Note: Descriptions are shown in the official language in which they were submitted.
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HIGH-DUCTILITY, HIGH-STRENGTH STEEL PRODUCT AND
PROCESS FOR PRODUCTION THEREOF
The present invention relate, to a steel product which has
high strength and high ductility and is superior in toughness
and resistance to collision and impact, particularly a steel
product, such as steel pipe, wire :rod, steel bar, steel section,
steel plate, and steel strip, having fine crystal grains, and
also to a process for production thereof.
Common practice to increase t:he strength of a steel product
is to add an alloying element such as Mn and Si, to perform heat
treatment such as controlled rolling, controlled cooling,
quenching, and tempering, and to add a precipitation hardening
element such as Nb and V. However, what is required of steel
products is not only high strengti~ but also high ductility and
high toughness. There has been a demand for a steel product
which has well-balanced strength and ductility/toughness.
Making grains finer is one of a few important means to
~.mprove both strength and duci:ility/toughness. This is
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accomplished. by performing austenite-ferrite transformation
from. fine austenite while preventing austenite grains from
becoming coarse, thereby giving fine ferrite grains, by working
which makes austenite grains finer, thereby giving fine ferrite
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grains, or by utilizing martensite and lower bainite that result
from quenching and tempering.
cane of these methods in general use for steel production
is controlled rolling which consists of strengthening in the
a.ustenite region and its ensuing austenite-ferrite
transformation to give rise to fine ferrite grains. Another
way in practice is to add a trace amount of Nb which suppresses
the recrystallization of austenite grains, thereby yielding
finer ferrite grains. Working at a temperature at which
austenite does not yet recrystallize permits austenite grains
to grow, giving rise to the deformation zone in grains , and finer
ferrite grains occur from this deformation zone. A recent
pract9_ce to obtain finer ferrite grains is controlled cooling
that is carried out during or after working.
The above-mentioned methods, however, need rebuilding of
the existing facilities and considerable remodeling of the
current process in the production of steel products, such as
steel pipes, having improved collision a,nd impact resistance
required for better automotive safety, an ever increasing
demand. Therefore, they are economically unfeasible.
In the meantime, steel products for line pipe need
resistance to stress corrosion cracking by sulfides, and this
o:bj ect; is achieved by hardness control through the reduction
of impurities or the adjustment of alloying elements. In
addition, conventional practices to improve fatigue resistance
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include heat treatment, such as thermal refining, induction
hardening, and carburizing, and addition of a large amount of
expensive alloying elements such as Ni, Cr, and Mo. The
cLisadvantage of these methods is poor weldability and high
production cost.
Steel pipes of small to medium diameter are produced mainly
by electric resistance welding with high frequency current.
The process for their production consists of continuously
feeding a flat strip steel, making it into a pipe stock using
a. forming roll, heating the opposing edges of the pipe stock
t.o a temperature above the melting point of steel by means of
high frequency current, and butt-welding the heated edges by
means of squeeze rolls.
This process, however, has a disadvantage of requiring
rolls that conform to the dimensions of the desired steel pipe;
therefore, it is not suitable for multi-product production in
small lots.
:Cn order to address this problem, there has been proposed
a new process in, for example, Japanese Patent Publication No.
246061990. This process consists of heating a flat strip steel
in a preheating furnace and a heating furnace, making the strip
steel into a pipe by electric resistance welding, heating the
pipe t:o a temperature above the A3 transformation point, and
rolling the heated pipe by a reducing mill so that it has a
predetermined outside diameter.
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This process, however, poses problems due to heating above
A3 point. Heating gives rise to scale which is bitten during
rolling. Heating also makescrystalgrainscoarse,aggravating
the ductility, strength, and toughness of the resulting steel
pipe.
A cold sizing process has been proposed in Japanese Patent
Zaid-open No. 33105/1988. This process is designed to reduce
the outside diameter of a hollow pipe stock, such as seamless
steel pipes and electric welded pipes , in the cold state by using
a series of reducing mills, each consisting of three rolls. The
disadvantage of this process is the necessity of a large-scale
mill to withstand high loads due to cold rolling and the
necessity of a lubricating facility to prevent rolls from
seizing. In addition, cold rolling gives rise to working
strain, which aggravates ductility and toughness.
It is an object of the present invention, which was
completed to address the above-mentioned problems, to provide
a steel product and a process for production thereof , said steel
product being superior in ductility, strength, toughness, and
resistance to collision and impact owing to fine ferrite crystal
grains.
Summary of the Invention
In accordance with the present invention, that object is achieved with a
steel pipe having high ductility and high strength which is characterized by
an
I I I
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average grain size not greater than 3 pm in the cross section perpendicular to
its
lengthwise direction and by a structure comprising ferrite or ferrite plus
pearlite
or ferrite plus cementite, having an elongation of 20% or more produced by
working and a tensile strength (TS:MPa) ;end elongation (EI:%) whose product
TSxEI is 20,000 or more and containing 0.2-1.3 wt % of Mn.
Brief Description of the Fi~re~
Fig. 1 is a graph showing the relation between the
elongation and tensile strength of steel pipes.
Fig. 2 is a graph showing the effect of tensile strain rate
on the relation between the tensile strength and ferrite grain
size of steel pipe.
Fig. 3 is a graph showing th~~ grain size of steel products
as the function of the temperatures at which rolling starts and
ends.
Fig. 4 is an electron photomicrograph showing the
metallographic structure of a sl:eel pipe in Example 1 of the
present invention.
Fig. 5 is a schematic diagram showing a test piece used
in test for resistance to stress corrosion cracking by sulfides .
Disclosure of the Invention
The present inventors carried out extensive studies on a
process for efficient production of high-strength steel pipes
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super:Lor in ductility, which led to the finding that it is
possible to produce desired steel pipes with balanced ductility
and strength by reducing steel pipes of specific composition
at a temperature of ferrite recrystallization.
The present invention is based on the experimental results
explained below.
'The experiment was carried out on electric welded steel
pipes (42.7 mm in dia. and 2.9 mm thick) containing 0.09 wt~
C, 0.9:0 wt~ Si, 0.80 wt~ Mn, and 0.04 wt~ A1. After heating
at various temperatures ranging from 750 to 400°C, they were
passed through a reducing mill at a rolling speed of 200 m/min
so that their outside diameter was reduced variously to
33 . 2-1.5 . 0 mm. The rolled pipes were tested for tensile strength
(TS) and elongation (El) . The relation between elongation and
tensile strength is shown in Fig. 1 (black dots) . Incidentally,
white dots in Fig. 1 represent the relation between elongation
and tensile strength of electric welded pipes in various sizes
without reducing. Elongation (E1) is expressed in terms of
values calculated from
El = Elo X (.,/- (ao/a) ) 0 . 4
(where Elo is the actually measured elongation, ao is 292 mm2,
and a is the sectional area (mma) of the specimen.)
