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On the determining role of microstructure of niobium-microalloyed steels with differences in impact toughness
Materials Science and Engineering A 491 (2008) 55–61
On the determining role of microstructure of niobium-microalloyed steels with differences in impact toughness R. Anumolu a,b , B. Ravi Kumar a , R.D.K. Misra a,b,∗ , T. Mannering c , D. Panda c , S.G. Jansto d a b
Center for Structural and Functional Materials, University of Louisiana at Lafayette, LA 70504-4130, USA Department of Chemical Engineering, University of Louisiana at Lafayette, Lafayette, LA 70504-4130, USA c Nucor-Yamato Steel, P.O. Box 1228, 5929 East State Highway 18, Blytheville, AR 72316, USA d Reference Metals, 1000 Old Pond Road, Bridgeville, PA 15017, USA Received 13 December 2007; accepted 2 January 2008
Abstract The relationship between microstructure and impact toughness was investigated for niobium-microalloyed steels with similar yield strength. The nominal steel composition was similar and any variation in processing history was unintentional. The general microstructure of the investigated steel was similar and consisted of 85% polygonal ferrite and 15% pearlite. Despite these similarities, they exhibited variation in toughness and were classified as high- and low-toughness steels. Detailed microstructural investigation including stereological analysis and electron microscopy implied that toughness is strongly influenced by mean intercept length of polygonal ferrite and pearlite colony, and their distribution, interlamellar spacing, and degenerated pearlite. © 2008 Elsevier B.V. All rights reserved. Keywords: Microalloyed steels; Impact toughness; Microstructure
1. Introduction Microalloying of carbon–manganese steels with niobium is now being widely used to produce high-strength structural beams with good toughness and weldability [1–4]. To obtain good toughness, the carbon level of the microalloyed steels is reduced, and the decrease in strength due to reduction in the carbon content is compensated by the addition of microalloying elements such as niobium [5,6]. A further increase in strength is achieved by precipitation strengthening through microalloying with Nb, Ti, and V individually or in combination [5–9]. The improvement in the properties of microalloyed steels is primarily a consequence of fine ferrite grain size produced by the transformation of austenite, where recrystallisation is suppressed by niobium carbide precipitation [6].
∗
Corresponding author at: Department of Chemical Engineering, University of Louisiana at Lafayette, Madison Hall Room 217, P.O. Box 44130, Lafayette, LA 70504-4130, USA. E-mail address: [email protected] (R.D.K. Misra). 0921-5093/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.msea.2008.01.008
In previous studies, we described the relationship between microstructure and toughness behavior as a function of cooling rate of niobium- and vanadium-microalloyed steels of yield strength ∼60 ksi (420 MPa). It was suggested that small differences in cooling rate have a significant influence on the resulting microstructure [2]. The objective of this study is to understand key microstructural differences in niobium-microalloyed steels that were processed under seemingly identical industrial processing history but exhibited differences in toughness behavior at similar yield strength. The microstructures were quantified in terms of stereological parameters notably mean ¯ ␣ ), pearlite colony (L ¯ p) intercept length of polygonal ferrite (L and their distribution, and pearlite interlamellar spacing. The stereological analysis was carried out in conjunction with the electron microscopy examination of precipitation behavior and fine microstructural features. 2. Materials The Nb-microalloyed steels presented here had chemical composition consistent with ASTM grade A992 (Table 1). In
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R. Anumolu et al. / Materials Science and Engineering A 491 (2008) 55–61
Table 1 Chemical composition range of Nb-microalloyed steels
3. Experimental procedure
Elements
Nb-microalloyed steel (wt.%)
C Mn V Nb Si P S N
0.030–0.100 0.500–1.500 0.001 0.020–0.050 0.15–0.25 0.010–0.020 0.015–0.025 0.009–0.01
Standard tensile tests were conducted at room temperature on longitudinal specimens machined according to ASTM E8 specification [10] and impact toughness was measured using standard Charpy V-notch impact test (ASTM E23) [10]. Specimens for microstructural investigation were mechanically polished using standard metallographic procedure and etched with 2% nital solution to reveal the microstructure and examined by Leitz optical microscope. The metallographic measurements were made on at least 20 fields-of-view in order to obtain representative data. ¯ p ) were ¯ ␣ ) and pearlite colony size (L Ferrite grain size (L ¯ estimated in terms of mean intercept length (L) determined by the following expression [11,12]:
general, the thermomechanical processing schedule involved casting of beam blanks or dog bones of beams, which were subsequently reheated, followed by a series of successive roughing and finishing reductions. The processing details are not discussed here due to proprietary reasons.
