U.S. patent application number 12/223690 was filed with the patent office on 2009-01-22 for fire resistant high strength rolled steel material and method of production of the same.
Invention is credited to Hiroshi Kita, Teruhisa Okumura, Kohichi Yamamoto, Suguru Yoshida.
Application Number | 20090020190 12/223690 |
Document ID | / |
Family ID | 38345311 |
Filed Date | 2009-01-22 |
United States Patent
Application |
20090020190 |
Kind Code |
A1 |
Okumura; Teruhisa ; et
al. |
January 22, 2009 |
Fire Resistant High Strength Rolled Steel Material and Method of
Production of The Same
Abstract
A fire resistant high strength rolled steel material superior in
fire resistance and toughness used for a structural member of a
building etc. containing, by mass %, C: 0.005% to less than 0.04%,
Mn: 0.8 to 1.7%, Si: 0.05 to less than 0.4%, Nb: 0.02 to 1%, Ti:
0.005 to 0.02%, N: 0.005% or less, B: 0.0003 to 0.003%, and Al:
0.005% to 0.03%, and a balance of Fe and unavoidable impurities,
and having the ratio Ti/N of 2 to 8, the value C-Nb/7.74 of 0.02%
or less, and having the ratio of the 0.2% yield strength at
600.degree. C. to the yield strength at room temperature of 0.50 or
more.
Inventors: |
Okumura; Teruhisa; (Osaka,
JP) ; Kita; Hiroshi; (Osaka, JP) ; Yamamoto;
Kohichi; (Kanagawa, JP) ; Yoshida; Suguru;
(Chiba, JP) |
Correspondence
Address: |
KENYON & KENYON LLP
ONE BROADWAY
NEW YORK
NY
10004
US
|
Family ID: |
38345311 |
Appl. No.: |
12/223690 |
Filed: |
February 8, 2007 |
PCT Filed: |
February 8, 2007 |
PCT NO: |
PCT/JP2007/052658 |
371 Date: |
August 5, 2008 |
Current U.S.
Class: |
148/522 ;
148/330 |
Current CPC
Class: |
C21D 8/0263 20130101;
C22C 38/004 20130101; C22C 38/12 20130101; C21D 8/0463 20130101;
C22C 38/02 20130101; B21B 3/00 20130101; C21D 8/00 20130101; C22C
38/04 20130101; C22C 38/14 20130101 |
Class at
Publication: |
148/522 ;
148/330 |
International
Class: |
C21D 8/00 20060101
C21D008/00; C22C 38/04 20060101 C22C038/04; C22C 38/16 20060101
C22C038/16 |
Foreign Application Data
Date |
Code |
Application Number |
Feb 8, 2006 |
JP |
2006-030962 |
Claims
1. A fire resistant high strength rolled steel material containing,
by mass %, C: 0.005% to less than 0.04%, Mn: 0.8 to 1.7%, Si: 0.05
to less than 0.4%, Nb: 0.02 to 1%, Ti: 0.005 to 0.02%, N: 0.005% or
less, B: 0.0003 to 0.003%, Al: 0.005% to 0.03%, and the balance of
Fe and unavoidable impurities, and having the ratio Ti/N of 2 to 8,
the value C--Nb/7.74 of 0.02% or less by mass %, and having the
ratio of the 0.2% yield strength at 600.degree. C. to the yield
point strength (when the yield point is unclear, 0.2% yield
strength) at room temperature of 0.50 or more.
2. A fire resistant high strength rolled steel material as set
forth in claim 1, further containing, by mass %, one or more of Cr:
0.4% or less, Cu: 1% or less, and Ni: 0.7% or less.
3. A method of production of a fire resistant high strength rolled
steel material comprising preparing a cast slab containing, by mass
%, C: 0.005% to less than 0.04%, Mn: 0.8 to 1.7%, Si: 0.05 to less
than 0.4%, Nb: 0.02 to 1%, Ti: 0.005 to 0.02%, N: 0.005% or less,
B: 0.0003 to 0.003%, and Al: 0.005% to 0.03%, and the balance of Fe
and unavoidable impurities, and having the ratio Ti/N of 2 to 8,
the value C--Nb/7.74 of 0.02% or less by mass %, heating this to a
temperature region of 1250 to 1350.degree. C., then starting
rolling and rolling to give a cumulative reduction rate at
1000.degree. C. or less of 30% or more to thereby give the ratio of
a 0.2% yield strength at 600.degree. C. and a yield point strength
(when yield point is unclear, a 0.2% yield strength) at room
temperature of 0.50 or more.
4. A method of production of a fire resistant high strength rolled
steel material as set forth in claim 3, wherein said cast slab
further contains, by mass %, one or more of Cr: 0.4% or less, Cu:
1% or less, and Ni: 0.7% or less.
5. A method of production of a fire resistant high strength rolled
steel material comprising preparing a cast slab containing, by mass
%, C: 0.005% to less than 0.04%, Mn: 0.8 to 1.7%, Si: 0.05 to less
than 0.4%, Nb: 0.02 to 1%, Ti: 0.005 to 0.02%, N: 0.005% or less,
B: 0.0003 to 0.003%, and Al: 0.005% to 0.03%, and a balance of Fe
and unavoidable impurities, and having the Ti/N of 2 to 8, the
value C--Nb/7.74 of 0.02% or less by mass %, heating this to a
temperature region of 1250 to 1350.degree. C., then starting
rolling, and cooling, after the end of said rolling, at 800 to
500.degree. C. in temperature range by a 0.1 to 10.degree. C./sec
average cooling rate to thereby give a ratio of 0.2% yield strength
at 600.degree. C. and yield point strength (when yield point is
unclear, 0.2% yield strength) at room temperature of 0.50 or
more.
6. A method of production of a fire resistant high strength rolled
steel material as set forth in claim 5, wherein said cast slab
further contains, by mass %, one or more of Cr: 0.4% or less, Cu:
1% or less, and Ni: 0.7% or less.
