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Influence of molybdenum content on transformation behavior of high performance bridge steel during continuous cooling
Materials and Design 49 (2013) 465–470
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Materials and Design journal homepage: www.elsevier.com/locate/matdes
Short Communication
Influence of molybdenum content on transformation behavior of high performance bridge steel during continuous cooling Jun Chen ⇑, Shuai Tang, Zhenyu Liu, Guodong Wang The State Key Laboratory of Rolling and Automation, Northeastern University, P.O. Box 105, No. 11, Lane 3, Wenhua Road, HePing District, Shenyang 110819, People’s Republic of China
a r t i c l e
i n f o
Article history: Received 1 November 2012 Accepted 6 January 2013 Available online 21 January 2013
a b s t r a c t The continuous-cooling-transformation (CCT) diagrams of high performance bridge steel with different molybdenum content were plotted by means of a combined method of dilatometry and metallography. The results show that the molybdenum addition of 0.17 wt% does not noticeably alter the transformation behavior, whereas 0.38 wt% significantly. In addition, the molybdenum addition of 0.38 wt% completely eliminates the formation of polygonal ferrite (PF) and significantly lower the granular ferrite (GF) transformation starting temperatures throughout the range of cooling rates studied. At lower cooling rates, with the increase of the molybdenum content, the martensite/austenite (M/A) constituents are noticeably refined, whereas the effects are not obvious at higher cooling rates. Moreover, the molybdenum addition of 0.38 wt% can significantly increase the Vickers hardness, but the Vickers hardness increments (by comparison of Mo-0.17wt% steel and Mo-0.38wt% steel) are sharply reduced at the cooling rate of 30 °C/s, indicating that at higher cooling rate, the molybdenum usage can be saved and the higher strengthen can be also gained. It could be found the GF transformation starting temperature is linear with the cooling rate. The empirical equation was established to calculate GF transformation starting temperatures, and the calculated values are in good agreement with measured ones. Ó 2013 Elsevier Ltd. All rights reserved.
1. Introduction The bridge design and the constructions of large-scale and longspan steel bridges have been widely developed recently [1], so the high performance bridge steels with higher strength, ductility, toughness, weldability and corrosion resistance, etc., attract more and more researchers’ attentions. To balance these mechanical properties, it is of significance to develop optimal microstructure, which can be achieved by suitably designed chemical compositions and thermomechanical control process (TMCP) [2–4]. The good weldability and low temperature toughness are primarily attributed to lowering the carbon content in the chemical compositions [5,6]. The achievement of high strength can be gained by combined additions of manganese, nickel, chromium, copper, molybdenum, niobium, vanadium or titanium, etc., which can enhance the strength by grain refinement, solid-solution strengthening, precipitation hardening and the modification of transformation [2,7–10]. However, for a given steel composition, the microstructure can be significantly affected by TMCP, so it is necessary to well understand the influence of alloying elements on transformation during continuous cooling, which is good to the achievement of energy saving and emission reduction.
⇑ Corresponding author. Tel.: +86 024 8368 1628; fax: +86 024 2390 6472. E-mail address: [email protected] (J. Chen). 0261-3069/$ - see front matter Ó 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.matdes.2013.01.017
It is well known that the molybdenum plays a vital role in steels, such as, exerting a vigorous effect on hardenability [11], supplying the sufficient nucleation sites to Nb(C,N) with a high dislocation density by soluted molybdenum, restraining the growth of Nb(C,N) during aging by the molybdenum segregated at the precipitates/matrix interfaces [12], reducing the carbon diffusion coefficient in austenite and the diffusion ability of the niobium, shifting the ferrite transformation and perlite transformation fields to the right [13–15] and lowering the transformation temperatures due to its hardenability effect [16], etc., indicating the influence of molybdenum on microstructure is pronounced. Many investigations have been made to describe the effect of molybdenum on microstructure and transformation temperatures [6,12–15,17]. However, for the high grade bridge steel with nickel, chromium and copper, the influence of molybdenum content on transformation behavior during continuous cooling has not been reported in details. Furthermore, it is of significance for avoiding the formation of brittle granular bainite with larger M/A constituents, improving its strength and toughness and saving the molybdenum additions to understand the influence of molybdenum on transformation behavior. In the present paper, the effects of molybdenum content on transformation behavior of high performance bridge steel during continuous cooling were investigated by simulation of the thermo-mechanical control process. The continuous-cooling-transformation (CCT) diagrams with different molybdenum content were
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plotted. Moreover, the bainite ferrite (BF) transformation starting temperatures were estimated according to the Lever Rule. So the bainite transformation region was refined. In addition, the empirical equation for the calculation of granular ferrite (GF) transformation starting temperatures was also established. 2. Materials and experimental procedure The chemical compositions of steels investigated in the present work are given in Table 1. These steels were vacuum melted ingots and hot rolled to plates with the thickness of approx. 