Note: Descriptions are shown in the official language in which they were submitted.
CA 02525147 2005-11-08
,
DESCRIPTION
Fe-Cr ALLOY BILLET AND METHOD FOR PRODUCTION THEREOF
FIELD OF THE INVENTION
The present invention relates to an iron base alloy (in the specification
simply referred to as "Fe-Cr alloy") billet containing Cr in the range of 5 to
17 % and a method of manufacturing the same, in more detail, an Fe-Cr alloy
billet that can largely reduce a surface treatment of a billet before
manufacturing of seamless steel pipes that are manufactured by blooming, and
a method of manufacturing the billet.
BACKGROUND ART
In recent years, for use in oil wells and chemical industries, a demand
for steel pipes made of a Fe-Cr alloy is high, and in order to manufacture it
efficiently with high quality, the production according to a hot seamless
steel
pipe manufacturing method is increasing. However, in the manufacture of
the Fe-Cr alloy seamless steel pipes, on an external surface of an obtained
steel
pipe, in some cases, surface defects such as scale flaws are generated.
Such surface defects, in many cases, are caused owing to scale defects on
a billet surface prior to tube-making. That is, owing to descaling failure in
a
manufacturing process of a billet, scales are left without being removed, the
scales are squeezed in or rolled together to be the scale defects, and when
the
billet is subjected to tube-making with the scale defects remained thereon,
the
surface defects are caused.
Accordingly, an improvement in a method of descaling in the
manufacturing process of billet is forwarded. However, at present time, it is
difficult to assuredly remove scale residue. Accordingly, in order to prevent
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the surface defects from occurring on a steel pipe after hot tube-making,
almost
all the billets are subjected to a surface inspection before tube-making, and
based on the results, the surface treatment is applied.
Normally, a biIlet used for manufacturing the Fe-Cr alloy seamless steel
pipe is, as shown in Figs. 1 and 2 that are described later, manufactured by
blooming a cast steel made of the same alloy. The cast steel ,after being
heated to substantiaIly 1200 C, is processed by the blooming by means of a
box
type or grooved roll. At that time, with a multi-stage roll, while gradually
reducing it and making a diameter of the material smaller, the cast steel is
finished into a billet shape.
In the blooming, in order to remove the scales generated on the cast
steel owing to heating,. the descaling with high-pressure water is applied.
However, frequently, a descaling failure is caused, remaining scales are
squeezed in or rolled together with a surface of the cast steel, and thereby
the
scale defects are caused on the surface of the billet.
In order to reduce the scale defects, descaling capability, for iiistance, an
increase in a flow rate and ejection pressure of descaling water is enhanced.
However, since as the descaling proceeds, a temperature of a bulk material
becomes lower, the manufacture of the billet itself is disturbed, that is,
also in
the enhancement of the descaling capability, there is a limit. From these.
situations, at present time, it is difficult to assuredly remove the scale
residue
on the surface of the billet.
In order to cope with the above problems, there have been proposed
various countermeasures of heating equipment. Japanese Patent Application
Publication No. 07-258740 proposes, a continuous heating method
characterized in that when the cast steel such as a slab or billet is
continuously heated with a combustion burner, the generation of oxidation
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scale is suppressed during heating, the cast steel after the heating is
oxidized to generate scales excellent in peelability, and thereby surface
defects
axe removed. However, when the proposed method is applied, a large-scale
improvement and remodeling of a continuous heating furnace become
necessary.
Furthermore, in Japanese Patent Application Publication No.57-2831, a
method in which before the blooming, SiC is coated to impart oxidizability and
thereby to improve the peelability of the scales is disclosed. However,
according to the method disclosed here, coating equipment to coat SiC becomes
necessary. Furthermore, the coating operation becomes an off-line operation,
resulting in lowering the production efficiency.
Accordingly, either of the countermeasures proposed in Japanese Patent
Application Publication Nos. 07-258740 and 57-2831 cannot be brought into
actual operation as they are, and also from the capability thereof viewpoint,
the complete descaling is difficult. Accordingly, after the manufacture of the
billet, the surface treatment prior to tube-making has not yet been omitted.
As a method of surface treatment of the billet before tube-making, there
is a conventional method in which flaws are detected by ultrasonic defect
detection or the like and portions in concern are externally ground by use of
a
grinder or a peeler. However, since locations where the flaws occur and the
frequency thereof are different from one billet to another billet, automated
operation is difficult; as a result, the surface treatment before tube-making
normally becomes an off-line operation. Accordingly, the manufacture of the
seamless steel pipes from the billet is low in the production efficiency and a
work environment of the billet treatment is bad.
In the case of the treatment of the billet being automated, irrespective of
locations and rate of incidence of flaws, in some cases, whether flaws are
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present or not, it is necessary to uniformly grind all billet surfaces to
remove
and treat. In this case, the yield of the billet is remarkably deteriorated.
