U.S. patent application number 13/636857 was filed with the patent office on 2013-06-27 for high-strength electric resistance welded steel tube and production method therefor.
This patent application is currently assigned to JFE STEEL CORPORATION. The applicant listed for this patent is Masatoshi Aratani, Norimasa Hirata, Yoshikazu Kawabata, Saiji Matsuoka. Invention is credited to Masatoshi Aratani, Norimasa Hirata, Yoshikazu Kawabata, Saiji Matsuoka.
Application Number | 20130160889 13/636857 |
Document ID | / |
Family ID | 44673378 |
Filed Date | 2013-06-27 |
United States Patent
Application |
20130160889 |
Kind Code |
A1 |
Aratani; Masatoshi ; et
al. |
June 27, 2013 |
HIGH-STRENGTH ELECTRIC RESISTANCE WELDED STEEL TUBE AND PRODUCTION
METHOD THEREFOR
Abstract
A high-strength electric resistance welded steel tube has a
composition including, in terms of percent by mass, C: 0.05 to
0.20%, Si: 0.5 to 2.0%, Mn: 1.0 to 3.0%, P: 0.1% or less, S: 0.01%
or less, Al: 0.01 to 0.1%, N: 0.005% or less, and the balance being
Fe and unavoidable impurities; and a structure which is a dual
phase structure including a ferrite phase and a martensite phase,
with a volume ratio of the martensite phase being 20 to 60%,
wherein a tensile strength TS is 1180 MPa or more, an elongation El
in a tube axis direction is 10% or more, and a yield ratio is less
than 90%; and after application of a 2% prestrain and baking
finishing that includes a heat treatment of 170.degree. C..times.10
min, a strength increase (BH value) is 100 MPa or more and a yield
ratio is 90% or more.
Inventors: |
Aratani; Masatoshi;
(Handa-shi, JP) ; Kawabata; Yoshikazu; (Handa-shi,
JP) ; Matsuoka; Saiji; (Kurashiki-shi, JP) ;
Hirata; Norimasa; (Handa-shi, JP) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
Aratani; Masatoshi
Kawabata; Yoshikazu
Matsuoka; Saiji
Hirata; Norimasa |
Handa-shi
Handa-shi
Kurashiki-shi
Handa-shi |
|
JP
JP
JP
JP |
|
|
Assignee: |
JFE STEEL CORPORATION
Tokyo
JP
|
Family ID: |
44673378 |
Appl. No.: |
13/636857 |
Filed: |
March 23, 2011 |
PCT Filed: |
March 23, 2011 |
PCT NO: |
PCT/JP2011/057928 |
371 Date: |
November 29, 2012 |
Current U.S.
Class: |
138/177 ;
219/78.16 |
Current CPC
Class: |
C21D 9/08 20130101; C21D
2211/008 20130101; C22C 38/04 20130101; C21D 9/46 20130101; C21D
2211/005 20130101; C22C 38/06 20130101; B21C 37/08 20130101; C21D
8/0205 20130101 |
Class at
Publication: |
138/177 ;
219/78.16 |
International
Class: |
F16L 9/02 20060101
F16L009/02; B23K 11/00 20060101 B23K011/00 |
Foreign Application Data
Date |
Code |
Application Number |
Mar 24, 2010 |
JP |
2010-068499 |
Jan 27, 2011 |
JP |
2011-015076 |
Claims
1. A high-strength electric resistance welded steel tube having a
composition including, in terms of percent by mass, TABLE-US-00006
C: 0.05 to 0.20% Si: 0.5 to 2.0% Mn: 1.0 to 3.0% P: 0.1% or less S:
0.01% or less Al: 0.01 to 0.1% N: 0.005% or less,
and the balance being Fe and unavoidable impurities, and a
structure which is a dual phase structure including a ferrite phase
and a martensite phase, with a volume ratio of the martensite phase
being 20 to 60%, wherein a tensile strength TS is 1180 MPa or more,
an elongation El in a tube axis direction is 10% or more, and a
yield ratio is less than 90%; and after application of a 2%
prestrain and baking finishing that includes a heat treatment of
170.degree. C..times.10 min, a strength increase (BH value) is 100
MPa or more and a yield ratio is 90% or more.
2. The high-strength electric resistance welded steel tube
according to claim 1, wherein the composition further includes, in
terms of percent by mass, at least one selected from Cu: 1.0% or
less, Ni: 1.0% or less, Cr: 0.5% or less, Mo: 0.5% or less, Nb:
0.05% or less, Ti: 0.05% or less, W: 0.05% or less, and B: 0.0050%
or less.
3. The high-strength electric resistance welded steel tube
according to claim 1, wherein the composition further includes, in
terms of percent by mass, Ca: 0.0050% or less and/or REM: 0.0050%
or less.
4. A method for producing a high-strength electric resistance
welded steel tube, the method comprising a hot rolling process of
hot-rolling a steel into a hot-rolled sheet; a cold-rolling process
of pickling the hot-rolled sheet and cold-rolling the pickled
hot-rolled sheet to prepare a cold-rolled sheet; an annealing
process of annealing the cold-rolled sheet into a cold-rolled
annealed sheet so as to prepare a material for a steel tube; and a
tube production process of continuously forming the material for a
steel tube into a substantially cylindrical open tube and
electric-resistance-welding the open tube to prepare an electric
resistance welded tube, wherein the steel has a composition
including, in terms of percent by mass, TABLE-US-00007 C: 0.05 to
0.20% Si: 0.5 to 2.0% Mn: 1.0 to 3.0% P: 0.1% or less S: 0.01% or
less Al: 0.01 to 0.1% N: 0.005% or less,
and the balance being Fe and unavoidable impurities, in the
hot-rolling process, the hot rolling is conducted at a finishing
temperature equal to or higher than an Ar.sub.a transformation
point and at a coiling temperature of 500 to 700.degree. C. to
prepare the hot-rolled sheet, in the annealing process, after
soaking is performed at a temperature in a two-phase temperature
region ranging from an Ac.sub.1 transformation point to an Ac.sub.3
transformation point, the sheet is cooled at an average cooling
rate of 10.degree. C./s or more to a temperature in the range of
600 to 750.degree. C. and then rapidly cooled at a cooling rate of
500.degree. C./s or more from the temperature in the range of 600
to 750.degree. C. to room temperature, and then soaking is
performed in the temperature range of 150 to 300.degree. C., the
forming is performed by a roll forming method involving a cage roll
method, and the electric resistance welded tube has a tensile
strength TS of 1180 MPa or more, an elongation El in a tube axis
direction of 10% or more, and a yield ratio less than 90%, and
exhibits, after application of a 2% prestrain and baking finishing
that includes a heat treatment of 170.degree. C..times.10 min, a
strength increase (BH value) of 100 MPa or more and a yield ratio
of 90% or more.
5. The method for producing a high-strength electric resistance
welded steel tube according to claim 4, wherein the composition
further includes, in terms of percent by mass, at least one
selected from Cu: 1.0% or less, Ni: 1.0% or less, Cr: 0.5% or less,
Mo: 0.5% or less, Nb: 0.05% or less, Ti: 0.05% or less, W: 0.05% or
less, and B: 0.0050% or less.
6. The method for producing a high-strength electric resistance
welded steel tube according to claim 4, wherein the composition
further includes, in terms of percent by mass, Ca: 0.0050% or less
and/or REM: 0.0050% or less.
Description
CROSS REFERENCE TO RELATED APPLICATIONS
[0001] This application is the U.S. National Phase application of
PCT International Application No. PCT/JP2011/057928, filed Mar. 23,
2011, and claims priority to Japanese Patent Application No.
2010-068499, filed Mar. 24, 2010, and 2011-015076, filed Jan. 27,
2011, the disclosures of each of which are incorporated herein by
reference in their entireties for all purposes.
FIELD OF THE INVENTION
[0002] The present invention relates to high-strength electric
resistance welded steel tubes suitable for use in crash members for
automobiles such as door impact beams, cross members, and pillars,
and, in particular, to a high-strength electric resistance welded
steel tube having both excellent formability and shock
absorption.
