- Email: [email protected]
Effect of inclusions on the formation of acicular ferrite in Ti-bearing non quenched-and-tempered steel
Journal of University of Science and Technology Beijing Volume 14, Number 6, December 2007, Page 501
Materials
Effect of inclusions on the formation of acicular ferrite in Ti-bearing non quenched-and-temperedsteel Ying Yang'*",Chaobin
Fuming Wang'),Zhanbing Yang'), and Bo Song')
1) Metallurgical and Ecological Engineering School, University of Science and Technology Beijing, Beijing 100083, China 2) Technology Centre, Anshan Iron and Steel Group Corporation, Anshan 114009, China
3) Technology Centre, Xinyu Iron and Steel Group, Xinyu 336501, China (Received 2006-04-06)
Abstract: Nucleation of acicular ferrite and its influence factors in non quenched-and-tempered steel was studied by using TEM and thermodynamic calculation. The results show that the complex particles with a center made of Ti oxide, A1,0,, and silicate and an outside made of a small quantity of mixture of TiN and MnS are able to act as femte nucleation nuclei. The acicular ferrite percentage changes little with Ti. When the oxygen content was 80 ppm, the volume percentage of acicular ferrite decreased due to an increase in allotriomorphic ferrite. The larger the cooling rate and the shorter the incubation time, the finer the titanium oxide and the higher the nucleation ratio of acicular femte.
Key words: acicular ferrite; titanium; inclusions; non quenched-and-tempered steel
[This work was financialfysupported by the National Natural Science Foundation of China (N0.50574010) and the National Doctorate Fund of the Ministry of Education of China (N0.20060008015).]
1. Introduction Non quenched-and-tempered steel has advantages of saving energy and materials, low cost, and good properties, therefore, it is widely used to make parts like connecting rod and cranked axle with good strength and toughness in automobile and tractor machine-making industry. Non quenched-and-tempered steel is named as "green steel" due to its saving energy, high efficiency, and environmental protection. However, one apparent defect for this kind of steel is the deficiency in toughness which limits its applications. Recently, it has been reported that interlocking acicular ferrite with high angle boundary can enhance the strength and toughness of the steel with refined grain size [ 1-21. Acicular ferrite nucleates mainly at the surface of nonmetallic inclusions. Many inclusions can act as heterogeneous nucleation sites for acicular ferrite, such as TiN [3], Al-rich inclusions [4-61, MnS [4], and titanium oxides [7-111, among which Ti,03 is the most effective one [7-81. Barbaro [12] reported that the main influencing factors on the formation of acicular ferrite are the distribution, size, and type of inclusions and the size of prior austenite grains. And a11 these factors are decided by cooling rate and the Corresponding author: Fuming Wang, E-mail: [email protected]
composition of the steel. In the present work, the types of inclusions and the nucleation of acicular ferrite have been discussed on the basis of TEM analysis and thermodynamic calculation. Systematic experiments have been conducted to study the influence of Ti, 0, and the size distribution of inclusions on the formation of acicular ferrite.
2. Experimental procedures The standard for microalloyed non quenched-andtempered steel [I31 was referred for designing the chemical compositions of experimental steels. Different steel samples were made in a Si-Mo furnace of high temperature. The ingots of No.1-No.3 and No.4No.7 with a size of @45mmx80 mm were obtained by quenching the crucibles with different samples directly from 1600and 1200°C using water, respectively. The chemical compositions of the experimental steels are shown in Table 1. The ingots were forged into bars of 910 mm with the process of starting forging temperature at 12OO0C, final forging temperature at 950"C, and air cooling at the rate of about 40"C/min. The specimens were cut from the ingots and their microstructures were examAlso available online a t www.sciencedireet.com
J. Univ. Sci. Technol. Beijing, Vo1.14,No.6, Dec 2007
502
croscope to study the shape and size of the particles. The EDS analysis was used to study the compositions of the particles in order to make semi-quantitative analysis of the phase (the Cu peaks in the spectrum came from the Cu grid used to prepare the carbon replica).
