The present invention relates to a high-strength rail used for overseas freight
railways intended to improve delayed fracture resistance.
[Background Art]
[0002]
In accordance with economic development, efforts are being made to newly
10 exploit natural resources such as coal. Specifically, mining in a district with harsh
natural environments that has been thus far left unexploited is underway. Accordingly,
in overseas freight railways that transport resources, the track environment is becoming
significantly harsher. As a result, there has been a demand for better than ever wear
resistance for rails. From the above-described background, there has been a demand for
15 the development of a rail having better wear resistance than high-strength rails currently
in use.
[0003]
Rails described below have been developed to improve the wear resistance or
surface damage resistance of rail steels. A principal property of the above-described
20 rails is that, to improve the wear resistance, by increasing the amount of carbon in steel,
the volume fiaction of cementite in a pearlite lamellar is increased and the strength is
increased (for example, refer to Patent Documents 1 and 2). Alternatively, to improve
the surface damage resistance as well as the wear resistance, the metallographic structure
is consists of bainite, and the strength is increased (for example, refer to Patent
P
- - p E - z E6 -9 - - -- - - - - - - --- - -
[0004]
In the technique disclosed in Patent Document 1, a rail having excellent wear
resistance in which the volume fraction of cementite in a lamellar in a pearlite structure is
increased using hyper-eutectoid steel (C: more than 0.85% to 1.20%), can be provided.
[0005]
In the technique published in Patent Document 2, a rail having excellent wear
resistance in which the volume fraction of cementite in a lamellar in a pearlite structure is
increased using hyper-eutectoid steel (C: more than 0.85% to 1.20%), and similarly, the
hardness is controlled, can be provided.
10 [0006]
In the technique disclosed in Patent Document 3, a rail having improved wear
resistance and surface damage resistance in which the amount of carbon is set in a range
of 0.2% to 0.5%, and Mn and Cr are added so as to form the metallographic structure
with bainite and to improve the strength, can be provided.
[0007]
In the techniques disclosed in Patent Documents 1 to 3, the volume fraction of
cementite in the pearlite structure is increased, and simultaneously, the strength is
increased. Alternatively, the metallographic structure is formed with bainite so as to
further increase the strength. Therefore, the wear resistance can be improved.
20 However, when the strength was increased, the risk of the occurrence of delayed fracture
due to residual hydrogen in steel heightened, and there was a problem in that rail
breakage became likely to occur.
[0008]
Therefore, there has been a demand for the development of a high-strength rail
3
the above-described problem, high-strength rails described below have been developed.
The main characteristics of these rails are as follows. Hydrogen accumulation places
are dispersed by increasing hydrogen trapping sites in steel. In addition, in the rails,
delayed fracture is suppressed by refining the structure or by suppressing the
5 precipitation of carbides in grain boundaries (for example, refer to Patent Documents 4 to
[0009]
In the techniques disclosed in Patent Docurnents 4 and 5, the delayed fracture
resistance is improved by dispersing A-based inclusions (for example, MnS) or C-based
10 inclusions (for example, SiO2 or CaO) that are hydrogen trap sites in a pearlite structure,
and furthermore by controlling the amount of hydrogen in steel.
[OO 1 01
In the technique disclosed in Patent Document 6, a rail having excellent delayed
fracture resistance in which Nb is added so as to refine the bainite structure and to
15 prevent the precipitation of carbides in grain boundaries.
[OOll]
However, in the techniques disclosed in Patent Docurnents 4 and 5, the
inclusions that are the trap sites of residual hydrogen are coarsened depending on the
component system, and the delayed fracture resistance of pearlite steel does not
20 sufficiently improve. Additionally, there is a problem in that the inclusions serve as
initiation points of fatigue or fracture depending on the types of the inclusions, and rail
breakage becomes likely to occur. In addition, in the technique disclosed in Patent
Document 6, there are problems in that the structure is not sufficiently refined or the ~
precipitation of carbides in grain boundaries is not sufficiently suppressed due to the 1
4
of an alloy.
[Related Art Documents]
[Patent Documents]
[OO 1 21
5 [Patent Document 11 Japanese Unexamined Patent Application,'First
Publication No. H08-144016
[Patent Document 21 Japanese Unexamined Patent Application, First
Publication No. H08-246 100
[Patent Document 31 Japanese Unexamined Patent Application, First
10 Publication No. H09-296254
[Patent Document 41 Japanese Unexamined Patent Application, First
Publication No. 2007-27771 6
[Patent Document 51 Japanese Unexamined Patent Application, First
Publication No. 2008-50684
15 [Patent Document 61 Japanese Unexamined Patent Application, First
Publication No. H08-158014
[Patent Document 71 Japanese Unexamined Patent Application, First
Publication No. H08-246100
[Patent Document 81 Japanese Unexamined Patent Application, First
20 Publication No. H09- 1 1 1352
[Patent Document 91 Japanese Unexamined Patent Application, First
Publication No. H08-092645
[Summary of the Invention]
[Problems to be Solved by the Invention]
-
-- --~ ~ ~ ~ ~ ~ ~ E . ~ @ ~ ~ ~ K-ZE - Z Q T- E C E- G- -~ ~-.-- ---
The present invention has been made in consideration of the above-described
problems. An object of the present invention is to provide a rail having improved
delayed fiacture resistance required particularly for rails in overseas freight railways.
[Means for Solving the Problem]
[OO 1 41
(1) According to an aspect of the present invention, there is provided a rail
including, in a steel rail, by mass%: C: 0.70% to 1.20%; Si: 0.05% to 2.00%; Mn: 0.10%
to 2.00%; Al: 0.0020% to 0.0100%, and a balance consisting of Fe and impurities,
wherein 95% or more of a head surface section, which is a range fiom surfaces of head
10 comer sections and a head top section of the steel rail as a starting point to a depth of 20
mm, is a perlite or a bainite structure, and the structure contains 20 to 200 MnS-based
sulfides formed around an Al-based oxide as a nucleus and having a grain size in a range
of 1 pm to 10 pm per square millimeter of an area to be inspected on an arbitrary
horizontal cross section of the structure.
15 [00 1 51 I
(2) In addition, the rail according to the above (1) may further include, by
(3) In addition, the rail according to any one of the above (2) may further
include, by mass%, H: 2.0 ppm or less.
[00 1 61
(4) In addition, the rail according to any one of the above (I) to (3) may further
include, by mass%, one or two or more of the follows (a) to (n):
(a) Mg: 0.0005% to 0.0200%;
(b) Ca: 0.0005% to 0.0200%;
6
(d) Cr: 0.01% to 2.00%;
(e) Mo: 0.01% to 0.50%;
(f) Co: 0.01% to 1.00%;
(g) B: 0.0001% to 0.0050%;
(h) Cu: 0.01% to 1.00%;
(i) Ni: 0.01% to 1.00%;
(j) V: 0.005% to 0.50%;
(k) Nb: 0.001% to 0.050%;
(1) Ti: 0.0050% to 0.0500%;
(m) Zr: 0.0001% to 0.0200%;
(n) N: 0.0060% to 0.0200%.
[Effect of the Invention]
[00 171
According to the aspect of the present invention, it is possible to improve the
15 delayed fracture resistance of a rail used for oversea freight railways and to significantly
improve the service life by controlling the components and structure of the rail, and
furthermore, by controlling the form or number of MnS-based sulfides formed around an
Al-based oxide in steel as a nucleus.
