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Microstructure and Transformation Characteristics of Acicular Ferrite in High Niobium-Bearing Microalloyed Steel
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ScienceDirect JOURNAL OF IRON AND STEEL RESEARCH, INTERNATIONAL. 2010, 17(6): 53-59
Microstructure and Transformation Characteristics of Acicular Ferrite in High Niobium-Bearing Microalloyed Steel YANG Jing-hong' ,
LIU Qing-you 2 ,
SUN Dong-bai",
LI Xiang-yang"
O. Luoyang Ship Material Research Institute, Luoyang 471039, Henan, China; 2. China Iron and Steel Research Institute Group, Beijing 100081, China; 3. School of Materials Science and Engineering, University of Science and Technology Beijing, 10083, China) Abstract: The transformation behavior and microstructural characteristics of a low carbon high niobium-bearing microalloyed pipeline steel were investigated by deformation dilatometry and microstructure observation. The continuous cooling transformation curves of the test steel were constructed. The results showed that high niobium content and deformation enhanced the formation of acicular ferrite; the microstructures changed from polygonal ferrite, quasi-polygonal ferrite to acicular ferrite with increasing cooling rates from O. 5 to 50 'C Is and was dominated by acicular ferrite in a broadened cooling rate higher than 5 ·C/s. The chaotic microstructure consisted of non-equiaxed ferrite and interwoven ferrite laths with high density dislocations and subunits. The results of isothermal holding treatment showed that acicular ferrite microstructure was formed at 550 - 600 'C and consisted of highly misoriented plate packets having internal low angle boundaries. With increasing the holding time or temperature, some low misorientation boundaries changed to high misorientation owing to the movement of dislocations and coarsening of grain. Key words: microalloyed steel; acicular ferrite; transformation; continuous cooling transformation
In recent years, the demands for high performance pipeline steels which have not only high strength but also good low temperature toughness have been increasing urgently. As the good properties of steels can be obtained by controlling the chemical compositions and process parameters, several researchers have studied the relationship between microstructural features and mechanical properties of pipeline steels[I-4]. The second West-East natural gas transmission project in China is a long distance transmission. project with strategic importance following the first one. API X80 grade steels have been selected to make the pipeline which has a diameter of 1219 mm and gas transmission pressure of 12 MPa in the project. At present, the microstructure of acicular ferrite (AF) that commonly adopted in X80 steel, is achieved by the addition of molybdenum[S-7], which increases the production cost remarkably. Besides, because
the X80 plates/strips used for the West-East pipeline is relatively thick, it is necessary to increase the deformation amount below the non-recrystallizaion temperature (Tnr ) to ensure a final fine microstructure. It has been well known that niobium can not only improve the T nr , which makes it possible to apply larger deformation below T nr , but also improve the hardenability of austenite and thus enhance the formation of bainite rnicrostructurel'i! , therefore, the high niobium-bearing steel technology, aiming to replace molybdenum or decrease molybdenum content, has been developed for producing X80 grade pipeline steel in China. In this study, the phase transformation behavior and characteristics in high niobium-bearing steel during continuous cooling and isothermal holding were examined by dilatometry analysis and microstructure observation in addition to an orientation imaging microscopy (OIM) technique based on elec-
Foundation Item: Item Sponsored by National Key Technologies Research and Development Program of China (2006BAE03A15) Biography: YANG Jing-hong0977-), Male, Doctor; E-mail: [email protected]; Received Date: March 16, 2009
To study the microstructure and transformation characteristics of the steel, the continuous cooling and isothermal transformation were investigated after two pass and single pass deformation compression tests on a Gleeble-1500D thermomechanical simulator. The thermomechanical process of continuous cooling and isothermal holding treatments are illustrated in Fig. 1.
tron back-scattered diffraction (EBSD).
1
Vol. 17
Journal of Iron and Steel Research, International
• 54 •
Materials and Experimental Procedure
The chemical compositions of the steel which were taken from the slab industrially produced are given in Table 1. The cylindrical samples of 8 mm X 12 mm were cut from slab after solution treatment at 1200 'C Xl h and then quenching. Table 1 C
0.034
Mn
0.17
Si 1. 80
p 0.011
%
Chemical composition of the test steel S 0.0017
Als
Cu
0.25
0.016
(a) 1200 'C, 6 min
Ni O. 18
Cr 0.26
N
Ti
Nb
0.0038
0.011
0.12
(b) 1200 'C, 6 min
Fast cooling ~====l660-650 'C l: 10-1000 s
Quenching
Time
Fig. 1
Diagram of thermomechanical process for continuous cooling transformation (a) and isothermal holding treatment (b)
The diametrical change of the deformed samples during continuous cooling or isothermal holding under different processes was recorded by an attached quartz dilatometer. The microstructures in the center of the samples were observed by optical microscope after being etched with 4 % Nital. A more detailed metallographic examination was performed on selected specimens using both scanning electron microscope (SEM) and transmission electron microscope (TEM). To examine the orientation relationship between grains, EBSD analyses were carried out through SEM. Thin foils for TEM and EBSD were prepared using standard twin-jet electropolishing in an electrolyte consisting of perchloric acid of 5 % and methanol of 95 % in volume percent.
