U.S. patent application number 14/428217 was filed with the patent office on 2015-08-20 for hot-rolled steel sheet and method for manufacturing the same.
This patent application is currently assigned to JFE STEEL CORPORATION. The applicant listed for this patent is JFE STEEL CORPORATION. Invention is credited to Sota Goto, Chikara Kami.
Application Number | 20150232970 14/428217 |
Document ID | / |
Family ID | 50277943 |
Filed Date | 2015-08-20 |
United States Patent
Application |
20150232970 |
Kind Code |
A1 |
Kami; Chikara ; et
al. |
August 20, 2015 |
HOT-ROLLED STEEL SHEET AND METHOD FOR MANUFACTURING THE SAME
Abstract
A high-strength hot-rolled steel sheet is provided having a high
low-temperature toughness and a low yield ratio that is suitable
for a steel pipe material. The hot-rolled steel sheet has a
composition that contains C: 0.03% to 0.10%, Si: 0.01% to 0.50%,
Mn: 1.4% to 2.2%, P: 0.025% or less, S: 0.005% or less, Al: 0.005%
to 0.10%, Nb: 0.02% to 0.10%, Ti: 0.001% to 0.030%, Mo: 0.01% to
0.50%, Cr: 0.01% to 0.50%, and Ni: 0.01% to 0.50%. The composition
of the hot-rolled steel sheet has an Moeq value in a range of 1.4%
to 2.2%. The hot-rolled steel sheet includes an inner layer, which
has a microstructure that contains a main phase and a second phase,
and an outer layer, which has a microstructure that contains a
tempered martensite phase or a tempered martensite phase and a
tempered bainite phase.
Inventors: |
Kami; Chikara; (Chiba,
JP) ; Goto; Sota; (Handa, JP) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
JFE STEEL CORPORATION |
Tokyo |
|
JP |
|
|
Assignee: |
JFE STEEL CORPORATION
Tokyo
JP
|
Family ID: |
50277943 |
Appl. No.: |
14/428217 |
Filed: |
September 11, 2013 |
PCT Filed: |
September 11, 2013 |
PCT NO: |
PCT/JP2013/005388 |
371 Date: |
March 13, 2015 |
Current U.S.
Class: |
148/506 ;
148/330; 148/332; 148/335; 148/602 |
Current CPC
Class: |
C22C 38/001 20130101;
C22C 38/02 20130101; C21D 6/005 20130101; C21D 2211/008 20130101;
C22C 38/04 20130101; C22C 38/46 20130101; C22C 38/54 20130101; C21D
2211/002 20130101; C21D 6/004 20130101; C22C 38/42 20130101; C21D
9/46 20130101; C22C 38/48 20130101; C22C 38/58 20130101; C22C 38/50
20130101; C22C 38/06 20130101; C22C 38/002 20130101; C21D 8/1222
20130101; C21D 8/1261 20130101; C21D 6/008 20130101; C21D 8/0263
20130101; C22C 38/44 20130101 |
International
Class: |
C22C 38/58 20060101
C22C038/58; C21D 9/46 20060101 C21D009/46; C21D 6/00 20060101
C21D006/00; C22C 38/54 20060101 C22C038/54; C22C 38/50 20060101
C22C038/50; C22C 38/00 20060101 C22C038/00; C22C 38/46 20060101
C22C038/46; C22C 38/44 20060101 C22C038/44; C22C 38/42 20060101
C22C038/42; C22C 38/06 20060101 C22C038/06; C22C 38/04 20060101
C22C038/04; C22C 38/02 20060101 C22C038/02; C21D 8/02 20060101
C21D008/02; C22C 38/48 20060101 C22C038/48 |
Foreign Application Data
Date |
Code |
Application Number |
Sep 13, 2012 |
JP |
2012-201266 |
Claims
1. A hot-rolled steel sheet having a composition comprising, on a
mass percent basis: C: 0.03% to 0.10%, Si: 0.01% to 0.50%, Mn: 1.4%
to 2.2%, P: 0.025% or less, S: 0.005% or less, Al: 0.005% to 0.10%,
Nb: 0.02% to 0.10%, Ti: 0.001% to 0.030%, Mo: 0.01% to 0.50%, Cr:
0.01% to 0.50%, Ni: 0.01% to 0.50%, and a remainder of Fe and
incidental impurities, the hot rolled steel sheet comprising: an
inner layer having a microstructure that contains a main phase and
a second phase, the main phase being bainitic ferrite having an
average grain size of 10 .mu.m or less, the second phase having an
area fraction in a range of 1.4% to 15% and containing massive
martensite having an aspect ratio of less than 5.0, and an outer
layer having a microstructure that contains (i) a tempered
martensite phase or (ii) a tempered martensite phase and a tempered
bainite phase.
2. The hot-rolled steel sheet according to claim 1, wherein the
composition of the steel sheet has an Moeq value, which is defined
by the following formula (1), in a range of 1.4% to 2.2% by mass:
Moeq (%)=Mo+0.36Cr+0.77Mn+0.07Ni (1) wherein, Mn, Ni, Cr, and Mo
denote the corresponding element contents (% by mass).
3. The hot-rolled steel sheet according to claim 1, wherein the
chemical composition of the steel sheet further comprises, on a
mass percent basis, at least one of the following: Cu: 0.50% or
less, V: 0.10% or less, and B: 0.005% or less.
4. The hot-rolled steel sheet according to claim 1, wherein the
chemical composition of the steel sheet further comprises, on a
mass percent basis, Ca: 0.0005% to 0.0050%.
5. The hot-rolled steel sheet according to claim 1, wherein the
massive martensite has a maximum size of 5.0 .mu.m or less and an
average size in the range of 0.5 to 3.0 .mu.m.
6. The hot-rolled steel sheet according to claim 1, wherein a
hardness of the hot-rolled steel sheet at a depth of 0.5 mm from a
surface thereof in a thickness direction is 95% or less of a
maximum hardness in the thickness direction.
7. A method for manufacturing a hot-rolled steel sheet, comprising:
subjecting steel to a hot-rolling step, a cooling step, and a
coiling step to form the hot-rolled steel sheet, wherein: the steel
contains, on a mass percent basis, C: 0.03% to 0.10%, Si: 0.01% to
0.50%, Mn: 1.4% to 2.2%, P: 0.025% or less, S: 0.005% or less, Al:
0.005% to 0.10%, Nb: 0.02% to 0.10%, Ti: 0.001% to 0.030%, Mo:
0.01% to 0.50%, Cr: 0.01% to 0.50%, Ni: 0.01% to 0.50%, and a
remainder of Fe and incidental impurities, the hot-rolling step
includes: heating the steel to a heating temperature in a range of
1050.degree. C. to 1300.degree. C., rough-rolling the heated steel
to form a sheet bar, and finish-rolling the sheet bar such that a
cumulative rolling reduction at a temperature of 930.degree. C. or
less is 50% or more, thereby forming a hot-rolled steel sheet, the
cooling step includes first cooling, second cooling, third cooling,
and fourth cooling in this order, the first cooling being started
immediately after completion of the finish rolling, and including
cooling the hot-rolled steel sheet to a martensitic transformation
start temperature or less at an average cooling rate of 100.degree.
C./s or more with respect to surface temperature, the second
cooling including, after completion of the first cooling, holding
the hot-rolled steel sheet for 1 s or more at a surface temperature
of 600.degree. C. or more, the third cooling including, after
completion of the second cooling, cooling the hot-rolled steel
sheet to a cooling stop temperature in a range of 600.degree. C. to
450.degree. C. at an average cooling rate in a range of 5.degree.
C. to 30.degree. C./s with respect to a temperature at half a
thickness of the hot-rolled steel sheet, and the fourth cooling
including (i) cooling the hot-rolled steel sheet from the cooling
stop temperature of the third cooling to a coiling temperature at
an average cooling rate of 2.degree. C./s or less with respect to
the temperature at half the thickness of the hot-rolled steel sheet
or (ii) alternatively holding the hot-rolled steel sheet at a
temperature in the range extending from the cooling stop
temperature of the third cooling to the coiling temperature for 20
s or more, and the coiling step includes coiling the hot-rolled
steel sheet at a surface temperature of 450.degree. C. or more.
8. The method for manufacturing a hot-rolled steel sheet according
to claim 7, wherein the composition has an Moeq value, which is
defined by the following formula (1), in a range of 1.4% to 2.2% by
mass: Moeq (%)=Mo+0.36Cr+0.77Mn+0.07Ni (1) wherein, Mn, Ni, Cr, and
Mo denote the corresponding element contents (% by mass).
9. The method for manufacturing a hot-rolled steel sheet according
to claim 7, wherein the hot-rolled steel sheet further contains, on
a mass percent basis, at least one of the following: Cu: 0.50% or
less, V: 0.10% or less, and B: 0.0005% or less.
10. The method for manufacturing a hot-rolled steel sheet according
to claim 7, wherein the hot-rolled steel sheet further contains Ca:
0.0005% to 0.0050% by mass.
Description
TECHNICAL FIELD
[0001] Embodiments of the present disclosure relate to a
high-strength hot-rolled steel sheet with a low yield ratio
suitable as a material for spiral steel pipes and
electric-resistance-welded (ERW) pipes for use in line pipes, and a
method for manufacturing the high-strength hot-rolled steel sheet
with a low yield ratio. In particular, embodiments of the present
disclosure relate to maintaining a low yield ratio and high
low-temperature toughness while preventing a decrease in yield
strength after pipe manufacturing.
BACKGROUND ART
[0002] Spiral steel pipes are manufactured by helically winding a
steel sheet. Large-diameter steel pipes can be efficiently
manufactured using this process. Thus, in recent years, spiral
steel pipes have been widely used as line pipes for crude oil and
natural gas transport. In particular, in long-distance pipelines,
transport pressure is being increased to improve transportation
efficiency. Furthermore, since many oil wells and gas wells are
located in cold districts, long-distance pipelines often pass
through cold districts. Thus, there is a demand for high-strength
and high-toughness line pipes. There is also a demand for line
pipes having a low yield ratio from the perspective of buckling
resistance and earthquake resistance. The yield ratio of spiral
steel pipes in the longitudinal direction is not significantly
changed by pipe manufacturing and is substantially the same as the
yield ratio of the hot-rolled steel sheet material. Thus, in order
to lower the yield ratio of line pipes made of spiral steel pipes,
the yield ratio of the hot-rolled steel sheet material must be
lowered.