[Brief Description of the Drawings]
20. [00 1 81
[Figure 11 FIG. 1 is a view illustrating a relationship between the number of
fine (grain size in a range of 1 pm to 10 pm) MnS-based sulfides formed around an
Al-based oxide in steel as a nucleus and the threshold stress value of delayed fracture.
[Figure 21 FIG. 2 is a view illustrating the names of surface locations on a
7
according to the invention and regions in which a pearlite structure or a bainite structure
is required.
[Figure 31 FIG. 3 is a view illustrating a location at which the fine (grain size
in a range of 1 pm to 10 pm) MnS-based sulfides formed around an Al-based oxide as a
5 nucleus are measured.
[Figure 41 FIG. 4 is a view illustrating a relationship between the numbers of
fine (grain size in a range of 1 pm to 10 pm) MnS-based sulfides formed around an
Al-based oxide as a nucleus and the threshold stress values of delayed fracture in
Invention Rail Steels (reference signs A1 to A49) and Comparative Rails (reference signs
10 a7 to a20) described in Tables 1 and 2.
[Figure 51 FIG. 5 is a view illustrating the numbers of fine (grain size in a
range of 1 pm to 10 pm) MnS-based sulfides formed around an Al-based oxide as a
nucleus and the threshold stress values of delayed fracture in Invention Rail Steels
(reference signs A1 0 to A1 5, A1 8 to A23, A27 to A29, A3 1 to A44, and A46 to A48)
15 described in Table 1 using a relationship between the control of an S addition amount, the
optimization of the S content and the control of S and H addition amounts.
[Figure 61 FIG. 6 is a pattern diagram illustrating a delayed fracture test
method.
[Embodiment of the Invention]
20 [00 191
Hereinafter, an embodiment of the present invention will be described in detail
using the accompanying drawings. However, the present invention is not limited to the
below description, and a person skilled in the art can easily understand that the form and
detail of the present invention can be modified in various manners within the purport and
-
-Ee~REGB&-O&Z-~&&~~~&kf~r&@eintevrctation-of_iheePresentzin~i~isT - -
8
not limited to the descriptions of the embodiment described below.
As the embodiment, a rail having excellent delayed fracture resistance
(hereinafter, sometimes, referred to as a rail according to the embodiment) will be
described in detail. Hereinafter, the unit of a composition, mass%, will be simply
5 expressed as %.
[0020]
First, the present inventors studied a method of improving the delayed fracture
of a rail using inclusions that are hydrogen trap sites. As a result of studying the cheap
inclusions having a small effect on the various properties of the rail, it was clarified that a
10 soft MnS-based sulfide formed from S contained as an impurity of iron and (A) Mn
generally added as a strengthening element has no effect on toughness or fatigue
properties and (B) is cheap, and therefore the MnS-based sulfides are promising
hydrogen trap sites.
[002 11
Next, to use MnS-based sulfides as the hydrogen trap sites, the formation state
of the MnS-based sulfides was investigated. As a result, it was found that the
MnS-based sulfides are classified into relatively large MnS-based sulfides and relative
small MnS-based sulfides having a grain size of 5 pm or less.
[0022]
2 0 To make the MnS-based sulfides effectively serve as the hydrogen trap sites, it is
necessary to increase,the surface area between the MnS-based sulfides that are the trap
sites and base metal in contact with the MnS-based sulfide, that is, to refine the
MnS-based sulfides. Therefore, first, the forming behaviors of the large MnS-based
sulfides were investigated. As a result of analyzing steel in the middle of solidification,
-_lIEE&E%-&a&%~~e~- % ~ ~ ~ e - f o r m e d - f i o m - a l i ~ i d ~ i ~ t ~
9
and coarsen in the liquid phase before the steel is solidified (gamma iron).
[0023]
The inventors studied a method for refining the MnS-based sulfides formed in
the liquid phase. As a result, it was found that, to refine the MnS-based sulfides, stable
5 nuclei accelerating the formation of the MnS-based sulfides in the liquid phase are
required. Based on the above-described finding, an attention was paid to an oxide that
is stable at a high temperature, and fine oxides were selected to use the oxides as the
nuclei. Steel containing 1 .O% of carbon was melted, and a variety of oxide-forming
elements were added, thereby investigating the forming behaviors of oxides and
10 MnS-based sulfides. As a result, it was found that, when a certain amount of Al is
added, and an Al-based oxide is finely dispersed in a liquid phase, it is possible to make
the Al-based oxide having a close lattice constant to the lattice constant of MnS serve as
a formation nucleus of the MnS-based sulfides, and consequently, it is possible to refine
the MnS-based sulfides.
15 [0024]
Next, the inventors studied the A1 content for finely forming the Al-based oxide
in a liquid phase. As a result, it was found that, to prevent the formation of a coarse
Al-based oxide having an adverse effect on the various properties of the rail and to form
a sufficient amount of a fine Al-based oxide in a liquid phase, it is important to control
20 the Al addition amount to be in a certain range.
[0025]
On the basis of the above-described finding, the inventors investigated the
delayed fracture as described below. That is, steel containing 1 .O% of carbon
(0.2%Si-1 .O%Mn-0.0080%S) and 2.5 ppm of hydrogen as base components and having
-
L E ~ ~ ~ ~ ~ ~ ~ ~ &-3- & i t i ~ k t - &0~0 4~~ ~&, ~~~~e r~e ~~mae 1n- td~- d-- ~~d ~.d u ~ e
10
steel pieces. Next, rail rolling and a heat treatment were carried out on the steel pieces,
thereby manufacturing rails having a pearlite or bainite structure in the head surface
section (a range from the outer surface of the head section as the starting point to a depth
of 20 mrn). A three-point bend test in which tensile stress was applied to the head
5 section was carried out on the rails obtained as described above, and the delayed fracture
was evaluated. The delayed fracture was evaluated using a three-point bend (span
length: 1.5 m) method so that the tensile stress acted on the head section. The stress
condition was set in a range of 200 MPa to 500 MPa, the stress application time was set
to 500 hours, and the maximum value of the stress in a case in which the steel piece was
10 not broken when the stress had been applied over 500 hours was considered as the
. .
threshold stress value of delayed fracture.
[0026]
As a result of the delayed fracture test, for the steel containing 0.0010% of Al
that is the content in a case in which A1 is intentionally not added during ordinary rail
15 refining, the threshold stress value of delayed fracture was 220 MPa, and for the steel
containing 0.0040% of A1 that is the content in a case in which Al is intentinally added
during ordinary rail refining, the threshold stress value of delayed fracture was 330 MPa.
That is, it was found that, when the addition amount of A1 is increased, the number of
fme MnS-based sulfides formed around an Al-based oxide as a nucleus increases, and the
20 delayed fracture resistance improves.
[0027]
Furthermore, the inventors studied a method for further improving the delayed
fracture. Steel containing 1 .O% of carbon (0.2%Si-l.O%Mn-0.0040%A1) and 2.5 ppm
of hydrogen as base components and having changed the S addition amounts of 0.0080%
11
carried out, thereby manufacturing rails having a pearlite or bainite structure in the head
surface section. A three-point bend test in which tensile stress was applied to the head
section was carried out using the rails, and the delayed fracture was evaluated.
[0028]
As a result, for the S addition amount of 0.0080%, the threshold stress value of
delayed fracture.was 330 MPa, and for S addition amount of 0.0150% of S, the threshold
stress value of delayed fracture was 380 MPa. That is, it was confirmed that, when the
S content is increased, the number of fine MnS-based sulfides formed around an
Al-based oxide that is a hydrogen trap site as a nucleus further increases, and the delayed
10 fracture resistance improves.
