TECHNICAL FIELD
[0001] The present invention relates to an intermediate material for stainless steel for
edged tools, which is used in, for example, razors, cutters, kitchen knives, and knives.
BACKGROUND ART
[0002] Martensitic stainless steel has been widely used as a material for edged tools such
as razors, cutters, kitchen knives, and knives. In particular, a strip of high-carbon
martensitic stainless steel containing approximately 13% by mass of Cr and approximately
0.65% by mass of C is known to be most suitable as a material for razors. The high-carbon
martensitic stainless steel used in such applications (hereinafter referred to as
"stainless steel for edged tools") is usually subjected to hardening and tempering
before use. The stainless steel for edged tools is required to have high hardness
when in use.
[0003] The stainless steel for edged tools is usually manufactured through the following
manufacturing processes.
[0004] First, a raw material is melted and cast thereby to manufacture a material. Next,
the material is hot-rolled thereby to manufacture an intermediate material. The material
may be hot-forged or hot-rolled through a blooming process.
[0005] Next, the intermediate material is subjected to an initial annealing thereby to manufacture
an annealed material. The annealed material is repeatedly subjected to cold rolling
followed by strain-removal annealing the necessary number of times thereby to manufacture
a cold-rolled steel strip having an intended thickness. Then, the cold-rolled steel
strip is subjected to hardening and tempering to produce stainless steel for edged
tools.
[0006] Furthermore, the stainless steel for edged tools is subjected to processing processes
such as sharpening and cutting, and thus becomes an end-product. It is noted that,
in general, trading in the market of stainless steel for edged tools is often conducted
in the form of either an annealed material or a cold-rolled steel strip.
[0007] For the above-described stainless steel for edged tools, there has been proposed
a technique of achieving high hardness by heat treatment for a short time during hardening.
For example, as a representative example,
JP-A-5-39547 (Patent Literature 1) discloses that heat treatment during hardening can be performed
for a short time by controlling the carbide density of steel for stainless razors.
CITATION LIST
PATENT LITERATURE
NON-PATENT LITERATURE
SUMMARY OF INVENTION
PROBLEMS TO BE SOLVED BY THE INVENTION
[0010] As described above, with respect to the shortened hardening processing time and increased
hardness of the stainless steel for edged tools, there have been proposed various
techniques that focus on the features of the cold-rolled steel strip.
[0011] However, there have been few studies that focus on the features of the intermediate
material after hot rolling and before annealing. The relationship between the features
of the intermediate material and the properties of the annealed material for stainless
steel for edged tools after annealing and before hardening, which is commercially
available as a semi-finished product, has not been satisfactorily elucidated. The
relationship between the features of the intermediate material and the carbide distribution
of the cold-rolled steel strip has also not been satisfactorily elucidated.
[0012] For this reason, there has been a problem that poor knowledge on what the features
of the intermediate material should be like inhibits excellent hardening properties
that the stainless steel for edged tools originally has from being satisfactorily
elicited.
[0013] An object of the present invention is to provide an intermediate material for stainless
steel for edged tools having an excellent carbide distribution, of which the hardness
can be increased by heat treatment for a short time during hardening.
SOLUTIONS TO THE PROBLEMS
[0014] The present inventors studied the relationship between the carbide distribution that
influences the hardenability and hardness of stainless steel for edged tools and the
intermediate material for stainless steel for edged tools that influences the carbide
distribution.
[0015] First, the present inventors have ascertained that, among the features of the intermediate
material for stainless steel for edged tools, the strain amount before annealing influences
the carbide distribution after annealing of the intermediate material.
[0016] Then, the present inventors have found that, by allowing strain to be retained in
a final pass of hot rolling for the intermediate material for stainless steel for
edged tools substantially including an FCC phase, the carbide distribution after annealing
can be improved when a KAM value by an SEM-EBSD method is 0.5° or more or when a half-value
width of a (200) plane of the FCC phase in X-ray diffraction becomes 0.3° or more,
and thus have accomplished the present invention.
