[Technical Field of the Invention]
[OOOl]
The present invention relates to low-oxygen clean steel and a stcel product
produced from the low-oxygen clean steel, and particularly, to low-oxygen clean steel
obtained by casting low-oxygen clean molten steel deoxidized with Al or A1-Si, and a
low-oxygen clean steel product produced from the low-oxygen clean steel.
Priority is claimed on Japanese Patent Application No. 201 3-091725, filed
April 24,2013, the content of which is incorporated herein by reference.
[Related Art]
[0002]
Conventionally, steel having excellent mechanical characteristics has been
required as a steel for a steel rod or a wire rod. Usually, in steel available for these
uses, breakage and fatigue breakage resulting from nonmetallic inclusions easily occur
with ail increase in strc~~gth.T he nonmetallic inclusions are mainly A1203 containing
inclusions generated in the course of deoxidation.
[0003]
As for the A1203-containing inclusions, particles of the AlzO3-based
inclusions form clusters, or components such as CaO are incorporated, and thus the
melting point of the inclusion particles is lowered. Therefore, the particles aggregate
together and easily increase in size. The inclusions having a size increased due to the
aggregation cause deterioration in performance of a steel. Accordingly, various
methods of preventing the increase in size of the inclusions have been examined.
Many methods of decreasing the size of inclusions by suppressing the formation of
clusters due to the aggregation of inclusion particles have been proposed.
[OO04]
For example, Patent Documents 1 to 6 disclose a method of reducing FeO
binders of alumina clusters by adding a minute amount of REM to a steel. This
method is effective in reducing the FeO binders, however, the generation of coarse
CaO-Alz03-based inclusions caused by a minute amount of Ca or CaO inevitably
mixed in the steel cannot be prevented only by adding REM.
[OOOS]
Patent Document 7 discloses a method of reducing FeO binders of alumina
clusters by adding Mg. However, in this method, similarly to the method disclosed in
Patent Documents 1 to 6, coarse CaO-A1103-Mg0-based inclusions are generated by a
minute amount of Ca or CaO and a minute amount of Mg or MgO which are inevitably
mixed in from a refractory material for refining.
[OOO6]
Patent Document 8 discloses a method of preventing the generation of coarse
inclusions by further dcoxidation of steel, in which "On (dissolved oxygen) ilr the steel
is controlled and removed with Al, in order of Ti and REM. However, in this method,
since "On is intentionally allowed to remain in the steel, an increase in the degree of
oxidation of slag cannot be avoided in a secondary refining process, and thus this
method is not applied to the production of low-oxygen clean steel.
[OOO7]
Patent Document 9 discloses a method of preventing the generation of clusterlike
inclusions which cause press cracks by complex deoxidation with Al+Ti+REM.
However, in the method dcscribed in Patent Document 9, deoxidatiou with Ti is
essentially similar to the method described in Patent Document 8, and thus the method
described in Patent Document 9 cannot be applied to thc production of low-Ti steel.
In addition, the method described in Patent Document 9 cannot be applied to the
production of high-cleanliness steel since it is difficult to intentionally form inclusions
having 50% or grcater of A1203 under strong-deoxidation refining.
[OOOS]
Patent Document 10 discloses a steel which contains SiO2-based stretching
inclusions and in which REIbI is added as a deoxidizing agent to decrease T.0 (total
oxygen in the steel). However, in steel including steel for a suspension spring and a
bearing, A1 is essentially added to provide fine crystal grains. Therefore, the base of
the composition of the inclusions changes from SiO2 to A1203 due to the deoxidation
with Al. Accordingly, the technology described in Patent Document 10 cannot be
applied to Al-added steel.
[0009]
Patent Document 11 discloses a method of improving, when REM-containing
molten steel is cast, producibility upon casting by adding REM in accordance with "0
and "S" in the molten stcel. However, this method is a method for preventing the
generation of REM sulfide when the REM is added, and its object is not the
modification of inclusions. Accordingly, the target value of REM is significantly
high.
[OOlO]
Patent Document 12 discloses high-cleanliness steel having excellent fatigue
properties and cold workability. However, the characteristics of Patent Document 12
relate to the adjustment of the composition of oxide-based inclusions in Si-deoxidized
steel, and do not relate to the modification of A1203-bascd inclusions by the addition of
REM.
Patent Document
[OOll]
[Patent Document 11 Japanese Unexamined Patent Application, First
Publication No. 2004-052076
[Patent Document 21 Japanese Unexamined Patent Application, First
Publication No. 2004-052077
[Patent Document 31 Japanese Unexamined Patent Application, First
Publication No. 2005-002420
[Patent Document 41 Japanese Unexamined Patent Application, First
Publication No. 2005-002421
[Patent Document 51 Japanese Unexamined Patent Application, First
publication No. 2005-002422
[Patent Document 61 Japanese Unexamined Patent Application, First
Publication No. 2005-002425
[Patent Document 71 Japanese Unexamined Patent Application, First
Publication No. 2005-002419
[Patent Document 81 Japanese Unexamined Patent Application, First
Publication No. 2007-186744
[Patent Document 91 Japanese Unexamined Patent Application, First
Publication No. 2006-0971 10
[Patent Document 101 Japanese Unexamined Patent Application, First
Publication No. S63-140068
[Patent Document 111 Japanese Unexamined Patent Application, First
Publication No. 2005-060739
[Patent Document 121 Japanese IJnexamincd Patent Application, First
Publication No. 2005-029888
Disclosure of the Invention Problems to be Solved by the Invention
I [0012]
I
I
As described above, conventionally, various methods of improving
mechanical characteristics of a steel for a steel rod or a wire rod have been proposed.
However, basically, all these methods are methods of suppressing the generation of
inclusions or decreasing the size of inclusions.
[0013]
In recent years, a steel for a steel rod or a wire rod has been required to be
further improved in mechanical characteristics. In order to meet such a requirement,
it is necessary to examine improvement measures based on a viewpoint different from
that of conventional methods.
[0014]
In order to improve mechanical characteristics, particularly, fatigue properties
of a steel for a steel rod or a wire rod, the inventors of the invention have conducted
intensive studies focusing on "the modification of inclusions", which has not been
considered in conventional methods.
[00 151
The invention is contrived in view of the above-described circumstances.
i An object ofthe invention is to improve mechanical characteristics by suppressing the
I generation of inclusions and by modifying the inclusions. Specifically, the object is
to improve mechanical characteristics, particularly, ratigue properties by suppressing
the generation of Ca0-A1203-based inclusions which easily aggregate together and
increase in size in Al-deoxidized steel and Al-Si-deoxidized steel containing A1203
inclusions, by modifying the inclusions, and by controlling the form of the inclusions.
In addition, an object of the invention is to provide steel in which the above-described
problem has been solved, and a steel product formed of the steel.
Means for Solving the Problem
[0016]
The inventors of the invention have thought that in order to suppress the
generation and coarsening of Ca0-Alz03-based inclusions which easily increase in size,
it is effective to previously decrease the amount of Ca0-Alz03-based inclusions to he
generated by suppressing the mixing of Ca or a Ca-containing material in molten steel,
and it is also effective to modify residual Ca0-AI2O3-based inclusions into inclusions
having another component composition by adding some inclusion modifying materials.
The inventors of the invention have analyzed changes in properties of the inclusions
and in characteristics of the steel by adding various substances as an inclusion
modifying material. As a result, they have obtained the following knowledge.
[0017]
That is, it has been found that the inclusions can be modified by adding, to
molten steel in which T.0 (total oxygen) has been sufficiently decreased by 1)crrorrning
A1 deoxidation or A1-Si deoxidation while suppressing the mixing of Ca or a Cacontaining
material in the molten steel, a minute amount of REM (rare-earth elements)
such as La, Ce, Pr, and Nd before the end of the deoxidation.
