U.S. patent application number 11/215413 was filed with the patent office on 2006-01-26 for high-strength steel for welded structures excellent in high temperature strength and method of production of the same.
Invention is credited to Tatsuya Kumagai, Yasushi Mizutani, Ryuji Uemori, Yoshiyuki Watanabe.
Application Number | 20060016526 11/215413 |
Document ID | / |
Family ID | 35655876 |
Filed Date | 2006-01-26 |
United States Patent
Application |
20060016526 |
Kind Code |
A1 |
Mizutani; Yasushi ; et
al. |
January 26, 2006 |
High-strength steel for welded structures excellent in high
temperature strength and method of production of the same
Abstract
The present invention provides a high-strength steel for welded
structures excellent in strength at a high temperature of a
temperature range of 600.degree. C. to 800.degree. C. and a method
of production of the same, in particular 490 MPa class
high-strength steel for welded structures excellent in high
temperature strength containing 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 and in accordance with need one or more of Cu,
Ni, Cr, V, Ti, Ca, REM, and Mg, having a weld crack susceptible
formulation P.sub.CM defined as
P.sub.CM=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 Fe and
unavoidable impurities, the microstructure being mainly composed of
a mixed structure of ferrite and bainite, and the fraction of
bainite being 20 to 90%.
Inventors: |
Mizutani; Yasushi;
(Kimitsu-shi, JP) ; Watanabe; Yoshiyuki;
(Kimitsu-shi, JP) ; Uemori; Ryuji; (Kimitsu-shi,
JP) ; Kumagai; Tatsuya; (Tokai-shi, JP) |
Correspondence
Address: |
Kenyon & Kenyon
One Broadway
New York
NY
10004
US
|
Family ID: |
35655876 |
Appl. No.: |
11/215413 |
Filed: |
August 29, 2005 |
Related U.S. Patent Documents
|
|
|
|
|
|
Application
Number |
Filing Date |
Patent Number |
|
|
PCT/JP05/13101 |
Jul 21, 2004 |
|
|
|
11215413 |
Aug 29, 2005 |
|
|
|
Current U.S.
Class: |
148/654 ;
148/334 |
Current CPC
Class: |
C21D 8/0226 20130101;
C21D 2211/002 20130101; C21D 2211/005 20130101; C21D 8/00 20130101;
C22C 38/02 20130101; C22C 38/04 20130101; C22C 38/12 20130101 |
Class at
Publication: |
148/654 ;
148/334 |
International
Class: |
C21D 8/00 20060101
C21D008/00; C22C 38/22 20060101 C22C038/22 |
Foreign Application Data
Date |
Code |
Application Number |
Jul 21, 2004 |
JP |
2004-213511 |
Claims
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
P.sub.CM defined as
P.sub.CM=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 claim 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
excellent in high temperature strength as set forth in claim 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 .mu.m 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 claim
1 or 2 to a range of 1100 to 1250.degree. C., rolling it at a
temperature of 850.degree. C. or more with a cumulative amount of
reduction at 1100.degree. C. or less of 30% or more, and cooling it
by air cooling or accelerated cooling from a temperature of
800.degree. C. or more to a temperature of 650.degree. C. or less.
Description
RELATED APPLICATION
[0001] This is a continuation-in-part application of International
Application No. PCT/JP2005/013101, filed on Jul. 8, 2005, based on
the Japanese priority application No. 2004-213511, filed on Jul.
21, 2004.
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.degree. C. to 800.degree. 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.degree. C. The allowable
service temperature is considered to be about 500.degree. 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.degree. C. or more at the time of fires. This is because steel
materials fall in yield strength at 350.degree. 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.degree. C. and 700.degree. 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.degree. 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.degree. 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.degree. C. Development of steel
materials for use at 700 to 800.degree. 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.degree. C. to
800.degree. 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.
[0010] (1) A 490 MPa class high-strength steel for welded
structures excellent in high temperature strength, comprising as
steel compositions, by wt %, [0011] C: 0.005% to less than 0.040%,
[0012] Si: 0.5% or less, [0013] Mn: 0.1 to less than 0.5%, [0014]
P: 0.02% or less, [0015] S: 0.01% or less, [0016] Mo: 0.3 to 1.5%,
[0017] Nb: 0.03 to 0.15%, [0018] Al: 0.06% or less, and [0019] N:
0.006% or less, [0020] having a weld crack susceptible formulation
P.sub.CM defined as
P.sub.CM=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%.