[0029]
In addition to the control of the MnS-based sulfide, the inventors studied a
method of further improving the delayed fracture resistance. As a result, it was
confiied that, when the amount of hydrogen is controlled to 2.0 ppm or less by
15 intensifying the secondary refining (degassing) of molten steel or applying a
dehydrogenation treatment in a steel piece, the threshold stress value of delayed fracture
improves up to 450 MPa, and the delayed fracture resistance further improves.
[0030]
FIG. 1 illustrates a relationship between the number of fine (grain size in a range
20 of 1 pm to 10 pm) MnS-based sulfides formed around an Al-based oxide in steel as a
nucleus and the threshold stress value of delayed fracture. The number of fine
MnS-based sulfides formed around an Al-based oxide as a nucleus was measured using
an optical microscope or a scanning electron microscope after taking a sample at a
location 10 mm to 20 mrn deep from the surface of the rail head section and polishing the
12
of 1 pm to 10 pm) was converted to the number of the grains per square millimeter after
the measurement.
[003 11
When the A1 addition amount is increased, the number of fine M~S-based
5 sulfides increases, and the threshold stress value increases as illustrated in FIG. I. In
addition, when the S content is further increased, the number of fine MnS-based sulfides
further increases, and the threshold stress value increases. In addition, when the amount
of hydrogen in steel is controlled to 2.0 ppm or less, the threshold stress value further
improves.
[0032]
That is, the, present invention relates to a rail intended to improve the delayed
fracture resistance of a rail used for overseas freight railways and to significantly
improve the service life by controlling the components and the structure of the rail steel,
controlling the form or number of MnS-based sulfides formed around an Al-based oxide
15 in steel as a nucleus, further increasing the S addition amount, and reducing the amount
of hydrogen.
[0033]
Next, the reasons for limiting the steel composition of the rail according to the
present invention will be described. Hereinafter, the unit of the steel composition,
20 mass%, will be simply expressed as %.
[0034]
(1) The reasons for limiting the chemical components of steel
The reasons for limiting the chemical components of steel in the
above-described numeric ranges in the rail according to the embodiment will be
P -kTOFWEiE,B$,~@h~@T,=~-~ -X h
C is an effective element for accelerating pearlitic transformation and ensuring
the wear resistance. In addition, C is a necessary element for maintaining the strength
of the bainite structure. When the C content is less than 0.70%, in the present
5 component system, it is not possible to maintain the minimum strength or wear resistance
required for rails. Further, a soft pro-eutectoid ferrite structure in which strain is likely
to be stored is formed, and delayed fracture becomes likely to occur. In addition, when
the C content exceeds 1.20%, a large amount of a pro-eutectoid cementite structure
having low toughness is formed, and delayed fracture becomes likely to occur.
10 Therefore, the C addition amount is limited in a range of 0.70% to 1.20%. Meanwhile,
to stabilize the formation of the pearlite structure or the bainite structure and improve the
delayed fracture resistance, the C addition amount is desirably set to 0.80% to 1.10%.
[0036]
Si is an element that forms a solid solution in ferrite in the pearlite structure or
15 the base ferrite structure in the bainite structure, increases the hardness (strength) of the
rail head section, and improves the wear resistance. Furthermore, Si is an element that
suppresses the formation of a pro-eutectoid cementite structure having low toughness and
suppresses the occurrence of delayed fracture in hyper-eutectoid steel. However, when
the Si content is less than 0.05%, the above-described effects cannot be sufficiently
20 expected. On the other hand, when the Si content exceeds 2.00%, the number of surface
defects are generated during hot rolling. Furthermore, a martensite structure having low
toughness is formed in the head surface section, and delayed fracture becomes likely to
occur. Therefore, the Si content is limited in a range of 0.05% to 2.00%. Meanwhile,
to stabilize the formation of the pearlite structure or the bainite structure and improve the
14
[003 71
Mn is an element that improves the hardenability, stabilizes the formation of
pearlite, and simultaneously, decreases the lamellar spacing in the pearlite structure.
Furthermore, Mn is an element that stabilizes the formation of bainite, simultaneously,
5 decreases the transformation temperature, ensures the hardness of the pearlite structure or
the bainite structure, and improves the wear resistance. However, when the 'Mn content
is less than 0.10%, the effect is small. In addition, the formation of a soft pro-eutectoid
ferrite structure in which strain is likely to be stored is induced, and it becomes difficult
to ensure the wear resistance or the delayed fracture resistance. On the other hand,
10 when Mn content exceeds 2.00%, the hardenability significantly increases, a martensite
structure having an adverse effect on toughness is formed in the head surface section, and
delayed fracture becomes likely to occur. Therefore, the Mn content is limited to be in a
range of 0.10% to 2.00%. Meanwhile, to stabilize the formation of the pearlite structure
or the bainite structure and improve the delayed fracture resistance, the Mn addition
15 amount is desirably set to 0.20% to 1.50%.
[003 81
A1 acts as a formation nucleus of a MnS-based sulfide in a liquid phase, and is
an essential element for finely dispersing the MnS-based sulfide. When the A1 content
is less than 0.0020%, the amount of an Al-based oxide formed is small, and A1 does not
20 sufficiently act as a formation nucleus of a MnS-based sulfide in a liquid phase.
Therefore, it becomes difficult to finely disperse the MnS-based sulfide. As a result, it
also becomes difficult to ensure the delayed fracture resistance. In addition, when the
A1 content exceeds 0.01 00%, A1 becomes excessive, the Al-based oxide is formed in a
cluster form, and rail breakage becomes likely to occur due to stress concentration.
- = = R = ~ T - z F P w & m-&w & ~ & ~ - h ~ f o r e ,
15
the A1 addition amount is limited to 0.0020% to 0.0100%. Meanwhile, to function as a
formation nucleus of a MnS-based sulfide, and prevent the clustering of an Al-based
oxide, the A1 addition amount is desirably set to 0.0030% to 0.0080%. Meanwhile,
during ordinary rail refining, less than 0.0020% of Al is interfused from a raw material or
5 refractory. Therefore, the A1 addition amount in a range of 0.0020% or more represents
the addition of A1 in a refining step.
S is an element inevitably contained in steel. When refining is carried out in a
converter, the S content is reduced up to 0.0030% to 0.0300%. However, there is a
10 correlation between the S content and the formation amount of the MnS-based sulfide,
and, when the S content increases, the number of fine MnS-based sulfides formed around
an Al-based oxide as a nucleus increases, and therefore, in an embodiment of the present
invention, the S content is set to 0.0100% to 0.0250%. When the S content is less than
0.01 00%, an increase in the formation amount of a fine MnS-based sulfide cannot be
15 expected. On the other hand, when the S content exceeds 0.0250%, stress concentration
or structure embrittlement occurs due to the coarsening of the MnS-based sulfide or an
increase in the formation density, and rail breakage becomes likely to occur. Therefore,
the S content has been limited in a range of 0.0100% to 0.0250%. Meanwhile, to
accelerate the formation of a fine MnS-based sulfide and prevent the coarsening of the
20 MnS-based sulfide, the S addition amount is desirably set to 0.0120% to 0.0200%.
[0040]
H is an element causing delayed fracture. When the H content in a steel piece
(bloom) before rail hot-rolling exceeds 2.0 ppm, the H content piled up in the interfaces
between MnS-based sulfides and the base metal increases, and delayed fracture becomes
16
preferably set to 2.0 ppm or less. Meanwhile, the lower limit of the H content is not
limited; however, when secondary refining (degassing) capability in the refining step or
the dehydrogenation treatment capability of the bloom is taken into account, the H
content of approximately 1.0 ppm is considered to be the limit in actual manufacturing.