12 mm using two-high 450 mm experimental hot rolling mill. The specimens were cut from these plates and machined to round ladder dilatometry samples with the diameter of 6 mm and 10 mm and the length of 15 mm and 30 mm in the middle and edge, respectively. The Gleeble 3800 thermo-mechanical simulator was adopted to test transformation temperatures of Mo-free, Mo-0.17wt% and Mo0.38wt% steels. The round ladder samples were reheated to 1200 °C and held for 300 s, then cooled to the deformation temperature of 870 °C and held for 15 s to eliminate temperature gradient. The true strain and strain rate is 0.7 and 5 s1, respectively. The samples were immediately cooled to room temperature with different cooling rates of 0.5, 2, 5, 10, 15, 20, 25, 30 °C/s after compression deformation. The surface underneath the thermocouple was polished and etched in 4% nital solution for the observation of OM (LEICA DMIRM). The continuous-cooling-transformation (CCT) diagrams were plotted by transformation temperatures determined on the dilatometric curves with the help of metallographic observation and Lever Rule [18]. The Vickers-hardness of samples with different cooling rates for tested steels was tested using Vickers-durometer of HV-50 with the load of 10 kg. The testing standard followed the guidelines of ISO 6507-1: 2005(E) [19], while a diamond indenter is forced into the surface of the tested samples, and then the diagonal length (d1 and d2) of the indentation left in the surface after removal of the diamond indenter is measured. At last, the Vickers-hardness (PHV) can be calculated using the equation of HV = 0.1891 F/d2 (F-Force, d-Mean value of d1 and d2). 3. Results and discussion 3.1. Transformation behavior On the basis of the dilatometric curves, metallographic observation and Lever Rule, the continuous-cooling-transformation (CCT) diagrams for Mo-free, Mo-0.17wt% and Mo-0.38wt% steels are given in Fig. 1. Furthermore, the transformation field was divided into polygonal ferrite (PF), granular ferrite (GF) and bainite ferrite (BF) transformation fields based on ferrite morphologies [2]. In addition, the martensite transformation starting temperatures (Ms) for Mo-free, Mo-0.17wt% and Mo-0.38wt% steels were estimated as approx. 407 °C, 406 °C and 402 °C using Eq. (1) [20], respectively. It can be seen that the CCT diagrams for Mo-free steel are quite similar to those for Mo-0.17wt% steel (by comparison of Fig. 1a and b). However, the molybdenum addition of 0.38 wt% can significantly alter the appearance of the CCT diagrams (by comparison of Fig. 1c, a and b), showing no PF transformation field and
lower decrements of GF and BF transformation starting temperatures with increasing cooling rates investigated, and Kong et al. also indicated that the addition of element Mo could shift the ferrite transformation starting line to the right obviously [13], which is in good agreement with the result in this study. Moreover, the PF and GF transformation fields are reduced with increasing molybdenum content, conversely, the BF and martensite transformation fields are expanded. But Kong et al. did not refine the bainite transformation field and indicate the influence of Mo content on bainite transformation field [14]. In addition, for GF transformation, the range of transformation starting temperatures throughout the range of cooling rates studied is 664–567 °C, 630–543 °C and 585–545 °C for Mo-free, Mo-0.17wt% and Mo-0.38wt% steels, respectively, indicating that the molybdenum addition significantly lower GF transformation starting temperatures. And the formation of PF and GF can be completely suppressed throughout the range of cooling rates studied and at the cooling rates higher than 25 °C/s in Mo-0.38wt% steel, respectively. It has been reported that the molybdenum tending to segregate to prior austenite grain boundaries leads to lower grain boundary energies [21,22], which can lower the nucleation rates of all ferrite morphological types heterogeneously nucleating at austenite grain boundaries [22] and enhance the solute draglike effect (SDLE) [23], leading to the lower ferrite growth rate [24]. Moreover, the molybdenum addition can increase carbon diffusion activation energy in austenite and the carbon diffusion coefficient is decreased [14], which also lowers the ferrite growth rate. So the formation of PF mainly dominated by carbon diffusion is restricted due to the molybdenum addition. Furthermore, the molybdenum addition of 0.17 wt% can only lower the PF transformation temperatures, but that of 0.38 wt% completely eliminates the formation of PF throughout the range of cooling rates studied, showing the molybdenum addition of 0.38 wt% is more effective to lower ferrite nucleation rate and growth rate. The bainite nucleates like martensite but with the partition of the interstitial carbon [25], so the GF transformation is slowed down and the GF transformation starting temperatures are also lowered due to aforementioned lower carbon diffusion coefficient due to the molybdenum addition [14]. Further, the GF transformation starting temperatures can be significantly lowered, when molybdenum addition is up to 0.38 wt%, indicating that the hardenability of austenite can be significantly increased by the molybdenum addition of 0.38 wt%. In addition, from Fig. 1, it can be found that the GF transformation field is shifted to the right by the increase of molybdenum content, indicating that the GF transformation is also restricted by the molybdenum addition.
MsðKÞ ¼ 764:2 302:6C 30:6Mn 16:6Ni 8:9Cr þ 2:4Mo 11:3Cu þ 8:58Co þ 7:4W 14:5Si
ð1Þ
3.2. Transformation microstructures Some typical metallographic pictures of samples with different cooling rates are shown in Fig. 2. The microstructure composed of PF and GF is observed at the lowest cooling rate in Mo-free and Mo-0.17wt% steels, but for Mo-0.38wt% steel, the PF microstructure is not observed. In addition, it can be seen that the M/A
Table 1 Chemical compositions of tested steels (wt%). Steel
C
Si
Mn
P
S
Als
Ni
Cr
Cu
Ti
Mo-free Mo-0.17wt% Mo-0.38wt%
0.052 0.051 0.056
0.43 0.44 0.49
1.56 1.60 1.57
0.0079 0.0067 0.0017
0.0012 0.0011 0.0085
0.012 0.011 0.025
0.34 0.33 0.54
0.46 0.46 0.45
0.45 0.44 0.47
0.024 0.026 0.029
Mo
Nb
0.17 0.38
0.041 0.044 0.042
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(a) 800
(b)800 700 PF
PF o
Temperature, C
o
Temperature, C
700
600
GF BF
500
400
GF
500 BF Ms
400
Ms
300 25 30 20 15
300
600
5
10
1
10
10
2
2
25 30 20 15
o
0.5 C/s
10
200
3
1
2
5
10 2
10
o
0.5 C/s 3
10
10 o
o
Cooling time from 870 C, s
Cooling time from 870 C, s
(c) 800 600 GF
o
Temperature, C
700
500 400
BF
Ms
300 25 30 20 15
200 100
10
2
5
10
100
o
0.5 C/s
1000 o
Cooling time from 870 C, s Fig. 1. Continuous cooling transformation diagrams of (a) Mo-free steel, (b) Mo-0.17wt% steel and (c) Mo-0.38wt% steel.
