RELATED APPLICATION
TECHNICAL FIELD
[0002] The present invention relates to a high-strength steel for welded structures used
for buildings, civil engineering, offshore structures, shipbuilding, various storage
tanks, and other general welded structures and superior in high temperature strength
at a temperature range of 600°C to 800°C and in a relatively short time of about 1
hour and a method of production for the same. The present invention mainly covers
steel plate, steel pipe, and steel shapes, etc.
BACKGROUND ART
[0003] The strength of general steel materials for welded structures falls starting around
350°C. The allowable service temperature is considered to be about 500°C. Therefore,
when using these steel materials for buildings, offices, homes, vertical parking structures,
and other structures, they are required to be covered with fire-resistant coverings
to ensure safety in the event of fires. The Building Standards Law in Japan requires
that the temperature of steel materials not rise to 350°C or more at the time of fires.
This is because steel materials fall in yield strength at 350°C or so to about 2/3
of that at an ordinary temperature or below the necessary strength. Such a fire-resistant
covering has a large influence on construction costs.
[0004] To solve these problems, "fire-resistant steel" provided with yield strength at the
time of high temperatures is being developed (for example,
Japanese Patent Publication (A) No. 2-77523 and
Japanese Patent Publication (A) No. 10-68044). The yield strengths at 600°C and 700°C supposedly can be maintained at least at
2/3 of the standard minimum yield strength at the ordinary temperature. However, only
the yield strength at specific temperatures is shown. The yield strength at higher
temperatures is not alluded to as well. In particular, a temperature of over 700°C
falls in the temperature region for partially starting transformation depending on
the steel compositions. Therefore, a stable production of practical steel has been
extremely difficult - so much so that a rapid drop in the yield strength is feared.
[0005] Previously, the present inventors discovered a steel enabling high temperature strength
at 700 to 800°C to be secured and a method of production of the same (for example,
Japanese Patent Publication (A) No. 2004-43961). This requires, in terms of the steel compositions, the addition of B. This facilitates
control of the microstructure and enables achievement of a low yield ratio in particular
for steels for building structures. However, as is generally known, B has both not
only advantages, but also disadvantages, such as increasing the quenchability. For
example, at the time of small heat input welding, the heat affected zone (HAZ) remarkably
hardens, so the toughness desgrades. Conversely when the welding heat input becomes
too large, as the B precipitates at the austenite grain boundaries, the quenchability
of B cannot be effectively utilized, the microstructure becomes coarse, and the toughness
desgrades. Thus, there is the problem that the range of the welding heat input is
limited.
[0006] A steel for building structures is required to have a low yield ratio from the viewpoint
of earthquake resistance. The JIS standard for "Rolled Steel Materials for Building
Structures" regulates the yield ratio of 80% or less. Previous inventions of the present
inventors focused on this point. However, the amended Japanese Building Standards
Law enforced since June 2000 has changed what had previously been provisions on use
to provisions on performance and called for early use of new technologies and materials.
Regarding steel materials for building use, Article 37 of the Building Standards Law
allows use of JIS materials for building structures in Paragraph 1 and use of steel
materials assessed for performance in accordance with various performance requirements
and certified by the Minister of Land, Infrastructure, and Transport in Paragraph
2. Therefore, the present inventors engaged in intensive studies on steel materials
excellent in high temperature strength of course and also weldability and weld zone
toughness in a broad range of input heat without being bound by the JIS provisions
on yield ratio for steel materials for building use and thereby completed the present
invention.
DISCLOSURE OF THE INVENTION
[0007] As explained above, when utilizing steel materials for buildings, since ordinary
steel materials are low in high temperature strength (yield strength), they cannot
be used without coverings or with reduced fire-resistant coverings and have had to
be given expensive fire-resistant coverings. Further, even newly developed steel materials
have fire-resistant temperatures limited to a guarantee of 600 to 700°C. Development
of steel materials for use at 700 to 800°C without fire resistant coverings and thereby
enabling elimination of the fire resistant covering step has therefore been desired.
[0008] An object of the present invention is to provide a high-strength steel for welded
structures excellent in high temperature strength in a temperature range of 600°C
to 800°C and a method of production able to stably supply that steel on an industrial
basis.
