Note: Descriptions are shown in the official language in which they were submitted.
CA 02441720 2003-11-12
NSC-M256
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HIGHLY IMPACT-RESISTANT STEEL PIPE
AND METHOD FOR PRODUCING THE SAME
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a highly
impact-resistant steel pipe used as a member, such as a
material for a door impact beam, a bumper, a bumper
reinforcement or the like of an automobile, that requires
an impact absorption energy, and a method for producing
the steel pipe. Though the term "a steel pipe" is
generally regarded as a steel pipe having a circular
sectional shape, it is regarded as a steel pipe having a
round or square sectional shape in this specification.
Here, the round shape includes a circular shape, an
elliptical shape, etc. and the square shape includes not
only a polygonal shape such as a triangular shape, a
tetragonal shape, a pentagonal shape, etc. but also an
irregular sectional shape.
2. Description of the Related Art
A high strength electric-resistance-welded
steel pipe is mostly used for a member of a door impact
beam disposed for absorbing the shock of a lateral
collision of an automobile. Such a steel pipe is
required to have an excellent impact absorbing capacity
that is secured by a high tensile strength and a low
proof stress and, when a steel pipe is one produced by
electric-resistance-welding, the steel pipe is further
required not to undergo the deterioration of strength and
toughness at the welded portion. Such an impact
absorbing capacity is expressed by a yield ratio defined
by the ratio of a proof stress to a tensile strength.
However, in general, the proof stress of a steel pipe
increases as the tensile strength thereof increases and,
therefore, the tensile strength and the yield ratio of a
conventional steel pipe has been 1,500 to 1,600 MPa and
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70 to 80%, respectively, at the least. For that reason,
a steel pipe having a higher strength and a lower yield
ratio has been desired for further promoting weight
reduction and improving collision safety.
Meanwhile, a proof stress has so far been
generally obtained by measuring a stress imposing a
permanent strain of 0.2% on a test piece as it is
stipulated in JIS and a yield ratio has been calculated
on the basis of the 0.2%-proof stress. Fig. 1 is a graph
schematically showing the stress-strain curve of a steel
and the impact energy that can be absorbed by the
deformation of an impact absorbing member at the time of
collision is represented by the area of the hatched
portion. As a steel pipe has so far been evaluated by a
0.2%-proof stress, an impact absorbing member has been
designed with the area of the portion hatched by the
oblique lines sloping to the right regarded as an impact
energy absorbing capacity. On the contrary, according
the research by the present inventors, it has been
clarified that, in the case of a steel material not
having a clearly visible yield point, namely having a
stress-strain curve with a sloping shoulder, a
considerable amount of impact energy is absorbed before a
permanent strain of 0.2% is imposed on the steel pipe at
the time of collision as shown in Fig. 1 and therefore
the impact absorbing capacity of the steel material has
been underestimated when it is evaluated by a yield ratio
calculated on the basis of a 0.2%-proof stress stipulated
in JIS.
Needless to say, an electric-resistance-welded
portion and the vicinity thereof are also required to
have not only the strength but also the toughness that
governs an impact energy absorbing capacity. To meet the
requirements, various developments have been used in an
effort to improve those properties as shown in Japanese
Unexamined Patent Publication No. H7-18374. However,
most electric-resistance-welded steel pipes tend to
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undergo deterioration of strength and toughness in the
vicinity of the electric-resistance-welded portions
because the steel pipes are produced by electric-
resistance-welding.
SUMMARY OF THE INVENTION
The object of the present invention is, by solving the
aforementioned conventional problems, to provide: a highly
impact-resistant steel pipe having the material properties
of a higher strength and a lower yield ratio than a
conventional steel pipe, not undergoing the deterioration
of toughness in the vicinity of an electric-resistance-
welded portion in the case of an electric-resistance-welded
steel pipe, and further having an ultra-low weight and a
high collision safety; and a method for producing the steel
pipe.
A highly impact-resistant steel pipe according to
the present invention is established for solving the
aforementioned problems and is characterized in that: the
tensile strength (hereunder referred to as TS) of the
steel pipe is 1,700 MPa or more; and the yield ratio
(hereunder referred to as YR) thereof, the yield ratio
being the ratio of the 0.1%-proof stress (hereunder
referred to as YS) to the tensile strength TS (YS/TS), is
72% or less. Likewise, the present invention also provides
a steel pipe having a TS that is 1,800 MPa or more and a
YR that is 70% or less, a TS that is 1,900 MPa or more and
a YR that is 68% or less and a TS that is 2,000 MPa or
more and a YR that is 66% or less. Here, it is preferable
that the dislocation density of a steel pipe after a
tensile test according to JIS is in the range from 1010 to
1014/mm-2 .
