U.S. patent application number 09/758322 was filed with the patent office on 2001-05-31 for martensitic stainless steel for seamless steel pipe.
Invention is credited to Nakanishi, Tetsuya, Tanida, Mutsumi.
Application Number | 20010001966 09/758322 |
Document ID | / |
Family ID | 15206726 |
Filed Date | 2001-05-31 |
United States Patent
Application |
20010001966 |
Kind Code |
A1 |
Tanida, Mutsumi ; et
al. |
May 31, 2001 |
Martensitic stainless steel for seamless steel pipe
Abstract
A martensitic stainless steel for seamless steel pipes,
excellent in descaling property and machinability, having the
following chemical composition, by weight %; 0.025 to 0.22% of C,
10.5 to 14% of Cr, 0.16 to 1.0% of Si, 0.05 to 1.0% of Mn, 0.05% or
less of Al, 0.100% or less of N, 0.25% or less of V, 0.020% or less
of P, 0.004 to 0.015% of S, and the balance Fe and impurities, This
steel may include 0.0002 to 0.0050% of B, and/or 0.0005 to 0.005%
of Ca. In this case, the upper limit of S may be extended up to
0.018%. Preferably, Al is limited to less than 0.01%.
Inventors: |
Tanida, Mutsumi;
(Wakayama-shi, JP) ; Nakanishi, Tetsuya;
(Wakayama-shi, JP) |
Correspondence
Address: |
Platon N. Mandros
Burns, Doane, Swecker & Mathis, L.L.P.
P.O. Box 1404
Alexandria
VA
22313-1404
US
|
Family ID: |
15206726 |
Appl. No.: |
09/758322 |
Filed: |
January 12, 2001 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
09758322 |
Jan 12, 2001 |
|
|
|
PCT/JP00/03151 |
May 17, 2000 |
|
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Current U.S.
Class: |
148/325 ;
420/34 |
Current CPC
Class: |
C21D 8/105 20130101;
C22C 38/001 20130101; C22C 38/04 20130101; C22C 38/002 20130101;
C22C 38/40 20130101; C22C 38/18 20130101 |
Class at
Publication: |
148/325 ;
420/34 |
International
Class: |
C22C 038/00; C22C
038/18 |
Foreign Application Data
Date |
Code |
Application Number |
May 18, 1999 |
JP |
137782/1999 |
Claims
1. A martensitic stainless steel for seamless steel pipes,
excellent in descaling property and machinability, characterized by
consisting of, by weight %, 0.025 to 0.22% of C, 10.5 to 14% of Cr,
0.16 to 1.0% of Si, 0.05 to 1.0% of Mn, 0.05% or less of Al, 0.100%
or less of N, 0.25% or less of V, 0.020% or less of P, 0.004 to
0.015% of S, and the balance Fe and impurities.
2. A martensitic stainless steel for seamless steel pipes,
excellent in descaling property and machinability, characterized by
consisting of, by weight %, 0.025 to 0.22% of C, 10.5 to 14% of Cr,
0.16 to 1.0% of Si, 0.05 to 1.0% of Mn, 0.0002 to 0.0050% of B,
0.05% or less of Al, 0.100% or less of N, 0.25% or less of V,
0.020% or less of P, and 0.004 to 0.018% of S, with the remainder
being Fe and impurities, where % is weight %.
3. A martensitic stainless steel for seamless steel pipes,
excellent in descaling property and machinability, according to
claim 1 or 2, wherein Al as an impurity is suppressed to less than
0.01%.
4. A martensitic stainless steel for seamless steel pipes,
excellent in descaling property and machinability, according to
claim 1 or 2, wherein Al as an impurity is suppressed to 0.005% or
less.
5. A martensitic stainless steel for seamless steel pipes,
excellent in descaling property and machinability, characterized by
consisting of, by weight %, 0.025 to 0.22% of C, 10.5 to 14% of Cr,
0.16 to 1.0% of Si, 0.05 to 1.0% of Mn, 0.05% or less of Al, 0.0005
to 0.005% of Ca, 0.100% or less of N, 0.25% or less of V, 0.020% or
less of P, 0.004 to 0.018% of S, and the balance Fe and
impurities.
6. A martensitic stainless steel for seamless steel pipes,
excellent in descaling property and machinability, characterized by
consisting of, by weight %, 0.025 to 0.22% of C, 10.5 to 14% of Cr,
0.16 to 1.0% of Si, 0.05 to 1.0% of Mn, 0.0002 to 0.0050% of B,
0.05% or less of Al, 0.0005 to 0.005% of Ca, 0.100% or less of N,
0.25% or less of V, 0.020% or less of P, 0.004 to 0.018% of S, and
the balance Fe and impurities.