This converted value was used in consideration of the size effect
of the specimen.
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It is noted from Fig. 1 that the specimens obtained by
reducing after heating at 750-400°C exhibit higher elongation
for the same strength than electric welded pipes without
reducing. In other words , the present inventors found that it
is possible to produce high-strength steel pipes with balanced
ductility and strength by reducing steel pipes of specific
composition at a temperature ranging from 750°C to 400°C.
rZoreover, it was found that the above-mentioned steel pipe
has f9_ne ferrite grains not greater than 3 ~,tm. In order to
examine resistance to collision and impact, the present
inventors established the relation between tensile strength
(TS) and ferrite grain size, with the strain rate greatly changed
over a broad range (2000 s-1). The results are shown in Fig.
2. It is noted from Fig. 2 that the tensile strength remarkably
increases with the decreasing ferrite grain size not more than
3 Eun, preferably not more than 1 ).un, and this tendency is
significant in the case of high strain rate as is experienced
in deformation by collision and impact. In other words, it was
found that steel pipes with fine ferrite grains are superior
in ductile-strength balance and have greatly improved
rESSistance to collision and impact.
The present invention is based on the above-mentioned
findings.
The present invention covers a steel product with high
ductility and high strength which is characterized in that it
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has an average grain size lower than 3 ).Lm, preferably lower than
1. ).lm, in the cross section perpendicular to its lengthwise
d~irec~tion, that it has a structure composed mainly of ferrite
or fe~_rite plus pearlite or ferrite plus cementite, and that
it has an elongation 20~ or more and a product of tensile strength
(TS in MPa) and elongation (El in ~) which is 10000 or more.
The present invention also covers a steel pipe with high
ductility and high strength which is characterized in that it
has an average grain size lower than 3 ).~,m, preferably lower than
1 ~.Lm, in the cross section perpendicular to its lengthwise
direction, that it has a structure composed mainly of ferrite
or ferrite plus pearlite or ferrite plus cementite, that it has
a:n elo:ngation greater than 20~ and a product of tensile strength
('rS in. MPa) and elongation (E1 in ~) which is 10000 or more,
and that it has a percent ductile fracture by Charpy impact test
of 95~~k or more, preferably 100, in the cross section
perpendicular to its lengthwise direction.
The present invention also covers a process for producing
a steel product, preferably a steel pipe, with high ductility
and high strength, said process comprising rolling a steel
product containing C not more than 0.60 wt~ at a temperature
for ferrite recrystallization with a reduction of area greater
than 20~. Said rolling may be carried out by the aid of
lubrication.
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The present invention also covers a steel pipe with high
ductility and high strength characterized in that it has a
composition of C 0.005-0.30, Si 0.01-3.0~, Mn 0.01-2.0~, A1
0.001-0.10 on a weight basis, with the remainder being Fe and
unavoidable impurities , and that it has a structure of ferrite
ar a structure of ferrite and a second phase other than ferrite
not more than 30~ in terms of areal ratio, with said ferrite
having a grain size not greater than 3 Elm, preferably not greater
than 1 ~lxn .
:Ln the present invention, the above-mentioned composition
m.ay be C 0.005-0.30, Si 0.01-3.0~, Mn 0.01-2.0~, A1 0.001-
0.10~,, and one or more selected from Cu not more than 1~, Ni
not mare than 2~, Cr not more than 2~, and Mo not more than 1~,
with l.he remainder being Fe and unavoidable impurities;
the above-mentioned composition may be C 0.005-0.30, Si
0.01-3.0~, Mn 0.01-2.0~, A1 0.001-0.10$, and one or more
selected from Nb not more than 0.1~, V not more than 0.3~, Ti
notmo.re than 0.2~, andB not more than 0. 004, with the remainder
being Fe and unavoidable impurities;
or the above-mentioned composition may be C 0.005-0.30,
Si Ø01-3.0~, Mn 0.01-2.0~, A1 0.001-0.10, and one or more
selected from REM not more than 0 . 02~ and Ca not more than 0 . 01~,
with the remainder being Fe and unavoidable impurities.
The above-mentioned composition may be C 0.005-0.30, Si
0.01-3.0~, Mn 0.01-2.0~, A1 0.001-0.10, and one or more
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selected from Cu not more than 1~, Ni not more than 2~, Cr not
more than 2~, and Mo not more than 1~ and one or more selected
from Nb not more than 0.1~, V not more than 0.3~, Ti not more
than 0.2~, and B not more than 0.004.
The above-mentioned composition may be C 0.005-0.30, Si
0.01-3.0~, Mn 0.01-2.0~, A1 0.001-0.10, and one or more
s~elect:ed from Cu not more than 1$, Ni not more than 2~, Cr not
more than 2~, and Mo not more than l~ ,and one or more selected
from REM not more than 0.02 and Ca not more than 0.01, with
tlZe remainder being Fe and unavoidable impurities.
The above-mentioned composition may be C 0.005-0.30, Si
0.01-3.0~, Mn 0.01-2.0~, Al 0.001-0.10~,and one or more
selected from Nb not more than 0.1~, V not more than 0.3~, Ti
not more than 0.2~, and B not more than 0.004, and one or more
selected from REM not more than 0. 02~r and Ca not more than 0 . 01~,
with the remainder being Fe and unavoidable impurities.
Moreover, the above-mentioned composition may be C
0.005-0.30, Si 0.01-3.0~, Mn 0.01-2.0~, A1 0.001-0.10, one
or. more selected from Cu not more than 1~, Ni not more than 2~,
Cr not more than 2~, and Mo not more than 1~, one or more selected
from Nb not more than 0.1~, V not more than 0.3~, Ti not more
than 0.2~, and B not more than 0.004; and one or more selected
from REM not more than 0.02 and Ca not more than 0.01, with
th.e remainder being Fe and unavoidable impurities.
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The present invention also covers a process for producing
a stee'1 pipe with high ductility and high strength, said process
<:omprising heating a pipe stock having any of the above-
mentioned compositions at (Acl + 50°C) to 400°C, preferably
750-400°C, and reducing the heated pipe stock at (Acl + 50°C)
to 400°C, preferably 750-400°C, such that the cumulative
diameter reduction is 20~ or more. The rolling is preferably
carried out such that at least one pass reduces the diameter
by 6~ or more per pass and the cumulative diameter reduction
is 605 or more. In addition, the reducing mentioned above is
preferably carried out by the aid of lubrication.
