U.S. patent application number 10/519981 was filed with the patent office on 2006-03-09 for material for sliding parts having self lubricity and wire material for piston ring.
Invention is credited to Etsuo Fujita, Kunichika Kubota, Yoshiki Masugata, Yoshihiro Minagi.
Application Number | 20060048865 10/519981 |
Document ID | / |
Family ID | 29996943 |
Filed Date | 2006-03-09 |
United States Patent
Application |
20060048865 |
Kind Code |
A1 |
Fujita; Etsuo ; et
al. |
March 9, 2006 |
Material for sliding parts having self lubricity and wire material
for piston ring
Abstract
A material for use as self-lubricating sliding parts, which
consists of a steel containing, by mass, from not less than 0.4% to
less than 1.5% of C (carbon), 0.1 to 3.0% of Si, 0.1 to 3.0% of Mn,
from inclusive zero to 0.5% of Cr, 0.05 to 3.0% of Ni, 0.3 to 2.0%
of Al, 0.3 to 20% in total (Mo+W+V) of at least one element
selected from the group consisting of Mo, W (tungsten) and V
(vanadium), and 0.05 to 3.0% of Cu, wherein there can be observed
graphite particles having an average particle size of not more than
3 .mu.m in a section of the metal structure of the steel. The steel
is also used as piston rings. In the steel, the graphite particles
observed in the structural section occupy an area rate of not less
than 1% in the overall area of the structural section, and have an
average particle size of not more than 3 .mu.m. The steel may
further contain not more than 0.3% of S and/or not more than 0.01%
of Ca. The steel is subjected to nitriding treatment for use.
Inventors: |
Fujita; Etsuo; (Yasugi,
JP) ; Kubota; Kunichika; (Yasugi, JP) ;
Masugata; Yoshiki; (Yonago, JP) ; Minagi;
Yoshihiro; (Yasugi, JP) |
Correspondence
Address: |
ANTONELLI, TERRY, STOUT & KRAUS, LLP
1300 NORTH SEVENTEENTH STREET
SUITE 1800
ARLINGTON
VA
22209-3873
US
|
Family ID: |
29996943 |
Appl. No.: |
10/519981 |
Filed: |
June 30, 2003 |
PCT Filed: |
June 30, 2003 |
PCT NO: |
PCT/JP03/08309 |
371 Date: |
January 3, 2005 |
Current U.S.
Class: |
148/332 ;
420/91 |
Current CPC
Class: |
C22C 38/44 20130101;
C22C 38/04 20130101; C22C 38/60 20130101; C22C 38/08 20130101; C22C
38/02 20130101; C22C 38/34 20130101; C22C 38/58 20130101; C22C
38/12 20130101; C22C 38/06 20130101 |
Class at
Publication: |
148/332 ;
420/091 |
International
Class: |
C22C 38/42 20060101
C22C038/42; C22C 38/44 20060101 C22C038/44 |
Foreign Application Data
Date |
Code |
Application Number |
Jul 1, 2002 |
JP |
2002-191722 |
Claims
1. A material for use as self-lubricating sliding parts, which
consists of a steel comprising, by mass, from not less than 0.4% to
less than 1.5% of C (carbon), 0.1 to 3.0% of Si, 0.1 to 3.0% of Mn,
from inclusive zero to 0.5% of Cr, 0.05 to 3.0% of Ni, 0.3 to 2.0%
of Al, 0.3 to 20% in total (Mo+W+V) of at least one element
selected from the group consisting of Mo, W (tungsten) and V
(vanadium), and 0.05 to 3.0% of Cu, wherein there can be observed
graphite particles having an average particle size of not more than
3 .mu.m in a section of the metal structure of the steel.
2. A material according to claim 1, wherein the graphite particles
observed in the structural section occupy an area rate of not less
than 1% in the overall area of the structural section, and have an
average particle size of not more than 3 .mu.m.
3. A material according to claim 1, wherein no vanadium carbides
are observed in the structural section.
4. A material according to claim 1, wherein the steel contains, by
mass, 0.3 to 5.0% in total (Mo+W) of at least one element selected
from the group consisting of Mo and W, and less than 0.1% of V.
5. A material according to claim 1, wherein the steel contains, by
mass, 0.7 to 2.0% of Al.
6. A material according to claim 1, wherein the steel contains, by
mass, 1.5 to 3.0% of Mo.
7. A material according to claim 1, wherein the steel contains, by
mass, not more than 10% of Co.
8. A material according to claim 1, wherein the steel contains, by
mass, not more than 0.3% of S (sulfur).
9. A material according to claim 8, wherein the steel further
contains, by mass, not more than 0.01% Ca.
10. A material according to claim 1, wherein the steel has been
subjected to nitriding treatment to use as sliding parts.
11. A wire material for use as piston rings, which consists of a
steel comprising, by mass, from not less than 0.4% to less than
1.5% of C (carbon), 0.1 to 3.0% of Si, 0.1 to 3.0% of Mn, from
inclusive zero to 0.5% of Cr, 0.05 to 3.0% of Ni, 0.3 to 2.0% of
Al, 0.3 to 20% in total (Mo+W+V) of at least one element selected
from the group consisting of Mo, W (tungsten) and V (vanadium), and
0.05 to 3.0% of Cu, wherein there can be observed graphite
particles having an average particle size of not more than 3 .mu.m
in a section of the metal structure of the steel, and wherein
sulfide inclusions observed in the structural section, being
parallel to the periphery of the piston ring, are distributed such
that straight lines each passing through a major axis of the
respective sulfide inclusion cross one another within a cross angle
of not more than 30 degrees which angle is referred to as a degree
of parallelism.
12. A wire material according to claim 11, wherein graphite
particles observed in a section of the metal structure occupy an
area rate of not less than 1% in the overall area of the structural
section, and have an average particle size of not more than 3
.mu.m.
13. A wire material according to claim 11, wherein the steel
contains, by mass, not more than 10% of Co.
14. A wire material according to claim 11, wherein the steel
contains, by mass, not more than 0.3% of S (sulfur).
15. A wire material according to claim 14, wherein the steel
further contains, by mass, not more than 0.01% of Ca.
16. A wire material according to claim 11, wherein the steel has
been subjected to nitriding treatment to use as piston rings.
Description
TECHNICAL FIELD
[0001] The present invention relates to material for use as sliding
parts, e.g. piston rings, cylinder liners and vanes, which are
incorporated in automotive engines or other internal combustion
engines, as well as usual plain bearings, roller bearings, ball
bearings, gears and dies.
BACKGROUND ART
[0002] Materials having excellent wear resistance property have
been applied so far to sliding parts such as cylinder liners and
vanes. Materials for piston rings used in internal combustion
engines, especially automotive engines, have been changed from cast
steel to steel wire, which is processed to ring forms. In usual,
the piston rings are produced in such a manner that an ingot with a
predetermined composition is hot-worked to wire by forging,
hot-rolling or the like, the thus obtained wire is further formed
to have a given cross-sectional form corresponding to a small
sectional form of a piston ring by drawing or the like, the formed
wire is conditioned to have a given hardness, and subsequently the
wire is subjected to bending working so as to be a ring form with a
predetermined radius of curvature.
[0003] Presently, there are three types of piston ring, which are a
top ring, a second ring and an oil ring. These are attached to a
piston in the above order from the side of a combustion chamber of
an internal combustion engine. Since the top and oil rings are
operated under especially severe conditions, employment of steel
material has been developed for advanced functionalization in
Japan. Such an employment of steel material is a response to
demands for further improvement of internal combustion engines in
these days. That is, advanced functionalization of internal
combustion engines are requested in response to progress of
researches on post-internal combustion engines such as electric
vehicles. A demand for enhancement of sliding performance has been
also intensified in order to deal with diesel engines, which are
driven under severe conditions, since diesel engines with a higher
internal pressure make an environmental burden smaller than
gasoline engines with the provisions that light oil is upgraded and
that exhaust gas filters are more functionalized.
[0004] By the way, recent researches have been directed also to
internal phenomena of engines and elucidated that second rings made
of cast steel are subjected to heaviest abrasion among the three
piston rings, as reported by the under-mentioned non-patent
literature No. 1.
[0005] The other reasons for application of steel material to
piston rings in place of cast iron are that in order to employ thin
rings, which enables advanced environmental performance of engines,
for reduction of a power loss due to a friction force, it is
necessary to provide material excellent in mechanical strength and
wear resistance property, and that employment of harmless nitriding
treatment also accelerates application of steel rings, because of
correspondence to severe environmental controls as well as easiness
of technology transfer and development of ring-manufacturing
process, in place of chrome plating, which has been mainly employed
as surface-finishing of cast iron rings.
