-
Accepted Manuscript
Effects of annealing cooling rates on mechanical properties, microstructure andtexture in continuous annealed IF steel
Ruwen Zheng, Renbo Song, Wuyan Fan
PII: S0925-8388(16)32753-0
DOI: 10.1016/j.jallcom.2016.09.018
Reference: JALCOM 38852
To appear in: Journal of Alloys and Compounds
Received Date: 19 June 2016
Revised Date: 14 August 2016
Accepted Date: 2 September 2016
Please cite this article as: R. Zheng, R. Song, W. Fan, Effects of annealing cooling rates on mechanicalproperties, microstructure and texture in continuous annealed IF steel, Journal of Alloys and Compounds(2016), doi: 10.1016/j.jallcom.2016.09.018.
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http://dx.doi.org/10.1016/j.jallcom.2016.09.018
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Effects of annealing cooling rates on mechanical properties,
microstructure and texture in continuous annealed IF steel
Ruwen Zheng, Renbo Song*, Wuyan Fan
School of Materials Science and Engineering, University of Science and Technology Beijing, Beijing 100083, China 5
Abstract1
The mechanical properties, microstructure and texture of interstitial-free (IF) steel
processed through continuous annealing routes under five cooling rates (5 oC/s, 50
oC/s, 200 oC/s, 500 oC/s and 1000 oC/s) have been studied. The cooling rates of 200
oC/s, 500 oC/s and 1000 oC/s are defined as ultra-fast cooling rates. This paper focuses 10
on the differences of mechanical properties, microstructure and texture evolution
between ultra-fast and common cooling rates (5 oC/s, 50 oC/s) for the first time. The
overall structural features of specimens were observed by optical microscopy and
nano-sized precipitates were observed by transmission electron microscope (TEM) on
carbon extraction replicas. Texture measurement was carried out using D8 advance 15
X-Ray Diffraction (XRD) and the (110), (200) and (112) pole figures were
determined. The yield strength increases with increasing cooling rates, from 96 MPa
under the cooling rate of 5 oC/s to 136 MPa under the cooling rate of 1000 oC/s , while
the ultimate tensile strength changes very little (around 280 MPa). The samples
annealed under the cooling rates of 5 oC/s and 500 oC/s have low rave values (2.13 and 20
2.21 respectively). By contrast, the samples in cases of 50 oC/s, 200 oC/s and 1000
*Corresponding author. Tel:+86-10-82377990; e-mail: [email protected]
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oC/s possess higher rave values (2.32, 2.40 and 2.51 respectively). Three morphologies
of TiN particles can be observed in all samples. TiN, with CaO and SiO2 acting as
heterogeneous nuclei, is prone to have big size. The size of TiN precipitates deceases
as increasing of cooling rates. FeTiP is found near TiC precipitates under the cooling 25
rate of 5 oC/s. The sample annealed under the cooling rate of 5 oC/s has the weakest
{111} and {111} orientations. The intensity of {111}
component increases as the cooling rate increases from common cooling rates of 5
oC/s and 50 oC/s to ultra-fast cooling rates of 200 oC/s, 500 oC/s and 1000 oC/s. The
intensity of {011} orientation decreases as the cooling rate increases from 5 30
oC/s to 500 oC/s. The relationship between r value, microstructure and texture
components has been studied in this paper.
Key Words: Interstitial-free steel; Precipitation; Texture; Cooling rates; Continuous
annealing
1. Introduction 35
Application of IF steels in the automotive industry has seen a significant increase
over the past couple of decades due to their excellent deep-drawing property[1].
Titanium (Ti) and niobium (Nb) are added to these steels, singly or in combination, to
scavenge carbon and nitrogen by the formation of carbides and nitrides and thus
remove interstices from the matrix[2-4]. This clear matrix is essential for increased 40
formability and is suitable for galvanneal coating, which is required for automotive
body and home appliances[5]. The main trends in the development of IF steels are
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towards high-strength and high-formability varieties. P and Mn have been added in IF
steels as solution strengthening elements to improve strength performance.
