Materials Transactions, Vol. 48, No. 12 (2007) pp. 3079 to 3087
Special Issue on Growth of Ecomaterials as a Key to Eco-Society III
#2007 The Japan Institute of Metals
Interaction between Phosphorus Micro-Segregation and Sulfide Precipitation
in Rapidly Solidified Steel—Utilization of Impurity Elements
in Scrap Steel
Zhongzhu Liu1 , Yoshinao Kobayashi2 , Mamoru Kuwabara1 and Kotobu Nagai2
1
2
Graduate School of Engineering, Nagoya University, Nagoya 464-8603 Japan
National Institute for Materials Science, Tsukuba 305-0047, Japan
Copper is one of the main residual elements in steel, especially in recycled scrap steel. Sulfur and phosphorus are two of the main
impurities in steel, and it may result in a large emission of slag and CO2 to remove them from steel. Utilization of these elements has been an
important and difficult matter for metallurgist. In the present paper, the as-cast steels containing different concentrations of copper, sulfur and
phosphorus are prepared by strip casting process or laboratory rapid solidification process. The effect of phosphorus addition on sulfide
precipitation is investigated and discussed with respect to the morphology, size, and composition of sulfide. Both experimental results and
mathematical calculation showed that the addition of phosphorus retards the sulfide precipitation at high temperature, promotes more copper
bearings and smaller sulfides precipitation at low temperature. On the other hand, sulfide precipitates are shown to reduce the micro-segregation
degree of phosphorus in steel, which may be because some phosphorus dissolves in sulfide and sulfide particles provide more interfaces for
phosphorus to distribute. [doi:10.2320/matertrans.MK200704]
(Received June 1, 2007; Accepted July 30, 2007; Published September 12, 2007)
Keywords: phosphorus micro-segregation, manganese sulfide, copper sulfide, precipitation, rapid solidification, low carbon steel
1.
Introduction
Copper is one of the major residual elements in steel
because it is difficult to be removed during the steelmaking
process. With the scrap steel recycled continuously, the
concentration of copper in steel has been increasing gradually. Sulfur and phosphorus are undesirable impurities in steel
since they may lead to low toughness, poor weldability and so
on. Sulfides in steel also cause problems due to their size and
morphology. Large sulfides usually result in poor mechanical
properties, and non-spherical sulfides cause some properties
with anisotropy.
On the other hand, some novel processes, such as thin slab
continuous casting and compact rolling process/thermomechanical treatment,1,2) have recently become popular
throughout the world, and some other novel processes such
as cross-rolling have been studied.3) Such novel processes
have encouraged people to reconsider some elements’ roles
in steel with more comprehensive views. For example, the
harmful effects in the conventional continuous casting and
rolling process that are caused by some impurities could be
reduced, and these impurities may become beneficial
elements in such novel processes. Phosphorus, which is
usually considered to be an impurity in steel, has been
reported to have great effects on phase transformation and to
refine the prior-austenite grain size during the solidification
process.4–6)
Copper and sulfur in steel may form copper sulfide,
especially during rapid solidification process.7–9) Sulfide
usually has different solubility in different iron phase. Since
phosphorus has a great effect on the phase transformation
temperature and process, some interaction between phosphorus and sulfide precipitation may exist. In this paper, the
effect of phosphorus on sulfide precipitation, as well as the
effect of sulfide on phosphorus micro-segregation are investigated and discussed.
2.
Experimental Procedures
2.1 Materials and casting conditions
The chemical compositions for four kinds of steel samples,
which are designated as LPS and HPS as well as CuS2P and
Cu2P steel, are shown in Table 1. The contents of the
impurities in these steels are designed to be slightly higher
than those in normal low carbon steel to simulate the scrap
steel. The LPS and HPS samples are cast by a twin drum
caster at the Mitsubishi Heavy Industries Ltd., Hiroshima
R&D Center. The casting speed is about 0.333 m/s, the
casting temperature is about 1846 K. The width of mold is
0.6 m and the thickness of samples is about 2:2 103 m.
The CuS2P and Cu2P samples are prepared in an induction
heating furnace under vacuum atmosphere in the laboratory.
