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Effect of rare earths on impact toughness of a low-carbon steel
Materials and Design 33 (2012) 306–312
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Effect of rare earths on impact toughness of a low-carbon steel Hong-Liang Liu, Cheng-Jun Liu ⇑, Mao-Fa Jiang School of Materials and Metallurgy, Northeastern University, Shenyang 110004, China
a r t i c l e
i n f o
Article history: Received 7 September 2010 Accepted 21 June 2011 Available online 5 July 2011 Keywords: A. Ferrous metals and alloys E. Fracture F. Microstructure
a b s t r a c t Studies of an industrial low-carbon steel (B450NbRE) suggest that the impact toughness is unexpectedly low under its practical service, probably resulting from the unstable recovery of rare earths (RE) in steelmaking. The purpose of this work is to investigate the effect of RE on the impact toughness in low-carbon steel. The B450NbRE steels with content of 0.0012–0.0180 wt.% RE were produced by vacuum induction furnace. The impact toughness and microstructure were investigated after hot rolled. The Gleeble-1500 thermal simulator was used to validate the effect of RE on the microstructure. The results indicate that the microstructure of hot-rolled steels is characterized by polygonal ferrite, quasi-polygonal ferrite, bainite and pearlite. The impact toughness increases with RE contents reaching the peak with content of 0.0047 wt.% RE, such a change exhibits the same rule as the case of the ferrite amount. However, this improvement in impact toughness is not only due to an increase in ferrite amount, but also the fine grained structure and the cleaner grain boundaries. And content of 0.0180 wt.% RE is excessive. Such an addition of the RE resulted in the martensite precipitates at the grain boundaries, which are extremely detrimental to impact toughness. Ó 2011 Elsevier Ltd. All rights reserved.
1. Introduction The effects of rare earths (RE) on steelmaking have been extensively studied [1–5], focusing on the purification of liquid steel and the modification of inclusions by the addition of RE into steels. Since RE has a strong affinity for oxygen and sulfur, it has been used to deoxidize and desulfurize in steels. Some studies also demonstrated that RE can be used to modify the type of sulfides, so that the sulfides can often keep globular and do not deform into stringers during rolling, and the inclusion spacing also increases, resulting in a significant improvement in the toughness [2,6]. However, RE is no longer popularly used in steelmaking, because steel can now be readily purified using modern refining techniques, and calcium is effective in the low-cost modification of inclusions. However, as the cleanliness of steels has been greatly improved, the RE is realized to be crucial as microalloying element in steels. It has also reported that the RE can improve significantly the corrosion resistance of low-carbon steels [7,8]. The 09CuPTiRE, 16MnRE and BNbRE steels have been well developed in China and widely used for over 30 years [9,10]. Recently, a new B450NbRE steel for the middle-beam of train has been developed in Baotou Iron and Steel Corp. [11], in which Mn, Ni, Nb and V are alloyed to obtain the desired strength and toughness, while Cu and RE are employed to improve the corrosion resistance. Nevertheless, the impact toughness of the B450NbRE steel is unexpectedly low under its ⇑ Corresponding author. Tel./fax: +86 24 83681589. E-mail address: [email protected] (C.-J. Liu). 0261-3069/$ - see front matter Ó 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.matdes.2011.06.042
practical service, probably resulting from the unstable recovery of RE in steelmaking [12]. In the present work, the effect of RE additions on the microstructures and properties of the B450NbRE steel is systematically investigated, focusing on the establishment of the relationship between RE content and the impact toughness. 2. Experiments The B450NbRE steels were prepared by vacuum induction melting furnace. The type 20# steel with a chemical composition (wt.%) of 0.2C, 0.05Si, 0.12Mn and balance Fe was used as the starting material, in which Si, Mn, Nb, V, graphite, pure iron, the RE, among others, were subsequently added. The final compositions are summarized in Table 1. Here, the RE is composed of 50 wt.% La and 50 wt.% Ce in this study. The as-made B450NbRE steels were forged into 75 mm 75 mm ingots, and reheated to 1200 °C and held for more than 30 min, then hot rolled and the finish rolling temperature at above 950 °C, eventually air cooled to room temperature (Fig. 1). Finally, the B450NbRE steel was hot rolled into thick plates with dimensions of 12 mm 180 mm. Charpy v-notch impact test specimens with dimensions of 10 mm 10 mm 55 mm were prepared by spark cutting from the thick plates and then mechanically polished. The impact toughness tests were conducted according to ASTM E23 [13], with an Amsler RKP 450 testing machine at 20 °C and 40 °C, respectively. Metallographic observations were carried out on the hot-rolled specimens subjected to impact tests. The thermal simulator
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H.-L. Liu et al. / Materials and Design 33 (2012) 306–312 Table 1 Chemical composition of B450NbRE steels (wt.%). No.
C
Si
Mn
Ni
Cu
V
Nb
RE
1 2 3 4 5 6
0.088 0.092 0.094 0.078 0.088 0.082
0.578 0.530 0.522 0.498 0.562 0.562
1.590 1.588 1.629 1.516 1.591 1.544
0.33 0.33 0.32 0.30 0.33 0.31
0.32 0.34 0.32 0.35 0.34 0.32
0.089 0.093 0.085 0.087 0.091 0.093
0.044 0.045 0.040 0.037 0.041 0.046
0.0012 0.0031 0.0047 0.0087 0.0140 0.0180
1200 ,30min
Temperature /
reduction of 80%
950
air cooling
Time / s Fig. 1. Schematic of hot-rolled B450NbRE steels.