Further, the present inventors: measured the
properties of Charpy absorbed energy in the vicinity of
electric-resistance-welded portions in an attempt to solve
the aforementioned problems; found that oxides containing
Si and Mn remained on the fractured surface at a portion
where Charpy absorbed energy decreased and
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those components were one of the causes of the
deterioration of toughness; and confirmed that the
deterioration of toughness in the vicinity of an electric-
resistance-welded portion could be prevented when a
specific relationship between Si and Mn, namely according to
the expression Mn/8 - 0.07 :_5 Si :_!5 Mn/8 + 0.07, was secured.
The present invention also provides a highly impact-
resistant electric-resistance-welded steel pipe
characterized by having a high strength of 1,700 MPa or
more in tensile strength and being produced by controlling
the Si amount in the steel of the steel pipe in the range
from Mn/8 - 0.07 to Mn/8 + 0.07.
A highly impact-resistant steel pipe according to
the present invention: preferably contains, in mass, 0.19
to 0.35% C, 0.1 to 0.3% Si, 0.5 to 1.6% Mn, not more than
0.025% P, not more than 0.02% S, 0.010 to 0.050% Al, 2 to
35 ppm B, and 0.005 to 0.05% Ti as indispensable
components; and may further contain, in mass, arbitrary
components selected from among the group of 0.005 to
0.050% Nb, 0.005 to 0.070% V, 0.005 to 0.5% Cu, 0.005 to
0.5% Cr, 0.1 to 0.5% Mo, 0.1 to 0.5% Ni, not more than
0.01o Ca, and not more than 0.1% rare earth metals (REMs).
Further, it is preferable that 950 or more of the
microstructure of the steel pipe is transformed into
martensite by induction hardening and the prior austenite
grain size number of the steel pipe is #6 or more.
Here, the sectional shape includes both a round shape
and a square shape.
The present invention also provides a highly
impact-resistant steel pipe characterized in that it
essentially consists of, in mass, 0.19 to 0.35% C, 0.5 to
1.60% Mn, not more than 0.025% P, not more than 0.01% S,
0.010 to 0.050% Al, 2 to 35 ppm B, and 0.005 to 0.05% Ti, Si
in an amount that is controlled in the range from Mn/8 - 0.07
to Mn/8 + 0.07 (mass %), as indispensable compounds and
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unavoidable impurities; the tensile strength TS of said steel
pipe is 1,700 MPa or more; and the YR thereof, said YR being
the ratio of the 0.1%-proof stress YS to said tensile
strength TS (YS/TS), is 72% or less.
The present invention also provides a highly
impact-resistant steel pipe characterized in that it
essentially consists of, in mass, 0.19 to 0.35% C, 0.5 to
1.60% Mn, not more than 0.025% P, not more than 0.01% S,
0.010 to 0.050% Al, 2 to 35 ppm B, and 0.005 to 0.05% Ti, Si
in an amount that is controlled in the range from Mn/8 - 0.07
to Mn/8 + 0.07 (mass %), as indispensable compounds and
unavoidable impurities; the tensile strength TS of said
steel pipe is 1,800 MPa or more; and the YR thereof, said YR
being the ratio of the 0.1%-proof stress YS to said tensile
strength TS (YS/TS), is 70% or less.
The present invention also provides a highly
impact-resistant steel pipe characterized in that it
essentially consists of, in mass, 0.19 to 0.35% C, 0.5 to
1.60% Mn, not more than 0.025% P, not more than 0.01% S,
0.010 to 0.050% Al, 2 to 35 ppm B, and 0.005 to 0.05% Ti, Si
in an amount that is controlled in the range from Mn/8 - 0.07
to Mn/8 + 0.07 (mass %), as indispensable compounds and
unavoidable impurities; the tensile strength TS of said
steel pipe is 1,900 MPa or more; and the YR thereof, said YR
being the ratio of the 0.1%-proof stress YS to said tensile
strength TS (YS/TS), is 68% or less.