¯ = VV␣ × LT L N␣
Fig. 1. Scanning electron micrographs of the fracture surface of high-toughness steels.
(1)
R. Anumolu et al. / Materials Science and Engineering A 491 (2008) 55–61
where N is the number of ferrite grains or pearlite colonies intercepted by the test lines and LT is the line length of the test lines. Transmission electron microscopy was carried out on thin foils of Nb-microalloyed steels. These foils were prepared by cutting thin wafers from the steel samples, and grinding them to ∼100 m in thickness. Three millimeter discs were punched from the wafers and electropolished using a solution of 10% perchloric acid in acetic acid electrolyte. Foils were examined with a Hitachi 7600 TEM/STEM operated at 100 kV. Transmission electron microscope was used to determine interlamellar spacing, and study fine-scale microstructural features. The fracture surface of low- and high-toughness samples was examined with a field emission scanning electron microscope (JEOL 6300F).
57
4. Results and discussion The average yield and tensile strength of low- and highimpact toughness steels were similar at ∼54 ksi (378 MPa) and ∼70 ksi (490 MPa) respectively, where low- and high-toughness refers to 86 ± 41 ft-lbs (117 N m) and 203 ± 26 ft-lbs (275 N m) respectively. Examination of the fracture surface of Charpy test specimens with high-toughness indicated that the entire fracture surface was 100% ductile and characterized by microvoid coalescence (Fig. 1). However, the low-toughness steel was characterized by a combination of both ductile and cleavage fracture. The center of the fracture surface was quasi-cleavage and the outer regions were ductile with numerous voids (Fig. 2). These observations were consistent with the impact toughness data confirming that the microstructural differences must have con-
Fig. 2. Scanning electron micrographs of the fracture surface of low-toughness steels.
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R. Anumolu et al. / Materials Science and Engineering A 491 (2008) 55–61
Fig. 4. Scanning electron micrographs of (a) high- and (b) low-toughness steels. Fig. 3. Light micrographs of (a) high- and (b) low-toughness steels.
tributed to the observed variation in toughness of the examined microalloyed steels. Representative light and scanning electron micrographs of Nb-microalloyed steels with high- and low-toughness are presented in Figs. 3 and 4, respectively. In general, the microstructural constituents for both the steels were polygonal ferrite and pearlite. The results of the quantitative metallographic analysis are summarized in Fig. 5 and Table 2. The extent of pearlite was similar in both the steels at ∼15%. However, there were differences in ferrite grain size and pearlite colony size Table 2 Quantitative metallographic analysis of Nb-microalloyed steels Stereological parameters
Low-toughness steel
High-toughness steel
Ferrite Volume fraction (%) Mean intercept length, L␣ (m)
84.47 ± 0.018 20.8 ± 4.1
85.93 ± 0.022 17.8 ± 3.6
Pearlite Volume fraction (%) Mean intercept length, Lp (m)
15.51 ± 0.01 13.77 ± 4.6
14.06 ± 0.02 12.87 ± 0.09
and their frequency of size distribution on comparing the two steels. The average ferrite grain size and pearlite colony size was lower for high-toughness steels. Also, transmission electron micrographs suggested that interlamellar spacing was lower for high-toughness steels (∼175 nm) as compared to low-toughness steels (∼200 nm). Representative TEM micrographs of Nb-microalloyed steels illustrating fine microstructural features (nature of pearlite, precipitation of NbC, strain-induced precipitation, and dislocation density) for high- and low-toughness steels are presented in Figs. 6 and 7, respectively. It was observed that steels with high-toughness contained a very high fraction of broken cementite particles (Fig. 6a), where as low-toughness steel generally consisted of lamellar pearlite with occasional presence of degenerated pearlite. The combination of ferrite and broken cementite morphology is referred as degenerated pearlite. Degenerated pearlite is formed by nucleation of cementite at ferrite/austenite interface followed by carbide-free ferrite layers enclosing the cementite particles in the transformation temperature range between normal pearlite and upper bainite [13]. Similar to lamellar pearlite, degenerated pearlite is also formed by diffusion process and considering its morphology, the difference is attributed to the insufficient carbon diffusion to develop con-
R. Anumolu et al. / Materials Science and Engineering A 491 (2008) 55–61
59
Fig. 5. Quantitative metallographic analysis of high- and low-toughness steels in terms of (a) mean intercept length of ferrite grains and (b) mean intercept length of pearlite colony.
