U.S. patent application number 11/071170 was filed with the patent office on 2005-09-08 for martensitic stainless steel.
This patent application is currently assigned to Daido Tokushuko Kabushiki Kaisha. Invention is credited to Hamano, Shuji, Shimizu, Tetsuya.
Application Number | 20050194067 11/071170 |
Document ID | / |
Family ID | 34747679 |
Filed Date | 2005-09-08 |
United States Patent
Application |
20050194067 |
Kind Code |
A1 |
Hamano, Shuji ; et
al. |
September 8, 2005 |
Martensitic stainless steel
Abstract
A martensitic stainless steel of this invention, aimed at
achieving excellent corrosion resistance and cold workability and a
desirable level of toughness, while keeping the hardness equivalent
to that of conventional martensitic stainless steel, which consists
essentially of, in % by mass, C: less than 0.15%, Si: 0.05% or more
and less than 0.20%, Mn: 0.05-2.0%, P: 0.03% or less, S: 0.03% or
less, Cu: 0.05-3.0%, Ni: 0.05-3.0%, Cr: 13.0-20.0%, Mo: 0.2-4.0%,
V: 0.01-1.0%, Al: 0.030% or less, Ti: less than 0.020%, O: 0.020%
or less, N: 0.40-0.80%, and the balance of Fe and inevitable
impurities.
Inventors: |
Hamano, Shuji; (Nagoya-shi,
JP) ; Shimizu, Tetsuya; (Nagoya-shi, JP) |
Correspondence
Address: |
ARENT FOX PLLC
1050 CONNECTICUT AVENUE, N.W.
SUITE 400
WASHINGTON
DC
20036
US
|
Assignee: |
Daido Tokushuko Kabushiki
Kaisha
|
Family ID: |
34747679 |
Appl. No.: |
11/071170 |
Filed: |
March 4, 2005 |
Current U.S.
Class: |
148/325 ; 420/41;
420/61 |
Current CPC
Class: |
C22C 38/60 20130101;
C22C 38/42 20130101; C22C 38/46 20130101; C22C 38/44 20130101; C22C
38/48 20130101; C22C 38/02 20130101; C22C 38/52 20130101; C22C
38/04 20130101 |
Class at
Publication: |
148/325 ;
420/041; 420/061 |
International
Class: |
C22C 038/44 |
Foreign Application Data
Date |
Code |
Application Number |
Mar 4, 2004 |
JP |
2004-061159 |
Claims
What is claimed is:
1. A martensitic stainless steel consisting essentially of, in % by
mass, C: less than 0.15%, Si: 0.05% or more and less than 0.20%,
Mn: 0.05-2.0%, P: 0.03% or less, S: 0.03% or less, Cu: 0.05-3.0%,
Ni: 0.05-3.0%, Cr: 13.0-20.0%, Mo: 0.2-4.0%, V: 0.01-1.0%, Al:
0.030% or less, Ti: less than 0.020%, O: 0.020% or less, N:
0.40-0.80%, and the balance of Fe and inevitable impurities.
2. The martensitic stainless steel as claimed in claim 1, further
containing any one or more of steel components which consist of Co:
0.05-4.0%, W: 0.020-0.20%, Ta: 0.020-0.20%, and Nb:
0.010-0.20%.
3. The martensitic stainless steel as claimed in claim 1, further
containing any one or more of steel components which consist of B:
0.001-0.01%, Mg: 0.001-0.01%, Ca: 0.001-0.01%, and Zr:
0.020-0.20%.
4. The martensitic stainless steel as claimed in claim 2, further
containing any one or more of steel components which consist of B:
0.001-0.01%, Mg: 0.001-0.01%, Ca: 0.001-0.01%, and Zr:
0.020-0.20%.
5. The martensitic stainless steel as claimed in claim 1, further
containing any one of or both of steel components which consist of
Te: 0.005-0.05% and Se: 0.02-0.20%.
6. The martensitic stainless steel as claimed in claim 2, further
containing any one of or both of steel components which consist of
Te: 0.005-0.05% and Se: 0.02-0.20%.
7. The martensitic stainless steel as claimed in claim 3, further
containing any one of or both of steel components which consist of
Te: 0.005-0.05% and Se: 0.02-0.20%.
8. The martensitic stainless steel as claimed in claim 4, further
containing any one of or both of steel components which consist of
Te: 0.005-0.05% and Se: 0.02-0.20%.
9. The martensitic stainless steel as claimed in claim 1, having a
value of W.sub.C/W.sub.N of less than 0.30, where W.sub.C (%) is C
content, and W.sub.N (%) is N content.
10. The martensitic stainless steel as claimed in claim 2, having a
value of W.sub.C/W.sub.N of less than 0.30, where W.sub.C (%) is C
content, and W.sub.N (%) is N content.