The present invention also provides a highly
impact-resistant steel pipe characterized in that it
essentially consists of, in mass, 0.19 to 0.35% C, 0.5 to
1.60% Mn, not more than 0.025% P, not more than 0.01% S,
0.010 to 0.050% Al, 2 to 35 ppm B, and 0.005 to 0.05% Ti, Si
in an amount that is controlled in the range from Mn/8 - 0.07
to Mn/B + 0.07 (mass %), as indispensable compounds and
unavoidable impurities; the tensile strength TS of said
steel pipe is 2,000 MPa or more; and the YR thereof, said YR
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being the ratio of the 0.1%-proof stress YS to said tensile
strength TS (YS/TS), is 66% or less.
The invention also provides a highly impact-resistant steel
pipe characterized in that it essentially consists of, in mass,
0.19 to 0.35% C, 0.5 to 1.60% Mn, not more than 0.025% P, not more
than 0.01% S, 0.010 to 0.050% Al, 2 to 35 ppm B, and 0.005 to
0.05% Ti, Si in an amount of 0.10 to 0.30% and Si is controlled in
the range from Mn/8 - 0.07 to Mn/8 + 0.07 (mass o), as
indispensable components and further one or more optional
components selected from among the group consisting of 0.005 to
0.050% Nb, 0.005 to 0.07001 V, 0.005 to 0.5% Cu, 0.1 to 0.5% Mo,
0.1 to 0.5% Ni, not more than 0.01o Ca, not more than 0.1% rare
earth metals (REMs) and mixtures thereof, the balance being iron
and unavoidable impurities; the tensile strength TS of said steel
pipe is 1,700 MPa or more; and the YR thereof, said YR being the
ratio of the 0.1%-proof stress YS to said tensile strength TS
(YS/TS), is 72% or less.
The invention also provides a highly impact-resistant steel
pipe characterized in that it essentially consists of, in mass,
0.19 to 0.35% C, 0.5 to 1.60% Mn, not more than 0.02511 P, not more
than 0.01% S, 0.010 to 0.050% Al, 2 to 35 ppm B, and 0.005 to
0.05% Ti, Si in an amount of 0.10 to 0.30% and Si is controlled in
the range from Mn/B - 0.07 to Mn/8 + 0.07 (mass o), as
indispensable components and further one or more optional
components selected from among the group consisting of 0.005 to
0.050% Nb, 0.005 to 0.070% V, 0.005 to 0.5% Cu, 0.1 to 0.5% Mo,
0.1 to 0.5% Ni, not more than 0.01% Ca, not more than 0.1% rare
earth metals (REMs) and mixtures thereof, the balance being iron
and unavoidable impurities; the tensile strength TS of said steel
pipe is 1,800 MPa or more; and the YR thereof, said YR being the
ratio of the 0.1%-proof stress YS to said tensile strength TS
(YS/TS), is 70% or less.
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The invention also provides a highly impact-resistant steel
pipe characterized in that it essentially consists of, in mass,
0.19 to 0.35% C, 0.5 to 1.60% Mn, not more than 0.025% P, not more
than 0.01% S, 0.010 to 0.050% Al, 2 to 35 ppm B, and 0.005 to
0.05% Ti, Si in an amount of 0.10 to 0.30% and Si is controlled in
the range from Mn/8 - 0.07 to Mn/8 + 0.07 (mass o), as
indispensable components and further one or more optional
components selected from among the group consisting of 0.005 to
0.050% Nb, 0.005 to 0.070o V, 0.005 to 0.5% Cu, 0.1 to 0.5% Mo,
0.1 to 0.5% Ni, not more than 0.01% Ca, not more than 0.1o rare
earth metals (REMs) and mixtures thereof, the balance being iron
and unavoidable impurities; the tensile strength TS of said steel
pipe is 1,900 MPa or more; and the YR thereof, said YR being the
ratio of the 0.1%-proof stress YS to said tensile strength TS
(YS/TS), is 68% or less.
