ORIGINAL
[Document ~ a m eD]e scription
[Title of Invention] NON-POST-HEAT TREATED STEEL AND NON-POSTHEAT
TREATED STEEL MEMBER
[Technical Field]
[00011
The present invention relates to non-post-heat treated steel and a nonpost-
heat treated steel member produced by using this steel. More
specifically, the present invention relates to non-post-heat treated steel
suitable for use as a starting material of a non-post-heat treated steel member
such as a connecting rod for an automobile engine which is fracture-split after
being formed in a predetermined shape through hot forging, and in which high
fatigue strength is required, and also relates to a non-post-heat treated steel
member such as the above connecting rod produced by using the above steel.
[Background Art]
[00021
A fracture splitting method has been used as a producing method of
connecting rods for automobile engines.
[00031
A connecting rod is split into two pieces at its large end portion
connected to a crackshaft, and in general, one of these pieces including a rod
portion connected to a piston is referred to as a "rod body", and the other
including a semi-circular portion at the large end portion is referred to as a
"cap" or the like.
[00041
The fracture splitting method is also called as a "cracking method" that
hot-forges a material into a shape with the rod body and the cap integrated,
* that is, the same shape as that when assembled to the crankshaft, and
thereafter, the hot-forged material is split into two portions (parts) of the rod
body and the cap. Splitting into two parts is carried out as if they are
fractured by applying impact load; thus this is referred to as the "fracture
splitting".
[00051
The fracture splitting method eliminates hot-forging the rod body and
the cap separately, and allows tight fitting between fine recesses and
projections existing on brittle fracture surfaces which are formed at the time of
splitting; therefore, it is possible to eliminate "positioning pins" embedded for
preventing positioning deviation between the rod body and the cap.
Specifically, the fracture splitting method can significantly decrease producing
processes, thereby greatly reducing production cost.
[0006]
In order to apply the fracture splitting method, it is essential to have a
property of being split in a brittle manner by applying impact load. In
general, a cutout is so introduced as to generate stress concentration at a
portion which is desired to be fractured, and thus deformation of a product is
concentrated in the vicinity of the fracture portion; and if a degree of plastic
deformation is great until the fracture occurs, both pieces of the product
cannot be smoothly fitted to each other at their fracture surfaces after being
split. If a number of voids unique to ductile fracture occur on the fracture
surfaces, this hinders smooth fitting, and thus it is preferable to attain brittle
fractures like "cleavage fracture", which helps to attain smooth fracture
surfaces. Specifically, it is preferable to split the product with almost no
energy of impact stress absorbed.
[00071
@ High fatigue strength is also required in connecting rods, and thus nonpost-
heat treated steel that has high strength as it is hot forged is desired.
[00081
Various studies have been conducted on non-post-heat treated steel
excellent in fracture splitting performance with low toughness and high
strength for years. Particularly, investigation has been conducted on highstrength
low-toughness non-post-heat treated steel having a duplex
microstructure of ferrite and perlite (referred to as "ferrite + perlite
microstructure", hereinafter) that improves a disadvantage of non-post-heat
treated steel having a perlite microstructure containing carbon of
approximately 0.7% in mass%, as described in Patent Document 1, which has
been practiced as the steel for a cracking connecting rod in Europe for the first
time. This is because perlite is harder than ferrite, and inherently has a low
toughness property, so that perlite is preferable for fracture-splitting, but
endurance ratio (ratio between tensile strength and fatigue limit) is so small
that it is hard to attain high fatigue strength, and perlite is also poor in drill
workability for drilling bolt holes due to its high hardness.
[00091
V-based non-post-heat treated steel to which vanadium is added is
representative of non-post-heat treated steel including the ferrite + perlite
microstructure and having high strength. V-based non-post-heat treated
steel is widely applied to products for mechanical structures in which high
strength and high toughness are required, but low toughness suitable to
cracking connecting rods cannot be attained if V-based non-post-heat treated
steel is used as it is. To address this disadvantage, there have been suggested
and disclosed various methods of providing V-based non-post-heat treated steel
with a low toughness property.
Loo 101
e In an invention described in Patent Document 2, phosphorus that
segregates at grain boundaries to encourage brit'tleness is positively added,
and the phosphorus content is defined so as to enhance the fracture splitting
performance. In an invention described in Patent Document 3, the form and
the number of sulfide-based inclusions are controlled so as to enhance the
fracture splitting performance. Inventions described in Patent Documents 4
to 6 are directed to attaining non-post-heat treated steel having a highstrength
low-toughness property by adding titanium. Patent Document 4
describes that TiN inclusions each having a diameter of 5 pm or more are
dispersed, thereby enhancing a cracking property, and obtaining appropriate
rough fracture surfaces. Patent Document 5 describes that the fracture
splitting performance can be improved by controlling the form and the number
of sulfide-based inclusions, and controlling "amount of effective Ti" defined
based on the titanium content and the nitrogen content, that is, "amount of
remaining Ti obtained by subtracting TIN from amount of Ti in the steel" to be
0.003% or more in mass%. In the invention described in Patent Document 6,
non-post-heat treated steel having a low-toughness and a low-ductility along
with a free-cutting property is attained in such a manner that Ti is added, Zr
is compositely added along with Ti if necessary, and Ca of 0.0005 to 0.01% is
further added, and at this time, balance of amount of Ti, Zr, and S in the steel
is specified so as to generate sufficient amount of (Mn, Ca) S that is complex
sulfide of Mn and Ca even after Ti combines, or Ti and Zr combine with S to
generate sulfide.
[Citation List]
[Patent Document]
Loo 111
[Patent Document 11 US5135587A
@ [Patent Document 21 JP2004-277840A
[Patent Document 31 JP2000-73141A
[Patent Document 41 JP2004-277817A
[Patent Document 51 JP2009-1557244
[Patent Document 61 JP2005-240149A
[Non Patent Document]
[00121
[Non Patent Document 11 W. J. Liu et al.: Metall. Trans. A, 20A (1989) 1907
[Summary of Invention]
[Technical Problem]
[00131
As aforementioned, disclosed are various techniques that attain high
strength non-post-heat treated steel excellent in fracture splitting performance
on the basis of V-based non-post-heat treated steel.
[OO 141
The steel whose "P content is more than 0.070%" disclosed in Patent
Document 2 has difficulties to stably secure hot workability.
100 151
In the steel "that contains 100 to 4000 I mm2of sulfide-based inclusions
each having a width of 1 pm or more, and also having average aspect ratio of
10 or less" disclosed in Patent Document 3, it is difficult to industrially
produce the steel with good reproducibility, since a producing method of
realizing the above number and form of the sulfide-based inclusions is not
necessarily apparent.
LOO 161
The steel "in which there exist, in number density, 5 I mm2 or more of
TiN inclusions whose maximum diameter is 5 pm or more", disclosed in Patent
@ Document 4, has a problem that TiN inclusions have such high hardness that
machinability, especially drill workability is likely to be deteriorated.
