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Influence of Prior Austenite Deformation and Non-Metallic Inclusions on Ferrite Formation in Low-Carbon Steels
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ScienceDirect JOURNAL OF IRON AND STEEL RESEARCH, INTERNATIONAL. 2010, 17(6): 36-42
Influence of Prior Austenite Deformation and Non-Metallic Inclusions on Ferrite Formation in Low-Carbon Steels ZHANG Chi,
XIA Zhi-xin ,
YANG Zhi-gang,
LIU Zhi-hao
(Department of Materials Science and Engineering, Tsinghua University, Beijing 100084, China) Abstract: Effects of prior austenite deformation and non-metallic inclusions on the ferrite nucleation and grain refinement of two kinds of low-carbon steels have been studied. The ferrite nucleation on MnS and VCC,N) is observed. The combination of thermomechanical processes with adequate amounts of non-metallic inclusions formed in low-carbon steels could effectively refine the grain size and the microstructure. Ferrite nucleated on the single MnS or VCC,N) inclusions and complex MnS+VCC,N) inclusion. The proper addition of elements S and V could effectively promote the formation of ferrite and further refinement of ferrite grains. Key words: austenite deformation; non-metallic inclusion; ferrite nucleation; low-carbon steel
The ultra-low-carbon bainitic (ULCE) and high strength low alloy (HSLA) steel plates possessing excellent combination of strength, toughness and weldability have been developed to replace conventional medium-carbon steels in past few decades[l-Z]. These steels were commonly micro-alloyed with titanium, niobium and vanadium for grain refinement and carbide precipitation hardening. In addition, thermomechanical technology IS well known as an indispensable processing technology for these steels[3].
between austenite and ferrite may effectively reduce the energy barrier of IF transformation[lZ-lS]; the last one is resulted from the thermal strain energy around inclusions induced by their difference in thermal expansion coefficient between inclusions and matrix[16-17]. For HSLA and ULCB steels produced
Since the positive effects of non-metallic inclusions on intragranular ferrite (IF) in steels are disclosed, much attention has been paid to improve the toughness of the heat-affected zones (HAZ) of welded structure[4-S]. Plenty of research has been subject to microstructure refinement of HSLA steels using IF[6-7]. Three kinds of nucleation mechanisms of IF on non-metallic inclusions have also been put forward. One is attributed to chemical element depleted zone promoting the driving force of phase transformation around inclusions like manganese depleted zone (MDZ) and carbon depleted zone (CDZ) et al[S-Il]; the second is the lower interface energy
steels, few literatures could be available on the integrated effects of prior austenite deformation with varying deformation ratio and the amount of non-metallic inclusions on the ferrite nucleation, their grain refinement and microstructure evolution. In this study, compressive austenite deformation under various applied deformed volume has been carried out on two kinds of low carbon experimental steels containing different amount of non-metallic inclusions, aiming at investigating the integrative effects and operating mechanisms of austenite deformation and non-metallic inclusions on the ferrite nucleation and microstructure.
by accelerated cooling or direct quenching, prior compressive deformation of austenite has been proved to be an effective way to refine ferrite grain size, and considerably correlated investigation has been performed[lS-ZO]. However, for low carbon
Foundation Item: Item Supported by National Natural Science Foundation of China (50871059); Specialized Research Foundation for Doctoral Program of Higher Education (20070003006) Biography: ZHANG Chi0973-), Male. Doctor. Associate Professor;
E-mail: chizhang@mail. tsinghua, edu. en;
Received Date: May 12. 2009
Issue 6
1
Influence of Prior Austenite Deformation and Non-Metallic Inclusions on Low-Carbon Steels
• 37 •
Experimental Procedure
