WITHDRAWN: Effect of pressing temperature on microstructure and tensile behavior of low carbon steels processed by equal channel angular pressing

Materials Science and Engineering A325 (2002) 31 – 37 www.elsevier.com/locate/msea

Effect of pressing temperature on microstructure and tensile behavior of low carbon steels processed by equal channel angular pressing Dong Hynk Shin a,*, Jong-Jin Pak a, Young Kuk Kim a, Kyung-Tae Park b, Yong-Seog Kim c b

a Department of Metallurgy and Materials Science, Hanyang Uni6ersity, Ansan, Kyunggi-Do 425 -791, South Korea Department of Ad6anced Materials Science and Engineering, Taejon National Uni6ersity of Technology, Taejon 300 -717, South Korea c Department of Metallurgy and Materials Science, Hongik Uni6ersity, Seoul 121 -791, South Korea

Received 28 November 2000; received in revised form 2 April 2001

Abstract Two grades of low carbon steels, one containing vanadium and the other without vanadium, were subjected to equal channel angular (ECA) pressing at a temperature range of 623– 873 K. For steel without vanadium, the ECA pressing at 623 K resulted in ultrafine ( :0.3 mm) ferrite grains with high angle boundaries. At higher pressing temperature, coarser grains with low angle boundaries were formed, which indicates that the recovery occurs at a significant rate during the pressing. For the steel containing vanadium, submicrometer order ferrite grains and high dislocation density were preserved up to pressing temperature of 873 K. The enhanced thermal stability of the steel containing vanadium was attributed to its peculiar microstructure consisted with fine ferrite grains with uniformly distributed nanosized cementite particles. In addition, the tensile behaviors of the ECA pressed steels were characterized and discussed based on the microstructure. © 2002 Elsevier Science B.V. All rights reserved. Keywords: Low carbon steels; Equal channel angular pressing; Ultrafine grain size; Microstructure; Thermal stability

1. Introduction The equal channel angular (ECA) pressing is being considered as one of the most viable techniques which are capable of producing ultrafine grained (UFG) materials without residual porosity. In the process, materials are subjected to a severe plastic deformation by passing them repeatedly through a die, which has two channels of same cross section meeting at an angle. Although several techniques have been developed to impose the severe plastic deformation on metals, the ECA pressing has some advantages over the other techniques: (a) capability of producing large bulk UFG materials and (b) large plastic shear deformation per pass [1].

* Corresponding author. Tel./fax: + 82-31-4005224. E-mail address: [email protected] (D.H. Shin).

Even with the potential for industrial applications of the ECA pressing, only a limited number of works have been reported on the ECA pressing of commercial alloys [2]. Recent studies on Al alloys have identified four processing parameters that influence the microstructure of the alloy underwent the ECA pressing. The parameters include the angle between two channels [3], rotation of the sample between pressings [4,5], the speed [6] and temperature [7] of the ECA pressing. According to their studies, shear deformation was promoted when the angle of channel is 90° and more homogeneous microstructure was resulted expeditiously by rotating the sample by 90° around its longitudinal axis between each pressing step. Slower pressing speed led to a more equilibrated microstructure and increment in the pressing temperature to larger grains. Recently, the authors reported microstructural characteristics and mechanical properties of a low carbon steel processed by the ECA pressing technique [8–11].

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As-pressed sample subjected to total effective strain of 4 at 623 K resulted in a grain refinement of ferritic steel from 30 to 0.3 mm and a increase of yield strength (YS) increase from 310 MPa to over 900 MPa. Annealing of the pressed sample at 753 K, however, led to a slight decrease in YS to 700 MPa. So far, the studies on the ECA pressing of steel have been limited only at low temperatures. The employment of higher pressing temperature might reduce the pressing pressure considerably and simplify the processes by excluding annealing treatment. In the present investigation, microstructural evolutuion and mechanical properties of UFG low carbon steels have been studied as a function of the ECA pressing temperature in a range from 623 to 873 K. The ECA pressing was conducted on two grades of low carbon steels, one containing vanadium and the other without vanadium. In addition, the effect of the static annealing temperature on the microstructure of the as-pressed sample was also investigated for comparison. The tensile properties of thus produced steels were measured.

