U.S. patent application number 11/990477 was filed with the patent office on 2009-10-08 for high strength thick-gauge electric-resistance welded steel pipe excellent in hardenability, hot workability and fatigue strength and method of production of the same.
Invention is credited to Takahiro Ichiyama, Tetsuo Ishitsuka, Motofumi Koyuba, Hiroyuki Mimura, Naoki Takasugi.
Application Number | 20090250146 11/990477 |
Document ID | / |
Family ID | 37771616 |
Filed Date | 2009-10-08 |
United States Patent
Application |
20090250146 |
Kind Code |
A1 |
Ishitsuka; Tetsuo ; et
al. |
October 8, 2009 |
High Strength Thick-Gauge Electric-Resistance Welded Steel Pipe
Excellent in Hardenability, Hot Workability and Fatigue Strength
and Method of Production of the Same
Abstract
The present invention provides high strength thick-gauge
electric-resistance welded steel pipe excellent in hardenability,
hot workability, and fatigue strength and a method of production of
the same, that is, thick-gauge electric-resistance welded steel
pipe containing, by mass %, C: 0.25 to 0.4%, Si: 0.01 to 0.50%, Mn:
0.8 to 1.5%, P: 0.05% or less, S: 0.05% or less, Al: 0.05% or less,
Ti: 0.005 to 0.05%, B: 0.0005 to 0.01%, N: 0.001 to 0.05%, and a
balance of Fe and unavoidable impurities, having a critical cooling
rate V.sub.c expressed by equation <1> of less than
30.degree. C./s, and having a ratio of thickness t and outside
diameter D, t/D, of over 0.15 to 0.30 in range: log
Vc=2.94-0.75.beta. <1> where, .beta.=2.7C+0.4Si+Mn
Inventors: |
Ishitsuka; Tetsuo; (Chiba,
JP) ; Mimura; Hiroyuki; (Tokyo, JP) ; Koyuba;
Motofumi; (Yamaguchi, JP) ; Takasugi; Naoki;
(Tokyo, JP) ; Ichiyama; Takahiro; (Yamaguchi,
JP) |
Correspondence
Address: |
KENYON & KENYON LLP
ONE BROADWAY
NEW YORK
NY
10004
US
|
Family ID: |
37771616 |
Appl. No.: |
11/990477 |
Filed: |
August 17, 2006 |
PCT Filed: |
August 17, 2006 |
PCT NO: |
PCT/JP2006/316539 |
371 Date: |
February 13, 2008 |
Current U.S.
Class: |
148/645 ;
148/330 |
Current CPC
Class: |
B60G 21/055 20130101;
C22C 38/002 20130101; C21D 9/50 20130101; C22C 38/12 20130101; C22C
38/22 20130101; C22C 38/26 20130101; B60G 2206/427 20130101; C21D
2211/008 20130101; C22C 38/04 20130101; C22C 38/08 20130101; C21D
8/10 20130101; C22C 38/06 20130101; C22C 38/02 20130101; C21D 8/105
20130101; C21D 9/08 20130101; C22C 38/001 20130101; C22C 38/14
20130101; C22C 38/28 20130101; C21D 6/004 20130101; C21D 9/14
20130101; B60G 2206/8201 20130101; C22C 38/32 20130101 |
Class at
Publication: |
148/645 ;
148/330 |
International
Class: |
C21D 8/02 20060101
C21D008/02; C22C 38/00 20060101 C22C038/00 |
Foreign Application Data
Date |
Code |
Application Number |
Aug 22, 2005 |
JP |
2005-239953 |
Aug 22, 2005 |
JP |
2005-240130 |
Claims
1. High strength thick-gauge electric-resistance welded steel pipe
excellent in hardenability, hot workability, and fatigue strength
containing, by mass %, C: 0.25 to 0.4%, Si: 0.01 to 0.50%, Mn: 0.8
to 1.5%, P: 0.05% or less, S: 0.05% or less, Al: 0.05% or less, Ti:
0.005 to 0.05%, B: 0.0005 to 0.01%, N: 0.001 to less than 0.004%,
and a balance of Fe and unavoidable impurities, having a critical
cooling rate Vc expressed by equation <1> of less than
30.degree. C./s, and having a ratio of thickness t and outside
diameter D, t/D, of over 0.15 to 0.30 in range: Log
Vc=2.94-0.75.beta. <1> Where, .beta.=2.7C+0.4Si+Mn
2. High strength thick-gauge electric-resistance welded steel pipe
excellent in hardenability, hot workability, and fatigue strength
as set forth in claim 1, characterized by further containing, by
mass %, one or more of Cr: 0.1 to 1%, Mo: 0.05 to 1%, V: 0.01 to
0.5%, and Ni: 0.1 to 1%: where, in equation <1>,
.beta.=2.7C+0.4Si+Mn+0.45Ni+0.8Cr+2Mo
3. High strength thick-gauge electric-resistance welded steel pipe
excellent in hardenability, hot workability, and fatigue strength
as set forth in claim 1, characterized by further containing, by
mass %, Nb: 0.01 to 0.1%.
