U.S. patent application number 12/736153 was filed with the patent office on 2011-01-06 for fire-resistant steel material superior in weld heat affected zone reheat embrittlement resistance and low temperature toughness and method of production of same.
Invention is credited to Yasushi Hasegawa, Masaki Mizoguchi, Yoshiyuki Watanabe.
Application Number | 20110002808 12/736153 |
Document ID | / |
Family ID | 42128759 |
Filed Date | 2011-01-06 |
United States Patent
Application |
20110002808 |
Kind Code |
A1 |
Mizoguchi; Masaki ; et
al. |
January 6, 2011 |
FIRE-RESISTANT STEEL MATERIAL SUPERIOR IN WELD HEAT AFFECTED ZONE
REHEAT EMBRITTLEMENT RESISTANCE AND LOW TEMPERATURE TOUGHNESS AND
METHOD OF PRODUCTION OF SAME
Abstract
The present invention provides a fire-resistant steel material
superior in weld heat affected zone reheat embrittlement resistance
and low temperature toughness when welded by large heat input and
exposed to fire and a method of production of the same, that is, a
material containing C: 0.012 to 0.050%, Mn: 0.80 to 2.00%, Cr: 0.80
to 1.90%, and Nb: 0.01 to less than 0.05%, restricting Cu to 0.10%
or less, containing suitable quantities of Si, N, Ti, and Al,
restricting the contents of Mo, B, P, S, and O, and having a
balance of Fe and unavoidable impurities, having contents of C, Mn,
Cr, Nb, and Cu satisfying -1200C-20Mn+30Cr-330Nb-120Cu.gtoreq.-80,
having a steel structure as observed by an optical microscope of an
area fraction of 80% or more of a ferrite phase, and having a
balance of the steel structure of a bainite phase, martensite
phase, and mixed martensite-austenite structure.
Inventors: |
Mizoguchi; Masaki; (Tokyo,
JP) ; Hasegawa; Yasushi; (Tokyo, JP) ;
Watanabe; Yoshiyuki; (Tokyo, JP) |
Correspondence
Address: |
KENYON & KENYON LLP
ONE BROADWAY
NEW YORK
NY
10004
US
|
Family ID: |
42128759 |
Appl. No.: |
12/736153 |
Filed: |
October 15, 2009 |
PCT Filed: |
October 15, 2009 |
PCT NO: |
PCT/JP2009/068150 |
371 Date: |
September 14, 2010 |
Current U.S.
Class: |
420/83 ; 148/645;
420/104; 420/106; 420/110; 420/112; 420/84; 420/90; 72/201 |
Current CPC
Class: |
C21D 2211/005 20130101;
C21D 2211/008 20130101; C22C 38/06 20130101; C22C 38/04 20130101;
C22C 38/001 20130101; C21D 9/50 20130101; C22C 38/02 20130101; C21D
8/0226 20130101; C21D 1/25 20130101; C22C 38/28 20130101; C22C
38/26 20130101 |
Class at
Publication: |
420/83 ; 148/645;
420/90; 420/104; 420/110; 420/106; 420/112; 420/84; 72/201 |
International
Class: |
C22C 38/00 20060101
C22C038/00; C21D 8/00 20060101 C21D008/00; C22C 38/20 20060101
C22C038/20; C22C 38/18 20060101 C22C038/18; C22C 38/12 20060101
C22C038/12; C22C 38/22 20060101 C22C038/22; C22C 38/08 20060101
C22C038/08; C22C 38/32 20060101 C22C038/32; B21B 27/06 20060101
B21B027/06 |
Foreign Application Data
Date |
Code |
Application Number |
Oct 27, 2008 |
JP |
2008-275900 |
Claims
1. A fire-resistant steel material superior in weld heat affected
zone reheat embrittlement resistance and low temperature toughness
characterized by containing, by mass %, C: 0.012% to 0.050%, Si:
0.01% to 0.50%, Mn: 0.80% to 2.00%, Cr: 0.80% to 1.90%, Nb: 0.01%
to 0.05%, N: 0.001% to 0.006%, Ti: 0.010% to 0.030%, and Al: 0.005%
to 0.10%, further limiting the contents of Cu, Mo, B, P, S, and O
to: Cu: 0.10% or less, Mo: less than 0.01%, B: less than 0.0003%,
P: less than 0.02%, S: less than 0.01%, and less than 0.01%, and
having a balance of Fe and unavoidable impurities, having contents
of C, Mn, Cr, Nb, and Cu [mass %] satisfying
-1200C-20Mn+30Cr-330Nb-120Cu.gtoreq.-80, having a steel structure
as observed by an optical microscope of an area fraction of 80% or
more of a ferrite phase, and having a balance of the steel
structure of a bainite phase, martensite phase, and mixed
martensite-austenite structure.
2. A fire-resistant steel material superior in weld heat affected
zone reheat embrittlement resistance and low temperature toughness
as set forth in claim 1 characterized by further containing, by
mass %, one or both of V: 0.40% or less and Ni: 1.00% or less.
3. A fin-resistant steel material superior in weld heat affected
zone reheat embrittlement resistance and low temperature toughness
as set forth in claim 1 characterized by further containing, by
mass %, Zr: 0.010% or less, Mg: 0.005% or less, Ca: 0.005% or less,
Y: 0.050% or less, La: 0.050% or less, and Ce: 0.050% or less.
4. A method of production of a fire-resistant steel material
superior in weld heat affected zone reheat embrittlement resistance
and low temperature toughness characterized by heating a steel slab
having the steel compositions as set forth in claim 1 to 1150 to
1300.degree. C. temperature, then hot working or hot rolling it at
800.degree. C. to 900.degree. C. temperature by a reduction ratio
of 50% or more, then allowing it to cool.
5. A method of production of a fire-resistant steel material
superior in weld heat affected zone reheat embrittlement resistance
and low temperature toughness characterized by applying the method
of production as set forth in claim 4, then treating the steel
material at a 400.degree. C. to less than 650.degree. C.
temperature range for 5 minutes to 360 minutes for tempering heat
treatment.
Description
TECHNICAL FIELD
[0001] The present invention relates to a fire-resistant steel
material superior in weld heat affected zone reheat embrittlement
resistance and low temperature toughness and a method of production
of the same.
BACKGROUND ART
[0002] Steel structures of buildings etc. are required to exhibit
strength for a certain period to prevent collapse and enable
residents to escape when exposed to fire. However, in general,
steel materials fall in strength when exposed to high temperatures.
Therefore, in the past, as a countermeasure, the technique has been
employed of providing the steel material with a fire-resistant
covering for the purpose of keeping down the rise of temperature of
the steel material at the time of fire.
[0003] On the other hand, in recent years, due to environmental
issues and issues of aesthetics etc., art has been proposed for
forming steel structures without using fire-resistant coverings.
Various scales of fires and ambient temperatures etc. may be
envisioned, so when not providing the steel materials with
fire-resistant coverings, the steel materials supporting the
strength of the structures are required to be made as high as
possible in strength at high temperatures. The property of not
easily falling in strength even at high temperatures is called the
"fire-resistant performance".
[0004] For steel materials providing such fire-resistant
performance, in the past Mo has been positively utilized. Mo is an
element useful for raising the high temperature strength by
precipitation strengthening. However, in recent years, the price of
Mo has skyrocketed, so art based on alloy designs not relying on
addition of Mo has been proposed (for example, see PLTs 1 to
4).
[0005] Further, when a steel structure is exposed to fire, the heat
affected zone (below, sometimes called the "HAZ") of the weld joint
sometimes cannot keep up with the deformation and breaks. The low
level of deformation ability of the HAZ when exposed to a high
temperature (below, sometimes called "HAZ reheat embrittlement") is
particularly remarkable in steel containing Mo or B. For this
reason, steel has been proposed using solution strengthening by Nb
to raise the high temperature strength and suppress the addition of
Mo and B (for example, see PLT 5).
