Microstructure and Mechanical Properties of Microalloyed

High-Strength Transformation-Induced Plasticity Steels

X.D. WANG, B.X. HUANG, L. WANG, and Y.H. RONG

The high strength of transformation induced plasticity (TRIP) steels with tensile strength from
800 to 1000 MPa were designed based on grain refinement and precipitation strengthening
through microalloying with Nb, Nb/V, and Nb/Mo in a Fe-0.2C-1.5Si-1.5Mn cold-rolled TRIP
steel. The origins of alloying strengthening for three grades of 860, 950, and 1010 MPa TRIP
steels obtained in this work were revealed by the combination of Thermo-Calc and transmission
electron microscopy (TEM). The addition of Nb in Nb, Nb/V, and Nb/Mo TRIP steels can
effectively refine the austenite grain in the hot-rolled process by the NbC carbides retarding
austenite recrystallization and, in turn, refine final microstructure after intercritical annealing.
The addition of Nb/V can precipitate partially fine and dispersive (Nb,V)C carbides in ferrite
grains instead of coarse NbC carbides; therefore, the precipitation strengthening plays an
important role in the increase of TRIP steel strength. The addition of Nb/Mo cannot only
precipitate fully fine and dispersive (Nb,Mo)C carbides in ferrite grains but also increase the
volume fraction of bainite accompanying the decrease of volume fraction of ferrite, leading to
the drastic increase of both the yield strength and tensile strength.

DOI: 10.1007/s11661-007-9366-4

The Minerals, Metals & Materials Society and ASM International 2007

I. INTRODUCTION II. EXPERIMENTAL PROCEDURES

THE tensile strength of transformation induced A. Composition of Steels and the Heat-Treatment
plasticity (TRIP) steels typically ranges from 600 to Process
800 MPa. In order to attain a drastic weight reduction The chemical composition of the TRIP steels with Nb,
and safety improvement of vehicle, high-strength TRIP Nb/V, and Nb/Mo microalloying in the present work as
steels with tensile strength of above 800 MPa are well as the reference TRIP steel (R-TRIP) are listed in
required for automobile applications such as pillars Table I. They were melted in a medium frequency
and reinforcement bars.[1] High-strength steels with a furnace in the laboratory in the Technology Center of
good balance of high strength and good ductility can be Shanghai Baosteel. The ingots were forged to a thick-
achieved by grain refinement strengthening, precipita- ness of 35 mm. Then, the slabs of R-TRIP and the other
tion strengthening, and phase transformation strength- three steels were reheated to a temperature of 1200 C
ening. Therefore, the combination of strengthening and 1250 C for 1 hour, respectively, and hot rolled to a
methods mentioned previously can be used to design a thickness of 3 mm with the finishing temperature of
new TRIP steel to meet the higher strength TRIP grade. 860 C. The steel sheets were finally air cooled to room
The most effective way to improve the strength and temperature and cold rolled to a thickness of 1 mm. The
ductility as well as weldability is the addition of intercritical annealing was carried out at 800 C for
microalloying elements such as Ti, Nb, V, and Mo. 70 seconds, and then the steels were slowly cooled to
Many investigations have been focused on the addition 690 C with a cooling rate of 10 C/s, isothermally held
of Nb and Ti[2–6] in TRIP steels, but there are few at 400 C for 4 minutes followed by water quenching, as
reports on Nb/V[7,8] and Nb/Mo[9] complex microalloy- shown in Figure 1.
ing for hot- or cold-rolled TRIP steels. The aim of this
work is to develop 800 to 1000 MPa grade cold-rolled
TRIP steels using grain refinement of ferrite and
precipitation in both ferrite and austenite, and to reveal B. Test and Characterization Procedures
the origins of microalloying strengthening. The mechanical properties were measured on a Zwick
T1-FR020TN A50 tensile testing machine (ZWICK
Company, Germany) at the strain rate of 10-3 s-1 with
specimens of gage length of 50 mm, gage width of
Drs. X.D. WANG and B.X. HUANG, and Y.H. RONG, Professor 12.5 mm, and thickness of 1.0 mm at room temperature.
are with the School of Materials Science and Engineering, Shanghai The tensile specimens were machined with their axis
Jiao Tong University, Shanghai 200240, P.R. China. Contact e-mail: oriented parallel to the rolling direction. The micro-
yhrong@sjtu.edu.cn Dr. L. WANG is with the Baosteel Research and
Development technology Center, Shanghai 201900, P.R. China.
structures were studied by optical microscopy (OM) and
Manuscript submitted January 31, 2007. scanning electron microscopy (SEM) after conventional
Article published online November 21, 2007 Nital etching. Transmission electron microscopy (TEM)