4. High strength thick-gauge electric-resistance welded steel pipe
excellent in hardenability, hot workability, and fatigue strength
as set forth in claim 1, characterized by further containing, by
mass %, Ca: 0.0002 to 0.005%.
5. A method of production of high strength thick-gauge
electric-resistance welded steel pipe excellent in hardenability,
hot workability, and fatigue strength characterized by heating
electric-resistance welded steel pipe having the ingredients as set
forth in claim 1 to 800 to 1200.degree. C. and stretch reducing
rolling it by a cross-section reduction rate of 40 to 80% in range.
Description
TECHNICAL FIELD
[0001] The present invention relates to high strength thick-gauge
electric-resistance welded steel pipe excellent in hardenability,
hot workability, and fatigue strength suitable for a hollow
stabilizer for securing running stability of an automobile and
method of production of the same.
BACKGROUND ART
[0002] As one measure for improvement of the fuel economy of
automobiles, the chasses of automobiles are being reduced in
weight. The stabilizer easing the rolling of the chassis at the
time of cornering of the automobile and securing stability of the
chassis at the time of high speed running may be mentioned as being
part of this. In the past, stabilizers have been produced by
working a steel bar or other solid material into the required
shape, but to lighten the weight, they are increasingly being
produced using seamless steel pipe, electric-resistance welded
steel pipe, or other hollow members.
[0003] As electric-resistance welded steel pipe for a stabilizer,
WO2002/070767 discloses electric-resistance welded steel pipe for a
stabilizer restricted in composition so as to obtain a uniform
metal structure of the electric-resistance welded part and matrix
material part, a small difference in hardness between the
electric-resistance welded part and matrix material part, and
excellent workability. Further, Japanese Unexamined Patent
Publication No. 2004-011009 discloses electric-resistance welded
steel pipe for a hollow stabilizer limiting the contents of Ti and
N to secure hardenability.
[0004] Japanese Unexamined Patent Publication No. 2004-009126
proposes electric-resistance welded steel pipe for a hollow
stabilizer having a ratio t/D of the thickness t and outside
diameter D of the steel pipe of 20% or more and a tensile strength
of 400 to 755 N/mm.sup.2 and discloses to increase the thickness by
stretch reducing rolling.
[0005] Further, Japanese Unexamined Patent Publication No.
2003-201543 proposes high strength steel pipe for an automobile
structural member obtained by stretch rolling a material pipe,
having a tensile strength of over 580 MPa, having a yield ratio of
70% or less, and excellent in workability to withstand
hydroforming, while Japanese Unexamined Patent Publication No.
2004-292922 proposes a method of production of high tension steel
pipe specifying the heating temperature, diameter reduction rate,
etc. at the time of stretch rolling to obtain superior bending,
stretching, pipe end flattening, and other composite secondary
workability.
[0006] Further, Japanese Unexamined Patent Publication No.
2005-076047 discloses a method of production of a hollow stabilizer
forming a material pipe into a stabilizer shape by cold bending,
quenching this shaped steel pipe, and heat treating it for
tempering, which method of production of a hollow stabilizer rolls
said material pipe at a rolling temperature 600 to 850.degree. C.
and a cumulative diameter reduction rate of 40% or more after heat
treating the master steel pipe so as to improve the fatigue
resistance characteristics.
[0007] Further, Japanese Patent No. 3,653,871 discloses
electric-resistance welded steel pipe for quenching use containing,
by mass %, C: 0.15 to 0.3%, Mn: 0.5 to 2.0%, and Cu: 0.05 to 0.30%,
one or more elements selected from Si.ltoreq.0.41%, P.ltoreq.0.02%,
Al.ltoreq.0.03%, Nb.ltoreq.0.020%, B.ltoreq.0.001%,
Ti.ltoreq.0.01%, and Cr.ltoreq.0.42%, unavoidable impurities
limited to 0.ltoreq.Ni+Mo<0.15% and S.ltoreq.0.003%, and the
balance of Fe to give excellent workability and a high residual
strength rate after penetration of water due to corrosion.
[0008] However, for example, a stabilizer is produced by further
stretch rolling electric-resistance welded steel pipe to obtain
thick-gauge electric-resistance welded steel pipe having the
required thickness/outside diameter ratio and then 1) forming it
into the required shape by bending or other cold forming, heating
and water cooling this for quenching, then tempering it or 2)
heating the thick-gauge electric-resistance welded steel pipe,
forming it to the required shape by pressing or other hot forming,
then water cooling it for quenching, then tempering it. The latter
method of hot forming is superior compared with the former cold
forming in that the forming work is easier and even complicated
shapes can be handled, so this is advantageous as a production
process.
[0009] However, with this method, the material is shaped after
heating, so the time until quenching becomes long--leading to a
drop in the temperature of the formed member, contact between the
press die and the master steel pipe (electric-resistance welded
steel pipe) causes a drop in temperature, and the formation of
heating scale causes uneven temperature, so it becomes difficult to
secure a sufficient hardened state overall and insufficient
hardening is liable to occur. A steel material for steel pipe
better in hardenability is therefore necessary. For this reason,
the general practice is to use B-containing steel having a high
hardenability for the stabilizer steel, but B-containing steel has
the major problems that it is poor in hot workability and is
susceptible to cracking and flaws at the time of hot forming.
