DESCRIPTION
ROLLED STEEL BAR FOR HOT FORGING
TECHNICAL FIELD
roo011
The present invention relates to a rolled steel bar for hot forging. More
particularly, the present invention relates to a rolled steel bar for hot forging
that can be used suitably as a starting material for high-strength, non-refined,
and hot-forged parts of automobiles, industrial machines, and the like.
BACKGROUND ART
roo021
In recent years, fiom the viewpoint of reducing COz emissions, the need
for improving the fuel economy has increased, and for machine structural
parts that are. used for automobiles, industrial machines, and the like, it has
been desired to increase the strength of the parts for the purpose of decreasing
the size of the parts.
100031
Also, from the viewpoint of reducing the production cost, there has
mainly been used a hot-forged part (hereinafter, a hot-forged part that is
produced without being subjected to heat treatment of quenching and
tempering is referred to as a "non-refined hot-forged part"), in which a steel
bar produced by hot rolling (hereinafter, an as hot-rolled steel bar that is
produced by hot rolling is referred to as a "rolled steel bar") is subjected to a
forming process by means of hot forging without subsequently being subjected
to heat treatment of quenching and tempering, that is, "refining treatment," so
as to give a desired strength to the steel bar.
[0004]
Many of the hot-forged parts are subjected to the forming process by
mainly being rolled-down in the axial direction of the rolled steel bar, which is
a starting material.
[00051
However, some of the hot-forged parts are subjected to the forming
process by mainly being rolled-down in the direction perpendicular to the axis
of rolled steel bar, that is, in the direction perpendicular to the rolling direction
scarcely by being rolled-down in the axial direction of the rolled steel bar. For
the hot-forged parts subjected to the forming process by being rolled-down in
such a direction, the state of distribution of inclusions andlor precipitates
formed in the hot rolling (that is, the state of distribution in the rolled steel
bar of inclusions andlor precipitates elongated in the axial direction) remains
even after the hot forging. Therefore, there is a tendency for the fatigue
strength against the stress in the direction perpendicular to the axis of hotforged
part to decrease (hereinafter, the fatigue strength against the stress in
the direction perpendicular to the axis of hot-forged part is referred to as the
"transverse fatigue strength").
[0006]
As the tensile strength of hot-forged part is increased, the transverse
fatigue strength can also be increased. However, the increase in tensile
strength of the non-refined hot-forged part produced without being subjected
to refining treatment leads to a decrease in tool service life in the cutting
process carried out after hot forging. For this reason, there arise problems of
increasing cutting costs and an increase in cutting time.
[00071
Therefore, it is not necessarily desirable that the transverse fatigue
strength of hot-forged part be improved by increasing the tensile strength.
Cooosl
6.4. , i'. . .
In such a situation, Patent Document 1 (JP8-92687A) and Patent
Document 2 (JP~-287677A) disclose "High strength and high toughness nonrefined
steel for hot forging and its production method" and "High strength
non-refhed steel for hot forging", respectively, as described below.
roo091
That is, Patent Document 1 (JP8-92687A) discloses a "high strength and
high toughness non-refined steel for hot forging" configured such that in a
steel containing, by mass percent, Si: 2% or less (excluding o%), S: 0.10% or
less (excluding o%), N: 0.02% or less (excluding o%), 0: 0.010% or less
(excluding 0%), and unavoidable impurities, the steel further contains, by
mass percent, C: 0.10 to 0.6%, Mn: 0.3 to 2.5%, Cr: 0.05 to 2.5%, V: 0.03 to
0.5%, Al: 0.060% or less (excluding 0%), and Ti: 0.005 to 0.03%, and still
further contains, by mass percent, as necessary one or more kinds selected
from a group of Pb: 0.3% or less (excluding o%), Ca: 0.01% or less (excluding
O%), Te: 0.3% or less (excluding o%), Bi: 0.3% or less (excluding o%), Zr: 0.1%
or less (excluding O%), Hf: 0.1% or less (excluding O%), Y: 0.1% or less
(excluding o%), rare earth metals: 0.1% or less (excluding 0%), and Mg: 0.1% or
less (excluding 0%), the balance being Fe and unavoidable impurities, wherein
1 x 102 to 1 x 106 / mm2 of inclusions each having an average crystal grain size
of 0.1 to 5 pm are contained, and the inclusions are Ti oxideslnitrides, MnS,
and composite compounds consisting mainly of the Ti oxideslnitrides and MnS;
and a production method therefor.
