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Rapidly Solidified Steel Droplets with B and P Addition
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J. Mater. Sci. Technol., 2013, 29(10), 995e998
Rapidly Solidified Steel Droplets with B and P Addition Na Li1)*, Junwei Zhang1), Qian Xu2), Lulu Zhai1), Shengli Li1), Jiguang Li1) 1) School of Material and Metallurgy, University of Science and Technology Liaoning, Anshan 114051, China 2) Tianjin Iron and Steel Group Co., Ltd, Tianjin 300301, China [Manuscript received September 3, 2012, in revised form December 4, 2012, Available online 7 June 2013]
Low carbon steels with B and P additions were remelted by electromagnetic levitation and solidified in a vacuum drop tube. The droplet volumes were set to be 2 mm 2 mm 2 mm (TM) and 5 mm 5 mm 5 mm (FM), respectively. The microstructure of rapidly solidified steel droplets (cooled in silicon oil) with P and both B and P addition was observed. The microstructures of B-bearing droplet samples were more uniform than those of B-free ones, for both TM and FM samples. The distribution of C and P along the diameter of each sample was detected. The well-distribution of C and P was detected in B-bearing droplet samples. So it could be deduced that B was also well distributed in the steels. It was B atoms that promoted the well-distribution of C and P, which further improved the uniformity of microstructure under the condition of rapid solidification. The micro-hardness of Bbearing samples was higher than that of B-free samples, and the hardening mechanism was discussed in detail. KEY WORDS: Rapid solidification; Boron; Phosphorus; Droplet; Micro-hardness
1. Introduction Rapid solidification is a significant research subject in the field of material science and condensed physics[1e4] and plays a major role in material engineering and crystal growth[5], which can remarkably increase the solid solution of alloying elements, produce fine microstructures and reduce or eliminate the segregation of alloying elements. However, the segregation of P and C was observed in rapidly solidified strip-casting steel strips[6]. Trace element B is known to affect grain size even when it presents in low concentrations[7,8]. In most instances, it is assumed that element B is concentrated at the grain boundaries[9,10] where it enters into the precipitates to suppress the microcavity formation and the diffusivity along the grain boundaries is decreased. So it is considered to be a beneficial additive which can improve mechanical properties at high temperature[11,12], and a very low concentration of B (in the order of magnitude of 105) may increase considerably the hardenability of low carbon steels[13,14]. The B segregation to grain boundaries is well documented. While the macro-segregation of B and the effect of B on the macrosegregation of other elements in rapidly solidified carbon steels are not clear yet.
Corresponding author. Assoc. Prof., Ph.D.; Tel.: þ86 412 5929535; Fax: þ86 412 5929525; E-mail address: [email protected] (N. Li). 1005-0302/$ e see front matter Copyright Ó 2013, The editorial office of Journal of Materials Science & Technology. Published by Elsevier Limited. All rights reserved. http://dx.doi.org/10.1016/j.jmst.2013.05.012 *
In this work, the steel droplets with the volume of about 2 mm 2 mm 2 mm (TM) and 5 mm 5 mm 5 mm (FM), respectively were solidified rapidly in silicon oil after being remelted in a short drop tube. The characteristic of macrosegregation of C and P in both B-bearing and B-free samples were observed and analysed. 2. Experimental Low carbon steels with P and both B and P addition were prepared by 2 kg-high-frequency vacuum induction furnace, and the composition was listed in Table 1. Small samples with the size of 2 mm 2 mm 2 mm and 5 mm 5 mm 5 mm, respectively were taken from the bulk. All the sides of the small samples were grinded and then cleaned with alcohol. The dry samples were remelted with suspension-type vacuum furnace and the melted droplets were then solidified in silicone oil. Both the vacuum furnace and the silicone oil were placed in vacuum drop tube. The drop heights were set to be about 0.2 m. The TM samples were remelted with containerless processing and the FM samples were remelted in crucibles (since they were too big to be remelted with containerless processing) in the vacuum furnace and solidified in silicone oil together with the crucibles. The schematic diagram of the experimental device is shown in Fig. 1. The microstructures were observed by optical microscopy (Axioskop2); the elements P, C and B were examined by using electron probe micro-analysis (EPMA-1610), and the microhardness of all the samples were measured by using a Vickers micro-hardness tester (AMH43) under a load of 9.8 N and a 10-s holding time.
