U.S. patent application number 11/905522 was filed with the patent office on 2008-04-17 for martensitic free cutting stainless steel.
This patent application is currently assigned to DAIDO TOKUSHUKO KABUSHIKI KAISHA. Invention is credited to Koichi ISHIKAWA, Tetsuya SHIMIZU.
Application Number | 20080089804 11/905522 |
Document ID | / |
Family ID | 39145129 |
Filed Date | 2008-04-17 |
United States Patent
Application |
20080089804 |
Kind Code |
A1 |
ISHIKAWA; Koichi ; et
al. |
April 17, 2008 |
Martensitic free cutting stainless steel
Abstract
The present invention relates to a martensitic free cutting
stainless steel, comprising: by weight percent, C: 0.10 to 1.20%,
Si: 0.10 to 2.00%, Mn: 0.80 to 2.00%, S: 0.10 to 0.30%, Cr: 10.5 to
18.0%, Pb: 0.03 to 0.30%, Te: 0.01 to 0.10%, B: 0.0005 to 0.010%,
and O: 0.005 to 0.030%, with the reminder being substantially Fe
and inevitable impurities; wherein, among the inevitable
impurities, contents of P and N are, P: 0.005 to 0.10%, and N:
.ltoreq.0.050%; wherein respective formulae of
3.0.ltoreq.[Mn]/[S].ltoreq.15.0, 0.10.ltoreq.[Te]/[S], and
10.ltoreq.[S]/[O].ltoreq.40 are satisfied; and wherein a sulfide
having a circle equivalent diameter of 2.0 .mu.m or more and an
aspect ratio of 10 or less is present in a total amount of 0.50 to
10% by area ratio.
Inventors: |
ISHIKAWA; Koichi;
(Nagoya-shi, JP) ; SHIMIZU; Tetsuya; (Nagoya-shi,
JP) |
Correspondence
Address: |
BACON & THOMAS, PLLC
625 SLATERS LANE
FOURTH FLOOR
ALEXANDRIA
VA
22314
US
|
Assignee: |
DAIDO TOKUSHUKO KABUSHIKI
KAISHA
Nagoya-shi
JP
|
Family ID: |
39145129 |
Appl. No.: |
11/905522 |
Filed: |
October 2, 2007 |
Current U.S.
Class: |
420/41 |
Current CPC
Class: |
C22C 38/20 20130101;
C22C 38/44 20130101; C22C 38/54 20130101; C22C 38/001 20130101;
C22C 38/22 20130101; C22C 38/38 20130101; C22C 38/42 20130101; C22C
38/60 20130101; C22C 38/002 20130101; C22C 38/02 20130101; C22C
38/04 20130101; C21D 6/002 20130101; C21D 2211/008 20130101; C21D
9/18 20130101; C22C 38/32 20130101; C22C 38/34 20130101; C22C 38/58
20130101 |
Class at
Publication: |
420/041 |
International
Class: |
C22C 38/60 20060101
C22C038/60; C22C 38/00 20060101 C22C038/00 |
Foreign Application Data
Date |
Code |
Application Number |
Oct 3, 2006 |
JP |
2006-271360 |
Jul 3, 2007 |
JP |
2007-174797 |
Claims
1. A martensitic free cutting stainless steel, comprising: by
weight percent, C: 0.10 to 1.20%, Si: 0.10 to 2.00%, Mn: 0.80 to
2.00%, S: 0.10 to 0.30%, Cr: 10.5 to 18.0%, Pb: 0.03 to 0.30%, Te:
0.01 to 0.10%, B: 0.0005 to 0.010%, and O: 0.005 to 0.030%, with
the remainder being substantially Fe and inevitable impurities;
wherein, among the inevitable impurities, contents of P and N are:
P: 0.005 to 0.10%, and N: .ltoreq.0.050%; wherein respective
formulae of 3.0.ltoreq.[Mn]/[S].ltoreq.15.0, 0.10.ltoreq.[Te]/[S],
and 10.ltoreq.[S]/[O].ltoreq.40 are satisfied; and wherein a
sulfide having a circle equivalent diameter of 2.0 mm or more and
an aspect ratio of 10 or less is present in a total amount of 0.50
to 10% by area ratio.
2. The martensitic free cutting stainless steel according to claim
1, which further comprises: by weight percent, at least one element
selected from the group consisting of Cu: 0.01 to 2.0%, Ni: 0.01 to
2.0%, and Mo: 0.01 to 1.0%.
3. The martensitic free cutting stainless steel according to claim
1, which further comprises: by weight percent, Bi: 0.01 to
0.30%.
4. The martensitic free cutting stainless steel according to claim
2, which further comprises: by weight percent, Bi: 0.01 to
0.30%.
5. The martensitic free cutting stainless steel according to claim
1, which further comprises: by weight percent, at least one element
selected from the group consisting of Ca: 0.0001 to 0.05%, Mg:
0.0001 to 0.02%, and REM: 0.0001 to 0.02%.
6. The martensitic free cutting stainless steel according to claim
2, which further comprises: by weight percent, at least one element
selected from the group consisting of Ca: 0.0001 to 0.05%, Mg:
0.0001 to 0.02%, and REM: 0.0001 to 0.02%.
7. The martensitic free cutting stainless steel according to claim
3, which further comprises: by weight percent, at least one element
selected from the group consisting of Ca: 0.0001 to 0.05%, Mg:
0.0001 to 0.02%, and REM: 0.0001 to 0.02%.
8. The martensitic free cutting stainless steel according to claim
4, which further comprises: by weight percent, at least one element
selected from the group consisting of Ca: 0.0001 to 0.05%, Mg:
0.0001 to 0.02%, and REM: 0.0001 to 0.02%.
9. The martensitic free cutting stainless steel according to claim
1, further comprising, by weight percent, at least one element
selected from the group consisting of: W: 0.01 to 2.0%. Nb: 0.01 to
0.50% Ta: 0.01 to 0.50%, and V: 0.01 to 0.5%.
10. The martensitic free cutting stainless steel according to claim
2, further comprising, by weight percent, at least one element
selected from the group consisting of: W: 0.01 to 2.0%, Nb: 0.01 to
0.50% Ta: 0.01 to 0.50%, and V: 0.01 to 0.5%.
11. The martensitic free cutting stainless steel according to claim
3, further comprising, by weight percent, at least one element
selected from the group consisting of: W: 0.01 to 2.0%. Nb: 0.01 to
0.50% Ta: 0.01 to 0.50%, and V: 0.01 to 0.5%.
12. The martensitic free cutting stainless steel according to claim
4, further comprising, by weight percent, at least one element
selected from the group consisting of: W: 0.01 to 2.0%, Nb: 0.01 to
0.50% Ta: 0.01 to 0.50%, and V: 0.01 to 0.5%.
13. The martensitic free cutting stainless steel according to claim
5, further comprising, by weight percent, at least one element
selected from the group consisting of: W: 0.01 to 2.0%. Nb: 0.01 to
0.50% Ta: 0.01 to 0.50%, and V: 0.01 to 0.5%.
14. The martensitic free cutting stainless steel according to claim
6, further comprising, by weight percent, at least one element
selected from the group consisting of: W: 0.01 to 2.0%, Nb: 0.01 to
0.50% Ta: 0.01 to 0.50%, and V: 0.01 to 0.5%.
15. The martensitic free cutting stainless steel according to claim
7, further comprising, by weight percent, at least one element
selected from the group consisting of: W: 0.01 to 2.0%. Nb: 0.01 to
0.50% Ta: 0.01 to 0.50%, and V: 0.01 to 0.5%.
16. The martensitic free cutting stainless steel according to claim
8, further comprising, by weight percent, at least one element
selected from the group consisting of: W: 0.01 to 2.0%, Nb: 0.01 to
0.50% Ta: 0.01 to 0.50%, and V: 0.01 to 0.5%.
17-32. (canceled)
Description
FIELD OF THE INVENTION
[0001] The present invention relates to a martensitic free cutting
stainless steel. More particularly, it relates to a martensitic
free cutting stainless steel which does not contain Se which is one
of free cutting elements.
BACKGROUND OF THE INVENTION
[0002] Conventionally, as the elements for improving the
machinability of a stainless steel, for example, S, Pb, Se, and Te
have been known.
