Docun~en'tI ype] Specification
[Title ofthe Invention] ROLLED STEEL BAR FOR MACHINE STRUCTURAL
USE AND METHOD OF PRODlJClNG TIjE SAME
[Technical Ficld of the Invention]
[000 1 1
The present invention relates to a rolled steel bar for machine structural use
which is suitable as a material of a mechanical component or a structural member
(hereinafter, referred to as "mechanical structural member") produced by hot forging or
the like, and a method of producing the same.
Priority is claimed on Japanese Patent Application No. 2014-137878, filed on
July 3,2014, the content of which is incorporated herein by reference.
[Related Art]
[OOO2]
In a mechanical structural member used in a vehicle, an industrial machine, or
the like, not only high strength but also excellent ductility and toughness may be
required. Jn this case, it is preferable that a metallographic structure of the
mechanical structural member is tempered martensite. Therefore, in many cases, the
mechanical structural member is formed by performing a refining heat treatment such
as quenching and tempering, and machining a hot forged steel bar which is a material
of the mechanical structural member.
On the other hand, in a mechanical structural member in which high
toughness o: ductility are not necessary, in general, machining is performed after hot
forging without performing a refining heat treatment from thc viewpoint of production
costs. In a case where a metallographic structure of steel (non-heattreated steel)
which is produced without performing a refining heat treatinent, is a composite
structure including ferrite and pearlite, excellent machinability and a high yield ratio
are obtained. In a case where the metallographic structure includes bainite, the
machinability deteriorates, and the yield ratio decreases. Therefore, in many cases, a
metallographic structure of rolled or normalized stccl is a composite structure
including ferrite and pearlite.
[0003]
In addition, fatigue resistance may be required for a mechanical structural
member.
In this case, a mechanical structural member having a metallographic
structure, which is a composite structure including ferrite and pearlite, has a problem in
that soft ferrite causes fatigue fracture. In order to solve the problem, for example,
Patent Documents 1 to 3 disclose steel or a hot-forged product in which fatigue
resistance is improved by hardening ferrite and reducing the difference in hardness
between ferrite and pearlite due to solid solution strengthening by addition of Si and
precipitation strengthening by addition of V or the like.
However, in Patent Docunlent 1, it is necessary thzt steel contain more than
0.30% of V. In a casc where the steel contains a large amount of V, even ir the
heating temperature during hot forging is sufficiently high, V is not sufficiently solidsoluted.
In this case, undissolved V carbide remains, which causes a problem in that
the strength and ductility of the mechanical structural member deteriorate.
In addition, in Patent Document 2, it is necessary that steel contains 0.01% or
higher of Al. I-Iowever, A1 has a problem in that A1 fonns a hard oxide in the steel
that significantly deteriorates the machinability thereof. :
In addition, in Patent Document 3, it is necessary that steel contains 1.0% or
higher of MI? and 0.20% or higher of Cr. However, Mn and Cr have a problem in that
they promote bainite transformation and thereby deteriorating machinability and
decreasing the yiclcl ratio.
[00041
On the other hand, for example, Patent Document 4 discloses a steel in which
fatigue resistance (fatigue strength) is improved by solid solution strengthening using
Si instead of V, which is an expensive element and due to refinement of lamellar
spacing by addition of Cr.
However, in a case where steel contains a certain amount or less of Si, fatigue
resistance can be improved. However, in a case where steel contains a large amount
of Si, there is a problem in that a decarburized layer is formed on a surface of steel and
the fatigue resistance of the steel as a mechanical structural member deteriorates. In
addition, in Patent Document 4, it is necessary that steel contains 0.10% or higher of Cr.
However, Cr promotes bainite transformation and thereby deteriorating machinability
and decreasing the yield ratio.
[Prior Art Document]
[Patent Document]
[OOOS]
[Patent Document 11 Japanese Unexanlined Patent Application, First
PublicationNo. H7-3386
[Patent Document 21 Japanese Unexamined Patent Application, First
Publication No. H9-143610
[Patent Document 31 Japanese Unexamined Patent Application, First
Publication No. HI 1-152542
[Patent Document 41 Japanese Unexamined Patent Application, First
Publication No. 1-110-226847
[Disclosure or the Invention]
[Problems to be Solved by the invention]
[OO06]
As described above, in the related art, a mechanical structural member having
excellent fatigue resistance, which contains a large amount of Si without containing Cr
and Al to reduce the costs, has not been provided.
The present inventors perfonned a thorough investigation and found that, in
order to improve the fatigue resistance of a mechanical structural member, in particular,
it is important to control the hardness of a surface of the mechanical structural member.