[0041]
In addition, to the rail having the above-described component composition, Mg,
Ca, REM, Cr, Mo, Co, B, Cu, Ni, V, Nb, Ti, Zr, and N may be added as necessary for the
. purpose of the improvement of the delayed fiacture resistance by the fine dispersion of
the Al-based oxide and the MnS-based sulfide, the improvement of the wear resistance
10 by an increase in the hardness (strength) of the pearlite structure or the bainite structure,
the improvement of the toughness, the prevention of the softening of the heat affected
zones, the control of the cross-sectional hardness distribution inside the rail head section,
and the like.
[0042]
Here, Mg and Ca suppress the clustering of the Al-based oxide, and finely
disperses the MnS-based sulfide. REM breaks the connecting section of the clustering
of the Al-based oxide, and finely disperses the MnS-based sulfide. Cr and Mo increase
the equilibrium transformation point, decrease the lamellar spacing of the pearlite
structure or refine the bainite structure, and improve the hardness. Co refines the base
20 ferrite structure on an worn surface, and increases the hardness of the worn surface. B
decreases the dependency of the pearlite transformation temperature on the cooling rate,
and makes the hardness distribution in the rail head section uniform. In addition, B
improves the hardenability of the bainite structure, and improves the hardness. Cu
forms a solid solution in ferrite in the pearlite structure or the bainite structure, and
17
or the bainite structure, and simultaneously, prevents the softening of the heat affected ,
zone in a welded joint. V, Nb and Ti suppress the growth of austenite grains using a
carbide or nitride generated during hot rolling or in the subsequent cooling process.
Furthermore, V, Nb and Ti improve the toughness and hardness of the pearlite structure
5 or the bainite structure using precipitation hardening. In addition, V, Nb and Ti stably
generate a carbide or nitride during reheating, and prevent the softening of the heat
affected zone in a welded joint. Zr increases the equiaxial grain ratio of a solidification
structure, thereby suppressing the formation of a segregation band in the central part of
the cast bloom, and suppressing the formation of a pro-eutectoid cementite structure or
10 martensite structure. The main purpose is that N segregates in austenite grain
boundaries, thereby accelerating pearlitic transformation or bainitic transformation, and
refining the pearlite structure or bainite structure.
Mg is a strong deoxidizing element, and is an element that, when added, reforms
15 an Al-based oxide to a MgOAl-based oxide (spinel) or MgO, thereby preventing the
clustering or coarsening of the Al-based oxide, and accelerating the finely-dispersed
formation of fine MnS-based sulfide. However, when the MgO content is less than
3
0.0005%, the effect is weak. Therefore, to obtain the above-described effect, the lower
limit of the MgO content is desirably set to 0.0005%. On the other hand, when the
20 MgO content exceeds 0.0200%, a coarse MgO oxide is generated, and rail breakage
becomes likely to occur due to stress concentration. Therefore, the limit of the MgO
content is desirably set to 0.0005% to 0.0200%.
Ca is a strong deoxidizing element, and is an element that, when added, reforms
18
coarsening of the Al-based oxide, and accelerating the finely-dispersed formation of fine
MnS-based sulfide. However, when the Ca content is less than 0.0005%, the effect is
weak. Therefore, to obtain the above-described effect, the lower limit of the Ca content
is desirably set to 0.0005%. On the other hand, when the Ca content exceeds 0.0200%,
5 a coarse Ca oxide is generated, and rail breakage becomes likely to occur due to stress
concentration. Therefore, the limit of the Ca content is desirably set to 0.0005% to
0.0200%.
[0045]
REM is the strongest deoxidizing element, and is an element that reduces the
10 clustered Al-based oxide which is obtained by addition so as to refine the Al-based oxide,
thereby accelerating the finely-dispersed formation of fine MnS-based sulfide.
However, when the REM content is less than 0.0005%, the effect is small, and REM does
not act sufficiently as a formation nucleus of the MnS-based sulfide. Therefore, when
the REM content exceeds 0.0500%, a hard REM oxysulfide (REM202S) is generated,
15 and rail breakage becomes likely to occur due to stress concentration. Therefore, the
limit of the REM addition amount is desirably set to 0.0005 to 0.0500%.
[0046]
Meanwhile, REM refers to a rare earth metal such as Ce, La, Pr or Nd. The
addition amount limits the total content of all EMS. When the total of all contents is
20 within the above-described range, the same effects can be obtained irrespective of the
number of REMs - singular or multiple (two or more).
[0047]
Cr is an element that increases the equilibrium transformation temperature, and
decreases the lamellar spacing in the pearlite structure by increasing the degree of
19
temperature, and improves the hardness (strength) of the pearlite structure or bainite
structure. However, when the Cr content is less than 0.01%, the effect is small, and the
effect that improves the hardness of the rail steel is not observed. On the other hand,
when the Cr content exceeds 2.00%, the hardenability significantly improves, and a
5 martensite structure having an adverse effect on toughness is formed in the rail head
surface section and the like such that delayed fracture becomes likely to occur.
Therefore, the Cr addition amount is desirably limited to be in a range of 0.01% to
2.00%.
[0048]
10 Similarly to Cr, Mo is an element that increases the equilibrium transformation
temperature, and decreases the lamellar spacing in the pearlite structure by increasing the
degree of undercooling. In addition, Mo is an element that stabilizes bainitic
transformation and improves the hardness (strength) of the pearlite structure or bainite
structure. However, when the Mo content is less than 0.01%, the effect is small, and the
15 effect that improves the hardness of the rail steel is not observed. On the other hand,
when Mo is excessively added so that the Mo content exceeds 0.50%, the transformation
rate significantly decreases, and a martensite structure having an adverse effect on to
toughness is formed in the rail head surface section and the like such that delayed
fracture becomes likely to occur. Therefore, the Mo addition amount is desirably
20 limited to be in a range of 0.01% to 0.50%.
[0049]
Co is an element that forms a solid solution in ferrite in the pearlite structure or
the base ferrite structure in the bainite structure, and further refines a fine ferrite structure
formed by the contact with a wheel on the worn surface of the rail head surface section,
-
- - - ~ ~ o ~ ~ E ~ ~ ~ n ~ ~ ~ ~ & ~ - & ~ & ~ ~ ~ & % c t m e - m d - i m pwerar-~revsisitannceg. - t h e -
2 0
However, when the Co content is less than 0.01%, the refining of the ferrite structure is
not accelerated, and the effect that improves the wear resistance cannot be expected.
On the other hand, when the Co content exceeds 1.00%, the above-described effects are
saturated, and therefore the refining of the ferrite structure in accordance with the content
5 is not achieved, and economic efficiency decreases due to an increase in the alloy
addition costs. Therefore, the Co addition amount is desirably limited to be in a range
of 0.01% to 1.00%.
[0050]
B is an element that forms iron boroncarbide (Fe23(CB)6) in austenite grain
10 boundaries, and reduces the dependency of the pearlitic transformation temperature on
the cooling rate through the pearlitic transformation-accelerating effect. In addition, as
a result, a more uniform hardness distribution is supplied to the inside of the rail from the
surface of the head section, and it is possible to extend the service life of the rail.