Fig. 6. Representative transmission electron micrograph of high-toughness steel illustrating fine microstructural features: (a) nature of pearlite, (b) precipitation of NbC in ferrite, (c) strain-induced precipitation, and (d) dislocation density.
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R. Anumolu et al. / Materials Science and Engineering A 491 (2008) 55–61
Fig. 7. Representative transmission electron micrograph of low-toughness steel illustrating fine microstructural features: (a) nature of pearlite, (b) precipitation of NbC in ferrite, (c) strain-induced precipitation, and (d) dislocation density.
tinuous lamellae [14]. It is suggested that the interface between ferrite and cementite in degenerated pearlite is wider than the conventional pearlite, thus the ferrite grain boundary area of the controlled-rolled steels that contain degenerated pearlite is higher as compared to the conventionally processed steel [15]. Degenerated pearlite is believed to promote toughness [15]. Representative illustrations of strain-induced precipitation on dislocations, fine-scale precipitates in ferrite, and dislocation density (Figs. 6 and 7) suggest similarity in the precipitation behavior of both high-toughness (Fig. 6b and c) and lowtoughness (Fig. 7b and c) steels. The above results suggest that even in the case of structural beams subjected to less severe deformation, strain induced precipitation occurred on dislocations, while the fine precipitates in ferrite are precipitated during cooling. The precipitation of microalloying elements occurs during various stages of thermomechanical processing of steels. At soaking temperatures the microalloying elements are taken into solution depending on the limitation imposed by the solubility product. For carbide forming elements, the solubility in austenite at any given temperature depends on the C content of the steel. When the temperature is lowered during cooling, super-
saturation of these solute elements increases and precipitation begins at favorable kinetic conditions. Deformation of austenite introduces lattice defects such as dislocations and vacancies which may assist the diffusion process and control the kinetics of precipitation. As a consequence, strain induced precipitation occurs at defects. In a manner similar to the precipitation behavior, no significant differences in dislocation density could be discernable in the two steels (Figs. 6d and 7d). From the above observations, four factors can be identified for the observed variation in toughness of steels processed under seemingly identical conditions. The impact toughness controlling factors as derived from the study are ferrite grain size, pearlite colony size, interlamellar spacing, lamellar pearlite versus non-lamellar pearlite (degenerated pearlite). While the effect of grain size on impact toughness of microalloyed steels is well established [16], it seems apparent that pearlite colony size and interlamellar spacing also plays a role in determining toughness. An important difference between low- and high-toughness microalloyed steels is the presence of degenerated pearlite in high-toughness steels. The finer cementite in degenerated pearlite as compared to the conventional
R. Anumolu et al. / Materials Science and Engineering A 491 (2008) 55–61
pearlite is expected not only to give higher yield strength but also improves toughness because coarse pearlite deforms inhomogeneously with strain localized in narrow slip bands, where as fine degenerated pearlite exhibits uniform strain distribution during deformation [17]. Thus, the differences in toughness can be attributed to ferrite grain size, pearlite colony size, interlamellar spacing, lamellar pearlite versus non-lamellar pearlite (degenerated pearlite). 5. Conclusions Niobium-microalloyed steels characterized by similar yield strength but with differences in impact toughness were investigated to examine the underlying microstructural factors that contributed to the differences. This behavior occurred in spite of no intentional differences in processing history. The general microstructure of the niobium-microalloyed steel was similar for high- and low-toughness steels and consisted of 85% polygonal ferrite and 15% pearlite. Stereological analysis of the microstructure and electron microscopy studies implied that differences in toughness are related to average ferrite grain size and pearlite colony size, interlamellar spacing and degenerated pearlite. Acknowledgement The University of Louisiana at Lafayette gratefully acknowledges financial support from CBMM, Brazil.
