U.S. patent application number 16/329163 was filed with the patent office on 2019-07-04 for h section and method for manufacturing same.
This patent application is currently assigned to NIPPON STEEL & SUMITOMO METAL CORPORATION. The applicant listed for this patent is NIPPON STEEL & SUMITOMO METAL CORPORATION. Invention is credited to Kazutoshi ICHIKAWA, Masaki MIZOGUCHI, Tetsuya SEIKE, Hirokazu SUGIYAMA.
Application Number | 20190203309 16/329163 |
Document ID | / |
Family ID | 62626651 |
Filed Date | 2019-07-04 |
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United States Patent
Application |
20190203309 |
Kind Code |
A1 |
MIZOGUCHI; Masaki ; et
al. |
July 4, 2019 |
H SECTION AND METHOD FOR MANUFACTURING SAME
Abstract
This H section includes, as a chemical composition, C, Si, Mn,
Nb, V, Ti, and N; in which the H section includes, as a
metallographic structure, ferrite of 60 area % to less than 100
area %, an average grain size of this ferrite is 1 .mu.m to 30
.mu.m, a thickness of a flange is 20 mm to 140 mm, tensile yield
stress is 385 MPa to 530 MPa, and Charpy absorbed energy at
-20.degree. C. is 100 J or more.
Inventors: |
MIZOGUCHI; Masaki; (Tokyo,
JP) ; ICHIKAWA; Kazutoshi; (Tokyo, JP) ;
SUGIYAMA; Hirokazu; (Tokyo, JP) ; SEIKE; Tetsuya;
(Tokyo, JP) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
NIPPON STEEL & SUMITOMO METAL CORPORATION |
Tokyo |
|
JP |
|
|
Assignee: |
NIPPON STEEL & SUMITOMO METAL
CORPORATION
Tokyo
JP
|
Family ID: |
62626651 |
Appl. No.: |
16/329163 |
Filed: |
December 21, 2017 |
PCT Filed: |
December 21, 2017 |
PCT NO: |
PCT/JP2017/045965 |
371 Date: |
February 27, 2019 |
Current U.S.
Class: |
1/1 |
Current CPC
Class: |
C21D 8/0226 20130101;
B21B 1/088 20130101; C22C 38/001 20130101; C22C 38/46 20130101;
C22C 38/00 20130101; C22C 38/28 20130101; C22C 38/04 20130101; C21D
2211/005 20130101; C22C 38/06 20130101; C22C 38/44 20130101; C22C
38/12 20130101; C21D 8/005 20130101; C22C 38/002 20130101; C22C
38/42 20130101; C21D 2211/001 20130101; C22C 38/26 20130101; C21D
2211/008 20130101; C22C 38/24 20130101; C22C 38/48 20130101; C22C
38/50 20130101; C22C 38/58 20130101; C22C 38/16 20130101; C22C
38/14 20130101; C22C 38/08 20130101; C22C 38/02 20130101 |
International
Class: |
C21D 8/00 20060101
C21D008/00; C22C 38/02 20060101 C22C038/02; C22C 38/04 20060101
C22C038/04; C22C 38/00 20060101 C22C038/00; C22C 38/06 20060101
C22C038/06; C22C 38/14 20060101 C22C038/14; C22C 38/12 20060101
C22C038/12; C22C 38/08 20060101 C22C038/08; C22C 38/16 20060101
C22C038/16; C22C 38/42 20060101 C22C038/42; C22C 38/44 20060101
C22C038/44; C22C 38/46 20060101 C22C038/46; C22C 38/48 20060101
C22C038/48; C22C 38/50 20060101 C22C038/50 |
Foreign Application Data
Date |
Code |
Application Number |
Dec 21, 2016 |
JP |
2016-248181 |
Claims
1. An H section, wherein a steel comprises, as a chemical
composition, by mass %, C: 0.05% to 0.160%, Si: 0.01% to 0.60%, Mn:
0.80% to 1.70%, Nb: 0.005% to 0.050%, V: 0.05% to 0.120%, Ti:
0.001% to 0.025%, N: 0.0001% to 0.0120%, Cr: 0% to 0.30%, Mo: 0% to
0.20%, Ni: 0% to 0.50%, Cu: 0% to 0.35%, W: 0% to 0.50%, Ca: 0% to
0.0050%, Zr: 0% to 0.0050%, Al: limited to 0.10% or less, B:
limited to 0.0003% or less, and a remainder including Fe and
impurities, wherein C, Mn, Cr, Mo, V, Ni, and Cu in the chemical
composition satisfy 0.30.ltoreq.Ceq.ltoreq.0.48, when
Ceq=C+Mn/6+(Cr+Mo+V)/5+(Ni+Cu)/15, the steel includes, as a
metallographic structure, by area fraction, ferrite of 60% to less
than 100%, a mixed structure MA of martensite and austenite being
limited to 3.0% or less, and a structure other than the ferrite and
the MA being limited to 37% or less, an average grain size of the
ferrite is 1 .mu.m to 30 .mu.m, a shape is an H-shape and a
thickness of a flange is 20 mm to 140 mm, when being seen in a cut
section orthogonal to a rolling direction, at a position of (1/6)F
from an end surface of the flange in the width direction, tensile
yield stress is 385 MPa to 530 MPa and maximum tensile strength is
490 MPa to 690 MPa, when a length of the flange in a width
direction is F, and at the position of (1/6)F and at a position of
(114)t.sub.2 from an outer surface of the flange in a thickness
direction, absorbed energy in a Charpy test at -20.degree. C. is
100 J or more, when the thickness of the flange is t.sub.2.
2. The H section according to claim 1, comprising, as the chemical
composition, by mass %, Nb: more than 0.02% to 0.050%.
3. The H section according to claim 1, comprising, as the chemical
composition, by mass %, N: more than 0.005% to 0.0120%.
4. The H section according to claim 1, comprising, as the chemical
composition, by mass %, Cu: limited to less than 0.03%.
5. The H section according to claim 1, comprising, as the chemical
composition, by mass %, Al: limited to less than 0.003%.
6. The H section according to claim 1, wherein the thickness of the
flange is 25 mm to 140 mm.
7. A method for manufacturing the H section according to claim 1,
comprising: steelmaking to obtain a molten steel which has the
chemical composition according to claim 1; casting the molten steel
after the steelmaking to obtain a steel piece; heating the steel
piece after the casting to 1,100.degree. C. to 1,350.degree. C.;
hot rolling on the steel piece after the heating to form a shape,
when being seen in a cut section orthogonal to a rolling direction,
into an H-shape under conditions, wherein a cumulative rolling
reduction at a position of (1/6)F from an end surface of a flange
in a width direction within more than 900.degree. C. and
1,100.degree. C. or less is 20% or more, the cumulative rolling
reduction at the position within 730.degree. C. to 900.degree. C.
is 15% or more, and the rolling ends at 730.degree. C. or more; and
cooling a hot rolled material after the hot rolling by natural
cooling.
Description
TECHNICAL FIELD OF THE INVENTION
[0001] The present invention relates to a thick H section, which
has excellent strength and low temperature toughness, and a method
for manufacturing the same. Priority is claimed on Japanese Patent
Application No. 2016-248181, filed on Dec. 21, 2016, the content of
which is incorporated herein by reference.
RELATED ART
[0002] Recently, buildings such as high-rise buildings are becoming
large and high, and thick steels have been utilized as strength
members required for the structure. However, generally, in ferrous
materials, as the thickness of a product increases, it becomes
difficult to ensure the strength, and it is also difficult to
ensure the toughness.
[0003] In regard to such a problem, in Patent Document 1, a
technology of ensuring the toughness by utilizing an effect of
refining prior austenite grains due to a Ca--Al-based oxide and
obtaining a steel, in which high strength is ensured by applying
accelerated cooling, is proposed.
[0004] In addition, in Patent Document 2, a technology of ensuring
the toughness by utilizing the effect of refining prior austenite
grains due to a Mg--S-based inclusion and obtaining a steel, in
which high strength is ensured by applying accelerated cooling, is
proposed.
[0005] However, when a thick steel plate is manufactured, if
accelerated cooling is applied after hot rolling, the cooling rate
becomes slow inside the steel plate compared to its surface, so
that a significant difference is caused between the surface and the
inside in the temperature history during cooling, and a difference
is caused in mechanical properties such as strength, ductility,
toughness, depending on a portion of a steel.
[0006] In addition, for large buildings, it is desired to use thick
H sections. However, these H sections are unique in shape.
Universal rolling and the like are applied to form a steel piece
into an H-shape. However, rolling conditions (temperature and
rolling reduction) are limited in universal rolling. Therefore, in
a case where an H section is manufactured, particularly in a case
where a thick H section having a thickness of a flange of 20 mm or
more is manufactured, it is not easy to control mechanical
properties compared to general thick steel plates (thick steel
plates).
[0007] In regard to such a problem, in Patent Documents 3 and 4,
methods of ensuring homogeneous mechanical properties of a steel
piece by performing natural cooling after hot rolling of the steel
piece, in which the amount of C is reduced and B is added, are
proposed.
[0008] In addition, in Patent Documents 5 to 8, methods for
manufacturing a thick H section or an H section for the purpose of
high strength, high toughness, and the like are disclosed.