7. A martensitic stainless steel for seamless steel pipes,
excellent in descaling property and machinability, according to
claim 5 or 6, wherein Al as an impurity is suppressed to less than
0.01%.
8. A martensitic stainless steel for seamless steel pipes,
excellent in descaling property and machinability, according to
claim 5 or 6, wherein Al as an impurity is suppressed to 0.005% or
less.
9. A seamless steel pipe excellent in descaling property and
machinability that is produced by inclined rolling method from the
steel according to either one of claims 1 to 8.
Description
TECHNICAL FIELD
[0001] The present invention relates to a steel used for making a
material of seamless steel pipes, such as oil well pipes or line
pipes, and particularly to a martensitic stainless steel
characterized by having excellent descaling property and
machinability.
BACKGROUND ART
[0002] Martensitic stainless steels defined as SUS 410, SUS420 and
others in JIS (Japanese Industrial Standards) have high strength
and excellent corrosion resistance even in a corrosive environment
containing CO.sub.2, and thereby have been used as materials for
seamless steel pipes, such as oil well pipes, line pipes,
geothermal well pipes and others.
[0003] The seamless steel pipe is generally produced by means of an
inclined rolling method, such as Mannesmann plug mill process and
Mannesmann mandrel mill process, a hot extrusion method such as
Ugine-Sejournet process, or a hot press method such as Erhart
pushbench process. It has been known that reducing S (sulfur) in
addition to keeping down Cr equivalent [Cr+4Si-(22 C+0.5 Mn+1.5
Ni+30 N)] of a steel was desirable to prevent surface defects, such
as cracks and scabs (or laps), which are likely to result from
these hot workings.
[0004] In oil well pipes and the like, it is often the case that
each of the pipes is provided with connecting screws at both ends.
The martensitic stainless steel originally has a large cutting
resistance, and the steel, having the reduced S content as
described above, is likely to experience a seizure between a
cutting tool and a cutting work in the same manner as austenitic
stainless steels. This results in a shortened life of the cutting
tool and greatly reduces the efficiency of production.
[0005] Publication of the unexamined patent application
Sho-52-127423 discloses a martensitic stainless steel excellent in
machinability, including 0.003 to 0.40% of rare earth element.
However, according to test result of the present inventors, the
rare earth element has no effect to improve machinability and
besides that it increases inclusions in the steel, particularly
deteriorating the quality of threaded portion. In this steel, S
(sulfur) is limited to 0.03% or less on the grounds that it impairs
corrosion resistance and hot-workability. In addition, the
hot-workability is merely evaluated based entirely on the condition
of scabs created during rolling the steel into a plate, and it is
not clear whether the hot-workability for forming a seamless steel
pipe is sufficient or not.
[0006] Publication of the unexamined patent application Hei-5-43988
discloses a martensitic stainless steel including 13.0 to 17.0% of
Cr, and optionally less than about 0.5% of S (preferably 0.1 to 0.5
to improve machinability). However, this steel includes about 1.5
to 4.0% of Cu. Since Cu is a component, which significantly
deteriorates the hot-workability of steel, such a steel, including
a large quantity of Cu, is not a suitable material for producing
the seamless steel pipe by the inclined rolling method or the
like.
[0007] Publication of the unexamined patent application
Hei-9-143629 discloses an invention of a material pipe for steel
pipe joint couplings, in which 0.005 to 0.050% of S is included as
well as 5.0 to 20.0% of Cr so as to arrange "Mn/S" in 35 to 110. In
this invention, the hot forging process is applied to produce the
above material pipe for couplings, on the basis of the recognition
that a Cr steel seamless pipe of high S conteny cannot be produced
by the inclined rolling method such as the Mannesmann processes,
due to its inferior hot-workability, That is, the material pipe
disclosed in the publication is a short size pipe, which is
produced by a hot forging process. In addition, while Al content is
defined to 0.010 to 0.035% in the claim of the publication, actual
Al content is unclear because there is no description of the Al
content of the steel as examples. Since Al creates oxide compounds
including Al.sub.2O.sub.3, which is hard and has a high melting
point, it accelerates wear on cutting tools, it is generally
required to limit the Al content or to control the oxide
composition by other components, such as Ca. However, these are not
considered in the invention of this publication.
