TECHNICAL FIELD
The present invention relates to a clad steel plate having
excellent thermal conductivity which can be used for cookware
􀀘􀀃 and the like.
BACKGROUND ART
Unlike direct heating with gas or heating elements,
electromagnetic cookers indirectly heat a target object by
􀀔􀀓􀀃 virtue of electromagnetic induction. In an electromagnetic
cooker, temperature will not increase except for at the target
object, reducing risks of a burn and a fire. Further the
emission of carbon dioxide is reduced. These characteristics
appear to make electromagnetic cookers more popular.
􀀔􀀘􀀃 For a material of a pan and the like used with an
electromagnetic cooker, a thin stainless steel-clad steel plate
including a stainless steel mating material and a low-carbon
steel base material, and a clad steel plate having a stainless
steel mating material and a base material made of aluminum or
􀀕􀀓􀀃 an aluminum alloy are often used in view of corrosion
resistance and induction heating properties.
An electromagnetic cooker represents a safe and clean
heating means as described above, but suffers from a slower
heating rate and longer cooking time as compared with gas
􀀕􀀘􀀃 heating due to a limited power of the device as well as
difficult broth penetration into food materials. Accordingly,
a cookware product has recently been proposed including a
2
stainless steel-clad steel plate having improved induction
heating properties.
Meanwhile, a clad steel plate is known to have a use other
than cookware. For example, Patent Document 1 proposes a two-
􀀘􀀃 layer or three-layer clad steel plate including a base material
made of a mild steel having a carbon content of 0.005% or less
and a mating material(s) made of stainless steel or nickel or a
nickel alloy, the ratio (Al/N) of the amount of aluminum to the
amount of nitrogen in the base material being 6 or more, and
􀀔􀀓􀀃 the amount of nitrogen contained in the mating material(s)
being 0.01% or less.
Patent Document 2 proposes a stainless steel-clad steel
plate having an outer layer material of stainless steel and a
base material of low-carbon steel, the content of acid-soluble
􀀔􀀘􀀃 Al in the base material being 0.10 to 1.5% by weight.
Patent Document 3 proposes a clad steel material including
an upper material, an intermediate material, and a lower
material, the upper material including a ferrite-based
stainless steel containing 10.0 to 30.0% by weight of Cr and
􀀕􀀓􀀃 having a plate thickness of 0.3 to 3.0 mm, the intermediate
material including aluminum with a purity of 99% or more and
having a plate thickness of 1.0 to 10.0 mm, and the lower
material including a steel plate with a plate thickness of 3.0
to 30.0 mm.
􀀕􀀘􀀃 Patent Document 4 proposes a method of manufacturing a
patterned metal plate or a rainbow-colored metal plate, the
method including cold-rolling or skin pass-rolling a clad
3
material with a rolling roll having a pattern on a surface
thereof, the clad material having a thin coating of a metal on
a surface of an internal base material, the metal being softer
than the base material.
􀀘􀀃 Further, Patent Document 5 proposes a metal plate for a
cookware product and a method of manufacturing the same, the
metal plate having a large number of independent protrusions
formed on a surface of the metal plate corresponding to an
inner surface of the cookware product, and a flat continuous
􀀔􀀓􀀃 groove portion being formed between the respective independent
protrusions.
Patent Document 1: Japanese Examined Patent Application
Publication No. H05-14610
Patent Document 2: Japanese Unexamined Patent Application,
􀀔􀀘􀀃 Publication No. H11-77888
Patent Document 3: Japanese Unexamined Patent Application,
Publication No. S64-40188
Patent Document 4: Japanese Unexamined Patent Application,
Publication No. H02-263501
􀀕􀀓􀀃 Patent Document 5: Japanese Unexamined Patent Application,
Publication No. 2002-65469
DISCLOSURE OF THE INVENTION
Problems to be Solved by the Invention
􀀕􀀘􀀃 According to the clad steel plate of Patent Document 1,
the composition of the mild steel as the base material are
defined, and the clad steel plate is subjected to temper
4
rolling mainly for the purpose of improving workability, i.e.,
for the purpose of reducing development of a crack and wrinkle
upon drawing process or extensive bending process when used as
a kitchenware material such as a pan and kettle. However,
􀀘􀀃 Patent Document 1 does not provide information about a surface
profile of the clad steel plate.
According to the stainless steel-clad steel plate of
Patent Document 2, the contents of acid-soluble Al, C, Ti, and
N in the low carbon steel as the base material are defined in
􀀔􀀓􀀃 view of induction heating properties and workability. However,
Patent Document 2 does not provide information about the
surface profile or heat transfer properties of the steel sheet.
