U.S. patent application number 11/060417 was filed with the patent office on 2005-08-25 for high strength seamless steel pipe and its manufacturing method.
Invention is credited to Sakamoto, Makoto.
Application Number | 20050183799 11/060417 |
Document ID | / |
Family ID | 34858011 |
Filed Date | 2005-08-25 |
United States Patent
Application |
20050183799 |
Kind Code |
A1 |
Sakamoto, Makoto |
August 25, 2005 |
High strength seamless steel pipe and its manufacturing method
Abstract
Anon-heat treated seamless steel pipe comprising, by weight, C:
0.10 to 0.25%, Si: 0.05 to 1.00%, Mn: 0.50 to 2.50%, P: not more
than 0.03%, S: not more than 0.05%, Cr: 0.40 to 1.50%, Mo: 0.05 to
1.50%, V: 0.02 to 0.30%, Al: 0.003 to 0.10%, B: 0.0003 to 0.01%,
and N: 0.001 to 0.02%, in which the balance comprises Fe and
impurities. According to this non-heat treated seamless steel pipe,
a ferrite area ratio is not more than 10%, and a carbon equivalent
Ceq. (%) defined by the following equation is 0.45 to 0.85%.
Ceq.=C+Mn/6+(Cr+Mo+V)/5+(Cu+Ni)/15
Inventors: |
Sakamoto, Makoto; (Naga-gun,
JP) |
Correspondence
Address: |
CLARK & BRODY
1090 VERMONT AVENUE, NW
SUITE 250
WASHINGTON
DC
20005
US
|
Family ID: |
34858011 |
Appl. No.: |
11/060417 |
Filed: |
February 18, 2005 |
Current U.S.
Class: |
148/593 ;
420/106 |
Current CPC
Class: |
C21D 8/0226 20130101;
C22C 38/22 20130101; C22C 38/04 20130101; C21D 8/10 20130101; C22C
38/02 20130101 |
Class at
Publication: |
148/593 ;
420/106 |
International
Class: |
C22C 038/22; C21D
009/08 |
Foreign Application Data
Date |
Code |
Application Number |
Feb 19, 2004 |
JP |
JP2004-043279 |
Claims
What is claimed is:
1. A high strength non-heat treated seamless steel pipe comprising,
by weight, C: 0.10 to 0.25%, Si: 0.05 to 1.00%, Mn: 0.50 to 2.50%,
P: not more than 0.03%, S: not more than 0.05%, Cr: 0.40 to 1.50%,
Mo: 0.05 to 1.50%, V: 0.02 to 0.30%, Al: 0.003 to 0.10%, B: 0.0003
to 0.01%, and N: 0.001 to 0.02%, wherein the balance comprises Fe
and impurities, a ferrite area ratio is not more than 10%, and a
carbon equivalent Ceq. (%) defined by the following equation is
0.45 to 0.85%. Ceq.=C+Mn/6+(Cr+Mo+V)/5+(Cu+Ni)/15
2. The high strength non-heat treated seamless steel pipe according
to claim 1, wherein a part of Fe is replaced with at least one kind
selected from a group consisting of Ti: 0.005 to 0.2%, Cu: 0.05 to
1.5%, Ni: 0.05 to 1.5%, and Nb: 0.005 to 0.2%.
3. A method of manufacturing the high strength non-heat treated
seamless steel pipe according to claim 1, wherein a temperature of
finish rolling at a manufacturing step is not less than 900.degree.
C. and the following relational expression is satisfied. Ceq.
(%).times.finish rolling temperature (.degree. C.).gtoreq.450
4. The method of the high strength non-heat treated seamless steel
pipe according to claim 3, wherein a part of Fe is replaced with at
least one kind selected from a group consisting of Ti: 0.005 to
0.2%, Cu: 0.05 to 1.5%, Ni: 0.05 to 1.5%, 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 pipe for
machine structural use and the like, and more particularly, it
relates to a non-heat treated seamless steel pipe which can be used
as being made into a hot-manufactured pipe, and has high strength,
high toughness and excellent weldability.
