Technical Field
[0001] The present disclosure relates to an electric resistance welded steel pipe for a
mechanical structural part, and a method of producing the same.
Background Art
[0002] Mechanical structural parts made of electric resistance welded steel pipes as
materials have been utilized.
For example, Patent Document 1 discloses a steel material for an automotive
underbody part, which is one kind of mechanical structural parts. Specifically, Patent
Document 1 discloses, as a steel material for an automotive underbody part which has high
fatigue characteristics, which does not require a high heat treatment cost and which also has
an excellent forming processability, a steel material for an automobile underbody part having
excellent fatigue characteristics, characterized in that the steel material is one to which Nb and
Mo are added compositely, and that a difference between a Vickers hardness at a central
portion in a sheet thickness direction and the maximum value of a Vickers hardness within a
region 0.5 mm from a surface, after being subjected to bending forming that achieves a
bending radius R of a sheet outer surface of from 2 to 5 times the sheet thickness, is from 50
to 150 points.
[0003] Patent Document 1: Japanese Patent Application Laid-Open (JP-A) No. 2008-63656
SUMMARY OF INVENTION
Technical Problem
[0004] In recent years, a reduction in weight is demanded for mechanical structural parts.
In order to achieve a reduction in weight of a mechanical structural part, a processed
portion X that consists of at least one of a drawing-processed portion or a steel pipe
bending-processed portion, in the mechanical structural part, is required to have an excellent
tensile strength as well as an excellent fatigue limit.
In the present disclosure, the term “steel pipe bending-processed portion” refers to a
portion that has been subjected to steel pipe bending. The term “steel pipe bending” refers
to bending which is performed on at least one portion in the pipe axis direction of a steel pipe,
and in which the pipe axis (namely, the central axis) of the steel pipe is bent.
2
In contrast, the “bending forming that achieves a bending radius R of the sheet outer
surface of from 2 to 5 times the sheet thickness” in Patent Document 1 is bending in which a
steel sheet constituting one portion in the circumferential direction of a steel pipe is bent
strongly (namely, under a condition in which a bending radius R of the sheet outer surface is 5
times or less the sheet thickness), and is not bending in which the central axis of the steel pipe
is bent (see FIG. 2 in Patent Document 1). Thus, the bending forming in Patent Document 1
does not correspond to the “steel pipe bending” in the present disclosure. The bending
forming in Patent Document 1 is a strong bending forming (namely, bending forming with a
small bending radius R) for forming an ear portion of an axle beam, which is an automotive
underbody part (for the description above, see FIG. 1 to FIG. 6 in Patent Document 1). The
technique disclosed in Patent Document 1 utilizes the fact that the hardness at a surface layer
of the steel sheet is significantly increased with respect to the hardness at the central portion
in the wall thickness direction of the steel sheet, when the above-described strong bending
forming is performed (see paragraphs 0022 and 0023 in Patent Document 1). In particular,
the paragraph 0022 describes that a strong bending forming that achieves a bending radius R
of the sheet outer surface of from 2 to 5 times the sheet thickness is essential, because an
increase in hardness due to work hardening will be insufficient when the bending radius R of
the sheet outer surface exceeds 5 times the sheet thickness.
However, Patent Document 1 is silent about the drawing-processed portion and the
steel pipe bending-processed portion.
[0005] An object of one embodiment of the present disclosure is to provide: an electric
resistance welded steel pipe for a mechanical structural part, which includes a processed
portion X that consists of at least one of a drawing-processed portion or a steel pipe
bending-processed portion, and in which the processed portion X has an excellent tensile
strength and fatigue limit; and a method of producing the electric resistance welded steel pipe
for a mechanical structural part.
Solution to Problem
[0006] Means for solving the problem described above includes the following embodiments.
<1> An electric resistance welded steel pipe for a mechanical structural part, the pipe
comprising a processed portion X that consists of at least one of a drawing-processed portion
or a steel pipe bending-processed portion,
wherein the processed portion X comprises a base metal portion and an electric
resistance welded portion,
wherein the base metal portion has a chemical composition consisting of, in terms
of % by mass:
3
from 0.150 to 0.230% of C,
from 0 to 0.50% of Si,
from 0.50 to 1.65% of Mn,
from 0 to 0.030% of P,
from 0 to 0.010% of S,
from 0.010 to 0.050% of Nb,
from 0.10 to 0.60% of Mo,
from 0.005 to 0.060% of Al,
from 0 to 0.0060% of N,
from 0 to 0.030% of Ti,
from 0 to 0.100% of V,
from 0 to 0.5% of Cr,
from 0 to 0.500% of Cu,
from 0 to 0.500% of Ni,
from 0 to 0.0030% of B,
from 0 to 0.0030% of Ca,
from 0 to 0.0040% of Mg, and
a balance consisting of Fe and impurities,
wherein an area ratio of tempered bainite with respect to an entire microstructure of
the base metal portion is 80% or more,
wherein the base metal portion has a tensile strength of from 850 to 1,000 MPa,
wherein a yield elongation of 0.2% or more is observed in a tensile test of the base
metal portion, and
wherein a ratio of a Vickers hardness at a position at a depth of 50 μm from an outer
surface of the base metal portion with respect to a Vickers hardness at a central portion in a
wall thickness direction of the base metal portion is 95% or more.
<2> The electric resistance welded steel pipe for a mechanical structural part according to
<1>, which is an electric resistance welded steel pipe for an automotive underbody part.
<3> The electric resistance welded steel pipe for a mechanical structural part according to
<1> or <2>, wherein the processed portion X has an outer circumference length of from 50 to
500 mm, and has a maximum wall thickness of from 1.0 to 5.0 mm.
