Inclusion evaluation method

Abstract

To provide an inclusion evaluation method capable of suppressing variation in measurement of dimensions of non-metallic inclusions, to improve stability of evaluation of the non-metallic inclusions.SOLUTION: An inclusion evaluation method comprises: performing destructive test on a test piece 1 made of a metallic material to cause a fracture in the test piece 1 starting from a non-metallic inclusion 3; performing dry etching on a fracture surface 9 resulting from the destructive test to make the non-metallic inclusion 3 that was the origin of the fracture on the fracture surface 9 apparent due to the difference in etching rate with a material covering the non-metallic inclusions 3; and measuring the dimensions of the non-metallic inclusion 3 that was the origin of the fracture.SELECTED DRAWING: Figure 5

Description

[Technical Field]

[0001] The present invention relates to an inclusion evaluation method for evaluating inclusions contained in a metallic material. [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] However, during destructive testing, the non-metallic inclusion that caused the fracture to begin may be covered by impurities or the base material of the test piece, which causes variation in the measurement of the non-metallic inclusion's dimensions and limits the stability of the evaluation. [Prior art documents] [Patent documents]

[0006] [Patent Document 1] Japanese Patent Application Laid-Open No. 2009-65789 Summary of the Invention [Problem to be solved by the invention]

[0007] The problem to be solved is that there is variation in the measurement of the dimensions of non-metallic inclusions, which limits the stability of the evaluation. [Means for solving the problem]

[0008] The present invention is a method for carrying out a destructive test on a test piece made of a metallic material, causing a fracture in the test piece originating from a non-metallic inclusion, dry etching the fracture surface obtained by the destructive test, and detecting the non-metallic inclusion that was the origin of the fracture on the fracture surface. and Due to the difference in etching rate between the non-metallic inclusion and the material covering it, The material is removed to remove the non-metallic inclusions. The present invention provides an inclusion evaluation method for making non-metallic inclusions that have become apparent and have become the origins of the fractures and measuring the dimensions of the non-metallic inclusions. [Effects of the Invention]

[0009] The present invention can suppress variations in the measurement of the dimensions of non-metallic inclusions and improve the stability of evaluation. [Brief explanation of the drawings]

[0010] [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] FIG. 2 is a graph showing the relationship between the tempering temperature and the hardness of the test piece according to the example. [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 schematic diagram showing dry etching of the test piece according to the example. [Figure 6] 6(A) and (B) are electron microscope photographs showing an example of the appearance of nonmetallic inclusions according to the example. [Figure 7] 7(A) and (B) are electron microscope photographs showing other examples of the manifestation of nonmetallic inclusions according to the example. [Figure 8] FIG. 8 is a conceptual extreme value statistical graph of the dimensions of non-metallic inclusions according to the example. DETAILED DESCRIPTION OF THE INVENTION

[0011] The present invention achieves the objectives of suppressing variation in the measurement of the dimensions of non-metallic inclusions and improving the reliability of evaluation of non-metallic inclusions by dry etching the fracture surface of a destructive test of a metallic test piece and utilizing the difference in etching rate to make the non-metallic inclusions apparent.

[0012] That is, the inclusion evaluation method involves performing a destructive test on a test piece 1 made of a metallic material, causing a fracture in the test piece 1 starting from a non-metallic inclusion 3, dry-etching a fracture surface 9 resulting from the destructive test, revealing the non-metallic inclusion 3 that is the fracture starting point on the fracture surface 9 due to the difference in etching rate between this non-metallic inclusion 3 and the material 11 that covers it, and measuring the dimensions of the non-metallic inclusion 3 that is the fracture starting point.

[0013] Various types of dry etching can be applied, but ion milling, for example, may also be used.

[0014] In ion milling, the test piece 1 is irradiated with an ion beam 14 while rotating the test piece 1 around a rotation axis that intersects with the fracture surface 9. The irradiation angle of the ion beam 14 with respect to the rotation axis 13 is preferably 30 degrees or less. In this case, the irradiation angle may be 0 degrees.