constituents are significantly refined by increasing molybdenum content (by comparison of Fig. 2a, e and i). The M/A constituents can be also refined greatly (by comparison of Fig. 2b, f and j and Fig. 2a, e and i) by increasing cooling rate, which is in good agreement with Mazancová and Mazanec’s report [26]. However, the effect of increasing molybdenum addition to refine M/A constituent is not pronounced at higher cooling rates, such as 10 °C/s or higher one than 10 °C/s. When the cooling rate is increased to 20 °C/s, a gradual increase in molybdenum addition is accompanied by a rise in the volume fraction of BF and martensite (by comparison of Fig. 2c, g and k). When the cooling rate is increased to 30 °C/s, only a few GF can be observed in Mo-free and Mo-o.17wt% steels and the formation of GF can be completely suppressed in Mo0.38wt% steel. Furthermore, the ferrite lath is sufficiently refined with increasing molybdenum content. For the tested steels in the present work, the carbon concentration exceeds the solubility limit in PF and GF, so the carbon can be rejected into residual austenite during PF and GF transformation, and the carbon-enriched austenite may be transformed into martensite below Ms during further cooling, which is the reason why the M/A constituents can be observed in Fig. 2. The molybdenum addition can significantly reduce the carbon diffusion, so the larger block carbon-enriched austenite can be hardly formed in Mo0.38wt% steel, showing the M/A constituents are significantly refined by increasing molybdenum addition at the lowest cooling rate. And Kong et al. has also indicated that the M/A constituents can be refined by decreasing the carbon diffusion [14]. However, at higher cooling rates, such as 10 °C/s or higher, the carbon diffusion may be mainly controlled by lower GF transformation
temperatures, so increasing molybdenum addition to refine M/A constituent is not obvious, as observed in Fig. 2b, f and j. Moreover, the GF transformation temperatures can be lowered at higher cooling rates, so the carbon diffusion is also reduced; showing fine M/A constituents at higher cooling rates (by comparison of Fig. 2b, f and j and Fig. 2a, e and i). 3.3. Vickers-hardness The effects of cooling rates on Vickers-hardness of tested steels are shown in Fig. 3. From Fig. 3, it can be seen that the Vickershardness can be increased with the increase of molybdenum content due to transformation strengthening and higher dislocation strengthening, and the changes of Vickers-hardness are in good agreement with the microstructure evolution. For Mo-0.38wt% steel, the Vickers-hardness slightly increases at first, then sharply increases and almost remains unchanged at last with increasing cooling rates, showing an ‘‘S’’ shape, whereas for Mo-0.17wt% and Mo-free steels, the changes of Vickers-hardness are almost the same with increasing cooling rates and the plateau is not observed at higher cooling rates. As aforementioned molybdenum addition of 0.38 wt% noticeably increasing hardenability, so the highest Vickers hardness can be gained at lower cooling rate, i.e., 20 °C/s, for Mo-0.38wt%. However, for Mo-0.17wt% and Mo-free steels, if the cooling rates are further increased, it can be thought the highest Vickers hardness may be also gained and the Vickers hardness vs cooling rate may be also showed as ‘‘S’’ shape. The molybdenum addition of 0.38 wt% can significantly increase the Vickers hardness at the cooling rates ranged from
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J. Chen et al. / Materials and Design 49 (2013) 465–470
(b)
(a)
10µm
(c)
10µm
(f)
(e)
10µm
(i)
10µm
(g)
10µm
(j)
10µm
(d)
(h)
10µm
10µm
(k)
10µm
10µm
(l)
10µm
10µm
Fig. 2. Typical optical microstructure of (a–c) and (d) Mo-free steel, (e–g) and (h) Mo-0.17wt% steel and (i–k) and (l) Mo-0.38wt% steel with different cooling rates, (a), (e), (i) 0.5 °C/s, (b), (f), (j) 10 °C/s, (c), (g), (k) 20 °C/s and (d), (h), (l) 30 °C/s.
0.5 °C/s to 25 °C/s, whereas the Vickers hardness increments (by comparison of Mo-0.17wt% steel and Mo-0.38wt% steel) are sharply reduced at the cooling rate of 30 °C/s. However, Cizek et al. indicated that the molybdenum addition of 0.309 wt% appeared to be less influence on strengthening [2], but we found that the molybdenum addition of 0.38 wt% could greatly increase strengthening. The Vickers hardness can be mainly increased through enhanced hardenability and associated modification of the resultant transformation microstructure based on the analysis of Sections 3.1 and 3.2. The transformation temperatures for Mo-0.38wt% steel are further lower than that for Mo-0.17wt% at lower cooling rates, so the Vickers hardness can be sufficiently increased due to microstructure refinement, dislocation strengthening and transformation hardening, etc. However, the transformation temperature for Mo-0.17wt% steel at the cooling rate of 30 °C/s is similar to that for Mo-0.38wt% steel at
the cooling rate of 20 °C/s, so the effects of microstructure refinement, dislocation strengthening and transformation hardening, etc., can be also gained for Mo-0.17wt% steel at the cooling rate of 30 °C/s, resulting in the Vickers hardness increments (by comparison of Mo-0.17wt% and Mo-0.38wt% steels) are sharply reduced at the cooling rate of 30 °C/s. This phenomenon indicates that the molybdenum content can be lowered as the cooling rate is increased. 3.4. Empirical equation It is found that the GF transformation starting temperature is almost linear with the cooling rate for Mo-free, Mo-0.17wt% and Mo0.38wt% steels, as shown in Fig. 4. So the empirical equation for the calculation of GF transformation starting temperatures was established, as follows:
J. Chen et al. / Materials and Design 49 (2013) 465–470
indicating that Eq. (2) can be used to estimate GFs of tested steels with different molybdenum additions.
400 Mo-free steel Mo-0.17wt% steel Mo-0.38wt% steel
Vickers-hardness, HV
375
469
350
4. Conclusions
325
The continuous-cooling-transformation (CCT) diagrams of high performance bridge steel with different molybdenum content were plotted by means of a combined method of dilatometry and metallography. The influence of molybdenum content and cooling rate on transformation behavior, transformation microstructure, and Vickers-hardness was discussed in details. In addition, the empirical equation for estimating the granular ferrite transformation starting temperature was established. The following conclusions have been drawn in this study.
300 275 250 225 200
0
1
10
10 o
CR, C/s
1. The molybdenum addition of 0.17 wt% cannot noticeably alter the transformation behavior; conversely, when the molybdenum addition is up to 0.38 wt%, the transformation behavior can be significantly altered. The PF and GF transformation fields are reduced with the increase of molybdenum content; whereas, the BF and martensite transformation fields are expanded. In addition, the GF transformation starting temperatures throughout the range of cooling rates studied is 664– 567 °C, 630–543 °C and 585–545 °C for Mo-free, Mo-0.17wt% and Mo-0.38wt% steels, respectively, indicating that the GF transformation starting temperatures can be pronouncedly lowered by the molybdenum addition of 0.38 wt%. 2. The M/A constituents can be obviously refined with the increase of molybdenum content at lower cooling rates; whereas, the effects are not obvious at higher cooling rates. However, the ferrite lath can be refined with the increase of molybdenum content at higher cooling rates. In addition, increasing cooling rates can significantly refine M/A constituents and ferrite lath. 3. The molybdenum addition of 0.17 wt% cannot greatly increase the Vickers hardness. When the molybdenum addition is up to 0.38 wt%, the Vickers hardness can be noticeably increased, but Vickers hardness increments (by comparison of Mo0.17wt% steel and Mo-0.38wt% steel) are sharply reduced at the cooling rate of 30 °C/s. So it can be thought that the molybdenum content can be lowered as the cooling rate is increased. 4. The empirical equation for estimating GF transformation starting temperatures was established, as follows:
700
o
GF transformation start temperature, C
Fig. 3. Vickers-hardness vs cooling rate (CR).
