Note: Descriptions are shown in the official language in which they were submitted.
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STEEL PRODUCTS EXCELLENT IN MACHINABILITY
AND MACHINED STEEL PARTS
TECHNICAL FIELD
The present invention relates to steel products which
exhibit excellent machinability, as well as to machined
steel parts. More particularly, the invention relates to
steel products which exhibit excellent machinability and are
suitable for steel stocks of structural steel parts for a
variety of machinery such as transportation machinery
including automobiles, machinery for industrial use,
construction machinery, and the like, and to a variety of
machined structural steel parts for machinery, such as
crankshafts, connecting rods, gears, and the like.
TECHNICAL BACKGROUND
In conventional manufacture of structural steel parts
for a variety of machinery such as transportation machinery,
machinery for industrial use, construction machinery, and
the like, such steel parts are generally either (a) formed
roughly into predetermined shapes through hot working, then
formed into desired shapes through machining, followed by
thermal refining through quenching and tempering, or (b)
subjected to hot working, and then quenching and tempering,
followed by machining.
However, as structural parts for machinery have been
improved to be of high strength, the cost for machining has
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been increased accordingly. Therefore, for ease of
machining and for lowering the cost, there is an increased
demand for free cutting steel having excellent machinability.
It is well known that the machinability of steel is
improved through addition of free-cutting elements
(machinability-improving elements) such as Pb, Te, Bi, Ca
and S, singly or in combination. For this reason, in order
to improve machinability of steels such as steels for
machine structural use, there has been employed the method
of incorporating the above free-cutting elements into the
steels. However, when the free-cutting elements are merely
incorporated into steels for machine structural use and the
like, in many cases the desired mechanical properties (for
example, toughness and fatigue strength) cannot be secured.
Under these circumstances, a technique comprising hot
working and then machining, followed by quenching and
tempering, as described in (a) above is disclosed in Patent
Application Laid-open (Xokai ) Nos. 2-111842 and 6-279849.
This technique involves "hot rolled steel products endowed
with excellent machinability and hardenability" in which C
is present in steel as graphite and the machinability of the
steel is improved through utilization of the notch effect
and lubrication effect of graphite, as well as the "method
of manufacturing steels for machine structural use with
excellent machinability."
However, in the steel products disclosed in Patent
Application Laid-open (Kokai ) No. 2-111842, it is essential
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that B be incorporated into the steel so that boron nitride
particles (BN) serve as nuclei for precipitation, to thereby
facilitate graphitization, and thus the steel becomes
susceptible to cracks when solidified. In contrast, in the
method disclosed in Patent Application Laid-Open (Kokai ) No.
6-279849, graphitization in steel is accelerated under the
as-hot-rolled condition, through addition of Al and through
limitation of O (oxygen) content in steel to a low level.
This method requires more than five hours for treatment of
graphitization after hot rolling, and thus is not very
economical.
In contrast, a technique comprising hot working, and
then quenching and tempering, followed by machining, as
described in (b) above is disclosed, for example, in Patent
Application Laid-open (Kokai) No. 6-212347. This involves
"hot forged steel products having high fatigue strength and
a method of manufacturing the same" in which steel having a
specific chemical structure is quenched immediately after
hot forging, followed by tempering, to thereby precipitate
TiC. However, in the hot forged steel products obtained by
this method, the ratio of N to Ti (N/Ti) is merely specified
as less than 0.1, and therefore excellent machinability
cannot always be secured. Briefly, if the content of N in
steel containing 0.01 to 0.20 wt.% of Ti is merely specified
such that N/Ti is less than 0.1, hard TiN may often be
formed in a great amount, causing degradation of
machinability, and further causing degradation of toughness.
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In TETSU-TO-HAGANE (vol. 57 (1971) S484), it is
reported that machinability may be improved through
incorporation of Ti into deoxidation-adjusted free-cutting
steel. However, this publication also describes that
incorporation of a great amount of Ti produces a great
amount of TiN, resulting in incresed wear of tools and
disadvantages in terms of machinability. For example, the
life of the drill to a steel having the following
composition based on % by weight, C: 0.45%; Si: 0.29%; Mn:
0.78%; P: 0.017%; S: 0.041%; Al: 0.006%; N: 0.0087%; Ti:
0.228%; O: 0.004%; and Ca: 0.001%, is adversely short, and
therefore, machinability of above-mentioned steel is poor.
Consequently, it is concluded that machinability of steel is
not improved through simple addition of Ti.
DISCLOSURE OF THE INVENTION
In view of the foregoing, an object of the present
invention is to provide steel products which have excellent
machinability and thus are suitable for steel st~ocks of
structural steel parts for a variety of machinery such as
transportation machinery including automobiles, machinery
for industrial use, construction machinery, and the like,
and to provide a variety of machined structural steel parts
for machinery, such as crankshafts, connecting rods, gears,
and the like.
The gist of the present invention will be summarized
below.
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(I) A steel product which exhibits excellent
machinability and which has the following chemical
composition based on % by weight: C: 0.05% to 0.6%; S:
0.002% to 0.2%; Ti: 0.04% to 1.0%; N: 0.008% or less; Nd: 0%
to 0.1%; Se: 0% to 0.5%; Te: 0% to 0.05%; Ca: 0% to 0.01%;
Pb: 0% to 0.5%; and Bi: 0% to 0.4%; wherein the maximum
diameter of titanium carbosulfide contained in the steel is
not greater than 10 ~m, and its amount expressed in the
index of cleanliness of the steel is equal to or more than
0.05%.
(II) A non-heat-treated type steel product, according
to (I) above, which has the following chemical composition
based on % by weight, C: 0.2% to 0.6%; Si: 0.05% to 1.5%;
Mn: 0.1% to 2.0%; P: 0.07% or less; S: 0.01% to 0.2%; Al:
0.002% to 0.05%; Cu: 0% to 1.0%; Ni: 0% to 2.0%; Cr: 0% to
2.0%; Mo: 0% to 0.5%; V: 0% to 0.3%; Nb: 0% to 0.1%; and the
balance: Fe and unavoidable impurities, wherein at least 90%
of the microstructure of the steel is constituted by ferrite
and pearlite.
(III) A non-heat-treated type steel product, according
to (I) above, which has the following chemical composition
based on % by weight, C: 0.05% to 0.3%; Si: 0.05% to 1.5%;
Al: 0.002% to 0.05%; Cu: 0% to 1.0%; Mo: 0% to 0.5%; V: 0%
to 0.30%; Nb: 0% to 0.1%; B: 0% to 0.02%; and the balance:
Fe and unavoidable impurities, wherein fn3, expressed by the
following equation, has a value of 2.5% to 4.5%; and at
least 90% of the microstructure of the steel is constituted
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by bainite, or ferrite and bainite:
fn3 = 0.5Si(%) + Mn(%) + 1.13Cr(%)+ 1.98Ni(%).
(IV) A heat-treated type steel product, according to
(I) above, which has the following chemical composition
based on % by weight, C: 0.1% to 0.6%; Si: 0.05% to 1.5%;
Mn: 0.4% to 2.0%; Al: 0.002% to 0.05%; Cu: 0% to 1.0%; Ni:
0% to 2.0%; Cr: 0% to 2.0%; Mo: 0% to 0.5%; V: 0% to 0.3%;
Nb: 0% to 0.1%; B: 0% to 0.02%; and the balance: Fe and
unavoidable impurities, wherein at least 50% of the
microstructure of the steel is constituted by martensite.
(V) A machined steel part made of the steel product as
described in (I) above.
(VI) A machined steel part made of the non-heat-
treated type steel product as described in (II) above.
(VII) A machined steel part made of the non-heat-
treated type steel product as described in (III) above.
(VIII) A machined steel part made of the heat-treated
type steel product as described in ~IV) above.
The expression "titanium carbosulfide" as used herein
encompasses titanium sulfide.
The expression ~m~x;mllm diameter (of titanium
carbosulfide)" as used herein refers to "the longest
diameter among the diameters of respective titanium
carbosulfide entities."
The index of cleanliness of the steel is determined by
"the microscopic testing method for the non-metallic
inclusions in steel~ prescribed in JIS G 0555, and performed
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by means of an optical microscope at x400 magnification and
60 visual fields.
The term "non-heat-treated type steel product" as used
herein refers to a steel product manufactured without
"quenching and tempering" which are so-called "thermal
refining," and includes "steel which may be used under the
as-cooled condition after hot working" as well as "steel
obtained through aging corresponding to tempering after hot
working and cooling." The term "heat-treated type steel
product" refers to steel products obtained through
"quenching and tempering".
Ratios referred to in terms of microstructure denote
those observed under a microscope, i.e. area percentage.
In (II) above, "at least 90% of the microstructure of
the steel is constituted by ferrite and pearlite" means that
the total of the respective contents of ferrite and pearlite
in the microstructure where ferrite and pearlite coexist is
at least 90%.
In (III) above, "at least 90% of the microstructure of
the steel is constituted by bainite" means that the bainite
content in the microstructure where no ferrite exists is at
least 90%, and "at least 90% of the microstructure of the
steel is constituted by ferrite and bainite" means that the
total of the respective contents of ferrite and bainite in
the microstructure where ferrite and bainite coexist is at
least 90%.
In (IV) above, "at least 50% of the microstructure of
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the steel is constituted by martensite" means that the
martensite content in the microstructure is at least 50%.
In addition, the above (IV) is directed to a "heat-treated
type steel product" which has undergone quenching and
tempering. Likewise, the above mentioned martensite refers
to martensite which has undergone tempering, i.e. "tempered
martensite," and will hereinafter be referred to simply as
"martensite".
BEST MODE FOR CARRYING OUT THE INVENTION
The present inventors conducted various experiments to
investigate the effects of the chemical composition and the
microstructure of steel products on machinability and
mechanical properties.
As a result, the present inventors found that
machinability of a steel product is improved by (a) addition
of a proper amount of Ti to the steel, (b) transformation of
sulfides to titanium carbosulfides for controlling
inclusions in the steel, and (c) minute dispersion of the
titanium carbosulfides in the steel.
The present inventors continued the studies to find
the facts (d) to (p) as follows:
(d) Titanium carbosulfide is formed in steel when Ti
is intentionally added to steel containing an adequate
amount of S.
