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 strength, elongation, and 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 strength, elongation,
and toughness. 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 being used in an increasing number of
20 cases.
In general, strength, elongation, and toughness are
in a contradictory relationship. If raising the strength,
the elongation and the toughness fall. To achieve all of
strength, elongation, and toughness, in the rolling
25 process, it is necessary to roll the steel at what is
called the "y-non-recrystallization temperature range",
that is, a 750 to 850°C or so low temperature, and cause
the formation of fine ferrite grains.
In the past, various methods for causing an
30 improvement in the strength, elongation, and toughness of
steel plate have been proposed. For example, there are
the arts which are disclosed in PLT's 1 to 5.
PLT 1 describes steel plate of a plate thickness of
40 mm or more which is excellent in arrestability of
35 brittle cracks.
PLT 2 describes steel plate which is defined in the
Vicker's hardness of the steel plate and is excellent in
- 2 -
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
5 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
to the final pass 15 seconds or less.
PLT 4 describes a method of production of steel
10 plate which has excellent strength and toughness by
setting the rolling conditions so as to satisfy a
predetermined relationship between the rolling
temperature and rolling reduction at each rolling pass
and enjoying the effects of refinement of the
15 recrystallized 7-grains and rolling in the nonrecrystallization
region to the maximum extent to as to
refine the final microstructure.
PLT 5 describes a method of production of steel
plate which is excellent in strength and toughness by
20 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
reduction in the non-recrystallization region 70% or
more.
25
Citations List
Patent Literature
PLT 1: Japanese Patent Publication (A) No. 2007-
302993
30 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-
35 269924
PLT 5: Japanese Patent Publication (A) No. 11-181519
Summary of Invention
Technical Problem
However, the above PLT's 1 to 5 had the following
problems.
5 The method of production which is described in PLT 1
requires low-temperature rolling (CR) until a larger
thickness of steel plate. If low-temperature rolling, the
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. Furthermore, at the time of
accelerated cooling, air-cooling is required in the
middle. The productivity of accelerated cooling is low.
15 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
20 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 desci-ibed in
25 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 is
30 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
35 large restrictions in terms of facilities and is not
practical.
Therefore, the present invention has as its task to
- 4 -
reduce the drop in productivity due to the need for lowtemperature
rolling in the prior art and to provide a
method of production of steel plate for welded structure
use which is excellent in strength, elongation, and
5 toughness which can be applied to steel plate which has a
yield stress of 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
10 of a method of production of steel plate which enables
refinement of the microstructure, even without lowtemperature
rolling, by just high-temperature rolling
and, furthermore, which applies accelerated cooling which
changes the cooling rate in stages so as to enable the
15 second phases to be hardened while securing ferrite.
Solution to Problem
The inventors studied in depth the method of
production of steel plate. As a result, they discovered
20 manufacturing conditions which enable the microstructure
to be refined by utilization of refinement by yrecrystallization
even with rolling at a high temperature
of 850 to 950°C or so referred to as the "7-
recrystallization temperature range" and realized a
25 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
(below, also referred to as the "second stage rolling".
Further, the first stage of hot rolling also being
30 referred to as the "first stage rolling"), the rolling
reduction per pass is made larger than the conventional
manufacturing process and the time between passes is
optimized. If increasing the rolling reduction per pass,
the number of passes decreases, so the productivity
35 becomes higher. With the conventional low-temperature
rolling in the 7-non-recrystallization temperature range,
- 5 -
the rolling reaction force becomes large, so the rolling
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 y-
5 recrystallization temperature range, by making the
rolling reduction 10 to 25% and, furthermore, making the
time between passes 3 to 25 seconds, it is possible to
make use of the refinement by y-recrystallization and
refine the microstructure.
10 Furthermore, it was learned that by dividing the
accelerated cooling after rolling into two stages with
different cooling rates and cooling in two stages making
the cooling rate in the cooling of the first half (below,
also referred to as the "first stage cooling") slow and
15 making the cooling rate in the cooling of the second half
(below, also referred to as the "second stage cooling")
fast, it is possible to harden the second phases while
securing the ferrite and possible to produce steel plate
which is excellent in strength, elongation, and
20 toughness.
