[Technical Field]
[OOOl]
5 The present invention relates to a carbon steel sheet in which an amount of C is
more than 0.25% and less than 0.50% in terms of mass%, and particularly relatcs to the
carbon steel sheet to be shaped by punching, hole expanding, forging, or the like.
Priority is claimed on Japanese Patent Application No. 2013-092408, filed
April 25,2013, the content of which is incorporated herein by reference.
10 [Background Art]
[0002]
When a mechanical component having a complex shape is manufactured
conventionally, in many cases, each of a plurality of components is first manufactured
individually, and then, they are combined to obtain the shape of the product. In this
15 case, parts having a complex shape such as a gear are often cut before being combined.
On the other hand, in recent years, in order to reduce manufacturing costs, forming
components having a shape similar to that of product by punching, hole expanding,
forging, or the like is promoted. As a result, a number of the components can be
I
I reduced and manufacturing can be performed with fewer processes. When a large
I
I
20 deformation is applied, a hot working in which deformation resistance is low is
employed, and when it is necessary to work with good accuracy of shape, a cold
working is employed. If the steel sheet is worked to be a complcx shape similar to that
of the product, the steel sheet needs a workability better than in thc conventional case in
which cach of a plurality of parts is manufactured, and then, they are combined. That
is, in a conventional steel sheet, if the steel shcct is punched, expanded, or forged so as
to be a complex shape, the steel sheet may become cracked or the dimensional accuracy
of the product may be deteriorated. In addition, of course, the product after working
I may require properties such as toughness, strength, wear resistance equal to or more
5 than the conventional art. In order to solve the problems, Patent Documents 1 to 3
propose techniques as follows.
[0003]
Patent Document 1 proposes a steel reclining seat gear of which a raw material
is a steel sheet excellent in notched tensile elongation ratio, in which C: 0.15% to 0.50%
10 and S: 0.01% or less in telms of mass%, and a relationship WP] 5 6 x WB] + 0.005 is
satisfied. Patent Document 1 focuses on a strong correlation between punchability and
the notched tensile elongation ratio, and proposes that the notched tensile elongation
ratio and the punchability can be enhanced by increasing a grain size of a carbide
dispersed in the steel sheet.
15 [0004]
Patent Document 2 proposes a high carbon steel which includes C: 0.70% to
1.20% in terms of mass%, and in which a grain size of carbide dispersed in ferrite
matrix is controlled. Since the notched tensile elongation ratio of the steel, which has
a close relationship with the punchability, is enhanced, the steel is excellent in
20 punchability. In addition, since a configuration of MnS is controlled by further
i including Ca in the steel, the pnnchability of the steel is further enhanced.
I [0005]
Patent Document 3 proposes a steel for gear excellent in cold forgeability,
which includes C: 0.10% to 0.40% and S: 0.010% or less in terms of mass%, in which
shape of the inclusion is categorized in accordance with ASTM-D method, and in which
the shape and the number of the inclusions are set within a range.
[0006]
In addition, in order to control an amount and/or a configuration of inclusions
5 in the steel, Ca and/or REM (Rare Earth Metal) has been added. The inventors have
proposed a technique in which Ca and REM were added to a thick steel plate for
structure including 0.08% to 0.22% of C in terms of mass% to control oxide (inclusion)
formed in the steel as a mixture phase state of high-melting phase and low-mclting
phase for preventing the oxide (inclusion) from elongation during rolling and for
10 preventing erosion of a continuous-casting nozzle and an internal inclusion defect from
occurring.
[Prior Art Documents]
[Patent Documents]
[0007]
15 [Patent Document 11 Japanese unexamined patent application, First Publication
No. 2000-265238
[Patent Document 21 Japanese unexarnined patent application, First Publication
No. 2000-265239
[Patent Document 31 Japanese unexamined patent application, First Publication
20 No. 2001-329339 ~ [Patent Document 41 Japanese unexamined patent application, First Publication
I No. 2011-68949
[Summary of Invention]
[Technical Problem]
I [OOOS]
The above-described four documents identify the cause of a starting point of
cracking which deteriorates workability, specifically punchability and forgeability, and
propose a countermeasure thereon. Patent Document 1 recognizes that micro voids
5 grown from carbide is the starting point of cracking and intends to increase a grain size
I I of the carbide to prevent the micro void from joining. Similar to that idea, Patent
1
Document 2 proposes increasing a grain size of the carbide. In addition, Patent
Document 2 focuses on that MnS in the steel sheet (elongated during rolling) acts as the
starting point of cracking, and proposes including Ca to prevent MnS in the steel from
10 forming. Patent Document 3 recognizes that an elongated oxide type inclusion (B-type
of the ASTM-D method) and a non-clongated oxide type inclusion (D-type of ASTM-D
method) cause deterioration of the forgeability, and defines the size, the length, and the
total number thereof in accordance with the categorization of ASTM-D method.
15 However, in thc above-described prior art, problems regarding workability and
toughness of the product after working remain as follows.
[OO lo]
In the steel described in Patent Document 1, although the punchability is
enhanced by controlling the grain size of the carbide, the composition or configuration
20 of the inclusions are not controlled, and thus, MnS elongated during rolling the steel
remains in the steel. Therefore, cracking occurs in the steel during worlting under a
severe worlting condition so as to be a more complex shape, in which the elongated
MnS (which is categorized as an A-type inclusion, since the MnS is elongated in a
working direction) acts as the starting point. Even if manufacturing is terminated
without causing cracking, the elongated MnS remaining in the product deteriorates the
toughness of the product after working.
[OOll]
In the steel described in Patent Document 2, including Ca causes spheroidizing
5 of the shape of MnS, and thus, the number of the A-type inclusion decreases. On the
other hand, the inventors found that, in the steel described in Patent Document 2,
although A-type inclusions decreased, a granular inclusions discontinuously forming a
line along with the worlting direction in a group (hereinafter B-type inclusions) and
inclusions that are unevenly dispersed (hereinafter C-type inclusions) remain in the steel
10 in a large number. In addition, it was found that the inclusions acted as the starting
points of fractures which deteriorate the workability and the toughness of the product.
Moreover, the steel described in Patent Document 2 includes Ti. However, there is a
1 problem that, if a coarse carbonitride including Ti (categorized as C-type inclusion)
forms independently in the steel, the carbonitride including Ti acts as the starting point
15 of fracture, and thus, the workability and the toughness tend to deteriorate.
[0012]
Although Patent Document 3 defines the size, the lenglh, and the total number
of the elongated oxide type inclusions and the non-elongated oxide type inclusions,
Patent Document 3 discloses no specific method to archive the definition.
20 [0013]
In Patent Document 4, the number density of the inclusions is controlled by
adding Ca and/or REM. However, the amount of C of the steel described in Patent
Document 4 is 0.08 mass% to 0.22 mass%, and thus, sufficient strength (tensile strength,
wear resistance, hardness, and the like) may not be obtained if the steel is used as a raw
material for machine structural component having a complex shape. Patent Document
4 does not disclose a method for controlling the number density of the inclusion in the
steel for which it is necessary to include more than 0.25 mass% of C.
[0014]
5 The present invention is invented in view of the above-described problem, and
has an object to provide a carbon steel sheet including more than 0.25% and less than
0.50% of C in terms of mass% and having a workability suitable for manufacturing a
product having a complex shape such as a gear.
[Method for Solving the Problem]
10 [0015]
The present invention focuses on A-typc inclusions, B-type inclusions, and
C-type inclusions as the main starting points of fractnre, deteriorating properties such as
workability of the steel sheet, the toughness of the product, and the like. A steel sheet
excellent in workability is provided by decreasing the amount of each of the A-typc
15 inclusions, the B-type inclusions, and the C-type inclusions. A product manufactured
by the steel sheet according to the present invention, in which the number of the
inclusions acting as the starting point of cracking is small, has high toughness
Therefore, reducing inclusions can enhance the workability of the steel sheet and the
toughness of the product (manufactured with the steel using as raw material).
20 The gist of the invention is as follows.
[0016]
(1) 111 a steel sheet according to one embodiment of the present invention, a
chemical con~positionc omprises, by mass%: C: more than 0.25% and less than 0.50%;
Si: 0.10% to 0.60%; Mn: 0.40% to 0.90%; Al: 0.003% to 0.070%; Ca: 0.0005% to
!
I 0.0040%; REM: 0.0003% to 0.0050%; Cu: 0% to 0.05%; Nb: 0% to 0.05%; V: 0% to
0.05%; Mo: 0% to 0.05%; Ni: 0% to 0.05%; Cr: 0% to 0.50%; B: 0% to 0.0050%; P:
limited to 0.020% or less; S: limited to 0.0070% or less; Ti: limited to 0.050% or less;
I 0: limited to 0.0040% or less; N: limited to 0.0075% or less; and remainder including
I
I 1 5 iron and impurity, amounts of each elements by mass% in the chemical composition
I I
satisfy both of expression 1 and expression 2, a number density of carbonitrides
including Ti which exists indcpendently and has a long side ol: 5pm or more is limited to
5 piecesimm2 or less,
0.3000 5 { Ca I 40.88 + ( REM / 140 ) 12 ) I ( S i 32.07 ) : expression 1,
10 and
Ca 5 0.0058 - 0.0050 x C : expression 2.
