Non-destructive inspection method for steel materials

Abstract

Identify the factors that cause changes in the magnetic permeability of the inspection object and evaluate the surface treatment state of the inspection object with high precision. The non-destructive inspection method for steel materials according to one aspect of the present invention includes a preparation step (S1), a placement step (S3), an eddy current generation step (S4), a frequency change step (S5), an impedance calculation step (S6), and an evaluation step (S7). In the preparation step (S1), a non-destructive inspection device is prepared. In the placement step (S3), the inspection object is placed. In the eddy current generation step (S4), eddy currents are generated in the inspection object. In the frequency change step (S5), the penetration depth of the alternating magnetic field penetrating the inspection object is continuously changed. In the impedance calculation step (S6), the impedance value of the inspection object at each penetration depth is calculated. In the evaluation step (S7), the ratio of the impedance value of the inspection object at each penetration depth to the impedance value of the steel material before surface treatment at each penetration depth is calculated, and the change factor of the magnetic permeability of the inspection object is determined based on the calculation result of the ratio, and the surface treatment state is evaluated.

Description

Technical Field

[0001] The present invention relates to a non-destructive inspection method for steel. Background Art

[0002] Conventionally, various non-destructive inspection methods have been proposed. Among various non-destructive inspection methods, steel that has been subjected to surface treatments such as shot peening and heat treatment in the manufacturing process of steel is used as an inspection object, and it is determined whether the above surface treatment has been properly performed on the inspection object without destroying the inspection object.

[0003] For example, there is the following non-destructive inspection method: an alternating current is passed through a coil disposed on the surface of steel as an inspection object, and the impedance of the coil is measured, thereby measuring the residual stress distribution of the inspection object after shot peening (see Patent Document 1). In this non-destructive inspection method, data related to impedance of a plurality of samples (steel after shot peening) with different residual stress generation states are acquired in advance, and the acquired data is compared with the data related to impedance of the inspection object, thereby evaluating the inspection object.

[0004] Prior Art Documents

[0005] Patent Documents

[0006] Patent Document 1: Japanese Published Patent "Japanese Unexamined Patent Application Publication No. 2008-002973" Summary of the Invention

[0007] Problems to be Solved by the Invention

[0008] However, in the above non-destructive inspection method, although it is possible to measure the generation state of the residual stress of the steel after shot peening, for example, it is impossible to determine whether the carbon content of the steel after shot peening is normal. Therefore, for example, in the case where a shot peening process is performed after a carburizing and quenching process, it is impossible to determine whether a defect occurred in the carburizing and quenching process or in the shot peening process. Thus, in the above non-destructive inspection method, there is a problem that even if the steel as the inspection object is determined to be a defective product, it is impossible to determine in which processing step the defect occurred, and there is room for improvement in the evaluation accuracy of the surface state of the inspection object.

[0009] One aspect of the present invention has been completed in view of the above problems, and an object thereof is to provide a non-destructive inspection method for steel that can evaluate the surface state of an inspection object with high accuracy.

[0010] Solutions to the Problems

[0011] One method for non-destructive inspection of steel involved in one aspect of the present invention, which is completed to solve the above problems, includes a preparation process, an arrangement process, an eddy current generation process, a frequency change process, an impedance calculation process, and an evaluation process.

[0012] In the preparation process, a non-destructive inspection device having a frequency variable circuit and a coil is prepared. The frequency variable circuit can change the frequency of an alternating current. The coil can excite an alternating magnetic field through the alternating current.

[0013] In the arrangement process, the inspection object is arranged in such a way that the alternating magnetic field excited by the coil penetrates into the interior of the inspection object. The inspection object is steel after surface treatment.

[0014] In the eddy current generation process, the alternating magnetic field penetrates into the interior of the inspection object, thereby generating eddy currents in the inspection object.

[0015] In the frequency change process, the frequency variable circuit continuously changes the frequency of the alternating current from a low frequency to a high frequency, thereby continuously changing the penetration depth of the alternating magnetic field into the inspection object.

[0016] In the impedance calculation process, based on the potential difference between both ends of the coil and the current value flowing through the coil, the impedance value of each penetration depth of the inspection object is calculated.

[0017] Then, in the evaluation process, the ratio of the impedance value of each penetration depth of the inspection object calculated in the impedance calculation process to the impedance value of each penetration depth of the steel before surface treatment is calculated, and based on the calculation result of the ratio, the change factors of the magnetic permeability of the inspection object are determined, and the surface treatment state is evaluated. The change factors of the magnetic permeability are, for example, the carbon content and nitrogen content of the inspection object, the size and shape of the inspection object, the hardness of the inspection object, etc.

[0018] Effects of the Invention

[0019] According to the non-destructive inspection method of steel of the present invention, the factors causing changes in the magnetic permeability of the inspection object can be determined, and the surface treatment state of the inspection object can be evaluated with high precision. Description of the Drawings

[0020] Figure 1 is a circuit diagram of the non-destructive inspection device according to Embodiment 1 of the present invention.

[0021] Figure 2 is a schematic diagram showing the alternating magnetic field generated by the coil according to Embodiment 1.

[0022] Figure 3 is a flowchart for explaining the non-destructive inspection method of steel according to Embodiment 1.

[0023] Figure 4 is a flowchart showing the process of the manufacturing process according to Embodiment 1.

[0024] Figure 5 is a flowchart showing the process of the setting process according to Embodiment 1.

[0025] Figure 6 is a flowchart showing the process of the evaluation process according to Embodiment 1.

[0026] Figure 7 (a) of is a graph showing an example of the calculation result of the impedance ratio of the evaluation process according to Embodiment 1, Figure 7 (b) of is a diagram showing an example of the display result of the display device in the notification process according to Embodiment 1.

[0027] Figure 8 is a diagram showing the change in magnetic permeability of the steel material after surface treatment according to Embodiment 1.

[0028] Figure 9 is a graph showing an example of the calculation result of the impedance ratio according to Modification 1 of Embodiment 1.

[0029] Figure 10 is a flowchart showing the process of the evaluation process according to Embodiment 2 of the present invention.

[0030] Figure 11 (a) of is a graph showing an example of the calculation result of the impedance ratio of the evaluation process according to Embodiment 2, Figure 11 (b) of is a diagram showing an example of the display result of the display device in the notification process according to Embodiment 2.

