Z665
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DESCRIPTION
Title o£ Invention: Steel For Machine Structural Purposes
For Surface Hardening Use and Steel Parts For Machine
5 Structural Purposes and Method of Production of Same
Technical Field
The present invention relates to steel for machine
structural purposes for surface hardening use which is
10 applied to power transmission parts of automobiles etc.,
in particular parts which require high surface fatigue
strength under a corrosive environment, for example,
gears, continuously variable transmissions, constant
velocity joints, hubs, bearings, to steel parts for
15 machine structural purposes, and to a method of
production of those parts.
Background Art
Parts for machine structural purposes, for example,
20 gears of automatic transmissions and sheaves of
continuously variable transmissions, constant velocity
joints, hubs, and other power transmission parts are
required to have a high surface fatigue strength.
In the past, for the above parts, case hardened
25 steels with C of around 0.2% (JIS SCr420, SCM420, etc.)
have generally been used for the material, while a
hardened layer of martensite with C of around 0.8% has
been formed on the surface of the parts by carburized
quenching so as to raise the surface fatigue strength in
30 use.
However, carburization is treatment which takes a
long time of 5 to 10 hours, in some cases more than 10
hours, along with the austenite transformation at the
high temperature of around 950°C, so heat treatment
35 deformation (quenching distortion) due to the crystal
grain coarsening unavoidably becomes greater.
For this reason, in the case of parts for which a
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high precision is demanded, the parts have to be ground,
honed, and otherwise finished after carburized quenching.
Further, steel parts for machine structural purposes
are used in environments in which corroslon resistance is
5 demanded. In this case, the solution employed has been
(a) using as the material a steel material which is
excellent in corrosion resistance in addition to surface
fatigue strength, (b) improving the part properties by
heat treatment, or (c) combining the steel material and
10 heat treatment to improve the part properties.
In recent years, there has been rising demand for
reducing the noise of automobile engines etc. To satisfy
the demands for improvement of the corrosion resistance
and reduction of noise, there are limits with the
15 conventional method of carburization of SCr420, SCM420,
etc.
Therefore, development of steel materials which are
excellent in strength and corrosion resistance by using
induction hardening and ferritic nitrocarborizing and
20 other heat treatment is being studied. In induction
hardening, only the surface layer part is heated in a
short time for transformation to austenite and quenching,
so it is possible to obtain a surface hardened part with
small quenching distortion.
25 However, if trying to obtain a hardness equal to
that of a carburized quenched material by just induction
hardening, 0.8% or more of C becomes necessary. If
increasing the amount of C of the steel material, the
hardness of the inside, where no improvement of the
30 surface fatigue strength is required, rises and the
machineability remarkably deteriorates, so it is not
possible to just increase the amount of C of the steel
material.
For this reason, there are limits to raising the
35 surface fatigue strength by just induction hardening.
Ferritic nitrocarborizing is a surface hardening
method which causes nitrogen and carbon to permeate by
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diffusion into the steel material surface at the
temperature region of 500 to 650°C below the
transformation point so as to form a hardened layer and
improve the wear resistance, seizing resistance, fatigue
5 resistance, etc.
At the surface layer of the steel material, the
nitrogen which penetrates the layer by diffusi9i;; f'JtlUS
nitrides. In general, at the surfacemost layer of the
steel material, a compound layer which is comprised of
10 FesN, Fe4N, and other nitrides is formed. At the inside
from that, a nitrided layer in which N penetrates by
diffusion is formed.
Ferritic nitrocarborizing can be performed at a low
temperature. Further, the treatment time is 0.5 to 5
15 hours or shorter than carburization, so this is often
used for production of steel parts where a low distortion
is required. Furthermore, at the surface layer of the
soft nitrided steel material, the N concentration becomes
higher and the corrosion resistance is improved.
20 However, with just ferritic nitrocarborizing
treatment, the depth of the hardened layer is small, so
it is difficult to apply ferritic nitrocarborizing to the
production of transmission gears etc. to which a high
surfae pressure is applied.
25 Recently, as a technique for making up for the
defects of induction hardening and ferritic
nitrocarborizing and obtaining better mechanical
properties, in particular raising the surface fatigue
strength, the technique of ferritic nitrocarborizing,
30 then induction hardening has been experimented with.
PLT 1 proposes the method of combining induction
hardening and gas ferritic nitrocarborizing to make up
for the respective defects and obtain better mechanical
properties, in particular improve the softening
35 resistance to obtain a higher surface fatigue strength.
However, with the above method, the surface hardness
is high, but the concentration of N in the nitrided layer
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is low, so the hardness at a high temperature is low, a
sufficient softening resistance cannot be obtained at the
surface of a gear etc. which becomes a high temperature
at the time of operation, and a high surface fatigue
5 strength cannot be obtained.
PLT 2 proposes a method of production of combining
induction hardening and fsi^i^itic^; nitrocarborizing to
produce a part for machine structural purposes which is
excellent in mechanical strength. With this method, to
10 make nitrides form a solid solution, induction heating of
900 to 1200°C is performed.
However, there are insufficient elements which have
a high affinity with N and which break down and diffuse
nitrides, so high temperature heating is required.
15 Therefore, an oxide layer is formed at the steel material
surface and the mechanical properties deteriorate.
Further, with the above method, formation of a thick
compound layer is not considered, so it is not possible
to obtain a good surface fatigue strength under a high
20 surface pressure.
PLT 3 proposes the art of combining induction
hardening and nitriding to obtain excellent mechanical
properties. However, with this art, nitriding is
performed at a high temperature of 600°C or more, so the
25 compound layer is thin and, furthermore, the N
concentration is low, so the amount of N which diffuses
upon breakdown of the compound at the time of induction
hardening is small.
In the above art, it is possible to form a compound
30 layer by nitriding, but it is difficult to form a thick,
high N concentration nitrided layer. For this reason,
even if combined with induction hardening, it is not
possible to form a nitrided layer with a high softening
resistance and a good surface fatigue strength.
35 PLT 4 proposes a part for machine structural
purposes which is excellent in mechanical strength and
machineability which is produced by ferritic
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nitrocarborizing, then induction hardening and suitably
balancing the amounts of Mn and S.
However, in the above parts for machine structural
purposes, the mechanical strength, for example^ the
5 durability under a lubrication environment containing
water, is insufficient. In a lubrication environment
containing water, corrosion which occurs at the sliding
surfaces of the steel part forms a starting point for
early breakage of the steel part.
10 For this reason, in steel materials which are used
in such environments, to secure the mechanical strength,
in particular good fatigue properties, it is necessary to
improve the corrosion resistance more.
Normally, gears and other power transmission parts
15 are obtained by forging, then machining the steel
materials (materials) to finish them to the required
shapes then surface hardening them in the next step to
obtain the completed parts.
The proposals in the above PLT's 1 to 4 are arts
20 aimed at raising the strength of the working surfaces by
treating medium carbon steel containing alloy elements
for surface hardening. For this reason, in the above art,
the hardness of the steel material (material) rises more
than necessary. Further, machineability is not
25 considered. Therefore, deterioration of the
machineability cannot be avoided. As a result, the
productivity falls and the manufacturing cost rises.
Therefore, in a steel material which is used for
production of the above steel parts, the task is to
30 suppress the rise in hardness in the steel material after
forging and to secure machineability by addition of
elements facilitating machining so as to thereby achieve
an improvement in the surface fatigue strength of the
surface hardened part.
35
Citations List
Patent Literature
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PLT 1: Japanese Patent Publication (A) No. 06-
172561
PLT 2: Japanese Patent Publication (A) No. 07-
090363
5 PLT 3: Japanese Patent Publication (A) No.
2007-077411
PLT 4: WO2010/082685
Summary of Invention
10 Technical Problem
The present invention, in view of the above
problems, has as its task the development of a technique
for making up for the defects of the low surface hardness
and internal hardness of the steel material by just
15 induction hardening or ferritic nitrocarborizing. It has
as its object to solve the above task by provision of
steel for machine structural purposes for surface
hardening use which is used as a material for a steel
part for machine structural purposes excellent in surface
20 fatigue strength and corrosion resistance which is
provided with a high surface hardness, temper softening
resistance, and machineability which cannot be obtained
by a conventional soft nitrided induction hardened steel
part and, furthermore, is provided with a sufficient
25 lubrication film for the working surfaces, steel parts
for machine structural purposes which are produced by
that steel, and a method of production of those parts.
Solution to Problem
30 To improve the surface fatigue strength of a steel
part, it is effective to (i) improve the surface
hardness, (ii) increase the depth of the steel hardened
layer, and (iii) improve the softening resistance for
maintaining the high temperature strength at working
35 surfaces which become high in temperature (around 300°C).
Further, to prevent a drop in the productivity of
steel parts, it is necessary to raise the machineability
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of the steel material (material) so as to improve the
part workability. Furthermore, to prevent seizing or
sticking of the working surfaces of steel parts, it is
effective to form a sufficient lubrication film at the
5 working surfaces of the steel parts.
TherefOrQ, thO invent*!^* studied improvement of the
machineability and corrosion resistance by surface
hardening of the steel parts by a combination of ferritic
nitrocarborizing and induction heat treatment and
10 adjustment of the composition of ingredients of the steel
material. As a result, they obtained the following
discoveries (a) to (e).
(a) To increase the softening resistance, it is
effective to form a nitrided layer with a high N
15 concentration. With just nitriding treatment, even if
forming a compound layer, formation of a nitrided layer
with a high N concentration and a large thickness is
difficult and increasing the softening resistance is
impossible.
20 To increase the softening resistance, it is
necessary to use the compound layer which is formed at
the time of ferritic nitrocarborizing (layer comprised of
FesN, Fe4N, or other nitrides) as the N source and break
down the nitrides by the subseguent induction heating so
25 as to make a sufficient amount of N diffuse into the
steel material.
FIG. 1 shows by comparison the distribution of
hardness at a cross-section from the surface in a core
direction of a steel material treated by ferritic
30 nitrocarborizing (see "dotted line" in the figure) and
the distribution of hardness at a cross-section from the
surface in a core direction of a steel material treated
by ferritic nitrocarborizing, then induction hardening
(see "solid line" in the figure).
35 As shown in FIG. 1, due to the ferritic
nitrocarborizing, the surfacemost layer of the steel
material is formed with an extremely hard compound layer.
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but the thickness is small. Further, due to the induction
heating, the nitrides break down at the compound layer of
the surfacemost layer, N diffuses tO the insidC/ and the
hardness of the surfacemost layer falls somewhat, but the
5 hardened layer which i§ effective foi Improvement of the
surface fatigue strength (nitrided layer) increases (see
"solid line" in figure).
Note that, in a steel material which is treated by
induction hardening, the surface layer is martensite and
10 the core part is ferilte-pearlite.