Citation List
[0006] Patent Literature
[0007] PLT 1: Japanese Patent Publication (A) No. 2002-115022
[0008] PLT 2: Japanese Patent Publication (A) No. 2007-211278
[0009] PLT 3: Japanese Patent Publication (A) No. 2007-224415
[0010] PLT 4: Japanese Patent Publication (A) No. 2008-88547
[0011] PLT 5: Japanese Patent Publication (A) No. 2008-121081
SUMMARY OF INVENTION
Technical Problem
[0012] In recent years, buildings have become larger in size and
higher in stories. In particular, when welded structures become
larger in size, the steel materials used become larger in size and
the required welding efficiency becomes higher. Due to this, the
heat input at the time of welding becomes higher. With large heat
input welding, the rise in temperature of the HAZ at the time of
welding becomes remarkable and the cooling rate falls.
[0013] For this reason, coarsening of the grains of the old
austenite (below, sometimes referred to as "old .gamma.") and
precipitation of carbides etc. at the old .gamma.-grain boundaries
of the HAZ are promoted. As a result, the drop in reheat
embrittlement and toughness of the HAZ becomes remarkable.
[0014] Further, to raise the high temperature strength of a steel
material, after hot rolling, it is preferable to perform
accelerated cooling to suppress the formation of bainite. On the
other hand, if performing accelerated cooling, due to the
temperature control at the time of cooling or nonuniformity of the
cooling, the steel material sometimes deforms. Therefore, as the
method of production of a steel material, the method of not
performing accelerated cooling after hot rolling, but allowing
natural cooling is preferable.
[0015] However, when allowing natural cooling after hot rolling, a
bainite structure becomes difficult to obtain. This is
disadvantageous in obtaining high temperature strength.
Furthermore, if increasing the amount of addition of alloy elements
to secure high temperature strength without accelerated cooling,
there was the problem that grain boundary precipitation etc. caused
reheat embrittlement of the HAZ.
[0016] The present invention was made in consideration of the above
problem and has as its object the provision of a fire-resistant
steel material superior in HAZ reheat embrittlement resistance and
low temperature toughness even in the case of large heat input
welding and a method of production of the same.
Solution to Problem
[0017] The inventors engaged in detailed studies through
experimentation and analysis on the chemical compositions and
production conditions for preventing the reheat embrittlement of
large heat input HAZ and securing low temperature toughness of the
HAZ. As a result, they learned that to secure both reheat
embrittlement resistance and low temperature toughness of the HAZ,
control of the contents of the C, Mn, Cr, Nb, and Cu is extremely
important.
[0018] The gist of the present invention, based on this discovery,
is as follows: [0019] (1) A fire-resistant steel material superior
in weld heat affected zone reheat embrittlement resistance and low
temperature toughness characterized by containing, by mass %,
[0020] C: 0.012% to 0.050%, [0021] Si: 0.01% to 0.50%, [0022] Mn:
0.80% to 2.00%, [0023] Cr: 0.80% to 1.90%, [0024] Nb: 0.01% to
0.05%, [0025] N: 0.001% to 0.006%, [0026] Ti: 0.010% to 0.030%, and
[0027] Al: 0.005% to 0.10%, further limiting the contents of Cu,
Mo, B, P, S, and O to: [0028] Cu: 0.10% or less, [0029] Mo: less
than 0.01%, [0030] B: less than 0.0003%, [0031] P: less than 0.02%,
[0032] S: less than 0.01%, and [0033] O: less than 0.01%, and
having a balance of Fe and unavoidable impurities, having contents
of C, Mn, Cr, Nb, and Cu [mass %] satisfying
[0033] -1200C-20Mn+30Cr-330Nb-120Cu.gtoreq.-80,
having a steel structure as observed by an optical microscope of an
area fraction of 80% or more of a ferrite phase, and having a
balance of the steel structure of a bainite phase, martensite
phase, and mixed martensite-austenite structure. [0034] (2) A
fire-resistant steel material superior in weld heat affected zone
reheat embrittlement resistance and low temperature toughness as
set forth in the above (1) characterized by further containing, by
mass %, one or both of [0035] V: 0.40% or less and [0036] Ni: 1.00%
or less. [0037] (3) A fire-resistant steel material superior in
weld heat affected zone reheat embrittlement resistance and low
temperature toughness as set forth in the above (1) or (2)
characterized by further containing, by mass %, [0038] Zr: 0.010%
or less, [0039] Mg: 0.005% or less, [0040] Ca: 0.005% or less,
[0041] Y: 0.050% or less, [0042] La: 0.050% or less, and [0043] Ce:
0.050% or less. [0044] (4) A method of production of a
fire-resistant steel material superior in weld heat affected zone
reheat embrittlement resistance and low temperature toughness
characterized by heating a steel slab having the steel compositions
as set forth in any of the above (1) to (3) to 1150 to 1300.degree.
C. temperature, then hot working or hot rolling it at 800.degree.
C. to 900.degree. C. temperature by a reduction ratio of 50% or
more, then allowing it to cool. [0045] (5) A method of production
of a fire-resistant steel material superior in weld heat affected
zone reheat embrittlement resistance and low temperature toughness
characterized by applying the method of production as set forth in
the above (4), then treating the steel material at a 400.degree. C.
to less than 650.degree. C. temperature range for 5 minutes to 360
minutes for tempering heat treatment.
Advantageous Effects of Invention
[0046] According to the present invention, a fire-resistant steel
material having a high yield strength at 600.degree. C. temperature
even if exposed to fire, suppressed in reheat embrittlement at the
weld heat affected zone, and superior in low temperature toughness
of the base material and weld heat affected zone is obtained.
Further, it becomes possible to produce a fire-resistant steel
material superior in weld heat affected zone reheat embrittlement
resistance and low temperature toughness by a high productivity
method of production using the steel as hot rolled.
[0047] Therefore, the contribution of the fire-resistant steel
material of the present invention to the securing of the safety of
buildings using it is extremely great. The contribution to the
industry is extremely remarkable.
BRIEF DESCRIPTION OF DRAWINGS
[0048] FIG. 1 is a view showing the effects of C, Mn, Cr, Nb, and
Cu on the reheat embrittlement resistance of the HAZ.
EMBODIMENTS OF INVENTION
[0049] As one feature of the present invention, the positive use of
Cr may be mentioned. Even if adding Cr, this will not contribute
much at all to the yield strength or tensile strength at room
temperature and the high temperature strength. However, the
addition of Cr results in a remarkable alleviation of HAZ reheat
embrittlement.
[0050] This is considered to be due to the fact that Cr forms
several nm to tens of nm clusters of carbides. Due to the formation
of fine carbides of Cr, formation of coarse carbides causing
embrittlement of the grain boundaries and segregation of C at the
grain boundaries are suppressed.
[0051] Further, to secure high temperature strength, it is
necessary to introduce dislocations into the structure of the steel
material. For introduction of dislocations, formation of
martensite, bainite, and other hard phases is effective. It is
necessary to add certain amounts of C, Mn, and Nb as elements
improving the hardenability.
[0052] On the other hand, to obtain a sufficient low temperature
toughness at the time of large heat input welding, it is necessary
to limit the amount of C to a level lower than steel materials for
general use, that is, to 0.05% or less. Further, by limiting the
amount of C to 0.05% or less, low temperature toughness of the base
material can also be secured. Further, the C and Nb contributing to
the formation of carbides lower the reheat embrittlement
resistance. Further, Cu improves the hardenability, but the HAZ
reheat embrittlement becomes remarkable.