Further, B-containing steel sometimes falls in fatigue strength--an
important characteristic for a stabilizer.
[0010] Further, the trend toward reduction of the weight of
automobile chasses is accelerating. Further higher strength
electric-resistance welded steel pipe for stabilizers is being
sought.
DISCLOSURE OF THE INVENTION
[0011] The stabilizer use electric-resistance welded steel pipes
and high strength steel pipes described in WO2002/070767, Japanese
Unexamined Patent Publication No. 2004-011009, Japanese Unexamined
Patent Publication No. 2004-009126, Japanese Unexamined Patent
Publication No. 2003-201543, Japanese Unexamined Patent Publication
No. 2004-292922, Japanese Unexamined Patent Publication No.
2005-076047, Japanese Unexamined Patent Publication No.
2005-076047, Japanese Patent No. 3653871, etc. are useful as steel
pipe for automobile structural members, but, as explained above,
cannot sufficiently handle the problems arising from changes in the
process in the method of production of automobile structural
members. Further, they cannot be said to be sufficient in terms of
fatigue characteristics either.
[0012] The present invention, in consideration of the above
problems, has as its object the provision of high strength
thick-gauge electric-resistance welded steel pipe having sufficient
hardenability and excellent in hot workability and fatigue strength
and a method of production of the same.
[0013] The thick-gauge electric-resistance welded steel pipe of the
present invention was devised to achieve this object. It increases
the C as much as possible to improve the strength (hardness) to an
extent not impairing the weldability and toughness, strictly limits
the range of the N content to improve the hot workability and
fatigue strength, and adjusts the composition of the steel material
so that the critical cooling rate V.sub.c becomes a specific range
so as to thereby secure the hardenability. Further, in the
production of the thick-gauge electric-resistance welded steel pipe
of the present invention, the heating temperature and the
cross-section reduction rate are set to specific ranges for the
stretch reducing rolling of the electric-resistance welded steel
pipe.
[0014] The gist of this is as follows:
[0015] (1) High strength thick-gauge electric-resistance welded
steel pipe excellent in hardenability, hot workability, and fatigue
strength containing, by mass %, C: 0.25 to 0.4%, Si: 0.01 to 0.50%,
Mn: 0.8 to 1.5%, P: 0.05% or less, S: 0.05% or less, Al: 0.05% or
less, Ti: 0.005 to 0.05%, B: 0.0005 to 0.01%, N: 0.001 to 0.05%,
and a balance of Fe and unavoidable impurities, having a critical
cooling rate Vc expressed by equation <1> of less than
30.degree. C./s, and having a ratio of thickness t and outside
diameter D, t/D, of over 0.15 to 0.30 in range:
(log Vc=2.94-0.75.beta. <1> [0016] where,
.beta.=2.7C+0.4Si+Mn
[0017] (2) High strength thick-gauge electric-resistance welded
steel pipe excellent in hardenability, hot workability, and fatigue
strength as set forth in (1), characterized by further containing,
by mass %, one or more of Cr: 0.1 to 1%, Mo: 0.05 to 1%, V: 0.01 to
0.5%, and Ni: 0.1 to 1%:
[0018] where, in equation <1>,
.beta.=2.7C+0.4Si+Mn+0.45Ni+0.8Cr+2Mo
[0019] (3) High strength thick-gauge electric-resistance welded
steel pipe excellent in hardenability, hot workability, and fatigue
strength as set forth in (1) or (2), characterized by further
containing, by mass %, Nb: 0.01 to 0.1%.
[0020] (4) High strength thick-gauge electric-resistance welded
steel pipe excellent in hardenability, hot workability, and fatigue
strength as set forth in any one of (1) to (3), characterized by
further containing, by mass %, Ca: 0.0002 to 0.005%.
[0021] (5) A method of production of high strength thick-gauge
electric-resistance welded steel pipe excellent in hardenability,
hot workability, and fatigue strength characterized by heating
electric-resistance welded steel pipe having the ingredients as set
forth in any one of (1) to (4) to 800 to 1200.degree. C. and
stretch reducing rolling it by a cross-section-reduction rate of 40
to 80% in range.
[0022] The thick-gauge electric-resistance welded steel pipe of the
present invention is extremely excellent in hardenability, so in
the production of a stabilizer or other automobile structural
member, for example, it is possible to obtain a sufficient
hardening effect immediately after hot forming. Further, the
quenching means is not limited to water cooling. It is also
possible to obtain a sufficient hardening effect by oil quenching
which has a smaller cooling rate than water cooling.
[0023] Further, since the hot workability is excellent, cracks or
flaws are difficult to occur even with hot forming when producing
automobile members. In addition, the fatigue strength is excellent,
so there is high durability against repeated load. Further, the
strength is high, so the stabilizers and other automobile
structural members may be lightened in weight more.
BRIEF DESCRIPTION OF THE DRAWINGS
[0024] FIG. 1 is a view of the relationship between hardness of a
steel material for thick-gauge electric-resistance welded steel
pipe after quenching and tempering and the amount of C.