2 2. 1
Experimental Results
Continuous cooling transformation Fig. 2 shows the microstructures of the test steel after two-pass deformation and transformation at different cooling rates. It can be found that with an increase in the cooling rate, the grain size becomes smaller and complicate. The microstructure
of the steel is composed of polygonal ferrite (PF), quasi-polygonal ferrite (QF), and a small amount of degenerated pearlite at the cooling rate of O. 5 'C / s. The degenerated pearlite, consisting of ferrite and non-lamellar cementite, is the transformation product of retained austenite at relatively low temperature. With increasing the cooling rate, the amount of PF decreases and the acicular ferrite (AF) appears. When the cooling rate is increased further, the fraction of acicular ferrite in the mixed microstructure increases. The microstructure is fully composed of acicular ferrite when the cooling rate is higher than 5 'C / s, leading to an AF dominated microstructure in the steel. The microstructures become finer at higher cooling rates and effective grain size reaches 2 - 5 fLm. It can be seen from Fig. 2 that the AF dominated microstructure in the steel is characterized by fine non-equiaxed ferrite and interwoven non-parallel ferrite laths in various sizes and with random distribution, which is often described as a "chaotic arrangement" of laths; the prior austenite grain boundary network is eliminated and matrix is dispersed with fine
Issue 6
Microstructure and Transformation Characteristics of Acicular Ferrite in Microalloyed Steel
(a) 0.5
Fig. 2
·C/s;
(b) 1 ·C/s;
(c) 5
·C/s;
(e) 30
·C/s;
(f) 50
·C/s.
Microstructures of specimens after two-pass deformation and then cooling at different cooling rates
martensite/austenite (M-A) islands. The M-A islands are carbon enriched and are attributed to the partitioning of carbon to the austenite during the transformation to acicular ferrite. With accelerated cooling, part of the carbon-enriched austenite is transformed to martensite and some remaining austenite coexists with the martensite. Moreover, the microstructural characteristics of AF presents an assemblage of interwoven non-parallel ferrite laths with high density tangled dislocations and subunits. The microstructure of specimen at the cooling rate of 30 'C / s is observed by TEM, as shown in Fig. 3. It can be seen that the high density dislocations are distributed in AF grains and there are dispersed M- A islands and subgrains in the matrix phase. According to both the temperature-dilatation
Fig.3
(d) 15 ·C/s;
• 55 •
curves and microstructure observations, the continuous cooling transform~tion curves of-t he test steel is constructed and shown in Fig. 4. It can be seen that the AF microstructure can be obtained in a wide cooling rate range by high niobium addition. The experimental results demonstrated that for the experimental steel, the AF dominated microstructure can be obtained easily by a two-stage controlled rolling with the deformation in the austenite recrystallization region plus the austenite non-recrystallization region and controlled cooling at a cooling rate higher than 5 'C/ s. It demonstrates that two-stage rolling of thermomechanical control process (TMCP) is optimum to obtain microstructure of AF, and can be further applied in industrial production for the lowcarbon high-niobium-bearing steel.
TEM observation showing morphology of ferrite laths (a) and substructure in acicular ferrite (b)
2. 2 1100
~
~ Eo<
700 500
300 100
Fig. 4
CCT diagram of test steel under two-pass deformation condition
(a) Held at 550 'C;
Fig.5
Isothermal holding transformation
The microstructures of specimens deformed by compression of 40 % at 850 'C and then isothermal holding at 550, 600, and 650 'C for lOs are shown in Fig. 5. For a specimen isothermally held at 650 'C for 10 s , the microstructure is martensite obviously owing to quenching. At an isothermal holding temperature of 600 'C, acicular ferrite was formed from deformed austenite, and its volume fraction is approximately 35 %. At an isothermal holding temperature of 550 'C for 10 s, the volume fraction of acicular ferrite is greatly increased though there are some bainite microstructures in the matrix. The time-dilatation curve shows that the transformation of acicular ferrite was quickly completed at a maximum
900
P ~
Vol. 17
Journal of Iron and Steel Research. International
• 56 •
(b) Held at 600 'C;
(c) Held at 650 ·C.
Microstructure of specimens held at different temperatures for 10 s after deformation at 850 "c by 40%
of 5 s (Fig. 6), showing the characteristics of diffusionless transformation-'". The results of isothermally holding at different temperatures confirm that acicular ferrite forms well in the region of 550 - 600 ·C. The microstructures of the specimens isothermally held for different times at 550 'C are shown in Fig. 7. Acicular ferrite nucleated mainly on the nucleation sites within the austenite grain, such as dislocations and other defects. According to the time-dilatation curves of the specimen isothermally held at different conditions, the transformation of acicular ferrite proceeds very
(a) 10 s;
Fig.7
(b) 100 s;
V
1.08
5s
J
1.06
~ :=
1.04
Cl
1.02 '---'-_ _--'500 502
'--_ _........._ _--'0....1 506 508
504
Time/s
Fig. 6
Diametrical change of deformed samples isothermally held at 550 "c for 10 s
(c) 600 s.