Fig. 1. A schematic illustration of ECA pressing die and (b) of the ECA pressing set-up for elevated temperatures.

2. Experimental procedures Two grades of low carbon steel (Fe–0.15C– 1.1lMn– 0.26Si and Fe– 0.ISC–1.17Mn–0.06V– 0.008N in wt.%) were used in the present study. The former steel is designated as CS and the latter as CSV, respectively. The steels were manufactured by the conventional thermomechanical processing route at a steel mill. Ingots of both steels were homogenized at 1523 K for 1 hr and then size-rolled to fabricate the plates of 350×150× 50 mm3. CS steel was austenitized at 1473 K for 1 h and then air-cooled. The mean linear intercept size of the ferrite grain and pearlite colony was  30 mm. CSV steel was oil-quenched to room temperature after the same austenitization treatment followed by a normalization treatment for homogeneous distribution of fine vanadium containing carbides. The normalization treatment consisted of soaking at 1173 K for 1 h and followed by air-cooling. Mean linear intercept size of ferrite grain and pearlite colony of CSV steel was 10 mm. In spite of different grain sizes between the two grade steels, the area fraction of the constituent phases, i.e. ferrite of 85% and pearlite of the remainder, was nearly the same in both steels. After machining the plate into cylindrical samples of ƒ 18× 130 mm2, the ECA pressing was conducted up to four passes. ECA pressing die was designed to yield an effective strain of 1 by single pass: the inner contact angle (ƒ) and the outer contact angle („) between the two channels of the die were 90 and 20°, respectively as illustrated schematically in Fig. 1(a). During the ECA pressing, the sample was rotated 180° around its longitudinal axis between each pass. This pattern of the pressing was selected since it restores the original segment of the microstructure at an even number of passes [12,13]. Prior to the ECA pressing, samples were soaked at 623 K for 10 mm. for an uniform temperature distribution. The samples were then heated at a heating rate of 10 K min − 1 to the pressing temperatures. The ECA pressing temperature was changed from 623 to 873 K. The pressing speed was 2 mm s − 1 and it took :2 min. for each pressing step. After each pressing, the samples were removed immediately from the exit die and then inserted into the inlet die for the next pressing step. Set-up for the ECA pressing at the elevated temperatures is shown schematically in Fig. 1(b)). The pressing die was placed tightly inside the cylindrical steel block. The heating bars were placed vertically in the steel block for heating the pressing die. The die temperature was controlled by a thermocouple inserted to the vicinity of the area where the sample undergoes the shear deformation during the pressing. The temperature was monitored continuously and controlled within 95 K of the set temperature.

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Fig. 2. TEM micrographs with SAD patterns for the CS steel after ECA pressing at (a) 623, (b) 753, (c) 813, and (d) 873 K.

The microstructural examination of samples was performed using a transmission electron microscopy (Jeol, JEM 2010) operated at 200 kV. Thin foils for the TEM observation were prepared by a twin-jet polishing at ambient temperatures. The etchant was 20% perchloric acid and 80% methanol and the applied potential was 40 V. Tensile tests were conducted using an Instron machine with an intial stain rate of 1.33× 10 − 3 s − 1 on the full scale tensile samples of 25.4 mm gage length at room temperature.

