Method for inclusion evaluation

Abstract

To provide a method for evaluating inclusions to enlarge an observation surface in a test piece, thereby enabling accurate measurement of the size and the type of nonmetallic inclusions that cause fatigue fracture in a desired steel material in a short time and at low cost.SOLUTION: A method for evaluating an inclusion evaluation includes: performing a tensile test on a test piece obtained by introducing hydrogen into a precursor metal piece having an internal hardness of more than 400 HV and a surface hardness of 250-400 HV, to cause fracture originating from a nonmetallic inclusion affected by the introduced hydrogen, and measuring the size and / or identifying the type of the nonmetallic inclusion acting as the fracture origin.SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] The present invention relates to an inclusion evaluation method. [Background technology]

[0002] Several methods for evaluating inclusions have been proposed.

[0003] For example, in Patent Document 1, a critical volume of 400 mm 3 The document describes a method for measuring the diameter of non-metallic inclusions that are fracture initiation points, for predicting the maximum inclusion diameter in steel materials by ultrasonic fatigue testing, in which a plurality of test specimens as described above are taken, charged with hydrogen, and then stress is applied to each test specimen by ultrasonic vibration to fracture the test specimen, and the diameter of the non-metallic inclusions that are fracture initiation points on the fracture surface of each fractured test specimen is measured.

[0004] For example, Patent Document 2 describes an inclusion evaluation method in which a tensile test is performed on a test piece made of a metal material into which hydrogen has penetrated, and fractures are caused in the test piece with non-metallic inclusions affected by the hydrogen as the fracture origins. Thereafter, the type of non-metallic inclusions that are present in the critical volume of the test piece and that are the fracture origins are identified and their dimensions are measured, a distribution function of the dimensions of the non-metallic inclusions is calculated, and the cleanliness of the metal material is evaluated based on this distribution function.

[0005] For example, Non-Patent Document 1 describes an ultrasonic fatigue testing method. [Prior art documents] [Patent documents]

[0006] [Patent Document 1] Patent Publication No. 2021-60428 [Patent Document 2] Japanese Patent Application Laid-Open No. 2010-217076 [Non-patent literature]

[0007] [Non-Patent Document 1] Yoshiyuki Furuya, Transactions of the Japan Society of Mechanical Engineers, Series A, (2007), 73, pp.957-964 Summary of the Invention [Problem to be solved by the invention]

[0008] One method for evaluating inclusions is the point counting method in JIS G0555 (Appendix 1), but because this method relies on optical microscopic observation of the (2D) cross section of a test piece, it is difficult to evaluate the largest inclusion contained within the steel volume (in 3D).In addition, the number of fields that can be observed is limited by the amount of work required, so evaluation accuracy may be insufficient, especially in materials such as spring steel, which have a high level of cleanliness and a low frequency of large inclusions.

[0009] Although the ultrasonic fatigue testing method (for example, Non-Patent Document 1) results in a larger evaluation volume than the point calculation method, due to the limitations of the ultrasonic output of the ultrasonic fatigue testing machine, even with the use of measures such as hydrogen charging, the evaluation volume is limited to 800 mm 3 Furthermore, because ultrasonic fatigue testing requires that the resonant frequency of the test piece match the excitation frequency of the equipment (usually 20 kHz), the diameter of the test part needs to be a weight for resonance, and the vicinity of the surface of the material cannot be evaluated.

[0010] In recent years, therefore, there has been an inclusion evaluation method (Patent Document 2) that performs a tensile test on a test piece made of a metal material into which hydrogen has penetrated, causes fractures in the test piece originating from non-metallic inclusions affected by the hydrogen, identifies the type of non-metallic inclusion that is present in the risk volume of the test piece and is the origin of the fracture, measures its dimensions, calculates a distribution function of the dimensions of the non-metallic inclusions, and evaluates the cleanliness of the metal material using this distribution function. However, although this method places few restrictions on the test load, if a test specimen is to be produced from the rolled material in order to evaluate the entire cross section of the steel ingot, the test specimen must be thick. This increases the hydrogen charging time and worsens the surface roughness due to corrosion by the electrolyte, making it difficult to evaluate the portion near the surface of the steel ingot before rolling.