Microstructure of specimens held at 550 "c for different times after deformation at 850 "c by 40%
Issue 6
Microstructure and Transformation Characteristics of Acicular Ferrite in Microalloyed Steel
quickly by transformation behavior associated with shear transformation similar to that of bainitic ferrite. After the transformation of acicular ferrite is finished, however, actually the dilatation slightly increases with increasing holding time. This is thought to be the result of the coarsening of pre-existing acicular ferrite grains owing to the relatively high reaction temperature and the formation of some polygonal ferrite and quasi-polygonal ferrite at prioraustenite grain boundaries as shown in Fig. 7 (b). Fig. 8 is the typical grain map for the steel isothermally held at 550 'C for 100 s from EBSD. Comparing the microstructure in Fig. 8 with that in Fig. 7 (b), the optical images define a much finer and interlocked microstructure. The grain maps are composed of monochromatic areas, and each point belonging to these areas exhibits a misorientation lower than a certain tolerance angle with its neighbor points. In the case of the grain map shown in Fig. 8, the tolerance angle imposed is 15°, which is taken as the .criterion of considering the microstructural unit that controls the brittle-fracture propagation in acicular ferrite microstructure'Y". Each one of these monochromatic areas in the grain map defines a crystallographic packet and each crystallorgraphic packet is generally formed by several ferrite plates, which can possess different spatial orientations and consequently are difinitely different from the morphological packets formed by parallel plates. The formation of an acicular ferrite packet is more obvious from the change of the average grain size with the tolerance angle. Fig. 9 shows the typical sections of the boundary misorientation maps obtained from the EBSD data of the present steel deformed at different temperatures
Fig. 8 Typical EBSD grain micrograph of acicular ferrite isothermally held at 550 t for 100 s
or holding times. In the boundary maps, the grain boundaries are distinguished by grey level according to the misorientation. The bold lines refer to high misorientation angle boundaries (HABs) greater than 15°. The slim lines represent low-angle boundaries (LABs) lower than 15°. It is seen that the initial grains of sample held at 550 'C for 10 s are finer while low-angle boundaries are more at an early stage of transformation. When the holding time is increased, the fraction of HABs increases and LABs decreases which is owing to the absorption of dislocations moving into boundaries and some sub-boundaries changed into high-angle boundaries. Holding at a higher temperature of 600 'C for 10 s , there are allotriomorphic ferrites or polygonal ferrite with high angle boundaries in austenite grain boundaries and the number of low misorientation angle boundaries is less to that held at 550 'C for 10 s obviously.
(a) Isothermally holding at 550 'C for 10 s; (b) Isothermally holding at 550 'C for 100 s; (c) Isothermally holding at 600 'C for 10 s.
Fig. 9
• 57 •
EBSD micrographs showing LABs and DABs in samples after different treatments
• 58 •
3
Journal of Iron and Steel Research. International
Discussion
3.1
lhmsfonnation characteristics of acicular ferrite The formation of acicular ferrite in pipeline steels is closely related to a heavy deformation in the austenite non-recrystallization region. The deformation of austenite below non-recrystallization temperature promotes the formation of acicular ferrite and quasi-polygonal ferrite, inhibits pearlite and bainite formation, and furthermore, enlarges the AF region significantly in the steel (Fig. 2). The high density of dislocations in the deformed austenite appears to be beneficial to the formation of AF instead of bainite. The main differences between bainite and acicular ferrite formation are found to be the nucleation sites and growth directions. Bainite nucleates at austenite grain boundaries and grows as a sheaf of parallel plates in the same growth direction within the austenite grains, whereas acicular ferrite generally nucleates intragranularly and grows as primary plates in the same or different orientations and thus, prior austenite grain boundaries become indistinct; then, a new set of secondary plates nucleates at the austenite/ primary AF' s interface. Consequently, AF is characterized by a chaotic arrangement of plates, showing a fine-grained interwoven morphology. as shown in Fig. 5 and Fig. 7. The acicular ferrite transformation is promoted and the identical acicular ferrite is formed in the range of 550 - 600 'C for the test steel, which is lower when compared with other studies[9] and is believed to be owing to the compositions of the steel. High niobium content increases the non-recrystallization temperature and allows greater reduction to be introduced into the austenite before transformation so as to achieve high volume fraction of AF in the pipeline steel; at the same time, niobium enhances acicular ferrite transformation through decreasing the transformation temperature and retarding the transformation of bainite microstructure. 3. 2
Microstructure characteristics of acicular ferrite Acicular ferrite is regarded as the most desirable microstructural feature in view of strength and toughness in mild and low alloyed steel weld metals. This is owing to its nature of small grain size and high angle boundaries[1l-12]. The evolution and diversity of mechanical properties for base metal demand the same properties to the weld. Fig. 7 shows that acicular ferrite grains with an
Vol. 17
apparently irregular or "feathery" shape are present independently within an austenite grain, and the acicular ferrite grain consists of a great amount of dislocations and subunits (Fig. 3). The adjacent acicular ferrite grains in the shape of a parallelogram show low misorientation angle and several acicular ferrite grains consist of a so-called crystallographic packet[IO,13]. The closed packets show a high angle boundaries (Fig. 9). This demonstrates the difference of crystallographic characteristics of the acicular ferrite in the steels and welds. The transformation of acicular ferrite shows characteristics of diffusionless transformation (Fig. 6); however, from Fig. 7 and Fig. 9, it can be seen that with the holding time or temperature increasing, there is little bainite microstructure and acicular ferrite laths is broaden. At the same time, the ratio of the number of high angle boundaries (larger than 15°) to that of low angle boundaries is large and the grain aspect ratios decreased, which is owing to the dislocation recovery or boundries coarsening. Thus, it shows a characteristics of diffusion in acicular ferrite growth.
