Name of Document] DESCRIPTION
[Title of the Invention] GAS-CARBURIZED STEEL PART EXCELLENT IN
SURFACE FATIGUE STRENGTH, STEEL PRODUCT FOR GAS
CARBURIZING; MANUFACTURING METHOD OF
5 GAS-CARBURIZED STEEL PART
[Technical Field]
[0001] The present invention relates to a gas-carburized steel part
excellent in surface fatigue strength, and further relates to a steel product for
gas carburizing used for manufacturing the above gas-carburized steel part
10 and a manufacturing method of a gas-carburized steel part using the steel
product.
[Background Art]
[0002] Steel parts such as a gear and a bearing are used in a severe
environment where they are subjected to a large load by transmission of
15 torque or the like. For this reason, high fatigue strength and abrasion
resistance are required for the above-described steel parts. After being
formed into a shape to be used, these steel parts are subjected to a surface
hardening treatment to have the required high fatigue strength and abrasion
resistance provided thereto while securing the internal toughness.
20 [0003] Si in steel exhibits high resistance to temper softening in high
carbon martensite, so that for achieving an increase in strength of surface
fatigue strength, the content of Si is desirably increased. For example, in
Patent Document 1, there has been disclosed a technique in which the content
of Si in steel is set to 0.5 to 3.0% and vacuum carburizing is performed.
25 However, the vacuum carburizing has disadvantages in that a continuous
treatment is difficult to be performed, tarring occurs, it is difficult to control
2
part properties, and so on, and thus it is difficult to achieve mass production.
[0004] In contrast to this, gas carburizing does not have these
disadvantages, and as a surface hardening treatment intended for mass
production, the gas carburizing is more preferable than the vacuum
5 carburizing.
[0005] However, Si in the steel deteriorates carburizing performance in
the gas carburizing. The deterioration of carburizing performance means
that under the same carburizing condition, the depth of a hardened layer
obtained by the carburizing is inferior to that of a case hardened steel used
10 normally such as a JIS steel SCr420.
[0006] For example, Non-Patent Document 1 has reported that with an
increase in content of Si, a gas-carburized depth is decreased, and the upper
limit of the content of Si to which gas. carburizing is applicable is 1.2%. For
this reason, the development of a technique making gas carburizing possible
15 on a high Si-containing steel has been desired.
[Prior Art Document]
[Patent Document]
[0007]
[Patent Document 1] Japanese Laid-open Patent Publication No.
20 2008-280610
[Non-Patent Document]
[0008]
[Non-Patent Document 1] "Testu-to-Hagane" Vol. 58 (1972) No. 7 (June
1, 1972, published by The Iron and Steel Institute of Japan), page 926
25 [Disclosure of the Invention]
[Problems to Be Solved by the Invention]
[0009] In consideration of the above circumstances, the present invention
has an object to provide a steel part preventing resistance to temper softening
from decreasing and having excellent surface fatigue strength, Further, the
present invention has an object to provide a steel product for gas carburizing
5 suitable for manufacturing the above steel part and a manufacturing method
of a gas-carburized steel part.
[Means for Solving the Problems]
[0010] When the content of Si in a steel product is increased as described
above, the resistance to temper softening improves, but the gas carburizing
10 performance deteriorates. Then, the present inventors earnestly studied a
method in which the gas carburizing performance does not deteriorate even
though the content of Si is increased.
[0011] As a result, there obtained the knowledge in which in order to
improve the resistance to temper softening, Expression (1) below is required
15 to be satisfied when the contents of Si, Mn, and Cr (mass%) in the steel
product are set to [Si%], [Mn%], and [Cr%].
32 ? 3.5[Si%] + [Mn%] + 3[Cr%] > 9 ... (1)
[0012] Further, there obtained the knowledge in which in order to prevent
the gas carburizing performance from deteriorating even though the content of
20 Si is° increased, an alloy shortage layer satisfying Expression (2) below is
required to exist in a range from the surface of the steel product to 2 to 50 μm
in depth.
3.5[Si%] + [Mn%] + 3[Cr°ro] < 9_ (2)
[0013] The present invention has been made based on the
25 above-described knowledge, and the gist of the present invention is as
follows.
4
[0014] (1) A gas-carburized steel part having a gas-carburized layer
containing C: 0.50 mass% or more on a surface thereof includes: a
composition of a base metal contains, in mass%, C: 0.1 to 0.4%, Si:
exceeding 1.2 to 4.0%, Mn: 0.2 to 3.0%, Cr: 0.5 to 5.0%, Al: 0.005 to 0.1%,
5 S: 0.001 to 0.3%, N: 0.003 to 0.03%, and 0: limited to 0.0050% or less, and
P: limited to 0.025% or less, when the contents of Si, Mn, and Cr (mass%) are
set to [Si%], [Mn%], and [Cr%], Expression (1) below is satisfied, and an
alloy shortage layer satisfying Expression (2) below exists in a range from the
surface to 2 to 50 μm in depth.
10 32 >- 3.5[Si%] + [Mn%] + 3[Cr%] > 9 ...' (1)
3.5[Si%] + [Mn%] + 3[Cr%] < 9 ... (2)
(2) The gas-carburized steel part according to (1), in which the composition of
the base metal further contains, in mass%, one type or two types or more of
Nb: 0.01 to 0.3%, Ti: 0.01 to 0.3%, and V: 0.01 to 0.3%.
15 (3) The gas-carburized steel part according to (1), in which the composition of
the base metal further contains, in mass%, one type or two types or more of
Ni: 0.2 to 3.0%, Cu: 0.2 to 3.0%, Co: 0.2 to 3.0%, Mo: 0.05 to 0.4%, W: 0.05
to 0.4%, and B: 0.0006 to 0.005%.
(4) The gas-carburized steel part according to (1), in which the composition of
20 the base metal further contains, in mass%, one type or two types or more of
Nb: 0.01 to 0.3%, Ti: 0.01 to 0.3%, and V: 0.01 to 0.3%, and one type or two
types or more of Ni: 0.2 to 3.0%, Cu: 0.2 to 3.0%, Co: 0.2 to 3.0%, Me: 0.05
to 0.4%, W: 0.05 to 0.4%, and B: 0.0006 to 0.005%.
(5) A steel product for gas carburizing used for manufacturing a carburized
25 steel part, the steel part for gas carburizing includes: in mass%, C: 0.1 to 0.4%,
Si: exceeding 1.2 to 4.0%, Mn: 0.2 to 3.0%, Cr: 0.5 to 5.0%, Al: 0.005 to
5
0.1%, S: 0.001 to 0.3%, N: 0.003 to 0.03%, and 0: limited to 0.0050% or less,
and P: limited to 0.025% or less, in which when the contents of Si, Mn, and
Cr (mass%) are set to [Si%], [IVln%], and [Cr%], Expression (1) below is
satisfied.
