Consideration of Magnetic Measurements for Characterisation of Ferrite–Martensite Commercial Dual-Phase (DP) Steel and Basis for Optimisation of the Operating Magnetic Field for Open Loop Deployable Sensors

Mohsen Aghadavoudi Jolfaei,Lei Zhou,Claire Davis

doi:10.3390/met11030490

Abstract

The magnetic properties of commercial dual-phase (DP) steels (DP600, DP800 and DP1000 grades) were evaluated using initial permeability, incremental permeability and coercivity and correlated with the key microstructural differences between the grades. The ferrite grain sizes and ferrite fractions have been compared with the magnetic parameters obtained from minor and major magnetisation loops within each DP grade. It has been revealed that the incremental permeability increases with the applied magnetic field amplitude to reach a peak and then drops at a higher magnetic field, with the values being different for the three DP grades at a lower field and converging to a similar permeability value at the high field. The effects of ferrite grain size and phase fraction on the incremental permeability are considered, and it has been shown that the influence of ferrite grain boundaries on magnetic permeability is more dominant than the effect of ferrite fraction in commercial DP steel samples. An analysis of the correlation between coercivity and initial permeability with tensile strength shows that the initial permeability provides a slightly better prediction of strength for the steels examined, which is believed to be due to the fact that a combination of reversible and irreversible domain components affect the coercivity value, while the initial permeability is predominantly affected by reversible domain movements. Based on the trend between incremental permeability and applied magnetic field and the commercial EM sensor (EMspec) operating parameters, the effect of lift-off and hence magnetic field strength on the sensitivity to DP steel properties can be assessed.

Highlights

Dual-phase (DP) steels are generally characterised by a continuous soft ferrite matrix and small islands of martensite
The detailed study of metallurgy and characteristic magnetic properties of commercial DP600, DP800 and DP1000 steel samples indicate that the magnetic properties of incremental permeability, initial permeability and coercivity are affected by both ferrite grain size and phase balance
The magnetic permeability is influenced by microstructural features as intrinsic parameters and/or extrinsic parameters

Summary

Introduction

Dual-phase (DP) steels are generally characterised by a continuous soft ferrite matrix and small islands of martensite (tempered martensite and/or bainite). It is important to consider how well the different magnetic properties that can be measured characterise DP steel and to compare the sensitivity of those parameters (i.e., coercivity and permeability) to microstructure features. There have been many investigations on the empirical relationships between minor/major loop measurements and the assessment of mechanical properties such as hardness and tensile strength (UTS) [10,11,12] or minor/major loop to assess grain size and precipitates [6] as an inspection method for the cold rolling process [16,17] and creep detection [18]; few studies are available on detailed correlations between the measured magnetic permeability at different applied magnetic fields and the sensitivity to different microstructural features. To optimise the sensitivity of the measurement, the incremental permeability–magnetic field data can be considered rather than using a single magnetic field value, which may provide better sensitivity to microstructure and mechanical properties for DP steels

Materials and Methods

Microstructure

Findings

Conclusions