Precipitation behavior in IF steels is quite complex and TiN, TiS, TiC and NbC 45
precipitates are quite common in Ti-IF steels[6-8]. Characterization of TiN, TiC and
Ti (C, N) in titanium-alloyed ferritic chromium steels focusing on different particle
morphologies has been studied by Michelic[9]. A detailed analysis of TiN morphology
and composition shows that there are four subcategories, pure TiN, TiN with an oxide
nuclei, TiN with a carbide or carbonitride layer and TiN clusters (agglomerated single 50
TiN), which differ significantly in size and morphology in ferrite stainless steels[9].
Ghosh and his group[10] have studied the thermodynamics of precipitation and
textual development in IF steels. Above 900 oC, the TiCN phase mainly contains N
apart from Ti, which is simply TiN. However, at around 900 oC, independent TiC
formation is likely to take place and the amount of carbon is considerably increased in 55
this phase below 900 oC[10]. The precipitation behavior of FeTiP-type precipitates in
IF steels has been studied[2-4, 11]. P has been added to normal IF steels as solid
solution strengthening elements and this leads to the formation of FeTiP precipitates
in annealing process. FeTiP prevents the growth of {111} recrystallization texture.
FeTiP usually precipitates at the grain boundary and the segregation of phosphorus 60
atoms at grain boundaries has been studied before[12]. During the annealing process,
non-equilibrium segregation of phosphorus atoms at the grain boundaries is related to
the moving dislocation lines[12-14].
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The strong {111} recrystallization textures are beneficial for deep-drawing
property and their formation mechanism and texture evolution have been studied in 65
several literatures[15-20]. Path of crystal rotation during rolling and the impacts of
alloying elements and second phase particles on texture formation in low carbon
steels have been studied[21]. It is reported that the precipitates increase the
recrystallization temperature and hinder the recrystallization texture[22]. In addition,
different texture components have different impacts on r and △r values. The {011} 70 component contributes to the increase of r value greatly in BCC materials, but
it also increases △r value. The {111} and {111} components can increase r value without increasing △r value[23, 24].
Previous studies focused on mechanical properties and microstructure under
different continuous annealing temperature and holding time. As for cooling rates, 75
few studies have been made, especially under ultra-fast cooling rates. The present
industrial production continuous annealing cooling rate is lower than 50 oC/s and
precipitation behavior and texture evolution under ultra-fast cooling rates have not
been studied. While apart from common cooling rates (5 oC/s and 50 oC/s), ultra-fast
cooling rates (200 oC/s, 500 oC/s and 1000 oC/s) were also carried out in this 80
experiment. This paper makes comparison studies between effects of common cooling
rates (5 oC/s and 50 oC/s) and ultra-fast cooling rates (200 oC/s, 500 oC/s and 1000
oC/s) on mechanical properties (focusing on r values), microstructure and texture
components. Considering the low rave values in cases of 5 C/s and 1000 C/s and their
different major texture components, a relationship between the r value, △r value and 85
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texture components in IF steel has been established. By controlling annealing process,
the preferred texture components, which increase r value and decrease △r value, can be obtained. Thus the formability of the material can be improved. Studying different
continuous annealing cooling rates also supplies the IF continuous annealing studies
and improves continuous annealing efficiency. 90
2. Experimental and analytical methods
The starting material was a Ti-IF steel of 1 mm in thickness and its chemical
compositions are listed in Tab.1. P has been added as solid solution strengthening
elements. The steel was prepared by hot rolling and cold rolling and the processing
parameters are given in Tab.2. 95
Tab. 1 Chemical composition of the IF steel (wt%)
C Si Mn S P Ti Nb 0.0015 0.022 0.09 0.0061 0.015 0.072 0.012
Tab. 2 Processing parameters of the IF steel
Hot finish rolling temperature
(oC)
Hot coiling temperature
(oC)
Cold rolling reduction
% 930 630 82
There were five stages in the annealing cycles: (1) HS (heating section): heating
from room temperature to soaking temperature (810 oC); (2) SS (soaking section): 100
holding at soaking temperature for 180 s; (3) CS (cooling section): cooling to ambient
temperature (cooling rates range from 5 oC/s to 1000 oC/s); (4) OA (over aging
section): holding at overaging temperature (240 oC) for 120 s; (5) AC (air cooling): air
cooling to room temperature. The continuous annealing process was simulated in the
CCT-AY heat treatment system under the cooling rates of 5 oC/s and 50 oC/s. As for 105
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the cooling rates of 200 oC/s, 500 oC/s and 1000 oC/s, the steels were cooled in
different mediums. The different annealing tests were performed on laboratory scale.