METALLURGICAL AND MATERIALS TRANSACTIONS A VOLUME 39A, JANUARY 2008—1

 Table I. Chemical Composition of the TRIP Steels (Mass Percent)

Steel C Mn Si P S Nb Mo V
R-TRIP 0.19 1.46 1.53 0.005 0.004 — — —
Nb 0.17 1.49 1.56 0.006 0.004 0.044 — —
Nb/V 0.17 1.50 1.50 0.006 0.004 0.043 — 0.089
Nb/Mo 0.18 1.50 1.50 0.006 0.005 0.045 0.13 —

211a peaks. The carbon concentration was estimated by

both the measurement of lattice constant from 220c and
311c peaks and using the equation proposed by Dyson
and Holmes.[10] Thermodynamic calculations were per-
formed to determine the precipitation temperature of
Nb- and V- carbides using the Thermo-Calc software. In
the calculation process, the SSOL database was used,
and the phases including liquid, fcc, bcc, hcp, and all
kinds of carbides in the studied alloying system neglect-
ing S and P elements were selected.

III. RESULTS
A. Variation of Microstructures with Alloying Elements
Fig. 1—Schematic diagram of heat-treatment process.
The microstructure of the studied steels mainly
comprises polygonal ferrite (PF), bainite (B), and
retained austenite (Ar). An identification method and
specimens were prepared by mechanical polishing and its detailed description of the microstructure of the
electropolishing in a twin-jet polisher using 4 pct per- R-TRIP steel were given elsewhere.[11,12] The effect of
chloric acid solution. The TEM investigation was alloying elements on the microstructure of the steels is
carried out in a JEM-100CX (JEOL Company, Japan) shown in Figure 2, in which the large gray areas
operated at 100 kV to identify the microstructure and represent PF, and the fine plumbeous areas represent
precipitates. The volume fraction of retained austenite B and Ar in the OM micrographs. It can be seen
was measured by X-ray diffraction (XRD) with Cu Ka from Figure 2 that the average ferrite grain size of
radiation based on a direct comparison method of Nb-containing TRIP steels including Nb/V and Nb/Mo
comparing with the integrated intensity of the 220c and ones is smaller than that of Nb-free steel, although the

Fig. 2—OM micrographs of steels for (a) R-TRIP, (b) Nb, (c) Nb/V, and (d) Nb/Mo.

2—VOLUME 39A, JANUARY 2008 METALLURGICAL AND MATERIALS TRANSACTIONS A

average grain size of Nb/Mo steel is somewhat larger The significant difference of microalloying TRIP steels
than that of Nb or Nb/V steels. from the R-TRIP steel is the precipitation of carbides in
The SEM micrographs in Figure 3 show the typical the former. Figure 6(a) shows the carbides with the two
morphologies of different microstructures, including average particle sizes of 40 ± 10 nm (polygonal car-
polygonal ferrite and precipitates in both ferrite interior bides) and 5 ± 2 nm (spherical carbides) in Nb/V TRIP
and the ferrite grain boundaries (Figure 3(a)) as well as steel, respectively, and they were identified as NbC or
the bainite and retained austenite (Figure 3(b)). It is (Nb,V)C with fcc lattice by SAED, as shown by the
clear from Figure 3(b) that the partially bainite trans- inserted diffraction pattern in Figure 6(b). Moreover, the
formation from austenite exhibits ‘‘incomplete reaction orientation relationship between the ferrite matrix and
phenomenon’’[13] due to the existence of T0 line.[14] The the NbC or (Nb,V)C carbides is identified by SAED as
identification of phases in different microstructures for ½112
a ==½011
MC ; ð110Þa ==ð100ÞMC . The center dark-field
the R-TRIP steel was further carried out by TEM. It can image with strong diffraction spot of g200 clearly shows
be seen from Figure 4 that the islandlike austenite with both fine carbides and coarse ones.
fcc structure forms in the interior and at grain bound- It is clear from Figure 7 that the large number of
aries of ferrite. The selected area electron diffraction (Nb,Mo)C carbides with fcc lattice are identified by
(SAED) pattern (Figure 4(c)) has been obtained from SAED, mainly precipitate at the dislocations in ferrite
the islandlike austenite in the ferrite interior and is the matrix (Figure 7(a)), and exhibit uniform distribution
same as obtained from retained austenite islands located (Figure 7(b)). The average size of (Nb,Mo)C precipitates
at the ferrite grain boundary. Therefore, the dark-field was determined to be 5 ± 2 nm (Figure 7(b)). From the
image (Figure 4(b)) shows both the islandlike austenite diffraction pattern of (Nb,Mo)C carbides (Figure 7(c)),
in the interior and at the grain boundary of ferrite. This it can be determined that the orientation relationship
implies that they belong to an original austenite. between the ferrite matrix and the fine (Nb,Mo)C
Comparing the bright-field image (Figure 5(a)) with carbides is ½012
a ==½110
MC ; ð100Þa ==ð110ÞMC , which is
dark-field images ( Figures 5(b) and (c)), the filmlike different from the orientation relationship between the
austenite between the bainite plates with bcc structure ferrite matrix and the NbC or (Nb,V)C carbides.
growing from the ferrite boundaries was also identified
with the combination of SAED (Figure 5(d)). More-
B. Mechanical Properties
over, there are no carbides between bainite plates or the
bainite interior, which results from the suppressing effect The mechanical properties of the studied steels are
of Si on carbides in R-TRIP steel. It is worth noting that summarized in Table II. All steels exhibit a good
Si in TRIP steel can only suppress cementite (Fe3C) but combination of strength and ductility. It is clear from
not other kinds of carbides[15,16] and, thus, bainitic Table II that both the yield strength (R0.2) and tensile
carbides probably exist in Nb/V or Nb/Mo TRIP steel. strength (Rm) of the Nb-containing steels are improved
This consideration needs to be verified by experiment. compared with the Nb-free (R-TRIP) steel; namely, the