<4> A method of producing the electric resistance welded steel pipe for a mechanical
structural part according to any one of <1> to <3>, the method comprising:
a step of preparing an as-rolled electric resistance welded steel pipe, wherein:
the as-rolled electric resistance welded steel pipe comprises a base metal portion A
4
and an electric resistance welded portion A; the base metal portion A has a chemical
composition consisting of, in terms of % by mass:
from 0.150 to 0.230% of C,
from 0 to 0.50% of Si,
from 0.50 to 1.65% of Mn,
from 0 to 0.030% of P,
from 0 to 0.010% of S,
from 0.010 to 0.050% of Nb,
from 0.10 to 0.60% of Mo,
from 0.005 to 0.060% of Al,
from 0 to 0.0060% of N,
from 0 to 0.030% of Ti,
from 0 to 0.100% of V,
from 0 to 0.5% of Cr,
from 0 to 0.500% of Cu,
from 0 to 0.500% of Ni,
from 0 to 0.0030% of B,
from 0 to 0.0030% of Ca,
from 0 to 0.0040% of Mg, and
a balance consisting of Fe and impurities,
an area ratio of bainite with respect to an entire microstructure of the base metal
portion A is 80% or more,
the base metal portion A has a tensile strength of from 600 to 800 MPa, and
a total elongation of 13.0% or more is observed in a tensile test of the base metal portion A;
a cold working step of subjecting at least one portion in a pipe axis direction of the
as-rolled electric resistance welded steel pipe to cold working that is at least one of cold
drawing or cold steel pipe bending and that is performed under a condition of a maximum
area reduction rate of from 10 to 40%; and
a tempering step of subjecting the as-rolled electric resistance welded steel pipe that
has been subjected to the cold working to tempering at a tempering temperature of from 450
to 650°C, to obtain the electric resistance welded steel pipe for a mechanical structural part.
<5> The method of producing the electric resistance welded steel pipe for a mechanical
structural part, according to <4>, wherein the as-rolled electric resistance welded steel pipe
has an outer diameter of from 50 to 150 mm, and has a wall thickness of from 2.0 to 4.0 mm.
5
Advantageous Effects of Invention
[0007] One embodiment of the present disclosure provides: an electric resistance welded
steel pipe for a mechanical structural part, which includes a processed portion X that consists
of at least one of a drawing-processed portion or a steel pipe bending-processed portion, and
in which the processed portion X has an excellent tensile strength and fatigue limit; and a
method of producing the electric resistance welded steel pipe for a mechanical structural part.
BRIEF DESCRIPTION OF DRAWINGS
[0008] FIG. 1 is a plan view showing a fatigue test specimen used in Examples.
DESCRIPTION OF EMBODIMENTS
[0009] In the present disclosure, any numerical range indicated using the expression “from *
to” represents a range in which numerical values described before and after the “to” are
included in the range as a lower limit value and an upper limit value.
In the present disclosure, the symbol “%” indicating the content of a component
(element) refers to “% by mass”.
In the present disclosure, the content of C (carbon) may be referred to as “C content”.
The same may apply to the contents of other elements.
Further, in the present disclosure, the definition of the term “step” includes not only
an independent step, but also a step which is not clearly distinguishable from another step, as
long as the intended purpose of the step is achieved.
[0010] In the present disclosure, the term “as-rolled electric resistance welded steel pipe”
refers to an electric resistance welded steel pipe that has not been subjected to a heat treatment
other than a seam heat treatment, after pipe-making. In other words, the “as-rolled” in the
“as-rolled electric resistance welded steel pipe” means “in a state of as-roll-formed”.
The term “pipe-making” refers to a process of roll-forming a hot-rolled steel sheet
that has been uncoiled from a hot coil to form an open pipe, and subjecting abutting portions
of the resulting open pipe to electric resistance welding to form an electric resistance welded
portion.
The term “hot coil” refers to a hot-rolled steel sheet that has been produced using a
hot strip mill, and coiled in the form of a coil.
The term “roll forming (to roll-form)” refers to subjecting a hot-rolled steel sheet that
has been uncoiled from a hot coil to bending continuously, to form the sheet in the form of an
open pipe.
[0011] The hot-rolled steel sheet produced using a hot strip mill is different from a steel
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plate produced using a plate mill in that the hot-rolled steel sheet is a continuous steel sheet.
A steel plate cannot be used in roll forming which is a continuous bending, since the
steel plate is not a continuous steel sheet.
An electric resistance welded steel pipe is clearly distinguished from a welded steel
pipe (such as a UOE steel pipe) produced using a steel plate, in the points described above.
[0012] In the processed portion X in the electric resistance welded steel pipe according to
the present disclosure, or in an as-rolled electric resistance welded steel pipe in the present
disclosure, the base metal portion refers to a portion other than the electric resistance welded
portion and a heat affected zone of the pipe.
The term “heat affected zone (hereinafter, also referred to as “HAZ”) refers to a
portion affected by heat caused by electric resistance welding (affected by heat caused by the
electric resistance welding and a seam heat treatment in a case in which the seam heat
treatment is performed after the electric resistance welding).
[0013] [Electric Resistance Welded Steel Pipe for Mechanical Structural Part]
The electric resistance welded steel pipe for a mechanical structural part according to
the present disclosure (hereinafter, also simply referred to as “electric resistance welded steel
pipe”) is an electric resistance welded steel pipe for a mechanical structural part that includes
a processed portion X that consists of at least one of a drawing-processed portion or a steel
pipe bending-processed portion,
wherein the processed portion X comprises a base metal portion and an electric
resistance welded portion,
wherein the base metal portion has a chemical composition consisting of, in terms
of % by mass:
from 0.150 to 0.230% of C,
from 0 to 0.50% of Si,
from 0.50 to 1.65% of Mn,
from 0 to 0.030% of P
from 0 to 0.010% of S
from 0.010 to 0.050% of Nb
from 0.10 to 0.60% of Mo
from 0.005 to 0.060% of Al
from 0 to 0.0060% of N
from 0 to 0.030% of Ti
from 0 to 0.100% of V
from 0 to 0.5% of Cr
7
from 0 to 0.500% of Cu
from 0 to 0.500% of Ni
from 0 to 0.0030% of B
from 0 to 0.0030% of Ca
from 0 to 0.0040% of Mg, and
a balance consisting of Fe and impurities,
wherein an area ratio of tempered bainite with respect to an entire microstructure of
the base metal portion is 80% or more,
wherein the base metal portion has a tensile strength of from 850 to 1,000 MPa,
wherein a yield elongation of 0.2% or more is observed in a tensile test of the base
metal portion, and
wherein a ratio of a Vickers hardness at a position at a depth of 50 μm from an outer
surface of the base metal portion with respect to a Vickers hardness at a central portion in a
wall thickness direction of the base metal portion is 95% or more.