61
References [1] S. Shanmugam, N. Ramisetti, R.D.K. Misra, T. Mannering, D. Panda, S. Jansto, Mater. Sci. Eng. A 460 (2007) 335. [2] S. Shanmugam, R.D.K. Misra, T. Mannering, D. Panda, S. Jansto, Mater. Sci. Eng. A 437 (2006) 436. [3] M. Tanniru, S. Shanmugam, R.D.K. Misra, Mater. Sci. Technol. 21 (2005) 159. [4] S. Shanmugam, M. Tanniru, R.D.K. Misra, Mater. Sci. Technol. 21 (2005) 165. [5] R.D.K. Misra, G.C. Weatherly, J.E. Hartmann, A.J. Boucek, Mater. Sci. Technol. 17 (2001) 1119–1129. [6] S.W. Thomson, G. Krauss, Metall. Trans. 20A (1989) 2279–2288. [7] S. Shanmugam, M. Tanniru, R.D.K. Misra, T. Mannering, D. Panda, S. Jansto, Mater. Sci. Technol. 21 (2005) 883. [8] S. Shanmugam, N. Ramisetti, R.D.K. Misra, J. Hartmann, S.G. Jansto, Mater. Sci. Eng. A 478 (2008) 26–37. [9] C.P. Reip, S. Shanmugam, R.D.K. Misra, Mater. Sci. Eng. A 424 (2006) 307. [10] Annual Book of ASTM Standards, ASTM Designation, E8 and E23, vol. 03.01, Philadelphia, PA, 1995, p. 142. [11] R.T. DeHoff, F.N. Rhines (Eds.), Quantitative Microscopy, McGraw-Hill Inc., USA, 1968. [12] E.R. Weibel, Stereological Methods, vol. 2, Academic Press, London, 1980. [13] Y. Ohmori, Trans. Iron Steel Inst. Jpn. 11 (1971) 339–348. [14] Y. Ohmori, R.W.K. Honeycombe, Proc. ICSTIS (Suppl.) Trans. Iron Steel Inst. Jpn. 11 (1971) 1160–1165. [15] T. Yamane, K. Hisayuki, Y. Kawazu, T. Takahashi, Y. Kimura, Tsukuda, J. Mater. Sci. 37 (2002) 3875–3879. [16] R.D.K. Misra, K.K. Tenneti, G.C. Weatherly, G. Tither, Metall. Trans. 34A (2003) 2341–2351. [17] H. Joung Sim, Y. Bum Lee, W.J. Nam, J. Mater. Sci. 39 (2004) 1849–1851.
On the determining role of microstructure of niobium-microalloyed steels with differences in impact toughness R. Anumolu a,b , B. Ravi Kumar a , R.D.K. Misra a,b,∗ , T. Mannering c , D. Panda c , S.G. Jansto d a b
Center for Structural and Functional Materials, University of Louisiana at Lafayette, LA 70504-4130, USA Department of Chemical Engineering, University of Louisiana at Lafayette, Lafayette, LA 70504-4130, USA c Nucor-Yamato Steel, P.O. Box 1228, 5929 East State Highway 18, Blytheville, AR 72316, USA d Reference Metals, 1000 Old Pond Road, Bridgeville, PA 15017, USA Received 13 December 2007; accepted 2 January 2008
Abstract The relationship between microstructure and impact toughness was investigated for niobium-microalloyed steels with similar yield strength. The nominal steel composition was similar and any variation in processing history was unintentional. The general microstructure of the investigated steel was similar and consisted of 85% polygonal ferrite and 15% pearlite. Despite these similarities, they exhibited variation in toughness and were classified as high- and low-toughness steels. Detailed microstructural investigation including stereological analysis and electron microscopy implied that toughness is strongly influenced by mean intercept length of polygonal ferrite and pearlite colony, and their distribution, interlamellar spacing, and degenerated pearlite. © 2008 Elsevier B.V. All rights reserved. Keywords: Microalloyed steels; Impact toughness; Microstructure
1. Introduction Microalloying of carbon–manganese steels with niobium is now being widely used to produce high-strength structural beams with good toughness and weldability [1–4]. To obtain good toughness, the carbon level of the microalloyed steels is reduced, and the decrease in strength due to reduction in the carbon content is compensated by the addition of microalloying elements such as niobium [5,6]. A further increase in strength is achieved by precipitation strengthening through microalloying with Nb, Ti, and V individually or in combination [5–9]. The improvement in the properties of microalloyed steels is primarily a consequence of fine ferrite grain size produced by the transformation of austenite, where recrystallisation is suppressed by niobium carbide precipitation [6].