3. Results Fig. 2 shows TEM micrographs with selected-area diffraction (SAD) patterns of the CS steel pressed at 623, 753, 813 and 873 K, respectively. The area of the SAD analysis was :3 mm in diameter. The pressing speed was kept at 2 mm s − 1 and it took : 2 min. per each pressing step. Nearly equiaxed ferrite grains of 0.3 mm were formed after the pressing at 623 K (Fig. 2(a)). A detailed examination of the ferrite grains at a higher magnification [10] revealed ultrafine grains with high dislocation density inside and extensive extinction contours near the grain boundaries. These microstructural characteristics indicate that the grain boundaries

in the as-pressed sample are in a non-equilibrium state. In addition, the SAD pattern with number of rings and spots shows an evidence of high angle grain boundaries. The sample pressed at 753 K also had equiaxed ferrite grains with a slightly increased size of 0.4 mm. The SAD pattern revealed a Ž1 1 0 diffraction pattern with extra spots, indicating grain boundaries of low misorientation angles. Several interesting features were noticed in the sample pressed at 813 K (Fig. 2(c)): (a) well-defined grain boundaries, (b) notable grain growth, and (c) thick grain boundary region. In the samples pressed at 873 K (Fig. 2(d)), most of the boundaries were well defined also, but their appearance was quite different from that observed in the sample pressed at 813 K. In some grains, the formation of low angle grain boundaries (marked A) was observed. In addition, there are curved grain boundaries (marked B) which might have moved from the region of low dislocation density to the region of high dislocation density. These facts suggest that recovery is actively involved at the pressing temperature. Although it was not shown in Fig. 2(d), dislocation-free large grains were observed in some areas of the sample. These large grains appeared to be formed by the dynamic recrystallization during the ECA pressing.

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Fig. 3. TEM micrographs with SAD patterns for the CSV steel after ECA pressing at (a) 623, (b) 753, (c) 813, and (d) 873 K.

The CSV steel containing 0.06 wt.% vanadium was also pressed at the same temperatures. The general features of microstructure in the CSV steel pressed at 623 K were comparable to the CS steel, except slightly finer equiaxed grains of less than 0.3 mm as shown in Fig. 3(a). The sample pressed at 753 K (Fig. 3(b)) also shows equiaxed ferrite grains having the characteristics of non-equilibrium boundaries, i.e., poorly developed grain boundaries and the existence of extensive extinction contours. The microstructures of the CSV steel pressed at 813 and 873 K (Fig. 3(c) and (d), respectively) were quite different from those of the CS steel. Nearly equiaxed grains with relatively high dislocation density were observed predominantly. The SAD patterns suggest that most of the grain boundaries are of low angle boundaries in nature. The large grains with low dislocation density were not observed in the CSV steels. Increase in the pressing temperature for the CSV steel has resulted in slightly larger grains, but the dislocation density in ferrite grains remained high. The dislocation density was comparable to that of the CS steel pressed at 623 K (Fig. 2(a)). It is of interest to note that such fine ferrite grains could be obtained in the CSV sample at a relatively high temperature, which is over 0.9 of the eutectoid transformation temperature (896 K) of steel.

Figs. 4 and 5 represent the stress–strain curves of CS and CSV steels pressed at different pressing temperatures, respectively. The tensile curves of CS and CSV steels are almost identical in the as-received condition. As shown in Fig. 4, YS of the CS steel pressed at 623 K was measured to be 937 MPa, a significant increase

Fig. 4. Stress – strain curves of the CS steel after ECA pressing at 623, 753, 813 and 873 K.

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over 70% enhancement compared to that of the as-received state.

4. Discussion

Fig. 5. Stress – strain curves of the CSV steel after ECA pressing at 623, 753, 813, and 873 K.

Fig. 6. Effects of static annealing and pressing temperatures on the grain size of the CS and CSV steels.

from that of as-received sample (310 MPa). As the pressing temperature was increased to 753 K, YS decreased significantly to 600 MPa. Further increase in the pressing temperature resulted in a gradual decrease in the YS value. When pressed at 873 K, YS of the CS steel was 480 Mpa – equivalent to 55% enhancement compared to that of as-received condition. In case of the CSV steel, YS of the sample pressed at 623 K was similar to that of the CS steel as shown in Fig. 5. However, the drastic drop in the YS was not observed in the CSV steel with increasing the pressing temperature. YS of the CSV steel pressed at 753 K was as high as 800 MPa and the elongation 16%. The YS of CSV steel pressed at 873 K was 640 Mpa – equivalent to

