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Microstructure evolution and strength-reduction in area balance of ultrafine-grained steels processed by warm caliber rolling
Scripta Materialia 55 (2006) 751–754 www.actamat-journals.com
Microstructure evolution and strength-reduction in area balance of ultrafine-grained steels processed by warm caliber rolling Shiro Torizuka, Eijiro Muramatsu, S.V.S. Narayana Murty* and Kotobu Nagai National Institute for Materials Science, Steel Research Center, 1-2-1 Sengen, Tsukuba, Ibaraki 305 0047, Japan Received 5 February 2006; revised 13 March 2006; accepted 30 March 2006 Available online 21 July 2006
Ultrafine grained steels with different carbon contents were produced through warm caliber rolling and evaluated for their stress– strain behavior along with the reduction in area. It was found that the reduction in area–tensile strength balance is far better than the conventional ferrite + pearlite steels and even superior to bainitic steels for all materials tested in the present study. Ó 2006 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved. Keywords: Ultrafine grained steel; Strength; Reduction in area
Ultrafine grained steels have an excellent combination of high yield strength and low ductile–brittle transition temperature can be achieved by grain refinement. On the other hand, it is known that uniform elongation and total elongation decrease with decreasing ferrite grain size. Ultrafine grain refinement to 1 lm deteriorates the uniform elongation in the tensile tests of steels [1–3]. As a result of the decrease in strain (work)-hardening, the tensile strength becomes very close to the yield strength of the ultrafine grained structures. The concept of a strain-hardening design using second phases has been proposed to improve the strength–ductility balance of the ultrafine-grained steels [4]. Ohmori et al. [5] have successfully demonstrated the applicability of strain hardening design using cementite in low carbon steels. They noted that the balance of yield strength and uniform elongation for ultrafine-grained structures could be improved by the dispersion of cementite particles [5]. While uniform elongation is a measure of ductility of the material, reduction in area in tensile tests is also an important measure of ductility [6]. However, measurement of the reduction in area requires standard tensile specimen which in turn requires bulk material for the fabrication of the specimen. Reduction in area is affected by second phases and inclusions. Ultrafine grained steels usually consist of ferrite and dispersed cementite particles. These cementite particles are formed through the fragmentation of pearlite by the large strain deforma* Corresponding author. Tel./fax: +81 29 859 2192; e-mail: susarla. [email protected]
tion process and get dispersed in the ultrafine grained ferrite matrix. Therefore, the volume fraction of the cementite as a second phase in ultrafine grained steel is far less than the volume fraction of pearlite of starting ferrite + pearlite steels. Therefore, ultrafine grained steels are expected to have a higher reduction in area, compared to conventional grain size ferrite + pearlite steels. The aim of the study was (i) to produce ultrafine grained microstructures in steels with different carbon contents and (ii) to evaluate the stress–strain behavior along with the reduction in area of these ultrafine grained steels. The data are compared to the mechanical properties of coarse grained steel with the same composition. The chemical composition of the steels used in the present study corresponds to 0.3Si–1.5Mn–0.01P– 0.002S with varying carbon contents, viz. 0.05, 0.15, 0.30, 0.45% (all in weight percentage). The composition of steel with 0.02C has a slightly different chemical composition with 0.3Si–0.2Mn–0.01P–0.002S (wt.%). Round bars with a diameter of / 115 mm were produced by hot forging, and pieces 600 mm in length were sampled as rolling material for warm multi-pass caliber rolling. The rolling process is shown schematically in Figure 1. The rolling was performed in two stages. In the first stage, the material was heated to 1173 K and subsequently rolled in the temperature range of 1023–993 K (which is near the Ar3 transformation point) to 80 mm square (h) in 10 passes (stage-I: cumulative reduction in area, 40%). This was followed by 21 passes of warm rolling from 80 mm h to a final 18 mm h (stage-II:
1359-6462/$ - see front matter Ó 2006 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.scriptamat.2006.03.067
752
S. Torizuka et al. / Scripta Materialia 55 (2006) 751–754
Figure 1. Schematic illustration of the caliber rolling process employed in the present study [7].