4
Conclusions 1) Niobium enhances the formation of acicular
ferrite, and the microstructure of low-carbon high Nb-bearing steel is dominated by acicular ferrite at a broadened cooling rate higher than 5 'C / sunder continuous cooling conditions. 2) Acicular ferrite in test steel is formed at 550600 'C, and a great amount of dislocations and subunits are present. The formation process of acicular microstructure includes nucleation and coarsening. 3) Acicular ferrite consists of highly misoriented plates packets having internal low angle boundaries. With increasing the holding time or temperature, some low misorientation boundaries are changed to high misorientations owing to dislocations moving and grain boundaries coarsening. References : [lJ
[2J
[3J
Yakubtsov I A. Poruks P, Boyd J D. Microstructure and Mechanical Properties of Bainitic Low Carbon High Strength Plate Steels [JJ. Materials Science and Engineering, 2008, 480AO/ 2): 109. XIAO Fu-ren , LIAO Bo, SHAN Yi-yin , et al, Challenge of Mechanical Properties of an Acicular Ferrite Pipeline Steel [JJ. Materials Science and Engineering, 2006, 431AO/2): 4l. Sang Yong Shin, Hwang Byoungchul , Kim Sangho , et al. Fracture Toughness Analysis in Transition Temperature Region of API X70 Pipeline Steels [JJ. Materials Science and Engineering, 2006, 429AO/2): 196.
Issue 6 [4J
[5J
[6J
[7J
[8J
[9J
Microstructur~ and Transformation Characteristics of Acicular Ferrite in Microalloyed Steel
Zhao M C, Shan Y Y, Xiao F R, et al. Acicular Ferrite Formation During Hot Plate Rolling for Pipeline Steels [1]. Materials Science and Technology, Z003, 19(3): 355. Yakubtsov I A, Boyd J D. Effect of Alloying on Microstructure and Mechanical Properties of Bainitic High Strength Plate Steels [J]. Materials Science and Technology, Z008, Z4(Z): ZZI. Saha Podder A, Pandit A S, Murugaiyan A, et al. Phase Transformation Behavior in Two C-Mn-Si Based Steels Under Different Cooling Rates [1]. Ironmaking and Steelmaking, Z007, 34(1): 83. TANG Zheng-hua , Stumpf Waldo. The Role of Molybdenum Additions and Prior Deformation on Acicular Ferrite Formation in Microallyed Nb-Ti Low-Carbon Line-Pipe Steels [1]. Materials Characterization, Z008, 59(6): 717. Rees G I, Bhadeshia H K D H, Perdrix J. et al. The Effect of Niobium in Solid Solution on the Transformation Kinetics of Bainite [1]. Materials Science and Engineering, 1995, 194A (Z): 179. Kim Y M, Lee H, Kim N J. Transformation Behavior and Mi-
[10J
[l1J
[lZJ
[13 J
• 59 •
crostructural Characteristics of Acicular Ferrite in Linepipe Steels [1]. Materials Science and Engineering, Z008, 478AO/ Z): 361. Daz-Fuentes M, Iza-Mendia A, Gutierrez 1. Analysis of Different Acicular Ferrite Microstructures in Low-Carbon Steels by Electron Backscattered Diffraction. Study of Their Toughness Behavior [1]. Metallurgical and Materials Transaction, Z003, 34M11): Z505. ZHAO Ming-chun , YANG Ke, SHAN Vi-yin. Comparison on Strength and Toughness Behaviors of Microalloyed Pipeline Steels With Acicular Ferrite and Ultrafine Ferrite [1]. Materials Letters. Z003, 57(9/10): 1496. Terasaki H, Komizo Y. In Situ Observation of Morphological Development for Acicular Ferrite in Weld Metal [1]. Science and Technology of Welding and Joining, Z006, 11(5): 561. Gourgues A F, Flower H M, Lindley T C. Electron Backscattering Diffraction Study of Acicular Ferrite, Bainite, and Martensite Steel Microstructures [1]. Materials Science and Technology, ZOOO, 160), Z6.
