Metallogr. Microstruct. Anal. DOI 10.1007/s13632-015-0191-7
TECHNICAL ARTICLE
Effect of Micro-alloying Element Boron on the Strengthening of High-Strength Steel Q690D Wei-jian Liu • Jing Li • Cheng-bin Shi Chun-miao Wang
•
Received: 19 December 2014 / Revised: 5 February 2015 / Accepted: 11 February 2015 Ó Springer Science+Business Media New York and ASM International 2015
Abstract The effect of boron addition on strengthening of steel Q690D was studied. The metallographic microstructures of Q690D plate samples after controlled rolling and cooling process and roughing rolling in bench-scale were observed by optical microscope and transmission electron microscope, and the mechanical properties were determined. The results showed that the microstructure of Q690D became finer distinctly with increase in the content of soluble boron in steel. The yield strength and tensile strength also increased with the increase of soluble boron. Most of boron in steel Q690D existed in solid solution state. The main precipitates in Q690D steel were the Ti(C, N) and NbC. The amount of BN was small. Among the strengthening performance of steel Q690D given by boron addition, refinement strengthening gave the greatest contribution on the strengthening effect. Keywords Boron Lath bainite Transformation strengthening Recrystallization Refinement strengthening
Introduction In the early twentieth century, boron has been added into steels as a kind of micro-alloying element for improving W. Liu (&) J. Li C. Shi State Key Laboratory of Advanced Metallurgy, University of Science and Technology Beijing, Beijing 100083, People’s Republic of China e-mail:
[email protected] C. Wang Qinggong College, Hebei United University, Tangshan 063000, People’s Republic of China
the hardenability [1–3]. The range of boron addition amount is about 5–30 ppm, and too much or too little will reduce the effect of improving hardenability [4]. In recent decades, boron-bearing steel has been developed rapidly. The application of boron in low-carbon steel, ultra-lowcarbon steel, and low-alloy steel is maturing gradually [5–9]. The mechanism of hardenability improvement by boron addition has been discussed deeply by many researchers [10, 11]. The mutual effect of boron and other alloying elements is also an important factor influencing the effect of boron. In low-carbon steel, combined addition of niobium, boron, or molybdenum, boron obtains higher hardenability than single addition of boron [5, 12]. Shen et al. [13]. concluded that titanium could protect the hardenability of boron in C–Mn–B steel. Many studies [14, 15] have reported that boron addition in steel can play a role of grain coarsening. It is due to the formation of massive BN decreases the pinning effect of fine precipitates, such as Nb(C,N). Few studies [16, 17] have discussed the grain refinement caused by boron addition. The refining effect caused by boron addition needs to be further considered. Q690D plates with different boron contents after thermo-mechanical control process (TMCP) were taken as experimental specimens. Q690D steel is adopted by the Chinese standards (GB/T 1591-2008), and the equivalent steel grade is grade 100w [690w] (A 709/A 709M-2007 in ASTM standards). Q690D steel is categorized into the high-strength structural steel, and it is also a kind of lowcarbon bainitic steel with excellent mechanical properties such as high strength and high toughness. Q690D plates are extensively applied to the engineering machinery and coal mine machinery, such as hydraulic support, port crane, and platform truck. TMCP could be described as a combination of controlled rolling and controlled cooling. It is beneficial
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for increasing the mechanical properties of steels and saving the energy. The kind of technology is widely used in steel industry for producing steels with high mechanical properties [18, 19]. In this study, the influence of boron addition on the strengthening in Q690D steel was studied. The optical microscope (OM) and transmission electron microscope (TEM) were conducted, and the strength of steel samples was determined.