cumulative reduction in area, 95%). In stage-II, an ultrafine grained ferrite structure was formed by multi-pass warm caliber rolling at 823 K. Immediately following each stage, caliber rolled rods were water quenched. This caliber rolling procedure resulted in microstructures consisting of ultrafine ferrite grains in the 0.02C steel and of ultrafine ferrite grains along with dispersed spheroidized cementite in 0.05, 0.15, 0.30 and 0.45C steels [7]. Microstructure of caliber rolled bars changes with cumulative strain from work hardened to ultrafine grained structure [7]. Round bar tensile test specimens with a parallel section length of 24.5 mm and a diameter of /3.5 mm and full size 2 mm V-notch Charpy test specimens were machined from the center of the cross section in the rolling direction (longitudinal direction of bars). The specimens were tested in a standard tensile testing machine at a cross head speed of 0.5 mm/min. Scanning electron microscopic observations were made after etching the specimens with 1.5% Nital to reveal the microstructure. Figure 2(a)–(j) shows typical microstructures of caliber rolled bars at a fixed strain of 3.0 for 0.02, 0.05, 0.15, 0.3 and 0.45 carbon contents in both longitudinal direction and cross section. It can be seen from these microstructures that some of the grains are slightly elongated in the direction of rolling and cementite is uniformly distributed in the ferrite matrix with the distribution becoming better with increasing carbon content. It can be clearly seen that the average grain sizes of specimens measured by linear intercept method with 0.02 and 0.05 C are slightly coarser (0.7 and 0.6 micron), respectively, while the grain sizes of specimens with carbon contents of 0.15, 0.30 and 0.45 are nearly the same (0.5 micron) at the same cumulative reduction. The average size of cementite particles in all specimens is in the range of 100–200 nm [5]. Figure 3(a)–(e) shows the boundary maps of the specimens with different carbon contents (viz. 0.02, 0.05, 0.15, 0.30, 0.45%) taken at the center of the caliber rolled specimens. High angle grain boundaries of 15° or higher are revealed by red lines1, while 5° 6 h 6 15° are represented as dark blue lines. Low angle dislocation boundaries (h < 1.5°) are represented by sky blue lines. Although some sub grains and ferrite grains elongated in the rolling direction have been retained, a large num-
1
For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.
Figure 2. Microstructures of caliber rolled specimens with different carbon contents in longitudinal direction (L section) and cross section (C section): (a, b) 0.02C; (c, d) 0.05C; (e, f) 0.15C; (g, h) 0.30C and (i, j) 0.45C.
Figure 3. Boundary maps of the caliber rolled specimens with various carbon contents.
ber of equiaxed ultrafine ferrite grains in the submicron range surrounded by high angle grain boundaries were observed in all the specimens. This clearly indicates that the changes in the original microstructure consist of not only the elongation of the original ferrite grains in the rolling direction, but also the generation of new ultrafine ferrite grains. From the boundary maps, it was noted that more than 70% of the boundaries are high angled
S. Torizuka et al. / Scripta Materialia 55 (2006) 751–754
Figure 4. Nominal stress–strain curves of ultrafine-grained steel with carbon content in the range 0.02–0.45 wt.%.
ones. Therefore, it is clear from these microstructures that caliber rolling produces bulk UFG materials having uniform microstructures predominantly consisting of high angle grain boundaries. Hence, this method is appropriate for comparison of mechanical properties of materials of various carbon contents. Figure 4 shows the nominal stress–strain curves of ultrafine grained steels with carbon contents in the range of 0.02–0.45%. When carbon content is 0.02%, lower yield strength is 700 MPa and the total elongation is as small as 5%. Uniform elongation does not appear in the stress–strain curve. In other words, the stress– strain curve of 0.02C is characterized by no uniform elongation. This indicates that immediately after yielding, plastic instability occurs. However, when carbon content is slightly increased to 0.05%, yield strength increased to 830 MPa and total elongation drastically increased to 14%. Further, uniform elongation of 3% appears in this stress–strain curve. No plastic instability immediately after yielding as observed for 0.02C material is noticed. This is attributed to strain hardening by cementite dispersion. When carbon content is 0.15, strain hardening is evident. Strain hardening becomes more evident with a carbon content of 0.3%. Ultimate tensile strength is clearly observed in steels with carbon content of 0.3 and 0.45%. The tensile strength of 1000 MPa and total elongation of 20% is obtained at the carbon content of 0.45%. The failure stress also increases with an increase in carbon content monotonically. The volume fraction of cementite is virtually zero in steel with carbon content of 0.02%, resulting in no uniform elongation. The presence of cementite is clearly very effective in increasing the uniform elongation. Therefore, it is clear that ultrafine grained steels get more strain hardened as the carbon content increases. Figure 5 shows the true stress–true strain curves as well as work hardening rate as a function of true strain which was obtained from the room temperature tensile test results presented in Figure 4. The effect of carbon content (or the volume fraction of cementite) on the true stress is clear from the comparison of data shown in Figure 5 for various compositions. An increase in the carbon content causes an increase in the true stress. The strain hardening rate, on the other hand, increases with carbon content with a drastic effect from 0.02C to 0.05C and thereafter steadily up to 0.3C. Since the onset of plastic instability in tension is governed by Hart’s criwhere r is the flow stress and e is the terion [8], r P dr de dr strain rate and de is the strain hardening rate, a higher
753
Figure 5. True stress–true strain curves as well as work hardening rate as a function of true strain.