[0015] In the inclusion evaluation method, hydrogen may be made to penetrate into the test piece 1, and a destructive test may be performed on the test piece 1 into which hydrogen has penetrated.

[0016] Alternatively, the test piece 1 may be cut out from a wire rod and have an unprocessed outer surface 3 formed by the outer surface of the wire rod, and the position of the non-metallic inclusion 3 that was the origin of the fracture from the outer surface of the test piece 1 may be measured. [Example]

[0017] 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.

[0018] In the inclusion evaluation method, a destructive test is first performed on a test piece 1 made of a metallic material, and a fracture is caused in the test piece 1 starting from a non-metallic inclusion 3 (Fig. 4).

[0019] The test piece 1 of this example is cut out from a wire rod of spring steel, e.g., SAE9254, as a metallic material. This test piece 1 is a round bar with a circular cross section corresponding to the shape of the wire rod, and the outer peripheral surface 3a is an unmachined surface that is the outer peripheral surface of the wire rod itself. Both sides of the test piece 1 in the axial direction constitute gripping portions 5. The axial direction refers to the direction along the axis of the test piece 1.

[0020] In this example, the axial length of the test piece 1 is 150 mm, the diameter of the test piece 1 is 9.8 mm, and the gauge length and the axial length of the gripping portion 5 are each 50 mm.

[0021] However, the shape and size of the test piece 1 are not limited to this. For example, a JIS No. 4 test piece may be used. Furthermore, the metal material may be other than spring steel.

[0022] The test piece 1 is set to various hardnesses by heat treatment. 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.

[0023] 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 HV280. 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.

[0024] 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. In this example, the hardness of the test piece 1 can be set to a value other than HV280 by adjusting the tempering temperature.

[0025] FIG. 2 is a graph showing the relationship between the tempering temperature and the hardness of the test piece.

[0026] As shown in Figure 2, when test piece 1 is tempered at 400°C, 455°C, 580°C, and 700°C, the hardness of test piece 3 can be HV600, HV500, HV370, and HV280, respectively. The tempering time is approximately 30 minutes at each temperature. Note that the straight lines in Figure 2 are approximate straight lines.

[0027] After the heat treatment, hydrogen is allowed to penetrate into the test piece 1. Hereinafter, the act of allowing hydrogen to penetrate is referred to as "hydrogen charging." Note that the hydrogen charging may be omitted.

[0028] FIG. 3 is a conceptual diagram showing hydrogen charging to the test piece 1.

[0029] Hydrogen charging is performed by, for example, immersing the test piece 1 in a hydrogen charging solution 7 for a predetermined time, as shown in Figure 3. For example, the test piece 1 is immersed in a 20 mass% ammonium thiocyanate aqueous solution at 50°C for 48 hours.

[0030] 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.

[0031] Alternatively, the metal material may be hydrogen-charged before forming the test piece 1. In this case, the metal material before hydrogen charging is set to the same hardness as the test piece 1 by quenching and tempering.

[0032] The hydrogen-charged test specimen 1 is subjected to a tensile test as a destructive test. This tensile test causes fractures to occur in the test specimen 1, with the fractures originating from the non-metallic inclusions 3. 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.

[0033] FIG. 4 is a conceptual diagram showing a tensile test on the test piece 1.

[0034] In the tensile test of this example, the test piece 1 was held on both sides and pulled at a tension rate of 20 mm / min to cause fracture originating from a nonmetallic inclusion 3 between the gauge marks of the tensile test piece 1. Note that fracture originating from a nonmetallic inclusion 3 refers to fracture in which the nonmetallic inclusion 3, which is the fracture origin, is exposed on the fracture surface 9 of the test piece 1.