Fig. 3—SEM micrographs of Nb/V steel: (a) lower magnification image and (b) larger magnification image.

Fig. 4—TEM micrographs of retained austenite at different sites in R-TRIP steel: (a) bright-field image of intergranular austenite and interferrit-
ic austenite, (b) g111 center dark-field image of intergranular austenite and interferritic austenite, and (c) SAED pattern.

METALLURGICAL AND MATERIALS TRANSACTIONS A VOLUME 39A, JANUARY 2008—3

Fig. 5—TEM micrographs of bainite and filmlike retained austenite in R-TRIP steel: (a) bright-field image of bainite and filmlike austenite,
(b) g111 center dark-field image of filmlike austenite, (c) g110 center dark-field image of bainite lath, and (d) SAED pattern.

Fig. 6—TEM micrographs of carbides in steel Nb/V: (a) bright-field image and (b)g200 center dark-field image and SAED pattern inserted.

Fig. 7—TEM micrographs of dispersed carbides inside ferrite grains of Nb/Mo steel: (a) bright-field image, (b) g111 center dark-field image, and
(c) SAED pattern.

tensile strength of microalloying steel reaches 863 MPa retained austenite of R-TRIP steel reach 12 and
for Nb alloying, 950 MPa for Nb/V alloying, and 1.13 pct, respectively, corresponding to the highest
1010 MPa for Nb/Mo alloying. The elongations of elongation (30 pct). Although the fAr of Nb, Nb/V,
Nb/V (22 pct) and Nb/Mo (20 pct) steels are lower than and Nb/Mo steel is lower than that of R-TRIP steel,
those of Nb (29 pct) or R-TRIP (30 pct) steels. It also their CAr is still relatively high, i.e., near or above 1 wt
can be seen from Table II that the volume fraction (fAr) pct, and thus the retained austenite will contribute to
of retained austenite and the carbon content (CAr) in the about 8 pct of their elongations by the TRIP effect.[17]