[0031] Figure 12 is a flowchart showing the process of the evaluation process according to Embodiment 3 of the present invention.

[0032] Figure 13 (a) of is a graph showing an example of the calculation result of the impedance ratio of the evaluation process according to Embodiment 3, Figure 13 (b) of is a diagram showing an example of the display result of the display device in the notification process according to Embodiment 3. Detailed Embodiment

[0033] 〔Embodiment 1〕

[0034] Next, with reference to Figures 1 - 9 the non-destructive inspection method according to Embodiment 1 of the present invention will be described.

[0035] [Non-destructive inspection equipment]

[0036] The non-destructive inspection device 1 of the first embodiment is as follows Figure 1 As shown, the oscillator 10, the detector 20 and the measuring device 30 are provided. The oscillator 10 has an AC power source 11 and a frequency variable circuit 12. The frequency variable circuit 12 is connected to the AC power source 11 and changes the frequency of the AC current output from the AC power source 11.

[0037] The detector 20 has a coil 21 to be described later. One end side of the coil 21 ( Figure 1 The other end side of the coil 21 ( Figure 1 The detector 20 (see point B) is connected to an I / V conversion circuit 34 described later. The detector 20 is used when evaluating the inspection object M described later (determining whether it is good or bad in this embodiment). Figure 3 ).also, Figure 1 The circuit symbol representing the coil 21 within the dotted line represents an electrical equivalent circuit of the coil 21 .

[0038] like Figure 2 As shown, the coil 21 is formed by winding a plurality of conductive wires into a cylindrical shape. In Embodiment 1, a wire formed by bundling a plurality of thin wires into one wire is used as the wire, thereby increasing the resonant frequency of the coil 21. In addition, a component (core coil) in which a wire is wound around a hollow cylindrical core may be used as the coil 21. In addition, a single wire may be used as the wire.

[0039] In the method for manufacturing coil 21 of Embodiment 1, first, a wire material obtained by twisting and braiding hundreds of copper enameled wires is wound around a resin cylinder, and then the wound wire material is bonded with epoxy resin and the cylinder is removed, thereby manufacturing coil 21.

[0040] In addition, as a method for making the coil 21, there is also a method in which a wire covered with a thermosetting resin is used and the wire is wound, and then the wire is fixed to a coil-shaped shape by heating with hot air, a drying furnace, etc. As such, as long as the wire can maintain the coil-shaped shape, the method for making the coil is not particularly limited.

[0041] The measuring device 30 includes an amplifier circuit 31, an absolute value circuit 32, a low pass filter (LPF) 33, an I / V conversion circuit 34, an absolute value circuit 35, an LPF 36, a control unit 37, and a display device 38. The measuring device 30 measures a change in the value of the impedance of the coil 21 based on a signal indicating the electrical characteristics of the alternating current flowing through the coil 21.

[0042] One end side of the amplifier circuit 31 (the left side in Figure 1 is connected to both ends of the coil 21 ( Figure 1 points A and B), and the other end side (the right side in Figure 1 ) is connected to the absolute value circuit 32. A signal of the potential difference between both ends of the coil 21 is input to the amplifier circuit 31. After the signal input to the amplifier circuit 31 is amplified, it is input to the absolute value circuit 32.

[0043] The absolute value circuit 32 is a full-wave rectifier circuit. The potential difference signal input to the absolute value circuit 32 is full-wave rectified and then converted to DC by the LPF 33. The potential difference signal converted by the LPF 33 is input to the control unit 37.

[0044] The I / V conversion circuit 34 is connected to the other end side of the coil 21 ( Figure 1 point B). A signal representing the current value of the current flowing through the coil 21 is input to the I / V conversion circuit 34 and thus converted to a signal representing a potential difference. Then, after being full-wave rectified by the absolute value circuit 35, it is converted to DC by the LPF 36. The signal converted by the LPF 36 is input to the control unit 37.

[0045] The control unit 37 includes a microprocessor, an interface circuit, a memory, and a program for operating them, etc., but is not shown in the figure. The control unit 37 is connected to the frequency variable circuit 12, the LPF 33, and the LPF 36. A signal representing the electrical characteristics of the coil 21, that is, a signal of the frequency of the alternating current flowing through the coil 21, a signal of the current value at each frequency, and a potential difference signal are input to the control unit 37. The control unit 37 calculates the impedance value at each frequency based on the signal representing the electrical characteristics of the coil 21.

[0046] In addition, the control unit 37 has a function of outputting a signal for automatically and continuously changing the frequency to the frequency variable circuit 12. In addition, in the first embodiment, in a state where the steel material M to be inspected is disposed inside the coil 21, the frequency is changed by the frequency variable circuit 12 through the control output from the control unit 37 (refer to Figure 2 ). In addition, it is assumed that the frequency of the alternating current can also be changed manually.

[0047] In addition, the control unit 37 calculates the impedance value Z 2 at each continuously changed frequency, and calculates the impedance ratio γ 2 of the calculated impedance value Z 0 to the impedance value Z 2 of the specified steel material (reference steel material) before surface treatment 2 / Z 0 ). In addition, the control unit 37 calculates the impedance value Z at each penetration depth in the case of good products1 The impedance Z of the steel before surface treatment 0 and the impedance ratio γ 1 (Z 1 / Z 0 ). Then, the impedance ratio γ of the inspection object M 2 is compared with the impedance ratio γ of the non-defective product, 1 thereby also functioning to determine the quality of the surface state of the inspection object M.

[0048] The display device 38 is a device that displays the evaluation result (quality determination result) of the control unit 37 as shown in (b) in the notification process (S8) described later. In the display device 38, in addition to displaying the result of the quality determination of the inspection object M, it also displays, as shown in (a) of Figure 7 , a graph showing the relationship between the impedance ratio γ of the inspection object M Figure 7 and the penetration depth. 2

[0049] [Eddy current control method]

[0050] Next, the eddy current control method in the non-destructive inspection device 1 will be described. First, an alternating current is applied from the AC power supply 11 to the coil 21 of the non-destructive inspection device 1. When an alternating current is applied to the coil 21, as described later, the alternating magnetic flux excited by the coil 21 penetrates into the inspection object M disposed inside the coil 21 (refer to Figure 2 ). As a result, eddy currents are generated in the inspection object M.