[0006] According to one proposal, a steel piston ring is made
resistant to abrasion and seizure by subjecting its surface, which
is brought into contact with a cylinder liner, to surface-treatment
such as nitriding (see the patent literature No. 1). There has been
proposed also another method for improvement of wear resistance
property without surface-treatment (see the patent literature No.
2). [0007] *Non-patent literature No. 1: Hideki Saitoh et al.
"Researches on abrasion of diesel engines under severe driving
conditions", Research report lectures (1999), Kyushu Branch of
Japan Society Of Mechanical Engineers (Internet <URL:
http://www.ns.kogakuin.ac.Jp/.sup.-wwa1013/EGR/nagasaki/nagasaki.html>-
) [0008] *Patent literature No. 1: JP-10-030726 A [0009] *Patent
literature No. 2: JP-58-046542 B2
[0010] Steel piston rings are outstandingly excellent in mechanical
properties and wear resistance property than those of cast iron
rings, but have poor seizure resistance property, which is one of
reasons that second rings of steel have not been made good
progress. Researches for improvement of seizure resistance property
are based on reformation of a surface, e.g. nitriding a surface of
a steel ring, which comes in contact with a cylinder liner, as
reported in the patent literature No. 1. But, such unsolved
problems still remain as a cost for surface-treatment and aluminum
adhesion which occurs at the interface between a piston and a
piston ring.
[0011] There has been proposed another method for improvement of
seizure resistance property without surface-treatment, as disclosed
in the patent literature No. 2, according to which an alloy
contains not less than 10% of Cr in order to increase a rate of
chromium carbide in a steel matrix. Although an significant
improvement in wear resistance property can be attained by such an
increase of chromium carbide, there are some concerns in
productivity, which include a small improvement in seizure
resistance property and deterioration in machinability. As
countermeasures against such disadvantages, the patent literature
No. 2 proposes surface-treatments such as PVD, whereas an increase
of a manufacturing cost cannot be avoided according to such
surface-treatment.
[0012] Moreover, in the case of using light oil or even gasoline as
fuel, sometimes they contain comparatively much amount sulfur
depending upon those quality. When such fuel containing a much
amount of sulfur is used in internal combustion engines, piston
rings are exposed to a sulfuric dewing atmosphere due to formation
of SO.sub.4.sup.2- from sulfur contained in the fuel. Therefore,
piston rings are also required to be resistant to sulfuric acid
corrosion, and thus the requirement to piston rings for improved
corrosion resistance property is becoming more strict than it
was.
DISCLOSURE OF INVENTION
[0013] The present invention is proposed under the above problems,
and an object of which is to provide material for sliding parts and
wire material for piston rings, wherein the material for sliding
parts should be provided with excellent wear resistance property by
improving seizure resistance property and nitriding treatment, and
wherein the wire material should have excellent property of
corrosion resistance to sulfuric acid, excellent productivity and a
low friction coefficient.
[0014] The inventors have researched and examined sliding motions
of sliding parts, which are exposed to a sliding atmosphere in a
state of fluid lubrication, represented by environments of piston
rings, in detail. As a result, the inventors have discovered an
optimum metal structure suitable for improvement of seizure
resistance property and a decrease in a friction coefficient as
well as a chemical composition suitable for realization of the
metal structure resistant to sulfuric acid corrosion.
[0015] Thus, according to a first aspect of the invention, there is
provided a material for use as self-lubricating sliding parts,
which consists of a steel comprising, by mass, from not less than
0.4% to less than 1.5% of C (carbon), 0.1 to 3.0% of Si, 0.1 to
3.0% of Mn, from inclusive zero to 0.5% of Cr, 0.05 to 3.0% of Ni,
0.3 to 2.0% of Al, 0.3 to 20% in total (Mo+W+V) of at least one
element selected from the group consisting of Mo, W (tungsten) and
V (vanadium), and 0.05 to 3.0% of Cu, wherein there can be observed
graphite particles having an average particle size of not more than
3 .mu.m in a section of the metal structure of the steel.
[0016] According to one preferable embodiment, the graphite
particles observed in a section of the metal structure occupy an
area rate of not less than 1% in the overall area of the structural
section, and have an average particle size of not more than 3
.mu.m. More preferably, no vanadium carbides are observed in the
structural section.
[0017] Further, preferably the steel contains 0.3 to 5.0% in total
of at least one element selected from the group consisting of Mo
and W, and less than 0.1% V. A preferable Al content is within a
range of 0.7 to 2.0%. Preferably, the steel may further contain 1.5
to 3.0% of Mo and/or not more than 10% of Co. Preferably, S
(sulfur) and Ca contents of the steel are controlled to be not more
than 0.3% and not more than 0.01%, respectively. Preferably, the
steel is subjected to nitriding treatment to use as sliding
parts.
[0018] According to a second aspect of the invention, there is
provided a wire material for use as piston rings, which consists of
a steel comprising, by mass, from not less than 0.4% to less than
1.5% of C (carbon), 0.1 to 3.0% of Si, 0.1 to 3.0% of Mn, from
inclusive zero to 0.5% of Cr, 0.05 to 3.0% of Ni, 0.3 to 2.0% of
Al, 0.3 to 20% in total (Mo+W+V) of at least one element selected
from the group consisting of Mo, W (tungsten) and V (vanadium), and
0.05 to 3.0% of Cu, wherein there can be observed graphite
particles having an average particle size of not more than 3 .mu.m
in a section of the metal structure of the steel. A piston ring
made of the wire material has a metal structure in which sulfide
inclusions observed in the structural section, being parallel to
the periphery of the piston ring, are distributed such that
straight lines each passing through a major axis of the respective
sulfide inclusion cross one another within a cross angle of not
more than 30 degrees which angle is referred to as a degree of
parallelism. According to one preferable embodiment, the graphite
particles observed in a section of the metal structure occupy an
area rate of not less than 1% in the overall area of the structural
section, and have an average particle size of not more than 3
.mu.m.
[0019] The wire material for use as piston rings preferably
contains not more than 10 mass % of Co, and further contains not
more than 0.3 mass % of S (sulfur) and not more than 0.01 mass % of
Ca. Preferably it is subjected to nitriding treatment to use as
piston rings.
[0020] A key aspect of the invention is realization of the
particular metal structure of steel, in which fine graphite
particles are precipitated in a steel matrix by a proper rate, in
order to improve seizure resistance property of steeland lower a
friction coefficient of sliding parts such as piston rings. Namely,
the present invention aims at improvement of durability fully
taking peculiar frictional motions between piston rings and
cylinders into consideration. According to the invention metal
structure, it is possible to attain satisfactory advantageous
effects in the above posed subjects which are to improve seizure
resistance property of steel and lower a friction coefficient, and
on which the prior art has been unsatisfactory even in connection
with non-surface-treatment material or nitrided material which is
advantageous in a cost of surface-treatment.
[0021] With regard to the invention metal structure with graphite
precipitates, the present inventors sought for chemical components
which enable fully rapid precipitation of fine graphite particles
and also be effective in improvement of workability and
machinability of steel in order to establish the metal structure as
industrially applicable means. Fruitful results are that further
improvement in the above effects could be attained by addition of a
single element of sulfur or of sulfur and Ca to the steel. This is
another key feature of the present invention.
[0022] First, the invention metal structure with graphite
precipitates will be described.
[0023] A sliding part is mainly designed for fluid lubrication,
wherein a fluid film such as oil or water is constantly formed
between mechanical elements which are brought into sliding contact
with each other under severe conditions. Formation of the fluid
film leads to application of buoyancy to a relatively moving fluid,
as noted by relative motion of an airplane in the air. The fluid
film between sliding parts becomes thicker as viscosity rise of the
fluid or an increase of a relative velocity, so that mechanical
elements are protected from abrasion.
[0024] However, most of internal combustion engines are the
reciprocation type that a relative velocity between a piston ring
and a cylinder becomes zero in the vicinity of upper and lower dead
points, so that sliding parts suffer often wear and/or seizure due
to break of the fluid film, resulting in abnormal motion of
internal combustion engines. Furthermore, since inhibition of oil
inflow into a combustion chamber, so-called as "oil scraping",
becomes an important factor for advanced performance of piston
rings, it is difficult to maintain the fluid film in a state
suitable for the purpose.