After 3 kg of electrolytic iron is melted, the alloying elements
(Mn, Si, Cu, S) are added to the melted iron. The melt is then
cast into a water cooled copper mould. A specimen with a
3 60 200 mm steel plate on one side and a 5 60 200 mm steel plate on the other side can be obtained as shown
in Fig. 1. More detail description of the preparation of
samples could be found in the previous papers.10,11)
2.2 Analysis methods
The precipitates in the samples are observed by Scanning
Electron Microscopy (SEM) and Transmission Electron
Microscopy (TEM). Specimens for SEM observation are
etched by 3 vol% nitric acid (Nital) or 10% acetylacetone-1%
tetramethylammonium chloride-methyl alcohol (hereinafter
abbreviated to 10% AA electrolyte). In the latter case, the
sample is mirror polished and etched in the 10% AA
electrolyte at a controlled potential.12) The SEM observation
is performed on a LEO 1550 microscope with highresolution. Carbon extraction replicas are prepared through
the standard procedures for TEM observation. The replicas
are floated on molybdenum grids and a beryllium specimen
3080
Z. Liu, Y. Kobayashi, M. Kuwabara and K. Nagai
Table 1
The chemical composition of the samples (mass%).
Sample
C
Si
Mn
P
S
Cu
O
Ae4 , K
Ae3 , K
Thickness,
103 m
LPS
0.096
0.26
0.61
0.013
0.016
0.12
0.004
1754.7
1120.1
2.2
HPS
CuS2P
0.088
0.074
0.25
0.15
0.56
0.68
0.081
0.19
0.017
0.0593
0.12
0.89
0.004
<0:004
1708.2
1633.2
1171.5
1229.3
2.2
5.0
Cu2P
0.08
0.15
0.60
0.20
0
1.0
<0:004
1643.0
1234.3
5.0
(a)
Induction furnace
(b)
3mm
plate
Quartz
nozzle
(a)
5mm
plate
(b)
C
D
TD
Water cooled
copper mould
ND
B
Fig. 1 The lab rapid solidification device for preparation of CuS2P and
Cu2P samples.
holder is used to avoid the possible detection of copper from
the grid and the specimen holder. Although it could be
avoided to detect the Cu peak from the grids, unfortunately
there is some overlap between the S K peak and the Mo L
peak. The value of the ratio between any element (Mn, Cu,
Fe) and S based on the present Energy Dispersive X-Ray
Spectroscopy (EDS) analysis data is a little lower than the
real value in the particle since the S peak reflects the Mo
content to some extent. The TEM observation is performed
with a JEM-2000FXII microscope operated at 200 kV and
coupled to an EDS.
The distribution of phosphorus in CuS2P and Cu2P
samples are investigated with an Electron-Probe MicroAnalyzer (EPMA). The analysis is conducted on an area of
1:024 1:024 cm2 by 512 512 points with a beam size of
1.0 mm.
1 µm
(c)
(d)
Fig. 2 Spherical sulfides in LPS sample. (a) Morphology; EDS of (b) Point
B, (c) Point C, and (d) Point D.
(a)
020
3.
200 220
Experimental Results
3.1 Sulfide precipitates
Sulfides with two kinds of morphology are found in LPS
sample, spherical and plate-like, as shown in Figs. 2 and 3.
The size of spherical sulfides varies considerably, from
approximately 1 107 m to 1:6 106 m. The large spherical sulfides consist mainly of Mn and S, with some Cu and
Fe. When the size of the particles decreased, the content of
Cu in sulfides increased as shown in Fig. 2.
The plate-like sulfides consist mainly of Cu and S, with
small amounts of Fe and Mn, as shown in Fig. 3. The short
and long axes of the plate-like sulfide are between
ð5{10Þ 108 m and ð3{14Þ 107 m, respectively.
Fig. 3(c) shows that the diffraction pattern of the plate-like
copper sulfide has a face centered cubic (f.c.c) structure.
About 8% (in number) of the sulfides have a plate-like
morphology in LPS sample.