Here, T.O is a total amount of the dissolved oxygen in the steel and the
undissolved oxygen contained in the inclusions etc.
[00 1 S]
Specifically, the generation oECa0-A1203-based inclusions is suppressed by
adding REM as described above. Furthermore, it has been round that CaO of thc
Ca0-A1203-based inclusions generated in a small amount is reduced by REM, and thus
the Ca0-A1203 inclusions are modified into Alz03-based andlor REM203-based
inclusions or composite inclusions including these inclusions.
[0019]
The invention is based on the above-described knowledge and the gist thereof
is as follows.
[0020]
(1) According to an aspect of the invention, there is provided low-oxygen
clean stecl containing C, Si, Mn, P, and S as chemical components, and further
containing, by mass %, 0.005% to 0.20% ofhl, greater than 0% to 0.0005% of Ca,
0.00005% to 0.0004% of REM, and greater than 0% to 0.003% of T.0, wherein the
FEM content, the Ca content, and the T.O content satisfy the following Expressions 1
and 2; nonmetallic inclusions which have a maximum predicted diameter of 1 p to 30
pm measured using an extreme value statistical method under the condition in which a
prediction area is 30,000 mm2, and contain A1201 and REM oxide are dispersed in the
steel, an average proportion of the A1203 in the nonmetallic inclusions is greater than
50%, the REM is one or two or more of rare-earth elements La, Ce, Pr, and Nd, and the
% steel is Al-deoxidized steel or Al-Si-deoxidized steel.
1 0.155REMlCa14.00 . . . Expression 1 I 1 CalT.010.50 . . . Expression 2
I
(2) The low-oxygen clean steel according to (1) may further satisfy the
)
t following Expression 3.
!
0.055REMlT.050.50 . . . Expression 3
(3) The low-oxygen clean steel according to (1) or (2) may contain, by
mass %, 1.20% or less of C, 3.00% or less of Si, 16.0% or less of Mn, 0.05% or less of
P, and 0.05% or less of S as the chemical components with the remainder Fe and
impurities.
(4) The low-oxygen clean steel according to (3) may further contain, by
mass %, one or two or morc of 3.50% or less of Cr, 0.85% or less of Mo, 4.50% or less
of Ni, 0.20% or less of Nb, 0.45% or less of V, 0.30% or less of W, 0.006% or less of B,
0.06% or less ofN, 0.25% or less of Ti, 0.50% or less of Cu, 0.45% or less of Pb,
0.20% or less of Bi, 0.01% or less of Te, 0.20% or less of Sb, and 0.01% or less of Mg
as the chemical components.
(5) According to another aspect of the invention, there is provided a lowoxygen
clean steel product produced by processing the low-oxygen clean steel
according to (I) or (2).
(6) According to still another aspect of the invention, there is provided a lowoxygen
clean steel product produced by processing the low-oxygen clean steel
according to (3).
(7) According to still another aspect of the invention, there is provided a lowoxygen
clean steel product produced by processing the low-oxygen clean steel
according to (4).
[Effects of the Invention]
[0021]
According to the above-described aspect of the invention, it is possible to
provide low-oxygen clean steel which has excellent fatigue properties and in which
nonmetallic inclusions containing A1203 and REM oxide which have a high melting
point and hardly aggregate are dispersed in the steel. The nonmetallic inclusions may
contain REM sulfide, MgO, or both of REM sulfide and MgO.
Brief Description of Drawings
[0022]
FIG. 1 is a diagram showing the relationship between a maximum grain
diameter (-\/area (pm)) of nonmetallic inclusions and fatigue strength (MPa) (Non-
Patent Document Yultitaka Muraltami, "Metal Fatigue, Effects of Micro-Defects and
Inclusions").
FIG. 2 is a diagram showing the relationship between an REM content (ppm)
and steel piece extreme value statistics (maximum predicted diameter) (pm)
FIG. 3 is a diagram showing the relationship between a ratio of REM to Ca
and steel piece extreme value statistics (pm).
FIG. 4 is a diagram showing the relationship between a ratio of REM to T.0
and steel piece extreme value statistics (pm)
FIG. 5 is a diagram showing the relationship between the ratio of Ca to T.0
and steel piece extreme value statistics (pm) analyzed when REM is appropriately
added (0.00005% to 0.0004%), when REM is excessively added (greater than
0.0004%), and when REM is not added (the REM content is less than 0.00005%).
FIG 6 shows diagrams showing forms (SEM reflected electron images) of
nonmetallic inclusions existing in steel. FIGS. 6(a) and 6(b) show forms of
nonmetallic inclusions of invention examples ("No. 2-1" in Tables 2-1 and 2-2 to be
shown later), and FIGS. 6(c) and 6(d) show forms of nonmetallic inclusions of
comparative examples ("'No. 2-2" in Tables 2-1 and 2-2 to be shown later).
FIG. 7 shows an aspect of the production of a radial rolling fatigue test piece.
FIG. 7(a) shows a shape of the material ofthe radial rolling fatigue test piece, FTG. 7(b)
shows an aspect of the collection of the radial rolling fatigue test piece, and FIG. 7(c)
shows a final shape of the collected radial rolling fatigue test piece
FIG. 8 is a diagram showing the relationship between steel piece extreme
- 9 -
value statistics (maximum predicted diameter) obtained through an extreme value
statistical method and the shortest breaking life obtained through a radial fatigue test.
FIG. 9 is a diagram showing a shape of a test piece produced lor evaluation of
rotary bending fatigue properties.
FIG. 10 is a diagram showing the relationship between maximum stress and
the number of times of endurance, obtained through an Ono-type rotary bending test.
Description of Embodiments
[0023]
Low-oxygen clean steel according to an embodiment of the invention
(hereinafter, may be referred to as the low-oxygen clean steel according to this
embodiment) will be described in detail.
The low-oxygen clean steel according to this embodiment contains C, Si, Mn,
P, and S as fundamental elements, further contains, by mass %, 0.005% to 0.20% ofAl,
greater than 0% to 0.0005% of Ca, 0.00005% to 0.0004% of REM, and greater than
0% to 0.003% of T.O, and if necessary, contains other elements.
In the low-oxygen clean steel according to this embodiment, the REM content,
the Ca content, and thc T.0 content satisfy the following Expressions 1 and 2, and
preferably satisfy the following Expression 3. In the steel, nonmetallic inclusions
which have a maximum predicted diainetcr of 1 pm to 30 pm measured using an
extreme value statistical method under the condition in which the prediction area is
30000 mm2, and containAlz03 and REM oxide are dispersed. The average
I j proportion of the A1203 in the nonmetallic inclusions is greater than 50%. The lowoxygen
clean steel according to this embodiment is Al-deoxidized steel or A1-Sideoxidized
steel.
0.155REMiCa54.00 . . . Expression 1
Ca/T.Oi0.50 . . . Expression 2
0.055REM/T.0_<0.50 . . . Expression 3
[0024]
Here, REM is one or two or more of rare-earth elements La, Ce, Pr, and Nd
[0025]
As described above, in the low-oxygen clean steel according to this
embodiment, nonmetallic inclusions containing fine A1203 and REM oxide are
dispersed by the "suppressioi~o f the generation of the inclusions" and the
"modification of the generated inclusions".
The effect of the "suppression of the generation of the inclusions" is obtained
by controlling the A1 content, the Ca content, and the T.O content within predetermined
ranges.
The effect of the "modification of the generated inclusions" is obtained by a
minute amount of E M of 0.00005 mass % to 0.0004 mass % (to be described later in
detail). The inclusion modification effect of REM is obtained through a reducing
action by REM with respect to CaO or CaO of Ca0-A1203.