PRIOR ART DOCUMENT
Patent Document
[0009] [Patent Document 1] Japanese Patent No. 5655984
[0010] [Patent Document 2] Japanese Patent No. 5867651
[0011] [Patent Document 3] Japanese Unexamined Patent Application,
First Publication No. 2003-328070
[0012] [Patent Document 4] Japanese Unexamined Patent Application,
First Publication No. 2011-106006
[0013] [Patent Document 5] Japanese Unexamined Patent Application,
First Publication No. H11-158543
[0014] [Patent Document 6] Japanese Unexamined Patent Application,
First Publication No. H11-335735
[0015] [Patent Document 7] Japanese Unexamined Patent Application,
First Publication No. 2016-84524
[0016] [Patent Document 8] Japanese Unexamined Patent Application,
First Publication No. H10-68016
DISCLOSURE OF THE INVENTION
Problems to be Solved by the Invention
[0017] In the related art, in thick H sections having a thickness
of a flange of 20 mm or more, it has not been easy to control
mechanical properties. Therefore, such thick H sections have been
required to satisfy toughness no more than at room temperature or
at 0.degree. C. However, recently, in consideration of usage in a
cold district and the like, thick H sections are required to have
excellent toughness at a lower temperature. Furthermore, in
consideration of strength per unit weight as structural materials,
thick H sections are also required to have yield stress
(specifically, yield strength or 0.2% proof stress) of 385 MPa or
more.
[0018] The present invention has been made in consideration of such
circumstances, and an object thereof is to provide a thick H
section, which has excellent strength and low temperature
toughness, and a method for manufacturing the same.
Means for Solving the Problem
[0019] The gist of the present invention is as follows.
[0020] (1) According to an aspect of the present invention, there
is provided an H section; in which a steel includes, as a chemical
composition, by mass %, C: 0.05% to 0.160%, Si: 0.01% to 0.60%, Mn:
0.80% to 1.70%, Nb: 0.005% to 0.050%, V: 0.05% to 0.120%, Ti:
0.001% to 0.025%, N: 0.0001% to 0.0120%, Cr: 0% to 0.30%, Mo: 0% to
0.20%, Ni: 0% to 0.50%, Cu: 0% to 0.35%, W: 0% to 0.50%, Ca: 0% to
0.0050%, Zr: 0% to 0.0050%, Al: limited to 0.10% or less, B:
limited to 0.0003% or less, and a remainder including Fe and
impurities; in which C, Mn, Cr, Mo, V, Ni, and Cu in the chemical
composition satisfy 0.30.ltoreq.Ceq.ltoreq.0.48, when
Ceq=C+Mn/6+(Cr+Mo+V)/5+(Ni+Cu)/15 is established; the steel
includes, as a metallographic structure, by area fraction, ferrite
of 60% to less than 100%, a mixed structure MA of martensite and
austenite being limited to 3.0% or less, and a structure other than
the ferrite and the MA being limited to 37% or less; an average
grain size of the ferrite is 1 .mu.m to 30 .mu.m; a shape is an
H-shape and a thickness of a flange is 20 mm to 140 mm, when being
seen in a cut section orthogonal to a rolling direction; at a
position of (1/6)F from an end surface of the flange in the width
direction, tensile yield stress is 385 MPa to 530 MPa and maximum
tensile strength is 490 MPa to 690 MPa, when a length of the flange
in a width direction is F; and at the position of (1/6)F and at a
position of (1/4)t.sub.2 from an outer surface of the flange in a
thickness direction, absorbed energy in a Charpy test at
-20.degree. C. is 100 J or more, when the thickness of the flange
is t.sub.2.
[0021] (2) The H section according to (1) may include, as the
chemical composition, by mass %, Nb: more than 0.02% to 0.050%.
[0022] (3) The H section according to (1) or (2) may include, as
the chemical composition, by mass %, N: more than 0.005% to
0.0120%.
[0023] (4) The H section according to any one of (1) to (3) may
include, as the chemical composition, by mass %, Cu: limited to
less than 0.03%.
[0024] (5) The H section according to any one of (1) to (4) may
include, as the chemical composition, by mass %, Al: limited to
less than 0.003%.
[0025] (6) In the H section according to any one of (1) to (5), the
thickness of the flange may be 25 mm to 140 mm.
[0026] (7) According to another aspect of the present invention,
there is provided a method for manufacturing the H section
according to any one of (1) to (6), including steelmaking to obtain
a molten steel which has the chemical composition according to any
one of (1) to (5); casting the molten steel after the steelmaking
to obtain a steel piece; heating the steel piece after the casting
to 1,100.degree. C. to 1,350.degree. C.; hot rolling on the steel
piece after the heating to form a shape, when being seen in a cut
section orthogonal to a rolling direction, into an H-shape under
conditions, in which a cumulative rolling reduction at a position
of (1/6)F from an end surface of a flange in a width direction is
20% or more within more than 900.degree. C. and 1,100.degree. C. or
less, the cumulative rolling reduction at the position is 15% or
more within 730.degree. C. to 900.degree. C., and the rolling ends
at 730.degree. C. or more; and cooling a hot rolled material after
the hot rolling by natural cooling.
Effects of the Invention
[0027] According to the aspect of the present invention, it is
possible to provide a thick H section, which has excellent strength
and low temperature toughness, and a method for manufacturing the
same.
BRIEF DESCRIPTION OF THE DRAWINGS
[0028] FIG. 1 is a schematic cross-sectional view explaining a
position at which a test piece of an H section according to an
embodiment of the present invention is collected.
[0029] FIG. 2 is a flowchart showing a method for manufacturing an
H section according to the embodiment of the present invention.
EMBODIMENT OF THE INVENTION
[0030] Hereinafter, a suitable embodiment of the present invention
will be described in detail. However, the present invention is not
limited to only the configurations disclosed in the present
embodiment, and various changes can be made within a range not
departing from the gist of the present invention. In addition, in
the following numerical limitation ranges, the lower limit value
and the upper limit value are included in the range thereof. In
numerical values expressed with "more than" or "less than", the
value is not included in the numerical value range thereof. The
symbol "%" related to the amount of each element means "mass
%".
[0031] As described above, until now, thick H sections having a
thickness of a flange of 20 mm or more have been required to
satisfy toughness no more than at room temperature or at 0.degree.
C. However, currently, in consideration of usage in a cold district
and the like, thick H sections are required to have excellent
toughness at a lower temperature, such as approximately -20.degree.
C. Furthermore, in consideration of strength per unit weight as
structural materials, thick H sections are also required to have
yield stress (specifically, yield strength or 0.2% proof stress) of
385 MPa or more.
[0032] Therefore, in regard to a thick H section (there may
hereinafter be a case where it is disclosed as a steel),
particularly in regard to a flange which is an important portion in
the structure of an H section, the inventors have found the
following knowledge by investigating a steel composition (a
chemical composition of a steel) affecting the strength and the low
temperature toughness, and the influence of a steel structure (a
metallographic structure of a steel). In the present embodiment,
strength means tensile yield stress and the maximum tensile
strength, and low temperature toughness means absorbed energy in a
Charpy test at -20.degree. C.
[0033] First, an excessively increase in hardenability caused by
adding an alloying element encourages the generation of a
martensite-austenite mixed structure (hereinafter, it will be
disclosed as an MA) in a steel, and which leads to deterioration in
low temperature toughness. Particularly, since B in an alloying
element noticeably tends to encourage the generation of an MA, it
is effective that B is not actively added and is limited to the
level of impurities or less.
[0034] In addition, in order to realize the high yield stress
(yield strength or 0.2% proof stress) and to improve the toughness
at -20.degree. C. at the same time, it is effective that Nb is
added. Since Nb increases the strength of a steel through
precipitation strengthening, there is no need to excessively
increase the hardenability, and strength of a steel can be
increased without encouraging the generation of an MA. In addition,
Nb has effects of suppressing recrystallization of austenite during
hot rolling, accumulating strain in a steel caused through rolling,
and bringing grain refinement of ferrite after transformation.
[0035] In addition, in order to improve the toughness at
-20.degree. C., it is effective that V is added. V has effects of
being precipitated as carbonitride (VC, VN, or a composite
thereof), functioning as a nucleation site of ferrite, and bringing
grain refinement of ferrite.
[0036] In addition, when Mn is added, strength and low temperature
toughness are further improved. Furthermore, for achieving both the
high strength and the low temperature toughness, it is important to
control the area fraction of ferrite, the area fraction of MA, the
average grain size of ferrite, and the like as the steel structure
after the steel composition is controlled.
[0037] In order to stably control a steel structure, when hot
rolling a steel piece in which the steel composition is controlled,
it is necessary to apply sufficient rolling strain to each of a
recrystallization temperature range and a non-recrystallization
temperature range of austenite. Specifically, in a temperature
range of more than 900.degree. C. and 1,100.degree. C. or less, hot
rolling in which the cumulative rolling reduction is 20% or more is
performed. Moreover, in a temperature range of 900.degree. C. or
less, hot rolling in which the cumulative rolling reduction is 15%
or more is performed. Through rolling at a temperature more than
900.degree. C., austenite grains are subjected to grain refinement
to deteriorate the hardenability, so that the generation amount of
an MA, and the like are kept low. Through rolling at 900.degree. C.
or less, ferrite is subjected to grain refinement by applying a
large amount of strain to the inside of a steel so as to increase
the frequency of nucleation of ferrite.
[0038] In addition, in order to stably control a steel structure,
when cooling after hot rolling, it is preferable to have a small
difference between the cooling rates of a surface of a steel and
inside thereof. In a case where a steel is natural cooled without
accelerated cooling after hot rolling, the cooling rate is
decreased in both a surface and the inside of the steel, so that
the difference therebetween is also reduced. For example, in an H
section having a thickness of a flange of 20 mm, if a steel is
subjected to natural cooling after hot rolling, both the average
cooling rates from 800.degree. C. to 500.degree. C. of a surface of
a steel and inside thereof become 1.degree. C./sec or less.