In place of the uniform grinding of the surface of the billet like this, as to
an automated treatment that specifies positions of flaws, for instance,
Japanese Patent Application Publication No. 10-277912 proposes a method of
treating surface flaws characterized in that after marking on a cast steel,
image data thereof is collected, and from the image data, surface flaw data is
extracted. However, according to the proposed method of treating surface
flaws, a lot of equipment and expenses are necessary; accordingly, it is not
suitable for a method of treating the billet.
As mentioned above, in manufacturing the billet for use in the
manufacture of the seamless steel pipe, in order to prevent the scale defect
from occurring on the surface thereof, various proposals have been submitted.
However, the complete descaling is difficult, that is, the surface treatment
after the manufacture of the billet has not yet been omitted.
Furthermore, in surface treatment of the billet, the operation is usually
performed off-line, the production efficiency is low and work environment is
bad. Even when the treatment is automated, the production yield is lowered
and huge equipment expense is necessary.
Accordingly, a manufacturing method that can omit or reduce the
surface treatment of the billet, in particular, a manufacturing method that
can
largely reduce the surface treatment of the billet after blooming for use in
the
manufacture of Fe-Cr alloy seamless steel pipes is demanded to be developed.
SUNIlVIARY OF THE INVENTION
The present invention is carried out in accordance with the
abovementioned problems of conventional technologies and a demand for
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development of a manufacturing method, and intends to provide a Fe-Cr alloy
billet that can largely reduce the treatment of the billets before tube-making
in
the case of seamless steel pipe, being manufactured from a Fe-Cr alloy cast
steel by means of the blooming, and a method of manufacturing the billet.
In view of that the descaling methods that have been used and proposed
so far cannot completely remove the scale defects generated on a surface of
the
billet, the present inventors hit on an idea of not removing the scales, but
positively covering the billet surface with the scale, thereby suppressing the
surface defects.
In order to embody the idea in the Fe-Cr alloy billet, the blooming of the
cast steel adopted in the process of manufacturing the Fe-Cr alloy billet was
studied in detail.
Figs. 1(a) through 1(c) are diagrams for explaining a blooming process of
the cast steel in a manufacturing process of the billet, and situations of
change
in cross section of the cast steel accompanying the blooming process. Fig.
1(a) shows a cross section of the cast steel before the blooming, Fig. 1(b)
showing a cross section of the cast steel in the middle process of the
blooming,
and Fig.1(c) showing a cross section of the billet after the blooming. The
blooming is performed at both first and second stand. In the first stand, with
a grooved roll, for instance, a box type roll and in the second stand with a
grooved roll, reverse rolling is respectively carried out.
A cast steel 1 in the blooming, after being heated to substantially 1200
C, is gradually reduced for every reduction surface at the first stand. As
shown in Fig. 1(b), it is processed into the cast steel 1 having a rectangular
cross section. In the next place, the cast steel 1 having a rectangular cross
section is charged at the second stand, rolled so as to gradually make the
cross
section smaller and, as shown in Fig. 1(c), finished in a shape like a final
billet
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2.
Fig. 2 is a diagram of one example for explaining in detail situations of
change in a shape of the cross section of the cast steel in the blooming
process
in the manufacture of the billet. In the blooming process shown in Fig. 2, the
cross section of the cast steel 1 is gradually reduced and finally tni.shed to
a
billet 2 after rolling ten passes. In the rolling process, the cast steel 1
before the blooming is placed so as being laid on the shorter side
(corresponding to Fig. 1(a)), and processed so as to be the cast steel 1
having a
rectangular cross section after the rolli.ng of a seven pass at the first
stand
(corresponding to Fig. 1(b)). Next, the cast steel having the rectangular
cross
section is subjected to the eighth through tenth roIli.ng at the second stand
and
finished into the final billet 2 (corresponding to Fig. 1(c)).
In a page shown in Fig. 2, the first, second, fourth, sixth, eighth and
tenth passes show the rolling in the vertical reduction direction, and the
third,
fifth, seventh and ninth passes show the rolling in the horizontal reduction
direction. In an actual rolling, the steel strip is rotated 90 to change a
rolli.ng
reduction direction.
The cast steel 1 shown in Fig. 1(a) is divided into a high reduction rate
surface 3 and a low reduction rate surface 4, the high reduction rate surface
3
showing a surface that becomes higher in the reduction rate in the blooming,
the low reduction rate surface 4 showing other surface thereof. In the
ordinary blooming, as shown in Fig. 2, the cast steel before the blooming is
disposed in the longitudinal direction; accordingly, the high reduction rate
surface 3 becomes a surface of shorter side in the slab-shaped cast steel ,
the
low reduction rate surface 4 becoming a surface of longer side.
However, when by the blooming process shown in Figs. 1(a) through 1(c)
and Fig. 2, the cast steel 1 is reduced for every reducing surfaces at the
first
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stand and further rolled at the second stand to be finished into the billet 2,
and,
in an external surface of the billet 2, an area ratio of a portion that was
the
high reduction rate surface 3 to a portion that was the low reduction rate
surface 4 in the cast steel 1 becomes almost the same.