If ferritic nitrocarborizing is used to form a
thickness 10 \xnx or more compound layer, induction
hardening may be used to form a nitrided layer with a
high N concentration and large thickness. The compound
15 layer which is formed by ferritic nitrocarborizing
becomes fragile depending on the nitriding conditions and
causes the mechanical properties to deteriorate, so
usually the thickness of the compound layer is reduced.
However, in the present invention, conversely a
20 compound layer of a required thickness is formed and the
properties of the compound layer are actively utilized.
This point is a characterizing feature of the present
invention.
That is, if the compound layer of the required
25 thickness which is formed on the surfacemost layer of the
steel material by ferritic nitrocarborizing is treated by
induction hardening, the structure of the surface layer
of the steel material becomes martensite with a high N
concentration, and the softening resistance at the time
30 of a high temperature is strikingly increased.
(b) To use ferritic nitrocarborizing to form a
thick compound layer, the amount of S which inhibits
bonding of Fe and N is reduced. If making S form a solid
solution in the steel material independently, the S
35 concentrates at the steel material surface and obstructs
the penetration of N.
To prevent the concentration of S at the steel
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material surface, a required amount of Mn is added to the
steel material to immobilize the S as MnS. The amount of
Mn is preferably an amount which satisfies Mn/S>70.
(«) To raise the mchlneslDility of the steel
5 material, an element which facilities machining which
suppresses the rise of hardness of the steel material and
further which contributes to improvement of the surface
fatigue strength as well is added to an extent whereby
the surface fatigue strength after surface hardening
10 treatment does not deteriorate. To suppress a rise in
hardness of the steel material, it is effective to not
excessively add Mn, N, and other alloy elements.
Furthermore, Al and B, which contribute to
improvement of the surface fatigue strength as well while
15 improving the machineability, are compositely added. B
bonds with the N in the steel material to form BN and
contributes to improvement of the machineability at the
time of working the steel material. B forms BN in the
process of cooling with a slow cooling speed, for
20 example, cooling after forging, and does not harden the
steel material, so does not cause deterioration of the
machineability.
At the time of induction heating, the BN breaks down
to form solute B. At the time of quenching, the hardness
25 of the surface layer of the steel material is improved
and the surface fatigue strength of the surface layer of
the steel material is improved.
Al is present in a solute state and remarkably
improves the machineability of the steel material. Al
30 also does not have any effect on the rise of hardness of
the steel material. Al bonds with N to form AIN at the
time of ferritic nitrocarborizing and raises the N
concentration near the surface layer, so also contributes
to improvement of the surface fatigue strength.
35 Furthermore, if compositely adding Al and B, B forms
BN effective for improvement of the machineability and
consumes the N in the steel material. As a result, a
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greater amount of solute Al is present in the steel
material and the machineability is synergistically
improved.
(d) To prevent seizing and sticking of the working
5 surfaces of steel parts, it is effective to provide an
oil reservoir which continuously forms a film of a
lubricant. In the present invention, ferritic
nitrocarborizing is used to form a compound layer at the
surface layer of the steel material then induction
10 heating is used for transformation of this to austenite
and quenching to form a nitrided layer.
FIG. 2 is a view which shows the state of a nitrided
layer which is formed at the surface of a steel material.
FIG. 2 (a) shows the state of the structure of a nitrided
15 layer which is observed by an optical microscope, while
FIG. 2(b) shows the state of the structure of a nitrided
layer which is observed by a scan type electronic
microscope.
The nitrided layer is a hard porous layer in which a
20 large number of pores are formed due to breakdown of the
compound layer and which function as oil reservoirs.
Since there is a hard porous layer at the surface layer
of the steel material, the lubrication effect at the
surface layer of the steel material is improved and the
25 wear resistance and durability are improved more.
By controlling the ferritic nitrocarborizing
conditions and induction heating conditions, it is
possible to form 5000/mm^ or more pores of a circle
equivalent diameter of 0.1 to 1 |xm which function
30 effectively as oil reservoirs in a region of a depth of 5
fj,m or more from the surface.
(e) To improve the corrosion resistance at the
surface of a steel part, it is effective to raise the N
concentration at the surface layer part by nitriding or
35 ferritic nitrocarborizing treatment. Furthermore,
composite addition of Cr and Sn is effective.
At the recessed parts at the surface of a steel
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part, the N which dissolves out from the steel and the
moisture in the usage environment react to form ammonium
ions which deposit at relief parts on the surface of the
steel part. Due to this buildup, the pH of the steel
5 material surface rises and corrosion becomes more
difficult.
Due to the synergistic effect of this and increased
stability of the passivation film which is formed at the
surface of the steel part due to the composite addition
10 of Or and Sn the higher the pH, the corrosion resistance
of the surface of the steel part is improved. Note that,
when compositely adding Cr and Sn, the Or is preferably
0.05% or more and the Sn is preferably 20% or more of the
Cr.
15 The present invention was completed based on the
above discoveries and has as its gist the following:
(1) Steel for machine structural purposes for
surface hardening use characterized by containing, by
mass%,
20 C: 0.30 to 0.60%,
Si: 0.02 to 2.0%,
Mn: 0.35 to 1.5%,
Al: 0.001 to 0.5%,
Cr: 0.05 to 2.0%,
25 Sn: 0.001 to 1.0%,
S: 0.0001 to 0.021%,
N: 0.0030 to 0.0055%,
Ni: 0.01 to 2.0%,
Cu: 0.01 to 2.0%,
30 P: 0.030% or less, and
0: 0.005% or less,
having a balance of Fe and unavoidable impurities, and
having contents of Sn, Cu, Ni, Mn, and S satisfying the
following formulas (1) and (2);
35 -0.19<0.12xSn+Cu-0.1xNi<0.15... (1)
600.2xCr... (3)
5 (3) Steel for machine structural purposes for
surface hardening use as set forth in (1) Ql (2)
characterized in that the steel further contains, by
mass%, one or more of
B: 0.0003 to 0.005%,
10 W: (i.6025 to 0.5%,
Mo: 0.05 to 1.0%,
V: 0.05 to 1.0%,
Nb: 0.005 to 0.3%, and
Ti: 0.005 to 0.2%.
15 (4) Steel for machine structural purposes for
surface hardening use as set forth in any one of (1) to
(3) characterized in that the steel further contains, by
mass%, one or more of
Ca: 0.0005 to 0.01%,
20 Mg: 0.0005 to 0.01%,
Zr: 0.0005 to 0.05%, and
Te: 0.0005 to 0.1%.
(5) A steel part for machine structural purposes
which is produced by nitriding or ferritic
25 nitrocarborizing then induction hardening steel for
machine structural purposes for surface hardening use as
set forth in any one of (1) to (4), the steel part
characterized in that
(a) a surface layer of a depth of 0.4 mm or more
30 from the surface is a nitrided layer and
(b) a Vicker's hardness is 650 or more at the
surface layer of a depth of 0.2 mm from the surface when
treating the steel by tempering at 300°C.
(6) A steel part for machine structural purposes as
35 set forth in (5) characterized in that at a layer inside
the nitrided layer at a depth of 5 lim or more from the
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surface, there are 5000/mm^ or more of pores of a circle
equivalent diamoter of 0.1 t^ 1 ^m.
(7) A steel part for machine structural purposes as
set forth in (5) or (6) characterized in that a layer
5 which corresponds to the compound layer which is formed
by nitriding or ferritic nitrocarborizing at the surface
side in the nUiiUsd layer Is removed by machining.
(8) A method of production of a steel part for
machine structural purposes chai^aetsi^ized by forming
10 steel for machine structural purposes for surface
hardening use as set forth in any one of (1) to (4) into
a predetermined shape by forging or/and cutting, then
treating the shaped steel member by nitriding or ferritic
nitrocarborizing at 550 to 650°C for 0.5 to 5 hours, then
15 treating it by induction heating at 900 to 1100°C for 0.05
to 5 seconds for quenching, and (a) forming a nitrided
layer at a surface layer of a depth of 0.2 mm or more
from the surface and (b) making a Vicker's hardness 650
or more at the surface layer of a depth of 0.2 mm from
20 the surface when tempering at 300°C.
(9) A method of production of a steel part for
machine structural purposes as set forth in (8)
characterized by performing the induction hardening, then
grinding away the surface layer by an amount of grinding
25 (mm) satisfying the following formula (4):
0.05x{l-0.006x(600-nitriding or ferritic
nitrocarborizing temperature (°C))}x{(nitriding or
ferritic nitrocarborizing time (hr)+1)/3}0.2xCr... (3)
35 First, the reasons for limitation of the composition
of ingredients of the invention steel will be explained.
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Below, the % relating to the composition of ingredients
means mass%.
C: 0.30 to 0.60%
C is important for obtaining the strength of steel.
5 In particular, it is necessary for reducing the fraction
o£ £ errite as the structure before induction hardening,
improving the hardening ability at the time of induction
hardening, and deeply forming the hardened layer.
If less than 0.30%, the ferrite fraction rises and
10 the hardening at the time of induction hardening becomes
insufficient, so the lower limit is made 0.30%. If over
0.60%, the machineability and the forgeability at the
time of fabrication of parts remarkably fall and,
furthermore, the possibility of quenching cracks
15 occurring becomes larger at the time of induction
hardening, so the upper limit is made 0.60%. Preferably,
it is 0.34 to 0.56%.
Si: 0.02 to 2.0%
Si acts to raise the softening resistance of the
20 quenched layer so as to improve the surface fatigue
strength. To obtain the effect of addition, 0.02% or more
is added. Preferably, it is 0.07% or more. If over 2.0%,
the decarburization at the time of forging becomes
remarkable, so 2.0% is made the upper limit. If
25 considering the manufacturability, 1.3% or less is
preferable.
Mn: 0.35 to 1.5%
Mn is effective for improving the hardenability and
increasing the softening resistance to thereby improve
30 the surface fatigue strength. Further, Mn immobilizes the
S in the steel as MnS to prevent the phenomenon of
obstruction of penetration by N due to the concentration
of S at the steel material surface and promotes the
formation of a thick compound layer due to the ferritic
35 nitrocarborizing. Furthermore, Mn has the action of
causing a drop in the fraction of ferrite in the
structure before induction hardening and raising the
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hardening ability at the time of induction hardening.
To obtain the effect of addition, 0.35% or more is
added; but to reliably immobilize S as MnS to rendQr it
harmless, an amount which satisfies Mn/S>60 is added.
5 Preferably, it is 0,50% or more.
If adding Mn in a large amount, the hardenability at
thQ time of indUCti*^ harden m g will be improved ana
therefore the hardness will rise and the surface fatigue
strength will be improved, but if over 1.5%, the hardness
10 of the Steel material itself will greatly rise and the
machineability of the steel material before ferritic
nitrocarborizing will remarkably deteriorate. As a
result, the productivity will fall, so the upper limit is
made 1.5%. Preferably, it is 1.2% or less.