[0051] In the non-destructive inspection device 1 of the first embodiment, the control unit 37 outputs a control signal to the frequency variable circuit 12, whereby the frequency of the alternating current can be continuously changed from a low frequency to a high frequency. Moreover, by continuously changing the frequency of the alternating current by the frequency variable circuit 12, the penetration depth of the alternating magnetic flux penetrating into the inspection object M can be continuously changed. Specifically, the frequency of the alternating current is continuously changed, for example, from a low frequency of about 10 kHz to a high frequency of about 20 MHz. As a result, the penetration depth of the alternating magnetic flux penetrating into the inspection object M changes from 0 μm to 150 μm.

[0052] Here, there is a relationship shown in the following formula (1) between the penetration depth of the above-described alternating magnetic flux penetrating into the inspection object M and the frequency of the alternating current. In the first embodiment, using the relational expression of formula (1), the penetration depth of the alternating magnetic flux penetrating into the inspection object M is determined based on the frequency of the alternating current.

[0053] [Equation 1]

[0054]

[0055] In the above formula (1), δ represents the penetration depth [m], f represents the alternating current frequency [Hz], σ represents the conductivity [S / m] of the inspection object M, and μ represents the magnetic permeability [H / m] of the inspection object M.

[0056] The control unit 37 continuously changes the penetration depth at which the alternating magnetic field penetrates the inspection object M, and obtains the ratio of the potential difference between both ends of the coil 21 to the current value flowing through the coil 21, thereby calculating the impedance value for each penetration depth of the inspection object M. This impedance value becomes different values due to changes in the magnetic permeability of the steel after surface treatment and other reasons.

[0057] The magnetic permeability of the above inspection object M varies according to various parameters such as the carbon content and nitrogen content of the inspection object M, the magnitude and direction of elastic deformation, the magnitude of plastic deformation, the atomic arrangement state (phase transformation), size and shape, and hardness. In the non-destructive inspection method for steel in Embodiment 1, the steel after surface treatment is used as the inspection object M, and various evaluations are performed on the inspection object M by utilizing the relationship between the above parameters and the magnetic permeability (refer to Figure 3 ).

[0058] [Manufacturing Process]

[0059] Here, the flow of the manufacturing process of the steel in Embodiment 1 will be described with reference to the flowchart shown in Figure 4 . In addition, the flowchart shown in Figure 4 is an example and is not limited thereto.

[0060] In the manufacturing process of the steel in Embodiment 1, first, after performing the casting process (S11), a machining process such as cutting (S12) is performed. In this machining process (S12), for example, the steel is machined into a desired shape (cylindrical shape, gear shape, etc.). In addition, the type of machining can be appropriately changed.

[0061] Next, a carburizing and quenching process (S13) is performed. In this carburizing and quenching process (S13), a carburizing and quenching treatment is performed on the steel, whereby the carbon amount on the surface of the steel increases and the magnetic permeability of the steel decreases. In addition, the carbon amount is just an example of the factors that cause changes in the magnetic permeability, and the magnetic permeability of the inspection object M varies according to the above-mentioned multiple parameters.

[0062] After performing the carburizing and quenching process (S13), a shot peening process (S14) is performed. In this shot peening process (S14), a shot peening device (not shown) is used to project small spherical projection materials onto the surface of the steel, thereby providing surface modification of the steel. After this shot peening, the magnetic permeability of the surface of the steel increases.

[0063] Next, a finishing process (S15) is performed. In this finishing process (S15), finishing treatments such as brushing, polishing, and barrel finishing are appropriately performed on the steel material.

[0064] [Non-destructive Inspection Method]

[0065] Next, with reference to Figure 3 the flowchart shown, a non-destructive inspection method for steel material based on the non-destructive inspection device 1 of Embodiment 1 will be described. In addition, Figure 3 the flowchart shown is an example and is not limited to the order of this flowchart.

[0066] In the non-destructive inspection method for steel material of Embodiment 1, first, a preparation process (S1) of preparing the steel material to be the inspection object M and preparing the above non-destructive inspection device 1 is performed. As the steel material to be the inspection object M, for example, it is assumed to be steel materials used for structural components (spur gears, gears, etc.) of automobiles, airplanes, construction machinery, etc., and steel materials used for springs, dies, tools, etc.

[0067] In Embodiment 1, the steel material that has been subjected to carburizing and quenching treatment in the carburizing and quenching process (S13) and then shot peening treatment in the shot peening process (S14) is used as the inspection object M.

[0068] In addition, regarding the material of the steel material as the inspection object M, for example, chromium molybdenum steel (JIS standard: SCM420) is used. Regarding the shape of the steel material as the inspection object M, the steel material machined into a cylindrical shape in the machining process (S12) is used.

[0069] Next, after the preparation process (S1), a setting process (S2) is performed. In the setting process (S2), each process is performed in the order of Figure 5 the flowchart shown. In addition, Figure 5 the flowchart shown is an example and is not limited thereto.

[0070] First, the steel material before surface treatment (reference steel material) is arranged at a specified position (S21) of the non-destructive inspection device 1. Specifically, the reference steel material is arranged at the center of the circular cross-section inside the cylindrical coil 21, and a state is set such that the alternating magnetic field excited by the coil 21 can penetrate into the interior of the steel material before surface treatment.

[0071] Next, various measurement conditions such as the arrangement position of the reference steel material and the frequency range of the alternating magnetic field flowing through the coil 21 are set (S22). Here, considering the relationship between the above-mentioned multiple parameters and the magnetic permeability, measurement conditions that can appropriately evaluate the surface treatment state of the inspection object M are set.

[0072] Then, AC magnetic penetration is made into the reference steel material, and thereby, by continuously changing the frequency of the alternating current by the variable frequency circuit 12, the penetration depth of the AC magnetic penetration into the reference steel material is continuously changed. At this time, the impedance value Z of each penetration depth of the reference steel material is measured 0 (S23), and the impedance value Z of each penetration depth of the measured reference steel material 0 is stored in a database (not shown) (S24).

