J Mater Sci DOI 10.1007/s10853-014-8280-6
ULTRAFINEGRAINED MATERIALS
Microstructure and texture evolution in low carbon steel deformed by differential speed rolling (DSR) method Kotiba Hamad • Rachmad Bastian Megantoro Young Gun Ko
•
Received: 27 February 2014 / Accepted: 28 April 2014 Ó Springer Science+Business Media New York 2014
Abstract The study examined the microstructural and textural evolution of low carbon steel samples fabricated using a differential speed rolling (DSR) process with respect to the number of operations. For this purpose, the samples were deformed by up to 4-pass of DSR at room temperature with a roll speed ratio of 1:4 for the lower and upper rolls, respectively. The DSR technique applied to low carbon steel samples resulted in a microstructure composed of ultrafine ferrite grains, approximately 0.4 lm in size, after 4-pass with a high-angle grain boundary fraction of *65 %. The microstructural features of the ferrite phase indicated the occurrence of continuous dynamic recrystallization, beginning with the formation of a necklace-like structure of ultrafine equiaxed grains around the elongated grains, which were formed in the early stages of deformation, and ending with ultrafine recrystallized grains surrounded by boundaries with high angles of misorientations. In the pearlite phase, the microstructural changes associated with DSR deformation were presented by the occurrence of bending, kinking, and breaking of the cementite lamellar plates. In addition, the evolution of texture after DSR processing was affected by shear deformation and rolling deformation, leading to the formation of a texture composed of fractions of components with shear texture orientations such as {110} h001i (Goss) and orientations close to {112} h111i, in addition to rolling texture components consisting mainly of a-fiber and c-fiber.
K. Hamad R. B. Megantoro Y. G. Ko (&) School of Materials Science and Engineering, Yeungnam University, Gyeongsan 712-79, Republic of Korea e-mail:
[email protected]
Introduction In recent years, bulk metals with ultrafine-grained structures, i.e., a grain size B1 lm, have attracted considerable interest owing to their unique physical and mechanical properties [1]. This type of material can be fabricated effectively by applying severe plastic deformation (SPD) to metals with coarse grain sizes using special processes, in which intensive strain is imposed on the bulk samples, resulting in a submicron structure. Several SPD processing techniques are currently available but the following four processes have attracted the most attention: equal-channel angular pressing (ECAP) [2, 3], high pressure torsion (HPT) [4, 5], accumulative roll-bonding (ARB) [6, 7], and asymmetric rolling (ASR) [8–10]. ASR is relatively easy to implement and the shortcomings of ECAP and HPT processing do not apply to this technology. In ASR, the material, in the form of a sheet, is rolled between two working rolls with different characteristics, and simple shear deformation (the main characteristic of SPD processing) is introduced through the thickness of the processed sheet by the asymmetry of the processing conditions applied to both sides of the sheet. In general, asymmetry is created using three methods: differential speed rolling (DSR), differential diameter rolling (DDR), and differential fraction rolling (DFR), which modify the properties of the rolled sheets. Among them, DSR provides better control over the microstructural parameters than DDR and DFR [11]. The DSR process has been applied successfully to fabricate several metallic materials, such as Mg, Ti, Al, Cu, and steel, as well as to improve their practical applications [12–21]. The results available to date for these materials show that processing by DSR generally produces an ultrafine-grained microstructure with improved strength
