Document Typ" Specification
[Title of the invention] ROLLED S'TEEL BAR FOR MACHINE S'TRUCI'UIII\L
USE AND METHOD OF PIZODUCING TI-IE SAME
[Technical Field of the Invention]
[OOOl]
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 stmctural member") produced by hot forging or
the like, and a method of producing the same.
Priority is claimed on Japanese Patent Application No. 2014-137736, 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 he
required. In this case, it is preferable that a metallographic structure of the
mechanical structural rilemher 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 hot forged a steel bar which is a material of
the mechanical structural member.
On the other hand, in a mechanical structural member in which high
toughness or ductility are not necessary, in general, machining is performed afler hot
forging without performing a refining heat treatment from the viewpoint of production
costs. In a case where a metallographic structure of steel (non-heattreatecl steel),
which is produced without performing a refining heat treatment, is a composite
structure including ferritc and pearlite, cxccllent machinability and a high yield ratio
are obtained. In a case where the metallographic structurc includes bainite, the
machinability dctcriorates, and the yield ratio decreases. Therefore, in many cases, a
metallographic structure of rolled or normalized stecl is a composite structure
including ferrite and pearlite.
[0003]
In addition, fatigue resislance may be required for a mechanical structural
member.
, . In this case, a mechanical stmct~rralm ember having a metallograph~c
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.
IIowever, in Patent Document 1, it is necessary that steel contain more than
0.30% of V. In a case where the steel contains a large amount of V, even if the
heating temperature during hot forging is sufficiently high, V is not sufficiently solidsoiuted.
In this case, undissolved V carbide remains, which causes a problem in that
the wrength and ductility of the mechanical structural member deteriorate.
111 addition, in Patent Document 2, it is necessary that steel contains 0.01% or
higher of Al. However, A1 has a problem in that A1 forms a hard oxide in the stcel
that significantly deteriorates the machinability thereof.
In addition, in Patent Document 3, it is necessary that steel contains 1.0% or
higher of Mn and 0.20% or highel of Cr. However, Mn and Cr have a problem in that
thcy promote bainitc transfo~mationa nd thereby deteriorating machinability and
decreasing the yield ratio.
100041
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 stecl 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 Unexamined Patent Application, First
Publication No. H7-3386
[Patent Document 21 Japanese Unexamined Patent Application, First
Publication No. H9-143610
[Patent Document 31 Japanese Unexamined Patent Application, First
Publication No. H11-152542
[Patent Document 41 Japanese Unexamined Patent Application, First
Publication No. Hl0-226847
[Disclosure of the Invention]
[Problems to bc Solved by the Invention]
[0006]
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 A1 to reduce the costs, has not been provided.
The present inventors performed 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 s~ufaceo f 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 ofa
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 member 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 eEective to control a structure of
a surrace part of a rolled steel bar (rolled steel bar for machine structural use) which is
a malerial of the mechanical structural mcmben
However, it was found that, in a case whcrc a rolled steel bar, which contains
a large amount of Si without containing Cr 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.
[0009]
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 a result, the
present inventors discovered that the decarburization of a surface of the mechanical
structural member occurs due to ihe rolled steel bars which are the material of the
mechanical structural member.
Further, the present inventors clarified that decarburization of a surface of a
rolled steel bar is derived from decarburization of a cast piece which is promoted in a
temperature range of an aly dual phase region in which ferrite (a) and austenite (y) are
present together during cooling after continuous casting or during heating before hot
rolling, and investigated countermeasures. The present inventors clarified that, by
increasing the C content in the steel to reduce the temperature range of an aly dual
phase region (a temperature difference between the Aj temperature and the A,
temperature) in which decarburization is promoted and reducing the size of a cast piece
during casting, a period of time during which the temperature of the cast piece is in the
aly dual phase region is reduced and the decarburization of a surface of a rolled steel
bar can be reduced. In addition. it was also found that, by reducing the size of the
billet, a blooming step for adjusting the size of a billet aftel casting can be removcd.
Fui-thcr, the present invclltors discovered an optimum component composition
(chemical composition) and production conditions of a rolled steel bar with which the
strength of a mechanical structural member, which is formcd 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 (fatigue limit ratio) can bc obtained in the mechanical structural member
which is obtained by hot-forging the rolled steel bar.
[OOlO]
The present invention has been made based on the above-described findings.
The summary ofthe present invention is as follows.