[0003] For example, S generally forms a sulfide type inclusion such
as MnS or MnSe. Accordingly, stress concentrates to the inclusion
upon forming chips, thereby to improve the machinability. Further,
for example, Pb is present in the form of a simple substance in the
steel, and serves as a lubricant between a tool and chips. As a
result, the machinability is improved.
[0004] In recent years, there has been an increase in components
requiring accurate finishing for ensuring the dimensional accuracy,
components in complicated shapes with a large machining allowance,
or the like. For the production of these components, the
machinability is demanded to be improved as much as possible. For
this reason, the content of the elements for improving the
machinability tends to be increased. Further, these elements may be
added not alone, but in the combination thereof.
[0005] For example, JP-A-2002-38241 discloses a free cutting
stainless steel which contains, by mass percent, C: 0.50% or less,
Si: 0.05 to 2.00%, Mn: 0.05 to 1.00%, S: 0.05 to 0.50%, Se: 0.02 to
0.20%, Te: 0.01 to 0.10%, and Cr: 10.00 to 30.00%, in which
component ratios of a Mn/S ratio: 2 or less, a Se/S ratio: 0.2 or
more, and a Te/S ratio: 0.04 or more are satisfied, with the
balance including Fe and inevitable impurities.
[0006] Further, for example, JP-A-8-134602 discloses a martensitic
stainless steel which contains, by weight percent, C: 0.5% or less,
Si: 0.05 to 2.00%, Mn: 0.10 to 3.00%, P: 0.20% or less, Ni: 2.00%
or less, Cr: 12.0 to 25.0%, Mo: 0.10 to 3.00%, S: 0.40 to 0.50%,
Al: 0.10% or less, N: 0.10% or less, 0.60 to 200 ppm, Pb: 0.03 to
0.30%, and Te: 0.02 to 0.15%, with the balance including Fe and
inevitable impurities, and has a Mn/S ratio of 4.5 to 6.5, and a
Te/S ratio of less than 0.07.
[0007] Herein, when the content of S which is the main element for
improving the machinability is excessive, the hot workability or
the cold workability of the stainless steel may be
deteriorated.
[0008] Whereas, the machinability is improved by the stress
concentration to the sulfide. For this reason, the size or form of
the formed sulfide directly affects the machinability. Further,
when the formed sulfide is too large, it operates as the breakage
starting point, resulting in the deterioration of the strength of
the stainless steel. Especially, when the formed sulfide extends
extremely in one direction, the anisotropy occurs in the stainless
steel, resulting in the deterioration of the toughness. Also from
these viewpoints, it is important to control the size and the form
of the sulfide.
[0009] However, for conventional free cutting stainless steels, the
balance therebetween has not been sufficiently achieved. Actually,
there have not been obtained the free cutting stainless steels
excellent in all of the machinability, the hot workability, the
cold workability, and the toughness.
[0010] Incidentally, the free cutting stainless steel of
JP-A-2002-38241 has a Mn/S ratio of as small as 2.0 or less, and
hence it is inferior in hot workability. This tends to cause an
increase in manufacturing cost as well. Whereas, when Se is
essentially added for the purpose of improving the machinability,
the inclusions increase in size. Therefore, the sulfides extend
long in the longitudinal direction of the steel material.
Accordingly, anisotropy occurs in the toughness, the fatigue
strength, or the like, and thus the characteristics are largely
degraded. In addition, unfavorably, selenium ions detrimental to a
human body are generated due to the corrosion of selenides.
[0011] On the other hand, the free cutting stainless steel of
JP-A-8-134602 has a largely increased S content, and whereby it has
been improved in machinability due to the increase in size of
sulfides. However, sulfides are excessively formed, and in
addition, the inclusions increase in size. Therefore, the sulfides
extend long in the longitudinal direction of the steel material.
Accordingly, anisotropy occurs in the toughness, the fatigue
strength, or the like, and thus the characteristics are largely
degraded. Further, it also causes the deterioration of the
corrosion resistance, the hot workability, or the cold
workability.
SUMMARY OF THE INVENTION
[0012] It is an object of the invention to provide a martensitic
free cutting stainless steel excellent in the machinability, the
hot workability, the cold workability, and the toughness.
BRIEF DESCRIPTION OF THE DRAWINGS
[0013] FIG. 1 is a view schematically showing the test piece
collection direction in the Charpy impact test.
DETAILED DESCRIPTION OF THE INVENTION
[0014] Namely, the present invention relates to the following (1)
to (6).
[0015] (1) A martensitic free cutting stainless steel,
comprising:
[0016] by weight percent,
[0017] C: 0.10 to 1.20%,
[0018] Si: 0.10 to 2.00%,
[0019] Mn: 0.80 to 2.00%,
[0020] S: 0.10 to 0.30%,
[0021] Cr: 10.5 to 18.0%,
[0022] Pb: 0.03 to 0.30%,
[0023] Te: 0.01 to 0.10%,
[0024] B: 0.0005 to 0.010%, and
[0025] O: 0.005 to 0.030%, with the reminder being substantially Fe
and inevitable impurities;
[0026] wherein, among the inevitable impurities, contents of P and
N are:
[0027] P: 0.005 to 0.10%, and
[0028] N: .ltoreq.0.050%;
[0029] wherein respective formulae of
[0030] 3.0.ltoreq.[Mn]/[S].ltoreq.15.0,
[0031] 0.10.ltoreq.[Te]/[S], and
[0032] 10.ltoreq.[S]/[O].ltoreq.40 are satisfied; and
[0033] wherein a sulfide having a circle equivalent diameter of 2.0
.mu.m or more and an aspect ratio of 10 or less is present in a
total amount of 0.50 to 10% by area ratio.
[0034] (2) The martensitic free cutting stainless steel according
to (1), which further comprises:
[0035] by weight percent,
[0036] at least one element selected from the group consisting
of
[0037] Cu: 0.01 to 2.0%,
[0038] Ni: 0.01 to 2.0%, and
[0039] Mo: 0.01 to 1.0%.
[0040] (3) The martensitic free cutting stainless steel according
to (1) or (2), which further comprises:
[0041] by weight percent,
[0042] Bi: 0.01 to 0.30%.
[0043] (4) The martensitic free cutting stainless steel according
to any one of (1) to
[0044] (3), which further comprises:
[0045] by weight percent,
[0046] at least one element selected from the group consisting
of
[0047] Ca: 0.0001 to 0.05%,
[0048] Mg: 0.0001 to 0.02%, and
[0049] REM: 0.0001 to 0.02%.
[0050] (5) The martensitic free cutting stainless steel according
to any one of (1) to (4), which further comprises:
[0051] by weight percent,
[0052] W: 0.01 to 2.0%,
[0053] (6) The martensitic free cutting stainless steel according
to any one of (1) to (5), which further comprises:
[0054] by weight percent,
[0055] at least one element selected from the group consisting
of
[0056] Nb: 0.01 to 0.50%,
[0057] Ta: 0.01 to 0.50%, and
[0058] V: 0.01 to 0.50%.
[0059] A martensitic free cutting stainless steel in accordance
with the present invention satisfies the foregoing component
composition, and S, Mn, Te, and O satisfy the foregoing respective
formulae, whereby the good balance is achieved. Accordingly, in the
steel, a sulfide having a specific size and form is present in a
specific area ratio. Therefore, the martensitic free cutting
stainless steel in accordance with the invention is excellent in
machinability, hot workability, cold workability, and toughness in
comparison with the conventional martensitic free cutting stainless
steels.
[0060] In this regard, when at least one element selected from the
group consisting of Cu, Ni, and Mo is/are contained in a specific
ratio, it becomes more likely to improve the corrosion
resistance.
[0061] Further, when Bi is contained in a specific ratio, it
becomes more likely to improve the machinability.
[0062] Still further, when at least one element selected from the
group consisting of Ca, Mg, and REM are contained in a specific
ratio, it becomes more likely to improve the hot workability.
[0063] Moreover, when W is contained in a specific ratio, it
becomes more likely to improve the strength and the corrosion
resistance.
[0064] Additionally, when at least one element selected from the
group consisting of Nb, Ta, and V are contained in a specific
ratio, it becomes more likely to improve the toughness.
[0065] Hereinafter, a martensitic free cutting stainless steel in
accordance with one embodiment of the present invention (which may
be referred to as "this stainless steel") will be described in
details. The stainless steel of the invention contains the
following elements, with the reminder being substantially Fe and
inevitable impurities. The types of the addition elements, the
component ratio, the reason for limitation, and the like are as
follows. Incidentally, the unit of the component ratio is weight
percent. Herein, in the present specification, all the percentages
defined by weight are the same as those defined by mass,
respectively.
