DESCRIPTION
Title of Invention
METHOD OF PRODUCTION OF STEEL PLATE
5
Technical Field
The present invention relates to a method of
production of steel plate, more particularly relates to a
method of production of steel plate for welded structure
10 use which is high in rolling productivity and excellent
in low temperature toughness.
Background Art
The steel plate which is used in ships, buildings,
15 tanks, offshore structures, line pipe, and other welded
structures is required to provide low temperature
toughness to suppress brittle fracture of the structures.
In particular, steel plate with a yield stress of 315 MPa
to 550 MPa and a plate thickness of 10 mm to 40 mm is
20 being used in an increasing number of cases.
In general, low temperature toughness is improved by
rolling in the rolling process at a low temperature of
about 750 to 850°C or so, which is called the "y-nonrecrystallization
temperature range", and making th'e
25 grains finer.
In the past, various methods for causing an
improvement in the low temperature toughness of steel
plate have been proposed. For example, there are the arts
which are disclosed in PLT's 1 to 5.
30 PLT"1 describes steel plate of a plate thickness of
40 mm or more which is excellent in arrestability of
brittle cracks.
PLT 2 describes steel plate which is defined in the
Vicker's hardness of the steel. plate and is excellent in
35 workability and a method of production of the same.
PLT 3 describes a method of producing a steel
material with little variation in material quality which
makes the time between passes from the completion of the
fifth last pass in the final rolling to the start before
the fourth last pass 30 seconds or more and which makes
the times between passes from before the fourth last pass
5 to the final pass 15 seconds or less.
PLT 4 describes a method of production of steel
plate which has excellent strength and toughness by
setting the rolling conditions so as to satisfy a
predetermined relationship between the rolling
10 temperature and rolling reduction at each rolling pass
and enjoying the effects of refinement of the
recrystallized y-grains and rolling in the non--
recrystallization region to the maximum extent to as to
refine the final microstructure,
15 PLT 5 describes a method of production of steel
plate which is excellent in strength and toughness by
using two rolling mills for tandem rolling with less than
5 seconds between passes so as to promote
recrystallization and by making the cumulative rolling
20 reduction in the non-recrystallization region 70% or
more.
25
30
Citations List
Patent Literature
PLT 1: Japanese Patent Publication (A) No. 2007-
302993
PLT 2: Japanese Patent Publication (A) No. 2006-
19381
PLT 3: Japanese Patent Publication (A) No. 2002-
249822
PLT 4: Japanese Patent Publication (A) No. 2004-
269924
PLT 5: Japanese Patent Publication (A) No. 11-181519
35 Summary of Invention
Technical Problem
However, the above PLT's 1 to 5 had the following
problems.
The method of production which is described in PLT 1
requires low-temperature rolling (CR) at a larger
thickness of steel plate. If low-temperature rolling, the
5 grains can be made finer and the low temperature
toughness is improved. However, if low-temperature
10
rolling, time is taken for waiting for the temperature to
drop after the end of high-temperature rolling, so the
rolling productivity falls.
The method of production which is described in PLT 2
requires low-temperature rolling, so the productivity is
low. Furthermore, the steel plate which is covered is
high strength steel with a yield stress of 600 MPa or
more. Steel plate with a yield stress of 315 MPa to 550
15 MPa and a plate thickness of 10 mm to 40 mm which the
present invention covers differs in microstructures, so
this cannot be applied.
If making the time between passes 30 seconds or more
like in the method of production which is described in
20 PLT 3, it was learned, as a result of study by the
inventors, that the recrystallized y coarsened.
The method of production which is described in PLT 4
manages the rolling temperature by the surface
temperature, so the variation in material quality ils
25 large. On top of this, the time until recrystallization
is not defined, so it is difficult to refine the
recrystallized y-grains.
Tandem rolling using two rolling mills like in the
method of production which is described in PLT 5 has
30 large restrictions in terms of facilities and is not
practical.
Therefore, the present invention has as its cask to
reduce the drop in productivity due to the need for lowtemperature
rolling in the prior art and to provide a
35 method of production of steel plate for welded structure
use which is excellent in low temperature toughness which
can be applied to steel plate which has a yield stress of
- 4 -
315 MPa to 550 MPa and a plate thickness of 10 mm to 40
mm, which does not require special facilities, and which
is small in variation of material quality. Specifically,
it has as its task the provision of a method of
5 production of steel plate which enables refinement of the
microstructure, even without low-temperature rolling, by
just high-temperature rolling.
10
Solution to Problem
They inventors studied in depth the method of
production of steel plate. As a result, they discovered
manufacturing conditions which enable the microstructure
to be refined by utilization of refinement by yrecrystallization
even with rolling at a high temperature
15 of 850 to 950°C or so referred to as the "'-
recrystallization temperature range" and realized a
method of production of steel plate which can realize
both rolling productivity and low temperature toughness.
Specifically, in a second stage of the hot rolling
20 (below, also referred to as the "second stage rolling".
Further, the first stage of hot rolling also being
referred to as the "first stage rolling"), the rolling
reduction per pass is made larger than the conventipnal
manufacturing process and the time between pass;<_es is
25 optimized. If increasing the rolling reduction per pass,
the number of passes decreases, so the productivity
becomes higher. With the conventional low-temperature
rolling in the y-non-recrystallization temperature range,
the rolling reaction force becomes large, so the rolling
30 reduction was kept down to less than 10%.
However, according to studies by the inventors, it
was learned that in high-temperature rolling in the yrecrystallization
temperature range, by making the
rolling reduction 10 to 25% and, furthermore, making the
35 time between passes 3 to 25 seconds, it is possible to
make use of the refinement by y-recrystallization and
refine the microstructure.