[OOll]
(1) According to a first aspect of the present invention, 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: 0.005%
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: 0.10% or lower, Al: lower than 0.01%,
and N: 0.0060% or less, K1 obtained from the following Expression 1 is 0.95 to 1.05,
K2 obtained from the following Expression 2 is more than 35, K3 obtained from the
following Expression 3 is 10.7 or more, the Mn content and the S content satisfy the
following Expression 4, and a total decarburized depth in surface layer is 500 pnl or
less,
K l=C+Si/7+Mn/5+1.54xV (Expression I ) ,
K2=139-28.6xSi+l05xMn-833xS-l3420xN (Expression 2),
1<3=137xC-44.0xSi (Expression 3), and
MidS?8.0 (Expression 4)
C, Si, Mn, V, S, and N in Expressions 1 to 4 represent the contents of the
respective clcments by mass%.
LO01 21
(2) 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]
(3) According to another aspect of the present invention, a method of
producing a rolled steel bar for machine structural use, the rolled steel bar for machine
structural use being the rolled steel bar for machine structural use according to (I) to
(2) includes: making molten steel having the chemical composition according to (1) or
(2); continuously casting the molten steel to obtain a cast piece having a crosssectional
area of 40000 cm2 or less; and subsequently to the continuous casting,
heating the cast piece to a temperature range of 1 OOO°C to 11 50°C and holding the cast
piece in the temperature range for 7000 seconds or shorter and performing a steel bar
rolling.
[Effects of the Invention]
[OOI 41
In the rolled steel bar for machine structural use according to the aspects of
the present invention in which the Cr content and the A1 content are limited and which
includes a large amount of Si to reduce the costs, the formation of a deep decarburized
layer can bc prevented. A mecha~~icsatlr uctural member which is produced by hotforging
the rolled stecl bar has excellent fatigue resistance and thus remarlcably
contributes to the industry. In addition, under the production conditions according to
the aspects of the present invention, a blooming step can be removed from the
production steps of the rolled steel bar. Therefore, the production costs can be
reduced, and the contribution to the industry is extremely significant.
[Embodiment of the Invention]
[00 151
Arolled 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% to 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 stecl bar for machine
structural use, the impurities includes Cr: 0.10% or lower, Al: lower than 0.01%, and
N: 0.0060% or lower, K1 obtained from "Kl=C+Sil7+Mn/5+1.54xV" is 0.95 to 1.05,
K2 obtained from "K2=139-28.6xSi+105xMn-833xS-l342OxNis more than 35, K3
obtained from "K3=137xC-44.OxSi" is 10.7 or more, the Mn content and the S content
satisfy Mn/S?8.0, and the total decarburized depth in surface 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 content 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 content is expressed by a range of 0.45% to 0.65%, for example, the range
represents 0.45% or higher and 0.65% or lower.
[0017]
(C: 0.45% to 0.65%)
C is an element which can increase the tensile strength oofthe steel at low cost
In addition, C is an element and decreases the A3 temperature of the steel
Decarburization of a surface of a cast piece is promoted when the temperature ofthe
cast piece is in an aly dual phase region (that is, a temperature range of the A3
temperature to the A1 temperature) during cooling after continuous cooling or during
heating before hot rolling. Therefore, decarburization of the surface of the cast piece
is reduced by increasing the C content, and thereby narrows the temperature range of
the uly dual phase region.
In the rolled steel bar according to the embodiment, the C content is set to be
0.45% or higher in order to narrow the temperature range of the uiy dual phase region
and to thereby secure the strength. On the other hand, in a case where the rolled stcel
bar according to the embodiment is continuously cast immediately aftel being formed
by hot forging, the higher the C content in the steel, the lower the yield ratio. The
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 strnctural member.
The C content is preferably 0.60% or lower.
[0018] i
(Si: Higher than 1.00% to 1.50%)
Si is an element that is inexpensive and is effective for contributing to high
strengthening of the stccl. In order to obtain the effect, thc Si content is set to be
higher than 1.00%. The Si content is preferably 1.1 0% or higher. On the other hand,
in a case where the Si content is excessively high, the decarburized depth of surface
layer is excessively large, hot ductility deteriorates, defects are likely to occur during
steel bar rolling or hot forging. As the Si content increases, the temperature range of
the a17 dual phase region become broader. Therefore, the Si content is set to be
1 .SO% or lower.