strain hardening rate is essential for larger uniform elongation in the material. Ohmori et al. [5] have not clarified the effect of presence and absence of cementite on the true stress–true strain curves of ultrafine grained steels. In the present study, this is clarified by the selection of virtually no cementite steel with those containing various fractions of cementite. Figure 6((a) and (b)) shows the variation 2 2 of percent ðd0 df Þ 100 as age elongation and reduction in area d2 0
a function of the volume fraction of cementite. Here, the volume fractions of cementite particles for the steels with the various carbon contents were evaluated roughly by the thermodynamic equilibrium calculations. It can be clearly seen from Figure 6(a), that while the uniform elongation gradually increases as the volume fraction of cementite increases, the non uniform elongation drastically increases from the specimen with no cementite (0.02C) to the specimen with about 0.5 volume per cent cementite (0.05C). This rapid rise in non uniform elongation is followed by a slight decrease with a further increase in the volume fraction of cementite. On the
Figure 6. Variation of (a) percentage elongation and (b) reduction in area with volume fraction of cementite for the steels studied.
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S. Torizuka et al. / Scripta Materialia 55 (2006) 751–754
Figure 7. Relationship between reduction in area (RA) and tensile strength of ferrite + pearlite steel (j) [9], bainite steel (n) [10], tempered martensite (d) [9] and ultrafine-grained steel ().
other hand, the total elongation rises drastically with the introduction of cementite and thereafter increases gradually. It can be seen from Figure 6(b) that the reduction in area slightly decreases with an increase in the volume fraction of cementite. It may be noted that uniform elongation is enhanced by the presence and volume fraction of second phase cementite particles due to enhanced work hardening. However, second phase dispersion helps in increasing the non uniform elongation at small volume fractions, but slightly decreases at large fractions. On the other hand, the reduction in area monotonically decreases with second phase dispersion even at large volume fractions. While reduction in area and non uniform elongation are closely related to each other, they are not same. Figure 7 shows the relationship between the reduction in area and tensile strength of ultrafine grained steels with the carbon contents of 0.02–0.45%. The test data of conventional ferrite + pearlite steel [9], tempered martensitic steel [9] and bainite steel [10] are also plotted in Figure 7 for the purpose of comparison. This figure presents the reduction in area of 0.02C, 0.05C, 0.15C and 0.45C steels. It is clear from this figure that 0.02C steel has a strength of 700 MPa and a reduction in area of 80%. For the case of 0.05C steel, the strength has increased to 825 MPa with a slight decrease in reduction in area to 76%. On the other hand, the strength of 0.15C steel has increased to 850 MPa and a reduction in area of 70%. Finally, the 0.45C steel has strength of almost 1000 MPa while the reduction is still about 60%. For the case of conventional 0.15C ferrite + pearlite steel, the strength is 450 MPa and the reduction in area is 82%. For the case of conventional S45C ferrite + pearlite steel, when the tensile strength is 650 MPa, the reduction in area is 55%. The usual trend for conventional steels is that as the pearlite volume fraction increases, the strength increases; however, this is accompanied by a drastic decrease in reduction in area. Therefore, the balance of reduction in area and tensile strength for ultrafine grain steels is far better than the conventional ferrite + pearlite steels, bainitic steels and even superior