BACKGROUND OF THE INVENTION
[0003] In recent years, for the purposes of achieving enhanced
safety of automobiles and in particular ensuring safety of
occupants, shock absorbing members for absorbing impact energy upon
collision are installed in automotive bodies. For example, a
high-strength steel tube having a desired high strength and a
martensitic structure induced by a quenching treatment has been
applied to door impact beams, i.e., shock absorbing members, as
described in Patent Literature 1.
[0004] Patent Literature 1 discloses a method for producing an
electric resistance welded steel tube for machine structural use,
the method including quenching a steel tube containing C: 0.15 to
0.22%, Mn: 1.5% or less, Si: 0.5% or less, Ti: 0.04% or less, B:
0.0003 to 0.0035%, N: 0.0080% or less and one or more selected from
Ni: 0.5% or less, Cr: 0.5% or less, and Mo: 0.5% or less, wherein
the electric resistance welded steel tube for machine structural
use has a tensile strength of 120 kgf/mm.sup.2 or more. According
to the technology described in Patent Literature 1, a high-strength
steel tube that has a tensile strength of 120 kgf/mm.sup.2 or more
and an excellent elongation of 10% or more, that can be used for
reinforcing automobiles, and that can be applied to door impact
bars (door impact beams) and center cores for bumpers can be
obtained by performing a heat treatment once.
[0005] Steel sheets having a tensile strength of 120 kgf/mm.sup.2
or more are also disclosed in Patent Documents 2 to 7 which
disclose the technologies related to high-strength cold-rolled
steel sheets that are used in automotive structural members and
have a tensile strength of 900 MPa or more. These steel sheets all
have a dual phase structure containing a ferrite phase and a
martensite phase or a structure containing a bainite phase and a
retained austenite phase in addition to these phases, and the upper
limits of the area fractions of the bainite phase and the retained
austenite phase are defined. According to these literatures, it is
because of this structure that the steel sheets exhibit both
formability and high strength.
PATENT LITERATURE
[0006] PTL 1: Japanese Unexamined Patent Application Publication
No. 3-122219 [0007] PTL 2: Japanese Unexamined Patent Application
Publication No. 2010-255094 [0008] PTL 3: Japanese Unexamined
Patent Application Publication No. 2010-126787 [0009] PTL 4:
Japanese Unexamined Patent Application Publication No. 2009-242816
[0010] PTL 5: Japanese Unexamined Patent Application Publication
No. 2009-203550 [0011] PTL 6: Japanese Unexamined Patent
Application Publication No. 2007-100114 [0012] PTL 7: Japanese
Unexamined Patent Application Publication No. 2005-163055
SUMMARY OF THE INVENTION
[0013] The technology described in Patent Literature 1 does not
present a serious problem in the cases where steel tubes are used
straight without being subjected to any working, such as in the
cases of door impact beams. However, steel tubes that are used in
other automotive shock absorbing members such as cross members and
pillars that require complicated forming to make various shapes are
required to exhibit excellent formability in addition to the high
strength.
[0014] The technologies described in Patent Literatures 2 to 5 have
problems in that, because of the low cooling rate after holding of
heat during annealing, precipitation of carbides occurs, the solute
C content in the ferrite becomes insufficient, the strength
increase (bake hardening value or BH value) caused by a
prestrain-baking finishing treatment is small, and a BH value of
100 MPa or more is not reliably achieved.
[0015] The technology described in Patent Literature 6 does not
consider the cooling rate from the holding of heat during annealing
to the start of water quenching. For example, when the time taken
up to the start of water quenching is long due to the layout of the
production line and thus the cooling rate is low, the C content
distribution proceeds between ferrite and austenite and thus the
amount of the solute C remaining in the ferrite presumably
contributing to the bake hardenability is insufficient. Thus,
Patent Literature 6 does not describe or anticipate that the BH
value of 100 MPa or more is ensured.
[0016] In the technology described in Patent Literature 7, the
cooling rate during finish annealing is low, e.g., 550.degree.
C./min at maximum in Examples, and the elongation is only about 8%.
The elongation is generally low and 11% at maximum. Accordingly,
when a steel sheet produced by the technology described in Patent
Literature 7 is formed into an electric resistance welded steel
tube, the elongation will further decrease due to the processing
strain applied during tube forming and the resulting steel tube
does not reliably achieve an elongation of 10% or more.
[0017] Under these requirements, the present invention provides a
high-strength electric resistance welded steel tube that has
excellent formability and that can ensure excellent shock
absorption suitable for use in automotive shock absorbing members
and a method for producing the high-strength electric resistance
welded steel tube.
[0018] Note that "high strength" refers to a tensile strength TS of
1180 MPa or more.
[0019] Moreover, "excellent formability" refers to an elongation El
of 10% or more and preferably 12% or more in the tube axis
direction and a yield ratio (=0.2% proof stress/tensile
strength.times.100(%)) of less than 90% determined by a tensile
test using a JIS No. 12 tensile test specimen (GL: 50 mm) defined
by Japanese Industrial Standards (JIS). Furthermore, "excellent
shock absorption" refers to the case in which the strength increase
(bake hardening value or BH value), i.e., the difference between
the 0.2% proof stress after heat-treating (baking finishing) a 2%
prestrained tube at 170.degree. C. for 10 minutes and the strength
upon application of a 2% prestrain, is 100 MPa or more and the
yield ratio in the tube axis direction is 90% or more. The BH value
is defined in FIG. 2.
[0020] The inventors of the present application have conducted
extensive studies to find ways to improve the formability of
electric resistance welded steel tubes while maintaining the high
strength. As a result, the inventors have found that an electric
resistance welded tube having excellent formability can be produced
by using, as a material for a steel tube, a steel sheet
(cold-rolled steel sheet) having a ferrite-martensite dual phase
structure, excellent formability, and a desired bake hardenability
and employing a tube production method with which a tube can be
formed without significantly degrading the excellent formability of
the material for a steel tube. After this electric resistance
welded tube is worked to have a desired component shape, a heat
treatment (baking finishing) is performed to increase the strength
so that the proof stress is improved and the resulting component
can reliably achieve excellent shock absorption.
[0021] The present invention has been made based on the
above-described findings and conducting further studies. The
summary of the present invention according to exemplary embodiments
is as follows:
(1) A high-strength electric resistance welded steel tube having a
composition including, in terms of percent by mass, C: 0.05 to
0.20%, Si: 0.5 to 2.0%, Mn: 1.0 to 3.0%, P: 0.1% or less, S: 0.01%
or less, Al: 0.01 to 0.1%, N: 0.005% or less, and the balance being
Fe and unavoidable impurities, and a structure which is a dual
phase structure including a ferrite phase and a martensite phase,
with a volume ratio of the martensite phase being 20 to 60%, in
which a tensile strength TS is 1180 MPa or more, an elongation El
in a tube axis direction is 10% or more, and a yield ratio is less
than 90%; and after application of a 2% prestrain and baking
finishing that includes a heat treatment of 170.degree. C..times.10
min, a strength increase (BH value) is 100 MPa or more and a yield
ratio is 90% or more. (2) In the high-strength electric resistance
welded steel tube of (1), the composition further includes, in
terms of percent by mass, at least one selected from Cu: 1.0% or
less, Ni: 1.0% or less, Cr: 0.5% or less, Mo: 0.5% or less, Nb:
0.05% or less, Ti: 0.05% or less, W: 0.05% or less, and B: 0.0050%
or less. (3) In the high-strength electric resistance welded steel
tube of (1) or (2), the composition further includes, in terms of
percent by mass, Ca: 0.0050% or less and/or REM: 0.0050% or less.