(2) In the steel sheet according to the above-described (I), the chemical
composition may further comprise one or more of, by mass%: Cu: 0.01% to 0.05%; Nb:
0.01% to 0.05%; V: 0.01% to 0.05%; Mo: 0.01% to 0.05%; Ni: 0.01% to 0.05%; Cr:
15 0.01% to 0.50%; and B: 0.0010% to 0.0050%.
(3) In the steel sheet according to the above-described (1) or (2), the steel sheet
may further include a composite inclusion which includes Al, Ca, 0 , S, and REM, and
an inclusion in which the carbonitride including Ti is adhered on the composite
inclusion.
20 (4) In the steel sheet according to the above-described (I) or (2), the amounts
of the each elements by mass% in the chemical composition may satisfy expression 3,
I ~ 18 x (REMi140) - 0116 ? 0 : expression 3.
(5) In the steel sheet according to the above-described (3), the amounts of the
each elements by mass% in the chemical composition satisfy expression 4,
18 x (REMl140) - 011 6 2 0 : expression 4.
[Effect of the invention]
[0017]
I
According to the above-described embodiments of the present invention, a steel
!
!
5 sheet excellent in punchability, hole expansibility, forgeability, and the like and in
toughness after working can be provided by reducing a number density of A-type
inclusions, a number density of B-type inclusions, a number density of C-type
inclusions, and a number density of coarse carbonitrides including Ti, which has angnlar
shape and is present independently, in the steel.
10 [Brief Description of the Drawing]
[Figure 11 A graph indicating a relationship between a total chemical equivalent
of Ca and REM combining with S and number density ofA-type inclusions.
[Figure 21 A graph indicating a relationship between an amount of Ca in a steel
15 and the total number density of B-type inclusions and C-type inclusions.
[Figure 31 A graph indicating a relationship between an amount of C in a steel
and tensile strength of the steel.
[Embodiments of the Invention]
[0019]
20 Hereinafter, a preferable embodiment of the present invention will be described.
IIowever, the present invention is not limited to the construction disclosed in the present
'~ embodiment. Various modifications can be made on the present invention without
departing from the spirit or scope of the present invention.
At first, inclusions included in the steel according to the present invention will
be described.
[0021]
Decreasing worlcability of the steel sheet is caused by non-metallic inclusions,
5 carbonitrides, and the like. If stress is applied to the steel sheet, they act as starting
points of cracking of the steel sheet. The inclusions are oxides, sulfides, or the like
which exist in a molten metabor forms during solidification of the molten metal. The
size of the inclusions (long side) is from several micrometers to several hundred
micrometers if it is elongated by rolling. Therefore, in order to enhance the
10 workability of the steel sheet, it is important to decrease the number of inclusions. As
described above, a state in which the size as well as the number of the inclusions in the
steel sheet is small, i.e. a state in which "cleanliness of the steel is high" is preferred.
[0022]
Although the shape, the distribution state, and the like of the inclusions are
15 various, in JIS G 0555, the inclusions are distinguished as A-type inclusions, B-type
inclusions, and C-type inclusions. Hereinafter, in the present embodiment, inclusions
are categorized as three types in accordance with the definition described below.
A-type inclusion: non-metallic inclusions in the steel, which are plastically
delormed by working. It has high elongation and is frequently elongated along to a
20 working direction in the worked steel sheet. In the present embodiment, inclusions of
j which an aspect ratio (size in long axis / size in short axis) is 3.0 or more are defined as
i the A-type inclusions.
B-type inc1usion:non-metallic inclusions in the steel which are granular
inclusions discontinuously forming a line along with the working direction in a group.
It frequently has an angular shape and has low elongation. In the present embodiment,
inclusions which form inclusion groups in which three or more of the inclusions form a
line along to the working direction, in which clearance between the inclusions is 50pm
or less, and in which the aspect ratio (size in long axis I size in short axis) of the
5 inclusions are less than 3.0 is defined as the B-type inclusion.
C-type inc1usion:inclusions unevenly dispersing without plastic deformation.
The C-type inclusions frequently have angular shapes or spheroidal shapes and have
low elongation. In the present embodiment, inclusions of which an aspect ratio (size
in long axis I size in short axis) is 3.0 or less, and which disperse in a random manner
10 are defined as the C-type inclusion.
Although the carbonitride including Ti which is very hard and which has an
angular shape is categorized by the C-type inclusions in general, the carbonitride
including Ti may be distinguished from the C-type inclusions in the present
embodiment. If the carbonitride including Ti exists independently, the influence of the
15 carbonitride including Ti over the preference of the steel sheet is larger than that of the
other C-type inclusions (C-type inclusions not being the carbonitride including Ti).
"Carbonitride including Ti existing independently" is a carbonitride including Ti which
exists in a state in which the carbonitride including Ti does not adhere to inclusions not
including Ti. On the other hand, if the carbonitride including Ti exists in a state in
20 which the carbonitride including Ti adheres to other inclusion (for example, composite
inclusions including Al, Ca, 0, S, and REM), the influence of the carbonitride including
Ti over the preference of the steel sheet is substantially the same as that of the other
C-type inclusions. In the present embodiment, the carbonitride including Ti adhering
to the other inclusions is assumed as the C-type inclusions not being carbonitride
1 including Ti. I
I In the present embodiment, "number density of C-type inclusions" is a total ol
"number density of the C-type inclusions which is not carbonitrides including Ti
(including the carbonitrides including Ti adhering to the C-type inclusions)" and
I 5 "number density of the carbonitrides including Ti existing independently". The
carbonitrides including Ti can be distinguished tkom the other C-type inclusions based
on the shape and the color thereof.
[0023]
In the steel sheet according to the present embodiment, only inclusions having
10 1pm or more of grain size (in a case of inclusions having substantially spheroidal shape)
or 1 pm or more of size in long axis (in a case of deformed inclusions) are taken into
account. Even ilinclusions having a grain size or a size in long axis of less than 1 pm
is included in the steel, the influcncc thereof over the workability of the steel is small,
and therefore, such inclusions are not talten into account in the present embodiment.
15 In addition, the long axis described above is defined as a longest line in lines connecting
nonadjacent vertexes of out!ine form of cross section in the observed section of the
inclusions. In a similar way, the size in short axis described above is defined as a
shortest line in the lines connecting the nonadjacent vertexes of the outline lorm of the
cross section in the observed section of thc inclusions. In addition, a long side
20 described below is defined as a longest line in lines connecting adjacent vertexes of the
outline form of the cross section in the observed section of the inclusions. Hereinafter,
"grain size (in a case of inclusions having substantially spheroidal shape) or size in long
axis (in a case of deformed inclusions)" may be abbreviated as "grain size or size in
long axis"
[0024] i
Conventionally, in order to control the number of inclusions in the steel andlor
a configuration of the inclusions, Ca andlor REM (Rare Earth Metal) has been added
1
therein. As described above, the inventors have proposed a technique in Patent
1
I
i 5 Document 4, in which Ca and REM is added to a structural thick steel plate including
0.08% to 0.22% of C in terms of mass% to control oxides (inclusions) formed in the
steel so as to be a mixed phase of a high-melting phase and a low-melting phase, and
thus, the oxides (inclusions) is prevented from elongating during rolling and an erosion
of a continuous-casting nozzle and an internal inclusion defect are prevented from
10 occurring.
[0025]
In addition, the inventors have studied a condition rcgarding a steel including
more than 0.25% and less than 0.50% of C in terms of mass%, which could reduce the
above-described A-type inclusions, B-type inclusions, and C-type inclusions by
15 including Ca and REM. Consequently, a condition which could concurrently reduce
the A-type inclusions, and the B-type inclusions and the C-type inclusions has been
founded. The concrete content thereof is described as follows.
[0026]
(Regarding A-type inclusion)
20 The inventors studied about further adding Ca and REM for the steel including
more than 0.25% and less than 0.50% of C in terms of mass%. Consequently, it was
found that when an amount of each elements in the chemical composition in terms of
mass% satisfied below Expression I, the A-type inclusions in the steel, particularly MnS
constructing the A-type inclusions, could be significantly reduced.
[0027]
0.3000 5 { Ca 140.88 + ( REM / 140 ) 12) / ( S / 32.07 ) :Expression I
[0028]
I An experiment on which the finding was based is described as follows. ~ 5 In a vacuum melting furnace, multiple types of steels including chemical
compositions in which an amount of C was 0.45% in terms of mass% and the amounts
of total 0 (T.O.), N, S, Ca, and REM were varied within ranges disclosed in Table 1
were manufactured as 50kg ingots. These ingots were hot-rolled under a condition in
which a finish rolling temperature was 860°C and were air-cooled to obtain hot-rolled
10 steels.