[0014] The electric resistance welded steel pipe according to the present disclosure includes
the processed portion X, and the processed portion X has an excellent tensile strength
(specifically, the tensile strength of the base metal portion in the processed portion X is from
850 to 1,000 MPa), and the processed portion X also has an excellent fatigue limit.
Such effects are achieved by the combination of:
the chemical composition of the base metal portion in the processed portion X;
the above-described microstructure of the base metal portion mainly composed of
tempered bainite;
the fact that the above-described yield elongation is observed in a tensile test of the
base metal portion; and
the fact that the above-described Vickers hardness ratio is 95% or more.
The electric resistance welded steel pipe according to the present disclosure will be
described below in specific detail.
[0015] The electric resistance welded steel pipe according to the present disclosure includes
a processed portion X that consists of at least one of a drawing-processed portion or a steel
pipe bending-processed portion.
As described above, the “steel pipe bending” in the “steel pipe bending-processed
portion” as used herein, is bending which is performed on at least one portion in the pipe axis
direction of a steel pipe, and in which the pipe axis (namely, the central axis) of the steel pipe
is bent.
In other words, the processed portion X that consists of at least one of the
8
drawing-processed portion or the steel pipe bending-processed portion is at least one portion
in the pipe axis direction of the electric resistance welded steel pipe.
The processed portion X includes a base metal portion and an electric resistance
welded portion.
The processed portion X is a portion formed by performing a processing and
tempering in the order mentioned, on at least one portion in the pipe axis direction of an
as-rolled electric resistance welded steel pipe (namely, an electric resistance welded steel pipe
that has not been subjected to a heat treatment other than a seam heat treatment, after
pipe-making) including a base metal portion A and an electric resistance welded portion A.
The “base metal portion A” and the “electric resistance welded portion A” as used
herein refer to the base metal portion and the electric resistance welded portion in an as-rolled
electric resistance welded steel pipe, respectively.
The “processing” refers to at least one of drawing or steel pipe bending. The
processed portion X may be a portion formed by performing both drawing and steel pipe
bending on the same portion of an as-rolled electric resistance welded steel pipe.
The “tempering” refers to a heat treatment of heating to a temperature equal to or less
than Ac1 point. Production method X to be described later can be referred to for one
example of tempering conditions.
[0016] The processed portion X may be one portion in the pipe axis direction of the electric
resistance welded steel pipe according to the present disclosure, or may be the entirety of the
electric resistance welded steel pipe according to the present disclosure.
An electric resistance welded steel pipe of an embodiment in which the entirety of
the pipe is the processed portion X may be, for example, an electric resistance welded steel
pipe produced by performing drawing over the entire length of an as-rolled electric resistance
welded steel pipe, followed by tempering.
An electric resistance welded steel pipe of an embodiment in which one portion in
the pipe axis direction is the processed portion X may be, for example, an electric resistance
welded steel pipe produced by performing steel pipe bending (namely, a processing to bend
the pipe axis) on one portion in the pipe axis direction of an as-rolled electric resistance
welded steel pipe, and then tempering the entirety of the steel pipe.
[0017] The processed portion X is distinguishable in appearance from a non-processed
portion. In other words, a portion that consists of at least one of the drawing-processed
portion or the steel pipe bending-processed portion, in appearance, is the processed portion X
in the present disclosure. It is also possible to regard a portion of the base metal portion
having a tensile strength of from 850 to 1,000 MPa as the processed portion X.
9
[0018] The details of the chemical composition of the base metal portion in the processed
portion X will be described later. The base metal portion contains both Nb and Mo, as one
of the features of the chemical composition of the base metal portion. Nb and Mo
contributes to precipitation strengthening to be described later.
[0019] In the base metal portion in the processed portion X, the area ratio of tempered
bainite with respect to the entire microstructure thereof is 80% or more.
This feature also contributes to the precipitation strengthening to be described later.
This feature indicates that the electric resistance welded steel pipe according to the
present disclosure is produced by subjecting an as-rolled electric resistance welded steel pipe
in which the area ratio of bainite with respect to the entire microstructure of the base metal
portion A is 80% or more, to a processing and tempering.
[0020] The base metal portion in the processed portion X has a tensile strength of from 850
to 1,000 MPa.
As described above, the electric resistance welded steel pipe according to the present
disclosure is an electric resistance welded steel pipe produced by subjecting an as-rolled
electric resistance welded steel pipe to a processing (namely, at least one of drawing or steel
pipe bending) and tempering in the order mentioned.
In general, the strength tends to decrease in an electric resistance welded steel pipe
that has been subjected to tempering, as compared to an electric resistance welded steel pipe
(namely, an as-rolled electric resistance welded steel pipe) before being subjected to
tempering. The reason for this is thought to be because a strain introduced into a steel
structure decreases due to tempering.
However, despite the fact that the electric resistance welded steel pipe according to
the present disclosure is an electric resistance welded steel pipe that has been subjected to
tempering, an excellent tensile strength (namely, from 850 to 1,000 MPa) is ensured as the
tensile strength of the base metal portion in the processed portion X. The following reasons
1 and 2 can be considered as the reasons for achieving an excellent tensile strength (namely,
from 850 to 1,000 MPa).
Factor 1: Because a plastic strain is introduced into the steel structure due to the
processing, which is at least one of drawing or steel pipe bending, to cause work hardening.
Factor 2: Because composite carbides containing Nb and Mo are precipitated during
tempering on dislocations formed in the steel structure due to introduction of the plastic strain
to cause precipitation strengthening.