to tempered martensitic steels tested in the present study. Generally bainitic steels or acicular ferrite steels have good tensile strength and reduction in area balance which is far superior to conventional ferrite + pearlite steels. However, ultrafine ferrite steels dispersed with cementite have a superior tensile strength-reduction in area than that of bainite and acicular ferrite steels. It is worth noting that the reduction in area is controlled by size and volume fraction of second phase particles such as pearlite or cementite. Ductile fracture is controlled by crack initiation and its propagation [11]. Inclusions and second phase particles such as cementite or interfaces act as nucleation sites for crack initiation. The decrease in the volume fraction of second phase due to ultrafine grain refinement from initial pearlite to uniformly dispersed cementite helps in achieving this superior tensile strength-reduction in area balance through effective second phase dispersion strengthening. In summary: 1. The most notable feature of ultrafine grained steels is their high reduction in area coupled with high tensile strength, with the combination being far superior to conventional ferrite + pearlite steels, bainitic steels, and even tempered martensitic steels. 2. Decrease in second phase volume fraction (pearlite) and its effective dispersion in the ferrite matrix as fine cementite particles coupled with ultrafine ferrite grains resulted in superior mechanical properties. The authors appreciated the assistance of Mrs. Etsuko Tsuchiya and Mrs. Tomoko Nozawa in conducting mechanical property tests and microstructure analysis reported in this study. [1] K. Nagai, J. Mater. Process. Technol. 117 (2001) 329. [2] N. Tsuji, Y. Ito, Y. Saito, Y. Minamino, Scripta Mater. 47 (2002) 893. [3] Howe: Mater. Sci. Technol. 16 (2000) 1264. [4] T. Hayashi, K. Nagai, Trans. Jpn. Soc. Mech. Eng. 68A (2002) 1553. [5] A. Ohmori, S. Torizuka, K. Nagai, ISIJ Int. 44 (2004) 1063. [6] S. Torizuka, E. Muramatsu, K. Nagai, CAMP-ISIJ 18 (2005) 1745. [7] A. Ohmori, S. Torizuka, K. Nagai, N. Koseki, Y. Kogo, Tetsu-to-Hagane 89 (2003) 781. [8] E.W. Hart, Acta Metall. 15 (1967) 351. [9] NIMS data base, Materials Information Technology Station, National Institute for Materials Science, Tsukuba, Japan. [10] E. Gondo, M. Araki, T. Yoshimura, N. Egachi, Seitetsu Kenkyu 303 (1980) 75. [11] P.F. Thomason, J. Inst. Met. 96 (1968) 360.
Microstructure evolution and strength-reduction in area balance of ultrafine-grained steels processed by warm caliber rolling Shiro Torizuka, Eijiro Muramatsu, S.V.S. Narayana Murty* and Kotobu Nagai National Institute for Materials Science, Steel Research Center, 1-2-1 Sengen, Tsukuba, Ibaraki 305 0047, Japan Received 5 February 2006; revised 13 March 2006; accepted 30 March 2006 Available online 21 July 2006
Ultrafine grained steels with different carbon contents were produced through warm caliber rolling and evaluated for their stress– strain behavior along with the reduction in area. It was found that the reduction in area–tensile strength balance is far better than the conventional ferrite + pearlite steels and even superior to bainitic steels for all materials tested in the present study. Ó 2006 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved. Keywords: Ultrafine grained steel; Strength; Reduction in area
Ultrafine grained steels have an excellent combination of high yield strength and low ductile–brittle transition temperature can be achieved by grain refinement. On the other hand, it is known that uniform elongation and total elongation decrease with decreasing ferrite grain size. Ultrafine grain refinement to 1 lm deteriorates the uniform elongation in the tensile tests of steels [1–3]. As a result of the decrease in strain (work)-hardening, the tensile strength becomes very close to the yield strength of the ultrafine grained structures. The concept of a strain-hardening design using second phases has been proposed to improve the strength–ductility balance of the ultrafine-grained steels [4]. Ohmori et al. [5] have successfully demonstrated the applicability of strain hardening design using cementite in low carbon steels. They noted that the balance of yield strength and uniform elongation for ultrafine-grained structures could be improved by the dispersion of cementite particles [5]. While uniform elongation is a measure of ductility of the material, reduction in area in tensile tests is also an important measure of ductility [6]. However, measurement of the reduction in area requires standard tensile specimen which in turn requires bulk material for the fabrication of the specimen. Reduction in area is affected by second phases and inclusions. Ultrafine grained steels usually consist of ferrite and dispersed cementite particles. These cementite particles are formed through the fragmentation of pearlite by the large strain deforma* Corresponding author. Tel./fax: +81 29 859 2192; e-mail: susarla. [email protected]