[0025] Nevertheless, the inventors have reviewed various fluid
lubricating modes during sliding motion for effective use of fluid
lubrication in order to overcome the above problems. In short, the
fluid lubricating mode comprises three actions, i.e. (1) wedging,
(2) expansion/contraction, and (3) squeezing. The squeezing action
(3) is effective even in the state that a relative velocity becomes
zero. The squeezing action can be explained as follows, on the
presumption that a solid plate is sliding on a base plate in
presence of a fluid. On such a presumption, distribution of a
pressure, which acts on a surface of the solid plate facing to the
base plate, is under the boundary condition that a pressure becomes
zero at an edge of the solid plate. The pressure distribution shall
be varied according to a domed function in order to generate a
positive pressure distribution necessary for maintenance of
lubrication. Such pressure distribution is represented by the
formula of: .differential. 2 .differential. x 2 .times. P +
.differential. 2 .differential. y 2 .times. P < 0 Formula
.times. .times. 1 ##EQU1##
[0026] In the formula 1, P is a pressure, x is a sliding direction,
and y is a distance along a direction perpendicular to the sliding
direction. Reynolds equation, i.e. a principal equation for fluid
lubrication, is represented by the formula of: .differential.
.differential. x .times. ( .rho. .times. .times. h 3 12 .times.
.eta. .times. .differential. P .differential. x ) + .differential.
.differential. y .times. ( .rho. .times. .times. h 3 12 .times.
.eta. .times. .differential. P .differential. y ) = .times. u 2
.times. .differential. ( .rho. .times. .times. h ) .differential. x
+ .times. .rho. .times. .times. h 2 .times. .differential. u
.differential. x + .times. .differential. ( .rho. .times. .times. h
) .differential. t Formula .times. .times. 2 ##EQU2##
[0027] In Formula 2, .rho. is a density of a fluid, .eta. is a
thickness of a fluid film, .eta. is a viscosity coefficient, t is a
time, and u is a relative velocity. A condition necessary for
generation of a positive pressure in the fluid film can be written
by Formula 3 according to Formulas 1 and 2. u 2 .times.
.differential. ( .rho. .times. .times. h ) .differential. x + .rho.
.times. .times. h 2 .times. .differential. u .differential. x +
.differential. ( .rho. .times. .times. h ) .differential. t < 0
Formula .times. .times. 3 ##EQU3##
[0028] Formula 3 has three terms. First and second terms, which
involve the relative velocity u, correspond to the wedging and the
expansion/contraction, respectively, mentioned as the above. The
third term, which does not involve the relative velocity u,
corresponds to the squeezing action, which is expected to be
effective even under the condition that the relative velocity
becomes zero between a piston ring and a cylinder.
[0029] The condition that the third term is negative has the
physical meaning that a fluid film rapidly decreases in thickness
with the provision that the fluid density is constant, resulting in
generation of a positive pressure in the fluid film. Such a
phenomenon is practically realized by abruptly applying a vertical
load to a solid plate, which is sliding on a base plate, so as to
squeeze the fluid film. Consequently, a high positive pressure is
simultaneously generated by squeezing the fluid film, and the
sliding plate hardly comes in direct contact with the base plate.
In short, the squeezing action is realized.
[0030] The inventors have found that the squeezing action is
intensified by reforming a sliding surface to a structure, which
includes many fine pores. Fine pores in the sliding surface retain
a fluid therein and instantaneously supply the fluid therefrom to a
dry surface even under the condition that the fluid film is
collapsed at a relative velocity being zero. A significant decrease
in thickness of the fluid film, which is originated in movement of
the fluid, leads to the squeezing action. As a result, seizure is
inhibited in the vicinity of upper and lower dead points during
reciprocating motion, and a friction coefficient is also
decreased.
[0031] The graphite-precipitated structure according to the present
invention is determined in order to achieve the above actions and
effects. That is, graphite particles not only act as a solid
lubricant but also promote formation of oil-retaining pores after
dropout thereof. The pores realize the squeezing action suitable
for retention of an oil film. The squeezing action, which ensures
formation of a stable oil film regardless of pressure fluctuations,
is intensified by presence of pores on a sliding surface, as
mentioned above. Precipitation of graphite particles is exactly
effective for intensification of the squeezing action, and the
effectiveness is ensured for normal sliding parts, e.g. sliding
bearings, roller bearings or ball bearings and also for sliding
parts, e.g. piston rings, cylinder liners, shims of valve lifters,
cams, gears, dies or cutting blades, which are difficult to
constantly form such a fluid lubricant film due to significant
fluctuations of a pressure.
[0032] The graphite-precipitated structure is also effective for
inhibition of adhesive wear, which have become a problem recently,
in the case where it is applied to a piston ring attached to an
aluminum piston. Since aluminum is scarcely soluble in carbon,
adhesive reaction is suppressed.
[0033] The invention material for use as sliding parts has the
structure that graphite particles are distributed therein. It is
important to control graphite particles, which are observed in a
structural section, to a size of not more than 3 .mu.m in average.
If an average size exceeds 3 .mu.m, graphite particles are often
damaged at peripheries during sliding motion, and graphite debris
unfavorably invade sliding planes. The distribution of graphite
particles is more effective under the condition that graphite
particles observed in a section of the metal structure occupy an
area rate of not less than 1% in the overall area of the structural
section. With regard to relatively large graphite particles of not
less than 1 .mu.m, it is more preferable to make the graphite
particles to have an average size of not more than 5 .mu.m or an
area rate of not more than 5% in the overall area of the structural
section.
[0034] Formation of pores, which is effective for lubrication but
leads to reduction of a fluid film due to the above function and
become ineffective in the end, has not been regarded as an
important factor for a fluid lubricating design. However, the
invention means is effective especially for internal combustion
engines, which involve reciprocating motion with difficulty to
continuously form a fluid film. For instance, graphite precipitates
become more effective under the condition that irregular frictional
behaviors, wherein the temporary state that a fluid film is
collapsed in the vicinity of upper and lower dead points at a
relative velocity being nearly zero is turned to a state with a
plenty of lubricating oil, are repeated between piston rings and
cylinders.
[0035] A lubricating design, which enables retention of a fluid
film, is important especially under the condition that the fluid
film breaks temporarily due to structural reasons, represented by
relative motion between piston rings and cylinder liners.
Regardless of the lubricating design, possibility of solid contact
rises in correspondence with changes in rotating speeds of engines
or structures of sliding parts. In this regard, application of
material, wherein graphite particles effective for solid
lubrication are dispersed, to such irregular sliding parts ensures
sufficient lubrication under various sliding conditions.
[0036] An alloy design of the invention steel, which is formed to
sliding parts, will be understood from the following
explanation:
[0037] There are reports on graphitic steels from of old, but most
of such reports are about Si or Ni-alloyed steels. Besides, it
takes several tens hours or longer to hold steels at an elevated
temperature of not lower than 600.degree. C. for precipitation of
graphite particles. According to the present invention, an element
Al, which accelerates decomposition of cementite, is alloyed
together with Ni at a proper ratio in order to complete
precipitation of graphite particles in a several hours.
[0038] Since carbon is generally precipitated as semi-stable
cementite in a steel matrix in prior to precipitation of graphite,
cementite shall be decomposed and changed to stable graphite in a
graphitizing process. Conventional graphitic steels have the
disadvantage that cementite is hardly decomposed, so that a fairly
long time is required for precipitation of graphite particles. On
the other hand, the present invention employs an alloy design,
wherein elements such as Cr, which impede decomposition of
cementite, are controlled so as to complete decomposition of
cementite in a short time, even when semi-stable cementite is
precipitated. As a result, carbon is precipitated as graphite at
once without substantial formation of cementite.
[0039] A diffusion velocity of pores is raised by addition of Al,
which has a high diffusion velocity in steel. The higher diffusion
velocity accelerates aggregation of pores, which serve as sites for
precipitation of graphite particles. Consequently, precipitation of
graphite particles is completed in a short time due to the effect
of Al and the rapid aggregation of pores. Furthermore,
precipitation of graphite particles in a surface layer only is
facilitated by nitriding or the like.
[0040] Addition of Al is also suitable for an alloy design of a
nitriding hardened steel, since Al is a nitriding hardening
element. Another element Cr, which has the same nitriding hardening
effect, unfavorably impedes precipitation of graphite particles as
a fundamental technological gist of the present invention and also
causes significant degradation of corrosion resistance to sulfuric
acid. In this sense, addition of Cr is avoided as much as possible,
but Al is alloyed as the most important element for the
purpose.
[0041] Nitriding of graphite steel has been regarded as material
inappropriate for precipitation of graphite, since precipitation of
graphite particles in the nitriding steel is accompanied with the
disadvantage that a nitrided layer is embrittled by presence of
graphite particles of not less than 10 .mu.m in size, which act as
faults, in the nitrided layer. According to the present invention,
the disadvantage is suppressed by reforming graphite precipitates
to fine particles.