The present invention. was made based on the above
discoveries and, furthermore, in consideration of the
chemical compositions of steel which is excellent in
5 productivity and low temperature toughness. Its gist is
as follows:
(1) A method of production of steel plate
characterized by
preparing a steel slab which contains, by
10 mass%,
15
C: 0.04 to 0.16%,
Si: 0.01 to 0.5%,
Mn: 0.2 to 2.5%,
P: 0.03% or less,
S: 0.02% or less,
Al: 0.001 to 0.10%,
Nb: 0.003 to 0.02%,
Ti: 0.003 to 0.05%, and
N: 0.001 to 0.008%,
20 which contains, as optional elements, one or more of
Cu: 0.03 to 1.5%,
Ni: 0.03 to 2.0%,
Cr: 0.03 to 1.5%,
Me: 0.01 to 1.0%,
25 V: 0.003 to 0.2%,
B: 0.0002 to 0.005%,
Ca: 0.0005 to 0.01%,
Mg: 0.0005 to 0.01%, and
REM: 0.0005 to 0.01%,
30 which has a carbon equivalent Ceq of the following
formula (A) of 0.2 to 0.5%, and which has a balance of Fe
and unavoidable impurities,
heating this to 1000 to 1200°C, then
rolling by first stage rolling at a plate
35 thickness center temperature of 950 to 1200°C, a
cumulative rolling reduction of 50 to 95%, and a number
of passes of 4 to 16 passes, then
rolling by second stage rolling at a plate
thickness center temperature of 850 to 950°C, a number of
passes of 2 to 8 passes, a rolling reduction at each pass
of 10 to 25%, and a time between passes of 3 to 25
5 seconds, then
cooling by accelerated cooling from a plate
thickness center temperature of 750°C or more by a 1 to
50°C/s cooling rate down to 650°C or less
so as to obtain to steel plate which has a
10 plate thickness of 10 to 40 mm, a yield stress of 315 to
15
550 MPa, a microstructure of a mixed microstructure of
ferrite and bainite or of ferrite, pearlite, and bainite,
and an average grain size at the plate thickness center
part of 5 to 20 μm:
Ceq=C+Mn/6+(Cu+Ni)/15+(Cr+Mo+V)/5.. (A)
(2) A method of production of steel plate as set
forth in (1) characterized by tempering at 300 to 650°C
after the accelerated cooling ends.
20 Advantageous Effects of Invention
The method of production of.steel plate for welded
structure use of the present invention does not include
low-temperature rolling, so the temperature waitingj,time
is short. Further, the rolling reduction is large, so the
25 number of passes is small and the rolling productivity is
high.
Further, according to the method of production of
the present invention, by utilizing the refinement by yrecrystallization
so as to refine the microstructure by
30 high-temperature rolling in the y-recrystallization
temperature range, it is possible to produce steel plate
for welded structure use which is excellent in low
temperature toughness.
35 Description of Embodiments
First, a preferable method of production of steel
plate for welded structure use of the present invention
will be explained.
First, molten steel which has been, adjusted to the
desired chemical compositions is smelted by a known
5 smelting method using a converter etc. and is cast into a
steel slab by continuous casting or another known casting
method.
During the cooling at the time of casting, or after
the cooling, the steel slab is heated to 1000 to 1200°C in
10 temperature. If the heating temperature of the steel slab
is less than 1000°C, the solubilization becomes
insufficient. If the heating temperature exceeds 1200°C,
the heated y-grains coarsen and refinement in the
subsequent rolling process becomes difficult.
15 Furthermore, in the period before the start of the hightemperature
rolling, time is taken for waiting for the
temperature to fall, so the productivity becomes lower.
The preferable range of heating temperature is 1050 to
1150°C.
20 Next, first stage hot rolling (first stage rolling)
is performed by a plate thickness center temperature of
950 to 1200°C, a cumulative rolling reduction of 50 to
95%, and a number of passes of 4 to 16.
If the plate thickness center temperature t=eeds
25 1200°C, the recrystallized 7-grains cannot be made finer.
If the plate thickness center temperature becomes less
than 950°C, the productivity falls. The preferable plate
thickness center temperature is 1000 to 1150°C.
If the cumulative rolling reduction becomes less
30 than 50%, the recrystallization does not sufficiently
proceed and the recrystallized y-grains cannot be wade
finer. If the cumulative rolling reduction exceeds 95%,
the rolling load becomes larger and the productivity
falls. The preferable cumulative rolling reduction is 60%
35 to 900.
If the number of passes becomes less than 4, the
recrystallized 7-grains cannot be made finer. If the
number of passes exceeds 16, the productivity falls. The
preferable number of passes is 5 to 14.
5 Next, second stage hot rolling (second stage
rolling) is performed by a plate thickness center
temperature of 850 to 950°C, a rolling reduction per pass
of 10 to 25%, a time between passes of 3 to 25 seconds,
and a number of passes of 2 to 8 passes.
10 If the plate thickness center temperature exceeds
950°C, the recrystallized 7-grains cannot be made finer.
If the plate thickness center temperature becomes less
than 850°C, the productivity falls. The preferable plate
thickness center temperature is 870 to'930°C.
15 If the rolling reduction per pass becomes less than
10%, the number of passes increases, so the productivity
falls. If the rolling reduction per pass exceeds 25%, the
load of the rolling mills becomes extremely large, so
realization becomes difficult. The preferable rolling
20 reduction per pass is 13 to 22%.
To make the rolling reduction, per pass 10% or more
and improve the productivity, the time between passes
becomes an important factor.
If the rolling reduction per pass is 10 to 25% in
25 range and the time between passes becomes less than'3
seconds, the next pass is proceeded to within the
incubation period which is required for nucleation in
recrystallization or during the initial stage of
recrystallization, so recrystallization does not
30 sufficiently proceed. If the time between passes exceeds
25 seconds, before the next pass is proceeded to, the
primary recrystallization ends and the secondary
recrystallization, which is driven by the grain boundary
energy, is started, so the recrystallized 7-grains
35 coarsen. That is, if the time between passes does not
become 3 to 25 seconds in range, the task of the present
invention, that is, refinement of the microstructure by
high-temperature rolling, cannot be achieved. The
preferable time between passes is 5 to 23 seconds.