Description
TECHNICAL FIELD
[0001] The present invention relates to a fire resistant high
strength rolled steel material used for the structural member of a
building and a method of production of the same.
BACKGROUND ART
[0002] So-called "fire resistant steel" is a steel material for
building use which has a predetermined strength even when the
building catches fire etc. and the material becomes a high
temperature. Here, fire resistant steel envisioning a temperature
of a building at the time of a fire of 600.degree. C. and able to
maintain its strength at that temperature will be explained.
[0003] Now, the methods of strengthening steel materials include
mainly the 1) method of grain refinement of the ferrite, the 2)
solution strengthening method using alloy elements, the 3)
dispersion strengthening method using a hardening phase, and the 4)
method using fine precipitates. Deformation of a steel material,
seen microscopically, is caused by movement of dislocations in the
crystal grains. Each of these methods is a method strengthening the
resistance to such movement of dislocations.
[0004] Therefore, first the 1) method of grain refinement of the
ferrite will be explained.
[0005] The dislocations moving in the crystal grains stop once at
the grain boundaries, then move to the adjoining crystal grains, so
the crystal grain boundaries provide resistance to the movement of
dislocations. Therefore, if the crystal grains become finer, the
frequency of moving dislocations meeting crystal grain boundaries
rises, so the resistance increases. This method of strengthening is
the 1) method of grain refinement of the ferrite.
[0006] Note that, generally, the strength is evaluated by the
following equation known as the Hall-Petch equation:
.sigma.=.sigma..sub.0+k.times.d.sup.-0.5
[0007] Here, .sigma. is the yield strength, .sigma..sub.0 is the
constant stress value, the relative constant k is called the
locking parameter and is an indicator of the resistance at the
crystal grain boundaries, and d is the crystal grain size.
[0008] Next, the 2) the solution strengthening method using alloy
elements will be explained.
[0009] When there are solute atoms of different sizes such as alloy
elements on the plane of movement of the dislocations, called the
"slip plane", a resistance acts with respect to the movement of the
dislocations. Further, due to the distribution of the alloy
elements in the steel, an elastic stress field is formed and acts
as a drag resistance with respect to movement of the dislocations.
It is known that the size of the drag resistance is affected by the
solute atom concentration, misfit due to the solute/solvent atom
size, and diffusion coefficient of the solute atoms.
[0010] The method of strengthening by increasing this alloy element
on the slip plane resistance or the drag resistance by alloy
elements is referred to as the 2) solution strengthening method
using alloy elements.
[0011] Further, as the solution strengthening method strengthening
the structure by the increase of the drag resistance, there is the
technology of utilizing of the drag effect of the dissolved Nb. The
technology of utilizing the drag effect of this dissolved Nb is
used for the production of thin-gauge fire resistant steel and is
described in for example Japanese Patent Publication (A) No.
2000-054061 and Japanese Patent Publication (A) No.
2000-248335.
[0012] The drag effect of dissolved Nb is the phenomenon of the
dissolved Nb concentrating at dislocations and other lattice
defects and forming resistance to movement of defects and
dislocations to thereby improve the strength.
[0013] The present inventors discovered the possibility of the drag
resistance due to this dissolved Nb effectively functioning in the
temperature region up to about 600.degree. C. and thereby developed
the Nb-based fire resistant steel of the present invention, but
discovered that to make the drag effect of this dissolved Nb fully
function and achieve fire resistant steel having sufficient fire
resistance, it is necessary to satisfy the following
conditions.
[0014] First, the amount of dissolved C must be made a low value.
This is because if the amount of dissolved C is high, it forms NbC
and reduces the amount of dissolved Nb.
[0015] Second, B has to be added. Part of the contained Nb cannot
be maintained in the solid solution state, but segregates at the
crystal grain boundaries and cannot concentrate at the dislocations
and other lattice defects, but if adding B, the B segregates at the
crystal grain boundaries instead of Nb and helps the Nb maintain
its dissolved state.
[0016] Third, the amount of dissolved N must be reduced. This is
because the added B reacts with N and ends up producing BN so loses
the ability to segregate at the crystal grain boundaries. To reduce
the amount of dissolved N, the means of adding Ti to cause the
formation of TiN and reduce the amount of dissolved N is used.
[0017] Furthermore, the 3) dispersion strengthening method using a
hardening phase will be explained.
[0018] A macrostructure comprised of a hard phase and soft phase
mixed together (complex phase structure) generally changes in
strength according to the volume fractions of the phases. This is
due to the fact that, compared with a soft phase, the dislocations
in the crystal grains of the hard phase tend to be difficult to
move during deformation, that is, the resistance required for
deformation is large. The method of increasing the resistance using
the presence of this hard phase is the 3) the dispersion
strengthening method using a hardening phase.
[0019] For example, with a composite phase structure made of
ferrite and pearlite, if the hard phase, that is, the pearlite,
increases in volume fractions, the soft phase, that is, the ferrite
structure, relatively falls and the strength rises.
[0020] Finally, the 4) method of using fine precipitates will be
explained.
[0021] If the precipitate is distributed on the slip plane at the
time of movement of dislocations in the crystal grains, it blocks
the dislocations and acts as resistance to movement of the
dislocations. The method of strengthening by increasing the
resistance due to precipitates will be referred to as the 4) method
of using fine precipitates.
[0022] In this conventional fire resistant steel, the 4) method of
using fine precipitates by adding Mo to form Mo carbides is used.
The fire resistant steel strengthened by the 4) method of using
fine precipitates using Mo and its method of production etc. are
described in Japanese Patent Publication (A) No. 2005-272854 and
Japanese Patent Publication (A) No. 09-241789.
[0023] In these conventional fire resistant steels, the amount of C
contained is a high value of about 0.1%, so the property of alloy
elements not dissolving and precipitates ending up being produced
is utilized.