[OO 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 case where the
Mn content is excessively high, bainite that decreases the yield ratio may also be
present in a stmcture of a hot-forged product. Therefore, the Mn 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
transformation. In order to prevent bainite transformation during cooling after hot
forging, the P content is set to be 0.005% or higher. On thc other hand, in a case
where the P content is excessively high, hot ductility deteriorates, and defects may be
initiated in the billet. Thercfore, the upper limit of the 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 erfect, 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 0.100%, 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 be 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
i t is not neccssary that the rolled stcel bar includes these elements, the lowcr limits
thereof are 0%.
Here, the impurities rcfer to elements that are incorporated from raw malerials
such as ore or scrap, or incorporated in various cnvironrnents of the production proccss
when the steel is industrially produced, aid 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 A1 oxide and to thereby deteriorate the machinability of the steel.
Accordingly, the lower the Al content, thc 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%.
[OOZS]
(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. Therelore, it is preferable that the N content is low. In
a case where thc N content is higher tkan 0.0060%, in particular, the contribution of V
to precipitation strengthening decreases significantly. Therefore, the N content is
limited to 0.0060% or lower. On the other hand, in a case where the amount of N is
reduced, the costs increase due to steelmalung technical reasons. Therefore, the
lower limit of the N content may be set as 0.0020%.
LO0261
(Cr: 0.10% 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, the better. 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, craclts are likely to occur in a specific direction. In a
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
spheroidizing MnS, it is preferable that each of the Ca content, the Zr content, andlor
the Te content is 0.0005% or higher. On the other hand, in a case where the Ca
content, the Zi content, or the Te content is excessively high, a coarse oxide of Ca, Zr,
- 13 -
or Tc is formed, and thus the n~achinabilityd eteriorates. Therefore, even in a case
where the steel contains Ca, Zr, or Te, it is preferable that each ofthe Ca content, the
Zr content, and the Te content is 0.0030% or lower.
[0028]
Ti: 0.010% to 0.050%
Ti is an element 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 rollcd steel bar according to the embodiment, it is necessary that not
only the amounts of thc 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 I).
Kl=C+Sil7+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 K1. In a case where a mechanical structural member is produced by hotforging
a rolled steel bar having a KI value of 0.95 or more, a structure of the
mechanical structural member includes pearlite, which is a major component, and
fcrritc, and thc mcchanical struct~~rmale mber has a tcnsilc strc~lgtiol f higher than 900
MPa, a 0.2% proof stress of 570 MPa or higher, and a fatigue limit ratio (fatigue
limititensile strength) of 0.45 or higher. On thc other hand, in a case where J35)
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+105xMn-833xS-13420xN (Expression 2)
COO321
In the experiment, 17 rolled steel bars, which contained 0.52% to 0.54% of C
and were different lrom each other in the amounts of Si, Mn, P, S, and N, were used.
The hot ductility 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 test piece at a rate of 0.05 mmls to break the
test piece. Regression computation was performed by using the values of reduction in
area at the holding temperatures (tensile temperatures) of 95OoC, 1 10O0C, and 1200°C
as dependent variables and using the ilmounts of the alloy elements as independent
variables, and significant independent variables were averaged to obtain K2
(Expression 2).
As a result, in a case whcre this K2 value is more than 35, defects or cracks do
not occur during the casting of the billet and the hot forging or the rolled steel bar.
Accordingly, the hot ductility index 1<2 is set to be more than 35.
The upper limit of K2 is not necessarily limited and is determined based on
the ranges ofthe respective amounts of Si, Mn, S, and N. For example, the upper
limit of I(2 may be set as 100.
As can be seen from Expression 2, Si, S, and N are factors that detctiorate hot
ductility, 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 MdS is lower than 8.0,
harmful FeS is formed. Even if the K2 value is more than 35, in a case where MdS
is lower than 8.0, the characteristics deteriorate.
[0033]
(K3Y 0.7)
K3 is an index indicating the width of the temperature range of the aly dual
phase region affecting the surface decarburization, and is obtained from the following
(Expression 3).