996
N. Li et al.: J. Mater. Sci. Technol., 2013, 29(10), 995e998
Table 1 Composition of droplet samples (wt%) Sample No.
C
Mn
Si
S
P
B
Fe
1 2
0.035 0.038
0.179 0.181
0.036 0.028
0.004 0.005
0.089 0.097
e 0.0033
Bal. Bal.
3. Results and Discussion 3.1. Microstructural characterization Fig. 2 shows the microstructure of droplet sample TM 1 (Fig. 2(a)), TM 2 (Fig. 2(b)), FM 1 (Fig. 2(c)) and FM 2 (Fig. 2(d)) solidified in drop tube, respectively. The microstructures are mainly fine ferrites. The convection heat transfer outside the melt was ignored because of the vacuum condition in drop tube, and because the volume of the droplets was rather large, it was regarded that the solidification process happened after the liquid drops met the silicone oil. For all the samples, the microstructures near the surface are similar to those at the centre, which indicates that the solidification speed is approximately the same from the surface to the centre. The microstructures of TM samples are rather fine as shown in Fig. 2(a) and (b). The total crystallization latent heat was rather low due to its smaller volume, and the heat was transferred to the cooling medium soon. The formation of crystals and the following growing process were finished in a short time and the microstructures were rather fine. With increasing volume, the total crystallization latent heat, which was directly proportional to the volume of the melt, increased obviously and the size of microstructures of FM samples increases correspondingly as shown in Fig. 2(c) and (d). It can be seen from Fig. 2 that the microstructures of Bbearing samples (Fig. 2(b) and (d)) were more uniform than those of B-free samples (Fig. 2(a) and (c)). For TM samples, the microstructure becomes rather finer by adding a certain amount of B. It was reported that the addition of B can increase the hardenability of steel[15], therefore the addition of trace element B may result in the uniform microstructure and properties. 3.2. Distribution of alloying elements The distribution of C and P were detected along the diameter of each droplet sample, as shown in Figs. 3 and 4. For all the samples, the distributions of both C and P are more uniform in B-bearing samples than those in B-free samples. Fig. 3 shows the distribution profile of C and P along the diameter of the droplets TM 1 (Fig. 3(a)) and TM 2 (Fig. 3(b)). For TM 1, the distributions of both C and P show a tendency of
segregation in the centre and side regions, which reflect the heat transmission condition during solidification and subsequent cooling process. It is presumed that the solute-solvent redistribution comes into being during the dropping and initial solidifying period, which leads to the fairly higher C and P concentration near the surface of the droplets. Then the solute contents decrease because of the high solidification rate, followed by the centre segregation of solutes. So the distributions of C and P in sample TM 1 are rather sensitive to the outer solidification conditions. With the same relative intensity as vertical coordinate, it is obviously shown that B atoms can promote the uniformity of both C and P in sample TM 2 as shown in Fig. 3(b), which is also corresponding to the finer and more uniform microstructure shown in Fig. 2. In B-bearing cast steels, an amount of borides and borocarbides were observed[16]. So the well-distributed borides or borocarbide may act as the crystal nucleus during the solidification process and then lead to the finer and more uniform microstructure. Sample TM 2 with proper B addition becomes not so sensitive to the cooling conditions. It also should be noticed that, the distributions of C and P become more uniform when the volume of the samples increased by comparing those in TM samples (Fig. 3) and FM ones (Fig. 4), respectively. In FM samples the concentration variation is not so obvious as that in TM samples, and there is only slight segregation in sample FM 1. For FM samples, the latent heat of solidification increases obviously with the sample volume increased, which lead to a less sensitive system compared with TM samples during rapid solidification. Meanwhile, the distribution of C and P in B-bearing FM samples (Fig. 4(b)) shows also more uniform than that in B-free TM samples (Fig. 3(b)). The effects of B on the distributions of C and P in rapidly solidified steel samples are also notable as shown in Fig. 4 when the volumes of the samples are increased. Thus B may affect the elements distributions and microstructures by influencing the latent heat of solidification, the heat transfer rate or both. The segregation of B in cast steels was investigated by means of particle-tracking autoradiography (PTA)[15,17] and secondary ion mass spectrometry (SIMS)[18] techniques even though only trace amount of B element was added in the steels. The results indicated that the equilibrium segregation of boron disappeared during high temperature austenization, while the non-equilibrium segregation along grain boundaries of a quenched boron steel occurred during cooling from a higher temperature by diffusion from their neighbouring regions, because the density of B detected in the grains was nearly equal to that at the grain boundaries and its neighbouring regions. So it can be deduced that B distributed uniformly in the steel at high temperature even though some B may segregate at the grain boundaries at room temperature. Meanwhile, B also can improve the uniform distribution of other elements such as C and P in steel under the condition of rapid solidification. 3.3. Micro-hardness
Fig. 1 Schematic diagram of the vacuum drop tube.