(1) C: 0.10 to 1.20%
[0066] C is an element which is solid-solved in the base metal, and
enhances the hardness. Accordingly, in order to obtain a sufficient
hardness, the lower limit of the content of C is set at 0.10% or
more.
[0067] However, when the content of C exceeds 1.20%, a large
quantity of carbides in a simple substance form which are harmful
for the improvement of the machinability are generated, or the
deterioration of the corrosion resistance is caused. The upper
limit of the content of C is preferably 1.00% or less, and more
preferably 0.50% or less.
(2) Si: 0.10 to 2.00%
[0068] Si operates as a deoxidizer of the steel. In order to obtain
the effect, the lower limit of the content of Si is set at 0.10% or
more. The lower limit of the content of Si is preferably 0.2% or
more.
[0069] However, when the content of Si is excessive, the amount of
6-ferrite to be formed increases. Thus, it is observed that the hot
workability tends to be deteriorated. Accordingly, the upper limit
of the content of Si is set at 2.00% or less. When the hot
workability is important, the upper limit of the content of Si is
preferably 1.20% or less, and more preferably 0.50% or less.
(3) Mn: 0.80 to 2.00%
[0070] Mn operates as a deoxidizer of the steel. In addition, Mn
forms a compound effective for the improvement of the machinability
by the coexistence with S. However, when the content of Mn is
small, S is in excess amounts. Accordingly, the hot workability
becomes deteriorated. For this reason, the lower limit of the
content of Mn is set at 0.80% or more.
[0071] On the other hand, among the compounds for improving the
machinability, especially, MnS largely reduces the corrosion
resistance, and inhibits the cold workability, and in addition,
excessively reduces the Ms point. Accordingly, the upper limit of
the content of Mn is set at 2.00% or less. Particularly, when the
machinability and the corrosion resistance are important, the upper
limit of the content of Mn is preferably 1.50% or less, and more
preferably 1.20% or less.
(4) S: 0.10 to 0.30%
[0072] S binds to Mn, Cr, or the like to form a compound effective
for the improvement of the machinability. In order to obtain the
effect, the lower limit of the content of S is set at 0.10% or
more. The lower limit of the content of S is preferably 0.13% or
more, and more preferably 0.15% or more.
[0073] On the other hand, when the content of S is excessive, it is
observed that the hot workability tends to be deteriorated.
Accordingly, the upper limit of the content of S is set at 0.30% or
less. The upper limit of the S content is preferably 0.25% or less,
and more preferably 0.20% or less from the viewpoints of achieving
the excellent balance with the hot workability.
(5) Cr: 10.5 to 18.0%
[0074] Cr is an element for improving the corrosion resistance. In
order to obtain the effect, the lower limit of the content of Cr is
set at 10.5% or more. The lower limit of the content of Cr is
preferably 11.0% or more, and more preferably 12.0% or more.
[0075] On the other hand, when the content of Cr is excessive, the
manufacturing cost increases, and in addition, the hot workability
is also deteriorated. Accordingly, the upper limit of the content
of Cr is set at 18.0% or less. The upper limit of the content of Cr
is preferably 15.0% or less, and more preferably 14.0% or less.
(6) Pb: 0.03 to 0.30%
[0076] Pb is an element effective for the improvement of the
machinability. In order to obtain the effect, the lower limit of
the content of Pb is set at 0.03% or more. The lower limit of the
content of Pb is preferably 0.10% or more, and more preferably
0.13% or more from the viewpoints of ensuring a sufficient amount
for the improvement of the machinability, and the like.
[0077] On the other hand, when the content of Pb is excessive, it
is observed that the hot workability tends to be deteriorated.
Accordingly, the upper limit of the content of Pb is set at 0.30%
or less. The upper limit of the content of Pb is preferably 0.25%
or less, and more preferably 0.23% or less from the viewpoints of
facilitating the improvement of the hot workability, and the
like.
(7) Te: 0.01 to 0.10%
[0078] Te is an element effective for improving the machinability,
and it also suppresses the extension of sulfide due to rolling. In
order to obtain the effect, the lower limit of the content of Te is
set at 0.01% or more.
[0079] On the other hand, when the content of Te is excessive, it
is observed that the hot workability tends to be deteriorated.
Accordingly, the upper limit of the content of Te is preferably
0.10% or less. The upper limit of the content of Te is preferably
0.08% or less, and more preferably 0.05% or less from the
viewpoints of facilitating the improvement of the hot workability,
and the like.
(8) B: 0.0005 to 0.010%
[0080] B is an element effective for improving the hot workability.
In order to obtain the effect, the lower limit of the content of B
is set at 0.0005% or more. The lower limit of the content of B is
preferably 0.0010% or more.
[0081] On the other hand, when the content of B is excessive, the
manufacturing cost increases. Accordingly, the upper limit of the
content of B is set at 0.010% or less. The upper limit of the
content of B is preferably 0.008% or less.
(9) O: 0.005 to 0.030%
[0082] O is an element which is involved in the formation of a
sulfide necessary for the improvement of the machinability.
Accordingly, in order to obtain the effect, the lower limit of the
content of 0 is set at 0.005% or more. The lower limit of the
content of 0 is preferably 0.007% or more.
[0083] On the other hand, when the content of 0 is excessive, an
oxide which is not effective for the improvement of the
machinability becomes more likely to be formed.
[0084] Accordingly, the upper limit of the content of 0 is set at
0.030% or less. The upper limit of the content of 0 is preferably
0.020% or less.
(10) P: 0.005 to 0.10%
[0085] P is segregated in the grain boundary, and enhances the
sensitivity for the grain boundary corrosion. In addition, it
incurs the deterioration of the toughness. Therefore, the content
thereof is preferably low. Accordingly, the upper limit of the
content of P is set at 0.10% or less. The upper limit of the
content of P is preferably 0.050% or less.
[0086] On the other hand, a larger reduction of the content of P
than necessary incurs an increase in manufacturing cost.
Accordingly, in view of the refining technology or the like, the
lower limit of the content of P is set at 0.005% or more.
(11) N: 0.050% or Less
[0087] N forms a nitride which is harmful for the improvement of
the machinability. Therefore, the content thereof is desirably
controlled to the minimum. Accordingly, the upper limit of the
content of N is set at 0.050% or less. The upper limit of the
content of N is preferably 0.030% or less, although it depends on
the even balance with the manufacturing cost.
[0088] The stainless steel of the present invention may further
arbitrarily contain one or two or more elements selected from the
following elements in addition to the foregoing essential elements.
The component ratio, the reason for limitation of each element, and
the like are as follows.
<1> At Least One Element Selected from Cu: 0.01 to 2.0%, Ni:
0.01 to 2.0%, and Mo: 0.01 to 1.0%
[0089] Cu is an element which is effective for the improvement of
the corrosion resistance, especially the corrosion resistance in a
reducing acid environment. In order to obtain the effect, the lower
limit of the content of Cu is set at 0.01% or more. The lower limit
of the content of Cu is preferably 0.05% or more, and more
preferably 0.10% or more.
[0090] On the other hand, when the content of Cu is excessive, it
is observed that the hot workability tends to be deteriorated.
Accordingly, the upper limit of the content of Cu is set at 2.0% or
less. The upper limit of the content of Cu is preferably 1.0% or
less, and more preferably 0.8% or less.
[0091] Ni is an element which is effective for enhancing the
corrosion resistance imparted by Cr. In order to obtain the effect,
the lower limit of the content of Ni is set at 0.01% or more. The
lower limit of the content of Ni is preferably 0.05% or more, and
more preferably 0.10% or more.
[0092] On the other hand, when the content of Ni is excessive, the
manufacturing cost increases. Accordingly, the upper limit of the
content of Ni is set at 2.0% or less. The upper limit of the
content of Ni is preferably 1.0% or less, and more preferably 0.5%
or less.
[0093] Mo is an element which is capable of improving the corrosion
resistance and the strength. In order to obtain the effect, the
lower limit of the content of Mo is set at 0.01% or more. The lower
limit of the content of Mo is preferably 0.05% or more, and more
preferably 0.10% or more.
[0094] On the other hand, when the content of Mo is excessive, the
hot workability is deteriorated, and in addition, the manufacturing
cost increases. Accordingly, the upper limit of the content of Mo
is set at 1.0% or less. The upper limit of the content of Mo is
preferably 0.60% or less, and more preferably 0.50% or less.
