U.S. patent application number 12/449512 was filed with the patent office on 2011-10-27 for steel for welded structures excellent in high temperature strength and low temperature toughness and method of production of same.
Invention is credited to Ryuji Uemori, Yoshiyuki Watanabe.
Application Number | 20110262298 12/449512 |
Document ID | / |
Family ID | 42339627 |
Filed Date | 2011-10-27 |
United States Patent
Application |
20110262298 |
Kind Code |
A1 |
Watanabe; Yoshiyuki ; et
al. |
October 27, 2011 |
STEEL FOR WELDED STRUCTURES EXCELLENT IN HIGH TEMPERATURE STRENGTH
AND LOW TEMPERATURE TOUGHNESS AND METHOD OF PRODUCTION OF SAME
Abstract
By heating a steel material comprising C: 0.003 to 0.05%, Si:
0.60% or less, Mn: 0.6 to 2.0%, P: 0.020% or less, S: 0.010% or
less, Cr: 0.20 to 1.5%, Nb: 0.005 to 0.05%, Al: 0.060% or less, and
N: 0.001 to 0.006%, further limiting, as an impurity, Mo to 0.03%
or less, having a balance of iron and unavoidable impurities, and
having a weld cracking parameter P.sub.CM value defined by
P.sub.CM=C+Si/30+Mn/20+Cu/20+Ni/60+Cr/20+Mo/15+V/10+5B of 0.22% or
less, to 1000 to 1300.degree. C. in temperature, finishing the hot
rolling at 800.degree. C. or more in temperature, then cooling,
steel for welded structures excellent in high temperature strength
and low temperature toughness can be inexpensively provided.
Inventors: |
Watanabe; Yoshiyuki; (Tokyo,
JP) ; Uemori; Ryuji; (Tokyo, JP) |
Family ID: |
42339627 |
Appl. No.: |
12/449512 |
Filed: |
January 15, 2009 |
PCT Filed: |
January 15, 2009 |
PCT NO: |
PCT/JP2009/050906 |
371 Date: |
August 10, 2009 |
Current U.S.
Class: |
420/105 ;
148/645; 420/104; 72/201 |
Current CPC
Class: |
C21D 8/0263 20130101;
C22C 38/22 20130101; C22C 38/42 20130101; C21D 2211/005 20130101;
C22C 38/38 20130101; C22C 38/48 20130101; C22C 38/02 20130101; C21D
2211/004 20130101; C22C 38/28 20130101; C22C 38/44 20130101; C22C
38/06 20130101; C21D 8/0226 20130101; C22C 38/24 20130101; C22C
38/04 20130101; C22C 38/001 20130101; C22C 38/26 20130101; C22C
38/58 20130101; C22C 38/46 20130101; C22C 38/004 20130101 |
Class at
Publication: |
420/105 ;
148/645; 72/201; 420/104 |
International
Class: |
C22C 38/22 20060101
C22C038/22; B21B 1/26 20060101 B21B001/26; C22C 38/18 20060101
C22C038/18; C21D 8/02 20060101 C21D008/02 |
Claims
1. A method of production of steel for welded structures excellent
in high temperature strength and low temperature toughness
characterized by comprising heating a steel material comprising, by
mass %, C: 0.003 to 0.05%, Si: 0.60% or less, Mn: 0.6 to 2.0%, P:
0.020% or less, S: 0.010% or less, Cr: 0.20 to 1.5%, Nb: 0.005 to
0.05%, Al: 0.060% or less, and N: 0.001 to 0.006%, further
limiting, as an impurity, Mo to 0.03% or less, having a balance of
iron and unavoidable impurities, and having a weld cracking
parameter P.sub.CM value defined by
P.sub.CM=C+Si/30+Mn/20+Cu/20+Ni/60+Cr/20+Mo/15+V/10+5B of 0.22% or
less, to 1000 to 1300.degree. C. in temperature, finishing the hot
rolling at a temperature of 800.degree. C. or more, and then
cooling.
2. A method of production of steel for welded structures excellent
in high temperature strength and low temperature toughness as set
forth in claim 1, characterized by, after finishing said hot
rolling, starting accelerated cooling from 750.degree. C. or more
in temperature, and stopping the accelerated cooling at 550.degree.
C. or less.
3. A method of production of steel for welded structures excellent
in high temperature strength and low temperature toughness as set
forth in claim 1 characterized by further containing, by mass %,
one or both of V: 0.01 to 0.10% and Ti: 0.005 to 0.025%.
4. A method of production of steel for welded structures excellent
in high temperature strength and low temperature toughness as set
forth in claim 1, characterized by further containing, by mass %,
one or more of Ni: 0.05 to 0.50%, Cu: 0.05 to 0.50%, B: 0.0002 to
0.003%, and Mg: 0.0002 to 0.005%.