The micro-hardness of the original cast samples and all the TM and FM samples were measured and the average test results are shown in Table 2. It can be seen from Table 2 that the micro-hardness of rapidly solidified samples is much higher than that of original cast samples. The micro-hardness of FM samples with larger volume shows lower values than that of TM samples. Under each condition, the micro-hardness of B-bearing samples is higher than that of B-free samples, while this tendency is weakened as the volume of droplet samples increases. So adding only trace amount of B in
N. Li et al.: J. Mater. Sci. Technol., 2013, 29(10), 995e998
997
Fig. 2 Microstructures of droplet sample TM 1 (a), TM 2 (b), FM 1 (c) and FM 2 (d).
Fig. 3 Distribution of C and P from one surface to another throughout the droplet samples TM 1 (a) and TM2 (b).
Fig. 4 Distribution of C and P from one surface to another throughout the droplet samples FM 1 (a) and FM 2 (b).
998
N. Li et al.: J. Mater. Sci. Technol., 2013, 29(10), 995e998
Table 2 Micro-hardness of each sample (HV) Sample No.
Cast
TM
FM
1 2
118.22 135.31
157.33 184.15
150.49 159.86
(4) The enhanced micro-hardness of B-bearing samples may be due to the precipitation of carbides and/or borides and the solid solution of more C and P as a result of competing with the local position at grain boundaries with B.
the steel can strengthen the material, even under the rapidly solidified conditions. The investigation on B-bearing cast steel has discovered that the addition of B element in the low carbon steel can form the borides having high hardness and improve the wear resistance of low carbon steel[19,20]. B is easy to segregate at the grain boundaries even with a trace amount. It is well known that C and P also prefer to segregate at the grain boundaries in steel, so all the elements, B, C and P may compete for the local positions at the grain boundaries. B promotes the well-distribution of both C and P as mentioned above, which means that the precipitations that contain either C, B and P are uniform, and there must be a certain amount of C, B and P which well-distributed in the grains. The carbides and borides precipitated at grain boundaries can enhance the grain boundary hardening significantly[10,11]. Meanwhile, the solid solution of more C and P as the results of B addition also can increase the micro-hardness of the steel. The enhancing result may be affected by the solidification and cooling conditions more or less.
Acknowledgements This work was supported by the National Natural Science Foundation of China (No. 51074210), and the Open Foundation of Key Laboratory of Materials Forming and Microstructure & Properties Control, Colleges and Universities in Liaoning Province (USTLKL2012-01).
4. Conclusions
[10]
(1) Vacuum drop tube with the height of 0.2 m was used to gain rapidly solidified droplet steel samples with different volume and P, B addition. The microstructures were observed and the distribution of C and P was detected. (2) The solidification speed was increased greatly when the volume of samples decreased, which showed rather fine microstructures. The microstructures of B-bearing droplet samples were more uniform than those of B-free ones, for both TM (2 mm 2 mm 2 mm) and FM (5 mm 5 mm 5 mm) samples. (3) C and P were well-distributed in B-bearing droplet samples than in B-free ones for both TM and FM samples. It indicated that B atoms promoted the uniformity of other alloying elements such as C and P.