K3=137xC-44.0~3 (Expression 3)
By adjusting K3 to be 10.7 or higher in the steel composition of the rolled
steel bar according to the embodiment, the temperature range of the aly dual phase
region can be narrowed, for example, 80°C or lower. In this case, the decarburization
occurring on the surface of the cast piece during cooling afler continuous casting or
during heating before hot rolling can be reduced. As a result. the decarburization of
the surface of the rolled steel bar is reduced, and deterioration in the fatigue resistance
or the mechanical structural lucnlber obtained after hot-forging can be prevented
From the viewpoint of reducing the decarburi~ationi,t is preferable that the
temperature range of the aly dual phase rcgion is narrow. Thercforc, it is not
necessary to set the upper limit of thc K3. However, in a case where the K3 value is
high and the temperature range of the aly dual phase rcgion is narrow, the structure
after hot forging consists of only pearlite, and the yield ratio may decrease. Therefore,
the upper limit of K3 may be set as 60.
[0034]
(MdS28.0)
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, Mn/S is set to be 8.0 or higher. In a case
where Mn/S is 8.0 or higher, the above-described K2 value is controlled by hot
ductility. Accord'mgly, MdS is not particularly limited as long as it is 8.0 or higher,
and the upper limit thereof is determined based on the minimum value of the S content
and the maximum value of the Mn content.
[0035]
Next, the decarburized depth and the structure of the rolled steel bar according
to the embodiment will be described.
COO361
[Total Decarburized Depth in Surface Layer]
As described above, the decarburized depth of the rolled steel bar (total
decarburized depth in surface layer) affccts the fatigue resistance ol: a mechanical
struclural member obtained by hot-forging the rolled stccl bar. I11 a inechanical
structural me~nbctrh at is formed by hot-forging a rolled steel bar having a total
decarburized depth in surface layer of more than 500 pnm, the fatigue rcsistance
(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
decarburizaiion depending on steel components. Accordingly, the total decarburized
depth in surfacc 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 present).
In the embodiment, the total decarburized depth in surface layer of the rolled
steel bar is defined as the average value of decarburized depths in surface layer
measured at 12 positions in total when decarburized depths are measured at four
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 which 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).
[0037]
It is not necessary to limit the structure (metallographic structure) of the rolled
steel bar according to the embodiment. However, as described above, it is preferable
that the mechanical structural member has a composite structure (ferrite-pearlite
structure) including ferrite and pearlite. In a case where the structure of the
mechanical structural member is a structure including ferrite and pearlite, the structure
of the rolled steel bar is also a structure including ferrite and pcarlite in many cases.
[0038]
Next, an example of a method of producing thc rolled stcel bar according to
the embodiment will be described.
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); continuously casting the mokten
steel to obtain a cast piece having a cross-sectional area of40000 cm2 or less (casting
step); and hot-rolling (also referred to as steel bar rolling) the cast piece obtained by
casting (steel bar rolling step). In the mcthod of producing the rolled steel bar
according to the embodiment, the casting cross-sectional area of the cast piece is
sufficiently small at 40000 cm2 or less. Therefore, blooming for reducing the crosssectional
area is not performed before the steel bar rolling.
[0039]
As the casting cross-sectional area during the continuous casting is small, a
period of time during which the temperature of the cast piece is in the aly dual phase
region is reduced, and the surface decarburization is prevented. The prescnt inventors
performed an investigation and found that: in a case where the steel having the abovedescribed
chemical composition was cast to have a cross-sectional area of 196000 cm2,
the decarburized depth of surface layer was 1.8 mm at a maximum; however, in a case
where the steel having the above-described chemical composition was cast to have a
cross-sectional area of 40000 cm2, the decarburized depth of surface layer was 0.7 mm
at a maximum. In addition, in a case wllere the cross-sectional area was 40000 cm2
during casting, the decarburized depth of surface layer was not more than 500 pm in a
rolled steel bar having a diameter of '70 inm which was produced by hot-rolling the
cast piece under conditions described below without blooming. As described above,
in a case where the decarburized depth of surface layer of a rolled steel bar is 500 ym
or less, a hot-forged product (mechanical structural member) produced by hot-forging
the rolled steel bar has a small decrease in fatigue strength caused by surface
decarburization. Accordingly, it is preferable that the casting cross-sectional area in
the casting step is limited to 40000 cm2 or less. In a case where the casting crosssectional
area exceeds 40000 cm2 ,.i t .is difficult to perform the steel bar rolling without
blooming. During thc casting, conditions other than the casting cross-sectional area
may be the same as those of an ordinary method.
[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 the 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 1 150°C. The reason for this is that, in a case where the
billet is heated to a temperature of higher than 1 15O0C, 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 teinperature (1000°C to 11 50°C) is set to be
7000 seconds or shorter. In order to sufliciently solid-solute V, it is preferable that the
holding time is set to be 10 seconds or longer.