5. A method of production of steel for welded structures excellent
in high temperature strength and low temperature toughness as set
forth in claim 1, characterized by further containing, by mass %,
one of Ca: 0.0005 to 0.004% and an REM: 0.0005 to 0.008%.
6. A steel for welded structures excellent in high temperature
strength and low temperature toughness characterized by being
obtained by heating a steel material comprising, by mass %, C:
0.003 to 0.05%, Si: 0.60% or less, Mn: 0.6 to 2.0%, P: 0.020% or
less, S: 0.010% or less, Cr: 0.20 to 1.5%, Nb: 0.005 to 0.05%, Al:
0.060% or less, and N: 0.001 to 0.006%, further limiting, as an
impurity, Mo to 0.03% or less, having a balance of iron and
unavoidable impurities, and having a weld cracking parameter
P.sub.CM value defined by
P.sub.CM=C+Si/30+Mn/20+Cu/20+Ni/60+Cr/20+Mo/15+V/10+5B of 0.22% or
less, to 1000 to 1300.degree. C. in temperature, finishing the hot
rolling at a temperature of 800.degree. C. or more, and then
cooling.
Description
TECHNICAL FIELD
[0001] The present invention mainly targets fire-resistant steel
for building structures aimed at maintaining the proof strength at
the time of fires and other high temperature conditions, but is not
limited to building applications and can also be applied to steel
for welded structures for offshore structures, ships, bridges,
various storage tanks, and a broad range of other applications.
Note that the strength level of the steel plate mainly covered is a
yield strength of 235 to 475 MPa and a tensile strength of 400 to
640 MPa, i.e., the classes generally called "40 kg" and "50 kg
steels".
PRIOR ART
[0002] So-called "fire-resistant steel" is disclosed in Japanese
Patent Publication (A) No. 2-77523 and numerous other publications.
However, almost all contain Mo. It is true that Mo is an element
extremely effective in securing the high temperature proof strength
of steel, but at the same time it is an expensive element.
[0003] In this regard, steel for general structures for which
standards are set by the Japan Industrial Standard (JIS) etc. fall
in strength starting from about 350.degree. C., so the allowable
temperature is about 350.degree. C. That is, when using such a
steel material for buildings, offices, homes, multistory parking
structures, and other structures, to secure safety at the time of a
fire, it is obligatory to apply a sufficient fire-resistant
coating. Japanese building laws stipulate that at the time of a
fire, the temperature of steel materials not reach 350.degree. C.
or more. This is because with such steel materials, at 350.degree.
C. or so, the proof strength becomes about 2/3 that of ordinary
temperature or falls below the required strength. For this reason,
when utilizing a general steel material for a structure, it is
necessary to apply a fire-resistant coating so that the temperature
of the steel material does not reach 350.degree. C.
[0004] To eliminate or reduce this fire-resistant coating,
fire-resistant steel enhanced in high temperature proof strength at
high temperature tensile tests of 600.degree. C. etc. (below, when
not particularly clearly indicated, a "high temperature" indicates
600.degree. C. and a "high temperature strength" indicates a high
temperature proof strength) has been coming into use.
[0005] In general, fire-resistant steel has Mo added to it for the
purpose of maintaining the high temperature strength. However, the
market for Mo greatly fluctuates. While depending on the amount of
addition as well, in many cases it results in a higher cost
compared with the cost of fire-resistant coating. For this reason,
development and commercialization of inexpensive fire-resistant
steel to which Mo is not added have been awaited.
[0006] The present invention has as its object to obtain steel for
welded structures excellent in high temperature strength without
adding expensive Mo and also excellent in low temperature
toughness--one of the basic performances of steel materials. For
this purpose, by limiting the steel compositions to a specific
range and further limiting the method of production, there is
provided a method able to supply fire-resistant steel--excellent in
high temperature strength, suppressed in weld cracking parameter,
and securing low temperature toughness--industrially stably and
further at a low cost.
[0007] According to the present invention, steel for welded
structures having sufficient proof strength even at the time of a
fire or other environment exposed to a high temperature can be
supplied in large amounts inexpensively, so this can contribute to
the improvement of safety of welded steel structures for a broad
range of applications.
[0008] The point of the present invention is that to stably secure
a high temperature strength at 600.degree. C., instead of expensive
Mo, a relatively small amount of C and co-addition of Cr and Nb are
used for transformation strengthening and precipitation
strengthening using Cr or Nb precipitates (carbonitrides).
[0009] That is, the inventors discovered that by addition and
inclusion of a suitable amount of Cr in an Mo-free composition, the
hardenability of the steel is improved, the transformation
temperature falls, and the hard structure including cementite
becomes bainitic.
