791
J. Mater. Sci. Technol., Vol.21 No.6, 2005
• Research Articles
Effect of Cooling History on Yield Ratio of Fine-grain
Ferrite/Pearlite Steel
Fuxian ZHU1)† , Xin LI1) , Yanchun LIU1) , Xianghua LIU1) , Guangfu SHE1) and Zhongping ZHANG2)
1) State Key Laboratory of Rolling and Automation, Northeastern University, Shenyang 110004, China
2) Panzhihua Steel Group Co. Ltd, Panzhihua 617067, China
[Manuscript received May 2, 2005, in revised form September 2, 2005]
Effect of different cooling history on yield ratio of HP295 steel for welded gas cylinder was studied and the result
implied that both fine microstructure and good mechanical properties can be attained by using two-stage cooling
and controlling the coiling temperature. The yield ratio of HP295 steel sheet was reduced to less than 0.8 by twostage cooling process. The pass percent of yield ratio was enhanced from less than 90% to 100%. The mechanical
properties satisfied the criteria of GB6653-1994.
KEY WORDS: HP295 steel sheet; Yield ratio; Two-stage cooling; Coiling temperature
1. Introduction
Table 1 Composition of the test HP295 steel (wt pct)
Grain refinement is the only way to enhance strength
and impact toughness. It has become a new invest
hotspot for metallurgy industry to produce fine-grain or
ultra-fine grain high strength steel with a low cost by
thermo-mechanical controlled processing (TMCP), compact strip production (CSP), ultra-fast cooling process
and so on. In the meantime, grain refinement increases
yield ratio of steel, which deteriorates the cold forming
performance, and also arouses the attention of domestic
and foreign specialists[1,2] .
The present study aims at finding the effective cooling history to reduce the yield ratio of fine-crystal ferrite/pearlite steel under industrial condition. All the
works were taken with the original chemical composition,
heating and rolling process.
No.
A
B
C
0.15
0.14
Si
0.06
0.06
Mn
0.78
0.79
P
0.013
0.018
S
0.011
0.012
Als
0.062
0.025
Ceq.
0.28
0.27
2. Experimental
The chemical composition of HP295 test steel
sheet is summarized in Table 1. The billet size is
200 mm×1080 mm×11000 mm. It was reheated at
1200◦ C for 120∼150 min; the rough rolling began at
1150◦ C; the finish rolling began at less than 1000◦ C and
the final rolling temperature was about 850◦ C.
In 1450 mm hot rolling mill group of Panzhihua steel
co. Ltd, China, the length of laminar flow cooling zone
is 62.4 m, and there are 12 groups of laminar flow cooling nozzle at both upper and lower side of roller way. In
this experiment, cooling the steel sheets was operated by
opening and closing different nozzles. The effect of forestage, aft-stage and two-stage cooling on yield ratio were
compared and analysed in detail. Different cooling histories are taken and summarized in Fig.1, where curve I
and III stand for traditional one-stage cooling, and curve
II stands for two-stage cooling.
The whole experiment was controlled and the data
were recorded by computer; the temperature at each
cooling stage was measured by infrared thermome
ter; the test samples were made according to the
† Prof., to whom correspondence should be addressed,
E-mail: [email protected].
Fig.1 Diagrammatic sketch of different cooling path
GB/T2975 and GB/T228; the mechanical properties of
the samples were measured by an Instron tensile tester
and the microstructures were observed with a Leica picture analyzer and EM-400 transmission electronic microscope (TEM).
3. Results and Discussion
The experimental conditions of steel A and the mechanical properties are summarized in Table 2. The cooling history of sample A1 was through path III (fore-stage
cooled and coiled at 640◦ C), which is shown in Fig.1.
Sample A2 was cooled through path II (two-stage cooled
and coiled at 640◦ C) and the cooling history of sample
A3 was fore-stage cooled and coiled at 690◦ C. According to the data in Table 2, the mechanical properties of
steel A all satisfied the criteria of GB6553, but its yield
ratio is obviously different. For example, the yield ratio
was reduced form 0.82 to 0.80 by using two-stage cooling
instead of fore-stage cooling and the yield ratio was
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J. Mater. Sci. Technol., Vol.21 No.6, 2005
Table 2
No.
Process parameters and mechanical properties of steel A
Thickness/mm FRBT1) /◦ C FRT2) /◦ C
CT3) /◦ C Cooling path
σs /MPa
σb /MPa
σs
σb
δ5 /%
DG4) (level)
A1
2.2
980
845
640
fore-stage
415
505
0.82
37
12.0
A2
2.2
980
845
640
two-stage
405
505
0.80
35
11.5
A3
2.2
950
840
690
fore-stage
365
485
0.75
41
11.0
Notes: 1) FRBT–finish rolling beginning temperature, 2) FRT–final rolling temperature, 3) CT–coiling temperature,
4) DG–degree of grain
Table 3 Process parameters and mechanical properties of steel B
No.
