U.S. patent application number 09/771589 was filed with the patent office on 2001-10-11 for super fine granular steel pipe and method for producing the same.
This patent application is currently assigned to Kawasaki Steel Corporation. Invention is credited to Furukimi, Osamu, Hashimoto, Yuji, Hira, Takaaki, Itadani, Motoaki, Kanayama, Taro, Matsuoka, Saiji, Morita, Masahiko, Nishimori, Masanori, Okabe, Takatoshi, Tanaka, Nobuki, Toyooka, Takaaki, Yorifuji, Akira.
Application Number | 20010027831 09/771589 |
Document ID | / |
Family ID | 27570295 |
Filed Date | 2001-10-11 |
United States Patent
Application |
20010027831 |
Kind Code |
A1 |
Toyooka, Takaaki ; et
al. |
October 11, 2001 |
Super fine granular steel pipe and method for producing the
same
Abstract
A steel pipe containing fine ferrite crystal grains, which has
excellent toughness and ductility and good ductility-strength
balance as well as superior collision impact resistance, and a
method for producing the same are provided. A steel pipe containing
super-fine crystal grains can be produced by heating a base steel
pipe having ferrite grains with an average crystal diameter of di
(.mu.m), in which C, Si, Mn and Al are limited within proper
ranges, and if necessary, Cu, Ni, Cr and Mo, or Nb, Ti, V, B, etc.
are further added, at not higher than the Ac.sub.3 transformation
point, and applying reducing at an average rolling temperature of
.theta.m (.degree. C.) and a total reduction ratio Tred (%) within
s temperature range of from 400 to Ac.sub.3 transformation point,
with di, .theta.m and Tred being in a relation satisfying a
prescribed equation.
Inventors: |
Toyooka, Takaaki; (Aichi,
JP) ; Yorifuji, Akira; (Aichi, JP) ;
Nishimori, Masanori; (Aichi, JP) ; Itadani,
Motoaki; (Aichi, JP) ; Hashimoto, Yuji;
(Aichi, JP) ; Okabe, Takatoshi; (Aichi, JP)
; Kanayama, Taro; (Aichi, JP) ; Morita,
Masahiko; (Okayama, JP) ; Matsuoka, Saiji;
(Okayama, JP) ; Tanaka, Nobuki; (Aichi, JP)
; Furukimi, Osamu; (Chiba, JP) ; Hira,
Takaaki; (Chiba, JP) |
Correspondence
Address: |
YOUNG & THOMPSON
745 SOUTH 23RD STREET 2ND FLOOR
ARLINGTON
VA
22202
|
Assignee: |
Kawasaki Steel Corporation
Hyogo
JP
|
Family ID: |
27570295 |
Appl. No.: |
09/771589 |
Filed: |
January 30, 2001 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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|
09771589 |
Jan 30, 2001 |
|
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09254024 |
Feb 26, 1999 |
|
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09254024 |
Feb 26, 1999 |
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PCT/JP98/02811 |
Jun 24, 1998 |
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Current U.S.
Class: |
148/333 |
Current CPC
Class: |
C21D 8/10 20130101; C21D
2201/00 20130101 |
Class at
Publication: |
148/333 |
International
Class: |
C22C 038/44 |
Foreign Application Data
Date |
Code |
Application Number |
Jun 26, 1997 |
JP |
9-170790 |
Jul 22, 1997 |
JP |
9-196038 |
Aug 20, 1997 |
JP |
9-223315 |
Aug 25, 1997 |
JP |
9-228579 |
Sep 5, 1997 |
JP |
9-240930 |
May 15, 1998 |
JP |
10-133933 |
Claims
What is claimed is:
1. A super fine granular steel pipe with high collision impact
property and high workability having a composition containing, by
weight, 0.005 to 0.3%C, 0.01 to 3.0%Si, 0.01 to 2.0%Mn, 0.001 to
0.10%Al, and balance Fe with unavoidable impurities, and a cross
section perpendicular to a longitudinal direction of the steel pipe
after reducing contains super fine grains of a ferrite having an
average crystal grain size of 3 .mu.m or less, and an absorbed
energy up to a strain rate of 30% by performing high speed tensile
tests at a strain rate of 2000s-1 is 202 MJ/m.sup.3 or more, which
is obtained in a method for producing a steel pipe, comprising
heating or soaking a base steel pipe having an outer diameter of
ODi (mm) and having ferrite grains with an average crystal diameter
of di (.mu.m) in the cross section perpendicular to the
longitudinal direction of the steel pipe, and then applying
reducing at an average rolling temperature of .theta.m(.degree. C.)
and a total reduction ratio Tred(%) to obtain a product pipe having
an outer diameter of ODf (mm), wherein, said reducing comprises
performing it in a temperature range of 400.degree. C. or more but
not more than the heating or soaking temperature, and in such a
manner that said average crystal diameter of di (.mu.m), said
average rolling temperature of .theta.m(.degree. C.), and said
total reduction ratio Tred (%) are in a relation satisfying
equation (1) as
follows:di.ltoreq.(2.65-0.003.times..theta.m).times.10.su-
p.((0.008+.theta.m/50000).times.Tred) (1)wherein, di represents the
average crystal diameter of the base steel pipe (.mu.m); .theta.m
represents the average rolling temperature (.degree. C.)
(=(.theta.i+.theta.f)/2, wherein .theta.i is a temperature of
starting rolling (.degree. C.), and .theta.f is a temperature of
finishing rolling (.degree. C.)); and Tred represents a total
reduction ratio (%) (=ODi-ODf).times.100/ODi, where, ODi is an
outer diameter of a product pipe (mm)).
2. A super fine granular steel pipe as claimed in claim 1, further
containing one or more selected from a group consisting of 1% or
less of Cu, 2% or less of Ni, 2% or less of Cr, 1% or less of Mo,
or furthermore one or more selected from a group consisting of 0.1%
or less of Nb, 0.5% or less of V, 0.2% or less of Ti, 0.005% or
less of B, or furthermore one or more selected from a group
consisting of 0.02% or less or REM, 0.01% or less of Ca.
3. A super fine granular steel pipe with high resistance against
sulfide stress corrosion crack and high workability having a
composition containing, by weight, 0.005 to 0.1%C, 0.01 to 0.5%Si,
0.01 to 1.8%Mn, 0.001 to 0.10%Al, and balance Fe with unavoidable
impurities, and a cross section perpendicular to a longitudinal
direction of the steel pipe after reducing contains super fine
grains of a ferrite having an average crystal grain size of 3 .mu.m
or less, and in a test that a tensile stress corresponding to 120%
of yield strength is applied to a C-ring test specimen in an NACE
bath, no cracks generate during a test period of 200 hr, which is
obtained in a method for producing a steel pipe, comprising heating
or soaking a base steel pipe having an outer diameter of ODi (mm)
and having ferrite grains with an average crystal diameter of di
(.mu.m) in the cross section perpendicular to the longitudinal
direction of the steel pipe, and then applying reducing at an
average rolling temperature of .theta.m(.degree. C.) and a total
reduction ratio Tred(%) to obtain a product pipe having an outer
diameter of ODf (mm), wherein, said reducing comprises performing
it in a temperature range of 400.degree. C. or more but not more
than the heating or soaking temperature, and in such a manner that
said average crystal diameter of di (.mu.m), said average rolling
temperature of .theta.m(.degree. C.), and said total reduction
ratio Tred (%) are in a relation satisfying equation (1) as
follows:di.ltoreq.(2.65-0.003.times..theta.m).times.10.su-
p.((0.008+.theta.m/50000).times.Tred) (1)wherein, di represents the
average crystal diameter of the base steel pipe (.mu.m); .theta.m
represents the average rolling temperature (.degree. C.)
(=(.theta.i+.theta.f)/2, wherein .theta.i is a temperature of
starting rolling (.degree. C.), and .theta.f is a temperature of
finishing rolling (.degree. C.)); and Tred represents a total
reduction ratio (%) (=ODi-ODf).times.100/ODi, where, ODi is an
outer diameter of a product pipe (mm)).
4. A super fine granular steel pipe as claimed in claim 3, further
containing one or more selected from a group consisting of 0.5% or
less of Cu, 0.5% or less of Ni, 0.5% or less of Cr, 0.5% or less of
Mo, furthermore one or more selected from a group consisting of
0.1% or less of Nb, 0.1% or less of V, 0.1% or less of Ti, 0.004%
or less of B, or furthermore one or more selected from a group
consisting of 0.02% or less or REM, 0.01% or less of Ca.
5. A super fine granular steel pipe with high fatigue resistance
property and high workability having a composition containing, by
weight, 0.06 to 0.30%C, 0.01 to 1.5%Si, 0.01 to 2.0%Mn, 0.001 to
0.10%Al, and balance Fe with unavoidable impurities, and a cross
section perpendicular to a longitudinal direction of the steel pipe
after reducing contains super fine grains of a ferrite having an
average crystal grain size of 3 .mu.m or less, and a fatigue
strength at a load stress for 10.sup.6 endurance cycles is not less
than 220 Mpa in a cantilever type oscillation fatigue test, which
is obtained in a method for producing a steel pipe, comprising
heating or soaking a base steel pipe having an outer diameter of
ODi (mm) and having ferrite grains with an average crystal diameter
of di (.mu.m) in the cross section perpendicular to the
longitudinal direction of the steel pipe, and then applying
reducing at an average rolling temperature of .theta.m(.degree. C.)
and a total reduction ratio Tred(%) to obtain a product pipe having
an outer diameter of ODf (mm), wherein, said reducing comprises
performing it in a temperature range of 400.degree. C. or more but
not more than the heating or soaking temperature, and in such a
manner that said average crystal diameter of di (.mu.m), said
average rolling temperature of .theta.m(.degree. C.), and said
total reduction ratio Tred (%) are in a relation satisfying
equation (1) as
follows:di.ltoreq.(2.65-0.003.times..theta.m).times.10.su-
p.((0.008+.theta.m/50000).times.Tred) (1)wherein, di represents the
average crystal diameter of the base steel pipe (.mu.m); .theta.m
represents the average rolling temperature (.degree. C.)
(=(.theta.i+.theta.f)/2, wherein .theta.i is a temperature of
starting rolling (.degree. C.), and .theta.f is a temperature of
finishing rolling (.degree. C.)); and Tred represents a total
reduction ratio (%) (=ODi-ODf).times.100/ODi, where, ODi is an
outer diameter of a product pipe (mm)).
6. A super fine granular high carbon steel pipe as claimed in claim
5, further containing one or more selected from a group consisting
of 1% or less of Cu, 2% or less of Ni, 2% or less of Cr, 1% or less
of Mo, or furthermore one or more selected from a group consisting
of 0.1% or less of Nb, 0.5% or less of V, 0.2% or less of Ti,
0.005% or less of B, or furthermore one or more selected from a
group consisting of 0.02% or less or REM, 0.01% or less of Ca.
Description
TECHNICAL FIELD OF THE INVENTION
[0001] The present invention relates to a steel pipe containing
super-fine crystal grains, which has excellent strength, toughness
and ductility and superior collision impact resistance and a method
for producing the same.
BACKGROUND ART
[0002] The strength of steel materials have been increased
heretofore by adding alloying elements such as Mn and Si, and by
utilizing, for instance, controlled rolling, controlled cooling,
thermal treatments such as quenching and tempering, or by adding
precipitation hardening elements such as Nb and V. In the case of a
steel material, however, not only strength but also high ductility
and toughness are required. Hence, a steel material with balanced
strength and ductility as well as toughness has been demanded.
[0003] The reduction in crystal size is important in that it is one
of the few means for increasing not only strength, but also both of
ductility and toughness at the same time. Crystal grains
sufficiently reduced in size can be realized by, for example, a
method which comprises preventing coarsening of austenite grains
and obtaining fine ferritic crystal grains from fine austenite
grains by utilizing the austenite--ferrite transformation; a method
which comprises obtaining fine ferrite grains from fine austenite
grains realized by working; or a method which comprises utilizing
martensite or lower bainite resulting from quenching and
tempering.
[0004] In particular, controlled rolling comprising intense working
in the austenitic region and reducing size of ferrite grains by
using the subsequent austenite--ferrite transformation is widely
utilized for the production of steel materials. Furthermore, a
method for further reducing the size of ferrite grains by adding a
trace amount of Nb and thereby suppressing the recrystallization of
austenite grains is also known in the art. By working in a
temperature in the non-recrystallizing temperature region,
austenite grains grow as to form a transgranular deformation band,
and ferrite grains generate from the deformation band as to further
reduce the size of the ferrite grains. Furthermore, controlled
cooling which comprises cooling during or after working is also
employed.
[0005] However, the fine grains available by the methods above have
lower limits in the grain size of about 4 to 5 .mu.m. Furthermore,
the methods are too complicated to be applied to the production of
steel pipes. In the light of such circumstances, a method
comprising simple process steps and yet capable of further reducing
the grain size of ferrite crystals for improving the toughness and
ductility of steel pipes has been required. Moreover, concerning
the recent increasing demand for steel pipes having superior
collision impact resistances to achieve the object of improving
safety of automobiles, limits in cutting cost has been found so
long as the methods enumerated above are employed, because they
required considerable modification in process steps inclusive of
replacing the equipment and the like.
[0006] Furthermore, the improvement in resistances against sulfide
stress corrosion cracks of steel pipes for use in line pipes, at
present, hardness control is performed to lower the concentration
of impurities and control the concentration of alloy elements.