[0003] Facing such demands, for example, Patent Literature 1
describes a method for manufacturing a hot-rolled steel sheet
having high low-temperature toughness, a low yield ratio, and high
tensile strength for use in line pipes, in a technique described in
Patent Literature 1, a hot-rolled steel, sheet is manufactured by
heating a steel slab to a temperature in the range of 1180.degree.
C. to 1300.degree. C., the steel slab containing, on a weight
percent basis, C: 0.03% to 0.12%, Si: 0.50% or less, Mn: 1.70% or
less, Al: 0.070% or less, and at least one of Nb: 0.01% to 0.05%,
V: 0.01% to 0.02%, and Ti: 0.01% to 0.20%, hot-rolling the steel
slab at a rough-rolling finishing temperature in the range of
950.degree. C. to 1050.degree. C. and a finishing delivery
temperature in the range of 760.degree. C. to 800.degree. C.,
cooling the hot-rolled sheet at a cooling rate in the range of
5.degree. C. to 20.degree. C./s, starting air cooling at a
temperature of more than 670.degree. C., holding the temperature
for 5 to 20 s, cooling the hot-rolled sheet at a cooling rate of
20.degree. C./s or more, and coiling the hot-rolled sheet at a
temperature of 500.degree. C. or less. The technique described in
Patent Literature 1 can be used to manufacture a hot-rolled steel
sheet having a tensile strength of 60 kg/mm.sup.2 or more (590 MPa
or more), a yield ratio of 85% or less, and high toughness
represented by a fracture transition temperature of -60.degree. C.
or less.
[0004] Patent Literature 2 describes a method for manufacturing a
high-strength hot-rolled steel sheet with a low yield ratio for use
in pipes. A technique described in Patent Literature 2 is a method
for manufacturing a hot-rolled steel sheet that includes heating
steel to a temperature in the range of 1000.degree. C. to
1300.degree. C., the steel containing C: 0.02% to 0.12%, Si: 0.1%
to 1.5%, Mn: 2.0% or less, Al: 0.01% to 0.10%, and Mo+Cr: 0.1% to
1.5%, completing hot rolling at a temperature in the range of
750.degree. C. to 350.degree. C., cooling the hot-rolled steel
sheet to a coiling temperature at a cooling rate in the range of
10.degree. C. to 50.degree. C./s, and coiling the hot-rolled steel
sheet at a temperature in the range of 480.degree. C. to
600.degree. C. The technique described in Patent Literature 2 can
be used to manufacture a hot-rolled steel sheet composed mainly of
ferrite, containing martensite having an area fraction in the range
of 1% to 20%, having a yield ratio of 85% or less, and having a
small decrease in yield strength after pipe manufacturing, without
performing rapid cooling from the austenite temperature range.
[0005] Patent Literature 3 describes a method for manufacturing an
electric-resistance-welded (ERW) pipe having high low-temperature
toughness and a low yield ratio. In a technique described in Patent
Literature 3, an electric-resistance-welded (ERW) pipe is
manufactured by hot-rolling a slab that contains, on a mass percent
basis, C: 0.01% to 0.09%, Si: 0.50% or less, Mn: 2.5% or less, Al:
0.01% to 0.10%, Nb: 0.005% to 0.10%, and one or two or more of Mo:
0.5% or less, Ca: 0.5% or less, Ni: 0.5% or less, and Cr; 0.5% or
less such, that the Mn, Si, P, Cr, Ni, and Mo content relation Mneq
satisfies 2.0 or more, cooling the hot-rolled sheet to a
temperature in the range of 500.degree. C. to 650.degree. C. at a
cooling rate of 5.degree. C./s or more, coiling the hot-rolled
sheet, holding the hot-rolled sheet at a temperature in this
temperature range for 10 min or more, cooling the hot-rolled sheet
to a temperature of less than 500.degree. C., and forming the
hot-rolled steel sheet into a electric-resistance-welded (ERW)
pipe. The technique described in Patent Literature 3 can be used to
manufacture an electric-resistance-welded (ERW) pipe that has a
microstructure containing bainitic ferrite as a main phase, 3% or
more martensite, and optionally 1% or more retained austenite, has
a fracture transition temperature of -50.degree. C. or less, and
has high low-temperature toughness and high plastic strain
absorbing capability.
[0006] Patent Literature 4 describes a high-toughness steel plate
having a low yield ratio. A technique described in Patent
Literature 4 can be used to manufacture a high-toughness steel
plate having a low yield ratio by heating a slab containing C:
0.03% to 0.15%, Si: 1.0% or less, Mn: 1.0% to 2.0%, Al: 0.005% to
0.060%, Ti: 0.008% to 0.030%, N: 0.0020% to 0.010%, and O: 0.010%
or less to a temperature preferably in the range of 950.degree. C.
to 1300.degree. C., hot-rolling the slab at a rolling reduction of
10% or more in the temperature range of (Ar3 transformation
point+100.degree. C.) to (Ar3 transformation point+150.degree. C.)
and at a finish-rolling temperature of 800.degree. C. to
700.degree. C., starting accelerated cooling of the hot-rolled
plate at a temperature of (the finish-rolling
temperature--50.degree. C.) or more, water cooling the hot-rolled
plate to a temperature in the range of 400.degree. C. to
150.degree. C. at an average cooling rate in the range of 5.degree.
C. to 50.degree. C./s, and then air cooling the hot-rolled plate.
The hot-rolled plate has a mixed microstructure of ferrite having
an average grain size in the range of 10 to 50 .mu.m and bainite in
which martensite-austenite constituent are dispersed and constitute
1% to 20% by area. The shape (rod-like or massive, as described
below) of the martensite-austenite constituent is not
described.
CITATION LIST
Patent Literature
[0007] PTL 1: Japanese Unexamined Patent Application Publication
No. 63-227715
[0008] PTL 2: Japanese Unexamined Patent Application Publication
No. 10-176239
[0009] PTL 3: Japanese Unexamined Patent Application Publication
No. 2006-299413
[0010] PTL 4: Japanese Unexamined Patent Application Publication
No. 2010-59472
SUMMARY OF INVENTION
Technical Problem
[0011] However, in the technique described in Patent Literature 1,
because of the high cooling rate before and after air cooling,
particularly after air cooling, the cooling rate and the cooling
stop temperature must be rapidly and properly controlled. In
particular, the manufacture of hot-rolled steel sheet with a large
thickness needs large-scale cooling equipment. Furthermore, a
hot-rolled steel sheet manufactured by using the technique
described in Patent Literature 1 has a microstructure composed
mainly of soft polygonal ferrite, and it is difficult to achieve
the desired high strength.
[0012] The technique described in Patent Literature 2 has a problem
in that a decrease in yield strength after pipe manufacturing is
still observed, and it is sometimes difficult to meet the recent
demand for high steel pipe strength.
[0013] The technique described in Patent Literature 3 cannot
consistently meet a recent high low-temperature toughness
specification for cold districts represented by a fracture
transition temperature vTrs of -80.degree. C. or less.
[0014] A steel plate manufactured by using the technique described
in Patent Literature 4 has low toughness represented by a fracture
transition temperature vTrs as low as approximately -30.degree. C.
to -41.degree. C. and cannot meet the recent demand for further
improved toughness.
[0015] In recent years, there has been another demand for materials
for high-strength thick-walled steel pipes in order to efficiently
transport crude oil. However, there are problems of increased
amounts of alloying elements due to reinforcement and necessity of
rapid cooling in a process of manufacturing a hot-rolled steel
sheet due to an increased thickness. Since hot-rolled steel sheets
are conveyed through a water cooling zone having a limited length
at a high speed before coiling, hot-rolled steel sheets having a
greater thickness require stronger cooling. Thus, the steel sheets
have excessively high surface hardness.
[0016] In particular, for example, in the manufacture of a
hot-rolled steel sheet having a large thickness of 10 mm or more,
the hot-rolled steel sheet is conveyed at a high speed in the range
of 100 to 250 mpm (meter per minute) in finish rolling and is
conveyed through a cooling zone at substantially the same high
speed after the finish rolling. Thus, hot-rolled steel sheets
having a greater thickness require cooling with a higher heat
transfer coefficient. This results in hot-rolled steel sheets
having excessively high surface hardness, higher hardness on the
surface than in the interior thereof, and an uneven hardness
distribution. Such an uneven hardness distribution can be
responsible for variations in the characteristics of steel pipes.
Such an uneven surface hardness distribution results from the
holding of a steel sheet surface in a transition boiling
temperature range (a boundary between film boiling and nucleate
boiling) in the cooling process. To avoid this, it is necessary to
maintain the steel sheet surface temperature at more than
500.degree. C. In the case of steel sheets having a large
thickness, however, because of an excessively low internal cooling
rate, desired inner layer microstructures cannot be formed.
Although the surface hardness can be made uniform by decreasing the
steel sheet surface temperature below the transition boiling range,
this results in a maximum cross section hardness of more than 300
points in terms of HV 0.5. Such increased hardness results in not
only undesired pipe shapes after pipe manufacturing but also
undesired characteristics of steel pipes and even impossibility of
pipe manufacturing.
[0017] Embodiments of the present disclosure aim to solve such
problems of the related art and provide a material for steel pipes,
particularly a high-strength hot-rolled steel sheet that is
suitable for spiral steel pipes, that can maintain its strength
after spiral pipe manufacturing, and that has high low-temperature
toughness and a low yield ratio, without performing complicated
heat treatment or large-scale modification of equipment. In
particular, embodiments of the present disclosure provide a
high-strength hot-rolled, steel sheet having a thickness of 8 mm or
more (more preferably 10 mm or more) and 50 mm or less (more
preferably 25 mm or less) and having high low-temperature toughness
and a low yield ratio. The term "high-strength", as used herein,
refers to a yield strength of 480 MPa or more at an angle of 30
degrees with the rolling direction and a tensile strength of 600
MPa or more in the sheet width direction. The term "high
low-temperature toughness", as used herein, refers to a fracture
transition temperature vTrs of -80.degree. C. or less in a Charpy
impact test. The term "low yield ratio", as used herein, refers to
a case where a steel sheet has a continuous yielding type
stress-strain curve and a yield ratio of 85% or less. The term
"steel sheets" includes steel sheets and steel strips.