(4) A method for producing a high-strength electric resistance
welded steel tube, the method including a hot rolling process of
hot-rolling a steel into a hot-rolled sheet; a cold-rolling process
of pickling the hot-rolled sheet and cold-rolling the pickled
hot-rolled sheet to prepare a cold-rolled sheet; an annealing
process of annealing the cold-rolled sheet into a cold-rolled
annealed sheet so as to prepare a material for a steel tube; and a
tube production process of continuously forming the material for a
steel tube into a substantially cylindrical open tube and
electric-resistance-welding the open tube to prepare an electric
resistance welded tube. The steel has a composition including, in
terms of percent by mass, C: 0.05 to 0.20%, Si: 0.5 to 2.0%, Mn:
1.0 to 3.0%, P: 0.1% or less, S: 0.01% or less, Al: 0.01 to 0.1%,
N: 0.005% or less, and the balance being Fe and unavoidable
impurities. In the hot-rolling process, the hot rolling is
conducted at a finishing temperature equal to or higher than an
Ar.sub.3 transformation point and at a coiling temperature of 500
to 700.degree. C. to prepare the hot-rolled sheet. In the annealing
process, after the cold-rolled sheet is heated to and soaked at a
temperature in a two-phase temperature region ranging from an
Ac.sub.1 transformation point to an Ac.sub.3 transformation point,
the sheet is cooled at an average cooling rate (defined as "average
cooling rate 1") of 10.degree. C./s or more to a temperature in the
range of 600 to 750.degree. C. and then rapidly cooled at an
average cooling rate (defined as "average cooling rate 2") of
500.degree. C./s or more from the temperature in the range of 600
to 750.degree. C. to room temperature, and then a tempering
treatment that includes re-heating the sheet to a temperature in
the range of 150 to 300.degree. C. is performed so as to prepare a
cold-rolled annealed sheet. The forming is performed by a roll
forming method involving a cage roll method. The electric
resistance welded tube has a tensile strength TS of 1180 MPa or
more, an elongation El in a tube axis direction of 10% or more, and
a yield ratio less than 90%, and exhibits, after application of a
2% prestrain and baking finishing that includes a heat treatment of
170.degree. C..times.10 min, a strength increase (BH value) of 100
MPa or more and a yield ratio of 90% or more. (5) In the method for
producing a high-strength electric resistance welded steel tube of
(4), the composition further includes, in terms of percent by mass,
at least one selected from Cu: 1.0% or less, Ni: 1.0% or less, Cr:
0.5% or less, Mo: 0.5% or less, Nb: 0.05% or less, Ti: 0.05% or
less, W: 0.05% or less, and B: 0.0050% or less. (6) In the method
for producing a high-strength electric resistance welded steel tube
in (4) or (5), the composition further includes, in terms of
percent by mass, Ca: 0.0050% or less and/or REM: 0.0050% or
less.
[0022] According to the present invention, a high-strength electric
resistance welded steel tube that has excellent formability
suitable for use in shock absorbing members of automotives and that
can reliably achieve excellent shock absorption after being formed
into an actual component shape can be produced at low cost and thus
the present invention provides remarkable industrial advantages.
Moreover, the high-strength electric resistance welded steel tube
according to the present invention can be used not only in door
impact beams but also in all types of automotive parts such as
automotive shock absorbing components, e.g., cross members and
pillars, that require formability, and automotive body parts.
BRIEF DESCRIPTION OF THE DRAWINGS
[0023] FIG. 1 is a schematic diagram showing one example of a
facility for producing an electric resistance welded tube, the
facility employing a CBR roll forming method suitable for
implementing an embodiment of the present invention.
[0024] FIG. 2 is a schematic diagram showing the definition of a
strength increase (BH value) after baking finishing.
DETAILED DESCRIPTION OF EMBODIMENTS OF THE INVENTION
[0025] The reasons for limitations on the composition of a
high-strength electric resistance welded steel tube are first
described. Hereinafter, mass % is simply denoted as % unless
otherwise noted.
C: 0.05 to 0.20%
[0026] Carbon (C) strengthens the steel and the C content in the
present invention should preferably be 0.05% or more to ensure a
desired strength. When the C content exceeds 0.20%, the weldability
is degraded. Thus, in the present invention, the C content is
preferably limited to be in the range of 0.05 to 0.20% and more
preferably in the range of 0.08 to 0.18%.
Si: 0.5 to 2.0%
[0027] Silicon (Si) serves as a deoxidizing agent, strengthens the
steel by forming a solid solution, accelerates formation of
ferrite, and is thus an important element for ensuring excellent
formability. Silicon also causes solid solution strengthening of
the ferrite phase to thereby suppress the martensite phase fraction
and achieve a desired high strength. The Si content needs to be
0.5% or more in order to attain these effects. In contrast, when
the Si content exceeds 2.0%, large amounts of silicon oxides occur
in the steel sheet surface and the chemical conversion treatability
is thereby degraded. Accordingly, in the present invention, the Si
content is advantageously limited to be in the range of 0.5 to 2.0%
and preferably in the range of 1.0 to 1.8%.
Mn: 1.0 to 3.0%
[0028] Manganese (Mn) improves hardenability, promotes formation of
the martensite phase, and increases the strength of the steel. The
Mn content of 1.0% is required in embodiments of the present
invention in order to reliably achieve a desired strength. In
contrast, when the Mn content exceeds 3.0%, segregation is
accelerated, slab cracks tend to occur during casting, and the
amount of the martensite phase increases excessively, thereby
degrading the formability. Accordingly, the Mn content is limited
to be in the range of 1.0 to 3.0% and preferably in the range of
1.5 to 2.5%.
P: 0.1% or less
[0029] Phosphorus (P) is an impurity in the present invention and
the P content is preferably as low as possible to avoid adverse
effects on formability. However, excessively decreasing the P
content increases the refining cost. Accordingly, the P content is
limited to 0.1% or less which does not substantially cause adverse
effects. Preferably, the P content is 0.05% or less.
S: 0.01% or less
[0030] As with phosphorus (P), sulfur (S) is an impurity in the
present invention and the S content is preferably as low as
possible to avoid adverse effects on formability. However,
excessively decreasing the S content increases the refining cost.
Accordingly, the upper limit of the S content is set to 0.01% and
preferably 0.005% or less.
Al: 0.01 to 0.1%
[0031] Aluminum (Al) serves as a deoxidizing agent and the Al
content needs to be 0.01% or more in order to achieve this effect.
When the Al content exceeds 0.1%, saturation occurs and the effect
that corresponds to the content cannot be anticipated. Accordingly,
the Al content is limited to be in the range of 0.01 to 0.1% and
preferably in the range of 0.01 to 0.08%.
N: 0.005% or less
[0032] Nitrogen (N) strengthens the steel but decreases the
formability and the content of nitrogen as an impurity is
preferably decreased as much as possible. However, excessively
decreasing the N content increases the refining cost. Accordingly,
the N content is limited to 0.005% or less which does not have
substantial adverse effect. Preferably, the N content is 0.004% or
less.
[0033] While the components described heretofore are the basic
components, at least one selected from Cu: 1.0% or less, Ni: 1.0%
or less, Cr: 0.5% or less, Mo: 0.5% or less, Nb: 0.05% or less, Ti:
0.05% or less, W: 0.05% or less, and B: 0.0050% or less and/or at
least one selected from Ca: 0.0050% or less and REM: 0.0050% or
less may be contained in addition to the basic composition.
[0034] Copper (Cu), nickel (Ni), chromium (Cr), molybdenum (Mo),
niobium (Nb), titanium (Ti), tungsten (W), and boron (B) all
increase the strength of the steel and one or more of these
elements can be selected as needed and added.
Cu: 1.0% or less
[0035] Copper (Cu) increases the strength of the steel and improves
the corrosion resistance, and may be contained as needed. These
effects can be achieved at a Cu content of 0.05% or more but the
hot workability is degraded at a Cu content exceeding 1.0%.
Accordingly, when copper is to be used, the Cu content is
preferably limited to 1.0% or less and more preferably 0.08 to
0.5%.
Ni: 1.0% or less
[0036] Nickel (Ni) increases the strength of the steel and improves
the corrosion resistance and may be contained as needed. These
effects can be achieved at a Ni content of 0.05% or more. However,
since nickel is an expensive element, incorporation of a large
quantity of Ni exceeding 1.0% increases the cost of the raw
material. Accordingly, when the nickel is to be used, the Ni
content is preferably limited to 1.0% or less and more preferably
0.08 to 0.5%.