Furthermore, B improves the hardenability of the bainite structure, and improves the
15 hardness of the bainite structure. However, when the B content is less than 0.0001%,
the effect is not sufficient, and there is no improvement in the hardness distribution in the
rail head section. On the other hand, when the B content exceeds 0.0050%, coarse iron
boron carbide is formed, and rail breakage becomes likely to occur due to stress
concentration. Therefore, the B addition amount is desirably limited in a range of
20 0.0001% to 0.0050%.
[005 11
Cu is an element that forms a solid solution in ferrite in the pearlite structure or
the base ferrite structure in the bainite structure, and improves the hardness (strength)
through solid solution strengthening, thereby improving the wear resistance. However,
--- 7 -7- ---
kP&e-~c~n&~~-l&~~%i~~&-~~I$~ct-cmotbe-e~ected-~n-the-other
hand, when the Cu content exceeds 1.00%, a martensite structure having an adverse
effect on toughness is formed in the rail head surface section and the like due to the
significant improvement of hardenability, and delayed fracture becomes likely to occur.
Therefore, the Cu addition amount is desirably limited to be in a range of 0.01 % to '
5 1.00%.
[0052]
Ni is an element that improves the toughness of the pearlite structure or the
bainite structure, and simultaneously, improves the hardness (strength) through solid
solution strengthening, thereby improving the wear resistance. Furthermore, Ni forms
10 Ni3Ti intermetallic compound together with Ti, finely precipitates in the heat affected
zones, and suppresses softening through precipitation strengthening. In addition, Ni is
an element that suppresses the intergranular embrittlement in Cu-added steel. However,
when the Ni content is less than 0.01%, the effect is significantly small. On the other
hand, when the Ni content exceeds 1.00%, a martensite structure having an adverse effect
15 on toughness is formed in the rail head surface section and the like due to the significant
improvement of hardenability, and delayed fracture becomes likely to occur. Therefore,
the Ni addition amount has been limited in a range of 0.01% to 1.00%.
[0053]
V is an element that precipitates in a form of a V carbide or V nitride in a case in
20 which ordinary hot rolling or a heat treatment in which steel is heated to a high
temperature is carried out. The precipitated V carbide or V nitride refines austenite
grains using the pining effect, and improves the toughness of the pearlite structure or the
bainite structure. Furthermore, the V nitride and V carbide formed in a cooling process
after hot rolling increases the hardness (strength) of the pearlite structure or the bainite
-~:mzz~m-he~g prF&;bc7P. P-i&taf &h Z@Z@meaz wcai resistance. mai ion,
since V forms a V carbide or V nitride in a relatively high temperature range in a heat
affected zone reheated in a temperature range that is equal to or lower than Acl point, V
is an effective element for preventing the softening of the heat affected zone in a welded
joint. However, when the V content is less than 0.005%, the above-described effect
5 cannot be sufficiently expected, and the toughness or hardness (strength) does not
improve. On the other hand, when the V content exceeds 0.50%, the precipitation
hardening of the V carbide or nitride becomes excessive, the pearlite structure or the
bainite structure embrittles, and the toughness of the rail decreases. Therefore, the V
addition amount is desirably limited to be in a range of 0.005% to 0.50%.
10 [0054]
Similarly to V, Nb is an element that, in a case in which ordinary hot rolling or a
heat treatment in which steel is heated to a high temperature is carried out, the Nb carbide
or Nb nitride refines austenite grains using the pining effect, and improves the toughness
of the pearlite structure or the bainite structure. Furthermore, the Nb nitride and Nb
15 carbide formed in the cooling process after hot rolling increases the hardness (strength)
of the pearlite structure or the bainite structure using precipitation hardening, and
improves the wear resistance. Furthermore, the Nb nitride and Nb carbide formed in the
cooling process after hot rolling increases the hardness (strength) of the pearlite structure
or the bainite structure using precipitation hardening. In addition, since Nb stably forms
20 an Nb carbide or Nb nitride in a wide temperature range from a low-temperature range to
a high-temperature range in a heat affected zone reheated in a temperature range that is
equal to or lower than Ac 1 point. Therefore, Nb is an effective element for preventing
the softening of the heat affected zone in a welded joint. However, when the Nb content
is less than 0.001%, the above-described effect cannot be expected, and the toughness or
the Nb content exceeds 0.050%, the precipitation hardening of the Nb carbide or nitride
becomes excessive, the pearlite structure or the bainite structure embrittles, and the
toughness of the rail decreases. Therefore, the Nb addition amount is desirably limited
in a range of 0.001% to 0.050%.
5' [0055]
Ti is an element that precipitates in a form of a Ti carbide or Ti nitride in a case
in which ordinary hot rolling or a heat treatment in which steel is heated to a high
temperature is carried out. The Ti carbide or Ti nitride refmes austenite grains using the
pining effect, and improves the toughness of the pearlite structure or the bainite structure.
10 Furthermore, the Ti nitride and Ti carbide formed in the cooling process after hot rolling
increases the hardness (strength) of the pearlite structure or the bainite structure using
precipitation hardening, and improves the wear resistance. In addition, Ti refines
structures in a heat affected zone heated up to the austenite range using the fact that the
Ti carbide or Ti nitride precipitated during reheating in welding does not melt, and is an
15 effective element for preventing the embrittlement of a welded joint section. However,
when the Ti content is less than 0.0050%, the above-described effect cannot be
sufficiently obtained. On the other hand, when the Ti content exceeds 0.0500%, a
coarse Ti carbide or Ti nitride is formed, and rail breakage becomes likely to occur due to
stress concentration. Therefore, the Ti addition amount is desirably limited in a range of
20 0.0050% to 0.0500%.
[0056]
Since the ZrO2-based inclusion has favorable lattice consistency with gamma-Fe,
the ZrO2-based inclusion serves as a solidification nucleus of a high-carbon rail in which
the gamma-Fe is a solidified primary phase, and increases the equiaxial grain ratio of a
24
segregation band in the central part of the cast bloom, and suppresses the formation of a
martensite structure or pro-eutectoid cementite structure formed in a rail segregation
section. However, when the Zr content is less than 0.0001%, the number of the
Zr02-based inclusions decreases, and the Zr02-based inclusion does not sufficiently
5 serve as a solidification nucleus. As a result, a martensite or pro-eutectoid cementite
structure is formed in the segregation section, and the toughness of the rail is decreased.
On the other hand, when the Zr content exceeds 0.0200%, a large amount of a coarse
Zr02-based inclusion is formed, and rail breakage becomes likely to occur due to stress
concentration. Therefore, the Zr addition amount is desirably limited in a range of
10 0.0001% to 0.0200%.
[0057]
N is an effective element for improving the toughness by mainly refining
structures through segregation in the austenite grain boundaries and accelerating of the
pearlitic transformation or the. bainitic transformation fiom the austenite grain boundaries.
15 In addition, N is an element that accelerates the precipitation of VN or A1N when being
added together with V or Al. VN or A1N is effective for improving the toughness of the
@ pearlite structure or the bainite structure by refining austenite grains using the pining
effect in a case in which ordinary hot rolling or a heat treatment in which steel is heated
to a high temperature is carried out. However, when the N content is less than 0.0060%,
20 the above-described effect is weak. On the other hand, when the N content exceeds
0.0200%, it becomes difficult to form a solid solution in steel, air bubbles serving as the
starting point for fatigue damage are generated, and rail breakage becomes likely to occur.
Therefore, the N addition amount is desirably limited in a range of 0.0060% to 0.0200%.
25
melting steel in an ordinarily-used melting 'furnace such as a converter or an electric
furnace, casting an ingot from the molten steel, blooming or continuously casting the
ingot, and then hot-rolling the ingot. Furthermore, a heat treatment is carried out for the
purpose of controlling the metallographic structure in the rail head top section as
5 necessary.