Solution to Problem
[0018] In order to achieve the objects, the present inventors
extensively studied various factors that can affect steel pipe
strength and steel pipe toughness after pipe manufacturing. As a
result, the present inventors found that strength reduction due to
pipe manufacturing is caused by a decrease in yield strength due to
the Bauschinger effect on the inner surface side of the pipe
subjected to compressive stress and by the loss of yield elongation
on the outer surface side of the pipe subjected to tensile
stress.
[0019] As a result of further investigation, the present inventors
found that the use of a steel sheet having a microstructure that
contains fine bainitic ferrite as a main phase and hard massive
martensite finely dispersed in the bainitic ferrite can suppress
strength reduction after pipe manufacturing, particularly after
spiral pipe manufacturing, and provide a steel pipe having a low
yield ratio of 85% or less and high toughness. The present
inventors found that such a microstructure can improve the work
hardening ability of steel pipe materials, that is, steel sheets,
sufficiently increase strength owing to work hardening on the outer
surface side of the pipe during pipe manufacturing, and suppress
strength reduction after pipe manufacturing, particularly after
spiral pipe manufacturing. Furthermore, the present inventors found
that finely dispersed massive martensite can significantly improve
toughness.
[0020] The present inventors also found that the surface
microstructure of steel sheets composed of a tempered martensite
single phase or a mixed phase of tempered martensite and tempered
bainite is effective in preventing an uneven increase in the
surface hardness of the steel sheets and providing steel pipes
having the desired pipe shape and uniform ductility after pipe
manufacturing.
[0021] Embodiments of the present disclosure have been accomplished
on the basis of these findings after further consideration.
[0022] (1) A hot-rolled steel sheet having a composition
containing, on a mass percent basis, C: 0.03% to 0.10%, Si: 0.01%
to 0.50%, Mn: 1.4% to 2.2%, P: 0.025% or less, S: 0.005% or less,
Al: 0.005% to 0.10%, Nb: 0.02% to 0.10%, Ti: 0.001% to 0.030%, Mo:
0.01% to 0.50%, Cr: 0.01% to 0.50%, Ni: 0.01% to 0.50%, and a
remainder of Fe and incidental impurities, wherein the hot-rolled
steel sheet includes an inner layer having a microstructure that
contains a main phase and a second phase, the main phase being
bainitic ferrite having an average grain size of 10 .mu.m or less,
the second phase having an area fraction in the range of 1.4% to
15% and containing massive martensite having an aspect ratio of
less than 5.0, and the hot-rolled steel sheet includes an outer
layer having a microstructure that contains a tempered martensite
phase or a tempered martensite phase and a tempered bainite
phase.
[0023] (2)The hot-rolled steel sheet according to (1), wherein the
composition has Moeq defined by the following formula (1) in the
range of 1.4% to 2.2% by mass:
Moeq (%)=Mo+0.36Cr+0.77Mn+0.07Ni (1)
[0024] (wherein Mn, Mi, Cr, and Mo denote the corresponding element
contents (% by mass)).
[0025] (3) The hot-rolled steel sheet according to (1) or (2),
further containing, on a mass percent basis, one or two or more
selected from Cu: 0.50% or less, V: 0.10% or less, and B: 0.0005%
or less.
[0026] (4) The hot-rolled steel sheet according to any one of (1)
to (3), further containing Ca: 0.0005% to 0.0050% by mass.
[0027] (5) The hot-rolled steel sheet according to any one of (1)
to (4), wherein the massive martensite has a maximum size of 5.0
.mu.m or less and an average size in the range of 0.5 to 3.0
.mu.m.
[0028] (6) The hot-rolled steel sheet according to any one of (1)
to (5), wherein the hardness of the hot-rolled steel sheet at a
depth of 0.5 mm from a surface thereof in the thickness direction
is 95% or less of the maximum hardness in the thickness
direction.
[0029] (7) A method for manufacturing a hot-rolled steel sheet
including subjecting steel to a hot-rolling step, a cooling step,
and a coiling step to form the hot-rolled steel sheet, wherein the
steel contains, on a mass percent basis, C: 0.03% to 0.10%, Si:
0.01% to 0.50%, Mn: 1.4% to 2.2%, P: 0.025% or less, S: 0.005% or
less, Al: 0.005% to 0.10%, Nb: 0.02% to 0.10%, Ti: 0.001% to
0.030%, Mo: 0.01% to 0.50%, Cr: 0.01% to 0.50%, M: 0.01% to 0.50%,
and a remainder of Fe and incidental impurities, the hot-rolling
step includes heating the steel to a heating temperature in the
range of 1050.degree. C. to 1300.degree. C., rough-rolling the
heated steel to form a sheet bar, and finish-rolling the sheet bar
such that the cumulative rolling reduction at a temperature of
930.degree. C. or less is 50% or more, thereby forming a hot-rolled
steel sheet, the cooling step includes first cooling, second
cooling, third cooling, and fourth cooling in this order, the first
cooling being started immediately after completion of the finish
rolling and including cooling the hot-rolled steel sheet to a
martensitic transformation start temperature (Ms point) or less at
an average cooling rate of 100.degree. C./s or more with respect to
surface temperature, the second cooling including, after completion
of the first cooling, holding the hot-rolled steel sheet for 1 s or
more at a surface temperature of 600.degree. C. or more, the third
cooling including, after completion of the second cooling, cooling
the hot-rolled steel sheet to a cooling stop temperature in the
range of 600.degree. C. to 450.degree. C. at an average cooling
rate in the range of 5.degree. C. to 30.degree. C./s with respect
to the temperature at half the thickness of the hot-rolled steel
sheet, the fourth cooling including cooling the hot-rolled steel
sheet from the cooling stop temperature of the third cooling to a
coiling temperature at an average cooling rate of 2.degree. C./s or
less with respect to the temperature at half the thickness of the
hot-rolled steel sheet or alternatively holding the hot-rolled
steel sheet at a temperature in the range of the cooling stop
temperature of the third cooling to the coiling temperature for 20
s or more, and the coiling step includes coiling the hot-rolled
steel sheet at a surface temperature of 450.degree. C. or more.
[0030] (8) The method for manufacturing a hot-rolled steel sheet
according to (7), wherein the composition has Moeq defined by the
following formula (1) in the range of 1.4% to 2.2% by mass:
Moeq (%)=Mo+0.36Cr+0.77Mn+0.07Ni (1)
[0031] (wherein Mn, Ni, Cr, and Mo denote the corresponding element
contents (% by mass)).
[0032] (9) The method for manufacturing a hot-rolled steel sheet
according to (7) or (8), wherein the hot-rolled steel sheet further
contains, on a mass percent basis, one or two or more selected from
Cu: 0.50% or less, V: 0.10% or less, and B: 0.0005% or less.
[0033] (10) The method for manufacturing a hot-rolled steel sheet
according to any one of (7) to (9), wherein the hot-rolled steel
sheet further contains Ca: 0.0005% to 0.0050% by mass.
Advantageous Effects of Invention
[0034] Embodiments of the present disclosure provide a
high-strength hot-rolled steel sheet having high low-temperature
toughness and a low yield ratio that is particularly suitable as a
material for spiral steel pipes. The hot-rolled steel sheet can
maintain strength after pipe manufacturing, does not have an uneven
surface hardness distribution, has low cross section hardness, has
the desired pipe shape and uniform ductility in the pipe
manufacturing, and has a yield strength of 480 MPa or more at an
angle of 30 degrees with the rolling direction, a tensile strength
of 600 MPs or more in the sheet width direction, a fracture
transition temperature vTrs of -80.degree. C. or less in a Charpy
impact test, and a yield ratio of 85% or less. A high-strength
hot-rolled steel sheet with a low yield ratio according to
embodiments can be easily manufactured at low cost without
particular heat treatment. Thus, embodiments of the present
disclosure have significant industrial advantages. The embodiments
of the present disclosure also have the advantage that
electric-resistance-welded (ERW) pipes for use in line pipes laid
using a reel barge method or line pipes that require earthquake
resistance can be easily manufactured at low cost. The embodiments
of the present disclosure also have the advantage that a
high-strength hot-rolled steel sheet with a low yield ratio
according to embodiments can be used as a material for
manufacturing high-strength spiral steel pipe piles that serve as
architectural members and harbor structure members having high
earthquake resistance. The embodiments of the present disclosure
also have the advantage that spiral steel pipes manufactured using
such a hot-rolled steel sheet can be applied to high-value-added
high-strength steel pipe piles because of their low yield ratios in
the longitudinal direction of the pipes.
BRIEF DESCRIPTION OF DRAWINGS
[0035] FIG. 1 is a schematic explanatory view-illustrating the
relationship between the formation of massive martensite and second
cooling in cooling after hot rolling.
DESCRIPTION OF EMBODIMENTS
[0036] The reason for limiting the composition of a hot-rolled
steel sheet according to embodiments will be described below.
Unless otherwise specified, the mass percentage is simply expressed
in %.
[0037] C: 0.03% to 0.10%
[0038] C can precipitate as carbide and contribute to increased
strength of steel sheets by precipitation hardening. C is also an
element that can contribute to improved toughness of steel sheets
by decreasing the crystal grain size. Furthermore, C can dissolve
in steel, stabilize austenite, and promote the formation of
untransformed austenite. These effects require a C content of 0.03%
or more. However, a C content of more than 0.10% tends to result in
the formation of coarse cementite at grain boundaries and low
toughness. Thus, the C content is limited to the range of 0.03% to
0.10%, preferably 0.04% to 0.09%.
[0039] Si: 0.01% to 0.50%
[0040] Si can contribute to increased strength of steel sheets by
solid-solution hardening. Si can also contribute to a low yield
ratio by the formation of a hard second phase (for example,
martensite). These effects require a Si content of 0.01% or more.