[00171
In the steel "in which amount of effective Ti defined by f = [Ti] - [N] x 48
I 14 is 0.003% or more in mass%, and there exist 100 to 4000 I mm2 of sulfidebased
inclusions each having a width of 1 pm or more, and an average aspect
ratio of the sulfide is 15 or less" disclosed in Patent Document 5, there is a
problem that it is not taken into consideration that Ti combines with S to
generate Ti-based sulfide (TiS), so that the amount of effective Ti is
substantially reduced; therefore, an effect attained by Ti cannot always be
achieved.
[0018]
In the steel disclosed in Patent Document 6, it is taken into
consideration that Ti and Zr combine with S to generate TiS and Zr-based
sulfide (z~S)b, ut (Mn, Ca) S plays a chief role in improvement of machinability,
and it is essential to contain Ca. There has been a problem that Ca often
causes nozzle clogging in a casting process, which hinders smooth production
on the actual production basis.
[00191
In the light of the current economic situation and global situation, it is a
fact that significant increase in price of rare metals such as V and Ti occurs,
and it becomes difficult to stably acquire rare metals. Consequently, it has
been desired to realize low-toughness high-strength non-post-heat treated steel
to which the fracture splitting method is applicable even if the contents of the
above elements are reduced as much as possible, specifically, the upper limit of
the V content is set to be 0.150% or less, and the upper limit of the Ti content
is set to be 0.200%.
[00201
0 An object of the present invention, which has been made in order to
solve the problems according to the conventional art, is to provide non-postheat
treated steel suitable for use as a starting material of a non-post-heat
treated steel member such as a connecting rod for an automobile engine which
is fracture-split after being formed in a predetermined shape through hot
forging, and in which high fatigue strength is required, and also provide to a
non-post-heat treated steel member such as the above connecting rod produced
by using the above steel.
[Solution to Problem]
[00211
The present inventors have prepared Ti-V-based non-post-heat treated
steels having various compositions, and have carefully examined a relation
between the compositions, strength, toughness, and machinability.
[oossl
As a result, the findings (a) to (f) were obtained as follows.
[00231
(a) Ti effective to strengthening and low-toughening can be grasped by
focusing on amount of effective Ti. Note that Ti in the steel is consumed not
only in TIN, but also in TiS. Hence, the "amount of effective Ti" should be
defined not based on residual amount of Ti obtained by subtracting amount of
Ti consumed in generation of TiN from amount of Ti contained in the steel, as
specified in the above Patent Document 5, but based on residual amount of Ti
obtained by subtracting amount of Ti used in generation of TiN and TiS from
amount of Ti contained in the steel.
Loo241
(b) If the amount of effective Ti which is defined by the above described
(a) (simply referred to as "amount of effective Ti", hereinafter) has a negative
0 value, high-strength and low-toughness can be stably attained at high
reproducibility. To the contrary, if the amount of effective Ti has a value of 0
or more, strength and toughness sensitively react to slight change in condition
of the hot forging process, and thus they easily change, which makes it difficult
to stably attain high strength and low toughness at high reproducibility.
[00251
(c) The amount of effective Ti having a negative value means that no free
dissolved Ti exists as a chemical component. Considering that connecting
rods are produced through hot forging, for example, if starting material steel is
heated and maintained at a temperature of approximately 1423 to 1523 K
(1150 to 1250 "C) before hot forging, TiS particles are partially melted, so that
dissolved Ti exists in the austenite host phase; therefore amount of this
dissolved Ti can be calculated by using a formula of a solubility product of TiS.
[0026]
(d) If dissolved Ti, which is generated in the austenite host phase due to
TiS particles partially melted during the steel is heated and maintained for
hot forging (referred to as "during hot forging" for simplicity, hereinafter),
precipitates as composite carbide containing V during a cooling process
subsequent to hot forging, significantly great effects of increasing strength and
enhancing the fracture splitting performance are attained compared to the
case of precipitation of simple carbide including only a single metal element
like V or Ti.
[0027]
(e) The amount of effective Ti influences the amount of dissolved Ti
generated in the austenite host phase due to TiS particles partially melted
during hot forging as set forth in (dl. If the amount of effective Ti is
appropriately controlled, it is possible to reproducibly and stably attain a
property equivalent to that of actually-used high-strength low-toughness nonpost-
heat treated steel having the V content of 0.150% or more even if the
upper limit of the V content is set to be not more than 0.150%, and the upper
limit of the Ti content is set to be 0.200%.
[00281
(0 Machinability and fatigue strength of a steel material having a ferrite
+ perlite microstructure can be integrally grasped based on the hardness of the
steel material, and they have a correlation with carbon equivalent. A
conventional regression formula may be applicable to find the carbon
equivalent. Both high fatigue strength and excellent machinability can be
attained by controlling the carbon equivalent to have a value within an
appropriate range.
[00291
The present invention has been accomplished based on the above
findings, and includes non-post-heat treated steel set forth below (1) and (2),
and a non-post-heat treated steel member set forth below (3) produced by use
of the above non-post-heat treated steel.
[00301
(1) Non-post-heat treated steel having a chemical composition
comprising: in mass%, C: 0.27 to 0.40%, Si: 0.15 to 0.70%, Mn: 0.55 to 1.50%,
P: 0.010 to 0.070%, S: 0.05 to 0.15%, Cr: 0.10 to 0.60%, V: 0.030% or more and
less than 0.150%, Ti: more than 0.100%, not more than 0.200%, Al: 0.002 to
0.050%, and N: 0.002 to 0.020%; the balance being Fe and impurities, wherein
Et represented by Formula <1> is less than 0, and Ceq represented by
Formula <2> is more than 0.60 and less than 0.80:
Et = [Ti] - 3.4 [N] - 1.5 [S] <1>
Ceq = [Cl + ([Sil 1 10) + ([Mnl 1 5) + (5[Cr] 1 22) + (33[V] 1 20) - (5[Sl 1
7) * * * <2>
@ where a symbol of each element enclosed by [ 1 in Formulas and <2>
denotes a content in mass% of the element in the steel.
[003 11
(2) The non-post-heat treated steel set forth in (1) further containing, in
mass%, one or more of Cu: 0.40% or less, and Ni: 0.30% or less, in lieu of part
of Fe.
100321
(3) A non-post-heat treated steel member including the chemical
composition set forth in (1) or (21, wherein a Charpy impact value at a room
temperature is 1.0 to 7.0 JIcm2, and fatigue strength is 450 MPa or more.
[Advantageous Effects of Invention]
[00331
The non-post-heat treated steel of the present invention can be suitably
used as a starting material of a non-post-heat treated steel member such as a
connecting rod for an automobile engine which is fracture-split after being
formed in a predetermined shape through hot forging, and in which high
fatigue strength is required. The non-post-heat treated steel member of the
present invention is excellent in cracking property and fatigue resistance
property, and can be used as a connecting rod for an automobile engine or the
like.