[0041]
According to the production method including the above-described steps, the
rolled steel bar according to thc embodiment can be obtained. In addition, by forging
the rolled steel bar, a structural member 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 rolled steel bar is forged at 1000°C to
1300°C. In a casc where a mechanical structural member is formed by forging, a
material of the mechanical structural membcr is hot-forged after high-frequency
heating in many cases. Since the high-frequency heating, the heating time for the
temperature to reach a predetermined value is short, extreme decarburization is less
likely to occur on the surface layer of the material (rolled steel bar).
[Examples]
[0042]
[Example 11
By continuous casting Steel A having a chemical composition shown in Table
1, plural cast pieces having a cross-sectional area of 26244 cm2 (cross-section size:
162x162 mm), a cross-sectional area of 40000 cm2 (cross-section size: 200x200 mm),
or a cross-sectional area of 75000 cm2 (cross-section size: 250x300 mm) were obtained.
Steel A includes C and Si such that the K3 value is near the lower limit. In this
composition, decarburization is likely to occur. The remainder of Table 1 includes Fe
and impuritics. 3
As shown in Table 2, these cast pieces were heated to 11 50°C or 1 20OoC,
were held at this temperature for 7000 seconds or 10000 seconds, and then were hotrolled
to produce rolled steel bars having a diameter of 70 mm. Then, these rolled
stccl bas were air-cooled at room temperature. The total decarburized depths in
surface layer of the rolled steel bars were obtained using the abovc-dcscribcd mcthod.
Table 2 shows the results of measuring the cross-sectional areas of the cast
pieces and the total decarburized depths in surface layer of the rolled steel bars.
100441
[Table 21
pable 1)
Steel
A
Component (mass%)
C I Si IMnI P I S I V I C r l A1 1 N
0.48 1 1.25 10.62 10.017 10.051 10.11 10.07 10.006 10.0055
K2
52
MnIS
12.2
K3
10-.8-
KI
0.95
[0045]
It can be seen from Samples A1 to A3 that, by adjusting the casting crosssectional
area of each of Samples No. A1 to A3 to be 40000 cm2 or less, the total
decarburized depth in surface layer of the rolled steel bar can be reduced to be 500 pm
or less even in a case where heating conditions during steel bar rolling are a high
temperature and a long time (115OoCx7000 seconds), in which decarburization is
promoted. Furthet; it can be seen from the results of Sample No. A4 that, even in a
case where the heating tcmperatnre at the start of steel bar rolling is set as 1 150°C,
when a holding time is 10000 seconds which is longer than 7000 seconds, the total
decarburized depth in surface layer of the rolled steel bar is excessively deep. In
addition, it can be seen from the result of Sample No. AS that, in a case where the
heating temperature during the steel bar rolling is set as 1200°C, the total decarburized
depth in surface layer of tbe rolled stecl bar is excessively deep. Therefore,
supposedly, it is preferable that the holding temperature at the start of steel bar rolling
is 1000°C to 1150°C and the holding time is 7000 seconds or shorter.
[0046]
[Exaniplc 21
Steels (Nos. I3 to AH) having chemical compositions shown in Tablc 3 were
made and then were continuously cast. As a result, cast pieces having a crosssectional
area of 40000 cm2 were obtained. Thc remainder of Table 3 includcs Fc and
impurities. These cast pieces were hot-rolled without blooming to produce rolled
steel bars having a diameter of 40 mm. As shown in Table 4, the cast pieces were
hot-rolled at a heating temperature of 1150°C to 1200°C for a holding time or2000
seconds to 7000 seconds. After the hot rolling, the rolled steel bars were air-cooled.
LO0471
[Table 31
Underlined values represents that the values are out of the range of the present invention.
The total decarburized depths it1 surfacc layer of the rolled stccl bars wcrc
obtained using the above-described method. The results are shown in Table 4.
Next, each of the rolled steel bars 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 thiclcness 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 thickness of 10 mm (thickness as forged),
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 mmlmin.
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 ym after
hot forging were provided (Test Nos. 2 and 3). In addition, all the comers 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 microstructures of the forged flat sheets after hot
forging, the 0.2% proof stresses, the tensile strengths, the yield raQos (0.2% proof
stressltensile strength), and the fatigue limit ratios (fatigue limitltensile strenglh) at lo7
times obtained by the tension compression test.
