Deformation mechanism and microstructure evolution of medium-Mn AHSS under various loading conditions

https://doi.org/10.1016/j.ijmecsci.2021.106812Get rights and content

Highlights

  • Martensitic transformation is more promoted by equibiaxial tension than by uniaxial tension and in-plane torsion.

  • Strain aging effect and deformation bands are prevalent in uniaxial tension but suppressed in equibiaxial tension and torsion.

  • The bcc phases exhibit a common texture evolution while austenite exhibits complex texture evolution.

  • The calibrated Olson–Cohen model well captures the evolution of austenite under different loading conditions.

Abstract

Third-generation advanced high-strength steels possess an exceptional combination of high strength and ductility owing to their high work-hardening rate during deformation. In this study, the mechanical properties and microstructure/texture evolution of medium-Mn steel under various loading conditions (uniaxial tension, in-plane torsion, and equibiaxial tension) were investigated. Interrupted electron back-scattered diffraction and X-ray diffraction characterizations were carried out to track the microstructure and texture evolution of individual phases. Digital image correlation was employed for the in-situ measurement of formation and expansion of deformation bands, i.e., the Lüders and the Portevin-Le Châtelier (PLC) bands. The dynamic strain aging effect in terms of the formation of PLC bands and stress serrations was significantly affected by the loading conditions and stress level. Multiple types of stress serrations were observed in all tested specimens at different deformation stages. The texture evolution of the steel under various deformation modes was further tracked. Ferrite and fresh martensite exhibited the plain texture evolution like most body-centered cubic metals, whereas austenite showed a more complex texture evolution. The martensitic transformation notably decreased the RD||<001> intensity of the austenite; austenite debris was rotated with ferrite grains, while some intact austenite grains self-rotated to a stable orientation, which is unfavorable for martensitic transformation. The obvious intragranular orientation gradient observed in the austenite intact grains implies that considerable deformation occurred in the austenite. Finally, the experimental results indicate that equibiaxial tension primarily promoted the martensitic transformation, and the classical Olson–Cohen model was calibrated to capture the volume fraction evolution of austenite under various stress states.

Introduction

The current environmental challenges and rigorous fuel-efficiency requirements are increasing the demand for lightweight metals and alloys. Third-generation advanced high-strength steels (3GAHSS), such as medium‑manganese (Mn) steels, are of significant interest in the automotive industry for reducing the overall product weight while maintaining sufficient strength and crash performance [1, 2]. The high fraction of retained austenite (RA, up to ∼80% [3, 4] and stabilized by enrichment with Mn) in the medium-Mn steels can be tuned to yield various plasticity-enhancing mechanisms, such as transformation-induced plasticity (TRIP) and twinning-induced plasticity (TWIP) effects, to realize a favorable combination of strength, ductility, and formability [5]. RA can be dynamically transformed into martensite during deformation through the TRIP effect, which promotes a high work-hardening rate and delays deformation localization and necking [6, 7]. The well-tailored multiphase microstructure and the superior plasticity-enhancing mechanisms, on the one hand, provide excellent mechanical properties and formability of medium-Mn steels; on the other hand, they lead to the complex plastic behavior and microstructure evolution of metal sheets during forming operations [8].

The microstructure of the as-received medium-Mn steels mainly comprises body-centered cubic (BCC) ferrite and bainite and face-centered cubic (FCC) RA. The ferrite (α) phase dominates the ductility; martensite islands, which can be continuously generated via the strain-induced martensitic transformation (MT) of the metastable RA, strengthen the material [9]. The mechanical stability of the austenite and the micromechanical interaction among multiphase structures are crucial for the synchronous improvement of strength and plasticity [10]. Therefore, the austenite stability has attracted considerable interest and has been widely investigated, particularly for austenitic stainless steels and first-generation AHSS TRIP steels [11, 12]. Austenite stability is affected by deformation conditions, such as temperature, strain rate, deformation mode, strain path [13], [14], [15], and microstructural morphology and composition (e.g., carbon concentration) [16, 17]. Most components of AHSS are fabricated through sheet metal forming operations, which generally involve complex loading conditions. Investigations have revealed that the deformation, formability, and failure mode of AHSS are significantly affected by loading conditions [18, 19]. For instance, Powell et al. [20] studied the effects of deformation modes, including tension, torsion, and compression, on the work-hardening behaviors of types 301 and 304 austenitic stainless steels. They clarified that the deformation modes substantially affect the MT of these steels and that uniaxial tension promotes MT more than compression and torsion. Wu et al. [21] reported that for medium-Mn (10 wt. %) steel under a linear strain path, MT was much more strongly promoted by uniaxial tension than by biaxial tension; however, when the steel underwent a stamping process, the region under biaxial tension following a bilinear strain path showed the maximum transformation rate. Currently, the effects of deformation modes on the austenite stability and MT are not well understood yet; relevant research on 3GAHSS is insufficient.