Two distinct characteristics are observed in HPS sample
compared to LPS sample. First is the presence of spherical
sulfides as shown in Fig. 4. Most of these spherical sulfides
100 nm
(b)
(c)
500 nm
Fig. 3
200 nm
Plate-like sulfides in LPS sample, f.c.c structure.
were mainly Cu2x S (Fig. 4(b) and (d)), and their size was
less than 1:5 107 m, which is far less than the size of the
sulfides in LPS sample. The second is the absence of platelike sulfide in HPS sample.
Interaction between Phosphorus Micro-Segregation and Sulfide Precipitation in Rapidly Solidified Steel—Utilization of Impurity Elements
(a)
(b)
3081
(a)
E
100 nm
(c)
(d)
100 nm
Fig. 4 Morphology and EDS of spherical sulfides in HPS sample. (a) TEM
image and (b) EDS spectrum from extraction replica specimen; (c) SEM
image and (d) EDS spectrum, electrolytically etched by 10% AA
electrolyte.
1.0
(b)
Ni / N total or Σ Ni / N total
0.9
0.8
0.7
HPS− Ni / N total
LPS− Ni / N total
HPS− Σ Ni / N total
LPS− Σ N i / N total
0.6
0.5
0.4
0.3
0.2
0.1
0.0
0
Fig. 5
100
200
300
400
500
Particle Size , X10−9 m
600
700
The size distribution of sulfides in LPS and HPS samples.
The size distribution of spherical sulfides in LPS and HPS
samples is comparably shown in Fig. 5. The spherical sulfide
in LPS sample has a wide size distribution, which is from
smaller than 1 107 m to larger than 1:6 106 m. On the
other hand, the size distribution in HPS sample is from
smaller than 1 107 m to about 5 107 m. Most of the
sulfides in HPS sample are smaller than 1:5 107 m.
Copper sulfides with a fine size of less than 50 nm are also
reported in other sample containing 0.12% phosphorus.7,8) It
seems that the fine sulfide, especially copper sulfide,
precipitates more easily in high phosphorus steel.
The above results show the effects of phosphorus on
sulfide precipitation in the present samples as follows:
(1) The size of spherical sulfide particles becomes smaller
in HPS sample compared to that in LPS sample, indicating
that phosphorus retards the sulfide precipitation at high
temperature;
(2) The composition of the spherical sulfides was mainly
MnS in LPS sample while it was mainly Cu2x S in HPS
sample, indicating that phosphorus suppresses MnS precipitation and promotes Cu2x S precipitation.
Fig. 6 The distribution of solute elements revealed by EPMA in Cu2P (a)
and CuS2P (b) samples.
(3) The plate-like Cu2x S seldom appears in HPS sample,
indicating that phosphorus has some effect on the copper
sulfide morphology.
3.2 Phosphorus micro-segregation
The distribution of phosphorus in CuS2P and Cu2P
samples, the former contains sulfide forming elements and
the later does not contain, are investigated by EPMA.
Figure 6 shows the distribution of concentration of Mn, Cu, P
and S elements. Whether in CuS2P sample or in Cu2P
sample, most of these elements are rich in the interdendrite
Relative Cumulative Freqyency, RCF
3082
Z. Liu, Y. Kobayashi, M. Kuwabara and K. Nagai
4.
1.0
Discussion
0.9
4.1
0.8
0.7
0.6
0.5
0.4
0.3
0.2
Cu2P
CuS2P
0.1
0.0
−1
0
1
3
2
(CP, i- CP, mean)/CP, mean
5
4
Fig. 7 The concentration deviation from the mean value revealed by
EPMA.
region, and the dots with high concentration of elements
portray the sketch of dendrite structure. That means the main
micro-segregation of solute elements in the as-cast sample is
caused during the liquid/solid transformation process,
although there are several solid phase transformations
following the liquid/solid transformation.
Two parameters, K1 and K2 , are used to evaluate the microsegregation degree of phosphorus in present paper. They are
defined as the following equations, respectively.
Cp;i Cp;mean
K1 ¼
ð1Þ
Cp;mean
Cp;max Cp;min
ð2Þ
K2 ¼
Cp;mean
Where Cp;i is the concentration of phosphorus for point i;
Cp;mean is the mean concentration of phosphorus for all the
512 512 points; Cp;min and Cp;max are the minimum and
maximum concentration of phosphorus in the 512 512
points.
