U.S. patent application number 10/806391 was filed with the patent office on 2004-12-02 for non-heat treated seamless steel tube.
Invention is credited to Kondo, Kunio.
Application Number | 20040238075 10/806391 |
Document ID | / |
Family ID | 33399586 |
Filed Date | 2004-12-02 |
United States Patent
Application |
20040238075 |
Kind Code |
A1 |
Kondo, Kunio |
December 2, 2004 |
Non-heat treated seamless steel tube
Abstract
A non-heat treated seamless steel tube has a composition
consisting of, by weight, C: 0.10 to 0.25%, Si: 0.05 to 1.0%, Mn:
0.5 to 2.5%, P: not more than 0.03%, S: not more than 0.05%, Cr:
0.5 to 2.0%, V: 0.03 to 0.3%, Al 0.003 to 0.10%, N: 0.001 to 0.02%,
O: not more than 0.003%, and the balance comprises Fe and
impurities, in which the carbon equivalent Ceq. (%) defined by the
following equation is 0.60 to 0.85%.
Ceq.=C+Mn/6+(Cr+Mo+V)/5+(Ni+Cu)/15
Inventors: |
Kondo, Kunio; (Sanda-shi,
JP) |
Correspondence
Address: |
CLARK & BRODY
1750 K STREET NW
SUITE 600
WASHINGTON
DC
20006
US
|
Family ID: |
33399586 |
Appl. No.: |
10/806391 |
Filed: |
March 23, 2004 |
Current U.S.
Class: |
148/334 ; 420/91;
420/98 |
Current CPC
Class: |
C22C 38/38 20130101;
C22C 38/24 20130101; C22C 38/02 20130101; C22C 38/06 20130101 |
Class at
Publication: |
148/334 ;
420/091; 420/098 |
International
Class: |
C22C 038/22; C22C
038/42 |
Foreign Application Data
Date |
Code |
Application Number |
Mar 26, 2003 |
JP |
JP2003-084410 |
Claims
What is claimed is:
1. A non-heat treated seamless steel tube having a composition
consisting of, by weight, C: 0.10 to 0.25%, Si: 0.05 to 1.0%, Mn:
0.5 to 2.5%, P: not more than 0.03%, S: not more than 0.05%, Cr:
0.5 to 2.0%, V: 0.03 to 0.3%, Al: 0.003 to 0.10%, N: 0.001 to
0.02%, and O: not more than 0.003%, wherein the balance comprises
Fe and impurities, and the carbon equivalent Ceq. (%) defined by
the following equation is 0.60 to 0.85.
Ceq.=C+Mn/6+(Cr+Mo+V)/5+(Ni+Cu)/15
2. The non-heat treated seamless steel tube according to claim 1,
wherein a part of Fe is replaced with at least one kind selected
from, Ni: 0.05 to 1.5%, Mo: 0.05 to 1.5%, Cu: 0.05 to 1.5%, and B:
0.0003 to 0.01%.
3. The non-heat treated seamless steel tube according to claim 1,
wherein a part of Fe is replaced with one or 2 kinds selected from,
Ti: 0.005 to 0.2%, and Nb: 0.005 to 0.2%.
4. The non-heat treated seamless steel tube according to claim 1,
wherein a part of Fe is replaced with at least one kind selected
from, Ni: 0.05 to 1.5%, Mo: 0.05 to 1.5%, Cu: 0.05 to 1.5%, and B:
0.0003 to 0.01%, and replaced with one or 2 kinds selected from,
Ti: 0.005 to 0.2%, and Nb: 0.005 to 0.2%.
Description
BACKGROUND OF THE INVENTION
[0001] 1. Field of the Invention
[0002] The present invention relates to a seamless steel tube used
for a machine structure and the like, and more particularly, it
relates to a non-heat treated seamless steel tube which can be used
as being made into the hot-manufactured tube, and has high
strength, high toughness and excellent weldability.
[0003] 2. Description of the Background Art
[0004] Conventionally, a seamless steel tube used for usage
requiring high strength and toughness has been manufactured as
follows. That is, billets are hot-work pierced and rolled to
manufacture the seamless steel tube. Then, the steel tube is
hardened and tempered, whereby predetermined levels of strength and
toughness are applied thereto to be a product.