The present inventors also found that the above-mentioned
process permits the production of a steel pipe with high strength
and high toughness and superior resistance to stress corrosion
cracking if the composition of the pipe stock is specified in
an adequate range. This finding led the present inventors to
conceive to utilize the process for the production of line pipes .
Zine pipes conventionally have the content of impurities ,
such as S, reduced and the hardness controlled by means of
alloying elements for improvement in resistance to stress
corrosion cracking. Such conventional methods are limited in
s-treng~thening and pose a problem with high production cost.
Specifying the composition of the pipe stock in an adequate range
and performing the reduction in the ferrite recrystallizing
region yield a line pipe with high strength and high toughness,
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owing to dispersed fine ferrite and fine carbide, superior in
resistance to stress corrosion cracking resistance due to
limited alloying elements, leading to reduced hardening by
~Telding and less crack generation and propagation.
Accordingly, the present invention covers a process for
producing a steel pipe superior in ductility and resistance to
collision and impact as well as resistance to stress corrosion
cracking resistance, said process comprising heating a pipe
stock at (Aci + 50°C) to 400°C, preferably 750-400°C; and
reducing the heated pipe stock at (Acl + 50°C) to 400°C,
preferably 750-400°C, such that the cumulative diameter
reduction is 20~ or more, said pipe stock having a composition
of C 0.005-0.10, Si 0.01-0.5~, Mn 0.01-1.8~, A1 0.001-0.10,
one or more selected from Cu not more than 0.5~, Ni not more
than 0.6~, Cr not more than 0.5~, and Mo not more than 0.5~,
and one or more selected from Nb not more than 0.1~, V not more
than 0.1~, Ti not more than 0.1~, and B not more than 0.004,
or further one or more selected from REM not more than 0.02
and Ca. not more than 0.01, with the remainder being Fe and
unavoidable impurities.
The present inventors also found that the above-mentioned
process permits the production of a steel pipe with high strength
and high toughness and superior fatigue resistance if the
composition of the pipe stock is specified in an adequate range.
This finding led the present inventors to conceive to utilize
~
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l:he process for the production of steel pipes with high fatigue
resistance. Specifying the composition of the pipe stock in
an adequate range and performing the reduction in the ferrite
recrystallizing region yield a steel pipe with high strength
and high toughness, owing to dispersed fine ferrite and fine
precipitation, superior in fatigue resistance due to limited
alloying elements, leading to reduced hardening by welding and
less crack generation and propagation.
Accordingly, the present invention covers a process for
producing a steel pipe superior in ductility and strength as
well as fatigue resistance, said process comprising heating a
pipe stock at (Acl + 50°C) to 400°C, preferably 750-
400°C, and
reducing the heated pipe stock at (Acl + 50°C) to 400°C,
preferably 750-400°C, such that the cumulative diameter
reduction is 20~ or more, said pipe stock having a composition
of C 0.06-0.30, Si 0.01-1.5~, Mn 0.01-2.0~, and Al 0.001-0.10,
with the remainder being Fe and unavoidable impurities.
l:n the present invention, the above-mentioned composition
may be C 0. 06-0 . 30~, Si 0. 01-1. 5~, Mn 0. 01-2. 0~, A1 0. 001-0. 10~,
a:nd one or more selected from Cu not more than 1. 0~, Ni not more
than 2.0~, Cr not more than 2.0~, and Mo not more than 1.0~,
with t:he remainder being Fe and unavoidable impurities; the
above-mentioned composition may be C 0.06-0.30, Si 0.01-1.5~,
Mm 0.01-2.0~, A1 0.001-0. 10~, and one or more selected from Nb
not more than 0. 1~, V not more than 0.3~, Ti not more than 0.2~,
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and B not more than 0.004, with the remainder being Fe and
unavoidable impurities; or the above-mentioned composition may
be C 0.06-0.30, Si 0.01-1.5~, Mn 0.01-2.0~, A1 0.001-0.10,
a.nd one or more selected from REM not more than 0.02 and Ca
n.ot more than 0. 01~, with the remainder being Fe and unavoidable
impurities; the above-mentioned composition may be C 0.06-
0.30~, Si 0.01-1.5~, Mn 0.01-2.0~, A1 0.001-0.10, one or more
seleci~ed from Cu not more than 1.0~, Ni not more than 2.0~, Cr
not mare than 2.0~, and Mo not more than 1.0~, and one or more
selected from Nb not more than 0.1~, V not more than 0.3~, Ti
not more than 0 . 2$, and B not more than 0 . 004, with the remainder
being Fe and unavoidable impurities; the above-mentioned
composition may be C 0.06-0.30, Si 0.01-3.O~k, Mn 0.01-2.0~,
A1 0 . 001-0. 10~, one or more selected from Nb not more than 0. 1~,
V,not more than 0.3~, Ti not more than 0.2~, and B not more than
0 . 004$ , and one or more selected from REM not more than 0 . 02~
a:ad Ca not more than 0.01, with the remainder being Fe and
u:navoi.dable impurities; the above-mentioned composition may be
C 0.06-0.30, Si 0.01-1.5~, Mn 0.01-2.0~, A1 0.001-0.10, one
o:r more selected from Cu not more than 1.0~, Ni not more than
2 . 0 ~ , and Mo not more than 1 . 0 ~ , Mo not more than' 1 . 0 ~ , and one
o:r more selected from REM not more ,than 0. 02~ and Ca not more
than U.01~, with the remainder being Fe and unavoidable
impurities, or the above-mentioned composition may be C
0"06-0.30, Si 0.01-1.5~, Mn 0.01-2.0~, A1 0.001-0.10~k, one or
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more selected from Cu not more than 1. 0%, Ni not more than 2. 0%,
Cr not more than 2.0%, and Mo not: more than 1.0%, one or more
selected from Nb not more than 0.1%, V not more than 0.3%, Ti
not more than 0. 2%, and B not more than 0.004%, and one or more
selected from REM not more than 0 . C12% and Ca not more than 0 . O1 % ,
with the remainder being Fe and unavoidable impurities.
10 Best Mode for Carrying out the -Cnvention
The following explanation shows the process of producing
steel products according to the present invention.
The steel product of the pre~;ent invention has a structure
composed mainly of ferrite or ferrite plus pearlite or ferrite
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plus cementite; therefore, it is not specifically restricted
i.n it;s chemical composition so long as it has the structure
mentioned above. Apreferred composition to give the structure
of fei°rite or ferrite plus pearlite or ferrite plus cementite
is one which contains C not more than 0.60 wt~, preferably not
more i~han 0.20 wt~., more preferably not more than 0.10 wt~.