Goal of the ECA pressing is to introduce severely plastic strains into materials without changing the cross-section area of samples. Accumulation of the plastic deformation from a simple shear is more effective when pressed at low temperatures. However, it would be beneficial to conduct the ECA pressing at elevated temperatures for the practical point of view as stated in the previous section. In recent investigations on aluminum alloys [7], two prominent effects of the pressing temperature on the microstructure were noted. Firstly, grain size increases with the pressing temperature. Secondly, the grains processed at higher pressing temperatures are separated by boundaries of low angle misorientation. This type of grain boundary was also observed in the present investigation. While the CS samples pressed at 623 K appeared to have equiaxed grains with high angle boundaries, the samples pressed at temperatures above 753 K had low angle boundaries. It should be noted that even the equiaxed grains appeared to be similar in the samples pressed at 623 and 753 K, they have different types of grain boundaries. The microstructure of the sample pressed at 813 K was manifested by two features: (a) low dislocation density inside individual grains and (b) relatively thick grain boundary region. These phenomena may indicate that recovery process associated with annihilation of dislocations at grain boundaries is reasonably fast at the pressing temperature. The formation of relatively sharp grain boundaries in the sample pressed at 873 K suggests that the recovery process is more accelerated at the pressing temperatures. In ECA pressing, the rate of recovery should increase with the pressing temperature and that provides a greater opportunity for the annihilation of dislocations at the subgrain boundaries rather than to impinge upon [7]. Therefore, the evolution of the high angle grain boundaries should become more difficult at the higher pressing temperatures. Earlier work demonstrated a thermal stability of the ultrafine grains of the CS steel ECA pressed at 623 K during static annealing treatments at elevated temperatures [10]. It is, therefore, appropriate to compare the influence of the static annealing and the pressing temperatures on the microstructure and tensile properties of the ECA pressed samples. Fig. 6 shows a composite plot, illustrating the effect of static annealing and pressing temperatures on the grain size of the CS and CSV steels. Marked ‘annealed’ represents the sample ECA pressed followed by a static annealing treatment and ‘pressed’ denotes the sample underwent the ECA press-

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ing at the temperatures, respectively. Inspection of Fig. 6 shows that grain size increases with the annealing and pressing temperatures. Several interesting features were noted in the microstructural changes occurring during the static annealing treatment of the CS steel pressed at 623 K. Firstly, the ultrafine ferrite grains showed little growth when annealed in a temperature range of 693– 783 K for 1 h. Although the holding time for the pressing is shorter than that of the annealing, the effect of pressing temperature on the grain size of the CS and CSV samples was similar to that of the static annealing temperature in the range from 693 to 783 K. The samples did not recrystallized when pressed at the temperature range [10]. The high rate of recovery at the pressing temperatures should have prevented the recrystallization and the evolution of the high angle boundaries. Secondly, a microstructure consisted of coarse recrystallized grains (5 mm) and recovered ultrafine grains was resulted when annealed at a temperature range of 813–873 K for 1 h [9,10]. By contrast, pressing at the same temperature range led to smaller grains ( 1 mm) with low angle boundaries due to higher rate of recovery. From these results, it is evident that ECA pressing at low temperatures followed by static annealing at elevated

Fig. 7. SEM micrographs of the CS (a) and CSV (b) steels pressed at 873 K.