[0053] Next, B, which forms nitrides at the grain boundaries and
causes remarkable reduction of the reheat embrittlement resistance,
is limited in content to less than 0.0003% and is preferably not
added. For Mo as well, to suppress the precipitation of Mo carbides
and Laves phases at the grain boundaries, Mo is not positively
added and is restricted in content to less than 0.01%. On the other
hand, Ti is effective for alleviation of reheat embrittlement. The
reason is that carbides and nitrides of Ti precipitate inside the
grains as well whereby the carbides and nitrides precipitating at
the grain boundaries are reduced.
[0054] Furthermore, the inventors etc. engaged in detailed studies
through experimentation and analysis on the effects of the various
alloy elements of fire-resistant steel on the HAZ reheat
embrittlement. Specifically, they produced fire-resistant steels
having various compositions containing C: 0.010 to 0.050%, Si: 0.01
to 0.50%, Mn: 0.80 to 2.00%, Cr: 0.80 to 1.90%, Nb: 0.01 to less
than 0.05%, N: 0.001 to 0.006%, Ti: 0.010 to 0.030%, Al: 0.005 to
0.10%, and Cu: 0 to 0.10% and having a balance of Fe. Note that,
for the method of production, a process not involving accelerated
cooling, but allowing natural cooling after hot rolling was
employed.
[0055] Test pieces were obtained from the obtained fire-resistant
steels and subjected to heat cycles envisioning welding by a heat
input of 10 kJ/mm. The "heat cycle envisioning welding by a heat
input of 10 kJ/mm" is a heat history of heating from room
temperature to 1400.degree. C. by 20.degree. C./s, holding at
1400.degree. C. for 2 seconds, then cooling during which making the
cooling rate from 800.degree. C. to 500.degree. C. 3.degree. C./s.
After that, the test pieces were raised from room temperature to
600.degree. C. temperature over 60 minutes, held at 600.degree. C.
for 30 minutes, then subjected to tensile tests at 600.degree. C.
and measured for the area reduction of the broken parts of the test
pieces. The area reduction values were used as indicators of the
HAZ reheat embrittlement. 20% or more was considered good.
[0056] As a result, by multiple linear regression analysis, it was
learned that the HAZ reheat embrittlement resistance can be defined
by -1200C-20Mn+30Cr-330Nb-120Cu. Further, as shown in FIG. 1, it
was learned that to secure HAZ reheat embrittlement resistance, it
is necessary that the contents of C, Mn, Cr, Nb, and Cu satisfy the
following formula using the contents of the elements (mass %):
-1200C-20Mn+30Cr-330Nb-120Cu.gtoreq.-80
Note that, when Cu is not included, the elements must satisfy
-1200C-20Mn+30Cr-330Nb.gtoreq.-80.
[0057] Here, an upper limit of -1200C-20Mn+30Cr-330Nb-120Cu is not
set since the higher the value, the better the HAZ reheat
embrittlement resistance. However, based on the lower limit values
of the contents of C, Mn, Nb, and Cu and the upper limit value of
the content of Cr, the upper limit of -1200C-20Mn+30Cr-330Nb-120Cu
becomes 23.3.
[0058] As explained above, in particular, by controlling the
amounts of addition of C, Mn, Cr, Nb, Ti, Cu, Mo, and B, it is
possible to secure high temperature strength of the base material
and achieve both reheat embrittlement resistance and low
temperature toughness of the HAZ at the time of large heat input
welding.
[0059] Further, with the system of compositions of the present
invention, by performing 800.degree. C. or more hot rolling or hot
working, then allowing the steel to naturally cool, a
fire-resistant steel material having a room temperature tensile
strength of 400 MPa to 610 MPa is obtained. In particular, the
yield stress at 600.degree. C. temperature becomes 157 MPa or more
when the room temperature tensile strength is 400 to 489 MPa in
range and becomes 217 MPa or more when the room temperature tensile
strength is 490 to 610 MPa in range.
[0060] Further, after the process of natural cooling to room
temperature after hot rolling, by tempering at 400.degree. C. to
650.degree. C. temperature, it becomes possible to lower only the
room temperature tensile strength without lowering the high
temperature strength and to improve the low temperature toughness
of the base material.
[0061] Below, the present invention will be explained in
detail.
[0062] First, the reasons for limitation of the ranges of essential
chemical compositions defined for working the present invention
will be explained. Note that, in the following explanation, the
amounts of addition of the elements are all expressed by mass
%.
[0063] [C: 0.012% to 0.050%]
[0064] C is an element effective for improving the hardenability of
the steel material and is added in an amount of 0.012% or more.
Note that, from the viewpoint of sufficiently securing
hardenability, addition of 0.015% or more or 0.020% or more is more
preferable. On the other hand, if adding over 0.050% of C, at the
HAZ at the time of large heat input welding, many mixed
martensite-austenite structures (below, sometimes called "MA
phases") or precipitated carbides are formed. As a result,
sometimes the low temperature toughness of the HAZ is remarkably
degraded. In addition, sometimes the amount of carbides
precipitating at the grain boundaries of the HAZ at the time of a
fire is increased and reheat embrittlement of the HAZ is invited.
For this reason, the range of addition of C was defined as 0.012%
to 0.050%. To secure strength, C is preferably added in an amount
of 0.020% or more. On the other hand, to raise the low temperature
toughness of the HAZ, the upper limit of the amount of C is
preferably set to 0.040% or less.
[0065] [Si: 0.01% to 0.50%]
[0066] Si is a deoxidizing element and an element contributing to
the improvement of the hardenability. At least 0.01% or more is
added. On the other hand, if adding Si over 0.50%, sometimes the
amount of formation of the MA phase of the HAZ at the time of large
heat input welding is increased and the low temperature toughness
is reduced. For this reason, the range of addition of Si is defined
as 0.01% to 0.50%. To raise the strength, addition of 0.05% or more
of Si is preferable. Further, to raise the toughness of the HAZ,
making the upper limit of the amount of Si 0.30% or less is
preferable.
[0067] [Mn: 0.80% to 2.00%]
[0068] Mn is effective for improving the hardenability. To secure
the 400 MPa or more room temperature tensile strength aimed at by
the present invention, addition of 0.80% or more is necessary. On
the other hand, Mn is liable to segregate at the grain boundaries
and aggravate HAZ reheat embrittlement, so the upper limit of the
amount of addition was set at 2.00%. To raise the strength,
addition of 1.00% or more of Mn is preferable. On the other hand,
to secure HAZ reheat embrittlement resistance, the upper limit of
the amount of Mn is preferably made 1.60% or less. To raise the HAZ
low temperature toughness, the upper limit of the amount of Mn is
preferably set to 1.50% or less.
[0069] [Cr: 0.80% to 1.90%]
[0070] Cr does not contribute much at all to the room temperature
yield strength and tensile strength and, further, does not
contribute much at all to the improvement of the high temperature
strength when using materials of the system of chemical
compositions of the present invention to produce a steel material
as hot rolled. This was found by research of the inventors. On the
other hand, Cr forms fine Cr carbides and thereby consumes carbon
atoms without itself contributing to HAZ reheat embrittlement and
has the effect of suppressing HAZ reheat embrittlement due to
coarsening of Nb or V carbides.
[0071] In the present invention, in particular, to suppress reheat
embrittlement, 0.80% or more of Cr is added. The preferable lower
limit of the amount of Cr is 0.90% or more, while the more
preferable lower limit is 1.00% or more. Further, if adding Cr over
1.90%, the HAZ toughness falls due to the hardening of the HAZ and
the increase of the MA phase, so the upper limit is made 1.90%. The
preferable upper limit of the amount of Cr is 1.80% or less and the
more preferable upper limit is 1.50% or less.