5 32 3.5[Si%] + [Mn%] + 3[Cr%] > 9 ... (1)
(6) The steel product for gas carburizing according to (5), further includes: in
mass%, one type or two types or more of Nb: 0.01 to 0.3%, Ti: 0.01 to 0.3%,
and V: 0.01 to 0.3%.
(7) The steel product for gas carburizing according to (5), further includes: in
10 mass%, one type or two types or more of Ni: 0.2 to 3.0%, Cu: 0.2 to 3.0%,
Co: 0.2 to 3.0%, Me: 0.05 to 0.4%, W: 0.05 to 0.4%, and B: 0.0006 to
0.005%.
(8) The steel product for gas carburizing according to (5), further includes: in
mass%, one type or two types or more of Nb: 0.01 to 0.3%, Ti: 0.01 to 0.3%,
15 and V: 0.01 to 0.3%, and one type or two types or more of Ni: 0.2 to 3.0%,
Cu: 0.2 to 3.0%, Co: 0.2 to 3.0%, Me: 0.05 to 0.4%, W: 0.05 to 0.4%, and B:
0.0006 to 0.005%.
(9) A steel product for gas carburizing used for manufacturing a carburized
steel part, in which a composition of a base metal contains, in mass%, C: 0.1
20 to 0.4%, Si: exceeding 1.2 to 4.0%, IVln: 0.2 to 3.0%, Cr: 0.5 to 5.0%, Al:
0.005 to 0.1%, S: 0.001 to 0.3%, N: 0.003 to 0.03%, and 0: limited to
0.0050% or less, and P: limited to 0.025% or less, when the contents of Si,
Mn, and Cr (mass%) are set to [Si%], [Mn%], and [Cr%], Expression (1)
below is satisfied, and an alloy shortage layer satisfying Expression (2) below
25 exists in a range from its surface to 2 to 50 pam in depth.
32 >_ 15[Si%] + [Mn%] + 3[Cr%] > 9 ... (1)
3.5[Si%] + [Mn%] + 3[Cr%] < 9 ... (2)
(10) The steel product for gas carburizing according to (9), in which the
composition of the base metal further contains, in mass%, one type or two
types or more of Nb: 0.01 to 0.3%, Ti: 0.01 to 0.3%, and V: 0.01 to 0.3%.
5 (11) The steel product for gas carburizing according to (9), in which the
composition of the base metal further contains, in mass%, one type or two
types or more of Ni: 0.2 to 3.0%, Cu: 0.2 to 3.0%, Co: 0.2 to 3.0%, Mo: 0.05
to 0.4%, W: 0.05 to 0.4%, and B: 0.0006 to 0.005%.
(12) The steel product for gas carburizing according to (9), in which the
10 composition of the base metal further contains, in mass%, one type or two
types or more of Nb: 0.01 to 0.3%, Ti: 0.01 to 0.3%, and V: 0.01 to 0.3%, and
one type or two types or more of Ni: 0.2 to 3.0%, Cu: 0.2 to 3.0%, Co: 0.2 to
3.0%, Mo: 0.05 to 0.4%, W: 0.05 to 0.4%, and B: 0.0006 to 0.005%.
(13) A manufacturing method of a gas-carburized steel part using a steel
15 product for gas carburizing containing, in mass%, C: 0.1 to 0.4%, Si:
exceeding 1.2 to 4.0%, Mn: 0.2 to 3.0%, Cr: 0.5 to 5.0%, Al: 0.005 to 0.1%,
S: 0.001 to 0.3%, N: 0.003 to 0.03%, and 0: limited to 0.0050% or less, and
P: limited to 0.025% or less and in which when the contents of Si, Mn, and Cr
(mass%) are set to [Si%], [Mn%], and [Cr%], Expression (1) below is
20 satisfied, the manufacturing method includes: performing primary carburizing
in which the steel product for gas carburizing is subjected to a heat treatment
under an atmosphere where an oxide layer is formed; removing the oxide
layer formed on a surface; and then, performing secondary carburizing in a
carburizing atmosphere.
25 32 ? 3.5 [Si%] + [Mn%] + 3 [Ct% o] > 9 ... (1)
(14) The steel product for gas carburizing according to (13), in which by the
primary carburizing, an alloy shortage layer satisfying Expression (2) below
is formed in a range from the surface of the steel product for gas carburizing
to 2 to 50 p.m in depth.
3.5[Si%] + [Mn%] + 3[Cr%] < 9 ... (2)
5 (15) The manufacturing method of the gas-carburized steel part according to
(13), in which the steel product for gas carburizing further contains, in mass%,
one type or two types or more of Nb: 0.01 to 0.3%, Ti: 0.01 to 0.3%, and V:
0.01 to 0.3%.
(16) The manufacturing method of the gas-carburized steel part according to
10 (13), in which the steel product for gas carburizing further contains, in mass%,
one type or two types or more of Ni: 0.2 to 3.0%, Cu: 0.2 to 3.0%, Co: 0.2 to
3.0%, Mo: 0.05 to 0.4%, W: 0.05 to 0.4%, and B: 0.0006 to 0.005%.
(17) The manufacturing method of the gas-carburized steel part according to
(13), in which the steel product for gas carburizing further contains, in mass%,
15 one type or two types or more of Nb: 0.01. to 0.3%, Ti: 0.01 to 0.3%, and V:
0.01 to 0.3%, and one type or two types or more of Ni: 0.2 to 3.0%, Cu: 0.2 to
3.0%, Co: 0.2 to 3.0%, Mo: 0.05 to 0.4%, W: 0.05 to 0.4%, and B: 0.0006 to
0.005%.
[Effect of the Invention]
20 [0015] According to the present invention, it is possible to provide a steel
part preventing resistance to temper softening from decreasing and having
excellent surface fatigue strength. Further, according to the present
invention, it is possible to provide a steel product for gas carburizing suitable
for manufacturing the above steel part and a manufacturing method of a
25 gas-carburized steel part.
[Brief Description of the Drawings]
8
[0016] FIG 1 is a view showing one example of distribution of 3.5[Si%]
+ [Mn%] + 3 [Cr%] from a surface.
FIG 2 is a macrophotograph of structure of a surface layer, of a steel product
of Reference example No. 29, obtained after primary carburizing.
5 FIG 3 is a microphotograph of structure of the surface layer, of the steel
product of Reference example No. 29, obtained after secondary carburizing.
FIG 4 is a macrophotograph of structure of a surface layer, of a steel product
of Invention example No. 9, obtained after primary carburizing.
FIG 5 is a macrophotograph of structure of the surface layer, of the steel
10 product of Invention example No. 9, obtained after secondary carburizing.