In addition, the present inventors found that, in order to control the hardness of a
surface of a mechanical structural member, it is effective to control a structure of a
surface part of a rolled steel bar (rolled steel bar for machine structural use) which is a
material of the mechanical structural member.
[0007]
The present invention has been made in consideration of the above-described
circumstances, and an object thereof is to provide a rolled steel bar for machine
structural use which is suitable as a material of a mechanical structural melnber in
which high strength and excellent fatigue resistance are required, and a method of
producing the same.
[Means for Solving the Problem]
[OOOS]
As described above, in order to improve the fatigue. resistance of a mechanical
structural member, it is important to control the hardness of, in particular, a surface of
the mechanical structural member. To that end, it is effective to control a structure of
a surface part of a rolled steel bar (rolled steel bar for machine structural use) which is
a material ofthe mechanical structural nicmber.
However, it was found that, in a case where a rolled steel bar, which contains
a large amount of Si without containiiig CI- to reduce cost, is used as a material of a
mechanical structural member, decarburization of a surface of the mechanical
structural member is significant, the hardness decreases, and the fatigue resistance
deteriorates.
[OO09]
Therefore, the present inventors investigated the effect of decarburization on
fatigue resistance and the reason for decarburization in a mechanical structural member
which is formed of a rolled steel bar containing a large amount of Si. As aresult, the
present inventors discovered that the decarburization of a surface of the mechanical
structural member occurs due to the rolled steel bars which are the material of the
mechanical structural member. In addition, the present inventor made it clear that
decarburization of a surface of a rolled steel bar can be reduced by removing a
decarburized layer of cast piece which is used for manufacturing a rolled steel bar and
succeeded to improve the fatigue resistance of the mechanical structural member.
Further, the PI-esent inventors discovered an optimum chemical co~nposition
and production conditions of a roiled steel bar with which the strength of a mechanical
structural member, which is formed by hot-forging the rolled steel bar, can be
improved while securing the hot ductility of the rolled steel bar which requires during
hot forging.
In addition, the present inventors also discovered that excellent fatigue
resistance can be obtained in the mechanical structural member which is obtained by
hot-forging the rolled steel bar.
[OOl 01
'I'he present invention has bce,m~~ad c based on the above-described findings,
'The summary of thc present invention is as follows.
[001 l]
(1) According to a first aspect of the present iwerition, a rolled steel bar for
machine structural use having a chemical composition including, by mass%, C: 0.45%
to 0.65%, Si: higher than 1.00% to 1.50%, Mn: higher than 0.40% to 1.00%, P: 0.00594
to 0.050%, S: 0.020% to 0.100%, V: 0.08% to 0.20%, Ti: 0% to 0.050%, Ca: 0% to
0.0030%, Zr: 0% to 0.0030%, Te: 0% to 0.0030%, and a remainder including Fe and
impurities, in which the impurities include Cr: 0.1 0% or lower, AI: lower than 0.01%,
and N: 0.0060% or lower, 1C1 obtained from the following Expression 1 is 0.95 to 1.05,
K2 obtained from the following Expression 2 is more than 35, the Mn content and the
S content satisfy the following Expression 3, and a total clecarburized depth of a
surface layer is 500 pm or less,
Kl=C+Sil7+M11/5+1.54xV (Expression I),
K2=139-28.6xSiil05xMn-833xS-13420xN (Expression 2), and
Mn/S>8.0 (Expression 3)
C, Si, Mn, V, S, and N in Expressions 1 to 3 represent the contents of the
respective elements in mass%.
[OO 121
The rolled steel bar for machine structural use according to (I), wherein the
chemical composition may further include, by mass%, one or more selected from the
group consisting of Ti: 0.010% to 0.050%, Ca: 0.0005% to 0.0030%, Zr: 0.0005% to
0.0030%, and Te: 0.0005% to 0.0030%.
[0013]
According to another aspect of the present invention, a method of producing a
rolled steel bar for machine structural usc, tlie rolled steel bar for machine structural
use being the rolled steel bar according to (1) to (2) includes: malting molten steel
having the cheniical composition according to (I) or (2); continuously casting the
molten steel to obtain a cast piece; blooming the cast piece to obtain a steel piece;
scarfing all faces of the steel piece after the blooming at 2 mm or more from a surface;
and performing a steel bar rolling after holding the steel piece after tlie scarfing at a
heating temperature of 1000°C to 1150°C for 7000 seconds or shorter.
[Effects of the Iuvention]
[0014]
In the rolled steel bar for machine structural use according to the aspects of
the present invention in which the Cr content and the Al content are limited and which
includes a large amount of Si to reduce the costs, the thickness of a decarburized layer
of a surface layer can be prevented. A mechanical structural member which is
produced by hot-forging the rolled steel bar has excellent fatigue resistance and thus
remarkably contributes to the industry.