Cr: 0.5% or less
[0037] Chromium (Cr) improves the hardenability and thus increases
the strength of the steel, and improves the corrosion resistance.
Chromium may be contained as needed. These effects are achieved at
a Cr content of 0.05% or more. However, the formability decreases
at a Cr content exceeding 0.5%. Accordingly, when chromium is to be
used, the Cr content is preferably limited to 0.5% or less and more
preferably 0.05 to 0.4%
Mo: 0.5% or less
[0038] Molybdenum (Mo) improves the hardenability and increases the
strength of the steel through precipitation strengthening, and may
be contained as needed. These effects are achieved at a Mo content
of 0.05% or more. However, the ductility decreases and the cost of
raw material increases at a Mo content exceeding 0.5%. Accordingly,
when molybdenum is to be used, the Mo content is preferably limited
to 0.5% or less and more preferably 0.1 to 0.4%.
Nb: 0.05% or less
[0039] Niobium (Nb) reduces the size of crystal grains and
increases the strength of the steel through precipitation
strengthening, and may be contained as needed. Such effects are
achieved at a Nb content of 0.005% or more but the ductility
decreases at a Nb content exceeding 0.05%. Accordingly, when
niobium is to be used, the Nb content is preferably limited to
0.05% or less and more preferably 0.008 to 0.03%.
Ti: 0.05% or less
[0040] Titanium (Ti) reduces the size of crystal grains and
increases the strength of the steel through precipitation
strengthening, and may be contained as needed. Such effects are
achieved at a Ti content of 0.005% or more but the ductility
decreases at a Ti content exceeding 0.05%. Accordingly, when
titanium is to be used, the Ti content is preferably limited to
0.05% or less and more preferably 0.008 to 0.03%.
W: 0.05% or less
[0041] Tungsten (W) increases the strength of the steel through
precipitation strengthening and may be contained as needed. Such an
effect is achieved at a W content of 0.01% or more but the
ductility decreases at a W content exceeding 0.05%. Accordingly,
when tungsten is to be used, the W content is preferably limited to
0.05% or less and more preferably 0.01 to 0.03%.
B: 0.0050% or less
[0042] Boron (B) improves the hardenability, thereby helping adjust
the martensite fraction to be within a particular range and
increases the strength of the steel, and may be contained as
needed. Such effects are achieved at a B content of 0.0005% or
more. However, saturation occurs and effects corresponding to the
content cannot be anticipated at a B content exceeding 0.0050%,
which is economically disadvantageous. Accordingly, when boron is
to be used, the B content is preferably limited to 0.0050% or less
and more preferably 0.001 to 0.003%.
Ca: 0.0050% or less and/or REM: 0.0050% or less
[0043] Calcium (Ca) and a rare earth element (REM) improve the
ductility through morphological control of sulfide-based inclusions
and may be contained as needed. Such an effect is achieved at a Ca
content and a REM content of 0.0020% or more. However, at a Ca
content and a REM content exceeding 0.0050%, the amount of
inclusions becomes excessively large and the cleanness of the steel
is decreased. Accordingly, when calcium and the rare earth element
are to be used, the Ca content and the REM content are both
preferably limited to 0.0050% or less and more preferably 0.0020 to
0.0040%.
[0044] The balance other than the components described above is Fe
and unavoidable impurities.
[0045] Next, the reasons for limitations on the structure of the
steel tube of embodiments of the present invention are
described.
[0046] A steel tube of the present invention preferably has a dual
phase structure including 20 to 60% of a martensite phase in terms
of volume ratio with the remainder being a ferrite phase. Because
of this structure, a desired high strength, excellent formability,
and excellent bake hardenability are all attained.
[0047] A desired high strength is not achieved at a martensite
phase fraction less than 20 vol % because the ferrite phase is
dominant in the structure. At a martensite phase fraction exceeding
60 vol %, the martensite phase becomes dominant and a desired
formability may not be ensured. Accordingly, the martensite phase
fraction in the structure is limited to be in the range of 20 to
60% in terms of a volume ratio and preferably 40 to 55% in terms of
volume ratio.
[0048] Next, a preferable method for producing the steel tube of
the present invention is described.
[0049] In the present invention, a steel is subjected to a
hot-rolling process, a cold-rolling process, and an annealing
process to form a material for a steel tube, and the material for a
steel tube is subjected to a tube production process to form an
electric resistance welded tube.
[0050] The method for producing the steel is not particularly
limited. Preferably, a molten steel having the above-described
composition is refined by a common refining method using a
converter or the like and formed into a slab or the like by a
continuous casting method or an ingoting-rolling method so as to
form a steel.
[0051] The steel is subjected to a hot-rolling process through
which the steel is hot-rolled into a hot-rolled sheet.
[0052] The steel may be reheated after cooling or, when the steel
holds a particular quantity of heat, may be directly sent to be
hot-rolled without reheating. When reheating is to be performed,
the heating temperature is preferably 1000 to 1250.degree. C. When
the heating temperature during reheating is less than 1000.degree.
C., deformation resistance is high and the load imposed on a
rolling machine is excessively large, thereby possibly making
rolling difficult. In contrast, when the heating temperature
exceeds 1250.degree. C., the crystal grains become coarse and the
ductility decreases significantly.
[0053] Hot rolling includes rough rolling and finish rolling. The
conditions of the rough rolling are any as long as a sheet bar
having particular dimension and shape is obtained. The finish
rolling involves rolling at a finishing temperature equal to or
higher than the Ar.sub.3 transformation point of a steel strip,
i.e., the material to be rolled. After the finish rolling, the
steel strip is coiled at a coiling temperature of 500 to
700.degree. C.
[0054] When the finishing temperature is lower than the Ar.sub.3
transformation point, finishing rolling involves rolling at an
(.alpha.+.gamma.) two-phase region and the structure is a mixed
grain structure in which significantly coarse crystal grains and
fine crystal grains are mixed. Thus, when a cold-rolling process
and an annealing process are performed thereafter, satisfactory
formability may not be reliably obtained and rough surfaces occur
as a result of working such as press forming and bending work.
Accordingly, the finishing temperature of the hot-rolling is
limited to a temperature equal to or higher than the Ar.sub.3
transformation point. At a coiling temperature less than
500.degree. C., a hard phase is generated during cooling, the roll
load increases during cold-rolling, and thus the productivity is
decreased. When the coiling temperature is high exceeding
700.degree. C., a non-transformed austenite transforms into
pearlite and thus formability is decreased. Thus, the coiling
temperature is limited to be in the range of 500 to 700.degree. C.
The coiling temperature is preferably 650.degree. C. or less.
[0055] The hot rolled sheet obtained through the hot-rolling
process is next subjected to a cold-rolling process of pickling the
hot-rolled sheet and then cold-rolling the pickled sheet into a
cold-rolled sheet. The conditions of the cold-rolling process such
as reduction during cold rolling are not particularly defined.
[0056] The resulting cold-rolled sheet is subjected to an annealing
process to form a cold-rolled annealed sheet.
[0057] The annealing process is crucial in the present invention in
order to reliably achieve the desired formability and the desired
bake hardenability (BH). The annealing process is preferably
conducted in a continuous annealing line.
[0058] In the annealing process, after the cold-rolled sheet is
heated to a temperature in a two-phase temperature range ranging
from the Ac.sub.1 transformation point to the Ac.sub.3
transformation point and soaked thereat, the sheet is cooled
(average cooling rate 1) at an average cooling rate of 10.degree.
C./sec or more to a temperature in the range of 600 to 750.degree.
C. and then rapidly cooled (average cooling rate 2) from the
temperature in the range of 600 to 750.degree. C. to room
temperature at an average cooling rate of 500.degree. C./s or more.
The sheet is then subjected to a tempering treatment of reheating
the sheet to a temperature in the range of 150 to 300.degree. C.
and thereby made into a cold-rolled annealed sheet. Note that in
order to stably achieve the desired high strength and the bake
hardenability, the cooling rate (average cooling rate 1) from the
soaking temperature to the temperature at the start of rapid
cooling is preferably 15.degree. C./s or more and the average
cooling rate (average cooling rate 2) in the rapid-cooling
treatment is preferably 800.degree. C./s or more, more preferably
1000.degree. C./s or more, and most preferably 1100.degree. C./s or
more.