If the number of passes becomes less than 2, the
5 recrystallized y-grains cannot be made finer. If the
number of passes exceeds 8, the productivity falls. The
preferable number of passes is 3 to 7.
After the above hot rolling, accelerated cooling is
performed from a plate thickness center 'temperature of
10 750°C or more by` a 1 to 50°C/s cooling rate down to a
temperature of 650°C or less.
If the plate thickness center temperature it the
time of start of cooling becomes less than 750°C, the
ferrite transformation proceeds, so a ferrite fine-
15 grained microstructure is hard to obtain.
If the cooling rate is less than 1°C/s, a fine
microstructure is difficult to obtain, while if the
cooling rate is over 50°C/s, a 20% or more ferrite
percentage becomes difficult to obtain.
20 If the cooling stop temperature exceeds 650°C, it
becomes difficult to obtain a fine microstructure.
The preferable conditions for accelerated cooling
are a plate thickness center temperature at the time of
start of first stage cooling of 770°C or more, .i cooling
25 rate of 5 to 40°C/s, and a cooling stop temperature of
600°C or less.
Note that, control of the production using the plate
thickness center temperature of the steel plate is also a
feature of the method of production of steel plate of the
30 present invention. By using the plate thickness center
temperature, compared with when using the surface
temperature of the steel plate, even when the plate
thickness changes etc., it is possible to suitably
control the manufacturing conditions and possible to
35 efficiently produce good quality steel plate with little
- 10 -
variations in material quality.
In the rolling process, usually, in the period from
heating to rolling, the surface temperature etc. of the
steel plate is measured while calculating the temperature
5 distribution inside of the steel plate. The rolling is
performed while predicting the rolling reaction force
from the results of calculation of the temperature
distribution. In this way, it is possible to easily find
the steel plate center temperature during rolling. Even
10 when performing accelerated cooling, the accelerated
cooling is controlled while predicting the temperature
distribution at the inside of the plate thickness in the
same way.
After the accelerated cooling, the steel may, if
15 necessary, be tempered at 300 to 650°C.
With tempering at less than 300°C, the effect of
tempering is hard to obtain. If the tempering temperature
exceeds 650°C, the amount of softening becomes larger and
securing strength becomes difficult.
20 The preferable tempering temperature is 400 to 600°C.
The method of production of the present invention
can be applied to the production of steel plate with a
plate thickness of 10 to 40 mm and a yield stress of 315
to 550 MPa. In particular, it can be applied to the'
25 production of steel plate of a yield stress of the 315
MPa class, 355 MPa class, or 390 MPa class for ship hull
structures.
In steel plate with a plate thickness of less than
10 mm, the plate shape deteriorates, so accelerated
30 cooling cannot be applied. In steel plate with a plate
thickness of over 40 mm, to secure toughness, lowtemperature
rolling becomes essential, so simultaneous
achievement of good productivity is not possible.
In production of steel plate with a yield stress of
35 less than 315 MPa, accelerated cooling is not required,
so the present invention does not have to be applied. In
- 11 -
production of steel plate with a yield stress of over 550
MPa, to secure toughness, low-temperature rolling becomes
essential, so simultaneous achievement of good
productivity is not possible.
5 According to the above manufacturing conditions, it
is possible to utilize the refinement due to yrecrystallization
and refine the microstructure even with
high-temperature rolling. Furthermore, the method of
production of the present invention does not require low-
10 temperature rolling, so the temperature waiting time is
short, and further the rolling reduction is large in the
rolling, so the number of passes is also small. The
method of production is excellent in rolling
productivity.
15 The chemical compositions of the steel plate to
20
which the method of production of the present invention
is applied is as follows considering the strength,
toughness, heat affected zone (HAZ) toughness,
weldability, etc.
C is added in an amount of 0.040 or more so as to
secure the strength and toughness of the base material.
If the content of C exceeds 0.160, it becomes difficult
to secure a good HAZ toughness, so the content of C is
made 0.160 or less. To secure the strength of the base
25 material, the lower limit of the content of C may be set
to 0.06% or 0.08%. Further, to improve the HAZ toughness,
the upper limit of the content of C may be set to 0.15%
or .0.14 % .
Si is effective as a deoxidizing element and
30 strengthening element, so 0.01% or more is added. If the
content of Si exceeds 0.5%, the HAZ toughness greatly
deteriorates, so the amount of addition of Si is made
005% or less. To reliably perform the deoxidation, the
lower limit of the content of Si may be set to 0.05% or
35 0.10%. Further, to improve the HAZ toughness, the upper
limit of the content of Si may be set to 0.40% or 0.34%.
Mn is added in 0.2% or more so as to secure the
12 -
strength and toughness of the base material. If the
content of Mn exceeds 2.5%, the center segregation
becomes remarkable and the base material at the part
where center segregation occurs and the HAZ deteriorate
5 in toughness, so the content of Mn is made 2.5% or less.
To improve the strength and toughness of the base
material, the lower limit of the content of Mn may be set
to 0.6% or 0.8%. To prevent deterioration of the material
qualities due to center segregation, the upper limit of
10 the content of Mn may be set to 2.0%, 1.8%, or 1.6%.
P is an impurity element. To stably secure the HAZ
toughness, the content of,P has to be reduced to 0.03% or
less. To improve the HAZ toughness, the content of P may
be made 0.02% or less or 0.015% or less.
15 S is an impurity element. To stably secure the
properties of the base material and the HAZ toughness,
the content of S has to be reduced to 0.02% or less. To
improve the properties of the base material and the HAZ
toughness, the content of S may be made 0.01% or less or
20 0.008% or less.