[0011] The present invention provides an inclusion evaluation method that can enlarge the observation surface of a test piece, thereby enabling the size and type of non-metallic inclusions that cause fatigue fracture in a desired steel material to be accurately measured in a short time and at low cost. [Means for solving the problem]

[0012] The present invention includes the following (1) to (5). (1) An inclusion evaluation method in which hydrogen is allowed to penetrate into a precursor metal piece having an internal hardness of over 400 HV and a surface hardness of 250 to 400 HV, and a tensile test is performed on the resulting test piece, thereby causing fractures to occur originating from non-metallic inclusions affected by the penetrated hydrogen, and measuring the dimensions and / or identifying the type of the non-metallic inclusions that are the fracture origins. (2) The inclusion evaluation method according to (1) above, wherein the amount of hydrogen contained in the test piece is 0.50 to 6.00 ppm. (3) The precursor metal piece is obtained by heat treating a pre-heat-treatment steel piece and then removing α mm of its surface in the thickness direction, The pre-heat treatment steel billet is obtained by removing β mm from the surface of the test steel material in the thickness direction, The inclusion evaluation method described in (1) or (2) above, wherein the area of ​​the observation surface of the test piece is 62% or more of the area of ​​the cross section of the test steel material, which is wider than the observation surface by a thickness of α + β mm. (4) The inclusion evaluation method according to any one of (1) to (3) above, wherein the cross-sectional diameter of the evaluation portion of the test piece exceeds 10 mm. (5) An inclusion evaluation method described in any one of (1) to (4) above, in which hydrogen is introduced into the precursor metal piece by performing an electrolytic hydrogen charging method using the precursor metal piece as the negative electrode, a platinum electrode as the anode, and a sodium citrate buffer solution with a concentration of 0.01 mol / L or more. [Effects of the Invention]

[0013] According to the present invention, the observation surface of a test piece can be enlarged, and therefore, an inclusion evaluation method can be provided that can accurately measure the size and type of non-metallic inclusions that cause fatigue fracture in a desired steel material in a short time and at low cost. [Brief explanation of the drawings]

[0014] [Figure 1] FIG. 2 is a side view (schematic diagram) of a test piece. [Figure 2] 1 is a flowchart showing a procedure of an embodiment. [Figure 3] This shows an SEM image and a photograph of the appearance of the observation surface when the fracture originated from a non-metallic inclusion. [Figure 4] This shows an SEM image and a photograph of the appearance of the observation surface when the fracture originated in the surface layer. DETAILED DESCRIPTION OF THE INVENTION

[0015] The inclusion evaluation method of the present invention will now be described. The inclusion evaluation method of the present invention is an inclusion evaluation method in which hydrogen is allowed to penetrate into a precursor metal piece having an internal hardness of more than 400 HV and a surface hardness of 250 to 400 HV, and a tensile test is performed on the resulting test piece to generate fractures originating from non-metallic inclusions affected by the penetrated hydrogen, and the dimensions and / or type of the non-metallic inclusions that are the fracture origins are measured.

[0016] In the inclusion evaluation method of the present invention, examples of non-metallic inclusions include SiO2, Al2O3, and the like.

[0017] When a tensile test is performed on a test specimen, the specimen is broken, and the fracture surface (observation surface) is observed using a scanning electron microscope (SEM), it is sometimes possible to discover what is believed to be the largest non-metallic inclusion contained in the test specimen.Theoretically, the test specimen is thought to break at a plane that includes the point where the largest non-metallic inclusion is present, so it is thought that such largest non-metallic inclusion is present on the fracture surface. If nonmetallic inclusions are found on the fracture surface, their shape can be confirmed and their size can be measured.In addition, the type (composition) of the nonmetallic inclusions can be confirmed using energy dispersive X-ray fluorescence spectroscopy (EDX). Furthermore, by conducting tensile tests on a large number of test specimens, collecting data on the size of the largest nonmetallic inclusion that is considered to be contained in the test specimens, and calculating a distribution function, it is possible to estimate from the distribution function, based on extreme value statistics, the size of the largest nonmetallic inclusion that can occur when a steel material is produced under the same production conditions as those used to produce the test specimens.