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and hardness [14–17, 22]. For example, a high yield tensile strength of *470 MPa was achieved in 6061 Al alloy when processed using the DSR method after a thickness reduction of 70 % compared to *90 and *365 MPa for their coarse-grained and ECAP-processed counterparts, respectively [22]. Also, the microhardness tests of AZ31 magnesium alloy after processing by DSR [12] showed that the use of the warm DSR deformation for 2-pass using the 180° rotation of the sample around the rolling direction (RD) between passages would be more beneficial for producing higher microhardness values in the DSR-deformed alloy samples than those fabricated by various turns of HPT [23] and many pass numbers of ECAP [24]. By a careful examination of the published literature on the DSR processing, it can be noted that no information related to the microstructural parameters and texture that developed in ultrafine-grained low carbon steel materials fabricated via the DSR process are available. Shin and coauthors, however, synthesized low carbon steel with an ultrafine ferrite structure using ECAP [25–27]. In each of their investigations, the experiments were carried out using an iron alloy containing 0.15 wt% C, the samples were pressed at 623 K, and each sample was rotated by 180° between consecutive passes through the ECAP die. The processing procedure used in these studies led to low carbon steel samples possessing an ultrafine microstructure with a grain size of *0.3 lm after 4-pass. Fukuda et al. [28] performed experiments on low carbon steel samples (0.08 wt% C), where the samples were pressed up to a total of three passes at room temperature and rotated 90° between the consecutive pressings. The deformed structure of the processed materials was characterized by the formation of ultrafine grains in the ferrite phase surrounded by boundaries with high angles of misorientations, resulting in high strength and hardness of the fabricated samples. Recently, the annealing behavior of the ultrafine-grained microstructure in 0.09 wt% carbon steel samples produced by ECAP was also investigated [29]. It follows, therefore, that several sets of data on the fabrication of ultrafine-grained low carbon steels are now available but they refer exclusively to processing using ECAP. This study was motivated in part by the fact that low carbon steels with an ultrafine-grained microstructure can be used as structural materials in many applications, and in particular, by the expected capability of DSR process to produce low carbon steel sheets with high strength without need to change the chemical composition of the materials. Therefore, the objectives of this study were to use the DSR method to fabricate sheets of 0.18 wt% carbon steel with ultrafine microstructure and to study the microstructure and texture introduced after different numbers of DSR operations.
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Experimental procedures The material used in this study was a low carbon steel sheet whose chemical composition was Fe 0.18 %, C 0.012 %, Si 0.5 %, Mn 0.007 %, Cr (wt%). After it was machined into plates, 70 mm in length, 30 mm in width, and 4 mm in thickness, the material was homogenized at 1100 °C for 3 h followed by furnace cooling to obtain an equiaxed ferrite–pearlite microstructure with a grain size of *100 lm as shown in Fig. 1a. A series of lubricated DSR operations, up to four, were performed using two working rolls with an identical diameter of 220 mm, which revolved at a roll speed ratio of 1:4 for the lower and upper rolls, respectively, under the condition that the velocity of the lower roll was fixed at 5 rpm. The samples were subjected to a 30 % thickness reduction per pass, so that the total thickness reduction was *75 % after 4-pass DSR. To clarify the effects of the asymmetry in the speed, the microstructure and texture of the DSR-processed samples were compared with those of the samples processed by equal speed rolling (ESR) under the same rolling conditions (thickness reduction per pass, number of operations). The microstructural and crystallographic features of the processed samples were examined using electron backscatter diffraction (EBSD) measurement in a field-emission scanning electron microscope (FE-SEM) with an accelerating voltage of 15 kV. The EBSD data was analyzed using a TSL OIM 6.1.3 analyzer. The preparation of the samples for the EBSD studies consisted of conventional mechanical polishing on sheets cut in the normal direction–rolling direction (ND–RD) plane of the fabricated samples and etching using a picric acid in ethanol solution. For microstructure changes in pearlite phase, the samples used for EBSD measurements were observed utilizing a FE-SEM with an accelerating voltage of 10 kV.