The present invention was made based on the above
discoveries and, furthermore, in consideration of the
chemical compositions of steel which is excellent in
productivity, strength, elongation, and toughness. Its
25 gist is as follows:
(1) A method of production of steel plate
characterized by
preparing a steel slab which contains, by
mass%,
30
35
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
- 6 -
5
10
15
20
25
N: 0.001 to 0.008%,
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%,
Moo 0.01 to 1.0%,
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%,
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
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
seconds, then
cooling by first stage cooling from a plate
thickness center temperature of 750°C or more by a 0.5 to
8-°C/s cooling rate down to 630 to 700°C, then
cooling by second stage cooling by a 10 to
50°C/s cooling rate down to 550°C or less in temperature
30 so as to obtain to steel plate which has a
plate thickness of 10 to 40 mm, a yield stress of 315 to
550 MPa, a microstructure of a mixed microstructure of
one or more of soft phase ferrite and hard phase
pearlite, bainite, and martensite, an area percentage of
35 ferrite at the plate thickness center part of 70 to 95%,
an average Vicker's hardness of the hard phases of 250 to
- 7 -
500, and an average grain size 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
5 after the accelerated cooling ends.
Advantageous Effects of Invention
The method of production of steel plate for welded
structure use of the present invention does not include
10 low-temperature rolling, so the temperature waiting time
is short. Further, the rolling reduction is large, so the
number of passes is small and the rolling productivity is
high.
Further, according to the method of production of
15 the present invention, by utilizing the refinement by yrecrystallization
so as to refine the microstructure by
high-temperature rolling in the y-recrystallization
temperature range and, furthermore, by making the
accelerated cooling after rolling two-stage cooling of a
20 first stage of slow cooling and a second stage of rapid
cooling so as to secure ferrite while hardening the
second phases, it is possible to produce steel plate for
welded structure use which is excellent in strength,
elongation, and toughness.
25
Description of Embodiments
First, a preferable method of production of steel
plate for welded structure use of the present invention
will be explained.
30 First, molten steel which has been adjusted to the
desired chemical compositions is smelted by a known
smelting method using a converter etc. and is cast into a
steel slab by continuous casting or another known casting
method.
35 During the cooling at the time of casting, or after
the cooling, the steel slab is heated to 1000 to 1200°C in
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 austenite grains coarsen and refinement in the
5 subsequent rolling process becomes difficult.
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
10 1150°C.
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.
15 If the plate thickness center temperature exceeds
1200°C, the recrystallized y-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.
20 If the cumulative rolling reduction becomes less
than 50%, the recrystallization does not sufficiently
proceed and the recrystallized y-grains cannot be made
finer. If the cumulative rolling reduction exceeds 95%,
the rolling load becomes larger and the productivity
25 falls. The preferable cumulative rolling reduction is 60%
to 90%.
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
30 preferable number of passes is 5 to 14.
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,
35 and a number of passes of 2 to 8 passes.
- 9 -
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
5 thickness center temperature is 870 to 930°C.
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
10 realization becomes difficult. The preferable rolling
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.
15 If the rolling reduction per pass is 10 to 25% in
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
20 recrystallization, so recrystallization does not
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
25 energy, is started, so the recrystallized y-grains
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
30 preferable time between passes is 5 to 23 seconds.
If the number of passes becomes less than 2, the
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.
35 After the above hot rolling, first stage cooling is
performed from a plate thickness center temperature of
- 10 -
750°C or more by a 0.5 to 8°C/s cooling rate down to a
plate thickness center temperature of 630 to 700°C in
range, then second stage cooling is performed by a 10 to
50°C/s cooling rate down to a temperature of 550°C or
5 less.
If the plate thickness center temperature at the
time of start of cooling becomes less than 750°C, the
ferrite transformation proceeds, so a ferrite finegrained
microstructure is hard to obtain.
10 If the cooling rate in the first stage cooling is
less than 0.5°C/s, a fine microstructure is not obtained,
while if the cooling rate is over 8°C/s, a 70% or more
ferrite area percentage cannot be obtained.
If the cooling rate in the second stage cooling is
15 less than 10°C/s, the hardness of the hard phases will not
become an average Vicker's hardness of 250 or more. If
over 50°C/s, the hardness of the hard phases will not
become an average Vicker's hardness of 500 or less.
If the cooling stop temperature exceeds 550°C, the
20 hardness of the hard phases will not become an average
Vicker's hardness of 250 or more.
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, a cooling
25 rate 1 to 7°C/s, an end temperature of first stage cooling
of 640 to 690°C, a cooling rate of second stage cooling of
15 to 45°C/s, and a cooling stop temperature of 500°C or
less.
Note that, control of the production using the plate
30 thickness center temperature of the steel plate is also a
feature of the method of production of steel plate of the
present invention. By using the plate thickness center
temperature, compared with when using the surface
temperature of the steel plate, even when the plate
35 thickness changes etc., it is possible to suitably
- 11 -
control the manufacturing conditions and possible to
efficiently produce good quality steel plate with little
variations in material quality.
