Description
Title of Invention: ROLLED STEEL BAR OR WIRE ROD FOR HOT
FORGING
Technical Field
[OOOl]
The present invention relates to a steel bar or wire
rod, and more specifically relates to a rolled steel bar
or wire rod for hot forging.
Background Art
[0002]
Machine parts such as gears and pulleys are used for
automobiles or industrial machinery. Majority of these
machine parts are produced by the following method. A
starting material composed of alloy steel for machine
structural use is prepared. The starting material has,
for example, a chemical composition corresponding to
SCr420, SCM420, or SNCM 420 in Japanese Industrial
Standard (JIS). The starting material is, for example, a
hot rolled steel bar or wire rod. The starting material
is subjected to hot forging to produce an intermediate
product. The intermediate product is subjected to
normalizing as needed. Further, the intermediate product
is subjected to cutting. The intermediate product after
cutting Is subjected to a casehardening treatment. The
casehardening treatment may be, for example, carburizing
- 2 -
quenching, carbonitriding quenching, or induction
quenching. The casehardened intermediate product is
subjected to tempering at a tempering temperature of not
more than 200°C. The intermediate product after
tempering is subjected to shotpeening as needed. Through
the above described processes, machine parts are produced.
[0003]
In recent years, to cope with increases in the fuel
efficiency of automobiles and the output power of engines,
machine parts have been reduced in weight and size. Tkius,
loads imposed on machine parts have increased compared
with the past. For that reason, those machine parts are
required to have excellent bending fatigue strength,
surface fatigue strength (contact fatigue strength), and
wear resistance.
[0004]
On the other hand, the reduction of production cost
of machine parts is also demanded. Specifically,
omission of additional processes such as shotpeening is
demanded to reduce production cost. Further, the
proportion of the cost of cutting in the production cost
is large. Therefore, to reduce production cost, high
machinaoillty is required for rolled steel bar or wire
rod for hot forging, which is to be used for the starting
material for machine parts.
[0005]
Accordingly, for a rolled steel bar or wire rod for
hot forging which provldes the startlng material for
machine parts, excellent machinability is required in
addition to excellent bending fatigue strength, surface
fatigue strength and wear resistance.
[0006]
Techniques to improve the properties of steel to be
used as the starting material for machine parts are
proposed in JP60-21359A, JP7-242994A, and JP7-126803A.
[0007]
In steel for gears disclosed in JP60-21359A, Si and
P contents are specified as Si: not more than 0.1% and P:
not more than 0.01%. JP60-21359A describes that such
specification allows the steel for gear to have high
strength, as well as toughness and high reliability.
[0008]
Steel for gears disclosed in JP7-242994A contains
Cr: 1.50 to 5.0%, and further contains, as needed, Si:
0.40 to 1.0% while satisfying 7.5% > 2.2 x Si(%) + 2.5 x
Mn ( % ) + Cr ( % ) + 5.7 x Mo ( % ) . JP7-242994A describes that
having such a chemical composition allows the steel for
gear to have excellent tooth surface strength.
[0009]
Steel for carburized gears disclosed in JP7-126803A
contains Si: 0.35 to not more than 3.0%, V: 0.05 to 0.53,
etc. JP.7-126803A describes that, having such a chemical
composition, the steel for gear can have a high bending
fatlyue strength and a high surface fatlgue strength.
Disclosure of the Invention
- 4 -
[00101
However, in JP60-21359A, no investigation has been
made on the surface fatigue strength. For that reason,
the steel for gear disclosed in JP60-21359A may have low
surface fatigue strength. In JP7-242994A, no
investigation has been made on the bending fatigue
strength. For that reason, the steel for gear disclosed
in JP7-242994A may have low bending fatigue strength.
The steel for gear disclosed in JP7-126803A contains V.
V increases the hardness of steel after hot forging. For
that reason, the steel after hot forging may have reduced
machinability. In short, none of JP60-21359A, JP7-
242994A, and JP7-126803A discloses a steel having
excellent bending fatigue strength, surface fatigue
strength, and wear resistance, as well as excellent
machinability.
[OOll]
It is an object of the present invention to provide
a rolled steel bar or wire rod for hot forging which has
excellent bending fatigue strength, surface fatigue
strength, wear resistance, and machinability even after
hot forging .
[OOI 21
A rolled steel bar or wire rod for hot forg: ng
according to the present invention has a chemical
compos:tion comprising, by mass%, C: 0.1 to d.25%, S i :
0.30 to 0.60%, Mn: 0.50 to 1.0%, S: 0.003% to 0.05%, Cr:
1.50 to 2.00%, Mo: not more than 0.10% (including O%,,
- 5 -
Al: 0.025 to 0.05%, N: 0.010 to 0.025%, and the balance
being Fe and impurities, wherein the impurities contain
P: not more than 0.025%, Ti: not more than 0.003%, and 0
(oxygen): not more than 0.002%, respectively, and wherein
fnl defined by Formula (1) is 1.60 to 2.10. The above
described rolled steel bar or wire rod for hot forging
has a structure consisting of a ferrite-pearlite
structure, a ferrite-pearlite-bainite structure, or a
ferrite-bainite structure. A maximum value/a minimum
value of average ferrite grain size, which is obtained by
making a measurement in 15 visual fields each having an
area of 62500 p n q n a cross section, is not more than
2.0.
fnl = Cr + 2 x Mo (1)
where each symbol of elements in Formula (1) is
substituted by a content (mass%) of a corresponding
element.
[0013]
The steel bar or wire rod for hot forging according
to the present invention has excellent bending fatigue
strength, surface fatigue strength, wear resistance, and
machinability.
;@0:4;
The rolled steel bar or wire rod for hot forging
according to the present invention may contain Nb: not
more than 0.08% by mass% in place of a part of Fe.
Brief Description of Drawings
- 6 -
[0015]
[Figure 11 Figure 1 is a s i d e view of a small r o l l e r
specimen f o r a r o l l e r p i t t i n g t e s t , which is f a b r i c a t e d
i n Examples.
[Figure 21 Figure 2 is a s i d e view o f a notched Ono-type
r o t a t i n g bending f a t i g u e specimen, which i s f a b r i c a t e d i n
Examples.
[Figure 31 Figure 3 is a diagram showing a c a r b u r i z i n g
quenching c o n d i t i o n i n Examples.
[Figure 41 Figure 4 is a f r o n t view o f a l a r g e r o l l e r f o r
a r o l l e r p i t t i n g t e s t i n an Example.
D e s c r i p t i o n of Embodiments
[0016]
The p r e s e n t i n v e n t o r s have conducted i n v e s t i g a t i o n
and r e s e a r c h on bending f a t i g u e s t r e n g t h , s u r f a c e f a t i g u e
s t r e n g t h , wear r e s i s t a n c e , and m a c h l n a b i l i t y o f a r o l l e d
s t e e l b a r o r wire rod f o r hot f o r g i n g ( h e r e a f t e r , simply
r e f e r r e d t o as a s t e e l bar o r wire r o d ) . As a r e s u l t ,
t h e p r e s e n t i n v e n t o r s have obtained t h e f o l l o w i n g
f i n d i n g s .
[0017]
( a ) As t h e S i content i n c r e a s e s , t h e s u r f a c e f a t i g u e
s t r e n g t h and t h e wear r e s i s t a n c e of s t e e l ir!r.reases.
F u r t h e r , as the C r content and the Mo c o n t e n t i n c r e a s e ,
t h e bendlng f a t i g u e s t r e n g t h , t h e s u ~ f a c ef a t i g u e
s t r e n g t h , and the wear r e s i s t a n c e of s t e e ; i n c r e a s e .
