Specification
[Title ofthe Invention] ROLLED STEEL BAR OR ROLLED WIRE ROD FOR
COLD-FORGED COMPONENT
[Technical Field of the Invention]
[0001]
The present invention relates to a rolled steel bar or rolled wire rod that is
suitable as a material of a cold-forged component and is excellent in cold forgeability.
Particularly, the present invention relates to a rolled steel bar or rolled wire rod that is
suitable as a material of a high-strength cold-forged component and is excellent in cold
forgeability and in which the HRC hardness is 34 or greater after quenching and
tempering.
Priority is claimed on Japanese Patent Application No. 2014-233971, filed on
November 18,2014, the content of which is incorporated herein by reference.
[Related Art]
[0002]
Cold forging is good for the surface texture and dimensional accuracy of
components after forging. Components manufactured by cold forging are
manufactured at lower cost than components manufactured by hot forging, and the
yield ratio thereof is high. Accordingly, cold forging is widely applied to
manufacture of components for various industrial machines including vehicles, such as
gears, shafts, and bolts, or building structures.
[0003]
In recent years, downsizing and weight reduction have proceeded in
components for a mechanical structure used in vehicles, industrial machines, and the
like, and an increase in size has proceeded in building structures. From such a
- 1 -
background, components manufactured by cold forging are required to have a further
increase in strength.
[0004]
For these cold-forged components, a carbon steel for a mechanical structure
specified in JIS G 4051, an alloy steel for a mechanical structure specified in JIS G
4053, and the like have been used. These steels, in general, are adjusted so as to have
a predetermined strength or hardness by repeatedly performing a step including
spheroidizing annealing and drawing or cold drawing of the steel which is hot product
rolled into a steel bar shape or a wire rod shape, and by being formed into a component
shape by cold forging and performing a heat treatment such as quenching and
tempering.
[0005]
The above-described steel for a mechanical structure has a relatively high
carbon content of approximately 0.20% to 0.40%, and can be used as a high-strength
component through a thermal refining treatment. Meanwhile, as for the abovedescribed
steel for a mechanical structure, the strength of a steel bar or wire rod that is
a rolled steel that is used as a forging material is increased. Therefore, in a case
where the steel is not softened by adding the cold drawing and the subsequent
spheroidizing annealing step in the course of manufacturing, problems are generated
during manufacturing, such as wear or cracking of the die easily occurring during cold
forging for component formation, and component cracking.
[0006]
Particularly, in recent years, there has been a tendency that components have a
more complicated shape with an increased strength. The more complicated the
component shape, the higher the possibility of the occurrence of cracking. Thus, in
- 2 -
order to further soften the steel in which a high strength is obtained by quenching and
tempering, before cold forging, measures are employed such as increasing the time of
the spheroidizing annealing treatment or repeating the cold drawing step and the
spheroidizing annealing step more than once.
[0007]
However, these measures include a lot of costs such as personnel cost and
equipment cost, and a large energy loss occurs. Accordingly, a steel that can be
produced even in a case where the step is omitted or the time of the step is reduced is
required.
[0008]
Based on such a background, in order to omit the spheroidizing annealing
treatment or reduce the time of the spheroidizing annealing treatment, a proposal has
been made about a boron steel or the like produced in such a way that the strength of a
rolled steel that is used as a forging material is reduced by reducing contents of alloy
elements such as C, Cr, and Mn, and then a reduction in the hardenability caused by
reducing the alloy elements is compensated by adding boron.
[0009]
For example, Patent Document I discloses a hot-rolled steel for cold forging
having an excellent grain coarsening resistance and excellent cold forgeability, and a
method of manufacturing the hot-rolled steel for cold forging. Specifically, Patent
Document 1 discloses a hot-rolled steel for cold forging having an excellent grain
coarsening resistance and excellent cold forgeability in which 0.10% to 0.60% of C,
0.50% or less of Si, 0.30% to 2.00% of Mn, 0.025% or less of P, 0.025% or less of S,
0.25% or less ofCr, 0.0003% to 0.0050% ofB, 0.0050% or less ofN, and 0.020% to
0.100% ofTi are contained, and TiC or Ti(CN) having a diameter of 0.2 Jl111 or less is
- 3 -
contained at 20 pieces/100 flm2 or greater in matrix of the steel, and a method of
manufacturingthe hot-rolled steel for cold forging.
[001 0]
Patent Document 2 discloses a steel for a mechanical structure for cold
working, and a method of manufacturing the steel for a mechanical structure for cold
working. Specifically, a steel for a mechanical structure for cold working that
contains C, Si, Mn, P, S, AI, N, and Cr, and in which a metallographic structure has
pearlite and pro-eutectoid ferrite, a total area fraction of the pearlite and pro-eutectoid
ferrite to entire structure is 90% or greater, the relationship between an area fraction A
of the pro-eutectoid ferrite and Ae represented by Ae=(0.8-Ceq) x96.75 (where
Ceq=[C]+O.l x[Si]+0.06x[Mn]+O.ll x[Cr] ([(element name)] means the amount
(mass%) of each element)) is A>Ae, and the average grain size of ferrite in the proeutectoid
ferrite and pearlite is 15 to 25 f!m, and a method of manufacturing the same.
In addition, it is disclosed that in the steel for a mechanical structure for cold working
of Patent Document 2, sufficient softening can be realized by perfmming a normal
spheroidizing treatment.
[0011]
il
According to the technology disclosed in Patent Document 1, the hardness of
'i the rolled steel can be reduced. Therefore, cold forging can be performed at low cost,
and a grain coarsening resistance during quenching heating can be provided.
However, in the steel of Patent Document 1, the Cr content of the steel is low, and thus
the hardenability is low and there is a limit on increasing the strength of the component.
[0012]
The steel for a mechanical structure for cold working disclosed in Patent
Document 2 can be softened by performing a normal spheroidizing aunealing
- 4 -
treatment and can be applied to a high-strength component. However, the balance
between the amounts of the chemical compositions of the steel is not optimized, and
the ferrite fraction of the structure of the rolled steel is substantially small. Therefore,
there is a problem in that in a case where the steel as-product-rolled or in which
spheroidizing annealing treatment in a short period of time is performed, is used when
cold forging is performed on the component, cracking occurs and the component
cannot be manufactured at low cost.
[Prior Art Document]
[Patent Document]
[0013]
[Patent Document 1] Japanese Patent (Granted) Publication No. 3443285
[Patent Document 2] Japanese Unexamined Patent Application, First
Publication No. 2013-227602
[Disclosure of the Invention]
[Problems to be Solved by the Invention]
[0014]
The present invention is made in view of the current situation, and an object
thereof is to provide a rolled steel for a high-strength cold-forged component, which
has a steel bar shape or a wire rod shape and which has excellent hardenability and
cold forgeability. Here, excellent hardenability means that HRC hardness in a center
portion is 34 or greater after performing quenching and tempering. Excellent cold
forgeability means that the occurrence of cracking is effectively suppressed during cold
forging even in a case where a spheroidizing annealing treatment is omitted or the time
ofthe spheroidizing annealing treatment is reduced, before cold forging.
[Means for Solving the Problem]
i-: - 5 -
fi
---c--- ------
[0015]
The inventors have conducted various examinations in order to solve the
above-described problems, and as a result, found the. following knowledge.
[0016]
(a) In a case where cold forgeability is secured so that component formation is
possible even if a spheroidizing annealing treatment is omitted or the time of the
spheroidizing annealing treatment is reduced, the tensiie strength of the steel (rolled
steel bar or rolled wire rod) as-product-rolled is required to be 750 MPa or less. In
addition, the internal structure excluding a surface layer portion in which a
decarburized layer may be generated is a ferrite-pearlite structure, and the ferrite
fraction thereof is required to be greater than 40%.
[0017]
(b) In order to secure a high component strength by quenching and tempering,
the C content is required to be increased to increase quenched hardness (hardness after
quenching), and alloy elements such as Mn and Cr are required to be contained to
increase hardenability. That is, sufficient quenched hardness and hardenability
necessary for the sufficient quenched hardness are required to be secured for use in a
high-strength cold-forged component.
[0018]
(c) In order to improve cold forgeability and secure hardness after quenching
by an improvement of hardenability, it is necessary to control the internal structure in
sufficient consideration of the balance between amounts of elements such as C, Si, Mn,
and Cr.
[0019]
The present invention is completed based on the above-described knowledge,
- 6 -
and the gist thereof is as follows.