Mo-free steeel Mo-0.17wt% steel Mo-0.38wt% steel
675 650 625 600 575 550 525 500
0
5
10
15
25
30
o
Cooling rate, C/s Fig. 4. GF transformation starting temperatures vs cooling rate.
700
650
o
Measured GFs, C
675
GFs ð CÞ ¼ ð30:1CR 609:5ÞMo2 ð8:7CR 40:4ÞMo 3:0CR þ 657:8 625 600
Acknowledgments
575
This work is supported by the Fundamental Research Funds for the Central Universities (N110607003, N100507002 and N110307002) and the Scholarship Award for Excellent Doctoral Student granted by Ministry of Education of PR China.
550 550
575
600
625
650
675
700
o
Calculated GFs, C
References
Fig. 5. Comparisons of measured GFs and calculated GFs.
GFs ð CÞ ¼ ð30:1CR 609:5ÞMo2 ð8:7CR 40:4ÞMo 3:0CR þ 657:8
ð2Þ
where the CR is cooling rate, °C/s, and Mo is molybdenum content, wt%. The comparisons of calculated values and measured ones are given in Fig. 5. It can be seen that the calculated GF transformation starting temperatures are in good agreement with measured ones,
[1] Shim CS, Whang JW, Chung CH, Lee PG. Design of double composite bridges using high strength steel. Procedia Eng 2011;14:1825–9. [2] Cizek P, Wynne BP, Davies CHJ, Muddle BC, Hodgson PD. Effect of composition and austenite deformation on the transformation characteristics of low-carbon and ultralow-carbon microalloyed steels. Metall Mater Trans A 2002;33: 1331–49. [3] Olasolo M, Uranga P, Rodriguez-Ibabe JM, López B. Effect of austenite microstructure and cooling rate on transformation characteristics in a low carbon Nb–V microalloyed steel. Mater Sci Eng A 2011;528:2559–69. [4] Zhao MC, Yang K, Xiao FR, Shan YY. Continuous cooling transformation of undeformed and deformed low carbon pipeline steels. Mater Sci Eng A 2003;355:126–36. [5] Guo J, Shang CJ, Yang SW, Guo H, Wang XM, He XL. Weather resistance of low carbon high performance bridge steel. Mater Des 2009;30:129–34.
470
J. Chen et al. / Materials and Design 49 (2013) 465–470
[6] Anijdan SHM, Rezaeian A, Yue S. The effect of chemical composition and austenite conditioning on the transformation behavior of microalloyed steels. Mater Charact 2012;63:27–38. [7] Zhang CL, Cai DY, Wang YH, Liu MQ, Liao B, Fan YC. Effects of deformation and Mo, Nb, V, Ti on continuous cooling transformation in Cu–P–Cr–Ni–Mo weathering steels. Mater Charact 2008;59:1638–42. [8] Li L, Ding H, Du LX, Wen JL, Song HM, Zhang PJ. Influence of Mn content and hot deformation on transformation behavior of C–Mn steels. J Iron Steel Res Int 2008;15:51–5. [9] Liu SK, Zhang J. The influence of the Si and Mn concentration on the kinetics of the transformation in Fe–C–Si–Mn alloys. Metall Mater Trans A 1990;21: 1517–25. [10] Manohar PA, Chandra T, Killmore CR. Continuous cooling transformation behavior of microalloyed steels containing Ti, Nb, Mn and Mo. ISIJ Int 1996;36: 1486–93. [11] Paxton HW, Bain EC. Alloying elements in steels. OH: Metals Park, ASM; 1966. p. 274. [12] Uemori R, Chijiiwa R, Tamehiro H, Morikawa H. AP–FIM study on the effect of Mo addition on the microstructure in Ti–Nb steel. Appl Surf Sci 1994;76– 77:225–60. [13] Kong JH, Zhen L, Guo B, Li PH, Wang AH, Xie CS. Influence of Mo content on microstructure and mechanical properties of high strength pipeline steel. Mater Des 2004;25:723–8. [14] Kong JH, Xie CS. Effect of molybdenum on continuous cooling bainite transformation of low-carbon microalloyed steel. Mater Des 2006;27: 1169–73. [15] García de Andrés C, Capdevila C, Madariaga I, Gutiérrez I. Role of molybdenum in acicular ferrite formation under continuous cooling in a medium carbon microalloyed forging steel. Scripta Mater 2005;45:709–16.
[16] Ghosh A, Mishra B, Das S, Chatterjee S. Microstructure, properties, and age hardening behavior of a thermomechanically processed ultralow carbon Cubearing high-strength steel. Metal Mater Trans A 2005;36:703–13. [17] Tang ZH, Stumpf W. The role of molybdenum additions and prior deformation on acicular ferrite formation in microalloyed Nb–Ti low carbon line-pipe steels. Mater Charact 2008;59:717–28. [18] Yuan XQ, Liu ZY, Jiao SH, Liu XH, Wang GD. Effects of nanoprecipitates in austenite on ferrite transformation starting temperature during continuous cooling in Nb–Ti micro-alloyed steels. ISIJ Int 2007;47:1658–65. [19] ISO international standard. Standard test method for Vickers-hardness test of metallic materials. ISO 6507–1; 2005(E). [20] Capdevila C, Caballero FG, Carcía De Andrés C. Determination of Ms temperature in steels: a bayesian neural network model. ISIJ Int 2002;42: 894–902. [21] Enomoto M, White CL, Aaronson HI. Evaluation of the effects of segregation on austenite grain boundary energy in Fe–C–X alloys. Metall Trans A 1988;19: 1807–18. [22] Essadiqi E, Jonas JJ. Effect of deformation on the austenite-to-ferrite transformation in a plain carbon and two microalloyed steels. Metall Trans A 1988;19:417–26. [23] Chen JK, Vandermeer RA, Reynolds Jr WT. Effects of alloying elements upon austenite decomposition in low-c steels. Metall Mater Trans A 1994;25: 1367–79. [24] Essadiqi E, Jonas JJ. Effect of deformation on ferrite nucleation and growth in a plain carbon and two microalloyed steels. Metall Trans A 1989;20:987–98. [25] Bhadeshia HKDH. The bainite transformation: unresolved issues. Mater Sci Eng A 1999;273–275:58–66. [26] Mazancová E, Mazanec K. Physical metallurgy characteristics of the M/A constituent formation in granular bainite. J Mater Process Tech 1997;64: 287–92.