<2> Bi: 0.01 to 0.30%
[0095] Bi is an element which is capable of further improving the
machinability. Accordingly, it may be optionally added. In order to
obtain the effect, the lower limit of the content of Bi is set at
0.01% or more. The lower limit of the content of Bi is preferably
0.05% or more.
[0096] On the other hand, when the Bi content is excessive, the hot
workability becomes more likely to be deteriorated. Accordingly,
the upper limit of the content of Bi is set at 0.30% or less. The
upper limit of the content of Bi is preferably 0.20% or less.
<3> At Least One Element Selected from Ca: 0.0001 to 0.05%,
Mg: 0.0001 to 0.02%, and REM: 0.0001 to 0.02%
[0097] Ca, Mg, and REM are elements which are effective for the
improvement of the hot workability. Accordingly, they may be
optionally added. In order to obtain the effect, all of the content
of Ca, the content of Mg, and the content of REM are set at 0.0001%
or more, respectively.
[0098] On the other hand, when the content of Ca, the content of
Mg, and the content of REM are excessive, the effect is saturated.
In addition, it is observed that the hot workability adversely
tends to be deteriorated. Accordingly, the content of Ca is set at
0.05% or less, and preferably 0.01% or less. The content of Mg is
set at 0.02% or less, and preferably 0.01% or less. The content of
REM is set at 0.02% or less.
<4> W: 0.01 to 2.0%
[0099] W has an effect of further improving the corrosion
resistance and the strength, Accordingly, it may be optionally
added. In order to obtain the effect, the content of W is set at
0.01% or more. The lower limit of the content of W is preferably
0.05% or more, and more preferably 0.10% or more.
[0100] On the other hand, when the content of W is excessive, not
only the hot workability is deteriorated, but also an increase in
cost is incurred. Accordingly, the content of W is set at 2.0% or
less. The upper limit of the content of W is preferably set at 1.0%
or less.
<5> At Least One Element Selected from Nb: 0.01 to 0.50%, Ta:
0.01 to 0.50%, and V: 0.01 to 0.50%
[0101] Nb, Ta, and V each form a carbonitride to thereby refine the
grain size, and thus have an effect of enhancing the toughness. In
order to obtain the effect, all of the content of Nb, the content
of Ta, and the content of V are set at 0.01% or more, respectively.
All of the content of Nb, the content of Ta, and the content of V
are preferably 0.05% or more, and more preferably 0.10% or more,
respectively.
[0102] On the other band, when the content of Nb, the content of
Ta, and the content of V are excessive, an increase in cost is
incurred. Accordingly, all of the content of Nb, the content of Ta,
and the content of V are set at 0.50% or less, respectively. All of
the content of Nb, the content of Ta, and the content of V are
preferably 0.40% or less, respectively.
[0103] Herein, the stainless steel of the invention satisfies the
respective formulae of 3.0.ltoreq.[Mn]/[S].ltoreq.15.0,
0.10.ltoreq.[Te]/[S], and 10.ltoreq.[S]/[O].ltoreq.40.
Incidentally, each parenthesis [ ] in the formulae indicates the
weight percent of each element. Hereinafter, the technical meanings
of respective formulae will be described.
3.0.ltoreq.[Mn]/[S].ltoreq.15.0
[0104] When the value of [Mn]/[S] is less than 3.0, the content of
Mn is extremely low, or the content of S is extremely high.
Accordingly, production of the stainless steel of the invention
tends to become difficult. Therefore, the lower limit of the value
of [Mn]/[S] is set at 3.0 or more. The lower limit of the value of
[Mn]/[S] is preferably 4.0 or more.
[0105] On the other hand, when the value of [Mn]/[S] exceeds 15.0,
the content of Mn in the sulfide is so high that it becomes
difficult to ensure the content of Cr in the sulfide. Thus, it is
observed that the corrosion resistance tends to be remarkably
deteriorated. Therefore, the upper limit of the value of [Mn]/[S]
is set at 15.0 or less. The upper limit of the value of [Mn]/[S] is
preferably 10.0 or less.
0.10.ltoreq.[Te]/[S]
[0106] When the value of [Te]/[S] is less than 0.10, the content of
Te is too low, or the content of S is too high. Accordingly, the
sulfide extends, and the spindle shape which is an effective shape
for the machinability is difficult to be formed, thereby resulting
in the deterioration of the machinability. Additionally, anisotropy
occurs in the toughness, the fatigue strength, or the like. Like
this, it is observed that the characteristics tend to be largely
degraded. Therefore, the lower limit of the value of [Te]/[S] is
set at 0.10 or more.
[0107] On the other hand, the upper limit of the value of [Te]/[S]
is not particularly limited. However, it is preferably 0.50 or less
from the viewpoint of the hot workability.
10.ltoreq.[S]/[O].ltoreq.40
[0108] When the value of [S]/[O] is less than 10, a low proportion
of [S] makes it difficult to obtain sufficient machinability.
Whereas, a high proportion of [0] results in an increase in amount
of hard oxides. As a result, it is observed that the machinability
tends to be deteriorated. Therefore, the lower limit of the value
of [S]/[O] is set at 10 or more. The lower limit of the value of
[S]/[O] is preferably 15 or more.
[0109] On the other hand, in the case where the value of [S]/[O]
exceeds 40, when the proportion of [S] is high, it is observed that
the manufacturability of this stainless steel tends to be
deteriorated. When the proportion of [O] is low, it is observed
that a sulfide having a size effective for the machinability tends
to be difficult to obtain. From these viewpoints, the upper limit
of the value of [S]/[O] is preferably 30 or less.
[0110] Further, in the stainless steel of the invention, sulfides
having a circle equivalent diameter of 2.0 .mu.m or more and an
aspect ratio of 10 or less are present in a total amount of 0.50%
or more by area ratio. As a result, an excellent machinability can
be exerted.
[0111] The upper limit of the above-mentioned area ratio is 10.0%
or less. When it exceeds the value, anisotropy occurs in the
toughness, the fatigue strength, or the like. Like this, it is
observed that the characteristics tend to be largely degraded.
[0112] Indeed, when the above-mentioned specific sulfides are
present in an amount equal to or more than the above-mentioned area
ratio in the stainless steel of the invention, it does not matter
if there are sulfides which are not the above-mentioned specific
sulfides such as sulfides having a circle equivalent diameter of
less than 2.0 .mu.m, and sulfides having an aspect ratio of more
than 10.
[0113] The above-mentioned area ratio can be determined in the
following manner. Namely, for the mirror-polished surface of the
stainless steel of the invention, typical microphotographs are
taken at a 200-fold magnification in 50 visual fields. Then, color
extraction of the sulfide (inclusion) is carried out. Thus, the
circle equivalent diameter and the aspect ratio of each sulfide are
measured by image processing. Out of these, the total area ratio of
the sulfides having a circle equivalent diameter of 2 .mu.m or more
and an aspect ratio of 10 or less can be determined.
[0114] Further, the aspect ratio indicates the value of the longer
diameter of the sulfide/shorter diameter of the sulfide.
[0115] Then, a typical method for manufacturing the stainless steel
of the invention will be described.
[0116] By the use of, for example, a high frequency induction
furnace, respective raw materials are dissolved and molten into a
steel ingot, and then, cooled to manufacture an ingot so as to
achieve a stainless steel satisfying the above-mentioned component
composition and respective formulae. Subsequently, the resulting
ingot is subjected to hot forging or hot rolling, followed by a
heat treatment such as annealing is. Consequently, the stainless
steel of the invention can be manufactured.
[0117] In the foregoing manufacturing process, as the heating
temperature during the hot forging or hot rolling, for example, a
temperature range of 950 to 1250.degree. C. can be specifically
exemplified.
[0118] Further, one example of the heat treatment in the
manufacturing process can be shown as follows. Annealing can be
carried out in the following manner. For example, heating is
carried out at 750 to 900.degree. C. for 3 to 5 hours. Then,
furnace cooling is carried out to around 600.degree. C. at a rate
of 10 to 20.degree. C./hour. Thereafter, air cooling is carried
out.
[0119] Moreover, in the manufacturing process, pickling or
polishing for removal of the surface oxide layer may be optionally
carried out, and cold rolling may be optionally carried out.