[0020]
(1) A rolled steel bar or rolled wire rod for a cold-forged component according
to an aspect of the present invention that has a chemical composition consisting of, in
mass%: C: 0.24% to 0.36%; Si: less than 0.40%; Mn: 0.20% to 0.45%; S: less than
0.020%; P: less than 0.020%; Cr: 0.70% to 1.45%; AI: 0.005% to 0.060%; Ti: greater
than 0.020% to 0.060%; B: 0.0003% to 0.0040%; N: 0."0020% to 0.0080%; Cu: 0% to
0.50%; Ni: 0% to 0.30%; Mo: 0% to 0.050%; V: 0% to 0.050%; Zr: 0% to 0.050%; Ca:
0% to 0.0050%; and Mg: 0% to 0.0050% with the remainder of Fe and impurities, in
which Y1 and Y2 represented by the following Formulas <1> and <2>, satisfy a
relationship represented by the following Formula <3>, a tensile strength is 750 MPa
or less, an intemal structure is a ferrite-pearlite structure, and a ferrite fraction is 40%
or greater in the intemal structure.
Yl=[Mn] x[Cr]
Y2=0.134x(D/25 .4-(0.50x.Y[C]) )/(0.50x.Y[C])
Y1>Y2
Formula <1>,
Formula <2>, and
Formula <3>,
where [C], [MnJ, and [Cr] in the formulas represent respective ammmts of
elements in mass%, and D represents a diameter of the rolled steel bar or rolled wire
rod in the unit of mm.
[0021]
(2) In the rolled steel bar or rolled wire rod for a cold-forged component
according to (1 ), the chemical composition may contain, in mass%, one or more
selected from the group consisting ofCu: 0.03% to 0.50%, Ni: 0.01% to 0.30%, Mo:
0.005% to 0.050%, and V: 0.005% to 0.050%.
[0022]
- 7 -
(3) In the rolled steel bar or rolled wire rod for a cold-forged component
according to (l) or (2), the chemical composition may contain, in mass%, one or more
selected from the group consisting of Zr: 0.003% to 0.050%, Ca: 0.0005% to 0.0050%,
and Mg: 0.0005% to 0.0050%.
[0023]
The "impurities" in the remainder of "Fe and impurities" are components
unintentionally contained in the steel, and refer to materials mixed from ore as a raw
material, scrap, a manufacturing environment, or the like in the industrial iron and steel
manufacturing.
[0024]
The rolled steel bar or rolled wire rod refers to a rolled steel with a steel bar
shape or a wire rod shape as-hot-product-rolled. Hereinafter, in this specification of
the present invention, the "rolled steel bar or rolled wire rod" may be collectively
expressed as a "rolled bar and wire rod" or a "rolled steel". The hot product rolling
may be expressed as "hot rolling".
[Effects of the Invention]
[0025]
A rolled bar and wire rod (rolled steel bar or rolled wire rod) for a cold-forged
component according to the aspect of the present invention has a tensile strength of
750 MPa or lower, and an internal metallographic structure thereof is a ferrite-pearlite
structure having a ferrite fraction of 40% or greater. In addition, the rolled bar and
wire rod has excellent cold forgeability, and hardenability since the amount of elements
;)
are controlled. Therefore, using the rolled bar and wire rod of the present invention
as a material, a component can be formed by cold forging even in a case where a
spheroidizing annealing treatment is omitted or the time of the spheroidizing annealing
- 8 -
il
i :I
'I
.~ 1 ~ :
treatment is reduced, and a high-strength cold-forged component having an HRC
hardness of 34 or greater can be obtained through quenching and tempering.
[Brief Description of the Drawings]
[0026]
FIG. 1 is a diagram showing a shape of a bolt formed by forging in examples.
FIG. 2 is a diagram showing the relationship between: a Cr content and a Mn
content; and hardenability.
[Embodiments of the Invention]
[0027]
Hereinafter, a rolled steel bar or rolled wire rod for a cold-forged component
according to an embodiment of the present invention (may be referred to as a rolled bar
and wire rod according to this embodiment) will be described in detail. In the
following description, the symbol "%"related to each element content means "mass%".
[0028]
(A) Chemical Composition (chemical elements)
[0029]
C: 0.24% to 0.36%
C is an element that increases hardenability of a steel to contribute to a
strength improvement. In order to obtain this effect, the C content is controlled to be
0.24% or greater. In a case of further increasing quenched hardness of a cold-forged
component, the C content is preferably controlled to be 0.26% or greater. In a case
where the C content is greater than 0.36%, the cold forgeability is reduced .
Accordingly, the C content is controlled to be 0.36% or less. In a case of further
increasing the cold forgeability, the C content is preferably controlled to be 0.33% or
less.
- 9 -
I il
[0030]
Si: Less Than 0.40%
In order to reduce the tensile strength of a rolled steel after hot rolling (asrolled),
the Si content is preferably as low as possible: Accordingly, the Si content
may be 0%. Meanwhile, since Si strengthens ferrite by solid solution strengthening,
Si may be contained in order to obtain an effect of increasing the tempered hardness of
a cold-forged component. However, since the cold forgeability is significantly
reduced in a case where the Si content is 0.40% or greater, it is necessary to control the
Si content to be less than 0.40% even in a case where Si is contained. From the
viewpoint of cold forgeability, the Si content is preferably less than 0.30%, and more
preferably less than 0.20%. The Si content is even more preferably 0.10% or less in
consideration of the tensile strength of a rolled steel.
[0031]
Mn: 0.20% to 0.45%
Mn is an element that increases hardenability of a steel, and in order to obtain
this effect, the Mn content is controlled to be 0.20% or greater. It is preferable that
Mn content is 0.25% or greater in order to further increase the hardenability. In a
case where the Mn content is greater than 0.45%, a ferrite transformation start
temperature is lowered during cooling after finish rolling, and thus the ferrite fi·action
is reduced and bainite is generated. As a result, the cold torgeability of the steel is
reduced. Therefore, the Mn content is controlled to be 0.45% or less. In a case of
improving the cold forgeability, the Mn content is preferably 0.42% or less, more
preferably 0.40% or less, and even more preferably 0.35% or less.
[0032]
S: Less Than 0.020%
- 10 -
S is contained as impurities. S is an element that reduces cold forgeability,
and the S content is preferably as low as possible. Particularly, in a case where the S
content is 0.020% or greater, MnS has an elongated coarse form, and the cold
forgeability is significantly reduced. Accordingly, the S content is limited to be less
than 0.020%. The S content is preferably less than 0.010%.
[0033]
P: Less Than 0.020%
P is contained as impurities. P is an element that reduces cold forgcability
and is segregated in the grain boundary in heating to an austenite temperature range to
cause cracking during quenching. Accordingly, the P content is preferably low.
Particularly, in a case where the P content is 0.020% or greater, the cold forgeability is
significantly reduced or cracking significantly occurs. Thus, the P content is less than
0.020%, and preferably less than 0.010%.
[0034]
Cr: 0.70% to 1.45%
Cr is an element 1hat increases hardenability of a steel as in a case of Mn. In
order to obtain this effect, 1he Cr content is controlled to be 0.70% or greater. In
order to stably obtain high hardenability, the Cr content is preferably 0.80% or greater,
and more preferably 0.90% or greater. In a case where 1he Cr content is greater than
1.45%, the hardenability increases. However, a ferrite transformation start
temperature is lowered during cooling after finish rolling, and thus the ferrite fraction
is reduced and bainite is generated. As a result, the cold forgeability of the steel is
reduced. Therefore, the Cr content is controlled to be 1.45% or less. In order to
further increase the cold forgeability, the Cr content is preferably 1.30% or less, and
more preferably 1.20% or less.
- 11 -
\I
[0035]
AI: 0.005% to 0.060%
AI is an element having a deoxidizing action. In addition, AI is an element
that acts to form AIN by combining with N, refine austenite grains during hot rolling
and suppress the generation of bainite by a pinning effect of AIN. In order to obtain
these effects, the AI content is controlled to be 0.005% or greater. In a case of more
securely suppressing the generation of bainite, the AI content is preferably 0.015% or
greater, and more preferably 0.020% or greater. In a case where the AI content is
greater than 0.060%, the effects of AI are saturated. In addition, coarse AIN is
generated and the cold forgeability is thus reduced. Therefore, the AI content is
controlled to be 0.060% or less. From the viewpoint of increasing the cold
forgeability, the AI content is preferably 0.050% or less, and more preferably 0.045%
or less.