The clad steel material according to Patent Document 3
includes ferrite-based stainless steel and aluminum. However,
􀀔􀀘􀀃 in the clad steel material according to Patent Document 3, the
ferrite-based stainless steel has poor corrosion resistance,
and aluminum suffers from poor abrasion resistance, resulting
in possibly reduced cookware lifetime. Further, Patent
Document 3 does not provide information about the surface
􀀕􀀓􀀃 profile of the clad steel material.
The method of manufacturing a patterned metal plate or a
rainbow-colored metal plate via rolling process according to
Patent Document 4 is intended to improve designability by
performing rolling so that fine unevenness is formed on the
􀀕􀀘􀀃 surface of the clad plate. However, Patent Document 4 does not
provide information about heat transfer properties of the
patterned metal plate and the rainbow-colored metal plate.
5
In the metal plate for a cookware product and the method
of manufacturing the same according to Patent Document 5, a
large number of independent protrusions are formed on a surface
of the metal plate corresponding to the inner surface of the
􀀘􀀃 cookware product, and a flat continuous groove portion is
formed between the respective independent proportions. A steel
plate having an oxide film formed on a surface thereof may also
be used as the above metal plate. The above metal plate for a
cookware product may be manufactured by rolling a metal plate
􀀔􀀓􀀃 with an emboss roll. This metal plate for a cookware product
is suitable for manufacturing a cookware product having a
cooking surface which is resistant to burn dry. However,
thermal conductivity is not mentioned in Patent Document 5.
A cookware material having thermal conductivity superior
􀀔􀀘􀀃 to that of a conventional clad steel plate and capable of
electromagnetic induction heating has been desired.
An object of the present invention is to provide a clad
steel plate having excellent thermal conductivity which can be
suitably used for cookware and others.
􀀕􀀓􀀃 Means for Solving the Problems
The present inventors conducted extensive studies about a
clad steel plate having good thermal conductivity. During the
course of the studies, the present inventors focused on a plate
thickness ratio of a base material and mating materials in a
􀀕􀀘􀀃 three-layer clad steel plate having the base material and the
mating materials, the base material including low-carbon steel,
and the mating materials including stainless steel and being
6
each disposed on either surface of the base material. When a
three-layer clad steel plate having a plate thickness ratio L
of 1.0 to 5.0 wherein "the plate thickness ratio L = the
thickness of the base material / the total thickness of the
􀀘􀀃 mating materials" (Equation 1) was used as a basic material, a
clad steel plate was obtained having good thermal conductivity
and further excellent adhesiveness between the base material
and the mating materials. Moreover, the present inventors also
found that a plurality of protruded portions and depressed
􀀔􀀓􀀃 portions provided on a surface of at least one of the mating
materials of the basic material can further improve thermal
conductivity. Then the present invention has been completed.
Specifically, the present invention can provide the followings.
An embodiment of the present invention is a three-layer
􀀔􀀘􀀃 clad steel plate including: a carbon steel base material; and
stainless steel mating materials each disposed on either
surface of the base material, a plate thickness ratio L
represented by Equation (1) being 1.0 or more to 5.0 or less,
and a plurality of protruded portions and depressed portions
􀀕􀀓􀀃 being formed on at least one surface of the clad steel plate,
the plate thickness ratio L = the thickness of the base
material / the total thickness of the mating materials ∙∙∙
Equation (1),
wherein the thickness of the base material and the thicknesses
􀀕􀀘􀀃 of the mating materials are those at the protruded portions.
Further, the area of the plurality of protruded portions
is preferably 20 to 80% relative to the area of the surface of
7
the clad steel plate on which the protruded portions are formed.
Moreover, the plurality of protruded portions and
depressed portions preferably have a depression-protrusion
difference of 0.02 mm or more to 0.2 mm or less in the plate
􀀘􀀃 thickness direction.
Effects of the Invention
According to the clad steel plate of an embodiment of the
present invention, a plate thickness ratio L of 1.0 to 5.0
defined as the ratio of the thickness of the base material to
􀀔􀀓􀀃 the total thickness of the mating materials in the three-layer
clad steel plate including the low-carbon steel base material
and the stainless steel mating materials each disposed on
either surface of the base material can provide a clad steel
plate having a good heat transfer rate and excellent
􀀔􀀘􀀃 adhesiveness between the base material and the mating materials.
Further, provision of the plurality of protruded portions and
depressed portions on a surface of at least one of the mating
materials of the three-layer clad steel plate can reduce a
cross-sectional area of the three-layer clad steel plate,
􀀕􀀓􀀃 allowing for a further improved heat transfer coefficient.