[0018] Furthermore, the inclusion evaluation method of the present invention allows for a larger area of ​​the observation surface (fracture surface) of the test piece used in the tensile test. Conventionally, inclusion evaluation methods in which a tensile test is performed on a test piece of a metal material into which hydrogen has penetrated, and fracture is initiated in the test piece from non-metallic inclusions affected by hydrogen, the maximum diameter of the metal material was considered to be approximately 7 mm. Making the diameter larger than this would require a longer hydrogen penetration treatment, which would result in damage near the surface of the metal material due to the influence of hydrogen ions, etc., and cause fracture to initiate near the surface during the tensile test, making it difficult for fracture to initiate from inclusions. The inclusion evaluation method of the present invention uses a metallic material with a diameter of more than 7 mm, and is less likely to cause damage to the surface of the metallic material even when subjected to a long-term hydrogen penetration treatment, and fracture can be initiated from a nonmetallic inclusion in a tensile test. Therefore, nonmetallic inclusions can be efficiently found on the fracture surface, and as a result, the size and type of nonmetallic inclusions can be accurately measured in a short time and at low cost.

[0019] <Test steel> In the inclusion evaluation method of the present invention, it is preferable to first prepare a test steel material manufactured under desired manufacturing conditions. The inclusion evaluation method of the present invention uses a specific test piece obtained by infiltrating hydrogen into a precursor metal piece, and the method for producing the precursor metal piece is not limited. However, it is preferable that the precursor metal piece is obtained by processing a test steel material, subjecting the pre-heat-treated steel piece to heat treatment, and then further processing the pre-heat-treated steel piece.

[0020] The type, size, and shape of the test steel material are not particularly limited. The test steel material may be, for example, spring steel, bearing steel, carbon steel, etc. The same applies to the steel piece before heat treatment, precursor metal piece, and test piece described below. However, since the test steel is processed to obtain test pieces, and then the test pieces are subjected to a tensile test, it is preferable that the shape of the test steel is close to the shape of a test piece that can be subjected to a tensile test. Specifically, a steel bar is preferable, and a cylindrical steel bar or a rectangular steel bar is more preferable.

[0021] <Steel before heat treatment> Next, the test steel is preferably processed to obtain a pre-heat-treated steel slab, which preferably has a shape similar to that of the test piece, but slightly larger than the test piece. Specifically, it is preferable to remove the entire surface of the pre-heat-treatment steel billet by a thickness of about 0.1 to 1.0 mm as necessary, so that the billet has the same shape as the test piece to be used for the tensile test. In other words, it is preferable that the pre-heat-treatment steel billet has a shape that is larger than the test piece by a thickness of about 0.1 to 1.0 mm. Note that the precursor metal billet is obtained by removing the entire surface of the pre-heat-treatment steel billet, and the shape of the precursor metal billet is, in principle, the same as the shape of the test piece.

[0022] Here, the pre-heat treatment steel slab is obtained by removing the entire surface of the test steel material by β mm in the thickness direction, where β is preferably 4 mm or less.

[0023] <Precursor metal piece> The resulting pre-heat-treated steel slab is subjected to a heat treatment. The type of heat treatment is not particularly limited as long as the heat treatment can control the surface carbon content of the pre-heat-treated steel slab, and may be, for example, a conventionally known heat treatment. Examples of heat treatments include quenching, gas carburizing, and tempering. Quenching can be performed using a known muffle furnace or gas carburizing furnace, in which the steel slab before heat treatment is held in the furnace for approximately 0.1 to 10 hours in an air atmosphere adjusted to an austenite single phase temperature (A3 temperature) or higher, for example, approximately 850 to 1100°C. Gas carburizing is a process in which a known gas carburizing furnace is used, and acetylene, propane, ethanol, or the like is introduced as a carburizing gas into the furnace adjusted to a temperature above the austenite single phase temperature (A3 temperature), for example, about 850 to 1100°C, and the pre-heat-treated steel billet is heated for about 0.1 to 10 hours. This process allows the carbon monoxide generated by the incomplete combustion of the carburizing gas to be used to control the surface carbon content of the pre-heat-treated steel billet.