[0073] In addition, data related to the impedance value Z of the reference steel material after various surface treatments are performed may be pre-stored in the database 0 . Various surface treatments are, for example, shot peening, quenching, nitriding, carburizing, tempering, annealing, surface machining, grinding, and low-temperature annealing (temper) treatment

[0074] Next, a determination method to be used in the evaluation process (S7) described later is selected (S25). The determination methods are three determination methods: (1) region determination, (2) peak determination, and (3) area determination described later. Any one of these three determination methods is selected

[0075] Next, the same processes as the above S21 to S24 are performed on at least one or more steel materials (good products) that have been well surface-treated. Thus, the impedance value Z of the good products is measured 1 , and the data related to this impedance value Z 1 is stored in the database. And, the impedance value Z of each penetration depth of the reference steel material is calculated 0 and the impedance ratio γ 1 of the impedance value Z of each penetration depth of the good products 1 (Z 1 / Z 0 ) is calculated, and this impedance ratio γ 1 is stored in the database

[0076] Then, based on the calculated impedance ratio γ 1 of the good products, the range of the threshold value used in the determination method selected in S25 is set (S26). The accuracy of the evaluation of the surface treatment state of the inspection object M can be adjusted by appropriately setting the type of the determination method and the range of the threshold value. Through the above processes, the setting process (S2) ends. In addition, it is preferable that the arrangement process (S3) to the evaluation process (S7) described later are performed immediately after the setting process (S2). This is because the influence of interference factors such as the surrounding temperature can be reduced, and thus the accuracy of the evaluation can be improved

[0077] Next, return to Figure 3, a steel material configuration process (S3) for configuring the inspection object M is performed. Specifically, the steel material serving as the inspection object M is arranged at the center of the circular cross-section inside the cylindrical coil 21 in a state where the alternating current excited by the coil 21 can penetrate into the inspection object M. In addition, the configuration method is not limited to this, as long as the alternating current of the coil 21 can penetrate into the inspection object M. In addition, the inspection object M can also be arranged at a position facing the coil 21.

[0078] After the configuration process (S3), an eddy current generation process (S4) for generating eddy currents in the inspection object M is performed. Specifically, the control unit 37 operates the AC power supply 11 by means of the frequency variable circuit 12. When the AC power supply 11 operates, an alternating magnetic field is excited in the coil 21 (refer to Figure 2 ). The alternating magnetic field of the coil 21 penetrates into the inspection object M, thereby generating eddy currents inside the inspection object M.

[0079] Next, a frequency change process (S5) for continuously changing the penetration depth of the alternating magnetic field penetrating into the inspection object M is performed. Specifically, the control unit 37 outputs a control signal to the frequency variable circuit 12, thereby continuously changing the frequency of the alternating current output from the AC power supply 11. As a result, the penetration depth of the alternating magnetic field penetrating into the inspection object M continuously changes. In this case, depending on the internal composition of the inspection object M, even if the same alternating magnetic field is provided to the inspection object M, the penetration depth of the alternating magnetic field penetrating into the inspection object M varies.

[0080] In addition, in Embodiment 1, the surface state of the inspection object M was inspected in such a way that the penetration depth of the alternating magnetic field penetrating into the inspection object M changed from 0 μm to 150 μm (refer to Figure 7 (a) and Figure 8 ).

[0081] After the frequency change process (S5), an impedance calculation process (S6) for calculating the value Z of the above impedance for each penetration depth of the inspection object M is performed 2 . Specifically, the control unit 37 calculates the value Z of the above impedance based on the potential difference between both ends of the coil 21 ( Figure 1 points A and B) and the current value flowing through the coil 21 2 .

[0082] Next, an evaluation process (S7) for inspecting the quality of the surface state of the inspection object M is performed. Specifically, each process is carried out in the order of the flowchart shown in Figure 6 . In addition, Figure 6 the flowchart shown is an example and is not limited to this.

[0083] In Embodiment 1, it is assumed that the area determination among the above three determination methods is used. First, the control unit 37 calculates the impedance value Z at each penetration depth of the inspection object M calculated by the impedance calculation process (S6). 2 and the impedance value Z at each penetration depth of the steel material (reference steel material) before surface treatment measured in the setting process (S2). 0 of the impedance ratio γ 2 (Z 2 / Z 0 ). Then, it is determined whether the calculated impedance ratio γ of the inspection object M 2 is within the threshold range (S31). Here, the threshold range uses the data set in the setting process (S2).

[0084] When the impedance ratio γ of the inspection object M 2 is within the threshold range (S31: Yes), the control unit 37 determines that the inspection object M is a good product (S32). On the other hand, when the impedance ratio γ of the inspection object M 2 is not within the threshold range (S31: No), the control unit 37 determines that the inspection object M is a defective product (S33).

[0085] Next, after the evaluation process (S7), a notification process (S8) for notifying whether the inspection object M is a good product or a defective product is performed. In this notification process (S8), it is displayed on the display device 38 whether the inspection object M is a good product.

[0086] In Embodiment 1, in the notification process (S8), on the display device 38 as Figure 7 shown in (a) and Figure 8 as shown, a graph is displayed with the penetration depth depicted on the horizontal axis and the impedance ratio γ of the inspection object M depicted on the vertical axis. 2 In addition, on the display device 38 as Figure 7 shown in (b) of, the determination result of good product determination is displayed.

[0087] In Figure 7 the example shown in (b) of, it is shown that the number of measurements of the inspection object M is 7, and 6 of these 7 are determined to be good products and 1 is determined to be a defective product. The set number of points indicates that the impedance ratio γ has been calculated at a total of 151 points for the penetration depth between 0 μm and 150 μm. 2 .

[0088] Figure 8 is a diagram showing the change in the magnetic permeability of the steel material after surface treatment. When carburizing and quenching treatment is performed in the carburizing and quenching process (S13), the carbon content on the surface of the steel material increases. When the carbon content of the steel material increases, the magnetic permeability decreases. Due to the influence of such carburizing, as Figure 8As shown, after the carburizing and quenching treatment, the impedance ratio γ of the object M to be inspected 2 becomes smaller (refer to Figure 8 the circular mark).

[0089] On the other hand, when shot peening is performed in the shot peening process (S14), the magnetic permeability of the steel increases due to the surface modification of the steel. Due to the influence of such shot peening (SP), after the shot peening treatment, the impedance ratio γ of the object M to be inspected 2 becomes larger (refer to Figure 8 the square mark).

[0090] If the particle size of the projection material projected onto the surface of the steel is increased during shot peening or the degree of shot peening (SP) is increased, the magnetic permeability of the surface of the steel further increases due to the influence of SP (refer to Figure 8 the diamond mark).