DISCLOSURE OF THE INVENTION
[0024] However, in recent years, the soaring price of Mo has
resulted in the technique of the use of Mo as the main factor in
the strengthening method of alloy elements starting to lose price
competitiveness.
[0025] Therefore, the inventors engaged in intensive research on
inexpensive fire resistant steel using inexpensive Nb instead of
expensive Mo as the dissolved element and methods of production of
the same. As a result, they discovered that there were the
following problems in making steel containing Nb as a dissolved
element a fire resistant steel able to be used for thick steel
materials.
[0026] The first problem is that problems arise in toughness if the
amounts of addition of Ti and Al are outside predetermined ranges
when applying the drag effect of dissolved Nb to thick fire
resistant steel. When producing thick-gauge fire resistant steel,
this toughness becomes a problem when the thickness of the steel
plate is 7 mm or more, in particular when the thickness of the
steel plate is 12 mm or more.
[0027] The second problem is the definition of the suitable amount
of dissolved C for efficiently obtaining the drag effect of Nb.
[0028] The third problem is the definition of the surface
properties, in particular the amount of addition of Si for
preventing surface defects due to rolled-in scale at the time of
reheating in a heating furnace.
[0029] The present invention adjusts the balance of ingredients of
C, Nb, B, and Ti and the content of the deoxidizing elements (Si,
Al) to achieve the target room temperature yield point, high
temperature strength, high toughness, and good surface
properties.
[0030] The inventors engaged in intensive research and development
and as a result discovered methods for solving the above
problems.
[0031] First, for the first problem, the inventors discovered that
by making the content of B 0.0003 to 0.003%, controlling the
content of Al to 0.005% to 0.03%, and adjusting the contents of Ti
and N so that Ti/N is 2 to 8 in range, it is possible to secure the
targeted toughness.
[0032] Next, for the second problem, the inventors discovered that
to prevent the dissolved Nb from forming carbides such as NbC
precipitates and to remain in solid solution so as to concentrate
at the dislocations and other lattice defects, it is necessary to
make the value of C-Nb/7.74 for example 0.02 or less. This
corresponds to dissolved C of 0.02% or less
[0033] Finally, for the third problem, the inventors discovered
that when making Ti/N an amount of 2 to 8 in range, to secure the
strength of the matrix material and suppress the formation of scale
defects, it is sufficient to make the content of Si less than
0.4%.
[0034] Furthermore, the inventors discovered that if the dissolved
C is 0.02% or less, the solid solution of Nb causes the drag
resistance against disclocations to increase and promises great
solution strengthening. They discovered that this drag resistance
is affected by the concentration of the solute atoms, misfit due to
the solute/solvent atom size, and diffusion coefficient of the
solute atoms and that under these conditions, Nb has a great
effect. In addition, they discovered that the strengthening effect
due to the drag effect of dissolved Nb is about 5 to 8 times the
strengthening effect due to the addition of Mo in conventional fire
resistant steel and that it is possible to secure the equivalent
high temperature strength by addition of a smaller amount of
alloy.
[0035] Above, according to the present invention, by adjusting the
balance of the ingredients C, Nb, B, Ti, Al, and Si, it is possible
to achieve the targeted room temperature yield point, high
temperature strength, and high toughness and good surface
properties.
[0036] Based on this discovery, according to the present invention,
there is provided a fire resistant high strength rolled steel
material containing, by mass %, C: 0.005% to less than 0.04%, Mn:
0.8 to 1.7%, Si: 0.05 to less than 0.4%, Nb: 0.02 to 1%, Ti: 0.005
to 0.02%, N: 0.005% or less, B: 0.0003 to 0.003%, and Al: 0.005% to
0.03%, and a balance of Fe and unavoidable impurities, and having
the ratio, Ti/N of 2 to 8, the value C--Nb/7.74 of 0.02% or less by
mass %, and the ratio of the 0.2% yield strength at 600.degree. C.
to the yield point strength at room temperature of 0.50 or
more.
[0037] Note that when the room temperature yield point is unclear,
the 0.2% yield strength is used, but for calculation of the 0.2%
yield strength, the offset method of JIS Z 2241 is used.
[0038] This fire resistant high strength rolled steel material may
further contain, by mass %, one or more of Cr: 0.4% or less, Cu: 1%
or less, and Ni: 1.0% or less.
[0039] Further, according to the present invention, there is
provided a method of production of a fire resistant high strength
rolled steel material comprising preparing a cast slab containing,
by mass %, C: 0.005% to less than 0.04%, Mn: 0.8 to 1.7%, Si: 0.05
to less than 0.4%, Nb: 0.02 to 1%, Ti: 0.005 to 0.02%, N: 0.005% or
less, B: 0.0003 to 0.003%, and Al: 0.005% to 0.03%, and a balance
of Fe and unavoidable impurities, and having the ratio of Ti/N of 2
to 8, the value C--Nb/7.74 of 0.02% or less, heating this to a
temperature region of 1250 to 1350.degree. C., then starting
rolling and rolling to give a cumulative reduction rate at
1000.degree. C. or less of 30% or more to thereby give the ratio of
the 0.2% yield strength at 600.degree. C. to the yield point
strength at room temperature of 0.50 or more.
[0040] Furthermore, according to the present invention, there is
provided a method of production of a fire resistant high strength
rolled steel material comprising preparing a cast slab containing,
by mass %, C: 0.005% to less than 0.04%, Mn: 0.8 to 1.7%, Si: 0.05
to less than 0.4%, Nb: 0.02 to 1%, Ti: 0.005 to 0.02%, N: 0.005% or
less, B: 0.0003 to 0.003%, and Al: 0.005% to 0.03%, and a balance
of Fe and unavoidable impurities, and having the ratio Ti/N of 2 to
8, the value C--Nb/7.74 of 0.02% or less by mass %, heating this to
a temperature region of 1250 to 1350.degree. C., then starting
rolling, and cooling, after the end of said rolling, at 800 to
500.degree. C. in temperature range by a 0.1 to 10.degree. C./sec
average cooling rate to thereby give the ratio of the 0.2% yield
strength at 600.degree. C. to the yield point strength at room
temperature of 0.50 or more.