Moreover, when the percentage (protrusion area percentage)
of the area of the protruded portions relative to the surface
area of a mating material is 20 to 80%, or the depressionprotrusion
difference is 0.02 to 0.2 mm in the plate thickness
􀀕􀀘􀀃 direction, even better thermal conductivity can be retained,
allowing for prolonged use.
8
BRIEF DESCRIPTION OF THE DRAWINGS
Fig. 1 shows the appearance of a clad steel plate produced
in an Example.
Fig. 2 schematically shows a cross section of a clad steel
􀀘􀀃 plate for illustrating the protrusion area percentage and
depression-protrusion difference of the clad steel plate.
Fig. 3 shows a two-dimensional profile of the surface of the
clad steel plate from an Example on which a depressionprotrusion
pattern is formed.
􀀔􀀓􀀃 Fig. 4 shows a two-dimensional profile of the surface of the
clad steel plate from another Example on which a depressionprotrusion
pattern is formed.
Fig. 5 schematically shows a heat transfer profile of a flat
clad steel plate.
􀀔􀀘􀀃 Fig. 6 schematically shows a heat transfer profile of a clad
steel plate having a depression-protrusion pattern formed on a
surface of a mating material.
Fig. 7 schematically shows a heat transfer profile of another
flat clad steel plate.
􀀕􀀓􀀃 Fig. 8 schematically shows a method of manufacture in which
embossing is performed to form depressions and protrusions on a
surface of a clad steel plate by the rolling process.
PREFERRED MODE FOR CARRYING OUT THE INVENTION
􀀕􀀘􀀃 Below, the embodiments of the present invention will be
described. The descriptions of these shall not limit the
present invention.
9
An embodiment of the present invention is a three-layer
clad steel plate including: low carbon steel as a base
material; and stainless steel as mating materials (hereinafter
may also be referred to as "outer layers") each disposed on
􀀘􀀃 either surface of the base material, in which a plate thickness
ratio L defined as the thickness of the base material / the
total thickness of the outer layers is 1.0 to 5.0 with regard
to the thicknesses of the base material and the mating
materials, and a plurality of protruded portions and depressed
􀀔􀀓􀀃 portions are formed on at least one surface of the clad steel
plate.
(Clad steel plate)
The three-layer clad steel plate according to an
embodiment of the present invention (hereinafter may also be
􀀔􀀘􀀃 referred to as the "three-layer clad steel plate") may be
manufactured mainly via hot rolling, annealing, and cold
rolling. Specifically, metal plates as raw materials of the
three layers are laminated, and a release material such as Ni
foil is further laminated on the both sides of these, which is
􀀕􀀓􀀃 then placed inside a bag made of a foil or thin sheet of a
metal having oxidation resistance such as stainless steel.
Subsequently, the bag is vacuum pumped, and then filled with an
inert gas such as nitrogen gas, and then heated from the
outside of the bag to allow laminated metal plates to be
􀀕􀀘􀀃 diffusion bonded. After diffusion bonded, hot rolling may be
performed to obtain a desired plate thickness, and then
annealing and cold rolling may be further repeated to obtain a
10
flat three-layer clad steel plate. It is noted that the last
step before the embossing process described below is preferably
an annealing step.
There is no particular limitation for the type of the base
􀀘􀀃 material of the three-layer clad steel plate. Steel plates
such as those made of low carbon steel, medium carbon steel,
high carbon steel, alloy steel, and the like can be used as the
base material. When good press-formability is required, deep
drawing steel plates such as those made of low carbon steel
􀀔􀀓􀀃 alloyed with Ti, low carbon steel alloyed with Nb, and the like
are preferred as an underlying steel plate. Further, highstrength
steel plates alloyed with P, Si, Mn, and the like may
be used.
When low carbon steel is used as the base material, for
􀀔􀀘􀀃 example, SPCC as defined in JIS G 3141 is preferred.
Specifically, those may be used having a carbon concentration
of 0.15% by mass or less, a manganese concentration of 0.60% by
mass or less, a phosphorus concentration of 0.10% by mass or
less, and a sulfur concentration of 0.05% by mass or less.
􀀕􀀓􀀃 There is no particular limitation for the type of
stainless steel used for the mating materials of the threelayer
clad steel plate. Ferrite-based, austenite-based, or
biphasic stainless steel plates may be used as the mating
materials, depending on usage environments of a clad steel
􀀕􀀘􀀃 plate. When the usage environments of the clad steel plate are
relatively mild with regard to corrosion, a ferrite-based
stainless steel plate, which is less expensive, can be used.
11
In usage environments where acid resistance and workability are
important, an austenite-based stainless steel plate may be
preferably used as the mating materials. Alternatively, when
high strength and pitting resistance are required, a biphasic
􀀘􀀃 stainless steel plate may be used as the mating materials.