FIG 6 is a graph in which fatigue lives of the steel product of Invention
example No. 9 and the steel product of Reference example No. 29 are
compared with respect to respective carburizing treatments in a roller pitting
fatigue test.
15 [Mode for Carrying out the Invention]
[0017] When the content of Si in steel is increased, resistance to temper
softening improves, but gas carburizing performance deteriorates. As a
result that the present inventors examined the cause of the deterioration of gas
carburizing performance, it turned out that an oxide layer formed on a surface
20 in the early stage of gas carburizing and mainly composed of one type or two
types or more of Si, Mn, and Cr affects the deterioration of gas carburizing
performance.
[0018] That is, it is conceivable that Si, Mn, and Cr oxides stably exist in
an atmosphere at the time of carburizing and the oxides exist in a layer state,
25 and thereby gas carburization reaction of a. steel product is inhibited. Then,
from the examination conducted by the present inventors, it turned out that
9
when the contents of Si, Mn, and Cr (mass%) are set to [Si%], [Mn%], and
[Cr%] and 3.5[Si%] + [Mn%] + 3[Cr%] > 9 is established, the oxide layer is
formed on the surface of the steel product to thereby deteriorate the gas
carburizing performance.
5 [0019] Then, in order to improve the deterioration of gas carburizing
performance caused by the oxide layer to invent a steel part having excellent
surface fatigue strength, the present inventors thought that after the oxide
layer being formed, the oxide layer is removed by a mechanical method (for
example, mechanical polishing), thereby making gas carburizing possible
10 even though the content of Si is large.
[0020] Then, in accordance with this idea, steel products based on
components causing the deterioration of carburizing performance by the
effect of the oxide layer were used to be subjected to a heat treatment under
an atmosphere of the oxide layer being formed (primary carburizing). Next,
15 the steel product from which the oxide layer has been removed and the steel
product from which the oxide layer has not been removed were subjected to a
gas carburizing treatment (secondary carburizing) to examine their gas
carburizing performance.
[0021] As a result, it turned out that the carburizing performance of the
20 steel, product from which the oxide layer has not been removed hardly
changes, but the carburizing performance of the steel product from which
only the oxide layer has been removed is improved. This is conceivable that
when the oxide layer is formed in the primary carburizing, solid-solution
alloy elements (Si, Mn, and Cr) in the steel are consumed by oxidation, and
25 consequently a shortage layer ofthe alloy elements formed on the surface of
the steel product greatly affects the carburizing performance.
10
[0022] That is, when only the oxide layer is removed after the primary
carburizing, on the surface of the steel product exposed to the atmosphere, the
solid solution amount of the alloy elements has been decreased and the
contents of the alloy elements (Si, Mn, and Cr) contributing to oxidation are
5 decreased. For this reason, it is conceivable that a new oxide layer is not
formed easily.
[0023] From the above, from the steel product based on the components
causing the deterioration of carburizing performance by the formation of the
oxide layer: 3.5 [Si%] + [Mn%] + 3[Cr%] > 9, only the oxide layer was
10 removed after the primary carburizing, and then a steel product having an
alloy shortage layer defined by a chemical composition range: 3.5[Si%] +
[Mn%] + 3[Cr%] < 9 exist in a range from its surface to 2 to 50 p.m in
depth was obtained. It turned out that the alloy shortage layer exists on the
surface of the above steel product, thereby making the gas carburizing
15 (secondary carburizing) possible.
[0024] Further, as a result of the examination of a correlation between the
content of Si and the gas carburizing performance, it turned out that when the
content of Si in the steel product (base metal) exceeds 1.2°/x, by the gas
carburizing, a steel part excellent in surface fatigue strength can be obtained.
20 [0025] A steel product for gas carburizing of the present invention,
(which is sometimes called a "present invention steel product" hereinafter),
has been made based on the above knowledge, and contains, in mass%, C: 0.1
to 0.4%, Si: exceeding 1.2 to 4.0%, Mn: 0.2 to 3.0%, Cr: 0.5 to 5.0%, Al:
0.005 to 0.1%, S: 0.001 to 0.3%, N: 0.003 to 0.03%, and 0: limited to
25 0.0050% or less and P: limited to 0.025% or less, in which when the contents
of Si, Mn, and Cr (mass%) are set to [Si%], [Mn%], and [Cr%], Expression
11
(1) below is satisfied.
32 ? 3.5[Si%] + [Mn%] + 3[Cro/o] > 9- (1)
[0026] Further, the steel product for gas carburizing of the present
invention can also employ a form in which an alloy shortage layer satisfying
5 Expression (2) below exists in a range from the surface to 2 to 50 μm in
depth.
3.5[Si%] + [Mn%] + 3[Cr%] < 9 ... (2)
[0027] First, the reason for defining the chemical composition of the steel
product for gas carburizing of the present invention will be explained.
10 Hereinafter, % in the chemical composition means mass%.
[0028] C: 0.1 to 0.4%
C is an element necessary for maintaining the strength of the steel. The
content of C determines the strength of a core portion and also affects the
depth of an effective hardened layer. The lower limit of C is set to 0.1% in
15 order to secure the required strength of the core portion. On the other hand,
if the content of C is too large, the toughness decreases, so that the upper limit
of C is set to 0.4%. Cis preferably 0.15 to 0.25%.
[0029] Si: exceeding 1.2 to 4.0%
Si is an element effective for deoxidation of the steel, and is an element
20 effective for providing the necessary strength and hardenability, and further is
an element effective for improving the resistance to temper softening. In
order to obtain the addition effect of Si, the lower limit of Si is set to be larger
than 1.2%. On the other hand, if the content of Si exceeds 4.0%,
decarburizing at the time of forging becomes remarkable, so that the upper
25 limit of Si is set to 4.0%. Si is preferably 1.2 to 2,5%.
[0030] Mn: 0.2 to 3.0%
12
Mn is an element effective for deoxidation, and is an element effective
for providing the necessary strength and hardenability. Further, Mn is an
element that fixes S being an impurity element inevitably mixed in the steel as
MnS to make it harmless. In order to secure the addition effect of Mn, the
5 lower limit of Mh is set to 0.2%. On the other hand, if the content of Mn
exceeds 3.0%, even though a sub-.zero treatment is performed, retained
austenite stably exists and the strength is decreased, and thus the upper limit
of Mn is set to 3.0%. Mn is preferably 0.5 to 1.5%.
[0031] Cr: 0.5 to 5.0%
10 Cr is an element effective for improving the hardenability, and is an
element effective also for improving the resistance to temper softening. In
order to obtain the addition effect of Cr, the lower limit of Cr is set to 0.5%.