[0010] Due to this, the ordinary temperature and high temperature
strengths rise and the matrix is transformed at a relatively low
temperature resulting in a fine bainitic structure. Because of
this, the inventors discovered that at the time of a high
temperature, carbonitrides of Cr and Nb alone or together resulting
from the addition of Cr and Nb precipitate extremely finely in the
matrix and a high temperature strength can be secured and
maintained at a high level and thereby reached the present
invention.
[0011] In the above way, fire-resistant steel not containing Mo is
in itself extremely epochmaking. At the same time, since no Mo with
its high hardenability is contained, this leads to improvement of
the basic performance of steel for welded structures (strength and
toughness) of course and also conversely the weldability and gas
cutting performance.
[0012] The present invention defines the amounts of not only Cr and
Nb, but also individual elements such as C, Si, and Mn and the weld
cracking parameter P.sub.CM and further limits the production
conditions so as to not only achieve both excellent high
temperature strength and low temperature toughness without using
expensive Mo, but also secure various usage performances for steel
for welded structures. Its gist is as follows:
[0013] (1) A method of production of steel for welded structures
excellent in high temperature strength and low temperature
toughness characterized by comprising heating a steel material
comprising, by mass %, [0014] C: 0.003 to 0.05%, [0015] Si: 0.60%
or less, [0016] Mn: 0.6 to 2.0%, [0017] P: 0.020% or less, [0018]
S: 0.010% or less, [0019] Cr: 0.20 to 1.5%, [0020] Nb: 0.005 to
0.05%, [0021] Al: 0.060% or less, and [0022] N: 0.001 to 0.006%,
further limiting, as an impurity, Mo to 0.03% or less, having a
balance of iron and unavoidable impurities, and having a weld
cracking parameter P.sub.CM value defined by [0023]
P.sub.CM=C+Si/30+Mn/20+Cu/20+Ni/60+Cr/20+Mo/15+V/10+5B of 0.22% or
less, to 1000 to 1300.degree. C. in temperature, finish rolling
temperature of 800.degree. C. or more, and then cooling.
[0024] (2) A method of production of steel for welded structures
excellent in high temperature strength and low temperature
toughness as set forth in claim 1, characterized by, after
finishing said hot rolling, starting accelerated cooling from
750.degree. C. or more in temperature, and stopping the accelerated
cooling at 550.degree. C. or less.
[0025] (3) A method of production of steel for welded structures
excellent in high temperature strength and low temperature
toughness as set forth in (1) or (2) characterized by further
containing, by mass %, one or both of [0026] V: 0.01 to 0.10% and
[0027] Ti: 0.005 to 0.025%.
[0028] (4) A method of production of steel for welded structures
excellent in high temperature strength and low temperature
toughness as set forth in any one of (1) to (3), characterized by
further containing, by mass %, one or more of [0029] Ni: 0.05 to
0.50%, [0030] Cu: 0.05 to 0.50%, [0031] B: 0.0002 to 0.003%, and
[0032] Mg: 0.0002 to 0.005%.
[0033] (5) A method of production of steel for welded structures
excellent in high temperature strength and low temperature
toughness as set forth in any one of (1) to (4), characterized by
further containing, by mass %, one of [0034] Ca: 0.0005 to 0.004%
and [0035] an REM: 0.0005 to 0.008%.
[0036] (6) A steel for welded structures excellent in high
temperature strength and low temperature toughness characterized by
being obtained by heating a steel material comprising, by mass %,
[0037] C: 0.003 to 0.05%, [0038] Si: 0.60% or less, [0039] Mn: 0.6
to 2.0%, [0040] P: 0.020% or less, [0041] S: 0.010% or less, [0042]
Cr: 0.20 to 1.5%, [0043] Nb: 0.005 to 0.05%, [0044] Al: 0.060% or
less, and [0045] N: 0.001 to 0.006%, further limiting, as an
impurity, No to 0.03% or less, having a balance of iron and
unavoidable impurities, and having a weld cracking parameter
P.sub.CM value defined by [0046]
P.sub.CM=C+Si/30+Mn/20+Cu/20+Ni/60+Cr/20+Mo/15+V/10+5B of 0.22% or
less, to 1000 to 1300.degree. C. in temperature, finish rolling
temperature of 800.degree. C. or more, and then cooling.
BEST MODE FOR CARRYING OUT THE INVENTION
[0047] The ranges of addition of the different alloy elements
defined in the present invention will be explained first.
[0048] C: 0.003 to 0.05%
[0049] C is limited to an extremely low level in high strength
steel. This is closely related to the other elements and to the
method of production. Even among the steel compositions, C has the
greatest effect on the properties of a steel material. A lower
limit of 0.003% is the smallest value for securing strength and
preventing the weld and other heat affected zones from softening
more than necessary.
[0050] If the amount of C is too great, the hardenability rises
more than necessary and the balance of strength and toughness of
the steel material, the weldability, etc. are adversely affected.
Further, as explained later, depending on the targeted plate
thickness and strength, the accelerated cooling is stopped at a
relatively low temperature in some cases. To suppress excessive
hardening near the top and bottom surfaces of the steel material at
that time or fluctuations in property in the plate thickness
direction, the upper limit was made 0.05%.
[0051] From the fluctuations in operation and balance with the
other elements, to avoid a drop in strength, the lower limit is
preferably made 0.005%, more preferably 0.01%. Further, to avoid
excessive hardening by accelerated cooling and fluctuations in
property, the upper limit is preferably made 0.04%, more preferably
0.03%.
[0052] Si: 0.60% or less
[0053] Si is an element included in steel for deoxidation, but if
overly added, the weldability and HAZ toughness deteriorate, so the
upper limit was made 0.60%. Steel can be deoxidized by Ti and Al as
well, so the content may be determined by the balance with these
elements. However, from the viewpoint of the HAZ toughness,
hardenability, etc., the lower the better. Zero addition is also
possible. For this reason, the upper limit may be limited to 0.40%,
0.20%, or 0.10%. Note that when a steelmaking plant produces steel,
even when using Ti and Al for deoxidation without the addition of
Si, 0.01% or more of Si is generally included.
[0054] Mn: 0.6 to 2.0%
[0055] Mn is an element essential for securing room temperature
strength and toughness. The lower limit is 0.6%. Preferably, the
content is 0.8% or more or 1.0% or more. However, if the amount of
Mn is too large, the hardenability rises and the weldability and
HAZ toughness are degraded. Not only that, but also center
segregation at the continuously cast slab is enhanced, so the upper
limit was made 2.0%. Preferably, the content is made 1.8% or less,
more preferably 1.6% or less or 1.4% or less.
[0056] P: 0.020% or less
[0057] P, if small in amount, tends to reduce the intergranular
fractures at the HAZ, so the smaller, the better. If the content is
large, it degrades the low temperature toughness of the base
material and the weld zone, so the upper limit is made 0.020%.
0.015% or less, 0.010% or less, or 0.008% or less is more
preferable. Of course, zero addition is also possible.
[0058] S: 0.010% or less
[0059] S is preferably small in amount from the viewpoint of the
low temperature toughness of the base material. If the content is
large, the low temperature toughness of the base material and the
weld zone is degraded, so the upper limit is made 0.010%. 0.008% or
less, 0.006%, or 0.004% is more preferable. Of course, zero
addition is also possible.
[0060] Cr: 0.20 to 1.5%
[0061] Cr is one of the most important elements in the present
invention. To secure high temperature strength, together with Nb,
addition of Cr is essential. This is because due to the effect of
improvement of hardenability by Cr, the transformation temperature
falls and the hard structure containing cementite becomes bainitic,
so the room temperature and high temperature strengths are raised
and further, because at the time of high temperature, precipitation
strengthening by precipitates of Cr (carbonitrides) is
utilized.
[0062] To obtain these effects, the content of Cr has to be a
minimum of 0.20%. Preferably, it is 0.35% or more. 0.50% or more or
0.8% or 1.0% or more is more preferable. However, if the amount of
addition is too great, deterioration of the toughness and
weldability of the base material and weld zone is caused and
economy is also lost, so the upper limit was made 1.5%. Preferably,
it may be 1.3% or less.
[0063] Nb: 0.005 to 0.05%
[0064] Nb, along with Cr, is the most important element in the
present invention. In the same way as Cr, this is because
precipitation strengthening by precipitates (carbonitrides) of Nb
is utilized to secure high temperature strength.
[0065] For this reason, at least 0.005% is necessary. Preferably,
the amount of addition is 0.010% or more. However, if the amount of
addition is too great, this causes deterioration in the toughness
of the weld zone, so the upper limit was made 0.05%. Preferably,
the amount of addition is 0.045% or less, more preferably 0.030% or
less. Note that addition of Nb also contributes to raising the
non-recrystallization temperature of austenite and bringing out the
effect of controlled rolling at the time of hot rolling to its
maximum extent.
[0066] Due to the above addition of Cr and Nb, it is possible to
secure high temperature strength even under Mo-free conditions.
Therefore, in the present invention, Mo is not intentionally added.
Further, even when Mo is unintentionally mixed in as an impurity,
it is restricted to 0.03% or less.
[0067] Al: 0.060% or less
[0068] Al is an element generally included in steel for
deoxidation. Deoxidation is also performed by Si and Ti, so the
amount should be determined by the balance with these elements.
However, if the amount of Al becomes large, not only will the
cleanliness of the steel become poorer, but also the toughness of
the weld metal will deteriorate, so the upper limit is made 0.060%.
Preferably, it may be 0.040% or less. The smaller the amount the
better. Zero addition is also possible. Note that when a
steelmaking plant produces steel, even when not using Al for
deoxidation, 0.001% or more of Al is generally included.
[0069] N: 0.001 to 0.006%
[0070] N is included in the steel as an unavoidable impurity, but
bonds with Nb to form carbonitrides to increase the strength.
Further, it forms TiN to enhance the properties of the steel as
explained above. For this reason, as an amount of N, a minimum of
0.001% is required. Preferably, the amount may be 0.0015% or more.
However, addition of an amount of N is harmful to the weld heat
affected zone toughness and weldability. In the present invention
steel, the upper limit is 0.006%. More preferably it may be 0.0045%
or less.
[0071] Next, the reasons for addition of V and Ti which may be
included in accordance with need will be explained.
[0072] V: 0.01 to 0.10%
[0073] V has substantially the same effects as Nb. The role of V in
the present invention is to complement the Nb. However, V has a
smaller effect than Nb and also has an effect on the hardenability,
so upper and lower limits were set. The lower limit was made 0.01%
as the smallest amount at which the effect of addition of V can be
reliably obtained. Preferably, the lower limit may be 0.025% or
more. The upper limit was made 0.10% considering also the effects
on the later explained weld cracking parameter P.sub.CM.
Preferably, the upper limit is 0.08% or less, more preferably 0.05%
or less.
[0074] Ti: 0.005 to 0.025%
[0075] Ti is preferably added for improving the toughness of the
base material and weld heat affected zone. The reason why is that
Ti, when the amount of Al is low (for example 0.003% or less),
bonds with O to form precipitates mainly comprised of
Ti.sub.2O.sub.3. These become nuclei for formation of intragranular
ferrite and improve the toughness of the weld heat affected
zone.
[0076] Further, Ti bonds with N to form TiN which finely
precipitates in the steel material and is effective for suppressing
coarsening of the .gamma. grains at the time of heating and
refining the rolled structure. Further, the fine TiN present in a
steel material refines the weld heat affected zone structure and
improves the toughness. To obtain these effects, Ti has to be a
minimum of 0.005%. However, if too great, it forms TiC which
degrades the low temperature toughness and weldability, so the
upper limit was made 0.025%. Preferably, it is 0.020% or less.
[0077] Next, the reasons for addition of Ni, Cu, B, and Mg will be
explained.
[0078] The main purpose for further adding these elements to the
basic compositions is to improve the strength, toughness, and other
properties without detracting from the excellent characteristics of
the invention steels. Therefore, the amounts of addition by nature
should be self restricted.
[0079] Ni: 0.05 to 0.50%
[0080] Ni, if not added in excess, improves the strength and
toughness of the base material without having a detrimental effect
on the weldability. To bring out these effects, addition of at
least 0.05% is essential.
[0081] On the other hand, excessive addition is not only expensive,
but also is not preferable for the weldability. Further, if adding
a large amount of Ni, the possibility of inducing stress corrosion
cracking (SCC) in liquid ammonia has been pointed out. According to
experiments of the inventors, addition of up to 1.0% does not
greatly degrade the weldability or SCC in liquid ammonia and rather
has a greater effect in improving the strength and toughness, but
giving priority to economy, the upper limit was made 0.50%.
Further, when giving priority to economy, the upper limit may also
be set to 0.35%.
[0082] Cu: 0.05 to 0.50%
[0083] Cu exhibits substantially the same effects and phenomena as
Ni. The upper limit of 0.50% is set since in addition to
deterioration of the weldability, excessive addition results in Cu
cracks at the time of hot rolling and therefore difficult
production. The lower limit should be made the smallest amount by
which the substantial effect can be obtained and therefore is
0.05%. When giving priority to economy, the upper limit may also be
set to 0.30%.
[0084] B: 0.0002 to 0.003%
[0085] B segregates at the austenite grain boundaries and
suppresses formation of ferrite to thereby improve the
hardenability and contribute to improvement of the strength. To
obtain this effect, a minimum of 0.0002% or more is required.
[0086] However, with addition of too much, not only would the
effect of improvement of the hardenability become saturated, but
also B precipitates harmful to the toughness might be formed, so
the upper limit is made 0.003%. Preferably, it may be 0.002% or
less. Note that in cases such as steel for storage tanks etc. where
stress corrosion cracking is a concern, reduction of the hardness
of the base material and weld heat affected zone often becomes the
point (for example, to prevent sulfide stress corrosion cracking
(SSCC), in terms of Rockwell hardness, HRC.ltoreq.22
(HV.ltoreq.248) is considered essential). In such a case, addition
of B, which increases the hardenability, is not preferable. Note
that B has the above effect of improving the strength, but there is
the problem that addition of B causes deterioration of the heat
affected zone toughness and other material quality, so to avoid
these problems, it is more preferable to limit B to 0.0003% or less
or not add it.