[0005] In the manufacturing steps of the seamless steel tube, since
heat treatment is needed after the tube is formed, its cost is
increased and its delivery is delayed. In order to solve the above
problem, there is increased the demand for a non-heat treated
seamless steel tube having high strength and high toughness without
needing the heat treatment.
[0006] The non-heat treated seamless steel tube having the high
strength and high toughness is disclosed in Japanese Unexamined
Patent Publication No. 05-202447, Japanese Unexamined Patent
Publication No. 09-25541, Japanese Unexamined Patent Publication
No. 10-130783, Japanese Unexamined Patent Publication No.
10-204571, Japanese Unexamined Patent Publication No. 10-324946,
Japanese Unexamined Patent Publication No. 11-36017, Japanese
Unexamined Patent Publication No. 2000-328192, Japanese Unexamined
Patent Publication No. 2001-247931, and Japanese Unexamined Patent
Publication No. 2001-262275, for example.
[0007] Japanese Unexamined Patent Publication No. 05-202447,
Japanese Unexamined Patent Publication No. 09-25541, Japanese
Unexamined Patent Publication No. 10-130783, Japanese Unexamined
Patent Publication No. 10-205671, Japanese Unexamined Patent
Publication No. 10-324946, Japanese Unexamined Patent Publication
No. 11-36017, and Japanese Unexamined Patent Publication No.
2000-328192 each disclose a method of adjusting components or a
hot-manufacturing method in order to implement non-heat treated
steel tube having high strength and high toughness. In addition,
the above Japanese Unexamined Patent Publications are in common
with each other in that carbon (C) not less than 0.2% is added by
weight to design a medium carbon group component. According to the
prior art, since carbon not less than 0.2% is added, the toughness
is not sufficient in the strength level. Especially, at a welding
part which was hardened, the toughness is lowered and/or a weld
crack is generated.
[0008] Japanese Unexamined Patent Publication No.2001-323338
discloses a method of implementing hot workability, machinability
and toughness for steel in which a broad range of added amounts of
carbon is added. However, since vanadium (V) is not added in the
steel tube in this case, the sufficient strength cannot be
provided.
[0009] Japanese Unexamined Patent Publication No. 2001-247931 and
Japanese Unexamined Patent Publication No. 2001-262275 disclose
technique for providing the strength and toughness by restricting a
hot manufacturing temperature so as to control a metal
constitution, and technique for providing the hot workability, for
the steel in which a broad range of added amounts of carbon is
added. However, in order to implement low-temperature manufacturing
disclosed in the above documents, it is necessary to convert the
facility because a motor power is insufficient in the conventional
facility. Furthermore, a facility such as a reheating furnace is
needed in order to reheat the tube to be manufactured after cooled
it once.
[0010] Still further, according to Japanese Unexamined Patent
Publication No. 2001-247931, although a broad range of added
amounts of carbon is defined in CLAIM, the carbon amount is 0.2% or
more by weight in embodiments.
SUMMARY OF THE INVENTION
[0011] It is an object of the present invention to provide a
non-heat treated seamless steel tube having high strength, high
toughness and excellent weldability. Especially, it is an object to
implement both high strength and high toughness and prevent a crack
from being generated at a welding part and toughness from being
lowered without strictly restricting the degree of manufacturing
processing and a manufacturing temperature.
[0012] The inventor of the present invention found that the
following matters were effective in order to attain the above
objects.
[0013] (1) A carbon content is lowered. Then, in order to
compensate the strength because the carbon content is lowered,
manganese (Mn), chromium (Cr) and vanadium (V) are added together.
Thus, high strength can be provided and preferable toughness can be
provided, including the welding part.
[0014] (2) The carbon content is lowered and then the carbon
equivalent (Ceq.) is adjusted in a predetermined range.
