Methods for evaluating the surface characteristics of steel materials.

Abstract

Provided is a surface characteristic evaluation method for evaluating the residual stress of steel subjected to surface modification processing while taking the depth direction into consideration. This method includes a step for preparing a surface characteristic evaluation device, a step for disposing steel subjected to surface modification processing such that the steel is permeated by AC magnetism generated by a coil included in the surface characteristic evaluation device, a step for generating eddy currents in a subject, a step for consecutively changing the AC magnetism applied to the coil, a step for calculating an impedance Z1 for each frequency from the potential difference across the coil and the value of the current flowing through the coil, a step for carrying out calculation on the basis of the impedances Z1, and a step for evaluating the residual stress of the steel on the basis of the result obtained from the calculation.

Description

Steel surface characteristic evaluation method

[0001] The present invention relates to a method for evaluating residual stress in a steel material that has been subjected to a surface modification treatment.

[0002] Various heat treatments (carburizing and quenching, nitriding heat treatment, induction hardening, etc.) and shot peening are widely known as surface modification treatments for steel materials. Surface modification treatments impart residual stress near the surface of the steel material, improving the fatigue strength of the steel material. The treatment conditions for surface modification treatments are considered so that the desired residual stress is imparted to the depth from the steel surface depending on the steel material's intended use. To accurately evaluate whether this surface modification treatment has been performed appropriately, the depth direction must also be taken into consideration.

[0003] A method for measuring the fatigue strength of steel material is disclosed in Patent Document 1. In Patent Document 1, the depth at which a predetermined compressive residual stress of a steel material that has been subjected to shot peening reaches a peak value is evaluated. However, the evaluation method disclosed in Patent Document 1 requires that measurement conditions be set for each measurement object and surface modification treatment conditions. Therefore, accurate evaluation cannot be performed due to variations in individual materials, etc.

[0004] Another method for measuring the fatigue strength of steel is disclosed in Patent Document 2. In Patent Document 2, the frequency of the excitation current passed through an excitation coil in contact with the steel material is sequentially changed to sequentially change the penetration depth (magnetic permeability) of the magnetic flux on the steel material surface, while sequentially measuring the output voltage value of the detection coil, thereby calculating the distribution of compressive residual stress in the steel material. However, the output voltage value of the detection coil includes a voltage component due to changes in magnetic permeability and a voltage component due to the impedance of the detection coil itself. Therefore, if the impedance characteristics of the detection coil itself change due to changes in the surrounding environment (temperature, noise, etc.), the reliability of the measured value decreases. Furthermore, the excitation coil in this measurement device must be designed taking into account the phenomenon in which the detection signal changes depending on the distance from the steel material (lift-off phenomenon), but this perspective is not disclosed. As such, the measurement device in Patent Document 2 cannot accurately evaluate the compressive residual stress of steel material.

[0005] JP-A No. 07-092140 JP-A No. 05-203503

[0006] In view of the above, an object of the present invention is to provide a surface characteristic evaluation method that can accurately evaluate the residual stress of a steel material that has been subjected to a surface modification treatment, taking into account the distribution in the depth direction.

[0007] One aspect of the present invention is a surface property evaluation method for evaluating the residual stress of a steel material that has undergone a surface modification treatment. This surface property evaluation method includes the following steps (1) to (6). These steps may be performed separately, or two or more may be performed simultaneously. (1) A preparation step of preparing a surface property evaluation device including an oscillator equipped with an AC power supply and a frequency variable circuit, a detector connected to the oscillator and equipped with a coil, and a measuring instrument connected to the frequency variable circuit and the detector. (2) A test object placement step of placing a steel material that has undergone a surface modification treatment as a test object so that an AC magnetic field excited by the coil penetrates into the test object. (3) An eddy current generation step of activating an AC power supply to excite an AC magnetic field in the coil, causing the AC magnetic field to penetrate into the test object, thereby generating an eddy current in the test object. (4) A frequency change step of continuously changing the frequency of the AC current using a frequency variable circuit, thereby continuously changing the penetration depth of the AC magnetic field into the test object. (5) An impedance calculation step of detecting the potential difference between both ends of the coil and the value of the current flowing through the coil and calculating the impedance Z1 for each frequency. (6) An evaluation step of evaluating the residual stress of the test object based on the impedance Z1.