[0029]
The inclusions in the hot-rolled steel sheets were observed by optical
microscope at 400-fold magnification (if shapes of the inclusions were measured in
detail, observed at 1000-fold magnification) in 60 view fields in total, in which
15 observed sections were cross-sections parallel to rolling direction and plate thickness
direction of the hot-rolled steel sheets. In each of the view fields, inclusions whose
grain size were 1pm or more (if a shape of the inclusions were spherical) or inclusions
whose long axis were 1 pm or more (if shapes of the inclusions were deformed) were
observed to categorize the inclusions as the A-type inclusions, the B-type inclusions and
20 the C-type inclusions, and number densities thereof were measured. In addition, the
number density of carbonitrides including Ti existing independently and having an
angular shape, among the C-type inclusions, was measured. Moreover, the
carbonitrides including Ti, composite inclusions including REM, MnS, Ca-A1203 type
inclusions, and the like can be identified by observing stlucture of the hot-rolled steel
sheet using EPMA (Electron Probe Micro Analysis) or SEM (Scanning Electron
Microscope) having EDX (Energy Dispersive X-Ray Analysis).
[0030]
1
I
Furthermore, as an index of workability of the hot-rolled steel sheets obtained
i 1 5 as described above, a charpy impact value at room temperature (about 25°C) was
measured. The charpy impact value is a value indicating the toughness of the steel
sheet. The more the inclusions there are, which act as a starting point of cracking or
the larger the sizes of the inclusions are, the lower the charpy impact value is.
Therefore, there is a strong correlation between the charpy impact value and the
10 worltability. When various worlts are performed, although a value of a limit strain
which causes cracking varies depending on each methods of the working, the value of a
limit strain has a correlation with the charpy impact value.
The results of the above-described experiment showed that there was a
15 correlation between the charpy impact value and the number density of the inclusions.
Specifically, it became clear that if a number density of the A-type inclusions in the steel
was more than 6 pieces!mm2, the charpy impact value was greatly deteriorated. In
addition, it became clear that more than 6 pieceslmm2 of a total number density of the
B-type inclusions and the C-type inclusions violently deteriorated the charpy impact
20 value. Furthermore, regarding the carbonitrides including Ti which are the C-type
inclusions, it became clear that if a number density of the coarse carbonitrides including
,
I Ti, which existed independently and which had 5pm or more of long side, was more
than 5 pieces!rnn~~,thec harpy impact value was greatly deteriorated.
I [Table 11
i [0033]
Next, the inventors studied a specific method for archiving the number density
of the inclusions as described above.
5 In steel, it is assumed that Ca combines with S to form CaS, and REM
i
I combines with S and 0 to form REM202S (oxysulfide). R1, which is a total chemical
equivalent of Ca and REM combining with S, can be expressed as
R1= {Ca/40.88+(REM/ 140)12} /(S/32.07)
in which an atomic weight of S is assumed as 32.07, an atomic weight of Ca is assumed
10 as 40.88, an atomic weight of REM is assumed as 140, and an amount of each elements
in a chemical composition in terms of mass% is used.
[0034]
Thus, a relationship between the number densities of the A-type inclusions
measured in the above-described hot-rolled steel sheets and the above-described R1 of
15 the each hot-rolled steels was examined. The results are shown in Figure 1. In the
Figure 1, a circular symbo1,represents a result ofa steel including a chemical
composition which includes Ca and does not include REM (hereinafter, referred as
!;
"single incorporation of Ca") and a quadrangular symbol (described as "REM + Ca" in
the Figure 1) represents a result of a steel including a chemical composition which
20 includes both of Ca and REM (hereinafter, referred as "compositely incorporation of
E M and Can). In a case of the single incorporation of Ca, the amount of REM was
j
I assumed as 0 to calculate the above-described R1. From the figure 1, it became clear
that in both case of the single incorporation of Ca and the compositely incorporation of
REM and Ca, there was a correlation between the number density of the A-type
inclusions and the above-described R1.
[0035]
Specifically, when the value of the above-described Rl is 0.3000 or more, the
number density of the A-type inclusions decreases to be 6 pieces/mm2 or less.
5 Consequently, a charpy impact value enhances.
A size in long axis of the A-type inclusion in the steel in the case of the single
incorporation of Ca is longer than that in the case of the compositely incorporation of
REM and Ca. It is assumed that, in the case of the single incorporation of Ca, CaO -
A1203 type low-melting oxide forms as the A-type inclusion and the oxide is elongated
10 during rolling. Therefore, in view of the size in long axis of the inclusions which has
an adverse effect on characteristics of the steel sheet, the compositely incorporation of
REM and Ca is more desirable than the single incorporation of Ca.
[0036]
Consequently, it was found that, when the above-described expression I was
15 satisfied and the REM and Ca were compositely included, the number density of the
A-type inclusions in the steel preferably decreased to 6 pieces/mm2 or less.
When the value of Rl is 1.000, as an average composition, 1 equivalence of Ca
and REM combining with S in the steel are exist in the steel. However, in practice,
even if the value of R1 is 1.000, MnS may form at micro segregation portion between
20 dendrite branches. When the value of R1 is 2.000 or more, forming MnS at the micro
segregation portion between the dendrite branches can be preferably prevented fiom
causing. On the other hand, if a large amount of Ca and REM are included and the
value of Rl is more than 5.000, coarse B-type inclusions or coarse C-type inclusions
having more than 20pm of maximum length tend to form. Therefore, it is preferable
I
that the value of R1 is 5.000 or less. That is, it is preferable that an upper limit of the
right side of the above-described expression I is 5.000.
[0037]
(Regarding B-type inclusion and C-type inclusion)
i
I
i
j 5 As described above, the number density of the B-type inclusions and C-type
inclusions having less than 3 of the aspect ratio (size in long axis / size in short axis)
and having 1 pm or more of the grain size or the size in long axis was measured by
observing the above-described observing surface of the hot-rolled steel sheet. As a
result, the inventors found that, in both cases of the single incorporation of Ca and the
10 compositely incorporation of REM and Ca, the greater the amount of Ca was, the larger
the number density of the B-type inclusions and C-type inclusions was. On the other
hand, the inventors found that the amount of REM did not strongly effect on the number
density of the inclusions.
[0038]
15 Figure 2 shows a relationship between the amount of Ca in the steel and the
total number density of the B-type inclusions and the C-type inclusions in both cases of
the single incorporation of Ca and the compositely incorporation of REM and Ca. In
the figure 2, the circular symbol shows the result in the single incorporation of Ca, and
the quadrangular symbol (which is illustrated as "REM + Ca" in the figure 2) shows the
20 result in the compositely incorporation of REM and Ca. From the figure 2, it became
clear that, in both cases of the single incorporation of Ca and the compositely
incorporation of REM and Ca, the greater the amount of Ca in the stcel was, the greater
the total numbcr density of the B-type inclusions and the C-type inclusions was. In
addition, when the anlount of Ca in the case of the single incorporation of Ca and the
amount of Ca in the case of the compositely incorporation of REM and Ca were equal,
the total number densities of the B-type inclusions and the C-type inclusions thereof
were substantially equal. That is, it was found that, if REM and Ca were compositely
included in the steel, REM did not affect the total number density of the B-type
5 inclusions and the C-type inclusions.
[0039]
As described above, in order to decrease the A-type inclusions, it is preferable
to increase the amount of Ca and the amount of REM in the steel within the
above-described range. On the other hand, if the amount of Ca is increased to reduce
10 the A-type inclusions, as described above, a problem of increasing the B-type inclusions
and the C-type inclusions is caused. That is, in the case of the single incorporation of
Ca, it is not possible to concurrently reduce the A-type inclusions, and the B-type
inclusions and the C-type inclusions. On the other hand, in the case of the compositely
incorporation of REM and Ca, the amount of Ca can be reduced while the chemical
15 equivalent (the value of R1) of REM and Ca combining with S is secured, and thus, the
case is preferable. That is, it was found that, in the case of the compositely
incorporation of WM and Ca, the number density ofthe A-type inclusions could be
preferably decreased without increasing the total number density of the B-type
inclusions and the C-type inclusions.
20 [0040]
I It is assumed that the reason why the total number density of the B-type
i !
i inclusions and the C-type inclusions depends on the amount of Ca is as follows.
;
[0041]
As described above, in the case of the single incorporation of Ca, Ca - AI2O3
type inclusions form in the steel. The inclusions are low-melting oxides, and thus the
inclusions are liquid phase in molten stcel and tend not to aggregate and unite in the
molten steel. That is, it is difficult to flotation-separate the Ca - A1203 type inclusions
from the molten steel. Therefore, a large amount of the inclusions having a size of
5 several micrometers disperse and remain in the slab, and thus, the total number density
of the B-type inclusions and the C-type inclusions increases.
[0042]
In addition, as described above, in the case of the compositely incorporation of
REM and Ca, the total number density of the B-type inclusions and the C-type
10 inclusions increases depend on the amount of Ca in a same manner. A melting point of
an inclusion, of which REM content is large, is higher than the melting point of the Ca -
AI2O3 type inclusion, and the inclusion having a REM content is large exists as solid
state in the molten steel. However, in the case of the compositely incorporation of
REM and Ca, a inclusion of which Ca content is large forms around the inclusion ol'
15 which REM content is large, in which the inclusion of which REM content is large acts
as a core. The inclusion is called Ca - REM composite inclusion. In this case, the
inclusion of which Ca content is large is liquid phase in the molten steel. That is, a
i
I surface of the Ca - REM composite inclusion is liquid phase in the molten steel, and it is
1
assumed that a behavior of aggregation and union thereof is similar to that of the Ca -
20 AI2O3 type inclusion which forms in the case of the single incorporation of Ca.
j Therefore, it is assumed that a large amount of the Ca - REM composite inclusions
~ disperse and remain in the slab, and the total number density of the B-type inclusions
and the C-type inclusions increases.
