AA590
- 1 -
,
4p| DESCRIPTION
Title of Invention: Non-Quenched and Tempered Steel for I
Hot Forging Use and Hot Forged Non-Quenched and Tempered f
t
5 Product and Method of Production of Same f
I
I
Technical Field j
i
[0001] The present invention relates to a non-quenched
and tempered steel material for hot forging use which is r
10 to be worked into a machine part of an automobile, I
industrial machinery, etc. without quenching and i.
tempering after hot forging, a hot forged non-quenched j
and tempered part which uses the same, and a method of
production of the same, in particular a material which
15 has a high strength and a high endurance ratio as hot
forged without quenching and tempering and which can be
induction hardened.
Background Art
20 [0002] In the past, most machine and structural parts
of automobiles, industrial machinery, etc. are given high ?
strength and high toughness by reheating and by quenching f
and tempering after hot forging into a part shape from a •
steel bar material. In recent years, from the viewpoint 25 of reducing production costs, the quenching and tempering •
step is increasingly being eliminated. For example, as f
seen in PLT 1 etc., non-quenched and tempered steel which
can give high strength and high toughness as hot forged
even without quenching and tempering has been proposed.
30 Among these as well, in crankshafts and other shaft
parts, the steel is hot worked, then cooled to impart a
predetermined strength, further machined etc. to work it
into a predetermined shape, then treated at necessary ;
locations by induction hardening to form case hardened j'
35 layers so as to impart wear resistance and fatigue
strength. ;
[0003] PLT 1 describes using a steel material to which )
•
- 2 - i
t
•0 Si is added in over 1.0% and to which S, V, and N are
added in large amounts so as to obtain hot forged non- quenched and tempered steel which has a strength of a
conventional quenched and tempered material or more and a •
5 low temperature toughness as hot forged. However, it does not describe anything regarding the fatigue strength and I
the endurance ratio of this non-quenched and tempered l
steel.
[0004] Up to here as well, there have been several 10 reports of non-quenched and tempered steel for induction I
hardening use. For example, the invention which is described in PLT 2 is an invention which makes the ?.
structure before induction hardening one of a bainite
rate of 75% or more so as to prevent a drop in the case
15 hardness and depth of the case hardening layer due to the f
formation of residual ferrite after induction hardening.
However, there is no description at all regarding the fatigue strength and the endurance ratio of the nonquenched
and tempered steel for induction hardening use
20 as set forth in PLT 2. These properties are not
considered at all. f
[0005] Further, for example, the invention which is
set forth in PLT 3 is an invention which reduces the '<
amount of residual austenite after induction hardening.
25 However, in the non-quenched and tempered steel for
induction hardening use as set forth in PLT 3, there is
no description at all regarding the fatigue strength and
the endurance ratio. These properties are not considered
at all. Further, to improve the cuttability, it is
30 described that S, Pb, Bi, Te, Se, and Ca may be added in
suitable amounts, but it is learned that if the tensile
strength is 1100 MPa or more, the effect of improvement
of the cuttability is small.
[0006] In the application of these high strength non-
35 quenched and tempered steels to steel parts for machine ;
and structural use, what actually poses an obstacle is j
achieving the contradictory properties of higher fatigue
- 3 - j
I
K
*pk strength and cuttability. In general, fatigue strength is [
considered to be dependent on the tensile strength. If
making the tensile strength higher, the fatigue strength ;
becomes higher. On the other hand, the rise in tensile
5 strength causes a drop in cuttability. Most steel parts for machine and structural use require cutting after hot forging. This leads to a large increase in the cutting
costs. In general, when making the tensile strength
higher to raise the fatigue strength of a steel part for
10 machine and structural use, it is learned that if the ;
tensile strength is over 1300 MPa, the cuttability
remarkably falls. The cutting and production costs
greatly increase, so increasing the strength to a tensile [
strength of 1300 MPa is difficult in practice. Therefore,
15 in these parts for machine and structural use, the ?
increase in cutting costs due to the drop in cuttability
becomes a bottleneck in achieving higher fatigue j
strength. Both higher fatigue strength and cuttability are sought.