On the other hand, if the content of Cr exceeds 5.0%, the hardness increases
and the cold workability deteriorates, and thus the upper limit of Cr is set to
15 5.0%. Cris preferably 0.8 to 2.5%.
[0032] Al: 0.005 to 0.1%
Al is an element effective for deoxidation, and is an element that
precipitates as nitrides to produce an effect of making crystal grains fine. In
order to obtain the addition effect of Al, the lower limit of Al is set to 0.005%.
20 On the other hand, if the content of Al exceeds 0.1%, precipitates are
coarsened to cause embrittlement, and thus the upper limit of Al is set to 0.1%.
Al is preferably 0.01 to 0.05%.
[0033] S: 0.001 to 0.3%
S is an impurity element to be inevitably mixed in the steel, but is an
25 element effective for improving the machinability. In order to secure the
required machinability, the lower limit of S is set to 0.001%. On the other
13
hand, if the content of S exceeds 0.3%, the forgeability deteriorates
remarkably, and thus the upper limit of S is set to 0.3%. S is preferably
0.001 to 0.1%.
[0034] N: 0.003 to 0.03%
5 N is an element to be inevitably mixed in the steel, but is an element that
forms chemical compounds with Al to produce an effect of making crystal
grains fine. In order to obtain the effect of making crystal grains fine, the
lower limit of N is set to 0.003%. On the other hand, if the content of N
exceeds 0.03%, the forgeability deteriorates remarkably, and thus the upper
10 limit of N is set to 0.03%.
[0035] 0: 0.0050% or less
0 exists in the steel as oxide-based inclusions such as alumina and
titania. If the content of 0 is large, the oxides are increased in size, and
damage starting from the oxides is caused in a power transmission part, and
15 thus 0 is required to be limited to 0.0050% or less. The smaller the content
of 0 is, the more preferable it is, and thus 0 is desirably 0.0020% or less, and
is more desirably 0.0015% or less when the long life is desired.
[0036] P: 0.025% or less
P is a component to be contained in the steel as an impurity, and
20 segregates in grain boundaries to decrease the toughness, and thus P is
required to be decreased as much as possible and is limited to 0.025% or less.
The smaller the content of P is, the more preferable it is, and thus P is
desirably 0.020% or less, and is more desirably 0.015% or less when the long
life is desired
25 [0037] Besides, to the present invention steel product, one type or two
types or more of Nb, Ti, and V may also be added for the purpose of making
14
crystal grains finer and preventing crystal grains from coarsening.
[0038] Nb: 0.01 to 0.3%
Ti: 0.01 to 0.3%
V: 0.01 to 0.3%
5 Nb, Ti, and V form chemical compounds with C or N to exhibit an effect
of making crystal grains fine, and thus 0.01% or more of one type or two
types or more of Nb, T, and V is added. However, even though each of the
elements is added in excess of the above-described upper limit, the effect of
making crystal grains fine is saturated, and additionally, the productivities of
10 hot rolling, hot forging, cutting, and so on are decreased, and thus the upper
limit of each of Nb, Ti, and V is set to 0.3%. Each of N, Ti, and V is
preferably 0.02 to 0.1%.
[0039] To the present invention steel product, one type or two types or
more of Ni, Cu, Co, Mo, W, and B may also be added for the purpose of
15 further improving the hardenability.
[0040] Ni: 0.2 to 3.0%
Cu: 0.2 to 3.0%
Co: 0.2 to 3.0%
Mo: 0.05 to 0.4%
20 W: 0.05 to 0.4%
B: 0.0006 to 0.005%
Ni, Cu, and Co are elements effective for improving the hardenability.
In order to obtain the addition effect of Ni, Cu, and Co, 0.2% or more of each
of Ni, Cu, and Co is added, but if the added amount of each of Ni, Cu, and Co
25 exceeds 3.0%, the addition effect is saturated to be disadvantageous
economically, and thus the upper limit of each of Ni, Cu, and Co is set to
15
3.0%. Each of Ni, Cu, and Co is preferably 0.2 to 2.0%.
[0041] Mo, W, and B are also elements effective for improving the
hardenability. In order to obtain the addition effect of Mo, W, and B, 0.05%
or more of each of Mo and W is added, and 0.0006% or more of l3 is added.
5 However, if the added amount of each of Mo and W exceeds 0.4%, the
addition effect is saturated to be disadvantageous economically, and thus the
upper limit of each of Mo and W is set to 0.4%. If the added amount of B
exceeds 0.005%, B chemical compounds precipitate in grain boundaries and
the toughness decreases, and thus the upper limit of B is set to 0.005%.
10 [0042] Each of Mo and W is preferably 0.10 to 0.3%. B is preferably
0.0006 to 0.001 %.
[0043] Next, the chemical composition (Si, Mn, and Cr) of the steel
product forming the oxide layer to deteriorate the carburizing performance
will be explained.
15 [0044] As a result that the present inventors performed elemental analysis
of the oxide layer by using a characteristic X ray, it was confirmed that Si, Mn,
Cr, and 0 exist in the oxide layer.
[0045] Si, Mn, and Cr are elements each having a strong tendency to
oxidation. As for the components other than Si, Mn, and Cr, the elements
20 each-having a weak tendency to oxidation (for example, Ni and Cu) do not
oxidize, thus having no effect on the formation of the oxide layer, and the
elements each having a strong tendency to oxidation (for example, Ti and V)
have the minute contents as compared to those of Si, Mn, and Cr, thus having
a negligibly small effect on the formation of the oxide layer.
25 [0046] Thus, Si, Mn, and Cr are most connected with the formation of the
oxide layer deteriorating the carburizing performance, and in the chemical
16
composition of the present invention steel product, the condition of the
chemical composition that forms the oxide layer to deteriorate the carburizing
performance can be set only with Si, Mn, and Cr.
[0047] The steel product containing C: 0.1 to 0.4%, Al: 0.005 to 0.1%, S:
5 0.001 to 0.1%, N: 0.003 to 0.03%, 0: limited to 0.005% or less, and P: limited
to 0.025% or less, Si within a range of 0.1 to 4.0%, Mn within a range of 0.1
to 3.0%, and Cr within a range of 0.1 to 5.0% was forged to be subjected to a
heat treatment, and then by machining, a cylindrical test piece having a
diameter of 30 mm was made to be subjected to gas carburizing.
10 [0048] The concentration of C in the uppermost surface layer on the level
where the deterioration of gas carburizing performance is caused under the
same gas carburizing condition (950°C a carbon potential 0.8) was subjected
to multiple regression analysis with each of the concentrations of Si, Mn, and
Cr as a factor, and as the critical condition reaching the concentration of C in
15 the case of the normal gas carburizing being performed, Expression (1')
below was obtained.