The invention also provides a highly impact-resistant steel
pipe characterized in that it essentially consists of, in mass,
0.19 to 0.35% C, 0.5 to 1.60% Mn, not more than 0.025% P, not more
than 0.01% S, 0.010 to 0.050% Al, 2 to 35 ppm B, and 0.005 to
0.05% Ti, Si in an amount of 0.10 to 0.30% and Si is controlled in
the range from Mn/B - 0.07 to Mn/8 + 0.07 (mass o), as
indispensable components and further one or more optional
components selected from among the group consisting of 0.005 to
0.050% Nb, 0.005 to 0.070a V, 0.005 to 0.5% Cu, 0.1 to 0.5% Mo,
0.1 to 0.5% Ni, not more than 0.01% Ca, not more than 0.1% rare
earth metals (REMs) and mixtures thereof, the balance being iron
and unavoidable impurities; the tensile strength TS of said steel
pipe is 2,000 MPa or more; and the YR thereof, said YR being the
ratio of the 0.1%-proof stress YS to said tensile strength TS
(YS/TS), is 66% or less.
The present invention also provides a method for
producing a highly impact-resistant steel pipe as described above,
characterized in that said steel pipe containing, in mass, 0.19 to
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0.35% C, 0.10 to 0.30% Si, 0.5 to 1.60% Mn, not more than 0.025%
P, not more than 0.01% S, 0.010 to 0.050% Al, 2 to 35 ppm B, and
0.005 to 0.05% Ti as indispensable components, and further one or
more components selected from among the group of 0.005 to 0.050%
Nb, 0.005 to 0.070% V, 0.005 to 0.5% Cu, 0.1 to 0.5% Mo, 0.1 to
0.5% Ni, not more than 0.01% Ca, and not more than 0.1% rare earth
metals (REMs), is subjected to induction heating and then water
quenching.
Further, a method for producing a highly impact-
resistant steel pipe according to the present invention
is characterized in that said steel pipe containing, in
mass, 0.19 to 0.35% C, 0.1 to 0.3% Si, 1.0 to 1.6% Mn,
not more than 0.025% P, not more than 0.02% S, 0.010 to
0.050% Al, 2 to 35 ppm B, and 0.005 to 0.05% Ti as
indispensable components, and arbitrary components
selected from among the group of 0.005 to 0.050% Nb,
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0.005 to 0.070% V, 0.005 to 0.5% Cu, 0.005 to 0.5% Cr,
0.1 to 0.5% Mo, 0.1 to 0.5% Ni, not more than 0.01% Ca,
and not more than 0.1% rare earth metals (REMs) is
subjected to induction heating and then water quenching.
Here, it is preferable that the cooling rate of said
water quenching is 100 C/sec. or higher and the cooling
water temperature of said water quenching is 35 C or
lower.
As explained above, the present invention has been
developed with intent to secure a material having more
higher TS and a lower YR than a conventional material. A
material having such a high TS is generally obtained by
subjecting it to water quenching after heating and thus
making the structure thereof composed of martensite. In
prior technologies, the material property of a low yield
ratio has been obtained by making soft austenite and/or
ferrite remain partially in a hard martensite structure
and thus lowering a proof stress. However, with such
prior technologies, as has been explained above, the
tensile strength and the yield ratio have been 1,500 to
1,600 MPa and 70 to 80%, respectively, at the least.
In contrast, in the present invention, while more
higher TS than ever is secured by eliminating retained
austenite and/or retained ferrite in a martensite
structure, YS and YR are lowered by increasing the
dislocation density in a hard martensite structure more
than ever and thus causing the deformation to occur
easily under a stress. A dislocation density in a steel
pipe according to the present invention is in the range
from 1010 to 101 /mm-2 and is extremely high whereas a
dislocation density in a conventional steel pipe is in
the range from 108 to 109/mm-2. By securing such a high
dislocation density, a highly impact-resistant steel pipe
according to the present invention can have a lower YR
than a conventional steel pipe while more higher TS is
maintained. Moreover, by employing a 0.1%-proof stress
that has not been used as a YS value for calculating a YR
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value, an impact absorbing capacity can be evaluated more
properly and the weight of a door impact beam can be
reduced, almost to the limit.
Further, as explained above, it is thought that the
oxides containing Si and Mn, the oxides being formed at
an electric-resistance-welded portion, cause the
toughness of the welded portion to deteriorate, and the
present invention makes it possible to exclude the oxides
from an electric-resistance-welded portion and completely
prevent the toughness of the welded portion from
deteriorating, while a high tensile strength of 1,700 MPa
or more is maintained as shown in the data of the after-
mentioned examples, by controlling an Si amount in the
range from Mn/8 - 0.07 to Mn/8 + 0.07.
BRIEF DESCRIPTION OF THE DRAWINGS
Fig. 1 is a graph schematically showing the stress-
strain curve of a steel.