[0021] The above-described microstructure in which the area ratio of tempered bainite is
80% or more in the base metal portion is thought to contribute to effectively providing the
10
effect of the precipitation strengthening. Specifically, in a case in which the area ratio of
bainite in the base metal portion of the as-rolled electric resistance welded steel pipe is 80%
or more, it is thought that Nb and Mo can be kept dissolved in the steel at a stage before the
tempering. As a result, it is thought to be possible to allow composite carbides containing
Nb and Mo to precipitate during tempering, and to cause precipitation strengthening to occur
effectively.
[0022] In the electric resistance welded steel pipe according to the present disclosure, a yield
elongation of 0.2% or more is observed in a tensile test of the base metal portion in the
processed portion X. The “yield elongation of 0.2% or more” as used herein refers to a
substantial yield elongation.
The fact that a yield elongation of 0.2% or more is observed in a tensile test of the
base metal portion in the processed portion X means a decrease in residual strain. Therefore,
the introduction of the strain due to repeated stress is reduced by satisfying this feature,
eventually leading to an improvement in the fatigue characteristics of the processed portion X.
Further, the above-described feature indicates that the electric resistance welded steel
pipe according to the present disclosure is produced by subjecting an as-rolled electric
resistance welded steel pipe to a processing and tempering.
In contrast to the electric resistance welded steel pipe according to the present
disclosure, a substantial yield elongation (namely, a yield elongation of 0.2% or more) is not
observed in an as-rolled electric resistance welded steel pipe (namely, an electric resistance
welded steel pipe that has not been subjected to a heat treatment other than a seam heat
treatment, after pipe-making).
[0023] In the electric resistance welded steel pipe according to the present disclosure, the
ratio (hereinafter, also referred to as “hardness ratio [depth 50 μm/wall thickness center]”) of
the Vickers hardness at a position at a depth of 50 μm from the outer surface of the base metal
portion with respect to the Vickers hardness at the central portion in the wall thickness
direction of the base metal portion, in the processed portion X, is 95% or more.
In the electric resistance welded steel pipe according to the present disclosure, the
occurrence of fatigue cracks from the side of the outer surface is reduced, by satisfying a
hardness ratio [depth 50 μm/wall thickness center] of 95% or more, on the premise that the
tensile strength and the yield elongation described above are satisfied. Therefore, this
feature contributes to improving the fatigue strength of the processed portion X.
[0024] The above-described feature that the hardness ratio [depth 50 μm/wall thickness
center] is 95% or more means that the formation of a decarbonized layer is inhibited in a
region including the outer surface of the base metal portion. The decarbonized layer is
11
generally formed by “hardening”, which is a heat treatment of heating to a temperature higher
than the Ac1 point.
The above-described feature is achieved by producing an electric resistance welded
steel pipe by subjecting an as-rolled electric resistance welded steel pipe to tempering (a heat
treatment of heating to a temperature equal to or less than the Ac1 point), not to hardening (a
heat treatment of heating to a temperature higher than the Ac1 point).
[0025] As described above, the tensile strength, the microstructure, the yield elongation and
the hardness ratio [depth 50 μm/wall thickness center], among the features of the electric
resistance welded steel pipe according to the present disclosure, are closely related not only to
the chemical composition, but also to production conditions for producing the electric
resistance welded steel pipe according to the present disclosure. One example of the
production method for producing the electric resistance welded steel pipe according to the
present disclosure will be described later as the production method X.
[0026] A mechanical structural part for which the electric resistance welded steel pipe for a
mechanical structural part according to the present disclosure is used is preferably an
automotive underbody part, a bearing or a motor cover, and more preferably an automotive
underbody part.
Examples of the automotive underbody part include axle beams, trading arms,
suspension members, linking materials, and torsion beams.
[0027] < Chemical Composition of Base Metal Portion >
The chemical composition (namely, the chemical composition in the present
disclosure) of the base metal portion in the processed portion X of the electric resistance
welded steel pipe according to the present disclosure will be described below.
[0028] C: from 0.150 to 0.230%
C is an element that binds Nb and Mo to form fine composite carbides of Nb and Mo,
and increases the tensile strength and the fatigue limit of the processed portion X. In a case
in which the C content is less than 0.150%, it may result in a failure to obtain this effect.
Further, low temperature coiling may have to be performed in a step of producing a hot-rolled
steel sheet by hot rolling, in order to increase the tensile strength, in a case in which the C
content is less than 0.150%. The low temperature coiling may cause variations in the tensile
strength. Accordingly, the C content is 0.150% or more. The lower limit of the C content
is preferably 0.160%, and more preferably 0.170%.
In a case in which the C content is higher than 0.230%, however, pearlite and
martensite may form and grow markedly in the microstructure, possibly causing not only a
decrease in cold workability, but also a decrease in fatigue limit. Accordingly, the C content
12
is 0.230% or less. The upper limit of the C content is preferably 0.220%, and more
preferably 0.210%.
[0029] Si: from 0 to 0.50%
Si is an optional element. In other words, the Si content may be 0%, or may be
higher than 0%.
Si is an element that contributes to the deoxidation of steel. The lower limit of the
Si content is preferably 0.01%, and more preferably 0.05%, from the viewpoint of more
effectively obtaining such an effect.
In a case in which the Si content is higher than 0.50%, however, it may cause a
decrease in the toughness of the base metal as well as an excessive formation of Si oxide
during electric resistance welding, possibly resulting in reduced mechanical properties of the
processed portion X. Accordingly, the Si content is 0.50% or less. The upper limit of the
Si content is preferably 0.48%, and more preferably 0.40%.
However, an attempt to excessively reduce the Si content leads to a higher production
cost. Accordingly, the lower limit of the Si content is preferably 0.01%, and more preferably
0.05%, in view of industrial productivity.
[0030] Mn: from 0.50 to 1.65%
Mn is an element that increases the tensile strength and the fatigue limit of the
electric resistance welded steel pipe. In a case in which the Mn content is less than 0.50%, it
may result in a failure to obtain this effect. Accordingly, the Mn content is 0.50% or more.