15 600.2
Sn is present together with Cr to stabilize the
15 passivation film of Cr oxides and contribute to the
improvement of the corrosion resistance of the steel
material. To obtain this effect of improvement of the
corrosion resistance, 0.001% or more is added.
Preferably, it is 0.002% or more. More preferably, it is
20 0.003% or more.
If adding a large amount of Sn, the hot ductility
falls and flaws occur in the casting and/or rolling at
the time of production of the steel material, so the
upper limit is made 1.0%. The point where the formation
25 of flaws is certain is preferably 0.3% or less, more
preferably 0.1% or less.
N dissolved out from the steel material and ammonium
ions from the moisture in the usage environment are
produced and deposit at the relief shapes at the surface
30 of a steel part to thereby make the pH of the surface
rise and make corrosion difficult, and the passivation
film which is formed on the surface of the steel part
stabilizes the higher the pH, so the corrosion resistance
is improved by this synergy.
35 The corrosion resistance of the steel material is
improved with a small addition of Cr and Sn, but if
adding an amount of S which satisfies Sn/Cr>0.2 to the
- 20 -
amount of Cr, it is possible to form a stable passivation
film with a smll^r amount o£ Cr.
Note that, Sn is an element which, like Cu, lowers
the hot ductility, so to secure the hot ductility of the
5 steel material, it is necessary that the elements
improving the hot ductility, including Ni, satisfy the
following formula (1). This point will be explained
later.
-0.15^0.12xSn+Cu-0.1xNi<0.15... (1)
10 Ni: 0.01 to 2.0%
Ni contributes to improve the toughness. To obtain
the effect of improvement of the toughness, 0.01% or more
is added. Preferably, it is 0.07% or more. If over 2.0%,
the machineability of the steel material deteriorates, so
15 2.0% is made the upper limit. Preferably, it is 1.6% or
less. Further, Ni improves the hot ductility, so the
above formula (1) must be satisfied.
Cu: 0.01 to 2.0%
Cu strengthens the ferrite to raise the
20 hardenability and, furthermore, raise the corrosion
resistance. If less than 0.01%, the effect of addition
does not appear, so the lower limit is made 0.01%.
Preferably, it is 0.08% or more.
If over 2.0%, the improvement of the mechanical
25 properties becomes saturated, so the upper limit is made
2.0%. Preferably, it is 1.6% or less. Further, Cu is an
element which lowers the hot ductility in the same way as
Sn, so the above formula (1) must be satisfied.
Sn is an element which raises the corrosion
30 resistance while lowering the hot workability. Cu, in the
same way as Sn, is an element which causes a drop in the
hot workability. On the other hand, Ni is an element
which raises the hot workability.
In the invention steels which simultaneously contain
35 Sn, Cu, and Ni, to maintain the required hot workability,
prevent cracks and flaws at the production, continuous
casting, hot rolling, and hot forging of the steel
- 21 -
material, and to secure productivity, it iS important tO
suitably balance the amount of Sn, the amount of Cu, and
the amount of Ni.
That is, the above formula (1) is a relationship Qt
5 composition which means securing the necessary hot
workability under a suitable balance of the amount of Sn,
the amount of Cu, and the amount of Ni.
Note that, the coefficient of Sn ^0.12) is a
coefficient which determines the extent of inhibition of
10 hot ductility compared with Cu, while the coefficient of
Ni (0.1) is a coefficient which determines, by
experience, the extent of improvement of the hot
ductility.
In the above formula (1), the lower limit is
15 determined by the smallest values of Sn and Cu and the
largest value of Ni, but the upper limit was set to 0.15
from the viewpoint of maintaining the hot workability
necessary for securing the productivity. The above
formula (1) will be explained in detail below.
20 Sn is an element which becomes a cause of cracks and
flaws at the time of hot temperature working. If Sn and
Cu are copresent, the high temperature ductility
remarkably falls whereby cracks and flaws occur at the
time of high temperature working and inferior parts
25 result. That is, if trying to improve the corrosion
resistance by the addition of Sn, the manufacturability
of the steel material is sometimes impaired.
For this reason, in the actual process of production
of a steel material, the occurrence of cracks and flaws
30 becomes a problem in the step of the blooming the cast
slab.
The hot workability of a cast slab which contains Sn
is inferior to the hot workability of a general cast
slab, so in the present invention, the most severe
35 blooming is simulated to evaluate the hot workability.
The upper limit of the relationship of composition having
the amounts of Sn, Cu, and N as variables is determined
- 22 -
to prevent cracks at the time of bar rolling and flaws
after rolling.
The tested steels contained, by mass%, C: 0.28 to
0.63%, Si: 0.01 to 2.10%, Mn: 0.30 to 2.10%, Al: 0.010 to
5 5.50%, Cr: 0.10 to 2.50%, S: 0.005 to 0.022%, N: 0.0031
to 0.0060%, and B: 0.0001 to 0.0052% as a basic
composition and had 1.05% or less of Sn, 1.50% or less of
Cu, and 2.10% or less of Ni added in various
combinations. These were melted by vacuum melting and
IS cast into 15 kg ingots to evaluate the hot rolling
characteristics.
From the ingots, a test material of a cross-section
of 80 mm square was cut out from the steel material and a
thermocouple was attached to the center part. The test
15 material was loaded into a 1050°C electric furnace. When
the center part of the test material reached 1045°C, the
material was taken out after 30 minutes and the test
material was rolled when the surface temperature reached
1000°C.
20 The hot rolling was performed for three continuous
passes to successively reduce the plate thickness of 80
mm to 64 mm, 51 mm, and 41 mm, then the material was aircooled
to room temperature.
When the test material cracked in the rolling, the
25 test was suspended at the point of time of cracking. A
test material which did not crack was cooled, then a
cross-section of the test material was cut out and
observed for surface flaws. In the judgment of surface
flaws, materials with flaws of less than a depth of 100
30 |am were evaluated as "flawless" while ones with flaws of
over 100 |j,m were evaluated as "flawed".
From the result of the above observation, it was
learned that if the value of "0.12xSn+Cu-0.IxNi" exceeds
0.15, cracks and flaws occur in the process of production
35 of the steel material or process of production of a steel
part.
- 23 -
That is, in the invention steels, if setting the
amounts of Sn, Cu, and Ni to be in the ingJiviidual ISngeS
of composition and satisfy 0.12xSn+Cu-0,lxNi<0.15, it is
possible to prevent cracks and flaws in the process of
5 production of steel materials Ql the piOCSSS Of
production of steel parts.
S: 0.0001 to 0.021%
S' is an element which improves the machineability,
but Qn the Other hand concentrates st thQ steQl material
10 surface and prevents penetration of N into the steel
material at the time of ferritic nitrocarborizing and
inhibits ferritic nitrocarborizing. While added to
improve the machineability, if over 0.021%, the extent of
inhibition of ferritic nitrocarborizing becomes
15 remarkable and, furthermore, the forgeability
deteriorates, so the upper limit is made 0.021%.
Preferably, it is 0.015% or less. The lower limit is
made the industrial limit of 0.0001%. Preferably, it is
0.001% or more. Further, S more preferably satisfies
20 Mn/S>70 to be immobilized as MnS and rendered harmless.
N: 0.0030 to 0.0055%
N forms various types of nitrides and refines the
austenite structure at the time of induction quenching.
To obtaining this refining effect, 0.0030% or more is
25 added. Preferably, it is 0.0035% or more.
If adding a large amount of N, the hardness rises
and, further, AIN is produced and the solute Al which is
effective for improvement of the machineability is
decreased, so the machineability deteriorates. Further,
30 the ductility in the high temperature region falls,
coarse AlN and coarse BN are produced, the steel material
becomes remarkably brittle, and cracks form at the time
of rolling and forging.
For this reason, the upper limit is made 0.0055%.
35 Preferably, it is 0.0050% or less.
P: 0.030% or less
P is an impurity element which is unavoidably mixed
- 24 -
in. It segregates at the grain boundaries and causes a
drop in the toughness. For this reason, it has to be
reduced as much as possible and is made 0.030% or less.
Preferably, it is 0.015% or less.
5 The lower limit is not particularly set, but
reduction to less than 0.0001% is difficult industrially,
so unavoidably 0.6061% or more is included.
0: 0.005% or less
0 is an element which is unavoidably present as
10 AI2O3, Si02, and other oxide-based inclusions. If the
amount of 0 is large, the above oxides become larger in
size and function as starting points for breakage of
power transmission parts, so the amount is made 0.005% or
less. Preferably, it is 0.002% or less. If aiming at a
15 higher lifetime, 0.001% or less is preferable.
The lower limit is not particularly set, but
reduction to less than 0.0001% is difficult industrially,
so unavoidably 0.0001% or more is contained.
Next, the reasons for limitation of the composition
20 of the selective ingredients of the invention steels will
be explained.
[Steel Strengthening Elements]
B: 0.0003 to 0.005%
B bonds with the N in the steel and is present as BN
25 at the time of working the steel material so as to
contribute to the improvement of the machineability.
Further, B forms BN during cooling with a slow cooling
speed, so acts to prevent the steel material from
hardening. Furthermore, at the time of induction heating
30 performed after working the steel material, the BN breaks
down whereby B is produced and the hardenability is
greatly improved so the surface fatigue strength is
improved.
To obtain the effect of addition, 0.0003% or more is
35 added. Preferably, it is 0.0008% or more. If over 0.005%,
the effect of addition becomes saturated. Furthermore, it
becomes a cause of cracks at the time of rolling or
- 25 -
forging, so 0.005% is made the upper limit. Preferably,
it is 0.004% or less.
W: 0.0025 to 0.5%
W raises the hardenability and contributes to the
5 improvement of the surface fatigue characteristics, but
on the other hand raises the hardness to degrade the
machineability. For this reason, the amount of addition
of W is limited. To obtain the effect of addition,
0.0025% or more is added, but to achieve a large
10 improvement in the surface fatigue strength, 0.01% or
more is added. Preferably, it is 0.03% or more. If over
0.5%, the effect of addition is saturated and the
machineability deteriorates, so 0.5% is made the upper
limit. Preferably, it is 0.2% or less.
15 Mo: 0.05 to 1.0%
Mo raises the softening resistance of the quenched
layer to contribute to improvement of the surface
pressure fatigue strength, further, makes the quenched
layer tougher, and contributes to an improvement of the
20 bending fatigue strength. To obtain the effect of
addition, 0.05% or more is added. Preferably, it is 0.10%
or more. If over 1.0%, the effect of addition becomes
saturated, so 1.0% is made the upper limit. Preferably,
it is 0.7% or less.
25 V: 0.05 to 1.0%
V precipitates and disperses as nitrides in the
steel material and contributes to the refinement of the
austenite structure at the time of induction hardening.
To obtain the effect of addition, 0.05% or more is added.
30 Preferably, it is 0.10% or more. If over 1.0%, the effect
of addition becomes saturated, so the upper limit is made
1.0%. Preferably, it is 0.7% or less.