3.5[Si%] + [Mn%] + 3[Cr%] = 9 ... (1')
[0049] That is, when the value of 3.5[Si%] + [Mn%] + 3[Cr%] exceeds 9,
the carburizing performance deteriorates and a decrease in concentration of C
20 is seen. The deterioration of gas carburizing performance caused by the
oxide layer begins to appear from the point when the value of 3.5[Si%] +
[Mn%] + 3[Cr%] exceeds 9, and as the value of 3.5[Si%] + [Mn%] + 3[Cr%]
increases, the carburizing performance deteriorates.
[0050] On the other hand, Si, Mn, and Cr are elements effective for
25 providing the strength and hardenability of the steel product. Further, Si and
Cr are elements effective for improving the resistance to temper softening.
17
In order to obtain the strength and resistance to temper softening required for
the steel parts such as a gear and a bearing, the value of 3.5[Si%] + [Mn%] +
3[Cr°/a] in the base metal is required to exceed 9. However, according to the
upper limits of the contents of Si, Mn, and Cr, 32 > 3.5[Si%] + [Mn%] +
5 3 [Cr%] is established. Thus, in the steel product for gas carburizing of the
present invention, Expression (1) below is set to be satisfied.
32 ? 3.5[Si%] + [Mn%] + 3[Cr%] > 9 ... (1)
[0051] However, as described above, when the value of 3.5[Si%] +
[Mn%] + 3 [Cr%] exceeds 9, the oxide layer is formed on the surface of the
10 steel product, and thus the carburizing performance deteriorates. Then, the
steel product for gas carburizing of the present invention is desirably a form
that has the alloy shortage layer improving the carburizing performance on
the surface.
[0052] The alloy shortage layer is a region in which the alloy elements
15 are consumed by oxidation and that is formed in the surface layer portion of
the steel product and in which the concentration of the alloy is decreased. In
the present invention, the existence of the above alloy shortage layer
contributes to the improvement of carburizing performance.
[0053] In the present invention steel product, the alloy shortage layer is
20 defined as a region where the contents of Si, Mn, and Cr (%) satisfy
Expression (2) below.
3.5[Si%] + [Mn%] + 3[Cr%] < 9 ... (2)
[0054] The steel product for gas carburizing of the present invention has
the alloy shortage layer satisfying Expression (2) formed on the surface
25 thereof, and thereby the gas carburizing performance is improved. The alloy
shortage layer as above can be obtained in a manner that the steel product
18
having the value of 3.5[Si%] + [Mn%] + 3[Cr%] in the base metal in excess
of 9 is subjected to the primary carburizing, and thereafter the oxide layer
formed in the primary carburizing is removed. Thus, even though the steel
product having had the alloy shortage layer satisfying Expression (2) formed
5 on the surface thereof is subjected to the gas carburizing (secondary
carburizing) again, a new oxide layer deteriorating the gas carburizing
performance is not formed.
[0055] As above, for improving the gas carburizing performance in the
secondary carburizing, the alloy shortage layer is required to exist, namely,
10 the contents of Si, Mn, and Cr (%) are required to satisfy Expression (2)
above.
[0056] However, in order to sufficiently improve the carburizing
performance, the value of the left side of Expression (2) above is preferably
smaller, and 3.5 [Si%] + [Mn%] + [Cr%] < 7 is preferable. Incidentally, the
15 preferable lower limit of the value of the left side is determined according,to
the equilibrium with the atmosphere of the gas carburizing, and thus it varies
depending on the gas carburizing condition, but is preferably 1.0 < 3.5 [Si%] +
[Mn%] + 3[Cr%].
[0057] The steel product for gas carburizing of the present invention has
20 the alloy shortage layer satisfying Expression (2) below in a range from the
surface to 2 to 50 μm in depth. The lower limit of the thickness of the alloy
shortage layer is set to 2 μm so as to prevent that by the diffusion of the alloy
elements from the base metal, the concentration of the alloy shortage layer is
increased and the oxide layer deteriorating the carburizing performance is
25 formed. As for the above numerical value of 2 μm, the carburizing
condition of the gas carburizing temperature: 950°C and the carburizing time:
19
120 minutes is supposed.
[0058] However, when the gas carburizing temperature is high or the gas
carburizing is performed for a long time, the diffusion length of the alloy
elements becomes long, and thus in order to securely improve the carburizing
5 performance, the thickness of the alloy shortage layer is preferably 10 μm or
more.
[0059] Conversely, when the gas carburizing temperature is low and the
carburizing time is short, it is possible to improve the gas carburizing
performance even though the thickness of the alloy shortage layer is 2 μm or
10 less. On the other hand, if the thickness of the alloy shortage layer is too
thick, an incompletely hardened layer is also thickened, the hardness is
decreased, and the fatigue strength is decreased, and thus the upper limit of
the thickness of the alloy shortage layer is set to 50 μm.
[0060] Next, the present invention steel product and a manufacturing
15 method of a steel part using the present invention steel product will be
explained.
[0061] First, by an ordinary method, a steel having a composition in the
present invention range is melted and cast, and an obtained steel billet or steel
ingot is hot worked and shaped to obtain a steel product for gas carburizing.
20 The hot working is hot rolling or hot forging, and may be performed a.
plurality of times, or the hot working may also be performed in a manner to
combine hot rolling and hot forging.
[0062] The shaping may be performed by hot forging, cold working such
as cold forging, or cutting, or may also be performed in a manner to combine
25 hot forging, cold working, and cutting. The obtained steel product for gas
carburizing is subjected to a heat treatment (primary carburizing) in which an
20
oxide layer is formed, and then the oxide layer is removed.
[0063] The heat treatment (primary carburizing) is required to be
performed in an atmosphere where Fe is not oxidized but Si, Mn, and Cr are
selectively oxidized. The atmosphere is a H2/H20 atmosphere, a CO/CO2
5 atmosphere, an inert gas atmosphere containing a minute amount of oxygen,
or a low-oxygen partial pressure atmosphere represented by an atmosphere
mixed with these atmospheres.
[0064] According to the chemical composition of the steel product, the
necessary atmosphere is changed, but when the atmosphere is adjusted to the
10 direction of the atmosphere temperature being high and to the direction of the
oxygen partial pressure in the atmosphere being high, the alloy shortage layer
strongly tends to thick, and thus in the present invention, it is desirable to
adjust the atmosphere to the direction of the atmosphere temperature being
high and/or to the direction of the oxygen partial pressure in the atmosphere
15 being high.
[0065] In an example, by using a carburizing atmosphere, the formation
of the alloy shortage layer is achieved. The removing of the formed oxide
layer is performed by a mechanical removing method represented by shot
peening, a chemical removing method such as chemical polishing or
20 electrolytic polishing, or a method combined with these methods.