[0024] After subjecting the pre-heat-treated steel billet to the heat treatment as described above, it is preferable to process the surface of the pre-heat-treated steel billet to obtain a precursor metal billet, the shape of which is, in principle, the same as that of the test piece. Specifically, it is preferable to remove the entire surface of the pre-heat-treatment steel billet after heat treatment by a thickness of about 0.1 to 1.0 mm to obtain a precursor metal billet. Here, the precursor metal piece is obtained by removing α mm from the entire surface of the pre-heat-treated steel material in the thickness direction. That is, α (mm) is preferably 0.1 to 1.0 mm as described above. Furthermore, α (mm) is more preferably 0.1 to 0.5 mm, and even more preferably 0.1 to 0.2 mm.

[0025] As mentioned above, if the precursor metal piece is obtained by heat treating a pre-heat-treated steel piece and then removing α mm of its surface in the thickness direction, and if the pre-heat-treated steel piece is obtained by removing β mm of its surface in the thickness direction from a test steel material, it is preferable that α + β ≦ 4 (mm) be satisfied. In this case, a very wide area of ​​a particular cross section of the test steel can be evaluated in the test specimen. In addition, the area of ​​the observation surface of the test specimen may be 50% or more, 60% or more, or 62% or more of the cross-sectional area of ​​the test steel material, which is wider by a thickness of α + β mm. When the test steel is cylindrical and the evaluation portion of the test piece is also cylindrical, the cross-sectional area of ​​the evaluation portion perpendicular to the central axis may be 50% or more, 60% or more, or 62% or more of the cross-sectional area of ​​the test steel perpendicular to the central axis.

[0026] The surface roughness (Rz) of the obtained precursor metal piece is preferably 0.01 to 4.0 μm, and more preferably 0.01 to 3.0 μm, where the surface roughness (Rz) is a value measured by the stylus method specified in JIS-B0601.

[0027] The resulting precursor metal piece was cut perpendicular to the central axis at a portion that would later become the evaluation portion of the test piece, and the Vickers hardness was measured for the interior (near the central axis) and the surface (up to 0.05 mm from the outer edge). The Vickers hardness was measured four times with a test force of 300 gf, and the average value was used. In this case, the internal hardness is more than 400 HV, preferably more than 400 HV and not more than 500 HV, and more preferably 430 to 470 HV. The surface layer has a hardness (surface layer hardness) of 250 to 400 HV, preferably 270 to 390 HV, and more preferably 350 to 390 HV. Thus, in the inclusion evaluation method of the present invention, the cross-sectional hardness of the surface layer of the precursor metal piece, which will later become the evaluation portion of the test piece, is lower than that of the interior.

[0028] <Test piece> The test piece is made by infiltrating hydrogen into the precursor metal piece described above. The method for infiltrating hydrogen into the precursor metal piece is not particularly limited as long as it is a treatment that can embrittle the nonmetallic inclusions contained in the precursor metal piece.

[0029] The method for introducing hydrogen into the precursor metal piece is preferably an electrolytic hydrogen charging method using the precursor metal piece as the negative electrode, a platinum electrode as the positive electrode, and a sodium citrate buffer solution with a concentration of 0.01 mol / L or more. As the buffer solution, acetate buffer solution, phosphate buffer solution, phthalate buffer solution, etc. can also be used. In this case, the decrease in the pH of the electrolyte during hydrogen charging can be suppressed, so even if hydrogen charging is performed for a relatively long period of time, the surface of the precursor metal piece is less likely to be damaged. For example, if the precursor metal piece is cylindrical and has a large cross-sectional diameter, hydrogen charging for a relatively long period of time is required to allow hydrogen to penetrate to near the central axis of the precursor metal piece. In the present invention, the specific electrolyte as described above is used, so the decrease in pH of the electrolyte during hydrogen charging is suppressed, and the surface of the precursor metal piece is less likely to be damaged.