[oo;8]
(b) On the other hand, if the Mo content is
excessively high, production of bainite is promoted in
steel after hot forging, and in steel after subjected tg
hot forging and further to normalizing. Similarly, even
when Mo is not contained, if the Cr content is
excessively high, production of bainite is promoted.
Bainite deteriorates the machinability of steel.
Therefore, it is preferable to suppress the production of
bainite, thereby suppressing the deterioration of the
machinability of steel.
[0019]
(c) As so far described, to obtain excellent bending
fatigue strength, surface fatigue strength and wear
resistance as well as excellent machinability, it is
preferable to adjust Si, Mo, and Cr contents.
Particularly, to increase machinability while increasing
bending fatigue strength, surface fatigue strength and
wear resistance, it is preferable to adjust the total
amount of Cr and Mo contents. Specifically, if the
chemical composition of steel satisfies Formula (2), it
is possible to obtain excellent bending fatigue strength,
surface fatigue strength, wear resistance, and
machinabi lity:
1.60 I Cr + 2 x Mo 52.10 (2)
where, each symbol of elements in Formula (2) is
substituted by the content (mass?) of a corresponding
element.
[0020]
(d) I f t h e v a r i a t i o n of t h e g r a i n s i z e i n a s t e e l
bar or wire rod is l a r g e , the bending f a t i g u e s t r e n g t h
w i l l d e c l i n e . When the v a r i a t i o n of grain s i z e is large,
the surface f a t i g u e s t r e n g t h may also d e c l i n e . As an
index t o show the degree of v a r i a t i o n of grain s i z e , a
r a t i o of average f e r r i t e grain s i z e is defined as follows.
Fifteen v i s u a l f i e l d s each having an area of 62500 pn2
are selected from a region excluding any decarburized
layer i n the o u t e r l a y e r of a cross section of the s t e e l
bar or wire rod. Image analysis is performed on each of
the selected 15 v i s u a l f i e l d s . S p e c i f i c a l l y , an average
f e r r i t e grain s i z e is measured in each visual f i e l d . The
average f e r r i t e grain s i z e i n each visual f i e l d is
measured according t o t h e i n t e r c e p t method s p e c i f i e d i n
JIS GO551 (2005).
[0021]
O u t of the average f e r r i t e grain s i z e s determined i n
each of the 15 v i s u a l f i e l d s , a maximum value and a
minimum value are s e l e c t e d . Then the maximum value/the
minimum value is determined. The determined value is
defined as a r a t i o of average f e r r i t e grain s i z e . That
is, the r a t i o of average f e r r i t e grain s i z e i s defined by
the following Formula ( 3 ) :
A r a t i o of average f e r r i t e grain s i z e = the maximum
value among average ferrite grain s i z e s obtained in 15
visua; fle:as/tne mlnimum value amony average f e r r i t e
gralris s i z e s obtained in 15 visual f i e l d s . ( 3
When the ratio of average ferrite grain size is not
more than 2.0, the variation of grains in steel is small.
As a result, the bending fatigue strength and surface
fatigue strength of steel are high.
[0022]
The rolled steel bar or wire rod for hot forging
according to the present invention has been completed
based on the above described findings. Hereafter, the
rolled steel bar or wire rod for hot forging according to
the present invention will be described in detail.
Hereafter, "%'I indicating the content of an element
constituting a chemical composition represents "mass%".
[0023]
[Chemical composition]
The chemical composition of the steel bar or wire
rod according to the present invention contains the
following elements.
[0024]
C: 0.1 to 0.25%
Carbon (C) increases carburizing-quenching or
carbonitriding-quenching hardenability. Therefore, C
increases the strength of steel. Particularly, C
increases the strength of the core portion of a machine
part after carburizing and quenching or carbonitriding
and quenching. On the other hand, when C is excessively
contained, the amount of deformation of a machine part
after carb~rlzing and quenching or carbonltrlding and
quenching wil- remarkably increase. Therefore, the :3
- 10 -
c o n t e n t s h a l l be 0 . 1 t o 0.25%. The lower l i m i t of t h e C
c o n t e n t is p r e f e r a b l y more t h a n 0.1%, more p r e f e r a b l y n o t
l e s s t h a n 0.15%, and f u r t h e r p r e f e r a b l y n o t l e s s t h a n
0.18%. The upper l i m i t of t h e C c o n t e n t is p r e f e r a b l y
l e s s t h a n 0.25%, more p r e f e r a b l y n o t more t h a n 0.23%, and
f u r t h e r p r e f e r a b l y n o t more t h a n 0.20%.
[0025]
S i : 0.30 t o 0.60%
S i l i c o n ( S i ) improves t h e h a r d e n a b i l i t y of s t e e l .
S i f u r t h e r improves t h e temper s o f t e n i n g r e s i s t a n c e of
s t e e l . T h e r e f o r e , S i i n c r e a s e s t h e s u r f a c e f a t i g u e
s t r e n g t h and wear r e s i s t a n c e of s t e e l . On t h e o t h e r hand,
i f S i is e x c e s s i v e l y c o n t a i n e d , t h e s t r e n g t h of s t e e l
a f t e r h o t f o r g i n g w i l l become e x c e s s i v e l y h i g h . As a
r e s u l t . , t h e m a c h i n a b i l i t y of s t e e l w i l l d e t e r i o r a t e .
F u r t h e r , i f S i is e x c e s s i v e l y c o n t a i n e d , t h e bending
f a t i g u e s t r e n g t h w i l l d e c l i n e . T h e r e f o r e , t h e S i c o n t e n t
s h a l l be 0.30 t o 0.60%. The lower l i m i t of t h e S i
c o n t e n t is p r e f e r a b l y more t h a n 0.30%, more p r e f e r a b i y
n o t l e s s t h a n 0.40%, and f u r t h e r p r e f e r a b l y not l e s s t h a n
0.45%. The upper l i m i t of t h e S i c o n t e n t is p r e f e r a b l y
l e s s t h a n 0.60%, more p r e f e r a b l y n o t more t h a n 0.57%, and
f u r t h e r p r e f e r a b l y not more t h a n 0.55%.
100261
Mn: 0.50 t o 1.0%
Manganese (Mn) improves t h e h a r d e n a b i l i t - y of s t e e l
and i n c r e a s e s t h e s t r e n g t h of s t e e l . T h e r e f o r e , Mn
increases t h e s t r e n g t h of t h e c o r e p o r t i o n c f a machine
- 1: -
part subjected to carburizing and quenching, or
carbonltriding and quenching. On the other hand, if Mn
is excessively contained, the machinability of steel
after hot forging will deteriorate. Further, when Mn is
excessively contained, Mn oxide will be produced on the
surface of steel. That will result in an increase in the
depth of an abnormally carburized layer after carburizing
and quenching, or carbonitriding and quenching. The
abnormally carburized layer is, for example, a boundary
oxidation layer and a slack quenched layer. As the depth
of the abnormally carburized layer increases, the bending
fatigue strength and the pitting strength of steel will
decline. Pitting is one form of fracture due to surface
fatigue. Therefore, as the pitting strength declines,
the surface fatigue strength decllnes as well. Therefore,
the Mn content shall be 0.50 to 1.0%. The lower limit of
the Mn content is preferably more than 0.50%, more
preferably not less than 0.55%, and further preferably
not less than 0.60%. The upper limit of the Mn content
is preferably less than 1.0%, more preferably not more
than 0.95%, and further preferably not more than 0.9%.