REFERENCES [1] [2] [3] [4] [5] [6] [7] [8] [9]
[11] [12] [13] [14] [15] [16] [17] [18] [19] [20]
H.J. Trodahl, Adv. Mater. 13 (2001) 1031e1033. P.K. Galenko, D.A. Danilov, Phys. Rev. E 69 (2004) 051608. Y. Ruan, F.P. Dai, B.B. Wei, Chin. Sci. Bull. 52 (2007) 2630e2635. M. Leonhardt, W. Löser, H.G. Lindenkreuz, Acta Mater. 50 (2002) 725e734. M.C. Flemings, Metal. Trans. 5 (1974) 2121e2134. N. Li, Z.Y. Liu, G.P. Zhou, X.H. Liu, G.D. Wang, Int. J. Miner. Metall. Mater. 17 (2010) 417e422. Y.R. Cho, B.S. Seong, Iron Steel Technol. 1 (2004) 46e51. M.H. Baarman, Scand. J. Metall. 27 (1998) 148e158. B.J. Thomas, G. Henry, Boron in Steel, Metall. Soc. of AIME, Warrendale, PA, 1980, pp. 80e83. S.Y. Cui, Z.X. Zhang, Y.L. Xu, J. Li, X.S. Xiao, C.C. Zhu, J. Mater. Sci. Technol. 27 (2011) 1059e1064. X.X. Yao, Mater. Sci. Eng. A 271 (1999) 353e359. N. Li, W.R. Sun, Y. Xu, S.R. Guo, D.Z. Lu, Z.Q. Hu, Mater. Lett. 60 (2006) 2232e2235. R.A. Grange, T.M. Garvey, Trans. ASM 37 (1946) 136e141. H.K.D.H. Bhadeshia, J.W. Christian, Metall. Trans. A 21 (1990) 767e797. X.L. He, Y.Y. Chu, X.L. Zhang, Z.S. Yu, Q.P. Li, X.G. Yin, Acta Metal. Sin. 13 (1977) 235e245 (in Chinese). H.G. Fu, Z.H. Li, Z.Q. Jiang, J.D. Xing, Mater. Lett. 61 (2007) 4504e4507. Y.Y. Chu, X.L. He, Z.S. Yu, Y.G. Ou, Acta Metall. Sin. 16 (1980) 380e385 (in Chinese). L. Karlsson, H. Nordén, H. Odelius, Acta Metall. 36 (1988) 1e12. H.G. Fu, Z.Q. Jiang, Acta Metall. Sin. 42 (2006) 545e548 (in Chinese). X.D. Song, Z.Q. Jiang, H.G. Fu, Foundry Technol. 27 (2006) 805e808.
J. Mater. Sci. Technol., 2013, 29(10), 995e998
Rapidly Solidified Steel Droplets with B and P Addition Na Li1)*, Junwei Zhang1), Qian Xu2), Lulu Zhai1), Shengli Li1), Jiguang Li1) 1) School of Material and Metallurgy, University of Science and Technology Liaoning, Anshan 114051, China 2) Tianjin Iron and Steel Group Co., Ltd, Tianjin 300301, China [Manuscript received September 3, 2012, in revised form December 4, 2012, Available online 7 June 2013]
Low carbon steels with B and P additions were remelted by electromagnetic levitation and solidified in a vacuum drop tube. The droplet volumes were set to be 2 mm 2 mm 2 mm (TM) and 5 mm 5 mm 5 mm (FM), respectively. The microstructure of rapidly solidified steel droplets (cooled in silicon oil) with P and both B and P addition was observed. The microstructures of B-bearing droplet samples were more uniform than those of B-free ones, for both TM and FM samples. The distribution of C and P along the diameter of each sample was detected. The well-distribution of C and P was detected in B-bearing droplet samples. So it could be deduced that B was also well distributed in the steels. It was B atoms that promoted the well-distribution of C and P, which further improved the uniformity of microstructure under the condition of rapid solidification. The micro-hardness of Bbearing samples was higher than that of B-free samples, and the hardening mechanism was discussed in detail. KEY WORDS: Rapid solidification; Boron; Phosphorus; Droplet; Micro-hardness
1. Introduction Rapid solidification is a significant research subject in the field of material science and condensed physics[1e4] and plays a major role in material engineering and crystal growth[5], which can remarkably increase the solid solution of alloying elements, produce fine microstructures and reduce or eliminate the segregation of alloying elements. However, the segregation of P and C was observed in rapidly solidified strip-casting steel strips[6]. Trace element B is known to affect grain size even when it presents in low concentrations[7,8]. In most instances, it is assumed that element B is concentrated at the grain boundaries[9,10] where it enters into the precipitates to suppress the microcavity formation and the diffusivity along the grain boundaries is decreased. So it is considered to be a beneficial additive which can improve mechanical properties at high temperature[11,12], and a very low concentration of B (in the order of magnitude of 105) may increase considerably the hardenability of low carbon steels[13,14]. The B segregation to grain boundaries is well documented. While the macro-segregation of B and the effect of B on the macrosegregation of other elements in rapidly solidified carbon steels are not clear yet.