[0008] With respect to oil well pipes of 13Cr stainless steel
(martensitic stainless steel), the API (American Petroleum
Institute) Standards require "no scale on an inner surface of the
pipe". In the 13Cr stainless steel, it is difficult to remove scale
uniformly. Particularly a low sulfur martensitic stainless steel
has a significantly low descaling property due to the high adhesion
between the scale and the surface of the steel, and the scale is
thereby apt to remain on the surface.
DISCLOSURE OF INVENTION
[0009] The present invention has been addressed for the purpose of
the improving machinability and descaling property of martensitic
stainless steel while keeping up its inherent mechanical property
and corrosion resistance.
[0010] The present inventors have significantly improved the
machinability and the descaling property while maintaining its
fundamental characteristics by most suitably selecting alloying
elements and content thereof composing the martensitic stainless
steel.
[0011] As described above, heretofore, in the martensitic stainless
steel, the S content has been limited as low as possible in order
to improve its hot-workability. However, according to the result of
inventors' detailed studies, an optimal content of S can yield not
only enhanced machinability but also improved the descaling
property of the steel. On the other hand, the deterioration of
hot-workability and associated difficulty in the production of
seamless steel pipes (problem of cracks and scabs occurring during
piercing) can be settled by improving pipe-producing techniques.
For example, piecing with low reduction in roll gorge, or piecing
by cone-type rolls piercing mill, which was developed by the
present applicant, makes it possible to produce, by the inclined
rolling method, a high quality seamless steel pipe equal to the
conventional seamless steel pipes of low S steel. Further,
improvement of material quality, i.e., improvement of
hot-workability, can also be achieved by adding B (boron).
[0012] Suppressing Al content or adding a suitable amount of Ca can
further enhances the effect of improving the machinability by
adding a suitable amount of S.
[0013] A subject matter of the present invention, based on the
above knowledge, is defined as the following martensitic stainless
steel. Hereinafter, % in each component's content stands for weight
%.
[0014] (1) A martensitic stainless steel for seamless steel pipes,
excellent in descaling property and machinability, the martensitic
stainless steel consisting of 0.025 to 0.22% of C, 10.5 to 14% of
Cr, 0.16 to 1.0% of Si, 0.05 to 1.0% of Mn, 0.05% or less of Al,
0.100% or less of N, 0.25% or less of V, 0.020% or less of P, and
0.004 to 0.015% of S, and the balance being Fe and impurities.
[0015] (2) A martensitic stainless steel for seamless steel pipes,
excellent in descaling property and machinability, the martensitic
stainless steel consisting of 0.025 to 0.22% of C, 10.5 to 14% of
Cr, 0.16 to 1.0% of Si, 0.05 to 1.0% of Mn, 0.0002 to 0.0050% of B,
0.05% or less of Al, 0.100% or less of N, 0.25% or less of V,
0.020% or less of P, and 0.004 to 0.018% of S, and the balance
being Fe and impurities.
[0016] (3) A martensitic stainless steel for seamless steel pipes,
excellent in descaling property and machinability, in which 0.0005
to 0.0050% of Ca is further included in the above steel (1) or
(2)
[0017] When Ca is included, the S content of the above steel (1)
can also be 0.004 to 0.018%.
[0018] As described above, since Al creates Al.sub.2O.sub.3 and
thereby deteriorates machinability, the Al content in the steels
(1) to (3) is preferably 0.01% or less, and more preferably 0.005%
or less. In the steels (1) to (3), up to 0.6% of Ni may also be
included as an impurity. However, as described later, since Ni
adversely affects sulfide cracking resistance of the steel and
deteriorates descaling property, the Ni content should be
preferably 0.2% or less and more preferably 0.1% or less.
[0019] "Martensitic stainless steel", herein, means a steel the
major structure of which is martensite, and small amounts (up to
about 5% by area rate) of other structure, such as ferrite,
bainite, pearlite, may be mixed therein.
BRIEF DESCRIPTION OF DRAWINGS
[0020] FIGS. 1 and 2 are tables showing the respective chemical
compositions of steel used in a test.
[0021] FIGS. 3 and 4 are tables showing the results of various
tests.
BEST MODE FOR CARRYING OUT THE INVENTION
[0022] The martensitic stainless steel of the present invention has
overall excellent characteristics as seamless steel pipes by the
synergism of the respective components described above. Each effect
of the components is as follows.