[0039] In a case where the cooling rate after hot rolling is slow,
generally, it is not easy to ensure the yield stress and the low
temperature toughness at the same time. However, both the yield
stress and the low temperature toughness can be achieved by
optimally controlling the steel composition and manufacturing
conditions. For example, as a steel composition, the C content is
0.05% to 0.160%, B is not added and is limited to the level of
impurities or less, Nb and V are actively added, the amounts of the
alloying elements such as Mn, Ti, and N are appropriately
controlled, and a carbon equivalent Ceq is controlled to be a range
of 0.30 to 0.48. Furthermore, as the steel structure, the area
fraction of ferrite, the area fraction of MA, the average grain
size of ferrite, and the like are elaborated by optimally
controlling the manufacturing conditions. As a result, a thick H
section having excellent strength and low temperature toughness can
be obtained.
[0040] Hereinafter, an H section according to the present
embodiment will be described. First, the steel composition and
reasons for limiting numerical ranges will be described in
detail.
[0041] The H section according to the present embodiment includes,
as a chemical composition, base elements and includes optional
elements as necessary. The remainder includes Fe and
impurities.
[0042] In the chemical composition of the H section according to
the present embodiment, C, Si, Mn, Nb, V, Ti, and N are the base
elements (main alloying elements).
[0043] (C: 0.05% to 0.160%)
[0044] C (Carbon) is an element which is effective for
strengthening of a steel. Therefore, the lower limit for the C
content is set to 0.05%. Preferably, the lower limit for the C
content is set to 0.060%, 0.070%, or 0.080%. On the other hand,
when the C content exceeds 0.160%, and which leads to deterioration
in low temperature toughness. Therefore, the upper limit for the C
content is set to 0.160%. In order to further improve the low
temperature toughness, the upper limit for the C content is
preferably set to 0.140%, 0.130%, or 0.120%.
[0045] (Si: 0.01% to 0.60%)
[0046] Si (Silicon) is a deoxidizing element and is an element
which also contributes to improvement of strength. Therefore, the
lower limit for the Si content is set to 0.01%. Preferably, the
lower limit for the Si content is set to 0.05%, 0.10%, or 0.15%. On
the other hand, when the Si content exceeds 0.60%, the generation
of an MA is encouraged, and which leads to deterioration in low
temperature toughness. Therefore, the upper limit for the Si
content is set to 0.60%. In order to further improve the low
temperature toughness, the upper limit for the Si content is
preferably set to 0.40% or 0.30%.
[0047] (Mn: 0.80% to 1.70%)
[0048] Mn (Manganese) is an element which contributes to
improvement of strength. Therefore, the lower limit for the Mn
content is set to 0.80%. In order to further increase the strength,
the lower limit for the Mn content is preferably set to 1.0%, 1.1%,
or 1.2%. On the other hand, when the Mn content exceeds 1.70%, the
hardenability is excessively increased, generation of an MA is
encouraged, and the low temperature toughness is impaired.
Therefore, the upper limit for the Mn content is set to 1.70%.
Preferably, the upper limit for the Mn content is set to 1.60% or
1.50%.
[0049] (Nb: 0.005% to 0.050%)
[0050] Nb (Niobium) is an element which suppresses
recrystallization of austenite at the time of hot rolling,
contributes to grain refinement of ferrite by accumulating work
strain in a steel, and contributes to improvement of strength
through precipitation strengthening. Therefore, the lower limit for
the Nb content is set to 0.005%. Preferably, the lower limit for
the Nb content is set to 0.010%, more than 0.020%, 0.025%, or
0.030%. However, when the Nb content exceeds 0.050%, and which may
lead to significant deterioration in low temperature toughness.
Therefore, the upper limit for the Nb content is set to 0.050%.
Preferably, the upper limit for the Nb content is set to 0.045%,
0.043%, or 0.040%. In a case where Nb is not intentionally added,
the Nb content included as an impurity is set to less than 0.005%.
In order to set the Nb content to 0.005% or more, Nb is
intentionally included in a steel.
[0051] (V: 0.05% to 0.120%)
[0052] V (Vanadium) is an element which has effects of being
precipitated as carbonitride inside grains of austenite, acting as
transformation nuclei with respect to ferrite, and refining ferrite
grains. Therefore, the lower limit for the V content is set to
0.05%. Preferably, the lower limit for the V content is set to more
than 0.05%, 0.06%, or 0.07%. However, when the V content exceeds
0.120%, the low temperature toughness may be impaired by coarsening
precipitates. Therefore, the upper limit for the V content is set
to 0.120%. Preferably, the upper limit for the V content is set to
0.110% or 0.100%.
[0053] (Ti: 0.001% to 0.025%)
[0054] Ti (Titanium) is an element which forms TiN and fixes N in a
steel. Therefore, the lower limit for the Ti content is set to
0.001%. In order to further refine austenite due to a pinning
effect of TiN, the lower limit for the Ti content is preferably set
to 0.005%, 0.007%, or 0.010%. On the other hand, when the Ti
content exceeds 0.025%, coarse TiN is generated and the low
temperature toughness is impaired. Therefore, the upper limit for
the Ti content is set to 0.025%. Preferably, the upper limit for
the Ti content is set to 0.020%, 0.015%, or 0.012%.
[0055] In addition, in a case where Al is not actively added, Ti
serves as a deoxidizing element, thereby remaining N which is not
bonded to Ti. However, this N is precipitated as V carbonitride
having Ti oxide as nuclei. That is, since Ti serves as a
deoxidizing element and Ti oxide is precipitated, precipitation of
V carbonitride is promoted, and low temperature toughness can be
improved.
[0056] (N: 0.0001% to 0.0120%)
[0057] N (Nitrogen) is an element which forms TiN or VN and
contributes to grain refinement of a structure or precipitation
strengthening. Therefore, the lower limit for the N content is set
to 0.0001%. Preferably, the lower limit for the N content is set to
0.0020%, 0.0035%, more than 0.0050%, or 0.0060%. However, when the
N content exceeds 0.0120%, the low temperature toughness is
deteriorated, and surface cracking at the time of casting or
material defect due to strain aging of a manufactured steel is
caused. Therefore, the upper limit for the N content is set to
0.0120%. Preferably, the upper limit for the N content is set to
0.0110%, 0.0100%, or 0.0090%.
[0058] The H section according to the present embodiment contains
impurities as a chemical composition. "Impurities" indicate
elements incorporated from ore or scrap as a raw material, or the
manufacturing environment, when a steel is industrially
manufactured. For example, impurities mean elements such as Al, B,
P, S, and O. In these impurities, it is preferable that Al and B
are limited as follows in order to sufficiently exhibit the effects
of the present embodiment. In addition, since it is preferable that
the amounts of impurities are small, there is no need to limit the
lower limit value, and the lower limit value for impurities may be
0%.
[0059] (Al: 0.10% or less)
[0060] Al (Aluminum) is an element used as a deoxidizing element.
However, when the Al content exceeds 0.10%, oxide is coarsened and
becomes origins of brittle fracture, so that low temperature
toughness is deteriorated. Therefore, the upper limit for the Al
content is limited to 0.10%. In addition, in a case where Al is not
actively used as a deoxidizing element, Ti serves as the
deoxidizing element, and Ti oxide is precipitated in a steel. This
Ti oxide functions as a nucleation site of V carbonitride, refines
the grain size of ferrite, and contributes to improvement of low
temperature toughness. Therefore, the upper limit for the Al
content may be limited to less than 0.003%, 0.002%, or 0.001% in a
case of including Al as impurities, instead of using Al as a
deoxidizing element. Generally, in order to set the Al content to
0.003% or more, Al is intentionally included in a steel.
[0061] (B: 0.0003% or less)
[0062] B (Boron) increases the hardenability, encourages the
generation of an MA, and deteriorates the low temperature
toughness. Therefore, in the present embodiment, B is not actively
added and is limited to the level of impurities or less. The upper
limit for the B content is limited to 0.0003%. Preferably, the
upper limit for the B content is limited to less than 0.0003%,
0.0002%, or 0.0001%. Generally, in order to set the B content to
more than 0.0003%, B is intentionally included in a steel.
[0063] (P: 0.03% or less, S: 0.02% or less, and 0: 0.005% or
less)
[0064] P (Phosphorus), S (Sulfur), and 0 (Oxygen) are impurities. P
and S encourage the weld cracking through solidifying segregation
and deteriorates the low temperature toughness. Preferably, the
upper limit for the P content is limited to 0.03%, 0.02%, or 0.01%.
In addition, preferably, the upper limit for the S content is
limited to 0.02% or 0.01%. O deteriorates the low temperature
toughness by being solid-solubilized in a steel, and deteriorates
the low temperature toughness through coarsening of oxide
particles. Preferably, the upper limit for the O content is limited
to 0.005%, 0.004%, or 0.003%.
[0065] In addition to the base elements and the impurities
described above, the H section according to the present embodiment
may contain optional elements. For example, instead of a part of Fe
which is the remainder as described above, Cr, Mo, Ni, Cu, W, Ca,
Zr, Mg, and/or REM may be contained as optional elements. These
optional elements may be contained in accordance with their
purpose. Thus, there is no need to limit the lower limit values for
these optional elements, and the lower limit values may be 0%. In
addition, even if these optional elements are contained as
impurities, the foregoing effects are not impaired.
[0066] (Cr: 0% to 0.30%)
[0067] Cr (Chromium) is an element which contributes to improvement
of strength. As necessary, the Cr content may be 0% to 0.30%. In
order to further improve the strength, the lower limit for the Cr
content is preferably set to 0.01%, 0.05%, or 0.10%. On the other
hand, when the Cr content exceeds 0.30%, the generation of an MA
may be encouraged and the low temperature toughness may be
deteriorated. Therefore, preferably, the upper limit for the Cr
content is set to 0.30%, 0.25%, or 0.20%.
