Elsevier

Acta Materialia

Volume 84, 1 February 2015, Pages 110-123
Acta Materialia

Mechanisms of subgrain coarsening and its effect on the mechanical properties of carbon-supersaturated nanocrystalline hypereutectoid steel

https://doi.org/10.1016/j.actamat.2014.10.027Get rights and content

Abstract

Carbon-supersaturated nanocrystalline hypereutectoid steels with a tensile strength of 6.35 GPa were produced from severely cold-drawn pearlite. The nanocrystalline material undergoes softening upon annealing at temperatures between 200 and 450°C. The ductility in terms of elongation to failure exhibits a non-monotonic dependence on temperature. Here, the microstructural mechanisms responsible for changes in the mechanical properties were studied using transmission electron microscopy (TEM), TEM-based automated scanning nanobeam diffraction and atom probe tomography (APT). TEM and APT investigations of the nanocrystalline hypereutectoid steel show subgrain coarsening upon annealing, which leads to strength reduction following a Hall–Petch law. APT analyzes of the Mn distribution near subgrain boundaries and in the cementite give strong evidence of capillary-driven subgrain coarsening occurring through subgrain boundary migration. The pronounced deterioration of ductility after annealing at temperatures above 350°C is attributed to the formation of cementite at subgrain boundaries. The overall segregation of carbon atoms at ferrite subgrain boundaries gives the nanocrystalline material excellent thermal stability upon annealing.

Introduction

Cold-drawn pearlitic steel wires are important engineering materials for a variety of applications such as automobile tire cords, suspension bridge and power cables, piano strings, and springs due to their ultrahigh strength. In 1995 it was reported that severe cold-drawing of pearlite yields a tensile strength of 5 GPa [1]. In the following years the tensile strength of cold-drawn pearlitic steel wires has been increased to 6.35 GPa [2] and very recently even up to 7 GPa [3]. The extraordinary strength has made the materials attractive not only for engineering applications but also for studying basic relationships between structure and mechanical properties of nanoscaled alloys. During the past 50 years great efforts have been made to understand the microstructural evolution and its effect on strength upon cold drawing [3], [4], [5], [6], [7], [8], [9]. The most frequently reported finding is deformation-induced cementite decomposition [10], [11], [12], [13], [14], [15], [16], [17], [18], [19] and its “unexpected” consequence on strain hardening, i.e. the decomposition of the hard phase—cementite—surprisingly does not adversely affect the material’s strength. On the contrary, the tensile strength continuously increases upon cold drawing [3], [4], [20], even when the cementite has been significantly dissolved [3], [18], [19]. It is worth noting that the mechanism of deformation-induced cementite decomposition is still under dispute. Different from the assumption that the decomposition takes place upon cold drawing, due to the interaction between dislocations and carbon [3], [18], [19], Takahashi et al. [21] suggested that it mainly occurs upon low-temperature aging after cold drawing. With the development of characterization techniques such as Mössbauer spectroscopy [10], field ion microscopy (FIM) [11], [15], [22], [23], [24], [25] and atom probe tomography (APT) [12], [13], [14], [15], [16], [18], [19] a deeper understanding of the mechanisms of cementite decomposition and their effects on microstructure and strength has been achieved. Among these characterization techniques APT is able to provide nano- and atomic-scale information on the carbon distribution in both cementite and ferrite with high compositional accuracy and statistical significance [2], [19]. Recently, Li et al. [3] observed that above a true drawing strain of 4.19 the original lamellar ferrite/cementite structure in a hypereutectoid steel wire is gradually replaced by a 2-D nanoscaled ferrite subgrain structure upon further drawing. The dissolved carbon atoms were found to be segregated at ferrite subgrain boundaries (SGBs), suppressing dynamic recovery and thus stabilizing the dislocation structure. Hence, the heavily deformed wires are no longer hypereutectoid pearlitic steels but carbon-supersaturated nanocrystalline hypereutectoid steels. At a true drawing strain of 6.52 the subgrain size has been reduced to below 10 nm, which provides a tensile strength of up to 7 GPa [3].