[0008] In one aspect of the present invention, the internal strain caused by the surface modification treatment is detected by penetrating an AC magnetic field excited in a coil into the steel material. The depthwise distribution of this strain can be detected by continuously changing the frequency of the AC current applied to the coil. Furthermore, by using the impedance Z1, the signal corresponding to the coil impedance and the S / N ratio (S: evaluation voltage, N: noise other than the evaluation voltage) can be increased, thereby improving the evaluation accuracy. In this way, the residual stress of a steel material that has undergone a surface modification treatment can be accurately evaluated taking into account the depthwise direction.

[0009] In one embodiment, the method may further include a reference impedance measurement step of measuring a reference impedance Z0 for each frequency using a steel material before surface modification as a test object. Then, in the evaluation step, the ratio between the reference impedance Z0 and the impedance Z1 may be calculated for each frequency, and the residual stress of the steel material after surface modification may be evaluated based on this calculated value group. By using the ratio of impedances measured using the steel material before and after surface modification as a test object, voltage drift due to changes in ambient temperature and humidity can be reduced. Furthermore, it is possible to extract only the changes in electromagnetic properties due to the surface modification. As described above, the residual stress of the steel material after surface modification can be accurately evaluated by taking into account the distribution in the depth direction.

[0010] In one embodiment, the evaluation step may calculate the inductive reactance X for each frequency using the impedance Z, and evaluate the residual stress of the steel material that has been subjected to the surface modification treatment based on these inductive reactances X. By calculating the inductive reactance, which is the Y-axis component of the impedance (the imaginary component of the complex impedance), and performing evaluation based on this inductive reactance, it is possible to evaluate only the magnetic permeability of the test object. As a result, the accuracy of the evaluation is improved.

[0011] In one embodiment, the penetration depth of the AC magnetic field may be converted from the frequency of the AC current using the following formula, and the depth-wise distribution of residual stress in the steel material that has been subjected to the surface modification treatment may be evaluated based on the converted penetration depth. This makes it possible to accurately grasp and evaluate the depth-wise distribution of strain.

[0012] In one embodiment, the group of calculated values ​​may be plotted on a coordinate system having the following coordinate axes A and B. The distribution of strain inside the steel material caused by the surface modification treatment can be visually grasped. Coordinate axis A: Indicates the ratio of the reference impedance Z0 measured using the steel material before the surface modification treatment as the test object to the impedance Z1 measured using the steel material after the surface modification treatment as the test object. Coordinate axis B: Indicates the frequency.

[0013] In one embodiment, in the step of evaluating the residual stress, the residual stress of the surface-treated steel material may be evaluated based on the locus of a graph of a group of calculated values ​​plotted on a coordinate system having coordinate axes A and B and one or more calculated values ​​including the extreme values ​​of this locus. The residual stress can be evaluated from both the aspect of the distribution of strain inside the steel material caused by the surface modification treatment and the value of the calculated value at the depth where the extreme value occurs.

[0014] In one embodiment, in the evaluation step, the quality of the surface modification treatment may be determined by comparing one or more calculated values, including the extreme values, with a preset threshold value. Whether the surface modification treatment has been properly performed can be determined taking into account the depth direction.

[0015] According to one aspect and embodiment, a surface characteristic evaluation method can be provided that can accurately evaluate the residual stress of a steel material that has been subjected to a surface modification treatment, taking into account the depth direction. This makes it possible to accurately evaluate the degree of the surface modification treatment applied to the steel material, taking into account the distribution in the depth direction.

[0016] FIG. 1 is a circuit diagram for explaining one embodiment of the present invention. FIG. 2 is a schematic diagram for explaining eddy currents generated in a coil in one embodiment of the present invention. FIG. 3 is a flowchart for explaining a surface property evaluation method in one embodiment of the present invention. FIG. 4 is a graph for explaining an example in one embodiment of the present invention. FIG. 5 is a graph showing residual stress of a steel material used in an example in one embodiment of the present invention. FIG. 6 is a circuit diagram for explaining another embodiment (modified example 1) of the present invention. FIG. 7 is a circuit diagram for explaining another embodiment (modified example 2) of the present invention.

[0017] An embodiment of the present invention will be described with reference to the drawings. In the following description, up, down, left, and right directions refer to the directions in the drawings unless otherwise specified.

[0018] (Surface Property Evaluation Apparatus) A surface property evaluation apparatus 1 according to one embodiment includes an oscillator 10 , a detector 20 , and a measuring instrument 30 .