[0059] When the heating and soaking temperature is outside the
two-phase temperature region ranging from the Ac.sub.1
transformation point to the Ac.sub.3 transformation point, a
(ferrite+martensite) structure having a desired structural fraction
cannot be reliably obtained in the subsequent rapid cooling. When
the cooling rate (average cooling rate 1) from the heat holding
temperature to the temperature at the start of rapid cooling is
less than 10.degree. C./s, distribution of the C content proceeds
between ferrite and austenite, the amount of solute C in the
ferrite presumably contributing to bake hardenability becomes
small, and thus the desired bake hardenability is not obtained.
When the temperature at the start of rapid cooling is outside the
range of 750.degree. C. to 600.degree. C., a (ferrite+martensite)
structure having a desired structural fraction cannot be obtained.
When the temperature at the start of rapid cooling exceeds
750.degree. C., the ductility decreases. When the temperature at
the start of rapid cooling is less than 600.degree. C., a desired
high strength cannot be reliably obtained. The soaking time at the
above-described temperature is preferably 30 s or longer.
[0060] When the cooling rate (average cooling rate 2) from the
temperature in the range of 600 to 750.degree. C. to room
temperature is less than 500.degree. C./s on average, the amount of
transformed martensite is small, a (ferrite+martensite) structure
having a desired structural fraction cannot be formed, a desired
high strength cannot be reliably achieved, and a desired bake
hardening value of 100 MPa or more is not obtained due to a small
amount of solute C in the ferrite presumably contributing to the
bake hardenability. The cooling rate in the rapid-cooling treatment
is the average cooling rate from the temperature at the start of
rapid cooling to 200.degree. C.
[0061] The method of the rapid cooling treatment is not
particularly limited but jet flow water is preferably used for
cooling from the viewpoint of suppressing variation in the material
in the steel sheet width direction and longitudinal direction.
[0062] In the annealing process of the present invention, a
tempering treatment in which the sheet is reheated to a temperature
in the range of 150 to 300.degree. C. is preferably performed after
the rapid cooling treatment so as to further improve the toughness.
The toughness-improving effect is not anticipated at a tempering
temperature less than 150.degree. C.
[0063] The ductility decreases due to the low-temperature tempering
brittleness at a reheating temperature exceeding 300.degree. C.
Accordingly, the temperature range for reheating is limited to 150
to 300.degree. C.
[0064] The resulting cold-rolled annealed sheet may be subjected to
skinpass rolling if needed. The rolling reduction of skinpass
rolling is preferably 0.2% or more and 1.0% or less. At a rolling
reduction of skinpass rolling less than 0.20, a shape-correcting
effect is not obtained. At exceeding 1.0%, deterioration of
elongation becomes significant.
[0065] The cold-rolled annealed sheet (cold-rolled annealed steel
strip) that have gone through the processes described above is used
as a material for a steel tube, and a tube production process is
conducted on the material for a steel tube to produce an electric
resistance welded steel tube. The tube production process involves
continuously forming the material for a steel tube into a
substantially cylindrical open tube and electric-resistance-welding
the open tube to form an electric resistance welded tube.
[0066] In the present invention, forming in the tube production
process is performed by a roll forming method involving a cage roll
method. The roll forming method involving the cage roll method
refers to a forming technique with which small rolls called cage
rolls are arranged along the tube outer surface so as to form a
tube smoothly. Among the roll forming method involving the cage
roll method, the roll forming method employing a chance-free bulge
roll (CBR) method is preferred. According to this method, the
strain applied to the strip during forming can be minimized and
deterioration of the properties of the material caused by work
hardening can be suppressed.
[0067] An example of a production facility for producing electric
resistance welded tubes employing a CBR roll forming method is
shown in FIG. 1. According to the CBR roll forming method, two
edges of a strip 1 is preliminarily formed with edge bend rolls 2,
the central part of the strip is bend-worked by using center bend
rolls 3 and cage rolls 4 so as to form an element tube having a
vertically long oval figure, and four positions of the base tube in
the tube circumferential direction are over-bent with fin pass
rolls 5, followed by reducing so as to conduct stretch forming of
the tube side portions and the bend and return forming of the
over-bent portion to thereby make a round element tube (refer to
Kawasaki Steel Giho Vol. 32 (2000), pp. 49 to 53). The CBR roll
forming method is characterized in that the strain applied to the
material (strip) is small and the variation in strain applied in
the tube circumferential direction is small compared with a
conventional breakdown (BD) method. While the round element tube
obtained as such is being pressed with squeeze rolls 7, the butting
edges are welded by welding means (high-frequency resistance
welding) 6 so as to form an electric resistance welded tube 8.
[0068] A steel sheet (material for steel tubes) which is obtained
by the production method described above and has high strength,
excellent formability, and excellent bake hardenability is used to
form a tube through the tube production process described above.
Thus, the strain applied during the tube production can be
minimized, the work hardening can be suppressed, and a
high-strength electric resistance welded steel tube that has
excellent formability and capable of ensuring excellent shock
absorption after being processed into a component can be
produced.
[0069] The resulting high-strength electric resistance welded steel
tube has a tensile strength TS of 1180 MPa or more, an elongation
El in the tube axial direction of 10% or more, and a yield ratio of
less than 90%. After the steel tube is subjected to a 2% prestrain
and a baking finishing treatment of heat-treating the prestrained
steel tube at 170.degree. C. for 10 minutes, the strength increase
(BH value) is 100 MPa or more and the yield ratio is 90% or
more.
[0070] When the elongation of the electric resistance welded tube
in the tube axial direction is less than 10%, the formability of
the tube is degraded and it becomes difficult to form a desired
shape. Preferably, the elongation is 12% or more. When the yield
ratio of the electric resistance welded tube exceeds 90%, the
formability of the tube is degraded and it becomes difficult to
form a desired shape. The yield ratio is preferably 85% or
less.
[0071] When the BH value of the electric resistance welded tube
after baking finishing is less than 100 MPa, the energy absorbed
upon collision becomes small and the tube does not satisfy the
requirements for shock absorbing members. Preferably, the BH value
is 110 MPa or more. The tube production process employed in
producing the electric resistance welded tube of the present
invention can minimize the strain applied during the tube
production and the variation in strain applied in the tube
circumferential direction is also decreased. Thus, in the electric
resistance welded tube of embodiments of the present invention, the
variation in BH value among positions in the tube circumferential
direction (i.e., the difference between the maximum value and the
minimum value) is small and the BH values at the respective
positions in the tube circumferential direction excluding the
resistance welded portion are uniform and within the range of 100
to 130 MPa. When the yield ratio of the electric resistance welded
tube is less than 90%, the electric resistance welded tube absorbs
less energy upon collision and does not satisfy the requirements
for shock absorbing members.
[0072] In the present invention, the heat treatment condition for
baking finishing is preferably set to 170.degree. C..times.10 min.
However, this condition is the minimum heat treatment condition for
obtaining the strength increase (BH value) of 100 MPa or more after
the baking finishing. The electric resistance welded tube of
embodiments of the present invention will exhibit an strength
increase (BH value) of 100 MPa or more after baking finishing under
any other favorable conditions. As for the heat treatment
conditions under which an strength increase (BH value) of 100 MPa
or more is obtained after the baking finishing, a heating
temperature in the range of 170 to 250.degree. C. is preferably
held for 10 to 30 minutes. When the heating temperature is less
than 170.degree. C., the solute C required to yield the desired
strength increase diffuses into dislocations and does not
sufficiently pin the dislocations. As a result, the desired
strength increase (BH value) is not reliably achieved after the
baking finishing. In contrast, when the temperature is excessively
high exceeding 250.degree. C., not only the productivity decreases,
but also the tube may come to be heated in the blue brittleness
range, possibly resulting in deterioration of the material.