[0017] According to an aspect of the present invention, an intermediate material for stainless
steel for edged tools, which substantially includes an FCC phase and is a material
after hot rolling and before annealing, has a composition of, in % by mass, 0.46 to
0.72% of C, 0.15 to 0.55% of Si, 0.45 to 1.00% of Mn, 12.5 to 13.9% of Cr, 2:0 to
2.0% of Mo and W, and a remainder of Fe and impurities. In addition, a KAM value by
an SEM-EBSD method in a position at 1/4 in depth of a plane thickness from a surface
of a rolled plane is 0.5° or more.
[0018] According to another aspect of the present invention, in an intermediate material
for stainless steel for edged tools which is a material after hot rolling and before
annealing, a half-value width of a (200) plane of a FCC phase in X-ray diffraction
in a position at 1/4 in depth of a plane thickness from a surface of a rolled plane
is 0.3° or more.
EFFECTS OF THE INVENTION
[0019] The stainless steel for edged tools manufactured using the intermediate material
for stainless steel for edged tools according to the present invention can be increased
in hardness by heat treatment for a short time during hardening. Therefore, the intermediate
material for stainless steel for edged tools according to the present invention is
most suitable especially in applications such as razors having a thin thickness.
BRIEF DESCRIPTION OF THE DRAWINGS
[0020]
FIG. 1 is a schematic diagram illustrating a position at which a test piece is sampled
and an evaluated plane.
FIG. 2 is a drawing-substitute photograph illustrating an example of a metal structure
of an annealed material of an intermediate material for stainless steel for edged
tools according to the present invention.
FIG. 3 is a drawing-substitute photograph illustrating an example of a metal structure
of an annealed material of an intermediate material for stainless steel for edged
tools according to a comparative example.
FIG. 4 is a drawing-substitute photograph illustrating an example of a metal structure
of an annealed material of an intermediate material for stainless steel for edged
tools according to the present invention.
DESCRIPTION OF EMBODIMENTS
[0021] As described above, one of the important characteristics of the present invention
is that the carbide distribution in the intermediate material after annealing is improved
by controlling the residual strain amount in the intermediate material before annealing.
[0022] First, the KAM (kernel average misorientation) value which is most characteristic
is described below.
< KAM value by SEM-EBSD method is 0.50° or more>
[0023] In the present invention, the residual strain plays an important role. With respect
to a KAM value defined in the present invention, for example, the KAM value by the
SEM (scanning electron microscope)-EBSD (electron backscatter diffraction) method
is described as a measurement method of the residual strain in Non-Patent Literature
1. According to the studies conducted by the present inventors, it has been confirmed
that the KAM value by the SEM-EBSD method of the intermediate material for stainless
steel for edged tools having the above-described composition is correlated with the
carbide distribution of the annealed material for stainless steel for edged tools
obtained using the above-described intermediate material.
[0024] Specifically, when the KAM value by the SEM-EBSD method of the intermediate material
for stainless steel for edged tools is less than 0.50°, the intermediate material
can be said to be in a state of having small residual strain. When annealing is thereafter
performed, coarse carbides are likely to be precipitated at grain boundaries, compared
to a material having a large residual strain amount. As a result, for example, toughness
decreases after hardening and tempering performed when used in edged tools. Therefore,
an average value of the KAM values by the SEM-EBSD method needs to be 0.50° or more.
It is noted that the larger KAM value causes larger residual strain, and is therefore
preferred. However, when the KAM value exceeds 2.00°, the residual strain is likely
to become large in variation depending on the position. Therefore, the upper limit
of the KAM value is preferably 2.00° or less.
[0025] Next, the half-value width is described below.
<Full-width at half maximum of (200) plane of FCC phase in X-ray diffraction is 0.3°
or more>
[0026] In the present invention, the residual strain plays an important role. It is known
that there is a correlation between the half-value width and the residual strain.