[0021] (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 [0022] Cu: 0.05 to
1.0%, [0023] Ni: 0.05 to 1.0%, [0024] Cr: 0.05 to 1.0%, [0025] V:
0.01 to 0.1%, [0026] Ti: 0.005 to 0.025%, [0027] Ca: 0.0005 to
0.004%, [0028] REM: 0.0005 to 0.004%, and [0029] Mg: 0.0001 to
0.006%
[0030] (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 .mu.m or less.
[0031] (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.degree. C., rolling it at a
temperature of 850.degree. C. or more with a cumulative amount of
reduction at 1100.degree. C. or less of 30% or more, and cooling it
by air cooling or accelerated cooling from a temperature of
800.degree. C. or more to a temperature of 650.degree. C. or
less.
BEST MODE CARRYING OUT THE INVENTION
[0032] Below, details of the present invention will be
explained.
[0033] 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.degree. 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.
[0034] 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.
[0035] First, the reasons for limiting the steel compositions as in
the claims in the present invention will be explained.
[0036] 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%.
[0037] 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.degree. 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.
[0038] 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.degree. 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.sub.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.
[0039] 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.
[0040] 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.sub.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.
[0041] 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.degree. C.
and 800.degree. 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%.
[0042] 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%.
[0043] 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.
[0044] 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.
[0045] 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.
[0046] 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.
[0047] 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.
[0048] 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%.
[0049] 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.
[0050] 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.sub.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%.
[0051] 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.sub.2O.sub.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%.
[0052] 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.
[0053] 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%.
[0054] 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.
[0055] 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.sub.CM is limited to 0.15% or less. Here, P.sub.CM is
defined by the following formula as an index of the weld crack
susceptibility:
P.sub.CM=C+Si/30+Mn/2O+Cu/2O+Ni/60+Cr/20+Mo/15+V/10+5B
[0056] In general, the lower the P.sub.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.sub.CM of 0.15% or less
is an extremely low value.
[0057] 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.
[0058] 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.
[0059] 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 .mu.m or less. Depending on the plate thickness or
steel ingredients, sufficient toughness is obtained even over 120
.mu.m 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 .mu.m or
less.
[0060] 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.
[0061] The reheating temperature of the ingots or slabs having the
predetermined steel compositions is limited to the range of 1100 to
1250.degree. C. The lower limit 1100.degree. 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.degree. C.
[0062] 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.degree. C.
or less of 30% or more. The rolling end temperature is limited to
850.degree. 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.
[0063] 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.degree. C. or more to a
temperature of 650.degree. C. or less.
[0064] Note that, in the present invention, "high temperature
strength" targets 600.degree. C. to 800.degree. 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.gtoreq.-0.0033.times.T+2.80 in the range of a steel material
temperature T (.degree. C.) of 600.degree. C. to 800.degree. C.
EXAMPLES
[0065] 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, Table 2 shows the production