The lower limit of the Mn content is preferably 0.60%, and more preferably 0.70%.
In a case in which the Mn content is higher than 1.65%, however, Mn oxide may be
formed excessively during electric resistance welding, possibly resulting in reduced
mechanical properties of the processed portion X. Accordingly, the Mn content is 1.65% or
less. The upper limit of the Mn content is preferably 1.60%, and more preferably 1.50%.
[0031] P: from 0 to 0.030%
P is an element that can be contained as an impurity. In a case in which the P
content is higher than 0.030%, a decrease in electric resistance weldability and/or a decrease
in toughness may occur. Accordingly, the P content is 0.030% or less. The upper limit of
the P content is preferably 0.015%, and more preferably 0.010%.
The P content may be 0%, or may be higher than 0%.
An attempt to excessively reduce the P content leads to a higher production cost.
Accordingly, the lower limit of the P content is preferably 0.001%, and more preferably
0.005%, in view of industrial productivity.
[0032] S: from 0 to 0.010%
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S is an element that can be contained as an impurity. In a case in which the S
content is higher than 0.010%, a decrease in electric resistance weldability and/or a decrease
in toughness may occur. Accordingly, the S content is 0.010% or less. The upper limit of
the S content is preferably 0.005%, and more preferably 0.003%.
The S content may be 0%, or may be higher than 0%.
An attempt to excessively reduce the S content leads to a higher production cost.
Accordingly, the lower limit of the S content is preferably 0.0001%, and more preferably
0.0005%, in view of industrial productivity.
[0033] Nb: from 0.010 to 0.050%
Nb is contained along with Mo, and is an element that binds to C to form fine
composite carbides containing Nb and Mo due to tempering to be described later. This
allows for markedly increasing the fatigue limit of the processed portion X while maintaining
the tensile strength of the processed portion X at a high level. In a case in which the Nb
content is less than 0.010%, it may result in a failure to obtain this effect. Accordingly, the
Nb content is 0.010% or more. The lower limit of the Nb content is preferably 0.015%, and
more preferably 0.020%.
In a case in which the Nb content higher than 0.050%, however, the toughness of the
base metal and the toughness of the welded portion after the electric resistance welding are
decreased. Accordingly, the Nb content is 0.050% or less. The upper limit of the Nb
content is preferably 0.045%, and more preferably 0.040%.
[0034] Mo: from 0.10 to 0.60%
Mo is contained along with Nb, and is an element that binds to C to form fine
composite carbides containing Nb and Mo due to tempering to be described later. This
allows for markedly increasing the fatigue limit of the processed portion X while maintaining
the tensile strength of the processed portion X at a high level. In a case in which the Mo
content is less than 0.10%, it may result in a failure to obtain this effect. Accordingly, the
Mo content is 0.10% or more. The lower limit of the Mo content is preferably 0.15%, and
more preferably 0.20%.
In a case in which the Mo content is higher than 0.60%, however, not only
workability but also toughness is decreased. Accordingly, the Mo content is 0.60% or less.
The upper limit of the Mo content is preferably 0.55%, and more preferably 0.50%.
[0035] Al: from 0.005 to 0.060%
Al is an element that contributes to the deoxidation of steel. In a case in which the
Al content is less than 0.005%, it may result in a failure to obtain this effect. Accordingly,
the Al content is 0.005% or more. The lower limit of the Al content is preferably 0.010%,
14
and more preferably 0.020%.
In a case in which the Al content is higher than 0.060%, however, alumina-based
oxides remain excessively in the processed portion X, resulting in a decrease in the
mechanical properties of the processed portion X. Accordingly, the Al content is 0.060% or
less. The upper limit of the Al content is preferably 0.045%, and more preferably 0.040%.
[0036] N: from 0 to 0.0060%
N is an element that can be contained as an impurity. In a case in which the N
content is higher than 0.0060%, N may be dissolved in the resulting steel material to cause in
a decrease in workability. Accordingly, the N content is 0.0060% or less. The upper limit
of the N content is preferably 0.0055%, and more preferably 0.0050%.
The N content may be 0%, or may be higher than 0%.
An attempt to excessively reduce the N content leads to a higher production cost.
Accordingly, the lower limit of the N content is preferably 0.0001%, and more preferably
0.0005%, in view of industrial productivity.
[0037] Ti: from 0 to 0.030%
Ti is an optional element. In other words, the Ti content may be 0%, or may be
higher than 0%.
Ti is an element that binds to C and/or N in steel to form Ti nitride or Ti carbonitride,
and can thereby increase the toughness of the resulting steel material through the effect of
refining the structure. The lower limit of the Ti content is preferably higher than 0%, more
preferably 0.005%, and still more preferably 0.007%, from the viewpoint of more effectively
obtaining such an effect.
In a case in which the Ti content is higher than 0.030%, however, coarse Ti nitride
and/or coarse Ti carbonitride may be formed, possibly resulting in a decrease in the fatigue
limit and the toughness of the processed portion X. Accordingly, the Ti content is 0.030% or
less. The upper limit of the Ti content is preferably 0.025%, and more preferably 0.020%.
[0038] V: from 0 to 0.100%
V is an optional element. In other words, the V content may be 0%, or may be
higher than 0%.
V is an element that binds to C and/or N in steel to form at least one selected from
the group consisting of V carbide, V nitride and V carbonitride, and can thereby increase the
toughness of the resulting steel material. The lower limit of the V content is preferably
higher than 0%, more preferably 0.005%, and still more preferably 0.010%, from the
viewpoint of more effectively obtaining such an effect.
In a case in which the V content is higher than 0.100%, however, at least one selected
15
from the group consisting of coarse V carbide, coarse V nitride and coarse V carbonitride may
be formed, possibly resulting in a decrease in the fatigue limit and the toughness of the
processed portion X. Accordingly, the V content is 0.100% or less. The upper limit of the
V content is preferably 0.090%, and more preferably 0.080%.