Loo 101
Patent Document 2 (JP6-287677A) discloses a high strength non-refined
steel for hot forging containing, by mass percent, C: 0.25 to 0.50%, Si: 0.40 to
2.00%, Mn: 0.50 to 2.50%, Cr: 0.10 to 1.00%, S: 0.03 to 0.10%, V: 0.05 to 0.30%,
and N: 0.0050 to 0.0200%, further containing one or two kinds of Al: 0.005 to
. -. . .
.' .:A ' - .. . ; . . . . * * ' -
' ,. , .. - .
- . O.&O% ixnd_ii 6.002.b O.WA,~ d % ~ + t ~ r -c&t&iing Ca: 0.0004 to
. - - . . , . -
' .. ..-... . I .
0.0050% as necessary, the balance being Fe and unavoidable impurities,
wherein
the carbon equivalent Ceq.(%) expressed by the formula of
is 0.83 to 1.23%, and
the bainite transformation index Bt expressed by the formula of
is 0 or less.
LIST OF PRIOR ART DOCUMENT(S)
roo111
Patent Document 1: JP8-92687A
Patent Document 2: JP6-287677A
DISCLOSURE OF THE INVENTION
PROBLEMS TO BE SOLVED BY THE INVENTION
[00121
According to the technique disclosed in Patent Document 1 (JP8-92687A),
a non-refined hot-forged part can be provided with a tensile strength of 90
kgflmm2 (882.6 MP~)or higher. However, in the case of a steel containing
0.005% or more of Ti as an essential element as in the technique proposed in
Patent Document 1 (JP8-92687A), if 1 x 102 to 1 x 106 / mmz of inclusions each
having an average crystal grain size of 0.1 to 5 pm that are Ti oxidestnitrides,
MnS, and composite compounds consisting mainly of the Ti oxidestnitrides and
MnS are merely contained, the transverse fatigue strength is decreased by the
Ti nitrides arranged in the axial direction of the hot-forged part in the case
where a rolled steel bar is used by being rolled-down in the direction
perpendicular to the axis thereof and by being formed by means of hot forging.
[00131
According to the technique disclosed in Patent Document 2 (JP~-
287677A), a non-refined hot-forged part can be provided with a tensile
strength of 900 MPa or higher. Moreover, the non-refined hot-forged part is
excellent in machinability because the part consists of a mixed structure of
ferrite and pearlite (hereinafter, referred to as ltferrite/pearlite") in which the
formation of bainite is avoided. However, the steel specifically disclosed in
Patent Document 2 (JP~-287677k) contains at least 0.033% of S. In the case
where a large amount of S is contained in a steel, there is a possibility that the
transverse fatigue strength may be decreased by coarse MnS arranged in the
axial direction of the hot-forged part in the case where a rolled steel bar is
used by being rolled-down in the direction perpendicular to the axis thereof
and by being formed by means of hot forging.
[00141
The present invention has been made in view of the above-described
present situation, and accordingly an objective thereof is to provide a rolled
steel bar for hot forging from which a high-strength, non-refined, and hotforged
part having a tensile strength of 900 MPa or higher and a transverse
endurance ratio (fatigue strength / tensile strength) of 0.47 or higher can be
obtained.
[00151
The transverse endurance ratio is a value obtained by dividing the
fatigue strength against the stress in the direction perpendicular to the axis of
hot-forged part by the tensile strength in the direction perpendicular to the
axis of hot-forged part.
MEANS FOR SOLVING THE PROBLEMS
100 161
The present inventors conducted various studies to solve the abovedescribed
problems. As a result, the findings of the following items (a) to (0
were obtained.
[00171
(a) For a non-refined hot-forged part, in order to obtain a high transverse
endurance ratio, the internal structure (that is, a structure excluding a nearsurface
portion where a decarburized layer may be formed at the heating stage
at the time of hot forging) must be made ferritelpearlite. On the other hand,.
in the case where either one or both of bainite and martensite are intermixed
in the internal structure, a high transverse endurance ratio cannot be obtained.
[0018l
(b) In order to avoid the formation of bainite after hot forging and to
assure a tensile strength of 900 MPa or higher in the non-refined hot-forged
part, the contents of elements for improving the hardenability must be
controlled precisely.
[00191
(c) For a hot-forged part that is formed by rolling-down a rolled steel bar
in the direction perpendicular to the axis of the rolled steel bar, in order to
obtain a high transverse fatigue strength, it is effective to contain a
precipitation strengthening element. However, it is undesirable to add Ti
liable to form coarse nitrides at the solidification time.
[00201
(d) On the other hand, unlike Ti, V does not form coarse nitrides at the
solidification time. Therefore, the N content can also be increased, whereby a
high transverse fatigue strength can be provided by precipitating the carbides,
nitrides, or carbo-nitrides of V in the cooling process at the time of hot forging.
