Full length articleJoint investigation of strain partitioning and chemical partitioning in ferrite-containing TRIP-assisted steels
Graphical abstract
Introduction
Driven by the increasing demands for safe and lightweight automotive materials, significant research efforts have been devoted to the development of advanced high strength steels (AHSS) with good combination of strength and ductility [1], [2], [3]. Ductility enhancement through the martensitic transformation of retained austenite, the so-called transformation induced plasticity (TRIP) effect, triggered the design of TRIP-assisted multiphase steels, such as conventional TRIP steel, quenched and partitioned (Q&P) steel and medium manganese steel [4], [5], [6], [7], [8], [9].
Numerous studies show that the premise for an effective TRIP behavior is a high volume fraction of retained austenite that transforms during deformation [10], [11], [12], [13]. However, incomplete transformation of the retained austenite is a prevalent problem for TRIP-assisted steels, especially for those with a matrix structure consisting of soft and hard microstructure regions [14], [15], [16], [17]. The mechanical stability of the retained austenite is a crucial factor controlling the transformation ratio [18,19]. A number of studies have revealed the important effects of the chemical composition [20], size [12], morphology [21], distribution [22] and crystal orientation [23] of the retained austenite on its mechanical stability, while no systematic quantitative research illuminated the specific relation between the strain distribution in the matrix, the strain partitioning among the different microstructure components and the mechanical stability of the retained austenite [24], [25], [26]. For TRIP-assisted multiphase steels, the strength difference among adjacent phases results in strain partitioning and as a consequence in an inhomogeneous strain and stress distribution during deformation [27,28]. Since the retained austenite, depending on its composition and stability, requires a certain loading condition to undergo martensitic transformation, the strain partitioning among the microstructure components has a crucial influence on the TRIP effect [29,30]. Also, strain localization and interfacial damage initiation in such multiphase microstructures depend on such load partitioning effects [31], [32], [33]. These findings underline the necessity to conduct studies where strain partitioning, the TRIP effect and damage initiation in TRIP-assisted multiphase steels are jointly considered [34]. A quantitative study of these key features requires in-situ (or quasi in-situ) tensile testing in conjunction with high resolution electron back-scattered diffraction (EBSD) and microscopic digital image correlation (µ-DIC) strain mapping [20,35,36]. By combining µ-DIC with EBSD probing, we investigated the strain partitioning and the TRIP effect in a ferrite-containing Q&P steel [20]. We found that a strong strain contrast exists between the ferrite and martensite. The ferrite/martensite (F/M) fraction ratio and martensite morphology have crucial influences on the strain partitioning and the TRIP effect. Our previous work also revealed that the transformation ratio of retained austenite in the TRIP steel with a matrix consisting of ferrite, bainite and martensite is much larger than that in the TRIP steel with a matrix composed of only ferrite and martensite [16]. It was inferred that this phenomenon may result from the strain partitioning between the soft and hard matrix structures. The introduction of bainite in a ferrite-containing Q&P steel could improve the deformation uniformity among the matrix structures, thus enhancing the TRIP effect. However, to understand, prove and apply these effects, quantitative results about the strain partitioning and deformation-driven transformation effects in TRIP steels with different matrix structures are required, together with an improved probing and understanding of the role of the atomic-scale chemical partitioning across the hetero-interfaces.
Additionally, understanding of the origin of the mechanical partitioning among the main microstructural components requires to also map the chemical partitioning between them. This applies particularly for the strongest austenite stabilizer, viz. carbon, which has also the highest influence on the hardness of the different microstructure components [37]. For this reason it is important to map the carbon partitioning at near atomic-scale across the different microstructure regions in the TRIP-assisted multiphase steel [38,39].
In this work, we present results from quasi in-situ tensile tests in conjunction with high resolution EBSD and µ-DIC mapping conducted on two types of ferrite-containing TRIP-assisted steels, namely, one with martensitic and one with bainitic matrix. This procedure enables a quantitative investigation of strain partitioning and the TRIP effect. Combination of the strain maps and EBSD results allows us to identify correlations between size, orientation, structure, load partitioning, TRIP response and interfacial fracture, revealing the significance of strain partitioning on optimal utilization of the TRIP effect. The microstructure and micromechanical probing is combined with chemical probing: since X-ray diffraction (XRD) and energy dispersive spectrum analysis (EDS) cannot reveal differences in carbon concentration across these hetero-interfaces at near atomic-scale [40,41]. We study chemical partitioning here by atom probe tomography (APT) [42], [43], [44], [45]. Based on these investigations, we develop a microstructure design strategy to optimize the TRIP effect for yielding a good compromise between strength, damage tolerance and ductility of ferrite-containing TRIP-assisted steels.
Section snippets
Experimental details
A low-C, low-Si, Al-added steel with chemical composition of Fe-0.18C-0.53Si-1.95Mn-1.46Al-0.08P (wt.%) was melted in a vacuum induction furnace and cast into a 150 kg ingot. Subsequently, the ingot was forged into slabs with a cross section of 60 × 60 mm. The transformation temperatures of the undeformed samples were measured using a Formastor-FII (FTF-340) dilatometer. The corresponding results are Ac1=712°C, Ac3=1037°C, Ar1=634°C, Ar3=914°C, Ms =421°C and Mf =172°C, respectively [16,20
Mechanical properties
The strength levels of the two samples are similar, with a yield strength and ultimate tensile strength (UTS) of about 540 MPa and 930 MPa, respectively. The total elongation and uniform elongation of sample B are 6%−8% larger than those of sample M. Sample B has a higher work hardening rate than sample M for a true strain regime above 4.4%, leading to its higher uniform elongation reaching up to about 16.5%. More detailed property results can be found in [16].
Microstructural characterization
Fig. 1 shows the SE images of the
Effect of carbon partitioning on stability features of retained austenite
As the APT results show, the carbon partitioning can result in a carbon gradient in the retained austenite. This means that correspondingly a stability gradient exists in the retained austenite, since austenite stabilization depends in the present system mainly on the carbon content. Retained austenite can perform different deformation mechanisms, such as dislocation glide, mechanical twinning and martensitic transformation, depending on the stacking fault energy (SFE) controlled by the
Conclusions
The target of this work was to obtain a better understanding of the interplay between chemical (carbon) and mechanical partitioning in multiphase TRIP steels. We applied two hot rolling direct quenching and partitioning (HDQ&P) processes to a low-C low-Si Al-added steel and obtained two types of ferrite-containing TRIP-assisted steels with different constituents, one with ferrite, bainite, martensite and retained austenite and a second one with ferrite, martensite and retained austenite. We
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 authors are grateful for support from the Natural Science Foundation of Chongqing, China (No. cstc2018jcyjAX0221), the Fundamental Research Funds for the Central Universities (No. SWU117054), the State Scholarship Fund of Chinese Scholarship Council (CSC) (No. 201506080070) and the “Zeng Sumin Cup” Project from School of Materials and Energy in Southwest University.
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