[0059]
(2) The reason for limiting the metallographic structure
The reason for limiting at least one portion of the head surface section of the rail
to the pearlite structure of the bainite structure, in the rail according to the present
10 invention will be described in detail.
[0060]
First, the reason for limiting the structure to the pearlite structure or the bainite
structure will be described.
In the rail head surface section that comes into contact with a wheel, it is most
15 important to ensure wear resistance and rolling fatigue damage resistance. As a result'
of investigating the relationship between the metallographic structure and the
above-described properties, it was confirmed that the properties were most favorable in a
pearlite structure and a bainite structure. Furthermore, regarding delayed fracture
resistance as well, it was confirmed by tests that, when a pearlite structure and a bainite
20 structure are used, the delayed fracture resistance does not degrade. Therefore, the
structure in the head surface section of the rail has been limited to a pearlite structure or a
bainite structure for the purpose of ensuring wear resistance, rolling fatigue damage
resistance and delayed fracture resistance.
[-~ P - ~ ~ p - F006~1] ~ T 7 F ~ ~ ~ g ~ 7-+ x l ; &-P7~- ---~- - - ,--:-
eLdi*sti c ve- e o t p'e i t 6 r u c m f l d d t h T b ~ t ~ ~ t u rise n ot
26
particularly limited, but the pearlite structure is desirable for tracks in which wear
resistance is important, and the bainite structure is desirable for tracks in which rolling
fatigue damage resistance is important. In addition, a mixed structure of both structures
may be used.
[0062]
FIG. 2 illustrates the names of surface locations on a cross section of the head
section of the rail having excellent delayed fracture resistance according to the present
invention and regions in which the pearlite structure or the bainite structure is required.
A rail head section 3 includes a head top section 1 and head comer sections 2 located at
10 both ends of the head top section 1. One of the head comer sections 2 is a gauge comer
(G.C.) section that mainly comes into contact with a wheel.
[0063]
A range from the surfaces of the head corner sections 2 and the head top section
1 as the starting point to a depth of 20 mm is called a head surface section (3a, hatched
15 section). As illustrated in FIG. 2, when the pearlite structure or the bainite structure is
disposed in the head surface section that is the range from the surfaces of the head comer
sections 2 and the head top section 1 as the starting point to a depth of 20 mm, in the rail,
wear resistance and rolling fatigue damage resistance are ensured, and delayed fracture
resistance is improved.
20 [0064]
Therefore, it is desirable to dispose the pearlite structure or the bainite structure
in the head surface section at which the rail mainly comes into contact with a wheel, and
delayed fracture resistance is required. Other sections not requiring the above-described
properties may include metallographic structures other than the pearlite structure and the
[0065]
The hardness of the above-described metallographic structures is not particularly
limited. The hardness is desirably adjusted depending on the conditions of a track to be
constructed. Meanwhile, the hardness Hv is desirably controlled in a range of
5 approximately 300 to 500 in terms of hardness to sufficiently ensure wear resistance or
rolling fatigue damage resistance. A desirable method for obtaining the pearlite
structure or the bainite structure having a hardness Hv in a range of 300 to 500 is that an
appropriate alloy is selected, and accelerated cooling is carried out on a high-temperature
rail head section in which a hot-rolled or reheated austenite region is present. When the
10 method described in Cited Documents 7, 8,9 or the like is used as the method for the
accelerated cooling, it is possible to obtain a predetermined structure and hardness.
[0066]
The metallographic structure of the head surface section of the rail according to
the present invention is desirably made up of the above-limited pearlite structure or
15 bainite structure. However, depending on the component system of the rail or the heat
treatment manufacturing method, there is a case in which an extremely small amount of a
pro-eutectoid ferrite structure, pro-eutectoid cementite structure or martensite structure
that occupies 5% or less of the above-described structures in terms of area ratio is
interfused. However, even when the above-described structure is interfused, there is no
20 large adverse effect on the delayed fiacture resistance of the rail or the wear resistance
and rolling fatigue damage resistance of the head surface section as long as the amount of
the structure is small. Therefore, the metallographic structure of the head surface
section of the rail according to the present invention may include an extremely small
amount, 5% or less, of the pro-eutectoid ferrite structure, the pro-eutectoid cementite
2 8
the head surface section of the rail according to the embodiment may include 95% or
more of the pearlite structure or the bainite structure or a mixed structure of the pearlite
structure and the bainite structure. To ensure delayed fracture resistance, and
sufficiently improve wear resistance or rolling fatigue damage resistance, it is desirable
5 to form 98% or more of the metallographic structure of the head surface section with the
pearlite structure or the bainite structure. Meanwhile, in the microstructure column in
Tables land 2, all described structures other than the pearlite structure or the bainite
structure have an amount of more than 5% in terms of area ratio.
[0067]
10 (3) The reason for limiting the number per unit area of the MnS-based sulfides
formed around an Al-based oxide as a nucleus and having a grain size in a range of 1 pm
to 10 pm
[0068]
In an embodiment of the present invention, the reason for limiting the grain size
15 of the MnS-based sulfide grain formed around an Al-based oxide as a nucleus on an
arbitrary horizontal cross section that is an evaluation subject in the rail in a range of 1
pm to 10 pm will be described in detail.
As a result of a variety of melting tests, when the grain size of the MnS-based
sulfide grain formed around an Al-based oxide as a nucleus exceeds 10 pm, the effect of 1
20 the grain as a hydrogen trap site decreases due to a decrease in the surface area per unit 1
volume. Inaddition, when the grain size of the MnS-based sulfide around an Al-based
oxide as a nucleus lower than the 1 pm, the effect of the grain as a hydrogen trap site
increases, but the amount of the MnS-based sulfide becomes excessive, the
metallographic structure becomes brittle. When the grain size of the MnS-based sulfide I
29
it is possible to ensure the surface area of interfaces between the base metal and
inclusions, the MnS-based sulfides formed around an Al-based oxide as a nucleus
become capable of serving as sufficient hydrogen trap sites. Furthermore, since
inclusions are finely dispersed, it is possible to decrease the amount of hydrogen trapped
5 by the respective inclusions. As a result, the delayed fracture resistance improves.
Therefore, the grain size of the MnS-based sulfide grain formed around an Al-based
oxide as a nucleus has been limited'in a range of 1 pm to 10 p.m.
Meanwhile, the grain size of the MnS-based sulfide grain formed around an
Al-based oxide as a nucleus can be obtained by measuring the cross-sectional area,
10 converting the cross-sectional area to an equivalent circle cross section, and computing
the grain size. Therefore, the results are applied to the above-described limitation.
Next, in the embodiment of the present invention, the reason for limiting the
number of MnS-based sulfides formed around'an Al-based oxide as a nucleus and having
15 a grain size in a range of 1 pm to 10 pm on an arbitrary horizontal cross section of the
rail in a range of 20 to 200 per square millimeter of an area to be inspected will be
described in detail.
[0070]
When the MnS-based sulfides formed around an Al-based oxide as a nucleus
20 and having a grain size in a range of 1 pm to 10 pm is less than 20 per square millimeter
of an area to be inspected, it becomes difficult to ensure the surface area of interfaces
between the base metal and inclusions, and the inclusions do not function as sufficient
hydrogen trap sites. In addition, when the MnS-based sulfides formed around an
Al-based oxide as a nucleus and having a grain size in a range of 1 pm to 10 pm per
30
becomes excessive, the metallographic structure becomes brittle, and rail breakage
becomes likely to occur. Therefore, the MnS-based sulfides formed around an Al-based
oxide as a nucleus and having a grain size in a range of 1 pm to 10 pm per square
millimeter of an area to be inspected has been limited to be in a range of 20 to 200.