4—VOLUME 39A, JANUARY 2008 METALLURGICAL AND MATERIALS TRANSACTIONS A

 Table II. Mechanical Properties and Retained Austenite (Nb,V)C and (Nb,Mo)C are expected to precipitate at
Parameters of Different TRIP Steels lower temperature than that of NbC[19,21] by the addition
of V or Mo so that the fine and dispersive (Nb,V)C and
Steel R0.2 (MPa) Rm (MPa) El (Pct) fAr (Pct) CAr (Pct) (Nb,Mo)C carbides can be obtained.
R-TRIP 463 781 30 12.0 1.13 Figure 6 shows that there are two average sizes of
Nb 539 863 29 9.4 1.08 NbC or (Nb,V)C in Nb/V TRIP steel. The coarse
Nb/V 563 950 22 8.4 0.98 carbides (average size of 40 nm) are probably NbC and
Nb/Mo 580 1010 20 6.4 1.10 mainly precipitate from austenite during hot rolling and
Average values of R0.2, Rm, and El are listed. The measurement from ferrite during intercritical annealing due to ferrite
errors in R0.2, Rm, and El are ±11 MPa, ±13 MPa, and ±1, with lower solubility of carbon than that in austenite.
respectively. The calculation errors in fAr and CAr are estimated to be The former have no orientation relationships with the
in the range ±0.2 and ± (0.01 to 0.03), respectively. ferrite matrix, while the latter are found and present
the same orientation relationships with ferrite matrix as
the fine carbides, which precipitate from ferrite during
cooling from the intercritical annealing temperature to
IV. DISCUSSION room temperature.[22] For (Nb,Mo)C carbides, they
only precipitate from ferrite during cooling from the
A. Effect of Microalloying on Ferrite Grain Size intercritical annealing temperature to room temperature
It can be seen from Figure 2 that the ferrite grain sizes due to their much lower precipitation temperature than
of Nb-containing TRIP steels including Nb/V and Nb/ that of NbC,[21] and thus their particles always are fine
Mo ones are all smaller than that of Nb-free steel, and (about 5 nm). As a result, the precipitation approaches
its reason will be analyzed as follows. The calculated corresponding to the precipitate sizes of NbC, (Nb,V)C,
result by Thermo-Calc software indicates that the NbC and (Nb,Mo)C carbides in the hot-rolling process or in
carbide precipitation temperature in the present Nb steel the intercritical temperature and subsequent cooling
is approximately 1220 C. This means that during process for TRIP steels are mainly summarized as
reheating at 1250 C before hot rolling, the Nb com- follows: (1) the carbides are coarse when they precipitate
pletely dissolves in austenite, and then during the from austenite during the hot rolling or ferrite during
subsequent cooling and hot-rolling process, the NbC intercritical annealing; (2) the carbides are fine when
carbides precipitation will occur. This may suggest that they precipitate from ferrite during the cooling process
the Nb-containing carbides formed at high temperature after intercritical annealing; and (3) the undissolved
can effectively refine the austenite grain size in the hot NbC carbides will remain as coarse particles if the
rolling process, which results from the fact that these homogeneous annealing temperature prior to hot rolling
carbides enriched on the austenite grain boundaries and is less than its dissolving temperature.[18]
dislocations can effectively retard austenite recrystalli-
zation and impede the motion of dislocations, and in
C. Effect of Microalloying on the Mechanical Properties
turn refine the microstructure during cooling from hot
rolling temperature to room temperature. It is worth Table II shows that both R0.2 and Rm of the Nb TRIP
pointing out that if the homogeneous annealing tem- steel are improved compared with the Nb-free (R-TRIP)
perature prior to hot rolling is less than the dissolving steel. Although the carbon content of the former is less
temperature of NbC, 1220 C, for 0.17 wt pct C TRIP than that of the latter, their increments, ‡R0.2 and ‡Rm,
steel, the undissolved NbC carbides will exist and of the Nb TRIP steel are 76 and 82 MPa, respectively,
abnormally grow so that they weaken the refinement indicating that the addition of Nb markedly increases
of austenite grains and the precipitation strengthening the strength of TRIP steel. The effect of Nb addition on
of the Nb carbides.[18] The effect of grain refinement of strength mainly lies in the refinement of ferrite grains,
Nb/Mo microalloyed steel (Figure 2(d)) is weaker than while the precipitation strengthening of Nb carbides is
that of Nb microalloyed or Nb/V microalloyed steel weak, because the NbC carbides mainly precipitate from
(Figure 2(b) and (c)), which may be attributed to the high temperature and their sizes are coarse.[18] As a
fact that the addition of Mo into the Nb/Mo steel result, it can be safely concluded that for Nb/V and
decreases the driving force of precipitation and the Nb/Mo TRIP steels, the grain refinement strengthening
diffusivity of the NbC carbide forming species in TRIP of ferrite also contributes to the improvement of
steel with Nb[19,20] and weakens the retardation of the strength due to the addition of Nb. Besides, for Nb/V
recrystallization of austenite. However, the average steel, the precipitation strengthening of (Nb,V)C also
grain size of Nb/Mo microalloyed steel is less than that plays an important role in the increase of strength,
of Nb-free steel, because the addition of Mo into the especially the fine and dispersive (Nb,V)C in ferrite
Nb/Mo steel still retards the recrystallization due to the grains (Figure 6), which results in the further increase of
strong solute drag effect of Mo in steel.[19] both ‡R0.2 (24 MPa) and ‡Rm (87 MPa) as compared to
the Nb TRIP steel. For Nb/Mo steel compared with Nb
steel, both ‡R0.2 and ‡Rm drastically increase and are 41
B. Effect of Microalloying on Carbide Precipitation and 147 MPa, respectively, which are attributed to not
Temperature only fully fine and dispersive (Nb,Mo)C carbides in
In the alloying design in this work, an important idea ferrite grains (precipitation strengthening) but also the
is that in Nb TRIP steel, the complex carbides of increase of volume fraction of bainite accompanying the