[Brief Description of Drawings]
Lo0341
[Figure 11 Figure 1 is a drawing showing hyperbolas indicating respective
solubility products of TiS at 1523 K (1250 "C) and at 1423 K (1150 "C), and a
straight line corresponding to a stoichiometric ratio of TiS, as an example for
explaining concentrations of Ti and S dissolved in austenite during hot forging.
! @
I In the drawing, an abscissa [S] represents the S content in mass%, that is, the
I concentration of S (mass%), and an ordinate [Ti] represents the Ti content in
mass%, that is, the concentration of Ti (mass%).
[Figure 21 Figure 2 is a drawing showing a shape of a test specimen used in a
tensile test in Example. The unit of the dimension is mm.
[Figure 31 Figure 3 is a drawing showing a shape of a test specimen used in an
Ono type rotating bending fatigue test in Example. The unit of the dimension
is mm.
[Figure 41 Figure 4 is a drawing showing a shape of a test specimen used in an
impact test in Example. The unit of the dimension is mm.
[Description of Embodiment]
[00351
Detail description will be provided on requirements of the present
invention, hereinafter.
LO0361
(A) Chemical composition of steel:
A symbol "%" of content of each element denotes "mass %", hereinafter.
[00371
C: 0.27 to 0.40%
C is the most important element to decide strength of a steel material,
and the C content should be 0.27% or more in order to secure strength of a
mechanical product to be used in a non-post-heat treated state in which no
heat treatment is applied after being formed into a product member through
hot forging as in the present invention. On the other hand, the C content of
more than 0.40% causes increase in rate of a perlite microstructure, and
decrease in endurance ratio, which deteriorates the fatigue resistance property
@ and machinability. Accordingly, the C content is defined to be 0.27 to 0.40%.
It is preferable to set the C content to be 0.30% or more.
[0038]
Si: 0.15 to 0.70%
Si is an element that contributes to increase in strength as a solidsolution
strengthening element, and is effective for effectively carrying out
deoxidation of the steel. These effects can be attained with the Si content of
0.15% or more. The excessive Si content rather deteriorates hot workability
including a hot forging property as well as machinability, and the Si content of
more than 0.70% results in saturation of the above effects; therefore it is
preferable to set the upper limit thereof to be 0.70%. Accordingly, the Si
content is defined to be 0.15 to 0.70%. Note that it is preferable to set the Si
content to be 0.20% or more, and to be 0.60% or less.
[00391
Mn: 0.55 to 1.50%
Mn contributes to increase in strength as a solid-solution strengthening
element. Mn combines with S to generate MnS, and has an effect of
enhancing machinability. In order to attain these effects, the Mn content
should be 0.55% or more. The Mn content of more than 1.50% rather
saturates the effects, increases the cost, and causes excessive hardenability,
which may generate a bainitic microstructure that deteriorates the fracture
splitting performance and machinability. Accordingly, the Mn content is
defined to be 0.55 to 1.50%. It is preferable to set the Mn content to be 0.60%
or more, and 1.40% or less.
[00401
P: 0.010 to 0.070%
P is an element that contributes to increase in strength as a solidsolution
strengthening element, and is likely to segregate at the grain
0 boundaries, which deteriorates toughness of the steel. This property has a
preferable effect for the fracture splitting performance. In order to attain this
effect, the P content should be 0.010% or more. The excessive P content
deteriorates hot workability including the hot forging property, and the P
content of more than 0.070% causes significant deterioration of hot workability.
Accordingly, the P content is defined to be 0.010 to 0.070%. It is preferable to
set the P content to be 0.030% or more, and 0.060% or less.
[00411
S: 0.05 to 0.15%
S is an impurity usually contained in steel, and combines with Mn and
Ti to generate sulfide such as MnS and TiS, and has an effect to enhance
machinability; thus S is positively added. As described later, TiS determines
the amount of effective Ti, and becomes partially melted at a temperature for
hot machining, which determines the amount of dissolved Ti in the austenite
host phase; for this reason, enough S should be added to generate sufficient
amount of TiS. Accordingly, the S content of 0.05% or more is needed. The
excessive S content, particularly, the S content of more than 0.15% generates
segregation defect in cast pieces, and deteriorates hot workability including
the hot forging property. Accordingly, the S content is defined to be 0.05 to
0.15%. It is preferable to set the S content to be 0.07% or more, and 0.13% or
less.
Lo0421
Cr: 0.10 to 0.60%
Cr contributes to increase in strength as a solid-solution strengthening
element. This effect can be attained with the Cr content of 0.10% or more.
To the contrary, the Cr content of more than 0.60% rather generates a bainitic
microstructure that deteriorates the fracture splitting performance and
a machinability. Accordingly, the Cr content is defined to be 0.10 to 0.60%. It
is preferable to set the Cr content to be 0.15% or more, and 0.50% or less.
Lo0431
V: 0.030% or more and less than 0.150%
V precipitates in a ferrite matrix as carbide, and has an effect to
contribute to increase in strength as a precipitation strengthening element,
and to enhance the fracture splitting performance. Carbide containing both V
and Ti is generated by adding V along with Ti, thereby further enhancing the
effect to increase strength and enhance the fracture splitting performance.
The V content should be 0.030% or more to attain such an effect. Meanwhile,
V is a rare metal element, and extremely expensive among alloy elements to be
added in the steel, and currently, the price of V is significantly increased in the
market, and stable obtainment of V has become more difficult. Accordingly, it
is preferable to reduce the V content as much as possible, and thus in the
present invention, the upper limit of the V content is defined to be less than
0.150% that is approximately one half of the upper limit of the V content in
conventional common V-containing non-post-heat treated steel. It is
preferable to set the V content to be 0.050% or more.
Lo0441
Ti: more than 0.100%, 0.200% or less
Ti combines with nitrogen to generate TiN, and TiN functions as pinning
particles for suppressing coarsen of grains during hot working. Ti combines
with S to generate TiS, and TiS has an effect of enhancing machinability. TiS
becomes partially melted at a temperature for hot working, thereby generating
dissolved Ti and dissolved S in the austenite host phase. Amount of the
dissolved Ti and the dissolved S is dependent on the solubility product of TiS.
A formula for finding the solubility product of TiS will be described later. Ti
and S having greater amount than this solubility product at the above
i
1 @ temperature generates TiS particles. Since the solubility product becomes
I
greater as the temperature increases, if Ti and S existing as TiS particles at a
room temperature are maintained at a temperature for hot forging, the TiS
particles become partially melted from the surfaces of the TiS particles; and
consequently amount of Ti and S dissolved in the austenite host phase
becomes increased. Specifically, TiS particles have an effect of decomposing
itself at a temperature for hot forging and increasing amount of dissolved Ti.