[0043]
The Ca - A1103 type inclusion is elongated by rolling to be the A-type inclusion
if the grain size or the size in the long axis thereof is more than about 4pm. On the
other hand, if the grain size or the size in long axis of the Ca - A1203 type inclusion is
less than about 4pm, the Ca - A1203 type inclusion is hardly elongated (ratio of size in
5 long axis / size in short axis thereof remains to less than 3) by the rolling, and thus, the
Ca - A1203 type inclusion becomes the B-type inclusion or the C-type inclusion after the
rolling. In addition, the inclusion of which REM content is large, which forms in the
case of the compositely incorporation of REM and Ca, is hardly elongated by the rolling.
Furthermore, the inclusion having large Ca content, which forms around the inclusion
10 having large REM, is also hardly elongated through the rolling. That is, in the case of
the compositely incorporation of REM and Ca, the inclusion of which REM content is
large prevents the inclusion of which Ca content is large from elongation, and thus,
inclusions become mainly the B-type inclusions and the C-type inclusions.
[0044]
15 Moreover, the inventors found that the number density of the B-type inclusions
and the C-type inclusions yas affected by an amount of C in the steel. Hereinafter, the
effect of the amount of C in the steel is described.
1 4 [0045]
Ingots including 0.26% of C in terms of mass% were manufactured and the
!:
20 number density of the B-type inclusions and the C-type inclusions thereof was
I
i measured by the experiment of which the method is same to the above-described
~ method. Then, an experimental result of the steel including 0.26% of C and an
experimental result of the above-described steel including 0.45% of C were compared.
[0046]
I
As a result of the comparison, it became clear that the total number density of
the B-type inclusions and the C-type inclusions related to the amount of Ca and the
amount of C. Specifically, the inventors found that, even if the amount of Ca was the
same, the greater the amount of C was, the more the total number density of the B-type
5 inclusions and the C-type inclusions was. More specifically, it was found that, in order
to reduce the total number density of the B-type inclusions and the C-type inclusions to
6 pieces/mm2 or less, it was necessary that the amounts of each elements in terms of
mass% in the chemical composition were controlled within a range expressed by the
follow expression TI.
10 [0047]
Ca 5 0.0058 - 0.0050 x C :Expression I1
[0048]
The expression 11 indicates that it is necessary to vary an uppcr limit of the
amount of Ca depending on the amount of C, i.e. it is necessary that the more thc
15 amount of C is, the lower the upper limit of the amount of Ca is. Although the lower
limit of the right side of the above-described expression I1 is not limited, the substantial
lower limit ofthe right side ofthe above-described expression 11 is 0.0005, which is the
lower limit of the amount of Ca in terms of mass%.
[0049]
$
20 It is assumed that the reason why increasing the amount of C increases the total
1 number density of the B-type inclusions and the C-type inclusions is that increasing the
i
C concentration in the molten steel extends the range of solidification temperature,
which is from liquidus temperature to solidus temperature, to increase the length of the
dendrite structure. That is, it is assumed that since the dendrite structure grows long,
inclusions are easily captured between the dendrite branches (inclusions are hardly
effused from between the dendrite branches). Thereforc, there is a tendency that the
more the amount of C in the steel, the longer the dendrite structure during solidification
grows, and thus, in order to satisfy the above-described expression 11, it is necessary that
5 the more the amount of C in the steel, the lower the upper limit of the amount of Ca is.
The phase of the steel having the above-described carbon concentration range
(C: more than 0.25% and less than 0.50%) during solidification is liquid phase + 6
phase at peritectic temperature or more and is liquid phase + y phase at the peritectic
temperature or lower. That is, a degree of microsegregation of solute element such as
10 S at the peritectic temperature or lower differs from that at the peritectic temperature or
higher. It should be noted that S has an effect on capturing inclusions since S is a
surface-active element, and that a solid/liquid distribution coefficient of S in a case
where the phase is liquid phase +y phase is lower than that of S in a case where the
phase is liquid phase + 6 phase. The lower the solidlliquid distribution coefficient of S
15 is, the less an amount of S distributed to the solid phase is and the more an amount of S
distributed to the liquid phqse is. When a large amount of S which is the
surface-active element is distributed to the liquid phase, an interface energy between the
liquid phase and the solid phase decreases, and thus, the inclusions become to be easily
captured by the interface between the liquid phase and the solid phase.
20 When a temperature of the steel is the peritectic temperature or lower (i.e. a
phase ofthe steel is liquid phase + y phase), S is distributed to the liquid phase in
comparatively large content. Thus, the degree of microsegregation of S between the
dendrite branches (y phase) becomes high. Therefore, it is assumed that the inclusions
are easily captured in particular at the peritectic temperature or lower. In addition, the
highcr the C concentration is, the easier the inclusions are captured between the dendrite
branches, since the higher the C concentration is, the less the 6 phase is and the more the
y phase is. The expression I1 was defined based on the evaluation including the
~ above-described effect and on the observing result. When the C concentration in the
i I
! 5 steel is more than 0.25% and less than 0.50% which is higher than the peritectic point,
the expression I1 is valid.
[OOSO]
As described above, it was found that both the A-type inclusions, and the
B-type inclusions and the C-type inclusions can be advantageously decreased by
10 including a proper amount of REM and Ca depending of the amount of C. In addition
to these findings, the inventors studied about a configuration of the carbonitrides
I :
i including Ti which easily became to a starting point of cracking.
[0051]
(Regarding carbonitride including Ti)
15 If Ti is mixed from auxiliary raw material such as alloy, scrap, and the like, the
carbonitride including Ti su,ch as TiN forms in the steel. The carbonitride including Ti
has high hardness and has an angular shape. Therefore, if the coarse carbonitride
including Ti independently forms in the steel, the charpy impact energy of the steel and
then the workability of the steel are deteriorated, since the carbonitride tends to act as
20 the starting point of fracture
[00521
As described above, a relationship between an amount of the carbonitride
including Ti and the workability of the steel sheet was studied, and as a result, it was
found that if the number density of the carbonitrides including Ti existing independently
l
and having 5pm or more of the long side was 5 pieces/mm2 or less, fracture hardly
occurred and the worltability was prevented from detcrioration. Here, the carbonitride
including Ti includes Ti carbide, Ti nitride and Ti carbonitride. In addition, if Nb
which is optionally element is included, the carbonitride including Ti includes TiNb
5 carbide, TiNb nitride and TiNb carbonitride, and the like.
[0053]
In order to decrease such coarse carbonitride including Ti, it is considered to
decrease an amount of Ti. However, in a range of C concentration of the stccl
according to the present embodiment, the carbonitride including Ti easily forms even if
10 the amount of Ti is extremely small and the carbonitride including Ti, which is once
formed, easily coarsen during heat treatment of the steel. Thereforc, if the C
concentration is more than 0.25% and less than 0.50%, the number density of the
carbonitrides including Ti may be increased to more than 5 pieces/mm2 due to Ti mixed
as impurity to deteriorate the workability of the steel, even if Ti is not included as a
15 composition of the steel. As a method for solving the problem, it is considered to
prevent Ti from being mixed during manufacturing stage to control the amount of Ti to
about 1Oppm. However, in view of equipment capacity and manufacturing efficiency,
it is not preferable to employ such a method.
Therefore, the inventors studied another method for reducing the adverse effect
20 due to such coarse carbonitrides including Ti, and thus, the inventors found that the
compositely incorporation of REM and Ca is effective.
When IaM and Ca are compositely included, at first, composite inclusions
including Al, Ca, 0, S, and REM form in the steel, and then, the carbonitrides including
Ti compositely and prcfercntially form on the composite inclusions including
By compositely and preferentially forming the carbonitrides including Ti on the
composite inclusions including REM, the carbonitrides including Ti which form
independently in the steel and which have angular shape can be reduced. That is, the
I
I number density of the coarse carbonitrides including Ti existing ~ independently and I
I 5 having 5pm or more of long side can be preferably reduced to 5 pieces /mm2 or less.
[0054]
The carbonitrides including Ti which compositely form on the composite
inclusions including REM hardly act as starting points of fracture. Regarding the
reason for this, it is assumed that angular shape portions of the carbonitrides including
10 Ti are reduced by compositely precipitating the carbonitrides including Ti on the
composite inclusions including REM. For example, the shape of the carbonitride
including Ti is cubic or rectangular parallelepiped, and thus, if the carbonitride
including Ti exists independently in the steel, all of 8 points of vertexes of the
carbonitride including Ti contact with matrix. The vertex acts as the starting point of
15 fracture, and thus, the carbonitride including Ti, which has 8 points of vertexes, has 8
points of starting points of fracture. On the other hand, for example, if the carbonitride
including Ti compositely precipitates on the composite inclusion including REM and
half of the shape of the carbonitridc including Ti contacts with the matrix, only 4 points
I
of the carbonitride including Ti contact with the matrix. That is, the vertexes of the
20 carbonitride including Ti contacting with the matrix are reduced from 8 points to 4
points. As a result, the starting points of fracture due to the carbonitride including Ti
are reduced from 8 points to 4 points.