[0120] The application of the stainless steel of the invention as
described above is not particularly limited. For the applications
of the stainless steel of the invention, specifically, for example,
the stainless steel of the invention can be preferably used for
members required to be subjected to cold cutting processing (such
as finishing processing), and to have a corrosion resistance, a
high strength, and the like, such as a motor shaft, a pump shaft, a
valve component, a screw, a bolt, and a nut.
EXAMPLES
[0121] Next, the invention will be specifically described by way of
Examples.
1. Manufacturing of Stainless Steels in Accordance with Examples
and Comparative Examples
[0122] First, respective stainless steels of component compositions
shown in Tables 1 to 3 were molten into their respective 50-kg
steel ingots by the use of a high frequency induction furnace.
Then, the steel ingots were cooled to manufacture respective
ingots.
[0123] Then, respective ingots were heated to 1000 to 1200.degree.
C., and processed into round bar steels with a diameter of 60 mm
and with a diameter of 20 mm, and square bar steels with a width of
60 mm and a height of 30 mm by hot forging.
[0124] Then, the bar steels were further heated at 860.degree. C.
for another one hour, followed by slow cooling (an annealing
treatment). As a result, stainless steels in accordance with
Examples and Comparative Examples to be subjected to each test were
obtained.
[0125] Incidentally, the stainless steels in accordance with
Comparative Example 1 and Comparative Example 2 are SUS410 and
SUS416 specified according to JIS, respectively. The stainless
steels in accordance with Comparative Example 9 and Comparative
Example 10 are SUS420J2 and SUS420F specified according to JIS,
respectively.
[0126] Further, the stainless steels in accordance Comparative
Examples 1 to 8 are for the comparison with the stainless steels in
accordance with Examples 1 to 15, and 31 to 35. The stainless
steels in accordance Comparative Examples 9 to 16 are for the
comparison with the stainless steels in accordance with Examples 16
to 30, and 36 to 40.
2. Sulfide Characteristics
[0127] Then, for the sulfides present in each stainless steel in
accordance with Examples and Comparative Examples, the circle
equivalent diameter, the aspect ratio, and the area ratio were
measured.
[0128] The measurement method was as follows. Samples with a length
of 10 mm per side were collected from the 20-mm round bar steels,
and each sample was embedded in a resin so that the longitudinal
direction is the measurement surface. Then, for the mirror-polished
surface of each stainless steel, typical microphotographs were
taken at a 200-fold magnification in 50 visual fields by means of
an optical microscope.
[0129] Thereafter, color extraction of each sulfide (inclusion) was
carried out. Thus, the circle equivalent diameter and the aspect
ratio of each sulfide were measured by image processing. Out of
these, the total area ratio of the sulfides having a circle
equivalent diameter of 2 .mu.m or more and an aspect ratio of 10 or
less was determined. The results are shown in Tables 4 to 6.
TABLE-US-00001 TABLE 1 C Si Mn P S Cu Ni Cr Mo Pb Te Ex. 1 0.13
0.25 0.98 0.027 0.18 0.05 0.12 12.3 0.05 0.18 0.03 2 0.14 0.37 1.03
0.022 0.23 0.14 0.09 12.2 0.13 0.23 0.04 3 0.16 0.34 1.11 0.018
0.16 0.11 0.34 12.8 0.08 0.19 0.03 4 0.17 0.58 1.35 0.008 0.19 0.25
0.28 11.9 0.45 0.15 0.05 5 0.11 1.08 0.89 0.056 0.22 0.01 0.59 13.2
0.23 0.08 0.03 6 0.13 0.14 1.57 0.032 0.27 0.10 -- 13.8 0.18 0.27
0.08 7 0.12 1.76 1.23 0.087 0.13 0.67 1.03 12.9 -- 014 0.02 8 0.10
0.43 0.97 0.031 0.18 1.43 0.21 12.1 -- 0.17 0.03 9 0.15 0.35 0.83
0.032 0.10 0.03 0.01 11.8 -- 0.19 0.01 10 0.17 0.41 1.11 0.027 0.25
-- -- 12.5 -- 0.23 0.03 11 0.12 0.58 1.37 0.047 0.11 -- -- 12.3 --
0.20 0.05 12 0.14 0.97 0.99 0.029 0.28 0.78 0.02 11.9 0.81 0.03
0.04 13 0.13 1.22 0.85 0.055 0.26 1.55 0.49 12.4 0.22 0.21 0.03 14
0.15 0.45 1.81 0.025 0.27 -- 0.55 13.6 0.02 0.29 0.06 15 0.16 0.23
1.18 0.033 0.29 -- 0.12 12.2 0.06 0.16 0.04 Comp. 1 0.13 0.35 0.97
0.023 0.003 0.17 0.28 12.7 0.11 -- -- Ex. 2 0.12 0.39 1.03 0.034
0.26 0.23 0.14 12.1 0.08 -- -- 3 0.14 0.67 1.76 0.028 0.27 0.41
0.59 12.2 0.05 0.09 0.02 4 0.11 0.33 0.81 0.048 0.07 0.08 0.29 12.9
0.23 0.05 0.07 5 0.15 0.41 1.56 0.023 0.10 0.29 0.11 12.1 0.07 0.27
0.003 6 0.13 0.62 0.89 0.041 0.13 0.08 0.19 12.9 0.10 0.05 0.02 7
0.16 0.29 2.23 0.029 0.45 0.31 0.27 13.1 0.02 0.26 0.04 8 0.12 0.55
0.53 0.033 0.31 0.22 0.45 12.0 0.46 0.02 0.14 B O N Bi Ca, Mg, REM
W Nb, Ta, V Fe Ex. 1 0.007 0.008 0.028 -- -- -- V: 0.15 Bal. 2
0.004 0.012 0.019 -- -- -- -- Bal. 3 0.006 0.009 0.025 -- -- -- --
Bal. 4 0.005 0.015 0.025 -- -- -- -- Bal. 5 0.009 0.007 0.033 -- --
-- Nb: 035 Bal. 6 0.003 0.008 0.020 -- Mg: 0.0021 0.18 -- Bal. 7
0.010 0.011 0.022 -- -- -- -- Bal. 8 0.005 0.009 0.047 0.12 Ca:
0.0024 -- -- Bal. 9 0.003 0.006 0.021 -- -- -- V: 0.15 Bal. 10
0.007 0.024 0.031 -- -- -- -- Bal. 11 0.006 0.005 0.019 0.19 -- --
Ta: 032 Bal. 12 0.004 0.009 0.028 -- REM: 0.013 -- -- Bal. 13 0.008
0.010 0.022 -- -- -- V: 0.38 Bal. 14 0.003 0.013 0.020 -- Mg:
0.0014 -- Nb: 0.12 Bal. 15 0.009 0.011 0.019 -- -- -- -- Bal. Comp.
1 -- 0.009 0.023 -- -- -- -- Bal. Ex. 2 -- 0.010 0.031 -- -- -- --
Bal. 3 0.005 0.034 0.024 -- -- -- -- Bal. 4 0.009 0.011 0.019 -- --
-- -- Bal. 5 0.004 0.009 0.018 -- -- -- -- Bal. 6 0.008 0.003 0.028
-- -- -- -- Bal. 7 0.005 0.008 0.027 -- -- -- -- Bal. 8 0.006 0.015
0.022 -- -- -- -- Bal. (*) Unit of component ratio: wt % (*)
Comparative Example 1 is SUS410. (*) Comparative Example 2 is
SUS416.