[0003] 2. Description of the Background Art
[0004] Conventionally, a seamless steel pipe for usage requiring
high strength and high toughness has been manufactured as follows.
That is, the seamless steel pipe is manufactured through a step of
piercing a billet and a step of rolling in a hot process. Then, the
steel pipe is quenched and tempered, whereby predetermined levels
of strength and toughness are provided in a product.
[0005] According to the manufacturing method of the seamless steel
pipe, since heat treatment is needed after the pipe is
manufactured, its cost is increased and it takes time until the
product is delivered. In order to solve the above problem, there is
an increasing demand for a non-heat treated seamless steel pipe
having high strength and high toughness.
[0006] Conventionally, the non-heat treated seamless steel pipe
used in a hydraulic cylinder and the like is mostly 540 MPa steel.
This "540 MPa steel" means steel having strength in which tensile
strength is not less than 540 MPa and yield stress is not less than
390 MPa. When the steel pipe used in the hydraulic cylinder is
highly pressurized and reduced in weight, high strength heat
treated steel which went through quenching and tempering steps is
used. In order to provide a high strength non-heat treated seamless
steel pipe, some attempts have been made, that is, carbon loadings
are increased or a large amount of expensive alloy element is
applied, for example. However, weldability is lowered and a cost is
increased in the above attempts.
[0007] The above problems will be described in more detail with
reference to the conventional related art.
[0008] Non-heat treated seamless steel pipes having high strength
and high toughness are disclosed in Japanese Unexamined Patent
Publication No. 03-162524, 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.
2001-323338, Japanese Unexamined Patent Publication No.
2001-247931, and Japanese Unexamined Patent Publication No.
2001-262275, for example.
[0009] The Japanese Unexamined Patent Publication No. 03-162524
discloses a method of manufacturing a high tensile seamless steel
pipe having excellent low-temperature toughness. More specifically,
a material is heated up to 1150 to 1300.degree. C. and precooling
process by forcible cooling and reheating process are performed
while the pipe is produced and then final forcible cooling is
performed for it to miniaturize crystal grains in order to increase
the toughness and the strength. According to this method, a
precooling apparatus is needed when the pipe is produced and a
forcible cooling apparatus is needed after the pipe is produced,
which is a problem.
[0010] In each of the Japanese Unexamined Patent Publication No.
05-202447, the Japanese Unexamined Patent Publication No. 09-25541,
the Japanese Unexamined Patent Publication No. 10-130783, the
Japanese Unexamined Patent Publication No. 10-205671, the Japanese
Unexamined Patent Publication No. 10-324946, the Japanese
Unexamined Patent Publication No. 11-36017, and the Japanese
Unexamined Patent Publication No. 2000-328192, component adjustment
and a method of manufacturing a pipe in a hot working process in
order to provide the non-heat treated steel pipe having the high
strength and the high toughness are disclosed. In addition, it is
in common among the above Japanese Unexamined Patent Publications
that carbon (C) is added 0.2% or more to design a medium
carbonaceous component. Thus, since carbon is added 0.2% or more,
the toughness is not sufficient with respect to the strength.
Especially, at a welding part, the toughness is lowered because of
quench hardening or a weld crack is generated.
[0011] The Japanese Unexamined Patent Publication No. 2001-323338
discloses a method of implementing hot workability, machinability
and toughness in steel having a broad range of carbon loadings.
However, since vanadium (V) is not added in a steel pipe disclosed
in this document, sufficient strength cannot be provided.
[0012] The Japanese Unexamined Patent Publication No. 2001-247931
and the Japanese Unexamined Patent Publication No. 2001-262275
disclose technique for providing the strength and the toughness and
technique for providing the hot workability by restricting a
manufacturing temperature in a hot working process so as to control
a metal structure in steel having a broad range of carbon contents.