Figure 7 shows the Relative Cumulative Frequency (RCF)
vs. K1 in CuS2P and Cu2P samples. The concentrations of
phosphorus for analyzed points in CuS2P sample are closer to
the mean value than those in Cu2P sample, which shows the
segregation degree of phosphorus in CuS2P sample is lower
than that in Cu2P sample. This tendency is also supported by
the data listed in Table 2, where it is shown that the microsegregation degree evaluated by K2 in CuS2P sample (5.45)
is also lower than that in Cu2P sample (5.95). In addition, the
concentration range, that is the difference between Cp;max and
Cp;min , in CuS2P sample is lower than that in Cu2P sample as
listed in Table 2.
Effect of phosphorus on the precipitation of spherical sulfide
Phosphorus is a well known ferrite stabilizing element and
has a great effect on the phase transformation temperature.
According to the Fe-C phase diagram containing different
phosphorus concentration,4,10) phosphorus decreases the
liquidus and solidus of steel and also has a great effect on
Ae4 and Ae3 temperature. The addition of 0.1 mass% P in
steel may lower Ae4 by 55 K and raise Ae3 by 70 K.4) Since
phosphorus segregates very easily during solidification, it
may have a greater effect on the local transformation
temperature compared to that at the equilibrium phase state.
The sulfide in the present as-cast steels may be formed in
quite a different stage, for example, in the last stage during
solidification, during the = transformation, in the -Fe
region, and during the = transformation and so on. It is not
so easy to determine when a specified sulfide is formed, but
for the sulfides with a large size they are usually formed at
high temperature, for example during solidification or =
transformation process. A lot of manganese sulfide particles
with a large size are observed in LPS sample while few in
HPS sample. Phosphorus may exert some influences on these
sulfides formation due to its effect on the phase transformation temperature, the Equilibrium Distribution Coefficient (EDC) of sulfur between liquid and solid phase, the
activity coefficient of sulfur and the growth behavior of
sulfide.
A modified Clyne and Kurz’s13) mode is used to calculate
the redistributation of solute elements during solidification.
Half of the area of the secondary dendrite spacing is selected
as the calculation domain and is divided into N (N ¼ 30)
nodes as shown in Fig. 8. The secondary dendrite arm space
is set as 1:5 105 m in the present calculation based on
experimental data for both LPS and HPS samples. The
calculation is carried out by the direct finite difference
method which is described in detail in a previous paper.10,14)
The TL , TS , TA4 and TA3 temperatures are calculated by the
following equations,4,15) respectively:
TL ¼ 1809 78½%C 7:6½%Si 4:9½%Mn
34:4½%P 38½%S 4:7½%Cu; K
ð3Þ
TS ¼ 1809 175½%C 20:5½%Si 6:5½%Mn
500½%P 700½%S; K
ð4Þ
TA4 ¼ 1665 þ 1122½%C 60½%Si þ 12½%Mn
550½%P 160½%S; K
0:5
TA3 ¼ 1183 230½%C
ð5Þ
þ 44:7½%Si
30½%Mn þ 700½%P 20½%Cu; K
ð6Þ
Table 2 The summary of the parameters of EPMA data.
Sample
Mean
CP , %
SD
SEM
Minimum
CP , %
Maximum
CP , %
CP Rang,
%
K2
CuS2P
0.274
0.11166
2:18 104
0.088
1.582
1.494
5.45
0.040
1.664
1.624
5.95
0.15088
2:95 104
pffiffiffiffiffiffiffiffi
Pn
2
1
SD
Note: The standard deviation SD ¼ Var, where Var ¼ n1 i¼1 ðCp;i Cp;mean Þ ; The standard error of the mean SEM ¼ pffiffin ; and n ¼ 512 512 ¼ 262144.
Cu2P
0.273
Interaction between Phosphorus Micro-Segregation and Sulfide Precipitation in Rapidly Solidified Steel—Utilization of Impurity Elements
Center of
dendrite
Inter-dendrite
Nodes from the center of dendrite
1
2
3
......