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Short Communication
Influence of molybdenum content on transformation behavior of high performance bridge steel during continuous cooling Jun Chen ⇑, Shuai Tang, Zhenyu Liu, Guodong Wang The State Key Laboratory of Rolling and Automation, Northeastern University, P.O. Box 105, No. 11, Lane 3, Wenhua Road, HePing District, Shenyang 110819, People’s Republic of China
a r t i c l e
i n f o
Article history: Received 1 November 2012 Accepted 6 January 2013 Available online 21 January 2013
a b s t r a c t The continuous-cooling-transformation (CCT) diagrams of high performance bridge steel with different molybdenum content were plotted by means of a combined method of dilatometry and metallography. The results show that the molybdenum addition of 0.17 wt% does not noticeably alter the transformation behavior, whereas 0.38 wt% significantly. In addition, the molybdenum addition of 0.38 wt% completely eliminates the formation of polygonal ferrite (PF) and significantly lower the granular ferrite (GF) transformation starting temperatures throughout the range of cooling rates studied. At lower cooling rates, with the increase of the molybdenum content, the martensite/austenite (M/A) constituents are noticeably refined, whereas the effects are not obvious at higher cooling rates. Moreover, the molybdenum addition of 0.38 wt% can significantly increase the Vickers hardness, but the Vickers hardness increments (by comparison of Mo-0.17wt% steel and Mo-0.38wt% steel) are sharply reduced at the cooling rate of 30 °C/s, indicating that at higher cooling rate, the molybdenum usage can be saved and the higher strengthen can be also gained. It could be found the GF transformation starting temperature is linear with the cooling rate. The empirical equation was established to calculate GF transformation starting temperatures, and the calculated values are in good agreement with measured ones. Ó 2013 Elsevier Ltd. All rights reserved.
1. Introduction The bridge design and the constructions of large-scale and longspan steel bridges have been widely developed recently [1], so the high performance bridge steels with higher strength, ductility, toughness, weldability and corrosion resistance, etc., attract more and more researchers’ attentions. To balance these mechanical properties, it is of significance to develop optimal microstructure, which can be achieved by suitably designed chemical compositions and thermomechanical control process (TMCP) [2–4]. The good weldability and low temperature toughness are primarily attributed to lowering the carbon content in the chemical compositions [5,6]. The achievement of high strength can be gained by combined additions of manganese, nickel, chromium, copper, molybdenum, niobium, vanadium or titanium, etc., which can enhance the strength by grain refinement, solid-solution strengthening, precipitation hardening and the modification of transformation [2,7–10]. However, for a given steel composition, the microstructure can be significantly affected by TMCP, so it is necessary to well understand the influence of alloying elements on transformation during continuous cooling, which is good to the achievement of energy saving and emission reduction.
⇑ Corresponding author. Tel.: +86 024 8368 1628; fax: +86 024 2390 6472. E-mail address: [email protected] (J. Chen). 0261-3069/$ - see front matter Ó 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.matdes.2013.01.017
It is well known that the molybdenum plays a vital role in steels, such as, exerting a vigorous effect on hardenability [11], supplying the sufficient nucleation sites to Nb(C,N) with a high dislocation density by soluted molybdenum, restraining the growth of Nb(C,N) during aging by the molybdenum segregated at the precipitates/matrix interfaces [12], reducing the carbon diffusion coefficient in austenite and the diffusion ability of the niobium, shifting the ferrite transformation and perlite transformation fields to the right [13–15] and lowering the transformation temperatures due to its hardenability effect [16], etc., indicating the influence of molybdenum on microstructure is pronounced. Many investigations have been made to describe the effect of molybdenum on microstructure and transformation temperatures [6,12–15,17]. However, for the high grade bridge steel with nickel, chromium and copper, the influence of molybdenum content on transformation behavior during continuous cooling has not been reported in details. Furthermore, it is of significance for avoiding the formation of brittle granular bainite with larger M/A constituents, improving its strength and toughness and saving the molybdenum additions to understand the influence of molybdenum on transformation behavior. In the present paper, the effects of molybdenum content on transformation behavior of high performance bridge steel during continuous cooling were investigated by simulation of the thermo-mechanical control process. The continuous-cooling-transformation (CCT) diagrams with different molybdenum content were
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plotted. Moreover, the bainite ferrite (BF) transformation starting temperatures were estimated according to the Lever Rule. So the bainite transformation region was refined. In addition, the empirical equation for the calculation of granular ferrite (GF) transformation starting temperatures was also established. 2. Materials and experimental procedure The chemical compositions of steels investigated in the present work are given in Table 1. These steels were vacuum melted ingots and hot rolled to plates with the thickness of approx. 