[0009] The present invention achieves the above object by limiting the steel compositions,
microstructure, etc. to suitable ranges so overcome the above problems and has as
its gist the following.
- (1) A 490 MPa class high-strength steel for welded structures excellent in high temperature
strength, comprising as steel compositions, by wt%,
C: 0.005% to less than 0.040%,
Si: 0.5% or less,
Mn: 0.1 to less than 0.5%,
P:0.02% or less,
S: 0.01% or less,
Mo: 0.3 to 1.5%,
Nb: 0.03 to 0.15%,
Al: 0.06% or less, and
N: 0.006% or less,
having a weld crack susceptible formulation PCM defined as PCM=C+Si/30+Mn/20+Cu/20+Ni/60+Cr/20+Mo/15+V/ 10+5B of 0.15% or less, substantially not
containing B, and a remainder of Fe and unavoidable impurities, the microstructure
mainly composed of a mixed structure of ferrite and bainite, and the percentage of
bainite being 20 to 90%.
- (2) A 490 MPa class high-strength steel for welded structures excellent in high temperature
strength as set forth in (1), further comprising by wt% at least one of
Cu: 0.05 to 1.0%,
Ni: 0.05 to 1.0%,
Cr: 0.05 to 1.0%,
V: 0.01 to 0.1%,
Ti: 0.005 to 0.025%,
Ca: 0.0005 to 0.004%,
REM: 0.0005 to 0.004%, and
Mg: 0.0001 to 0.006%
- (3) A 490 MPa class high-strength steel for welded structures superior in high temperature
strength as set forth in (1) or (2), wherein an average circle equivalent diameter
of the prior austenite of a cross-section parallel to the rolling direction at a 1/4
thickness position is 120 µm or less.
- (4) A method of production of a 490 MPa class high-strength steel for welded structures
excellent in high temperature strength comprising the steps of; reheating semi-finished
products or cast products comprised of the steel compositions as set forth in (1)
or (2) to a range of 1100 to 1250°C, rolling it at a temperature of 850°C or more
with a cumulative amount of reduction at 1100°C or less of 30% or more, and cooling
it by air cooling or accelerated cooling from a temperature of 800°C or more to a
temperature of 650°C or less.
BEST MODE CARRYING OUT THE INVENTION
[0010] Below, details of the present invention will be explained.
[0011] For high temperature strength, complex addition of Mo and Nb to promote precipitation
of stable carbonitrides at the time of high temperatures and increase of the dislocation
density by conversion to bainite of the microstructure and delay of dislocation recovery
by solute Mo and Nb are effective. In particular, to realize strength at an extremely
high temperature of 700 to 800°C aimed at by the present invention, as an extension
of the prior discoveries, addition of a large amount of Mo is essential, but this
runs counter to the objective of securing excellent weldability and weld zone toughness
of welded structure steels. Achievement of these with high temperature strength is
extremely difficult.
[0012] According to research of the present inventors, by introducing suitable alloy elements
and controlling the microstructure, in particular by obtaining heat stability of the
matrix structure at a high temperature and a suitable coherent precipitation strengthening
effect and dislocation recovery delaying effect, it is possible to achieve both excellent
weldability and weld zone toughness and high temperature strength.
[0013] First, the reasons for limiting the steel compositions as in the claims in the present
invention will be explained.
[0014] C has the most remarkable effect on the properties of the steel material, so has
to be controlled to a narrow range. 0.005% to less than 0.040% is the range of limitation.
With an amount of C of less than 0.005%, the strength is insufficient, while with
0.040% or more, in the present invention with the large amount of addition of Mo,
the weldability and weld zone toughness are degraded and, when the cooling rate after
the end of rolling is excessive, the percentage of formation of bainite increases
and the risk of the strength becoming excessive rises. Further, to stably maintain
the mixed matrix structure of bainite and ferrite thermodynamically at the time of
high temperature heating corresponding to a fire and maintain the coherency with the
complex carbonitride precipitates of Mo, Nb, V, and Ti to secure a strengthening effect,
C has to be made less than 0.040%.