[Embodiment of the Invention]
[OOlS]
A rolled steel bar for machine structural use according to an embodiment of
the present invention (hereinafter, also referred to as "rolled steel bar according to the
embodiment") has a chemical composition including, by mass%, C: 0.45% to 0.65%,
Si: higher than 1.00% to 1.50%, Mn: higher than 0.40% to 1.00%, P: 0.005% to
0.050%, S: 0.020% to 0.100%, V 0.08% lo 0.20%, and a remainder including Fe and
impurities, and optionally further includes Ti: 0.050% or lower, Ca: 0.0030% or lower,
Zr: 0.0030% or lower, and Te: 0.0030% or lower. In the rolled steel bar for machine
structural use, the impurities includes Cr: 0.10% or lower, Al: lower than 0.01%, and
I\i: 0.0060% or lowel; I8.0, and the total dccarburized depth in
surrace layer is 500 pm or less.
[0016]
First, the chemical composition of the rolled steel bar according to the
embodiment will be described. Hereinafter, " % regarding the chemical composition
represents "mass%. In a case where the amount is expressed by a range in the
following description, the range includes an upper limit and a lower limit. That is, in
a case where the amount is expressed by a range of 0.45% to 0.6596, for example, the
range represents 0.45% or higher and 0.65% or lower.
[00 171
(C: 0.45% to 0.65%)
C is a useful element which can increase the tensile strength of the steel at low
cost. In order to obtain the effect, the C content is set to be 0.45% or higher. On the
other hand, the higher the C content in the steel, the lower thc yield ratio of the
mechanical structural nlember obtained by forging a bot,rolled steel bar. 'l'he yield
ratio is a value obtained by dividing a 0.2% proof stress by a tensile strength. When
the yield ratio decreases, in a case where the 0.2% proof stress is a predetermined
value, the tensile strength increases excessively, which causes deterioration in
machinability. Accordingly, the C content is set to be 0.65% or less in order to
prevent a decrease in the yield ratio of the mechanical structural member. The C
content is preferably 0..60% or lower. !
[OOI 81
(Si: I-ligher than 1.00% to 1 SO%)
Si is an element that is inexpensive and is effective for contributing to highstrengthening
of the steel. In order to obtain the efkct, the Si content is set to be
higher than 1.00%. The Si content is preferably 1.10% or higher. On the other hand,
in a case where the Si content is excessively high, the decarburized layer depth of
surface layer is excessively large, hot ductility deteriorates, and defects are likely to
occur during steel bar rolling or hot forging. Therefore, the Si content is set to be
1.50% or lower.
100 191
(Mn: Higher than 0.40% to 1.00%)
Mn is a solid solution strengthening element that can increase the strength of
the steel while preventing a decrease in ductility as compared to Si and V. In addition,
Mn is an element that is bonded to S to form MnS and to thereby improve
machinability. When the Mn content is low, S forms FeS at an austenite grain
boundary and deteriorates hot ductility. Therefore, cracks or defects are likely to be
initiated. Accordingly, in order to prevent the formation of FeS and to secure hot
ductility, the Mn content is higher than 0.40%. On the other hand, in a ease where the
Mn content is excessively high, bainite that decreases the yield ratio may also be
present in a structure of a hot-forged product. Therefore, the MII content is set to be
1.00% or lower. The Mn content is preferably 0.95% or lower and is more preferably
0.90% or lower.
[0020]
(P: 0.005% to 0.050%)
P is an element that promotes ferrite transformation to prevent bainite
I
transformation. In order to prevent bainite transformation during cooling after hot
forging, the P content is set to be 0.005% or higher. On the other hand, in a case
where the P content is excessively higll, hot ductility deteriorates, and defects may bc
initiated in the billet. Therefore, the upper limit orthe P content is limited to 0.050%.
The P content is preferably 0.040% or lower.
[0021]
(S: 0.020% to 0.100%)
S is an element that forms manganese sulfide (MnS) to improve machinability,
and contributes to improvement of machinability. In order to obtain the effect, the S
content is set to be 0.020% or higher. On the other hand, in a case where the S
content is higher than 0.10.0%, a large amount of coarse MnS is dispersed in the steel,
hot ductility deteriorates, and defects may be initiated in the billet. Therefore, the
upper limit of the S content is limited to 0.100%.
