Elsevier

Materials Characterization

Volume 71, September 2012, Pages 49-57
Materials Characterization

Through process texture evolution and magnetic properties of high Si non-oriented electrical steels

https://doi.org/10.1016/j.matchar.2012.06.006Get rights and content

Abstract

A detailed understanding of microstructural changes in a sequence of thermomechanical processing allows the improvement of magnetic properties in FeSi strips. The current contribution considers the texture evolution in non-oriented electrical steels of high Si content. Hot band strips of various textures were subjected to cold rolling and recrystallization annealing. The findings suggest that the crystallographic orientations observed after cold rolling are correlated with the hot band texture. In contrast, the evolution of recrystallization textures was more likely affected both by the hot and cold rolling microstructural features. The evolution of recrystallization textures is discussed on the basis of crystal plasticity calculations while the magnetic properties are correlated with the crystalline anisotropy energy density.

Highlights

► Low stored energy grains ensure improved magnetic properties (MP) in BCC metals. ► Texture randomization of magnetically unfavorable orientations improves the MP. ► Magnetically favorable texture of hot band improves the MP after recrystallization.

Introduction

The magnetic properties of electrical steels such as magnetization curves, permeability and specific losses are, to a large extent, correlated with the microstructure and crystallographic texture [1], [2], [3]. The resulting microstructural characteristics of materials are conditioned by the thermomechanical processing (TMP) [1], [2], [3], [4], [5], [6], [7] which involves a slab reheating, hot rolling, cold rolling and final recrystallization (RX) annealing. Microstructure and texture evolutions during hot rolling depend on the rolling temperature and the coiling condition [4], [5], [6]. Textures produced by hot rolling and/or partial recrystallization of austenite give rise to different types of transformation textures [6], [8]. This resulting texture affects the final properties of the cold rolled and subsequently recrystallized sheets [8]. Hot rolling conducted mainly in the austenite region leads to a heterogeneous final microstructure with a low magnetic induction whereas rolling in the two-phase region ensures improved magnetic properties [9], [10]. Reducing the amount of deformation and decreasing the hot rolling temperature typically induce an enhanced magnetic induction in non-oriented electrical steels [9]. It was also found that hot band annealing results in enhanced permeability in Fe–Si steels [11], [12]. In addition to the TMP parameters, the chemical composition [9], [12], [13], by way of its influence on phase transformations, plays an important role in both microstructure and texture evolutions. High Si containing grades, i.e. steels without phase transformations, typically have coarse grained hot band microstructures which enhance shear band formation during cold rolling. These shear bands are fertile fields for Goss oriented grains during subsequent recrystallization annealing [14].

The relevant texture components which evolve during thermomechanical processing of electrical steels are aligned along the < 110 >//RD (α-fibre), hhlhl+1hl+2hl (α*-fibre), < 100 >//ND (θ-fibre), < 100 >//RD (η-fibre) and < 111 >//ND (γ) fibres (see Fig. 1). However, the most frequently observed α, α* and γ-fibre texture components, which evolve during both the hot and cold rolling operations, are unfavorable for magnetic properties since they have hard magnetization directions [1], [15], [16]. In addition to a specific texture, a coarse grained microstructure is required to minimize the magnetic losses. Varying the final annealing conditions produces a wide range of microstructures with different textures [14], [17], [18], [19], [20], [21]. In low carbon grades, a strong temperature gradient, induced by rapid heating, combined with diffusion controlled grain growth [17], [18], [19], [21], [22] serves to produce a homogeneous microstructure of columnar grains which have an appropriate texture for magnetic applications. However, in modern vacuum degassed steels, the amount of carbon is too low to promote carbon induced grain growth, thus rapid heating leads to microstructure refinement [14], [20]. Although, the columnar type of microstructures produced by various methods tends to enhance the magnetically favorable θ and η-fibre texture components, the industrial application of these methods is limited. The same is true of cross rolling [1], two stage cold rolling and surface heat treatment technologies [22], [23], [24], [25], [26], [27], [28].