[0035] In this example, the hardness of the test specimen 1 before hydrogen charging is less than HV400, so the load on the testing machine (not shown) is smaller than when the hardness before hydrogen charging is HV400 or more. As a result, tests can be performed more simply with a smaller testing machine than when the hardness before hydrogen charging is HV400 or more, or the testing machine can be protected. In particular, in this example, the hardness of the test specimen 1 before hydrogen charging is HV280, so the load on the testing machine is small.

[0036] In addition, when a test piece 1 having a hardness of less than HV400 contains non-metallic inclusions 3 of 10 μm or larger, heat treatment and hydrogen charging make it more likely that fracture will occur in a tensile test, originating from the largest non-metallic inclusion 3 among the non-metallic inclusions 3 of 10 μm or larger.

[0037] Therefore, when a tensile test is performed on a test piece 1 having a hardness of less than HV400, it is preferable to heat treat the test piece 1 containing nonmetallic inclusions 3 of 10 μm or more to reduce the hardness to less than HV400, hydrogen-charge the test piece 1 having a hardness of less than HV400, and then perform a destructive test on the hydrogen-charged test piece 1.

[0038] There is no upper limit to the size of the nonmetallic inclusions 3 of 10 μm or more, but even if the size is, for example, 500 μm, fracture is likely to occur starting from the largest nonmetallic inclusion 3 in the test piece 1.

[0039] However, in this example, even if the largest non-metallic inclusion 3 contained in the test specimen 1 is less than 10 μm, it is possible to cause fracture originating from the non-metallic inclusion 3 in the tensile test of the test specimen 1.

[0040] The dimension of the nonmetallic inclusion 3 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 3 may be expressed as the major axis, minor axis, or an average diameter of these, instead of the equivalent circle diameter.

[0041] After the tensile test, the fracture surface 9 of the test piece 1 caused by the tensile test is dry-etched, and the non-metallic inclusion 3 that was the origin of the fracture on the fracture surface 9 is made visible by the difference in etching rate between this non-metallic inclusion 3 and the material 11 covering it.

[0042] The substance 11 covering the nonmetallic inclusion 3 is the base material (metal material) due to ductile deformation of the test piece 1 or impurities that have adhered to the nonmetallic inclusion 3.

[0043] When the hardness of the test specimen 1 is HV280 or less as in this example, the base material substance 11 is likely to cover the nonmetallic inclusions 3 in a large amount when hydrogen charging and tensile testing are performed on the test specimen 1. However, even if the test specimen 1 has a different hardness, the substance 11 can also serve as the base material. Impurities are substances that adhere to the test specimen 1 during fracture, and refer to substances other than the base material of the test specimen 1. When a fatigue test is performed as a destructive test, the substance 11 as an impurity is likely to adhere to the nonmetallic inclusions 3.

[0044] The etching rate difference refers to the difference in etching speed. The visualization refers to the visualization of the contours of the nonmetallic inclusions 3 on the fracture surface 9. This visualization can be achieved by removing the material 11 covering the nonmetallic inclusions 3, so that the contours of the nonmetallic inclusions 3 are larger than before the removal.

[0045] FIG. 5 is a conceptual diagram showing dry etching of the test piece 1. As shown in FIG.

[0046] The dry etching in this embodiment is performed by ion milling. However, the dry etching can also be achieved by various techniques other than ion milling, such as ion etching, sputter etching, and plasma etching.

[0047] 5, during ion milling, the test piece 1 is rotated around a rotation axis 13 that intersects with the fracture surface 9 while being irradiated with an ion beam 14. The irradiation angle of the ion beam 14 with respect to the rotation axis 13 is 30 degrees or less, and preferably 0 degrees.

[0048] The irradiation angle in this embodiment refers to the angle between the central axis 15 of the ion beam 14 and a reference axis 16 parallel to the rotation axis 13. However, the irradiation angle may also be the angle between the rotation axis 13 itself and the central axis 15.