That is, in the low-oxygen clean steel according to this embodimenl, it is
important to control the amounts of Al, Ca, and T.O to 0.005 mass %to 0.20 mass %,
to greater than 0 mass % to 0.0005 mass %, and to greater than 0 mass % to 0.003
mass %, respectively, from the viewpoint of the suppression of thc generation of the
inclusions, and it is important to coiltrol the amount of REM to 0.00005 mass % to
0.0004 mass % from the viewpoint of the modification orthe generated inclusions.
[0026]
lisually, steel contains C, Si, Mn, l', S, and if necessary, other elements with
the remainder Fe and impurities. In the low-oxygen clean steel according to this
embodiment, the above-described inclusion modification effect by REM is exhibited
without being affected by molten steel components such as C, Si, and Mn other than Al,
Ca, REM, and T.O. That is, it is not necessary to restrict the contents of elements
other than Al, Ca, REM, and T.O. The inventors of the invention have confirmed this
fact by way of experiment in an actual operation. The reasons for restriction of the
respective contents will be described later.
Furthermore, the inventors of the invention have found that it is important for
Al, Ca, REM, and T.0 existing in a minute amount in the molten steel to be controlled
not only in the content of each element, but also in the content ratio in order to
appropriately maintain the mutual action and reaction between the elements and to
maximize the inclusion modification effect by REM. Specifically, they have found
that it is effective to control the ratio of REM to Ca, the ratio of REM to T.0, and the
ratio of Ca to T.0 as indices of the content ratios. The reasons for restriction of these
content ratios will be described later.
[0028]
First, the reasons for restriction of the component composition (chemical
components) will be described. Hereinafter, % means mass %. In the low-oxygen
clean steel according to this embodiment, chemical components are preferably within
the following ranges in specimen of steel sampled from molten steel before casting
based on JIS G 0417 or specimen of steel after casting
[0029]
Al: 0.005% to 0.20%
A1 is a deoxidizing element and is an element which makes crystal grains of
steel finer. In order to obtain these effects, the lower limit of the Al content is 0.005%.
The lower limit of the A1 content is preferably 0.010%.
[0030]
When A1 is contained in molten steel, the molten steel inevitably becomes Al.
deoxidized molten steel and Alz03-containing inclusions are gcnerated in the molten
steel. When the A1 content in the moltcn steel is greater than 0.20%, the inclusions
are generated in a large amount and remain in the steel and the fatigue properties of the
steel deteriorate. Therefore, the upper limit of the Al content is 0.20%. The upper
limit of the A1 content is preferably 0.10%.
[003 l]
Ca: greater than 0% to 0.0005%
Ca is a deoxidizing element and is an element which forms CaO-AlzO3-based
inclusions which easily aggregate together and have a low melting point through a
deoxihation reaction. When the Ca content in molten steel is greater than 0.0005%,
A1203-based inclusions are made into CaO-AI2O3-based composite inclusions having a
low melting point and coarsen. The CaO-AlzO3-based inclusions coarsening and
remaining in the steel are not liquefied at a rolling temperature and remain in a coarse
state in the steel. Thc amount of Ca is preferably as small as possible, but 0.0005% or
less of Ca is permissible. Accordingly, the upper limit of the Ca content is 0.0005%.
The upper limit of the Ca content is preferably 0.0003%, and more preferably
0.00025%.
In the current steelmaking method in which refining is performed by bringing
slag containing CaO into contact with an upper portion of molten steel in a ladle, Ca is
inevitably incorporated in the molten steel, and thus Ca cannot be con~pletcly
eliminated from the steel. Therefore, the lower limit of the Ca content is greater than
0%.
In the low-oxygen clean steel according to this embodiment, the generation of
Ca0-A1203-based inclusions can be suppressed under the condition in which there is a
minute amount of Ca incorporated inevitably in the molten steel
[0032]
In this embodiment, the Ca content is adjusted before the addition of REM.
The method of suppressing the Ca content to 0.0005% or less in the course of refining
will be described later.
[0033]
REM: 0.00005% to 0.0004%
REM is an important element which modifies Ca0-Al2O3-based inclusions by
reducing CaO in molten steel and CaO in inclusions. The molten steel suficiently
deoxidized with Al or A1-Si contains 0.00005% to 0.0004% of REM (rare-earth
element, one or two or more of La, Ce, Pr, and Nd) in order to obtain the inclusion
modification effect. The inclusion modification effect cannot be obtained when the
REM content is 0.00005% or less.
[0034]
When the mollcrr steel contains greater than 0.0004% REM, the inclusions
increase in size. The detailed mechanism thereof is not clear, but is thought to be that
when the molten steel contains greater than 0.0004% REM, a compound phase having
a low melting point and a high REM concentration appears in the inclusions and
promotes the aggregation olthe inclusions, and thus the inclusions increase in sizc.
Therefore, the upper limit of the REM content is 0.0004%. The upper limit of the
REM content is preferably 0.0003%, and more preferably 0.0002%.
The range of the REM content is based on the result of the evaluation on the
relationship between steel piecc extreme value statistics (maximum predicted
diameter) of the nonmetallic inclusions in the low-oxygen clean steel according to this
embodiment calculated through an extreme value statistical method and fatigue
strength.
[0036]
FIG. 1 is a diagram showing the relationship between a maximum diameter
(-\/area (pm)) of the nonmetallic inclusions and fatigue strength (MPa). From FIG. 1,
it is found that the fatigue strength is improved with a decrease in grain diameter
(-\/area (pm)) of the nonmetallic inclusions.
[0037]
The component composition and the form (dimensions, shape) of the
nonmetallic inclusions have a large influence on the fatigue strength of steel. The
component composition and the form (dimensions, shape) of the nonmetallic
inclusions will be described later.
[0038]
FIG. 2 shows the relationship between an E M content (ppm) and steel piece
extreme value statistics (pm). The steel piece extreme value statistics (pm) provide
an estimated value (maximum predicted diameter) of the maximum diameter of the
inclusions existing in a predetermined test amount (prediction area) of a steel, which is
obtained through an extreme value statistical method. In this embodiment, the steel
piece extreme value statistics are calculated through an extreme value statistical
I method with a prediction area of 30,000 mm2.
[003 91
From FIG. 2, it is found that the REM content at which the steel piece extreme
value statistics (pm) are 30 pm or lcss is 4 ppm (0.0004%) or less. In any analysis
targct steel, T.O was 5 ppm to 20 ppm and was within a preferable range of this
embodimcnt. In the low-oxygen clean stcel according to this embodiment, as
described above, thc upper limit of the REM content is 0.0004% based on the above
description.
[0040]
In addition, according to FIG. 2, the inclusion modification effect of REM is
exhibited when the REM content is 0.5 ppm or greater. Accordingly, the lower limit
of the REM content is 0.00005%. That is, the REM content is 0.00005% to 0.0004%.
The REM content is preferably 0.00005% to 0.0003%, and morc preferably 0.00005%
T.O: greater than 0% to 0.003%
0 is an element which exists in molten steel and forms an oxide.
Accordingly, in producing steel which has excellent mechanical characteristics and in
which a small amount of inclusions are finely dispersed, the T.O content is required to
be controlled. In addition, it is also important to control the T.O content in the
relationship with the contents of Ca and REM, which are constituent elemerlts of oxide
inclusions, in molten steel.
[0042]
When the T.0 content of molten steel is greater than 0.003%, oxide inclusions
are generated in a large amount and remain in the steel, and mechanical characteristics,
particularly, fatigue properties of the steel deteriorate. Therefore, the T.0 content is
0.003% or less. The T.0 content is preferably 0.002% or less, and more preferably
0.001% or less.
Although the amount of T.0 is preferably as small as possible, the lower limit
thereof is greater than 0% since it is difficult to adjust the amount of T.0 to 0%.