The testing materials in this research comprise of two kinds of experimental steels designated as experimental steel A and B, respectively. Their nominal chemical compositions are shown in Table 1. A is a kind of commercial steel produced by rolling. B is produced by cast with additions of S and V on the basis of A. The specimens with size of 16 mmX 25 mmX 60 mm used in deformation test were sectioned from experimental steel plate with the dimension of 60 mm along the rolling direction (RD) and the dimension of 25 mm along the transverse direction (TD). The prior compressive deformation of austenite was carried out on a Gleeble 1500 thermomechanical simulator at a constant strain of 5 S-I. The deformation method is shown in Fig. 1[21J • Table 1 Chemical composition of experimental steels (mass percent, Steel designation
C
Si
Mn
Steel A
0.08
0.90
Steel B
0.11
0.89
%)
P
S
V
Cr
2. 20
0.011
0.005
0.020
0.85
2.20
0.010
0.045
0.110
1. 03
Holding time 5s, 1 min and 10 min
Deformation ratio: Hl%, 20%, 30%,40%, 50%,60%
Fig. 2 Schematic diagram of heat treatment parameters
2
Results and Discussion
2. 1 Microstructure comparison between experimental steel A and B under same applied conditions For the sake of comparing the difference between experimental steel A and B under the same applied conditions. Prior to the microstructure observation, experimental steel A and B specimens were compressive deformed with ratios of 10 %- 60 %, and the holding time at 900 'C and 650 'C is 5 min. The counterpart results of the contents and the average grain size of ferrite between experimental steel A and B are presented in Fig. 3 and Fig. 4. It is shown in Fig. 3 that ferrite volume of experimental steel A similarly kept continuing increase with the increasing of deformation ratio. However, it displays a phenomena that ferrite content in experimental steel A is always a little higher than that in experimental steel B under the same treating proce-
oor----------------, 50
Fig. 1 Hot deformation method Before hot compressive deformation, the specimens were heated to 1250 'C at a heating rate of 10 'C / s and held for 3 min, which was determined according the precipitating temperature at about 1100 'C of VN in austenite'{'". And then cooling to 900 'C, the heat treated specimens were performed various kinds of heat treatment process with different deformed ratios of 10 %- 60 %. After deformation, the specimens were cooled to 650 'C at rate of 20 'C/ s and isothermal held for 5 min for ferrite phase transformation. The schematic illustration of correlative processing parameters is shown in Fig. 2.
~ oS ~
40
~
I': 0
'l:l 30
al
<1:1 4l
.§ ;;
/
20
10 '-'-_ _-'-_ _-'-_ _""'"---_ _-'--_ _.....
10
Fig. 3
20 30 40 50 Deformation volume of austeniteJ%
60
Comparison of ferrite volume fraction between experimental steel A and B under the same thermal-mechanical treatment conditions
Journal of Iron and Steel Research. International
• 38 •
18
J
16
gs
14
'E ~ .... 0
'Ii!
c::
'~
e ell
12
~
« 10 ......_ _......._ _'--_--'_ _......._ _........ 10
W
ro
~
~
00
Defonnation volume of austeniteJ%
Fig. 4
Comparison between experimental steel A and B under applied same conditions
dure , which was possibly resulted from the reason that the experimental steel A had lower carbon content than experimental steel B and more easily carried out the ferrite phase transformation. It indicates that the effect of carbon is much stronger than that of inclusions on ferrite formation in low carbon steel. Therefore, there is more ferrite in experimental steel A than experimental steel B. As shown in Fig. 4, the average grain sizes of ferrite in both experimental steel A and B decreased with the increasing of deformation ratio. It apparently displays that the average grain size of ferrite in experimental steel B is obviously smaller than that in experimental steel A under the same applied conditions, which maybe mainly attribute to more inclusions in experimental steel B pinned the grain boundary of original austenite grains and new formed ferrite grains and restricted the occurrence of dynamic recrystallization operation during the isothermal treatment at 650 'C [22] .
2. 2 Microstructure characterization of experimental steel B Fig. 5 presents typical microstructure morphology of experimental steel B in four kinds of different processing conditions. It is shown in Fig. 5 (a) that the microstructure is mainly composed of grain boundary allotriomorphic ferrite (GBF) and bainite. Moreover, the ferrite volume fraction (white area) is a little low. Since it is impossible to form adequate V(C, N) precipitates in 5 s , few IF was observed in austenite grains. Due to longer holding time (10 min) for V(C,N) precipitates formation in
Vol. I?
