Materials Science Forum Vols. 561-565 (2007) pp. 1023-1026
online at http://www.scientific.net
© (2007) Trans Tech Publications, Switzerland
Online available since 2007/10/02
Control of Carbides and Graphite in Ni-hard Type
Cast Iron for Hot Strip Mills
Sergio Villanueva Bravo 1,a, Kaoru Yamamoto 2,b,
Hirofumi Miyahara c and Keisaku Ogi 3,d
1
Dept. of Materials Science and Engineering, Kyushu University, Fukuoka, 819-0395, JAPAN
(present work: Autonomous San Luis Potosi University, San Luis Potosi, Mexico)
2
(present work: Kurume National College of Technology, Fukuoka, 830-8555, JAPAN)
3
(present work: Oita National College of Technology, Oita, 870-0152, JAPAN)
a
[email protected], b [email protected],
c
[email protected], d [email protected]
Keywords: solidification, carbides, graphite, cast iron
Abstract The carbide and graphite formation and redistribution of alloy elements during
solidification were investigated on Ni-hard type cast iron (Fe-C-Si-Ni-Cr-Mo) to develop higher
quality rolls for hot steel strip mills. By the control of Ni and Si contents of iron, eutectic graphite
flakes crystallize even in cast irons containing strong carbide formers such as V, Nb and Cr. The
crystallization of Ni-hard type cast iron with V and Nb proceeds in the order of primary , + MC,
+ M3C and + graphite eutectic. Since the influence of each alloying element on graphite formation
is estimated based on the solubility of C in molten iron, the change in graphite forming tendency of
residual liquid is evaluated by the parameter expressing the solubility limit of C to molten iron. The
amount of graphite increases with the decreasing of solubility parameter. In addition, inoculation with
ferrosilicon effectively increases the graphite flakes.
Introduction
As for the roll materials for hot steel rolling, alloy cast irons, which disperse a large amount of
carbide in the matrix, are widely used because of their superior abrasion resistance [1]. For
applications requiring a high degree of strength, hardness and wear resistance Ni-hard type cast iron
is among the most effective material available [2]. Ni-hard type cast iron castings have shown
outstanding in a variety of severe applications including work rolls for hot steel milling. High
chromium cast iron and high-speed-steel type alloy are also widely used in steel plant, and Ni-hard
type cast iron is generally used in finishing stands [3]. However, the recent development in milling
technique demands the remarkable improvement and the precise control of roll quality. Since the roll
consumption in hot rolling of wire and bar represents around 10% of the total processing cost [4], the
effective alloying design is desirable to develop the proper solidification microstructure.
Ni-hard type cast iron consists of matrix, M3C carbide and graphite, and the matrix transforms to
martensite during cooling to room temperature in the mold. Distribution of graphite flakes or particles
can improve flaking and scoring resistance. On the contrary, it is well known that the addition of Nb
and V to white cast iron promotes the formation of MC type carbide and enhance the abrasion
resistance [5,6]. Therefore, the dispersion of MC type carbide in the matrix should improve the wear
resistance of Ni-hard type cast iron. However, even in V/Nb added alloys graphite flakes have to be
remained to keep the lubricant effect.
In the present study, the influences of V and Nb additions on the solidification sequence and
structure, and also on the redistribution of alloying elements during solidification have been
investigated for Ni-hard type cast iron. Furthermore, the effect of inoculation with a ferrosilicon has
also been studied in order to homogeneously distribute more amount of graphite.
Experimental Procedure
The alloy compositions used in the present study are listed in table 1. Blocks of 99.99% pure Fe, Cr
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and Ni and ferroalloys of Fe-5.3mass%C
Table 1 Chemical composition of the alloys.
(hereby abbreviate to %), Fe-75%Si,
Fe-64%Nb, Fe-83%V, Fe-62%Mo were
charged in an alumina crucible. Then, the
charge was melted in a carbon resistance
furnace under an argon atmosphere. Different
amounts of Nb and V were added to a typical
Ni-hard type cast iron for hot steel milling
rolls, which is shown as alloy No. 1. Since the
V and Nb are strong carbide formers, the
contents of Si and Cr were also systematically
changed depending on the V and Nb additions.
The as-cast specimens were smoothly
polished, and the amount and shape of
graphite were examined by using an optical
microscope. The specimens are also etched and examined metallographically. For the etching, two
reagents were used to identify the carbide in each specimen; etching with Pickral reagent makes M3C
brown, and the Murakami's solution colors MC carbides dark brown.
Thermal analyses were carried out to measure the temperature changes of these specimens in a SiC
electric resistance furnace. The specimen was placed inside of an alumina-silica crucible, then heated
at 10 K/min until 1723 K in argon atmosphere. After keeping the melt bath for 10 minutes to dissolve
the alloying elements completely, the specimen was cooled at 10 K/min until 1173 K, and
subsequently quenched into water. The solidification sequence was also evaluated based on the
), was calculated for all the tested specimens to
quenched structure. The solubility parameter (
predict the graphite forming tendency.
Results and discussion
Solidification sequence and microstructure
Figure 1 shows the thermal analysis results of
specimen No.1 and No.3. Specimen No.1 solidifies
in the order of primary austenite ( ), (L -> + M3C)
eutectic and (L -> + Gr) eutectic. The eutectic
reaction of (L -> + MC) occurs around 1453K in
the specimen No.3, while the addition of 0.53%Nb
and 1.86%V has little influence on the starting
temperature of primary , (L -> + M3C) eutectic
and (L -> + Gr) eutectic.