ined by optical microscope and scanning electron microscope (SEM) after being polished and etched in 3~01%nital. Extracted carbon replicas were made from the specimens of 2' and 5' which were quenched directly from 1600 and 1200°C, respectively, and were analyzed in a high-resolution transmission electron mi-
Table 1. Chemical compositions of the experimental steels No. 1" 2' 3" 4' 5"
6" 7"
C
0.32 0.32 0.37 0.33 0.35 0.33 0.35
S
0.045 0.046 0.042 0.045 0.046 0.043 0.045
Si
0.20 0.21 0.20 0.20 0.19 0.23 0.20
Mn 1.20 1.30 1.20 1.20 1.30 1.22 1.25
P 0.016 0.015 0.016 0.016 0.016 0.016 0.016
3. Results and discussion 3.1. Analysis of the precipitates containing Ti in the steels In practice, A1 is unavoidable in molten steel. The phase stability diagram calculation of the Al-Ti-0 system in molten iron was carried out by using the software of MATLAB to confirm the possibility of the formation of all kinds of inclusions. Fig. 1 shows that neither T i 0 nor TiO, can form at 1873 K, the final stable inclusion should be A120,.Ti0, within this composition range for the studied system. Titanium oxides, aluminum oxides, and silicates form the complex inclusions due to the existence of some elements in the steels, such as Si and Mn.
Also,
Ti
0.0020 0.0020 <0.0005 0.0020 0.0020 0.0028
-
-
0.0083 <0.0050 -
0.0160 0.0250 0.0400
wt% [OISO,
N
0.0038 0.0038 0.0075 0.0040 0.0035 0.0038 0.0038
0.0070 0.0070 0.0070 0.0070 0.0070 0.0070 0.0070
spherical in shape, as shown in Figs. 2 and 3. Figs. 2(b)-(d) and Figs. 3(b)-(d) are the EDS spectra of the corresponding sites in Figs. 2(a) and 3(a), respectively. In Fig. 2, the particle is about 300 nm, the core of the particle is titanium oxide, site B is the complex phase composed of titanium oxide, a small amount of aluminum oxide and manganese sulfide. And the surface of the particle is MnS, as shown at site C. All above shows that titanium oxide and a little aluminum oxide are at the center of most of the complex inclusions, with manganese sulfide at the surface at 1600°C. Fig. 3 shows another complex particle, with a size of about 200 nm. The center of the particle consists of the complex inclusions of titanium oxide, aluminum oxide, and silicate; sites B and C are titanium nitride and manganese sulfide, respectively. Since the specimen was quenched from 1200°C, the titanium nitride precipitated during the solidification. And it is obvious that manganese sulfide and titanium nitride precipitate on the complex inclusions of titanium oxide, aluminum oxide, and silicate in the temperature range above 12oo0c,
3.2. Nucleus formation of acicular ferrite (AF)
Fig. 1. Phase stability diagram of the Fe-Al-Ti-0 system in molten iron at 1873 K.
Typical complex particles in the specimens being quenched directly from 1600 and 12OO0C,respectively, were observed by TEM, which were found to be
The microstructure of the forged sample was analyzed by SEM. Fig. 4 shows the particle and its EDS spectrum, which is the nucleus of AF. The particle consists of titanium oxide, aluminum oxide, complex silicate, a small of amount of manganese sulfide, and titanium nitride, with a size of 0.6 pm. As shown in Fig. 4, the inclusion is located at the crossing of 4 ferrite plates, so the nonmetallic inclusion containing titanium can serve as the nucleus of AF.
3.3. Influence of titanium oxide on the microstruc-
...
Y. Yang et al., Effect of inclusions on the formation of acicular ferrite in Ti-bearing
ture of the nonquenched and tempered steel Fig. 5 shows the microstructures of samples 1'-7'. Figs. 5(a) and (d) are the microstructures of the specimens without Ti addition, most of the ferrites form on the boundaries of prior austenite grains. The others in Fig. 5 are the microstructures of Ti-bearing steels, and most of the microstructures are fine interlocking acicular ferrites.