In the rolling process, usually, in the period from
5 heating to rolling, the surface temperature etc. of the
steel plate is measured while calculating the temperature
distribution inside of the steel plate. The rolling is
performed while predicting the rolling reaction force
from the results of calculation of the temperature
10 distribution. In this way, it is possible to easily find
the steel plate center temperature during rolling. Even
when performing accelerated cooling, the accelerated
cooling is controlled while predicting the temperature
distribution at the inside of the plate thickness in the
15 same way.
After the accelerated cooling, the steel may, if
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
20 exceeds 650°C, the amount of softening becomes larger and
securing strength becomes difficult.
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 plater with a
25 plate thickness of 10 to 40 mm and a yield stress of 315
to 550 MPa. In particular, it can be applied to the
production of steel plate of a yield stress of the 315
MPa class, 355 MPa class, or 390 MPa class for ship hull
structures.
30 In steel plate with a plate thickness of less than
10 mm, the plate shape deteriorates, so accelerated
cooling cannot be applied. In steel plate with a plate
thickness of over 40 mm, to secure toughness, lowtemperature
rolling becomes essential, so simultaneous
35 achievement of good productivity is not possible.
In production of steel plate with a yield stress of
- 12 -
less than 315 MPa, accelerated cooling is not required,
so the present invention does not have to be applied. In
production of steel plate with a yield stress of over 550
MPa, to secure toughness, low-temperature rolling becomes
5 essential, so simultaneous achievement of good
productivity is not possible.
According to the above manufacturing conditions, it
is possible to utilize the refinement due to yrecrystallization
and refine the microstructure even with
10 high-temperature rolling. Furthermore, the method of
production of the present invention does not require lowtemperature
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
15 method of production is excellent in rolling
productivity.
The chemical compositions of the steel plate to
which the method of production of the present invention
is applied is as follows considering the strength,
20 elongation, toughness, heat affected zone (HAZ)
toughness, weldability, etc.
C is added in an amount of 0.04% or more so as to
secure the strength and toughness of the base material.
If the content of C exceeds 0.16%, it becomes difficult
25 to secure a good HAZ toughness, so the content of C is
made 0.16% or less. To secure the strength of the base
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%
30 or 0.14%.
Si is effective as a deoxidizing element and
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
35 0.5% or less. To reliably perform the deoxidation, the
lower limit of the content of Si may be set to 0.05% or
0.10%. Further, to improve the HAZ toughness, the upper
- 13 -
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
strength and toughness of the base material. If the
content of Mn exceeds 2.5%, the center segregation
5 becomes remarkable and the base material at the part
where center segregation occurs and the HAZ deteriorate
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
10 to 0.6% or 0.8%. To prevent deterioration of the material
qualities due to center segregation, the upper limit of
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
15 less. To improve the HAZ toughness, the content of P may
be made 0.02% or less or 0.015% or less.
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
20 improve the properties of the base material and the HAZ
toughness, the content of S may be made 0.01% or less or
0.008% or less.
Al is an element which performs deoxidation and is
necessary for reducing the impurity element 0. In
25 addition to Al, Mn and Si also contribute to deoxidation.
However, even when Mn or Si is added, if the content of
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
30 the base material and the HAZ are degraded in toughness,
so the amount of addition of Al is made 0.10% or less. To
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
35 content of Al may be made 0.08% or 0.06%.
Nb, by addition of 0.003% or more, contributes to
improvement of the strength and toughness of the base
- 14 -
material. However, if the content of Nb exceeds 0.02%,
the HAZ toughness and the weldability fall, so the
content of Nb is made 0.02% or less. To enable the effect
of refinement by Nb to be exhibited better, the lower
5 limit of the content of Nb may also be set to 0.005%. To
improve the HAZ toughness and weldability, the upper
limit of the content of Nb may be made 0.015% or 0.012%.
Ti forms TiN by addition and suppresses the
enlargement of the austenite grain size at the time of
10 heating the steel slab. If the austenite grain size
becomes large, the grain size after transformation also
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 0.003% or
15 more. However, if the content of Ti exceeds 0.05%, TiC is
formed and the HAZ toughness falls, so the content of Ti
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%.
20 N forms TiN and suppresses the enlargement of the
austenite grain size at the time of heating the steel
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.
25 In addition to the above-mentioned additive
elements, as optional elements which can be added in
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
1.0%, V: 0.03 to 0.2%, and B: 0.0002 to 0.005% may be
30 contained. By adding these elements, the base material
can be improved in strength and toughness. In accordance
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%, 0.5%, or 0.3%, the upper limit of the
35 content of Cr to 1.0%, 0.5%, or 0.3%, the upper limit of
the content of Me to 0.3%, 0.2%, or 0.1%, the upper limit
of the content of V to 0.1%, 0.07%, or 0.05%, and the
- 15 -
upper limit of the content of B to 0.003%, 0.002, or
0.001%.