[0027]
S: 0.003 to 0.05%
Sulf~r (S) combines with Mn to form YnS. MnS
improves the machinability of steel. On the other hand,
if S is excessively contained, coarse MnS wi.1 be formed.
Coarse YnS decreases the bendlng fatlgue streriqth and the
surface fatigue strength of steel. Therefore, the S
- 12 -
c o n t e n t s h a l l be 0.003 t o 0.05%. The lower l i m i t of t h e
S c o n t e n t is p r e f e r a b l y more t h a n 0.003%, f u r t h e r
p r e f e r a b l y n o t less t h a n 0.005%, and f u r t h e r p r e f e r a b l y
n o t l e s s t h a n 0.01%. The upper l i m i t of t h e S c o n t e n t is
p r e f e r a b l y l e s s t h a n 0.05%, more p r e f e r a b l y n o t more than
0.03%, and f u r t h e r p r e f e r a b l y n o t more t h a n 0.02%.
[0028]
C r : 1.50 t o 2.00%
Chromium ( C r ) improves t h e h a r d e n a b i l i t y of s t e e l
and t h e temper s o f t e n i n g r e s i s t a n c e of s t e e l . As a
r e s u l t , C r i n c r e a s e s t h e bending f a t i g u e s t r e n g t h , t h e
s u r f a c e f a t i g u e s t r e n g t h , and t h e wear r e s i s t a n c e of
s t e e l . On t h e o t h e r hand, i f C r is e x c e s s i v e l y c o n t a i n e d ,
t h e p r o d u c t i o n o f b a i n i t e is promoted i n s t e e l a f t e r h o t
f o r g i n g o r a f t e r n o r m a l i z i n g . As a r e s u l t , t h e
m a c h i n a b i l i t y of s t e e l d e t e r i o r a t e s . T h e r e f o r e , t h e C r
c o n t e n t s h a l l be 1.50 t o 2.00%. The lower l i m i t of t h e
Cr c o n t e n t is p r e f e r a b l y more t h a n 1.50%, more p r e f e r a b l y
n o t l e s s t h a n 1.70%, and f u r t h e r p r e f e r a b l y n o t l e s s t h a n
1.80%. The upper l i m i t of t h e C r c o n t e n t is p r e f e r a b l y
l e s s t h a n 2.00%, more p r e f e r a b l y n o t more t h a n 1.95%, and
f u r t h e r p r e f e r a b l y not more t h a n 1.90%.
[0029;
Mo: not more than 0.10% ( i n c l u d i n g 0%)
Molybdenum (Mo) may not be c o n t a i n e d . No improves
t h e h a r d e n a b i L l t y and t h e temper s o f t e n i n g r e s i s t a n c e of
s t e e l . A s a r e s u i t , Mo i n c r e a s e s the ending f a t i g d e
s t r e n g t h , t h e s u r f a c e f a r i g u e s t r e n g y h , and t h e wear
- 13 -
resistance of steel. On the other hand, if Mo is
excessively contained, the production of bainlte is
promoted in steel after hot forging or normalizing. As a
result, the machinability of steel deteriorates.
Therefore, the Mo content shall be not more than 0.10%
(including 0%). The lower limit of the Mo content is
preferably not less than 0.02%. The upper limit of the
Mo content is preferably less than 0.10%, and more
preferably not more than 0.08%, and further preferably
not more than 0.05%.
[ 0 0 3 0 1
Al: 0.025 to 0.05%
Aluminum (Al) deoxidizes steel. Further, A1
combines with N to form A1N. A1N suppresses the
coarsening of austenitic grains due to carburizing and
heating. On the other hand, if A1 is excessively
contained, A1 forms coarse A1 oxides. Coarse A1 oxides
decrease the bending fatigue strength of steel.
Therefore, the A1 content shall be 0.025 to 0.05%. The
lower limit of the A1 content is preferably more than
0.025%, more preferably not less than 0.027%, and further
preferably not less than 0.030%. The upper limit of AL
is preferably less than 0.05%, more preferably not more
than 0.045%, and further preferably not more than 0.04%.
:0031:
Ii: 0.010 to 0.025%
Nitrogen (N) combines with A; or Nb to form A1N or
NbN. A1N or NbN suppresses the coarsening of austenitic
- 14 -
grains due to carburizing and heating. On the other hand,
if N is excessively contained, stable production in steel
making process becomes difficult. Therefore, the N
content shall be 0.010 to 0.025%. The lower limit of the
N content is preferably more than 0.010%, more preferably
not less than 0.012%, and further preferably not less
than 0.013%. The upper limit of N is preferably less
than 0.025%, more preferably not more than 0.020%, and
further preferably not more than 0.018%.
[0032]
The balance of the chemical composition of the steel
bar or wire rod according to the present invention is
comprised of Fe and impurities. The impurities as used
herein represent elements which are mixed from ores and
scrap to be used as the raw material of steel., or the
environments of production processes. In the present
invention, the contents of PI Ti, and 0 (oxygen) as
impurities are limited as follows.
[ 0 0 3 3 ]
P: not more than 0.025%
Phosphorus (P) segregates at grain boundaries and
embrittles the grain boundaries. As a result, P
decreases the fatigue strength of steel. Therefore, the
P content is preferab:~ as low as possibl.e. The P
content shall be not more than 0.025%. '?he P content is
preferably less than 0.025%, and more preferably not more
than 0.020%.
10034 :
Ti: not more than 0.003%
Titanium (Ti) combines with N to form coarse TiN.
Coarse TiN decreases the fatigue strength of steel.
Therefore, the Ti content is preferably as low as
possible. The Ti content shall be not more than 0.003%.
The Ti content is preferably less than 0.003%, and
further preferably not more than 0.002%.
[0035]
0 (Oxygen) : not more than 0.002%
Oxygen (0) combines with A1 to form oxide based
inclusions. Oxide based inclusions decrease the bending
fatigue strength of steel. Therefore, the 0 content is
preferably as low as possible. The 0 content shall be
not more than 0.002%. The 0 content is preferably less
than 0.002%, and more preferably not more than 0.001%.
[0036]
The chemical composition of the steel bar or wire
rod according to the present invention satisfies Formula
( 2 ) .
1.60 < Cr + 2 x Mo 5 2.10 (2
Where, each symbol of elements in Formula (2) is
substituted by the content (mass%) of a corresponding
element.
[0037]
As so far described, both Cr and Mo Improve the
hardenability and the temper softening resistance of
steel. As a result, Cr and Mo increase the bendlng
fatigue strength, the surface fatigue strength, and the
- 16 -
wear resistance of steel. Comparing Mo with Cr, Mo
achieves the same level of advantageous effects
(increases in the bending fatigue strength, the surface
fatigue strength, and the wear resistance) as Cr by an
amount half of the Cr content. Therefore, it is defined
that fnl = Cr + 2Mo. Each symbol of elements in fnl is
substituted by the content (mass%) of a corresponding
element (Cr or Mo) .
[0038]
When fnl is less than 1.60, at least one or more
kinds of the bending fatigue strength, the surface
fatigue strength and the wear resistance of steel will
decline. On the other hand, when fnl exceeds 2.10, the
production of bainite is promoted in steel after hot
forging or normalizing. As a result, the machinability
of steel deteriorates. When fnl is 1.60 to 2.10, it is
possible to increase the bending fatigue strength, the
surface fatigue strength, and the wear resistance of
steel while suppressing the deterioration of the
machinability of steel. The lower limit of fnl is
preferably not less than 1.80. The upper limit of fnl is
preferably less than 2.00.