Al is an element which performs deoxidation and is
necessary for reducing the impurity element 0. In
addition to Al, Mn and Si also contribute to deoxidation.
However, even when Mn or Si is added, if the content- of
25 Al is less than 0.001%, it is not possible to stably
reduce 0. However, if the content of Al exceeds 0.10%,
alumina-based coarse oxides and clusters are formed and
the. base material and the HAZ are degraded in toughness,
so the amount of addition of Al is made 0.10% or less. To
30 reliably perform deoxidation, the lower limit of the
content of Al may be made 0.01% or 0.015%. To suppress
the formation of coarse oxides, the upper limit of the
content of Al may be made 0.08% or 0.06%.
Nb, by addition of 0.003% or more, contributes to
35 improvement of the strength and toughness of the base
material. However, if the content of Nb exceeds 0.02%,
the HAZ toughness and the weldability fall, so the
13 -
content of Nb is made 0.02% or less. To enable the effect
of refinement by Nb to be exhibited better, the lower
limit of the content of Nb may also be set to 0.005%. To
improve the HAZ toughness and weldability, the upper
5 limit of the content of Nb may be made 0.015% or 0.0120.
Ti forms TiN by addition and suppresses the
enlargement of the austenite grain size at the time of
heating the steel slab. If the austenite grain size
becomes large, the grain size after transformation also
10 becomes large and the toughness falls. To obtain a grain
size of a magnitude required for preventing a drop in the
toughness, Ti has to be added in an amount of 0.0030 or
more. However, if the content of Ti exceeds 0.050, TiC is
formed and the HAZ toughness falls, so the content of Ti
15 is made 0.05% or less. To improve the HAZ toughness, the
upper limit of the content of Ti may be made 0.03% or
0.02%.
N forms TiN and suppresses the enlargement of the
austenite grain size at the time of heating the steel
20 slab, so 0.001% or more is added. If the content of N
exceeds 0.008%, the steel material becomes brittle, so
the content of N is made 0.008% or less.
In addition to the above-mentioned additive
elements, as optional elements which can be added in
25 accordance with need, by mass%, one or more of Cu.- 0.03
to 1.5%, Ni: 0.03 to 2.0%, Cr: 0.03 to 1.5%, Mo: 0.01 to
1.00, V: 0.03 to 0.2%, and B: 0.0002 to 0.005% may be
contained. By adding these elements, the base material
can be improved in strength and toughness. In accordance
30 with need, the upper limit of the content of Cu may be
set to 1.0%, 0.5% or 0.3%, the upper limit of the content
of Ni to 1.0%, 0.50, or 0.30, the upper limit of the
content of Cr to 1.00, 0.5%, or 0.30, the upper limit of
the content of Mo to 0.3%, 0.2%, or 0.1%, the upper limit
35 of the content of V to 0.1%, 0.07%, or 0.05%, and the
upper limit of the content of B to 0.0030, 0.002, or
0.001%.
14
If these elements are added excessively, the HAZ
toughness and the weldability deteriorate, so the upper
limits of the contents are defined as explained above.
Furthermore, as other optional elements, by mass.,
5 one or more of Ca: 0.0005 to 0.01%, Mg: 0.0005 to 0.01%,
and REM: 0.0005 to 0.01% may also be contained. By adding
these elements, the HAZ toughness is improved.
To improve the strength and toughness of the base
material etc., these optional elements may be
10 intentionally added. However, to reduce the alloy costs
etc., these optional elements need not be added at all.
These elements, even when,not intentionally added, may be
contained in the steel as unavoidable impurities such as
Cu: ,0.050 or less, Ni: 0.05% or less, Cr: 0.05% or less,
15 Mo: 0.03% or less, V. 0.01% or less, B: 0.0004% or less,
Ca: 0,0008% or less, Mg: 0.0008% or less: and REM:
0.0008% or less. Even when these elements are contained
in the steel as unavoidable impurities, there is no
effect on the method of production of steel plate of the
20 present invention.
The steel plate which is produced by the method of
production of steel plate for welded structure use of the
present invention is given a carbon equivalent which is
found by the above formula (A) of 0,2 to 0.5%. When!^the
25 optional elements are contained as unavoidable
impurities, their contents are entered to find the carbon
equivalent.
If the carbon equivalent is less than 0.2%, the
strength which is demanded from the steel plate which is
30 produced by the method of production of the present
invention cannot be satisfied. If the carbon equivalent
is over 0.5%, the elongation, toughness, and weldability
which are demanded from the steel plate which is produced
by the method of production of the present invention
35 cannot be satisfied. To secure strength, the lower limit
of the carbon equivalent may be set to 0.25%, 0.28%, or
0.30%. To improve the HAZ toughness and weldability, the
- 15
lower limit of the carbon equivalent may also be set to
0.43%, 0040, or 0.38%.
The microstructure of the steel plate which is
produced by the method of production of steel plate for
5 welded structure use of the present invention is a mixed
microstructure of the ferrite and bainite or of
ferrite/pearlite and bainiteo By becoming such a
microstructure, the strength and toughness which are
demanded from the steel plate which is produced by the
10 method of production of the present invention are
secured.
The steel plate which is produced by the method of
production of steel plate for welded structure use of the
present invention has an average grain size at the plate
15 thickness center part of 5 to 20 μm. As a result, the
toughness which is demanded from the steel plate which is
produced by the method of production of steel plate for
welded structure use of the present invention is
satisfied.
20 The steel plate which is produced by the method of
production of steel plate for welded structure use of the
present invention has a ferrite area percentage at the
plate thickness center part of 20 to 80%. As a result,
the steel plate which is produced by the method of
25 production of steel plate for welded structure use of the
present invention becomes excellent in elongation,
toughness, and strength.
Examples
30 The chemical compositions of the molten steel was
adjusted in the steelmaking process, then the steel was
continuously cast to produce each steel slab.