[0025] FIG. 2 is a view of the relationship between the reduction
in area at 850.degree. C. and the N content.
[0026] FIG. 3 is view for explaining the method of the fatigue
test.
BEST MODE FOR WORKING THE INVENTION
[0027] The inventors investigated to improve the hardenability, hot
workability, and fatigue strength of thick-gauge
electric-resistance welded steel pipe for a stabilizer and further
to increase the strength thereof.
[0028] First, the inventors studied the strength and investigated
the hardness after water-cooled quenching and tempering for steel
materials for electric-resistance welded steel pipe (Test Materials
A, B, C) and comparative steel materials (Conventional Materials A,
B) of the compositions shown in Table 1 changed in amounts of C.
FIG. 1 shows the relationship between the change in hardness after
water-cooled quenching and tempering and the amount of C.
[0029] As will be understood from FIG. 1, due to the increase in
the amount of C, the hardness increases and for example rises by
about 10% or more from the hardness level of the conventional
materials and the strength increases. Note that the effect becomes
remarkable if C: 0.25% or more.
TABLE-US-00001 TABLE 1 (mass %) C Si Mn Cr Mo B N Ti Conven- 0.22
0.20 0.55 0.35 -- 0.0015 0.0050 0.015 tional material A Conven-
0.26 0.20 0.83 0.36 0.15 0.0016 0.0038 0.016 tional material B Test
0.28 0.25 0.80 0 0.15 0.0011 0.004 0.014 Material to to to A 1.1
0.35 0.30 Test 0.33 Same Same Same Same Same Same 0.015 Material as
as as as as as B above above above above above above Test 0.37 Same
Same 0.15 Same Same Same 0.015 Material as as to as as as C above
above 0.30 above above above
[0030] Next, the inventors studied the hardenability.
[0031] FIG. 1 simultaneously shows the relationship between the
hardnesses of 100% and 90% martensite structures and the amount of
C. From Test Materials A, B, and C, it is learned that if hardening
to at least a 90% martensite structure, a hardness of 10% or more
of the conventional material can be secured. Therefore, as an
indicator of the hardenability, for example, it is sufficient to
use the critical cooling rate Vc (.degree. C./sec) giving a 90%
martensite structure known in the past from Tetsu to Hagane, 74
(1988), p. 1073. This is usually expressed by the following
equation <1>:
log Vc=2.94-0.75.beta. <1>
where
.beta.=2.7C+0.4Si+Mn,
or
.beta.=2.7C+0.4Si+Mn+0.45Ni+0.8Cr+2Mo.
[0032] From FIG. 1, it is clear that with water quenching, a 90% or
higher martensite structure is obtained, but oil quenching has a
much smaller cooling rate than water quenching. Usually, the
cooling rate reached when oil quenching steel pipe of a size used
for a stabilizer is 30.degree. C./s. Therefore, to secure a 90%
martensite structure even with oil quenching, in the present
invention, the critical cooling rate Vc is made less than
30.degree. C./sec.
[0033] As explained above, to improve the strength of the material
steel pipe, the amount of C is increased and the ingredients are
selected so that the critical cooling rate Vc shown in <1>
becomes less than 30.degree. C./sec.
[0034] Next, the inventors studied the methods for improving the
hot workability and fatigue strength.
[0035] The inventors investigated the causes for the poor
workability of the B-containing steel in the 600 to 900.degree. C.
temperature region where hot forming is performed and as a result
found the fact that the amount of N in the steel has a major effect
on the hot deformation resistance. That is, the inventors
fabricated test materials comprised of 0.3C-1.1Mn-0.020Ti-0.0013B
steel changed in N content from 0.01% to 0.001% in range, conducted
single-axis tensile tests at 850.degree. C. within the temperature
range of hot forming, and measured the reduction in area at that
time.
[0036] FIG. 2 shows the relationship between the reduction of area
at 850.degree. C. and the N content.
[0037] As will be understood from FIG. 2, the smaller the content
of N, the larger the reduction of area, that is, the better the hot
workability. If the content of N falls to 0.005%, the reduction of
area rises to 40%, the line where hot forming generally becomes
possible, while if the content of N is less than 0.004%, the
reduction of area reaches 50% or more where hot forming can be
performed without problem. The inventors discovered that the reason
was that due to the N content falling, the amount of TiN
precipitating in the temperature region for hot forming falls. That
is, in general, B-containing steel must contain Ti with its high
effect of immobilizing N so as to suppress the precipitation of BN
reducing the effect of B in improving the hardenability. Due to
this, with B-containing steel, the inventors found that the hot
workability was poor since TiN precipitated in the hot forming
temperature region.
[0038] Further, the inventors discovered that massive precipitation
of TiN causes a drop in the fatigue strength--an important
characteristic of a stabilizer. Further, massive precipitation of
TiN is also disadvantageous for toughness. On the other hand, due
to the suitable presence of TiN, the growth of .gamma. grains is
suppressed and improvement of toughness is contributed to.