Results Microstructure evolution of low carbon steel during DSR Microstructure changes in ferrite phase An image quality (IQ) map, which is the relative index of the Kikuchi pattern quality, shown in Fig. 1b for the initial sample suggested that locations with a higher IQ parameter (close to white) correspond to the ferrite phase, whereas pearlite phase corresponds to the locations with a lower IQ parameter (close to black). The IQ parameters taken along the dashed line in Fig. 1b were presented in Fig. 1d showing the low and high IQ regions along this line. Pearlite, which is being composed of fine lamellae of
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Fig. 1 a Optical image of the as-homogenized microstructure of low carbon steel, b quality map image of the as-homogenized of low carbon steel, c inverse pole figure of the as-homogenized of low
carbon steel after excluding the pearlite phase (black areas), and d image quality (IQ) parameters along the dashed line in (b)
ferrite and cementite, does not provide a good pattern because of the overlapping of patterns from two different phases. Therefore, analysis of EBSD was performed only on ferrite phases after partitioning out the data related to the pearlitic phases as shown in Fig. 1c (black regions). Figure 2 shows the general structure taken in the ferrite phase of the samples after (a) 1-pass DSR, (b) 2-pass DSR, (c) 4-pass DSR, and (d) 4-pass ESR, where the grain orientations detected are color-coded that correspond to the various orientations of the grains according to the unit tangle attached to Fig. 2. These structures correspond to the distributions of the crystallographic directions observed parallel to the RD of the processed sample. All images were recorded at the position of the sheet in the RD–ND plane corresponding approximately to the upper layer contacting to the roll with the high speed. A careful examination of the microstructures shown in Fig. 2 showed that the number of grains with ultrafine sizes increased with increasing the amount of deformation introduced during DSR processing. Also, the nucleation of ultrafine grains around coarse grains was observed after 1-pass of DSR processing (Fig. 2a). In addition, the number of ultrafine grains increased with increasing the number of DSR operations, as shown in Fig. 2b and c, respectively. To examine the effect of roll speed ratio, the microstructure developed after 4-pass of ESR (roll speed ratio of 1:1) was compared with that developed after the DSR at the same rolling conditions. As displayed in Fig. 2d, the ESRdeformed sample showed a bimodal grain size distribution consisting of a large fraction of elongated grains with a small fraction of newly formed ultrafine grains, whereas the ultrafine-grained microstructure with a more homogeneous distribution of grain sizes was observed in the sample processed by DSR suggesting that additional
deformation was imposed during processing by DSR. This microstructure obtained by ESR processing was nearly same to that obtained after 2-pass by DSR with a roll speed ratio of 1:4. Significant grain refinement occurred when processing by DSR after 4-pass showing an ultrafine-grained microstructure with a mean grain size of *0.4 lm. To show the ultrafine-grained microstructure achieved by the DSR, the confidence index (CI) map of the processed samples by DSR after 4-pass was recorded over an area of approximately 2 9 1 lm and step sizes of 0.08 lm as shown in Fig. 3a, where different levels of brightness correspond to the values of the CI and image. Figure 3b shows the distribution of the CI and IQ values along the dashed line that crosses the zones with the ferrite phase, where the high CI indicate a high level of grain refinement. Figure 3b shows the high level of grain refinement of the 4-pass deformed samples. Figure 4 shows the grain-to-grain misorientations between the ferrite grains measured using EBSD for the distributions at the selected position in the RD–ND plane of the fabricated samples after deformation through 1-pass DSR, 2-pass DSR, 3-pass DSR, and 4-pass ESR. The distributions displayed in Fig. 4 showed large fractions of grain boundaries with a low angle of misorientation from 2° to 15° (low angles grain boundaries, LAGBs) present in the sample fabricated by 1-pass DSR, and the subsequent increase of high-angle grain boundaries (HAGBs),[15°, as the number of DSR operations increased, which is directly connected with the evolution of new ultrafine grains. In addition, the roll speed ratio was significant toward the evolution of HAGBs, where higher fractions of HAGBs were achieved after DSR deformation for 4-pass compared to those achieved after 4-pass ESR at the same rolling conditions. Figure 4 shows
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J Mater Sci Fig. 2 Inverse pole figure of the low carbon steel samples processed by a 1-pass DSR, b 2pass DSR, c 4-pass DSR, and d 4-pass ESR
Fig. 3 a Confidence map of the low carbon steel sample processed by 4-pass DSR and b confidence index along the dashed line in (a)
that at the highest amount of deformation introduced in DSR processing after 4-pass, the angular characteristics of the boundaries developed in the fabricated microstructure were similar to those predicted by Mackenzie for the randomly misoriented polycrystalline aggregates of cubic metals [30], which is indicated by the solid curve in Fig. 4. In addition, the average misorientation angle of the fabricated samples after 4-pass DSR (*28°), was nearly similar to that of the
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predicted random distribution (29.5°). These results also showed that the average misorientation angle increased with increasing number of operations, where values of 17.28° and 20.6° were obtained after 1-pass DSR and 2-pass DSR, respectively. In contrast, the average misorientation angle of the ESR-processed sample, with a value of *22.87°, was less than that of the DSR-processed sample at the same rolling conditions.