20 [0007] For example, PLT 4 describes an invention which
secures cuttability by suppressing the rise in strength
after hot forging and increases the depth of the case
hardening layer due to induction hardening so as to
improve the fatigue strength of the part as a whole.
25 [0008] Further, as the means for achieving both higher
fatigue strength and cuttability, raising the ratio of
the fatigue strength and the tensile strength, that is,
the endurance ratio (=fatigue strength/tensile strength),
is effective. For example, PLT 5 proposes that it is
30 effective to make the metal structure mainly bainite and ;
to reduce the high carbon martensite-austenite and
residual austenite in the structure. However, the
endurance ratio is at most 0.56 or less. There is a limit
to raising the strength without reducing the cuttability.
35 The fatigue strength is also low in each case.
[0009] PLT 6 describes a crankshaft and method of
production of the same which can obtain a high wear
|!;-
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*P resistance and fatigue strength and achieve high
machineability. In this method, the micrometallic
structure of the hot forged product before a soft
nitriding treatment is made a mainly bainite (70% or
5 more) structure and, further, this hot forged product is
soft nitrided under 550 to 650°C temperature conditions so
as to improve the fatigue strength and other mechanical
properties of the crankshaft. To suitably increase the
internal hardness after soft nitriding and obtain a high
10 fatigue strength, the ingredients of the steel material ;
are prescribed using a parameter Hg which expresses the amounts of C, Si, Mn, Cr, Mo, and V of the steel material
by a specific relationship. However, soft nitriding
treatment usually has to be performed by exposure to an
15 atmosphere containing nitrogen and heating in a
temperature region of the austenization temperature or <
less. The equipment and cost become more expensive :
compared with case hardening treatment using induction hardening. Further, a steel material designed for soft
20 nitriding treatment over a certain time period contains a
large amount of Si, so in induction hardening by
instantaneous induction heating at only the surface,
residual austenite remains in the internal structure and
high fatigue strength cannot be obtained.
25
Citations List
Patent Literature
[0010] PLT 1: Japanese Patent Publication No. 1-198450
Al
30 PLT 2: Japanese Patent Publication No. 63-100157 Al <
PLT 3: Japanese Patent Publication No. 11-286744 Al
PLT 4: Japanese Patent Publication No. 2005-68518 Al
PLT 5: Japanese Patent Publication No. 4-176842 Al
PLT 6: Japanese Patent Publication No. 2010-189697 Al
35
Summary of Invention
Technical Problem
>
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^f [0011] Therefore, the present invention advantageously
solves the above such problem and has as its object the
provision of a non-quenched and tempered steel for hot
forging use which enables induction hardening and a hot
5 forged non-quenched and tempered part and method of >
production of the same which control the structure inside J
a part by cooling after hot forging so as to suppress the
drop in cuttability accompanying high strength and to
improve the fatigue strength. ;
10 j
Solution to Problem i
[0012] The present invention discovers that by using
steel comprised of high carbon steel given a high case hardness by induction hardening and containing a large
15 amount of added Mo, causing the precipitation of a large
amount of Mo carbonitrides in the cooling process after ;
hot forging, and reducing the density of dislocations and other defects in the matrix, it is possible to obtain a '
hot forged non-quenched and tempered part which has a
20 high endurance ratio, suppresses the drop in cuttability •
accompanying high strength, and improves the fatigue
strength and thereby completed the present invention.
Here, the "carbonitrides" which is used in the *
Description means carbonitrides and carbides.
25 [0013] The gist of the present invention is as
follows:
[0014] (1) Non-quenched and tempered steel for hot
forging use able to be treated by induction hardening
which comprises, by mass%, C: 0.45 to 0.60%, Si: 0.02 to
30 0.15%, Mn: 1.50 to 3.00%, P: 0.0002 to 0.150%, S: 0.001
to 0.200%, Cr: 0.02 to 1.00%, Al: 0.001 to 0.300%, V:
0.01 to 0.30%, Mo: 0.03 to 1.00%, and N: 0.0020 to
0.0070% and which has a balance of Fe and unavoidable
impurities.