[0073] When the holding time is as short as less than 10 minutes,
the diffusion time is insufficient and the required amount of
solute C cannot reach dislocations. Thus, the desired strength
increase (BH value) cannot be reliably achieved after baking
finishing. In contrast, when the holding time is longer than 30
minutes, the productivity is decreased. Preferably, the holding
time is 25 minutes or shorter.
EXAMPLES
[0074] Molten steel samples indicated in Table 1 are refined in a
converter and continuously casted into slabs (steels). These slabs
(steels) are subjected to a hot-rolling process under conditions
indicated in Table 2 to form hot-rolled sheets (thickness: 2.4 to
3.0 mm), followed by pickling. The hot-rolled sheets were subjected
to a cold-rolling process of cold-rolling the sheets into
cold-rolled sheets, and the cold-rolled sheets were subjected to an
annealing process under conditions shown in Table 2 to form
cold-rolled annealed sheets (thickness: 1.2 to 1.8 mm). As a
result, materials for steel tubes were obtained. Test specimens
were taken from the obtained materials for steel tubes and
structural observation and a tensile test were carried out. The
test methods were as follows.
Structural Observation
[0075] Test specimens for structural observation were taken from
the materials for steel tubes. Sections of the test specimens taken
in the rolling direction were polished, corroded with nital, and
observed with a scanning electron microscope (2000.times.
magnification). Photographs of 10 or more areas of observation were
taken, the types of the structures such as ferrite and martensite
were identified with an image analyzer, and the structural
fractions (volume ratios) of the respective phases were
calculated.
(2) Tensile Test
[0076] JIS No. 12 tensile test specimens (gauge length: 50 mm) were
taken from the materials for steel tubes according to JIS Z 2201 so
that the tensile direction matched the rolling direction. A tensile
test was carried out according to JIS Z 2241 to determine the 0.2%
proof stress YS (MPa), the tensile strength TS (MPa), and the
elongation El (%). The yield ratio YR was calculated and the
strength and formability were evaluated.
[0077] The results are shown in Table 3.
[0078] Each of the materials for steel tubes was formed by a CBR
roll forming method into a substantially cylindrical open tube.
While pressing the butting edges with squeeze rolls, the butting
edges were electric resistance welded by high-frequency resistance
welding. As a result, an electric resistance welded tube (48.6 mm
in outer diameter and 1.2 to 1.8 mm in thickness) was obtained.
Some of the steel tubes were formed by a BD forming method in the
tube production process.
[0079] The resulting electric resistance welded tube was subjected
to structural observation, tensile test, and baking finishing test
to evaluate the structure, the tensile characteristics, and the
bake hardenability. The test methods were as follows.
(1) Structural Observation
[0080] Test specimens for structural observation were taken from
each steel tube. Sections of the specimens taken in the tube axial
direction were polished, corroded with nital, and observed with a
scanning electron microscope (2000.times. magnification).
Photographs of 10 or more areas of observation were taken, the
types of the structures such as ferrite and martensite were
identified with an image analyzer, and the structural fractions
(volume ratios) of the respective phases were calculated as
averages of 10 or more areas of observation.
(2) Tensile Test
[0081] JIS No. 12 tensile test specimens (gauge length: 50 mm) were
taken from the steel tubes according to JIS Z 2201 so that the
tensile direction matched the tube axis direction, and a tensile
test was conducted according to JIS Z 2241 to calculate the 0.2%
proof stress YS (MPa), the tensile strength TS (MPa), and the
elongation El (%). The yield ratio YR was calculated and the
strength and formability were evaluated.
(3) Baking Finishing Test
[0082] JIS No. 12 tensile test specimens were taken from the steel
tubes according to JIS Z 2201 so that the tensile direction matched
the tube axis direction. A 2% tensile strain was applied as a
prestrain and a heat treatment at 170.degree. C. was conducted for
10 minutes to perform baking finishing. The tensile test specimens
were taken at particular positions in the tube circumferential
direction (eleven positions 30.degree. spaced from each other in
the circumferential direction while assuming the electric
resistance welded portion to be 0.degree.; the electric resistance
welded portion was excluded).
[0083] A tensile test was conducted on the treated specimens. The
0.2% proof stress YS and the tensile strength TS after the baking
finishing were determined and the yield ratio
(=(YS/TS).times.100(%)) after the baking finishing was calculated.
The bake hardening value (BH value) was calculated as shown in FIG.
2 by determining the difference between the 0.2% proof stress after
baking finishing and the strength after application of a 2% strain.
The maximum value and the minimum value of the BH value were
determined from among the positions in the circumferential
direction. YS and TS are each an arithmetic mean of the values at
the positions in the circumferential direction.
[0084] The results are shown in Table 4.
[0085] In all of examples of the present invention, an electric
resistance welded tube that has a high strength, i.e., a tensile
strength TS of 1180 MPa or more and excellent formability, i.e., an
elongation El in the tube axial direction of 10% or more and a
yield ratio (=(0.2% proof stress/tensile strength).times.100(%)) in
the tube axis direction of less than 90%, and exhibits excellent
shock absorption, i.e., a BH value of 100 MPa or more and a yield
ratio in the tube axis direction of 900 or more, after application
of a prestrain of 2% or more and a heat treatment at 170.degree.
C..times.10 min (baking finishing). In all of the examples of the
present invention, the variation in BH value among the positions in
the circumferential direction is small and the BH values fall
within the range of 100 to 130 MPa.
[0086] In contrast, comparative examples outside the range of the
present invention have an insufficient strength, low formability,
or an insufficient BH value.
[0087] The influence of the baking finishing conditions was also
studied.
[0088] JIS No. 12 tensile test specimens were taken from the steel
tube No. 1 (Example of the present invention) shown in Table 2
according to JIS Z 2201 so that the tensile direction matched the
tube axis direction. A 2% tensile strain was applied as a prestrain
and a heat treatment was performed while varying the heating
temperature and holding time within the ranges of 100 to
250.degree. C. and 5 to 30 minutes to perform baking finishing. The
tensile test specimens were taken at particular positions in the
tube circumferential direction (eleven positions 30.degree. spaced
from each other in the circumferential direction while assuming the
electric resistance welded portion to be 0.degree.; the electric
resistance welded portion is excluded). A tensile test was
conducted on the bake-finished specimens. The 0.2% proof stress YS
and the tensile strength TS after the baking finishing were
determined and the yield ratio (=(YS/TS).times.100(%)) after the
baking finishing was calculated. The bake hardening value (BH
value) was calculated as shown in FIG. 2 by determining the
difference between the 0.2% proof stress after baking finishing and
the strength after application of a 2% tensile strain. The maximum
value and the minimum value of the BH value were determined from
among the positions in the circumferential direction. YS and TS are
each an arithmetic mean of the values at the positions in the
circumferential direction. The results are shown in Table 5.
[0089] When the heating temperature of the heat treatment is less
than 170.degree. C., i.e., outside the range of the preferable
baking finishing, a BH value of 100 MPa cannot be reliably achieved
unless excessively long baking finishing is conducted without
considering the decrease in productivity. The excessively long
baking finishing refers to the baking finishing that takes more
than 30 minutes. Even when the heating temperature is 170.degree.
C. or more, a BH value of 100 MPa or more is not always achieved if
the holding time is 5 minutes, i.e., less than 10 minutes, and a
desired BH value cannot be stably achieved.