[0041] Note that in these methods of production, if the yield point
at room temperature is unclear the 0.2% yield strength is used.
[0042] In these methods of production, said cast slab may further
contain, by mass %, one or more of Cr: 0.4% or less, Cu: 1% or
less, and Ni: 1.0% or less.
[0043] According to the present invention, there may be provided a
steel material superior in fire resistance which has a high
strength and high toughness and brings out the drag effect of
dissolved Nb to the maximum extent and thereby give a yield
strength of 1/2 or more of that at room temperature even at
600.degree. C. by just solid solution of Nb without adding at all
the Mo generally added for fire resistant steel.
BRIEF DESCRIPTION OF THE DRAWINGS
[0044] FIG. 1 is a view showing a suitable range in the
relationship of Nb and C.
[0045] FIG. 2 is a view showing a suitable range in the
relationship of Ti and N.
[0046] FIG. 3 is a view for explaining the drag effect of Nb,
wherein (a) is a view of the case of adding Nb and B and (b) is a
view of the case of simply adding only Nb.
[0047] FIG. 4 is a schematic view showing an example of the layout
of apparatuses for working the method of the present invention.
[0048] FIG. 5 is a view showing the cross-sectional shape of an
H-beam and the sampling position of mechanical test pieces.
BEST MODE FOR CARRYING OUT THE INVENTION
[0049] Below, the ranges of ingredients in the fire resistant steel
of the present invention and the conditions for control of the
ranges of ingredients will be explained. Note that the ranges of
ingredients are shown by mass %.
[0050] C raises the hardenability. In order to give the strength
required for a structural steel material, 0.005% or more is
necessary. Preferably, the C content is 0.01% or more.
[0051] However, to obtain a strengthening effect due to the drag
effect of the dissolved Nb, it must be less than 0.04%. If 0.04% or
more, there will be a high possibility of a large amount of Nb
ending up precipitating as NbC and the amount of dissolved Nb
contributing to solution strengthening ending up being reduced. To
obtain the strengthening effect due to the drag effect of dissolved
Nb, 0.02% or less is preferable.
[0052] Note that, as explained later, if C--Nb/7.74 is 0.02% or
less in range, the amount of dissolved Nb is secured. Further, by
reducing the C content, there is also the effect that the later
added B prevents the precipitation of Fe.sub.23(CB).sub.6.
[0053] Mn raises the hardenability. To secure the strength and
toughness of the matrix material, 0.8% or more has to be added, but
Mn is an element causing center precipitation when producing cast
slabs by continuous casting. If added over 1.7%, the hardenability
excessively rises at the precipitated parts and the toughness
deteriorates. Considering the above, the range of content was made
0.8% to 1.7%.
[0054] If Si becomes 0.4% or more, it produces the low melting
point Fe.sub.2SiO.sub.4 compound during the reheating of the cast
slab, causes worse scale peeling, and forms surface defects, but to
secure the strength of the matrix material and for preparatory
deoxidation in the case of restricting the amount of addition of Al
as explained later, 0.05% or more has to be added. If the later
mentioned Ti/N is 2 to 8 in range, to secure the strength of the
matrix material and suppress the formation of scale defects, the
content of Si should be made less than 0.4%, so the Si content was
made 0.05% to less than 0.4%. To further improve the surface
properties by the prevention of scale defects, the Si content is
preferably made 0.2% or less.
[0055] Nb is an element important in the present invention. The
copresence of dissolved Nb and B remarkably raises the
hardenability and thereby increases the room temperature yield
point. Further, for employing the drag effect to increase the high
temperature strength, 0.02% or more is added. However, if over 1%,
the effect of addition of Nb is saturated, so the upper limit was
made 1%. In the present invention, it is possible to draw out the
effect of the dissolved Nb required for fire resistant steel to the
maximum extent, so in general is 0.1% or less. If the balance with
the other ingredients is good, a 0.05% or less amount of addition
of Nb gives a sufficient effect. To secure a sufficient fire
resistance by the drag effect of Nb, it is necessary to not only
define the amount of addition of Nb, but also satisfy the following
conditions for sufficiently obtaining the amount of dissolved
Nb.
[0056] When Nb is dissolved, the drag effect of the dissolved Nb is
improved and contributes to strength. However, Nb is a strong
carbide forming element, so if C is present, forms NbC. The
dissolved Nb is reduced and the strengthening mechanism due to the
drag effect ends up being weakened.
[0057] In the present invention, to obtain dissolved Nb sufficient
for strengthening, the inventors found that, for the relationship
between the amount of addition of Nb to the amount of addition of
C, it is necessary to make C--Nb/7.74 0.02 mass % or less. Here,
when C--Nb/7.74 is 0.02% or less in range, the Nb and C separate,
the required amount of dissolution of Nb can be secured, and the
solution strengthening required for the fire resistance is
contributed to.
[0058] Organizing the above, the amounts of addition of Nb and C
and the suitable range of balance of addition becomes as shown in
FIG. 1. That is, the amount of addition of C has to be to 0.005% or
more to secure the strength (b) and has to be less than 0.04% to
secure the toughness (c). To secure the high temperature strength,
the amount of addition of Nb has to be 0.02% or more (a) and the
amount of addition of Nb to the amount of addition of C has to be
restricted so that Nb becomes (C-0.02).times.7.74 or more (d).
[0059] N forms NbN and BN nitrides and reduces the hardenability by
Nb and B. Further, it causes the formation of high carbon
island-shaped martensite at the lath boundaries of the bainite
phase to degrade the toughness. Therefore, the N content was
restricted to 0.005% or less. Note that the unavoidable impurities
generally include 20 to 30 ppm or so of N, so N is preferably
suppressed to 0.003% or less.