[0039] Cr: from 0 to 0.5%
Cr is an optional element. In other words, the Cr content may be 0%, or may be
higher than 0%.
Cr is an element that increases the hardenability of the resulting steel material, and
thereby increases the strength of the steel material. The lower limit of the Cr content is
preferably higher than 0%, more preferably 0.1%, and still more preferably 0.15%, from the
viewpoint of more effectively obtaining such an effect.
In a case in which the Cr content is higher than 0.5%, however, Cr oxide may be
formed in the electric resistance welded portion, possibly resulting in a decrease in the
toughness of the electric resistance welded portion. Accordingly, the Cr content is 0.5% or
less. The upper limit of the Cr content is preferably 0.4%, and more preferably 0.3%.
[0040] Cu: from 0 to 0.500%
Cu is an optional element. In other words, the Cu content may be 0%, or may be
higher than 0%.
Cu is an element capable of increasing the strength of the resulting steel material.
The lower limit of the Cu content is preferably higher than 0%, more preferably 0.001%, and
still more preferably 0.010%, from the viewpoint of more effectively obtaining such an effect.
In a case in which the Cu content is higher than 0.500%, however, not only the
toughness of the steel material is decreased due to an excessive strengthening, but also cracks
are more likely to occur on the surface of a slab during slab casting due to a liquid metal
embrittlement effect of Cu. Accordingly, the Cu content is 0.500% or less. The upper limit
of the Cu content is preferably 0.400%, and more preferably 0.300%.
[0041] Ni: from 0 to 0.500%
Ni is an optional element. In other words, the Ni content may be 0%, or may be
higher than 0%.
Ni is an element capable of increasing the strength of the resulting steel material and
capable of increasing the toughness of the steel material, as well, by dissolving in the steel
material. Ni is also an element capable of inhibiting the liquid metal embrittlement effect of
Cu in a Cu-containing steel. The lower limit of the Ni content is preferably higher than 0%,
more preferably 0.001%, and still more preferably 0.010%, from the viewpoint of more
effectively obtaining such effects.
16
In a case in which the Ni content is higher than 0.500%, however, the weldability of
the steel material may be decreased. Accordingly, the Ni content is 0.500% or less. The
upper limit of the Ni content is preferably 0.450%, and more preferably 0.400%.
[0042] B: from 0 to 0.0030%
B is an optional element. In other words, the B content may be 0%, or may be
higher than 0%.
B is an element capable of increasing the hardenability of the resulting steel material
and thereby increasing the strength of the steel material, by dissolving in the steel material.
The lower limit of the B content is preferably higher than 0%, more preferably 0.0001%, and
still preferably 0.0005%, from the viewpoint of more effectively obtaining such an effect.
In a case in which the B content is higher than 0.0030%, however, coarse nitrides
may be formed to cause a decrease in the fatigue limit of the steel material. Accordingly, the
B content is 0.0030% or less. The upper limit of the B content is preferably 0.0025%, and
more preferably 0.0020%.
[0043] Ca: from 0 to 0.0030%
Ca is an optional element. In other words, the Ca content may be 0%, or may be
higher than 0%.
Ca is an element capable of increasing the toughness of the resulting steel material by
controlling the form of sulfide-based inclusions. The lower limit of the Ca content is
preferably higher than 0%, more preferably 0.0001%, and still more preferably 0.0010%,
from the viewpoint of more effectively obtaining such an effect.
In a case in which the Ca content is higher than 0.0030%, however, coarse Ca oxide
may be formed to cause a decrease in the toughness of the steel material. Accordingly, the
Ca content is 0.0030% or less. The upper limit of the Ca content is preferably 0.0025%, and
more preferably 0.0020%.
[0044] Mg: from 0 to 0.0040%
Mg is an optional element. In other words, the Mg content may be 0%, or may be
higher than 0%.
Mg is an element capable of increasing the toughness of the weld heat affected zone
(HAZ) by forming fine oxides. The lower limit of the Mg content is preferably higher than
0%, more preferably 0.0001%, and still more preferably 0.0010%, from the viewpoint of
more effectively obtaining such an effect.
In a case in which the Mg content is higher than 0.0040%, however, coarse oxides
may be formed to cause a decrease in the toughness of the resulting steel material.
Accordingly, the Mg content is 0.0040% or less. The upper limit of the Mg content is
17
preferably 0.0035%, and more preferably 0.0030%.
[0045] Balance: Fe and Impurities
In the chemical composition of the base metal portion in the processed portion X, the
balance excluding the respective elements described above is Fe and impurities.
The term “impurities” as used herein refers to components which are contained in
raw materials (such as ores and scraps), or components which are mixed during production
steps and are not intentionally incorporated into the steel.
Examples of the impurities include all elements other than the elements described
above. Only one kind, or two or more kinds of elements may be contained as the impurities.
Examples of the impurities include O, Sb, Sn, W, Co, As, Pb, Bi, H, and REM. The
term “REM” as used herein refers to a rare earth element(s), namely, at least one element
selected from the group consisting of Sc, Y, La, Ce, Pr, Nd, Pm, Sm, Eu, Gd, Tb, Dy, Ho, Er,
Tm, Yb and Lu.
Among the elements described above, the content of O is preferably controlled so as
to be 0.006% or less.
In general, Sb, Sn, Co or As can be contained, for example, in a content of 0.1% or
less, Pb or Bi can be contained, for example, in a content of 0.005% or less, and H can be
contained, for example, in a content of 0.0004% or less, as the impurities.
The contents of other elements need not be particularly controlled, as long as the
contents are within usual ranges.
[0046] < Microstructure of Base Metal Portion >
The microstructure of the base metal portion in the processed portion X of the
electric resistance welded steel pipe according to the present disclosure will be described
below.
Roughly speaking, the microstructure of the base metal portion is a microstructure
mainly composed of tempered bainite.
Specifically, the area ratio of tempered bainite with respect to the entire
microstructure of the base metal portion is 80% or more.