[0015] Si is an element contained in steel for deoxidation. It has a substitution type solid
solution hardening action, so is effective for improving the base material strength
at ordinary temperature, but there is no effect of improvement of the over 600°C high
temperature strength. Further, if added too much, the weldability and weld zone toughness
deteriorate, so the upper limit was made 0.5%. Steel can be deoxidized even with only
Ti and Al. The lower the content the better from the viewpoint of the weld zone toughness,
quenchability, etc. Addition is not necessarily required.
[0016] Mn is an element essential for securing strength and toughness. As a substitutional
type solid solution strengthening element, Mn is effective for raising the strength
at room temperature, but the effect of improvement is not that large for over 600°C
high temperature strength. Therefore, in steel containing a relatively large amount
of Mo like in the present invention, the content must be made less than 0.5% from
the viewpoint of improvement of the weldability, that is, the reduction of P
CM. Keeping the upper limit of the Mn low is also advantageous from the viewpoint of
the center segregation of the continuously cast slab. Note that, for the lower limit,
at least 0.1% has to be added for securing the strength and toughness of the base
material.
[0017] P and S are impurities in the steel of the present invention, the lower the better.
P segregates at the grain boundaries and encourages grain boundary fracture, while
S forms a sulfide such as MnS and causes deterioration of the toughness of the base
material and weld zone, so the upper limits are made 0.02% and 0.01%, respectively.
[0018] Mo is an essential element along with Nb from the viewpoint of achieving and maintaining
high temperature strength in the steel of the present invention. Simply for the high
temperature strength, the greater the amount added, the more advantageous, but this
should be limited if considering also the base material strength and weldability and
the weld zone toughness. In the present invention with the C being kept low, if within
the later explained range of P
CM (0.16% or less), Mo may be contained up to an amount of 1.5%. As the lower limit,
to stably secure high temperature strength even with complex addition with Nb or addition
of V and Ti effective for improving the high temperature strength explained later,
its addition of 0.3% or more is necessary.
[0019] Nb is an element added complexly together with Mo. First, as a general effect of
Nb, it raises the recrystallization temperature of austenite and is useful in bringing
out to the maximum extent the effect of controlled rolling at the time of hot rolling.
Further, it also contributes to increased fineness of the heated austenite at the
time of reheating before rolling. Further, it has the effect of improvement of the
high temperature strength by suppressing precipitation hardening and dislocation recovery.
Complex addition with Mo contributes to even greater improvement of the strength.
If less than 0.03%, the effect of suppressing precipitation hardening and dislocation
recovery at 700°C and 800°C is small. If over 0.15%, the degree of hardening is reduced
with respect to the amount of addition. Not only is this not preferably economically,
the weld zone also deteriorates in toughness. For these reasons, Nb is limited to
the range of 0.03 to 0.15%.
[0020] Al is an element generally included in steel for deoxidation, but sufficient deoxidation
is achieved by just Si or Ti. In the present invention, no lower limit is set (including
0%). However, if the amount of Al becomes larger, not only does the cleanliness of
the steel become poorer, but also the toughness of the weld zone deteriorates, so
the upper limit was made 0.06%.
[0021] N is contained in steel as an unavoidable impurity, but when adding Nb and the later
explained Ti, it bonds with the Nb to form a carbonitride to increase the strength
and forms TiN to improve the properties of the steel. Therefore, as the amount of
N, a minimum of 0.001% is necessary. However, an increase in the amount of N is harmful
to the weld zone toughness and weldability. In the present invention, the upper limit
is therefore made 0.006%. Note that the upper limit does not necessarily have any
limitative significance in terms of characteristics and is set in the range confirmed
by the present inventors.
[0022] Next, the reasons for addition and amounts of addition of the Cu, Ni, Cr, V, Ti,
Ca, REM, and Mg able to be contained in accordance with need will be explained.
[0023] The main purpose of adding these elements to the basic compositions is to improve
the strength, toughness, and other characteristics without detracting from the excellent
features of the steel of the present invention. Therefore, the amounts of addition
by nature should be naturally limited.
[0024] Cu improves the strength and toughness of the base material without having a remarkably
detrimental effect on the weldability and weld zone toughness. To realize these effects,
its addition of at least 0.05% is essential. On the other hand, excessive addition
not only causes the weldability to deteriorate, but also leads to increased risk of
occurrence of Cu cracks at the time of hot rolling, so the upper limit is set to 1.0%.