If these elements are added excessively, the HAZ
toughness and the weldability deteriorate, so the upper
5 limits of the contents are defined as explained above.
Furthermore, as other optional elements, by mass%,
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.
10 To improve the strength and toughness of the base
material etc., these optional elements may be
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
15 contained in the steel as unavoidable impurities such as
Cu: 0.05% or less, Ni: 0.05% or less, Cr: 0.05% or less,
Me: 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
20 in the steel as unavoidable impurities, there is no
effect on the method of production of steel plate of the
present invention.
The steel plate which is produced by the method of
production of steel plate for welded structure use of the
25 present invention is given a carbon equivalent which is
found by the above formula (A) of 0.2 to 0.5%. When the
optional elements are contained as unavoidable
impurities, their contents are entered to find the carbon
equivalent.
30 If the carbon equivalent is less than 0.2%, the
strength which is demanded from the steel plate which is
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
35 which are demanded from the steel plate which is produced
by the method of production of the present invention
cannot be satisfied. To secure strength, the lower limit
- 16 -
of the carbon equivalent may be set to 0.25%, 0.28%, or
0.300. To improve the HAZ toughness and weldability, the
lower limit of the carbon equivalent may also be set to
0.43%, 0.4%, or 0.38%.
5 The microstructure of the steel plate which is
produced by the method of production of steel plate for
welded structure use of the present invention is a mixed
microstructure of the soft phase ferrite and the hard
phases pearlite, bainite, and martensite. By becoming
10 such a microstructure, the strength, elongation, and
toughness which are demanded from the steel plate which
is produced by the method of production of the present
invention are secured.
The steel plate which is produced by the method of
15 production of steel plate for welded structure use of the
present invention has a ferrite area percentage at the
plate thickness center part of 70 to 95%, a Vicker's
hardness of the hard phases of an average of 250 to 500,
and an average grain size of 5 to 20 pm.
20 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.
25 Examples
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,
30 rolled by plate rolling to obtain thickness 10 to 40 mm
steel plate, then the steel plate was water cooled. In
the steel plate of Test No. 25, air cooling was performed
instead of water cooling (comparative example).
After this, in accordance with need, the steel plate
35 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
- 17 -
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 Cast slab symbol Chemical composition ( masso)
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 0.5 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
0 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
1 0.14 0.28 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 slab symbol Chemical composition ( mass%)
Cu Ni Cr Mo 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) 0.289
B (0.01) (0.02) (0.01) (0.002) (0.001) (0.0001) (0.0002) (0.0001) (0.0001) 0.398
C 0.2 0.2 0.2 0.1 0.03 0.001 (0.0001) (0.0002) (0.0001) 0.316
D (0.01) (0.02) (0.01) (0.001) (0.001) (0.0001) (0.0001) 0.001 0.001 0.364
E 0.3 0.3 (0.02) (0.001) (0.001) (0.0001) (0.0001) (0.0001) (0.0002) 0.361
F (0.01) (0.01) (0.01) (0.001) (0.002) (0.0001) 0.001 (0.0001) (0.0001) 0.317
Inv. cast G 0.03 (0.01) (0.01) (0.002) (0.001) (0.0002) (0.0001) (0.0002) (0.0001) 0.429
slab H (0.01) (0.02) (0.01) (0.001) 0.04 0.002 (0.0001) (0.0001) (0.0002) 0.296
I (0.02) (0.01) (0.03) (0.002) (0.001) (0.0001) (0.0002) (0.0001) 0.001 0.299
J (0.01) (0.02) (0.01) (0.001) (0.001) 0.003 (0.0001) (0.0001) 0.002 0.301
K (0.01) (0.02) 0.4 0.2 (0.001) (0.0001) (0.0001) 0.001 (0.0001) 0.326
L 0.2 0.2 0.2 0.2 0.05 0.001 0.001 0.001 0.001 0.300
M (0.01) (0.01) 0.2 0.1, (0.001) (0.0001) (0.0001) (0.0001) (0.0002) 0,385
N (0.01) (0.02) (0.01) (0.001) (0.001) (0.0002) (0.0002) (0.0001) (0.0001) 0.444
0 (0.02) (0.01) (0.01) (0.001) (0.002) (0..0001) 0.001 0.002 (0.0001) 0.398
P (0.01) (0.02) (0.01) (0.002) (0.001) (0.0001) (0.0001) (0.0001) (0.0001) 0.041
C t
Q 0.2 0.2 (0.02) (0.001) (0.001) 0.002 (0.0001) (0.0002) 0.001 0.361
omp. cas
slab
R (0.01) (0.01) (0.01) (0.002) (0.001) (0.0001) 0.002 (0.0001) (0.0002) 0.487
S 0.3 1.2 0.3 0.3 0.08 (0.0001) (0.0002) (0.0001) (0.0001) 0.569
T (0.01) (0.02) (0.01) (0.001) (0.001) (0.0001) (0.0001) 0.001 (0.0002) 0.344
* Parentheses indicate analysis values of amounts contained as unavoidable impurities
- 20 -
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 Vicker's
hardness was obtained based on JIS Z 2244 (2009) by
measuring the hard phases by a test load of 10 gf at 20
20 points and finding the average value of the same.