∗
Corresponding author at: Department of Chemical Engineering, University of Louisiana at Lafayette, Madison Hall Room 217, P.O. Box 44130, Lafayette, LA 70504-4130, USA. E-mail address: [email protected] (R.D.K. Misra). 0921-5093/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.msea.2008.01.008
In previous studies, we described the relationship between microstructure and toughness behavior as a function of cooling rate of niobium- and vanadium-microalloyed steels of yield strength ∼60 ksi (420 MPa). It was suggested that small differences in cooling rate have a significant influence on the resulting microstructure [2]. The objective of this study is to understand key microstructural differences in niobium-microalloyed steels that were processed under seemingly identical industrial processing history but exhibited differences in toughness behavior at similar yield strength. The microstructures were quantified in terms of stereological parameters notably mean ¯ ␣ ), pearlite colony (L ¯ p) intercept length of polygonal ferrite (L and their distribution, and pearlite interlamellar spacing. The stereological analysis was carried out in conjunction with the electron microscopy examination of precipitation behavior and fine microstructural features. 2. Materials The Nb-microalloyed steels presented here had chemical composition consistent with ASTM grade A992 (Table 1). In
56
R. Anumolu et al. / Materials Science and Engineering A 491 (2008) 55–61
Table 1 Chemical composition range of Nb-microalloyed steels
3. Experimental procedure
Elements
Nb-microalloyed steel (wt.%)
C Mn V Nb Si P S N
0.030–0.100 0.500–1.500 0.001 0.020–0.050 0.15–0.25 0.010–0.020 0.015–0.025 0.009–0.01
Standard tensile tests were conducted at room temperature on longitudinal specimens machined according to ASTM E8 specification [10] and impact toughness was measured using standard Charpy V-notch impact test (ASTM E23) [10]. Specimens for microstructural investigation were mechanically polished using standard metallographic procedure and etched with 2% nital solution to reveal the microstructure and examined by Leitz optical microscope. The metallographic measurements were made on at least 20 fields-of-view in order to obtain representative data. ¯ p ) were ¯ ␣ ) and pearlite colony size (L Ferrite grain size (L ¯ estimated in terms of mean intercept length (L) determined by the following expression [11,12]:
general, the thermomechanical processing schedule involved casting of beam blanks or dog bones of beams, which were subsequently reheated, followed by a series of successive roughing and finishing reductions. The processing details are not discussed here due to proprietary reasons.
¯ = VV␣ × LT L N␣
Fig. 1. Scanning electron micrographs of the fracture surface of high-toughness steels.
(1)
R. Anumolu et al. / Materials Science and Engineering A 491 (2008) 55–61
where N is the number of ferrite grains or pearlite colonies intercepted by the test lines and LT is the line length of the test lines. Transmission electron microscopy was carried out on thin foils of Nb-microalloyed steels. These foils were prepared by cutting thin wafers from the steel samples, and grinding them to ∼100 m in thickness. Three millimeter discs were punched from the wafers and electropolished using a solution of 10% perchloric acid in acetic acid electrolyte. Foils were examined with a Hitachi 7600 TEM/STEM operated at 100 kV. Transmission electron microscope was used to determine interlamellar spacing, and study fine-scale microstructural features. The fracture surface of low- and high-toughness samples was examined with a field emission scanning electron microscope (JEOL 6300F).
57
4. Results and discussion The average yield and tensile strength of low- and highimpact toughness steels were similar at ∼54 ksi (378 MPa) and ∼70 ksi (490 MPa) respectively, where low- and high-toughness refers to 86 ± 41 ft-lbs (117 N m) and 203 ± 26 ft-lbs (275 N m) respectively. Examination of the fracture surface of Charpy test specimens with high-toughness indicated that the entire fracture surface was 100% ductile and characterized by microvoid coalescence (Fig. 1). However, the low-toughness steel was characterized by a combination of both ductile and cleavage fracture. The center of the fracture surface was quasi-cleavage and the outer regions were ductile with numerous voids (Fig. 2). These observations were consistent with the impact toughness data confirming that the microstructural differences must have con-
Fig. 2. Scanning electron micrographs of the fracture surface of low-toughness steels.