[ 0 0 3 9 1
The chemical composition of the rolled steel bar or
wire rod for hot forging according to the present
invention may contain Nb in place of a part of Fe.
:0040]
Nb: not rnore than 0.08%
- 17 -
Niobium (Nb) is a selective element. Nb combines
with C and N to form Nb carbides, Nb nitrides or Nb
carbonitrides. Nb carbides, Nb nitrides and Nb
carbonitrides suppress, as with A1 nitrides, the
coarsening of austenitic grains during carburizing and
heating. If even a slight amount of Nb is contained, the
above described effect will be achieved. On the other
hand, when Nb is excessively contained, Nb carbides, Nb
nitrides and Nb carbonitrides will become coarse. For
that reason, it is not possible to suppress the
coarsening of austenitic grains during carburizing and
heating. Therefore, the Nb content shall be not more
than 0.08%. The lower limit of the Nb content is
preferably not less than 0.01%. The upper limit of the
Nb content is preferably less than 0.08%, and more
preferably not more than 0.05%.
[0041]
[Microstructure]
The microstructure of the steel bar or wire rod
according to the present invention consists of a ferritepearlite
structure, a ferrite-pearlite-bainite structure,
or a ferrite-bainite structure. Here, the "ferritepearlite
structure" represents a two-phase structure
whose matrix (parent phase) consists of ferrite and
pearlite. The "ferrite-pearlite-bainite structure"
represents a three-phase structure whose matrix consists
of ferrite, pearl ite, and bainite. The "£err :*_e-bainit-e
structure" represents a two-phase structure whose matrix
consists of ferrite and bainite.
[0042]
In short, the microstructure of the steel bar or
wire rod according to the present invention does not
contain martensite. Martensite is hard, and it
deteriorates the ductility of steel. Therefore, when a
steel bar or wire rod containing martensite is conveyed
or straightened, the steel bar or wire rod is likely to
have cracks. Since the microstructure of the steel bar
or wire rod according to the present invention does not
contain martensite, cracks are not likely to occur during
straightening or conveyance.
[0043]
Each phase described above is identified by the
following method. A sample is cut out which includes a
central portion of a section (cross section)
perpendicular to the longitudinal direction of the steel
bar or wire rod. The surface (including the central
portion) of the cut out sample is mirror polished. The
polished surface is etched with Nital. The etched
surface is subjected to microstructure observation by an
optical microscope of 400-times magnification.
Specifically, 15 visual fields are arbitrarily selected
from a region in the etched surface excluding any
decarburized layer of the outer layer of the steel bar or
wire rod. ':hen, each visual field is observea to
]dent: fy :he microst ructure. If bainite is Ir~clxded in
- 19 -
any of the 15 visual fields, it is judged that the
microstructure of,the steel includes bainite. Similar
judgment is made for ferrite and pearlite as well. The
size of each visual field is 250 p x 250 p = 62500 pn2.
[0044]
Further, in the above described microstructure, the
ratio of average ferrite grain size defined by Formula
(3) is not more than 2.0, in a cross section.
[0045]
Image analysis is performed on each of the above
described 15 visual fields. Specifically, a ferrite
phase is identified in each visual field. The ferrite
grain size in the identified ferrite phase is measured.
An average ferrite grain size of each visual field is
measured according to the intercept method specified in
JIS GO551 (2005).
[0046]
Among average ferrite grain sizes (15 in total)
determined in each of the 15 visual fields, the maximum
and the minimum values are selected. Then, based on the
above described Formula ( 3 ) , the ratio of average ferrite
grain size = (the maximum value of ferrite average grains
size/the minimum value of average ferrite grain size) is
determined.
I00471
When the grain size is non-uniform in a steel
material afcer hot rolling (that is, an as-hot rolled
material), the grain size will remain to be non-uniform
- 20 -
even after hot forging or carburizing and quenching which
is a post process. If the grain size is non-uniform, the
bending fatigue strength and the surface fatigue strength
will decline. Therefore, the grain size in the as-hot
rolled material is preferably as uniform as possible. To
evaluate the degree of uniformity of grain size, it is
preferable to evaluate the ratio of average ferrite grain
size. The ferrite grain size can be observed easier than
that of pearlite or bainite by means of etching.
Therefore, investigating the degree of uniformity of
average ferrite grain size (that is, the ratio of average
ferrite grain size) facilitates the evaluation of the
degree of uniformity of the grain size in structure.
Further, fatigue fracture occurs starting from a location
of lowest strength. For that reason, using the maximum
value/the minimum value of average ferrite grain size
rather than the standard deviation of average ferrite
grain size as an index is more suitable for evaluating
the bending fatigue strength and the surface fatigue
strength.
[0048]
If the microstructure consists of various mixed
structures including ferrite as described above and the
ratio of average ferrite grain size is not more than 2.0,
the variation of grain size in the steel bar or wire rod
is small. As a result, the bending fatigue sttrength and
the surface fatigue strength of stee; after hot forging,
or carburizing and quenching ~ncrease. The ratio of
- 2 1 -
average ferrite grain size is preferably not more than
1. b.
[0049]
On the other hand, if the ratio of average ferrite
grain size exceeds 2.0, one or more kinds of the bending
fatigue strength and the surface fatigue strength of
steel will decline.
[0050]
[Production method]
An example of the method for producing the steel bar
or wire rod according to the present invention, and an
example of the method for producing a machine part
represented by a gear and a pulley will be described.
Note that the production method will not be limited to
the following.
[0051]
Molten steel having the above described chemical
composition and satisfying Formula (2) is produced. The
molten steel is used to produce a cast piece (slab or
bloom) by a continuous casting process. In the
continuous casting process, the cast piece in the course
of solidification is subjected to rolling reduction.
Next, the cast piece 1s heated. The heating temperature
in this occasion is 1250 to 1300°C and the heating time
is not less than 10 hours. The heated cast piece is
billeted by a billeting machine to produce a billet.
COO52 1
The billet is hot rolled to produce a steel bar or
wire rod for hot forging. Specifically, the billet is
heated. In this occasion, the heating temperature is
1150 to 1200°C, and the heating time is not less than 1.5
hours. The heated billet is hot rolled to produce a
steel bar or wire rod. The finishing temperature in the
hot rolling is 900 to 1000°C. Water cooling is not
performed before finish rolling. After the finish
rolling, the steel bar or wire rod is cooled at a cooling
rate not more than that of spontaneous cooling in the air
(hereafter, simply referred to as spontaneous cooling)
until the surface temperature reaches not more than 600°C.
In the hot rolling, a reduction of area ( % ) , which is
defined by Formula (4), shall be not less than 87.5%:
Reduction of area = {l - (sectional area of steel
bar or wire rod/sectional area of billet)) x 100. ( 4 )
[0053]
The steel bar or wire rod after finish rolling needs
not to be cooled to the room temperature at a cooling
rate not more than that of spontaneous cooling. After
the surface temperature of the steel bar or wire rod
reaches not more than 600°C, the steel bar or wire rod
may be cooled at a cooling rate more than that of
spontaneous cooling, such as by air cooling, mist cooling,
water cooling, and the like.
[0054]
The above described heating temperature represenLs
an average value of ln-furnace texperature of a heating
- 23 -
furnace. The above described heating time represents the
time period in a furnace at the above described heating
temperature. The finishing temperature represents the
surface temperature of the steel bar or wire rod just
after finish rolling. The finish rolling represents, for
example, rolling in the final stand among a plurality of
stands used for rolling in a continuous mill. The
cooling rate after finish processing represents the
surface cooling rate of the steel bar or wire rod.