[0030] In the electrolytic hydrogen charging method described above, the charging current density is 0.01 to 3 mA / cm 2 The charge time is preferably 48 to 168 hours.

[0031] The surface area of ​​the platinum electrode is preferably at least twice, more preferably at least four times, and even more preferably at least five times the surface area of ​​the precursor metal piece, in which case the entire precursor metal piece is charged almost uniformly.

[0032] As described above, hydrogen can be infiltrated into the precursor metal piece to obtain a test piece.

[0033] The shape of the test piece is not particularly limited, as long as it is roughly rod-shaped and can be used in a conventionally known tensile testing machine, has gripping portions at both ends, an evaluation portion (parallel portion) in the center, the central axes of the gripping portion and the evaluation portion coincide, the cross-sectional shape of the evaluation portion does not change in the longitudinal direction, and the cross-sectional area of ​​the evaluation portion in the direction perpendicular to the longitudinal direction is smaller than the cross-sectional area of ​​the gripping portion. It is preferable that the test specimen is not a conventional shouldered test specimen (Reference: Fujita et al., Iron and Steel, Vol. 95, Issue 12, pp. 870-879) or a threaded test specimen, because stress tends to concentrate in the gripping area. The gripping portion may have a shape that allows it to be gripped by a hydraulic chuck, and may also have a tapered shape.

[0034] The shape of the test piece may be, for example, that shown in Figure 1. Figure 1 is a side view (schematic diagram) of the test piece. As shown in Figure 1, the test piece 1 is generally rod-shaped. It has a gripping portion 2 at each end and an evaluation portion 3 in the center, with the central axes of the gripping portion 2 and the evaluation portion 3 coinciding. In the embodiment shown in Figure 1, both the gripping portion 2 and the evaluation portion 3 are cylindrical. As in the embodiment exemplified here, it is preferable that both the gripping portion 2 and the evaluation portion 3 be cylindrical. The evaluation section may also be called the parallel section. In addition, the cross-sectional shape of the evaluation part 3 does not change in the longitudinal direction, and the cross-sectional diameter (b) does not change either. In the test piece of the embodiment shown in Figure 1, the cross-sectional shape of the grip part 2 does not change, and the cross-sectional diameter (a) does not change either. Furthermore, the diameter b of the cross section of the evaluation part 3 in the direction perpendicular to the longitudinal direction is smaller than the diameter a of the cross section of the grip part 2. Therefore, the cross-sectional area of ​​the evaluation part 3 is smaller than the cross-sectional area of ​​the grip part 2.

[0035] The surface roughness (Rz) of the test piece is preferably 4.0 μm or less, and more preferably 3.0 μm or less. The surface roughness (Rz) means a value measured by the stylus method specified in JIS-B0601.

[0036] The amount of hydrogen contained in the test piece is preferably 0.50 to 6.00 ppm, and more preferably 1 to 4 ppm. The amount of hydrogen contained in the test piece is measured as follows. After obtaining the test piece, a cylindrical sample 2 mm thick is cut out from the parallel section in the longitudinal direction (within 30 minutes after hydrogen charging) and subjected to hydrogen thermal desorption analysis. The cylindrical sample is heated from room temperature to 350°C at 100°C / h, and the mass of hydrogen desorbed from the cylindrical sample is measured using a quadrupole mass analyzer (Q-mass). After hydrogen thermal desorption analysis, the mass of the cylindrical sample is measured to determine the amount of hydrogen contained in the entire test piece. The sample is not limited to a cylindrical shape, but may be a prismatic shape.

[0037] <Tensile test> The test piece obtained by infiltrating hydrogen into the precursor metal piece as described above should be subjected to a tensile test as soon as possible because there is a possibility that hydrogen may be desorbed from the test piece. After the hydrogen infiltration treatment such as hydrogen charging is completed, it is preferable to subject the test piece to a tensile test within approximately 30 minutes. After the tensile test, the surface roughness of the test specimen and the amount of hydrogen contained in the test specimen may be measured.