[0068] (Mo: 0% to 0.20%)
[0069] Mo (Molybdenum) is an element which is solid-solubilized in
a steel and contributes to improvement of strength. As necessary,
the Mo content may be 0% to 0.20%. In order to further improve the
strength, the lower limit for the Mo content is preferably set to
0.01%, 0.05%, or 0.10%. However, when the Mo content exceeds 0.20%,
the generation of an MA may be encouraged, and which may lead to
deterioration in low temperature toughness. Therefore, the upper
limit for the Mo content is preferably set to 0.20%, 0.17%, or
0.15%.
[0070] (Ni: 0% to 0.50%)
[0071] Ni (Nickel) is an element which is solid-solubilized in a
steel and contributes to improvement of strength. As necessary, the
Ni content may be 0% to 0.50%. In order to further improve the
strength, the lower limit for the Ni content is preferably set to
0.01%, 0.05%, or 0.10%. However, when the Ni content exceeds 0.50%,
the hardenability may be increased, the generation of an MA may be
encouraged, and the low temperature toughness may be deteriorated.
Therefore, the upper limit for the Ni content is preferably set to
0.50%, 0.30%, or 0.20%.
[0072] (Cu: 0% to 0.35%)
[0073] Cu (Copper) is an element which contributes to improvement
of strength. As necessary, the Cu content may be 0% to 0.35%.
However, addition of Cu may encourage the generation of an MA and
may deteriorate the low temperature toughness. Therefore,
preferably, the Cu content is limited to 0.30% or less, 0.20% or
less, or 0.10% or less. Alternatively, the Cu content may be
limited to less than 0.03% or less than 0.01% which is the level of
impurities.
[0074] (W: 0% to 0.50%)
[0075] W (Tungsten) is an element which is solid-solubilized in a
steel and contributes to improvement of strength. As necessary, the
W content may be 0% to 0.50%. Preferably, the lower limit for the W
content is set to 0.001%, 0.01%, or 0.10%. However, when the W
content exceeds 0.50%, the generation of an MA may be encouraged
and the low temperature toughness may be deteriorated. Therefore,
preferably, the upper limit for the W content is set to 0.50%,
0.40%, or 0.30%. In a case where W is not intentionally added, the
W content included as an impurity is set to less than 0.001%. In
order to set the W content to 0.001% or more, W is intentionally
included in a steel.
[0076] (Ca: 0% to 0.0050%)
[0077] Ca (Calcium) is an element which is effective for
controlling the form of sulfide, suppresses generation of coarse
MnS, and contributes to improvement of low temperature toughness.
As necessary, the Ca content may be set 0% to 0.0050%. Preferably,
the lower limit for the Ca content is set to 0.0001%, 0.0005%, or
0.0010%. On the other hand, when the Ca content exceeds 0.0050%,
the low temperature toughness may be deteriorated. Therefore,
preferably, the upper limit for the Ca content is set to 0.0050%,
0.0040%, or 0.0030%.
[0078] (Zr: 0% to 0.0050%)
[0079] Zr (Zirconium) is an element which is precipitated as
carbide, nitride, or a composite thereof and contributes to
precipitation strengthening. As necessary, the Zr content may be
set 0% to 0.0050%. Preferably, the lower limit for the Zr content
is set to 0.0001%, 0.0005%, or 0.0010%. On the other hand, when the
Zr content exceeds 0.0050%, carbide or nitride of Zr may be
coarsened and the low temperature toughness may be deteriorated.
Therefore, preferably, the upper limit for the Zr content is set to
0.0050%, 0.0040%, or 0.0030%. In a case where Zr is not
intentionally added, the Zr content included as an impurity is set
to less than 0.0001%. In order to set the Zr content to 0.0001% or
more, Zr is intentionally included in a steel.
[0080] (Mg: 0% to 0.0050%, and REM: 0% to 0.0050%)
[0081] Mg (Magnesium) and REM (rare earth metal) are elements which
contribute to improvement of toughness of a base material or
toughness of a welded heat-affected zone (HAZ). As necessary, the
Mg content may be 0% to 0.0050%, and the REM content may be 0% to
0.0050%. Preferably, the lower limit for the Mg content is set to
0.0005%, 0.0010%, or 0.0020%, and the lower limit for the REM
content is set to 0.0005%, 0.0010%, or 0.0020%. On the other hand,
preferably, the upper limit for the Mg content is set to 0.0040%,
0.0030%, or 0.0025%, and the upper limit for the REM content is set
to 0.0040%, 0.0030%, or 0.0025%.
[0082] (Ceq: 0.30 to 0.48)
[0083] In the H section according to the present embodiment, from
the view of ensuring the strength, the carbon equivalent Ceq is
controlled. Specifically, when the Ceq is set as the following
Expression 1, C, Mn, Cr, Mo, V, Ni, and Cu, by mass %, in the
chemical composition of an H section satisfy
0.30.ltoreq.Ceq.ltoreq.0.48. If the Ceq is less than 0.30, strength
becomes insufficient. Therefore, the lower limit for Ceq is set to
0.30. Preferably, the lower limit for Ceq is set to 0.32%, 0.34%,
or 0.35%. On the other hand, when Ceq exceeds 0.48, the low
temperature toughness is deteriorated. Therefore, the upper limit
for Ceq is set to 0.48. Preferably, the upper limit for Ceq is set
to 0.45%, 0.43%, or 0.40%. When the Ceq is calculated by the
following Expression 1, the Ceq may be calculated by substituting 0
for the value in Expression 1, for an element in which the amount
in a steel is equal to or less than a detection limit
Ceq=C+Mn/6+(Cr+Mo+V)/5+(Ni+Cu)/15 (Expression 1)
[0084] The steel composition described above may be measured by a
general analysis method for a steel. For example, the steel
composition may be measured by using ICP-AES (inductively coupled
plasma-atomic emission spectrometry). C and S may be measured by
using a combustion-infrared absorption method, N may be measured by
using an inert gas fusion-thermal conductivity method, and O may be
measured by using an inert gas fusion-non dispersive infrared
absorption method.
[0085] Next, the steel structure and reasons for limiting numerical
ranges of the H section according to the present embodiment will be
described in detail.
[0086] In the H section according to the present embodiment, the
steel structure includes ferrite of 60% to less than 100% by area
fraction, a mixed structure MA of martensite and austenite being
limited to 3.0% or less, and a structure other than ferrite and the
MA being limited to 37% or less. In addition, the average grain
size of ferrite is set to 1 .mu.m or more and 30 .mu.m or less.
[0087] (Area Fraction of ferrite: 60% to less than 100%)
[0088] Ferrite is a main constituent phase in the steel structure
of the H section according to the present embodiment. If the area
fraction of ferrite is less than 60%, low temperature toughness is
deteriorated. Therefore, the lower limit for the ferrite fraction
is set to 60%. Preferably, the lower limit for the ferrite fraction
is set to 65%, 70%, or 75%. On the other hand, controlling the area
fraction of ferrite to 100% is physically difficult because it
involves formation of pearlite or bainite. Therefore, the upper
limit for the ferrite fraction is set to less than 100%. In order
to preferably control the strength and the low temperature
toughness, the upper limit for the ferrite fraction is preferably
set to 90%, 85%, or 80%.
[0089] (Area Fraction of MA: 3.0% or less)
[0090] If generation of an MA is encouraged, low temperature
toughness is deteriorated. In the H section according to the
present embodiment, strength of a steel is increased without
encouraging the generation of an MA. Therefore, the MA fraction is
limited to 3.0% or less. Preferably, the upper limit for the MA
fraction is set to 2.5%, 2.0%, or 1.5%. The smaller the MA
fraction, the more preferable. Therefore, the lower limit for the
MA fraction may be 0%.
[0091] (Area Fraction of Structure Other than ferrite and MA: 37%
or less)
[0092] The steel structure of the H section according to the
present embodiment includes bainite, pearlite, and the like as
structures other than the ferrite and the MA, as described above.
If a structure other than ferrite and an MA is excessively
included, low temperature toughness is deteriorated. Therefore, the
area fraction of a structure other than ferrite and an MA (the
remainder of ferrite and an MA, as described above) is limited to
37% or less. Preferably, the fraction of a structure other than
ferrite and an MA is set to 35% or less, 30% or less, or 25% or
less. The smaller the fraction of a structure other than ferrite
and an MA, the more preferable. Therefore, the lower limit therefor
may be 0%.
[0093] (Average Grain Size of ferrite: 1 .mu.m to 30 .mu.m)
[0094] It is preferable that an average grain size of ferrite is
refined. When the grain size of ferrite exceeds 30 .mu.m, the low
temperature toughness is deteriorated. Therefore, the upper limit
for the grain size of ferrite is set to 30 .mu.m. Preferably, the
upper limit for the grain size of ferrite is set to 25 .mu.m, 22
.mu.m, or 18 .mu.m. On the other hand, it is industrially difficult
to control the grain size of ferrite to be less than 1 .mu.m.
Therefore, the lower limit for the grain size of ferrite is set to
1 .mu.m. Preferably, the lower limit for the grain size of ferrite
is set to 3 .mu.m, 5 .mu.m, or 10 .mu.m.
[0095] The steel structure described above may be discriminated
through observation using an optical microscope. For example, FIG.
1 is a schematic cross-sectional view orthogonal to a rolling
direction of an H section. However, the steel structure is observed
by setting a portion in the vicinity of an evaluation portion 7
illustrated in FIG. 1 as an observed section. Specifically, in FIG.
1, the steel structure is observed by setting a portion in the
vicinity of the evaluation portion 7 at a position of (1/6)F from
an end surface 5a of a flange in a width direction and a position
of (1/4)t.sub.2 from an outer surface 5b of the flange in a
thickness direction, as the observed section. This observed section
is a section parallel to the end surface 5a of the flange in the
width direction.