However, a Si content of more than 0.50% results in significant
formation of oxidized scale containing fayalite and a poor steel
sheet appearance. Thus, the Si content is limited to the range of
0.01% to 0.50%, preferably 0.20% to 0.40%.
[0041] Mn: 1.4% to 2.2%
[0042] Mn can dissolve in steel, improve quenching hardenability,
and promote the formation of martensite. Mn is also an element that
can lower the bainitic ferrite transformation start temperature and
contribute to improved toughness of steel sheets by decreasing the
microstructure size. These effects require a Mn content of 1.4% or
more. However, a Mn content of more than 2.2% results in a heat
affected zone having low toughness. Thus, the Mn content is limited
to the range of 1.4% to 2.2%. The Mn content preferably ranges from
1.6% to 2.0% in terms of stable formation of massive
martensite.
[0043] P: 0.025% or Less
[0044] P can dissolve in steel and contribute to increased strength
of steel sheets, but lowers toughness. Thus, in the embodiments, P
is preferably minimized as an impurity. However, a P content of up
to 0.025% is acceptable. Thus, the P content is limited to 0.025%
or less, preferably 0.015% or less. Since an excessively low P
content results in high refining costs, the P content is preferably
approximately 0.001% or more.
[0045] S: 0.005% or Less
[0046] S in steel can form coarse sulfide inclusions, such as MnS,
and induce cracking of slabs. S also lowers the ductility of steel
sheets. Such phenomena are noticeable at a S content of more than
0.005%. Thus, the S content is limited to 0.005% or less,
preferably 0.004% or less. Although the S content may be 0%, an
excessively low S content results in high refining costs. Thus, the
S content is preferably approximately 0.0001% or more.
[0047] Al: 0.005% to 0.10%
[0048] Al can act as a deoxidizing agent. Al is an element that is
effective in fixing N, which is responsible for strain aging. These
effects require an Al content of 0.005% or more. However, an Al
content of more than 0.10% results in a high oxide content of steel
and low toughness of base materials and welds. When steel, such as
a slab, or a steel sheet is heated in a furnace, Al tends to form a
nitride surface layer, which may increase the yield ratio. Thus,
the Al content is limited to the range of 0.005% to 0.10%,
preferably 0.08% or less.
[0049] Nb: 0.02% to 0.10%
[0050] Nb can dissolved in steel or precipitate as carbonitride,
can suppress coarsening and recrystallization of austenite grains,
and allows rolling of austenite in a un-recrystallization
temperature range. Nb is also an element that can form fine carbide
or carbonitride precipitates and contribute to increased strength
of steel sheets. During cooling after hot rolling, Nb can
precipitate as carbide or carbonitride on dislocations introduced
by hot rolling, act as a nucleus for .gamma..fwdarw..alpha.
transformation, promote the formation of bainitic ferrite in
grains, and contribute to the formation of fine massive
untransformed austenite, which results in the formation of fine
massive martensite. These effects require a Nb content of 0.02% or
more. However, an excessively high Nb content of more than 0.10%
may result in high deformation resistance in hot rolling, thus
making hot rolling difficult. Furthermore, an excessively high Nb
content of more than 0.10% results in a bainitic ferrite main phase
having a high yield strength, thereby making it difficult to
achieve a yield ratio of 85% or less. Thus, the Nb content is
limited to the range of 0.02% to 0.10%, preferably 0.03% to
0.07%.
[0051] Ti: 0.001% to 0.030%
[0052] Ti can fix N as nitride and contribute to the prevention of
cracking of slabs. Furthermore, Ti can form fine carbide
precipitates and increase the strength of steel sheets. These
effects require a Ti content of 0.001% or more. However, a high Ti
content of more than 0.030% results in an excessively high bainitic
ferrite transformation point and low toughness of steel sheets.
Thus, the Ti content is limited to the range of 0.001% to 0.030%,
preferably 0.005% to 0.025%.
[0053] Mo: 0.01% to 0.50%
[0054] Mo can contribute to improved quenching hardenability and
promote the formation of martensite by moving C from bainitic
ferrite to untransformed austenite and thereby improving the
hardenability of the untransformed austenite. Furthermore, Mo is an
element that can dissolve in steel and contribute to increased
strength of steel sheets by solid-solution hardening. These effects
require a Mo content of 0.01% or more. However, a Mo content of
more than 0.50% results in the formation of an excessive amount of
martensite and low toughness of steel sheets. Furthermore, a large
amount of expensive Mo results in high material costs. Thus, the Mo
content is limited to the range of 0.01% to 0.50%, preferably 0.10%
to 0.40%.
[0055] Cr: 0.01% to 0.50%
[0056] Cr has the effects of delaying .gamma..fwdarw..alpha.
transformation, contributing to improved quenching hardenability,
and promoting the formation of martensite. These effects require a
Cr content of 0.01% or more. However, a Cr content of more than
0.50% tends to result in a frequent occurrence of defects in welds.
Thus, the Cr content is limited to the range of 0.01% to 0.50%,
preferably 0.20% to 0.45%.
[0057] Ni: 0.01% to 0.50%
[0058] Ni can contribute to improved quenching hardenability and
promote the formation of martensite. Furthermore, Ni is an element
that can contribute to further improved toughness. These effects
require a Si content of 0.01% or more. However, such effects level
off at a Ni content of more than 0.50% and are not expected to be
proportional to the Ni content beyond this threshold. A Ni content
of more than 0.50% is therefore economically disadvantageous. Thus,
the Ni content is limited to the range of 0.01% to 0.50%,
preferably 0.30% to 0.45%.
[0059] These components are base components. In embodiments, the
amounts of these components are preferably adjusted in the ranges
described above such that Moeq defined by the following formula (1)
ranges from 1.4% to 2.2%:
Moeq (%)=Mo+0.36Cr+0.77Mn+0.07Ni (1)
[0060] (wherein Mn, Ni, Cr, and Mo denote the corresponding element
contents (% by mass)).
[0061] Moeq is an indicator of the quenching hardenability of
untransformed austenite that remains in a steel sheet after the
cooling step. Moeq of less than 1.4% results in insufficient
quenching hardenability of untransformed austenite, which results
in transformation of untransformed austenite into pearlite or the
like during the subsequent coiling step. Moeq of more than 2.2%
results in the formation of an excessive amount of martensite and
low toughness. Thus, Moeq is preferably limited to the range of
1.4% to 2.2%. Moeq of 1.5% or more results in a low yield ratio and
further improved ductility. Thus, Moeq is more preferably 1.5% or
more.
[0062] In addition to the components described above, if necessary,
a hot-rolled steel sheet according to embodiments may contain one
or two or more selected from Cu: 0.50% or less, V: 0.10% or less,
and B: 0.0005% or less, and/or Ca: 0.0005% to 0.0050%.
[0063] One or two or more selected from Cu: 0.50% or less, V: 0.10%
or less, and %: 0.0005% or less
[0064] Cu, V, and B are elements that can contribute to
reinforcement of steel sheets and can be used as required.
[0065] V and Cu can contribute to reinforcement of steel sheets by
solid-solution hardening or precipitation hardening. B can
segregate at grain boundaries and contribute to reinforcement of
steel sheets due to Improved quenching hardenability. In order to
produce these effects, Cu: 0.01% or more, V: 0.01% or more, and/or
B: 0.0001% or more are preferred. However, steel sheets having a V
content of more than 0.10% have low weldability. Steel sheets
having a B content of more than 0.0005% have low toughness. Steel
sheets having a Cu content of more than 0.50% have poor hot
workability. Thus, when steel sheets contain these elements, Cu:
0.50% or less, V: 0.10% or less, and/or B: 0.0005% or less are
preferred.
[0066] Ca: 0.0005% to 0.0050%
[0067] Ca is an element that can contribute to morphology control
of sulfide by which coarse sulfide becomes spherical sulfide. Steel
sheets can contain Ca, if necessary, in order to produce these
effects, Ca: 0.0005% or more is preferred. However, steel sheets
having a Ca content of more than 0.0050% have low cleanliness.
Thus, when steel sheets contain Ca, Ca: 0.0005% to 0.0050% is
preferred.
[0068] The remainder other than the components described above is
be and incidental impurities. The incidental impurities may be N:
0.005% or less, 0: 0.005% or less, Mg: 0.003% or less, and/or Sn:
0.005% or less.
[0069] The reason for limiting the microstructure of a
high-strength hot-rolled steel sheet with a low yield ratio
according to embodiments will be described below.
[0070] A high-strength hot-rolled steel sheet with a low yield
ratio according to embodiments has a composition as described above
and has different microstructures on an outer surface layer
(hereinafter also referred to simply as an outer layer) in the
thickness direct ion and on an inner surface layer (hereinafter
also referred to simply as an inner layer) in the thickness
direction. Steel pipes formed of a steel sheet having such
different microstructures at different positions in the thickness
direction can have a low yield ratio and uniform ductility. The
term "an outer surface layer (outer layer) in the thickness
direction", as used herein, refers to a region having a depth of
less than 1.5 mm from the front and pack sides of a steel sheet in
the thickness direction. The term "an inner surface layer (inner
layer) in the thickness direction", as used herein, refers to a
region having a depth of 1.5 mm or more from the front and hack
sides of a steel sheet in the thickness direction.
[0071] The outer surface layer (outer layer) in the thickness
direction has a single-phase microstructure composed of a tempered
martensite phase or a mixed microstructure composed of a tempered
martensite phase and a tempered bainite phase. Such a
microstructure allows the steel sheet to have low hardness on the
outer surface thereof in the thickness direction and be provided
with high uniform ductility. Since pipe forming is a bending
deformation, processing strain in the thickness direction increases
with distance from the center of the steel sheet in the thickness
direction and increases with the thickness of the steel sheet.
Thus, it is important to control the outer layer
microstructure.