[00211 1. .1 _ m1m1m ,
(e) By containing a minute amount of S, MnS that has been thought to
exert an adverse influence on the transverse fatigue strength can be dispersed
finely in the steel bar without being coarsened. Therefore, the formation
nucleus of ferrite is increased even within the austenite grains after hot
forging, so that the formation of bainite can be restrained.
lo0221
(D As a result, a hot-forged part having a tensile strength of 900 MPa or
higher and a transverse endurance ratio of 0.47 or higher after hot forging can
be obtained.
roo231
T h present invention has been completed on the basis 01 the abovedescribed
findings, and the gist thereof is rolled steel bars for hot forging
described below.
loo241
(1) A rolled steel bar for hot forging having a chemical composition
consisting, by mass percent, of C: 0.27 to 0.37%, Si: 0.30 to 0.75%, Mn: 1.00 to
1.45%, S: 0.008% or more and less than 0.030%, Cr: 0.05 to 0.30%, Al: 0.005 to
0.050%, V: 0.200 to 0.320%, and N: 0.0080 to 0.0200%, the balance being Fe
and impurities, wherein
the contents of P, Ti and 0 in the impurities are, by mass percent, P:
0.030% or less, Ti: 0.0040% or less, and 0: 0.0020% or less; and
Y1 expressed by the following formula is 1.05 to 1.18.
Y1= C + (1110)Si + (115)Mn + (5122)Cr + 1.65V - (517)s ... <1>
where, C, Si, Mn, Cr, V, and S in the above formula represent, respectively,
the content by mass percent of each of the elements.
[00251
(2) A rolled steel bar for hot forging having a chemical composition
consisting, by mass percent, of C: 0.27 to 0.37%, Si: 0.30 to 0.75%, Mn: 1.00 to
1.45%, S: 0.008% or more and less than 0.030%, Cr: 0.05 to 0.30%, Al: 0.005 to
0.050%, V: 0.200 to 0.320%, and N: 0.0080 to 0.0200%, and one or more
elements selected from Cu: 0.30% or less, Ni: 0.30% or less, and Mo: 0.10% or
less, the balance being Fe and impurities, wherein
the contents of P, Ti and 0 in the impurities are, by mass percent, P:
0.030% or less, Ti: 0.0040% or less, and 0: 0.0020% or less; and
Y2 expressed by the following formula <2> is 1.05 to 1.18.
Y2 = C + (1110)Si + (115)Mn + (5122)Cr + 1.65V - (517)s + (115)Cu +
(115)Ni + (114)Mo . .. <2>
where, C, Si, Mn, Cr, V, S, Cu, Ni, and Mo in the above formula <2> represent,
respectively, the content by mass percent of each of the elements.
lo0261
The term "impurities" indicate those impurities that are mixed from raw
materials such as ore and scrap or production environments when ferrous
materials are produced on an industrial scale.
ADVANTAGEOUS EFFECT(S) OF THE INVENTION
loo271
By using the rolled steel bar for hot forging of the present invention as a
starting material, a high-strength, non-refined, and hot-forged part having a
tensile strength of 900 MPa or higher and a transverse endurance ratio of 0.47
or higher can be obtained.
MODE FOR CARRYING OUT THE INVENTION
[00281
In the following, the requirements of the present invention are explained
in detail. The ,symbol "%" for the content of each element in the explanation
below means "% by mass".
[00291
C: 0.27 to 0.37%
C (carbon) is an element for strengthening a steel, and therefore 0.27%
or more of C must be contained. On the other hand, if the C content exceeds
0.37%, although the tensile strength after hot forging increases, the transverse
endurance ratio decreases in some cases. Therefore, the C content is set to
0.27 to 0.37%. The C content is preferably 0.29% or more and preferably
0.35% or less.
-[00301
Si: 0.30 to 0.75%
Si (silicon) is a deoxidizing element, and also an element necessary for
strengthening ferrite by means of solid-solution strengthening and for
enhancing the tensile strength after hot forging. In order to achieve these
effects, 0.30% or more of Si must be contained. On the other hand, if the Si
content exceeds 0.75%, not only these effects are saturated, but also the
surface decarburization of rolled steel bar becomes remarkable. Therefore,
the Si content is set to 0.30 to 0.75%. The Si content is preferably 0.35% or
more and preferably 0.70% or less.
Lo03 11
Mn: 1.00 to 1.45%
Mn (manganese) is an element for strengthening ferrite and pearlite by
means of solid-solution strengthening and for enhancing the tensile strength
after hot forging, and therefore 1.00% or more of Mn must be contained. On
the other hand, if the Mn content exceeds 1.45%, not only these effects are
saturated, but also the hardenability is enhanced, and bainite is formed after
hot forging, so that the transverse-fatigue strength is decreased in some cases.