[0042] Reformation of graphite precipitates to fine particles may
be achieved by either one of (1) introduction of work strains to
divide graphite precipitates, (2) inclusion of Al.sub.2O.sub.3 or
the like and (3) dispersion of BN, TiC or the like, which serves as
a site for precipitation of graphite. However, the method (1) puts
restrictions on manufacturing conditions, and the method (2) needs
difficult processing for dispersion of Al.sub.2O.sub.3 or the like.
The remaining method (3) also needs difficult processing as for
high-carbon steel, since proper dispersion of BN, TiC is achieved
only by strict control of trace components. As for the known
reformation, dispersion of TiC is disclosed by JP-11-246940 A, and
precipitation of BN as a site for precipitation of graphite is
disclosed by Iwamoto et al., "Iron and Steel" vol. 84 (1998), p.
57. But, any method requires heat-treatment for precipitation of a
secondary phase in a high-temperature zone of 1000.degree. C. or
higher in order to raise a diffusion velocity, so that it is hardly
applicable to high-alloy steel, wherein alloying elements are
likely to significantly aggregate, due to difficulty in uniform
distribution of fine graphite particles.
[0043] The inventors have studied precipitation of fine graphite
particles from various aspects and discovered that precipitation of
a Cu--Al intermetallic compound in a steel matrix is effective for
the purpose. The Cu--Al intermetallic compound, i.e. a secondary
phase, which serves as a site for precipitation of graphite, is
precipitated at a relatively low temperature of not higher than
800.degree. C., so as to enable formation of a stable structure
with fine graphite particles in a short time. Since Cu and Al
contents are controlled at levels for inhibiting embrittlement
according to the present invention, a graphitic structure is formed
as a lubricant phase without degradation of mechanical strength.
Moreover, the additive Cu is also effective for improvement of
corrosion-resistance to sulfuric acid.
[0044] Although there are many proposals heretofore on distribution
of graphite particles in cast steel for lubrication, material for
use as sliding parts has been changed from cast steel to
surface-treated steel in response to deterioration of environments,
to which various sliding parts are exposed. But, in a case of
cylinder blocks, which mostly comprise aluminum parts, cast steel
with a graphite-precipitated structure is still used as material
for inner walls of cylinder liners aiming at the above effects. A
sliding part of the present invention is characterized by the alloy
design suitable for employment of steel material, which still has
properties of cast steel, in order to impart mechanical strength,
wear resistance property and corrosion-resistance to sulfuric acid,
which are necessary to cope with deterioration of environments, as
well as sliding properties. Chemical components of the sliding part
will be understood from the following explanation:
[0045] Carbon is an important element, a part of which
solution-hardens a steel matrix, another part of which is
precipitated as carbides and the remaining of which is precipitated
as graphite. Carbide and graphite precipitates improve wear
resistance and seizure resistance properties. At least 0.4 mass %
of C is necessary for realization of such effects. However, excess
carbon of not less than 1.5 mass % unfavorably lowers a melting
temperature of the carbides, so that a metal structure is hardly
homogenized by diffusion annealing, e.g. heating an ingot at around
1200.degree. C. for several tens hours, for elimination of
solidification segregation. In this sense, a carbon content is
determined within a range of not less than 0.4 mass % but less than
1.5 mass %, preferably 0.5 mass % or more but less than 1.3 mass
%.
[0046] Si is added as a conventional deoxidizing agent and also as
an accelerator for precipitation of graphite. Si is also effective
for improvement of corrosion resistance to sulfuric acid. In this
regard, a lower limit of Si is determined at 0.1 mass %. Moreover,
the additive Si suppresses softening of steel during annealing, and
the effect of Si is important especially in low-alloy steel. A Si
content is preferably determined at a value of not less than 1.0
mass % in order to raise high-temperature strength without
annealing softening. However, an upper limit of Si is controlled to
3.0 mass %, since excess Si unfavorably raises a Al temperature.
Therefore, the Si content is determined within a range of 0.1 to
3.0 mass %, preferably 0.5 to 3.0 mass %, more preferably 1.0 to
3.0 mass %.
[0047] Mn is added as the same deoxidizing agent as Si. At least
0.1 mass % of Mn is necessary for deoxidation, but excess Mn
impedes precipitation of graphite. In this sense, an upper limit of
Mn is controlled to 3.0 mass %, and a Mn content is determined
within a range of 0.1 to 3.0 mass %.
[0048] Cr is an effective nitriding hardening element, but
unfavorably suppresses decomposition of semi-stable cementite and
strongly impedes precipitation of graphite. Also Cr significantly
deteriorates corrosion resistance property to sulfuric acid.
Therefore, an upper limit of Cr content is controlled to 0.5 mass.
In this sense, a Cr content is determined within a range of 0 to
0.5 mass %, preferably 0 to 0.3 mass %.
[0049] Ni is an accelerator for precipitation of graphite and also
effective for inhibition of red shortness, which often occurs in
Cu-alloyed steel, but unfavorably raises solubility of carbon in
Fe, resulting in poor workability in an annealed state. Therefore,
a Ni content is determined within a range of 0.05 to 3.0 mass %,
preferably 0.6 to 1.5 mass %.
[0050] Al is an element effective for raising nitriding hardness as
well as Cr. Since an increase of Cr is necessarily avoided in the
invention alloy design, nitriding hardness is ensured at a value
suitable for the purpose by addition of Al. The element Al acts as
a graphite former, promotes diffusion of pores and also forms a
Cu--Al phase, which serves as a site for precipitation of graphite,
together with Cu. Namely, Al is an effective element for
precipitation of fine graphite particles in a short time, so that
an Al content shall be not less than 0.3 mass %. An upper limit of
Al is controlled to 2.0 mass %, since an increase of Al raises an
Al temperature as well as Si. Therefore, an Al content is
determined within a range of 0.3 to 2.0 mass %, preferably 0.7 to
2.0 mass %.
[0051] Mo is a carbide former, which does not impede precipitation
of graphite so much in comparison with Cr but improves
heat-resistance of steel. Molybdenum carbide restrains a steel
matrix at a thermoforming step, which follows a bending step in a
piston ring-manufacturing process, resulting in improvement of
dimensional stability. However, excess Mo impedes decomposition of
cementite as well as Cr.
[0052] Nevertheless, the effect of Mo on impedance of
graphitization is weak, but the additive Mo remarkably improves
heat-resistance and dimensional stability during heat-treatment.
Especially in a piston ring-manufacturing process, which involves
heat-treatment of fine wire, the effect of Mo on dimensional
stability is important for suppressing deviations of abutment
profiles. In this sense, Mo is added at a ratio of 0.3 mass % or
more. On the other hand, an upper limit of Mo is controlled to 20
mass %, since precipitation of graphite is impeded as an increase
of Mo. V and W have the same effects as Mo. Therefore, at least one
element selected from the group consisting of Mo, W and V is added
at a ratio within a range of 0.3 to 20 mass % in total.
[0053] It is preferable to form a metal structure, wherein vanadium
carbide is not observed in a structural section, even in vanadium
alloyed steel, since precipitation of graphite is significantly
impeded by vanadium carbide. In the case where V is added solely or
together with both Mo and W, a ratio of V is preferably controlled
to a value less than 0.1 mass % with 0.3 to 5.0 mass % in total of
Mo and W. The element Mo intensifies a squeezing action of graphite
and promotes formation of a fluid film at a high pressure,
resulting in improvement of seizure resistance property and a
decrease in a kinetic friction coefficient. Furthermore, sulfuric
acid corrosion resistance property is improved by addition of Mo.
Therefore, the amount of a single additive Mo is preferably
controlled within a range of 1.5 to 3.0 mass %.
[0054] Cu is an important element as well as Al, for precipitation
of a Cu--Al intermetallic phase and rapid formation of a stable
structure with fine graphite particles. The additive Cu is also
effective for improvement of sulfuric acid corrosion-resistance. In
this sense, it is necessary to control a ratio of Cu in relation
with Al, and a Cu content is determined at a value of not less than
0.05 mass %, preferably not less than 0.2 mass % for realizing
effects of Cu and the Cu--Al phase. However, excess Cu causes an
increase of hardness in an annealed state and degrades workability
of steel, so that an upper limit of Cu is controlled to 3.0 mass %.
Therefore, a Cu content is determined within a range of 0.05 to 3.0
mass %, preferably 0.2 to 3.0 mass %.