Ultra-fast cooling rates could be applied on industrial scale and the Kobe Steel Group
Japan has already applied ultra-fast cooling rates on steel production. The continuous
annealing curve is shown in Fig.1. 110
–
Fig.1 Continuous annealing curve of the IF steel
The mechanical properties of the samples annealed under different cooling rates,
including yield strength (YS), ultimate tensile strength (UTS), elongation, n and r
values were investigated at room temperature in three directions of 0°, 45° and 90° in 115
the tension test. Further TEM equipped with an energy dispersive spectrometer (EDS)
was used to study the precipitation behavior. Specimens for TEM were prepared by
carbon extraction replicas. Macroscopic textures were measured on an X-ray
diffractometer and three pole figures {110}, {200}, and {112} were obtained.
Orientation distribution functions (ODFs) were calculated from these pole figures 120
using the Bunge method[25]. The α, γ, η, ζ, θ, and ε-fiber intensities were also plotted
in this study.
3. Results
3.1 Mechanical properties under different cooling rates
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It can be clearly seen that their mechanical properties (shown in Tab.3) are 125
different under different cooling rates. The yield strength turns out gradually upward
trend with the increase of cooling rates, while the ultimate tensile strength changes
little (around 280 MPa). The yield ratio increases with the increase of cooling rates.
The yield ratio is the ability to resist deformation from yielding to plastic instability
and the lake of adequate strain hardening results in high yield ratio[26, 27]. The 130
mechanical results also reveal that the cooling rates have little effect on elongation
and n values.
The r value is defined as true plastic strain ratio of width to thickness under
uniaxial tension and low r value indicates poor deep-drawing property [28].
rave=(r0+2r45+r90)/4 (3-1) 135
△r=(r0-2r45+r90)/2 (3-2) The rave values of specimens annealed under the cooling rates of 5
oC/s and 500
oC/s are significantly lower than others. The rave value is 2.40 when the cooling rate is
200 oC/s and it decreases to 2.21 under the cooling rate of 500 oC/s.
In addition, LDR (limiting drawing ratio) value is usually used to evaluate deep 140
drawability and theoretically can be expressed as follows:
LDR = �exp ��2fexp�−�����1 + �� 2� � + exp �2���1 + �� 2� � − 1 (3-3) Where f is the factor of drawing efficiency, and when f equals to 0.9, the calculated
results are in good agreement with the experimental results[26]. Based on the
equation, the LDR values of samples annealed under different cooling rates (form 5 145
oC/s to 1000 oC/s) are 2.53, 2.59, 2.61, 2.18 and 2.22 respectively. This result reveals
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that the deep property of sample annealed under the cooling rate of 200 oC/s is higher
than others.