REFERENCE SIGNS LIST
[0090] 1 strip [0091] 2 edge bend roll [0092] 3 center bend roll
[0093] 4 cage roll [0094] 5 fin pass roll [0095] 6 welding means
[0096] 7 squeeze roll [0097] 8 electric resistance welded tube
[0098] 9 cutter [0099] 10 open tube
TABLE-US-00001 [0099] TABLE 1 Steel Chemical composition (mass %)
No. C Si Mn P S Al N Cu, Ni, Cr, Mo, Nb, Ti, W, B Ca, REM Reference
A 0.130 1.40 2.2 0.018 0.0013 0.034 0.0020 -- -- Example B 0.100
1.40 2.4 0.018 0.0013 0.034 0.0020 -- -- Example C 0.180 1.40 2.2
0.018 0.0013 0.034 0.0030 -- -- Example D 0.140 0.80 2.3 0.018
0.0013 0.034 0.0020 -- -- Example E 0.130 1.40 2.2 0.018 0.0013
0.034 0.0020 Ti: 0.015, Nb: 0.021 -- Example F 0.130 1.40 2.2 0.018
0.0013 0.034 0.0040 Cr: 0.15, Mo: 0.10 -- Example G 0.130 1.40 2.2
0.018 0.0013 0.034 0.0020 V: 0.11 -- Example H 0.130 1.40 2.2 0.018
0.0013 0.034 0.0020 Ni: 0.10, Cu: 0.10, B: 0.0015 -- Example I
0.130 1.40 2.2 0.018 0.0013 0.034 0.0020 W: 0.10 Ca: 0.0030 Example
J 0.103 1.40 2.2 0.018 0.0013 0.034 0.0020 -- REM: 0.0030 Example K
0.040 1.40 2.2 0.018 0.0013 0.034 0.0020 -- -- Comparative Example
L 0.250 1.40 2.2 0.018 0.0013 0.034 0.0020 -- -- Comparative
Example M 0.120 0.40 2.2 0.018 0.0013 0.034 0.0020 -- --
Comparative Example N 0.120 2.10 2.2 0.018 0.0013 0.034 0.0020 --
-- Comparative Example O 0.120 1.40 0.5 0.018 0.0013 0.034 0.0020
-- -- Comparative Example P 0.120 1.40 3.1 0.018 0.0013 0.034
0.0020 -- -- Comparative Example Q 0.130 1.40 2.2 0.018 0.0013
0.034 0.0020 -- Ca: 0.0025 Example R 0.135 1.40 2.2 0.0009 0.0010
0.048 0.0030 -- -- Example S 0.145 1.43 2.1 0.015 0.0009 0.035
0.0038 Ti: 0.015 -- Example
TABLE-US-00002 TABLE 2 Hot-rolling process Steel Heating Finishing
Coiling Cold-rolling process tube Steel Transformation point
(.degree. C.) temperature temperature temperature Thickness
Reduction Thickness No. No. Ac.sub.1 Ac.sub.3 Ar.sub.3 (.degree.
C.) (.degree. C.) (.degree. C.) (mm) (%) (mm) 1 A 740 899 859 1200
900 700 3.0 40 1.8 2 B 738 908 868 1200 900 700 3.0 40 1.8 3 C 740
886 846 1200 900 700 3.0 40 1.8 4 D 722 870 830 1200 900 700 3.0 40
1.8 5 E 740 899 859 1200 900 700 3.0 40 1.8 6 F 743 903 863 1200
900 700 3.0 40 1.8 7 G 740 899 859 1200 900 700 3.0 40 1.8 8 H 739
898 858 1200 900 700 3.0 40 1.8 9 I 741 901 861 1200 900 700 3.0 40
1.8 10 J 740 899 859 1200 900 700 3.0 40 1.8 11 K 740 932 892 1200
900 700 3.0 40 1.8 12 L 740 871 831 1200 900 700 3.0 40 1.8 13 M
711 858 818 1200 900 700 3.0 40 1.8 14 N 761 934 894 1200 900 700
3.0 40 1.8 15 O 758 902 862 1200 900 700 3.0 40 1.8 16 P 731 902
862 1200 900 700 3.0 40 1.8 17 A 740 899 859 1200 900 700 3.0 40
1.8 18 B 738 908 868 1200 900 700 3.0 40 1.8 19 C 740 886 846 1200
900 700 3.0 40 1.8 20 Q 740 899 859 1200 900 700 3.0 40 1.8 21 R
740 898 858 1200 900 700 2.4 50 1.2 22 S 742 897 857 1250 910 580
3.6 50 1.8 23 A 740 899 859 1200 900 560 3 40 1.8 24 A 740 899 859
1200 900 600 3 40 1.8 25 A 740 899 859 1200 900 650 3 40 1.8
Annealing process Tube Tube Heating and cooling conditions
Tempering produc- size Aver- Quenching Aver- Cooling treatment tion
(mm) Heating Hold- age start age stop Heating process Outer Steel
temper- ing cooling temper- cooling temper- temper- Roll diam- tube
Steel ature time rate 1 ature rate 2 ature ature forming eter No.
No. (.degree. C.) (s) (.degree. C./s) (.degree. C.) (.degree. C./s)
(.degree. C.) (.degree. C.) method (mm .phi.) Reference 1 A 860 500
11 680 1000 RT 200 C B R 48.6 Example 2 B 860 500 12 680 1000 RT
200 C B R 48.6 Example 3 C 860 500 15 680 1000 RT 200 C B R 48.6
Example 4 D 860 500 15 680 1000 RT 200 C B R 48.6 Example 5 E 860
500 19 680 1000 RT 200 C B R 48.6 Example 6 F 860 500 19 680 1000
RT 200 C B R 48.6 Example 7 G 860 500 15 680 1000 RT 200 C B R 48.6
Example 8 H 860 500 15 680 1000 RT 200 C B R 48.6 Example 9 I 860
500 10 680 1000 RT 200 C B R 48.6 Example 10 J 860 500 10 680 1000
RT 200 C B R 48.6 Example 11 K 860 500 11 680 1000 RT 200 C B R
48.6 Comparative Example 12 L 860 500 12 680 1000 RT 200 C B R 48.6
Comparative Example 13 M 860 500 11 680 1000 RT 200 C B R 48.6
Comparative Example 14 N 860 500 12 680 1000 RT 200 C B R 48.6
Comparative Example 15 O 860 500 15 680 1000 RT 200 C B R 48.6
Comparative Example 16 P 860 500 15 680 1000 RT 200 C B R 48.6
Comparative Example 17 A 860 500 15 680 1000 RT 200 B D 48.6
Comparative Example 18 B 860 500 15 680 1000 RT 200 B D 48.6
Comparative Example 19 C 860 500 15 680 1000 RT 200 B D 48.6
Comparative Example 20 Q 860 500 12 680 1000 RT 200 C B R 48.6
Example 21 R 830 500 6 680 1000 RT 150 C B R 48.6 Comparative
Example 22 S 850 500 7 670 550 RT 355 C B R 48.6 Comparative
Example 23 A 860 500 16 680 1100 RT 200 C B R 48.6 Example 24 A 860
500 16 680 1100 RT 200 C B R 48.6 Example 25 A 860 500 16 680 1100
RT 200 C B R 48.6 Example Steel Hot-rolling process Cold-rolling
process tube Steel Transformation point (.degree. C.) Heating
Finishing Coiling Thickness Reduction Thickness No. No. Ac.sub.1
Ac.sub.3 Ar.sub.3 temperature temperatur temperature (mm) (%) (mm)
26 A 740 899 859 1200 900 700 3 40 1.8 27 A 740 899 859 1200 900
700 3 40 1.8 28 A 740 899 859 1200 900 700 3 40 1.8 29 A 740 899
859 1200 900 700 3 40 1.8 30 A 740 899 859 1200 900 700 3 40 1.8 31
A 740 899 859 1200 900 700 3 40 1.8 32 A 740 899 859 1200 900 700 3
40 1.8 Annealing process Tube Heating and cooling conditions
Tempering produc- Tube Aver- Quenching Aver- Cooling treatment tion
size Heating age start age stop Heating process Outer Steel temper-
Hold- cooling temper- cooling temper- temper- Roll diam- tube Steel
ature ing rate 1 ature rate 2 ature ature forming eter No. No.
(.degree. C.) time (s) (.degree. C./s) (.degree. C.) (.degree.