Therefore, by strictly controlling the N content, which had not
been that strictly controlled in the past, it becomes possible to
obtain B-containing steel with desirable hot workability, fatigue
strength, and toughness.
[0039] In this way, the thick-gauge electric-resistance welded
steel pipe of the present invention is raised in C content and
suppressed in N content to a small amount so as to improve the
strength and improve the hot workability and fatigue strength and
is suitably controlled in other ingredients so as to lower the
critical cooling rate Vc and improve the hardenability.
[0040] Below, the chemical ingredients of the thick-gauge
electric-resistance welded steel pipe of the present invention will
be explained.
[0041] C is an element entering into solid solution or
precipitating in a base material and increasing the strength of the
steel.
[0042] Automobile structural members higher in strength than before
have to have a 90% martensite structure and a hardness of Hv400, so
C has to be contained in an amount of 0.25% or more, but if
contained over 0.4%, the workability and the weldability
deteriorate, so the content is made 0.25 to 0.4% in range.
[0043] Si is an alloy element contributing to solution
strengthening. To obtain this effect, 0.01% or more must be
contained. Further, there is the effect of increasing the tempering
softening resistance. To obtain this effect, 0.25% or more must be
added. On the one hand, if added over 0.5%, the toughness falls.
Therefore, the content is made 0.01 to 0.50% in range. Note that
preferably the content is 0.25 to 0.35%.
[0044] Mn is an element for increasing the hardenability. If the
content is less than 0.8%, the effect of increasing the
hardenability cannot be sufficiently secured, while if over 1.5%,
the weldability and weld zone soundness are detrimentally affected,
so the content is made 0.8 to 1.5% in range.
[0045] Al is an element required as a deoxidation material for
molten steel. Further, it is an element for immobilizing N.
Therefore, the amount has a major effect on the crystal grain size
or the mechanical properties. If the content is over 0.05%, the
crystal grain size coarsens and the toughness drops, the
nonmetallic inclusions increase, and flaws easily occur on the
product surface, so the content is made 0.05% or less. Note that
preferably the content is 0.03% or less.
[0046] B is an element which, when added in a fine amount, greatly
improves the hardenability of the steel material. Further, there is
the effect of grain boundary strengthening. If the content is less
than 0.0005%, it is not possible to expect the effect of improving
the hardenability. On the one hand, if over 0.01%, coarse
B-containing phases tend to be produced and embrittlement easily
occurs. Therefore, the content is made 0.0005% to 0.01%. Note that
preferably the content is over 0.0010 to 0.0020%.
[0047] N is an element having the effects of causing the
precipitation of nitrides or carbonitrides and increasing the
strength. However, in B-containing steel, the drop in hardenability
due to the precipitation of BN, as explained above, the drop in the
hot workability and fatigue strength due to the precipitation of
TiN due to the Ti added to prevent the precipitation of BN, and
further the drop in toughness become problems. On the other hand,
TiN has the effect of suppressing coarsening of the .gamma. grains
at the time of high temperature and increasing the toughness. For
this reason, to optimize the balance of the hot workability,
fatigue strength, and toughness, the content is made 0.001 to
0.005% in range. Note that preferably the content is 0.002 to less
than 0.004%.
[0048] Ti acts to immobilize the N in the steel as TiN and suppress
the precipitation of BN so as to stably and effectively improve the
hardenability by the addition of B. Therefore, addition in an
amount of 3.42 times the N content is necessary at the minimum as
in the stoichiochemical level of TiN. The range of the Ti content
is automatically determined from the above range of N content.
However, there is also the amount precipitating as carbides, so to
more reliably immobilize the N, the content is made higher than the
theoretical value of 0.005 or more, while if over 0.05%, the
toughness tends to deteriorate, so the content is made 0.005 to
0.05% in range. Note that preferably the content is 0.01 to
0.02%.
[0049] P is an element having a detrimental effect on the weld
crack resistance and toughness, so is limited to not more than
0.05%. Note that preferably the content is 0.03% or less.
[0050] S has an effect on the formation of nonmetallic inclusions
in a steel material, degrades the bendability, flattenability, and
other workability of steel pipe, and becomes a cause of
deterioration of increase of toughness and an increase in
anisotropy and reheating crack susceptibility. Further, this also
has a detrimental effect on the soundness of the weld zone.
Therefore, the content is limited to 0.05% or less. Note that
preferably the content is 0.01% or less.
[0051] The thick-gauge electric-resistance welded steel pipe of the
present invention may contain, in accordance with need, one or more
of Cr, Mo, V, and Ni and/or at least one of Ca and Nb.
[0052] Cr is an element improving the hardenability. Further, it
has the effect of causing the precipitation of M.sub.23C.sub.6 type
carbides in the base material and has the action of increasing the
strength and increasing the fineness of the carbides. If the
content is less than 0.1%, these actions and effects cannot be
sufficiently expected. Further, if over 1%, defects easily occur at
the time of electric-resistance welding. Therefore, the content is
made 0.1 to 1% in range. Note that preferably the content is 0.1 to
0.6%.
[0053] Mo is an element having the effect of improving the
hardenability and has the effect of causing solution strengthening.