35 [0015] (2) The non-quenched and tempered steel for hot
forging use able to be treated by induction hardening as
set forth in the above (1) characterized by, furthermore,
- 6 -
^ comprising, by mass%, one or more of Ca: 0.0002 to
0.0100%, Te: 0.0002 to 0.1000%, and Zr: 0.0002 to
0.2000%.
[0016] (3) An induction hardenable hot forged non-
5 quenched and tempered part which has the steel {
ingredients described in the above (1) or (2), which has !
a steel structure of an area ratio 95% or more bainite
structure, and which has Mo carbonitrides which are
dispersed in the steel with an average size of 4 ran to 11 I
10 nm. |
[0017] (4) A method of production of a hot forged non- i
quenched and tempered part characterized by heating a i
steel material comprised of the composition of ?
ingredients as set forth in the above (1) or (2) to 1000°C
15 to 1250°C, hot forging it, then, after that hot forging,
cooling it by an average cooling rate down to 200°C by I
0.05°C/sec to 0.80°C/sec, then treating the locations
where strength is required by induction hardening. j:
20 Advantageous Effects of Invention ;
[0018] The steel of the present invention is optimal
as a material for an induction hardenable hot forged non- i
quenched and tempered steel part use which suppresses an
increase in cutting costs while providing high fatigue
25 strength. Further, according to the method of production
of the present invention, it is possible to produce an
induction hardenable hot forged non-quenched and tempered j
part which has a high endurance ratio and high fatigue j
strength. Furthermore, when used as a part for automobile I
30 use or industrial machinery use, the hot forged non- i
quenched and tempered part of the present invention can be induction hardened, so much higher strength of the <
part becomes possible and lighter weight, reduced fuel consumption, and lower cost of the vehicle can be
35 contributed to.
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jf Description of Embodiments
[0019] The inventors engaged in intensive studies on
the range of steel ingredients, type of structure, and
heat treatment conditions for the above-mentioned object
5 and discovered the following: That is, i
(a) By making fine Mo carbonitrides disperse in an area
ratio 95% or more bainite structure, there is a higher
endurance ratio (=fatigue strength/tensile strength) than
conventional non-quenched and tempered steel. The larger
10 the effect of the cooling rate after hot forging and the
smaller the cooling rate, the more the endurance ratio is
improved. This is because the smaller the cooling rate,
the longer the time in the temperature region at which Mo
carbonitrides precipitate and the greater the amount of
15 precipitation, so the more the tensile strength and
fatigue strength rise, but the smaller the cooling rate,
the coarser the carbonitrides and the more remarkably the
tensile strength drops, while the more the fatigue
strength rises or is maintained without falling. In
20 general, with the precipitation of carbonitrides of Mo
etc. which are used for precipitation strengthening, not ;
only the fatigue strength, but also the tensile strength
rises and the cuttability is remarkably lowered, so •
higher fatigue strength and good cuttability cannot both
25 be achieved. It was learned that by coarsening the
• precipitates and controlling the size of Mo carbonitrides ;
to 4 nm to 11 nm, it is possible to raise the fatigue
strength without raising the tensile strength which
affects the cuttability. However, the main structure has
30 to be made a bainite structure and the balance of the
pro-eutectoid ferrite and residual austenite structure
have to be made an area ratio of less than 5%.
(b) V, in the same way as Mo, forms carbonitrides and
improves the ration of endurance, but, with high N, forms '
35 stable V nitrides at a higher temperature, forms nuclei
of pro-eutectoid ferrite in the cooling process after hot
forging, and leads to a drop in strength and endurance i
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y ratio. To sufficiently utilize the effect of improvement
of the endurance ratio by V carbonitrides, low N is a
necessary condition.
[0020] The present invention was first made by further
5 study based on these findings.
[0021] Below, the present invention will be explained
in detail. First, the reasons for limitation of the
above-mentioned range of ingredients of steel will be
explained.
10 [0022] C: 0.45 to 0.60%
C is an important element which determines the
strength of steel. Compared with other alloy elements,
the alloy cost is low. The alloy cost of the steel
material can be reduced by adding a large amount of C.
15 Further, the case hardening after the induction hardening
treatment is determined by the amount of C in the steel.