That is, a cross section of the billet 2 after the blooming shown in Fig.
1(c) is equally divided into four portions of two high reduction rate surfaces
3'
(portion reduced with high reduction rate of the cast steel 1) and two low
reduction rate surfaces 4' (portion reduced with low reduction rate of the
cast
steel 1) and a central angle 0 (an angle occupying in a surface portion of the
billet 2) of the high reduction rate surface 3' shown in the same drawing
becomes 900.
Fig. 3 is a perspective view showing an entire configuration of the billet
after the blooming. In the rolling with the grooved roll at the first stand, a
center portion of the low reduction rate surface 4 is not directly restrained
by a
reduction roll, or, even when restrained, is only slightly restrained compared
to
other portions. Accordingly, in the billet 2 after the blooming, as shown in
Fig.
3, wrinkles 5 are generated in the longitudinal direction of the billet.
As the grooved roll that is used in the blooming, a box type roll, a
diamond type roll or an oval type roll can be illustrated. However, the box
type roll is effective in preventing the cast steel from inclining/falling.
Accordingly, in view of the stability of the blooming, the box type roll is
adopted
in many cases.
Accordingly, on the basis of the wrinkles 5 of the billet 2 after the
blooming, the high reduction rate surface 3' can be specified in a range of a
central angle of 45 (0/2) with a surface h that is orthogonal to the
wrinkles 5
as a center of the billet 2.
Based on the knowledge of the high reduction rate surface of the cast.
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steel and the biIlet, the manufacturing process of the Fe-Cr alloy billet was
further studied in more detail and the following findings (a) through (e) were
obtained.
(a) In order to prevent the scale defects from occurring on the surface of the
Fe-Cr alloy billet, it is difficult to completely remove the scales generated
on
the steel strip before the blooming.
(b) Complete removal of the scales generated on the cast steel was given up
and generation pattern of the scales which are uniikely to be squeezed in or
rolled together during the blooming was studied. As a result, scales
generated and adhered to the cast steel over a large covering area were found
unlikely to be squeezed in or rolled together during the blooming.
(c) Specifically, in the process of manufacturing the billet, there is no need
for
descali.ng with a high-pressure water descaler.
(d) Furthermore, as the rolling of a first pass in the blooming (first stand)
is
begun from the high reduction rate surface of the cast steel , the generated
scales can be more closely adhered to the cast steel .
(e) Still furthermore, as heating conditions (atmosphere, heating
temperature and holding time) of the cast steel were adjusted, the scales are
unlikely to exfoliate during the blooming and can be generated over a larger
covering area of the cast steel .
The present invention was achieved based on the above findings and a
Fe-Cr alloy billet according to the (1) below and methods of manufacturing the
Fe-Cr alloy billet according to (2) through (4) below are gist of the
invention.
(1) An Fe-Cr alloy billet, characterized in that a high reduction rate surface
is
covered with a scale layer with an area ratio of 70 %, 80 %, 90 % or more.
(2) A manufacturing method of the Fe-Cr alloy billet, the manufacturing
method of the Fe-Cr alloy billet by the blooming, without applying the
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descaling of the steel strip.
(3) A manufacturing method of the Fe-Cr alloy billet, wherein in a
manufacturing method of a Fe-Cr alloy billet by the blooming, after a scale
having a thickness of 1000 m or more is formed on the cast steel, without
applying the descaling, the blooming is performed.
(4) In the manufacturing method of the Fe-Cr alloy billet according to (3), it
is
preferable to firstly reduce the high reduction rate surface of the cast steel
-
Furthermore, the cast steel is preferably held in an atmosphere containing
2.5 % by volume or more of steam, and at a temperature of 1200 C or more for
2 hr or more to generate the scale.
In the present invention, the "Fe-Cr alloy" means an iron base alloy
containing 5 to 17 % of Cr and, whereby necessary, other alloy elements such
as Ni and Mo may be contained.
The "high reduction rate surface" according to the invention means, in
the cast steel, a surface where when the blooming is applied to form into a
billet shape, the reduction rate becomes higher, and, in the billet, a portion
that was the high reduction rate surface in the cast steel before the rolling.
Normally, in the cast steel having a slab shape, the high reduction rate
surface
becomes a shorter side surface.
The "high reduction rate surface" in the billet, as shown in Fig. 3, simply
on the basis of the wrinkles, can be specified in a range where a central
angle is
45 (0/2) with a central surface orthogonal to the wrinkles with respect to a
center of the billet. In order to more accurately specify the "high reduction
rate surface" in the billet, results of macro-observation of a cross section
of the
billet can be used.
Fig. 4 is a diagram showing one example of observation results of
macro-photographs of the biIlet cross section. In the center portion of the
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macro-observation, as shown with an elliptic of dotted line, segregation
correlated with a direction of the cross section of the cast steel before the
blooming can be observed. That is, since a position where the segregation
occurs coincides with a final solidifying position of the cast steel , the
final
solidifying position depends on a shape of cross section made of a longer side
surface 4 and a shorter side surface 3 of the cast steel .