[0036]
Ti: Greater Than 0.020% and 0.060% or Less
Ti is an element that forms a carbide, a nitride, or a carbonitride by combining
with N or C, and has an effect of refining austenite grains during hot rolling by a
pinning effect. The refining of austenite grains suppresses the generation of bainite in
the course of cooling after finish rolling, and contributes to an increase in the ferrite
fraction. In addition, Ti also acts to increase an effect of improving hardenability by
B since Ti fixes, as TiN, N solid-dissolved in a steel, and thus suppresses the
generation ofBN. In order to obtain these effects, the Ti content is controlled to be
greater than 0.020%. The Ti content is preferably 0.030% or greater, and more
preferably greater than 0.035%. In a case where the Ti content is greater than 0.060%,
fine Ti carbides or Ti carbonitrides are precipitated in a large amount during finish
- 12 -
rolling, the ferrite is strengthened, and thus the tensile strength excessively increases.
Therefore, the Ti content is controlled to be 0.060% or less. The Ti content is
preferably 0.050% or less, and more preferably 0.045% or less.
[0037]
B: 0.0003% to 0.0040%
B is an element effective for increasing hardenability even in a case where it
is contained in a minute amount. In order to obtain this effect, the B content is
controlled to be 0.0003% or greater. In a case of further increasing the hardenability,
the B content is preferably 0.0005% or greater, and more preferably 0.0010% or
greater. In a case where the B content is greater than 0.0040%, the hardenability
improving effect is saturated, and the cold forgeability is reduced. In a case of further
improving the cold forgeability, the B content is preferably 0.0030% or less, and more
preferably 0.0025% or less.
[0038]
N: 0.0020% to 0.0080%
N forms a nitride or a carbonitride by combining with AI, or Ti, and has an
effect of refining of austenite grains in hot rolling. In order to obtain the eflcct, theN
content is controlled to be 0.0020% or greater, and preferably 0.0030% or greater. In
a case where the N content is too high, the effect of refining of austenite grains is
saturated, and N combines with B and forms a nitride, thereby weakening the
hardenability improving effect of B. Thus, theN content is controlled to be 0.0080%
or less. In order to stably improve the hardenability, theN content is preferably less
than 0.0070%, and more preferably 0.0060% or less.
[0039]
Fmihermore, in the bar according to this embodiment, it is also necessary to
- !3 -
control the balance between the amounts of elements in addition to the actual amounts
thereof. Specifically, Yl represented by the following Formula <1> and Y2
represented by the following Formula <2> satisfy the relationship represented by
Formula <3>.
Yl=[Mn] x[Cr]
Y2=0.134x(D/25 .4-(0.SOx..J[C]))/(O.SOx..J[C])
Yl>Y2
Formula
Formula<2>
Formula<3>
In the formulas, [C], [Mn], and [Cr] represent the respective amounts thereof
in mass%, and D represents the diameter (mm) of the rolled bar and wire rod.
[0040]
In a case ofYl>Y2, hardenability such that HRC hardness is 34 or greater in
a center portion after a thermal refining treatment, is obtained by general quenching
and tempering (for example, after heating in a temperature range of 880°C to 900°C,
quenching is performed by oil cooling, and tempering is performed at 400°C to 600°C).
[0041]
Formulas <1> to <3> will be described.
As described above, Yl is a value represented as a product of the masses
(mass%) of Mn and Cr contained in the steel, and is a parameter of hardenability
required for a rolled bar and wire rod for a high-strength cold-forged component.
Y2 is a parameter representing the relationship between D and [C] having an
influence on the fraction of the martensite structure obtained, in a case where a rolled
bar and wire rod having a diameter ofD (mm) is heated to a temperature equal to or
higher than an Ac3 point and quenched by oil cooling, at a position ofD/2 (mm) from
the surface that is a center portion of the rolled bar and wire rod. The cooling rate in
the quenching by oil cooling varies depending on the diameter D of the rolled bar and
- 14 -
wire rod, and in general, the cooling rate is approximately 10 to 40 °C/sec.
The Ac3 point can be calculated from a known calculation formula, for
example, Ac3=912.0-230.5xC+ 31.6xSi-20.4xMn-39.8xCu-18.1 xNi-
14.8xCr+l6.8xMo based on the chemical composition. Otherwise, theAc3 point can
be experimentally estimated from a change of an expansion ratio of the steel measured
during temperature rise by heating.
[0042]
After the thermal refining treatment by quenching and tempering, in order to
obtain HRC hardness of 34 or greater in the center portion, it is necessary to control
the quenched hardness before the tempering in the center portion (D/2 portion) of the
rolled bar and wire rod to be 45 or greater in terms ofHRC hardness. In addition, in
order to control the quenched hardness to be 45 or greater in terms of HRC hardness,
the C content, the Mn content, and the Cr content having a large influence on the
quenched hardness are required to be adjusted.
In a case where the structure is martensite, the hardness thereof is almost
determined by the C content, and in a case where the C content is in the range of the
rolled bar and wire rod according to this embodiment, the hardness becomes 45 or
greater in terms ofHRC hardness. Therefore, in order to secure quenched hardness of
45 or greater in terms ofHRC hardness, the structure after quenching may be
controlled to be martensite in a major part (90% or greater in terms of a structure
fraction).
[0043]
As a result of the examination of the inventors, it has been found that 90% or
greater of martensite is obtained after quenching in the center portion of the rolled bar
and wire rod by controlling each of the Mn content and the Cr content to be a
- 15 -
---,~-·-~--·-~- -------
predetermined value or greater. Specifically, in a case where Yl represented as a
product of the contents of Mn and Cr and which increases the hardenability, is larger
than the parameter Y2 representing the relationship between D and [C] having an
influence on the fraction of the martensite structure obtained in the center portion of
the rolled bar and wire rod, the structure of the center portion ofthe rolled bar and wire
rod after quenching includes 90% or greater of martensite. Accordingly, in the rolled
bar and wire rod according to this embodiment, Yl> Y2 is satisfied. In a case of
Yl<::Y2, an incompletely quenched structure such as bainite or ferrite is generated
during quenching, and thus 90% or greater of martensite cannot be secured. In this
case, the strength and the hydrogen embrittlement resistance are reduced.
FIG. 2 is a diagram showing the relationship between: a Cr content and a Mn
content; and hardenability in a case where the diameter of a rolled bar and wire rod is
15 mm and a C content is 0.30%. In FIG. 2, in a case where the Mn content and the
Cr content are above a border line B, Y1> Y2 is satisfied, and martensite occupies 90%
or greater of the structure of the center portion of the rolled bar and wire rod after
quenching.
[0044]
As a specific standard ofhardenability, in a steel hardenability test method
(one end quenching method) of .TIS G 0561, a so-called Jominy test, Hardness J 7 mm
at a position separated from a quenched end by at least 7 mm may be 45 or greater in
terms ofHRC hardness.
[0045]
Since the hardness of the rolled bar and wire rod after quenching also depends
on the diameter D of the rolled bar and wire rod, the diameter D of the rolled bar and
wire rod is preferably small from the viewpoint of hardenability. In a case where the
- 16 -
rolled bar and wire rod is applied to a high-strength cold-forged component, the rolled
bar and wire rod preferably has a diameter of approximately 6 to 3 5 mm, and more
preferably 8 to 16 mm.
[0046]
The rolled bar and wire rod according to this embodiment basically contains
the above-described chemical compositions with the remainder of Fe and impurities.
However, if necessary, at least one or more selected from Cu, Ni, Mo, V, Zr, Ca, and
Mg may be contained in place of a part of Fe of the remainder. Since these elements
are not necessarily required to be contained, the lower limits thereof are 0%. Here,
the "impurities" are components unintentionally contained in the steel, and refer to
materials mixed from ore as a raw material, scrap, a manufacturing environment, or the
like in the industrial iron and steel manufacturing.
[0047]
Hereinafter, actions and effects of arbitrary elements Cu, Ni, Mo, V, Zr, Ca,
and Mg, and preferable contents thereof in a case where the elements are contained
will be described.