[0072] Note that, in the present invention, the greater the amounts
of elements such as C, Mn, Nb, Ni, and Cu aggravating HAZ reheat
embrittlement added, the more preferable it is to increase the
amount of addition of Cr to counter this.
[0073] [Nb: 0.01% to less than 0.05%]
[0074] Nb increases the hardenability of the steel material and
contributes to the improvement of the dislocation density as well.
It precipitates as carbides or nitrides and contributes to the
improvement of the room temperature tensile strength and high
temperature strength as well, so 0.01% or more is added. However,
if adding 0.05% or more of Nb, the drop in the HAZ toughness and
the HAZ reheat embrittlement caused by the coarse precipitation of
NbC at the grain boundaries become remarkable, so the amount of
addition is restricted to 0.01% to less than 0.05%. To raise the
room temperature tensile strength, it is preferable to add Nb in an
amount of 0.02% or more. On the other hand, to suppress a drop in
the HAZ toughness and reheat embrittlement resistance, it is
preferable to make the upper limit of the amount of Nb less than
0.03%.
[0075] [N: 0.001% to 0.006%]
[0076] N forms nitrides with various types of alloy elements to
contribute to the improvement of the high temperature strength, so
0.001% or more is added. The preferable lower limit of the amount
of N is 0.002% or more, while the more preferable one is 0.003% or
more. However, if adding a large amount of N, the nitrides
precipitating at the grain boundaries of the HAZ become coarser and
the HAZ reheat embrittlement becomes remarkable, so the upper limit
was made 0.006%. The preferable upper limit of the amount of N is
0.005% or less.
[0077] [Ti: 0.010% to 0.030%]
[0078] Ti precipitates as carbides and nitrides and contributes to
the improvement of the room temperature tensile strength and high
temperature strength. Further, Ti precipitates in the HAZ as
carbides and nitrides not only at the grain boundaries, but also
inside the grains and thereby consumes the carbon and nitrogen. As
a result, Ti suppresses the coarse precipitation of carbides or
nitrides of other, alloy elements at the grain boundaries and
contributes to the suppression of HAZ reheat embrittlement. To
obtain these effects, addition of 0.010% or more of Ti is
necessary. The preferable lower limit of the amount of Ti is 0.015%
or more, while the more preferable lower limit is 0.020%. On the
other hand, if adding Ti over 0.030%, the base material remarkably
falls in low temperature toughness, so the upper limit was made
0.030%. The preferable upper limit of the amount of Ti is 0.025% or
less.
[0079] [Al: 0.005% to 0.10%]
[0080] Al is an element required for deoxidation of the steel
material. In particular, in a steel material containing Cr, Al is
added as a main deoxidizing element to prevent oxidation of the Cr
during the refining. This effect of enabling control of the
concentration of oxygen in the molten steel is obtained by addition
of 0.005% or more, so the lower limit value of Al was made 0.005%.
The preferable lower limit of the amount of Al is 0.020% or more,
while the more preferable one is 0.030% or more. On the other hand,
if the content of Al exceeds 0.10%, coarse oxide clusters are
formed and the toughness of the steel material is impaired in some
cases, so the upper limit value was set at 0.10%. The preferable
upper limit of the amount of Al is 0.075% or less, while the more
preferable upper limit is 0.050% or less.
[0081] [Cu: 0.10% or less]
[0082] Cu is effective for improvement of the room temperature
tensile strength and high temperature strength by the improvement
of the hardenability, but in the present invention is an element
causing remarkable HAZ reheat embrittlement. Therefore, while
inclusion of a small amount due to factors in industrial production
is unavoidable, it is preferable to refrain from deliberate
addition. The allowable upper limit is set to 0.10%. The amount of
Cu is preferably limited to 0.05% or less.
[0083] [Mo: less than 0.01%]
[0084] Mo contributes to the improvement of the room temperature
tensile strength and high temperature strength by improvement of
the hardenability and precipitation strengthening. However, Mo
easily coarsely precipitates as carbides or Laves phases at the HAZ
grain boundaries and results in remarkable HAZ reheat
embrittlement, so addition of Mo is not preferable in the present
invention. Therefore, while inclusion of a small amount due to
factors in industrial production is unavoidable, it is preferable
to refrain from deliberate addition. From the leeway in industrial
production, the upper limit of the amount of addition is set to
less than 0.01%.
[0085] [B: less than 0.0003%]
[0086] B contributes to the improvement of the room temperature
tensile strength and high temperature strength by improvement of
the hardenability and precipitation strengthening. However, B
nitrides easily coarsely precipitate at the HAZ grain boundaries
and result in remarkable HAZ reheat embrittlement, so addition of B
is not preferable in the present invention. Therefore, while
inclusion of a small amount due to factors in industrial production
is unavoidable, it is preferable to refrain from deliberate
addition. From the leeway in industrial production, the upper limit
of the amount of addition is set to less than 0.0003%.
[0087] [P: less than 0.02%]
[0088] P is an impurity which remarkably reduces the low
temperature toughness of the base material and further also results
in remarkable HAZ reheat embrittlement at the time of a fire, so
the upper limit of the amount of addition is set to less than
0.020%. The preferable upper limit of the amount of P is 0.01% or
less.
[0089] [S: less than 0.01%]
[0090] S is an impurity which remarkably reduces the low
temperature toughness of the base material and further also results
in remarkable HAZ reheat embrittlement at the time of a fire, so
the upper limit of the amount of addition is set to less than
0.01%. The preferable upper limit of the amount of S is 0.005% or
less.
[0091] [O: less than 0.01%]
[0092] O is an impurity which remarkably reduces the low
temperature toughness of the base material and further also results
in remarkable HAZ reheat embrittlement at the time of a fire, so
the upper limit of the amount of addition is set to less than
0.010%. The preferable upper limit of the amount of O is 0.005% or
less, while the more preferable limit is 0.003% or less.
[0093] In the present invention, in addition to the above essential
elements, the elements explained below may further be selectively
added.
[0094] Below, the reasons for limitation of the ranges of addition
of the optional elements in the present invention will be
explained.
[0095] [V: 0.40% or less]
[0096] V forms carbides due to reheating at the time of a fire and
thereby is extremely effective for improving the high temperature
strength, so addition of 0.03% or more is preferable. On the other
hand, if adding over 0.40% of V, the carbides precipitating at the
grain boundaries of the HAZ will coarsen and remarkable HAZ reheat
embrittlement will result, so the amount of addition is preferably
limited to 0.40% or less. Further, the amount of addition of V is
more preferably 0.05% to 0.20% in range.
[0097] [Ni: 1.00% or less]
[0098] Ni is effective for improvement of the room temperature
tensile strength and high temperature strength by the improvement
of the hardenability, but causes remarkable HAZ reheat
embrittlement. Therefore, while inclusion of a small amount due to
factors in industrial production is unavoidable, it is preferable
to refrain from deliberate addition. The allowable upper limit is
set to 1.00%. The preferable upper limit of the amount of Ni is
0.40% or less, while the more preferable limit is 0.20% or
less.
[0099] [Zr: 0.010% or less]
[0100] Zr precipitates as carbides and nitrides and contributes to
the improvement of the room temperature tensile strength and high
temperature strength. To obtain this effect, 0.002% or more of Zr
is preferably added. On the other hand, if adding over 0.010% of
Zr, the carbides precipitating at the grain boundaries coarse and
the HAZ reheat embrittlement becomes remarkable, so the upper limit
of the amount of addition of Zr is preferably made 0.010% or less.
The preferable upper limit of the amount of Zr is 0.005% or
less.