[0043]
Next, the reasons why the ratio of lEM to Ca and the ratio of Ca to T.0 are
rcstricted to 0.15 to 4.00 and to 0.50 or less, respectively, and the reasons why the ratio
of REM to T.0 is preferably 0.05 to 0.50 in the low-oxygen clean steel according to
this embodiment will be described.
[0044]
REM is an element which reduces CaO in inclusions to act for modification of
the inclusions and suppression of coarsening. Therefore, the ratio of REM to Ca
which is a ratio of the REM content to Ca content is an important index for
maximizing the inclusion modification effect of REM.
[0045]
FIG. 3 shows the relationship between the ratio of REM to Ca and steel piece
extreme value statistics (pm).
[0046]
From FIG. 3, it is found that the steel piece extreme value statistics (pm) are
30 pm or less when the ratio of REM to Ca is 0.15 to 4.00. When the ratio of REM to
Ca is lcss than 0.15, the inclusions containing Ca0-A1203 as a main component are not
sufficiently modified. As a result, the inclusions have a grain diameter (steel piece
cxtremc value statistics) greater than 30 pm, and thus coarsen and remain in the steel,
and the mechanical characteristics thereof are not improved.
[0047]
When the ratio olREM to Ca is greater than 4.00, the steel piece extreme
value statistics (pm) are greater than 30 pm. The reason for this is assumed to be that
since the molten steel has an excessively high REM content, the concentration of REM
oxide in the inclusions to be generated excessively increases, and thus the composition
of the inclusions is out of an appropriate range. The detailed mechanism thereof is
not clear, but is presumed to be that when the REM concentration in the inclusions
excessively increases, the inclusions aggregate together due to the generation of a lowmelting-
point phase in the inclusions, and as a result, the steel piece extreme value
statistics (pm) are increased.
[0048]
From the above description, the ratio of REM to Ca is 0.15 to 4.00. The
ratio of REM to Ca is preferably 0.20 to 3.00, and more preferably 1.00 to 3.00.
[0049]
CalT.0: 0.50 or less (CalT.010.50)
The ratio of Ca to T.0 which is a ratio of the Ca content to the T.0 content is
an important index for suppressing the generation and coarsening of Ca0-A1203-based
inclusions and for maximizing the inclusion modification effect of REM
[0050]
FIG. 5 shows the relationship between the ratio of Ca to T.0 and stccl piece
extreme value statistics (pm) analyzed when REM is appropriately added (steel having
an REM content of 0.00005% to 0.0004%), when REM is excessively added (steel
having an REM content greater than 0.0004%), and when REM is not added (the REM
content is less than 0.00005%).
[0051]
From FIG 5, it is found that in the case of the appropriate addition of REM
indicated by 0 in FIG. 5, the steel piece extreme value statistics are 30 pm or less when
the ratio oECa to T.0 is 0.50 or less. The reason for this is presumed to be that when
the ratio of Ca to T.O is 0.50 or less, the Ca0 activity of the inclusions is maintained at
a high level, the reducing reaction of Ca0 by REM easily occurs, and thus the
coarsening of the nonmetallic inclusions is suppressed
[0052]
Accordingly, the ratio of Ca to T.O is 0.50 or less. The ratio of Ca to T.O is
preferably 0.10 to 0.40. When the Ca content is 0.00025% or less, the ratio of Ca to
T.O is preferably 0.20 or less in order to suppress the coarsening of the inclusions by
Ca.
[0053]
[0054]
The ratio of REM to T.O is an effective index for sufficiently exhibiting the
inclusion modification effect of REM. Accordingly, in addition to the ratio of REM
to Ca and the ratio of Ca to T.O that have been described above, the ratio of REM to
T.O is preferably 0.05 to 0.50 in order to prominently exhibit the inclusion
modification effect of REM.
When the ratio of REM to T.O is greater than 0.50, CaO contributing as a
binding agent in the aggregation of the inclusions and CaO of Ca0-A1203 are reduced
immediately after the addition of REM, but a large amount of unreacted REM (REM
I itself, which is a strong deoxidizing element) remains and excessively reduces Alz03.
1 As a result, REM203-A1203 inclusions are generated in a large amount ~ and coarsen.
! Therefore, there is no contribution to an improvement in mechanical characteristics.
[0055]
When the ratio of REM to T.O is lcss than 0.05, there is no sufficient
contribution to the reduction of CaO and Ca0 of Ca0-A1203, which contribute as a
binding agent of the inclusions, and thus the inclusion modification effect is not
sufficiently exhibited. Therefore, the effect of finely dispersing the nonmetallic
inclusions in the steel is not obtained, and thus there is no contribution to an
improvement in mechanical characteristics. Accordingly, the ratio of REM to T.O is
preferably 0.05 to 0.50, and more preferably 0.10 to 0.40.
[0056]
FIG. 4 shows the relationship between the ratio of REM to T.O and steel piece
extremc value statistics in steel having 0.003% or less oET.0. In FIG. 4, all of the
REM content, the ratio of REM to Ca, the ratio of Ca to T.O, etc. are within the ranges
of the low-oxygen c!ean steel according to this embodiment.
[0057]
When REM satisfying the ratio of REM to T.O of 0.05 to 0.50, and preferably
0.10 to 0.40 is contained in clean molten steel having 0.003% or less of T.O, the REM
sufficiently reduces CaO contributing as a binding agent in the aggregation of the
inclusions and Ca0 of Ca0-A1203 (that is, the inclusion modification effect is
sufficiently exhibited). As a result, the inclusions do not aggregate and the
nonmetallic inclusions are more finely dispersed.
[OOSS]
Next, preferable contents of C, Si, and Mn, which are fundamental elements
of molten steel, and P and S, which are impurity elements, will be described. As
described above, in the low-oxygen clean steel according to this embodiment, the
inclusion modification effect by REM is exhibited without being affected by steel
components such as C, Si, and Mn other than Al, Ca, REM, and T.O. Therefore, it is
not necessary to restrict the contents of elements other than Al, Ca, REM, and T.0.
when obtaining the effect of this embodiment. However, in practical stcel, the
contents of C, Si, Mn, etc. are prelerably controllcd to secure predetermined
characteristics. Hereinafter, a preferable component composition (chemical
components) will be described based on the component composition of the practical
steel.
[0059]
C: 1.20% or less
C is an effective element for securing the strength or hardness of steel after
hardening. Types of steels which are not required to have such strength or hardness
are not essentially required to contain C. Accordingly, the lower limit of the C
content is not particularly restricted. However, since C is a fundamental element of
steel and it is difficult to adjust the content thereof to 0%, the content of C cannot be
0%.
In the case of increasing the strength or the hardness, the C content is
preferably 0.001% or greater. However, when the C content is greater than 1.20%
cracks are generated upon hardening or the steel becomes too hard, whereby the life of
a cutting tool is deteriorated. Therefore, the upper limit of the C content is preferably
1.20%. The upper lin~iot f the C content is more preferably 1.00%.
[0060]
Si: 3.00% or less
Si is an elfective element for securing the strength or hardness by improving
hardenability of steel. Types of steels which are not required to have such strength or
hardness are not essentially required to contain Si. Accordingly, the lower limit of the
Si content is not particularly restricted. However, since Si is a fundamental element
of steel and it is difficult to adjust the content thereof to 0%, the content of 3i cannot
beO%.
In the case or increasing the strength or the hardness of steel, the Si content is
preferably 0.001% or greater. However, when the Si content is greater than 3.00%,
the effect is saturated and the hardness of the steel excessively increases, whereby the
life of a cutting tool is deteriorated. Therefore, the upper limit of the Si content is
preferably 3.00%. The upper limit of the Si content is more preferably 2.50%.
[0061]
Mn: 16.0% or less
Mn is an effective ekment for securing the strength or hardness by improving
hardenability of steel. Types of steels which are not required to have such strength or
hardness are not essentially required to contain Mn. Accordingly, the lower limit of
the Mn content is not particularly restricted. However, since Mn is a fundamental
element of steel and it is difficult to adjust the content thcreof to 0%, the content of Mn
cannot beO%.