Fig. 5 (b), there are much higher volume of ferrites observed, even some IF in austenite grains could be observed. Owing 30% prior austenite deformation, the microstructure in Fig. 5 (c) is apparently different from the above two. The volume fraction of ferrites increases markedly. Due to shorter isothermal holding time, however, there are few V(C,N) precipitates formed in the matrix and the ferrite is almost distributed on the grain boundary and has higher average grain size. In contrast with Fig. 5 (c) and (d), it is easily found out that the average grain size of ferrite in Fig. 5 (d) is more refined and the volume fraction of ferrite in the latter is clearly higher. Moreover, there is much higher volume fraction of IF formed in original austenite grains. That was probably due to enough V(C,N) precipitates formed in Fig. 5 (d), more ferrite nucleation sites were provided. Prior austenite deformation provides adequate deformed storage energy for ferrite phase transformation, which significantly shortens the incubating period of ferrite nucleation. That directly results in more refined average grain size and more IF attained simultaneously. The specimen in Fig. 5 (d) was promised to be present the best combination of strength and toughness. From Fig. 5 (a) and (b), it could be inferred that MnS and V (C, N) inclusions could positively contribute to the intragranular nucleation of ferrite and increase their amounts. As shown in Fig. 5 (a) and (c), prior austenite deformation containing few inclusions would result in more GBF formation and much higher average grain size. That is possibly due to the deformation energy storage generated from the deformation process, which accelerate ferrite nucleation. Based on Fig. 5 (b) and (d), the viewpoint of the prior austenite deformation promoting the effect of MnS and V(C,N) inclusions induced intragranular ferrite formation and ferrite grain refinement could be directly displayed in some degree[23-26]. Therefore, it could be concluded that prior austenite deformation could maximize the effects of non-metallic inclusions on ferrite and IF nucleation. The volume fractions of GBF and IF in each image of Fig. 5 were measured quantitatively, as summarized in Fig. 6. It is more directly revealed that under the situation of few inclusions in the austenite grains and no deformation, the ferrite mostly formed at the grain boundary and hardly in the grains. If there are enough time for the formation of V(C, N)
Issue 6
Influence of Prior Austenite Deformation and Non-Metallic Inclusions on Low-Carbon Steels
precipitates or exert 30 % austenite deformation, intragranular nucleation of ferrite is benefited, but volume fraction of ferrite in the microstructure is still very low. Only under the condition of adequate
(a) Direct cooling at 900 'C for 5 s; 900 'C for 5 s;
Fig. 5
39 •
MnS and V ( C, N) precipitates formed and 30 % compressive deformation conducted, it is possible to form more ferrite and much higher volume fraction of IF. It indicates that prior austenite deformation have
(b) Direct cooling at 900 'C for 10 min; (c) 30% compressive deformation at (d) 30% compressive deformation at 900 ·C for 10 min.
Optical microstructures of experimental steel B in different heat treatment processes
strongly changed the internal structure of original austenite grains and provided more energy storage for intragranular ferrite nucleation and phase transformation.
No
MnS+
inclusion V(C,N) No deformation GBF-Grain boundary ferrite;
Fig. 6
No
MnS+
inclusion V(C,N) 30%deformation IF-Intragranular ferrite.
Ferrite volume percentage of experimental steel B for four kinds of processes
2. 3 Ferrite nucleation behavior on the inclusions in experimental steel B 2.3.1 Ferrite nucleation on the surface of MnS preci pitates It is well known that there may be three kind of inclusions constitute in experimental steel B, i. e. single MnS precipitate, single V (C, N) precipitate and their complex. In order to identify the actual situation of ferrite nucleation in experimental B, the specimens with 30 % compressive deformation at a
• 40 •
Vol. 17
Journal of Iron and Steel Research, International
deformed rate of 10 S-l were observed by SEM. The specimens were held for 5 min at 650 'C for full transformation of ferrite. The SEM image is shown as Fig. 7. The EDS analysis reveals that the inclusion in the ferrite is MnS precipitate. It is a pity that there are not V(C, N) inclusions observed in the specimens using SEM with considerable efforts. The possible reason is that due to the smaller size VeC,N) inclusions
have come off during preparing the SEM sample, or the magnification of SEM is still not enough. 2. 3. 2 Ferrite nucleation on the surface of MnS and complex MnS+VeC,N) inclusions In order to further confirm whether the ferrite has nucleated on the other ferrites or MnS+ vee, N) complex, the specimens were observed by TEM, as shown in Fig. 8 and Fig. 9. (b)
S
Mn Fe
Fe
~Mn
Mn
o
A 1
'I. 2
345
6
~-~ 7
EnergylkeV Ca) SEM image;
Fig. 7
(a) Bright field image;
Cb) EDS spectrum.
SEM image of ferrite nucleation at MnS inclusion
(b) Dark field image;
CC) Selected area diffraction patterns CSADP);
Cd) Identification of diffraction patterns.
Fig. 8
Identification of ferrite nucleated on inclusions with TEM
8
Issue 6
Influence of Prior Austenite Deformation and Non-Metallic Inclusions on Low-Carbon Steels
(a) Bright field image;
Fig. 9
(b) Dark field image of inclusion in the middle part; (d) Identification of SADP. inclusion on the edge;
• 41 •
(c) Dark field image of
Identification of ferrite nucleated on inclusion with TEM
TEM observation revealed that the ferrite had nucleated on different morphology inclusions. In order to identify the compositions and constitute of the inclusions. phase identification of inclusions in Fig. 8 (a) has been conducted. The dark field image and diffraction patterns of Fig. 8 Ca) and the identification are presented in Fig. 8 (b) to (d). Through measuring the distance between diffraction dots and comparing with ASTM cards. it could be identified that the inclusion in Fig. 8 (a) is VCC.N). There are some parallel crystal-band axis between ferrite and V CC. N) precipitate with low energy shown as Fig. 8 Cd)[15.27], which could confirm one kind of ferrite nucleation mechanism of crystal lattice interface energy's theory operated on between ferrite and inclusions[28]. The similar experimental procedure identify that the inclusions in Fig. 9 Ca) is composed of MnS located in the edge and VCN .C) in the middle. which displays a different dark images like Fig. 9 Cb) and Cc). It also proves that the V CC. N) precipitate could nucleate in the MnS inclusion, which is similar to Miyamoto and Maki's reports[15.28].