[0015] The metal structure of the non-heat treated steel is
normally ferrite and perlite steel. However, when it is highly
carbonized in order to satisfy the demand for the high strength,
the toughness is lowered. Then, the inventor of the present
invention lowers the carbon content and adds Mn, Cr and V together
in order to compensate the strength. Thus provided metal structure
of the non-heat treated steel becomes a bainite-based structure, so
that the high strength and high toughness can be secured. In
addition, although the term "bainite based structure" definitely
includes a structure in which bainite exists 100%, it also includes
a mixture structure of bainite and ferrite in which volume % of
ferrite is 50 or less.
[0016] The essential features of the present invention are as
follows.
[0017] (1) To control the carbon content so as to be less than
0.2%.
[0018] (2) To add manganese (Mn), chromium (Cr) and vanadium (V)
together.
[0019] (3) To provide the bainite based metal structure, in which
the carbon equivalent Ceq. defined by the following equation
satisfies a range from 0.60-0.85, as a component composition range
in which high strength and high toughness can be implemented.
Ceq.=C+Mn/6+(Cr+Mo+V)/5+(Ni+Cu)/15
[0020] When the Ceq. is in the range of 0.60-0.85, the strength is
secured. As a method of satisfying the range, there are various
kinds of adjustments in added amounts of an alloy element. Above
all, the inventors of the present invention found that when Mn, Cr
and V were added together at the same time, the strength and the
toughness were preferably in balance. In other words, when the
carbon content is the same, as compared with the case the intended
carbon equivalent Ceq. is attained by adding Mn and Cr only, the
more preferable toughness can be provided in the case V is added
while the amount of Mn and Cr is lowered to attain the intended
carbon equivalent Ceq. Therefore, regarding Mn, Cr and V, it is
important to add them together.
[0021] In view of the above points, the non-heat treated seamless
steel tube of the present invention has a composition consisting
of, by weight, C: 0.10 to 0.25%, Si: 0.05 to 1.0%, Mn: 0.5 to 2.5%,
P: not more than 0.03%, S: not more than 0.05%, Cr: 0.5 to 2.0%, V:
0.03 to 0.3%, Al: 0.003 to 0.10%, N: 0.001 to 0.02%, and
[0022] O: not more than 0.003%, in which the balance comprises Fe
and impurities, and the carbon equivalent Ceq. (%) defined by the
following equation is 0.60 to 0.85.
Ceq.=C+Mn/6+(Cr+Mo+V)/5+(Ni+Cu)/15
[0023] A part of Fe may be replaced with one or 2 kinds or more
selected from Ni: 0.05 to 1.5%, Mo: 0.05 to 1.5%, Cu: 0.05 to 1.5%,
and B: 0.0003 to 0.01%.
[0024] Alternatively, a part of Fe may be replaced with one or 2
kinds selected from Ti: 0.005 to 0.2%, and Nb: 0.005 to 0.2%.
[0025] Furthermore, a part of Fe may replaced with one or 2 kinds
or more selected from Ni: 0.05 to 1.5%, Mo: 0.05 to 1.5%, Cu: 0.05
to 1.5%, and B: 0.0003 to 0.01%, and replaced with one or 2 kinds
or more selected from Ti: 0.005 to 0.2% and Nb: 0.005 to 0.2%.
[0026] The above restricted reason is described hereinafter.
[0027] C: 0.10 to 0.25%
[0028] C is an element which increases the strength but lowers the
toughness and weldability. Therefore, in order to secure the
toughness and weldability at high level, it is necessary to limit
the content of C in a range to 0.25% by weight. In view of the
toughness and weldability, the lower content of C is preferable but
when it is less than 0.10% by weight, it is difficult to secure the
strength. Therefore, the content of C is limited in the range of
0.10 to 0.25% by weight. In view of the most preferable balance
between the strength and the toughness, the content of C is
preferably in a range of 0.13 to less than 0.20%, more preferably
in a range of 0.13 to 0.17%.
[0029] Si: 0.05 to 1.0%
[0030] Si serves as a deoxidizer and has a property improving the
strength. However, when the content is less than 0.05%, the effect
cannot be provided and when it exceeds 1.0%, the toughness is
lowered. Thus, the content of Si is limited to a range of 0.05 to
1.0% by weight. In view of securing the most preferable balance
between the strength and the toughness, the content of Si is
preferably in a range of 0.1 to 0.4%.