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Fig. 4 Grain-to-grain misorientation distributions of the low carbon steel samples processed by DSR
Misorientation profiles were calculated along the dashed lines; T1, T2, and T3, presented in Fig. 2a–c, respectively, showing the point-to-point and point-to-origin misorientations through several grains, including elongated and equiaxed grains, after 1-pass and 2-pass DSR and through
equiaxed grains after 4-pass DSR. Figure 5 shows changes in the point-to-point and point-to-origin misorientations profiles of the DSR-processed samples after the various numbers of operations. The point-to-origin misorientations of the samples after 1-pass DSR tend to rise by increasing the distance from the elongated grain boundary accompanied by discontinuous changes at LAGBs inside the elongated grain suggesting significant evolution of orientation gradient in initial grains after 1-pass DSR [31] (Fig. 5a). Further deformation by DSR (2-pass) led to an increase the misorientation angle distributed along T2 and the formation of subgrains bounded mostly by LAGBs, as shown by the point-to-point profile (Fig. 5b). With further deformation by 4-pass DSR, the point-to-point misorientation changes and misorientations angle along the dashed line T3 were larger than those of T1 and T2, indicating the ultrafine grained microstructure obtained after processing by 4-pass DSR. Microstructure changes in pearlite phase Figure 6 shows the changes in the pearlite microstructure of the low carbon steel samples processed by DSR after
Fig. 5 Misorientation profile along dashed lines shown in the Fig. 2 for the low carbon steel samples processed by a 1-pass DSR, b 2-pass DSR, c 4-pass DSR, and d 4-pass ESR
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Fig. 6 Pearlite morphology for a the initial low carbon steel sample and the samples processed by b 1-pass DSR, c 2-pass DSR, and d 4-pass DSR
different number of operations. As shown in Fig. 6a for the initial sample, the initial pearlite colonies consisted of a well-defined cementite lamellar structure. After 1-pass DSR, the cementite lamellae deformed in a regular manner, as shown in Fig. 6b, and the cementite plates remained parallel to each other, but they were severely bent and kinked along bending bands, as shown by the arrows in Fig. 6b, resulting in a wavy and curled cementite structure. More bending and kinking of the cementite plates were observed after 2-pass DSR, and a discrete form of cementite was formed, indicating the occurrence of the cementite lamella breakage along the bending bands during DSR deformation, where some short bars and ellipses could be observed after 2-pass DSR (Fig. 6c). After 4-pass DSR, the cementite plates were almost fully fractured (Fig. 6d). Texture evolution of low carbon steel during DSR The orientation distribution function (ODF) sections of the Euler space when u2 = 45° obtained from the inverse
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Table 1 Main fibers characterizing the texture in bcc materials Texture
Fiber
Fiber position
Rolling texture
a
//to U, u1 = 0° and u2 = 45°
c
//to u1, U = 54.7° and u2 = 45°
pole figure (IPF) images shown in Fig. 2 were used to observe the main texture of the deformed samples under various rolling conditions without overlapping. The texture evaluation of the deformed samples was performed in the ferrite phase after the partitioning of the EBSD data, whereas there was no textural characterization in the pearlite phase. The fibers characterizations and textures components of body-centered cubic (bcc) materials found in ODF sections of the Euler space when u2 = 45° are shown in Table 1 and Fig. 7, respectively. Figure 8 displays the ODF sections of the deformed samples taken from the RD–ND plane and to ease the reading of the ODFs, positions of the major texture components and fibers were indicated.