In many engineering applications such as suspension bridges and power cables cold-drawn hypereutectoid steel wires are subjected to hot-dip galvanization or blueing (a heat treatment to simulate the hot-dip galvanized process, up to 550°C for 15 min after cold drawing) to improve their anti-corrosion property [26], [27]. Such processes may reduce the tensile strength because the temperature during galvanizing can approach 500°C [26], [27]. Thus, it is essential to study the thermal stability of heavily cold-drawn pearlite as well as the microstructural mechanisms associated with strength reduction during annealing. The strength reduction of cold-drawn pearlite during annealing has been reported in Refs. [21], [26], [27], [28], [30]. Some results obtained by microstructural investigations using TEM and APT can be found in Refs. [13], [26], [28], [29], [30]. It is known that for the same heat-treatment condition the annealed microstructure of a material strongly depends on its microstructure prior to annealing. For cold-drawn pearlitic steels this prior microstructure, depending on the drawing strain d, can be either a heterophase-dominated lamellar structure at low strains or a nanosized carbon-supersaturated ferrite subgrain-dominated dislocation structure at extremely high strains [3]. The materials investigated in the above-mentioned studies were mainly subjected to relatively low drawing strain, where the lamellar structure still prevails. The observations were often performed under relatively short and not sufficiently systematic annealing conditions. In this sense, the strength–microstructure relationships during annealing of cold-drawn pearlite have not yet been systematically studied, especially for wires with extremely high drawing strains.

Here we study the microstructure–property relationships of annealed carbon-supersaturated nanocrystalline hypereutectoid (0.98 wt.% C) steels produced from severely cold-drawn pearlite by a true strain d of 6.0. This initial microstructure prior to annealing is significantly different from the materials studied in previous papers in which heterophase boundaries are still dominant [13], [21], [26], [27], [28], [29], [30]. The present work focuses mainly on the evolution of the nanoscaled subgrain structure in ferrite during annealing. More specifically, first, systematic investigations have been performed on the evolution of the nanosized ferrite subgrain structure during annealing. Second, quantitative analyzes of the subgrain structures in terms of area fractions of low-angle and high-angle grain boundaries have been performed as a function of the annealing temperature Ta using scanning nanobeam diffraction and the software ASTAR [31]. Third, subgrain coarsening is for the first time experimentally studied at the atomic scale and understood through triple-junction-controlled migration of subgrain boundaries. Finally, the effect of Ta on the ductility of annealed wires is also studied. The relationship between tensile strength and ferrite subgrain size can be described by a Hall–Petch law. On the basis of these investigations the effects of microstructural evolution on the mechanical properties of the carbon-supersaturated nanocrystalline steel upon annealing are discussed.

Section snippets

Material and processing

The original pearlitic steel wires subjected to heavy cold drawing were of hypereutectoid composition (Fe–0.98C–0.31Mn–0.20Si–0.20Cr–0.01Cu–0.006P–0.007S in wt.% or Fe–4.40C–0.30Mn–0.39Si–0.21Cr–0.003Cu–0.01P–0.01S in at.%), and were provided by Suzuki Metal Industry Co. Ltd. Before cold drawing the wires were austenitized at 950°C for 80 s followed by pearlitic transformation in a lead bath at 580°C for 20 s and subsequent quenching in water. After this treatment specimens were subjected to cold

Changes of strength and ductility upon annealing

Fig. 1(a) displays true tensile stress–strain curves of annealed cold-drawn wires at various Ta for 30 min. For each annealing condition three specimens were measured up to the point of fracture. The obtained average ultimate tensile strengths σUTS are presented as a function of Ta in Fig. 1(b), where data with lower drawing strains taken from the literature [21], [26], [27], [28], [30] are also plotted for comparison. It can be seen that the tensile strength at room temperature increases with d

Discussion

As mentioned in Section 1, the microstructures prior to annealing are strongly dependent on the drawing strain d. They differ from each other not only in the defect density and interlamellar spacing, but also in the morphology and the volume fractions of the phase constituents. For the wires with low and moderate drawing strains lamellar structures are still dominant despite the cementite decomposition [13], [21], [26], [27], [28], [29], [30]. For the present case with extremely high drawing

Conclusions

After annealing at temperatures between 250 and 450°C for 30 min, the carbon-supersaturated nanocrystalline hypereutectoid steel produced from severely cold-drawn pearlite exhibits significant temperature-dependent changes in microstructure, tensile strength and ductility. Based on TEM and APT investigations conducted on various annealed samples the mechanisms responsible for the microstructural evolution and its effect on the mechanical properties are discussed.

No strength reduction is observed

Acknowledgments

The authors thank Dr. H. Yarita, Suzuki Metal Industry Co. Ltd., for providing the cold-drawn specimens. We are grateful to the Deutsche Forschungsgemeinschaft for co-sponsoring some of this research (SFB 602 and KI230/34–1).

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