From the observation results of the macro-photograph of the cross
section shown in Fig. 4, a surface approximately in parallel with the eIliptic
of
dotted line is the longer side surface 4, the "lower reduction rate surface",
and
a surface orthogonal to the elliptic of dotted line is the shorter side
surface 3,
the "higher reduction rate surface". Accordingly, since, in the billet, even
after the roIling, the segregation correlated with a direction of cross
section of
the cast steel before the blooming remains, from a distribution of the
segregation shown by the elliptic dotted line, the "high reduction rate
surface"
in the billet can be specified.
As mentioned above, the area ratios of the high reduction rate surface
and the low reduction rate surface on an external surface of the billet after
the
manufacture become almost the same, and the cross section of the billet is
equally divided into four portions of two high reduction rate surfaces and two
low reduction surfaces. Accordingly, a value of an "area rate of the high
reduction rate surface" (a ratio of area of scales in the high reduction rate
surface) stipulated according to the invention, when multiplied by 1/2, can be
replaced by a "total area rate (of billet)" (a ratio of area of scales in an
entire
area of the billet).
That is, in the invention, "70 % or more in the area rate of the high
reduction rate surface" can be stipulated in other words as "35 % or more of
total area rate", "80 % or more in the area rate of the high reduction rate
CA 02525147 2008-06-18
surface" can be stipulated in other words as "40 % or more of total area
rate",
and "90 % or more in the area rate of the high reduction rate surface" can be
stipulated in other words as "45 % or more of total area rate".
BRIEF DESCRIPTION OF THE DRAWINGS
Figs 1(a) through 1(c) are diagrams for explaining a blooming process of
a steel strip in a manufacturing step of a billet, and situations of a change
in a
cross section of the cast steel accompanying therewith.
Fig. 2 is a diagram of one example for explaining in detail situations of a
change in a shape of the cross section of the cast steel in the blooming
process
in manufacture of the billet.
Fig. 3 is a perspective view showing an entire constitution of the billet
after the blooming.
Fig. 4 is a diagram showing one example of observation results of
macro-photographs of the cross section of the billet.
Fig. 5 is a diagram showing relationship between a rate of incidence of
defects on a surface of a billet that uses a test sample A and a thickness of
scale
of the cast steel,.
Fig. 6 is a diagram showing relationship between a rate of incidence of
defects of a surface of a billet that similarly uses a test sample B and a
thickness of scale on the cast steel ~.
Fig. 7 is a diagram showing relationship between a rate of incidence of
defects of a surface of a biIlet that similarly uses a test sample C and a
thickness of scale of the ; cast steel.
Fig. 8 is a diagram showing relationship between a thickness of scale of
the cast steel and a holding temperature when an amount of steam in an
atmosphere of a heating furnace is varied.
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BEST MODE FOR CARRYING OUT THE INVENTION
In a Fe-Cr alloy billet according to the present invention, a high
reduction rate surface thereof is covered with a scale layer at an area rate
of
70 %, 80 %, 90 % or more. In other words, it is covered with the scale layer
at
a total area rate of 35 % or more, 40 % or more or 45 % or more.
As shown in examples described later, in the case of the high reduction
rate surface being covered with the scale layer at an area rate of 70 % or
more,
a surface treatment rate can be reduced by substantially 50 % in comparison
with comparative examples where the descaling is applied.
In the Fe-Cr alloy billet according to the invention, there is tendency
that the higher the area rate for the high reduction rate surface is, the
lower
the surface treatment rate of the biIlet is. For instance, in the case of the
high
reduction rate surface being covered with the scale layer at the area rate of
80 % or more, the treatment rate becomes substantially 30 % of that of a
comparative example, and similarly in the case of being covered with the scale
layer at the area rate of 90 % or more, the treatment rate becomes
substantially 20 % of that of the comparative example. Accordingly, the area
rate of the high reduction rate surface covered by the scale correlates well
with
the rate of incidence of defects on a surface of the billet.
In the manufacturing method according to the invention, in the
blooming of the cast steel, in order to remove scales generated during heating
of the steel strip, the descaling with a high pressure water descaler is not
applied. The reason for this is that as mentioned above, since a technology
for
completely removing the scale has not yet been established, it is intended to
avoid that the scale remains incompletely or irregularly and is squeezed in
and
rolled together to cause the scale flaw.
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In the manufacturing method according to the invention, although
whether the blooming of the cast steel is started from a high reduction rate
surface or from a low reduction rate surface is not speculated, it is
preferably
started from the high reduction rate surface of the, cast steel,. This is
because
when the high reduction rate surface is rolled at the first pass of the
blooming,
the scale generated on the cast steel can be press-bonded sufficiently onto
the
high reduction rate surface.
Furthermore, a reason for press-bonding the scale onto the high
reduction rate surface is because when the scale is squeezed in the high
reduction rate surface with the scale insufficiently remained, the scale flaw
is
likely to be caused. In the invention, when the scale is closely attached at
the
area rate of 70 % or more, in a process of the blooming after that, the scale
becomes unlikely to be squeezed in a matrix of the cast steel. The tendency
becomes more remarkable as the higher the area rate with which the scale
covers becomes higher.