Therefore, the Mn content is set to 1.00 to 1.45%. The Mn content is
preferably 1.10% or more and preferably 1.40% or less.
Lo0321
S: 0.008% or more and less than 0.030%
S (sulfur) is an important element in the present invention. S combines
with Mn to form MnS, and increases the formation nucleus of ferrite within
the austenite grains after hot forging as well, so that the formation of bainite
can be restrained. Further, the machinability is also improved by MnS.
Therefore, 0.008% or more of S must be contained. On the other hand, if the
S content becomes 0.030% or more, MnS takes an elongated coarse form, so
that the transverse fatigue strength is decreased, and the transverse
endurance ratio is decreased. Therefore, the S content must be controlled
precisely, and the S content is set to 0.008% or more and less than 0.030%.
The S content is preferably 0.010% or more and preferably 0.027% or less.
Coo331
Cr: 0.05 to 0.30%
Like Mn, Cr (chromium) is an element for strengthening ferrite and
pearlite by means of solid-solution strengthening and for enhancing the tensile
strength after hot forging, and therefore 0.05% or more of Cr must be
contained. On the other hand, if the Cr content exceeds 0.30%, not only these
effects are saturated, but also the hardenability is enhanced, and bainite is
formed after hot forging, so that the transverse fatigue strength is decreased in
some cases. Therefore, the Cr content is set to 0.05 to 0.30%. The Cr content
is preferably 0.08% or more and preferably 0.20% or less. The Cr content is
further preferably less than 0.20%.
roo341
Al (aluminum) not only has a deoxidizing function but also has functions
of combining with N to form AlN, restraining the growth of austenite grains at
the time of hot forging owing to the pinning effect thereof, and restraining the
formation of bainite. Therefore, 0.005% or more of A1 must be contained. On
the other hand, if the A1 content exceeds 0.050%, the above-described effects
are saturated. Therefore, the Al content is set to 0.005 to 0.050%. The A1
content is preferably 0.010% or more.
[0035]
V: 0.200 to 0.320%
V (vanadium) combines with C and N to form carbides and nitrides or
carbo-nitrides, and has a func-ti on -o.f .- effectively increasing the transverse
endurance ratio of hot-forged part. Therefore, 0.200% or more of V is
contained. On the other hand, if the V content exceeds 0.320%, not only the
above-described effect is saturated, but also the cost rises. Therefore, the V
content is set to 0.200 to 0.320%. The V content is preferably 0.220% or more .
and preferably 0.300% or less.
100361
N: 0.0080 to 0.0200%
N (nitrogen) is an important element in the present invention. N has
functions of combining with V to form nitrides or carbo-nitrides and effectively
increasing the transverse endurance ratio of hot -forged part, and also
combining with A1 to form AlN and restraining the growth of austenite grains
at the time of hot forging owing to the pinning effect thereof and restraining
the formation of bainite. Therefore, 0.0080% or more of N must be contained.
However, if the N content increases and especially exceeds 0.0200%, a pinhole
is formed in a steel in some cases. Therefore, the N content is set to 0.0080 to
0.0200%. The N content is preferably 0.0090% or more and preferably
0.0150% or less.
lo0371
The rolled steel bar for hot forging of the present invention is a steel
having a chemical composition consisting of the above-described elements
ranging &om C to N with the balance being Fe and impurities, in which the
contents of P, Ti and 0 in the impurities are P: 0.030% or less, Ti: 0.0040% or
less, and 0: 0.0020% or less; and Y1 expressed by the formula is 1.05 to
1.18.
100381
As described already, the term "impurities" indicate those impurities
that are mixed from raw materials such as ore and scrap or production
environments when ferrous materials are produced on an industrial scale.
roo391
Hereunder, there is explained the reasons for restricting the contents of
P, Ti and 0 in the impurities to the above-described ranges, respectively, in
the present invention.
[00401
P: 0.030% or less
P (phosphorus) is an element contained as an impurity in a steel. In
particular, if the P content exceeds 0.030%, the segregation becomes
remarkable, and the fatigue strength is decreased in some cases. Therefore,
the P content in the impurities is set to 0.030% or less. The P content in the
impurities is preferably 0.025% or less. It is desirable that the P content'
contained as an impurity be as low as possible as far as not raising the cost in
the steel making process.