If the content is less than 0.05%, these effects cannot be
sufficiently expected. On the one hand, if over 1%, coarse carbides
easily precipitate and the toughness is deteriorated, so the
content is made 0.05 to 1% in range. Note that preferably the
content is 0.1 to 0.5%.
[0054] Ni is an element having the effect of improving the
hardenability and toughness. If the content is less than 0.1%, this
effect cannot be expected, while if over 1%, there is a possibility
of residual .gamma. even after quenching and the fatigue durability
is degraded. Therefore, the content is made 0.1 to 1% in range.
Note that preferably the content is 0.015 to 0.5%.
[0055] V is an element having the effect of improving the
hardenability and has the effect of precipitation strengthening by
the V carbonitrides. If the content is less than 0.01%, these
effects cannot be sufficiently expected, while if over 0.5%, coarse
carbides easily precipitate and the toughness is degraded, so the
content is made 0.01 to 0.5% in range. Note that preferably the
content is 0.02 to 0.05%.
[0056] Nb has the effect of precipitation strengthening by Nb
carbonitrides and further has the effect of increasing the fineness
of the former austenite grains and increasing the toughness.
Further, there is the effect of suppression of the decarbrization
of the surface.
[0057] If the content is less than 0.01%, the effect of improvement
of the strength and toughness is not sufficient, while if contained
over 0.1%, the carbides increase and the toughness drops.
Therefore, the content is made 0.01 to 0.1% in range. Note that
preferably the content is 0.02 to 0.04%.
[0058] Ca is an element having the effect of making the oxides and
sulfides spherical in shape and improving the workability. If the
content is less than 0.0002%, these effects cannot be sufficiently
expected, while if over 0.005%, the oxides in the steel increase
and the toughness is degraded, so the content is made 0.0002 to
0.005% in range. Note that preferably the content is 0.0002 to
0.004%.
[0059] Next, the reason for making the ratio of the thickness t
(mm) of the steel pipe and the outside diameter D (mm) of the steel
pipe, that is, t/D, is made a range of over 0.15 to 0.30 in the
thick-gauge electric-resistance welded steel pipe of the present
invention will be explained.
[0060] To lighten the weight of the stabilizer, the smaller the t/D
the better. However, the smaller the t/D, the greater the main
stress applied at the time of use, so the more the fatigue
characteristics drop. On the other hand, if the t/D becomes larger,
the effect of reducing the weight becomes smaller. In addition,
production of electric-resistance welded steel pipe becomes
difficult. To secure the minimum extent of the fatigue strength,
the lower limit of t/D is set to over 0.15, while from the
viewpoint of the producibility and reduction of weight, the upper
limit is set to 0.30.
[0061] The method of production of the thick-gauge
electric-resistance welded steel pipe of the present invention will
be explained.
[0062] Molten steel produced to have the required chemical
composition is cast to a slab or is formed into an ingot once, then
is hot rolled to a slab. This cast or rolled (slab is then hot
rolled to obtain a hot rolled steel sheet.
[0063] This hot rolled steel sheet is formed into an
electric-resistance welded steel pipe by the method of production
of ordinary electric-resistance welded steel pipe, for example, hot
or cold electric-resistance welding.
[0064] The thick-gauge electric-resistance welded steel pipe of the
present invention has a ratio of thickness/outside diameter of the
steel pipe explained above, that is, t/D, of over 0.15 to 0.30.
When the electric-resistance welded steel pipe making machine has
the ability to produce electric-resistance welded steel pipe having
such a range of thickness/outside diameter ratio, it is possible to
use the hot rolled steel sheet to directly produce the thick-gauge
electric-resistance welded steel pipe of the present invention.
[0065] However, the greater the thickness in electric-resistance
welded steel pipe, the smaller the outside diameter of the pipe,
and the higher the strength of the steel material of the steel
pipe, the harder the production. In general, an electric-resistance
welded steel pipe having a thickness/outside diameter ratio t/D of
0.15 or less can be produced by an ordinary electric-resistance
welded steel pipe making machine, but if the t/D is over 0.15, the
production capacity is exceeded, so with an ordinary
electric-resistance welded steel pipe making machine, it is often
difficult to directly produce a thick-gauge electric-resistance
welded steel pipe of the present invention having a t/D of over
0.15 to 0.30.
[0066] Therefore, an ordinary electric-resistance welded steel pipe
making machine is used to produce electric-resistance welded steel
pipe having a thickness/outside diameter ratio of 0.15 or less
(also called a "master pipe" here), then this is hot stretch rolled
to produce thick-gauge electric-resistance welded steel pipe having
a thickness/outside diameter ratio of over 0.15 to 0.30.
[0067] The stretch reducing rolling may be performed using a
stretch reducer etc.
[0068] A stretch reducer is a rolling mill provided with a
plurality of rolling stands having three or four rolls around a
rolling axis in series along the rolling axis. By adjusting the
roll speeds and rolling forces of the rolling stands of this
rolling mill, it is possible to control the tension in the axial
direction of the steel pipe (rolling direction) and the compression
force in the circumferential direction and thereby increase the
thickness/outside diameter ratio in stretch reducing rolling.