However, in order to implement low-temperature manufacturing
disclosed in the above documents, it is necessary to convert a
facility because a motor power is insufficient in the conventional
facility. Furthermore, a reheating furnace and the like is
additionally needed in order to reheat a pipe after cooling it once
to manufacture the pipe.
[0013] Still further, according to the Japanese Unexamined Patent
Publication No. 2001-247931, although a broad range of carbon
contents is defined in claims, carbon is added 0.2% or more in the
description in the embodiment.
[0014] According to a method disclosed in the Japanese Unexamined
Patent Publication No. 2001-262275, although a pipe is cooled down
once and reheated again to 900.degree. C. after the pipe is
manufactured, V, Ti, Nb and the like are precipitated when the
temperature is lowered once, and these precipitations grow in the
subsequent reheating process. As a result, the precipitations
become large, so that the toughness is lowered and sufficient
strength is not provided.
SUMMARY OF THE INVENTION
[0015] It is an object of the present invention to provide a
non-heat treated seamless steel pipe having both high strength and
high toughness and having preferable weldability. Especially, it is
an object of the present invention to provide a high strength
non-heat treated seamless steel pipe in which the tensile strength
is not less than 640 Mpa and the yield stress is not less than 490
Mpa.
[0016] It is another object of the present invention to provide a
method of manufacturing the non-heat treated seamless steel pipe
having the above characteristics.
[0017] The inventor of the present invention found that the
following things were effective in order to attain the above
objects.
[0018] (1) A carbon content was lowered. Then, in order to
compensate strength because the carbon content was lowered,
manganese (Mn), chromium (Cr) and vanadium (V) were added together.
Thus, high strength and preferable toughness were provided in the
pipe including the welding part.
[0019] (2) A ferrite area ratio was lowered to a predetermined
level or less. Thus, predetermined strength was provided. The
"ferrite area ratio" was measured as an area ratio of a ferrite
crystal existing in the steel pipe to its visual field, by means
such as an optical microscope.
[0020] (3) The carbon content was lowered and then a carbon
equivalent (Ceq.) is adjusted in a predetermined range.
[0021] The standard is that a metal structure of the non-heat
treated steel comprises ferrite-pearlite steel. However, when it
was highly carbonized in order to meet the demand for high
strength, it was found that the toughness was lowered. Then, the
inventor of the present invention lowers the carbon content and
added 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 the high
toughness can be secured. Not only the term "bainite-based
constitution" includes a structure in which bainite exists by 100%,
but also it includes a mixture consisting of bainite and ferrite in
which volume % of ferrite is 50 or less.
[0022] The essential features of the present invention are as
follows, that is,
[0023] (1) the carbon content is controlled to be 0.25% or
less,
[0024] (2) manganese (Mn), chromium (Cr) and vanadium (V) are added
together,
[0025] (3) the ferrite area ratio is controlled to be 10% or less,
and
[0026] (4) the bainite based metal structure is provided, in which
the carbon equivalent Ceq. (%) defined by the following equation
satisfies a range from 0.45 to 0.85, as a component composition
range to implement the high strength and the high toughness.
Ceq.=C+Mn/6+(Cr+Mo+V)/5+(Cu+Ni)/15
[0027] When the ferrite area ratio is not more than 10% and the
Ceq. is in the range of 0.45 to 0.85, the strength is secured. A
method of satisfying the above ranges is implemented by adjusting
various kinds of contents of alloy elements and adjusting the
temperature of finish rolling in the manufacturing process of the
pipe. Above all, the inventor 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 provided in a balanced
manner. In other words, when the carbon content is the same, as
compared with a case the intended carbon equivalent Ceq. is
attained by adding Mn and Cr only, the more preferable toughness
can be provided in a case V is added while the amount of Mn and Cr
is reduced to attain the intended carbon equivalent Ceq. Therefore,
regarding Mn, Cr and V, it is important to add them together.