Next, the steel slab was reheated and, furthermore,
rolled by plate rolling to obtain thickness 10 to 40 mm
35 steel plate, then the steel plate was water cooled. In
the steel plate of Test No. 18, air cooling was performed
instead of water cooling (comparative example).
16
After this, in accordance with need, the steel plate
was heat treated to produce yield strength 315 MPa to 550
MPa steel plate. Tables 1 to 2 show the chemical
compositions of different steel plates. The underlines in
5 Table 1 show contents which are outside the scope of the
present invention. The parentheses in Table 2 show
analysis values of the amounts contained as unavoidable
impurities.
Table 1
Class
Cass ytmb
chemical composition (mass o )
C Si Mn P S Al Nb Ti N
A 0.10 0.02 0.8 0.003 0.012 0.03 0.008 0.010 0.007
B 0.06 0.12 2.0 0.007 0.003 0.08 0.-012 0.006 0.004
C 0.14 0.18 0.5 0.007 0.003 0.04 0.015 0.014 0.004
D 0.16 0.30 1.2 0.009 0.006 0.05 0.007 0.016 0.002
E 0.05 0.08 1.6 0.012 0.002 0.07 0.018 0.020 0.004
F 0.08 0.04 1.4 0.005 0.009 0.01 0.015 0.019 0.001
Inv. cast G 0.14 0.32 1.7 0.007 0.007 0.05 0.005 0.010 0.005
slab H 0.10 0.16 1.1 0.015 0.005 0.02 0.015 0.008 0.008
I 0.09 0.10 1.2 0.009 0.002 0.05 0.015 0.011 0.003
J 0.13 0.40 1.0 0.004 0.004 0.06 0.006 0.007 0.005
K 0.12 0.12 0.2 0.005 0.005 0.04 0.010 0.012 0.003
L 0.15 0.34 0.2 0.017 0.008 0.02 0.009 0.015 0.001
M 0.04 0.32 1.7 0.006 0.007 0.05 0.010 0.017 0.002
N 0.04 0.06 2.4 0.008 0.005 0.04 0.020 0.012 0.005
0 0.11 0.12 1.7 0.008 0.001 -0.01 0.004 0.015 0.004.
P 0.02 0.08 0.1 0.004 0.005 0.04 0.001 0.001 0.002
Q 0.18 0.22 0.9 0.007 0.005 0.06 0.016 0.012 0.005
Comp. cast
slab
R 0.05 0.40 2.6 0.005 0.004 0.05 0.009 0.009 0.004
S 0.10 0.46 1.4 0.003 0.006 0.03 0.012 0.008 0.002
T 0.14 0.2 _8 -1.2 0.005 0.003 0.02 0.030 0.060 0.003
* Underlines indicate outside scope of the present invention.
Table 2 (Continuation of Table 1)
Class
Cast solab
Chemical composition (mass)
Cu 'Ni Cr No V B Ca Mg REM Ceq.
A 0.4 0.4 (0.01) (0.001) (0.001) (0.0002) 0.001 (0.0002) (0.0001) 0.2843
B (0.01) (0.02) (0.01) (0.002) (0.001) (0.0001) (0.0002) (0.0001) (0.0001) 0.3887
C 0.2 0.2 0.2 0.1 0.03 0.001 (0.0001) ("0.0002) (0.0001) 0.3160
D (0.01) (0.02) (0.01) (0.001) (0.001) (0.0001) (0.0001) 0.001 0.001 0.3556
E 0.3 0.3 (0.02) (0.001) (0.001) (0.0001) (0.0001) (0.0001) (0.0002) 0.3523
F (0.01) (0.01) (0.01) (0.001) (0.002) (0.0001) 0.001 (0,.0001) (0.0001) 0.3094
Inv.
t
G 0.03 (0.01) (0.01) (0.002) (0.001) (0.0002) (0.0001) (0.0002) (0.0001) 0.4221
cas
slab H (0.01) (0.02) (0.01) (0.001) 0.04 0.002 (0.0001) (0.0001) (0.0002) 0.2871
I (0.02) (0.01) (0.03) (0.002) (0.001) (0.0001) (0.0002) (0.0001) 0.001 0.2841
J (0.01) (0.02) (0.01) (0.001) (0.001) 0.003 (0.0001) (0.0001) 0.002 0.2923
K (0.01) (0.02) 0.4 0.2 (0..001) (0.0001) (0.0001) 0.001 (0.0001) 0.3211
L 0.2 0.2 0.2 0.2 0.05 0.001 0.001 0.001 0.001 0.3000
M (0.01) (0.01) 0.2 0.1 (0.001) (0.0001) (0.0001) (0.0001) (0.0002) 0.3818
N (0.01) (0.02) (0.01) (0.001) (0.001) (0.0002) (0.0002) (0.0001) (0.0001) 0.4356
0 (0.02) (0.01) (0.01) (0.001) (0.002) (0.0001) 0.001 0.002 (0.0001) 0.3887
P (0.01) (0.02) (0.01) (0.002) (0.001) (0.0001) (0.0001) (0.0001) (0.0001) 0.0321
Comp. Q 0.2 0.2 (0.02) (0.001) (0.001) 0.002 (0.0001) (0.0002) 0.001 0.3523
cast R (0.01) (0.01) (0.01) (0.002) (0.001) (0.0001) 0.002 (0.0001) (0.0002) 0.4794
slab S 0.3 1.2 0.3 0.3 0.08 (0.0001) (0.0002) (0.0001) (0.0001) 0.5693
T (0.01) (0.02) (0 31) (0.001) (0.001) (0.0001) (0.0001) 0.001 (0.0002) 0.3356
9
* Parentheses indicate analysis values of amounts contained as unavoidable impurities
19
The produced steel plates were measured for
microstructure phase percentages, average grain size, and
mechanical properties.
