Elsevier

Acta Materialia

Volume 59, Issue 2, January 2011, Pages 658-670
Acta Materialia

Deformation and fracture mechanisms in fine- and ultrafine-grained ferrite/martensite dual-phase steels and the effect of aging

https://doi.org/10.1016/j.actamat.2010.10.002Get rights and content

Abstract

Three ferrite/martensite dual-phase steels varying in the ferrite grain size (12.4, 2.4 and 1.2 μm) but with the same martensite content (∼30 vol.%) were produced by large-strain warm deformation at different deformation temperatures, followed by intercritical annealing. Their mechanical properties were compared, and the response of the ultrafine-grained steel (1.2 μm) to aging at 170 °C was investigated. The deformation and fracture mechanisms were studied based on microstructure observations using scanning electron microscopy and electron backscatter diffraction. Grain refinement leads to an increase in both yield strength and tensile strength, whereas uniform elongation and total elongation are less affected. This can be partly explained by the increase in the initial strain-hardening rate. Moreover, the stress/strain partitioning characteristics between ferrite and martensite change due to grain refinement, leading to enhanced martensite plasticity and better interface cohesion. Grain refinement further promotes ductile fracture mechanisms, which is a result of the improved fracture toughness of martensite. The aging treatment leads to a strong increase in yield strength and improves the uniform and total elongation. These effects are attributed to dislocation locking due to the formation of Cottrell atmospheres and relaxation of internal stresses, as well as to the reduction in the interstitial carbon content in ferrite and tempering effects in martensite.

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

References (61)

  • R. Song et al.

    Mater Sci Eng A

    (2006)
  • R. Song et al.

    Acta Mater

    (2005)
  • R. Ueji et al.

    Acta Mater

    (2002)
  • R.Z. Valiev et al.

    Prog Mater Sci

    (2006)
  • Y. Saito et al.

    Acta Mater

    (1999)
  • Y. Ivanisenko et al.

    Acta Mater

    (2003)
  • R. Song et al.

    Acta Mater

    (2005)
  • H. Azizi-Alizamini et al.

    Scr Mater

    (2007)
  • D.H. Shin et al.

    Mat Sci Eng A

    (2005)
  • D.L. Bourell et al.

    Acta Metall

    (1983)
  • M. Delincé et al.

    Acta Mater

    (2007)
  • Z.H. Jiang et al.

    Mater Sci Eng A

    (1995)
  • Y.I. Son et al.

    Acta Mater

    (2005)
  • M. Calcagnotto et al.

    Mater Sci Eng A

    (2010)
  • M. Calcagnotto et al.

    Mater Sci Eng A

    (2010)
  • J. Lian et al.

    Mater Sci Eng A

    (1991)
  • J.D. Kang et al.

    Scr Mater

    (2007)
  • Y.L. Su et al.

    Mater Sci Eng

    (1987)
  • H. Ghadbeigi et al.

    Mater Sci Eng A

    (2010)
  • P. Uggowitzer et al.

    Mater Sci Eng

    (1982)
  • Howe AA, et al. ECSC contract 7210.PR/167. Luxembourg: EC;...
  • P.D. Hodgson et al.

    Mater Sci Forum

    (1998)
  • Yada H, Matsumura Y, Nakajima K. US patent 4466,842. Tokyo, Japan: Nippon Steel Corporation;...
  • A. Ohmori et al.

    ISIJ Int

    (2004)
  • K.T. Park et al.

    Metall Mater Trans A

    (2002)
  • T. Hanamura et al.

    ISIJ Int

    (2004)
  • Z.L. Zhang et al.

    Mater Sci Forum

    (2007)
  • Hayami S, Furukawa T. In: Korchynsky M, editor. Microalloying 75 proceedings of an international symposium on...
  • Speich GR, Miller RL. In: Kot RA, Morris JW, editors. Structure and properties of dual-phase steels. New York: The...
  • J.M. Moyer et al.

    Metall Trans A

    (1975)
  • Cited by (0)

    View full text