3.4. Influence factors on the formation of AF (1) Influence of Ti on the formation of AF. The volume fraction of AF shown in Fig. 5 are obtained by quantitative determination. The volume fractions of AF for samples 5' (0.016wt% Ti), 6' (0.025wt% Ti), and 7' (0.040wt% Ti) are 44~01% 43vol%, and 46vol%, respectively. It has no obvious difference among them, therefore, the change of Ti content in the steels has little influence on the amount of AF when the oxygen content in the studied steels are definite and limited. The amount of nucleated inclusions and the prior austenite grain size are two important parameters for controlling the formation of acicular ferrite [12]. First, an increase in Ti content enhances the amount of inclusions, which in turn enhances the heterogeneous nucleation sites within the austenite grains and promotes the formation of AF. Second, an increase in Ti content makes the prior austenite grains finer. Although the volume fraction of titanium oxide is enough to induce the nucleation of AF, the prior austenite grains are too fine to form AF
503
[14]. The inclusion like TiN pins the boundaries of prior austenite grains to limit its growth, which is unfavorable for the formation of AF. Therefore, the quantity of AF in samples 5#, 6' and 7' has no obvious difference. (2) Influence of 0 on AF. Samples 2' (0,38 ppm) and 3' (0,75 ppm) were quenched directly from 1600°C. The volume fraction of AF in the above samples are 68~01%and 3vol%, respectively. When 0 is about 80 pprn and Ti is very low, the volume fraction decreases sharply, allotriomorphic ferrites precipitate on the boundaries of prior austenite grains. The number of nucleation sites on the boundaries or within the prior austenite grains is an important factor for the formation of AF and allotriomorphic ferrite. Small quantities of AF were formed in sample 3' since the 0 content is 75 ppm while Ti is very low, and the prior austenite grain is finer [15-161. The decrease in the quantity of titanium oxide is favorable for the formation of allotriomorphic ferrites on the boundaries of prior austenite grains and then it holds back the formation of AF at the surface of inclusions. In addition, TiN can promote the nucleation of AF [13]. In the present work, the content of N is very low. High 0 and low N are unfavorable for the formation of AF. Because most of Ti and 0 react to reach equilibrium, while the remaining Ti can not react with N to form TiN at the solidification temperature, the nucleation rate of AF decreases.
Fig. 2. Complex particle and its EDS analysis at different positions extracted from sample 5: (a) TEM micrograph; (b) EDS spectrum of site A; (c) EDS spectrum of site B; (d) EDS spectrum of site C.
J. Univ. Sci. Technol. Beijing, Vo1.14, N0.6, Dec 2007
504
Fig. 3. Complex particle and its EDS analysis at different positions extracted from sample 5: (a) TEM micrograph; (b) EDS spectrum of site A; (c) EDS spectrum of site B; (d) EDS spectrum of site C.
Fig. 4. Secondary electron image of the precipitationnucleus of AF (a) and EDS of position A (b) for sample 5.
(3) Influence of the size distribution of inclusions on AF. Sample 2' was quenched directly from 1600°C. The volume fraction of AF is 68vol%, which is more than those of samples 5' (44vol%), 6' (43~01%)and 7' (46vol%), respectively, hence it can be shown that the size distribution of inclusions has effect on the amount of AF. With an increase in cooling rate, the degree of super-cooling increases and the incubation time decreases, therefore, the quantity of the oxides increases and their size decreases, and an increased precipitation of MnS is observed. With slow cooling of liquid steel, the oxides become bigger and their quantity decreases.
In the present work, samples 5'-7' were furnace cooled to 1200°C and then the inclusions formed at 1200°C were not as dispersed and fine as those in the samples rapidly being cooled from 1600°C. And segregations occur in the samples being furnace cooled. Since AF formation requires very fine and higher quantities of deoxidation products with uniform distribution in every austenite grain [17], the quantity of AF in samples 5'-7' is relatively lower than that in sample 2'.
4. Conclusions (1) In the samples quenched at 1600"C, most of the
...