[0071]
The above-described MnS-based sulfides formed around an Al-based oxide as a
nucleus refer to an inclusion having an Al-based oxide in the vicinity of the central part
of the MnS-based sulfide grain and an MnS-based sulfide coating the surrounding of the
Al-based oxide. The presence ratio between the Al-based oxide and the MnS-based
10 sulfide is not particularly limited, but the presence ratio of the Al-based oxide is desirably
30% or less in terms of area ratio to ensure the ductility of the inclusion and to suppress
the fracture of the rail.
[0072]
Regarding the Al-based oxide that is a nucleus and the MnS-based sulfide
15 coating the surrounding of the Al-based oxide, the inclusion may include elements other
than the Al-based oxide and the MnS-based sulfide. Other elements may be partially
interfused. To more stably improve the delayed fracture resistance using the MnS-based
sulfides formed around an Al-based oxide as a nucleus and having a grain size in a range
of 1 pm to 10 pm, the area ratio of Al2O3 is desirably 60% or more in the Al-based oxide
20 that is a nucleus, and the area ratio of MnS is desirably 80% or more in the MnS-based
sulfide coating the surrounding of the Al-based oxide.
[0073]
The number of MnS-based sulfides formed around an Al-based oxide as a
nucleus and having a grain size in a range of 1 pm to 10 pm was measured from a sample
Each cut-out sample was mirror-polished, on an arbitrary cross-section, MnS-based
sulfides formed around an Al-based oxide as a nucleus were inspected using an optical
microscope or a scanning microscope, the number of inclusions having the above-limited
size was counted, and the number was converted to the number per unit cross-section.
5 The representative values of individual rails described in examples are the average values
of numbers measured at 20 visual fields.
The determination of the MnS-based sulfide grain formed around an Al-based
oxide as a nucleus was carried out by sampling a typical inclusion in advance, and
10 carrying out an electron probe micro-analysis (EPMA), and then the inclusion was
specified. The differentiation of inclusions was carried out using properties (form or
color) in the optical microscopic or scanning microscopic photographs of the specified
inclusion as basic information.
The measurement location of the MnS-based sulfide grain is not particularly
15 limited, but the MnS-based sulfide grain is desira. b,l..y... measured in a range of 10 mrn to 20
. .
rnm deep from the rail head surface section as illustrated in FIG. 3.
(4) The control method of the Al-based oxide
Regarding the control of the fine Al-based oxide that serves as a nucleus of the
20 MnS-based sulfide grain, a preferable example of a manufacturing method will be
described.
[0076]
A1 is a strong deoxidizing element, and, when A1 particles which is called as
metallic aluminum is added to molten steel, the metallic aluminum reacts with free
ng. -
3 2
and consequently coarsens an Al-based oxide. When a coarsened Al-based oxide is
present, rail breakage becomes likely to occur due to stress concentration. Therefore,
preventing the coarsening of the Al-based oxide is important for improving delayed
fracture resistance.
5 [0077]
A method for preventing the coarsening of the Al-based oxide can be
appropriately selected. For example, it is possible to preliminarily deoxidize molten
steel in advance using an element having a stronger oxidizing force than A1 (Mg or REM),
decrease the oxygen amount as much as possible so as to decrease the A1 content to the
10 necessary minimum content, and refine the Al-based oxide. In addition, on the contrary
to the above-described method, for example, it is also possible to inject a batch of a
necessary amount of A1 for deoxidation in a state in which a large amount of free oxygen
is contained in molten steel without carrying out preliminary deoxidation, accelerate the
formation or levitation of coarse N2O3 clusters, and use the residual fine Al-based oxide.
15 In addition, for the purpose of controlling the formation of a coarse Al-based oxide
through reoxidation from slag, it is also possible to intensify slag ejection in addition to
the above-described deoxidation control.
[0078]
As a method of removing the coarsened Al-based oxide, to levitate the Al-based
20 oxide, it is preferable to apply blowing of Ar in a ladle after refining, blowing of fine air
bubbles in a tundish before casting or the like. In addition, for the purpose of
suppressing the agglomeration of the Al-based oxide or accelerating the levitation of the
coarse Al-based oxide during casting, it is possible to apply electromagnetic stirring in a
tundish.
3 3
In addition to the above-described control in molten steel, a strong rolling
reduction may be added to solid-phase steel in which the MnS-based sulfide is yet to be
formed through hot-rolling. The strong rolling reduction during hot-rolling can finely
crush the coarsened Al-based oxide. When the Al-based oxide is finely crushed, the
5 MnS-based sulfides are also dispersedly formed, and the delayed fracture resistance
further improves.
[OOSO]
(5) The method of controlling the S content
Regarding the method of controlling the S content for controlling the number of
10 fine MnS-based sulfides, a preferable example of a manufacturing method will be
described.
A large amount of S is contained as an impurity in a molten iron. It is normal
to control the S content in a converter. In a converter, CaO is added, and S is ejected
into slag in a form of CaS. When refining is carried out in an ordinary converter, the S
15 content is reduced to 0.0030% to 0.0300%. When the S content is controlled to more
than 0.0100% to 0.0250% by controlling the desulfurization treatment time or the CaO
content in the converter, and the number of the MnS-based sulfides formed around an
Al-based oxide as a nucleus and having a grain size in a range of 1 pm to 10 pm is
increased, it is possible to improve the delayed fracture resistance.
2 0 [OOSl]
(6) The method of controlling the H content
Regarding the control of the H content further improving the delayed fracture
resistance, a preferable example of a manufacturing method will be described.
H is contained in a molten iron as an impurity. It is normal to control the H
3 4
refining, a ladle is put into a vacuum state, and H in steel is exhausted. The H content
can be controlled to 2.0 ppm or less by controlling the treatment time during the
secondary refining, and it may further improve the delayed fracture resistance.
[0082]
Hydrogen intrudes from the atmosphere after the above-described refining, and
there is a case in which the amount of hydrogen in a bloom after casting is increased. In
such a case, it may apply a method in which the bloom is cooled slowly or reheated,
thereby diffusing hydrogen inside the bloom outside.
[Example]
[0083]
Next, examples of the present invention will be described.
Table 1 describes the chemical components and various properties of Invention
Rails. Table 1 describes the chemical component values, the microstructures of the
head surface sections, the hardness of the head surface sections and the number of the
15 MnS sulfide-based inclusions formed around an Al-based oxide as a nucleus and having a
grain size in a range of 1 pm to 10 pm. Furthermore, Table 1 also describes the results
of the delayed fracture tests (limit stress values) carried out using a method illustrated in
FIG. 6. The microstructures of the head surface sections include microstructures into
which a small amount, 5% or less in terms of area ratio, of a pro-eutectoid ferrite
20 structure, pro-eutectoid cementite structure or martensite structure is interfused.
[0084]
Table 2 describes the chemical components and various properties of
Comparative Rails. Table 2 describes the chemical component values, the
microstructures of the head surface sections, the hardness of the head surface sections
.. . . . . . - .