[0091] The change in magnetic permeability caused by the carburizing and quenching treatment and the shot peening treatment described above can be used to determine in which process of the carburizing treatment process (S13) and the shot peening process (S14) a defect has occurred in the evaluation process (S7) of Embodiment 1.

[0092] For example, when it is determined that the magnetic permeability of the object M to be inspected is larger than that of a good product, it is determined that the carbon content of the object M to be inspected is low and a defect has occurred in the carburizing and quenching process (S13). On the other hand, when it is determined that the magnetic permeability of the object M to be inspected is smaller than that of a good product, it is determined that a defect has occurred in the shot peening process (S14) and the magnetic permeability has not been sufficiently increased.

[0093] According to the non-destructive inspection method of the steel described in Embodiment 1 above, by the non-destructive inspection device 1, after generating eddy currents in the object M to be inspected, the penetration depth of the alternating magnetic field penetrating into the object M to be inspected is continuously changed, whereby the impedance value Z of the object M to be inspected at each penetration depth calculated in the impedance calculation process (S6) can be calculated 2 and the ratio of the impedance value Z 0 at each penetration depth of the steel before surface treatment (reference steel) to the impedance value Z 2 at each penetration depth of the object M to be inspected, that is, the impedance ratio γ 2 can be obtained. Then, based on the calculation result of the impedance ratio γ

[0094] the change factor of the magnetic permeability of the object M to be inspected can be determined, and the surface treatment state of the object M to be inspected can be evaluated with high precision. 1 For example, the impedance ratio γ of a good product 2By making a comparison, it is possible to highly accurately evaluate (judge good or bad) the surface treatment state of the inspection object M. In addition, by focusing on the carbon amount state which is one of the factors causing changes in the magnetic permeability of the inspection object M, it is possible to determine in which of the carburizing treatment process (S13) and the shot peening process (S14) a defect has occurred.

[0095] In addition, according to the non-destructive inspection method for steel materials of Embodiment 1, it is possible to perform evaluation in such a way that the optimal determination method to be executed in the evaluation process (S7) is selected from the three determination methods of (1) area determination, (2) peak determination, and (3) area determination in the setting process (S2). Therefore, it is possible to improve the accuracy of the evaluation of the surface treatment state. As in Embodiment 1, in the case where it is assumed that generation occurs over a wide range of the penetration depth of the inspection object M (refer to Figure 7 (a)), by using area determination, good evaluation can be performed.

[0096] In addition, in the setting process (S2), the range of the threshold value to be used in the evaluation process (S7) is appropriately set, whereby it is possible to adjust the accuracy of the evaluation of the surface treatment state according to the needs of the user.

[0097] According to the above non-destructive inspection method for steel materials, in the evaluation process (S7), it is possible to perform evaluation in such a way that the optimal method is selected from the three determination methods of (1) to (3) above according to the inspection object M, thereby improving the accuracy of the evaluation. For example, in the case of assuming the difference between the impedance ratio γ 1 of non-defective products and the impedance ratio γ 2 of defective products over the entire area of the penetration depth of the inspection object M, by using the distribution of the ratio of (1) for evaluation (area determination), it is possible to reliably evaluate the surface treatment state.

[0098] 〔Modification Example 1〕

[0099] Refer to Figure 9 to describe the non-destructive inspection method for steel materials according to Modification Example 1 of Embodiment 1. In Modification Example 1, the difference from Embodiment 1 is that the steel material after the carburizing and quenching process (S13) shown in Figure 4 is used as the reference steel material.

[0100] Figure 9 is a graph showing an example of the calculation result of the impedance ratio γ 2 according to Modification Example 1. According to Figure 9 it can be seen that according to the difference in the particle size of the projection material used in the shot peening process (S14), the impedance ratio γ 2For different results. Specifically, when increasing the size of the particle diameter of the projection material, the magnetic permeability of the inspection object M increases, and the impedance ratio γ of the inspection object M 2 increases over a wide range of the penetration depth. In contrast, when decreasing the size of the particle diameter of the projection material, the magnetic permeability of the inspection object M decreases, and the impedance ratio γ of the inspection object M 2 decreases over a wide range of the penetration depth.

[0101] When using the steel material after the carburizing and quenching process (S13) as the reference steel material in this way, it is possible to determine more accurately whether the shot peening process (S14) has been carried out normally.

[0102] 〔Modification Example 2〕

[0103] Next, a non-destructive inspection method for the steel material according to Modification Example 2 of Embodiment 1 will be described. In Modification Example 2, the size and shape of the inspection object M are evaluated.

[0104] [Evaluation of Size and Shape]

[0105] When evaluating the size and shape of the inspection object M, in the Figure 5 set process (S2) shown, a steel material (S21) before surface treatment having a desired size and shape is arranged. Hereinafter, in the same manner as in Embodiment 1, the measurement conditions (S22) are set, the impedance value of the steel material (reference steel material) before surface treatment is measured (S23), and the impedance value Z of the reference steel material is stored 0 (S24). Next, one determination method is selected from the three determination methods of (1) region determination, (2) peak determination, and (3) area determination (S25).

[0106] Next, the same processing as S21 to S24 described above is performed on at least one or more steel materials (good products) having a desired size and shape. Thus, the impedance value Z of the good product is measured 1 , and the data related to this impedance value Z 1 is stored in the database. And, the impedance value Z of each penetration depth of the reference steel material is calculated 0 and the impedance value Z of each penetration depth of the good product 1 to calculate the impedance ratio γ 1 (Z 1 / Z 0 ), and this impedance ratio γ 1 is stored in the database. Then, based on the calculated impedance ratio γ of the good product 1 , the range of the threshold value used in the determination method selected in S25 is set (S26).

[0107] Then, the same operation as in Embodiment 1 is performed Figure 3The processes of S3 to S6 are performed, enabling the evaluation of the dimensions and shape of the inspection object M in the evaluation process S7. Thus, when the dimensions and shape of the inspection object M do not meet the desired conditions, it can be determined that the inspection object M is a defective product.

[0108] 〔Modification Example 3〕

[0109] Next, a non-destructive inspection method for steel materials according to Modification Example 3 of Embodiment 1 will be described. In Modification Example 3, the hardness of the inspection object M is evaluated.