Nb: 0.005 to 0.3%
Nb precipitates and disperses as nitrides in the
35 steel material and contributes to the refinement of the
austenite structure at the time of induction hardening.
To obtain the effect of addition, 0.005% or more is
- 26 -
added. Preferably, it is 0.010% or more. If over 0.3%,
the effect of addition becomes saturated, so the upper
limit is made 0.3%. Preferably, it is 0.1% or less.
Ti: 0.005 to 0.2%
5 Ti precipitates and disperses as nitrides in the
steel material and contributes to the refinement of the
austenite structure at the time of induction hardening.
To obtain the effect of addition, 0.005% or more is
added. Preferably, it is 0.010% or more. If over 0.2%,
10 the precipitates coarsen and the steel material is
embrittled, so the upper limit is made 0.2%. Preferably,
it is 0.1% or less.
[Elements Improving Bending Strength]
In steel parts, when improvement of the bending
15 fatigue strength is sought, one or more of Ca: 0.0005 to
0.01%, Mg: 0.0005 to 0.01%, Zr: 0.0005 to 0.05%, and Te:
0.0005 to 0.1% is added.
These elements all suppress stretching of MnS to
make the bending fatigue strength higher and prevent
20 bending fatigue fracture of gears and fatigue fracture of
spline bases of shaft parts.
To obtain an effect of suppression of stretching of
MnS, Ca, Mg, Zr, and Te are added in 0.0005% or more.
Preferably, these elements are 0.0010% or more.
25 Even if added in large amounts, the effect of
addition becomes saturated, so the upper limit is made
0.01% for Ca and Mg, 0.05% for Zr, and 0.1% for Te.
Preferably, the limit is 0.005% or less for Ca and Mg,
0.01% or less for Zr, and 0.05% or less for Te.
30 Further, the invention steel may contain Pb, Bi, Zn,
REM, Sb, etc. in ranges not impairing ira properties in
addition to the above ingredients.
Next, the thickness and hardness of the nitrided
layer of steel parts for machine structural purposes of
35 the present invention (hereinafter sometimes referred to
as "the invention steel parts") will be explained.
The invention steel parts are steel parts obtained
- 27 -
by nitriding or ferritic nitrocarborizing, then induction
hardening the invention steel and (a) have a surface
lSyS5? Af & depth o£ 0.4 mm or more from the surface
comprised of a nitrided layer and (b) have a Vicker's
5 hardness of 650 or more at the surface layer of a depth
Of 0.2 iiiiii from thQ surface wh&w t^w^^i^mg at 300°G.
When grinding the surface layer of the steel part
for use, the nitrogen compound layer which is produced by
the nitriding or ferritic nitrocarborizing is not
10 completely removed. To leave an N-containing layer which
contributes to securing the surface hardness and
improving the corrosion resistance at the surface layer
of the steel part, the surface layer of a depth of 0.4 mm
or more from the surface is made a nitrided layer.
15 If the thickness of the nitrided layer is
insufficient, the thickness of the surface layer having
the sufficient hardness will be thin, spalling of
internal fracture will occur before fracture at the
surface starting point, the lifetime will become shorter,
20 and, further, the effect of improvement of the corrosion
resistance will not be exhibited.
Surface fatigue fracture is fracture at a surface
starting point at a working surface made high in
temperature (300°C or so) , so it is possible to raise the
25 surface fatigue strength to prevent surface fatigue
fracture. For improvement of the surface fatigue
strength, maintaining a high temperature strength, that
is, increasing the tempering softening resistance, is
effective. For this reason, the Vicker's hardness is made
30 650 or more at the surface layer of a depth of up to 0.2
mm from the surface of the steel part at the time of
tempering at 300°C.
If the Vicker's hardness is less than 650 at the
surface layer of a depth of up to 0.2 mm from the surface
35 of the steel part at the time of tempering at 300°C, a
high surface pressure cannot be withstood.
- 28 -
In actual steel parts, the fact of being a steel
part which was soft nitrided, then induction hardened can
be discerned by taking a microsample from the steel part,
corroding it by a Nital corrosive solution, then
5 observing the stri^gtuts undsi an optlcal Eiicroscope and
making a judgment by that structure, t^Q OliStlibUtiOn Of
hardness from the surface to the core part, and,
furthermore, the distribution of N from the surface to
the core part which was measured by EPMA.
10 Next, the structure of the surface layer of the
invention steel parts will be explained.
The invention steel parts are steel parts which are
nitrided or soft nitrided, then induction hardened which
have 5000/min^ or more pores with a circle equivalent
15 diameter of 0.1 to 1 |ain present in a layer of a depth of
5 )am or more from the surface.
In steel parts (gears) which fracture due to surface
fatigue due to rolling, the lubrication of the working
surfaces is important. If the lubrication at the working
20 surfaces is insufficient, seizing and sticking will occur
at contact of the steel materials with each other and the
surface fatigue strength will fall.
To secure a sufficient lubrication film at the
working surfaces, it is effective to provide oil
25 reservoirs so that a film of a lubricant is formed
continuously at the working surfaces.
The invention steel parts are characterized by
forming at the surface layer of the steel material a
compound layer mainly comprised of FesN, Fe4N, and other
30 Fe nitrides by ferritic nitrocarborizing, then using
induction hardening to transform the structure to
austenite and quench the steel to form a nitrided layer.
The nitrided layer is formed by the N which is
produced by breakdown of the compound layer which was
35 formed at the time of ferritic nitrocarborizing and by
the N which permeates by diffusion in the steel material
at the time of ferritic nitrocarborizing. At this time.
- 29 -
the compound layer becomes a hard porous layer with a
large number of pores present dispersed in it after
induction hardening.
The pores function as oil reservoirs to improve the
5 lubrication effect and further improve the wear
resistance and durability.
If 5000/ram^ or more pores of a circle equivalent
diameter of 0.1 to 1 urn are present in a layer ^1; S 'ilspth
of 5 |am or more from the surface, th^ p9t§g Will
10 effectively function as oil reservoirs. To form pores
which effectively function as oil reservoirs, it is
necessary to control the ferritic nitrocarborizing
conditions and/or induction heating conditions.
Even with just ferritic nitrocarborizing, pores can
15 be formed in the compound layer and the pores function as
oil reservoirs, but the compound layer would be extremely
brittle and could not withstand a large load, so the
surface fatigue strength is not improved.
If the pores become coarse, they become starting
20 points for pitching and other surface fatigue and result
in a drop of the surface fatigue strength, so the circle
equivalent diameter of the pores was made 1 \im or less.
If the pores are too small in circle equivalent diameter,
they will not sufficiently function as oil reservoirs, so
25 the lower limit of the circle equivalent diameter was
made 0.1 \im.
If the number of pores is too small, they will not
effectively function overall as oil reservoirs, so
5000/mm^ or more have to be present in a layer of a depth
30 of 5 nm or more from the surface of the steel part.
During the period until the lifetime is reached
under normal operation, the gear surfaces and other
sliding surfaces normally become worn by 5 pm or more, so
to extend the lifetime of a steel part, there must be
35 5000/mm^ or more pores of a circle equivalent diameter 0.1
to 1 (om present at a layer of a depth of 5 f^m or more
- 30 -
from the surface of the steel part.
The size and number of pores depends on the ferritic
nitrocarborizing conditions and/or induction heating
conditions. To form an effectively functioning porous
5 layer, it is necessary to select ferritic
nitrocarborizing conditions and/or induction hardening
conditions which give a high surface fatigue strength.
It is preferable to perform nitriding or ferritic
nitrocarborizing at 550 to 650°C for 0.5 to 5 hours, then
10 perform induction heating at 900 to 1100°C for 0.05 to 5
seconds.
In the invention steel parts, the surface layer is
martensite, but the core part has to be left as ferritepearlite.
15 If quenching only the surface layer to transform it
to martensite, compressive stress will remain at the
surface layer and the surface fatigue strength will rise.
If transforming even the core part to martensite, the
compressive stress which remains at the surface layer
20 will decrease and the surface fatigue strength will fall.
In the invention steel parts, it is preferable to
machine away the layer in the nitrided layer which
corresponds to the nitrogen compound layer.
The location where a nitrogen compound layer is
25 formed at the time of nitriding or ferritic
nitrocarborizing becomes martensite including pores by
the subsequent induction hardening, but the layer inside
from the nitrogen compound layer (hereinafter referred to
as the "nitrogen diffusion layer") becomes martensite
30 without pores. The two structures differ in the point of
hardness.
In the invention steel parts, after heat treatment,
sometimes the parts are ground to relieve stress, but if
this grinding is insufficient, the martensite of the
35 nitrogen compound layer and the martensite of the
nitrogen diffusion layer will appear at the surface in a
mixed manner after grinding and the surface hardness will
- 31 -
become uneven.
Therefore, when grinding the surface o£ invention
Steel parts for use, to eliminate unQVQnnQss of surface
hardness, it is necessary to secure an amount of grinding
5 of at least the thickness of the nitrogen compound layer.
The amount of grinding for removing the nitrogen
compound layer will be explained. The thickness of the
nitrogen compound layer depends on the temperature and/or
time o£ nitriding or ferritic nitrocarborizing.
10 ThersfCirs, the relationship between the temperature
and/or time of nitriding or ferritic nitrocarborizing and
the thickness of the nitrogen compound layer which is
measured after nitriding or ferritic nitrocarborizing was
investigated. The results are shown in FIG. 4.
15 The abscissa shows the 0.05x{1-0.006x(600-nitriding
or ferritic nitrocarborizing temperature
(°C))}x{(nitriding or ferritic nitrocarborizing time
(hr)+l)/3} based on the results of investigation of the
effects of temperature (°C) and time (hr) in nitriding or
20 ferritic nitrocarborizing.
As shown in FIG. 4, a relationship of the thickness
of the nitrogen compound layer (mm)=0.05x{1-0.006x(600-
nitriding or ferritic nitrocarborizing temperature
(°C))}x{(nitriding or ferritic nitrocarborizing time
25 (hr)+l)/3} was obtained.
For this reason, in removing by grinding a layer of
the surface of a steel part, it is sufficient to secure
an amount of removal by grinding of the thickness (mm)
found by the above relationship or more. Further, the
30 upper limit of the amount of removal by grinding was made
the 0.2 mm at which the effect of addition of N to the
steel material by nitriding or ferritic nitrocarborizing
can be obtained.
Therefore, the amount of grinding for removing the
35 nitrogen compound layer preferably satisfies the
following formula (4):
- 32 -
0.05x{l-0.006x(600-nitriding or ferritic
nitrocarborizing temperature (°C))}x{(nitriding or
zmg time (hr)+1)/31:^amount of
removal by grinding (mm)<0.2mm... (4)
5 Next, the method of production of the invention
Steel parts will be explained.
The invention steel parts are produced by forming
the invention steel into a predetermined shape by forging
and/or cutting and then nitriding or ferritic
10 nitrocarborizing the formed steel member.