[0019] The oscillator 10 includes an AC power supply 11 and a frequency variable circuit 12. The frequency variable circuit 12 is connected to the AC power supply 11 and is capable of changing the frequency of the AC current output from the AC power supply 11.

[0020] The detector 20 includes a coil 21. One end (point A) of the coil 21 is connected to the AC power supply 11, and is supplied with an AC current output from the AC power supply 11. The circuit symbol within the dashed line indicating the coil 21 in FIG. 1 indicates the electrical equivalent circuit of the coil 21.

[0021] The measuring instrument 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 means 37, and a display 38. The control means 37 also includes a storage means inside it or in a region not shown. Specifically, the control means 37 can be configured with a microprocessor, an interface circuit, a memory, and a program for operating these (all not shown), etc.

[0022] The amplifier circuit 31 is connected to points A and B, which are both ends of the coil 21. A signal representing the potential difference between points A and B is input to the amplifier circuit 31 and amplified. This amplified signal is full-wave rectified by an absolute value circuit 32 and then converted to DC by an LPF 33. This converted signal is input to the control means 37.

[0023] The I / V conversion circuit 34 is connected to the other end (point B) of the coil 21. A signal indicating the current value of the current flowing through the coil 21 is input to the I / V conversion circuit 34 and converted into a signal indicating a potential difference. The signal is then full-wave rectified by an absolute value circuit 35 and converted into DC by an LPF 36. The converted signal is input to a control means 37.

[0024] The control means 37 is connected to the frequency variable circuit 12, the LPF 33, and the LPF 36, and receives as input the frequency of the AC current applied to the coil 21 and the signal corresponding to this frequency that has passed through the LPFs 33 and 36. Calculations are performed based on these input signals, and the surface characteristics of the specimen are evaluated based on the calculation results. The frequency of the AC current may be changed manually, or the frequency may be changed automatically by providing the control means 37 with a function to output a signal that continuously changes the frequency to the frequency variable circuit 12. In this embodiment, the latter method is used.

[0025] The display 38 displays the evaluation result by the control means 37 or issues a warning.

[0026] (Evaluation Method) Next, a method for evaluating the surface characteristics of a specimen using the surface characteristic evaluation device 1 of this embodiment will be described. In the following, a case will be described in which shot peening (hereinafter referred to as SP treatment) is selected as the surface modification treatment, and the compressive residual stress of a steel material that has been subjected to the SP treatment is evaluated as the degree of the surface modification treatment.

[0027] S01: Preparation Step: Prepare the surface property evaluation device 1 and the aforementioned steel material (untreated product) before SP treatment. In this embodiment, a 40 mm diameter × 30 mm diameter piece of chromium-molybdenum steel (SCM420H specified in JIS G4053) that had been carburized and quenched was prepared.

[0028] S02: First Placement Step (Unprocessed Article) An unprocessed article is set in the detector 20 as a test object. There are no particular limitations on the method for setting the test object, as long as the AC magnetic field penetrates into the test object when it is excited in the coil 21 in the next step. In this embodiment, the unprocessed article is set at the center of the circular cross section of the coil 21, and so that the entire unprocessed article is located inside the coil 21. Setting the test object in this manner can reduce variations in the direction perpendicular to the depth direction inside the test object due to the material, thereby improving measurement accuracy.

[0029] S03: Untreated Product Measurement Process (Reference Impedance Measurement Process) As a first eddy current generation process, the control means 37 outputs a signal to the frequency variable circuit 12 to control the frequency of the AC current output from the AC power supply 11, and activates the AC power supply 11. Activation of the AC power supply 11 excites AC magnetic field in the coil 21 (see FIG. 2). Because the test object is set on the inner periphery of the coil 21, this AC magnetic field penetrates the test object. The penetration of the AC magnetic field generates eddy currents on the surface of the test object. The eddy currents generate a demagnetizing field against the AC magnetic field. At this time, the magnetic permeability changes depending on the magnitude of the residual stress. Therefore, the magnitude of the magnetic flux resulting from this demagnetizing field and the AC magnetic field varies depending on the magnitude of the residual stress, which indicates the degree of surface modification treatment. In other words, depending on the magnitude of the residual stress, the signals indicating the electrical characteristics of the coil when AC current flows through the coil (the signal indicating the potential difference between the coils (between A and B) and the signal indicating the current value after flowing through the coil) change. The control means 37 calculates the impedance Z0 at that frequency from the signal input to the control means 37 through the amplifier circuit 31, absolute value circuit 32, and LPF 33, and the signal input to the control means 37 through the I / V conversion circuit 34, absolute value circuit 35, and LPF 36. Furthermore, the depth to which the AC magnetic field penetrates into the subject depends on the frequency of the AC current. Therefore, in a first frequency changing step, the control means 37 changes the frequency of the AC current output from the AC power supply 11. While changing the frequency of the AC current, in a first detection step, a signal indicating the electrical characteristics of the AC current is detected for each frequency, and the impedance Z0 of the coil 21 is calculated from this signal and stored in the storage means.