[0007] Conventionally, fatigue resistance has been improved by
employing thermal treatments such as quench hardening and
tempering, induction hardening, and carburizing, or by adding
expensive alloy elements such as Ni, Cr, Mo, etc. in large amounts.
However, these methods has problems of impairing the weldability,
and furthermore, of increasing the cost.
[0008] A high strength steel pipe having a tensile strength of over
600 MPa is produced by using a carbon-rich material containing
carbon (C) at a concentration of 0.30% or more, or by a material
containing C at a high concentration and other alloy elements added
at large quantities. In the case of high strength steel pipes thus
increased in strength by methods above, however, the elongation
properties tend to be impaired. Thus, in general, the application
of intense working is avoided; in case intense working is
necessary, intermediate annealing is performed during working, and
further thermal treatments such as normalizing, quenching and
tempering, etc., is applied. However, the application of additional
thermal treatment such as intermediate annealing makes the process
complicated.
[0009] In the light of the circumstances above, a method which
allows intense working of high strength steel pipe without applying
intermediate annealing is demanded, and also, further reduction in
crystal grains is desired for the improvement in workability of
high strength steel pipes.
[0010] An object of the present invention is to advantageously
solve the problems above, and to provide a steel pipe improved in
ductility and collision impact resistance without incorporating
considerable change in production process. Another object of the
present invention is to provide a method for producing the same
steel. Further, another object of the present invention is to
provide a steel pipe and a method for producing the same, said
steel pipe containing super fine grains having excellent toughness
and ductility which are ferrite grains 3 .mu.m or less in size,
preferably, 2 .mu.m, and more preferably, 1 .mu.m or less in
size.
[0011] A still another object of the present invention is to
provide a high strength steel pipe containing superfine crystal
grains, which is improved in workability and having a tensile
strength of 600 MPa or more, and to a method for producing the
same.
DISCLOSURE OF THE INVENTION
[0012] The present inventors extensively and intensively performed
studies on a method of producing high strength steel pipes having
excellent ductility, yet at a high production speed. As a result,
it has been found that a highly ductile high strength steel pipe
having well-balanced strength and ductility properties can be
produced by applying reducing to a steel pipe having a specified
composition in a temperature range of ferrite recovery or
recrystallization.
[0013] First, the experimental results from which the present
invention is derived are described below.
[0014] A seam welded steel pipe (.phi.42.7 mm D.times.2.9 mm t)
having a composition of 0.09 wt % C- 0.40 wt %Si--0.80 wt %Mn--0.04
wt %Al was heated to each of the temperatures in a range of from
750 to 550.degree. C., and reducing was performed by using a
reducing mill to obtain product pipes differing in outer diameter
in a range of .phi.33.2 to 15.0 mm while setting the output speed
of drawing to 200 m/min. After rolling, the tensile strength (TS)
and elongation (E1) were measured on each of the product pipes, and
the relation between elongation and strength was shown graphically
as is shown in FIG. 1 (plotted by solid circles in the figure). In
the figure, the open circles show the relation between elongation
and strength of seam welded steel pipes of differing size which
were obtained by welding but without applying rolling.
[0015] For the values of elongation (E1), a reduced value obtained
by the following equation:
E1=E10.times.({square root}(a0/a)).sup.0.4
[0016] (where, E10 represents the observed elongation, a0 is a
value equivalent to 292 mm.sup.2, and a represents the cross
section area of the specimen (mm.sup.2)).
[0017] Referring to FIG. 1, it can be seen that higher elongation
can be obtained if the base steel pipe is subjected to reducing in
the temperature range of from 750 to 550.degree. C. as compared
with the elongation of an as-welded seam welded steel pipe at the
same strength. That is, the present inventors have been found that
a high strength steel pipe having good balance in ductility and
strength can be obtained by heating a base steel pipe having a
specified composition to a temperature range of 750 to 400.degree.
C. and applying reducing.
[0018] Furthermore, it has been found that the steel pipe produced
by the production method above contain fine ferrite grains 3 .mu.m
or less in size. To investigate the collision impact resistance
properties, the present inventors further obtained the relation
between the tensile strength (TS) and the grain size of ferrite
while greatly changing the strain rate to 2,000 s.sup.-1. As a
result, it has been found that the tensile strength considerably
increases with decreasing the ferrite grain diameter to 3 .mu.m or
less, and that the increase in TS is particularly large at the
collision impact deformation in case the strain rate is high. Thus,
it has been found additionally that the steel pipe having fine
ferrite grains exhibits not only superior balance in ductility and
strength, but also considerably improved collision impact
resistance properties.
[0019] The present invention, which enables a super fine granular
steel pipe further reduced in grain size to 1 .mu.m or less,
provides a method for producing steel comprising heating or soaking
a base steel pipe having an outer diameter of ODi (mm) and having
ferrite grains with an average crystal diameter of di (.mu.m) in
the cross section perpendicular to the longitudinal direction of
the steel pipe, and then applying drawing at an average rolling
temperature of .theta.m (.degree. C.) and a total reduction ratio
Tred (%) to obtain a product pipe having an outer diameter of ODf
(mm),
[0020] wherein, said drawing comprises performing it in the
temperature range of 400.degree. C. or more but not more than the
heating or soaking temperature, and in such a manner that said
average crystal diameter of di (.mu.m), said average rolling
temperature of .theta.m (.degree. C.), and said total reduction
ratio Tred (%) are in a relation satisfying equation (1) as
follows:
di.ltoreq.(2.65-0.003.times..theta.m).times.10.sup.((0.008+.theta.m/50000)-
.times.Tred) (1)
[0021] where, di represents the average crystal diameter of the
base steel pipe (.mu.m); .theta.m represents the average rolling
temperature (.degree. C.) (=(.theta.i+.theta.f)/ 2; where .theta. i
is the temperature of starting rolling (.degree. C.), and .theta. f
is the temperature of finishing rolling (.degree. C.)); and Tred
represents the total reduction ratio (%) (=ODi-ODf).times.100/ ODi;
where, ODi is the outer diameter of the base steel pipe (mm), and
ODf is the outer diameter of the product pipe (mm)). In the present
invention, the reducing is preferably performed in the temperature
range of from 400 to750.degree. C. It is also preferred that the
heating or soaking of the base steel pipe is performed at a
temperature not higher than the Ac.sub.3 transformation
temperature. It is further preferred that the heating or soaking of
the base steel pipe is performed at a temperature in a range
defined by (Ac.sub.1+50.degree. C.) by taking the Ac.sub.1
transformation temperature as the reference temperature.
Furthermore, the drawing is preferably performed under
lubrication.
[0022] Preferably, the reducing process is set as such that it
comprises at least one pass having a reduction ratio per pass of
6%, and that the cumulative reduction ratio is 60% or more.
[0023] Furthermore, the method for producing super fine granular
steel pipe containing super fine grains having an average grain
size of 1 .mu.m or less according to the present invention
preferably utilizes a steel pipe containing 0.70 wt % or less of C
as the base steel pipe, and it preferably a steel pipe containing
by weight, 0.005 to 0.30% C, 0.01 to 3.0% Si, 0.01 to 2.0% Mn,
0.001 to 0.10% Al, and balance Fe with unavoidable impurities. In
the present invention, furthermore, the composition above may
further contain at least one type selected from one or more groups
selected from the groups A to C shown below:
[0024] Group A: 1% or less of Cu, 2% or less of Ni, 2% or less of
Cr, and 1% or less of Mo;
[0025] Group B: 0.1% or less of Nb, 0.5% or less of V, 0.2% or less
of Ti, and 0.005% or less of B; and
[0026] Group C: 0.02% or less of REM and 0.01% or less of Ca.
[0027] Additionally, the present inventors have found that, by
restricting the composition of the base steel pipe in a proper
range, a steel pipe having high strength and toughness and yet
having superior resistance against stress corrosion cracks can be
produced by employing the above method for producing steel pipes,
and that such steel pipes can be employed advantageously as steel
pipes for line pipes.
[0028] In order to improve the stress corrosion crack resistance
properties, conventionally, steel pipes for use in line pipes have
been subjected to hardness control comprising reducing the content
of impurities such as S or controlling the alloy elements. However,
such methods had limits in improving the strength, and had problems
of increasing the cost.
[0029] By further restricting the composition of the base steel
pipe to a proper range, and by applying reducing to the base steel
pipe in the ferritic recrystallization region, fine ferrite grains
and fine carbides can be dispersed as to realize a steel pipe with
high strength and high toughness. At the same time, the alloy
elements can be controlled as such to decrease the weld hardening,
while suppressing the generation and development of cracks as to
improve the stress corrosion crack resistance.
[0030] That is, the present invention provides a steel pipe having
excellent ductility and collision impact resistance, yet improved
in stress corrosion crack resistance by applying drawing under
conditions satisfying equation (1) to abase steel pipe containing,
by weight, 0.005 to 0.10% C, 0.01 to 0.5% Si, 0.01 to 1.8% Mn,
0.001 to 0.10% Al, and further containing at least, one or more
types selected from the group consisting of 0.5% or less of Cu,
0.5% or less of Ni, 0.5% or less of Cr, and 0.5% or less of Mo; or
furthermore one or more selected from the group consisting of 0.1%
or less of Nb, 0.1% or less of V, 0.1% or less of Ti, and 0.004% or
less of B; or further additionally, one or more selected from the
group consisting of 0.02% or less of REM and 0.01% or less of
Ca;.and balance Fe with unavoidable impurities.
[0031] Furthermore, the present inventors have found that, by
restricting the composition of the base steel pipe in a further
proper range, a steel pipe having high strength and toughness, and
yet having superior fatigue resistant properties can be produced by
employing the above method for producing steel pipes, and that such
steel pipes can be employed advantageously as high fatigue strength
steel pipes.
[0032] By restricting the composition of the base steel pipe to a
proper range, and by applying drawing to the base steel pipe in the
ferritic recovery and recrystallization region, fine ferrite grains
and fine precipitates can be dispersed as to realize a steel pipe
with high strength and high toughness. At the same time, the alloy
elements can be controlled as such to decrease the weld hardening,
while suppressing the generation and development of fatigue cracks
as to improve the fatigue resistance properties.
[0033] That is, the present invention provides a steel pipe having
excellent ductility and collision impact resistance, yet improved
in fatigue resistant properties by applying drawing under
conditions satisfying equation (1) to abase steel pipe containing,
by weight, 0.06 to 0.30% C, 0.01 to 1.5% Si, 0.01 to 2.0% Mn, 0.001
to 0.10% Al, and balance Fe with unavoidable impurities.
[0034] Additionally, it is possible to obtain a high strength steel
pipe having excellent workability, characterized in that it has a
composition containing, by weight, more than 0.30% to 0.70% C, 0.01
to 2.0% Si, 0.01 to 2.0% Mn, 0.001 to 0.10% Al, and balance Fe with
unavoidable impurities, and a texture consisting of ferrite and a
second phase other than ferrite accounting for more than 30% in
area ratio, with the cross section perpendicular to the
longitudinal direction of the steel pipe containing super fine
grains of said ferrite having an average crystal grain size of 2
.mu.m or less; otherwise, with the cross section perpendicular to
the longitudinal direction of the steel pipe containing super fine
grains of said ferrite having an average crystal grain size of 1
.mu.m or less.
BRIEF DESCRIPTION OF THE DRAWINGS
[0035] FIG. 1 is a graph showing the relation between elongation
and tensile strength of the steel pipe;
[0036] FIG. 2 is a graph showing the influence of tensile strain
rate on the relation between the tensile strength and the grain
size ferrite crystals of the steel pipe;
[0037] FIG. 3 is the electron micrograph showing the metallic
texture of the steel pipe obtained as an example according to the
present invention;
[0038] FIG. 4 is a schematically drawn diagram of an example of
equipment line according to a preferred embodiment of the present
invention;
[0039] FIG. 5 is a schematically drawn diagram of an example of a
production equipment for solid state pressure welded steel pipes
and a production line for continuous production according to a
preferred embodiment of the present invention;
[0040] FIG. 6 is a graph showing the relation between the total
reduction ratio and the average crystal grain size of the base
steel pipe, which are the parameters that affect the size reduction
of crystal grains of the product pipe; and
[0041] FIG. 7 is a schematically drawn explanatory diagram showing
the shape of the test specimen for use in sulfide stress corrosion
crack resistance test.
[0042] (Explanation of Symbols)
[0043] 1 Flat strip
[0044] 2 Pre-heating furnace
[0045] 3 Forming and working apparatus
[0046] 4 Induction heating apparatus for pre-heating edges
[0047] 5 Induction heating apparatus for heating edges
[0048] 6 Squeeze roll
[0049] 7 Open pipe
[0050] 8 Base steel pipe
[0051] 14 Uncoiler
[0052] 15 Joining apparatus
[0053] 16 Product pipe
[0054] 17 Looper
[0055] 18 Cutter
[0056] 19 Pipe straightening apparatus
[0057] 20 Thermometer
[0058] 21 Reducing mill
[0059] 22 Soaking furnace (seam cooling and pipe heating
apparatus)
[0060] 23 Descaling apparatus
[0061] 24 Quenching apparatus
[0062] 25 Re-heating apparatus
[0063] 26 Cooling apparatus
BEST MODE FOR CARRYING OUT THE INVENTION
[0064] In the present invention, a steel pipe is used as the
starting material. There is no particular limitation concerning the
method for producing the base steel pipe. Thus, favorably
employable is an electric resistance welded steel pipe (seam welded
steel pipe) using electric resistance welding, a solid state
pressure welded steel pipe obtained by heating the both edge
portions of an open pipe to a temperature region of solid state
pressure welding and effecting pressure welding, a forge welded
steel pipe, or a seamless steel pipe obtained by using Mannesmann
piercer.