A.nothesr preferred composition is one which contains Si not more
than 2.0 wt~, Mn not more than 2.0 wt~, A1 not more than 0.10
wt~ , Cu not more than 1 . 0 wt~ , Ni not more than 2 . 0 wt~ , Cr not
more than 3.0 wt~, Mo not more than 2.0 wt~, Nb not more than
0 . 1 wt~ , V not more than 0 . 5 wt~ , Ti not more than 0 . 1 wt~ , and
B not more than 0.005 wt~. And, the structure may contain, in
addition to ferrite, pearlite, and cementite, not more than 30
volt of bainite without restriction. Needless to say, the
structure composed mainly of ferrite plus pearlite or the
structure composed mainly of ferrite plus cementite may contain
a small amount of cementite or pearlite, respectively.
According to the present invention, the steel product is
heated to a temperature, preferably, 800°C or lower, and then
rolled into a desired shape. The heating method is not
specifically restricted; however, induction heating is
desirable because of its high heating speed and its ability to
suppress the growth of crystal grains . The heating temperature
is preferably 800°C or lower at which crystal grains do not
become coarse, so that the grain size in the raw material is
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}:ept not greater than 20 (,im. This results in fine ferrite grains
not greater than 3 ~.~.m, preferably not greater than 1 ~.tm, after
subsequent ferrite recrystallization. The lower limit of the
heating temperature is 400°C, preferably 550°C, because with
heating under 400°C, the steel product presents difficulties
i.n rolling due to increase in deformation resistance.
Consequently, the heating temperature for rolling is 400-800°C,
preferably 600-700°C. Heating is carried out such that the
a.ustenitic change is 25~ or less.
The rolling temperature is restricted to a range in which
ferrite recrystallization takes place. In the present
invention, this range is preferably 400-750°C, depending on the
chemical composition of the steel blank used. Rolling at a
tempex:ature higher than this range gives rise to a two-phase
structure of ferrite plus austenite containing a large amount
of austenite or a single-phase structure of austenite. The
resulting product does not have the structure composed mainly
of ferrite or ferrite plus pearlite or ferrite plus cementite.
O:n the other hand, rolling at a temperature exceeding 750°C
causes ferrite grains to grow remarkably after
recrystallization. This is detrimental to the desired fine
g:rains not greater than 3 Win, preferably not greater than 2 ~..Lm.
Rolling at a temperature lower than 400°C is difficult to carry
out,due to blue shortness, with decrease in ductility and
toughness on account of insufficient recrystallization and
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residual deformation strain. Therefore, the rolling
temperature is 400-750°C, preferably 560-720°C, more
preferably 600-700°C. At 560-720°C, the grain size will be not
grreater than 1 (am, and at 600-700 °C, the grain size will be not
greater than 0.8 ~tm. Fig. 3 schematically shows the relation
between the grain size and the rolling temperature (at the start
and end of rolling).
Rolling is carried out such that the reduction of area is
greater than 20~. In the present invention, the reduction of
area is defined as the value calculated by the formula (Ao -
A) /A x 100 , where Ao is the cross sectional area before rolling
and A is the cross sectional area after rolling. With a
reduction of area less than 20~, rolling does not make
recrystallized grains finer because of insufficient strain.
The reduction of area is preferably greater than 50~.
After rolling, the steel product is cooled to room
temperature. Cooling may be natural air cooling or any of known
forced air cooling, water cooling, and mist cooling. The latter
i;s des:irable to suppress the growth of grains. The cooling rate
i;s preferably greater than 1°C/s.
An appropriate rolling method may be selected according
to the shape of the stock. For steel pipe stocks, reducing by
means of a plurality of grooved rolls , called as a reducer, is
desirable. Stocks adequate for this process include electric
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resistance welded pipes, forge-welded steel pipes, and solid
phase pressure-welded steel pipes.
l~ccording to the present invention, rolling is carried out
with lubrication. Lubricated rolling ensures uniform
distr_Lbution of strain and grain size in the thickness
direcl~ion. Rolling without lubrication tends to cause
concentrated strain in the surface and uneven grain size
distribution in the thickness direction. Ordinary rolling
oils, such as mineral oil and synthetic ester, may be used for
lubricated rolling. They are not specifically restricted.
The above-mentioned process yields a high-toughness,
high-ductility steel product which has a structure composed
mainly of ferrite or ferrite plus pearlite or ferrite plus
cementite, and which has an average grain size not greater than
3 Eun, preferably not greater than 1 E.lm, in the cross section
perpendicular to the lengthwise direction of the steel product.
The steel product of the present invention may have a structure
which contains not more than 30~ of bainite in addition to
ferrite, pearlite, and cementite. The steel product will
increase in strength but decrease in toughness and ductility
if it contains bainite more than specified above and martensite.
With an average grain size in excess of 3 E.Lm, the steel
product will lose a balance between strength and
toughness/ductility; that is, it does not meet the requirement
tY.iat elongation is 20~ or more and the product of tensile
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strength (TS: MPa) and elongation (E1: ~k) is 10000 or more. A
large average grain size leads to brittle cracking that occurs
in, the cross section in the lengthwise direction of the steel
pipe during Charily impact test at -100°C. This implies a
failure to meet the requirement for toughness that the percent
ductile fracture is 95~ or more, preferably 100. With an
average grain size not greater than 3 Vim, preferably not greater
than .1 ~.m, the steel pipe is less vulnerable to brittle cracking
in the cross section perpendicular to the lengthwise direction
and is superior in toughness.
The process of the present invention for producing steel
products will be described in more detail in the following, with
stress placed on steel pipes.
The present invention employs steel pipes as the stock.
(here are no specific restrictions on the process of producing
steel pipe stocks. Adequate examples include electric
resistance welded steel pipes produced by electric resistance
with high frequency current, solid-phase pressure-welded steel
pipes produced by pressure welding after heating edges to a
temperature suitable for solid-phase pressure-welding,
forge-welded steel pipes, and seamless steel pipes produced by
Mannesmann piercing rolling.
The following explains the reason why the chemical
composition is restricted for the steel pipes as stock and
product.
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C . 0.005-0.30
C is an element which dissolves in the basic metal to form
a solid solution or precipitates in the form of carbide in the
basic metal, thereby increasing the strength of steel.