VCC,N) inclusions have obviously increased the volume fraction of ferrite. In addition. there is more high volume fraction of IF formed in original austenite grains but the average size of the grains basically keeps stable. 2) The ferrite volume in the experimental steel B with more MnS and VCC, N) inclusions has a remarkably continuous increase with the increase of deformation ratio in the range from 10 % to 60 %• and the average grain size of ferrite is refined. 3) Microstructure observation of experimental steel Busing SEM and TEM has verified that the ferrite nucleats on the single MnS or V(C.N) inclusions and complex MnS+ V(C,N) inclusion. 4) The suitable addition of element S and V could effectively promote the formation of ferrite and further refinement of ferrite grains, because the more inclusions induce the heterogeneous nucleation operation. References : [IJ
Garcia C I. Lis A K. Pytel S M. et al. Ultra-Low Carbon Bainitic Plate Steels: Processing. Microstructure and Properties
3
[2J
[J]. ISS Trans. 1992. 13: 103. Hulka K, Hesterkamp F, Nachtel L. Microstructure and Prop-
[3J
erties of HSLA Steels [M]. Warrendale: TMS, 1988. Ouchi C. Development of Steel Plates by Intensive Use of TM-
Conclusions 1) With 30% compressive deformation, the two
kinds of experimental steels containing MnS and
• 42 •
[4J
[5J
[6J
[7J
[8J
[9J
[10J
[l1J
[12J
[13J
[14J
[15 J
[16J
Journal of Iron and Steel Research, International
CP and Direct Quenching Processes [JJ. ISIJ Int. 2001, 41 (6):542. Lee J L, Pan Y T. The Formation of Intragranular Acicular Ferrite in Simulated Heat Affected Zone [JJ. ISIJ Int. 1995, 35(8): 1027. Zhang Z. Farrar R A. Role of Non-Metallic Inclusions in Formation of Acicular Ferrite in Low Alloy Weld Metals [JJ. Mater Sci Technol , 1996, 12(3): 237. Madariaga I. Gutierrez I. Role of the Particle-Matrix Interface on the Nucleation of Acicular Ferrite in a Medium Carbon Microalloyed Steel [JJ. Acta Mater. 1999.47(3): 951. Shim J H. Cho Y W. Chung S H. et al. Nucleation of Intragranular Ferrite at Ti 2 0 a Particle in Low Carbon Steel [JJ. Acta Mater. 1999. 47(9): 2751. Mills A R, Thewlis G. Whiteman J A. Nature of Inclusions in Steel Weld Metals and Their Influence on Formation of Acicular Ferrite [JJ. Mater Sci Technol , 1987. 3(2): 1051. Shigesato G. Sugiyama M. Development of in Situ Observation Technique Using Scanning Ion Microscopy and Demonstration of Mn Depletion Effect on Intragranular Ferrite Transformation in Low-Alloy Steel [n J Electron Microsc, 2002. 51(6): 359. Kim H S. Lee H G, Oh K S. MnS Precipitation in Association With Manganese Silicate Inclusions in Si/Mn Deoxidized Steel [JJ. Metall Mater Trans. 2001. 32A(6): 1519. Shim J H. Oh Y J. Sun J Y. et al. Ferrite Nucleation Potency of Non-Metallic Inclusions in Medium Carbon Steels [JJ. Acta Mater. 2001. 49(2): 2115. Yang Z G. Enomoto M. A Discrete Lattice Plane Analysis of Coherent f. c. c. /Bl Interfacial Energy [JJ. Acta Mater, 1999. 47(8): 4515. Yang Z G, Enomoto M. Calculation of the Interfacial Energy of BI-Type Carbides and Nitrides With Austenite [JJ. Metall Mater Trans. 2001. 32A(2): 267. Yang Z G. Enomoto M. Discrete Lattice Plane Analysis of Baker-Nutting Related Bl Compound/Ferrite Interfacial Energy [JJ. Mater Sci Eng. 2002. A332: 184. Miyamoto G. Shinyoshi T • Yamaguchi J. et al. Crystallography of Intragranular Ferrite Formed on [MnS+ V(C. N) J Complex Precipitate in Austenite [JJ. Scripta Mater. 2003, 48(4): 371. Pan T. Yang Z G. Bai B Z. et al. Study of Thermal Stress and Strain Energy in y-Fe Matrix Around Inclusion Caused by
[17J
[18J
[19J
[20J
[21]
[22J
[23J
[24J [25J
[26J
[27J