[0087] Mg: 0.0002 to 0.005%
[0088] Mg has the action of controlling the growth of the austenite
grains in the weld heat affected zone and refining so as to
strengthen and toughen the weld zone. To obtain this effect, Mg has
to be 0.0002% or more. On the other hand, if the amount of addition
increases, the effect on the amount of addition becomes smaller, so
this is not a wise course in terms of cost, so the upper limit was
made 0.005%. Preferably, it may be 0.0035% or less.
[0089] Next, the reasons for addition of Ca or REM will be
explained.
[0090] Ca: 0.0005 to 0.004%
[0091] REM: 0.0005 to 0.008%
[0092] The Ca and REM control the shape of the MnS and improve the
low temperature toughness of the base material. In addition, they
reduce the hydrogen induced cracking (HIC, SSC, and SOHIC)
susceptibility under a wet hydrogen sulfide environment. To obtain
these effects, a minimum of 0.0005% is necessary.
[0093] However, addition of too much conversely causes the
cleanliness of the steel to deteriorate and raises the base
material toughness and hydrogen induced cracking (HIC, SSC, and
SOHIC) susceptibility under a wet hydrogen sulfide environment, so
the upper limits of the amounts of addition were respectively made,
for Ca and REM, 0.004% and 0.008%. Preferably, the limits may be
made 0.003% and 0.006% or less. Note that Ca and REM have
substantially equivalent effects, so it is sufficient to add either
of these in the above range. Addition of both is also possible.
[0094] Even if limiting the individual elements of the steel,
unless the system of compositions as a whole is suitable, excellent
characteristics cannot be obtained. In the present invention, from
the contents of the different elements (mass %), the value of the
weld cracking parameter P.sub.CM, defined by the following formula,
is limited to 0.22% or less.
[0095] P.sub.CM=C+Si/30+Mn/20+Cu/20+Ni/60+Cr/20+Mo/15+V/10+5B
[0096] P.sub.CM is a parameter expressing the weldability. The
lower, the better the weldability. In JIS G 3106 "Rolled Steels for
Welded Structure", while differing depending on the strength level
and the plate thickness, at the strictest, it is limited to 0.24%
or less.
[0097] According to the broad range of various weld crack tests of
the inventors, P.sub.CM is limited to 0.22% or less as a condition
able to reliably prevent weld cold cracking even under harsher
restraint conditions and environmental conditions. Note that the
lower limit is not particularly set, but is restricted naturally
from the ranges of limitation of the compositions.
[0098] Next, the production conditions will be explained.
[0099] The reason for limiting the heating temperature before the
hot rolling to 1000 to 1300.degree. C. is to keep the austenite
grains at the time of heating small and refine the rolled
structure. 1300.degree. C. is the upper limit temperature at which
the austenite will not become extremely coarse at the time of
heating. If the heating temperature exceeds this, the austenite
grains become coarse mixed grains. The structure after
transformation also becomes coarse, so the steel remarkably
deteriorates in toughness.
[0100] On the other hand, if the heating temperature is too low,
depending on the plate thickness, not only does securing the later
mentioned finish rolling temperature become difficult, but also the
non-recrystallization temperature of the austenite is raised. From
the viewpoint of the solubility of Nb for bringing out
precipitation strengthening, the lower limit was made 1000.degree.
C. The most preferable heating temperature range is 1050 to
1250.degree. C.
[0101] The steel material heated under the above-mentioned
conditions is hot rolled at 800.degree. C. or more, then cooled.
The cooling means is not particularly an issue. The material may
also be allowed to stand in the atmosphere for cooling, but by
accelerated cooling from a temperature of 750.degree. C. or more to
a temperature of 550.degree. C. or less, it is possible to improve
the characteristics of the steel material more.
[0102] If the finish rolling temperature falls below 800.degree.
C., in the invention steels, where the amount of C is relatively
small, the ferrite is liable to precipitate by transformation and
ferrite is liable to be worked (rolled). This is not preferable
from the viewpoint of securing the low temperature toughness. For
this reason, the finish rolling temperature is limited to
800.degree. C. or more. Preferably, it may be 820.degree. C. or
more.
[0103] The relatively low strength so-called "40 kg class steel"
(for example, JIS standard SM400 and SN400 steel) after being hot
rolled at 800.degree. C. or more can satisfy a predetermined
strength even if allowed to stand in the atmosphere for
cooling.
[0104] However, even with 50 kg class steel (for example, JIS
standard SM490 and SN490 steel) or 40 kg class steel, if the plate
thickness becomes greater, it becomes difficult to secure stability
of the strength as cooled by standing in the atmosphere, so
accelerated cooling from a temperature of 750.degree. C. or more
after hot rolling at 800.degree. C. or more is preferable.
Accelerated cooling after rolling improves the characteristics of
the steel material and does not harm the excellent features of the
present invention.