[0072] An uneven cooling history of a hot-rolled steel sheet, for
example, cooling of a hot-rolled steel sheet through a transition
boiling region results in a local increase in hardness and uneven
hardness. These problems can be avoided when the outer layer has a
single-phase microstructure composed of a tempered martensite phase
or a mixed microstructure composed of a tempered martensite phase
and a tempered bainite phase. The mixture ratio of the tempered
martensite phase to the tempered bainite phase of the mixed
microstructure is not particularly limited. From the perspective of
temper softening treatment, the area fraction of the tempered
martensite phase preferably ranges from 60% to 100%, and the area
fraction of the tempered bainite phase preferably ranges from 0% to
40%. The microstructure can be formed under certain manufacturing
conditions, in particular, at a cumulative rolling reduction of 50%
or more at a temperature of 930.degree. C. or less in finish
rolling, and by sequentially performing a first cooling, second
cooling, third cooling, and fourth cooling in a cooling step after
the completion of the finish rolling. The first cooling includes
cooling the hot-rolled steel sheet to a martensitic transformation
start temperature (Ms point) or less at an average cooling rate of
100.degree. C./s or more with respect to surface temperature. The
second cooling includes, after the completion of the first cooling,
holding the hot-rolled steel sheet for 1 s or more at a surface
temperature of 600.degree. C. or more. The third cooling includes,
after the completion of the second cooling, cooling the hot-rolled
steel sheet to a cooling stop temperature in the range of
600.degree. C. to 450.degree. C. at an average cooling rate in the
range of 5.degree. C. to 30.degree. C./s with respect to the
temperature at half the thickness of the hot-rolled steel sheet.
The fourth cooling includes cooling the hot-rolled steel sheet from
the cooling stop temperature of the third cooling to a coiling
temperature at an average cooling rate of 2.degree. C./s or less
with respect to the temperature at half the thickness of the
hot-rolled steel sheet or alternatively holding the hot-rolled
steel sheet at a temperature in the range of the cooling stop
temperature of the third cooling to the coiling temperature for 20
s or more. The microstructure and area fraction can be identified
and calculated by observing and measuring using the methods
described below in the examples.
[0073] The hardness of a steel sheet at a depth of 0.5 mm from a
surface thereof in the thickness direction is preferably 95% or
less of the maximum hardness in the thickness direction. The fact
that the hardness of a hot-rolled steel sheet at a depth of 0.5 mm
from a surface thereof in the thickness direction is not equal to
the maximum hardness in the thickness direction is important in
ensuring the workability of the hot-rolled steel sheet and the
desired pipe shape after pipe manufacturing. The maximum hardness
in the thickness direction preferably corresponds to a Vickers
hardness HV 0.5 of 165 points or more and 300 points or less, more
preferably 280 points or less. This hardness can be achieved under
certain manufacturing conditions, in particular, by performing a
first cooling and a second cooling in a cooling step after the
completion of finish rolling, the first cooling including cooling
the hot-rolled steel sheet to a martensitic transformation start
temperature (Ms point) or less at an average cooling rate of
100.degree. C./s or more with respect to surface temperature, the
second cooling including, after the completion of the first
cooling, holding the hot-rolled steel sheet for 1 s or more at a
surface temperature of 600.degree. C. or more. The hardness can be
measured using the method described below in the examples.
[0074] The inner surface layer (inner layer) in the thickness
direction has a microstructure composed of a main phase and a
second phase. The main phase is a bainitic ferrite phase. The
second phase is formed of massive martensite having an aspect ratio
of less than 5.0 dispersed in the main phase. The main phase herein
refers to a phase having an occupied area of 50% by area or more.
The bainitic ferrite preferably has an area fraction of 85% or
more, more preferably 88.3% or more. The bainitic ferrite main
phase has a substructure having a high dislocation density and
contains needle-shaped ferrite and acicular ferrite. The bainitic
ferrite does not include polygonal ferrite having a very low
dislocation density or semi(quasi)-polygonal ferrite including a
substructure, such as fine subgrains. In order to achieve the
desired high strength, the bainitic ferrite main phase must contain
fine carbonitride precipitates. The bainitic ferrite main phase has
an average grain size of 10 .mu.m or less. An average grain size of
more than 10 .mu.m results in insufficient work hardening ability
in a region having a low strain of less than 5% and a decrease in
yield strength due to bending in spiral pipe manufacturing. The
desired low-temperature toughness can be achieved by decreasing the
average grain size of the main phase even when the steel sheet
contains much martensite.
[0075] The second phase in the inner layer has an area fraction in
the range of 1.4% to 15% and is formed of massive martensite having
an aspect ratio of lass than 5.0. Massive martensite in in
embodiments of the present disclosure is martensite formed from
untransformed austenite at prior .gamma. grain boundaries or within
prior .gamma. grains in a cooling process after rolling. In the
embodiments of the present disclosure, such massive martensite is
dispersed at prior .gamma. grain boundaries or between bainitic
ferrite grains of the main phase. Martensite is harder than the
main phase, can introduce a large number of mobile dislocations
into bainitic ferrite during processing, and allows yield behavior
of a continuous yielding type. Since martensite, which has higher
tensile strength than bainitic ferrite, a low yield ratio can be
achieved. When the martensite is massive martensite having an
aspect ratio of less than 5.0, the martensite can introduce more
mobile dislocations into adjacent bainitic ferrite and effectively
improve ductility. Martensite having an aspect ratio of 5.0 or more
becomes rod-like martensite (non-massive martensite) and cannot
achieve the desired low yield ratio. Nevertheless, rod-like
martensite having an area fraction of less than 30% of the total
amount of martensite is allowable. The massive martensite
preferably has an area fraction of 70% or more of the total amount
of martensite. The aspect ratio can be measured using the method
described below in the examples.
[0076] Such effects require dispersion of massive martensite having
an area fraction of 1.4% or more. It is difficult to achieve the
desired low yield ratio with massive martensite having an area
fraction of less than 1.4%. When the massive martensite has an area
fraction of more than 15%, the low-temperature toughness is
significantly decreased. Thus, the area fraction of massive
martensite is limited to the range of 1.4% to 15%, preferably 10%
or less. In addition to massive martensite, the second phase may
contain bainite having an area fraction of approximately 7.0% or
less.
[0077] The microstructure can be formed under certain manufacturing
conditions, in particular, at a cumulative rolling reduction of 50%
or more at a temperature of 930.degree. C. or less in finish
rolling, and by sequentially performing a first cooling, second
cooling, third cooling, and fourth cooling in a cooling step after
the completion of the finish rolling. The first cooling includes
cooling the hot-rolled steel sheet to a martensitic transformation
start temperature (Ms point) or less at an average cooling rate of
100.degree. C./s or more with respect to surface temperature. The
second cooling includes, after the completion of the first cooling,
holding the hot-rolled steel sheet for 1 s or more at a surface
temperature of 600.degree. C. or more. The third cooling includes,
after the completion of the second cooling, cooling the hot-rolled
steel sheet to a cooling stop temperature in the range of
600.degree. C. to 450.degree. C. at an average cooling rate in the
range of 5.degree. C. to 30.degree. C./s with respect to the
temperature at half the thickness of the hot-rolled steel sheet.
The fourth cooling includes cooling the hot-rolled steel sheet from
the cooling stop temperature of the third cooling to a coiling
temperature at an average cooling rate of 2.degree. C./s or less
with respect to the temperature at half the thickness of the
hot-rolled steel sheet or alternatively holding the hot-rolled
steel sheet at a temperature in the range of the cooling stop
temperature of the third cooling to the coiling temperature for 20
s or more.
[0078] The massive martensite preferably has a maximum sire of 5.0
.mu.m or less and an average size in the range of 0.5 to 3.0 .mu.m.
Coarse massive martensite having an average size of more than 3.0
.mu.m tends to act as a starting point of brittle fracture or
promote crack propagation and lowers the low-temperature toughness.
Excessively fine massive martensite grains having an average size
of less than 0.5 .mu.m result in a decreased number of mobile
dislocations introduced into adjacent bainitic ferrite. Massive
martensite having a maximum size of more than 5.0 .mu.m results in
low toughness. Thus, the massive martensite preferably has a
maximum size of 5.0 .mu.m or less and an average size in the range
of 0.5 to 3.0 .mu.m. The term "diameter", as used herein in the
context of the dimensions of massive martensite, refers to half the
sum of the length along the major axis and the length along the
minor axis. The maximum diameter is the "maximum" size of the
massive martensite. The arithmetic mean of the "diameters" of
grains is the "average" size of the massive martensite. At least
100 martensite grains are subjected to the measurement.
[0079] The microstructure can be formed under certain manufacturing
conditions, in particular, at a cumulative rolling reduction of 50%
or more at a temperature of 930.degree. C. or less in finish
rolling, and by sequentially performing a first cooling, second
cooling, third cooling, and fourth cooling in a cooling step after
the completion of the finish rolling. The first cooling includes
cooling the hot-rolled steel sheet to a martensitic transformation
start temperature (Ms point) or less at an average cooling rate of
100.degree. C./s or more with respect to surface temperature. The
second cooling includes, after the completion of the first cooling,
holding the hot-rolled steel sheet for 1 s or more at a surface
temperature of 600.degree. C. or more. The third cooling includes,
after the completion of the second cooling, cooling the hot-rolled
steel sheet to a cooling stop temperature in the range of
600.degree. C. to 45.degree. C. at an average cooling rate in the
range of 5.degree. C. to 30.degree. C./s with respect to the
temperature at half the thickness of the hot-rolled steel sheet.
The fourth cooling includes cooling the hot-rolled steel sheet from
the cooling stop temperature of the third cooling to a coiling
temperature at an average cooling rate of 2.degree. C./s or less
with respect to the temperature at half the thickness of the
hot-rolled steel sheet or alternatively holding the hot-rolled
steel sheet at a temperature in the range of the cooling stop
temperature of the third cooling to the coiling temperature for 20
s or more.
[0080] The microstructure, area fraction, and average grain size
can be identified and calculated by observing and measuring using
the methods described below in the examples.
[0081] A method for manufacturing a high-strength hot-rolled steel
sheet with a low yield ratio according to the embodiments will be
described below.
[0082] In the embodiments of the present disclosure, steel having a
composition as described above is subjected to a hot-rolling step,
a cooling step, and a coiling step to form a hot-rolled steel
sheet.
[0083] The steel may be manufactured by any method. Preferably,
molten steel having a composition as described above is smelted
using a known melting method, such as using a converter or an
electric furnace, and the molten steel, is formed into steel, such
as a slab, using a known casting method, such as a continuous
casting process.