[0031] Mn: 0.5 to 2.5%
[0032] Mn is an element which increases the strength without
lowering the toughness by being added together with Cr and V to the
steel in which the content of C is lowered. In order to secure a
predetermined strength, the content has to be not less than 0.5%.
Meanwhile, when it exceeds 2.5%, the weldability and toughness are
lowered. Therefore, the content of Mn is limited to the range of
0.5% to 2.5%. In view of securing the most preferable balance
between the strength and the toughness, the content of Mn is
preferably in a range of 1.5 to 2.0% by weight.
[0033] P: not more than 0.03%
[0034] P is an impurity element which incrassates in the vicinity
of final solidification point at the time of solidification and
segregates at a grain boundary to lower the hot workability and the
toughness. Therefore, it is preferably lowered as much as possible.
However, since it is permissible until 0.03%, the content of P is
determined to not more than 0.03%. However, in order to secure the
higher toughness, it is preferably not more than 0.02% and it is
more preferably not more than 0.01% by weight.
[0035] S: not more than 0.05%
[0036] S is also an impurity element like P, which segregates at a
grain boundary at the time of solidification to lower the hot
workability and the toughness. Therefore, it is preferably lowered
as much as possible. However, since it is permissible until 0.05%,
the content of S is determined to not more than 0.05% by weight.
However, the machinability is lowered in some cases when it is
lowered too much. Therefore, a lower limit value of S is preferably
set at 0.01% when the machinability is emphasized. Meanwhile, when
the toughness is emphasized more than the machinability, the
content of S is preferably not more than 0.02% and it is more
preferably not more than 0.01% by weight.
[0037] Cr: 0.5 to 2.0%
[0038] Cr is an element which increases the strength without
lowering the toughness by being added together with Mn and V to the
steel in which the content of C is lowered. In order to secure a
predetermined strength, the content has to be not less than 0.5% by
weight. Meanwhile, when it exceeds 2.0%, the weldability and
toughness are lowered. Therefore, the content of Cr is limited to
the range of 0.5% to 2.0%. In view of securing the most preferable
balance between the strength and the toughness, the content of Cr
is preferably in a range of 0.9 to 1.5% by weight.
[0039] V: 0.03 to 0.3%
[0040] V is added because it segregates fine V- carbide to increase
the strength. When it is added together with Mn and Cr, the
toughness can be prevented from being lowered while high strength
is maintained. In order to obtain this effect, V has to be added by
not less than 0.03%. Meanwhile, when it exceeds 0.3%, the toughness
is lowered. Therefore, the content of V is limited to the range of
0.03 to 0.3%. In view of securing the most preferable balance
between the strength and the toughness, the content of V is
preferably in a range of 0.05 to 0.15% by weight.
[0041] Al: 0.003 to 0.10%
[0042] Al is an element which serves as a deoxidizer. In order to
obtain this effect, the content has to be not less than 0.003%.
When it exceeds 0.10%, aluminum group inclusions are increased and
surface defects could frequently occur. Therefore, the content of
Al is limited to the range of 0.003 to 0.10%. In addition, in order
to secure a stable surface quality, it is preferable in a range of
0.003 to 0.05% by weight.
[0043] N: 0.001 to 0.02%
[0044] N is an element which miniaturizes crystal grains together
with Al and Ti to improve the toughness. However, when it is less
than 0.001%, the effect is small. Meanwhile, when it exceeds 0.02%,
the toughness is lowered to the contrary. Therefore, the content of
N is limited to a range of 0.001 to 0.02% by weight.
[0045] O: not more than 0.003%
[0046] lowers the toughness and fatigue strength when the content
exceeds 0.003% by weight. Therefore, the content of is set to be
not more than 0.003%.
[0047] One or 2 kinds or more selected from Ni: 0.05 to 1.5%, Mo:
0.05 to 1.5%, Cu: 0.05 to 1.5%, and B: 0.0003 to 0.01% All of Ni,
Mo, Cu and B are elements which improve a hardening property and
increase the strength of the steel, and one kind or two or more
kinds can be selected from them and added according to need. The
effect can be provided when 0.05% or more is added regarding Ni, Mo
and Cu and 0.0003% or more is added regarding B. Meanwhile, in a
case 1.5% or more is added regarding Ni, Mo and Cu, the strengthen
effect is saturated and cost is increased in the case of Ni, the
weldability and toughness are lowered in the case of Mo, and the
hot workability is lowered in the case of Cu. Therefore, the upper
limit values of Ni, Mo and Cu are set at 1.5% by weight,
respectively. When B is added more than 0.01%, since the toughness
is lowered, the upper limit value is set at 0.01%.