11. The martensitic stainless steel as claimed in claim 3, having a
value of W.sub.C/W.sub.N of less than 0.30, where W.sub.C (%) is C
content, and W.sub.N (%) is N content.
12. The martensitic stainless steel as claimed in claim 4, having a
value of W.sub.C/W.sub.N of less than 0.30, where W.sub.C (%) is C
content, and W.sub.N (%) is N content.
13. The martensitic stainless steel as claimed in claim 5, having a
value of W.sub.C/W.sub.N of less than 0.30, where W.sub.C (%) is C
content, and W.sub.N (%) is N content.
14. The martensitic stainless steel as claimed in claim 6, having a
value of W.sub.C/W.sub.N of less than 0.30, where W.sub.C (%) is C
content, and W.sub.N (%) is N content.
15. The martensitic stainless steel as claimed in claim 7, having a
value of W.sub.C/W.sub.N of less than 0.30, where W.sub.C (%) is C
content, and W.sub.N (%) is N content.
16. The martensitic stainless steel as claimed in claim 8, having a
value of W.sub.C/W.sub.N of less than 0.30, where W.sub.C (%) is C
content, and W.sub.N (%) is N content.
17. The martensitic stainless steel as claimed in claim 1, having a
mean crystal grain size of the prior austenitic grain in the
tempered martensitic structure of 50 .mu.m or less.
18. The martensitic stainless steel as claimed in claim 2, having a
mean crystal grain size of the prior austenitic grain in the
tempered martensitic structure of 50 .mu.m or less.
19. The martensitic stainless steel as claimed in claim 3, having a
mean crystal grain size of the prior austenitic grain in the
tempered martensitic structure of 50 .mu.m or less.
20. The martensitic stainless steel as claimed in claim 4, having a
mean crystal grain size of the prior austenitic grain in the
tempered martensitic structure of 50 .mu.m or less.
21. The martensitic stainless steel as claimed in claim 5, having a
mean crystal grain size of the prior austenitic grain in the
tempered martensitic structure of 50 .mu.m or less.
22. The martensitic stainless steel as claimed in claim 6, having a
mean crystal grain size of the prior austenitic grain in the
tempered martensitic structure of 50 .mu.m or less.
23. The martensitic stainless steel as claimed in claim 7, having a
mean crystal grain size of the prior austenitic grain in the
tempered martensitic structure of 50 .mu.m or less.
24. The martensitic stainless steel as claimed in claim 8, having a
mean crystal grain size of the prior austenitic grain in the
tempered martensitic structure of 50 .mu.m or less.
25. The martensitic stainless steel as claimed in claim 9, having a
mean crystal grain size of the prior austenitic grain in the
tempered martensitic structure of 50 .mu.m or less.
26. The martensitic stainless steel as claimed in claim 10, having
a mean crystal grain size of the prior austenitic grain in the
tempered martensitic structure of 50 .mu.m or less.
27. The martensitic stainless steel as claimed in claim 11, having
a mean crystal grain size of the prior austenitic grain in the
tempered martensitic structure of 50 .mu.m or less.
28. The martensitic stainless steel as claimed in claim 12, having
a mean crystal grain size of the prior austenitic grain in the
tempered martensitic structure of 50 .mu.m or less.
29. The martensitic stainless steel as claimed in claim 13, having
a mean crystal grain size of the prior austenitic grain in the
tempered martensitic structure of 50 .mu.m or less.
30. The martensitic stainless steel as claimed in claim 14, having
a mean crystal grain size of the prior austenitic grain in the
tempered martensitic structure of 50 .mu.m or less.
31. The martensitic stainless steel as claimed in claim 15, having
a mean crystal grain size of the prior austenitic grain in the
tempered martensitic structure of 50 .mu.m or less.
32. The martensitic stainless steel as claimed in claim 16, having
a mean crystal grain size of the prior austenitic grain in the
tempered martensitic structure of 50 .mu.m or less.
Description
RELATED APPLICATION
[0001] This application claims the priority of Japanese Patent
Application No. 2004-061159 filed on Mar. 4, 2004.
BACKGROUND OF THE INVENTION
[0002] 1. Field of the Invention
[0003] This invention relates to a high-hardness martensitic
stainless steel excellent in corrosion resistance.
[0004] 2. Description of the Related Art
[0005] Martensitic stainless steel such as SUS420J2 and SUS440C
have generally been used in fields in need of certain levels of
corrosion resistance, hardness and wear resistance, including
cylinder liner, shaft, bearing, gear, pin, bolt, screw, roll,
turbine blade, mold, die, valve, valve seat, cutting tool and
nozzle and so on.
[0006] However, the martensitic stainless steel, which contains a
large amount of C in view of ensuring a necessary level of
hardness, is inferior to austenitic stainless steel represented by
SUS304 and SUS316 in corrosion resistance, and cannot be used under
outdoor environments where water drops or aqueous solution may
adhere. This is partially solved by providing surface treatment
such as plating, but a problem arises in that any scratch or
peeling-off of the plated film may allow corrosion to proceed.
[0007] Another problem is that the martensitic stainless steel is
extremely low in the cold workability due to eutectic carbide
produced therein. On the other hand, the austenitic stainless steel
represented by SUS304 and SUS316 are excellent in the corrosion
resistance but far inferior to the martensitic stainless steel in
the hardness, showing only a hardness of as small as HRC40 or
around after cold working.
[0008] The present applicant previously disclosed, in Japanese
Laid-Open Patent Publication "Tokkai" No. 2002-256397, a
martensitic stainless steel equivalent to or superior to SUS420J2
in terms of cold workability and temper hardness, and equivalent to
or superior to SUS316 in terms of corrosion resistance. Our
previous martensitic stainless steel has, however, not paid a
special consideration on the toughness which would be necessary for
use as the mechanical components listed in the above.
[0009] It is therefore an object of the present invention to
provide a martensitic stainless steel which is equivalent to the
conventional martensitic stainless steel in terms of hardness,
excellent in corrosion resistance and cold workability, and also
satisfactory in toughness.
SUMMARY OF THE INVENTION
[0010] Aiming at solving the aforementioned problems, a martensitic
stainless steel of this invention consists essentially of, in % by
mass, C: less than 0.15%, Si: 0.05% or more and less than 0.20%,
Mn: 0.05-2.0%, P: 0.03% or less, S: 0.03% or less, Cu: 0.05-3.0%,
Ni: 0.05-3.0%, Cr: 13.0-20.0%, Mo: 0.2-4.0%, V: 0.01-1.0%, Al:
0.030% or less, Ti: less than 0.020%, O: 0.020% or less, N:
0.40-0.80%, and the balance of Fe and inevitable impurities.
[0011] This invention makes it possible for a martensitic stainless
steel to ensure a necessary level of temper hardness, to improve
corrosion resistance and cold workability, and to ensure a
necessary level of toughness, by reducing the C content, by
increasing the N content, by reducing also the Si, Al and Ti
contents, and by adding V. The following paragraphs will describe
reasons for the compositional limitations.
[0012] C (Carbon): less than 0.15%
[0013] C is an interstitial element, and contributes to improvement
in the strength, and improvement in the temper hardness through
bonding with Cr, Mo, W, V, Nb and Ta, described later. Addition in
an amount of 0.01% or more is preferable in view of obtaining these
effects. On the other hand, any excessive addition lowers amount of
solubility of N, and allows coarse primary carbides to generate,
and this not only degrades the cold workability after annealing,
corrosion resistance and toughness after hardening-and-tempering,
but also increases residual austenite content to thereby result in
degraded temper hardness. The amount of addition is therefore
limited to less than 0.15%, and more preferably 0.14% or less.
[0014] Si (Silicon): 0.05% or more and less than 0.20%
[0015] Si is a deoxidizer element, and is effective for suppressing
Al possibly produces AlN which is causative of an extreme lowering
in the toughness and ductility. Addition in an amount of 0.05% or
more is necessary in view obtaining these effects. Whereas, any
excessive addition not only extremely lowers the toughness and
ductility, but also adversely affects the hot workability, so that
the amount of addition is therefore limited to less than 0.20, and
more preferably 0.18% or less.
[0016] Mn (Manganese): 0.05-2.0%
[0017] Mn is an element effective for increasing amount of
solubility of N, and is also effective as a deoxidizing and
desulfurizing element. Addition in an amount of 0.05% or more, and
more preferably 0.10% or more, is necessary in view of obtaining
these effect. Whereas, any excessive addition not only increases
amount of residual austenite content, and this not only degrades
the temper hardness but also degrades corrosion resistance. The
amount of addition is therefore limited to 2.0% or less, and more
preferably 1.0% or less.
[0018] P (Phosphorus): 0.03% or less
[0019] P is an element possibly lowers the hot workability, grain
boundary strength, toughness and ductility, and is preferably
suppressed to a lower level. The amount of addition is limited to
0.03% or less. It is to be, however, noted that any effort of
excessively lowering in the content will raise the cost.
[0020] S (Sulfur): 0.03% or less
[0021] S is an element possibly degrades the corrosion resistance,
toughness and ductility during cold working, and also degrades the
hot workability, and is preferably suppressed to a lower level. The
amount of addition of S is set to 0.03% or less, and preferably
0.02% or less. It is to be, however, noted that any effort of
excessively lowering in the content will raise the cost.
[0022] Cu (Copper): 0.05-3.0%
[0023] Cu is an element capable of improving not only the toughness
during cold working, but also the corrosion resistance. The
addition in an amount of 0.05% or more, and more preferably 0.08%
or more, is necessary in view of obtaining these effects. Whereas,
any excessive addition increases residual austenite content, and
this not only results in lowered temper hardness but also in
degraded hot workability. The amount of addition is therefore
limited to 3.0% or less, and more preferably 1.0% or less.
[0024] Ni (Nickel): 0.05-3.0%
[0025] Ni is a potent austenite stabilizing element, and is
therefore effective for suppressing nitrogen blow. It also
contributes to improvements in the corrosion resistance and
toughness. Addition in an amount of 0.05% or more, and more
preferably 0.08% or more, is necessary in view of obtaining these
effects. Whereas, any excessive addition increases the hardness
after annealing, to thereby results in degraded cold workability.
It not only extremely lowers the corrosion resistance, toughness
and ductility due to increase in the insolubilized Cr carbonitride
during hardening, but also lowers the temper hardness due to
increase in residual austenite content. The amount of addition is
therefore limited to 3.0% or less, and more preferably 1.0% or
less.
[0026] Cr (Chromium): 13.0%-20.0%
[0027] Cr is an element capable of increasing amount of solubility
of N, and can therefore contribute to increase not only in the
strength, but also in the oxidation resistance and corrosion
resistance. It also contributes to increase in the hardness through
bonding with C and N during tempering to thereby produce fine
carbonitride grains. Addition in an amount of 13.0% or more, and
more preferably 14.0% or more, is necessary in view of obtaining
these effects. Whereas, any excessive addition increases residual
austenite content and thereby lowers the temper hardness. The
amount of addition is therefore limited to 20.0% or less, and more
preferably 19.0% or less.
[0028] Mo (Molybdenum): 0.2-4.0%
[0029] Mo increases amount of solubility of N to thereby improve
the corrosion resistance, and improves the hardness as a solid
solution hardening element. It also contributes to improvement in
the hardness through bonding with C and N during tempering.
Addition in an amount of 0.2% or more, and more preferably 0.4% or
more, is necessary in view of obtaining these effects. Whereas, any
excessive addition will make it difficult to ensure an austenitic
phase effective for suppressing nitrogen blow, and will also result
in degradation of the toughness and ductility due to increase in
insolubilized Cr carbonitride during hardening. The amount of
addition is therefore limited to 4.0% or less, and more preferably
3.5% or less.
[0030] V (Vanadium): 0.01-1.0%
[0031] V contributes to micronization of the crystal grains through
bonding with C and N, and contributes also to improvement in the
toughness as a solute element. Addition in an amount of 0.01% or
more, and more preferably 0.02% or more, is necessary in view of
obtaining these effects. Whereas, any excessive addition allows
large amounts of carbide, oxide and nitride to remain in the steel,
to thereby degrade the toughness. The amount of addition is
therefore limited to 1.0% or less, and more preferably 0.8% or
less.
[0032] Al (Aluminum): 0.030% or less
[0033] Al is an element effective as a deoxidizing element,
similarly to Si and Mn. Addition in an amount of 0.001% or more is
preferable in view of obtaining the effect. This invention is,
however, aimed at increasing amount of solubility of N, and any
excessive addition thereof is undesirable because it will extremely
degrade the toughness and ductility due to production of AlN. The
amount of addition is therefore necessarily limited to 0.030% or
less, and more preferably 0.025% or less in view of ensuring a
desirable level of toughness.
[0034] Ti (Titanium): less than 0.020%
[0035] Ti allows large amounts of oxide and nitride to remain in
the steel, to thereby extremely degrade the corrosion resistance
and toughness. Addition in an amount of less than 0.020%, and more
preferably 0.018 or less, is necessary in view of ensuring a
desirable level of toughness.
[0036] O (Oxygen): 0.020% or less
[0037] O is preferably suppressed to a lower level because it
allows a large amount of oxide to remain in the steel, to thereby
extremely degrade the corrosion resistance and toughness. The
amount addition is therefore limited to 0.020% or less, and more
preferably 0.010% or less.
[0038] N (Nitrogen): 0.40-0.80%
[0039] N is an interstitial element, and one of most important
elements in this invention because it can extremely improve the
hardness and corrosion resistance of the martensitic stainless
steel, and can further improve the hardness during tempering
through formation of fine Cr nitride. Addition in an amount of
0.40% or more, and preferably 0.42% or more, is necessary in view
of obtaining these effects. Whereas, any excessive addition induces
generation of nitrogen blow, and allows insolubilized Cr
carbonitride to remain during hardening. This not only results in
an extreme degradation in the corrosion resistance, toughness and
ductility, but also results in degradation of the hardness after
hardening-and-tempering, due to increased amount of residual
austenite. The amount of addition is therefore limited to 0.80% or
less, and more preferably 0.70% or less.
[0040] Next, the martensitic stainless steel of this invention can
further contain any one or more of steel components which consist
of Co: 0.05-4.0%, W: 0.020-0.20%, Ta: 0.020-0.20%, and Nb:
0.010-0.20%. The following paragraphs will describe reasons for the
compositional limitations.
[0041] Co (Cobalt): 0.05-4.0%
[0042] Co is a potent austenite stabilizing element, and is
therefore effective for suppressing nitrogen blow. It also
contributes to improvements in the corrosion resistance. It is also
effective for ensuring a desirable level of hardness during
hardening, because it can raise the Ms point to thereby reduce
amount of residual austenite. Addition in an amount of 0.05% or
more, and more preferably 0.07% or more, is preferable in view of
obtaining these effects. Whereas, any excessive addition not only
results in increase in the cost, but also in degradation in the
corrosion resistance, toughness and ductility, due to increase in
the insolubilized Cr carbonitride during hardening. It is therefore
preferable to limit the amount of addition to 4.0% or less, and
more preferably 2.0% or less.
[0043] W (Tungsten): 0.020-0.20%
[0044] W contributes to improvement in the hardness as a solid
solution hardening element, or through bonding with C and N during
tempering. Addition in an amount of 0.020% or more, and more
preferably 0.040% or more, is preferable in view of obtaining the
effect. Whereas, any excessive addition may degrade the toughness
and ductility. It is therefore preferable to limit the amount of
addition to 0.20% or less, and more preferably 0.15% or less.
[0045] Ta (Tantalum): 0.020-0.20%
[0046] Ta contributes to micronization of the crystal grain through
bonding with C and N. Addition in an amount of 0.020% or more, and
more preferably 0.040% or more, is preferable in view of obtaining
this effect. Whereas, any excessive addition may allow large
amounts of carbide, oxide, and nitride to remain in the steel,
similarly to Ti, to thereby degrade the toughness. It is therefore
preferable to limit the amount of addition to 0.20% or less, and
more preferably 0.15% or less.
[0047] Nb (Niobium): 0.010-0.20%
[0048] Nb contributes to micronization of the crystal grain through
bonding with C and N. Addition in an amount of 0.010% or more, and
more preferably 0.020% or more, is preferable in view of obtaining
this effect. Whereas, any excessive addition may allow large
amounts of carbide, oxide, and nitride to remain in the steel,
similarly to Ti, to thereby degrade the toughness. It is therefore
preferable to limit the amount of addition to 0.20% or less, and
more preferably 0.10% or less.
[0049] Next, the martensitic stainless steel of this invention can
further contain any one or more of steel components which consist
of B: 0.001-0.01%, Mg: 0.001-0.01%, Ca: 0.001-0.01%, and Zr:
0.020-0.20%. The following paragraphs will describe reasons for the
compositional limitations.
[0050] B (Boron): 0.001-0.01%
[0051] B contributes to improvement in the toughness, and is also
effective for improving the hot workability. Addition in an amount
of 0.001% or more is preferable in view of obtaining this effect.
Whereas, any excessive addition may adversely affect the hot
workability. It is therefore preferable to limit the amount of
addition to 0.01% or less, and more preferably 0.008% or less.
[0052] Mg (Magnesium): 0.001-0.01%
[0053] Mg is effective for improving the hot workability. Addition
in an amount of 0.001% or more is preferable in view of obtaining
this effect. Whereas, any excessive addition may adversely affect
the hot workability. The amount of addition is preferably limited
to 0.01% or less, and more preferably 0.008% or less.
[0054] Ca (Calcium): 0.001-0.01%
[0055] Ca is effective for improving the hot workability, and also
for improving the machinability. Addition in an amount of 0.001% or
more is preferable in view of obtaining these effects. Whereas, any
excessive addition may adversely affect the hot workability. It is
therefore preferable to limit the amount of addition to 0.01% or
less, and more preferably 0.008% or less.
[0056] Zr (Zinc): 0.020-0.20%
[0057] Zr contributes to improvement in the toughness. Addition in
an amount of 0.020% or more, and more preferably 0.030% or more, is
preferable in view of obtaining the effect. Whereas, any excessive
addition may adversely affect the toughness and ductility. It is
therefore preferable to limit the amount of addition to 0.20% or
less, and more preferably 0.15% or less.
[0058] Next, the martensitic stainless steel of this invention can
further contain either of, or both of steel components which
consist of Te: 0.005-0.05% and Se: 0.02-0.20%. The following
paragraphs will describe reasons for the compositional
limitations.
[0059] Te (Tellurium): 0.005-0.05%
[0060] Te contributes to improvement in the machinability. Addition
in an amount of 0.005% or more, and more preferably 0.01% or more,
is preferable in view of obtaining the effect. Whereas, any
excessive addition may adversely affect the toughness and hot
workability. It is therefore preferable to limit the amount of
addition to 0.05% or less, and more preferably 0.04% or less.
[0061] Se (Selenium): 0.02-0.20%
[0062] Se contributes to improvement in the machinability. Addition
in an amount of 0.02% or more, and more preferably 0.05% or more,
is preferable in view of obtaining the effect. Whereas, any
excessive addition may adversely affect the toughness. It is
therefore preferable to limit the amount of addition to 0.20% or
less, and more preferably 0.15% or less.
[0063] Next, the martensitic stainless steel of this invention
preferably has a value of W.sub.C/W.sub.N of less than 0.30, and
more preferably 0.29 or less, where W.sub.C (%) is C content, and
W.sub.N (%) is N content. The ratio of contents of C and N, both
are interstitial elements, largely affects the hardness and
corrosion resistance. A value of W.sub.C/W.sub.N of 0.30 or more
may result in a degraded corrosion resistance, and may also fail in
ensuring a necessary level of hardness.
[0064] Next, the martensitic stainless steel of this invention
preferably has a mean crystal grain size of the prior austenitic
grain in the tempered martensitic structure of 50 .mu.m or less,
and more preferably 40 .mu.m or less. The size of the prior
austenitic grain affects the toughness. A mean crystal grain size
exceeding 50 .mu.m may result in a degraded toughness.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
[0065] The experiments below were conducted in order to confirm the
effects of this invention.
[0066] Alloys having chemical compositions listed in Table 1 were
melted in a pressurizable high-frequency induction furnace,
homogenized under heating, and hot-forged to thereby produce 24-mm
diameter round rods. The rods were annealed by being heated at a
temperature of Ac3+50.degree. C. for 4 hours, cooled at a cooling
rate of 15.degree. C./h down to 650.degree. C., and then allowed to
cool in the air.
[0067] Test samples were collected after these processes, and
subjected to measurements of anneal hardness, and limit
compressibility for crack generation by compression test.
[0068] 1. Measurement of Anneal Hardness
[0069] Hardness of the samples after annealing was measured as
Rockwell B-scale hardness using a Rockwell hardness test specified
by JIS-Z2245.
[0070] 2. Measurement of Limit Compressibility for Crack
Generation
[0071] Limit compressibility for crack generation was measured by a
compression test. Compression test pieces were columns of 15 mm in
diameter and 22.5 mm high, and were compressed using a 600-t
hydraulic press machine. Ten each test pieces were measured under
the individual reduction ratios, and a reduction ratio at which the
number of test pieces causing crack generation was reduced to 5 or
less (50% or less) was defined as limit compressibility for crack
generation.
[0072] Next, the test pieces were hardened by oil quenching after
being kept at 100.degree. to 1100.degree. C. for one hour,
subjected to sub-zero treatment in liquid nitrogen, and tempered by
being kept at 450.degree. C. for one hour and then allowed to cool
in the air.
[0073] Test samples were collected after these processes, and
subjected to measurement of hardening-and-temper hardness, salt
spray test, measurement of pitting corrosion potential, and Charpy
impact test. Mean crystal grain size of the prior austenitic grain
was also measured.
[0074] 3. Measurement of Hardening-and-Temper Hardness
[0075] Hardness of the samples after hardening and tempering was
measured as Rockwell C-scale hardness using a Rockwell hardness
test specified by JIS-Z2245.
[0076] 4. Salt Spray Test
[0077] The test was conducted conforming to a method specified by
JIS-Z2371. After the test, the test pieces were evaluated by a
four-level rating based on ratios of corroded area, where A: not
corroded, B: corroded only in less than 5% area, C: 5-20%, both
ends inclusive, and D: over 20%.
[0078] 5. Measurement of Pitting Corrosion Potential
[0079] Pitting corrosion potential (mV) was measured conforming to
a method specified by JIS-G0577.
[0080] 6. Charpy Impact Test
[0081] Charpy impact test was conducted using 10R notch test pieces
(depth of notch=2 mm, R diameter=10 mm) cut out from the product,
conforming to a method specified by JIS-Z2242, so as to obtain
Charpy impact values.
[0082] 7. Measurement of Mean Crystal Grain Size of Prior
Austenitic Grain
[0083] Ten fields of view of 0.1 mm.sup.2 were randomly observed
under an optical microscope (ca. 400.times. magnification), so as
to measure crystal grain sizes of the prior austenitic grain in the
tempered martensite structure, and thereby a mean value was
determined.
[0084] Similar test was conducted as Comparative Example 1, using
SUS440C, a representative of currently-available material. The
SUS440C (Comparative Example 1) was melted in a high-frequency
induction furnace, homogenized under heating, and hot-forged to
thereby produce a 24-mm diameter round rod. The rods were annealed
by being heated at 850.degree. C. for 4 hours, cooled at a cooling
rate of 15.degree. C./h down to 650.degree. C., and then allowed to
cool in the air. The rods were then hardened by oil quenching after
being kept at 1050.degree. C. for one hour, subjected to sub-zero
treatment in liquid nitrogen, and tempered by being kept at
200.degree. C. for one hour and then allowed to cool in the
air.
[0085] Similar test was also conducted as Comparative Example 13,
using SUS316. The SUS316 (Comparative Example 13) was melted in a
high-frequency induction furnace, homogenized under heating, and
hot-forged to thereby produce a 24-mm diameter round rod. The rod
was then solution-treated by keeping it at 1050.degree. C. for one
hour and by water quenching. Test samples were collected after
these processes, and subjected to the above-described salt spray
test and measurement of pitting potential.
1TABLE 1 C Si Mn P S Cu Ni Cr Mo Co W V Al Inventive Example 1 0.08
0.09 0.09 0.019 0.002 0.11 0.17 17.0 1.02 0.14 0.017 Inventive
Example 2 0.03 0.12 0.16 0.018 0.003 0.15 0.22 16.2 3.02 1.52 0.51
0.022 Inventive Example 3 0.01 0.15 0.49 0.020 0.005 0.50 0.50 17.9
0.99 0.02 0.008 Inventive Example 4 0.14 0.14 0.11 0.021 0.006 0.08
0.13 13.4 2.56 0.10 0.20 0.009 Inventive Example 5 0.05 0.18 0.30
0.019 0.004 0.09 0.14 15.5 1.54 0.98 0.49 0.007 Inventive Example 6
0.10 0.15 0.16 0.022 0.005 0.20 0.19 16.1 1.99 0.21 0.027 Inventive
Example 7 0.01 0.10 1.01 0.022 0.005 0.17 0.22 18.5 1.02 0.15 0.09
0.32 0.023 Inventive Example 8 0.03 0.15 1.51 0.019 0.002 0.11 0.17
14.2 2.01 0.14 0.017 Inventive Example 9 0.04 0.10 0.51 0.019 0.007
0.11 0.16 16.1 1.51 0.12 0.20 0.022 Inventive Example 10 0.03 0.15
0.15 0.023 0.004 2.47 0.08 15.0 1.98 0.05 0.06 0.008 Inventive
Example 11 0.10 0.16 0.13 0.021 0.001 0.12 2.48 14.9 0.49 0.07 0.31
0.020 Inventive Example 12 0.13 0.14 0.15 0.020 0.005 0.15 0.15
14.9 1.99 0.02 0.008 Inventive Example 13 0.08 0.14 0.17 0.021
0.007 0.15 0.15 16.0 1.99 0.78 0.008 Comparative Example 1 1.01
0.20 0.40 0.020 0.041 0.10 0.15 16.3 0.02 Comparative Example 2
0.21 0.15 0.31 0.018 0.002 0.21 0.21 15.0 1.00 0.60 0.03 0.009
Comparative Example 3 0.09 0.24 0.30 0.017 0.017 0.17 0.15 15.0
0.49 0.02 0.008 Comparative Example 4 0.05 0.15 2.51 0.017 0.009
0.16 0.16 11.2 1.01 0.02 0.009 Comparative Example 5 0.03 0.14 0.32
0.028 0.008 0.17 0.17 16.0 5.02 0.008 Comparative Example 6 0.04
0.15 0.30 0.017 0.011 0.18 0.16 16.0 0.44 0.56 0.02 0.059
Comparative Example 7 0.05 0.14 0.12 0.023 0.013 0.16 0.10 18.1
0.52 0.04 0.011 Comparative Example 8 0.02 0.15 0.26 0.021 0.012
0.19 0.20 21.9 0.50 0.04 0.010 Comparative Example 9 0.08 0.15 0.13
0.018 0.009 0.20 0.21 15.0 1.05 0.11 0.04 0.03 Comparative Example
10 0.03 0.14 0.31 0.019 0.010 4.21 0.21 14.1 0.49 0.03 0.010
Comparative Example 11 0.03 0.10 0.10 0.021 0.010 0.12 3.50 15.9
0.52 0.05 0.06 0.02 0.009 Comparative Example 12 0.08 0.13 0.37
0.019 0.017 0.13 0.14 15.0 0.05 1.32 0.009 Comparative Example 13
0.04 0.34 1.12 0.021 0.010 0.21 11.60 18.1 2.11 Ti Nb O N B Mg Ca
Ta Zr Te Se C/N Inventive Example 1 0.010 0.003 0.71 0.11 Inventive
Example 2 0.049 0.004 0.59 0.090 0.05 Inventive Example 3 0.050
0.003 0.50 0.02 Inventive Example 4 0.049 0.003 0.51 0.003 0.27
Inventive Example 5 0.004 0.52 0.003 0.10 Inventive Example 6 0.051
0.004 0.60 0.003 0.003 0.17 Inventive Example 7 0.015 0.005 0.65
0.003 0.02 Inventive Example 8 0.003 0.66 0.02 0.09 0.05 Inventive
Example 9 0.096 0.003 0.48 0.002 0.003 0.08 Inventive Example 10
0.003 0.64 0.11 0.05 Inventive Example 11 0.070 0.004 0.42 0.003
0.002 0.003 0.10 0.11 0.02 0.24 Inventive Example 12 0.050 0.004
0.45 0.003 0.003 0.11 0.28 Inventive Example 13 0.004 0.56 0.14
Comparative Example 1 0.02 50.50 Comparative Example 2 0.003 0.49
0.003 0.003 0.43 Comparative Example 3 0.002 0.60 0.003 0.15
Comparative Example 4 0.005 0.34 0.10 0.15 Comparative Example 5
0.005 0.52 0.003 0.003 0.12 0.06 Comparative Example 6 0.003 0.59
0.11 0.11 0.07 Comparative Example 7 0.051 0.004 0.52 0.03 0.12
0.10 Comparative Example 8 0.004 0.54 0.003 0.11 0.04 Comparative
Example 9 0.031 0.63 0.003 0.12 0.13 Comparative Example 10 0.003
0.52 0.003 0.06 Comparative Example 11 0.003 0.59 0.10 0.03 0.11
0.05 Comparative Example 12 0.002 0.50 0.11 0.10 0.16 Comparative
Example 13 0.02 2.00
[0086] Results of the measurements are shown in Table 2.
2 TABLE 2 Limit compressibility Hardening- Mean Anneal for crack
and-temper Salt Pitting Impact crystal hardness generation hardness
spray potential value grain size (HRB) (%) (HRC) test (mV) (J/cm2)
(.mu.m) Inventive Example 1 92 >80 61 A 0.52 15 22 Inventive
Example 2 90 >80 59 A 0.65 16 24 Inventive Example 3 92 >80
58 A 0.42 22 31 Inventive Example 4 90 >80 60 A 0.45 15 22
Inventive Example 5 91 >80 59 A 0.49 17 24 Inventive Example 6
91 >80 61 A 0.61 15 26 Inventive Example 7 88 >80 59 A 0.52
15 22 Inventive Example 8 87 >80 58 A 0.58 17 38 Inventive
Example 9 90 >80 59 A 0.49 17 21 Inventive Example 10 92 >80
58 A 0.52 16 46 Inventive Example 11 90 >80 58 A 0.41 15 22
Inventive Example 12 91 >80 58 A 0.50 21 23 Inventive Example 13
91 >80 61 A 0.54 16 25 Comparative Example 1 95 45 60 D -0.11 20
24 Comparative Example 2 99 55 57 D -0.09 4 47 Comparative Example
3 85 65 60 C 0.28 2 43 Comparative Example 4 82 >80 51 D 0.06 16
46 Comparative Example 5 90 70 50 A 0.45 3 96 Comparative Example 6
88 65 58 C 0.11 3 32 Comparative Example 7 90 60 56 C 0.17 2 23
Comparative Example 8 89 70 52 A 0.39 9 42 Comparative Example 9 90
65 60 D -0.09 3 45 Comparative Example 10 89 >80 55 A 0.38 15 47
Comparative Example 11 120 >80 51 A 0.35 19 45 Comparative
Example 12 89 70 61 C 0.12 3 24 Comparative Example 13 -- -- -- A
0.41 -- --
[0087] It is found from Table 2 that all of the steels of Inventive
Examples according to this invention are excellent in the corrosion
resistance and cold workability, and are satisfactory in the
toughness, while keeping the hardness equivalent to that of the
conventional martensitic stainless steel. In other words, the
steels of Inventive Examples are far superior to SUS440C
(Comparative Example 1) in the cold workability, equivalent or
superior to SUS 316 (Comparative Example 13), an austenitic
stainless steel, in the corrosion resistance, and equivalent to SUS
440C (Comparative Example 1) in the impact value, while keeping the
temper hardness of HRC58 or above.
[0088] Next, the hardening conditions in Example 3 and Example 6
were altered in three ways so as to vary the mean crystal grain
sizes, and impact values of the individual samples were measured.
Results are shown in Table 3.
3 TABLE 3 Mean crystal grain size Impact value .mu.m J/cm2
Inventive Example 3(a) 24 21 Inventive Example 3(b) 31 22 Inventive
Example 3(c) 98 13 Inventive Example 6(a) 22 17 Inventive Example
6(b) 26 15 Inventive Example 6(c) 92 10
[0089] It is known from Table 3 that examples (a) and (b), having
mean grain sizes of the prior austenitic grain smaller than those
in examples (c) were found to have large impact values and
therefore have excellent toughness.
[0090] It is to be understood that the embodiments described in the
foregoing paragraphs are merely for explanatory purposes, and that
this invention can of course be embodied in any types of
improvements and modifications based on knowledge of those skilled
in the art without departing from the spirit of the invention.
[0091] As is obvious from the above, the martensitic stainless
steel of this invention is suitable for use as components in need
of certain levels of, hardness, wear resistance, corrosion
resistance, cold workability and toughness, including cylinder
liner, shaft, bearing, gear, pin, bolt, screw, roll, turbine blade,
mold, die, valve, valve seat, cutting edge and nozzle.
* * * * *