The dissolved Ti precipitates during the cooling process subsequent to hot
forging, and if V exists at this time, the dissolved Ti is likely to precipitate as
carbide combined with V. Carbide containing both V and Ti has an effect to
further increase strength, and further enhance the fracture splitting
performance, compared to simple carbide including only V or Ti. In order to
attain such an effect, the Ti content should be more than 0.100%. Meanwhile,
Ti is a rare metal element as similar to V, is a relatively expensive element
among alloy elements to be added in the steel, and is likely subjected to
influence of the market, which makes it difficult to obtain Ti at stable cost.
Hence, it is preferable to reduce the Ti content as much as possible, and the
upper limit of the Ti content is defined to be 0.200% in the present invention.
It is preferable to set the Ti content to be 0.110% or more, and 0.190% or less.
[00451
Al: 0.002 to 0.050%
A1 is an element effective for deoxidation of the steel, and the Al content
should be 0.002% or more so as to attain this effect. The A1 content of more
than 0.050% excessively generates hard aluminum particles, which
significantly deteriorates machinability. Accordingly, the Al content is
defined to be 0.002 to 0.050%. It is preferable to set the A1 content to be
0.004% or more, and 0.040% or less.
[00461
N: 0.002 to 0.020%
N is an effective element that combines with Ti to generate TIN serving
as pinning particles for suppressing coarsen of grains during hot working.
The N content should be 0.002% or more to attain this effect. Because TiN is
hard, the N content of more than 0.020% excessively generates TiN particles,
which significantly deteriorates machinability, and Ti contained in the steel is
consumed in TiN before generating TiS because of generation of TIN. In the
present invention, it is necessary to partially melt TiS during hot forging so as
to generate dissolved Ti; therefore, generation of TiS is essential, and it is
unfavorable to generate great amount of TiN prior to generation of TiS.
Accordingly, the N content is defined to be 0.002 to 0.020%. It is preferable to
set the N content to be 0.015% or less.
Coo471
One of the non-post-heat treated steels of the present invention has a
chemical composition comprising the aforementioned elements from C to N
with the balance being Fe and impurities, and the chemical composition
satisfies the conditions of Et and Ceq described later.
[00481
The term "Impurities" denotes those impurities which come from ores
and scraps as row materials, manufacturing environments, and so on during
industrially producing the steel.
[00491
The other of the non-post-heat treated steels of the present invention
contains one or more of Cu and Ni in lieu of part of Fe, and has a chemical
composition that satisfies the conditions of Et and Ceq.
[00501
I
!
I 0 Hereinafter, description will be provided on the operational advantages
of Cu and Ni that are optional elements, and on the reason for limiting the
contents thereof.
[00511
Cu: 0.40% or less
Cu contributes to increase in strength as a solid-solution strengthening
element, and Cu may be added so as to attain this effect. The Cu content of
more than 0.40% likely causes hot crack, or likely generates a bainitic
microstructure, which may deteriorate the fracture splitting performance and
machinability. Accordingly, the amount of Cu is defined to be 0.40% or less if
contained. It is preferable to set the amount of Cu to be 0.30% or less if
contained.
[00521
On the other hand, in order to stably attain the solid-solution
strengthening effect resulted from addition of Cu, it is preferable to set the
amount of Cu to be 0.05% or more, and more preferably 0.10% or more, if
contained.
[00531
Ni: 0.30% or less
Ni contributes to increase in strength as a solid-solution strengthening
element, and has an effect of suppressing hot crack caused by containing Cu,
and thus Ni may be added so as to attain this effect. The Ni content of more
than 0.30%, however, only results in increase in cost. Accordingly, the
amount of Ni is defined to be 0.30% or less if contained. It is preferable to set
the amount of Ni to be 0.20% or less if contained.
[00541
Meanwhile, in order to stably attain the solid-solution strengthening
effect caused by Ni, it is preferable to set the Ni content to be 0.05% or more,
@ and more preferably 0.10% or more if contained. It is preferable to set the
amount of Ni to be not less than one half of the amount of Cu if Ni is contained
along with Cu in order to suppress hot crack caused by containing Cu.
Cu and Ni may be contained in combination, or either of them may be
contained on the standalone basis, in the above range. If Cu and Ni are
contained in combination, the total amount of Cu and Ni may be 0.70%,
obtained as a sum of the respective upper limit of Cu and Ni, but it is
preferable to set this total amount to be 0.50% or less, more preferably 0.35%
or less.
100561
Et: less than 0
In the non-post-heat treated steel of the present invention, Et
represented by the following Formula <1> should be less than 0. As described
above, a symbol of each element enclosed by [ 1 in Formula <1> denotes a
content in mass% of the element in the steel.
Et = [Ti] - 3.4 [N] - 1.5 [S] <1>
[00571
The above will be described, hereinafter.
[00581
The gist of the present invention is to obtain high-strength non-postheat
treated steel having high fatigue strength and excellent in fracture
splitting performance in which V containing non-post-heat treated steel is used
as a base material, Ti is contained in this base material, the total content of V
and Ti is set to be approximately 0.3% at most, and the amount of effective Ti
is controlled, wherein the steel is heated and maintained during hot forging so
as to supply dissolved Ti existing in the austenite host phase through partial
I
I
melting of TiS particles. Hereinafter, description will be provided on Et of
Formula <1> that represents the amount of effective Ti.
[00591
As aforementioned, the "amount of effective Ti " in the present invention
denotes not the residual amount of Ti obtained by subtracting the amount of Ti
consumed in generation of TiN from the amount of Ti contained in steel, as
specified in Patent Document 5, but the residual amount of Ti obtained by
subtracting the amount of Ti used for generation of TiN and TiS from the
amount of Ti contained in the steel. This is because Ti contained in the steel
is consumed not only by TiN but also by TiS sulfide.
[0060]
Taking atomic weights of N and S into consideration, the amount of Ti
used for generation of TiN and TiS is 3.4 (= 48 / 14) times as much as the N
content, and 1.5 (= 48 / 32) times as much as the S content, respectively.
Hence, the amount of effective Ti (Et) is defined by the above Formula .
[0061]
Et of less than 0 (referred to as "Et < O", hereinafter) denotes that the
amount of effective Ti has a negative value, that is, no free dissolved Ti exists
as a chemical component, in other words, this means that entire Ti in the steel
combines with N and S to generate TIN and TiS. In order to stably attain
high-strength and low-toughness at high reproducibility, it is essential to
satisfy this condition.
Lo0621
In order to generate carbide containing both V and Ti that contribute to
strengthening and low-toughening in the cooling process subsequent to hot
forging, it is necessary that dissolved Ti exists in the austenite host phase. If
Et < 0 is satisfied, it seems that no dissolved Ti exists, but if starting material
steel is heated and maintained at a temperature of approximately 1423 to
1523 K (1150 to 1250 'C) before hot forging, for example, TiS particles are
! partially melted at this time, which allows dissolved Ti to exist in the
austenite host phase, and this Ti contributes to generation of carbide
containing both V and Ti. The solubility product of TiN is extremely smaller
than that of TiS, and thus melting of TiN particles is small enough to be
ignored with respect to melting of TiS particles.
LO0631
If Et < 0 is satisfied, entire Ti contained in the steel is fixed as TiN and
TiS in a state at a room temperature before heating in the above hot forging.
Heating and maintaining the steel during hot forging makes it easy to
appropriately adjust the amount of dissolved Ti to be generated in the
austenite host phase through partial melting of TiS particles, thereby stably
attaining high-strength and low-toughness at high reproducibility. To the
contrary, if Et 2 0 is satisfied, the amount of dissolved Ti in the austenite host
phase is so great that such a problem may be caused that bainite is likely to be
generated, resulting in hindrance of low toughening. Because of the great
amount of dissolved Ti, Ti generates simple carbide (Tic) by itself in addition
to melting into VC. Contrary to VC, Tic easily precipitates in austenite as
well, so that Tic is subjected to strong influence of machining-induced
precipitation; specifically, Tic is subjected to influences of the temperature
range and the working ratio of hot forging, which changes the size and the
distribution condition of precipitating Tic particles, resulting in great
variation in strength of connecting rods. Accordingly, Et should be defined to
be Et < 0.
Lo0641
Ceq: more than 0.60 and less than 0.80
In the non-post-heat treated steel of the present invention, Ceq
represented by the following Formula <2> should be more than 0.60 and less
than 0.80 (referred to as "0.60 < Ceq < 0.8OU, hereinafter). As described above,
1 a symbol of each element enclosed by [ 1 in Formula <2> denotes a content in I
1 mass% of the element in the steel.
I
I
Ceq = [C] + ([Si] 1 10) + ([Mn] 1 5) + (5[CrI I 22) + 33[V] 1 20) - (5[Sl I 7) ... <2>
Lo0651
The above will be described, hereinafter.
[00661
Ceq defined by Formula <2> is an empirical formula representing
hardness of the non-post-heat treated steel by using a regression equation of
contents of major additive elements, and it is generally referred to as "carbon
equivalent1'.
[00671
If a value of Ceq is greater, hardness of the starting material becomes
greater, and thus machinability becomes deteriorated. On the other hand, if
a value of Ceq is smaller, hardness of the starting material becomes smaller,
and thus fatigue strength becomes deteriorated. Such non-post-heat treated
steel that contains both Ti and V of the above amounts has a ferrite matrix
reinforced by carbide containing V and Ti; therefore, sufficient fatigue strength
can be attained even though Ceq is less than 0.80, and Ceq should be less than
0.80 in order to secure machinability. Meanwhile, Ceq of 0.60 or less
significantly deteriorates fatigue strength. Accordingly, Ceq is defined to be
0.60 < Ceq < 0.80. It is preferable to set Ceq to be 0.65 or more, more
preferably 0.67 or more.
[00681
In the production process of the non-post-heat treated steel of the
present invention, the steel is deoxidized with A1 so as to stabilize
deoxidization, and composite carbide is generated with Ti along with V. In
the production process of the non-post-heat treated steel of the present
invention, in order to prevent deterioration of the yield rate of Ti, Ti is added
after the steel is sufficiently deoxidized with Al for example, that is, the steel is
i I
I melted while adding Al and Ti in the order of A1 and Ti.
[0069]
(B) Non-post-heat Treated Steel Member
The non-post-heat treated steel member of the present invention has the
chemical composition set forth in (A), its Charpy impact value at a room
temperature is 1.0 to 7.0 J/cm2, and its fatigue strength is 450 MPa or more.
This Charpy impact value denotes a value obtained by using V notch test
specimens set forth in JIS Z 2242 (2005), and this value is referred to as "vERT",
hereinafter. The fatigue strength denotes a maximum stress that causes no
fracture in 107 cycles of repetitively applying stress in the Ono type rotating
bending fatigue test using smooth specimens, and the fatigue strength is
referred to as "ow", hereinafter.
[00701
VERTre presents amount of energy per lcm2 that is consumed as the work
of plastic deformation at the time of fracturing, and thus this may be used as
an index of evaluating the fracture splitting performance. In the member
having the chemical composition set forth in the above (A), if VERT becomes
more than 7.0 J/cm2, ductile fracture is generated at the time of fracturesplitting,
which may hinder tight fitting. Because smaller VERT more likely
causes brittle fracture, it is preferable that VERTis as small as possible in the
light of the fracture splitting performance and fitting after fracture, and VERT
of 1.0 J/cm2 or more causes no inconvenience during transportation and the
like.
[007 11
If ow is 450 MPa or more, it is possible to secure sufficient fatigue
strength required in connecting rods for automobile engines. It is preferable
!
i
i I @ that ow is as great as possible, but the upper limit thereof is approximately
1
550 MPa in the case of the member having the chemical composition set forth
in the above (A).
Loo721
The non-post-heat treated steel member of the present invention, whose
VERT is 1.0 to 7.0 JIcm2 and ow is 450 MPa or more, may be produced by
forming the steel having the chemical composition set forth in the above (A)
through hot forging after the steel is maintained at a temperature T that
allows the value y represented by using the following Formulas <3> and <4> to
be more than 0.001, and not more than 0.020.
y = { ~+ t(E t2 + 6 x a (~))0.5)2/ <3>
a ( ~=) 1 0 {-(176401 T) + 8.201 .. . <4>
where a(T) represents a function of the temperature T, and this
temperature is indicated as an absolution temperature (unit: K).
[00731
The present invention employs the following features: that the chemical
composition of the steel is adjusted to be "Et < On, entire Ti in the steel is fmed
as TiN and TiS in a state at a room temperature before hot forging, TiS
particles are allowed to be partially melted so as to elute dissolved Ti in the
austenite host phase by heating and maintaining the steel during hot forging
for forming the steel into a predetermined shape, and this dissolved Ti
contributes to generation of carbide containing both V and Ti during the
cooling process subsequent to the hot forging.
[00741
The above Formulas <3> and <4> define the amount of dissolved Ti that
exists in the austenite host phase due to TiS particles partially melted by
heating and maintaining the steel during the above hot forging, and "y"
represents the amount of the above dissolved Ti. Note that a unit for the
1 @ amount of dissolved Ti is mass%, and thus this is equivalent to Ti I
concentration in mass%.
Lo0751
How much TiS particles become melted in austenite, that is, how much
amount of the dissolved Ti has an equilibrium relationship with the TiS
particles may be calculated based on the solubility product. One of
representative formulas regarding the solubility product of TiS is the following
Formula <5> set forth in non-Patent Document 1.
log [Ti] [S] = -(17640 / T) + 8.20 <5>
Herein, [Ti] and [S] denote respective contents in the steel in mass% of
Ti and S both of which are involved in generation of TiS.
Lo0761
The above Formula <5> may be converted into the following Formula
<6>.