METALLURGICAL AND MATERIALS TRANSACTIONS A VOLUME 39A, JANUARY 2008—5

 into conventional TRIP steel. The tensile strength of
860 MPa (Nb steel), 950 MPa (Nb/V steel), and
1010 MPa (Nb/Mo steel) were obtained; meanwhile,
the origins of alloying strengthening were analyzed in
detail and summarized as follows.
(1) The addition of Nb can effectively refine the aus-
tenite grain in the hot-rolling process by the NbC
carbides retarding austenite recrystallization, and
in turn refine final microstructure, which results
in the increase of the yield strength and tensile
strength of TRIP steels, especially the yield
strength.
(2) The addition of Nb/V can precipitate partially fine
and dispersive (Nb,V)C carbides in ferrite grains
instead of coarse NbC carbides; therefore, in addi-
Fig. 8—SEM micrograph of Nb/Mo steel. tion to the grain refinement strengthening of fer-
rite, the precipitation strengthening plays an
important role in increasing the tensile strength of
decrease of volume fraction of ferrite (phase transfor-
TRIP steel.
mation strengthening). It can be found from Figure 8
(3) The addition of Nb/Mo cannot only precipitate
that the volume fraction of bainite reaches about 35 pct,
fully fine and dispersive (Nb,Mo)C carbides in fer-
while the volume fraction of bainite in typical TRIP steel
rite grains but also increase the volume fraction of
is less than 30 pct.[11] There are three reasons why the
bainite accompanying the decrease of volume frac-
addition of Mo increases the volume fraction of bainite.
tion of ferrite; thus, the combination of precipita-
(1) The addition of Mo lowers the equilibrium temper-
tion strengthening and the phase transformation
ature (T50) of 50 pct ferrite and 50 pct austenite
strengthening results in the drastic increase of
formation in intercritical annealing (for Nb steel,
strength.
T50 = 795 C; and for Nb/Mo steel, T50 = 784 C),
while the practically intercritical annealing is 800 C and
over T50 (784 C), and thus it leads to the increase of
volume fraction of austenite. (2) The addition of Mo can ACKNOWLEDGMENT
suppress the formation of ferrite with about 6 pct
volume fraction[11] during slow cooling from 800 C to This work is financially supported by the National
690 C (Figure (1)).[23] (3) The addition of Mo is Natural Science Foundation of China (Grant
favorable to the formation of bainite.[24] The preceding No.50571060).
work indicates that the grain refinement strengthening
of ferrite markedly increases the yield strength of TRIP
steels, while the precipitation strengthening of carbides
markedly increases the tensile strength, which is consis- REFERENCES
tent with the principle of alloying strengthening. 1. Y. Sakuma: Proc. of ‘‘Int. Conf. on Advanced High Strength Sheet
Among the four kinds of TRIP steels mentioned Steels for Automotive Applications,’’ CO, 2004, Association for
Iron & Steel Technology, Warrendale, PA, 2004, pp. 11–18.
above, the elongation of Nb/Mo steel is the least due to 2. E.V. Pereloma, I.B. Timokhina, K.F. Russell, and M.K Miller:
(1) plenty of fine carbides located at dislocations thereby Scripta Mater., 2006, vol. 54, pp. 471–76.
impeding the motion of dislocations, and (2) the smallest 3. K.I. Sugimoto, T. Muramatsu, S.I. Hashimoto, and Y. Mukai:
volume fractions of both soft ferrite and retained J. Mater. Process. Technol., 2006, vol. 177, pp. 390–95.
4. W. Bleck, A. Frehn, E. Kechagias, J. Ohlert, and K. Hulka:
austenite. The R-TRIP steel has the largest elongation Mater. Sci. Forum, 2003, vols. 426–432, pp. 43–48.
due to the highest volume fraction of carbon-enriched 5. E.V. Pereloma, I.B. Timokhina, and P.D. Hodgson: Mater. Sci.
retained austenite and a lack of carbide precipitation. Eng. A, 1999, vols. 273–275, pp. 448–52.
For the other two kinds of TRIP steels, their elongations 6. K. Nagayama, T. Terasaki, K. Tanaka, F.D. Fischer, T. Antretter,
are between those of the Nb/Mo and the R-TRIP steels. G. Cailletaud, and F. Azzouz: Mater. Sci. Eng. A, 2001, vol. 308,
pp. 25–37.
This is consistent with their intermediate values in terms 7. W. Shi, L. Li, C.X. Yang, R.Y. Fu, L. Wang, and P. Wollants:
of retained austenite fraction and fine carbide precipi- Mater. Sci. Eng. A, 2006, vol. 429, pp. 247–51.
tates. 8. C. Scott, P. Maugis, P. Barges, and M. Goune: Proc. of ‘‘Int. Conf.
on Advanced High Strength Sheet Steels for Automotive Applica-
tions,’’ CO, 2004, Association for Iron & Steel Technology,
Warrendale, PA, 2004, pp. 181–93.
V. CONCLUSIONS 9. S. Hashimoto, S. Ikeda, K.I. Sugimoto, and S. Miyake: ISIJ Int.,
2004, vol. 44, pp. 1590–98.
Based on the general principle of alloying strength- 10. D.J. Dyson and B. Holmes: J. Iron Steel Inst., 1970, vol. 208,
ening including grain refinement strengthening, precip- pp. 469–74.
11. X.D. Wang, B.X. Huang, Y.H. Rong, and L. Wang: Mater. Sci.
itation strengthening, and phase transformation Eng. A, 2006, vols. 438–440, pp. 300–05.
strengthening, three kinds of microalloying TRIP steels 12. E. Girault, P. Jacques, and Ph. Harlet: Mater. Charact., 1998,
were designed by the addition of Nb, Nb/V, and Nb/Mo vol. 40, pp. 111–18.