[0060] Al deoxidizes molten steel and is added to sufficiently
obtain room temperature and high temperature strength. Therefore,
0.005% or more has to be added. However, in particular in the case
of steel shapes or thick-gauge steel plate, when adding over 0.03%,
this forms island-shaped martensite to degrade the toughness.
Further, this has a detrimental effect on the high temperature
strength of the weld zone, so the amount has to be 0.03% or less.
When further toughness of the matrix material as a thick-gauge
steel material or reheating embrittlement characteristics of the
weld zone are required, this may be limited to 0.015% or less. If
further limited to less than 0.01%, the maximum effect can be
obtained from the viewpoint of the amount of addition of Al.
[0061] There are two main effects of addition of Ti.
[0062] First, this is added for increasing the fineness of the
.gamma.-grains by precipitation of TiN and for suppressing the
precipitation of BN and NbN to increase the amount of dissolved B
and improve the effect of B in raising the hardenability by the
reduction of the dissolved N. Due to this, the room temperature
yield point and high temperature strength are raised. If the amount
of addition is less than 0.005%, the amount of precipitation of TiN
is insufficient and these effects are not exhibited, so the lower
limit of the amount of Ti was made 0.005%. Excessive Ti over 0.02%
precipitates as coarse Ti(CN) which degrades the toughness of the
matrix material and weld heat affected zone, so the amount was
limited to 0.02% or less.
[0063] Second, this is added to reduce the amount of dissolved N
weakening the drag effect of Nb.
[0064] The inventors engaged in intensive research and as a result
learned that, by mass %, a Ti/N of 2 to 8 in range is suitable. If
Ti/N is less than 2, the dissolved N is not immobilized as TiN,
while if Ti/N is over 8, the excessive Ti forms coarse Ti(CN) which
degrades the toughness. By limiting the Ti/N in this way, the
toughness as a thick-gauged steel material can be sufficiently
secured and the hardenability by B can be utilized to the maximum
to obtain high temperature strength as a fire resistant steel. If
making Ti/N 2.5 to 6, further preferable characteristics can be
obtained.
[0065] Organizing the above, the ranges of suitable amounts of
addition of Ti and Nb are as shown in FIG. 2. That is, the amount
of addition of Ti has to be 0.005% or more to secure the amount of
precipitation of TiN (a) and has to be 0.02% or less to suppress
the precipitation of coarse Ti(CN) (b). The N content has to be
0.005 or less (c), while Ti/N as to be 2 (e) to 8 (d).
[0066] There are two objects of the addition of B.
[0067] The first object is, through composite addition with Nb,
further raising the hardenability and contributing to the rise in
strength. The inventors engaged in intensive research and as a
result found that if less than 0.0003%, this effect is not
sufficient, while if over 0.003%, iron-boron compounds are produced
and the hardenability is reduced.
[0068] The second object is to draw out the drag effect of Nb to
the maximum extent. As shown in FIG. 3(b), part of the Nb contained
in the steel cannot maintain its dissolved state in the ferrite and
segregates at the crystal grain boundaries 8, so the drag effect
cannot be exhibited. However, as shown in FIG. 3(a), if adding B,
B, in place of Nb, segregates at the crystal grain boundaries 8 to
suppress the segregation of Nb. It thereby assists Nb in
maintaining its dissolved state in ferrite. For this object as
well, the content of B should be made 0.0003 to 0.003%.
[0069] In the sense of achieving both the first and the second
objects to the maximum extent, 0.001 to 0.002% of B is preferably
added.
[0070] Cr is effective for strengthening the matrix material by the
improvement of the hardenability. However, excessive addition over
0.4% is harmful from the viewpoint of the toughness and
hardenability, so the upper limit was made 0.4%.
[0071] Cu is effective for strengthening the matrix material by the
improvement of the hardenability. However, excessive addition over
1% is harmful from the viewpoint of the toughness and
hardenability, so the upper limit was made 1%.
[0072] Ni is effective for strengthening the matrix material by the
improvement of the hardenability. However, the upper limit was made
1.0% from the viewpoint of economy.
[0073] The P and S contained as unavoidable impurities are not
particularly limited in amount, but their solidification
segregation cause weld cracks and a drop in toughness, so they
should be reduced as much as possible. The amount of P is
preferably 0.03% or less, while the amount of S is preferably 0.02%
or less.
[0074] A cast slab having the above composition and having a
balance of Fe and unavoidable impurities is heated to a temperature
region where the surface temperature becomes 1250 to 1350.degree.
C., then is started to be rolled. The reason for reheating to a
temperature region where the surface temperature of the cast slab
becomes 1250 to 1350.degree. C. is that heating to 1250.degree. C.
or more is preferable for making the Nb dissolve in a short time
and obtaining the dissolved Nb required for strengthening the
matrix material and heating to 1250.degree. C. or more is required
for facilitating plastic deformation when producing steel shapes by
hot working. Note that, from the performance of the heating furnace
and economy, the upper limit of the heating temperature was made
1350.degree. C.
[0075] The cast slab heated to a temperature region of a surface
temperature of 1250 to 1350.degree. C. in this way is hot rolled.
In this hot rolling, the steel is rolled at a cumulative reduction
rate at 1000.degree. C. or less of 30% or more, whereby it is made
finer in .gamma. grains by work recrystallization. Due to this, it
is possible to make the steel higher in toughness and higher in
strength.
[0076] After the end of this hot rolling, the steel is cooled at a
temperature range of 800 to 500.degree. C. by a 0.1 to 10.degree.
C./sec average cooling rate. The reason for making the temperature
range of the cooling 800 to 500.degree. C. is to secure the
dissolved Nb. Further, the reason for making the cooling rate 0.1
to 10.degree. C./sec is that if the average cooling rate is less
than 0.1.degree. C./sec, the hardenability becomes insufficient,
while if the average cooling rate is over 10.degree. C./sec,
martensite is produced and the toughness of the matrix material is
remarkably lowered.