To form a nitrided layer with a high N concentration
to obtain a high surface fatigue strength, a compound
layer which breaks down and becomes a source of supply of
N at the time of induction heating (a layer mainly
15 comprised of FesN, Fe4N, and other Fe nitrides) is formed
on the steel surface by ferritic nitrocarborizing.
To cause a sufficient amount of N to diffuse into
the steel material to thickly form a nitrided layer which
Is hard and has a high tempering softening resistance, it
20 is necessary to make the thickness of the compound layer
after ferritic nitrocarborizing 10 |im or more.
If the ferritic nitrocarborizing temperature is a
high temperature, the compound layer becomes thin and,
furthermore, the N concentration in the compound layer
25 becomes lower, so the ferritic nitrocarborizing
temperature is made 650°C or less. If the ferritic
nitrocarborizing temperature is 650°C or less, it is
possible to prevent heat treatment distortion of the
steel material, grain boundary oxidation, etc.
30 Preferably, it is 620°C or less.
To form a thick compound layer, the lower limit of
the ferritic nitrocarborizing temperature is made 550°C.
Preferably, it is 580°C or more.
The thickness of the nitrided layer becomes greater
35 along with the ferritic nitrocarborizing time, so the
- 33 -
ferritic nitrocarborizing time has to be at least 0.5
hour. If over 5 hours, the saturation point is reached in
the increase of thickness of the nitrided layer^ so th^
ferritic nitrocarborizing time is made 5 hours or less.
5 Preferably, it is 1 to 4 hours.
The cooling after the ferritic nitrocarborizing is
performed by air-cooling, N2 gas cooling, oil cooling, and
other methods. For the ferritic nitrocarborizing, gas
ferritic nitrocarborizing or salt bath ferritic
10 nitrocarborizing can be used. Th^ IHQthOU Of Supplying
nitrogen to the steel material surface and forming a 10
fxm or more compound layer at the surface layer of the
steel material may be either of ferritic nitrocarborizing
or nitriding.
15 The nitriding in the present invention is not a
method like ferritic nitrocarborizing of treatment by a
mixed atmosphere of NH3 and CO2 (in some cases, including
N2 as well), but is a method which is industrially
differentiated from it as a surface hardening method
20 which uses only NH3 for long treatment.
To break down the compound layer which was formed by
the ferritic nitrocarborizing at the surface layer of the
steel material and make the produced N diffuse into the
steel material so as to form a nitrided layer with a high
25 N concentration in the layer at a region of a depth of
0.4 mm or more from the surface obtain a high hardness of
a Vicker's hardness of 650 or more at the layer of a
depth of 0.2 mm from the surface by tempering at 300°C,
after the ferritic nitrocarborizing, it is necessary to
30 use induction heating to transform the structure to
austenite and perform quenching.
Regarding the heating at the time of induction
hardening, the breakdown and formation of a solid
solution by the nitrogen compound layer have to be
35 considered. To secure a state of a single phase of
austenite and the nitrogen compound layer forming a solid
solution, the heating temperature has to be made 900 to
- 34 -
1100°C and the holding time has to be made 0.05 to 5
seconds.
If the heating temperature is less than 900°C. the
breakdown and formation of a solid solution by the
5 nitrogen compound become insufficient and the required
hardness can no longer be secured. If the heating
temperature exceeds 1100°C, the N diffuses up to the
inside, the concentration of N at the surface becomes
less than 0.5%, a Vicker's hardness of 650 or more at the
10 time of tOmpering at 300'C «AMot be secured, and,
furthermore, due to the increase in the oxide layer, the
mechanical properties deteriorate.
If the holding time is shorter than 0.05 second, the
breakdown of the compound layer and the diffusion of the
15 produced N become insufficient, while if over 5 seconds,
the N diffuses up to the inside, the concentration of N
at the surface becomes less than 0.5%, and a Vicker's
hardness of 650 or more at the time of tempering at 300°C
cannot be secured,
20 The frequency of the high frequency is around 400
kHz for small sized steel parts and 5 kHz or so for large
sized steel parts. The coolant which is used for the
quenching is preferably water, a polymer quenching agent,
or other water-based coolant with a large cooling
25 ability.
After induction hardening, the steel part is
preferably tempered at a low temperature of around 150°C
in the same way as a general carburized quenched part so
as secure toughness.
30
Examples
Next, examples of the present invention will be
explained, but the conditions in the examples are
illustrations of the conditions which were employed for
35 confirming the workability and effects of the present
invention. The present invention is not limited to these
- 35 -
conditions. The present invention can employ various
conditions so long as not departing from the gist of the
present invention and achieving the object of the present
invention.
5 (Example 1)
Steel materials of the compositions of ingredients
which are shown in Table 1 and Table 2 (continuation of
Table 1) (invention examples) and in Table 3 (comparative
examples) were produced.
- 3G -
Table 1
Composition of ingredients (niass%)
Ex. Steel no. Class
C Si Mn P S Al Cr Sn 0 Ni Cu N B
~ a ~Inv. ex."0.350.250.880.0l"4 0.007 0.1200.10"0.07 0.0010.01 0.010.0040^
2 b Inv. e^. 0.540.200.790.0150.0100.0556.11 Q.10 0.0010.010.010.0039
_2 c Inv. ex. 0.57 1 . 3 5 0 . 8 1 0 . 0 1 5 0.0110.027 0.70 0.15 0 . 0 0 2 0 . 0 1 0 . 0 2 0 . 0 0 46
4 d Inv. ex. 0 . 6 0 1 . 9 6 0 . 3 6 0 . 0 1 4 0 . 0 0 5 0 . 0 3 5 0 . 1 2 0.03 0.001 0.01 0.01 0.00490.0003
5 e Inv. ex. 0 . 3 1 0 . 2 5 0 . 8 6 0 . 0 0 9 0 . 0 0 9 0 . 0 6 9 1.20 0.25 0.004 0.02 0 . 0 1 0 . 0 0 42
6 f Inv. ex. 0 . 5 5 0 . 0 5 1 . 4 5 0 . 0 1 5 0 . 0 0 6 0 . 2 0 0 0 . 1 0 0.03 0 . 0 0 3 0 . 0 1 0 7 0 3 0 . 0 0 5 0~
7 g Inv. ex. 0 . 4 0 0 . 2 5 0 . 6 8 0 . 0 0 2 0 . 0 0 9 0 . 1 2 6 1 . 8 5 0.40 0 . 0 0 2 0 . 0 1 0 . 0 3 0 . 0 0 4 5 0 . 0 0 10
8 h Inv. ex. 0.550.051.490.0120.0210.3000.10 0.04 0.0010.010.020.00460.0025
9 i Inv. ex. 0 . 5 5 0 . 2 5 0 . 8 1 0 . 0 1 5 0 . 0 1 1 0 . 0 3 0 0 . 1 0 0.05 0 . 0 0 2 0 . 0 1 0 . 0 1 0 . 0 0 4 1 0 . 0 0 04
10 j Inv. ex. 0 . 5 3 0 . 2 5 0 . 7 5 0 . 0 1 4 0 . 0 0 9 0 . 0 2 0 0 . 1 5 0.04 0 . 0 0 4 0 . 0 1 0 . 0 2 0 . 0 0 34
11 k Inv. ex. 0 . 5 5 0 . 2 0 0 . 8 8 0 . 0 1 5 0 . 0 1 2 0 . 0 3 0 0 . 1 2 0.04 0 . 0 0 3 0 . 0 2 0 . 0 2 0 . 0 0 3 1 0 . 0 0 04
12 1 Inv. e x . 0 . 5 5 0 . 3 5 0 . 7 2 0 . 0 1 4 0 . 0 1 0 0 . 0 2 0 0 . 1 5 0.05 0 . 0 0 3 0 . 0 1 0 . 0 1 0 . 0 0 5 5 0 . 0 0 03
13 m Inv. ex. 0 . 4 5 0 . 2 5 0 . 9 0 0 . 0 1 5 0 . 0 1 1 0 . 1 9 2 0 . 1 3 0.05 0 . 0 0 2 0 . 0 1 0 . 0 3 0 . 0 0 5 5 0 . 0 0 07
14 n Inv. ex. 0 . 5 g i ; i . 2 0 0 . 8 6 0 . 0 1 4 0 . 0 1 1 0 . 0 4 0 0 . 1 5 0.04 0 . 0 0 2 0 . 0 2 0 . 0 1 0 . 0 0 4 6 0 . 0 0 03
15 o Inv. ex. 0 . 4 9 0 . 2 7 0 . 8 5 0 . 0 1 5 0.011 0 . 0 4 0 0 . 1 8 0.05 0.003 0 . 0 2 0 . 0 1 0.00460.0006
16 p Inv. ex. 0.55 0.34 1.490.015 0.005 0.099 1.13 0.24 0.001 0.02 0.02 0.0044 0.0009