(Gleeble-1500) was used to validate the effect of the RE on the microstructure. The thermal simulation test specimens were cut from the hot-rolled thick plates, and reheated to 1200 °C for 3 min holding. Then they were rolled with a reduction of 20% or 40% at 950 °C followed by 30 s holding time, and eventually the specimens were cooled down to 200 °C with the cooling rate of 0.5 °C/s and 1 °C/s, respectively, as schematically indicated in Fig. 2. The heat treatment was also used to validate the microstructure with high RE content. The specimens with dimension of 10 mm 10 mm 15 mm were cut from the hot-rolled thick plates. Then they were reheated to 700–900 °C for 30 min holding, and eventually air cooled to room temperature. Optical microscopy (Olympus BX51 and Neophot 32) was used for metallographic observations. The impact fracture surface features and heat treatment microstructure were examined by scanning electron microscopy (SEM EVO 50) equipped with energy dispersive X-ray spectroscopy (EDX). The microstructure was characterized with transmission electron microscopy (TEM TECNAI
G20). Thin foils used for TEM characterizations were cut from the Charpy impact test specimens, and then mechanically thinned down to 100 lm thick, and finally polished in acetic acid containing 10% perchloric acid by conventional double-jet method.
3. Results 3.1. Impact toughness of hot-rolled steels The relationship between RE content and the impact toughness of the B450NbRE steel at both 20 °C and 40 °C are presented in Fig. 3. The dimensionless method is used to facilitate the comparison in the present work. The impact toughness increases with RE content reaching the peak with a content of 0.0047 wt.% RE. Subsequently, it decreases with further increasing RE content. An appropriate RE content can improve significantly the impact toughness of the B450NbRE steel. The impact fracture surface features of the specimens with contents of 0.0012, 0.0047 and 0.0180 wt.% RE are shown in Fig. 4 for comparison. The fracture surface of the No. 3 specimen containing 0.0047 wt.% RE consists of dimples, showing a typical ductile fracture characteristic [14,15] (Fig. 4b). However, the fracture surfaces of Nos. 1 and 6 specimens are cleavage and quasi-cleavage fractures, respectively, and tearing ridges with obvious secondary cracks are also observed in the No. 6 specimen. The result shows that crack initiation and propagation is relatively easy in the No. 6 specimen.
3.2. Microstructure of hot-rolled steels The microstructure of the hot-rolled steel with different RE content is shown in Fig. 5. It can be inferred that all the microstructure is composed of ferrite, bainite and pearlite. For the No. 3 specimen, the microstructure is mostly fine polygonal ferrite (Fig. 5c), with the average grain size of 21.1 lm. While for the other specimens, the microstructure is quasi-polygonal ferrite, with the grain size much larger than the No. 3 specimen. In addition to the differences in the shape of the ferrite, the differences in the amount of the ferrite are also significant. Statistical analysis of microstructure suggests that the ferrite amount firstly increases with RE content, and then decreases after the RE content reaches a certain level. The maximum ferrite amount is 92.4 vol.% with a content of 0.0047 wt.% RE, as shown in Fig. 6. These microstructure character-
5
1200 ,3 min /s
Temperature /
950 30 s
reduction of 20%,40%
0.5,1 10
/s
/s
Impact toughness
10
-40 20
4
3
2
1
0 0.000
Time / s Fig. 2. Schematic of the hot deformation tests with the cooling rate of 0.5 and 1 °C/ s.
0.004
0.008
0.012
0.016
0.020
RE content / % Fig. 3. Effect of RE content on the impact toughness of B450NbRE steels at 20 and 40 °C using the dimensionless method.
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Fig. 4. SEM fractographs of (a) No. 1 specimen (RE = 0.0012 wt.%) fracture surface, (b) No. 3 specimen (RE = 0.0047 wt.%) tough fracture, and (c) No. 6 specimen (RE = 0.0180 wt.%) fracture surface with obvious secondary crack.
Fig. 5. Microstructure of hot-rolled B450NbRE steels: Nos. (a) 1, (b) 2, (c) 3, (d) 4, (e) 5, and (f) 6 specimens.
and d). However, the specimen with an intermediate RE content of 0.0047 wt.% contains high-density dislocations in grain interiors and at the flat and clean GBs (Fig. 7b and e). With the increase of the RE content, the alloyed elements become much denser in GBs. Consequently, the movements of GBs are inhibited with the pinning effect of alloy-enriched area, which leads to the fine ferrite grains. As the RE content increases to 0.0180 wt.%, a small volume of the secondary phase becomes visible at GBs (Fig. 7c), and upper bainite has been detected in grains. These might be straightly related to the formation of secondary cracks observed in the impact specimen with the same RE content. Besides, high-density dislocations and dislocation substructures are found in ferrites (Fig. 7f).
94
Ferrite amount / %
92 90 88 86 84 82
3.3. Secondary phase at grain boundaries 80 0.000
0.004
0.008
0.012
0.016
0.020
RE content / % Fig. 6. Relationship between the ferrite amount and RE content of hot-rolled B450NbRE steels.
istics of B450NbRE steels agree well with the impact toughness variations. The original microstructures of hot-rolled B450NbRE steel specimens with different RE content were observed by TEM and representatively given in Fig. 7 for comparison. For the specimen with the lowest content of 0.0012 wt.% RE, the ferrite grain presents quasi-polygonal morphologies, with low density dislocations at the bent grain boundaries (GBs) and in grain interiors (Fig. 7a
In order to demonstrate the fact of RE accumulation at GBs and explain the toughness variation of different RE content specimens, the study on the secondary phase precipitates at GBs was conducted for B450NbRE steels. A peculiar phase was only detected in No. 6 specimen with high RE additions. TEM micrographs of the secondary phase in hot-rolled specimen are presented in Fig. 8. The SAD pattern confirms that the secondary phase is bcctype microstructure and it is different from ferrite. Its probably to be martensite. In order to prove this prediction, the hot-rolled B450NbRE steel with a content of 0.0180 wt.% RE is reheated at different temperature and air cooling, the fine particles are presented at the original quasi-polygonal ferrite GBs, the EDX indicate that it is cementite (Fig. 9). This demonstrates that the secondary phase at GBs is martensite.