[0069] That is, in the stretch reducing rolling, the steel pipe is
reduced in outside diameter by the reduction force on the outside
diameter, while the thickness is increased. On the other hand, the
thickness is reduced due to the tension acting on the axial
direction of the steel pipe. The final thickness is determined by
the balance of the two. The thickness of the thus stretch reducing
rolled steel pipe is mainly determined by the tension between the
rolling stands, so it is necessary to find the tension between
rolling stands for obtaining the target thickness from rolling
theory etc. and set the roll speeds of the rolling stands on which
this tension acts.
[0070] As explained above, the present invention heats said
electric-resistance welded steel pipe (master pipe) to 800 to
1200.degree. C. and cross-section reduction rate 40 to 80% and hot
stretch rolls it to obtain thick-gauge electric-resistance welded
steel pipe having a thickness/outside diameter ratio of over 0.15
to 0.30.
[0071] Here, the "cross-section reduction rate" is the (outside
diameter of the steel pipe before stretch reducing-outside diameter
of the steel pipe after stretch reducing)/outside diameter of steel
pipe before stretch reducing.times.100 (%).
[0072] If the heating temperature of the electric-resistance welded
steel pipe at the time of stretch reducing rolling is less than
800.degree. C., the deformation resistance is large, while if over
1200.degree. C., there is remarkable formation of heat scale and
the surface properties deteriorate. Therefore, the heating
temperature is made 800 to 1200.degree. C. in range.
[0073] Further, if the cross-section reduction rate at the time of
stretch reducing rolling is less than 40%, the compression force is
insufficient. It is therefore difficult to make thick-gauge
electric-resistance welded steel pipe having a thickness/outside
diameter ratio of over 0.15 to 0.30 from electric-resistance welded
steel pipe (master pipe) having a thickness/outside diameter ratio
of 0.15 or less. On the other hand, if the cross-section reduction
rate is over 80%, there is remarkable formation of surface defects
on the steel pipe due to stretch reducing rolling and securing a
uniform surface becomes difficult. Therefore, the cross-section
reduction rate in the stretch reducing rolling is made 40 to
80%.
[0074] Note that whether the thick-gauge electric-resistance welded
steel pipe of the present invention was produced by stretch rolling
can be judged by observation of the angularity of the inside
surface of the cross-section vertical to the pipe axial direction
(C section) or by measurement of thickness.
[0075] For example, the stretch reducer used for the stretch
reducing rolling, as explained above, is a rolling mill provided
with a plurality of rolling stands having three rolls or four rolls
around the rolling axis in series along the rolling axis. Normally,
the rolls of the adjoining rolling stands (for example, the N and
N+1 rolling stands) are offset in phase. In the case of three-roll
rolling stands, the rolls are arranged offset in phase by exactly
60.degree. while in the case of four-roll rolling stands, the rolls
are arranged offset in phase by exactly 45.degree..
[0076] Therefore, the inside shape of the cross-section vertical to
the axial direction of the thick-gauge electric-resistance welded
steel pipe produced by stretch reducing rolling (C section) is
hexagonal when the stretch reducer is provided with three-roll
rolling stands and is octagonal when it is provided with four-roll
rolling stands.
[0077] Further, when the phase of the rolls in four continuous
rolling stands of a stretch reducer (for example, N, N+1, N+2, and
N+3 rolling stands) are offset by 30.degree., 60.degree., and
90.degree. in the case of three-roll rolling stands and offset by
22.5.degree., 45.degree., and 67.5.degree. in the case of four-roll
rolling stands, the inner shape of the cross-section vertical to
the axial direction of the thick-gauge electric-resistance welded
steel pipe after stretch rolling (C section) is dodecagonal in the
case of provision of three-roll rolling stands and hexadecagonal in
the case of four-roll rolling stands.
[0078] In this way, it is learned that when the inner shape of the
cross-section vertical to the axial direction of the thick-gauge
electric-resistance welded steel pipe is formed in such a polygonal
shape, this thick-gauge electric-resistance welded steel pipe was
produced by stretch reducing rolling.
EXAMPLES
[0079] Various steels having the compositions shown in Table 2 were
melted and cast into slabs. Each slab was heated to 1150.degree. C.
and hot rolled at a rolling finish temperature of 890.degree. C.
and a coiling temperature of 630.degree. C. to obtain steel sheet
having a sheet thickness of 6 mm. This hot rolled steel sheet was
slit to predetermined widths and welded by high frequency
electric-resistance welding to obtain electric-resistance welded
steel pipe (master pipe) with an outside diameter of 90 mm. Next,
high frequency induction heating was used to heat this steel pipe
to 980.degree. C., then this was stretch reducing rolled to obtain
thick-gauge electric-resistance welded steel pipe having a
thickness of 7 mm and an outside diameter of 35 mm.
[0080] Further, the electric-resistance welded steel pipe produced
by the Steel No. 1 of Table 2 was changed in cross-section
reduction rate in the stretch reducing rolling to produce
thick-gauge electric-resistance welded steel pipe having a
thickness of 5 to 7.5 mm and an outside diameter 30 to 35 mm.