35
nucleus and having a grain size in a range of 1 pm to 10 p.m. Furthermore, Table 2 also
describe the results of the delayed fracture tests (limit stress values) carried out using a
method illustrated in FIG. 6. In the microstructures of the head surface sections in Table
2, regarding Comparative Examples into which more than 5% in terms of area ratio of a
5 pro-eutectoid ferrite structure, pro-eutectoid cementite structure or martensite structure is
, interfused, the pro-eutectoid ferrite structure, pro-eutectoid cementite structure or
martensite is also described in the column of the microstructure of the head surface
section.
[0085]
The manufacturing conditions of Invention Rails and Comparative Rails
described in Tables 1 and 2 are as described below.
Molten steel -+ component adjustment (converter and secondary refining:
degassing) -+ casting (bloom) + reheating (1250°C) -+ hot rolling (finishing
temperature 950°C) -, heat treatment (initial temperature 800°C, accelerated cooling) +
15 air cooling

39
[OOSS]

The method of determining the amount of hydrogen for Invention Rails and
Comparative Rails described in Tables 1 and 2 is as described below.
5 (1) Analysis step: molten steel was sampled from the inside of a mold during the
casting of a bloom.
(2) Sample holding method: after sampling, the sample was rapidly cooled +
immersed in liquid nitrogen.
(3) Analysis method: thermal conductivity method
Sample size: a cylinder with a diameter of 6 mrn and a thickness of 1 mm
Heating temperature: 1900°C (the sample was induction-heated on a graphite
crucible)
Atmosphere: inert gas (Ar)
Carrier gas: N2
Analyzer: thermal conductivity detector
[0089]

The microstructures of the head surface section and hardness of the head surface
section of Invention Rails and Comparative Rails described in Tables 1 and 2 were
20 determined by observing structures at a location 3 mm deep from the surface of the rail
head surface section. In addition, the hardness was measured using a Vickers hardness
meter. The measurement method is as described below.
(1) Preliminary treatment: after the cutting of the rail, a horizontal cross section
was polished.
40
(3) Measurement device: Vickers hardness meter (load 98 N)
(4) Measurement location: a location 3 rnm deep from the surface of the rail
head surface section
(5) Number of measurements: measurements were carried out at 5 or more
5 points, and the average value was considered to be the representative value of the rail.
[0090]

The MnS-based sulfides formed around an Al-based oxide as a nucleus in
10 Invention Rails and Comparative Rails described in Tables 1 and 2 were measured at a
location 10 rnm to 20 rnrn deep fi-om the surface of the rail head surface section as
illustrated in FIG. 3. The measurement method is as described below.
(1) Preliminary treatment: cutting of the rail + polishing of a horizontal cross
section
15 (2) Measurement method: MnS-based sulfides formed around an Al-based oxide
as a nucleus were inspected using an optical microscope or a scanning microscope, the
number of inclusions having the above-limited size is counted, the number was converted
to the number per unit cross-section, and the average values of numbers, which are
measured at 20 visual field, was considered to be the representative value.
(3) Preliminary measurement: a typical inclusion was sampled, an electron probe
micro-analysis (EPMA) was carried out, and an inclusion was specified. The
differentiation of inclusions was carried out using properties (form or color) in the optical
microscopic photographs of the specified inclusion as basic information during the
optical microscopic or scanning microscopic observation.
4 1

The conditions for the delayed fracture test of Invention Rails and Comparative
Rails described in Tables 1 and 2 are as described below.
(1) Rail shape: 136 pound rail (67 kglm)
(2) Delayed fracture test
Test method: three-point bending (span length: 1.5 my refer to FIG. 6)
Test position: a load was applied to the rail bottom section (tensile stress acts on
the head section).
Stress conditions: 200 MPa to 500 MPa (on the surface of the rail head section)
Stress application time: 500 hours
(3) Limit stress value: the maximum value of the stress in a case in which the
steel piece was not broken when a predetermined stress had been applied over 500 hours.
[0092]
Details of Invention Rails and Comparative Rails described in Tables 1 and 2 are
15 as described below.
(1) Invention Rails (49 pieces)
Reference signs A1 to A49: rails having a chemical component value, a
microstructure of the head surface section, hardness of the head surface section, and the
number of Mn sulfide-based inclusions formed around an Al-based oxide as a nucleus
20 and having a grain size in a range of 1 pm to 10 pm within the range of the present
invention
(2) Comparative Rails (20 pieces)
Reference signs a1 to a6 (6 pieces): rails having C, Si and Mn addition amounts
or a microstructure of the head surface section outside the range of the present invention
42
Mn sulfide-based inclusions formed around an Al-based oxide as a nucleus and having a
grain size in a range of 1 pm to 10 pm outside the range of the present invention
[0093]
As described in Tables 1 and 2, compared with Comparative Rails (reference
5 signs a1 to a6), Invention Rails (reference signs A1 to A49) have C, Si and Mn addition
amounts of steel converged within the limited ranges, and therefore the formation of a
pro-eutectoid ferrite structure, pro-eutectoid cementite structure or martensite structure is
suppressed, and it is possible to control the head surface section to include a pearlite
structure or a bainite structure, thereby improving the delayed fracture resistance.
[0094]
In addition, as described in Tables 1 and 2 and furthermore illustrated in FIG. 4,
compared with Comparative Rails (reference signs a7 to a20), Invention Rails (reference
signs A1 to A49) have A1 addition amount of steel converged within the limited range in
addition to the C, Si, and Mn addition amount, it is possible to suppress the number of
15 Mn sulfide-based inclusions formed around an Al-based oxide as a nucleus and having a
grain size in a range of 1 pm to 10 pm and to improve the delayed fracture resistance.
[0095]
In addition, as described in Tables land 2 and furthermore illustrated in FIG. 5,
when Invention Rails (reference signs A10 to A1 5, A1 8 to A23, A27 to A29, A3 1 to A44,
20 and A46 to A49) are compared from the viewpoint of the S content and the H content, it
is possible to further improve the delayed fracture resistance with the same number of
MnS-based sulfides formed around an Al-based oxide as a nucleus by controlling the S
content so as to suppress the number of MnS-based sulfides formed around an Al-based
oxide as a nucleus and having a grain size in a range of 1 pm to 10 pm, and furthermore,
43
[Description of the Reference Symbols]
[0096]
1 : HEAD TOP SECTION
2: HEAD CORNER SECTION
5 3: RAIL HEAD SECTION
3a: HEAD SURFACE SECTION (RANGE FROM THE SURFACES OF
HEAD CORNER SECTIONS AND HEAD TOP SECTION AS STARTING POINT TO
A DEPTH OF 20 mm, HATCHED SECTION)

Claims
[Claim 11
A rail comprising, in a steel rail, by mass%:
C: 0.70% to 1.20%;
5 Si: 0.05% to 2.00%;
Mn: 0.10% to 2.00%;
Al: 0.0020% to 0.0100%, and
a balance consisting of Fe and'impurities,
wherein 95% or more of a head surface section, which is a range from surfaces
10 of head comer sections and a head top section of the steel rail as a starting point to a
depth of 20 mm, is a pearlite or a bainite structure, and
the structure contains 20 to 200 MnS-based sulfides formed around an Al-based
oxide as a nucleus and having a grain size in a range of 1 pm to 10 pm per square
millimeter of an area to be inspected on an arbitrary horizontal cross section of the
15 structure.
[Claim 21
The rail according to Claim 1, further comprising, by mass%,
S: 0.0100% to 0.0250%.
[Claim 31
2 0 The rail according to Claims 2, further comprising, by mass%,
H: 2.0 ppm or less.
[Claim 41
The rail according to any one of Claims 1 to 3, further comprising, by mass%,
one or two or more of: