U.S. patent application number 13/070629 was filed with the patent office on 2011-09-29 for heat resistant steel for exhaust valve.
This patent application is currently assigned to Daido Tokushuko Kabushiki Kaisha. Invention is credited to Mototsugu Osaki, Takashi Tsuyumu, Shigeki Ueta.
Application Number | 20110236247 13/070629 |
Document ID | / |
Family ID | 44260241 |
Filed Date | 2011-09-29 |
United States Patent
Application |
20110236247 |
Kind Code |
A1 |
Osaki; Mototsugu ; et
al. |
September 29, 2011 |
HEAT RESISTANT STEEL FOR EXHAUST VALVE
Abstract
The present invention provides a heat resistant steel for an
exhaust valve, containing: more than 0.50% by mass but less than
0.80% by mass of C, more than 0.30% by mass but less than 0.60% by
mass of N, 17.0% by mass or more but less than 25.0% by mass of Cr,
4.0% by mass or more but less than 12.0% by mass of Ni, 7.0% by
mass or more but less than 14.0% by mass of Mn, 2.0% by mass or
more but less than 6.0% by mass of Mo, more than 0.5% by mass but
less than 1.5% by mass of Si, and 0.025% by mass or more but less
than 1.0% by mass of Nb, with the balance consisting of Fe and
unavoidable impurities, in which a content of P contained in the
unavoidable impurities is regulated to less than 0.03% by mass, a
total content of C and N is from 0.85% by mass to 1.3% by mass, and
a ratio of the content of Nb to the content of C is 0.05 or more
but less than 1.8.
Inventors: |
Osaki; Mototsugu;
(Nagoya-shi, JP) ; Ueta; Shigeki; (Nagoya-shi,
JP) ; Tsuyumu; Takashi; (Wako-shi, JP) |
Assignee: |
Daido Tokushuko Kabushiki
Kaisha
Nagoya
JP
Honda Motor Co., Ltd.
Tokyo
JP
|
Family ID: |
44260241 |
Appl. No.: |
13/070629 |
Filed: |
March 24, 2011 |
Current U.S.
Class: |
420/41 ; 420/47;
420/64; 420/66 |
Current CPC
Class: |
C22C 38/58 20130101;
C22C 38/001 20130101; C21D 9/0068 20130101; F01L 3/02 20130101;
F01L 2820/00 20130101; C22C 38/44 20130101; C22C 38/48 20130101;
F01L 2303/00 20200501; C22C 38/02 20130101 |
Class at
Publication: |
420/41 ; 420/47;
420/64; 420/66 |
International
Class: |
C22C 38/58 20060101
C22C038/58; C22C 38/48 20060101 C22C038/48; C22C 38/44 20060101
C22C038/44; C22C 38/52 20060101 C22C038/52; C22C 38/54 20060101
C22C038/54 |
Foreign Application Data
Date |
Code |
Application Number |
Mar 25, 2010 |
JP |
2010-070720 |
Claims
1. A heat resistant steel for an exhaust valve, comprising: more
than 0.50% by mass but less than 0.80% by mass of C, more than
0.30% by mass but less than 0.60% by mass of N, 17.0% by mass or
more but less than 25.0% by mass of Cr, 4.0% by mass or more but
less than 12.0% by mass of Ni, 7.0% by mass or more but less than
14.0% by mass of Mn, 2.0% by mass or more but less than 6.0% by
mass of Mo, more than 0.5% by mass but less than 1.5% by mass of
Si, and 0.025% by mass or more but less than 1.0% by mass of Nb,
with the balance consisting of Fe and unavoidable impurities,
wherein a content of P contained in the unavoidable impurities is
regulated to less than 0.03% by mass, wherein a total content of C
and N is from 0.85% by mass to 1.3% by mass, and wherein a ratio of
the content of Nb to the content of C is 0.05 or more but less than
1.8.
2. The heat resistant steel for an exhaust valve according to claim
1, which further comprises 0.001% by mass or more but less than
0.01% by mass of Mg and Ca in total.
3. The heat resistant steel for an exhaust valve according to claim
1, which further comprises at least one selected from the group
consisting of: 0.001% by mass or more but less than 0.03% by mass
of B, and 0.001% by mass or more but less than 0.1% by mass of
Zr.
4. The heat resistant steel for an exhaust valve according to claim
2, which further comprises at least one selected from the group
consisting of: 0.001% by mass or more but less than 0.03% by mass
of B, and 0.001% by mass or more but less than 0.1% by mass of
Zr.
5. The heat resistant steel for an exhaust valve according to claim
1, which further comprises 0.01% by mass or more but less than 5.0%
by mass of Co.
6. The heat resistant steel for an exhaust valve according to claim
2, which further comprises 0.01% by mass or more but less than 5.0%
by mass of Co.
7. The heat resistant steel for an exhaust valve according to claim
3, which further comprises 0.01% by mass or more but less than 5.0%
by mass of Co.
8. The heat resistant steel for an exhaust valve according to claim
4, which further comprises 0.01% by mass or more but less than 5.0%
by mass of Co.
Description
FIELD OF THE INVENTION
[0001] The present invention relates to a heat resistant steel for
an exhaust valve.
BACKGROUND OF THE INVENTION
[0002] An inlet valve for introducing a mixed gas of fuel and air
into a cylinder and an exhaust valve for discharging combustion gas
to the outside of the cylinder have been used in an engine. Of
these, the exhaust valve is exposed to high-temperature combustion
gas, so that a material having excellent high-temperature
properties (for example, high-temperature hardness, fatigue
properties, wear resistance and oxidation resistance) has been used
in the exhaust valve. As the material for the exhaust valve, there
has been known a Ni base superalloy (for example, NCF751), an
austenitic heat resistant steel (for example, SUH35) or the
like.
[0003] Ni base superalloys are a material in which a .gamma.' phase
is allowed to be precipitated by aging treatment, thereby enhancing
strength and wear resistance thereof at high temperature. Ni base
superalloys are expensive, but have extremely high heat resistance.
Accordingly, a valve using the same has been mainly used in a
high-power engine exposed to a temperature of 800.degree. C. or
more.
[0004] On the other hand, austenitic heat resistant steels are a
material in which M.sub.23C.sub.6 type carbides are allowed to be
precipitated, thereby enhancing strength and wear resistance
thereof at high temperature. The austenitic heat resistant steels
are inferior to the Ni base superalloys in high-temperature
properties, but are inexpensive. Accordingly, a valve using the
same has been mainly used in an engine for which high heat
resistance is not required.
[0005] With respect to such materials suitable for the exhaust
valve, various proposals have hitherto been made. For example,
JP-A-2004-277860 discloses a heat resistant alloy for an exhaust
valve comprising, by weight %, C: 0.01% to 0.2%, Si: 1% or less,
Mn: 1% or less, Ni: 30% to 62%, Cr: 13% to 20%, W: 0.01% to 3.0%,
Al: 0.7% to less than 1.6%, Ti: 1.5% to 3.0%, B: 0.001% to 0.01%,
P: 0.02% or less and S: 0.01% or less, with the balance consisting
of Fe and unavoidable impurities.
[0006] Further, JP-A-9-279309 discloses a Fe--Cr--Ni heat resistant
alloy comprising, by weight %, C: 0.01% to 0.10%, Si: 2% or less,
Mn: 2% or less, Cr: 14% to 18%, Nb+Ta: 0.5% to 1.5%, Ti: 2.0% to
3.0%, Al: 0.8% to 1.5%, Ni: 30% to 35%, B: 0.001% to 0.01%, Cu:
0.5% or less, P: 0.02% or less, S: 0.01% or less, O: 0.01% or less
and N: 0.01% or less, with the balance consisting of Fe and
unavoidable impurities and having a predetermined component
balance.
[0007] Furthermore, JP-A-2001-323323 discloses a method for
manufacturing an automotive engine valve comprising subjecting a Fe
base heat resistant steel having a Fe-0.53% C-0.2% Si-9.2% Mn-3.9%
Ni-21.5% Cr-0.43% N composition to solution treatment at
1,100.degree. C. to 1,180.degree. C., and forging a bevel portion
of the valve at 700.degree. C. to 1,000.degree. C., followed by
aging treatment.
[0008] This document describes that when the Fe base heat resistant
steel having the predetermined composition is subjected to the
solution treatment, the forging and the aging treatment under the
predetermined conditions, the hardness of a valve face portion can
be adjusted to HV 400 or more.
[0009] Due to the recent rising cost of raw materials, the
production cost of exhaust valves is significantly affected by
fluctuations of the raw material cost. In particular, the Ni base
superalloy has a large Ni content, so that the raw material cost
and production cost of the exhaust valves made of Ni base
superalloy are largely affected by the price of Ni. Accordingly,
materials in which the amount of Ni is decreased to decrease the
fluctuation band of the raw material cost have been desired.
However, in the Ni base superalloy, Ni is an element for forming a
.gamma.' phase as a reinforcing phase, so that a further decrease
in the amount of Ni results in a difficulty of achieving high
strength utilizing the y' phase.
[0010] On the other hand, the carbide precipitation type austenitic
heat resistant steel is hard to be affected by the price of Ni, but
has a problem of being poor in high temperature properties,
compared to the .gamma.' precipitation type Ni base superalloy.
There has been also known a material obtained by increasing the
strength of SUH35 in order to solve this problem (for example,
overseas standard LV21-43 steel (SUH 35+1W, 2Nb)). However,
regarding the LV21-43 steel, problems of difficult texture control
and poor hot workability still remain.
SUMMARY OF THE INVENTION
[0011] A problem that the invention is to solve is to provide a
heat resistant steel for an exhaust valve, which has a relatively
small Ni content, has high mechanical properties (for example,
tensile strength, fatigue strength, wear resistance, hardness and
the like), and moreover, has an excellent corrosion resistance.
[0012] Namely, the present invention provides the following items 1
to 4.
[0013] 1. A heat resistant steel for an exhaust valve,
comprising:
[0014] more than 0.50% by mass but less than 0.80% by mass of
C,
[0015] more than 0.30% by mass but less than 0.60% by mass of
N,
[0016] 17.0% by mass or more but less than 25.0% by mass of Cr,
[0017] 4.0% by mass or more but less than 12.0% by mass of Ni,
[0018] 7.0% by mass or more but less than 14.0% by mass of Mn,
[0019] 2.0% by mass or more but less than 6.0% by mass of Mo,
[0020] more than 0.5% by mass but less than 1.5% by mass of Si,
and
[0021] 0.025% by mass or more but less than 1.0% by mass of Nb,
[0022] with the balance consisting of Fe and unavoidable
impurities,
[0023] wherein a content of P contained in the unavoidable
impurities is regulated to less than 0.03% by mass,
[0024] wherein a total content of C and N is from 0.85% by mass to
1.3% by mass, and
[0025] wherein a ratio of the content of Nb to the content of C is
0.05 or more but less than 1.8.
[0026] 2. The heat resistant steel for an exhaust valve according
to item 1 above, which further comprises 0.001% by mass or more but
less than 0.01% by mass of Mg and Ca in total.
[0027] 3. The heat resistant steel for an exhaust valve according
to item 1 or 2 above, which further comprises at least one selected
from the group consisting of
[0028] 0.001% by mass or more but less than 0.03% by mass of B,
and
[0029] 0.001% by mass or more but less than 0.1% by mass of Zr.
[0030] 4. The heat resistant steel for an exhaust valve according
to any one of items 1 to 3 above, which further comprises 0.01% by
mass or more but less than 5.0% by mass of Co.
[0031] Both of C and N are austenite-stabilizing elements, and at
the same time, also elements for forming an MX type carbonitride
(including an MC type carbide). In the invention, the (C+N) amount
(total content of C and N) and the Nb/C ratio (ratio of the content
of Nb to the content of C) are regulated in specific ranges, so
that the MX type carbonitrides (including the MC type carbides)
having a proper size are formed in proper amounts in the material
after solution treatment. Accordingly, grains are not coarsened
after the solution treatment, and coarse primary crystal MX type
carbonitrides also do not remain. Further, since M.sub.23C.sub.6
type carbides are precipitated in proper amounts in the material by
aging treatment, high-temperature properties are improved.
Furthermore, since a solid-solution hardening element is limited to
Mo, the high-temperature properties are improved.
BEST MODE FOR CARRYING OUT THE INVENTION
[0032] Some embodiments of the invention will be described in
detail below. Herein, in the present specification, all the
percentages defined by mass are the same as those defined by
weight, respectively.
[1. Heat Resistant Steel for Exhaust Valve]
[0033] The heat resistant steel for an exhaust valve according to
the invention comprises the following elements with the balance
consisting of Fe and unavoidable impurities. The kind of additive
elements, the component range thereof and the reason for limitation
thereof are as follows. In an embodiment, the heat resistant steel
according to the invention comprises the following main constituent
elements and optional secondary constituent element(s), with the
balance consisting of Fe and unavoidable impurities. In another
embodiment, the heat resistant steel according to the invention
consists essentially of the following main constituent elements and
optional secondary constituent element(s), with the balance
consisting of Fe and unavoidable impurities. In still another
embodiment, the heat resistant steel according to the invention
consists of the following main constituent elements and optional
secondary constituent element(s), with the balance consisting of Fe
and unavoidable impurities.
[1.1. Main Constituent Elements]
[0034] (1) 0.50<C<0.80 mass %
[0035] C is an austenite-stabilizing element, and inhibits the
formation of a sigma phase or a Laves phase as a harmful phase.
Further, C is preferentially bound to Nb to produce an MC type
carbide. A proper amount of the MC type carbide having a proper
size inhibits grains from being coarsened during solution treatment
and improves strength properties. Further, a proper amount of the
MC type carbide having a proper size acts as a hard phase to
improve wear resistance. Furthermore, C is bound to Cr to produce
M.sub.23C.sub.6 type carbides, thereby improving the wear
resistance and the strength properties. In order to obtain such
effects, it is necessary that the C content exceeds 0.50 mass %.
The C content preferably exceeds 0.52 mass %.
[0036] On the other hand, an excessive C content results in an
excessive carbide amount that causes deterioration of
processability. It is therefore necessary that the C content is
less than 0.80 mass %. The C content is more preferably less than
0.70 mass %, and furthermore preferably less than 0.67 mass %.
(2) 0.30<N<0.60 mass %
[0037] N is an austenite-stabilizing element, and acts as an
alternative element for austenite-forming elements such as Ni and
Mn. Further, N acts for reinforcement of a matrix as an
interstitial solid-solution hardening element, because of its small
atomic radius. Furthermore, N acts in complex with substitutional
solid-solution hardening elements such as Mo and W to contribute to
improvement on strength. In addition, N is substituted for a C site
of the MC type carbide to form an MX type carbonitride. In order to
obtain such effects, it is necessary that the N content exceeds
0.30 mass %. It is more preferable that the N content exceeds 0.35
mass %.
[0038] On the other hand, an excessive N content results in a
difficulty of allowing N to be dissolved in the matrix. It is
therefore necessary that the N content is less than 0.60 mass %.
The N content is more preferably less than 0.50 mass %, and
furthermore preferably less than 0.47 mass %.
(3) 17.0.ltoreq.Cr<25.0 mass %
[0039] Cr has a function of forming a protective oxide coating of
Cr.sub.2O.sub.3 in an operating temperature range of the exhaust
valve. Cr is therefore an element indispensable for improving
corrosion resistance and oxidation resistance. Further, Cr binds to
C to form the Cr.sub.23C.sub.6 carbide, thereby contributing to
improvement of the strength properties. In order to obtain such
effects, it is necessary that the Cr content is 17.0 mass % or
more. The Cr content is more preferably 18.0 mass % or more, and
furthermore preferably 19.5 mass % or more.
[0040] On the other hand, an excessive Cr content causes
destabilization of austenite, because Cr is a ferrite-stabilizing
element. Further, the excessive addition of Cr promotes the
formation of the sigma phase or the Laves phase that are an
embrittlement phase, thereby causing deterioration of hot
workability and strength property. It is therefore necessary that
the Cr content is less than 25.0 mass %. The Cr content is more
preferably less than 23.5 mass %, and furthermore preferably 22.5
mass % or less.
(4) 4.0.ltoreq.Ni<12.0 mass %
[0041] Ni is added as an austenite-stabilizing element. In order to
stabilize austenite, it is necessary that the Ni content is 4.0
mass % or more. The Ni content is more preferably 4.5 mass % or
more, and furthermore preferably 5.1 mass % or more.
[0042] On the other hand, an excessive Ni content causes an
increase in cost. It is therefore necessary that the Ni content is
less than 12.0 mass %. The Ni content is more preferably less than
11.5 mass %, and furthermore preferably 10.5 mass % or less.
(5) 7.0.ltoreq.Mn<14.0 mass %
[0043] Mn is added as an austenite-stabilizing element. Mn not only
acts as an alternative element for expensive Ni, but also has an
effect of enhancing solubility of N. In order to obtain such
effects, it is necessary that the Mn content is 7.0 mass % or more.
The Mn content is more preferably 7.5 mass % or more, and
furthermore preferably 8.0 mass % or more.
[0044] On the other hand, an excessive Mn content causes
deterioration of high-temperature properties due to a decrease in
melting point. It is therefore necessary that the Mn content is
less than 14.0 mass % The Mn content is more preferably 12.5 mass %
or less, and furthermore preferably less than 11.0 mass %.
(6) 2.0.ltoreq.Mo<6.0 mass %
[0045] Mo acts as a solid-solution hardening element for a .gamma.
phase of the matrix, and is an element effective for improvement of
high-temperature strength. In order to obtain such an effect, it is
necessary that the Mo content is 2.0 mass % or more. The Mo content
is more preferably 2.9 mass % or more, and furthermore preferably
3.3 mass % or more.
[0046] On the other hand, an excessive Mo amount causes an increase
in deformation resistance. Further, the formation of the sigma
phase or the Laves phase, that are an embrittlement phase, is
promoted and thus the hot workability and fatigue properties are
deteriorated. It is therefore necessary that the Mo content is less
than 6.0 mass %. The Mo content is more preferably less than 5.1
mass %, and furthermore preferably 4.5 mass % or less.
[0047] Incidentally, there is also a technique of adding W besides
Mo as the solid-solution hardening element. However, in the
invention, the technique is limited to the addition of Mo. The
amount of solid-solution hardening due to the solid-solution
hardening element such as Mo or W largely depends on the atomic
weight of the element. Mo is smaller in the atomic weight than W,
and larger in the number of atoms per unit mass %. Accordingly, Mo
provides a larger amount of solid-solution hardening. For this
reason, when it is intended to obtain the equivalent amount of
solid-solution hardening by the addition of W, precipitation of the
Laves phase becomes dominant, resulting in a failure to obtain an
effect equivalent to that due to Mo. Accordingly, in order to
maximally obtain the effect of solid-solution hardening, in the
invention, the technique is limited to the addition of Mo.
(7) 0.5<Si<1.5 mass %
[0048] Si is an effective element as a deoxidizing agent at the
time of dissolution and for imparting the oxidation resistance in a
high-temperature region. Further, Si has an effect of improving
strength as a solid-solution hardening element. In order to obtain
such effects, it is necessary that the Si content exceeds 0.5 mass
%. The Si content is more preferably 0.55 mass % or more.
Furthermore preferably, the Si content exceeds 0.60 mass %.
[0049] On the other hand, an excessive Si amount conversely results
in a decrease in the strength properties. Further, an oxide of Si
is liable to delaminate. When Si oxides are produced in large
amounts, oxide layers delaminate, thereby deteriorating the
oxidation resistance. It is therefore necessary that the Si content
is less than 1.5 mass %. The Si content is more preferably less
than 1.1 mass %, and furthermore preferably less than 0.9 mass
%.
[0050] Incidentally, there is a problem that a Si-containing Fe
base alloy is generally liable to corrode in high-temperature
environments where Pb coexists. Accordingly, Si-free materials have
hitherto been used in steels for exhaust valves. However, according
to the present fuel circumstances (production of lead-free
gasoline), lead corrosion resistance has become out of the
question. In the invention, therefore, Si is positively added to
make efficient use of it for improvement of the oxidation
resistance and the strength properties. This point is one of major
characteristics of the invention.
(8) 0.025.ltoreq.Nb<1.0 mass %
[0051] Nb binds to C and N to cause precipitation of MX type
carbonitrides (including MC type carbides, hereinafter the same). A
proper amount of the MX type carbonitride having a proper size
inhibits grains from being coarsened after the solution treatment,
which is effective for improvement of the high-temperature strength
properties. In order to obtain such an effect, it is necessary that
the Nb content is 0.025 mass % or more.
[0052] On the other hand, the addition of excessive Nb promotes the
production of ferrite and generates coarse MX type carbonitrides in
large amounts. The coarse carbonitrides partially remain even after
the solution treatment, which causes deterioration of the hot
workability. Further, the fatigue properties also deteriorate. It
is therefore necessary that the Nb content is less than 1.0 mass %.
The Nb content is more preferably less than 0.9 mass %, and
furthermore preferably less than 0.8 mass %.
[0053] Incidentally, elements for forming the MX type carbides
include Ti, V and the like, as well as Nb. However, in the
invention, the forming element is limited to Nb. The reason for
this is as follows.
[0054] Ti has a strong bonding force to C and N, and relatively
coarse primary crystal MX type carbonitrides (primary carbides) are
precipitated in large amounts. The primary carbides are not
dissolved even by the solution treatment, so that the coarse
carbonitrides exert a significant influence on deterioration of the
fatigue properties and impact properties.
[0055] Further, V is effective for improvement of the strength
properties. However, V has a strong bonding force to O, so that a V
oxide is formed to significantly deteriorate the oxidation
resistance of the material.
[0056] Accordingly, from the balance of the strength properties and
the oxidation resistance, the forming element of the MX type
carbonitrides is limited to Nb.
(9) P<0.03 mass %
[0057] The addition of P stimulates a refinement effect of the
carbide, and is effective for improvement of the high-temperature
strength properties. However, the addition of excessive P
significantly decreases the melting point to deteriorate the high
temperature strength and the hot workability. Further, the
precipitated carbide is coarsened depending on aging treatment
conditions. Regarding the fatigue properties, the coarse carbide
becomes a starting point of breakage to cause deterioration of
properties. It is therefore necessary that the P content is
regulated to less than 0.03 mass %. The present application aims at
improvement of the high-temperature strength properties by the
increases in the solid-solution hardening element amount and the
carbide amount, so that a smaller P content is preferred in order
to inhibit deterioration of processability as much as possible.
[1.2. Secondary Constituent Elements]
[0058] The heat resistant steel for an exhaust valve according to
the invention may further contain any one or two or more of the
following elements, in addition to the above-mentioned
elements.
(1) 0.001.ltoreq.(Mg, Ca)<0.01 mass %
[0059] Both of Mg and Ca can be added as a
deoxidizing/desulfurizing agent at the time of melting of the
alloy. Mg and/or Ca contribute to improvement of the hot
workability of the alloy. In order to obtain such effects, it is
necessary that the total content of Mg and Ca is 0.001 mass % or
more.
[0060] On the other hand, an excessive content of Mg and/or Ca
tends to deteriorate the processability rather than to improve it.
It is therefore necessary that the total content of Mg and Ca is
less than 0.01 mass %.
(2) 0.001.ltoreq.R<0.03 mass % (3) 0.001.ltoreq.Zr<0.1 mass
%
[0061] Both of B and Zr segregate in grain boundaries to reinforce
the boundaries. In order to obtain such an effect, it is necessary
that the contents of B and Zr are each 0.001 mass % or more.
[0062] On the other hand, excessive contents of B and Zr results in
impairing the hot workability. It is therefore necessary that the B
content is less than 0.03 mass %. Further, it is necessary that the
Zr content is less than 0.1 mass %. Any one or both of B and Zr may
be added.
(4) 0.01.ltoreq.Co<5.0 mass %
[0063] Co acts as an austenite-stabilizing element, and is used as
an alternative element for Ni. Further, Co contributes to
improvement of the strength properties. In order to obtain such
effects, it is necessary that the Co content is 0.01 mass % or
more.
[0064] On the other hand, an excessive Co amount results in high
cost. It is therefore necessary that the Co content is less than
5.0 mass %.
[0065] In this regard, with regard to each element contained in the
steel of the invention, according to an embodiment, the minimal
amount thereof present in the steel is the smallest non-zero amount
used in the Examples of the developed steels as summarized in Table
1. According to a further embodiment, the maximum amount thereof
present in the steel is the maximum amount used in the Examples of
the developed steels as summarized in Table 1.
[1.3. Component Balance]
[0066] The heat resistant steel for an exhaust valve according to
the invention satisfies the following conditions, in addition to
that the component elements are within the above-mentioned
ranges.
(1) 0.85.ltoreq.C+N.ltoreq.1.3 mass %
[0067] As described above, each of C and N is a strong
austenite-stabilizing element, and effectively acts as an
alternative element for expensive Ni on cost reduction. Further,
both of C and N have a function of forming the MX type
carbonitrides.
[0068] A proper amount of the MX type carbonitride having a proper
size inhibits grains from being coarsened after the solution
treatment, which is effective for improvement of the
high-temperature strength properties. In order to obtain such
effects, it is necessary that the (C+N) content (total content of C
and N) is 0.85 mass % or more. The (C+N) content is more preferably
0.87 mass % or more, and furthermore preferably 0.9 mass % or
more.
[0069] On the other hand, an excessive (C+N) content results in
forming coarse MX type carbonitrides in large amounts. The coarse
carbonitrides partially remain even after the solution treatment,
which causes deterioration of the hot workability. It is therefore
necessary that the (C+N) content is 1.3 mass % or less. The
(C.sub.+N) content is more preferably 1.20 mass % or less, and
furthermore preferably 1.15 mass % or less.
(2) 0.05.ltoreq.Nb/C<1.8
[0070] A proper amount of the MX type carbonitride having a proper
size has a role of preventing grains from being coarsened due to a
pinning effect. In order to obtain such an effect, it is necessary
that the ratio (Nb/C) of the Nb content (mass %) to the C content
(mass %) is 0.05 or more. The Nb/C ratio is more preferably 0.07 or
more, and furthermore preferably 0.1 or more.
[0071] On the other hand, when Nb becomes relatively excessive to
C, Nb preferentially binds to C, and the coarse primary crystal MX
type carbonitrides are precipitated in a large amount. The coarse
primary crystal MX type carbonitrides do not disappear completely
even after the solution treatment, which causes deterioration of
the fatigue properties. Further, C is depleted to decrease the
amount of the M.sub.23C.sub.6 type carbides precipitated, which are
effective for improvement of the wear resistance and the strength
properties. It is therefore necessary that the Nb/C ratio is less
than 1.8. The Nb/C ratio is more preferably less than 1.5, and
furthermore preferably 1.3 or less.
[2. Manufacturing Method of Heat Resistant Steel for Exhaust
Valve]
[0072] A manufacturing method of the heat resistant steel for an
exhaust valve according to the invention comprises a
melting/casting step, a homogenized heat treatment step, a forging
step, a solution treatment step and an aging step.
[2.1. Melting/Casting Step]
[0073] The melting/casting step is a step of melting and casting
the raw materials blended to a predetermined composition. A melting
method of the raw materials and a casting method of molten metal
are not particularly limited, and various methods can be used. Any
melting conditions may be used as long as they are conditions under
which the molten metal which has homogeneous components and is
possible to be casted is obtained.
[2.2. Homogenized Heat Treatment Step]
[0074] The homogenized heat treatment step is a step of subjecting
an, ingot obtained in the melting/casting step to homogenized heat
treatment. The homogenized heat treatment is performed in order to
homogenize components of the ingot.
[0075] As the conditions for the homogenized heat treatment,
optimum conditions are selected depending on the components.
Usually, the heat treatment temperature is from 1,100.degree. C. to
1,250.degree. C. Further, the heat treatment time is from 5 hours
to 25 hours.
[2.3. Forging Step]
[0076] The forging step is a step of plastically deforming the
ingot subjected to the homogenized heat treatment to a
predetermined shape. A forging method and forging conditions are
not particularly limited, and any method and conditions may be used
as long as a desired shape can be efficiently produced.
[2.4. Solution treatment Step]
[0077] The solution treatment step is a step of subjecting the
material obtained in the forging step to solution treatment. The
solution treatment is performed in order to allow the coarse
primary crystal MX type carbonitrides to disappear.
[0078] As the solution treatment conditions, optimum conditions are
selected depending on the components. In general, with an increase
in the solution treatment temperature, the remaining amount of
primary carbides decreases and the amount of fine intragranular
carbides precipitated in the aging treatment increases. This is
therefore effective for improvement of the fatigue properties.
However, when the treatment is performed at a temperature higher
than 1,200.degree. C., precipitation of grain boundary reaction
carbides is promoted, leading to deterioration of properties.
Accordingly, the solution treatment is preferably performed at
1,000.degree. C. to 1,200.degree. C. for 20 minutes or more,
followed by oil cooling treatment.
[2.5. Aging Step]
[0079] The aging step is a step of subjecting the material after
the solution treatment to aging treatment. The aging treatment is
performed in order to allow the M.sub.23C.sub.6 type carbides to be
precipitated.
[0080] As the aging treatment conditions, optimum conditions are
selected depending on the components. The aging treatment is
preferably performed at 700.degree. C. to 850.degree. C. for 2
hours or more, followed by air cooling treatment, although
depending on the components.
[3. Operation of Heat Resistant Steel for Exhaust Valve]
[0081] Both of C and N are the austenite-stabilizing elements, and
at the same time, also the elements for forming the MX type
carbonitride. In the invention, the (C+N) amount and the Nb/C ratio
are respectively controlled in specific ranges, so that the MX type
carbonitrides having a proper size are formed in proper amounts in
the material after the solution treatment. Accordingly, the grains
are not coarsened after the solution treatment, and the coarse
primary crystal MX type carbonitrides also do not remain. Further,
since the M.sub.23C.sub.6 type carbides are precipitated in proper
amounts in the material by the aging treatment, the
high-temperature properties are improved. Furthermore, since the
solid-solution hardening element is limited to Mo, the
high-temperature properties are improved.
[0082] Further, the amount of Si added is controlled in a specific
range, so that the oxidation resistance is improved, and the
solid-solution hardening is also achieved.
[0083] Furthermore, the amount of Ni is increased compared to
conventional austenitic heat resistant steels, so that the .gamma.
phase is stabilized to improve toughness.
EXAMPLES
Examples 1 to 24 and Comparative Examples 1 to 16
[1. Preparation of Specimens]
[0084] Alloys having compositions shown in Tables 1 and 2 were each
melted in a high-frequency induction furnace to obtain 50 kg of an
ingot. To each ingot prepared by melting, homogenized heat
treatment was performed at 1,180.degree. C. for 16 hours. Then, the
ingot was forged to a rod stock having a diameter of 18 mm. To the
material forged, solution treatment (ST) was further performed.
Solution treatment was conducted under conditions at 1,150.degree.
C. for 30 minutes, followed by oil cooling (Examples 1 to 24) or
conditions at 1,050.degree. C. for 30 minutes, followed by oil
cooling (Comparative Examples 1 to 16). Further, to the material
after the solution treatment (ST), aging treatment (AG) was
performed under conditions of 750.degree. C. for 4 hours, followed
by air cooling.
TABLE-US-00001 TABLE 1 Component (mass %) C N Si Mn Cr Ni Mo Nb P
Others Nb/C C + N Example 1 0.62 0.46 0.75 9.3 20.6 6.5 4.1 0.71
0.010 1.15 1.08 Example 2 0.53 0.42 0.62 8.5 20.3 5.8 5.7 0.72
0.009 1.36 0.95 Example 3 0.58 0.38 0.71 7.2 19.3 6.9 2.3 0.44
0.008 0.76 0.96 Example 4 0.62 0.51 0.67 8.7 24.1 5.5 4.8 0.84
0.014 1.35 1.17 Example 5 0.63 0.44 0.73 11.5 21.3 5.4 2.5 0.72
0.008 1.14 1.07 Example 6 0.62 0.40 0.72 7.2 20.9 10.4 3.8 0.69
0.010 1.11 1.02 Example 7 0.60 0.39 0.59 8.7 23.6 4.3 4.6 0.62
0.009 1.03 0.99 Example 8 0.61 0.44 0.63 8.9 20.9 5.9 4.1 0.71
0.007 Mg: 0.005 1.16 1.05 Example 9 0.55 0.43 0.73 9.2 20.1 5.8 3.9
0.73 0.007 Ca: 0.005 1.33 0.98 Example 10 0.55 0.47 0.67 9.7 21.4
6.2 4.2 0.66 0.012 B: 0.015 1.20 1.02 Example 11 0.57 0.42 0.68 9.3
21.3 5.9 4.3 0.68 0.011 Zr: 0.05 1.19 0.99 Example 12 0.62 0.44
0.65 9.3 21.4 5.2 3.8 0.71 0.010 Co: 2.5 1.15 1.06 Example 13 0.59
0.42 0.71 9.2 19.8 11.7 4.2 0.70 0.009 1.19 1.01 Example 14 0.51
0.40 0.68 8.6 20.8 6.2 5.4 0.71 0.009 1.39 0.91 Example 15 0.59
0.38 1.31 8.9 19.3 6.3 3.4 0.72 0.008 1.22 0.97 Example 16 0.73
0.32 0.71 9.1 20.1 5.9 3.8 0.68 0.010 0.93 1.06 Example 17 0.61
0.43 0.63 9.3 20.9 4.6 4.1 0.71 0.013 1.16 1.04 Example 18 0.55
0.48 0.53 8.3 21.8 5.8 4.2 0.68 0.007 1.24 1.03 Example 19 0.56
0.45 0.61 9.5 17.2 5.9 3.9 0.70 0.009 1.25 1.01 Example 20 0.57
0.54 0.73 8.1 17.8 6.0 4.1 0.71 0.013 1.25 1.11 Example 21 0.61
0.44 0.74 8.3 21.4 5.9 4.0 0.94 0.009 1.54 1.05 Example 22 0.53
0.41 0.69 9.3 20.1 5.9 3.8 0.08 0.010 0.15 0.94 Example 23 0.62
0.38 0.60 9.2 20.9 6.2 3.9 0.31 0.011 0.50 1.00 Example 24 0.53
0.42 0.68 9.3 21.1 9.8 4.0 0.69 0.009 1.30 0.95
TABLE-US-00002 TABLE 2 Component (mass %) C N Si Mn Cr Ni Mo Nb P
Others Nb/C C + N Comparative Example 1 0.49 0.40 0.09 8.9 21.2 3.9
-- -- 0.009 0.00 0.89 Comparative Example 2 0.50 0.45 0.08 9.0 21.0
4.0 -- 2.03 0.010 W: 1.0 4.06 0.95 Comparative Example 3 0.61 0.45
0.72 9.2 21.6 5.2 1.5 0.69 0.009 1.13 1.06 Comparative Example 4
0.63 0.42 0.69 8.6 20.4 6.3 3.9 1.45 0.011 2.30 1.05 Comparative
Example 5 0.59 0.43 0.72 9.5 21.6 6.1 1.9 0.67 0.014 W: 1.1 1.14
1.02 Comparative Example 6 0.61 0.40 0.82 9.0 21.0 6.0 -- 0.69
0.008 W: 3.5 1.13 1.01 Comparative Example 7 0.63 0.51 0.78 9.1
19.2 6.3 4.0 -- 0.009 V: 0.82 1.30 1.14 Comparative Example 8 0.59
0.42 0.66 9.0 21.2 6.1 4.0 -- 0.010 Ti: 0.78 1.32 1.01 Comparative
Example 9 0.61 0.40 0.71 9.2 21.1 5.9 4.2 0.69 0.120 1.13 1.01
Comparative Example 10 0.62 0.42 0.32 9.9 21.2 6.2 4.1 0.66 0.007
1.06 1.04 Comparative Example 11 0.60 0.38 1.67 9.1 21.0 6.0 3.9
0.68 0.014 1.13 0.98 Comparative Example 12 0.72 0.41 0.70 9.2 20.9
6.1 4.0 0.03 0.008 0.04 1.13 Comparative Example 13 0.51 0.53 0.70
10.2 22.3 6.0 4.1 0.98 0.008 1.92 1.04 Comparative Example 14 0.51
0.31 0.70 9.0 20.9 6.1 3.9 0.73 0.008 1.43 0.82 Comparative Example
15 0.73 0.59 0.70 10.5 22.1 5.9 4.0 0.88 0.008 1.21 1.32
Comparative Example 16 0.55 0.38 0.71 9.0 21.2 6.1 6.5 0.71 0.010
1.29 0.93
[2. Test Methods]
[2.1. Hardness]
[0085] The hardness at ordinary temperature was measured using the
C scale of a Rockwell hardness tester. Further, the hardness at
800.degree. C. was measured at a measuring load of 5 kg using a
high-temperature Vickers hardness tester.
[2.2. Tensile Test]
[0086] A test piece having a test portion diameter of 8 mm and a
test piece length of 90 mm was cut out of each material. The
tensile test was performed at 800.degree. C. using this test piece
to measure the tensile strength.
[2.3. Fatigue Test]
[0087] A test piece having a parallel portion diameter of 8 mm and
a test piece length of 90 mm was cut out of each material. The
Ono-type rotary bending fatigue test was performed at 800.degree.
C. using this test piece to measure 10.sup.7-cycle fatigue
strength.
[2.4. Oxidation Test]
[0088] A cylindrical test piece having a diameter of 8 mm and a
length of 17 mm was prepared from each material. This test piece
was continuously heated for 400 hours in an atmospheric atmosphere
of 850.degree. C., and air-cooled. The weight increase by oxidation
was calculated from the difference in weight between before and
after the test, and taken as the index of oxidation resistance.
[Results]
[0089] The results thereof are shown in Tables 3 and 4. The
following are known from Tables 3 and 4.
[0090] (1) In all of Examples 1 to 24, a high-temperature hardness
at 800.degree. C. of about 200 HV or more is obtained, and the
specimens have sufficient high-temperature wear resistance required
at the time of use in an exhaust valve application. Further, in all
of Examples 1 to 24, the specimens have a tensile strength of 370
MPa or more at 800.degree. C. These are affected by solid-solution
hardening due to Mo and hardening due to the carbides
(particularly, the M.sub.23C.sub.6 type carbides).
[0091] (2) In all of Examples 1 to 24, the specimens have a
10.sup.7-cycle fatigue strength of 240 MPa or more, and are also
excellent in the high-temperature fatigue properties as carbide
precipitation type austenitic heat resistant steels used for
exhaust valve materials. The reason for this is considered to be
that the grains are prevented from being coarsened due to the
pinning effect of the Nb-based MX type carbonitrides, and that the
size and amount of the coarse carbides (initial crystal carbides)
contributing to early breakage are optimized by the Nb/C
standard.
[0092] (3) Comparative Examples 1 and 2 are conventional steels.
Comparative Example 1 is SUH35 and Comparative Example 2 is
LV21-43. Both of them have low mechanical properties, fatigue
properties and oxidation resistance at high temperature.
[0093] In both of Comparative Examples 3 and 4, the mechanical
properties and fatigue strength at high temperature are low,
because the Mo content is small in Comparative Example 3, and
because the Nb content is excessive in Comparative Example 4.
[0094] In Comparative Examples 5 and 6 in which a part or all of Mo
is substituted by W, the mechanical properties at high temperature
are low, and particularly, the fatigue strength is low.
[0095] Ti and V are MX type carbonitride-forming elements similarly
to Nb, and have been considered to have an effect equal to that of
Nb. However, in Comparative Examples 7 an 8 in which these elements
are added, the weight increase by oxidation is large, compared to
Examples 1 to 24. The reason for this is considered to be that Ti
and V have a strong bonding force to 0 compared to Nb, resulting in
easy occurrence of the production of oxides. Further, in
Comparative Example 8, the 10.sup.7-cycle fatigue strength is low.
The reason for this is considered to be that stable and coarse
carbonitrides are produced because the bonding force of Ti to C and
N is strong, which causes early breakage under repeated stress.
Accordingly, Ti and V cannot be alternative elements for Nb.
[0096] In Comparative Example 9, the fatigue strength is low,
because the P content is excessive.
[0097] In both of Comparative Examples 10 and 11, the oxidation
resistance is low, because the Si content is small in Comparative
Example 10, and because the Si content is excessive in Comparative
Example 11.
[0098] In both of Comparative Examples 12 and 13, the fatigue
strength is low, because the Nb/C ratio is low in Comparative
Example 12, and because the Nb/C ratio is excessively high in
Comparative Example 13.
[0099] In Comparative Example 14, the mechanical properties and
fatigue strength at high temperature are low, because the (C+N)
content is small.
[0100] In Comparative Example 15, the fatigue strength is low,
because the (C+N) content is excessively large.
[0101] Further, in Comparative Example 16, the fatigue strength is
low, because the Mo content is excessively large.
[0102] In contrast, from the oxidation resistance test at
850.degree. C. for 400 hours in an atmospheric atmosphere, it has
been shown that the specimens in Examples 1 to 24 have good
oxidation resistance in comparison with Comparative Examples 1 to
16.
[0103] Further, as shown in Tables 3 and 4, the high-temperature
hardness, the fatigue properties and the oxidation resistance which
are desired for exhaust valves are optimized in good levels, when
the components are in the more preferred ranges thereof.
Specifically, the high-temperature hardness at 800.degree. C. is
210 or more, the 10.sup.7-cycle fatigue strength at 800.degree. C.
is 260 MPa or more, and the weight increase by oxidation after the
oxidation test at 800.degree. C. for 400 hours is 1.3 mg/cm.sup.2
or less.
[0104] From the above results, it has been shown that the heat
resistant steels according to the invention are excellent in the
high-temperature properties and useful as materials for exhaust
valves.
TABLE-US-00003 TABLE 3 High- 10.sup.7-Cycle Oxidation Temperature
Tensile Fatigue Resistance Aging Hardness Strength Strength
800.degree. C. .times. Hardness 800.degree. C. 800.degree. C.
800.degree. C. 400 h (HRC) (Hv) (MPa) (MPa) (mg/cm.sup.2) Example 1
36.9 215 416 265 1.29 Example 2 37.2 218 420 258 1.31 Example 3
35.3 201 406 250 1.30 Example 4 35.4 200 403 248 1.18 Example 5
35.6 201 404 252 1.38 Example 6 36.1 207 411 256 1.26 Example 7
36.2 210 408 251 1.20 Example 8 36.4 208 404 256 1.31 Example 9
36.1 208 407 257 1.29 Example 10 35.9 209 405 255 1.32 Example 11
36.4 211 408 258 1.28 Example 12 36.6 209 410 255 1.22 Example 13
36.2 203 404 256 1.24 Example 14 37.0 216 418 260 1.32 Example 15
35.2 199 398 243 1.21 Example 16 37.1 214 411 254 1.31 Example 17
35.4 202 401 246 1.32 Example 18 36.0 205 410 255 1.35 Example 19
34.9 198 389 243 1.40 Example 20 35.3 200 394 246 1.45 Example 21
35.6 203 405 250 1.28 Example 22 36.8 210 412 261 1.28 Example 23
36.9 212 412 264 1.29 Example 24 35.6 202 404 251 1.28 *Heat
treatment: solution treatment/1050.degree. C. .times. 0.5 h, oil
cooling, aging treatment/750.degree. C. .times. 4 h, air cooling
*Fatigue test: Ono-type rotary bending fatigue test (the number of
rotations/3,500 rpm)
TABLE-US-00004 TABLE 4 High- 10.sup.7-Cycle Oxidation Temperature
Tensile Fatigue Resistance Aging Hardness Strength Strength
800.degree. C. .times. Hardness 800.degree. C. 800.degree. C.
800.degree. C. 400 h (HRC) (Hv) (MPa) (MPa) (mg/cm.sup.2)
Comparative 32.0 166 310 169 3.44 Example 1 Comparative 33.2 167
321 187 2.48 Example 2 Comparative 33.8 187 337 207 1.43 Example 3
Comparative 33.5 189 352 216 1.65 Example 4 Comparative 33.3 179
331 181 1.55 Example 5 Comparative 33.6 181 335 148 1.78 Example 6
Comparative 31.4 191 349 215 2.32 Example 7 Comparative 32.2 193
339 168 1.98 Example 8 Comparative 35.0 190 395 198 1.98 Example 9
Comparative 34.9 192 380 235 2.31 Example 10 Comparative 34.8 188
375 230 2.13 Example 11 Comparative 34.2 188 370 223 1.35 Example
12 Comparative 36.5 187 374 220 1.36 Example 13 Comparative 31.3
160 305 170 1.37 Example 14 Comparative 36.6 189 380 215 1.35
Example 15 Comparative 37.0 220 415 210 1.36 Example 16 *Heat
treatment: solution treatment/1050.degree. C. .times. 0.5 h, oil
cooling, aging treatment/750.degree. C. .times. 4 h, air cooling
*Fatigue test: Ono-type rotary bending fatigue test (the number of
rotations/3,500 rpm)
[0105] Although embodiments of the invention have been described in
detail above, the invention should not be construed as being
limited to the embodiments set forth above in any way, and various
modifications are possible without departing from the gist of the
invention.
[0106] This application is based on Japanese patent application No.
2010-070720 filed Mar. 25, 2010, the entire contents thereof being
hereby incorporated by reference.
[0107] The heat resistant steel for an exhaust valve according to
the invention can be used in exhaust valves of various engines.
* * * * *