[0055]
In addition, in consideration that the carbonitride including Ti precipitates on
specific crystal face of the composite inclusion including REM, it is assumed that the
reason why the carbonitride including Ti tends to compositely and preferentially
precipitates on the composite inclusion including REM is that lattice consistency
between the specific crystal face of the composite inclusion including REM and the
5 carbonitride including Ti is good.
An adverse effect of the composite of the carbonitride including Ti and the
inclusion including REM (i.e. the inclusion in which the carbonitride including Ti
adheres on the surface of the composite inclusion including Al, Ca, 0, S, and REM) is
smaller than that of the carbouitride including Ti existing independently, and thus, it is
10 recognized that the composite of the carbonitride including Ti and the inclusion
including REM is not the carbonitride including Ti existing independently and is the
C-type inclusion.
[0056]
Next, a chemical composition of the steel sheet according to the present
15 emhodimcnt will be described.
[0057]
At first, a limited range and a reason of the limitation regarding a basic
composition of the steel sheet according to the present embodiment will be described.
The term " % described herein is "mass%.
20 [0058]
(C: more than 0.25% and less than 0.50%)
C (carbon) is an important element for securing strength (hardness) of the steel
sheet. The strength of the steel sheet is secured by setting the amount of C to more
than 0.25%. When the amount of C is 0.25% or less, hardenability of the steel sheet
decreases, and thus, strength which is necessary for products made by using the steel
sheet as a material, for example gears and the like, cannot be obtained. On the other
hand, if the amount oC C is 0.50% or more, since long time is required for heat treatment
for securing workability, the workability of the steel sheet may be deteriorated unless
5 otherwise the time for the heat treatment is elongated. In addition, iCthe amount of C
increases, the total number density of the B-type inclusions and the C-type inclusions
increases. It is assumed that the reason of this is that, if the amount of C is high, the
dendrite structure grows long during solidification of the molten steel, and thus, the
inclusions are easily captured between the dendrite branches. Therefore, the amount of
10 C is controlled to more than 0.25% and less than 0.50%.
It is preferable that the lower limit of C is 0.27%. Generally, the higher the
amount of C is, the higher the hardness and the tensile strength after performing heat
treatments (quenching and tempering) increase. Specifically, when the amount of C is
0.27% or more, 1300MPa or more of strength can be sufficiently secured after
15 performing the quenching and the low-temperature tempering. Figure 3 is a graph
showing a relationship between the amount of C and the tensile strength. The
inventors measured the tensile strength of the steel sheets which satisfied the condition
of the steel sheet according to the present embodiment except for the amount of C, and
which had various amount of C. As a result, it was found that, when the amount of C
20 was 0.27% or more, the steel certainly had 1300MPa or more of tensile strength. In
addition, in the steel sheet according to the present embodiment, it is preferable that the
lower limit of the amount of C be 0.30%. In the steel sheet according to the present
embodiment, it is preferable that the upper limit of the amount of C is 0.48%.
[0059]
! (Si: 0.10% to 0.60%)
Si (silicon) acts as a deoxidizing agent, and Si is an element effective for
increasing hardenability to enhance the strength (hardness) of the steel sheet. If the
1 amount of Si is less than 0.10%, the above-described effect cannot be obtained. On the
I
1 5 other hand, if the amount of Si is more than 0.60%, a deterioration of surface property
of the steel sheet duc to a scale flaw during hot rolling may be caused. Therefore, the
amount of Si is controlled to be 0.10% to 0.60%. It is preferable that the lower limit of
the amount of Si is 0.15%. It is preferable that the upper limit of the amounl of Si is
0.55%.
10 [0060]
(Mn: 0.40% to 0.90%)
Mn (manganese) is an element which acts as a deoxidizing agent and an
element effective for increasing hardenability to enhance the strength (hardness) of the
steel sheet. If the amount of Mn is less than 0.40%, the above-described effect cannot
15 be obtained sufficiently. On the other hand, if the amount of Mn is more than 0.90%,
the workability of the steel sheet may deteriorate. Therefore, the amount of Mn is
controlled to 0.40% to 0.90%. It is preferable that the lower limit of Mn is 0.50%. It
is prererable that the upper limit of Mn is 0.75%.
[0061]
20 (Al: 0.003% to 0.070%)
A1 (aluminum) is an clement which acts as a deoxidizing agent and an element
I effectivc for fixing N to enhance the workability of the steel sheet. If the amount of A1
I
is less than 0.003%, the above-described effect cannot be obtained sufficiently, and thus,
it is necessary that 0.003% or more of A1 is included. On the other hand, if tbe amount
I ofAl is more than 0.070%, the above-described effect saturates and coarse inclusions
increase. The workability may be deteriorated by the coarse inclusions, or the surface
flaw may tend to be easily occurred by the coarse inclusions. Therefore, the amount or
A1 is controlled to be 0.003% to 0.070%. It is preferable that the lower limit ofAl is
I
1
I
I
5 0.010%. It is preferable that the upper limit ofAl be 0.040%.
[0062]
(Ca: 0.0005% to 0.0040%)
Ca (calcium) is an element effective lor controlling configuration of the
inclusions to enhance the workability of the steel sheet. If the amount of Ca is less
10 than 0.0005%, the above-described effect cannot he obtained sufficiently. Although
REM can control the configuration of the inclusions, if the amount of Ca is less than
0.0005%. nozzle clogging may occur during continuous casting to prcvent the operation
from stable and inclusions having large specific gravity may accumulate at lower
surface side of the slab to deteriorate the workability of the steel sheet, in a same
15 manner as a case of the single incorporation of REM described as follows. On the
other hand, if the amount of Ca is more than 0.0040%, coarse low-melting oxides such
as, for example, CaO - AlzOi type inclusions and/or inclusions such as CaS type
inclusion which easily elongate during rolling may easily form to deteriorate the
! workability of the steel sheet. In addition, if the amount of Ca is more than 0.0040%,
20 nozzle refractor erosion may easily occur and deteriorate stability of the operation of the
I continuous casting. Therefore, the amount of Ca is controlled to 0.0005% to 0.0040%.
A lower limit of the amount of Ca is preferably 0.0007% and more preferably 0.0010%.
An upper limit of the amount orC is preferably 0.0030% and more preferably 0.0025%.
[0063]
I Moreover, it is necessary that the upper limit of the amount of Ca is controlled
I i
I depending on the amount of C. Specifically, it is necessary that the amount of C and
!
the amount of Ca in terms of mass% in the chemical composition are controlled within a
range expressed by the below expression 111. If the amount of Ca does not satisfy the
5 below expression 111, the total number density of the B-type inclusions and the C-type
inclusions becomes more than 5 pieces/mm2.
[0064]
Ca 5 0.0058 - 0.0050 x C : Expression 111
[0065]
10 (REM: 0.0003% to 0.0050%)
REM (Rare Earth Metal) indicates rare earth elements and is a generic name
I
for 17 elements consisting of scandium Sc (atomic number 21), yttrium Y (atomic
number 39), and lanthanoid (15 elements from lanthanum of which atomic number is 57
to lutetium of which atomic number is 71). The steel sheet according to the present
15 embodiment includes one or more elements selected from the 17 elements. Generally,
in view of availability, REM is often selected from Ce (cerium), La (lanthanum), Nd
(neodymium), and Pr (praseodymium). Adding misch metal which is a mixture of
these elements into the steel is extensively used. A main composition of the misch
metal is Ce, La, Nd, and Pr. In the steel sheet according to the present embodiment, a
20 total amount of these rare earth elements included in the steel sheet is recognized as the
1 amount of REM. In the above-described method for calculating R1 which is a total
chemical equivalent of Ca and E M , since an average atomic weight of the misch metal
is about 140, it is recognized that the atomic weight of REM is 140.
[0066]
REM is an element cffective for controlling the configuration of the inclusions
to enhance the workability of the steel sheet. If the amount of E M is less than
0.0003%, the above-described effect cannot be obtained sufficiently, and a problem
which is the same as the case of the single incorporation of Ca occurs. That is, if the
5 amount of REM is less than 0.0003%, CaO - A1203 type inclusions and part of CaS may
be elongated by rolling to deteriorate the property of the steel sheet (worltability and
toughness after working). In addition, if the amount of REM is less than 0.0003%, the
composite inclusions including Al, Ca, 0 , S, and REM, on which the carbonitrides
including Ti tend to preferentially composite, are low, and thus, the carbonitrides
10 including Ti which form independently in the steel sheet increase to easily deteriorate
the workability. On the other hand, if the amount of REM is more than 0.0050%,
nozzle clogging tends to occur during continuous casting. In addition, if the amount of
REM is more than 0.0050%, the number density of the formed REM-type inclusions
(oxides, or oxysulfides) becomes comparatively high, and thus, the REM-type
15 inclusions accumulate at lower surface side of the slab curbed during continuous casting
the slab. This causes an internal defect in the product obtained by rolling the slab, and
this may deteriorate the workability ofthe steel sheet. Therefore, the amount of REM
is controlled to 0.0003% to 0.0050%. The lower limit of the amount of REM is
preferably 0.0005%, and more preferably 0.0010%. The upper limit of the amount of
20 REM is preferably 0.0040% and more preferably 0.0030%.
[0067]
Moreover, it is necessary that the amount of Ca and the amount of REM arc
controlled depending on the amount of S. Specifically, it is necessary that the amount
of each elements in the chemical composition in terms of mass% arc controlled within a
range expressed by the below expression IV. If the amount of Ca, the amount of REM,
and the amount of S do not satisfy the below expression IV, the number density of the
A-type inclusions becomes more than 6 pieces/mm2. When the value of the right side
of the below expression IV is 2 or more, the configuration of the inclusions can be
5 controlled more preferably. Furthermore, although the upper limit ofthe below
expression IV is not limited, if the value of the right side of the below expression IV is
more than 5, the coarse B-type inclusions or the coarse C-type inclusions having more
than 20pm of maximum length tend to occur. Therefore, it is preferable that the upper
limit of the below expression IV is 5.