steel samples 1# with soluble boron of 0.0008% and 2# with soluble boron of 0.0018% were taken as the study object for the most distinct differences of strength and soluble boron content. For observing the size of prior-austenite after roughing rolling, the roughing rolling in bench-scale was conducted. The experimental material is a slab with the thickness of 100 mm. The rolling temperatures are the same as the
Experimental Experimental Procedure The Q690D plate samples were taken from an industrial production process. The process route is as follows: hot metal pretreatment ? top and bottom combined blown converter ? ladle furnace (LF) refining ? vacuum degassing (VD) ? slab continuous casting ? 3,500 mm steckel mill ? laminar flow cooling. TMCP technology was employed in the production process. TMCP process was divided into two-phase rolling. The holding temperature was 1,200 °C. The start rolling temperature of roughing rolling was 1,030 °C. The reduction amount was 110 mm in the process of roughing rolling. The start rolling temperature of finish rolling was 890 °C, and the finish rolling ended at 820 ± 20 °C. The reduction amount was 50 mm. The final thickness was 20 mm. The rolling speed was 1.15 m/s. After rolling, plates were cooled with 15 °C/s of cooling rate until 430 °C. After controlled rolling and cooling process, the samples were cut off. The composition of each samples were measured, as shown in Table 1. The yield strength and tensile strength of each steel samples changed with the increase of soluble boron are shown in Fig. 1. It can be concluded that both the yield strength and the tensile strength rise with the increase of soluble boron content from Fig. 1. The strength with soluble boron between 0.0014 and 0.0018% is higher than the soluble boron content between 0.0008 and 0.0012% obviously. Hence, Table 1 Chemical compositions of steel samples (mass%)
a
Bt means total boron in steel; Bs means acid soluble boron in steel
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Fig. 1 Relationship between boron content and yield strength of steel (a), and tensile strength of steel (b)
C
Si
Mn
S
P
Bta
Bsa
Ti
Nb
Mo
N
1#
0.052
0.19
1.58
0.001
0.012
0.0012
0.0008
0.019
0.051
0.205
0.0045
2# 3#
0.055 0.052
0.18 0.18
1.61 1.57
0.001 0.001
0.014 0.010
0.0020 0.0017
0.0018 0.0014
0.020 0.018
0.049 0.055
0.205 0.207
0.0048 0.0043
4#
0.052
0.19
1.61
0.001
0.011
0.0018
0.0016
0.018
0.052
0.204
0.0047
5#
0.049
0.26
1.62
0.002
0.012
0.0020
0.0015
0.019
0.054
0.200
0.0045
6#
0.051
0.21
1.60
0.001
0.011
0.0016
0.0010
0.019
0.051
0.204
0.0040
7#
0.050
0.25
1.63
0.001
0.012
0.0018
0.0015
0.018
0.056
0.210
0.0045
8#
0.052
0.25
1.59
0.001
0.010
0.0017
0.0012
0.019
0.055
0.219
0.0047
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industrial production process. The experimental slab was deformed according to the proportion of reduction in actual roughing rolling. The final thickness of specimen is 39 mm. After roughing rolling in bench-scale, the steel samples 1# and 2# were water-quenched to the room temperature when the temperature was 890 °C. The cooling rate is about 80 °C/s. Microscopic Observation The metallographic specimens were prepared by grinding and polishing. The etching solution for revealing the microstructure of steel samples was alcohol solution containing 4% of nitric acid. The microstructure of steel samples 1# and 2# after roughing rolling in bench-scale was etched by the saturated aqueous picric acid at 80 °C for revealing the prior austenite boundaries. The electrolyte for TEM film specimens was alcohol solution containing 5% of perchloric acid. The method of carbon extraction replica specimens was used for observing the precipitates.
Results and Discussion Transformation Strengthening The metallographic microstructure of steel samples 1# and 2# is shown in Fig. 2. The microstructure of both steel samples is lath bainite. The elongated prior austenite grain boundaries can be seen in the microstructure of steel sample 2#. Theoretically, the requirement of improving bainite hardenability can be reached as long as the alloy elements have high ability of suppressing the formation of ferrite and have a small influence on bainite transformation [20]. Many researchers [21, 22] have demonstrated that the addition of boron could improve the amount of bainite. Boron in solid solution is liable to segregate on the grain boundaries with higher diffusion speed. The boron segregation to austenite grain boundaries reduces the grain
boundary energy and reduces the quantity of preferential nucleation sites for ferrite [23]. The austenite/ferrite transformation is inhibited, and the amount of bainite increases. Because of the characteristic of grain boundary segregation, a little boron addition can improve hardenability greatly. Only boron in solid solution can play the role in improving hardenability. Soluble boron in low-alloyed steel is mainly boron in solid solution and Fe–C–B compounds, such as Fe23(C, B)6 or Fe3(C, B). Many studies [5, 24] indicate that Fe–C–B compounds are restrained when other alloyed elements such as niobium, titanium, and molybdenum existed in steel. Therefore, the majority of soluble boron can exist as boron in solid solution. Boron addition is beneficial for obtaining more bainite [25, 26]. In this study, the full bainite microstructure can be achieved with soluble boron of 0.0008%. Further improving the content of boron has negligible effect on transformation strengthening induced