To obtain the necessary strength, the lower limit is made
0.4 5%. However, if adding a large amount of C, at the
time of bainite transformation, residual austenite and
20 martensite-austenite is formed with C concentrated at the
lath boundaries and the endurance ratio falls, so the
upper limit is made 0.60%. Note that, "steel able to be
treated by induction hardening" of the present invention
means steel able to be given at least a strength which
25 case hardening after induction hardening treatment
reguires. Therefore, in the present invention, this is
steel which has a 0.45% or more amount of C. To obtain a
higher strength, an amount of C over 0.5% is preferable.
[0023] Si: 0.02 to 0.15%
30 Si is an element which increases the amount of
residual austenite in steel by bainite transformation in
the cooling process after hot forging. When performing
treatment by induction hardening which heats only the
case layer, residual austenite remains at the non-heated
35 parts. If the amount of Si exceeds 0.15%, the fatigue
strength and the endurance ratio remarkably fall.
Therefore, the amount is restricted to 0.15% or less.
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TJ However, if kept down to less than 0.02%, the production
cost becomes massive, so the lower limit is made 0.02%.
[0024] Mn: 1.50 to 3.00%
Mn is an element which accelerates bainite
5 transformation and is an important element for making the
structure bainite in the cooling process after hot i
forging. Furthermore, it bonds with S to form sulfides
and has the effect of improving the cuttability. To
achieve these effects, the lower limit is made 1.50%. On
10 the other hand, if adding over 3.00% of an amount of Mn,
the base material becomes hard and brittle, so conversely
the cuttability remarkably falls. The upper limit is made
3.00. In particular, an amount of Mn over 2.0% is
preferable since it results in an area ratio 95% bainite
15 structure even at a slow cooling rate.
[0025] P: 0.0002 to 0.150%
P is contained in steel as unavoidable impurities
usually in an amount of 0.0002% or more, so the lower
limit is made 0.0002% or more. If added in a large
20 amount, P is an element which segregates at the prior
austenite grain boundaries and aggravates cracking after
induction hardening, so the upper limit is made 0.150%.
Preferably, it is 0.100% or less, more preferably 0.050%
or less.
25 [0026] S: 0.001 t o 0.200%
S forms sulfides with Mn and has the effect of
improving the cuttability. To achieve this effect, the
lower limit is made 0.001%. S forms sulfides with Mn and j
has the effect of improving the cuttability. Further, it
30 also has the effect of suppressing the growth of
austenite grains and maintaining a high toughness. To
achieve these effects, the lower limit is made 0.001%.
However, while also dependent on the amount of Mn, if
added in a large amount, the mechanical properties become
35 large in anisotropy, so the upper limit is made 0.200%.
[0027] Cr: 0.02 to 1.00%
Cr, like Mn, is an element which is effective for
r
'-
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jr promoting bainite transformation. To achieve these
effects, the lower limit is made 0.02%. However, if §
adding a large amount of Cr, Fe-based carbides are I
stabilized and, in the case of induction hardening, the 1
5 case hardening falls, so the upper limit is made 1.00%. I
[0028] Al: 0.001 to 0.300% |
Al precipitates and disperses as nitrides in the §
steel, so has the effects of preventing coarsening of the |
austenite structure at the time reheating for forging and
10 preventing coarsening of the subsequent bainite
structure. Furthermore, Al has the effect of bonding with
oxygen and depositing on the tool surface to prevent tool
wear at the time of machining. To achieve these effects,
the lower limit is made 0.001%. Preferably, the content
15 is made 0.050% or more, more preferably 0.100%. On the ;
other hand, if over 0.300%, a large amount of hard
inclusions are formed and either the endurance ratio or
cuttability falls. Therefore, the upper limit is made
0.300%.
20 [0029] V: 0 . 0 1 t o 0.30%
V is an element which is effective for promoting
bainite transformation. Further, it is an element which
is effective for forming carbonitrides, strengthening the
bainite structure by precipitation strengthening, and
25 raising the endurance ratio. To achieve these effects, ;.