[0130] TABLE-US-00002 TABLE 2 C Si Mn P S Cu Ni Cr Mo Pb Te Ex. 16
0.33 0.32 1.02 0.019 0.16 0.08 0.11 12.7 0.04 0.17 0.04 17 0.35
0.29 1.11 0.028 0.19 -- -- 12.2 -- 0.20 0.05 18 0.37 0.28 1.32
0.032 0.21 0.12 0.37 12.5 0.07 0.18 0.03 19 0.32 0.61 0.98 0.044
0.18 0.26 0.22 12.3 0.13 0.23 0.04 20 0.36 1.02 1.18 0.009 0.24
0.33 0.64 11.8 0.21 0.12 0.03 21 0.33 0.22 0.81 0.077 0.12 -- --
15.2 -- 0.09 0.02 22 0.34 0.55 1.71 0.033 0.29 0.87 1.11 14.1 0.78
0.25 0.07 23 0.30 1.65 0.97 0.029 0.15 1.52 0.32 13.1 0.08 0.16
0.05 24 0.31 0.41 0.83 0.025 0.16 -- -- 12.2 -- 0.05 0.05 25 0.33
0.38 1.08 0.019 0.10 0.03 0.05 12.8 0.01 0.17 0.01 26 0.34 0.52
1.41 0.011 0.25 -- -- 13.4 -- 0.22 0.03 27 0.32 0.89 0.95 0.034
0.19 -- -- 12.8 -- 0.08 0.04 28 0.37 1.15 0.81 0.058 0.13 0.31 0.32
11.4 0.22 0.11 0.05 29 0.35 0.39 1.45 0.031 0.22 0.41 0.22 11.9
0.12 0.28 0.03 30 0.33 0.44 1.11 0.049 0.27 0.21 0.12 12.2 0.06
0.19 0.08 Comp. 9 0.33 0.33 0.55 0.028 0.002 0.12 0.09 12.8 0.05 --
-- Ex. 10 0.35 0.38 1.12 0.035 0.19 0.16 0.15 12.9 0.07 -- -- 11
0.37 0.49 1.54 0.044 0.24 0.09 0.33 11.9 0.13 0.06 0.03 12 0.34
0.38 1.25 0.029 0.05 0.43 0.39 12.8 0.08 0.10 0.08 13 0.36 0.54
0.82 0.024 0.26 0.38 0.41 13.2 0.37 0.24 0.004 14 0.33 0.19 1.41
0.035 0.11 0.07 0.12 12.3 0.21 0.28 0.02 15 0.31 0.22 2.19 0.078
0.41 0.29 0.07 12.2 0.11 0.11 0.03 16 0.33 0.65 0.49 0.033 0.35
0.22 0.11 12.6 0.22 0.01 0.10 B O N Bi Ca, Mg, REM W Nb, Ta, V Fe
Ex. 16 0.006 0.007 0.032 -- -- -- V: 0.43 Bal. 17 0.003 0.009 0.021
-- -- -- -- Bal. 18 0.005 0.010 0.018 -- -- -- -- Bal. 19 0.007
0.008 0.023 -- -- -- -- Bal. 20 0.004 0.007 0.031 0.08 Ca: 0.0008
-- Nb: 0.13 Bal. 21 0.009 0.010 0.022 -- Mg: 0.0011 0.78 -- Bal. 22
0.006 0.021 0.041 -- -- -- -- Bal. 23 0.004 0.006 0.025 -- -- -- --
Bal. 24 0.006 0.008 0.022 -- -- -- V: 0.07 Bal. 25 0.004 0.006
0.019 0.14 -- -- Ta: 0.32 Bal. 26 0.008 0.010 0.025 -- REM: 0.008
-- -- Bal. 27 0.005 0.009 0.020 -- -- -- -- Bal. 28 0.007 0.011
0.032 -- -- -- V: 0.31 Bal. 29 0.003 0.022 0.016 -- -- -- -- Bal.
30 0.009 0.016 0.019 -- Ca: 0.0032 -- Mb: 0.19 Bal. Comp. 9 --
0.007 0.022 -- -- -- -- Bal. Ex. 10 -- 0.011 0.027 -- -- -- -- Bal.
11 0.006 0.041 0.017 -- -- -- -- Bal. 12 0.008 0.010 0.020 -- -- --
-- Bal. 13 0.005 0.008 0.022 -- -- -- -- Bal. 14 0.003 0.002 0.029
-- -- -- -- Bal. 15 0.007 0.007 0.031 -- -- -- -- Bal. 16 0.009
0.011 0.025 -- -- -- -- Bal. (*) Unit of component ratio: wt % (*)
Comparative Example 9 is SUS420J2. (*) Comparative Example 10 is
SUS420F.
[0131] TABLE-US-00003 TABLE 3 C Si Mn P S Cu Ni Cr Mo Pb Ex. 31
0.14 0.34 1.19 0.029 0.17 0.09 0.04 12.3 0.04 0.15 32 0.14 0.29
1.03 0.033 0.19 0.05 0.18 12.8 0.14 0.18 33 0.18 0.41 1.33 0.027
0.22 0.03 0.08 12.1 0.06 0.14 34 0.16 0.24 0.92 0.038 0.16 0.18
0.32 13.3 0.21 0.19 35 0.15 0.37 1.55 0.021 0.20 0.33 0.22 12.4
0.09 0.20 36 0.32 0.27 1.19 0.028 0.17 0.03 0.07 12.3 0.03 0.16 37
0.38 0.43 1.07 0.045 0.23 0.07 0.26 12.7 0.02 0.20 38 0.33 0.38
1.28 0.034 0.20 0.14 0.17 12.1 0.10 0.18 39 0.33 0.29 1.15 0.031
0.16 0.02 0.11 12.1 0.01 0.15 40 0.35 0.48 1.45 0.026 0.18 0.09
0.34 12.2 0.18 0.10 Te B O N Bi Ca, Mg, REM W Nb, Ta, V Fe Ex. 31
0.03 -- 0.009 0.027 -- -- -- -- Bal. 32 0.02 -- 0.011 0.032 -- --
-- -- Bal. 33 0.04 0.001 0.008 0.026 -- -- -- -- Bal. 34 0.03 0.001
0.013 0.029 -- -- -- -- Bal. 35 0.04 0.002 0.007 0.019 -- -- -- --
Bal. 36 0.04 -- 0.008 0.029 -- -- -- -- Bal. 37 0.03 -- 0.014 0.035
-- -- -- -- Bal. 38 0.06 0.001 0.009 0.014 -- -- -- -- Bal. 39 0.03
0.001 0.010 0.026 -- -- -- -- Bal. 40 0.04 0.002 0.013 0.026 -- --
-- -- Bal. (*) Unit of component ratio: wt %
[0132] TABLE-US-00004 TABLE 4 Area ratio of [Mn]/[S] [Te]/[S]
[S]/[O] sulfide (%) Ex. 1 5.44 0.17 22.5 1.77 2 4.48 0.17 19.2 1.33
3 6.94 0.19 17.8 1.03 4 7.11 0.26 12.7 1.44 5 4.05 0.14 31.4 1.56 6
5.81 0.30 33.8 1.90 7 9.46 0.15 11.8 0.98 8 5.39 0.17 20.0 1.32 9
8.30 0.10 16.7 0.64 10 4.44 0.12 10.4 1.57 11 12.45 0.45 22.0 0.89
12 3.54 0.14 31.1 2.31 13 3.27 0.12 26.0 1.63 14 6.70 0.22 20.8
1.43 15 4.07 0.14 26.4 1.59 Comp. 1 -- -- 0.3 -- Ex. 2 -- -- 25.6
1.68 3 6.52 0.07 7.9 1.57 4 11.57 1.00 6.4 0.41 5 15.60 0.03 11.1
0.67 6 6.85 0.15 43.3 1.02 7 4.96 0.09 56.3 2.03 8 1.71 0.45 20.7
1.89 (*1) [X] denotes the ratio (wt %) of element X.
[0133] TABLE-US-00005 TABLE 5 Area ratio of [Mn]/[S] [Te]/[S]
[S]/[O] sulfide (%) Ex. 16 6.38 0.25 22.9 2.31 17 5.84 0.26 21.1
1.38 18 6.29 0.14 21.0 1.78 19 5.44 0.22 22.5 1.47 20 4.92 0.13
34.3 1.55 21 6.75 0.17 12.0 1.04 22 5.90 0.24 13.8 1.83 23 6.47
0.33 25.0 1.22 24 5.19 0.31 20.0 1.09 25 10.80 0.10 16.7 0.67 26
5.64 0.12 25.0 1.65 27 5.00 0.21 21.1 1.32 28 6.23 0.38 11.8 1.02
29 6.59 0.14 10.0 1.25 30 4.11 0.30 16.9 1.83 Comp. 9 -- -- 0.3 --
Ex. 10 -- -- 17.2 1.31 11 6.42 0.13 5.9 1.88 12 25.00 1.60 5.0 0.34
13 3.15 0.02 32.5 1.74 14 12.82 0.18 55.0 0.62 15 5.34 0.07 58.6
2.44 16 1.40 0.29 31.8 2.12 (*1) [X] denotes the ratio (wt %) of
element X.