[0055]
An example of the method for producing a machine
part by using a rolled steel bar or wire rod for hot
forging is as follows.
[0056]
A rolled steel bar or wire rod for hot forging is
subjected to hot forging to produce an intermediate
product having a rough shape. The intermediate product
may be subjected to thermal refining treatment. The
thermal refining treatment is for example normalizing.
The intermediate product is machined into a predetermined
shape. The machining is for example cutting or piercing.
[0057]
The intermediate product after machining may be
subjected to a casehardening treatment. The
casehardening treatment is, for example, carburizing
treatment, nitriding treatment, or induction quenching
treatment, etc. The casehardened intermediate product is
sub:ected to finish processing to produce a machine part.
- 24 -
[0058]
The steel bar or wire rod which has been produced by
the above described processes has excellent bending
fatigue strength, surface fatigue strength, and wear
resistance as well as excellent machinability even after
hot forging .
Example 1
[0059]
Steels A to C having chemical compositions shown in
Table 1 were melted by a 70-ton converter.
[Table 11
[00601
The molten steels of Steels A to C were used to
produce cast pieces (blooms) of 400 mm x 300 mm by a
continuous casting process. The produced blooms were
spontaneously cooled to 600°C in the atmosphere. Note
that the cast piece in the course of solidification was
subjected to rolling reduction in a continuous casting
step.
[0061]
Next, production conditions shown in Table 2 were
set.
[Table 21
Specifically, a "heating temperature" column in a
"cast piece" column of Table 2 shows heating temperatures
- 2 6 -
(OC) of the cast piece at each condition. A "heating
time" column in the "cast piece" column of Table 2 shows
heating times (minutes) of the cast piece at each
condition. Similarly, a "heating temperature" column in
a "billet" column of Table 2 shows heating temperatures
(OC) of the billet at each condition. A "heating time"
column in the "billet" column shows heating times
(minutes) of the billets at each condition. A "water
cooling before finish rolling" column in a "rolling
condition" column shows whether or not water cooling of
the billet was performed before finish rolling at each
condition. "With" in the column indicates that water
cooling was performed. And "without" indicates that
water cooling was not performed. A "finishing
temperature" column in the "rolling condition" column
shows finishing temperatures (OC) at each condition. A
"cooling condition" column in the "rolling condition"
column shows cooling conditions after finish rolling at
each condition.
[0063]
Steel bars of Test numbers 1 to 10 shown in Table 3
were produced based on the steels shown in Table 1 and
the production conditions shown in Table 2.
[Table 3j
Specifically, in each Test number, the cast pieces
of the steels shown in Table 3 were heated at the
production conditions (heating temperature and heating
- 28 -
time of cast piece) shown in Table 3. The heated cast
piece was billeted to produce a billet of 180 mm x 180 mm.
The produced billet was cooled to the room temperature
(25OC) .
[0065j
Next, the billet was heated at the production
conditions (heating temperatures and hating times of the
billet) shown in Table 3. The heated billet was hot
rolled at the production conditions (water cooling before
finish rolling, finishing temperature, cooling condition)
shown in Table 3 to produce steel bars having a diameter
of 50 mm and a diameter of 70 mm. The steel bars after
rolling were spontaneously cooled as-is to the room
temperature in the atmosphere. That is, the steel bars
were as-hot rolled members.
[3066]
[Microstructure observation test]
A steel bar of 50 mm diameter was cut perpendicular
to its longitudinal direction. A sample including a
central portion of the cut section was cut out. In the
surfaces of the sample, the surface corresponding to the
above described central portion was polished into a
mirror surface. And the polished surface was etched with
Nital. The etched surface was observed in 15 visual
fields with an optical microscope of 400-times
magnification. The 15 visual fields were arbitrarily
selected from a region excluding any decarbur'zed layer
ir! the outer layer. The size of each visual field was
- 29 -
250 p x 250 p. Microstructure was observed in each
visual field.
[0067]
As a result of the microstructure observation test,
the microstructure of any Test number did not include
martensite. The microstructure of each Test number was
any one of a ferrite-pearlite structure, a ferritepearlite-
bainite structure, and a ferrite-bainite
structure. The results of the microstructure observation
are shown in a "microstructure" column in Table 3. "F +
P" in the table indicates that the microstructure of
corresponding Test number was a ferrite-pearlite
structure. And "F + P + B" indicates a ferrite-pearlitebainite
structure, and "F + B" indicates a ferritebainite
structure.
[0068]
[Average ferrite grain size measurement]
Average ferrite grain sizes of the above described
15 visual fields were measured according to the intercept
method specified in JIS GO551 (2005) .
[0069]
Among the average ferrite grain sizes (15 in total)
of each visual fields, a maximum value and a minimum
values were determined. Then, a ratio of average ferrite
grain size (= maximum value/minimum value) was determined
based on Formula (3). The ratios of average ferri~e
grain size are shown in Table 3.
[0070]
[Preparation of surface fatigue strength specimen and
bending fatigue strength specimen]
The steel bar of each Test number was heated at
1200°C for 30 minutes. Next, the steel bar was subjected
to hot forging with the finishing temperature being not
less than 950°C to produce a round bar having a diameter
of 35 mm. The round bar of 35mm diameter was machined to
prepare a small roller specimen for roller pitting
(hereafter, simply referred to as a small roller
specimen) shown in Figure 1, and a notched Ono-type
rotating bending fatigue specimen shown in Figure 2 (the
dimensional unit in the drawing is mm for both Figures 1
and 2). The small roller specimen shown in Figure 1
included a test portion (a columnar portion having a
diameter of 26 mm and a width of 28 mm) in its center.
[0071]
Each prepared specimen was subjected to carburizing
and quenching at the conditions shown in Figure 3 by
using a gas carburizing furnace. After quenching, the
specimen was subjected to tempering at 150°C for 1.5
hours. The small roller specimen and the Ono-type
rotating bending fatigue specimen were subjected to
finish processing of the gripping portion for the purpose
of removing heat treatment strain.
[0072]
[Surface fatigue strength test;
In the roller pitting test, the above described
small roller specimen and a large roller having a shape
shown in Figure 4 (the dimensional unit in the drawing is
mm) were combined. The large roller shown in Figure 4
was made of steel which satisfied the specification of
SCM420H in JIS, and was fabricated through general
production processes, that is, processes of normalizing,
specimen machining, eutectic carburizing with a gas
carburizing furnace, low temperature tempering and
polishing.
[0073]
A roller pitting test by use of the small roller
specimen and the large roller was conducted at conditions
shown in Table 4.
[Table 41
TABLE4
As shown in Table 4, the number of revolutions of
Test machine
Specimen
Maximum interfacial pressure
Nu m ber of tests
Slip factor
Number of revolutions of
small roller
Circumferential speed
Lubricant oil temperature
Oil used
the small roller speclmen was 1000 rpm, a slip factor was
Roller pitting test machine
Small roller of 26 mm dia.
Large roller of 130 mm dia. (contact portion 700 mmR)
4000 MPa
Six
-40%
1000 rpm
Small roller: 1.36 mls, Large roller: 1.90 mls
90°C
Oil for automatic transmission
-40%, a contact interfacial pressure between the large
roller and the small roller specimen during test was 4000
- 32 -
MPa, and the number of repetition was 2.0 x lo7 cycles.
When the rotational speed of the large roller was V1
m/sec and the rotational speed of the small roller
specimen was V2 m/sec, the slip factor ( % ) was determined
by the following formula:
Slip factor = (V2 - V1) /V2 x 100.