12 mm using two-high 450 mm experimental hot rolling mill. The specimens were cut from these plates and machined to round ladder dilatometry samples with the diameter of 6 mm and 10 mm and the length of 15 mm and 30 mm in the middle and edge, respectively. The Gleeble 3800 thermo-mechanical simulator was adopted to test transformation temperatures of Mo-free, Mo-0.17wt% and Mo0.38wt% steels. The round ladder samples were reheated to 1200 °C and held for 300 s, then cooled to the deformation temperature of 870 °C and held for 15 s to eliminate temperature gradient. The true strain and strain rate is 0.7 and 5 s1, respectively. The samples were immediately cooled to room temperature with different cooling rates of 0.5, 2, 5, 10, 15, 20, 25, 30 °C/s after compression deformation. The surface underneath the thermocouple was polished and etched in 4% nital solution for the observation of OM (LEICA DMIRM). The continuous-cooling-transformation (CCT) diagrams were plotted by transformation temperatures determined on the dilatometric curves with the help of metallographic observation and Lever Rule [18]. The Vickers-hardness of samples with different cooling rates for tested steels was tested using Vickers-durometer of HV-50 with the load of 10 kg. The testing standard followed the guidelines of ISO 6507-1: 2005(E) [19], while a diamond indenter is forced into the surface of the tested samples, and then the diagonal length (d1 and d2) of the indentation left in the surface after removal of the diamond indenter is measured. At last, the Vickers-hardness (PHV) can be calculated using the equation of HV = 0.1891 F/d2 (F-Force, d-Mean value of d1 and d2). 3. Results and discussion 3.1. Transformation behavior On the basis of the dilatometric curves, metallographic observation and Lever Rule, the continuous-cooling-transformation (CCT) diagrams for Mo-free, Mo-0.17wt% and Mo-0.38wt% steels are given in Fig. 1. Furthermore, the transformation field was divided into polygonal ferrite (PF), granular ferrite (GF) and bainite ferrite (BF) transformation fields based on ferrite morphologies [2]. In addition, the martensite transformation starting temperatures (Ms) for Mo-free, Mo-0.17wt% and Mo-0.38wt% steels were estimated as approx. 407 °C, 406 °C and 402 °C using Eq. (1) [20], respectively. It can be seen that the CCT diagrams for Mo-free steel are quite similar to those for Mo-0.17wt% steel (by comparison of Fig. 1a and b). However, the molybdenum addition of 0.38 wt% can significantly alter the appearance of the CCT diagrams (by comparison of Fig. 1c, a and b), showing no PF transformation field and
lower decrements of GF and BF transformation starting temperatures with increasing cooling rates investigated, and Kong et al. also indicated that the addition of element Mo could shift the ferrite transformation starting line to the right obviously [13], which is in good agreement with the result in this study. Moreover, the PF and GF transformation fields are reduced with increasing molybdenum content, conversely, the BF and martensite transformation fields are expanded. But Kong et al. did not refine the bainite transformation field and indicate the influence of Mo content on bainite transformation field [14]. In addition, for GF transformation, the range of transformation starting temperatures throughout the range of cooling rates studied is 664–567 °C, 630–543 °C and 585–545 °C for Mo-free, Mo-0.17wt% and Mo-0.38wt% steels, respectively, indicating that the molybdenum addition significantly lower GF transformation starting temperatures. And the formation of PF and GF can be completely suppressed throughout the range of cooling rates studied and at the cooling rates higher than 25 °C/s in Mo-0.38wt% steel, respectively. It has been reported that the molybdenum tending to segregate to prior austenite grain boundaries leads to lower grain boundary energies [21,22], which can lower the nucleation rates of all ferrite morphological types heterogeneously nucleating at austenite grain boundaries [22] and enhance the solute draglike effect (SDLE) [23], leading to the lower ferrite growth rate [24]. Moreover, the molybdenum addition can increase carbon diffusion activation energy in austenite and the carbon diffusion coefficient is decreased [14], which also lowers the ferrite growth rate. So the formation of PF mainly dominated by carbon diffusion is restricted due to the molybdenum addition. Furthermore, the molybdenum addition of 0.17 wt% can only lower the PF transformation temperatures, but that of 0.38 wt% completely eliminates the formation of PF throughout the range of cooling rates studied, showing the molybdenum addition of 0.38 wt% is more effective to lower ferrite nucleation rate and growth rate. The bainite nucleates like martensite but with the partition of the interstitial carbon [25], so the GF transformation is slowed down and the GF transformation starting temperatures are also lowered due to aforementioned lower carbon diffusion coefficient due to the molybdenum addition [14]. Further, the GF transformation starting temperatures can be significantly lowered, when molybdenum addition is up to 0.38 wt%, indicating that the hardenability of austenite can be significantly increased by the molybdenum addition of 0.38 wt%. In addition, from Fig. 1, it can be found that the GF transformation field is shifted to the right by the increase of molybdenum content, indicating that the GF transformation is also restricted by the molybdenum addition.
MsðKÞ ¼ 764:2 302:6C 30:6Mn 16:6Ni 8:9Cr þ 2:4Mo 11:3Cu þ 8:58Co þ 7:4W 14:5Si
ð1Þ
3.2. Transformation microstructures Some typical metallographic pictures of samples with different cooling rates are shown in Fig. 2. The microstructure composed of PF and GF is observed at the lowest cooling rate in Mo-free and Mo-0.17wt% steels, but for Mo-0.38wt% steel, the PF microstructure is not observed. In addition, it can be seen that the M/A
Table 1 Chemical compositions of tested steels (wt%). Steel
C
Si
Mn
P
S
Als
Ni
Cr
Cu
Ti
Mo-free Mo-0.17wt% Mo-0.38wt%
0.052 0.051 0.056
0.43 0.44 0.49
1.56 1.60 1.57
0.0079 0.0067 0.0017
0.0012 0.0011 0.0085
0.012 0.011 0.025
0.34 0.33 0.54
0.46 0.46 0.45
0.45 0.44 0.47
0.024 0.026 0.029
Mo
Nb
0.17 0.38
0.041 0.044 0.042
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(a) 800
(b)800 700 PF
PF o
Temperature, C
o
Temperature, C
700
600
GF BF
500
400
GF
500 BF Ms
400
Ms
300 25 30 20 15
300
600
5
10
1
10
10
2
2
25 30 20 15
o
0.5 C/s
10
200
3
1
2
5
10 2
10
o
0.5 C/s 3
10
10 o
o
Cooling time from 870 C, s
Cooling time from 870 C, s
(c) 800 600 GF
o
Temperature, C
700
500 400
BF
Ms
300 25 30 20 15
200 100
10
2
5
10
100
o
0.5 C/s
1000 o
Cooling time from 870 C, s Fig. 1. Continuous cooling transformation diagrams of (a) Mo-free steel, (b) Mo-0.17wt% steel and (c) Mo-0.38wt% steel.