[0048]
Cu: 0.50% or Less
Cu is an element that increases hardenability, and may be contained. In
order to stably obtain this effect, the Cu content is preferably 0.03% or greater, and
more preferably 0.05% or greater. In a case where the Cu content is greater than
0.50%, the hardenability excessively increases, and bainite is generated after finish
rolling. Thus, the cold forgeability is reduced. Accordingly, even in a case where
Cu is contained, the Cu content is controlled to be 0.50% or less. The Cu content in a
case where Cu is contained from the viewpoint of improving the cold forgeability is
- 17 -
I
preferably 0.30% or less, and more preferably 0.20% or less.
[0049]
Ni: 0.30% or Less
Ni is an clement that increases hardenability, and may be contained. In order
to stably obtain this effect, the Ni content is preferably 0.01% or greater, and more
preferably 0.03% or greater. In a case where the Ni content is greater than 0.30%, the
effect ofNi is saturated. In addition, the hardenability excessively increases, and
bainite is generated after finish rolling. Thus, the cold forgeability is reduced.
Accordingly, even in a case where Ni is contained, the Ni content is controlled to be
0.30% or less. The Ni content in a case where Ni is contained from the viewpoint of
improving the cold forgeability is preferably 0.20% or less, and more preferably 0.10%
or less.
[0050]
Mo: 0.050% or Less
Mo is an element that strengthens a steel by solid solution strengthening, and
significantly improves hardenability of a steel. Mo may be contained in order to
obtain this effect. In order to stably obtain this effect, the Mo content is preferably
0.005% or greater. In a case where the Mo content is greater than 0.050%, bainite or
martensite is generated after finish rolling, and the cold forgeability is reduced.
Accordingly, even in a case where Mo is contained, the Mo content is controlled to be
0.050% or less. The Mo content in a case where Mo is contained from the viewpoint
of improving the cold forgeability is preferably 0.030% or less, and more preferably
0.020% or less.
[0051]
V: 0.050% or Less
- 18 -
V is an element that forms a carbide, a nitride, or a carbonitride by combining
with C and N. In addition, V is an element that improves hardenability of a steel even
in a case where it is contained in a minute amount. Accordingly, V may be contained.
In order to stably obtain these effects, the V content is preferably 0.005% or greater.
In a case where the V content is greater than 0.050%, the strength of a rolled steel
increases due to the precipitated carbide or nitride, and the cold forgeability is reduced.
Accordingly, even in a case where Vis contained, the V content is controlled to be
0.050% or less. The V content in a case where Vis contained from the viewpoint of
improving the cold forgeability is preferably 0.030% or less, and more preferably
0.020% or less.
[0052]
Zr: 0.050% or Less
Zr is an element that acts to improve hardenability of a steel even in a case
where it is contained in a minute amount. A minute amount of Zr may be contained
to achieve the above object. In order to stably obtain this effect, the Zr content is
preferably 0.003% or greater. In a case where the Zr content is greater than 0.050%,
coarse nitrides are generated, and the cold forgeability is reduced. Accordingly, even
in a case where Zr is contained, the Zr content is controlled to be 0.050% or less. The
Zr content in a case where Zr is contained is preferably 0.030% or less, and more
preferably 0.020% or less from the viewpoint of improving the cold forgeability.
[0053]
Ca: 0.0050% or Less
Ca forms a sulfide by combining with S, and acts as a production nucleus of
MnS. MuS with CaS as a production nucleus is finely dispersed and becomes a
production nucleus for precipitation of ferrite during cooling after finish rolling.
- 19 -
:I
Accordingly, in a case where MnS dispersed finely is present, the ferrite fraction
increases. That is, in a case where Ca is contained, the ferrite fraction increases, and
thus Ca may be contained. In order to stably obtain this effect, the Ca content is
preferably 0.0005% or greater. In a case where theCa content is greater than
0.0050%, the effect is saturated, and Ca reacts with oxygen in the steel together with
AI, and thus generates a coarse oxide. Thus, the cold forgeability is reduced.
Accordingly, even in a case where Ca is contained, theCa content is controlled to be
0.0050% or less. TheCa content in a case where Ca is contained is preferably
0.0030% or less, and more preferably 0.0020% or less from the viewpoint of
improving the cold forgeability.
[0054]
Mg: 0.0050% or Less
Mg is an element that forms a sulfide by combining with S, and acts as a
production nucleus ofMnS. Mg has an effect of finely dispersing MnS. In a case
where MnS is finely dispersed, ferrite is precipitated with MnS, dispersed during
cooling after finish rolling, as a production nucleus. Thus, the ferrite fraction is
improved. Mg may be contained in order to obtain this effect. In order to stably
obtain this effect, the Mg content is preferably 0.0005% or greater. In a case where
the Mg content is greater than 0.0050%, the effeet of Mg is saturated. In addition,
since the adding yield of Mg is low and the adding of Mg deteriorates the
manufacturing cost, the amount ofMg in a case where Mg is contained is preferably
0.0030% or less, and more preferably 0.0020% or less.
il
[0055]
(B) Tensile Strength of Steel
[0056]
- 20 -
---------~~·--~
The rolled bar and wire rod according to this embodiment has excellent cold
forgeability. Therefore, even in a case where a spheroidizing annealing treatment
after product rolling is omitted or performed in a short period of time, a reduction in
the life of the die during cold forging, or cracking of the component during formation
does not occur. This is because by controlling not only the chemical compositions of
the steel adjusted as described above, but also the manufacturing conditions of the
rolled steel, the structure of the rolled steel and the precipitates are controlled to be
suitable for cold forging, and the strength of the steel is reduced. In this embodiment,
excellent cold forgeability means that, for example, cracking does not occur even in a
case where a round bar of q, 10.5 mmx40 mmL cut out from the rolled bar and wire rod
is processed into a bo It shown in FIG. 1.
[0057]
1n a case where the tensile strength is greater than 7 50 MPa, the possibility of
the occunence of cracking of the component during cold forging is increased.
Therefore, in the rolled bar and wire rod according to this embodiment, it is necessary
to control the tensile strength to be 750 MPa or less after controlling the structure as
will be described later.
Even in a case where the tensile strength is greater than 750 MPa, cracking of
the component does not easily occur during cold forging in a case where a
spheroidizing annealing treatment is performed for a long period of time of
approximately 20 hours or repeatedly performed more than once (for example, 10
hoursx2 times). However, the rolled bar and wire rod according to this embodiment
is provided to secure cold forgeability even in a case where the spheroidizing annealing
treatment is omitted or the time of the spheroidizing annealing treatment is reduced
such that the heat treatment is completed in at least 10 hours. In order to achieve this
- 21 -
object, an upper of the tensile strength in the rolled bar and wire rod according to this
embodiment is limited. The tensile strength.ofthe rolled bar and wire rod is
preferably 700 MPa or less, and more preferably 650 MPa or less.
[0058]
(C) About Internal Structure of Steel
[0059]
The rolled bar and wire rod according to this embodiment has excellent cold
forgeability. Therefore, a reduction in the life of the die during cold forging, or
cracking of a formed component does not occur even in a case where a conventional
spheroidizing annealing treatment after product rolling requiring approximately 20
hours is omitted or performed in about half the time, or the spheroidizing annealing
treatment that has been performed more than once is performed once. This is because
the metallographic structure of the rolled bar and wire rod is controlled to have a form
suitable for cold forging by not only adjusting the chemical compositions ofthe steel,
but also controlling the manufacturing conditions of the rolled bar and wire rod.
[0060]
Specifically, in the rolled bar and wire rod according to this embodiment, the
structrue (internal structure) of a portion, which excludes a surface layer portion
ranging up to 100 fill from the surface in which a decarburized layer may be generated,
is a ferrite-pearlite structure, and the fraction of the ferrite is 40% or greater. Here,
the ferrite-pearlite structure means a structrue that is a mixed structure in which ferrite
and pearlite occupy 95% or greater of the entire structure in terms of an area fraction (a
structure in which a total of the area fraction of the ferrite and the area fraction of the
pearlite is 95% or greater). In the measurement of the ferrite fraction, a ferrite phase
between lamella cementites included in the pearlite is not included as the ferrite. The
- 22 -
mixed structure in which ferrite and pearlite occupy 95% or greater of the entire
structure in terms of an area fraction means that a total of area fractions of structures
such as martensite and bainite other than the ferrite and the pearlite is less than 5%.
In order to obtain good cold forgeability, the mixed structure of ferrite and pearlite is
required to be 95% or greater in the entire structure in terms of an area fraction, and is
preferably 100%.