[0110] [Evaluation of Hardness]

[0111] When evaluating the hardness of the inspection object M, in the Figure 5 shown setting process (S2), the steel material before surface treatment (reference steel material) (S21) is arranged. Next, similar to Embodiment 1, the measurement conditions are set (S22), and the impedance value Z of the reference steel material is measured 0 (S23), and the impedance value Z of the reference steel material is stored 0 (S24). Then, one determination method is selected from the three determination methods: (1) area determination, (2) peak determination, and (3) area determination (S25).

[0112] Next, at least one or more steel materials (good products) with the desired hardness are subjected to the same processes as S21 to S24 described above. Thus, the impedance value Z of the good product is measured 1 , and the data related to this impedance value Z 1 is stored in the database. Also, the impedance value Z of each penetration depth of the reference steel material is calculated 0 and the impedance value Z of each penetration depth of the good product 1 to obtain the impedance ratio γ 1 (Z 1 / Z 0 ), and this impedance ratio γ 1 is stored in the database. Then, based on the calculated impedance ratio γ of the good product 1 , the range of the threshold value used in the determination method selected in S25 is set (S26).

[0113] Then, similar to Embodiment 1, the Figure 3 processes of S3 to S6 are performed, enabling the evaluation of the hardness of the inspection object M in the evaluation process (S7). Thus, when the hardness of the inspection object M does not meet the desired conditions, it can be determined that the inspection object M is a defective product.

[0114] 〔Embodiment 2〕

[0115] Next, referring to Figure 10 , Figure 11The non-destructive inspection method of the steel material according to Embodiment 2 will be described with reference to (a) and (b) below. In addition, for the sake of convenience of explanation, components having the same functions as those described in the above Embodiment 1 are denoted by the same reference numerals, and their descriptions are appropriately omitted. In the non-destructive inspection method of the steel material according to Embodiment 2, peak determination is used in the determination method of the evaluation process.

[0116] As Figure 10 shown, in the evaluation process (S7a) of Embodiment 2, the control unit 37 determines whether the impedance ratio γ 2 at a specific penetration depth of the inspection object M is within the range of the threshold value (S41). Specifically, the evaluation (good or bad determination in Embodiment 2) of the inspection object M is performed by limiting to the part where the value of the impedance ratio γ 2 of the inspection object M is at the penetration depth at the peak (refer to the enclosed line in (a) of Figure 11 ).

[0117] When the impedance ratio γ 2 of the inspection object M is within the range of the threshold value (S41: Yes), the control unit 37 determines that the inspection object M is a good product (S42). On the other hand, when the impedance ratio γ 2 of the inspection object M is not within the range of the threshold value (S41: No), the control unit 37 determines that the inspection object M is a defective product (S43).

[0118] In Embodiment 2, in the notification process (S8), the display device 38 displays the determination result of the good product determination as shown in Figure 11 (b). In the display example shown in Figure 11 (b), it is shown that the number of measurements of the inspection object M is 7, 6 of which are determined to be good products and 1 is determined to be a defective product. In addition, it is shown that the evaluation was performed by limiting the penetration depth range to 10 μm to 30 μm in the peak determination.

[0119] According to the non-destructive inspection method of the steel material according to Embodiment 2 described above, it is also possible to evaluate the surface treatment state of the inspection object M with high accuracy in the same manner as in Embodiment 1. In particular, in Embodiment 2, the evaluation process (S7a) is performed using peak determination. As a result, when the difference between the impedance ratio γ 1 assumed to be a good product and the impedance ratio γ 2 of a defective product appears significantly at the peak of the impedance ratio γ 2 at a specific penetration depth, a higher-precision evaluation can be performed.

[0120] [Embodiment 3]

[0121] Next, with reference to Figure 12 , Figure 13(a) and (b) are used to illustrate the non-destructive inspection method for steel materials in Embodiment 3. In addition, for the sake of convenience in explanation, components having the same functions as those described in Embodiment 1 above are denoted by the same reference numerals, and their explanations are appropriately omitted. In the non-destructive inspection method of Embodiment 3, area determination is used in the evaluation process.

[0122] In the evaluation process (S7b) of Embodiment 3, as Figure 12 shown, the surface treatment state of the inspection object M is evaluated (good or bad determination). The control unit 37 determines whether the integral value of the impedance ratio γ 2 in a specific value range at the penetration depth of the inspection object M (refer to the black filling in (a) of Figure 13 ) is within the threshold range (S51). When the integral value of the impedance ratio γ 2 in a specific value range at the penetration depth of the inspection object M is within the threshold range (S51: Yes), the control unit 37 determines that the inspection object M is a good product (S52). On the other hand, when the integral value of the impedance ratio γ 2 in a specific value range at the penetration depth of the inspection object M is not within the threshold range (S51: No), the control unit 37 determines that the inspection object M is a defective product (S53).

[0123] In Embodiment 3, in the notification process (S8), the display device 38 displays the determination result of the good product determination as shown in Figure 13 (b). In the display example shown in Figure 13 (b), it is shown that the integral value in the range of the penetration depth of 10 μm to 30 μm is calculated and evaluated in the area determination.

[0124] According to the non-destructive inspection method for steel materials in Embodiment 3 described above, it is also possible to evaluate the surface treatment state of the inspection object M with high precision in the same manner as in Embodiment 1. In particular, in Embodiment 3, the area determination is used in the evaluation process (S7b). Thus, in a case where it is difficult to perform evaluation regardless of which of the above-mentioned area determination and peak determination methods is used, by using the integral value of the value of the impedance ratio γ 2 in a specific value range at the penetration depth for evaluation, it is possible to distinguish the minute difference between the impedance ratio γ 1 of a good product and the impedance ratio γ 2 of a defective product.

[0125] 〔Other Embodiments〕

[0126] In the above-described embodiment, it is assumed that the evaluation is performed in such a manner that the optimal determination method to be used in the evaluation process (S7) is selected from the three determination methods of (1) region determination, (2) peak determination, and (3) area determination in the setting process (S2), but it is not limited thereto. For example, two of the three determination methods of (1) to (3) above may be used, or all three determination methods may be used. Additionally, it may be set to perform the evaluation in such a way that the determination method is appropriately changed according to the penetration depth.