by bainite transformation. Refining Strengthening In Fig. 2, the grain boundary of deformed prior-austenite can be observed in steel sample 2#. It indicates that the recrystallization of steel sample 2# is insufficient. The microstructure of steel sample 2# is finer than steel sample 1#. It shows that boron addition in Q690D steel can play a role in promoting the grain refinement. To further observe the microstructure of Q690D steel, the TEM analysis was conducted for observing the film specimens of steel samples 1# and 2#, as shown in Fig. 3. The bainite laths of both steel samples 1# and 2# arrange in parallel. The thickness of bainite laths for both steel samples was counted. The results are illustrated in Fig. 4. From Fig. 4, the thickness of bainite laths in steel sample 2# is about 200–300 nm, and the values of steel sample 1# are 400–700 nm. The bainite lath of steel sample 2# is finer than steel sample 1# distinctly. Grain refinement is related with the effect of boron on the dynamic recrystallization. The occurrence of dynamic recrystallization resulted in the refinement of austenite
Fig. 2 Metallographic microstructure of the steel samples with different soluble boron content after controlled rolling and cooling: steel sample 1# with 0.0012% of Bt content and 0.0008% of Bs content; steel sample 2# with 0.0020% of Bt content and 0.0018% of Bs content
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Metallogr. Microstruct. Anal. Fig. 3 Microstructure of the samples of Q690D by TEM: steel sample 1# with 0.0012% of Bt content and 0.0008% of Bs content; steel sample 2# with 0.0020% of Bt content and 0.0018% of Bs content
Fig. 4 Thickness distribution of bainite laths for steel samples
Fig. 5 The recrystallization of steel samples 1# and 2# quenched after roughing rolling in bench-scale: steel sample 1# with 0.0012% of Bt content and 0.0008% of Bs content; steel sample 2# with 0.0020% of Bt content and 0.0018% of Bs content
grain in the process of roughing rolling at the starting temperature of 1,030 °C, which belonged to the recrystallization region. Figure 5 shows the microstructure for steel samples 1# and 2# after roughing rolling in bench-scale.
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According to Fig. 5, it is found that the recrystallization of steel sample 1# is complete and steel sample 2# is incomplete. The difference between the two steel samples is caused by the different content of boron. Boron addition
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can restrain dynamic recrystallization caused by the delay of nucleation and growth of dynamic recrystallization [27, 28]. The development of dynamic recrystallization depends on the movement of grain boundary. The movement of grain boundary can be inhibited by the solute dragging effect of alloy elements in solid solution and pinning effect of fine precipitates of alloyed elements, such as Nb(C, N), Ti(C, N), and BN [29, 30]. Due to the most boron in steel Q690D existed in solid solution, the inhibiting effect of boron on recrystallization is solute dragging effect. Mavropoulos and Jonas [31] reported that niobium had the highest effectiveness of retarding the recrystallization of austenite compared with titanium and vanadium. Many investigations have discussed the combined effect of boron and niobium on retarding the recrystallization [32, 33]. The diffusion coefficient of niobium at grain boundaries decreases with the increase of boron addition. As a result, the dragging effect of grain boundaries caused by niobium enhances. In addition, the formation of Nb–B compounds decreases the motivating force of recrystallization. The insufficient recrystallization leads to the persistence of more defects in steel. Accordingly, the amount of nucleation sites for bainite rises. It is beneficial for refinement of bainite laths. Consequently, the steel sample 2# has the finer grain than steel sample 1# which is due to insufficient recrystallization as shown in Fig. 3. The yield strength and tensile strength heighten with the refinement of grain.
was calculated using Thermo-Calc (TCFE6 database). The results are shown in Fig. 6. The morphology of precipitates in steel sample 2# is shown in Fig. 7. Through the statistics of 50 view fields, the average size of TiN is about 60–70 nm, and the NbC is about 20–30 nm. Because the precipitates containing niobium and titanium are liable to dissolve mutually, the elements of niobium and titanium were found in the EDS spectrum for TiN and NbC. The fine precipitates can pin the grain boundaries. Thus, the fine precipitates can play a role of grain refining and precipitation strengthening [34]. From Fig. 6 and the average sizes of main precipitates, it can be known that there are a large amount of NbC and TiN. These precipitates distribute dispersively. It can be deduced that the precipitates contribute to strengthening greatly. As discussed above, the amount of BN is small due to titanium addition. The BN precipitates were not found. In addition, the size of BN is large generally. Therefore, the contribution of the boron-containing precipitates to strengthening is small. In steel Q690D, the full lath bainite can be achieved with soluble boron of 0.0008%. Trace boron addition can play a role in transformation strengthening. Further improving the content of boron, the refining strengthening will play the leader role of strengthening. BN has little contribution to strengthening caused by the small amount.