[0096] The steel structure is observed by polishing and etching the
observed section described above. Polishing is performed until the
observed section becomes a specular section, and etching is
performed by using an etching solution suitable for identifying the
constituent phase. For example, if the observed section finished to
be a specular section is etched with a nital solution such that the
steel structure is manifested, pearlite or bainite is colored.
Therefore, ferrite, martensite, and austenite can be identified. In
addition, if the observed section finished to be a specular section
is etched with a Le Pera solution such that the steel structure is
manifested, the constituent phases other than martensite and
austenite are colored black. Therefore, the mixed structure MA of
martensite and austenite can be identified.
[0097] In the H section according to the present embodiment, the
fractions of ferrite and an MA are obtained from the nital-etched
observed section, and the remainder is regarded as the fractions of
the structures of pearlite and bainite. The MA fraction is obtained
from the Le Pera-etched observed section. Specifically, on a
photograph of an optical microscope of 200 magnifications (as
necessary, a plurality of visual fields) captured in the
nital-etched observed section, measurement points are disposed in a
lattice shape having one side of 25 .mu.m, and it is discriminated
whether or not the structure is ferrite or an MA at not less than
1,000 measurement points. The value obtained by dividing the number
of measurement points determined to be ferrite or an MA by the
number of all of the measurement points is regarded as the fraction
of ferrite or an MA.
[0098] Similarly, on a photograph of an optical microscope of 200
magnifications (as necessary, a plurality of visual fields)
captured in the Le Pera-etched observed section, measurement points
are disposed in a lattice shape having one side of 25 .mu.m, and it
is discriminated whether or not the structure is an MA at not less
than 1,000 measurement points. The value obtained by dividing the
number of measurement points determined to be an MA by the number
of all of the measurement points is regarded as the MA fraction.
Then, the fraction of ferrite is obtained by subtracting the total
fraction of pearlite, bainite, and the MA fraction obtained as
above from 100%.
[0099] In addition, in the H section according to the present
embodiment, the average grain size of ferrite is obtained by an
intercept method in conformity with JIS G 0551 (2013) using the
photograph of an optical microscope of 200 magnifications captured
in the nital-etched observed section described above.
[0100] Next, mechanical properties of the H section according to
the present embodiment will be described in detail.
[0101] In the H section according to the present embodiment, test
pieces are collected from a region including the evaluation portion
7 illustrated in FIG. 1, as a position at which the average
mechanical properties (strength and low temperature toughness) can
be obtained, and mechanical properties are evaluated.
[0102] First, the evaluation portion 7 in FIG. 1 will be described.
FIG. 1 is a schematic cross-sectional view orthogonal to the
rolling direction of an H section. In FIG. 1, an X-axis direction
is defined as the width direction of the flange, a Y-axis is
defined as the thickness direction of the flange, and a Z-axis
direction is defined as the rolling direction.
[0103] As illustrated in FIG. 1, when the length of the flange in
the width direction is F and the thickness of the flange is
t.sub.2, the center of the evaluation portion 7 is at the position
of (1/6)F from the end surface of the flange in the width direction
and the position of (1/4)t.sub.2 from the outer surface of the
flange in the thickness direction. The outer surface of the flange
in the thickness direction is a surface on one side in the
thickness direction of the flange, is a surface on a side which
does not come into contact with a web 6, and is the surface 5b
illustrated in FIG. 1. In addition, the end surface of the flange
in the width direction is the end surface 5a illustrated in FIG.
1.
[0104] A test piece used when low temperature toughness is
evaluated through the Charpy test is collected at the position of
the evaluation portion 7 such that the longitudinal direction of
the test piece becomes parallel to the rolling direction. In
addition, a surface on which a notch is formed in the test piece is
any of surfaces parallel to the end surface 5a of the flange in the
width direction. In addition, the test piece may be collected at
any position as long as it is the position of (1/6)F from the end
surface 5a of the flange in the width direction and the position of
(114)t.sub.2 from the outer surface 5b of the flange in the
thickness direction.
[0105] A test piece used when yield stress (yield strength or 0.2%
proof stress) and tensile strength (maximum tensile strength) are
evaluated through a tension test is collected such that the
position of (1/6)F from the end surface 5a of the flange in the
width direction becomes the center of the test piece in the
thickness direction, in FIG. 1. In a test piece, the longitudinal
direction of the test piece may be parallel to the rolling
direction such that the entirety of the flange in the thickness
direction is cut out. The test piece may be collected at any
position as long as it is the position of (1/6)F from the end
surface 5a of the flange in the width direction.
[0106] In the H section according to the present embodiment, as
mechanical properties, the yield stress at a normal temperature
becomes 385 MPa or more, the tensile strength becomes 490 MPa or
more, and the Charpy absorbed energy at -20.degree. C. becomes 100
J or greater. When the strength is excessively high, the low
temperature toughness may be impaired. Therefore, the upper limit
for the yield stress is preferably set to 530 MPa and the upper
limit for the tensile strength is preferably set to 690 MPa. In
addition, since it is industrially difficult that the Charpy
absorbed energy at -20.degree. C. exceeds 500 J, the upper limit
for the Charpy absorbed energy at -20.degree. C. may be set to 500
J. A normal temperature indicates 20.degree. C.
[0107] When mechanical properties of the H section according to the
present embodiment are evaluated, the tension test is performed in
conformity with JIS Z 2241 (2011), and the Charpy test is performed
in conformity with JIS Z 2242 (2005). When a yielding phenomenon is
recognized in a stress-strain curve obtained from the tension test,
yield strength is obtained as the yield stress. When no yielding
phenomenon is recognized in the stress-strain curve, 0.2% proof
stress is obtained as the yield stress.
[0108] Next, the shape of the H section according to the present
embodiment will be described in detail.
[0109] In the H section according to the present embodiment, the
thickness t.sub.2 of the flange is 20 mm to 140 mm. For example, in
a high-rise building structure, thick H sections are demanded as
strength members. Therefore, the lower limit for the thickness of
the flange is set to 20 mm. Preferably, the lower limit for the
thickness of the flange is set to 25 mm, 40 mm, or 56 mm On the
other hand, if the thickness t.sub.2 of the flange exceeds 140 mm,
the working amount at the time of hot working becomes insufficient,
so that it is difficult to achieve both the strength and the low
temperature toughness. Therefore, the upper limit for the thickness
of the flange is set to 140 mm. Preferably, the upper limit for the
thickness of the flange is set to 125 mm, 89 mm, or 77 mm. For
example, it is preferable that the thickness t.sub.2 of the flange
is 25 mm to 140 mm. The thickness t.sub.1 of the web of an H
section is not particularly regulated. However, it is preferable
that the thickness t.sub.1 of the web of an H section is 20 mm to
140 mm, and it is more preferable that the thickness t.sub.1 of the
web of an H section is 25 mm to 140 mm.
[0110] In addition, in a case where an H section is manufactured
through hot rolling, it is preferable that the ratio
(t.sub.2/t.sub.1) of thickness of flange/thickness of web is 0.5 to
2.0. If the ratio (t.sub.2/t.sub.1) of thickness of
flange/thickness of web exceeds 2.0, the web may be deformed into a
wavy shape. On the other hand, in a case where the ratio
(t.sub.2/t.sub.1) of thickness of flange/thickness of web is less
than 0.5, the flange may be deformed into a wavy shape.
[0111] In technologies in the related art, in thick H sections
having a thickness of a flange of 20 mm or more, it has been
difficult to achieve both the strength and the toughness. However,
even though the H section according to the present embodiment is a
thick H section having a thickness of a flange of 20 mm or more,
the steel composition and the steel structure are optimally
controlled. Therefore, it is possible to achieve both the strength
and the low temperature toughness.
[0112] Next, a preferable method for manufacturing the H section
according to the present embodiment will be described in
detail.
[0113] The method for manufacturing an H section according to the
present embodiment includes steelmaking, casting, heating, hot
rolling, and cooling.
[0114] In the steelmaking, the chemical composition of a molten
steel is adjusted to obtain the steel composition described above.
In the steelmaking, a molten steel manufactured by performing
converter refining or secondary refining may be used, or a molten
steel melted in an electric furnace may be used as a raw material.
In the steelmaking, as necessary, deoxidation processing or vacuum
degassing may be performed.
[0115] In the casting, a steel piece is obtained by casting a
molten steel after the steelmaking. Casting is performed by a
continuous casting method, an ingot method, or the like. From the
viewpoint of productivity, it is preferable to adopt continuous
casting. It is preferable that the shape of a steel piece is a beam
blank having a shape close to that of an H section to be
manufactured, but the shape thereof is not particularly limited. In
addition, from the viewpoint of productivity, it is preferable that
the thickness of a steel piece is 200 mm or more. In consideration
of reduction of segregation or homogeneity in the heating
temperature before hot rolling is performed, it is preferable that
the thickness thereof is 350 mm or less.
[0116] In the heating, a steel piece after the casting is heated to
1,100.degree. C. to 1,350.degree. C. If the heating temperature of
a steel piece is less than 1,100.degree. C., deformation resistance
at the time of finish rolling is increased. Therefore, the lower
limit for the heating temperature is set to 1,100.degree. C. In
order to sufficiently solid-solubilize elements which forms carbide
or nitride, such as Nb, the lower limit for the heating temperature
is preferably set to 1,150.degree. C. On the other hand, if the
heating temperature exceeds 1,350.degree. C., scale on a steel
piece surface is liquefied, so that manufacturing is hindered.