(Continued From Page 46) References : [lJ
[ZJ
[3J
[4J
LIU Jian-hua, MA Dang-shen , ZHANG Zhan-pu , et al. Influence of Niobium Content on Structure and Mechanical Properties of Steel Cr8WMoZVZSiNb [1]. Special Steel, Z008, Z9 (6): 55 (in Chinese). Bensely A. Prabhakaran A. Mohan Lal D, et al. Enhancing the Wear Resistance of Case Carburized Steel by Cryogenic Treatment [1]. Cryogenics, Z005, 45, 747. Mohan Lal D, Renganarayanan S, Kalanidhi A. Cryogenic Treatment to Augment Wear Resistance of Tool and Die Steel [1]. Cryogenics, ZOOl, 41: 149. Zhirafar S, Rezaeian A, Pugh M. Effect of Cryogenic Treatment on The Mechanical Properties of 4340 Steel [1]. J Mater Process Tech. Z007, 186: Z98.
[5J
[6J
[7J [8J
[9J
Molinari A, pellizzari M. Gialanella S, et al. Effect of Cryogenic Treatment on the Mechanical Properties of Tool Steels [J]. Journal of Materials Processing Technology. ZOOl, 118: 350. MA Dang-shen , LIU Iian-hua , CHEN Zai-zhi , et al. Effect of Heat Treatment Process on Microstructure and Mechanical of Cr8MoZVSi and DZ Cold Work Steel [1]. Iron and Steel, Z008, 43(9): 67 (in Chinese). George A Roberts, Robert A Cary. Tool Steels [MJ. 4th ed. New York: American Society for Metals. 1980. Das D, Dutta A K, Ray K K. On the Enhancement of Wear Resistance of Tool Steels by Cryogenic Treatment [1]. Philosophical Magazine Letters, Z008, 88(1): 801. DONG Yun. LIN Xiao-ping , XIAO Hong-sheri. Deep Cryogenic Treatment of High-Speed Steels and Its Mechanism [1]. Heat Treatment Met, 1998(3): 55.
ScienceDirect JOURNAL OF IRON AND STEEL RESEARCH, INTERNATIONAL. 2010, 17(6): 53-59
Microstructure and Transformation Characteristics of Acicular Ferrite in High Niobium-Bearing Microalloyed Steel YANG Jing-hong' ,
LIU Qing-you 2 ,
SUN Dong-bai",
LI Xiang-yang"
O. Luoyang Ship Material Research Institute, Luoyang 471039, Henan, China; 2. China Iron and Steel Research Institute Group, Beijing 100081, China; 3. School of Materials Science and Engineering, University of Science and Technology Beijing, 10083, China) Abstract: The transformation behavior and microstructural characteristics of a low carbon high niobium-bearing microalloyed pipeline steel were investigated by deformation dilatometry and microstructure observation. The continuous cooling transformation curves of the test steel were constructed. The results showed that high niobium content and deformation enhanced the formation of acicular ferrite; the microstructures changed from polygonal ferrite, quasi-polygonal ferrite to acicular ferrite with increasing cooling rates from O. 5 to 50 'C Is and was dominated by acicular ferrite in a broadened cooling rate higher than 5 ·C/s. The chaotic microstructure consisted of non-equiaxed ferrite and interwoven ferrite laths with high density dislocations and subunits. The results of isothermal holding treatment showed that acicular ferrite microstructure was formed at 550 - 600 'C and consisted of highly misoriented plate packets having internal low angle boundaries. With increasing the holding time or temperature, some low misorientation boundaries changed to high misorientation owing to the movement of dislocations and coarsening of grain. Key words: microalloyed steel; acicular ferrite; transformation; continuous cooling transformation
In recent years, the demands for high performance pipeline steels which have not only high strength but also good low temperature toughness have been increasing urgently. As the good properties of steels can be obtained by controlling the chemical compositions and process parameters, several researchers have studied the relationship between microstructural features and mechanical properties of pipeline steels[I-4]. The second West-East natural gas transmission project in China is a long distance transmission. project with strategic importance following the first one. API X80 grade steels have been selected to make the pipeline which has a diameter of 1219 mm and gas transmission pressure of 12 MPa in the project. At present, the microstructure of acicular ferrite (AF) that commonly adopted in X80 steel, is achieved by the addition of molybdenum[S-7], which increases the production cost remarkably. Besides, because
the X80 plates/strips used for the West-East pipeline is relatively thick, it is necessary to increase the deformation amount below the non-recrystallizaion temperature (Tnr ) to ensure a final fine microstructure. It has been well known that niobium can not only improve the T nr , which makes it possible to apply larger deformation below T nr , but also improve the hardenability of austenite and thus enhance the formation of bainite rnicrostructurel'i! , therefore, the high niobium-bearing steel technology, aiming to replace molybdenum or decrease molybdenum content, has been developed for producing X80 grade pipeline steel in China. In this study, the phase transformation behavior and characteristics in high niobium-bearing steel during continuous cooling and isothermal holding were examined by dilatometry analysis and microstructure observation in addition to an orientation imaging microscopy (OIM) technique based on elec-
Foundation Item: Item Sponsored by National Key Technologies Research and Development Program of China (2006BAE03A15) Biography: YANG Jing-hong0977-), Male, Doctor; E-mail: [email protected]; Received Date: March 16, 2009
To study the microstructure and transformation characteristics of the steel, the continuous cooling and isothermal transformation were investigated after two pass and single pass deformation compression tests on a Gleeble-1500D thermomechanical simulator. The thermomechanical process of continuous cooling and isothermal holding treatments are illustrated in Fig. 1.
tron back-scattered diffraction (EBSD).