[0084] The steel is subjected to the hot-rolling step.
[0085] The hot-rolling step includes heating steel having a
composition as described above to a heating temperature in the
range of 1050.degree. C. to 1300.degree. C., rough-rolling the
heated steel to form a sheet bar, and finish-rolling the sheet bar
such that the cumulative rolling reduction at a temperature of
930.degree. C. or less is 50% or more, thereby forming a hot-rolled
steel sheet.
[0086] Heating temperature: 1050.degree. C. to 1300.degree. C.
[0087] Steel used in embodiments essentially contains Nb and Ti, as
described above. In order to achieve the desired high strength by
precipitation hardening, coarse carbide and nitride must be once
dissolved in steel and then finely precipitated. Thus, the steel is
heated to a heating temperature of 1050.degree. C. or more. At a
heating temperature of less than 1050.degree. C., the elements
remain undissolved, and the resulting steel sheet cannot have the
desired strength. A high heating temperature of more than
1300.degree. C. results in coarsening of crystal grains and steel
sheets having low toughness. Thus, the heating temperature for the
steel is limited to the range of 1050.degree. C. to 1300.degree.
C.
[0088] The steel heated to the heating temperature is subjected to
rough rolling to form a sheet bar. The steel may be subjected to
rough rolling under any conditions, provided that the sheet bar has
the desired size and shape.
[0089] The sheet bar is then subjected to finish rolling to form a
hot-rolled steel sheet having the desired site and shape. In the
finish rolling, the cumulative rolling reduction at a temperature
of 930.degree. C. or less is 50% or more.
[0090] Cumulative rolling reduction at a temperature of 930.degree.
C. or less: 50% or more
[0091] The cumulative rolling reduction at a temperature of
930.degree. C. or less is 50% or more in order to decrease the size
of bainitic ferrite and finely disperse massive martensite in the
inner layer microstructure. A cumulative rolling reduction of less
than 50% at a temperature of 930.degree. C. or less results in an
insufficient rolling reduction and a lack of a fine bainitic
ferrite main phase in the inner layer microstructure. This also
results in insufficient dislocations that act as precipitation
sites for NbC and the like, which promotes nucleation in
.gamma..fwdarw..alpha. transformation, and insufficient formation
of bainitic ferrite in grains. If is therefore impossible to keep a
large number of finely dispersed massive untransformed .gamma.
grains for forming massive martensite. Thus, in the finish rolling,
the cumulative rolling reduction at a temperature of 930.degree. C.
or less is limited to 50% or more. The cumulative rolling reduction
is preferably 80% or less. Such effects level off at a rolling
reduction of more than 80%. Furthermore, a rolling reduction of
more than 80% may result in a frequent occurrence of separation and
low absorbed energy in a Charpy impact test.
[0092] The finishing temperature of the finish rolling preferably
ranges from 850.degree. C. to 760.degree. C. in terms of steel
sheet toughness, steel sheet strength, and rolling load. When the
finishing temperature of the finish rolling is as high as more than
850.degree. C., the rolling reduction per pass must be increased to
achieve the cumulative rolling reduction of 50% or more at a
temperature of 930.degree. C. or less, which sometimes results in
increased rolling load. When the finishing temperature of the
finish rolling is as low as less than 760.degree. C., this
sometimes results in the formation of ferrite during rolling,
coarsening of the microstructure and precipitates, and decreases in
low-temperature toughness and strength.
[0093] The hot-rolled steel sheet is then subjected to the cooling
step.
[0094] The cooling step includes first cooling, second cooling,
third cooling, and fourth cooling in this order. The first cooling
is started immediately after the completion of the finish rolling
and including cooling the hot-rolled steel sheet to a martensitic
transformation start temperature (Ms point) or less at an average
cooling rate of 100.degree. C./s or more with respect to surface
temperature. The second cooling includes, after the completion of
the first cooling, holding the hot-rolled steel sheet for 1 s or
more at a surface temperature of 600.degree. C. or more. The third
cooling includes, after the completion of the second cooling,
cooling the hot-rolled steel sheet to a cooling stop temperature in
the range of 600.degree. C. to 450.degree. C. at an average cooling
rate in the range of 5.degree. C. to 30.degree. C./s with respect
to the temperature at half the thickness of the hot-rolled steel
sheet. The fourth cooling includes cooling the hot-rolled steel
sheet from the cooling stop temperature of the third cooling to a
coiling temperature at an average cooling rate of 2.degree. C./s or
less with respect to the temperature at half the thickness of the
hot-rolled steel sheet or alternatively holding the hot-rolled
steel sheet at a temperature in the range of the cooling stop
temperature of the third cooling to the coiling temperature for 20
s or more. The coiling step includes coiling the hot-rolled steel
sheet at a surface temperature of 450.degree. C. or more.
[0095] Cooling is started immediately, preferably within 15 s,
after the completion of the finish rolling.
[0096] In the first cooling, the hot-rolled steel sheet is cooled
to a martensitic transformation start temperature (Ms point) or
less at an average cooling rate of 100.degree. C./s or more with
respect to surface temperature. The cooling rate in the first
cooling is the average cooling rate in the temperature range of
600.degree. C. to 450.degree. C. with respect to surface
temperature. In the first cooling, a single-phase microstructure
composed of a martensite phase or a mixed micro-structure composed
of a martensite phase and a bainite phase is formed on the steel
sheet outer layer. The average cooling rate in the first cooling
has no particular upper limit. Depending on the capacity of a
cooling apparatus, the hot-rolled steel sheet can be cooled at a
higher cooling rate. The holding time at the martensitic
transformation start temperature (Ms point) or less with respect to
surface temperature depends on the desired surface microstructure
and is 10 s or less, preferably 7 s or less. Holding the hot-rolled
steel sheet at a temperature of the Ms point or less for a long
time results in an excessively high occupied area of a single phase
formed of a martensite phase or a mixed microstructure composed of
a martensite phase and a bainite phase, which results in a lower
thickness percentage of the desired microstructure.
[0097] In the second cooling after the first cooling, the
hot-rolled steel sheet is held for 1 s or more at a surface
temperature of 600.degree. C. or more utilizing internal
recalescence without cooling or heating. In the second cooling, the
martensite phase and the bainite phase are tempered, and the outer
layer microstructure becomes a single-phase microstructure composed
of the tempered martensite phase or a mixed microstructure composed
of the tempered martensite phase and the tempered bainite phase. A
steel sheet surface temperature of less than 600.degree. C. and a
holding time of less than 1 s result in insufficient tempering of
the outer layer microstructure. Thus, in the second cooling, the
hot-rolled steel sheet is held at a surface temperature of
600.degree. C. or more for 1 s or more, preferably 600.degree. C.
or more for 2 s or more. The holding time at a temperature of
600.degree. C. or more has no particular upper limit. However, in
order to satisfy the third cooling conditions at half the thickness
of the hot-rolled steel sheet and suppress the formation of
polygonal ferrite, the holding time is preferably 6 s or less. The
steel sheet surface temperature may be increased to 600.degree. C.
or more using any method, for example, utilizing internal heat in
the thickness direction or using an external heater. After the
outer layer microstructure of the steel sheet is formed by the
first cooling and the second cooling, the third cooling is
performed to form an inner layer microstructure of the steel sheet,
which includes a bainitic ferrite main phase and a massive
martensite second phase.
[0098] The average cooling rate of the third cooling at half the
thickness of the hot-rolled steel sheet ranges from 5.degree. C. to
30.degree. C./s in the polygonal ferrite formation temperature
range, which ranges from 750.degree. C. to 600.degree. C. An
average cooling rate of less than 5.degree. C./s results in an
inner layer microstructure composed mainly of polygonal ferrite
rather than the desired microstructure composed of a bainitic
ferrite main phase. Rapid cooling at an average cooling rate of
more than 30.degree. C./s results in insufficient concentration of
an alloying element in untransformed austenite, which makes it
difficult to finely disperse a desired amount of massive martensite
by the subsequent cooling and to provide a hot-rolled steel sheet
having the desired low yield ratio and desired high low-temperature
toughness. Thus, the cooling rate at half the thickness of the
hot-rolled steel sheet is limited to the range of 5.degree. C. to
30.degree. C./s, preferably 5.degree. C. to 25.degree. C./s. The
temperature at half the thickness of the hot-rolled steel sheet can
be calculated by heat-transfer calculation based on the steel sheet
surface temperature and the temperature and amount of cooling
water.
[0099] The cooling stop temperature in the third cooling ranges
from 600.degree. C. to 450.degree. C. A cooling stop temperature
above this temperature range makes it difficult to form the desired
inner layer microstructure composed of a bainitic ferrite main
phase. A cooling stop temperature below this temperature range
results in substantial completion of transformation of
untransformed .gamma. and an insufficient amount of massive
martensite.
[0100] In embodiments, the first to third cooling is followed by
the fourth cooling. FIG. 1 schematically illustrates the
temperature at half the thickness of the hot-rolled steel sheet in
the fourth cooling in the temperature range from the cooling stop
temperature of the third cooling to the coiling temperature. As
illustrated in FIG. 1, the fourth cooling is slow cooling. Slow
cooling in this temperature range allows alloying elements, each as
C, to be farther diffused into untransformed .gamma., thereby
stabilizing untransformed .gamma. and facilitating the formation of
massive martensite in the subsequent cooling. Such slow cooling is
performed by cooling the hot-rolled steel sheet from the cooling
stop temperature of the third cooling to the coiling temperature at
an average cooling rate of 2.degree. C./s or less, preferably
1.5.degree. C./s or less, with respect to the temperature at half
the thickness of the hot-rolled steel sheet or by holding the
hot-rolled steel sheet at a temperature in the range of the cooling
stop temperature of the third cooling to the coiling temperature
for 20 s or more. Cooling from the cooling stop temperature of the
second cooling to the coiling temperature at an average cooling
rate of more than 2.degree. C./s results in insufficient diffusion
of alloying elements, such as C, into untransformed .gamma.,
insufficient stabilization of the untransformed .gamma., and
formation of rod-like untransformed .gamma. remaining between
bainitic ferrite grains, as in cooling indicated by a dotted line
in FIG. 1, thus making it difficult to form the desired massive
martensite.