(e) The formation of titanium carbosulfide in the
steel decreases the amount of production of MnS.
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(f) If the S content in steel is constant, titanium
carbosulfides are superior to MnS in terms of effect of
improving machinability. This is because the titanium
carbosulfide has a melting point lower than that of MnS and
thus achieves an increased lubrication effect on tool faces
during machining.
(g) In order to cause titanium carbosulfide to fully
exert its machinability-improving effect, it is important to
restrict the N content to as low as 0.008% or less in order
to suppress precipitation of TiN.
(h) The restriction of the N (nitrogen) content leads
to a decrease in the TiN content in steel. Therefore, it
becomes possible to improve, among mechanical properties,
especially the toughness.
(i) In order to improve machinability by making use of
titanium carbosulfides, it is important to optimize the size
of titanium carbosulfides and their amounts expressed in the
index of cleanliness of the steel (hereinafter referred to
simply as " index of cleanliness").
(j) Titanium carbosulfide produced during steelmaking
is not solubule in the steel matrix at heating temperatures
for ordinary hot working or for ordinary quenching in a
thermal refining process. For this reason, titanium
carbosulfides exert a so-called "pinning effect" in the
austenite region, which is effective in preventing the
enlargement of austenite grains. Needless to say, titanium
carbosulfides are not solubule in the steel matrix at
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heating temperatures for ordinary tempering in a thermal
refining process, or hot working, or for aging process
corresponding to tempering.
(k) Steel product, containing at least 90% of ferrite
and pearlite in the microstructure, very rarely suffers an
occurrence of bend due to transformation-induced strain and
residual stress.
(1) Steel containing at least 90% of bainite and
ferrite exclusively, or ferrite and bainite in the
microstructure, exhibits an excellent balance between
strength and toughness.
(m) Steel containing at least 50% of martensite in the
microstructure exhibits an extremely excellent balance
between strength and toughness.
~ n) In non-heat-treated type steel products, having a
certain chemical composition and containing at least 90% of
ferrite and pearlite in the microstructure, an excellent
balance between strength and toughness can be obtained, if
the steel satisfies any of the following: (1) ferrite
accounts for 20% to 70% based on area percentage, (2)
ferrite grain size of at least 5 according to JIS grain size
number, or (3) the average lamellar spacing of pearlite is
0.2 ~m or less.
(o) If the value of fnl expressed by equation (1)
below is greater than 0%, and/or the value of fn2 expressed
by equation (2) below is greater than 2, the machinability-
improving effect of titanium carbosulfides is improved. In
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addition, if the value of fn2, expressed by equation (2)
below, is greater than 2, the pinning effect of titanium
carbosulfide is improved and excellent strength and
toughness is obtained.
fnl = Ti(%) - 1.2S(%)..... (1)
fn2 = Ti(%)/S(%)-----(2)-
(p) The value of fn3, expressed by equation (3) below,
governs a certain relationship between the microstructure
and toughness of non-heat-treated type steel which has a
certain chemical composition. If this value is within a
certain range, at least 90% of the microstructure is bainite
exclusively, or ferrite and bainite.
fn3 = 0.05Si(%) + Mn(%) + 1.13Cr(%) +1.98Ni(%)..... (3).
The present invention has been accomplished based on
the above findings.
Requirements of the present invention will now be
described in detail. The symbol "%" indicative of the
content of each element means "% by weight."
(A) Chemical Composition of Steel Products
C :
C binds to Ti together with S to form titanium
carbosulfide and to have an effect of improving
machinability. Also, C is an element effective for securing
strength. However, if the carbon content is less than 0.05%,
these effects cannot be obtained. On the other hand, if the
carbon content is in excess of 0.6%, toughness will be
impaired. Therefore, the carbon content shall be from 0.05%
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to 0.6%.
In non-heat-treated type steel containing at least 90%
of ferrite and pearlite in the microstructure of the steel
(hereinafter referred to as "steel products under Condition
X" for purpose of simplicity), the carbon content shall be,
desirably, from 0.2% to 0.6%, more desirably, from 0.25% to
0.5%.
In non-heat-treated type steel containing at least 90%
of bainite exclusively, or ferrite and bainite in the
microstructure (hereinafter referred to as "steel products
under Condition Y" for purpose of simplicity), the carbon
content shall be, desirably, from 0.05% to 0.3%, more
desirably, from 0.1% to 0.24%.
In heat-treated type steel containing at least 50% of
martensite in the microstructure (hereinafter referred to as
"steel products under Condition Z" for purpose of
simplicity), the carbon content shall be, desirably, from
0.1% to 0.6%.
S :
S binds to Ti together with C to form titanium
carbosulfide and to have an effect of improving
machinability. However, if the sulfur content is less than
0.002%, the effect cannot be obtained.
Conventionally, S has been incorporated into free-
cutting steel in order that the machinability is improved by
forming MnS. According to the studies of the present
inventors, the above-mentioned machinability-improving
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effect of MnS relies on the effect of improving lubrication
between the chips and the face of a tool during machining.
To make matters worse, MnS may become large and cause a
large macro-streak-flaw for steel products, resulting in a
defect.
In the present invention, the machinability-improving
effect of S is obtained by forming titanium carbosulfide
through incorporation of adequate amounts of C and Ti.
Therefore, as mentioned above, the sulfur content is
required to be not less than 0.002%. By contrast, if the
sulfur content is in excess of 0.2%, although no effect is
provided for machinability, coarse MnS is produced in the
steel again, which leads to problems such as a macro-streak-
flaw. In addition, since hot workability is considerably
impaired, plastic working becomes difficult and toughness
may be impaired. Therefore, the sulfur content shall be
from 0.002% to 0.2%.
In "steel products under Condition X," the sulfur
content shall be, desirably, from 0.01% to 0.2%, more
desirably, from 0.02% to 0.17%.
In "steel products under Condition Y," the sulfur
content shall be, desirably, from 0.005% to 0.17%.
Ti:
In the present invention, Ti is an important alloy
element to control inclusion. If the titanium content is
less than 0.04%, S is not fully incorporated into the
titanium carbosulfide and thus improved machinability is not
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obtained. By contrast, if the titanium content is in excess
of 1.0%, not only the cost increases as machinability-
improving effect saturates, but also the toughness and hot-
workability decrease excessively. Therefore, the titanium
content shall be from 0.04% to 1.0%.
In "steel products under Condition X,'~ the titanium
content shall be, desirably, from 0.08% to 0.8%.
In "steel products under Condition Y," the titanium
content shall be, desirably, from 0.06% to 0.8%.
In "steel products under Condition Z," the titanium
content shall be, desirably, from 0.06% to 0.8%.
N: 0.008% or less
In the present invention, it is very important to
restrict the nitrogen content to a low level. Briefly, N,
having strong affinity with Ti, easily binds to Ti to form
TiN, thereby immobilizing Ti. Therefore, the addition of a
great amount of N impedes the full exertion of the above-
mentioned machinability-improving effect of titanium
darbosulfide. Moreover, coarse TiN impairs toughness and
machinability. Therefore, the nitrogen content shall be
0.008% or less. In order to enhance the effect of titanium
carbosulfide, the upper limit of the nitrogen content shall
be, desirably, 0.006%.
Nd:
Nd may be omitted. Nd, if added, becomes Nd2S3 serving
as a chip breaker to have an effect of improving
machinability. Further, since Nd2S3is finely produced in
14
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molten steel in a dispersing manner at relatively high
temperatures, the growth of austenite grains, due to heat,
is restricted during hot working or quenching in the
subsequent process and thus the microstructure becomes fine,
resulting in high strength and toughness of steel. To
reliably obtain this effect, the neodymium content shall be,
desirably, not less than 0.005%. However, if the neodymium
content is in excess of 0.1%, Nd2S3becomes coarse and could
impair toughness. Therefore, the neodymium content shall be
from 0% to 0.1%. Desirably, the upper limit of the
neodymium content shall be 0.08%.
Se:
Se may be omitted. Se, if added, has an effect of
further improving the machinability of steel. To reliably
obtain this effect, the selenium content shall be, desirably,
not less than 0.1%. However, when the selenium content is
in excess of 0.5%, not only the above-mentioned effect
saturates, but also fatigue strength and/or toughness
decrease as coarse inclusions are produced. Therefore, the
selenium content shall be from 0% to 0.5%.
Te:
Te may be omitted. Te, if added, has an effect of
further improving machinability of steel. To reliably
obtain this effect, the tellurium content shall be,
desirably, not less than 0.005%. However, when the
tellurium content is in excess of 0.05%, not only the above-
mentioned effect saturates, but also fatigue strength and/or
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toughness of the steel decrease as coarse inclusions are
produced. Further, addition of a great amount of Te leads
to decreased hot-workability. Specifically, if the
tellurium content is in excess of 0.05%, scratches are
formed in the surfaces of steel products which have
undergone hot working. Therefore, the tellurium content
shall be from 0% to 0.05%.
Ca:
Ca may be omitted. Ca, if added, has an effect of
remarkably improving machinability of steel. To reliably
obtain this effect, the calcium content shall be, desirably
not less than 0.001%. However, when the calcium content is
in excess of 0.01%, not only the above-mentioned effect
saturates, but also fatigue strength and/or toughness
decrease as coarse inclusions are produced. Therefore, the
calcium content shall be from 0% to 0.01%.
Pb:
Pb may be omitted. Pb, if added, has an effect of
further improving the machinability of steel. To reliably
obtain this effect, the lead content shall be, desirably,
not less than 0.05%. However, when the lead content is in
excess of 0.5%, not only the above-mentioned effect
saturates, but also fatigue strength and/or toughness
decrease as coarse inclusions are produced. Further,
addition of a great amount of Pb leads to decreased hot-
workability. Specifically, if the lead content is in excess
of 0.5%, scratches are formed in the surfaces of steel
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products which have undergone hot working. Therefore, the
lead content shall be from 0% to 0.5%.
Bi:
Bi may be omitted. Bi, if added, has an effect of
further improving the machinability of steel. To reliably
obtain this effect, the bismuth content shall be, desirably,
not less than 0.05%. However, when the bismuth content is
in excess of 0.4~, not only the above-mentioned effect
saturates, but also fatigue strength and/or toughness
decrease as coarse inclusions are produced. Further,
addition of a great amount of Bi leads to decreased hot-
workability, resulting in scratches which are formed in the
surfaces of steel products which have undergone hot working.
Therefore, the bismuth content shall be from 0% to 0.4%.
As far as machinability is concerned, no particular
restriction is imposed on any elements other than C, S, Ti,
N, Nd, Se, Te, Ca, Pb and Bi used for "steel products
excellent in machinability" in the present invention.
However, there are often requirements for other properties
in addition to machinability. These requirements include
rare occurrence of bend or residual stress due to
transformation-induced strain, excellent balance between
strength and toughness, and so on. In such cases, the
requirements are satisfied by determining the chemical
composition of the above-mentioned elements other than C, S,
Ti, N, Nd, Se, Te, Ca, Pb and Bi, in relation to the
microstructures of steel products.
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The chemical composition of the elements other than C,
S, Ti, N, Nd, Se, Te, Ca, Pb and Bi will next be described
for each case of the above-mentioned "steel products under
Condition X", "steel products under Condition Y" and "steel
products under Condition Z".
(A-1) In the case of non-heat-treated type steel
products containing at least 90% of ferrite and pearlite in
the microstructure ("steel products under Condition X")
si:
Si is an element effective for deoxidizing a steel and
for strengthening the ferrite phase. Further, the increased
silicon content improves lubrication on the surface of the
chips during machining and thus the service life of the tool
is extended, resulting in improved machinability. However,
if the silicon content is less than 0.05%, the effect of the
addition is insignificant, whereas if the silicon content is
in excess of 1.5%, not only the above-mentioned effect
saturates, but also toughness is impaired. Therefore, the
silicon content shall be, desirably, from 0.05% to 1.5%,
more desirably, from 0.3% to 1.3%, most desirably, from 0.5%
to 1.3%.
Mn:
Mn is an element effective for improving fatigue
strength through solid-solution strengthening. However, if
the manganese content is less than 0.1%, the effect is
difficult to obtain, whereas if the manganese content is in
excess of 2.0%, in the case of "steel products under
18
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Condition X", endurance ratio (fatigue strength/tensile
strength) and yield ratio (yield strength/tensile strengthJ
may be impaired. Therefore, the manganese content shall be,
desirably, from 0.1% to 2.0%, more desirably, from 0.4% to
2.0%, and most desirably, from 0.5% to 1.7%.
P:
P may be intentionally added. This is because P has an
effect of improving tensile strength and fatigue strength in
"steel products under Condition X". In order to reliably
obtain this effect, the phosphorus content shall be,
desirably, not less than 0.01%. However, if the phosphorus
content is in excess of 0.07%, toughness decreases
remarkably and hot-workability is impaired. Therefore, the
phosphorus content shall be, desirably, not greater than
0.07%. If P is added intentionally, the phosphorus content
shall be, desirably, from 0.015% to 0.05%.
A1:
A1 is an element effective for deoxidizing a steel.
However, if the aluminum content is less than 0.002%, the
desired effect is difficult to obtain, whereas if the
aluminum content is in excess of 0.05%, the effect is
saturated and machinability is also impaired. Therefore,
the aluminum content shall be, desirably, from 0.002% to
0.05%, more desirably, from 0.005% to 0.03%.
Cu :
Cu may be omitted. Cu, if added, has an effect of
improving strength, especially fatigue strength of a steel,
19
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through precipitation strengthening. To reliably obtain
this effect, the copper content shall be, desirably, not
less than 0.2%. However, when the copper content is in
excess of 1.0%, hot-workability is impaired, and moreover as
precipitates become coarse, the above-mentioned effect
saturates or decreases. In addition, the cost increases.
Therefore, the copper content shall be, desirably, from 0%
to 1.0~.
Ni:
Ni may be omitted. Ni, if added, has an effect of
improving strength. To reliably obtain this effect, the
nickel content shall be, desirably, not less than 0.02%.
However, when the nickel content is in excess of 2.0%, this
effect saturates and thus the cost increases. Therefore,
the nickel content shall be, desirably, from 0% to 2.0%.
Cr:
Cr may be omitted. Cr, if added, has an effect of
improving fatigue strength through solid-solution
strengthening. To reliably obtain this effect, the chromium
content shall be, desirably, not less than 0.02%. However,
if the chromium content is in excess of 2.0%, in "steel
products under Condition X", endurance ratio and yield ratio
may be impaired. Therefore, the chromium content shall be,
desirably, from 0% to 2.0%. In the case where Cr is added,
the chromium content shall be, desirably, from 0.05% to 1.5%.
Mo:
Mo may be omitted. Mo, if added, has an effect of
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improving strength, especially fatigue strength of a steel,
since the microstructure composed of ferrite and pearlite
becomes fine. To reliably obtain this effect, the
molybdenum content shall be, desirably, not less than 0.05%.
However, when the molybdenum content is in excess of 0.5%,
the microstructure through hot working becomes abnormally
coarse, resulting in lowered fatigue strength. For that
reason, the molybdenum content shall be, desirably, from 0%
to 0.5%.
V:
V may be omitted. V, if added, has an effect of
improving strength, especially fatigue strength of a steel,
since V precipitates as fine nitride or carbonitride. To
reliably obtain this effect, the vanadium content shall be,
desirably, not less than 0.05%. However, when the vanadium
content is in excess of 0.3%, the precipitates become coarse,
resulting in saturation, or even impairment, of the above-
mentioned effect. In addition, the material costs increase.
Therefore, the vanadium content shall be, desirably, from 0%
to 0.3%.
Nb:
Nb may be omitted. Nb, if added, has an effect of
preventing coarsening of austenite grains, to thereby
enhance strength, especially fatigue strength of a steel,
since Nb precipitates as fine nitride or carbonitride. To
reliably obtain this effect, the niobium content shall be,
desirably, not less than 0.005%. However, when the niobium
CA 02243123 1998-07-1~
content is in excess of 0.1%, not only does the above-
mentioned effect saturate, but also coarse hard carbonitride
may be produced to damage tools, resulting in lowered
machinability. Therefore, the niobium content shall be,
desirably, from 0% to 0.1%. More desirably, the upper limit
of niobium content shall be 0.05%.
fnl, fn2:
As mentioned above, if the value of fnl expressed by
the equation (1) is greater than 0%, and/or the value of fn2
expressed by the equation (2) is greater than 2, the
machinability-improving effect of titanium carbosulfides is
enhanced. In addition, if the value of fn2, expressed by
the equation (2), is greater than 2, the pinning effect of
titanium carbosulfides is enhanced, to thereby improve
tensile strength and fatigue strength. Therefore, it is
desired that the value of fnl shall be greater than 0%, or
alternatively, the value of fn2 shall be greater than 2. No
particular limitation is imposed on the upper limits of the
values of fnl and fn2, and they may be determined so as to
comply with compositional requirements.
Incidentally, O (oxygen) as an impurity element forms
hard oxide-type inclusions, by which the machine tool may be
damaged, resulting in lowered machinability. In particular,
the oxygen content in excess of 0.015% may considerably
impair machinability. Consequently, in order to maintain
excellent machinability, the amount of O as an impurity
element shall be, desirably, 0.015% or less. More desirably,
CA 02243123 1998-07-1~
the oxygen content shall be 0.01% or less.
(A-2) In the case of non-heat-treated type steel
products in which bainite or a combination of ferrite and
bainite accounts for at least 90% of the microstructure of
the steel ("steel products under Condition Y")
si:
Si has an effect of deoxidizing a steel and improving
hardenability. Furthermore, in "steel products under
Condition Y", the increased silicon content improves
lubrication on the surface of the chips during machining and
thus the service life of the tool is extended, resulting in
improved machinability. However, when the silicon content
is less than 0.05%, the above-mentioned effects are poor,
whereas if the silicon content is in excess of 1.5%, not
only do the above-mentioned effects saturate, but also
toughness is impaired. Therefore, the silicon content shall
be, desirably, from 0.05% to 1.5%. More desirably, the
silicon content shall be from 0.5% to 1.3%.
Al:
Al is an element having powerful deoxidizing effect on
a steel. To secure this effect, the aluminum content shall
be, desirably, not less than 0.002%. However, when the
aluminum content is in excess of 0.05%, the effect saturates
and the only result is increased cost. Therefore, the
aluminum content shall be, desirably, from 0.002% to 0.05%,
more desirably, from 0.005% to 0.04%.
Cu :
CA 02243123 1998-07-1~
Cu may be omitted. Cu, if added, has an effect of
improving machinability as well as strength of the steel
without lowering toughness. To reliably obtain this effect,
the copper content shall be, desirably, not less than 0.2%.
However, when the copper content is in excess of 1.0%, not
only is hot workability impaired, but also precipitates may
become coarse, resulting in saturated the above-mentioned
effect or lowered toughness. In addition, the cost
increases. Therefore, the copper content shall be,
desirably, from 0% to 1.0%.
Mo:
Mo may be omitted. Mo, if added, has an effect of
improving hardenability and strength of a steel by rendering
the microstructure of the steel very fine. To reliably
obtain this effect, the molybdenum content shall be,
desirably, not less than 0.05%. However, when the
molybdenum content is in excess of 0.5%, the microstructure
obtained through hot working becomes abnormally coarse,
resulting in lowered toughness. For this reason, the
molybdenum content shall be, desirably, from 0% to 0.5%.
V:
V may be omitted. V, if added, has an effect of
improving strength, since V precipitates as fine nitride or
carbonitride, and moreover, has an effect of improving
lubrication on the surface of the chips during machining.
To reliably obtain these effects, the vanadium content shall
be, desirably, not less than 0.05%. However, when the
24
CA 02243123 1998-07-1~
vanadium content is in excess of 0.30%, as the precipitates
become coarse, the above-mentioned effect may saturate or
toughness may decrease. In addition, the cost increases.
Therefore, the vanadium content shall be, desirably, from 0%
to 0.30%.
Nb:
Nb may be omitted. Nb, if added, has an effect of
preventing coarsening of austenite grains and improving
strength and toughness of the steel, since Nb precipitates
as fine nitride or carbonitride. To reliably obtain this
effect, the niobium content shall be, desirably, not less
than 0.005%. However, when the niobium content is in excess
of 0.1%, not only does the above-mentioned effect saturate,
but also coarse hard carbonitride may be produced to damage
tools, inviting degraded machinability. Therefore, the
niobium content shall be, desirably, from 0% to 0.1%.
B:
B may be omitted. B, if added, has an effect of
improving strength and toughness of a steel due to increased
hardenability. To secure this effect, the boron content
shall be, desirably, not less than 0.0003%. However, when
the boron content is in excess of 0.02%, not only may the
above-mentioned effect saturate, but also toughness may
decrease. Therefore, the boron content shall be, drsirably,
from 0% to 0.02%.
fn3:
As described above, the value of fn3, expressed by the
CA 02243123 1998-07-1~
aforementioned equation (3), is correlated to the
microstructure and toughness of a non-heat-treated type
steel product having a certain chemical composition. When
the value is in the range of 2.5 - 4.5%, the primary
microstructure of the non-heat-treated type steel product
comes to be bainite, or a combination of ferrite and bainite,
thus achieving well-balanced strength and toughness.
Si, Mn, Cr and Ni, which form the terms of the
equation for fn3, have the effect of enhancing hardenability
of the steel. When the value of fn3 is less than 2.5%,
intended improvement in hardenability cannot be obtained,
with toughness being sometimes degraded. In contrast, the
values of fn3 in excess of 4.5% result in excessive
hardenability, which may in turn degrade toughness.
Therefore, it is desired that the value of fn3 expressed by
the equation (3) shall be from 2.5% to 4.5%. In this
connection, the contents of the respective elements other
than Si are not particularly limited, so long as the above-
mentioned fn3 falls within the range of 2.5-4.5%. However,
desirably, Mn, Cr and Ni shall be contained in amounts of
0.4-3.5%, 3.0% or less, and 2.0%
or less, respectively.
fnl, fn2:
In the case of "steel products under Condition Y", as
mentioned above, the machinability-improving effect of
titanium carbosulfides is enhanced when the value of fnl
expressed by the equation (1) is greater than 0%, and/or the
26
CA 02243123 1998-07-1~
value of fn2 expressed by the equation (2) is greater than 2.
Furthermore, when the value of fn2, expressed by the
equation (2), is greater than 2, the pinning effect of
titanium carbosulfides increases as well, to thereby improve
tensile strength and fatigue strength. Therefore, it is
desired that the value of fnl shall be greater than 0%, or
alternatively, the value of fn2 shall be greater than 2.
The upper limits of the values of fnl and fn2 are not
particularly limited, and they may be determined based on
compositional requirements.
Incidentally, O (oxygen) as an impurity element forms
hard oxide-type inclusions, by which the machine tool may be
damaged, resulting in lowered machinability. In particular,
the oxygen content in excess of 0.015% may invite
significant degradation in machinability. Therefore, even
in the case of "steel products under Condition Y", in order
to maintain excellent machinability, the amount of O as an
impurity element shall be, desirably, 0.015% or less. More
desirably, the oxygen content shall be 0.01% or less.
Moreover, from the viewpoint of securing toughness of
the steel, phosphorus (P) as an impurity element shall be,
desirably, suppressed to 0.05% or less.
(A-3) In the case of heat-treated type steel products
in which martensite accounts for at least 50% of the
microstructure of the steel ("steel products under Condition
Z " )
si:
CA 02243123 1998-07-1~
Si has an effect of deoxidizing a steel and improving
hardenability. Furthermore, in the case of "steel products
under Condition Z", increased silicon content improves
lubrication on the surface of the chips during machining and
thus the service life of the tool is extended, resulting in
improved machinability. However, if the silicon content is
less than 0.05%, the above-mentioned effects are poor,
whereas if the silicon content is in excess of 1.5%, not
only the above-mentioned effects saturate, but also
toughness is impaired. Therefore, the silicon content shall
be, desirably, from 0.05% to 1.5%.
Mn:
Mn improves hardenability of a steel and improves
fatigue strength through solid-solution strengthening.
However, if the manganese content is less than 0.4%, these
effects are difficult to obtain, whereas if the manganese
content is in excess of 2.0%, not merely these effects
saturate, but also the steel becomes excessively hard to
cause degradation in toughness. Accordingly, the manganese
content shall be, desirably, from 0.4% to 2.0%.
Al:
Al is an element having strong deoxidizing effect on a
steel. In order to secure this effect, the aluminum content
shall be, desirably, not less than 0.002%. However, if the
aluminum content is in excess of 0.05%, the effect saturates
and the only result is increased costs. Therefore, the
aluminum content shall be, desirably, from 0.002% to 0.05%,
28
CA 02243123 1998-07-1
more desirably, from 0.005~ to 0.04%.
Cu :
Cu may be omitted. Cu, if added, has an effect of
improving strength without lowering toughness, and in
addition, enhances machinability. To secure these effects,
the copper content shall be, desirably, not less than 0.2%.
However, when the copper content is in excess of 1.0%, hot
workability is impaired and precipitates become coarse,
resulting in saturated above-mentioned effect or even
impairing the effect. In addition, the cost increases.
Therefore, the copper content shall be, desirably, from 0%
to 1.0%.
Ni:
Ni may be omitted. Ni, if added, has an effect of
improving hardenability of a steel. To secure this effect,
the nickel content shall be, desirably, not less than 0.02%.
However, when the nickel content is in excess of 2.0%, this
effect saturates and thus the cost increases. Therefore,
the nickel content shall be, desirably, from 0% to 2.0%.
Cr:
Cr may be omitted. Cr, if added, has an effect of
enhancing hardenability of a steel, and also improves
fatigue strength through solid-solution strengthening. To
reliably obtain these effects, the chromium content shall be,
desirably, not less than 0.03%. However, when the chromium
content is in excess of 2.0~, not only do the above-
mentioned effects saturate, but also the steel becomes
29
CA 02243123 1998-07-1~
excessively hard, resulting in lowered toughness. Therefore,
the chromium content ~hall be, desirably, from 0% to 2.0%.
Mo:
Mo may be omitted. Mo, if added, has an effect of
improving hardenability of a steel. To reliably obtain this
effect, the molybdenum content shall be, desirably, not less
than 0.05%. However, when the molybdenum content is in
excess of 0.5%, not only does the above-mentioned effect
saturate but also the steel becomes excessively hard,
resulting in lowered toughness and increased cost. For this
reason, the molybdenum content shall be, desirably, from 0%
to 0.5%.
V:
V may be omitted. V, if added, has an effect of
improving strength, especially fatigue strength of a steel,
since V precipitates as fine nitride or carbonitride. To
reliably obtain this effect, the vanadium content shall be,
desirably, not less than 0.05%. However, when the vanadium
content is in excess of 0.3%, the precipitates become coarse,
resulting in saturation, or even impairment, of the above-
mentioned effect. In addition, the material costs increase.
Therefore, the vanadium content shall be, desirably, from 0%
to 0.3%-
Nb:
Nb may be omitted. Nb, if added, has an effect ofpreventing coarsening of austenite grains, to thereby
enhance strength, especially fatigue strength and toughness
CA 02243123 1998-07-1~
of a steel, since Nb precipitates as fine nitride or
carbonitride. To reliably obtain these effects, the niobium
content shall be, desirably, not less than 0.005%. However,
when the niobium content is in excess of 0.1%, not only do
the above-mentioned effects saturate, but also coarse hard
carbonitride may be produced to damage tools, resulting in
lowered machinability. Therefore, the niobium content shall
be, desirably, from 0% to 0.1%. More desirably, the upper
limit of niobium content shall be 0.05%.
B:
B may be omitted. B, if added, has an effect of
improving strength and toughness of a steel due to increased
hardenability. To secure this effect, the boron content
shall be, desirably, not less than 0.0003%. However, when
the boron content is in excess of 0.02%, not only may the
above-mentioned effect saturate, but also toughness may be
lowered. Therefore, the boron content shall be, drsirably,
from 0% to 0.02%.
fnl, fn2:
Also in "steel products under Condition Z", as
aforementioned, if the value of fnl expressed by the
equation (1) is greater than 0%, and/or the value of fn2
expressed by the equation (2) is greater than 2, the
machinability-improving effect of titanium carbosulfides is
enhanced. In addition, if the value of fn2, expressed by
the equation (2), is greater than 2, the pinning effect of
titanium carbosulfides is enhanced, to thereby improve
CA 02243123 1998-07-1~
tensile strength and fatigue strength. Therefore, it is
desired that the value of fnl shall be greater than 0%, or
alternatively, the value of fn2 shall be greater than 2. No
particular limitation is imposed on the upper limits of the
values of fnl and fn2, and they may be determined so as to
comply with compositional requirements.
Incidentally, O (oxygen) as an impurity element forms
hard oxide-type inclusions, by which the machine tool may be
damaged, resulting in lowered machinability. In particular,
the oxygen content in excess of 0.015% may considerably
impair machinability. Consequently, also in "steel products
under Condition Z", in order to maintain excellent
machinability, the amount of O as an impurity element shall
be, desirably, 0.015% or less. More desirably, the oxygen
content shall be 0.01% or less.
Moreover, from the point of securing toughness of the
steel, P (phosphorus) as an impurity element shall be,
desirably, suppressed to 0.05% or less.
(B) The size and the index of cleanliness in terms of
titanium carbosulfides
In order to improve machinability of steel products
having chemical compositions described in (A) above through
use of titanium carbosulfides, it is important that the size
and the index of cleanliness in terms of titanium
carbosulfides be optimized. As described herein above, the
expression "titanium carbosulfides" encompasses titanium
sulfides.
CA 02243123 1998-07-1~
In the case in which the amount expressed by the index
of cleanliness in terms of titanium carbosulfide having a
maximum diameter of not more than 10 ~m is less than 0.05%,
titanium carbosulfides cannot exhibit their machinability-
improving effect. The above-mentioned index of cleanliness
shall be, desirably, not less than 0.08%. When the above-
mentioned index of cleanliness in terms of titanium
carbosulfides is excesslvely large, fatigue strength may
sometimes be degraded. Therefore, the upper limit of the
above-mentioned index of cleanliness in terms of titanium
carbosulfides shall be, desirably, approximately 2.0~.
The reason why the size of titanium carbosulfide is
limited i.e., why the maximum diameter of titanium
carbosulfide is set to 10 ~m---is that sizes in excess of
10 ~m reduce fatigue strength and/or toughness. Desirably,
the m~x;mllm diameter of titanium carbosulfide shall be 7 ~m.
However, in view that too small a maximum diameter of
titanium carbosulfides provides insignificant machinability-
improving effect, the lower limit of the maximum diameter of
titanium carbosulfide shall be, desirably, about 0.5 ~m.
The form of titanium carbosulfide is basically
determined by the amounts of Ti, S and N contained in the
steel. In order to bring the size and the index of
cleanliness in terms of titanium carbosulfides within the
predetermined ranges, it is important to prevent
overproduction of titanium oxides. To this end, according
to a preferred steelmaking process, steel is first
CA 02243123 1998-07-1~
sufficiently deoxidized with Si and Al, then Ti is added;
since in some cases, satisfaction of the compositional
requirements for the steel mentioned in (A) is not
sufficient by itself.
Titanium carbosulfides can be discerned from other
inclusions based on their color and shape through mirror-
like polishing of test pieces cut from steel products and
through observation of the polished surface under an optical
microscope at x400 or higher multiplication. That is,
titanium carbosulfides have a very pale gray color and a
granular (spherical) shape corresponding to B-type
inclusions according to JIS (Japanese Industrial Standards).
Detailed determination of titanium carbosulfides may also be
performed through observation of the aforementioned mirror-
like-polished surface under an electron microscope equipped
with an analytical device such as EDX (energy dispersive X-
ray spectrometer).
The index of cleanliness in terms of titanium
carbosulfides is determined as described hereinabove; i.e.,
in accordance with "the microscopic testing method for the
non-metallic inclusions in steel" prescribed in JIS G 0555,
and performed by means of an optical microscope at x400
magnification and 60 visual fields.
(C) Microstructure of steel products
So far as machinability is concerned, "steel products
excellent in machinability" of the present invention can be
obtained by simply prescribing the amounts of C, S, Ti, N,
34
CA 02243123 1998-07-1~
Nd, Se, Te, Ca, Pb and Bi as described in (A) above and also
prescribing the size and the index of cleanliness in terms
of titanium carbosulfide as described in (B) above. However,
when the steel is required to meet other characteristics in
addition to machinability, the microstructure of steel
products may be additionally prescribed as well.
First, in the case in which not less than 90% of the
microstructure of a steel product is constituted by ferrite
and pearlite, occurrence of bend and residual stress
attributed to transformation-induced strain does not raise a
critical issue. Therefore, if not less than 90% of the
microstructure of a steel product is made to be constituted
by ferrite and pearlite, reformation (straightening step) as
a finish step can be eliminated, leading to reduced costs.
Moreover, in the case in which the steel product is a non-
heat-treated type steel product, there can be saved
considerable energy and cost which would otherwise be
required for thermal refining.
In order to make not less than 90% of the
microstructure of a non-heat-treated type steel product to
be constituted by ferrite and pearlite, a semi-finished
product having a chemical composition described in (II)
above may first be heated to 1050-1300~C, then subjected to
hot working such as hot forging to finish at a temperature
not lower than 900~C, and subsequently subjected to air
cooling or atmospheric cooling at a cooling rate of not more
than 60~c/min for at least a period until the temperature
CA 02243123 1998-07-1~
reaches 500~C. In the present specification, the expression
"cooling rate" refers to the cooling rate as measured on the
surface of the steel product.
In the case of non-heat-treated type steel products
having the above microstructure, well-balanced excellent
strength and toughness can be obtained when at least one of
the following conditions are met: ferrite accounts for 20-
70% in terms of the area percentage; ferrite grain size is 5
or more as expressed by the JIS grain size number; the
average lamellar spacing of pearlite is 0.2 ~Im or less.
Next, in the case of steel products in which not less
than 90% of the microstructure is constituted by bainite or
a combination of ferrite and bainite, well-balanced strength
and toughness are appreciable. Therefore, if well-balanced
strength and toughness are required, not less than 90% of
the microstructure of a steel product should be made to be
constituted by bainite, or a combination of ferrite and
bainite. Moreover, in the case in which the steel product
is a non-heat-treated type steel product, there can be saved
considerable energy and cost which would otherwise be
required for thermal refining.
In order to make not less than 90% of the
microstructure of a non-heat-treated type steel product to
be constituted by bainite, or by a combination of ferrite
and bainite, a semi-finished product having a chemical
composition described in (III) above may first be heated to
1050-1300~C, then subjected to hot working such as hot
36
CA 02243123 1998-07-1~
forging to finish at a temperature not lower than 900~C, and
subsequently subjected to air cooling or atmospheric cooling
at a cooling rate of not more than 60~C/min for at least a
period until the temperature reaches 300~C.
In the case of non-heat-treated type steel products,
the greater the working ratio of the steel products during
hot working, the finer the microstructure of the steel
products, thus exhibiting a better balance between strength
and toughness. Therefore, the working ratio during hot
working shall be, desirably, not less than 1.5. The
expression "working ratio" is used to refer to the ratio
Ao/A where Ao represents a sectional area before working and
A represents a sectional area after working.
When the prior austenite grain size in the
microstructure is 4 or more as expressed by the JIS grain
size number, a non-heat-treated type steel product in which
not less than 90% of the microstructure is constituted by
bainite or a combination of ferrite and bainite (i.e., a
"steel product under Condition Y") can be consistently
imparted with well-balanced strength and toughness. As used
herein, the expression "prior austenite grains" in a non-
heat-treated type steel product refers to austenite grains
right before bainite or ferrite is generated therefrom as a
result of transformation under heat and hot working. Prior
austenite grains in a non-heat-treated type steel product in
which not less than 90% of the microstructure is constituted
by bainite or a combination of ferrite and bainite can be
CA 02243123 1998-07-1~
readily determined through corrosion with nital and
observation under an optical microscope.
When aging treatment is performed by the application
of heat under conditions of 200-700~C for 20-150 minutes
following hot working and cooling, a particularly excellent
balance between strength and toughness can be obtained.
Finally, in the case of a steel product in which not
less than 50% of the microstructure is constituted by
martensite, balance between strength and toughness becomes
more excellent. Therefore, when more excellent balance
between strength and toughness is required, not less than
50% of the microstructure should be made to be constituted
by martensite. Moreover, in the case in which the steel
product is a heat-treated type steel product, remarkably
excellent balance between strength and toughness can be
obtained.
In order to make not less than 50% of the
microstructure of a heat-treated type steel product to be
constituted by martensite, a semi-finished product having a
chemical composition described in (IV) above may be treated
as follows. Briefly, the semi-finished product is first
heated to 1050-1300~C, then subjected to hot working such as
hot forging at a working ratio of 1.5 or more and to
finishing at a temperature not lower than 900~C.
Subsequently the finished steel material is subjected to air
cooling or atmospheric cooling at a cooling rate of not more
than 60~C/min for at least a period until the temperature
CA 02243123 1998-07-1~
reaches 300~C. Subsequently, the steel product is heated to
a temperature range of 800-950~C, maintained for 20-150
minutes, then quenched by use of a cooling medium such as
water or oil, followed by heating to 400-700~C, maintained
for 20-150 minutes, and then subjected to air cooling,
atmospheric cooling, or alternatively, depending on cases,
water cooling or oil cooling followed by tempering. The
quenching treatment may be performed by way of so-called
"direct quenching," in which steel products are quenched
directly from the austenite region or austenite-ferrite dual
phase region after hot working.
In order for a heat-treated type steel product to
secure remarkably excellent strength and toughness in a well
balanced manner, it is preferred that not less than 80% of
the microstructure be made martensite. The remainding
portion of the microstructure other than martensite is
constituted by microstructure resulting from tempering of
ferrite, pearlite or bainite in the case in which an
austenite region undergoes quenching, microstructure
resulting from tempering of ferrite in the case in which an
austenite-ferrite dual-phase region undergoes quenching, or
microstructure resulting from temperering of austenite which
has remained untransformed even when quenching was performed
(so-called retained austenite). Substantially 100% of the
microstructure may represent martensite.
When the prior austenite grain size is not less than 5
according to the JIS grain size number, a heat-treated type
39
CA 02243123 1998-07-1~
steel product in which not less than 50~ of the
microstructure is constituted by martensite (i.e., a "steel
product under Condition Z") can be consistently imparted
with extremely well-bslanced strength and toughness. As
used herein, the expression "prior austenite grains" in a
heat-treated type steel product refers to austenite grains
right before being subjected to quenching. Prior austenite
grains in a heat-treated type steel product in which not
less than 50% of the microstructure is constituted by
martensite can be readily identified as follows, for example.
A steel product is quenched or is quenched and then tempered,
and a sample steel piece is cut out. The test piece is
etched with aqueous solution of picric acid to which a
surfactant has been added. The etched surface of the test
piece is observed under an optical microscope.
(Examples)
The present invention is described concretely using
examples, which should not be construed as limiting the
present invention thereto.
(Example 1)
Steels having chemical compositions shown in Tables 1
to 4 were manufactured through a melting process in a 150 kg
vacuum melting furnace or a 3-ton vacuum melting furnace.
Steels 1, 6, and 36 to 40 were manufactured through a
melting process in the 3-ton vacuum melting furnace, and
other steels were manufactured through a melting process in
CA 02243123 1998-07-1~
the 150 kg vacuum melting furnace. In order to prevent the
generation of titanium oxides, all steels other than steels
36 and 38 underwent adjustment of the size and the index of
cleanliness of titanium carbosulfide. This adjustment was
carried out by adding Ti, after various elements had been
added, subsequent to sufficient deoxidization with Si and Al.
For steels 36 and 38, Ti was added to a molten steel during
deoxidation with Si and Al.
Steels 1 to 36 in Tables 1 to 3 are examples of the
present invention, and contain each component element in an
amount falling in a range specified by the present invention.
In contrast, steels 37 to 46 in Table 4 are comparative
examples, in which any of component elements falls outside a
range specified by the present invention.
T a b I e 1
SteelChemical composition (percent bY weight) Balance: Fe and unavoidable impurities
C Si Mn P S Ti Al N Cu Ni Cr Mo V Nb B Nd Se Te Ca Pb Bi fnl fn2 fn3
1 0.48 0.47 0.85 0.020 0.055 0.09 0.020 0.0028 - - 0.05 - - - - - - - - - - O.OZ 1.64 1.14
2 0.23 0.21 1.60 0.020 0.012 0.06 0.021 0.0015 - - 0.50 - - - - 0.045 - - - - - 0.05 5.00 2.27
3 0.37 0.21 1.01 0.021 0.157 0.81 0.019 0.0021 - - O.Z2 - 0.08 - - - 0.17 - - - - 0.62 5.16 1.36
4 0.47 0.43 0.83 0.020 0.012 0.16 0.018 0.0019 - - - - - 0.03 - - - 0.02 - - - 0.15 13.2 1.05
0.39 0.11 0.98 0.020 0.114 0.40 0.021 0.0034 0.02 - - 0.09 - - - - - 0.003 - - 0.26 3.51 1.04 o
6 0.27 0.23 0.78 0.019 0.161 0.86 0.021 0.0023 - - 0.44 0.02 - - 0.0003 - - - - 0.15 - 0.67 5.34 2.52 r7 0.41 0.70 0.87 0.019 0.063 0.20 0.021 0.0020 - 0.15 - 0.13 0.04 - - - - - - - 0.12 0.12 3.19 1.52
8 0.30 1.52 0.76 0.018 0.050 0.66 0.020 0.0013 - - - - 0.06 - - 0.037 - - - - - 0.60 13.3 1.52
9 0.23 0.48 1.86 0.019 0.069 0.24 0.021 0.0012 - - 0.27 - - - - - - - 0.16 3.45 2.90
0.45 0.04 1.14 0.020 0.010 0.61 0.021 0.0034 - - - - 0.04 - - - - - - - 0.60 59.4 1.16
11 0.23 0.80 1.02 0.020 0.146 0.22 0.021 0.0038 - - 1.15 0.05 - - - - - - 0.003 0.12 - 0.04 1.51 2.72
12 0.25 0.01 2.16 0.019 0.015 0.56 0.020 0.0038 0.02 - - - - - - - - - - 0.09 0.54 38.4 2.17
f n 1 = T i (%) - 1 . 2 S (%) ~ f n 2 = T i (%) / S (%)
f n 3 = O . 5 S i (%) + M n (%) + 1 . 1 3 C r (%) + 1 . 9 8 N i (~/O)
CA 02243123 1998-07-lS
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43
T a b 1 e 1 3
Steel Chemical composition (percent by weight) Balance: Fe and unavoidable imPurities
C Si Mn P S Ti Al N Cu Ni Cr Mo V Nb B Nd Se Te Ca Pb Bi fnl fn2 fn3
0.23 1.57 0.40 0.020 0.133 0.67 0.020 0.0032 - 0.12 0.05 - - - - - - 0 03 - - - 0 51 5.0Z 1.48
26 0.10 0.15 0.50 0.019 0.141 0.78 0.019 0.0030 - - 0.55 0.51 0.24 - - - - - 0.003 - - 0.61 5.53 1.20
27 0.28 0.40 1.50 0.019 0.011 0.19 0.020 0.0025 - 0.11 - - 0.06 0.0011 - - - 0.12 - 0.17 17.0 1.82
28 0.21 0.05 0.70 0.018 0.044 0.27 0.019 0.0018 - - 1.34 - - - - - 0.12 - - - - 0.22 6.23 2.24 D
29 0.20 0.10 1.54 0.020 0.015 0.63 0.020 0.0022 - - - 0.05 G.07 0.0005 - - - - - - 0.62 41.5 1.59 ~
0.25 0.20 0.80 0.018 0.110 0.73 0.021 0.0027 0.04 - 1.20 - - 0.07 - - - - - 0.14 - 0.60 6.64 2.26
31 0.21 l.OS 1.00 0.019 0.143 0.44 0.020 0.0015 - 0.05 0.45 - - - - - - - 0.002 - - 0.27 3.09 2.13
32 0.23 0.45 0.80 0.020 0.025 0.28 0.020 0.0013 - - 0.05 0.20 0.12 0.02 0.0010 - 0.21 0.02 - - - 0.25 11.1 1.08
33 0.14 0.21 1.51 0.019 0.032 0.87 0.020 0.0031 - - - 0.04 0.15 - 0.0009 - - - - - - 0.83 27.1 1.32
34 0.26 0.33 1.34 0.019 0~114 0.39 0.020 0.0027 - - 0.64 - - - - 0.038 - - - 0.06 - 0.25 3.43 2.23
0.29 1.60 0.44 0.018 0.065 0.24 0.018 0.0018 - 0.10 0.15 - 0.06 - 0.0016 - - - - - 0.10 0.16 3.65 1.32
36 0.51 0.22 0.95 0.020 O.U73 0.25 0.021 0.0070 0.11 - - - - - - 0.050 - - - - 0.09 0.16 3.45 1.06
f n 1 = T i (%) - 1 . 2 S (%) ~ f n 2 = T i (%) / S (%)
f n 3 = O . 5 S i (%) + M n (~,~) + 1 . 1 3 C r (%) + 1 . 9 8 N i (%)
CA 02243123 1998-07-15
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CA 02243123 1998-07-1~
Next, each of the steels was hot forged, such that the
steel was heated to a temperature of 1250~C and then
finished at a temperature of 1000~C, to obtain a round bar
having a diameter of 60 mm. The hot-forged round bars were
cooled to a temperature of 300~C, at a cooling rate of
5~C/min to 35~C/min by air cooling or atmospheric cooling,
thereby adjusting their microstructures, so as to obtain a
tensile strength of about 845 MPa to 870 MPa. For steels 6,
7, 9, 11, 29 to 36, 40, 45 and 46, the hot-forged round bars
were cooled as above and then heated at a temperature of
770~C to 900~C for 1 hour, followed by water quenching. The
water-quenched round bars were tempered at a temperature of
550~C to 560~C (followed by air cooling), so as to adjust
their microstructures and strengths.
Test pieces were obtained from each of the round bars
at a position 15 mm deep from the surface (at a position
described as a R/2 site, where R denotes the radius of the
round bar). The obtained test pieces were JIS No. 14A
tensile test pieces, Ono-type rotating bending fatigue test
pieces (diameter of straight portion: 8 mm; length of
straight portion: 18.4 mm), and JIS No. 3 impact test pieces
(2 mm U-notch Charpy test pieces), which were used for
testing tensile strength, fatigue strength (fatigue limit),
and toughness (impact value), respectively, at room
temperature.
A test piece was obtained from each of the round bars
at a position described as a R/2 site in accordance with Fig.
46
CA 02243123 1998-07-1~
3 of JIS G 0555. In each of the obtained test pieces, the
mirror-like polished surface to be observed measured 15 mm
(width) by 20 mm (height). The polished surface was
observed through an optical microscope at 400 magnifications
over a range of 60 visual fields. Through the observation,
the index of cleanliness in terms of titanium carbosulfides
in the steel was measured such that titanium carbosulfides
were distinguished from other inclusions, and also the
maximum diameter of titanium carbosulfides was obtained.
Subsequently, the mirror-like polished surface of each test
piece was etched with nital. The etched surface was
observed through the optical microscope at 100
magnifications so as to observe the state of microstructure,
i.e. to obtain the occupancy rate (area percentage) of
individual constituent phases of microstructure, at the R/2
site.
Also, a drilling test was conducted for evaluation of
machinability. Specifically, each of the round bars having
a diameter of 60 mm was cut to obtain round bar blocks, each
having a length of 55 mm. The blocks were drilled 50 mm
deep in the length direction. The number of bores were
counted and drilled until the drilling tool became disabled
due to failure of the top cutting edge. The number of the
drilled bores was defined as a machinability index
indicative of machinability of steel. The drilling test was
conducted through use of a 6 mm-diameter straight shank
drill of high speed tool steel, JIS SKH59, and a water-
47
CA 02243123 1998-07-15
soluble lubricant, at a feed of 0.20 mm/rev and a revolution
of 980 rpm.
Tables 5 to 8 show the results of the above tests.
Tables 5 to 8 also contain quenching and tempering
conditions for steels 6, 7, 9, 11, 29 to 36, 40, 45 and 46.
48
CA 02243123 1998-07-lS
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CA 02243123 1998-07-15
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CA 02243123 1998-07-1~
As seen from Tables 5 to 8, in test Nos. 1 to 35, the
machinability indices are in excess of 200. The tested
steels 1 to 35 contain C, S, Ti and N in amounts falling
within respective ranges, as specified in the present
invention and have a maximum diameter of titanium
carbosulfides, not greater than 10 ~m and a index of
cleanliness in terms of titanium carbosulfide not lower than
0.05%. By contrast, in test No. 36, the machinability index
is as low as 51, since the tested steel 36 has a index of
cleanliness in terms of titanium carbosulfide lower than
0.05% despite its C, S, Ti and N contents, falling within
respective ranges as specified in the present invention. In
test Nos. 37, 39 and 40, the machinability indices are as
low as 58, 40 and 45, respectively, since some of the C, Ti
and N contents of the tested steels 37, 39 and 40 fall
outside the corresponding range as specified in the present
invention. In test No. 38, the machinability index is as
low as 31, since the S content of the tested steel 38 falls
outside the corresponding range, as specified in the present
invention, and also the tested steel 38 has a index of
cleanliness in terms of titanium carbosulfide lower than
0.05%.
As described above, when machinability is evaluated
while the tensile strength is maintained at substantially
the same level, the steels, according to the present
invention, show excellent machinability.
In test Nos. 41 to 46, in which the Nd, Se, Te, Ca, Pb
CA 02243123 1998-07-1~
and Bi contents of the tested steels 41 to 46, respectively,
fall outside respective ranges as specified in the present
invention, machinability is favorable, but fatigue strength
and/or toughness is inferior to that of test Nos. 2 to 7, in
which the tested steels 2 to 7 contain these elements in
amounts falling within respective ranges, as specified in
the present invention.
As seen from Tables 5 to 8, in the steels according to
the present invention, excellent balance between
machinability and fatigue strength is attained when the
maximum diameter of a titanium carbosulfide is 0.5 ~lm to 7
~m, and the index of cleanliness in terms of titanium
carbosulfide is 0.08% to 2.0%. Further, when bainite or a
combination of ferrite and bainite accounts for at least 90%
of microstructure, good balance between strength and
toughness is established. When martensite accounts for at
least 50% of microstructure, balance between strength and
toughness becomes extremely excellent.
(Example 2)
Steels 47 to 54 having chemical compositions shown in
Table 9 were manufactured through a melting process in a 150
kg vacuum melting furnace or a 3-ton vacuum melting furnace.
Steels 47 to 49 were manufactured through a melting process
in the 3-ton vacuum melting furnace, and other steels were
manufactured through a melting process in the 150 kg vacuum
melting furnace. In order to prevent the generation of
titanium oxides, the steels underwent adjustment of the size
CA 02243123 1998-07-1~
and the index of cleanliness of titanium carbosulfide. This
adjustment was carried out by adding Ti, after various
elements had been added, subsequent to sufficient
deoxidization with Si and Al. Steels 47 to 54 in Table 9
are examples of the present invention, and contain each
component element in an amount falling in a range specified
by the present invention.
CA 02243123 1998-07-15
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56
CA 02243123 1998-07-1~
Next, each of the steels was hot forged, such that the
steel was heated to a temperature of 1250~C and then
finished at a temperature of 1000~C, to obtain a round bar
ha~ing a diameter of 60 mm. The hot-forged round bars were
cooled to a temperature of 400~C, at a cooling rate of
5~C/min to 35~C/min by air cooling or atmospheric cooling,
thereby adjusting tensile strength through attainment of a
microstructure which is primarily composed of ferrite and
pearlite.
Test pieces for use in various tests were obtained
from each of the round bars at a position as deep as R/2
from the surface of the round bar in a manner similar to
that of Example 1. The obtained test pieces were JIS No.
14A tensile test pieces, Ono-type rotating bending fatigue
test pieces (diameter of straight portion: 8 mm; length of
straight portion: 18.4 mm), and JIS No. 3 impact test pieces
(2 mm U-notch Charpy test pieces), which were used for
testing tensile strength, fatigue strength (fatigue limit),
and toughness (impact value), respectively, at room
temperature.
A test piece was obtained from each of the round bars
at a position described as a R/2 site in accordance with Fig.
3 of JIS G 0555. In each of the obtained test pieces, the
mirror-like polished surface to be observed measured 15 mm
(width) by 20 mm (height). The polished surface was
observed through an optical microscope at 400 magnifications
over a range of 60 visual fields. Through the observation,
CA 02243123 1998-07-1~
.~
the index of cleanliness in terms of titanium carbosulfides
in the steel was measured such that titanium carbosulfides
were distinguished from other inclusions, and also the
m~x; mllm diameter of titanium carbosulfides was obtained.
Subsequently, the mirror-like polished surface of each test
piece was etched with nital. The etched surface was
observed through the optical microscope at 100
magnifications so as to observe the state of microstructure,
i.e. to obtain the occupancy rate (area percentage) of
individual constituent phases of microstructure, at the R/2
site. In test Nos. 51 to 53 corresponding to the tested
steels 51 to 53, the ferrite grain size number as specified
in JIS was measured, and the average lamellar spacing of
pearlite was obtained from photographs taken through a
scanning electron microscope.
Also, a drilling test was conducted for evaluation of
machinability. The test conditions and the evaluation
method were similar to those of Example 1.
Table 10 shows the results of the above tests.
58
CA 02243123 1998-07-lS
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C O ~ ~ C~ C
59
CA 02243123 1998-07-1~
As seen from Table 10, in the case of non-heat-treated
type steel products in which ferrite and pearlite account
for at least 90% of microstructure, good balance between
strength and toughness is obtained when at least one of the
following conditions is satisfied: the area percentage of
ferrite is 20% to 70%; the grain size of ferrite in terms of
JIS grain size number is not smaller than 5; and the average
lamellar spacing of pearlite is 0.2 ~m or less. Moreover,
the machinability index assumes a relatively large value
when the value of fnl represented by the aforementioned
equation (1) is greater than 0%, and/or the value of fn2
represented by the aforementioned equation (2) is greater
than 2. When the value of fn2, expressed by the equation
(2), is greater than 2, fatigue strength is also relatively
high.
(Example 3)
Steels 55 to 59 having chemical compositions shown in
Table 11 were manufactured through a melting process in a
150 kg vacuum melting furnace or a 3-ton vacuum melting
furnace. Steels 55 and 56 were manufactured through a
melting process in the 3-ton vacuum melting furnace, and
other steels were manufactured through a melting process in
the 150 kg vacuum melting furnace. In order to prevent the
generation of titanium oxides, the steels underwent
adjustment of the size and the index of cleanliness of
titanium carbosulfide, in this example too. This adjustment
was carried out by adding Ti, after various elements had
CA 02243123 1998-07-15
been added, subsequent to sufficient deoxidization with Si
and Al. Steels 55 to 59 in Table 11 are examples of the
present invention, and contain each component element in an
amount falling in a range specified by the present invention.
61
CA 02243123 1998-07-15
c~ o t-- ~ o c~
c~ . o ~ . c~
C~_ _ C'J O ~
t-- _ O C'J O
o o o o o
. _ _ o
c~I I I
o o
~ o
c~I I o
,~ I o I o
o O
._ c~
~_a) o
3 ~I l O
e
V~i l l l I
O
o I o
o o
C ~
o
~3 o o
~ CD
CL. ~ O
Z I I I . I
o o
C~
~ ~ o I . _
~ ~ Z
x
~ O\ ~
D ~ n ~ ~ Ln ~~
bO O o o O U~ ~
O
zI I I .
I I I . _
c~ o ~ C~
o o o o o I I C')
,zooooOc~_
c o o o o o
._ O -- ~C~~ L~
._ ~ o o o o o +
o o o o o o
e a~ c~ Ln ~ _ ,0
o-- X C~ _C~ O\
o o o o o O
Ln ~D Ln c~
O O O O O ~ +
C~ O O O O O
O O O O O
L~ ~r L~ O
c~r ~ '~ \~ U)
~: . . . . . O~
OC) _ CO _ 00
~-- ~ X _ ~ _ --
_ ~ o
~_ ~ O ~ -- 11 11
O -- --
'' O O O O O
Lt~L~ L~LOO L~
V~
62
CA 02243123 1998-07-1~
Next, each of the steels was hot forged, such that the
steel was heated to a temperature of 1250~C and then
finished at a temperature of 1000~C, to obtain a round bar
having a diameter of 60 mm. The hot-forged round bars were
cooled to a temperature of 300~C, at a cooling rate of
5~C/min to 35~C/min by air cooling or atmospheric cooling,
thereby adjusting tensile strength through attainment of a
microstructure which is primarily composed of bainite, or
ferrite and bainite. In the case of steels 57 and 58, aged
steel was also tested (test Nos. 60 and 61). Specifically,
the hot-forged round bars of steels 57 and 58 were cooled as
above and then aged, i.e. heated at a temperature of 560~C
for 1 hour, followed by air cooling.
Test pieces for use in various tests were obtained
from each of the round bars at a position as deep as R/2
from the surface of the round bar in a manner similar to
that of Example 1. The obtained test pieces were JIS No.
14A tensile test pieces, Ono-type rotating bending fatigue
test pieces (diameter of straight portion: 8 mm; length of
straight portion: 18.4 mm), and JIS No. 3 impact test pieces
(2 mm U-notch Charpy test pieces), which were used for
testing tensile strength, fatigue strength (fatigue limit),
and toughness (impact value), respectively, at room
temperature.
A test piece was obtained from each of the round bars
at a position described as a R/2 site in accordance with Fig.
3 of JIS G 0555. In each of the obtained test pieces, the
63
CA 02243123 1998-07-1~
mirror-like polished surface to be observed measured 15 mm
(width) by 20 mm (height). The polished surface was
observed through an optical microscope at 400 magnifications
over a range of 60 visual fields. Through the observation,
the index of cleanliness in terms of titanium carbosulfides
in the steel was measured such that titanium carbosulfides
were distinguished from other inclusions, and also the
maximum diameter of titanium carbosulfides was obtained.
Subsequently, the mirror-like polished surface of each test
piece was etched with nital. The etched surface was
observed through the optical microscope at 100
magnifications so as to observe the state of microstructure,
i.e. to obtain the occupancy rate (area percentage) of
individual constituent phases of microstructure, at the R/2
site.
Also, a drilling test was conducted for evaluation of
machinability. The test conditions and the evaluation
method were similar to those of Example 1.
Table 12 shows the results of the above tests. Table
12 also contains the conditions of aging treatment conducted
on steels 57 and 58 in test Nos. 60 and 61.
64
CA 02243123 1998-07-15
C
O O ~~,
o
~ ~ ~ C_
o a~
C ~ ~'_
~ bC
~ ~C ~ ~ b - I I I I I I I ~- ~
C ~ , ~ C
e O ~a
. _ . _
._ ~ O ~ ) ~ C
~3 ~ C'J C'3 0 C'J O o c~
_C ~~ o
:~ .
tu
~ cd eca \ O
~-- _C ~-- ~ o
o O CQ 0 ~ CD -- C-- ~c~ ~ ='
~- cd ~--
--~ ~C t~ ~~~~~~~3~
V c ~ ~~ ~ r-- O C~ ~ ~ ~ ~ C
c
c_ o
C ~ ~ ~ ~ ~ o C~ C~ ~C c
._bc~ Q ~, CD 'i' c~ ~ 0 0 ~D ~ c
aC, ~ 0 0 0 oo 0 0 0 ~ ~-
E-- ~-- (V C
~ o o o o o o o
al -- ~a~
C~
~ ~o C~ o o o~ o o
C.> ~ o~ Lr~1-- o o C~ o o U~
_ o
o ~, ~ o o o o o o o C
~_ _ ~'_
C~
\ o~ C~o o C~ o o
C--~
V~ -- ~
c a~ o c
~ o C _ C~-- o ~ -- o
c_a~ c o o o o o o o cCD
~ c ~ c~ c
~c~ ~, e ~ _ C~ 0 ~ ~ C~ ~ ~q COD
.e ~ ~~
C
CLJ ~ C~ ~-- 0 C~ C ~ ~o
c~ c c a~
~ ~ C
o a~ o
C~_ Z
CA 02243123 1998-07-1~
As seen from Table 12, in the case of non-heat-treated
type steel products in which bainite, or ferrite and bainite
account for at least 90% of microstructure, good balance
between strength and toughness is obtained by conducting
aging treatment after hot working and subsequent cooling.