Thickness/mm
Cooling path
σs /MPa
σb /MPa
σs /σb
δ5 /%
r
N
FGS1) /µm
VOP2) /%
B1
2.5
fore-stage
366
450
0.813
36
0.80
0.19
7.4
17.36
B2
2.5
two-stage
352
449
0.784
38
0.84
0.20
8.8
15.53
B3
2.5
aft-stage
353
447
0.790
40
0.84
0.19
10.1
16.45
B4
2.5
two-stage
353
467
0.756
39
Notes: 1) FGS–ferrite grain size, 2) PCP–volume fraction of perlite grain
0.80
0.21
10.5
16.31
Fig.2 Microstructures of steel A with different cooling history: (a) for A2, (b) for A3
Fig.3 Microstructures of steel B with different cooling history: (a) for B1, (b) for B2, (c) for B3, (d) for B4
J. Mater. Sci. Technol., Vol.21 No.6, 2005
793
Fig.4 Pearlite profile of hot strips under different cooling process: (a) for steel B1, (b) for steel B2
reduced to about 0.75 by increasing the coiling temperature from 640◦ C to 690◦ C. Thus it can be seen that yield
ratio can be adjusted by changing the cooling history.
The microstructures of steel A produced by modifying the cooling history are shown in Fig.2. By using
two-stage cooling and coiling at 640◦ C, equiaxed ferrite
grains were obtained and the average ferrite grain size
was about 8.46 µm and the volume fraction of ferrite
grain was 87.04% (Fig.2(a)); by using two-stage cooling
and coiling at 690◦ C, the average ferrite grain size was
12.3 µm, slightly coarser than the former, and the volume fraction of ferrite grain was 79.80% (Fig.2(b)). It
can be concluded that, the main reason of lower yield
ratio gained by two-stage cooling is the existence greater
amount of ferrite grains, and increasing coiling temperature may lead to the bigger ferrite grains.
The mechanical propertyies of steel B are summarized in Table 3. The finish rolling began at 980◦ C and
finished at 840◦ C. Sample B1, B2 and B3 were cooled
through fore-stage, two-stage and aft-stage cooling separately and coiled at 660◦ C; sample B4 was coiled at 690◦ C
followed the two-stage cooling. It can be seen from Table
3 that the changing trend of yield ratio in steel B was
similar to that in steel A. The yield ratio can be reduced
to 0.75∼0.78 through two-stage cooling.
Figure 3 shows the microstructures of steel B produced with different cooling history, and there was evident difference among them.
Two-stage cooling is to open the fore-end nozzles
while finish rolling ended and cool the steel to 720◦ C
at 30◦ C/s, then the steel is air cooled for 3∼4 s by closing middle nozzles; Open the aft-end nozzles to cool the
steel sheets to coiling temperature. This cooling process is usually used in producing double phase steels and
it is seldom adopted in producing ferrite/pearlite steels.
There are no similar reports about it except that it had
been practiced in the study of HP325 steel sheets[3] .
By comparing Fig.3(a) and (b), it can be seen obviously that ferrite grains in steel B1 was rather fine, but
the grains were not uniform, and for the big proportion of
fine grains, the average grain size was rather small, which
was 7.44 µm. Otherwise, there was no enough time for
these fine grains to grow up by using fore-stage cooling;
the effect of ferrite precipitation process on pearlite shape
was comparatively weak, so the pearlite was coarse, bad
dispersivity with high percentage, which is about 17.36%.
While for steel B2 it was air-cooled at the critical ferrite
transformation temperature scope, which was advantageous to getting sufficient ferrite precipitation. Though
the ferrite grain size was slightly bigger, about 8.77 µm,
the distribution of ferrite grain was very symmetrical and
most of these ferrite grains were equiaxed and the size of
pearlite was small, well disseminated with a lower proportion of about 15.53%.
The microsrtucture of pearlite in steel B1 and B2 is
shown in Fig.4. It can be seen that the pearlite in steel
B2 is small and the carbide layer is thicker than that in
steel B1. For the two-stage cooling, the aim of the first
stage cooling is to enhance the driving force of ferrite
nucleation. The steel was cooled at the critical temperature, which was advantageous for ferrite to transformation. Stopping water cooling and air cooling for about
3∼4 s is to accelerate the precipitation of ferrite and increase its volume fraction. Because of the precipitation
of ferrite, carbon enriched in austenite, the second stage
cooling can restrain the growing of ferrite and accelerate
the phase transformation from the austenite to pearlite
with higher carbon concentration and advanced strength
properties. The pearlite was also dispersed uniformly.
The morphology of dislocations in ferrite grains in
steel B1 and B2 are shown in Fig.5. The dislocation density in the ferrite grains in steel B2 is slightly lower by
comparing the two TEM microstructures. This may be
the direct reason for the lower yield strength. While the
pearlite with higher carbon concentration can increase
the tensile strength of the steel, so the yield ratio is reduced.