[0134] TABLE-US-00006 TABLE 6 Area ratio of [Mn]/[S] [Te]/[S]
[S]/[O] sulfide (%) Ex. 31 7.00 0.18 18.9 1.21 32 5.42 0.11 17.3
1.33 33 6.05 0.18 27.5 1.53 34 5.75 0.19 12.3 1.28 35 7.75 0.20
28.6 1.44 36 7.00 0.24 21.3 1.23 37 4.65 0.13 16.4 1.08 38 6.40
0.30 22.2 1.12 39 7.19 0.19 16.0 1.33 40 8.06 0.22 13.8 1.43 (*1)
[X] denotes the ratio (wt %) of element X.
3. Evaluation Test
[0135] Then, by using the resulting respective round bar steels and
square bar steels, relative evaluations were carried out for the
machinability (machinability in turning, machinability in
drilling), the hot workability, the cold workability, and the
toughness (anisotropy). At this step, as the reference data, the
corrosion resistance and the quenching/tempering hardness, which
are the basic characteristics of the stainless steel, were also
measured, respectively.
1) Machinability
[0136] The machinability evaluations were carried out for two of
the machinability in turning and the machinability in drilling.
[0137] The machinability in turning was evaluated in the following
condition. By the use of a 60-mm round bar steel, the tool wear
amount and the chip shape were relatively evaluated.
[0138] Namely, by the use of a carbide tool (UTi20T) as a turning
machining tool, turning was carried out without a lubricant under
the conditions of a cutting speed of 150 mm/min, a depth of cut per
revolution of 1.0 mm, and a feed per revolution of 0.2 mm/rev.
[0139] Herein, the tool wear amount was evaluated by measuring the
wear amount of the average tool flank wear (see, JIS B170 section
"(5) Tool damage No. 5005"). The case where the tool wear amount
was 50 .mu.m or less was rated as "A"; the case of 51 to 100 .mu.m
was rated as "B"; and the case of 101 .mu.m or more was rated as
"C".
[0140] Further, the chip shape was confirmed by visual observation.
The one which was good in chip breakability was rated as "A"; the
one which was broken into coils of approximately several turns was
rated as "B"; and the one which was inferior in breakability and
provided continuous chips was rated as "C". Incidentally,
continuous chips unfavorably make automation difficult.
[0141] On the other hand, the machinability in drilling was
evaluated in the following condition. By the use of a test piece
with a width of 60 mm, a height of 30 mm, and a length of 200 mm,
such a cutting speed as to result in a tool life (undrillable) of
5000 mm and the chip shape were relatively evaluated.
[0142] Namely, by the use of a high speed steel drill SKH51
(diameter of 5 mm), drilling was carried out without a lubricant
under the conditions of a hole depth of 15 mm, and a feed per
revolution of 0.07 mm/rev. Thus, the tool life distance was
measured by changing the cutting speed.
[0143] Herein, the case of a cutting speed of more than 50 m/min, a
high speed, was rated as "A"; the case of a cutting speed of 20 to
50 m/min, an intermediate speed, was rated as "B"; and the case of
a cutting speed of less than 20 m/min, a low speed, was rated as
"C".
[0144] Further, the chip shape was confirmed by visual observation
of the initial chips when drill machining was carried out at a
cutting speed of 20 m/min. The one which was good in chip
breakability was rated as "A"; the one which was broken into coils
of approximately several turns was rated as "B"; and the one which
was inferior in breakability and provided continuous chips was
rated as "C". Incidentally, continuous chips unfavorably make
automation difficult.
2) Hot Workability
[0145] The hot workability was evaluated by the appearance after
hot forging of each ingot into a 20-mm round bar steel and the
high-temperature high-speed tensile test (Gleeble test).
[0146] The appearance after forging was evaluated based on the
degree of occurrence of crack. The one with no crack was rated as
"A"; the one with slight crack of such a degree as to be able to be
cut by a grinder, occurred therein, was rated as "B"; and the one
with large crack occurred therein was rated as "C".
[0147] The high-temperature high-speed tensile test was carried out
in the following condition. Each high-temperature high-speed
tensile test piece with a diameter of 6 mm and a length of 110 mm
was cut out from the steel in the as-casted state, and relatively
evaluated based on the value of reduction of area at 1000.degree.
C.
3) Cold Workability
[0148] The cold workability evaluation was carried out in the
following condition. Namely, for each stainless steel, a
cylindrical test piece with a diameter of 12 mm and a height of 18
mm was prepared from a 20-mm round bar steel was prepared. Then,
for each test piece, a single-step compression test was carried out
by a 600-t hydraulic press to measure the limit compressibility.
Incidentally, a higher limit compressibility indicates excellent
cold workability.
4) Toughness (Anisotropy)
[0149] The toughness evaluation was carried out at room temperature
(24.degree. C.) according to JIS Z2202. A material was collected as
shown in FIG. 1 from a square bar steel with a width of 60 mm and a
height of 30 mm, and roughly machined. Then, the material was
heated to 950.degree. C., and kept for 30 minutes. Then, the
material was oil cooled, and thus subjected to a quenching
treatment. Then, the material was heated to 180.degree. C., and
kept for 1 hour. Then, the air-cooled and tempered test piece was
machined into a JIS No. 4 test piece, so that a Charpy impact test
in the L and T directions was carried out.
5) Corrosion Resistance
[0150] The corrosion resistance test was carried out in accordance
with a high-temperature high-humidity test. Namely, for each
stainless steel, a cylindrical test piece with a diameter of 10 mm
and a height of 50 mm was prepared from a 20-mm round bar steel.
Then, the surface of the test piece was polished with emery papers
of up to No. 400 count, and degreased and washed. Then, these
respective test pieces were held in a constant-temperature
constant-humidity bath at a temperature of 50.degree. C., and a
relative humidity of 98% RH for 96 hours. Then, for each test piece
after holding, whether rust occurred or not was visually observed.
The stainless steels in accordance with Examples 1 to 15, and 31 to
35 were compared with the stainless steel in accordance with
Comparative Example 2. On the other hand, the stainless steels in
accordance with Examples 16 to 30, and 36 to 40 were compared with
the stainless steel in accordance with Comparative Example 10. The
case where rust occurred in a very small amount was judged as "A+";
the case where rust occurred in a comparable amount was judged as
"A"; and the case where rust occurred in a large amount was judged
as "B".
6) Quenching/Tempering Hardness
[0151] For each stainless steel, a cylindrical test piece with a
diameter of 20 mm and a height of 10 mm was prepared from a 20-mm
round bar steel. Then, each sample was heated to 950.degree. C.,
and kept for 30 minutes. Then, it was oil cooled, and thus
subjected to a quenching treatment. Then, each test piece was
heated to 180.degree. C., and kept for 1 hour. Then, it was air
cooled and subjected to a tempering treatment. Thereafter, the
hardness was measured in terms of Rockwell hardness (C scale).
[0152] The summary of the results are shown in Tables 7 to 9.
TABLE-US-00007 TABLE 7 Machinability Machinability Machinability
Hot workability in turning in drilling Forging Reduction Limit
Toughness (J/cm.sup.2) Corrosion Hardness Tool Chip Cutting Chip
outward of area compressibility L T resistance (HRC) abrasion shape
speed shape appearance (%) (%) direction direction Ex. 1 A 41.2 A A
A A A 61 70 39 11 2 A 40.8 A A A A A 62 73 41 13 3 A 39.1 A A A A A
58 71 40 10 4 A 40.3 A A A A A 67 72 38 10 5 A 38.9 A A A A A 61 68
43 11 6 A 41.7 A A A A A 56 69 41 9 7 A 40.6 A A A A A 72 73 45 12
8 A 41.0 A A A A A 63 71 37 11 9 A 40.1 A A A A A 75 71 41 10 10 A
41.1 A A A A A 62 69 38 13 11 A 39.2 A A A A A 71 73 42 12 12 A
40.7 A A A A A 59 72 39 10 13 A 41.1 A A A A A 60 71 40 9 14 A 39.9
A A A A A 57 70 38 13 15 A 40.4 A A A A A 59 71 41 14 Comp. 1 .sup.