As described above, the above-described microstructure effectively contributes to
obtaining precipitation strengthening by Mo and Nb during tempering, and eventually
contributes to improving the tensile strength and the fatigue strength.
The area ratio of tempered bainite is preferably 85% or more, and more preferably
90% or more, from the viewpoint of further improving the tensile strength and the fatigue
strength of the processed portion X.
[0047] The concept of each of bainite and tempered bainite in the present disclosure
18
encompasses granular bainite, upper bainite, lower bainite, and auto-tempered martensite.
In all of these structures, changes in structural forms due to tempering in the present
disclosure are small. Therefore, there is no need to strictly distinguish bainite from tempered
bainite in the present disclosure.
[0048] The area ratio of tempered bainite with respect to the entire microstructure of the
base metal portion may be 100%, or may be less than 100%.
The balance in a case in which the area ratio of tempered bainite is less than 100%
preferably contains at least one selected from the group consisting of ferrite (for example,
polygonal ferrite, acicular ferrite, etc.) and pearlite.
The concept of pearlite as used herein encompasses degenerate pearlite.
[0049] The area ratio of tempered bainite with respect to the entire microstructure of the
base metal portion is determined as follows.
A sample for observing the microstructure is collected from a central position in the
wall thickness direction of the base metal portion, in a C-cross section (namely, a cross
section perpendicular to the pipe axis direction of the processed portion X) of the processed
portion X. The size of an observation surface in the sample is set to 3 mm × 3 mm.
Subsequently, the observation surface of the sample is subjected to mirror polishing.
Thereafter, the observation surface of the sample is etched with 3% nitric acid alcohol (Nital
etchant). The etched observation surface is observed with a light microscope at a
magnification of 500 times. Arbitrary five visual fields in the observation surface are
specified, and photographic images of the respective specified visual fields are formed. The
region of each visual field is set to 200 μm × 200 μm.
The area of tempered bainite in all the photograph images of the five visual fields is
determined, the thus obtained area is divided by the total area of the entire five visual fields,
and multiplied by 100, to obtain the area ratio (%) of tempered bainite with respect to the
entire microstructure of the base metal portion.
[0050] < Tensile Strength of Base Metal Portion >
The base metal portion in the processed portion X has a tensile strength of from 850
to 1,000 MPa.
The fact that the tensile strength of the base metal portion in the processed portion X
is 850 MPa or more contributes to ensuring the strength as an electric resistance welded steel
pipe for a mechanical structural part. The tensile strength is preferably 900 MPa or more.
The fact that the tensile strength of the base metal portion in the processed portion X
is 1,000 MPa or less contributes to the ease of production of an electric resistance welded
steel pipe for a mechanical structural part (for example, the ease of production of a hot-rolled
19
steel sheet as the material, the ease of roll forming when roll-forming a hot-rolled steel sheet
to perform pipe-making, and the like). The tensile strength is preferably 950 MPa or less.
[0051] The tensile strength of the base metal portion in the processed portion X is measured
as follows.
A tensile test specimen is collected from a thickness region corresponding to 70%
with respect to the total thickness, with the central portion in the wall thickness direction as
the center thereof, in the base metal portion in the processed portion X. The type of the
tensile test specimen to be collected is selected from round-bar test specimens defined in JIS
Z 2241 (2011), depending on the thickness of the base metal portion. In a case in which a
test specimen with suitable dimensions is not defined in JIS Z 2241 (2011), a test specimen
(proportional test piece) which is proportionally reduced in size of any of the defined test
specimens may be used.
Using the collected tensile test specimen, a tensile test is carried out at normal
temperature (20 ± 15°C) in the atmosphere, in accordance with JIS Z 2241 (2011). The
tensile strength (TS) is determined from the resulting stress-strain curve.
[0052] < Yield Elongation of Base Metal Portion >
In the electric resistance welded steel pipe according to the present disclosure, a yield
elongation of 0.2% or more is observed in a tensile test of the base metal portion.
The “tensile test of the base metal portion” as used herein refers to a tensile test for
measuring the tensile strength of the base metal portion. The test method thereof is as
described in the section of the tensile strength of the base metal portion.
As described above, this feature (namely, the fact that a yield elongation of 0.2% or
more is observed in the tensile test of the base metal portion) contributes to reducing the
introduction of the strain due to repeated stress, and eventually contributes to improving the
fatigue strength of the processed portion X.
[0053] < Hardness Ratio [Depth 50 μm/Wall Thickness Center] >
In the electric resistance welded steel pipe according to the present disclosure, the
hardness ratio [depth 50 μm/wall thickness center] (namely, the ratio of the Vickers hardness
at a position at a depth of 50 μm from the outer surface of the base metal portion with respect
to the Vickers hardness at the central portion in the wall thickness direction of the base metal
portion) is 95% or more.
As described above, this feature (namely, the fact that the hardness ratio [depth 50
μm/wall thickness center] is 95% or more) contributes to reducing the occurrence of fatigue
cracks from the outer surface side, and eventually contributes to improving the fatigue
strength of the processed portion X.
20
The hardness ratio [depth 50 μm position/wall thickness central portion] is preferably
120% or less, and more preferably 115% or less. In a case in which the hardness ratio [depth
50 μm position/wall thickness central portion] is 120% or less, the stress can be received by
the entire wall thickness (particularly, the concentration of stress to the central portion in the
wall thickness direction can be reduced), and as a result, the fatigue strength of the processed
portion X can further be improved.
[0054] The Vickers hardness at a position at a depth of 50 μm from the outer surface of the
base metal portion is determined as follows. Five points which are located on a line
corresponding to a position at a depth of 50 μm from the outer surface of the base metal
portion and spaced apart at a pitch of 0.5 mm, in a C cross section of the processed portion X,
are specified as measurement points. The Vickers hardness is measured at each of the five
measurement points under the condition of a load of 100 gf, in accordance with JIS Z 2244
(2009). The arithmetic mean value of the measured values at the five measurement points is
defined as the “Vickers hardness at a position at a depth of 50 μm”.