According to the studies conducted by the present inventors, it has been confirmed
that the half-value width in X-ray diffraction of the intermediate material for stainless
steel for edged tools having the above-described composition is correlated with the
carbide distribution of the annealed material for stainless steel for edged tools
obtained using the above-described intermediate material.
[0027] Specifically, when the half-value width of the (200) plane of the FCC phase in X-ray
diffraction of the intermediate material for stainless steel for edged tools is less
than 0.3°, the intermediate material can be said to be in a state of having small
residual strain. When annealing is thereafter performed, coarse carbides are likely
to be precipitated at grain boundaries, compared to a material having a large residual
strain amount. As a result, for example, toughness decreases after hardening and tempering
performed when used in edged tools. Therefore, the half-value width in X-ray diffraction
of the (200) plane of the FCC phase needs to be 0.3° or more. It is noted that the
larger half-value width causes larger residual strain, and is therefore preferred.
However, when the half-value width exceeds 1.0°, the residual strain is likely to
become large in variation depending on the position. Therefore, the upper limit of
the half-value width is preferably 1.0° or less.
<Position at 1/4 in depth of plate thickness from surface of rolled plane>
[0028] In the present invention, the measurement of the KAM value by the SEM-EBSD method
or the measurement of the half-value width of the (200) plane of the FCC phase in
X-ray diffraction as described above is performed in a position at 1/4 in depth of
the plane thickness from the surface of the rolled plane.
[0029] The "rolled plane" in the present invention refers to, as illustrated in FIG. 1,
a plane with which a rolling roll is in contact during rolling of the intermediate
material for stainless steel for edged tools. The reason why the rolled plane side
is used for evaluation is that, since the strain amount introduced by rolling is non-uniform
in a thickness direction, settling the evaluated plane and the thickness enables the
evaluation to be performed under the same conditions.
[0030] In addition, the position at 1/4 in depth of the plane thickness from the surface
is selected in the present invention because the vicinity of the surface has large
strain introduced during hot rolling thereby to decrease the particle diameter of
crystals generated by recrystallization, and is therefore not suitable for the measurement
of the KAM value and the half-value width. It is also because the middle position
of the plate thickness, on the other hand, has less rolling reduction during the final
pass so that a difference in strain amount due to whether the final pass is performed
or not is small compared to the position at 1/4 of the plate thickness, and accordingly,
a difference in KAM value or in half-value width is unlikely to be produced.
[0031] With respect to the half-value width of the (200) plane of the FCC phase in X-ray
diffraction, the position at 1/4 in depth of the plane thickness from the surface
is selected from the same reason as described above. The vicinity of the surface has
large strain introduced during hot rolling thereby to decrease the particle diameter
of crystals generated by recrystallization, and is therefore not suitable for the
measurement of the half-value width. On the other hand, the middle position of the
plate thickness has less rolling reduction during the final pass so that a difference
in strain amount due to whether the final pass is performed or not is small compared
to the position at 1/4 of the plate thickness, and accordingly, a difference in half-value
width is unlikely to be produced.
[0032] The (200) plane of the FCC phase is selected in the measurement of the half-value
width because the above-described orientation has a peak that provides the highest
intensity in X-ray diffraction in the alloy system of the composition defined in the
present invention. The peak intensity is low outside the (200) plane, and therefore
the effect by a difference in strain amount on the half-value width is smaller compared
to the (200) plane. Thus, the measurement of the half-value width on the (200) plane
is sufficient.
[0033] Next, the alloy composition that provides fundamental characteristics defined in
the present invention is described below. The content of each element is in % by mass.
<C: 0.46 to 0.72%>
[0034] The C content is 0.46 to 0.72% for achieving the hardness sufficient as an edged
tool and minimizing the crystallization of eutectic carbides during casting and solidification.
When the C content is less than 0.46%, the hardness sufficient as an edged tool cannot
be obtained. When the content exceeds 0.72%, the increase of the crystallization amount
of the eutectic carbides in a balance with the Cr amount causes edge chipping during
sharpening. The lower limit of the C content is preferably 0.50%, and more preferably
0.65%. The upper limit of the C content is preferably 0.70%.