Among the mechanical properties, the yield stress
and the elongation were tested using the test pieces of
the entire thickness while the Charpy fracture appearance
transition temperature (vTrs) was tested using a test
25 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 2241
(1998) "Tensile Test Method of Metal Materials". Two
30 pieces each were tested and measured and the averages
found. The tensile test pieces were made the No. lB test
pieces of JIS Z 2201 (1998).
The Charpy fracture appearance transition
temperature (vTrs) was found using 2 mm V-notch Charpy
35 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.
-- 21 -
The temperatures at the 50% brittle fracture rates were
measured.
The results of measurement of the steel plates are
shown together with the methods of production in Tables 3
5 to 8. Note that, the temperatures and cooling rates in
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,
10 In the present embodiment, the total elongation was
made 20% or more, 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
15 of the present invention or properties 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
( mm)
Tempe
(°C)
Start
temp.
( O C)
C)
End
temp,
C)
Cumulative
rolling
reduction
(%)
No. of
passes
Start
temp.
oC)
- End
temp,
(oC )
Rolling
reduction
per pass
(%)
Time
between
passes
(s)
No. of
passes
Final
thickness
( mm)
1 A 200 1150 1100 1005 87.5 10 942 883 *1 *2 4 12
2 F 150 1090 1062 975 87 8 938 899 *1 *2 2 12
3 B 213 1070 1035 982 86.9 11 925 892 *1 *2 2 20
4 C 180 1110 1072 990 77.8 7 941 922 *1 *2 3 20
5 D 150 1070 1025 974 64.7 5 899 854 *1 *2 4 30
6 K 200 1090 1052 988 70 7 935 883 *1 *2 5 30
Inv.
7 N 180 1060 993 971 65 7 924 891 *1 *2 4 40
ex.
8 E 248 1050 1002 985 73.8 7 941 936 *1 *2 3 40
9 G 240 1115 1055 976 75.8 9 906 874 *1 *2 2 35
10 H 180 1090 1037 963 77.2 9 948 879 *1 *2 4 25
11 I 160 1040 1005 966 75 8 941 886 *1 *2 5 15
12 J 180 1035 984 949 80 9 929 897 *1 *2 2 22
13 L 150 1025 988 962 82.7 8 942 863 *1 *2 3 18
* Values of *1 indicated in Table 7, values of *2 indicated in Table 8.
Table 4
First Second
Temper- Grain Elon- Producstage
stage Microossttrruuccttuurree percentage Strength Toughness
cooling cooling
ing Hess size gation tivity
Fracture
Cast
Class
Test
slab
s- t
E d E d
Ferrite Pearlite Bainite Martensite Hard Average Total
appear-
No'
s Y of
ar
5p eed
n
Speed
n
Temp.
p' area area area area phase grain
Yield
elonance
Rolling
temP
-
tem
'
temP stress transi- time
(° (°C/s)
(°c)
(°C/s) (° (°C) percentage percentage percentage percentage hard- size
(MPa)
gation
tion (s)
(%) (a) (s) (%) ness (pm) (8)
temp.
(°C)
1 A 858 8 652 45 120 450 Be 0 8 4 285 8 321 38 -92 124.4
2 F 824 5 634 38 360 - 92 1. 5 2 443 11 346 33 -76 103.4
3 H 853 3 640 26 280 500 89 1 8 2 465 16 491 27 -68 133.8
4 C 879 4 676 32 350 - 77 4 19 G 302 12 373 30 -72 120.2
5 1 816 7 693 24 60 550 73 6 10 11 258 9 352 29 -84 158
I
6 K 831 1 664 18 320 - 86 3 8 3 263 18 317 37 -69 146.4
nv.
ex
7 N 846 0.7 642 15 40 - 93 0 0 7 484 17 510 35 -65 157.1
.