Therefore, the upper limit for the heating temperature is set to
1,350.degree. C. In the heating, a steel piece which is not cooled
to the room temperature after the casting may be used.
[0117] In the hot rolling, rough rolling, intermediate rolling, and
finish rolling are performed with respect to a steel piece after
the heating. In rough rolling, forming is performed such that the
shape seen in a cut section orthogonal to the rolling direction
becomes a substantial H-shape. With respect to this steel piece
having a substantial H-shape, hot rolling, in which the surface
temperature of the steel is a temperature range of more than
900.degree. C. and 1,100.degree. C. or less and the cumulative
rolling reduction is 20% or more, is perfonned. Moreover, hot
rolling, in which the surface temperature of the steel is a
temperature range of 730.degree. C. to 900.degree. C. and the
cumulative rolling reduction is 15% or more, is performed. In this
hot rolling, forming is performed such that the shape seen in the
foregoing cut section ultimately becomes an H-shape.
[0118] In the temperature range of more than 900.degree. C. and
1,100.degree. C. or less, in order to reduce the generation amount
of bainite or an MA through grain refinement of austenite grains,
the cumulative rolling reduction is set to 20% or more. Preferably,
the lower limit for the cumulative rolling reduction in the
temperature range of more than 900.degree. C. and 1,100.degree. C.
or less is set to 25%, 30%, or 35%. As necessary, the upper limit
for the cumulative rolling reduction in the temperature range of
more than 900.degree. C. and 1,100.degree. C. or less may be set to
60%.
[0119] In the temperature range of 730.degree. C. to 900.degree.
C., in order to refine ferrite, the cumulative rolling reduction is
set to 15% or more. Preferably, the lower limit for the cumulative
rolling reduction in the temperature range of 730.degree. C. to
900.degree. C. is set to 20%, 25%, or 30%. As necessary, the upper
limit for the cumulative rolling reduction in the temperature range
of 730.degree. C. to 900.degree. C. may be set to 50%.
[0120] When rolling is performed at a temperature below 730.degree.
C., and which may lead to deterioration in low temperature
toughness. Therefore, a finish temperature of rolling (finish
rolling temperature) is set to 730.degree. C. or more on the
surface temperature of a steel. Preferably, the upper limit for the
finish rolling temperature is set to 750.degree. C.
[0121] In the hot rolling, rough rolling, intermediate rolling, and
finish rolling are performed. However, for example, rolling at the
temperature range of more than 900.degree. C. to 1,100.degree. C.
may be performed by any of rough rolling, intermediate rolling, and
finish rolling. Similarly, rolling at the temperature range of
730.degree. C. to 900.degree. C. may be performed by any of rough
rolling, intermediate rolling, and finish rolling. In the method
for manufacturing an H section according to the present embodiment,
the cumulative rolling reduction in the foregoing temperature range
may be controlled.
[0122] In addition, the cumulative rolling reduction in the
foregoing temperature range is obtained with reference to the
thickness of the flange at a position corresponding to (1/6)F from
the end surface 5a of the flange in the width direction illustrated
in FIG. 1. For example, the cumulative rolling reduction in the
temperature range of more than 900.degree. C. and 1,100.degree. C.
or less is set to the rolling reduction calculated from the
difference between the thickness of the flange at the time when the
surface temperature of a steel is 1,100.degree. C. and the
thickness of the flange immediately before the temperature reaches
900.degree. C. Similarly, the cumulative rolling reduction in the
temperature range of 730.degree. C. to 900.degree. C. is set to the
rolling reduction calculated from the difference between the
thickness of the flange at the time when the surface temperature of
a steel is 900.degree. C. and the thickness of the flange at the
time when the surface temperature of a steel is 730.degree. C.
[0123] In the hot rolling, the methods for rough rolling,
intermediate rolling, and finish rolling are not particularly
limited. For example, breakdown rolling is performed as the rough
rolling, universal rolling or edging rolling is performed as the
intermediate rolling, universal rolling is performed as the finish
rolling, so that the shape seen in a cut section orthogonal to the
rolling direction may be formed into an H-shape.
[0124] In the hot rolling, water cooling may be performed between
rolling passes. Water cooling performed between the rolling passes
is cooling performed for the purpose of controlling temperature in
a temperature range which is more than the temperature at which
austenite is phase-transformed. Bainite or an MA is not generated
in a steel due to water cooling performed between the rolling
passes.
[0125] In addition, in the hot rolling, dual heat rolling may be
performed. Dual heat rolling means a rolling method in which a
steel piece is cooled to a temperature of 500.degree. C. or less
after primary rolling, and then, the steel piece is heated to
1,100.degree. C. to 1,350.degree. C. and secondary rolling is
performed again. In dual heat rolling, the plastic deformation
amount is reduced in hot rolling, and the decrease in temperature
in the rolling also becomes small. Therefore, a second heating
temperature can be lowered.
[0126] In the cooling, a hot rolled material after the hot rolling
is cooled. In the method for manufacturing an H section according
to the present embodiment, a hot rolled material is subjected to
natural cooling in the atmosphere as it is after ending of the hot
rolling. In a case where a hot rolled material is subjected to
natural cooling in the atmosphere, the average cooling rates of a
surface of a steel and inside thereof from 800.degree. C. to
500.degree. C. become 1.degree. C./sec or less. Since the cooling
rates of a surface of a steel and inside thereof are uniform by
performing natural cooling on a hot rolled material in the
atmosphere, unevenness in mechanical properties depending on the
portions of a steel is suppressed. In the method for manufacturing
an H section according to the present embodiment, natural cooling
means that cooling is performed in the atmosphere without forcibly
performing cooling until the temperature of a steel reaches to
400.degree. C. or less from immediately after hot rolling.
[0127] In technologies in the related art, a hot rolled material
has been subjected to accelerated cooling in order to achieve both
the strength and the toughness. Accordingly, unevenness in
mechanical properties has been caused between a surface of a steel
and inside thereof. On the other hand, in the method for
manufacturing an H section according to the present embodiment,
although a hot rolled material is subjected to natural cooling in
the atmosphere, the steel composition and the steel structure are
optimally controlled. Therefore, it is possible to achieve both the
strength and the low temperature toughness without causing
unevenness in mechanical properties between a surface of a steel
and inside thereof.
[0128] Since the method for manufacturing an H section according to
the present embodiment does not require an advanced steelmaking
technology or accelerated cooling, it is possible to reduce a
manufacturing load and to shorten a construction period. Therefore,
the H section according to the present embodiment can improve
reliability of a large building without impairing economic
feasibility.
EXAMPLE 1
[0129] Next, the effects of the aspect of the present invention
will be specifically described in more detail using Examples.
However, the conditions in Examples are merely examples of
conditions employed to check the feasibility and the effects of the
present invention, and the present invention is not limited to
these examples of conditions. The present invention can employ
various conditions as long as the conditions do not depart from the
gist of the present invention and the object of the present
invention is achieved.
[0130] Steels respectively having the chemical compositions shown
in Table 1 to Table 3 were smelted, and steel pieces having a
thickness of 240 mm to 300 mm were manufactured through continuous
casting. The steels were smelted by using a converter, and their
compositions were adjusted by performing primary deoxidation and
adding an alloying element. As necessary, vacuum degassing was
performed. H sections were manufactured by heating the obtained
steel pieces and performing hot rolling. The steel compositions
indicated as compositions No. 1 to No. 48 were obtained by
chemically analyzing samples respectively collected from the
manufactured H sections. Although it is not shown in Tables, P was
0.03% or less, S was 0.02% or less, and 0 was 0.005% or less in all
of the Examples. The blank for the chemical composition in Tables
indicates that it was not actively added to the steel, or its
amount was equal to or less than the detection limit.
[0131] FIG. 2 illustrates processes for manufacturing an H section.
Hot rolling of a steel piece heated by a heating furnace 1 was
performed in a universal rolling apparatus array including a rough
rolling mill 2a, an intermediate rolling mill 2b, and a finish
rolling mill 2c. A hot rolled material was subjected to natural
cooling as it is until the temperature reached to 400.degree. C. or
less after ending of hot rolling. Both the average cooling rates of
a surface of the hot rolled material and inside thereof from an
ending temperature of hot rolling to 500.degree. C. were 1.degree.
C./sec or less. In the case where water cooling was performed
between passes of hot rolling, spray cooling of an outer surface of
the flange was performed by using water cooling apparatuses 3
provided in the front and the rear of the intermediate universal
rolling mill (intermediate rolling mill) 2b. In this case, reverse
rolling was performed.
[0132] Table 4 to Table 6 show the manufacturing conditions and the
manufactured results. The rolling reductions at the time of hot
rolling shown in Table 4 to Table 6 are cumulative rolling
reductions in the temperature ranges at a position corresponding to
(1/6)F from the end surface 5a of the flange in the width direction
illustrated in FIG. 1.
[0133] In regard to the manufactured H sections, as described
above, the Charpy test was performed at -20.degree. C. by using the
test piece collected from the evaluation portion 7 illustrated in
FIG. 1, and the low temperature toughness was evaluated. In
addition, the tension test was performed at a normal temperature
(20.degree. C.) by using the test piece in which the position of
(1/6)F from the end surface 5a of the flange in the width direction
became the center in the thickness direction, and the tensile
properties were evaluated. In addition, the structure was observed
by using the sample having a portion in the vicinity of the
evaluation portion 7 illustrated in FIG. 1 as the observed section,
and the steel structure was evaluated.
[0134] The tension test was performed in conformity with JIS Z 2241
(2005). In the case where the stress-strain curve of the tension
test indicated yielding behavior, a yield point was regarded as the
yield stress. In the case where no yielding behavior was indicated,
0.2% proof stress was regarded as the yield stress. A Charpy impact
test was performed in conformity with JIS Z 2242 (2005). The Charpy
impact test was performed at -20.degree. C.