Further, there is no particular limitation for the surface
finishing process of stainless steel plates as the mating
materials of the three-layer clad steel plate, and any known
means may be used. The thicknesses of two mating materials
􀀔􀀓􀀃 each disposed on either surface of the base material may be the
same or different. When the clad steel plate is to be
subjected to plastic working such as bending, stainless steel
plates with different thickness may also be used as the mating
materials depending on the method of working for plastic
􀀔􀀘􀀃 working and the shape after working.
(Embossing process)
The clad steel plate according to an embodiment of the
present invention has a plurality of protruded portions and
depressed portions on at least one surface thereof. Formation
􀀕􀀓􀀃 of a depression-protrusion pattern including protruded portions
and depressed portions on a surface of the clad steel plate can
further improve thermal conductivity from one side, e.g., a
heated side, of the clad steel plate to the other side, e.g., a
non-heated side. As a means of providing such a depression􀀕􀀘􀀃
protrusion pattern, for example, embossing process may be used.
Specifically, the rolling method, the press working, and the
like may be used.
12
With regard to a method of embossing, the rolling method
using a emboss roll has superior productivity. Even when an
emboss roll used for rolling is worn or damaged, the profile of
the emboss roll can be reworked by performing cutting or
􀀘􀀃 etching process. Therefore, the rolling method is also
preferred in view of cost reduction.
When the rolling method as described above is used, a
four-stage rolling machine, for example, as shown in Fig. 8 may
be used. This four-stage rolling machine is configured to
􀀔􀀓􀀃 include a stepped roll 40 as an upper work roll having a
depression-protrusion pattern and a flat roll 41 as a lower
work roll having no depression-protrusion pattern, both of
which have back-up rolls 42 and 43, respectively. The stepped
roll 40 has a roll profile in which a plurality of large􀀔􀀘􀀃
diameter portions and small-diameter portions are arranged in
the axis direction and the circumferential direction. A
predetermined depression-protrusion pattern is transferred to a
clad steel plate 46 fed to the four-stage rolling machine
through a pay-off reel 44 from the roll profile on the stepped
􀀕􀀓􀀃 roll 40 to obtain a clad steel plate 47 having a plurality of
protruded portions and depressed portions formed on a surface
thereof (hereinafter may also be referred to as a "depressionprotrusion
clad steel plate"). Then, the depression-protrusion
clad steel plate 47 having a plurality of protruded portions
􀀕􀀘􀀃 and depressed portions is retrieved in a take-up reel 45. It
is noted that when a depression-protrusion pattern is formed on
the both sides of the clad steel plate 46, a stepped roll (not
13
shown) instead of the flat roll 41 may be used as the lower
roll to perform the aforementioned rolling operations.
(Protrusion area percentage)
The percentage of the area of protruded portions on the
􀀘􀀃 clad steel plate relative to the surface area of the clad steel
plate on which the protruded portions are formed (hereinafter
may also be referred to as the "protrusion area percentage") is
preferably 20 to 80%. Fig. 2 is a schematic diagram for
illustrating the definition of the protrusion area percentage
􀀔􀀓􀀃 with reference to a structure where a plurality of protruded
portions and depressed portions are configured to be lined on a
surface of a steel plate. In general, the protrusion area
percentage refers to a percentage of the area of protruded
portions occupying a surface of a steel plate. As used herein,
􀀔􀀘􀀃 the protrusion area percentage is defined as a percentage of
the sum of an area W(1), W(2), ··· W(n) of the corresponding
protruded portions relative to the overall surface area W at a
height lower by 10% from a protrusion height H (H - h = 0.9 H)
wherein the protrusion height H is a distance between the top
􀀕􀀓􀀃 of a protruded portion and the bottom of a depressed portion,
and h (= 0.1 × H) is 10% of the protrusion height H as shown in
Fig. 2. That is, the protrusion area percentage can be
expressed as follows.
􀀕􀀘􀀃 The protrusion area percentage can be calculated as
follows. First, a two-dimensional profile where the vertical
14
axis corresponds to the plate thickness direction as shown in
Figs. 3 and 4 is obtained from a region arbitrary selected from
a surface of a clad steel plate. Then, a distance (the
protrusion height H) is measured between the top of a protruded
􀀘􀀃 portion and the bottom of a depressed portion for the protruded
portions shown in the above two-dimensional profile. The
protrusion height H is determined based on a distance at each
of the depressed portions located in either side of a protruded
portion. A reference point is then determined by subtracting
􀀔􀀓􀀃 0.1 times from the resulting protrusion height. Subsequently,
the sum of the area of a portion of each protruded portion
included between the reference points is divided by the overall
area of the selected region to calculate a protrusion area
percentage. In Figs. 3 and 4, an example of a portion used for
􀀔􀀘􀀃 measuring the area of a protruded portion is indicated as W(i).