[0065] The chemical composition of the base steel pipe or product
steel pipe is limited in accordance with the following reasons. C:
0.07% or less:
[0066] Carbon is an element to increase the strength of steel by
forming solid solution with the matrix or by precipitating as a
carbide in the matrix. It also precipitates as a hard second phase
in the form of fine cementite, martensite, or bainite, and
contributes in increasing ductility (uniform elongation). To
achieve a desired strength and to obtain the effect of improved
ductility by utilizing cementite and the like precipitated as the
second phase, C must be present at a concentration of 0.005% or
more, and preferably, 0.04% or more. Preferably, the concentration
of C is in a range not more than 0.30%, and more preferably, 0.10%
or less. In view of these requirements, the concentration of C is
preferably confined in a range of from 0.005 to 0.30%, and more
preferably, in a range of from 0.04 to 0.30%.
[0067] To improve the stress corrosion crack resistance of the
steel pipe to make it suitable for use in line pipes, the
concentration of C is preferably controlled to a range of 0.10% or
less. If the concentration exceeds 0.10%, the stress corrosion
crack resistance decreases due to the hardening of the welded
portion.
[0068] To improve the fatigue resistance properties of the steel
pipe to make it suitable for use as a high fatigue strength steel
pipe, the concentration of C is preferably controlled to a range of
from 0.06 to 0.30%. If the concentration is lower than 0.06%, the
fatigue resistance properties decrease due to insufficiently low
strength.
[0069] To achieve a desired strength of 600 MPa or more, the
concentration of C must exceed 0.30%. However, if C should be
incorporated at a concentration exceeding 0.70%, the ductility is
inversely impaired. Thus, the concentration of C should be in a
range exceeding 0.30% but not more than 0.70%. Si: 0.01 to
3.0%:
[0070] Silicon functions as a deoxidizing element, and it increases
the strength of the steel by forming solid solution with the
matrix. This effect is observed in case Si is added at a
concentration of at 0.01% or more, preferably at 0.1% or more, but
an addition in excess of 3.0% impairs ductility. In case of high
strength steel pipe, the upper limit in concentration is set at
2.0% by taking the problem of ductility into consideration. Thus,
the concentration of Si is set in a range of from 0.01 to 3.0%, or
of from 0.01 to 2.0%. Preferably, however, the range is from 0.1 to
1.5%.
[0071] To improve the stress corrosion crack resistance of the
steel pipe to make it suitable for use in line pipes, the
concentration of Si is preferably controlled to 0.5% or less. If
the concentration exceeds 0.5%, the stress corrosion crack
resistance decreases due to the hardening of the welded
portion.
[0072] To improve the fatigue resistance properties of the steel
pipe to make it suitable for use as a high fatigue strength steel
pipe, the concentration of Si is preferably controlled to 1.5% or
less. If the concentration exceeds 1.5%, the fatigue resistance
properties decrease due to the formation of inclusions. Mn:. 0.01
to 2.0%:
[0073] Manganese increases the strength of steel, and accelerates
the precipitation of a second phase in the form of fine cementite,
or martensite and bainite. If the concentration is less than 0.01%,
not only it becomes impossible to achieve the desired strength, but
also fine precipitation of cementite or the precipitation of
martensite and bainite is impaired. If the addition should exceed
2.0%, the strength of the steel is excessively increased to
inversely impair ductility. Thus, the concentration of Mn is
limited in a range of from 0.01 to 2.0%. From the viewpoint of
realizing balance strength and elongation, the concentration of Mn
is preferably is in a range of from 0.2 to. 1.3%, and more
preferably, in a range of from 0.6 to 1.3%.
[0074] To improve the stress corrosion crack resistance of the
steel pipe to make it suitable for use in line pipes, the
concentration of Mn is preferably controlled to 1.8% or less. If
the concentration exceeds 1.8%, the stress corrosion crack
resistance decreases due to the hardening of the welded portion.
Al: 0.001 to 0.10%:
[0075] Aluminum provides fine crystal grains. To obtain such fine
crystal grains, Al should be added at a concentration of at least
0.001%. However, an addition in excess of 0.10% increases
oxygen-containing inclusions which impair the clarity. Thus, the
concentration of Al is set in a range of from 0.001 to 0.10%, and
preferably, in a range of from 0.015 to 0.06%. In addition to the
basic steel composition above, at least one type of an alloy
element selected from one or more groups of A to C below may be
added.
[0076] Group A: Cu: 1% or less, Ni: 2% or less, Cr: 2% or less, and
Mo: 1% or less:
[0077] Any element selected from the group of Cu, Ni, Cr, and Mo
improves the quenching property of the steel, and increase the
strength. Thus, one or two or more elements can be added depending
on the requirements. These elements lowers the transformation
point, and effectively generate fine grains of ferrite or of second
phase. However, the upper limit for the concentration of Cu is set
at 1%, because Cu incorporated in a large quantity impairs the hot
workability. Ni increases not only the strength, but also
toughness. However, the effect of Ni saturates at an addition in
excess of 2%, and an addition in excess increases the cost. Hence,
the upper concentration limit is set at 2%. The addition of Cr or
Mo in large quantities not only impairs the weldability, but also
increases the total expense. Thus, their upper limits are set to 2%
and 1%, respectively.
[0078] Preferably, the concentration range for the elements in
Group A is from 0.1 to 0.6% for Cu, from 0.1 to 1.0% for Ni, from
0.1 to 1.5% for Cr, and from 0.05 to 0.5% for Mo.
[0079] To make the steel pipes useful for line pipes by improving
the resistance against stress corrosion cracks, the concentration
of Cu, Ni, Cr, and Mo is each restricted to be 0.5% or lower. If
any of them is added in large quantities as to exceed the
concentration of 0.5%, hardening occurs on the welded portion as to
degrade the stress corrosion crack resistance.
[0080] Group B: Nb: 0.1% or less, V: 0.5% or less, Ti: 0.2% or
less, and
[0081] B: 0.005% or less:
[0082] Any element of the group consisting of Nb, V, Ti, and B
precipitates as a carbide, a nitride, or a carbonitride, and
contributes to the production of fine crystal grains and to a
higher strength. In particular, for steel pipes which have joints
and which are heated to high temperatures, these elements function
effectively in producing fine crystal grains during heating for
joining, or as precipitation nuclei for ferrite during cooling.
They are therefore effective in preventing hardening at joint
portions. Thus, one or two or more elements can be added depending
on the requirements. However, since their addition in large
quantities leads to the degradation in weldability and toughness,
the upper limits for the concentration of the elements are set as
follows: 0.1% for Nb; 0.5%, preferably 0.3% for V; 0.2% for Ti; and
0.005%, preferably 0.004% for B. More preferably, the concentration
range for the elements in Group B is from 0.005 to 0.05% for Nb,
0.05 to 0.1% for V, from 0.005 to 0.10% for Ti, and from 0.0005 to
0.002% for B.
[0083] To make the steel pipes useful for line pipes by improving
the resistance against stress corrosion cracks, the concentration
of Nb, V, and Ti is each restricted to be 0.1% or lower. If any of
them should be added in large quantities as to exceed the
concentration of 0.1%, hardening occurs on the welded portion as to
degrade the stress corrosion crack resistance.
[0084] Group C: REM: 0.02% or less, and Ca: 0.01% or less:
[0085] REM and calcium Ca control the shape of inclusions and
improve the workability. Any element of this group precipitates as
a sulfide, an oxide, or a sulfate, and prevents hardening from
occurring on the joint portions of steel pipes. Thus, one or more
elements can be added depending on the requirements. However, if
the addition should exceed the limits of 0.02% for REM and 0.01%
for Ca, too many inclusions form as to lower clarity, and
degradation in ductility occurs as a result. It should be noted
that an addition of less than 0.004% for REM, or an addition of
less than 0.001% of Ca exhibits small effect. Hence, it is
preferred that REM are added as such to give a concentration of
0.004% or more, and that Ca is added to 0.001% or more.
[0086] The base steel pipes and product steel pipes contain, in
addition to the components described above, balance Fe with
unavoidable impurities. Allowable as the unavoidable impurities are
0.010% or less of N, 0.006% or less of O, 0.025% or less of P, and
0.020% or less of S.
[0087] N: 0.010% or less:
[0088] Ni is allowed to a concentration of 0.010%; a quantity
necessary to be combined with Al to produce fine crystal grains.
However, an incorporation thereof in excess of this limit impairs
the ductility. Hence, it is preferred that the concentration of N
is lowered to 0.010% or lower, and more preferably, the
concentration thereof is controlled to be in a range of from 0.002
to 0.006%.
[0089] O: 0.006% or less:
[0090] O impairs clarity by forming oxides. Their incorporation is
not desirable, and its allowable limit is 0.006%.
[0091] P: 0.025% or less:
[0092] P is preferably not incorporated, because it impairs the
toughness by segregation in grain boundaries. The allowable limit
thereof is 0.025%.
[0093] S: 0.020% or less:
[0094] S is preferably not incorporated, because it increases
sulfides and leads to the degradation of clarity. The allowable
limit thereof is 0.020%.
[0095] Description on the structure of the product pipes is given
below.
[0096] 1) The steel pipe according to the present invention has
excellent ductility and collision impact resistance properties, and
comprises a texture based on ferrite grains having an average
crystal diameter of 3 .mu.m or less.
[0097] If the size of the ferrite grains exceeds 3 .mu.m, no
apparent improvement can be obtained in ductility as well as in
collision impact resistance properties i.e., the resistance
properties against impact weight. Preferably, the average crystal
size of ferrite grains is 1 .mu.m or less.
[0098] The average crystal diameter of the ferrite grains in the
present invention is obtained by observation under an optical
microscope or an electron microscope. More specifically, a cross
section obtained by cutting the steel pipe perpendicular to the
longitudinal direction thereof, and the observation was made on the
etched surface using Nital etchant. Thus, the diameter of the
equivalent circle was obtained for 200 or more grains, and the
average thereof was used as the representative value.
[0099] The structure based on ferrite grains as referred in the
present invention includes a structure containing solely ferrite
and having no precipitation of a second phase, and a structure
containing ferrite and a second phase other than ferrite.
[0100] Mentioned as the second phase other than ferrite are
martensite, bainite, and cementite, which may precipitate alone or
as a composite of two or more thereof. The area ratio of the second
phase should account for 30% or less. The second phase thus
precipitated contributes to the increase in uniform elongation in
case of deformation. Thus, it improves the ductility and the
collision impact resistance properties. However, such an effect
becomes less apparent if the area ratio of the second phase exceeds
30%.
[0101] 2) The high strength steel pipe according to the present
invention comprises a structure based on ferrite and a second phase
accounting for more than30% in area ratio, and contains grains
having an average crystal diameter of 2 .mu.m or less as observed
on a cross section cut perpendicular to the longitudinal direction
of the steel pipe. As the second phase other than ferrite,
mentioned are martensite, bainite, and cementite, which may
precipitate alone or as a composite of two or more thereof. The
area ratio of the second phase should account for more than 30%.
The second phase thus precipitated contributes to the increase in
strength and in uniform elongation as to improve the strength and
ductility. However, such an effect is small if the area ratio of
the second phase is 30% or less. The area ratio of the second phase
other than ferrite is therefore preferred to be more than 30% but
not more than 60%. If the area ratio should exceed 60%, the
ductility is impaired due to the coarsening of cementite
grains.
[0102] If the average crystal diameter should exceed 2 .mu.m,
distinct improvement in ductility is no longer observed, and hence,
there is no apparent improvement in the workability. Preferably,
the average grain diameter of ferrite is 1 .mu.m or less.
[0103] The average crystal grain diameter according to the present
invention was obtained by observation under an optical microscope
or an electron microscope. More specifically, a cross section
obtained by cutting the steel pipe perpendicular to the
longitudinal direction thereof, and the observation was made on the
etched surface using Nital etchant. Thus, the diameter of the
equivalent circle was obtained for 200 or more grains, and the
average thereof was used as the representative value. The grain
diameter of the second phase is obtained by taking the boundary of
pearlite colony as the grain boundary in case pearlite is the
second phase, and, by taking the packet boundary as the grain
boundary in case bainite or martensite is the second phase.
[0104] An example of the steel pipe according to the present
invention is given in FIG. 3.
[0105] The method of producing the steel pipe according to the
present invention is described below.
[0106] The base steel pipe of the composition described above is
heated in a temperature range of Ac.sub.3 to 400.degree. C.,
preferably, to a range of (Ac.sub.1+50.degree. C.) to 400.degree.
C., and more preferably, to a range of 750 to 400.degree. C.
[0107] If the heating temperature exceeds the AC.sub.3
transformation point, not only degradation of the surface
properties, but also the coarsening of crystal grains occurs.
Accordingly, the heating temperature for the base steel pipe is
preferably set at a temperature not higher than the Ac.sub.3
transformation point, preferably, not higher than the
(Ac.sub.1+50.degree. C.), and more preferably, not higher than
750.degree. C. On the other hand, if the heating temperature is
lower than 400.degree. C., a favorable rolling temperature cannot
be realized. Thus, the heating temperature is preferably not lower
than 400.degree. C.
[0108] Then, the heated base steel pipe is subjected to
drawing.
[0109] Although not limiting, drawing is preferably performed by
using a three-roll type reducing mill. The reducing mill preferably
comprises a plurality of stands, such that rolling is performed
continuously. The number of stands can be determined depending on
the size of the base steel pipe and the product steel pipe.
[0110] The rolling temperature for reducing is in a range
corresponding to the ferrite recovery and recrystallization
temperature range, i.e., from Ac.sub.3 to 400.degree. C., but
preferably, in a range of (Ac.sub.1+50.degree. C.) to 400.degree.
C., and more preferably, in a range of from 750 to 400.degree. C.
If the rolling temperature should exceed the Ac.sub.3
transformation point, no super fine crystal grains would become
available, and ductility does not increase as expected in the
expense of decreasing strength. Thus, the rolling temperature is
set at a temperature not higher than Ac.sub.3 transformation point,
preferably, at a temperature not higher than (Ac.sub.1+50.degree.
C.) and more preferably, not higher than 750.degree. C. If the
rolling temperature should be lower than 400.degree. C., on the
other hand, the material becomes brittle due to blue shortness
(brittleness), and may undergo breakage.
[0111] Furthermore, at rolling temperatures lower than 400.degree.
C., not only the deformation resistance of the material increases
as to make the rolling difficult, but also the working strain tends
to remain due to insufficient recovery and recrystallization of the
material. Thus, the drawing is performed in a limited temperature
range of from Ac.sub.3 to 400.degree. C., preferably, in a range of
(Ac.sub.1+50.degree. C.) to 400.degree. C., and more preferably, in
a range of from 750 to 400.degree. C. Most preferably, the
temperature range is from 600 to 700.degree. C.
[0112] The cumulative reduction ratio in diameter during drawing is
set at 20% or higher.
[0113] If the cumulative reduction ratio in diameter, which is
equivalent to {[(outer diameter of the base steel pipe)-(outer
diameter of the product pipe)]/(outer diameter of the base steel
pipe).times.100}, should be lower than 20%, the crystal grains
subjected to recovery and recrystallization tend to be
insufficiently reduced in size. Such a steel pipe cannot exhibit
superior ductility. Furthermore, the production efficiency becomes
low due to the low rate of pipe production. Accordingly, in the
present invention, the cumulative reduction ratio in diameter is
set at 20% or higher. However, at a cumulative reduction ratio of
60% or higher, not only an increase in strength due to work
hardening occurs, but also fine structure becomes prominent. Thus,
even in a steel pipe having a component system containing the alloy
elements at a lower concentration than the aforementioned
composition range, well balanced strength and ductility can be
imparted thereto. It can be understood therefrom that, more
preferably, the cumulative reduction ratio in diameter is set at
60% or higher.
[0114] In performing drawing, it is preferred that the rolling
comprises at least one pass having a diameter reduction ratio per
pass of 6% or higher.
[0115] If the diameter reduction ratio per pass during drawing
should be set lower than 6%, fine crystal grains which result from
recovery and recrystallization processes tend to be insufficiently
reduced in size. On the other hand, with a diameter reduction ratio
per pass of 6% or higher, an elevation in temperature occurs by the
heat of working, which prevents the drop in temperature from
occurring. Thus, the diameter reduction ratio per pass is
preferably set at 8% or higher, so that high effect is obtained in
realizing finer crystal grains.
[0116] The drawing process of the steel pipe according to the
present invention realizes a rolling under biaxial strain, which is
particularly effective in obtaining fine crystal grains. In
contrast to this, the rolling of a steel sheet is under uniaxial
strain because free end is present in the direction of sheet width
(i.e., in the direction perpendicular to the rolling direction).
Thus, the reduction in grain size becomes limited.
[0117] In the present invention, it is preferred that drawing is
performed under lubricating conditions, By performing the drawing
under lubrication, the strain distribution in the thickness
direction becomes uniform that the distribution of crystal size
distribution also becomes uniform in the thickness direction. If
non-lubricating rolling should be performed, strain concentrates
only on the surface layer portion of the material as to disturb the
uniformity of the crystal grains in the thickness direction. The
lubricating rolling can be carried out by using a rolling oil well
known in the art, for instance, a mineral oil or a mineral oil
mixed with a synthetic ester can be used without any
limitations.
[0118] After reducing, the steel material is cooled to room
temperature. Cooling can be performed by using air cooling, but
from the viewpoint of suppressing the grain growth as much as
possible, any of the cooling methods known in the art, for
instance, water cooling, mist cooling, or forced air cooling, is
applicable. The cooling rate is 1.degree. C./sec or more, and
preferably, 10.degree. C./sec or more. Furthermore, stepwise
cooling such as holding in the midway of cooling, can be employed
depending on the requirements on the properties of the product.
[0119] In the method according to the present invention, drawing as
described below can be applied to the base steel pipe by stably
maintaining the crystal grain diameter of the product pipe to 1
.mu.m or less, or to 2 .mu.m or less in case of a high strength
steel pipe.
[0120] Let the average crystal grain diameter of the ferrite
grains, or, of that inclusive of the second phase in case of a high
strength steel pipe, be di (.mu.m), as observed in the cross
section cut perpendicular to the longitudinal direction of the
steel pipe at an outer diameter of ODi (mm). The base steel pipe is
then heated or soaked, and is subjected to drawing at an average
rolling temperature of .theta.m (.degree. C.) and at a total
reduction ratio in diameter of Tred (%) as to obtain a finished
product pipe having an outer diameter of ODf (mm).
[0121] The reducing is preferably applied by using a plurality of
pass rollers called a reducer. An example of an equipment line
suitable for carrying out the present invention is shown in FIG. 4.
In FIG. 4 is shown a rolling apparatus 21 comprising a plurality of
stands having a pass. The number of stands of the rolling mill is
determined properly depending on the combination in the diameter of
the base steel pipe and the product pipe. For the pass rolls, any
type selected from the rolls well known in the art,, for instance,
two rolls, three rolls, or four rolls, can be favorably
applied.
[0122] There is no particular limitation concerning the heating or
soaking method, however, it is preferred that heating using a
heating furnace or induction heating is employed. In particular,
induction heating method is preferred from the viewpoint of high
heating rate and of high productivity, or from the viewpoint of its
ability of suppressing the growth of crystal grains. (In FIG. 4 is
shown a re-heating apparatus 25 of an induction heating type.) The
heating or soaking is performed at a temperature not higher than
the Ac.sub.3 transformation point corresponding to a temperature
range at which no coarsening of crystal grain occurs, or, at a
temperature not higher than (Ac.sub.1+50.degree. C.), by taking the
Ac.sub.1 transformation point of the base steel pipe as the
standard, or more preferably, in the temperature range of from 600
to 700.degree. C. In the present invention, as a matter of course,
the product pipe results with fine crystal grains even if the
heating or soaking of the base steel pipe should be performed at a
temperature deviating from the temperature range above.
[0123] In case the second phase in the texture of the base steel
pipe is pearlite, layered cementite incorporated in pearlite
undergoes size reduction by separation by performing rolling in the
temperature range above. Thus, the workability of the product pipe
is improved because better elongation properties are acquired.
Similarly, in case the second phase in the structure of the base
steel pipe is bainite, the bainite undergoes recrystallization
after working as to form a fine bainitic ferrite structure. Thus,
the workability of the product pipe is improved because of the
improved elongation properties.
[0124] The reducing is performed at a temperature range of
400.degree. C. or more but not more than the heating or soaking
temperature. Preferably, the temperature is not higher than
750.degree. C. The temperature region over the Ac.sub.3
transformation point, or over (Ac.sub.1+50.degree. C.), or over
750.degree. C., corresponds to the ferrite-austenite two-phase
region rich in austenite, or a single phase region of austenite.
Thus, it is difficult to obtain a ferritic texture or a texture
based on ferrite by working. Moreover, the effect of producing fine
crystal grains by ferritic working cannot be fully exhibited. If
drawing should be carried out at a temperature higher than
750.degree. C., ferrite grains grow considerably after
recrystallization as to make it difficult to obtain fine grains. In
case drawing is performed at a temperature lower than 400.degree.
C., on the other hand, difficulties are found in carrying out the
drawing because the temperature range corresponds to the blue
brittleness region, or ductility and toughness decrease because
working stress tends to remain due to insufficient
recrystallization. Thus, drawing temperature is set at a
temperature not lower than 400.degree. C. but not higher than the
Ac.sub.3 transformation point, or at a temperature not higher than
(Ac.sub.1+50.degree. C.), and preferably, at a temperature not
higher than 750.degree. C. More preferably, the temperature range
is from 560 to 720.degree. C., and most preferably, from 600 to
700.degree. C.
[0125] The reducing is performed in the temperature range described
above, and under the conditions satisfying equation (1), where di
(.mu.m) represents the average ferrite crystal diameter as observed
in the cross section perpendicular to the longitudinal direction of
the base steel pipe; .theta.m (.degree. C.) represents the average
rolling temperature in the drawing; and Tred (%) represents the
total reduction ratio.
[0126] In case di, .theta.m, and Tred do not satisfy the relation
expressed by equation (1), the ferrite crystals of the resulting
product pipe cannot be micro-grained as such to yield an average
diameter (diameter as observed in the cross section perpendicular
to the longitudinal direction of the steel pipe) of 1 .mu.m or
less. Similarly, the resulting high strength steel pipe cannot
yield micro-grains as such having an average diameter (diameter as
observed in the cross section perpendicular to the longitudinal
direction of the steel pipe) of 2 .mu.m or less.
[0127] Product steel pipes differing in diameter were produced by
rolling a JIS STKM 13A equivalent base steel pipe (having an ODi of
60.3 mm and a wall thickness of 3.5 mm) by using a rolling
apparatus consisting of serially connected 22 stands of 4-roll
rolling mill, and under the conditions of an output speed is 200
m/min, an average rolling temperature of 550 or 700.degree. C. The
influence of the total reduction ratio in diameter and the average
crystal diameter of the base steel pipe on the crystal grain
diameter of the finished product pipe is shown in FIG. 6. The
conditions shown by the hatched region satisfy the relation
expressed by equation (1), and the base steel pipes with conditions
falling in this region are capable of providing product pipes
comprising crystal grains 1 .mu.m or less in diameter.
[0128] After rolling, a product pipe 16 is preferably cooled to a
temperature of 300.degree. C. or lower. The cooling can be
performed by air cooling, but with an aim to suppress the grain
growth as much as possible, any of the cooling methods known in the
art, for instance, water cooling, mist cooling, or forced air
cooling, can be applied by using a quenching apparatus 24. The
cooling rate is 1.degree. C./sec or higher, and preferably,
10.degree. C./sec or higher.
[0129] In the present invention, a cooling apparatus 26 may be
installed on the input side of a rolling apparatus 21, or in the
midway of the rolling apparatus 21 to control the temperature.
Furthermore, a descaling apparatus 23 may be provided on the input
side of the rolling apparatus 21.
[0130] The base steel pipe for use as the starting material in the
present invention may be any steel pipe selected from a seamless
steel pipe, a seam welded steel pipe, a forge welded steel pipe, a
solid pressure welded steel pipe, and the like. Furthermore, the
production line of the super fine granular steel pipe according to
the present invention may be connected to the production line for
the base steel pipe described hereinbefore. An example of
connecting the production line to the production line of the solid
pressure welded steel pipe is shown in FIG. 5.
[0131] A flat strip 1 output from an uncoiler 14 is connected to a
preceding hoop by using a joining apparatus 15, and after being
preheated by a pre-heating furnace 2 via a looper 17, it is worked
into an open pipe 7 by using a forming apparatus 3 composed of a
plurality of forming rolls. The edge portion of the open pipe 7
thus obtained is heated to a temperature region lower than the
fusion point by an edge preheating induction heating apparatus 4
and an edge heating induction heating apparatus 5, and is butt
welded by using a squeeze roll 6 to obtain a base steel pipe 8.
[0132] Then, as described above, the base steel pipe 8 is heated or
soaked to a predetermined temperature by using a soaking furnace
22, descaled by a descaling apparatus 23, rolled by using a rolling
apparatus 21, cut by a cutter, and straightened by a pipe
straightening apparatus 19 to finally provide a product pipe 16.
The temperature of the steel pipe is measured by using a
thermometer 20.
[0133] Similarly in the case of drawing, as described above,
rolling is preferably performed under lubrication.
[0134] Thus, in accordance with the production method described
above, a steel pipe consisting of super-fine ferrite grains 1 .mu.m
or less in average crystal grain size as observed in the cross
section cut perpendicular to the longitudinal direction of the
steel material can be obtained. Furthermore, the production method
above is effective in producing steel pipes, such as seam welded
steel pipes, forge welded steel pipes, solid pressure welded steel
pipes, etc., having a uniform hardness in the seam portion.
[0135] It is also possible to produce, without performing an
intermediate annealing, a high strength steel pipe having a texture
comprising ferrite and a second phase other than ferrite accounting
for more than 30% in area ratio, and yet consisting of super-fine
ferrite grains 2 .mu.m or less in average crystal grain size as
observed in the cross section cut perpendicular to the longitudinal
direction of the steel material.
