Magnetic behaviour versus tensile deformation mechanisms in a non-oriented Fe–(3 wt.%)Si steel

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Abstract

This study presents an extended in situ magnetic characterisation of a non-oriented (NO) Fe–(3 wt.%)Si steel. An appropriate experimental device was created and magnetic measurements were performed under uniaxial tensile stresses approaching and exceeding the macroscopic elastic limit σe and in the corresponding unloaded states. Both Barkhausen noise and B–H hysteresis loops were measured. The sensitivity to stress was found to be qualitatively similar to that of polycrystalline iron. The different stages of the tensile deformation (perfectly elastic stage, microplastic yielding stage, the two strain-hardening stages) were clearly identified by the magnetic parameters. In the plastic strain domain, the coercive field Hc and the inverse of the initial relative permeability 1/μri linearly increase, while the maximal relative permeability μrmax and the Barkhausen noise peak height BNmax linearly decrease with the applied stress σ. The remnant induction Br keeps a low and constant value. Furthermore, a linear dependence of 1/μri, Hc, μrmax and BNmax on the kinematic hardening X was found. By using measurements on prestrained specimens under reloaded elastic stresses, an accurate identification of the effect of dislocations acting as pinning sites and of the magnetoelastic effect of long-range internal stresses was proposed.

Introduction

A strong coupling exists between mechanical and magnetic properties of ferromagnetic materials. In order to better understand the physical mechanisms involved, with the practical aim of developing magnetic methods for nondestructive evaluation of the mechanical state, this research theme has received increasing attention in recent years.

This study is in line with several previous works on the effect of plastic deformation on the magnetic behaviour of electrical steel [1], [2]. The pinning effect of the dislocations alone cannot account for the strong degradation of the magnetic properties observed at the very beginning of the strain-hardening. Therefore, considerations on the magnetoelastic anisotropy induced by the long-range internal stresses have been proposed [3], [4]. However, the magnetic measurements have been performed ex situ, on plastically strained unloaded specimens. Significant differences between under stress and stress-removed measurements have been reported on carbon–steel alloys by Makar and Tanner [5], [6], [7]. On these bases, it has appeared as necessary to continue this work by performing in situ magnetic measurements on electrical steel, to validate and to develop the given inferences on the effect of internal stresses.

An appropriate experimental device was developed, which enables magnetic measurements on uniaxial tensile strained sheet and strip specimens, under stress and after removal of the stress, below and after the macroscopic elastic limit. Barkhausen noise and magnetic hysteresis loops were detected. A brief description of the benchmark and some preliminary results were previously reported in [8], but this paper offers a detailed presentation and further interesting results of our works on NO Fe–(3 wt.%)Si steel. Firstly, the mechanical properties under a uniaxial tensile test are given. The benchmark is described in the next section. The results of the magnetic measurements are then discussed. An accurate identification of the effect of dislocations acting as pinning sites and of the magnetoelastic effect of long-range internal stresses is proposed in the last section.

Section snippets

Material presentation and mechanical properties under monotonous uniaxial tensile test

A “fully-process” non-oriented (NO) Fe–(3 wt.%)Si steel sheet of 0.35 mm thick was used for this study. The material, presenting a body centred cubic (bcc) ferritic phase was, therefore, received in a recrystallised state, with a weak {111}〈uvw〉 recrystallisation texture, typical for NO electrical laminations. The grain structure is isotropic in the volume and the average diameter of the grains is 47 μm.

Mechanical behaviour was characterised under uniaxial tensile tests performed at room

Magnetic measurements test apparatus

The experimental device enabling in situ magnetic measurements on tensile strained samples is presented in Fig. 2. It consists of a single sheet tester type device adapted to a Zwick mechanical testing machine controlled by microcomputer. The samples were strips 20 mm wide and 250 mm long cut out in the rolling direction RD and vacuum annealed at 720 °C for 2 h in order to eliminate the residual stresses which originate from the manufacturing process [9]. Two ferrite yokes (25×25 mm2 section)

Magnetic behaviour in the loaded and corresponding unloaded states

Fig. 5 shows the results of the magnetic measurements in the loaded states (under-stress measurements σ>0), with stresses approaching and exceeding the macroscopic elastic limit σe, and in the corresponding unloaded states (stress-removed measurements σ=0). Like Degauque [20], we take into consideration the inverse of the initial permeability 1/μri. The magnetic parameters are plotted with respect to the level of the applied stress σ, because it has been suggested that the stress is a more

Magnetic behaviour of prestrained specimens under reloaded elastic stresses

The results of the magnetic measurements carried out on three different specimens, prestrained at 1.75, 6, and 11.6%, under reloaded elastic stresses are shown in Fig. 8. All magnetic properties are entirely recovered, reaching the same level as for the initial loaded state, either the plastic prestrain corresponds to the first hardening stage or to the second one. This recovery has been also found by Neurath [19] for the total losses measured at 60 Hz on prestrained specimens of grain-oriented

Conclusions

This work presents, firstly, a simple experimental device allowing magnetic measurements on magnetic sheet and strip samples under an applied uniaxial tensile stress approaching and exceeding the macroscopic elastic limit σe. It consists of a double-yoke SST test frame adapted to a universal testing machine. The feeding-acquisition system makes it possible to detect both the Barkhausen noise and the magnetic hysteresis loops under an imposed magnetising current waveform.

Secondly, an in situ

Acknowledgements

This work was financially supported by the Pôle Régional “Multifonctionalité des Matériaux et Optimisation”, Picardie, France (project no. 99.3). The authors wish to thank Dr Florence Ossart from LMT-ENS Cachan France for help in FEM modelling and useful discussions. They also thank the technical staff of the Roberval laboratory, especially J.P. Wtyklo, for assistance in experimentation.

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