Surface characteristics evaluation method for steel material

A surface characteristics evaluation method of evaluating a residual stress in a subject made of a steel material subjected to a surface modification treatment, comprising:

• a preparation step of preparing a surface characteristics evaluation apparatus, wherein the surface characteristics evaluation apparatus comprises
• an oscillator including an AC power supply and a frequency changing circuit,
• a detector including a coil connected to the oscillator, an amplifier amplifying a potential difference between opposite ends of the coil, a first absolute value circuit connected to the amplifier, an I/V conversion circuit connected to one end of the coil and a second absolute value circuit connected to the I/V conversion circuit, and
• a measuring instrument connected to the frequency changing circuit and the first and second absolute value circuits of the detector;
• a subject placement step of placing the subject in such a manner that an alternating magnetic field induced by the coil permeates into the subject;
• an eddy current generation step of generating an eddy current in the subject by activating the AC power supply to make the coil induce the alternating magnetic field and allowing the alternating magnetic field to permeate into the subject;
• a frequency change step of successively changing the frequency of the alternating current by the frequency changing circuit, thereby successively changing a depth of permeation of the alternating magnetic field into the subject;
• an impedance calculation step of calculating a magnitude of impedance Z1 for each of different frequencies by detecting a signal representing a potential difference outputted from the first absolute value circuit and a signal representing a value of a current flowing through the coil from the second absolute value circuit;
• a reference impedance measurement step of preliminary measuring a magnitude of reference impedance Z0 at each of different frequencies for a subject yet to be subjected to the surface modification treatment;
• and
• an evaluation step of evaluating the residual stress in the subject based on the magnitude of the impedance Z1,
• wherein in the evaluation step, impedance ratios between the magnitude of the reference impedance Z0 and the magnitude of the impedance Z1 are computed for each of the different frequencies, and the residual stress in the subject subjected to the surface modification treatment is evaluated based on the impedance ratios.