[0038] The tensile test may be carried out by a conventionally known method. It is preferable to grip the gripped portion of the test piece with a hydraulic chuck and carry out the tensile test at a crosshead speed of about 1 mm / min.

[0039] When a tensile test is performed on a test piece, fracture occurs starting from a non-metallic inclusion affected by the hydrogen that has penetrated. Theoretically, the test piece is thought to fracture at a plane containing the point where the largest non-metallic inclusion is located, and therefore, such largest non-metallic inclusion is thought to be present on the fracture surface. The fracture surface of the test piece can be examined for the presence of nonmetallic inclusions by observing secondary electron images and backscattered electron images using a scanning electron microscope (SEM). The size and shape of the nonmetallic inclusion that is the fracture origin can be measured using, for example, a scanning electron microscope (SEM). The type or composition of the nonmetallic inclusion that initiated the fracture can be identified using energy dispersive X-ray fluorescence spectroscopy (EDX). [Example]

[0040] The present invention will be described with reference to examples, but the present invention is not limited to the examples described below.

[0041] The example was carried out according to the flow chart shown in FIG.

[0042] In the examples and comparative examples, a wedge-shaped test piece or a tapered test piece was used. Both the wedge-shaped and tapered test specimens are generally cylindrical overall, and the cross sections perpendicular to the longitudinal direction are circular at all points. Both the wedge-shaped and tapered test specimens have a parallel section in the center, with two gripping sections sandwiching it, and the central axes of the parallel section and gripping sections are aligned. The parallel section is machined to be thinner than the gripping sections. The parallel section is identical to the evaluation section.

[0043] First, a cylindrical test steel material with a cross-sectional diameter of 19 mm was prepared, and then this test steel material was machined to obtain a rod-shaped pre-heat treatment steel piece whose cross-sectional diameter at all longitudinal points was 0.3 mm larger than that of the wedge-shaped or tapered test piece described above.

[0044] The cylindrical test steel material and the pre-heat treatment steel billet were made of developed steel X, or SUP12, SUP10, SUP7 or SUJ2, the composition of which is specified in JIS-G4801 or JIS-G4805. The composition of developed steel X is as follows: C: 0.40% by mass Si: 1.80% by mass Mn: 0.30% by mass Ni: 0.50% by mass Cr:1.00% by mass V:0.20% by mass B:0.0015% by mass Nb:0.03% by mass

[0045] Next, the steel billets before heat treatment were quenched using a muffle furnace or a gas carburizing furnace. Here, the quenching using the muffle furnace was carried out in air at 850°C for 30 minutes. In addition, quenching using a gas carburizing furnace involved heating pre-heat-treated steel pieces for 30 minutes in a furnace adjusted to 850°C with acetylene flowing in as the carburizing gas, and the surface carbon content of the pre-heat-treated steel pieces was controlled by the carbon monoxide generated by the incomplete combustion of the carburizing gas. If pre-heat-treated steel pieces made of pure iron were charged into a gas carburizing furnace and carburized under the same conditions, the surface carbon content of the pre-heat-treated steel pieces would be 0.3 mass%. In the comparative examples, salt bath quenching was performed. This method involves immersing the pre-heat-treatment steel pieces in a heated sodium chloride-barium chloride solution and heating them. This method results in almost no change in the surface carbon content of the pre-heat-treatment steel pieces.

[0046] After the pre-heat-treatment steel pieces were quenched as described above, the surfaces of the pre-heat-treatment steel pieces were ground so that their cross-sectional diameters were reduced by 0.3 mm, to produce precursor metal pieces having the same shape as the wedge-shaped test pieces or precursor metal pieces having the same shape as the tapered-shaped test pieces.

[0047] By grinding in this way, a precursor metal piece was obtained with a parallel section having a cross-sectional diameter of 15 mm. The precursor metal piece and the test piece were identical in shape, and since the cross-sectional diameter of the cylindrical test steel material prepared initially was 19 mm, it was possible to evaluate 62% of the specific cross section of the cylindrical test steel material in the test piece (the cross section of the parallel section (evaluation section) of the test piece).

