Materials

& Design
Materials and Design 27 (2006) 1169–1173
www.elsevier.com/locate/matdes

Short communication

Effect of molybdenum on continuous cooling bainite

transformation of low-carbon microalloyed steel
a,b,* a
Junhua Kong , Changsheng Xie
a
Department of Materials Science and Engineering, Huazhong University of Science and Technology, Wuhan 430074, PR China
b
Technology Centre, Iron and Steel Research Institute, Wuhan Iron and Steel (Group) Company, 28# Yejin Road, Qingshan, Wuhan 430080, PR China

Received 9 November 2004; accepted 9 February 2005

Available online 31 March 2005

Abstract

Through simulation of thermomechanical processing/on-line accelerated cooling processing and observation of microstructure,
the effect of molybdenum on continuous cooling bainite transformation of ultra-low carbon microalloyed steel was studied. The
continuous cooling transformation curves of the trial steels with or without molybdenum addition were also determined. The result
showed that the separate temperature of bainite was obviously reduced and the size of microstructure became smaller as 0.40 wt%
Mo was added to the steel. At the same time, the martensitic structure, which formed at some cooling conditions, became finer and
dispersed more uniformly. The deformed austenite would transform to finer bainite even when the cooling rate was not too high.
 2005 Elsevier Ltd. All rights reserved.

Keywords: Molybdenum; (A) Microalloyed steel; (F) Bainite; Transformation

1. Introduction exceeding ferrite critical transformation point, the prod-

uct was mainly bainite.
Low carbon microalloyed steel was widely used in When the carbon content of the steel reduced, the sta-
mechanical engineering, pressure vessels and pipelines bility of deformed austenite decreased, the bainite trans-
transporting oil and natural gas for its high strength, formation advanced. Therefore, the CCT of the steel
good toughness and weldability. Many studies have after deformation was mainly bainitic transformation.
been reported about the microstructure evolvement of Through simulation of thermomechanical processing
the steel after deformation in continuous cooling condi- (TMP)/on-line accelerated cooling (OLAC) processing
tion [1,2]. They showed that deformation could enhance [3] and observation of microstructure, the effect of
transformation-driving force, accelerate the continuous molybdenum on continuous cooling bainite transforma-
cooling transformation (CCT), and shift the pearlite tion of ultra-low carbon microalloyed steel was studied,
transformation line of CCT to its right. As the deforma- and CCT curves of the tested steels with or without
tion increased, transformation- driving force enhanced. molybdenum addition was determined.
And when the cooling rate increased to the extent, i.e.,

2. Experimental procedure

*
Corresponding author. Tel.: +86 27 86806779; fax: +86 27
The steel was molten in a 50 kg vacuum furnace and
86805219. rolled to the plate with a thickness of 15 mm through
E-mail address: junhuakong@yahoo.com.cn (J. Kong). controlling the rolling process and cooling rate.

0261-3069/$ - see front matter  2005 Elsevier Ltd. All rights reserved.
doi:10.1016/j.matdes.2005.02.006
1170 J. Kong, C. Xie / Materials and Design 27 (2006) 1169–1173