[0030] S04: Surface Modification Treatment Step After the untreated product is removed from the detector 20, compressive residual stress is imparted by SP treatment to obtain the above-mentioned steel material that has been subjected to surface modification treatment (surface-modified product).

[0031] S05: Surface Modification Treated Product Measurement Step The steel material (surface modification treated product) that has been surface modified in step S04 is set on the detector 20 as a second test object placement step. Next, a second eddy current generation step, a second frequency change step, and a second detection step similar to step S03 are performed, and the impedance Z1 of the coil 21 for each frequency is calculated. The frequency in this step is the same as that in step S03. Since the aforementioned demagnetizing field differs depending on the magnitude of the compressive residual stress, which indicates the degree of surface modification, the degree of surface modification of the test object can be evaluated by evaluating the electrical characteristics of the alternating current flowing through the coil.

[0032] S06: Judgment step (evaluation step) The ratio of impedance Z1 to impedance Z0 (Z1 / Z0) is calculated for each frequency by the control means 37 to obtain a group of calculated values. By using the impedance ratio (Z1 / Z0) to evaluate the surface characteristics, it is possible to reduce voltage drift caused by changes in the measurement environment (temperature, humidity, etc.). In addition, since it is possible to extract only the changes in the electromagnetic characteristics of the test object due to the surface modification treatment, the accuracy of the evaluation of the surface characteristics is improved.

[0033] A graph (coordinate system) is prepared with the impedance ratio (Z1 / Z0) on the vertical axis and frequency on the horizontal axis, and points corresponding to the calculated values, which are the impedance ratios (Z1 / Z0) calculated for each frequency, are plotted on the graph. Since frequency corresponds to the depth from the surface of the test object, the depthwise distribution of compressive residual stress, which indicates the degree of surface modification treatment, can be visually grasped from the locus on the graph.

[0034] The depth from the surface of the steel material may be calculated from the frequency and used as the horizontal axis. The relationship between the frequency and the depth from the surface of the steel material can be calculated from a calibration curve that shows the relationship between the frequency and the depth from the surface of the steel material, which is created from (Equation 2). The correction coefficient k in (Equation 2) is a value that varies depending on the shape (e.g., the volume of the test object) and properties (e.g., whether or not a heat treatment has been performed as a preliminary step), the conditions of the SP treatment (e.g., the particle size, hardness, injection time, and injection pressure of the shot), etc., and is calculated in advance by experiment.

[0035]

[0036] Based on the obtained distribution, the quality of the surface modification treatment is judged by the control means 37. An example of a judgment method is shown below, but the present invention is not limited to this method.

[0037] A group of thresholds and an allowable range for determining whether or not the SP treatment has been properly performed are calculated in advance. A plurality of steel products that have been properly SP treated (good products) and a plurality of steel products that have not been satisfactorily SP treated (defective products) are measured using the operations in steps S01 to S04 described above, and a group of thresholds and an allowable range that indicate the impedance ratio and the corresponding frequency or depth from the steel surface are determined based on these measurements.

[0038] This set of thresholds is compared with the set of calculated values. For example, several frequencies (e.g., six) are selected from the frequencies at which the locus of the graph of the set of calculated values ​​from which the impedance ratios are calculated reaches an extreme value, as well as any frequencies in the vicinity thereof, and the impedance ratios at the selected frequencies are compared with the thresholds. If all of the compared values ​​are within the allowable range of the thresholds, it is determined that "SP processing has been performed properly." If even one value deviates from the threshold range, it is determined that "SP processing has been performed improperly."

[0039] Another determination method involves determining whether or not a graph showing a group of calculated values ​​(a graph showing the relationship between the frequency or depth from the steel surface and the impedance ratio) is within the allowable range of a similar graph shown by a group of threshold values, and determining whether or not an extreme value in the graph of the impedance ratio is within the allowable range of extreme values ​​in the group of threshold values. If both are within the allowable range of the group of threshold values, it is determined that "SP processing has been performed appropriately," and if either of them deviates from the allowable range of the group of threshold values, it is determined that "SP processing is inappropriate."