tion process and get dispersed in the ultrafine grained ferrite matrix. Therefore, the volume fraction of the cementite as a second phase in ultrafine grained steel is far less than the volume fraction of pearlite of starting ferrite + pearlite steels. Therefore, ultrafine grained steels are expected to have a higher reduction in area, compared to conventional grain size ferrite + pearlite steels. The aim of the study was (i) to produce ultrafine grained microstructures in steels with different carbon contents and (ii) to evaluate the stress–strain behavior along with the reduction in area of these ultrafine grained steels. The data are compared to the mechanical properties of coarse grained steel with the same composition. The chemical composition of the steels used in the present study corresponds to 0.3Si–1.5Mn–0.01P– 0.002S with varying carbon contents, viz. 0.05, 0.15, 0.30, 0.45% (all in weight percentage). The composition of steel with 0.02C has a slightly different chemical composition with 0.3Si–0.2Mn–0.01P–0.002S (wt.%). Round bars with a diameter of / 115 mm were produced by hot forging, and pieces 600 mm in length were sampled as rolling material for warm multi-pass caliber rolling. The rolling process is shown schematically in Figure 1. The rolling was performed in two stages. In the first stage, the material was heated to 1173 K and subsequently rolled in the temperature range of 1023–993 K (which is near the Ar3 transformation point) to 80 mm square (h) in 10 passes (stage-I: cumulative reduction in area, 40%). This was followed by 21 passes of warm rolling from 80 mm h to a final 18 mm h (stage-II:
1359-6462/$ - see front matter Ó 2006 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.scriptamat.2006.03.067
752
S. Torizuka et al. / Scripta Materialia 55 (2006) 751–754
Figure 1. Schematic illustration of the caliber rolling process employed in the present study [7].
cumulative reduction in area, 95%). In stage-II, an ultrafine grained ferrite structure was formed by multi-pass warm caliber rolling at 823 K. Immediately following each stage, caliber rolled rods were water quenched. This caliber rolling procedure resulted in microstructures consisting of ultrafine ferrite grains in the 0.02C steel and of ultrafine ferrite grains along with dispersed spheroidized cementite in 0.05, 0.15, 0.30 and 0.45C steels [7]. Microstructure of caliber rolled bars changes with cumulative strain from work hardened to ultrafine grained structure [7]. Round bar tensile test specimens with a parallel section length of 24.5 mm and a diameter of /3.5 mm and full size 2 mm V-notch Charpy test specimens were machined from the center of the cross section in the rolling direction (longitudinal direction of bars). The specimens were tested in a standard tensile testing machine at a cross head speed of 0.5 mm/min. Scanning electron microscopic observations were made after etching the specimens with 1.5% Nital to reveal the microstructure. Figure 2(a)–(j) shows typical microstructures of caliber rolled bars at a fixed strain of 3.0 for 0.02, 0.05, 0.15, 0.3 and 0.45 carbon contents in both longitudinal direction and cross section. It can be seen from these microstructures that some of the grains are slightly elongated in the direction of rolling and cementite is uniformly distributed in the ferrite matrix with the distribution becoming better with increasing carbon content. It can be clearly seen that the average grain sizes of specimens measured by linear intercept method with 0.02 and 0.05 C are slightly coarser (0.7 and 0.6 micron), respectively, while the grain sizes of specimens with carbon contents of 0.15, 0.30 and 0.45 are nearly the same (0.5 micron) at the same cumulative reduction. The average size of cementite particles in all specimens is in the range of 100–200 nm [5]. Figure 3(a)–(e) shows the boundary maps of the specimens with different carbon contents (viz. 0.02, 0.05, 0.15, 0.30, 0.45%) taken at the center of the caliber rolled specimens. High angle grain boundaries of 15° or higher are revealed by red lines1, while 5° 6 h 6 15° are represented as dark blue lines. Low angle dislocation boundaries (h < 1.5°) are represented by sky blue lines. Although some sub grains and ferrite grains elongated in the rolling direction have been retained, a large num-
1
For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.
Figure 2. Microstructures of caliber rolled specimens with different carbon contents in longitudinal direction (L section) and cross section (C section): (a, b) 0.02C; (c, d) 0.05C; (e, f) 0.15C; (g, h) 0.30C and (i, j) 0.45C.