The microstructure phase percentages were obtained
5 by using an optical microscope to observe the
microstructure at a plate thickness center position by a
magnification of 500X and finding the average values of
the area percentages of the different phases to the total
field region by image analysis.
10 The. average grain size was obtained by using the
EBSP (electron back scattering pattern) method to measure
500 μm x 500 μm regions by a 1 μm pitch, defining the
boundary where the difference in crystal orientation with
the adjoining grains is 15° or more as the grain boundary,
15 and finding the average value of the grain sizes at that
time.
Among the mechanical properties, the yield stress
was tested using the test pieces of the entire thickness
while the Charpy fracture appearance transition
20 temperature (vTrs) was tested using a test piece taken
from the center part of plate thickness. The results were
used as representative values of the steel plates.
The tensile test was performed based on JIS Z X241
(1998) "Tensile Test Method of Metal Materials", Two
25 pieces each were tested and measured and the averages
found. The tensile test pieces were made the No. 1B test
pieces of JIS Z 2201 (1998).
The Charpy fracture appearance transition
temperature (vTrs) was found using 2 mm V-notch Charpy
30 impact test pieces based on JIS Z 2242 (2005) "Charpy
Impact Test Method of Metal Materials". Three pieces each
were tested for each temperature for five temperatures.
The temperatures at the 50% brittle fracture rates were
measured.
35 The results of measurement of the steel plates are
shown together with the methods of production in Tables 3
to 8. Note that, the temperatures and cooling rates in
20 -
the methods of production are values at plate thickness
center positions. They were found from the actually
measured surface temperatures by heat conduction analysis
by the known differential method.
5 In the present embodiment, the fracture appearance
transition temperature was made -60°C or less, and a
rolling time of 200 s or less was defined as "good". The
underlines in Tables 3 to 8 show conditions which are
outside the scope of the present invention or properties
10 and productivity of the steel plate outside of the values
defined as "good".
Table 3
Heating First stage rolling Second stage rolling
Class
Test
no.
Slab
symbol
Slab
thickness
O
Temp .
o
( C)
Start
tempp..
o
( C)
End
tempp..
o
(C)
Cumul -
ative
rolling
reduction
(a)
No . of
passess
Start
temp .
o
( C)
End
temp.
(
o
C)
Rolling
reduction
per pass
(o)
Time
between
passes
(s)
No. of
passes
Final
thickness
(mm)
1 A 200 1140 1090 1010 87.5 10 945 885- *1 *2 4 12
2 F 150 1100 1056 988 87 8 932 901 *1 *2 2 12
3 B 213 1080 1030 985 86 . 9 11 920 890 *1 *2 2 20
4 C 180 1120 1085 995 77.8 7 940 923 *1 *2 3 20
5 D 150 1065 1020 980 64 . 7 5 900 855 *1 *2 4 30
Inv . 6 K 200 1095 1056 973 70 7 932 886 *1 *2 5 30
ex . 7 N 180 1050 996 974 65 7 928 895 *1 *2 4 40
8 E 248 1040 1005 982 73.8 7 943 938 * 1 *2 3 40
9 G 240 1120 1060 982 75 . 8 9 910 881 * 1 *2 2 35
10 H 180 1080 1026 968 77 . 2 9 947 876 * 1 *2 4 25
11 I 160 1030 1000 960 75 8 945 882 *1 *2 5 15
12 J 180 1040 990 955 80 9 935 904 *1 *2 2 22
13 L 150 1020 985 993 82 . 7 8 945 855 * 1 *2 3 18
* Values of *1 indicated in Table 7, values of *2 indicated in Table 8.
Table 4
Grain Product-
Cooling Tempering Microstructure percentage Strength Toughness
size ivity
Cast- Ferrite Pearlite Bainite Fracture
Test Average
Class
No.
slab Start End
Speed Temp.
area area area
grain
yield appearance
Rolling
symbol temp. temp. percent- percent- percent- stress transition
(°C) (°C)
°
( C/s) (°C) age age age
size
(MPa) temp. time (s)
(%) (PM)
(°C)
1 A 865 520 33 - 72 5 23 6 325 -108 123.9
2 F 810 120 44 550 68 6 26 8 389 -85 101.9
3 B 858 390 18 - 35 0 65 15 520 -72 134.9
4 C 887 80 27 500 46 10 44 9 395 -81 121.3
5 D 824 610 19 - 79 17 4 12 346 -77 156.9
I
6 K 835 170 25 450 68 7 25 11 327 -82 146.6
nv.
ex
N 852 480 17 - 58 3 39 13 344 -79 157.6
.
8 E 902 160 14 550 65 2 33 9 365 -95 175.7
9 G 853 620 8 - 67 15 18 9 425 -70 164.1
10 H 842 440 16 - 55 3 42 7 346 -88 177.4
11 I 846 150 20 530 64 2 34 11 352 -83 175.5
12 J 865 550 19 - 61 7 32 12 348 -79 139.4
13 L 821 500 14 - 58 5 37 13 324 -74 160.7
Table 5
Heating First stage rolling Second stage rolling
lass
Test
No.
Cast
-
slab
symbol
Slab
thickness
(mm)
emp.
(°C)
Start
temp.
(°C)
End
temp.
(°C)
Cumulative
rolling
reduction
(o)
o. of
passes
Start
temp.
(°C)
End
temp.