In the case of increasing the strength or the hardness, the Mn content is
preferably 0.001% or greater. I-Iowever, when the Mn content is greater than 16.0%,
quenching cracks are generated upon hardening or the steel becomes too hard, whereby
the life of a cutting tool 1s deteriorated. Therefore, the upper limit of the MI] content
is preferably 16.0%. The upper limit of the Mn content is more preferably 12.0%.
When a certain amount of C (for example, 0.1% or greater) is contained, the strength
of practical steel can be secured even when the Mn content is 2.0% or less.
[0062]
P: 0.05% or less
P is an impurity element, and when the P content is too large, the toughness of
steel deteriorates. Therefore, the P content is preferably restricted to 0.05% or less,
and more preferably to 0.03% or less. IIowever, large refining cost is required to
decrease the P content to 0.0001% or less. Therefore, the lower limit of the P content
9 in practical steel is approximately 0.0001%.
I !
j [0063] ~ S: 0.05% or less
Similarly to P, S is an impurity element, and when the S content is too large,
the toughness of steel deteriorates. Therefore, the S content is preferably restricted to
0.05% or less, and more preferably to 0.03% or less. Large refining cost is required
to decrease the S content to C.0001% or less. Therefore, the lower limit of the S
content in practical steel is approximately 0.0001%.
The low-oxygen clean steel according to this embodiment may further contain
one or two or more of 3.50% or less of Cr, 0.85% or less of Mo, 4.50% or less of Ni,
0.20% or less of Nb, 0.45% or less of V, and 0.30% or less of W other than the abovedescribed
elements in such a range as not to damage the characteristics thereof. Since
it is not necessary to essentially contain these elements, the lower limits thereof are 0%.
[0065]
Cr: 3.50% or lcss
Cr is an effective element for securing the strength or hardness by improving
hardenability of steel. The Cr content is preferably 0.01% or greater to obtain this
effect. When the Cr content is greater than 3.50%, toughness and ductility deteriorate.
Thus, the upper limit of the Cr content when Cr is contained is 3.50%. The upper
limit of the Cr content is preferably 2.50%.
100661
Mo: 0.85% or less
Mo is an effective element for securing the strength or hardness by improving
hardenability or steel. In addition, Mo is an element which forms carbide to
contribute to an improvement in temper softening resistance. The Mo content is
preferably 0.001% or greater when obtaining these cffccts. When the Mo content is
greater than 0.85%, a supercooling structure which causes deterioration in toughness
and ductility is easily generated. Thus, the upper limit of the Mo content when Mo is
contained is 0.85%. The upper limit of the Mo content is preferably 0.65%.
Ni: 4.50% or less
Ni is an effective element for securing the strength or hardness by improving
hardenability. The Ni content is preferably 0.005% or greater to obtain this effect.
When the Ni content is greater than 4.50%, toughness and ductility deteriorate. Thus,
the upper limit of the Ni content when Ni is contained is 4.50%. The upper limit of
the Ni content is preferably 3.50%.
[0068]
Nb: 0.20% or less
Nb is an element which forms carbide, nitride, or carbonitride to contribute to
the prevention of coarsciiing of crystal grains and an improvement in temper softening
resistance. The Nb content is preferably 0.001% or greater when obtaining these
effects. When the Nb content is greater than 0.20%, toughness and ductility
deteriorate. Thus, the upper limit of the Nb content when Nb is contained is 0.20%.
The upper limit of the Ni content is preferably 0.10%.
[0069]
V: 0.45% or less
V is an element which forms carbide, nitride, or carbonitride to contribute to
the prevention of coarsening of crystal grains and an improvement in temper softening
resistance. The V content is preferably 0.001% or greater when obtaining these
effects. When the V content is greater than 0.45%, toughness and ductility deteriorate.
Thus, the upper limit of the V content when V is contained is 0.45%. The upper limit
of the V content is prcferably 0.35%.
LO0701
W: 0.30% or less
W is an effective element for securing the strength or hardness by improving
hardenability of steel. In addition, W is an element which forms carbide to contribute
to an improvement in temper softening resistance. The W content is preferably
0.001% or greater when obtaining these effects. When the W content is greater than
0.30%, a supercooling structure which causes deterioration in toughness and ductility
is easily generated. Thus, the upper limit of the W content when W is contained is
0.30%. The upper limit of the W content is preferably 0.20%.
[0071]
The low-oxygen clean steel according to this embodiment may further contain,
by mass %, one or two or more of 0.006% or less of B, 0.06% or less of N, 0.25% or
less of Ti, 0.50% or less of Cu, 0.45% or less of Pb, 0.20% or less ofBi, 0.01'% or less
of Te, 0.20% or less of Sb, and 0.001% or less of Mg other than the above-described
elements in such a range as not to damage the characteristics thereof. Since it is not
necessary to essentially contain these elements, the lower limits thereof are 0%.
[0072]
B: 0.006% or less
B is an element which increases hardcnability of steel to contribute to an
improvement in strength. In addition, B is an element which is segregated in
austenite grain boundaries to suppress the grain boundary segregation of 1' and to
improve fatigue strength. The B content is preferably 0.0001% or greater when
obtaining these effects. When the B content is greater than 0.006%, the effect is
saturated and embrittlement is caused. Therefore, the upper limit ofthe B content is
0.006% when B is contained. The upper limit of the B content is preferably 0.004%.
[0073]
N: 0.06% or less
N is an element which forms fine nitride to provide fine crystal grains and
contributes to an improvemei~ti n strength and toughness. The N content is preferably
0.001% or greater when obtaining these effects. When the N content is greater than
0.06%, nitride is generated in an excessive amount, and thus toughness deteriorates.
Therefore, the upper limit of the N content is 0.06% when N is contained. The upper
limit of the N content is preferably 0.04%.
[0074]
Ti: 0.25% or less
Ti is an element which forms fine Ti nitride to provide fine crystal grains and
contributes to an improvement in strength and toughness. The Ti content is
preferably 0.0001% or p,reater when obtaining these effects. When the Ti corltcnt is
greater than 0.25%, Ti nitride is generated in an excessive amount, and thus toughness
deteriorates. Therefore, the upper limit of the Ti content is 0.25% when Ti is
contained. The upper limit of the Ti content is preferably 0.15%.
[0075]
Cu: 0.50% or less
Cu is an element which increases corrosion resistance of steel. The Cu
content is preferably 0.01 % or greater to obtain this effect. When the Cu content is
greater than 0.50%, hot ductility deteriorates, and thus cracks or flaws are caused
Therefore, the upper limit of the Cu content is 0.50% when Cu is contained. The
upper limit of the Cu content is preferably 0.30%.
[0076]
Pb: 0.45% or less
1 Pb is an element which contributes to an improvement in machinability oS
I
steel. The Pb content is preferably 0.001% or greater to obtain this effect. When the
Pb content is greater than 0.45%, toughness deteriorates. Therefore, the upper limit
of the Pb content is 0.45% w!~en Pb is contained. The upper limit of the Pb content is
preferably 0.30%.
I
1
[0077]
I
! Bi: 0.20% or less
Bi is an element which contributes to an improvement in machinability of
stcel. The Bi content is preferably 0.001% or greater to obtain this effcct. When the
Bi content is greater than 0.20%, toughness deteriorates. Therefore, the upper limit of
the Bi content is 0.20% when Bi is contained. The upper limit of the Bi content is
preferably 0.10%.