[0101] [Mg: 0.005% or less]
[0102] Mg controls the form of the sulfides in the steel material
and has the effect of reducing the drop in base material toughness
due to sulfides. To obtain such an effect, 0.0005% or more of Mg is
preferably added. On the other hand, even if adding over 0.005% of
Mg, the effect becomes saturated, so when adding Mg, the upper
limit is preferably made 0.005% or less. The preferable upper limit
of the amount of Mg is 0.002% or less.
[0103] [Ca: 0.005% or less]
[0104] Ca controls the form of the sulfides in the steel material
and has the effect of reducing the drop in base material toughness
due to sulfides. To obtain such an effect, 0.0005% or more of Ca is
preferably added. On the other hand, if adding over 0.005% of Ca,
the effect becomes saturated, so when adding Ca, the upper limit is
preferably made 0.005% or less. The preferable upper limit of the
amount of Ca is 0.003% or less.
[0105] [Y: 0.050% or less]
[0106] Y controls the form of the sulfides in the steel material
and has the effect of reducing the drop in base material toughness
due to sulfides. To obtain such an effect, 0.001% or more of Y is
preferably added. On the other hand, if adding over 0.050% of Y,
the effect becomes saturated, so when adding Y, the upper limit is
preferably made 0.050% or less. The preferable upper limit of the
amount of Y is 0.030% or less.
[0107] [La: 0.050% or less]
[0108] La controls the form of the sulfides in the steel material
and has the effect of reducing the drop in base material toughness
due to sulfides. To obtain such an effect, 0.001% or more of La is
preferably added. On the other hand, if adding over 0.050% of La,
the effect becomes saturated, so when adding La, the upper limit is
preferably made 0.050% or less. The preferable upper limit of the
amount of La is 0.020% or less.
[0109] [Ce: 0.050% or less]
[0110] Ce controls the form of the sulfides in the steel material
and has the effect of reducing the drop in base material toughness
due to sulfides. To obtain such an effect, 0.001% or more of Ce is
preferably added. On the other hand, if adding over 0.050% of Ce,
the effect becomes saturated, so when adding Ce, the upper limit is
preferably made 0.050% or less. The preferable upper limit of the
amount of Ce is 0.020% or less.
[0111] In the present invention, due to the above limits on the
chemical compositions, a fire-resistant steel material having a
high yield strength at 600.degree. C. temperature even when exposed
to fire and simultaneously suppressed in reheat embrittlement of
the heat affected zone of the weld joint and superior in base
material and weld joint low temperature toughness can be
realized.
[0112] Next, the structure of the steel material of the present
invention will be explained.
[0113] In general, a steel material may be considered to exhibit
high temperature strength due to dislocation strengthening due to
dislocations present in the steel material and precipitates
blocking movement of dislocations. Therefore, when the temperature
of the steel material exceeds 550.degree. C. and dislocations merge
and are eliminated due to upward movement of the dislocations,
sometimes the high temperature strength rapidly declines.
[0114] For this reason, to secure a high temperature strength, it
is effective that the steel material have a sufficient margin of
dislocations at the time before being exposed to fire, that is, at
room temperature, or include large amounts of structures blocking
movement of dislocations, specifically precipitates and crystal
grain boundaries.
[0115] Further, while explained in detail in the method of
production explained later, in the present invention, from the
viewpoint of the productivity of products with stable mechanical
properties, the fire-resistant steel material is produced as hot
rolled without using accelerated cooling. For this reason, the
steel material structure (metal structure), as observed by an
optical microscope, is a structure having an area fraction of 80%
or more of a ferrite phase and a balance of a bainite phase,
martensite phase, and mixed martensite-austenite structures (MA
phase). To secure the toughness of the base material, the area
fraction of the ferrite phase is preferably made 85% or more.
Further, to secure strength, the area fraction of the ferrite phase
is preferably made 97% or less.
[0116] A steel material having the chemical composition of the
present invention and having a steel structure made the above
structure, as explained in detail later, is hot worked or hot
rolled at 800.degree. C. to 900.degree. C. temperature by a large
reduction ratio. Due to such production conditions, the
precipitates blocking dislocations in the steel material are made
to finely disperse and, further, the structure can be made a finer
grain, so a greater high temperature strength is obtained.
[0117] Next, the mechanical properties of the steel material of the
present invention will be explained.
[0118] In the fire-resistant steel material of the present
invention, by applying to the steel material of the above steel
compositions and steel structure the various steps of the
conditions shown in the method of production explained below, it
becomes possible to provide fire-resistant steel plate having the
mechanical properties explained below.
[0119] [Room Temperature Tensile Strength and 600.degree. C. Yield
Stress]
[0120] In the fire-resistant steel material of the present
invention, the room temperature tensile strength is 400 MPa to 610
MPa. The yield stress at 600.degree. C. temperature is 157 MPa or
more when the room temperature tensile strength is 400 MPa to 489
MPa and is 217 MPa or more when the room temperature tensile
strength is 490 MPa to 610 MPa. Due to this, in building
applications, a fire-resistant steel material securing the various
requirements in building design and having a sufficient margin of
safety in fires can be realized.
[0121] [600.degree. C. Breakage Area Reduction Value]
[0122] In the fire-resistant steel material of the present
invention, the reheat embrittlement resistance is evaluated by
using a test piece given a heat history envisioning welding by a
heat input of 5 kJ/mm and 10 kJ/mm and measuring the area reduction
value of the broken part at 600.degree. C. temperature. In the
present invention, a fire-resistant steel material having an area
reduction value of the broken part at a 600.degree. C. temperature
of 20% or more is obtained. Due to this, a fire-resistant steel
material having sufficient deformation ability when the weld joint
HAZ is reheated to the envisioned temperature 600.degree. C. of a
fire can be realized.
[0123] [Method of Production of Fire-Resistant Steel Material]
[0124] Below, the reasons for limitation of the method of
production of a fire-resistant steel material of the present
invention superior in high temperature strength of the base
material and reheat embrittlement resistance and low temperature
toughness of the weld heat affected zone will be explained.
[0125] The method of production of a fire-resistant steel material
of the present invention is a method heating a steel slab having
the above-mentioned steel compositions to 1150.degree. C. to
1300.degree. C. temperature, then hot working or hot rolling it at
a 800.degree. C. to 900.degree. C. temperature by a reduction ratio
of 50% or more, then allowing it to cool.
[0126] In the method of production of the present invention, to
secure the requirements in building design and obtain a sufficient
safety margin against fire in a fire-resistant steel material used
for building applications, as explained above, a steel slab having
a chemical composition having as its required conditions giving a
room temperature tensile strength of 400 MPa to 610 MPa, a high
yield strength at 600.degree. C., an area reduction value of the
broken part at 600.degree. C. of the weld HAZ of the steel material
of 20% or more, a superior reheat embrittlement resistance, secured
low temperature toughness even at the HAZ due to welding by a heat
input of 5 kJ/mm, and secured base material toughness is used as
the material. Further, the steel slab may be hot worked or hot
rolled at a prescribed temperature and reduction amount to produce
a fire-resistant steel material satisfying all of the above
properties.
[0127] [Reduction Ratio in Hot Working or Hot Rolling]
[0128] As explained above, the high temperature strength of a steel
material is considered to be achieved by dislocation strengthening
by dislocations present in the steel material and the precipitates
blocking movement of dislocations, so if the temperature exceeds
550.degree. C. and the dislocations merge and are eliminated due to
upward movement of the dislocations, the high temperature strength
sometimes is reduced. Therefore, to secure a good high temperature
strength, it is effective to provide a sufficient extra margin of
amount of dislocations at room temperature or include a large
number of precipitates, crystal grain boundaries, or other
structures blocking movement of dislocations.