[0048] One or two kinds selected from Ti: 0.005 to 0.2% and Nb:
0.005 to 0.2%
[0049] Both Ti and Nb are elements which contribute to miniaturize
the structure by forming carbide to improve the toughness, and
segregate in a matrix to increase the strength. Therefore, the
above one kind or two kinds can be added according to need. The
effect can be obtained when 0.005% or more are added regarding both
elements. Meanwhile, when they are added more than 0.2%, the
toughness is lowered. Therefore, both of them are limited to a
range of 0.005 to 0.2% by weight.
[0050] Carbon equivalent Ceq. (%): 0.60 to 0.85%
Ceq.=C+Mn/6+(Cr+Mo+V)/5+(Ni+Cu)/15
[0051] The alloy element contents are defined by weight %,
respectively. In addition to the restriction of the above component
composition, in view of preferably keeping the strength, toughness
and weldability, it is preferable that the Ceq. is limited to the
range of 0.60 to 0.85%. When the Ceq. is lower than 0.60%, the
strength cannot be secured and when the Ceq. is more than 0.85%,
the toughness is lowered and the weld crack is likely to be
generated. In addition, in view of high strength, it is preferably
0.65 to 0.85% and more preferably it is in a range of 0.70 to
0.85%.
[0052] The balance comprises Fe and impurities.
[0053] Ca, Mg and REM (rare-earth metal) in which 0.01% is set as
an upper limit value, respectively can be contained in the
impurities. Although these elements do not largely affect the
strength, toughness and weldability, since they prevent a nozzle of
a tundish from clogging at the time of casting of a round billet
especially, they are added in some cases. When the content of the
elements exceed 0.01%, a surface nature state deteriorates and
yield is lowered. Therefore, they may be added as impurities by
setting 0.01% as their upper limit.
[0054] The seamless steel tube according to the present invention
can be manuractured as follows. That is, the molten steel having
the above composition is treated in a converter, an electric
furnace or a vacuum melting furnace, and solidified by a continuous
casting method or ingot making method. Then, the solidified object
becomes a steel tube material as it is or through blooming. Then,
the steel tube goes through a normal manufacturing process of the
seamless steel tube, then it is air-cooled.
[0055] Although the cooling after the hot rolling is preferably air
cooling by natural cooling, warm cooling such as air blast cooling
(cooling with a wind shelter cover) or wind cooling (cooling with
some wind) may be performed.
EXAMPLES
[0056] Steels having chemical compositions shown in a table 1 are
melted, then casted into ingots and then formed into billets by
casting. The billets are heated to 1250.degree. C. and formed into
tubes by a Mannessman mandrel type of mill. Thus, the seamless
steel tubes each having an outer diameter of 150 mm and a wall
thickness of 24.2 mm are provided. The steel tubes are rolled and
then air-cooled. Then, mechanical characteristics (tensile
characteristics and impact characteristics by a Charpy test) and
weld crack properties of these steel tubes are examined as being
made into the tube.
[0057] In addition, referring to the weld crack properties, welding
is performed by a shielded metal arc welding method with heat input
of 20 kJ/cm without preheating, based on JIS Z 3158 "Y-shaped weld
cracking test" and cracking existence is examined. The results of
the examinations are also shown in the table 1. In the table,
reference character TS (Ma) designates a tensile strength. In
addition, reference character 2uE20 (J) designates a Charpy
breaking energy value based on JIS Z 2202 and JIS Z 2242, as an
index of toughness, which corresponds to a Charpy breaking energy
value (J) in a test at 20.degree. C. for a 2mm U-notch chip.
1TABLE 1 Nos. C Si Mn P S Cr V Al N O Ni inventive 1 0.15 0.19 1.80
0.015 0.020 1.20 0.10