[0055] By the way, sulfur is conventionally added as an organized
extreme-pressure additive to engine oil, which is supplied to an
internal combustion engine, for improvement of lubrication and
inhibition of seizure. The inventors have hit upon inclusion of
sulfide MnS in a steel matrix on the contrary. The sulfide serves
as a sulfur source for forming an in situ sulfide film on a fresh
plane, which is exposed by frictional heat, and the sulfide film
effectively improves lubricating performance. According to the
invention means, excellent lubricating performance is almost
permanently ensured due to distribution of the lubricant in the
steel material without necessity of adding a plenty of a lubricant
for improvement of lubricity at predetermined parts or without
disappearance of lubricating performance, which often occurs during
exchange of engine oil containing the extreme-pressure
additive.
[0056] Another conventional means for an increase of chromium
carbide in steel for use as a piston ring aims at reduction of a
surface area of a piston ring, which comes in contact with a
cylinder liner, and enhancement of wear resistance property of the
piston ring, to which a sliding energy is applied at a high rate
per unit area, in order to balance abrasion between the piston ring
and the cylinder liner. Although seizure resistance property is
improved by distribution of chromium carbide, distribution of
chromium carbide is directed to prevention of partial bearing from
abnormal rising, by such a situation, which is essentially caused
by non-uniform contact, as to promote abrasion of the cylinder
liner for increase of a contact area. In short, the distribution of
chromium carbide makes the piston ring compatible with the cylinder
liner at the beginning of attachment, but becomes ineffective on
abrasion properties, e.g. adhesive abrasion, with durability.
[0057] Excessive improvement of wear resistance property leads to
the situation that the cylinder liner is attacked by the piston
ring. If the cylinder liner is extremely attacked, a clearance
unfavorably becomes larger, resulting in an increase of a blowby
rate, which corresponds to a volume of exhaust gas. On the other
hand, the additive sulfur has effects on improvement of seizure
resistance property due to a decrease in a friction coefficient
without accelerated abrasion of the steel material, so that an
internal combustion engine is driven for a long while without
substantial change of a clearance.
[0058] Namely, the invention material for use as sliding parts is
further improved in seizure resistance property by addition of
sulfur at a proper ratio. The element sulfur is mostly formed to
MnS by reaction with Mn, and the reaction product MnS acts on
engine oil as a lubricant to exhibit lubricity. Consequently, a
friction coefficient is decreased, and seizure resistance property
is improved.
[0059] Seizure is the phenomenon that rubbing surfaces are clung
together due to transfer of atoms therebetween. The transfer of
atoms is promoted by thermal oscillation in the state that the
rubbing surfaces are heated at a high temperature due to frictional
heat. A temperature of the rubbing surface is represented by a
monotonously increasing function in relation with a friction
energy, i.e. (a friction coefficient.times.bearing.times.a slip
velocity. That is, as a decrease in a friction coefficient, a
temperature hardly rises, resulting in improvement of seizure
resistance property. Addition of sulfur is effective for such a
decrease in the friction coefficient, but excess sulfur causes
degradation of mechanical properties with the fear that steel wire
would be broken down in a drawing step for manufacturing steel
piston rings. Therefore, an upper limit of sulfur is controlled to
0.3 mass %. A sulfur content is preferably determined within a
range of 0.01 to 0.3 mass %, more preferably 0.03 to 0.3 mass
%.
[0060] The inventors have also found that an increase of a forging
rate, which is applied to material containing up to 0.3 mass % of
sulfur in a manufacturing process, effectively improves mechanical
properties of sliding parts. That is, the mechanical properties are
upgraded as an increase of the forging rate. Especially when steel
piston rings are manufactured by bending steel wire, the increase
of a forging rate advantageously prevents the steel wire from
fracture and breakage during bending.
[0061] The forging rate is defined by a sectional ratio of an ingot
to a product profile in a piston ring-manufacturing process. The
forging rate is represented by a ratio of (a sectional area of an
unforged ingot)/(a sectional area of a bent product), with respect
to a section of steel material perpendicular to a forging or
drawing direction or a small section of a piston ring as a final
product. But, a sectional reduction ratio from steel wire to a
piston ring product is negligible small for realization of the
above effects, so that the forging rate may be evaluated by a ratio
of (a sectional area of an unforged ingot)/(a sectional area of
steel wire, which is forged and drawn but unbent). As the forging
rate is higher, the material is more heavily forged.
[0062] Steel, which distributes sulfide MnS therein, originally has
the cast structure that there are many spheroidal or spindled
sulfide inclusions with random orientation at triple points of
grain boundaries in a cellular solidification structure.
Orientation of the sulfide inclusions are gradually changed as an
increase of the forging rate, resulting in improvement of
mechanical properties.
[0063] As an increase of the forging ratio, sulfide inclusions are
more oriented along a longitudinal direction of steel wire and
elongated in a state corresponding to a peripheral stress, which is
mainly applied to a piston ring. Consequently, unfavorable effects
of sulfide inclusions on mechanical properties are substantially
eliminated. Degradation of mechanical properties is typically
prevented by reforming sulfide inclusions to a shape with an aspect
ratio (a major axis size/a minor axis size) of 3 or more. In other
words, poor orientation of sulfide inclusions with an aspect ratio
of 3 or more along a peripheral direction leads to degradation of
mechanical properties.
[0064] Concretely, distribution of sulfide inclusions, especially
sulfide inclusions with an aspect ratio of 3 or more, which are
microscopically observed on a surface structure parallel to a
periphery of a piston ring, is controlled to the state that a
parallelism (an angle at an acute side) between straight lines,
each of which passes through a major axis of a separate sulfide, is
held within a range of not more than 30 degrees, in order to
provide steel wire useful as piston rings or material useful as
sliding parts. In this sense, the forging rate is preferably
determined at a value of 500 or more.
[0065] FIG. 5 is a set of schematic views, which illustrate
microstructures of unforged steel with a forging rate of 1 (as
cast) and forged steel with a forging rate of 500 in an unetched
state observed by an optical microscope, and schemes for explaining
measurement of a parallelism of sulfide inclusions. Two of sulfide
inclusions with an aspect ratio of 3 or more are arbitrarily
selected, an angle at an acute side between straight lines (a
line-A and a line-B), each of which passes through a major axis of
a separate sulfide, is measured, and the measurement is repeated
over a whole of the microscopic view. The same measurement is
further repeated for at least ten microscopic views. A maximum
value among the measured angles is evaluated as the parallelism. In
the case where there is no intersection (as noted in the forged
steel with a forging rate of 500 in FIG. 5), a line-A' parallel to
the line-A may be regarded as an auxiliary line. Herein, sulfide,
which is observed as a connected particle in a 400 times
microscopic view, is regarded as a separate inclusion, and a
straight line, which passes through a major axis of the separate
inclusion, is determined as a measuring line.
[0066] In FIG. 5, the unforged steel with a forging rate of 1 has
the structure that sulfide inclusions are distributed with a
parallelism more than 30 degrees, but the forged steel with a
forging rate of 500 has the structure that any parallelism is
controlled to a value of not more than 30 degrees. In fact, the
figure of 30 degrees is a designed value according to rupture
mechanics. FIG. 6 is a graph, which illustrates analytical results
by G. R. Irwin, "Analysis of Stresses and Strains Near the End of a
Crack Transversing a Plate", Trans. ASME, Ser. E, J. Appl. Mech.,
Vol. 24, No. 3 (1957), pp. 361-364, for explaining how to change a
stress intensity factor in the state that cracks propagate along a
direction different from a stress direction. The analytical results
are represented by the formula of:
K.sub.1=(1-cos.sup.2.beta.).sigma. {square root over (.pi..alpha.)}
Formula 4
[0067] In Formula 4, K.sub.1 is a a stress intensity factor, .beta.
is an angle between a stress direction and a crack-propagating
direction, .sigma. is a stress, and .sigma. is a length of a crack.
Formula 4 indicates that propagation of a crack, which is
perpendicular to a stress direction (at .beta.=90 degrees), is
facilitated, while a crack, which extends along a stress direction
(at .beta.=0), does not propagate so much. Facilitation of
propagation (i.e. an abrupt increase of a stress intensify factor)
corresponds to an angle of 30 degrees. Since the inclusions can be
regarded as cracks due to poor kinetic bonding strength, it is
understood that distribution of the inclusion with controlled
deviation of orientation within a range of not more than 30
degrees, i.e. orientation arrangement of elongated inclusions, is
important to inhibit propagation of cracks.
