or less. Inclusion evaluation method.

9. The inclusion evaluation method according to any one of claims 1 to 3, The masking is a resin coating. Inclusion evaluation method.

10. A method for manufacturing a test piece made of a metallic material, which has gripping portions at both ends and an effective portion provided in an intermediate portion between the gripping portions, and which is subjected to a destructive test after hydrogen penetration, comprising: The outer surface is formed by cutting out a wire made of a metal material and the outer surface of the wire is the outer surface of the wire, a surface layer is removed from the outer surface in at least a portion of the intermediate portion; a masking layer is formed on the outer surface of at least the gripping portion to suppress the penetration of hydrogen; the part of the intermediate portion from which at least the surface layer has been removed is not subjected to the masking and is made the effective portion; Test specimen manufacturing method.

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