EXAMPLE 1
[0136] Base steel pipes whose chemical composition is shown in
Table 1 were each heated to temperatures given in Table 2 by using
an induction heating coil, and, by using three-roll structure
rolling mills, they were rolled under conditions shown in Table 2
to provide product pipes. In Table 2, a solid state pressure welded
steel pipe was obtained by pre-heating a 2.6 mm thick hot rolled
flat strip to 600.degree. C., continuously forming the resulting
flat strip into an open pipe by using a plurality of rolls,
pre-heating the both edge portions of the open pipe 1,000.degree.
C. by means of induction heating, and further heating the both edge
portions to the non-melting temperature region of 1,450.degree. C.
by induction furnace, at which the both ends were butted by using a
squeeze roll, where solid phase pressure welding was carried out.
Thus was obtained a steel pipe 42.7 mm in diameter and 2.6 mm in
thickness. On the other hand, a seamless steel pipe was produced by
heating a continuously cast billet, followed by producing a pipe by
using a Mannesmann mandrel type mill.
[0137] Tensile properties, collision impact properties, and
structure of the product pipes were investigated, and the results
are given in Table 2. Tensile properties were measured-on a JIS No.
11 test piece. Yield stress was obtained by taking the lower yield
point in case the yield phenomenon is clearly observed, but 0.2% PS
was used for the other cases.
[0138] For the value of elongation, a reduced value was obtained in
accordance with the following equation by taking the size effect of
the test piece into consideration:
E1=E10.times.({square root}(a0/a)).sup.0.4
[0139] (where, E10 represents the observed elongation, a0 is a
value equivalent to 292 mm.sup.2, and a represents the cross
section area of the specimen (mm.sup.2)).
[0140] The collision impact properties were obtained by performing
high speed tensile tests at a strain rate of 2,000 s.sup.-1. Then,
the absorbed energy up to a strain of 30% was obtained from the
observed stress--strain curve to use as the collision impact
absorption energy for evaluation.
[0141] The collision impact property is represented by a
deformation energy of a material at a strain rate of from 1,000 to
2,000 s.sup.-1 practically corresponding to the collision of an
automobile, and is superior for a higher value.
[0142] From Table 2, it can be understood that the specimens
falling in the scope of the present invention (Nos. 1 to 16 and
Nos. 19 to 22) exhibit excellent balance in ductility and strength.
Moreover, high tensile strength is observed for these specimens
having higher strain rate, and these specimens are also high in
collision impact absorption energy. On the other hand, the
specimens falling out of the scope of claims according to the
present invention, i.e., Comparative Examples No. 17, No. 18, and
No.23, suffer low values for either ductility or strength. These
specimens suffer not only poor balance in strength--ductility, but
also low collision impact property.
[0143] Comparative Example Nos. 17 and 18 furthermore yield a
reduction ratio falling outside the range according to the present
invention, show coarsening in ferrite grains, and suffer poor
balance in strength--ductility and low collision impact absorption
energy.
EXAMPLE 2
[0144] Base steel pipes whose chemical composition is shown in
Table 3 were each heated to temperatures given in Table 4 by using
an induction heating coil, and, by using three-roll structure
rolling mills, they were rolled under conditions shown in Table 4
to provide product pipes. The base steel pipes were produced in the
same procedure as that described in Example 1.
[0145] Tensile properties, collision impact properties, and
structure of the product pipes were investigated in the same manner
as in the Example, and the results are given in Table 4.
[0146] From Table 4, it can be understood that the specimens
falling in the scope of the present invention (Nos. 2-1 to 2-3,
Nos. 2-6 to 2-8, and Nos. 2-10 to Nos. 2-14) exhibit excellent
balance in ductility and strength. Moreover, high tensile strength
is observed for these specimens with higher strain rate, and these
specimens are also high in collision impact absorption energy. On
the other hand, the specimens falling out of the scope according to
the present invention, i.e., Comparative Examples No. 2-4,No. 2-5,
and No. 2-9, suffer low values for either ductility or strength.
These specimens suffer not only poor balance in
strength--ductility, but also low collision impact property.
[0147] The present invention provides steel pipes having not only a
never achieved good balance in ductility and strength, but also
excellent collision impact resistance properties. Furthermore, the
steel pipes according to the present invention exhibit superior
properties in secondary working, for instance, bulging such as
hydroforming, and are therefore suitable for use in bulging.
[0148] Among the steel pipes according to the present invention,
the welded steel pipes (seam welded steel pipes) and the solid
phase pressure welded steel pipes subjected to seam cooling yield a
hardened seam portion having a hardness at the same level as that
of the mother pipe after rolling, and show further distinguished
improvement in bulging.
EXAMPLE 3
[0149] Base steel pipes whose chemical composition is shown in
Table 5 were each heated to temperatures given in Table 6 by using
an induction heating coil, and, by using three-roll structure
rolling mills, they were rolled under conditions shown in Table 6
to provide product pipes. The base steel pipes 110 mm in diameter
and 4.5 mm in thickness were produced from hot rolled sheet steel
produced by controlled rolling and controlled cooling.
[0150] Tensile properties, collision impact properties, the
structure of the product pipes, and sulfide stress corrosion crack
resistance were investigated, and the results are given in Table 6.
Similar to Example 1, tensile properties were measured on a JIS No.
11 test piece. For the elongation, a reduced value was obtained in
accordance with the following equation by taking the size effect of
the test piece into consideration: E1=E10.times.({square
root}(a0/a).sup.0.4 (where, E10 represents the observed elongation,
a0 is a value equivalent to 292 mm.sup.2, and a represents the
cross section area of the specimen (mm.sup.2)).
[0151] Similar to Example 1 again, the collision impact properties
were obtained by performing high speed tensile tests at a strain
rate of 2,000 s.sup.-1. Then, the absorbed energy up to a strain of
30% was obtained from the observed stress--strain curve to use as
the collision impact absorption energy for evaluation.
[0152] The collision impact property is represented by a
deformation energy of a material at a strain rate of from 1,000 to
2,000 s.sup.-1 practically corresponding to the collision of an
automobile, and is ~superior for a higher value.
[0153] The sulfide stress corrosion crack resistance was evaluated
on a C-ring test specimen shown in FIG. 7. Thus, a tensile stress
corresponding to 120% of the yield strength was applied to the
specimen in an NACE bath (containing 0.5% acetic acid and 5% brine
water, saturated with H.sub.2S, and at a temperature of 25.degree.
C. and a pressure of 1 atm) to investigate whether cracks generated
or not during a test period of 200 hr. The C-ring specimens were
cut out from the mother body of the product tube in the T direction
(the circumferential direction). The test was performed on 2 pieces
each under the same condition.
[0154] From Table 6, it can be understood that the specimens
falling in the scope of the present invention (Nos. 3-1 to 3-3,
Nos. 3-5 to 3-8, No. 3-10, and No. 3-12) exhibit excellent balance
in ductility and strength. Moreover, high tensile strength is
observed for these specimens having higher strain rate, and these
specimens are also high in collision impact absorption energy.
Furthermore, they have excellent resistance against sulfide stress
corrosion cracks, and are therefore superior when used in line
pipes. On the other hand, the specimens failing out of the scope
according to the present invention, i.e., Comparative Examples No.
3-4, No. 3-9, and No. 3-11, suffer low values for either ductility
or strength. These specimens suffer not only poor balance in
strength--ductility, but also low collision impact property.
Furthermore, breakage was found to occur on these specimens in the
NACE bath, showing degradation in sulfide stress corrosion crack
resistance.
[0155] Comparative Example No. 3-4 yields a reduction ratio falling
outside the range according to the present invention, shows
coarsening in ferrite grains, suffers poor balance in
strength--ductility and low collision impact absorption energy, and
exhibits an impaired sulfide stress corrosion crack resistance.
[0156] Comparative Example No. 3-9 and No. 3-11 are produced at a
rolling temperature falling out of the range according to the
present invention. Hence, they show coarsening in ferrite grains,
suffer poor balance in strength--ductility and low collision impact
absorption energy, and exhibit impaired sulfide stress corrosion
crack resistance.
EXAMPLE 4
[0157] Base steel pipes whose chemical composition is shown in
Table 7 were each heated to temperatures given in Table 8 by using
an induction heating coil, and, by using three-roll structure
rolling mills, they were rolled under conditions shown in Table 8
to provide product pipes. The base steel pipes for use in the
present example were produced by first forming a hot rolled hoop
using a plurality of, forming rolls to obtain open pipes. Then,
seam welded steel pipes 110 mm in diameter and 2.0 mm in thickness
were produced by welding the both edges of each of the resulting
open pipes using induction heating. Otherwise, seamless pipes 110
mm in diameter and 3.0 mm in thickness were produced by heating the
continuously cast billets, and then producing pipes therefrom by
using a Mannesmann mandrel type mill.
[0158] Tensile properties, collision impact properties, the
structure, and the fatigue resistance properties of the product
pipes were investigated, and the results are given in Table 8.
Tensile properties, collision impact, properties, and the structure
were evaluated in the same manner as in Example 1.
[0159] For the fatigue properties, the product pipes were used as
they are for the test specimens, to which cantilever type
oscillation fatigue test was performed (oscillation speed: 20 Hz).
Thus, fatigue strength was obtained.
[0160] From Table 8, it can be understood that the specimens
falling in the scope the present invention (No. 4-1, No. 4-3, and
Nos. 4-6 to 4-9) exhibit excellent balance in ductility and
strength. Moreover, high tensile strength is observed for these
specimens with higher strain rate, and these specimens are also
high in collision impact absorption energy. Furthermore, they yield
excellent fatigue resistance properties suitable for use as high
fatigue strength steel pipes. On the other hand, the specimens
falling out of the scope of claims according to the present
invention, i.e., Comparative Examples No. 4-2, No. 4-4, and No.
4-5, suffer low values for fatigue strength.
[0161] Comparative Example No. 4-2 is produced without applying the
rolling according to the present invention, Comparative Example No.
4-5 of yields a reduction ratio falling out of the claimed range,
and Comparative Example No. 4-4 is rolled at a temperature range
out of the claimed range. Hence, they show coarsening in ferrite
grains, suffer poor balance in strength--ductility and low
collision impact absorption energy, and exhibit impaired fatigue
resistance properties.
EXAMPLE 5
[0162] A starting steel material Al whose chemical composition is
shown in Table 9 was hot rolled to provide a 4.5 mm thick flat
strip. By using the production line shown in FIG. 5, the flat strip
1 was preheated to 600.degree. C. in a preheating furnace 2, and
was continuously formed into an open pipe by using a forming
apparatus 3 composed of a plurality of groups of forming rolls. The
edge portions of each of the open pipes 7 thus obtained were heated
to 1,000.degree. C. by an edge preheating induction heating
apparatus 4, and were then heated to 1,450.degree. C. by using an
edge heating induction heating apparatus 5, where they were butted
and solid phase pressure welded by using squeeze rolls 6 to obtain
base steel pipes 8 having a diameter of 88.0 mm and a thickness of
4.5 mm.
[0163] Then, each of the base steel pipes was subjected to seam
cooling, and was heated or soaked to a predetermined temperature
shown in Table 10 by using a pipe heating apparatus 22, and a
product pipe having the predetermined outer diameter was produced
therefrom by using a rolling apparatus 21 composed of a plurality
of three-roll structured rolling mill. The number of stands was
varied depending on the outer diameter of the product pipe; i.e., 6
stands were used for a product pipe having an outer diameter of
60.3 mm, whereas 16 stands were used for those having an outer
diameter of 42.7 mm.
[0164] In the rolling step above, the product pipe of No. 5-2 was
subjected to lubrication rolling by using a rolling oil based on
mineral oil mixed with a synthetic ester.
[0165] The product pipes were air cooled after rolling.
[0166] Crystal grain diameter, tensile properties, and impact
resistance properties were investigated for each of the product
pipes thus obtained, and the results are given in Table 10. The
crystal grain diameter was obtained by microscopic observation
under a magnification of 5,000 times of at least 5 vision fields
taken on a cross section (C cross section) perpendicular to the
longitudinal direction of the steel pipe, thus measuring the
average crystal grain diameter of ferrite grains. Tensile
properties were measured on a JIS No. 11 test piece. For the
elongation, a reduced value was obtained in accordance with the
following equation by taking the size effect of the test piece into
consideration: E1=E10.times.({square root}(a0/a).sup.0.4 (where,
E10 represents the observed elongation, a0 is a value equivalent to
100 mm.sup.2, and a represents the cross section area of the
specimen (mm.sup.2)) . Impact properties (toughness) were evaluated
by subjecting the actual pipe to Charpy impact tests, and by using
the ductile rupture ratio in C cross section at a temperature of
-150.degree. C. Charpy impact test on an actual pipe was performed
by applying impact to an actual pipe V- notched for 2 mm in a
direction perpendicular to the longitudinal direction of the pipe,
and the ratio of ductile rupture was obtained therefrom.
[0167] From Table 10, it can be understood that the specimens
falling in the scope of the present invention (No. 5-2, Nos. 5-4 to
5-7, Nos. 5-9 to 5-11, and No. 5-13) consist of fine ferrite grains
1 .mu.m or less in average crystal diameter, have high elongation
and toughness, and exhibit excellent balance in strength,
toughness, and ductility. In case of specimen No. 5-2 subjected to
lubrication rolling, small fluctuation was observed in crystal
grains along the direction of pipe thickness. On the other hand,
the specimens falling out of the scope according to the present
invention, i.e., the Comparative Examples (No. 5-1, No. 5-3, No.