Here, the data of the two-dimensional profile is linearly
approximated for protruded portions and depressed portions of a
clad steel plate, and the slope of the resulting straight line
is further corrected horizontally to identify a reference line
􀀕􀀓􀀃 M. Portions located above the reference line M on the surface
are considered as protruded portions to be measured.
A protrusion area percentage of less than 20% increases a
proportion of portions where depressed portions are formed with
large-diameter portions of a stepped roll upon forming a
􀀕􀀘􀀃 depression-protrusion pattern by the rolling method, resulting
in an increased burden to the roll due to increased rolling
load. This will reduce the life time of the roll, and result
15
in increased manufacturing cost. On the other hand, when the
protrusion area percentage is more than 80%, an effect for
improving thermal conductivity is small.
Depression-protrusion difference
􀀘􀀃 The depression-protrusion difference is preferably 0.02 to
0.2 mm in the plate thickness direction of a depressionprotrusion
clad steel plate. The depression-protrusion
difference corresponds to the protrusion height H as shown in
Fig. 2. The depression-protrusion difference as used herein is
􀀔􀀓􀀃 computed as follows. Two-dimensional profiles at 5 positions
over an area of an arbitrary region (for example, within the
range of 100 mm2) on a surface of a steel plate were measured
for determining a protrusion height at each position. The mean
value of the protrusion heights at these points was considered
􀀔􀀘􀀃 as the depression-protrusion difference. When the depressionprotrusion
difference is less than 0.02 mm, the clad steel
plate may be more susceptible to wear during use, and thus
prolonged use may be difficult. On the other hand, a
depression-protrusion difference of more than 0.2 mm may
􀀕􀀓􀀃 increase burden to a roll due to increased rolling load upon
forming a depression-protrusion pattern by the rolling method.
This will reduce the life time of the roll, and result in
increased manufacturing cost. Further, the clad steel plate
may be significantly work-hardened, which may have a
􀀕􀀘􀀃 significant impact on workability after depressions and
protrusions are formed.
A clad steel plate can retain good thermal conductivity,
16
and can be used for a prolonged time when it has a protrusion
area percentage of 20 to 80% and a depression-protrusion
difference of 0.02 to 0.2 mm as described above.
(Depression-protrusion pattern)
􀀘􀀃 As described above, a clad steel plate can be used in
which a depression-protrusion pattern including protruded
portions and depressed portions are formed by embossing process.
A pattern of the depression-protrusion pattern may be regular,
or may be partially or entirely random. For example, a random
􀀔􀀓􀀃 depression-protrusion pattern may be formed as shown in Fig. 1.
The cross section of two-dimensional profiles of a clad
steel plate having a depression-protrusion pattern on a surface
thereof are shown in Figs. 3 and 4. Fig. 3 shows the twodimensional
profile of a clad steel plate having a depression􀀔􀀘􀀃
protrusion pattern where the protrusion area percentage is
about 80%, and the depression-protrusion difference is about
0.06 mm. Fig. 4 shows the two-dimensional profile of a clad
steel plate having a depression-protrusion pattern where the
protrusion area percentage is about 55%, and the depression􀀕􀀓􀀃
protrusion difference is about 0.20 mm.
The depression-protrusion patterns in Figs. 3 and 4
represent examples where the protrusion area percentage falls
within the range of 20 to 80%, and the depression-protrusion
difference falls within the range of 0.02 to 0.20 mm.
􀀕􀀘􀀃 Formation of a plurality of depressed portions on a surface of
a clad steel plate reduces the cross-sectional area of the clad
steel plate, increasing the heat transfer rate of the clad
17
steel plate. Further, a depression-protrusion pattern formed
on a surface of a clad steel plate in a highly random manner in
the pattern direction can allow origins of water-boiling to be
widely distributed, leading to uniform boiling, for example,
􀀘􀀃 throughout the inside of a cookware product when that
depression-protrusion clad steel plate is used as a material of
the cookware product. This is effective in view of shortened
cooking time because heat is uniformly transferred to food
materials. Further, according to an embodiment of the present
􀀔􀀓􀀃 invention, a relatively large depression-protrusion pattern may
be formed on a clad steel plate. In particular, when the
depression-protrusion pattern is formed by the rolling method,
mechanical strength may also be enhanced by virtue of work
hardening. Therefore, a clad steel plate having excellent
􀀔􀀘􀀃 abrasion resistance which can withstand prolonged use can be
obtained.