[OOSOl
[Table 41
*I FP: Fenite and pearlite structures
Test Nos. 2 and 3 are reference ewrnples in which the decarburized layer was removed by grinding after ]lot forging
Note
4
Total Decarburized Depth
in Surface Layer
~rm
Holding
Test Time Fatigue
0.2% Proof
Stress
see
No. Microstructure* l
1
Tensile
Steel Strength
Limit Ratio
Heating
Temperature
No. Yield Ratio
MPa O C MPa
[OOSll
[Table 51
Note
*2FP: Fenite and pearlite stluctures, B: bainite structure
* represents that the evaluation was not able to be performed.
Rolled Steel Bar
Total Decarburized
Depth in
Surface Layer
~rm
355
*
*
Test
No.
21
22
2
Forged Flat Sheet
Steel
No.
K
L
M
Microstmcture*2
FP
*
*
Fatigue
Limit Ratio
0.44
*
*
0.2% proof
Stress
MPa
548
*
*
Heating
Temperature
"C
1150
1150
1150
Time
sec
7000
7000
7000
~ ~ ~ ~ i l ~
Strength
MPa
794
*
*
Ratio
0.69
*
*
[0052]
'Tcst Nos. 4 to 1 1 and 20 of 'fable 4 are Examples according to the present
invention. All the total decarburized depth in surface layer of the rolled steel bars
were 500 pm or less. In addition, in the forged flat sheets obtained by forging the
rolled steel bars, the tensile strengths were 91 1 MPa or higher, the 0.2% proof stresses
were 592 MPa or higher, and the fatigue limit ratios (fatigue limit/tensile strength)
obtained by the tension compression fatigue test were 0.46 or higher. In addition,
from a comparison between Test Nos. 2 and 3 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.
[0053]
Test Nos. 12 to 19 of Table 4 are Comparative Examples in which the
decarburized depth of the rolled steel bar was more than 500 pm. Each of these
examples does not satisfy at least one of tensilc strength: 900 MPa or higher, 0.2%
proof stress: 570 MPa or higher, and fatigue limit ratio: 0.45 or more.
[0054]
Test Nos. 21 to 44 of Table 5 are Comparative Examples of Steels Nos. K to
AH in which the any of the steel component (chemical composition), Mn/S, K1, K2, or
K3 is out of the range of the present invention.
In Test Nos. 22,23,24,28,29,33,35, and 36 using Steel Nos L, M, N, R, S,
W, Y, and Z in which h4/S 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. 21 (Steel No. T<), the C coiltent, the Si content, and the 1<1 value
were low, and the tensile strength and the 0.2% proof stress did not reach 900 MPa and
570 MPa, which were desired values, respectively.
In Test No. 25 (Steel No. O), not only ferrite and pearlite but also bainite were
present together in the microstructure of the forged product. InTest No. 25, the 0.2%
proof stress did not reach 570 MPa that was a desired value. The reason for this is
presnmed to be that, since the structure had a large amount of Mn, not only ferrite and
pearlite (FP) structures but also the bainite (B) structure were present together.
In Test No. 26 (Steel No. P) in which the K3 value was low, during the hot
rolling, the heating temperature was 1150°C and the holding time was 7000 seconds.
The decarburized depth of surface layer of the rolled steel bar was more than 500 pm,
and the tensile strength, the 0.2% proof stress, and the fatigue limit ratio were low due
to the decarburization.
[0055]
In Test No. 27 (Steel No. Q) in which the K1 value was low, the tensile
strength and the 0.2% proof stress were low.
In Test No. 30 (Steel No. T), since the C content was high, the tensile strength
was high, hut the 0.2% proof stress and the fatigue limit ratio were low.
In Test No. 3 1 (Steel No. U), the V content was low, and K1 was low.
Therefore, the tensile strength and the 0.2% proof stress were lower than 900 MPa and
570 MPa, which were desired values, respectively.
In Test No. 32 (Steel No. V), the V content was high. Therefore, the tensile
strength and the fatigue limit ratio were satisfactory, but the 0.2% proof stress was low
due to the presence of the bainite structure.
[0056]
In 'l'cst No. 23 (Steel No. M), MnIS was low. Therefore, cracks and defects
occuned during forging. In Steel No. J, MnIS was low. Therefore, cracks and
dcfects occurred during forging.
In Test No. 24 (Steel No. N), the Si content was high, and K2 was low.