<Si: 0.15 to 0.55%>
[0035] Si is added as a deoxidizing agent during smelting. For obtaining sufficient deoxidizing
effect, Si is retained in an amount of 0.15% or more. On the other hand, when the
content exceeds 0.55%, the increase of the inclusion amount causes edge chipping during
sharpening. Therefore, the Si content is set to be 0.15 to 0.55%. Also, Si has the
effect of increasing the tempering softening resistance. When Si is added in an amount
of 0.20% or more, the hardness can be further increased. Therefore, the lower limit
of the Si content is preferably 0.20%, and the upper limit of the Si content is preferably
0.35%.
<Mn: 0.45 to 1.00%>
[0036] Mn is added as a deoxidizing agent during smelting in a similar manner to Si. For
obtaining sufficient deoxidizing effect, Mn is retained in an amount of 0.45% or more.
On the other hand, when the content exceeds 1.00%, hot workability decreases. Therefore,
the Mn content is set to be 0.45 to 1.00%. The lower limit of the Mn content is preferably
0.65%, and the upper limit of the Mn content is preferably 0.85%.
<Cr: 12.5 to 13.9%>
[0037] The Cr content is 12.5 to 13.9% for achieving sufficient corrosion resistance and
minimizing crystallization of eutectic carbides during casting and solidification.
When the Cr content is less than 12.5%, sufficient corrosion resistance as stainless
steel cannot be obtained. When the content exceeds 13.9%, the increase of the crystallization
amount of the eutectic carbides causes edge chipping during sharpening. The lower
limit of the Cr content is preferably 13.0%, and the upper limit of the Cr content
is preferably 13.6%.
<Mo+W/2: 0 to 2.0%>
[0038] Mo and W may not be added (0%). However, these elements improve corrosion resistance,
and therefore can be added as necessary to an upper limit of 2.0%. When the Mo+W/2
content exceeds 2.0%, solid solution strengthening and deformation resistance are
increased. Accordingly, hot workability deteriorates. Therefore, the content of Mo+W/2
is set to be 0 to 2.0%.
[0039] Other than the elements described above, Fe and impurities are contained.
[0040] Examples of typical impurity elements include P, S, Ni, V, Cu, Al, Ti, N, and O.
Mixing-in of these elements is unavoidable. However, the contents of the impurity
elements are preferably controlled in the following ranges: P≤0.03%, S≤0.005%, Ni≤0.15%,
V≤0.2%, Cu≤0.1%, Al≤0.01%, Ti≤0.01%, N≤0.05%, and O≤0.05%.
[0041] The following describes an intermediate material for stainless steel for edged tools
according to the present invention and a typical method for manufacturing an annealed
material using the intermediate material.
[0042] First, a material for stainless steel for edged tools is manufactured by melting
and casting. Examples of the melting include vacuum melting, air melting, vacuum arc
remelting, and electroslag remelting. Examples of the casting include die casting
and continuous casting, by which the material is obtained. The obtained material may
be subjected to homogenization heat treatment as necessary. The material may be further
subjected to a blooming process by hot forging or hot rolling.
[0043] Thereafter, the material is subjected to hot rolling. The hot rolling is performed
so that the rolling reduction is 80% or more, and the temperature of the material
after hot rolling (material temperature) is 1000 to 1250°C. Then, in final hot rolling,
hot rolling is performed at a material temperature of 900°C or less and a rolling
reduction of 10% or more. Accordingly, an intermediate material for stainless steel
for edged tools is manufactured.
[0044] The temperature in the final hot rolling is set at 900°C or less in order to introduce
residual strain into the material. In the temperature range exceeding 900°C, dynamic
recovery and recrystallization are likely to occur. For this reason, residual strain
is unlikely to be introduced. Also, the rolling reduction is set at 10% or more because,
at the rolling reduction less than 10%, residual strain is not sufficiently introduced,
thereby causing carbides to concentrate on grain boundaries during annealing.