In a manufacturing method according to the invention, the scale having
a thickness of 1000 m or more that becomes a defect with difficulty in the
blooming and is unlikely in causing a defect on the surface of the billet
after
the manufacture is generated on the cast steel - The thickness of the scale
20, can be obtained by controlling heating conditions (atmosphere, heating
temperature and holding time) of the cast steel.
Figs. 5 through 7 are diagrams showing relationship, in the case of the
descaling being not applied, between the rate of incidence of defects on a
surface of a Fe-Cr alloy billet and a thickness of the scale of the cast steel
. As
test samples, 5 to 17 % Cr-containing alloys A, B and C shown in Table 1 are
used. Fig.5 shows relationship with test sample A, Fig. 6 showing
relationship with test sample B, and Fig. 7 showing relationship with test
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sample C, respectively.
Table 1
Test Content of chemical component (mass %)
sample C Si Mn P S Cr Ni Mo Fe
A 0.18 0.25 0.5 0.015 0.007 5.0 - - Bal.
B 0.18 0.25 0.5 0.015 0.008 13.0 - - Bal.
C 0.18 0.25 0.5 0.014 0.008 17.0 - - Bal.
As specific conditions, test samples A, B and C are heated at a
temperature of 1200 C in an air atmosphere heating furnace with a holding
time varied to alter the thicknesses of a high reduction rate surface and a
low
reduction rate surface of the cast steel. The test samples each are measured
for the rate of incidence of defects on a surface of the billet. The reason
for the
air atmosphere heating furnace being set at a temperature of 1200 C is due to
the fact that the heating temperature is appropriate for reducing the
deformation resistance in the blooming.
Furthermore, the measurement of the rate of incidence of defects on the
surface of the billet is carried out by detecting the surface defects, after
removing the scale on the billet surface by means of shot blasting, by use of
a
flaw detecting method with a leak detector of magnetic flux. The rate of
incidence of defects is expressed in terms of a number ratio (number of
billets'
where defects are detected/total billets number).
From results shown in Figs. 5 through 7, it is found that as the scale
becomes thicker, the rate of incidence of defects decreases. When the
thickness of the scale of the high reduction rate surface is 1000 m or more,
the
rate of incidence of defects becomes 35 % or less, and furthermore when it is
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1200 m or more, the rate of incidence of defects becomes 25 % or less. The
results, as explained in examples described later, show that the rate of
incidence of defects is reduced to one half, furthermore substantially to one
third that of the comparative example that is reproduced by a conventional
method.
From this, in the invention, before the blooming, the thickness of the
scale on the cast steel is necessarily 1000 m or more, and furthermore
desirably 1200 m or more.
A detail of the mechanism is not clear; however, it is assumed that when
the rate of incidence of defects on the billet surface is intended to be
suppressed, in order to cover the billet surface stretched by the blooming
with
the scale layer having an area rate as large as possible, a certain amount of
scale, that is, a scale thickness is effective to be secured.
Fig. 8 is a diagram showing relationship between the thickness of the
scale on the cast steel and a holding temperature when an amount of steam in
an atmosphere of a heating furnace is varied. In the drawing, an amount of
steam contained in an atmosphere gas was varied as 0, 2.5, 10 and 20 % by
volume %.
With 13 % Cr-containing alloy B shown in the Table 1 as a test sample
and with an atmosphere gas of 10 % C02-5 % 02-Bal. N2 as a basis, a
concentration of steam contained in the atmosphere gas was varied in the
range of 0 to 20 %. At this time, the cast steel was heated at a temperature
of
1200 C and a holding time was varied, and a thickness of the scale generated
on the cast steel was measured.
The thickness of the scale was measured by, after the cast steel was
oxidized at a holding time between 1 to 6 hr, cutting a test sample followed
by
processing to a micro-sample further followed by observing a cross section.
CA 02525147 2008-06-18
Furthermore, scale structures at this time are shown in Table 2.
From results shown in Fig. 8, in order to obtain a scale having a
thickness of 1000 m or more in an atmosphere that does not contain steam,
heating of substantially 6 hr is necessary. The atmosphere that does not
contain steam is substantially the same as air atmosphere.
On the other hand, by allowing containing 2.5 % or more of steam in the
atmosphere, an oxidation speed can be very much improved. In order to
effectively obtain a thickness of 1200 m or more, in an atmosphere containing
2.5 % or more of steam, the cast steel has only to be held for 2 hr or more at
a
temperature of 1200 C.
Table 2
Steam in atmosphere Structure of scale
(%) External layer scale Internal layer scale
0 Fe203 FeCr204
(not containing) Fe304 Fe304
Fe203 FeCr204
2.5 to 20 Fe304 FeO
FeO
As shown in Table 2, scale structures of all are constituted of
two-Zayered structure including an external layer scale and an internal layer
scale. In the invention, the external layer scale is a scale generated outside
of
a surface of an original cast steel and the internal layer scale is a scale
generated inside of the surface of the original cast steel.