1
Vol. 17
Journal of Iron and Steel Research, International
• 54 •
Materials and Experimental Procedure
The chemical compositions of the steel which were taken from the slab industrially produced are given in Table 1. The cylindrical samples of 8 mm X 12 mm were cut from slab after solution treatment at 1200 'C Xl h and then quenching. Table 1 C
0.034
Mn
0.17
Si 1. 80
p 0.011
%
Chemical composition of the test steel S 0.0017
Als
Cu
0.25
0.016
(a) 1200 'C, 6 min
Ni O. 18
Cr 0.26
N
Ti
Nb
0.0038
0.011
0.12
(b) 1200 'C, 6 min
Fast cooling ~====l660-650 'C l: 10-1000 s
Quenching
Time
Fig. 1
Diagram of thermomechanical process for continuous cooling transformation (a) and isothermal holding treatment (b)
The diametrical change of the deformed samples during continuous cooling or isothermal holding under different processes was recorded by an attached quartz dilatometer. The microstructures in the center of the samples were observed by optical microscope after being etched with 4 % Nital. A more detailed metallographic examination was performed on selected specimens using both scanning electron microscope (SEM) and transmission electron microscope (TEM). To examine the orientation relationship between grains, EBSD analyses were carried out through SEM. Thin foils for TEM and EBSD were prepared using standard twin-jet electropolishing in an electrolyte consisting of perchloric acid of 5 % and methanol of 95 % in volume percent.
2 2. 1
Experimental Results
Continuous cooling transformation Fig. 2 shows the microstructures of the test steel after two-pass deformation and transformation at different cooling rates. It can be found that with an increase in the cooling rate, the grain size becomes smaller and complicate. The microstructure
of the steel is composed of polygonal ferrite (PF), quasi-polygonal ferrite (QF), and a small amount of degenerated pearlite at the cooling rate of O. 5 'C / s. The degenerated pearlite, consisting of ferrite and non-lamellar cementite, is the transformation product of retained austenite at relatively low temperature. With increasing the cooling rate, the amount of PF decreases and the acicular ferrite (AF) appears. When the cooling rate is increased further, the fraction of acicular ferrite in the mixed microstructure increases. The microstructure is fully composed of acicular ferrite when the cooling rate is higher than 5 'C / s, leading to an AF dominated microstructure in the steel. The microstructures become finer at higher cooling rates and effective grain size reaches 2 - 5 fLm. It can be seen from Fig. 2 that the AF dominated microstructure in the steel is characterized by fine non-equiaxed ferrite and interwoven non-parallel ferrite laths in various sizes and with random distribution, which is often described as a "chaotic arrangement" of laths; the prior austenite grain boundary network is eliminated and matrix is dispersed with fine
Issue 6
Microstructure and Transformation Characteristics of Acicular Ferrite in Microalloyed Steel
(a) 0.5
Fig. 2
·C/s;
(b) 1 ·C/s;
(c) 5
·C/s;
(e) 30
·C/s;
(f) 50
·C/s.
Microstructures of specimens after two-pass deformation and then cooling at different cooling rates
martensite/austenite (M-A) islands. The M-A islands are carbon enriched and are attributed to the partitioning of carbon to the austenite during the transformation to acicular ferrite. With accelerated cooling, part of the carbon-enriched austenite is transformed to martensite and some remaining austenite coexists with the martensite. Moreover, the microstructural characteristics of AF presents an assemblage of interwoven non-parallel ferrite laths with high density tangled dislocations and subunits. The microstructure of specimen at the cooling rate of 30 'C / s is observed by TEM, as shown in Fig. 3. It can be seen that the high density dislocations are distributed in AF grains and there are dispersed M- A islands and subgrains in the matrix phase. According to both the temperature-dilatation
Fig.3
(d) 15 ·C/s;
• 55 •
curves and microstructure observations, the continuous cooling transform~tion curves of-t he test steel is constructed and shown in Fig. 4. It can be seen that the AF microstructure can be obtained in a wide cooling rate range by high niobium addition. The experimental results demonstrated that for the experimental steel, the AF dominated microstructure can be obtained easily by a two-stage controlled rolling with the deformation in the austenite recrystallization region plus the austenite non-recrystallization region and controlled cooling at a cooling rate higher than 5 'C/ s. It demonstrates that two-stage rolling of thermomechanical control process (TMCP) is optimum to obtain microstructure of AF, and can be further applied in industrial production for the lowcarbon high-niobium-bearing steel.