[0049] If the irradiation angle is about 30 degrees, the non-metallic inclusions 3 can be efficiently exposed due to the difference in etching rate. However, if the irradiation angle is smaller than 30 degrees, particularly 0 degree, the ion beam 14 can be reliably irradiated onto the non-metallic inclusions 3 from directly above, and the non-metallic inclusions 3 can be more reliably exposed due to the difference in etching rate.

[0050] The test piece 1 can be rotated by supporting the test piece 1 on a rotor (not shown) and driving the rotor with an electric motor or the like. The rotation speed of the test piece 1 in this embodiment is about 25 rpm. The ion beam 14 is irradiated onto the fracture surface 9 of the test piece 1 by an ion gun 17. The ion beam 14 in this embodiment is, for example, an argon ion beam, and its voltage is about 6 kV. The irradiation time of the ion beam 14 in this embodiment (the time for performing dry etching) is about 4 to 8 hours.

[0051] The rotation speed of the test piece 1, the type of ion beam 14, the voltage, the irradiation time, etc. can be set appropriately depending on the difference in etching rate between the non-metallic inclusion 3 and the material 11 covering it, etc.

[0052] 6(A) and (B) are electron microscope photographs showing an example of the appearance of nonmetallic inclusions 3. In the example of FIGS. 6(A) and (B), substances 11 covering the nonmetallic inclusions 3 are impurities.

[0053] 6(A) and (B) show the state of the impurity substance 11 before and after dry etching, observed using an electron microscope (SEM). Before dry etching, the nonmetallic inclusions 3 were covered with substance 11, as shown in FIG. 6(A), and after dry etching, substance 11 was removed, as shown by the two-dot chain line in FIG. 6(B).

[0054] As a result, the original boundary between the non-metallic inclusion 3 and the base material of the test piece 1 is clearly defined. When the substance 11 is an impurity, the dry etching is performed for approximately 8 hours.

[0055] 7(A) and (B) are electron microscope photographs showing other examples of the appearance of nonmetallic inclusions 3. In the examples of Fig. 7(A) and (B), the substance 11 covering the nonmetallic inclusions 3 is the base material resulting from ductile deformation of the test specimen 1.

[0056] 7(A) and (B), similar to FIGS. 6(A) and (B), the state of substance 11, which is the base material of test piece 1, was observed using an electron microscope before and after dry etching. Before dry etching, substance 11 covered nonmetallic inclusions 3, as shown in FIG. 7(A), and after dry etching, substance 11 was removed, as shown in FIG. 7(B).

[0057] As a result, the buried portions of the nonmetallic inclusions 3 are exposed, and the original boundary between the nonmetallic inclusions 3 and the base material of the test piece 1 is clearly defined. When the substance 11 is the base material of the test piece 1, the dry etching is performed for approximately four hours.

[0058] Therefore, it is possible to determine whether the substance 11 is an impurity or the base material of the test piece 1 based on the duration of the dry etching.

[0059] After dry etching, the type of nonmetallic inclusion 3 that caused the fracture is identified. In this example, the nonmetallic inclusion 3 is an Al-Ca-Si-Mg-O inclusion. However, the type of nonmetallic inclusion 3 varies depending on the metal material.

[0060] Here, identification means specifying with a certain degree of certainty the type of nonmetallic inclusion 3. Therefore, in addition to direct identification by detecting the components of the nonmetallic inclusion 3, indirect identification is also possible.

[0061] 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 3 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.

[0062] Furthermore, when applying the inclusion evaluation method to multiple test specimens 1 of the same type, the components of non-metallic inclusions 3 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 3 that were the origin of fracture are of the same type as the non-metallic inclusions 3 in some of the test specimens 1 whose components were detected.

[0063] Furthermore, when applying the inclusion evaluation method to multiple test specimens 1 of the same type, the distribution line 19 described below is determined, and the components of non-metallic inclusions 3 are detected in some of the test specimens 1. If the non-metallic inclusions 3 are located within the confidence interval of the distribution line 19, 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 3 in some of the test specimens 1 whose components have been detected.