(Plate thickness ratio L)
The plate thickness ratio L of a clad steel plate is
defined by the following Equation (1) using the thickness of a
􀀕􀀓􀀃 base material and the total thickness of mating materials:
the plate thickness ratio L = the thickness of the base
material / the total thickness of the mating materials ∙∙∙
Equation (1).
The mating materials are each disposed on either surface of the
􀀕􀀘􀀃 base material, and thus the total thickness of the mating
materials each disposed on either surface is used in Equation
(1). The thickness of the base material and the thicknesses of
18
the mating materials are those at protruded portions.
A plate thickness ratio L of less than 1.0 means that the
proportion of mating materials in a clad steel plate is large.
When mating materials are made of a material (stainless steel)
􀀘􀀃 having a relatively lower heat conductivity than a base
material (carbon steel), the heat conductivity of the resulting
clad steel plate may be decreased. Moreover, mating materials
may be detached from a base material at a working site due to
different ductility between the base material and the mating
􀀔􀀓􀀃 materials when comprehension stress is exerted on a clad steel
plate during work. On the other hand, a plate thickness ratio
of more than 5.0 means that the proportion of mating materials
is excessively small. This may result in breakage of the
mating materials (stainless steel plates) when a clad steel
􀀔􀀘􀀃 plate is subjected to plastic working such as bending.
Therefore, the plate thickness ratio L preferably falls within
the range of 1.0 to 5.0. The lower limit thereof is more
preferably 1.5, even more preferably 2.0. The upper limit
thereof is more preferably 4.0, even more preferably 3.5.
􀀕􀀓􀀃
EXAMPLES
Below, Examples of the present invention will be described.
The present invention shall not be limited to the following
Examples.
􀀕􀀘􀀃 First, the following four types of three-layer clad steel
plates were manufactured. It is noted that the four types of
three-layer clad steel plates manufactured each have a plate
19
thickness of 0.8 mm. Hereinafter, the "plate thickness ratio
L" may also be referred to as the "plate thickness ratio."
(1) A three-layer clad steel plate including a base material
made of a cold rolled steel plate of 0.64-mm thick SPCC (JIS G
􀀘􀀃 3141) (hereinafter referred to "SPCC") and mating materials
made of 0.08-mm SUS304 each disposed on either surface thereof;
and having a plate thickness ratio of 4.0.
(2) A three-layer clad steel plate including a base material
made of 0.56-mm thick SPCC and mating materials made of 0.12-mm
􀀔􀀓􀀃 SUS304 each disposed on either surface thereof; and having a
plate thickness ratio of 2.3.
(3) A three-layer clad steel plate including a base material
made of 0.48-mm thick SPCC and mating materials made of 0.16-mm
SUS304 each disposed on either surface thereof; and having a
􀀔􀀘􀀃 plate thickness ratio of 1.5.
(4) A three-layer clad steel plate including a base material
made of 0.72-mm thick SPCC and mating materials made of 0.04-mm
SUS304 each disposed on either surface thereof; and having a
plate thickness ratio of 9.
􀀕􀀓􀀃 Next, among these, the three-layer clad steel plates
having a plate thickness ratio of 4.0, 2.3, and 1.5 were
subjected to embossing process to form protruded portions and
depressed portions on a surface of the respective three-layer
clad steel plate using a four-stage rolling machine as shown in
􀀕􀀘􀀃 Fig. 8. Thereby, test pieces were obtained for the depressionprotrusion
clad steel plates of Examples 1 to 6. Working
conditions for the embossing process were as follows: the
20
diameter of a stepped roll in the four-stage rolling machine
was 110 mm, and the rolling load was set according to the
depression-protrusion differences shown in Table 1, and the
rolling rate was 0.5 m/min.
􀀘􀀃 Comparative Examples 1 and 2 are three-layer clad steel
plates without embossing process.
It is noted that the plate thickness ratio at protruded
portions of the depression-protrusion clad steel plates did not
differ from that of the three-layer clad steel plates before
􀀔􀀓􀀃 forming a depression-protrusion pattern.
The protrusion area percentage and depression-protrusion
difference of the depression-protrusion pattern formed on a
surface of the depression-protrusion clad steel plate
manufactured were measured from a profile obtained by expanding
􀀔􀀘􀀃 a region with an area of 100 mm2 arbitrarily selected from the
clad steel plate using a profile measuring machine (contracer).
Examples of the two-dimensional profile, the two-dimensional
profiles of the clad steel plates from Examples 1 and 4 in
which the vertical axis corresponds to the plate thickness are
􀀕􀀓􀀃 shown in Figs. 3 and 4, respectively. Various properties of
the depression-protrusion clad steel plates manufactured are
shown in Table 1.