TEM observation showing morphology of ferrite laths (a) and substructure in acicular ferrite (b)
2. 2 1100
~
~ Eo<
700 500
300 100
Fig. 4
CCT diagram of test steel under two-pass deformation condition
(a) Held at 550 'C;
Fig.5
Isothermal holding transformation
The microstructures of specimens deformed by compression of 40 % at 850 'C and then isothermal holding at 550, 600, and 650 'C for lOs are shown in Fig. 5. For a specimen isothermally held at 650 'C for 10 s , the microstructure is martensite obviously owing to quenching. At an isothermal holding temperature of 600 'C, acicular ferrite was formed from deformed austenite, and its volume fraction is approximately 35 %. At an isothermal holding temperature of 550 'C for 10 s, the volume fraction of acicular ferrite is greatly increased though there are some bainite microstructures in the matrix. The time-dilatation curve shows that the transformation of acicular ferrite was quickly completed at a maximum
900
P ~
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Journal of Iron and Steel Research. International
• 56 •
(b) Held at 600 'C;
(c) Held at 650 ·C.
Microstructure of specimens held at different temperatures for 10 s after deformation at 850 "c by 40%
of 5 s (Fig. 6), showing the characteristics of diffusionless transformation-'". The results of isothermally holding at different temperatures confirm that acicular ferrite forms well in the region of 550 - 600 ·C. The microstructures of the specimens isothermally held for different times at 550 'C are shown in Fig. 7. Acicular ferrite nucleated mainly on the nucleation sites within the austenite grain, such as dislocations and other defects. According to the time-dilatation curves of the specimen isothermally held at different conditions, the transformation of acicular ferrite proceeds very
(a) 10 s;
Fig.7
(b) 100 s;
V
1.08
5s
J
1.06
~ :=
1.04
Cl
1.02 '---'-_ _--'500 502
'--_ _........._ _--'0....1 506 508
504
Time/s
Fig. 6
Diametrical change of deformed samples isothermally held at 550 "c for 10 s
(c) 600 s.
Microstructure of specimens held at 550 "c for different times after deformation at 850 "c by 40%
Issue 6
Microstructure and Transformation Characteristics of Acicular Ferrite in Microalloyed Steel
quickly by transformation behavior associated with shear transformation similar to that of bainitic ferrite. After the transformation of acicular ferrite is finished, however, actually the dilatation slightly increases with increasing holding time. This is thought to be the result of the coarsening of pre-existing acicular ferrite grains owing to the relatively high reaction temperature and the formation of some polygonal ferrite and quasi-polygonal ferrite at prioraustenite grain boundaries as shown in Fig. 7 (b). Fig. 8 is the typical grain map for the steel isothermally held at 550 'C for 100 s from EBSD. Comparing the microstructure in Fig. 8 with that in Fig. 7 (b), the optical images define a much finer and interlocked microstructure. The grain maps are composed of monochromatic areas, and each point belonging to these areas exhibits a misorientation lower than a certain tolerance angle with its neighbor points. In the case of the grain map shown in Fig. 8, the tolerance angle imposed is 15°, which is taken as the .criterion of considering the microstructural unit that controls the brittle-fracture propagation in acicular ferrite microstructure'Y". Each one of these monochromatic areas in the grain map defines a crystallographic packet and each crystallorgraphic packet is generally formed by several ferrite plates, which can possess different spatial orientations and consequently are difinitely different from the morphological packets formed by parallel plates. The formation of an acicular ferrite packet is more obvious from the change of the average grain size with the tolerance angle. Fig. 9 shows the typical sections of the boundary misorientation maps obtained from the EBSD data of the present steel deformed at different temperatures
Fig. 8 Typical EBSD grain micrograph of acicular ferrite isothermally held at 550 t for 100 s
or holding times. In the boundary maps, the grain boundaries are distinguished by grey level according to the misorientation. The bold lines refer to high misorientation angle boundaries (HABs) greater than 15°. The slim lines represent low-angle boundaries (LABs) lower than 15°. It is seen that the initial grains of sample held at 550 'C for 10 s are finer while low-angle boundaries are more at an early stage of transformation. When the holding time is increased, the fraction of HABs increases and LABs decreases which is owing to the absorption of dislocations moving into boundaries and some sub-boundaries changed into high-angle boundaries. Holding at a higher temperature of 600 'C for 10 s , there are allotriomorphic ferrites or polygonal ferrite with high angle boundaries in austenite grain boundaries and the number of low misorientation angle boundaries is less to that held at 550 'C for 10 s obviously.
(a) Isothermally holding at 550 'C for 10 s; (b) Isothermally holding at 550 'C for 100 s; (c) Isothermally holding at 600 'C for 10 s.