[0022]
(V: 0.08% to 0.20%)
V is an element that forms V carbide and/or V nitride to contribute to
precipitation strengthening of the steel, and has an effect of increasing the yield ratio of
the steel. In order to obtain the effect, the V content is set to be 0.08% or higher. On
the other hand, V is an expensive alloy element and promotes undesirable bainite
transformation during cooling after hot forging. Accordingly, in order to reduce the
costs and to prevent bainite transformation, the V content is set to he 0.20% or lower.
The V content is preferably 0.15% or lower.
[0023]
The rolled steel bar according to the embodiment has the above-described
chemical composition and contains a remainder including Fe and impurities.
However, the rolled steel bar according to the embodiment optionally further includes
Ca, Te, Zr, and Ti in the following ranges instead of a portion of Fe. However, since
it is not necessary that the rolled steel bar irtcludcs these elements, the lower limits
thereof are 0%.
Here, the impurities refer to elements that are incorporated frotn raw materials
such as ore or scrap, or incorporated in various environments of the production process
when the steel is industrially produced, and the impurities are allowed to be included in
the steel in a range where there are no adverse effects in the present invention. The
amounts of, in particular, Al, N, and Cr among the impurities, are limited to the
following ranges.
[0024]
(Al: Lower than 0.01%)
A1 is an impurity. In a case where A1 is present in the steel, A1 is bonded to
oxygen to form hard Al oxide and to thereby deteriorate the machinability of the steel.
Accordingly, the lower the Al content, the better. In a case where the Al content is
0.01% or higher, the machinability deteriorates significantly. Therefore, the A1
content is limited to lower than 0.01%.
[0025]
(N: 0.0060% or lower)
N is an impurity. In a case where N is present in the steel, N is bonded to V
to form V nitride. The V nitride is coarser than V carbide and has a small
contribution to precipitation strengthening as compared to V carbide. Accordingly, as
the N content increases, the amount of V nitride increases, and the amount of V carbide
decreases accordingly. As a result, the contribution of V to precipitation
strengthening decreases. In order to obtain the effect of sufficient precipitation
strengthening even in a case where the V content is low, it is preferable that the total
amount of V nitride is small. Therefore, it is preferable that the N content is low. In
a case where the N content is higher than 0.0060%, the contribution of V to
precipitation strengthening decreases significantly. Thcrefore, the N content is
limited to 0.0060% or lower. On the other hand, in a case where the amount of N is
reduced excessivel~: the costs significantly increase due to steelmalting technical
reasons. Therefore, the lower limit of the N content may be set as 0.0020%.
100261
(Cr: 0.1 0% or lower)
Cr is an impurity. Cr has little effect on the strength but promotes bainite
transformation during cooling after hot forging. Therefore, in a case where the Cr
content increases, the yield ratio of a mechanical structural member obtained by hotforging
the rolled steel bar decreases. 'The lower the Cr content is, the better it is. In
a case where the Cr content is higher than 0.10%, the effect thereof is significant.
Therefore, the Cr content is limited to 0.10% or lower
[0027]
(Ca: 0.0005% to 0.0030%)
(Zr: 0.0005% to 0.0030%)
(Te: 0.0005% to 0.0030%)
Ca, Te, and Zr are elements that refine and spheroidize MnS particles (that is,
control the form of a sulfide). In a case where MnS is stretched, the anisotropy of hot
ductility increases. Therefore, cracks are likely to occur in a specific direction. In a
I
case where it is necessary to control the initiation of cracks, the steel may contain one
or more selected from Ca, Zr, and Te. In order to obtain the effect of refining and
I
I ! spheroidizing MnS, it is preferable that each of the Ca content, the Zr content, and/or
I the Te content is 0.0005% or higher. On the other hand, in a case where the Ca
content, the Zr content, or the Te content is excessively high, a coarse oxide of Ca, Zr,
or Te is formed, and thus thc macliinability dcterioratcs Thercfore, cvcn in a case
where the stecl contains Ca, Zr, or Te, it is preferable that each of the Ca content, the
Zr content, and the Te content is 0.0030% or lower.
[0028]
(Ti: 0.010% to 0.050%)
Ti is an elcment that forms Ti nitride in the steel. Ti nitride has an effect of
refining grains of the structure of the steel. In order to obtain this effect, it is
preferable that the Ti content be 0.010% or higher. On the other hand, Ti nitride is
hard, which may decrease the tool life during cutting. Therefore, in a case where the
steel contains Ti, the Ti content IS set to be 0.050% or lower.
[0029]
In the rolled steel bar according to the embodiment, it is necessary that not
only the amounts of the above-described respective elements but also the amounts of C,
Si, Mn, V, S, and N satisfy the following relationships. In the following expressions,
C, Si, Mn, V, S, and N represent the amounts of the respective elements in mass%.