Solid
liquid
Table 4 Selected interaction coefficients in dilute solutions of ternary iron
base alloys at 1873 K.19Þ
Element j
C
eSj
0.11
j
eMn
ePj
28 29 30
Cl
t+dt
Cl
t
3083
Mn
0:07
0.12
0.0
0.12
0.0
0.0
6.45
"Sj
j
"Mn
Si
0.063 0:026
P
S
0.029
Cu
0:028 0:0084
0:0035
0.062
7.76
5:87
4.1
3:3
2:7
0.5
0
0
5:87
7.0
14.2
0
8.4
"Pj
—
0.024
0:048
0.028
2:35
4.1
—
6.03
CO
CS
t+dt
1.25
CSt
0
dfS
fS
1.0
Fig. 8 Schematic showing the solute redistribution with complete liquid
mixing and some solid-state diffusion.
The calculated composition is based on 0.088%C0.25%Si-0.56%Mn-0.017%S-0.12%Cu. Phosphorus is set
as 0.013%P and 0.081% for low P steel and high P steel,
respectively. For such composition, the steels are solidified
completely as a phase and then proceed to a solid =
transformation. The values of physical properties used for
calculation are listed in Table 3.15–17)
The liquid/solid Equilibrium Distribution Coefficients
(EDC) of solute elements in multi-component systems may
be different from those in binary systems because of the
possible existence of solute interactions. It is difficult to
calculate the influence of these interactions on EDC of solute
elements, but it is possible to estimate the effect of an
addition of one alloying element on the EDC of another
element in iron phase as shown in eq. (7).18)
ln kij ¼ ln
ki3
¼ ð1 kj2 Þ "ij NjL
ki2
ð7Þ
Where kij is the Distribution Interaction Coefficient (DIC),
which shows the effect of addition of j element on the EDC of
i element in iron; ki3 is the EDC of i element in Fe-i-j system;
ki2 is the EDC of i element in Fe-i system, which is equal to
k=L in Table 3 in present calculation; "ij is the interaction
coefficient as listed in Table 4;19) NjL is the mole fraction of j
element in liquid phase. The effects of P on EDC of S, as well
as that of S on P are considered in present calculation. There
Table 3
DIC of S or P, k ij
1.20
δS
1.15
1.10
1.05
kSP
kPS
1.00
0.0
0.5
1.0
1.5
2.0
2.5
Concentration of P or S, C / %
Fig. 9 The interactive effect on the distribution coefficient between P and S
elements.
P
is equal to
is no such interaction between P and Mn since "Mn
zero. That interactive effect between P and S is shown in
Fig. 9. Phosphorus and sulfur could increase each other’s
EDC, which means they could decrease each other’s microsegregation to some extent.
Figure 10 shows the temperature evolution for low P steel
and high P steels during solidification. The liquidus and
solidus temperature for the high P steel are lower than those
for the low P steel, and this tendency is remarkable especially
at the solidus temperature. The effect of phosphorus on phase
transformation temperature is recently proved by an ‘‘in-situ’’
observation of the = transformation by the present
authors.20) The same LPS and HPS samples are firstly hold
at 1723 K for 1200 s and then cooled to an ambient
temperature with a 0.33 K/s cooling rate. The observed
starting temperatures of = transformation in LPS and HPS
samples are close at about 1698–1701 K. However, as the
transformation continues, the transformation process in the
The equilibrium distribution coefficient and diffusion coefficient of the solute elements for mathematical calculation.15{17Þ
elements
k=L
D , cm2 /s
D at 1790
K, cm2 /s
D at 1750
K, cm2 /s
C
Si
0.19
0.77
0:0127 expð19450=RTÞ
8:0 expð59500=RTÞ
5:36 105
4:34 107
4:73 105
2:96 107
Mn
0.76
0:76 expð53640=RTÞ
2:14 107
1:52 107
2:9 expð55000=RTÞ
7
3:92 107
6
1:79 106
7
4:51 107
P
S
Cu
0.23
0.05
0.90
3.0
4:56 expð51300=RTÞ
25 expð62000=RTÞ
5:58 10
2:48 10
6:72 10
3084
Z. Liu, Y. Kobayashi, M. Kuwabara and K. Nagai
0.050
low P Steel
Temperature, T / K
1790
TS
1780 TL
1770
high P Steel
1760
1750
1740
Concentration of S, CS / %
1800
1730
0.045
0.040
0.035
0.030
0.025
0.020
0.015
0.010
0.005
0.000
0
0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0
Concentration of S, CS / %
1.0
Concentration of Mn, C Mn / %
10
15
20
25
30
0.050
Fig. 10 The temperature evolution during solidification.