[0028] In view of the above points, the non-heat treated seamless
steel pipe of the present invention comprises, by weight, C: 0.10
to 0.25%, Si: 0.05 to 1.00%, Mn: 0.50 to 2.50%, P: not more than
0.03%, S: not more than 0.05%, Cr: 0.40 to 1.50%, Mo: 0.05 to
1.50%, V: 0.02 to 0.30%, Al: 0.003 to 0.10%, B: 0.0003 to 0.01%,
and N: 0.001 to 0.02%, in which the balance comprises Fe and
impurities, the ferrite area ratio is not more than 10%, and the
carbon equivalent Ceq. (%) defined by the following equation is
0.45 to 0.85%.
Ceq.=C+Mn/6+(Cr+Mo+V)/5+(Cu+Ni)/15
[0029] According to the method of manufacturing the non-heat
treated seamless steel pipe, a temperature of finish rolling at a
manufacturing step is not less than 900.degree. C. and the
following relational expression is satisfied.
Ceq. (%).times.finish rolling temperature (.degree.
C.).gtoreq.450
[0030] In addition, a part of Fe may be replaced with one or more
kinds selected from Ti: 0.005 to 0.2%, Cu: 0.05 to 1.5%, Ni: 0.05
to 1.5%, and Nb 0.005 to 0.2%.
[0031] The above specified reason will be described
hereinafter.
[0032] C: 0.10 to 0.25%
[0033] C is an effective element to secure quenching ability and it
is an important element to constitute the metal structure with a
mixture of bainite and ferrite (pearlite) in a state after hot
rolling and to provide necessary strength. In this respect,
although it is necessary to contain carbon 0.10% or more, since the
toughness and weldability are lowered if the carbon is too much, an
upper limit value of the carbon content is set at 0.25%. In order
to provide the most preferable balance between the strength and the
toughness, the content of C is preferably in the range of 0.15 to
0.23%.
[0034] Si: 0.05 to 1.00%
[0035] Si serves as a deoxidizer at the time of melting. When Si
content is less than 0.05%, deoxidization is not sufficiently
implemented, so that non-metallic inclusions are increased.
However, when the Si content is too much, the toughness is lowered,
in addition, scale formation is increased and a surface property
deteriorates at the time of heating the steel material. Thus, an
upper limit value of the Si content is set at 1.00%. The most
preferable Si content is in a range of 0.15 to 0.40%.
[0036] Mn: 0.50 to 2.50%
[0037] Mn serves as a deoxidizer at the time of melting similar to
Si. In addition, it 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 the predetermined strength, the content has to be 0.50% or
more. Meanwhile, when it exceeds 2.50%, the weldability and the
toughness are lowered. Therefore, the content of Mn is limited to
the range of 0.50% to 2.50%. In view of securing the most
preferable balance between the strength and the toughness, the
content of Mn is preferably in a range of 0.16 to 1.50%.
[0038] P: not more than 0.03%
[0039] P is an impurity element which incrassates in the vicinity
of final coagulating point at the time of coagulation 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 be 0.03% or less. However, in order to secure the
higher toughness, it is preferably 0.02% or less and it is more
preferably 0.01% or less.
[0040] S: not more than 0.05%
[0041] S is also an impurity element like P, which segregates at a
grain boundary at the time of coagulation 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 be 0.05% or less. 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 0.02% or less and it is more preferably 0.018% or
less.
[0042] Cr: 0.40 to 1.50%
[0043] Cr is an effective element in improving the quenching
ability, and it 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 the
predetermined strength, the content has to be 0.40% or more.
Meanwhile, when it exceeds 1.5%, the weldability and the toughness
are lowered. Therefore, the content of Cr is limited to the range
of 0.40% to 1.50%. In view of securing the most preferable balance
between the strength and the toughness, the content of Cr is
preferably in a range of 0.40 to 1.2%.