[0030]
(Kl : 0.95 to 1.05)
K1 is a carbon equivalent that is an index indicating the strength and is
obtained from the following (Expression 1).
K1=C+Si/7+Mn/5+1,54xV (Expression 1)
The tensile strength of a mechanical structural member that is formed by hotforging
the rolled steel bar according to the embodiment is affected by the carbon
equivalent KI. In a case where a mechanical 3tructural member is produced by hotforging
a rolled steel bar having a K1 value of 0.95 or more, a structure of the
mechanical structural member includes pearlite, which is a major component, and
ferrite, and the ~i~echanicsatlr uctural member has a te~lsiles trength of higher than 900
MPa, a 0.2% proof' stress of 570 MPa or higher, and a fatigue limit ratio (hligue
liniitltensile strength) of 0.45 or higher. On the other hand, in a case where 1<1 is
higher than 1.05, bainite is formed in the mechanical slructural member, which
decreases the yield ratio. Accordingly, the carbon equivalent 1<1 is limited to 0.95 to
1.05.
[003 11
(K2>3 5)
K2 is an index indicating hot ductility that is obtained from.an experiment
described below by the present inventors, and is obtained from the following
(Expression 2).
K2=139-28.6xSi+l05xMn-833xS-l3420xN (Expression 2)
[0032]
In the experiment, 17 rolled steel bars, which contained 0.52% to 0.54% of C
and were different from each other in the amounts of Si, Mn, I?, S, and N, were used.
The hot ductiliw of a test piece having a diameter of 10 mm and a length of 100 mm,
which was obtained by cutting and processing each of the rolled steel bars, was
evaluated. The hot ductility was evaluated based on values of reduction in area after
breaking which was obtained using a method including: heating and melting the center
of the test piece; holding the test piece at various temperatures immediately after the
test piece was solidified; and drawing the lest piece at a rate of 0.05 mmis to break the
test piece. Regression computation was performed by using the values of reduction in
area at the holding temperatures (tensile temperatures) of 950°C, 1 10O0C, and 1200°C
as dependent variables and using the amounts of the alloy elements as independent
variables, and significant independent variables were averaged to obtain 1<2
(Expression 2).
As a result, in a case where this K2 value is more than 35, dcfects or cracks do
not occur during the casting of the billet and ihe hot forging of the rolled steel bar.
Accordingly, the hot ductility index 1<2 is set to be more than 35.
The upper limit of 1<2 is not necessarily limited and is determined based on
the ranges of the respective amounts of Si, Mn, S, and N. For example, the upper
limit of K2 may be set as 100.
As can be seen from Expression 2, Si, S, and N are factors that deteriorate hot
ductilitv, and Mn is a factor that improves hot ductility. Therefore, basically, it is
necessary that the K2 value is satisfied in consideration a balance between the above
factors. However, as described below, in a case where MnlS is lower than 8.0,
harmful FeS is formed. Even if the K2 value is more than 35, in a case where MniS
is lower than 8.0, the characteristics deteriorate.
[0033]
(MnlSr8.0)
As described above, S is bonded to Mn to form MnS. However, in a case
where the S content is excessively high with respect to the Mn content, not only MnS
but also FeS are formed at an austenite grain boundary. As a result, in this case, hot
ductility deteriorates significantly, and cracks occur during hot forging. Accordingly,
in order to prevent the formation of FeS, MnlS is set to be 8.0 or higher. In a case
where MnlS is 8.0 or higher, the above-described 1<2 value is controlled by hot
ductility. Accordingly, Mn/S is not particularly limited as long as it is 8.0 or higher,
and the upper limit thereof is determined based on the minimunl value of the S content
and the maximum value of the Mn content.
LO0341
Next, the decarburized dcpth and the structure of the rolled steel bar according
to the embodiment will be described.
100351
[Total Decarburized Depth in Surface Layer]
As described above, the decarburized depth of the rolled steel bar (total
decarburized depth in surface layer) affects the fatigue resistance of a mechanical
structural member obtained by hot-forging the rolled steel bar. In a mechanical
structural member that is formed by hot-forging a rolled steel bar having a total
decarburized depth in surface layer of more than 500 pm, the fatigue resistance
(fatigue limit ratio) deteriorates. As the total decarburized depth in surface layer
increases, tensile strength, proof stress, and fatigue limit ratio may decrease due to
decarburization depending on steel components. Accordingly, the total decarburized
depth in surface layer of the rolled steel bar is set to be 500 pm or lower. The lower
limit is 0 pm (that is, a decarburized layer may not be piesent).