[0075]
During testing, a lubricant (commercially available
oil for automatic transmission) was sprayed at an oil
temperature of 90°C to the contact portion (surface of
the test portion) between the large roller and the small
roller specimen in the counter direction to the
rotational direction. The roller pitting test was
conducted at the conditions described above to evaluate
the surface fatigue strength.
[0076]
For each Test number, six tests were conducted in
the roller pitting test. After testing, an S-N curve was
created in which contact stress was plotted for the
ordinate and the number of repetition until the
occurrence of pitting was plotted for the abscissa.
Among the contact stress of the specimens in which
pitting did not occur until the number of repetition of
2.0 x lo7, the highest contact stress was defined as the
surface fatigue strength of that Test number. It was
defined that pitting occurred when among portions of the
small roller specimen where the surface was damaged, the
area of the largest portion reached not less than 1 mm'.
- 33 -
[0077]
Table 3 shows s u r f a c e f a t i g u e s t r e n g t h s obtained by
t h e t e s t . I n t h e s u r f a c e f a t i g u e s t r e n g t h of Table 3,
t h e s u r f a c e f a t i g u e s t r e n g t h of Test number 1 was s e t as
a r e f e r e n c e value (100%) . Then, t h e s u r f a c e f a t i g u e
s t r e n g t h of each Test number was r e p r e s e n t e d by a r a t i o
( % ) with r e s p e c t t o t h e r e f e r e n c e v a l u e . I f t h e s u r f a c e
f a t i g u e s t r e n g t h was not l e s s than 120%, it was judged
t h a t e x c e l l e n t s u r f a c e f a t i g u e s t r e n g t h was o b t a i n e d .
[0078]
[Wear r e s i s t a n c e e v a l u a t i o n ]
The amount of wear of t h e t e s t p o r t i o n of t h e small
r o l l e r specimen was measured f o r t h e specimens i n which
t h e number of r e p e t i t i o n i n t h e r o l l e r p i t t i n g t e s t
reached 1 . 0 x l o 6 . S p e c i f i c a l l y , a maximum height
roughness (Rz) was determined according t o JIS B0601
( 2 0 0 1 ) . Here, a s m a l l e r Rz value i n d i c a t e s a h i g h e r wear
r e s i s t a n c e . A s u r f a c e roughness t e s t e r was used t o
measure t h e amount of wear. Table 3 shows t h e amounts of
wear. In t h e amount of wear i n Table 3, t h e amount of
wear of Test number 1 was s e t as a r e f e r e n c e value ( 1 0 0 % ) .
Then, t h e amount of wear of each Test number was
r e p r e s e n t e d by a r a t i o ( % ) with r e s p e c t t o t h e r e f e r e n c e
v a l u e . When t h e amount of wear was not more than 80%, it
was judged t h a t e x c e l l e n t wear r e s i s t a n c e was o b t a i n e d .
[0079]
[Bending f a t i g u e s t r e n g t h t e s t ]
The bending fatigue strength was determined by the
Ono-type rotating bending fatigue test. The number of
tests in the Ono-type rotating bending fatigue test was
decided to be eight for each Test number. The test was
conducted with the number of revolutions during testing
being 3000 rpm, and otherwise in an ordinary manner.
Among the stresses of the specimens which have not broken
off until the numbers of repetition of 1.0 x lo4 cycles
and 1.0 x 10' cycles, the highest stresses were defined
as a medium-cycle and high-cycle rotating bending fatigue
strengths, respectively.
[0080]
Medium-cycle and high-cycle bending fatigue
strengths are shown in Table 3. In the medium-cycle and
high-cycle bending fatigue strengths, the medium-cycle
and high-cycle bending fatigue strengths of Test number 1
were set as reference values (100%). Then, the mediumcycle
and high-cycle bending fatigue strengths of each
Test number were represented by ratios ( % ) with respect
to the reference values. It was judged that exceilent
bending fatigue strength was obtained if the bending
fatigue strength was not less than 115% for both the
medium cycle and high cycle.
[0081]
[Cutting test]
A cutting test was conducted to eva~- uate
machinability. A cutting specimen was obtained by the
following method. A steel bar of 70 mrn diameter of each
- 35 -
Test number was heated at a heating temperature of 1250°C
for 30 minutes. The heated steel bar was hot forged at a
finishing temperature of not less than 950°C to obtain a
round bar of 60 mm diameter. From this round bar, a
cutting specimen having a diameter of 55 mm and a length
of 450 mm was obtained by machining. By using this
cutting specimen, a cutting test was conducted at the
following conditions.
[0082]
Cutting test (lathe turning)
Tip: Base metal material is P20 grade cemented
carbide without coating
Test conditions: Circumferential speed is 200 m/min,
feed is 0.30 mm/rev, a depth of cut is 1.5 mm, with the
use of water-soluble cutting oil
Measured item: Amount of major cutting edge wear of
flank face after 10 minutes of cutting time
[0083]
Table 3 shows obtained amounts of major cutting edge
wear. In Table 3, the amount of major cutting edge wear
of Test number 2 (Steel B was used) was set as a
reference value (100%) . Then, the amount of major
cutting edge wear of each Test number was represented by
a ratio ( % ) with respect to the reference value. It was
judged that excellent machinability was obtained when the
amount of major cutting edge wear was not more than 80%.
[0084 J
[Evaluation results]
With r e f e r e n c e t o Table 3, t h e chemical compositions
of t h e s t e e l b a r s of Test numbers 4 and 9 ( S t e e l C) were
w i t h i n t h e range of t h e p r e s e n t i n v e n t i o n , and f n l
s a t i s f i e d Formula ( 2 ) . F u r t h e r , t h e r a t i o s of average
f e r r i t e g r a i n s i z e of Test numbers 4 and 9 were both not
more than 2.0. As a r e s u l t , f o r Test number 4 and 9, t h e
medium-cycle and high-cycle bending f a t i g u e s t r e n g t h s
were not l e s s than 115%, and t h e s u r f a c e f a t i g u e s t r e n g t h
was not l e s s than 120%. F u r t h e r , t h e amount of wear was
not more than 80%. Furthermore, t h e amount o f major
c u t t i n g edge wear was not more than 80%. Therefore,
s t e e l b a r s of Test numbers 4 and 9 e x h i b i t e d e x c e l l e n t
bending f a t i g u e s t r e n g t h , s u r f a c e f a t i g u e s t r e n g t h , wear
r e s i s t a n c e and m a c h i n a b i l i t y .
[0085]
On t h e o t h e r hand, t h e chemical composition o f t h e
s t e e l bar of Test number 1 ( S t e e l A) corresponded t o
SCr420tI i n J I S . For t h a t reason, t h e S i c o n t e n t and t h e
C r content of Test number 1 were l e s s than t h e lower
l i m i t s of t h e S i content and t h e C r content of t h e
p r e s e n t i n v e n t i o n . F u r t h e r , f n l of Test number 1 was
l e s s than the lower l i m i t of Formuia ( 2 ) . As a r e s u l t ,
Test number 1 e x h i b i t e d low bending f a t i g u e s t r e n g t h ,
s u r f a c e f a t i g u e s t r e n g t h , and wear r e s i s t a n c e .
[0086]
The chemical composition of t h e s t e e l bar of Test
number 2 ( S t e e l B) corresponded t o SCM420H i n JIS.
Therefore, t h e S i content and t h e Cr content of Test
- 3'1 -
number 2 were less than the lower limits of the Si
content and the Cr content of the present invention.
Further, the Mo content of Test number 2 exceeded the
upper limit of the Mo content of the present invention.