H.-L. Liu et al. / Materials and Design 33 (2012) 306–312
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Fig. 7. Bright field TEM micrographs of hot-rolled B450NbRE steels: (a) and (d) No. 1 specimen (RE = 0.0012 wt.%) with low-density dislocations in grain interiors and the bent grain boundaries; (b) and (e) No. 3 specimen (RE = 0.0047 wt.%) with high-density dislocations in grain interiors; (c) and (f) No. 6 specimen (RE = 0.0180 wt.%) with a small volume of second phase at the grain boundaries, and dislocations substructures in grain interiors.
Fig. 8. Bright field TEM micrographs of No. 6 specimen (RE = 0.0180 wt.%) after hot rolled: (a) and (d) secondary phase precipitates at grain boundaries, (c) SAD pattern analysis for the region shown in (b).
Fig. 9. SEM micrographs showing heat treatment microstructure of No. 6 specimen (RE = 0.0180 wt.%) at 850 °C. (a) and (b) ultrafine grain size ferrite at grain boundaries. (c) EDS results for the particle in (b).
3.4. Microstructure of steels subjected to thermal simulation tests The evolution of microstructure of the steels closely depends on their chemical compositions and the thermo-mechanical processing [16–21]. To detect more clearly the effect of the RE content on the microstructure of B450NbRE steels, the other experiment was conducted using the Gleeble-1500 thermal simulator. The processing parameters are given in Fig. 2. The microstructures of B450NbRE steels with the deformation ratio of 20% or 40%, and the cooling rate of 0.5 °C/s and 1 °C/s are compared in Figs. 10 and 11. The primary microstructure is charac-
terized by polygonal ferrite, quasi-polygonal ferrite, bainite and degenerate pearlite. Similarly, It can be seen that the ferrite grain size of the No. 3 specimen with a content of 0.0047 wt.% RE, is finer than the others. Furthermore, with increasing the content of RE, the pearlite content decreases and ultimately it disappears, but the bainite amount increases. The ferrite amount was also measured as a function of the content of RE (Fig. 12). It is found that such a change exhibits the same rule as the case of the hot-rolled microstructure (Fig. 6). Its of interest to be found that boundaries inside the quasipolygonal ferrite are evolved from the prior austenite GBs, which
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Fig. 10. Microstructures of steels subjected to thermal simulation tests with a reduction of 20%: (a), (b), and (c) correspond to Nos. 1, 3, and 6 specimens (RE = 0.0012, 0.0047 and 0.0180 wt.%) with a cooling rate of 0.5 °C/s; (d), (e), and (f) correspond to Nos. 1, 3, and 6 specimens with a cooling rate of 1 °C/s.
Fig. 11. Microstructures of steels subjected to thermal simulation tests with a reduction of 40%: (a), (b) and (c) correspond to Nos. 1, 3, and 6 specimens (RE = 0.0012, 0.0047 and 0.0180 wt.%) with a cooling rate of 0.5 °C/s; (d), (e), and (f) correspond to Nos. 1, 3, and 6 specimens with a cooling rate of 1 °C/s.
has high RE contents (Figs. 10c and 11c). However, this fact is not easy to be found in the hot-rolled microstructures.
4. Discussion 4.1. Effect of RE content on the microstructure As stated above, the microstructures of hot-rolled steels are similar to those of steels subjected to thermal simulation tests, and they are all characterized by a combination of polygonal ferrite, quasi-polygonal ferrite, pearlite and bainite. For the two cases, the ferrite amount increases with the RE content reaching the peak with content of 0.0047 wt.% RE, and then it decreases with increasing RE content. The quasi-polygonal ferrite formed at high temperatures (below Ae3) [22], and it nucleates heterogeneously at the austenite GBs (Figs. 10a and 11a). Apparently, the nucleation of the quasi-polyg-
onal ferrite should be related to the diffusion of solute atoms [23]. The increase in the deformation temperature and the strain imposed, as well as the decrease in the cooling rate would enhance the diffusion of solute atoms, and thus promote the nucleation and growth of quasi-polygonal ferrites. Considering the different testing conditions of hot rolling and thermal simulation experiments, it is now readily to understand why the major phases of hot-rolled steel are composed of quasi-polygonal ferrites rather than thermal simulation tested steels. Owing to the partitioning of the alloy atoms during the growth of the ferrite [24], the excess carbon and solute atoms would concentrate into the retained austenite, causing the retained austenite changing into pearlite or bainite in different transformation temperature ranges (Figs. 5, 10 and 11). However, the formation of pearlite relies on the nucleation of cementite, which also involves diffusion of the alloy atoms. As further decreasing of transformation temperature to Bs, the retained austenite with high carbon concentration transforms into bainite.
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75
75
reduction of 20% reduction of 40%
reduction of 20% reduction of 40% 70
Ferrite amount / %
Ferrite amount / %
70
65
60
65
60
55
55
50
50 0.0012
0.0047
0.0180
0.0012
0.0047
0.0180
RE content / %
RE content / %
Fig. 12. Relationship between ferrite amount and RE content subjected to thermal simulation tests: (a) 0.5 °C/s and (b) 1 °C/s.