6—VOLUME 39A, JANUARY 2008 METALLURGICAL AND MATERIALS TRANSACTIONS A

13. H.K.D.H. Bhadeshia and D.V. Edmonds: Acta Metall., 1980, 20. I.B. Timokhina, P.D. Hodgson, and E.V. Pereloma: Proc. of ‘‘Int.
vol. 28, pp. 1265–73. Conf. on TRIP-Aided High Strength Ferrous Alloys,’’ Ghent, June
14. B.C. De Cooman : Curr. Opin. Solid State Mater. Sci., 2004, vol. 8, 2002, Wissenschaftsverlag Mainz GmbH, Aachen, Germany,
pp. 285–303. 2002, pp. 153–58.
15. G.M. Michal and J.A. Sloane: Metall. Trans. A, 1986, vol. 17A, 21. S. Hashimoto, S. Ikeda, K. Sugimoto, and S. Miyake: Proc. of
pp. 1287–94. ‘‘Int. Conf. on Advanced High Strength Sheet Steels for Automotive
16. D.V. Edmonds, K. He, F.C. Rizzo, B.C. DeCooman, D.K. Applications,’’ CO, 2004, Association for Iron & Steel Technology,
Matlock, and J.G. Speer: Mater. Sci. Eng. A, 2006, vols. 438–440, Warrendale, PA, 2004, pp. 195–203.
pp. 25–34. 22. G.M. Michal and I.E. Locci: Scripta Metall., 1988, vol. 22,
17. X.D. Wang, B.X. Huang, Y.H. Rong, and L. Wang: J. Mater. Sci. pp. 1801–06.
Technol., 2006, vol. 22, pp. 625–28. 23. M. Bouet, J. Root, E. Es-Sadeqi, and S. Yue: Mater. Sci. Forum,
18. X.D. Wang, B.X. Huang, Y.H. Rong, and L. Wang: Proc. 3rd Int. 1998, vols. 284–286, pp. 319–26.
Conf. on Advanced Structural Steels, Gyeongju, Korea, Aug. 2006, 24. W.B. Lee, S.G. Hong, C.G. Park, and S.H. Park: Metall. Mater.
Committee of ICASS, Gyeongju, Korea, 2006, pp. 1152–55. Trans. A, 2002, vol. 33A, pp. 1689–98.
19. T. Wada, H. Wada, J.F. Elliot, and J. Chipman: Metall. Trans.,
1972, vol. 3, pp. 2865–72.

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