As shown in Fig.2, the specimen No.1 is
composed of dendritic primary , ledeburite and
graphite, a typical structure of Ni-hard type cast
iron for roll. On the other hand, MC carbide
particles (eutectic and pre-eutectic) distribute
among the dendritic
in all the specimens
containing Nb and V. However, few amount of MC
carbide is observed in the specimen No.2, because
of very low addition of MC former. The more the
addition of MC formers, the more amount of
vermicular type MC carbide crystallizes as in
specimens No.6. A small amount of primary MC
crystallizes in the specimen No.10 and No.11
because of higher addition of Nb, indicating the
chemical composition of these specimens are on the
Fig. 1 Thermal analysis for Ni-Hard type cast iron
(a)
(b)
(c)
Fig.2 Microstructures
of specimen No.1(a),
No.6(b), and No.10(c).
Materials Science Forum Vols. 561-565
(a)
liquidus plane of primary MC. Since primary MC
tends to segregate in the manufacturing process
(centrifugal casting etc.) of rolls, the Nb content
should be lowered below 1.8%.
The shape and amount of graphite change
depending on the composition of specimen as
shown in Fig.3. Graphite tends to crystallize in (c)
granular rather than flaky shape in Ni-hard type
irons with carbide formers. Though the higher Si
and lower Cr, Nb and V additions tends to promote
the graphite formation, any element shows no
simple relation with the amount of graphite in the
present multi-component alloys. The effect of each
element on the graphite formation tendency is
quantitatively evaluated based on the influence of
element(i) on the solubility of C in molten iron (m’i).
The synthetic influence of alloying elements on
graphite formation was appropriately estimated by
the following solubility parameter (eq.1), a simple
summation of CLi , in the case of high alloy cast iron
such as high speed steel type cast irons for roll [7].
1025
(b)
Fig.3 Graphite distribution in specimen
No.1(a), No.6(b) and
No.10(c).
=0.07 (%Cr) + 0.14 (%V) + 0.07 (%Nb)
– 0.06 (%Ni) – 0.31 (%Si) + 0.02 (%Mo)
… (1)
Where, CLi is the content of each element in
molten iron. The amounts of graphite in the
specimens cooled at 10K/min are plotted against the Fig.4 Relation between the amount of graphite
solubility parameter
as shown in Fig.4. In and the solubility parameter
this calculation the chemical composition of
specimen was applies as CLi. This result shows that Table 2 Partition coefficients of Ni, Cr, Mo and Si
the amount of graphite tends to increase with the to primary and + M3C eutectic
decrease in solubility parameter, however the
scatter of data is still large.
As described in the results on thermal analysis
experiment, eutectic graphite crystallizes after the
ledeburite formation; it seems the stable eutectic
reaction (L -> + Gr) occurs after the unstable
eutectic (L -> + M3C). This should happen
because of the change in the composition of
residual liquid during solidification. For example, the composition changes of residual liquid is
calculated by Scheil’s equation and the partition coefficients of alloying elements to primary and +
M3C eutectic given in Table 2 [8]. While the Ni and Si content decreases and the Cr content increases
by the crystallization of primary , Ni and Si increase and Cr decreases with the proceed of + M3C
eutectic. The Mo content of residual liquid continuously increases during solidification as shown in
Fig. 5. By using the data in Fig. 5, the changes in solubility parameter of residual liquid was evaluated
as shown in Fig. 6. Firstly the solubility parameter increases by the formation of primary, it decreases
with the crystallization of + M3C eutectic and takes the value of -0.52 at the solid fraction of 0.96,
where + Graphite eutectic starts. Similarly the changes in solubility parameter during solidification
and the fraction solid for graphite formation could be evaluated on other specimens by using Scheil’s
equation and partition coefficients of each alloying element to primary , + MC and + M3C
eutectic.
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The effect of inoculation on graphite
Crystallization
The effect of inoculation on graphite formation
was evaluated on the specimen No.8. A series of 0.2
to 1.0 % Si were inoculated by using Fe-75% Si alloy
particles. The amount of graphite increases from
0.7vol% to about 3vol% by the inoculation of 0.2 to
1.0%Si. The higher the inoculation, the more the
graphite formation. Graphite particles crystallize in
irregular nodular shape and distribute among +
M3C eutectic even in inoculated specimens.
Summary
The effects of addition of 0.02 to 1.82%Nb and
0.8 to 1.96%V were investigated on the
microstructure of Ni-hard type cast iron. The
following conclusions were obtained.
Fig.5 The changes in Ni, Si, Cr and Mo contentin
residual liquid during solidification of
specimen No.1.
(1) By the addition of Nb and V, + MC eutectic
reaction appears between the primary and +
M3C eutectic. The solidification sequence is
interpreted based on the changes in chemical
composition of residual liquid during
solidification. The amount of MC carbides
increases with increase in Nb and V contents.
(2) + graphite eutectic crystallizes at the final stage
of solidification. The influences of alloy
elements on the amount of graphite are Fig.6 The change in solubility parameter during
evaluated based on the Solubility parameter.
solidification of specimen No.1.
The amount of graphite increases almost
linearly with decreasing of Solubility parameter (
).
(3) The inoculation with Fe-75%Si alloy particles effectively increases the amount of graphite, and
higher amount of inoculation results in more uniform distribution of graphite.
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