[0077] The characterizing feature of the steel ingredients of the
present invention is the ability to secure sufficient hardenability
even with an average cooling rate of 0.1.degree. C./sec. The
invention may also be applied to a thick-gauge steel material, for
example, extremely thick H-section steel having a flange thickness
of 125 mm. Further, in the present invention, due to the addition
of B and Nb, the start of transformation is delayed in the
continuous cooling process. By using the above cooling rate, the
untransformed .gamma. is maintained as with the rapid cooling until
a relatively low temperature. By the drop in the diffusion rate of
Nb, NbC does not precipitate and Nb dissolves in the supersaturated
state.
[0078] The fire resistant high strength rolled steel material of
the present invention is suitably used as a structural member of a
building etc. Specifically, it is embodied as H-section steel,
I-section steel, angle steel, channel-section steel, unequal side
unequal thickness angle steel, and other steel shapes, for example,
thick gauge steel plate of a thickness of 7 mm or more.
[0079] Further, for example, under the above conditions, when
producing H-section steel as an example of the fire resistant high
strength rolled steel material of the present invention, the
H-section steel has sufficient strength and toughness even at the
flange thickness 1/2 part and width 1/2 part where the mechanical
test characteristics are hardest to guarantee.
[0080] Further, due to the strengthening effect based on the drag
effect of the dissolved Nb, it is possible to obtain high strength
fire resistant rolled H-section steel having a superior fire
resistance ability and toughness. Furthermore, this H-section steel
is superior in high temperature characteristics, so when used for a
fire resistant material for building use, a sufficient fire
resistant object with a coating thickness of 50% or less of the
past can be achieved.
EXAMPLES
[0081] Below, the effects of the present invention will be shown
further by examples.
[0082] A cast slab of each of the various steel types shown in
Table 1 was heated and rolled. Specifically, prototype steel was
produced in a converter, alloy ingredients were added, Ti and B
were added, then continuous casting was used to cast a 240 to 300
mm thick cast slab. The cast slab was heated, then hot rolled to
obtain H-section steel (web height 414 mm.times.flange width 405
mm.times.web thickness 18 mm.times.flange thickness 28 mm).
[0083] In the rolling, in the universal rolling mill train shown in
FIG. 4, the material to be rolled (cast slab) 5 exiting from the
heating furnace 1 is passed through the rough rolling mill 2,
intermediate rolling mill 3, and finishing rolling mill 4.
[0084] In the rolling mills, as shown in FIG. 5, H-section steel
having an H cross-sectional shape comprised of a web 6 and a pair
of flanges 7 was rolled.
[0085] Note that in water cooling between rolling passes, water
cooling systems were provided before and after the intermediate
rolling mill 3, and the flange outside surfaces were repeatedly
spray cooled and reverse rolled. Accelerated cooling after rolling
was performed after the end of rolling by the final rolling mill 4
by a cooling system provided at the back surface by spray cooling
of the flange outside surfaces.
[0086] In the steel materials (H-section steels), test pieces were
taken from the positions of the center (1/2t.sub.2) of thickness
t.sub.2 of the flange 7 and half of the total length B of the
flange width (1/2B) to investigate the mechanical
characteristics.
[0087] This location was judged as optimum for evaluating the
mechanical test characteristics of the H-section steel because the
flange 1/2B part is lowest in mechanical characteristics of the
H-section steel.
[0088] As the mechanical test characteristics of the steel material
(H-section steel), room temperature (21.degree. C.) yield point
(yield point stress YP (MPa), when unclear, the 0.2% yield strength
is used) and the tensile strength (TS (MPa)), 600.degree. C. 0.2%
yield strength (600YS (MPa)), the ratio of the 600.degree. C. yield
strength (600YS) and room temperature (21.degree. C.) yield point
(yield point stress YP) (600YS/YP ratio (%)), impact value
(vE0.degree. C. (J)), and yield ratio (YR) are shown.
[0089] As the passing standards for the mechanical test
characteristics, a room temperature (21.degree. C.) tensile
strength TS of 400 MPa or more, a yield point (YP) of a high
strength of 235 MPa or more, a 600.degree. C. 0.2% yield strength
(600YS) of 50% or more of the room temperature (21.degree. C.)
yield point (yield point stress YP), and a 0.degree. C. Charpy
impact absorption energy value (vE0) of 47J or more were demanded.
This is because if these passage standards, the material can be
judged as suitable as a steel material for a fire resistance.