RE, together with the other alloying elements, plays an important role in the transformation. The covalent atomic radius of La and Ce are 0.1877 nm and 0.1825 nm respectively, but that of iron is 0.1210 nm. As solute atoms, the diffusion of RE is difficult. Furthermore, our recent work has indicated that RE improves the solubility of Nb in austenite in the pipeline steel, and inhibits the precipitation [25]. The interaction coefficients between RE and C, Nb, V, Ti are also negatives [26]. This could be provided as a further evidence that the addition of RE would impede cementite nucleation, including the diffusion process of carbon and solute atoms. Consequently, with the increase of RE content, the pearlite gradually gets decreased or even disappeared (Figs. 10 and 11). The formation of quasi-polygonal ferrite is also inhibited with increasing the RE content. Though the electrolytic analysis indicates that the solid solubility of RE is limited, our recent work has demonstrated that the free RE increases with increasing the RE content [27]. More recently, it is found that the free RE in austenite is much more than that it is detected after hot-rolled in pipeline steels. Therefore, the bainite transformation benefits from RE additions. In a word, the pearlite content of the hot-rolled steel reduces with increasing RE to its final disappearance, and correspondingly the ferrite amount increases. On the other hand, the bainite amount increases but the ferrite decreases with further increasing RE content. 4.2. Effect of RE content on grain boundary characteristic The formation of quasi-polygonal ferrite is reconstructive transformation process involving diffusion of alloy atoms, and normally the grains can grow freely across the austenite GBs (Figs. 10a and 11a). However, it was here found that, as the RE content is high enough, boundaries occurring inside the quasi-polygonal ferrite have been found (Figs. 10c and 11c). And it was considered to be the prior austenite GBs which did not get disappeared through the transformation, in contrast with other microstructure in this study. However, there is still a lot of work to do to prove it to be prior austenite GBs. McLean and Northcott [28] have indicated that the driving force of whether the solution elements are distributed in grain interiors or at GBs depends on the distortion energy difference. As indicated above, the atomic radius of RE is much larger than that of iron. So, RE should be distributed at GBs theoretically. It also detected the RE enriched at GBs in the other low carbon steels [27,29–31]. The appropriate RE accumulation at GBs is good for boundaries’ cleanness (Fig. 7b), thus its beneficial for the increase of the impact toughness. Besides, the grain growth is also inhibited by the drag-
ging effect of the enriched RE at the GBs, which necessarily leads to fine grains. The corresponding impact fracture surface would exhibit typically ductile characteristics, i.e., dimples (Fig. 4b). However, the ferrite transformation is blocked by too much RE enrichment at GBs. The quasi-polygonal ferrite reconstructive transformation is changed, namely, ferrites are just able to grow towards grain interiors, and unable to get across freely the austenite GBs theoretically (Figs. 10c and 11c). Such a phenomenon is difficult to be detected in hot-rolled steels with high-content ferrite and fine-grained structures. Of the above, the addition of RE makes the alloy elements in the solid solution more stable and the RE is inclined to accumulate at GBs. The excess carbon and solute atoms concentrated at the retained austenite make the austenite much more stable, which postpones the diffusion transformation and makes the martensite transformation occurred. It can be seen that there is some martensite presented as the second-phase at GBs (Fig. 7), which is extremely detrimental for the impact toughness and causes the fracture face becoming quasi-cleavage type.
4.3. Effect of RE content on the impact toughness It is obvious that the addition of the RE has an important effect in inhibiting the pearlite transformation, thus getting the volume fraction of ferrite increased. Generally speaking, the impact toughness of the steel would get increased with the reduction of the pearlite. However, recent research on the relationship between microstructure and impact fracture surface has demonstrated that there might be no direct correlation between the ferrite amount and impact toughness. The impact toughness also correlate well with the grain size and GBs features. An appropriate RE content induces the fine grained structure and clean GBs, which is the reason why the impact toughness of B450NbRE steel increased. Nonetheless, a volume of the martensite precipitates at GBs with excess addition of RE, which is detrimental to the impact toughness [32].
5. Conclusions The microstructure of hot-rolled steels are similar to those of steels subjected to thermal simulation tests, and they are all characterized by polygonal ferrite, quasi-polygonal ferrite, pearlite and bainite. For the two cases, the pearlite content reduces with increasing RE content, and correspondingly the ferrite amount increases. However, the bainite amount increases and the ferrite amount decreases with further increasing RE content. The maximum ferrite amount is 92.4 vol.% with a content of 0.0047 wt.% RE.
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The impact toughness increases with RE content reaching the peak with content of 0.0047 wt.% RE, and then it decreases with further increasing RE content. An appropriate RE content can improve significantly the impact toughness of the B450NbRE steel significantly. The improvement of impact toughness of RE-micoralloyed steels is closely related to the pearlite decreases, fine grained structure changes and GBs cleaning induced by an appropriate RE content. With further increasing RE content to 0.0180 wt.%, the martensite as the secondary phase precipitates at GBs induces a decreased impact toughness of the steel. Acknowledgments This work was financially supported by the Chinese Ministry of Education (109048) and the Ministry of Science and Technology of China (2006BAB02B03 and 2006BAE03A04). The authors are grateful for these supports. References [1] Wilson WG, Kay DAR, Vahed A. Use of thermodynamics and phase equilibriums to predict the behavior of the rare earth elements in steel. J Met 1974;26:14–23. [2] Luyckx L, Bell JR, Mclean A, Korchynsky M. Sulfide shape control in high strength low alloy steels. Metall Trans 1970;1:3341–50. [3] Alan D, Kay R. High temperature thermodynamics and applications of rare earth oxides and sulphides in ferrous metallurgy. Miner Process Extr Metall Rev 1992;10:307–23. [4] Shi LQ, Chen JZ, Northwood DO. Inclusion control in a 16Mn steel using a combined rare earth and calcium treatment. J Mater Eng 1991;13:273–9. [5] Waudby PE. Rare earth additions to steel. Int Metall Rev 1978;23:74–98. [6] Garrison Jr WM, Maloney JL. Lanthanum additions and the toughness of ultrahigh strength steels and the determination of appropriated lanthanum additions. Mater Sci Eng A 2005;403:299–310. [7] Hinton BRW. Corrosion inhibition with rare earth metal salts. J Alloys Compd 1992;180:15–25. [8] Wang C, Jiang F, Wang FH. The characterization and corrosion resistance of cerium chemical conversion coatings for 304 stainless steel. Corros Sci 2004;46:75–89. [9] Zhao TC, Fang YC. Microstructure and mechanical properties of dual-phase weathering steel 09CuPTiRe. J Mater Sci 2004;39:4393–6. [10] Li CL, Wang YS, Chen JJ, Liu CJ, Jiang MF. Effects of rare earth on structure and mechanical properties of chean BNbRE steel. J Rare Earths 2005;23:470–3. [11] Chen JJ, Jiang MF, Li K. Development of B450NbRE climate-resistant steel with high strength. Chin Rare Earths 2006;27:73–5. [12] Liu HL, Liu CJ, Jiang MF. Effects of rare earth elements on thermal simulation microstructure of B450NbRE steel. Chin J Rare Metals 2011;35:53–8.