Precipitation Strengthening
Conclusions
Because of the efficient titanium addition, the Ti/N ratio of Q690D steel is about 4.1, which is higher than the ideal Ti/ N ratio of 3.42. Almost all of [N] react with titanium. Thus the amount of BN and NbN is small in Q690D. The residual titanium reacted with C generates small amount of TiC. The Nb-containing precipitates are mainly NbC. The sequences and amount of precipitates in steel sample 2#
(1)
(2)
Most of soluble boron in Q690D steel existed as boron in solid solution. With the increase in the content of soluble boron in steel, the thickness of bainite laths turns to be finer and the yield strength and tensile strength improve. Q690D steel can be strengthened by boron addition. 0.0008% of boron addition can make the full lath
Fig. 6 Equilibrium precipitation predicted by Thermo-Calc for the steel samples. As discussed above, the main precipitates in steel Q690D are Ti(C, N) and NbC
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Fig. 7 TEM micrograph and EDS spectrum of precipitates in steel sample 2#
(3)
bainite microstructure of Q690D steel to be achieved. Further improving boron content has negligible effect on the transformation strengthening, and the increase of strengthening was attributed to the grain refining caused by the inhibition of boron on recrystallization. The grain refinement is caused by the inhibition effect of boron on dynamic recrystallization. Boron addition also improves the inhibition of niobium on dynamic recrystallization. The insufficient dynamic recrystallization leads to more defects remained in steel. These defects are the nucleation sites for lath bainite.
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(4)
The main precipitates in Q690D steel are Ti(C, N) and NbC. BN has little contribution to strengthening caused by the small amount.
References 1. M.E. Bush, P.M. Kelly, Strengthening mechanisms in bainitic steels. Acta Metall. 19, 1363–1371 (1971) 2. K. Yamamoto, T. Hasegawa, J.I. Takamura, Effect of boron on intra-granular ferrite formation in Ti-oxide bearing steels. ISIJ Int. 36, 80–86 (1996) 3. A. Brown, J.D. Garnish, R.W.K. Honeycombe, The distribution of boron in pure iron. Met. Sci. 8, 317–324 (1974)
Metallogr. Microstruct. Anal. 4. G.F. Melloy, P.R. Summon, P.P. Podgursky, Optimizing the boron effect. Metall. Trans. B 4, 2279–2289 (1973) 5. T. Hara, H. Asahi, R. Uemori, Role of combined addition of niobium and boron and of molybdenum and boron on hardnenability in low carbon steels. ISIJ Int. 44, 1431–1440 (2004) 6. D.Q. Bai, S. Yue, T.M. Maccagno, Effect of deformation and cooling rate on the microstructures of low carbon Nb–B steels. ISIJ Int. 38, 371–379 (1998) 7. E. El-Kashif, K. Asakura, K. Shibata, Effect of cooling rate after recrystallization on p and b segregation along grain boundary in if steels. ISIJ Int. 43, 2007–2014 (2003) 8. I.B. Timokhina, A.I. Nosenkov, A.O. Humphreys, Effect of alloying elements on the microstructure and texture of warm rolled steels. ISIJ Int. 44, 717–724 (2004) 9. G. Shigesato, T. Fujishiro, T. Hara, Grain boundary segregation behavior of boron in low-alloy steel. Metall. Trans. A 45A, 1876–1882 (2014) 10. R.M. Goldhoff, J.W. Spretnak, Distribution of boron in gammairon grains. Metall. Trans. A 209, 1278–1283 (1957) 11. L. Karlsson, H. Norden, H. Odelius, Non-equilibrium grain boundary segregation of boron in austenitic stainless steel-I. Large scale segregation behaviour. Acta Metall. 