5-8, and No. 5-12), exhibit coarsened crystal grains and suffer
degradation in ductility and toughness. It has been found that the
texture of the product pipes falling in the scope of claims of the
present invention consists of ferrite and pearlite grains, ferrite
and cementite grains, or ferrite and bainite grains.
EXAMPLE 6
[0168] A steel material B1 whose chemical composition is shown in
Table 9 was molten in a converter, and billets were formed
therefrom by continuous casting. The resulting billets were heated,
and seamless pipes 110.0 mm in diameter and 6.0 mm in thickness
were obtained therefrom by using a Mannesmann mandrel type mill.
The seamless pipes thus obtained were re-heated to temperatures
shown in Table 11 by using induction heating coils, and product
pipes having the outer diameter shown in Table 11 were produced
therefrom by using a three-roll structured rolling mill. The number
of stands was varied depending on the outer diameter of the product
pipe; i.e., 18 stands were used for a product pipe having an outer
diameter of 60.3 mm, 20 stands were used for a product pipe 42.7 mm
in diameter, 24 stands were used for a product pipe 31.8 mm in
diameter, and 28 stands were used for those having an outer
diameter of 25.4 mn.
[0169] The characteristic properties of the product pipes were each
investigated and are shown in Table 11. Thus, investigations were
made in the same manner as in Example 5 on the structure, crystal
grain size, tensile properties, and toughness.
[0170] From Table 11, it can be understood that the specimens
falling in the scope of the present invention (No. 6-1, No. 6-3,
No. 6-6, No. 6-7, and No. 6-9) consist of fine ferrite grains 1
.mu.m or less in average crystal diameter, have high elongation and
toughness, and exhibit excellent balance in strength, toughness,
and ductility. On the other hand, the specimens falling out of the
scope according to the present invention, i.e., the Comparative
Examples (No. 6-2, No. 6-4, No. 6-5, and No. 6-8), exhibit
coarsened crystal grains and suffer degradation in ductility and
toughness.
[0171] It has been found that the texture of the product pipes
falling in the scope of claims of the present invention consists of
ferrite and pearlite grains, ferrite and cementite grains, or
ferrite and bainite grains.
EXAMPLE 7
[0172] Starting steel materials whose chemical composition is shown
in Table 12 were each heated to temperatures given in Table 13 by
using an induction heating coil, and, by using three-roll structure
rolling mills, they were rolled under conditions shown in Table 13
to provide product pipes. The number of stands was varied depending
on the type of the pipe; i.e., 24 stands were used for seamless
pipes, whereas 16 stands were used for solid phase pressure welded
pipes and seam welded pipes.
[0173] In Table 13, a solid state pressure welded steel pipe was
obtained by pre-heating a 2.3 mm thick hot rolled flat strip to
600.degree. C., continuously forming the resulting flat strip into
an open pipe by using a plurality of rolls, pre-heating the both
edge portions of the open pipe to 1,000.degree. C. by means of
induction heating, further heating the both edge portions by
induction furnace to a temperature of 1,450.degree. C., i.e., to a
temperature below the melting, at which the both ends were butted
by using a squeeze roll, and carrying out solid phase pressure
welding. Thus was obtained the steel pipes having the predetermined
outer diameter. On the other hand, seamless steel pipes were
produced by heating a continuously cast billet, and producing
therefrom the seamless pipes 110.0 mm in diameter and 4.5 mm in
thickness by using a Mannesmann mandrel type mill.
[0174] The characteristic properties of the product pipes were each
investigated and are shown in Table 13. Thus, investigations were
made in the same manner as in Example 1 on the structure, crystal
grain size, tensile properties, and toughness.
[0175] From Table 13, it can be understood that the specimens
falling in the scope of the present invention consist of fine
ferrite grains 1 .mu.m or less in average crystal diameter, have
high elongation and toughness, and exhibit excellent balance in
strength, toughness, and ductility. It has been found that the
structure of the product pipes falling in the scope of claims of
the present invention consists of ferrite and pearlite grains, or
of ferrite, pearlite, and bainite grains, or of ferrite and
cementite grains, or of ferrite and martensite grains.
EXAMPLE 8
[0176] Each of the starting steel materials whose chemical
composition is shown in Table 14 was hot rolled to provide a 4.5 mm
thick flat strip. By using the production line shown in FIG. 5, the
flat strip I was preheated to 600.degree. C. in a preheating
furnace 2, and was continuously formed into an open pipe by using a
forming apparatus 3 composed of a plurality of groups of forming
rolls. The edge portions of each of the open pipes 7 thus obtained
were heated to 1,000.degree. C. by an edge preheating induction
heating apparatus 4, and were then heated to 1,450.degree. C. by
using an edge heating induction heating apparatus 5, where they
were butted and solid phase pressure welded by using squeeze rolls
6 to obtain base steel pipes 8 having a diameter of 110.0 mm and a
thickness of 4.5 mm.
[0177] Then, each of the base steel pipes was subjected to seam
cooling, and was heated or soaked to a predetermined temperature
shown in Table 15 by using a pipe heating apparatus 22, and a
product pipe having the predetermined outer diameter was produced
therefrom by using a rolling apparatus 21 composed of a plurality
of three-roll structured rolling mill. The number of stands was
varied depending on the outer diameter of the product pipe; i.e., 6
stands were used for a product pipe having an outer diameter of
60.3 mm, whereas 16 stands were used for those having an outer
diameter of 42.7 mm.
[0178] In the rolling step above, the product pipe of No. 1-2 was
subjected to lubrication rolling by using a rolling oil based on
mineral oil mixed with a synthetic ester.
[0179] The product pipes were air cooled after rolling.
[0180] Crystal grain diameter and tensile properties were
investigated for each of the product pipes thus obtained, and the
results are given in Table 15. The crystal grain diameter was
obtained by microscopic observation under a magnification of 5,000
times of at least 5 vision fields taken on a cross section (C cross
section) perpendicular to the longitudinal direction of the steel
pipe, thus measuring the average crystal grain diameter of ferrite
grains. Tensile properties were measured on a JIS No. 11 test
piece. For the elongation, a reduced value was obtained in
accordance with the following equation by taking the size effect of
the test piece into consideration: E1=E10.times.({square
root}(a0/a)).sup.0.4 (where, E10 represents the observed
elongation, a0 is a value equivalent to 100 mn.sup.2, and a
represents the cross section area of the specimen (mm.sup.2)).
[0181] From Table 15, it can be understood that the specimens
falling in the scope of the present invention (No. 1-2, Nos. 1-4 to
1-7, and No. 1-10) consist of fine grains 2 .mu.m or less in
average crystal diameter, have high elongation and toughness, yield
a tensile strength of 600 MPa or higher, and exhibit excellent
balance in strength, toughness, and ductility.
[0182] In case of specimen No. 1-2 subjected to lubrication
rolling, small fluctuation was observed in crystal grains along the
direction of pipe thickness. On the other hand, the specimens
falling out of the scope according to the present invention, i.e.,
the Comparative Examples (No. 1-1, No. 1-3, No. 1-8, and No. 1-9),
exhibit coarsened crystal grains and suffer degradation in
ductility.
[0183] It has been found that the texture of the product pipes
falling in the scope of claims of the present invention comprises
ferrite, and cementite which accounts for more than 30% in area
ratio as a second phase.
EXAMPLE 9
[0184] Each of the base steel pipes whose chemical composition is
shown in Table 16 was re-heated by an induction heating coil to
temperatures shown in Table 17, and product pipes each having the
outer diameter shown in Table 17 were each obtained therefrom by
using a three-roll structure rolling mill apparatus. The number of
stands used in the rolling mill was 16.
[0185] The characteristic properties of the product pipes were each
investigated and are shown in Table 17. Thus, investigations were
made in the same manner as in Example 8 on the texture, crystal
grain size, and tensile properties.
[0186] From Table 17, it can be understood that the specimens (Nos.
2-1 to 2-6) falling in the scope of the present invention consist
of fine ferrite grains 2 .mu.m or less in average crystal diameter,
yield a tensile strength of 600 MPa or higher, have high
elongation, and exhibit excellent balance in strength and
ductility. On the other hand, the specimens falling out of the
scope according to the present invention, i.e., the Comparative
Examples (No. 2-7 and No. 2-8), exhibit coarsened crystal grains
and suffer degradation in strength that a targeted tensile strength
is not obtained.
[0187] It has been found that the texture of the product pipes
falling in the scope of the present invention comprises ferrite,
and a second phase containing pearlite, cementite, bainite, or
martensite, which accounts for more than 30% in area ratio.
[0188] As described above, the present invention provides high
strength steel pipes considerably improved in balance of ductility
and strength. Moreover, the steel pipes according to the present
invention exhibit superior properties in secondary working, for
instance, bulging such as hydroforming. Hence, they are
particularly suitable for use in bulging.
[0189] Among the steel pipes according to the present invention,
the welded steel pipes and the solid state pressure welded steel
pipes subjected to seam cooling yield a hardened seam portion
having a hardness at the same level as that of the mother pipe
after rolling, and show further distinguished improvement in
bulging.
1TABLE 1 Steel Chemical Composition (wt %) Ac.sub.1 Ac.sub.3 No. C
Si Mn P S Al N O .degree. C. .degree. C. Note A 0.09 0.40 0.80
0.012 0.005 0.035 0.0035 0.0025 770 900 Invention B 0.08 0.07 1.42
0.015 0.011 0.036 0.0038 0.0036 760 875 Invention C 0.06 0.21 0.35
0.013 0.008 0.028 0.0025 0.0028 775 905 Invention D 0.11 0.22 0.45
0.017 0.013 0.018 0.0071 0.0035 775 885 Invention E 0.21 0.20 0.50
0.016 0.013 0.024 0.0043 0.0030 770 855 Invention F 0.03 0.05 0.15
0.021 0.007 0.041 0.0026 0.0038 780 905 Invention G 0.09 0.15 0.52
0.024 0.003 0.004 0.0025 0.0026 775 890 Invention
[0190]
2 TABLE 2-1 Conditions of reduction rolling Base steel pipe Temp.
of Temp. of Cumulative Final Outer Heating starting finishing
reduction No. of rolling Outer diameter Steel diameter temp.
rolling rolling ratio Total No. pass 6% speed of pipe product No.
No. Type mm .degree. C. .degree. C. .degree. C. % of pass or more
m/min mm 1 A Solid phase pressure 42.7 750 710 690 65 14 9 200 15.0
welded pipe 2 A Solid phase pressure 42.7 700 670 660 65 14 9 200
15.0 welded pipe 3 A Solid phase pressure 42.7 650 635 620 65 14 9
200 15.0 welded pipe 4 A Solid phase pressure 42.7 700 655 630 40 7
4 140 25.5 welded pipe 5 A Solid phase pressure 42.7 650 605 590 40
7 4 140 25.5 welded pipe 6 A Solid phase pressure 42.7 700 660 630
30 5 3 120 29.7 welded pipe 7 A Solid phase pressure 42.7 650 615
590 30 5 3 120 29.7 welded pipe 8 A Solid phase pressure 42.7 700
660 640 22 3 2 110 33.2 welded pipe 9 A Solid phase pressure 42.7
650 615 585 22 3 2 110 33.2 welded pipe 10 A Solid phase pressure
42.7 650 620 580 22 7 0 110 33.2 welded pipe Characteristics of
pipe product Tensile strength Elongation High speed tensile
Collision Impact Ferrite grain Area ratio of Type of TS El strength
absorped energy diameter second phase second MPa % MPa MJ
.multidot. m.sup.-3 .mu.m % phase* Miscellaneous Note 525 44 728
242 2.0 10 C Invention 575 43 780 260 2.0 11 C Invention 622 40 864
292 1.0 11 C Invention 537 43 761 257 1.0 11 C Invention 580 38 799
267 1.5 11 C Invention 512 40 724 241 1.5 11 C Invention 562 38 799
268 1.0 11 C Invention 493 42 712 230 1.0 11 C Invention 541 39 755
249 1.5 11 C Invention 537 36 751 242 1.5 11 C Invention
[0191]
3 TABLE 2-2 Conditions of reduction rolling Base steel pipe Temp.
of Temp. of Cumulative Final Outer Heating starting finishing
reduction No. of rolling Outer diameter Steel diameter temp.
rolling rolling ratio Total No. pass 6% speed of pipe product No.