[0066] The oxide layer is removed, and then the steel product for gas
carburizing is subjected to a gas carburizing treatment (secondary
carburizing) to manufacture a gas-carburized steel part. Si has an effect of
increasing the activity of C in the, steel product, and decreases the carburizing
25 amount, and thus as the condition of the secondary carburizing treatment, it is
desirable to increase the carbon potential. Further, as the treatment
21
temperature is higher, before and after the carburizing treatment, cutting may
also be performed.
[0067] Further, before performing the cold working, spheroidizing may
also be performed in order to improve the formability. The spheroidizing is
5 desirably performed at 700 to 800°C.
[Example]
[0068] Next, an example of the present invention will be described, but
the conditions of the example are condition examples employed for
confirming the applicability and effects of the present invention, and the
10 present invention is not limited to these condition examples. The present
invention can employ various conditions as long as the object of the present
invention is achieved without departing from the spirit of the invention.
[0069] Steel products 1 to 32 each having a chemical composition shown
in Table 1 were subjected to forging and a heat treatment, and thereafter by
15 machining, small roller test pieces each having a cylinder portion having a
diameter of 26 mm and a width of 28 nun, and large roller test pieces each
having a diameter of 130 mm and a width of 18 mm, and a crowning with R. _
150 mm on its outer circumference were made.
[0070] [Table 1]
20
[0071 ] As for the made roller test pieces, the test pieces No. 1 to 19 and
21 were subjected to a carburizing treatment (heat treatment) A to be prepared
for a test. The test pieces No . 22 to 27, and 29 to 32 were subjected to a
carburizing treatment (heat treatment) B to be prepared for a test . The test
25 pieces No. 20 and 28 were subjected to a carburizing treatment (heat
treatment) C to be prepared for a test.
22
5
10
15
20
25
[0072] Carburizing treatment (heat treatment) A:
The test pieces were subjected to "RX gas carburizing of 950°C - carbon
potential 0.8 x 1 hour > oil hardening (primary carburizing)." Next, the test
pieces were subjected to shot peening under the condition of "a shot diameter
of 0.2 mm, a shot speed of 70 m/s, and an are height of 0.6 mm A."
Thereafter, the test pieces were subjected to " RX gas carburizing of 950°C -
carbon potential 0.8 x 2 hours > oil hardening" (secondary carburizing) and
subsequently were subjected to a tempering treatment of 150°C x 90 minutes
to be prepared for a test.
[0073] Carburizing treatment (heat treatment) B:
The test pieces were subjected to "RX gas carburizing of 850°C - carbon
potential 0.8 x 1 hour .> oil hardening (primary carburizing)." Next, the test
pieces were subjected to shot peening under the condition of "a shot diameter
of 0.2 mm, a shot speed of 70 m/s, and an are height of 0.6 mm A."
Thereafter, the test pieces were subjected to "RX gas carburizing of 950°C -
carbon potential 0.8 x 2 hours -> oil hardening" (secondary carburizing) and
subsequently were subjected to a tempering treatment of 150°C x 90 minutes
to be prepared for a test.
[0074] Carburizing treatment (heat treatment) C:
The test pieces were subjected to "RX gas carburizing of 950°C - carbon
potential 0.8 x 150 hours > oil hardening" (primary carburizing). Next, the
test pieces were subjected to shot peening under the condition of "a shot
diameter of 0.2 mm, a shot speed of 70 m/s, and an are height of 0.6 mm A."
Thereafter, the test pieces were subjected to "RX gas carburizing of 950°C -
carbon potential 0.8 x 2 hours 4 oil hardening" (secondary carburizing) and
subsequently were subjected to a tempering treatment of 150°C x 90 minutes
23
to be prepared for a test.
[0075] In the carburizing treatment A and the carburizing treatment B, it
is intended that due to the temperature different of the primary carburizing,
the diffusion length is changed, namely the thickness of the alloy shortage
5 layer is changed. The temperature of the primary carburizing and the
oxygen partial pressure are lower in the carburizing treatment B than in the
carburizing treatment A, and the thickness of the alloy shortage layer becomes
thinner in the carburizing treatment B than in the carburizing treatment A.
Perhaps the sufficient alloy shortage layer is not formed in the carburizing
10 treatment B.
[0076] Incidentally, the evaluation of the value of 3.5[Si%] + [l\/Ln%] +
3 [Cr%] on the uppermost surface and the evaluation of the concentration of C
of the uppermost surface were conducted in a manner to, by an EPNLA,
measure the concentration distribution of Si, Mn, Cr, and C and to use the
15 concentrations at the position of 30 pm from the surface.
[0077] In order to evaluate the surface fatigue strength, a roller pitting
fatigue test was conducted by using the large roller test pieces and the small
roller test pieces.
[0078] In the roller pitting fatigue test, the large roller was pressed
20 against the small roller with a contact pressure set to 2500 a of Hertzian
stress. Circumferential speed directions of both the rollers at a contact
portion were set the same and a slip ratio was set to -40% (was set that the
circumferential speed of the contact portion of the large roller is 40% larger
than that of the small roller), and then the rollers were rotated. Then, the
25 number of rotations until pitting occurs in the small roller was set to the life.
[0079] The temperature of a gear oil supplied to the above-described
contact portion was set to 80°C. The occurrence of pitting was detected by
an attached vibration meter. The vibration was detected, and then the
rotations of both the rollers were stopped to confirm the occurrence of pitting
and the number of rotations. In the case when pitting does not occur even
5 though the number of rotations reaches 10 million times, the small roller can
be evaluated to sufficiently have the surface fatigue strength, and thus the test
was stopped at 10 million times.
[0080] The results of the above evaluation and test are collectively shown
in Table 2.
to [0081] [Table 2]
[0082] In FIG 1, the distribution of "3.5[Si%] + [Mn%] + 3[Cr%]" of a
surface layer portion of Invention example No. 1 is shown as an example of
the distribution of the concentration of a surface layer. As shown in FIG 1, a
15 decrease in the alloy concentration is seen in the surface layer portion, and the
alloy shortage layer is positioned from the surface to 2.9 pm.
[0083] As shown in Table 2, in Invention examples No. 1 to 20, the
concentration of C of the uppermost surface layer after the second carburizing
(secondary carburizing) becomes higher than the content of C in the steel
20 product (base metal), but in Comparative examples No. 22 to 27, the
concentration of C of the uppermost surface layer is substantially the same as
the content of C in the steel product (base metal) even though Comparative
examples No. 22 to 27 each have the same chemical composition as that of
Invention examples No. 1, 5 to 7, 12, and 13.