17 q Inv. ex. 0 . 6 0 0 . 2 0 0 . 8 8 0 . 0 1 4 0 . 0 1 1 0 . 0 5 0 0 . 1 3 0.05 0.002 0 . 0 2 0 . 0 1 0.0045 0.0003
18 r Inv. ex. 0 . 5 6 0 . 3 1 0 . 8 0 0 . 0 1 3 0.010 0 . 2 0 0 0 . 1 5 0.04 0.002 1 . 9 0 0 . 3 3 0 . 0 0 5 0 0.0003
19 s Inv. ex. 0.45 0.25 0.88 0.015 0.012 0.010 0.18 0.04 0.004 0.01 0.01 0.0049 0.0005
20 t Inv. ex. 0.57 0.10 0.90 0.013 0.012 0.031 1.00 0.25 0.003 0.01 0.15 0.0039 0.0003
21 u Inv. ex. 0 . 6 0 0 . 2 5 0 . 8 0 0 . 0 1 5 0 . 0 1 0 0 . 0 2 5 0 . 1 1 0.05 0 . 0 0 4 0 . 0 6 0 . 0 1 0 . 0 0 51
22 V Inv. e x . 0 . 5 6 0 . 1 0 0 . 9 0 0 . 0 1 5 0 . 0 0 5 0 . 0 3 1 1 . 0 0 0.25 0 . 0 0 3 1 . 4 0 0 . 2 5 0 . 0 0 3 9 0 . 0 0 12
23 w Inv. ex. 0 . 5 5 0 . 2 5 0 . 6 9 0 . 0 1 3 0 . 0 0 9 0 . 0 1 0 0 . 1 1 0.05 0 . 0 0 4 0 . 5 5 0 . 2 0 0 . 0 0 5 1 0 . 0 0 10
24 X Inv. ex. 0 . 5 3 0 . 2 5 0 . 9 2 0 . 0 1 4 0 . 0 1 3 0 . 0 3 1 0 . 1 5 0.06 0 . 0 0 3 0 . 0 1 0 . 0 1 0 . 0 0 45
25 y Inv. ex. 0 . 56 0 .20 0 . 81 0 . 013 0 . Oil 0. 050 0 .11 0 . 30 0 . 004 0 . 01 0 . 02 0 . 0052 0 . 0010
26 z Inv. ex. 0.550.250.950.0130.012 0.0500.01 0.05 0.003 0.02 0.02 0.00490.0003
27 aa Inv. ex. 0 . 55 0 .20 0 . 96 0 . 014 0 . 013 0 . 045 0 .11 0 . 05 0 . 003 0 . 01 0 . 03 0 . 0046
28 ab Inv. ex. 0 . 55 0 . 25 0 . 93 0 . 015 0 . 012 0 . 035 0 .15 0 . 06 0 . 003 0 . 03 0 . 02 0 . 0049 0 . 0003
29 ac Inv. ex. 0 . 53 0 .25 0 . 78 0 . 014 0 • 005 0 . 040 0 .12 0 . 05 0 . 003 0 . 01 0 . 02 0 . 0053 0 . 0003
30 ad Inv. ex. 0 . 51 0 .20 0. 89 0 . 013 0 . 004 0 • 030 0 .11 0 . 04 0 . 003 0 . 01 0 . 02 0 . 0051 0 . 0010
31 ae Inv. ex. 0.55 0.25 0.85 0.015 0.010 0.035 0.15 0. 06 0.004 0.01 0 .03 0.0054 0.0003
32 af Inv. ex. 0 . 50 1. 30 0 . 36 0 . 015 0 . 005 0 . 041 0 .15 0 . 05 0 . 003 0 . 01 0 . 03 0 . 0038 0 . 0020
33 ag Inv. ex. 0.53 0.25 0.89 0.015 0.012 0.123 0.10 0.05 0.003 1.50 0.28 0.0046
34 ah Inv. ex. 0.560.230.790.0140.0110.1020.11 0.06 0.0020.010.030.0046
35 ai Inv. ex. 0 . 55 0 .21 0 . 81 0 .014 0 .010 0 . 050 0 .12 0 .05 0 . 004 0 .03 0 .02 0 .0041 0 .0010
36 ak Inv. ex. 0 . 53 0 . 05 1. 45 0 . 015 0 . 020 0. 050 0 .15 0 . 06 0 . 003 0 . 01 0 . 02 0 . 0051 0 . 0003
37 al Inv. ex. 0.55 0.25 0.80 0.013 0.010 0.120 0.10 0.05 0.002 0.020.02 0.0043 0.0010
38 am Inv. ex. 0 . 53 0 .20 0. 80 0 . 015 0 . Oil 0 • 023 0.15 0 . 06 0 . 003 0 . 80 0 . 20 0 . 0045 0 . 0009
39 an Inv. ex. Q . 56 Q .35 0 .85 0 .014 0 .012 0 .020 0 .15 Q . 06 0 .003 0 . 02 0 .02 0 . 0052
40 ao Inv. ex. 0.550.530.500.0150.0070.0340.16 0.06 0.0020.100.100.0048
41 ap Inv. ex. 0.530.05 0.400.015 0.005 0.49 0.11 0.06 0.0020.250.110.0051
42 aq Inv. ex. 0 . 30 1. 95 0 . 90 0 . 014 0 . 005 0 . 027 0 .50 0 .15 0 . 003 0 . 30 0 .15 0 . 0040 0 . 0010
43 ar Inv. ex. 0 . 53 0 .20 0. 83 0 . 009 0 . 010 0 . 020 0 . 05 0 . 01 0 . 003 0 . 01 0 . 01 0 . 0051 0 . 0010
44 as Inv. ex. 0 .55 0 . 25 0 .79 0 . Oil 0 . 013 0 . 030 0 .10 0 . 003 0 . 004 0 . 03 0 • 03 0 . 0046 0 . 0015
45 at Inv. ex. 0.53 0.30 0.85 0.009 0.008 0.020 0.11 0.002 0.005 0.01 0.02 0.0039 0.0010
461 au |lnv. ex.|o .55 0 .26 0 . 82 0 . 010 0 . 006 0 . 01o|o . ISjO . 001 0 . 005 0 . 01 0 . 01 0700351
- 37 -
Table 2 (continuation of Table 1)
Composition of ingredients (mass!)
^ Steel „, I I I I r i I i ,^ „ _ 0.12*Sn+
^^- no. ^^^^^ W MO V Nb Ti Ca Mg Zr Te ""^^ "'^/'^^ Cu-0. l*Ni
1 ' a Inv. ex. " - - — -—— ^^^^ . ^^^^
2 b Inv. ex. 79 0.91 0.02
3 c Inv. ex. 74 0.21 Q.04
4 d Inv. ex. 72 0.25 0.01
5 e Inv. ex. 96 0.21 0.04
6 f Inv. ex. 242 0.30 0.03
7 g Inv. ex. 76 0.22 0.08
J h Inv. ea, ^J2_ 71 O.JO 0.02
9 i Inv. ex. 0.45 74 0.50 0.02
10 j Inv. ex. 0.50 83 0.27 0.02
11 k Inv. ex. 0^ 73 0.33 0.02
12 1 Inv. ex. 0.06 " 72 " 0.33 0.02
13 m Inv. ex. 0.17 82 0.38 0.04
14 n Inv. ex. 0.060.09 0.05 ' 80 0.27 ' 0.01
15 o Inv. ex. 0.003 0.50 0.005 77 0.28 0.01
16 p Inv. ex. 0.26 0.006 298 0.21 0.05
17 q Inv. ex. 0.100.06 80 0.38 0.01
18 r Inv. ex. 80 0.27 0.14
19 s Inv. ex. 73 0.22 0.01
20 t Inv. ex. 75 0.25 0.18
21 u Inv. ex. 80 0.45 0.01
22 V Inv. ex. 0.09 0.090.15 0.01 0.10 180 0.25 0.14
23 w Inv. ex. 0.01 0.200.10 0.02 0.05 77 0.45 0.15
24 x Inv. ex. - 0.0007 71 0.40 0.02
25 y Inv. ex. 0.01 0.0006 74 2.73 0.06
26 z Inv. ex. 0.0010 79 4.17 0.02
27 aa Inv. ex. 0.0007 74 0.45 0.04
28 ab Inv. ex. 0.01 0.009 78 0.40 0.02
29 ac Inv. ex. 0.006 156 0.42 0.03
30 ad Inv. ex. 0.01 0.05 223 0.36 0.02
31 ae Inv. ex. 0.05 0.080 85 0.40 0.04
32 af Inv. ex. 0.600.08 0.04 0.03 0.005 0.0008 72 0.33 0.04
33 ag Inv. ex. 0.25 0.01 0.03 0.001 0.02 74 0.50 0.14
34 ah Inv. ex. 0.10 0.0005 0.001 72 0.55 0.04
35 ai Inv. ex. 0.12 0.009 81 0.42 0.02
36 ak Inv. ex. 0.10 0.20 0.003 0.01 73 0.40 0.03
37 al Inv. ex. 0.050.10 0.10 0.0005 0.002 80 0.50 0.02
38 am Inv. ex. 0.05 0 . 1 0 0 . 1 5 0.01 0.01 0.0006 0.0006 0.0010 0.050 73 0.40 0.13
39 an Inv. ex. 0.20 0.02 0.09 71 0.40 0.03
40 ao Inv. ex. 0.090.11 0.007 0.04 71 0.38 0.10
41 ap Inv. ex. 0.11 0.12 0.01 80 0.55 0.09
42 aq Inv. ex. 0 . 0 5 0 . 0 7 0 . 0 9 0 . 0 1 0 . 0 2 0 . 0 0 0 6 0 . 0 0 0 6 0.01 0.001 180 0.30 0.14
43 ar Inv. ex. 83 0.20 0.01
44 as Inv. ex. 61 0.03 0.03
45 at Inv. ex. 106 0.02 0.02
46 1 au |lnv. ex.| | ~ | | | | | 137 | 0 . 0 1 | 0.01
- 38 -
Table 3
Composition of ingredients (inass%)
S t e e l „, I i I I \ I f I I I I I
Ex. Class
no. C Si Mn P S Al Cr Sn 0 Ni Cu N B
TT ba Coi^. ex. 0728i;23O00.0300.0100.015oTlT 0.03 0030.010.010.0052
48 bb Comp. ex. 0.55 0.80 0.29 0.009 0. 005 0.025 0.15 0.04 0.002 0. 01 0. 01 0.0040 0.0010
49 be Comp. ex. 0 . 4 5 0 . 2 5 0 . 5 0 0 . 0 1 0 0 . 0 1 2 0 . 4 5 0 0 . 1 0 0.01 0 . 0 0 1 0 . 0 1 0 . 1 2 0 . 0 0 4 6 0 . 0 0 11
50 bd Comp. ex. 0 . 6 3 0 . 2 5 0 . 9 0 0 . 0 1 6 0 . 0 1 0 0 . 0 7 4 0 . 4 9 0.12 0,0020,20O.OZ0.00550.0003
51 be Comp. ex. 0 . 5 0 0 . 7 0 2 . 1 2 0 . 0 2 5 0 . 0 1 3 0 . 0 5 1 1 . 2 3 ~ 0 . 2 3 0.002 0 . 0 1 0 . 0 3 0.0046
52 bf Comp. ex. 0 . 5 0 1 . 1 2 1 . ^ 0 0 . 0 1 9 0 . 0 2 2 0 . 0 9 8 0 . 8 2 0.30 0.005 0 . 0 1 0 . 0 1 0 . 0 0 5 1 0 . 0 0 30
53 bg Comp. ex. 0.43 0.20 1.45 0.022 0.019 0.081 2.50 0.60 0.001 0.10 0.13 0.004~ 0.0006
54 bh comp. ex. 0 . 5 3 1 . 0 3 0 . 8 0 0 . 0 1 0 0 . 0 1 0 0 . 0 2 0 0.04 0.10 0 . 0 0 5 0 . 0 1 0 . 1 3 0 . 0 0 48
55 bi Comp. ex. 0.53 2.10 0.85 0.005 0. Oil 0.030 0.52 0.13 0 . 0 0 1 0 . 0 1 0 . 0 2 0.0052
56 bj Comp. ex. 0 . 5 5 0 . 2 5 0 . 8 8 0 . 0 0 2 0 . 0 1 2 0 . 0 4 0 0 . 2 0 1.05 0 . 0 0 1 0 . 0 1 0 . 0 3 0.0055 0.0005
57 bk Comp. ex. 0.45 0.20 0.90 0.016 0.010 0.025 0.15 0.05 0.002 0 . 0 1 0 . 0 2 0.0039 0.0052
58 bl Comp. ex. 0.50 0 . 0 3 0 . 7 8 0 . 0 1 6 0 . 0 0 9 0.11 0.11 0.05 0.002 2.10 0.37 0.0049 0.0003
59 bm Comp. ex. 0.440.681.230.0120.0130.0130.91 0.20 0.003 0.010.010.0043 0.0003
60 bn Comp. ex. 0.500.151.200.0140.012 5.50 0.10~.03 0.003Ol07020.0055
61 bo Comp. ex. 0 . 5 0 0 . 4 5 1 . 2 0 0 . 0 1 3 0 . 0 1 1 0 . 0 2 6 0 . 1 2 0.05 0 . 0 0 6 0 . 0 1 0 . 0 3 0.0054
62 bp Comp. ex. 0.55 0 . 0 1 1 . 4 5 0 . 0 3 0 0 . 0 1 5 0 . 0 5 2 1.90 0.35 0 . 003 0 • 01 0 . 02 0. 0060 0 • 0005
63 bq Comp. ex. 0 . 6 0 1 • 3 0 1 • 4 9 0 . 0 1 5 0 . 0 2 6 0 • 0 5 0 0 . 1 1 0.30 0 . 0 0 2 0 . 1 0 0 . 0 6 0 . 0 0 44
64 br Comp. ex. 0 .55 0 . 15 0 .25 0 .015 0 . 015 0 • 030 0 .15 <0 • OOl|o . 002|o . 03|o. lo|o . 0055|o • 0010
Composition of i n g r e d i e n t s (mass%)
Ex. St^^^ Class \ \ 1 1 . 1 \ 1 1 Mn/SSn/Cr °-^ff.";.