[Ti] = 10 1-(17640 / T) + 8.201 / [s] . - a <6>
[00771
Assuming [Ti] [S] = a(T), this may be substituted into Formula <5> so as
to obtain Formula <4>, and this may also be converted into the following
Formula <7>.
[Ti] = a(T) l [S] -. <7>
[0078]
If the temperature T of a(T) is decided, a(T) becomes a constant, and
thus a relation between [Ti] and [S] gives a hyperbola.
[00791
Detailed description will be provided with reference to Figure 1 showing
the solubility product of TiS at 1523 K (1250 oC) and at 1423 K (1150 oC),
hereinafter.
[00801
As an example, two hyperbolas in the first quadrant are obtained based
on Formula <7> for the cases of T = 1523 K and 1423 K as shown in Figure 1,
where [S] is defined as the x axis, and [Ti] - 3.4 [N] is defined as the y axis.
The reason for defining not [Ti] but [Ti] - 3.4 [N] as the y axis is based on that
it is considered that TiN is generated before TiS is generated, and thus the
amount of Ti in mass% used for generation of TiS is determined by subtracting
the amount of Ti consumed in TiN from the amount of Ti existing in the steel.
Such treatment is required because [Ti] and [S] represented in Formula <5>
indicate the contents of Ti and S in the steel, both of which are involved in
generation of TiS. Although, as for [S], MnS may be deemed as particles in
which S is consumed, it is appropriate to consider that TiS is generated prior
to generation of MnS; therefore, it is supposed herein that entire S contained
in the steel primarily becomes involved in generation of TiS.
[00811
The stoichiometric ratio of TiS is 1 : 1, and if this is converted into a
mass ratio, it becomes 1.5 : 1; thus Figure 1 shows a straight line passing
through the origin with inclination of 1.5. If Ti and S generate TiS, or if Ti
and S are melted from TiS, the ratio of 1.5 : 1 is maintained, and thus it may
be considered that it moves along this straight line of Figure 1.
[00821
If the amount of Ti (Ti content in mass% involved in generation of TiS)
and the amount of S (S content in mass% involved in generation of TiS) in the
steel are represented by the point Ao in Figure 1, the point Ao lies above the
two hyperbolas, which means that the product of the amount of Ti and the
amount of S in the steel is greater than the solubility product at any
temperature, and thus TiS is generated at both 1523 K and 1423 K. A region
equivalent to a portion below the hyperbolas indicates a range of the amount of
Ti and the amount of S that can be dissolved in austenite, and if a straight line
1 @
I passing through the point Ao with inclination of 1.5 is drawn, a point at which
I
this straight line intersects the hyperbola representing the solubility product
at 1523 K is defined as AI, and a point at which this straight line intersects the
hyperbola representing the solubility product at 1423 K is defined as A2,
respectively, the respective coordinates of these points A1 and A2 indicate
respective amounts of Ti and S that can be dissolved in austenite at the
respective temperatures. Putting the focus on the dissolved Ti now, it is
necessary to find values of the y coordinates of the point A1 and the point A2.
A method of finding an intersection point between the straight line passing
through the point Ao with inclination of 1.5 and the hyperbola in the first
quadrant that represents the solubility product at 1523 K or 1423 K is nothing
but an arithmetical procedure to find a coordinate of the intersection point
assuming that equations for this straight line and the hyperbola are equal. If
the equation is solved, two values are obtained for the value y because there is
f before the radical sign, but the value y in the first quadrant is physically
significant value, and thus a solution having a sign of plus before the radical
sign is employed.
Lo0831
Taking [Ti] - 3.4 [Nl - 1.5 [S] = Et into account, it is found that the value
y that is the y coordinate of the intersection point can be given by Formula <3>.
Lo0841
If the steel is maintained at the temperature T that allows the above
value y, that is, the amount of dissolved Ti in mass% in the austenite host
phase to be more than 0.001, and not more than 0.020, and then is formed
through hot forging, it is possible to provide the non-post-heat treated steel
member including the chemical composition set forth in (A) with the properties
that VERT is 1.0 to 7.0 JIcm2, and ow is 450 MPa or more at high stable
reproducibility.
If the value y represented by using Formulas <3> and <4> is not more
than 0.001 (i.e. 0.001 or less), the amount of dissolved Ti in the austenite host
phase may be too small to satisfy both properties that VERTis 1.0 to 7.0 JIcm2,
and ow is 450MPa or more at the same time. On the other hand, if the value
y is more than 0.020, the amount of dissolved Ti is so great that such a
problem may be caused that the bainitic microstructure is encouraged to grow,
or simple Tic carbide precipitates, resulting in great variation of strength.
[OOB~I
The steel, which is a material to be treated, is maintained at the
temperature T during hot forging, and the temperature T denotes a
temperature at a central portion of the material to be treated.
[00871
A time period in which the temperature of the material to be treated
becomes uniform is enough for a time period for maintaining the material at
the temperature T, and this time period may be variously changed depending
on the heating method, the heat capacity of heating facility and the like; and it
is preferable to heat and maintain a billet for one minute or more when the
billet is heated and maintained by high frequency induction heating, and it is
preferable to heat and maintain a billet for 15 minutes or more when the billet
is heated and maintained in a furnace, for example. It is preferable to set the
above temperature T to be within a range of 1423 to 1523 K (1150 to 1250 "C).
[00881
The steel is preferably subjected to allowing cooling in the atmosphere
after hot forging, for example. Rapid cooling using water or oil after hot
forging is unfavorable in the present invention. Wing cooling (forced air
cooling) for cooling the hot forged member by sending air to the member with
fans may be employed, and in this case, it is necessary to prevent a cooling
1I @
I speed from being excessively high so as not to generate great amount of the
bainitic microstructure. Bainitic microstructure in an area ratio of up to 10%
may be contained in the ferrite + perlite microstructure because moderate
generation of the bainitic microstructure never greatly affects the fracture
splitting performance. The critical cooling speed for generation of the bainitic
microstructure is varied depending on the selection of the chemical
composition of the steel, and as far as it is within the range of the present
invention, the cooling speed after hot forging may be adjusted to be within a
range of approximately 10 to 100 OCImin, thereby substantially attaining the
ferrite + perlite microstructure where mixture of the bainitic microstructure
does not matter.
[00891
More detailed description will be provided on the present invention
based on the following Example, hereinafter.
[Example]
[00901
Each Steel 1 to 20 having the chemical composition shown in Table 1
was melted by 50 kg in a vacuum furnace, and produced into an ingot. In
Table 1, "Et" and "Ceq" respectively represented by Formulas and <2> are
also described.
[00911
Each Steel 1 to 11 in Table 1 has a chemical composition within the
range specified by the present invention. To the contrary, each Steel 12 to 20
in Table 1 has a chemical composition deviating from the range specified by
the present invention, and Steel 20 is equivalent to the steel for a cracking
connecting rod set forth in Patent Document 1, which has already been
practiced in Europe.