[00411
Ti: 0.0040% or less
In the present invention, Ti (titanium) is an element the content of
which must be restricted. However, Ti is mixed unavoidably from ore, scrap,
and the like. In particular, if the compounding ratio of scrap is increased by
attaching importance to the holding-down of raw material cost, the mixing
amount of Ti increases in spite of an impurity. If the mixing amount of Ti
increases, and coarse Ti nitrides are formed, and the Ti nitrides are arranged
undesirably in the axial direction of the hot-forged part. In particular, if the
Ti content exceeds 0.0040%, the transverse fatigue strength is decreased, and
the transverse endurance ratio of 0.47 or higher cannot be obtained.
Therefore, the Ti content in the impurities is set to 0.0040% or less. The Ti
content in the impurities is prefer~bly0 .0035% or less, further preferably less
than 0.0030%.
lo0421
0: 0.0020% or less
0 (oxygen) is an impurity element that mainly exists as an oxide-based
inclusion in a steel, and decreases the transverse fatigue strength. If the 0
content increases and exceeds 0.0020%, the generation frequency of coarse
oxides increases, so that the transverse fatigue strength is decreased, and the
transverse endurance ratio is decreased. Therefore, the 0 content in the
impurities is set to 0.0020% or less. The 0 content in the impurities is
preferably 0.0015% or less.
Coo431
The reason for restricting Y1 expressed by formula <1> is described later
together with the reason for restricting Y2 expressed by formula <2>.
lo0441
The rolled steel bar for hot forging of the present invention may contain
one or more kinds of elements selected &om a group of Cu, Ni and Mo as
necessary in lieu of a part of the Fe. In this case, Y2 expressed by the formula
<2> is 1.05 to 1.18.
100451
Hereunder, the operational advantages and the reasons for restricting
the contents of Cu, Ni and Mo, which are optional elements, are explained.
100461
Cu: 0.30% or less
Cu (copper) is an element for strengthening ferrite and pearlite by
means of solid-solution strengthening. For this reason, Cu may be contained.
However, if the Cu content exceeds 0.30%, not only this effect is saturated, but
also the hardenability is enhanced, and bainite is formed after hot forging, so
that the transverse fatigue strength is decreased in some cases. Therefore,
the upper limit was placed on the content of Cu, if contained, and the content
of Cu, if contained, is set to 0.30% or less. The content of Cu, if contained, is
preferably 0.20% or less.
[00471
On the other hand, in order to stably achieve the above-described effect
of Cu, the content of Cu is preferably 0.03% or more, further preferably 0.05%
or more.
[0048]
Ni: 0.30% or less
Ni (nickel) is an element for strengthening ferrite and pearlite by means
of solid-solution strengthening. For this reason, Ni may be contained.
However, if the Ni content exceeds 0.30%, not only this effect is saturated, but
also the hardenability is enhanced and bainite is formed after hot forging, so
that the transverse fatigue strength is decreased in some cases. Therefore,
the upper limit was placed on the content of Ni, if contained, and the content of
Ni, if contained, is set to 0.30% or less. The content of Ni, if contained, is
preferably 0.20% or less.
[00491
On the other hand, in order to stably achieve the above-described effect
of Ni, the content of Ni is preferably 0.03% or more, further preferably 0.05%
or more.
[OO~O]
Mo: 0.10% or less
Mo (molybdenum) is an element for strengthening ferrite and pearlite by
means of solid-solution strengthening. For this reason, Mo may be contained.
However, if the Mo content exceeds 0.10%, bainite is formed after hot forging,
so that the transverse fatigue strength is decreased in some cases. Therefore,
the upper limit was placed on the content of Mo, if contained, and the content
of Mo, if contained, is set to 0.10% or less. The content of Mo, if contained, is
preferably 0.08% or less.
Coo511
On the other hand, in order to stably achieve the above-described effect
of Mo, the content of Mo, if contained, is preferably 0.03% or more.
[00521
Only any one kind of Cu, Ni and Mo may be contained, or two or more
kinds of these elements may be contained compositely. The total content of
Cu, Ni and Mo is preferably 0.30% or less.
Coo531
Y1 or Y2: 1.05 to 1.18
In order to provide the non-refined hot-forged part with a tensile
strength of 900 MPa or higher, the rolled steel bar for hot forging that is a
starting material for the non-refined hot-forged part must be configured so
that, in the case where Cu, Ni and Mo are not contained, Y1 [= C + (111O)Si +
(115)Mn + (5122)Cr + 1.65V - (517)SI expressed by formula is 1.05 to 1.18,
or in the case where one or more kinds of Cu, Ni and Mo are contained, Y2 [= C
expressed by formula <2> is 1.05 to 1.18.
[0054] at.'
