Microstructure and texture evolution during tensile deformation of symmetric/asymmetric-rolled low carbon microalloyed steel
Introduction
Grain ultra-refinement technologies have been widely applied to various kinds of metals including pure copper [1], [2], aluminum [3] and titanium [4], body-centered cubic (bcc) steel [5], metastable titanium alloys [6], [7], etc., because they give rise to good mechanical properties such as high yield and ultimate tensile strength, and good toughness, compared with the conventional microcrystalline materials. Nevertheless, nanocrystalline (NC) and ultrafine-grained (UFG) metals often exhibit a rather low tensile plasticity, i.e. a uniform elongation of only a few percent, which limits their widespread practical applications in industry. Recent attention has focused on the strategies and procedures related to the adjustment of both the tensile strength and plasticity of NC and UFG metals [8].
In the search for new approaches to prevent the plastic instability in NC and UFG metals, of particular interest are the underlying mechanisms under plastic tensile deformation. Wang et al. [1] first proposed that the inhomogeneous structure induced high levels of strain hardening that stabilized the tensile deformation. Alternatively, Fang et al. [2] reported a NG metal film adherent on a coarse-grained (CG) substrate of the same metal with a graded grain-size transition between them that increased the tensile plasticity of NC metals through the mechanically driven growth of NC grains during deformation.
The above provide new insights into a grain ultra-refinement design pathway towards the ultra-strong and highly ductile structural metallic materials based on the concept of ‘gradient’ architectures. Asymmetric rolling (AsR) has been reported as a process that can introduce shear deformation [9], [10], [11], [12], [13], [14], [15], [16], [17], [18], which is one of the major features of severe plastic deformation (SPD) processing, in addition to deformation through plane strain compression similar to symmetric rolling (SR). The asymmetry may be introduced by using either rolls with different radii [9], [10], [12], [14], [15], [16], [17], or different angular speed [12], [13]. Until now, studies have focused mainly on aluminum [9], [15] and its alloys [10], interstitial-free steel [11], [12], [13], [14], and pure magnesium [16], [17]. In our preliminary work [18], a gradient ultrafine ferrite (<1 μm) and martensite structure with pronounced strain hardening was produced in a commercial low carbon microalloyed steel by taking advantage of AsR and dynamic transformation.
In this work, detailed microstructural and texture evolutions of both the AsR and SR specimens during uniaxial tensile deformation were investigated by microstructural observations using scanning electron microscopy and electron backscattered diffraction. Meanwhile, a realistic micromechanical modeling based approach by means of a Representative Volume Element (RVE) was also employed to predict the plastic strain and stress partitioning during plastic deformation.
Section snippets
Experimental
The as-received hot rolled steel plate was used for the investigation [18]. Billets of 30 mm×65 mm cross-section and 5 mm thickness were cut from the as-received steel plate and then solution-treated at 1200 °C for 1 h, followed by air-cooling to room temperature. The solution-treated billets were machined slightly to remove the surface scale, and a hole for thermocouple (1.5 mm in diameter) was drilled to record a real-time temperature change, as schematically illustrated in Fig. 1. After reheating
Micro-modeling setup
A realistic microstructure based finite element (FE) model was developed using two-dimensional (2D) RVE models. Microstructural attributes such as the shape, size, volume fraction, and spatial distribution of the reinforcement phase (martensite) in the soft matrix (ferrite) were incorporated in the models [19]. In the 2D RVE model, an ideal plane stress is applied and the martensite is randomly distributed in the ferrite matrix, according to the realistic microstructure.
The mechanical
The as-received material
The microstructure of the as-received steel plate is shown in Fig. 2a, and consists of coarse-grained ferrite with a ~25 μm grain size and ~10% pearlite. The initial texture of this material in Fig. 2b, exhibited dominant hot rolling textures of ε-fiber 〈110〉//TD (~43.6%) and γ-fiber 〈111〉//ND (~26.8%) with a maximum intensity of 2.0 multiples of a random distribution (MRD). The true stress (σ)–strain (ε) curve of the sample is shown in Fig. 3a, where both true yield and ultimate tensile stress
Summary
- (1)
AsR leads to a relatively higher ductility and strain hardening ability with an improved yield and ultimate tensile strength over SR by increasing the capacity of the duplex microstructure to accommodate the plastic stability during deformation.
- (2)
AsR promotes ductile fracture by inhibiting the formation of cleavage and brittle fracture as well as martensite cracks due to the finer ferrite grain size, beneficial shear fiber textures and the more dispersed martensite.
- (3)
The orientations of the tensile
Acknowledgments
The authors gratefully acknowledge the financial support of the National Natural Science Foundation of China, China (Grant no. 51401050). This work was also supported by grants through the Australian Research Council (ARC), Australia Laureate Fellowship (Prof. Hodgson) and the Alfred Deakin Research Fellowship (Dr. Cai).
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