[0061]
In the internal structure, in a case where the ferrite fraction is less than 40%,
good cold forgeability cannot be secured even in a case where the tensile strength is
7 50 MPa or less. Thus, problems are caused such as cracking occurring in the
component during formation or a reduction in the life of the die. The ferrite fraction
is preferably 45% or greater, and more preferably 50% or greater. The upper limit of
the ferrite fraction is not particularly specified. However, in order to control the
ferrite fraction to be greater than 80% as-hot-rolled, it is necessary to spheroidize the
lamella cementite that forms the pearlite structure, and for this, it is necessary to
perform a soaking treatment for a long period oftime after rolling. Accordingly, the
cost rises, and this is difficult to industrially realize. Therefore, the upper limit of the
ferrite fi'action may be 80%.
In a case where the mixed structure of ferrite and pearlite is less than 95% in
the entire structure in terms of an area fraction, there is a concern that the tensile
strength of the rolled bar and wire rod may be greater than 750 MPa due to hard
structures such as martensite and bainite. In addition, since the hard structures
become fracture origins, there is a concern that the cold forgeability may be reduced.
[0062]
The identification ofthe structures and the calculation of the area fraction are
- 23 -
performed, for example, as follows.
A rolled bar and wire rod is cut into a length of 1 0 nnu. Then, resin
embedding is performed such that a cross-section serves as a test surface, and mirror
polishing is performed. Next, the surface is corroded with a 3% nitric acid alcohol
(nita! etchant) to cause a microstructure to emerge. Thereafter, microstructure
photographs of 5 fields of view are taken using an optical microscope at 500-fold
magnification at a position corresponding to a D/4 position (D: diameter of the rolled
steel) of the rolled steel bar or rolled wire rod to identity the "phase". Using image
analysis software, ferrite area fractions of the respective fields of view are measured as
ferrite fractions, and the average value thereof is obtained. The fraction of a total of
ferrite and pearlite is obtained by obtaining a pearlite fraction in the same manner, and
adding the ferrite fraction and the pearlite fraction.
[0063]
(D) Preferable Manufacturing Process
In the rolled bar and wire rod according to this embodiment, it is important to.
control not only the chemical compositions of the steel, but also the structure as-rolled.
Accordingly, rolled bar and wire rods having chemical compositions and a structure
within the range of the present invention are included in the rolled bar and wire rod
according to this embodiment regardless of the manufacturing methods thereof.
However, in a case where a manufacturing process including the following
steps is applied to a steel having predetermined chemical compositions, a structure asrolled
can be stably controlled to be in a preferable range. Hereinafter, preferable
manufacturing conditions will be described in detail.
[0064]

- 24 -
---- ----- ----- ~
First, a molten steel in which chemical compositions such as C, Si, Mn, and
Cr are adjusted and that is melted by a converter, a normal electric furnace, or the like
is cast to obtain a steel ingot or a cast piece. The obtained steel ingot or cast piece is
bloomed to obtain a steel piece (material for product rolling). At tlris time, a heating
temperature before blooming is preferably 1200°C or higher in order to dissolve coarse
carbonitrides or carbides such as Ti(C,N), and TiC generated during solidification.
[0065]

Then, the steel piece is heated prior to the rolling. In this case, the heating
temperature is preferably 1 050°C or lower as long as the rolling is possible. In a case
where the heating temperature is too high, the fme carbonitrides or carbides
precipitated in the steel piece are dissolved and coherently precipitated along with
ferrite transformation during cooling after the product rolling. Accordingly, the
strength after the product rolling increases, and there is a concern that the cold
forgeability may be reduced.
[0066]

After the heating, a steel bar or wire rod having a predetermined diameter is
obtained by the product rolling including finish rolling. The finish rolling is rolling
that is performed by a finish rolling mill array in a final step of the product rolling. In
the finish rolling, a working speed Z is preferably 5 to 15/sec, and the finish rolling is
preferably performed in a rolling temperature range of750°C to 850°C. The working
speed Z is a value obtained using the following Formula (i) from a reduction of area of
the steel by fmish rolling and a finish rolling time. Regarding the finish rolling
temperature, a temperature at an outlet side of the finish rolling mill array may be
- 25 -
measured using an infrared radiation thermometer.
[0067]
Z={ -ln(l-R) }/t ... (i)
Here, R is a reduction of area of the steel by finish rolling, and t is a finish
rolling time (sec). In represents a natural logarithm.
[0068]
The reduction of area R is obtained using R=(A0-A)/A0 from a cross-sectional
area Ao before finish rolling of the rolled bar and wire rod and a cross-sectional area A
after finish rolling.
[0069]
The finish rolling time tis a period of time (sec) during which the rolled bar
and wire rod passes through the fmish rolling mill array, and can be obtained by
dividing the distance from a first rolling mill to a last rolling mill in the finish rolling
mill array by the average transfer speed of the rolled bar and wire rod.
[0070]
In a case where the finish rolling temperature is below 750°C or the working
speed of the finish rolling is too high, ferrite transforms from unrecrystallized austenite
grams. In this case, the structure after cooling is excessively refined, and thus the
strength excessively increases, and the cold forgeability is reduced. In contrast, in a
case where the temperature of the finish rolling is above 850°C or the working speed is
low, austenite grains after re-crystallization become coarse, and a ferrite transformation
start temperature is lowered. In this case, the ferrite fraction of the structure after
cooling is reduced, and the cold forgeability is reduced.
[0071]

- 26 -
After the finish rolling is completed, cooling is preferably performed at an
average cooling rate of 0.2 to 5 °C/sec until the surface temperature of the rolled steel
goes down to 500°C.
In a case where the average cooling rate to 500°C is lower than 0.2 °C/sec, a
time of transformation from austenite to ferrite is long, and thus there is a concern that
decarburization may occur in the surface layer portion of the rolled steel. In a case
where the average cooling rate is higher than 5°C/sec, there is a concern that hard
structures such as martensite and bainite may be formed.
[0072]
With a manufacturing process including the above-described manufacturing
steps, it is possible to stably obtain a rolled bar and wire rod having such a tensile
strength and internal structure that hardenability for obtaining quenched hardness at a
level suitable for use in a high-strength cold-forged component is secured, and good
cold forgeability can be realized even in a case where a spheroidizing annealing
treatment is omitted or the time of the spheroidizing annealing treatment is reduced.
By performing cold forging, quenching, and tempering on the rolled steel bar
or wire rod according to this embodiment, a high-strength cold-forged component can
be obtained.
[Examples]
[0073]
Hereinafter, the present invention will be described in detail using examples,
but is not limited to these examples.
[0074]
Even in a case where steels have the same chemical compositions, structures
thereof vary according to the manufacturing process. Accordingly, the requirements
- 27 -
------------
of the present invention may not be satisfied even in a case where the chemical
compositions of the present invention are satisfied. Therefore, first, structures and
characteristics of steels, obtained by manufacturing steels having the same chemical
compositions under different manufacturing conditions, were evaluated. Next, steel
ingots having different chemical compositions were melted, and rolled steels were
manufactured under the same conditions to evaluate structures and characteristics of
the obtained steels.
[0075]
Specifically, first, steels having chemical compositions shown in Table 1 were
melted by an electric furnace, and the obtained steel ingots were heated at 1200°C and
bloomed into steel pieces with 162 mm square. In the steels having the chemical
compositions shown in Table 1, AO, AI, and A2 have the same chemical compositions,
and BO, Bl, and B2 have the same chemical compositions. In Table 1, the symbol"-"
represents that the element content is at an impurity level, and the element can be
judged to be not substantially contained.
[0076]
Regarding these steels, manufacturing conditions of the steps until the product
rolling with respect to the steel piece after blooming to a wire rod having a
predetermined diameter were changed to obtain steel bars or wire rods.
That is, in Invention Examples AO and BO shown in Table 1, steel pieces with
162 mm square were used as materials for product rolling. These steel pieces were
heated at 1 040°C, and then subjected to product rolling at a finish rolling temperature
of 820°C so as to obtain a predetermined diameter, and thus a rolled steel bar or rolled
wire rod were produced. In this case, the working speed of the finish rolling was in a
range of 5 to 15/sec, and after the finish rolling was completed, cooling was performed
- 28 -
in such a way that the average cooling rate to 500°C was 0.4 °C/sec.