[28J
Vo!.17
Thermal Coefficient Difference [JJ. Acta Mater Sinica , 2003. 39(0): 1037. Enomoto M. Nucleation of Phase Transformations at Intragranular Inclusions in Steel [JJ. Metall Mater. 1998, 4(2), 115. Gibbs R K. Hodgson P D. Parker B A. The Formation of the Highly Crystallographic Bainitic [CJ //Liaw P K, eds, Proceedings of the Morris E. Fine Symposium. Warrendale, TMS, 1991, 73. Bakkaloglu A. Effect of Processing Parameters on the Microstructure and Properties of an Nb Microalloyed Steel [JJ. Mater Lett, 2000, 56(3), 200. Chiou C S, Yang J R, Huang C Y. The Effect of Prior Compressive Deformation of Austenite on Toughness Property in an Ultra-Low Carbon Bainitic Steel [JJ. Mater Chern Phys , 2001. 690/2/3): 113. Wang J P. Yang Z G. Bai B Z. et al. Grain Refinement and Microstructural Evolution of Grain Boundary Allotriomorphic Ferrite/Granular Bainite Steel After Prior Austenite Deformation [JJ. Mater Sci Eng, 2004. 369AO/2): 112. Misra R D K. Nathani H. Hartmann J E. et al. Microstructural Evolution in a New 770 MPa Hot Rolled Nb- Ti Microalloyed Steel [JJ. Mater Sci Eng, 2005, 394AO/2), 339. Lee J L. Pan Y T. Effect of Sulfur Content on the Microstructure and Toughness of Simulated Heat-Affected Zone in TiKilled Steels [JJ. Metall Trans. 1993, 24A(6): 1399. Tomita Y. Saito N. Improvement in HAZ Toughness of Steel by TiN-MnS Addition [J]. ISIJ Int. 1994. 34(0): 829. Maloney J L, Garrison W M. The Effect of Sulfide Type on the Fracture Behavior of HY180 Steel [JJ. Acta Mater. 2005, 53(2), 533. GUO A M. LI S R. GUO J. et al. Effect of Zirconium Addition on the Impact Toughness of the Heat Affected Zone in a High Strength Low Alloy Pipeline Steel [JJ. Mater Charact , 2008. 59(2): 134. PAN T • YANG Z G • ZHANG C, et al. Kinetics and Mechanisms of Intragranular Ferrite Nucleation on Non-Metallic Inclusions in Low Carbon Steels [JJ. Mater Sci Eng. 2006. 438440A: 1128. Maki T, Furuhara T. Effect of Oxygen Potential on Cohesion of Pulverized Iron Oxide Under a Reducing Atmosphere [JJ. Tetsu-to-Hagane , 2005. 91(1), 16.
ScienceDirect JOURNAL OF IRON AND STEEL RESEARCH, INTERNATIONAL. 2010, 17(6): 36-42
Influence of Prior Austenite Deformation and Non-Metallic Inclusions on Ferrite Formation in Low-Carbon Steels ZHANG Chi,
XIA Zhi-xin ,
YANG Zhi-gang,
LIU Zhi-hao
(Department of Materials Science and Engineering, Tsinghua University, Beijing 100084, China) Abstract: Effects of prior austenite deformation and non-metallic inclusions on the ferrite nucleation and grain refinement of two kinds of low-carbon steels have been studied. The ferrite nucleation on MnS and VCC,N) is observed. The combination of thermomechanical processes with adequate amounts of non-metallic inclusions formed in low-carbon steels could effectively refine the grain size and the microstructure. Ferrite nucleated on the single MnS or VCC,N) inclusions and complex MnS+VCC,N) inclusion. The proper addition of elements S and V could effectively promote the formation of ferrite and further refinement of ferrite grains. Key words: austenite deformation; non-metallic inclusion; ferrite nucleation; low-carbon steel
The ultra-low-carbon bainitic (ULCE) and high strength low alloy (HSLA) steel plates possessing excellent combination of strength, toughness and weldability have been developed to replace conventional medium-carbon steels in past few decades[l-Z]. These steels were commonly micro-alloyed with titanium, niobium and vanadium for grain refinement and carbide precipitation hardening. In addition, thermomechanical technology IS well known as an indispensable processing technology for these steels[3].