100781
Te: 0.01% or less
Te is an element which contributes to an improvement in machinability of
steel. The Te content is preferably 0.0001% or greater to obtain this effect. When
the Te content is greater than 0.01%, toughness deteriorates. Therefore, the upper
limit ol'the Te content is 0.01% when Te is containcd. The upper limit ofthe Te
content is preferably 0.005%
100791
Sb: 0.20% or less
Sb is an element which contributes to an improvement in corrosion resistance
based on sulfuric acid resistance and hydrochloric acid resistance and an improvement
in machinability. The Sb content is preferably 0.001% or greater when obtaining
these effects. When the Sb content is greater than 0.20%, toughness deteriorates.
Therefore, the upper limit of the Sb content is 0.20% when Sb is contained. The
upper limit of the Sb content is preferably 0.10%.
[0080]
Mg: 0.01% or lcss
Mg is an element which contributes to an improvement in machinability of
steel. The Mg content is preferably 0.0001% or greater to obtain this effect. When
the Mg content is greater than 0.01%, toughness deteriorates. Therefore, the upper
limit of the Mg content is 0.01% when Mg is contained. The upper limit of the Mg
content is preferably 0.005%.
[0083]
Next, the nonmetallic inclusions existing in a finely dispersed manner in the
low-oxygen clean steel according to this embodiment will be described.
[0082]
The low-oxygen clean steel according to this embodiment is obtained by
adding, by mass %, 0.00005% to 0.0004% of REM to molten steel which contains
0.005% to 0.20% ofAl, 0.0005% or less of Ca, and 0.003% or less of T.0 and in which
the ratio of Ca to T.0 is 0.50 or less. The low-oxygen clean steel according to this
embodiment satisfies (xl) the ratio of REM to Ca is 0.15 to 4.00 and (y) the ratio of Ca
to T.O is 0.50 or less, and preferably further satisfies (x2) the ratio of REM to T.O is
0.05 to 0.50.
[0083]
Molten steel having the following chemical components: 0.005% to 0.20% of
A1; 0.0005% or less of Ca; and 0.003% or less of T.0, in which the ratio of Ca to T.O
is 0.50 or less, is used when obtaining the low-oxygen clean steel according to this
embodiment. In such molten steel, the amount of CaO existing in the molten steel
and the amount of Ca0-A1203 inclusions are small.
[00841
When REM is added to the molten steel in the above state in an amount oS
0.00005% to 0.0004% such that the above-described (xl) (preferably further (x2)) is
satisfied, the WM reduces CaO, acting as a binding agent to promote the aggregation
of the inclusions, FeO, compounds such as Fe0-A1203, and CaO in Ca0-A1203
inclusions. As a result, (i) the Ca0-A1203 inclusions are modified into A1201-based
andlor REM203-based inclusions, and (ii) the aggregation of Al203-based inclusions,
A1203-MgO-based inclusions, and REM20,-based inclusions is suppressed, whereby
the inclusions do not coarsen.
That is, fine nonmetallic inclusions are generated in the molten steel by
adding REM as described above. Accordingly, the low-oxygen clean stecl according
to this embodiment obtained by casting the molten steel in which the fine nonmetallic
inclusions exist is capable of obtaining a structure in which the nonmetallic inclusions
are finely dispersed. The nonmetallic inclusions are fine and have a size of 30 pm or
less even in terms of the maximum predicted diameter obtained using an extreme value
statistical method with a prediction area of 30,000 rnm2. In addition, since the
nonmetallic inclusions are fine, fatigue fracture hardly occurs thererrom as is apparent
fracture-n~echanically. Therefore, the mechanical characteristics, particularly, fatigue
properties of the low-oxygen clean steel according to this embodiment are significantly
improved. This is the most important characteristic of the low-oxygen clean steel
according to this embodiment.
In this embodiment, the maximum predicted diameter of the inclusions is a
value estimated using, for example, the extreme value statistical method described in
"Metal Fatigue, Effects of Micro-Defects and Inclusions" (written by Yukitaka
Murakami, Yokendo, published in 1993, P.223 to 239). The maximum predicted
diamcter (-\/area (max)) of the inclusions is calculated through the expression: -\/area
(max) = (a2 + b2 )1 12 where a is a major axis and b is a minor axis perpendicular to the
major axis.
[OOS6]
FIG. 6 shows typical forms of nonmetallic inclusions existing in steel (SEM
reflected electron image). These are forms of nonmetallic inclusions detected when
evaluating steel piece extreme value statistics in examples to be described later. FIGS.
6(a) and 6(b) show forms of nonmetallic inclusions of invention examples (No. 2-1 in
Tables 2-1 and 2-2 to be shown later) (type of steel: suspension spring A), and FIGS.
6(c) and 6(d) show representative forms of nonmetallic inclusions of comparative
examples (No. 2-2 in 'Tables 2-1 and 2-2 to be shown later) (type of steel: suspension
spring A).
[OOS7]
The diameter (see blaclc borders) of the nonmetallic inclusions of the
comparative examples shown in FIGS. 6(c) and 6(d) is on the order oftens of-pm.
The diameter (see black borders) of the nonmetallic inclusions of the invention
examples shown in FIGS. 6(a) and 6(b) is on the order of several-pm. "Fine
nonmetallic inclusions" exist in various shapes in the low-oxygen clean steel according
to this embodiment as shown in FIGS. 6(a) and 6(b). Since the nonmetallic
inclusions are finc due to the modification with REM, fatigue fracturc hardly occurs
therefrom. The inventors of the invention have confirmed this fact by way of
experiment in an actual operation with respect to main types of stccls which are used in
spring steel, bearing steel, case hardening steel etc.
LOOSS]
The above-described fact that fatigue fracture hardly occurs from the fine
nonmetallic inclusions also relates to the component composition of the nonmetallic
inclusions. Hereinafter, the component composition of the nonmetallic inclusions
will be described.
[0089]
Table 1 shows component compositions of the above-described nonmetallic
inclusions shown in FIGS. 6(a) to 6(d). Table 1 also shows component compositions,
which are separately observed from those in FIGS. 6(a) to 6(d), of nonmetallic
inclusions (Invention Examples 3 to 12) of the low-oxygen clean steels according to
this embodiment and nonmetallic inclusions (Comparative Examples 3 to 6) of
comparative steels. The component composition of the nonmetallic inclusions was
measurcd as follows.
[0090]
The average composition of one inclusion detected by an optical microscope
is measured using an energy dispersive X-ray analysis method to analyze the
composition of Mg, Al, Si, Ca, La, Ce, Nd, Mn, Ti, and S. Since Mn and Ca form
both of oxide and sulfide, S is allowed to form sulfide in order of MnS and CaS and
the remaining Ca and Mn are analyzed as oxide. When obtaining the average of the
inclusion composition, the number average may bc taken after examining the
compositions of a plurality of inclusions as described above.
[0091]
The nonmetallic inclusions shown in FIG. 6 have a difference in contrast
therebetween. This shows that the nonmetallic inclusions have a mixed phase of
oxide and sulfide, but the fact that the nonmetallic inclusions have a mixed phase does
not have a dominant influence on fatigue properties. This is consistent with the
relationship between the grain diameter of the nonmetallic inclusions and the fatigue
strength shown in FIG. 1.

[0093]
The inclusion compositions of FIGS. 6(a) and 6(b) are shown in Invention
Examples 1 and 2 of Tahle 1, and the inclusion compositions of FIGS. 6(c) and 6(d)
are shown by mass % in Comparative Examples 1 and 2 of Tahle 1. In Comparative
Examples 1 and 2 and further Comparative Examples 3 to 6, the inclusions are not
modified with REM. However, in Invention Examples 1 and 2 and further Invention
Examples 3 to 12, the inclusions are modified with REM.
[0094]
As is apparent from Table 1, A1203 andlor CaO are main components in
Comparative Examples 1 and 2 and further Comparative Examples 3 to 6. All03 and
REM oxide are main components in Invention Examples 1 and 2 and further Invention
Examples 3 to 12. In addition, the average proportion of A1203 in the inclusions in
each example exceeds 50%.