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Effect of rare earths on impact toughness of a low-carbon steel Hong-Liang Liu, Cheng-Jun Liu ⇑, Mao-Fa Jiang School of Materials and Metallurgy, Northeastern University, Shenyang 110004, China
a r t i c l e
i n f o
Article history: Received 7 September 2010 Accepted 21 June 2011 Available online 5 July 2011 Keywords: A. Ferrous metals and alloys E. Fracture F. Microstructure
a b s t r a c t Studies of an industrial low-carbon steel (B450NbRE) suggest that the impact toughness is unexpectedly low under its practical service, probably resulting from the unstable recovery of rare earths (RE) in steelmaking. The purpose of this work is to investigate the effect of RE on the impact toughness in low-carbon steel. The B450NbRE steels with content of 0.0012–0.0180 wt.% RE were produced by vacuum induction furnace. The impact toughness and microstructure were investigated after hot rolled. The Gleeble-1500 thermal simulator was used to validate the effect of RE on the microstructure. The results indicate that the microstructure of hot-rolled steels is characterized by polygonal ferrite, quasi-polygonal ferrite, bainite and pearlite. The impact toughness increases with RE contents reaching the peak with content of 0.0047 wt.% RE, such a change exhibits the same rule as the case of the ferrite amount. However, this improvement in impact toughness is not only due to an increase in ferrite amount, but also the fine grained structure and the cleaner grain boundaries. And content of 0.0180 wt.% RE is excessive. Such an addition of the RE resulted in the martensite precipitates at the grain boundaries, which are extremely detrimental to impact toughness. Ó 2011 Elsevier Ltd. All rights reserved.
1. Introduction The effects of rare earths (RE) on steelmaking have been extensively studied [1–5], focusing on the purification of liquid steel and the modification of inclusions by the addition of RE into steels. Since RE has a strong affinity for oxygen and sulfur, it has been used to deoxidize and desulfurize in steels. Some studies also demonstrated that RE can be used to modify the type of sulfides, so that the sulfides can often keep globular and do not deform into stringers during rolling, and the inclusion spacing also increases, resulting in a significant improvement in the toughness [2,6]. However, RE is no longer popularly used in steelmaking, because steel can now be readily purified using modern refining techniques, and calcium is effective in the low-cost modification of inclusions. However, as the cleanliness of steels has been greatly improved, the RE is realized to be crucial as microalloying element in steels. It has also reported that the RE can improve significantly the corrosion resistance of low-carbon steels [7,8]. The 09CuPTiRE, 16MnRE and BNbRE steels have been well developed in China and widely used for over 30 years [9,10]. Recently, a new B450NbRE steel for the middle-beam of train has been developed in Baotou Iron and Steel Corp. [11], in which Mn, Ni, Nb and V are alloyed to obtain the desired strength and toughness, while Cu and RE are employed to improve the corrosion resistance. Nevertheless, the impact toughness of the B450NbRE steel is unexpectedly low under its ⇑ Corresponding author. Tel./fax: +86 24 83681589. E-mail address: [email protected] (C.-J. Liu). 0261-3069/$ - see front matter Ó 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.matdes.2011.06.042
practical service, probably resulting from the unstable recovery of RE in steelmaking [12]. In the present work, the effect of RE additions on the microstructures and properties of the B450NbRE steel is systematically investigated, focusing on the establishment of the relationship between RE content and the impact toughness. 2. Experiments The B450NbRE steels were prepared by vacuum induction melting furnace. The type 20# steel with a chemical composition (wt.%) of 0.2C, 0.05Si, 0.12Mn and balance Fe was used as the starting material, in which Si, Mn, Nb, V, graphite, pure iron, the RE, among others, were subsequently added. The final compositions are summarized in Table 1. Here, the RE is composed of 50 wt.% La and 50 wt.% Ce in this study. The as-made B450NbRE steels were forged into 75 mm 75 mm ingots, and reheated to 1200 °C and held for more than 30 min, then hot rolled and the finish rolling temperature at above 950 °C, eventually air cooled to room temperature (Fig. 1). Finally, the B450NbRE steel was hot rolled into thick plates with dimensions of 12 mm 180 mm. Charpy v-notch impact test specimens with dimensions of 10 mm 10 mm 55 mm were prepared by spark cutting from the thick plates and then mechanically polished. The impact toughness tests were conducted according to ASTM E23 [13], with an Amsler RKP 450 testing machine at 20 °C and 40 °C, respectively. Metallographic observations were carried out on the hot-rolled specimens subjected to impact tests. The thermal simulator
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H.-L. Liu et al. / Materials and Design 33 (2012) 306–312 Table 1 Chemical composition of B450NbRE steels (wt.%). No.
C
Si
Mn
Ni
Cu
V
Nb
RE
1 2 3 4 5 6
0.088 0.092 0.094 0.078 0.088 0.082
0.578 0.530 0.522 0.498 0.562 0.562
1.590 1.588 1.629 1.516 1.591 1.544
0.33 0.33 0.32 0.30 0.33 0.31
0.32 0.34 0.32 0.35 0.34 0.32
0.089 0.093 0.085 0.087 0.091 0.093
0.044 0.045 0.040 0.037 0.041 0.046
0.0012 0.0031 0.0047 0.0087 0.0140 0.0180
1200 ,30min
Temperature /
reduction of 80%
950
air cooling
Time / s Fig. 1. Schematic of hot-rolled B450NbRE steels.