[0064] Furthermore, when the dimensions of the non-metallic inclusions 3 are a predetermined size, such as 10 μm or more, they may be assumed to be the same type of non-metallic inclusions 3.

[0065] Before or after, or instead of, such identification, the dimensions of the nonmetallic inclusions 3 are measured. In this example, the dimensions of the nonmetallic inclusions 3 are measured by observing the fracture surface using an electron microscope and measuring the major axis, minor axis, and equivalent circle diameter.

[0066] In this example, the largest non-metallic inclusion 3 in the test specimen 1 is revealed by removing the substance 11, which reduces variation in the measurement of the size of the non-metallic inclusion 3 and improves the stability of the evaluation of the non-metallic inclusion 3.

[0067] In this example, the position of the largest nonmetallic inclusion 3 in the test specimen 1, which was the fracture initiation point, from the outer peripheral surface 3a of the test specimen 1 is measured. This position is obtained as a distance in the radial direction. Here, since the outer peripheral surface 3a of the test specimen 1 is the outer peripheral surface of the wire, this example makes it possible to obtain information on the position of the nonmetallic inclusion 3 from the outer peripheral surface of the wire. This position information cannot be obtained by conventional methods of processing materials into test specimen shapes conforming to various standards.

[0068] In the evaluation of this embodiment, a distribution function of the dimensions of the measured non-metallic inclusions 3 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.

[0069] 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 3 that initiated the fracture are measured in advance. Then, as shown in Fig. 8, an extreme value statistical graph is generated in which the dimensions of the nonmetallic inclusions 3 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 Fig. 8 only conceptually illustrates the extreme value statistical graph.

[0070] Based on this extreme value statistical graph, a distribution line 19 can be obtained as a regression line. Using this distribution line 19, the size of the largest non-metallic inclusion 3 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 3 contained in the metallic material. In this embodiment, the cleanliness is determined by the size of the largest non-metallic inclusion 3 in the metallic material.

[0071] In this way, the inclusion evaluation method of this embodiment, like the fatigue test, allows for accurate prediction of the largest nonmetallic inclusion 3. In particular, since the size of the nonmetallic inclusions 3 can be measured with high accuracy by revealing the nonmetallic inclusions 3, the accuracy of the cleanliness evaluation based on this can also be improved. [Explanation of symbols]

[0072] 1 test piece 3 Non-metallic inclusions 9 Fracture Surface 11 Substance 13 Rotation axis 14 Ion beam

Claims

1. A destructive test is performed on a test piece made of a metallic material, and a fracture originating from a non-metallic inclusion is generated in the test piece, Dry etching is performed on the fracture surface obtained by the destructive test; removing the substance by utilizing the difference in etching rate between the non-metallic inclusion that became the origin of the fracture on the fracture surface and the substance covering the non-metallic inclusion, thereby exposing the non-metallic inclusion; measuring the size of the non-metallic inclusion that is the origin of the fracture; Inclusion evaluation method.

2. The inclusion evaluation method of claim 1, The dry etching is ion milling. Inclusion evaluation method.

3. The inclusion evaluation method according to claim 2, the test piece is irradiated with an ion beam while being rotated around a rotation axis intersecting the fracture surface, and the irradiation angle of the ion beam with respect to the rotation axis is 30 degrees or less. Inclusion evaluation method.

4. The inclusion evaluation method according to claim 3, The irradiation angle is 0 degrees. Inclusion evaluation method.

5. The inclusion evaluation method according to any one of claims 1 to 4, Hydrogen is allowed to penetrate the test piece, The destructive test is performed on the test piece into which hydrogen has penetrated. Inclusion evaluation method.

6. The inclusion evaluation method according to any one of claims 1 to 5, the test piece is cut out from a wire rod and has an unprocessed outer circumferential surface formed by the outer circumferential surface of the wire rod; measuring the position of the non-metallic inclusion, which is the origin of the fracture, from the outer peripheral surface of the test piece; Inclusion evaluation method.

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