8 E 894 2 653 21 200 600 89 1 8 2 352 16 332 34 -68 175.1
9 G 829 6 638 12 460 - 78 7 14 0 298 6 365 32 -84 163.6
10 H 823 5 672 23 540 - 77 2 21 0 278 10 354 30 -76 176.5
11 I 818 0.8 645 35 420 - 85 4 9 2 281 18 325 33 -60 174.8
12 J 831 4 666 31 510 - 81 6 23 0 274 14 335 32 -70 141.2
13 L 809 3 689 42 490 - 74 5 18 3 266 12 356 27 -63 159.7
Table 5
Heating First stage rolling Second stage rolling
lass
Test
No
Cast
slab
symbol
Slab
thickness
(mm)
emp.
°C)
Start
tem
p°
(°C)
End
tem
p`
(°C)
Cumulative
rolling
reduction
(%)
o. of
passes
Start
tem
p°
(°C)
End
tempp.°
(°C)
Rollin
g
reduct
ion
per
pass
(%)
Time
passes
(s)
o. of
passes
Final
thickness
(mm)
14 F 250 1170 1144 962 77.6 17 943 781 *1 *2 14 12
15 G 120 1140 1096 1042 80 7 949 915 *1 *2 3 12
16 0 150 1150 1012 968 87 8 933 -885 *1 *2 2 12
17 H 180 1250 1195 1038 83.9 10 925 885 *1 *2 2 20
18 I 200 1110 1062 1016 77.5 8 942 911 *1 *2 4 20
19 J 245 1095 1045 1012 90.2 13 926 916 *1 *2 1 20
20 K 252 1080 1028 979 80.2 11 942 939 *1 *2 3 30
21 L 130 1160 1095 1058 42.1 3 936 876 *1 *2 5 30
22 M 274 1110 1074 1008 83.6 10 942 912 *1 *2 3 30
23 N 235 1185 1116 1011 69.4 8 950 873 *1 *2 3 40
24 J 180 1074 1011 979 65 7 922 886 *1 *2 4 40
Comp. 25 0 248 1165 1095 1056 75.8 7 938 932 *1 *2 2 40
ex. 26 P 120 1105 1091 1009 82 8 936 875 *1 *2 3 12
27 Q 150 1040 984 952 63.3 5 922 862 *1 *2 7 20
28 R 240 1020 982 971 82.5 10 941 938 *1 *2 2 30
29 S 180 1030 1005 978 72.2 6 948 911 *1 *2 3 30
30 T 180 1040 993 962 55.6 5 929 881 *1 *2 4 40
31 M 250 1175 1140 987 74.8 10 936 921 *1 *2 2 38
32 0 195 1180 1132 1014 77.9 11 949 856 *1 *2 4 27
33 J 175 1070 1021 971 88.6 12 924 872 *1 *2 6 14
34 B 270 1045 1030 979 81.9 11 937 852 *1 *2 7 32
35 N 220 1135 1105 1018 80.9 11 921 867 *1 *2 3 36
36 I 200 1090 1035 986 70 11 935 892 *1 *2 3 34
37 L 180 1140 1090 999 80.6 13 940 884 *1 *2 4 16
* Values of *1 indicated in Table 7, values of *2 indicated in Table 8.
* Underlines indicate outside scope of the present invention.
Table 6
First
stage
cooling
Second
stage
cooling
Tempering Microstructure percentage Hardness
Grain
size
Strength
Elongation
Toughness
Productivity
Class
Test
No.
Cast
slab
symbol
tart
temp.
(°C)
peed
(°C/s)
nd
temp.
( °C)
peed
(°C/s)
nd
temp,
(°C)
emp.
( °C)
Ferrite
area
percentage
(9)
Pearlite
area
percentage
(4)
Bainite
area
percentage
(9)
Martensite
area
percentage
(8)
ard
phase
hardness
Average
grain
size
(μm)
ield
stress
(MPa)
Total
elongation
(°a)
Fracture
appearance
transition
temp.