Corresponding author. Assoc. Prof., Ph.D.; Tel.: þ86 412 5929535; Fax: þ86 412 5929525; E-mail address: [email protected] (N. Li). 1005-0302/$ e see front matter Copyright Ó 2013, The editorial office of Journal of Materials Science & Technology. Published by Elsevier Limited. All rights reserved. http://dx.doi.org/10.1016/j.jmst.2013.05.012 *
In this work, the steel droplets with the volume of about 2 mm 2 mm 2 mm (TM) and 5 mm 5 mm 5 mm (FM), respectively were solidified rapidly in silicon oil after being remelted in a short drop tube. The characteristic of macrosegregation of C and P in both B-bearing and B-free samples were observed and analysed. 2. Experimental Low carbon steels with P and both B and P addition were prepared by 2 kg-high-frequency vacuum induction furnace, and the composition was listed in Table 1. Small samples with the size of 2 mm 2 mm 2 mm and 5 mm 5 mm 5 mm, respectively were taken from the bulk. All the sides of the small samples were grinded and then cleaned with alcohol. The dry samples were remelted with suspension-type vacuum furnace and the melted droplets were then solidified in silicone oil. Both the vacuum furnace and the silicone oil were placed in vacuum drop tube. The drop heights were set to be about 0.2 m. The TM samples were remelted with containerless processing and the FM samples were remelted in crucibles (since they were too big to be remelted with containerless processing) in the vacuum furnace and solidified in silicone oil together with the crucibles. The schematic diagram of the experimental device is shown in Fig. 1. The microstructures were observed by optical microscopy (Axioskop2); the elements P, C and B were examined by using electron probe micro-analysis (EPMA-1610), and the microhardness of all the samples were measured by using a Vickers micro-hardness tester (AMH43) under a load of 9.8 N and a 10-s holding time.
996
N. Li et al.: J. Mater. Sci. Technol., 2013, 29(10), 995e998
Table 1 Composition of droplet samples (wt%) Sample No.
C
Mn
Si
S
P
B
Fe
1 2
0.035 0.038
0.179 0.181
0.036 0.028
0.004 0.005
0.089 0.097
e 0.0033
Bal. Bal.
3. Results and Discussion 3.1. Microstructural characterization Fig. 2 shows the microstructure of droplet sample TM 1 (Fig. 2(a)), TM 2 (Fig. 2(b)), FM 1 (Fig. 2(c)) and FM 2 (Fig. 2(d)) solidified in drop tube, respectively. The microstructures are mainly fine ferrites. The convection heat transfer outside the melt was ignored because of the vacuum condition in drop tube, and because the volume of the droplets was rather large, it was regarded that the solidification process happened after the liquid drops met the silicone oil. For all the samples, the microstructures near the surface are similar to those at the centre, which indicates that the solidification speed is approximately the same from the surface to the centre. The microstructures of TM samples are rather fine as shown in Fig. 2(a) and (b). The total crystallization latent heat was rather low due to its smaller volume, and the heat was transferred to the cooling medium soon. The formation of crystals and the following growing process were finished in a short time and the microstructures were rather fine. With increasing volume, the total crystallization latent heat, which was directly proportional to the volume of the melt, increased obviously and the size of microstructures of FM samples increases correspondingly as shown in Fig. 2(c) and (d). It can be seen from Fig. 2 that the microstructures of Bbearing samples (Fig. 2(b) and (d)) were more uniform than those of B-free samples (Fig. 2(a) and (c)). For TM samples, the microstructure becomes rather finer by adding a certain amount of B. It was reported that the addition of B can increase the hardenability of steel[15], therefore the addition of trace element B may result in the uniform microstructure and properties. 3.2. Distribution of alloying elements The distribution of C and P were detected along the diameter of each droplet sample, as shown in Figs. 3 and 4. For all the samples, the distributions of both C and P are more uniform in B-bearing samples than those in B-free samples. Fig. 3 shows the distribution profile of C and P along the diameter of the droplets TM 1 (Fig. 3(a)) and TM 2 (Fig. 3(b)). For TM 1, the distributions of both C and P show a tendency of
