| 摘要 |
PROBLEM TO BE SOLVED: To quantitatively evaluate the degree of residual distortion as plasticity by measuring a magnetic field of the surface of a steel product to determine a distribution range in terms of a difference between the maximum and the minimum values of a magnetic signal within a zone of a specified size. SOLUTION: A specified tensile load is applied to a test object 10 is applied to measure the degree of residual distortion with aπtype displacement meter or the like. The surface magnetic field of the test object 10 after loaded is measured, for example, by a superconducting quantum interference device fluxmeter having a concentric type secondary differentiation detection coil and zoned by 5×5 cm<2> to define a difference between the maximum value and minimum value within the zone. A distribution range as a difference between the maximum and minimum values of a space differential value of each zone positioned on a specified line in the longitudinal direction of a narrow part 10b of the test object 10 correlates with the degree of residual distortion in a plastically deformed area. Therefore, for example, the surface of a steel material of the existing structure is scanned by the fluxmeter to extract a space differential value of each zone on a line along the direction of drawing the steel product. Thus, by learning the distribution range of the space differential values, the residual distortion can be obtained quantitatively.
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