Figure 3. Boundary maps of the caliber rolled specimens with various carbon contents.
ber of equiaxed ultrafine ferrite grains in the submicron range surrounded by high angle grain boundaries were observed in all the specimens. This clearly indicates that the changes in the original microstructure consist of not only the elongation of the original ferrite grains in the rolling direction, but also the generation of new ultrafine ferrite grains. From the boundary maps, it was noted that more than 70% of the boundaries are high angled
S. Torizuka et al. / Scripta Materialia 55 (2006) 751–754
Figure 4. Nominal stress–strain curves of ultrafine-grained steel with carbon content in the range 0.02–0.45 wt.%.
ones. Therefore, it is clear from these microstructures that caliber rolling produces bulk UFG materials having uniform microstructures predominantly consisting of high angle grain boundaries. Hence, this method is appropriate for comparison of mechanical properties of materials of various carbon contents. Figure 4 shows the nominal stress–strain curves of ultrafine grained steels with carbon contents in the range of 0.02–0.45%. When carbon content is 0.02%, lower yield strength is 700 MPa and the total elongation is as small as 5%. Uniform elongation does not appear in the stress–strain curve. In other words, the stress– strain curve of 0.02C is characterized by no uniform elongation. This indicates that immediately after yielding, plastic instability occurs. However, when carbon content is slightly increased to 0.05%, yield strength increased to 830 MPa and total elongation drastically increased to 14%. Further, uniform elongation of 3% appears in this stress–strain curve. No plastic instability immediately after yielding as observed for 0.02C material is noticed. This is attributed to strain hardening by cementite dispersion. When carbon content is 0.15, strain hardening is evident. Strain hardening becomes more evident with a carbon content of 0.3%. Ultimate tensile strength is clearly observed in steels with carbon content of 0.3 and 0.45%. The tensile strength of 1000 MPa and total elongation of 20% is obtained at the carbon content of 0.45%. The failure stress also increases with an increase in carbon content monotonically. The volume fraction of cementite is virtually zero in steel with carbon content of 0.02%, resulting in no uniform elongation. The presence of cementite is clearly very effective in increasing the uniform elongation. Therefore, it is clear that ultrafine grained steels get more strain hardened as the carbon content increases. Figure 5 shows the true stress–true strain curves as well as work hardening rate as a function of true strain which was obtained from the room temperature tensile test results presented in Figure 4. The effect of carbon content (or the volume fraction of cementite) on the true stress is clear from the comparison of data shown in Figure 5 for various compositions. An increase in the carbon content causes an increase in the true stress. The strain hardening rate, on the other hand, increases with carbon content with a drastic effect from 0.02C to 0.05C and thereafter steadily up to 0.3C. Since the onset of plastic instability in tension is governed by Hart’s criwhere r is the flow stress and e is the terion [8], r P dr de dr strain rate and de is the strain hardening rate, a higher
753
Figure 5. True stress–true strain curves as well as work hardening rate as a function of true strain.
strain hardening rate is essential for larger uniform elongation in the material. Ohmori et al. [5] have not clarified the effect of presence and absence of cementite on the true stress–true strain curves of ultrafine grained steels. In the present study, this is clarified by the selection of virtually no cementite steel with those containing various fractions of cementite. Figure 6((a) and (b)) shows the variation 2 2 of percent ðd0 df Þ 100 as age elongation and reduction in area d2 0
a function of the volume fraction of cementite. Here, the volume fractions of cementite particles for the steels with the various carbon contents were evaluated roughly by the thermodynamic equilibrium calculations. It can be clearly seen from Figure 6(a), that while the uniform elongation gradually increases as the volume fraction of cementite increases, the non uniform elongation drastically increases from the specimen with no cementite (0.02C) to the specimen with about 0.5 volume per cent cementite (0.05C). This rapid rise in non uniform elongation is followed by a slight decrease with a further increase in the volume fraction of cementite. On the
Figure 6. Variation of (a) percentage elongation and (b) reduction in area with volume fraction of cementite for the steels studied.