Fig. 9
• 57 •
EBSD micrographs showing LABs and DABs in samples after different treatments
• 58 •
3
Journal of Iron and Steel Research. International
Discussion
3.1
lhmsfonnation characteristics of acicular ferrite The formation of acicular ferrite in pipeline steels is closely related to a heavy deformation in the austenite non-recrystallization region. The deformation of austenite below non-recrystallization temperature promotes the formation of acicular ferrite and quasi-polygonal ferrite, inhibits pearlite and bainite formation, and furthermore, enlarges the AF region significantly in the steel (Fig. 2). The high density of dislocations in the deformed austenite appears to be beneficial to the formation of AF instead of bainite. The main differences between bainite and acicular ferrite formation are found to be the nucleation sites and growth directions. Bainite nucleates at austenite grain boundaries and grows as a sheaf of parallel plates in the same growth direction within the austenite grains, whereas acicular ferrite generally nucleates intragranularly and grows as primary plates in the same or different orientations and thus, prior austenite grain boundaries become indistinct; then, a new set of secondary plates nucleates at the austenite/ primary AF' s interface. Consequently, AF is characterized by a chaotic arrangement of plates, showing a fine-grained interwoven morphology. as shown in Fig. 5 and Fig. 7. The acicular ferrite transformation is promoted and the identical acicular ferrite is formed in the range of 550 - 600 'C for the test steel, which is lower when compared with other studies[9] and is believed to be owing to the compositions of the steel. High niobium content increases the non-recrystallization temperature and allows greater reduction to be introduced into the austenite before transformation so as to achieve high volume fraction of AF in the pipeline steel; at the same time, niobium enhances acicular ferrite transformation through decreasing the transformation temperature and retarding the transformation of bainite microstructure. 3. 2
Microstructure characteristics of acicular ferrite Acicular ferrite is regarded as the most desirable microstructural feature in view of strength and toughness in mild and low alloyed steel weld metals. This is owing to its nature of small grain size and high angle boundaries[1l-12]. The evolution and diversity of mechanical properties for base metal demand the same properties to the weld. Fig. 7 shows that acicular ferrite grains with an
Vol. 17
apparently irregular or "feathery" shape are present independently within an austenite grain, and the acicular ferrite grain consists of a great amount of dislocations and subunits (Fig. 3). The adjacent acicular ferrite grains in the shape of a parallelogram show low misorientation angle and several acicular ferrite grains consist of a so-called crystallographic packet[IO,13]. The closed packets show a high angle boundaries (Fig. 9). This demonstrates the difference of crystallographic characteristics of the acicular ferrite in the steels and welds. The transformation of acicular ferrite shows characteristics of diffusionless transformation (Fig. 6); however, from Fig. 7 and Fig. 9, it can be seen that with the holding time or temperature increasing, there is little bainite microstructure and acicular ferrite laths is broaden. At the same time, the ratio of the number of high angle boundaries (larger than 15°) to that of low angle boundaries is large and the grain aspect ratios decreased, which is owing to the dislocation recovery or boundries coarsening. Thus, it shows a characteristics of diffusion in acicular ferrite growth.
4
Conclusions 1) Niobium enhances the formation of acicular
ferrite, and the microstructure of low-carbon high Nb-bearing steel is dominated by acicular ferrite at a broadened cooling rate higher than 5 'C / sunder continuous cooling conditions. 2) Acicular ferrite in test steel is formed at 550600 'C, and a great amount of dislocations and subunits are present. The formation process of acicular microstructure includes nucleation and coarsening. 3) Acicular ferrite consists of highly misoriented plates packets having internal low angle boundaries. With increasing the holding time or temperature, some low misorientation boundaries are changed to high misorientations owing to dislocations moving and grain boundaries coarsening. References : [lJ
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[3J
Yakubtsov I A. Poruks P, Boyd J D. Microstructure and Mechanical Properties of Bainitic Low Carbon High Strength Plate Steels [JJ. Materials Science and Engineering, 2008, 480AO/ 2): 109. XIAO Fu-ren , LIAO Bo, SHAN Yi-yin , et al, Challenge of Mechanical Properties of an Acicular Ferrite Pipeline Steel [JJ. Materials Science and Engineering, 2006, 431AO/2): 4l. Sang Yong Shin, Hwang Byoungchul , Kim Sangho , et al. Fracture Toughness Analysis in Transition Temperature Region of API X70 Pipeline Steels [JJ. Materials Science and Engineering, 2006, 429AO/2): 196.
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Microstructur~ and Transformation Characteristics of Acicular Ferrite in Microalloyed Steel
Zhao M C, Shan Y Y, Xiao F R, et al. Acicular Ferrite Formation During Hot Plate Rolling for Pipeline Steels [1]. Materials Science and Technology, Z003, 19(3): 355. Yakubtsov I A, Boyd J D. Effect of Alloying on Microstructure and Mechanical Properties of Bainitic High Strength Plate Steels [J]. Materials Science and Technology, Z008, Z4(Z): ZZI. Saha Podder A, Pandit A S, Murugaiyan A, et al. Phase Transformation Behavior in Two C-Mn-Si Based Steels Under Different Cooling Rates [1]. Ironmaking and Steelmaking, Z007, 34(1): 83. TANG Zheng-hua , Stumpf Waldo. The Role of Molybdenum Additions and Prior Deformation on Acicular Ferrite Formation in Microallyed Nb-Ti Low-Carbon Line-Pipe Steels [1]. Materials Characterization, Z008, 59(6): 717. Rees G I, Bhadeshia H K D H, Perdrix J. et al. The Effect of Niobium in Solid Solution on the Transformation Kinetics of Bainite [1]. Materials Science and Engineering, 1995, 194A (Z): 179. Kim Y M, Lee H, Kim N J. Transformation Behavior and Mi-
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