Note that it is known that Cu cracks themselves can be avoided by suitable addition
of Ni in accordance with the amount of Cu. The weldability is also related to the
amount of C and other alloy element, so the upper limit does not necessarily have
any limitative significance.
[0025] Ni exhibits an effect substantially the same as Cu and in particular has a large
effect on the improvement of the toughness of the base material. To reliably enjoy
these effects, addition of at least 0.05% is essential. On the other hand, excess
addition causes the weldability to deteriorate even with Ni. Since it is a relatively
expensive element, the economy is impaired, so in the present invention, the upper
limit is made 1.0% considering also targeting 490 MPa class steel.
[0026] Cr improves the strength of the base material, so can be added in accordance with
need. To enable clear differentiation with the entry of trace amounts as trap elements
from scrap etc. and reliably obtain the effects, addition of a minimum of 0.05% or
more is necessary. Too great an addition, like with other elements, causes the weldability
and weld zone toughness to deteriorate, so the upper limit is set at 1.0%.
[0027] As explained above, Cu, Ni, and Cr are effective not only from the viewpoint of the
mechanical properties of the base material, but also the weather resistance. For this
purpose, they are preferably positively added in a range not greatly detracting from
the weldability and weld zone toughness.
[0028] V has substantially the same effect and action as Nb including improvement of the
high temperature strength, but the effect is small compared with Nb. Further, V, as
will be understood from the fact that it is also included in the expression of P
CM, also influence the quenchability and weldability. Therefore, to reliably obtain
the effect of addition of V, the lower limit is made 0.01%. To eliminate any detrimental
effect, the upper limit is made 0.1%.
[0029] Ti, like Nb, V, etc., is effective in improving the high temperature strength. In
addition, when in particular the demands on the base material and weld zone toughness
are severe, its addition is preferable. The reason is that when the amount of Al is
small (for example, 0.003% or less), Ti bonds with O to form a precipitate mainly
comprised of Ti
2O
3 which form nuclei for the production of in-grain transformed ferrite and improve
the weld zone toughness. Further, Ti bonds with N and finely precipitates in the slab
as TiN. It suppresses the coarsening of the austenite grains at the time of heating
and is effective for increasing the fineness of the rolled structure. Further, the
fine TiN present in the steel plate increases the fineness of the structure of the
weld heat affected zone at the time of welding. To enjoy these effects, the content
of Ti has to be a minimum of 0.005%. However, if too great, it forms TiC and causes
the low temperature toughness and weldability to deteriorate, so the upper limit is
made 0.025%.
[0030] Ca and REM traps the impurity S and act to improve the toughness and suppress cracking
due to diffused hydrogen at the weld zone. If too great in amount, however, coarse
inclusions are formed and the toughness is detrimentally affected, so both elements
are limited o the range of 0.0005 to 0.004%, respectively. The two elements have substantially
equivalent effects, so to obtain the above effect, it is sufficient to add either
of the two.
[0031] Mg acts to suppress the growth of austenite grains and increase fineness in HAZ (heat
affected zone) and increases the toughness of the weld zone. To obtain such an effect,
Mg has to be at least 0.0001%. On the other hand, if the amount of addition is increased,
the extent of the effect with regard to the amount of addition becomes smaller and
economy is lost, so the upper limit is made 0.006%.
[0032] Note that in the present invention, B is not intentionally added. The point is that
it is not substantially contained over the level included as an impurity in the steelmaking
process. B remarkably improves the quenchability by addition in a small amount, so
when used for high-strength steel, it is advantageous in terms of control of the microstructure
or improvement of the strength and simultaneously has the risk of deterioration of
the weldability and weld zone toughness. The present invention avoids intentional
addition of B and is made substantially B-free for the purpose of greatly improving
not only the high temperature characteristics, but also the performance when used
as welded structure steel.
[0033] Even if limiting the individual ingredients of the steel as explained above, if the
system of the compositions as a whole is not suitable, the characteristic feature
of the present invention, that is, the excellent characteristics, is not obtained.