If Y1 or Y2 becomes more than 1.18, the hardness after hot forging is
increased, so that the machinability is decreased in some cases. Further, the
hardenability is enhanced, and bainite is formed after hot forging, so that the
transverse endurance ratio may be decreased. On the other hand, if Yl or Y2
becomes less than 1.05, the non-refined hot-forged part the starting material of
which is the rolled steel bar for hot forging cannot be provided with a tensile
strength of 900 MPa or higher.
[00561
Y1 or Y2 is preferably 1.08 or'more and preferably 1.16 or less.
Lo0561
The rolled steel bar for hot forging of the present invention can be
produced by the procedure described below. A cast piece having the chemical
composition defined in the present invention is heated, for example, at a
temperature of 1200 to 1300°C for 120 to 180 minutes, and thereafter is
bloomed to prepare a slab measuring 180 mm x 180 mm. Subsequently, the
slab is heated at a temperature of 1150 to 1250°C for 90 to 150 minutes, and is
rolled in the temperature range of 1100 to 1000°C into a steel bar having a
predetermined size, for example, a diameter of 40 mm.
[00571
Then, the rolled steel bar for hot forging of the present invention having
the predetermined size, for example, a diameter of 40 mm is cut to a length of
100 mm. The cut steel bar is heated to a temperature of 1200 to 1250°C with
a high-frequency heating device, thereafter being press-forged in the direction
perpendicular to the axis of the rolled steel bar in the temperature range of
1150 to 1100°C by using a hot forging machine to a thickness of 18 mm, and is
cooled in the temperature range of 800 to 550°C at a cooling rate of 30 to
50°C/min. Thereby, a non-refined hot-forged part having a tensile strength of
900 MPa or higher and a transverse endurance ratio of 0.47 or higher can be
obtained easily.
[0068]
The "heating temperature" of the cast piece and the slab represents the
temperature in the furnace at the time when the cast piece and the slab are
heated.
[0059]
The steel bar rolling temperature represents the surface temperature of
the workpiece.
[OO~O]
The "heating temperature" of the steel bar at the time when the highfrequency
heating device is used represents the surface temperature of steel
bar. The press-forging temperature at the time when the hot forging machine
is used and the temperature at which the steel bar is cooled at a cooling rate of
30 to 50°Clmin after forging also represent the surface temperature of the
workpiece.
[0061]
The "cooling rate" in the temperature range of 800 to 550°C after forging
is a value obtained by dividing the temperature difference of 250°C by the time
required for decreasing the surface temperature of workpiece from 800°C to
550°C.
100621
The following examples illustrate the present invention more specif!ically.
The present invention is not limited to these examples.
[0063]
Cast pieces each having a cross section of 300 mm x 400 mm consisting
of steels A to U having chemical compositions given in Table 1 were heated at
1250°C for 120 minutes, and thereafter were bloomed to prepare slabs each
measuring 180 mm x 180 mm. Subsequently, the slabs were heated at
1200°C for 90 minutes, and were hot-rolled in the temperature range of 1100
to 1000°C to produce steel bars each having a diameter of 40 mm.
[0064]
The steels A to 3 given in Table 1 are steels the chemical compositions of
which are within the range defined in the present invention. On the other
hand, the steels K to U are steels the chemical compositions of which are out of
the range defined in the present invention.
COO651
Od d* mo oa md md mmddmdmd dmwdmdtd- dmdodmmNoa dmdm2Q ~
~ 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0
o o o o o o o o o o o o o o o o o o o d d 0 s A a,
Next, forged products each having a thickness of 18 mm was produced
by hot forging by using the steel bars each having a diameter of 40 mm as
starting materials.
[0067]
Specifically, first, each of the steel bars each having a diameter of 40 mm
was cut to a length of 110 mm.
[00681
Next, the steel bar having a diameter of 40 mm and a length of 110 mm
was heated to 1260°C with a high-frequency heating device, thereafter being
subjected to hot forging in which the steel bar was rolled down in the direction --- -- - - - - L -
perpendicular to the axis of steel bar by a press at a temperature of 1150 to -9.
1100°C to finish a forged product having a thickness of 18 mm, and was cooled
to room temperature by allowing the forged product to cool in the atmosphere.
The cooling rate in the temperature range of 800 to 550°C was 30°C/min.
[0069]
For the forged product, the micro-structure, tensile property, and fatigue
property were examined by the methods of the following items to <3>,
respectively.
[OO~O]
Examination of micro-structure of forged product
A specimen having a transverse section of 10 mm x 10 mm was cut out
from a 112 position in the width direction and a 112 position in the thickness
direction of the forged product having a thickness of 18 mm. Next, the
specimen was embedded in a resin so that the transverse section was a surface
to be examined, the transverse section being mirror polished, and thereafter
the mirror polished surface was etched with 3% alcohol nitride (nital) to cause
the micro-structure to appear. Subsequently, the image of micro-structure
was captured in five visual fields by using an optical microscope having a
magnification of x600 to identlfy the "phase".