Tab. 3 Mechanical properties of the IF steels
Steel Cooling
rate
(oC/s)
Direction
(deg)
r rave r n nave Elongation
(%)
YS
(MPa)
UTS
(MPa)
1#
5
0 2.41
2.13
0.81
0.29
0.30
52.4 96 284
45 1.72 0.31 50 103 282
90 2.66 0.31 54 100 271
2#
50
0 2.83
2.32
1
0.30
0.30
53.5 103 280
45 1.82 0.30 50 105 287
90 2.81 0.31 48 102 276
3#
200
0 2.93
2.40
0.91
0.30
0.30
50.2 119 277
45 1.94 0.29 49.3 126 283
90 2.77 0.30 51.2 118 277
4#
500
0 2.53
2.21
0.75
0.30
0.29
50.3 123 284
45 1.84 0.29 47.6 132 289
90 2.64 0.29 48 127 280
5#
1000
0 3.03
2.51
0.97
0.30
0.29
51.6 126 275
45 2.02 0.29 48.6 136 285
90 2.96 0.28 48 132 277
3.2 Microstructure under different cooling rates 150
3.2.1Grain sizes
The optical micrographs of IF steels annealed under different cooling rates are
presented in Fig.2 and the effect of cooling rates on the ferrite grain sizes is shown in
Fig.3. All samples are made up of equiaxed ferrite grains. The results show that grain
sizes decrease with increasing cooling rates. The mean grain size reaches its 155
maximum (15.13 µm) under the cooling rate of 5 oC/s and decreases to the minimum
value of 11.75 µm under the cooling rate of 1000 oC/s. According to statistics
calculation, the average grain aspect ratio (17:10) in steels under the cooling rates of 5
oC/s and 50 oC/s is a little higher than that (about 3:2) in steels annealed under the
ultra-fast cooling rates of 200 oC/s, 500 oC/s and 1000 oC/s. 160
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165
170
175
180
185
190
Fig. 2 Optical micrographs of IF steels under different cooling rates
(a) 5 oC/s; (b) 50 oC/s; (c) 200 oC/s; (d) 500 oC/s; (e) 1000 oC/s
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Fig. 3 Influence of cooling rates on ferrite grain sizes 195
3.2.2 Precipitation behavior
Fig.4 shows the presence of precipitates in samples annealed under different
cooling rates. A general observation made from the TEM micrographs is that cuboid
TiN is the most common precipitates under all different cooling rates and TiC
precipitates are in elliptical shape. Three morphologies of TiN precipitates: (1) pure 200
TiN; (2) TiN clusters (Fig.4d); (3) TiN with an oxide nuclei (Fig.5); can be observed.
TiN particles (with oxide nuclei), which grew under the cooling rates of 5 oC/s and 50
oC/s, are greatly larger than those grown under ultra-fast cooling rates. The sizes of
pure TiN observed under different cooling rates are shown in Tab.4. and they decrease
with increasing cooling rates. Fig.6 reveals elliptical TiC precipitates and the presence 205
of FeTiP which formed on the pre-existing TiC particle under the cooling rate of 5
oC/s only. The schematic development (including morphologies and forming stage) of
different precipitates is shown in Fig.7.
210
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215
220 225
230
235
Fig. 4 TEM extraction replica micrographs showing presences of different precipitates under
various cooling rates 240
(a) 5 oC/s; (b) 50 oC/s; (c) 200 oC/s; (d) 500 oC/s; (e) 1000 oC/s; (f) non-annealed sample
245
250
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255
260
265
270
Fig.5 TEM micrographs showing the presence of large size TiN precipitates formed under the 275
cooling rates of 5 oC/s (a) and 50 oC/s (b) and corresponding EDS spectra (c) and (d)
Tab.4 Different sizes of pure TiN precipitates under different cooling rates
Common cooling rates
Ultra-fast cooling rates Non-annealed sample
Cooling rates/ oC·s-1
5 50 200 500 1000 none
TiN sizes/nm 20~25 20 10~15 10~15 10~12 10
280
285
290
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295
300
305
310
Fig. 6 TEM extraction replica micrographs showing presences FeTiP and TiC precipitates formed under
the cooling rate of 5 oC/s (a) and corresponding EDS spectra FeTiP (b) and TiC (c)
Fig. 7 Schematic development of various morphologies of different precipitates over different process
315
3.3 Texture evolution under different cooling rates
The ideal orientations of texture components in BCC materials are shown
schematically for φ2=0° and 45° ODF sections in Fig.8 and Tab.5. For IF steel, the
vital fibers are the α-fiber (fiber axis parallel to the rolling direction including
components of {001} , {112} and {111} ), γ-fiber (fiber axis 320
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parallel to the normal direction including components of {111} and {111}
), ε-fiber (fiber axis parallel to the transverse direction including
components of {001} , {112} , {111} and {011} ), η-fiber
(fiber axis parallel to the rolling direction including components of {001}
and {011} ), θ-fiber (fiber axis parallel to the normal direction 325
including components of {001} and {001} ) and ξ-fiber (fiber axis
parallel to the normal direction including components of {011} , {011} ,
{011} and {011} [18].