0.01% or more in content is necessary. On the other hand,
if over 0.30%, the effects become saturated, so the upper
limit is made 0.30%. ?
[0030] Mo: 0.03 to 1.00%
30 Mo is not only an element which is effective for
promoting bainite transformation, but has the largest !
solid solubility in austenite compared with V, Ti, Nb, or >
other alloy elements which give precipitation j
strengthening by alloy carbides and gives a large amount [
35 of precipitation of Mo carbonitrides in the cooling process. This is not preferable since the precipitation I
r ;
of carbonitrides of Mo etc. which are generally used for »
i
:
1
- 11 - f
^ precipitation strengthening causes not only the fatigue
strength, but also the tensile strength to rise and the i
cuttability to be remarkably lowered. However, it was
learned that if the size of the Mo carbonitrides is
5 controlled to 4 nm to 11 nm, it is possible to raise only
the fatigue strength without raising the tensile strength
which affects the cuttability, that is, the fatigue
strength and the endurance ratio can be raised. To
achieve these effects, 0.03% or more in content is
10 necessary. On the other hand, if over 1.00%, the effects
become saturated, so the upper limit is made 1.00%.
[0031] N: 0.0020 to 0.0070%
N generally forms nitrides with V which are utilized
for preventing coarsening of the austenite structure at
15 the time of hot forging, but V nitrides form nuclei for ;
pro-eutectoid ferrite and conversely promote the
transformation of pro-eutectoid ferrite and cause a drop
in strength and endurance ratio. To suppress the
formation of V nitrides, the upper limit of the amount of
20 N is made 0.0070%. Further, this is usually included, as
an unavoidable impurity in steel, in 0.0020% or more, so
the lower limit is made 0.0020%. r
[0032] Content of One or More of Ca: 0.0002 to
0.0100%, Te: 0.0002 to 0.1000%, and Zr: 0.0002 to 0.2000%
25 Ca, Te, and Zr all form oxides, form nuclei for
precipitation of Mn sulfides and have the effect of
uniform fine dispersion of Mn sulfides. Further, each
element forms a solid solution in Mn sulfides, causes a
drop in the transformation ability, suppresses the
30 elongation of the shape of the Mn sulfides after rolling
or hot forging, and reduces the anisotropy of mechanical
properties. To achieve these effects, the lower limits of :
Ca, Te, and Zr are made 0.0002%. On the other hand, if Ca
exceeds 0.0100%, Te exceeds 0.1000%, and Zr exceeds i
35 0.2000%, conversely these oxides, sulfides, and other
hard inclusions are formed in large amounts and the i;
endurance ratio and cuttability fall. Therefore, the
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^ upper limit of Ca is made 0.0100%, the upper limit of Te
is made 0.1000%, and the upper limit of Zr is made
0.2000%. [0033] Next the reasons for limitation of the steel
5 structure of hot forged non-quenched and tempered part
which is explained above will be explained.
[0034] (Area Ratio 95% or More Bainite Structure)
The structure is prescribed to be an area ratio 95% I
or more bainite structure because if the main structure
10 is a bainite structure, while there is a high endurance
ratio, if the remaining structure of ferrite, residual
austenite, or martensite-austenite has an area ratio of
5% or more, the endurance ratio remarkably falls. The :
smaller the remaining structure, the higher the endurance
15 ratio, preferably an area ratio of 97% or more.
[0035] (Average Size of Mo Carbonitrides Dispersed in
Steel of 4 nm to 11 nm)
The average size of Mo carbonitrides in the bainite
structure is prescribed as 4 nm or more because if the
20 average size is less than 4 nm, there is a high fatigue
strength, but simultaneously the tensile strength is also
high and the value of the endurance ratio is small, so
both higher fatigue strength and cuttability cannot be
realized. More preferably, the average size is 8 nm or
25 more. Further, the upper limit value of the average size i
of the Mo carbonitrides is prescribed as 11 nm because
with an average size of over 11 nm, not only the tensile
strength, but also the fatigue strength remarkably falls, !
so higher fatigue strength cannot be achieved. Note that,
30 Mo carbonitrides are pin shaped. The "size" of Mo
carbonitrides which is used in the Description is the i.
length in the longitudinal direction.