A.sup.+ 39.8 C C C C A 86 81 80 68 Ex. 2 -- 40.3 B B B B A 70 62 43
5 3 A 40.8 C B C B A 63 59 39 10 4 A 41.1 B B B B A 73 70 42 11 5 A
39.8 B B B B A 55 58 38 4 6 A 40.1 B B B B A 58 69 41 9 7 A 39.9 A
A A A B 46 40 39 3 8 A 40.5 B A B A B 41 62 40 10
[0153] TABLE-US-00008 TABLE 8 Machinability Machinability
Machinability Hot workability in turning in drilling Forging
Reduction Limit Toughness (J/cm.sup.2) Corrosion Hardness Tool Chip
Cutting Chip outward of area compressibility L T resistance (HRC)
abrasion shape speed shape appearance (%) (%) direction direction
Ex. 16 A 52.2 A A A A A 61 62 21 10 17 A 51.4 A A A A A 55 58 18 12
18 A 52.8 A A A A A 62 63 20 9 19 A 50.9 A A A A A 60 59 17 11 20 A
51.7 A A A A A 59 57 23 13 21 A 52.5 A A A A A 65 61 19 12 22 A
51.0 A A A A A 58 59 21 10 23 A 50.1 A A A A A 61 60 20 11 24 A
51.9 A A A A A 63 61 23 11 25 A 50.4 A A A A A 67 58 22 14 26 A
51.2 A A A A A 56 59 19 12 27 A 50.9 A A A A A 60 60 20 11 28 A
52.7 A A A A A 64 62 23 10 29 A 52.2 A A A A A 59 59 20 13 30 A
50.8 A A A A A 57 60 19 12 Comp. 9 .sup. A.sup.+ 52.1 C C C C A 81
76 41 33 Ex. 10 -- 51.8 B B B B A 63 51 19 3 11 A 52.2 C B C B A 56
48 21 10 12 A 51.0 B B B B A 62 62 23 11 13 A 50.9 B B B B A 54 50
19 4 14 A 52.1 B B B B A 57 59 22 9 15 A 50.4 A A A A B 39 31 20 3
16 A 51.3 B A B A B 35 52 18 10
[0154] TABLE-US-00009 TABLE 9 Machinability Machinability
Machinability Hot workability in turning in drilling Forging
Reduction Limit Toughness (J/cm.sup.2) Corrosion Hardness Tool Chip
Cutting Chip outward of area compressibility L T resistance (HRC)
abrasion shape speed shape appearance (%) (%) direction direction
Ex. 31 A 40.2 A A A A A 63 72 39 11 32 A 39.9 A A A A A 68 69 41 10
33 A 41.8 A A A A A 72 74 39 12 34 A 41.0 A A A A A 69 71 38 9 35 A
40.5 A A A A A 70 73 40 11 36 A 51.0 A A A A A 61 58 21 10 37 A
52.1 A A A A A 58 61 20 11 38 A 50.4 A A A A A 59 60 19 12 39 A
51.7 A A A A A 64 58 22 13 40 A 52.8 A A A A A 61 62 18 9
[0155] The tables indicate the followings. Namely, the stainless
steel in accordance with Comparative Example 1 is SUS410 specified
according to JIS, and contains a very small amount of S which is
one of free cutting elements, and is not a so-called free cutting
stainless steel. Accordingly, it contains almost no sulfides, and
is very excellent in corrosion resistance, and has almost no
anisotropy, and is also excellent in toughness. Further, it is also
excellent in hot workability and cold workability. However, it is
very inferior in machinability.
[0156] On the other hand, the stainless steel in accordance with
Comparative Example 2 is SUS416 specified according to JIS. As
compared with SUS410, it contains S which is one of free cutting
elements, added therein (it also contains no Pb nor Te which is
another free cutting element). Accordingly, the machinability is
improved by the presence of sulfides which are inclusions. However,
this is due to a mere increase in amount of S to be added to
SUS410. Therefore, anisotropy occurs, so that the toughnesses in
the L direction and in the T direction are badly balanced. This is
presumably due to the fact that there occurred a sulfide which
extended in the form of a string in the longitudinal direction.
Further, the cold workability is also deteriorated as compared with
SUS410.
[0157] In addition, the stainless steel in accordance with
Comparative Example 9 is SUS420J2 specified according to JIS. This
is also not a free cutting stainless steel as with the SUS410.
Accordingly, it contains almost no sulfides, and is very excellent
in corrosion resistance, and has almost no anisotropy, and is also
excellent in toughness. Further, it is also excellent in hot
workability and cold workability. However, it is very inferior in
machinability.
[0158] Furthermore, the stainless steel in accordance with
Comparative Example 10 is SUS420F specified according to J1S. As
compared with SUS420J2, it contains S which is one of free cutting
elements, added therein (it contains no Pb nor Te which is another
free cutting element). Accordingly, the machinability is improved
by the presence of sulfides which are inclusions. However, this is
due to a mere increase in amount of S to be added to SUS420J2.
Therefore, anisotropy occurs, so that the toughnesses in the L
direction and in the T direction are badly balanced. This is
presumably due to the fact that there occurred a sulfide which
extended in the form of a string in the longitudinal direction.
Further, the cold workability is also deteriorated as compared with
SUS420J2.
[0159] The stainless steels in accordance with Comparative examples
3 and 11 each contain 0 in a larger proportion than the range
specified in this application. Accordingly, as compared with the
stainless steels in accordance with Examples 1 to 15, and 31 to 35,
and the stainless steels in accordance with Examples 16 to 30, and
36 to 40, the machinability is lower. This is presumably due to the
fact that excessive addition of 0 resulted in the formation of
oxides which are not effective for the improvement of the
machinability.
[0160] As indicated, the stainless steels in accordance with
Comparative Examples 4 and 12 each contain S which is one of free
cutting elements in a smaller proportion than the range specified
in this application. Accordingly, the total area ratio of a
specific sulfide is also lower than the range specified in this
application. As compared with the stainless steels in accordance
with Examples 1 to 15, and 31 to 35, and the stainless steels in
accordance with Examples 16 to 30, and 36 to 40, the effect of
improving the machinability cannot be sufficiently obtained.
[0161] As indicated, the stainless steels in accordance with
Comparative Examples 5 and 13 each contain Te which is one of free
cutting elements in a smaller proportion than the range specified
in this application. Accordingly, as compared with the stainless
steels in accordance with Examples 1 to 15, and 31 to 35, and the
stainless steels in accordance with Examples 16 to 30, and 36 to
40, the effect of improving the machinability cannot be
sufficiently obtained. Further, it is indicated that the cold
workability is deteriorated, and that the toughness is also
deteriorated due to the anisotropy.
[0162] The stainless steels in accordance with Comparative Examples
6 and 14 each contain 0 in a smaller proportion than the range
specified in this application. Accordingly, as compared with the
stainless steels in accordance with Example 1 to 15, and 31 to 35,
and the stainless steels in accordance with Examples 16 to 30, and
36 to 40, the machinability is deteriorated. This is presumably due
to the fact that too small amount of 0 inhibited the sufficient
formation of sulfides with such a size as to improve the
machinability.
[0163] As indicated, the stainless steels in accordance with
Comparative Examples 7 and 15 each contain S in an extremely larger
proportion than the range specified in this application.
Accordingly, the machinability is comparable to that of each
stainless steel in accordance with Examples 1 to 15, and 31 to 35,
and that of each stainless steel in accordance with Examples 16 to
30, and 36 to 40. However, the anisotropy has occurred, resulting
in a deterioration of the toughness. Further, the cold workability
is also extremely deteriorated.
[0164] The stainless steels in accordance with Comparative Examples
8 and 16 each has, especially, an extremely smaller value of
[Mn]/[S] than the range specified in this application. Accordingly,
the machinability is deteriorated, and in addition, the hot
workability is also inferior as compared with each stainless steel
in accordance with Examples 1 to 15, and 31 to 35, and each
stainless steel in accordance with Examples 16 to 30, and 36 to
40.
[0165] In contrast to these, the stainless steels in accordance
with Examples 1 to 40 satisfy the component composition specified
in this invention. In addition, S, Mn, Te, and O each satisfy the
above-mentioned respective formulae. Furthermore, a specific
sulfide is present in a specific area ratio. Accordingly, the
machinability, the hot workability, the cold workability, and the
toughness were excellent.
[0166] Up to this point, the martensitic free cutting stainless
steels in accordance with the invention were described. However,
the invention is not limited to the embodiments and the examples,
and various modifications may be made without departing from the
scope thereof.
[0167] The present application is based on Japanese Patent
Application No. 2006-271360 filed on Oct. 3, 2006 and Japanese
Patent Application No. 2007-174797 filed on Jul. 3, 2007, and the
contents thereof are incorporated herein by reference.
[0168] Furthermore, all the documents cited herein are incorporated
by reference in their entireties.
* * * * *