Typically, the as-received medium-Mn steel sheet is subjected to complex thermomechanical operations, which inevitably give rise to the crystallographic texture of the steel. The crystallographic orientation and texture evolution of austenite grains also play key roles in austenite stability [22]. Tirumalasetty et al. [23] reported that strain-induced MT is associated with grain rotation capabilities and the experienced stress states of austenite grains. They also found that austenite grain rotation was affected by its surrounding microstructure and was a significant factor contributing to ductility enhancement. Cakmak et al. [24] reported that during the torsional deformation of 304 L stainless steel, the austenite grains belonging to the C texture component underwent preferential MT. The texture evolution of martensite was consistent with that of other monolithic BCC materials deformed under the same conditions. Fundamental studies on the texture-transformation relation are crucial for a better understanding on the micromechanics of the TRIP medium-Mn steels.

Apart from the adequate compromise between the strength and formability of medium-Mn steels, plastic instability and deformation localization phenomena, such as the Lüders banding and the Portevin-Le Châtelier (PLC) banding, have been frequently reported for 3GAHSS [25]. They are generally associated with non-uniform deformation, abnormal work hardening, and reduced total elongation to failure [26]. The Lüders bands, commonly referred to as the static strain aging effect, manifest as yield point elongation and formation of a stress plateau in engineering stress-strain curves [27, 28]. By contrast, the PLC bands, normally attributed to the dynamic strain aging (DSA) effect, are associated with heterogeneous plastic deformation and stress serrations in the post-yielding stage of the stress–strain curve [29]. A well-known interpretation of these strain aging effects is the pinning and unpinning effects of solute atoms on mobile dislocations [30]. The induced deformation bands reduce the ductility of steel and lead to undesirable traces on the surface of the final product. Because of these notorious effects, the formation of macroscopic deformation bands and strain aging effects have been studied for many years [26, 31]. For instance, it was reported that the localized strain triggered by the Lüders band could cause unstable MT in medium-Mn steel [16], and the PLC effect results in the discontinuous strain-induced MT [32]. Recently, Yang et al. [33] reported that a cold-rolled 7Mn steel annealed at 700 °C exhibited type A PLC bands throughout the entire deformation period, while the same steel annealed at 720 °C exhibited a more complicated type A+B bands first, and subsequently, type A bands. For medium-Mn steels, although the deformation bands and stress serrations were recently reported, studies concerning these phenomena are limited.

The application of full-field strain measurements, e.g., the digital image correlation (DIC) technique, enables favorable spatial resolution to correlate the strain patterns with specific macro- and micromechanical mechanisms in terms of various types of serrations and microstructure evolutions [34, 35]. However, most studies have focused on the loading condition of uniaxial tension, whereas a limited number of studies on more complex stress states. Romhanji et al. [36] reported that the DSA of an AlMg7 sheet was significantly suppressed in biaxial loading. Because of the multiphase microstructure and the transformation of RA into martensite, the mechanisms of deformation banding and DSA are more complicated than those in single-phase materials [28]. For instance, the volume fraction and stability of austenite were demonstrated to influence the Lüders strain [16] profoundly; the localized deformation led to significantly different crystallographic textures in the bands for high-Mn steel [37].