[00711
<2> Examination of tensile property of forged product
A No.14A test specimen (diameter of parallel part: 5mm) specified in JIS
22201 (1998) of JIS Handbook [I] Ferrous Materials and Metallurgy I issued
by Japanese Standards Association on January 21, 2011, was sampled from a
112 position in the thickness direction of the forged product having a thickness
of 18 mm so that the longitudinal direction of test specimen was the width
direction of forged product, that is, the direction perpendicular to the axis of
forged product, and that the center of the parallel part of test specimen lies at
- the 112 position in the width direction of forged product. A tension test was
conducted at room temperature with the gage length being 25 mm to
determine the tensile strength. The target of the tensile strength of forged
product was set so as to be 900 MPa or higher.
100721
<3> Examination of fatigue property of forged product
Also, both the ends of the width of forged product having a thickness of
18 mm were milled to remove scale and to finish both the ends to flat planes.
Next, both of the milled ends of forged product and a commercially available
SlOC steel material specified in JIS G4051 (2009) were welded to each other
by electron beam welding to prepare a plate material having a width of 130
mm. Subsequently, an Ono type rotating bending fatigue test specimen
having a diameter of parallel part of 8 mm and a length of 106 mm was
sampled from a 112 position in the thickness direction of the plate material so
that the longitudinal direction of test specimen is the width direction of plate
material, that is, the direction perpendicular to the axis of forged product, and
that the center of the parallel part of test specimen lies at the 112 position in
the width direction of plate material.
Lo07 31
Then, a rotating bending fatigue test was conducted at room
temperature in the atmosphere under the condition that the stress ratio is -1
with the number of tests being eight. The lowest value of stress amplitude
endured with the number of repetitions being 1.0 x 107 or larger was set so as
to be a fatigue strength. Further, a transverse endurance ratio was
determined by dividing the fatigue strength by the tensile strength. The
target of the transverse endurance ratio of forged product was set so as to be
0.47 or higher.
- .
[007d
Table 2 summarizes the results of the tests. The mark "0" in the
"evaluation" column of Table 2 indicates that both of the tensile strength and
the transverse endurance ratio of forged product met the targets, and the mark
" x" indicates that at least one property did not meet the target.
[00751
Table 2
lo0761
Table 2 reveals that test Nos. 1 to 10, which are the example
embodiments of the present invention that meet the conditions defined in the
present invention, are evaluated as "0". That is, it is apparent that, for test
Nos. 1 to 10, all of the micro-structures of forged products produced by using
the steel bars as starting materials are ferritelpearlite, and the target tensile
strength of 900 MPa or higher and the target transverse endurance ratio of
0.47 or higher are attained.
lo0771
Test
No.
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
"F+P"
indicates mixed structure of ferritelpearlite and bainite.
Mark * indicates deviation from conditions deked in present invention.
Mark $ indicates that target is not met.
Steel
A
B
C
D
E
F
G
H
I
J
*K
*L
*M
*N
*O
*P
*Q
*R
*T
*U
in
Microstructure
F+P
F+P
F+P
F+P
F+P
F+P
F+P
F+P
F+P
F+P
F+P
F+P+B
F+P+B
F+P
F+P+B
F+P+B
F+P
F+P
*SF+P
F+P
F+P
micro-structure
Evaluation
0
0
0
0
0
0
0
0
0
0
x
x
x
x
x
x
X
x
x
x
x
structure,
Remarks
Example
embodiment
of present
invention
Comparative
example
and "F+P+BM
Forged
Tensile
strength
(MPa)
943
1020
973
1012
968
1039
953
970
953
948
921
1083
1023
1001
1034
1092
$868
964
$874
965
1015
column
product
Fatigue
strength
( m a )
450
486
480
490
470
495
455
465
460
445
405
440
410
440
420
445
410
405
420
410
460
indicates
Transverse
endurance
ratio
0.48
0.48
0.49
0.48
0.49
0.48
0.48
0.48
0.48
0.47
$ 0.44
$0.41
$ 0.40
$0.44
$ 0.41
$0.41
0.47
$0.42
0.48
$ 0.42
$ 0.45
ferritelpearlite
In contrast, for test Nos. 11 to 21, which are comparative examples that
do not meet the chemical compositions defined in the present invention, either
the tensile strength or the transverse endurance ratio of forged product does
not meet the target.
[00781
In test No. 11, the V content of the used steel K is 0.177%, being lower
than the range defined in the present invention. Therefore, the transverse
endurance ratio of forged product is as low as 0.44.