J Mater Sci Fig. 7 Fibers characterizations and textures components of the bcc materials found in the u2 = 45° orientation distribution function (ODF) sections of the Euler
Fig. 8 Orientation distribution function (ODF) sections at u2 = 45° for the low carbon steel samples processed by a 1pass DSR, b 2-pass DSR, c 4pass DSR, and d 4-pass ESR
Figure 8a showed that the texture developed after 1-pass DSR was characterized by the formation of some components with orientations belonging to shear and rolling textures such as {110} h001i (Goss) and {001} h110i (rotated cube), respectively (Table 1, Fig. 7). With increasing amount of deformation introduced by the DSR process, the overall texture intensity increased significantly, and a fiber texture was formed and became more pronounced compared to that after 1-pass of DSR deformation (Fig. 8b). The fiber texture in bcc materials, which is the main components of the rolling texture resulting from the strain plane deformation, consists of the a-fiber with
h110i parallel to the RD and the c-fiber with h111i along the ND of the rolling plane. The former consists of orientations, such as {001} h110i, {114} h110i, {113} h110i, and {112} h110i as well as {111} h110i, which also belongs to the c-fiber, as does {111} h112i (Fig. 8b) [8, 9]. After deformation by 4-pass DSR (Fig. 8c), the texture of the fabricated sample consisted mainly of a-fiber, with a weak intensity of the rotated cube component, as well as intensified components with orientations belonging to c-fiber to accommodate the deformation imposed by DSR [20]. On the other hand, the texture components with shear orientations, such as {110} h001i and orientations close to
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{112} h111i, were recorded after DSR deformation as shown in the u2 = 45° section of the Euler space (Fig. 8a–c). After 1-pass DSR, the intensity of the shear component {112} h111i increased, and with further increasing of the thickness reduction (2-pass and 4-pass), the shear component {110} h001i appeared due to the increasing shear deformation with increasing the thickness reduction by the DSR process [16, 17, 32]. In contrast, the texture of the sample deformed by 4-pass ESR (Fig. 8d), was characterized by a stronger a-fiber and weaker c-fiber compared to the texture obtained after 4-pass DSR, indicating a larger amount of the deformation imposed by DSR compared to that imposed by ESR at the same rolling conditions.
Discussion Grain refinement mechanisms This study has demonstrated that deformation by DSR process after a high thickness reduction of 75 % achieved by 4-pass can result in an ultrafine-grained microstructure in the ferrite phase, with mean grain size of *0.4 lm surrounded by high angle boundaries. In addition, the deformed structure had a texture characterized by strong components such as {111} h110i, and {111} h112i, forming an intensified c-fiber in order to accumulate large amounts of the deformation introduced into the sample during DSR processing. In addition to the evolution of the ultrafine-grained microstructure in the ferrite phase, there was also a significant change in the morphology of the pearlite phase with deformation. In order to understand the microstructural evolution of the low carbon steel samples during processing by DSR, it is important to examine the mechanisms of grain refinement developed for ferrite as well as the morphological changes in pearlite. Figure 9 shows the mechanism that is responsible for the grain refinement of the ferrite phase. As shown in Fig. 9a, after DSR deformation for 1-pass, the microstructure consisted of elongated grains in the RD surrounded by equiaxed ultrafine grains that were bounded by HAGBs, forming a necklace-like structure. This structure is normally formed as a result of the nucleation of grains by a continuous dynamical recrystallization (CDRX) process occurred during SPD [33]. A careful examination of the deformed microstructure after 1-pass of DSR showed that the elongated grains have different orientations surrounded by LAGBs leading to the formation of subgrains (Fig. 9b). To examine the slight differences in orientations between the subgrains inside the elongated grains after 1-pass DSR, superimposed bcc unit cells were constructed for the subgrains (Fig. 9c) showing significant evolution of the