Table 1 structure of Steel B containing 0.40 wt% Mo was finer

Chemical compositions of the steels (wt%) than that of Steel A Mo-free. The difference was not
No. Mo C Si Mn P S Nb Ti so obvious when the cooling rate was over 30 C/s as
Steel A 0 0.02 0.30 1.8 0.007 0.003 0.070 0.021 comparing Fig. 1(h-1) with Fig. 1(h-2), Fig. 1(j-1) with
Steel B 0.40 0.02 0.30 1.8 0.007 0.003 0.070 0.021 Fig. 1(j-2).
Therefore, adding 0.40 wt% Mo to the steel could
make the microstructures under all cooling rates fine
and not sensitive to the change of cooling rate. On the
Table 2
The temperature of each transformation
contrary, the microstructure of the Mo-free steel became
finer and more homogeneous as the cooling rate
Deformed step 1 2 3
increased.
Deformed temperature (C) 1050 900 810 SEM observations were presented in Fig. 2. Fig. 2
(i-1) and (i-2) were the SEM photos of Steel A and Steel
B samples under the cooling rate 40 C/s. Fig. 2(d-1)
The chemical compositions of the steels were given in and (d-2) were the SEM photos of Steel A and Steel B
Table 1. samples under the cooling rate 10 C/s. The island struc-
Sampled from the middle of the plate, then ture in Fig. 2(i-2) and (d-2) was much more than that in
machining to round specimen with the dimension / Fig. 2(i-1) and (d-1) because of the addition of 0.40 wt%
8 · 12 mm. First the specimen was heated and Mo to the steel. Moreover, these island structures were
austenitized on THERMECMASTOR-Z equipment at finer and distributed more uniformly in the matrix com-
1170 C, and kept 600 s at this temperature, then com- pared with the Steel A, the Mo-free steel.
pressed three times with reduction was 40%, 30%, 25%
in turn and strain rate was 5 per second. After deforma-
3.2. Determination of transformation temperature and
tion it was cooled down from 810 to 200 C at different
bainite CCT curve
cooling rate. The deformation temperature of each step
was showed in Table 2, and the cooling rate in Table 3.
The results of transformation temperature of the steel
tested in 10 different cooling rates were shown in Table
4, and bainite CCT curve of the steels with 0.40 wt% Mo
3. Results
or Mo-free was shown in Fig. 3.
From Table 4 it could be seen, the Bs and Bf points of
3.1. Characteristics of microstructure
Steel A exceeded that of Steel B for about 40 C when
the cooling rate was between 1 and 15 C/s. No obvious
Optical microscopic observations of the specimen
difference of Bs and Bf points was found when the cool-
were presented in Fig. 1. It was found that the micro-
ing rate was over 15 C/s.
structures at all tested cooling rates were bainite with
As shown in Fig. 3, the austenite transformation
very few polygonal ferrite.
curve in moderate temperature shifted to the lower posi-
Fig. 1(a-1), (c-1), (e-1), (h-1) and (j-1) showed the
tion when 0.40 wt% Mo was added to the steel. Thus in
microstructures of Steel A, a Mo-free steel, under the
the same cooling condition, Bs and Bf points were re-
cooling rate of 1, 5, 15, 30 and 60 C/s. It could be
duced to some degree compared with Mo-free steel.
seen from these pictures that the bainite structure be-
came finer as the cooling rate increased after deforma-
tion. But the microstructures of Steel B, a steel
containing 0.40 wt% Mo, were all finer under each 4. Analysis and discussion
cooling rate as shown in Fig. 1(a-2), (c-2), (e-2), (h-2)
and (j-2). The specimen was deformed for three steps after
Comparing Fig. 1(a-1) with Fig. 1(a-2), Fig. 1(c-1) austenitized in 1170 C and transformed in succedent
with Fig. 1(c-2), Fig. 1(e-1) with Fig. 1(e-2), it could cooling process. When the steel contained relative high
be found that under the same cooling rate, their micro- percentage of alloy elements, such as manganese, molyb-
structures showed obvious difference that the micro- denum, etc., deformed austenite transformed to bainite

Table 3
The cooling rate of V810/200
Cooling rate V1 V2 V3 V4 V5 V6 V7 V8 V9 V10
C/s 1 2 5 10 15 20 25 30 40 60
 J. Kong, C. Xie / Materials and Design 27 (2006) 1169–1173 1171