In Invention Examples AOI and BOI.shown in Table I, steel pieces with 162
mm square were used as materials for product rolling. These steel pieces were heated
at I 040°C, and then subjected to product rolling at a finish rolling temperature of
850°C so as to obtain a predetermined diameter, and thus a rolled steel bar or rolled
wire rod were produced. In this case, the working speed ofthe finish rolling was in a
range of 5 to 15/sec, and after the fmish rolling was completed, cooling was performed
in such a way that the average cooling rate to 500°C was 0.4 °C/sec.
[0077]
In Comparative Examples A1, A2, B1 and B2, steel pieces with 162 mm
square were used as materials for product rolling, and a heating temperature and finish
rolling temperature were changed shown in table 1, and thus a rolled steel were
produced. Other conditions were the same as those of AO and BO.
Specifically, in Comparative Examples A1 and B1, steel pieces were heated at
1 050°C prior to product rolling, and then subjected to product rolling at a finish rolling
temperature of 920 to 950°C so as to obtain a predetermined diameter, and thus a
rolled steel bar or rolled wire rod were produced. In this case, the working speed of
the finish rolling was in a range of 5 to 15/sec, and after the finish rolling was
completed, cooling was performed in such a way that the average cooling rate to
500°C was 0.4 °C/sec.
In addition, in Comparative Examples A2 and B2, steel pieces were heated at
1150°C prior to product rolling, and then subjected to product rolling at a finish rolling
temperature of 830°C so as to obtain a predetermined diameter, and thus a rolled steel
bar or rolled wire rod were produced. In this case, the working speed of the finish ·
rolling was in a range of 5 to 15/sec, and after the finish rolling was completed, cooling
- 29 -
---------------- ----------
was performed in such a way that the average cooling rate to 500°C was 0.4 °C/sec.
[0078]
Next, rolled steels were produced from steel pieces having chemical
compositions shown in No. I to 25 in Table 2, using the following method. In Table 2,
the symbol"-" represents that the element content is at an impurity level, and the
element can be judged to be not substantially contained.
[0079]
That is, steels having chemical compositions shown in Table 2 were melted by
an electric furnace, and the obtained steel ingots were heated at 1200°C and bloomed
into steel pieces with 162 mm square. These steel pieces were used as materials for
product rolling. Next, the materials for product rolling were heated at 1030°C to
1 050°C, and then subjected to product rolling at a finish rolling temperature adjusted
to be between 750°C to 850°C. In this case, the working speed of the finish rolling
was in a range of 5 to 15/sec in all of the cases, and after the finish rolling was
completed, cooling was performed in such a way that the average cooling rate to
500°C was 0.4 to 2 °C/sec.
- 30 -
• : -::.::.-,_::~-~:::.::::z.;-;:-<:-': ,c,oc_o."'~'""' ,- ·,",·,:.-..-~=_o::o::;~ <:C'C•c~-~"--"P.<-~"'-7~•'--" o• --...,.-_--..-.----~ -=~~~.--.- --
[0080]
[Table 1]
mass%: remainder of Fe and impurities
Heating Stee I Finish c Si Mn p s Cr AI Ti N B Cu Ni Mo v Ca Mg Zr Temperature Rolling
No. of Product Temperature Rolling
AO 0. 32 0.03 0. 38 0. 009 0. 010 1. 10 0030 0. 036 0. 0038 0. 0023 - - - - - - - 1040°C 820°C
Invention
Examples
A01 0. 32 0.03 0. 38 0. 009 0. 010 1. 10 0.030 0. 036 0. 0038 0. 0023 - - - - - - - 1040°C 850°C
A1 0. 32 0.03 0. 38 0. 009 0. 010 1. 10 0. 030 0. 036 0. 0038 0. 0023 - - - - - - - 1050°C 950°C
Comparative
Examples
A2 0. 32 0.03 0. 38 0.009 0. 010 1. 10 0. 030 0 036 0. 0038 0. 0023 - - - - - - - 1150°C 830°C
80 0. 30 0. 04 0.42 0. 008 0. 010 1. 05 0. 039 0. 039 0.0046 0.0020 0. 08 0. 07 - - - - - 1040°C 820°C
Invention
Examples
801 0. 30 0.04 0.42 0. 008 0. 010 1. 05 0. 039 0. 039 0. 0046 0. 0020 0. 08 0.07 - - - - - 1040°C 850°C
B1 0. 30 0.04 0. 42 0008 0. 010 1. 05 0. 039 0 039 0. 0046 0. 0020 0. 08 0. 07 - - - - - 1050°C 920°C
Comparative
Examples
B2 0. 30 0.04 0.42 0.008 0. 010 1. 05 0039 0. 039 0. 0046 0. 0020 0. 08 0. 07 - - - - - 1150°C 830°C
- 31 -
[piJ&\J
[lo.b\e 2-]
Steel
No.
1
1'
2
3
4
"'" 5
OCJ.J
+-' "- 6
<=E 7 CJ.)"' >X
<=W 8
9
10
11
12
13
14
15
16
17
CJ.) >" 18
·- CJ.) +-'- 19 '"-'E"- "'"' 20 C. X
EW 21 0
'-' 22
23
24
25
c Si
o_ 30 0.06
0. 29 0.06
0. 29 0.05
0. 35 0. 06
0. 32 0. 05
0. 29 0. 06
0. 28 0.22
0. 26 0.35
0. 31 0.07
0. 30 0.04
0. 28 0.04
o_ 27 0.06
0. 26 0.07
0. 27 0. 05
0. 27 0. 04
0. 26 0. 07
0. 22 0.05
0. 40 0.05
0. 32 0.04
0. 33 0. 08
0. 28 0. 05
0. 30 0. 20
0. 34 0. 05
0.28 o_ 06
0. 32 0.06
0. 30 0.05
Mn p s Cr AI
0.30 0.010 0.006 0.98 0. 042
0.29 0. 009 0.005 1. 02 0. 035
0.39 0. 009 0.007 1. 00 0. 038
0.32 0. 012 0. 009 1. 25 0. 035
0.44 0. 010 0.005 0.97 0. 034
o_ 34 0. 009 0.013 1. 39 0. 039
0.38 0. 008 0.006 0.85 0. 041
0.27 0. 007 0.005 1. 15 0.035
0. 31 0. 010 0.010 1. 05 0.036
0.30 0. 011 0.006 1. 09 0. 040
0.29 0. 007 0.009 1. 00 0.045
0.28 0. 012 0.010 0.95 0. 033
0.32 0. 007 0.009 0.98 0.030
0.35 0. 008 0. 008 0. 99 0.027
0.27 0.009 0.006 0.88 0. 035
0.29 0.010 0.007 0. 77 0. 028
0.30 0.007 0.010 0.95 0. 033
0.40 0. 010 0.011 1. 05 0. 038
0.82 0. 014 0.008 0.99 0. 034
0.40 0. 010 0. 033 1. 00 0.038
0.33 0.012 0.009 0. 55 0. 028
0.39 0.009 0.010 1. 25 0. 030
0.42 0.008 0.007 1.22 0. 025
0.38 0.012 0. 010 0. 90 0.030
0.40 0. 012 0.010 1. 50 0.031
0.34 0. 010 0. 011 1. 05 0. 032
Ti N
0. 034 0.0035
0. 036 0.0041
0. 039 0.0046
0. 038 0. 0046
0. 035 0.0041
0.039 0.0055
0. 038 0.0040
0. 044 0.0064
0.035 0.0043
0. 031 0.0045
0.024 0.0031
0.036 0.0039
0. 031 0.0041
0.052 0.0069
0.036 0.0040
0. 032 0.0045
0. 033 0.0046
0. 039 0.0048
0. 032 0.0046
0. 039 0.0050
0. 035 0.0049
0. 075 0.0037
0. 015 0.0032
0.030 0.0044
0.035 0.0036
0.036 0.0038
'1.'2.-
J-3""-
B
0.0016
0.0021
0.0019
0. 0017
0. 0022
0. 0029
0. 0024
0. 0031
0. 0024
0.0016
0. 0013
0. 0009
0. 0016
0. 0018
0. 0018
0.0021
0.0017
0.0019
0. 0015
0. 0019
0. 0017
0. 0022
0. 0025
0. 0002
0. 0024
0. 0021
Cu Ni
- -
- -
- -
- -
- -
- -
- -
- -
0. 10 -
0.09 0.08
- -
- -
- -
- -
- -
- -
- -
- -
- -
- -
- -
- 0.05
- -
0.05 -
0.05 0.05
- -
mass%: remainder of Fe and impurities
Mo v Ca Mg Zr
- - - - -
- - - - -
- - - - -
- - - - -
- - - - -
- - - - -
- - - - -
- - - - -
- - - - -
- - - - -
- 0.015 - - -
0.010 - - - -
- - 0.0013 . - -
- - - 0.0005 0.016
- - - - -
- - - - -
- - - - -
- - - - -
- - - - -
- - - - -
- - - - -
- - - - -
- - - - -
- - - - -
- - - - -
- 0. 10 - - -
[0082]
With respect to the rolled steel bars or rolled wire rods produced by the abovedescribed
method, diameter, tensile strength, ferrite fraction, the sum of a ferrite fraction
and a pearlite fraction, hardness after quenched, hardness after quenching and tempering,
cold forgeability were investigated.