The Vickers hardness at the central portion in the wall thickness direction of the base
metal portion is determined as follows. Five points which are located on a line
corresponding to the central portion in the wall thickness direction of the base metal portion
and spaced apart at a pitch of 0.5 mm, in a C cross section of the processed portion X, are
specified as measurement points. The Vickers hardness is measured at each of the five
measurement points under the condition of a load of 100 gf, in accordance with JIS Z 2244
(2009). The arithmetic mean value of the measured values at the five measurement points is
defined as the “Vickers hardness at the central portion in the wall thickness direction”.
The hardness ratio [depth 50 μm/wall thickness center] is determined by dividing the
“Vickers hardness at a position at a depth of 50 μm” by the “Vickers hardness at the central
portion in the wall thickness direction”, and multiplying by 100.
[0055] < Outer Circumference Length of Processed Portion X >
The processed portion X preferably has an outer circumference length of from 50 to
500 mm.
The outer circumference length within the range of from 50 to 500 mm is roughly a
length that corresponds to the outer circumference length of a steel pipe having an outer
diameter of from 16 to 160 mm.
The outer circumference length of the processed portion X is more preferably from
50 to 400 mm, and still more preferably from 100 to 300 mm.
[0056] < Maximum Wall Thickness of Processed Portion X >
The processed portion X preferably has a maximum wall thickness of (namely, a
21
maximum value of the wall thickness of the processed portion X) of from 1.0 to 5.0 mm,
more preferably from 1.5 to 4.5 mm, and still more preferably from 2.0 to 4.0 mm.
[0057] [One Example of Method of Producing Electric Resistance Welded Steel Pipe for
Mechanical Structural Part (Production Method X)]
One example of the method of producing the electric resistance welded steel pipe
according to the present disclosure (hereinafter, referred to as “production method X”) will be
described below.
The production method X to be described below is a method of producing an electric
resistance welded steel pipe of each Example to be described later.
[0058] The production method X includes:

CLAIMS
1. An electric resistance welded steel pipe for a mechanical structural part, the pipe
comprising a processed portion X that consists of at least one of a drawing-processed portion
or a steel pipe bending-processed portion,
wherein the processed portion X comprises a base metal portion and an electric
resistance welded portion,
wherein the base metal portion has a chemical composition consisting of, in terms
of % by mass:
from 0.150 to 0.230% of C,
from 0 to 0.50% of Si,
from 0.50 to 1.65% of Mn,
from 0 to 0.030% of P,
from 0 to 0.010% of S,
from 0.010 to 0.050% of Nb,
from 0.10 to 0.60% of Mo,
from 0.005 to 0.060% of Al,
from 0 to 0.0060% of N,
from 0 to 0.030% of Ti,
from 0 to 0.100% of V,
from 0 to 0.5% of Cr,
from 0 to 0.500% of Cu,
from 0 to 0.500% of Ni,
from 0 to 0.0030% of B,
from 0 to 0.0030% of Ca,
from 0 to 0.0040% of Mg, and
a balance consisting of Fe and impurities,
wherein an area ratio of tempered bainite with respect to an entire microstructure of
the base metal portion is 80% or more,
wherein the base metal portion has a tensile strength of from 850 to 1,000 MPa,
wherein a yield elongation of 0.2% or more is observed in a tensile test of the base
metal portion, and
wherein a ratio of a Vickers hardness at a position at a depth of 50 μm from an outer
surface of the base metal portion with respect to a Vickers hardness at a central portion in a
wall thickness direction of the base metal portion is 95% or more.
46
2. The electric resistance welded steel pipe for a mechanical structural part according to
claim 1, which is an electric resistance welded steel pipe for an automotive underbody part.
3. The electric resistance welded steel pipe for a mechanical structural part according to
claim 1 or 2, wherein the processed portion X has an outer circumference length of from 50 to
500 mm, and has a maximum wall thickness of from 1.0 to 5.0 mm.
4. A method of producing the electric resistance welded steel pipe for a mechanical
structural part according to any one of claims 1 to 3, the method comprising:
a step of preparing an as-rolled electric resistance welded steel pipe, wherein:
the as-rolled electric resistance welded steel pipe comprises a base metal portion A
and an electric resistance welded portion A; the base metal portion A has a chemical
composition consisting of, in terms of % by mass:
from 0.150 to 0.230% of C,
from 0 to 0.50% of Si,
from 0.50 to 1.65% of Mn,
from 0 to 0.030% of P,
from 0 to 0.010% of S,
from 0.010 to 0.050% of Nb,
from 0.10 to 0.60% of Mo,
from 0.005 to 0.060% of Al,
from 0 to 0.0060% of N,
from 0 to 0.030% of Ti,
from 0 to 0.100% of V,
from 0 to 0.5% of Cr,
from 0 to 0.500% of Cu,
from 0 to 0.500% of Ni,
from 0 to 0.0030% of B,
from 0 to 0.0030% of Ca,
from 0 to 0.0040% of Mg, and
a balance consisting of Fe and impurities,
an area ratio of bainite with respect to an entire microstructure of the base metal
portion A is 80% or more,
the base metal portion A has a tensile strength of from 600 to 800 MPa, and
47
a total elongation of 13.0% or more is observed in a tensile test of the base metal portion A;
a cold working step of subjecting at least one portion in a pipe axis direction of the
as-rolled electric resistance welded steel pipe to cold working that is at least one of cold
drawing or cold steel pipe bending and that is performed under a condition of a maximum
area reduction rate of from 10 to 40%; and
a tempering step of subjecting the as-rolled electric resistance welded steel pipe that
has been subjected to the cold working to tempering at a tempering temperature of from 450
to 650°C, to obtain the electric resistance welded steel pipe for a mechanical structural part.
5. The method of producing the electric resistance welded steel pipe for a mechanical
structural part, according to claim 4, wherein the as-rolled electric resistance welded steel pipe
has an outer diameter of from 50 to 150 mm, and has a wall thickness of from 2.0 to 4.0 mm.