Fig. 2 is a graph showing the relationship between
cooling rates and YR values.
Fig. 3 is a graph showing the relationship between
cooling water temperatures and YR values.
Fig. 4 is a graph showing the measurement result of
the toughness at upset welded portions.
Fig. 5 is a graph showing the relationship between
the prior austenite grain size numbers and the
occurrence of cracks at impact bending tests.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
The present invention is hereunder explained in
detail. A highly impact-resistant steel pipe according
to the present invention is
produced by subjecting a steel pipe containing, in mass,
0.19 to 0.35% C, 0.1 to 0.3% Si, 0.5 to 1.6% Mn, not more
than 0.025% P, not more than 0.02% S, 0.010 to 0.050% Al,
2 to 35 ppm B, and 0.005 to 0.05% Ti as indispensable
components, and arbitrary components selected from among
the group of 0.005 to 0.050% Nb, 0.005 to 0.070% V, 0.005
to 0.5% Cu, 0.005 to 0.5% Cr, 0.1 to 0.5% Mo, 0.1 to 0.5%
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Ni, not more than 0.01% Ca, and not more than 0.1% rare
earth metals (REMs), to induction heating and then water
quenching. The reason for regulating the amount of each
component is explained below.
C is a component indispensable for strengthening
martensite itself and thus enhancing hardness and must be
added by at least 0.19% for securing a TS value of 1,700
MPa or more. However, an excessive C amount makes a
martensite structure brittle and baking cracks that cause
fracture during quenching occur. Therefore, a C amount
is set at not more than 0.35%. Further, it is preferable
to set a C amount: at about 0.21% for obtaining a steel
pipe having a TS of 1,700 MPa or more and a YR of 72% or
less
at about 0.24% for a steel pipe having a TS of 1,800 MPa
or more and a YR of 70% or less
at about 0.28% for a steel pipe having a TS of 1,900 MPa
or more and a YR of 68% or less
and at about 0.30% for a steel pipe having a TS of 2,000
MPa or more and a YR of 66% or less.
Si, Mn and Ti are components for accelerating a
transformation from austenite into martensite at the time
of quenching. However, when the amounts of Si, Mn and Ti
are lower than the amounts specified in the ranges of 0.1
to 0.3% Si, 0.5 to 1.6% Mn and 0.005 to 0.05% Ti,
respectively, hardenability deteriorates, retained
austenite and/or retained ferrite appear, and thus
desired material properties are not obtained. On the
other hand, when the amounts of Si, Mn and Ti exceed the
above ranges, baking cracks and segregation are
undesirably caused. In particular, a preferable Mn
amount is 1.0% or more for stabilizing hardenability.
Note that Ti has the function of improving hardenability
by fixing N.
B is a component that suppresses the precipitation
of ferrite. However, when B combines with N contained in
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a steel as a component of a gas and forms BN, the effect
is lost. Therefore, a B content is set at 2 ppm or more.
On the other hand, when a B content exceeds 35 ppm, B
forms segregated inclusions. P and S form segregated
inclusions and make a martensite structure brittle.
Therefore, the contents of P and S must be 0.025% or less
and 0.02% or less, respectively. Al is a deoxidizing
agent. However, when an Al amount is less than 0.010%,
the deoxidizing effect is insufficient. On the contrary,
when an Al amount exceeds 0.050%, the oxides undesirably
form intergranular inclusions.
Nb and V are precipitation hardening components that
enhance strength by forming precipitates in a martensite
structure and thus preventing dislocations from passing
through the precipitates. Cu, Cr, Mo and Ni are solid
solution hardening components that enhance strength by
dissolving in martensite crystals and thus preventing
dislocations from passing through the dissolved
components. Note that Cr and Mo function also as
precipitation hardening components. Those components,
though they contribute to the enhancement of strength,
cause the cost to increase and, moreover, form segregated
inclusions when they are excessively added. Therefore,
their appropriate amounts are 0.005 to 0.050% Nb, 0.005
to 0.07% V, 0.005 to 0.5% Cu, 0.005 to 0.5% Cr, 0.1 to
0.5% Mo and 0.1 to 0.5% Ni.
Ca and rare earth metals (REMs) are the components
that contribute to the control of inclusion shape.