[0127] In the above-described embodiment, the steel material after being subjected to shot peening as a surface treatment is used as the inspection object M, but it is not limited thereto. In addition to this, steel materials after being subjected to, for example, quenching treatment, nitriding treatment, carburizing treatment, tempering treatment, annealing treatment, surface machining treatment, grinding treatment, or stress relieving treatment as surface treatments can also be used as the inspection object M. In these cases, it is also possible to store the impedance ratio γ 0 of the reference steel material and the impedance value Z 1 of the good product in the database and appropriately set the range of the threshold value used in the evaluation, so as to evaluate the above-described surface treatment state of the inspection object M with high precision. 1

[0128] In addition, it is also possible to evaluate the arrangement state (phase transformation) of the atoms of the inspection object M, the magnitude of the plastic deformation of the inspection object M, and the magnitude and direction of the elastic deformation of the inspection object M as the surface treatment state of the inspection object M.

[0129] That is, the steel material after being subjected to carburizing and quenching treatment in the carburizing and quenching process (S13) contains retained austenite. When shot peening is performed on it, it transforms into martensite and the content of retained austenite decreases. When the content of martensite increases, the magnetic permeability of the steel material increases. Utilizing this relationship, the phase transformation of the inspection object M can be evaluated as the surface treatment state of the inspection object M.

[0130] In addition, regarding the steel material after being subjected to shot peening in the shot peening process (S14), the magnetic permeability of the steel material changes with the magnitude of the plastic deformation of the steel material. Utilizing this relationship, it is also possible to evaluate the magnitude of the plastic deformation of the inspection object M.

[0131] In addition, regarding the steel material after being subjected to shot peening in the shot peening process (S14), the magnetic permeability of the steel material changes with the magnitude and direction of the elastic deformation of the steel material. Utilizing this relationship, it is also possible to evaluate the magnitude and direction of the elastic deformation of the inspection object M.

[0132] In addition, the magnitude relationship of the above-mentioned factors causing changes in permeability is "phase change of the object under inspection M > plastic deformation of the object under inspection M > magnitude and direction of elastic deformation of the object under inspection M". By pre-storing these relationships in a database, it is possible to evaluate the surface treatment state of the object under inspection M, and it is also possible to determine in which process a defect has occurred.

[0133] The present invention is not limited to the above-described embodiments, and various modifications can be made within the scope shown in the claims. Embodiments obtained by appropriately combining technical means separately disclosed in different embodiments are also included in the technical scope of the present invention.

[0134] (Summary)

[0135] A non-destructive inspection method for steel according to one aspect of the present invention includes a preparation process, an arrangement process, an eddy current generation process, a frequency change process, an impedance calculation process, and an evaluation process.

[0136] In the preparation process, a non-destructive inspection device having a frequency variable circuit and a coil is prepared. The frequency variable circuit can change the frequency of an alternating current. The coil can excite an alternating magnetic field by the alternating current.

[0137] In the arrangement process, the object under inspection is arranged in such a way that the alternating magnetic field excited by the coil penetrates into the interior of the object under inspection. The object under inspection is steel after surface treatment.

[0138] In the eddy current generation process, the alternating magnetic field is made to penetrate into the interior of the object under inspection, thereby causing the object under inspection to generate eddy currents.

[0139] In the frequency change process, the frequency variable circuit continuously changes the frequency of the alternating current from a low frequency to a high frequency, thereby continuously changing the penetration depth of the alternating magnetic field into the object under inspection.

[0140] In the impedance calculation process, based on the potential difference between both ends of the coil and the current value flowing through the coil, the impedance value of each penetration depth of the object under inspection is calculated.

[0141] Then, in the evaluation process, the ratio of the impedance value of each penetration depth of the object under inspection calculated in the impedance calculation process to the impedance value of each penetration depth of the steel before surface treatment is calculated, and based on the calculation result of the ratio, the factors causing changes in the permeability of the object under inspection are determined, and the surface treatment state is evaluated. The factors causing changes in permeability are assumed to include, for example, the carbon content and nitrogen content of the object under inspection, the size and shape of the object under inspection, the hardness of the object under inspection, etc.

[0142] According to the non-destructive inspection method of steel described above, by means of the non-destructive inspection device, after generating eddy currents in the inspection object, the penetration depth of alternating current magnetic field penetrating into the inspection object is continuously changed. Thus, it is possible to calculate the ratio (impedance ratio) of the impedance value of each penetration depth of the inspection object calculated in the impedance calculation process to the impedance value of each penetration depth of the steel before surface treatment. Then, based on the calculation result of this ratio, the factors causing the change in the magnetic permeability of the inspection object can be determined, and the surface treatment state of the inspection object can be evaluated with high precision.

[0143] For example, taking the steel after carburizing and quenching process and then shot peening process as the inspection object, when it is determined that the magnetic permeability of the inspection object is larger than that of the good product, it is determined that the carbon content of the inspection object is low and there is a problem in the carburizing and quenching process. On the other hand, when it is determined that the magnetic permeability of the inspection object is smaller than that of the good product, it is determined that there is a problem in the shot peening process and the magnetic permeability cannot be sufficiently increased. In this way, the surface treatment state of the inspection object can be evaluated with high precision.

[0144] In addition, in the non-destructive inspection method of steel according to one aspect of the present invention, it is characterized in that the factors causing the change in the magnetic permeability include any one of the carbon content of the inspection object, the magnitude and direction of the elastic deformation of the inspection object, the magnitude of the plastic deformation of the inspection object, and the atomic arrangement state of the inspection object.

[0145] According to the non-destructive inspection method of steel described above, for example, evaluating the carbon content state (carbon content) of the inspection object as a factor causing the change in magnetic permeability, it is possible to determine, by using the change in magnetic permeability caused by the carbon content state of the good product and the defective product, that there is a problem in the carburizing and quenching process, etc., and the inspection object can be evaluated more accurately. In addition, the carbon content state of the inspection object can also be evaluated by calculating the carbon content of the inspection object based on the ratio. Further, the carbon content state of the inspection object can also be evaluated based on the correlation between the ratio and the carbon content state of the inspection object.

[0146] In addition, in the non-destructive inspection method of steel according to one aspect of the present invention, it is characterized in that the surface treatment at least includes shot peening treatment, quenching treatment, nitriding treatment, carburizing treatment, tempering treatment, annealing treatment, surface machining treatment, grinding treatment, and low-temperature annealing treatment.

[0147] According to the non-destructive inspection method of steel described above, it is at least possible to determine whether shot peening treatment, quenching treatment, nitriding treatment, carburizing treatment, tempering treatment, annealing treatment, surface machining treatment, grinding treatment, and low-temperature annealing treatment are carried out normally.