[0045] When such hot rolling is performed, pearlite transformation does not sufficiently
occur. For this reason, the intermediate material is substantially an FCC phase. It
is noted that "the intermediate material substantially includes an FCC phase" described
in the present invention means that 80% by volume or more of the FCC phase is measured
by an X-ray diffraction apparatus. At this time, the remainder is martensite formed
during cooling. A specific evaluation method therefor is described below in later-described
examples.
[0046] The intermediate material for stainless steel for edged tools manufactured by the
above-described manufacturing method is subjected to an annealing process at 800 to
860°C for one to 100 hours. Accordingly, there is manufactured an annealed material
of stainless steel for edged tools containing precipitated carbides.
[0047] Furthermore, a cold-rolled steel strip having a thickness of less than 0.5mm for
stainless steel for edged tools can be manufactured using the above-described annealed
material by repeating cold rolling and annealing.
[0048] When the cold-rolled steel strip for stainless steel for edged tools is subjected
to hardening, tempering, and sharpening to provide an edged tool, the cold-rolled
steep strip may be subjected to the sub-zero treatment after hardening and the coating
of the surface after tempering, as necessary.
Examples
[0049] The present invention is further described below in detail with reference to the
following examples.
[0050] A steel ingot (material) having a chemical composition shown in Table 1 was produced
by melting.
[Table 1]
(% by mass) |
|
C |
Si |
Mn |
Cr |
Mo |
W |
Remainder |
Composition 1 |
0.69 |
0.33 |
0.75 |
13.22 |
0.01 |
0.02 |
Fe and unavoidable impurities |
Composition 2 |
0.50 |
0.50 |
0.89 |
13.39 |
1.30 |
0.06 |
Same as above |
[0051] The steel ingot was subjected to a hot blooming process to produce a hot rolling
material with a width of 350 mm and a thickness of 50 mm. There were produced two
rolling materials having the composition of Composition 1, and one rolling material
having the composition of Composition 2.
[0052] The hot rolling material of Composition 1 was heated to 1200°C, and was subjected
to hot rolling at a total rolling reduction ratio of 95% (the temperature of the material
after this hot rolling (material temperature) was 1050°C). Thereafter, final hot rolling
was performed at a material temperature of 850°C and a rolling reduction ratio of
15% thereby to produce an intermediate material A according to the present invention.
[0053] As a comparative example, an intermediate material B was produced in a process in
which the final hot rolling process was omitted. In this process, the hot rolling
material of Composition 1 was heated to 1200°C to be subjected to hot rolling. As
a result, the intermediate material B was produced in which the material temperature
of hot rolling was 1050°C and the total rolling reduction ratio was 95%.
[0054] Furthermore, the hot rolling material of Composition 2 was heated to 1200°C, thereby
subjected to hot rolling at a total rolling reduction ratio of 95% (the temperature
of the material after this hot rolling (material temperature) was 1050°C). Thereafter,
final hot rolling was performed at a material temperature of 850°C and a rolling reduction
ratio of 15% thereby to produce an intermediate material C according to the present
invention.
[0055] A test piece was sampled in the vicinity of the center in width for each of the intermediate
materials 1A, B, and C for stainless steel for edged tools. The sampling position
of the test piece is a position illustrated in FIG. 1. A vertical section 2 is an
evaluated plane of a metal structure observation plane. A rolled plane 3 is an evaluated
plane for the EBSD and the X-ray diffraction.
[0056] The metal structure was observed on the vertical section of the sampled test piece.
Also, the position at 1/4 in depth of the plate thickness from the rolled plane of
the test piece to be used in the EBSD and the X-ray diffraction was prepared by mirror
polishing followed by electrolytic polishing. Table 2 shows the KAM value by the EBSD
method, the half-value width, and the FCC amount by X-ray diffraction for each sample.
[0057] In the above-described metal structure observation, the vertical section of the test
piece was polished to be a mirror finished surface and then corroded with an aqueous
solution of ferric chloride to perform observation using an optical microscope.