between austenite and ferrite may effectively reduce the energy barrier of IF transformation[lZ-lS]; the last one is resulted from the thermal strain energy around inclusions induced by their difference in thermal expansion coefficient between inclusions and matrix[16-17]. For HSLA and ULCB steels produced
Since the positive effects of non-metallic inclusions on intragranular ferrite (IF) in steels are disclosed, much attention has been paid to improve the toughness of the heat-affected zones (HAZ) of welded structure[4-S]. Plenty of research has been subject to microstructure refinement of HSLA steels using IF[6-7]. Three kinds of nucleation mechanisms of IF on non-metallic inclusions have also been put forward. One is attributed to chemical element depleted zone promoting the driving force of phase transformation around inclusions like manganese depleted zone (MDZ) and carbon depleted zone (CDZ) et al[S-Il]; the second is the lower interface energy
steels, few literatures could be available on the integrated effects of prior austenite deformation with varying deformation ratio and the amount of non-metallic inclusions on the ferrite nucleation, their grain refinement and microstructure evolution. In this study, compressive austenite deformation under various applied deformed volume has been carried out on two kinds of low carbon experimental steels containing different amount of non-metallic inclusions, aiming at investigating the integrative effects and operating mechanisms of austenite deformation and non-metallic inclusions on the ferrite nucleation and microstructure.
by accelerated cooling or direct quenching, prior compressive deformation of austenite has been proved to be an effective way to refine ferrite grain size, and considerably correlated investigation has been performed[lS-ZO]. However, for low carbon
Foundation Item: Item Supported by National Natural Science Foundation of China (50871059); Specialized Research Foundation for Doctoral Program of Higher Education (20070003006) Biography: ZHANG Chi0973-), Male. Doctor. Associate Professor;
E-mail: chizhang@mail. tsinghua, edu. en;
Received Date: May 12. 2009
Issue 6
1
Influence of Prior Austenite Deformation and Non-Metallic Inclusions on Low-Carbon Steels
• 37 •
Experimental Procedure
The testing materials in this research comprise of two kinds of experimental steels designated as experimental steel A and B, respectively. Their nominal chemical compositions are shown in Table 1. A is a kind of commercial steel produced by rolling. B is produced by cast with additions of S and V on the basis of A. The specimens with size of 16 mmX 25 mmX 60 mm used in deformation test were sectioned from experimental steel plate with the dimension of 60 mm along the rolling direction (RD) and the dimension of 25 mm along the transverse direction (TD). The prior compressive deformation of austenite was carried out on a Gleeble 1500 thermomechanical simulator at a constant strain of 5 S-I. The deformation method is shown in Fig. 1[21J • Table 1 Chemical composition of experimental steels (mass percent, Steel designation
C
Si
Mn
Steel A
0.08
0.90
Steel B
0.11
0.89
%)
P
S
V
Cr
2. 20
0.011
0.005
0.020
0.85
2.20
0.010
0.045
0.110
1. 03
Holding time 5s, 1 min and 10 min
Deformation ratio: Hl%, 20%, 30%,40%, 50%,60%
Fig. 2 Schematic diagram of heat treatment parameters
2
Results and Discussion
2. 1 Microstructure comparison between experimental steel A and B under same applied conditions For the sake of comparing the difference between experimental steel A and B under the same applied conditions. Prior to the microstructure observation, experimental steel A and B specimens were compressive deformed with ratios of 10 %- 60 %, and the holding time at 900 'C and 650 'C is 5 min. The counterpart results of the contents and the average grain size of ferrite between experimental steel A and B are presented in Fig. 3 and Fig. 4. It is shown in Fig. 3 that ferrite volume of experimental steel A similarly kept continuing increase with the increasing of deformation ratio. However, it displays a phenomena that ferrite content in experimental steel A is always a little higher than that in experimental steel B under the same treating proce-
oor----------------, 50
Fig. 1 Hot deformation method Before hot compressive deformation, the specimens were heated to 1250 'C at a heating rate of 10 'C / s and held for 3 min, which was determined according the precipitating temperature at about 1100 'C of VN in austenite'{'". And then cooling to 900 'C, the heat treated specimens were performed various kinds of heat treatment process with different deformed ratios of 10 %- 60 %. After deformation, the specimens were cooled to 650 'C at rate of 20 'C/ s and isothermal held for 5 min for ferrite phase transformation. The schematic illustration of correlative processing parameters is shown in Fig. 2.
~ oS ~
40
~
I': 0
'l:l 30
al
<1:1 4l
.§ ;;
/
20
10 '-'-_ _-'-_ _-'-_ _""'"---_ _-'--_ _.....