In the embodiment, the total decarburized depth in surface layer of the rolled
steel bar is defined as the average valuc of decarburized depths in surface layer
measured at 12 positions in total when decarburized depths are measured at Sour
positions at an angle interval of 90 degrees in a circumferential direction of each of
three cross-sections, the three cross-sections being obtained by cutting the rolled steel
bar at the center thereof in a longitudinal direction and at two positions at a length of
114 of the total length from two opposite ends thereof. The decarburized depth of
surface layer is defined as the depth at \vhich the carbon content measured at a straight
line moving to the inside from the surface is 90% or higher of the constant carbon ;
content measured at the inside (internal carbon content), and can be measured using an
electron probe micro analyzer (EPMA).
[003 61
It is not necessary to limit the structure (metallographic structure) of the rolled
steel bar according to the embodiment. However, as dcscribed abovc, it is preferable
that the mechanical structural member has a composite structure (ferrite-pearlite
structure) including ferrite and pearlitc. In a case where the structure of the
mechanical structural member is a structure including ferrite and pearlite, thc structure
of the rolled steel bar is also a structure including ferrite and pearlite in many cases.
[0037]
Next, an example of a method of producing the rolled steel bar according to
the embodiment will be described.
[0038]
The rolled steel bar according to the embodiment is produced using a method
including: malting molten steel having the above-described chemical composition
using an ordinary method (molten steel making step); a cast piece by an
ordinary method, for example, continuous casting (casting step); blooming the cast
piece to obtain a steel piece (blooming step), scarfing all faces of the steel piece
(scarfing step), and hot-rolling (also referred to as steel bar rolling) the steel piece after
scarfing (steel bar rolling step).
In a case where the steel piece in which all faces are scarfed at 2 mm or more
during the scarfing step is subjected to the steel bar rolling step, not only
decarburization of the steel bar, but also decarburization of the mechanical structural
member which is produced by forging the rolled steel bar are suppressed. As a result,
deterioration of the fatigue resistance of the mechanical structural member can be
prevented.
[0039]
A deep decarburized layer which is formed on ihc surfacc of the steel piecc
having Si as mentioned above remains not only in the rolled stecl bar but also in the
hot forgings (the mechanical structural nicmher) produced using the steel bar as a
material. I11 addition, the deep decarburized layer deteriorates the ~iiechanical
properties of the mechanical structural member, in particular, the fatigue resistance.
For example, the present inventors performed an investigation and found that: in a case
where the steel piece having large amount of Si was cast to have a cross-sectional area
of 196000 cm2 and was bloomed to have the cross-sectional area of 26244 cm2, the
decarburized depth of surface layer was 1.8 mm at a maximum. Accordingly,
although the depth of the dccarburization depends on the size of the cast piece and the
size of the steel piece, in a case ihe steel piece is produced through the blooming step,
as long as the steel piece is hot rolled after bloon~inga nd scarfing the face at 2 mm or
more from the surface, it is possible to set the total decarburized depth in surface layer
to 500 pm or less. On the other hand, when the scarfing amount is excessive, a
reduction of the weight, an increase of the surface detcrioration, an increase of the
scarfing cost, and an increase of the scarfing time are worried. Therefore, it is
preferable to set the scarfing amount to be 10 mm or less. It is preferable that the
scarfing is performed on all faces of the steel piece
The object of the scarfing the steel piece is to remove the deep decarburized
layer formed during the continuous casting. In subsequent steps, when the conditions
I
are controlled properly, the deep decarburized layer which deteriorates the fatigue
resistance of the hot forging is not formed. According to the size of the steel piece,
1 blooming is performed again after blooming and scarfing; however, the heating time in
i
i
i
blooming is set to 900 seconds or shorter
Scarfing of the steel picce may be performed by themomechanilally scarfing
the surface of the stecl picce using combuslion gas and oxygen. In addition, the
scarfing may bc performed in a state that the steel piece has a high temperature or ill a
state that the steel picce is cooled. In a case where the bloomi~lg is perfonned again
after the blooming and the scarfing, it is preferable that the scarfing is performed in a
state in which the steel piece is not cooled and has a high temperature. On the other
hand, machining using a grinder and the like, is inefficient, therefore, is not included in
the method of the present embodiment.
[0040]
In the steel bar rolling (hot rolling) step, in order to promote solid solution of
V into the steel, it is necessary to heat the billet to 1000°C or higher and to perform hot
rolling. By dissolving V to be solid-soluted during the heating of the steel bar rolling,
the size of V carbide that reprecipitates in the rolled steel bar after hot rolling is small.