C./s) (.degree. C.) (.degree. C.) method (mm.phi.) Reference 26 A
910 500 15 680 1000 RT 200 --* -- Comparative Example 27 A 700 500
15 680 800 RT 200 --* -- Comparative Example 28 A 860 500 15 580
600 RT 200 --* -- Comparative Example 29 A 860 500 15 800 1000 RT
200 --* -- Comparative Example 30 A 860 500 15 680 50 RT 200 --* --
Comparative Example 31 A 860 500 15 680 1000 RT 100 --* --
Comparative Example 32 A 860 500 15 680 1000 RT 350 --* --
Comparative Example *) --* : Tube was not produced Cooling rate 1:
Cooling rate in the temperature range from the heat holding
temperature to the temperature at the start of quenching Cooling
rate 2: Cooling rate from the temperature at the start of quenching
to 200.degree. C.
TABLE-US-00003 TABLE 3 Material for steel tube (cold-rolled
annealed sheet) Properties of electric Structure Structure
Martensite Tensile characteristics Martensite Steel phase 0.2%
proof Tensile Yield ratio phase tube Steel fraction stress YS
strength TS YR Elongation El fraction No. No. Type* (Vol %) (MPa)
(MPa) (%) (%) Type* (Vol %) 1 A F + M 52 870 1245 70 17 F + M 52 2
B F + M 55 831 1190 70 18 F + M 55 3 C F + M 58 880 1265 70 16 F +
M 58 4 D F + M 52 835 1201 70 17 F + M 52 5 E F + M 51 877 1255 70
16 F + M 51 6 F F + M 53 869 1245 70 16 F + M 53 7 G F + M 52 839
1210 69 17 F + M 52 8 H F + M 54 833 1189 70 16 F + M 54 9 I F + M
55 840 1211 69 18 F + M 55 10 J F + M 48 870 1245 70 18 F + M 48 11
K F + P + M 10 591 845 70 22 F + P + M 10 12 L F + M 65 933 1336 70
12 F + M 65 13 M F + M 52 633 910 70 19 F + M 52 14 N F + M 55 909
1311 69 11 F + M 55 15 O F + M 45 770 1101 70 18 F + M 45 16 P F +
M 65 929 1340 69 12 F + M 65 17 A F + M 52 883 1262 70 16 F + M 52
18 B F + M 55 844 1210 70 16 F + M 55 19 C F + M 58 901 1285 70 15
F + M 58 20 Q F + M 52 875 1239 71 20 F + M 51 21 R F + M 60 850
1227 69 16 F + M 60 22 S F + M 70 860 1220 70 8 F + M 70 23 A F + M
52 869 1243 70 18 F + M 52 24 A F + M 53 858 1239 69 18 F + M 53 25
A F + M 51 865 1226 71 19 F + M 51 26 A F + M 80 955 1340 71 6 Tube
was not produced 27 A F + P 0 630 756 83 18 Tube was not produced
28 A F + M 20 655 925 71 15 Tube was not produced 29 A F + M 80 945
1350 70 6 Tube was not produced 30 A F + P 0 628 765 82 18 Tube was
not produced 31 A F + M 53 905 1245 73 8 Tube was not produced 32 A
F + M 53 1001 1255 80 8 Tube was not produced *)F: ferrite, M:
martensite, B: bainite, P: pearlite
TABLE-US-00004 TABLE 4 Properties of electric resistance welded
tubes Tensile characteristics Properties after application of 2%
prestrain .fwdarw. finishing 0.2% Yield Steel proof Tensile Yield
ratio Elongation 0.2% proof Tensile ratio YR tube Steel stress YS
strength TS YR El stress YS strength TS YR BH value No. No. (MPa)
(MPa) (%) (%) (MPa) (MPa) (%) Minimum Maximum Reference 1 A 1002
1265 79 14 1325 1355 98 110 125 Example 2 B 956 1210 79 15 1272
1301 98 112 122 Example 3 C 1027 1285 80 13 1345 1376 98 105 125
Example 4 D 951 1213 78 14 1280 1300 98 110 123 Example 5 E 1065
1276 83 12 1339 1365 98 112 115 Example 6 F 1023 1265 81 13 1321
1345 98 110 118 Example 7 G 1001 1233 81 14 1305 1333 98 112 120
Example 8 H 996 1206 83 13 1271 1296 98 115 122 Example 9 I 987
1236 80 14 1295 1321 98 115 125 Example 10 J 978 1265 77 15 1320
1346 98 112 118 Example 11 K 756 856 88 18 921 945 97 30 35
Comparative Example 12 L 1239 1356 91 8 1410 1443 98 110 125
Comparative Example 13 M 780 925 84 16 988 1016 97 115 125
Comparative Example 14 N 1121 1321 85 8 1375 1421 97 100 125
Comparative Example 15 O 905 1121 81 15 1201 1233 97 105 119
Comparative Example 16 P 1159 1353 86 8 1410 1441 98 102 125
Comparative Example 17 A 1246 1288 97 8 1350 1371 98 90 125
Comparative Example 18 B 1187 1235 96 8 1289 1315 98 90 135
Comparative Example 19 C 1256 1305 96 7 1356 1389 98 90 135
Comparative Example 20 Q 999 1258 79 17 1326 1352 98 112 125
Example 21 R 985 1233 80 12 1246 1356 92 56 65 Comparative Example
22 S 978 1239 79 5 1238 1345 92 45 56 Comparative Example 23 A 1003
1266 79 15 1328 1352 98 110 124 Example 24 A 989 1256 79 16 1321
1348 98 112 124 Example 25 A 979 1248 78 17 1329 1339 99 113 123
Example Properties of electric resistance welded tubes Tensile
characteristics Properties after finishing baking Steel 0.2% proof
Tensile Yield Elongation 0.2% Tensile Yield ratio YR tube Steel
stress YS strength TS ratio YR El proof strength TS YR BH value No.
No. (MPa) (MPa) (%) (%) (MPa) (MPa) (%) Minimum Maximum Reference
26 A Tube was not produced Comparative Example 27 A Tube was not
produced Comparative Example 28 A Tube was not produced Comparative
Example 29 A Tube was not produced Comparative Example 30 A Tube
was not produced Comparative Example 31 A Tube was not produced
Comparative Example 32 A Tube was not produced Comparative
Example
TABLE-US-00005 TABLE 5 Properties before baking Conditions of
baking finishing finishing * 0.2% Heating Hold- Properties after
baking finishing Test Steel proof Tensile Yield tempera- ing 0.2%
proof Tensile Yield speci- tube stress YS strength ratio ture time
stress YS strength ratio BH value (MPa) men No. No. (MPa) (MPa) YR
(.degree. C.) (min) (MPa) (MPa) YR Minimum Maximum A1 1 870 1245 70
100 10 1184 1352 88 2 7 A2 1 870 1245 70 100 15 1184 1354 87 2 8 A3
1 870 1245 70 100 20 1185 1350 88 3 6 A4 1 870 1245 70 100 25 1186
1355 88 4 8 A5 1 870 1245 70 100 30 1187 1352 88 5 10 A6 1 870 1245
70 150 10 1215 1350 90 33 105 A7 1 870 1245 70 150 15 1232 1355 91
50 60 A8 1 870 1245 70 150 20 1249 1360 92 67 79 A9 1 870 1245 70
150 25 1265 1358 93 83 95 A10 1 870 1245 70 150 30 1279 1355 94 97
115 A11 1 870 1245 70 170 5 1275 1355 94 93 115 A12 1 870 1245 70
170 10 1282 1354 95 100 110 A13 1 870 1245 70 170 15 1322 1352 98
140 121 A14 1 870 1245 70 170 20 1325 1355 98 110 125 A15 1 870
1245 70 170 25 1329 1360 98 147 126 A16 1 870 1245 70 170 30 1332
1365 98 150 132 A17 1 870 1245 70 200 5 1281 1365 94 99 132 A18 1
870 1245 70 200 10 1336 1357 98 154 167 A19 1 870 1245 70 200 15
1335 1366 98 153 170 A20 1 870 1245 70 200 20 1334 1362 98 152 165
A21 1 870 1245 70 250 5 1281 1362 94 99 165 A22 1 870 1245 70 250
10 1338 1365 98 156 165 A23 1 870 1245 70 250 15 1340 1355 99 158
165 A24 1 870 1245 70 250 20 1338 1358 99 156 170 *) Underlined
conditions are outside the preferable conditions for baking
finishing
* * * * *