However, their excessive addition causes harmful
segregation that leads to the destruction of a martensite
structure. Therefore, appropriate addition amounts of Ca
and rare earth metals (REMs) are 0.01% or less and 0.1%
or less, respectively. Note that Nb, V, Cu, Cr, Mo, Ni,
Ca and rare earth metals (REMs) are not indispensable
components but components selectively added as occasion
demands. As the rare earth metals (REMs), for example,
Y, La, Ce and Sm may be used.
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In the present invention, a steel having an
aforementioned composition is formed into a steel pipe by
electric-resistance-welding, thereafter the steel pipe is
transferred into a work coil for high frequency heating
and heated to a temperature of 900 C or higher by
induction heating, and subsequently the heated steel pipe
is quenched with water from the state of austenite. In
that case, either of the method wherein the steel pipe
passes through a fixed work coil and a fixed water
quenching device while the steel pipe is continuously
transferred on a conveyer or the method wherein the steel
pipe is fixed and a work coil and a water quenching
device move along the steel pipe may be employed.
The transformation from austenite to martensite
occurs instantaneously with the water quenching and, at
the same time, an expansion of about 7 to 8% is generated
due to the transformation strain and the dislocation
density in the martensite structure increases
drastically. Here, a dislocation density is determined
by observing a specimen after subjected to a tensile test
according to JIS with a transmission electron microscope,
measuring the numbers of dislocations in ten visual
fields, the area of each visual field being 1 m x 1 m,
and averaging the measured numbers. The unit of a
dislocation density is dislocations/mm-2 because the
dislocation density is expressed by the dislocation
length per unit volume.
A dislocation density in a steel pipe according to
the present invention is in the range from 1010 to
1014/mm-2 and is extremely high whereas a dislocation
density in a conventional steel pipe is in the range from
108 to 109/mm-z. By securing such a high dislocation
density, a yield point lowers and therefore a highly
impact-resistant steel pipe according to the present
invention can have a lower YR than a conventional steel
pipe while a higher TS is maintained.
It is preferable to control a cooling rate at water
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quenching to 100 C/sec. or higher for obtaining such a
highly impact-resistant steel pipe having a high TS and a
low YR. Fig. 2 is a graph showing the relationship
between cooling rates and YR values and it is understood
from the figure that a YR value decreases abruptly by
controlling a cooling rate to 100 C/sec. or higher. This
is presumably because a transformation strain occurs
abruptly by rapid cooling and a dislocation density
increases.
Further, it is preferable to control a cooling water
temperature, at water quenching, to 35 C or lower for
obtaining such a highly impact-resistant steel pipe
having a high TS and a low YR. As shown in Fig. 3, a YR
value increases as a cooling water temperature rises.
This is presumably because quenching becomes insufficient
as a cooling water temperature rises and an ideal
martensite transformation is hard to secure.
A highly impact-resistant steel pipe according to
the present invention produced by aforementioned method
can have a lower YR than a conventional steel pipe while
a far higher strength is maintained, and therefore, when
the steel pipe is used as a member, such as a material
for a door impact beam, a bumper, a bumper reinforcement
or the like of an automobile, that requires an impact
absorption energy, an excellent impact absorbing capacity
can be elicited. Further, by employing a 0.1%-proof
stress in the present invention instead of a 0.2%-proof
stress in prior art, the impact absorbing capacity
according to the present invention increases by a degree
corresponding to the area hatched with horizontal lines
in Fig. 1 in comparison with the impact absorbing
capacity calculated with a 0.2%-proof stress, and
therefore the impact absorbing capacity according to the
present invention correlates more accurately with an
impact absorbing capacity at an actual collision. By so
doing, together with a high strength, a weight reduction
can be achieved. As a result, it becomes possible to
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provide a shock absorbing member having both an ultra-low
weight and a high collision safety.
Meanwhile, a highly impact-resistant steel pipe
according to the present invention is a steel pipe that
secures a high strength of 1,700 MPa or more in tensile
strength by inevitably containing 0.10 to 0.30% Si and
0.5 to 1.6% Mn. Thus a transformation from austenite
into martensite is accelerated. Further, the most
important feature of the present invention is that the
toughness of an electric-resistance-welded portion is
prevented from deteriorating by controlling the amounts
of Si and Mn so as to satisfy the expression Mn/8 - 0.07
s Si s Mn/8 + 0.07. Fig. 4 is a graph showing the
measurement result of the toughness at upset welded
portions and it is confirmed from the figure that a
relative Charpy absorbed energy is the largest when an Si
amount is in the above range.