no. W Mo V Nb Ti Ca Mg Zr Te Cu-O.lNi
"47~ ba Comp. ex. - ~ 80 0.27 0.01
48 bb Comp. ex. 0.02 58 0.27 0.01
49 be Comp. ex. 0.01 42 0.10 0.12
50 bd Comp. ex. 90 0.24 0.01
51 be Comp. ex. 0.30 0.30 167 0.19 0.06
52 bf Comp. ex. 0.07 0.20 0.20 73 0.37 0.05
53 bg Comp. ex. 0.25 0.15 0.0020 76 0.24 0.19
54 bh Comp. ex. 0.10 0.04 80 2.50 0.14
55 bi Comp. ex. 0.60 0.0010 0.0006 77 0.25 0.03
56 bj Comp. ex. 0.10 0.10 73 5.25 0.16
57 bk Comp. ex. 0.10 0.10 90 0.33 0.03
58 bl Comp. ex. 87 0.45 0.17
59 bm Comp. ex. 0.04 0.23 0.05 95 0.22 0.03
60 bn Comp. ex. 0 .15 0 . 50 0 .15 100 0.30 0.02
61 bo Comp. ex. 0.07 0.02 109 0.42 0.04
62 bp comp. ex. 0.01 0.0006 0.10 97 0.18 0.06
63 bq Comp. ex. 0.10 0.30 57 2.73 0.09
~64 I br |comp. ex.| | |o.5o| |o .Ol|o • 00081 | | | 17 |#####| #VRLUE'.
To evaluate the hot rollability of a steel material,
5 a test material of a cross-section of 80 mm square was
cut out from the steel material, a thermocouple was
attached to the center part, and the material was loaded
into a 1050°C electric furnace.
When the center part of the test material reached
10 1045°C, the material was taken out from the electric
furnace after 30 minutes and was hot rolled when the
surface temperature reached 1000°C. The hot rolling was
performed for three continuous passes to successively
reduce the plate thickness of 80 mm to 64 mm, 51 mm, and
- 39 -
41 mm, then the material was air-cooled to room
temperature.
With a test material which cracked in the rollin(j,
the test was suspended at the point of time of cracking.
5 Other test materials were cooled, then a cross-section of
the rolled material was cut out and observed for surface
flaws. In the judgment of flaws, materials with flaws of
less than a depth of 100 )Lim were evaluated as "flawless"
while ones with flaws of over 100 ^m were evaluated as
10 "flawed".
As a result of observation, the steel materials of
the compositions which are shown in Table 1 and Table 2
(continuation of Table 1) (Invention Examples 1 to 46)
were found to be "flawless, so were then evaluated for
15 mechanical properties. In the steel material of the
compositions of Examples 53 and 56 to 58 which are shown
in Table 3, the materials were found to be "flawed".
As shown in Table 3, the steel materials of Examples
53, 56, and 58 had Sn, Cu, and Ni not satisfying
20 0.12xSn+Cu-0.1xNi<0.15. In Example 57, B (selective
element) exceeds the upper limit 0.005% of the range of
the present invention (see Table 3).
Regarding Examples 53 and 56 to 58 where were judged
to be "flawed" in the hot rolling, there was remarkable
25 occurrence of flaws at the stage of production of the
steel material and there was a problem in
manufacturability, so subsequent evaluation of the
properties was not performed (see Table 6).
In Example 55, the decarburization became remarkable
30 at the hot forging stage of the steel material. After
that, even if applying ferritic nitrocarborizing and
induction hardening, the hardness of the surface layer
part was insufficient, so subsequent evaluation of the
properties was not performed.
35 Regarding the other examples, the materials were
forged and annealed, then cylindrical test pieces of
- 40 -
diameters of 45 mm and lengths of 100 mm wm f&hn&Aled
for evaluation of machineability and then were' evaluated
for machineability. Note that, the test pieces for
evaluation of machineability were forged and annealed and
5 were evaluated for msclilneability in the ststs 6£ the
forged and annealed steel material.
The machineability was evaluated by a deep hole
drilling tQSt by MQL (minim™ quantity lubrication) using
an NC machining center used for production of gears,
10 crankshafts, and other auto parts.
Table 4 shows the drilling conditions. Holes were
drilled under the drilling conditions which are shown in
Table 4 and the number of holes until the drill broke was
measured. When reaching 1000 or more holes, the
15 machineability was judged as being good and the test was
ended.
Table 4
Machining c o n d i t i o n s Drill Others
C u t t i n g 65 m/min Size (|)5 nimx Hole 90 mm
speed length 168 forming
Feed 0.17 mm/rev Material mm depth Total number
L u b r i c a t i o n Mist Ceramic Judgment of holes
l u b r i c a t i o n coated of until d r i ll
(MQL) Projection cemented lifetime breakage
carbide {Test ended
105 mm at 1000
holes)
Table 5 and Table 6 show r e s u l t s of m a c h i n e a b i l i ty
20 evaluation t e s t s . As shown i n Table 5, i n t h e i n v e n t i on
examples of Examples 1 t o 46, t h e number of d r i l l e d holes
was over 1000 i n each case and t h e r e f o r e t he
m a c h i n e a b i l i t y was e x c e l l e n t .
On t h e o t h e r hand, as shown i n Table 6, i n t h e
25 comparative examples of Examples 50 t o 52 and 59, t h e
number of d r i l l e d holes f a i l e d t o reach 1000 and
t h e r e f o r e t h e m a c h i n e a b i l i t y was i n s u f f i c i e n t .
- 41 -
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- 42 -
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- 43 -
As the roller pitching test pieces, small roller
test pieces which have a cylindrical part of a diameter
of 26 Iran and a width of 28 mm and 1 arge roller test
pieces which have a diamet^^t «£ 130 M and a Width Of 18
5 nun were fabricated and evaluated for surface fatigue
strength. As test pieces for measurement of 300°C
tempering hardness, cylindrical test pieces of a diarriQtGr
of 26 mm and a length of 100 mm were fabricated and
evaluated for tempering softening resistance.
10 The small roller test pieces, large roller test
pieces, and cylindrical test pieces were nitrided by
ferritic nitrocarborizing (nitriding by nitriding gas
composition: N2 (0.45 NmVhr)+NH3 (0 . 5 NmVhr)+CO2 (0 , 05
Nm'^/hr) , then, after nitriding, cooling by N2 gas) , then
15 were induction hardened (frequency: 100 kHz).
The coolant used at the time of induction hardening
was tap water or a boiler quencher. After quenching, the
test piece was tempered at 150°C for 1 hr.
Due to the ferritic nitrocarborizing, at the
20 surfacemost layers of the test pieces, Fe-N compounds
were formed. Inside, nitrogen diffusion layers were
formed. The Fe-N compounds were mainly s compounds and y'
compounds.
E compounds are formed along the surfaces of the test
25 pieces in substantially even quality film shapes and form
clear boundaries from the matrix phase. The depth from
the surface to the boundary is defined as the thickness
of the nitrogen compound layer. Nitrogen diffuses and
permeates to the inside from the nitrogen compound layer
30 and forms a region with a high N concentration. The
region up to where the N concentration becomes the same
as the N concentration before ferritic nitrocarborizing
is defined as the nitrided layer.
The N diffusion and permeation region cannot be
35 differentiated by an optical microscope, to EPMA line
analysis is used for measurement of the N concentration
- 44 -
and quantitative differentiation. The thicknesses of the
nitrogen compound layer and nitrided layer become greater
the higher the temperature and/or the longer the time of
the ferritic nitrocarborizing.
5 The Pe-W compounds and matrix phase transformed to
austenite due to the induction heating, then transformed
to martensite at the subsequent quenching.
The nitrogen which was introduced by the ferritic
nitrocarborizing formed a solid solution in the austenite
10 at the heating stage. With short time induction heating,
leaving aside the surface, the concentration of nitrogen
at the inside did not change much at all compared with
the time of ferritic nitrocarborizing, so the martensite
has nitrogen forming a solid solution in it at a
15 concentration substantially the same as the time of
ferritic nitrocarborizing. The concentration of nitrogen
in martensite can be measured by EPMA line analysis.
If the nitrogen concentration in the steel material
becomes higher, the hardness, 300°C tempering hardness,
20 and the corrosion resistance are improved. The durability
of the steel material in the roller pitching test has a
positive correlation with the 300°C tempering hardness. In
the present invention, the target lower limit value was
made a Vicker's hardness of 650.
25 Due to the above heat treatment, the surface layer
part is formed with fine pores. These pores function as
pits which containing lubrication oil at the surface of
the steel part and improve the lubricating ability. In
the present invention, to secure a sufficient lubricating
30 ability, the lower limit of the pore density was set to
5000/mm^.
The pores are sometimes better to be removed,
depending on the usage environment, to improve the
mechanical properties. For example, in a usage
35 environment in which foreign matter is mixed, the
toughness of the regions including the pores becomes
insufficient and the pores form starting points for
- 45 -
peeling and breakage.
That is, there are sometimes cases where grinding
away the compound layer which is produced by the ferritic
nitrocarborizing after induction hardening is preferable.
5 However, if completely grinding away the nitrided layer,
the effect of ferritic nitrocarborizing is lost, so it is
necessary to leave the nitrided layer in a predetermined
thickness.
Each cylindrical test piece for measurement of the
10 300°C tempering hardness was soft nitrided, then induction
hardened under the same conditions as the small roller
test pieces and the large roller test pieces.