25 [0084] Invention examples No. 1 to 20 each can endure 10 million
rotations in the roller pitting fatigue test to thus have the excellent surface
25
5
1 0
15
20
25
fatigue strength. In Comparative example No. 21, the concentration of Si is
lower than that determined in the present invention, so that the surface fatigue
strength is low. In Comparative example No. 28, the thickness of the alloy
shortage layer exceeds the thickness defined in the present invention, so that
the he surface fatigue strength is low.
[0085] Reference examples No. 29 to 33 are examples where no
deterioration of gas carburizing performance is seen before and after shot
peening because the contents of Si, Mn, and Cr in the steel product (base
metal) do not satisfy the condition of 3.5 [Si%] + [Mn%] + 3 [Cr%] > 9 and the
oxide layer inhibiting the carburizing performance is not formed.
[0086] That is, as shown in Reference examples No. 29 to 33, in the steel
product having the contents of Si, Mn, and Cr in the base metal satisfying
3.5[Si%] + [Mn%] + 3[Cr%] 9, the gas carburizing performance does not
deteriorate. However, the above steel product cannot sufficiently obtain the
strength and resistance to temper softening required for the steel parts such as
a gear and a bearing. This point is a technical reason why in the present
invention steel product, the contents of Si, Mn, and Cr in the steel product
(base metal) are defined to satisfy 3.5[Si%] + [Mn%] + 3[Cr%] 9.
[0087] From the above results, it is obvious that in Invention examples
No. Ito 20, the carburizing performance has been improved.
[0088] This is attributed to the fact that (a) mass% of the chemical
composition of the steel product (base metal) falls within a predetermined
range (C: 0.1 to 0.4%, Si: exceeding 1.2 to 4.0%, lVIn: 0.2 to 3.0%, Cr: 0.5 to
5.0%, Al: 0.005 to 0.1%, S: 0.001 to 0.3%, N: 0.003 to 0.03%, and 0:
0.0050% or less and P: 0.025% or less), (b) the contents of Si, Mn, and Cr
(%) in the steel product (base metal) satisfy the condition of 3.5[Si%] +
26
[Mn%] + 3[Cr%] > 9, and (c) the thickness of the alloy shortage layer existing
in the surface layer and having the contents of Si, Mn, and Cr (%) satisfying
3.5[Si%] + [Mn%] + 3[Cr%] <_ 9 is 2 to 50 μm.
[0089] The structure of the surface layer after the primary carburizing and
5 the structure of the surface layer after the secondary carburizing observed in
the steel product of Invention, example No. 9 and the steel product of
Reference example No. 29 were as follows. As shown in FIG 2, in the steel
product of Reference example No. 29, an incompletely hardened layer made
of mainly pearlite was formed in the surface layer obtained after the primary
10 carburizing. Further, as shown in FIG. 3, in the steel product of Reference
example No. 29, an incompletely hardened layer made of mainly pearlite was
further formed in the surface layer obtained after the secondary carburizing.
[0090] On the other hand, as shown in FIG 4, in the steel product of
Invention example No. 9, martensite was formed in the surface layer obtained
15 after the primary carburizing. Further, as shown in FIG. 5, in the steel
product of Invention example No. 9, a hardened layer made of mainly
martensite is further formed after the secondary carburizing, and as compared
to Reference example No. 29 (FIG. 3), the amount of the incompletely
hardened layer being formed was decreased.
20 [0091] Further, the fatigue life in the roller pitting fatigue test in the case
when the steel product of Invention example No. 9 was subjected to three
treatments of gas carburizing, invented method application, and vacuum
carburizing, and the fatigue life in the roller pitting fatigue test in the case
when the steel product of Reference example No. 29 was subjected to two
25 treatments of gas carburizing and vacuum carburizing were compared. A
result of the comparison is shown in FIG. 6. Incidentally, in both the cases
27
of Invention example No. 9 and Reference example No. 29, the condition of
the "gas carburizing" is the same as that of the above-described carburizing
treatment A. The "invented method application" performed on the steel
product of Invention example No. 9 is the above-described carburizing
5 treatment A. Consequently, the steel product of Reference example No. 29
was not able to obtain the sufficient fatigue life. The steel product of
Invention example No. 9 was able to, by the "invented method application,"
obtain the fatigue life equal to that by the vacuum carburizing.
[Industrial Applicability]
10 [0092] The present invention greatly contributes to achievement of high
power, improvement of fuel efficiency, and so on of an automobile, a
construction vehicle, an industrial machine, and so on to thus have large
industrial applicability.
28
[Name of Document] What Is Claimed
[Claim 1] A gas-carburized steel part having a gas-carburized layer
containing C: 0.50 mass% or more on a surface thereof comprising:
a composition of a base metal contains, in mass%,
5 C: 0.I to 0.4%,
Si: exceeding 1.2 to 4.0%,
Mn: 0.2 to 3.0%,
Cr: 0.5 to 5.0%,
Al: 0.005 to 0.1%,
10 S: 0.001 to 0.3%,
N: 0.003 to 0.03%, and
0: limited to 0.0050% or less, and
P: limited to 0.025% or less,
when the contents of Si, Mn, and Cr (mass%) are set to [Si%], [Mn%], and
15 [Cr%], Expression (1) below is satisfied, and
an alloy shortage layer satisfying Expression (2) below exists in a range from
the surface to 2 to 50 μm in depth.
32 ? 3.5[Si%] + [Mn%] + 3[Cr%] > 9 ... (1)
3.5[Si%] + [Mn%] + 3[Cr%] < 9- (2)
20 [Claim 21 The gas-carburized steel part according to claim 1, wherein
the composition of the base metal further contains, in mass%, one type or two
types or more of
Nb: 0.01 to 0.3%,
Ti: 0.01 to 0.3%, and
25 V: 0.01 to 0.3%.
[Claim 3] The gas-carburized steel part according to claim 1, wherein
the composition of the base metal further contains, in mass%, one type or two
29
types or more of
Ni: 0.2 to 3.0%,
Cu: 0.2 to 3.0%,
Co: 0.2 to 3.0%,
5 Mo: 0.05 to 0.4%,
W: 0.05 to 0.4%, and
B: 0.0006 to 0.005%.
[Claim 4] The gas-carburized steel part according to claim 1, wherein
the composition of the base metal further contains, in mass%, one type or two
10 types or more of
Nb: 0.01 to 0.3%,
Ti: 0.01 to 0.3%, and
V: 0.01 to 0.3%, and one type or two types or more of
Ni: 0.2 to 3.0%,
15 Cu:0.2to3.0%,
Co: 0.2 to 3.0%,
Me: 0.05 to 0.4%,
W: 0.05 to 0.4%, and
B: 0.0006 to 0.005%.