(Gleeble-1500) was used to validate the effect of the RE on the microstructure. The thermal simulation test specimens were cut from the hot-rolled thick plates, and reheated to 1200 °C for 3 min holding. Then they were rolled with a reduction of 20% or 40% at 950 °C followed by 30 s holding time, and eventually the specimens were cooled down to 200 °C with the cooling rate of 0.5 °C/s and 1 °C/s, respectively, as schematically indicated in Fig. 2. The heat treatment was also used to validate the microstructure with high RE content. The specimens with dimension of 10 mm 10 mm 15 mm were cut from the hot-rolled thick plates. Then they were reheated to 700–900 °C for 30 min holding, and eventually air cooled to room temperature. Optical microscopy (Olympus BX51 and Neophot 32) was used for metallographic observations. The impact fracture surface features and heat treatment microstructure were examined by scanning electron microscopy (SEM EVO 50) equipped with energy dispersive X-ray spectroscopy (EDX). The microstructure was characterized with transmission electron microscopy (TEM TECNAI
G20). Thin foils used for TEM characterizations were cut from the Charpy impact test specimens, and then mechanically thinned down to 100 lm thick, and finally polished in acetic acid containing 10% perchloric acid by conventional double-jet method.
3. Results 3.1. Impact toughness of hot-rolled steels The relationship between RE content and the impact toughness of the B450NbRE steel at both 20 °C and 40 °C are presented in Fig. 3. The dimensionless method is used to facilitate the comparison in the present work. The impact toughness increases with RE content reaching the peak with a content of 0.0047 wt.% RE. Subsequently, it decreases with further increasing RE content. An appropriate RE content can improve significantly the impact toughness of the B450NbRE steel. The impact fracture surface features of the specimens with contents of 0.0012, 0.0047 and 0.0180 wt.% RE are shown in Fig. 4 for comparison. The fracture surface of the No. 3 specimen containing 0.0047 wt.% RE consists of dimples, showing a typical ductile fracture characteristic [14,15] (Fig. 4b). However, the fracture surfaces of Nos. 1 and 6 specimens are cleavage and quasi-cleavage fractures, respectively, and tearing ridges with obvious secondary cracks are also observed in the No. 6 specimen. The result shows that crack initiation and propagation is relatively easy in the No. 6 specimen.
3.2. Microstructure of hot-rolled steels The microstructure of the hot-rolled steel with different RE content is shown in Fig. 5. It can be inferred that all the microstructure is composed of ferrite, bainite and pearlite. For the No. 3 specimen, the microstructure is mostly fine polygonal ferrite (Fig. 5c), with the average grain size of 21.1 lm. While for the other specimens, the microstructure is quasi-polygonal ferrite, with the grain size much larger than the No. 3 specimen. In addition to the differences in the shape of the ferrite, the differences in the amount of the ferrite are also significant. Statistical analysis of microstructure suggests that the ferrite amount firstly increases with RE content, and then decreases after the RE content reaches a certain level. The maximum ferrite amount is 92.4 vol.% with a content of 0.0047 wt.% RE, as shown in Fig. 6. These microstructure character-
5
1200 ,3 min /s
Temperature /
950 30 s
reduction of 20%,40%
0.5,1 10
/s
/s
Impact toughness
10
-40 20
4
3
2
1
0 0.000
Time / s Fig. 2. Schematic of the hot deformation tests with the cooling rate of 0.5 and 1 °C/ s.
0.004
0.008
0.012
0.016
0.020
RE content / % Fig. 3. Effect of RE content on the impact toughness of B450NbRE steels at 20 and 40 °C using the dimensionless method.
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Fig. 4. SEM fractographs of (a) No. 1 specimen (RE = 0.0012 wt.%) fracture surface, (b) No. 3 specimen (RE = 0.0047 wt.%) tough fracture, and (c) No. 6 specimen (RE = 0.0180 wt.%) fracture surface with obvious secondary crack.
Fig. 5. Microstructure of hot-rolled B450NbRE steels: Nos. (a) 1, (b) 2, (c) 3, (d) 4, (e) 5, and (f) 6 specimens.
and d). However, the specimen with an intermediate RE content of 0.0047 wt.% contains high-density dislocations in grain interiors and at the flat and clean GBs (Fig. 7b and e). With the increase of the RE content, the alloyed elements become much denser in GBs. Consequently, the movements of GBs are inhibited with the pinning effect of alloy-enriched area, which leads to the fine ferrite grains. As the RE content increases to 0.0180 wt.%, a small volume of the secondary phase becomes visible at GBs (Fig. 7c), and upper bainite has been detected in grains. These might be straightly related to the formation of secondary cracks observed in the impact specimen with the same RE content. Besides, high-density dislocations and dislocation substructures are found in ferrites (Fig. 7f).
94
Ferrite amount / %
92 90 88 86 84 82
3.3. Secondary phase at grain boundaries 80 0.000
0.004
0.008
0.012
0.016
0.020
RE content / % Fig. 6. Relationship between the ferrite amount and RE content of hot-rolled B450NbRE steels.
istics of B450NbRE steels agree well with the impact toughness variations. The original microstructures of hot-rolled B450NbRE steel specimens with different RE content were observed by TEM and representatively given in Fig. 7 for comparison. For the specimen with the lowest content of 0.0012 wt.% RE, the ferrite grain presents quasi-polygonal morphologies, with low density dislocations at the bent grain boundaries (GBs) and in grain interiors (Fig. 7a
In order to demonstrate the fact of RE accumulation at GBs and explain the toughness variation of different RE content specimens, the study on the secondary phase precipitates at GBs was conducted for B450NbRE steels. A peculiar phase was only detected in No. 6 specimen with high RE additions. TEM micrographs of the secondary phase in hot-rolled specimen are presented in Fig. 8. The SAD pattern confirms that the secondary phase is bcctype microstructure and it is different from ferrite. Its probably to be martensite. In order to prove this prediction, the hot-rolled B450NbRE steel with a content of 0.0180 wt.% RE is reheated at different temperature and air cooling, the fine particles are presented at the original quasi-polygonal ferrite GBs, the EDX indicate that it is cementite (Fig. 9). This demonstrates that the secondary phase at GBs is martensite.
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Fig. 7. Bright field TEM micrographs of hot-rolled B450NbRE steels: (a) and (d) No. 1 specimen (RE = 0.0012 wt.%) with low-density dislocations in grain interiors and the bent grain boundaries; (b) and (e) No. 3 specimen (RE = 0.0047 wt.%) with high-density dislocations in grain interiors; (c) and (f) No. 6 specimen (RE = 0.0180 wt.%) with a small volume of second phase at the grain boundaries, and dislocations substructures in grain interiors.