Fig. 1. The optical microscope pictures of Steel A and Steel B samples transformed in different cooling rates: (a-1) Steel A (V1 = 1 C/s); (a-2) Steel B
(V1 = 1 C/s); (c-1) Steel A (V3 = 5 C/s); (c-2) Steel B (V3 = 5 C/s); (e-1) Steel A (V5 = 15 C/s); (e-2) Steel B (V5 = 15 C/s); (h-1) Steel A
(V8 = 30 C/s); (h-2) Steel B (V8 = 30 C/s); (j-1) Steel A (V10 = 60 C/s); (j-2) Steel B (V10 = 60 C/s).

in accelerated cooling condition for manganese and transformation. Therefore, the production of trans-
molybdenum could restrain the formation of polygonal formation was mainly bainite with very few polygonal
ferrite and pearlite obviously, but hardly retard bainite ferrites.
1172 J. Kong, C. Xie / Materials and Design 27 (2006) 1169–1173

Fig. 2. The SEM photos of Steel A and Steel B samples transformed in different cooling rates: (i-1) Steel A (V9 = 40 C/s); (i-2) Steel B (V9 = 40 C/
s); (d-1) Steel A (V4 = 10 C/s); (d-2) Steel B (V4 = 10 C/s).

Fig. 3. Transformed austenite continuous cooling curve of the steels with 0.40% Mo and Mo-free.

The deformed austenite transformed to a great deal activation energy in austenite increased and the carbon
of parallel lathing bainite, which had the same orienta- diffusion coefficient decreased. But bainite transforma-
tion in the view of crystal. As the strong carbide-forming tion was a half-diffusion one mainly dominated by car-
element Mo was added to the steel, the carbon diffusion bon diffusion. Because the carbon diffusing speed was
 J. Kong, C. Xie / Materials and Design 27 (2006) 1169–1173 1173

Table 4 tensite start temperature (Ms point) of Mo-contained

Transformation temperatures of the samples steel reduced, and the formed M island or M-A island
Cooling rate (C/s) Steel A Steel B structure became finer and dispersed in the lathing of
Bs (C) Bf (C) Bs (C) Bf (C) bainite or at the interface of the grains equably. These
1 685 596 640 564 island structures would be helpful for strengthening of
2 660 607 621 569 the steel.
5 648 576 621 546
10 648 562 607 508
15 641 532 607 515
20 627 515 607 516
5. Conclusions
25 607 505 614 540
30 596 501 590 502 (1) When 0.40 wt% Mo was added to the low-carbon
40 573 478 560 493 microalloyed steel, bainite transformation was slo-
60 578 467 550 483 wed down and Bs, Bf points reduced, so the micro-
Bs: bainite transformation start temperature; Bf: bainite tranformation structure of bainite formed from CCT became
finish temperature. finer accordingly. Ms point of the steel decreased
as the adding of Mo, M island or M-A island struc-
ture became finer and dispersed more equably in the
reduced, bainite transformation was slowed down and lathing of bainite or at the interface of the grains.
Bs point decreased [4]. Therefore, if some quantitative (2) When 0.40 wt% Mo was added to the low-carbon
Mo were added to the steel, bainite transformation microalloyed steel, the deformed austenite would
would start and finish in a lower temperature range, transform to finer bainite even if the continuous
and as the Bs point decreased, the microstructure of bai- cooling rate was not high.
nite produced from transformation would be finer than
that of Mo-free steel.
Compared Steel B, the Mo-containing steel with Steel
A, the Mo-free steel, it could be seen that when the cool- Acknowledgements
ing rate increased in Mo-free steel, the carbon diffusing
speed was reduced, and bainite transformation was de- The present work was financially supported by a
layed to a lower temperature effected by cooling rate, China National ‘‘the ten-fifth’’ project. Authors are
thus the obtained bainite was finer than that in smaller grateful to Prof. Zheng LinÕs help and Mr. Wu Lixin,
cooling rate. But even if the cooling rate was not high Ms. Zhen JingÕs corporation in the experiments.
in Mo-containing steel, the deformed austenite would
transform to finer bainite, because the restraining effect
of Mo on the carbon diffusing activity, which was simi-
lar to the influence of cooling rate. References
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