constituents are significantly refined by increasing molybdenum content (by comparison of Fig. 2a, e and i). The M/A constituents can be also refined greatly (by comparison of Fig. 2b, f and j and Fig. 2a, e and i) by increasing cooling rate, which is in good agreement with Mazancová and Mazanec’s report [26]. However, the effect of increasing molybdenum addition to refine M/A constituent is not pronounced at higher cooling rates, such as 10 °C/s or higher one than 10 °C/s. When the cooling rate is increased to 20 °C/s, a gradual increase in molybdenum addition is accompanied by a rise in the volume fraction of BF and martensite (by comparison of Fig. 2c, g and k). When the cooling rate is increased to 30 °C/s, only a few GF can be observed in Mo-free and Mo-o.17wt% steels and the formation of GF can be completely suppressed in Mo0.38wt% steel. Furthermore, the ferrite lath is sufficiently refined with increasing molybdenum content. For the tested steels in the present work, the carbon concentration exceeds the solubility limit in PF and GF, so the carbon can be rejected into residual austenite during PF and GF transformation, and the carbon-enriched austenite may be transformed into martensite below Ms during further cooling, which is the reason why the M/A constituents can be observed in Fig. 2. The molybdenum addition can significantly reduce the carbon diffusion, so the larger block carbon-enriched austenite can be hardly formed in Mo0.38wt% steel, showing the M/A constituents are significantly refined by increasing molybdenum addition at the lowest cooling rate. And Kong et al. has also indicated that the M/A constituents can be refined by decreasing the carbon diffusion [14]. However, at higher cooling rates, such as 10 °C/s or higher, the carbon diffusion may be mainly controlled by lower GF transformation
temperatures, so increasing molybdenum addition to refine M/A constituent is not obvious, as observed in Fig. 2b, f and j. Moreover, the GF transformation temperatures can be lowered at higher cooling rates, so the carbon diffusion is also reduced; showing fine M/A constituents at higher cooling rates (by comparison of Fig. 2b, f and j and Fig. 2a, e and i). 3.3. Vickers-hardness The effects of cooling rates on Vickers-hardness of tested steels are shown in Fig. 3. From Fig. 3, it can be seen that the Vickershardness can be increased with the increase of molybdenum content due to transformation strengthening and higher dislocation strengthening, and the changes of Vickers-hardness are in good agreement with the microstructure evolution. For Mo-0.38wt% steel, the Vickers-hardness slightly increases at first, then sharply increases and almost remains unchanged at last with increasing cooling rates, showing an ‘‘S’’ shape, whereas for Mo-0.17wt% and Mo-free steels, the changes of Vickers-hardness are almost the same with increasing cooling rates and the plateau is not observed at higher cooling rates. As aforementioned molybdenum addition of 0.38 wt% noticeably increasing hardenability, so the highest Vickers hardness can be gained at lower cooling rate, i.e., 20 °C/s, for Mo-0.38wt%. However, for Mo-0.17wt% and Mo-free steels, if the cooling rates are further increased, it can be thought the highest Vickers hardness may be also gained and the Vickers hardness vs cooling rate may be also showed as ‘‘S’’ shape. The molybdenum addition of 0.38 wt% can significantly increase the Vickers hardness at the cooling rates ranged from
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J. Chen et al. / Materials and Design 49 (2013) 465–470
(b)
(a)
10µm
(c)
10µm
(f)
(e)
10µm
(i)
10µm
(g)
10µm
(j)
10µm
(d)
(h)
10µm
10µm
(k)
10µm
10µm
(l)
10µm
10µm
Fig. 2. Typical optical microstructure of (a–c) and (d) Mo-free steel, (e–g) and (h) Mo-0.17wt% steel and (i–k) and (l) Mo-0.38wt% steel with different cooling rates, (a), (e), (i) 0.5 °C/s, (b), (f), (j) 10 °C/s, (c), (g), (k) 20 °C/s and (d), (h), (l) 30 °C/s.
0.5 °C/s to 25 °C/s, whereas the Vickers hardness increments (by comparison of Mo-0.17wt% steel and Mo-0.38wt% steel) are sharply reduced at the cooling rate of 30 °C/s. However, Cizek et al. indicated that the molybdenum addition of 0.309 wt% appeared to be less influence on strengthening [2], but we found that the molybdenum addition of 0.38 wt% could greatly increase strengthening. The Vickers hardness can be mainly increased through enhanced hardenability and associated modification of the resultant transformation microstructure based on the analysis of Sections 3.1 and 3.2. The transformation temperatures for Mo-0.38wt% steel are further lower than that for Mo-0.17wt% at lower cooling rates, so the Vickers hardness can be sufficiently increased due to microstructure refinement, dislocation strengthening and transformation hardening, etc. However, the transformation temperature for Mo-0.17wt% steel at the cooling rate of 30 °C/s is similar to that for Mo-0.38wt% steel at
the cooling rate of 20 °C/s, so the effects of microstructure refinement, dislocation strengthening and transformation hardening, etc., can be also gained for Mo-0.17wt% steel at the cooling rate of 30 °C/s, resulting in the Vickers hardness increments (by comparison of Mo-0.17wt% and Mo-0.38wt% steels) are sharply reduced at the cooling rate of 30 °C/s. This phenomenon indicates that the molybdenum content can be lowered as the cooling rate is increased. 3.4. Empirical equation It is found that the GF transformation starting temperature is almost linear with the cooling rate for Mo-free, Mo-0.17wt% and Mo0.38wt% steels, as shown in Fig. 4. So the empirical equation for the calculation of GF transformation starting temperatures was established, as follows:
J. Chen et al. / Materials and Design 49 (2013) 465–470
indicating that Eq. (2) can be used to estimate GFs of tested steels with different molybdenum additions.
400 Mo-free steel Mo-0.17wt% steel Mo-0.38wt% steel
Vickers-hardness, HV
375
469
350
4. Conclusions
325
The continuous-cooling-transformation (CCT) diagrams of high performance bridge steel with different molybdenum content were plotted by means of a combined method of dilatometry and metallography. The influence of molybdenum content and cooling rate on transformation behavior, transformation microstructure, and Vickers-hardness was discussed in details. In addition, the empirical equation for estimating the granular ferrite transformation starting temperature was established. The following conclusions have been drawn in this study.
300 275 250 225 200
0
1
10
10 o
CR, C/s
1. The molybdenum addition of 0.17 wt% cannot noticeably alter the transformation behavior; conversely, when the molybdenum addition is up to 0.38 wt%, the transformation behavior can be significantly altered. The PF and GF transformation fields are reduced with the increase of molybdenum content; whereas, the BF and martensite transformation fields are expanded. In addition, the GF transformation starting temperatures throughout the range of cooling rates studied is 664– 567 °C, 630–543 °C and 585–545 °C for Mo-free, Mo-0.17wt% and Mo-0.38wt% steels, respectively, indicating that the GF transformation starting temperatures can be pronouncedly lowered by the molybdenum addition of 0.38 wt%. 2. The M/A constituents can be obviously refined with the increase of molybdenum content at lower cooling rates; whereas, the effects are not obvious at higher cooling rates. However, the ferrite lath can be refined with the increase of molybdenum content at higher cooling rates. In addition, increasing cooling rates can significantly refine M/A constituents and ferrite lath. 3. The molybdenum addition of 0.17 wt% cannot greatly increase the Vickers hardness. When the molybdenum addition is up to 0.38 wt%, the Vickers hardness can be noticeably increased, but Vickers hardness increments (by comparison of Mo0.17wt% steel and Mo-0.38wt% steel) are sharply reduced at the cooling rate of 30 °C/s. So it can be thought that the molybdenum content can be lowered as the cooling rate is increased. 4. The empirical equation for estimating GF transformation starting temperatures was established, as follows:
700
o
GF transformation start temperature, C
Fig. 3. Vickers-hardness vs cooling rate (CR).