[0129] Here, in the method of production of the present invention,
from the viewpoint of productivity of a product with stable
mechanical properties in actual production, the object is to
produce a fire-resistant steel material as hot rolled without using
accelerated cooling. Therefore, the steel structure as a whole does
not become a high dislocation density bainite or martensite. The
steel structure becomes one with the low dislocation density
ferrite structures accounting for 80% or more of the area fraction
of the steel structure as observed by an optical microscope and
with a balance of less than 20% of bainite, martensite, and MA.
[0130] Therefore, to secure a high high-temperature strength in the
present invention, relying on just the increase of the fraction of
bainite or martensite in the steel material is insufficient. It is
necessary to make the precipitates blocking dislocation finely
disperse and make the structure finer in grain.
[0131] The inventors discovered by experimentation and analysis
that to realize fine dispersion of precipitates in the steel
material and finer grain of the structure, when hot rolling steel
slabs having the chemical composition of the present invention, it
is effective to increase the reduction ratio at a 800.degree. C. to
900.degree. C. temperature, specifically, making the reduction
ratio 50% or more, more preferably 70% or more.
[0132] Further, by introducing a large amount of dislocations at
the temperature region right before transformation from austenite
to ferrite or bainite, these dislocations will become sites for
formation of nuclei for precipitation and these dislocations will
become sites for formation of nuclei for ferrite or bainite
transformation. Due to this, it was learned that it is possible to
realize both fine dispersion of precipitates and finer grains of
the structure.
[0133] Note that, in general, if the amount of reduction is made
large in the austenite region, the higher transformation
temperature will cause the bainite fraction to fall and the ferrite
fraction to rise in some cases, but in the chemical compositions of
the present invention, the amount of C is kept low, so it is clear
that bainite transformation easily occurs and a drop in the bainite
fraction can be suppressed.
[0134] [Heating Temperature Before Hot Working or Hot Rolling]
[0135] As explained above, in the method of production of the
present invention, effective utilization of the precipitation of
the alloy elements is important. As the means for stably and
reliably obtaining precipitation of such alloy elements, it is
necessary to heat the steel slabs before hot working or hot rolling
to 1150.degree. C. to 1300.degree. C. Such heat treatment heats the
steel slabs to a 1150.degree. C. or more temperature to thereby
cause the carbides or nitrides of the various alloy elements, for
example, NbC, NbN, VC, TiC, ZrC, Cr.sub.23C.sub.6, etc., to
completely or as much as possible form solid solutions and thereby
improve the hardenability after hot rolling and increase the amount
of precipitation after hot working or hot rolling.
[0136] When not heating before hot working or hot rolling, the C,
Cr, Nb, V, Ti, Zr, and other alloy elements already precipitate as
coarse precipitates before hot rolling, so reduction of the
dislocation density of the steel material due to the drop in
hardenability after hot working or hot rolling or reduction of the
amount of precipitation strengthening due to the reduction in fine
carbides or nitrides precipitating after hot working or hot rolling
is sometimes invited.
[0137] On the other hand, if making the heating temperature before
hot working or hot rolling over 1300.degree. C., the increase in
oxide scale at the surface of the steel material becomes
remarkable, so the upper limit of the heating temperature is set to
1300.degree. C.
[0138] [Tempering Heat Treatment]
[0139] In the method of production of the present invention, it is
also possible to allow the steel material to naturally cool to room
temperature after hot rolling, then apply a step of tempering heat
treatment to the steel material. By applying tempering heat
treatment to the steel material, it becomes possible to promote the
precipitation of alloy elements remaining in the solid solution
state without completely precipitating by just the natural cooling
after hot rolling and further increase the number of precipitates
suppressing the reduction in dislocations at the time of fire.
[0140] In such tempering treatment, the temperature can be
determined by suitable selection between 400.degree. C. to
650.degree. C. By deciding this based on the required room
temperature tensile strength and type of precipitated alloy
elements, the effect of the present invention can be further
enhanced.
[0141] Further, the same applies to the time of the tempering heat
treatment. When the change in structure at the time of tempering is
governed by the dispersion of the substance, raising the
temperature and prolonging the time have similar effects, so the
time can be suitably determined between 5 minutes to 30 minutes in
accordance with the tempering temperature.
[0142] As explained above, the method of production of a
fire-resistant steel material of the present invention heats a
steel slab having steel compositions of the above defined range to
1150.degree. C. to 1300.degree. C. temperature, then hot works or
hot rolls it at 800.degree. C. to 900.degree. C. temperature by a
reduction ratio of 50% or more, then allows the result to naturally
cool. According to this method of production, it is possible to
produce a fire-resistant steel material having a high yield
strength at a 600.degree. C. temperature even when exposed to fire,
simultaneously suppressed in reheat embrittlement at the heat
affected zone of the weld joint, and giving superior low
temperature toughness of the base material and weld joint.
Therefore, it becomes possible to produce a fire-resistant steel
material for buildings superior in high temperature strength and
superior in weld joint reheat embrittlement resistance by an
economical compositions using less alloy elements and a high
productivity method of production stopping at hot rolling.
Examples
[0143] Below, examples of the fire-resistant steel material of the
present invention and method of production of the same will be
given and the present invention explained more specifically, but
the present invention is of course not limited to the following
examples and can be worked suitably changed within a scope
complying with the gist of the invention explained before and after
this. These are all included in the technical scope of the present
invention.
[0144] [Preparation of Fire-Resistant Steel Material]
[0145] The deoxidation and desulfurization and the chemical
compositions of the molten steel in the steelmaking process were
controlled and the steel continuously cast to prepare slabs of the
chemical compositions shown in the following Table 1. Further,
under the production conditions shown in Table 2, the slabs were
reheated and hot worked to the respective plate thicknesses, then
heat treated under different conditions to prepare fire-resistant
steel materials of the invention examples and comparative
examples.
[0146] Specifically, first, the slabs were reheated at 1150.degree.
C. to 1300.degree. C. temperatures for 1 hour, then immediately
started to be rough rolled to obtain steel plates of plate
thicknesses of 100 mm at a 1050.degree. C. temperature. Further,
under the conditions shown in Table 2, they were made into
thick-gauge steel plates of final thicknesses of 15 mm to 35 mm or
were forged or rolled into steel shapes of complicated
cross-sectional shapes with maximum thicknesses of 15 mm to 35 mm.
The finishing temperatures were controlled to 800.degree. C. or
more. The reduction ratios at the 800.degree. C. to 900.degree. C.
temperature at that time were controlled to the values shown in
Table 1 while performing final rolling. Further, after the end of
the rolling, the plates were immediately allowed to cool to thereby
prepare the fire-resistant steel materials of the invention
examples and comparative examples.
[0147] [Evaluation and Testing]
[0148] The fire-resistant steel materials of the invention examples
and comparative examples prepared by the above method were
evaluated and tested as follows:
[0149] First, regarding the room temperature tensile test, this was
performed based on JIS Z 2241. When an upperyield point appeared on
the stress-strain curve, the upper yield point was defined as the
room temperature yield strength, while when one did not appear, the
0.2% yield strength was defined as the room temperature yield
strength.
[0150] Further, regarding the high temperature tensile test, this
was performed based on JIS G 0567 at a 600.degree. C. temperature.
The measured 0.2% yield strength was made the 600.degree. C. yield
strength.
[0151] Further, the 600.degree. C. tensile area reduction value of
the HAZ (weld heat affected zone) was evaluated by a heat cycle
applying a heat history envisioning a heat input of 5 kJ/mm and 10
kJ/mm to the steel slab. After applying the heat cycle, the slab
was raised from room temperature to 600.degree. C. temperature over
60 minutes, was held at 600.degree. C. for 30 minutes, then was
subjected to a tensile test at 600.degree. C. The area reduction
value of the broken part of the test piece was measured and used as
an indicator of reheat embrittlement of the HAZ. The threshold
value of the indicator was made 20% or more.