[0023] C (Carbon) may enhance strength of steel. In order to obtain
such an effect, the C content is required to be 0.025% or more. On
the other hand, more than 0.22% of C deteriorates corrosion
resistance of steel and allows cracks to occur during
quenching.
[0024] Cr (Chromium) is a primary component of steel for enhancing
corrosion resistance. Particularly Cr of 10.5% or more improves
resistance to pitting corrosion and crevice corrosion, and it
further significantly enhances corrosion resistance under an
environment containing CO.sub.2. On the other hand, more than 14%
of the content allows .delta.-ferrite to be created during workings
under high temperature because chromium is an element to form
ferrite, resulting in deteriorated hot-workability. In addition, an
excessive amount of chromium causes an increased ferrite in the
steel, and thereby deteriorates the strength of the steel after the
heat treatment (tempering treatment described hereinafter) which
assures stress-corrosion cracking resistance. Based on these
reasons, the chromium content was defined in the range of 10.5 to
14%.
[0025] Si (Silicon) is an element required as a deoxidizer in order
to remove oxygen which deteriorates the hot-workability. If the
content is less than 0.16%, the deoxidizing effect is insufficient,
and no improvement in hot-workability is obtained. On the other
hand, excessive amount of Si causes a deteriorated toughness of the
steel. Thus, the upper limit of Si content is defined in 1.0%.
[0026] Mn (Manganese) is also an element required as a deoxidizer
in steel making and contributes to enhance the strength of the
steel. Mn also stabilizes sulfur in the steel as MnS and improves
the hot-workability. In less than 0.05% of the manganese contents,
the deoxidizing effect is insufficient, resulting in a poor effect
of improvement in the hot-workability. However, since excessive
manganese content causes a deteriorated toughness of the steel, the
upper limit should be defined in 1.0%. Regarding the importance of
toughness, the Mn content is preferably selected as low as
possible, for example 0.30% or less in the range of 0.05% or
more.
[0027] Al (aluminum) is effective as a deoxidizer of steel. Thus,
in case of necessity, Al is added to the steel of the present
invention. However, since aluminum creates oxide compounds mostly
comprised of hard and high melting point Al.sub.2O.sub.3, which
accelerate wear on cutting tools, as described above, its content
is preferably as little as possible. In addition, an excessive
amount of aluminum deteriorates cleanliness of steel and a choking
of an immersed nozzle during continuous casting.
[0028] For the above reasons, when aluminum is added, its content
must be limited to 0.05% or less. It is recommendable that aluminum
is not positively added and its content nor is in the range of less
than 0.01% or, more preferably, not more than 0.005%. In case of a
steel containing Calcium, the aluminum content may be selected in a
relative high range of 0.05% or less because calcium oxide forms
low melting point oxide compounds in cooperation with the oxides of
aluminum, silicon, manganese, and others, and thereby offsets the
adverse effect of aluminum.
[0029] N (nitrogen) may be included up to 0.100% because it reduces
the chromium equivalent and thereby improves hot-workability.
However, more than 0.100% of N deteriorates the toughness of steel.
Although N may not be positively added, its content is preferably
selected in the range of 0.020 to 0.100% when its effect of
strengthening and improving the hot-workability of the steel is
expected.
[0030] Generally, in martensitic stainless steels, S (sulfur) has
heretofore been considered as an impurity, which deteriorates
hot-workability and should be limited as low as possible. In
contrast, this sulfur is positively utilized in the present
invention. However, when the after-mentioned B and/or Ca are not
added, more than 0.015% of the sulfur causes a significant
deterioration in hot-workability. Therefore, it will be difficult
to prevent the occurrence of scabs during piercing by an inclined
rolling mill in the producing process of seamless pipes, even if
the producing conditions are improved.
[0031] Sulfur concentrates in the boundary surface between the
scale and the substrate after the steel is processed into a pipe so
that the removing property of the scale on the outer and the inner
surfaces (descaling property) is significantly improved. Thus, the
S content is defined in the range of 0.004 to 0.015%. When one or
both of B and Ca are added, the upper limit of S is extended up to
0.018%.
[0032] P (phosphor) is an impurity of steel, and its high content
deteriorates the toughness of steel pipe products. The allowable
upper limit is 0.020% to secure toughness and it is preferable to
be as little as possible, in the range of not more than 0.020%, and
specifically not more than 0.018%.