Y. Yung et ul., Effect of inclusions on the formation of acicular ferrite in Ti-bearing
particles are complex inclusions with a core of titanium oxide and aluminum oxide and a surface of manganese sulfide. In the samples quenched at 1200°C, the core of complex inclusions of titanium oxide, alu-
505
minum oxide, and silicate, and the surface of a small quantity of titanium nitride and manganese sulfide are observed. The nuclei of AF are mainly these complex inclusions.
Fig. 5. Optical micrographs for samples 1'-7#.
(2) With the addition of Ti in the steels, AF is formed, instead of ferrite nets along the prior austenite. (3) With Ti increase in the steels, the amount of AF has no obvious change. When the oxygen content is about 80 ppm, acicular ferrites decrease due to the increase in allotriomorphic ferrites. The cooling rate and incubation time have great influence on the distribution of nucleation particles.
References [I] J. Yin and C. Xu, Ferrite formation at intragranular MnS
in mould cast non-quenched and tempered steels, Heat Treat. (in Chinese), I9(2004),No.1, p.18. [2] S. Ohkita, Improvement of HAZ toughness in HSLA steel by introducing finely dispersed Ti-oxide, Weld, Res. Suppl., 5(1987),No.10, p.301. [3] X.J. Zhuo, H.S. Kim, Y.B. Kang, e f al., Nucleation of intragranular ferrite of manganese titanium oxide and MnS inclusions in Si-Mn-Ti deoxidized steel, [in] CSM 2005 Annual Meeting Proceeding (in Chinese), CSM, Beijing, 2005, p.687. [4] Z. Zhang and R.A. Farrar, Role of nonmetallic inclusions in formation of acicular ferrite in low alloy weld metals, Mater. Sci. Technol., 12(1996),p.237.
506 [5] A.R. Bhatti, M.E. Saggese, J.A. Whiteman, et al., Analysis of inclusions in submerged arc welds in microalloyed steels, Weld. J., 63(1984), p.224. [6] J.M. Dowling, J.M. Corbett, and H.W. Ken, Inclusion phases and nucleation of acicular ferrite in submerged arc welds in high strength low alloy steels, Metall. Trans. A , 17A(1986), p.1611. [7] J.-S. Byun, J.-H. Shim, Y.W. Cho, et al., Non-metallic inclusion and intra-granular nucleation of ferrite in Tikilled C-Mn steel, Acta Muter., 51(2003), No.4, p.1593. [8] Z.T. Ma and D. J a k e , Characteristics of oxide precipitation and growth during solidification of deoxidized steel, ZSZJ Znt., 38( 1998), No.1, p.46. [9] J.H. Shim, Y.J. Oh, J.Y. Suh, et al., Ferrite nucleation potency of nonmetallic inclusions in medium carbon steel, Acta Muter., 49(2001), p.2115. [lo] J.H. Shim, Y.W. Cho, and S.H. Chung, Nucleation of intragranular ferrite at Ti,O, particle in low carbon steel, Acta Muter., 47( 1999), No.9, p.275 1.
J. Univ. Sci. Technol. Beijing, Vo1.14, N0.6, Dec 2007 [I I] J.S. Byun, J.H. Shim, J.Y. Suh, et al., Inoculated acicular ferrite microstructure and mechanical properties, Muter. Sci. Eng., A319-321(2001), p.326. [12] F.J. Barbaro, P. Krauklis, and K.E. Easterling, Formation of acicular ferrite at oxide particle in steels, Muter. Sci. Technol., 5(1989), No.11, p.1057. [ 131 C.R. Dong, Microattoyed Non-quenched and -tempered Steel, The Metallurgical Industry Press, Beijing, 2000. [I41 H. Gotl, K. Miyazawa, and K. Tanaka, Effect of cooling rate on composition of oxides precipitated during solidification of steels, ZSIJ Znt., 35( 1995), No.6, p.708. [15] S.C. Han, Physical metallurgy for acicular ferrite, Dev. Appl. Muter., 10(1995), No.6, p.2. [16] S.-Y. Lee, Effects of titanium and oxygen content on microstructure in low carbon steel, Muter. Trans., 43(2002), No.3, p.518. [ 171 G.C. Jiang, Clean Steels and Secondary Metallurgy, Press for Science and Technology, Shanghai, 1995.