[Table 1]
Plate thickness
ratio L
Depressionprotrusion
difference (mm)
Protrusion area
percentage (%)
21
Example 1 4.0 0.06 80
Example 2 2.3 0.06 80
Example 3 1.5 0.06 80
Example 4 4.0 0.20 55
Example 5 2.3 0.20 55
Example 6 1.5 0.20 55
Comparative
Example 1
1.5 - -
Comparative
Example 2
9.0 - -
(Tests for evaluating heat transfer properties)
Tests for evaluating a heat transfer rate were performed
according to the following procedure. A test piece having a
􀀘􀀃 standardized and predetermined surface area (A) and thickness
(B) was produced, and thermocouples were each attached to
either surface thereof, i.e., either a front or back surface
thereof. Then, the test piece was heated on an electromagnetic
cooker at a constant power (E) to measure temperature at the
􀀔􀀓􀀃 both surfaces, i.e., the front and back surfaces of the test
piece. A temperature difference (D) was obtained from the
measured temperature values, and a heat transfer rate (C) was
computed by the following equation (2). The larger is the heat
transfer rate (C), the larger is the amount of heat transferred
􀀔􀀘􀀃 from a heated side to a non-heated side through a test piece.
Therefore, a larger heat transfer rate indicates that heat
transfer is facilitated.
22
C = (E × B)/(A × D) ··· Equation (2)
In the heating test, a square test piece with a side
length of 50 mm was cut out from a depression-protrusion clad
steel plate having a depression-protrusion pattern, and this
􀀘􀀃 was placed on an electromagnetic cooker set at a power of 500 W,
and then heated until a heated surface of the test piece
reached a target temperature. The target temperature was set
at 100ºC, 300ºC, and 500ºC because a heat transfer rate changes
depending on heating temperature.
􀀔􀀓􀀃 Heat transfer properties were evaluated based on the
computed heat transfer rate (W/m·K). The evaluation results
are shown in Table 2. Evaluation criteria were as follows:
those having a heat transfer rate improved by 50% or more
relative to Comparative Example 1 were designated as " very
􀀔􀀘􀀃 good "; those improved by 25% or more but less than 50% were
designated as " good "; those improved by 5% or more but less
than 25% were designated as " fair "; and those improved by
less than 5% were designated as " poor."
(Tests for evaluating workability)
􀀕􀀓􀀃 Workability was evaluated according to the following
procedure. A rectangular test piece with a longer side length
of 50 mm and a shorter side length of 20 mm was cut out from a
depression-protrusion clad steel plate having a depressionprotrusion
pattern. The above rectangular test piece was
􀀕􀀘􀀃 subjected to contact bending at the center portion of the
longer side thereof, and then the presence or absence of a
crack at the working site around the center portion was
23
visually observed for evaluation. The evaluation results are
shown in Table 2. A test piece having no observable crack was
designated as " good "; those having an observable crack was
designated as " poor."
􀀘􀀃 [Table 2]
Evaluation of thermally conductivity
Evaluatio
n of
workabili
ty
100ºC 300ºC 500ºC
Heat
transfer
rate
(W/m􀣭
K)
Evaluat
ion
Heat
transfer
rate
(W/m􀣭
K)
Evaluat
ion
Heat
transfer
rate
(W/m􀣭
K)
Evaluat
ion
Example 1 53.7
very
good
50.5 good 44.2 good good
Example 2 43.2 fair 41.6 fair 37.9 fair good
Example 3 37.6 fair 36.6 fair 33.7 fair good
Example 4 56.7
very
good
53.3
very
good
46.6 good good
Example 5 45.6 good 43.9 good 40.0 good good
Example 6 39.7 fair 38.7 fair 35.6 fair good
Comparative
Example 1
35.7 􃸫 34.8 􃸫 32.0 􃸫 good
Comparative
Example 2
61.0
very
good
60.0
very
good
42.0 good poor
(Evaluation results of thermal conductivity)
24
As shown in Table 1, Examples 1 to 6 are depressionprotrusion
clad steel plates each having a depressionprotrusion
pattern formed on a surface of a steel plate and
having a plate thickness ratio L of 1.0 to 5.0. As shown in
􀀘􀀃 Table 2, the depression-protrusion clad steel plates from
Examples 1 to 6 were found to have better thermal conductivity
as compared with the clad steel plate having no depressionprotrusion
pattern from Comparative Example 1.
A depression-protrusion clad steel plate, which is a
􀀔􀀓􀀃 composite material, has a heat transfer property in which a
thermal gradient θ varies across the plate thickness direction.
