Deformation mechanism and microstructure evolution of medium-Mn AHSS under various loading conditions
Graphical abstract
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
References (61)
- et al.
A novel two-step intercritical annealing process to improve mechanical properties of medium Mn steel
Acta Mater.
(2019) - et al.
Experimental investigation on a novel medium Mn steel combining transformation-induced plasticity and twinning-induced plasticity effects
Int. J. Plast.
(2016) - et al.
Three-dimensional finite element analysis of representative volume elements for characterizing the effects of martensite elongation and banding on tensile strength of ferrite-martensite dual-phase steels
Int. J. Mech. Sci.
(2019) - et al.
Transformation-induced plasticity in ferrous alloys
J. Mech. Phys. Solids
(2005) - et al.
On the fracture characteristics of advanced high strength steels during hydraulic bulge test
Int. J. Mech. Sci.
(2021) - et al.
Constitutive modeling of evolving plasticity in high strength steel sheets
Int. J. Mech. Sci.
(2016) - et al.
Rate-dependent isotropic‒kinematic hardening model in tension‒compression of TRIP and TWIP steel sheets
Int. J. Mech. Sci.
(2018) - et al.
Finite element analysis of the effects of martensitic phase transformation in TRIP steel sheet forming
Int. J. Mech. Sci.
(2005) - et al.
Investigation of the mechanical behaviour of multi-phase TRIP steels using finite element methods
Int. J. Mech. Sci.
(2008) - et al.
An evaluation of fracture properties of type-304 austenitic stainless steel at high deformation rate using the small punch test
Int. J. Mech. Sci.
(2018)