[0105] Accelerated cooling inherently increases the cooling rate in
the transformation region and thereby refines the structure and
simultaneously raises the strength and toughness. Therefore, unless
started before the start of transformation or at least started
before the end of transformation, it has substantially no meaning.
For this reason, the accelerated cooling start temperature is
limited to 750.degree. C. or more. This accelerated cooling has to
be performed down to a temperature of 550.degree. C. or less in
order to obtain this effect. With a temperature over 550.degree.
C., the transformation does not sufficiently proceed at the time of
accelerated cooling and the refinement of the structure becomes
insufficient. The preferable start temperature of the accelerated
cooling is 760.degree. C. or more. The preferable range of stop
temperature of the accelerated cooling is 520 to 300.degree. C.
[0106] Note that the cooling rate at the time of accelerated
cooling depends on the steel compositions and the intended strength
or low temperature toughness level, but the average cooling rate
from the accelerated cooling start temperature to 550.degree. C. at
a position of 1/4 the plate thickness from the surface in the
direction of plate thickness is preferably made 3.degree. C./sec or
more.
[0107] Further, even if tempering after rolling at the Ac1
temperature or less, the excellent features of the present
invention are not impaired. This cancels out the unevenness of
cooling and improves the uniformity of quality in the plate, so is
rather preferable.
EXAMPLES
[0108] Steel plates of various steel compositions (thickness 19 to
100 mm) were produced by a converter--continuous casting--plate
rolling process and investigated for properties.
[0109] Table 1 shows the steel compositions of the comparative
steels and the invention steels, while Table 2 shows the production
conditions and properties of steel plates.
[0110] The steel plates produced in accordance with the present
invention (invention steels) all have good properties. As opposed
to this, it was learned that the steel plates not produced
according to the present invention (comparative steels) were
inferior in one or more of the propereties.
[0111] Comparative Steel 11 is high in the amount of C, so compared
with the invention steels, both the base material and simulated HAZ
are inferior in low temperature toughness.
[0112] Comparative Steel 12 does not have any Nb added. Further,
Comparative Steel 13 is low in the amount of Cr. Both are therefore
low in high temperature strength.
[0113] Comparative Steel 14 is low in the amount of C, so is low in
high temperature strength.
[0114] Comparative Steel 15 is high in the amount of Cr, so both
the base material and simulated HAZ are inferior in toughness.
[0115] Comparative Steel 16 is high in Nb and inferior in HAZ
toughness.
[0116] Comparative Steels 17-1 to 3 are the same in compositions as
the Invention Steel 5. However, Comparative Steel 17-1 is low in
finish rolling temperature and as a result an accelerated cooling
start temperature cannot be secured and ends up becoming low, so is
low in both room temperature and high temperature strength.
Comparative Steel 17-2 is low in accelerated cooling start
temperature, so is low in both room temperature and high
temperature strength. Comparative Steel 17-3 is high in accelerated
cooling stop temperature, so is low in both room temperature and
high temperature strength.
[0117] Comparative Steel 18 has individual elements and a method of
production within the scope of the present invention and has an
ordinary temperature and a high temperature strength or toughness
etc. satisfying the characteristics required for the 490 MPa class,
but has a high P.sub.CM, so cracks occurred in terms of the
weldability (y-groove weld cracking test).
TABLE-US-00001 TABLE 1 Chemical compositions (mass %) Class Steel C
Si Mn P S Cr Nb Al N Mo Others P.sub.CM.sup.1) Inv. 1 0.003 0.31
0.95 0.006 0.003 0.81 0.018 0.028 0.030 0.01 0.011Ti, 0.0010B 0.107
steel 2 0.01 0.16 1.31 0.004 0.002 0.65 0.020 0.033 0.024 0.01