[0044] Mo: 0.05 to 1.50%
[0045] Mo is an effective element to improve the quenching ability
and also it is an important element to secure necessary strength.
In this respect, it is necessary to contain Mo 0.05% or more, but
if it is applied too much, the strength is increased too much and
the toughness is lowered. In this respect, its upper limit value is
set at 1.50%. A preferable content of Mo is in a range of 0.10 to
1.00%.
[0046] V: 0.02 to 0.30%
[0047] V is an effective element to form carbide or nitride to
miniaturize austenitic crystal grains. In addition, 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 0.02% or more. Meanwhile, when it
exceeds 0.3%, the toughness is lowered. Therefore, a content of V
is limited to the range of 0.02 to 0.30%. In view of securing the
most preferable balance between the strength and the toughness, the
content of V is preferably in a range of 0.03 to 0.15%.
[0048] Al: 0.003 to 0.10%
[0049] Al is an element which serves as a deoxidizer. In order to
obtain this effect, a content has to be 0.003% or more. When it
exceeds 0.10%, aluminum inclusions are increased and a surface
defect 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 preferably in a range of
0.003 to 0.05%.
[0050] B: 0.0003 to 0.01%
[0051] B is an effective element to provide the quenching ability
and it is also an important element to constitute the metal
structure with the mixture of bainite and ferrite (pearlite) in a
state after the hot rolling and to provide the necessary strength.
In order to provide this effect, a content of B has to be 0.0003%
or more, however, if it is added too much, the toughness is
lowered. Thus, an upper limit value of the content is set at
0.01%.
[0052] N: 0.001 to 0.02%
[0053] 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, a content of N
is limited to the range of 0.001 to 0.02%.
[0054] One or more kinds selected from Ti: 0.005 to 0.2%, Cu: 0.05
to 1.5%, Ni: 0.05 to 1.5%, and Nb: 0.005 to 0.2%.
[0055] Both Ti and Nb are elements which form the carbide to fine
the structure to improve the toughness and segregate in a base to
increase the strength. The effect can be obtained when either of
them is added 0.005% or more. Meanwhile, when they are added more
than 0.2%, the toughness is lowered. Therefore, both of them is
limited to the range of 0.005 to 0.2%.
[0056] Both Cu and Ni are elements which improve the quenching
ability to increase the strength of the steel. The effect can be
provided when they are added 0.05% or more. Meanwhile, when they
are added beyond 1.5%, Ni causes a strength increasing effect to be
saturated and increases the cost and Cu lowers the hot workability.
Therefore, upper limits of Ni and Cu are set at 1.5%.
[0057] The balance: Fe and impurities.
[0058] Ca, Mg and REM (rare-earth metal) in which 0.01% is set as
their upper limit values can be contained in the impurities.
Although these elements do not largely affect the strength, the
toughness and the weldability, since they prevent a nozzle of a
tundish from clogging at the time of casting a round billet
especially, they are added in some cases. When a content of each
element exceeds 0.01%, the surface property deteriorates and yield
is lowered. Therefore, they may be added as impurities by setting
0.01% as their upper limits.
[0059] Ferrite area ratio.ltoreq.10%
[0060] The ferrite area ratio measured as an area ratio of a
ferrite crystal existing in the steel pipe to its visual field, by
the means such as the optical microscope has to be 10% or less for
the following reason. That is, soft ferrite grains lowers the
material strength when its area ratio is increased. Since the
predetermined strength is not provided if the ferrite area ratio
becomes a certain value or more, it is set at 10% or less.