As a result, during heating for hot-forging tlie rolled steel bar, the solid solution of V
carbide is easy, and the amount of undissolved V carbide that causes a decrease in the
strength and ductility of the mechanical structural member is reduced. In a case
where the heating temperature is lower than 100O0C, V is not sufficiently solid-soluted.
On the other hand, it is necessary that the upper limit of the heating temperature during
the steel bar rolling is set as 1150°C. The reason for this is that, in a case where the
billet is heated to a temperature of higher than 1150°C, the rate of surface
decarburization increases rapidly. In addition, in a case where the holding time at the
heating temperature increases, the decarburization is promoted. Accordingly, in order
to reduce the total decarburized depth in surface layer of the rolled steel bar to 500 pm
or less, the holding time at the heating temperature (lOOO°C to 11 50PC) is set to be
7000 seconds or shorter. In order to sufficiently solid-solute \I, it is preferable that the
lower limit of holding time is set to be 10 seconds.
[OOill]
According to the production mctliod including the above-dcscribed steps, the
rolled steel bar according to the embodi~iientc an be obtained. In addition, by forging
the rolled steel bar, a structural meniber having excellent fatigue resistance can be
obtained. Forging conditions may be the same as conditions under which a rolled
steel bar is usually forged. For example, the heating temperature is 1000°C to
1300°C. In a case where a mechanical structural member is fomied by forging, a
material of the mechanical structural member is hot-forged after high-frequcncy
heating in many cayes. Since the high-frequency heating, the heating time for the
temperature to reach a predetermined value is short, extreme decarburization is less
liltely to occur on the surface layer of the material (rolled steel bar).
[Examples]
[0042]
[Example 11
By casting Steel A having a chemical composition shown in Table 1, cast
piece having a size of 350x560 mm was obtained. Steel A includes a low amount of
C and a high amount of Si, in which decarburization is likely to occur. The remainder
of Table 1 includes Fe and impurities. All of the faces of the cast piece are scarfed
under the conditions in which target scarfing amount is 1 mm, 2 mm, or 3 mm
immediately after heating the cast piece to 1300°C and blooming the cast piece to have
a cross section size of 280x280 mm, is rolled to have the cross section size of 162x162
mm, and is cooled to obtain a steel piece as a material of a rolled steel bar.
t The steel pieces were heated to 1 150°C or 123OoC, were held at this
temperature for 5000 seconds or 10000 seconds, and then were hot-rolled to produce
rolled steel bars having a diameter of 50 mm. Then, these rolled steel bars were aircooled
at rooin temperature Thc total decarburized depths in surface layer ol'the
rolled steel bars were obtained using the above-described method.
Table 2 shows the results of scarfed depth, heating condition during the steel
bar rolling, and the measured total decarburized depths in surfacc layer of the rolled
steel bars.
[DO431
[Table 11
[0044]
[Table 21
P
ICI
pp
0.97
MdS
191
Component (mass%)
7C -P S V I CT 1 Al N
K2
85
-
0.45 1.50 0.84 0.020 OW 0.09 0.08 0.005 0.0047
No. Note
Comparative Exampie
'Total Decarbukd Deptlof
Surface Layer
4vemge Scarfmg
Depth
Example
Exawle
Comparative Exaqle
Stccl Bar Rolhg
Comparative Examplr
Heating
Te~weraturc
Holding
Tim
[00451
It can be seen from Samples Al to A3 that, by adjusting the scarred depth to
be 2.0 mn~or more, the decarburized depth of the rolled steel bar can he reduced to he
500 pn or less even in a case where heating conditions during steel bar rolling arc a
high temperature and a long time such as I 150"Cx7000 seconds, in which
decarburization is promoted.
The object of the scarfing the steel piece is to remove the deep decarburized
layer formed duriug the continuous casting. In subsequent steps, the deep
decarburized layer which deteriorates the fatigue resistance of ihe hot forging is not
formed.
Sample No. A4 in table 2 is an exanlple in which holding time is excessively
long at 1 150°C and the total decarburized depth in surface layer is large. In addition,
sample No. A5 is an example in which heating temperature is 1230°C and the total
decarburized depth of the surface layer is large.
[0046]
[Example 21
Steels (Nos. U to AD) having chemical compositions shown in Tablc 3 were
made and then were continuously cast and obtain cast pieces. The cast pieces were
bloomed to obtain steel pieces. The steel pieces excluding Test Nos. 12 to 19 were
scarfed under conditions in which target scarfing amount is 3 mm. The remainder of
Table 3 includes Fe and impurities. These scarfed steel pieces were hot-rolled to
produce rolled steel bars having a diameter of 45 mm. Some steel pieces (Test Nos.