[0036] Next the reasons for limitation of the method
of production of the hot forged non-quenched and tempered
35 part which is explained above will be explained.
[0037] (Heating Steel Material to 1000°C or less and to
1250°C or more)
•
- 13 -
jF The steel material comprised of the above-mentioned
composition of ingredients is prescribed as being heated
to 1000°C or less and to 1250°C or more so as to cause
carbonitrides of Mo and V to sufficiently precipitate in
5 the cooling process by heating before hot forging so as
to make Mo, V sufficiently form a solution in the steel.
If less than the heating temperature 1000°C, Mo and V
cannot be made to sufficiently form a solution in the •
steel, the amount of precipitation strengthening in the
10 subsequent cooling process is small, and the fatigue
strength and the endurance ratio become small. On the
other hand, if raising the heating temperature more than
necessary, growth of the austenite grains is promoted,
the structure transformed in the subsequent cooling
15 process becomes coarse, and conversely the endurance
ratio falls. Therefore, the upper limit of the heating
temperature is made 1250°C. f.
[0038] (Hot Forging, Then Cooling Down to 200°C by an
Average Cooling Rate of 0.05°C/sec to 0.80°C/sec)
20 The average cooling rate after the hot forging down I
to 200°C or less is prescribed as 0.05°C/sec to 0.80°C/sec
so as to lengthen the time at the temperature region at
which the Mo carbonitrides precipitate, increase the
amount of precipitation in the cooling process, and •-
25 control the size of carbonitrides. With an average
cooling rate of 0.80°C/sec or more, the amount of
precipitation of Mo carbonitrides is not sufficiently
obtained and the effect of improvement of strength and
endurance ratio is small. In particular, to coarsen the ;:
30 Mo carbonitrides and obtain a high endurance ratio,
preferably the average cooling rate is made 0.50°C/sec or
less. More preferably, it is made 0.30°C/sec or less. On
the other hand, if the average cooling rate is less than
0.05°C/sec, an area ratio 5% or more pro-eutectoid ferrite
35 is formed at the bainite lath boundary, and the fatigue
i
j
- 14 -
jr strength and endurance ratio remarkably fall.
[0039] Note that, an induction hardenable hot forged
non-quenched and tempered part which has a high fatigue
strength can be obtained by the present invention, but to
5 secure sufficiently cuttability, the tensile strength is
preferably 1300 MPa or less.
[0040] The present invention will be explained in
detail below by examples. Note that, these examples are
for explaining the technical significance and effects of
10 the present invention and do not limit the scope of the
present invention.
Examples
[0041] Steel of each chemical composition which is
15 shown in Table 1 was smelted in a 150 kg vacuum melting
furnace. This was rolled to a steel bar with a diameter
of 100 mm, then a test piece for forging use was cut out
and was forged and heat treated under the conditions
which are shown in Table 2. The cooling method down to
20 200°C after hot forging was air cooling or furnace
cooling, while the cooling rate was controlled by •
changing the diameter of the test piece. The average
cooling rate was found by dividing the value of the
temperature of the test piece after hot forging minus
25 200°C by the time required for cooling down to 200°C after
hot forging. In addition, for comparison, the
conventional steel S55C was melted and heat treated to
give a tensile strength of the same extent as the present
invention. The test piece was heated to 1100°C, then water
30 cooled down to room temperature, then heat treated again
at 450°C for 1 hour. Note that, the underlined parts in I
Table 1 and Table 2 show conditions outside the scope of
the present invention.
[0042] From the center parts of these forged
35 materials, a JIS Z 2201 No. 14 tensile test piece and a
JIS Z 2274 No. 1 rotating bending fatigue test piece were
- 15 - :
JF taken and the tensile strength and fatigue strength
•
found. Here, the fatigue strength is defined as the
stress amplitude which is endured without breaking at 107 [
rotations in a rotating bending fatigue test. Further, 1
5 the "endurance ratio" (fatigue strength/tensile strength) I
is found as the ratio of the fatigue strength and tensile I
strength.