20 [Claim 5] A steel product for gas carburizing used for manufacturing a
carburized steel part, the steel part for gas carburizing comprising:
in mass%,
C: 0.1 to 0.4%,
Si: exceeding 1.2 to 4.0%,
25 Mn: 0.2 to 3.0%,
Cr: 0.5 to 5.0%,
30
Al:: 0.005 to 0.1%,
S: 0.001 to 0.3%,
N: 0.003 to 0 .03%, and
0: limited to 0.0050% or less, and
5 P: limited to 0.025% or less, wherein
when the contents of Si, Mn, and Cr (mass%) are set to [Si%], [Mn%], and
[Cr%], Expression (1) below is satisfied.
32 > 3.5 [Si%] + [Mn%] + 3[Cr%] > 9 ... (1)
[Claim 6] The steel product for gas carburizing according to claim 5,
10 further comprising:
in mass%, one type or two types or more of
Nb: 0.01 to 0.3%,
Ti: 0.01 to 0.3%, and
V: 0.01 to 0.3%.
15 [Claim 71 The steel product for gas carburizing according to claim 5,
further comprising:
in mass%, one type or two types or more of
Ni: 0.2 to 3.0%,
Cu: 0.2 to 3.0%,
20 Co: 0.2 to 3.0%,
Mo: 0.05 to 0.4%,
W: 0.05 to 0 .4%, and
B: 0.0006 to 0.005%.
[Claim 8] The steel product for gas carburizing according to claim 5,
25 further comprising:
in mass%, one type or two types or more of
31
fib: 0.01 to 0.3%,
Ti: 0.01 to 0.3%, and
V: 0.01 to 0.3%, and one type or two types or more of
Ni: 0.2 to 3.0%,
5 Cu: 0.2 to 3.0%,
Co: 0.2 to 3.0%,
Me: 0.05 to 0.4%,
W: 0.05 to 0.4%, and
B: 0.0006 to 0.005%.
10 [Claim 9] A steel product for gas carburizing used for manufacturing a
carburized steel part, wherein
a composition of a base metal contains , in mass%,
C: 0.1 to 0.4%,
Si: exceeding 1.2 to 4.0%,
15 Mn: 0.2 to 3.0%,
Cr: 0.5 to 5.0%,
Al: 0.005 to 0.1%,
S: 0.001 to 0.3%,
N: 0.003 to 0.03%, and
20 0: limited to 0.0050% or less, and
P: limited to 0.025% or less,
when the contents of Si, Mn, and Cr (mass%) are set to [Si%], [Mn%], and
[Cr%], Expression ( 1) below is satisfied, and
an alloy shortage layer satisfying Expression (2) below exists in a range from
25 its surface to 2 to 50 lun in depth.
32 ? 3.5[Si%] + [Mn%] + 3[Cr%] > 9 ... (1)
32
3.5 [Si%] + [Mn%] + 3[Cr%] 9 ... (2)
[Claim 10] The steel product for gas carburizing according to claim 9,
wherein
the composition of the base metal further contains, in mass%, one type or two
5 types or more of
Nb: 0.01 to 0.3%,
Ti: 0.01 to 0.3%, and
V: 0.01 to 0.3%.
[Claim 11] The steel product for gas carburizing according to claim 9,
10 wherein
the composition of the base metal further contains, in mass%, one type or two
types or more of
Ni: 0.2 to 3.0%,
Cu: 0.2 to 3.0%,
15 Co: 0.2 to 3.0%,
Me: 0.05 to 0.4%,
W: 0.05 to 0.4%, and
B: 0.0006 to 0.005%.
[Claim 12] The steel product for gas carburizing according to claim 9,
20 wherein
the composition of the base metal further contains, in mass%, one type or two
types or more of
Nb: 0.01 to 0.3%,
Ti: 0.01 to 0.3%, and
25 V: 0.01 to 0.3%, and one type or two types or more of
Ni: 0.2 to 3.0%,
33
Cu: 0.2 to 3.0%,
Co: 0.2 to 3.0%,
Mo: 0.05 to 0.4%,
W: 0.05 to 0.4%, and
5 B: 0.0006 to 0.005%.
[Claim 13] A manufacturing method of a gas-carburized steel part using a
steel product for gas carburizing containing, in mass%,
C: 0.1 to 0.4%,
Si: exceeding 1.2 to 4.0%,
10 Mn: 0.2 to 3.0%,
Cr: 0.5 to 5.0%,
Al: 0.005 to 0.1%,
S: 0.001 to 0.3%,
N: 0.003 to 0.03%, and
15 0: limited to 0.0050% or less, and
P: limited to 0.025% or less and in which
when the contents of Si, IVIn, and Cr (mass%) are set to [Si%], [4n%], and
[Cr%], Expression (1) below is satisfied, the manufacturing method
comprising:
20 performing primary carburizing in which the steel product for gas carburizing
is subjected to a heat treatment under an atmosphere where an oxide layer is
formed;
removing the oxide layer formed on a surface; and
then, performing secondary carburizing in a carburizing atmosphere.
25 32 >_ 3.5[Si%] + [Mn%] + 3[Cr%] > 9 ... (1)
[Claim 14] The steel product for gas carburizing according to claim 13,
34
wherein
by said primary carburizing , an alloy shortage layer satisfying Expression (2)
below is formed in a range from the surface of the steel product for gas
carburizing to 2 to 50 gm in depth.
5 3.5[Si%] + [Mn%] + 3[Cr%] < 9 ... (2)
[Claim 15 ] The manufacturing method of the gas-carburized steel part
according to claim 13, wherein
the steel product for gas carburizing further contains , in mass%, one type or
two types or more of
10 Nb: 0.01 to 0.3%,
Ti: 0.01 to 0 .3%, and
V: 0.01 to 0.3%.
[Claim 161 The manufacturing method of the gas-carburized steel part
according to claim 13, wherein
15 the steel product for gas carburizing further contains , in mass%, one type or
two types or more of
Ni: 0.2 to 3.0%,
Cu: 0.2 to 3.0%,
Co: 0.2 to 3.0%,
20 Mo: 0. 05 to 0.4%,
W: 0.05 to 0.4%, and
B: 0.0006 to 0.005%.
[Claim 17] The manufacturing method of the gas -carburized steel part
according to claim 13, wherein
25 the steel product for gas carburizing further contains , in mass%, one type or
two types or more of
35
Nb: 0.01 to 0.3%,
Ti: 0.01 to 0.3%, and
V: 0.01 to 0.3%, and one type or two types or more of
Ni: 0.2 to 3.0%,
5 Cu: 0.2 to 3.0%,
Co: 0.2 to 3.0%,
Mo: 0.05 to 0.4%,
W: 0.05 to 0.4%, and
B: 0.0006 to 0.005%.