[0068] Since sulfur is a representative element unfavorable for
mechanical properties of steel, proper means for improvement of
strength is desired to make steel material applicable to piston
rings. For instance, a proposal of JP-07-258792 A, which allows at
most 1 mass % of sulfur, principally relates to cast steel, which
is formed to a cylinder liner or else with an insufficient forging
rate. However, a practical process for manufacturing steel piston
rings at a economical cost is achieved by metal forming such as
drawing, rolling and bending. If such steel containing up to 1 mass
% of sulfur is processed to wire for use as piston rings by the
metal forming, the steel may be broken down in a drawing step due
to shortage of material strength necessary for the metal forming.
As a result, it is difficult to manufacture steel piston rings with
high reliability.
[0069] According to the present invention as mentioned the above,
material, which contains not more than 0.3 mass % of sulfur, is
preferable for use as sliding parts in order to further improve
seizure resistance property. Controlled addition of sulfur is
typically meaningful in wire material, which is formed to a product
profile with a high forging rate, for use as piston rings.
[0070] The effect of sulfur is more enhanced by addition of Ca
together with sulfur. The element Ca, which has a strong reducing
power, is included in MnS, so that Ca is likely to ooze out onto a
seized surface. Ooze of Ca inhibits formation of oxides on the
seized surface but facilitates formation of lubricious sulfides.
However, excess Ca is unfavorable for hot-workability, so that an
upper limit of Ca is preferably controlled to 0.01 mass %. A Ca
content is preferably determined within a range of 0.0001 to 0.01
mass %, more preferably 0.0005 to 0.01 mass %, for achievement of
the above effects.
[0071] Addition both of sulfur and Ca is also effective for
improvement of machinability and grindability other than seizure
resistance property. Especially, distribution of MnS and
precipitation of graphite particles improve machinability of steel.
Due to the improved machinability, a corner of steel material is
machined to an objective profile with a small radius of curvature,
so that piston rings with a high oil-scraping power can be
manufactured with ease.
[0072] The invention material for use as sliding parts and piston
rings may contain Co for improvement of corrosion-resistance,
especially sulfuric acid corrosion-resistance. The element Co as
well as Mo intensifies a squeezing effect of graphite and promotes
formation of a fluid film at a high pressure, resulting in
improvement of seizure resistance property and a decrease in a
kinetic friction coefficient. Such effects of Co are noted at a
ratio of not less than 0.5 mass %. But Co is an expensive element,
and further improvement is not expected by excess Co. Therefore, a
Co content is preferably controlled to not more than 10 mass %,
more preferably within a range of 2 to 5 mass %.
[0073] The invention steel material for use as sliding parts and
piston rings contains the above elements at specified ratios and
the balance being substantially Fe. Other elements are controlled
to not more than 10 mass %, preferably not more than 5 mass %, in
total.
[0074] The invention steel material may further contain one or more
of the following elements within specified ranges of: [0075]
P.ltoreq.0.1 mass %, Mg.ltoreq.0.01 mass %, B.ltoreq.0.01 mass %,
Zr.ltoreq.0.1 mass %,
[0076] A preferable condition of the present invention is to make
the metal structure to contain nonmetallic inclusions occupying an
area rate of not more than 2.0% in the overall area of the
structural section, whereby preventing fracture in a drawing
process during forming steel material to wire and occurrence of
breakage during forming the wire to a coil. The specified structure
is typically suitable for a piston ring-manufacturing process
accompanied with forming and processing fine wire, in order to
establish a manufacturing process with high productivity.
[0077] Nitriding further improves seizure resistance and wear
resistance properties, as an additional effect in the present
invention. Nitriding may be combined with other surface treatment
such as PVD or Cr-plating, since excellent seizure resistance
property is imparted to steel material regardless of
surface-treatment. Take a piston ring as an example. Such
surface-treatment is conventionally applied to a main sliding
surface of the piston ring, which comes in contact with a cylinder
liner, but un-applicable to its friction surface, which cmes in
contact with a piston. In short, inhibition of adhesive abrasion
can not be expected by the conventional surface-treatment. However,
the invention material, which has excellent seizure resistance
property and resists to adhesive reaction without necessity of
surface-treatment, is extremely useful as piston rings.
[0078] The invention material may be subjected to intercalation
processing, whereby foreign molecules or ions are inserted into a
laminar molecular structure of graphite for further improvement of
sliding characteristics by immersion in a CuCl.sub.2 solution for
instance, due to its metal structure with a graphite phase.
Moreover, graphite particles in the intercalation-processed state
act as a polymerization catalyst. Therefore, the material is
reformed to a state suitable for polymerization of a lubricating
oil by a polymer coat (coating with a polymer film) or
intercalation-processing as pre-treatment, in order to provide
sliding parts, which maintain self-lubricity originated in
polymerizing reaction during sliding motion.
BRIEF DESCRIPTION OF DRAWINGS
[0079] FIG. 1 is a microphotograph illustrating distribution of
graphite particles, which are observed on a section of the
invention material (Specimen No. 3);
[0080] FIG. 2 is another microphotograph illustrating distribution
of graphite particles, which are observed on a different section of
the same material as the material of FIG. 1;
[0081] FIG. 3 is a microphotograph illustrating distribution of
graphite particles, which are observed on a section of a
comparative material (Specimen No. 14);
[0082] FIG. 4 is another microphotograph illustrating distribution
of graphite particles, which are observed on a different section of
the same material as the material of FIG. 3;
[0083] FIG. 5 is a set of schematic views of microstructures for
explaining parallelism of sulfide inclusions;
[0084] FIG. 6 is a graph for explaining an effect of an angle
between a stress direction and a crack-propagating direction on a
stress intensify coefficient;
[0085] FIG. 7 is a schematic view for explaining a frictional
abrasion test at a super-high pressure;
[0086] FIG. 8 is a schematic view for explaining a reciprocating
abrasion test; and
[0087] FIG. 9 is Stribeck's diagram illustrating a relationship
between a reciprocal number of a load and a kinetic friction
coefficient for explanation of lubrication.
EMBODIMENTS
[0088] The other features of the present invention will be clearly
understood from the following examples.
EXAMPLE 1
[0089] Several steels were melted in a high-frequency induction
furnace in the open air, adjusted to chemical compositions in Table
1, and cast to ingots of 220 mm.times.220 mm in section size. In
Table 1, Specimen No. 1 to 6 satisfy definitions of the present
invention. Specimen No. 11 to 16 are comparative steels, wherein
Specimen No. 16 corresponds to JIS SUS440B, used for conventional
piston rings. TABLE-US-00001 TABLE 1 Chemical compositions (mass %)
Specimen Mo + No. C Si Mn Cr Al S Mo W V Mo + W W + V Ca Ni Cu Co
Fe 1 0.42 1.85 2.83 0.25 0.31 0.29 0.5 <0.01 <0.01 0.5 0.5
0.0002 2.96 0.1 <0.01 bal. 2 0.66 0.93 1.21 0.45 0.78 0.13 2.0
2.9 1.2 4.9 6.1 0.0010 0.2 2.8 <0.01 bal. 3 0.79 1.52 1.98 0.31
1.48 0.06 7.2 5.1 <0.01 12.3 12.3 0.0030 0.3 1.1 <0.01 bal. 4
0.98 2.32 1.03 0.21 1.03 0.12 7.5 10.0 0.5 17.5 18.0 0.0060 0.5 1.3
<0.01 bal. 5 1.42 2.78 1.01 0.01 1.91 0.03 2.9 3.0 <0.01 7.9
7.9 0.0098 2.1 0.8 <0.01 bal. 6 0.88 1.55 1.05 0.03 1.03
<0.01 1.05 <0.01 <0.01 1.05 1.1 <0.0001 0.8 0.8
<0.01 bal. 11 0.55 1.49 0.71 0.21 <0.01 <0.01 1.0 2.0
<0.01 3.0 3.0 0.0003 <0.05 <0.05 <0.01 bal. 12 1.00
0.25 0.31 1.48 1.53 <0.01 1.8 3.4 <0.01 5.2 5.2 <0.0001
<0.05 <0.05 <0.01 bal. 13 0.55 1.51 0.72 1.01 <0.01
0.35 4.8 9.9 <0.01 14.7 14.7 0.0002 <0.05 0.8 <0.01 bal.
14 0.78 1.23 0.46 0.03 1.34 0.06 4.2 2.1 <0.01 6.3 6.3 0.0050
2.53 <0.05 <0.01 bal. 15 0.55 1.35 0.55 0.60 <0.01 0.31
0.9 0.9 <0.01 1.8 1.8 <0.0001 1.44 <0.05 <0.01 bal. 16
0.80 0.31 0.40 17.5 <0.01 <0.01 1.0 <0.01 <0.01 1.0 1.0
<0.0001 <0.05 <0.05 <0.01 bal.