(°C)
Rolling
reduction
per pass
(o)
Time
between
passes
(s)
o. of
passes
Final
thickness
(mm)
14 F 250- 1180 1142 965 77.6 17 948- 783 1 *2 14 12
15 G 120 1130 1095 1050 80 7 945 918 *1 *2 3 12
16 H 180 1240 i"_80 1025 83.9 10 928 895 *1 *2 2 20
17 I 200 1090 1050 1020 77.5 8 948 915 *1 *2 4 20
18 J 245 1090 1054 1019 90.2 13 928 920 *.1 *2 1 20
19 K 252 1070 1025 985 80.2 11 949 942 *1 *2 3 30
20 L 130 1150 1080 1065 42.1 3 935 873 *1 *2 5 30
21 M 274 1120 1085 1005 83.6 10 944 916 *1 *2 3 30
22 N 235 1182 1110 1004 69.4 8 948 875 *1 *2 3 40 JI
Comp. 23 0 248 1158 1092 1060 75.8 7 941 935 , *1 *2 2 40
ex. 24 P 120 1110 1096 1004 82 8 933 877 *1 *2 3 12
25 Q 150 1020 985 958 63.3 5 925 865 *1 *2 7 20
26 R 240 1030 990 975 82.5 10 948 940 *1 *2 2 30
27 S 180 1040 1010 972 72.2 6 943 912 *1 *2 3 30
28 T 180 1020 986 965 55.6 5 940 875 *1 *2 4 40
29 M 250 1180 1145 993 74.8 10 940 926 *1 *2 2 38
30 0 195 1175 1128 1010 77.9 11 946 853 *1 *2 4 27
31 J 175 1075 1026 975 88.6 12 930 875 *1 *2 6 14
32 B 270 1050 1035 984 81.9 11 942 857 *1 *2 7 32
33 N 220 1130 1100 1013 80.9 11 915 862 *1 *2 3 36
*
*
Underlines indicate outside scope of the present invention.
Values of *1 indicated in Table 7, values of *2 indicated in Table 8.
I
Table 6
Cooling
Tempering
Microstructure percentage
Grain
ss ize
Strength Toughness
Productivit
y
Class
Test
No.
-Castslab
symbol
Start
temp.
(°C)
End
temp.
(°C)
peed
°
( C/s)
emp.
°
( C)
Ferrite
area
percentage
(%)
Pearlite
area
percentage
(a)
Bainite
area
percentage
(o)
Average
grain
size
(μm)
Yield
stress
(MPa)
Fracture
transition
temp.
(°C)
olling
time (s)
14 F 760 350 24 - 74 3 23 7 343 -87 281.8
15 G 902 320 62 - 15 0 85 6 612 -14 117.2
16 H 863 480 13 - 45 6 49 30 426 -5 359.4
17 I 680 180 17 600 86 2 12 26 306 -24 165.3
18 J 890 580 12 - 73 12 15 28 337 -28 168.9
19 K 911 490 16 - 43 4 53 32 359 -19 167.2
20 L 842 200 25 500 35 0 65 37 454 -3 418.2
21 M 880 720 12 - 88 12 0 26 310 -11 164.8
22 N 843 320 14 450 32 4 64 27 426 -15 431.4
Comp. 23 0 Air cooling - 87 13 0 39 335 -6 148.9
ex. 24 P 845 570 22 - 97 3 0 45 245 12 110.4
25 Q 836 240 16 550 45 11 44 12 493 -6 100.7
26 R 912 440 9 - 56 2 42 14 346 -2 158.4
27 S 878 380 24 - 5 0 95 11 673 -9 111.1
28 T 844 110 13 600 42 7 51 13 526 -1 186.8
29 M 890 610 9 - 72 10 18 24 367 -28 170.7
30 0 826 580 14 - 64 9 27 22 379 -37 256.7
31 J 833 120 23 570 43 3 54 37 342 -9 189.2
32 B 825 200 13 450 38 2 60 42 365 -7 261,1
33 N 840 580 10 - 40 6 54 39 415 -4 223.2
* Underlines indicate outside scope of the present invention or deviated from prescribed values.
Table 7
Cl T t
*1) Rolling reduction at each pass (°s)
ass es no.
1 2 3 4 5 6 7 8 9 10 it 12 13
1 - 20.0 15. 0' 17.6 14.3
2 21.1 20.0
3 17.9 13.0
4 20.0 21.9 20.0
5 11.6 10.6 14.3 16.7
6 13.3 15.4 13.6 10.5 11.8
Inv. ex. 7 11.1 10.7 10.0 11.1
8 15.4 16.4 13.0
9 22.4 22.2
10 12.2 11.1 12.5 10.7
11 17.5 18.2 18.5 18.2 16.7
12 22.2 21.8
13 11.5 13.0 10.0
14 14.3 14.6 12.2 11.1 9.4 10.3 11.5 8.7 9.5 10.5 11.8 13.3 7.7
15 20.8 21.1 20
16 17.2 16.7
17 15.6 15.8 15.6 14.8
18 16.7
19 16 14.3 16.7
20 19.8 20 15.4 18.2 16.7
21 15.6 10.5 11.8
22 19.4 17.2 14.6
C
23 20 16.7
omp. ex.
24 18.2 22.2 14.3
25 18.2 11.1 12.5 11.4 12.9 14.8 13
26 14.3 16.7
27 12 18.2 16.7
28 18.8 15.4 12.7 16.7
29 22.2 22.4
30 11.6 10.5 11.8 10
31 5 5.3 5.6 59. 6.3 6.7
32 6.1 6.5 7 5 5.3 5.6 5.9
33 4.8 5 5 . 3
* Underlines shown outside of the scope of the present invention.
Table 8
l
*2) Time between passes (s)
C ass Test no..
1-2 2-3 3-4 4-5 5-6 6-7 7-8 8-9 9-10 10-11 11-12 12-13
1 3.7 46 3.8
2 11.3
3 4.1
4 8.1 3.6
5 3.4 4.5 16.8
6 5.8 7.5 3.4 4.5
Inv. ex. 7 5.9 4.2 12.5
8 3.9 4.6
9 15.6
10 15.1 16.7 14.8
11 16.4 19.2 18.3 18.8
12 24.1
13 22.5 23.2
14 3.9 4.6 4.1 6.5 3.8 3.5 9.8 6.6 8.3 11 6.1 5.5
15 5.1 4.3
16 14.7
17 17.9 3.8 5.4
18
19 2.5 2.8
20 7.3 9.6 3.6 4.3
21 4.2 5.6
22 28.5 32
23 6.5
Comp. ex.