Materials
Effect of inclusions on the formation of acicular ferrite in Ti-bearing non quenched-and-temperedsteel Ying Yang'*",Chaobin
Fuming Wang'),Zhanbing Yang'), and Bo Song')
1) Metallurgical and Ecological Engineering School, University of Science and Technology Beijing, Beijing 100083, China 2) Technology Centre, Anshan Iron and Steel Group Corporation, Anshan 114009, China
3) Technology Centre, Xinyu Iron and Steel Group, Xinyu 336501, China (Received 2006-04-06)
Abstract: Nucleation of acicular ferrite and its influence factors in non quenched-and-tempered steel was studied by using TEM and thermodynamic calculation. The results show that the complex particles with a center made of Ti oxide, A1,0,, and silicate and an outside made of a small quantity of mixture of TiN and MnS are able to act as femte nucleation nuclei. The acicular ferrite percentage changes little with Ti. When the oxygen content was 80 ppm, the volume percentage of acicular ferrite decreased due to an increase in allotriomorphic ferrite. The larger the cooling rate and the shorter the incubation time, the finer the titanium oxide and the higher the nucleation ratio of acicular femte.
Key words: acicular ferrite; titanium; inclusions; non quenched-and-tempered steel
[This work was financialfysupported by the National Natural Science Foundation of China (N0.50574010) and the National Doctorate Fund of the Ministry of Education of China (N0.20060008015).]
1. Introduction Non quenched-and-tempered steel has advantages of saving energy and materials, low cost, and good properties, therefore, it is widely used to make parts like connecting rod and cranked axle with good strength and toughness in automobile and tractor machine-making industry. Non quenched-and-tempered steel is named as "green steel" due to its saving energy, high efficiency, and environmental protection. However, one apparent defect for this kind of steel is the deficiency in toughness which limits its applications. Recently, it has been reported that interlocking acicular ferrite with high angle boundary can enhance the strength and toughness of the steel with refined grain size [ 1-21. Acicular ferrite nucleates mainly at the surface of nonmetallic inclusions. Many inclusions can act as heterogeneous nucleation sites for acicular ferrite, such as TiN [3], Al-rich inclusions [4-61, MnS [4], and titanium oxides [7-111, among which Ti,03 is the most effective one [7-81. Barbaro [12] reported that the main influencing factors on the formation of acicular ferrite are the distribution, size, and type of inclusions and the size of prior austenite grains. And a11 these factors are decided by cooling rate and the Corresponding author: Fuming Wang, E-mail: [email protected]
composition of the steel. In the present work, the types of inclusions and the nucleation of acicular ferrite have been discussed on the basis of TEM analysis and thermodynamic calculation. Systematic experiments have been conducted to study the influence of Ti, 0, and the size distribution of inclusions on the formation of acicular ferrite.
2. Experimental procedures The standard for microalloyed non quenched-andtempered steel [I31 was referred for designing the chemical compositions of experimental steels. Different steel samples were made in a Si-Mo furnace of high temperature. The ingots of No.1-No.3 and No.4No.7 with a size of @45mmx80 mm were obtained by quenching the crucibles with different samples directly from 1600and 1200°C using water, respectively. The chemical compositions of the experimental steels are shown in Table 1. The ingots were forged into bars of 910 mm with the process of starting forging temperature at 12OO0C, final forging temperature at 950"C, and air cooling at the rate of about 40"C/min. The specimens were cut from the ingots and their microstructures were examAlso available online a t www.sciencedireet.com
J. Univ. Sci. Technol. Beijing, Vo1.14,No.6, Dec 2007
502
croscope to study the shape and size of the particles. The EDS analysis was used to study the compositions of the particles in order to make semi-quantitative analysis of the phase (the Cu peaks in the spectrum came from the Cu grid used to prepare the carbon replica).