[0061] Carbon equivalent Ceq. (%): 0.45 to 0.85%
Ceq.=C+Mn/6+(Cr+Mo+V)/5+(Cu+Ni)/15
[0062] The chemical symbols on the right-hand side show contents of
alloy elements, which are defined by weight %, respectively. In
addition to the restrictions of the above component composition and
the ferrite area ratio, in view of preferably keeping the strength,
the toughness and the weldability, it is preferable that the Ceq.
is limited to a range of 0.45 to 0.85%. When the Ceq. is lower than
0.45%, 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 the balance between the
strength and the toughness, it is preferably in a range of 0.46 to
0.85% and more preferably, it is in a range of 0.46 to 0.70%. In
addition, when the carbon equivalent Ceq. is calculated, an element
which is not contained in the steel may be calculated such that its
element content is 0.
[0063] The seamless steel pipe according to the present invention
can be manufactured as follows. That is, the steel having the above
composition is melted in a steel converter, an electric furnace or
a vacuum melting furnace, and solidified by a continuous casting
method or an ingot casting method. Then, the solidified object
becomes a steel pipe material as it is or through blooming. Then,
the steel pipe material goes through a normal manufacturing process
of the seamless steel pipe and it is air-cooled.
[0064] Although the cooling after the hot rolling is preferably air
cooling by natural cooling, warm cooling such as air blasting
(cooling with a wind shelter cover) or wind cooling (cooling with
some wind) may be performed.
[0065] Characteristic point of the method of the present invention
lies in the fact that the temperature of the finish rolling in the
manufacturing step is not less than 900.degree. C. and the
relational expression such as Ceq. (%).times.finish rolling
temperature (.degree. C.).gtoreq.450 is satisfied. In order to
provide the bainite structure while the carbon content is kept low
and the alloy element content to be added is kept low, the finish
rolling temperature has to be not less than 900.degree. C. in the
manufacturing step. In addition, in order to provide the strength
of the steel pipe at the predetermined level or more, the value of
the Ceq. (%).times.finish rolling temperature (.degree. C.) has to
be 450 or more.
[0066] According to the present invention, unlike the method
disclosed in the Japanese Unexamined Patent Publication No.
2001-262275, since the rolling is completed without lowering the
temperature once in the manufacturing step, the precipitates are
fine because it does not become large by the reheating, so that
both toughness and strength can be preferably provided.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
Embodiment
[0067] Steel (sample numbers 1 to 30) having chemical composition
shown in a table 1 was melted, then poured into an ingot and then
formed into a billet by casting. The billet was heated up to
1250.degree. C. and formed into a pipe by a Mannesmann mandrel type
of mill. Thus, a seamless steel pipe having an outer diameter of
406.4 mm and a wall thickness of 12.7 mm was provided The steel
pipe was rolled and then air-cooled. Then, mechanical
characteristics of the steel pipe (sample numbers 1 to 30) was
examined and shown in a table 2. In addition to the mechanical
characteristics, a ferrite area ratio, a finish rolling
temperature, a value of the finish rolling temperature.times.Ceq.,
a billet heating temperature, evaluation for each steel pipe are
shown in the table 2.