[0152] Further, the Charpy test of the base material was performed
by taking a 2 mm V-shaped impact test piece from the plate
thickness 1/2 t of each steel material based on JIS Z 2202 and
performing an impact test based on the method of JIS Z 2242. At
this time, the threshold value of the absorption energy was made
27J considering the earthquake resistance of building
structures.
[0153] Further, the Charpy test of the HAZ was performed by
applying to each steel material a heat cycle envisioning welding by
a heat input of 5 kJ/mm and a heat input of 10 kJ/mm, then taking a
2 mm V-shaped impact test piece based on JIS Z 2202 and performing
an impact test based on the method of JIS Z 2242. At this time, the
threshold value of the absorption energy was made 27J considering
the earthquake resistance of building structures.
[0154] Note that, the "heat history envisioning welding by a heat
input of 5 kJ/mm" is a heat cycle of heating from room temperature
to 1400.degree. C. by 20.degree. C./s, holding at 1400.degree. C.
for 1 second, then cooling during which cooling from 800.degree. C.
to 500.degree. C. in range by 15.degree. C./s. Further, the "heat
history envisioning welding by a heat input of 10 kJ/mm" is a heat
cycle of heating from room temperature to 1400.degree. C. by
20.degree. C./s, holding at 1400.degree. C. for 2 seconds, then
cooling during which cooling from 800.degree. C. to 500.degree. C.
in range by 3.degree. C./s.
[0155] Further, regarding the structure of the steel material, the
structure of the steel material was observed by an optical
microscope. From the results, the total of the area fractions of
bainite, martensite, and MA was calculated and the area fraction of
ferrite was found.
[0156] A list of the chemical compositions of the fire-resistant
steel materials of the invention examples and comparative examples
is shown in the following Table 1 and a list of the production
conditions and mechanical properties of the steel materials is
shown in the following Table 2.
[0157] Note that, in Table 1, Steel Type Nos. 1 to 21 are invention
examples having steel compositions defined by the present
invention, while Steel Type Nos. 22 to 34 are comparative examples
given steel compositions outside the defined range of the present
invention. The value of formula: -1200C-20Mn+30Cr-330Nb-120Cu is
shown as the HAZ reheat embrittlement coefficient.
[0158] Further, in Table 2, the produced plate thickness, heating
temperature, hot rolling conditions (finishing temperature,
reduction ratio), tempering temperature, room temperature tensile
strength (room temperature TS), room temperature yield strength
(room temperature YS), 600.degree. C. yield strength (600.degree.
C. YS), HAZ 600.degree. C. tensile test breakage area reduction
value (HAZ reheat embrittlement area reduction value), 0.degree. C.
base material Charpy absorption energy, and 0.degree. C. HAZ Charpy
absorption energy are shown.
[0159] Further, in Table 2, regarding the strength level, steels of
a room temperature tensile strength of 400 to 489 MPa were
displayed as the 400 MPa class, while steels of a room temperature
tensile strength of 490 to 610 MPa were displayed as the 500 MPa
class.
[0160] Further, in Table 1 and Table 2, values outside the range of
the present invention are shown underlined.
TABLE-US-00001 TABLE 1 Steel type Chemical composition, (mass %)
no. C Si Mn P S Cr Nb Ti N Al 1 0.049 0.05 0.90 0.005 0.004 1.50
0.020 0.011 0.0040 0.050 2 0.050 0.45 1.00 0.005 0.004 1.80 0.020
0.025 0.0039 0.049 3 0.040 0.30 1.01 0.007 0.005 1.30 0.031 0.011
0.0039 0.049 4 0.039 0.30 1.00 0.010 0.005 1.90 0.030 0.010 0.0039
0.049 5 0.031 0.29 1.20 0.010 0.006 1.30 0.010 0.019 0.0030 0.040 6
0.030 0.29 1.21 0.005 0.006 1.50 0.010 0.019 0.0030 0.040 7 0.030
0.29 1.55 0.005 0.003 1.01 0.023 0.021 0.0031 0.040 8 0.029 0.30
1.54 0.006 0.003 1.01 0.020 0.021 0.0032 0.045 9 0.030 0.30 1.56
0.007 0.003 1.00 0.021 0.025 0.0029 0.045 10 0.030 0.30 1.56 0.004
0.004 1.00 0.020 0.025 0.0028 0.045 11 0.030 0.30 1.40 0.004 0.007
1.31 0.033 0.021 0.0029 0.035 12 0.031 0.30 1.39 0.004 0.007 1.50
0.032 0.020 0.0029 0.035 13 0.025 0.29 1.35 0.006 0.004 1.00 0.031
0.010 0.0044 0.020 14 0.025 0.05 1.35 0.010 0.004 0.99 0.030 0.010
0.0045 0.020 15 0.024 0.29 1.20 0.012 0.003 0.90 0.048 0.011 0.0044
0.020 16 0.024 0.29 1.20 0.004 0.005 0.89 0.049 0.027 0.0048 0.020
17 0.020 0.10 1.50 0.007 0.005 0.81 0.048 0.028 0.0050 0.071 18
0.020 0.12 1.52 0.003 0.005 0.80 0.040 0.028 0.0051 0.073 19 0.019
0.10 0.82 0.004 0.004 1.02 0.040 0.012 0.0055 0.075 20 0.015 0.11
1.20 0.005 0.005 1.02 0.041 0.024 0.0050 0.080 21 0.012 0.05 1.80
0.005 0.005 1.50 0.040 0.028 0.0019 0.080 22 0.065 0.05 1.00 0.006
0.006 1.00 0.040 0.020 0.0040 0.050 23 0.003 0.30 1.20 0.006 0.006
1.02 0.021 0.011 0.0040 0.050 24 0.030 0.78 0.85 0.006 0.006 0.80
0.021 0.020 0.0030 0.049 25 0.029 0.09 0.45 0.005 0.006 1.20 0.020
0.015 0.0029 0.048 26 0.030 0.25 2.50 0.006 0.006 0.82 0.049 0.012
0.0031 0.040 27 0.025 0.20 1.40 0.005 0.005 0.44 0.048 0.010 0.0030
0.041 28 0.030 0.20 1.40 0.004 0.002 2.30 0.040 0.012 0.0040 0.041
29 0.040 0.25 1.20 0.005 0.003 0.81 0.060 0.011 0.0031 0.040 30
0.025 0.30 1.55 0.005 0.004 0.99 0.150 0.025 0.0029 0.042 31 0.030
0.31 1.56 0.004 0.003 1.30 0.030 0.040 0.0029 0.030 32 0.030 0.25
1.40 0.004 0.002 1.01 0.030 0.021 0.0020 0.030 33 0.025 0.29 1.55
0.006 0.005 1.00 0.031 0.013 0.0020 0.050 34 0.029 0.28 1.54 0.006
0.006 1.01 0.030 0.020 0.0021 0.050 35 0.045 0.30 1.85 0.005 0.004
0.81 0.048 0.010 0.0033 0.040 36 0.020 0.31 1.09 0.005 0.004 1.90
0.250 0.010 0.0030 0.040 Steel HAZ reheat type Chemical
composition, (mass %) embrittlement no. V Ni Cu Mo B O Other
coefficient Remarks 1 0.20 0.0010 Zr 0.005 -38.4 2 0.34 0.0010 Zr
0.005 -32.6 3 0.11 0.0011 -39.4 4 0.12 0.0002 0.0012 Mg 0.002 -19.7
5 0.15 0.10 0.0013 -37.5 6 0.20 0.0011 -18.5 7 0.09 0.0011 -44.3 8
0.09 0.009 0.0013 -41.9 9 0.05 0.0014 -44.1 10 0.05 0.10 0.05
0.0002 0.0015 -49.8 Inv. ex. 11 0.10 0.0002 0.0020 Y 0.030 -35.6 12