After that, the test piece was tempered at 300°C for
600 minutes and was measured for hardness from the
15 surface to the core part at a cut cross-section using a
Vicker's hardness meter. The surface layer was martensite
and the core part was ferrite-pearlite. The nitrogen
concentration at a location of 0.2 mm from the surface
was measured by EPMA.
20 The density of pores with a circle equivalent
diameter of 0.1 to 1 ^m was found by ferritic
nitrocarborizing and induction hardening a test piece
under the same conditions as the case of a small roller
test piece and large roller test piece, cutting it at a
25 plane perpendicular to the rolling direction, burying the
cut section in a resin, polishing this to a mirror
surface, and processing the surfacemost layer structure
by image processing.
The number of pores was measured at a magnification
30 of 3000X for 40 or more fields of 50 |Jin^ size each, the
measurement value was converted to a number of pores per
mm^, and it was judged if there were 5000 or more pores.
As shown in Table 5, in the invention examples of
Examples 1 to 46, the thickness of the nitrided layer
35 which was measured by EPMA was 0.4 mm or more. The
targeted 300°C tempering hardness and pore density were
- l e -
obtained.
Further, since the thickness of the nitrided layer
was 0.4 mm or more, even if removing the maximum amount
of grinding of 0.2 mm by grinding, a thickness of 0.2 mm
5 or more can be secured. A 300°C tempering hardness and the
99£tvgion rseistance could be Mint^inQd.
Further, in the invention examples of Examples 1 to
46, in the roller pitching fatigue tests under a 30%
water-containing lubrication oil environment, a
10 durability of 1,000,000 cycles (lO^X) was secured under
3500 MPa stress conditions.
On the other hand, as shown in Table 6, in Examples
47 to 48, 51, 54, and 60 to 64 of the comparative
examples of Examples 47 to 64, breakage occurred before
15 reaching 1,000,000 cycles under the above stress
conditions, so the durability in a corrosive environment
was insufficient.
The above heat treated large roller test pieces and
small roller test pieces were used for a standard surface
20 fatigue test of the "roller pitching fatigue test".
The roller pitching fatigue test was performed by
pushing against small roller test pieces by various hertz
stress surface pressures and against large roller test
pieces to make the peripheral speed directions of the two
25 rollers the same and by making the slip rate at the
contact parts -40% (peripheral speed of large roller test
piece becomes 40% larger than peripheral speed of small
roller test pieces) for rotation.
The temperature of the lubrication oil which was fed
30 to the contact parts (gear oil) was made 90°C and the
contact stress between the large roller test pieces and
the small roller test pieces was made 3500 MPa.
The cutoff in the tests was made 1,000,000 cycles
(lO^X) . When no pitching occurred at the small roller test
35 pieces and 1,000,000 cycles were reached, it was judged
that durability of the small rollers was secured.
Pitching was detected by a vibration meter which was
- 47 -
attached to the test machine. After vibration was
detected, the rotation of the two rollers was stopped ^fi'd
the occurrence of pitching and number of rotations were
confirmed.
5 Note that, it running the tests in an environment in
which the test pieces easily corrode, it is believed
possible to evaluate the durability in an actual
corrosive environment. In the lubrication oil, 30% water
was mixed.
10 As shown in Table 5, in each of the invention
examples of Examples 1 to 46, durability of 1,000,000
cycles (lO^X) was exhibited in an environment of a
lubrication oil which contains 30% water (corrosive
environment).
15 On the other hand, as shown in Table 6, in each of
Examples 47 to 49, 51, 58, and 60 to 64 among the
comparative examples of Examples 47 to 64, breakage
occurred before reaching 1,000,000 cycles. The durability
in an environment of a lubrication oil which contains 30%
20 water (corrosive environment) was insufficient.
The compositions of ingredients of the steel
materials which are used in Examples 1 to 46, as shown in
Table 1 and Table 2, are within the range of the present
invention. Further, the conditions of ferritic
25 nitrocarborizing and induction heating and guenching are
also in the range of the present invention.
On the other hand, the compositions of ingredients
of the steel materials which are used in Examples 47 to
64, as shown in Table 3, have one or more ingredients
30 outside the range of the present invention. In
particular, in a steel material which is insufficient in
ingredients which contribute to the 300°C tempering
hardness, for example, C, Si, etc. and in a steel
material which has a composition of ingredients which
35 contribute to the corrosion resistance and heat treatment
conditions outside the range of the present invention
(for example, a steel material with a Sn/Cr not
- 48 -
satisfying the range of the present invention and a Steel
material with nitriding conditions not satisfying the
tange of the present invention), the results of a rollsr
pitching test under a lubricating environment which
5 contains 30% water (corrosive environment) failed to
reach the targets and the durability was poor.
Further, if Mn/S is less than 70 and thG ferritic
nitrocarborizing conditions (temperature and time) are
not in the range of the present invention, the amount of
10 penett^tign Of N in the ferritic nitrocarborizing is
insufficient and similarly the durability is poor.
In the end, in the comparative examples of Examples
47 to 64, as shown in Table 6, (a) the surface fatigue
strength in a corrosive environment fails to reach the
15 target 1,000,000 cycles (lO^X), (b) even if a good
durability is exhibited, the machineability is
insufficient, or (c) numerous flaws occur or
decarburization becomes remarkable at the time of
production of the steel material and the mechanical
20 properties cannot be evaluated.
In Examples 49, 51, and 62, the Sn/Cr is low. This
is believed to be one cause of the insufficient fatigue
durability in a corrosive environment.
In Examples 48, 49, 63, and 64, the Mn/S is low and
25 concentration of S at the surface cannot be prevented, so
the thickness of the compound layer at the time of
ferritic nitrocarborizing is thin, the thickness of the
nitrided layer of the steel part after induction
hardening becomes a thin one of less than 0.4 mm, the N
30 concentration up to a depth of 0.2 mm from the surface is
low, and the Vicker's hardness with 300°C tempering is
less than 650. This is believed to be one cause of the
insufficient fatigue durability in a corrosive
environment.
35 In Example 47, the C concentration is low, so the
300°C tempering hardness is insufficient, while in Example
- 49 -
62, the Si concentration is low, so the 300°C tempering
hardness is insufficient. This is believed to be one
cause of the insufficient fatigue durability in a
corrosive environment.
5 II^ EMAilaple 54, Or is lower than the lower limit of
the range of the present invention, so it is believed the
corrosion resistance becomes insufficient and the fatigue
durability in a corrosive environment is insufficient. In
Example 60, Al exceeds the upper limit of the range of
10 the present invention, so it is believed that the Al
precipitates coarsen and the surface fatigue strength did
not reach the target under a corrosive environment.
In Example 61, the oxygen concentration exceeds the
upper limit of the range of the present invention, so
15 oxides are excessively formed. It is believed that these
oxides act as starting points of fatigue fracture at the
time of a fatigue test and result in insufficient
durability.
The machineability was evaluated by running a hole
20 drilling test under the conditions which are shown in
Table 4. In Examples 50 to 52 and 59, the target was
achieved.
In Example 50, C exceeds the range of the present
invention, in Examples 51 and 52, Mn exceeds the range of
25 the present invention, and in Example 59, Ni exceeds the
range of the present invention. These are considered the
causes for raising the hardness at ordinary temperature
and lowering the drillability.
(Example 2)
30 Even if the composition of ingredients of a steel
material is in the range of the present invention, if the
temperature and time of the ferritic nitrocarborizing do
not satisfy the range of the present invention, the
amount of penetration of N in the ferritic
35 nitrocarborizing is insufficient and the durability is
inferior.
Table 7 shows examples (invention examples and
- 50 -
comparative examples). In the comparative example of
Example 63, the ferritic nitrocarborizing temperature is
low, so a sufficient thickness of a nitrided layer is not
formed and the fatigue durability in a corrosive
5 environment is insufficient.
In the comparative example of Example 64, the
induction hardening temperature is less than 900°C, a
sufficient hardness cannot be obtained, and the fatigue
durability in a corrosive environment is insufficient. In
10 the invention examples of Examples 7 and 9, the fatigue
durability in a corrosive environment is sufficient.
- 51 -
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30 Claim 3
Steel for machine structural purposes for surface
hardening use as set forth in claim 1 or 2 characterized
in that said steel further contains, by mass%, one or
more of
35 B: 0.0003 to 0.005%,
W: 0.0025 to 0.5%,
- 55 -
Mo: 0.05 to 1.0%,
V: 0.05 to 1.0%,
Nb: 0.005 to 0.3%, and
Ti: 0.005 to 0.2%.
5
Claim 4
Steel for machine structural purposes for surface
hardening use as set forth in any one of claims 1 to 3
characterized in that said steel further contains, by
10 mass%, one or more of
Ca: 0.0005 to 0.01%,
Mg: 0.0005 to 0.01%,
Zr: 0.0005 to 0.05%, and
Te: 0.0005 to 0.1%.
15
Claim 5
A steel part for machine structural purposes which
is produced by nitriding or ferritic nitrocarborizing
then induction hardening steel for machine structural
20 purposes for surface hardening use as set forth in any
one of claims 1 to 4, said steel part characterized in
that
(a) a surface layer of a depth of 0.4 mm or more
from the surface is a nitrided layer and
25 (b) a Vicker's hardness is 650 or more at the
surface layer of a depth of 0.2 mm from the surface when
treating the steel by tempering at 300°C.
Claim 6
30 A steel part for machine structural purposes as set
forth in claim 5 characterized in that at a layer inside
said nitrided layer at a depth of 5 |Jin or more from the
surface, there are 5000/mm^ or more of pores of a circle
equivalent diameter of 0.1 to 1 |am.
35
Claim 7
- 56 -
A steel part for machine structural purposes as set
forth in claim 5 or 6 characterized in that a layer which
corresponds to the compound layer which is formed by
nitriding or ferritic nitrocarborizing at the surface
5 side in said nitrided layer is removed by machining.
Claim 8
A method of production of.a steel part for machine
structural purposes characterized by forming steel for
10 machine structural purposes for surface hardening use as
set forth in any one of claims 1 to 4 into a
predetermined shape by forging or/and cutting, then
treating the shaped steel member by nitriding or ferritic
nitrocarborizing at 550 to 650°C for 0.5 to 5 hours, then
15 treating it by induction heating at 900 to 1100°C for 0.05
to 5 seconds for quenching, and (a) forming a nitrided
layer at a surface layer of a depth of 0.2 mm or more
from the surface and (b) making a Vicker's hardness 650
or more at the surface layer of a depth of 0.2 mm from
20 the surface when tempering at 300°C.
Claim 9
A method of production of a steel part for machine
structural purposes as set forth in claim 8 characterized
25 by performing said induction hardening, then grinding
away the surface layer by an amount of grinding (mm)
satisfying the following formula (4):
0.05x{l-0.006x(600-nitriding or ferritic
nitrocarborizing temperature (°C))}x{(nitriding or
30 ferritic nitrocarborizing time (hr)+1)/3}