A surface property evaluation method for evaluating surface treatment state of a metal product, comprising:
- a preparation step for preparing three test pieces consisting of a comparative sample whose surface is not treated, a reference sample whose surface is appropriately treated, and a test subject product for evaluation of performed surface treatment;
- an impedance distribution acquisition step for acquiring impedance distributions of the three test pieces;
- and
- an impedance distribution analysis step for evaluating surface treatment state of the test subject product, based on the impedance distributions acquired in the impedance distribution acquisition step;
- wherein the impedance distribution acquisition step comprises:
- a step for disposing each of the three test pieces inside a coil and applying an AC current to the coil to cause an AC magnetism excited by the coil to penetrate into each of the three test pieces;
- a step for varying the frequency of the AC current applied to the coil, thereby varying a penetration depth of the AC magnetism into each of the three test pieces;
- and
- a data acquisition step for acquiring the impedance distribution in a surface of each of the three test pieces by measuring coil impedances at multiple frequencies;
- wherein the impedance distribution analysis step comprises:
- a step for calculating an impedance ratio γ1 (Z1/Z0) at each frequency, wherein the impedance ratio γ1 of an impedance Z0 of the comparative sample and an impedance Z1 of the reference sample is acquired at each of the multiple frequencies in the data acquisition step to create a reference measurement value group Gγ1;
- a step for calculating at each frequency for the same multiple frequencies as in the reference measurement value group Gγ1 an impedance ratio γ2 (Z2/Z0), wherein the impedance ratio γ2 of an impedance Z2 of the test subject product and the impedance Z0 of the comparative sample is acquired in the data acquisition step to create an evaluation measurement value group Gγ2;
- and
- a step for evaluating the surface treatment state of the test subject product by comparing certain evaluation criteria, set based on the reference measurement value group Gγ1, with the evaluation measurement value group Gγ2;
- wherein the evaluation criteria are selected from among:
- an area width criterion in which the surface treatment state of the test subject product is evaluated by setting an allowable impedance ratio range at each frequency based on the impedance ratio γ1 at each frequency of the reference measurement value group Gγ1 and comparing said allowable impedance ratio range to the impedance ratio γ2;
- a peak criterion in which the surface treatment state of the test subject product is evaluated by setting an allowable peak position range based on the peak position of the reference measurement value group Gγ1 relative to frequency and comparing said allowable peak position range to the peak position in the impedance ratio γ2;
- and
- an integral area criterion in which the surface treatment state of the test subject product is evaluated by setting an allowable integral value range based on the integral value of the impedance ratio γ1 of the reference measurement value group Gγ1 in a predetermined frequency range, and comparing said allowable integral value range to the integral value of the impedance ratio γ2 in said frequency range.