58
R. Anumolu et al. / Materials Science and Engineering A 491 (2008) 55–61
Fig. 4. Scanning electron micrographs of (a) high- and (b) low-toughness steels. Fig. 3. Light micrographs of (a) high- and (b) low-toughness steels.
tributed to the observed variation in toughness of the examined microalloyed steels. Representative light and scanning electron micrographs of Nb-microalloyed steels with high- and low-toughness are presented in Figs. 3 and 4, respectively. In general, the microstructural constituents for both the steels were polygonal ferrite and pearlite. The results of the quantitative metallographic analysis are summarized in Fig. 5 and Table 2. The extent of pearlite was similar in both the steels at ∼15%. However, there were differences in ferrite grain size and pearlite colony size Table 2 Quantitative metallographic analysis of Nb-microalloyed steels Stereological parameters
Low-toughness steel
High-toughness steel
Ferrite Volume fraction (%) Mean intercept length, L␣ (m)
84.47 ± 0.018 20.8 ± 4.1
85.93 ± 0.022 17.8 ± 3.6
Pearlite Volume fraction (%) Mean intercept length, Lp (m)
15.51 ± 0.01 13.77 ± 4.6
14.06 ± 0.02 12.87 ± 0.09
and their frequency of size distribution on comparing the two steels. The average ferrite grain size and pearlite colony size was lower for high-toughness steels. Also, transmission electron micrographs suggested that interlamellar spacing was lower for high-toughness steels (∼175 nm) as compared to low-toughness steels (∼200 nm). Representative TEM micrographs of Nb-microalloyed steels illustrating fine microstructural features (nature of pearlite, precipitation of NbC, strain-induced precipitation, and dislocation density) for high- and low-toughness steels are presented in Figs. 6 and 7, respectively. It was observed that steels with high-toughness contained a very high fraction of broken cementite particles (Fig. 6a), where as low-toughness steel generally consisted of lamellar pearlite with occasional presence of degenerated pearlite. The combination of ferrite and broken cementite morphology is referred as degenerated pearlite. Degenerated pearlite is formed by nucleation of cementite at ferrite/austenite interface followed by carbide-free ferrite layers enclosing the cementite particles in the transformation temperature range between normal pearlite and upper bainite [13]. Similar to lamellar pearlite, degenerated pearlite is also formed by diffusion process and considering its morphology, the difference is attributed to the insufficient carbon diffusion to develop con-
R. Anumolu et al. / Materials Science and Engineering A 491 (2008) 55–61
59
Fig. 5. Quantitative metallographic analysis of high- and low-toughness steels in terms of (a) mean intercept length of ferrite grains and (b) mean intercept length of pearlite colony.
Fig. 6. Representative transmission electron micrograph of high-toughness steel illustrating fine microstructural features: (a) nature of pearlite, (b) precipitation of NbC in ferrite, (c) strain-induced precipitation, and (d) dislocation density.
60
R. Anumolu et al. / Materials Science and Engineering A 491 (2008) 55–61
Fig. 7. Representative transmission electron micrograph of low-toughness steel illustrating fine microstructural features: (a) nature of pearlite, (b) precipitation of NbC in ferrite, (c) strain-induced precipitation, and (d) dislocation density.
tinuous lamellae [14]. It is suggested that the interface between ferrite and cementite in degenerated pearlite is wider than the conventional pearlite, thus the ferrite grain boundary area of the controlled-rolled steels that contain degenerated pearlite is higher as compared to the conventionally processed steel [15]. Degenerated pearlite is believed to promote toughness [15]. Representative illustrations of strain-induced precipitation on dislocations, fine-scale precipitates in ferrite, and dislocation density (Figs. 6 and 7) suggest similarity in the precipitation behavior of both high-toughness (Fig. 6b and c) and lowtoughness (Fig. 7b and c) steels. The above results suggest that even in the case of structural beams subjected to less severe deformation, strain induced precipitation occurred on dislocations, while the fine precipitates in ferrite are precipitated during cooling. The precipitation of microalloying elements occurs during various stages of thermomechanical processing of steels. At soaking temperatures the microalloying elements are taken into solution depending on the limitation imposed by the solubility product. For carbide forming elements, the solubility in austenite at any given temperature depends on the C content of the steel. When the temperature is lowered during cooling, super-