[0090] Each ingot was hot-worked to wire material of 9 mm.times.9
mm in section size at a forging rate of approximately 598, except
Specimen No. 13. Specimen No. 13 was forged but was not formed to a
test piece due to fracture during hot-working in succession to
forging.
[0091] The wire material was annealed and then subjected to quench
and tempering under predetermined conditions so as to moderate its
hardness to around 45HRC. A surface structure of the quenched and
tempered wire material was observed in an unetched state by an
optical microscope for measuring distribution of graphite
particles, i.e. an average particle size and an area rate of
graphite particles, which shared the surface structure. The
distribution of graphite particles was investigated by image
analysis of ten views, which were observed by a 1000 times optical
microscope. A size of a graphite particle was represented by a
diameter of a real circle, which had the same area as an inspected
graphite particle. Specimen No. 1 to 6 had the structure that
graphite particles of 0.3 to 2 .mu.m in average size were
distributed with the area rate of 0.5 to 5%.
[0092] FIGS. 1 to 4 are microphotographs illustrating distribution
of graphite particles in Specimen No. 3 and 14. Precipitation of
fine graphite particles is detected in a matrix of Specimen No. 3,
but graphite particles in a matrix of Specimen No. 14 are coarse.
The difference in particle size between Specimen No. 3 and 14 is
explained as follows: Since Specimen No. 4 contains Cu and Al at
proper ratios, fine Cu--Al intermetallic particles precipitate in
prior to precipitation of graphite and act as sites for
precipitation of graphite, resulting in fine graphite particles. On
the other hand, the Cu--Al intermetallic phase is ineffective in
Specimen No. 14 due to shortage of Cu or Al, so as to allow growth
of graphite to coarse particles. Table 2 shows distribution of
graphite particles in all the Specimens including Specimen No. 3
and 14. Distribution of graphite particles was not detected in any
matrix of Specimen No. 11, 12, 15 and 16.
[0093] Each specimen was ion-nitrided 5 hours at 530.degree. C. in
an atmosphere of H.sub.2:N.sub.2=1:1 and used as a test piece for
evaluation of seizure resistance and wear resistance properties.
Seizure resistance property was evaluated by a frictional abrasion
test at a superhigh pressure, using a frictional abrasion tester
shown in FIG. 7 under the following conditions. A rotating torque
of an opposite part, which was held in abrasive contact with the
test piece, was measured. A time, at which the rotating torque
abruptly rose, was regarded as initiation of seizure, and a load at
the time was evaluated as a scuffing load. A kinetic friction
coefficient was calculated from a rotating torque of the opposite
part at a load of 10 MPa. In FIG. 7, the numeral 1 is a test piece,
the numeral 2 is an opposite part, and the mark F is a load,
respectively. TABLE-US-00002 A profile of a square of 5 mm .times.
5 mm a sliding surface: in size A friction velocity: 2 m/second A
pressure applied an initial pressure of to a friction surface: 1.5
MPa an increase rate of 0.5 MPa/minute A lubricant oil: motor oil
#30 The lubricating oil was dropped at a rate of 10 cm.sup.3/minute
only at an initial stage but stopped thereafter. An opposite part:
JIS FC250 (grey cast iron with hardness of 100 HRB)
[0094] Wear resistance property was evaluated by a reciprocating
abrasion test, wherein a test piece of 8 mm in diameter and 20 mm
in length was rubbed with an opposite part (FC250) of 20 mm in
diameter by reciprocating motion for measuring a wearout width of
the test piece. A reciprocating abrasion tester is schematically
illustrated in FIG. 8, while the other abrasion conditions are
under-mentioned. In FIG. 8, the numeral 1 is a test piece, the
numeral 2 is an opposite part, the mark F is a load, and the mark
OIL is a lubricating oil, respectively. TABLE-US-00003 A pressing
load: 500 N A sliding distance 130 mm per cycle: A maximum sliding
0.5 m/second velocity: A lubricating oil motor oil #30 (dropped):
An opposite part: JIS FC250 (grey cast iron with hardness of 100
HRB)
[0095] Table 2 shows measurement results of scuffing loads, kinetic
friction coefficients and wearout widths together with hardness of
nitrided layers. TABLE-US-00004 TABLE 2 Distributon of graphite
Average Hardness of particle nitrided Scuffing Kinetic Width of
Specimen size Area rate Vanadium layer load friction wearout No.
(.mu.m) (%) carbide (HV) (MPa) coefficient (mm) Note 1 0.4 0.8 no
853 12.5 0.11 0.55 Inventive examples 2 0.5 0.9 yes 890 11.5 0.12
0.65 3 0.8 3.5 No 930 10.5 0.07 0.5 4 0.7 2.6 No 910 13.5 0.08 0.55
5 1.9 4.1 No 950 10.5 0.07 0.51 6 0.7 1.9 No 895 12.6 0.06 0.51 11
undetected 0 No 445 6.5 undetected 1.53 Comparative 12 undetected 0
No 773 7.0 undetected 0.71 examples 14 6.2 5.8 No 723 6.0
undetected 0.64 15 undetected 0 No 563 7.0 undetected 1.62 16
undetected 0 No 1032 7.5 undetected 0.53
[0096] Results in Table 2 prove that all the Specimen No. 1 to 6,
which satisfy the definitions of the present invention, are
excellent in seizure resistance and wear resistance properties due
to high scuffing loads and small widths of wearout. Especially,
Specimen No. 3 to 6 have small kinetic friction coefficients and
properties suitable for use as sliding parts. On the other hand,
all the comparative Specimens, which do not satisfy the specified
distribution of graphite particles in the present invention, are
inferior in seizure resistance property. Poor wear resistance
property of Specimen Nos. 11 or 15 is caused by insufficient
nitriding hardness due to shortage of Cr and Al as nitriding
hardening elements.
[0097] Specimens of Example 1, without nitriding treatment, were
subjected to a seizure test under the same condition as the above.
Results are shown in Table 3. Specimen No. 1 to 6, which satisfy
the specified distribution of graphite particles in the present
invention, have excellent seizure resistance property without
necessity of surface treatment. On the other hand, the comparative
Specimen No. 14 has a smaller scuffing load. When a sliding surface
of the Specimen No. 14 was microscopically observed after the
seizure test, fracture of graphite particles at peripheries were
detected. The observation result suggests invasion of graphite
debris into the sliding surface, resulting in degradation of
sliding characteristics. TABLE-US-00005 TABLE 3 Distribution of
graphite Average Specimen particle Area rate Scuffing load No. size
(.mu.m) (%) (MPa) Note 1 0.4 0.8 11.5 Invention 2 0.5 0.9 10.5
Specimen 3 0.8 3.5 11.0 4 0.7 2.6 13.0 5 1.9 4.1 12.0 6 0.7 1.9
11.5 11 Undetected 0 6.5 Comparative 14 6.2 5.8 6.0 Specimen
EXAMPLE 2
[0098] Each of Specimen No. 1 and 15 in Table 1 was hot-rolled to a
coil of 5.5 mm in diameter and then processed to a flat wire
profile of 1.5 mm.times.3.1 mm in section size by drawing and
cold-rolling. Specimen No. 1 was formed to the objective profile
without troubles, but Specimen No. 15 was broken in a drawing step
due to its poor cold-workability. A metal structure of each billet
of Specimen No. 1 and 15 was microscopically observed in an undrawn
state along a direction perpendicular to a rolling direction and
analyzed for measuring the area rate of nonmetallic inclusions. The
area rate of nonmetallic inclusions was 1.86% in Specimen No. 1,
but 2.23% in Specimen No. 15. Comparison of the observation results
indicates that breakage of Specimen No. 15 was caused by excess
nonmetallic inclusions at a ratio above 2.0% in addition to excess
sulfur.
EXAMPLE 3
[0099] Each of Specimen No. 1 to 6, 11 and 12 was processed to a
flat wire profile of 1.5 mm.times.3.1 mm in section size under the
same condition as Example 2, heated 30 minutes at 1000.degree. C.,
quenched and tempered to hardness of around 510 HV. The processed
test piece was machined 10 times by a grinding cutter at a
rotational frequency of 10000 r.p.m and a feed rate of 1 mm/second,
for investigating occurrence frequency of burrs. Table 4 shows test
results on the occurrence frequency of burrs. TABLE-US-00006 TABLE
4 Specimen Occurrence frequency of No. burrs Note 1 0 Invention 2 0
specimens 3 0 4 0 5 0 6 7 11 8 Comparative 12 10 specimens
[0100] Occurrence of burrs was detected in any of Specimen No. 11
and 12, but no burrs occurred in any of Specimen No. 1-5, to which
S was added at a proper ratio. The results prove that addition of S
has remarkable effects on suppression of burrs. Consequently,
piston rings can be manufactured with high productivity.