In a scale formed in an atmosphere that contains 2.5 % or more of steam,
the external layer scale is made of Fe203, Fe304 and FeO and the internal
layer
scale is made of FeCr204 and FeO. On the other hand, in a scale generated in
an atmosphere that does not contain steam, the external layer scale is made of
Fe203 and Fe304 and the internal layer scale is made of FeCr204 and Fe304.
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Although the scale structure may be any of the above modes, as a scale
structure that the scale defect cannot generate more easily, ones containing
FeO are preferable. This is because, owing to high deformability of FeO
itself,
the FeO is not likely to cause destruction such as crack even under a large
pressure, and furthermore, since the high temperature hardness thereof is
lower than that of the cast steel, the squeezing flaw is not likely to be
caused_
For instance, Fe203 is hardly deformed, and furthermore, Fe304, when it
is deformed by stretching experimentally at a very low speed at a temperature
of 800 C or more, can be stretched but cannot cope with a deformation speed
during the rolling, resulting in causing crack and peeling off. On the other
hand, FeO can deform in conformity with a deformation speed during the
rolling and does not cause crack.
In the case of FeO being present, FeO is preferably contained 30 % or
more as a thickness in the external layer scale when a cross section is
subjected to micro-observation. The thickness of FeO can be measured by
observing a color tone by means of the cross section micro-observation, by
mapping 02 (oxygen) by use of EPMA or by identifying in advance a structure
of the whole scale by use of X-ray diffraction.
Furthermore, when the steam concentration becomes more than 20 %,
effects of a rise of the scale generation speed and an increase in FeO ratio
are
gradually saturated. Accordingly, in consideration of damage of a furnace
wall and the like of the heating furnace, the upper limit of the steam
concentration is desirably set at substantially 25 %.
. In the invention, in order to secure a scale thickness on the cast steel of
1000 m or more, a heating temperature of the cast steel is desirably set at
1200 C or more. Furthermore, the heating temperature, from viewpoints not
only of scale generation but also of processability during the blooming, is
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desirably set at 1200 C or more. On the other hand, the upper limit of the
heating temperature, similarly, in consideration of the damage and the like of
the equipment, is desirably set at 1300 C or less.
In the invention, in order to secure the scale thickness on the cast steel
of 1000 gm or more, in the case of the heating temperature of the cast steel
being set at 1200 C or more, a holding time is preferably set at 2 hr or
more.
(Example 1)
Effects that a manufacturing method of a Fe-Cr alloy billet stipulated by
the present invention exhibits will be explained with reference to specific
Example 1 and Example 2. Test materials were 5 to 17 % Cr-containing alloys
A, B and C, and as a cast steel of starting material, a bloom CC material
having a short side of 280 mm x long side of 600 mm x length of 7400 mm was
used. The cast steel was subjected to heating at 1200 C for 6 hr in an
atmospheric heating furnace (not containing steam). Furthermore, after the
heating of the cast steel, the manufacture was carried out under two
conditions, that is, in one, the descaling was applied with a high-pressure
water descaler having a pressure of 100 kg/cm2 and in the other, the descaling
was not applied.
The blooming of the cast steel was performed at the first and second
stand respectively by reverse rolling. The first pass of the rolli.ng at the
first
stand was differentiated by whether the high reduction rate surface was
reduced or the low reduction rate surface was reduced. Thereafter, at the
first
stand, the cast steel was reduced to a cross sectional shape of substantially
short side of 250 mm x long side of 400 mm, followed by finishing, at the
second
stand, into a billet of a final size of a diameter of 225 0.
.After the billet was manufactured, a surface scale was removed by shot
blasting and flaw detection was performed by use of an NDI flaw detector due
18
CA 02525147 2008-06-18
to magnetic leakage flux flaw detecting method. Here, flaws having a depth
of 0.5 mm or more were detected. The flaw having a depth of 0.5 mm or more,
when subjected to rolling and tubing as it is without treating, becomes a flaw
on a surface of a steel tube; accordingly, it is necessary to treat a surface.
A
criterion was not determined on a length of defect. However, in consideration
of being stretched to a final product, a defect having a small length such as
several tens millimeters was checked.
The rate of incidence of defects was evaluated in terms of number ratio
(number of generated defects/total number). At the last, an area rate with
which the scale covers a surface of the billet was investigated. The area rate
of the scale was measured in such a manner that a cross section observation
sample was sampled from the high reduction rate surface of the billet for each
1 m, a length of peeled scale was observed by micro-observation, and {(average
length of peeled scale in a vertical direction x average length of peeled
scale in
a horizontal direction) / total area} was calculated as an area rate. As the
area rate of the scale, an average value of the area rates of all samples in
the
respective billets was used.