[00791
In test No. 12, although the contents of elements of the used steel L meet
the conditions defined in the present invention, Y1 is as high as 1.24, deviating
from the range dehed in the present invention. Therefore, in addition to
ferrite and pearlite, bainite is recognized in the micro-structure of forged
product, and the transverse endurance ratio is as low as 0.41.
[0080]
In test No. 13, the Ni content of the used steel M is 0.35%, exceeding the
range defined in the present invention. Therefore, in addition to ferrite and
pearlite, bainite is recognized in the micro-structure of forged product, and the
transverse endurance ratio is as low as 0.40.
[00811
In test No. 14, the Ti content of the used steel N is 0.0098%, exceeding
the range defined in the present invention. Therefore, the transverse
endurance ratio of forged product is as low as 0.44.
[00821
In test No. 15, the Mn content of the used steel 0 is 1.53%, exceeding the
range defined in the present invention. Therefore, in addition to ferrite and
pearlite, bainite is recognized in the micro-structure of forged product, and the
transverse endurance ratio is as low as 0.41.
[00831
In test No. 16, although the contents of elements of the used steel P meet
the conditions defined in the present invention, Y2 is as high as 1.23, deviating
from the range defined in the present invention. Therefore, in addition to
ferrite and pemlite, bainite is recognized in the micro-structure of forged
product, and the transverse endurance ratio is as low as 0.41.
[0084]
In test No. 17, although the contents of elements of the used steel Q
meet the conditions defined in the present invention, Y1 is as low as 0.96,
deviating &om the range defined in the present invention. Therefore, the
tensile strength of forged product is as low as 868 MPa.
. .
[00851
In test No. 18, the S content of the used steel R is 0.043%, exceeding the
range defined in the present invention. Therefore, the transverse endurance
ratio of forged product is as low as 0.42.
[0086]
In test No. 19, although the contents of elements of the used steel S meet
the conditions defined in the present invention, Y2 is as low as 0.99, deviating
from the range defined in the present invention. Therefore, the tensile
strength of forged product is as low as 874 MPa.
[00871
In test No. 20, the 0 content of the used steel T is 0.0031%, exceeding
Therefore, the range defined in the present invention. the transverse
endurance ratio of forged product is as low as 0.42.
[0088l
In test No. 21, the C content of the used steel U is 0.45%, exceeding the
range defined in the present invention. Therefore, the transverse endurance
ratio of forged product is as low as 0.45.
INDUSWa APPLICABILITY
[0089]
By using the rolled steel b& for hot forging of the present invention as a
starting material, a high-strength, non-refined, and hot-forged part having a
tensile strength of 900 MPa or higher and a transverse endurance ratio of 0.47
or higher can be obtained.
We claim:
A rolled steel bar for hot forging having a chemical composition
consisting, by mass percent, of C: 0.27 to 0.37%, Si: 0.30 to 0.76%, Mn: 1.00
to 1.46%, S: 0.008% or more and lees than 0.030%, Cr: 0.06 to 0.30%, Al:
0.006 to 0.050%, V: 0.200 to 0.320%, and N: 0.0080 to 0.0200%, the balance
being Fe and impurities, wherein
the contents of P, Ti and 0 in the impuritiee are, by mass percent, P:
0.030% or less, Ti: 0.0040% or lese, and 0: 0.0020% or less; and
Y1 expressed by the following formula is 1.05 to 1.18.
Y1= C + (1/10)Si + (1/6)Mn + (6/22)Cr + 1.66V - (617)s ... el,
where, C, Si, Mn, Cr, V, and S in the above formula <1> represent,
respectively, the content by maas percent of each of the elements.
2. A rolled steel bar for hot forging having a chemical composition consisting,
by mass percent, of C: 0.27 to 0.37%, Si: 0.30 to 0.76%, Mn: 1.00 to 1,45%,
S: 0.008% or more and lese than 0.030%, Cr: 0.06 to 0.30%, Al: 0.006 to
0.060%, V: 0.200 to 0.320%, and N: 0.0080 to 0.0200%, and one or more
elements selected from Cu: 0.30% or less, Ni: 0.30% or less, and Mo: 0.10%
or leas, the balance being Fe and impuritiee, wherein
the contents of P, Ti and 0 in the impurities are, by mass percent, P:
0.030% or less, Ti: 0.0040% or less, and 0: 0.0020% or less; and
Y2 expressed by the following formula <2> is 1.05 to 1.18.
where, C, Si, Mn, Cr, V, S, Cu, Ni, and Mo in the above formula <2>
represent, reepectively, the content by mass percent of each of the
elements.