Deformation and fracture mechanisms in fine- and ultrafine-grained ferrite/martensite dual-phase steels and the effect of aging
Introduction
Grain refinement of metals is essential as it is the only strengthening mechanism that simultaneously enhances the toughness of a material. In recent years, a variety of methods have been developed to produce ultrafine-grained (UFG) materials with a ferrite grain size of around 1 μm [1], [2]. These methods can be divided into advanced thermomechanical processing (ATMP) routes, which aim at improving conventional processing routes in commercial large-scale rolling mills, and severe plastic deformation (SPD) techniques, which are essentially confined to laboratory-scale sample dimensions. The ATMP methods cover deformation-induced ferrite transformation (DIFT) [3], large-strain warm deformation [4], intercritical hot rolling [5], multi-directional rolling [6] and cold-rolling plus annealing of martensitic steel [7]. The most important SPD techniques are equal-channel angular pressing (ECAP) [8], accumulative roll bonding [9] and high-pressure torsion [10].
It was consistently found that yield strength and tensile strength are drastically increased due to grain refinement, whereas uniform and total elongation are decreased. Also, Lüders straining becomes more pronounced. Furthermore, UFG steels exhibit a very low strain-hardening rate [11], which marks the main limitation with respect to commercial applications. In terms of toughness, a significant reduction in the ductile-to-brittle transition temperature has been repeatedly reported [12], [13]. For these reasons, it is of ongoing interest to overcome the restricted ductility of UFG materials without sacrificing strength and toughness. Among the possible ways to restore the strain hardenability of UFG materials are the fabrication of a bimodal grain size distribution [14] and the introduction of nanosized cementite particles into the microstructure [6]. Another aspect that might improve the applicability of UFG materials is their ability to show superplasticity [15]. Shin and Park [16] showed that replacing cementite as a second phase by martensite through an intercritical annealing treatment leads to a substantial increase in strain-hardening rate, causing a higher ultimate tensile strength with only slightly reduced uniform and total elongation. In this way, a UFG ferrite/martensite dual-phase (DP) steel was designed which shows superior mechanical properties compared to coarser grained conventional DP steels. The term “dual-phase steel” refers to a group of steels consisting of a soft ferrite matrix and 3–30 vol.% of hard martensite islands. These steels are widely used for automotive applications. DP steels have a number of unique properties, which include a low elastic limit, high initial strain-hardening rate, continuous yielding, high tensile strength and high uniform and total elongation. Moreover, DP steels exhibit a bake-hardening (BH) effect, i.e. the yield strength increases upon aging at paint-baking temperatures (∼170 °C) after forming, giving rise to improved dent and crush resistance. The austenite-to-martensite phase transformation bears the main influence on the mechanical properties of dual-phase steels [17], [18]. This phase transformation involves a volume expansion of 2–4% [19], causing an elastically and plastically deformed zone in the ferrite adjacent to martensite [20]. The deformed zone contains a high number of unpinned dislocations [21], giving rise to dislocation heterogeneities in the ferrite. The low elastic limit is thus suggested to be generated by the combined effects of the present elastic stresses that facilitate plastic flow and the additional dislocation, which is assumed to be partly mobile during early stages of yielding [22]. Dislocation–dislocation interactions, dislocation pile-ups at ferrite/martensite interfaces and the corresponding long-range elastic back stresses contribute to rapid strain hardening.
Previous studies on grain refinement in DP steels consistently revealed that, unlike in other metallic materials, the increases in yield strength and tensile strength are not counteracted by a significant reduction in the uniform and total elongation [23], [24], [25], [26], [27], [28]. This can partly be explained by the enhanced strain-hardening rate due to grain refinement as a result of the higher number of geometrically necessary dislocations (GNDs) along the ferrite–martensite boundaries [25]. Ultrafine-grained DP steels have been produced by applying a two-step processing route consisting of (1) a deformation treatment to produce UFG ferrite and finely dispersed cementite or pearlite and (2) a short intercritical annealing in the ferrite/austenite two-phase field followed by quenching to transform all austenite to martensite. Grain refinement in step (1) was achieved by ECAP [27], cold rolling [28], cold swaging [24] and large-strain warm deformation [29]. A single-pass processing route based on DIFT was proposed by Mukherjee et al. [26], [30].
As the number of investigations on this topic is very limited, more research is required to understand the mechanical response of DP steels to ferrite grain sizes close to or below 1 μm. Furthermore, as the microstructures described in the previous studies often differ in the martensite volume fraction, it has not been possible to interpret the grain size effect on the mechanical properties independently so far. Therefore, in this study we compare the deformation and fracture mechanisms of a coarse-grained (CG), a fine-grained (FG) and a UFG-DP steel, having about the same martensite volume fractions. In addition, the aging (BH) response of the UFG-DP steel is investigated, which has not been addressed so far. In conjunction with the mechanical data, the microstructure evolution during tensile straining and the fracture mechanisms are studied by using scanning electron microscopy (SEM) and high-resolution electron backscatter diffraction (EBSD). Furthermore, slip-band evolution during deformation was investigated by performing tensile tests which were interrupted at strain levels between 1% and 4%.
Section snippets
Materials processing
A plain carbon manganese steel of composition (in wt.%) 0.17 C, 1.49 Mn, 0.22 Si, 0.033 Al, 0.0033 N, 0.0017 P and 0.0031 S was produced by vacuum induction melting. A lean composition was selected in order to show that a stable ferrite grain size of around 1 μm can be achieved via thermomechanical processing without microalloying. Carbon enhances both grain refinement and grain size stability [31]. At the same time, the carbon content has to be low enough to ensure good weldability, which is
Microstructures
The microstructure obtained after hot deformation and air cooling followed by intercritical annealing (CG route) consists of a ferrite matrix with a grain size of 12.4 μm and 31.3 vol.% martensite (Table 1), the latter occurring partly as isolated islands, partly as aligned bands.
By applying multi-pass warm deformation at 700 °C (FG route) and at 550 °C (UFG route) between hot deformation and intercritical annealing, the ferrite grain size is reduced to 2.4 and 1.2 μm, respectively. The martensite
Discussion
For the sake of clarity, the discussion of the deformation and fracture mechanisms will focus on the as-quenched specimens only. The effect of BH on deformation and fracture behavior will be addressed in a separate section.
Conclusions
Three low-carbon dual-phase steels with nearly constant martensite fraction around 30 vol.% martensite and different ferrite grain sizes (1.2, 2.4 and 12.4 μm) were produced by applying hot deformation and large-strain warm deformation at different deformation temperatures, followed by intercritical annealing. Their deformation and fracture mechanisms were studied based on tensile test data and microstructure observations. The BH response was investigated for the UFG steel. The main conclusions
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