330
335
\ 340
Fig. 8 Schematic illustration of the important texture components in BCC materials
(a) φ2=0°(b) φ2=45°
345
350
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Tab.5 The important fibers and orientations for BCC materials
Fiber name Fiber axis Important components
α //RD {001}, {112}, {111}
γ //ND {111}, {111}
ε //TD {001}, {112}, {111}, {011}
η //RD {001}, {011}
θ //ND {001}, {001}
ξ //ND {011}, {011}, {011}, {011}
Fig.9 illustrates the ODFs of samples annealed under different cooling rates.
Changes of maximum intensity versus the annealing cooling rates for major texture
components are presented in Fig.10.It can be seen that the main texture orientations 355
were {111} , {111} and {011} with the maximum intensity of
4.09×R, 4.15×R and 2.59×R when the cooling rate was 5 oC/s. Fig.9(b) indicates that
the intensity of {111} orientation increased by 25%, from 4.09×R to 5.06×R,
under the cooling rate of 50 oC/s. There was also an increase in the intensity of {001}
orientation from 0.453×R to 1.54 ×R. It should be noted that the intensity of 360
{111} increased by 21%, from 4.49×R to 5.41×R, under the cooling rate of 200
oC/s. Fig. 9(d) indicates the intensity of {111} and {111} texture
components decreased by 9.8% and 6.5% under the cooling rate of 500 oC/s,
compared with those formed under the cooling rate of 200 oC/s. The intensity of {011}
orientation decreased a lot, from 1.77×R to 1.05×R. Finally, when the cooling 365
rate was 1000 oC/s, the intensity of {111} and {011} components
increased, while the intensity of {111} decreased.
370
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385
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405
410
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420
Fig. 9 ODFs of the IF steel annealed under different cooling rates
(a) 5 oC/s; (b) 50 oC/s; (c) 200 oC/s; (d) 500 oC/s; (e) 1000 oC/s 425
430
435
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440
445
Fig. 10 The maximum intensity of main texture components versus annealing cooling rates
4. Discussion
4.1 Effects of cooling rates on mechanical properties 450
The mechanical properties of samples annealed under different cooling rates are
shown in Fig.11. The yield strength and rave values increase with increasing cooling
rates, except that the sample annealed under the cooling rate of 500 oC/s possesses
lower r value (2.21). The elongation rates, n values and UTS change little under
different cooling rates. 455
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Fig. 11 Mechanical properties of samples annealed under different cooling rates
According to the measured mechanical properties, the yield strength increases as 460
the increase of cooling rates. The grain boundary strengthening mechanism is usually
described by Hall-Petch equation as follows:
σy=σ0+ky*d-1/2 (4-1)
where σy is the yield strength, d is the average grain diameter, σ0 is the friction stress
and ky is the Hall-Petch slope[23]. According to the grain sizes obtained under 465
different cooling rates, the grain diameters decrease as the increase of cooling rates.
The grain size of the sample annealed under the cooling rate of 5 oC/s is 15.2µm,
while under the cooling rate of 1000 oC/s, the value decreases to 11.7µm. Based on the
Hall-Petch equation, the decrease of grain diameters will result in the increase of yield
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strength. This is consistent with the increase of yield strength values as the increasing 470
of cooling rates in this paper.
The little change of UTS, elongation and n values can be explained as follows.
Solution strengthening is the major method to increase UTS in this IF steel. All the
samples contain the same amount of P as solution strengthening atoms and thus the
UTS changes little in different steels. Ferrite structure and large grains are responsible 475
for retaining the ductility in low carbon steels[19, 29]. Since all samples are composed
of 100% ferrites and the difference of grain sizes formed under the cooling rates from
5 oC/s to 1000 oC/s is small, the elongation changes little. Strain hardening exponent
(n) is an important parameter to assess the deformation characteristic of steel and it
depends mainly on the structure types in low carbon steels[19]. The ferrites lead to 480
more hardening by large amount of deformation compared with other structure types.