Moreover, the machinability index assumes a relatively large
value when the value of fnl expressed by the aforementioned
equation (1) is greater than 0%, and/or the value of fn2
expressed by the aforementioned equation (2) is greater than
2. When the value of fn2, represented by the equation (2),
is greater than 2, fatigue strength is also relatively high.
(Example 4)
Steels 60 to 64 having chemical compositions shown in
Table 13 were manufactured through a melting process in a
150 kg vacuum melting furnace or a 3-ton vacuum melting
furnace. Steels 60 and 61 were manufactured through a
melting process in the 3-ton vacuum melting furnace, and
other steels were manufactured through a melting process in
the 150 kg vacuum melting furnace. In order to prevent the
generation of titanium oxides, the steels underwent
adjustment of the size and the index of cleanliness of
titanium carbosulfide, in this example too. This adjustment
was carried out by adding Ti, after various elements had
been added, subsequent to sufficient deoxidization with Si
and Al. Steels 60 to 64 in Table 13 are examples of the
present invention, and contain each component element in an
amount falling in a range specified by the present invention.
66
T a b 1 e 1 1 3
Steel Chemical composition (percent by weight) Balance: Fe and unavoidable impurities
C Si Mn P S Ti Al N Cu Ni Cr Mo V Nb B Nd Se Te Ca Pb Bi fnl fn2 fn3
0.36 1.28 1.06 0.020 0.087 0.05 0.020 0.0020 - - 0.20 - - - 0.0003 - - - - 0.08 - -0.05 0.57 1.93
61 0.31 1.22 1.14 0.020 0.165 0.61 0.021 0.0027 - - 1.04 - - - - 0.038 - - - 0.14 - 0.41 3.70 2.93 o
62 0.20 0.93 1.13 0.021 0.192 0.54 0 021 0.0015 0.70 - 0.80 - - - - - - - 0.002 - 0.05 0.31 2.81 2.50 r
63 0.20 1.17 1.34 0.018 0.025 0.35 0.021 0.0013 - - 0.80 0.04 0.17 - - - 0.21 0.02 - - - 0.32 14.1 2.83
64 0 33 1.32 1.11 0.015 0.091 0.12 0.025 0.0015 - - 0.18 - - - 0.0003 - - - - 0.09 - 0.01 1.32 1.97
f n 1 = T i (%) - 1 . 2 S (%) ~ f n 2 = T i (%) / S (%)
f n 3 = O . 5 S i (%) + M n (%) + 1 . 1 3 C r (%) + 1 . 9 8 N i (%)
CA 02243123 1998-07-1~
Next, each of the steels was hot forged, such that the
steel was heated to a temperature of 1250~C and then
finished at a temperature of 1000~C, to obtain a round bar
having a diameter of 60 mm. The hot-forged round bars were
cooled to a temperature of 300~C, at a cooling rate of
5~C/min to 35~C/min by air cooling or atmospheric cooling.
Subsequently, the hot-forged round bars were heated at a
temperature of 850~C to 900~C for 1 hour, followed by water
quenching. The water-quenched round bars were tempered at a
temperature of 550~C (followed by air cooling) so as to
adjust their microstructures and strengths.
Test pieces for use in various tests were obtained
from each of the round bars at a position as deep as R/2
from the surface of the round bar in a manner similar to
that of Example 1. The obtained test pieces were JIS No.
14A tensile test pieces, Ono-type rotating bending fatigue
test pieces (diameter of straight portion: 8 mm; length of
straight portion: 18.4 mm), and JIS No. 3 impact test pieces
(2 mm U-notch Charpy test pieces), which were used for
testing tensile strength, fatigue strength (fatigue limit),
and toughness (impact value), respectively, at room
temperature.
A test piece was obtained from each of the round bars
at a position described as a R/2 site in accordance with Fig.
3 of JIS G 0555. In each of the obtained test pieces, the
mirror-like polished surface to be observed measured 15 mm
(width) by 20 mm ~height). The polished surface was
CA 02243123 1998-07-1~
observed through an optical microscope at 400 magnifications
over a range of 60 visual fields. Through the observation,
the index of cleanliness in terms of titanium carbosulfides
in the steel was measured such that titanium carbosulfides
were distinguished from other inclusions, and also the
m~X;mum diameter of titanium carbosulfides was obtained.
Subsequently, the mirror-like polished surface of each test
piece was etched with nital. The etched surface was
observed through the optical microscope at 100
magnifications so as to observe the state of microstructure,
i.e. to obtain the occupancy rate (area percentage) of
individual constituent phases of microstructure, at the R/2
site.
Also, a drilling test was conducted for evaluation of
machinability. The test conditions and the evaluation
method were similar to those of Example 1.
Table 14 shows the results of the above tests. Table
14 also contains quenching and tempering conditions for
steels 60 to 64.
69
CA 02243123 1998-07-15
bO
a~ --
~- ~ O O O O O
bCO ~ 2
o
$ C ~ ~ 't_
t~ ~ C
C~ ~C ~ C,~ O O C O O
O ~ ~ a) o ~ ~ ~ ~
c~.C ~- 3 _~ 0 a) 0CO C~ ._
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o _c~ X ~:r -
a CD ;D CD c~
cr s
a c~ ~ ~ ~ CD c
CA 02243123 1998-07-1~
As seen from Table 14, in the case of heat-treated
type steel products in which martensite accounts for at
least 50% of microstructure, extremely excellent balance
between strength and toughness is obtained. Moreover, the
machinability index assumes a relatively large value when
the value of fnl represented by the aforementioned equation
(1) is greater than 0%, and/or the value of fn2 represented
by the aforementioned equation ~2~ is greater than 2. When
the value of fn2, expressed by the equation (2), is greater
than 2, fatigue strength is also relatively high.
(Example 5)
A portion of each of steels 1, 6, 36 to 40, 47 to 49,
55, 56, 60 and 61, which were manufactured through a melting
process in a 3-ton vacuum melting furnace, as described in
examples 1 to 4, was hot forged, such that the steel was
heated to a temperature of 1250~C and then finished at a
temperature of 1000~C, followed by atmospheric cooling to
room temperature, thereby obtaining a square bar 125 mm
square.
Next, each of the square bars was hot die forged, such
that the square bar was heated to a temperature of 1250~C
and then finished at a temperature not less than 1000~C.
The hot-die-forged square bars were cooled to a temperature
of 300~C, at a cooling rate of 5~C/min to 35~C/min by air
cooling or atmospheric cooling, in order to obtain near net
shape products of crankshafts. The thus-obtained near net
shape products were machined to obtain finished crankshafts.
~ CA 02243123 1998-07-1~
For test Nos. 68, 69, 73, 79 and 80, the hot-die-forged
square bars were cooled as above and then heated at a
temperature of 890~C to 900~C for 1 hour, followed by water
quenching. The water-quenched square bars were tempered at
a temperature of 550~C ~followed by air cooling) to obtain
near net shape products of crankshafts. The thus-obtained
near net shape products were machined to obtain finished
crankshafts.
In machining the near net shape products in order to
obtain finished crankshafts, there was used the coated
carbide insert having the shape as defined by the
designation code CNMG12041N-UX in JIS. The machining was of
dry type and carried out at a cutting speed of 100 m/min, a
depth of cut of 1.5 mm, and a feed of 0.25 mm/rev.
Subsequently, an oil hole was drilled in each of the
crankshafts through use of a 6 mm-diameter straight shank
drill of high speed tool steel, JIS SKH59, and a water-
soluble lubricant, at a feed of 0.20 mm/rev and a revolution
of 980 rpm. In the oil-hole-drilling, there was counted the
number of drilled crankshafts until the drill became
disabled due to failure of the top cutting edge of the drill.
The number of the drilled crankshafts was defined as a
machinability index indicative of machinability of steel.
A test piece was obtained from each of the crankpins
(70 mm diameter) of the above-mentioned near net shape
products of crankshafts in accordance with Fig. 3 of JIS G
0555 and with respect to the reference line which passes a
CA 02243123 1998-07-1~
.
position as deep as 15 mm from the surface of the crankpin.
In each of the obtained test pieces, the mirror-like
polished surface to be observed measured 15 mm (width) by 20
mm (height). The polished surface was observed through an
optical microscope at 400 magnifications over a range of 60
visual fields. Through the observation, the index of
cleanliness in terms of titanium carbosulfides in the steel
was measured such that titanium carbosulfides were
distinguished from other inclusions, and also the maximum
diameter of titanium carbosulfides was obtained.
Subsequently, the mirror-like polished surface of each test
piece was etched with nital. The etched surface was
observed through the optical microscope at 100
magnifications so as to observe the state of microstructure,
i.e. to obtain the occupancy rate (area percentage) of
individual constituent phases of microstructure. Further,
test pieces were obtained from each of the crankshafts, in
parallel with the axial direction of the crankshaft. The
obtained test pieces were JIS No. 14A tensile test pieces,
Ono-type rotating bending fatigue test pieces (diameter of
straight portion: 8 mm; length of straight portion: 18.4 mm),
and JIS No. 3 impact test pieces (2 mm U-notch Charpy test
pieces), which were used for testing tensile strength,
fatigue strength (fatigue limit), and toughness (impact
value), respectively, at room temperature.
Table 15 shows the results of the above tests. Table
15 also contains quenching and tempering conditions for test
CA 02243123 1998-07-1
Nos. 68, 69, 73, 79, and 80.
As seen from Table 15, the near net shape products of
crankshafts manufactured from the steel products according
to the present invention show excellent machinability.
Moreover, the crankshafts manufactured from the steel
products according to the present invention are superior, in
balance between strength and toughness, to the crankshafts
manufactured from the steel products of the comparative
examples.
74
CA 02243123 1998-07-15
bCO
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CA 02243123 1998-07-1
INDUSTRIAL APPLICABILITY
Since the steel products of the present invention have
excellent machinability and excellent balance between
strength and toughness, they can be used as steel stocks of
structural steel parts for a variety of machinery such as
transportation machinery including automobiles, machinery
for industrial use, construction machinery, and the like.
Various kinds of structural steel parts for machinery can
relatively readily be manufactured from the steel products
of the present invention through machining.
76