No. Type mm .degree. C. .degree. C. .degree. C. % of pass or higher
m/min mm 11 B Seam welded steel 42.7 650 650 622 65 14 9 200 15.0
pipe 12 B Seam welded steel 42.7 600 590 580 65 14 9 200 15.0 pipe
13 C Seam welded steel 42.7 650 640 620 65 14 9 200 15.0 pipe 14 D
Seamless steel 110 700 695 670 77 17 10 150 25.6 pipe 15 E Seamless
steel 110 700 695 670 77 17 10 150 25.6 pipe 16 A Solid phase
pressure 42.7 550 540 528 85 14 9 200 15.0 welded pipe 17 C Seam
welded steel 42.7 -- -- -- 0 -- -- -- 42.7 pipe 18 C Seam welded
steel 42.7 650 630 615 11 3 1 80 38.0 pipe 19 F Seam welded steel
42.7 650 600 545 65 14 9 200 15.0 pipe 20 G Seam welded steel 42.7
750 705 690 65 14 9 200 15.0 pipe 21 G Seam welded steel 42.7 650
620 615 65 14 9 200 15.0 pipe 22 G Seam welded steel 42.7 750 710
685 41 7 4 140 25.3 pipe 23 G Seam welded steel 42.7 950 910 890 22
3 2 110 33.1 pipe Characteristics of pipe product Tensile strength
Elongation High speed tensile Collision impact Ferrite grain Area
ratio of Type of TS El strength absorbed energy diameter second
phase second MPa % MPa MJ .multidot. m.sup.-3 .mu.m % phase*
Miscellaneous Note 555 42 792 265 1.0 15 C Invention 611 37 850 289
1.0 15 C Invention 492 42 685 225 2.5 7 C Invention 475 52 666 219
2.0 9 C Invention 526 46 733 231 2.0 22 C + B Invention 688 30 892
299 2.5 12 C Invention 409 43 566 185 11.0 6 P ** Comparative 427
40 570 191 7.0 8 C Invention 552 29 744 248 3.0 0 -- Invention 431
48 611 202 3.0 13 C Invention 511 33 704 233 3.0 13 C Invention 425
47 604 206 3.0 12 C Invention 410 45 570 183 18.0 13 C Comparative
Note) *C: Cementite, B: Bainite, M: Martensite, P: Pearlite
**Without reduction rolling
[0192]
4TABLE 3 Steel Chemical composition (wt. %) No. C Si Mn P S Al N O
Cu Ni H 0.07 0.20 0.66 0.018 0.005 0.028 0.0022 0.0025 -- -- I 0.08
0.04 1.35 0.015 0.011 0.036 0.0041 0.0032 -- -- J 0.15 0.21 0.55
0.009 0.004 0.010 0.0028 0.0028 -- -- K 0.05 1.01 1.35 0.012 0.001
0.035 0.0030 0.0030 -- -- L 0.15 0.22 0.41 0.018 0.003 0.031 0.0036
0.0038 0.11 0.15 Steel Chemical composition (wt. %) Ac.sub.1
Ac.sub.3 No. Cr Mo V Nb Ti B Ca .degree. C. .degree. C. Note H --
-- -- 0.009 0.008 -- -- 765 895 Inven-tion I -- -- 0.10 -- -- --
0.002 755 885 Inven-tion J 0.21 0.53 -- -- -- -- -- 785 890
Inven-tion K 0.92 -- -- 0.015 0.011 0.0023 -- 790 905 Inven-tion L
-- -- -- -- -- -- 0.002 760 875 Inven-tion
[0193]
5 TABLE 4 Conditions of reduction rolling Base steel pipe Temp. of
Temp. of Cumulative Final Outer Heating starting finishing
reduction No. of rolling Outer diameter Steel diameter temp.
rolling rolling ratio Total No. pass 6% speed of pipe product No.
No. Type mm .degree. C. .degree. C. .degree. C. % of pass or more
m/min mm 2-1 H Solid phase pressure 42.7 730 700 640 65 14 9 200
15.0 welded pipe 2-2 Solid phase pressure 42.7 670 640 600 65 14 9
200 15.0 welded pipe 2-3 Solid phase pressure 42.7 620 600 560 65
14 9 200 15.0 welded pipe 2-4 Solid phase pressure 42.7 -- -- -- 0
-- -- -- 42.7 welded pipe 2-5 Solid phase pressure 42.7 670 640 600
11 3 1 80 38.0 welded pipe 2-6 I Solid phase pressure 42.7 700 670
620 41 7 4 140 25.3 welded pipe 2-7 Solid phase pressure 42.7 800
780 770 41 7 4 140 25.3 welded pipe 2-8 Solid phase pressure 42.7
850 830 820 41 7 4 140 25.3 welded pipe 2-9 Solid phase pressure
42.7 950 930 910 41 7 4 140 25.3 welded pipe 2-10 J Seamless steel
pipe 110 700 700 690 69 17 15 400 34.1 2-11 K Seam welded steel
42.7 720 690 650 65 14 9 200 15.0 pipe 2-12 L Seamless steel pipe
110 700 700 680 77 24 18 400 25.4 2-13 Seamless steel pipe 110 800
780 770 77 24 18 400 25.4 2-14 Seamless steel pipe 110 850 830 820
77 24 18 400 25.4 Characteristics of pipe product Tensile strength
Elongation High speed tensile Collision impact Ferrite grain Area
ratio of Type of TS El strength absorbed energy diameter second
phase second MPa % MPa MJ .multidot. m.sup.-3 .mu.m % phase*
Miscellaneous Note 530 43 734 242 2.0 8 C Invention 640 38 884 301
1.0 7 C Invention 730 32 931 318 2.0 8 C Invention 470 40 640 196
7.0 7 C ** Comparative 490 37 666 199 6.0 8 C Comparative 530 40
724 240 2.5 13 C Invention 500 44 682 223 2.5 12 C Invention 480 41
644 205 2.8 14 C + P Invention 390 40 532 130 6.5 15 P Comparative
663 42 885 298 1.5 23 C + B Invention 712 34 931 318 1.5 12 M
Invention 581 44 802 259 1.5 18 C Invention 556 46 757 236 2.0 20 C
Invention 500 40 658 210 2.5 21 C + P Invention Note) * C:
Cementite, B: Bainite, M: Martensite, P: Pearlite **Without
reduction rolling
[0194]
6TABLE 5 Steel Chemical composition (wt. %) No. C Si Mn P S Al N O
Cu Ni M 0.05 0.30 1.22 0.007 0.001 0.022 0.0030 0.0028 -- 0.20 N
0.08 0.51 1.41 0.008 0.001 0.028 0.0035 0.0019 0.12 0.18 O 0.06
0.28 0.95 0.009 0.001 0.025 0.0026 0.0025 -- 0.15 P 0.06 0.30 1.18
0.008 0.001 0.028 0.0031 0.0023 0.15 0.15 Q 0.04 0.10 1.50 0.006
0.001 0.018 0.0029 0.0023 -- -- Steel Chemical composition (wt. %)
Ac.sub.1 Ac.sub.3 No. Cr Mo V Nb Ti B Ca REM .degree. C. .degree.
C. Note M -- 0.05 0.05 0.05 0.011 -- -- -- 770 895 Inven-tion N
0.15 -- 0.02 0.02 0.007 0.0011 -- -- 760 890 Inven-tion O -- 0.06
0.02 0.03 0.009 -- 0.002 -- 770 900 Inven-tion P -- -- 0.04 0.03
0.009 -- -- 0.007 765 900 Inven-tion Q -- 0.06 0.06 0.04 -- -- --
-- 770 885 Inven-tion
[0195]
7 TABLE 6 Conditions of reduction rolling Base steel pipe Temp. of
Temp. of Cumulative Outer Heating starting finishing reduction No.
of Outer diameter Steel diameter temp. rolling rolling ratio Total
No. pass 6% of pipe product No. No. Type mm .degree. C. .degree. C.
.degree. C. % of pass or more mm 3-1 M Seam welded steel 110 720
700 680 45 10 7 60.5 3-2 pipe 660 650 640 45 10 7 60.5 3-3 610 600
590 45 10 7 60.5 3-4 660 650 640 8 3 1 101.6 3-5 N 660 650 640 45
10 7 60.5 3-6 O 720 700 690 69 17 15 34.1 3-7 800 780 770 69 17 15
34.1 3-8 850 830 820 69 17 15 34.1 3-9 950 920 900 69 17 15 34.1
3-10 P 720 690 650 69 17 15 34.1 3-11 950 920 900 69 17 15 34.1
3-12 Q 720 700 680 77 24 18 25.4 Characteristics of pipe product
Yield Tensile High speed Presence of strength strength Elongation
tensile Collision impact SSC Ferrite grain Area ratio of Type of
*** TS El strength absorption energy resistant diameter second
phase second Miscell- MPa MPa % MPa MJ .multidot. m.sup.3
cracks**** .mu.m % phase* aneous Note 507 616 41 786 258
.largecircle. .largecircle. 2.0 5 C Invention 565 642 38 838 275
.largecircle. .largecircle. 1.5 5 C Invention 616 692 35 906 293
.largecircle. .largecircle. 2.0 5 C Invention 506 582 43 761 199
.largecircle. X 10.0 5 C Comparative 637 724 35 943 307
.largecircle. .largecircle. 2.0 20 C Invention 560 625 42 815 270
.largecircle. .largecircle. 1.5 5 C Invention 538 611 43 772 250
.largecircle. .largecircle. 2.0 5 C Invention 521 593 45 733 230
.largecircle. .largecircle. 2.5 5 C Invention 431 538 39 668 180 X
.largecircle. 6.0 8 C + B Comparative 582 640 40 830 273
.largecircle. .largecircle. 1.5 5 C Invention 445 550 39 678 180 X
X 6.5 7 C + B Comparative 600 658 38 861 279 .largecircle.
.largecircle. 1.5 5 C invention Note) *C: Cementite, B: Bainite, M:
Martensite, P: Pearlite **Without reduction rolling ***0.2% PS
****No breakage .largecircle., breakage X
[0196]
8TABLE 7 Steel Chemical composition (wt. %) No. C Si Mn P S Al N O
Cu Ni R 0.09 0.02 0.73 0.011 0.003 0.032 0.0036 0.0025 -- -- S 0.11
0.15 1.28 0.007 0.001 0.028 0.0041 0.0025 0.12 0.18 T 0.14 0.35
0.91 0.008 0.001 0.025 0.0038 0.0033 -- -- U 0.12 0.25 1.36 0.008
0.001 0.028 0.0030 0.0028 -- -- V 0.21 0.20 0.48 0.009 0.001 0.025
0.0038 0.0031 0.12 0.12 Steel Chemical composition (wt. %) Ac.sub.1
Ac.sub.3 No. Cr Mo V Nb Ti B Ca REM .degree. C. .degree. C. Note R
-- -- -- -- -- -- -- -- 770 880 Inven-tion S 0.15 -- -- -- -- -- --
-- 755 850 Inven-tion T -- -- 0.02 0.021 0.007 0.0011 -- -- 770 870
Inven-tion U -- -- -- -- -- -- 0.003 -- 760 865 Inven-tion V 0.11
0.05 0.02 0.009 0.009 -- -- 0.006 765 840 Inven-tion
[0197]
9 TABLE 8 Conditions of reduction rolling Base steel pipe Temp. of
Temp. of Cumulative Outer Heating starting finishing reduction No.
of Outer diameter Steel diameter temp. rolling rolling ratio Total
No. pass 6% of pipe product No. No. Type mm .degree. C. .degree. C.
.degree. C. % of pass or more mm 4-1 R Seam welded steel 110 660
650 630 68 14 9 35.0 4-2 pipe 35.0 ** 35.0 4-3 S 110 605 600 590 68
14 9 35.0 4-4 880 860 830 68 14 9 35.0 4-5 660 650 640 18 4 2 90.0
4-6 700 690 670 77 17 10 25.6 4-7 T Seamless steel 110 660 650 630
77 17 10 25.6 4-8 U pipe 660 650 630 77 17 10 25.6 4-9 V 660 650
630 77 17 10 25.6 Characteristics of pipe product Yield Tensile
High speed Fatigue strength strength Elongation tensile Collision
impact strength Ferrite grain Area ratio of Type of *** TS El
strength absorbed energy **** diameter second phase second MPa MPa
% MPa MJ .multidot. m.sup.3 MPa .mu.m % phase* Note 466 550 47 742
198 220 1.5 14 C Invention 364 448 45 553 124 140 13.0 15 C
Comparative 531 612 40 821 223 250 1.5 18 C Invention 421 517 38
648 143 155 8.0 16 C + B Comparative 451 522 36 679 151 160 9.0 18
C Comparative 525 575 42 761 255 250 0.9 18 C Invention 507 596 40
795 196 235 2.0 16 C Invention 523 618 39 806 198 240 2.5 20 C
Invention 570 657 37 850 210 255 2.0 23 C Invention Note) *C:
Cementite, B: Bainite, M: Martensite, P: Pearlite **Without
reduction rolling ***0.2% PS ****Load stress for 10.sup.5 endurance
cycles
[0198]
10TABLE 9 Steel Chemical composition wt. % No. C Si Mn P S Al N A1
0.06 0.05 0.35 0.018 0.019 0.028 0.0025 B1 0.25 0.20 0.82 0.012
0.007 0.010 0.0028
[0199]
11TABLE 10 Outer dia- Crystal grain Base Conditions of reduction
rolling Outer Total meter of diameter of steel pipe Healing Temp.
of Temp. of Av. rolling diameter of reduction Equation (1) Steel
base pipe base pipe Ac.sub.1 Ac.sub.3 temp. starting finshing
rolling temp. pipe product ratio Left Right No. No. mm .mu.m
.degree. C. .degree. C. .degree. C. rolling .degree. C. .degree. C.