All samples formed under different cooling rates are all made up of ferrite structure
explains the little change of n values in this paper.
4.2 Precipitation behavior
According to the second phase solid solubility formula in steels, the TiN and TiC 485
solid solubility formulas based on phase analysis are[30]
lg{[Ti][N]} γ=3.94-15190/T (4-2)
lg{[Ti][C]} γ=5.33-10475/T (4-3)
Their precipitation temperatures are 1355 oC and 872 oC respectively based on
these two formulas. The Ti compounds precipitation temperature has been studied and 490
previous researchers has made schematic illustration regarding the sequence of
precipitates formation and their stability[31]. According to his research, TiN takes
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place at 1300 oC, while TiC and FeTiP at about 700 oC. It is also found that TiN is
more stable than TiC in their studies. Park has pointed out that some oxides can act as
heterogeneous nuclei for the formation of TiN in ferritic chromium during steel 495
making route[31]. Some TiN precipitation took place in liquid based on previous
studies and the process is controlled by solubility limit of Ti-N equilibrium. CaO and
SiO2 are slag particles and they formed at liquid stage. These oxides acted as
nucleation for TiN precipitates at high temperatures and these TiN present in both all
the cooling rates. 500
The formation of large size TiN (shown in Fig. 5), which has oxide nuclei, are
due to they have more time to grow under the common cooling rates. In order to
explain the different sizes of pure TiN formed under different cooling rates, diffusion
controlled growth mechanism needs to be considered. Free standing TiN forms during
re-heating and hot rolling and it is the interstitial nitrogen which is trapped first by Ti 505
and remained so. As for free standing TiC, they usually take place during coiling.
Comparing the sizes of pure TiN in non-annealed sample and the sample annealed
under the cooling rate of 1000 oC/s, we can learn that TiN grows during the holding
section. As the cooling rates decrease from 1000 oC/s to 5 oC/s, the pure TiN particles
have obvious tendency to grow bigger. The cooling rates may change the dimensions 510
growth number and ultimately affects the time exponents and transformation
process[32,33].
Apart from these, FeTiP precipitates are observed under the cooling rate of 5 oC/s.
The precipitation behavior of FeTiP has been studied before. In batch annealing, the
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size of FeTiP precipitate is big and the content of phosphorus atoms is high[2]. On the 515
contrary, in the continuous annealing process, FeTiP is observed on the pre-existing
TiC and the content of phosphorus is low[3]. It has been reported that FeTiP takes
place over TiC particles, suggesting that FeTiP forms only after the formation of
TiC[2]. This is consistent with the FeTiP precipitation behavior in this paper. It is
reported that phosphorus accumulates at the bottom of the edge dislocation since the 520
radius of phosphorus (0.109 nm) is smaller than that of α-Fe (0.124 nm)[13]. This is
illustrated in Fig.12. Phosphorus atoms are driven by moving dislocation lines and
accumulate at grain boundaries during continuous annealing process[13]. The FeTiP
precipitates are found near TiC particles (Fig.6) in this study. Based on the above
conclusions, we can assume that phosphorus, which concentrates at grain boundaries, 525
interacts with Ti and Fe atoms. Thus FeTiP forms on pre-existing TiC particles. This
mechanism of FeTiP formation is consistent with the previous result that FeTiP may
not be a strict stoichiometric compound [11]. It has been studied that the formation of
FeTiP precipitates strongly prevents the growth of {111} recrystallization texture
which is beneficial for deep-drawing property[2, 3]. 530
535
Fig. 12 Schematic of phosphorus and edge dislocation
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4.3 Effect of texture on r value
All steels possess high rave values except those annealed under the cooling rates
of 5 oC/s and 500 oC/s (2.13 and 2.21 respectively). It is well known that the forming 540
property can be characterized by the rave and △r values which are strongly affected by the texture components. In order to get good forming property, the material should
have high rave value and low △r value[34, 35]. D. Daniel[35] has studied the relationship between r values and texture components in low carbon steels and the
statistics are listed in Tab.6. It can be learnt that the {111} , {111} and 545
{011} components are the most favorable to increase the r value, so the
intensity of these three components formed under different cooling rates is plotted in
Fig.13. From Tab.6, we can learn that the {011} component contributes most
to the increase of r value, but it also increases △r value dramatically. The {111} and {111} components can increase r value significantly without increasing △r 550 value. Different texture components correspond to different rave and △r values. In addition to texture components, Chung[24] suggests that grain morphology also has
an effect on the mechanical property anisotropy and equaixed grains normally
correspond to the low △r value. Since all samples are made up of equiaxed ferrite grains and the difference of average grain aspect ratio is small, the effect of grain 555
morphology on △r value could be ignored.