CaO in Comparative Exan~ples1 is 16.5% and CaO in Comparative Example
2 is 24.3%. These are values higher than 10%. In the invention examples, CaO is
1.0% or less and significantly lower than in the comparative examples.
In the case of the inclusions of the invention cxamples, Ti02 and SiO2 are
almost undetected (for example, 1 .O% or less). When deoxidation is sufficiently
performed with A1 or A1-Si, the nonmetallic inclusions almost contain no Ti02 and
SiOz.
100951
Even when one or two or more of CaS, MnS, REM sulfide and MgO
(compound layer) exist in inclusions having A1203 and REM oxide as main
components, the influence on thc size of the inclusions is small. For example, in the
case of the nonmetallic inclusions of Invention Example 1 in Table 1,3 1.1 mass % of
MnS and 10.2 mass % of CaS (total: 41.3 mass %) additionally exist, and in thc case of
the nonmetallic inclusions of Invention Example 2, 11.2 mass % of MnS and 13.6
mass % of CaS (total: 24.5 mass %) additionally exist.
[0096]
Even when CaS and MnS exist in an amount of approximately 0% to 42% in
inclusions having A1203 and REM oxide as main components, the size of the inclusiolls
is kept small in the range of the analysis. In addition, fatigue properties are not
affected by the existence of CaS and MnS, and thus the influence of the existence of
CaS and MnS on fatigue properties is confirmed to be sufficiently small.
Apreferable low-oxygen clean steel according to this embodiment will be
described.
[0098]
Similarly to general steel, a low-oxygen clean steel according to an
embodiment is preferably obtained by rolling a steel piece obtained through a refining
process and a casting process or the like. As the processing processes such as the
casting process and rollilig, arbitrary methods can lie employed so as to proviilc a
desired shape and desired characteristics.
As for the low-oxygen clean steel according to this embodiment, it is
important to add, by mass %, 0.00005% to 0.0004% of E M to molten steel which
contains 0.005% to 0.20% ofAl, 0.0005% or less of Ca, and 0.003% or less of T.0 and
in which the ratio of Ca to T.O is 0.50 or less.
Therefore, in the refining process, the Ca content is preferably restricted and
REM is preferably contained in the molten steel through thc following method in the
following manner.
[0099]

Various types of auxiliary raw materials and alloy iron are added to molten
steel when the molten steel is subjected to refining and component adjustment. In
general, since the auxiliary raw materials and alloy iron contain Ca in various forms, it
is important to manage the timing of addition of the auxiliary raw materials and alloy
iron and the Ca content contained therein in order to adjust the Ca content to 0.0005%
or less.
[O 1001
Ca in the alloy iron is contained as an alloy component at a high ratio.
Accordingly, in the case of molten steel deoxidized with A1 or A1-Si, the yield of Ca in
the molten steel is high. Thus, it is necessary to avoid the addition of alloy iron
having a high Ca content.
[OlOl]
Therefore, the amount of Ca to be added is preferably decreased using, for
example, alloy iron having 1.0% or less of Ca. In addition, since quicklime, dolomite
etc. to be added as a slag-making material contain Ca in the form of mainly oxide, it is
incorporated in slag when floating separation is sufficiently conducted. However,
since the floating separation cannot be sufficiently conducted during the terminal
stages of secondary refining, the addition is avoided. Other than quicklime, dolomite,
CaO-containing recycle slag may be used as a slag-making material.
In addition, in Al-deoxidized steel and Al-Si-deoxidized steel, it is important
to suspend no CaO in the molten steel to suppress the generation of Ca0-Al20?-based
inclusions. During the terminal stages of secondary refining, stirring of molten stcel
and slag containing CaO in a large amount is suppressed. For examplc, strong
stirring by Ar blowing into a ladle should be avoided. When the molten steel is
stirred from the viewpoint of uniform REM concentration, a stirring method such as
electromagnetic stirring is used in which the slag is not incorporated into the molten
steel.
[0103]

CaO adheres to incl.~sionsh aving A12O3 as a main component and functions
as a binding agent to promote coarsening. REM acting to reduce the CaO is added in
an amount of 0.00005% to 0.0004% to molten steel in which ladle slag refining has
been completed by sufiicient deoxidation with A1 or Al-Si. It is not preferable that
REM be added before deoxidation with A1 or AI-Si be performed since the inclusions
coarsen.
[0 1041
For example, in a common secondary refining process including ladle
electrode heating and vacuum degassing, molten steel is deoxidized by ladle electrode
heating, and then REM is added to the molten stecl in the vacuum degassing process.
REM may be added to a tundish or the molten steel in a mold.
[OlOS]
Sincc REM is added to thc molten steel in a minute amount, the molten steel
is preferably stirred to unifonnize the REM concentration in the molten steel after the
addition. For the stirring ofthe molten steel, stirring in a vacuum chamber in the
vacuum degassing process, stirring in the tundish by flowing of the molten steel, and
electromagnetic stirring in the mold can be applied.
REM may be added in the form of any of pure metal such as Ce and La, alloy
of REM metals, or alloy with other alloys. When REM is added, the shape thercof is
preferably a lump-like shape, a grain shape, or a wire shape from the viewpoint of
yield.
[0 1061
A low-oxygen clean steel product according to this embodiment can be
produced by processing the low-oxygen clean steel according to this embodiment
using an arbitrary method.
Examples
[0 1071
I Next, examples of the invention will be described. Conditions in the
I ~ invention examples are just an examples employed to confirm the feasibility and
1 effects ofthe invention. Therefore, the invention is not restricted to these examples
The invention can employ various conditions so long as not departing from the gist of
the invention while achieving the object of the invention.
[OlOS]
Example 1
A steel piece was produced by casting molten steel having a component
composition shown in Table 2-1. The slag composition and the conditions of
auxiliary raw materials at the time of refining are shown together in Table 2-2. In the
column of "conditions of auxiliary raw materials", a Ca source (CaSi or FeSi) to be put
into the molten steel and the mass percent of Ca in FcSi are shown. The component
composition includes the remainder Fe and impurities.
[0 1091
Using the above-described steel piece, steel piece extreme value statistics
(maximum predicted diameter) (pm) of nonmetallic inclusions in a prediction area of
30,000 rrm2 were estimated through an extreme value statistical method. The results
are shown together in Table 2-2. When thc steel piece extreme value statistics are 30
pm or less, the level is set to pass (G: GOOD), when the steel piece extreme value
statistics are greater than 30 pm to 37 pm, the level is set to B (BAD), and when the
steel piece extreme value statistics are greater than 37 pm, the level is set to VB
(VERY BAD). FIGS. 6(a) and 6(b) show the forms of the nonmetallic inclusions of
Invention Example No. 2-1, and FIGS. 6(c) and 6(d) show the forms of the
nonmetallic inclusions of C~~nparativEex ample 2-2.
[OllO]
[Table 2- 11

[Olll]
[Tablc 2-21

[0112]
In Table 2, the steel piece extreme value statistics ofNo. 1-1 are 18.8 pm (<30
m ) In No. 2-2, since REM is not added, the inclusions were not modified and the
steel piece extreme value statistics were 3 1.0 pm. In No. 2-2, fatigue fracture occurs
from the inclusions.
[0113]
The steel piece extreme value statistics of the inclusions in the table were
calculated using an extreme value statistical method in the following manner.