temperatures leads to relatively large recystallized grains with high angle boundaries. The ECA pressing at the elevated temperatures, however, promotes the formation of fine recovered grains with low angle boundaries. As the recovery rate is critical for the grain refinement in ECA pressing, the addition of Zr [13–17] and Sc [18–21] to an aluminum alloys has been attempted to control the rate. In the present experiments, vanadium was added for the same role in the steel. The characteristics of the microstructure of the two types of the steel, CS (Fig. 2(a)) and CSV (Fig. 3(a)), appeared to be similar when ECA pressed at 623 K. However, the effect of vanadium addition became evident at higher temperatures. Although the grain size increased with the pressing temperature, its morphology, i.e., fine equiaxed grains with high dislocation density (Fig. 3), preserved in the CSV steel. Increase in the recrystallization temperature of steels with the addition of strong carbide formers such as V and Nb has been recognized in various studies [22]. The addition of 0.06% of vanadium was reported to increase the temperature by 100 K. Thus the increased recrystallization temperature in the CSV sample must have contributed to the high dislocation density in the ferrite matrix as well as the preservation of the equiaxed grains. Fig. 7 shows SEM micrographs of microstructures of both grade steels pressed at 873 K. The most striking difference in microstructures between CS and CSV steels at higher pressing temperature is that the uniform distribution of the nano-sized particles throughout the ferrite phase. As shown in Fig. 7(a), the microstructure of the sample of CS steel pressed at 873 K shows pearlite colonies in the ferrite phase. However, in the CSV steel, the colonies disappeared almost completely and nanosized particles at the ferrite grain boundaries were noted (Fig. 7(b)). These nano-sized particles were identified as cementite (Fe3C) rather than particles associated with vanadium. As shown in Figs. 2 and 3, under the identical pressing conditions, the dislocation density of steels containing vanadium remains relatively high compared to steels without vanadium. It is considered that the precipitation of nano-sized Fe3C particles at ferrite grain boundaries, which was not occurred in CS steel but in CSV steel, is induced by the high dislocation density. The recent investigation attributed the spheroidization of pearlitic cementite of wire-drawn eutectoid steel to high dislocation density and dissolved carbon from severely deformed pearlitic cementite [23–25]. A dense dislocation structure was preserved in the CSV steel at 873 K, while the CS steel exhibited a typical recovered structure with low dislocation density at 873 K (Fig. 2(d)). Accordingly, in CSV steel, dissolved carbon atoms are possible to diffuse away from the pearlite colony along dislocation core as well as non-equilibrium boundaries to precipitate as the Fe3C particles.

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higher than that of the samples ECA pressed at the annealing temperatures. The strength of steel with vanadium pressed at 873 K was higher than that of steel without vanadium pressed at 753 K, probably due to the precipitation of fine cementite particles and higher dislocation density. Acknowledgements This work was performed with support from Korea Ministry of Science and Technology though ‘2000 National Research Laboratory Program’. References

Fig. 8. Effects of static annealing and pressing temperatures on the tensile properties of the CS and CSV steels.

Fig. 8 shows both sets of YS and elongation data of the CS and CSV steels as a function of the static annealing and pressing temperature. The strength of CSV steel pressed at 873 K was higher than that of CS steel pressed at 753 K, probably due to the precipitation of the fine Fe3C particles and higher dislocation density. The YS of the samples ECA pressed at 623 K followed by the static annealing is consistently higher than that of the sample pressed at the same annealing temperature. Elongation of the CS steel increased with the pressing temperature and finally recovered the value at 873 K close to that of the as-received condition. Similarly, for the CSV steel, elongation increased gradually with the annealing temperature. Smaller elongation of the CSV steel than that of the CS steel is believed to be resulted form the high dislocation density.

5. Conclusions (1)ECA pressing of a low carbon steel at 623 K resulted in ultrafine ferrite grains ( 0.3 mm) with high angle boundaries. When pressed at higher temperatures, coarser grains with low angle boundaries were formed probably due to a rapid recovery process. (2)Addition of vanadium to the steel preserved the high dislocation density up to pressing temperature of 873 K. In the steel pressed at the temperature, the pearlite colonies became completely dissolved and reprecipitated as nano-sized cementite particles distributed uniformly. The grain size of the ferrite matrix was less than 1 mm. (3)The YS of the samples ECA pressed at 623 K followed by a static annealing treatment is consistently

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