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Tab. 6 The correlation between texture components and r values in BCC materials
Texture Components r r
{001} 0.4 -0.8
{112} 2.1 -2.7
{111} 2.6 0
{111} 2.6 0
{554} 2.6 1.1
{011} 5.1 8.9
The FeTiP precipitate was observed under the cooling rate of 5 oC/s and its
formation result in the weak intensity of {111} and {111} components
(4.09×R and 4.15×R respectively). According to the analysis, the {111} and
{111} components are favorable to increase r values. Then the weak texture 565
components of {111} and {111} are responsible for the low rave value
(2.13) in the sample annealed under the cooling rate of 5 oC/s.
The sample annealed under the cooling rate of 500 oC/s also has a lower rave
value (2.21) and lower △r value (0.75) compared with those annealed under the cooling rates of 200 oC/s and 1000 oC/s. As shown in the Fig.13, we can learn that the 570
sample annealed under 500 oC/s has the weakest {011} fiber. According to the
relationship between texture components and r value, the strong {011}
component significantly contributes to the increase of r and △r value. Thus, the lower r and △r value in the sample annealed under the cooling rate of 500 oC/s is attributed to the weak intensity of {011} . 575
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Fig. 13 The intensity of major texture components under different cooling rates
5. Conclusions
(1) The yield strength, r and △r values are strongly influenced by cooling rates. The yield strength increases as increasing of cooling rates from 5 oC/s to 1000 oC/s. The r
values also show a rising trend as the increase of cooling rates, except that the sample 600
annealed under the cooling rate of 500 oC/s possesses lower r value. In addition, in the
case of 500 oC/s, the sample also has the lowest △r value. The ultimate tensile strength, n values and elongation rates change little under all different cooling rates.
(2) The size of pure TiN precipitates decreases as increasing of cooling rates. The
oxides, like CaO and SiO2, act as heterogeneous nuclei for TiN and these TiN have 605
larger size under the cooling rates of 5 oC/s and 50 oC/s. FeTiP can be observed on the
pre-existing TiC particles under the cooling rate of 5 oC/s and the presence of FeTiP
precipitate prevents the growth of recrystallization {111} texture.
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(3) The intensity of {111} orientation increases as the cooling rate increases
from 5 oC/s to 1000 oC/s while the intensity of {011} decreases as the cooling 610
rate increases from 5 oC/s to 500 oC/s. There is a texture transition in the γ-fiber: from
{111} to {111} under the cooling rate of 1000 oC/s. The r and △r values have strong connection with texture components. The low rave under the cooling rate
of 5 oC/s is attributed to the weak {111} and {111} components. The
weak {011} orientation is responsible for the low rave and △r values under the 615 cooling rate of 500 oC/s.
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Highlights:
(1) The best formability steel can be obtained under the cooling rate of 200 oC/s.
(2) FeTiP precipitates on parts of pre-existing TiC under the cooling rate of 5 oC/s.
(3) The size of pure TiN decreases as increasing of cooling rates.
(4) The intensity of {011} orientation decreases with increasing cooling rates.
(5) The weak {011} orientation is responsible for low r and △r values.