Cementite, martensite, and bainite that precipitate in the form
of fine grains as the hard secondary phase contribute to
ductility (uniform elongation). For the desired strength and
ductility due to cementite that precipitates as the secondary
p~haseJ the content of C is 0.005 or more, preferably 0.04 or
more. C in excess of 0.30 increases strength so much as to
adversely affect ductility. Therefore, the content of C is
limited to 0.005-0.30, preferably 0.04-0.30. Moreover, the
content of C is not more than 0. 10~ for the improvement of line
pipe iri resistance to stress corrosion cracking. C in excess
of 0.10 makes the weld zone hard, thereby adversely affecting
resistance to stress corrosion cracking.
For the steel pipe to have high fatigue strength and
improved fatigue resistance characteristics, the content of C
is preferably 0.06-0.30. A content less than 0.06 leads to
poor fatigue resistance characteristics due to strength.
Si . 0.01-3.0~
Si is an element that functions as a deoxidizer and also
forms a solid solution in the basic metal to increase the
strength of steel. It produces its effect when its content is
0 . 01~ or more, preferably 0. l~k or more. With a content in excess
'' CA 02281316 1999-09-02
22
of 3.0~, it adversely affects ductility. Therefore, the
content of Si is limited to 0.01-3.0~, preferably 0.1-1.5~.
Incidentally, the content of Si is not more than 0.5~ for
line pipes to have improve resistance to stress corrosion
cracking. Si in excess of 0 . 5~ makes the weld zone hard, thereby
adversely affecting resistance to stress corrosion cracking.
For the steel pipe to have high fatigue strength and
improved fatigue resistance characteristics, the content of Si
is preferably not more than 1.5~. A content in excess of 1.5~
leads to poor fatigue resistance characteristics because it
forms inclusions.
NLn . 0.01-2.0~
Nfn is an element to increase the strength of steel. In
the present invention, it also causes cementite as the secondary
phase to precipitate in the form of fine grains and promotes
the precipitation of martensite and bainite. With an amount
less than 0.01, it does not increase the strength, nor does
iii promote the precipitation of cementite, martensite, and
bainite. With an amount in excess of 2.0~, it adversely affects
ductility due to unduly increased excessive strength.
Therefore, the amount of Mn is limited to 0.01-2.0~. From the
standpoint of strength-elongation balance, it is 0.2-1.3~,
preferably 0.6-1.3~.
Incidentally, the content of Mn is preferably not more than
1.8~ for line pipes to have improved resistance to stress
' CA 02281316 1999-09-02
23
corrosion cracking. Mn in excess of 1.8~ makes the weld zone
hard, thereby adversely affecting resistance to stress
corrosion cracking.
A1 . 0.001-0.10
i~11 helps form fine grains . The content of A1 is at least
0~.001'~ for desired fine grains. With a content in excess of
0.10, it increases the amount of oxygen-based inclusions,
thereby adversely affecting cleanliness. Therefore, the
content of A1 is limited to 0.001-0.10, preferably 0.015-
0.06~,.
Furthermore, the above-mentioned composition for the
steel pipe stock may contain additionally one or more of the
following alloying elements.
Cu : not more than 1~, Ni : not more than 2~, Cr : not more than
2~, and Mo . not more than l~k.
These elements improve the hardenability of steel and
increase the strength of steel. They may be used alone or in
combiriation with one another according to need. They lower the
transformation point and give rise to fine ferrite grains and
make t:he secondary phase fine grains . The content of Cu is not
more 'than 1~, preferably 0.1-0.6~, because excessive Cu
adversely affects hot workability. The content of Ni is not
more than 2~, preferably 0.1-1.0~, because excessive Ni is
wasted without further effect of increasing strength and
improving toughness. The contents of Cr and Mo are not more
' CA 02281316 1999-09-02
,.
24
than 2~ and 1~, respectively, preferably 0.1-1.5~ and 0.05-
0.5~, respectively; excessive Cr and Mo adversely affect
weldability and ductility only to be wasted.
Incidentally, each of the contents of Cu, Ni, Cr, and Mo
i.s not more than 0. 5~ for line pipes to have improved resistance
to stress corrosion cracking. When used in excess of 0. 5~, they
make the weld zone hard, thereby adversely affecting resistance
to stress corrosion cracking.
Nb : not more than 0.1~, V : not more than 0.3~, Ti : not more
than U.2~, and B . not more than 0.004.
These elements precipitate in the form of carbide,
nitride, or carbonitride, contributing to fine grains and high
strength. For steel pipes having joints heated at a high
temperature, they make grains finer during heating and they also
function as nuclei for ferrite precipitation during cooling,
thereby preventing the weld zone from becoming hard. They may
be used alone or in combination with one another according to
need. When used excessively, they adversely affect weldability
and toughness. Therefore, the content of Nb is not more than
0.. 1~, ~~referably 0. 005-0 . 05~; the content of V is not more than
0,.3~, preferably 0.05-0.1~; the content of Ti is not more than
0 ~ 2~, ~~referably 0. 005-0 . 10~; and the content of B is not more
than 0.004, preferably 0.0005-0.002.
Incidentally, each content of Ni, V, and Ti is not more
than 0,.1~ for line pipes to have improved resistance to stress
CA 02281316 1999-09-02
,.
corrosion cracking. When used in excess of 0 . 1~, they adversely
a.ffec-ting resistance to stress corrosion cracking due to
precipitation hardening.
REM . not more than 0.02 and Ca . not more than 0.01.
F3oth REM and Ca adjust the form of inclusions and improve
workability. They also precipitate in the form of sulfide,
oxide or oxysulfide, thereby preventing the j oints of steel pipe
from becoming hard. They may be used alone or in combination
with tine another. When used excessively, they give rise to
excessive inclusions, which lower cleanliness and adversely
affect ductility. The content of REM is 0.004-0.02 and the
content of Ca is 0.001-0.01.
The above-mentioned composition for the steel pipe stock
and steel product may additionally contain Fe as a remainder
a.nd unavoidable impurities as follows.
Unavoidable impurities are N : not more than 0.010, O
not more than 0. 006, P : not more than 0. 025, and S : not more
than 0.020.
N . nat more than 0.010
N in an amount up to 0.010 is permissible, which is enough
to form fine grains in combination with Al; however, excessive
N adversely affects ductility. The content of N is not more
than 0.010$, preferably 0.002-0.006.
O . not more than 0.006
'' CA 02281316 1999-09-02
26
O in an amount up to 0.006 is permissible. The content
of O is as low as possible, because O forms oxides which adversely
affect cleanliness.
P . not more than 0.025
1? segregates at grain boundaries, thereby adversely
affecting toughness. The content of P is as low as possible,
although up to 0.025 is permissible.
S . not more than 0.020
S in an amount up to 0.020 is permissible. The content
of S :Ls as low as possible, because S forms sulfides which
adversely affect cleanliness.