(°C)
olling
time
(s)
14 F 764 8 643 38 380 - 87 3 10 0 338 8 339 36 -82 277
15 G 897 20 - 20 250 450 33 3 54 10 332 12 602 15 -10 116.6
16 0 841 4 651 58 130 - 86 0 14 0 558 14 658 12 7 101.7
17 H 853 2 633 19 110 400 68 5 22 5 297 28 405 19 -18 359.2
18 I 670 1 644 23 280 500 89 3 7 1 253 31 301 28 -20 166.3
19 J 879 5 639 14 350 - 67 13 20 0 189 31 324 17 -22 168.9
20 K 906 1 656 25 60 550 54 3 31 . 12 184 35 342 16 -13 167.2
21 L 845 7 641 17 300 - 48 7 43 2 243 40 431 15 2 417.8
22 M 684 5 695 24 600 - 90 - - 10 0 0 246 29 309 29 -8 165.1
23 N 84 6 3 677 12 140 - '53 2 39 6 248 31 411 -11 427.2
24 J 829 8 - 8 370 - 75 2 23 0 241 27 346 -21 158.9
Comp. 25 0 Air cooli ng - 88 12 0 0 220 38 336 -7 149.6
ex. 26 P 839 5 638 24 40 - 98 2 0 0 218 45 238
M
14 109.3
27 Q 641 2 641 35 130 450 57 - 9 22 12 267 18 512 -4 100.7
28 R 909 6 664 18 320 - 62 . 3 27 8 195 16 334 -4 159.1
29 S 869 4 673 21 210 550 28 1 56 15 324 13 631 13 -7 109.3
30 T 838 1 658 15 370 - 74 8 15 3 425 19 502 14 -3 188.5
31 M 878 8 685 14 480 - 66 5 27 2 232 33 357 16 -8 195.6
32 0 822 6 653 17 530 - 64 4 32 0 226 34 359 16 -7 254.8
33 J 864 0.7 681 28 410 - 59 3 34 4 229 39 381 15 -3 188.6
34 B 827 5 693 545 - 51 2 47 0 197 32 378 15 -7 260.9
35 N 841 7 662
P
450 - 48 3 43 6 189 29 381 15 -7 222.3
36 I 865 0.9 620 400 - 92 6 2 0 2-1 17 263 38 -72 169.8
37 839 5 710 21 380 - 42 4 49 5 192 22 408 16 -30 140.4
Ns
I
* Underlines indicate outside scope of the present invention or deviated from prescribed values.
- 26 -
Table 7
Class
Test *1) Rolling reduction at each pass (%)
No. 1 2 3 4 5 6 7 8 9 10 11 12 13
1 20 15 17.6 14.3
2 21.1 20
3 17.9 13
4 20 21.9 20
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 11.1
.
8 15.4 16.4 13
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 10
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 21.1 20
17 17.2 16.7
18 15.6 15.8 15.6 14.8
19 16.7
20 16 14.3 16.7
21 19.8 20 15.4 18.2 16.7
22 15.6 10.5 11.8
23 19.4 17.2 14.6
24 11.1 10.7 10 11.1
Comp. 25 20 16.7
ex. 26 18.2 22.2 14.3
27 18.2 11.1 12.5 11.4 12.9 14.8 13
28 14.3 16.7
29 12 18.2 16.7
30 18.8 15.4 12.7 16.7
31 22.2 22.4
32 11.6 10.5 11.8 10
33 5 5.3 5.6 5.9 6.3 6.7
34 6.1 6.5 7 5 5.3 5.6 5.9
35 4.8 5 5.3
36 15 17.6 19
37 17.1 17.2 16.7 20
* Underlines shown outside of the scope of the present invention.
27 -
Table 8
Class
Test *2) Time between passes (s)
No. 1-2 2-3 3-4 4-5 5-6 6-7 7-8 8-9 F9-10 10-11 11-12 12-13
1 4.2 3.5 3.9
2 9.8
3 5.2
4 8.6 4.2
5 3.6 5.8 14.2
6 6.2 6.5 3.8 4.9
Inv.
ex
7 6.2 3.8 13.1
.
8 3.8 5.3
9 14.6
10 15.5 14.8 17.2
11 16.1 18.8 19.1 17.9
12 24.4
13 23.1 22.8
14 4.1 5.2 3.8 7.2 3.9 3.3 10.1 7.8 8.8 12.1 6.8 5.4
15 5.6 4.4
16 11.5
17 14.3
18 17.2 5.1 3.8
19
20 2.4 2.9
21 7.1 8.5 4.2 5.4
22 5.3 4.2
23 33.5 31.2
24 6.2 5.1 10.5
Comp. 25 5.8
ex. 26 5.4 12.3
27 5.4 8.5 5.1 4.3 6.9 4.2
28 3.8
29 5.6 12.5
30 4.8 5.9 21.5
31 27.9
32 25.6 28.2 24.9
33 9.9 10.1 5.1 8.2 8.3
34 15.2 16.9 15.1 16.4 16.9 16.4
35 24.8 23.9
36 7.5 6.8
37 4.8 3.4 5.9
* Underlines shown outside of the scope of the present invention.
Test No. 1 to No. 13 are invention examples which
satisfy all of the conditions of the present invention
5 and are excellent in strength, elongation, toughness, and
productivity.
- 28 -
Test No. 14 to No. 37 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
5 second stage rolling passes and had a low second stage
rolling end temperature, so was long in rolling time and
low in productivity.
Test No. 15 was too fast in first stage cooling
rate, so was small in ferrite area percentage, high in
10 strength, and low in elongation and toughness.
Test No. 16 was too fast in second stage cooling
rate, so was high in hardness of the hard phases and
strength and was low in elongation and toughness.