The results are shown in Table 3 and Table 4.
[0083]
Tensile strength, ferrite fraction, hardness after quenching and tempering, cold
forgeability of rolled steel were investigated by the following method.
[0084]
<1> Investigation of Tensile Strength of Rolled Steel Bar or Rolled Wire Rod:
A 14A-test piece (diameter of parallel portion: 6 mm) specified in JIS Z 2241
was collected from a position of a center of the rolled steel bar or rolled wire rod such
that a longitudinal direction of the test piece was a rolling direction of the steel. The
gage length was set to 30 mm and a tensile test was performed at room temperature to
obtain the tensile strength.
[0085]
<2> Investigation of Ferrite Fraction and Pearlite Fraction of Rolled Steel:
The rolled steel bar or rolled wire rod was cut into a length of 10 mm. Then,
resin embedding was performed such that a cross-section served as a test surface, and
mirror polishing was performed. Next, the surface was corroded with a 3% nitric acid
alcohol (nita! etchant) to cause a microstructure to emerge. Thereafter, microstructure
photographs of 5 fields of view were taken using an optical microscope at 500-fold
magnification at a position corresponding to a D/4 position (D: diameter of the rolled
steel) of the rolled steel bar or rolled wire rod to identify the "phase". Using image
i
'I -- ... . ---- - - ------
analysis software, ferrite area fractions of the respective fields of view were measured
as ferrite fractions, and the average value thereof was obtained. In addition, a pearlite
fraction was obtained in the same manner to obtain a. total of the ferrite fraction and the
pearlite fraction.
[0086]
<3> Investigation of Quenched hardness
The rolled steel bar or rolled wire rod was cut into a length of200 mrnL, and
then heated at 880°C for 60 minutes in an Argas atmosphere and dipped in an oil tank
at 60°C to be quenched. Next, a test piece with a length of I 0 mm was collected from
a position of a center in a longitudinal direction of the quenched round bar, and then
polishing was performed on a cross-section as a test surface to measure HRC hardness
in a center portion of the cross-section.
[0087]
<4> Investigation of Tempered Hardness
The rest of the round bar quenched by the above-described method was
subjected to tempering in such a way that it was heated at 425°C for 60 minutes in the
atmosphere, and then taken out from the furnace to be cooled (air cooling in the
atmosphere). A test piece with a length of 1 0 mm was collected from a position of a
center of the round bar after the tempering, and then polishing was performed on a
cross-section as a test surface to measure HRC hardness in a center portion of the crosssection.
[0088]
<5> Investigation of Cold Forgeability
Tbe cold forgeability was evaluated after actually performing cold forging on a
bolt using the obtained rolled steel bar or rolled wire rod.
Specifically, a round bar of 10.5 nuux40 mrnL was cut out through
mechanical working from a position corresponding to a center portion of the cross
section of the rolled steel bar or rolled wire rod. Next, degreasing and pickling were
perfonned, and then a zinc phosphate treatment (75°C, dipping time: 600 seconds) and a
metallic soap treatment (80°C, dipping time: 180 seconds) were performed to attach a
lubrication-treated film including a zinc phosphate film and a metallic soap film to the
surface. The resulting material was used as a material for bolt forging. For bolt
forging, a die was designed such that working including: a first step of press- torming a
shaft portion by forging; and a second step of forming a bolt head portion and a flange
portion could be performed such that forging into a shape shown in FIG. 1 was possible,
and this die was mounted on a hydraulic forging press to perform cold forging. In FIG.
1, the unit of numerical values is nuu.
[0089]
Regarding the cold forgeability, whether cracking occurred in a surface of the
bolt during bolt formation was visually determined. The cold forgeability was
evaluated in such a way that a case where cracking occurred in the surface of the bolt
was evaluated as NG, and a case where cracking did not occur in any part was evaluated
as OK. The cracking in the surface of the bolt mainly occurred at a tip end of a flange
portion of a bolt head portion.
'i
!
[0090]
[Table 3]
Invention
Examples
Comparative
Examples
Invention
Examples
Comparative
Examples
,._
- +"-' '~ "C' D~.- !l§~ +-' "'~ u:> ·- C!
AO 15.0
A01 15.0
A1 15.0
A2 15.0
80 15. 0
801 15.0
81 15.0
82 15.0
->- >"-'
0.418 0. 146
0.418 0. 146
0.418 0.146
0.418 0. 146
0. 441 0. 155
0. 441 0. 155
0. 441 0. 155
0. 441 0. 155
C!)..c
-+-'~
·- b.O co
"'Co..
c"'::;;; (!)"-~ >--+-' u:>
615
625
775
792
598
601
764
779
·1.£
-~-
+"'- 'C
·- 0 >. -·- +-'
"' c "-+-' " "' ""' ·-
+-1.~ "''-' "'"' "'"' -
"'"' ..CCD..- "-"'~ .,·- -~+-I;# 0... ... ~ '-'Co "'co -{l ,._'-'~ +LJ....~ c" 0::: Cl." 0:::
"'"' "' CD:....C.. E:.-0 OC!)
LL.."- +-'00 "'"' "'"' OOJJ
LL.. ·- "' C::I: f-- :I: ,._
,._ "- 0 >-< LL.. L"L.'.
48 100 49 41 OK
45 96 49 41 OK
36 70 49 41 NG
41 90 49 41 NG
50 100 48 40 OK
48 97 48 40 OK
35 70 48 40 NG
42 90 48 40 NG
--·-"-~------~"·-~~·-~
'•";-~',''~'-~,-,'"' •; :;;·o,~·:;:_·_-__c-;~-_;:0 • • ~"·v·-~--
(.J)0°n]
e:-N::, X \.D
Invention
Examples
Comparative
Examples
-.""._ 0.z.
"'
1
1'
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
~
+"'' ~ "E"E'
"'~
Cl
12.0
12.0
15.0
20. 0
20. 0
25.0
15.0
15.0
15.0
15.0
15.0
15.0
15.0
15.0
20. 0
20. 0
15. 0
15.0
15.0
15.0
15.0
15.0
15.0
15.0
15.0
15.0
~~---
"' ·- c co:
>- >- "'"a._
§_3~.-~
t-0
0. 294 0.097 585
0. 296 0. 101 591
0. 390 0. 160 579
0.400 0.223 645
0. 427 0.239 645
0473 0. 356 616
0.323 0. 165 604
0. 311 0. 176 582
0. 326 0. 150 592
0.327 0 155 616
0. 290 0. 165 576
0. 266 0. 171 555
0. 314 0. 176 542
0. 347 0. 171 565
0 .. 238 0. 272 556
0.223 0. 280 532
0.285 0. 203 503
0.420 0. 116 778
0. 812 0. 146 790
0.400 0. 142 640
0. 182 0.165 522
0.488 0. 155 799
0. 512 0. 137 766
0. 342 0. 165 535
0.600 0. 146 815
0.357 0. 155 835
-"l-'J".~'
·;:~~
~ ~ ."..'"_~' w...
51
48
52
46
41
50
49
52
50
48
52
55
57
56
54
57
61
33
35
46
57
43
34
54
30
49
'J.)--
l£1.-
_;'lc:
·- 0
~ ....
"''-' a".'"_'~ ~ tLJ...C
·+-'""' ~ ~ :;;;<
w...
100
96
100
100
100
100
100
100
100
100
100
100
100
100
100
100
100
70
70
100
100
90
85
100
50
85
»
+' ""'"' ""'"' ·-
"'"' "'"' - .<::: "'~ ~"'~ -c:.c <.><:: 0 ""'(..> <::"' 0:: c. "0 0:: C"Q:'.:)"t!.:'- 2:; t"E-~":o:t'-: 23 "'-'":;~i',
0
L.L.