Therefore, cracks and defects occurred during forging.
In Test No. 34 (Steel No. X), the amounts of the respective elements were
within the range of the present invention, hut K3 was lower than 10.7%. Therefore,
the total decarburized depth in surface layer was large, and the 0.2% proof stress was
low.
In Test No. 28 (Steel No. R), K2 was low. Therefore, cracks and defects
occurred during forging.
In Test No. 29 (Steel No. S), Mn/S was low. Therefore, cracks and defects
occurred during forging.
[0057]
In Test No. 35 (Steel No. Y), the steel component was in the desired range and
the values ofK1, K2, and K3 were also within the range of the present invention;
however, the value of MdS was lower than 8.0. Therefore, cracks and largc defects
occurred during steel bar forging.
In Test No. 37 (Steel No. AA), K1 was satisfied, but the C content was low.
Therefore, the tensile strength and the 0.2% proof stress were lower than 900 MPa and
570 MPa, which were desired values, respectively.
In Test No. 38 (Steel No. AB), K1 was satisfied, but the Si content was low.
i Therefore the 0.2% proof stress was low.
In Test No. 39 (Steel No. AC), the MnIS value and the 1<2 value were satisfied,
but the Mn content was low. Therefore, cracks and large defects occ~irredd uring
forging.
In Test No. 40 (Steel No. AD), K1 wassatisfied, but the C content was high.
Therefore, the tensile strength was high, but the 0.2% proof stress and thc fatiguc limit
ratio were low.
In Test No. 41 (Steel No. AE), K1 was satisfied, but the V content was low.
Therefore the 0.2% proof stress and the fatigue limit ratio were low.
In Test No. 42 (Steel No. AF), the N content was high. Therefore, the
amount of V nitride increased, the contribution ofV to precipitation strengthening was
small, and the tensile strength, the 0.2% proof stress, arid the fatigue limit ratio were
low.
In Test No. 43 (Steel No. AG), the Cr content was high. Therefore, the
tensile strength and the fatiguc limit ratio were high, but the 0.2% proof stress was low
due to the presence of the hainite structure.
In Test No. 44 (Steel No. AH), K1 was high. Therefore, the 0.2% proof
stress was low due to the presence of the bainitc structure.
(Industrial Applicability]
[OOSS]
In the surface of the rolled steel bar for machine structural use according lo
the present invention in which the Cr content and the A1 content are limited and which
I
i includes a large amount of Si to reduce the costs, the formation of a deep decarburized
1
layer can be prevented. Amcchanical structural member which is produced by hoi-
I forging the rolled steel bar has excellent fatigue resistance and thus remarkably
contributes to the industry. In addition, under the production conditions according to
the aspects of the present invention, a blooming step can be removed from the
production steps of the iolled steel bar. Therehre, the production costs can be
seduced. and the contribution to the industry is extremely significant
[Document Type] CLAIMS
1. A rolled steel bar for machine structural use having a chemical
con~positionc omprising, 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% 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,
wherein the impurities include:
Cr: 0.10% or lower,
Al: lower than 0.01%, and
N: 0.0060% or lower,
K1 obtained from the following Expression 1 is 0.95 to 1.05,
K2 obtained from the following Expression 2 is more than 35,
K3 obtained from the following Expression 3 is 10.7 or more,
a Mn content and a S content satisfy tl~cfo llowing Expression 4,
a total decarburized depth in a surface layer is 500 pm or less,
KI=C+Si/7+Mn/5+1 .54xV (Expression I),
K2=139-28.6xSi+l05xMn-833xS-l3420xN (Expression 2),
1<3=1 ?7xC-~!~I..OxSi (Expression 3),
lvin/S>S.O (Expression 41, and
C, Si, i\Ail, V, S, and hi in the Expi.essions 1 to 4 represent the amounts of the
respective elements in mass%.
2. The rolled steel bar for machine structural use according to Claim 1,
wherein the chemical composition further comprising, 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%.
3. Amethod of producing the rolled steel bar for machine structural use
according to Claim 1 or 2, the method comprising:
making molten steel having the chemical composition according to Claim 1 or
2;
continuously casting the molten steel to obtain a cast piece having a crosssectional
area of 40000 cm2 or less; and
subsequently to the continuous casting, heating the cast piece to a temperature
range of 1000°C to 1 150°C and holding the cast piece in the temperature range for
7000 seconds or shorter and performing a steel bar rolling.