[0033] B (boron) is effective for preventing hot-workability from
being deteriorating due to the grain boundary segregation of sulfur
in steel. It also has effects for making crystal gains fine to
enhance toughness and lowering the melting point of oxide
compounds. Thus, boron may be added if necessary. When B is added,
its content is preferably selected in the range of 0.0002% or more
to assure the above effects. However, more than 0.0050% of boron
causes precipitation of carbide on grain boundaries and likely
deteriorates corrosion resistance of the steel. Thus, the upper
limit is defined in 0.0050%.
[0034] Calcium combines with sulfur and O (oxygen) to create
sulfide (CaS) and oxide (CaO), and then these transform the hard
and high melting point oxide compounds
(Al.sub.2O.sub.3--MnO--SiO.sub.2 oxide) into a low melting point
and soft oxide compounds which improves the machinability of steel.
These effects are exhibited when the calcium content is in the
range of 0.0005% or more, however, excessive calcium content
reduces the sulfur, which concentrates in the boundary surface
between the scale and the substrate, resulting in a deteriorated
scale removing property (descaling property). The excessive calcium
also causes inclusions on steel product after hot working. Summing
up these effects of calcium, when calcium is added, its content
should be defined in the range of 0.0005 to 0.005%. Calcium
addition is not always necessary as the same as the aforementioned
boron.
[0035] V (vanadium) contributes to enhance the strength of steel
through its precipitation hardening effect. It also serves for
improving machinability by lowering the melting point of the oxide
compounds. Thus, vanadium may be added at needed. However, when V
is added, the vanadium content should be limited up to 0.25%
because excessive vanadium deteriorates the toughness of the steel.
The vanadium content should preferably be selected in the range of
0.12 to 0.18% when a product having high strength is required.
[0036] Ni (nickel) is an element being mixed in steel to a certain
extent from scraps and others during steel making. In the present
invention, Ni may also be included as an inevitable impurity in the
range of 0.6% or less as defined in JIS. However, nickel increases
adhesion of scale, and deteriorates descaling property. This
adverse effect becomes significant when the nickel content is more
than 0.2%, thus, the nickel content is preferably suppressed to
0.2% or less. Further, the nickel content is more preferably
suppressed to 0.10% or less because a sulfide stress-corrosion
cracking is likely to occur in the steel containing nickel, when it
is used in an environment containing sulfide.
[0037] O (oxygen) is included in steel as an inevitable impurity.
Oxygen is combined with chromium, aluminum, silicone, manganese,
sulfur, and others to form oxides. While these oxides affect
machinability and mechanical property, the steel of the present
invention does not have that problem, even if the oxygen content is
in the range (about 10 to 200 ppm) as much as that normally
achieved by the conventional refining process for stainless
steel.
[0038] As described above, when one or more of B and Ca are added,
the upper limit of S can be extended up to 0.018%. That is,
increased sulfur further improves machinability and descaling
property of the steel while keeping up sufficient hot-workability.
While the stainless steel of the present invention may mix some
other structure as described above, this stainless steel is
substantially composed of martensite structure. This structure and
a predetermined mechanical property can be achieved by subjecting,
for example, to the following heat treatment after the steel has
been processed to a product (seamless steel pipe).
[0039] Quenching: heating at 920 to 1050.degree. C. for about 20
minutes, and then air-quenching (air-cooling or forced
air-cooling),
[0040] Tempering: heating at 625 to 750.degree. C. for about 30
minutes, and then air-cooling.
EXAMPLE
[0041] Three billets (outer diameter: 191 mm) of each steel, having
the chemical composition shown in FIG. 1 and FIG. 2, were prepared.
These billets were heated at 1230.degree. C. and then
piercing-rolled with 6.5% of the relative reduction in front of the
plug nose by an inclined roll piercer having a 10.degree. cross
angle. Each obtained hollow shell was extracting-rolled by a
mandrel mill, heated again, and fixed-size-rolled by a stretch
reducer, to produce seamless steel pipes, having 73.0 mm of outer
diameter, 5.51 mm of wall thickness, and 9700 mm of length. Five
steel pipes were produced from each billet. Thus, fifteen sample
steel pipes were obtained from each steel having respective ones of
compositions shown in FIGS. 1 and 2.
[0042] The above pipes were subjected to quenching at "980.degree.
C..times.20 minutes--air-cooling", and to tempering under the
following condition.