(a)
low P steel
high P steel
0.8
0.7
(b)
without considering
the solid back diffusion
with considering
the solid back dissusion
0.045
0.040
0.035
0.030
0.025
0.020
0.015
0.010
0.005
0.000
0.6
0
5
10
15
20
25
30
Node from the Center of Dendrite
0.5
0.4
0
5
Node from the Center of Dendrite
Solid fraction
0.9
(a)
without considering
the solid back diffusion
with considering
the solid back dissusion
5
10
15
20
25
30
Fig. 12 The concentration distribution of S with/without considering the
solid back diffusion in (a) LPS and (b) HPS samples.
Node from the Center of Dendrite
Concentration of S, CS / %
0.050
(b)
low P steel
high P steel
0.045
0.040
0.035
0.030
0.025
0.020
0.015
0.010
0.005
0.000
0
5
10
15
20
25
30
Node from the Center of Dendrite
Fig. 11 The concentration distribution of (a) Mn and (b) S in solid phase
just after solidification.
HPS becomes slower than that in the LPS, and the finishing
temperature of the = transformation of the HPS at about
1585 K is much lower than that of the LPS at about 1653 K.
The total times for the = transformation of the LPS and
HPS samples are about 142 s and 356 s, respectively. The
transformation process in the HPS is greatly retarded
compared to that in the LPS, especially in the last quarter
stage of the transformation process because of the addition of
phosphorus. This retardation effect is also believed existing
during solidification process.
During solidification, Mn and S continue to become richer
in retained liquid phase due to the redistribution between
liquid and solid phase. However, for the present calculation
composition, the actual product of Mn and S is still much
lower than the equilibrium solubility product for both low P
and high P steels. Therefore, MnS could not be formed in the
liquid phase in either steel.
The concentration distribution of Mn and S in the solid
phase just after solidification is shown in Fig. 11. In low P
steel and high P steel, the concentration distribution of Mn is
similar to each other, while that of S is quite different. The S
in low P steel is richer in the nodes closed to the interdendrite
area and shows a steep slope, while in high P steel it is a
gentler slope. That difference may be due to the effect of P on
the distribution coefficient of S or the back diffusion in solid
phase.
The concentration distribution of S in the solid phase just
after solidification with and without considering the solid
back diffusion is shown in Fig. 12. Without considering the
solid back diffusion, the concentration distribution of S is
close to each other in low P steel and high P steel. Only in the
last solidified area, the concentration of S in low P steel is
obviously higher than that in high P steel, which is caused
by the effect of P on the EDC of S in steel. That also means
the big difference in the concentration distribution of S
(Fig. 11(b)) in both steels is mainly caused by the solid back
diffusion. As shown in Fig. 10, high P steel has a wider
temperature range from liquidus temperature to solidus
Interaction between Phosphorus Micro-Segregation and Sulfide Precipitation in Rapidly Solidified Steel—Utilization of Impurity Elements
Activity Product, aMn*aS
0.050
low P steel
high P steel
0.045
0.040
activity product
0.035
0.030
0.025
0.020 equilibroum solubility product
0.015
0.010
0.005
0.000
0
5
10
15
20
25
30
Node from the Center of Dendrite
Fig. 13 The distribution of Mn and S activity product at solidus temperature in low P steel and high P steel.
temperature than that of low P steel. Since S is an element
with a high diffusion coefficient as shown in Table 3, where
the diffusion coefficient of S is more than ten times of that of
Mn, and there is more time for S to yield solid back diffusion
in high P steel compared with in low P steel, the segregation
degree of S in the last solidified zones is quite decreased in
high P steel as shown in Fig. 11. On the other hand, Mn has a
lower diffusion coefficient and the temperature change has
little effect on the distribution of its concentration.