0.18Cu, 0.18Ni 0.126 3 0.02 0.57 1.87 0.005 0.002 1.45 0.007 0.021
0.029 0.02 0.20Ni, 0.052V, 0.009Ti 0.215 4 0.02 0.22 1.45 0.007
0.002 0.41 0.012 0.023 0.051 0 0.062V 0.127 5 0.03 0.38 1.48 0.007
0.004 0.68 0.033 0.006 0.028 0.01 0.21Cu, 0.22Ni, 0.010Ti, 0.0012Mg
0.166 6 0.03 0.19 1.66 0.006 0.006 1.01 0.028 0.005 0.022 0.03
0.0009B, 0.0014Ca 0.176 7 0.03 0.44 0.62 0.007 0.003 0.22 0.047
0.045 0.043 0.01 0.32Cu, 0.32Ni, 0.062V, 0.0018REM 0.115 8 0.04
0.27 1.31 0.005 0.002 0.50 0.024 0.003 0.036 0 0.140 9 0.04 0.08
1.81 0.005 0.004 1.20 0.019 0.032 0.027 0 0.25Cu, 0.25Ni 0.210 10
0.05 0.24 1.89 0.006 0.005 0.56 0.021 0.016 0.032 0.02 0.014Ti,
0.0013B, 0.0012Ca 0.188 19 0.02 0.20 1.57 0.005 0.004 0.61 0.026
0.022 0.028 0.01 0.009Ti 0.136 Comp. 11 0.06 0.23 28 0.006 0.004
0.41 0.034 0.020 0.030 0.02 0.012Ti 0.153 steel 12 0.02 28 56 0.007
0.002 0.80 0 0.027 0.035 0.01 0.25Ni 0.153 13 0.03 29 27 0.008
0.008 0.14 0.020 0.028 0.026 0.01 0.0015Ca 0.111 14 0.001 33 57
0.006 0.004 0.69 0.018 0.031 0.032 0 0.125 15 0.04 25 31 0.008
0.005 1.72 0.025 0.024 0.027 0 0.200 16 0.04 31 25 0.006 0.004 0.51
0.065 0.033 0.025 0.02 0.139 17 0.03 03 48 0.007 0.004 0.68 0.028
0.006 0.028 0.01 0.21Cu, 0.22Ni, 0.010Ti, 0.0012Mg 0.166 18 0.04 39
83 0.007 0.005 1.38 0.030 0.031 0.033 0.02 0.25Cu, 0.25Ni, 0.070V,
0.011Ti 0.237 .sup.1)P.sub.CM = C + Si/30 + Mn/20 + Cu/20 + Ni/60 +
Cr/20 + Mo/15 + V/10 + 5B
TABLE-US-00002 TABLE 2 Acc. Acc. Root cracking Finish cooling
cooling Proof Simulated at y-crack Targeted Heating rolling start
stop Plate Yield Tensile strength HAZ test without strength temp.
temp. temp. temp. thick. strength strength vTrs at 600.degree. C.
toughness .sup.2), preheating Class Steel grade (.degree. C.)
(.degree. C.) (.degree. C.) (.degree. C.) (mm) (MPa) (MPa)
(.degree. C.) (MPa) .sup.1) vE0 (J) (room temp.) .sup.3) Inv. 1 400
MPa 1250 920 980 340 50 332 477 -98 182 163 None steel 2 490 MPa
1200 820 780 460 25 386 551 -84 250 124 None 3 400 MPa 1200 850 --
-- 40 395 548 -81 246 109 None 4 490 MPa 1280 860 820 480 50 431
545 -75 242 112 None 5 490 MPa 1200 900 860 430 32 433 563 -78 256
98 None 6 490 MPa 1100 950 930 450 100 338 518 -71 237 106 None 7
490 MPa 1100 930 900 300 80 376 522 -65 234 121 None 8 490 MPa 1050
870 840 410 60 386 536 -68 245 101 None 9 490 MPa 1150 810 -- -- 19
454 582 -75 261 126 None 10 490 MPa 1150 850 800 290 50 414 547 -67
243 95 None 19 490 MPa 1150 840 -- -- 28 298 452 -80 164 156 None
Comp. 11 490 MPa 1150 860 820 230 50 408 553 -12 248 18 None steel
12 400 MPa 1150 850 800 250 40 283 479 -78 142 141 None 13 490 MPa
1150 850 -- -- 40 376 529 -67 197 130 None 14 400 MPa 1200 850 --
-- 40 319 487 -86 151 139 None 15 490 MPa 1200 850 -- -- 40 362 541
-10 236 23 None 16 490 MPa 1200 900 -- -- 40 348 561 -55 251 14
None 17-1 490 MPa 1100 750 720 280 32 431 488 -82 198 111 None 17-2
490 MPa 1100 800 730 300 32 322 484 -79 195 106 None 17-3 490 MPa
1100 830 770 600 32 317 496 -80 197 99 None 18 490 MPa 1100 810 --
-- 40 363 527 -21 220 73 Yes .sup.1) Judgment criteria for passage:
400 MPa class steel: 157 MPa or more (235x(2/3)), 490 MPa steel,
217 MPa or more (325x(2/3)) .sup.2) Charpy impact absorption energy
of simulated heat cycle (conditions: after holding at 1400.degree.
C. .times. 10 sec, then cooling from 800 to 500.degree. C. by 100
sec) (average value of three samples) .sup.3) y-groove weld
cracking test (JIS Z 3158)
INDUSTRIAL APPLICABILITY
[0118] According to the present invention, steel for welded
structures excellent in high temperature strength and low
temperature toughness can be provided in large amounts
inexpensively. As a result, it becomes possible to reduce or
eliminate the fire-resistant coating for building structures.
Further, in applications other than buildings as well, since the
strength, toughness, and other basic performances are provided and
further high temperature strength is also provided, it becomes
possible obtain steel for welded structures able to be exposed to a
high temperature and to enhance much more the safety of
buildings.
* * * * *