Further, fnl of Test number 2 was less than the lower
limit of Formula (2). As a result, the bending fatigue
strength of Test number 2 was as low as less than 115%,
and the machinability thereof was also low.
[0087]
The chemical composition of Test number 3 (Steel C)
was within the range of the chemical composition of the
present invention. Further, fnl satisfied Formula (2) as
well. However, since the heating time of the cast piece
was too short (see Production condition 1 in Table 2 ) ,
the ratio of average ferrite grain size exceeded 2.0. As
a result, the medium-cycle and high-cycle bending fatigue
strengths of Test number 3 were as low as less than 115%.
[0088]
The chemical composition of Test number 5 was within
the range of the present invention, and also fnl
satisfied Formula (2). However, in Test number 5, water
cooling was performed before finish rolling (see
Production condition 3 in Table 2). For that reason, the
ratio of average ferrite grain size exceeded 2.0. As a
result, the medium-cycle and high-cycle bending fatigue
strengths of Test number 5 were as low as less than 115%.
[0089]
The chemical composition of Test number 6 was within
the range of the chemical composition of the present
invention, and also fnl satisfied Formula (2). However,
in Test number 6, the steel bar after finish rolling was
subjected to water cooling to 800°C (see Production
condition 4 in Table 2). For that reason, the ratio of
average ferrite grain size exceeded 2.0. As a result,
both the medium-cycle and high-cycle bending fatigue
strengths of Test number 6 were as low as less than 115%.
Further, the surface fatigue strength was as low as less
than 120%. Furthermore, the amount of wear exceeded 80%,
indicating low wear resistance.
[0090]
The chemical composition of Test number 7 was within
the range of the chemical composition of the present
invention, and also fnl satisfied Formula (2). However,
in Test number 7, the heating time of the cast piece was
too short, and also the heating time of the billet was
too short (see Production condition 5). As a result, the
ratio of average ferrite grain size exceeded 2.0. For
that reason, both the medium-cycle and high-cycle bending
fatigue strengths of Test number 7 were as low as less
than 1159.
The chemical composition of Test number 8 was within
the range of the chemical composition of the present
invention, and also fnl satisfied Formula (2). However,
in Test number 8, the heating temperature of the billet
- 39 -
was too high, and also the finishing temperature thereof
was too high (see Production condition 6). For that
reason, the ratio of average ferrite grain size exceeded
2.0. As a result, both the medium-cycle and high-cycle
bending fatigue strengths of Test number 8 were as low as
less than 115%. Further, the surface fatigue strength
was as low as less than 120%. Furthermore, the amount of
wear exceeded 80%, indicating low wear resistance.
[0092]
The chemical composition of Test number 10 was
within the range of the chemical composition of the
present invention, and also fnl satisfied Formula (2).
However, in Test number 10, the heating temperature of
the cast piece was too low (see Production condition 8).
For that reason, the ratio of average ferrite grain size
exceeded 2.0. As a result, the medium-cycle bending
fatigue strength was as low as less than 115%.
Example 2
[0093]
Molten steels having the chemical compositions of D
to S shown in Table 5 were produced in the same manner as
in Example I.
[Table 51
[0094]
Then, the steel bars of Test numbers 11 to 42 shown
in Table 6 were produced in the same production
conditions as in Example 1. The diameters of the steei
bars were 50 mm and 70 mrn. By using the produced steel
bars, the same tests as in Example 1 were conducted.
Then, the medium-cycle and high-cycle bending fatigue
strengths, surface fatigue strength, wear resistance, and
the amount of major cutting edge wear were determined,
respectively.
[Table 61
22
23
24
25
28
Obtained results are shown in Table 6. Referring to
27
28
29
Table 6, the chemical compositions of Test numbers 17, 19,
Comparative
hnmlive
Cornparalive
Comparative
. Comparative
21, 23, 31, 33, 41 and 42 were withln the range of the
Cornoardint
Comparative
Cornparalive
chemical composition of the present invention, and fnl
1
J
J
'K
.K
satisfied Formula (2) . Further, all of the ratios of
'L
'L
.M
average ferrite grain size of these Test numbers were not
4
7
5
7
5
2
6
2
FtPtE
F+P+B
F+P+E
F+P*E
F+P+B
F+B
F+E
FIE
'2 8
16
'2 4
17
'2 5
17
'2.7
17
11114
126
1114
126
11114
11112
11110
122
H I 3
122
115
124
11113
11 14
11113
120
125
140
135
135
130
1115
#I10
125
75
W
- 65
65
70
80
80
80
11105
1105
70
75
75
80
80
1110
more than 2.0. As a result, the medium-cycle and highcycle
bending fatigue strengths of these Test numbers
were not less than 115%, and the surface fatigue
strengths thereof were not less than 120%. Furthermore,
the amounts of wear were not more than 80%. Furthermore,
the amounts of major cutting edge wear were not more than
80%.
[0096]
On the other hand, the Si content and the Cr content
of the chemical composition of Test number 11 (Steel D)
were less than the lower limits of the Si content and the
Cr content of the present invention. As a result, the
surface fatigue strength of Test number 11 was less than
120%, and the amount of wear was more than 80%. Test
number 12 used the same Steel D as with Test number 11.
As a result, the surface fatigue strength and the wear
resistance were low. Further, in Test number 12, the
heating time of the cast piece was too short (Production
condition 1) . For that reason, the ratio of average
ferrite grain size exceeded 2.0. As a result, the
medium-cycle and high-cycle bending fatigue strengths
were as low as less than 115%.
[0097]
Although the chemical composition of Test number 13
(Steel E) was within the range of the chemical
composition of the present invention, fnl was less than
the lower limlt of Formula (2). As a result, the hiyhcycle
bending fatigue strength was as low as less than
- 43 -
115%. Test number 14 used the same Steel E as with Test
number 13. For that reason, the high-cycle bending
fatigue strength was low. Further, in Test number 14,
water cooling was performed before finish rolling
(Production condition 3). For that reason, the ratio of
average ferrite grain size exceeded 2.0. As a result,
the medium-cycle and high-cycle bending fatigue strengths
were lower than those of Test number 13.
[0098 J
The Si content of the chemical composition of Test
number 15 (Steel F) exceeded the upper limit of the Si
content of the present invention. As a result, the
medium-cycle and high-cycle bending fatigue strengths
were as low as less than 115%. Further, the amount of
major cutting edge wear was more than 80%, indicating low
machinability.
[0099]
Test number 16 used the same Steel F as with Test
number 15. As a result, the bending fatigue strength and
the machinability were low. Further, in Test number 16,
the steel bar after finish rolling was water-cooled to
800°C (Production condition 4). As a result, the ratio
of average ferrite grain size exceeded 2.0. For that
reason, Test number 16 showed a lower bending fatigue
strength than that of Test number 15. Moreover, the
surface fatigue strength thereof was less than 120%, and
the amount of wear thereof was more than 80%.
[OlOO]
The chemical composition of Test number 18 (Steel G)
was within the range of the chemical composition of the
present invention, and fnl satisfied Formula (2).
However, the heating time of the cast piece was too short
(Production condition 1). For that reason, the ratio of
average ferrite grain size exceeded 2.0. As a result,
the medium-cycle bending fatigue strength was as low as
less than 115%. Further, the surface fatigue strength
was as low as less than 120%.
[OlOl]
The chemlcal composition of Test number 20 (Steel H)
was within the range of the chemical composition of the
present invention, and fnl satisfied Formula (2).
However, water cooling was performed before finish
rolling (Production condition 3). For that reason, the
ratio of average ferrite grain size exceeded 2.0. As a
result, the medium-cycle and high-cycle bending fatigue
strengths were as low as less than 115%.