[0048] In addition, the portion of each precursor metal piece that would later become the parallel section was cut perpendicular to its longitudinal direction, and the Vickers hardness was measured for the interior (near the central axis) and surface (up to 0.05 mm from the outer edge) of the cut surface. Vickers hardness was measured four times with a test force of 300 gf, and the average value was calculated. The average values ​​obtained are shown in Table 1.

[0049] Next, each precursor metal piece was subjected to hydrogen charging. Hydrogen charging will now be described. An electrolytic solution having the composition shown in Table 1 is prepared in a beaker, and a precursor metal piece is immersed therein as a cathode and a platinum mesh is immersed therein as an anode. Then, a charging current density (mA / cm) shown in Table 1 is applied. 2 ) for the charging time shown in Table 1. The surface area of ​​the platinum mesh anode used was at least five times the surface area of ​​the precursor metal piece. In this case, the entire precursor metal piece was charged almost uniformly.

[0050] The precursor metal piece was subjected to hydrogen charging to form the test piece. The obtained test pieces were immediately (within a few minutes after hydrogen charging of the precursor metal pieces) subjected to tensile testing. Specifically, the test piece was gripped by a hydraulic chuck and subjected to a tensile test at a crosshead speed of 1 mm / min. The measured tensile strength (MPa) is shown in Table 1.

[0051] Furthermore, the surface roughness (μm) of the test piece after the tensile test was measured by the stylus method (JIS-B0601).

[0052] Next, immediately after the tensile test (within 30 minutes after hydrogen charging), a 2 mm thick cylindrical sample was cut from the parallel section in the longitudinal direction and subjected to hydrogen thermal desorption analysis. The cylindrical sample was heated at 100°C / h from room temperature to 350°C, and the mass of hydrogen desorbed from the cylindrical sample was measured using a quadrupole mass analyzer (Q-mass). After hydrogen thermal desorption analysis, the mass of the cylindrical sample was measured, and the amount of hydrogen contained in the entire test piece was determined. The results are shown in Table 1.

[0053] After the tensile test, the fracture surfaces of the test specimens were observed using a scanning electron microscope (SEM) with secondary electron images and backscattered electron images to identify whether the fracture originated from a nonmetallic inclusion or the surface layer. An example of an SEM image and external photograph of the observation surface (fracture surface) when the fracture origin is a nonmetallic inclusion is shown in Figure 3. The dimensions of the nonmetallic inclusion can be measured from these SEM images and external photographs. Figure 4 shows an example of an SEM image and a photograph of the appearance of the observation surface (fracture surface) when the fracture originated from the surface layer.

[0054] [Table 1]

Claims

1. The inclusion evaluation method involves conducting a tensile test on a test piece obtained by infiltrating hydrogen into a precursor metal piece having an internal hardness of over 400 HV and a surface hardness of 250 to 400 HV, thereby causing fractures to occur originating from non-metallic inclusions affected by the infiltrated hydrogen, and measuring the dimensions and / or identifying the type of the non-metallic inclusions that have become the fracture origins.

2. 2. The inclusion evaluation method according to claim 1, wherein the amount of hydrogen contained in the test piece is 0.50 to 6.00 ppm.

3. The precursor metal piece is obtained by heat treating a pre-heat-treated steel piece and then removing α mm of the surface of the pre-heat-treated steel piece in the thickness direction, The pre-heat treatment steel billet was obtained by removing β mm from the surface of the test steel in the thickness direction, 3. The inclusion evaluation method according to claim 1, wherein the area of ​​the observation surface of the test piece is 62% or more of the area of ​​the cross section of the test steel material, which is wider than the observation surface by a thickness of α+β mm.

4. The inclusion evaluation method according to claim 1 or 2, wherein the cross-sectional diameter of the evaluation portion of the test piece is greater than 10 mm.

5. 3. The inclusion evaluation method according to claim 1, wherein hydrogen is introduced into the precursor metal piece by electrolytic hydrogen charging using the precursor metal piece as the negative electrode, a platinum electrode as the anode, and a sodium citrate buffer solution having a concentration of 0.01 mol / L or more.

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