[0081] The obtained thick-gauge electric-resistance welded steel
pipe was heated to 960.degree. C., water cooled, quenched, and
tempered at 300.degree. C..times.1 hr and 350.degree. C..times.1
hr. Test pieces were taken from this steel pipe and subjected to
various types of tests so as to confirm the chararacteristics of
the thick-gauge electric-resistance welded steel pipe of the
present invention.
[0082] The hardness was obtained by measuring the center of
thickness by Hv9.8N at five points and finding the average.
[0083] The hot workability was evaluated using a single-axis
tensile test piece having a diameter of the parallel part of 6 mm,
applying tension at 850.degree. C., and finding the rate of
reduction of the cross-sectional area of the broken part.
[0084] Further, for the fatigue characteristics, the method of Bane
Ronbunshu (Springs Papers), 28 (1983), p. 46 was used to obtain a
fatigue test piece bent by a bending radius of 60 mm shown in FIG.
3. One end was fixed. A double flexure fatigue test was conducted
under stress conditions giving a primary stress amplitude of 600
MPa by a solid material of the same diameter to find the cycles
until break.
[0085] The results of these characteristics are shown in Table 2
and Table 3.
TABLE-US-00002 TABLE 2 Chemical composition (mass %) No. C Si Mn P
S Ni Cr Mo Nb V Ca Ti Inv. 1 0.30 0.25 1.30 0.015 0.003 0.015 ex. 2
0.28 0.25 1.10 0.015 0.004 0.015 3 0.30 0.25 1.30 0.015 0.003 0.30
0.015 4 0.30 0.25 1.30 0.015 0.003 0.50 0.03 0.015 5 0.30 0.25 1.30
0.015 0.003 0.20 0.015 6 0.30 0.25 1.30 0.015 0.003 0.10 0.20
0.0016 0.015 7 0.30 0.25 1.30 0.015 0.003 0.04 0.015 8 0.28 0.25
1.10 0.015 0.004 0.20 0.15 0.05 0.0032 0.015 9 0.30 0.01 1.30 0.015
0.004 0.0008 0.015 10 0.30 0.35 1.30 0.015 0.004 0.0008 0.015 11
0.30 0.50 1.30 0.015 0.004 0.0008 0.015 Comp. 12 0.22 0.20 0.55
0.015 0.003 0.35 0.015 ex. 13 0.26 0.20 0.83 0.015 0.004 0.36 0.15
0.015 14 0.12 0.25 2.00 0.015 0.003 0.015 Reduction of Cycles
Chemical composition (mass %) Vc area at 850.degree. C. until break
No. Al N B (.degree. C./sec) Hv9.8N (%) (.times.10.sup.3) Inv. 1
0.03 0.0035 0.0014 19.2 452 53 95 ex. 2 0.03 0.0036 0.0011 29.7 448
53 94 3 0.03 0.0035 0.0014 15.2 455 55 95 4 0.03 0.0036 0.0014 9.6
451 55 95 5 0.03 0.0035 0.0014 9.6 458 53 95 6 0.03 0.0035 0.0014
13.5 451 53 94 7 0.03 0.0035 0.0014 19.2 448 54 95 8 0.03 0.0036
0.0011 13.4 446 53 95 9 0.03 0.0023 0.0011 22.6 457 62 102 10 0.03
0.0023 0.0011 17.9 458 62 101 11 0.03 0.0023 0.0011 16.1 460 61 101
Comp. 12 0.03 0.0048 0.0015 64.9 384 43 82 ex. 13 0.03 0.0054
0.0012 19.5 451 37 76 14 0.03 0.0035 0.0015 13.2 296 53 93
TABLE-US-00003 TABLE 3 Outside Thick- Cross-section Cycles diameter
ness reduction until break No. (mm) (mm) rate (%) t/D
(.times.10.sup.3) Inv. a 30 7.0 68.1 0.23 138 ex. b 30 7.5 66.5
0.25 141 c 35 7.5 59.1 0.21 103 d 35 7.0 61.1 0.20 95 e 35 6.0 65.5
0.17 63 Comp. f 35 5.0 70.2 0.14 43 ex.
[0086] Steel Nos. 1 to 11 shown in Table 2 and having the chemical
ingredients of the present invention had excellent characteristics
of hardness, hot workability, and fatigue strength.
[0087] As opposed to this, Steel No. 12 had a large critical
cooling rate Vc, so was not sufficiently hardened and had an amount
of C of a low 0.22%, so did not give sufficient hardness. Steel No.
13 had too high an amount of N, so was poor in hot workability and
also somewhat low in fatigue characteristics. Steel No. 14 had an
insufficient amount of C, so even with tempering at 300.degree. C.,
the minimum necessary hardness required for an automobile
structural member could not be obtained.
[0088] The present invention Steel Pipe Nos. a to e shown in Table
3 had sufficient fatigue strengths having sufficient cycles until
break of over 50.times.10.sup.3.
[0089] As opposed to this, the Steel Pipe No. f was an example
where t/D was too small and sufficient fatigue strength could not
be obtained.
* * * * *