The following concerns the structure of the steel pipe as
the product.
The steel pipe of the present invention is characterized
by its structure composed of ferrite grains not larger than 3
~trn, preferably not larger than 1 dim, so that it is superior in
ductility and collision and impact resistance. With ferrite
grains coarser than 3 Nm, the steel pipe will not have remarkably
improved ductility and collision and impact resistance. The
ferrite grain size is expressed in terms of average value of
200 or more ferrite grains regarded as circles which are observed
under an optical or electron microscope when the cross section
perpendicular to the lengthwise direction of the steel pipe is
corroded with nitral solution.
' CA 02281316 1999-09-02
,,
27
In the present invention, the structure composed mainly
of ferrite includes the one which is composed of ferrite alone
without secondary phase and the one which is composed of ferrite
and a secondary phase other than ferrite. The secondary phase
other than ferrite includesmartensite,bainite,and cementite.
They may precipitate alone or in combination with one another.
The secondary phase should have a ratio of area not more than
30~. The secondary phase that has precipitated helps
elongation to occur evenly at the time of deformation, thereby
improving the ductility and collision and impact resistance of
the steel pipe. This effect becomes less significant as its
ratio of area exceeds 30~. Fig. 4 shows an example of the
structure of the steel pipe of the present invention.
fhe following concerns the process for producing the steel
pipe o~f the present invention.
The process starts with heating the steel pipe stock having
the above-mentioned composition. The heating temperature is
(Aci + 50°C) to 400°C, preferably 750-400°C. Heating
beyond
the upper limit deteriorates the surface properties and unduly
increases austenite, resulting is coarse grains. Therefore,
the heating temperature is not higher than (Acl + 50°C),
preferably not higher than 750 ° C. Heating below the lower limit
does not provide an adequate rolling temperature. Therefore,
the heating temperature is preferably 400°C or higher.
' CA 02281316 1999-09-02
28
The heated steel pipe stock subsequently undergoes
reducing preferably by a reducing mill of 3-roll type or 4-
roll type or any other types. Continuous reducing by a
plurality of stands is preferable. The number of stands depends
an the dimensions of the steel pipe stock and finished steel
pipe.
'.L'he rolling temperature for reducing is (Acl + 50 ° C) to
400°C, preferably ,750-400°C,. at which ferrite re-
crystallization takes place. A rolling temperature beyond the
upper limit causes ferrite grains to grow excessively after
recrystallization, thereby decreasing ductility. Therefore,
the rolling temperature is not higher than (Acl + 50°C),
preferably not higher than 750°C. On the other hand, a rolling
temperature below the lower limit brings about blue shortness,
which leads to brittleness and fracture during rolling. A
rolling temperature below 400°C causes such troubles as
ia~crea.sed deformation resistance, hence difficulties in
rolling of material, and insufficient recrystallization, hence
residual strain. Therefore, the rolling temperature for
reducing is (Acl + 50°C) to 400°C, preferably 750-400°C,
and
more preferably 600-700°C.
Reducing is carried out such that the cumulative diameter
reduction is 20~ or more, which is defined by (A - B) /A x 100,
where .A is the outside diameter of the base steel pipe and B
is the outside diameter of the product pipe. Failing to meet
CA 02281316 1999-09-02
29
this requirement results in a steel pipe poor in ductility
because of insufficient action by recrystallization to make
grains finer. Another problem is a low pipe forming rate and
hence low productivity. In the present invention, therefore,
the cumulative diameter reduction is greater than 20~.
However, if it exceeds 605, the resulting steel pipe will have
high strength and high ductility which are well balanced with
each other even though the content of the above-mentioned
alloying elements is low, on account of work hardening, leading
to increased strength, and finer structure. For this reason,
the cumulative diameter reduction is preferably 60~ or more.
Reducing is carried out such that at least one of rolling
passes accomplishes diameter reduction 6~ or more per pass.
Reducing with a diameter reduction smaller than 6~ per pass does
not produce the effect of making crystal grains finer by
recrystallization. Reducing with a diameter reduction of 6~
oz: more per pass generates heat, hence increases temperature,
keeping the desired rolling temperature. The diameter
reduction per pass is preferably 8~ or more for dynamic
recrystallization and finer crystal grains.
TIZe reducing of steel pipes according to the present
in.vent_ion provides biaxial stress, thereby producing a
significant effect of .making crystal grains finer. By
contrast, the rolling of steel plates merely provides uniaxial
stress,. with free ends existing in the rolling direction as well
CA 02281316 1999-09-02
as the widthwise direction (or the direction perpendicular to
the rolling direction) . Therefore, the rolling in this way is
limited in ability to make grains finer.
Also, the reducing of steel pipes according to the present
invention is preferably carried in the presence of a lubricant.
Z~ubricated rolling makes even the strain distribution in the
thickness direction and also makes even the grain size
d.istr:Lbution in the thickness direction. Rolling without
lubrication concentrates strain in the surface of the material
due to shear effect, resulting in uneven grain size in the
thickness direction. Any known rolling oil, such as mineral
oil and a mixture of mineral oil and synthetic ester, may be
used as a lubricant.
After reducing, the steel pipe is cooled to room
tempez:ature . Cooling may be natural air cooling or any of known
forced air cooling, water cooling, and mist cooling to suppress
the growth of grains. The cooling rate is preferably greater
than 1.0°C/s.
~..1, a 1
A steel raw material having the chemical composition shown
i:n Table 1 was made into flat strip steel of 3.2 mm in thickness
b:y hot. rolling. After preheating at 600°C, this strip steel
was continuously formed into an open pipe by means of a plurality
o:E forming rolls. The open pipe had its edges preheated to
1000°C by induction heating, and the edges were heated to 1300°C
" CA 02281316 1999-09-02
31
by induction heating and joined together by solid-phase
pressure welding using squeeze rolls. Thus there was obtained
a pipe stock, 31.8 mm in diameter and 3.2 mrii in wall thickness.
fnTith .its seam cooled, the pipe stock was induction-heated to
temperatures shown in Table 2. The heated pipe stock was
reduc<~d by means of a 3-roll reducing mill to form a product
steel pipe having the outside diameter shown in Table 2.
Incidentally; lubricated rolling with a mixture of mineral oil
and synthetic ester was performed on the product No. 1-2.