[0135] The structure was observed by using the method described
above, and the ferrite fraction, the MA fraction, and the fraction
of a structure other than the ferrite and the MA was measured by
using a photograph of an optical microscope. In addition, the
structure other than the ferrite and the MA was bainite or
pearlite. In addition, the average grain size of ferrite was
obtained by an intercept method in conformity with JIS G 0551
(2013) using a photograph of an optical microscope.
[0136] In regard to the tensile properties, a steel having yield
stress (YS) at a normal temperature of 385 MPa or more and tensile
strength (TS) of 490 MPa or more was determined as PASSED. In
addition, in regard to the low temperature toughness, a steel
having Charpy absorbed energy at -20.degree. C. (vE-20) of 100 J or
greater was determined as PASSED.
[0137] As shown in Tables 1 to 6, in Serial No. 1 to No. 8, Serial
No. 11 to No. 18, and Serial No. 34 to No. 43, which were the
examples in the present invention, all of the steel composition,
the steel structure, and mechanical properties satisfied the range
of the present invention.
[0138] On the other hand, in Serial No. 9 to No. 10, Serial No. 19
to No. 33, and Serial No. 44 to No. 50, which were comparative
examples, any of the steel composition, the steel structure, and
mechanical properties did not satisfy the range of the present
invention.
[0139] Serial No. 9 was an example in which rolling reduction
within more than 900.degree. C. and 1,100.degree. C. or less was
insufficient. Therefore, the ferrite fraction in the steel
structure became insufficient and the fraction of the structure
other than the ferrite and the MA became excessive, so that the
Charpy absorbed energy at -20.degree. C. became insufficient.
[0140] Serial No. 10 was an example in which rolling reduction
within 730.degree. C. to 900.degree. C. was insufficient.
Therefore, grain size of ferrite was coarsened, so that the Charpy
absorbed energy at -20.degree. C. became insufficient.
[0141] Serial No. 19 was an example in which rolling reduction
within more than 900.degree. C. and 1,100.degree. C. or less was
insufficient. Therefore, the fraction ferrite became insufficient,
the MA fraction became excessive, and the fraction of the structure
other than the ferrite and the MA became excessive, so that the
Charpy absorbed energy at -20.degree. C. became insufficient.
[0142] Serial No. 20 was an example in which the C content was
large. Serial No. 25 was an example in which the Nb content was
large. Serial No. 26 was an example in which the V content was
large. Serial No. 28 was an example in which the Al content was
large. Serial No. 29 was an example in which the Ti content was
large. Serial No. 30 was an example in which the N content was
large. Serial No. 31 was an example in which Ceq was excessive.
Therefore, the Charpy absorbed energy at -20.degree. C. became
insufficient in these examples.
[0143] Serial No. 21 was an example in which the C content was
small. Serial No. 24 was an example in which the Mn content was
small. Serial No. 32 was an example in which Ceq was insufficient.
Serial No. 46 was an example in which the Si content was small.
Therefore, YS and TS became insufficient in these examples.
[0144] Serial No. 22 was an example in which the Si content was
large. Serial No. 23 was an example in which the Mn content was
large and MA fraction was excessive. Therefore, the Charpy absorbed
energy at -20.degree. C. became insufficient in these examples.
[0145] Serial No. 27 was an example in which the V content was
small, so that grain size of ferrite is coarsened. Therefore, the
Charpy absorbed energy at -20.degree. C. became insufficient.
[0146] Serial No. 33 was an example in which the B content was
excessive and Ceq was excessive. Serial No. 49 was an example in
which the B was large, so that the MA fraction became excessive.
Therefore, the Charpy absorbed energy at -20.degree. C. became
insufficient in these examples.
[0147] Serial No. 44 and Serial No. 45 were examples in which the V
content was small, so that grain size of ferrite was coarsened.
Therefore, the Charpy absorbed energy at -20.degree. C. became
insufficient in these examples.
[0148] Serial No. 47 was an example in which the Nb content was
small, so that grain size of ferrite was coarsened. Therefore, YS
and TS became insufficient and the Charpy absorbed energy at
-20.degree. C. became insufficient.
[0149] Serial No. 48 was an example in which the Ti content was
small, so that grain size of ferrite was coarsened. Therefore, the
Charpy absorbed energy at -20.degree. C. became insufficient.
[0150] Serial No. 50 was an example in which the finish rolling
temperature was low. Therefore, the Charpy absorbed energy at
-20.degree. C. became insufficient.
TABLE-US-00001 TABLE 1 Serial Composition Chemical composition
[mass %] (remainder of Fe and impurities) No. No. C Si Mn Nb V Al
Ti N Cr Mo Ni Cu W B Ca Zr Ceq 1 1 0.158 0.55 0.82 0.045 0.098
0.095 0.024 0.0005 0.314 2 2 0.157 0.54 0.84 0.046 0.051 0.091
0.022 0.0007 0.307 3 3 0.131 0.38 1.55 0.047 0.099 0.032 0.012
0.0023 0.40 0.0018 0.409 4 4 0.130 0.09 1.59 0.041 0.102 0.028
0.010 0.0030 0.415 5 5 0.121 0.40 1.40 0.045 0.110 0.037 0.011
0.0070 0.20 0.20 0.403 6 6 0.119 0.44 0.95 0.040 0.115 0.031 0.012
0.0042 0.18 0.336 7 7 0.110 0.29 1.20 0.033 0.099 0.030 0.008
0.0019 0.20 0.0036 0.370 8 8 0.109 0.05 1.60 0.045 0.111 0.030
0.011 0.0071 0.398 9 8 0.109 0.05 1.60 0.045 0.111 0.030 0.011
0.0071 0.398 10 8 0.109 0.05 1.60 0.045 0.111 0.030 0.011 0.0071
0.398 11 9 0.100 0.41 1.22 0.049 0.089 0.012 0.002 0.0022 0.28
0.377 12 10 0.099 0.28 1.01 0.020 0.098 0.030 0.007 0.0080 0.19
0.0020 0.325 13 11 0.090 0.15 1.40 0.005 0.070 0.051 0.019 0.0114
0.337 14 12 0.090 0.12 1.35 0.035 0.090 0.040 0.009 0.0056 0.31
0.0025 0.354 15 13 0.080 0.56 1.55 0.030 0.099 0.019 0.012 0.0095
0.358 16 14 0.071 0.33 1.59 0.044 0.102 0.025 0.009 0.0072 0.18
0.392 17 15 0.060 0.07 1.30 0.033 0.088 0.032 0.010 0.0029 0.20
0.19 0.20 0.20 0.399
[0151] The blank means that an element is not intentionally added
or is equal to or less than a detection limit
[0152] The underline means that the value is beyond the range of
the present invention.
TABLE-US-00002 TABLE 2 Serial Composition Chemical composition
[mass %] (remainder of Fe and impurities) No No C Si Mn Nb V Al Ti
N Cr Mo Ni Cu W B Ca Zr Ceq 18 16 0.051 0.20 1.66 0.040 0.109 0.030
0.012 0.0030 0.18 0.25 0.402 19 16 0.051 0.20 1.66 0.040 0.109
0.030 0.012 0.0030 0.18 0.25 0.402 20 17 0.174 0.30 1.50 0.041
0.098 0.032 0.011 0.0034 0.10 0.464 21 18 0.040 0.10 1.42 0.020
0.101 0.028 0.011 0.0052 0.20 0.20 0.324 22 19 0.140 0.65 1.59
0.039 0.110 0.044 0.009 0.0070 0.10 0.447 23 20 0.139 0.39 1.80
0.038 0.111 0.031 0.010 0.0055 0.461 24 21 0.101 0.21 0.73 0.045
0.099 0.029 0.011 0.0039 0.19 0.21 0.21 0.308 25 22 0.130 0.38 1.60
0.061 0.113 0.040 0.012 0.0040 0.11 0.10 0.433 26 23 0.141 0.35
1.55 0.045 0.130 0.030 0.008 0.0051 0.425 27 24 0.099 0.35 1.54
0.039 0.044 0.021 0.008 0.0029 0.364 28 25 0.121 0.37 1.58 0.044
0.080 0.110 0.010 0.0027 0.400 29 26 0.120 0.38 1.55 0.047 0.090
0.052 0.028 0.0031 0.396 30 27 0.119 0.39 1.54 0.038 0.102 0.030
0.011 0.0131 0.396 31 28 0.150 0.45 1.61 0.045 0.112 0.030 0.012
0.0050 0.11 0.10 0.20 0.496 32 29 0.065 0.30 0.89 0.010 0.061 0.029
0.009 0.0044 0.20 0.266 33 30 0.070 0.35 1.10 0.035 0.051 0.030
0.012 0.0020 0.0005 0.264
[0153] The blank means that an element is not intentionally added
or is equal to or less than a detection limit
[0154] The underline means that the value is beyond the range of
the present invention.