Here, a relative Charpy absorbed energy is a
relative value defined by the ratio of a Charpy absorbed
energy at -40 C in the case of Si = Mn/8 + cz ((c = -0.30
to + 0.30) to a Charpy absorbed energy at -40 C in the
case of Si = Mn/8.
A steel pipe strengthened up to 1,700 MPa or more by
adding C and Mn in the steel as in the present invention
has a lower melting point than a steel pipe of a low
strength and the viscosity of the oxides formed on the
surface of the metal fused at the time of electric-
resistance-welding decreases relatively. Therefore, it
is particularly important to exclude the oxides by
controlling an amount of Si that affects the remaining of
the oxides at a welded portion, as mentioned above, to
prevent the toughness of the welded portion from
deteriorating.
Further, it is preferable
that 95% or more of the microstructure of a steel pipe is
transformed into martensite by induction hardening and
CA 02441720 2003-11-12
- 12 -
the prior austenite grain size number of the
steel pipe is #6 or more particularly for securing a low
temperature impact bending property. Fig. 5 is a graph
showing the result obtained by subjecting highly impact-
resistant electric-resistance-welded steel pipes (1,700
MPa in tensile strength) having various prior austenite
grain size numbers to impact bending tests and
observing the occurrence of cracks. From Fig. 5, it is
understood that steel pipes having an excellent low
temperature impact bending property are obtained by
securing minute crystals having prior austenite
grain size numbers of #6 or more. Here, crystals can be
fractionized by the effect of, for example, lowering a
hardening temperature, fractionizing the grains
of a pre-hardening structure, adding elements such as Nb,
V, Ti, etc., or the like. A prior austenite
grain size number may be measured by exposing the
boundaries of prior austenite grains in a base material
with a generally used austenite grain boundary exposing
liquid and thereafter employing a cutting method or an
image analysis method.
Meanwhile, the sectional shape includes both a round
shape and a square shape.
As methods of producing a square steel pipe having a
square sectional shape, there are the method wherein a
steel pipe is produced in an electric-resistance-welding
process and thereafter the steel pipe is formed into a
square sectional shape with forming rolls and the method
wherein a steel strip is bent continuously and then
formed into a square sectional shape at the time of
electric-resistance-welding.
In the latter case, it is possible that, when the
thickness of an ultra-high strength material is so
designed as to be thin, the upset allowance is hardly
secured at the time of welding and the risk of the
remaining of oxides increases. However, even in this
case, the present invention makes it possible to exclude
CA 02441720 2003-11-12
- 13 -
oxides and stabilize weld quality by controlling an
amount of Si that affects the remaining of oxides as
mentioned above and therefore to prevent the toughness of
a welded portion from deteriorating.
It has been estimated that, when an impact energy is
absorbed by the deformation of a door impact beam, the
impact to a corner is larger than that to a side. To
cope with that, in the former method, a welded portion is
detected before the roll forming and adjusted so that the
welded portion may not be located at a corner. In the
latter method, as a steel strip is bent and formed into a
square shape, the welded portion is never located at a
corner. As explained above, a corner is deviated from a
welded portion without fail and it is possible to make
the most of the high energy absorbing capacity of a base
material. Further, by employing a square steel pipe, it
is possible to make the section modulus larger than that
of a round steel pipe having an identical sectional area.
Therefore, the present invention, by employing a square
steel pipe having a square sectional shape, it is
possible to enhance an energy absorbing capacity, more
than in a round steel pipe, and to increase reliability.
The examples according to the present invention are
explained hereunder.
Example 1
Electric-resistance-welded steel pipes comprising
the steels having various compositions shown in Table 1
were produced, heated with induction heating by moving at
a constant speed on a conveyer and passing through wire
coils, and then rapidly cooled to an ordinary temperature
with an adjacent water quenching device. The cooling
rates and cooling water temperatures are shown Table 2.
The 0.1%-proof stresses and rupture strengths were
measured by subjecting cut-out test pieces to a tensile
tester. Further, the test pieces after being subjected
to the tensile tests were observed with a transmission
electron microscope. The resulting dislocation densities
CA 02441720 2003-09-19
- 14 -
are also shown in Table 2.