.degree. C. mm % side side 5-1 A1 88.0 3.8 770 900 400 395 412 404
42.7 51.5 3.8 9.67 5-2 450 445 458 452 60.3 31.5 3.8 4.45 5-3 670
660 641 651 60.3 31.5 3.8 3.20 5-4 670 660 638 649 42.7 51.5 3.8
8.45 5-5 810 775 748 762 42.7 51.5 3.8 5.74 5-6 8.2 450 445 462 454
42.7 51.5 8.2 9.75 5-7 600 590 592 591 42.7 51.5 8.2 9.19 5-8 670
660 639 650 60.3 31.5 8.2 3.21 5-9 670 660 636 648 42.7 51.5 8.2
8.47 5-10 735 720 702 711 31.8 63.9 8.2 13.57 5-11 780 760 737 749
31.8 63.9 8.2 11.85 5-12 13.1 450 445 458 452 42.7 51.5 13.1 9.75
5-13 445 440 466 453 31.8 63.9 13.1 15.86 Characteristics of pipe
product Crystal grain Yield strength Tensile strength Elongation
Real pipe Area ratio of diameter YS TS (EL) Charpy ductile rupture
ratio second phase .mu.m MPa MPa % % Structure* % Note Breakage
occurred during rolling Comparative 0.92 613 648 41 90 F + P P:8
Invention 2.25 496 538 32 40 F + C C:6 Comparative 0.55 431 518 48
100 F + C C:6 Invention 0.99 415 448 38 75 F + B B:8 Invention 0.95
552 597 41 90 F + P P:8 Invention 0.81 451 502 44 95 F + P P:6
Invention 5.12 451 485 28 0 F + C C:5 Comparative 0.68 439 506 46
100 F + C C:5 Invention 0.78 448 496 44 95 F + B B:8 Invention 0.90
413 462 43 90 F + B B:8 Invention 6.92 560 574 23 0 F + P P:8
Comparative 0.96 607 656 42 90 F + P P:8 Invention *: F represents
ferrite, P represents pearlite (inclusive of pseudo-pearlite), C
represents cementite, and B represents bainite.
[0200]
12TABLE 11 Outer dia- Crystal grain Base steel Conditions of
reduction rolling Outer Total meter of diameter of pipe Heating
Temp. of Temp. of Av. rolling diameter of reduction Equation (1)
Steel base pipe base pipe Ac.sub.1 A.sub.c.sub.3 temp. starting
finishing temp. pipe product ratio Left Right No. No. mm .mu.m
.degree. C. .degree. C. .degree. C. rolling .degree. C. .degree. C.
.degree. C. mm % side side 6-1 B1 110.0 6.3 765 830 625 615 591 603
60.3 45.2 6.3 6.78 6-2 735 720 690 705 60.3 45.2 6.3 5.33 6-3 735
720 684 702 42.7 61.2 6.3 12.14 6-4 15.2 560 550 553 552 42.7 61.2
15.2 14.53 6-5 675 665 640 653 42.7 61.2 15.2 3.44 6-6 680 670 637
654 31.8 71.1 15.2 21.70 6-7 785 765 726 746 31.8 71.1 15.2 17.59
6-8 28.1 680 670 637 654 31.8 71.1 28.1 21.70 6-9 680 675 634 655
25.4 76.9 28.1 28.75 Characteristics of pipe product Crystal grain
Yield point Tensile strength Elongation Real pipe Area ratio of
diameter YS TS (EL) Charpy ductile rupture ratio second phase .mu.m
MPa MPa % % Structure* % Note 0.82 589 660 42 95 F + P P:23
Invention 2.13 486 532 37 20 F + B B:25 Comparative 0.91 513 588 43
90 F + B B:20 Invention 2.36 601 643 41 20 F + P P:23 Comparative
3.22 564 602 34 10 F + C C:16 Comparative 0.57 592 671 44 100 F + C
C:16 Invention 0.88 568 623 46 90 F + B B:23 Invention 4.96 596 642
24 0 F + C C:18 Comparative 0.69 638 711 42 100 F + C C:18
Invention *: F represents ferrite, P represents pearlite (inclusive
of pseudo-pearlite), C represents cementite, and B represents
bainite.
[0201]
13TABLE 12 Steel Chemical composition (wt. %) No. C Si Mn P S Al N
Cu Ni Cr Mo V Nb Ti B Ca REM C1 0.09 0.40 0.80 0.012 0.005 0.035
0.0035 -- -- -- -- -- -- -- -- -- -- D1 0.21 0.20 0.50 0.016 0.013
0.024 0.0043 -- -- -- -- -- -- -- -- -- -- E1 0.15 0.21 0.55 0.009
0.004 0.010 0.0028 -- -- 0.21 0.53 -- -- -- -- -- -- F1 0.15 0.22
0.45 0.018 0.003 0.031 0.0036 0.11 0.15 -- -- -- -- -- -- 0.002 --
G1 0.08 0.04 1.35 0.015 0.011 0.036 0.0041 -- -- -- -- 0.10 -- --
-- 0.002 -- H1 0.05 1.01 1.35 0.012 0.001 0.035 0.0030 -- -- -- --
-- 0.015 0.011 0.0023 -- -- I1 0.14 0.30 1.30 0.011 0.003 0.028
0.0038 0.20 0.25 -- -- -- -- -- -- -- 0.008
[0202]
14 TABLE 13 Base steel pipe Outer Outer Crystal Conditions of
reduction rolling diameter Total dia- grain Heating Temp. of Temp.
of Av. of pipe reduction Equation (1) Steel meter diameter Ac.sub.1
Ac.sub.3 temp. starting finishing rolling product ratio Left Right
No. No. Type mm .mu.m .degree. C. .degree. C. .degree. C. rolling
.degree. C. rolling .degree. C. temp. .degree. C. mm % side side
7-1 C1 Solid phase 88.0 6.3 770 895 450 443 460 452 60.3 31.5 3.8
4.45 7-2 pressure 8.2 600 589 593 591 42.7 51.5 8.2 9.19 7-3 D1
welded 13.1 760 850 445 437 469 453 31.8 63.9 13.1 15.86 7-4 pipe
13.1 690 670 620 650 42.7 51.4 6.3 6.81 7-5 E1 Seam-less 110.0 6.3
785 880 625 610 596 603 60.3 45.2 6.3 6.78 7-6 steel pipe 15.2 785
762 730 746 31.8 71.1 15.2 17.59 7-7 F1 8.2 780 860 705 700 682 691
25.4 76.9 8.2 9.19 7-8 G1 Solid phase 42.7 3.8 755 875 700 670 620
645 25.4 40.5 3.8 5.02 7-9 pressure 6.7 610 595 588 592 15.1 64.6
6.7 9.19 welded 7-10 H1 Seam 5.5 775 900 720 690 653 672 15.1 64.6
5.5 9.19 welded steel pipe 7-11 I1 Solid phase 88.0 7.7 750 860 675
665 642 654 42.7 51.5 7.7 9.19 pressure welded pipe Characteristics
of pipe product Crystal grain Yield Strength Tensile strength
Elongation Real pipe Area ratio of diameter YS TS (EL) Charpy
ductile rupture ratio second phase .mu.m MPa MPa % % Structure* %
Note 0.87 632 665 44 100 F + P P:15 Invention 0.77 531 580 51 100 F
+ P P:15 Invention 0.92 661 692 42 95 F + P + PB:22 Invention B
0.75 511 548 49 100 F + P + PB:22 Invention B 0.80 688 713 37 100 F
+ P + PB:25 Invention B 0.85 588 630 40 95 F + P + PB:25 Invention
B 0.95 559 601 47 100 F + C C:11 Invention 0.95 526 572 44 100 F +
C C:10 Invention 0.91 535 581 48 100 F + C C:10 Invention 0.88 688
736 38 95 F + M M:15 Invention 0.85 463 523 46 100 F + C C:14
Invention *: F represents ferrite, P represents pearlite (inclusive
of pseudo-pearlite), C represents cementite, and B represents
bainite.
[0203]
15 TABLE 14 Steel Chemical composition (wt. %) No. C Si Mn P S Al A
0.43 0.32 1.53 0.008 0.003 0.015 B 0.53 0.21 0.85 0.011 0.004 0.025
C 0.35 0.35 1.31 0.013 0.003 0.031 D 0.33 0.35 0.86 0.012 0.003
0.022
[0204]
16 TABLE 15 Conditions of reduction rolling Base steel pipe Temp.
of Temp. of Outer Total Outer Crystal grain Heating starting
finishing Av. rolling diameter of reduction Equation (1) Steel
diameter diameter temp. rolling rolling temp. pipe product ratio
Left Right No. No. mm .mu.m Structure* .degree. C. .degree. C.
.degree. C. .degree. C. mm % side side 1-1 A 110 6 F + P 900 880
850 865 42.7 61 6 1.9 1-2 750 730 700 715 42.7 61 6 12 1-3 750 730
700 715 60.3 45 6 5.1 1-4 580 570 550 560 60.3 45 6 7.1 1-5 B 110 9
F + P 700 680 650 665 42.7 61 9 13 1-6 620 610 590 600 42.7 61 9 14
1-7 C 110 12 F + P 620 610 590 600 42.7 61 12 14 1-8 800 790 760
775 42.7 61 12 8.9 1-9 D 110 12 F + P 900 880 850 865 42.7 61 12
1.9 1-10 620 610 590 600 42.7 61 12 14 Characteristics of pipe
product Crystal grain Yield Strength Tensile strength Elongation
Structure of Second phase diameter YS** TS (EL) Area ratio .mu.M
MPa MPa % * % Note 7.5 504 641 37 P 65 Comparative 1.0 624 721 39 C
60 Invention 4.5 540 641 35 C, P 60 Comparative 1.5 685 773 37 C 60
Invention 1.5 660 759 40 C 65 Invention 1.0 687 782 38 C 65
Invention 1.5 610 700 40 C 40 Invention 8.0 520 618 37 C, P 40
Comparative 15 444 563 42 P 40 Comparative 1.5 553 633 43 C 35
Invention *: F represents ferrite, P represents pearlite (inclusive
of pseudo-pearlite), C represents cementile, and B represents
balnite. **0.2% PS
[0205]
17TABLE 16 Steel Chemical composition (wt. %) No. C Si Mn P S Al N
Cu Ni Cr Mo V Nb Ti B Ca REM O E 0.45 0.25 0.81 0.009 0.004 0.015
0.0028 0.15 0.20 0.12 0.08 -- -- -- -- -- -- 0.0023 F 0.36 0.26
0.97 0.008 0.003 0.021 0.0032 -- -- -- -- 0.08 0.02 0.02 0.009 --
-- 0.0019 G 0.48 0.25 0.78 0.014 0.006 0.018 0.0035 -- -- -- -- --
-- -- -- 0.002 0.004 0.0023 H 0.35 0.25 1.35 0.012 0.002 0.015
0.0036 0.12 0.10 0.10 0.05 0.05 0.01 0.01 0.001 0.002 -- 0.0022 I
0.33 0.15 0.51 0.013 0.004 0.028 0.0043 0.15 0.20 -- -- -- 0.01
0.01 -- -- -- -- 0.0025 J 0.32 0.15 0.53 0.011 0.003 0.036 0.0039
-- -- -- 0.20 0.10 -- -- -- -- -- 0.0021 K 0.09 0.02 0.73 0.011
0.003 0.032 0.0036 -- -- -- -- -- -- -- -- -- -- 0.0025 L 0.08 0.21
0.58 0.016 0.004 0.029 0.0045 -- -- -- -- -- 0.01 0.01 -- -- --
0.0019
[0206]
18 TABLE 17 Base steel pipe Conditions of reduction rolling Crystal
Temp. of Temp. of Av. Outer Total Outer grain Heating starting
finishing rolling diameter of reduction Equation (1) Steel diameter
diameter temp. rolling rolling temp. pipe product ratio Left Right
No. No. mm .mu.m Structure* .degree. C. .degree. C. .degree. C.
.degree. C. mm % side side 2-1 E 110 11 F + P 670 660 630 645 42.7
61 11 13.6 2-2 F 7 7 2-3 G 10 10 2-4 H 8 8 2-5 I 11 11 2-8 J 10 10
2-7 K 12 12 2-8 L 11 11 Characteristics of pipe product Crystal
grain Yield Strength Tensile strength Elongation Structure of
Second phase diameter YS** TS (EL) Area ratio .mu.m MPa MPa % * %
Note 1.5 659 761 39 C 65 Invention 1.5 667 753 40 45 Invention 1.5
623 739 40 65 Invention 1.0 701 796 38 45 Invention 1.5 603 678 42
40 Invention 1.5 622 708 41 35 Invention 2.5 469 539 45 11
Comparative 2.0 446 530 43 8 Comparative *: F represents ferrite, P
represents pearlite (inclusive of pseudo-pearlite), C represents
cementite, and B represents balnite. **0.2% PS
[0207] Applicability in Industry:
[0208] In accordance with the present invention, high strength
steel pipes having excellent ductility and impact resistance
properties can be obtained with high productivity and by a simple
process. Thus, the present invention extends the application field
of steel pipes and is therefore particularly effective in the
industry. Furthermore, the present invention reduces the use of
alloy elements and enables low cost production of high-strength
high-ductility steel pipes improved in fatigue resistance
properties, or high-strength high-toughness steel pipes for use in
line pipes improved in stress corrosion crack resistance. Moreover,
a high strength steel material containing super fine crystal grains
1 .mu.m or less in size is produced with superior in toughness and
ductility, thereby expanding the use of steel materials.
[0209] Also available easily and without applying intermediate
annealing is a steel material containing super fine crystal grains
2 .mu.m or less in size, which yields a tensile strength of 600 MPa
or more, and excellent toughness and ductility.
* * * * *