TABLE-US-00003 TABLE 3 Com- Serial position Chemical composition
[mass %] (remainder of Fe and impurities) No No C Si Mn Nb V M Ti N
Cr Mo Ni Cu W B Ca Zr Ceq 34 31 0.150 0.03 1.01 0.020 0.099 0.008
0.0006 0.338 35 32 0.139 0.51 1.20 0.015 0.078 0.009 0.0050 0.09
0.0022 0.0021 0.373 36 33 0.129 0.40 1.32 0.032 0.101 0.002 0.020
0.0033 0.369 37 34 0.121 0.10 0.99 0.045 0.089 0.015 0.0101 0.20
0.20 0.317 38 35 0.110 0.35 1.59 0.044 0.101 0.001 0.011 0.0077
0.395 39 36 0.100 0.34 1.62 0.042 0.051 0.012 0.0029 0.380 40 37
0.092 0.25 1.44 0.039 0.062 0.010 0.0020 0.344 41 38 0.082 0.41
1.00 0.019 0.110 0.001 0.025 0.0061 0.20 0.20 0.22 0.339 42 39
0.073 0.50 1.40 0.039 0.098 0.010 0.0080 0.11 0.0015 0.348 43 40
0.062 0.42 1.62 0.045 0.080 0.012 0.0073 0.20 0.388 44 41 0.099
0.35 1.50 0.020 0.040 0.012 0.0025 0.20 0.0019 0.370 45 42 0.102
0.36 1.48 0.031 0.031 0.001 0.008 0.0029 0.355 46 43 0.099 0.004
1.49 0.042 0.099 0.001 0.011 0.0061 0.367 47 44 0.111 0.35 1.35
0.004 0.109 0.001 0.009 0.0039 0.358 48 45 0.119 0.35 1.48 0.045
0.089 0.002 0.0004 0.0029 0.383 49 46 0.065 0.35 1.01 0.039 0.051
0.001 0.011 0.0033 0.20 0.20 0.22 0.0004 0.312 50 47 0.092 0.36
1.59 0.044 0.070 0.022 0.0025 0.371
[0155] The blank means that an element is not intentionally added
or is equal to or less than a detection limit
[0156] The underline means that the value is beyond the range of
the present invention.
TABLE-US-00004 TABLE 4 Rolling reduction Rolling within more
reduction Heating than 900.degree. C. within Finish Area Thickness
temperature and 1,100.degree. C. 730.degree. C. to rolling fraction
Serial Composition of flange of slab or less 900.degree. C.
temperature of ferrite No. No. [mm] [.degree. C.] [%] [%] [.degree.
C.] [%] 1 1 20 1310 50 35 740 75 2 2 20 1310 35 35 750 64 3 3 40
1310 35 30 762 68 4 4 40 1310 35 30 760 70 5 5 140 1310 25 22 801
75 6 6 140 1310 25 22 795 73 7 7 56 1310 33 30 752 77 8 8 77 1250
33 28 789 78 9 8 77 1250 13 28 796 55 10 8 77 1250 33 13 800 76 11
9 125 1310 28 25 822 79 12 10 125 1310 28 25 825 80 13 11 89 1200
30 27 848 83 14 12 89 1200 30 27 841 82 15 13 40 1150 35 30 790 84
16 14 40 1150 35 30 796 83 17 15 77 1310 33 28 849 86 Area fraction
other than Average Area ferrite and grain size Serial fraction of
MA of ferrite YS TS vE.sub.-20.degree. C. No. MA [%] [%] [.mu.m]
[MPa] [MPa] [J] Remarks 1 0.5 24.5 13.5 498 602 260 Present
invention 2 2.8 33.2 27.5 457 590 108 Present invention 3 2.5 29.5
16.0 464 570 248 Present invention 4 0.8 29.2 15.5 454 575 239
Present invention 5 1.3 23.7 18.9 414 521 208 Present invention 6
1.3 25.7 17.8 394 517 202 Present invention 7 0.8 22.2 12.6 425 555
255 Present invention 8 0.8 21.2 17.7 458 550 241 Present invention
9 2.8 42.2 24.5 461 553 77 Comparative Example 10 1.0 23.0 38.2 469
563 36 Comparative Example 11 0.5 20.5 18.8 413 532 199 Present
invention 12 0.3 19.7 20.1 414 529 189 Present invention 13 0.0
17.0 24.0 451 539 180 Present invention 14 2.3 15.7 26.6 425 544
177 Present invention 15 0.3 15.7 18.6 460 590 202 Present
invention 16 0.5 16.5 18.8 500 601 231 Present invention 17 1.0
13.0 27.8 423 552 169 Present invention
[0157] The underline means that the value is beyond the range of
the present invention.
TABLE-US-00005 TABLE 5 Rolling reduction Rolling within more
reduction Heating than 900.degree. C. within Finish Area Thickness
temperature and 1,100.degree. C. 730.degree. C. to rolling fraction
Serial Composition of flange of slab or less 900.degree. C.
temperature of ferrite No. No. [mm] [.degree. C.] [%] [%] [.degree.
C.] [%] 18 16 77 1310 33 28 840 88 19 16 77 1310 13 35 835 54 20 17
89 1310 30 27 821 62 21 18 89 1310 30 27 827 90 22 19 89 1310 30 27
833 65 23 20 56 1310 40 30 787 70 24 21 56 1310 40 30 777 75 25 22
56 1310 40 30 775 69 26 23 77 1310 33 28 780 68 27 24 77 1310 33 28
792 79 28 25 77 1310 33 28 799 75 29 26 77 1310 33 28 805 77 30 27
77 1310 33 32 783 79 31 28 77 1310 33 32 800 65 32 29 77 1310 33 32
774 85 33 30 77 1310 33 32 795 66 Area fraction Area other than
Average fraction ferrite and grain size Serial of MA MA of ferrite
YS TS vE.sub.-20.degree. C. No. [%] [%] [.mu.m] [MPa] [MPa] [J]
Remarks 18 1.0 11.0 24.5 418 548 135 Present invention 19 3.5 42.5
26.0 423 554 67 Comparative Example 20 0.8 37.2 18.2 434 543 88
Comparative Example 21 1.5 8.5 17.7 380 485 215 Comparative Example
22 3.5 31.5 18.4 446 551 54 Comparative Example 23 3.3 26.7 14.4
443 575 69 Comparative Example 24 0.3 24.7 14.3 383 484 275
Comparative Example 25 1.5 29.5 13.9 463 574 70 Comparative Example
26 1.3 30.7 15.0 462 555 92 Comparative Example 27 0.8 20.2 30.5
421 549 96 Comparative Example 28 0.5 24.5 16.1 448 556 48
Comparative Example 29 0.0 23.0 16.0 441 564 39 Comparative Example
30 0.5 20.5 15.2 452 561 88 Comparative Example 31 2.8 32.2 15.5
486 632 74 Comparative Example 32 0.3 14.7 14.8 369 481 236
Comparative Example 33 5.0 29.0 27.9 438 598 44 Comparative
Example
[0158] The underline means that the value is beyond the range of
the present invention.
TABLE-US-00006 TABLE 6 Rolling reduction Rolling within more
reduction Heating than 900.degree. C. within Finish Area Thickness
temperature and 1,100.degree. C. 730.degree. C. to rolling fraction
Serial Composition of flange of slab or less 900.degree. C.
temperature of ferrite No. No. [mm] [.degree. C.] [%] [%] [.degree.
C.] [%] 34 31 56 1250 40 30 820 77 35 32 56 1250 40 30 827 74 36 33
125 1310 28 25 830 69 37 34 125 1310 28 25 821 69 38 35 77 1310 33
28 812 72 39 36 77 1310 33 28 802 70 40 37 77 1310 33 28 798 72 41
38 89 1310 30 27 845 75 42 39 89 1310 30 27 856 76 43 40 89 1310 30
27 863 79 44 41 77 1310 33 28 790 73 45 42 77 1310 33 28 788 74 46
43 89 1310 30 27 833 65 47 44 56 1310 40 30 760 70 48 45 77 1310 33
30 792 75 49 46 77 1310 33 32 774 70 50 47 77 1310 25 35 720 69
Area fraction Area other than Average fraction ferrite and grain
size Serial of MA MA of ferrite YS TS vE.sub.-20.degree. C. No. [%]
[%] [.mu.m] [MPa] [MPa] [J] Remarks 34 0.3 22.7 16.8 456 562 334
Present invention 35 0.9 25.1 16.4 440 567 307 Present invention 36
1.3 29.7 18.5 415 519 280 Present invention 37 1.4 29.6 18.0 418
505 300 Present invention 38 0.8 27.2 13.2 468 581 355 Present
invention 39 1.2 28.8 27.9 444 561 114 Present invention 40 1.0
27.0 27.5 456 549 128 Present invention 41 2.5 22.5 14.4 438 539
332 Present invention 42 1.6 22.4 13.9 447 544 318 Present
invention 43 0.6 20.4 15.0 458 560 299 Present invention 44 1.5
25.5 33.6 450 570 77 Comparative Example 45 1.3 24.7 35.9 447 558
45 Comparative Example 46 2.5 32.5 18.0 375 487 115 Comparative
Example 47 1.3 28.7 31.2 379 479 92 Comparative Example 48 0.5 24.5
31.8 447 561 85 Comparative Example 49 4.6 25.4 27.9 492 602 54
Comparative Example 50 0.6 30.4 28.5 498 592 88 Comparative
Example
[0159] The underline means that the value is beyond the range of
the present invention.
INDUSTRIAL APPLICABILITY
[0160] According to the aspect of the present invention, it is
possible to provide a thick H section, which has excellent strength
and low temperature toughness, and a method for manufacturing the
same. Therefore, industrial applicability is high.
BRIEF DESCRIPTION OF THE REFERENCE SYMBOLS
[0161] 1: heating furnace
[0162] 2a: rough rolling mill
[0163] 2b: intermediate rolling mill
[0164] 2c: finish rolling mill
[0165] 3: water cooling apparatuses before and after intermediate
rolling mill
[0166] 4: H section
[0167] 5: flange
[0168] 5a: end surface of flange in width direction
[0169] 5b: outer surface of flange in thickness direction
[0170] 6: web
[0171] 7: tensile properties, low temperature toughness, and
evaluation portion of steel structure
[0172] F: length of flange in width direction
[0173] H: height
[0174] t.sub.1: thickness of web
[0175] t.sub.2: thickness of flange
* * * * *