The product pipe thus obtained was found to have the
characaeristic properties, i.e., structure, grain size,
tensile properties, and impact properties, as shown in Table
2. Grain size was determined by observing the cross section
(C) perpendicular to the lengthwise direction of the pipe under
a microscope (x5000) and expressed in terms of an average of
five or more observations. Tensile properties were measured
b:y using JIS No. l1 specimens. Incidentally, elongation (El)
is expressed in terms of values calculated from
E1 = Elo x (,/- (ao/a) ) 0 . 4
(where Elo is the actually measured elongation, ao is 100 mm2,
and a is the sectional area (mm2) of the specimen.) This
converted value was used in consideration of the size effect
of the specimen. Impact properties (toughness) was evaluated
in terms of percent ductile fracture of cross section C at -100 ° C
' CA 02281316 1999-09-02
32
measured in Charpy impact test with a 2-mm V notch in the
7_engt:hwise direction of the pipe.
It is noted from Table 2 that samples (Nos. 1-1 to 1-3)
i.n examples pertaining to the present invention are
characterized by a grain size of 2 j.Im, or fine grains not greater
than 3 /.lm, and also by~high elongation and toughness and
well-balanced strength and toughness/ductility. Sample No.
1-2, which underwent lubricated rolling, shows only a little
variation in grain size in the thickness direction. In
contrast, sample Nos . 1-4 and 1-5 (in comparative example) are
poor in. ductility and toughness due to coarse grains.
Incidentally, it was found that pearlite (P) includes, in
addition to the lamellar structure, pseudo pearlite which does
not form the lamellar structure.
E.x amp 7 a 2
1?~ steel raw material having the chemical composition shown
i:n Table 1 was made into flat strip steel of 3.2 mm in thickness
by hot rolling. This strip steel was continuously formed into
a:n open pipe by means of a plurality of forming rolls . The open
pipe had its edges preheated above the melting point by induction
heating, and the edges were butt-welded by using squeeze rolls .
Thus there was obtained a pipe stock, 31.8 mm in diameter and
3 . 2 mm in wall thickness . With its bead removed by a bead cutter,
the resulting electric welded pipe was heated again at the
temperature shown in Table 3 by induction heating. It was
' CA 02281316 1999-09-02
,.
.
33
reduced by means of a 3-roll reducing mill to form a finished
pipe having the outside diameter shown in Table 3.
'The finished pipe thus obtained was tested for
characteristic properties, i.e., structure, grain size,
tensile properties, and toughness, in the same manner as in
E:Kample 1. The results are shown in Table 3.
It is noted from Table 3 that samples (Nos . 2-2 , 2-3 , 2-5 ,
and 2-7) in examples pertaining to the present invention are
characterized by fine grains not greater than 3 ~m and also by
h~.gh elongation and toughness and well-balanced strength and
taughness/ductility. By contrast,samples (Nos.2-1,2-4,2-6,
2-8 , and 2-9 ) in comparative examples are poor in ductility and
toughness due to coarse grains.
~X~,m 1 e'
A steel having the composition shown in Table 1 was prepared
by using a converter, and this steel was made into a billet by
the continuous casting process. After heating, this billet was
made into a seamless pipe of 158 mm in outside diameter and 8
mm in wall thickness by using a Mannesmann mandrel mill. This
seamless pipe was heated again to the temperature shown in Table
4 by induction heating and then reduced by means of a 3-roll
reducing mill to form a product pipe having the outside diameter
shawn in Table 4.
' CA 02281316 1999-09-02
,.
34
'rhe product pipe thus obtained was tested for
characteristic properties in the same manner as in Examples 1
a.nd 2. The results are shown in Table 4.
:Lt is noted from Table 4 that samples (Nos . 3-1 , 3-2 , 3-4 ,
and 3--5) in examples pertaining to the present invention are
characterized by fine grains not greater than 3 ~m and also by
high elongation and toughness and well-balanced strength and
toughness/ductility. By contrast, samples (Nos. 3-3 and 3-
6) in comparative examples are poor in ductility and toughness
due to coarse grains.
Fr base steel pipe having the chemical composition shown
i:n Table 5 was heated by induction to a temperature shown in
Table 6 and then rolled into a finished steel pipe by means of
a 3-roll reducing mill under the rolling conditions shown in
Table 6.
The base steel pipe in Table 6 is either solid-phase
pressure-welded one or seamless one. The former was prepared
by preheating a 2.6 mm thick hot-rolled strip steel to 600°C,
continuously forming it into an open pipe by means of a plurality
of fornling rolls , preheating the edges of the open pipe to 1000 ° C
b5r induction, heating the edges to 1450 ° C below the melting point
b5r induction, and pressure-welding the edges by means of a
squeeze roll. It is 42.7 mm in diameter and 2.6 mm in wall
'' CA 02281316 1999-09-02
,.
thickness . The seamless pipe was prepared by using a Mannesmann
mandrel mill from a continuously cast billet (with heating).
'rhe product pipe thus obtained was tested for tensile
properties, collision and impact properties, and structure.
The results are shown in Table 6. Tensile properties were
measured by using JIS No.ll specimens. Incidentally,
elongation (E1) is expressed in terms of values calculated from
E1 = Elo X (.~/- (ao/a) ) 0 . 4
(where Elo is the actually measured elongation, ao is 292 mm2,
and a is the sectional area (mm2) of the specimen.) This
converted value was used in consideration of the size effect
of the specimen. Collision and impact properties were
evaluated in terms of the amount of energy which is absorbed
before the amount of strain reaches 30~ in the stress-strain
curve obtained by the high-speed tensile test at a strain rate
o:f 2000 s-i. Incidentally, collision and impact properties are
a measure of energy required to deform the material when an
automobile actually collides at a strain rate of 1000-2000 s~'1.
The larger the amount of this energy, the better the collision
and impact resistance.
It is noted from Table 6 that samples (Nos. 4-1 to 4-16
and 4-7.9 to 4-22 ) in examples pertaining to the present invention
have well-balanced ductility and strength, with a high tensile
strength at a high strain rate and a high energy absorption at
the time of collision and impact. By contrast, samples (Nos.
~ CA 02281316 1999-09-02
,.
36
46-17 , 4-18 , and 4-23 ) in comparative examples are poor in either
ductility or strength, poor in balance between strength and
ductility, and poor in collision and impact resistance.
Comparative samples (Nos.4-17 and 4-18), which do not
conform to the present invention in diameter reduction, have
coarse ferrite grains, unbalanced strength-ductility, and low
energy absorption at the time of collision and impact.
~xamo.~Le 5
1~ base steel pipe having the chemical composition shown
in Table 7 was heated by induction to a temperature shown in
Table 8 and then rolled into a product steel pipe by means of
a 3-roll reducing mill under the rolling conditions shown in
Table 8. Incidentally, the steel pipe stock was prepared in
the same manner as in Example 4.