10 [0068]
0.3000 5 { Ca 140.88 + ( REM / 140 ) / 2 ) / ( S / 32.07 ) : Expression IV
[0069]
In addition to the above-described basic composition, the steel sheet according
to the present embodiment includes impurity. The impurity indicates elements of P, S,
15 Ti, 0, N, Cd, Zn, Sb, W, Mg, Zr, As, Co, Sn, Pb, and the like mixed from auxiliary raw
material such as scrap or frqm manufacturing process. Since it is not essential to
include these elements, the lower limit of the amount of these elements is 0%. Among
them, P, S, Ti, 0, and N is limited as follows in order to preferably exercise the
above-described effect. In addition, it is preferable that the above-described impurity
20 except for P, S, 0, Ti, and N are limited to 0.01% or less. However, if0.01% or more
of these impurities are included, the above-described eflect is not lost. The term "%"
described herein is "mass%".
[0070]
(P: 0.020% or less)
P (phosphorus) has an activity of solute strengthening. On the other hand,
excess amount of P deteriorate the workability of the steel sheet. Therefore, the
amount of P is limited to 0.020% or less. The lower limit of P may be 0%. in view
of the conventional refining (including second refining), the lower limit of P may be
5 0.005%.
[0071]
(S: 0.0070% or less)
S (sulfur) is an impurity element which forms non-metallic inclusion to
deteriorate the workability of the steel sheet. Therefore, the amount of S is limited to
10 0.0070% or less, and preferably limited to 0.0050% or less. The lower limit of the
amount of S may be 0%. In view of the conventional refining (including second
refining), the lower limit of S may be 0.005%.
[0072]
(Ti: 0.050% or less)
15 Ti (titanium) is an element which forms the carbonitrides, which is hard and
has angular shape, to deteriorate the workability of the steel sheet. In the present
embodiment, although the harmful effect thereof on the workability can be relieved by
preferentially precipitating on the inclusions including REM as described above, if the
amount of Ti is more than 0.050%, the deterioration of the workability become obvious.
20 Therefore, the amount of Ti is limitcd to 0.050% or less. The lower limit of the
I amount of Ti may be 0%. In view of the conventional refining (including second
I refining), the lower limit of Ti may be 0.0005%.
(0:0. 0040% or less)
0 (oxygen) is an impurity element forming oxides (non-metallic inclusions),
which aggregate and coarsen to deteriorate the workability of the steel sheet.
Therefore, the amount of 0 is limited to 0.0040% or less. 'The lower limit of the
amount of 0 may be 0%. In view of the conventional refining (including second
5 refining), the lower limit of 0 may be 0.0010%. The amount of 0 of thc steel sheet
according to the present embodiment indicates a total amount of 0 (amount of T.0)
which is a total of the amount of zll 0 such as solid-solute 0 in the steel, 0 existing in
the inclusions, and the like
[0074]
10 In addition, it is preferable that the amount of 0 and the amount of E M in
i terms of mass% of each elements are controlled within the range expressed by the
below expression V. When the following expression V is satisfied, the number density
of the A-type inclusions further decreases, and thus, it is preferable. Although the
upper limit of the below expression V is not limited, the upper limit of the left side of
15 the below expression V is substantially 0.000643 in view of the upper limit and the
lower limit of the amount of 0 and the amount of REM.
[0075]
I
I lSx(REM1140)-O/16>_O ExpressionV
'I
[0076]
20 When the amount of 0 and the amount of REM is controlled based on the
I expression V to form mixed configuration of two kinds of composite oxides of
I
REM203- 11A1203 (in which molar ratio olREM203 and A1203 is 1 : 11) and REM203.
A1203 (in which molar ratio of EM2O3 and A1203 is 1 :I), the A-type inclusions more
preferably decrease. In the above expression V, "REM 1 140" expresses number of
moles of REM and "0 / 16" expresses number of moles of 0 . In order to form the
mixed configuration of REM203.11A1203 and REM203 .A1203, it is preferable that
REM is included with the amount thereof satisfying the above expression V. Il the
amount of REM is low such that the above expression V is not satisfied, mixed
5 configuration ofA1203 and REM203. 11AlzO3 may form. A1203 part included in the
mixed configuration and CaO may react to form CaO - A1203 type inclusions which
may be elongated by rolling
[0077]
(N: 0.0075% or less)
10 N (nitrogen) is an impurity element forming nitride (non-metallic inclusion) to
deteriorate the workability of the steel sheet. Therefore, the amount of N is limited to
0.0075% or less. The lower limit of the amount of N may bc 0%. In view of
conventional refining (including second refining), the lower limit of N may be 0.0010%
[0078]
15 In the steel sheet according to the present embodiment, the above-described
basic compositions are controlled and a remainder includes iron and above-described
impurity. On the other hand, in addition to the basic compositions, the steel sheet
according to the present embodiment may further include follow optional compositions
in the steel in place of the part of the iron in the remainder, as necessary.
That is, in addition to the above-described basic compositions and the impurity, I I
I the hot-rolled steel sheet according to the present embodiment may further include one
or more of Cu, Nb, V, Mo, Ni, and B as optional compositions. Hereinafter, a lim~ted
range and a reason of the limitation regarding optional compositions will be described.
I The term " % described herein is "mass%.
[OOSO]
(Cu: 0.05% or less)
I I Cu (copper) is an optional element having an effect of enhancing strength
i
1 5 (hardness) of the steel sheet. Therefore, as necessary, Cu may be included within a
range of 0.05% or less. In addition, when the lower limit of the amount of Cu is
0.01 %, the above-described effect can be obtained preferably. On the other hand, if
the amount of Cu is more than 0.05%, hot working cracking may occur during hot
rolling due to molten metal embrittlement (Cu cracking). Apreferable range of the
10 amount of Cu is 0.02% to 0.04%.
[OOSl]
(Nb: 0.05% or less)
Nb (niobium) is an optional element which forms carbonitrides and is effective
for preventing grain from coarsening and for enhancing the workability of the steel
15 sheet. Therefore, as necessary, Nb may be included within a range of0.05% or less.
In addition, when the lower limit of the amount ofNb is 0.01%, the above-described
effect can be obtained preferably. On the other hand, if the amount of Nb is more than
0.05%, coarse Nb carbonitrides may precipitate to deteriorate the workability of the
steel sheet. A preferable range of the amount of Nb is 0.02% to 0.04%.
20 [0082]
(V: 0.05% or less)
V (vanadium) is an optional element which forms carbonitrides similar to Nb
and is effective for preventing grains from coarsening and for enhancing the worlcability
of the steel sheet. Therefore, as necessary, V may be included within a range of 0.05%
or less. In addition, when the lower limit of the amount of V is 0.01%, the
above-described effect can be obtained preferably. On the other hand, if the amount of
V is more than 0.05%, coarse inclusions may form to deteriorate the worlcability of the
steel sheet. A preferable range of the amount is 0.02% to 0.04%.
5 [0083]
(Mo: 0.05% or less)
Mo (molybdenum) is an optional element which has an effect of enhancing
hardenability and enhancing resistance to temper softening to enhance strength
(hardness) of the steel sheet. Therefore, as necessary, Mo may be included within a
10 range of 0.05% or less. In addition, whcn the lower limit of the amount of Mo is
0.01%, the above-described effect can be obtained preferably. On the other hand, if
the amount of Mo is more than 0.05%, costs increase and the including effect saturates.
In addition, if the amount of Mo is more than 0.05%, the workability, pai-ticularly cold
workability of the steel sheet decreases, and thus, it becomes difficult to work the steel
15 sheet into complex shape (for example, gear shape). Therefore, the upper limit of the
amount of Mo is 0.05%. Apreferable range of the amount of Mo is 0.01% to 0.05%.
[0084]
(Ni: 0.05% or less)
Ni (nickel) is an optional element effective for enhancing hardenability to
20 enhance strength (hardness) and workability of the steel sheet. In addition, Ni is an
optional element having an effect of prevcnting the molten metal embrittlement (Cu
cracking) in a case of including Cu from occurring. Therefore, as necessary, Ni may
be included within a range of 0.05% or lcss. In addition, when the lower limit of the
amount of Ni is 0.01%, the above-described effect can be obtained prcferabiy. On the
other hand, if the amount of Ni is more than 0.05%, costs increases and the including
effect saturates, and thus, the upper limit of the amount of Ni is 0.05%. A preferable
range of the amount of Ni is 0.02% to 0.05%.