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orientation gradient, as revealed by the point-to-origin misorientations profile of the deformed sample after 1-pass DSR (Fig. 5a). With increasing number of operations by DSR, the misorientation angle was increased, due to the converting of LAGBs to HAGBs at the boundaries of the elongated grains that formed after 1-pass DSR, and more CDRX-ed grains appeared (Fig. 9d). With further deformation by 4-pass DSR (Fig. 9e), the regions containing ultrafine grains surrounded by high angle boundaries increased significantly and those of the elongated grains decreased, where a microstructure consisting of new ultrafine grains with high misorientation angles ranging from 15° to 60° was almost homogeneously developed over the entire area. Accordingly, when processing by 1-pass DSR, a uniform array of subgrains can be introduced within each coarse grain and the LAGBs of this subgrained structure evolve gradually with increasing strain imposed by DSR after 2-pass and 4-pass leading to an ultrafine structure consisting of very small grains separated by boundaries with high misorientations angles. To support this microstructural evolution with the additional deformation, the area fraction of the fully refined grains, which was defined as the grains with dimensions \1 lm, and the fraction of the HAGBs throughout the observed area was measured using image analysis software, and the results are presented in Fig. 9f. The fraction of ultrafine grains and HAGBs increased with increasing number of DSR operations and large increased in the fractions of ultrafine grains and HAGBs occurred beyond 4-pass and they measured *73 and *65 %, respectively. In low carbon steel materials, it is generally accepted that the microstructure development of ferrite after deformation by SPD methods can be similar to that of the extra low carbon steel or IF steel materials. Furthermore, DSRdeformed cementite in the pearlite phase shown in Fig. 6 is nearly similar to that obtained in the low carbon steel materials fabricated by different SPD methods such as ECAP [34, 35]. Shin et al. [34] examined the influence of the amount of strain imposed by ECAP on the pearlite microstructure of low carbon steel samples at a high pressing temperature of 623 K; they found that the cementite can be plasticity-deformed, leading to the formation of a globular cementite after 4-pass, suggesting that the strain imposed when processing by ECAP for 4-pass could operate a sufficient number of slip systems for the plastic deformation of cementite. In addition, Xiong et al. [35] deformed high carbon steel with a carbon content of 0.8 % by ECAP using the Bc route method at a high temperature of 923 K. After 4-pass, the microstructure of pearlite consisted of almost spheroidized cementite particles, with only a few lamellae still present. In this study, the plastic strain imposed by DSR after 4-pass led to deformation of the brittle cementite plates in pearlite phase
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Fig. 9 a Grain boundaries (GB’s) map for the low carbon steel sample processed by 1-pass DSR. b, c Inverse pole figure and with superimposed bcc unit cells for the low carbon steel sample processed by 1-pass DSR. d, e Inverse pole figure for the low carbon steel
sample processed by 2-pass and 4-pass DSR, respectively. f Fraction of ultrafine grains and HAGB as a function of the number of DSR operations. The fractions of HAGBs are calculated based on number of grain-weighted fraction
at room temperature without any cracking of the DSRfabricated sample, which indicates that a sufficient number of slip systems can be activated by DSR processing at room temperature resulting in improving the plastic deformability of cementite.
Figure 11a shows that the largest fraction of components was obtained for those with orientations belonging to a-fiber (rolling texture) after 4-pass ESR; also the fraction of the components with orientations belonging to a-fiber was much higher after ESR than those after 1-pass, 2-pass, and 4-pass DSR. On the other hand, the fraction of the shear texture components with orientations of {112} h111i and {110} h001i after DSR was higher than that after ESR deformation and they increased with increasing the number of DSR operations. Figure 11b shows the rolling texture and shear texture fractions of the fabricated samples, where the rolling texture was defined as the sum of the four different rolling texture components shown in Fig. 11a: {001} h110i, {112} h110i, {111} h110i, and {111} h112i, all these are well-known as conventional rolling texture components in bcc metals, whereas the shear texture was defined as the sum of two different shear texture components, {112} h111i and {110} h001i. The orientations that do not belong to any rolling and shear texture components were determined to be random texture. After 4-pass ESR,
Texture evolution in ferrite phase In bcc materials processed by the ASR process, the following two types of textures are generally observed: rolling texture, which is the result of strain plane deformation; and shear texture resulting from the shear deformation. Figure 10 shows the prevalence fibers variations consisting of a, c, and e, which appeared during the DSR process with a different number of operations. The results presented Fig. 10 indicated that only for the sample processed by 4-pass DSR, well-developed rolling component orientations: {001} h110i, {112} h110i, and {111} h112i were formed and clearly visible on shear components were also obtained.