segregation in the centre and side regions, which reflect the heat transmission condition during solidification and subsequent cooling process. It is presumed that the solute-solvent redistribution comes into being during the dropping and initial solidifying period, which leads to the fairly higher C and P concentration near the surface of the droplets. Then the solute contents decrease because of the high solidification rate, followed by the centre segregation of solutes. So the distributions of C and P in sample TM 1 are rather sensitive to the outer solidification conditions. With the same relative intensity as vertical coordinate, it is obviously shown that B atoms can promote the uniformity of both C and P in sample TM 2 as shown in Fig. 3(b), which is also corresponding to the finer and more uniform microstructure shown in Fig. 2. In B-bearing cast steels, an amount of borides and borocarbides were observed[16]. So the well-distributed borides or borocarbide may act as the crystal nucleus during the solidification process and then lead to the finer and more uniform microstructure. Sample TM 2 with proper B addition becomes not so sensitive to the cooling conditions. It also should be noticed that, the distributions of C and P become more uniform when the volume of the samples increased by comparing those in TM samples (Fig. 3) and FM ones (Fig. 4), respectively. In FM samples the concentration variation is not so obvious as that in TM samples, and there is only slight segregation in sample FM 1. For FM samples, the latent heat of solidification increases obviously with the sample volume increased, which lead to a less sensitive system compared with TM samples during rapid solidification. Meanwhile, the distribution of C and P in B-bearing FM samples (Fig. 4(b)) shows also more uniform than that in B-free TM samples (Fig. 3(b)). The effects of B on the distributions of C and P in rapidly solidified steel samples are also notable as shown in Fig. 4 when the volumes of the samples are increased. Thus B may affect the elements distributions and microstructures by influencing the latent heat of solidification, the heat transfer rate or both. The segregation of B in cast steels was investigated by means of particle-tracking autoradiography (PTA)[15,17] and secondary ion mass spectrometry (SIMS)[18] techniques even though only trace amount of B element was added in the steels. The results indicated that the equilibrium segregation of boron disappeared during high temperature austenization, while the non-equilibrium segregation along grain boundaries of a quenched boron steel occurred during cooling from a higher temperature by diffusion from their neighbouring regions, because the density of B detected in the grains was nearly equal to that at the grain boundaries and its neighbouring regions. So it can be deduced that B distributed uniformly in the steel at high temperature even though some B may segregate at the grain boundaries at room temperature. Meanwhile, B also can improve the uniform distribution of other elements such as C and P in steel under the condition of rapid solidification. 3.3. Micro-hardness
Fig. 1 Schematic diagram of the vacuum drop tube.
The micro-hardness of the original cast samples and all the TM and FM samples were measured and the average test results are shown in Table 2. It can be seen from Table 2 that the micro-hardness of rapidly solidified samples is much higher than that of original cast samples. The micro-hardness of FM samples with larger volume shows lower values than that of TM samples. Under each condition, the micro-hardness of B-bearing samples is higher than that of B-free samples, while this tendency is weakened as the volume of droplet samples increases. So adding only trace amount of B in
N. Li et al.: J. Mater. Sci. Technol., 2013, 29(10), 995e998
997
Fig. 2 Microstructures of droplet sample TM 1 (a), TM 2 (b), FM 1 (c) and FM 2 (d).
Fig. 3 Distribution of C and P from one surface to another throughout the droplet samples TM 1 (a) and TM2 (b).
Fig. 4 Distribution of C and P from one surface to another throughout the droplet samples FM 1 (a) and FM 2 (b).
998
N. Li et al.: J. Mater. Sci. Technol., 2013, 29(10), 995e998
Table 2 Micro-hardness of each sample (HV) Sample No.
