In this paper is described whether the X-ray stress measurement method can be employed for the skin-pass process control of cold-rolled steel strip by examining the relation between the reduction rate of the skin-passed steel strip and its surface residual stress measured by X-ray diffraction. We also examined the internal stress distribution of some kinds of skin-passed steel strip.
Measurement were carried out using parallel beam X-ray with sollar slits, and a scintillation counter was employed as a detector. JIS, SPC-1, and SPC-3 grades of cold-rolled steel strip were used in order to measure the relation between reduction rate and surface residual stress, and the reduction rate were varied from 0.3% to 2.5%.
For the materials undergoing the heaviest reduction, the diffraction angle differences from 2θ0 (2θ0: diffraction angle on φ=0) were measured and these values were plotted in the axis of the ordinate and the values of sin2φ in the axis of the abscissa. As the angle differences and the values of sin2φ showed a linear relation, we applied the two point method, usually called“Perpendicular-45°+η”method.
For the skin-pass reduction, each residual stress showed a similar inclination on the surface of the steel strip. In the case of light reduction, residual stress becomes compression and, in the case of heavy reduction, it becomes tension. (cf. Figs. 3, 4 and 5).
From the results of our experiments, we conclude as follows.
The surface residual stress of skin-passed materials depends upon their“stress-strain curves” before rolling, especially upon the behavior of the neighboring part of yield-point. Therefore, if the X-ray stress measurement method is to be employed for skin-pass process control, it is considered necessary to grasp a full picture of the form of the stress-strain curve and check deviations from the typical curve. And it is also essential to keep the deviations as small as possible and to establish the relation between the skin-pass reduction and the residual stress on the materials having the typical curve.