The frequencies of incidence of defects and the area rates of scales that
cover the high reduction rate surface of the biIlet at this time are shown in
Tables 3 through 5. Table 3 shows results of 5%o Cr-containing alloy A was
used as a test sample; Table 4 shows results of 13 % Cr-containing alloy B was
used as a test sample; and Table 5 shows results of 17 % Cr-containing alloy C
was used as a test sample.
In Example 1, in each case where any of the test samples was used,
thicknesses of scales formed on cast steel ~ immediately after taking out of a
heating furnace were substantially 1000 m, and the scale structure was made
. of an external layer scale of Fe203 and Fe3O4 and an internal layer scale of
19
CA 02525147 2005-11-08
FeCr2O4 and Fe304. Furthermore, thicknesses of scales covering surfaces of
the billets immediately after the manufacture were 150 m or more.
Table 3
Blooming State of billet
Test Rolled Rate of Scale area rate (%)
No. Descaling surface at the incidence of High All Group
first pass defects (%) reduction surface of
rate surface billet
Low
Al Applied reduction 92 50 25
rate surface Comparative
High example
A2 Applied reduction 97 48 24
rate surface
Low
Not A3 a pl ed reduction 47 73 36.5
rate surface Inventive
High example
Not A4 a pl ed reduction 35 83 41.5
rate surface
Note) Test sample: 5 % Cr-containing alloy A
CA 02525147 2005-11-08
Table 4'
Blooming State of billet
Test Rolled Rate of Scale area rate (%) Group
No. Descaling surface at incidence High All
the first of defects
pass (~o) reduction surface of
rate surface billet
Low
B1 Applied reduction 97 49 24.5
rate
surface Comparative
High example
B2 Applied reduction 93 47 23.5
rate
surface
Low
B3 Not reduction 45 71 35.5
applied rate
surface Inventive
High example
B4 Not reduction 33 82 41
applied rate
surface
Note) Test sample: 13 % Cr-containing alloy B
Table 5
Blooming State of billet
Test Rolled Rate of Scale area rate (%)
No. surface at incidence of High Group
Descaling the first defects reduction All surface of
pass (%) rate billet
surface
Low
Cl Applied reduction 94 50 25
rate surface Comparative
High example
C2 Applied reduction 98 45 22.5
rate surface
Low
Not C3 a pl ed reduction 44 70 35
rate surface Inventive
High example
Not C4 a pl ed reduction 32 80 40
rate surface
Note) Test sample: 17 % Cr-containing alloy C
21
CA 02525147 2005-11-08
As shown in Tables 3 through 5, in the case of the descaling being
applied in the blooming as comparative examples, the scale coverage was in
the range of 45 to 50 % by the area rate of the high reduction rate surface
(22.5
to 25 % in terms of the total area), the rate of incidence of defects was
nearly
the total number, and with the number rate of 92 to 98 % surface treatment
was necessary.
On the other hand, in the case of, among the inventive examples, the low
reduction rate surface being rolled in the first pass, the scale coverage of
the
high reduction rate surface was such high as in the range of 70 to 73 % by the
area rate of the high reduction rate surface (35 to 36.5 % in terms of the
total
area), the rate of incidence of defects were dropped as much as 44 to 47 %.
That is, one half that of the comparative examples. Furthermore, when the
high reduction rate surfaces were rolled at the first pass in the inventive
examples, the scale coverage was such high as in the range of 80 to 83 % in
the
area rate of the high reduction rate surface (40 to 41.5 % in terms of the
total
area), and at the same time, the rate of incidence of defects was reduced to
substantially one third that of the comparative examples, that is, 32 to 35 %.
From results shown in Tables 3 through 5, it is found that when the
scale coverage is substantially 70 % (35 % in terms of total area rate) in the
area rate of the high reduction rate surface, the rate of incidence of defects
is
reduced to substantially 50 % compared to the comparative example where the
descaling is applied, and furthermore, when the scale coverage is
substantially
80 % (40 % in terms of total area rate) in the area rate of the high reduction
rate surface, the rate of incidence of defects is reduced to substantially one
third compared to that of the comparative example.
This is assumed that although a detail of the mechanism is not clear,
22
CA 02525147 2008-06-18
when the scale is adhered with a certai_n area rate close to an entire area or
more, uneven scales that cause indentations or inclusions can be inhibited
from occurring.
(Example 2)
Steel strins obtained with test samples and cast steel of starting
materials under the same conditions as example 1 were heated in a heating
furnace. At this time, a moistening device was connected to the atmospheric
furnace to vary an atmosphere in the furnace, and heating was carried out at
1200 C for 6 hr.
The conditions of the blooming after the heating and measurement
conditions of the rate of incidence of defects and area rate with which scales
cover after the manufacture of billets were set as the same as that of
(Example l), and thereby an influence that the heating atmosphere affects on
the rate of incidence of defects of the billet was investigated. Results of
investigation are shown in Tables 6 through 8.