In particular, based on a previous invention (
Japanese Patent Application No. 2004-43961), since the invention is aimed at greatly improving the weldability and weld zone
toughness, the value of P
CM is limited to 0.15% or less. Here, P
CM is defined by the following formula as an index of the weld crack susceptibility:
P
CM=C+Si/30+Mn/20+Cu/20+Ni/60+Cr/20+Mo/15+V/10+5B
[0034] In general, the lower the P
CM, the better the weldability. If 0.22% or less, the preheating at the time of welding
(for preventing weld cold cracks) is said to be unnecessary. In high-strength steel,
in particular high-strength steel superior in high temperature strength like in the
present invention and substantially not containing B, which is an element remarkably
raising the quenchability, a P
CM of 0.15% or less is an extremely low value.
[0035] Further, in the present invention, the specific microstructure is also required.
With limiting just the steel compositions, superior weldability or weld zone toughness
as welded structure steel can be secured, but it is not possible to obtain satisfactory
high temperature characteristics or the basic characteristics as 490 MPa class steel,
in particular the strength. Therefore, to attain the object of the present invention,
the microstructure is limited to mainly a mixed structure of ferrite and bainite in
which the fraction of bainite is 20 to 90%. This is limited so as to clarify the characteristic
feature of the present invention based on the results of experiments by the present
inventors showing that if the percentage of bainite is low, securing 490 MPa class
room temperature strength and high temperature strength is difficult, while if the
fraction of bainite is too high, the risk of exceeding the range of strength of 490
MPa class steel defined by the JIS etc. increases and does not necessarily have any
limitative sense.
[0036] Note that these microstructures are assumed to represent a position of 1/4 thickness
in the direction of the thickness cross-section direction. Further, the term "bainite"
is widely used as the name of the structure among persons skilled in the art, but
in view of the diverse variations, some uncertainty may arise in terms of the specific
points in the region when measuring the fraction. In this case, there is also the
method of judgment by another structure, "ferrite", in the composition of the structure.
The fraction of ferrite in this case is 10 to 80%. The ferrite referred to here is
polygonal or pseudo-polygonal ferrite (not including acicular ferrite) not containing
any cementite.
[0037] The grain size of the austenite before transformation after rolling has to be suitably
limited in order to control the toughness of the steel containing a relatively high
percentage of Mo such as in the present invention (increasing the toughness). The
finer the grains of the austenite, the finer the final transformed microstructure
and the better the toughness. To obtain a toughness no different from ordinary steel
with low Mo, the austenite grain size at a position of 1/4 thickness in the plate
thickness cross-section direction is made an average circle equivalent diameter of
120 µm or less. Depending on the plate thickness or steel ingredients, sufficient
toughness is obtained even over 120 µm in some cases, while the grain size is limited
to enable toughness to be reliably and stably secured, but there is not necessarily
any limitative significance. Note that the austenite grain size is not necessarily
easy to judge in quite a few cases. In such a case, a notched impact test piece taken
from the steel plate in a direction perpendicular to the final rolling direction centered
at a 1/4 thickness position of the plate, for example, a JIS Z 2202 2 mm V-notch test
piece, is used. The fracture unit of brittle fracture at a sufficiently low temperature
is defined as the effective crystal grain size, able to be read as the "austenite
grain size", and the average circle equivalent diameter is measured. In this case
as well, similarly it must be 120 µm or less.
[0038] The above limited microstructure (microstructure, fraction of microstructure, prior
austenite grain size, etc.) and the high temperature characteristics and other excellent
characteristics aimed at by the present invention can be easily obtained by limiting
the method of production as follows.
[0039] The reheating temperature of the ingots or slabs having the predetermined steel compositions
is limited to the range of 1100 to 1250°C. The lower limit 1100°C is for making the
Mo and Nb and the V and Ti added according to need solute for the primary purpose
of securing the high temperature characteristics. To achieve this object, the higher
the reheating temperature, the better, but the heated austenite grains coarsen which
is not preferable from the viewpoint of the base material toughness, so the upper
limit is made 1250°C.
[0040] The rolling conditions are limited in order to directly control the austenite grain
size after rolling and before transformation to relatively fine grains as explained
above and for mainly securing toughness. Therefore, the rolling has to be performed
with an amount of cumulative reduction at 1100°C or less of 30% or more. The rolling
end temperature is limited to 850°C or more as the lower limit temperature for the
Mo and Nb or the V and Ti added in accordance with need to precipitate as carbides
under low temperature rolling.