ined by optical microscope and scanning electron microscope (SEM) after being polished and etched in 3~01%nital. Extracted carbon replicas were made from the specimens of 2' and 5' which were quenched directly from 1600 and 1200°C, respectively, and were analyzed in a high-resolution transmission electron mi-
Table 1. Chemical compositions of the experimental steels No. 1" 2' 3" 4' 5"
6" 7"
C
0.32 0.32 0.37 0.33 0.35 0.33 0.35
S
0.045 0.046 0.042 0.045 0.046 0.043 0.045
Si
0.20 0.21 0.20 0.20 0.19 0.23 0.20
Mn 1.20 1.30 1.20 1.20 1.30 1.22 1.25
P 0.016 0.015 0.016 0.016 0.016 0.016 0.016
3. Results and discussion 3.1. Analysis of the precipitates containing Ti in the steels In practice, A1 is unavoidable in molten steel. The phase stability diagram calculation of the Al-Ti-0 system in molten iron was carried out by using the software of MATLAB to confirm the possibility of the formation of all kinds of inclusions. Fig. 1 shows that neither T i 0 nor TiO, can form at 1873 K, the final stable inclusion should be A120,.Ti0, within this composition range for the studied system. Titanium oxides, aluminum oxides, and silicates form the complex inclusions due to the existence of some elements in the steels, such as Si and Mn.
Also,
Ti
0.0020 0.0020 <0.0005 0.0020 0.0020 0.0028
-
-
0.0083 <0.0050 -
0.0160 0.0250 0.0400
wt% [OISO,
N
0.0038 0.0038 0.0075 0.0040 0.0035 0.0038 0.0038
0.0070 0.0070 0.0070 0.0070 0.0070 0.0070 0.0070
spherical in shape, as shown in Figs. 2 and 3. Figs. 2(b)-(d) and Figs. 3(b)-(d) are the EDS spectra of the corresponding sites in Figs. 2(a) and 3(a), respectively. In Fig. 2, the particle is about 300 nm, the core of the particle is titanium oxide, site B is the complex phase composed of titanium oxide, a small amount of aluminum oxide and manganese sulfide. And the surface of the particle is MnS, as shown at site C. All above shows that titanium oxide and a little aluminum oxide are at the center of most of the complex inclusions, with manganese sulfide at the surface at 1600°C. Fig. 3 shows another complex particle, with a size of about 200 nm. The center of the particle consists of the complex inclusions of titanium oxide, aluminum oxide, and silicate; sites B and C are titanium nitride and manganese sulfide, respectively. Since the specimen was quenched from 1200°C, the titanium nitride precipitated during the solidification. And it is obvious that manganese sulfide and titanium nitride precipitate on the complex inclusions of titanium oxide, aluminum oxide, and silicate in the temperature range above 12oo0c,
3.2. Nucleus formation of acicular ferrite (AF)
Fig. 1. Phase stability diagram of the Fe-Al-Ti-0 system in molten iron at 1873 K.
Typical complex particles in the specimens being quenched directly from 1600 and 12OO0C,respectively, were observed by TEM, which were found to be
The microstructure of the forged sample was analyzed by SEM. Fig. 4 shows the particle and its EDS spectrum, which is the nucleus of AF. The particle consists of titanium oxide, aluminum oxide, complex silicate, a small of amount of manganese sulfide, and titanium nitride, with a size of 0.6 pm. As shown in Fig. 4, the inclusion is located at the crossing of 4 ferrite plates, so the nonmetallic inclusion containing titanium can serve as the nucleus of AF.
3.3. Influence of titanium oxide on the microstruc-
...
Y. Yang et al., Effect of inclusions on the formation of acicular ferrite in Ti-bearing
ture of the nonquenched and tempered steel Fig. 5 shows the microstructures of samples 1'-7'. Figs. 5(a) and (d) are the microstructures of the specimens without Ti addition, most of the ferrites form on the boundaries of prior austenite grains. The others in Fig. 5 are the microstructures of Ti-bearing steels, and most of the microstructures are fine interlocking acicular ferrites.