Phosphorus may also have some effect on the activity
coefficient of S in steel, as shown in Table 4.19) The
calculation shows phosphorus may slightly improve the
activity of S, but the activity coefficient product of Mn and S
is almost close to 1.0 whether in low P steel or high P steel.
Phosphorus not only decreases the micro-segregation of S
close to the interdendrite area and retards this microsegregation to lower temperature, which suppresses the
MnS formation, but also slightly improves the activity
coefficient of S, which contrarily promotes the MnS
formation. The synthetic influence of phosphorus on the
formation of MnS at the solidus temperature is shown in
Fig. 13. In a large number of nodes the activity product of Mn
and S is higher than the equilibrium product in high P steel,
but the product value is not so high. On the other hand, in a
small number of nodes that product is higher than the
equilibrium one in low P steel, but the product value is very
high compared to those in high P steel. Since the solidus
temperature is quite different, it is reasonable that sulfide in
high P steel will be formed from a wider region at lower
temperature while in low P steel it is formed from a narrow
region with high S concentration at higher temperature. The
higher the precipitation temperature, the quicker the growth
rate for sulfides, and the more sulfide precipitating at high
temperature, the higher tendency to allure sulfide precipitating from the matrix at low temperature. Thus, the retarding
effect of P on sulfide precipitation during solidification may
have some active effects on fine sulfide precipitation at low
temperature.
The above analysis is not only restricted to solidification
process, but also could be applied to = transformation
process, during which both of P and S are redistributed from
-phase to -phase similar to their behavior during solid-
3085
ification process. Thus phosphorus also could retard and
suppress sulfide formation during = transformation process. By these reasons, it seems reasonable that fine sulfides
are popular in HPS sample while sulfides are with various
sizes in LPS sample.
The sulfides in LPS and HPS have quite different compositions besides the difference in size as shown in Fig. 2 and
4. As discussed in a previous paper,8) MnS has advantages in
precipitation at high temperature, while Cu2x S does at low
temperature. Since phosphorus could suppress sulfide precipitation at high temperature, it seems reasonable that the
main composition of sulfide in LPS sample is MnS while in
HPS sample it is Cu2x S.
It is also interesting that the plate-like Cu2x S is found
mainly in LPS sample and seldom in HPS sample. Several
detailed investigations have been reported on the formation
of plate-like MnS in steel up to now. Matsubara21) mentioned
that plate-like MnS precipitated on {100} plane in austenite
and was semi-coherent with austenite. Recently, Furuhara22)
confirmed the semi-coherent relationship by High Resolution
TEM observation in austenite stainless steel. Kimura23) and
Yamamoto24) observed the precipitation and growth of platelike MnS in austenite over a certain temperature range by the
Confocal Scanning Laser Microscope. Since Cu2x S and
MnS have the similar f.c.c structure, and the values of the
structural parameters are also very close to each other
(5:564 108 m for Cu2x S and 5:224 108 m for MnS), it
is reasonable to speculate that the present plate-like Cu2x S
precipitates in austenite. Phosphorus significantly increases
the Ae3 temperature (Table 1) and therefore may promote
Cu2x S precipitation in -Fe instead of in -Fe. That is the
probable reason why few plate-like Cu2x S are found in high
phosphorus steel.
4.2
Effect of sulfide on the micro-segregation of phosphorus
As shown in Fig. 7 and Table 2, the micro-segregation
degree of phosphorus decreases in sample (CuS2P) containing sulfide particles. Although the micro-segregation of
elements mainly results from the redistribution during
solidification in a as-cast steel, its behavior also depends on
a lot of factors such as temperature history, solute concentration, interface and so on.25,26) In addition to free surface
and grain boundaries, the following interfaces are also
important to segregation behavior: stacking faults and phase
interfaces, including precipitate/matrix interfaces.