[0041] The cooling after rolling should also be limited from the viewpoint of control of
the structure. While depending on the steel compositions, when producing relatively
thin plates, even with the cooling rate of an extent of air cooling, a predetermined
microstructure can be obtained, but if thick plates, the cooling rate becomes slow
with air cooling and accelerated cooling becomes necessary in some cases. The accelerated
cooling in this case is, in steel plate production, most generally water cooling,
but it does not necessarily have to be water cooling. Further, the accelerated cooling
is meant to raise the cooling rate of the transformation region for controlling the
microstructure, so has to be performed from a temperature of 800°C or more to a temperature
of 650°C or less.
[0042] Note that, in the present invention, "high temperature strength" targets 600°C to
800°C. The quantitative target is a ratio p of the high temperature yield strength
to the ordinary temperature yield strength (=high temperature yield strength/ordinary
temperature yield strength) of p≥-0.0033×T+2.80 in the range of a steel material temperature
T (°C) of 600°C to 800°C.
EXAMPLES
[0043] Using the converter-continuous casting-plate rolling process, steel plates of various
ingredients (thickness of 12 to 80 mm) were produced, evaluated for their mechanical
properties and weldability and weld zone toughness, and investigated for the presence
of root cracks in a JIS-based y-groove weld crack test and for simulated HAZ toughness
corresponding to small input heat and extra large input heat welding by a weld simulating
thermal cycle. Table 1 shows the steel compositions of comparative examples and examples
of the present invention, the production conditions, the microstructure and results
of investigation of the various characteristics.
[0044] The examples of the present invention all satisfy the ranges of limitation of the
present invention and are extremely good in high temperature strength, simulated HAZ
toughness, and other various characteristics. As opposed to this, the comparative
examples have at least one of the steel compositions, production conditions, structure,
etc. outside the ranges of limitation of the present invention, so it is learned that
the characteristics are poor compared with the examples of the present invention.
That is, Comparative Example 19 has a low amount of C, so the fraction of bainite
is low and the ordinary temperature strength and high temperature strength (ratio)
are both low. Comparative Example 20 has a high amount of C, so the fraction of bainite
is high and the ordinary temperature strength is high. Further, the base material
toughness and the simulated HAZ toughness is also poor. Comparative Example 21 has
a low amount of Mo and is low in accelerated cooling start temperature as well, so
the fraction of bainite is low and due in part to this the high temperature strength
(ratio) is low. Comparative Example 22 has a low amount of Nb and is low in the heating
temperature and rolling end temperature as well and further is high in accelerated
cooling stop temperature, so is low in ordinary temperature strength and high temperature
strength (ratio). Comparative Example 23 has B added to it, so when using accelerated
cooling, the fraction of bainite is high and the base material toughness is poor.
Further, the simulated HAZ toughness is also poor. Comparative Example 24 has a high
amount of Mn and is high in P
CM and further is low in the cumulative amount of reduction at 1100°C or less, so the
fraction of bainite becomes high, the base material strength of the 490 MPa class
steel, and the base material toughness and simulated HAZ toughness are poor.
[0045] Note that for root cracks in the y-groove weld crack test did not occur even in cases
such as Comparative Example 24 where the P
CM is about 0.185% though higher than the range of limitation of the present invention.