[0101] The fourth cooling is preferably performed by stopping water
injection at the latter part of runout table. For a steel sheet
having a small thickness, the desired cooling conditions are
preferably ensured by completely removing cooling water remaining
on the steel sheet or installing a heat-insulating coyer.
Furthermore, the transport speed is preferably adjusted in order to
ensure a holding time of 20 s or more in the temperature range
described above.
[0102] After the fourth cooling, the hot-rolled steel sheet is
subjected to the coiling step.
[0103] The coiling step includes coiling the hot-rolled steel,
sheet at a surface temperature of 450.degree. C. or more. The
desired low yield ratio cannot be achieved at a coiling temperature
of less than 450.degree. C. Thus, the coiling temperature is
limited to 450.degree. C. or more. Through this step, the steel
sheet can be held for at least a predetermined time in a
temperature range where ferries and austenite coexist.
[0104] A hot-rolled steel sheet manufactured by using the method
described, above is used as a material for pipe manufacturing to
form spiral steel pipes and electric-resistance-welded (ERW) pipes
through common pipe manufacturing steps. The pipe manufacturing
steps are not particularly limited and may be common steps.
EXAMPLES
[0105] Molten steel having a composition listed in Table 1 was
formed into a slab (thickness: 220 mm) using a continuous casting
process. The slab was used as steel. The steel was subjected to a
hot-rolling step, in which the steel was heated to a heating
temperature listed in Table 2, rough-rolling the steel to form a
sheet bar, and finish-rolling the sheet bar under the conditions
listed in Table 2 to form a hot-rolled steel sheet (thickness: 8 to
25 mm). The hot-rolled steel sheet was subjected to a cooling step
immediately after the completion of the finish rolling. The cooling
step included first to fourth cooling listed in Table 2. After the
cooling step, the hot-rolled steel sheet was subjected to a coiling
step, which included coiling the hot-rolled steel sheet at a
coiling temperature listed in Table 2 and allowing the coil to
cool.
[0106] Test pieces were taken from the hot-rolled steel sheet and
were subjected to microstructure observation, a tensile test, an
impact test, and a hardness test.
[0107] The test methods are as follows:
(1) Microstructure Observation
[0108] A test piece for microstructure observation was taken from
the hot-rolled steel sheet such that a cross section thereof in the
rolling direction (L cross section) served as an observation
surface. After the test piece was polished and was etched with
nital, the microstructure of the test piece was observed and
photographed with an optical microscope (magnification ratio: 500)
or an electron microscope (magnification ratio: 2000). The type of
microstructure, the fraction (area fraction) of the microstructure
of each phase, and the average grain size were determined from the
photograph of the inner layer microstructure with an image
analyzing apparatus. For the outer layer, only the type of
microstructure was identified from the microstructure
photograph.
[0109] The average grain size of the bainitic ferrite main phase in
the inner layer microstructure was determined using an intercept
method in accordance with JIS G 0552. The aspect ratio of
martensite grains was calculated as the ratio (the length along the
major axis)/(the length along the minor axis) of the length of a
grain in the longitudinal direction or in a direction of the
maximum grain size (the length along the major axis) to the length
of the grain in a direction perpendicular to the longitudinal
direction (the length along the minor axis). Martensite grains
having an aspect ratio of less than 5.0 were defined as massive
martensite. Martensite grains having an aspect ratio of 5.0 or more
were referred to as "rod-like" martensite. The average sire of
massive martensite in the steel sheet was calculated by determining
half the sum of the length along the major axis and the length
along the minor axis of each massive martensite grain as the
diameter thereof and calculating the arithmetic mean of the
diameters. The maximum diameter of each massive martensite grain
was the maximum size of the massive martensite. At least 100
martensite grains were subjected to the measurement.
(2) Tensile Test
[0110] Test pieces for tensile test (full-thickness test pieces
specified in API-5L, (width: 38.1 mm, GL: 50 mm)) were taken from
the hot-rolled steel sheet such that the tensile direction was
perpendicular to the rolling direction (sheet width direction) or
at an angle of 30 degrees with the rolling direction. A tensile
test was performed in accordance with the ASTM A 370 specification
to determine tensile properties (yield strength YS and tensile
strength TS).
(3) Impact Test
[0111] V-notched test pieces were taken from the hot-rolled steel
sheet such that the longitudinal direction of the test pieces was
perpendicular to the rolling direction, and were subjected to a
Charpy impact test in accordance with the ASTM A 370 specification
to determine the fracture transition temperature vTrs (.degree.
C.).
(4) Hardness Test
[0112] Test pieces for hardness measurement were taken from the
hot-rolled steel sheet. The cross section hardness of the test
pieces was measured with a Vickers hardness tester (test force: 4.9
N) (load: 500 g). The cross section hardness of each of the test
pieces was continuously measured at intervals of 0.5 mm from a
surface of the steel sheet in the thickness direction. The hardness
at a depth of 0.5 mm from the surface of the steel sheet in the
thickness direction (depth direction) and the maximum hardness in
the thickness direction were determined. The hardness distribution
was judged to be good when the maximum hardness in the thickness
direction was 300 points or less, and the hardness at a depth of
0.5 mm from the surface was 95% or less of the maximum hardness in
the thickness direction.
[0113] A spiral steel pipe (outer diameter: 1067 mm.phi.) was then
manufactured by using a spiral pipe manufacturing process using the
hot-rolled steel sheet as a material for pipes. Test pieces for
tensile test (test pieces specified in API) were taken from the
steel pipe such that the tensile direction was the circumferential
direction of the pipe, and were subjected to a tensile test in
accordance with the ASTM A 370 specification to measure tensile
properties (yield strength YS and tensile strength TS). .DELTA.YS
(=YS of steel pipe-30-degree YS of steel sheet) was calculated from
the results to determine the strength reduction due to pipe
manufacturing. Table 3 shows the results.
TABLE-US-00001 TABLE 1 Steel Chemical components (% by mass) No. C
Si Mn P S Al N Nb Ti Mo Cr Ni Cu, V, B Ca Moeq* Note A 0.064 0.22
1.64 0.008 0.0011 0.036 0.0039 0.065 0.014 0.29 0.08 0.02 -- --
1.58 Example B 0.052 0.29 1.74 0.009 0.0006 0.035 0.0034 0.052
0.013 0.38 0.11 0.12 V: 0.022 -- 1.77 Example C 0.070 0.46 1.88
0.007 0.0012 0.033 0.0032 0.071 0.017 0.24 0.23 0.21 V: 0.039,
0.0021 1.79 Example B: 0.0001 D 0.041 0.42 1.46 0.009 0.0014 0.039
0.0032 0.033 0.021 0.29 0.48 0.06 V: 0.090 0.0023 1.59 Example E
0.083 0.38 1.91 0.010 0.0023 0.042 0.0042 0.097 0.009 0.26 0.41
0.20 B: 0.0004 -- 1.89 Example F 0.035 0.02 2.16 0.010 0.0015 0.035
0.0029 0.042 0.041 0.29 0.37 0.40 Cu: 0.25 0.0024 2.11 Example G
0.162 0.22 1.42 0.014 0.0019 0.035 0.0027 0.060 0.013 0.01 0.38
0.28 Cu: 0.29 0.0022 1.26 Comparative Example H 0.046 0.36 1.15
0.008 0.0025 0.051 0.0035 0.046 0.009 0.32 0.26 0.42 V: 0.022,
0.0024 1.33 Comparative B: 0.0002 Example I 0.051 0.17 1.57 0.007
0.0032 0.036 0.0038 0.051 0.012 0.09 -- -- V: 0.055, -- 1.30
Comparative B: 0.0001 Example J 0.040 0.17 1.65 0.009 0.0029 0.040
0.0046 0.042 0.015 -- -- 0.18 V: 0.025, -- 1.27 Comparative Cu:
0.15 Example K 0.079 0.42 1.60 0.011 0.0012 0.046 0.0033 0.129
0.021 0.31 0.19 0.11 B: 0.0003 0.0026 1.62 Comparative Example L
0.063 0.22 1.64 0.009 0.0009 0.035 0.0028 0.054 0.069 0.18 0.28
0.10 -- -- 1.55 Comparative Example M 0.091 0.14 1.62 0.012 0.0007
0.037 0.0034 0.056 0.017 0.11 0.05 0.01 V: 0.055 0.0019 1.38
Example *Moeq(%) = Mo + 0.36Cr + 0.77Mn + 0.07Ni
TABLE-US-00002 TABLE 2 Hot-rolling step Rough Cooling step Heating
rolling Finish rolling First cooling*2 Steel Heating Thickness
Finish rolling Rolling Cooling Average Cooling stop sheet Steel
temperature of sheet temperature reduction Thickness start cooling
rate temperature Ms No. No. (.degree. C.) bar (mm) (.degree. C.) *1
(%) (mm) time (s) *4 (.degree. C./s) *5 (.degree. C.) (.degree. C.)