Figure 14 shows a SEM/EDS line profile of Mn, P and S
elements across a sulfide particle in CuS2P sample. Phosphorus is shown co-existing with sulfide. It seems that
phosphorus segregates to the interfaces between sulfide and
matrix and/or some phosphorus dissolve in the sulfide.
Figure 15 shows the EDS spectrums of two sulfides on a
TEM extraction replica specimen. Some phosphorus is
observed co-existing with this sulfide. Although this phenomenon is not observed for all the sulfides, half of the
sulfides contain some degree of phosphorus.
Unfortunately there are few reports on the solubility of
phosphorus in sulfide and the segregation of phosphorus to
the interface between sulfide/iron. Recently a related
research work has been reported by Lauretta.27) The author
3086
Z. Liu, Y. Kobayashi, M. Kuwabara and K. Nagai
(a)
Analysis line
P
500 nm
S
(b)
(c)
Mn
Analysis line
P
Fig. 15 Phosphorus co-existed with sulfides in sample CuS2P. (a) TEM
image of sulfide particles; (b) EDS spectrums for two sulfides (exaction
replica specimen).
S
Mn
Fig. 14 Distribution of P along/around sulfide particles, SEM/EDS linescan profile. Sample CuS2P.
investigated the sulfidation of a Fe-based alloy containing
4.75%Ni, 0.99%Co, 0.89%Cr, and 0.66%P in 1.1% H2 S-H2
gas mixtures at 673–1274 K. After sulfidation for 4.5 hr at
different temperature, a sulfide layer is present on the metal.
In most experiments Ni, Co, and P are significantly enriched
at the metal-sulfide interface in the mineral schreibersite.
This mineral also formed at 1273 K but is not limited to the
metal-sulfide interface and is instead evenly distributed as
small inclusions through the sulfide layer.
It seems reasonable that the segregation degree of
phosphorus in a sample containing sulfide particles is
reduced if phosphorus has some solubility in sulfide particles
and sulfide particles provide more interfaces for phosphorus
to distribute. The area of these interfaces will greatly increase
especially when the sulfide particles has a fine size.
Once phosphorus could segregate to the interface between
sulfide/iron, this will also cause some influence on the sulfide
growth after precipitation. The sulfide growth behavior could
be described by Ostwald ripening model as the following
equation:28)
rt3 r03 ¼
8D½MVm
t
9RT
ð8Þ
where, rt is the particle radius at time t, r0 is the particle
radius at initial time, is the surface energy of the particlematrix interface, D is the diffusivity of the relevant atomic
species, [M] is the concentration of the relevant atomic
species in the matrix, Vm is the particle molar volume, R is the
gas constant, and T is the temperature. We can observe either
a low or low diffusion coefficients results in a low growth
rate. The grain boundary energy in -Fe is decreased from
0.795 J/m2 in pure Fe to 0.575 J/m2 in Fe containing 0.086%
P at 1723 K,29) it is also anticipated that phosphorus will
decrease the interface energy between sulfide/iron. Although
there are few data about the effect of phosphorus on interface
energy between sulfide/iron, it is reasonable that the
segregation of phosphorus to the interface between sulfide/
iron will restrict the growth of sulfide to some extent.
5.
Conclusions
Samples with different copper, sulfur and phosphorus
concentrations are prepared by rapid solidification using a
pilot strip caster or a laboratory rapid solidification caster.
The interaction between phosphorus and sulfide precipitation
are investigated. Phosphorus are shown to suppress manganese sulfide precipitation at high temperature while promote
copper sulfide precipitation with a finer size at low temperature, especially in an alpha-Fe phase. These effects result
from the effects of phosphorus on phase transformation
temperature, activity coefficient of sulfur and the growth
Interaction between Phosphorus Micro-Segregation and Sulfide Precipitation in Rapidly Solidified Steel—Utilization of Impurity Elements
behavior of sulfide. The segregation degree of phosphorus in
steel containing sulfide particles is reduced, which may be
due to the fact that phosphorus has some solubility in sulfide
particles and sulfide particles provide more interfaces for
phosphorus to distribute.
Acknowledgements
The authors would like to express their gratitude to Dr. F.
Yin, NIMS, and Dr. N. Yoshida, Sumitomo Metals, for their
fruitful discussions.
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