saturation of these solute elements increases and precipitation begins at favorable kinetic conditions. Deformation of austenite introduces lattice defects such as dislocations and vacancies which may assist the diffusion process and control the kinetics of precipitation. As a consequence, strain induced precipitation occurs at defects. In a manner similar to the precipitation behavior, no significant differences in dislocation density could be discernable in the two steels (Figs. 6d and 7d). From the above observations, four factors can be identified for the observed variation in toughness of steels processed under seemingly identical conditions. The impact toughness controlling factors as derived from the study are ferrite grain size, pearlite colony size, interlamellar spacing, lamellar pearlite versus non-lamellar pearlite (degenerated pearlite). While the effect of grain size on impact toughness of microalloyed steels is well established [16], it seems apparent that pearlite colony size and interlamellar spacing also plays a role in determining toughness. An important difference between low- and high-toughness microalloyed steels is the presence of degenerated pearlite in high-toughness steels. The finer cementite in degenerated pearlite as compared to the conventional
R. Anumolu et al. / Materials Science and Engineering A 491 (2008) 55–61
pearlite is expected not only to give higher yield strength but also improves toughness because coarse pearlite deforms inhomogeneously with strain localized in narrow slip bands, where as fine degenerated pearlite exhibits uniform strain distribution during deformation [17]. Thus, the differences in toughness can be attributed to ferrite grain size, pearlite colony size, interlamellar spacing, lamellar pearlite versus non-lamellar pearlite (degenerated pearlite). 5. Conclusions Niobium-microalloyed steels characterized by similar yield strength but with differences in impact toughness were investigated to examine the underlying microstructural factors that contributed to the differences. This behavior occurred in spite of no intentional differences in processing history. The general microstructure of the niobium-microalloyed steel was similar for high- and low-toughness steels and consisted of 85% polygonal ferrite and 15% pearlite. Stereological analysis of the microstructure and electron microscopy studies implied that differences in toughness are related to average ferrite grain size and pearlite colony size, interlamellar spacing and degenerated pearlite. Acknowledgement The University of Louisiana at Lafayette gratefully acknowledges financial support from CBMM, Brazil.
61
References [1] S. Shanmugam, N. Ramisetti, R.D.K. Misra, T. Mannering, D. Panda, S. Jansto, Mater. Sci. Eng. A 460 (2007) 335. [2] S. Shanmugam, R.D.K. Misra, T. Mannering, D. Panda, S. Jansto, Mater. Sci. Eng. A 437 (2006) 436. [3] M. Tanniru, S. Shanmugam, R.D.K. Misra, Mater. Sci. Technol. 21 (2005) 159. [4] S. Shanmugam, M. Tanniru, R.D.K. Misra, Mater. Sci. Technol. 21 (2005) 165. [5] R.D.K. Misra, G.C. Weatherly, J.E. Hartmann, A.J. Boucek, Mater. Sci. Technol. 17 (2001) 1119–1129. [6] S.W. Thomson, G. Krauss, Metall. Trans. 20A (1989) 2279–2288. [7] S. Shanmugam, M. Tanniru, R.D.K. Misra, T. Mannering, D. Panda, S. Jansto, Mater. Sci. Technol. 21 (2005) 883. [8] S. Shanmugam, N. Ramisetti, R.D.K. Misra, J. Hartmann, S.G. Jansto, Mater. Sci. Eng. A 478 (2008) 26–37. [9] C.P. Reip, S. Shanmugam, R.D.K. Misra, Mater. Sci. Eng. A 424 (2006) 307. [10] Annual Book of ASTM Standards, ASTM Designation, E8 and E23, vol. 03.01, Philadelphia, PA, 1995, p. 142. [11] R.T. DeHoff, F.N. Rhines (Eds.), Quantitative Microscopy, McGraw-Hill Inc., USA, 1968. [12] E.R. Weibel, Stereological Methods, vol. 2, Academic Press, London, 1980. [13] Y. Ohmori, Trans. Iron Steel Inst. Jpn. 11 (1971) 339–348. [14] Y. Ohmori, R.W.K. Honeycombe, Proc. ICSTIS (Suppl.) Trans. Iron Steel Inst. Jpn. 11 (1971) 1160–1165. [15] T. Yamane, K. Hisayuki, Y. Kawazu, T. Takahashi, Y. Kimura, Tsukuda, J. Mater. Sci. 37 (2002) 3875–3879. [16] R.D.K. Misra, K.K. Tenneti, G.C. Weatherly, G. Tither, Metall. Trans. 34A (2003) 2341–2351. [17] H. Joung Sim, Y. Bum Lee, W.J. Nam, J. Mater. Sci. 39 (2004) 1849–1851.