TABLE-US-00001 TABLE 1 600YS/ Chemical compositions YP C-Nb/ YP TS
600YS ratio vE0 No. C Si Mn P S Ti Al Nb B N Cr Cu Ni 7.74 Ti/N
(MPa) (MPa) (MPa) (%) (J) Remarks Inv. ex. 1 0.011 0.10 1.53 0.012
0.004 0.009 0.007 0.020 0.0018 0.0040 0.008 2.3 334 462 177 53 243
Inv. ex. 2 0.018 0.10 1.51 0.009 0.004 0.014 0.016 0.057 0.0023
0.0032 0.011 4.4 465 597 297 64 225 Inv. ex. 3 0.019 0.11 1.49
0.010 0.004 0.010 0.025 0.055 0.0020 0.0024 0.012 4.2 458 605 300
66 91 Inv. ex. 4 0.017 0.09 1.49 0.009 0.005 0.020 0.015 0.055
0.0009 0.0027 0.010 7.4 478 610 305 64 108 Inv. ex. 5 0.012 0.11
1.55 0.011 0.004 0.010 0.012 0.025 0.0005 0.0040 0.009 2.5 328 455
169 52 286 Inv. ex. 6 0.018 0.12 0.85 0.014 0.004 0.011 0.008 0.900
0.0009 0.0041 -0.098 2.7 305 429 195 64 216 Inv. ex. 7 0.014 0.37
1.60 0.013 0.003 0.012 0.006 0.059 0.0010 0.0039 0.006 3.1 457 573
278 61 265 Inv. ex. 8 0.013 0.21 1.50 0.012 0.004 0.005 0.007 0.030
0.0018 0.0020 0.009 2.5 396 511 215 54 128 Inv. ex. 9 0.013 0.10
1.52 0.012 0.004 0.008 0.005 0.030 0.0019 0.0039 0.009 2.1 374 489
203 54 276 Inv. ex.10 0.020 0.10 1.65 0.010 0.004 0.009 0.006 0.039
0.0017 0.0039 0.015 2.3 393 531 231 59 288 Inv. ex.11 0.037 0.10
1.62 0.011 0.004 0.008 0.005 0.140 0.0018 0.0039 0.019 2.1 418 529
243 58 298 Inv. ex.12 0.006 0.10 1.62 0.015 0.005 0.015 0.006 0.050
0.0018 0.0029 0.000 5.2 320 421 196 61 111 Inv. ex.13 0.014 0.14
0.97 0.013 0.004 0.010 0.006 0.090 0.0009 0.0042 0.64 0.40 0.002
2.4 468 587 273 58 146 Claim 2 Inv. ex.14 0.013 0.13 0.80 0.015
0.005 0.009 0.008 0.087 0.0010 0.0040 0.60 0.20 0.41 0.002 2.3 448
565 275 61 95 Claim 2 Comp. ex. 15 0.004 0.10 1.62 0.015 0.005
0.015 0.006 0.039 0.0017 0.0041 -0.001 3.7 315 396 187 59 55 C
insuf., TS insuf. Comp. ex. 16 0.040 0.15 1.60 0.015 0.005 0.018
0.008 0.160 0.0019 0.0032 0.019 5.6 436 543 235 54 45 C excess.,
tough. imsuf. Comp. ex. 17 0.018 0.40 1.55 0.013 0.006 0.009 0.010
0.035 0.0015 0.0035 0.013 2.6 425 550 259 61 106 Si excess., scale
Comp. ex. 18 0.012 0.10 0.70 0.012 0.005 0.012 0.010 0.025 0.0012
0.0040 0.009 3.0 286 385 153 53 86 Mn insuf., TS insuf. Comp. ex.
19 0.013 0.10 1.81 0.013 0.004 0.010 0.008 0.050 0.0019 0.0038
0.007 2.6 458 608 296 65 12 Mn excess., tough. insuf. Comp. ex. 20
0.018 0.10 1.57 0.014 0.005 0.022 0.005 0.030 0.0015 0.0042 0.014
5.2 423 525 237 56 28 Ti excess.. tough. insuf. Comp. ex. 21 0.020
0.10 1.62 0.015 0.005 0.009 0.035 0.039 0.0017 0.0039 0.015 2.3 402
535 233 58 40 Al excess.,tough. insuf. Comp. ex. 22 0.013 0.10 1.52
0.011 0.004 0.010 0.007 0.018 0.0018 0.0040 0.011 2.5 345 462 165
48 257 Nb insuf.. high temp. strength drop Comp. ex. 23 0.012 0.11
1.55 0.010 0.005 0.010 0.012 0.025 0.0003 0.0041 0.009 2.4 285 390
145 51 297 B insuf., strength insuf. Comp. ex. 24 0.012 0.11 1.55
0.010 0.005 0.010 0.012 0.025 0.0050 0.0039 0.009 2.6 351 465 197
56 19 B excess., tough. insuf. Comp. ex. 25 0.012 0.10 1.53 0.012
0.004 0.015 0.007 0.030 0.0018 0.0060 0.008 2.5 330 435 145 44 196
N excess., high temp. strength drop Comp. ex. 26 0.027 0.10 1.62
0.011 0.004 0.008 0.004 0.040 0.0018 0.0036 0.022 2.2 414 535 204
49 135 C-Nb/7.74 > 0.02. high temp. strength drop Comp. ex. 27
0.035 0.09 1.63 0.010 0.050 0.008 0.008 0.100 0.0018 0.0026 0.022
3.1 408 509 201 49 298 C-Nb/7.74 > 0.02, high temp. strength
drop Comp. ex. 28 0.012 0.10 1.52 0.012 0.004 0.007 0.005 0.040
0.0018 0.0038 0.007 1.8 405 489 196 48 146 Ti/N < 2, high temp.
strength drop Comp. ex. 29 0.015 0.10 1.52 0.012 0.005 0.018 0.005
0.030 0.0023 0.0021 0.011 8.6 422 518 235 56 45 Ti/N > 8.
toughness drop
[0090] Table 1 shows the chemical ingredients of the various steels
used in the Examples and the mechanical characteristics of the
H-section steels.
[0091] The Nos. 1 to 14H-section steels in the scope of the present
invention all satisfied the passage standards. The H-section steels
in the scope of the present invention had sufficient strength and
toughness even at the flange thickness 1/2t2 and width 1/2B parts
of the rolled steel shapes where the mechanical test
characteristics are hardest to guarantee and were superior in fire
resistance and toughness.
[0092] Comparative Example No. 17 satisfied the mechanical test
characteristics, but primary scale produced during the heating
remained in close contact to the surface until the final product
resulting in scale defects, so the steel was not of a level
suitable for the use as a steel material for building use.