10
Fig. 3
20 30 40 50 Deformation volume of austeniteJ%
60
Comparison of ferrite volume fraction between experimental steel A and B under the same thermal-mechanical treatment conditions
Journal of Iron and Steel Research. International
• 38 •
18
J
16
gs
14
'E ~ .... 0
'Ii!
c::
'~
e ell
12
~
« 10 ......_ _......._ _'--_--'_ _......._ _........ 10
W
ro
~
~
00
Defonnation volume of austeniteJ%
Fig. 4
Comparison between experimental steel A and B under applied same conditions
dure , which was possibly resulted from the reason that the experimental steel A had lower carbon content than experimental steel B and more easily carried out the ferrite phase transformation. It indicates that the effect of carbon is much stronger than that of inclusions on ferrite formation in low carbon steel. Therefore, there is more ferrite in experimental steel A than experimental steel B. As shown in Fig. 4, the average grain sizes of ferrite in both experimental steel A and B decreased with the increasing of deformation ratio. It apparently displays that the average grain size of ferrite in experimental steel B is obviously smaller than that in experimental steel A under the same applied conditions, which maybe mainly attribute to more inclusions in experimental steel B pinned the grain boundary of original austenite grains and new formed ferrite grains and restricted the occurrence of dynamic recrystallization operation during the isothermal treatment at 650 'C [22] .
2. 2 Microstructure characterization of experimental steel B Fig. 5 presents typical microstructure morphology of experimental steel B in four kinds of different processing conditions. It is shown in Fig. 5 (a) that the microstructure is mainly composed of grain boundary allotriomorphic ferrite (GBF) and bainite. Moreover, the ferrite volume fraction (white area) is a little low. Since it is impossible to form adequate V(C, N) precipitates in 5 s , few IF was observed in austenite grains. Due to longer holding time (10 min) for V(C,N) precipitates formation in
Vol. I?
Fig. 5 (b), there are much higher volume of ferrites observed, even some IF in austenite grains could be observed. Owing 30% prior austenite deformation, the microstructure in Fig. 5 (c) is apparently different from the above two. The volume fraction of ferrites increases markedly. Due to shorter isothermal holding time, however, there are few V(C,N) precipitates formed in the matrix and the ferrite is almost distributed on the grain boundary and has higher average grain size. In contrast with Fig. 5 (c) and (d), it is easily found out that the average grain size of ferrite in Fig. 5 (d) is more refined and the volume fraction of ferrite in the latter is clearly higher. Moreover, there is much higher volume fraction of IF formed in original austenite grains. That was probably due to enough V(C,N) precipitates formed in Fig. 5 (d), more ferrite nucleation sites were provided. Prior austenite deformation provides adequate deformed storage energy for ferrite phase transformation, which significantly shortens the incubating period of ferrite nucleation. That directly results in more refined average grain size and more IF attained simultaneously. The specimen in Fig. 5 (d) was promised to be present the best combination of strength and toughness. From Fig. 5 (a) and (b), it could be inferred that MnS and V (C, N) inclusions could positively contribute to the intragranular nucleation of ferrite and increase their amounts. As shown in Fig. 5 (a) and (c), prior austenite deformation containing few inclusions would result in more GBF formation and much higher average grain size. That is possibly due to the deformation energy storage generated from the deformation process, which accelerate ferrite nucleation. Based on Fig. 5 (b) and (d), the viewpoint of the prior austenite deformation promoting the effect of MnS and V(C,N) inclusions induced intragranular ferrite formation and ferrite grain refinement could be directly displayed in some degree[23-26]. Therefore, it could be concluded that prior austenite deformation could maximize the effects of non-metallic inclusions on ferrite and IF nucleation. The volume fractions of GBF and IF in each image of Fig. 5 were measured quantitatively, as summarized in Fig. 6. It is more directly revealed that under the situation of few inclusions in the austenite grains and no deformation, the ferrite mostly formed at the grain boundary and hardly in the grains. If there are enough time for the formation of V(C, N)
Issue 6
Influence of Prior Austenite Deformation and Non-Metallic Inclusions on Low-Carbon Steels
precipitates or exert 30 % austenite deformation, intragranular nucleation of ferrite is benefited, but volume fraction of ferrite in the microstructure is still very low. Only under the condition of adequate
(a) Direct cooling at 900 'C for 5 s; 900 'C for 5 s;
Fig. 5
39 •
MnS and V ( C, N) precipitates formed and 30 % compressive deformation conducted, it is possible to form more ferrite and much higher volume fraction of IF. It indicates that prior austenite deformation have
(b) Direct cooling at 900 'C for 10 min; (c) 30% compressive deformation at (d) 30% compressive deformation at 900 ·C for 10 min.