1 TABLE 1 C Si Mn P S Cr Mo V Al B N Ti Cu Ni Nb Ceq. Preferable
range 0.1/ 0.05/ 0.5/ 0.4/ 0.05/ 0.02/ 0.003/ .0003/ 0.001/ 0.25
1.0 2.5 .about.0.03 .about.0.05 1.5 1.5 0.3 0.1 .01 0.02 0.005/0.2
0.05/1.5 0.05/1.5 0.005/0.2 0.45/0.85 Sample 0.19 0.24 0.94 0.010
0.007 0.53 0.21 0.06 0.035 0.0014 0.0038 0.51 1 2 0.19 0.24 0.94
0.010 0.007 0.53 0.21 0.06 0.035 0.0014 0.0038 0.51 3 0.19 0.24
0.94 0.010 0.007 0.53 0.21 0.06 0.035 0.0014 0.0038 0.51 4 0.19
0.24 0.94 0.010 0.007 0.53 0.21 0.06 0.035 0.0014 0.0038 0.51 5
0.19 0.24 0.94 0.010 0.007 0.53 0.21 0.06 0.035 0.0014 0.0038 0.51
6 0.19 0.24 0.94 0.010 0.007 0.53 0.21 0.06 0.035 0.0014 0.0038
0.51 7 0.19 0.24 0.94 0.010 0.007 0.53 0.21 0.06 0.035 0.0014
0.0038 0.51 8 0.19 0.24 0.94 0.010 0.007 0.53 0.21 0.06 0.035
0.0014 0.0038 0.51 9 0.19 0.24 0.94 0.010 0.007 0.53 0.21 0.06
0.035 0.0014 0.0038 0.51 10 0.19 0.24 0.94 0.010 0.007 0.53 0.21
0.06 0.035 0.0014 0.0038 0.51 11 0.19 0.24 0.94 0.010 0.007 0.53
0.21 0.06 0.035 0.0014 0.0038 0.024 0.51 12 0.18 0.23 0.94 0.011
0.006 0.69 0.06 0.07 0.022 0.0009 0.0048 0.15 0.51 13 0.18 0.25
0.92 0.016 0.009 0.83 0.18 0.08 0.036 0.0011 0.0077 0.50 0.58 14
0.19 0.22 0.90 0.011 0.010 0.53 0.22 0.10 0.018 0.0013 0.0055 0.015
0.51 15 0.19 0.31 0.98 0.010 0.011 0.51 0.38 0.05 0.029 0.0019
0.0039 0.009 0.13 0.10 0.024 0.56 16 0.23 0.22 0.85 0.019 0.015
0.54 0.90 0.06 0.034 0.0005 0.0033 0.05 0.05 0.68 17 0.19 0.22 0.90
0.011 0.010 0.53 0.22 0.10 0.018 0.0013 0.0055 0.010 0.11 0.52 18
0.19 0.24 0.94 0.010 0.007 0.53 0.21 0.06 0.035 0.0014 0.0038 0.08
0.022 0.51 19 *0.28 0.22 0.84 0.015 0.010 0.08 0.05 0.02 0.023 *0
0.008 0.13 0.46 20 0.15 0.38 *0.49 0.011 0.012 1.15 0.49 0.05 0.024
0.0011 0.0056 0.030 0.57 21 0.15 0.21 *0.43 0.014 0.009 1.08 0.48
0.03 0.017 0.0011 0.0094 0.08 0.55 22 0.11 0.26 *0.42 0.013 0.008
0.58 0.94 0.07 0.018 0.0015 0.0076 0.50 0.028 0.53 23 0.11 0.26
*0.42 0.010 0.014 0.69 0.94 0.09 0.021 0.0005 0.0074 0.52 24 *0.06
0.35 1.00 0.014 0.014 0.80 0.30 0.32 0.019 0.0022 0.0076 0.51 25
*0.27 0.23 0.55 0.013 0.007 0.89 0.60 0.10 0.025 0.0009 0.0045 0.68
26 0.11 0.20 *2.60 0.013 0.012 0.54 0.21 *0.01 0.026 0.0009 0.0048
0.70 27 0.20 0.26 0.94 0.011 0.012 *0.25 0.20 0.05 0.031 0.0007
0.0051 0.46 28 0.15 0.26 0.68 0.012 0.009 0.25 *1.60 0.06 0.032
0.0011 0.0053 0.65 29 0.22 0.25 0.90 0.010 0.010 0.25 0.20 *0 0.029
0.0008 0.0053 0.46 30 0.25 0.25 0.90 0.014 0.008 0.25 *0.03 0.09
0.018 0.0008 0.0042 0.47 *Out of preferable range
[0068]
2 TABLE 2 Finish rolling Finish rolling Billet heating Tensile
strength Ferrite area ratio temperature temperature .times. Ceq.
temperature (TS) Yield stress (YS) Preferable range not more than
10% not less than 900 not less than 450 -- not less than 640 not
less than 490 Result Sampl .largecircle. 961 487 1250 718 549
.largecircle. 1 2 .largecircle. 971 492 1250 720 531 .largecircle.