[0043] From the forged material at a part of 1/4 ;
thickness in the L direction, a test piece for ;
10 examination of the structure was taken. The area ratio of
bainite was calculated by polishing the test piece to a
mirror surface, etching it by Le Pera etching, confirming
the rest of the structure other than bainite, that is, ^
the pro-eutectoid ferrite, residual austenite,
15 martensite-austenite, and other structures, capturing •
500X optical micrographs at 10 fields each, then
performing image analysis. I
[0044] The average size of the Mo carbonitrides was
obtained by finishing a test piece by the electrolytic
20 polishing method to a thin film, then using a '
transmission type electron microscope to capture 15000X
transmission type electron micrographs for 10 fields, E
finding the lengths in the longitudinal direction of the .
Mo carbonitrides observed in them by image analysis, and 25 finding their average value. [0045] The present invention steels of Nos. 1 to 18 ;
all have area ratio 95% or more bainite structures, Mo
carbonitrides of average sizes of 4.6 nm to 10.8 nm, and
endurance ratio of 0.58 or more high endurance ratio. To
30 secure cuttability, the tensile strength is 1300 MPa or
less, but as clear from a comparison with the same extent >
of tensile strength, higher fatigue strengths than the
guenched and tempered steel of the carbon steel of i
i
Conventional Example No. 28 are realized. •
35 [0046] As opposed to this, Comparative Example Nos.
23, 24, and 27 have large contents of either C, Si, or N. ;
Further, No. 21 is in the prescribed range of steel i
- 16 -
r
*~ composition, but has an average cooling rate outside the ;
prescribed value, has a large amount of ferrite, residual
austenite, etc. at the bainite lath boundaries, and has
an average size of Mo carbonitrides outside the
5 prescription, so is low in strength and endurance ratio.
-
Nos. 19 and 22 have steel compositions or heat treatment
conditions outside the prescription, are not sufficiently
strengthened by precipitation strengthening, and are low
in endurance ratio. No. 20 has the heating temperature •
10 made higher than necessary, so the bainite structure I
becomes coarser and conversely the endurance ratio is •
low. No. 25 has Mn added more than the necessary, the
tensile strength is high, and cutting becomes extremely
difficult. On the other hand, No. 26 has more than the 15 necessary amount of Al added and conversely falls in i
fatigue strength and endurance ratio. ;
[0047] As clear from this, the examples which satisfy
all of the conditions which are prescribed in the present i
invention are superior to the comparative examples and I
20 conventional examples in toughness and fatigue
characteristics.
:
i
- 17 -
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CLAIMS
1. Non-quenched and tempered steel for hot forging
use able to be treated by induction hardening which
• comprises, by mass%,
-^ 5 C: 0.45 to 0.60%,.
Si: 0.02 to 0.15%,
Mn: 1.50 to 3.00%,
P: 0.0002 to 0.150%,
S: 0.obi-to 0.200%,
10 Cr: 0.02 to 1.00%,
Al: 0.001 to 0.300%,
V: 0.01 to 0.30%,
Mo: 0.03 to 1.00%, and
N: 0.0020 to 0.0070% and which has a balance of Fe and
15 unavoidable impurities.
2. The non-quenched and tempered steel for hot
forging use able to be treated by induction hardening as
set forth in claim 1 characterized by, furthermore,
comprising, by mass%, one or more of Ca: 0.0002 to
20 0.0100%, Te: 0.0002 to 0.1000%, and Zr: 0.0002 to
0.2000%.
3. An induction hardenable hot forged non-quenched
-. and tempered part which has the steel ingredients
described in claim 1 or 2, which has a steel structure of
25 an area ratio 95% or more bainite structure, and which
,:;•' has Mo carbonitrides which are dispersed in the steel
A with an average size of 4 nm to 11 nm.
4. A method of production of a hot forged nonquenched
and tempered part characterized by heating a
30 steel material comprised of the composition of
ingredients as set forth in claim 1 or 2 to 1000°C tjD
1250°C, hot forging it, then, after that hot forging,
cooling it by an average cooling rate down to 200°C by
0.05°C/sec to 0.80°C/sec, then treating the locations
35 where strength is required by induction hardening.