Optical microstructures of experimental steel B in different heat treatment processes
strongly changed the internal structure of original austenite grains and provided more energy storage for intragranular ferrite nucleation and phase transformation.
No
MnS+
inclusion V(C,N) No deformation GBF-Grain boundary ferrite;
Fig. 6
No
MnS+
inclusion V(C,N) 30%deformation IF-Intragranular ferrite.
Ferrite volume percentage of experimental steel B for four kinds of processes
2. 3 Ferrite nucleation behavior on the inclusions in experimental steel B 2.3.1 Ferrite nucleation on the surface of MnS preci pitates It is well known that there may be three kind of inclusions constitute in experimental steel B, i. e. single MnS precipitate, single V (C, N) precipitate and their complex. In order to identify the actual situation of ferrite nucleation in experimental B, the specimens with 30 % compressive deformation at a
• 40 •
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Journal of Iron and Steel Research, International
deformed rate of 10 S-l were observed by SEM. The specimens were held for 5 min at 650 'C for full transformation of ferrite. The SEM image is shown as Fig. 7. The EDS analysis reveals that the inclusion in the ferrite is MnS precipitate. It is a pity that there are not V(C, N) inclusions observed in the specimens using SEM with considerable efforts. The possible reason is that due to the smaller size VeC,N) inclusions
have come off during preparing the SEM sample, or the magnification of SEM is still not enough. 2. 3. 2 Ferrite nucleation on the surface of MnS and complex MnS+VeC,N) inclusions In order to further confirm whether the ferrite has nucleated on the other ferrites or MnS+ vee, N) complex, the specimens were observed by TEM, as shown in Fig. 8 and Fig. 9. (b)
S
Mn Fe
Fe
~Mn
Mn
o
A 1
'I. 2
345
6
~-~ 7
EnergylkeV Ca) SEM image;
Fig. 7
(a) Bright field image;
Cb) EDS spectrum.
SEM image of ferrite nucleation at MnS inclusion
(b) Dark field image;
CC) Selected area diffraction patterns CSADP);
Cd) Identification of diffraction patterns.
Fig. 8
Identification of ferrite nucleated on inclusions with TEM
8
Issue 6
Influence of Prior Austenite Deformation and Non-Metallic Inclusions on Low-Carbon Steels
(a) Bright field image;
Fig. 9
(b) Dark field image of inclusion in the middle part; (d) Identification of SADP. inclusion on the edge;
• 41 •
(c) Dark field image of
Identification of ferrite nucleated on inclusion with TEM
TEM observation revealed that the ferrite had nucleated on different morphology inclusions. In order to identify the compositions and constitute of the inclusions. phase identification of inclusions in Fig. 8 (a) has been conducted. The dark field image and diffraction patterns of Fig. 8 Ca) and the identification are presented in Fig. 8 (b) to (d). Through measuring the distance between diffraction dots and comparing with ASTM cards. it could be identified that the inclusion in Fig. 8 (a) is VCC.N). There are some parallel crystal-band axis between ferrite and V CC. N) precipitate with low energy shown as Fig. 8 Cd)[15.27], which could confirm one kind of ferrite nucleation mechanism of crystal lattice interface energy's theory operated on between ferrite and inclusions[28]. The similar experimental procedure identify that the inclusions in Fig. 9 Ca) is composed of MnS located in the edge and VCN .C) in the middle. which displays a different dark images like Fig. 9 Cb) and Cc). It also proves that the V CC. N) precipitate could nucleate in the MnS inclusion, which is similar to Miyamoto and Maki's reports[15.28].
VCC,N) inclusions have obviously increased the volume fraction of ferrite. In addition. there is more high volume fraction of IF formed in original austenite grains but the average size of the grains basically keeps stable. 2) The ferrite volume in the experimental steel B with more MnS and VCC, N) inclusions has a remarkably continuous increase with the increase of deformation ratio in the range from 10 % to 60 %• and the average grain size of ferrite is refined. 3) Microstructure observation of experimental steel Busing SEM and TEM has verified that the ferrite nucleats on the single MnS or V(C.N) inclusions and complex MnS+ V(C,N) inclusion. 4) The suitable addition of element S and V could effectively promote the formation of ferrite and further refinement of ferrite grains, because the more inclusions induce the heterogeneous nucleation operation. References : [IJ
Garcia C I. Lis A K. Pytel S M. et al. Ultra-Low Carbon Bainitic Plate Steels: Processing. Microstructure and Properties
3
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Conclusions 1) With 30% compressive deformation, the two
kinds of experimental steels containing MnS and
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[5J
[6J
[7J
[8J
[9J
[10J
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[13J
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[15 J
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