When the heat transfer rate of the base material (SPCC) is
compared with that of the mating materials (SUS304), the heat
transfer rate of SUS304 is smaller than that of SPCC. This
􀀔􀀘􀀃 means that the thermal gradient θ1 of the mating materials
(stainless steel) > the thermal gradient θ2 of the base
material (carbon steel).
Figs. 5 and 6 both schematically show how heat is
transferred through a clad steel plate having a thickness ratio
􀀕􀀓􀀃 L of 4.0 in the heating test. A heating direction Z from a
heat source (not shown) to a heated surface X is shown in Figs.
5 and 6. In such heating, the difference D between a
temperature t1 of the heated surface X of a three-layer clad
steel plate 1 having no depression-protrusion pattern and a
􀀕􀀘􀀃 temperature t0 of the opposite surface is appropriately the
same at any position in the plate width direction as shown in
Fig. 5. However, in the depression-protrusion clad steel plate
25
2 having a depression-protrusion pattern, a temperature
difference D' at a portion (depressed portion) having a small
plate thickness is smaller than the temperature difference D at
a portion (protruded portion) having a large plate thickness
􀀘􀀃 because the thickness of the carbon steel is smaller in the
plate thickness direction as shown in Fig. 6. As described
above, the depression-protrusion clad steel plate having a
depression-protrusion pattern shows a thermal gradient varying
both in the plate thickness direction and the plate width
􀀔􀀓􀀃 direction.
Fig. 7 schematically shows how heat is transferred through
a three-layer clad steel plate 3 without a depressionprotrusion
pattern having a plate thickness ratio L of 1.5 in
the heating test. The plate thickness B of the three-layer
􀀔􀀘􀀃 clad steel plate 3 is the same as that of the three-layer clad
steel plate 1 in Fig. 5. The thermal gradient in the plate
thickness direction may also vary depending on the plate
thickness ratio. Accordingly, the temperature difference
between temperatures of the heated surface X and the opposite
􀀕􀀓􀀃 surface may also be different. When the plate thickness ratio
L is smaller than that of the three-layer clad steel in Fig. 5,
the proportion of the mating materials (stainless steel) having
a small heat transfer rate is large even in a case where the
plate thickness B is the same. Therefore, a temperature
􀀕􀀘􀀃 difference D'' will be larger than the temperature difference D.
As described above, a depression-protrusion clad steel
plate having better thermal conductivity can be obtained when a
26
depression-protrusion pattern having a large depressionprotrusion
difference and a small protrusion area percentage is
formed on a surface of the clad steel plate, and further a base
material having a large heat transfer rate such as carbon steel
􀀘􀀃 is used so as to have a high plate thickness ratio.
(Evaluation results of workability)
Results from workability evaluation by contact bending are
shown in Table 2 for the depression-protrusion clad steel
plates from Examples and the clad steel plates from Comparative
􀀔􀀓􀀃 Examples. Examples 1 to 6 and Comparative Example 1 did not
show a crack in stainless steel plates as the mating materials.
In contrast, Comparative Example 2 showed a crack in stainless
steel at the outer side of the test piece which was bent.
Comparative Example 2 has a plate thickness ratio (carbon
􀀔􀀘􀀃 steel/stainless steel) of 9.0 and a relatively small thickness
of the mating materials (stainless steel). When contact
bending is performed on a test piece, the tensile stress
exerted on the surface of a test piece will concentrate on a
mating material. Therefore, the concentrated stress exerted on
􀀕􀀓􀀃 the test piece of Comparative Example 2 presumably exceeded the
mechanical strength of the mating material (stainless steel),
resulting in a crack. Further, contact bending was found to be
possible when the plate thickness ratio (carbon steel/stainless
steel) was 7.0 or less.

I/We Claim:
1. A three-layer clad steel plate comprising: a carbon steel
base material; and stainless steel mating materials each
􀀘􀀃 disposed on either surface of the base material,
a plate thickness ratio L represented by Equation (1)
being 1.0 or more to 5.0 or less,
a plurality of protruded portions and depressed portions
being formed on at least one surface of the clad steel plate,
􀀔􀀓􀀃 the plate thickness ratio L = the thickness of the base
material / the total thickness of the mating materials ∙∙∙
Equation (1),
wherein the thickness of the base material and the
thicknesses of the mating materials are those at the protruded
􀀔􀀘􀀃 portions.
2. The clad steel plate according to claim 1, wherein the
area of the plurality of protruded portions is 20 to 80%
relative to the area of a surface of the clad steel plate on
􀀕􀀓􀀃 which the protruded portions are formed.
3. The clad steel plate according to claim 1 or 2, wherein
the plurality of protruded portions and depressed portions have
30
a depression-protrusion difference of 0.02 mm or more to 0.2 mm
or less in the plate thickness direction.