[0043] 80 ksi grade pipes (YS: 600 to 620 MPa, TS: 745 to 780
MPa)
[0044] 720.degree. C..times.30 minutes--air-cooling
[0045] 95 ksi grade pipes (YS: 680 to 700 MPa, TS: 830 to 850
MPa)
[0046] 700.degree. C..times.30 minutes--air-cooling
[0047] The structure, after the heat treatment of all sample steel
pipes, was substantially tempered martensite. The following tests
(or inspections) were performed on each obtained pipe. The test
results are shown FIG. 3 and FIG. 4.
[0048] (1) Inspection of the status of defect (scabs) on inner and
outer surfaces.
[0049] The defects were visually checked. The cases in which pipes
necessary for some repairs in order to remove scabs were 8 or more
(among the fifteen pipes), or pipes that can not be used as
commercial products, after the repairs, were 2 or more, are
indicated by a mark X, and other cases are indicated by a mark
.largecircle..
[0050] (2) Descaling test:
[0051] The inner and the outer surface of each pipe was descaled to
Sa21/2level of the ISO standard by suction shot blasting using
fused alumina particles (#16). The descaling property was evaluated
based on "descaling efficiency" determined by calculating the
number of pipes which could be processed per hour, in accordance
with the time which passed over the above descaling operation for
one pipe.
[0052] (3) Machinability test
[0053] A cutting test was performed by a process comprising
providing Buttress type threads of the API standards in each end of
the pipes after descaling, cutting off the threaded portion for
each threading, and repeatedly providing threads in each end of the
pipes. A chaser coated by CVD method was used as the cutting tool.
"Cutting efficiency" was determined by calculating the number of
pipes, which could be cut per hour, in accordance with the time
needed for the above one threading operation. The number of
threading, which was performed by one tool, was determined "Tool
life".
[0054] (4) Charpy impact test
[0055] A test piece of 10 mm.times.3.3 mm.times.55 mm which had 2
mm V notch was used. The test piece was cut out in the longitudinal
direction of a pipe, which was selected from each set of pipes of
the same chemical composition. The impact test was performed at
0.degree. C. of test temperature, and "absorbed energy" and
"ductile--brittle transition temperature (vTrs)" was
determined.
[0056] The steel A shown FIG. 1 is a conventional martensitic
stainless steel corresponding to SUS 420J2. The steels A1 to A3 are
steels made for comparison, all of which include S exceeding the
range of the present invention.
[0057] Referred to the test result in FIG. 3, the conventional
steel A had no flaw because it had low S content of 0.001%.
However, it had a significantly inferior machinability and low
descaling property.
[0058] On the other hand, while the comparative materials A1 to A3
were improved in machinability and descaling property, all of the
pipes included surface defects, which occurred during the pipe
production process, and thereby needed repairs. This was due to the
occurrence of scabs, which was due to their excessive content of S,
and could not be avoided despite applying the piecing conditions as
described above.
[0059] In the steels belonging to B group to F group, all of steels
corresponding to the present invention have the machinability and
descaling property superior to the comparative steels in each
group, and had no defects during the pipe production process. This
means that the steels of this invention also have excellent
hot-workability. Particularly, the steels including boron have no
surface defects, even if they have relatively high sulfur content,
and exhibit excellent machinability. In the steels in which the
nickel content was suppressed to 0.2% or less, descaling property
is further improved as compared to the steels including relatively
high nickel content.
[0060] As is apparent from FIG. 3, the steels of this invention,
the sulfur contents of which were arranged in a suitable range,
were on almost the same level of mechanical characteristics with
the conventional steels and the comparative steels in each
group.
[0061] The steels in FIG. 2 have relatively high aluminum content,
and steels of I group, J group and K group include calcium. The
test results of these sample members are shown in FIG. 4. It is
apparent from FIG. 4 that the steels of the G and H groups were
slightly inferior in machinability to the steels having lower
aluminum content described above. However, the steels of the I to K
groups including calcium had excellent machinability regardless of
the high aluminum content.
[0062] The steels in the group F in FIG. 1 and group K in FIG. 2
are high strength steels (95 ksi grade) including vanadium. As
shown in FIGS. 3 and 4, they had somewhat inferior toughness, but
had machinability superior to that of the steels which do not
include vanadium.
INDUSTRIAL APPLICABILITY
[0063] As show in the Example, the steel of the present invention
is remarkably superior to conventional martensitic stainless steel
in machinability and descaling property. In addition, it has
substantially the same hot-workability as that of the steel having
the low S content, and has no occurrence of surface defects during
the pipe production process. This steel is significantly useful for
materials of seamless steel pipes because of its mechanical
characteristics and corrosion resistance which are equivalent to
those of conventional martensitic stainless steels.
* * * * *