Figure 3(d) shows the microstructure of steel B4,
whose yield ratio is the lowest value in this experiments
as listed in Table 3. The result of metallographic analysis is as follows: average ferrite grain size is 10.50 µm;
pearlite volume fraction is 16.31% with slight zonal distribution. The reason for zonal distribution was that the
cooling intensity was not strong enough by measuring the
temperature after the first stage cooling, which is 745◦ C.
One of the effective methods to resolve this problem is to
open more nozzles at the first cooling stage, controlling
the finishing temperature to be at about 720◦ C. Another
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J. Mater. Sci. Technol., Vol.21 No.6, 2005
Fig.5 TEM photographs of test steels under different cooling process: (a) steel B1, (b) steel B2
method is to increase the finishing rolling temperature
with 10∼20◦ C, Controlling the finishing rolling temperature properly is not only beneficial to relieve zonal structure, but also helpful to reducing yield ratio[4,5] .
Figure 3(c) shows the microstructure of steel B3
which was cooled by aft-stage cooling and coiled at
660◦ C. Aft-stage cooling means that when fine rolling
is finished, closing the fore-end nozzles so that the steel
can be air-cooled to a certain temperature, then opening
the aft-end nozzles to cool the steel to coiling temperature rapidly. This kind of cooling history is shown as
curve I in Fig.1. This cooling mode cannot be used on
fine-crystal strengthen steels, because by air-cooling at
high temperature, the residual deformation is recovered;
the deformation induced transformation is weaken and
the ferrite grains are coarsened; carbon concentration in
ferrite grains is low, which increases the density of mobile dislocations, so the yield ratio is reduced and cold
forming property is enhanced. According to the characteristic of the 1450 rolling mill group in Panzhihua Steel
Co. Ltd, it is impossible to raise the rolling speed, so it
is hard to control finish rolling temperature. Considering the aim of the pass percent of yield ratio, aft-stage
cooling method can be used to counteract the negative
effect of low finish rolling temperature on yield ratio. A
good matching of yield strength and tensile strength can
be gained by using this cooling method.
The relationship between strength index of ferrite/pearlite steel and its concentration together with the
grain size is described as[1] :
σs (MPa) = 15.4[3.5 + 2.1(%Mn) + 5.4(%Si)+
1
23(%Nf) 2 + 1.13d −1/2 ]
(1)
σb (MPa) = 15.4[19.1 + 1.8(%Mn) + 5.4(%Si)+
0.25(%P) + 0.5d −1/2 ]
(2)
where (%Mn), (%Si) and (%Nf) are the mass percentage
of Mn, Si and solution nitrogen, respectively; (%P) is
the area percentage of pearlite; d is the grain size of ferrite. It can be seen from these equations that yield ratio
and tensile strength is in direct ratio to (d)−1/2 . Grain
refinement can enhance both yield strength and tensile
strength, and it contributes more to tensile strength, so
the finer the grain, the smaller the difference between
yield strength and tensile strength, and the higher the
yield ratio. The HP295 steel of Panzhihua Steel Co.
Ltd contains vanadium and titanium and the finishing
rolling temperature is correspondingly low, so the grains
are rather fine as the result of deformation accumulation, and the yield ratio is usually high. According to
the working condition of Panzhihua Steel Co. Ltd, the
simple method for reducing the yield ratio is to increase
the coiling temperature up to (670±10)◦ C. The practice
proved that the mechanical properties of the products all
satisfied the GB6653-1994 and the enterprises criterion.
Moreover, by modifying the rolling rules and controlling the finishing rolling and the cooling temperature,
the yield ratio can also be reduced. While these achievements can be gained at the cost of grain coarsening, so
the two-stage cooling combined with coiling at a proper
temperature is recommended.
Two-stage cooling technology can reduce yield ratio
by enhancing the ferrite percentage to decrease the yield
strength and well distributed pearlite to increase the tensile strength. Otherwise, the equiaxed ferrite grains have
a positive effect on enhancing strain hardening exponent
n and plastic strain ratio r, as shown in Table 3. This
result can also offer a reference to fine or ultra-fine grain
ferrite/pearlite steel production by CSP and so on.
4. Conclusions
(1) Using two-stage cooling and enhancing coiling
temperature properly can attain comparatively fine microstructures and good integrative mechanical properties.
(2) By adopting new process, the yield ratio was reduced effectively and the pass percent of HP295 steel
sheet in Panzhihua Steel Co. Ltd was enhanced from
less than 90% to 100%.
Acknowledgement
This work was supported by the National High-Tech Research and Development Program of China (863 Program)
(No. 2001AA332020-01).
REFERENCES
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[2] Haitao CHAI: The 8th annual convention of Chinese
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(in Chinese)
[3] Fuxian ZHU and Guodong WANG: Iron Steel, 1992,
27(2), 28. (in Chinese)
[4] Fuxian ZHU and Guodong WANG: Iron Steel, 1992,
27(9), 38. (in Chinese)
[5] Hongjin ZHAO and Fuxian ZHU: Iron Steel, 1992, 29(7),
36. (in Chinese)