Fig. 8. Bright field TEM micrographs of No. 6 specimen (RE = 0.0180 wt.%) after hot rolled: (a) and (d) secondary phase precipitates at grain boundaries, (c) SAD pattern analysis for the region shown in (b).
Fig. 9. SEM micrographs showing heat treatment microstructure of No. 6 specimen (RE = 0.0180 wt.%) at 850 °C. (a) and (b) ultrafine grain size ferrite at grain boundaries. (c) EDS results for the particle in (b).
3.4. Microstructure of steels subjected to thermal simulation tests The evolution of microstructure of the steels closely depends on their chemical compositions and the thermo-mechanical processing [16–21]. To detect more clearly the effect of the RE content on the microstructure of B450NbRE steels, the other experiment was conducted using the Gleeble-1500 thermal simulator. The processing parameters are given in Fig. 2. The microstructures of B450NbRE steels with the deformation ratio of 20% or 40%, and the cooling rate of 0.5 °C/s and 1 °C/s are compared in Figs. 10 and 11. The primary microstructure is charac-
terized by polygonal ferrite, quasi-polygonal ferrite, bainite and degenerate pearlite. Similarly, It can be seen that the ferrite grain size of the No. 3 specimen with a content of 0.0047 wt.% RE, is finer than the others. Furthermore, with increasing the content of RE, the pearlite content decreases and ultimately it disappears, but the bainite amount increases. The ferrite amount was also measured as a function of the content of RE (Fig. 12). It is found that such a change exhibits the same rule as the case of the hot-rolled microstructure (Fig. 6). Its of interest to be found that boundaries inside the quasipolygonal ferrite are evolved from the prior austenite GBs, which
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Fig. 10. Microstructures of steels subjected to thermal simulation tests with a reduction of 20%: (a), (b), and (c) correspond to Nos. 1, 3, and 6 specimens (RE = 0.0012, 0.0047 and 0.0180 wt.%) with a cooling rate of 0.5 °C/s; (d), (e), and (f) correspond to Nos. 1, 3, and 6 specimens with a cooling rate of 1 °C/s.
Fig. 11. Microstructures of steels subjected to thermal simulation tests with a reduction of 40%: (a), (b) and (c) correspond to Nos. 1, 3, and 6 specimens (RE = 0.0012, 0.0047 and 0.0180 wt.%) with a cooling rate of 0.5 °C/s; (d), (e), and (f) correspond to Nos. 1, 3, and 6 specimens with a cooling rate of 1 °C/s.
has high RE contents (Figs. 10c and 11c). However, this fact is not easy to be found in the hot-rolled microstructures.
4. Discussion 4.1. Effect of RE content on the microstructure As stated above, the microstructures of hot-rolled steels are similar to those of steels subjected to thermal simulation tests, and they are all characterized by a combination of polygonal ferrite, quasi-polygonal ferrite, pearlite and bainite. For the two cases, the ferrite amount increases with the RE content reaching the peak with content of 0.0047 wt.% RE, and then it decreases with increasing RE content. The quasi-polygonal ferrite formed at high temperatures (below Ae3) [22], and it nucleates heterogeneously at the austenite GBs (Figs. 10a and 11a). Apparently, the nucleation of the quasi-polyg-
onal ferrite should be related to the diffusion of solute atoms [23]. The increase in the deformation temperature and the strain imposed, as well as the decrease in the cooling rate would enhance the diffusion of solute atoms, and thus promote the nucleation and growth of quasi-polygonal ferrites. Considering the different testing conditions of hot rolling and thermal simulation experiments, it is now readily to understand why the major phases of hot-rolled steel are composed of quasi-polygonal ferrites rather than thermal simulation tested steels. Owing to the partitioning of the alloy atoms during the growth of the ferrite [24], the excess carbon and solute atoms would concentrate into the retained austenite, causing the retained austenite changing into pearlite or bainite in different transformation temperature ranges (Figs. 5, 10 and 11). However, the formation of pearlite relies on the nucleation of cementite, which also involves diffusion of the alloy atoms. As further decreasing of transformation temperature to Bs, the retained austenite with high carbon concentration transforms into bainite.
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75
75
reduction of 20% reduction of 40%
reduction of 20% reduction of 40% 70
Ferrite amount / %
Ferrite amount / %
70
65
60
65
60
55
55
50
50 0.0012
0.0047
0.0180
0.0012
0.0047
0.0180
RE content / %
RE content / %
Fig. 12. Relationship between ferrite amount and RE content subjected to thermal simulation tests: (a) 0.5 °C/s and (b) 1 °C/s.