Table 1
Class |
Steel |
Room temp. yield strength (MPa) |
Room temp. tensile stress (MPa) |
Ratio of yield strength to room temp. yield strength (p) |
vTrs (°C) |
Fraction of bainite in base material microstructure (%) |
prior austenite grain size(µm) |
Simulated HAZ toughness, vEo(J) |
Root Cracks |
600°C |
700°C |
800°C |
Heat history 1 |
Heat history 2 |
|
1 |
476 |
541 |
0.87 |
0.61 |
0.25 |
-45 |
54 |
50 |
89 |
69 |
No crack |
|
2 |
451 |
537 |
0.85 |
0.57 |
0.24 |
-31 |
52 |
71 |
82 |
62 |
No crack |
|
3 |
438 |
534 |
0.86 |
0.57 |
0.25 |
-36 |
61 |
63 |
97 |
84 |
No crack |
|
4 |
442 |
533 |
0.87 |
0.55 |
0.25 |
-40 |
29 |
47 |
87 |
64 |
No crack |
|
5 |
407 |
509 |
0.87 |
0.55 |
0.25 |
-35 |
37 |
74 |
83 |
67 |
No crack |
|
6 |
421 |
547 |
0.90 |
0.58 |
0.24 |
-31 |
65 |
83 |
79 |
65 |
No crack |
|
7 |
425 |
545 |
0.88 |
0.57 |
0.22 |
-34 |
57 |
109 |
78 |
69 |
No crack |
|
8 |
433 |
548 |
0.86 |
0.54 |
0.27 |
-37 |
60 |
68 |
80 |
88 |
No crack |
Inv. Ex. |
9 |
419 |
530 |
0.86 |
0.59 |
0.25 |
-30 |
42 |
55 |
82 |
65 |
No crack |
10 |
410 |
516 |
0.85 |
0.59 |
0.24 |
-32 |
48 |
61 |
96 |
63 |
No crack |
|
11 |
431 |
553 |
0.85 |
0.58 |
0.24 |
-30 |
51 |
97 |
88 |
69 |
No crack |
|
12 |
424 |
523 |
0.85 |
0.59 |
0.22 |
-28 |
45 |
60 |
90 |
83 |
No crack |
|
13 |
451 |
564 |
0.85 |
0.58 |
0.25 |
-35 |
64 |
64 |
79 |
86 |
No crack |
|
14 |
462 |
570 |
0.84 |
0.58 |
0.24 |
-32 |
67 |
52 |
86 |
71 |
No crack |
|
15 |
433 |
528 |
0.86 |
0.59 |
0.25 |
-38 |
59 |
67 |
82 |
68 |
No crack |
|
16 |
415 |
532 |
0.86 |
0.62 |
0.24 |
-35 |
65 |
55 |
83 |
74 |
No crack |
|
17 |
442 |
526 |
0.84 |
0.62 |
0.24 |
-32 |
62 |
58 |
91 |
67 |
No crack |
|
18 |
480 |
571 |
0.85 |
0.61 |
0.23 |
-37 |
76 |
46 |
88 |
65 |
No crack |
|
19 |
322 |
478 |
0.69 |
0.46 |
0.14 |
-47 |
16 |
59 |
78 |
96 |
No crack |
|
20 |
517 |
631 |
0.81 |
0.52 |
0.17 |
-3 |
96 |
62 |
31 |
22 |
No crack |
Comp. Ex. |
21 |
392 |
501 |
0.72 |
0.44 |
0.15 |
-49 |
18 |
51 |
80 |
56 |
No crack |
22 |
358 |
484 |
0.78 |
0.45 |
0.14 |
-21 |
47 |
70 |
83 |
61 |
No crack |
|
23 |
465 |
566 |
0.86 |
0.57 |
0.23 |
-3 |
95 |
68 |
13 |
16 |
No crack |
|
24 |
481 |
628 |
0.83 |
0.55 |
0.22 |
-1 |
93 |
132 |
38 |
19 |
No crack |
Tensile test piece: Thickness 40 mm or less, JIS Z 2201 1A (total thickness); thickness
over 50 mm, JIS Z 2201 4 (1/4 thickness), direction perpendicular to rolling direction
Charpy impact test piece: JIS Z 2202 2 mm V-notch, rolling direction
High temperature tensile test piece: rod (8 mm or 10 mmφ), 1/4 thickness position,
direction perpendicular to rolling direction
Heat history 1: 1400°C x 1 sec, cooling time 800→500°C 8 sec
Heat history 2: 1400°C x 30 sec, cooling time 800→500°C 330 sec |
INDUSTRIAL APPLICABILITY
[0046] The steel material produced by the steel compositions and method of production based
on the present invention satisfies the range of limitation of in terms of the microstructure
as well and is excellent in high temperature strength, weldability and weld zone toughness.
The development of welded structure steel having high temperature characteristics
far superior to the fire-resistant steel guaranteeing high temperature characteristics
up to the conventional 600°C or so can be stably mass produced on an industrial basis.
In particular, as building applications, a major increase in the buildings used for
and complete elimination of fire-resistant coverings can be expected.