Table 1
Inventive Inventive Inventive Inventive Inventive Inventive Inventive
Inventive
example 1 example 2 example 3 example 4 example 5 xam le 6 example 7 example 8
C(%) 0.21 0.24 0.28 0.28 0.30 0.30 0.35 0.30
Si(%) 0.22 0.23 0.21 0.21 0.20 0.20 0.21 0.20
(%) 1.41 1.43 1.41 1.41 1.44 1.44 1.40 1.00
P($) 0.021 0.021 0.018 0.018 0.016 0.016 0.020 0.018
SM 0.005 0.003 0.003 0.003 0.003 0.003 0.004 0.003
1(%) 0.025 0.030 0.025 0.025 0.026 0.026 0.023 0.025
B( ppm ) 13 11 10 10 10 10 11 10
Ti(%) 0.028 0.030 0.027 0.027 0.027 0.027 0.026 0.025
(%) - 0.032 0.032 0.032 - - - 0.030
(%) 0.033 0.030 0.035 0.035 - - - -
i(%) - - - - - - 0.3 -
CrM - - - - 0.15 0.15 0.15 -
Cu(%) 0.10 - - - - - - -
M(%) - - - 0.004 - - - -
Ca($) - - - - - 0.0027 0.0028 -
Table 2
Inventive Inventive Inventive Inventive Inventive Inventive Inventive
Inventive
example 1 example 2 example 3 example 4 example 5 example 6 example 7 example
8
Heating
tempera- 900 900 900 900 900 900 900 900
ture ( C)
Cooling
rate 150 200 200 170 200 200 200 200
( C/sec.)
Cooling
water
tempera- 30 30 30 30 30 30 30 30
ture ( C)
is-
location 101z 1013 1013 1013 1013 1016 1014 1010
density
(MM-Z)
0.1$-
roof
stress 1187 1209 1255 1247 1312 1303 1340 1290
(MPa)
Tensile
strength 1722 1861 1930 1908 2050 2083 2125 2030
(MPa)
Yield
ratio 69 65 65 65 64 63 63 64
($) I
As explained above, a highly impact-resistant steel
pipe according to the present invention not only has the
material properties of a higher strength and a lower
CA 02441720 2003-11-12
- 15 -
yield ratio than a conventional steel pipe but also
reflects more precisely an actual impact absorbing
capacity by employing a 0.1%-proof stress, and, as a
result, agrees better with a member, such as a material
for a door impact beam, a bumper, a bumper reinforcement
or the like of an automobile, that requires an impact
absorption energy. Further, a method according to the
present invention makes it possible to produce such a
highly impact-resistant steel pipe stably.
Example 2
Electric-resistance-welded steel pipes having
various compositions shown in Table 3 were produced and
the tensile strength, the ratio of the strength of a
welded portion to that of a body portion, the occurrence
of cracks at low temperature impact bending, and others
of each of the steel pipes were measured. The results
are shown in Table 3. The value of a= Si - Mn/8 and
the prior austenite grain size number of each of
the steel pipes are also shown in Table 3. The invention
examples 9 to 13 and the comparative examples 1 to 3 are
the examples of round-shaped electric-resistance-welded
steel pipes and the invention examples 14 and 15 and the
comparative example 4 are the examples of square-shaped
electric-resistance-welded steel pipes having quadratic
sectional shapes.
CA 02441720 2003-11-12
- 16 -
ro 0 Ln0 NU1D M
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CA 02441720 2003-11-12
- 17 -
Here, the amount of each component in Table 3 is
expressed in terms of mass $(except B which is expressed
in terms of mass ppm) and the balance of the components
consists of Fe and unavoidable impurities. In the column
of "Low temperature impulse bending," the mark 0
represents the case where no cracks occur when a steel
pipe is subjected to an impact bending test under the
condition of a low temperature of -60 C and the mark X
the case where cracks occur. The strength means a
tensile strength and the unit thereof is MPa. When the
box of a component is blank, it means the component is
not added.
Further, though it is not shown in Table 3, the
maximum values of the absorbed impact energy of the
invention examples 14 and 15 are somewhat larger than
those of the invention examples 9 to 13. This is
presumably because the section moduli of the invention
examples 14 and 15 are larger.
As explained above, a highly impact-resistant
electric-resistance-welded steel pipe having a round or
square sectional shape according to the present invention
not only is excellent in strength and toughness but also
does not undergo deterioration of strength and toughness
in the vicinity of an electric-resistance-welded portion
and is suitable for using as an impact absorbing member
such as a material for a door impact beam bumper, or a
bumper reinforcement, or the like.