In results of investigation, Table 6 shows results when 5 %
Cr-containing alloy A was used as a test sample, Table 7 shows results when
13 % Cr-containing alloy B was used as a test sample, and Table 8 shows
results when 17 % Cr-containing alloy Awas used as a test sample. In each of
cases where the above test samples were used in Example 2, a thickness of the
scale that covers the surface of the billet was 150 m or more.
23
CA 02525147 2005-11-08
Table fi
Blooming State of billet
Area rate of scale
Test Surface Steam in Rate of W Group
No. Descaling rolled at atmosphere incidence High Al1
the first of defect reduction
pass (%) (~o) rate surface of
surface billet
High
A5 Not reduction 0 35 83 41.5
applied rate
surface
High
A6 Not reduction 2.5 22 90 45
applied rate
surface
High
A7 Not reduction 5 21 92 46.5
applied rate
surface
High ^a>
A8 Not reduction 10 19 95 47.5
applied rate
surface
High
A9 Not reduction 20 20 93 46.5
applied rate >
surface ~
Low
A10 Not reduction 0 47 73 36.5
applied rate
surface
Low
All Not reduction 2.5 31 81 40.5
applied rate
surface
Low
A12 Not reduction 20 30 83 41.5
applied rate
surface
High
A13 Applied redua~ on 0 97 48 24 $
surface
Note) Test material: 5 % Cr-containing alloy A.
$ denotes Comparative example.
24
CA 02525147 2005-11-08
Table 7
Blooming State of billet
Area rate of scale
Test Rate of (%)
Surface Steam in Group
No. incidence High
Descaling rolled at the atmosphere All
of defect reduction
first pass (%) (oo) rate surface
surface of billet
High
Not B5 applied reduction 0 33 82 41
rate surface
High
Not B6 a pl ed reduction 2.5 24 90 45
rate surface
High
Not B7 a pl ed reduction 5 21 90 45
rate surface
Not High ~
B8 reduction 10 22 94 47
applied rate surface
High >
B9 Not reduction 20 22 93 46.5
applied rate surface
>
Low
Not B10 a pl ed reduction 0 45 71 35.5
rate surface
Low
Not B11 a pl ed reduction 2.5 30 81 40.5
rate surface
Low
Not B 12 a pl ed reduction 20 30 83 41.5
rate surface
High
B13 Applied reduction 0 93 47 23.5 $
rate surface
Note) Test material: 13 % Cr-containing alloy B.
$ denotes Comparative example.
CA 02525147 2008-06-18
Table 8
Blooming State of billet
Area rate of scale
Test Steam in Rate of (%) Group
Descaling rolled at the atmosphere incidence High ~ p
first pass o of defect reduction surface
(/a) (/o) rate of billet
surface
High
Not C5 applied reduction 0 32 80 40
rate surface
High
Not C6 applied reduction 2.5 22 90 45
rate surface
High
C7 a pli~d reduction 5 21 92 46.5
rate surface
High
C8 Not
reduction 10 19 95 47.5
applied rate surface
High ~
C9 Not reduction 20 20 93 46.5 0
applied rate surface ~
Not Low ~-+
C 10 applied reduction 0 44 70 35
rate surface
Low
Not C11 a plied reduction 2.5 31 81 40.5
rate surface
Low
Not C12 applied reduction 20 30 83 41.5
rate surface
High
C13 Applied reduction 0 98 45 22.5 $
rate surface
Note) Test material: 17 % Cr-containing alloy C
$ denotes Comparative example.
As shown in Tables 6 through 8, it is found that in the i.nventive
examples, as the concentration of steam in the atmosphere increases, the area
rate with which the scale covers the high reduction rate surface increases and
at the same time the rate of incidence of defects of the billet decreases.
This is
because a content of steam increases, the scale grows thicker on the cast
steel
and at the same time FeO that is unlikely to be squeezed in a mother material
26
CA 02525147 2008-06-18
during the blooming is much generated.
Among the inventive examples that use the respective test samples, as
shown in test Nos. A8 and A9, B8 and B9 and C8 and C9, when the , cast steel
before the blooming was held in an atmosphere contai.ni.ng 10 % or more of
steam in the concentration at a heating temperature of 1200 C or more for 2
hr or more to generate the scale, the area rate of the high reduction rate
surface that the scale covers can be more increased to 93 % or more, and can
be
reduced the rate of incidence of defects of the billet 22 % or less.
lo INDUSTRIAL APPLICABILITY
According to a Fe-Cr alloy billet manufacturing method of the present
invention, since the blooming is carried out with the high reduction rate
surface of the cast steel covered with the scale layer having a large area
rate,
the indentation and inclusion of the scale can be reduced. Thereby, in the
case
of a billet for use in seamless steel pipes being manufactured from a cast
steel
of a Fe-Cr alloy, surface treatment before tube-making can be largely reduced.
Accordingly, when the Fe-Cr alloy billet is adopted for tube-making of
seamless steel pipes, even the Fe-Cr alloy steel pipe relatively hard to
process,
being able to. manufacture at low manufacturing costs and with efficiently,
can
be widely applied in a field of manufacturing of hot seamless steel pipes.
27
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