Non-destructive measurement of microstructure and tensile strength in varying thickness commercial DP steel strip using an EM sensor

Abstract

It is desirable to be able to non-destructively evaluate the microstructural features and/or mechanical properties of steel strip. An electromagnetic sensor – sample FE model, developed in COMSOL multi-physics software, has been developed to determine the relationship between the low magnetic field relative permeability and microstructure (phase balance and grain size), and hence tensile strength, for commercial dual phase (DP) steel sheets of any thickness. The model for the effect of thickness on the sensor signal has been validated using DP steel sheets of 1 to 4 mm. It was found that the relative permeability is strongly dependent on the phase balance (fraction of ferrite and martensite) in the DP steels with an increasing relative permeability being seen for a higher fraction of ferrite. In addition, changes in the ferrite grain size also affect the relative permeability (increase in grain size increases relative permeability) and hence EM sensor signal, this being more significant for the higher ferrite content DP grades. Both these microstructural features affect the tensile strength (which decreases with an increase in ferrite fraction and increase in grain size), which results in a good correlation between the relative permeability and tensile strength. The results from the relative permeability – microstructure – tensile strength investigation and FE model to account for strip thickness have been combined to allow the prediction of tensile strength from EM sensor measurements on as-received commercial DP steel sheets of varying thickness and the approach has been tested.