conditions, and Table 3 shows the microstructure and results of
investigation of the various characteristics.
[0066] 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.sub.CM and further is low in the cumulative amount of
reduction at 1100.degree. 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.
[0067] 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.sub.CM is about 0.185% though higher than the range of
limitation of the present invention. TABLE-US-00001 TABLE 1
Chemical compositions (wt %, N&B: ppm) Class Steel C Si Mn P S
Mo Nb Al N Cu Invention 1 0.028 0.33 0.15 0.006 0.003 1.29 0.040
0.031 30 examples 2 0.020 0.14 0.18 0.004 0.003 0.80 0.039 0.004 53
3 0.018 0.15 0.33 0.008 0.003 0.50 0.120 0.035 34 0.95 4 0.026 0.22
0.30 0.003 0.001 1.10 0.040 0.033 32 5 0.035 0.10 0.38 0.004 0.004
1.12 0.032 0.003 42 6 0.038 0.14 0.20 0.008 0.005 0.80 0.050 0.004
26 7 0.035 0.20 0.40 0.008 0.003 0.40 0.140 0.020 52 8 0.026 0.08
0.36 0.004 0.005 0.50 0.056 0.035 26 9 0.027 0.15 0.22 0.006 0.007
1.10 0.055 0.022 47 10 0.024 0.20 0.20 0.008 0.008 1.18 0.048 0.006
36 11 0.026 0.25 0.20 0.004 0.003 1.25 0.059 0.033 33 12 0.027 0.14
0.28 0.008 0.002 0.80 0.080 0.025 29 13 0.024 0.05 0.22 0.006 0.003
1.45 0.100 0.030 33 14 0.027 0.15 0.20 0.005 0.002 0.90 0.040 0.004
38 0.30 15 0.006 0.28 0.18 0.004 0.003 1.30 0.050 0.030 29 0.50 16
0.029 0.12 0.49 0.004 0.007 0.90 0.039 0.044 29 17 0.035 0.20 0.15
0.007 0.003 0.60 0.039 0.006 45 18 0.028 0.09 0.12 0.007 0.005 1.30
0.035 0.012 37 Comparative 19 0.001 0.20 0.45 0.006 0.006 0.80
0.049 0.025 31 examples 20 0.045 0.19 0.30 0.008 0.005 0.65 0.050
0.026 30 21 0.032 0.20 0.31 0.008 0.005 0.24 0.050 0.025 28 22
0.033 0.20 0.30 0.008 0.005 0.55 0.010 0.024 32 23 0.032 0.18 0.30
0.007 0.008 0.54 0.048 0.026 35 24 0.034 0.18 0.80 0.008 0.004 1.20
0.049 0.026 24 Chemical compositions (wt %, N&B: ppm) Class
Steel Ni Cr V Ti Ca REM Mg B P.sub.CM Invention 1 0.133 examples 2
0.52 0.007 0.113 3 0.90 0.015 0.135 4 0.020 0.0015 0.122 5 0.033
0.009 0.135 6 0.058 0.112 7 0.30 0.012 0.093 8 0.75 0.045 0.021
0.0030 0.122 9 0.015 0.116 10 0.119 11 0.008 0.0020 0.128 12 0.20
0.031 0.0022 0.112 13 0.011 0.0015 0.133 14 0.15 0.008 0.0012 0.120
15 0.40 0.012 0.143 16 0.012 0.0011 0.118 17 0.045 0.0018 0.094 18
0.088 0.012 0.132 Comparative 19 0.50 0.010 0.109 examples 20 0.35
0.011 0.127 21 0.35 0.010 0.088 22 0.35 0.009 0.109 23 0.34 0.010
10 0.111 24 0.50 0.010 0.185
[0068] TABLE-US-00002 TABLE 2 Accelerated Rolling end Cumulative
cooling stop Plate Heating temp. reduction ratio at Accelerated
cooling temp. thickness Class Steel temp. (.degree. C.) (.degree.
C.) 1100.degree. C. or less (%) start temp. (.degree. C.) (.degree.
C.) (mm) Invention 1 1150 880 80 -- -- 12 example 2 1200 900 60 --
-- 32 3 1100 880 50 850 500 50 4 1150 910 70 -- -- 25 5 1100 870 50
-- -- 50 6 1100 900 40 880 480 70 7 1100 970 30 820 450 80 8 1100
950 50 820 530 50 9 1150 990 60 -- -- 32 10 1100 970 60 -- -- 32 11
1250 1000 60 -- -- 40 12 1100 960 50 -- -- 45 13 1150 920 60 850
580 32 14 1100 900 60 850 400 32 15 1150 880 50 820 550 40 16 1100
900 50 860 350 40 17 1050 860 50 810 480 40 18 1100 960 70 900 570
20 Comparative 19 1150 950 50 -- -- 60 example 20 1150 925 60 830
500 32 21 1150 940 50 780 520 45 22 1050 830 35 820 650 75 23 1250
850 60 850 570 32 24 1200 920 20 870 480 80
[0069] TABLE-US-00003 TABLE 3 Room Room Fraction of temp. temp.
bainite in base Simulated HAZ toughness, yield tensile Ratio of
yield strength to material vE.sub.o (J) strength stress room temp.
yield strength (p) vTrs microstructure Prior austenite Heat Heat
Root Class Steel (MPa) (MPa) 600.degree. C. 700.degree. C.
800.degree. C. (.degree. C.) (%) grain size(.mu.m) history 1
history 2 Cracks Inv. 1 476 541 0.87 0.61 0.25 -45 54 50 89 69 No
crack Ex. 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 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 Comp. 19 322 478 0.69 0.46 0.14 -47 16 59 78 96 No crack
Ex. 20 517 631 0.81 0.52 0.17 -3 96 62 31 22 No crack 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 mm.phi.), 1/4 thickness
position, direction perpendicular to rolling direction Heat history
1: 1400.degree. C. .times. 1 sec, cooling time
800.fwdarw.500.degree. C. 8 sec Heat history 2: 1400.degree. C.
.times. 30 sec, cooling time 800.fwdarw.500.degree. C. 330 sec
INDUSTRIAL APPLICABILITY
[0070] 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.degree. 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.
* * * * *