36, 1–12 (1988) 12. H. Asahi, Effects of Mo addition and austenitizing temperature on hardenability of low alloy B-added steels. ISIJ Int. 42, 1150–1155 (2002) 13. Y.S. Shen, S.S. Hansen, Effect of the Ti/N ratio on the hardenability and mechanical properties of a quenched-and-tempered C-Mn-B steel. Metall. Trans. A 28, 2027–2035 (1997) 14. M. Baarman, Notch toughness in hot-rolled low carbon steel wire rod. Wire Ind. 65, 74–77 (1998) 15. B. Yalamanchili, J.B. Nelson, P.M. Power, North Star Steel Texas’s experience with boron additions to low-carbon steel. Wire J. Int. 34, 90–94 (2001) 16. Y. Fujishiro, T. Hashimoto, H. Ohtani, Influence of boron and nitrogen contents on strength and toughness of controlled-rolled and acceleratedly-cooled low carbon steel. Tetsuto Hagane 75, 143–150 (1989) 17. E. El-Kashif, K. Asakura, T. Koseki, Effects of boron, niobium and titanium on grain growth in ultra high purity 18% Cr ferritic stainless steel. ISIJ Int. 44, 1568–1575 (2004) 18. Y.C. Liu, F.X. Zhu, Y.M. Li, Effect of TMCP parameters on the microstructure and properties of an Nb-Ti microalloyed steel. ISIJ Int. 45, 851–857 (2005) 19. Y. Nie, C.J. Shang, X. Song, Properties and homogeneity of 550-Mpa grade TMCP steel for ship hull. Int. J. Miner. Metall. Mater. 17, 179–184 (2010)
20. C.Y. Huang, J.R. Yang, S.C. Wang, Effects of boron, niobium and titanium on grain growth in ultra high purity 18% Cr ferritic stainless steel. Mater. Trans. JIM 34, 658 (1993) 21. A.O. Humphreys, D.S. Liu, M.R. Toroghinezhad, Effect of chromium and boron additions on the warm rolling behavior of low carbon steels. ISIJ Int. 42(Suppl), s52–s56 (2002) 22. Nobuhiro. Tsuji, Yukihiro. Matsubara, Tetsuo. Sakai, Effect of boron addition on the microstructure of hot-deformed Ti-added interstitial free steel. ISIJ Int. 37, 797–806 (1997) 23. X.M. Wang, X.L. He, Effect of boron addition on structure and properties of low carbon bainitic steels. ISIJ Int. 42, 38–46 (2002) 24. H. Tamehiro, M. Murata, R. Habu, Effect of combined addition of niobium and boron on thermomechanically processed steel. Tetsu to Hagane 72, 458–465 (1986) 25. K.Y. Zhu, C. Oberbillig, C. Musik, Effect of B and B ? Nb on the bainitic transformation in low carbon steel. Mater. Sci. Eng. A 528, 4222–4231 (2011) 26. D.T. Llewellyn, Boron in steels. Ironmak. Steelmak. 20, 338–343 (1993) 27. S.I. Kim, S.H. Choi, Y. Lee, Influence of phosphorous and boron on dynamic recrystallization and microstructures of hot-rolled interstitial free steel. Mater. Sci. Eng. A 406, 125–133 (2005) 28. S.I. Kim, Y. Lee, Influence of cooling rate and boron content on the microstructure and mechanical properties of hot-rolled high strength interstitial-free steels. Met. Mater. Int. 18, 735–744 (2012) 29. J. Jonas, I. Weiss, Effect of precipitation on recrystallization in microalloyed steels. Met. Sci. 13, 238–245 (1979) 30. I. Weiss, J. Jonas, Dynamic precipitation and coarsening of niobium carbonitrides during the hot compression of HSLA steels. Metall. Trans. A 11A, 403–410 (1980) 31. L. Mavropoulos, J. Jonas, Retardation of austenite recrystallization by the strain induced segregation of boron. Can. Metall. Q. 28, 159–169 (1989) 32. X.L. He, M. Djahazi, J.J. Jonas, The non-equilibrium segregation of boron during the recrystallization of Nb-treated HSLA steels. Acta Metall. Mater. 39, 2295–2308 (1991) 33. D. Anjana, K.D. Saikat, B.K. Jha, Strain hardening behavior and cold reducibility of boron-added low-carbon steel. J. Mater. Eng. Perform. 18, 109–115 (2009) 34. J. Fu, J. Zhu, L. Di, Study on the precipitation behavior of TiN in the microalloyed steel. Acta Metall. Sin. 36, 801–804 (2000)
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