3 .largecircle. 1004 509 1280 712 510 .largecircle. 4 .largecircle.
1011 512 1280 715 536 .largecircle. 5 .largecircle. 1019 516 1280
713 521 .largecircle. 6 .largecircle. 1044 529 1280 720 532
.largecircle. 7 .largecircle. 1031 522 1280 724 554 .largecircle. 8
.largecircle. 795 *403 1180 679 *470 X 9 .largecircle. 853 *432
1180 676 *473 X 10 .largecircle. 850 *431 1180 695 *479 X 11
.largecircle. 1023 518 1280 732 563 .largecircle. 12 .largecircle.
1050 536 1280 733 541 .largecircle. 13 .largecircle. 956 559 1250
770 565 .largecircle. 14 .largecircle. 998 509 1250 703 510
.largecircle. 15 .largecircle. 965 537 1250 695 527 .largecircle.
16 .largecircle. 920 624 1180 755 551 .largecircle. 17
.largecircle. 927 480 1180 685 501 .largecircle. 18 .largecircle.
985 504 1180 699 524 .largecircle. 19 X 950 *436 1250 *570 *364 X
20 .largecircle. 1025 584 1250 *496 *329 X 21 .largecircle. 980 534
1250 *503 *367 X 22 .largecircle. 965 513 1250 *565 *430 X 23
.largecircle. 1020 534 1280 *564 *440 X 24 .largecircle. 1034 528
1280 *638 *481 X 25 X 1035 703 1280 668 *476 X 26 .largecircle. 953
663 1280 651 *465 X 27 X 1026 469 1280 655 *432 X 28 X 1038 670
1280 678 *477 X 29 X 983 452 1250 *603 *415 X 30 X 963 456 1250
*586 *432 X *Out of preferable range
[0069] According to the sample numbers 1 to 30 shown in the tables
1 and 2, notable results will be described hereinafter.
[0070] (1) Sample Numbers 1 to 7, 11 to 18
[0071] The content of each component is within the preferable
range.
[0072] The carbon equivalent (Ceq) is within the preferable
range.
[0073] The ferrite area ratio is within the preferable range.
[0074] The finish rolling temperature is within the preferable
range.
[0075] The value of the finish rolling temperature.times.Ceq. is
within the preferable range.
[0076] According to these samples, the tensile strength (TS) is not
less than 640 Mpa and the yield stress (YS) is not less than 490
Mpa.
[0077] (2) Sample Numbers 8 to 10
[0078] The content of each component is within the preferable
range.
[0079] The carbon equivalent (Ceq) is within the preferable
range.
[0080] The ferrite area ratio is within the preferable range.
[0081] The finish rolling temperature is within the preferable
range.
[0082] The value of the finish rolling temperature.times.Ceq. is
below the preferable range.
[0083] According to these samples, the yield stress (YS) is less
than 490 MPa.
[0084] (3) Sample Numbers 19 to 30
[0085] The content of any of components is out of the preferable
range.
[0086] The carbon equivalent (Ceq) is within the preferable
range.
[0087] The ferrite area ratio is within the preferable range in
each of the sample numbers 20, 21, 22, 23, 24, and 26, and it is
out of the preferable range in each of the sample numbers 19, 25,
27, 28, 29, and 30.
[0088] The finish rolling temperature is within the preferable
range.
[0089] The value of the finish rolling temperature.times.Ceq. is
within the preferable range except for the sample number 19.
[0090] According to all these samples, the yield stress is less
than 490 Mpa, and according to the sample numbers 19 to 24, 29, and
30, the tensile strength is less than 640 MPa.
[0091] As described above, according to the present invention,
there can be provided the non-heat treated seamless steel pipe
which satisfies both high strength and high toughness.
[0092] Thus, the present invention can be effectively applied to
the non-heat treated seamless steel pipe for machine structural use
and the like.
* * * * *