[0102]
The chemical composition of Test number 22 (Steel I)
was within the range of the chemical composition of the
present invention, and fnl satisfied Formula (2).
However, the steel bar after finish rolling was water
cooled to 800°C (Production condition 4). For that
reason, the ratio of average ferrite grain size exceeded
2.0. As a result, the medium-cycle and high-cycle
bending fatigue strengths were as low as less than 115%.
[0103 j
The chemical composition of Test number 24 (Steel J)
was within the range of the chemical composition of the
present invention, and fnl satisfied Formula (2).
However, the heating time of the cast piece and the
heating time of the billet were too short (Production
condition 5). For that reason, the ratio of average
ferrite grain size exceeded 2.0. As a result, the
medium-cycle bending fatigue strength was as low as less
than 115%.
[0104]
The Cr content of the chemical composition of Test
number 25 (Steel K) exceeded the upper limit of the Cr
content of the present invention. For that reason, the
amount of major cutting edge wear was more than 80%,
indicating low machinability. It was inferred that an
excessive Cr content caused excessive production of
bainite in steel.
[ 01051
Test number 26 used the same Steel K as with Test
number 25. As a result, the machinability thereof was
low. Further, in Test number 26, the heating time of the
cast piece and the heating time of the billet were too
short (Production condition 5). For that reason, the
ratio of average ferrite grain size exceeded 2.0. As a
result, the medium-cycle and high-cycle bending fatigue
strengths were as low as less than 115%.
[0106]
The Cr content of the chemical composition of Test
number 27 (Steel L) was less than the lower limit of the
Cr content of the present invention. For that reason,
the medium-cycle and high-cycle bending fatigue strengths
were as low as less than 115%. Further, the surface
fatigue strength was as low as less than 120%.
[0107]
Test number 28 used the same Steel L as with Test
number 27. For that reason, the bending fatigue strength
was low. Further, in Test number 28, the heating
temperature of the billet was too high, and the finishing
temperature was also too high (Production condition 6).
For that reason, the ratio of average ferrite grain size
exceeded 2.0. As a result, the medium-cycle and highcycle
bending fatigue strengths were as low as less than
115%. Further, the surface fatigue strength was as low
as less than 120%.
[0108]
The Mo content of the chemical composition of Test
number 29 (Steel M) exceeded the upper limit of the Mo
content of the present invention. As a result, the
amount of major cutting edge wear of Test number 29
exceeded 80%, indicating low machinability. It was
inferred that an excessive Mo content caused excessive
production of bainite in steel.
[0109:
Test number 30 used the same Stee: K as with Test
number 29. As a result, the machlnability was low.
- 47 -
Further, in Test number 30, the heating temperature of
the cast piece was too low (Production condition 8). For
that reason, the ratio of average ferrite grain size
exceeded 2.0. As a result, the medium-cycle and highcycle
bending fatigue strengths were as low as less than
115%.
[0110]
The chemical composition of Test number 32 (Steel N)
was within the range of the chemical composition of the
present invention, and fnl satisfied Formula (2).
However, the heating temperature and finishing
temperature of the billet were too high (Production
condition 6). For that reason, the ratio of average
ferrite grain size exceeded 2.0. As a result, the
medium-cycle bending fatigue strength was as low as less
than 115%.
[Olll]
The chemical composition of Test number 34 (Steel 0 )
was within the range of the chemical composition of the
present invention, and fnl satisfied Formula (2).
However, the heating temperature of the cast piece was
too low (Production condition 8). For that reason, the
ratio of average ferrite grain size exceeded 2.0. As a
result, the medium-cycle bending fatigue strength was as
low as less than 115%.
10112,
The Mri content and the A1 content of the chemical
composition of Test number 35 (Steel P) were less than
- 48 -
the lower limits of the Mn content and the A1 content of
the present invention. For that reason, the medium-cycle
bending fatigue strength was as low as less than 115%.
Further, the surface fatigue strength was as low as less
than 120%.
[0113]
Test number 36 used the same Steel P as with Test
number 35. As a result, the medium-cycle bending fatigue
strength and the surface fatigue strength were low.
Further, in Test number 35, the steel bar after finish
rolling was water-cooled to 800°C (Production condition
4). For that reason, the ratio of average ferrite grain
size exceeded 2.0. As a result, the high-cycle bending
fatigue strength was as low as less than 115%. Further,
the medium-cycle bending fatigue strength was lower than
that of Test number 35.
[0114]
The Mn content and the A1 content of the chemical
composition of Test number 37 (Steel Q) exceeded the
upper limits of the Mn content and the A1 content of the
present invention. For that reason, the high-cycle
bending fatigue strength was as low as less than 115%.
Further, the amount of major cutting edge wear exceeded
80%, indicating low machinability.
[0115]
Test number 38 used the same Steel Q as with Test
number 37. As a result, the high-cycle bending fatigue
strength was low and also the machinability was low.
- 49 -
Further, in Test number 38, the heating temperature and
the finishing temperature of the billet were too high.
For that reason, the ratio of average ferrite grain sizs
exceeded 2.0. As a result, the medium-cycle bending
fatigue strength was as low as less than 115%. Further,
the high-cycle bending fatigue strength was lower than
that of Test number 37.
[0116]
Although the chemical composition of Test number 39
(Steel R) was within the range of the chemical
composition of the present invention, fnl exceeded the
upper limit of Formula (2). As a result, the
machinability of the steel of Test number 39 was low.
Test number 40 used the same Steel R as with Test number
39. For that reason, the machinability of the steel of
Test number 40 was low. Further, in Test number 40,
water cooling was performed before rolling (Production
condition 3). For that reason, the ratio of average
ferrite grain size exceeded 2.0. As a result, the
medium-cycle and high-cycle bending fatigue strengths
were lower than those of Test number 39.
[0117]
Although the embodiments of the present invention
have been described so far, the above described
embodiments are merely exampies to carry out the present
invention. Therefore, the present invention is not
limited to the above described embodiments, and can be
carrled out by appropriately modifying the above
- 50 -
described embodiments within a range not departing from
the spirit of the present invention.
We claim:
[Claim 11
A rolled steel bar or wire rod for hot forging,
comprising:
a chemical composition comprising, by mass%,
C: 0.1 to 0.25%,
Si: 0.30 to 0.60%,
Mn: 0.50 to 1.0%,
S: 0.003 to 0.05%,
Cr: 1.50 to 2.00%,
Mo: not more than 0.10% (~ncluding O % ) ,
Al: 0.025 to 0.05%,
N: 0.010 to 0.025%, and the balance being Fe and
impurities, wherein
the impurities contain P: not more than 0.025%,
Ti: not more than 0.003%, and 0 (oxygen): not more than
0.002%, respectively, and wherein
fnl defined by Formula (1) is 1.60 to 2.10; and
a structure consisting of a ferrite-pearlite
structure, a ferrite-pearlite-bainite structure, or a
ferrite-bainite structure, wherein
a maximum value/a minimum value of average ferrite
graln size, which is obtained by making a measurement in
15 visual fields each having an area of 62500 p2 in a
cross section, is not more than 2.0:
fnl = C r t 2 x Mo (1)
where each symbol of elements in Formula (1) is
substituted by a content (mass%) of a corresponding
element.
[Claim 21
The rolled steel bar or wire rod for hot forging
according to claim 1, wherein
fnl is not less than 1.80.
[Claim 31
The rolled steel bar or wire rod for hot forging
according to claim 1 or 2, further comprising, by mass%,
Nb: not more than 0.08% in place of a part of Fe.