Test No. 17 was too high in slab heating
15 temperature, so was small in ferrite area percentage,
large in average grain size, low in elongation and
toughness, and, furthermore, long in rolling time and low
in productivity.
Test No. 18 was too low in first stage cooling start
20 temperature, so was large in average grain size and low
in strength and toughness.
Test No. 19 was small in number of passes of second
stage rolling, so was small in ferrite area percentage,
large in average grain size, and low in hardness of the
25 hard phases, elongation, and toughness.
Test No. 20 was short in time between passes in
second stage rolling, so was small in ferrite area
percentage, large in average grain size, and low in
hardness of the hard phases, elongation, and toughness.
30 Test No. 21 was small in cumulative rolling
reduction of first stage rolling, so was small in ferrite
area percentage, large in average grain size, low in
hardness of the hard phases, elongation, and toughness,
and, furthermore, long in rolling time and low in
35 productivity.
Test No. 22 was too high in second stage cooling end
temperature, so was large in average grain size and was
- 29 -
low in the hardness of the hard phases, strength, and
toughness.
Test Nos. 23, 31, and 32 were long in time between
passes in second stage rolling, so were small in ferrite
5 area percentage, large in average grain size, and low in
hardness, elongation, and toughness. Furthermore, Test
No. 32 was long in rolling time and low in productivity.
Test No. 24 was low in second stage cooling rate, so
was large in average grain size and was low in the
10 hardness of the hard phases and toughness.
Test No. 25 used air cooling for cooling, so was
large in average grain size and was low in the hardness
of the hard phases and toughness.
Test Nos. 26 to 30 had ranges of chemical
15 compositions outside the scope of the present invention,
so the ferrite area percentage, the hardness of the hard
phases, strength, elongation, or toughness failed to
satisfy the requirements demanded from the steel which is
produced by the present invention.
20 Test Nos. 33 to 35 had small rolling reductions at
the passes in the second stage rolling, so were small in
ferrite area percentage, large in average grain size, and
low in the hardness of the hard phases, elongation, and
toughness. Nos. 34 and 35 had times between passes which
25 were within the prescribed 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.
Test No. 36 was low in end temperature of the first
stage cooling, so was low in the hardness of the hard
30 phases and strength.
Test No. 37 was high in end temperature of the first
stage cooling, so was low in ferrite area percentage,
large in average grain size, and low in the hardness of
the hard phases, elongation, and toughness.
35 From the above examples, it was confirmed that
according to the method of production of the present
invention, by utilizing the refining action of y-
30 -
recrystallization to refine the microstructure by hightemperature
rolling in the y-recrystallization temperature
range and, furthermore, by making the accelerated cooling
after rolling a two-stage cooling of a first stage of
5 slow cooling and a second stage of rapid cooling so as to
secure the ferrite while hardening the second phases,
steel plate which is excellent in strength, elongation,
and toughness is obtained.
Note that the present invention is not limited to
10 the above embodiments. It can be worked changed in
various ways within a scope not deviating from the gist
of the present invention.
Industrial Applicability
15 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
number of passes is small and the rolling productivity is
20 high. According to the present invention, by utilizing
the refinement by y-recrystallization so as to refine the
microstructure by high-temperature rolling in the yrecrystallization
temperature range and, furthermore, by
making the accelerated cooling after rolling two-stage
25 cooling of a first stage of slow cooling and a second
stage of rapid cooling so as to secure ferrite while
hardening the second phases, it is possible to provide a
method of production of steel plate for welded structure
use which is excellent in strength, elongation, and
30 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 applicability is
therefore large.

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 0.16%,
Si: 0.01 to 0.5%,
Mn: 0.2 to 2.5%,
P: 0.03% or less,
10 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%,
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%,
Moo 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 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
- 32 -
cooling by first stage cooling from a plate
thickness center temperature of 750°C or more by a 0.5 to
8°C/s cooling rate down to 630 to 700°C, then
cooling by second stage cooling by a 10 to 50°C/s
5 cooling rate down to 550°C or less in temperature
so as to obtain to steel plate which has a plate
thickness of 10 to 40 mm, a yield stress of 315 to 550
MPa, a microstructure of a mixed microstructure_of one or,
more of soft phase ferrite and hard phase pearlte,
10 bainite, and martensite, an area percentage of ferrite at
the plate thickness center part of '70 to 95%, an average
Vicker's hardness of the hard phases of 250 to 500, and
an average grain size of 5 to 20 }im:
Ceq=C+Mn/6+(Cu+Ni)/15+(Cr+Mo+V)/5... (A)
15 Claim 2
A method of production of steel plate as set forth
in claim 1 characterized by tempering at 300 to 650°C
after said accelerated cooling ends.