47 38 OK
46 38 OK
46 37 OK
51 42 OK
49 41 OK
48 41 OK
46 39 OK
45 38 OK
49 40 OK
48 40 OK
47 40 OK ·
46 40 OK
45 36 OK
46 37 OK
35 26 OK
33 24 OK
38 29 OK
55 46 NG
49 40 NG
48 39 NG
37 28 OK
49 41 NG
46 37 NG
36 26 OK
48 41 NG
49 42 .. NG
[0092]
From Table 3, in all of Test Nos. AO,AOI, BO and BOI, that were the invention
examples, the chemical compositions and the above-described Formulas to <3>
were satisfied, and the steel manufacturing conditions were appropriate. Thus, the
tensile strength was 750 MPa or less, and a ferrite-pearlite structure having a ferrite
fraction of 40% or greater was obtained. In addition, the quenched hardness was 45 or
greater in terms ofHRC hardness and hardness after quenching and tempering was 34
or greater in terms ofHRC hardness. In addition, there were no problems in cold
forgeability. As a result, the cold forgeability does not reach the target.
[0093]
On the other hand, in Test Nos. AI, A2, B I and B2, the tensile strength or the
ferrite fraction did not reach targets thereof.
[0094]
Test No. AI has the same chemical compositions as Test No. AO. However,
since the finish rolling temperature was high, that is, 950°C, the tensile strength is 750
MPa or greater, and the ferrite fraction is 40% or less. As a result, the cold forgeability
1s poor.
[0095]
Test No. A2 has the same chemical compositions as Test No. AO. However,
since the heating temperature of product rolling was high, that is, II50°C, the tensile
strength is 750 MPa or greater, and as a result, the cold forgeability is poor.
[0096]
Test No. B 1 has the same chemical compositions as Test No. BO. However,
since the finish rolling temperature is high, that is,. 920°C, the tensile strength is 7 50
MPa or greater, and the ferrite fraction is 40% or less. Thus, the cold forgcability is
[I
poor.
[0097]
Test No. B2 has the same chemical compositions as Test No. BO. However,
since the heating temperature of product rolling was high, that is, 1150°C, the tensile
strength is 7 50 MPa or greater. As a result, the cold forgeability is poor.
[0098]
In addition, from Table 4, in all of the rolled steel bars or rolled wire rods of
Test Nos. 1 to 13, that were the invention examples, since the chemical compositions
and the above-described Formulas <1> to <3> were satisfied, the tensile strength was
750 MPa or less, and a ferrite fraction was 40% or greater. In addition, the quenched
hardness of the center portion of the steel was 45 or greater in terms ofHRC hardness,
and there were no problems in cold forgeability.
[0099]
On the other hand, in the rolled steel bars or rolled wire rods of Test Nos. 14 to
25, since any one of the chemical compositions, or values ofYl and Y2 shown in the
above-described Formulas <1> and <2> did not satisfy the regulations of the present
invention, any one or more of the quenched hardness of the center portion of the steel,
the cold forgeability did not reach targets thereof.
[0100]
In Test Nos. 14 and 15, the chemical compositions satisfy the specified ranges
of the present invention, but the value ofYl is Y2 or less. Accordingly, the quenched
hardness of the center portion ofthe steel is less than 45 in terms ofHRC, and the
hardenability is not sufficient. As a result, the hardness after quenching and tempering
is less than 34 in terms ofHRC.
[0101]
'19
-..4-rIn
Test No. 16, since the C content is lower than the specified range of the
present invention, the quenched hardness of the center portion of the steel is less than 45
in terms ofHRC, and the quenched hardness is not sufficient. As a result, the hardness
after quenching and tempering is less than 34 in terms ofHRC.
[0102]
In Test No. 17, the C content is higher than the specified range of the present
invention, the tensile strength is 750 MPa or greater, arid the ferrite fraction is 40% or
less. Accordingly, the cold forgeability is poor.
[0103]
In Test No. 18, the Mn content is higher than the specified range of the present
invention, and a ferrite transformation start temperature is reduced. Accordingly, the
tensile strength is 750 MPa or greater, and the ferrite fraction is 40% or less, and the
cold forgeability is poor.
[0104]
In Test No. 19, the tensile strength is 750 MPa or less, and the ferrite fraction is
40% or greater. However, the S content is higher than the specified range ofthe
present invention, and thus MnS is coarse, and the cold forgeability is poor.
[0105]
In Test No. 20, the Cr content is lower than the specified range of the present
invention, the quenched hardness of the center portion of the steel is less than 45 in
terms of HRC, and the hardenability is not sufficient. As a result, the hardness after
quenching and tempering is less than 34 in terms ofl-IRC.
[0106]
In Test No. 21, the Ti content is higher than the specified range ofthe present
invention, the tensile strength is 750 MPa or greater, and the cold forgeability is poor .
. i
[0107]
In Test No. 22, the Ti content is lower than the specified range of the present
invention, the tensile strength is 750 MPa or greater, the ferrite fraction is 40% or less,
and the cold forgeability is poor.
[0108]
In Test No. 23, the B content is lower than the specified range of the present
invention, the quenched hardness of the center portion of the steel is less than 45 in
terms of HRC, and the hardenability is not sufficient. As a result, the hardness after
quenching and tempering is less than 34 in terms ofHRC.
[0109]
In Test No. , the Cr content is higher than the specified range of the present
invention, and bainite is generated in ratio of 50%. Accordingly, the tensile strength is
750 MPa or greater, the ferrite fraction is less than 40%, and the cold forgeability is
poor.
[0110]
In Test No. 25, the V content is higher than the specified range of the present
invention. Since V precipitates as a fme carbonitride or carbide, although the ferrite
fraction is 40% or greater, the tensile strength is 750 MPa or greater, and the cold
forgeability is poor.
[Industrial Applicability]
[0111]
Using a rolled bar and wire rod for a high-strength cold-forged component of
the present invention as a material, it is possible to obtain a high-strength cold-forged
component having excellent hardenability, in which formation can be performed by cold
forging even in a case where a spheroidizing annealing treatment is omitted or the time
of the spheroidizing annealing treatment is reduced.
[Brief Description of the Reference Symbols].
[0112]
B: BORDER LINE

[Document Type] CLAIMS
What is claimed is:
1. A rolled steel bar or rolled wire rod for a cold-forged component that has a
chemical composition consisting of, in mass%:
C: 0.24% to 0.36%;
Si: less than 0.40%;
Mn: 0.20% to 0.45%;
S: less than 0.020%;
P: less than 0.020%;
Cr: 0.70% to 1.45%;
AI: 0.005% to 0.060%;
Ti: greater than 0.020% to 0.060%;
B: 0.0003% to 0.0040%;
N: 0.0020% to 0.0080%;
Cu: 0% to 0.50%;
Ni: 0% to 0.30%;
Mo: 0% to 0.050%;
V: 0% to 0.050%;
Zr: 0% to 0.050%;
Ca: 0% to 0.0050%; and
Mg: 0% to 0.0050%
with a remainder of Fe and impurities,
wherein Y1 and Y2 represented by the following Formulas <1> and <2>,
satisfY a relationship represented by the following Formula <3>,
a tensile strength is 750 MPa or less,
an internal structure is a ferrite-pearlite stn\cture, and
a ferrite fraction is' 40% or greater in the intemal sti:ucture.
Yl=[Mn] x[Cr] Formula <1>,
Y2=0. 134~(D/25 .4-(0 .SOx..) [C]))/(0 .50 x..f[C]) Formula <2>, and
Yl>Y2 Formula <3>,
where [C], [Mn], and [Cr] in the formulas represent respective amounts of
elements in mass%, and D represents a diameter of the rolled steel bar or rolled wire rod
in the milt of mm.
2. The rolled steel bar or rolled wire rod for h cold-forged component
according to claim 1,
wherein the chemical composition contains, in mass%, one or more selected
from the gronp consisting of
Cu: 0.03% to 0.50%,
Ni: 0.01% to 0.30%,
Mo: 0.005% to 0.050%, and
V: 0.005% to 0.050%.
3. The rolled steel bar or rolled wire rod for a cold-forged component
according to claim 1 or 2,
wherein the chemical composition contains, in mass%, one or more selected
fi·om the group consisting of
Zr: 0.003% to 0.050%,
Ca: 0.0005% to 0.0050%, and
Mg: 0.0005% to 0.0050%.