3.4. Influence factors on the formation of AF (1) Influence of Ti on the formation of AF. The volume fraction of AF shown in Fig. 5 are obtained by quantitative determination. The volume fractions of AF for samples 5' (0.016wt% Ti), 6' (0.025wt% Ti), and 7' (0.040wt% Ti) are 44~01% 43vol%, and 46vol%, respectively. It has no obvious difference among them, therefore, the change of Ti content in the steels has little influence on the amount of AF when the oxygen content in the studied steels are definite and limited. The amount of nucleated inclusions and the prior austenite grain size are two important parameters for controlling the formation of acicular ferrite [12]. First, an increase in Ti content enhances the amount of inclusions, which in turn enhances the heterogeneous nucleation sites within the austenite grains and promotes the formation of AF. Second, an increase in Ti content makes the prior austenite grains finer. Although the volume fraction of titanium oxide is enough to induce the nucleation of AF, the prior austenite grains are too fine to form AF
503
[14]. The inclusion like TiN pins the boundaries of prior austenite grains to limit its growth, which is unfavorable for the formation of AF. Therefore, the quantity of AF in samples 5#, 6' and 7' has no obvious difference. (2) Influence of 0 on AF. Samples 2' (0,38 ppm) and 3' (0,75 ppm) were quenched directly from 1600°C. The volume fraction of AF in the above samples are 68~01%and 3vol%, respectively. When 0 is about 80 pprn and Ti is very low, the volume fraction decreases sharply, allotriomorphic ferrites precipitate on the boundaries of prior austenite grains. The number of nucleation sites on the boundaries or within the prior austenite grains is an important factor for the formation of AF and allotriomorphic ferrite. Small quantities of AF were formed in sample 3' since the 0 content is 75 ppm while Ti is very low, and the prior austenite grain is finer [15-161. The decrease in the quantity of titanium oxide is favorable for the formation of allotriomorphic ferrites on the boundaries of prior austenite grains and then it holds back the formation of AF at the surface of inclusions. In addition, TiN can promote the nucleation of AF [13]. In the present work, the content of N is very low. High 0 and low N are unfavorable for the formation of AF. Because most of Ti and 0 react to reach equilibrium, while the remaining Ti can not react with N to form TiN at the solidification temperature, the nucleation rate of AF decreases.
Fig. 2. Complex particle and its EDS analysis at different positions extracted from sample 5: (a) TEM micrograph; (b) EDS spectrum of site A; (c) EDS spectrum of site B; (d) EDS spectrum of site C.
J. Univ. Sci. Technol. Beijing, Vo1.14, N0.6, Dec 2007
504
Fig. 3. Complex particle and its EDS analysis at different positions extracted from sample 5: (a) TEM micrograph; (b) EDS spectrum of site A; (c) EDS spectrum of site B; (d) EDS spectrum of site C.
Fig. 4. Secondary electron image of the precipitationnucleus of AF (a) and EDS of position A (b) for sample 5.
(3) Influence of the size distribution of inclusions on AF. Sample 2' was quenched directly from 1600°C. The volume fraction of AF is 68vol%, which is more than those of samples 5' (44vol%), 6' (43~01%)and 7' (46vol%), respectively, hence it can be shown that the size distribution of inclusions has effect on the amount of AF. With an increase in cooling rate, the degree of super-cooling increases and the incubation time decreases, therefore, the quantity of the oxides increases and their size decreases, and an increased precipitation of MnS is observed. With slow cooling of liquid steel, the oxides become bigger and their quantity decreases.
In the present work, samples 5'-7' were furnace cooled to 1200°C and then the inclusions formed at 1200°C were not as dispersed and fine as those in the samples rapidly being cooled from 1600°C. And segregations occur in the samples being furnace cooled. Since AF formation requires very fine and higher quantities of deoxidation products with uniform distribution in every austenite grain [17], the quantity of AF in samples 5'-7' is relatively lower than that in sample 2'.
4. Conclusions (1) In the samples quenched at 1600"C, most of the
...
Y. Yung et ul., Effect of inclusions on the formation of acicular ferrite in Ti-bearing
particles are complex inclusions with a core of titanium oxide and aluminum oxide and a surface of manganese sulfide. In the samples quenched at 1200°C, the core of complex inclusions of titanium oxide, alu-
505
minum oxide, and silicate, and the surface of a small quantity of titanium nitride and manganese sulfide are observed. The nuclei of AF are mainly these complex inclusions.