Consequently, the complex deformation mechanism of the multiphase structure makes it challenging to understand the mechanical properties and microstructure evolution of medium-Mn steels under various loading conditions. One of the key challenges in the design and use of AHSS is to ensure a sufficient strength–ductility reserve maintained over a wide range of loading paths for various types of sheet metal forming operations and crash scenarios [2]. Thus, an in-depth understanding of the microscopic deformation mechanisms and microstructure evolution is urgently required for the emerging medium-Mn steels, not only for a better understanding of the mechanical properties and formability but also for the development of mechanism-based constitutive models for advanced numerical simulations. In this work, a series of experiments and crystallographic analyses were conducted to study the mechanical responses and microstructure evolution of commercial medium-Mn steel under various in-plane loading conditions (uniaxial tension, shear, and equibiaxial tension). Interrupted mechanical tests combined with DIC measurements were employed to track the localized strain patterns and obtain the equivalent stress–strain curves. The microstructure evolution and MT were then characterized by electron back-scattered diffraction (EBSD) and X-ray diffraction (XRD), followed by a detailed analysis and discussion on the roles of individual phases.

Section snippets

Material preparation and microstructure characterization

The as-received medium-Mn TRIP steel was provided by Baosteel (Baoshan Iron & Steel Co., Ltd.); its nominal chemical composition is listed in Table 1. The steel was produced by continuous casting, hot rolling, cold rolling, and batch annealing. The hot-rolled band with a thickness of 2.8 mm was obtained with a full martensite microstructure. It was then cold rolled to achieve a 50% thickness reduction followed by annealing for 48 h at 620 °C. The martensite microstructure was deformed during

Initial microstructure

First, the initial microstructure and crystallographic texture of the medium-Mn steel characterized by SEM-EBSD were examined, as shown in Fig. 1. Fig. 1(a) shows the phase map with yellow and green denoting the ferrite and the austenite phases, respectively, and Fig. 1(b) shows the orientation distribution functions (ODFs) of individual phases. RD, TD, and ND of each specimen were used as the specimen coordinate system for crystallographic orientation analysis. The phase map confirmed the

Discussion

Based on the EBSD and XRD characterizations of the interrupted tested specimens, the volume fraction evolution of austenite (fγ) with respect to deformation (the equivalent strain) was further summarized and plotted in Fig. 8. Five XRD measurements were carried out on the ND plane of the uniaxial tensile specimens, and 16 EBSD measurements on either the ND plane or the RD plane for uniaxial tensile or biaxial tensile (Nakajima) test samples. Because austenite has a different lattice structure

Conclusions

In this work, various mechanical tests and interrupted microstructure characterizations were carried out to study the mechanical responses and microstructure evolution of medium-Mn steel under various stress states, including uniaxial tension, equibiaxial tension, and in-plane torsion. The main findings include:

The studied steel exhibited both the Lüders and PLC bands, i.e., both the static and dynamic strain aging (DSA) effects. Various stress serrations, which are related to specific stress

Credit Author Statement

Changwei Lian: Experiments, Data analysis, Visualization, Writing-Original draft preparation.

Haiming Zhang: Conceptualization, Methodology, Funding acquisition, Data Curation, Validation, and Writing.

Jianping Lin:  Supervision, Funding acquisition, and Discussion.

Li Wang: Conceptualization, Resources, and Discussion.

Data availability

The raw data required to reproduce these findings are available to download from http://dx.doi.org/10.17632/j92rmrr88y.1. The processed data required to reproduce these findings are available to download from http://dx.doi.org/10.17632/j92rmrr88y.1.

Declaration of Competing Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgments

The work was funded by the National Natural Science Foundation of China with the projects of Nos. 52075329 and 51705317, the National Key R&D Program of China (2017YFB0304403), Shanghai Rising-Star Program (20QA1405300), as well as the Program of Shanghai Academic Research Leader (Grant No.19XD1401900). The authors greatly acknowledge the Instrumental Analysis Center of Shanghai Jiao Tong University and Institut für Umformtechnik und Leichtbau (IUL) in TU Dortmund for their supports on

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