A fast and non-destructive method to evaluate yield strength of cold-rolled steel via incremental permeability

Tensile test is one of the most widely used testing methods to measure the yield strength of the processed cold-rolled steel sheets due to its credible results. This testing method is destructive and time-consuming so it can only be used to detect the target parameters on the materials in the same batch. To solve this problem, an online and nondestructive testing device based on incremental permeability (IP) is proposed. IP is related to the reversible domain movements of ferromagnetic materials during the period of dynamic magnetization. This paper analyzed the relationship among IP, eddy-current (EC) impedance and microstructure, and discovered a new IP feature that indicates materials' average grain size and lattice friction. Experiments used the new feature to estimate the yield strength of steels and obtained an approximately accurate result. In addition, to raise the testing accuracy and realize the quantitative NDT of the yield strengths in cold-roll steel sheets, the rolling direction of the steel sheet was studied and analyzed in our research.