Fig. 5. Optical micrographs for samples 1'-7#.
(2) With the addition of Ti in the steels, AF is formed, instead of ferrite nets along the prior austenite. (3) With Ti increase in the steels, the amount of AF has no obvious change. When the oxygen content is about 80 ppm, acicular ferrites decrease due to the increase in allotriomorphic ferrites. The cooling rate and incubation time have great influence on the distribution of nucleation particles.
References [I] J. Yin and C. Xu, Ferrite formation at intragranular MnS
in mould cast non-quenched and tempered steels, Heat Treat. (in Chinese), I9(2004),No.1, p.18. [2] S. Ohkita, Improvement of HAZ toughness in HSLA steel by introducing finely dispersed Ti-oxide, Weld, Res. Suppl., 5(1987),No.10, p.301. [3] X.J. Zhuo, H.S. Kim, Y.B. Kang, e f al., Nucleation of intragranular ferrite of manganese titanium oxide and MnS inclusions in Si-Mn-Ti deoxidized steel, [in] CSM 2005 Annual Meeting Proceeding (in Chinese), CSM, Beijing, 2005, p.687. [4] Z. Zhang and R.A. Farrar, Role of nonmetallic inclusions in formation of acicular ferrite in low alloy weld metals, Mater. Sci. Technol., 12(1996),p.237.
506 [5] A.R. Bhatti, M.E. Saggese, J.A. Whiteman, et al., Analysis of inclusions in submerged arc welds in microalloyed steels, Weld. J., 63(1984), p.224. [6] J.M. Dowling, J.M. Corbett, and H.W. Ken, Inclusion phases and nucleation of acicular ferrite in submerged arc welds in high strength low alloy steels, Metall. Trans. A , 17A(1986), p.1611. [7] J.-S. Byun, J.-H. Shim, Y.W. Cho, et al., Non-metallic inclusion and intra-granular nucleation of ferrite in Tikilled C-Mn steel, Acta Muter., 51(2003), No.4, p.1593. [8] Z.T. Ma and D. J a k e , Characteristics of oxide precipitation and growth during solidification of deoxidized steel, ZSZJ Znt., 38( 1998), No.1, p.46. [9] J.H. Shim, Y.J. Oh, J.Y. Suh, et al., Ferrite nucleation potency of nonmetallic inclusions in medium carbon steel, Acta Muter., 49(2001), p.2115. [lo] J.H. Shim, Y.W. Cho, and S.H. Chung, Nucleation of intragranular ferrite at Ti,O, particle in low carbon steel, Acta Muter., 47( 1999), No.9, p.275 1.
J. Univ. Sci. Technol. Beijing, Vo1.14, N0.6, Dec 2007 [I I] J.S. Byun, J.H. Shim, J.Y. Suh, et al., Inoculated acicular ferrite microstructure and mechanical properties, Muter. Sci. Eng., A319-321(2001), p.326. [12] F.J. Barbaro, P. Krauklis, and K.E. Easterling, Formation of acicular ferrite at oxide particle in steels, Muter. Sci. Technol., 5(1989), No.11, p.1057. [ 131 C.R. Dong, Microattoyed Non-quenched and -tempered Steel, The Metallurgical Industry Press, Beijing, 2000. [I41 H. Gotl, K. Miyazawa, and K. Tanaka, Effect of cooling rate on composition of oxides precipitated during solidification of steels, ZSIJ Znt., 35( 1995), No.6, p.708. [15] S.C. Han, Physical metallurgy for acicular ferrite, Dev. Appl. Muter., 10(1995), No.6, p.2. [16] S.-Y. Lee, Effects of titanium and oxygen content on microstructure in low carbon steel, Muter. Trans., 43(2002), No.3, p.518. [ 171 G.C. Jiang, Clean Steels and Secondary Metallurgy, Press for Science and Technology, Shanghai, 1995.