Aghadavoudi Jolfaei, Mohsen (2019) Characterisation of advanced high strength strip steels using electromagnetic sensor system. PhD thesis, University of Warwick.

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Abstract

The mechanical properties of steel are strongly influenced by its microstructural features such as phase balance, grain size, dislocations and precipitates. In order to obtain accurate quality control of steel products, it is desirable to be able to monitor the mechanical properties non-destructively. It is known that the low frequency inductance (where the effect of eddy currents are negligible) measured using an EM sensor depends on the relative permeability of the sample and that the permeability is affected by microstructural parameters (i.e. phase fraction / distribution and, to a lesser extent, grain size are the important features in dual phase, DP, steel).

A variety of electromagnetic sensors have been reported for non-destructively assessing the state of steel microstructures including; monitoring the recovery and recrystallisation processes in-situ during heat treatment, phase transformation and detecting decarburisation in steel rod both on-line and off-line, etc. Recently it has been shown that electromagnetic sensors can measure the phase fraction in DP steel but the effect of strip thickness was not assessed.

This research work discusses the development of an EM sensor system that can be used to assess the microstructure (and hence mechanical properties) of commercially produced DP steels (in particular phase balance and grain size) with a range of thicknesses in a steel works test house environment, specifically, it focuses on employing an EM sensor system in the prediction of ultimate tensile strength for DP steels of any sheet thickness.

In this project, a set of heat treated DP600 grade of 1.4mm thickness and commercial DP steel samples, including DP600, DP800 and DP1000 with a range of strength levels and thicknesses, and produced in different strip mills, have been assessed. The sensor outputs have been correlated to microstructural phase fraction and mechanical properties.

Firstly, the magnetic properties of commercial DP steel samples were investigated through the major hysteresis loop and minor hysteresis loops. Measured coercivity from the major loop showed that the coercivity was affected by phase fraction (ferrite/martensite percentage) and ferrite grain size where the coercivity decreased with increased ferrite fraction.

Three types of minor loop configurations were used to derive incremental permeability values; the minor loop deviations from the initial magnetisation curve (μIc); the minor loop deviations from the main B-H loop (μBH) and the minor loop deviations from amplitude sweep (μi). It was found that although the incremental permeability values are not precisely the same for the three sets of measurements, similar trends for the DP samples can be observed where the incremental permeability values are affected by the phase fraction and ferrite grain size.

The effect of magnetic field on permeability for the DP steels was studied. It was shown that the incremental permeability increases with the applied field amplitude until reaching a maximum value at a certain applied field amplitude (i.e. very close to the coercivity values) and then drop at higher applied field amplitude and converge to a similar permeability value. The initial gradient and the peak position for the samples are different and would allow them to be distinguished from each other. It was observed in the commercial DP steels with a range of ferrite fraction (72 to 79%) and a range of average ferrite grain size (from 6 to 10μm), that the effect of ferrite grain boundaries on permeability is more significant than the effect of ferrite fraction within the range studied.

Finally, the measured magnetic properties were used to develop a link between microstructure and mechanical properties for DP steels, using a readily deployable EM sensor that can be used with large strip steel samples. The deployable sensor geometry and operation rely on a relatively low magnetic field being generated in the sample and therefore low field incremental permeability being the relevant material parameter being assessed. Initially, the effect of ferrite fraction for the laboratory heat-treated DP600 samples, with the same thickness (1.4mm), on EM sensor output signal (i.e. mutual real inductance) was investigated. It was found that the real inductance value at a low frequency (below approx.100 Hz) is dominated by differences in the relative permeability of the samples, showing an approximately linear trend of increasing low frequency inductance value with increasing ferrite content. The increasing amount of ferrite, which possesses a much higher relative permeability than martensite, showed higher real inductance value (in the range of 35 -70% ferrite fraction in these DP steels). The measured real inductance at a frequency of 10Hz was compared with the mechanical property (hardness). An approximately linear decrease in real inductance at 10 Hz with the hardness value was found for these samples. EM sensor measurements were then carried out for the commercial DP600, DP800 and DP1000 samples with different thicknesses (1 to 4 mm). The EM sensor system showed a significant effect of thickness on the signal with thicker strip showing a much higher mutual inductance value for the same microstructure. This is due to the skin depth (for this sensor, operation frequency and material characteristics) being larger than the sample thickness, therefore a thicker sample gives a large sensor response. To deal with this problem, a calibration curve (a plot of real inductance versus permeability for different thickness of material) was constructed using a FE model for the sensor and sample geometry. Therefore, 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). The model has been validated using commercial DP steel sheets of 1 to 4 mm.

It was found that the ferrite grain size affects the magnetic properties in DP steels as the grain boundaries act as effective pinning points to magnetic domain movement. Therefore, the magnetic permeability in DP steels is affected by ferrite grain size and ferrite fraction, both of which affect the tensile strength, therefore a single relationship between permeability and tensile strength results. The low field relative permeability, which is the permeability value derived from the EM sensor (e.g. U-shaped sensor), can therefore be used to predict the tensile strength in commercial DP steels.

The relationship between permeability and field was employed to develop the technique. Therefore, U-shaped sensor modification was carried out to increase the accuracy of tensile strength determination, this was done as part of a case study for Tata Steel Jamshedpur to evaluate DP steels.

Item Type:	Thesis (PhD)
Subjects:	T Technology > TA Engineering (General). Civil engineering (General) T Technology > TN Mining engineering. Metallurgy
Library of Congress Subject Headings (LCSH):	Steel -- Mechanical properties, Microstructure, Steel -- Heat treatment, Magnetic permeability
Official Date:	January 2019
Dates:	Date Event January 2019 UNSPECIFIED
Institution:	University of Warwick
Theses Department:	Warwick Manufacturing Group
Thesis Type:	PhD
Publication Status:	Unpublished
Supervisor(s)/Advisor:	Davis, Claire ; Zhou, Lei
Format of File:	pdf
Extent:	x, iv, xxii, 267 leaves : illustrations, charts
Language:	eng

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