0.10 0.09 0.0025 Y 0.030 -41.4 13 0.0024 -37.2 14 0.05 0.003 0.0024
-37.2 15 0.0019 -41.6 16 0.20 0.08 0.0019 Ce 0.015 -51.9 17 0.20
0.10 0.0024 Ca 0.002 -57.5 18 0.20 0.10 0.003 0.0025 Ca 0.002 -55.6
19 0.10 0.004 0.0025 -33.8 20 0.10 0.0001 0.0029 -36.9 21 0.35 0.09
0.009 0.0002 0.0040 La 0.020 -29.4 22 0.09 0.40 0.0020 -81.2 23
0.20 0.003 0.0010 -3.9 24 0.09 0.05 0.005 0.0010 -41.9 25 0.10
0.0012 -14.4 26 0.0013 -77.6 Comp. ex. 27 0.15 0.08 0.0014 -70.2 28
0.10 0.10 0.0020 -20.2 29 0.10 0.10 0.0020 -79.5 30 0.10 0.0015
-80.8 31 0.20 0.20 0.09 0.0030 -48.9 32 0.51 0.0032 -43.6 33 0.050
0.0033 -41.2 34 0.09 0.004 0.0019 0.0035 Ca 0.003 -45.2 35 0.10
0.05 0.0020 -88.5 36 0.11 0.0030 -71.3
TABLE-US-00002 TABLE 2 Produced Ord. Ord. Base Steel Strength plate
Finishing Red. Ferrite temp. temp. material type level thickness
Heating temp. ratio Tempering phase YS TS toughness no. [MPa] [mm]
[.degree. C.] [.degree. C.] [%] [.degree. C. .times. min] [%] [MPa]
[MPa] vE 0.degree. C. [J] 1 400 30 1220 850 60 91 271 450 267 2 500
30 1220 850 60 86 330 531 89 3 400 30 1220 850 60 92 307 475 288 4
400 30 1220 850 60 93 237 483 303 5 400 30 1220 850 60 93 297 487
192 6 400 30 1220 850 60 90 278 476 186 7 500 20 1250 840 75 87 420
574 162 8 500 20 1250 840 75 86 420 580 146 9 500 20 1250 840 75 85
454 585 158 10 500 20 1250 840 75 550.degree. C. .times. 30 min 85
455 582 165 11 500 20 1250 840 75 550.degree. C. .times. 30 min 88
383 578 201 12 500 20 1250 840 75 86 366 581 170 13 400 16 1180 840
75 95 352 482 284 14 400 16 1180 840 75 94 349 485 291 15 400 16
1180 840 75 96 367 473 299 16 500 12 1220 810 75 86 414 538 130 17
500 12 1220 810 75 600.degree. C. .times. 30 min 88 479 579 172 18
500 12 1220 810 75 600.degree. C. .times. 30 min 87 479 580 149 19
400 30 1250 870 60 95 355 420 289 20 400 30 1250 870 60 97 376 464
176 21 500 16 1250 810 75 550.degree. C. .times. 45 min 88 415 597
124 22 500 30 1220 850 60 580.degree. C. .times. 30 min 83 469 589
248 23 400 30 1220 850 60 99 339 460 331 24 400 30 1220 850 60 96
425 488 188 25 400 30 1220 850 60 97 360 397 320 26 500 20 1220 840
75 620.degree. C. .times. 30 min 87 388 599 206 27 500 20 1220 840
75 86 469 564 247 28 500 20 1220 840 75 85 410 597 230 29 500 20
1220 840 75 85 424 576 268 30 500 20 1220 840 75 620.degree. C.
.times. 30 min 82 449 600 154 31 500 20 1220 840 75 620.degree. C.
.times. 30 min 81 450 602 7 32 500 20 1220 840 75 600.degree. C.
.times. 30 min 83 419 580 124 33 500 20 1220 840 75 82 422 600 203
34 500 20 1220 840 75 85 438 593 165 35 500 20 1220 870 75 82 444
605 240 36 500 20 1220 870 75 84 427 580 259 Steel HAZ toughness
600.degree. C. HAZ reheat area type vE 0 [J] YS reduction value [%]
no. 5 kJ/mm 10 kJ/mm [MPa] 5 kJ/mm 10 kJ/mm Remarks 1 369 90 197 26
26 Invention 2 326 30 234 21 22 examples 3 346 99 180 31 30 4 340
157 183 38 35 5 337 145 190 31 29 6 362 55 193 44 40 7 311 53 218
30 28 8 307 61 234 23 22 9 295 73 228 26 26 10 294 68 234 20 22 11
311 84 221 28 30 12 294 150 224 26 26 13 346 119 168 48 44 14 355
139 183 42 41 15 361 112 161 48 47 16 331 29 219 24 20 17 291 82
219 36 37 18 293 66 225 35 35 19 372 167 160 54 49 20 363 178 161
66 60 21 299 108 218 64 61 22 286 7 241 7 6 Comparative 23 368 38
144 75 71 examples 24 352 16 167 42 40 25 382 39 149 67 60 26 286
202 224 15 13 27 299 37 231 6 4 28 303 20 240 45 40 29 318 26 229
19 19 30 299 17 247 16 14 31 292 9 257 29 18 32 310 8 255 16 16 33
282 130 252 13 12 34 284 48 233 9 5 35 294 84 258 6 5 36 290 17 237
28 9
[0161] [Results of Evaluation]
[0162] As shown in Table 1 and Table 2, the fire-resistant steel
materials of the invention examples produced by the steel
compositions and production conditions defined by the present
invention had a 600.degree. C. yield strength of 157 MPa or more in
the case of a room temperature tensile strength of 400 to 489 MPa
and of 217 MPa or more in the case of a room temperature tensile
strength of 490 to 610 MPa. At the same time, in the important
feature in the present invention, the 600.degree. C. tensile area
reduction value of the weld HAZ, as well, 20% or more is secured.
It is learned that high temperature deformation properties of the
HAZ are secured.
[0163] Furthermore, the fire-resistant steel materials of the
invention examples had base material and HAZ Charpy absorption
energies at 0.degree. C. of 27J or more, so it was learned that the
base material low temperature toughness and joint toughness
satisfied the needed performance. From these evaluation results, it
is clear that the fire-resistant steel materials of the present
invention are superior in high temperature strength and base
material and weld joint toughness.
[0164] Further, the fire-resistant steel materials of the invention
examples all included area fraction 80% or more ferrite phases.
Further, the total area fraction of the bainite phase, martensite
phase, and MA phase, with the ferrite phase providing the balance,
becomes less than 20% in the invention examples. Note that,
inclusions were observed in addition to the ferrite phase, bainite
phase, martensite phase, and MA phase, but the area fractions were
extremely small, so these could be ignored.
[0165] As compared with the above fire-resistant steel materials of
the invention examples, the steel materials of the comparative
examples failed to satisfy either the chemical composition or
production conditions defined by the present invention, so either
the 600.degree. C. yield strength (600.degree. C. YS), reduction
area at the broken part in the HAZ 600.degree. C. tensile test,
0.degree. C. base material Charpy absorption energy, or 0.degree.
C. HAZ Charpy absorption energy could not satisfy the targeted
characteristic.
[0166] From the results of the examples explained above, it is
clear that the fire-resistant steel material of the present
invention is superior in base material high temperature strength
and weld heat affected zone low temperature toughness and reheat
embrittlement resistance.
* * * * *