[00921
[Table 11
m - a - - m - b w a b m m m N b m b b m
g w m a N m m a b O b b - m a - - b m m m
O w w w b b w b b b b w m m b w w a a m a
0 0 0 ~ 0 0 0 0 0 0 0 0 0 0 0 0 0 ~ 0 0
C C C * * * C
b b b m m b a - W c 9 b a a b m - b - m m
N W N C 3 - - O C O b m W O b m W b W b m a E g g g g g g g 8 g g g g z g s g g g g z
0 0 + ~ 0 + 0 ~ ~ 0 0 + ~ ~ 0 0 0 0 ~ 0
C C * C *
-m
0.-
X
6
' L=
3 -g
a.3.
LL ..
- mm
B
E V
.-
t,,
0 a
.0-
E
2
3 -
N o m . - c 9 a o m m a O a m m m m m m m o
m w m w w m a - w m w w m m m w a a m r ~
0 0 0 0 0 0 - - - 0 0 0 0 0 0 0 0 0 0 0
u ~ ~ o o o ~ o o o o o ~ o o o o o ~ ~
0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0
b ~ m b b m w w m w m b m ~ b w m w w w
0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 ~
9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9
0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0
0 0 m m N m 0 - b m 0 0 a N 0 0 0 0 0
g F -- 0yry9yCTO WT Wy Tb bTmTWT0-N-Cy0y~ OY-Yb mT bqNi
o o o o o o o o o o o o o o o o o o p
C
0 0 N b m 0 0 a m W K 3 0 0 m m 0 0 0 0 0
b - a W m C 0 - b b W O W N b W W O a N e ?
~ > ~ 9 9 ° 9 T o o o T 9 9 T o o T T T o
o o o o o o o o o o o o p o o o o p o o
m - o m N - o N m * m m m m m a o o m ,
o L q q q * y q q q T T q q q q q q q q q T
~ 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0
O b W I O ~ b O O m O O I O N ~ b O - O O m
- O O - X - - N N ~ C Q - O C O ~ ~ - ~ Y N W
0 V ) " T T ' T ' T 9 T 7 7 9 9 9 T 9 - - 9
00000000000000p0p000
N m b o J N m m w N - m m m b N m m m w m
U 7 b W m m m l O W N W b l O W m m W m m m O
& 9 q q * q 9 9 * 9 9 9 9 9 9 q q 9 9 ~
0000000000000000000p
.0- C
C
.9c- - c
a.3
ln E
a,
5
nr.
'El
a.3
E
8 a
ln
h C !zg
a m0
=a 4 n 0
0 q5.a
s= 0
+ a.3 s5 N z% ha,
=E'
+ e
n a . 3 m C A-
-
3 V)
b w m - m m a ~ m ~ w ~ w ~ ~ o a a m m z,,c,,,\,,,l,,,~,,,,~,,,~,,za~g m ~ r n ~ m ~ c u ~ ~ ~ O - - m m a m o m o - a N W o N o o ~ o o , O l n
, O * m m m ~ ~ * ~ ~ s * ~ * * * s s ~ s ~
O O O O O O O O O O O O O O O ~ O O ~ ~ ~ ~ ~
- N O b W W b C O m O ~ N c 9 b m w b a m O
--.-.-Y,--.---N
-. --.oC
1 s3
sm
+ . B
C -a
- 2
a
" 0- e!
G G C
1 Each of the aforementioned ingots was once hot-forged into a square bar
55 mm on each side, and thereafter cooled down to a room temperature.
Subsequently, each square bar 55 mm on each side was heated at 1523 K
(1250 "C), and then was maintained for one hour, and thereafter was hotforged
into a round bar having a diameter of 35 mm while keeping the finish
temperature not to be below 1273 K (1000 "C). After the hot forging process
is completed, each bar was subjected to allowing cooling in the atmosphere.
[00941
In Steel 1, Steel 5 and Steel 7, different from the above procedure, each
square bar 55 mm on each side was heated at 1423 K (1150 "C), and then was
maintained for one hour, and thereafter was hot-forged into a round bar whose
diameter is 35 mm while keeping the finish temperature not to be below 1223
K (950 "C). After the hot forging process is completed, each bar was subjected
to allowing cooling in the atmosphere.
[00951
Various test specimens were collected from each round bar having a
diameter of 35 mm produced in the above manner, and an investigation was
conducted on a microstructure, a tensile property, a fatigue strength, an
impact property, and machinability thereof.
[0096]
The microstructure was observed in such a manner that test pieces
having observation surfaces parallel to a forging axis (longitudinal cross
sectional surface) of each round bar were cut out, were filled with resin and
then mirror-polished, and thereafter they were etched with a nital solution;
and then an observation was conducted on each test specimen at an R 1 2
portion ("R" indicates a radius of the round bar) thereof with an optical
microscope with magnification of lOOx and 400x.
Tensile test specimens, fatigue test specimens, and impact test
specimens were cut out in such a manner that a longitudinal direction of each
test specimen becomes parallel to the forging axis of each round bar.
Specifically, the tensile test specimens and the impact test specimens were cut
out from the R/2 portion of each round bar, and the fatigue test specimens
were cut out from the central portion of each round bar, and then the
respective test specimens were machined to have respective shapes in Figure 2,
Figure 3, and Figure 4.
[0098]
The tensile test was conducted by using the tensile test specimens
shown in Figure 2 under the control of the crosshead displacement at a room
temperature, and in the atmosphere, and the strain rate was adjusted to be
within a range of 10-3 to 10-4 Is. The tensile strength (TS) was found based on
the obtained data regarding "load versus elongation".
[00991
The fatigue test was conducted by using an Ono type rotating bending
fatigue test machine with smoothed test pieces of Figure 3 at a room
temperature, and in the atmosphere. The number of rotations was set to be
3400 rpm, and the maximum stress that causes no fracture in 107 cycles of
repetitively applying stress was defined as the fatigue strength (ow).
[o 1001
The impact test was conducted with a common method by using a
Charpy impact testing machine with V notched test specimens set forth in JIS
Z 2242 (2005) as shown in Figure 3 at a room temperature, and in the
atmosphere.
[OlOlI
' ~ e The machinability was evaluated in such a manner that a plate-like test
specimen having a thickness of 10 mm, a width of 30 mm, and a length of 300
mm was cut out from a central portion of each round bar, through holes each
having a depth of 10 mm were formed by a drill, and then the amount of corner
wear of the drill (amount of wear on the outermost circumference of the drill;
referred to as an average value of widths of wear surfaces generated on two
cutting blades of the drill) after drilling 100 through holes was formed.
[o 1021
The amount of corner wear of the drill was 0.53 mm when Steel 20
which is equivalent to the steel for a cracking connecting rod having a carbon
content of 0.7% set forth in Patent Document 1 was drilled,; therefore this
value was defined as a reference value, and the test specimen was judged to
have targeted preferable machinability when the amount of corner wear was
less than 50% of the reference value, that is, less than 0.265 mm.
[01031
The drilling test condition is as follows.