[0040] S07: Output step The result of the judgment of whether the surface modification treatment is successful or not is output to the display 38. The display 38 may only display the result of success or failure, or may sound an alarm when the result is failure. Alternatively, the display 38 may display the graph described above (a graph showing the relationship between the frequency (or depth from the surface of the steel material) and the impedance ratio).

[0041] Through the above steps, it is possible to evaluate the degree of surface modification treatment taking into account the depth direction from the surface of the specimen.

[0042] Next, the results of evaluating the surface characteristics of steel materials using the surface characteristic evaluation device 1 of this embodiment will be shown.

[0043] Shot peening was performed by spraying shot having an average particle size of 50 μm to 1000 μm (all manufactured by Shinto Kogyo Co., Ltd.) onto the above-mentioned carburized and quenched chromium molybdenum steel (φ40 mm × 30 mm) using a shot peening machine (manufactured by Shinto Kogyo Co., Ltd.) at a spray pressure of 0.3 MPa to achieve a coverage of 300% (see Table 1). This chromium molybdenum steel that had been subjected to the shot peening treatment was used as the specimen.

[0044]

[0045] The frequency of the AC current (used frequency) was set to 10 kHz to 20 MHz. The penetration depth of the AC magnetic field was calculated from the used frequency using the calibration curve.

[0046] The results are shown in Figure 4. It can be seen from Figure 4 that the depth at which the locus of the impedance ratio (Z / Z) shows an extreme value is around 10 µm for specimen A, around 25 µm for specimen B, and around 55 µm for specimen C.

[0047] Next, the compressive residual stress of specimens A to C was measured using an X-ray stress measurement device, and the results are shown in Figure 5. It can be seen from Figure 5 that the depth at which the compressive residual stress locus shows extreme values ​​is approximately 5 to 10 µm for specimen A, approximately 20 µm for specimen B, and approximately 50 µm for specimen C. Since there is a substantial correlation between the depths at which extreme values ​​are shown in Figures 4 and 5, it is suggested that the surface property evaluation device and surface property evaluation method of this embodiment can be used to evaluate the degree of surface modification treatment taking into account the depth direction.

[0048] (Modification 1) A surface property evaluation apparatus 2 according to another embodiment is shown in Figure 6. In the surface property evaluation apparatus 2 of this embodiment, the measuring instrument 30 in the surface property evaluation apparatus 2 of the first embodiment includes an amplifier circuit 31, an A / D conversion circuit 39a, an I / V conversion circuit 34, an A / D conversion circuit 39b, control means 37, and a display 38. The control means 37 has a built-in storage means 37a. Note that the storage means 37a may be provided external to the control means 37. Furthermore, since the configurations of the oscillator 10 and the detector 20 are the same as those of the first embodiment described above, a description thereof will be omitted, and the following description will focus on the differences from the first embodiment.

[0049] The amplifier circuit 31 is connected to points A and B, which are both ends of the coil 21. A signal representing the potential difference between points A and B is input to the amplifier circuit 31 and amplified. This amplified signal is converted from an analog voltage signal to a digital signal by an A / D conversion circuit 39a. This converted digital signal is input to the control means 37.

[0050] The I / V conversion circuit 34 is connected to the other end (point B) of the coil 21. A signal indicating the current value of the current flowing through the coil 21 is input to the I / V conversion circuit 34 and converted into an analog voltage signal. The analog voltage signal output from the I / V conversion circuit 34 is converted into a digital signal by the A / D conversion circuit 39b and input to the control means 37.

[0051] In the control means 37, the digital signals input from the A / D conversion circuits 39a and 39b are processed by digital signal processing. That is, the digital signals input from the amplifier circuit 31 and the I / V conversion circuit 34 are time-series signals that fluctuate in an AC manner, but they are converted into DC-like digital signals by digital calculations equivalent to the absolute value circuits 32 and 35 and LPFs 33 and 36 (FIG. 1) in the above-described embodiment. As a result, the analog voltage AC signals input to the A / D conversion circuits 39a and 39b are converted into digital values ​​proportional to the amplitude of the AC signals in the control means 37. Then, the impedance is calculated based on these digital values.