TABLE-US-00002 TABLE 2 Cumul. Size reduc- Cooling web tion rate
600/ Chemical compositions thickness/ Heat below after RT C-Nb/
flange temp. 1000.degree. C. rolling YP TS 600YS ratio vE0 No. C Si
Mn P S Ti Al Nb B N Cr Cu Ni 7.74 Ti/N thickness (.degree. C.) (%)
.degree. C./sec (MPa) (MPa) (MPa) (%) (J) Remarks Inv. 0.011 0.10
1.53 0.012 0.004 0.009 0.007 0.020 0.0018 0.0040 0.008 2.3 18/28
1300 40 -- 334 462 177 53 243 ex. 1 Comp. 0.011 0.10 1.53 0.012
0.004 0.009 0.007 0.020 0.0018 0.0040 0.008 2.3 18/28 1230 40 --
355 452 173 49 126 Heating ex. 30 temp.insuf., high temp. strength
drop Comp. 0.011 0.10 1.53 0.012 0.004 0.009 0.007 0.020 0.0018
0.0040 0.008 2.3 18/28 1260 25 -- 385 501 176 48 85 Below ex. 31
1000.degree. C. cumul. reduction insuf., high temp. strength drop
Inv. 0.014 0.14 0.97 0.013 0.004 0.010 0.006 0.090 0.0009 0.0042
0.64 0.40 0.002 2.4 18/28 1300 40 -- 468 587 273 58 146 ex. 13
Comp. 0.014 0.14 0.97 0.013 0.004 0.010 0.006 0.090 0.0009 0.0042
0.64 0.40 0.002 2.4 18/28 1200 40 -- 376 467 163 43 226 Heating ex.
32 temp. insuf., high temp. strength drop Comp. 0.014 0.14 0.97
0.013 0.004 0.010 0.006 0.090 0.0009 0.0042 0.64 0.40 0.002 2.4
18/28 1300 25 -- 316 451 143 45 107 Below ex. 33 1000.degree. C.
cumul. reduction insuf., high temp. strength drop Inv. 0.013 0.10
1.52 0.012 0.004 0.008 0.005 0.030 0.0019 0.0039 0.009 2.1 18/28
1300 -- 0.8 374 489 203 54 276 ex. 9 Inv. 0.013 0.10 1.52 0.012
0.004 0.008 0.005 0.030 0.0019 0.0039 0.009 2.1 18/28 1300 -- 8.0
420 565 271 65 108 ex. 34 Inv. 0.013 0.10 1.52 0.012 0.004 0.008
0.005 0.030 0.0019 0.0039 0.009 2.1 90/125 1300 -- 0.1 395 502 219
55 247 ex. 35 Comp. 0.013 0.10 1.52 0.012 0.004 0.008 0.005 0.030
0.0019 0.0039 0.009 2.1 18/28 1300 -- 15 459 618 325 71 21 Cooling
rate ex. 36 excess.. toughness insuf. Comp. 0.013 0.10 1.52 0.012
0.004 0.008 0.005 0.030 0.0019 0.0039 0.009 2.1 90/125 1300 -- 0.05
390 479 176 45 167 Cooling rate ex. 37 insuf.. high temp. strength
drop Inv. 0.013 0.13 0.80 0.015 0.005 0.009 0.008 0.087 0.0010
0.0040 0.60 0.20 0.41 0.002 2.3 18/28 1300 -- 0.8 448 565 275 61 98
ex. 14 Comp. 0.013 0.13 0.80 0.015 0.005 0.009 0.008 0.087 0.0010
0.0040 0.60 0.20 0.41 0.002 2.3 18/28 1300 -- 15 486 631 349 72 13
Cooling rate ex. 38 excess.. toughness insuf. Comp. 0.013 0.13 0.80
0.015 0.005 0.009 0.008 0.087 0.0010 0.0040 0.60 0.20 0.41 0.002
2.3 90/125 1300 -- 0.05 411 515 195 47 98 Cooling rate ex. 39
insuf.. high temp. strength drop
[0093] Next, the examples described in Table 2 will be
explained.
[0094] The steels of Nos. 1 and 13 of Table 1 were rolled at
different heating temperatures and cumulative reduction rates at
1000.degree. C. or less to obtain different H-section steels (web
height 414 mm.times.flange width 405 mm, web thickness 18
mm.times.flange thickness 28 mm) which were then investigated for
mechanical test characteristics. Nos. 1 and 13 of Table 2 are
examples of production of the present invention and satisfy the
standards of the characteristics of the present invention.
[0095] As shown by Nos. 30, 31, 32, and 33 of Table 2, when the
heating temperature is less than 1250.degree. C. and the cumulative
reduction rate at 1000.degree. C. or less is less than 30%, the
standards of the characteristics of the present invention cannot be
satisfied.
[0096] The steels of Nos. 9 and 14 of Table 1 were rolled by a
heating temperature of 1300.degree. C. and different average
cooling rates in the temperature range of 800 to 500.degree. C.
after rolling to obtain H-section steel (web height 414
mm.times.flange width 405 mm.times.web thickness 18 mm.times.28 mm
and web height 608 mm.times.flange width 477 mm.times.web thickness
90 mm.times.flange thickness 125 mm) which were then investigated
for mechanical test characteristics. Nos. 9, 14, 34, and 35 of
Table 2 are examples of production of the present invention and
satisfy the standards of the characteristics of the present
invention.
[0097] As shown by Nos. 36, 37, 38, and 39 of Table 2, when the
average cooling rate is outside of the range of 0.1 to 10.degree.
C./sec such as 0.05.degree. C./sec to 15.00.degree. C./sec, it is
not possible to satisfy the standards of the characteristics of the
present invention.
[0098] Note that in the examples, the typical rolled steel material
of H-section steel was studied, but the rolled steel materials
covered by the present invention is not limited to the H-section
steel of the above examples. It may also be applied to I-section
steel, angle steel, channel-section steel, unequal side unequal
thickness angle steel, and other steel shapes, thick gauge steel
plate, and other such steel materials. Production is also possible
even when the thickness is relatively large.
INDUSTRIAL APPLICABILITY
[0099] According to the present invention, it becomes possible to
produce steel shapes having fire resistance and toughness by
rolling. By utilizing the fire resistant steel material of the
present invention for the structural member of a building etc., a
great reduction in the costs can be realized by reduction of the
installation costs and shortening of the work period and
improvement of the reliability of large buildings, safety, and
improvement of the economicalness etc. can be achieved.
* * * * *