That is, the invention steel was cast by a curved continuous casting machine,
and then in a steel piece rolled at a surface reduction ratio of 1.8 or greater, a steel
sample was collected from a portion at a position of 114 from the loose surface side of
an L-cross-section of the steel piece (a cross-section including a center line of a loose
surface, a center line of a surface opposite thereto, and a center line of the steel piece)
and the steel piece extreme value statistics were calculated based on an extreme value
statistical method including measurement under the conditions of an test standard area
of 100 mm2 (area of 10 mm x 10 mm), a test field of 16 (that is, the number of tests is
16), and an area for perbrming prediction of 30,000 mm2. The calculatioii was
performed through the expression: dares (max) = ($+b2)'" where a is a major axis and
b is a minor axis perpendicular to the major axis. Here, thc loose surface is a surface
on the upper surface side in a horizontal portion from a curved portion of the curved
continuous casting machine.
[0114]
The estimation of the maximum predicted diameter (dares (max)) of the
inclusions using extreme value statistics is performed according to the method
described in, for example, "Metal Fatiguc, Effects of Micro-Defects and Inclusions"
(written by Yulcitaka Murakami, Yokendo, published in 1993, P.223 to 239). The used
method is a two-dimensional method of estimating maximum inclusions obscrved in a
certain area by two-dimensional examination.
[0115]
By using the above-described extreme value statistical method, the maximum
predicted diameter darea (max) of the inclusions in the prediction area (30,000 mm2)
was estimated from the test standard area (100 mm2) from the image of the nonmetallic
inclusions imaged by an optical microscope. Specifically, 16 pieces of data (data of
16 ficlds) on maximum diameters of the inclusions obtained through the observation
were plotted on extreme value probability paper in accordance with the method
described in the document to obtain a maximum inclusion distribution straight line
(linear function of the maximum inclusion and the extreme value statistical
standardization variable), and the maximum inclusion distribution straight line was
extrapolated to estimate a maximum predicted diameter -\/area (max) of the inclusions
in the area of 30,000 mm2.
[0116]
In addition, for (he specification of the nonmetallic inclusions, obsc~va tion
was performed at 1,000-fold magnification using an optical microscope to discriminate
the nonmetallic inclusions from a difference in contrast. The validity of the
discrimination method using the difference in contrast was previously confirmed by a
scanning electron microscope with an energy dispersive X-ray spectroscopic analyzer
A plurality of inclusions was analyzed to obtain an average conlposition ratio of the
inclusions.
[0117]
Example 2
One of characteristics required for a steel to which the invention steel is
applicd is contact fatibqe properties such as rolling fatigue properties and surfacc
fatigue properties. Therefore, evaluation of radial rolling fatigue properties was
perlormed in the following manner.
[0118]
Cast steel pieces obtained from a plurality of molten steels based on
components of steel type of SUJ2, in which Ca, REM, T.0, etc. were changed so as to
have different maximum predicted diameters of inclusions, were held for 25 hours to
30 hours at 1200°C to 1250°C in a heating furnace, and cementite sphcroidizing was
performed. Then, blooming was performed at 1000°C to 1200°C. The obtained
steel pieces were heated at 900°C to 1200°C and rolled up to 465 mm to provide a
material of a radial rolling fatigue test piece.
[0119]
FIG. 7 shows an aspect of the production of the radial rolling fatigue test piece.
FIG. 7(a) shows the shape of the material of the radial rolling fatigue test piece, FIG
7(b) shows an aspect of the collection of the radial rolling fatigue test piece, and FIG.
7(c) shows a final shapc of the collected radial rolling fatigue test piece.
[0120]
From the material of the radial rolling fatigue test piece (hereinafter, referred
to as "test piece") oC965 mm, a round bar (having a center hole at both ends and a
through hole of + 3 mm in one end portion at a position separate from the end surface
by 5 mm) having the shape (+: 12.2 mm, length: 150 mm) shown in FIG. 7(a) was
produced.
[0121]
This round bar was heated for 30 minutes at 840°C in an induction heating
furnace, and then subjected to quench with oil at 50°C. Thereafter, it was annealed
for 90 minutes at 180°C and air-cooled. From the round bar after the heat treatment,
both ends of the round bar were discarded as shown in FIG. 7(b), and four 22 mm-test
pieces having the final shape shown in FIG. 7(c) were collected from a center portion
thereof and provided to the radial rolling fatigue test.
[0122]
The radial rolling fatigue test was performed on 12 test pieces under the
conditions ofa test load of 600 kgf, a repetition rate of46240 cpm, and the number of
times of stopping of 1x10' using a radial rolling fatigue test machine (product name:
i "cylindrical fatigue life test machine" manufactured by NTN Corporation).
I
i FIG. 8 shows the relationship between the maximum predicted diameter (steel
piece extreme value statistics) of each test piece, obtained through the extreme value
statistical method, and the shortest breaking life obtained through the radial rolling
fatigue test. 8x lo7 or greater of the shortest breaking life is obtained when the steel
piece extreme value statistics are 30 pm or less.
[0124]
Example 3
Next, an Ouo-type rotary bending test was performed to evaluate rotary
I
I bending fatigue properties. FIG. 9 shows a shape of a test piece produced for
evaluation of the rotary bending fatigue properties.
[0125]
Using a test piecc produced with dimensions shown in FIG. 9, the Ono-type
rotary bending test was performed. The test piece was subjectcd to induction
hardening (frequency: 100 kHz). Tap water or a polymer quenching catalyzer was
used as a refrigerant in the induction hardening. After hardening, a tempering
treatment was performed for 1 hr at 1 50°C. Table 3 shows the test results, and FIG.
10 shows the relationship between maximum stress and the number of times of
endurance.

[0127]
From Table 3 and Fig.?, it is found that the invention steel has much better
rotary bending fatigue properties than the comparative steel.
[0128]
As described above, the invention steel has much better fatigue properties
than the conventional steel. Therefore, it is apparent that the life of a steel product
produced from the invention steel increases significantly.
[0129]
The improvement in mechanical characteristics of the invention steel was
verified while paying attention to fatigue properties having a large influence on
inclusions and a decrease in size of nonmetallic inclusion was confirmed in all of the
target steels. Accordingly, in the invention steel, in addition to the fatigue properties,
mechanical characteristics (toughness, ductility, etc.) necessary for casting, pressing,
and other processings are also presumed to be improved.
Industrial Applicability
[0130]
As described above, according to the invention, high-melting-point /\lz03-
REM oxide which has Ca0-A1203-based inclusions modified by adding a minute
amount of REM to Al-deoxidized molten steel or Al-Si-deoxidized molten steel and
hardly aggregates, and fine nonmetallic inclusions containing REM sulfide, MgO, or
both of REM sulfide and MgO exist in the steel. Accordingly, steel having excellent
fatigue properties can be provided and an improvement in other mechanical
characteristics can also be expected. As a result, since the life ol" a steel product
produced from the steel of the invention increases significantly, the invention is highly
applicable in steel production industries and in steel working industries.

[Document Type] Claims
1. A low-oxygen clean steel containing C, Si, Mn, P, and S as chemical
components, and further containing, by mass %, 0.005% to 0.20% ofAl, greater than
0% to 0.0005% of Ca, 0.00005% to 0.0004% of REM, and greater than 0% to 0.003%
of T.0,
wherein the REM content, the Ca content, and the T.0 content satisfy the
following Expressions 1 and 2,
nonmetallic inclusions which have a maximum predicted diameter of 1 pm to
30 pm measured using an extreme value statistical method under the condition in
which a prediction area is 30,000 mm2, and contain A1203 and REM oxide are
dispersed in the steel,
an average proportion of the A1203 in the nonmetallic inclusions is greater
than SO%,
the REM is one or two or more of rare-earth elements La, Ce, Pr, and Nd, and
the steel is Al-deoxidized steel or Al-Si-deoxidized steel,
0.15iREM/Ca_I4.00 . . . Expression 1,
Ca/T.O50.50 . . . Expression 2.
2. The low-oxygen clean steel according to claim 1,
wherein the following Expression 3 is satisfied,
0.05