1 A 1059 51 768 74 8 2.4 111 373 406 2 A 1091 55 759 55 25 7.6 145
372 406 3 A 1099 51 777 61 16 4.8 122 372 406 4 A 1261 58 762 70 14
4.2 123 371 406 5 A 1158 59 761 61 23 7.0 124 366 406 6 A 1247 58
772 69 16 4.8 117 361 406 7 A 1388 53 758 70 16 4.8 125 366 406 8 A
1281 57 759 19 14 4.2 124 362 406 9 A 1232 60 762 67 16 4.8 68 367
406 10 A 1252 59 768 70 14 4.2 119 674 406 11 A 1264 57 769 69 16
4.8 128 375 406 12 A 1155 59 773 64 21 6.4 131 377 406 13 A 1164 56
765 55 25 7.6 140 361 406 14 A 1270 57 766 67 19 5.8 135 388 406 15
B 1195 58 776 81 11 3.3 114 374 404 16 C 1185 51 782 78 10 3.0 117
347 390 17 D 1182 52 801 62 18 5.5 125 375 415 18 E 1168 55 763 64
16 4.8 127 341 380 19 F 1300 51 772 50 21 6.4 137 354 391 20 G 1206
52 734 66 16 4.8 125 329 363 21 H 1291 58 814 79 11 3.3 119 376 420
22 I 1241 59 780 58 25 7.6 147 385 420 23 J 1193 54 772 55 22 6.7
125 376 422 24 K 1199 56 785 76 11 3.3 115 360 396 25 L 1156 52 785
66 14 4.2 123 359 404 26 M 1176 55 773 68 14 4.2 123 361 398
Cooling step Second cooling*2 Third cooling*3 Fourth cooling*3
Coiling step Steel Final surface Holding Average Cooling stop
Average Holding Coiling sheet temperature time cooling rate
temperature cooling rate time temperature No. *6 (.degree. C.) *7
(s) *8 (.degree. C./s) (.degree. C.) *9 (.degree. C./s) *10 (s) *11
(.degree. C.) Note 1 608 1.4 18 551 1.5 -- 538 Example 2 613 2.7 28
558 0.5 -- 536 Example 3 603 2.0 22 555 -- 28 522 Example 4 605 1.8
25 556 4.5 -- 468 Comparative Example 5 601 2.4 29 551 -- 12 522
Comparative Example 6 617 1.9 22 454 2.0 -- 323 Comparative Example
7 617 1.8 19 550 1.0 -- 536 Comparative Example 8 604 2.0 15 557
2.0 -- 537 Comparative Example 9 418 2.3 19 437 -- 28 424
Comparative Example 10 684 2.0 27 551 1.0 -- 531 Comparative
Example 11 460 2.1 17 552 1.0 -- 529 Comparative Example 12 613 1.0
20 445 0.5 -- 460 Comparative Example 13 603 2.6 55 553 1.0 -- 540
Comparative Example 14 607 2.4 30 405 0.5 -- 521 Comparative
Example 15 621 2.0 21 533 1.0 -- 514 Example 16 603 1.8 21 527 1.0
-- 508 Example 17 628 2.1 27 550 1.0 -- 526 Example 18 632 2.0 30
501 1.0 -- 471 Example 19 618 2.4 28 486 0.5 -- 451 Example 20 632
1.8 22 543 1.0 -- 521 Comparative Example 21 645 1.6 23 561 0.5 --
541 Comparative Example 22 693 2.3 20 604 0.5 -- 576 Comparative
Example 23 697 2.6 18 607 0.5 -- 580 Comparative Example 24 628 1.7
15 535 0.5 -- 513 Comparative Example 25 634 1.8 15 561 1.0 -- 538
Comparative Example 26 660 2.1 21 584 0.5 -- 566 Example *1)
Cumulative rolling reduction (%) at a temperature of 930.degree. C.
or less *2Surface temperature control of the steel sheet
*3Temperature control at half the thickness of the steel sheet by
heat-transfer calculation *4) Average cooling rate in the range of
600.degree. C. to 450.degree. C. (For steel sheet No. 10. average
cooling rate in the range of cooling start temperature to first
cooling stop temperature) *5) By heat-transfer calculation *6) By
measurement with surface thermometer *7) Holding time at a surface
temperature of 600.degree. C. or more *8) Average cooling rate in
the range of 750.degree. C. to 600.degree. C. *9) Average cooling
rate in the range of third cooling stop temperature to fourth
coiling temperature *10) Holding time from third cooling stop
temperature to fourth coiling temperature *11) Surface
temperature
TABLE-US-00003 TABLE 3 Inner layer Outer layer Second phase
Hardness of BF Rod-like outer Average Massive martensite M Steel
layer/maximum Maximum Fraction grain Fraction Fraction sheet Steel
Type hardness hardness Type (% by size (% by Average Maximum Aspect
*4 (% by No. No. *1 *2 *3 HV0.5 *1 area (.mu.m) area) size (.mu.m)
size (.mu.m) ratio area) 1 A TM 91 261 BF + M 95.2 3.9 4.3 1.5 4.4
4.0 0.5 2 A TM 92 268 BF + M + B 95.6 4.9 3.4 1.2 4.2 3.5 0.5 3 A
TM + TB 93 264 BF + M 95.3 5.0 4.2 1.4 4.2 3.5 0.5 4 A TM 94 269 BF
+ M 95.6 4.7 0.9 0.3 2.9 2.5 3.5 5 A TM 94 267 BF + M 98.3 4.8 1.2
0.5 2.9 3.0 0.5 6 A TM 91 264 BF + M 95.2 4.7 1.3 0.8 3.4 3.0 2.5 7
A TM 92 268 BF + M 94.6 11.9 4.9 1.7 6.2 3.0 0.5 8 A TM 95 263 BF +
M 94.9 10.5 0.6 0.3 7.5 3.0 4.5 9 A TM 91 263 BF + M 95.8 10.8 3.2
1.2 5.5 3.0 1.0 10 A M + B 99 306 BF + M 95.9 4.5 1.6 0.6 2.9 2.5
2.5 11 A TM 96 294 BF + B 95.0 4.5 0.0 -- -- -- -- 12 A TM 98 304
BF 100.0 4.6 0.0 -- -- -- -- 13 A TM 94 262 BF + M 95.0 5.0 0.5 0.2
2.7 3.5 4.5 14 A TM 93 269 BF 98.9 5.1 0.2 0.2 2.4 3.0 0.9 15 B TM
93 264 BF + M 95.5 4.0 3.4 1.2 3.9 3.5 1.1 16 C TM 91 264 BF + M
94.6 4.4 4.9 1.7 4.8 4.5 0.5 17 D TM 93 262 BF + M + B 91.5 4.9 3.9
1.4 4.4 2.5 1.6 18 E TM + TB 93 269 BF + M + B 88.3 4.4 4.2 1.4 4.2
2.0 0.5 19 F TM + TB 92 262 BF + M 94.4 4.8 4.0 1.4 4.6 3.0 1.6 20
G TM + TB 91 262 BF + B 80.0 5.0 0.0 -- -- -- -- 21 H TM 95 263 BF
+ P 90.0 13.2 0.0 -- -- -- -- 22 I TM 93 263 BF 100.0 5.3 0.0 -- --
-- -- 23 J TM 92 264 BF 100.0 5.3 0.0 -- -- -- -- 24 K TM 93 262 BF
+ M 94.8 4.5 4.1 1.4 4.5 4.0 1.1 25 L TM 95 268 BF + M + F 94.6
11.1 3.9 1.4 5.3 3.5 0.5 26 M TM 94 263 BF + M + B 95.5 4.8 2.9 1.0
3.8 3.0 0.6 Inner layer Second phase Tensile Others Tensile
properties properties of Steel Type 30- Toughness steel pipe
Variation in sheet *1: % by YS TS YR degreeYS vTrs YS TS YR
strength No. area (MPa) (MPa) (%) *5 (MPa) (.degree. C.) (MPa)
(MPa) (%) .DELTA.YS*6 Note 1 -- 580 699 83 558 -115 571 671 85 13
Example 2 B: 0.5 586 697 84 563 -85 583 671 87 20 Example 3 -- 585
697 84 568 -110 576 669 86 7 Example 4 -- 621 714 87 606 -120 588
692 85 -18 Comparative Example 5 -- 595 684 87 585 -110 538 656 82
-47 Comparative Example 6 -- 595 709 86 578 -75 585 688 85 7
Comparative Example 7 -- 581 717 81 564 -65 585 696 84 21
Comparative Example 8 -- 678 770 88 639 -20 619 746 83 -20
Comparative Example 9 -- 586 715 82 571 -65 583 694 84 12
Comparative Example 10 -- 613 704 87 591 -120 579 681 85 -12
Comparative Example 11 B: 5.0 590 671 88 575 -60 537 647 83 -38
Comparative Example 12 -- 622 699 89 595 -110 544 672 81 -51
Comparative Example 13 -- 606 722 84 590 -70 580 698 83 -11
Comparative Example 14 -- 581 683 86 566 -80 539 657 82 -28
Comparative Example 15 -- 551 735 75 530 -105 568 710 80 38 Example
16 -- 547 739 74 537 -100 603 718 84 66 Example 17 B: 3.0 623 742
84 605 -95 614 714 86 9 Example 18 B: 7.0 610 735 83 592 -90 598
712 84 6 Example 19 -- 521 754 69 516 -110 584 730 80 68 Example 20
B: 20 548 615 89 529 -40 464 595 78 -65 Comparative Example 21 P:
10 534 607 88 509 -50 462 585 79 -47 Comparative Example 22 -- 566
636 89 565 -100 491 613 80 -75 Comparative Example 23 -- 606 666 91
600 -120 532 641 83 -67 Comparative Example 24 -- 643 739 87 628
-80 571 714 80 -57 Comparative Example 25 F: 1.0 621 739 84 609 -50
594 715 83 -16 Comparative Example 26 B: 1.0 606 721 84 579 -95 593
698 85 14 Example *1) F: Ferrite, P: Pearlite, B: Bainite, BF:
Bainitic ferrite, M: Martensite, TM: Tempered martensite, TB:
Tempered bainite *2) (Hardness at a depth of 0.5 mm from a
surface)/(Maximum hardness in the thickness direction) *3) Maximum
hardness in the thickness direction *4) (Amount of martensite
having an aspect raito of 5.0 or more)/(Total amount of martensite)
*5) Yield strength at 30 degrees with respect to rolling direction
*6.DELTA.YS = YS of steel pipe - 30-degree YS of steel sheet
[0114] All the examples provided high-strength high-toughness
hot-rolled steel sheets having a low yield ratio without particular
heat treatment. These hot-rolled steel sheets had a yield strength
of 480 MPa or more at an angle of 30 degrees with the rolling
direction, a tensile strength of 600 MPa or more in the sheet width
direction, high toughness represented by a fracture transition
temperature vTrs of -80.degree. C. or less, and a yield ratio of
85% or less. The comparative examples outside the scope of the
embodiments of the present disclosure could not provide hot-rolled
steel sheets having the desired characteristics because of low
toughness or a high yield ratio.
[0115] The examples provided hot-rolled steel sheets that had
little strength reduction due to pipe manufacturing even after
formed into steel pipes by pipe manufacturing and are suitable as
materials for spiral steel pipes and electric-resistance-welded
(ERW) pipes.
* * * * *