12 to 19 in table 4) were scarfed under condition in which the target scarfing amount is
1 mm and were hot-rolled lo produce rolled steel bars having a diameter of 45 mm, for
purposes of comparison. The steel pieces were hot-rolled at a heating temperature of
1100°C Sor a holdlng ttlne of 3600 seconds After the hot rolling, thc rolled steel bars
were a~r-cooledto normal ternperaturc
100471
[Table 31
Undertined vabmr ropreren,r (hat UleMilur aze out ofihe ruiEcof4c pmrant krrlb"
- 25
[0048]
Next, the total decarburized depths in surface layer of the rolled steel bars
obtained by hot rolling were measured using the above-described method.
Subsequently, each of the rolled steel bars having a diameter of45 mm was
heated to 1220°C by high-frequency heating, was held at 1220°C for 300 seconds, and
immediately was pressed in a diameter direction to be forged into a flat sheet having a
thickness of 10 mm. By cutting a side surface of the forged flat sheet, a test piece
which has a parallel body having a cross-sectional width of 15 mm, a thicltness of 10
., T , , . . rnm (tl3ichr.c~as foryxl), and a length of 20 mm was obtained and provided for a.
tension compression fatigue test under completely reversed tension and compression
and a tensile test. The tension compression fatigue test was performed according to
JIS Z 2273, in which a maximum load stress representing a lifetime of lo7 or more was
set as a fatigue limit. The tensile test was performed according to JIS Z 2241 at room
temperature at a rate of 20 mdmin.
The forged surface of the parallel body was as forged without working.
However, for reference, regarding Steels Nos. B and C, test pieces from which a
decarburized layer was removed by grinding the surface into a depth of 500 Fm after
hot forging were provided (Test Nos. 2 and 3 in Table 4). In addition, all the corners
of the cut portions of the test pieces were chamfered with a radius of 2 mm.
[0049]
Tables 4 and 5 show the total decarburized depth in surface layer of the rolled
steel bars before hot forging, the microst~uctureso f the forged flat sheets after hot
forging, the 0.2% proof stresses, the tensile strengths, the yield ratios (0.2% proof
stressltensile strength), and the fatigue limit ratios (fatigue iimitlte~lsiles trength) at lo7
times obtained by the tension compressioii test.
l00501
['Table 41
'72 FF: ~ e n i t ean d ~ ~ ~ rslniuicei~ uei8, :b uiiiiii: smicture
'j iepreselitr Uist tile eviiiiiaIioi~~ v a siio t able to be perioiiiled
[0052]
Test Nos. 4 to 11 and 20 of Table 4 are Examplcs according lo the present
invention. All the total decarburized depth in surface layer of the rolled steel bars,
which were scarfed all faces under conditions in which target scarfing amount is 3 mm,
were 500 pm or less. In addition, in the forged article (forged flat sheets) obtained by
forging the rolled steel bars, the tensile strengths were 948 MPa or higher, the 0.2%
proof stresses were 597 MPa or higher, and the fatigue limit ratios (fatigue limititensile
strength) obtained by the tension compression fatigue test were 0.47 or higher. In
addition, from a comparison between Test Nos. 2 and 3 in table 4 in which the
decarburized layer was removed by grinding after hot forging and Test Nos. 4 and 5, it
can be seen that, in a case where the decarburized depth in the rolled steel bar is 500
pm or less, a decrease in the fatigue limit ratio is 0.02 or less.
Test Nos. 12 to 19 of Table 4 are Coinparative Examples in which the
decarburized depth of the rolled steel bar was more than 500 pm. These are rolled
steel bar which are obtained by scarfing all faces at 1 mm and hot rolling. Each of
these examples does not satisfy at least onc of tensile strength: 900 MPa or higher,
0.2% proof stress: 570 MPa or higher, and fatigue limit ratio: 0.45 or more.
[00531
Test Nos. 21 to 39 of Table 5 are Comparative Examples produced using
Steels Nos. K to AD in which the any of the steel component (chemical composition),
MnIS, K1, or K2, is out of the range of the present invention.
In Test Nos. 22,23,24,27,28, and 32 using Steel Nos. L, M, N, Q, R, and V
in which MIS was lower than 8.0 or the K2 value was lower than 35%, cracks or large
defects occur during steel bar forging, and thus the evaluation was not performed after
hot forging. Therefore, the evaluation items of Table 5 are shown as "*".
In Test No. 33 (Steel No. W), the ICI value was low, and the tensilc strcngth
and the 0.2% proof stress did not reach 900 MPa and 570 MPa, which were desired
values, respectively.
In Test No. 21 (Steel No. K), the C content, the Si content, and the I