EXAMPLE 4
[0101] An ingot, which had the same chemical composition as
Specimen No. 1 in Table 1, was separately prepared. The ingot was
hot-worked to wire of 3.0 mm.times.3.0 mm in section size with a
forging rate, which was varied within a range of 1 to 10,000. The
hot-worked wire was conditioned to hardness of 400 HV by quench and
tempering. A parallelism of sulfide inclusions (with an aspect
ratio of 3 or more) in a surface structure, which was parallel to
an lengthwise direction of the elongated wire and served as a
periphery of a piston ring, was measured according to the
above-mentioned procedures.
[0102] The hardness-conditioned wire material was subjected to a
three-point flexure test with a span of 30 mm. A test piece, which
was bent with a deflection up to 10 mm without breakage, was
evaluated as the mark A, while a broken test piece was evaluated as
the mark B. The test results are meaningful for judging feasibility
whether quenched and tempered wire material is formed to a piston
ring with a predetermined curvature by roller bending or not. Table
5 shows the test results. TABLE-US-00007 TABLE 5 Evaluation
Specimen Forging Parallelism of No. rate (degrees) breakage 1-1 1
84.5 B 1-2 10 45.2 B 1-3 500 27.8 A 1-4 2000 11.5 A 1-5 10000 3.5
A
[0103] It is understood from the results in Table 5 that the metal
structure, wherein sulfide inclusions are distributed with a
parallelism of not more than 30 degrees, is excellent in mechanical
properties and effective for suppression of breakage during bending
wire material to a ring profile. The parallelism and the aspect
ratio of sulfide inclusions, which were observed on a surface
structure parallel to a periphery of a piston ring, were not
substantially changed between the wire material and a piston ring,
which was manufactured by bending the wire material.
[0104] The parallelism of sulfide inclusions, which are observed on
a surface structure of wire material, reflects a structure of a
piston ring, which is manufactured from the wire material by
bending. Distribution of sulfide inclusions with a parallelism of
not more than 30 degrees is effective for improvement of mechanical
properties of piston rings, without fears of fatigue fractures,
which often occur in conventional engines. In this sense, the
specified control of the parallelism is especially suitable for
wire material for use as piston rings.
EXAMPLE 5
[0105] Several steels were melted in a high-frequency induction
furnace in the open air, adjusted to chemical compositions in Table
6 and cast to ingots of 220 mm.times.220 mm in section size.
Specimen No. 22 contained Mo at a relatively higher ratio, and
Specimen No. 23 contained Co at a relatively higher ratio, as
compared with Specimen No. 21. TABLE-US-00008 TABLE 6 W, V <
0.01 mass %, Ca < 0.0001 mass % Chemical compositions (mass %)
Specimen No C Si Mn P S Ni Cr Mo Co Cu Al Fe 21 0.86 1.75 0.30
0.008 0.020 0.30 0.001 0.98 0.01 0.82 1.22 Bal. 22 0.86 1.69 0.30
0.010 0.022 0.30 0.001 1.97 0.01 0.76 1.20 Bal. 23 0.86 1.68 0.31
0.008 0.022 0.31 0.001 0.99 3.99 0.87 1.18 Bal.
[0106] Each ingot was hot-worked to wire material of 9 mm.times.9
mm in section size with a forging rate of approximately 598. The
wire material was annealed and then conditioned to hardness of
around 40 HRC by quench and tempering. Thereafter, a surface
structure of the quenched and tempered wire material was observed
in an unetched state for measuring distribution of graphite
particles (an average size of graphite particles and the area rate
of graphite particles, which shared the surface structure). The
distribution of graphite particles was investigated by analyzing 10
images, which were observed by a 1000 times optical microscope.
TABLE-US-00009 TABLE 7 Area rate of graphite Area rate of graphite
particles of 1 .mu.m or Average size of Average size Particles in
the more in size in the graphite of graphite overall area of the
overall area of the particles of Specimen particles structural
section structural section 1 .mu.m or more No. (.mu.m) (%) (%)
(.mu.m) 21 0.55 1.69 0.41 1.15 22 0.50 2.05 0.14 1.20 23 0.75 3.50
0.70 1.33
[0107] Any of Specimen No. 21-23 had a structure with fine graphite
precipitates. Graphite particles of not more than 1 .mu.m in size
were distributed the area rate of 1 to 4%, as noted in Table 7.
Relatively large graphite particles of 1 .mu.m or more had an
average particle size within a range of 1 to 1.5 .mu.m and the area
rate of less than 1% in any of Specimen No. 21 to 23. Namely, it is
understood that most of graphite precipitates are fine particles,
and an the area rate of the relatively large graphite particles to
all the graphite particles was controlled to a value less than
1/4.
[0108] Each specimen was used as a test piece for evaluation of a
kinetic friction coefficient. Evaluation of a kinetic friction
coefficient was performed, using the same frictional abrasion
tester shown in FIG. 7 (except friction velocity) under the
under-mentioned conditions. A kinetic friction coefficient was
calculated from a torque and a load applied to an opposite part, at
every moment when a load rose step by steP. FIG. 9 shows a
relationship between a reciprocal value of a load and a kinetic
friction coefficient. TABLE-US-00010 A profile of a sliding a
square of 5 mm .times. 5 mm surface: A friction velocity: 1
m/second A pressure onto a 1.5 MPa at an initial friction surface:
stage but raised at a rate of 0.5 MPa/minute A lubricating oil:
motor oil #30 continuously dropped at a rate of 10 cm.sup.3/minute
An opposite part: JIS FC250 (grey cast iron)
[0109] FIG. 9 is a diagram, so-called as "Stribeck's diagram", for
representing conditions of a load, which is applied to a
frictionally sliding part, in relation between by load
characteristics (abscissa) and a friction coefficient (ordinate).
Lubricating situations can be evaluated by the diagram. In a case
of the invention examples, the abscissa axis is represented by a
reciprocal value of a load, since a friction velocity is kept at a
constant value of 1 m/second. In each curve of FIG. 9, the side (a
low-stress side, i.e. a range indicated by the arrow) rightward
from a plot (an extreme value), where a friction coefficient is
smallest, corresponds to a region where fluid lubrication is
achieved without damage of a lubricating film. The left side (a
heavy-stress side) corresponds to a region where both of fluid and
solid lubrications occur due to direct contact of solid parts
together. The relation in FIG. 9 indicates that fluid lubrication
can be more achieved without damage of a fluid film even under a
heavy stress, as a plot (an extreme value), where a friction
coefficient is smallest, shifts leftwards in the diagram.
[0110] It is understood from the results in FIG. 9 that Specimen
No. 22, which contains Mo at a larger ratio than Specimen No. 21,
maintains a fluid film under a heavier load, and that Specimen No.
23, which contains Co at a larger ratio than Specimen No. 21, still
maintains a fluid film under a further heavier load. The improved
retention of the fluid film proves the effects of Mo and Co, i.e.
the above-mentioned squeezing effects.
[0111] A plot at the left end of each curve corresponds to a load,
when the frictional abrasion test was stopped due to occurrence of
seizure. The leftist plots of Specimen No. 22 and 23 shifted
leftwards (toward a heavier-stress side) due to addition of Mo and
Co in comparison with Specimen No. 21, resulting in further
improvement of seizure resistance property. Moreover, the kinetic
friction coefficients are decreased as a whole. In short, the
effects of graphite precipitates on sliding characteristics are
further intensified by addition of Mo and/or Co.
[0112] According to the present invention, steel material excellent
in seizure resistance property with a small friction coefficient is
bestowed with self-lubricity without necessity of
surface-treatment, so that the steel material is applicable to
various sliding parts with less energy loss caused by friction.
Moreover, the steel material is processed to piston rings, which
are less aggressive to cylinder liners and pistons, due to
simultaneously controlled distribution of sulfide inclusions.
Consequently, the present invention contributes to remarkable
improvements of internal combustion engines in environmental
ability and durability. Furthermore, the steel material is
processed to sliding parts or piston rings at a saved manufacturing
cost in a short lead time due to its excellent workability and
machinability. Namely, the present invention, which provides
material for use as sliding parts excellent from aspects both of
performance and processing, is a truly profitable technology in an
industrial point of view.
INDUSTRIAL APPLICABILITY
[0113] The material proposed by the present invention is useful as
sliding parts, such as piston rings, cylinder liners or vanes,
which are built in internal combustion engines of automotive
engines or the like, as well as sliding bearings, roller bearings,
ball bearings, gears and dies.
* * * * *
References