Since the volume fraction of < 100 >//ND orientations throughout processing chain is low, attaining the required θ and η-fibre orientations in recrystallization can be quite challenging. As such, tailoring the texture of the final product requires an optimized conventional processing strategy, which involves a thorough understanding of texture evolution along the processing route. The current contribution presents an analysis of the texture evolution in Fe–Si alloys, subjected to different TMP schedules. Texture evolution is discussed on the basis of experimental evidence, crystal plasticity calculations and crystallographic based models of magnetic properties.

Section snippets

Experimental Procedure

The materials used in the current study are Fe–2.4 wt.%Si and Fe–3.0 wt.%Si alloys. Both materials were rolled to a final thickness using a four stand high speed rolling mill at TU Freiberg. The investigated materials were reheated to 1260 °C and hot rolled to a thickness of ~ 2 mm. As Table 1 shows, the hot rolled strips were subjected to different thermomechanical treatments which resulted in four materials (designated A–D) with differing microstructural and textural features. After hot rolling,

Texture Evolution in Thermomechanical Processing Chain

Table 1 lists the hot rolling schedules, with various finishing temperatures, to which the investigated alloys were subjected. According to calculations [9], phase transformations do not occur in Fe–Si–0.01C alloys which have Si contents higher than 2.4 wt.%. This implies that the experimental hot rolling campaigns started and ended in the ferritic region. Fig. 2 reveals that, although the hot rolling was performed in the single phase region, the documented finishing temperatures (Table 1) gave

Conclusions

Results of crystal plasticity calculations reveal that low stored energy grains are more favorable for magnetic properties whereas recrystallization (RX) in Fe–Si steels occurs mainly in the high stored energy regions producing orientations with hard magnetization directions and correspondingly high anisotropy energy density.

Results from experiments and crystallographic based modeling of magnetic properties show that RX texture randomization in materials with magnetically unfavorable

Acknowledgments

This research was supported by the FWO-Odysseus Program within the project: Engineering of 3D microstructures in metals: bridging ten length scales of functionality. Dr. Tricia Bennett is gratefully acknowledged for the useful comments and fruitful discussion.

References (43)

  • J. Wang et al.

    Effect of heating rate on microstructure evolution and magnetic properties of cold rolled non-oriented electrical steel

    J Iron Steel Res Int

    (2010)
  • M. Mekhiche et al.

    A metallurgical and magnetic study of 100 textured soft magnetic sheets

    JMMM

    (1996)
  • F.J.G. Landgraf et al.

    Magnetic properties of silicon steel with as-cast columnar structure

    JMMM

    (2003)
  • P. Van Houtte

    Fast calculation of average Taylor factors and Mandel spins for all possible strain modes

    Int J Plast

    (2001)
  • J.J. Sidor et al.

    Modeling the crystallographic texture changes in aluminum alloys during recrystallization

    Acta Mater

    (2011)
  • K. Verbeken et al.

    Microtextural study of orientation change during nucleation and growth in a cold rolled ULC steel

    Scr Mater

    (2003)
  • P. Brissonneau

    Non-oriented Si–Fe sheets

    JMMM

    (1980)
  • F.J.G. Landgraf et al.

    Modelling the angular dependence of magnetic properties of a fully processed non-oriented electrical steel

    JMMM

    (2003)
  • L. Kestens et al.

    Texture control during the manufacturing of nonoriented electrical steels

  • E. Gomes et al.

    Effect of hot and cold rolling on grain size and texture in Fe–Si strips with Si-content larger than 2 wt.%

    Mater Sci Forum

    (2010)
  • K. Verbeken et al.

    Effect of hot and cold rolling on grain size and texture in Fe–2.4 wt.%Si strips

    IEEE Trans Magn

    (2008)
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