Mo-free steeel Mo-0.17wt% steel Mo-0.38wt% steel
675 650 625 600 575 550 525 500
0
5
10
15
25
30
o
Cooling rate, C/s Fig. 4. GF transformation starting temperatures vs cooling rate.
700
650
o
Measured GFs, C
675
GFs ð CÞ ¼ ð30:1CR 609:5ÞMo2 ð8:7CR 40:4ÞMo 3:0CR þ 657:8 625 600
Acknowledgments
575
This work is supported by the Fundamental Research Funds for the Central Universities (N110607003, N100507002 and N110307002) and the Scholarship Award for Excellent Doctoral Student granted by Ministry of Education of PR China.
550 550
575
600
625
650
675
700
o
Calculated GFs, C
References
Fig. 5. Comparisons of measured GFs and calculated GFs.
GFs ð CÞ ¼ ð30:1CR 609:5ÞMo2 ð8:7CR 40:4ÞMo 3:0CR þ 657:8
ð2Þ
where the CR is cooling rate, °C/s, and Mo is molybdenum content, wt%. The comparisons of calculated values and measured ones are given in Fig. 5. It can be seen that the calculated GF transformation starting temperatures are in good agreement with measured ones,
[1] Shim CS, Whang JW, Chung CH, Lee PG. Design of double composite bridges using high strength steel. Procedia Eng 2011;14:1825–9. [2] Cizek P, Wynne BP, Davies CHJ, Muddle BC, Hodgson PD. Effect of composition and austenite deformation on the transformation characteristics of low-carbon and ultralow-carbon microalloyed steels. Metall Mater Trans A 2002;33: 1331–49. [3] Olasolo M, Uranga P, Rodriguez-Ibabe JM, López B. Effect of austenite microstructure and cooling rate on transformation characteristics in a low carbon Nb–V microalloyed steel. Mater Sci Eng A 2011;528:2559–69. [4] Zhao MC, Yang K, Xiao FR, Shan YY. Continuous cooling transformation of undeformed and deformed low carbon pipeline steels. Mater Sci Eng A 2003;355:126–36. [5] Guo J, Shang CJ, Yang SW, Guo H, Wang XM, He XL. Weather resistance of low carbon high performance bridge steel. Mater Des 2009;30:129–34.
470
J. Chen et al. / Materials and Design 49 (2013) 465–470
[6] Anijdan SHM, Rezaeian A, Yue S. The effect of chemical composition and austenite conditioning on the transformation behavior of microalloyed steels. Mater Charact 2012;63:27–38. [7] Zhang CL, Cai DY, Wang YH, Liu MQ, Liao B, Fan YC. Effects of deformation and Mo, Nb, V, Ti on continuous cooling transformation in Cu–P–Cr–Ni–Mo weathering steels. Mater Charact 2008;59:1638–42. [8] Li L, Ding H, Du LX, Wen JL, Song HM, Zhang PJ. Influence of Mn content and hot deformation on transformation behavior of C–Mn steels. J Iron Steel Res Int 2008;15:51–5. [9] Liu SK, Zhang J. The influence of the Si and Mn concentration on the kinetics of the transformation in Fe–C–Si–Mn alloys. Metall Mater Trans A 1990;21: 1517–25. [10] Manohar PA, Chandra T, Killmore CR. Continuous cooling transformation behavior of microalloyed steels containing Ti, Nb, Mn and Mo. ISIJ Int 1996;36: 1486–93. [11] Paxton HW, Bain EC. Alloying elements in steels. OH: Metals Park, ASM; 1966. p. 274. [12] Uemori R, Chijiiwa R, Tamehiro H, Morikawa H. AP–FIM study on the effect of Mo addition on the microstructure in Ti–Nb steel. Appl Surf Sci 1994;76– 77:225–60. [13] Kong JH, Zhen L, Guo B, Li PH, Wang AH, Xie CS. Influence of Mo content on microstructure and mechanical properties of high strength pipeline steel. Mater Des 2004;25:723–8. [14] Kong JH, Xie CS. Effect of molybdenum on continuous cooling bainite transformation of low-carbon microalloyed steel. Mater Des 2006;27: 1169–73. [15] García de Andrés C, Capdevila C, Madariaga I, Gutiérrez I. Role of molybdenum in acicular ferrite formation under continuous cooling in a medium carbon microalloyed forging steel. Scripta Mater 2005;45:709–16.
[16] Ghosh A, Mishra B, Das S, Chatterjee S. Microstructure, properties, and age hardening behavior of a thermomechanically processed ultralow carbon Cubearing high-strength steel. Metal Mater Trans A 2005;36:703–13. [17] Tang ZH, Stumpf W. The role of molybdenum additions and prior deformation on acicular ferrite formation in microalloyed Nb–Ti low carbon line-pipe steels. Mater Charact 2008;59:717–28. [18] Yuan XQ, Liu ZY, Jiao SH, Liu XH, Wang GD. Effects of nanoprecipitates in austenite on ferrite transformation starting temperature during continuous cooling in Nb–Ti micro-alloyed steels. ISIJ Int 2007;47:1658–65. [19] ISO international standard. Standard test method for Vickers-hardness test of metallic materials. ISO 6507–1; 2005(E). [20] Capdevila C, Caballero FG, Carcía De Andrés C. Determination of Ms temperature in steels: a bayesian neural network model. ISIJ Int 2002;42: 894–902. [21] Enomoto M, White CL, Aaronson HI. Evaluation of the effects of segregation on austenite grain boundary energy in Fe–C–X alloys. Metall Trans A 1988;19: 1807–18. [22] Essadiqi E, Jonas JJ. Effect of deformation on the austenite-to-ferrite transformation in a plain carbon and two microalloyed steels. Metall Trans A 1988;19:417–26. [23] Chen JK, Vandermeer RA, Reynolds Jr WT. Effects of alloying elements upon austenite decomposition in low-c steels. Metall Mater Trans A 1994;25: 1367–79. [24] Essadiqi E, Jonas JJ. Effect of deformation on ferrite nucleation and growth in a plain carbon and two microalloyed steels. Metall Trans A 1989;20:987–98. [25] Bhadeshia HKDH. The bainite transformation: unresolved issues. Mater Sci Eng A 1999;273–275:58–66. [26] Mazancová E, Mazanec K. Physical metallurgy characteristics of the M/A constituent formation in granular bainite. J Mater Process Tech 1997;64: 287–92.