[0148] Further, a non-destructive inspection method for steel according to one aspect of the present invention is characterized in that, in an evaluation step, at least one of the following determination methods is executed to evaluate the surface treatment state of an object to be inspected: (1) determining whether the distribution of the determination ratio is within a threshold range; (2) determining whether the value of the ratio at a specific penetration depth is within a threshold range; and (3) determining whether the integral value of the ratio values in a specific value range of the penetration depth is within a threshold range.

[0149] According to the above non-destructive inspection method for steel, in the evaluation step, evaluation can be performed in such a manner that the optimal method is selected from the above three determination methods (1) to (3) according to the object to be inspected, thereby improving the accuracy of the evaluation. For example, in the case where it is assumed that a difference in impedance ratio between good products and defective products occurs over a wide range of the penetration depth of the object to be inspected, it is preferable to use the distribution of the ratio in (1) for evaluation. Further, in the case where it is assumed that a difference in impedance ratio between good products and defective products significantly appears at a specific part of the penetration depth, it is preferable to use the ratio at a specific penetration depth in (2) for evaluation. In addition, in the case where it is difficult to perform evaluation using either of the above determination methods (1) and (2), by using the integral value of the ratio values in a specific value range of the penetration depth for evaluation, a minute ratio difference can be determined.

[0150] Further, a non-destructive inspection method for steel according to one aspect of the present invention is characterized in that a setting step is performed before the above-described arrangement step, and in the setting step, the determination method to be executed in the evaluation step is selected, and a threshold range is set for the selected determination method.

[0151] According to the above non-destructive inspection method for steel, by appropriately setting the threshold range used in the evaluation step in the setting step, the accuracy of the evaluation can be adjusted according to the needs of the user.

[0152] Further, a non-destructive inspection method for steel according to one aspect of the present invention is characterized in that, in the evaluation step, the size and shape of the object to be inspected are evaluated as an evaluation of the surface treatment state.

[0153] According to the above non-destructive inspection method for steel, it is possible to evaluate whether the object to be inspected has a desired size and shape.

[0154] Further, a non-destructive inspection method for steel according to one aspect of the present invention is characterized in that, in the evaluation step, the hardness of the object to be inspected is evaluated as an evaluation of the surface treatment state.

[0155] According to the non-destructive inspection method of steel described above, it is possible to evaluate whether the inspection object has the desired hardness.

[0156] Description of reference numerals

[0157] 1: Non-destructive inspection device; 11: AC power supply; 12: Frequency variable circuit; 21: Coil; 37: Control unit; 38: Display device; M: Inspection object; S1: Preparation process; S2: Setting process; S3: Arrangement process; S4: Eddy current generation process; S5: Frequency change process; S6: Impedance calculation process; S7, S7a, S7b: Evaluation process; S8: Notification process.

Claims

1. A non-destructive inspection method for steel, characterized in that, it includes: A preparation process of preparing a non-destructive inspection device, the non-destructive inspection device having a frequency variable circuit capable of changing the frequency of an alternating current and a coil capable of exciting an alternating magnetic field by the alternating current; An arrangement process of using the surface-treated steel as an inspection object and arranging the inspection object in such a way that the alternating magnetic field excited by the coil penetrates into the interior of the inspection object, wherein the surface treatment includes at least carburizing and quenching treatment and shot peening treatment; An eddy current generation process of causing the alternating magnetic field to penetrate into the interior of the inspection object, thereby generating eddy currents in the inspection object; A frequency change process of continuously changing the frequency of the alternating current from a low frequency to a high frequency by the frequency variable circuit, thereby continuously changing the penetration depth of the alternating magnetic field into the inspection object; An impedance calculation process of calculating the impedance value of each penetration depth of the inspection object based on the potential difference between both ends of the coil and the current value flowing through the coil; and An evaluation process of calculating the ratio of the impedance value of each penetration depth of the inspection object calculated in the impedance calculation process to the impedance value of each penetration depth of the steel before the surface treatment, and determining the change factor of the magnetic permeability of the inspection object based on the calculation result of the ratio, and evaluating the surface treatment state, In the evaluation process, when it is determined that the magnetic permeability of the inspection object is greater than the magnetic permeability of a good product of the steel that has been well surface-treated, it is determined that the change factor of the magnetic permeability is the poor carburizing and quenching treatment. On the other hand, when it is determined that the magnetic permeability of the inspection object is smaller than the magnetic permeability of the good product, it is determined that the change factor of the magnetic permeability is the poor shot peening treatment, In the evaluation process, the surface treatment state of the inspection object is evaluated as follows: (1) When it is assumed that the difference in the ratio between the good product and the defective product occurs in a wide range of the penetration depth of the inspection object, determine whether the distribution of the ratio is within the threshold range; (2) When it is assumed that the difference in the ratio between the good product and the defective product significantly appears at a specific part of the penetration depth of the inspection object, determine whether the value of the ratio at the specific penetration depth is within the threshold range; (3) When it is difficult to evaluate using both the determination methods of (1) and (2), determine whether the integral value of the ratio value in a specific value range of the penetration depth is within the threshold range.

2. The non-destructive inspection method for steel according to claim 1, characterized in that, the change factor of the magnetic permeability includes any one of the carbon content of the inspection object, the magnitude and direction of the elastic deformation of the inspection object, the magnitude of the plastic deformation of the inspection object, and the atomic arrangement state of the inspection object.

3. The non-destructive inspection method for steel materials according to claim 1 or 2, characterized in that, the surface treatment at least includes the shot peening treatment, quenching treatment, nitriding treatment, carburizing treatment, tempering treatment, annealing treatment, grinding treatment.

4. The non-destructive inspection method for steel materials according to claim 1, characterized in that, a setting process is performed before the configuration process, and in the setting process, the determination method to be executed in the evaluation process is selected, and a threshold range is set for the selected determination method.

5. The non-destructive inspection method for steel materials according to claim 1 or 2, characterized in that, in the evaluation process, the size and shape of the inspection object are evaluated as the evaluation of the surface treatment state.

6. The non-destructive inspection method for steel materials according to claim 1 or 2, characterized in that, in the evaluation process, the hardness of the inspection object is evaluated as the evaluation of the surface treatment state.

7. The non-destructive inspection method for steel materials according to claim 1 or 2, characterized in that, the surface treatment at least includes surface machining treatment, low-temperature annealing treatment.

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