[008S]
5 (Cr: 0.50% or less)
Cr (chromium) is an element effective for enhancing hardenability to enhancc
strength (hardness) of the steel sheet. Therefore, as necessary, Cr may be included
within a range of 0.50% or less. In addition, when the lower limit of the amount of Cr
is 0.01%, the above-described effect can be obtained preferably. If the amount of Cr is
10 more than 0.50%, costs increases and the including effect saturates. Therefore, the
amount of Cr is controlled to 0.50% or less.
[0086]
(B: 0.0050% or less)
B (boron) is an element effective for enhancing hardenability to enhance
15 strength (hardness) of the steel sheet. Therefore, as necessary, B may be included
within a range of 0.0050% gr less. In addition, when the lower limit of the amount of
B is 0.0010%, the above-described effect can be obtained preferably. On the other
hand, if the amount of B is more than 0.0050%, Boron-type compound forms to
deteriorate the workability of the steel sheet, and thus, the upper limit thereof is
20 0.0050%. A preferable range of the amount of B is 0.0020% to 0.0040%.
! ~ [0087]
~ Next, a method for manufacturing the steel sheet according to the present
embodiment will be described.
[0088]
For the example, similar to the general steel sheet, the raw material of the steel
sheet according to the present embodiment is blast furnace molten iron, and a molten
steel manufactured by performing converter refining and second refining is
continuously-casted so as to be a slab, and then, the slab is hot-rolled, optionally
5 cold-rolled, andlor quenched so as to be the steel sheet. In this regard, during the
second refining in ladle after decarburization treatment in the converter, the composition
of the steel is controlled while controlling inclusions is performed by adding Ca and
REM. In addition to the blast furnace molten iron, molten steel obtained by iilelting
raw material of iron scrap in electric furnace may be used as the raw material.
10 [0089]
Ca and REM are added after controlling composition of other elements and
floating A1203 caused by Al deoxidization from the molten steel. If A1203 remains in
the molten metal in a huge amount, Ca and REM are consumed by reducing A1203.
Therefore, the amounts of Ca and REM used for fixing S decrease, and thus, Ca and
15 REM cannot sufficiently prevent from causing MnS.
[0090]
Since vapor pressure of Ca is high, Ca may be added as Ca-Si alloy, Fe-Ca-Si
alloy, Ca-Ni alloy, and the like in order to enhance yield ratio. In order to add the alloy,
20 an alloy wire constructed from the alloy may be used. REM ]nay be added as
I Fe-Si-REM alloy, misch metal, and the like. The misch metal is a mixture of
!
I rare-earth element. Specifically, the misch metal often includes 40% to 50% of Ce,
and 20% to 40% of La. For example, a misch metal consisting of 45% of Ce, 35% of
La, 9% of Nd, 6% of Pr, and other impurities is available.
[00911
Sequence of adding Ca and REM is not limited. On the other hand, if Ca is
added after adding REM, there is a tendency that sizes of the inclusions slightly
decrease. Therefore, it is preferable that Ca be added after adding REM.
5 [0092]
A1203 forms alter A1 deoxidization and a part of the A1203 is clustered.
However, when REM is added before adding Ca, a part of the cluster is reduced and
dissolved, and thus, a size of the cluster can be decreased. On the other hand, if Ca is
added before adding REM, AI2O3 may change to low-melting CaO - A1203 type
10 inclusion and the above-described A1203 cluster may change to one coarse CaO - A1203
type inclusion. Therefore, it is preferable that Ca be added afler adding REM.
[Examples]
[0093]
Effects of on embodiment of the present invention will be described in further
15 detail by examples. However, the condition in the examples is an example condition
employed to confirm the operability and the effects of the present invention, so the
present invention is not limited to the example condition. The present invention can
employ various types of conditions as long as the conditions do not depart from the
scope of the present invention and can achieve the object of the present invention.
20 [0094]
300 tons of molten steel having composition shown in Table 2A was melted by
using blast furnace molten iron as raw material, preliminary treating of molten iron,
decarburizing treating in converter, and then ladle refining to control composition. In
the ladle refining, at first, A1 was added to perform deoxidization, next, composition of
other elements such as Ti was controlled. Then, holding was performed during 5
minutes or longer to float A1203 caused by the A1 deoxidization, REM was added,
keeping was performed during 3 minutes to mix uniformly, and Ca was added. Misch
metal was used as REM. REM elements included in the misch metal were 50% of Ce,
j
5 25% of La, 10% of Nd, and a remainder of the misch metal was impurities. Therefore,
I
I
I a ratio of each REM elements included in the obtained steel sheet is substantially equal
to the ratio of each REM elements described above. Since vapor pressure of Ca was
high, Ca-Si alloy was added to increase yield rate.
[0095]
10 The above-described molten steel after refining was continuously-casted so as
to be a slab having a thickness of 250mm. Then, the slab was heated to 1250°C and
kept during 1 hour, hot-rolled with a finishing temperature of 850°C to make the
thickness as 5mm, and thereafter, coiled with a coiling temperature of 580°C. After
pickling the hot-rolled steel sheet, hot-rolled-sheet-annealing was performed at 700°C
15 during 72 hours. The hot-rolled steel sheet was quenched at 900°C during 30 minutes,
I
and further tempered at 100°C during 30 minutes.
In the hot-rolled steel sheet obtained after quenching and tempering,
composition and deformation behavior (ratio of size in long axis / size in short axis after
20 rolling; aspect ratio) of inclusions were examined. 60 view fields were observed using
optical microscope at 400-fold magnification (if shapes of the inclusions were measured
in detail, at 1000-fold magnification) in which observed sections were cross-sections
parallel to rolling direction and plate thickness direction. In each of the view fields,
inclusions whose grain sizes were 1 pm or more (if shapes of the inclusions were
spherical) or inclusions whose long axis were I pm or more (if shapes of the inclusions
were deformed) were observed to categorize thereof as the A-type inclusions, the B-type
inclusions and the C-type inclusions, and number densities thereof were measured. In
addition, a number density of a carbonitrides including Ti which precipitated
5 independently in the steel, had an angular shape, and had 5pm or more of long side, was
measured. Since the carbonitride including Ti differs from other C-type inclusion in
shape and color, the carbonitride including Ti can be distinguished by observation.
Alternatively, it is preferable that structure of the hot-rolled steel sheet is observed using
EPMA (Electron Probe Micro Analysis) or SEM (Scanning Electron Microscope)
10 having EDX (Energy Dispersive X-Ray Analysis). In this case, the carbonitrides
including Ti, the composite inclusions including REM, MnS, and CaO - A1203 type
inclusions in the inclusions can be identified.
[0097]
Evaluation criteria of the inclusions are as follows.
15 Regarding number density of the A-type inclusions, number density of the
B-type inclusions and number density of the C-type inclusions, in a case in which the
number density was more than 6 pieces/mm2, they were evaluated as B (Bad), in a case
in which the number density was more than 4 pieces/mm2 and 6 pieces/mm2 or less,
they were evaluated as G (Good), in a case in which the number density was more than
20 2 pieces/mm2 and 4 pieces/mm2 or less, they were evaluated as VG (Very Good), and in
a case in which the number density was more than 2 pieces/mm2 or less, they were
evaluated as GG (Greatly Good).
Regarding coarse inclusions which were B-type or C-type and of which
maximum length were 20pm or more, in a case in which the coarse inclusions were
more than 6 pieces/mm2, they were evaluated as B (Bad), in a case in which the coarse
inclusions were more than 3 picccs/mm2 and 6 pieces/mm2 or less, they were evaluated
as G (Good), and in a case in which the coarse inclusions were 3 pieces/mm2 or less,
they were evaluated as VG (Very Good).
5 Regarding carbonitrides including Ti which existed independently in the steel
and had 5pm or more of long side, in a case in which the number density was more than
5 pieces/mm2, they were evaluated as B (Bad), in a case in which the number density
was more than 3 pieces/~llma~nd 5 pieces/mm2 or less, they were evaluated as G (Good),
and in a case in which the number density was 3 pieces/mm2 or less, they were
1.0 evaluated as VG (Very Good).
[0098]
! Tensile strength (MPa), charpy impact value ( ~ / c ma~t )ro om temperature
!
I
(about 2S°C), and hole expansibility (%) d t h e hot-rolled steel sheet obtained after
quenching and tempering were evaluated. A steel sheet having 1200MPa or more of
15 tensile strength was recognized as a steel sheet satisfying evaluation criteria in tensile
strength. The charpy impact value at room temperature indicates toughness and is one
of indexes for evaluating worlcability of the steel sheet. In addition, toughness of the
product obtained by working the steel sheet can be evaluated by the charpy impact value.
A steel sheet having 6 ~/cm' or more of charpy impact value at room temperature was
20 recognized as a steel sheet satisfying evaluation criteria in toughness. The hole
expansibility is another index for evaluating workability. At first, a punched hole
having a diameter of 1Omm was made at a center of a steel sheet of 150mm x 150mm,
and then, the punched hole was stretched to expand by 60" of circular conic punch.
When a cracking penetrating the steel thickness was occurred in the steel sheet by the
stretching and expanding treatment, a hole diameter D (mm) was measured. Then, the
hole expansion value h (%) was calculated by an expression "h = ( D - 10 ) 1 10 x loo",
and a steel sheet having 80% or more of h (%) was recognized as a steel sheet satisfying
evaluation criteria in hole expansibility.