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Fig. 10 Intensity distributions along a a-fibers, b c-fiber, and c e-fibers of the deformed low carbon steel samples
the sample had 60.2 % rolling texture and 1.7 % shear texture, but *38 % were random orientations, suggesting that this sample shows a strong rolling texture. More quantitative texture information confirming the formation of a strong rolling texture after processing by ESR was concluded from the ODFs obtained from the EBSD data of the ESR-processed sample (Fig. 8d). On the other hand, the samples fabricated by DSR showed relatively higher fractions of shear texture and smaller fractions of rolling texture compared to the sample processed by ESR. In addition, the fraction of the shear texture of the DSR-fabricated samples increased with increasing number of operations, where the shear texture fractions of *4, *6.8, and *8.1 % were obtained in the DSR-fabricated samples after 1-pass, 2-pass, and 4-pass, respectively (Fig. 11b). Figure 11c shows the ratio between the shear texture and rolling texture of the deformed samples. As shown in
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Fig. 11c, the ratio decreased with increasing the number of operations (thickness reduction), where 38, 27, and 22 % of the texture components had shear orientations after 1-pass DSR, 2-pass DSR, and 4-pass DSR, respectively. This indicates that the shear texture can be obtained more effectively compared to the rolling texture when a high speed ratio and low thickness reduction are applied. However, the majority of the texture components in DSR-deformed samples still have rolling orientations. This behavior, in which a high fraction of rolling texture components is observed after DSR deformation, was attributed to the use of lubricated processing conditions. This can lead to a more homogenous distribution of imposed strain throughout the thickness direction of the processed sample, and it will not be accelerated in the surface layer contacting with the higher speed roll. Kamikawa et al. [36] examined the effect of using lubricants on the strain distribution
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Fig. 11 a Fraction of the individual texture components obtained for the deformed low carbon steel samples. b Fraction of rolling and shear texture components obtained for the deformed low carbon steel
samples and c ratio of the shear texture to the rolling texture for the deformed low carbon steel samples
throughout the thickness direction was investigated by on IF steel processed by ARB. They found that the microstructural parameters determined from EBSD date in different thickness locations of the non-lubricated samples showed a meaningful difference, depending on the thickness location. The microstructure, however, was found to be homogenous when a lubricant was applied due to the uniform distribution of the effective strain throughout the thickness direction of the lubricated sample.
structure consisting of almost equiaxed grains *0.4 lm in size after DSR deformation for 4-pass. The fraction of grain boundaries with misorientations higher than 15° increased with increasing the number of DSR operations, and the misorientation distributions after 4-pass DSR was close to those predicted by Mackenzie for randomly misoriented polycrystalline aggregates of cubic metals. Cementite plates in the pearlite phase were almost fully fractured after 4-pass DSR at room temperature. After 4-pass DSR, well-developed rolling component orientations: {001} h110i, {112} h110i, and {111} h112i were formed and clearly visible on shear components were also obtained.
2.
3. 4.
Summary This study examines the microstructural and textural evolution of low carbon steel samples fabricated by DSR process after several numbers of operations. Based on experimental results, the following conclusions can be drawn: 1.
Partial CDRX occurred in the ferrite phase during the early stages of deformation, after 1-pass and 2-pass DSR, resulting in a necklace structure and microstructures with a bimodal grain size distribution. These microstructures were evolved to a fully refined
Acknowledgements This work was supported by funding from the Yeungnam University Research Project (213-A-380010).
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