Effect of ferrite grain size on tensile deformation behavior of a ferrite-cementite low carbon steel
Introduction
The following Hall–Petch equation [1] holds generally for the yield strength as well as for flow stress in polycrystalline metals and alloys.where σ and D refer to the yield strength or flow stress and average grain size, respectively, and σ0 and k are constants. The above empirical equation was originally established for the experimental data in the grain size region of larger than 10 μm.
Recently low carbon steels with sub-micron grain size have been realized and they are called ultrafine-grained low carbon steels. Such grain refinement brings high strength even by lean alloying. The Eq. (1) has been reported to hold for the yield strength in the ultrafine-grained steels, but the value of k varies in cases [2], [3], [4], [5], [6], [7]. The influence of grain size on flow stress at a high strain rate has also been discussed in a few steels [8], [9], [10]. Therefore, the stress–strain relation that includes not only the yield stress but also flow stress or work-hardening behavior under a wide range of strain rate should be elucidated more carefully to explore further application of the ultrafine-grained alloys.
The reports on the ultrafine-grained metals reveal various types of tensile load-displacement or nominal stress–strain curves. For example, Yu et al. classifies them into four types according to characteristic shapes of the curves [6]. Fig. 1 depicts the classification together with the ideas how the yield strength (YS) is currently determined in each type [6]. Table 1 summarizes differences in some important aspects in yielding and work-hardening behaviors. The definition of “yield strength” is not consistent in these types. The constant k in Eq. (1) is not the same from type to type [6].
Work-hardening is another important subject for the ultrafine-grained alloys because the uniform elongation decreases with a decrease in grain size.
The present authors have studied ultrafine-grained low carbon steels with ferrite-pearlite (FP) microstructure to pursue a better combination of high strength and good ductility. The ultrafine-grained FP microstructure is believed to have advantages of both grain refinement strengthening and dual-phase strengthening [8], [9]. Here, the dual-phase strengthening means strengthening without serious decrease in ductility by the aid of harder second phase as is found in a dual-phase steel [11], [12].
It is however difficult to prepare ultrafine-grained FP steels with a ferrite grain size below 2 μm, since small-sized austenite does not transform to pearlite but decomposes into ferrite and dispersed cementite particles [13]. On the other hand, the ultrafine-grained ferrite microstructure with finely dispersed cementite particles (FC) is relatively easier to obtain with varied ferrite grain sizes from sub-micron to over 10 μm. To prepare larger size samples sufficient for mechanical testing, the present authors have utilized the warm bar rolling method, by which bar samples with homogeneous ultrafine-grained microstructure can be successfully obtained [5], [14], [15].
Examining the stress–strain relations for the FC specimens of grain size ranging from 0.47 to 13.6 μm, the present paper aims at revealing the grain size contribution to flow stress, work-hardening, strain rate sensitivity, and Lüders deformation in both static and dynamic tensile tests.
Section snippets
Experimental procedures
The FC specimens with various ferrite grain sizes were prepared by microstructural control for a JIS-SM490 steel (0.15 C, 0.4 Si, 1.5 Mn, 0.014 P, 0.004 S in mass %) [8], [16]. An ultrafine-grained FC specimen was obtained by austenitization at 1173 K for 3.6 ks and subsequent hot-rolling processes with accumulated area reductions of 91% at 773 K and 43.8% at 723 K followed by water quenching. Then, 0.47 and 0.7 μm FC specimens were prepared by water quenching after heat treatment for 3.6 ks at 743 and 773
Microstructure
Fig. 2 shows SEM micrographs of the FC specimens with ferrite grain sizes of 0.7 μm (a), 1.8 μm (b), 3.2 μm (c), and 13.6 μm (d). Fig. 3 represents an example of the orientation imaging microscopy (OIM) microstructure of the FC specimen with the ferrite grain size of 1.5 μm. Grain boundaries with misorientation angles larger than 15° are indicated by dark lines and those between 5 and 15° by gray lines. The fraction of high angle grain boundaries with misorientations of larger than 15° is about 60%
Conclusions
Tensile tests were performed with different strain rates at room temperature for ferrite-cementite (FC) specimens with a wide range of ferrite grain size to investigate the effects of ferrite grain size and strain rate on flow stress. The following conclusions can be obtained from this study.
- 1.
The lower yield stress and flow stress increase and uniform and total elongations decrease with a decrease in ferrite grain size to 0.47 μm.
- 2.
The stress–strain curves of the FC specimens are categorized into
Acknowledgements
The authors are grateful to Mr. M. Shinohara, student of Himeji Institute of Technology, for his help. Gratitude is also extended to Professor K. Kunishige and Dr. R. Ueji of Kagawa University for their help and advices. This work was supported by TOSTEM Foundation for Construction Materials Industry Promotion.
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