[0058] The measurement of the KAM value was performed using an SEM (Model No. "ULTRA 55")
manufactured by ZEISS, and an EBSD measurement and analysis system OIM (Orientation-Imaging-Micrograph)
manufactured by TSL. In each region delimited in a hexagon as a measurement region,
Kikuchi patterns formed by electrons reflected from electron beams incident on the
sample surface were obtained to measure the orientations in the region. The measured
orientation data were analyzed using the analysis software OIM Analysis of the above-described
system. The measurement area was 100 µm×100 µm. The distance between adjacent pixels
was 0.2 µm. The boundary having a misorientation between adjacent pixels of 5° or
more was considered as a crystal grain boundary.
[0059] It is noted that, as the KAM value, an average value of the misorientations between
an individual measurement point and a proximate measurement point excluding the crystal
grain boundary was calculated. This calculated average value was an average value
in all regions constituting the whole measurement plane.
[0060] Also, the measurement of the amount of the FCC phase in X-ray diffraction was performed
using RINT 2500 manufactured by Rigaku Corporation. Co was used as a line source.
The amount of the FCC phase was calculated using a diffraction line intensity ratio
obtained from each plane of (200)α, (211)α, (200)γ, (220)γ and (311)γ under the conditions
of a voltage of 40 kV and a current of 200 mA.
[0061] Next, the intermediate materials A to C for stainless steel for edged tools were
annealed at 840°C for 5 hours. Thereafter, from each of the annealed materials, a
test piece was sampled such that the vicinity of the center in width of the rolled
material illustrated in FIG. 1 was contained and the vertical section serving as the
evaluated plane 2 became a metal structure observation plane. The photographs of the
metal structures of the annealed intermediate materials A, B, and C are shown in FIG.
2 to FIG. 4 respectively.
[0062] In the metal structure observation, the evaluated plane was polished to be a mirror
finished surface and then corroded with an aqueous solution of ferric chloride to
perform observation using a scanning electron microscope.
[Table 2]
Material |
KAM value (°) |
Full-width at half maximum of (200) plane of FCC phase (°) |
Amount of FCC phase (% by volume) |
Remarks |
Intermediate material A |
0.77 |
0.340 |
100 |
Present invention |
Intermediate material B |
0.48 |
0.247 |
100 |
Comparative Example |
Intermediate material C |
1.11 |
0.395 |
84.24 |
Present invention |
[0063] When the intermediate material for stainless steel for edged tools was annealed,
more carbides after annealing were distributed in a grain, as seen from FIG. 2 and
FIG. 4, in a case where a KAM value was 0.5° or more or a half-value width of the
(200) plane of the FCC phase in X-ray diffraction was 0.3° or more. Thus, it can be
understood that this intermediate material had a favorable structure. On the other
hand, as seen from FIG. 3, in a case where a KAM value was less than 0.5 or a half-value
width of the (200) plane of the FCC phase was less than 0.3°, coarser carbides were
precipitated at a grain boundary. In this metal structure, carbides are unlikely to
decompose during hardening. For this reason, it is concerned that the coarse carbides
retained after hardening causes toughness to decrease.
[0064] From the results described above, it was confirmed that, when annealing is performed
to the intermediate material for stainless steel for edged tools having a KAM value
of 0.5° or more, or to the intermediate material for stainless steel for edged tools
having a half-value width of the (200) plane of the FCC phase in X-ray diffraction
of 0.3° or more, there can be achieved the metal structure of stainless steel for
edged tools that is suitable for the edged tools such as razors.
INDUSTRIAL APPLICABILITY
[0065] Stainless steel for edged tools manufactured using an intermediate material for stainless
steel for edged tools according to the present invention has a favorable carbide distribution.
Therefore, the present invention is applicable to razors or the like.
LIST OF REFERENCE NUMERALS
[0066]
- 1
- Intermediate material for stainless steel for edged tools
- 2
- Vertical section
- 3
- Rolled plane