The product steel pipe was tested for tensile properties,
colli:;ion and impact properties , and structure in the same way
as in Example 4. The results are shown in Table 8.
It is noted from Table 8 that samples (Nos . 5-1 to 5-3 and
5-7 to 5-10) in examples pertaining to the present invention
have well-balanced ductility and strength, with a high tensile
strength at a high strain rate and a high energy absorption at
t:he time of collision and impact. By contrast, samples (Nos.
5-4 to 5-6 ) in comparative examples are poor in either ductility
'' CA 02281316 1999-09-02
,> .
37
ar strength, poor in balance between strength and ductility,
a.nd poor in collision and impact resistance.
The present invention provides a steel pipe having
well-balanced ductility and strength and good collision and
impact properties, unlilce the conventional technology. This
steel pipe is suitable for bulging by hydroforming or the like.
Bulging will be very easy to perform in the case of electric
welded pipe or solid-phase pressure-welded pipe with the seam
cooled, because the hardened seam has the same level of hardness
as the pipe stock on account of reducing.
Erxa~mmle 6
A. base steel pipe, 110 mm in diameter and 4.5 mm in wall
thickness, having the chemical composition shown in Table 9 was
produced from hot-rolled steel plate which had undergone
controlled rolling and controlled cooling. The base steel pipe
was heated by induction to a temperature shown in Table 10 and
then reduced by using a 3-roll reducing mill under the condition
shown .in Table 10.
The product steel pipe was tested for tensile properties,
collision and impact properties, structure, and sulfide stress
corrosion cracking resistance. The results are shown in Table
10. Tensile properties were measured by using JIS No. l1
specimens in the same manner as in Example 4. Incidentally,
elongai~ion (E1) is expressed in terms of values calculated from
CA 02281316 1999-09-02
.>
r _
38
E1 = Elo X (,/- (ao/a) ) 0 . 4
(where Elo is the actually measured elongation, ao is 292 mm2,
and a is the sectional area (mm2) of the specimen.) This
converted value was used in consideration of the size effect
of the specimen.
Collision and impact properties were evaluated in terms
of the amount of energy which is absorbed before the amount of
strain reaches 30~ in the stress-strain curve obtained by the
high-speed tensile test at a strain rate of 2000 s-1.
Incidentally, collision and impact properties are a measure of
energy required to deform the material when an automobile
actually collides at a strain rate of 1000-2000 s-1. The greater
the amount of this energy, the better the collision and impact
resistance.
7:ncidentally, the sulfide stress corrosion cracking
resistance was evaluated by observing whether or not a C-ring
test piece shown in Fig. 5 breaks within 200 hours when it is
immersed under a tensile stress corresponding to 120 of yield
strength in an NACE bath (composed of 0.5~ acetic acid and 5~
sodium chloride, saturated with hydrogen sulfide) at 25 ° C and
1 atm. The C-ring test piece was cut out of the product pipe
in its circumferential direction. This test was duplicated for
each sample under the same conditions.
It is noted from Table 10 that samples (Nos. 6-1 to 6-
3, 6-6, 6-8 to 6-10) in examples pertaining to the present
'' CA 02281316 1999-09-02
.>
39
invention have well-balanced ductility and strength, high
t:ensi.le strength at high strain rate, and high energy absorption
at the time of collision and impact. They are also superior
i.n sulfide stress corrosion cracking resistance, and hence they
a.re suitable for use as line pipes .. By contrast, samples (Nos .
6-4 , E-5 , and 6-7 ) in comparative examples are poor in either
ductility or strength, poor in balance between strength and
ductility, poor in collision and impact properties, and poor
in su7.fide stress corrosion cracking resistance as indicated
by br~:akage in the NACE bath.
Samples (Nos . 6-4 and 6-7 ) in comparative examples , which
were reduced at a rolling temperature outside the range
specif=ied in the present invention, are poor in balance between
strenJth and ductility due to coarse ferrite grains, poor in
energy absorption at the time of collision and impact, and poor
i:n sulfide stress corrosion cracking resistance.
A base steel pipe having the chemical composition shown
in Table 11 was heated by induction to a temperature shown in
Table 12 and then rolled into a product steel pipe by means of
a 3-roll reducing mill under the rolling conditions shown in
Table 12. The base steel pipe in this example was either
electric resistance welded pipe of 110 mm in diameter and 2.0
mm in wall thicltness or seamless steel pipe of 110 mm in diameter
' CA 02281316 1999-09-02
r
and 3 . 0 mm in mall thickness . The former was prepared by forming
an open pipe from hot-rolled strip steel by means of a plurality
of forming rolls and then welding the edges by induction heating.
The latter was prepared by using a Mannesmann mandrel mill from
a. continuously cast billet with heating.
'.Che product pipe thus obtained was tested for tensile
properties, collision and impact properties, structure, and
fatigue resistance. The results are shown in Table 12.
Tensile properties and collision and impact properties were
measured in the same manner as in Example 4. Fatigue strength
was measured by subj ecting the finished pipe as a specimen to
cantilever reversed fatigue test (at a repeating rate of 20 Hz)
in the air.
It is noted from Table 12 that samples (Nos . 7-1, 7-3 , and
7-6 to 7-8) in examples have well-balanced ductility and
s'treng'th, high tensile strength at high strain rate, and high
energy absorption at the time of collision and impact. In
addition, they are superior in fatigue resistance. By
contrast, samples (Nos. 7-2, 7-4, and 7-5) in comparative
e~~amples are poor in fatigue strength. Sample No. 7-2 did not
undergo reducing, sample 7-5 had a ratio of reduction in diameter
which is outside the specified range, and sample No. 7-4 was
reduced. at a temperature outside the specified range.
Therefore, it is poor in balance between strength and ductility
' CA 02281316 1999-09-02
41
due to coarse ferrite grains, poor in energy absorption at the
tame of collision and impact, and poor in fatigue resistance.
f,xr~1_o~tati pn i n Irid ~gt'r,~
'The present invention provides a high-strength steel
product superior in toughness and ductility on account of
extremely fine grain size not greater than 3 ~tm. Therefore,
it will produce a significant industrial effect of expanding
the application area of steel products. The present invention
a:Lso provides a process for efficient and easy production of
high-strength steel pipe superior in ductility and impact
resistance. Therefore, it will produce a significant
industrial effect of expanding the application area of steel
pipe. The present invention permits the production of steel
pipes for line pipes which need high strength and toughness and
good stress corrosion cracking resistance. The present
invention also permits the economical production of high-
strength, high-ductility steel pipe having good fatigue
resistance, with the amount of alloying elements reduced.
CA 02281316 1999-09-02
42
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