Cast
TM
FM
1 2
118.22 135.31
157.33 184.15
150.49 159.86
(4) The enhanced micro-hardness of B-bearing samples may be due to the precipitation of carbides and/or borides and the solid solution of more C and P as a result of competing with the local position at grain boundaries with B.
the steel can strengthen the material, even under the rapidly solidified conditions. The investigation on B-bearing cast steel has discovered that the addition of B element in the low carbon steel can form the borides having high hardness and improve the wear resistance of low carbon steel[19,20]. B is easy to segregate at the grain boundaries even with a trace amount. It is well known that C and P also prefer to segregate at the grain boundaries in steel, so all the elements, B, C and P may compete for the local positions at the grain boundaries. B promotes the well-distribution of both C and P as mentioned above, which means that the precipitations that contain either C, B and P are uniform, and there must be a certain amount of C, B and P which well-distributed in the grains. The carbides and borides precipitated at grain boundaries can enhance the grain boundary hardening significantly[10,11]. Meanwhile, the solid solution of more C and P as the results of B addition also can increase the micro-hardness of the steel. The enhancing result may be affected by the solidification and cooling conditions more or less.
Acknowledgements This work was supported by the National Natural Science Foundation of China (No. 51074210), and the Open Foundation of Key Laboratory of Materials Forming and Microstructure & Properties Control, Colleges and Universities in Liaoning Province (USTLKL2012-01).
4. Conclusions
[10]
(1) Vacuum drop tube with the height of 0.2 m was used to gain rapidly solidified droplet steel samples with different volume and P, B addition. The microstructures were observed and the distribution of C and P was detected. (2) The solidification speed was increased greatly when the volume of samples decreased, which showed rather fine microstructures. The microstructures of B-bearing droplet samples were more uniform than those of B-free ones, for both TM (2 mm 2 mm 2 mm) and FM (5 mm 5 mm 5 mm) samples. (3) C and P were well-distributed in B-bearing droplet samples than in B-free ones for both TM and FM samples. It indicated that B atoms promoted the uniformity of other alloying elements such as C and P.
REFERENCES [1] [2] [3] [4] [5] [6] [7] [8] [9]
[11] [12] [13] [14] [15] [16] [17] [18] [19] [20]
H.J. Trodahl, Adv. Mater. 13 (2001) 1031e1033. P.K. Galenko, D.A. Danilov, Phys. Rev. E 69 (2004) 051608. Y. Ruan, F.P. Dai, B.B. Wei, Chin. Sci. Bull. 52 (2007) 2630e2635. M. Leonhardt, W. Löser, H.G. Lindenkreuz, Acta Mater. 50 (2002) 725e734. M.C. Flemings, Metal. Trans. 5 (1974) 2121e2134. N. Li, Z.Y. Liu, G.P. Zhou, X.H. Liu, G.D. Wang, Int. J. Miner. Metall. Mater. 17 (2010) 417e422. Y.R. Cho, B.S. Seong, Iron Steel Technol. 1 (2004) 46e51. M.H. Baarman, Scand. J. Metall. 27 (1998) 148e158. B.J. Thomas, G. Henry, Boron in Steel, Metall. Soc. of AIME, Warrendale, PA, 1980, pp. 80e83. S.Y. Cui, Z.X. Zhang, Y.L. Xu, J. Li, X.S. Xiao, C.C. Zhu, J. Mater. Sci. Technol. 27 (2011) 1059e1064. X.X. Yao, Mater. Sci. Eng. A 271 (1999) 353e359. N. Li, W.R. Sun, Y. Xu, S.R. Guo, D.Z. Lu, Z.Q. Hu, Mater. Lett. 60 (2006) 2232e2235. R.A. Grange, T.M. Garvey, Trans. ASM 37 (1946) 136e141. H.K.D.H. Bhadeshia, J.W. Christian, Metall. Trans. A 21 (1990) 767e797. X.L. He, Y.Y. Chu, X.L. Zhang, Z.S. Yu, Q.P. Li, X.G. Yin, Acta Metal. Sin. 13 (1977) 235e245 (in Chinese). H.G. Fu, Z.H. Li, Z.Q. Jiang, J.D. Xing, Mater. Lett. 61 (2007) 4504e4507. Y.Y. Chu, X.L. He, Z.S. Yu, Y.G. Ou, Acta Metall. Sin. 16 (1980) 380e385 (in Chinese). L. Karlsson, H. Nordén, H. Odelius, Acta Metall. 36 (1988) 1e12. H.G. Fu, Z.Q. Jiang, Acta Metall. Sin. 42 (2006) 545e548 (in Chinese). X.D. Song, Z.Q. Jiang, H.G. Fu, Foundry Technol. 27 (2006) 805e808.