RE, together with the other alloying elements, plays an important role in the transformation. The covalent atomic radius of La and Ce are 0.1877 nm and 0.1825 nm respectively, but that of iron is 0.1210 nm. As solute atoms, the diffusion of RE is difficult. Furthermore, our recent work has indicated that RE improves the solubility of Nb in austenite in the pipeline steel, and inhibits the precipitation [25]. The interaction coefficients between RE and C, Nb, V, Ti are also negatives [26]. This could be provided as a further evidence that the addition of RE would impede cementite nucleation, including the diffusion process of carbon and solute atoms. Consequently, with the increase of RE content, the pearlite gradually gets decreased or even disappeared (Figs. 10 and 11). The formation of quasi-polygonal ferrite is also inhibited with increasing the RE content. Though the electrolytic analysis indicates that the solid solubility of RE is limited, our recent work has demonstrated that the free RE increases with increasing the RE content [27]. More recently, it is found that the free RE in austenite is much more than that it is detected after hot-rolled in pipeline steels. Therefore, the bainite transformation benefits from RE additions. In a word, the pearlite content of the hot-rolled steel reduces with increasing RE to its final disappearance, and correspondingly the ferrite amount increases. On the other hand, the bainite amount increases but the ferrite decreases with further increasing RE content. 4.2. Effect of RE content on grain boundary characteristic The formation of quasi-polygonal ferrite is reconstructive transformation process involving diffusion of alloy atoms, and normally the grains can grow freely across the austenite GBs (Figs. 10a and 11a). However, it was here found that, as the RE content is high enough, boundaries occurring inside the quasi-polygonal ferrite have been found (Figs. 10c and 11c). And it was considered to be the prior austenite GBs which did not get disappeared through the transformation, in contrast with other microstructure in this study. However, there is still a lot of work to do to prove it to be prior austenite GBs. McLean and Northcott [28] have indicated that the driving force of whether the solution elements are distributed in grain interiors or at GBs depends on the distortion energy difference. As indicated above, the atomic radius of RE is much larger than that of iron. So, RE should be distributed at GBs theoretically. It also detected the RE enriched at GBs in the other low carbon steels [27,29–31]. The appropriate RE accumulation at GBs is good for boundaries’ cleanness (Fig. 7b), thus its beneficial for the increase of the impact toughness. Besides, the grain growth is also inhibited by the drag-
ging effect of the enriched RE at the GBs, which necessarily leads to fine grains. The corresponding impact fracture surface would exhibit typically ductile characteristics, i.e., dimples (Fig. 4b). However, the ferrite transformation is blocked by too much RE enrichment at GBs. The quasi-polygonal ferrite reconstructive transformation is changed, namely, ferrites are just able to grow towards grain interiors, and unable to get across freely the austenite GBs theoretically (Figs. 10c and 11c). Such a phenomenon is difficult to be detected in hot-rolled steels with high-content ferrite and fine-grained structures. Of the above, the addition of RE makes the alloy elements in the solid solution more stable and the RE is inclined to accumulate at GBs. The excess carbon and solute atoms concentrated at the retained austenite make the austenite much more stable, which postpones the diffusion transformation and makes the martensite transformation occurred. It can be seen that there is some martensite presented as the second-phase at GBs (Fig. 7), which is extremely detrimental for the impact toughness and causes the fracture face becoming quasi-cleavage type.
4.3. Effect of RE content on the impact toughness It is obvious that the addition of the RE has an important effect in inhibiting the pearlite transformation, thus getting the volume fraction of ferrite increased. Generally speaking, the impact toughness of the steel would get increased with the reduction of the pearlite. However, recent research on the relationship between microstructure and impact fracture surface has demonstrated that there might be no direct correlation between the ferrite amount and impact toughness. The impact toughness also correlate well with the grain size and GBs features. An appropriate RE content induces the fine grained structure and clean GBs, which is the reason why the impact toughness of B450NbRE steel increased. Nonetheless, a volume of the martensite precipitates at GBs with excess addition of RE, which is detrimental to the impact toughness [32].
5. Conclusions The microstructure of hot-rolled steels are similar to those of steels subjected to thermal simulation tests, and they are all characterized by polygonal ferrite, quasi-polygonal ferrite, pearlite and bainite. For the two cases, the pearlite content reduces with increasing RE content, and correspondingly the ferrite amount increases. However, the bainite amount increases and the ferrite amount decreases with further increasing RE content. The maximum ferrite amount is 92.4 vol.% with a content of 0.0047 wt.% RE.
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The impact toughness increases with RE content reaching the peak with content of 0.0047 wt.% RE, and then it decreases with further increasing RE content. An appropriate RE content can improve significantly the impact toughness of the B450NbRE steel significantly. The improvement of impact toughness of RE-micoralloyed steels is closely related to the pearlite decreases, fine grained structure changes and GBs cleaning induced by an appropriate RE content. With further increasing RE content to 0.0180 wt.%, the martensite as the secondary phase precipitates at GBs induces a decreased impact toughness of the steel. Acknowledgments This work was financially supported by the Chinese Ministry of Education (109048) and the Ministry of Science and Technology of China (2006BAB02B03 and 2006BAE03A04). The authors are grateful for these supports. References [1] Wilson WG, Kay DAR, Vahed A. Use of thermodynamics and phase equilibriums to predict the behavior of the rare earth elements in steel. J Met 1974;26:14–23. [2] Luyckx L, Bell JR, Mclean A, Korchynsky M. Sulfide shape control in high strength low alloy steels. Metall Trans 1970;1:3341–50. [3] Alan D, Kay R. High temperature thermodynamics and applications of rare earth oxides and sulphides in ferrous metallurgy. Miner Process Extr Metall Rev 1992;10:307–23. [4] Shi LQ, Chen JZ, Northwood DO. Inclusion control in a 16Mn steel using a combined rare earth and calcium treatment. J Mater Eng 1991;13:273–9. [5] Waudby PE. Rare earth additions to steel. Int Metall Rev 1978;23:74–98. [6] Garrison Jr WM, Maloney JL. Lanthanum additions and the toughness of ultrahigh strength steels and the determination of appropriated lanthanum additions. Mater Sci Eng A 2005;403:299–310. [7] Hinton BRW. Corrosion inhibition with rare earth metal salts. J Alloys Compd 1992;180:15–25. [8] Wang C, Jiang F, Wang FH. The characterization and corrosion resistance of cerium chemical conversion coatings for 304 stainless steel. Corros Sci 2004;46:75–89. [9] Zhao TC, Fang YC. Microstructure and mechanical properties of dual-phase weathering steel 09CuPTiRe. J Mater Sci 2004;39:4393–6. [10] Li CL, Wang YS, Chen JJ, Liu CJ, Jiang MF. Effects of rare earth on structure and mechanical properties of chean BNbRE steel. J Rare Earths 2005;23:470–3. [11] Chen JJ, Jiang MF, Li K. Development of B450NbRE climate-resistant steel with high strength. Chin Rare Earths 2006;27:73–5. [12] Liu HL, Liu CJ, Jiang MF. Effects of rare earth elements on thermal simulation microstructure of B450NbRE steel. Chin J Rare Metals 2011;35:53–8.
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