5 LO0991
In addition, a quantitative analysis for chemical composition of the obtained
hot-rolled steel sheet was performed using ICP - AES (Inductively Coupled
Plasma-Atomic Emission Spectrometry) or ICP - MS (Inductively Coupled
Plasma-Mass Spectrometry). There was a case in which a trace of REM element
10 among the REM elements is lower than analytical limit, and in this case, an amount of
the trace of REM was recognized to be proportional to the amount in misch metal (50%
of Ce, 25% oELa, and 10% of Nd) and was calculated by using a ratio with respect to
the analysis value of Ce, which has the largest amount.
[OlOO]
15 Results are shown in Table 2B. In the table, a value being out of range of the
present invention is underlined. All examples had construction satisfying the range of
the present invention, and thus, was excellent in the tensile strength, and the workability
indicated by the charpy impact value and the hole expansibility h. On the other hand,
comparative examples did not satisfy the condition defined according to the present
20 invention, and thus, did not have sufficient tensile strength or sufficient workability.
Regarding comparative example 1, the amount of Ca was lower than the lower
limit thereof, and thus, inclusions which hardly included Ca formed. Therefore, in
comparative example 1, many B-type inclusions, C-type inclusions, and coarse
inclusions folmed and the evaluation of the number density of the B-type inclusions +
the C-type inclusions and the evaluation of the number density of the coarse inclusions
were "B". In addition, nozzle clogging occurred during casting of the comparative
example 1.
Regarding comparative example 2, the amount of Ca was highcr than the upper
5 limit thereof, and thus, coarse CaO - A1103 type low-temperature oxides formed.
Therefore, the evaluation of the number density of the A-type inclusions, the evaluation
of the number density of the B-type inclusions + the C-type inclusions, and the
evaluation of the number density of the coarse inclusions were "B".
Regarding comparative example 3, the amount of REM was lower than the
10 lower limit thereof and the expression 3 was not satisfied, and thus, many coarse
carbonitrides including Ti formed independently in the matrix. Therefore, the
evaluation of the number density of the carbonitrides including Ti was "B".
Regarding comparative example 4, the amount of REM was higher than the
upper limit thereof, and thus, the evaluation of the number density of the B-type
15 inclusions + the C-type inclusions and the evaluation of the number density of the
coarse inclusions were "B". In addition, nozzle clogging occurred during casting of
the comparative example 4.
Regarding comparative example 5, the value of the right side of the expression
:; 1 was lower than 0.3, and thus, the evaluation of the number density of the A-type
': 20 inclusions was "B". In addition, the amount of C of the comparative example 5 was
i excess, and thus, the workability thereof was low. Therefore, the impact value of the
1 'I comparative example 5 was insufficient.
1
I
I Regarding comparative example 6, the expression 2 was not satisfied, and thus,
the evaluation of the number density of the B-type inclusions + the C-type inclusions
was "B".
Regarding comparative example 7, the amount of C was insufficient, and thus,
the tensile strength was insufficient.
Regarding comparative example 8, although the number density of the
5 inclusions was an adequate level, the amount of C was excess, and thus, the workability
was deteriorated. Therefore, the hole expansibility of the comparative example 8 was
non-acceptance.
Regarding comparative example 9, the amount of S was excess, and thus,
coarse MnS inclusions formed and the evaluation of the number density of the A-type
10 inclusions was "B". In addition, the impact value and the hole expansibility of the
comparative examplc 9 were insufficient.
I Regarding comparative example 10, the amount of Ti was excess, and thus, the
i evaluation of the number density of the carbonitrides including Ti was "B". Therefore,
i
i
I the impact value and the hole expansibility of the comparative example 10 were
I
i
15 insufficient.
Regarding comparative example 11, the amount of Ca was excess, and thus,
coarse inclusions of which CaO content was high formed and elongated. Therefore,
1 the evaluation of the number density of the A-type inclusions and the evaluation of the
I
!
I
L number density of the B-type inclusions and the C-type inclusions were "B". In
i
i 20 addition, regarding comparative example 11, CaO content was high, and thus, an effect
1
i of adhering the carbonitrides including Ti on the surface of the oxides was deteriorated.
i
1
i I Therefore, the evaluation of the number density of the carbonitrides including Ti of the
comparative example 11 was "B". As a result, the impact value and the hole
expansibility of the comparative example 11 were insufficient.
Regarding comparative example 12, the amount of REM was insufficient, and
thus, an effect of adhering thc carbonitrides including Ti on the surface of the oxides
was deteriorated. Therefore, the evaluation of the number density of the carbonitrides
including Ti of the comparative example 12 was "B". As a result, the impact value
5 and the hole expansibility of the comparative example 12 were insufficient.
Regarding comparative example 13, the amount of REM was excess, and thus,
the evaluation of the number density of the coarse inclusions was "B". Therefore, the
impact value and the hole expansibility of the comparativc example 13 were
insufficient.
10 Regarding comparative example 14, the amount of Mo was excess, and thus,
although the evaluation of the number density of the inclusions was good, the
workability was deteriorated. Therefore, the impact value and the hole expansibility of
the comparative example 14 were insufficient.
Regarding comparative example 15, the expression 1 was not satisfied, and
15 thus, the evaluation of the number density of the A-type inclusions was "B".
Therefore, the impact value and the hole expansibility of the comparative example 15
were insufficient.
! Regarding comparative example 16, the expression 2 was not satisfied, and
thus, the evaluation of the number density of the B-type inclusions + the C-type
20 inclusions was "B". Therefore, the impact value and the hole expansibility of the
comparative example 16 were insufficient.
I
I [0101]
[Table 2A]
[O 1021
[Table 2B]
[Industrial applicability]
[0103]
The amount of C, the amount of Ca, and the amount of REM of the steel sheet
5 according to the present invention satisfy the expression "0.3000 5 { Ca / 40.88 +
( REM I 140 ) I 2 ) 1 ( S 132.07 )" and the exprcssion "Ca 5 0.0058 - 0.0050 x C".
Therefore, the number density of the A-type inclusions having I pm or more of long side
of the steel sheet according to the present invention is limited to 6 pieces/mm2 or less,
and the total number density of the B-type inclusions and the C-type inclusions having
10 1 pm or more of long side of the steel sheet according to the present invention is limited
to 6 pieceslmm2 or less. In addition, Ti carbonitrides of the steel sheet according to the
1 present invention, which have 5pm or more of long side and exists independently, is
I
i limited to 5 pieceslmm2 or less. According to the above-described embodiment, the
A-type inclusions, the B-type inclusions, and the C-type inclusions in the steel are
I
15 decreased and the coarse carbonitrides including Ti existing independently is prevented
from forming, and thus, a steel sheet excellent in workability becomes available and the
present invention has high industrial applicability. The carbon steel sheet according to
the present invention can be used for manufacturing mechanical component having
various shapes such as gears, a clutch, and a washer of a vehicle, and the like.

[Document Type] Claims
1. A steel sheet, wherein
a chemical composition comprises, by mass%:
C: more than 0.25% and less than 0.50%;
5 Si: 0.10% to 0.60%;
Mn: 0.40% to 0.90%;
Al: 0.003% to 0.070%;
Ca: 0.0005% to 0.0040%;
REM: 0.0003% to 0.0050%;
I0 Cu: 0% to 0.05%;
Nb: 0% to 0.05%;
V: 0% to 0.05%;
Mo: 0% to 0.05%;
Ni: 0% to 0.05%;
15 Cr: 0% to 0.50%;
B: 0% to 0.0050%;
P: limited to 0.020% or less;
'1
I
S: limited to 0.0070% or less;
!
::
Ti: limited to 0.050% or less;
20 0: limited to 0.0040% or less;
I N: limited to 0.0075% or less; and
remainder including iron and impurity,
amounts of each elements by mass% in the chemical composition satisfy both
of expression 1 and expression 2,
a number density of carbonitrides including Ti which exists independently and
has a long side of 5pm or more is limited to 5 pieces/mm2 or less,
0.3000 5 { Ca140.88 + ( REM / 140 ) / 2 ) / ( S 132.07 ) : expression 1,
and
5 Ca 5 0.0058 - 0.0050 x C : expression 2.
2. The steel sheet according to claim 1, wherein
the chemical con~positionf urther comprises one or more of, by mass%:
Cu: 0.01% to 0.05%;
10 Nb: 0.01% to 0.05%:
V: 0.01% to 0.05%;
Mo: 0.01% to 0.05%;
Ni: 0.01% to 0.05%;
Cr: 0.01% to 0.50%; and
3. The steel sheet according to claim 1 or 2, wherein
the steel sheet further includes
a composite inclusion which includes Al, Ca, 0, S, and REM, and
20 an inclusion in which the carbonitride including Ti is adhered on the composite
inclusion.
I
4. The steel sheet according to claim 1 or 2, wherein
the amounts of thc each elements by mass% in the chemical composition
satisfy expression 3,
18 x(REMl140) - 01161 0 : expression 3.
5. The steel sheet according to claim 3, wherein
the amounts of the each elements by mass% in the chemical composition
satisfy expression 4,
18 x (REMl140) - 0116 1 0 : expression