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S. Torizuka et al. / Scripta Materialia 55 (2006) 751–754
Figure 7. Relationship between reduction in area (RA) and tensile strength of ferrite + pearlite steel (j) [9], bainite steel (n) [10], tempered martensite (d) [9] and ultrafine-grained steel ().
other hand, the total elongation rises drastically with the introduction of cementite and thereafter increases gradually. It can be seen from Figure 6(b) that the reduction in area slightly decreases with an increase in the volume fraction of cementite. It may be noted that uniform elongation is enhanced by the presence and volume fraction of second phase cementite particles due to enhanced work hardening. However, second phase dispersion helps in increasing the non uniform elongation at small volume fractions, but slightly decreases at large fractions. On the other hand, the reduction in area monotonically decreases with second phase dispersion even at large volume fractions. While reduction in area and non uniform elongation are closely related to each other, they are not same. Figure 7 shows the relationship between the reduction in area and tensile strength of ultrafine grained steels with the carbon contents of 0.02–0.45%. The test data of conventional ferrite + pearlite steel [9], tempered martensitic steel [9] and bainite steel [10] are also plotted in Figure 7 for the purpose of comparison. This figure presents the reduction in area of 0.02C, 0.05C, 0.15C and 0.45C steels. It is clear from this figure that 0.02C steel has a strength of 700 MPa and a reduction in area of 80%. For the case of 0.05C steel, the strength has increased to 825 MPa with a slight decrease in reduction in area to 76%. On the other hand, the strength of 0.15C steel has increased to 850 MPa and a reduction in area of 70%. Finally, the 0.45C steel has strength of almost 1000 MPa while the reduction is still about 60%. For the case of conventional 0.15C ferrite + pearlite steel, the strength is 450 MPa and the reduction in area is 82%. For the case of conventional S45C ferrite + pearlite steel, when the tensile strength is 650 MPa, the reduction in area is 55%. The usual trend for conventional steels is that as the pearlite volume fraction increases, the strength increases; however, this is accompanied by a drastic decrease in reduction in area. Therefore, the balance of reduction in area and tensile strength for ultrafine grain steels is far better than the conventional ferrite + pearlite steels, bainitic steels and even superior
to tempered martensitic steels tested in the present study. Generally bainitic steels or acicular ferrite steels have good tensile strength and reduction in area balance which is far superior to conventional ferrite + pearlite steels. However, ultrafine ferrite steels dispersed with cementite have a superior tensile strength-reduction in area than that of bainite and acicular ferrite steels. It is worth noting that the reduction in area is controlled by size and volume fraction of second phase particles such as pearlite or cementite. Ductile fracture is controlled by crack initiation and its propagation [11]. Inclusions and second phase particles such as cementite or interfaces act as nucleation sites for crack initiation. The decrease in the volume fraction of second phase due to ultrafine grain refinement from initial pearlite to uniformly dispersed cementite helps in achieving this superior tensile strength-reduction in area balance through effective second phase dispersion strengthening. In summary: 1. The most notable feature of ultrafine grained steels is their high reduction in area coupled with high tensile strength, with the combination being far superior to conventional ferrite + pearlite steels, bainitic steels, and even tempered martensitic steels. 2. Decrease in second phase volume fraction (pearlite) and its effective dispersion in the ferrite matrix as fine cementite particles coupled with ultrafine ferrite grains resulted in superior mechanical properties. The authors appreciated the assistance of Mrs. Etsuko Tsuchiya and Mrs. Tomoko Nozawa in conducting mechanical property tests and microstructure analysis reported in this study. [1] K. Nagai, J. Mater. Process. Technol. 117 (2001) 329. [2] N. Tsuji, Y. Ito, Y. Saito, Y. Minamino, Scripta Mater. 47 (2002) 893. [3] Howe: Mater. Sci. Technol. 16 (2000) 1264. [4] T. Hayashi, K. Nagai, Trans. Jpn. Soc. Mech. Eng. 68A (2002) 1553. [5] A. Ohmori, S. Torizuka, K. Nagai, ISIJ Int. 44 (2004) 1063. [6] S. Torizuka, E. Muramatsu, K. Nagai, CAMP-ISIJ 18 (2005) 1745. [7] A. Ohmori, S. Torizuka, K. Nagai, N. Koseki, Y. Kogo, Tetsu-to-Hagane 89 (2003) 781. [8] E.W. Hart, Acta Metall. 15 (1967) 351. [9] NIMS data base, Materials Information Technology Station, National Institute for Materials Science, Tsukuba, Japan. [10] E. Gondo, M. Araki, T. Yoshimura, N. Egachi, Seitetsu Kenkyu 303 (1980) 75. [11] P.F. Thomason, J. Inst. Met. 96 (1968) 360.