24 11.3 5.3
25 5.1 9.3 4.5 5.5 6.5 3.5
26 4.5
27 10.5 5.8
28 4.5 5.6 23.8
29 27.3
30 26.9 27.1 26.5
31 10.1 9.6 8.6 -4_-9 8.8
32 15.6 17.8 14.9 16.3 17.1 16.3
33 23.2 24.8
* Underlines shown outside of the scope of the present invention.
- 27
Test No. 1 to No. 13 are invention examples which
satisfy all of the conditions of the present invention
and are excellent in strength, toughness, and
productivity.
5 Test No. 14 to No. 33 are comparative examples with
the underlined conditions outside the scope of the
present invention.
Test No. 14 had a large number of first stage and
second stage rolling passes and had a low second stage
10 rolling end temperature, so was long in rolling time and
low in productivity.
Test No. 15 was too fast in cooling rate, so was
high in strength and low in toughness.
Test No. 16 was too high in slab heating
15 temperature, so was large in average grain size, low in
toughness, and, furthermore, long in rolling time and low
in productivity.
Test No. 17 was too low in cooling start
temperature, so was large in average grain size and low
20 in strength and toughness.
Test No. 18 was small in number of passes of second
stage rolling, so was large in average grain size and low
in toughness.
Test No. 19 was short in time between passes it
25 second stage rolling, so was large in average grain; size
and low in toughness.
Test No. 20 was small in cumulative rolling
reduction of first stage rolling, so was large in average
grain size, low in toughness, and, furthermore, long in
30 rolling time and low in productivity.
Test No. 21 was too high in cooling end temperature,
so was large in average grain size and was low strength
and toughness.
Test Nos. 22, 29, and 30 were long in time between
35 passes in second stage rolling, so were large in average
grain size, low in toughness, and, furthermore, long in
rolling time and low in productivity.
28
Test No. 23 used air cooling for cooling, so was
large in average grain size and was low in toughness.
Test Nos. 24 to 28 had ranges of chemical
compositionsoutside the scope of the present invention,
5 so the toughness was low.
Test Nos® 31 to 33 had small rolling reductions at
the passes in the second stage rolling, so were large in
average grain size and low in toughness. Nos, 33 and 33
had times between passes which were within the prescribed
10 range, but were somewhat long and had too small rolling
reductions in the passes, so were long in rolling time
and were low in productivity.
From the above examples, it was confirmed that
according to the method of production of the present
15 invention, by utilizing the refining action of yrecrystallization
to refine the microstructure by hightemperature
rolling in the y-recrystallization temperature
range, steel plate which is excellent in low temperature
toughness is obtained.
20 Note that the present invention is not limited to
the above embodiments. It can be worked changed in
various ways within a scope not deviating from the gist
of the present invention.
25 Industrial Applicability
The method of production of steel plate of the
present invention does not include a low-temperature
rolling process, so the temperature waiting time is
short. Further, the rolling reduction is large, so the
30 number of passes is small and the rolling productivity is
high. According to the present invention, it is possible
to utilize the refinement by y-recrystallization so as to
refine the microstructure even by high-temperature
rolling in the y-recrystallization temperature range and
35 possible to provide a method of production of steel plate
for welded structure use which is excellent in low
- 29 -
temperature toughness, so it is possible to apply the
invention to the production of steel plate which is used
for ships, buildings, tanks, offshore structures, line
pipe, and other welded structures. The industrial
5 applicability is therefore large.
3 0 --
CLAIMS
Claim 1
A method of production of steel plate characterized
by
5 preparing a steel slab which contains, by mass%,
C: 0.04 to 0.160,
Si: 0.01 to 0.5%,
Mn: 0.2 to 2.50,
P: 0.03% or less,
10 S: 0.020 or less,
Al: 0.001 to 0.10%,
Nb: 0.003 to 0.02%,
Ti: 0.003 to 0.05%, and
N: 0`.001 to 0.008%,
15 which contains, as optional elements, one or more of
Cu: 0.03 to 1.5%,
Ni: 0.03 to 2.0%,
Cr: 0.03 to 1.5%,
Moe 0.01 to 1.0%,
20 V: 0.003 to 0.2%,
B: 0.0002 to 0.005%,
Ca: 0.0005 to 0.01%,
Mg: 0.0005 to 0.01%, and
REM: 0.0005 to 0.01%,
25 which has a carbon equivalent Ceq of the following
formula (A) of 0.2 to 0.5%, and which has a balance of Fe
and unavoidable impurities,
heating the steel slab to 1000 to 1200°C, then
rolling by first stage rolling at a plate thickness
30 center temperature of 950 to 1200°C, a cumulative rolling
reduction of 50 to 95%, and a number of passes of 4 to 16
passes, then
rolling by second stage rolling at a plate thickness
center temperature of 850 to 950°C, a number of passes of
35 2 to 8 passes, a rolling reduction at each pass of 10 to
25%, and a time between passes of 3 to 25 seconds, then
- 31 -
cooling by accelerated cooling from a plate
thickness center temperature of 750°C or more by a 1 to
50°C/s cooling. rate down to 650°C or less
so as to obtain to steel plate which has a plate
5 thickness of 10 to 40 mm, a yield stress of 315 to 550
MPa, a microstructure of a mixed microstructure of
ferrite and bainite or of ferrite, pearlite, and bainite,
and an average grain size at the plate thickness center
part of 5 to 20 μm:
10 Ceq=C+Mn/6+(Cu+Ni)/15+(Cr+Mo+V)/5... (A)
Claim 2
A method of production of steel plate as set forth
in,claim l characterized by tempering at 300 to 650°C
after the accelerated cooling ends.