Drill: straight shank drill of SKH 51 having a diameter of 8 mm
Number of rotations: 754 rpm
Feed: 0.15 mmlrev
Lubricant: water-soluble lubricant
[O 1041
Each result of the above tests is comprehensively shown in Table 2.
The value "y" in Table 2 is a value calculated regarding the holding
temperature for each square bar 55 mm on each side obtained by using
Formulas , <3>, and <4>.
[01051
[Table 21
Table 2
[O 1061
As obvious from Table 2, all the round bars each having a diameter of 35
mm for the test numbers 1 to 11 and 21 to 23 produced by using Steel 1 to
Test
No.
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
2 1
22
23
'yn is a value obtained based on Et represented by the following Formula , and Formulas <3> and <4>.
Et = [Ti] - 3.4[N] - 1,5[SJ ...
y = {Et + (Et2 + 6 x a(T))On6}/2 ... <3>
a(T) = 1 O{.U7640rr)+s.20) ... <4>
"F", "PP"a, nd "Bn In the microstructure column denote ferrite, perlite, and baitnite, respectively.
"resent invention"denotes inventive Example, and "Comparison"denotes Comparative Example.
(*) represents deviation from the range of the chemical composition specified by the present invention, and (**)
represents deviation from the property value specified by the present invention.
(#) represents that the target is not achieved,
Steel
1
2
3
4
5
6
7
8
9
10
11
*I2
*I3
*I4
"15
*I6
*17
*I8
*I9
*20
1
5
7
Temperature
Temperature
T
1523K
(1250°C)
1523K
(1250°C)
1423K
(115OoC)
T denotes
y
0,0077
0.0109
0.0115
0.0145
0.0136
0,0163
0.0102
0.0106
0.0107
0.0114
0.0096
0,0089
0.0404
0,0365
0.0414
0.0107
0;0747
0.0178
0,0368
0.0050
0.0013
0.0028
0,0018
holding
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
F+P
F+P
F+P
F+P
F+P
F+P
F+P+B
P
F+P
F+P
F+P
temperature of the
TS
(MPa)
91 5
910
900
900
900
910
910
91 5
910
915
920
800
750
920
910
920
930
950
950
940
91 5
895
910
square
OW
(MPa)
460
455
450
460
450
460
455
460
455
455
460
" 395
'* 360
465
460
460
460
475
460
** 360
460
450
455
bar 55 mm on
v ERT
(Jlcm2)
3.2
3.1
2.8
4 .O
3.7
3.8
3.5
3,6
4.0
4.5
3.5
7.0
** 18.0
** 15.0
** 22,O
4.6
" 19,5
3.5
** 26,O
7.0
3.8
4.8
4.3
each slde,
Drill Corner Wear
(mm)
0.15
0.14
0.1 1
0.10
0.13
0.15
0.16
0,17
0.16
0.19
0,21
0.08
0.06
0.18
0.17
# 0.32
# 0.35
# 0.45
# 0.47
# 0.53
0.16
0.15
0.16
Note
C
0 =c
a>,
-C
C
C 8
a $?
C 8 'C m
Q
5 0
E
ww 'cc.
a, a,
t i- 2
0 Steel 11, which have the chemical composition specified in the present
invention, satisfy the specification that VERTis 1.0 to 7.0 JIcm2, and ow is 450
MPa or more; accordingly they are excellent in cracking property and fatigue
resistance, and have preferable machinability.
[01071
The test number 11 was conducted by using the round bar of Steel 11
having a diameter of 35 mm, and containing Cu of 0.27% and Ni of 0.15%, and
it is confirmed that a strength enhancing effect can be exerted by containing
the above contents of Cu and Ni.
[0108]
To the contrary, in the test number 12 using the round bar having a
diameter of 35 mm, the value of Ceq of Steel 12 is smaller than the lower limit
specified in the present invention, and thus ow does not satisfy the
specification that ow is 450 MPa or more, which results in poor fatigue
resistance.
[01091
In the test number 13 using Steel 13, the V content, Et, and Ceq among
the conditions of the chemical composition specified by the present invention
deviate from the range, and Ceq has a smaller value than the lower limit
thereof, and thus, ow does not satisfy the specification that ow is 450 MPa or
more, which results in poor fatigue resistance, and VERTh as a greater than the
specified upper limit, which results in poor fracture splitting performance.
[01101
In the test number 14 using Steel 14, Et deviates from the range of the
chemical composition specified by the present invention, and thus VERTh as a
value greater than the specified upper limit thereof, which results in poor
fracture splitting performance.
[Ollll
@ In the test number 15 using Steel 15, the S content and Et deviate from
the range of the chemical composition specified by the present invention, and
thus VERT has a value greater than the specified upper limit thereof, which
results in poor fracture splitting performance.
[01121
In the test number 16 using Steel 16, Ceq deviates from the range of the
chemical composition specified by the present invention, and Ceq has a value
greater than the specified upper limit thereof, and thus the amount of drill
corner wear is greater than the target value of 0.265 mm, resulting in poor
machinability.
[01131
In the test number 17 using Steel 17, the S content, Et, and Ceq deviate
from the range of the chemical composition specified by the present invention.
Because Et deviates from the range, VERT has a value greater than the
specified upper limit thereof, which results in poor fracture splitting
performance and Ceq has a value greater than the upper limit thereof, and
thus the amount of drill corner wear is also greater than the target value of
0.265 mm, resulting in poor machinability.
[OI 141
In the test number 18 using Steel 18, the V content and Ceq deviate
from the range of the chemical composition specified in the present invention,
and Ceq has a value greater than the upper limit thereof, and thus the amount
of drill corner wear is greater than the target value of 0.265 mm, resulting in
poor machinability.
[01151
In the test number 19 using Steel 19, the Ti content, Et, and Ceq deviate
from the range of the chemical composition specified by the present invention.
Because Et deviates from the range, VERT has a value greater than the
0 specified upper limit thereof, resulting in poor fracture splitting performance,
and because Ceq has a value greater than the upper limit thereof, the amount
of drill corner wear is greater than the target value of 0.265 mm, resulting in
poor machinability.
[OI 161
In the test number 20 using Steel 20 equivalent to the steel for a
cracking connecting rod set forth in Patent Document 1, the C content, the P
content, the Ti content, and Ceq deviate from the range of the chemical
composition. The concept of Et specified by the present invention cannot be
applied to this steel containing no Ti, but it is obvious that the test number 20
has poorer fatigue resistance and poorer machinability, compared to the
present invention.
[Industrial Applicability]
[01171
The non-post-heat treated steel of the present invention can be suitably
used as a starting material of a non-post-heat treated steel member such as a
connecting rod for an automobile engine which is fracture-split after being
formed in a predetermined shape through hot forging, and in which high
fatigue strength is required. The non-post-heat treated steel member of the
present invention is excellent in cracking property and fatigue resistance
property, and can be used as a connecting rod for an automobile engine or the
like.]