[0052] The surface property evaluation device 2 of this embodiment performs signal calculations using digital signal processing, making it less susceptible to noise, and therefore can perform evaluations with higher accuracy even in environments where noise is likely to occur.

[0053] (Modification 2) A surface property evaluation apparatus 3 according to another embodiment is shown in Fig. 7. In the surface property evaluation apparatus 3 according to this embodiment, a phase detection circuit 301, an absolute value circuit 302, and an LPF 303 are newly added to the measuring instrument 30 of the surface property evaluation apparatus 1 (Fig. 1) according to the first embodiment. Furthermore, since the configurations of the oscillator 10 and the detector 20 are the same as those in the first embodiment described above, their explanations will be omitted, and the following description will focus on the configuration newly added to the measuring instrument 30.

[0054] The phase detection circuit 301 is connected to the other end (point B) of the AC power supply 11 and the coil 21. A signal indicating the phase difference between the voltage applied by the AC power supply 11 and the current flowing through the coil 21 is output from the phase detection circuit 301, and after being full-wave rectified by an absolute value circuit 302, is converted to DC by an LPF 303. This converted signal is input to the control means 37. In other words, a voltage signal proportional to the phase difference between the voltage applied to the coil 21 and the current flowing through the coil 21 is input to the control means 37.

[0055] In the control means 37, in the measurement step S03 (FIG. 3), the impedance Z0 when an untreated product is used as the test object is calculated. Furthermore, the phase difference α0 when an untreated product is used as the test object is calculated from the signal input from the LPF 303. From the calculated impedance Z0 and phase difference α0, the inductive reactance X0 is calculated for each frequency using the formula X0 = Z0 × sin α0. Furthermore, in the measurement step S05 (FIG. 3), the impedance Z1 and phase difference α1 when a surface-modified product is used as the test object are similarly calculated, and the inductive reactance X1 is calculated for each frequency.

[0056] In the judgment step S06 (FIG. 3), the ratio (X1 / X0) of the inductive reactance X1 (surface-modified product) to the aforementioned inductive reactance X0 (untreated product) is calculated for each frequency by the control means 37. By using the inductive reactance ratio to evaluate the surface characteristics, it is possible to evaluate only the magnetic permeability of the test object. Although the inductive reactance has a smaller value than the impedance, it has excellent sensitivity to changes in electrical characteristics. When particularly precise evaluation is required, a more accurate evaluation can be performed by using the inductive reactance ratio.

[0057] The inductive reactance may be calculated by analog signal processing as in this embodiment, or by digital signal processing in the control means 37 using the circuit of FIG. 6. In this case, digital calculations equivalent to those of the phase detection circuit 301, absolute value circuit 302, and LPF 303 in the circuit of FIG. 7 are performed inside the control means 37, and the inductive reactance (X0, X1) is calculated based on the determined phase difference (α0, α1). That is, the control means 37 performs the following calculations a to c: a) The control means 37 calculates the phase difference (α0, α1) between the digital signal obtained by A / D conversion of the voltage between both ends of the coil by the A / D conversion circuit 39a and the digital signal obtained by A / D conversion of the signal of the current flowing through the coil by the A / D conversion circuit 39b. b) The impedance (Z0, Z1) is calculated from the digital signals input via the A / D conversion circuits 39a and 39b using the calculations described with reference to FIG. 6. c) Using the "phase difference" and "impedance" calculated in a and b above, calculate the inductive reactance (X0, X1).

[0058] In one embodiment, the evaluation of the degree of shot peening treatment when shot peening treatment is performed on carburized and quenched steel material has been described. However, the surface property evaluation device and surface property evaluation method of the present invention can also be used to evaluate cases where various heat treatments are performed as surface modification treatments. Furthermore, steel material that has only been subjected to shot peening treatment can also be evaluated.

[0059] REFERENCE SIGNS LIST 1 Surface property evaluation device 2 Surface property evaluation device of another embodiment (Modified example 1) 3 Surface property evaluation device of another embodiment (Modified example 2) 10 Oscillator 11 AC power supply 12 Frequency variable circuit 20 Detector 21 Coil 30 Measuring instrument 31 Amplifier circuit 32 Absolute value circuit 33 LPF 34 I / V conversion circuit 35 Absolute value circuit 36 ​​LPF 37 Control means 38 Display 39a A / D conversion circuit 39b A / D conversion circuit 301 Phase detection circuit 302 Absolute value circuit 303 LPF